O2-evolving Photocatalyst Research Articles

Cs,Rh-codoped WO3 with a core–shell structure responsive up to 600 nm as an O2-evolving photocatalyst for Z-schematic water splitting

Expanding the responsive wavelength of WO3 as an O2-evolving photocatalyst for Z-schematic solar water splitting by Cs,Rh-codoping.

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.

LaTaON2 Mesoporous Single Crystals for Efficient Photocatalytic Water Oxidation and Z-Scheme Overall Water Splitting.

LaTaON2 porous single crystals (PSCs), integrating structural coherence and porous microstructures, will warrant promising photocatalytic performance. The absence of grain boundaries in PSCs ensures rapid photocarrier transportation from bulk to the surface, thereby mitigating photocarriers' recombination. Porous microstructures not only provide ample reachable surface to host photochemical reactions but also reinforce photon-matter interactions by additional photon reflection/scattering. Here, we have synthesized LaTaON2 PSCs via a topotactic route and show significantly improved photocatalytic performance. Efficient water oxidation into O2 has been realized by LaTaON2 PSCs with an apparent quantum efficiency as high as 5.7% at 420 ± 20 nm. Stable overall water splitting into stoichiometric H2 and O2 has also been achieved in a Z-scheme setup using LaTaON2 PSCs as the O2 evolution photocatalyst. These results not only prove that PSCs facilitate photocarrier migrations, which in turn deliver exceptional photocatalytic performance, but also imply that PSCs are useful to reinvigorate conventional semiconductor photocatalysts toward efficient solar energy conversions.

An All‐in‐One Organic Semiconductor for Targeted Photoxidation Catalysis in Hypoxic Tumor

AbstractTumor hypoxia severely limits the therapeutic effects of photodynamic therapy (PDT). Although many methods for oxygen generation exist, substantial safety concerns, spatiotenporal uncontrollability, limited efficacy, and complicated procedures have compromised their practical application. Here, we demonstrate a biocompatiable all‐in‐one organic semiconductor to provide a photoxidation catalysis mechanism of action. A facile method is developed to produce gram‐level C5N2 nanoparticles (NPs)‐based organic semiconductor. Under 650 nm laser irradiation, the semiconductor split water to generate O2 and simultaneously produce singlet oxygen (1O2), showing that the photocatalyst for O2 evolution and the photosensitizer (PS) for 1O2 generation could be synchronously achieved in one organic semiconductor. The inherent nucleus targeting capacity endows it with direct and efficient DNA photocleavage. These findings pave the way for developing organic semiconductor‐based cancer therapeutic agents.

An All-in-One Organic Semiconductor for Targeted Photoxidation Catalysis in Hypoxic Tumor.

Tumor hypoxia severely limits the therapeutic effects of photodynamic therapy (PDT). Although many methods for oxygen generation exist, substantial safety concerns, spatiotenporal uncontrollability, limited efficacy, and complicated procedures have compromised their practical application. Here, we demonstrate a biocompatiable all-in-one organic semiconductor to provide a photoxidation catalysis mechanism of action. A facile method is developed to produce gram-level C5 N2 nanoparticles (NPs)-based organic semiconductor. Under 650 nm laser irradiation, the semiconductor split water to generate O2 and simultaneously produce singlet oxygen (1 O2 ), showing that the photocatalyst for O2 evolution and the photosensitizer (PS) for 1 O2 generation could be synchronously achieved in one organic semiconductor. The inherent nucleus targeting capacity endows it with direct and efficient DNA photocleavage. These findings pave the way for developing organic semiconductor-based cancer therapeutic agents.

(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.

(Invited) Surface Modification of Dye-Sensitized Hydrogen-Evolving Photocatalyst for Z-Scheme Solar Water Splitting

