Abstract
Titanium dioxide (TiO2) can protect photoelectrochemical (PEC) devices from corrosion, but the fabrication of high-quality TiO2 coatings providing long-term stability has remained challenging. Here, we compare the influence of Si wafer cleaning and postdeposition annealing temperature on the performance of TiO2/n+-Si photoanodes grown by atomic layer deposition (ALD) using tetrakis(dimethylamido)titanium (TDMAT) and H2O as precursors at a growth temperature of 100 °C. We show that removal of native Si oxide before ALD does not improve the TiO2 coating performance under alkaline PEC water splitting conditions if excessive postdeposition annealing is needed to induce crystallization. The as-deposited TiO2 coatings were amorphous and subject to photocorrosion. However, the TiO2 coatings were found to be stable over a time period of 10 h after heat treatment at 400 °C that induced crystallization of amorphous TiO2 into anatase TiO2. No interfacial Si oxide formed during the ALD growth, but during the heat treatment, the thickness of interfacial Si oxide increased to 1.8 nm for all of the samples. Increasing the ALD growth temperature to 150 °C enabled crystallization at 300 °C, which resulted in reduced growth of interfacial Si oxide followed by a 70 mV improvement in the photocurrent onset potential.
Highlights
Photoelectrochemical (PEC) solar fuel production from H2O and CO2 is one of the potential methods for storing solar energy in chemical form as hydrogen and hydrocarbons.[1]
Si oxide was not detected on the hydrofluoric acid (HF)-treated sample, which confirms the temporal passivation of Si surface against oxidation
We have studied the effect of standard cleaning treatments of Si wafers on the fabrication of the atomic layer deposition (ALD) TiO2 photoelectrode coating for photoelectrochemical applications
Summary
Photoelectrochemical (PEC) solar fuel production from H2O and CO2 is one of the potential methods for storing solar energy in chemical form as hydrogen and hydrocarbons.[1]. One viable approach to increase the efficiency is to use semiconductor materials of high-efficiency solar cells, such as Si, GaAs, and GaP, as photoelectrode materials, but because of their intrinsic chemical instability under PEC conditions, a protective coating is required.[2]. The stability of semiconductor photoelectrodes has been successfully increased by TiO2 thin film coatings grown by atomic layer deposition (ALD) using either amorphous[3,4] or crystalline[5] TiO2. The fabrication of high-quality TiO2 thin films providing long-term stability to the photoelectrodes has remained challenging and the stability of amorphous TiO2 (am.-TiO2) has shown to be controversial.[6]
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