Films of CoP have been electrochemically synthesized, characterized, and evaluated for performance as a catalyst for the hydrogen-evolution reaction (HER). The film was synthesized by cathodic deposition from a boric acid solution of Co2+ and H2PO2 – on copper substrates followed by operando remediation of exogenous contaminants. The films were characterized structurally and compositionally by scanning-electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman spectrophotometry. The catalytic activity was evaluated by cyclic voltammetry and chronopotentiometry. Surface characterization prior to electrocatalysis indicated that the film consisted of micrometer-sized spherical clusters located randomly and loosely on a slightly roughened surface. The composition of both the clusters and surface consisted of cobalt in the metallic, phosphide, and amorphous-oxide forms (CoO·Co2O3) and of phosphorus as phosphide and orthophosphate. The orthophosphate species, produced by air-oxidation, were eliminated upon HER electrocatalysis in sulfuric acid. The operando film purification yielded a functional electrocatalyst with a Co:P stoichiometric ratio of 1:1. After the HER, the surface was densely packed with micrometer-sized, mesa-like particles whose tops were flat and smooth. The CoP eletrodeposit exhibited an 85 mV overvoltage (η) for the HER at a current density of 10 mA cm–2 and was stable under operation in highly acidic solution, with an increase in η of 18 mV after 24 h of continuous operation. The comparative HER catalytic performance of CoP, film or nanoparticles, is as follows: ηPt < ηCoP film = ηCoP NP, ηNi2P < ηCoSe2 < ηMoS2 < ηMoSe2 .

Light absorbers with moderate band gaps (1−2 eV) are required for high-efficiency solar fuels devices, but most semiconducting photoanodes undergo photocorrosion or passivation in aqueous solution. Amorphous TiO 2 deposited by atomic-layer deposition (ALD) onto various n-type semiconductors (Si, GaAs, GaP, and CdTe) and coated with thin films or islands of Ni produces efficient, stable photoanodes for water oxidation, with the TiO 2 films protecting the underlying semiconductor from photocorrosion in pH = 14 KOH(aq). The links between the electronic properties of the TiO 2 in these electrodes and the structure and energetic defect states of the material are not yet wellelucidated. We show herein that TiO 2 films with a variety of crystal structures and midgap defect state distributions, deposited using both ALD and sputtering, form rectifying junctions with n-Si and are highly conductive toward photogenerated carriers in n-Si/TiO 2 /Ni photoanodes. Moreover, the photovoltage of these electrodes can be modified by annealing the TiO 2 in reducing or oxidizing environments. All of the polycrystalline TiO 2 films with compact grain boundaries investigated herein protected the n-Si photoanodes against photocorrosion in pH = 14 KOH(aq). Hence, in these devices, conduction through the TiO 2 layer is neither specific to a particular amorphous or crystalline structure nor determined wholly by a particular extrinsic dopant impurity. The coupled structural and energetic properties of TiO 2 , and potentially other protective oxides, can therefore be controlled to yield optimized photoelectrode performance.

An electrochemical liquid-liquid-solid (ec-LLS) process that produces large amounts of crystalline semiconductors with tunable nanostructured shapes without any physical or chemical templating agent is presented. Electrodeposition of Ge from GeO(2)(aq) solutions followed by dissolution into a liquid Hg electrode, saturation of the liquid alloy, and precipitation can yield polycrystalline Ge(s) under ambient conditions. A unique advantage of ec-LLS is that it involves precipitation under electrochemical control, where the applied bias precisely defines the flux of Ge into the liquid electrode. Fidelity of the saturation and precipitation of Ge from liquid electrodes affords a variety of material morphologies, including dense films of oriented nanostructured filaments with large aspect ratios (>10(3)). Electrodeposition involving a liquid electrolyte, a liquid electrode, and a solid deposit under ambient conditions represents a conceptually unexplored direct wet-chemical route for the preparation of bulk quantities of crystalline group-IV semiconductors without the time- and energy-intensive processing steps required in traditional preparations of semiconductor materials.

