Fabrication of a Core–Shell-Type Photocatalyst via Photodeposition of Group IV and V Transition Metal Oxyhydroxides: An Effective Surface Modification Method for Overall Water Splitting

The design of optimal surface structures for photocatalysts is a key to efficient overall water splitting into H₂ and O₂. A unique surface modification method was devised for a photocatalyst to effectively promote overall water splitting. Photodeposition of amorphous oxyhydroxides of group IV and V transition metals (Ti, Nb, Ta) over a semiconductor photocatalyst from corresponding water-soluble metal peroxide complexes was examined. In this method, amorphous oxyhydroxide covered the whole surface of the photocatalyst particles, creating a core–shell structure. The water splitting behavior of the novel core–shell-type photocatalyst in relation to the permeation behavior of the coating layer was investigated in detail. Overall water splitting proceeded successfully after the photodeposition, owing to the prevention of the reverse reaction. The photodeposited oxyhydroxide layers were found to function as molecular sieves, selectively filtering reactant and product molecules. By exploiting the selective permeability of the coating layer, redox reactions on the photocatalyst surface could be suitably controlled, which resulted in successful overall water splitting

Nature of Catalytic Active Sites Present on the Surface of Advanced Bulk Tantalum Mixed Oxide Photocatalysts

The most active photocatalyst system for water splitting under ultraviolet (UV) irradiation (270 nm) is the promoted 0.2% NiO/NaTaO₃:2% La photocatalyst with an optimized photonic efficiency of 56%, but fundamental issues about the nature of the surface catalytic active sites and their involvement in the photocatalytic process still need to be clarified. This is the first study to apply cutting-edge surface spectroscopic analyses to determine the surface nature of tantalum mixed oxide photocatalysts. Surface analysis with high-resolution X-ray photoelectron spectroscopy (1–3 nm) and high-sensitivity low-energy ion scattering spectroscopy (0.3 nm) indicates that the NiO and La₂O₃ promoters are concentrated in the surface region of the bulk NaTaO₃ phase. The NiO is concentrated on the NaTaO₃ outermost surface layers, while La₂O₃ is distributed throughout the NaTaO₃ surface region (1–3 nm). Raman and UV–vis spectroscopy revealed that the bulk molecular and electronic structures, respectively, of NaTaO₃ were not modified by the addition of the La₂O₃ and NiO promoters, with La₂O₃ resulting in a slightly more ordered structure. Photoluminescence spectroscopy reveals that the addition of La₂O₃ and NiO produces a greater number of electron traps resulting in the suppression of the recombination of excited electrons and holes. In contrast to earlier reports, La₂O₃ is only a textural promoter (increasing the BET surface area by ∼7-fold by stabilizing smaller NaTaO₃ particles) and causes an ∼3-fold decrease in the specific photocatalytic TOR_s (micromoles of H₂ per square meter per hour) rate because surface La₂O₃ blocks exposed catalytic active NaTaO₃ sites. The NiO promoter was found to be a potent electronic promoter that enhances the NaTaO₃ surface-normalized TOR_s by a factor of ∼10–50 and turnover frequency by a factor of ∼10. The level of NiO promotion is the same in the absence and presence of La₂O₃, demonstrating that there is no promotional synergistic interaction between the NiO and La₂O₃ promoters. This study demonstrates the important contributions of the photocatalyst surface properties to the fundamental molecular/electronic structure–photoactivity relationships of promoted NaTaO₃ photocatalysts that were previously not appreciated in the literature

Artificial Z‑Scheme Constructed with a Supramolecular Metal Complex and Semiconductor for the Photocatalytic Reduction of CO₂

A hybrid for the visible-light-driven photocatalytic reduction of CO₂ using methanol as a reducing agent was developed by combining two different types of photocatalysts: a Ru­(II) dinuclear complex (<b>RuBLRu′</b>) used for CO₂ reduction is adsorbed onto Ag-loaded TaON (Ag/TaON) for methanol oxidation. Isotope experiments clearly showed that this hybrid photocatalyst mainly produced HCOOH (TN = 41 for 9 h irradiation) from CO₂ and HCHO from methanol. Therefore, it converted light energy into chemical energy (Δ<i>G</i>° = +83.0 kJ/mol). Photocatalytic reaction proceeds by the stepwise excitation of Ag/TaON and the Ru dinuclear complex on Ag/TaON, similar to the photosynthesis Z-scheme

