Z-scheme Water Splitting Research Articles

Nanoparticulate TiN Loading to Promote Z-Scheme Water Splitting Using a Narrow-Bandgap Nonoxide-Based Photocatalyst Sheet.

Some oxide-based particulate photocatalyst sheets exhibit excellent activity during the water-splitting reaction. The replacement of oxide photocatalysts with narrow-bandgap photocatalysts based on nonoxides could provide the higher solar-to-hydrogen energy conversion efficiencies that are required for practical implementation. Unfortunately, the activity of nonoxide-based photocatalyst sheets is low in many cases, indicating the need for strategies to improve the quality of nonoxide photocatalysts and the charge transfer process. In this work, single-crystalline particulate SrTaO2N is studied as an oxygen evolution photocatalyst for photocatalyst sheets applied to Z-scheme water splitting, in combination with La5Ti2Cu0.9Ag0.1O7S5 and Au as the hydrogen evolution photocatalyst and conductive layer, respectively. The loading of SrTaO2N with CoOx provided increases activity during photocatalytic water oxidation, giving an apparent quantum yield of 15.7% at 420nm. A photocatalyst sheet incorporating CoOx-loaded SrTaO2N is also found to promote Z-scheme water splitting under visible light. Notably, the additional loading of nanoparticulate TiN on the CoOx-loaded SrTaO2N improves the water splitting activity by six times because the TiN promotes electron transfer from the SrTaO2N particles to the Au layer. This work demonstrates key concepts related to the improvement of nonoxide-based photocatalyst sheets based on facilitating the charge transfer process through appropriate surface modifications.

Metal-free carbazole-thiophene photosensitizers designed for a dye-sensitized H2-evolving photocatalyst in Z-scheme water splitting.

Dye-sensitized photocatalysts with molecular dyes and widegap semiconductors have attracted attention because of their design flexibility, for example, tunable light absorption for visible-light water splitting. Although organic dyes are promising candidates as metal-free photosensitizers in dye-sensitized photocatalysts, their efficiency in H2 production has far been unsatisfactory compared to that of metal-complex photosensitizers, such as Ru(II) tris-diimine-type complexes. Here, we demonstrate the substantial improvement of carbazole-thiophene-based dyes used for dye-sensitized photocatalysts through systematic molecular design of the number of thiophene rings, substituents in the thiophene moiety, and the anchoring group. The optimized carbazole-thiophene dye-sensitized layered niobate exhibited a quantum efficiency of 0.3% at 460nm for H2 evolution using a redox-reversible I- electron donor, which is six-times higher than that of the best coumarin-based metal-free dye reported to date. The dye-sensitized photocatalyst also facilitated overall water splitting when combined with a WO3-based O2-evolving photocatalyst and an I3-/I- redox shuttle mediator. The present metal-free dye provided a high dye-based turnover frequency for water splitting, comparable to that of the state-of-the-art Ru(II) tris-diimine-type photosensitizer, by simple adsorption onto a layered niobate. Thus, this study highlights the potential of metal-free organic dyes with appropriate molecular designs for the development of efficient water splitting.

Long-wavelength photoresponsive gallium zinc oxynitride for efficient oxygen evolution and Z-scheme water splitting reactions

Long-wavelength photoresponsive GaN:ZnO efficiently driving the oxygen evolution reaction expands the possibilities for effectively harnessing solar energy through water splitting.

Z-scheme water splitting utilizing CuLi1/3Ti2/3O2 as a hydrogen-evolving photocatalyst with photo-response up to 600 nm

CuLi1/3Ti2/3O2 modified with Cr2O3/Ru, which can utilize longer wavelength light in comparison with SrTiO3:Rh, a representative H2-evolving component, was successfully applied to Z-scheme water splitting with BiVO4 and Co(bpy)33+/2+.

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.

