The interaction of water with a magnetron-sputtered nickel oxide thin film on an n-type silicon photoanode is investigated in perspective to oxygen evolution. The substrate was exposed in-situ stepwise to gas phase water up to 10 L at liquid N 2 temperature and analyzed via X-ray and UV photoelectron spectroscopy in the so called frozen electrolyte approach. Photoemission of the pristine NiO x layer shows the presence of stoichiometric NiO and Ni 2 O 3 as well as of non-stoichiometric phases. In the monolayer range, molecular and dissociative adsorption is detected assigned to the NiO respective Ni 2 O 3 phase. Initially, the emission of the molecular adsorbed water species interacting with NiO is found at 0.8 eV lower binding energies as compared to water related emission for higher coverages with binding energies commonly assigned to H 2 O-H 2 O interaction. In addition to the chemical analysis, the electronic structure of the n-Si/SiO Most semiconducting materials, especially silicon, are unstable and degrade or passivate rapidly under photoanodic conditions in aqueous electrolytes.1 Nickel oxide thin films are promising catalysts for the hydrogen and the oxygen evolution reaction (OER) 2 and provide chemically stable coatings for silicon, while being transparent, antireflective and electronically conductive in addition.3-5 Elementary charge transfer processes located at a solid/liquid interface, are still scarcely approached by scientific studies on an atomistic scale.6 Especially the energetics of the photocatalytic dissociation of water on semiconducting materials is still unclear. In general, the determination of the chemical and electronic interaction at the interface between metal oxides and liquid electrolytes on an atomistic scale via surface science methods suffers from the so-called pressure gap. 7,8 One way to bridge the gap, model experiments at low temperatures can be utilized to monitor the surface chemistry by means of photoelectron spectroscopy under ultra-high vacuum. Regarding double layer formation, the frozen-electrolyte approach has already shown its potential in the past. 9 The principle mechanism of the oxygen evolution (OER) at nickel hydroxide based electrodes was investigated early on a more chemical basis 10 with various phase transitions involved. 11 It has been found that Ni(OH) 2 and NiOOH species at the surface are a prerequisite for any photocatalytic activity. However, the exact mechanism is still under debate 12 with the role of nickel oxide only little discussed. A first step for a better understanding is to study the reactivity of a pristine nickel oxide surface upon water adsorption. For NiO(100) it has been demonstrated that a perfect surface exhibits no reactive interaction with H 2 O unless oxygen vacancies induce the dissociation of water 13 or adsorbed O 2− ions at defect sites on a pre-oxidized surface enhance the reactivity. In contrast to NiO(100), theoretical studies predict dissociative water adsorption for NiO(111).14 Further to oxygen vacancies sub-coordinated ...

Nickel oxide/hydroxides [NiO x (OH) y ] are considered to be promising materials to replace noble metals for the oxygen evolution reaction in alkaline media. While several studies showed that iron impurities promote the activity of nickel-based catalysts, the effects of intrinsic nickel defects and the underlying mechanism remain unknown. In this work, X-ray photoelectron spectroscopy is combined with surface-enhanced Raman scattering to understand the reactivity of NiO x thin films, which were prepared at different temperatures and thus varied in their chemical composition and crystalline order. Raman spectroscopy was used to follow the characteristic oxidation of nickel species from Ni II (OH) 2 to Ni III OOH and Ni IV OO − under electrochemical conditions. A stronger oxide-to-hydroxide conversion, consistent with the post-electrochemistry study, was associated with the presence of initial Ni III impurities and oxygen vacancies and appears beneficial for the electrocatalytic activity.

Nickel oxide-based catalysts currently represent the state of the art in electrochemical water oxidation in alkaline pH. However, much of their functionality remains poorly understood, particularly regarding catalytically active sites...

The interaction between (001) n-Si and NiO x was investigated with regard to the oxygen evolution reaction (OER), applicable either for water splitting or CO 2 reduction. Thin layers of nickel oxide were deposited step by step by reactive sputter deposition and analyzed in-situ after each step using X-ray photoelectron spectroscopy (XPS). This was performed for silicon with different surface preparations: H-termination, thermally grown oxide (2 Å) and a monolayer of native oxide (4 Å). Upon contact formation the initial flatband like situation in the Si substrates changed to a 0.35 to 0.4 eV upward band bending for all three heterojunctions, with an alignment of the valence bands favorable to hole extraction. With near identical heterojunction performance and identical NiO x catalyst layers (η(10 mA/cm 2 ) = 0.44 ± 0.01 V vs. RHE on Ni) an equally identical performance for the OER would be expected. While the native oxide covered sample shows the expected performance in cyclic voltammetry measurements the others fall short of expectations. Using chopped light measurements, this under-performance could be attributed to a higher density of defect states at the silicon surface. Apparently a 4Å SiO 2 layer is sufficient protection to prevent the formation of defect states during NiO x deposition, thinner protective layers or none at all result in increased defect states, while thicker layers perform poorly due to their high resistance. Direct photoelectrochemical devices are an exciting approach to the storage of renewable energies. Depending on the preferred energy cycle they can be used either for water splitting or CO 2 reduction, in the end it is only a question of the employed catalysts and necessary overpotentials. A direct photoelectrochemical device has two main functional parts, which do not necessarily have to be physically separate. First, there is the photoabsorber, basically a solar cell, who has to supply the necessary photovolage U ph and photocurrent J ph to power the desired redox reaction. Second, the catalytic surface, which is in direct contact with the electrolyte and should be catalytically active, thus allowing high current densities at low overpotentials η. In an ideal case the total device performance should be a result of those two contributions only.Initial research on water splitting was focused on a single semiconducting material performing well in both functions. This was first demonstrated on a TiO 2 electrode by Fujishima and Honda in 1972. 1 In such a device the photoabsorber functionality is defined by the interface junction between semiconductor and electrolyte. Investigated materials were mostly wide bandgap materials, as they could, in theory, provide the necessary U ph .However, the double function of the semiconductor electrolyte interface makes an evaluation of such devices difficult, as it is hard to determine whether a lack of performance is a result from poor current voltage (IE) behavior of the junction or the poor catalytic performance of the material. Furthermore, t...

Magnetron sputtered nickel oxide thin films deposited on the native oxide of crystalline n-Si(100) wafers are studied in dependence of the substrate deposition temperature (600 °C, 400 °C, 200 °C, and room temperature) using X-ray and synchrotron excited photoemission spectroscopy as well as cyclic-voltammetry under illumination. We show that the substrate temperature during nickel oxide sputtering governs the composition of the pristine NiOx film and the OER performance. Two dedicated nickel oxide species are found with Ni2+ corresponding to stoichiometric NiO while Ni3+ indicates an oxygen rich NiOx (x > 1) phase. With decreasing deposition temperature, the ratio of Ni3+/Ni2+ in the pristine NiOx film increases. Information depth dependent synchrotron related photoemission spectroscopy further suggests that oxygen rich NiOx is found on top of the surface and at the grain boundaries. The OER onset potential improves from 1.55 V to 1.1 V in correlation to an increasing Ni3+/Ni2+ ratio in the pristine NiOx film and an increasing emission from a nickel oxyhydroxide phase (h-NiOx) after photo-assisted cyclic-voltammetry in alkaline solution. Upon electrochemical treatment, a reconditioning process is observed with the formation of h-NiOx that consists of Ni(OH)2 and NiOOH, while NiOx disappears.