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Bi2WO6 Nanocrystals with High Photocatalytic Activities Under Visible Light

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Abstract

Visible-light-induced photocatalyst Bi2WO6 with the size of ca. 30 nm has been synthesized via a facile template-free hydrothermal method. The effects of the pH values of the precursor suspensions and hydrothermal time on the photocatalytic activities of the Bi2WO6 nanoplates have been investigated in detail. The photocatalytic activity of as-prepared Bi2WO6 was about 8−10 times higher than that of the product prepared by solid-state reaction, which was evaluated by the degradation of RhB dye in water under visible light irradiation. Even under the illumination of a compact fluorescent lamp, the nanosized Bi2WO6 also exhibited high photocatalytic activity.

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... El más sencillo de estos óxidos con un n = 1 es el Bi 2 WO 6 , que resulta típicamente en una nanoestructura del tipo nanohojuelas, es de sencilla preparación, alta estabilidad y excelente actividadad fotocatalítica bajo irradiación de luz solar, que se ha constituido como un catalizador de alto interés en el ámbito de la fotocatálisis heterogénea y la química ambiental (Shang, et al., 2008). Este sólido, que además es un semiconductor con una energía interbandas de 2,5 eV, la cual le confiere actividad fotoquímica bajo irradiación en el rango visible del espectro electromagnético, tiene Revista de la UniveRsidad del ZUlia 3 a época. ...
... Ciencias Exactas, Naturales y de la Salud 23 una estructura cristalina ortorrómbica debido a la celda unitaria del cristal mostrada en la Figura 2 (Xu, et al., 2009, Shang, et al., 2008, Zhang y Zhou, 2012, Wolfe y Newnahm 1969, Lv, et al., 2016. ...
... El dopaje del tungstato de bismuto con elementos no metálicos como N ha demostrado ser prometedor no sólo para aumentar el rango de absorción de luz en la región visible, sino también para inhibir la recombinación de los pares electrón -hueco incrementando la velocidad de transferencia de los electrones fotogenerados a la superficie del catalizador que podría estar ligada a la mejora en la eficiencia de la actividad fotocatalítica (Shang, et al., 2008). La mejora en la respuesta en el visible de la fotocatálisis está relacionada con que el dopaje con nitrógeno resulta en la formación de un nivel de energía dopante en el fondo de la banda de conducción del Bi 2 WO 6 . ...
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After reviewing the chemical aspects of the environmental applications of heterogeneous photocatalysis under solar irradiation based on TiO2 and Bi2WO6. The mechanism for free radicals formation, strategies for improving energy conversion processes based on the solid materials and the modification of the catalyst surface by the adsorption of specific molecules are discussed. Aspects associated with the growing research on photocatalytic systems based on the simplest Aurivillius oxide: Bi2WO6 are studied, considering the key concepts that define the solar to chemical energy conversion and finally, various perspectives about environmental applications using the photoelectrocatalytic technology based on nanomaterials are exposed.
... The special electronic structure (hybrid of O 2p and Bi 6s) helps the valence band to be widely dispersed and results narrow band gap of the semiconductor (2.8 eV) able to absorb visible light. The hybrid system of the valence band is also favourable for the high mobility of photogenerated holes towards the surface of the semiconductor, which is crucial for photodegradation of pollutants [Shang, 2008;Zhang, 2014]. ...
... Among those methods, hydrothermal synthesis is widely used due to its simplicity of experimental process, effective to control the size and morphology, low cost, high yield, low temperature and large-scale method. The synthesis of Bi 2 WO 6 with various morphologies as nanoplates, flower-like shape, nanoparticles , nanocrystals [Shang, 2008], single crystals [Kania, 2016], nanocuboids [Kaur, 2016], microdiscs , hierarchical rose-like [He, 2012], hierarchical flower [Dumrongrojthanath, 2013], nanocages [Shang, 2009a] under different preparation methods has been reported. In fact the shape, size, structure, morphology and pore structure of the semiconductor could be controlled during the synthesis procedure by several variables; for instance, the pH of the precursor suspensions, temperature, thermal duration, surfactant, calcination, and so on Mi, 2012;Zhang, 2007b]. ...
Thesis
Au début des années 1970, Fujishima et Honda ont rapporté la première réaction induite par la lumière sur une électrode de TiO2 sur le dégagement d'oxygène et d'hydrogène dans une solution aqueuse. Depuis, l'application des réactions photocatalytiques s'est étendue dans différents domaines, en particulier dans l'assainissement de l'environnement en phase aqueuse et gazeuse. Les réactions photocatalytiques peuvent être utilisées pour la dégradation de polluants organiques (par exemple, COV, aromatiques, colorants) et inorganiques (par exemple, NO3-, Cr6+), pour la décomposition de l'eau en hydrogène et oxygène, la réduction du dioxyde de carbone, la désactivation des micro-organismes, ces exemples étant parmi les plus représentatifs.Les photocatalyseurs les plus courants sont des matériaux semi-conducteurs tels que TiO2, ZnO, Bi2WO6, WO3, Fe2O3, SnO2, CdS, ZnS, etc. Le TiO2 est un semi-conducteur de type n avec une bande interdite optique de 3,2 eV, aujourd'hui considéré comme le matériau de référence pour les applications photocatalytiques dans l'assainissement de l'environnement. D'autre part, Bi2WO6 est également un semi-conducteur de type n qui présente de bonnes caractéristiques d'absorption de la lumière visible, avec une bande interdite de 2,9 eV, une activité photocatalytique élevée et une bonne stabilité.Malgré le fait que les processus photocatalytiques présentent de nombreuses applications, leur mise en œuvre à grande échelle est limitée en raison de la faible absorption généralement de la plupart des semi-conducteurs sous lumière visible, des taux de recombinaison des porteurs de charge élevés et des phénomènes de photocorrosion qui affectent leur stabilité sous un éclairage à long terme.Afin d'augmenter la photoefficacité des semi-conducteurs, plusieurs approches ont été proposées; parmi elles, l'incorporation de matériaux carbonés comme additifs aux semi-conducteurs s'est révélée être une alternative intéressante. Une grande variété de matériaux carbonés a été utilisée à cet effet pour plusieurs réactions photocatalytiques, avec d'excellentes performances photocatalytiques des composites semi-conducteur/carbone. Cela a été attribué à plusieurs aspects : une meilleure absorption de la lumière visible du composite quand l’additif de carbone est incorporé, l'effet bénéfique du nanoconfinement des polluants ciblés dans la porosité des matériaux en carbone, les forts effets électroniques interfaciaux entre le carbone et le semi-conducteur, une séparation électron-trou efficace en raison de la faible longueur de diffusion à travers la porosité du carbone, ou la capacité de l'additif de carbone à agir comme accepteur des porteurs de charge photogénérés, réduisant ainsi le taux de recombinaison.Plus récemment, l'activité photochimique intrinsèque des matériaux carbonés a été démontrée, en raison de leur capacité à former des espèces réactives (par exemple, des radicaux hydroxyles, des superoxydes, des radicaux centrés sur le carbone) lorsqu'ils sont exposés à une illumination dans des environnements contrôlés. Lorsque les carbones sont illuminés, il est proposé que l'excitation électronique se produise dans les transitions π-π* et σ–π* impliquant des sites en zig-zag libres et des sites de type carbène. Une telle activité photocatalytique est influencée par la structure des pores (c'est-à-dire les effets de nanoconfinement), la chimie de surface et la composition du matériau carboné (c'est-à-dire la présence de groupes chromophores).Dans ce contexte, l'objectif principal de cette thèse était d'explorer l'application de matériaux carbonés d'origines et de caractéristiques différentes comme additifs aux semi-conducteurs pour la dégradation photocatalytique des polluants environnementaux. Une série de carbones issus de diverses sources (charbon bitumineux, polysaccharides, lignocellulosiques, nanotubes de carbone) ont été sélectionnés comme additifs en raison de leurs différentes caractéristiques. Le dioxyde de titane disponible dans le commerce et le tungstate de bismuth synthétisé en laboratoire ont été utilisés comme semi-conducteurs. Les photocatalyseurs semi-conducteur/carbone ont été préparés en incorporant une faible charge de l'additif de carbone (2% en poids) suivant deux approches: (i) mélange physique des deux composants, (ii) synthèse hydrothermale en un seul pot du semi-conducteur en présence de l'additif de carbone. Selon la nature des matériaux carbonés et la méthode de préparation, le mélange présentait des caractéristiques physico-chimiques, une réponse photoélectrochimique et une activité photocatalytique différentes.Tout d'abord, les spectres de réflectance diffuse des photocatalyseurs préparés ont été mesurés, pour étudier leurs caractéristiques optiques. Les données ont montré la présence de groupes fonctionnels optiquement actifs (c'est-à-dire impliquant des hétéroatomes O-, N-, S-) dans certains des matériaux carbonés, ce qui permet une augmentation de la capacité d'absorption de la lumière des composites dans le domaine visible. Les composites préparés par synthèse hydrothermale étaient de couleur claire et présentait une réflectance légèrement plus élevée que les composites obtenus par mélange physique des deux composants. Cette tendance était plus prononcée pour les composites impliquant des additifs de carbone hautement fonctionnalisés. La valeur énergétique de la bande interdite des mélanges déterminée par la méthode d'ajustement linéaire double est légèrement plus élevée quelle que soit la nature du carbone et la méthode de préparation que pour le semi-conducteur nu. Les valeurs de bande interdite peuvent être corrélées à la réflectance de l'échantillon: des valeurs de réflectance plus faibles représentent des bandes interdites plus élevées, et vice versa.Concernant la porosité des composites semi-conducteur/carbone, et quelles que soient les différentes caractéristiques poreuses des additifs de carbone utilisés, tous présentent des isothermes d'adsorption d'azote de type II(b), caractéristiques de matériaux à faible porosité. Ceci est associé à la faible teneur en additif carboné, comme mentionné ci-dessus. Néanmoins, la présence de l'additif de carbone a augmenté les paramètres de texture du composite semi-conducteur/carbone résultant; cet effet était plus remarquable dans le cas de ces matériaux de carbone avec une structure de pores bien développée. Concernant la dispersion des additifs de carbone dans les composites obtenus par mélange physique des deux composants, les particules des additifs se sont révélées être bien dispersées au sein des nanoparticules dominantes du semi-conducteur (typiquement de plus petites tailles), indiquant un bon contact entre le carbone et le semi-conducteur. Lorsque les composites ont été préparés par un procédé hydrothermal (par exemple, pour les composites Bi2WO6/carbone), les nanoparticules de semi-conducteur Bi2WO6 étaient plus grosses, en raison de la présence des additifs de carbone pendant la synthèse. Quoi qu'il en soit, une bonne dispersion de l'additif carboné a également été observée, suggérant un bon contact entre les deux phases. Les méthodes de préparation ont montré un effet sur la structure cristalline, avec des tailles de cristaux plus faibles des composites Bi2WO6/carbone synthétisés par méthode hydrothermale que le Bi2WO6 nu, soulignant une légère distorsion du réseau du semi-conducteur en raison de la présence de carbone lors de la synthèse. D'autre part, la nature acide/basique du carbone a également influencé la nature des composites semi-conducteur/carbone résultants. Tous les composites préparés par méthode hydrothermale présentent un caractère plus acide quelle que soit la nature du carbone, que les composés correspondants préparés par mélange physique.La réponse photoélectrochimique des matériaux semi-conducteurs/carbone a été évaluée en solution aqueuse. À cet effet, des électrodes ont été préparées sur un support conducteur (par exemple, un substrat en verre revêtu d'oxyde d'indium et d'étain) et analysées par voltamétrie cyclique, potentiel de circuit ouvert et chronoampérométrie. Les voltammogrammes des composites semi-conducteur/carbone mesurés dans le noir ont montré la forme typique du semi-conducteur, avec des densités de courant variables en fonction du type d'additif au carbone. Les densités de courant plus élevées dans toute la fenêtre électrochimique des voltammogrammes sont associées à la contribution de la double couche électrique formée dans la porosité des additifs carbonés. De plus, la méthode de préparation des composites a montré un effet sur les voltammogrammes. Les composites préparés par l’approche hydrothermale affichent des courants cathodiques plus élevés que leurs homologues obtenus par prélèvement physique mixte. Sous illumination, la photogénération de paires électron-trou dans la région de charge d'espace des composites a été confirmée par les photocourants anodiques enregistrés. L'amplitude des courants enregistrés dans le noir et du photocourant variait pour les différents matériaux. En général, les composites TiO2/carbone présentaient des valeurs de photocourant plus élevées que les composites Bi2WO6/carbone.Le potentiel de circuit ouvert des électrodes constituées de semiconducteurs uniquement a montré des résultats chargés positivement, indiquant le transfert d'électrons du semi-conducteur à la solution d'électrolyte lors de l'immersion dans l'électrolyte. Pour la plupart des électrodes à semi-conducteur/carbone, le potentiel de circuit ouvert s'est déplacé vers des valeurs négatives par rapport au semi-conducteur, indiquant le transfert d'électrons du carbone vers le semi-conducteur. Ceci était en accord avec le résultat observé dans le voltammogramme cyclique. Lorsque les électrodes ont été éclairées, le potentiel a chuté à des valeurs plus négatives en raison de l'augmentation de la population d'électrons dans la bande de conduction du semi-conducteur due à l'injection des électrons photogénérés. La goutte photopotentielle était reproductible sur plusieurs cycles marche/arrêt consécutifs. L'ampleur et la vitesse de la goutte photopotentielle dépendaient de la nature du carbone et de la méthode de préparation des composites. Toutes les électrodes TiO2/carbone présentaient une amplitude de chute photopotentielle plus élevée que le TiO2 nu, tandis