(Invited) Z-Scheme Water Splitting Using Dye-Sensitized Metal Oxide Nanosheets

Solar water splitting by a heterogeneous photocatalyst, producing H2 and O2, is a potential solution for realizing large-scale H2 production. While dye-sensitized metal oxides are good candidates as H2 evolution photocatalysts for solar-driven Z-scheme water splitting, their solar-to-hydrogen (STH) energy conversion efficiencies remains low because of uncontrolled charge recombination reactions. Here it is shown that modification of Ru dye-sensitized, Pt- intercalated HCa2Nb3O10 nanosheets (Ru/Pt/HCa2Nb3O10) with both amorphous Al2O3 and poly(styrenesulfonate) (PSS) improves the STH efficiency of Z-scheme overall water splitting by a factor of ~100, when the nanosheets are used in combination with a WO3-based O2 evolution photocatalyst and an I3 –/I– redox mediator, relative to an analogous system that uses unmodified Ru/Pt/HCa2Nb3O10. The Al2O3 and PSS modifiers, which have previously been shown to suppress back electron transfer reactions in a dye-sensitized H2 evolution photocatalyst, enabled operation of the Z-scheme system even at low intensity of simulated sunlight without a decrease in the STH values. By using the optimized photocatalyst, PSS/Ru/Al2O3/Pt/HCa2Nb3O10, a maximum STH of 0.12% and an apparent quantum yield of 4.1% at 420 nm were obtained, by far the highest among dye-sensitized water splitting systems and also comparable to conventional semiconductor-based suspended particulate photocatalyst systems.

Improved Photocatalytic O2 Evolution on a Sillén–Aurivillius Perovskite Oxychloride Bi6NbWO14Cl by Rh2O3 Additives and Surface Modifications

A Sillén–Aurivillius-type single-layered perovskite oxyhalide, Bi6NbWO14Cl, was recently reported as a stable and efficient O2-evolving photocatalyst for Z-scheme water splitting, even without any modifications. However, efforts have not yet been made to improve its photocatalytic activity. In this study, we synthesized a Bi6NbWO14Cl photocatalyst using Rh2O3 additives and modified its surface with Rh species to improve its photocatalytic activity. An appropriate amount of the Rh2O3 additive significantly enhanced the O2 evolution rate on Bi6NbWO14Cl from the aqueous AgNO3 solution, likely owing to enhanced charge transfer by the metallic Rh nanoparticles formed around the grain boundaries. In addition, surface modification with Rh species led to a further enhancement of the O2 evolution rate. The apparent quantum efficiency of the O2 evolution reached 11% at 420 nm, which is the best reported performance for oxyhalides prepared by solid-state reactions. These Rh species were characterized by X-ray absorption fine structure, X-ray photoelectron spectroscopy, and transmission electron microscopy, and their roles in the enhanced photocatalytic activities were discussed.

Mechanistic Insights on Permeation of Water over Iron Cations in Nanoporous Silicon Oxide Films for Selective H2 and O2 Evolution

Electrocatalysts encapsulated by an ultrathin and semipermeable oxide layer offer a promising avenue for efficient, selective, and cost-effective production of hydrogen through photoelectrochemical water splitting. This architecture is especially attractive for Z-scheme water splitting, for which a nanoporous oxide film can be leveraged to mitigate undesired, yet kinetically facile, reactions involving redox shuttles, such as aqueous iron cations, by limiting transport of these species to catalytically active sites. In this work, molecular dynamics simulations were combined with electrochemical measurements to provide a mechanistic understanding of permeation of water and Fe(III)/Fe(II) redox shuttles through nanoporous SiO2 films. It is shown that even for SiO2 pores with a width as small as 0.8 nm, water does not experience any energy barrier for permeating into the pores due to a favorable interaction with hydrophilic silanol groups on the oxide surface. In contrast, permeation of Fe(III) and Fe(II) into microporous SiO2 pores is limited due to high energy barriers, which stem from a combination of distortion and dehydration of the second and third ion solvation shells. Our simulations and experimental results show that SiO2 coatings can effectively mitigate undesired Fe(III)/Fe(II) redox reactions at underlying electrodes by attenuating permeation of iron cations, while allowing water to permeate and thus participate in water splitting reactions. In a broader context, our study demonstrates that selectivity of solvated cations can be manipulated by controlling the pore size and surface chemistry of oxide films.

