Silicon Oxides as a Competitive Material for Photoanodes

A water photosplitting process is an approach to electrochemical direct solar energy conversion into electricity, hydrogen fuel and chemical energy which is stored in the form of oxygen. A water photosplitting process requires oxidatively robust and inexpensive photoanodes. Nowadays the most often considered and studied material is titanium dioxide (TiO₂), but the competitive material for such a photoanode which is the silicon oxides, should be seriously considered because of their much more suitable bandgap (E_g) for sun light absorbtion (between 1,6 eV and 2,4 eV comparing to 3.2 eV for TiO₂) and their chemical stability in water solution.. Furthermore, a wide prevalence of silicon in the lithosphere, a source of its oxides, has to be bear in mind.

In the seventies and eighties it was proved that different bulk silicon oxide types (SiO_x) exist [1, 2]. These compounds are semiconductive and their electronic energy band gap is between 1.12 eV, for the bulk silicon, and 8.9 eV, for the bulk silica [3] Furthermore, it was shown that silicon oxide, SiO, might increase photoeffect of a mono crystalline silicon electrode and might protect such an electrode against corrosion in water solution [4, 5]. The aim is to find electrochemical conditions with the use of reagents usually available on the market, which would lead to the production of the photoactive material based on the silicon and silicon oxides, SiO_x [5-7].

In this presentation the formation of SiO_x films by electrodeposition [8] and its electronic surface characterization will be discussed. It will be pointed that photoelectrochemical water oxidation at the electrochemically obtained SiO_x films can be observed at the onset potentials lower than 0.2 V vs. NHE. Comparing this value to the dark water splitting potential, (1.23 V) means energy saving corresponding to ca. 1 V. The photoactivity of SiO_x photoanodes was highly reproducible and the observed photocurrent sustained for many hours. The electronic properties of the electrodeposited films will be discussed on the base of UV-Vis and XPS data. The main point of the discussion will be put on the determination of E_g for the multicomponents semiconductive films. The advantages and disadvantages of both techniques will be discussed. It will be shown that electrosynthesized films consist of Si, SiO and Si₂O₃. Additionally, the influence of morphology determined by AFM and SEM microscopy on photoactivity of the semiconductive films will be discussed as well.

 

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