Z-scheme water splitting system composed of water reduction and oxidation photocatalysts coupled with redox-reversible electron mediator has attracted considerable attention because of the promising activity for water splitting under visible light irradiation. To improve the activity of Z-scheme water-splitting photocatalyst, how to achieve the one-way electron transfer from O2 evolution photocatalyst to H2 evolution photocatalyst through electron mediator is crucial. Dye-sensitization is one of the promising approaches not only to enhance the light absorption ability but also to improve the charge separation efficiency at the dye-semiconductor interface. We recently found that the multilayering of Ru(II) dyes on the surface of water reduction catalyst Pt-TiO2 nanoparticle (nRu@Pt-TiO2, n = number of layers of Ru(II) dyes) significantly improved the photocatalytic H2 evolution activity.[1] In this work, to enhance the electron donation from iodide, we have synthesized two different 2Ru@Pt-TiO2 nanoparticles with different surface structures: P-2Ru@Pt-TiO2 having phosphate groups on the surface and ZrP-2 Ru@Pt-TiO2 with Zr4+-phosphate groups on the surface as well as our previously reported 2Ru@Pt-TiO2 without any surface phosphonates.[2] Figure shows the results of photocatalytic H2 evolution reactions in 0.5 M aqueous KI solutions of three kinds of dye-double-layered nanoparticle photocatalysts with different surface structures. The amount of H2 produced after 6 h of light irradiation increased in the following order: 2Ru@Pt-TiO2 < P-2Ru@Pt-TiO2 < ZrP-2Ru@Pt-TiO2 . Notably, the AQY of ZrP-2Ru@Pt-TiO2 in the first hour of reaction was approximately 1%. The Ru(III)/Ru(II) redox potential of Ru(II)dye with phosphonic acid groups was reported to be shifted positively to less than 0.1 V by immobilization to the TiO2 surface or by coordination bond formation with Zr4+ cations[3], suggesting that the binding of Zr4+ to the phosphonic acid groups of Ru(II) dye has little effect on the driving force of the electron donation reaction from the iodide. The zeta potential of ZrP-2Ru@Pt-TiO2 was estimated to be +7 mV without KI, and it negatively shifted to −6 mV in the presence of 0.5 M KI. On the other hand, the shift of P-2Ru@Pt-TiO2 was estimated to be smaller (from −7 to −12 mV). The negative shifts in the 0.5 M KI aqueous solution could be ascribed to the attraction of iodide anions to the nanoparticle surface by electrostatic interactions with the surface Zr4+ cations in the case of ZrP-2Ru@Pt-TiO2, and by hydrogen-bonding interactions with the surface phosphonic acids for P-2Ru@Pt-TiO2. These results suggest that the surface structure of Ru(II) dye-sensitized Pt-TiO2 nanoparticle photocatalyst has a great influence on the reactivity with the redox mediator. Details will be discussed. Figure 1

Boron-doped nitrogen-deficient carbon nitride-based Z-scheme heterostructures for photocatalytic overall water splitting

Photocatalytic overall water splitting can be achieved using Z-scheme systems that mimic natural photosynthesis by combining dissimilar semiconductors in series. However, coupling well-suited H2- and O2-evolving components remains challenging. Here, we fabricate a Z-scheme system for photocatalytic overall water splitting based on boron-doped, nitrogen-deficient carbon nitride two-dimensional (2D) nanosheets. We prepare ultrathin carbon nitride nanosheets with varying levels of boron dopants and nitrogen defects, which leads to nanosheets that can act as either H2- or O2-evolving photocatalysts. Using an electrostatic self-assembly strategy, the nanosheets are coupled to obtain a 2D/2D polymeric heterostructure. Owing to their ultrathin nanostructures, strong interfacial interaction and staggered band alignment, a Z-scheme route for efficient charge-carrier separation and transfer is realized. The obtained heterostructure achieves stoichiometric H2 and O2 evolution in the presence of Pt and Co(OH)2 co-catalysts, and the solar-to-hydrogen efficiency reaches 1.16% under one-sun illumination. Splitting water using suspensions of particulate carbon nitride-based photocatalysts may be a cheap way to produce hydrogen, but efficiencies have remained low. Now, Shen and colleagues use doped carbon nitride-based Z-scheme heterostructures to split water with a solar-to-hydrogen efficiency of 1.1% in the presence of metal-based co-catalysts.

Utilization of Perovskite-Type Oxynitride La0.5Sr0.5Ta0.5Ti0.5O2N as an O2-Evolving Photocatalyst in Z-Scheme Water Splitting

The particulate Z-scheme water splitting system involving a perovskite-type oxynitride as an O2-evolving photocatalyst has been first demonstrated using La0.5Sr0.5Ta0.5Ti0.5O2N accompanied by Ru-loaded SrTiO3:Rh and [Co(bpy)3]3+/2+ (bpy = 2,2′-bipyridine) as a H2-evolving photocatalyst and an electron mediator, respectively. O2 evolution was negligible when the bare oxynitride was employed. A Pt cocatalyst deposited on La0.5Sr0.5Ta0.5Ti0.5O2N promoted the reduction of [Co(bpy)3]3+ due to capturing electrons, resulting in an enhancement of O2 evolution. It has also been discovered that the [Co(bpy)3]3+/2+ electron mediator suppresses water formation over Pt particles.

