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International audience; H-2 is an environmentally-friendly fuel that would allow for a circular economy but its sustainable production, e.g. from solar energy and water, remains a challenge. A hybrid CuFexOy|TiO2-CoHEC (CoHEC = chloro([4,4 '-bipyridine]-2,6-dicarboxylic acid)bis(dimethylglyoximato)cobalt(iii)) photocathode for hydrogen evolution reaction (HER), with faradaic efficiencies of 54-88%, is described, the preparation of which uses only non-toxic and Earth-abundant elements, avoids etching treatments and limits the use of organic solvents. The semi-conducting CuFexOy light absorber is obtained by sol-gel synthesis followed by calcination at moderate temperature. Grafting the CoHEC cobaloxime catalyst at its surface results in H-2 evolution with an onset photocurrent potential of +860 mV; this process being stabilized by the presence of a thin layer of amorphous TiO2 deposited onto CuFexOy.

The need to satisfy the growing global population’s enormous energy demands is a major challenge for modern societies. Photoelectrochemical (PEC) water splitting (WS) is seen as a leading strategy for producing an extremely promising renewable store of energy – hydrogen (H2). However, PEC-WS is a complex process involving several sequential physicochemical reaction steps including light absorption, separation of photoexcited charges, and surface redox reactions. At present, FeO-based semiconductors represent a unique class of materials known to exhibit very high performance in all these processes. This review summarizes and critically discusses the major components of PEC-WS systems incorporating FeO-based light-harvesting systems, and outlines the progress that has been made, particularly over the last decade. Emphasis is placed on materials used as photoanodes (including hematite and nonhematite iron oxides, spinel iron ferrites, and pseudobrookite iron titanates) as well as materials used as cocatalysts and passivation layers – notably iron hydroxyoxides and their composites. We discuss strategies for overcoming the main limitations of the aforementioned materials via nanostructuring, elemental doping, surface decoration, and the formation of advanced hybrid nanoarchitectures. Finally, we use this knowledge to present a critical overview of the field and the future prospects of Fe-O semiconductors in PEC-WS applications.

Tin disulfide (SnS2) is attracting significant interest because of the abundance of its elements and its excellent optoelectronic properties in part related to its layered structure. In this work, we specify the preparation of ultrathin SnS2 nanoplates (NPLs) through a hot-injection solution-based process. Subsequently, Pt was grown on their surface via in situ reduction of a Pt salt. The photoelectrochemical (PEC) performance of such nanoheterostructures as photoanode toward water oxidation was tested afterwards. Optimized SnS2–Pt photoanodes provided significantly higher photocurrent densities than bare SnS2 and SnS2-based photoanodes of previously reported study. Mott–Schottky analysis and PEC impedance spectroscopy (PEIS) were used to analyze in more detail the effect of Pt on the PEC performance. From these analyses, we attribute the enhanced activity of SnS2–Pt photoanodes reported here to a combination of the very thin SnS2 NPLs and the proper electronic contact between Pt nanoparticles (NPs) and SnS2., This work was supported by the European Regional Development Funds and by the Spanish Ministerio de Economía y Competitividad through the project SEHTOP (ENE2016-77798-C4-3-R) and VALPEC (ENE2017-85087-C3). Y.Z., J.L., R.D., X.T., and T.Z. thank China Scholarship Council for scholarship support. Y.L. and N.G. thank the Swiss National Science Foundation (SNF) for funding under the Ambizione Energy grant no. PZENP2_166871. J.L. is a Serra Húnter Fellow and is grateful to ICREA Academia program and grants MINECO/FEDER ENE2015-63969-R and GC 2017 SGR 128. T.Z. and J.A. acknowledge funding from Generalitat de Catalunya 2017 SGR 327. ICN2 acknowledges support from the Severo Ochoa Programme (MINECO, grant no. SEV-2013-0295). IREC and ICN2 are funded by the CERCA Programme/Generalitat de Catalunya. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program.