Photoelectrochemical Hydrogen Evolution from Water Using Copper Gallium Selenide Electrodes Prepared by a Particle Transfer Method

Photocathodes prepared using p-type semiconductor photocatalyst powders of copper gallium selenides (CGSe) were investigated for visible-light-driven photoelectrochemical water splitting. The CGSe powders were prepared by solid-state reaction of selenide precursors with various Ga/Cu ratios. The CGSe photoelectrodes prepared by the particle transfer method showed cathodic photocurrent in an alkaline electrolyte. Pt modification was conducted for all the photoelectrodes by photoassisted electrodeposition. CGSe particles with a Ga/Cu ratio of 2, consisting of the CuGa₃Se₅ phase and an intermediate phase between CuGaSe₂ and CuGa₃Se₅, yielded the largest cathodic photocurrent. By surface modification with a CdS semiconductor layer, the photocurrent density and onset potential clearly increased, indicating enhancement of charge separation caused by the formed p-n junction with appropriate band alignment at solid–liquid interfaces. A multilayer structure on the particles was confirmed to be beneficial for enhancing the photocurrent, as in the case of thin-film photoelectrodes. A Pt/CdS/CGSe electrode (Ga/Cu = 2) was demonstrated to work as a photocathode contributing stoichiometric hydrogen evolution from water for 16 h under visible light irradiation

Core/Shell Structured La- and Rh-Codoped SrTiO₃ as a Hydrogen Evolution Photocatalyst in Z‑Scheme Overall Water Splitting under Visible Light Irradiation

The effects of preparation methods, calcination times, and La doping concentrations on the crystallinity, visible light absorption, and photocatalytic water splitting performance of Rh- and La-codoped SrTiO₃ (SrTiO₃:La/Rh) were investigated. Applying a two-step solid state reaction in which SrTiO₃ acted as a perovskite-type host produced core/shell structured SrTiO₃:La/Rh, the surface of which was enriched with the dopants. La doping suppressed the formation of oxygen vacancies and inactive Rh⁴⁺ species. Under visible light irradiation (λ > 420 nm), SrTiO₃:La/Rh exhibited 3.5 and 3.8 times higher rates of H₂ evolution in an aqueous methanol solution and during redox-free Z-scheme overall water splitting in combination with Ir/CoO_<i>x</i>/Ta₃N₅, respectively, compared to SrTiO₃:Rh. The solar-to-hydrogen efficiency of the Z-scheme system as measured under illumination with simulated sunlight (AM1.5G) was found to have improved by a factor of 3

Enhanced Water Oxidation on Ta₃N₅ Photocatalysts by Modification with Alkaline Metal Salts

Tantalum nitride (Ta₃N₅) is a promising nitride semiconductor photocatalyst for solar water splitting because it has band edge potentials capable of producing hydrogen and oxygen from water under visible light (λ < 590 nm). However, the photocatalytic performance of Ta₃N₅ has been far below expectations because insufficient crystallization upon thermal nitridation of the oxide precursors enhances undesirable charge recombination limiting the quantum efficiency of the photocatalytic reaction. This problem was successfully rectified in this study by modifying the surface of the starting Ta₂O₅ with a small amount of alkaline metal (AM) salts. Compared with conventional Ta₃N₅, Ta₃N₅ nitrided from AM salt-modified Ta₂O₅ had better crystallinity and smaller particles with smoother surfaces and, most importantly, demonstrated a 6-fold improvement in photocatalytic activity for O₂ evolution under visible light. AM salt modification was compatible with the loading of an O₂ evolution cocatalyst, such as CoO_<i>x</i>, yielding an apparent quantum efficiency of 5.2% at 500–600 nm. This indicates that the effects of AM modification were attributable to the changes in the crystallinity and the morphology of Ta₃N₅ rather than to catalytic effects. Detailed characterization of the Na₂CO₃-modified Ta₃N₅ suggested partial dissolution of Ta₂O₅ and nucleation of NaTaO₃ in the early stages of nitridation, which gave rise to the characteristic particle morphologies and improved the crystallinity of the nitridation products. This study demonstrates that a facile pretreatment of a starting material can improve the physical and photocatalytic properties of photocatalysts drastically, enabling the development of advanced photocatalysts for solar water splitting

