Photoassisted electrolysis of water by irradiation of a titanium dioxide electrode

Water oxidation by P25 TiO2 photoanodes in acidic solution

P25 TiO₂ photoanodes are used to photo-oxidise water in two different acids, 0.5 M H₂SO₄ and 1 M HClO₄. In the former acid, the linear sweep voltammogram, LSV, appears to exhibit two photocurrent waves, whilst only one in the latter. In 0.5 M H₂SO₄, the recorded LSV coupled with a low faradaic efficiency (0.58) for the photooxidation of water to O₂, f_O2, and a significant level of persulfate, f_S2O8 = 0.12, shows that the electrochemical kinetics are not simply those for water oxidation. In 1 M HClO₄, the LSV coupled with a high f_O2 value (0.91) suggest that the photocurrent is due to water oxidation. Photo-induced absorption spectroscopy, PIAS, measurements made using the P25 TiO₂ photoanode reveal a steady state absorbance change, ΔAbs_ss, associated with the steady-state concentration of surface accumulated holes, [h⁺]_ss, which varies with: (i) monitoring wavelength, with a peak at ca. 500 nm, and (ii) applied potential, flattening off at ca. 0.7 V vs Ag/AgCl. PIAS measurements, coupled with concomitant transient photocurrent (TC) measurements, on the P25 TiO₂ photoanode polarised at 1.3 v vs Ag/AgCl, in 1 M HClO₄, show that the oxidation of water is second order with respect the concentration of the surface-accumulated, photogenerated holes, [h⁺]_ss, which have a calculated turnover frequency of 19 s^-1, under 1 sun irradiation. This is the first reported example of the use of PIAS/TC to probe the photoelectrochemical kinetics exhibited by a mesoporous semiconductor photoanode derived from a powder, for water oxidation and the significance of such is discussed briefly.

Comprehensive insights into synthetic nitrogen fixation assisted by molecular catalysts under ambient or mild conditions

N2 is fixed as NH3 industrially by the Haber-Bosch process under harsh conditions, whereas biological nitrogen fixation is achieved under ambient conditions, which has prompted development of alternative methods to fix N2 catalyzed by transition metal molecular complexes. Since the early 21st century, catalytic conversion of N2 into NH3 under ambient conditions has been achieved by using molecular catalysts, and now H2O has been utilized as a proton source with turnover frequencies reaching the values found for biological nitrogen fixation. In this review, recent advances in the development of molecular catalysts for synthetic N2 fixation under ambient or mild conditions are summarized, and potential directions for future research are also discussed.

Near-Complete Suppression of Oxygen Evolution for Photoelectrochemical H2O Oxidative H2O2 Synthesis

Solar energy-assisted water oxidative hydrogen peroxide (H₂O₂) production on an anode combined with H₂ production on a cathode increases the value of solar water splitting, but the challenge of the dominant oxidative product, O₂, needs to be overcome. Here, we report a SnO_2-x overlayer coated BiVO₄ photoanode, which demonstrates the great ability to near-completely suppress O₂ evolution for photoelectrochemical (PEC) H₂O oxidative H₂O₂ evolution. Based on the surface hole accumulation measured by surface photovoltage, downward quasi-hole Fermi energy at the photoanode/electrolyte interface and thermodynamic Gibbs free energy between 2-electron and 4-electron competitive reactions, we are able to consider the photoinduced holes of BiVO₄ that migrate to the SnO_2-x overlayer kinetically favor H₂O₂ evolution with great selectivity by reduced band bending. The formation of H₂O₂ may be mediated by the formation of hydroxyl radicals (OH·), from 1-electron water oxidation reactions, as evidenced by spin-trapping electron paramagnetic resonance (EPR) studies conducted herein. In addition to the H₂O oxidative H₂O₂ evolution from PEC water splitting, the SnO_2-x/BiVO₄ photoanode can also inhibit H₂O₂ decomposition into O₂ under either electrocatalysis or photocatalysis conditions for continuous H₂O₂ accumulation. Overall, the SnO_2-x/BiVO₄ photoanode achieves a Faraday efficiency (FE) of over 86% for H₂O₂ generation in a wide potential region (0.6-2.1 V vs reversible hydrogen electrode (RHE)) and an H₂O₂ evolution rate averaging 0.825 μmol/min/cm² at 1.23 V vs RHE under AM 1.5 illumination, corresponding to a solar to H₂O₂ efficiency of ∼5.6%; this performance surpasses almost all previous solar energy-assisted H₂O₂ evolution performances. Because of the simultaneous production of H₂O₂ and H₂ by solar water splitting in the PEC cells, our results highlight a potentially greener and more cost-effective approach for "solar-to-fuel" conversion.

