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Cu2O-Sensitized TiO2 Nanorods with Nanocavities for Highly Efficient Photocatalytic Hydrogen Production under Solar Irradiation
Abstract
Hydrogen trititanate (H2Ti3O7) nanorods were synthesized by using hydrothermal method.
The transformation of the crystal structure from H2Ti3O7 to TiO2 occurred into either single
crystalline TiO2 (B) [calcined at 400 or 450 C] or bicrystalline TiO2 (B) with anatase phases
[calcined at 500 or 550 C] during calcination process. Calcination temperature from 450 to
550 C induced the both phase transformation and formation of large size nanocavities, and
the changes in the nanorods morphology were confirmed using HRTEM/TEM images.
Nanocomposites of Cu2O/TiO2 nanorods with different copper loading (CuxTNR) were
prepared by using the wet impregnation method with TiO2 nanorods [calcined at 500 C] and
copper nitrate as copper source. The structural, optical, surface elemental and morphological
properties of the synthesized catalysts were extensively characterized. Solar photocatalytic
hydrogen (H2) production experiment was carried out with aqueous-glycerol solution for 4 h.
The photocatalytic activity of TiO2 nanorods that are calcined at 500 C were exhibited very
high rate of H2 production, is ascribed to the improved separation of electron/hole pairs and
catalytic activity at bicrystalline TiO2 surface. For the first-time, we have achieved the higher
rate of H2 (50,339 μmol.h-1.g-1
cat) production under the set of the optimised conditions using
Cu1.5TNR nanorods containing nanocavities as catalyst under solar irradiation. This
enhancement in the activity can be attributed to the desirable absorption of UV-visible light
in natural solar spectrum and minimization of the recombination of electron-hole pairs,
multiple internal reflection of light within nanocavities, which improved the surface-interface
reactions. The present study clearly demonstrated that Cu loaded on titania nanorods
containing nanocavities were found to be more efficient and promising photocatalyst for H2
production under solar light irradiation.
... Espécies de cobre como CuO possuem valores de banda de valência e banda de condução dentro da band gap do TiO 2 , o que permite a transferências de elétrons, sendo que CuO tem se mostrado bastante promissor na supressão da recombinação eletrônica(GUI et al., 2014). Outros trabalho como o deKumar et al. (2015)que propuseram uma modificação na estrutura do TiO 2 , preparando nanotubos de óxido e posteriormente dopando com cobre mostraram resultados promissores na produção fotocatalítica de hidrogênio sobre luz solar, segundo os autores o carregamento com até 1,5% de Cu leva a formação de uma bicristalinidade nas nanocavidades causando o deslocamento da banda de condução para níveis mais altos de energia, assim retardando o processo de recombinação eletrônica. Mais tardeChen et al. (2018), mostrou que a dopagem com Cu pode ser tão eficiente quanto Pt, o que reduz significativamente os custos de produção dos fotocatalisadores. ...
Glycerol is the main by-product during the trans-esterification of vegetable oils to biodiesel.
In this study, we investigate the process of photocatalytic hydrogen production from glycerol
aqueous solution, with the use of cobalt doped TiO2 photocatalyst under solar light irradiation.
Cobalt doped TiO2 photocatalysts are prepared by impregnation method and these
catalysts are characterized by XRD, EDAX, DRS, TEM, EPR and XPS techniques. DRS studies
clearly show the expanded photo response of TiO2 into visible region on impregnation of Co2þ
ions on surface of TiO2. XPS studies also show change in the binding energy values of O1s, Ti
2p and Co 2p, indicating that Co2þ ions are in interaction with TiO2. Maximum hydrogen
production of 220 m mol h�1 g�1 is observed on 2 wt% cobalt doped TiO2 catalysts in pure water
under solar irradiation. A significant improvement in hydrogen production is observed in
glycerol: water mixtures; and maximum hydrogen production of 11,021 m mol h�1 g�1 is obtained
over 1 wt% cobalt doped TiO2 in 5% glycerol aqueous solutions. Furthermore, to
evaluate some reaction parameters such as cobalt wt% on TiO2, glycerol concentration,
substrate effect (alcohols) and pH of the solution on the hydrogen production activity are
systematically investigated. When the catalysts are examined under UV irradiation, a 3e4
fold increase in activity is observed where this activity seems to decrease with time; however,
a continuous activity is observed under solar irradiation on these catalysts. The decreased
activity could be ascribed the loss of cobalt ions under UV irradiation, as evidenced by EDAX
and TEM analysis. A possible explanation for the stable and continuous activity of cobalt
doped TiO2 photocatalysts under solar irradiation is proposed.
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Stable reduced TiO2 rutile nanorods with well-defined facets were prepared by a solvothermal route in the presence of Zn powder. The oxygen vacancy in the TiO2 nanorods, which can be tuned by the amount of Zn, results in a narrow band gap and visible-light photocatalytic activity.
CuO was introduced into porous TiO2 nanorod through impregnation method. Before the impregnation step, TiO2 nanorod was hydrothermally synthesized from TiO2 powder in aqueous NaOH solution and followed by thermal treatment at 450 °C. The structures and properties of impregnated samples were characterized using various techniques, including XRD, BET, XAS, TEM, and UV-DRS. Their photocatalytic performance on simultaneous hydrogen production from pure water and aqueous methanol solution was also investigated under solar light. It was found that CuO/TiO2 nanorod possessed a high surface area, good photocatalytic property and excellent hydrogen generation activity. Incorporation of Cu ions into the lattice framework of anatase TiO2 nanorod enhanced the efficiency in visible region at 438–730 nm. Moreover, the XAS results showed that some Cu ions formed solid solution in the TiO2 nanorod (CuxT1−xO2). However, the excessive incorporation of Cu ions did not improve any ability of anatase TiO2 nanorod for production of hydrogen from pure water splitting. This could be due to the excessive CuO agglomeration at outside-pores which blocked the sensitization of TiO2 nanorod. Only 1% Cu/TiO2 nanorod was found to be a remarkable and an efficient photocatalyst for hydrogen production under solar light from both pure water and sacrificial methanol splitting. The highest rate of hydrogen production of 139.03 μmol h−1 gcatalyst−1 was found in sacrificial methanol which was 3.24% higher than in pure water.
