Strain induced phase formation, microstructural evolution and bandgap narrowing in strained TiO2 nanocrystals grown by ball milling

Band gap narrowing in metal oxide semiconductor nanostructures is important and advantageous for various potential applications including visible light photocatalysis. We present a systematic study on the anomalous strain evolution, phase change and band gap narrowing in TiO2 nanocrystals (NCs) as a result of ball milling. In addition to the size reduction and strain evolution with milling time, we report the formation of a new phase of TiO2 with milling, as identified in the XRD pattern and Raman spectra for the first time. Besides the tetragonal anatase phase of TiO2 NCs, two additional peaks centered at 2θ = 31.28° and 41.60° evolve with milling, and it corresponds to the (1¯ 12) and (312) planes of Ti3O5, respectively. Further, our results show that the band gap of TiO2 NCs reduces with increasing strain and lowest band gap achieved in strained TiO2 is 2.71 eV, consistent with a recent theoretical calculation. The evolution of the crystallite size, strain, stress and energy density was evaluated from the line shape analysis of the XRD pattern using various models, such as uniform deformation model, uniform stress deformation model, uniform deformation energy density model and the results are compared with those obtained directly from HRTEM analyses. The increased d-spacing with milling time was attributed to the tensile strain in TiO2 NCs. Direct evidence of lattice strain and strain relaxation is provided from HRTEM imaging and differential scanning calorimetry analyses. Our results demonstrate strain engineering of TiO2 to achieve narrow bandgap in anatase TiO2 NCs, which are promising for visible light photocatalytic and other emerging applications.