Photocatalytic Overall Water Splitting by SrTiO3: Progress Report and Design Strategies

Overall water splitting (OWS) using particulate photocatalysts is a promising and straightforward route for producing sustainable and clean hydrogen. Strontium titanate (SrTiO3) is a well-documented photocatalyst in both one-step and two-step (Z-scheme) water splitting systems and presents state-of-the-art activities. Nevertheless, SrTiO3 still has inherent restraints, such as structural imperfections and insufficient optical absorption ability. This critical review summarizes the recent progress in photocatalytic overall water splitting using SrTiO3 photocatalyst, primarily focusing on the modification techniques that are being used to alleviate its drawbacks. Concretely, we outline cocatalyst loading, crystal-facet engineering, reduction of bulk defects, heteroatom doping, and constructing Z-Scheme water splitting systems. Following that, recent efforts in developing photocatalyst sheets and panel reactors for large-scale photocatalytic hydrogen generation are discussed. Lastly, current challenges regarding solar hydrogen generation by SrTiO3 photocatalyst are concluded, and future scope is proposed. The knowledge gained here may also be useful for developing other efficient photocatalysts.

Zr-doped BaTaO2N photocatalyst modified with Na–Pt cocatalyst for efficient hydrogen evolution and Z-scheme water splitting

Detailed investigation of photocatalysts, cocatalysts and redox mediators provides a Z-scheme overall water splitting system operating under long-wavelength visible light.

(Invited) Semipermeable Oxide Coatings for Selective Z-Scheme Photocatalytic Water Splitting

Similar to natural photosynthesis, Z-scheme photocatalytic water splitting relies on two different light absorbing components that are coupled by a redox active mediator that shuttles charge between them. Such a two-absorber system possesses several advantages over single absorber photocatalytic system, including higher theoretical solar-to-hydrogen conversion efficiency, relaxed band alignment requirements, and the potential for inherently safe operation whereby H2 and O2 evolution occur in separated compartments. However, a major disadvantage and challenge for Z-scheme photocatalysis is that the presence of a redox mediator introduces two undesirable back-reactions on top of parasitic H2 oxidation and O2 reduction reactions that can occur in a single absorber photocatalytic system. Previous research efforts have identified the use of semi-permeable oxide coatings as an attractive approach to suppress these thermodynamically favored redox reactions while still permitting the desired water splitting and mediator redox reactions to occur. Here, we present a combined experimental and computational approach based on model thin films that is used to (i) probe the performance limits of oxide-encapsulated photocatalysts, (ii) quantify the effects of coating defects on performance, and (iii) guide the rational design of coatings aimed at maximizing the solar-to-hydrogen conversion efficiency of a target photocatalytic system. This work specifically focusses on the development of silicon and titanium oxide coatings for Z-scheme water splitting based on a Fe(II)/Fe(III) mediator, showing that the best coatings can achieve selectivities > 90 % towards the H2 and O2 evolution reactions over undesired Fe(II)/Fe(III) back reactions. Another key finding from this work is that coating defects can have a significant influence on the performance of encapsulated electrodes, as revealed by scanning electrochemical microscopy (SECM) measurements that were used to locally quantify the parasitic back reaction rates around individual defects to determine their impact on the global selectivity of an encapsulated electrode.

Surface-modified, dye-sensitized niobate nanosheets enabling an efficient solar-driven Z-scheme for overall water splitting.