Development of cost-effective oxygen evolution catalysts is of capital importance for the deployment of large-scale energy-storage systems based on metal–air batteries and reversible fuel cells. In this direction, a wide range of materials have been explored, especially under more favorable alkaline conditions, and several metal chalcogenides have particularly demonstrated excellent performances. However, chalcogenides are thermodynamically less stable than the corresponding oxides and hydroxides under oxidizing potentials in alkaline media. Although this instability in some cases has prevented the application of chalcogenides as oxygen evolution catalysts and it has been disregarded in some others, we propose to use it in our favor to produce high-performance oxygen evolution catalysts. We characterize here the in situ chemical, structural, and morphological transformation during the oxygen evolution reaction (OER) in alkaline media of Cu2S into CuO nanowires, mediating the intermediate formation of Cu(OH)2. We also test their OER activity and stability under OER operation in alkaline media and compare them with the OER performance of Cu(OH)2 and CuO nanostructures directly grown on the surface of a copper mesh. We demonstrate here that CuO produced from in situ electrochemical oxidation of Cu2S displays an extraordinary electrocatalytic performance toward OER, well above that of CuO and Cu(OH)2 synthesized without this transformation., This work was supported by the European Regional Development Funds and by the Spanish Ministerio de Economía y Competitividad through the project SEHTOP (ENE2016-77798-C4-3-R). Y.Z., Junshan Li, R.D., X.H., and T.Z. thank the China Scholarship Council for the scholarship support. Y.L. and N.G. thank the Swiss National Science Foundation (SNF) for funding under the Ambizione Energy grant no. PZENP2_166871. Jordi Llorca is a Serra Hunter Fellow and is grateful to ICREA Academia program and to GC 2017 SGR 128. ICN2 acknowledges funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO project ENE2017-85087-C3. ICN2 is supported by the Severo Ochoa program from the Spanish MINECO (grant no. SEV-2017-0706) and is funded by the CERCA Programme/Generalitat de Catalunya. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program.

We present a strategy to produce porous NiTiO3/TiO2 nanostructures with excellent photocatalytic activity toward hydrogen generation. In a first step, nickel-doped TiO2 needle bundles were synthesized by a hydrothermal procedure. Through the sintering in air of these nanostructures, porous NiTiO3/TiO2 heterostructured rods were obtained. Alternatively, the annealing in argon of the nickel-doped TiO2 needle bundles resulted in NiOx/TiO2 elongated nanostructures. Porous NiTiO3/TiO2 structures were tested for hydrogen evolution in the presence of ethanol. Such porous heterostructures exhibited superior photocatalytic activity toward hydrogen generation, with hydrogen production rates up to 11.5 mmol h−1 g−1 at room temperature. This excellent performance is related here to the optoelectronic properties and geometric parameters of the material., This work was supported by the European Regional Development Funds and by the Spanish Ministerio de Economía y Competitividad through projects ENE2016-77798-C4-3-R and ENE2015-63969-R. Y. L. and N. G. thank the Swiss National Science Foundation (SNF) for funding under the Ambizione Energy grant no. PZENP2_166871. J.L. is a Serra Hunter fellow and is grateful to ICREA Academia program and GC 2017 SGR 128. T. Z., P.·Y. T. and J. A. acknowledge funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO project ENE2017-85087-C3. ICN2 acknowledges support from the Severo Ochoa Programme (MINECO, Grant no. SEV-2013-0295) and is funded by the CERCA Programme/Generalitat de Catalunya. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program. C. C. X., Y. Z and T. Z. thank the China Scholarship Council for scholarship support.

The search for ideal semiconductors for photoelectrochemical solar fuel conversion has recently recognized the spinel ferrites as promising candidates due to their optoelectronic tunability together with superb chemical stability. However, a systematic understanding of the main material factors limiting their performance is currently lacking. Herein, nanostructured thin-film electrodes of three representative spinels, namely CuFe2O4 (CFO), MgFe2O4 (MFO) and ZnFe2O4 (ZFO), are prepared by a solution-based approach and their photoelectrochemical (PEC) properties are comprehensively characterized. Annealing post-treatments together with the deposition of NiFeOx overlayers are found to improve the native n-type response, although a dominant bulk recombination (especially in MFO) limits the saturation photocurrents (below 0.4 mA cm−2 at 1.23 V vs. RHE). Likewise, prominent Fermi level pinning due to surface states at around 0.9 V vs. RHE in all cases appears to limit the photovoltage (to ca. 300 mV). Rapid-scan voltammetry is used to gain insight into the surface states and the operation of the overlayer. Interestingly, the NiFeOx is ineffective at mitigating Fermi level pinning, but clearly participates as an electrocatalyst to improve the overall performance. Generally, these results evidence the potential and current intrinsic limitations of the spinel ferrites—establishing a roadmap for the optimization of these materials as photoanodes for solar water oxidation.

Tunable ambipolar photoelectrochemical behavior emerges from microdomains of nanostructured p-type CuFeO2 and n-type Fe2O3 that arise from a single facile solution-processed thin fi lm. The switchable operation of this system is controlled by chemical, optical, or electronic inputs with a uniquely high photocurrent response (on the order of 1 mA cm(-2)), suitable for robust practical application as an oxygen photoregulator.

