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ZnO on rice husk: A sustainable photocatalyst for urban air purification
... The ever growing release of nitrogen oxides (NO x , x = 1,2) from fuel combustion and automobile exhausts is the root cause of various environmental problems and adverse effects on human health [1][2][3][4][5][6][7][8][9][10][11][12]. This concern has stimulated the interest towards effective actions aimed at the removal of gaseous NO x (De-NO x ) from the atmosphere, especially in urban and industrial areas [13][14][15][16]. ...
... Efficient and widely studied materials to trigger such processes are based on titanium(IV) oxide, but the wide TiO 2 band gap (E G ≈ 3.2 eV) requires activation by UV radiation, accounting for only 4% of the solar spectrum [3][4][5][14][15][16][21][22]. In addition, TiO 2 is considered potentially carcinogenic when inhaled [6] In this context, numerous efforts have been focused on the development of highly efficient Vis-light photocatalysts [1][2]14,[16][17]23] enabling a complete oxidation to harmless nitrate species with a limited (or absent) release of NO 2 , a toxic intermediate in the sequential oxidation NO → NO 2 − → NO 2 → NO 3 − [4,10], into the outer atmosphere [3,5,7,13,20,24]. In addition, a crucial goal is the development of supported nanosystems, featuring a lower tendency to sintering/deactivation than the widely studied powdered counterparts, and enabling an easier recovery after use [21,[25][26][27][28][29]. ...
... The observed aggregates, outgrowing from a more compact underlayer [mean thickness = (950 ± 200) nm)], were assembled into arrays with an inherently open structure. These features can indeed boost the system photocatalytic activity, thanks to the high interfacial area with the reaction environment, beneficial also for light-induced charge transfer at the system surface [5][6]16,21,27,45]. After RF-sputtering, the pristine Fe 2 O 3 morphology was preserved, and the deposit thickness did not undergo remarkable alterations [average pyramid height and underlayer thickness = (2000 ± 300) nm and (1000 ± 300) nm, respectively, for Fe 2 O 3 -CuO ( Fig. 2c and d); (2100 ± 300) nm and (1000 ± 200) nm, respectively, for Fe 2 O 3 -WO 3 ( Fig. 2e and f)]. ...
The increasing release and accumulation of harmful nitrogen oxides (NOx with x = 1,2) in industrial and urban environments renders the efficient removal of these atmospheric pollutants an urgent and obligatory issue. In this regard, the advantages yielded by photocatalytic oxidation processes have triggered the search for eco-friendly catalysts featuring an improved efficiency. In this work, we propose the use of heterostructures based on β-Fe2O3, a scarcely investigated iron(III) oxide polymorph, as viable De-NOx photocatalysts with appealing functional performances. The present materials were fabricated in supported form by chemical vapor deposition (CVD) of Fe2O3. A proof-of-principle investigation on the modulation of material performances by heterostructure formation is explored through Fe2O3 functionalization with CuO or WO3 by radio frequency (RF)-sputtering. The obtained results reveal a controllable dispersion of CuO or WO3 in close contact with β-Fe2O3, a crucial issue to profitably exploit their mutual interplay for De-NOx applications. A preliminary analysis in this regard revealed very encouraging conversion efficiency and selectivity towards nitrate formation, outstanding among non-titania oxide-based De-NOx photocatalysts. The improved photoactivity with respect to bare Fe2O3, CuO and WO3 was related to a higher oxygen defectivity and an enhanced separation of photogenerated charge carriers, enabled by the matched band edges in the target heterostructures.
... 5−12 Accordingly, several efforts have been undertaken to reduce NO emissions, especially in urban and industrialized areas. 1,3,13,14 In this context, photocatalytic technologies assisted by largely available natural resources, such as oxygen, water, and light, are low-cost and sustainable routes for NO degradation at ambient temperature. 8,10,15−20 These processes are initiated by the photoexcitation of suitable semiconductors, resulting in the generation of electron−hole pairs (e − /h + ). ...
... 1,2,8,18,21 Although various research works have been aimed at the development of De-NO x photocatalysts, 1,3,14 key problems still far from being completely overcome are related to the low activity and the generation of undesired/toxic byproducts, 1,5 in particular, the NO 2 intermediate. 6,13,20,22,23 Hence, an important goal to be fulfilled for an efficient NO x degradation, an actual open challenge, 2,4,10,[14][15][16]24 is the obtainment of a high selectivity toward harmless nitrates (NO 3 − ) as the major process products. 1,13,17,19,21 Over the last decade, semiconductor metal oxide nanosystems have been investigated as De-NO x photocatalysts, thanks to their favorable chemico-physical properties, relatively low cost, and good chemical stability. ...
... 1,13,17,19,21 Over the last decade, semiconductor metal oxide nanosystems have been investigated as De-NO x photocatalysts, thanks to their favorable chemico-physical properties, relatively low cost, and good chemical stability. 4,6,14 In fact, the assembly of high-area nanocrystalline and defective structures positively affects material reactivity and charge transport phenomena, paving the way to an improved photocatalytic activity. 4,9,13,20,22,25 Most literature works have involved the use of TiO 2 , 1,12,13,20,26 ZnO, 1,14,22,27 Material Characterization. ...
... However, the modified ZnO revealed efficient photocatalytic air purification. For instance, Pastor et al. (2019) recently presented ZnO@SiO 2 for enhanced indoor air purification under visible light irradiation. The ZnO@SiO 2 was prepared by calcination of homogenized mixture of ZnO and rice husk ash at 600 • C ( Fig. 4e and f). ...
... The results showed that the atrazine degradation followed first-order kinetics. Another observation compared the photocatalytic degradation rate of four polycyclic aromatic hydrocarbons (PAHs), and results showed that soil pH influenced the removal efficiency of PAHs (Zhang et al., 2008) (Yoo et al., 2020), and e-f) ZnO@SiO 2 for NO reduction, reproduced with permission from ref (Pastor et al., 2019). photocatalysts in contaminated soils, the percentage of pyrene degradation by TiO 2 in quartz sand, alluvial and red soils under sunlight irradiation were 78.3, 23.4, and 31.8%, ...
Photocatalysis appears to be an appealing approach for environmental remediation including pollutants degradation in water, air, and/or soil, due to the utilization of renewable and sustainable source of energy, i.e., solar energy. However, their broad applications remain lagging due to the challenges in pollutant degradation efficiency, large-scale catalyst production, and stability. In recent decades, massive efforts have been devoted to advance the photocatalysis technology for improved environmental remediation. In this review, the latest progress in this aspect is overviewed, particularly, the strategies for improved light sensitivity, charge separation, and hybrid approaches. We also emphasized the low efficiency and poor stability issues with the current photocatalytic systems. Finally, we provided future suggestions to further enhance the photocatalyst performance and lower its large-scale production cost. This review aims to provide valuable insights into the fundamental science and technical engineering of photocatalysis in environmental remediation.
... Zinc oxide (ZnO) semiconductor nanomaterials have already shown great potential as an eco-friendly photocatalyst for environmental pollution remediation due to their ability to degrade and mineralize organic toxic pollutants into CO 2 , H 2 O, and other light by-products by photocatalysis under UV light or solar light [4][5][6][7][8]. Nevertheless, these ZnO nanomaterials are often synthesized by complex and expensive routes; therefore, their integration in road infrastructures could be difficult. ...
... For all functionalized concrete samples, lower ZnO NS bandgap values (~3.13 ± 0.4 eV) than traditional ones of ZnO NWs grown on a silicon substrate [25], on tiling, and on rock samples [10] (usually from 3.20 to 3.23 eV) were recorded in line with previous results on other concrete blocks studied [5]. This variation caused by the morphology modification of ZnO NSs could be associated with their crystal quality, dislocations density, impurities, size, and thickness, for instance [5,7,22]. Indeed, NSHs are supposed to contain more oxygen defects, which could reduce the bandgap by acting as an indirect donor energy level below the conduction band [21,26,27]. ...
Stormwater runoff management and treatment are significant topics for designing a sustainable city. Therefore, photocatalytic, permeable, and removable concrete is a promising solution to reduce pollution through leaching with permeable and scalable road. The objective of this work was to develop cost-effective and greener photocatalytic concretes that can be easily scaled-up, and to demonstrate their photocatalytic activities. To achieve this, seedless hydrothermal ZnO nanostructures (NSs) in 2 h were employed to functionalize a concrete surface by a soft functionalization process, avoiding overconsumption of energy and chemical products. In this work, two different concretes were studied and used for the degradation of organic dye in water. The results demonstrated the universality of the proposed functionalization process by showing similar gap values, ZnO NSs morphologies, and XRD pattern, compared to the concrete functionalized by the traditional two-step hydrothermal synthesis. The XRD results certified the presence of the ZnO Würtzite phase on the concrete surface. The synthesis feasibility was attributed to the basic pH and O− groups’ presence in concrete. Then, their photocatalytic efficiency was proved for organic dye removal in water. An almost total degradation was recorded after 5 h under artificial solar light, even after several uses, demonstrating a similar efficiency to the photocatalytic concrete functionalized by the traditional two-step synthesis.
... A detailed observation of the main band at frequencies higher than 1300 cm −1 , Fig. 7, shows the appearance of a narrow peak at 1383 cm −1 in the pattern recorded on the sample after the photocatalytic test. This band, absent in the pristine sample, corresponds to the N\ \O stretching vibrations of NO 3 − (Ai et al., 2010;Pastor et al., 2019), confirming that the full photocatalytic oxidation of NO had taken. Therefore, it seems reasonable that the accumulation of nitrate on the photocatalyst surface was the reason for the decay in the NO removal efficiency on time and in repetitive runs. ...
... into account the above values, the selectivity of the De-NOx process (S) is estimated at around 90% for all the photocatalysts here studied, an outstanding value in line with those reported for the last advanced De-NOx photocatalysts recently reported(Balci Leinen et al., 2019;Folli et al., 2011;Pastor et al., 2019;Shang et al., 2019;Tan et al., 2017).The enhanced photochemical De-NOx behaviour must be related with the changes induced by the presence of Cr 3+ ions in the LDH N 2 adsorption-desorption isotherms for the ZnAl-LDH, ZnAlCr-LDH and ZnCr-LDH samples. The UV-Vis absorption spectra for the ZnAl-LDH, ZnAlCr-LDH and ZnCr-LDH samples. ...
The ZnAl-CO3, ZnAlCr-CO3 and ZnCr-CO3 LDH samples were studied as De-NOx photocatalysts in this work. Samples without Cr and increasing the presence of Cr3+ in the LDH framework in the 0.06, 0.15 and 0.3 Cr/Zn ratio were prepared by co-precipitation method, all of them constituted by pure LDH phase. The increase of chromium content in the LDH framework leads to lower crystallinity and higher specific surface area in the samples. Moreover, the CrO6 octahedron centres expand the photo-activity from UV to Visible light and assist to decrease the recombination rate of the electrons and holes. The favourable textural, optical and electronic properties of Cr-containing LDH samples explain the good NO removal efficiency (55%) and outstanding selectivity (90%) found for the analysed De-NOx process.
... The disadvantages lay in the fact that the photocatalytic efficiency is low, and some catalysts cannot be recovered or reused. Some metal-oxide based catalysts (including TiO 2 , ZnO, Fe 2 O 3 , etc. [10][11][12]) have low utilization rates under the visible light and low photo-generated charge separation efficiency, which limits their practical application. ...