While dye-sensitized metal oxides are good candidates as H2 evolution photocatalysts for solar-driven Z-scheme water splitting, their solar-to-hydrogen (STH) energy conversion efficiencies remain low because of uncontrolled charge recombination reactions. Here, we show that modification of Ru dye–sensitized, Pt-intercalated HCa2Nb3O10 nanosheets (Ru/Pt/HCa2Nb3O10) with both amorphous Al2O3 and poly(styrenesulfonate) (PSS) improves the STH efficiency of Z-scheme overall water splitting by a factor of ~100, when the nanosheets are used in combination with a WO3-based O2 evolution photocatalyst and an I3−/I− redox mediator, relative to an analogous system that uses unmodified Ru/Pt/HCa2Nb3O10. By using the optimized photocatalyst, PSS/Ru/Al2O3/Pt/HCa2Nb3O10, a maximum STH of 0.12% and an apparent quantum yield of 4.1% at 420 nm were obtained, by far the highest among dye-sensitized water splitting systems and comparable to conventional semiconductor-based suspended particulate photocatalyst systems.

Photocatalyst-Mediator Interface Modification by Surface-Metal Cations of a Dye-Sensitized H2 Evolution Photocatalyst.

To develop highly active H2 evolving dye-sensitized photocatalysts (DSPs) applicable for Z-scheme water splitting, we synthesized a series of Ru(II)-dye-double-layered DSPs, X'-RuCP6-Zr-RuP6@Pt-TiO2 (X'-DSP) with different surface-bound metal cations (X' = Fe2+, Y3+, Zr4+, Hf4+, and Bi3+). In 0.5 M KI aqueous solution, the photocatalytic H2 evolution activity under blue light irradiation (λ = 460 ± 15 nm) increased in the following order: nonmetal-modified DSP, H+-DSP (turn over number for 6 h irradiation = 35.2) < Fe2+-DSP (54.9) ≈ Bi3+-DSP (55.2) < Hf4+-DSP (65.5) ≈ Zr4+-DSP (68.3) ≈ Y3+-DSP (71.5), suggesting that the redox-inactive and highly charged metal cations tend to improve the electron donation from the iodide electron mediator. On the other hand, DSPs having heavy metal cations, Hf4+-DSP (18.4) and Bi3+-DSP (16.6), exhibited better activity under green light irradiation (λ = 530 ± 15 nm) than Zr4+-DSP (15.7) and H+-DSP (7.80), implying the contribution of a heavy atom effect of the surface-bound metal cation to partially allow the spin-forbidden metal-to-ligand charge-transfer excitation.

Mono-transition-metal-substituted polyoxometalates as shuttle redox mediator for Z-scheme water splitting under visible light

Z-scheme water splitting into H2and O2was demonstrated under visible light using a V-substituted Keggin-type polyoxometalate as shuttle redox mediator that is highly reversible and chemically stable under wide pH range of 2.5–6.5.

Innovation Synthesis of Nis Quantum Dots Modified Cds/Wo3 Heterostructures as High-Efficiency Bifunctional Photocatalysts for Construction of Visible Light Driven Z-Scheme Water-Splitting and Cr (Vi) Degradation

Innovation Synthesis of Nis Quantum Dots Modified Cds/Wo3 Heterostructures as High-Efficiency Bifunctional Photocatalysts for Construction of Visible Light Driven Z-Scheme Water-Splitting and Cr (Vi) Degradation

PbBi3O4X3 (X = Cl, Br) with Single/Double Halogen Layers as a Photocatalyst for Visible-Light-Driven Water Splitting: Impact of a Halogen Layer on the Band Structure and Stability