Examining the electrocatalytic performance of naturally-occurring metallic minerals is of interest for energy conversion applications given their unique atomic composition and formation history. Herein, we report the electrocatalytic function of an iron-based Gibeon meteorite for the oxygen evolution reaction (OER). After ageing under operational conditions in an alkaline electrolyte, an activity matching or possibly slightly superior to the best performing OER catalysts emerges, with stable overpotentials as low as 270 mV (for 10 mA cm(-2)) and Tafel slopes of 37 mV decade(-1). The Faradaic efficiency for the OER was unity and no deterioration in performance was detected during 1000 hours of OER operation at 500 mA cm(-2). Mechanistic studies suggest an operando surface modification involving the formation of a 3D oxy(hydroxide) layer with a metal atom composition of Co0.11Fe0.33Ni0.55, as indicated by Raman and XPS studies and trace Ir as indicated via elemental analysis. The growth of the catalyst layer was self-limiting to

p-Type delafossite CuFeO2 has recently been reported as a promising candidate for direct photoelectrochemical solar water reduction in alkaline conditions. However, despite its favorable optical band gap energy and light absorption, this material suffers from poor electron–hole separation that limits its optimum thickness to a few hundred nanometers. This limitation can be addressed by a host–guest strategy, where a mesoporous p-type scaffold is used to support a thin-film of the light absorber. Here we demonstrate this host–guest approach for the first time with CuFeO2 using p-type transparent CuAlO2 as a scaffold. Optimizing the scaffold layer thickness at 2 μm resulted in a 2.4-fold increase in photocurrent in the presence of O2—a sacrificial electron scavenger—reaching 2.4 mA cm−2 at +0.4 V vs. RHE. Moreover, comparing the performance to host–guest electrodes prepared with an insulating SiO2 scaffold as a control suggested that the observed improvement with CuAlO2 was due to a decreased recombination stemming from improved charge separation in addition to improved charge transport through the scaffold compared to the CuFeO2 film alone.

Templating nanocrystals into mesoporous electrodes using an organic porogen typically requires thermal oxidation at >400 degrees C to completely remove the organic material and afford good electronic properties. These oxidation conditions are incompatible with many NC materials. Here, we demonstrate that nitrocellulose can afford mesoporous CdS and CZTS nanocrystal thin film electrodes at only 250-300 degrees C in air or under argon. Remarkable control over the surface area and the average pore size (20-100 nm) in thin films are demonstrated by varying the ratio of preformed nanocrystals and nitrocellulose. Moreover the mesoporous electrodes exhibit excellent optoelectronic properties. Photoelectrochemical performance is enhanced due to the increased active surface area, showing an 8-fold photocurrent increase over compact nanocrystal films, and an IPCE over 70%.

Photoelectrochemical (PEC) cells have emerged as promising devices that afford the direct conversion of solar energy into electric power and/or chemical fuels. Apart from the obvious importance of the bulk properties of semiconductor materials employed as photoelectrodes, the semiconductor–liquid interface has proven to strongly govern surface-related processes, i.e. the stability, charge separation/recombination and catalytic activity. Because of this, numerous surface treatments have been reported in an effort to tailor the physicochemical properties of the semiconductor–liquid interface, and in turn, the overall PEC response. In this Perspective article we provide a brief conceptual overview of these surface engineering treatments, connecting the particular effects on the interfacial energetics with the respective consequences on the performance. The beneficial effects that arise from surface treatment are categorized as (i) the protection of the surface against photocorrosion, (ii) the passivation of deleterious surface states, (iii) the modification of the band edge positions or band bending, and (iv) the selective extraction of carriers and improved catalytic activity. State-of-the-art surface treatments such as the adsorption of organic molecules or ions, the deposition of semiconductor overlayers and metal nanoparticles or etching procedures are exemplified and described with respect to the observed beneficial effects. A common emerging theme from recent work is that one single surface treatment can lead to multiple distinct effects. Overall, we suggest that surface engineering holds the key for effectively managing the intrinsic common defects of native semiconductor photoelectrodes regardless of their nature, leading to improved light harvesting efficiency.