In recent studies of inorganic materials for energy applications, surface modification processes have been shown to be among the most effective methods to enhance the performance of devices. Here, we demonstrate a facile nano-decoration method which is generally applicable to anatase TiO2 nanostructures, as well as a nano-decorated hierarchical TiO2 nanostructure which improves the energy conversion efficiency of a dye-sensitized solar cell (DSSC). Using a facile sol-gel method, 0-D, 1-D, and 2-D type anatase TiO2 nanostructures were decorated with 200 nm long anatase TiO2 nanorods to create various hierarchical nanostructures. A structural analysis reveals that the branched nanorod has a highly crystalline anatase phase with anisotropic growth in the [001] longitudinal direction. When one of the hierarchical structures, a chestnut bur-like nanostructure, was employed in a dye-sensitized solar cell as a scattering layer, offering increased dye-loading properties, preserving a sufficient level of light-scattering ability and preserving superior charge transport and recombination properties as well, the energy conversion efficiency of the cell improved by 19% (from 7.16% to 9.09%) compared to a cell with a 0-D TiO2 sphere as a scattering layer. This generally applicable anatase nanorod-decorating method offers potential applications in various energy-conversion applications, especially in DSSCs, quantum-dot solar cells, photoelectrochemical water-splitting devices, photocatalysis, and lithium ion batteries.
Solar light induced interfacial charge transfer of electrons from TiO2 to CuO in a water-glycerol mixture produced 99 823 μmol h(-1) g(-1)catalyst of hydrogen gas. The dispersed CuO/TiO2 photocatalyst in solution exhibited uni-directional electron flow and capture at the Schottky barrier facilitating charge separation and electron transfer resulting in enhanced H2 production performance.
CuOx based junctions e.g. CuOx-TiO2 have been demonstrated to be very effective in photocatalytic H2 production, but the active component at the junction is less understood. We herein report that an as-prepared CuOx-TiO2 photocatalyst undergoes an in situ restructuring process during photocatalytic H2 production under solar light irradiation to form the active Cu2O-TiO2 interfacial structure on the surface. These results unambiguously demonstrate that Cu2O is the active copper species in CuOx-TiO2 photocatalysts.
Synthesis of narrow-dispersed nanocrystalline TiO2 was investigated by surfactant-aided solvothermal synthetic method in toluene solutions. Titanium isopropoxide (TIP) was used as precursor, which was decomposed at high temperature in the surfactant-dissolved solution. After the solution was thermally treated at 250degreesC for 20 h in an autoclave, low-dispersed TiO2 nanocrystalline particles with average size of < 6 nm were synthesized. When sufficient amount of TIP or surfactant was added in the solution, long dumbbell-shaped nanorods were formed, which may be due to the oriented growth of particles along [001] axis. Characterization of products was investigated by X-ray diffraction and transmission electron microscopy.
Ag2O/TiO2 catalysts with varying amounts of Ag2O 0.5, 1, 2, and 5 wt% loadings are prepared by impregnation method and Ag/TiO2 catalyst is prepared by photo deposition method. These catalysts are characterized by XRD, SEM-EDAX, DRS, XPS and TEM techniques. DRS studies clearly showing the expanded photo response of TiO2 into visible region on impregnation of Ag+ ions on surface layers of TiO2 due to the increased number of energy states created by the silver ions in TiO2 surface lattice. TEM images are showing the fine dispersion of silver particles on TiO2 surface. XPS of Ag2O/TiO2 calcined and used catalysts along with Ag2O/TiO2 reduced and Ag/TiO2 photo deposited catalysts are compared and the binding energy values of Ti 2p, O1s and Ag 3d are confirming that silver ions are in interaction with TiO2 in Ag2O/TiO2 calcined catalysts. EDAX analysis supports the presence of silver species on the surface layers of TiO2. Photocatalytic hydrogen production activity studies are conducted over Ag2O/TiO2, Ag2O/TiO2 reduced and Ag/TiO2 photo deposited catalysts in pure water and methanol:water mixtures under solar irradiation. Maximum hydrogen production of 145 μmoles/h is observed on 0.5wt% Ag2O/TiO2 catalyst in pure water and the maximum hydrogen production of 3350 μmoles/h is observed on 1wt% Ag2O/TiO2 catalyst in methanol:water mixtures. Whereas Ag2O/TiO2 reduced and Ag/TiO2 photo deposited catalysts are not showing any hydrogen production activity either in water or in methanol:water mixtures under solar irradiation. Based on the XPS, DRS, TEM, SEM-EDAX studies and the hydrogen production activity on these catalysts, a structure–activity correlation has been proposed wherein the interacted Ag+ ions on the surface layers of TiO2 are playing an important role in maintaining the hydrogen production activity under solar irradiation.