A Critical Review on Enhancement of Photocatalytic Hydrogen Production by Molybdenum Disulfide: From Growth to Interfacial Activities

Ultrathin 2D molybdenum disulfide (MoS₂ ), which is the flagship of 2D transition-metal dichalcogenide nanomaterials, has drawn much attention in the last few years. 2D MoS₂ has been banked as an alternative to platinum for highly active hydrogen evolution reaction because of its low cost, high surface-to-volume ratio, and abundant active sites. However, when MoS₂ is used directly as a photocatalyst, contrary to public expectation, it still performs poorly due to lateral size, high recombination ratio of excitons, and low optical cross section. Besides, simply compositing MoS₂ as a cocatalyst with other semiconductors cannot satisfy the practical application, which stimulates the pursual of a comprehensive insight into recent advances in synthesis, properties, and enhanced hydrogen production of MoS₂ . Therefore, in this Review, emphasis is given to synthetic methods, phase transitions, tunable optical properties, and interfacial engineering of 2D MoS₂ . Abundant ways of band edge tuning, structural modification, and phase transition are addressed, which can generate the neoteric photocatalytic systems. Finally, the main challenges and opportunities with respect to MoS₂ being a cocatalyst and coherent light-matter interaction of MoS₂ in photocatalytic systems are proposed.

A visible-light-induced photoelectrochemical water-splitting system featuring an organo-photocathode along with a tungsten oxide photoanode

An extremely low-biased water-splitting reaction occurred in a system containing a WO₃ photoanode and organo-photocathode.

Tantalum nitride films integrated with transparent conductive oxide substrates via atomic layer deposition for photoelectrochemical water splitting

Tantalum nitride, Ta₃N₅, is one of the most promising materials for solar energy driven water oxidation. One significant challenge of this material is the high temperature and long duration of ammonolysis previously required to synthesize it, which has so far prevented the use of transparent conductive oxide (TCO) substrates to be used which would allow sub-bandgap light to be transmitted to a photocathode. Here, we overcome this challenge by utilizing atomic layer deposition (ALD) to directly deposit tantalum oxynitride thin films, which can be fully converted to Ta₃N₅via ammonolysis at 750 °C for 30 minutes. This synthesis employs far more moderate conditions than previous reports of efficient Ta₃N₅ photoanodes. Further, we report the first ALD of Ta-doped TiO₂ which we show is a viable TCO material that is stable under the relatively mild ammonolysis conditions employed. As a result, we report the first example of a Ta₃N₅ electrode deposited on a TCO substrate, and the photoelectrochemical behavior. These results open the door to achieve efficient overall water splitting using a Ta₃N₅ photoanode.

A water splitting system using an organo-photocathode and titanium dioxide photoanode capable of bias-free H2 and O2 evolution

This study examined a water-splitting system comprising a TiO₂ photoanode and an organo-photocathode consisting of a p–n bilayer.

