Activity For Overall Water Splitting Research Articles

Hydrothermal-synthesized SrTiO3 photocatalyst codoped with rhodium and antimony with visible-light response for sacrificial H2 and O2 evolution and application to overall water splitting

The codoping effect of antimony on the photocatalytic activity of visible-light-driven SrTiO3 doped with rhodium (SrTiO3:Rh) was investigated. SrTiO3 doped with rhodium and antimony (SrTiO3:Rh/Sb) prepared by a hydrothermal method was found to be active for photocatalytic H2 evolution from an aqueous methanol solution and O2 evolution from an aqueous silver nitrate solution under visible light irradiation, although SrTiO3 doped with rhodium and no antimony was active only for the H2 evolution. Photocatalytic activities of SrTiO3:Rh/Sb were strongly dependent on the ratio of codopant to dopant (Sb/Rh). Diffuse reflection spectroscopy (DRS), electron spin resonance (ESR), Raman, and action spectrum analyses revealed the contribution of rhodium and antimony to visible-light response of SrTiO3:Rh/Sb. Unstable and reversible Rh3+ ions in oxidation state were the superior species for the H2 evolution. On the other hand, Rh3+ ions stabilized by codoping of antimony without the formation of Rh4+ ions and oxygen defects which would work as undesirable recombination sites between photogenerated electrons and holes played an important role in the O2 evolution. Moreover, when an IrOX cocatalyst was loaded on the surface of the SrTiO3:Rh/Sb photocatalyst, the photocatalytic activity of the O2 evolution drastically increased. The apparent quantum yield for the H2 evolution over Pt(0.3wt%)/SrTiO3:Rh(1%)/Sb(1%) and the O2 evolution over IrOX(3.0wt%)/SrTiO3:Rh(1%)/Sb(1%) at 420nm were 0.8% and 4.5%, respectively. The Z-scheme system composed of Ru(1.0wt%)/SrTiO3:Rh(2%) as a H2-evolving photocatalyst, IrOX(3.0wt%)/SrTiO3:Rh(1%)/Sb(1%) as an O2-evolving photocatalyst, and an Fe3+/Fe2+ redox couple as an electron mediator showed photocatalytic activity for overall water splitting under visible light irradiation.

Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ Electron Mediators for Overall Water Splitting under Sunlight Irradiation Using Z-Scheme Photocatalyst System

[Co(bpy)3](3+/2+) and [Co(phen)3](3+/2+) redox couples were revealed to play as electron mediators for Z-scheme photocatalyst systems composed of Ru/SrTiO3:Rh and BiVO4 powders for overall water splitting under visible light irradiation. These electron mediators were effective for only the combination of SrTiO3:Rh with BiVO4. They did not work when nondoped SrTiO3 and TiO2 of H2-evolving photocatalysts and WO3 of O2-evolving photocatalysts were employed. These results indicated that the affinity between photocatalysts and the Co-complex electron mediators was important. The photocatalytic activity depended on pH. Neutral pH conditions gave the highest activity for overall water splitting. Overall water splitting by the present system steadily proceeded for a long time. The Z-scheme photocatalyst system was also confirmed to split water under sunlight irradiation at the rates depending on weather. Moreover, overall water splitting by the Z-scheme photocatalyst system with the Co-complex electron mediator using a reaction cell in which the Ru/SrTiO3:Rh suspension was divided from BiVO4 suspension by a membrane filter resulted in H2 evolution separated from that of O2.

A Review of Synthesis Techniques for Gallium-Zinc Oxynitride Solar-Activated Photocatalyst for Water Splitting

Converting a practically limitless energy source, such as sunlight, into chemical energy with very little or no carbon footprint will pose a major challenge in the coming decades. The technology exists to convert solar energy into chemical fuels, such as hydrogen, through overall water splitting to produce hydrogen and oxygen. However, the photocatalytic efficiency is still below the feasibility limit. Many photocatalyst materials have been developed for solar hydrogen generation through water splitting in the last four decades. Gallium-zinc oxynitride (GaN:ZnO) solid solution is reported to be a suitable photocatalyst for overall water splitting and to have the highest photocatalytic activity. The traditional synthesis method contains difficulties and inefficiencies, such as long nitridation of starting materials at high temperatures and low Zn content of the synthesized photocatalyst. A number of experiments have been conducted in recent years to develop new synthesis approaches. In this article, a comprehensive review of various synthesis techniques of GaN:ZnO solid solution, along with their advantages and disadvantages, are presented. This information is essential for improving the efficiency of the synthesis techniques of GaN:ZnO solid solution and its photocatalytic activity for overall water splitting.

