Understanding Titanium Dioxide Passivation on Silicon Photoanodes in Photoelectrochemical Cells

Understanding Titanium Dioxide Passivation on Silicon Photoanodes in Photoelectrochemical Cells

Bras, J.A.

Smith, W.A. (mentor)
Dam, B. (mentor)
Savenij, T.J. (mentor)

Applied Sciences

Chemical Engineering

Materials for Energy Conversion and Storage

2015-07-02

Photoelectrochemical (PEC) water splitting is an elegant method to generate renewable hydrogen fuel from water by utilizing the abundant solar energy. Silicon (Si) as a semiconducting photoanode for PEC water splitting has received a growing interest due to the natural abundance of Si, its light absorbing ability, its excellent electronic properties and its low-cost processing. Although Si is a promising photoanode, its sensitivity towards corrosion in aqueous electrolytes hinders its use in PEC applications. To protect Si photoanodes against corrosion, thin layers (1-10 nm) of TiO2 were deposited onto FTO and Si by atomic layer deposition (ALD). Unannealed TiO2 was tested as well as TiO2 that was post-annealed in either air or vacuum environment. The properties that determine the photoelectrochemical activity as well as the stability were studied. The thickness and bandgap were determined by spectroscopic ellipsometry. X-ray diffraction crystallography (XRD) was used to analyse the crystal structure. Other physical, electronic and optical properties were characterized by ultraviolet-visible spectroscopy (UV-Vis), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The electrochemical properties were tested by electrochemical measurements, i.e. cyclic voltammetry and chronopotentiometry. It was found that the addition of very thin layers (<2 nm) of TiO2 on Si photoanodes that were post-annealed at 400°C in vacuum showed a high performance in PEC activity as well as stability. Moreover, the use of a Ni catalyst on the Si/TiO2 photoanodes showed an increase in reaction kinetics with photocurrent density reaching up to 25 mA/cm2. It was found that the stability of Si photoanodes is improved by a thin layer of TiO2 for 8 hours compared to 2 hours without TiO2 protection layer. To understand the charge transfer mechanism through the TiO2 layer, TiO2 layers were fabricated with different thicknesses of TiO2 and annealed in different annealing environments. These different TiO2 were evaluated regarding their PEC activity by cyclic voltammetry. As smaller thicknesses (<2 nm) of TiO2 enhance the PEC activity of Si photoanodes, compared to thicker layers of TiO2, it is implied that the mechanism of charge transport through TiO2 is tunneling. It was also shown that vacuum annealed TiO2 demonstrated a higher PEC activity compared to unannealed TiO2 in Si/TiO2/Ni photoanodes. The fact that annealing increases the PEC activity and removes defects, demonstrates that transport is not by defect states.

PEC
silicon
photoanodes
titanium dioxide
TiO2
passivation

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Student theses

master thesis

(c) 2015 Bras, J.A.