Enhanced Photocurrent of Transparent CuFeO2 Photocathodes by Self-Light-Harvesting Architecture

Structure-led manifestation of photocatalytic activity in magnetically recoverable spinel CuFe2O4nanoparticles and its application in degradation of industrial effluent dyes under solar light

An efficient removal of the photocatalysts used in the decontamination of water is crucial after its application beside its expected visible light sensitive activities. This study presents the synthesis of magnetically separable CuFe2O4nanoparticles (CFNPs) with enhanced photoactivity under AM 1.5 G sunlight. A simple two-step process involving co-precipitation and hydrothermal treatment is employed, with subsequent annealing at temperatures from 200 °C to 1000 °C to synthesize the CFNPs. The characteristic features of the highest photoactive tetragonal phase of CFNP are confirmed by powder XRD studies with Rietveld refinement. This scheme strategically controls the growth of a highly photoactive tetragonal phase with predominant (224) facets over other less active facets in cubic CuFe2O4. Mott-Schottky analysis confirms thep-type semiconducting nature of CFNPs. A favourable direct optical band gap of 1.73 eV, as well as photoluminescence emission quenching for visible photons, show that the (224) oriented CFNPs are good photocatalysts in the visible spectrum with demonstrated organic dye degradations, including methylene blue and others. A density functional theory-based approach validates that the adsorption of such dye is thermodynamically more favourable on (224) facets of CuFe2O4to facilitate the redox action by the excitons.

Optical and Electrical Modulation Strategies of Photoelectrodes for Photoelectrochemical Water Splitting

When constructing efficient, cost-effective, and stable photoelectrodes for photoelectrochemical (PEC) systems, the solar-driven photo-to-chemical conversion efficiency of semiconductors is limited by several factors, including the surface catalytic activity, light absorption range, carrier separation, and transfer efficiency. Accordingly, various modulation strategies, such as modifying the light propagation behavior and regulating the absorption range of incident light based on optics and constructing and regulating the built-in electric field of semiconductors based on carrier behaviors in semiconductors, are implemented to improve the PEC performance. Herein, the mechanism and research advancements of optical and electrical modulation strategies for photoelectrodes are reviewed. First, parameters and methods for characterizing the performance and mechanism of photoelectrodes are introduced to reveal the principle and significance of modulation strategies. Then, plasmon and photonic crystal structures and mechanisms are summarized from the perspective of controlling the propagation behavior of incident light. Subsequently, the design of an electrical polarization material, polar surface, and heterojunction structure is elaborated to construct an internal electric field, which serves as the driving force to facilitate the separation and transfer of photogenerated electron-hole pairs. Finally, the challenges and opportunities for developing optical and electrical modulation strategies for photoelectrodes are discussed.

Crystal Facet-Controlled Efficient SnS Photocathodes for High Performance Bias-Free Solar Water Splitting

To achieve a high solar-to-hydrogen (STH) conversion efficiency, delicate strategies toward high photocurrent together with sufficient onset potential should be developed. Herein, an SnS semiconductor is reported as a high-performance photocathode. Use of proper sulfur precursor having weak dipole moment allows to obtain high-quality dense SnS nanoplates with enlarged favorable crystallographic facet, while suppressing inevitable anisotropic growth. Furthermore, the introducing Ga2 O3 layer between SnS and TiO2 in SnS photocathodes efficiently improves the charge transport kinetics without charge trapping. The SnS photocathode reveals the highest photocurrent density of 28 mA cm-2 at 0 V versus the reversible hydrogen electrode. Overall solar water splitting is demonstrated for the first time by combining the optimized SnS photocathode with a Mo:BiVO4 photoanode, achieving a STH efficiency of 1.7% and long-term stability of 24 h. High performance and low-cost SnS photocathode represent a promising new material in the field of photoelectrochemical solar water splitting.