In the present investigation, hydrogen production via water splitting by nano-ferrites was studied using ethanol as the sacrificial donor and Pt as co-catalyst. Nano-ferrite is emerging as a promising photocatalyst with a hydrogen evolution rate of 8.275 μmol h(-1) and a hydrogen yield of 8275 μmol h(-1) g(-1) under visible light compared to 0.0046 μmol h(-1) for commercial iron oxide (tested under similar experimental conditions). Nano-ferrites were tested in three different photoreactor configurations. The rate of hydrogen evolution by nano-ferrite was significantly influenced by the photoreactor configuration. Altering the reactor configuration led to sevenfold (59.55 μmol h(-1)) increase in the hydrogen evolution rate. Nano-ferrites have shown remarkable stability in hydrogen production up to 30 h and the cumulative hydrogen evolution rate was observed to be 98.79 μmol h(-1). The hydrogen yield was seen to be influenced by several factors like photocatalyst dose, illumination intensity, irradiation time, sacrificial donor and presence of co-catalyst. These were then investigated in detail. It was evident from the experimental data that nano-ferrites under optimized reaction conditions and photoreactor configuration could lead to remarkable hydrogen evolution activity under visible light. Temperature had a significant role in enhancing the hydrogen yield.
The photocatalytic reforming of glycerol to hydrogen over palladium and gold modified TiO2 catalysts is reported. The rate of hydrogen production exceeds that measured for the photoreforming of methanol over both
catalysts with the palladium catalyst performing approximately four times better than the gold. The reaction occurs under
ambient conditions and neither system suffered from poisoning after extensive testing. This therefore offers a potentially
attractive route by which one of the major waste products from the manufacture of biodiesel can be converted into a useful
product.
Although photovoltaic cells have great potential for supplying carbon-free energy, they suffer
from the lack of an effi cient and cost-effective energy storage process that can supply energy
for transportation and nighttime use. A direct way to convert solar energy into chemical fuels
would solve this problem. Of several possible schemes, the photon-driven electrolysis of
water to produce hydrogen and oxygen has been studied most. Photoelectrolysis of water
can be achieved with either self-supported catalysts or with photoelectrochemical cells.
This article will introduce the basic principles of solar water splitting and highlight recent
developments with semiconductor light absorbers and co-catalysts. The role of combinatorial
approaches in identifying new metal oxide visible light-absorbing semiconductors will be
briefl y described, and the potential of using nanomaterials for more effi cient devices will be
discussed. Separate articles in this special issue will focus on recent developments in watersplitting
concepts.
Hydrogen can be produced at ambient conditions via an efficient, technologically simple, ecologically benign, and potentially very low-cost process, with the use of a Pt/TiO2 photocatalyst and three abundant and renewable sources: biomass, solar light, and water. The method combines photocatalytic splitting of water and light-induced oxidation of biomass compounds into a single process, able to produce hydrogen at room temperature and atmospheric pressure.
We herein present a useful and novel strategy to redesign photoanodes by building smart TiO2 nanorod networks in and on a film of P25 nanoparticles (NPs) to optimize the comprehensive performance of dye-sensitized solar cells (DSSCs). By using a doctor-blade method followed by a facile hydrothermal treatment, an interesting hierarchical double-layered film consisting of P25 NPs and TiO2 nanorods (NRs) was fabricated on a fluorine-doped tin oxide (FTO) substrate. In our strategy, P25 NPs (underlayer) with a large surface area can potentially enable a large amount of dye absorption while TiO2 NRs (overlayer) as the scattering part would effectively strengthen the light harvesting ability. Moreover, TiO2 NRs inserted into the P25 NP film also provide conducting networks for fast photogenerated electron transport. As demonstrated in photoanodes for DSSCs, this hierarchical double-layered photoanode indeed exhibits superior DSSC performance to that of a pure P25 NP film; the photovoltaic conversion efficiency increases up to 8.62% under illumination of one sun (AM 1.5 G, 100 mW cm−2), which is significantly better than 6.12% for the pure P25 NP photoanode. This work highlights the significance of the rational design of photoanode electrodes for enhanced energy conversion applications.
The surface of a TiO2 material was modified by loading NiOx in 10 wt% by impregnation with nickel nitrate followed by calcination in air at different temperatures. The TiO2 and NiOx/TiO2 samples prepared were applied for photocatalytic H-2 production from glycerol and water at 50 degrees C. The H-2 evolution was enhanced by NiOx loading and was dependent on calcination temperature. A maximum H-2 evolution was observed with 450 degrees C calcined NiOx/TiO2 sample. The properties of those TiO2 and NiOx/TiO2 samples were characterized by nitrogen adsorption, XRD, UV/vis, and XPS measurements to examine factors responsible for the enhancement of photocatalytic H-2 evolution with NiOx loaded and calcined TiO2 samples.
Mixtures of straight and defect-free multiwalled carbon nanotubes (MWCNTs), single walled carbon nanohorns (SWCNHs), and multiwalled carbon nanohorns (MWCNHs) were prepared by the arc discharge method. Functionalized Carbon Nanotubes (FCNTs) composed of MWCNTs, MWCNHS and SWCNHs were obtained using acid functionalization process. TiO2/FCNTs (CTP) nanohybrid photocatalysts were obtained with different amounts of FCNTs (2–20 wt%), by wet impregnation method. The photocatalytic activity of both pristine TiO2 as well as the nanohybrids were studied in aqueous glycerol solution under solar irradiation. Reaction parameters such as amount of catalyst and amount of FCNTs in nanohybrids were optimized to ensure high rates of H2 production. With a relatively low amount of pristine TiO2, a high rate of H2 production of 2134 μmol h−1 gcat−1 has been obtained and is likely due to several factors such as well dispersed catalyst, reduced particle–particle agglomeration, resulting in enhanced light absorption (less light shielding), improved separation of charge carriers and utilization for red-ox reactions. Even better results were obtained with the use of nanohybrid catalysts. The enhanced photocatalytic activities of the nanohybrid catalysts are ascribed to the synergetic effects of FCNTs that minimize light reflection which promotes enhanced solar light sensitization, good dispersion of the catalysts in the reaction solution, and improved surface-interface reactions of photogenerated charge carriers. In the present study 10 wt% FCNTs in nanohybrid (CTP-10) exhibited superior photocatalytic activity for H2 generation, nearly 3 times higher than that of pristine TiO2 catalyst. The CTP-10 photocatalyst exhibited photostability under the experimental conditions studied. The nanohybrid photocatalysts were characterized in order to confirm the crystal structure, morphology, surface chemical composition and optical properties. The present work demonstrates the unique beneficial photocatalytic properties of carbon nanotubes and nanohorn mixtures in nanohybrid photocatalysts, for enhanced H2 production.
