Unveiling the Electronic Structure of Grain Boundaries in Anatase with Electron Microscopy and First-Principles Modeling

Abstract

Polycrystalline anatase titanium dioxide has drawn great interest, because of its potential applications in highefficiency photovoltaics and photocatalysts. There has been speculation on the electronic properties of grain boundaries but little direct evidence, because grain boundaries in anatase are challenging to probe experimentally and to model. We present a combined experimental and theoretical study of anatase grain boundaries that have been fabricated by epitaxial growth on a bicrystalline substrate, allowing accurate atomic-scale models to be determined. The electronic structure in the vicinity of stoichiometric grain boundaries is relatively benign to device performance but segregation of oxygen vacancies introduces barriers to electron transport, because of the development of a space charge region. An intrinsically oxygen-deficient boundary exhibits charge trapping consistent with electron energy loss spectroscopy measurements. We discuss strategies for the synthesis of polycrystalline anatase in order to minimize the formation of such deleterious grain boundaries.

A post-deposition annealing approach for organic residue control in TiO2 and its impact on Sb2Se3/TiO2 device performance

“…However, more research efforts are required to understand the correlation between the USP precursor solution, residual organic species, and TiO 2 surface defects and, thus, their impact on the TiO 2 –Sb 2 Se 3 interface, related interface defect states, and final device performance. In this direction, a combination of advanced characterization methods, such as scanning transmission electron microscopy (STEM) 44 and transient SPV spectroscopy, 45 could be suitable techniques to gain in-depth insights into the impact of nanoscale carbon species on GBs electronic properties in TiO 2 and passivation of the electronic traps at the main interface.…”

Design Principles for Grain Boundaries in Solid-State Lithium-Ion Conductors

“…Aside from local electrostatic perturbations intrinsic to the GB structure, the segregation of charged defects can lead to the development of a space charge region and a corresponding electrostatic barrier, [41][42][43] but approximating the space charge potential for a given material requires a detailed analysis of defect energetics and is beyond the scope of the current study. Transport in the bulk of Li 3 OCl does not exhibit strong correlation, with a large value of t c = 8.9 ps.…”

“…This procedure has been used extensively in previous studies to successfully predict GB structures. 36,41,50 All geometry optimizations, AIMD and electronic structure calculations were carried out using the implementation of DFT within CP2K. 51 Geometry optimization is performed until the force on ions is less than 0.01 eV Å−1 .…”

Synthesis Control of Charge Separation at Anatase TiO2 Thin Films Studied by Transient Surface Photovoltage Spectroscopy

“…5,6 In turn, diffusion and recombination of photogenerated charge carriers depend on the average size of nanoparticles or grains and grain boundaries. 7 Photoinduced surface defects, such as oxygen vacancies, can act as trapping sites for charge carriers, which reduce the mobility, and reactivity of photogenerated holes. 8 sites even for photocatalytic purification processes driven by VIS light.…”

“…Different properties of materials can play a significant role in determining their photocatalytic activity. For example, surface states and facet orientations play an important role for adsorption of reactants and charge transfer during photoreactions. , In turn, diffusion and recombination of photogenerated charge carriers depend on the average size of nanoparticles or grains and grain boundaries . Photoinduced surface defects, such as oxygen vacancies, can act as trapping sites for charge carriers, which reduce the mobility, and reactivity of photogenerated holes .…”