Defect engineering of atomic layer deposited TiO2 for photocatalytic applications
Research output: Other conference contribution › Paper, poster or abstract › Scientific
|Publication status||Published - 28 Jun 2019|
|Publication type||Not Eligible|
|Event||EuroCVD 22-Baltic ALD 16 Conference - , Luxembourg|
Duration: 24 Jun 2019 → 28 Jun 2019
|Conference||EuroCVD 22-Baltic ALD 16 Conference|
|Period||24/06/19 → 28/06/19|
In our recent studies, we have reported means to thermally modify the defect structure of ALD grown am.-TiO2 thin film under oxidative  and reductive  conditions. TiO2 films were grown on silicon and fused quartz substrates by ALD at 200 °C using tetrakis(dimethylamido)titanium (TDMAT) and deionized water as precursors. The influence of heat treatment on the structure and properties of TiO2 was analyzed by X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS), X-ray diffraction (XRD), UV–Vis spectroscopy, current–voltage and PEC analysis.
Based on the results, the as-deposited am.-TiO2 is chemically unstable and visually black exhibiting both enhanced absorbance in the visible range and exceptionally high conductivity due to the trapped charge carriers (Ti3+). Heat treatment in air at 200 °C induces oxidation of Ti3+, decrease in absorbance and conductivity but has only a minor effect on the stability. However, a reasonable stability is obtained after oxidation at 300 °C, simultaneously with the crystallization of TiO2 into rutile. Furthermore, oxidation at 500 °C results in stable rutile TiO2 that produces the highest photocurrent for water oxidation. In contrast, reductive heat treatment in ultra-high vacuum (UHV) at 500 °C retains the amorphous phase for TiO2 but enhances the stability due to the formation of O– species via electron transfer from O to Ti. The schematic illustrations of the effect of oxidative and reductive heat treatments on the defect structure of ALD TiO2 are shown in Figures 1 and 2.
As a conclusion, ALD TiO2 has proven its diversity. Conductive as-deposited black TiO2 is photoelectrochemically unstable but it can be transformed into stable phases of photocatalytically active rutile or electrically “leaky” amorphous black TiO2 by heat treatment in oxidative or reductive conditions, respectively.
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