Enhanced Overall Water Splitting by a Zirconium-Doped TaON-Based Photocatalyst.

Solar-powered one-step-excitation overall water splitting (OWS) using semiconducting materials is a simple means of achieving scalable and sustainable hydrogen production. While tantalum oxynitride (TaON) is one of the few photocatalysts capable of promoting OWS via single-step visible-light excitation, the efficiency of this process remains extremely poor. The present work employed 15 nm amorphous Ta2 O5 ⋅3.3 H2 O nanoparticles as a new precursor together with Zr doping and an optimized nitridation duration to synthesize a TaON-based photocatalyst with reduced particle sizes and low defect densities. Upon loading with Ru/Cr2 O3 /IrO2 cocatalysts, this material exhibited stoichiometric water splitting into hydrogen and oxygen, with an order of magnitude improvement in efficiency. Our findings demonstrate the importance of inventing/selecting the appropriate synthetic precursor and of defect control for fabricating active OWS photocatalysts.

Sub-50 nm perovskite-type tantalum-based oxynitride single crystals with enhanced photoactivity for water splitting.

A long-standing trade-off exists between improving crystallinity and minimizing particle size in the synthesis of perovskite-type transition-metal oxynitride photocatalysts via the thermal nitridation of commonly used metal oxide and carbonate precursors. Here, we overcome this limitation to fabricate ATaO2N (A = Sr, Ca, Ba) single nanocrystals with particle sizes of several tens of nanometers, excellent crystallinity and tunable long-wavelength response via thermal nitridation of mixtures of tantalum disulfide, metal hydroxides (A(OH)2), and molten-salt fluxes (e.g., SrCl2) as precursors. The SrTaO2N nanocrystals modified with a tailored Ir-Pt alloy@Cr2O3 cocatalyst evolved H2 around two orders of magnitude more efficiently than the previously reported SrTaO2N photocatalysts, with a record solar-to-hydrogen energy conversion efficiency of 0.15% for SrTaO2N in Z-scheme water splitting. Our findings enable the synthesis of perovskite-type transition-metal oxynitride nanocrystals by thermal nitridation and pave the way for manufacturing advanced long-wavelength-responsive particulate photocatalysts for efficient solar energy conversion.

Photocatalyst releasing hydrogen from water.

Direct splitting of water using a particulate photocatalyst would be a good way to produce clean and recyclable hydrogen on a large scale, and in the past 30 years various photocatalysts have been found that function under visible light. Here we describe an advance in the catalysis of the overall splitting of water under visible light: the new catalyst is a solid solution of gallium and zinc nitrogen oxide, (Ga(1-x)Zn(x))(N(1-x)O(x)), modified with nanoparticles of a mixed oxide of rhodium and chromium. The mixture functions as a promising and efficient photocatalyst in promoting the evolution of hydrogen gas.

A microreactor functionalized with acoustic wave effects and a liquid phase catalytic reaction.

In a microreactor functionalized with the effects of acoustic waves such as surface acoustic waves (SAWs) and resonance oscillations (ROs) that can be generated on ferroelectric crystals by applying rf electric power, the acoustic wave-induced dynamic lattice displacement vertical to surface significantly enhanced the catalysis of Sc(OTf)(3) in aldol condensation reaction of benzaldehyde and acetophenone to chalcone, demonstrating that the functionalization provided the microreactor with reaction-controllable functions of promoting liquid phase catalytic reactions.

Origin of the overall water splitting activity of Ta3N5 revealed by ultrafast transient absorption spectroscopy.

Tantalum nitride (Ta3N5) is one of the few visible light absorbing photocatalysts capable of overall water splitting (OWS), by which the evolution of both H2 and O2 is possible. Despite favourable energetics, realizing the OWS or efficient H2 evolution in Ta3N5 prepared by the nitridation of tantalum oxide (Ta2O5) or Ta foil remains a challenge even after 15 years of intensive research. Recently our group demonstrated OWS in Ta3N5 when prepared by the short time nitridation of potassium tantalate (KTaO3). To obtain a mechanistic insight on the role of Ta precursor and nitridation time in realizing OWS, ultrafast dynamics of electrons (3435 nm probe) and holes (545 nm probe) is investigated using transient absorption spectroscopy. Electrons decay majorly by trapping in Ta3N5 prepared by the nitridation of Ta2O5, which do not show OWS. However, OWS activity in Ta3N5 prepared by 0.25 hour nitridation of KTaO3 is particularly favoured by the virtually absent electron and hole trapping. On further increasing the nitridation time of KTaO3 from 0.25 to 10 hour, trapping of both electron and hole is enhanced which concurrently results in a reduction of the OWS activity. Insights from correlating the synthesis conditions-structural defects-carrier dynamics-photocatalytic activity is of importance in designing novel photocatalysts to enhance solar fuel production.

