Multiscale modelling provides atomic-level insights into how oxygen vacancy defect nucleation leads to the formation of the visible light photocatalyst black titania.
Metal oxides are ubiquitous in everyday life and titania, arguably, has received the most scrutiny from materials scientists due to its functionality for applications such as photocatalytic water splitting, chemical sensors and nanomedicine1,2,3. What makes this material a daunting challenge to understand is that many of its desirable properties result from imperfections. Titania, formally TiO2 in its stoichiometric form, consists of Ti4+ cations and O2– anions, which both have closed electronic shells, such that a perfect crystal would be transparent and non-conductive (Fig. 1a). However, many defects common to TiO2, such as oxygen vacancies, OH groups or interstitial titanium atoms, not only disrupt the crystallinity but also create charge carriers that modify optical properties and electrical conductivity (Fig. 1b). Material scientists have spent decades trying to create the right defects to increase visible light optical absorption and electrical conductivity for photocatalytic applications. One example that has recently garnered interest is black TiO2, which contains substantial numbers of defects (Fig. 1c) and can absorb light in the visible range for water splitting, a holy grail for renewable energy4,5,6. How it forms, and what its structural composition is, are critical questions.
In the past decade, theory and modelling7,8,9 has been able to address the nature of these charge carriers and how they move through the TiO2 lattice. The current belief is that defects create localized Ti3+ centres instead of Ti4+ cations, where titanium atoms are reduced by an electron; this is known as a polaron (Fig. 1b). These polarons can hop from Ti to Ti sites, giving rise to electrical conductivity, while the change in their electronic state creates colour centres that leads to light absorption. When these polaronic defects form close to the surface, they enable chemical reactions by electron/hole exchange with adsorbed molecules. This is why TiO2 is prevalent in photocatalysis and many other applications that rely on chemistry driven by light and/or electricity.
Despite improvements in our capabilities to model these defects, studying how they move, react and respond to radiation beyond a local region is still daunting. As noted, polarons carry with them an electrical charge subject to Coulomb’s law (the force between any two charges falls off as the square of the inverse of the distance between them). In practice, this means that polarons can influence each other over nanometre distances and their chemical and dynamic behaviour can be coupled. Thus, simple models that do not consider these collective phenomena may not be able to explain critical materials properties. However, modern density functional techniques that model electronic structure are only able to handle about a few thousand atoms at present, and hence cannot comprehensively address these issues. Writing in Nature Materials, Sencer Selcuk, Xunhua Zhao and Annabella Selloni10 overcome these limitations by using reactive force fields that mimic bond-breaking/making processes and redox state changes. This approach, when parameterized from ab initio methods, can retain critical aspects of the physics/chemistry of the atomic interactions, and as a result serve as a much cheaper way to model dynamics and discover new collective dynamic phenomena. The authors use reactive force fields and DFT methods to provide a compelling picture of the formation mechanism, structural and optical properties of black TiO2.
Selcuk et al. show that black TiO2 is an example of just why long length and timescale defect interactions matters. When TiO2 is exposed to a large amount of reducing agents (in this case H2), a significant amount of charged defects may form as oxygen atoms are removed from the lattice (in the form of water, H2O), leaving behind Ti3+ sites and oxygen vacancies. If there is a lot of H2, these O vacancies will interact with each other and cause substantial changes in the atomic and electronic structure of TiO2. The authors show how oxygen vacancies, created by H2 adsorption, are thermodynamically driven from the surface facets of anatase (one polymorph of TiO2) nanoparticles into the bulk. However, on the (001) facets their migration is kinetically blocked by high barriers at the subsurface layers. As a result, the oxygen vacancies interact with each other, and oxygen atoms on neighbouring surface facets, to form an amorphous defect phase (Fig. 1c) on the nanoparticle surface. By the use of electronic structure calculations, the authors show that this structural transformation creates a pile-up of defect states in the bandgap, leading to optical absorptions across the entire visible spectrum. This study provides a compelling and comprehensive model for black TiO2 formation that accounts for the atomic and electronic structure, as well as the nucleation and growth of the material.
This work provides a model for how to control the formation and amorphization of the black TiO2 that is fruitful ground for further investigations. Selcuk et al. highlight the need to control the amount of anatase-(001) surface facets on the nanoparticles as well as the H2 gas pressure during preparation of the material. This can serve as a basis for future spectroscopic, kinetic and mechanism studies that will validate and extend these findings. The authors speculate that similar long-range defect interaction processes are at work in forming amorphous over-layers on TiO2 particles during photocatalytic water splitting, as well as on other oxides such as bismuth vanadate that are currently used as photoanodes. It would be interesting to use this approach to investigate this.
This investigation on the formation of black TiO2 represents how a collective, long-distance and long-timescale phenomenon may be captured at the atomic level and demonstrates the critical importance of defect interactions over great distances. We believe that phenomena of this type are ubiquitous in nature yet poorly understood, which emphasizes the importance of studies on materials structure/property under synthesis and operating conditions.
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Glezakou, VA., Rousseau, R. Shedding light on black titania. Nature Mater 17, 856–857 (2018). https://doi.org/10.1038/s41563-018-0150-1
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