Nanocatalysis has been a growing field over the past few decades with significant developments in understanding the surface properties of nanocatalysts. With recent advances in synthetic methods, size, shape and composition of the nanoparticles can be controlled in a well defined manner which facilitates achieving selective reaction products in multipath reactions. Nanoparticles with specific exposed crystal facets can have different reactivity than other facets for reaction intermediates, which favours selective pathways during the course of reaction. Heterogeneous catalysts have been studied extensively; nano-sized metal particles are absorbed on mesoporus supports, facilitating access to the large surface area of the nanoparticles and hence exposure of more catalytic sites. Photocatalysis is attractive area of catalysis, in which photoinduced charge carriers are used for a variety of catalytic applications. More interestingly, clean and renewable liquid fuels energy sources such as hydrogen and methyl alcohol can be generated using photocatalysts through water splitting and CO2 reduction, respectively. Herein, we highlight the progress of nanocatalysis through metal, bimetallic nanoparticle, metal-semiconductor hybrid nanostructures and oxide nanoparticles for various reactions.
It is no exaggeration to state that the energy crisis is the most serious challenge that we face today.
Among the strategies to gain access to reliable, renewable energy, the use of solar energy has
clearly emerged as the most viable option. A promising direction in this context is artificial photosynthesis.
In this article, we briefly describe the essential features of artificial photosynthesis in
comparison with natural photosynthesis and point out the modest success that we have had in splitting
water to produce oxygen and hydrogen, specially the latter.
Cu2O-decorated mesoporous TiO2 beads (MTBs) are developed as a low-cost, highly efficient photocatalyst for H2 production. MTBs with a high specific surface area of 189 m2 g−1, a large pore volume of 0.43 cm3 g−1 and a suitable pore size of 8.9 nm are decorated with band-structure-matched Cu2O nanocrystals through a simple, fast and low-cost chemical bath deposition process. The Cu2O nanocrystals serve as an electron–hole separation centre to promote H2 evolution. Under optimal operation conditions, an ultra-high specific H2 evolution rate of 223 mmol h−1 g−1 is achieved. The success is attributed to the structural advantages of the MTBs of high specific surface areas, large pore volumes and suitable pore sizes together with the much improved electron–hole separation and light utilisation of the Cu2O-decorated MTBs. The H2 evolution rates achieved with the Cu2O-decorated MTBs are one order of magnitude higher than those achieved by commercial P25 TiO2.
Nanomaterials and nanostructures hold promising potency to enhance the performance of solar cells by improving both light trapping and photo-carrier collection. Meanwhile these new materials and structures can be fabricated in a low-cost fashion, enabling cost-effective production of photovoltaics. In this review, we summarize the recent development of studies on intriguing optical properties of nanomaterials/nanostructures and efforts on building solar cell devices with these materials and structures. As the family of nanomaterials has great diversity, we highlighted a number of representative materials and structures, including nanowires, nanopillars, nanocones, nanodomes, nanoparticles, etc. And we have covered materials include crystalline Si, amorphous Si, CdS, CdSe, CdTe, ZnO, CuInSe2, etc. These materials and structures have different physical properties, such as band-gap, absorption coefficient, surface/bulk recombination rate, etc., as well as different synthesis/fabrication approaches. Works on these materials and structures have laid a solid foundation for developing a new generation photovoltaics.
Here, hierarchically meso/nanoporous TiO2 was successfully fabricated by a facile and efficient hydrolysis and calcination method using Ti(OC4H9)4 and K2S2O8 as precursors. The hydrosol was firstly prepared by drop-wise adding ethanol dissolved Ti(OC4H9)4 solution into acetone dissolved K2S2O8 solution under a vigorous stirring and heating condition. After being sufficiently hydrolyzed, the hydrosol was calcined to promote the crystallization of TiO2 and successfully dope S and C on TiO2. The calcination temperature significantly affects the doping of S and C on TiO2, crystallization of TiO2, formation of hierarchically meso/nanoporous structure of TiO2 and its light absorption capability. The S- and C-codoped TiO2 exhibits high photocatalytic H2 generation efficiency in a water/methanol sacrificial reagent system under the irradiation of UV light. The high photocatalytic efficiency is dependent on the comprehensively competing effects of the codoping of S and C, crystallization, specific surface area and light absorption capability. The S- and C-codoped TiO2 calcined at 600 °C demonstrates the highest photocatalytic H2 generation efficiency, which is ascribed to the balanced synergy of the abovementioned factors.
Developing sustainable methods of hydrogen production can have significant environmental and energy efficiency benefits. A potentially viable way forward is to produce hydrogen from water by combining solar energy and heterogeneous catalysts. Our results indicate that sub-nm gold particles can provide an enormous enhancement in photocatalytic hydrogen production under visible light. We have observed a 35 times increase in activity for Au modified CdS as compared to that of unmodified catalyst. This activity was much higher than that of any other co-catalysts tested, which included 2–4 nm Pt, Ru, Pd and Rh nanoparticles modified CdS. The activity of sub-nm Au was also much better than that of CdS modified with larger Au particles. Our first-principles calculations of unsupported Au clusters indicated that there is a substantial difference in both shape and electronic properties between charged and neutral bare Au clusters as well as between bare and ligand-protected ones. This is the first ever demonstration of the remarkable potential of sub-nm Au particles for H2 production.