que toutes les électrodes de Bi2WO6/carbone présentaient une amplitude de chute photopotentielle plus faible que le Bi2WO6 nu. En termes de séparation de charge, les semi-conducteurs seuls ont montré des valeurs de constante de vitesse plus rapides que les composites, quels que soient le type de semi-conducteur, la nature des additifs de carbone et le procédé de préparation des composites.Les transitoires de photocourant des électrodes à semi-conducteur/carbone enregistrés lors de l'éclairage marche/arrêt présentaient la forme typique des semi-conducteurs de type n, avec une augmentation du photocourant lors de l'activation de l'éclairage qui se rétractait à sa valeur d'origine lorsque la lumière était éteinte. Les amplitudes de photocourant des électrodes semi-conductrices/carbone impliquant des carbones faiblement fonctionnalisés présentaient des valeurs plus élevées que celles mesurées pour le semi-conducteur seul; en revanche, le carbone hautement fonctionnalisé a montré des valeurs inférieures ou égales à celles du semi-conducteur nu. La conductivité électrique et la mobilité électronique de la matrice de carbone se sont avérées être une force motrice pour augmenter l’amplitude du photocourant. Ceci était plus remarquable dans les composites TiO2/carbone que dans les composites Bi2WO6/carbone. Toutes les électrodes TiO2/carbone ont montré un photocourant anodique quel que soit le potentiel appliqué. D'autre part, la plupart des électrodes Bi2WO6/carbone présente un photocourant anodique pour des potentiels appliqués positifs et des photocourants cathodiques pour des potentiels appliqués négatifs. Cela suggère que les différents types de réactions photoélectrocatalytiques se sont produits à l'interface électrode/électrolyte en conditions d'éclairage, ce qui s'est avéré dépendant du potentiel appliqué pour le même matériau, en particulier pour Bi2WO6/carbone. Le photocourant anodique correspond à l'oxydation de l'eau, puisque l'eau est le seul donneur d'électrons au trou de l'électrolyte aqueux, tandis que le photocourant cathodique est attribué à la photoréduction de l'oxygène dissous dans l'électrolyte.Enfin, l'activité photocatalytique des matériaux a été évaluée pour la dégradation de la rhodamine B choisi comme polluant modèle. Le colorant a été imprégné sur les films photocatalytiques coulés sur un substrat en verre à partir de solutions aqueuses, et laissé sécher dans l'obscurité. Avant l'illumination des films photocatalytiques, leur capacité d'adsorption a été évaluée. Les données ont montré qu'une faible quantité (environ <5 μg) du colorant était adsorbée dans tous les composites étudiés. Cette faible capacité d'adsorption des catalyseurs est en accord avec la structure poreuse mesurée. Ensuite, le test photocatalytique a été réalisé en éclairant les films à l'aide d'une lumière solaire simulée. La décomposition photolytique du colorant en l'absence d'un photocatalyseur a montré une conversion non négligeable (environ 80%) après 60 minutes d''illumination. En présence d'un photocatalyseur, l'élimination complète du colorant a été observée après 10 minutes d'illumination; une légère différence de performance a été observée parmi les photocatalyseurs à des temps d'illumination courts. Tous les composites TiO2/carbone ont montré des performances légèrement meilleures que le semi-conducteur nu. D'autre part, les composites Bi2WO6/carbone préparés à partir de mélanges physiques ont montré des performances légèrement inférieures à celles du Bi2WO6. En revanche, les composites Bi2WO6/carbone synthétisés par un procédé hydrothermal ont montré une activité photocatalytique plus élevée que leur composite correspondant préparé par mélange physique. Des expériences réalisées en présence d'agents chimiques désactivateurs ont confirmé la formation de radicaux hydroxyles et de trous lors de l'irradiation des composites, vraisemblablement impliqués dans la dégradation photocatalytique du colorant. En raison de la nature acide de la plupart des catalyseurs, une forte interaction avec la forme cationique de la rhodamine B est attendue; en conséquence, la dégradation photocatalytique s'est réalisée par la voie de N-déséthylation, ce qui conduisait à un grand nombre d'intermédiaires. La différence de performance photocatalytique des composites était plus prononcée pour la formation et la dégradation de ces intermédiaires. Dans la plupart des cas, le semi-conducteur/carbone a montré une dégradation photocatalytique améliorée des intermédiaires par rapport au semi-conducteur nu. Cette amélioration était plus prononcée pour les catalyseurs comprenant des additifs de carbone acides ou/et microporeux.Les matériaux composites ont également montré une bonne performance photocatalytique pendant les cycles consécutifs (jusqu'à cinq cycles) avec une conversion rapide et presque complète du colorant dans tous les cycles. Ceci a confirmé l'absence de perte d'activité significative des catalyseurs après des cycles consécutifs, soulignant que les phénomènes de photocorrosion sont négligeables. La stabilité des photocatalyseurs a également été confirmée par l'absence de changements dans les photocatalyseurs après des cycles consécutifs et une irradiation à long terme (en termes de texture et de nature hydrophobe/hydrophile).L'activité photocatalytique des matériaux a également été évaluée pour la photoréduction du dioxyde de carbone (CO2) en milieu gazeux. Le CO2 gazeux a été introduit dans le photoréacteur contenant le film catalytique, suivi d'un éclairage à l'aide d'un simulateur solaire. La photoréduction de CO2 a été analysée par GC/MS et capteur numérique, montrant la production de gaz CO même si la quantité de CO est restée trop faible par rapport à la concentration initiale de CO2.
... The presence of a higher α-MnO2 content (3.78%) in the MnO2/GAC-3 compos and the increase in BET surface area led to better removal results [86]. Crystal size cou also influence the electron-hole recombination [87], thus generally decreasing the pho catalytic activity as the crystal sizes increased. Therefore, the MnO2/GAC-3 size of 12 nm was better than MnO2/GAC-1 (18.06 nm) and MnO2/GAC-2 (14.30 nm). ...
... The presence of a higher α-MnO 2 content (3.78%) in the MnO 2 /GAC-3 composite and the increase in BET surface area led to better removal results [86]. Crystal size could also influence the electron-hole recombination [87], thus generally decreasing the photocatalytic activity as the crystal sizes increased. Therefore, the MnO 2 /GAC-3 size of 12.79 nm was better than MnO 2 /GAC-1 (18.06 nm) and MnO 2 /GAC-2 (14.30 ...
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This work presents an efficient method for treating industrial wastewater containing aniline and benzothiazole, which are refractory to conventional treatments. A combination of heterogeneous photocatalysis operating in a fluidised bed reactor is studied in order to increase mass transfer and reduce reaction times. This process uses a manganese dioxide catalyst supported on granular activated carbon with environmentally friendly characteristics. The manganese dioxide composite is prepared by hydrothermal synthesis on carbon Hydrodarco® 3000 with different active phase ratios. The support, the metal oxide, and the composite are characterised by performing Brunauer, Emmett, and Teller analysis, transmission electron microscopy, X-ray diffraction analysis, X-ray fluorescence analysis, UV–Vis spectroscopy by diffuse reflectance, and Fourier transform infrared spectroscopy in order to evaluate the influence of the metal oxide on the activated carbon. A composite of MnO2/GAC (3.78% in phase α-MnO2) is obtained, with a 9.4% increase in the specific surface of the initial GAC and a 12.79 nm crystal size. The effect of pH and catalyst load is studied. At a pH of 9.0 and a dose of 0.9 g L−1, a high degradation of aniline and benzothiazole is obtained, with an 81.63% TOC mineralisation in 64.8 min.
... The crystal size varies between 11.24 and 21.65 nm ( Table 2). The nanoparticles with smallest crystallite sizes (~11-12 nm) were obtained through the reactions achieved at pH 8 and 10 (V1-V3 samples), while the pH variation at more extreme values (4 or 12) lead to a doubling of the crystallite dimension (~20 nm) of the nanoparticles (V4-V6 samples) If CeO2 NPs are intended to be use as photocatalysts, the ones with a smaller crystallit size are preferred, since the charge migration pathway to the surface is reduced compared to the ones with larger dimensions, thus resulting in a decrease in the recombination rat of the charge carriers [44,45]. Moreover, it can be observed that the increase in the reaction time (in the case of V3 and V6 samples) does not cause a significant change in the size o the crystallites. ...
... The nanoparticles with smallest crystallite sizes (~11-12 nm) were obtained through the reactions achieved at pH 8 and 10 (V1-V3 samples), while the pH variation at more extreme values (4 or 12) leads to a doubling of the crystallite dimension (~20 nm) of the nanoparticles (V4-V6 samples). If CeO 2 NPs are intended to be use as photocatalysts, the ones with a smaller crystallite size are preferred, since the charge migration pathway to the surface is reduced compared to the ones with larger dimensions, thus resulting in a decrease in the recombination rate of the charge carriers [44,45]. Moreover, it can be observed that the increase in the reaction time (in the case of V3 and V6 samples) does not cause a significant change in the size of the crystallites. ...
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Cerium oxide (CeO2) nanoparticles were synthesized with a chemical precipitation method in different experimental conditions using cerium nitrate hexahydrate (Ce(NO3)3·6H2O) as a precursor, modifying the solution pH, the reaction time, and Co atoms as dopants, in order to tune the band gap energy values of the prepared samples. The physical characteristics of the synthesized ceria nanoparticles were evaluated by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–Vis analyses and photoluminescence measurements. XRD data revealed a pure cubic fluorite structure of CeO2 NPs, the estimation of crystallite sizes by Scherrer’s formula indicates the formation of crystals with dimensions between 11.24 and 21.65 nm. All samples contain nearly spherical CeO2 nanoparticles, as well as cubic, rhomboidal, triangular, or polyhedral nanoparticles that can be identified by TEM images. The optical investigation of CeO2 samples revealed that the band gap energy values are between 3.18 eV and 2.85 eV, and, after doping with Co atoms, the Eg of samples decreased to about 2.0 eV. In this study, we managed to obtain CeO2 NPs with Eg under 3.0 eV by only modifying the synthesis parameters. In addition, by doping with Co ions, the band gap energy value was lowered to 2.0 eV. This aspect leads to promising results that provide an encouraging approach for future photocatalytic investigations.
... Bi 2 WO 6 photocatalysts were fabricated according to widely reported hydrothermal synthesis methods [4,9]. In a typical process, aqueous solutions of Na 2 WO 4 2H 2 O (5 mM) and Bi(NO 3 ) 3 5H 2 O (10 mM), were mixed together maintaining 1:2 M ratio until reaching 200 mL. ...
... The band gap of the synthesized Bi 2 WO 6 was E g ¼ 2.53 eV, calculated from Kubelka-Munk theory [42], as indicated by the linearization of the spectrum according to the Tauc plot presented in Fig. 1(F). These features are consistent with data reported by Shang and coworkers for their synthesis under similar conditions [9], representing the quality of the material that will be used in the kinetic measurements reported below. ...
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Glucose solar light photoinduced oxidation on Bi2WO6 and the chemical kinetics conditions for concurrent photoelectrochemical H2 production are reported. The results show that the conversion of this organic compound is determined by their surface concentration according to the Langmuir-Hinshelwood mechanism. The performance of Bi2WO6 powder for the photocatalytic oxidation of glucose is higher than that observed with TiO2-based materials. Glucose degradation and mineralization rates are similar; therefore, stable intermediates are not formed during glucose oxidation. Photoluminescence studies indicate that glucose promotes electron injection into the valence band of semiconductor. The initial glucose concentration in combination with the electrode potential used, determines the H2 production. In fact, electrode potential of 0.9 V vs. SHE and 60 ppm of glucose defines kobs values equal to kK with maximum H2 evolution rate: 3.05 μmol h⁻¹ cm⁻². This value arises from the transformation of the 97% of the Faradaic current measured. The phenomenological conditions for renewable energy applications have been envisaged.
... From this point onward, more and more photocatalytic materials, sensitive under visible light, have been discovered. For example, Sheng et al. reported that Fe2O3 had a 66% photodegradation efficiency for MB over a period of 120 min with the assistance of visible light [5]; Liang et al. degraded 74.2% of Orange II under 120 min of visible light by using a CdS photocatalyst [6]; and Meng et al. employed a Bi2WO6 photocatalyst to remove 95% of Rhb while exposed to 60 min of visible light [7]. ...
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In this study, La2O3/g-C3N4 heterojunction photocatalysts doped with different dosages of La2O3 were constructed by a facile ultrasound-assisted calcination approach. The as-prepared photocatalysts were characterized by XRD, FTIR, FESEM, TEM, XPS, PL and DRS to verify the composite photocatalysts’ purity and to investigate their structural, morphological and elemental composition, and their energy band. According to the results, a type of pure rod–sheet-shaped, heterostructured nanoparticle was successfully obtained. Decorated with 10% La2O3, 2 g/L of the composite sample had a 93% degradation rate for 20 mg/L tetracycline hydrochloride within 2 h under visible light at a pH of 7. After four successive photocatalytic runs, satisfactory stability and reusability was exhibited, with 70% of the tetracycline hydrochloride being removed in the final experiment. Electrons (e−), photogenerated holes (h+), superoxide radical anions (O2−) and hydroxyl radicals (OH) were the fundamental active species during the photocatalytic process and were investigated via quenching experiments. Furthermore, possible photocatalytic mechanisms were analyzed in this work.
... It is known that the time of diffusion of charge carriers from the bulk to the surface of the photocatalyst is directly proportional to the squared radius of the particle. 52,53 It means that a large crystallite size leads to an increase the diffusion time. The probability of recombination of electron/hole increases. ...