Cu2ZnSnS4 (CZTS) colloidal inks were employed to prepare thin-film photocathodes that served as a model system to interrogate the effect of different surface treatments, viz. CdS, CdSe, and ZnSe buffer layers along with methylviologen (MV) adsorption, on the photoelectrochemical (PEC) performance using aqueous Eu(3+) redox electrolyte. PEC experiments revealed that ZnSe and CdSe overlayers outperform traditional CdS, and the additional surface modification with MV was found to further boost the charge extraction. By analyzing the photocurrent onset behavior and measuring the open circuit photopotentials, insights are gained into the nature of the observed improvements. While a more favorable conduction band offset rationalizes the improvement offered by CdSe, charge transfer through midgap states is invoked for ZnSe. Improvement offered by MV treatment is clearly caused by both the shifting of the flat-band potential and a charge-transfer mediation effect. Overall, this work suggests promising alternative surface treatments for CZTS photocathodes for PEC energy conversion.

This paper addresses the origin of the decrease in the external electrical bias required for water photoelectrolysis with hematite photoanodes, observed following surface treatments of such electrodes. We consider two alternative surface modifications: a cobalt oxo/hydroxo-based (CoO x ) overlayer, reported previously to function as an efficient water oxidation electrocatalyst, and a Ga 2 O 3 overlayer, reported to passivate hematite surface states. Transient absorption studies of these composite electrodes under applied bias showed that the cathodic shift of the photocurrent onset observed after each of the surface modifications is accompanied by a similar cathodic shift of the appearance of long-lived hematite photoholes, due to a retardation of electron/hole recombination. The origin of the slower electron/hole recombination is assigned primarily to enhanced electron depletion in the Fe 2 O 3 for a given applied bias.

Visible light active photoelectrodes for hydrogen generation by solar photoelectrochemical water splitting have been under scrutiny for many decades. In particular the role of electron holes and charge transfer remains controversial. We have investigated the oxygen evolution of hematite in alkaline aqueous electrolyte under a bias potential during visible light illumination in a photoelectrochemical cell operando with soft X ray (O 1s) spectroscopy. Only under these conditions two new spectral signatures evolve in the valence band which we identify as an O 2p hole transition into the charge transfer band and an Fe 3d type hole into the upper Hubbard band. Quantitative analysis of their spectral weight and comparison with the photocurrent reveals that both types of holes contrary to earlier speculations and common perception contribute to the photocurrent. © 2012 American Chemical Society.

Herein, we report the performance of a photocatalytic system based on visible-light active MIL-125-NH2 mixed with nickel phosphide (Ni2P) nanoparticles. This combination boosts the H2 evolution rate to an outstanding value of 894 μmol h−1 g−1 under visible-light irradiation, which is among the highest H2 evolution rates reported to date for metal–organic frameworks (MOFs). The H2 generation rate exhibited by Ni2P/MIL-125-NH2 is almost 3 times higher than that of the Pt/MIL-125-NH2 system, highlighting the impact of the co-catalyst on photocatalytic water splitting. Additionally, our system outperforms the Ni2P/TiO2 system under UV-vis irradiation. The exceptional performance of Ni2P/MIL-125-NH2 is due to the efficient transfer of photogenerated electrons from MIL-125-NH2 to Ni2P, high intrinsic activity of Ni2P and exceptional synergy between them. This system exhibits the highest apparent quantum yields of 27.0 and 6.6% at 400 and 450 nm, respectively, ever reported for MOFs.

A series of TaOxNy photoelectrodes were deposited on F:SnO2 (FTO) substrates by DC reactive sputtering at room temperature, under a mixture of Ar, N-2 and O-2. The effects of the O-2 partial pressure during deposition (Po-2) on the films crystallinity, their chemical composition, their morphology as well as their absorption and photoelectrochemical properties have been investigated. The increase of PO2 led to the modification of film crystallinity, which evolved from a semicrystalline Ta3N5 structure to an amorphous state. The increase of the Po-2 also led to the increases of the oxygen content, of the bandgap energy and of the films roughness. Preliminary photoelectrochemical (PEC) investigations have been performed through current-voiltage measurements. An optimal Po-2 has been highlighted, corresponding to a tradeoff between, on the one hand, the amorphization of the films -which degrades PEC performances - and on the other, the tuning of the chemical composition as well as of the bandgap energy. (C) 2012 Published by Elsevier Ltd. Selection and/or peer review under responsibility of European Material Research Society (E-MRS)

Gas diffusion electrodes are essential components of common fuel and electrolysis cells but are typically made from graphitic carbon or metallic materials, which do not allow light transmittance and thus limit the development of gas-phase based photoelectrochemical devices. Herein, the simple and scalable preparation of F-doped SnO