Preparation of a novel Cd2Ta2O7 photocatalyst and its photocatalytic activity in water splitting

Abstract Cd 2 Ta 2 O 7 , which showed high photocatalytic activity for overall water splitting, was synthesized by sol–gel method. Its maximum activity increased from 46.2 μmol-H 2 /h and 23.0 μmol-O 2 /h into 173.0 μmol-H 2 /h and 86.3 μmol-O 2 /h when 0.2 wt.% NiO was loaded as co-catalyst. The band structure of Cd 2 Ta 2 O 7 was calculated by a plane-wave-based density functional theory (DFT). The valence band is mainly formed by O 2p AO's, and the conduction band predominantly consists of Ta 5d, O 2p and Cd 5s5p AO's. Cd atoms endue new features different from other tantalates, and its effect on the band structure of Cd 2 Ta 2 O 7 was discussed.

Preparation of amorphous and nanocrystalline sodium tantalum oxide photocatalysts with porous matrix structure for overall water splitting

Summary Herein, we report the preparation of a series of surfactant-free nanostructured sodium tantalum oxide using NaTa(OC 3 H 7 ) 6 as a single precursor. The reaction conditions for the novel synthetic method were optimized and the morphology and crystal structure of the prepared materials were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Condensation and polymerization of NaTa(OC 3 H 7 ) 6 under atmospheric pressure gave a porous amorphous structure that could be converted to crystalline NaTaO 3 while crystalline Na 2 Ta 2 O 6 nanocrystals with a 25 nm average particle size could be obtained from a hydrothermal method using NH 3 as a base catalyst. In addition, the photocatalytic behaviors of the prepared materials were investigated for overall water splitting into hydrogen and oxygen. Unexpectedly, porous amorphous sodium tantalum oxide showed much better catalytic activity over the crystalline one. The synthesized Na 2 Ta 2 O 6 nanocrystals also indicated promising activity for overall water splitting without any co-catalyst in comparison to bulk NaTaO 3 .

Facile temperature-controlled synthesis of hexagonal Zn2GeO4nanorods with different aspect ratios toward improved photocatalytic activity for overall water splitting and photoreduction of CO2

Single-crystalline hexagonal prism Zn(2)GeO(4) nanorods with different aspect ratios have been prepared via a solution phase route, which exhibits improved photocatalytic activities in overall water splitting and photoreduction of CO(2) due to the low crystal defects, high specific surface area and beneficial microstructure on the catalyst's surface.

Preparation of Core–Shell‐Structured Nanoparticles (with a Noble‐Metal or Metal Oxide Core and a Chromia Shell) and Their Application in Water Splitting by Means of Visible Light

Core-shell-structured nanoparticles, consisting of a noble metal or metal oxide core and a chromia (Cr(2)O(3)) shell, were studied as promoters for photocatalytic water splitting under visible light. Core nanoparticles were loaded by impregnation, adsorption or photodeposition onto a solid solution of gallium nitride and zinc oxide (abbreviated GaN:ZnO), which is a particulate semiconductor photocatalyst with a band gap of approximately 2.7 eV, and a Cr(2)O(3) shell was formed by photodeposition using a K(2)CrO(4) precursor. Photodeposition of Cr(2)O(3) on GaN:ZnO modified with a noble metal (Rh, Pd and Pt) or metal oxide (NiO(x), RuO(2) and Rh(2)O(3)) co-catalyst resulted in enhanced photocatalytic activity for overall water splitting under visible light (lambda>400 nm). This enhancement in activity was primarily due to the suppression of undesirable reverse reactions (H(2)-O(2) recombination and/or O(2) photoreduction) and/or protection of the core component from chemical corrosion, depending on the core type. Among the core materials examined, Rh species exhibited relatively high performance for this application. The activity for visible-light water splitting on GaN:ZnO modified with an Rh/Cr(2)O(3) core-shell configuration was dependent on both the dispersion of Rh nanoparticles and the valence state. In addition, the morphology of the Cr(2)O(3) photodeposits was significantly affected by the valence state of Rh and the pH at which the photoreduction of K(2)CrO(4) was conducted. When a sufficient amount of K(2)CrO(4) was used as the precursor and the solution pH ranged from 3 to 7.5, Cr(2)O(3) was successfully formed with a constant shell thickness (approximately 2 nm) on metallic Rh nanoparticles, which resulted in an effective promoter for overall water splitting.

The Synergistic Effects of Two Co-catalysts on Zn2GeO4 on Photocatalytic Water Splitting

Zinc orthogermanate was prepared via a hydrothermal method and a remarkable synergistic effect on the photocatalytic activity for overall water splitting was found for Zn2GeO4 co-loaded with noble metals (Pt, Rh, Pd, Au) and metal oxides (RuO2, IrO2). The photocatalytic activity of Pt-RuO2/Zn2GeO4 for overall water splitting is 2.2 times of Pt/Zn2GeO4 and 3.3 times of RuO2/Zn2GeO4. Photocatalytic half reactions evaluation of water splitting for H2 and O2 productions shows that Pt plays the major roles in H2 production and RuO2 promotes the O2 production. The roles and valence states of co-catalysts and the mechanism of photocatalytic reaction are discussed.