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Doping of TiO2 with various elements increases its photocatalytic activity due to the formation of new energy levels near the conduction band. Photocatalysis involving titanium dioxide is a heterogeneous process in which the surface of the catalyst also plays an important role. The structural properties of TiO2 are influenced by the synthesis method, the doping method, and the dopants. In this work, we compare different doping elements for the photocatalytic activity of titanium dioxide, which synthesized by the sol-gel method. As a doping method, low-temperature underwater plasma was used. Al, Cu, Mo, and W acting as electrodes were chosen as doping elements. The obtained samples were characterized by various techniques. The incorporation of elements leads to distortion of the TiO2 crystal lattice, changing its surface characteristics, and a decrease in the band gap. The introduction of aluminum and copper increases the photocatalytic activity to 70% while doping with Mo and W increases the activity to 96% for 60 minutes of visible light irradiation. Explanations of the effect of various doping elements on the photocatalytic activity of titanium dioxide are presented.
... The average diffusion time (Ʈ) of charge carriers from bulk to surface of a photocatalyst is expressed by Eq. (1)[59]. ...
... In Table 5 and Figure (ii) The low photocatalytic activity of the cobalt ferrite and the nanocomposites CFO/Ag-NPs is an issue, as the band gap is narrower than the other ferrites investigated. It is reported that the photocatalytic activity of photocatalysts is affected by crystal size [37], the method for preparation determining the size and by that the activity [38]. The change in the optical and the photocatalytic properties can be attributed to a change in the population of the Co 2+ at tetrahedral and octahedral sites [39]. ...
Article
Ferrites of the type MFe2O4, where M = Zn, Co, and Mg, were prepared via a sol-gel procedure using propylene oxide as a gelating agent. Silver nanoparticles with an average size of 6 nm were prepared in the presence of the ferrite powders in suspension using raffinose as a reducing agent in a basic water solution. Nine samples of the ferrite/silver nanoparticles nanocomposites were prepared in this way with different Ag content. The composites obtained were characterized by XRD, TEM, and UV/Vis spectroscopy. The size distribution of the particles was determined using the data from TEM. By TEM-EDAX, the location of the silver nanoparticles on the ferrite surfaces was determined. Using UV/Vis spectra, the band gap energies for the pure ferrites and the nanocomposites were calculated; the calculated band gap energies were influenced by the presence of the silver nanoparticles. The photocatalytic activity of the silver nanoparticles/ferrite nanocomposites for decomposition of Malachite Green in model water solutions under UV and visible light irradiation was determined. The ZnFe2O4/Ag-NPs nanocomposites were found to be active under visible light irradiation, while the MgFe2O4/silver nanoparticles composites especially active more than the pure MgFe2O4. The presence of silver nanoparticles did not influence the very low photocatalytic activity of CoFe2O4.
... The different shapes of Bi 2 WO 6 samples have been synthesized by tuning above parameters, which includes nanoplates, nanoparticles, and flower/sphere-like, nest/tyre/helix-like and nanocage-like superstructures, summarized in Figure 4C. [124][125][126][127][128][129] The different morphologies of Bi 2 WO 6 have obtained different structure properties related to the photocatalytic performance. Therefore, when those Bi 2 WO 6 were applied to degrade the organic pollutant and disinfect the bacteria, they exhibit structure/component-dependent photocatalytic performance. ...
Article
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Environmental pollution and energy crisis have become major challenges to sustainable development of human society. Solar-driven photocatalytic technology is regarded as an extremely attractive solution to environmental reme-diation and energy conversion. Unfortunately, practical applications of traditional photocatalysts are restricted owing to the poor absorption of visible light, insufficient charge separation and undefined reaction mechanism. Therefore , developing novel visible light photocatalysts and exploring their modification strategies are significant in the area of photocatalysis. Bi-based photocatalysts have attracted wide attention due to unique geometric structures , tunable electronic structure and decent photocatalytic activity under visible light. At present, Bi-based photocatalysts can be mainly classified as bismuth metal, binary oxides, bismuth oxyhalogen, multicomponent oxides and binary sulfides, and so forth. Although they can be used as independent photocatalysts for environmental purification and energy development, their efficiency is not ideal. Therefore, many efforts have been made to enhance their photocatalytic performance in the past few decades. Significant progresses in determining the fundamental properties of photocatalysts, improving the photocatalytic performance and understanding the photocatalytic mechanism in important reactions have been made benefited from the various new developed concepts and approaches. This review introduces the structural properties of Bi-based photocatalysts in detail and summarizes the design and modification strategy for improving the photocatalytic performance, including metal/ Peng Chen and Hongjing Liu contributed equally to this work.
... With the decrease of particle size, the diffusion time will be reduced [35,36]. Meanwhile, the recombination rate of the electrons and holes will also decrease because of the quick shift of electrons [37]. Furthermore, particle size directly impacts the specific surface area of photocatalysts, the surface area increases with decreasing of particle size [38]. ...
Article
Two-dimensionally layered structures are intriguing for the fundamental demand of opto-electronic devices and have broad applications due to their superior properties. In particular, layered heterostructures as promising materials in photocatalysis, have lots of active atoms exposed on the surface, which largely affect the photocatalytic performance. In this work, two-dimensional heterostructures were synthesized by a two-step hydrothermal method, in which monolayer BiPO4 chemically bonded on Bi2WO6 nano-sheets with Ag/Ag2O nanoparticles (BWO-AP). The two-step method significantly improved photocatalytic efficiency, compared with traditional one-step process. Compared to other controls, BWO-AP showed highest quantum efficiency and photocatalytic performances for O2 revolution in distilled water and Rh B degradation. It was ascribed to its fast photoinduced charge separation and transfer. Interestingly, there also was some amount of H2 generated in distilled water, and the increased Ag⁰ : Ag⁺ of BWO-AP after photocatalytic reaction could explain the consumption of photo-induced electrons from water splitting.
... As we all know, the red-shift of the wavelength always means band gap narrowing of the photocatalysts. Therefore, the plots of (ahn) 2 ...
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The three-dimensional flower-like Bi 2 WO 6 was synthesized through a one-step microwave method (the reaction temperature was 434 K and the reaction took 10 min) with the assistance of ethanolamine (EA). The prepared samples were characterized by X-ray diffraction, scanning electron microscopy, Fourier-Transform infrared spectroscopy, ultraviolet-visible spectroscopy, PL, X-ray photoelectron spectroscopy and Brunauer-Emmett-Teller analysis. Methyl orange was used as target pollutant to evaluate the photocatalysis property of samples. Furthermore, the influence of the mechanism of EA on the structure and catalytic performance of Bi 2 WO 6 was discussed. The detailed characterizations revealed that the threedimensional flower-like Bi 2 WO 6 was successfully synthesized with the assistance of EA. The results confirmed that EA significantly influenced the morphology of Bi 2 WO 6 products. The addition of EA can effectively alter the pressure of the reaction and improve the crystal phase and structure of Bi 2 WO 6 photocatalysts, enhancing the photocatalytic activity of samples and improving the photocatalytic efficiency. EA can serve as an assembling agent and structure-directing agent resulting in the formation of flower-like architectures. With the increase of the amount of EA, the as-prepared Bi 2 WO 6 sample gradually forms a flower-like structure, leading to a shorter time of light holes migrating to the surface of the catalyst. It makes the compound rate significantly decreased, and improves the photocatalytic efficiency of the sample.
... 46,47 However, long reaction time (up to 20-48 h) was usually needed for the hydrothermal/solvothermal stage. [37][38][39]48,49 In this study, a combined method of ultrasonic solvothermal treatment and high-temperature calcination was attempted to synthesize Bi 2 WO 6 with a shorter duration. The calcination temperatures were also investigated. ...
Article
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Bismuth tungstate (Bi2WO6) was successfully synthesized by a method combining ultrasonic solvothermal treatment and high-temperature calcination. The products were affirmed by X-ray diffraction, scanning electron microscopy, UV-vis diffuse reflection spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The characterization results indicated that calcination could improve the crystallinity and visible light utilization capacity of Bi2WO6. The photodegradation experiments showed that Bi2WO6 calcined at 450 °C for 3 h exhibited better photocatalytic activity for the degradation of norfloxacin and enrofloxacin under visible light irradiation than the catalyst prepared without calcination or calcined at other temperatures. Meanwhile, the effects of the amount of 450-Bi2WO6, the initial concentration of targets, and the pH of the solutions on the degradation were studied. Under the optimal conditions, the removal ratios reached to 92.95% (for norfloxacin) and 94.58% (for enrofloxacin) within 75 min. Furthermore, h+ and ·O2 - were identified to affect the photodegradation process significantly, and the possible photocatalytic mechanism was proposed. The as-prepared sample was verified to possess good stability and reusability, suggesting its potential application prospect in the treatment of fluoroquinolone antibiotics.
... Also, a low-energy barrier for electrical switching of ferroelastic domains, which involves a 90° polarization rotation, enables reversible control of in-plane strain in BWO thin films, which may be relevant for electromechanical and magnetoelectric device applications 6 . Besides its core appeal in ferroelectric applications, BWO is known for its excellent photocatalytic activity and ionic conductivity 7,8 . A recent theoretical study suggested that the band structure of BWO is Rashba spin-split 9 . ...
Article
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Aurivillius ferroelectric Bi2WO6 (BWO) encompasses a broad range of functionalities, including robust fatigue-free ferroelectricity, high photocatalytic activity, and ionic conductivity. Despite these promising characteristics, an in-depth study on the growth of BWO thin films and ferroelectric characterization, especially at the atomic scale, is still lacking. Here, we report pulsed laser deposition (PLD) of BWO thin films on (001) SrTiO3 substrates and characterization of ferroelectricity using the scanning transmission electron microscopy (STEM) and piezoresponse force microscopy (PFM) techniques. We show that the background oxygen gas pressure used during PLD growth mainly determines the phase stability of BWO films, whereas the influence of growth temperature is comparatively minor. Atomically resolved STEM study of a fully strained BWO film revealed collective in-plane polar off-centering displacement of W atoms. We estimated the spontaneous polarization value based on polar displacement mapping to be about 54 ± 4 μC cm −2 , which is in good agreement with the bulk polarization value. Furthermore, we found that pristine film is composed of type-I and type-II domains, with mutually orthogonal polar axes. Complementary PFM measurements further elucidated that the coexisting type-I and type-II domains formed a multidomain state that consisted of 90° domain walls (DWs) alongside multiple head-to-head and tail-to-tail 180° DWs. Application of an electrical bias led to in-plane 180° polarization switching and 90° polarization rotation, highlighting a unique aspect of domain switching, which is immune to substrate-induced strain.
... Also, a low-energy barrier for electrical switching of ferroelastic domains, which involves a 90° polarization rotation, enables reversible control of in-plane strain in BWO thin films, which may be relevant for electromechanical and magnetoelectric device applications 6 . Besides its core appeal in ferroelectric applications, BWO is known for its excellent photocatalytic activity and ionic conductivity 7,8 . A recent theoretical study suggested that the band structure of BWO is Rashba spin-split 9 . ...
Preprint
Aurivillius ferroelectric $Bi_2WO_6$ (BWO) encompasses a broad range of functionalities, including robust fatigue-free ferroelectricity, high photocatalytic activity, and ionic conductivity. Despite these promising characteristics, an in-depth study on the growth of BWO thin films and ferroelectric characterization, especially at the atomic scale, is still lacking. Here, we report pulsed laser deposition (PLD) of BWO thin films on (001) $SrTiO^3$ substrates and characterization of ferroelectricity using the scanning transmission electron microscopy (STEM) and piezoresponse force microscopy (PFM) techniques. We show that the background oxygen gas pressure used during PLD growth mainly determines the phase stability of BWO films, whereas the influence of growth temperature is comparatively minor. Atomically resolved STEM study of a fully strained BWO film revealed collective in-plane polar off-centering displacement of W atoms. We estimated the spontaneous polarization value based on polar displacement mapping to be about 54 $\pm$ 4 ${\mu}C cm^2$, which is in good agreement with the bulk polarization value. Furthermore, we found that pristine film is composed of type-I and type-II domains, with mutually orthogonal polar axes. Complementary PFM measurements further elucidated that the coexisting type-I and type-II domains formed a multidomain state that consisted of 90${\deg}$ domain walls (DWs) alongside multiple head-to-head and tail-to-tail 180${\deg}$ DWs. Application of an electrical bias led to in-plane 180${\deg}$ polarization switching and 90${\deg}$ polarization rotation, highlighting a unique aspect of domain switching, which is immune to substrate-induced strain.
... Where (r) is the grain's radius, and (D) is the diffusion coefficient of the carrier [59,60]. Therefore, a decrease in the grain radius would reduce the recombination of the photo-generated electron/hole pairs moving to the surface, and therefore improves the photocatalytic performance. ...
... With the decrease of particle size, the diffusion time will be reduced [35,36]. Meanwhile, the recombination rate of the electrons and holes will also decrease because of the quick shift of electrons [37]. Furthermore, particle size directly impacts the specific surface area of photocatalysts, the surface area increases with decreasing of particle size [38]. ...