A Novel Thermochemical Metal Halide Treatment for High-Performance Sb2Se3 Photocathodes

The fabrication of cost-effective photostable materials with optoelectronic properties suitable for commercial photoelectrochemical (PEC) water splitting represents a complex task. Herein, we present a simple route to produce Sb₂Se₃ that meets most of the requirements for high-performance photocathodes. Annealing of Sb₂Se₃ layers in a selenium-containing atmosphere persists as a necessary step for improving device parameters; however, it could complicate industrial processability. To develop a safe and scalable alternative to the selenium physical post-processing, we propose a novel SbCl₃/glycerol-based thermochemical treatment for controlling anisotropy, a severe problem for Sb₂Se₃. Our procedure makes it possible to selectively etch antimony-rich oxyselenide presented in Sb₂Se₃, to obtain high-quality compact thin films with a favorable morphology, stoichiometric composition, and crystallographic orientation. The treated Sb₂Se₃ photoelectrode demonstrates a record photocurrent density of about 31 mA cm^-2 at -248 mV against the calomel electrode and can thus offer a breakthrough option for industrial solar fuel fabrication.

Benchmark performance of low-cost Sb2Se3 photocathodes for unassisted solar overall water splitting

Determining cost-effective semiconductors exhibiting desirable properties for commercial photoelectrochemical water splitting remains a challenge. Herein, we report a Sb2Se3 semiconductor that satisfies most requirements for an ideal high-performance photoelectrode, including a small band gap and favourable cost, optoelectronic properties, processability, and photocorrosion stability. Strong anisotropy, a major issue for Sb2Se3, is resolved by suppressing growth kinetics via close space sublimation to obtain high-quality compact thin films with favourable crystallographic orientation. The Sb2Se3 photocathode exhibits a high photocurrent density of almost 30 mA cm-2 at 0 V against the reversible hydrogen electrode, the highest value so far. We demonstrate unassisted solar overall water splitting by combining the optimised Sb2Se3 photocathode with a BiVO4 photoanode, achieving a solar-to-hydrogen efficiency of 1.5% with stability over 10 h under simulated 1 sun conditions employing a broad range of solar fluxes. Low-cost Sb2Se3 can thus be an attractive breakthrough material for commercial solar fuel production.

Directly Photoexcited Oxides for Photoelectrochemical Water Splitting

Artificial photosynthesis promises to become a sustainable way to harvest solar energy and store it in chemical fuels by means of photoelectrochemical (PEC) cells. Although it is intriguing to shift the fossil-fuel-based economy to a renewable carbon-neutral one, which will alleviate environmental problems, there is still a long way to go before it rivals traditional energy sources. Existing solar water-splitting devices can be sorted into three categories: photovoltaic-powered electrolysis, PEC water splitting, and photocatalysis (PC). PEC and PC systems hold the potential to further reduce the cost of devices due to their simple structures in which photoabsorbers and catalysts are closely integrated. PC is expected to be the least expensive approach; however, additional costs and concerns are brought about by the subsequent explosive gas separation. At the heart of all devices, semiconductor photoabsorbers should be efficient, robust, and cheap to satisfy the strict requirements on the market. Therefore, this Review intends to give readers an overview on PEC water splitting, with an emphasis on oxide material-based devices, which hold the potential to support global-scale production in the future.

Recent Advances in Earth-Abundant Photocathodes for Photoelectrochemical Water Splitting

The conversion of solar energy into hydrogen through photoelectrochemical (PEC) water splitting is an attractive way to store renewable energy. Despite the intriguing concept of solar hydrogen production, efficient PEC devices based on earth-abundant semiconductors should be realized to compete economically with conventional steam reforming processes. Herein, recent milestones in photocathode development for PEC water splitting, particularly in earth-abundant semiconductors, in terms of new techniques for enhancing performance, as well as theoretical aspects, are highlighted. In addition, recent research into newly emerging low-cost p-type semiconductors in the PEC field, such as Cu₂ BaSn(S,Se)₄ and Sb₂ Se₃ , are scrutinized and the advantages and disadvantages of each material assessed.