Zinc Germanium Oxynitride: Influence of the Preparation Method on the Photocatalytic Properties for Overall Water Splitting

A series of oxynitride phases in the Zn−Ge−O−N solid solution, with the Zn/Ge ratio varying from 1.4 to 1.9, have been prepared according to different synthetic routes in order to determine the influence of the preparation step on the photocatalytic activity for overall water splitting. Preliminary tests were done in the presence of sacrificial agents to test separately the ability of each phase for photoreduction and photo-oxidation. Compositions with the lowest Zn/Ge ratios are active for photoreduction while compositions with a high crystallization state present a higher activity for photo-oxidation of water. Overall water splitting was achieved only with phases active for photoreduction and appears to be likely linked to a necessary condition implying a low Zn/Ge content.

Enhanced Photocatalytic Water Splitting of Hydrous LiCa2Ta3O10 Prepared by Hydrothermal Treatment

Abstract Hydration behavior of A′Ca2Ta3O10 (A′ = Cs, Rb, K, Na, and Li) was studied focusing on the photocatalytic activity for overall water splitting under UV-irradiation. Interlayer hydration was only observed for A′ = Na and Li, which possess a large enthalpy of hydration (ΔHh°). A Na phase readily yielded a stable hydrous phase even at ambient temperature, whereas a Li phase required hydrothermal treatment at ≥160 °C. This is consistent with the fact that the Na phase kept its space group (I4⁄mmm) unchanged during hydration, whereas the Li phase changed from I4⁄mmm to P4⁄mmm. Hydration of the Li phase doubled the rate of photocatalytic gas evolution (≥700 μmol-H2 h−1) in the presence of 0.5 wt % Ni load, which was the highest in the series of A′Ca2Ta3O10. It was demonstrated that the hydrated interlayer is an efficient structural modification to enhance the photocatalytic activity of the ion-exchangeable layered perovskite-type oxides.

Dependence of Activity and Stability of Germanium Nitride Powder for Photocatalytic Overall Water Splitting on Structural Properties

Germanium nitride (Ge3N4) powder prepared by thermal ammonolysis of GeO2 is examined as a photocatalyst for overall water splitting. Nitridation of GeO2 under a flow of NH3 at temperatures higher than 1123 K for 10 h results in production of either a single phase of β-Ge3N4 or a mixture of elemental Ge, α-Ge3N4, and β-Ge3N4, depending on the nitridation conditions. β-Ge3N4 exhibits activity for the stoichiometric decomposition of pure water into H2 and O2 under ultraviolet (UV) irradiation (λ > 200 nm) when loaded with nanoparticulate RuO2 as a cocatalyst. Improving the crystallinity of the β-Ge3N4 catalyst results in greater activity for overall water splitting and markedly reduced N2 release due to self-decomposition by photogenerated holes.

Photocatalytic Overall Water Splitting on Gallium Nitride Powder

Abstract Gallium nitride (GaN) powder was studied as a photocatalyst for overall water splitting. The photocatalytic activity of GaN for the reaction was found to be strongly dependent on the crystallinity of the material and the cocatalyst employed. Modification of well-crystallized GaN with Rh2−yCryO3 nanoparticles as a cocatalyst for H2 evolution resulted in the stable stoichiometric decomposition of H2O into H2 and O2 under ultraviolet irradiation (λ > 300 nm) by the band gap transition. RuO2 modification did not bring about appreciable H2 and O2 evolution. Low-crystallinity GaN exhibits negligible activity for overall water splitting, regardless of the kind of cocatalyst applied.

Modification of (Zn1+xGe)(N2Ox) Solid Solution as a Visible Light Driven Photocatalyst for Overall Water Splitting

Zinc germanium oxynitride, a solid solution between ZnO and ZnGeN2, is prepared by reaction of GeO2 and ZnO under a NH3 flow (20 mL min-1) at 1123 K. Samples nitrided for 5−15 h under these conditions exhibit a single phase of wurtzitic (Zn1+xGe)(N2Ox), and are responsive to visible light with a band gap of ca. 2.7−2.8 eV. The band gap decreases slightly with increasing nitridation time, that is, with increasing zinc content, attributable to p−d repulsion between Zn3d and N2p + O2p electrons in the upper valance band, which raises the top of the valance band. Nitridation for 15 h afforded (Zn1+xGe)(N2Ox) with the highest photocatalytic activity for overall water splitting. The optimized catalyst has high crystallinity due to complete reaction of the starting materials, without volatilization of zinc from the product under prolonged reduction. A range of cocatalysts is also examined, and Rh2-xCrxO3 is identified as the most effective cocatalyst for (Zn1+xGe)(N2Ox), which greatly causes an increase in the activity for hydrogen evolution. Modification of the optimized (Zn1.44Ge)(N2.08O0.38) sample by loading with Rh2-xCrxO3 (3.0 wt % Rh, 0.2 wt % Cr) results in an effective photocatalyst for overall water decomposition with a quantum efficiency of ca. 0.20% at 420 nm.