Article
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Bi2WO6 photocatalyst possesses great photocatalytic properties, modification with Ag, Ag2O, and Ag3PO4 further improve its photocatalytic activity. Photocatalyst utilized as suspended powders in wastewater treatment imposes restrictions for practical application. Therefore, the design and fabrication of photocatalyst film with high efficiency and stability are crucial for practical application. In this study, polyethylene glycol (PEG) modified AP-BWO (A: Ag, Ag2O; P: Ag3PO4; BWO: Bi2WO6) photocatalyst film coated on glass substrate was firstly synthesized with excellent photocatalytic activity and stability. Bi2WO6based films were modified with different PEG molecular weight, dosage and coating layers, followed by characteristic analysis (SEM, TEM, XPS, UVvis, PL, and photocurrent density) and photodegradation test. Photodegradation showed that 2-layered of 12 g L-1 PEG2000 modified AP-BWO film exhibited the highest photocatalytic activity. XPS and TEM indicated that PEG2000-AP-BWO film was fabricated successfully. SEM and UV-vis denoted the photocatalyst film showed small particle size and strong visible-light absorption. Furthermore, PL and photocurrent density confirmed its low recombination and fast charge separation of electronhole pairs. The photocatalytic films showed high activity and stability in both 10-cycle repeatability experiments and tandem-type photocatalytic recycle system. Therefore, PEG2000-AP-BWO film with enhanced photocatalytic performance and stability was a promising alternative for wastewater treatment.
... Bi 2 WO 6 crystallizes with perovskite-type [WO 4 ] 2− layers sandwiched between [Bi 2 O 2 ] 2+ layers [6], which is considered to be in favored of the efficient separation of photo-generated carriers during the photocatalytic process [7]. With preferable band composition (∼2.8 eV) and the particular layered structure Bi 2 WO 6 is a promising visible-light-driven catalyst material on the degradation of organic compounds and O 2 evolution from water [8,9]. ...
Article
As a promising structure in enhancing the photo-catalytic performance of the materials, hollow-shell structure still faces the great challenge of constructing an effective architecture for catalytic application. Herein, multi-shelled hollow microspheres of the visible-light photo-catalyst Bi2WO6 with shells up to the triple (BWO-MSH), have been synthesized by using a facile and effective carbonaceous template route. Intriguingly, the photo-catalytic reaction rate constant of BWO-MSH sample is nearly 16 times higher than the Bi2WO6 prepared through solid-state reaction (BWO-SSR). The single-, double-shelled Bi2WO6 microspheres (Broken) could only be synthesized with lower quality (synthesized with many broken species), and their photo-catalytic performance was nearly the same poor as the BWO-SSR sample. Furthermore, multi-shelled hollow microsphere of Bi2O3/Bi2WO6 (BO/BWO-MSH) composite was also synthesized. The present work not only extends the scope of high-performance complex mixed metal oxides with multi-shelled microstructures, but also brings a good reference to engineer other impressive photo-catalyst.
... Layered bismuth-based semiconductor materials have recently sparked remarkable interest, owing to their suitable bandgap, photochemical stability and flexible tunability [5,7,18]. Among, BiOI possesses showed the most intensive light absorption, due to its narrowest bandgap (1.7-1.9 eV) [5]. ...
Article
Limited light absorption and severe charge recombination have been widely acknowledged as the two main obstacles restricting the application of P25 in NO photocatalytic conversion. In this work, an advanced heterojunction photocatalyst of Bi/BiOI/black TiO2 was synthesized via an in-situ solid-state chemical reduction method. The black TiO2 extended the light absorption to the full spectral region. It constructed a direct Z-scheme heterojunction with BiOI nanosheets, not only promoting the charge separation, but also preserving the strong oxidation ability of photogenerated holes in black TiO2 and strong reducing ability of photogenerated electrons in BiOI. Thus, more •O2⁻ and •OH radical could form when Bi/BiOI/black TiO2 was subjected to visible light irradiation. Besides, the introduced Bi metal nanoparticles could also enhance the light utilization and served as a cocatalyst to accept the electrons. Therefore, enhanced NO photocatalytic oxidation activity was obtained, with the optimal sample Bi/BiOI/black TiO2 exhibiting a high NO conversion efficiency of ∼70% and NOx removal of ∼45%. This work may provide a refreshing perspective for the design of heterojunction photocatalysts for efficient NO photocatalytic purification and other photocatalytic applications.
... Bi 2 WO 6 Surface Modification. The Bi 2 WO 6 used in this research was synthesized through the hydrothermal method reported by Shang et al. 14 The major characteristics of the obtained photocatalyst are summarized in the Supporting Information ( Figure S1), whereas further details can be found in our previous publication. 15 The adsorption isotherm of trans-4-stilbenecarboxaldehyde (4SCA) (Aldrich, 99%) on Bi 2 WO 6 (A BET = 10 m 2 /g) was obtained using 10 (±0.01) mL (V) of an aqueous solution of the aldehyde at different concentrations and 10.00 (±0.01) mg of Bi 2 WO 6 (m s ) by stirring at pH = 3 in the dark for 48 h. ...
... Bismuth tungstate (Bi 2 WO 6 ), known as one of the simplest Aruivillius oxides, has aroused much attention for its excellent properties in physics and chemistry acting as excellent photocatalytic and solar energy transfer materials [9,10]. Since Kudo and Hijii obtained Bi 2 WO 6 successfully via solid state reaction [9], researchers have made much effort on preparing various structures of micro/nano-Bi 2 WO 6 , such as sol-gel and hydro/solvo-thermal method [11][12][13][14][15]. However, there are several disadvantages that limit the application of pure Bi 2 WO 6 , including low photo-absorption efficiency and high recombination of photo-induced electron and hole pairs. ...
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Bi2WO6 was modified by two-dimensional g-C3N4 (2D g-C3N4) via a hydrothermal method. The structure, morphology, optical and electronic properties were investigated by multiple techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy spectra (XPS), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Ultraviolet-visible diffuse reflection spectroscopy (DRS), photocurrent and electrochemical impedance spectroscopy (EIS), electron spin resonance (ESR), respectively. Rhodamine B (RhB) was used as the target organic pollutant to research the photocatalytic performance of as-prepared composites. The Bi2WO6/2D g-C3N4 exhibited a remarkable improvement compared with the pure Bi2WO6. The enhanced photocatalytic activity was because the photogenerated electrons and holes can quickly separate by Z-Scheme passageway in composites. The photocatalytic mechanism was also researched in detail through ESR analysis.
... Furthermore, the catalyst particle size and surface area are directly related in a photocatalytic setting. Therefore, the mean diffusion time from the bulk to the surface of the photocatalyst is given as follows for randomly generated charge carriers [41]: ...
Article
Herein, we report on the efficient visible-light driven BiPW12O40/BiOI (BPWBI) photocatalyst that was prepared by combining different weight percentages of BiPW12O40 (1.0, 3.0, 5.0%) with BiOI via facile one−step wet impregnation method. The prepared BPWBI photocatalysts were characterized by XRD, FTIR, TGA, BET, FESEM, EDS/Mapping, TEM, UV−Vis DRS, and PL. The photocatalytic performance of the materials was assessed for the degradation of Bisphenol A (BPA) under visible light. The optimum composite photocatalyst (1.0BPWBI) exhibited an efficiency of 94.2% on BPA after 120 min at the optimum conditions (catalyst dosage = 20 mg, pH =6.68, and constant pollutant concentration = 15 mg L⁻¹). In the presence of peroxodisulfate (PDS), the efficiency was improved to 98.7% in 80 min whereas the efficiency was drastically reduced in the presence of acid blue 25 (AB 25) dye. Photoluminescence (PL) measurements, confirmed that 1.0BPWBI exhibited enhanced electron-hole separation and slow recombination rate of electron-hole pairs. The trapping experiments deduced that h⁺, and •O2⁻ were the most active species responsible for the degradation of BPA. Moreover, the photodegradation pathway was hypothesized based on the LC-MS results on transformation products in the presence of 1.0BPWBI. The impressive photocatalytic performance and stability proved that 1.0BPWBI catalyst renders itself as a promising and valuable photocatalyst for wastewater treatment and can be further explored in the degradation of other organic pollutants for environmental remediation.
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Herein, we demonstrated for the first time the fabrication of a novel nanosheet-assembled Bi2MoO6 micronbox via a simple one-step method. In particular, this new Bi2MoO6 framework structure with ultra-large surface area can endow the sample with strong light harvesting ability and abundant surface active sites and it exhibits remarkably enhanced photocatalytic activity toward organic dye degradation and nitrogen fixation under solar-light irradiation, far exceeding that of conventional Bi2MoO6 with a different structure.
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Bismuth tungstate (Bi2WO6), with narrow band gap (2.7 eV), can degrade pollutants with remarkable photocatalytic effect under visible light and gain interest among researchers in recent years. The structure of Bi2WO6 photocatalysts are mainly layered multistage structure with strong adsorption capacity and weak photocatalytic activity. Usually, The Bi2WO6 photocatalysts are prepared by hydrothermal method, which is not conducive to the separation, treatment and reuse in use. Therefore, it is necessary to design and synthesize Bi2WO6 with spatial network structure to improve its catalytic activity and reusability, because the structure has the characteristics of high specific surface area, many active points and strong flexibility. In this review, the electrospinning method of prepared different kinds of Bi2WO6 will be introduced, in which the theoretical basis and experimental procedures will be emphatically analyzed. Besides, their photocatalytic degradation activities under visible light will be summarized. This article is protected by copyright. All rights reserved.
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A series of SnO2 nanocrystallines decorated g-C3N4 architectures were synthesized using a facile solvothermal method. The structural, morphological, and optical properties of the as-prepared nanocomposites were characterized in detail, indicating that SnO2 nanocrystallines with diameter ∼ 4 nm were well-dispersed on the surface of g-C3N4. The photocatalytic activity of the composites was investigated by degrading rhodamine B (RhB) under visible light irradiation. The CNS2 heterostructure exhibits enhanced photocatalytic activity than bare SnO2 and g-C3N4. Kinetic study revealed a promising degradation rate constant of 0.0593 min⁻¹ for the CNS2, which is 118 and 7 times higher than that of pure SnO2 and g-C3N4, respectively. What’s more, the CNS2 still retained the photocatalytic activity after three cycle measurements. The enhanced photocatalytic performances of the nanocomposite may be due to its large surface area (116.2 m²/g), appropriate ratio of SnO2/g-C3N4 and the compact structure of the junction between the SnO2 nanocrystallines and the g-C3N4, which inhibits the recombination of photogenerated electrons and holes.
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(00l) facets exposed plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) powders with single crystal grain were obtained successfully by molten salt method. The plane-like grains were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy, photoluminescence spectroscopy and X-ray photoelectron spectroscopy. With modification by platinum nanoparticles as cocatalyst, these materials were tested as photocatalysts for water splitting under full arc light irradiation. The H2 evolution activities of the plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) prepared by molten salt method were almost an order of magnitude higher than that of the ABi2Nb2O9 (A=Ca, Sr, Ba) prepared by traditional solid state method. The plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) powders also have higher O2 evolution activities than the ABi2Nb2O9 (A=Ca, Sr, Ba) powders prepared by traditional solid state method. The oxidation and reduction catalytic sites on the plane-like ABi2Nb2O9 (A = Ca, Sr, Ba) were discussed in detail. With the temperature of molten salt increasing, the variation tendencies of the photocatalytic activities of CaBi2Nb2O9, SrBi2Nb2O9 and BaBi2Nb2O9 for water splitting were different from each other due to the different growth rhythm of the plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) grains. The CaBi2Nb2O9 and SrBi2Nb2O9 with orthorhombic lattice had a higher photocatalytic activity than BaBi2Nb2O9 with tetragonal lattice because of the strong distortion of NbO6 octahedra in the perovskite-like slabs of CaBi2Nb2O9 and SrBi2Nb2O9.
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The radioresistance of tumor cells is considered to be an Achilles’ heel of cancer radiotherapy. Thus, an effective and biosafe radiosensitizer is highly desired but hitherto remain a big challenge. With the rapid progress of nanomedicine, multifunctional inorganic nanoradiosensitizers offer a new route to overcome the radioresistance and enhance the efficacy of radiotherapy. Herein, poly(vinylpyrrolidone) (PVP)-modified Bi2WO6 nanoplates with good biocompatibility were synthesized through a simple hydrothermal process and applied as a radiosensitizer for the enhancement of radiotherapy for the first time. On the one hand, the high-Z elements Bi (Z = 83) and W (Z = 74) endow the PVP-Bi2WO6 with better X-ray energy deposition performance, and thus enhancing radiation-induced DNA damages. On the other hand, Bi2WO6 semiconductors exhibit significant photocurrent and photocatalytic-like radiocatalytic activity under X-ray irradiation, giving rise to the effective separation of electron/hole (e−/h+) pairs and subsequently promoting the generation of cytotoxic reactive oxygen species (ROS), especially hydroxyl radical (OH). The γ-H2AX and clonogenic assays demonstrated that PVP-Bi2WO6 could efficiently increase cellular DNA damages and colony formations under X-ray irradiation. These versatile features endowed PVP-Bi2WO6 nanoplates with enhanced radiotherapy efficacy in animal models. In addition, Bi2WO6 nanoplates can also serve as good X-ray CT imaging contrast agents. Our findings provide an alternative nanotechnology strategy for tumor radiosensitization through simultaneous radiation energy deposition and radiocatalysis.