Layered oxyhalides with a Sillén(−Aurivillius) structure have attracted considerable attention as promising photocatalysts for efficient visible-light-induced water splitting due to the elevation of the O-2p valence band maximum (VBM) derived from the strong s–p interaction between Pb/Bi-6s and O-2p electrons and the electrostatic destabilization of oxygen anions in the fluorite layers. However, the role of the halogen layer, consisting of single or double layers, was not known. In this study, we examined a Sillén-type layered oxyhalide PbBi3O4X3 (X = Cl, Br) with alternating stacks of single/double halogen layers and skeletal Bi-based oxide layers. Density functional theory (DFT) calculations of PbBi3O4X3 revealed that the two inequivalent halogen layers contribute differently to the valence band density of states; the halogen p orbitals in double layers are higher in energy and responsible for the VBM, which is understood in terms of Madelung site potentials, where the electrostatic repulsion between halide ions is at play. Another finding from the DFT calculation is the spatial separation of the conduction band minimum and the VBM based on the asymmetry of halogen layers, which may promote the charge separation. While both PbBi3O4Cl3 and PbBi3O4Br3 possess band levels appropriate for visible-light-induced water splitting, only the former showed photocatalytic O2 evolution activity. The stability of PbBi3O4Cl3 is due to the significant contribution of the O-2p orbitals to its VBM, in stark contrast to PbBi3O4Br3, where Br-4p predominately contributes to the VBM. PbBi3O4Cl3 functions as a stable O2-evolving photocatalyst in Z-scheme water splitting under visible light irradiation. These results show that the electrostatic repulsion of halogen layers is a key factor for tuning the valence band structure of oxyhalides while maintaining sufficient stability for water-splitting photocatalysis.

A Z-scheme system constructed from WO3 modified TiO2 doped with Cr and Sb for visible light-driven overall water splitting

An artificial Z-scheme system using powder photocatalysts and a reversible redox mediator for the overall water splitting has been attracting attention for producing renewable H2 using solar energy. For the efficient use of this system, the development of efficient photocatalysts utilizing longer wavelength light is essential. TiO2 doped with Cr and Sb (TiO2:Cr/Sb) is one of the visible-light-active photocatalysts which has a bandgap of ca. 2.2 eV and absorbs light up to near 600 nm. However, TiO2:Cr/Sb powder in the Fe3+/Fe2+ reversible redox mediator showed negligible activity under visible light irradiation (λ &amp;gt; 460 nm). In this work, it was found that the photocatalytic performance for O2 evolution over TiO2:Cr/Sb modified with WO3 (WO3-TiO2:Cr/Sb) using the Fe3+/Fe2+ mediator was more than 15 times higher than that without modification. The apparent quantum efficiency of WO3-TiO2:Cr/Sb was estimated to be ca. 0.8% at 510 nm. The overall water splitting activity of Z-scheme water splitting (H2/O2 = 2), in combination with Ru/SrTiO3:Rh, was 9 times improved by the WO3 modification compared to that without modification. From the results of photoelectrochemical measurements, it was surmised that the loaded WO3 on the surface of TiO2:Cr/Sb may promote both reduction of Fe3+ to Fe2+ and oxidation of H2O to O2.

Toward practical solar-driven photocatalytic water splitting on two-dimensional MoS2 based solid-state Z-scheme and S-scheme heterostructure

Solar-driven splitting of water to hydrogen production is one of the utmost auspicious and cost-effective technologies providing sustainable fuel as it necessitates only sunlight and water. This substantial technology can be achieved in a two step-photoexcitation system using an artificial Z-scheme process. The key to comprehend this reaction is to discover an effective and robust photocatalyst which preferably makes sufficient use of solar energy. Benefiting from the ultrahigh surface area, appropriate band potentials and unique physiochemical properties of 2D layered structure, MoS2 has attained intensive research attention for hydrogen evolution reaction. The comprehensive review summarizes the recent progression in solid mediator Z-scheme based MoS2 particularly in the field of water splitting. The review initiates with the important bases of Z-scheme water splitting mechanism in addition to structural and optoelectronic properties of MoS2. In addition, detailed discussion on the solid-mediator Z-schemes to further improve the photocatalytic efficacy and limit the back (water-forming) reactions. The review mainly focuses on the solid-mediator based Z-schematic systems involving new tandem structures with an efficient synergistic effect to achieve both wide-range light absorptivity and charge pairs transference. In addition, a new S-scheme heterostructing is briefly introduced to understand the charge-transport route clearly and vividly. The aim of wide-spreading water splitting devices from lab-scale to wide-range potential applications has been considered as ‘artificial photosynthetic leaf-to-tree challenge’. Finally, a summary and outlook on the challenges and future research topics on MoS2 based Z-scheme heterostructure photocatalysts are presented.