Bi2MoO6 was one of the important bismuth-based semiconductors with a narrow bandgap, and has been widely used in selective oxidation catalysts, supercapacitors, and energy-storage devices. A series of Bi2MoO6/ZnO composite photocatalysts with different mass ratios were synthesized by the hydrothermal method. The synthesized samples were characterized by XRD, PL, UV-Vis, SEM, TEM, XPS, and BET analysis techniques. Under visible light conditions, Methylene blue (MB) was used as the target degradation product to evaluate its photocatalytic performance. The results showed that the degradation rate constant of Bi2MoO6/ZnO (0.4-BZO) was about twice that of the traditional photocatalysis of ZnO. The Bi2MoO6/ZnO composite catalyst maintained stable performance after four consecutive runs. The high photocatalytic activity of Bi2MoO6/ZnO was attributed to the efficient electron transport of the heterojunction, which accelerates the separation of electron-hole pairs and reduces the probability of carrier recombination near the Bi2MoO6/ZnO heterojunction. Bi2MoO6/ZnO nanocomposites have potential applications in the field of photodegradation.
... NSs growth could be also influenced by the textural properties (porosity, roughness, etc.) of concrete and local micro-turbulences [28][29][30]. The band gap variation could be caused by the morphological modifications of ZnO NSs, and could also be associated with their crystal quality, dislocations density, impurities, size, and thickness [30][31][32]. Indeed, nanosheets are supposed to contain more oxygen defects, which could reduce the band gap by acting as indirect donor energy levels below the conduction band [33][34][35]. ...
Semiconductor‐based photocatalysis is a well‐known and efficient process for achieving
water depollution with very limited rejects in the environment. Zinc oxide (ZnO), as a wide bandgap metallic oxide, is an excellent photocatalyst, able to mineralize a large scale of organic pollutants in water, under UV irradiation, that can be enlarged to visible range by doping nontoxic elements such as Ag and Fe. With high surface/volume ratio, the ZnO nanostructures have been shown to be prominent photocatalyst candidates with enhanced photocatalytic efficiency, owing to their being low‐cost, non‐toxic, and able to be produced with easy and controllable synthesis. Thus, ZnO nanostructures‐based photocatalysis can be considered as an eco‐friendly and sustainable process.
This paper presents the photocatalytic activity of ZnO nanostructures (NSs) grown on different substrates. The photocatalysis has been carried out both under classic mode and microfluidic mode. All tests show the notable photocatalytic efficiency of ZnO NSs with remarkable results obtained from a ZnO‐NSs‐integrated microfluidic reactor, which exhibited an important enhancement of photocatalytic activity by drastically reducing the photodegradation time. UV‐visible spectrometry and high‐performance liquid chromatography, coupled with mass spectrometry (HPLC‐MS), are simultaneously used to follow real‐time information, revealing both the photodegradation efficiency and the degradation mechanism of the organic dye methylene blue.
... However, due to low activity area and photocorrosion, its effect is inhibited in practical applications (Promdet et al. 2019). A large number of studies have been conducted to enhance the performance of ZnO, in which ZnO that have different forms have been confirmed by constructing bases, and co-precipitation methods have been proven to be effective preparation methods (Pan et al. 2015;Pastor et al. 2019;Guo et al. 2019;Ozer et al. 2017;He et al. 2020;Jayaraman et al. 2019;Koe et al. 2020;Sankar and Ashok 2020;Chen et al. 2021). ...
In this study, CaO prepared by calcination treatment from abandoned Achatina fulica shell was used as a raw material, and the floral CaO/ZnO photocatalytic composite material was prepared through co-precipitation method. SEM study showed ZnO with spindle-like petals in the range of 500–1000 nm grown on the surface of CaO carrier. The mapping image shows that the base component of the floral structure is mainly CaO, which is because CaO is not only in the reaction as a carrier, but also creates an alkaline environment in the methanol system, which is advantageous for co-precipitation. UV-vis spectroscopy shows that the visible light absorption of composites has red shifts; besides, PL, EIS, and photocurrent test showed that the composites have stronger electronic hole separation capabilities. The visible light degradation test of rhodamine B showed that CaO/ZnO photocatalytic composite could degrade 90% of the pollutants in 25 min, superior to CaO and ZnO, exhibiting recyclability properties, which is a potential candidate with cost-effective and sustainable photocatalysts.
... Therefore, research has been carried out to design alternative photocatalysts to TiO 2 . Several interesting alternative De-NO x advanced photocatalysts have been proposed lately, such as α-Fe 2 O 3 [18], g-C 3 N 4 [19], Fe 3 O 4 /mpg-C 3 N 4 (mpg: mesoporous graphitic) [20], BaWO 4 /g-C 3 N 4 [21], Cu 0.08 In 0.25 ZnS 1.41 [22], LaFeO 3 -SrTiO 3 [23], FeOOH-Carbon quantum dots [24], WO 3 / ZnO [25], ZnO@SiO 2 [26], Bi/Bi 2 O 2− x CO 3 [27], Au/La-Bi 5 O 7 I [28], BiSbO 4 [29] and Bi 2 MoO 6 [30], between others. ...
New efficient photocatalysts are required to remediate polluted urban atmospheres. We prepared a series of highly dispersed aqueous miscible organic (AMO) solvent treated NiTi layered double hydroxide (NixTi-AMO-LDHs (x = 2,3)) and studied their performance in the photocatalytic abatement of NOx gases. These photocatalysts can be prepared by a simple and scalable coprecipitation method at room temperature. NixTi-AMO-LDHs (x = 2,3) have been prepared with specific N2 BET surface areas and pore volumes up to to 492 m²g–1 and 1.37 cm³ g–1 respectively. Under sunlight simulation, the De-NOx performance of the conventional LDH and P25 (TiO2) benchmarks were surpassed by 40% and 17% respectively by the NixTi-AMO-LDHs. The NixTi-AMO-LDHs photocatalyst also exhibited outstanding reusability and unusually low release of the toxic NO2. The selectivity for the De-NOx process was investigated by in situ DRIFT measurements, the high surface area and pore volume was observed to play a significant role on the adsorption of the NO2 and N2O4 intermediates. The 2D character of the AMO-LDHs contributed to an enhancement in the production of radical species under illumination and reduction charge carrier recombination.
... In that way, efforts were made to replace TiO 2 by other photocatalysts, such as zinc oxide (ZnO). ZnO is indeed well known for its high photocatalytic activity (Liu et al., 2019;Shinde et al., 2017;Pastor et al., 2019), its good chemical stability, its non-toxicity (Li et al., 2008) and its low cost for both raw materials and nanostructure synthesis by hydrothermal method. Moreover, the hydrothermal route has already proven its easiness and its efficiency to synthesize ZnO nanostructures (ZnO NSs) onto non-conventional substrates as stainless-steel mesh film (Wang et al., 2012), flexible PDMS , polyester fibbers and other ceramics (Danwittayakul et al., 2013). ...
Atmospheric pollution is a major issue affecting environment and health. Therefore, semi-conductor-nanoparticles-based photocatalytic infrastructures appeared as a promising solution to improve urban air quality due to their abilities to mineralize toxic organic compounds. To avoid the direct use of free nanoparticles, as nanomaterials are suspected to be carcinogenic, this work intends to develop civil engineering materials functionalized by direct growth of ZnO nanostructures as a more environmentally friendly and biocompatible approach to reduce the air pollution. By scaling-up an innovative and low-cost hydrothermal direct growth synthesis, a few square meters paved with tiling and bitumen road were easily produced in order to evaluate their photocatalytic activity at large scale under solar lamp in a climatic chamber (Sense-City, 3200 m³) to reflect real atmospheric air purification situations. Observations provide insights into their ability to simultaneously remove various pollutants from a real car exhaust (O3, COX, NOX, VOCs) and their durability, as well as understandings of the weather conditions impact on the photocatalytic degradation of these pollutants and of the relationships between pollutants trend evolution. This study also seems to prove that, when exposed to sunlight, bitumen roads are responsible of pollutants emission and causes modifications in the Chapman cycle.
... However, due to low activity area and photocorrosion, its effect is inhibited in practical applications (Promdet et al. 2019). A large number of studies have been conducted to enhance the performance of ZnO, in which ZnO that have different forms of different forms have been con rmed by constructing bases and co-precipitation methods have been proven to be effective preparation methods (Pan et al. 2015;Pastor et al. 2019;Guo et al. 2019). ...
In this study, CaO prepared by calcination treatment from abandoned Achatina fulica shell was used as a raw material, and the nano-flower-like CaO/ZnO photocatalytic composite material was prepared through co-precipitation method. SEM study showed ZnO with spindle-like petals in the range of 500-1000 nm grown on the surface of CaO carrier. The mapping image shows that the base component of the nanometer flower is mainly CaO, which is because CaO is not only in the reaction as a carrier, but also creates an alkaline environment in the methanol system, which is advantageous for co-precipitation. UV-vis spectroscopy shows that the visible light absorption of composites has red shifts, besides, PL, EIS and photocurrent test showed that the composites have stronger electronic hole separation capabilities. The visible light degradation test of rhodamine B showed that CaO/ZnO photocatalytic composite could degrade 90% of the pollutants in 25 min, superior to CaO and ZnO, exhibiting recyclability properties, which is a potential candidate with cost-effective and sustainable photocatalysts.
... By comparison to conventional technologies for air purification, photocatalytic materials allow that air purifiers can be operated under ambient conditions to degrade a range of pollutants without much input power [89]. Photocatalytic materials composed of TiO 2 [90], ZnO [91], CdS [92], or Fe (III)-doped TiO 2 are extensively used for air purification systems [93], which are pure or doped metal oxide semiconductors. UV light is the most common light source for photocatalytic air purifiers. ...
Air pollution by toxic chemicals and pathogens (bacteria, viruses, and fungi) has become one of the most critical problems worldwide because it has strong negative impacts on human health, both indoors and outdoors. Photocatalysis is a promising candidate to purify the atmosphere by removing pollutants among several physicochemical and biological methodologies. In this review, we discuss photocatalytic materials used for air purification, advanced methodologies for photocatalysts synthesis, and principles of photocatalytic nanoparticles that decompose pollutants, which include inorganic and organic compounds as well as dangerous pathogens. Recent achievements, outlook, and future directions in the field are discussed.
... Photo catalytic degradation in conjunction with nanostructured oxides materials is an environmental friendly alternative for the treatment of these dyes [27][28] and has become choice for the wide range of researchers [29][30][31] where photo catalysts are used to remove the pollutants in the presence of light source. ZnO is playing dominating role as photo catalysts [32][33][34][35] while compared to other catalysts like [36][37] due to its low cost, higher absorption efficiency than titania, easy to grow, high stability in thermal, chemical and physical environments [38][39][40] and is utilized in wide range of applications. Moreover, as it is a direct wide band gap material having 3.37 eV of band gap it finds applications like in LEDs, laser diodes, photovoltaic, semiconductor heterojunction, transparent conductors, and so on [41][42][43][44]. ...