Although II-VI semiconductors such as CdS, CdTe, CdSe, ZnTe, and alloys thereof can have nearly ideal band gaps and band-edge positions for the production of solar fuels, II-VI photoanodes are wellknown to be unstable towards photocorrosion or photopassivation when in contact with aqueous electrolytes. Atomic-layer deposition (ALD) of amorphous, "leaky" TiO 2 films coated with thin films or islands of Ni oxide has been shown to robustly protect Si, GaAs, and other III-V materials from photocorrosion and therefore to facilitate the robust, solar-driven photoelectrochemical oxidation of H 2 O to O 2 (g). We demonstrate herein that ALD-deposited 140 nm thick amorphous TiO 2 films also effectively protect single crystalline n-CdTe photoanodes from corrosion or passivation. An n-CdTe/TiO 2 electrode with a thin overlayer of a Ni-oxide based oxygen-evolution electrocatalyst produced 435 AE 15 mV of photovoltage with a light-limited current density of 21 AE 1 mA cm À2 under 100 mW cm À2 of simulated Air Mass 1.5 illumination. The ALD-deposited TiO 2 films are highly optically transparent and electrically conductive. We show that an n-CdTe/ TiO 2 /Ni oxide electrode enables the stable solar-driven oxidation of H 2 O to O 2 (g) in strongly alkaline aqueous solutions, where passive, intrinsically safe, efficient systems for solar-driven water splitting can be operated.CdTe, with a 1.44 eV band gap, has been widely studied since the 1980s, 1-6 and is currently used primarily in thin-lm solar cells in which p-CdTe is deposited upon n-CdS to form a buried heterojunction device.7,8 CdTe has furthermore been investigated for use in photoelectrochemical (PEC) applications but is known to undergo a number of facile photooxidation or photocorrosion processes in various aqueous, as well as organic, media.1,2 Strongly alkaline or strongly acidic media have numerous benets for the electrolysis of water due to their high conductivity without the need for added electrolyte or buffering species and minimal pH gradients under operating conditions. Further, at high and low pH, viable permselective ionophoric membranes are available to separate the products of electrolysis, and the kinetics of water oxidation with suitable electrocatalysts is rapid, thus making possible the construction of a passive, intrinsically safe and efficient solar-driven water-splitting device.9-11 However, efficient photoanodes typically are unstable and rapidly corrode or passivate when operated in contact with electrolytes in these pH ranges.12,13 Recently, several reports have been published in which the protection of otherwise unstable materials is carried out using a protecting layer such as TiO 2 , MnO or metal-modied In-doped SnO 2 . 14-16Facile electron conduction is expected through the conduction band of TiO 2 , and hence TiO 2 has been developed as a Broader contextHigh-efficiency photoelectrochemical (PEC) solar-driven water splitting and/or carbon dioxide reduction will require the use of semiconductors capable of delivering a substantial amount ...

Using an electrochemical method under ambient conditions, crystallographically amorphous films of cobalt selenide have been deposited from aqueous solution onto planar Ti supports. These films have been evaluated as electrocatalysts for the hydrogen-evolution reaction. In 0.500 M H 2 SO 4 , the cobalt selenide films required an overpotential of $135 mV to drive the hydrogen-evolution reaction at a benchmark current density of À10 mA cm À2. Galvanostatic measurements indicated stability of the electrocatalytic films for >16 h of continuous operation at À10 mA cm À2. The facile preparation method, and the activity of the cobalt selenide films, suggest that electrodeposited metal chalcogenides are potentially attractive earth-abundant electrocatalysts for the hydrogen-evolution reaction.The development of a technology capable of efficiently producing molecular hydrogen from water is a critical step toward the realization of a sustainable, renewable, and carbonneutral source of energy. The electrochemical half-reaction for this process, known as the hydrogen-evolution reaction (HER), typically exhibits sluggish kinetics at most cathodes, but is effectively catalyzed by noble metals such as Pt and Pd, which provide high cathodic current densities at modest overpotentials.1,2 Earth-abundant materials capable of catalyzing the HER are being vigorously sought to reduce the cost and increase the scalability of water-splitting systems.3-6 Catalysts that operate effectively in acidic aqueous solutions are of particular interest to these efforts because acidic electrolytes are compatible with existing proton-exchange membranes and minimize the efficiency losses that can result from the formation of a pH gradient across the membrane. 7,8Effective catalysis of the HER in alkaline media has been demonstrated using earth-abundant transition metal alloys, most notably Ni-based alloys such as Ni-Mo. These highly active materials oen reach a current density of À10 mA cm À2 , a benchmark value for photoelectrochemical water-splitting systems, 3 with overpotentials <100 mV at pH $ 14, but are typically unstable in acidic media due to corrosive dissolution, at least when anodic current is passed through the material. 6,9-12Ni-Mo-N, Co 0.6 Mo 1.4 N 2 , MoB, Mo 2 C, Ni 2 P, Ni 5 P 4 and CoP have exhibited stable catalysis of the HER in strongly acidic aqueous electrolytes, but of these materials only the phosphides have been demonstrated to produce current densities of À10 mA cm À2 with overpotentials <200 mV. [13][14][15][16][17][18] Recently, transition metal chalcogenide materials, the prototypical example of which is MoS 2 , have attracted signicant attention as HER electrocatalysts in acidic media.19,20 Accordingly, signicant effort has been devoted to optimizing and engineering MoS 2 , and to a lesser extent MoSe 2 , WS 2 and WSe 2 , to improve their catalytic performance, resulting in overpotentials of <200 mV for effecting HER current densities of À10 mA cm À2 for certain preparations of MoS 2 . 21-24 These efforts include m...