The application of metal ion-implantation method has been made to improve the electronic properties of the TiO2 photocatalyst to realize the utilization of visible light. The photocatalytic properties of these unique TiO2 photocatalysts for the purification of water have been investigated. By the metal ion-implantation method, metal ions (Fe+, Mn+, V+, etc.) are accelerated enough to have the high kinetic energy (150 keV) and can be implanted into the bulk of TiO2. TiO2 photocatalysts which can absorb visible light and work as a photocatalyst efficiently under visible light irradiation were successfully prepared using this advanced technique. The UV-Vis absorption spectra of these metal ion-implanted TiO2 photocatalysts were found to shift toward visible light regions depending on the amount and the kind of metal ions implanted. They were found to exhibit an effective photocatalytic reactivity for the liquid-phase degradation of 2-propanol diluted in water at 295 K under visible light (λ>450 nm) irradiation. The investigation using XAFS analysis suggested that the substitution of Ti ions in TiO2 lattice with implanted metal ions is important to modify TiO2 to be able to adsorb visible light.
A series of novel V2O5/N,S–TiO2 composite photocatalysts were fabricated by a solid state reaction route. The investigated composite materials were successfully characterised by XRD, DRUV-vis spectra, SEM, TEM, XPS, FTIR, NH3-TPD, BET-surface area and photoresponse studies. The photocatalytic applications of the composite materials were evaluated for hydrogen production under visible light irradiation (λ ≥ 400 nm) and phenol degradation under direct solar light. The V2O5 component played a key role for the visible light activity of the composite system at longer wavelengths. Among all the prepared materials, V2O5/N,S–TiO2 activated at 500 °C showed promising activity towards hydrogen production (296.6 μmol h−1) under visible light and successively degraded 88% of 100 mg L−1 phenol solution under direct solar light irradiation in just 4 h. The present investigation is of great importance in view of energy and environmental applications.
1-D mesoporous TiO2 nanotube (TNT) with large BET surface area was successfully synthesized by a hydrothermal-calcination process, and employed for simultaneous photocatalytic H2 production and Cu2+ removal from water. Cu2+, across a wide concentration range of 8–800 ppm, was removed rapidly from water under irradiation. The removed Cu2+ then combined with TNT to produce efficient Cu incorporated TNT (Cu-TNT) photocatalyst for H2 production. Average H2 generation rate recorded across a 4 h reaction was between 15.7 and 40.2 mmol h−1 g−1catalyst, depending on initial Cu2+/Ti ratio in solution, which was optimized at 10 atom%. In addition, reduction process of Cu2+ was also a critical factor in governing H2 evolution. In comparison with P25, its large surface area and 1-D tubular structure endowed TNT with higher photocatalytic activity in both Cu2+ removal and H2 production.
Highly dispersed CuO was introduced into TiO2 nanotube (TNT) made by hydrothermal method via adsorption–calcination process or wet impregnation process, to fabricate CuO incorporated TNT photocatalysts (CuO-TNT) for hydrogen production. It was found that CuO-TNT possessed excellent hydrogen generation activity, which was constantly vigorous throughout 5 h reaction. Depending on the preparation method, hydrogen evolution rates over CuO-TNT were founded in the range of 64.2–71.6 mmol h−1 g−1catalyst, which was much higher than the benchmark P25 based photocatalysts, and even superior to some Pt/Ni incorporated TNT. This high photocatalytic activity of CuO-TNT was mainly attributed to the unique 1-D tubular structure, large BET surface area and high dispersion of copper component. Compared to wet impregnation, adsorption–calcination process was superior to produce active photocatalyst, since it was prone to produce photocatalyst with more highly dispersed CuO.
The advantage of copper doping onto TiO2 semiconductor photocatalyst for enhanced hydrogen generation under irradiation at the visible range of the electromagnetic spectrum has been investigated. Two methods of preparation for the copper-doped catalyst were selected – complex precipitation and wet impregnation methods – using copper nitrate trihydrate as the starting material. The dopant loading varied from 2 to 15%. Characterization of the photocatalysts was done by thermogravimetric analysis (TGA), temperature programmed reduction (TPR), diffuse reflectance UV-Vis (DR-UV-Vis), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). Photocatalytic activity towards hydrogen generation from water was investigated using a multiport photocatalytic reactor under visible light illumination with methanol added as a hole scavenger. Three calcination temperatures were selected – 300, 400 and 500°C. It was found that 10wt.% Cu/TiO2 calcined at 300°C for 30min yielded the maximum quantity of hydrogen. The reduction of band gap as a result of doping was estimated and the influence of the process parameters on catalytic activity is explained.
Pristine ETS-10 and anion-modified ETS-10 microporous titanosilicate catalysts were successfully synthesised by using inorganic alkaline-sols as precursors for the first time. Furthermore, use of halogen/organic-free medium and short time synthesis (within 5h) makes this methodology even more versatile and efficient. We have noticed that, presence of desired amount of alkali in synthesis gel promotes rapid crystallisation, opposed to huge alkali used in classical synthesis. The UV–vis diffuse reflectance (DR) spectrum of orange-red vis/ETS-10 catalyst shows complete shift of absorption edge compared to pristine ETS-10 and commercial N-doped TiO2. It was found that vis/ETS-10 catalyst showed ca. 8.79 times higher relative photonic efficiency for photocatalytic methanol reforming as compared with commercial N-doped TiO2. The observed higher photocatalytic efficiency of vis/ETS-10 catalyst might be attributed to visible light absorbing anionic species (S, C and N) and better semiconducting property of one-dimensional titanium chains within zeotype structure.
