The production of hydrogen from water using a catalyst and solar energy is an ideal future energy source, independent of fossil reserves. For an economical use of water and solar energy, catalysts that are sufficiently efficient, stable, inexpensive and capable of harvesting light are required. Here, we show that an abundant material, polymeric carbon nitride, can produce hydrogen from water under visible-light irradiation in the presence of a sacrificial donor. Contrary to other conducting polymer semiconductors, carbon nitride is chemically and thermally stable and does not rely on complicated device manufacturing. The results represent an important first step towards photosynthesis in general where artificial conjugated polymer semiconductors can be used as energy transducers.

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A metal-free polymeric photocatalyst for hydrogen production from water under visible light

Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

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

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.

/pdf/a-review-on-the-visible-light-active-titanium-dioxide-2qg81s4yao.pdf

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

Photoelectrochemical (PEC) cells offer the ability to convert electromagnetic energy from our largest renewable source, the Sun, to stored chemical energy through the splitting of water into molecular oxygen and hydrogen. Hematite (α-Fe(2)O(3)) has emerged as a promising photo-electrode material due to its significant light absorption, chemical stability in aqueous environments, and ample abundance. However, its performance as a water-oxidizing photoanode has been crucially limited by poor optoelectronic properties that lead to both low light harvesting efficiencies and a large requisite overpotential for photoassisted water oxidation. Recently, the application of nanostructuring techniques and advanced interfacial engineering has afforded landmark improvements in the performance of hematite photoanodes. In this review, new insights into the basic material properties, the attractive aspects, and the challenges in using hematite for photoelectrochemical (PEC) water splitting are first examined. Next, recent progress enhancing the photocurrent by precise morphology control and reducing the overpotential with surface treatments are critically detailed and compared. The latest efforts using advanced characterization techniques, particularly electrochemical impedance spectroscopy, are finally presented. These methods help to define the obstacles that remain to be surmounted in order to fully exploit the potential of this promising material for solar energy conversion.

Solar Water Splitting: Progress Using Hematite (α‐Fe2O3) Photoelectrodes

Reference LPI-ARTICLE-1978-012doi:10.1002/hlca.19780610740View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Hydrogen Evolution from Water by Visible Light, a Homogeneous Three Component Test System for Redox Catalysis

A bifunctional redox catalyst, composed of Pt and RuO2 co-deposited on a colloidal TiO2 carrier, is a highly potent mediator for water decomposition by visible light1. The system contains apart from the sensitizer (Ru(bipy)2+3) an electron relay—methylviologen. The latter is reduced on light excitation, and the photoreaction is coupled with catalytic steps2 generating H2 and O2 from water. To rationalize the surprisingly high efficiency of this photoredox system, we proposed a mechanism involving species adsorbed at the TiO2 surface. This led us to explore sensitizers which through suitable functionalization show an enhanced affinity for adsorption at the particle–water interface. We describe here the performance of electron relay-free systems capable of efficiently decomposing water into H2 and O2 under visible light illumination. A bifunctional redox catalyst composed of RuO2 and Pt co-supported on colloidal TiO2 particles is used. The only other component present is a sensitizer. Amphiphilic surfactant derivatives of Ru(bipy)2+3 exhibit extremely high activity in promoting the water cleavage process. Adsorption of the sensitizer at the TiO2 particle–water interface and electron ejection into the TiO2 conduction band are evoked to explain the observations. Exposure to UV radiation leads to efficient water cleavage in the absence of sensitizer.

Photochemical cleavage of water by photocatalysis

The Sabatier reaction: CO2+4H2 → CH4+2H2O(g) ΔG°298K = −27 kcal mol−1 (1) is an important catalytic process of wide industrial and academic interest1—3. It is applied to syngas conversion and the treatment of waste streams. Methane is one of the most important carbon resources of the world, serving as an energy vector as well as a feedstock for higher-value chemicals4—6. Despite its favourable thermodynamics, the eight-electron reduction of CO2 to CH4 by hydrogen is difficult to achieve: high-energy intermediates impose large kinetic barriers, and the formation of side products is common. Intensive investigations during the past decade have therefore been aimed at improving the activity and selectivity of methanation catalysts 1–3,8–11. Although significant progress has been made in this field, elevated temperatures (T>300 °C) and pressures (P> 1 atm) are still required for methane generation to proceed at significant rates and yields. Here we report the selective conversion of CO2 to methane at room temperature and atmospheric pressure, using highly dispersed Ru/RuOx loaded onto TiO2 as a catalyst. The reaction rate is sharply enhanced through photo-excitation of the support material.

Methanation and photo-methanation of carbon dioxide at room temperature and atmospheric pressure

Noble metal dispersions are suitable for mediating light-induced hydrogen1–5 and oxygen6–8 evolution from water. We report here a dramatic improvement of the hydrogen production rate when very finely dispersed platinum is used as a mediator in reaction (1). The first observations of the dynamics of intervention of the Pt particles in the redox events are presented. A centrifuged colloidal Pt catalyst stabilised by polyvinyl alcohol showed exceptionally high activity in promoting hydrogen evolution from water via 2MV+ + H2O→PtH2 + 2OH− + 2MV2+ (1) where MV+ stands for reduced methylviologen. The latter is produced photochemically in aqueous solution containing Ru(bipy)32+ as a sensitiser and EDTA as an electron donor. At 10−3 M Pt the reoxidation of MV+ requires only 15 µs and leads to quantitative formation of H2.

Hydrogen evolution from water induced by visible light mediated by redox catalysis

Nitrogen and sulfur co-doping has been achieved in the commercial TiO 2 nanoparticles of anatase TKP 102 (Tayca) by grinding it with thiourea and calcinating at 400 °C. The successful substitutional N-doping and cationic/anionic S-doping were validated by XPS measurements. Diffuse reflectance spectroscopy (DRS) showed a marked broadening of the absorption spectrum of the doped material towards the visible range. Phenol and dichloroacetate (DCA) oxidation and Escherichia coli inactivation were achieved under UV illumination using the N, S co-doped TiO 2 powders. Electron spin resonance (ESR) spin-trapping experiments showed that under UV light irradiation, the OH radicals were the main species responsible for photo-degradation of phenol and E. coli abatement. Photo-degradation of DCA was found to be due a direct interaction of the TiO 2 valence band holes ( h VB + ) with the DCA molecules. Moreover, under visible light (400–500 nm) illumination of N, S co-doped TiO 2 a complete inactivation of E. coli bacteria was observed. In contrast, under such conditions, phenol was only partially degraded, whereas DCA was not at all affected. ESR experiments performed with N, S co-doped TiO 2 powders illuminated with visible light and in the presence of singlet oxygen ( 1 O 2 ) quencher, TMP-OH, showed the formation of 1 O 2 . This suggests that superoxide radical ( O 2 − ) and its oxidation product, 1 O 2 , were responsible for E. coli inactivation by N, S co-doped TiO 2 nanoparticles under visible light.

Abatement of organics and Escherichia coli by N, S co-doped TiO2 under UV and visible light. Implications of the formation of singlet oxygen (1O2) under visible light

John Kiwi

Papers

Hydrogen Evolution from Water by Visible Light, a Homogeneous Three Component Test System for Redox Catalysis

Photochemical cleavage of water by photocatalysis

Methanation and photo-methanation of carbon dioxide at room temperature and atmospheric pressure

Hydrogen evolution from water induced by visible light mediated by redox catalysis

Abatement of organics and Escherichia coli by N, S co-doped TiO2 under UV and visible light. Implications of the formation of singlet oxygen (1O2) under visible light