Effects of Nitrogen/Fluorine Codoping on Photocatalytic Rutile TiO2 Crystal Studied by First-Principles Calculations.

Nitrogen/fluorine codoping of rutile TiO2 was recently reported to be effective for introducing visible-light absorption, and the resultant TiO2:N,F worked efficiently as an O2 evolution photocatalyst in a Z-scheme water-splitting system. Although an increase in the amount of nitrogen doped into rutile TiO2 lattice in the presence of fluorine was experimentally demonstrated, the role of fluorine in the system remained unclear. Here, we report a computational study on TiO2:N,F through the construction of supercell models with substitutional defects to reveal the atomic arrangement of the material and the electronic band structure. Calculations for all possible structures of nitrogen/fluorine and nitrogen/oxygen-vacancy relative positions revealed that the defect complexes were preferentially located on the (110) plane and that the distance between defects did not have a strong correlation with the formation energy. The present work also showed that although fluorine did not directly contribute to the narrowing of the band gap of TiO2:N,F, the fluorine activity of the synthetic atmosphere promotes the formation of substitutional defect complexes of nitrogen/fluorine for anion sites. This eventually increases the amount of nitrogen incorporated into the rutile TiO2 lattice and also results in reduction of the amount of oxygen vacancy, which is in qualitative agreement with our previous result of transient absorption measurement for rutile TiO2:N,F. The role of fluorine in TiO2:N,F is thus clarified through our systematic first-principles calculations.

Two-Dimensional Perovskite Oxynitride K2 LaTa2 O6 N with an H+ /K+ Exchangeability in Aqueous Solution Forming a Stable Photocatalyst for Visible-Light H2 Evolution.

Undoped layered oxynitrides have not been considered as promising H2 -evolution photocatalysts because of the low chemical stability of oxynitrides in aqueous solution. Here, we demonstrate the synthesis of a new layered perovskite oxynitride, K2 LaTa2 O6 N, as an exceptional example of a water-tolerant photocatalyst for H2 evolution under visible light. The material underwent in-situ H+ /K+ exchange in aqueous solution while keeping its visible-light-absorption capability. Protonated K2 LaTa2 O6 N, modified with an Ir cocatalyst, exhibited excellent catalytic activity toward H2 evolution in the presence of I- as an electron donor and under visible light; the activity was six times higher than Pt/ZrO2 /TaON, one of the best-performing oxynitride photocatalysts for H2 evolution. Overall water splitting was also achieved using the Ir-loaded, protonated K2 LaTa2 O6 N in combination with Cs-modified Pt/WO3 as an O2 evolution photocatalyst in the presence of an I3 - /I- shuttle redox couple.

Two‐Dimensional Perovskite Oxynitride K2LaTa2O6N with an H+/K+ Exchangeability in Aqueous Solution Forming a Stable Photocatalyst for Visible‐Light H2 Evolution

AbstractUndoped layered oxynitrides have not been considered as promising H2‐evolution photocatalysts because of the low chemical stability of oxynitrides in aqueous solution. Here, we demonstrate the synthesis of a new layered perovskite oxynitride, K2LaTa2O6N, as an exceptional example of a water‐tolerant photocatalyst for H2 evolution under visible light. The material underwent in‐situ H+/K+ exchange in aqueous solution while keeping its visible‐light‐absorption capability. Protonated K2LaTa2O6N, modified with an Ir cocatalyst, exhibited excellent catalytic activity toward H2 evolution in the presence of I− as an electron donor and under visible light; the activity was six times higher than Pt/ZrO2/TaON, one of the best‐performing oxynitride photocatalysts for H2 evolution. Overall water splitting was also achieved using the Ir‐loaded, protonated K2LaTa2O6N in combination with Cs‐modified Pt/WO3 as an O2 evolution photocatalyst in the presence of an I3−/I− shuttle redox couple.