Behavior and Energy States of Photogenerated Charge Carriers on Pt- or CoO_<i>x</i>‑Loaded LaTiO₂N Photocatalysts: Time-Resolved Visible to Mid-Infrared Absorption Study

Femtosecond to second time-resolved visible to mid-infrared absorption spectroscopy was applied to investigate the behavior of photogenerated electrons and holes on a Pt- or CoO_<i>x</i>-loaded LaTiO₂N photocatalyst. CoO_<i>x</i>-loaded catalyst exhibits the highest activity for water oxidation under visible light (<600 nm) excitation, and the quantum efficiency reaches up to ∼30%. Transient absorption spectra suggest that most of the photoexcited electrons in LaTiO₂N lose activity by deep trapping in the mid-gap states created at 0.74 eV (6000 cm^–1) below the conduction band. In this case, Pt loading was not so effective for H₂ evolution because the loaded Pt could not effectively capture the trapped electrons from LaTiO₂N. The electron transfer was slow, proceeding in 0–100 μs, and was thus ineffective. However, in the case of CoO_<i>x</i> loading, we have clearly observed, for the first time, that the holes are captured rapidly by CoO_<i>x</i> in a few picoseconds, and the lifetimes of electrons are dramatically prolonged to the second region. This implies that the photogenerated holes and electrons are separated effectively in CoO_<i>x</i> and LaTiO₂N, respectively. Furthermore, the electron trap becomes shallower, its depth decreasing from 0.74 eV (6000 cm^–1) to 0.49 eV (4000 cm^–1) upon CoO_<i>x</i> loading, suggesting that the reactivity of the trapped electrons increases. These perturbations of electrons and holes are what cause the dramatic increase in photocatalytic activity. We expected that coloading of Pt and CoO_<i>x</i> would further increase the activity, but it was significantly reduced. It was demonstrated that the undesirable process of recombination is accelerated under high loading and coloading

Protonated Oxide, Nitrided, and Reoxidized K₂La₂Ti₃O₁₀ Crystals: Visible-Light-Induced Photocatalytic Water Oxidation and Fabrication of Their Nanosheets

Protonated lanthanum titanium oxide H₂La₂Ti₃O₁₀ and oxynitride H₂La₂Ti₃O_{10–3/2<i>x</i>}N_<i>x</i> crystals were synthesized from the oxide, nitrided, and reoxidized layered K₂La₂Ti₃O₁₀ crystals prepared by solid-state reaction through proton exchange. Here, we investigated the holding time of nitridation of oxide K₂La₂Ti₃O₁₀ crystals influencing their crystal structure, shape, and absorption wavelength and band gap energy. The XRD and SEM results confirmed that the crystal structure and plate-like shape of the parent oxide were maintained after nitridation at 800 °C for 10 h, and the color of crystals was changed from white to dark green. However, no clear absorption edges were observed in the UV–vis diffuse reflectance spectra of the nitrided crystals due mainly to the reduced titanium species (Ti³⁺), which act as the recombination center of the photogenerated charge carriers. To decrease the amount of the reduced titanium species, the nitrided crystals were further reoxidized at 400 °C for 6 h. After partial reoxidation, the absorption intensity in the longer wavelength region was reduced, and the absorption edges appeared at about 449–460 nm. The photocatalytic activity for the water oxidation half-reaction was evaluated only for the protonated samples. The protonated reoxidized K₂La₂Ti₃O₁₀ crystals showed the O₂ evolution rate of 180 nmol·h^–1 (for the photocatalytic water oxidation) under visible-light irradiation, and the unexpected photocatalytic decomposition of N₂O adsorbed onto the photocatalyst surfaces was observed for the protonated oxide and protonated nitrided layered K₂La₂Ti₃O₁₀ crystals. Furthermore, lanthanum titanium oxide [La₂Ti₃O₁₀]^2– and oxynitride [La₂Ti₃O_{10–3/2<i>x</i>}N_<i>x</i>]^2– nanosheets were successfully fabricated by proton exchange and mechanical exfoliation (sonication) of the oxide, nitrided, and reoxidized K₂La₂Ti₃O₁₀ crystals. The TEM results revealed that the lateral sizes of the fabricated nanosheets grown along the ⟨001⟩ direction are 270–620 nm. Apparently, the colloidal suspensions of the fabricated nanosheets showed a Tyndall effect, implying their good dispersion and stability for several weeks in water

