Electrochemical Synthesis of Photoelectrodes and Catalysts for Use in Solar Water Splitting

The role of a metallic copper interlayer during visible photocatalytic hydrogen generation over a Cu/Cu2O/Cu/TiO2 catalyst

Cu/Cu₂O/Cu/TiO₂ catalyst was fabricated by in-situ photoreduction from Cu₂O/TiO₂, and it showed an excellent photocatalytic performance and high stability for H₂ evolution. The interlayer metallic Cu provided a bridge for electrons fast transfer from TiO₂ to Cu₂O.

Immobilising a cobalt cubane catalyst on a dye-sensitised TiO2 photoanode via electrochemical polymerisation for light-driven water oxidation

A cobalt cubane catalyst Co₄O₄(O₂CMe)₄(4-vinylpy)₄ was immobilised on a dye-sensitized TiO₂ electrode via electrochemical polymerization for light-driven water oxidation.

Improved photocathodic performance in Pt catalyzed ferroelectric BiFeO3 films sandwiched by a porous carbon layer

A porous carbon buffer layer loaded with Pt is very effective for enhancing the solar-driven H₂ production by the ferroelectric BiFeO₃ film.

Ab initio assessment of Bi1−xRExCuOS (RE = La, Gd, Y, Lu) solid solutions as a semiconductor for photochemical water splitting

The electronic properties of Bi_1−xRE_xCuOS (RE = La, Gd, Y and Lu) were computed by hybrid DFT to design new semiconductors for water splitting.

Preparation and characterization of CrFeWTiO2 photoanodes and their photoelectrochemical activities for water splitting

The chemical bath deposition (CBD) method was successfully applied to prepare WTiO₂ nanotube arrays co-deposited with chromium, iron and chromium–iron nanoparticles.

A Ni2P modified Ti4+ doped Fe2O3 photoanode for efficient solar water oxidation by promoting hole injection

Nickel phosphide (Ni₂P) was used as an excellent water oxidation cocatalyst for photoelectrochemical (PEC) water splitting, which could significantly promote the hole injection efficiency and suppress the back reaction of water oxidation over a Ti⁴⁺ doped Fe₂O₃ photoanode.

One-dimensional TiO2 Nanotube Photocatalysts for Solar Water Splitting

Hydrogen production from water splitting by photo/photoelectron-catalytic process is a promising route to solve both fossil fuel depletion and environmental pollution at the same time. Titanium dioxide (TiO2) nanotubes have attracted much interest due to their large specific surface area and highly ordered structure, which has led to promising potential applications in photocatalytic degradation, photoreduction of CO2, water splitting, supercapacitors, dye-sensitized solar cells, lithium-ion batteries and biomedical devices. Nanotubes can be fabricated via facile hydrothermal method, solvothermal method, template technique and electrochemical anodic oxidation. In this report, we provide a comprehensive review on recent progress of the synthesis and modification of TiO2 nanotubes to be used for photo/photoelectro-catalytic water splitting. The future development of TiO2 nanotubes is also discussed.

Seed-mediated phase-selective growth of Cu2GeS3 hollow nanoparticles with huge cavities

Although significant progress has been achieved in the synthesis of hollow nanoparticles (NPs), research on copper-based multinary chalcogenide (CMC) semiconductor NPs with hollow structures is still less developed.

Preparation of the TiO2/Graphic Carbon Nitride Core-Shell Array as a Photoanode for Efficient Photoelectrochemical Water Splitting

The photoelectrochemical (PEC) oxygen evolution reaction over a photoanode is a promising process for renewable energy. The fascinating properties of graphic carbon nitride (g-CN) in water splitting make the photoelectrode engineering of it for PEC use quite meaningful. In this work, we report the fabrication of the core-shell-structured TiO₂/g-CN composite film by hydrothermal growth for TiO₂ nanorod arrays and solvothermal growth for the g-CN layer. Herein, TiO₂ is used as an effective electron-transfer layer, and g-CN is used as a visible light absorption layer. Different reaction conditions were investigated in order to obtain the uniform TiO₂/g-CN nanorod core-shell structure. Outstanding photoelectrochemical performances of the optimized composites were obtained compared to that of pristine TiO₂ or g-CN because the high-quality heterojunction between g-CN and TiO₂ turned out to effectively reduce the recombination of charge carriers and improve the photoelectric conversion ability. Thus, the photocurrent density under visible light of TiO₂/g-CN reached 80.9 μA cm^-2, which is 21 times that of g-CN under 0.6 V (vs SCE). Finally, a systematic photoelectrocatalytic mechanism of charge carrier migration and the recombination path in the TiO₂/g-CN nanorod core-shell heterojunction was proposed, which can be considered to be a probable explanation of efficient PEC performance.