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Transition metal spinel type mixed oxides (FeCo2O4, MnCo2O4 and ZnCo2O4) nano-catalysts have been synthesized via co-precipitation route. The samples were characterized using XRD and FE-SEM techniques to determine structure and morphologies respectively. Furthermore, FTIR and BET analysis were carried out in order to confirm the structure and calculate surface areas of the samples. The photocatalytic potency of synthesized nano-catalysts have been examined for aqueous solution of methylene blue under visible light. It is found that photocatalytic activity of simple transition metal oxides was enhanced by making their binary/ternary metal oxides. FeCo2O4 exhibited highest photocatalytic efficiency among the prepared spinel structures while MnCo2O4 possess lowest efficiency. The photocatalytic behavior of mixed transition metal oxides can be accredited to its spinel structure, small particle size and large surface area. XRD and BET results revealed the small crystallite and particle size of spinel mixed oxides. The small crystallite size of spinel mixed oxides nano-structures contributes to increased photocatalytic activity by increasing the surface area of photocatalyst, decreasing the chance of reunion of charge carriers. It is observed that these binary oxide nano-structures have potential applications in heterogeneous photo decay of environmental contaminants and can act as propitious materials for resolving environmental issues and establishing green environment.
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The selective detection of dangerous and explosive gases with high sensitivity is serious problem to safety of humans and industrial processes. Small sized nanoparticles aggregated into a nanospheres offer plenty of advantages such as large specific area, greater mass transportation, and diffusion of gas molecules deep inside the material towards efficient gas sensing applications. Herein, In2O3 nanoparticles aggregated into nanospheres were synthesized successfully through one step microwave hydrothermal method. The structure, morphology and chemical states of prepared In2O3 nanospheres were studied by XRD, TEM and XPS measurements. Furthermore, obtained In2O3 nanospheres were applied as a gas sensing layer for acetaldehyde (CH3CHO) detection. Acetaldehyde is an important volatile organic compound which is highly reactive, toxic and carcinogenic agent. Gas sensor based on In2O3 nanospheres shows high sensitivity for a wide range of CH3CHO concentration (1–100 ppm). Concentration dependent resistances during the exposure and oxidation reaction of CH3CHO gas were investigated. The obtained results in this work indicate that In2O3 nanospheres are prospective candidates for the fabrication of CH3CHO gas sensors.
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Flower-like hierarchical CdS QDs/Bi/Bi2WO6 heterojunction photocatalyst was synthesized by an in-situ reduction and depositing process.The samples were systematically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (UV–vis DRS), photoelectrochemical measurements and photoluminescence (PL). The results indicate that plasmonic Bi nanoparticles as the electron-conduction mediator with the size of 10 nm promote the charge transfer effectively between CdS nanoparticles and Bi2WO6 nanosheets. The photocatalytic performance tests indicate that the CdS QDs/Bi/Bi2WO6 composite exhibits an extraordinary enhanced photocatalytic activity than pure Bi2WO6, Bi/Bi2WO6 and CdS QDs/Bi2WO6 for Rhodamine B and formaldehyde photodegradation, respectively. The superior photocatalytic performance of the CdS QDs/Bi/Bi2WO6 composite should be attributed to the improved visible light absorption property by coupling with CdS QDs and the plasmonic Bi, and the efficient charge transfer of metallic Bi nanoparticles.
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Bi 2 S 3 nanoribbons were dispersed in situ on Bi 2 WO 6 ultrathin nanosheets with exposed {001} facet via a simple one-set hydrothermal route forming Bi 2 S 3 /Bi 2 WO 6 heterojunctions with high active surface, in which the quantity of Bi 2 S 3 nanoribbons was tuned by changing the amount of thiourea in solution. The obtained composites were characterized by various measurements and their photocatalytic activities under visible-light irradiation were also investigated. The 7%-Bi 2 S 3 /Bi 2 WO 6 nanocomposites exhibited excellent photocatalytic activities in degradation of ofloxacin (OFL). A plausible degradation pathway for OFL was proposed via combining LC-MS and GC–MS detections with Gaussian calculations. The probable photocatalytic and formation mechanism of catalysts were discussed on the basis of the obtained experimental results and density function theory (DFT) calculations. An enhanced photocatalytic mechanism attributed to the synergistic effect between the exposed {001} facet and heterojunction with shape-crystalline integrity, significantly improving band structure and visible light absorption, increasing reactive sites, and suppressing the recombination of charges. The findings provide new insights for designing highly active and stable heterojunction catalysts to remove environmental pollutants and promote solar energy conversion efficiently.
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The rare earth ion Yb3+ doped Bi2WO6 photocatalysts were synthesized by hydrothermal method. Moreover, XRD, XPS, FESEM, TEM, Ramam, N2 adsorption-desorption isotherm measurements and UV-vis diffusion reflectance spectra were used to characterize the Yb3+ doped Bi2WO6 photocatalysts. The morphology, specific surface area, and pore volume distribution were greatly affected after Yb3+ ions doping. Photocatalytic performance of Bi2WO6 was effectively enhanced after Yb3+ ions doping, 6% Yb3+ doped Bi2WO6 had the best photocatalytic performance, and 96.2% Rhodamine B was degradated after irradiated 30 min, which was 1.29 times that of the pristine one. The enhanced photocatalytic performance was due to the increased specific surface area, decreased energy band gap and inhibition of photoelectron-hole recombination after Yb3+ ions doping.
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Photocatalysis (PC) technology has received global attention due to its high potential of addressing both environmental and energy issues using only solar light as energy input. However, large-scale commercialization of...
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The alginate‐based calcium tungstate composite was fabricated by a facile and eco‐friendly one‐step crosslinking process combining with the freeze‐drying method. Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, scanning electron microscope, X‐ray diffraction, thermogravimetric analysis, UV–vis diffuse reflectance spectrum and photoluminescence (PL) were employed to characterize the structures, morphologies, thermal stabilities, and optical properties of the synthesized composite materials. The obtained results demonstrate that the alginate‐based calcium tungstate composite presented a special porous network architecture with tetragonal scheelite‐type calcium tungstate microspheres attached. A possible synthesis mechanism was proposed based on the experimental data. Moreover, the PL data confirmed the broadband emission of the composite with its maximum at 438 nm that located in the blue emitting region. This study provides the alginate‐based calcium tungstate composite with unique morphology and good thermal stability as well as lays a foundation for its application in the field of PL.
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The massive use of non-renewable energy resources by humankind to fulfill their energy demands is causing severe environmental issues. Photocatalysis is considered one of the potential solutions for a clean and sustainable future because of its cleanliness, inexhaustibility, efficiency, and cost-effectiveness. Significant efforts have been made to design highly proficient photocatalyst materials for various applications such as water pollutant degradation, water splitting, CO2 reduction, and nitrogen fixation. Perovskite photocatalyst materials are gained special attention due to their exceptional properties because of their flexibility in chemical composition, structure, bandgap, oxidation states, and valence states. The current review is focused on perovskite materials and their applications in photocatalysis. Special attention has been given to the structural, stoichiometric, and compositional flexibility of perovskite photocatalyst materials. The photocatalytic activity of perovskite materials in different photocatalysis applications is also discussed. Various mechanisms involved in photocatalysis application from wastewater treatment to hydrogen production are also provided. The key objective of this review is to encapsulate the role of perovskite materials in photocatalysis along with their fundamental properties to provide valuable insight for addressing future environmental challenges.
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Pollution of surface and groundwater resources with antibiotics creates problems for human health, meanwhile, because of development of antibiotic resistance in bacteria. Chlortetracycline (CTC) is a tetracycline antibiotic, for which different removal methods were developed. Therefore, this study evaluated the removal efficiency of CTC from aqueous solutions using a photocatalytic process. In this study, Bi2WO6 was synthesized by microwave method and Ce loaded on Bi2WO6 by wet impregnation method. The prepared photocatalysts were characterized by XRD, FTIR, SEM, BET/BJH, DRS analysis. The degradation of CTC was investigated by the photocatalysis under solar light. Among the photocatalysts, Ce/Bi2WO6 (8%) had the highest CTC degradation efficiency. Response surface methodology (RSM) with central composite design (CCD) was used to investigate pH, time (min), mass of catalyst Ce/Bi2WO6 (8%) (g), and concentration of CTC (mg/L). In optimal conditions at pH 3.19, time 201.07 min, mass of Ce/Bi2WO6 (8%) of 0.05 g, and concentration of CTC 24.99 mg/L the photocatalytic degradation percentages of CTC were found to be 99.99%. Isothermic studies have shown that Fritz–Schlunder isotherms provide the best fit with the least error with experimental data.
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The development of high-efficient photocatalysts plays an important role in the sustainable utilization of solar energy. Hollow nanostructured photocatalysts are vital for solar light utilization and charge carrier separation in photocatalytic processes. Therefore, the construction of hollow semiconductor photocatalysts is a promising strategy for preparing novel high-efficient photocatalysts. This paper reviews common hollow semiconductor nanomaterials, such as oxides, sulfides, nitrides, C3N4, MOFs, and their composite photocatalysts. The characteristics of hollow-structure photocatalysts, the application of solar energy conversion, and their understanding of the photocatalytic mechanism are also reviewed. In addition, future challenges will focus on designing and majorizing broadband response hollow-structure photocatalysts to further enhance solar energy conversion. Hollow semiconductor photocatalysts will have potential applications in the natural environment, and these synthesized strategies can also provide new possibilities for synthesizing other high-performance semiconductor photocatalysts.
Conference Paper
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In this study, the effect of calcination temperature towards the optical, morphological properties and photocatalytic degradation performance of the synthesized LaFeO3 were investigated. The precursor and synthesized LaFeO3 calcined at 500-900 ºC were characterized using X-ray diffractometer (XRD), Thermogravimetric analysis (TGA), UV-Vis spectrophotometer, Fourier Transform Infrared Spectroscopy (FTIR) and BET. The photocatalytic activity of the synthesized LaFeO3 was performed on oily wastewater. The photocatalytic degradation performance of LaFeO3-600 is 67%, 72% and 63% for 1000ppm, 10000ppm and 20000ppm respectively. This study verified that calcination temperatures have a major impact on the optical, structural and morphological properties of nanoparticles.
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We report on Lu2(WO4)3: Yb³⁺, Tm³⁺ upconversion nanoparticles prepared via modified Pechini method, which is low-cost in comparison with analogs and allows achieving the homogenous rare earth ions (Yb³⁺ and Tm³⁺) dispersion in the host matrix. The Tm³⁺ doping concentration effect on structural and luminescence properties has been studied in detail. The structure of the nanoparticles was characterized by XRD and Raman spectroscopy. Both analytical methods confirmed the formation of single phase Lu2(WO4)3: Yb³⁺, Tm³⁺ nanoparticles without any impurities. It was found that thulium ions are uniformly incorporated into the tungstate structure up to a concentration of 1%. SEM images showed that prepared samples consisted of weakly agglomerated nanoparticles with an average size of about 40–80 nm. Emission and excitation spectra of all Lu2(WO4)3: xTm³⁺, 10% Yb³⁺ samples contained characteristic transitions inside Tm³⁺ ions. Upconversion spectra demonstrated intense infrared line (³H4–³H6) as well as weak blue ¹G4–³H6 and two red (¹G4–³F4, ³F2,3–³F6) bands obtained upon 974 nm excitation. Optimal Tm³⁺ doping concentration was found to be 0.3% for both ³H4–³H6 and ¹G4–³F4 transitions. Change of chromaticity coordinates along with Tm³⁺ amount increase was also observed.
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Non-noble metallic Bi/Bi2WO6 nanocomposites were synthesized by one-step semi-solvothermal method with water - N-N Dimethylformamide (DMF) under mild conditions. The DMF solvent not only prevents the formation of impurities [Bi6O6(OH)3(NO3)3·1.5H2O] from Bi(NO3)3·5H2O hydrolysis, but also in situ reduces Bi³⁺ ions to metallic Bi. The DMF percentage ranges from 0 to 100%, and the Bi/Bi2WO6 nanocomposites prepared in a 50% DMF precursor (50-BWO) solution were the most effective in the degradation of rhodamine B (RhB) under simulated sunlight irradiation. The RhB degradation efficiency was about 2.98 and 71.8 times higher than those of pristine Bi2WO6 and Bi microspheres, respectively. Then, the photocatalytic mechanisms of Bi/Bi2WO6 were proposed based on DFT calculations and experimental results. The enhanced photocatalytic activity of Bi/Bi2WO6 results mainly from two factors: the surface plasmonic resonance (SPR) of metallic Bi (which enhances light absorption) and the formation of Ohmic junction between Bi microspheres and Bi2WO6 nanosheets. The ohmic junction facilitates the effective separation of photogenerated carriers, enhancing efficiency. Additionally, the ·O2– and h⁺ radicals are the main active species in the degradation of RhB. Last, the durability and stability of Bi/Bi2WO6 were verified by cyclic experiments.
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Visible-light-driven AgPd/Bi2WO6 nanocomposites containing different atomic ratios of Ag:Pd were studied for photodegradation of rhodamine B (RhB) under visible light irradiation. The AgPd nanoparticles attached on the surface of Bi2WO6 nanoplates were prepared by photoreduction deposition method. Phase, morphology, vibrational mode, chemical composition, valence state and optical absorption were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS), transition electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV–visible spectroscopy. In this study, bimetallic AgPd nanoparticles were fully deposited on top of orthorhombic Bi2WO6 nanoplates. 10% Ag0.9Pd0.1/Bi2WO6 nanocomposites show the highest photodegradation of RhB illuminated by visible radiation. ⋅OH and ⋅O2⁻ are the main radicals that play the role in degrading of RhB over 10% Ag0.9Pd0.1/Bi2WO6 nanocomposites.