A nucleus-coupled electron transfer mechanism for TiO2-catalyzed water splitting

Based on first-principles calculations, we reveal that in the photocatalytic oxygen evolution reaction (OER) at the TiO2/water interface, the formation of an O-O bond always involves the anti-bonding σ2p* state elevated from the valence band into the conduction band of TiO2 regardless of a detailed reaction pathway. The role of photoholes is to deplete this anti-bonding state once it emerges into the band gap. The reaction barrier is thus determined by the onset where photoholes enter the reaction. This process represents a new reaction mechanism, termed nucleus-coupled electron transfer (NCET), where electron transfer is enabled by the movement of nuclei that promotes the reactive orbital to become the frontier orbital. The NCET mechanism for the OER is shown to exhibit an overall kinetic barrier surmountable at room temperature.

Multidisciplinary approaches to solar hydrogen

This review summarizes three different approaches to engineering systems for the solar-driven evolution of hydrogen fuel from water: molecular, nanomaterials and biomolecular. Molecular systems have the advantage of being highly amenable to modification and detailed study and have provided great insight into photophysics, electron transfer and catalytic mechanism. However, they tend to display poor stability. Systems based on nanomaterials are more robust but also are more difficult to synthesize in a controlled manner and to modify and study in detail. Biomolecular systems share many properties with molecular systems and have the advantage of displaying inherently high efficiencies for light absorption, electron-hole separation and catalysis. However, biological systems must be engineered to couple modules that capture and convert solar photons to modules that produce hydrogen fuel. Furthermore, biological systems are prone to degradation when employed in vitro. Advances that use combinations of these three tactics also are described. Multidisciplinary approaches to this problem allow scientists to take advantage of the best features of biological, molecular and nanomaterials systems provided that the components can be coupled for efficient function.

The design, fabrication, and photocatalytic utility of nanostructured semiconductors: focus on TiO2-based nanostructures

Recent advances in basic fabrication techniques of TiO2-based nanomaterials such as nanoparticles, nanowires, nanoplatelets, and both physical- and solution-based techniques have been adopted by various research groups around the world. Our research focus has been mainly on various deposition parameters used for fabricating nanostructured materials, including TiO2-organic/inorganic nanocomposite materials. Technically, TiO2 shows relatively high reactivity under ultraviolet light, the energy of which exceeds the band gap of TiO2. The development of photocatalysts exhibiting high reactivity under visible light allows the main part of the solar spectrum to be used. Visible light-activated TiO2 could be prepared by doping or sensitizing. As far as doping of TiO2 is concerned, in obtaining tailored material with improved properties, metal and nonmetal doping has been performed in the context of improved photoactivity. Nonmetal doping seems to be more promising than metal doping. TiO2 represents an effective photocatalyst for water and air purification and for self-cleaning surfaces. Additionally, it can be used as an antibacterial agent because of its strong oxidation activity and superhydrophilicity. Therefore, applications of TiO2 in terms of photocatalytic activities are discussed here. The basic mechanisms of the photoactivities of TiO2 and nanostructures are considered alongside band structure engineering and surface modification in nanostructured TiO2 in the context of doping. The article reviews the basic structural, optical, and electrical properties of TiO2, followed by detailed fabrication techniques of 0-, 1-, and quasi-2-dimensional TiO2 nanomaterials. Applications and future directions of nanostructured TiO2 are considered in the context of various photoinduced phenomena such as hydrogen production, electricity generation via dye-sensitized solar cells, photokilling and self-cleaning effect, photo-oxidation of organic pollutant, wastewater management, and organic synthesis.

Reaction of tris(bipyridine)ruthenium(III) with hydroxide and its application in a solar energy storage system