We have fabricated ZnO nano rods by hydrothermal method and successively doped them with tin (Sn) using different concentrations of 25, 50, 75 and 100 mg of tin chloride. XRD of the fabricated structures showed that ZnO possess hexagonal wurtzite phase. Scanning electron microscopy (SEM) was used to explore the morphology and it shows nanorod like morphology for all samples and no considerable change in the structural features were found. The dimension of nanorod is 200 to 300 nm. The doped materials were then investigated for their photo catalytic degradation of environmental pollutant Rhodamine B. The performance of doped ZnO is compared with the pristine ZnO. Scanning electron microscopy (SEM) was used to explore the morphology and it shows nanorod like morphology for all samples and no considerable change in the structural features were found. The dimension of nanorod is 200 to 300 nm. XRD of the fabricated structures showed that ZnO possess hexagonal wurtzite phase. Photo catalytic activity of rhodamine B was investigated under UV light and a maximum degradation efficiency of 85% was obtained. The optical property reveals the reduction in band gap of upto 17.14% for 100 mg Sn doped ZnO. The degradation is followed by the pseudo order kinetics. The produced results are unique in terms of facile synthesis of Sn doped ZnO and excellent photo degradation efficiency, therefore these materials can be used for other environmental applications.
... For the removal of pollutants, different techniques have been reported, including adsorption (Zhu, Zong, & Wu, 2018), coagulation (Moghaddam, Moghaddam, & Arami, 2010), chemical oxidation (Türgay, Ersöz, Atalay, Forss, & Welander, 2011), electrochemical processes (Siedlecka, Ofiarska, & Borzyszkowska, 2018), membrane separation processes (Fan, Gu, & Meng, 2018), advanced oxidation processes (AOPs) (Rajoriya, Bargole, George, & Saharan, 2018), and photo-oxidation (Alamelu, Raja, Shiamala, & Jaffar Ali, 2018;Cao, Xu, & Lin, 2012).Among of these methods, oxide semiconductor photocatalysis has attracted significant attention in wastewater purification thanks to its mild reaction conditions, high photocatalytic activity, and low energy consumption (Lee, Lai, Ngai, & Juan, 2016;You, Guo, Guo, & Liu, 2019). Due to photoelectric and antioxidant properties, the zinc oxides (ZnO) has gained widely researched compared with other photo-catalytic oxide semiconductor (Huang, Li, & Lin, 2014;Pastor, Balbuena, Cruz-Yusta, Pavlovic, & Sánchez, 2019;Sampaio, Bacsa, & Benyounes, 2015). The organic dyes can be decomposed to CO 2 and H 2 O by the ZnO photocatalyst (Singhal, Dixit, & Shukla, 2018). ...
Ag/ZnO photocatalysts with different silver content (0, 1, 2, and 3 mol%) were fabricated through a hydrothermal method. Characterization results of XRD showed the ZnO was a hexagonal wurtzite structure, and the Ag was a face-centered cubic (fcc) structure. The morphologies results suggested that the particles with nonuniform shapes (nano-rods, nano-spheres, and nano-sheets). UV–Vis DRS absorbance spectra unveiled a decrease in the band gap for Ag/ZnO with the introduction of Ag, indicating the improvement of visible light absorption. All Ag/ZnO samples demonstrated great photocatalytic activity (the highest reached 96.7% within 40 min) and were better than commercial ZnO and as-prepared ZnO samples. The introduction of Ag nanoparticles with the surface plasmon resonance (SPR) effect can significantly improve the photocatalytic performance for Ag/ZnO photocatalysts. Moreover, ·O2⁻ was proved to be the primary active group in the photoreaction. The Ag/ZnO photocatalysts also had good recycling stability and still maintained a high photodegradation rate (93.7%) after five recycles.
The emission of toxic VOCs (Volatile Organic Compounds) in the environment causes serious problems and, if from one side there are directives to reduce this threat, from the other side there are stringent necessities for their removal. Several methods have been proposed to remove toxic VOCs, but little attention has been paid towards their photooxidation using materials with improved photodetection. For these reasons, we account here on the good photocatalytic performance of semiconducting [email protected] nanostructures that revealed to be excellent for both ultraviolet and visible light detection, thanks to the wide ZnO band gap and visible Au surface plasmon resonance, towards the photooxidation of some toxic Volatile Organic Compounds. In particular, we tested these peculiar materials for the removal of some common VOCs as toluene, ethanol and formaldehyde under solar light irradiation. The [email protected] showed excellent performance in the total oxidation of toluene (95% of conversion) and formaldehyde (85%) with the consequent formation of only CO2 and water as by-products. This good activity was also confirmed in the photooxidation of ethanol that allowed to obtain the 72% of CO2 selectivity. Furthermore, in all tests the catalysts showed a good stability after several consecutive runs. The efficient electronic communication between the gold core and the zinc oxide shell permitted to enhance the overall photodetection of the solar irradiation, improving also the charge carriers separation. The [email protected] nanostructures can be considering promising candidates as photocatalysts for air purification.
There is a growing concern about the impact of air quality on human health. The industrialization based on the large use of fossil fuels has led to increased production of harmful air pollutants, such as volatile organic compounds (VOCs), carbon monoxide (CO), nitrogen and sulfur oxides (NO x , SO x ), pathogens, and particulate materials. Looking at cost‐effective ways to eliminate such pollutants, nanostructured materials that can act as photocatalysts for air purification are one of the potential energy‐efficient methods for pollution control, adding new functionalities to traditional building materials. In this chapter, an overview of the recent development of air purification photocatalytic systems based on nanostructured metal oxide semiconductors, carbon‐based materials, and heterostructures is provided as well as state‐of‐art material design strategies aimed at improving their performances.
Novel 4-nitrobenzenaminium benzenesulfonate (4NBASA) was synthesized and evaluated using Fourier Transform Infrared (FTIR), 1H and 13C Nuclear Magnetic Resonance (NMR), Ultra-Violet Visible (UV-Vis) spectroscopy, and Single-Crystal X-ray Diffraction (SC-XRD). In addition, theoretical calculations include optimized structure analysis. The (HOMO) high occupied molecular orbital and (LUMO) lowest unoccupied molecular orbital analysis, UV-Vis. parameters with the gas base model, MEP-Molecular Electrostatic Potential, and Non-Linear Optical (NLO) properties were accomplished. All theoretical computations were completed using the DFT/B3LYP functional and the 6-311G++ (d, p) basis set in the ground state. The assignments of estimated infrared vibrational frequencies were accomplished for the first time utilizing the VEDA4 program through the optimized structure, experimental and theoretical data correspond well. In particular, the title compound 4NBASA revealed potent bactericidal activity with maximum of (16.6±1.1mm) at 2.5 µg/mL against Staphlococcus aureus (MTCC 3615) and minimum zone of inhibition of (10.3±0.5mm) on Yersinia enterocolitica (MTCC 840) and also obtained excellent antifungal activity. Further, investigations are warranted to explore their promising NLO properties and other biochemical properties.
ZnO–CdS composites with different loadings of CdS were prepared by a simple wet chemical synthesis method for NOx degradation under visible light. The influence of CdS presence on the morphological and structural features of the synthesized composites was investigated through SEM, nitrogen adsorption–desorption isotherms, DRS, XRD, and FTIR analyses. XRD results and SEM images revealed the growth of ZnO nanoparticles along the “c” axis due to the presence of CdS leading to the formation of composites with needle/flower-like structures depending on the CdS content. The increased surface area and the enhanced visible light harvesting (e.g., ~ 2.4 eV) were observed for the photocatalyst containing CdS, as BET and DRS results show. The photocatalytic activity measurements evidenced that the NOx degradation using ZnO–CdS composites (60%) is higher comparatively with pristine ZnO (20%). This can be ascribed to the enlargement of surface area, the formation of flower-shaped structure, and the extended visible light response of the composites. Results indicate that there is on optimum value for CdS loading and the ZnO-15% CdS shows the best photoactivity for NO decomposition (60%). The proposed mechanism shows that the creation of heterojunction structure between CdS and ZnO and the subsequent charge separation can be responsible for the high photocatalytic performance of the as-prepared composites. Based on the results, this work represents a way forward in development of ZnO application in NOx abatement in urban environments.
Environmental air pollution is a major concern for society at present. Due to increasing road traffic, the presence of nitrogen oxides gases (NO x ) in urban areas exceeds the recommended limits for human health, as the harmful NOx are responsible for thousands of early deaths worldwide. The heterogeneous photocatalytic oxidation (PCO) assists as a simple, easy, efficient, and advanced method to be implemented in urban architecture in order to combat NOx pollution. The first successful applications have been obtained with the use of TiO 2 ‐based building materials. In the last few years, research on DeNO x materials has largely increased with the aim of finding the best photocatalyst, considering the efficiency, sustainability, and cost as key parameters. With the aim of increasing the light harvesting ability and selectivity in the PCO process for the photocatalyst, a wide range of new oxide nanomaterials are being studied, from the new functionalization of titania to new metal oxides/compounds (BiOI, Bi 2 MoO 6 , ZnO, SnO 2 , Fe 2 O 3 , LDHs, …). Apart from the inherent light activity ability of each compound, strategies considering changes in micro/nano‐morphology, doping, electronic heterojunctions, or oxygen vacancy engineering, among others, are successfully implemented in the search of enhanced DeNO x photocatalysts. The latest advances in this field are presented and critically discussed in relation to the current state‐of‐the‐art and emerging perspectives for future development.
Air pollution is a major component of environmental degradation in the 21st century worldwide due to the increased use of fossil fuels in various industries and their contribution to contaminating the natural atmosphere around us. Environmental pollution is posing a challenge to human existence and the continuation of human generation on this planet. Environmental degradation is considered a major affecting factor of climate change which includes both global warming driven by industrial emissions of green gases and large-scale shift in weather patterns. This problem is growing on a large scale throughout the world and poses challenges to survival and sustainability. Nanotechnology being universal technology can be used to solve this problem of environmental degradation by cleaning the degraded air on large scale and can counter the contribution of natural and industrial pollution. In this paper, the proposal of construction and use of nanotechnology based mega machine is analyzed and the prospectus and challenges of such machine production and maintenance for large scale air cleaning are discussed. The advantages, benefits, constraints, and disadvantages of production and use of such mega machines from various stakeholders’ points of view are identified. Finally, the idea of self-sustainable nanotechnology filters based mega machines is proposed using solar/wind energy supported systems for automatic and self-sustainable use.
The detrimental effects of gaseous nitrogen oxides (NOx, with x = 1 and 2) on both human health and the environment have triggered efforts aimed at the development of solar-activated photocatalysts for their efficient removal. In this regard, Fe2O3-WO3 and Fe2O3-CuO nanoheterostructures were prepared by a two-step vapor phase route. In particular, chemical vapor deposition (CVD) of β-Fe2O3, a scarcely investigated iron(III) oxide polymorph, was followed by radio frequency-sputtering of WO3 or CuO under mild conditions. The adopted strategy enabled the obtainment of nanoheterostructures with a peculiar pyramidal morphology and a uniform dispersion of CuO or WO3 onto the pristine iron(III) oxide. In this work, the chemical composition of the target systems was investigated by means of x-ray photoelectron and x-ray excited-Auger electron spectroscopies. In addition to the identification of elemental chemical states, the reported results confirmed the formation of pure and O-deficient systems, in which the direct interplay between the single components opens the door to air purification using the developed systems as photocatalysts.