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The escalation of pharmaceutical products development and sales in the last few decades have raised concerns about their effect on the environment, particularly water and consequently their removal from wastewater. Composites of cobalt oxide (Co3O4) and bismuth oxyiodide (BiOI) were synthesised and evaluated for degradation of ibuprofen (IBU) and effect of trimethoprim (TMP) under visible light irradiation. The properties of the as synthesised composites were evaluated using different techniques such as FTIR, SEM, TEM, XRD, BET, TGA, UV-Vis and PL. The self-assembled direct Z-scheme heterojunction showed high photocatalytic activity for degradation of ibuprofen (97.02 %) after adding 2 mg of TMP. TOC reduction of 85.68 ± 2.22 % at pH 11.3 was also calculated. The enhanced photocatalytic activity of the Z-scheme heterojunction was attributed to strong visible light absorption properties of the catalyst and improved separation of photoexcited charge carriers resulting from the built-in electric field formed between the BiOI microspheres and the Co3O4 wormy epitaxy established after equilibrium Fermi level was reached. The Z-scheme heterojunction is a promising material for the removal of pharmaceuticals in aquatic effluents.
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Heterostructure Pd/Bi2WO6 nanocomposites as visible-light-driven photocatalyst used for degradation of rhodamine B (RhB) were prepared by a sonochemical-assisted deposition method. Phase, morphology, atomic vibration mode, oxidation state of element and optical properties of pure Bi2WO6 and heterostructure Pd/Bi2WO6 nanocomposites were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, Fast-Fourier-Transform electron diffraction and X-ray photoelectron spectroscopy. The analyses showed a mixed phase of face-centered cubic Pd structure as a minor phase and orthorhombic Bi2WO6 structure as a major phase with Pd nanoparticles supported on top of Bi2WO6 nanoplates. The photocatalytic performance of Bi2WO6 and Pd/Bi2WO6 was tested through photodegradation of rhodamine B (RhB) under visible light irradiation. In this research, the loaded Pd nanoparticles greatly increased the photodegradation of RhB under visible light irradiation. Heterostructure 10% Pd/Bi2WO6 nanocomposites showed the highest photocatalytic activity for RhB degradation under visible light irradiation because of the formation of Schottky barriers and promotion of interfacial charge-transfer kinetics between metallic nanoparticles and semiconducting nanoplates.
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In a photocatalytic reaction, maintaining the high efficiency of photocatalyst under a low concentration of pollutants is a key challenge. In this work, a new 2D sodium titanate nanosheet encapsulated Ag2O-TiO2 (2D NTO/Ag2O-TiO2) p-n heterojunction photocatalyst is proposed to deal with this dilemma. Through a simple plasma electrolytic oxidation (PEO) treatment and ion exchange treatment, a classic Ag2O-TiO2 p-n heterojunction structure is prepared and used as the photoelectric conversion unit in the photocatalyst. Then, through a subsequent hydrothermal treatment, a 2D NTO film that serves as the adsorption unit in the photocatalyst can be produced on the surface of the Ag2O-TiO2 p-n heterojunction layer. Finally, the desired 2D NTO/Ag2O-TiO2 structure is formed. The photocatalyst exhibits superior photocatalytic performance including high degradation rate as well as excellent catalytic stability and durability by combining the high sunlight utilization efficiency and high photoelectric utilization efficiency of the Ag2O-TiO2 p-n heterojunction and the outstanding adsorption performance of the 2D NTO film. Therefore, the problem of photocatalytic slow kinetics under low pollutants concentration is perfectly solved. This work provides a new strategy for the structural design of high-performance photocatalysts.
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In the present study, additive-free, pH-driven, hydrothermal crystallization was used to obtain shape-tailored monoclinic BiVO 4 photocatalysts. The as-prepared BiVO 4 products were systematically characterized, uncovering their crystallographic, morphologic and optical properties, while their applicability was verified in the visible light-driven photodegradation of oxalic acid and rhodamine B. Monoclinic clinobisvanite was obtained in most cases, with their band gap values located between 2.1 and 2.4 eV. The morphology varied from large, aggregated crystals, individual microcrystals to hierarchical microstructures. It was found that the degradation efficiency values obtained in the case of oxalic acid were directly related to the presence of (040) crystallographic plane, while the degradation of rhodamine B was partially independent by the presence of this structural feature. The importance of (040) crystallographic plane was also demonstrated via the reduction of Cu 2+ to Cu, by analyzing the Raman spectra of the Cu containing samples, the mean primary crystallite size of Cu and Cu content. Furthermore, the presence of (040) crystallographic plane was directly proportional with the hydrodynamic properties of the powders as well.
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There is a need for photocatalysts with efficient photocarrier separation to address issues with environmental pollution. Photocarrier separation is largely determined by the orbital composition near the band edge. Here, we investigate Zn4B6O13 as an efficient photocatalyst for photodegradation of tetracycline. Theoretical calculations of Zn4B6O13 show that the valence band near the Fermi level is composed of d and p orbitals whereas the bottom of the conduction band is composed of s and p orbitals; thus, a large value of mh * /me * is derived from the band dispersion. The characteristics of this orbital composition promote separation of photoexcited carriers, leading to a high transfer efficiency of the catalyst. Moreover, photodegradation experiments demonstrate that the photocatalytic activity of Zn4B6O13 is approximately 5.2 times as high as that of SnO2. This study provides insights that might aid the development of novel borate-based environmental photocatalysts with superior performance.
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Yb-doped Bi2WO6 nanomaterials were successfully synthesized using a mild hydrothermal method. Remarkably, Yb-doped Bi2WO6 photocatalysts exhibited a higher photocatalytic activity than pristine Bi2WO6 by degrading RhB under visible light irradiation. The optimum mass ratio of Yb to Bi2WO6 was 2.0%, and it presented the highest photo-degradation efficiency. The crystal structures, morphologies, surface chemical states, and optical properties of the as-synthesized photocatalysts were carried out by XRD, SEM, XPS and DRS, respectively. The results of electron paramagnetic resonance spectroscopy, radical trapping experiments, photoluminescence spectroscopy and photocurrent measurements indicated that the Yb-doped Bi2WO6 samples produced more oxygen vacancies and superoxide anion active groups (.O2⁻), inhibited the recombination of the photo-generated electron-hole pairs, and enhanced the photo-degradation performance. As a result, the dopant Yb³⁺ could enhance photocatalytic performance of Bi2WO6 by introducing oxygen vacancies into its lattice. This study provides a useful strategy for developing effective visible light photocatalysts.
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In this paper, Bi2WO6 porous microspheres were prepared via a hydrothermal method and nano-TiO2 uniformly dotted on the surface of the Bi2WO6 microspheres using deposition technology to self-assemble a porous layer spherical TiO2-Bi2WO6 (TB) composite photocatalyst. The photocatalytic activity of the composite photocatalyst was evaluated using ethylene as the target degradation product. The results show that the photocatalytic degradation of ethylene by the TB composite photocatalyst was significantly improved, especially 10% TB. Samples containing 10% titanium dioxide had the highest photocatalytic activity for ethylene degradation. In addition, the degradation rate of ethylene was as high as 20.90%, which was 1.53 times higher than that of Bi2WO6 microspheres. The spherical structure of the porous layer gives the composite photocatalyst a high specific surface area, which can effectively promote the rapid migration of photogenerated carriers between TiO2 and Bi2WO6, and prevent the recombination of photogenerated electrons and photogenerated holes, thereby effectively improving the photocatalytic performance.
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The critical factor affecting the activity of photocatalytic hydrogen production is always the rapid recombination of photocarriers. In this paper, we construct an efficient and stable fluorinated-TiO2/Zn0.2Cd0.8S-diethylenetriamine (F-TiO2/ZCSD) heterostructure, and a novel structure modified by ZCSD nano-flowers was grown in situ on the surface of 2D F-TiO2 nanosheets via hydrothermal method. ZCSD has suitable valence and conduction band positions. Under visible light, photo-generated electrons are transferred to the F-TiO2 plane with defects. At this time, the potential well formed at the defect binds the electrons in it, which greatly restrains the recombination of the photo-induced electrons and the holes. Therefore, the photocatalytic hydrogen evolution rate of 20%F-TiO2/ZCSD is 13.5 mmol·g⁻¹·h⁻¹, which is 2.4 folds greater than that of ZCSD. Furthermore, the structure of the composite has not been damaged after six cycles of the test, which indicates that the composite has excellent photostability.
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Bi2MoO6 nanosheets with exposed (010) facets were selectively synthesized through hydrothermal method by adjusting the pH value in the presence of cetyltrimethyl ammonium bromide (CTAB) as the templates. The effects of CTAB content and hydrothermal conditions on the morphologies and crystal phases of the products were determined by using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy (DRS), Fourier-transform infrared spectroscopy (FTIR), Raman spectrometry, and Brunauer-Emmett-Teller surface area analyses. It is found that Bi2MoO6 nanosheets with relatively large particle sizes (plate length 0.5-3μm) and special anisotropic growth along the (010) plane can be obtained from an alkaline hydrothermal environment. The band gap of Bi2MoO6 can be fine-tuned from 2.30 to 2.57eV by adjusting the pH value of hydrothermal solution. The pH value has a significant effect on the composition of hydrothermal precursors, which results in Bi2MoO6 nanosheets with different ratio of (010) faces, especially the formation of Bi2O3 in the primary stage of the hydrothermal treatment is a key factor for the exposure of (010) facets. The visible-light-driven photocatalytic activities of the Bi2MoO6 products with different ratio of (010) facets exposed are investigated through the degradation of Rhodamine B, oxytetracycline, and tetracycline. Bi2MoO6 nanosheets synthesized at pH 10.0 with highest (010) facet exposed ratio exhibited highly efficient visible light photocatalytic activity for pollutant decomposition, which can be mainly attributed to the flake structures, the crystallinity and most importantly, the exposed (010) facet which generate high concentration of O²⁻.
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The TiO2 photoassisted degradation of the cationic dye rhodamine B (RhB) has been examined in aqueous dispersions under visible light irradiation at wavelengths longer than 470 nm in the presence and absence of the anionic surfactant sodium dodecylbenzenesulfonate (DBS). RhB degrades slowly via a pH-independent process in TiO2 dispersions containing no DBS. The surfactant DBS adsorbs strongly on the TiO2 particles and significantly accelerates RhB degradation with initial rates reaching maximal values at the critical micelle concentration of DBS (cmc = 1.2 mM). In the presence of DBS, rates decrease with increase in pH, an effect directly attributable to variations in the extent of adsorption of RhB with changes in the surface charge of TiO2 particles. The zeta (ζ)-potentials of TiO2 particles in RhB/DBS/TiO2 dispersions (pH 2.1) show that DBS significantly enhances RhB adsorption and correlates with an enhancement in the rate of photodegradation of RhB. The results confirm the heretofore presumed but valid notion that preadsorption on the surface of TiO2 particles is prerequisite for efficient photodegradation of RhB under visible light irradiation; moreover, the data infer that degradation occurs at the particle surface and not in the solution bulk. Present observations are consistent with a pathway in which excited RhB* injects an electron onto TiO2 (an electron-transfer mediator) that is subsequently scavenged by O2 to form the O2•- radical anion and ultimately the OH• radical, as evidenced by DMPO spin-trapping ESR experiments carried out under conditions otherwise similar to those in photodegradation which, we infer, participates in the RhB photodegradation.
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The civilian, commercial, and defense sectors of most advanced industrialized nations are faced with a tremendous set of environmental problems related to the remediation of hazardous wastes, contaminated groundwaters, and the control of toxic air contaminants. Problems with hazardous wastes at military installations are related in part to the disposal of chemical wastes in lagoons, underground storage tanks, and dump sites. Typical wastes of concern include heavy metals, aviation fuel, military-vehicle fuel, solvents and degreasing agents, and chemical byproducts from weapons manufacturing. In the civilian sector, the elimination of toxic and hazardous chemical substances such as the halogenated hydrocarbons from waste effluents and previously contaminated sites has become a major concern. General classes of compounds of concern include: solvents, volatile organics, chlorinated volatile organics, dioxins, dibenzofurans, pesticides, PCB's, chlorophenols, asbestos, heavy metals, and arsenic compounds. Advanced physicochemical processes such as semiconductor photocatalysis are intended to be both supplementary and complementary to some of the more conventional approaches to the destruction or transformation of hazardous chemical wastes such as high-temperature incineration, amended activated sludge digestion, anaerobic digestion, and conventional physicochemical treatment. 441 refs.
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To use solar irradiation or interior lighting efficiently, we sought a photocatalyst with high reactivity under visible light. Films and powders of TiO2-xNxhave revealed an improvement over titanium dioxide (TiO2) under visible light (wavelength < 500 nanometers) in optical absorption and photocatalytic activity such as photodegradations of methylene blue and gaseous acetaldehyde and hydrophilicity of the film surface. Nitrogen doped into substitutional sites of TiO2 has proven to be indispensable for band-gap narrowing and photocatalytic activity, as assessed by first-principles calculations and x-ray photoemission spectroscopy.
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The enthalpies of formation, from the constituent oxides, of CoWO4 and NiWO4 have been determined by direct synthesis calorimetry at . The following values of ΔHf° at 298 K (kJ mol−1) are reported: CoWO4, −60.8±2.6; NiWO4, −47.3±2.7. The results are compared with the data available in the literature.
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Heterogeneously dispersed semiconductor surfaces provide both a fixed environment to influence the chemical reactivity of a wide range of adsorbates and a means to initiate light-induced redox reactivity in these weakly associated molecules. Upon photoexcitation of several semiconductors nonhomogeneously suspended in either aqueous or nonaqueous solutions or in gaseous mixtures, simultaneous oxidation and reduction reactions occur. This conversion often accomplishes either a specific, selective oxidation or a complete oxidative degradation of an organic substrate present. The paper discusses the following: survey of reactivity (functional group transformations and environmental decontamination); mechanism of photocatalysis (photoelectrochemistry, carrier trapping, inhibition of electron hole recombination by oxygen, involvement of the hydroxy radical, adsorption effects, Langmuir-Hinshelwood kinetics, pH effects, temperature effects, and sensitization); and semiconductor pretreatment and dispersion (photocatalytically active semiconductors, photocatalyst preparation, and surface perturbation). 215 refs.