TiO2 nanowires (TiO2 NWs) were synthesized through the one step hydrothermal process in 10M NaOH (aq.) at 150°C for 72h followed by post-heat-treatment at 300–1000°C for 2h. SEM and XRD measurements revealed that as-synthesized TiO2 NWs were transformed into TiO2 (B) structure with maintaining 1D morphology at 300–400°C and further transformed into anatase structure at 500–800°C. At 900–1000°C, they partly transformed into rod-shaped rutile structure. The photocatalytic activity of prepared samples was evaluated with photocatalytic H2 evolution from aqueous methanol solution. The results indicated that the TiO2 NWs treated at appropriate post-heat-treatment temperature exhibited higher photocatalytic H2 evolution than that of starting TiO2 powder (Degussa P-25).
A series of nanocrystalline mesoporous ZrO2–TiO2 binary oxide photocatalysts with different wt% of ZrO2 and TiO2 were prepared by a sol–gel method. These binary oxide photocatalysts were characterized by XRD, N2 adsorption–desorption, DRS, FTIR, Raman spectroscopy, photoluminescence and TEM analyses. Detailed investigations revealed that the ZrO2–TiO2 catalysts are highly micro-crystalline in nature with a larger surface area than that of the pure TiO2 or ZrO2 catalysts since the added ZrO2 plays an important role in promoting the formation of nanoparticles with an anatase structure, high surface area and acidity. The photocatalytic reactivity of the catalysts was investigated for the degradation of 4-chlorophenol in an aqueous phase in which the ZrO2–TiO2 photocatalysts were found to exhibit remarkably higher photocatalytic reactivity than that of pure TiO2 and ZrO2. The catalytic activity of the binary oxide photocatalysts for the degradation of 4-chlorophenol was observed to be gradually enhanced with an increase in the ZrO2 content and reached an optimum at 12wt% of ZrO2 while maintaining the same percentage degradation with further loading of ZrO2 until 50wt%. Such high reactivity is due to the easy transfer of the photo-formed electrons from the conduction band surface trap states of ZrO2 to the conduction band of TiO2 through strong chemical interactions, thereby, preventing the radiative recombination of the photo-formed electrons and holes. The ZrO2–TiO2 catalysts were, thus, found to be highly active for the efficient degradation of 4-chlorophenol and, in fact, exhibited just as efficient activity as the commercial P-25, Degussa TiO2 catalysts, and a new reaction mechanism has, hereby, been proposed.
Cr-doped TiO2 nanorods with nanocavities were synthesized by a facile hydrothermal treatment and heating in air. The samples were characterized respectively by means of X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). XRD patterns indicated that all the samples were anatase crystalline. HRTEM results and the electron diffraction patterns illustrated that the TiO2 nanorods possessed the single-crystalline structure. TEM images confirmed that there were different types of nanocavities inside the nanorods, such as a circle, hexagon, and rectangle. XPS results suggested that Cr elements were successfully doped into the TiO2 nanorods after hydrothermal and most Cr congregated on the surface in the form of Cr2O3 after heating. The optical properties of the samples were studied with a UV−vis spectrometer. The photoelectrochemical activity of the Cr-doped TiO2 nanorods thin film was better than that of the commercial anatase TiO2 particulate thin film. The high photoelectrochemical activity of the synthesized Cr-doped TiO2 nanorods could be attributed to three factors: the doped Cr, one-dimensional nanostructure of the nanorod, and the increased light-harvesting abilities.
Hydrogen production from water under artificial solar light irradiation was performed over a series of Pt and Au/TiO2(anatase/rutile) photocatalysts. Different TiO2 supports with varying anatase/rutile contents were compared, based on either sol-gel synthesis or commercial TiO2. The influence of template promotion on sol-gel TiO2 synthesis has been studied using different porogens or templates. Among various factors influencing the hydrogen evolution efficiency, it was pointed out that the following parameters were crucial to enhance H-2 evolution: (i) the nature and content of the metallic co-catalyst, (ii) the surface, crystallographic, and porosity properties of the TiO2 anatase/rutile support, (iii) the anatase/rutile ratio, (iv) the metal-support interactions, and (v) the relative amount of methanol added as a sacrificial reagent. The influence of these different factors was studied in detail. In optimized conditions, important H-2 production efficiency (120 mu mol/min) was obtained over days without deactivation and with very low amounts of methanol.
Four Ag-based semiconductor oxides with visible-light absorption, α-AgGaO2, α-AgInO2, β-AgAlO2, and β-AgGaO2, were prepared to investigate the influences of chemical composition and the crystal structure on the electronic structures and photocatalytic properties of AgMO2 (M = Al, Ga, In). The catalytic efficiencies of these oxides were characterized by testing the photooxidation of gaseous 2-propanol to acetone under visible-light irradiation. The ranking of the activity was α-AgGaO2 > β-AgAlO2 > β-AgGaO2 > α-AgInO2. The electronic structures of these compounds were investigated in terms of density functional theory. These experimental and computational studies of these materials reveal the following: (1) regarding chemical compositions, the conduction bands constructed through hybridization of the Ag 5s5p states with Ga 4s4p states and Al 3s3p states are necessary for promotion of photocatalytic activities under visible light for α-phase and β-phase, respectively, and (2) regarding crystal structures, the Ag 4d states in the absence of crystal-field splitting in the α-phase are favorable for a well-dispersed valence band that is responsible for higher photocatalytic activity.