Hetero-type dual photoanodes for unbiased solar water splitting with extended light harvesting

Metal oxide semiconductors are promising photoelectrode materials for solar water splitting due to their robustness in aqueous solutions and low cost. Yet, their solar-to-hydrogen conversion efficiencies are still not high enough for practical applications. Here we present a strategy to enhance the efficiency of metal oxides, hetero-type dual photoelectrodes, in which two photoanodes of different bandgaps are connected in parallel for extended light harvesting. Thus, a photoelectrochemical device made of modified BiVO₄ and α-Fe₂O₃ as dual photoanodes utilizes visible light up to 610 nm for water splitting, and shows stable photocurrents of 7.0±0.2 mA cm⁻² at 1.23 V_RHE under 1 sun irradiation. A tandem cell composed with the dual photoanodes–silicon solar cell demonstrates unbiased water splitting efficiency of 7.7%. These results and concept represent a significant step forward en route to the goal of >10% efficiency required for practical solar hydrogen production.

Metal oxide semiconductors are promising photoelectrode materials for solar water splitting but their efficiency needs to be improved. Here, the authors report a hetero-type dual photoelectrode strategy in which two photoanodes of different band gaps are connected in parallel for extended light harvesting.

Synthesis and Characterization of an Earth-Abundant Cu2BaSn(S,Se)4 Chalcogenide for Photoelectrochemical Cell Application

Cu₂BaSnS_4-xSe_x films consisting of earth-abundant metals have been examined for photocathode application. Films with different Se contents (i.e., Cu₂BaSnS_4-xSe_x with x ≤ 2.4) were synthesized using a cosputter system with post-deposition sulfurization/selenization annealing treatments. Each film adopts a trigonal P3₁ crystal structure, with progressively larger lattice constants and with band gaps shifting from 2.0 to 1.6 eV, as more Se substitutes for S in the parent compound Cu₂BaSnS₄. Given the suitable bandgap and earth-abundant elements, the Cu₂BaSnS_4-xSe_x films were studied as prospective photocathodes for water splitting. Greater than 6 mA/cm² was obtained under illumination at -0.4 V versus reversible hydrogen electrode for Pt/Cu₂BaSnS_4-xSe_x films with ∼60% Se content (i.e., x = 2.4), whereas a bare Cu₂BaSnS_4-xSe_x (x = 2.4) film yielded ∼3 mA/cm² at -0.4 V/RHE.

Tuning the Photoelectrocatalytic Hydrogen Evolution of Pt-Decorated Silicon Photocathodes by the Temperature and Time of Electroless Pt Deposition

The electroless deposition of Pt nanoparticles (NPs) on hydrogen-terminated silicon (H-Si) surfaces is studied as a function of the temperature and the immersion time. It is demonstrated that isolated Pt structures can be produced at all investigated temperatures (between 22 and 75 °C) for short deposition times, typically within 1-10 min if the temperature is 45 °C or less than 5 min at 75 °C. For longer times, dendritic metal structures start to grow, ultimately leading to highly rough interconnected Pt networks. Upon increasing the temperature from 22 to 75 °C and for an immersion time of 5 min, the average size of the observed Pt NPs monotonously increases from 120 to 250 nm, and their number density calculated using scanning electron microscopy decreases from (4.5 ± 1.0) × 10⁸ to (2.0 ± 0.5) × 10⁸ Pt NPs cm^-2. The impact of both the morphology and the distribution of the Pt NPs on the photoelectrocatalytic activity of the resulting metallized photocathodes is then analyzed. Pt deposited at 45 °C for 5 min yields photocathodes with the best electrocatalytic activity for the hydrogen evolution reaction. Under illumination at 33 mW cm^-2, this optimized photoelectrode shows a fill factor of 45%, an efficiency (η) of 9.7%, and a short-circuit current density (|J_sc|) at 0 V versus a reversible hydrogen electrode of 15.5 mA cm^-2.

Charge Transport in Two-Photon Semiconducting Structures for Solar Fuels

Semiconducting heterostructures are emerging as promising light absorbers and offer effective electron-hole separation to drive solar chemistry. This technology relies on semiconductor composites or photoelectrodes that work in the presence of a redox mediator and that create cascade junctions to promote surface catalytic reactions. Rational tuning of their structures and compositions is crucial to fully exploit their functionality. In this review, we describe the possibilities of applying the two-photon concept to the field of solar fuels. A wide range of strategies including the indirect combination of two semiconductors by a redox couple, direct coupling of two semiconductors, multicomponent structures with a conductive mediator, related photoelectrodes, as well as two-photon cells are discussed for light energy harvesting and charge transport. Examples of charge extraction models from the literature are summarized to understand the mechanism of interfacial carrier dynamics and to rationalize experimental observations. We focus on a working principle of the constituent components and linking the photosynthetic activity with the proposed models. This work gives a new perspective on artificial photosynthesis by taking simultaneous advantages of photon absorption and charge transfer, outlining an encouraging roadmap towards solar fuels.