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Square Bi2WO6 nanoplates have been successfully synthesized by simple hydrothermal process. The effects of hydrothermal temperature and reaction time on morphologies and sizes of the nanoplates were investigated. These nanoplates are square geometric shapes having their basal plane as the (001) plane of orthorhombic Bi2WO6. On the basis of results of morphologies observation and selected area electron diffraction of series samples, a possible growth mechanism of the nanoplates is revealed. The square laminar shape could be attributed to anisotropic growth along the (001) plane, which is parallel to their intrinsic layer structure. UV−visible diffuse reflection spectra of the prepared Bi2WO6 nanoplates indicate they had absorption in the visible region, but a blue shift appeared compared to their bulk counterparts. Their photocatalytic activities are determined by rhodamine B degradation under visible light irradiation (λ > 400 nm). The reaction constant (k) of the best quality Bi2WO6 nanoplates is three times that of the sample prepared by solid-state reaction, which indicates much higher photocatalytic activities of the nanoplates performed under visible light irradiation.
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A new approach has been developed for the fabrication of visible light photocatalysts. Nanoclusters of MoS2 and WS2 are coupled to TiO2 by an in situ photoreduction deposition method taking advantage of the reducing power of the photogenerated electrons from TiO2 particles. The photocatalytic degradation of methylene blue and 4-chlorophenol in aqueous suspension has been employed to evaluate the visible light photocatalytic activity of the powders. The blue shift in the absorption onset confirms the size quantization of MS2 nanoclusters, which act as effective and stable sensitizers, making it possible to utilize visible light in photocatalysis. Quantum size effects alter the energy levels of the conduction and valence band edges in the coupled semiconductor systems, which favors the interparticle electron transfer. In addition, the coupled systems are believed to act in a cooperative manner by increasing the degree of charge carrier separation, which effectively reduces recombination.
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The visible-light-driven photocatalyst Bi2WO6 nanoparticles have been prepared by calcining amorphous complex precursor at a relatively low temperature of above 450oC. The effects of calcination temperature and time on the structures and properties of Bi2WO6 nanoparticles have been investigated in detail. The photocatalytic activity of the Bi2WO6 powders were evaluated by degradation of RhB molecules in water under visible light irradiation (λ>400nm). The results showed that the particle size and grain size of Bi2WO6 increased with the calcination temperature and time. The photocatalytic activity of the best sample was about 8.8 times higher than that of the sample prepared by traditional solid state reaction and the photo-degradations was a zero-order reaction. The best route to enhance the photocatalytic activity of Bi2WO6 was to prepare the sample at a lower temperature for a longer time, due to the samples with better crystallization and smaller particle size.
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Bi2WO6 powder photocatalyst was prepared using Bi(NO3)3 and Na2WO4 as raw materials by a simple hydrothermal method at 150°C for 24h, and then calcined at 300, 400, 500, 600 and 700°C for 2h, respectively. The as-prepared samples were characterized with UV-visible diffuse reflectance spectra, fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and N2 adsorption–desorption measurement. The photocatalytic activity of the samples was evaluated using the photocatalytic oxidation of formaldehyde at room temperature under visible light irradiation. It was found that post-treatment temperature obviously influenced the visible-light photocatalytic activity and physical properties of Bi2WO6 powders. At 500°C, Bi2WO6 powder photocatalyst showed the highest visible-light photocatalytic activity due to the samples with good crystallization and high BET surface area.
Article
A novel flower-like Bi2WO6 superstructure was successfully realized by a facile hydrothermal process without any surfactant or template. Based on the evolution of this morphology as a function of hydrothermal time, the formation mechanism was proposed to be as follows: (1) self-aggregation of nanoparticles; (2) formation of crystalline nanoplates by Ostwald ripening; and (3) organization of the in situ-formed nanoplates into spherical superstructures. The pretty flower-like superstructure of Bi2WO6 was retained after calcination at 550 °C for 4 h. Both the uncalcined and calcined Bi2WO6 exhibited excellent visible-light-driven photocatalytic efficiencies for the degradation of Rhodamine B (RhB), up to 84 and 97% within 60 minutes, respectively, which were much higher than those of TiO2 (P-25) and Bi2WO6 sample prepared by solid-state reaction (SSR-Bi2WO6). Close investigation indicated that plenty of pores with different sizes existed in the Bi2WO6 superstructures, which could serve as hierarchical transport paths for small molecules and might greatly improve their photocatalytic activities.
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Well-segregated bismuth sulfide (Bi2S3) nanorods with a high order of crystallinity have been successfully prepared from bismuth citrate by a simple reflux reaction in DMF at 160 °C. Bismuth citrate and thiourea act as the precursor materials. The products were characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy (TEM, HRTEM), scanning electron microscopy, selected area electron diffraction and energy-dispersive X-ray microanalysis. Bismuth citrate, which is known to be a linear polymer, may play a critical role as a precursor and a template for the growth of linear Bi2S3 nanorods.
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It has been revealed that magnesium acetylacetonate ([Mg{sup II}(acac)â]) irreversibly adsorbs on TiOâ from an EtOH and n-hexane mixed solution via a one-to-one stoichiometric ligand exchange of acac⁻ for Ti{sub s}-O⁻, whereas it hardly adsorbs on MgO. The successive oxidation of the resultant surface complex (Mg{sup II}(acac)-(OTi{sub s})) formed a MgOâ submonolayer on the surface of TiOâ. Six-time repetition of the adsorption-oxidation process achieved an approximate MgOâ monolayer owing to the selective adsorption of [Mg{sup II}(acac)â] on the portion of naked TiOâ. The rate of the TiOâ photoinduced oxidation of sodium dodecylbenzenesulfonate increased with the MgOâ submonolayer coverage (θ â 0.5) by a factor of 1.3.
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This review discusses salient features of interfacial electron transfer reactions in colloidal semiconductor solutions and thin films and their application for solar light energy conversion and photocatalytic water purification. This research is interdisciplinary and is situated at the limit between colloid science, and electrochemistry, and semiconductor physics. The section on colloidal semiconductors discusses material science aspects, optical properties, electronic properties, and charge carrier reactions in colloidal semiconductor solutions. The section on nanocrystalline semiconductor electrodes discusses some basic properties and preparation procedures of nanocrystalline TiO[sub 2] films, energetics and operations of the nanoporous solar cell, observations and importance of electronic states in the band gap region, photoinduced processes in nanocrystalline semiconductor electrodes, the electric field in a nanocrystalline electrode, and kinetic rate constants in the photosensitized solar cell. 156 refs.
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Photodegradation of a textile dye X3B using either UV (λ ≥ 320 nm) or visible light (λ ≥ 450 nm) over three catalysts of highly adsorptive TiO2 nanoparticles in water has been examined. All the adsorption isotherms demonstrated the Langmuir type behavior. The common observation was confirmed that for all the reactions induced by UV or visible light, the apparent initial rate of X3B loss in the aqueous phase increased with the initial equilibrated concentration of X3B. However, this correlation was changed when the rate was determined by the decreased concentration both in the aqueous phase and on the catalyst surface. This increase of real initial rate with the initial equilibrated concentration was observed only in the visible-light-induced reaction over TiO2 of Degussa P25. For all the other reactions, especially under UV irradiation, the real initial rate was found to increase only initially and then decrease with the initial equilibrated concentration. The result suggests that there is a screening effect by the adsorbed dye in the TiO2 photocatalytic reaction and a solution filter effect in the photosensitized reaction. Moreover, the photosensitized photodegradation of X3B was found to be also dependent on the physical properties of TiO2, but interestingly the relative activity among the catalysts was similar to that demonstrated in the photocatalytic reaction.
Article
Chemical oxygen demand (CODCr) and proton NMR, UV−vis, and spin trapping EPR spectroscopic evidence is presented to demonstrate the inverse photosensitized oxidative transformation of tetraethylated rhodamine (RhB) under visible illumination of aqueous titania dispersions. Both de-ethylation and oxidative degradation take place with the former proceeding in a stepwise manner to yield mono-, di-, tri-, and tetra-de-ethylated rhodamine species. Intermediates present after each de-ethylation step remain in a fast dynamic equilibrium between the titania particle surface and the bulk solution. The concentration of •OH radicals, formed from the inverse photosensitization process through the superoxide radical anion, increases upon addition of the anionic dodecylbenzene sulfonate surfactant (DBS) because a larger number of RhB excited states are able to inject an electron into the conduction band of the TiO2 particles. Also, intermediates that can no longer absorb the visible light, (i.e., once the dye solution is competely bleached) are unable to drive the photosensitized degradation further. A mechanism for the competitive photoreactions between degradation and de-ethylation is described, in which de-ethylation {ζ 0.0035} is mostly a surface occurring process, whereas degradation {ζ 0.0015}of the RhB chromophore is predominantly a solution bulk process.
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Bismuth tungstate hierarchical nest-like structures built by higher order nanoplate alignment have been successfully synthesized by a facile and economical method in the presence of polyvinyl pyrrolidone. The formation mechanism and effect of reaction time on the products were investigated. In addition, studies of the photocatalytic property demonstrate that the as-synthesized Bi2WO6 structures show excellent photocatalytic activity exposure to visible light irradiation. Furthermore, we first explored the electrochemical property of the Bi2WO6 nanostructure as an electrode in a lithium ion battery. Therefore, the preparation and properties studies of Bi2WO6 structures suggest potential future applications in photocatalysis by sunlight and as an electrode candidate in lithium ion batteries.
Article
BiVO4 photocatalysts for O2 evolution, which work under visible light irradiation, were prepared by an aqueous process. The BiVO4 photocatalysts were obtained by the reaction of layered potassium vanadate powder (KV3O8 and K3V5O14) with Bi(NO3)3 for 3 days in aqueous media at room temperature. Highly crystalline monoclinic and tetragonal BiVO4 were selectively synthesized by changing the ratio of vanadium to bismuth in the starting materials. X-ray diffraction and scanning electron microscopy measurements showed that the monoclinic BiVO4 was formed via a tetragonal BiVO4 intermediate. Tetragonal BiVO4 with a 2.9 eV band gap mainly possessed an ultraviolet absorption band while monoclinic BiVO4 with a 2.4 eV band gap had a characteristic visible light absorption band in addition to the UV band. The UV bands observed in the tetragonal and monoclinic BiVO4 were assigned to the band transition from O2p to V3d whereas the visible light absorption was due to the transition from a valence band formed by Bi6s or a hybrid orbital of Bi6s and O2p to a conduction band of V3d. The photocatalytic activity for O2 evolution from an aqueous silver nitrate solution under UV irradiation (300 < λ < 380 nm) on the tetragonal BiVO4 was comparable to that on the monoclinic BiVO4. The monoclinic BiVO4 also showed the high photocatalytic activity for the O2 evolution under visible light irradiation (λ > 420 nm). When the monoclinic BiVO4 was calcined at 700−800 K the activity was increased. The activity of this monoclinic BiVO4 was much higher than that of BiVO4 prepared by a conventional solid-state reaction. The quantum yield at 450 nm for the O2 evolution on the monoclinic BiVO4 was 9%.
Article
The photocatalytic oxidations of CN- and SO32- were studied at several semiconductor powders including TiO2, ZnO, CdS, Fe2O3, and WO3 using a xenon light source. TiO2, ZnO, and CdS were active photocatalysts for cyanide oxidation, while no oxidation was seen for Fe2O3 and WO3. The catalyzed oxidation of CN- at TiO2 using sunlight was also investigated. The rate constant for CN- oxidation at TiO2 in sunlight was 3.1 × 10-6 mol day-1 cm-2 illuminated surface. The product of the oxidation of CN- at TiO2 was quantitatively determined to be OCN-. TiO2, ZnO, CdS, and Fe2O3 photocatalyzed the oxidation of SO32-. The order of the catalytic activity was Fe2O3 ∼ ZnO ∼ CdS > TiO2. The rates of the photocatalytic oxidations were greater for SO32- than for CN- in the cases of TiO2, ZnO, and CdS. The chemical and photochemical stabilities for the most active catalysis were determined.
Article
A surfactant-templated approach was used to synthesize phosphated mesoporous titanium dioxide by incorporating phosphorus from phosphoric acid directly into the framework of TiO2. The resulting materials were characterized by XRD, nitrogen adsorption, TEM, XPS analysis, UV−vis spectroscopy, FT-IR spectroscopy, and isoelectric point measurements. The surface area of phosphated mesoporous TiO2 exceeded 300 m2/g after calcination at 400 °C. It was found that the incorporation of phosphorus could stabilize the TiO2 framework and increase the surface area significantly. This stabilization is attributed to two reasons:  the more complete condensation of surface Ti−OH in the as-prepared sample and the inhibition of grain growth of the embedded anatase TiO2 by the interspersed amorphous titanium phosphate matrix during thermal treatment. Both pure and phosphated mesoporous TiO2 show significant activities on the oxidation of n-pentane. The higher photocatalytic activity of phosphated mesoporous TiO2 can be explained by the extended band gap energy, large surface area, and the existence of Ti ions in a tetrahedral coordination.