Cu-doped TiO2 with varying amounts of Cu (0.2, 0.3, 0.5, 1, 2, and 5) are prepared by impregnation method and calcined at 350 and 450 °C for 5 h. These catalysts are characterized by X-ray diffraction, diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy energy-dispersive X-ray spectroscopy (EDAX), and transmission electron microscopy (TEM). The DRS studies are clearly showing the expanded photo response of TiO2 into the visible region on impregnation of copper ions. TEM images are depicting the fine dispersion of Cu particles on TiO2 surface. XPS studies are showing change in the binding energy values of Ti 2p, O 1s, and Cu 2p, indicating that copper ions are in interaction with TiO2. XPS results are also confirming that the oxidation state of copper is +2 in samples calcined at 350 °C and +1 in samples calcined at 450 °C. EDAX analysis supports the presence of copper species on the surface layers of TiO2. Photocatalytic hydrogen production activity studies are conducted over CuO/TiO2 and Cu2O/TiO2 catalysts in pure water and glycerol:water mixtures under solar irradiation. Maximum hydrogen production of 265 and 290 μmol h−1 is observed over 2 wt % CuO/TiO2 and Cu2O/TiO2 catalysts in pure water. A significant improvement in hydrogen production is observed in glycerol:water mixtures and maximum hydrogen production of 16,500 and 20,060 μmol h−1 is obtained over 0.5 wt % CuO/TiO2 and Cu2O/TiO2 catalysts in 5% glycerol aqueous solutions. No hydrogen production activity is observed on reduced catalysts under solar irradiation. Furthermore, when these catalysts are studied under UV irradiation, 2−3 fold increase in activity is observed on calcined catalysts, and the same level of activity is observed on reduced catalysts, but under these conditions the activity is limited by the dissolution of Cu ions into the solution. However, under solar irradiation a continuous and stable activity is observed over Cu2O/TiO2 catalyst. On the basis of the characterization and hydrogen production activity results, finely dispersed Cu in +1 oxidation state that is in interaction with TiO2 is proposed as a promising visible sensitive photocatalyst for the continuous production of hydrogen from glycerol:water mixtures.
Nitrogen-doped TiO2 (N−TiO2) nanocatalyst with spherical shape and homogeneous size has been synthesized through a chemical method using TiCl3 as precursor. The light absorption onset shifts from 380 nm on pure TiO2 to the visible region at 550 nm with N−TiO2. A clear decrease in the band gap and the nitrogen 2p states on the top of the valence band on N−TiO2 (compared to TiO2) is deduced from the optical absorption spectroscopy results. The chemical nature of N has been evolved as N−Ti−O in the anatase TiO2 lattice as identified by X-ray photoelectron spectroscopy (XPS). Photocatalytic decomposition of methylene blue has been carried out both in the UV and in the visible region and N−TiO2 shows higher activity than the Degussa P25 TiO2 photocatalyst in the visible region.
Titania, a wide band gap semiconductor, can generate powerful oxidants and reductants by absorbing photon energies. Titania has been extensively used in photoelectrochemical systems, such as dye-sensitized titania, a wide band gap semiconductor, can generate powerful oxidants and reductants by absorbing photon energies. To improve the photoreactivity of titania, several approaches, including doping and metal loading have been proposed. Nanocavities are isolated entities inside a solid and hence are very different from nanoporous, whose pores (often amorphous and irregular) connect together and open to the surface. Dense polyhedral nanocavities inside single-crystalline anatase titania nanorods were successfully synthesized by simply heating titanate nanorods. The size of the nanocavities is typically about 10 nm. The surfaces of the nanocavity polyhedron are determined to be the crystallographic low-index planes of the titania crystal. We found that these dense nanocavities significantly enhance the optical absorption coefficient of titania in the near-ultraviolet region, thereby providing a new approach to increasing the photoreactivity of the titania nanorods in the applications related to absorbing photons.
Photocatalytic water splitting with solar light is one of the most promising technologies for solar hydrogen production. From a systematic point of view, whether it is photocatalyst and reaction system development or the reactor-related design, the essentials could be summarized as: photon transfer limitations and mass transfer limitations (in the case of liquid phase reactions). Optimization of these two issues are therefore given special attention throughout our study. In this review, the state of the art for the research of photocatalytic hydrogen production, both outcomes and challenges in this field, were briefly reviewed. Research progress of our lab, from fundamental study of photocatalyst preparation to reactor configuration and pilot level demonstration, were introduced, showing the complete process of our effort for this technology to be economic viable in the near future. Our systematic and continuous study in this field lead to the development of a Compound Parabolic Concentrator (CPC) based photocatalytic hydrogen production solar rector for the first time. We have demonstrated the feasibility for efficient photocatalytic hydrogen production under direct solar light. The exiting challenges and difficulties for this technology to proceed from successful laboratory photocatalysis set-up up to an industrially relevant scale are also proposed. These issues have been the object of our research and would also be the direction of our study in future.
Cu2O/TiO2, Bi2O3/TiO2 and ZnMn2O4/TiO2 heterojunctions were studied for potential applications in water decontamination technology and their capacity to induce an oxidation process under VIS light. UV–vis spectroscopy analysis showed that the junctions-based Cu2O, Bi2O3 and ZnMn2O4 are able to absorb a large part of visible light (respectively, up to 650, 460 and 1000 nm). This fact was confirmed in the case of Cu2O/TiO2 and Bi2O3/TiO2 by photocatalytic experiments performed under visible light. A part of the charge recombination that can take place when both semiconductors are excited was observed when a photocatalytic experiment was performed under UV–vis illumination. Orange II, 4-hydroxybenzoic and benzamide were used as pollutants in the experiment. Photoactivity of the junctions was found to be strongly dependent on the substrate. The different phenomena that were observed in each case are discussed.