Synthesis of 3D mesoporous g-C3N4 for efficient overall water splitting under a Z-scheme photocatalytic system

Photocatalytic overall water splitting (H2/O2 = 2) on graphitic polymeric carbon nitride (g-C3N4) is still a challenge due to the presence of an insufficient number of active sites and the weak oxidative capacity of the valence band towards H2O. Herein, a 3D porous g-C3N4 photocatalyst was prepared by the SiO2 hard template method, and overall water splitting was achieved with a Z-scheme system. With the confinement effect of SiO2, 3D porous g-C3N4 possesses a large specific area and thin walls compared to bulk g-C3N4, which not only provides more active sites but also shortens the charge migration distance, thus enhancing the photocatalytic activity. With a specific surface area of 159.7 m2·g−1 and a 12.4 nm pore size, the mesoporous g-C3N4-15 showed the highest H2 evolution rate, reaching 2.5 (under full-wavelength light: λ > 320 nm) and 4.7 (under visible light: λ > 420 nm) times that of bulk g-C3N4. Under a Z-scheme system where BiVO4 and Fe2+/Fe3+ are used as the O2 evolution photocatalyst and carrier transfer mediator, respectively, the mesoporous g-C3N4-15 achieved overall water splitting under both full-wavelength (H2: 81.6 μmol∙h−1, O2: 40.4 μmol∙h−1) and visible (9.8 μmol∙h−1 and 4.4 μmol∙h−1) light irradiation with 1.8% apparent quantum yield (AQY) at λ = 420 nm. Besides high photocatalytic activity, the large specific surface area of g-C3N4-15 is beneficial for the adsorption and redox capability of Fe2+/Fe3+, which facilitates the formation of the Z-scheme system and promotes the photocatalytic activity.

Fe/Ru Oxide as a Versatile and Effective Cocatalyst for Boosting Z-Scheme Water-Splitting: Suppressing Undesirable Backward Electron Transfer.

The atificial Z-scheme is a promising and rational strategy for solar-to-chemical energy conversion such as water-splitting. In the Z-scheme, backward redox processes are an essential drawback that should be overcome to increase its efficiency. Here, we demonstrate that the simple co-loading of Fe/Ru oxide, (Fe,Ru)Ox, onto various photocatalysts effectively improves the efficiency of water oxidation by suppressing the undesirable backward oxidation of the redox reagent Fe2+. The (Fe,Ru)Ox co-loading on Bi4TaO8Cl afforded the highest water-splitting activity (apparent quantum efficiency of 1.6% at 420 nm) among the Z-scheme systems employing mixed-anion compounds as O2-evolving photocatalysts. The results of photoelectrochemical and electrochemical measurements along with time-resolved spectroscopy clarified the key roles of Fe/Ru oxide; the Ru oxide component functions as a "collector" of photogenerated carriers and active sites for surface redox reactions, while the Fe oxide component acts as a "blocker" against unfavorable Fe2+ oxidation. The versatile availability of Fe/Ru oxide has been demonstrated for other visible-light-responsive photocatalysts.

Solar Water Splitting under Neutral Conditions Using Z‐Scheme Systems with Mo‐Doped BiVO4 as an O2‐Evolving Photocatalyst

H2 and O2 gases are generated from water under visible light irradiation over cocatalysts loaded on different powdered photocatalysts in a Z-scheme system. Reduced graphene oxide (RGO) sheets help the Z-schematic electron transfer from O2-evolving photocatalysts to H2-evolving photocatalysts. More details can be found in article number 1900358 by Akihide Iwase, Akihiko Kudo, and co-workers.

Photoconductivity–Lifetime Product Correlates Well with the Photocatalytic Activity of Oxyhalides Bi4TaO8Cl and PbBiO2Cl: An Approach to Boost Their O2 Evolution Rates

Owing to the shallow valence band maximum generated upon hybridization of Bi/Pb 6s and O 2p orbitals, layered oxyhalides of Bi4TaO8Cl and PbBiO2Cl are stable and promising O2-evolving photocatalyst...

