Local Structural Distortions and Failure of the Surface-Stress 'Core-Shell' Model in Brookite Titania Nanorods

摘要

Structural refinements of whole-nanoparticle atomistic models using X-ray total-scattering data and the reverse Monte Carlo algorithm were demonstrated for mixtures of TiO2 brookite nanorods and irregularly shaped anatase nanoparticles, which exhibit enhanced photocatalytic properties. The brookite and anatase nanocrystals were shown to undergo lattice-volume expansion and contraction relative to their bulk prototypes. Detailed analysis of the refined structures focused on the nanorods because of their well-defined shapes and habits; the soundness of the obtained characteristics was assessed using first-principles calculations. The atomic positions in the nanorods differed from the bulk structure by an effective shift of the Ti Wyckoff position in the brookite unit cell, combined with additional static atomic displacements that increased progressively from the nanorod axis toward its side surfaces. A similar radial gradient was observed for distortions of the [TiO6] octahedra. A cross-sectional pattern of structural relaxations exhibited symmetry consistent with that imposed by the nanorod habits. Density functional theory calculations were in qualitative agreement with the experimental observations and additionally revealed a strong dependence of the structural relaxations, including lattice-volume changes, on sorbate coverage. The experimental and computed magnitudes of these relaxations appeared to be significantly larger than those predicted by a simple surface-stress model, with the difference attributed to a complex gradient of the local electrostatic potential within the nanorods. This study demonstrates the potential of using RMC refinements to obtain a detailed picture of atomic order in nanoparticles and highlights the existence of complex, spatially variable distortions that need to be considered while interpreting the electronic properties of these materials.