Pathway of Photocatalytic Oxygen Evolution on Aqueous TiO₂ Anatase and Insights into the Different Activities of Anatase and Rutile

Abstract

The photocatalytic oxidation of water to molecular oxygen is a key step toward the conversion of solar energy to fuels. Understanding the detailed mechanism and kinetics of this reaction is important for the development of robust catalysts with improved efficiency. TiO 2 is one of the best-known photocatalysts as well as a model system for the study of the oxygen evolution reaction (OER). Here we use hybrid density functional based energetic calculations and first-principles molecular dynamics simulations to investigate the pathway and kinetics of the OER on the majority (101) surface of anatase TiO 2 in a water environment. Our results show that terminal Ti−OH groups are stable intermediates at the aqueous (101) interface, in accord with the experimental observation that OH radicals are efficiently produced on anatase. Oxidation of Ti−OH gives rise to a second stable intermediate, a surface-bridging peroxo dimer ((O 2 2− ) br ) composed of one water and one surface lattice oxygen atom, consistent with the surface peroxo intermediates revealed by "in situ" measurements on rutile. Our calculations further predict that molecular oxygen evolves directly from (O 2 2− ) br through a concerted two-electron transfer, thus leading to oxygen exchange between TiO 2 and the adsorbed species. Oxygen exchange is found to be negligible on rutile, so that different OER pathways are likely to be operative on the two main TiO 2 polymorphs. This difference could explain the observed lower OER activity of anatase relative to rutile.

Diffusion and reaction pathways of water near fully hydrated TiO2 surfaces from ab initio molecular dynamics

“…120 The high reactivity of anatase (001) is undisputed, 41,68 while both dissociation and molecular adsorption have been reported on anatase (101). Dissociation pathways including surface defects 36,39 and excitation energy from light 35,121 have been discussed in this context. Arrouvel et al 68 calculated a small energy difference between dissociated and intact waters on anatase yet found low to none water reactivity on anatase (101) and (100); at elevated temperatures (above 450 K), the surfaces were dehydrated.…”

“…32 Computational work has focused on determining whether water dissociates or stays intact when adsorbed to low-energy TiO 2 a) Author to whom correspondence should be addressed: erik.brandt@ mmk.su.se surfaces, which is of particular interest to water splitting applications. [33][34][35] Both experiments and calculations have pointed out the key role of surface defects to water dissociation. Such defects are the results of, e.g., step dislocations 36,37 and oxygen vacancies at 38 or below 39,40 the TiO 2 surface, which expose undercoordinated titanium atoms that are highly reactive.…”

Computational Approaches to Photoelectrode Design through Molecular Functionalization for Enhanced Photoelectrochemical Water Splitting

“…Similar to the discussion on photocorrosion reaction pathways in Section 2.2, the water splitting reaction can be modeled approximately using first‐principles DFT and saddle‐point methods such as NEB without including an explicit water environment. In this approach, each intermediate step of the water splitting reaction is modeled using DFT to calculate thermodynamic energy barriers whereas NEB is used to calculate transition states and energies . Despite the exclusion of the water environment, this approach has provided useful insights about rate‐limiting steps and free‐energy barriers.…”

“…For example, Gao and co‐workers have calculated the Gibbs‐free‐energy profile for oxygen evolution on different crystal facets of anatase TiO 2 , suggesting that the experimentally observed sluggish kinetics of the oxygen evolution reaction (OER) arises from the thermodynamic energy barrier of the first deprotonation of water. Analogous studies were performed on different facets of anatase and rutile TiO 2 , mixed nickel–iron oxide, and silicon carbide photoelectrodes. In all these studies, the focus was on understanding the reaction pathways on non‐functionalized photoelectrode surfaces.…”

Electron Trap States and Photopotential of Nanocrystalline Titanium Dioxide Electrodes Filled with Single‐Walled Carbon Nanotubes

“…In particular, cyclic voltammetry measurements have been performed on TiO 2 films for the study of their electronic density of states, which is typically reflected in a charge accumulation region and certain trap states . Moreover, the detection of electronic states and intermediate species by electrochemical methods provides insights on reaction mechanisms, which are a topic of intense debate, particularly in the water oxidation reaction . Great differences in the TiO 2 energy levels and photocatalytic activity are found depending on the crystalline structure, particle morphology, and other external conditions such as electrolyte additives, oxygen purge, and pH …”