Z-scheme water splitting by microspherical Rh-doped SrTiO3 photocatalysts prepared by a spray drying method

Microspherical particles of Rh-doped SrTiO3 (SrTiO3:Rh) having size of 1–2 μm were synthesized by a spray drying (SD) method employing a water-soluble ammonium trilactatotitanate. When the calcination temperature increased up to 1200 °C, primary particles grew significantly while the microspherical morphology of the secondary particles was kept. Dynamic light scattering analysis proved well-dispersive nature of the SD sample. Photocatalytic activities of SrTiO3:Rh synthesized by the SD method, solid state reaction (SSR), and a polymerizable complex (PC) method were evaluated for H2 evolution using an electron donor such as methanol, Fe2+, or [Co(bpy)3]2+ and Z-scheme water splitting combined with BiVO4 of an O2-evolving photocatalyst and either Fe3+/2+, [Co(bpy)3]3+/2+, or reduced graphene oxide as an electron mediator. SrTiO3:Rh prepared by the SD method showed much higher activities for all Z-scheme water splitting than the samples synthesized by the SSR method. Activities of the SD sample for Z-scheme water splitting using electron shuttles, Fe3+/2+ and [Co(bpy)3]3+/2+, were slightly higher or comparable to those of the PC sample whereas the SD sample achieved higher activity in Z-scheme water splitting based on interparticle electron transfer. In addition, even when the mass of SrTiO3:Rh used for the Z-scheme system with Fe3+/2+ was decreased from 50 mg to 30 mg, the SD sample still showed high activity while the activity decreased for the SSR and PC samples. Thus, usefulness of the SD method to obtain highly active SrTiO3:Rh with well-dispersive nature has been demonstrated.

Enhanced water splitting through two-step photoexcitation by sunlight using tantalum/nitrogen-codoped rutile titania as a water oxidation photocatalyst

An improved efficiency of Z-scheme water splitting was demonstrated using tantalum/nitrogen-codoped rutile titania as an O2 evolution photocatalyst.

Microwave-Assisted Hydrothermal Synthesis of SrTiO3:Rh for Photocatalytic Z-scheme Overall Water Splitting

Developing a photocatalyst system for solar energy conversion to electric energy or chemical energy is a topic of great interest for fundamental and practical importance. In this study, hydrogen production by a new Z-scheme photocatalysis water-splitting system was examined over Rh-doped SrTiO3 (denoted as Rh:SrTiO3) with Ru nanoparticle as cocatalyst for H2 evolution and BiVO4 photocatalyst for O2 evolution under visible light irradiation, where Co(bpy)32+/3+ was used as electron mediator. The catalysts were characterized by powder X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), and Ultraviolet–visible spectroscopy. We present a fast and efficient method to synthesize Rh-doped SrTiO3 photocatalyst via microwave-assisted hydrothermal method. Our results showed a significant effect of Ti precursor on morphology of Rh:SrTiO3 prepared by microwave-assisted hydrothermal synthesis. The Ru/Rh:SrTiO3 prepared by TiCl4 precursor showed a nanoporous structure and high photocatalytic activity. The combination of Ru/Rh:SrTiO3 with BiVO4 achieves a high H2 evolution rate (317 μmoL g−1 h−1) and O2 evolution rate (168 μmol g−1 h−1) in 0.5 mM Co(bpy)32+/3+ solution under visible light irradiation.

Flux Synthesis of Layered Oxyhalide Bi4NbO8Cl Photocatalyst for Efficient Z-Scheme Water Splitting Under Visible Light.

An oxyhalide photocatalyst Bi4NbO8Cl has recently been proven to stably oxidize water under visible light, enabling the Z-scheme water splitting when coupled with another photocatalyst for water reduction. We herein report the synthesis of Bi4NbO8Cl particles via a flux method, testing various molten salts to improve its crystallinity and hence photocatalytic activity. The eutectic mixture of CsCl/NaCl with a low melting point allowed the formation of single-phase Bi4NbO8Cl at as low as 650 °C. Thus, synthesized Bi4NbO8Cl particles exhibited a well-grown and plate-like shape while maintaining surface area considerably higher than those grown with others fluxes. They showed three times higherO2 evolution rate under visible light than the samples prepared via a solid-state reaction. Time-resolved microwave conductivity measurements revealed greater signals (approximately 4.8 times) owing to the free electrons in the conduction band, indicating much improved efficiency of carrier generation and/or its mobility. The loading of RuO2 or Pt cocatalyst on Bi4NbO8Cl further enhanced the activity for O2 evolution because of efficient capturing of free electrons, facilitating the surface chemical reactions. In combination with a H2-evolving photocatalyst Ru/SrTiO3:Rh along with an Fe3+/Fe2+ redox mediator, the RuO2/Bi4NbO8Cl is an excellent O2-evolving photocatalyst, exhibiting highly effective water splitting into H2 and O2 via the Z-scheme.