The synthesis of pure ZnO along with nominal Mn and Co co-doping using a time-efficient and cost-effective wet chemical route to explore their detailed dielectric response is reported. Structural investigations revealed the development of hexagonal crystal symmetry of pristine ZnO. When 5% Mn is substituted, a slight decrease in lattice parameters is observed. However, Co doping did not affect the crystal symmetry as well as lattice parameters. The morphological study exposed that Mn and Co incorporation in ZnO significantly affected the shape, size, and distribution of particles. The maximum porosity is noticed when 5% of Mn is doped in pure ZnO. The elemental analysis confirmed the presence of all elements according to their stoichiometric formula. The relatively higher value of the dielectric constant is noticed in the present case while the value of loss tangent is explained based on oxygen vacancies generated during the auto-combustion process. Short-range electron transport is distinguished from long-range on the basis of relaxation peaks present in electric modulus. The effective contribution of conducting grains and insulating grain boundaries in different frequency regimes is explained by their impedance response.
The drawbacks of low utilization efficiency of solar energy and high recombination rate of photogenerated charge carriers limit the application of ZnO in photocatalytic reaction. In this study, a novel photocatalyst of ZnO decorated with polydopamine (PDA) and Au nanoparticles was successfully fabricated, using zeolite imidazolate frameworks (ZIF‐8) as ZnO precursor, followed by decorating with PDA and Au nanoparticles. The photocatalytic performance in degradation of rhodamine B (RhB) demonstrated that ZnO@PDA‐Au‐3 exhibited the most excellent photocatalytic activity. It could degrade 99 % of RhB in 24 min under UV‐vis light irradiation, in contrast with the degradation rate of 40 % and 16 % on ZnO@PDA and ZnO, respectively. The excellent catalytic activity was attributed to the contribution of PDA and Au. Particularly, Au not only promoted the absorption of visible light through localized surface plasmonic resonance (LSPR) effect, but also effectively facilitated the separation of photogenerated charge carriers though the Schottky barrier. A new kind of composite photocatalysts was successfully synthesized through a sequential process of calcinating ZIF‐8, assembling polydopamine (PDA) and reducing Au precursor. ZnO@PDA‐Au‐3 exhibited the highest degradation rate of 99 % within 24 min under UV‐vis light irradiation, while ZnO@PDA and ZnO displayed degradation rate about 40 % and 16 %, respectively. Au played a crucial role in the enhancement of the photocatalytic performance.
Nano-TiO2 photocatalysts with different genders of Spilosoma niveus wing template microstructures were prepared using tetrabutyl titanate ethanol solution as the precursor by ultrasonic impregnation and calcination. The microstructure of the material was characterized by SEM, XRD, BET, FT-IR, etc. In addition, the performance of the photocatalyst was evaluated by the tauc curve and photocatalytic performance test. Additionally, 82% and 73% of RhB were degraded by male and female TiO2 photocatalyst with wing template microstructure within 10 min, which was superior to 59% of that of same-scale TiO2 nanofiber tube. The TiO2 photocatalyst prepared in this study perfectly replicated the 3D Spilosoma niveus wing structure, reduced the band gap width, and had a positive effect on photodegradation of organic pollutants.
Various semiconductor-based nanomaterials were explored and advanced for effective visible-light absorption, charge separation, and charge transfer process in the field of photocatalysis. The potential nanomaterials used for environmental remediation can be generally classified as metal oxide, complex metal oxides, and metal sulfide-based photocatalysts. This chapter reviews the overall performance of these nanomaterials in photocatalysis for pollutants remediation present in our environment.
Although photocatalytic materials have been under extensive investigation for several decades, attempts to improve the current well-studied photocatalysts and develop novel photocatalysts for indoor air purification are still ongoing. The present work examines the progress made over the past three years in the application of photocatalysts for degrading gas-phase air pollutants. Three main issues are considered: (1) the basic principles of using photocatalytic degradation for air purification are briefly elucidated, including mass transfer, photocatalytic reaction mechanisms, degradation pathways, and strategies to enhance the photocatalytic efficiency; (2) novel photocatalytic systems with improved photocatalytic performance for the removal of air pollutants are discussed, focusing on state-of-the-art photocatalysts; and (3) future trends in the development of photocatalytic materials for commercialization are considered.
In this work the ability of Zn2-xCuxCr-CO3 layered double hydroxides (LDHs) as highly efficient DeNOx photocatalysts was studied. LDHs with x = 0, 0.2 and 0.4 were prepared using a coprecipitation method. The samples were characterized by different techniques such as XRD, XPS, FT-IR, ICP-MS, TG, SBET, SEM and Diffuse reflectance (DR). The increased amount of copper ions in the LDH layers gave rise to slight changes in the structure and morphology and an important variation of the optical properties of the LDHs. The prepared ZnCuCr-CO3 photocatalysts exhibited favourable conversion efficiency (51 %) and an extraordinary selectivity (97 %) for the photochemical NO abatement. The photochemical mechanism was elucidated from DOS, EPR, Femtosecond transient absorption and in-situ DRIFTS studies. The results suggested that the presence of Cu²⁺ ions in the LDH framework introduced new states in the valence band states, thus favouring the production and mobility of e⁻/h⁺ charge carriers and a greater production of ⋅O2⁻ and ⋅OH.
In this study, hierarchical porous ZnO photocatalysts were successfully prepared using bionic material rice husk as biological template. Bi2WO6/pure ZnO and Bi2WO6/hierarchical porous ZnO catalysts with different Bi2WO6 doping amounts were prepared via hydrothermal synthesis. The results show that the specific surface area of Bi2WO6/hierarchical porous ZnO catalyst with rice husk as biological template was 131.8505 m2/g, which was 101 m2/g higher than that of Bi2WO6/pure ZnO catalyst without rice husk as biological template. In addition, the hierarchical porous ZnO photocatalyst with rice husk template increased the degradation efficiency of microcystin (MC-LR) by 10.60% compared with commercially available ZnO photocatalyst. When the optimum doping amount was 0.2 mol/mol, the degradation efficiency of MC-LR by 0.2Bi2WO6/hierarchical porous ZnO was 79.0%, which was higher than that of the composite catalyst without rice husk template and single photocatalyst. This study provides a new idea for the degradation of MC-LR by using green photocatalyst.
Air pollution is a major component of environmental degradation in the 21st century worldwide due to the increased use of fossil fuels in various industries and their contribution to contaminating the natural atmosphere around us. Environmental pollution is posing a challenge to human existence and the continuation of human generation on this planet. Environmental degradation is considered a major affecting factor of climate change which includes both global warming driven by industrial emissions of green gases and large-scale shift in weather patterns. This problem is growing on a large scale throughout the world and poses challenges to survival and sustainability. Nanotechnology being universal technology can be used to solve this problem of environmental degradation by cleaning the degraded air on large scale and can counter the contribution of natural and industrial pollution. In this paper, the proposal of construction and use of nanotechnology based mega machine is analyzed and the prospectus and challenges of such machine production and maintenance for large scale air cleaning are discussed. The advantages, benefits, constraints, and disadvantages of production and use of such mega machines from various stakeholders’ points of view are identified. Finally, the idea of self-sustainable nanotechnology filters based mega machines is proposed using solar/wind energy supported systems for automatic and self-sustainable use.
A Mn3O4/BiOCl heterojunction photocatalyst was synthesized via a facile one-step hydrothermal synthesis method and characterized. The optical properties and electron characteristic are also investigated. The prepared Mn3O4/BiOCl was used to remove NO under the simulated solar light irradiation, and the long-period and multiple-cycles, capture experiments, ESR and in situ DRIFTS were used to investigate the stability of Mn3O4/BiOCl, the main active species and reaction products. The results showed that Mn3O4 nanoparticles are successfully deposited on micro-flower BiOCl to form a heterojunction interface between Mn3O4 and BiOCl. Compared with pure BiOCl, the surface oxygen vacancies of Mn3O4/BiOCl increases, and Mn3O4 acts as an electron acceptor to promote the transfer of electrons from BiOCl to Mn3O4. The enhancement of the optical properties is ascribed to a good energy band structure of Mn3O4/BiOCl, facilitating photogenerated carrier separation. Mn3O4/BiOCl achieves about 75% of NO removal efficiency within 10 min and exhibits superior inhibition ability for NO2 under light irradiation, but the photocatalytic activities gradually decrease due to the accumulation of products. When 5 or 10 vol% H2O is added into the simulated gas, the NO removal efficiency has been increased over Mn3O4/BiOCl, but the inhibition effect of NO2 is slightly weakened. The heterojunctions, optical properties and morphologies of Mn3O4/BiOCl are stable but the oxygen vacancies increase after reaction. The produced O2⁻ and OH radicals are active for the oxidation of NO to NO3⁻ under light irradiation, which is due to the Z-scheme heterojunctions of Mn3O4/BiOCl.
Heterojunction photocatalysts, which can alleviate the low carrier separation efficiency and insufficient light absorption capacity of a single catalyst, have received widespread attention. However, the specific interfacial structure of the heterojunction and its effect on the photocatalytic reaction is still unclear. Herein, a battery of zinc oxide/zinc sulfide ([email protected]) heterojunction microspheres with different degrees of sulfuration were successfully constructed via a facile hydrothermal method. The as-prepared photocatalysts shown decent aerobic nitric oxide (NO) oxidation performance under visible light irradiation, and the results of various characterization techniques illustrated that the superior photoactivity could be ascribed to the spatial separation of photoinduced electron-hole pairs due to the synergy of the internal electric field and the band offset. More importantly, density functional theory (DFT) calculations revealed that the heterojunction interface can significantly promote the generation of reactive oxygen species (ROS) and NO⁺ reaction intermediates and thus accelerate the photocatalytic reaction. Finally, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technology was used to time-dependently monitor the NO oxidation process, revealing the photocatalytic mechanism. This work investigated the role of the heterojunction interface in the gas-phase catalytic reaction, broadening the practical application of the [email protected] heterojunction.
This work presents the results of pulsed plasma-chemical modification of silicon dioxide nanopowder with zinc oxide nanoparticles (ZnO@SiO 2 ). The obtained ZnO@SiO 2 powders were characterized by transmission electron microscopy (TEM) and X-ray phase analysis. The size of the synthesized particles was in the range of 20–100[Formula: see text]nm. The photocatalytic characteristics of ZnO@SiO 2 were studied. When exposed to ultraviolet radiation, the methylene blue (MB) decomposes efficiently. Two samples characterized by the content of silicon tetrachloride in the initial mixture were synthesized. The band gap estimated from the absorption spectra calculated from the diffuse reflectance spectra for these samples was 2.4[Formula: see text]eV and 2.95[Formula: see text]eV for indirect transitions and 3.03[Formula: see text]eV and 3.24[Formula: see text]eV for direct allowed transitions.