Ultrafast fabrication of highly active BiVO4 photoanodes by hybrid microwave annealing for unbiased solar water splitting

Hybrid microwave annealing (HMA) with a silicon susceptor in a household microwave oven produces BiVO₄-based photoanodes of much improved performance in photoelectrochemical water oxidation in only 6 min relative to conventional thermal annealing in a traditional muffle furnace (FA) that needs a much longer time, 300 min. This technique can apply equally effectively to bare as well as modified BiVO₄ by Mo-doping, heterojunction formation with WO₃, and an oxygen evolution co-catalyst. Relative to FA, HMA forms BiVO₄ films of smaller feature sizes, higher porosity, and increased three dimensional roughness, which decrease the diffusion distance of holes to the surface and thereby increase mainly the bulk charge separation efficiency (η_bulk) of the photoanodes. Thus, the HMA-treated BiVO₄/WO₃ film achieves the state-of-the art η_bulk of ∼90% for water oxidation. Combination of a photoanode of NiOOH/FeOOH/BiVO₄/WO₃ (HMA, 6 min) with a 2p c-Si solar cell allows a solar to hydrogen conversion efficiency of ∼5.0% in unbiased overall water splitting, which is also comparable to the state-of-the-art for a similar material combination.

Protected Light-Trapping Silicon by a Simple Structuring Process for Sunlight-Assisted Water Splitting

Macroporous layers are grown onto n-type silicon by successive photoelectrochemical etching in HF-containing solution and chemical etching in KOH. This specific latter treatment gives highly antireflective properties of the Si surface. The duration of the chemical etching is optimized to render the surface as absorbent as possible, and the morphology of the as-grown layer is characterized by scanning electron microscopy. Further functionalization of such structured Si surface is carried out by atomic layer deposition of a thin conformal and homogeneous TiO2 layer that is crystallized by an annealing at 450 °C. This process allows using such surfaces as photoanodes for water oxidation. The 40 nm thick TiO2 film acts indeed as an efficient protective layer against the photocorrosion of the porous Si in KOH, enhances its wettability, and improves the light absorption of the photoelectrode. The macroporous dual-absorber TiO2/Si has a beneficial effect on water oxidation in 1 M KOH and leads to a considerable negative shift of the onset potential of ∼400 mV as well as a 50% increase in photocurrent at 1 V vs SCE.

Discovery of Fe-Ce Oxide/BiVO4 Photoanodes through Combinatorial Exploration of Ni-Fe-Co-Ce Oxide Coatings

An efficient photoanode is a prerequisite for a viable solar fuels technology. The challenges to realizing an efficient photoanode include the integration of a semiconductor light absorber and a metal oxide electrocatalyst to optimize corrosion protection, light trapping, hole transport, and photocarrier recombination sites. To efficiently explore metal oxide coatings, we employ a high-throughput methodology wherein a uniform BiVO4 film is coated with 858 unique metal oxide coatings covering a range of metal oxide loadings and the full (Ni-Fe-Co-Ce)Ox pseudoquaternary composition space. Photoelectrochemical characterization of the photoanodes reveals that specific combinations of metal oxide composition and loading provide up to a 13-fold increase in the maximum photoelectrochemical power generation for oxygen evolution in pH 13 electrolyte. Through mining of the high-throughput data we identify composition regions that form improved interfaces with BiVO4. Of particular note, integrated photoanodes with catalyst compositions in the range Fe(0.4-0.6)Ce(0.6-0.4)Ox exhibit high interface quality and excellent photoelectrochemical power conversion. Scaled-up inkjet-printed electrodes and photoanodic electrodeposition of this composition on BiVO4 confirms the discovery and the synthesis-independent interface improvement of (Fe-Ce)Ox coatings on BiVO4.

Profiling Photoinduced Carrier Generation in Semiconductor Microwire Arrays via Photoelectrochemical Metal Deposition

Au was photoelectrochemically deposited onto cylindrical or tapered p-Si microwires on Si substrates to profile the photoinduced charge-carrier generation in individual wires in a photoactive semiconductor wire array. Similar experiments were repeated for otherwise identical Si microwires doped to be n-type. The metal plating profile was conformal for n-type wires, but for p-type wires was a function of distance from the substrate and was dependent on the illumination wavelength. Spatially resolved charge-carrier generation profiles were computed using full-wave electromagnetic simulations, and the localization of the deposition at the p-type wire surfaces observed experimentally correlated well with the regions of enhanced calculated carrier generation in the volumes of the microwires. This technique could potentially be extended to determine the spatially resolved carrier generation profiles in a variety of mesostructured, photoactive semiconductors.