Photoelectrochemical Solar Cells Consisting of a Pt-Modified CdS Photoanode and an Fe(ClO₄)₂/Fe(ClO₄)₃ Redox Shuttle in a Nonaqueous Electrolyte

Photoelectrochemical photovoltaic cells (PEC PVs) consisting of an n-type CdS single-crystal electrode and a Pt black counter electrode in a nonaqueous electrolyte containing an Fe­(ClO₄)₂/Fe­(ClO₄)₃ redox shuttle were studied as a means of obtaining photovoltages above the onset voltage for water splitting with one-step photoexcitation. To improve the photovoltaic performance, the effects of the redox concentration on the cell performance were investigated by UV–vis absorption and PEC measurements and by assessing the electrolyte using hydrodynamic voltammetry. Under visible-light irradiation (420–800 nm) from a Xe lamp, a relatively high open-circuit voltage (<i>V</i>_OC) of approximately 1.6 V was obtained, resulting from the negative flat-band potential of the CdS and the positive redox potential of the Fe complexes. Upon optimization of the redox concentration, photocurrent for the Pt/CdS electrode was increased to approximately 30 mA cm^–2, and an incident photon-to-current conversion efficiency of up to 80% was achieved at 480 nm as a result of the promotion of the anodic reaction on the Pt surface. Under simulated sunlight, the PEC PV composed of Pt/CdS in a 20 mM Fe­(ClO₄)₂/Fe­(ClO₄)₃ electrolyte exhibited a <i>V</i>_OC of 1.38 V, a 3.54 mA cm^–2 short-circuit current, and a 2.8% photon-to-energy conversion efficiency

Effects of Se Incorporation in La₅Ti₂CuS₅O₇ by Annealing on Physical Properties and Photocatalytic H₂ Evolution Activity

Oxysulfoselenide semiconductor photocatalysts absorb light at longer wavelengths than the corresponding oxysulfides. However, the synthesis of oxysulfoselenides is challenging due to excessive particle growth and the limited availability of metal selenide precursors. In this study, a La₅Ti₂CuS₅O₇ (LTCSO) oxysulfide was annealed with Se powder in sealed, evacuated quartz tubes to obtain LTCSO:Se photocatalysts, and the properties of these materials were investigated. Se was found to be incorporated into the LTCSO upon heating at 973 K or higher, and the Se/(S + Se) ratio was increased to a maximum of 0.3 upon repeating the heat treatment twice. The addition of Se extended the absorption edge of the LTCSO and thus increased its photocatalytic H₂ evolution activity at longer wavelength. Even so, the apparent quantum yield at shorter wavelengths was reduced, which is similar to the results obtained for La₅Ti₂Cu­(S_1–<i>x</i>Se_<i>x</i>)₅O₇ (LTCS_1–<i>x</i>Se_<i>x</i>O) solid solutions. Overall water splitting was achieved by constructing photocatalyst sheets using LTCSO:Se and LTCS_1–<i>x</i>Se_<i>x</i>O as hydrogen evolution photocatalysts and BiVO₄ as an oxygen evolution photocatalyst. Heat treatment with Se is evidently an effective method for the transformation of oxysulfide photocatalysts to oxysulfoselenides that promote photocatalytic H₂ evolution and have longer absorption edge wavelengths