Advancement of photocatalytic systems that restricted formation and emission of toxic by-product NO2 remained a great challenge regarding NO removal. To circumvent this issue, in this study composites Bi4O5Br2/Bi2S3 were constructed through a facile anion exchange route that sulfurized pure Bi4O5Br2 (BB) in CS2 under room temperature. In-situ formed Bi2S3 (BS) was identified and intimately connected with BB to form n-p heterojunctions. Under visible light, n-p heterojunction composites BB-BS showed significantly improved NOx removal and selectivity for NO2⁻/NO3⁻ comparing to BB, although photocatalytic removal over NO was almost identical. Specifically, NOx removal by the best candidate BB-BS60 was 1.5 times that of BB, while selectivity for NO2⁻/NO3⁻ increased from 60% to 90%, causing extremely low formation of NO2. The generation of NO2⁻/NO3⁻ was identified by FT-IR spectra and XPS analyses. The variation of photocatalytic performance and relevant selectivity was systemically discussed and mainly related to boosting generation of radicals OH and O2⁻, strengthened visible-light absorption, and well-matched band structures of both components in the Z-scheme mode. Eventually, these ultra-stable composites could be utilized for successive five runs and catalytic performance was well preserved without surface cleaning. This work might shed light on the construction of suitable photocatalytic systems with significant improvement of total NOx removal and avoidance of toxic by-product NO2 formation or emission.
Marine Pollution Convention has been updated to prevent the excessive emissions from diesel machine. For high concentrated, large fluxed SO2 and NO photocatalytic oxidation, a novel dual stages oxidation-scrubbing process was proposed. The p-n junction ZnO-CuO/rGO ternary catalysts were prepared via the co-precipitation route and supported on reduced graphene oxide (rGO). The UV-vis spectra showed that the bandgap Eg of ZnO-CuO/rGO was notably narrowed to 2.3-2.6 eV for facilitating electron transfer, and its flat band potential of was -0.54 V versus Ag/AgCl. The PL spectra and photocurrent curves revealed that the formation of heterojunctions and the introduction of graphene promoted an efficient separation of the photo-induced charge carriers. The photocatalytic activity of the ZnO-CuO/rGO with 5wt% GO was evaluated for simultaneous DeSOx and DeNOx, and 97% desulfurization rate and 64% denitration rate could be reached under GHSV 2000 h⁻¹ for 270 min. In seawater scrubber, ZnO-CuO/rGO(5%) addition made SO3²⁻ conversion was 80.72%. The radical scavenging experiments demonstrated that holes (h⁺) and superoxide radicals (•O²⁻) contributed most to SO3²⁻ photocatalytic oxidation. Kinetic studies showed that SO3²⁻ oxidation on ZnO-CuO/rGO fitted the L-H model. The mechanism of photocatalytic oxidation in gas and liquid phases were also proposed to address the reaction pathway.
Synthesis of zinc oxide nanoparticles by pulse laser ablation method has been successed carried out. Synthesis was carried out in aquades medium with a repetition rate variation of 5 Hz, 10 Hz and 15 Hz pulse laser yielding brown colloids. The higher laser repetition rate, the colloidal color will be more dark brown. Characterization of zinc oxide nanoparticles includes UV-Vis, SEM-EDX, FTIR and XRD. The image of SEM shows that zinc oxide nanoparticles have a round shape. Measurement of particle distribution with imageJ software from SEM images showed that ZnO nanoparticles were 23.63 nm, 12.13 nm and 5.59 nm for 5 Hz, 10 Hz and 15 Hz shots. The EDX spectrum analysis results show that only Zn and O atoms in the ZnO nanoparticles colloid are synthesized. FTIR results showed that sprocket ZnO was formed at wave number 457 cm-1 and 545 cm-1. The XRD analysis results also show some peaks known as the ZnO phase. This indicates that ZnO nanoparticles have been formed. The testing of the antibacterial activity of ZnO nanoparticles using a liquid dilution method with nanoparticle concentrations of 40 ppm, 60 ppm and 80 ppm. The test results showed the percentage of degradation of Escherichia coli bacteria at concentrations of 40 ppm, 60 ppm and 80 ppm respectively at 89.60%, 97.76% and 98, 70%.
A novel ternary Z-scheme Ag3PO4/g-C3N4/ZnO photocatalyst has been prepared and was characterized by XRD, FTIR, XPS, SEM and TEM, UV-Vis DRS, Transient fluorescence spectra and EIS Nyquist plots to describe the morphology, structure and photocatalytic performance. The result of the characterization shows that the three substances successfully formed a composite Ag3PO4/g-C3N4/ZnO which exhibits enhanced performance and stability, due to the good electron transfer ability and suitable band structure. The degradation rate of tetracycline hydrochloride can reach 88.48% under the best conditions and more sensitive to sunlight. After using four times, the Ag3PO4/g-C3N4/ZnO still shows good degradation efficiency, indicating advanced photocatalytic properties and improved stability. Finally, a possible Z-scheme mechanism is proposed on the basis of the results of characterization and degradation experiments, showing that ZnO performs as electronic conductor between Ag3PO4 and g-C3N4, while Ag3PO4 and g-C3N4 generate h⁺ and •O2⁻. The resulting formation of Z-scheme structure showed decreased photocorrosion and improved stability.
This study systematically investigated the photocatalytic activity of dissolved state biochar (DSB) with different pyrolysis temperature to the degradation of atorvastatin (ATV), a medicine widely used to combat hyperlipidemia. It was found that the photocatalytic efficiency of DSB increased with the decrease of pyrolysis temperature, that is, DSB300 (DSB with 300 °C of pyrolysis temperature) had the greatest photocatalytic activity in same condition, which was attributed to the dual role of DSB300 as heterogeneous photocatalyst and photosensitizer. The mineral components were responsible for the heterogeneous photocatalytic activity of DSB300. Organic carbon components could synergistically enhance the heterogeneous photocatalytic activity by enhancement of electron-hole separation, and contribute to the formation of singlet oxygen (1O2) and triplet-excited state (3DSB*) as well. The identification of intermediate products and X-ray photoelectron spectroscopy (XPS) analysis of irradiated DSB300/ATV revealed that cross-coupling reaction between ATV and DSB existed in the photodegradation process of ATV. The detailed photodegradation pathways of ATV were proposed, which was triggered by oxygen insertion of pyrrole ring and hydroxyl addition. Meanwhile, the modification of DSB300 under irradiation was evidently attenuated with ATV as shown by multiple characterizations, which helped to keep the stability of DSB300 in photochemical reaction process.
In this work the ability of ZnAlFe-CO3 layered double hydroxides (LDHs) as highly efficient UV–Vis light photocatalysts for the photochemical oxidation of NO gas was studied. LDHs with 3.5 to 4.1 M²⁺/M³⁺ and 0.33 to 1.55 Fe/Al ratios were prepared by a coprecipitation method. The samples were characterized by different techniques such as XRD, XPS, FT-IR, ICP-MS, TG, SBET, SEM and Diffuse reflectance (DR). The increased presence of the Fe³⁺ ions gave rise to changes in the structure, morphology and optical properties of the LDHs. The prepared ZnAlFe-CO3 systems exhibited increased surface area and enhanced visible-light absorbance. The photochemical NO abatement resulted in outstanding conversion efficiency (56%) and selectivity (93%) for the iron containing samples, due to a decrease of the e⁻/h⁺ recombination, higher generation of O2– and OH radicals and their NO2 adsorption ability.
In this work, bismuth (Bi) nanoparticle anchored ZnWO4 microspheres (Bi/ZnWO4) were prepared and used as robust and efficient photocatalysts for NO removal at parts-per-billion level under visible light irradiation. The as-synthesized composite with a proper mass ratio of Bi (50%) displayed higher reaction rate (0.067 min-1) than its single counterparts ZnWO4 (0.004 min-1) and Bi (0.027 min-1), respectively. Due to the surface plasmon resonance (SPR) effect of Bi nanoparticles, the Bi/ZnWO4 composites showed broad light absorption in the visible spectrum. Moreover, the formation of Bi/ZnWO4 hetero-interface promoted the separation of photo-excited electron-hole pairs, which is demonstrated by the increased photocurrent density in comparison to the pristine materials. The above characteristics endowed the Bi/ZnWO4 composites with superior photocatalytic activity for NO removal. The radical scavanger tests revealed that the superoxide radical was the main active species to initiate NO oxidation, while the hydroxyl radical was not involved in the process. This study shows practical value in air pollutant abatement, because it provides an economical and feasible route to fabricate SPR-enhanced composite photocatalysts using earth abundant Bi material instead of noble metals.
A new composite of TiO2 (P25) and N-doped carbon quantum dots (P25/NCQD) was prepared by a hydrothermal method and was used for the first time as catalyst of the photo- oxidation of NO under UV and visible light irradiation. P25/NCQD composite exhibited a NO conversion (27.0%) more than two times higher of that observed for P25 (10%) under visible light and the selectivity of the process was increased from 37.4% to 49.3%. The composite also showed better photocatalytic performance than P25 in the UV region with increases of 36.3% on NO conversion and 16.8% on selectivity. Moreover, compared with P25, the photodegradation ratio of methylene blue was enhanced from 68% to 91% after UV irradiation for 1 h. NCQD played a crucial role on the photocatalytic activity improvement of P25/NCQD, increasing visible light absorption, slowing the recombination and improving the charge transfer.
Synthesis of monodispersed nanostructures would be attractive if the synthesis is simple, one step and cost effective. In this work, an eco-friendly approach for the synthesis of ZnO nanostructures using the aqueous extract of pomegranate peel has been described. ZnO nanoflakes were formed initially upon 15 min sunlight irradiation of an aqueous solution of Zn acetate in the presence of pomegranate peel extract. We observed that the nanoflakes formed are converted to ZnO nanopencils upon mild heating. The nanopencils have been characterized by diffused reflectance spectroscopy, X-ray diffraction analysis, photoluminescence spectroscopy, FT-Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. Furthermore, we examined the photocatalytic activity of the ZnO nanopencils and compared the performance with commercial ZnO and TiO2 (P25) nanoparticles towards the photocatalytic degradation of methylene blue under direct sunlight irradiation. The total carbon removal efficiency of the ZnO nanopencils for methylene blue is 95% which was achieved by 4.5 h.
Single phase of crystalline ZnO with controllable morphologies, including spindle-like, sunflower-like, dandelion-like and disk-like structures samples, have been successfully synthesised via a simple and effective low-temperature approach in a mixed solution of zinc nitrate and hexamethylenetetramine, with and without 2-aminoethanol as an accelerant. The ZnO crystal with different morphology can be obtained by simply varying the additive and reaction time. The samples were characterised by XRD, SEM, DRS and BET. The prepared ZnO with the sunflower-like superstructure showed excellent photocatalytic activity for the decomposition of NO<sub align="right"> x </sub> gas under UV light irradiation, being superior to that of a standard TiO<sub align="right"> 2 </sub> (Aeroxide P25) powder.
Mesocrystalline TiO 2 nanoparticles were synthesized using a hydrothermal approach. A simple two-step procedure at low temperature (<140 °C) allowed the nucleation of primary particles sized 2-4 nm and their subsequent assembly as almost spherical aggregates sized ≈20 nm. X-ray powder diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) studies, and HRTEM studies confirmed anatase as the unique TiO 2 crystalline phase. The mesocrystalline structure of the anatase aggregates was clearly evidenced by HRTEM and SAED results. The mesocrystalline nanopowders exhibit a mesoporous structure with a surface area and pore volume of 63.5 m 2 g −1 and 0.22 cm 3 g −1 , respectively. Ultraviolet (UV) and visible light (Vis) absorption ability were recorded. The combined high effectiveness and selectivity for the NO x abatement of the new mesocrystalline photocatalyst are reported. It is worth remarking that the maximised selectivity values reached for the NO x process are reported for the first time and could be associated with the mesoporous nature of the anatase photocatalyst.