Efficient overall water splitting under visible-light irradiation on (Ga(1-x)Zn(x))(N(1-x)O(x)) dispersed with Rh-Cr mixed-oxide nanoparticles: Effect of reaction conditions on photocatalytic activity.

The photocatalytic activity of (Ga(1-x)Zn(x))(N(1-x)O(x)) loaded with Rh-Cr mixed-oxide (Rh(2-y)Cr(y)O3) nanoparticles for overall water splitting under visible-light irradiation (lambda > 400 nm) is investigated with respect to reaction pH and gas pressure. The photocatalytic performance of the catalyst is found to be strongly dependent on the pH of the reactant solution but largely independent of gas pressure. The present photocatalyst exhibits stable and high photocatalytic activity in an aqueous solution of pH 4.5 for 72 h. The photocatalytic performance is much lower at pH 3.0 and pH 6.2, attributable to corrosion of the cocatalyst and hydrolysis of the catalyst. The dispersion of Rh(2-y)Cr(y)O3 as a cocatalyst on the (Ga(1-x)Zn(x))(N(1-x)O(x)) surface promotes hydrogen evolution, which is considered to be the rate-determining step for overall water splitting on this catalyst.

Characterization of Rh-Cr mixed-oxide nanoparticles dispersed on (Ga(1-x)Zn(x))(N(1-x)Ox) as a cocatalyst for visible-light-driven overall water splitting.

The structure of Rh-Cr mixed-oxide (Rh(2)(-)(y)Cr(y)O(3)) nanoparticles dispersed on (Ga(1)(-)(x)Zn(x))(N(1)(-)(x)O(x)) is characterized by electron microscopy and X-ray spectroscopy. The Rh(2)(-)(y)Cr(y)O(3) nanoparticle is an efficient cocatalyst for photocatalytic overall water splitting on the (Ga(1)(-)(x)Zn(x))(N(1)(-)(x)O(x)) solid solution and is loaded onto the catalyst by impregnation from an aqueous solution containing Na(3)RhCl(6).2H(2)O and Cr(NO(3))(3).9H(2)O followed by calcination in air. Impregnation of the (Ga(1)(-)(x)Zn(x))(N(1)(-)(x)O(x)) with 1 wt % Rh and 1.5 wt % Cr followed by calcination at 623 K for 1 h provides the highest photocatalytic activity. Structural analyses reveal that the activity of this photocatalyst is strongly dependent on the generation of trivalent Rh-Cr mixed-oxide nanoparticles with optimal composition and distribution.

Characterization of ruthenium oxide nanocluster as a cocatalyst with (Ga(1-x)Zn(x))(N(1-x)Ox) for photocatalytic overall water splitting.

The formation and structural characteristics of Ru species applied as a cocatalyst on (Ga(1)(-)(x)()Zn(x)())(N(1)(-)(x)()O(x)()) are examined by scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. RuO(2) is an effective cocatalyst that enhances the activity of (Ga(1)(-)(x)()Zn(x)())(N(1)(-)(x)()O(x)()) for overall water splitting under visible-light irradiation. The highest photocatalytic activity is obtained for a sample loaded with 5.0 wt % RuO(2) from an Ru(3)(CO)(12) precursor followed by calcination at 623 K. Calcination is shown to cause the decomposition of initial Ru(3)(CO)(12) on the (Ga(1)(-)(x)()Zn(x)())(N(1)(-)(x)()O(x)()) surface (373 K) to form Ru(IV) species (423 K). Amorphous RuO(2) nanoclusters are then formed by an agglomeration of finer particles (523 K), and the nanoclusters finally crystallize (623 K) to provide the highest catalytic activity. The enhancement of catalytic activity by Ru loading from Ru(3)(CO)(12) is thus shown to be dependent on the formation of crystalline RuO(2) nanoparticles with optimal size and coverage.

Overall water splitting on (Ga(1-x)Zn(x))(N(1-x)O(x)) solid solution photocatalyst: relationship between physical properties and photocatalytic activity.

