2.3. Evaluation of Photocatalytic Water Splitting 6507 2.3.1. Photocatalytic Activity 6507 2.3.2. Photocatalytic Stability 6507 3. UV-Active Photocatalysts for Water Splitting 6507 3.1. d0 Metal Oxide Photocatalyts 6507 3.1.1. Ti-, Zr-Based Oxides 6507 3.1.2. Nb-, Ta-Based Oxides 6514 3.1.3. W-, Mo-Based Oxides 6517 3.1.4. Other d0 Metal Oxides 6518 3.2. d10 Metal Oxide Photocatalyts 6518 3.3. f0 Metal Oxide Photocatalysts 6518 3.4. Nonoxide Photocatalysts 6518 4. Approaches to Modifying the Electronic Band Structure for Visible-Light Harvesting 6519

Semiconductor-based Photocatalytic Hydrogen Generation

Photoinduced reactivity of titanium dioxide

Recent years have seen a renewed interest in the harvesting and conversion of solar energy. Among various technologies, the direct conversion of solar to chemical energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclusively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise. Here we review recent progress in using plasmonic metallic nanostructures in the field of photocatalysis. We focus on plasmon-enhanced water splitting on composite photocatalysts containing semiconductor and plasmonic-metal building blocks, and recently reported plasmon-mediated photocatalytic reactions on plasmonic nanostructures of noble metals. We also discuss the areas where major advancements are needed to move the field of plasmon-mediated photocatalysis forward.

Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy

Nano-sized TiO 2 photocatalytic water-splitting technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy. Presently, the solar-to-hydrogen energy conversion efficiency is too low for the technology to be economically sound. The main barriers are the rapid recombination of photo-generated electron/hole pairs as well as backward reaction and the poor activation of TiO 2 by visible light. In response to these deficiencies, many investigators have been conducting research with an emphasis on effective remediation methods. Some investigators studied the effects of addition of sacrificial reagents and carbonate salts to prohibit rapid recombination of electron/hole pairs and backward reactions. Other research focused on the enhancement of photocatalysis by modification of TiO 2 by means of metal loading, metal ion doping, dye sensitization, composite semiconductor, anion doping and metal ion-implantation. This paper aims to review the up-to-date development of the above-mentioned technologies applied to TiO 2 photocatalytic hydrogen production. Based on the studies reported in the literature, metal ion-implantation and dye sensitization are very effective methods to extend the activating spectrum to the visible range. Therefore, they play an important role in the development of efficient photocatalytic hydrogen production.

A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production

Fujishima and Honda (1972) demonstrated the potential of titanium dioxide (TiO2) semiconductor materials to split water into hydrogen and oxygen in a photo-electrochemical cell. Their work triggered the development of semiconductor photocatalysis for a wide range of environmental and energy applications. One of the most significant scientific and commercial advances to date has been the development of visible light active (VLA) TiO2 photocatalytic materials. In this review, a background on TiO2 structure, properties and electronic properties in photocatalysis is presented. The development of different strategies to modify TiO2 for the utilization of visible light, including non metal and/or metal doping, dye sensitization and coupling semiconductors are discussed. Emphasis is given to the origin of visible light absorption and the reactive oxygen species generated, deduced by physicochemical and photoelectrochemical methods. Various applications of VLA TiO2, in terms of environmental remediation and in particular water treatment, disinfection and air purification, are illustrated. Comprehensive studies on the photocatalytic degradation of contaminants of emerging concern, including endocrine disrupting compounds, pharmaceuticals, pesticides, cyanotoxins and volatile organic compounds, with VLA TiO2 are discussed and compared to conventional UV-activated TiO2 nanomaterials. Recent advances in bacterial disinfection using VLA TiO2 are also reviewed. Issues concerning test protocols for real visible light activity and photocatalytic efficiencies with different light sources have been highlighted.

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A review on the visible light active titanium dioxide photocatalysts for environmental applications

Photo-enhanced hydrogenation of CO2 to mimic photosynthesis by CO co-feed in a novel twin reactor

CO2 photocatalytic reduction over Pt deposited TiO2 nanocrystals with coexposed {101} and {001} facets: Effect of deposition method and Pt precursors