Semiconductors-based photocatalysis is very efficient for the removal of kinetically-inert dilute NO, but the elimination of its atmospheric NO2 emission is still a great challenge. In this study, complete visible light NO oxidation mediated by O2 is achieved over a defect-engineered BiOCl with selectivity exceeding 99%. Well-designed oxygen vacancies on the prototypical (001) surface of BiOCl favored the formation of geometric-favorable superoxide radicals (•O2 ) in a side-on bridging mode under ambient condition, which thermodynamically suppressed the terminal end-on •O2 associated NO2 emission in case of higher temperatures, and thus selectively oxidized NO to nitrate. These findings can help us to understand the intriguing surface chemistry of photocatalytic NO oxidation and design highly efficient NOx removal systems.
Mesocrystalline TiO2 nanoparticles are synthesized by a hydrothermal approach. A simple two-step procedure at low temperature (< 140 ºC) let the nucleation of primary particles sized 2-4 nm and their subsequent assembly as almost spherical aggregates sized = 20 nm. X-ray powder diffraction (XRD), Raman and X-ray photoelectron (XPS) spectroscopies, and HRTEM studies confirm the anatase as unique TiO2 crystalline phase. The mesocrystalline structure of the anatase aggregates is clearly evidenced by HRTEM and SAED. The mesocrystalline nanopowders exhibit a mesopore structure with surface area and pore volume of 63.5 m2•g-1 and 0.22 cm3•g-1, respectively. Apart from ultraviolet (UV), visible light (Vis) absorption ability was recorded. The combined high effectiveness and selectivity on the NOx abatement of the new mesocrystalline photocatalyst is reported. It is to remark that the reached maximized selectivity through the NOx process is reported by the first time and could be associated with the mesopore nature of the anatase photocatalyst.
In this study, we report that layered double hydroxides (LDH) exhibited high photocatalytic activities in degrading NOx gases for the first time. ZnAl-CO3 LDHs with a 1.5–3.0 Zn/Al ratio were prepared by a coprecipitation method both with and without hydrothermal treatment. Syntheses were carried out with high and low metal concentrations, the latter being the most favorable in obtaining pure LDHs in the whole Zn/Al ratio range. The samples were characterized by different techniques such as PXRD, FT-IR, ICP mass, TGA, SBET, SEM and Diffuse reflectance (DR). The LDH particles grew as well-defined hexagonal nanolayers, whose size and crystallization depended on the synthetic procedure and the Zn/Al ratio. Those samples with lower crystallinity exhibit the highest specific surface area values (>50 m²·g⁻¹). The ZnAl-CO3 LDHs were UV light responsive with band–gap values close to 3.5 eV. The LDH photocatalysts show a high performance towards the photochemical oxidation process of NO gas, with efficiencies of around 55%. Remarkably, the ZnAl-CO3 photocatalysts exhibit an impressive selectivity towards the deNOx process, avoiding the emission of the toxic NO2 gas into the atmosphere. Interestingly, these promising deNOx results are repeated when working for a long irradiation period or with the highest concentration of NO in polluted atmospheres.
α-Fe2O3 catalyst supported on rice husk ash (RHA) with enhanced photochemical NO oxidation is reported. The Fe2O3/SiO2 composites are prepared through the calcination of RH and FeCl3 at temperatures of 600 and 900 °C, during 1 and 4 h. Fe2O3 crystalizes covering the cob-shaped silica skeleton. The thermal procedure determines the mineralogical composition, crystalline growth and surface area of samples. Samples obtained at 600 °C are α-Fe2O3 (hematite) with low levels of Fe2SiO4 and exhibit a microstructure constituted of meso/macropores, with surface area values of 19.6 and 16.1 m² g⁻¹. At 900 °C only pure hematite is obtained but the microstructure is denser with very small surface area. Increasing the time of calcination to 4 h, hexagonal plates and truncated pseudocubes large crystals are obtained. All the samples exhibit light absorption in the 300 to 900 nm UV–vis range. The photocatalytic oxidation of NO molecules is related to the surface area of the samples. In the best case an NO conversion value of 24% is found, the highest reported for hematite photocatalysts. Moreover, the selectivity of NO → NO2 → NO3⁻ oxidation process reaches values as high as 72% with the presence of large hexagonal plate hematite crystals. The existence of particles, which exhibit {001} facets, favors the adsorption of NO2 molecules and, therefore, the achievement of the photochemical oxidation process.
A novel mesoporous structure of zinc oxide was synthesized in hydrothermal autocalve in the presence of a functional ionic liquid (FIL) {[CH2CH2] O2 (mm)2}. This FIL with ether groups was used simultaneously as a designer templating agent and a source of the hydroxyl radical. The presence of this ionic liquid led to producing ethylene glycol in the reaction media, which adsorb on the surface of mesoporous hexagonal ZnO plates. These mesoporous structures can adsorb pollutant gases and increase photocatalytic oxidation of pollutant gases in compare with commercial ZnO nanoparticles and agglomerated nanoparticles synthesized in this work. XPS data confirmed ethylene glycol production by the ionic liquid, which could prove a role for ionic liquids as designers. The estimated BET surface area values of ZnO hexagonal mesoporous plates and agglomerated particles were 84 m²/g and 12 m²/g respectively. Optical properties of the mesoporous structures were analyzed by photoluminescence spectroscopy and diffuse reflectance UV–visible spectroscopy. The performance of these structures as efficient photocatalysts was further demonstrated by their removal of NOx, SO2 and CO under UV irradiation. The removal of NOx, SO2 and CO under UV irradiation was 56%, 81%, and 35% respectively, after 40 min of irradiation time. Reusability of the photocatalyst was determined; the results show no significant decrease of activity of photocatalyst. after five cycles.
A modified polymeric precursor method assisted by N-sources (urea or melamine) was used to obtain anion-doped ZnO nanoparticles. The influence of these molecules on the physical-chemical and photocatalytic properties of the as-synthesized samples was investigated. The ZnO nanoparticles exhibited a hexagonal wurtzite phase and crystallite sizes of approximately 20 nm. The addition of urea or melamine to the Zn2+ precursor solution improved the surface properties of the materials and resulted in controlled growth of the N-doped ZnO nanoparticles, with urea showing superior performance for this purpose. These changes led to improved photocatalytic performance in the degradation of methylene blue dye and ethionamide antibiotic under UVC irradiation. It was observed that the indirect mechanism involving *OH radical attack played the main role in both photodegradation reactions catalyzed by the as-synthesized ZnO samples, while the photosensitization mechanism had a negligible influence. The use of ESI-MS analyses showed that the MB dye molecules were broken up by the action of the ZnO photocatalyst, indicating the occurrence of a mineralization process.
Zinc oxide (ZnO), an inorganic compound that appears as a white powder, is used frequently as an ingredient in sunscreens. The aim of this review was to examine the toxicology and risk assessment of ZnO based upon available published data. Recent studies on acute, sub-acute, and chronic toxicities of ZnO indicated that this compound is virtually non-toxic in animal models. However, it was reported that ZnO nanoparticles (NP) (particle size, 40 nm) induced significant changes in anemia-related hematologic parameters and mild to moderate pancreatitis in male and female Sprague-Dawley rats at 536.8 mg/kg/day in a 13-week oral toxicity study. ZnO displayed no carcinogenic potential, and skin penetration is low. No-observed-adverse-effect level (NOAEL) ZnO was determined to be 268.4 mg/kg/day in a 13-week oral toxicity study, and a maximum systemic exposure dose (SED) of ZnO was estimated to be 0.6 mg/kg/day based on topical application of sunscreen containing ZnO. Subsequently, the lowest margin of safety (MOS) was estimated to be 448.2, which indicates that the use of ZnO in sunscreen is safe. A risk assessment was undertaken considering other routes of exposure (inhalation or oral) and major product types (cream, lotion, spray, and propellant). Human data revealed that MOS values (7.37 for skin exposure from cream and lotion type; 8.64 for skin exposure of spray type; 12.87 for inhalation exposure of propellant type; 3.32 for oral exposure of sunscreen) are all within the safe range (MOS > 1). Risk assessment of ZnO indicates that this compound may be used safely in cosmetic products within the current regulatory limits of 25% in Korea.
The utilization of photocatalytic technology to remove air pollutants has attracted global interest. However, it still suffers from low removal activities under visible light irradiation. In this study, we demonstrated that the switching of the semiconducting behavior from n-type to p-type can efficiently improve the photocatalytic activity of g-C3N4 for nitric oxide (NO) removal about 3.5 times. This is due to that such switching could change the majority of carriers in g-C3N4 from electrons to holes. Interestingly, the photocatalytic removal of NO in both n-type and p-type g-C3N4 is proceeded via hole oxidation. More importantly, p-type g-C3N4 displays strong stability in both photocatalytic performance and crystal structures. This study provides a new strategy to improve the photocatalytic activity of semiconductors for air pollution removal.
Spindle-shaped anatase TiO2 mesocrystals with a single-crystal-like structure were successfully produced through a novel green synthetic approach. These anatase mesocrystals could be easily transformed into porous single crystals or Ti³⁺ self-doped anatase mesocrystals upon heat treatment under certain atmospheres. It was revealed that the as-transformed Ti³⁺ self-doped anatase mesocrystals exhibited enhanced activity towards the visible-light-driven photocatalytic oxidation of NO. More information can be found in the Full Paper by H. Chen, J. Ye, et al. (DOI: 10.1002/chem.201605294).
Modified TiO2 nanoparticles were prepared by a Pickering emulsions route with the use of hydrophobic (oleylamine) and hydrophilic (polyethylene glycol) organic surfactants and their combination, as well. The attachment of modifiers to the TiO2 nanoparticles was proved via FT-IR and thermal analysis. Thicker modifier layer was formed on the TiO2 surface but lower average particle size was attained when amphiphilic modification was applied that was revealed by TEM analysis and dynamic light scattering, correspondingly. The amphiphilic modification permitted control of the hydrophobic properties of the TiO2 nanoparticles so that their incorporation in cement matrix led to preparation photocatalytic cement materials with superior De-NOx efficiency.
Mesocrystals are of great interest for a wide range of applications owing to their unique structural features and properties. However, the realization of well-defined metal oxide mesocrystals through a facile and green synthetic approach still remains a great challenge. Herein, we report a novel synthesis strategy for the production of spindle-shaped anatase TiO2 mesocrystals with a single-crystal-like structure, which was simply achieved via one-step hydrolysis reaction of TiCl3 in the green and recyclable media polyethylene glycol (PEG-400) without any additives. Such anatase mesocrystals were constructed from ultrasmall nanocrystal subunits (1.5~4.5 nm in diameter) and formed through oriented aggregation of the tiny nanocrystals pre-formed in the reaction system. Owing to their novel structural characteristics, the as-synthesized anatase mesocrystals could be easily fused in situ into porous single crystals by annealing in air. More significantly, after being annealed in vacuum, Ti3+ ions could be easily induced in the anatase crystal lattice, resulting in the formation of Ti3+ self-doped anatase mesocrystals. It was demonstrated that the thus-transformed Ti3+ self-doped anatase mesocrystals exhibited enhanced visible light activity toward photocatalytic oxidation of nitric oxide (NO) to NO3-, which could be largely attributed to their intrinsic Ti3+ self-doping nature as well as high crystallinity and high porosity of the mesocrystalline architecture.