The physical and photocatalytic properties of a novel solid solution between GaN and ZnO, (Ga(1-x)Zn(x))(N(1-x)O(x)), are investigated. Nitridation of a mixture of Ga(2)O(3) and ZnO at 1123 K for 5-30 h under NH(3) flow results in the formation of a (Ga(1-x)Zn(x))(N(1-x)O(x)) solid solution with x = 0.05-0.22. With increasing nitridation time, the zinc and oxygen concentrations decrease due to reduction of ZnO and volatilization of zinc, and the crystallinity and band gap energy of the product increase. The highest activity for overall water splitting is obtained for (Ga(1-x)Zn(x))(N(1-x)O(x)) with x = 0.12 after nitridation for 15 h. The crystallinity of the catalyst is also found to increase with increasing the ratio of ZnO to Ga(2)O(3) in the starting material, resulting in an increase in activity.

Activation of immobilized enzymes by acoustic wave resonance oscillation.

Acoustic wave resonance oscillation has been used successfully in the development of methods to activate immobilized enzyme catalysts. In this study, resonance oscillation effects were demonstrated for enzyme reactions on galactose oxidase (GAD), D-amino acid oxidase (DAAO), and L-amino acid oxidase (LAAO), all of which were immobilized covalently on a ferroelectric lead zirconate titanate (PZT) device that could generate thickness-extensional resonance oscillations (TERO) of acoustic waves. For galactose oxidation on immobilized GAD in a microreactor, TERO generation immediately increased enzyme activity 2- to 3-fold. Eliminating TERO caused a slight decrease in the activity, with ∼90% of the enhanced activity retained while the reaction proceeded. Contact of the enhanced enzyme with a galactose-free solution caused almost complete reversion of the activity to the original low level before TERO generation, indicating that, not only TERO-induced GAD activation, but also preservation of the increased activity, required a galactose substrate. Similar activity changes with TERO were observed for enzyme reactions on DAAO and LAAO. Kinetic analysis demonstrated that TERO helped strengthen the interactions of the immobilized enzyme with the reactant substrate and promoted formation of an activation complex.

Effects of distortion of metal-oxygen octahedra on photocatalytic water-splitting performance of RuO2-loaded niobium and tantalum phosphate bronzes.

Sodium, niobium, and tantalum phosphate bronzes Na(4)M(8)P(4)O(32) (M=Nb, Ta) are employed as photocatalysts for water splitting to reveal the effects of the distortion of metal-oxygen octahedra on the photocatalytic performance. Addition of RuO(2) as a co-catalyst leads to high, stable activity in the stoichiometric production of H(2) and O(2) under UV irradiation. The combination of highly crystallized phosphates and a high dispersion of RuO(2) particles result in high photocatalytic activity. The sodium niobium phosphate bronze Na(2)Nb(8)P(4)O(32), consisting of a framework built up from slabs of corner-sharing NbO(6) octahedra connected through isolated PO(4) tetrahedra, provide heavily distorted NbO(6) octahedra with large internal dipole moments. The results support the existing view that the activity correlates with the magnitude of the dipole moment. The heavy distortion of NbO(6) octahedra is shown to play a significant role in photocatalytic water splitting.

GaN:ZnO solid solution as a photocatalyst for visible-light-driven overall water splitting.

Photocatalytic overall water splitting has been studied extensively from the viewpoint of solar energy conversion. Despite numerous attempts, none have yielded satisfactory results for the development of photocatalysts, which work under visible light irradiation to efficiently utilize solar energy. We report here the first example of visible-light-driven overall water splitting on a novel oxynitride photocatalyst, a solid solution of GaN and ZnO with a band gap of 2.58-2.76 eV, modified with RuO2 nanoparticles. In contrast to the conventional non-oxide photocatalysts, such as CdS, the solid solution is stable during the overall water splitting reaction. This is the first example of achieving overall water splitting by a photocatalyst with a band gap in the visible light region, which opens the possibility of new non-oxide-type photocatalysts for energy conversion.

RuO2-loaded beta-Ge3N4 as a non-oxide photocatalyst for overall water splitting.

Germanium nitride beta-Ge3N4 dispersed with RuO2 nanoparticles is presented as the first example of a non-oxide photocatalyst for the stoichiometric decomposition of H2O into H2 and O2. All of the successful photocatalysts developed for overall water splitting over the past 30 years have been based on oxides of metals. The discovery of a non-oxide photocatalyst, such as nitrides and oxynitrides, achieving the same function is therefore expected to stimulate research on non-oxide photocatalysts. New opportunities for progress in the development of visible light-driven photocatalysis can thus be expected, as the higher valence band positions of metal nitrides compared to the corresponding metal oxides provide narrower band gaps, which are suitable for visible light activity.