Article
Nanosized Bi2WO6 was synthesized by a hydrothermal crystallization process. The as-prepared samples were characterized by X-ray diffraction, Brunauer–Emmet–Teller surface area and porosity measurements, transmission electron microscopy, Raman spectra, and diffuse reflectance spectroscopy. The photoactivities of the as-prepared samples for the rhodamine-B photodegradation were investigated systematically. As a result, the sample prepared at 180 °C exhibited the highest photochemical activity under visible-light irradiation. The further experiments revealed that the catalyst was active in a wide spectral range. Density functional theory calculations suggested that the visible-light response was due to the transition from the valence band formed by the hybrid orbitals of Bi 6s and O 2p to the conduction band of W 5d. The photoactivity of the catalyst in relationship with the hydrothermal temperature, the crystal and band structure were also discussed in detail.
Article
An oxide photocatalyst Bi2WO6 with corner-shared WO6 octahedral layered structure was synthesized. Its band gap was determined to be 2.69 eV from UV–vis diffuse reflectance spectra. The photocatalyst showed not only the activity for photocatalytic O2 evolution with the initial evolution rate of 2.0 mol/h but also the activity of mineralizing both CHCl3 and CH3CHO contaminants under visible light irradiation. Meanwhile, wavelength dependence of CH3CHO decomposition was observed, which indicated that the photocatalytic activity of the photocatalyst was in good agreement with its light-absorption ability.
Article
The behavior of semiconducting electrodes for photoelectrolysis of water is examined in terms of the physical properties of the semiconductor. The semiconductor‐electrolyte junction is treated as a simple Schottky barrier, and the photocurrent is described using this model. The approach is appropriate since large‐band‐gap semiconductors have an intrinsic oxygen overpotential which removes the electrode reaction kinetics as the rate‐limiting step. The model is successful in describing the wavelength and potential dependence of the photocurrent in WO 3 and allows a determination of the band gap, optical absorption depth, minority‐carrier diffusion length, flat‐band potential, and the nature of the fundamental optical transition (direct or indirect). It is shown for WO 3 that minority‐carrier diffusion plays a limited role in determining the photoresponse of the semiconductor‐electrolyte junction. There are indications that the diffusion length in this low carrier mobility material is determined by diffusion‐controlled bulk recombination processes rather than the more common trap‐limited recombination. It is also shown that the fundamental optical transition is indirect and that the band‐gap energy depends relatively strongly on applied potential and electrolyte. This effect seems to be the result of field‐induced crystallographic distortions in antiferroelectric WO 3 .
Article
Scientific studies on photocatalysis started about two and a half decades ago. Titanium dioxide (TiO2), which is one of the most basic materials in our daily life, has emerged as an excellent photocatalyst material for environmental purification. In this review, current progress in the area of TiO2 photocatalysis, mainly photocatalytic air purification, sterilization and cancer therapy are discussed together with some fundamental aspects. A novel photoinduced superhydrophilic phenomenon involving TiO2 and its applications are presented.
Article
Heterogeneous photocatalysis is a promising technique valuable for environmental purification. Nano-sized semiconductors such as ZnO and TiO2, which is one of the most basic functional materials, have emerged as effective photocatalyst materials. The surface photovoltage spectra (SPS) can be an effective method for quickly evaluating the photocatalytic activity of semiconductor materials since it can provide a rapid, non-destructive monitor of the semiconductor surface properties such as surface band bending, surface and bulk carrier recombination and surface states, mainly showing the carrier separation and transfer behavior with the aid of light, especially the electric-field-induced surface photovoltage spectra (EFISPS), in which SPS is combined with the electric-field-modified technique. In this review, the basic principles, measurement and applications of the SPS and EFISPS are mainly discussed together with some fundamental aspects like the electric properties of semiconductor surface and the principle of electric field effect. In particular, the applications of SPS to nano-sized semiconductors such as ZnO and TiO2 in heterogeneous photocatalysis are emphasized, which involve mainly evaluating the photocatalytic activity by analyzing semiconductor surface properties such as the separation efficiency of photoinduced carriers under illumination by the SPS measurement, highlighting our own contributions. The results show that the weaker the surface photovoltage signal is, the higher the photocatalytic activity is in the case of nano-sized semiconductor photocatalysts.
Article
Polycrystalline BaWO4 and PbWO4 thin films having a tetragonal scheelite structure were prepared at different temperatures. Soluble precursors such as barium carbonate, lead acetate trihydrate and tungstic acid, as starting materials, were mixed in aqueous solution. The thin films were deposited on silicon, platinum-coated silicon and quartz substrates by means of the spinning technique. The surface morphology and crystal structure of the thin films were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction, and specular reflectance infrared Fourier transform spectroscopy, respectively. Nucleation stages and surface morphology evolution of thin films on silicon substrates have been studied by atomic force microscopy. XRD characterization of these films showed that BaWO4 and PbWO4 phase crystallize at 500 °C from an inorganic amorphous phase. FTIR spectra revealed the complete decomposition of the organic ligands at 500 °C and the appearance of two sharp and intense bands between 1000 and 600 cm−1 assigned to vibrations of the antisymmetric stretches resulting from the high crystallinity of both thin films. The optical properties were also studied. It was found that BaWO4 and PbWO4 thin films have Eg=5.78 eV and 4.20 eV, respectively, of a direct transition nature. The excellent microstructural quality and chemical homogeneity results confirmed that soft solution processing provides an inexpensive and environmentally friendly route for the preparation of BaWO4 and PbWO4 thin films.
Article
An Aurivillius phase, Bi(2)SrTa(2)O(9), which consists of perovskite-like slabs and bismuth oxide sheets, was treated with 3 M hydrochloric acid for 72 h, and the resultant product was characterized. Scanning electron microscopy investigation indicated that no morphological change occurred during the acid treatment. X-ray diffraction (XRD) analysis revealed that the product exhibited tetragonal symmetry with a = 0.391 +/- 0.004 nm and c = 0.98 +/- 0.01 nm, and the a parameter is consistent with a typical value for cubic perovskite oxides. High-resolution electron microscopy (HREM) observations along both [001] and [010] showed that the structure of the perovskite-like slabs in Bi(2)SrTa(2)O(9) was retained after the acid treatment. The compositional analyses revealed the loss of a large portion of bismuth and a part of strontium (present in the bismuth oxide sheets due to B <--> Sr disorder) and the introduction of protons. These observations indicate that the bismuth oxide sheets in Bi(2)SrTa(2)O(9) were selectively leached and that protons were introduced into the interlayer space to form a protonated layered perovskite, H(1.8)[Sr(0.8)Bi(0.2)Ta(2)O(7)]. Though diffraction techniques (XRD and electron diffraction) demonstrated that an average structure of H(1.8)[Sr(0.8)Bi(0.2)Ta(2)O(7)] consisted of perovskite-like slabs stacked without displacement, HREM observation along [010] demonstrated that both a simple stacking sequence without displacement (P-type) and a stacking sequence with a relative displacement by (a + b)/2 (I-type) were present in H(1.8)[Sr(0.8)Bi(0.2)Ta(2)O(7)].
Article
The photocatalytic degradation of a series of (CH3)nNH(4-n)+ (0 < or = n < or = 4) was systematically studied in the UV-illuminated TiO2 aqueous suspensions at pH ranges of 3-11. By investigating the pH-dependent kinetics and analyzing intermediates and products, we elucidated the mechanistic pathways and the role of OH radicals in the photocatalytic oxidation. The deprotonated neutral species more rapidly degraded than their protonated counterparts for these homologous compounds because the OH radicals favorably reacted with the lone-pair electron on the nitrogen atom. Therefore, the photocatalytic degradation was highly enhanced at alkaline solutions for all substances except (CH3)4N+. The H-atom abstraction (from (CH3)4N+) by OH radicals initiated successive demethylation processes to generate tri-, di-, and monomethylammonium/amine as an intermediate and NH3/NH4+ as a final product. On the other hand, the OH-addition to the N-atom with the lone-pair electron led to NO2-/NO3- whose production was highly favored at alkaline conditions. The photocatalytic degradation rates of (CH3)4N+ were comparable at both acidic and alkaline conditions, which could not be explained by a simple electrostatic surface charge model. By using OH-scavenging tert-butyl alcohol as a diagnostic probe into the mechanism, it is suggested that the photocatalytic oxidation of (CH3)4N+ at acidic conditions proceeds through free OH radicals in the solution bulk, not on the surface of TiO2.
Article
ALTHOUGH the possibility of water photolysis has been investigated by many workers, a useful method has only now been developed. Because water is transparent to visible light it cannot be decomposed directly, but only by radiation with wavelengths shorter than 190 nm (ref. 1).
Article
Photocatalytic degradations of alachlor in TiO2 suspensions with and without the use of hydrogen peroxide were studied using two different monochromatic UV irradiations (300 and 350 nm). Direct photolysis of alachlor was a rather slow process, but the addition of TiO2 enhanced the reaction rates by 12 and 26 times using 300 and 350 nm UV irradiation, respectively. The results showed that a low H2O2 dosage in photocatalysis using 300 nm UV would enhance the rates by 3.3 times, but an overdose of H2O2 will retard the rate due to the hydroxyl radicals are consumed. However, this process is impracticable at 350 nm due to the absorption characteristic of H2O2. A neutral initial pH level was found to favor the H2O2 assisted photocatalysis at 300 nm UV illumination. Eleven major intermediates were identified by liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) and MS/MS. The major degradation mechanisms of H2O2-assisted alachlor photocatalysis include dechlorination, dealkylation, hydroxylation, cyclization, scission of C-O bond, and N-dealkylation. Bell-shaped evolution profiles of different intermediates were observed. Degradation pathways were proposed accordingly to illustrate series of degradation steps. The TOC analysis revealed the different stages of the reaction.
Article
Colloidal bismuth subcitrate (CBS, De-Nol) has been used for several decades for the treatment of gastric and duodenal ulcers, and Helicobacter pylori infection together with antibiotics. The solubility of CBS in water is found to be dramatically affected by pH, from >70 mg/mL at pH 7 to only about 1 mg/mL at pH 3. CBS was crystallized in dilute HCl at pH 3, and unique assembly of three basic bismuth citrate dimeric units ([Bi(cit)2Bi]2-) leads to the formation of two-dimensional sheets and 3D polymer.
Article
Light-harvesting macroporous channels have been successfully incorporated into a mesoporous TiO(2) framework to increase its photocatalytic activity. This bimodal porous material was characterized by X-ray diffractometry in both low-angle and wide-angle ranges, N(2) adsorption-desorption analysis, scanning and transmission electron microscopy, FT-IR, and diffuse reflectance spectroscopy. Ethylene photodegradation in gas-phase medium was employed as a probe reaction to evaluate the photocatalytic reactivity of the catalysts. The results reveal that sintering temperature significantly affects the structural stability and photocatalytic activity of titania. The catalyst which calcined at 350 degrees C possessed an intact macro/mesoporous structure and showed photocatalytic reactivity about 60% higher than that of commercial P25 titania. When the sample was calcined at 500 degrees C, the macroporous structure was retained but the mesoporous structure was partly destroyed. Further heating at temperatures above 600 degrees C destroyed both macro- and mesoporous structures, accompanied by a loss in photocatalytic activity. The high photocatalytic performance of the intact macro/mesoporous TiO(2) may be explained by the existence of macrochannels that increase photoabsorption efficiency and allow efficient diffusion of gaseous molecules.
Article
Visible-light-induced photodegradation of rhodamine B over nanosized Bi2WO6 has been observed. Bi2WO6 exhibited a high photoactivity to photodegrade rhodamine B in the central pH solution under visible irradiation (lambda > 420 nm). After five recycles for the photodegradation of rhodamine B, the catalyst did not exhibit any significant loss of activity, confirming the photocatalyst is essentially stable. The total organic carbon measurement displayed that a high degree of mineralization was achieved in the present photochemical system. The results of density functional theory calculation illuminated that the visible-light absorption band in the Bi2WO6 catalyst is attributed to the band transition from the hybrid orbitals of Bi6s and O2p to the W5d orbitals. The Bi2WO6-assisted photocatalytic degradation of rhodamine occurs via two competitive processes: a photocatalytic process and a photosensitized process. The transformation of rhodamine is mainly via the photocatalytic process. Kinetic studies by using electron spin resonance and the radical scavenger technologies suggest that *OH is not the dominant photooxidant. Direct hole transfers and O2*- could take part in Bi2WO6 photocatalysis. This study provided a possible treatment approach for organic pollutants by using visible light in aqueous ecosystems.
Article
Aurivillius structure Bi(2)MoO(6) (BG: 2.70 eV) that is a low-temperature phase showed an intense absorption band in the visible light region and photocatalytic activity for O(2) evolution from an aqueous silver nitrate solution under visible light irradiation, among various bismuth molybdates (Bi(2)MoO(6), Bi(2)Mo(2)O(9), and Bi(2)Mo(3)O(12)) synthesized by solid-state and reflux reactions. Bi(2)Mo(3)O(12) (BG: 2.88 eV) also showed photocatalytic activity for O(2) evolution under full-arc irradiation of a Xe lamp (lambda > 300 nm). The photocatalytic activity of the Aurivillius structure Bi(2)MoO(6) prepared by the reflux method was dependent on the annealing temperature after the preparation. The crystallinity was the important factor for the activity. Calculation by the density functional method indicated that the conduction band of Aurivillius structure Bi(2)MoO(6) was made up of Mo 4d orbitals. It turned out that the visible-light absorption of this photocatalyst was due to the transition from the valence band consisting of O 2p orbitals to the conduction band. The corner-sharing structure of the MoO(6) octahedra contributed to the visible light response and the photocatalytic performance because excitation energy and/or photogenerated electron and hole pairs began to migrate easily in the Aurivillius structure.