Coupling photocatalytic H2 evolution and phenol degradation have drawn much attention on H2 as clean energy and phenol as an organic pollutant to the environment. Such dual-function reaction can utilize the chemical potential of phenol oxidation to make up the chemical potential required for hydrogen evolution from water splitting. The production of H2 thus was enhanced via the phenol oxidation. However, H2 is still needed to be purified from the reaction products by traditional methods. In this study, we demonstrated the simultaneous separation of H2 using a photo twin-reactor under artificial sunlight, in which the photocatalytic efficiency was substantially increased due to the inhibition of backward reaction by separating H2 from the products directly. Three Rh-doped SrTiO3 (STO) photocatalysts calcined at 900, 1100, 1200 °C (named as STO:Rh900, STO:Rh1100, and STO:Rh1200, respectively) were prepared by solid-state fusion reaction, then photo-deposition method was applied to synthesize Pt loading STO:Rh. All photocatalysts were fully characterized by XRD, XPS, UV–vis, SEM, TEM, and DLS. A single reactor and a twin-reactor (Z-scheme system) were systematically designed by using Pt/STO:Rh for H2 evolution photocatalyst and WO3 for phenol oxidation photocatalyst, where Fe3+/Fe2+ pairs were served as electron transfer mediators to conduct the dual-function reaction. In the single reactor, the stoichiometric of the dual-function reaction was proposed and with high consistency to the experimental data. By using the twin-reactor, H2 production rate increased 2.7 times, reaching 1.90 μmol g−1 h−1, compared to that in the single reactor. Moreover, the H2 concentration of the gas-phase products increased from 70% (in the single reactor) to 94% owing to the separation function of the twin-reactor, which would significantly reduce the cost for further purification. The effect of phenol concentration on H2 production in the twin-reactor was also thoroughly investigated. The results showed that increased phenol initial concentration would enhance the production of H2. With 200 μmol L−1 phenol, the H2 yield (11.37 μmol g−1 in 6-h reaction) was increased by 20% compared to that of pure water splitting.

A dual-function photocatalytic system for simultaneous separating hydrogen from water splitting and photocatalytic degradation of phenol in a twin-reactor

InTaO4 is an efficient visible-light photocatalyst, which used to be synthesized by solid-state fusion at over 1100 °C. However, irregular morphology and severe agglomeration of particles were acquired due to nonuniform fusion of solid precursors. In this study, InTaO4 was synthesized by two sol-gel routes, the thermal hydrolysis and esterification methods. The precursors were indium (III) nitrate pentahydrate [In(NO3)3] and tantalum(V) butoxide [Ta(OC4H9)5] dissolved in solutions. The InTaO4 powders with a uniform grain size of 17.7 nm were successfully synthesized using the esterification method at a calcination temperature of 950 °C. A uniform InTaO4 thin film nearly 40 nm thick formed on an optical fiber at 1100 °C using the sol prepared by the esterification method. For the first time, InTaO4 was evaluated by the photocatalytic activity of CO2 photo reduction, which was conducted in aqueous solution under visible light irradiation. Cocatalyst NiO was loaded on the surface of InTaO4 to further enhance the methanol yield. The methanol yields of NiO/InTaO4 by esterification method were significantly higher than those by solid-state fusion. The esterification method provided homogeneous mixing of Ta(OC4H9)5 and In(NO3)3, resulting in nano-sized InTaO4 with uniform crystallinity and superior photocatalytic activity.

Sol-gel prepared InTaO4 and its photocatalytic characteristics

Deep oxidation of methanol was conducted using a novel Pt/BN catalyst in a steady-state-flow reactor. Hexagonal boron nitride (BN) is a graphitelike material in powder form with a specific surface area of 49 m2/g. A catalyst with 0.37 wt % Pt was prepared by the incipient wetness method using H2PtCl6 as a precursor dissolved in methanol. The reaction temperature ranged from −10 to 200 °C. The concentration of methanol varied from 1000 to 4000 ppmv, while the oxygen varied from 5 to 80% and was balanced with nitrogen. The volume hourly space velocity was approximately 50 000 h-1. Experimental results showed that a 50% methanol conversion could be achieved at room temperature (20 °C), reaching 95% conversion at 75 °C, while Pt/Al2O3, Dash 220 (a commercial Pt/alumina catalyst), and PtPd/CeO2/Al2O3 catalysts required considerably higher temperatures under the same conditions. The high oxidation activity of Pt/BN was attributed to the weak Pt−O bonding of the Pt clusters on the BN surface, which led to high o...

Jeffrey C.S. Wu

Papers

Photo-enhanced hydrogenation of CO2 to mimic photosynthesis by CO co-feed in a novel twin reactor

CO2 photocatalytic reduction over Pt deposited TiO2 nanocrystals with coexposed {101} and {001} facets: Effect of deposition method and Pt precursors

A dual-function photocatalytic system for simultaneous separating hydrogen from water splitting and photocatalytic degradation of phenol in a twin-reactor

Sol-gel prepared InTaO4 and its photocatalytic characteristics

Deep Oxidation of Methanol Using a Novel Pt/Boron Nitride Catalyst