Improved Photoelectrocatalytic Performance for Water Oxidation by Earth-Abundant Cobalt Molecular Porphyrin Complex-Integrated BiVO4 Photoanode

An earth-abundant, low-cost cobalt porphyrin complex (CoTCPP) is designed as a molecular catalyst to work on three-dimensional BiVO4 film electrode for water oxidation for the first time. Under illumination of a 100 mW cm(-2) Xe lamp, the CoTCPP-functionalized BiVO4 photoanode exhibits a 2-fold enhancement in photocurrent density at 1.23 V vs RHE and nearly a 450 mV cathodic shift at 0.5 mA cm(-2) photocurrent density relative to bare BiVO4 in 0.1 M Na2SO4 (pH = 6.8). Simultaneously, stoichiometric oxygen and hydrogen are generated with a faradic efficiency of 80% over 4 h. The activity and stability of the BiVO4 photoanode are dramatically increased by molecular CoTCPP, giving rise to higher performance than previously reported noble metal ruthenium complex-modified BiVO4 photoanode. By using hydrogen peroxide as the hole scavenger, we demonstrate that molecular CoTCPP catalyst greatly suppresses the hole-electron recombination on the surface of BiVO4 semiconductor, which offers a promising route toward high efficiency, low cost, practical solar fuel generation device.

Engineering MoSx/Ti/InP Hybrid Photocathode for Improved Solar Hydrogen Production

Due to its direct band gap of ~1.35 eV, appropriate energy band-edge positions, and low surface-recombination velocity, p-type InP has attracted considerable attention as a promising photocathode material for solar hydrogen generation. However, challenges remain with p-type InP for achieving high and stable photoelectrochemical (PEC) performances. Here, we demonstrate that surface modifications of InP photocathodes with Ti thin layers and amorphous MoS_x nanoparticles can remarkably improve their PEC performances. A high photocurrent density with an improved PEC onset potential is obtained. Electrochemical impedance analyses reveal that the largely improved PEC performance of MoS_x/Ti/InP is attributed to the reduced charge-transfer resistance and the increased band bending at the MoS_x/Ti/InP/electrolyte interface. In addition, the MoS_x/Ti/InP photocathodes function stably for PEC water reduction under continuous light illumination over 2 h. Our study demonstrates an effective approach to develop high-PEC-performance InP photocathodes towards stable solar hydrogen production.

Insight into the charge transfer in particulate Ta3N5 photoanode with high photoelectrochemical performance

Charge separation is one of the most critical factors for generating solar fuels via photoelectrochemical water splitting, but it is still not well understood. This work reveals the fundamental role of charge transfer in photoanodes for achieving high charge separation efficiency. Specifically, we fabricated a particulate Ta3N5 photoanode by a bottom-up method. By improving the charge separation with refined necking treatment, the photocurrent is increased by two orders of magnitude. The charge separation efficiency (ηsep) is analyzed by dividing it into charge generation efficiency (Φgene) and transportation efficiency (Φtrans). Necking treatment is found to substantially improve the electron transfer. Transient photovoltage (TPV) measurements based on the Dember effect is used to confirm the benefit of necking treatment in improving the charge transportation. The superior electron transfer in the necked-Ta3N5 electrode is further evidenced by the facile electron exchange reaction with the ferri/ferrocyanide redox couple. Moreover, cobalt phosphate is found to promote both charge separation and surface reaction, resulting in a photocurrent of 6.1 mA cm-2 at 1.23 V vs. RHE, which is the highest response for a particulate photoanode.

Hybrid Mn3O4–NiO nanocomposites as efficient photoelectrocatalysts towards water splitting under neutral pH conditions

Dual oxide Mn₃O₄–NiO nanocomposites synthesised by seed-mediated epitaxial growth have been exploited as electrocatalysts towards water splitting at an applied overpotential of 280 mV under neutral pH conditions.

Enhanced photoelectrochemical water splitting efficiency of hematite electrodes with aqueous metal ions as in situ homogenous surface passivation agents

Interactions between aqueous metal ions with hematite electrodes can in situ passivate surface states and thus enhance PEC efficiency.

Microwave-assisted fabrication of porous hematite photoanodes for efficient solar water splitting

Efficient PEC water splitting has been achieved over porous hematite photoanodes with wormlike networks.