Photocatalytic Properties of RuO2-Loaded β-Ge3N4 for Overall Water Splitting

Germanium nitride, β-Ge3N4, is studied as a non-oxide photocatalyst for overall water splitting. Nitridation of GeO2 at 1153 K for 10 h under a flow of NH3 results in the production of well-crystallized β-Ge3N4 particles with a phenacite structure belonging to the hexagonal crystal system. Although the as-prepared β-Ge3N4 displays negligible activity for overall water splitting, the material becomes photocatalytically active under ultraviolet (UV) irradiation (λ > 200 nm) when loaded with RuO2 nanoparticles as H2 evolution sites. The photocatalytic activity of RuO2-loaded β-Ge3N4 for overall water splitting is strongly dependent on the reaction conditions. The highest activity is obtained when the reaction is carried out in 1 M H2SO4 aqueous solution. The rates of both H2 and O2 evolution decreases as the reaction progresses, attributable to photoreduction of O2, collapse of the catalyst surface by elusion of Ge cations, and loosened interfacial contact between the β-Ge3N4 and the loaded RuO2 nanoparticle...

Effect of High-Pressure Ammonia Treatment on the Activity of Ge3N4 Photocatalyst for Overall Water Splitting

The photocatalytic activity of beta-Ge(3)N(4) powder for overall water splitting is successfully enhanced by ammonia treatment at 823 K for 5-24 h at ammonia pressures of 20 MPa or greater. The surface and bulk nitrogen content in the treated samples varies according to the treatment temperature and treatment time, related to the stability of beta-Ge(3)N(4) powder under pressurized ammonia. The change in nitrogen content resulted in a change in the photocatalytic activity for overall water splitting. A beta-Ge(3)N(4) powder treated at 823 K for 5 h under ammonia at 20 MPa exhibited a photocatalytic activity 4 times higher than that of the as-synthesized powder, attributable to a decrease in the density of anion defects in the bulk and surface.

Preparation of platinized strontium titanate covered with hollow silica and its activity for overall water splitting in a novel phase-boundary photocatalytic system

Platinum-loaded strontium titanate (Pt-SrTiO 3) (core) -silica (shell) powder was prepared by double-layer winding of a carbon and a silica layer on Pt-SrTiO 3 followed by heat treatment to remove the carbon layer. Scanning electron microscope (SEM) observation and analyses of the BET surface area suggested that the powder has a void space between Pt-SrTiO 3 (core) and silica (shell). When the surface of the powder was partially modified with a fluoroalkylsilylation agent, thus-obtained material assembled at a gas–water interface and acted as a photocatalyst for overall water splitting to produce hydrogen (H 2) and oxygen (O 2). Probably due to the suppression of a backward reaction, production of water from H 2 and O 2, on the platinum, the overall efficiency of this system was higher than that of the conventional suspension system. Moreover, while the Pt-SrTiO 3 powders directly covered with fluoroalkylethylsilyl groups showed low photostability, i.e., prolonged irradiation precipitated some of the surface-modified particles in water owing to photocatalytic decomposition of surface fluoroalkylethylsilyl groups, this material could retain its location at the phase boundary.

Overall Water Splitting on (Ga1-xZnx)(N1-xOx) 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.

Overall Water Splitting on Tungsten-Based Photocatalysts with Defect Pyrochlore Structure

Defect pyrochlore-type oxides, AMWO6 (A = Rb, Cs; M = Nb, Ta), loaded with nickel oxide showed photocatalytic activity for overall water splitting under ultraviolet-light irradiation. The conduction bands of the materials are thought to be composed of the W5d orbital hybridized with the Nb5d or Ta4d orbital.

Photocatalytic Water Splitting over La2Ti2O7Synthesized by the Polymerizable Complex Method

Highly donor-doped (110) layered perovskite materials, La2Ti2O7, with high surface areas were synthesized by the polymerizable complex (PC) method. Relative to La2Ti2O7 prepared by the solid state reaction (SSR) method, PC catalysts showed higher surface areas, crystallization at lower temperatures, higher phase purity, more uniform morphology and better-distributed nickel on the outer surface of La2Ti2O7. All these factors led to higher photocatalytic activity for overall water splitting under UV irradiation. The quantum yield of the reaction over La2Ti2O7 prepared by the PC method was as high as 27%, which was about twofold greater than that over La2Ti2O7 prepared by the SSR method.