The Impact of Different Si Surface Terminations in the (001) n-Si/NiOx Heterojunction on the Oxygen Evolution Reaction (OER) by XPS and Electrochemical Methods

Abstract

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...

Sputter Deposition of Transition Metal Oxides on Silicon: Evidencing the Role of Oxygen Bombardment for Fermi‐Level Pinning

“…Figure shows that the position of the Si 0 peak shifts to lower binding energy for any deposition method, indicating that the deposition of NiO leads to the formation of a depletion layer in the silicon with a built‐in voltage . The binding energies at the interfaces with RF and DC sputtered NiO saturate at ≈99.2 eV, corresponding to a built‐in potential of 0.4 V. This value is in line with what has been reported in the literature . In contrast, the binding energies obtained when NiO is deposited by MLO at T > 100 °C saturate at ≈99.0 eV, which is 0.6 eV lower than the expected value for the flat‐band condition.…”

“…Recently, the interest in interfacing TMOs with silicon has substantially increased. Silicon/TMO structures can be encountered not only in water‐splitting devices where the TMO layer is a catalytic layer, but also in solid‐state junction where the TMO layer could be implemented for creating a charge‐selective contact. For instance, n‐type materials as TiO 2 and ZnO could be implemented to create electron‐selective contacts on silicon, whereas p‐type materials or high‐work‐function materials, such as MoO 3 , WO 3 , V 2 O 5 , and NiO, could be implemented to create hole‐selective contacts .…”

“…We introduce this method as metal layer oxidation (MLO). We take as practical example, the deposition of p‐type NiO on n‐Si/SiO 2 and n‐Si/SiO 2 /Al 2 O 3 substrates, as NiO could be implemented in numerous applications including silicon‐based metal insulator semiconductor structure and photoelectrochemical water‐splitting devices …”

Importance of Nickel Oxide Lattice Defects for Efficient Oxygen Evolution Reaction

“…It is seen that the sample produced at RT (Figure a) needs an additional line shape for adequate fitting, which contains higher oxidized Ni III of ∼13% . This is also reflected in the oxygen signal (Figure b), which shows two lattice-bound oxygen components, Ni II –O and Ni III –O, at 529.2 and 530.7 eV. , The oxygen component at higher binding energies (532.0 eV) was attributed to deficient, interstitial, and peroxo-like species, referred to as oxygen vacancy (O V ). − The Ni 2p 3/2 region of NiO x prepared at 600 °C (Figure c) can be completely deconvoluted by the shape of the single crystal line, so the small amount of residual oxygen in Figure d is attributed to O V rather than Ni III moieties. The chemical composition in relation to the deposition temperature is summarized in Table S1.…”

Separating bulk and surface processes in NiOx electrocatalysts for water oxidation

“…S11 †), thereby conrming the highly defective nature of these materials. 31 Higher catalytic activities have previously been associated with highly defective and disordered phases of NiOOH. 32 Furthermore, the at band potential obtained from this analysis at 1.2 V RHE agrees well with previously literature reports.…”