(Invited) Oxide-Encapsulated (Photo)Electrocatalysts for Solar Fuels Production

Encapsulating active electrocatalysts with permeable oxide overlayers offers additional “control knobs” for tuning electrocatalyst selectivity and/or activity in ways that are not possible with conventional electrocatalysts.[1] In particular, nanoscopic oxide overlayers can possess selective transport characteristics that allow them to behave as ultrathin membranes that can block impurities and undesirable reactants while still permitting desired reactants and products to pass between the bulk electrolyte and active sites at the overlayer/catalyst buried interface.[1,2] In this presentation, I will describe recent investigations of silicon and titanium oxide-encapsulated thin film electrodes that allow for quantification of species permeabilities and to establish general design principles that govern the structure-property-performance relationships for nanoscopic oxide overlayers. First, I will show how the porosity and chemical composition of silicon oxide overlayers can be systematically tuned to control the relative permeabilities of protons and molecular oxygen, both of which are of great importance for water splitting.[3] Next, the selective transport properties of these coatings are reported in the presence of redox mediator species that are of interest for photocatalytic H2 production from Z-scheme water splitting.[4] Both silicon and titanium oxide overlayers are shown to be capable of selectively facilitating the desirable oxygen and hydrogen evolution reactions with selectivities of > 80% in the presence of a reversible redox mediator. Importantly, this work highlights the ability of ultrathin oxide overlayers to dramatically alter reaction selectivities, and presents design rules for overlayers that can be extended to many other (photo)electrocatalytic materials and reactions of interest to the solar fuels community.

(Invited) Dye-Sensitized Oxide Nanosheets for Visible-Light Water Splitting

Metal oxide nanosheets derived from the layered counterparts have attracted attention in various fields due to their unique structural properties. The anisotropic feature of nanosheets, having a thickness of ~1 nm and lateral dimensions ranging from several hundred nanometers to a few micrometer, is advantageous for heterogeneous photocatalysis, as the diffusion length of photogenerated carriers to the surface is shortened, leading to higher activity. It is also known that certain nanoparticulate metals or metal oxides on a semiconductor photocatalyst work as cocatalysts to promote water reduction and/or oxidation. In heterogeneous photocatalysis, the effect of cocatalyst size on the water-splitting performance had not been examined at sizes smaller than 1 nm due to the lack of an effective preparation method and a suitable photocatalyst. We have demonstrated that metal nanoclusters (such as Pt) of <1 nm in size could be deposited on the interlayer nanospace of KCa2Nb3O10 using the electrostatic attraction between a cationic metal complex and a negatively charged Ca2Nb3O10 – sheet, without the aid of any additional reagent. The material obtained exhibited 8 times greater photocatalytic activity for overall water splitting under band-gap irradiation than the previously reported analog using a RuO2 promoter. This study highlighted the superior functionality of <1 nm Pt nanoclusters for photocatalytic overall water splitting. With further modification by a ruthenium(II) photosensitizer, this material also worked as a H2 evolution photocatalyst in visible-light-driven Z-scheme water splitting, in combination with a WO3-based O2 evolution photocatalyst and a triiodide/iodide redox couple. Pt-intercalated HCa2Nb3O10 nanosheets further modified with amorphous AlO x clusters demonstrated a remarkable photocatalytic activity with a dye-based turnover number and frequency for H2 evolution of 4580 and 1960 h–1, respectively, which were by far the highest among dye-sensitized nonsacrificial photocatalytic systems ever reported.