Insights on the Stability and Cationic Nonstoichiometry of CuFeO2 Delafossite

CuFeO₂, the structure prototype of the delafossite family, has received renewed interest in recent years. Thermodynamic modeling and several experimental Cu-Fe-O system investigations did not focus specifically on the possible nonstoichiometry of this compound, which is, nevertheless, a very important optimization factor for its physicochemical properties. In this work, through a complete set of analytical and thermostructural techniques from 50 to 1100 °C, a fine reinvestigation of some specific regions of the Cu-Fe-O phase diagram under air was carried out to clarify discrepancies concerning the delafossite CuFeO₂ stability region as well as the eutectic composition and temperature for the reaction L = CuFeO₂ + Cu₂O. Differential thermal analysis and Tammann's triangle method were used to measure the liquidus temperature at 1050 ± 2 °C with a eutectic composition at Fe/(Cu + Fe) = 0.105 mol %. The quantification of all of the present phases during heating and cooling using Rietveld refinement of the high-temperature X-ray diffraction patterns coupled with thermogravimetric and differential thermal analyses revealed the mechanism of formation of delafossite CuFeO₂ from stable CuO and spinel phases at 1022 ± 2 °C and its incongruent decomposition into liquid and spinel phases at 1070 ± 2 °C. For the first time, a cationic off-stoichiometry of cuprous ferrite CuFe_{1- y}O_2-δ was unambiguous, as evidenced by two independent sets of experiments: (1) Electron probe microanalysis evidenced homogeneous micronic CuFe_{1- y}O_2-δ areas with a maximum y value of 0.12 [i.e., Fe/(Cu + Fe) = 0.47] on Cu/Fe gradient generated by diffusion from a perfect spark plasma sintering pristine interface. Micro-Raman provided structural proof of the existence of the delafossite structure in these areas. (2) Standard Cu additions from the stoichiometric compound CuFeO₂ coupled with high-temperature X-ray diffraction corroborated the possibility of obtaining a pure Cu-excess delafossite phase with y = 0.12. No evidence of an Fe-rich delafossite was found, and complementary analysis under a neutral atmosphere shows narrow lattice parameter variation with an increase of Cu in the delafossite structure. The consistent new data set is summarized in an updated experimental Cu-Fe-O phase diagram. These results provide an improved understanding of the stability region and possible nonstoichiometry value of the CuFe_{1- y}O_2-δ delafossite in the Cu-Fe-O phase diagram, enabling its optimization for specific applications.

Strategies for enhancing the photocurrent, photovoltage, and stability of photoelectrodes for photoelectrochemical water splitting

In this review, we survey recent strategies for photoelectrode optimization and advanced characterization methods towards efficient water splitting cells via feedback from these characterization methods.

Photoelectrodes based on 2D opals assembled from Cu-delafossite double-shelled microspheres for an enhanced photoelectrochemical response

Although a unique light-harvesting property was recently demonstrated in a photocathode based on 2-dimensional (2D) opals of CuFeO₂-shelled SiO₂ microspheres, the performance of a monolayer of ultra-thin CuFeO₂-shelled microspheres is limited by ineffective charge separation. Herein, we propose an innovative design rule, in which an inner CuFeO₂/outer CuAlO₂ double-shelled heterojunction is formed on each partially etched microsphere to obtain a hexagonally assembled 2D opal photoelectrode. Our Cu-delafossite double-shelled photocathode shows a dramatically improved charge separation capability, with a 9-fold increase in the photocurrent compared to that of the single-shelled counterpart. Electrochemical impedance spectroscopy clearly confirms the reduced charge transport/transfer resistance associated with the Cu-delafossite double-shelled photocathode, while surface photovoltage spectra reveal enhanced polarization of the photogenerated carrier, indicating improved charge separation capability with the aid of the heterojunction. Our finding sheds light on the importance of heterojunction interfaces in achieving optimal charge separation in opal architectures as well as the inner-shell/electrolyte interface to expedite charge separation/transport.