A novel method, that the precursor is burnt in argon without further reduction to directly prepare metallic catalyst, is developed using low-valued by-product. Rice husk not only contributed as catalyst support, but also as reductant and fuel. The XRD, TPR and TG-DTA analysis prove that Cu phase in the as-burnt catalyst is almost reduced to metallic Cu⁰. The as-prepared catalysts exhibit higher activity and methanol selectivity than those prepared by a conventional impregnation method. This method may open a new way to prepare metallic catalysts without further reduction, especially for some catalytic reactions promoted by K, Ca, Mg and Mn.
ZnO/CdO rodlike nanocopmposites were successfully synthesized by using the hydrothermal method in the ionic liquid medium (4-methylpyridine,2,2,2-trifluoroacetate). The heterojunction structure of photocatalyst and nanorod structures of ZnO attached to nanoparticels of CdO improved the performance of it compared to pure ZnO and nanoparticles of ZnO/CdO. The conventional hydrothermal method was performed for the synthesis of these ccomposites. The average thickness of nanorods was 53 nm. The samples were characterized by XRD measurements, scanning electron microscopy (SEM), photoluminescence spectroscopy (PL), diffuse reflectance spectroscopy (DRS), BET, X-ray photoelectron spectroscopy (XPS). The photocatalytic performance of the heterojunction photocatalyst was investigated by using photocatalytic reactor for degradation of air pollutants (SO2, NOX).
Commonly, the NOx emissions rates of diesel vehicles have been assumed to remain stable over the vehicle's lifetime. However, there have been hardly any representative long-term emission measurements. Here we present real-driving emissions of diesel cars and light commercial vehicles sampled on-road over 15 years in Zurich/Switzerland. Results suggest.deterioration of NOx unit emissions for Euro 2 and Euro 3 diesel technologies, while Euro 1 and Euro 4 technologies seem to be stable. We can exclude a significant influence of high-emitting vehicles. NOx emissions from all cars and light commercial vehicles in European emission inventories increase by 5-10% accounting for the observed deterioration, depending on the country and its share of diesel cars. We suggest monitoring the stability of emission controls particularly for high-mileage light commercial as well as heavy-duty vehicles.
As a green and sustainable technology, semiconductor-based heterogeneous photocatalysis has received much attention in the last few decades because it has potential to solve both energy and environmental problems. To achieve efficient photocatalysts, various hierarchical semiconductors have been designed and fabricated at the micro/nanometer scale in recent years. This review presents a critical appraisal of fabrication methods, growth mechanisms and applications of advanced hierarchical photocatalysts. Especially, the different synthesis strategies such as two-step templating, in situ template-sacrificial dissolution, self-templating method, in situ template-free assembly, chemically induced self-transformation and post-synthesis treatment are highlighted. Finally, some important applications including photocatalytic degradation of pollutants, photocatalytic H2 production and photocatalytic CO2 reduction are reviewed. A thorough assessment of the progress made in photocatalysis may open new opportunities in designing highly effective hierarchical photocatalysts for advanced applications ranging from thermal catalysis, separation and purification processes to solar cells.
Commonly, the NOx emissions rates of diesel vehicles have been assumed to remain stable over the vehicle's lifetime. However, there have been hardly any representative long-term emission measurements. Here we present real-driving emissions of diesel cars and light commercial vehicles' sampled on-road over fifteen years in Zurich/Switzerland. Results suggest deterioration of NOx unit emissions for Euro 2 and Euro 3 diesel technologies, while Euro 1 and Euro 4 technologies seem to be stable. We can exclude a significant influence of high-emitting vehicles. NOx emissions from all cars and light commercial vehicles in European emission inventories increase by 5% to 10% accounting for the observed deterioration, depending on the country and its share of diesel cars. We suggest monitoring the stability of emission controls particularly for high-mileage light commercial as well as heavy-duty vehicles.
A comparison is made in this study of the effectiveness of various commercial catalysts in the oxidation of NOx by heterogeneous photocatalysis. The following catalysts were considered: Aeroxide TiO2 P25, Aeroxide TiO2 P90, Hombikat UV-100, Kronos vlp7000, CristalACTIV PC105, CristalACTIV PC500, Kemira 650 and Anatasa Aldrich. All catalysts were deposited by a dip-coating technique onto borosilicate 3.3 glass plates. Optimization of catalyst load showed no significant enhancement of photoactivity, in general, above a deposited mass of 1.16 mg cm-2. Differences between photocatalyst activity were more apparent at longer illumination times. Photoactivity decreased in the presence of humidity and differences in the adsorbed products were detected. Photocatalyst activity was strongly influenced by specific surface area, with the best results obtained by the catalysts with the largest surface area, namely the PC500, Hombikat and Kronos. Photocatalyst stability was demonstrated in successive reuse cycles.
Today, a major issue about water pollution is the residual dyes from different sources (e.g., textile industries, paper and pulp industries, dye and dye intermediates industries, pharmaceutical industries, tannery and craft bleaching industries, etc.), and a wide variety of persistent organic pollutants have been introduced into our natural water resources or wastewater treatment systems. In fact, it is highly toxic and hazardous to the living organism; thus, the removal of these organic contaminants prior to discharge into the environment is essential. Varieties of techniques have been employed to degrade those organic contaminants and advanced heterogeneous photocatalysis involving zinc oxide (ZnO) photocatalyst appears to be one of the most promising technology. In recent years, ZnO photocatalyst have attracted much attention due to their extraordinary characteristics. The high efficiency of ZnO photocatalyst in heterogeneous photocatalysis reaction requires a suitable architecture that minimizes electron loss during excitation state and maximizes photon absorption. In order to further improve the immigration of photo-induced charge carriers during excitation state, considerable effort has to be exerted to further improve the heterogeneous photocatalysis under UV/visible/solar illumination. Lately, interesting and unique features of metal doping or binary oxide photocatalyst system have gained much attention and became favourite research matter among various groups of scientists. It was noted that the properties of this metal doping or binary oxide photocatalyst system primarily depend on the nature of the preparation method and the role of optimum dopants content incorporated into the ZnO photocatalyst. Therefore, this paper presents a critical review of recent achievements in the modification of ZnO photocatalyst for organic contaminants degradation.
The presence of NO
X
gases (NO+NO2) in the atmosphere is a major concern of society because of their associated adverse and harmful effects. In order to remove the NO
X
gases from the air, photocatalysis arises as an innovative and promising
technique. Through the use of photochemical oxidation processes the NO and NO−
2 gases are oxidised to NO−
3 form and thus removed from the air. In recent years new nanomaterials are being developed by researchers with the aim to enhance
their photocatalytic activity to combat the NO
X
pollution. The main focus is devoted to preparing new TiO2 based compounds with the highest specific surface area (SSA), different morphology and chemical modifications. In order to increase the SSA, different substrates
were used to disperse the TiO2 nanoparticles: organic and carbon fibres, mesoporous materials, clays composites and nanoporous microparticles. In the other hand, high photocatalytic performances were obtained with nanotubes, self-orderer nano-tubular films and nanoparticles with
the lowest size. Conversely, when TiO2 is doped with ions the oxide exhibited a better photocatalytic performance under visible light, which is related to the creation of intermediate energy states between the conduction band and the valence band. Alternatively, visible light photocatalysts
different from titanium oxide have been studied, which exhibit a good De-NO
X
efficiency working under λ > 400 nm visible light irradiation.
In this paper, an enhanced visible-light photocatalytic oxidation (PCO) of NO (∼400ppm) in the presence of the graphitic carbon nitride (g-C3N4) treated by the alkaline hydrothermal treatment is evaluated. Various g-C3N4 samples were treated in different concentrations of NaOH solutions and the sample treated in 0.12molL(-1) of NaOH solution possesses the largest BET specific surface area as well as the optimal ability of the PCO of NO. UV-vis diffuse reflection spectra (DRS) and photoluminescence (PL) spectra were also conducted, and the highly improved photocatalytic performance is ascribed to the large specific surface area and high pore volume, which provides more adsorption and active sites, the wide visible-light adsorption edge and the narrow band gap, which is favorable for visible-light activation, as well as the decreased recombination rate of photo-generated electrons and holes, which could contribute to the production of active species. Fluorescence spectra and a trapping experiment were conducted to further the mechanism analysis of the PCO of NO, illustrating that superoxide radicals (O2(-)) play the dominant role among active species in the PCO of NO.
Copyright © 2015 Elsevier B.V. All rights reserved.
Photocatalysis refers to the oxidation and reduction reactions on semiconductor surfaces, mediated by the valence band holes and conduction band electrons, which are generated by the absorption of ultraviolet or visible light radiation. Photocatalysis is widely being practiced for the degradation and mineralization of hazardous organic compounds to CO2 and H 2O, reduction of toxic metal ions to their non-toxic states, deactivation and destruction of water borne microorganisms, decomposition of air pollutants like volatile organic compounds, NOx, CO and NH 3, degradation of waste plastics and green synthesis of industrially important chemicals. This review attempts to showcase the well established mechanism of photocatalysis, the use of photocatalysts for water and air pollution control, visible light responsive modified-TiO2 and non-TiO2 based materials for environmental and energy applications, and the importance of developing reaction kinetics for a comprehensive understanding and design of the processes.
A series of self-nitrogen-doped and Zn-doped TiO2/C@SiO2 (ZTRH) nanoporous composites with high surface area and excellent biological hierarchical porous structure have been synthesized by sol-gel method at different temperatures. The porous SiO2 and activated carbon from the decomposition of rice husk (RH) were regarded as the template and porous catalytic carrier. The structure, crystallinity, morphology, and other physical-chemical properties of the samples were characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transforminfrared spectroscopy, N2 adsorption-desorption isotherms, and UV-vis diffuse reflectance spectroscopy. The feasibility of ZTRH nanoporous composites for decomposing pollutants was evaluated by rhodamine B under visible light irradiation. Compared with the traditional photocatalytic materials, ZTRH nanoporous materials not only performed high efficiency in pollutants degradation, but also exhibited good adsorption properties. Moreover, since RH ash acts as the catalyst support, the composites are easily recycled after catalysis reaction. The ZTRH nanoporous composites could be applied to the wastewater with different pH values and be regarded as a kind of promising recyclable photocatalyst in photodegradation of pollutants in water.
Gold nanoparticles (Au NPs) based catalysts have received much attention in the past two decades because of their unique catalytic properties in basic and applied research. Both experimental findings and theoretical predictions show that the size of Au NPs plays a crucial role in governing their catalytic capability, with smaller Au NPs typically exhibiting higher catalytic performance. Although mesoporous silica has been extensively used as Au NPs support, the diffusion of reactants and products in the pores has been a challenge. Herein, we report an alternative silica material, silica NPs from rice husks (RHs), which possess a rough surface, as potential support for Au NPs. Notably, in this study, RHs, a byproduct from rice production, was used as the silica source. Silica obtained from calcining HCl treated RHs was first modified by (3-aminopropyl)triethoxysilane (APTES), which was designed to play dual roles: one was to help absorb Au precursor (AuCl4-) onto silica surface and the other was to stabilize the resultant Au NPs obtained by reducing AuCl4- using sodium borohydride (NaBH4). Characterizations of the nanostructures revealed that Au NPs formed with a narrow size distribution of ca. 2-4 nm, which is very critical for catalytic applications. The reduction of 4-nitrophenol by NaBH4 with RH-silica supported Au NPs as a catalyst was systematically studied to demonstrate their excellent catalytic performance.
