Modification of Charge Trapping at Particle/Particle Interfaces by Electrochemical Hydrogen Doping of Nanocrystalline TiO₂

Abstract

Particle/particle interfaces play a crucial role in the functionality and performance of nanocrystalline materials such as mesoporous metal oxide electrodes. Defects at these interfaces are known to impede charge separation via slow-down of transport and increase of charge recombination, but can be passivated via electrochemical doping (i.e., incorporation of electron/proton pairs), leading to transient but large enhancement of photoelectrode performance. Although this process is technologically very relevant, it is still poorly understood. Here we report on the electrochemical characterization and the theoretical modeling of electron traps in nanocrystalline rutile TiO2 films. Significant changes in the electrochemical response of porous films consisting of a random network of TiO2 particles are observed upon the electrochemical accumulation of electron/proton pairs. The reversible shift of a capacitive peak in the voltammetric profile of the electrode is assigned to an energetic modification of trap states at particle/particle interfaces. This hypothesis is supported by first-principles theoretical calculations on a TiO2 grain boundary, providing a simple model for particle/particle interfaces. In particular, it is shown how protons readily segregate to the grain boundary (being up to 0.6 eV more stable than in the TiO2 bulk), modifying its structure and electron-trapping properties. The presence of hydrogen at the grain boundary increases the average depth of traps while at the same time reducing their number compared to the undoped situation. This provides an explanation for the transient enhancement of the photoelectrocatalytic activity toward methanol photooxidation which is observed following electrochemical hydrogen doping of rutile TiO2 nanoparticle electrodes.

Intrinsic intermediate gap states of TiO2 materials and their roles in charge carrier kinetics

“…The result showed that the perturbations in electrostatic potentials are responsible for a high density of strong trapping sites at grain boundaries, which was stated to be the main reason hampering electron interparticle transport. By pre-filling the boundary sites by an electrochemical reduction, the photocurrents were greatly increased due to an increase in electron transport along a nc-TiO 2 electrode [366].…”

Tunable Photo‐Electrochemistry of Patterned TiO2/BDD Heterojunctions

“…[ 14,15 ] Therefore, numerous efforts have been devoted for minimizing the recombination of the carriers or acquiring higher efficiencies of carrier separation in TiO 2 photocatalysts. [ 16–18 ] Among numerous p‐type materials, boron doped diamond (BDD) is a promising electrocatalyst. [ 24 ] This is because hydroxyl radicals (•OH) can be generated efficiently on the BDD surface during the advanced oxidation processes.…”

Engineering the Surface/Interface Structures of Titanium Dioxide Micro and Nano Architectures towards Environmental and Electrochemical Applications

“…Those TiO 2 materials with well-defined geometry and chemistry provide abundant possibilities to further tune the atomic scale structures. Therefore, different post-treatment techniques, including thermal annealing, laser irradiation, electrochemical cycling, and solution reaction, have been developed to yield TiO 2 materials with modified surface and interface structures [ 49 , 50 , 51 , 52 , 53 , 54 , 55 ].…”