Tensile strain induced narrowed bandgap of TiO2 films: Utilizing the two-way shape memory effect of TiNiNb substrate and in-situ mechanical bending

• Imposed tensile strain to anatase TiO2 nanofilm by using the two-way shape memory effect of NiTiNb substrate.
• Imposed tensile strain to rutile TiO2 thin film by in-situ mechanical bending.
• Tauc plot based on the PEC-tested auction spectrum was utilized to precisely determine the bandgap of TiO2.
• Tensile strain narrowed the bandgap of anatase TiO2 by 60 meV and rutile TiO2 by 70 meV.
• Tensile strain contributes to a 1.5 times larger photocurrent for the water oxidation reaction.

Elastic strain is one of the methods to alter the band gap of semiconductors. However, relevant experimental work is limited due to the difficulty in imposing strain. Two new methods for imposing tensile strain to TiO2 film were introduced here. One is by utilizing the two-way shape memory effect of NiTiNb substrate, and the other method is in-situ mechanical bending. The former method succeeded in imposing 0.4% tensile strain to anatase TiO2 nanofilm, and strain narrowed the bandgap of TiO2 by 60 meV. The latter method enabled rutile TiO2 thin film under the 0.5% biaxially tensile-strained state, which contributes to a narrowed bandgap with ΔEg of 70 meV. Also, photocurrents of both strained TiO2 films increased by 1.5 times compared to the strain-free films, which indirectly verified the previous DFT prediction proposed by Thulin and Guerra in 2008 that tensile strain could improve the mobility and separation of photo-excite carriers.

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