We have recently developed a single-step, low-temperature process for the catalytic production of fuels, such as hydrogen and/or alkanes, from renewable biomass-derived oxygenated hydrocarbons. This paper reviews our work in the development of this aqueous-phase reforming (APR) process to produce hydrogen or alkanes in high yields. First, the thermodynamic and kinetic considerations that form the basis of the process are discussed, after which reaction kinetics results for ethylene glycol APR over different metals and supports are presented. These studies indicate Pt-based catalysts are effective for producing hydrogen via APR. Various reaction pathways may occur, depending on the nature of the catalyst, support, feed and process conditions. The effects of these various factors on the selectivity of the process to make hydrogen versus alkanes are discussed, and it is shown how process conditions can be manipulated to control the molecular weight distribution of the product alkane stream. In addition, process improvements that lead to hydrogen containing low concentrations of CO are discussed, and a dual-reactor strategy for processing high concentrations of glucose feeds is demonstrated. Finally, various strategies are assembled in the form of a composite process that can be used to produce renewable alkanes or fuel-cell grade hydrogen with high selectivity from concentrated feedstocks of oxygenated hydrocarbons.
In this review, the photoluminescence (PL) performance and mechanism of nano-sized semiconductor materials, such as TiO2 and ZnO, are introduced, together with their attributes and affecting factors. Moreover, the applications of PL spectra in environmental photocatalysis are discussed in detail, viz. the inherent relationships between the PL intensity and photocatalytic activity are revealed on the basis of PL attributes, demonstrating that the PL spectra can reflect some important information such as surface defects and oxygen vacancies, surface states, photo-induced charge carrier separation and recombination processes in nano-sized semiconductor materials. Thus, the PL spectrum can provide a firm foundation in theory for designing and synthesizing new semiconductor photocatalysts with high activity, as well as quickly evaluating the photocatalytic activity of semiconductor samples.
Recent advances in the properties, synthesis, modifications and applications of one-dimensional single-crystalline Ti-O based nanostructures (including nanotubes, nanobelts, nanowires, and nanorods) are reviewed. The physical and chemical properties of one-dimensional nanostructured titanates, such as adsorption, stability, ion-exchange, optical, and proton conductivity properties, are described in connection with a particular application. The experimental parameters, morphologies, and mechanism of formation of one-dimensional nanostructured titanates produced by the alkaline hydrothermal method are critically discussed. Current progress in the modifications of one-dimensional single-crystalline Ti-O based nanostructures are discussed together with their improved performances. Examples of applications of one-dimensional nanostructured titanates in photocatalysis, lithium batteries, sensors, hydrogen production and storage, solar cells and biomedicine are presented.
Semiconductor photocatalysis has received much attention as a potential solution to the worldwide energy shortage and for counteracting environmental degradation. This article reviews state-of-the-art research activities in the field, focusing on the scientific and technological possibilities offered by photocatalytic materials. We begin with a survey of efforts to explore suitable materials and to optimize their energy band configurations for specific applications. We then examine the design and fabrication of advanced photocatalytic materials in the framework of nanotechnology. Many of the most recent advances in photocatalysis have been realized by selective control of the morphology of nanomaterials or by utilizing the collective properties of nano-assembly systems. Finally, we discuss the current theoretical understanding of key aspects of photocatalytic materials. This review also highlights crucial issues that should be addressed in future research activities.
This critical review shows the basis of photocatalytic water splitting and experimental points, and surveys heterogeneous photocatalyst materials for water splitting into H2 and O2, and H2 or O2 evolution from an aqueous solution containing a sacrificial reagent. Many oxides consisting of metal cations with d0 and d10 configurations, metal (oxy)sulfide and metal (oxy)nitride photocatalysts have been reported, especially during the latest decade. The fruitful photocatalyst library gives important information on factors affecting photocatalytic performances and design of new materials. Photocatalytic water splitting and H2 evolution using abundant compounds as electron donors are expected to contribute to construction of a clean and simple system for solar hydrogen production, and a solution of global energy and environmental issues in the future (361 references).
Single-crystal one-dimensional (1D) semiconductor architectures are important in materials-based applications requiring a large surface area, morphological control, and superior charge transport. Titania has widespread utility in applications including photocatalysis, photochromism, photovoltaics, and gas sensors. While considerable efforts have focused on the preparation of 1D TiO2, no methods have been available to grow crystalline nanowire arrays directly onto transparent conducting oxide (TCO) substrates, greatly limiting the performance of TiO2 photoelectrochemical devices. Herein, we present a straightforward low temperature method to prepare single crystal rutile TiO2 nanowire arrays up to 5 microm long on TCO glass via a non-polar solvent/hydrophilic substrate interfacial reaction under mild hydrothermal conditions. The as-prepared densely packed nanowires grow vertically oriented from the TCO glass substrate along the (110) crystal plane with a preferred (001) orientation. In a dye sensitized solar cell, N719 dye, using TiO2 nanowire arrays 2-3 microm long we achieve an AM 1.5 photoconversion efficiency of 5.02%.
Over the past decades, the tremendous effort put into TiO2 nanomaterials has resulted in a rich database for their synthesis, properties, modifications, and applications. The continuing breakthroughs in the synthesis and modifications of TiO2 nanomaterials have brought new properties and new applications with improved performance. Accompanied by the progress in the synthesis of TiO2 nanoparticles are new findings in the synthesis of TiO2 nanorods, nanotubes, nanowires, as well as mesoporous and photonic structures. Besides the well-know quantum-confinement effect, these new nanomaterials demonstrate size-dependent as well as shape- and structure-dependent optical, electronic, thermal, and structural properties. TiO2 nanomaterials have continued to be highly active in photocatalytic and photovoltaic applications, and they also demonstrate new applications including electrochromics, sensing, and hydrogen storage. This steady progress has demonstrated that TiO2 nanomaterials are playing and will continue to play an important role in the protections of the environment and in the search for renewable and clean energy technologies.
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