Irradiation of Ru(bipy)(3) (2+) (bipy = 2,2'-bipyridine) with light below 560 nm results in the formation of a charge-transfer excited state potentially capable of reducing water to dihydrogen with concomitant production of Ru(bipy)(3) (3+). The latter may be reduced by hydroxide [Formula: see text] to form dioxygen and regenerate the starting complex. The use of these reactions in a cell designed to bring about the photochemical decomposition of water is proposed.The stoichiometry, kinetics, and mechanism of the Ru(bipy)(3) (3+)-hydroxide reaction have been investigated by conventional and stopped-flow spectrophotometry. The dioxygen yield is a sharp function of pH, attaining its maximum value (about 80%) at pH 9. At low pH (3 and 4.8) the production of ruthenium(II) is first order with k(obsd) = (1.41 +/- 0.04) x 10(-4) sec(-1) (25 degrees , ionic strength mu = 1.00 M with sodium sulfate). In the intermediate pH range (7.9-10.0) complex kinetics are observed. In the hydroxide range 0.01-0.50 M, ruthenium(II) production is predominantly first order with k(obsd) = k(a)[OH(-)] + k(b)[OH(-)](2) sec(-1); k(a) = 148 M(-1) sec(-1) and k(b) = 138 M(-2) sec(-1) (25 degrees , mu = 1.00 M, sodium sulfate). For the k(a) term, the activation parameters are DeltaH(double dagger) = 15.3 +/- 1.0 kcal mol(-1) and DeltaS(double dagger) = 7 +/- 3 cal deg(-1) mol(-1) (1 cal = 4.184 J). An intermediate species (lambda(max) 800 nm) forms at the same rate as ruthenium(II) in this hydroxide range. It disappears with k(obsd) = 1.2 + 1.1 x 10(2) [OH(-)] sec(-1) at 25 degrees . Similarly absorbing (lambda(max) 750 to 800 nm) species are generated by the addition of hydroxyl radical to M(bipy)(3) (2+/3+) [M = Fe(II), Os(II), Ru(II), Cr(III), Ru(III)] in pulse radiolysis experiments. The kinetics above pH 7 are described in terms of rate-determining nucleophilic attack by hydroxide on the bound bipyridine ring. The hydroxide adduct so generated is tentatively identified with that observed in the pulse radiolysis experiments with Ru(bipy)(3) (2+).For reduction of Ru(bipy)(3) (3+) by hydrogen peroxide ruthenium(II) production is first order with k(obsd) = k(c)[HO(2) (-)] + k(d)[H(2)O(2)] where k(c) = 5.4 x 10(7) M(-1) sec(-1) and k(d) = 8.3 M(-1) sec(-1) (25 degrees , mu = 1.00 M, pH 3.5 to 9.7). This reaction produces dioxygen in 83 +/- 15% yield at pH 6.8 and in 1.0 N sulfuric acid.

n-Type Si-based photoelectrochemical cell: New liquid junction photocell using a nonaqueous ferricenium/ferrocene electrolyte

n-Type Si has been shown to serve as a stable photoanode in a cell for the conversion of light to electricity. The other components of the cell are a Pt cathode and an electrolyte consisting of an ethanol solution of [n-Bu(4)N]ClO(4) with a redox couple of ferricenium/ferrocene. Data from a two-compartment cell show that ferrocene is oxidized to ferricenium with 100 +/- 2% current efficiency at the Si photoanode. Furthermore, prolonged irradiation of the Si in a one-compartment cell yields constant photocurrent and output characteristics. The maximum open-circuit photopotential is approximately 700 mV, and the short-circuit quantum yield for electron flow at low light intensity exceeds 0.5. Conversion of monochromatic 632.8-nm light to electricity with approximately 2% power efficiency at an output voltage of approximately 200 mV has been sustained. These results represent a stable n-type Si-based photoelectrochemical cell.

Development of Visible-Light-Driven TiO2 and SrTiO3 Photocatalysts Doped with Metal Cations for H2 or O2 Evolution

This review paper represents photocatalytic properties of metal cation-doped TiO2 (rutile) and SrTiO3 photocatalysts for O2 evolution from an aqueous silver nitrate solution and H2 evolution from an aqueous methanol solution under visible light irradiation. Photocatalytic activities for the O2 evolution of Cr/Sb and Rh/Sb-codoped TiO2 are strongly dependent on the codoping ratio and the amount of doped chromium and rhodium. The codopant controls the oxidation number of doped chromium and rhodium. Rh-doped SrTiO3 in which the doped Rh species possesses a reversible redox property is active for the H2 evolution reaction under visible light irradiation. Overall water splitting under visible light irradiation proceeds with Z-scheme photocatalyst systems consisting of the Rh-doped SrTiO3 as a H2 evolution photocatalyst combined with BiVO4 as an O2 evolution photocatalyst and an Fe3+/Fe2+ electron mediator.

