Role of Oxide Defects in ALD grown TiO₂ Coatings on Performance as Photoanode Protection Layer

Photoelectrochemical (PEC) water splitting is one of the potential methods of utilizing solar energy. A major issue for the method and for renewable energy production is the development of an efficient, chemically stable and cost-effective semiconductor photoanode. Recently, titanium dioxide (TiO₂) coatings grown by atomic layer deposition (ALD) have appeared to be a promising approach to stabilize semiconductor photoanodes under PEC conditions. In particular, amorphous ALD grown TiO₂ has shown exceptional charge transfer properties compared to its crystalline form that are not properly understood yet. Therefore, we target to gain better understanding on the defect structure of ALD grown TiO₂ and utilize the information in the development of optimal photoanode protection layer for efficient solar water splitting.

In this work, structural, optical and photoelectrochemical properties of the ALD grown TiO₂ films were studied in as-deposited condition and after annealing in air at 500 °C. TiO₂ films were grown on n-type phosphorus-doped silicon and fused quartz by ALD at 200 °C using tetrakis(dimethylamido)titanium (TDMAT) and deionized water as precursors. The properties of TiO₂ were investigated by X-ray photoelectron spectroscopy (XPS), ellipsometry and UV/Vis/NIR spectrophotometry. In addition, results from X-ray diffraction (XRD), Raman spectroscopy and photoelectrochemical (PEC) cell are discussed.

Based on the results, as-deposited TiO₂ is amorphous and absorbs visible light as ''black'' TiO₂. After annealing in air at 500 °C TiO₂ crystallizes as rutile and becomes ''white'' TiO₂ that absorbs light only in the UV region. As-deposited TiO₂ contains significant amount of Ti^3+/2+ oxygen vacancies that are oxidized as Ti⁴⁺ upon annealing in air. In addition, nitrogen is found only in as-deposited titanium dioxide. As-deposited TiO₂ is not chemically stable under PEC conditions. In contrast, the annealed TiO₂ is chemically stable and showed 0.20 % ABPE efficiency for water splitting reaction.

As a conclusion, Ti³⁺ defects induce photocorrosion of ALD TiO₂ under PEC conditions. After annealing in air at 500 °C ALD TiO₂ is chemically stable and it can be used as a photoanode protection layer. In the future, research will be focused on optimizing the properties of ALD TiO₂/Si interface and studying the structure of the surface after deposition of nickel electrocatalysts on TiO₂/Si photoanode.