As an alternative to the gold standard TiO2 photocatalyst, the use of zinc oxide (ZnO) as a robust candidate for wastewater treatment is widespread due to its similarity in charge carrier dynamics upon bandgap excitation and the generation of reactive oxygen species in aqueous suspensions with TiO2. However, the large bandgap of ZnO, the massive charge carrier recombination, and the photoinduced corrosion-dissolution at extreme pH conditions, together with the formation of inert Zn(OH)2 during photocatalytic reactions act as barriers for its extensive applicability. To this end, research has been intensified to improve the performance of ZnO by tailoring its surface-bulk structure and by altering its photogenerated charge transfer pathways with an intention to inhibit the surface-bulk charge carrier recombination. For the first time, the several strategies, such as tailoring the intrinsic defects, surface modification with organic compounds, doping with foreign ions, noble metal deposition, heterostructuring with other semiconductors and modification with carbon nanostructures, which have been successfully employed to improve the photoactivity and stability of ZnO are critically reviewed. Such modifications enhance the charge separation and facilitate the generation of reactive oxygenated free radicals, and also the interaction with the pollutant molecules. The synthetic route to obtain hierarchical nanostructured morphologies and study their impact on the photocatalytic performance is explained by considering the morphological influence and the defect-rich chemistry of ZnO. Finally, the crystal facet engineering of polar and non-polar facets and their relevance in photocatalysis is outlined. It is with this intention that the present review directs the further design, tailoring and tuning of the physico-chemical and optoelectronic properties of ZnO for better applications, ranging from photocatalysis to photovoltaics.
Three types of N-doped (BiO)2CO3 hierarchical microspheres composed of 2D nanosheets were fabricated by a one-pot template-free hydrothermal method from bismuth citrate and an ammonia solution under different hydrothermal temperatures. The as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, nitrogen adsorption-desorption isotherms, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and UV-visible diffuse reflectance spectroscopy. Here, the ammonia worked as a nitrogen resource, doping (BiO)2CO3, and a solvent hydrolyzing the bismuth citrate. The temperature was found to influence the thickness of the nanosheets and the morphology of the N-doped (BiO)2CO3 microspheres. The fascinating morphology of N-doped (BiO)2CO3, including rose-like, hydrangea flower-like and peony flower-like microspheres, can be controllably fabricated by adjusting the hydrothermal temperature. The photocatalytic activities of the as-prepared samples were evaluated towards the removal of NO under visible-light irradiation. Compared to pure (BiO)2CO3, the N-doped (BiO)2CO3 hierarchical microspheres showed enhanced photocatalytic activity. In addition, the hydrangea flower-like and the peony flower-like N-doped (BiO)2CO3 microspheres displayed admirable photocatalytic activity. Finally, a possible formation mechanism of the various morphologies of N-doped (BiO)2CO3 mediated by the reaction temperature was proposed. This work could provide new insights into the controlled synthesis of photocatalytic nano/microstructures for potential environmental applications.
In-situ DRIFT spectra of (a) MnTiO2 and TiO2, and (b) Mn/ZrO2 and ZrO2 in NO + O2 at 250 °C.
MnOx accelerates the NO oxidation on the surface of the catalysts. TiO2 only provides the support surfaces for MnOx, while ZrO2 provides both the MnOx supports and the main adsorption sites for intermediate nitrates.
In the present paper, kinetics and mechanism of NO and oxygen coadsorption on TiO2 at room temperature, which is the key step of the catalytic removal of NOx pollutants from air, were studied. NO adsorption on TiO2 in the absence of oxygen is weak and reversible, but it is found to strongly increase in the presence of oxygen. The ratio between the amount of adsorbed NO and O2 in the course of adsorption is constant and close to three. A FTIR spectroscopic study reveals that the amount and composition of N-containing species on the TiO2 surface strongly depend on the contact time with the initial NO–O2 mixture and on its composition. At relatively small exposures, IR bands assigned to NO– and nitrosyl complexes Tin+–NO (n = 3–4) are predominant in the spectra. With increasing contact time, NO– disappears, and IR bands of NO3– and possibly NO2– appear and grow. The thermal stability of surface nitrates and nitrites correlates with their structure. IR spectra observed upon NO2 adsorption are similar to those after exposure to NO–O2 mixtures. Exposure of the sample with preadsorbed 14NO2 to gaseous 15NO results in a change in the IR spectra that suggests isotopic replacement of 14N with 15N in the adsorbed species. In the TPD profiles, after adsorption of NO–O2 and NO2, desorption peaks of NO and NO2 dominate which presumably arise from the thermal decomposition of NO3– (NO2–) and nitrosyls Tin+–NO. A multistep scheme for the interaction of NO and O2 with TiO2 is suggested which accounts for the results of both techniques applied.
Oxygen-deficient TiO2 was prepared with a low-temperature method and utilized in photocatalytic removal of gaseous NO at the 400 ppbv level in air under visible light (420 nm < lambda < 700 nm) irradiation. Catalysts synthesized at 200 degrees C (TiO2-200) exhibited the highest ability to remove the NO gas in air under visible light irradiation. A higher oxidation ability for NO2 to NO3- leads to a higher conversion of NOx on TiO2-200. The relationship between the physicochemical properties and the photocatalytic performance of the as-prepared catalyst is discussed. The large surface area of TiO2-200 can provide more active sites for the reaction. The oxygen vacancies of TiO2-200 can effectively expand the absorption of visible light and accelerate the separation of photogenerated electrons and holes. The first-principles density functional theory (DFT) calculation further confirms the role of oxygen vacancies on the narrowing of the band gap and separating of photogenerated electron-hole pairs.
Rose-like nanostructures of ZnO were prepared by a mild solution-phase route using a hydrothermal method in the presence of the chiral ionic liquid (CIL) ditetrabutylammonium tartrate, [TBA](2)[L-Tar] and Mg2+. Brunauer-Emmett-Teller analyses of specific surface area showed great enhancement in ZnO rosette nanostructures compared to amorphous and nanosheet morphologies (33.4 m(2)/g and 16 m(2)/g, respectively). Average pore size of these architectures is about 17.5 nm from Barrett-Joyner-Halenda model calculation. This CIL with the tartrate anion has been used as a designer templating agent and a source of the hydroxyl radical simultaneously. X-ray photoelectron spectroscopy (XPS) data confirm radical production by the ionic liquid. The effects of the morphology of ZnO hierarchical nanostructures on NOx, CO, and SO2 removal efficiencies were studied. It was found that the rose-like ZnO nanostructures perform much better than those of other samples and commercial ZnO in the removal of NOx, SO2, and CO under UV irradiation with photodegradation efficiency of similar to 23%, similar to 92%, and similar to 34% respectively.
In this study, the fundamental aspects governing the environmental behavior of nitrogen oxides were explored from two air quality-monitoring stations (AQM) in Seoul, Korea, over an 11-year period (1996–2006). The two stations were selected to represent an urban roadside (U-RS) and urban background station (U-BG). The mean concentration levels of NOx at the U-RS area exhibited a gradual decrease through years (from ∼290 ppb (1996) to 100 ppb (2003)), while those measured at the U-BG site maintained fairly stable values (∼70 ppb) throughout the years. The mean concentration levels of NO and NO2 at the U-BG site were almost the same (∼35 ppb), while the U-RS site showed that the mean value of NO (∼127 ppb) is significantly higher than that of NO2 (∼60 ppb). The observation of larger differences in NO level between the two sites reflects the direct impact of vehicular emissions at the U-RS site. Both correlation and factor analyses were conducted between the hourly data sets of NOx species (N=∼90,000) and relevant environmental variables for both the sites. The results of both tests confirm the prominent role of vehicular activities in determining the level of nitrogen oxides at urban roadside environments, although such effects appear to be rather intricate at U-BG. Our comparative analysis of NOx data sets collected over varying temporal scales indicate that their distributions in the urban environment can be distinguished more clearly in spatial factors (i.e., between urban background and urban roadside) than other criteria.
The photocatalytic characteristics of the TiO2/ZnO nanofibers synthesized by electrospinning followed by calcinating at different temperatures to alter the anatase-to-rutile ratio are investigated. The results demonstrate that the photocatalytic activity of TiO2/ZnO nanofibers is enhanced by optimizing the anatase/rutile ratio among the trade-off effects of the band-gap energy, the electron/hole recombination rate, and the surface area. When calcined at 650 °C, the TiO2/ZnO nanofibers with optimal anatase/rutile ratio (48:52) balancing these trade-off effects have the highest photocatalytic efficiency both in the degradation of RhB in liquid and conversion of NO gas.
Visible light activated nanostructured TiO2 with nitrogen and fluorine co-dopants were prepared by the surfactant assisted sol–gel method and immobilized on glass substrates by dip coating. The films were inserted inside a continuous flow photoreactor and examined for the photocatalytic oxidation of NO air pollutant with initial concentration of 200–800 ppbv. The modified catalysts exhibited significant photocatalytic activity under daylight illumination, with maximum percentage of NO removal equal to 24.2% and photooxidation rate up to 0.66 μg m−2 s−1. The reaction rates increased proportionally to the incident light intensity whereas for the strongly absorbed UV light a deviation from linearity was observed. Mass balance during photooxidation was confirmed by determining the amount of NO3− product residues onto the photocatalyst surface.
Copper modified SiO2 thin films have been synthetized by sol–gel/dip coating process and annealed at 450 °C. The presence of crystallized CuO is only detected for the highest level of introduced Cu [Cu/(Cu+Si)=0.2]. UV–Visible absorption edge increases with the copper content in the film. Optical gap (Eg) and Urbach energy (Eu) extracted from the UV–Visible absorption curves evidence the large effect of the presence of copper species on silica optical properties. The decrease of the optical gap agrees with the lowering of the UV–Visible absorption peak energy calculated at the Time Dependent Density Functional Theory (TD-DFT) level for silica clusters without and with a copper atom.
Photocatalyst materials were prepared as a hybrid between TiO2 /SiO2 via low temperature hydrothermal method (150°C) without further heat treatment. Porous silica from rice husk ash was used as a support for fine TiO2 particles which acted as a photocatalyst when radiated with a UV light. TiO2-deposited SiO2 was successfully prepared through hydrolysis of TiOSO4 solution by controlling synthesis parameters such as pH ,concentration of TiOSO4, temperature and time under hydrothermal treatment. The obtained products were characterized for physical and chemical properties by means of XRD, XRF, BET and TEM . It was found that pH had an influence on the crystallization of TiO2, and under an appropriated pH, only anatase presented along with amorphous phase. High crystallinity of nano-crystalline anatase ( about 5 nm) deposited on silica surface was observed through TEM. Adsorption and photocatalytic performances of the prepared catalyst were evaluated in methylene blue aqueous solution in the dark and under ultraviolet ray irradiation, respectively. Due to the synergetic functions of adsorption by porous substrate and decomposition by TiO2 photocatalyst, an enhancing of photocatalytic activity for decomposition of organic pollutants in water under UV rays was obtained.