Deposition of anatase titania onto carbon encapsulated magnetite nanoparticles

A novel magnetically separable photocatalyst (titania-coated carbon encapsulated magnetite: TCCEF) was prepared. The prepared composite photocatalyst was characterized with an x-ray diffractometer (XRD), a transmission electron microscope (TEM), a Fourier transform infrared spectrometer (FT-IR) and a vibrating sample magnetometer (VSM). The photocatalytic activity of the samples was determined by degrading model contaminated water, a phenol aqueous solution. The results were compared with single-phase titania (pure titania and Degussa P25) and Fe(3)O(4)/TiO(2), and enhanced photocatalytic activity was obtained. It is suggested that the enhanced photocatalytic activity is ascribed to two major factors. First, the encapsulation of magnetite into the carbon layer may inhibit the direct electrical contact of titania and magnetite, hence preventing the photodissolution of the iron oxide phase. Second, the enhanced hydroxyl groups on TCCEF may inhibit the recombination of electron-hole pairs. On the other hand, the magnetic photocatalyst can be easily recovered from a slurry with the application of an external magnetic field.

The improvement of spinach growth by nano-anatase TiO2 treatment is related to nitrogen photoreduction

The improvement of spinach growth is proved to relate to N2 fixation by nano-anatase TiO2 in this study. The results show that all spinach leaves kept green by nano-anatase TiO2 treatment and all old leaves of control turned yellow white under culture with N-deficient solution. And the fresh weight, dry weight, and contents of total nitrogen, NH4(+), chlorophyll, and protein of spinach by nano-anatase TiO2 treatment presented obvious enhancement compared with control. Whereas the improvements of yield of spinach were not as good as nano-anatase TiO2 treatment under N-deficient condition, confirming that nano-anatase TiO2 on exposure to sunlight could chemisorb N2 directly or reduce N2 to NH3 in the spinach leaves, transforming into organic nitrogen and improving the growth of spinach. Bulk TiO2 effect, however, was not as significant as nano-anatase TiO2. A possible metabolism of the function of nano-anatase TiO2 reducing N2 to NH3 was discussed.

Calculation of Zeta Potential from Electrokinetic Measurements on Titania Plugs

The O'Brien theory developed for the electrical conductivity and electro-osmosis in porous plugs consisting of nonconducting particles has been applied for the calculation of zeta-potentials from conductivity and streaming potential measurements at two pH values, 3.3 and 6.5 or 5.8, on plugs composed of titania (undoped and doped with W6+ and Li+ ions) particles. The particles have been prepared both by precipitation (continuous crystallization at constant supersaturation) and by the sol-gel method. It was found that O'Brien's theory is also applicable in n-type semiconductor particles, such as titania (doped and undoped). Specifically, for the correct determination of zeta-potentials measured in solutions of relatively low ionic strength, it is necessary to take into account surface conduction behind the shear plane, for all samples measured at pH 3.3, a value considerably lower than the isoelectric point (i.e.p.) of titania. Under these conditions the surface charge of the suspended titania particles is high, and consequently the contribution of surface conductance to the conductivity of the plug is significant. The effect of surface conduction behind the shear plane on the calculation of zeta-potential is enhanced for the Li+-doped preparations due to the increase of surface conductivity caused by doping with Li+. Measurements of the streaming potential for the samples prepared with the sol-gel method were made at pH 6.5 (5.8 for the undoped titania); i.e., near the pH corresponding to the i.e.p., correct values of zeta-potential may be obtained in suspensions even at low ionic strength without taking into account the polarization of the double layer due to surface conduction, by applying the primitive Smoluchowski theory. This treatment is valid because at pH values near the i.e.p. the surface charge of the particles is very low, and consequently surface conductance is negligible compared with the conductivity of the plug. Copyright 1999 Academic Press.