Simulating Constant Current STM Images of the Rutile TiO 2 (110) Surface for Applications in Solar Water Splitting
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Simulating Constant Current STM Images of the Rutile TiO2 (110) Surface for Applications in Solar Water Splitting F. F. Sanches1 , G. Mallia1 , and N. M. Harrison1,2
1 Thomas Young Centre, Department of Chemistry, Imperial College London, South Kensington London SW7 2AZ, UK 2 STFC Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK
ABSTRACT Solar water splitting has shown promise as a source of environmentally friendly hydrogen fuel. Understanding the interactions between semiconductor surfaces and water is essential to improve conversion efficiencies of water splitting systems. TiO2 has been widely adopted as a reference material and rutile surfaces have been studied experimentally and theoretically. Scanning Tunneling Microscopy (STM) is commonly used to study surfaces, as it probes the atomic and electronic structure of the surface layer. A systematic and transferable method to simulate constant current STM images using local atomic basis set methods is reported. This consists of adding more diffuse p and d functions to the basis sets of surface O and Ti atoms, in order to describe the long range tails of the conduction and valence bands (and, thus, the vacuum above the surface). The rutile TiO2 (110) surface is considered as a case study.
INTRODUCTION Solar water splitting has shown a lot of promise as an environentally friendly source of hydrogen fuel. However, solar-to-fuel efficiencies have to be improved significantly for this to become a viable alternative to fossil fuels. A fundamental understanding of photoanode surface and water interaction could be essential in improving these efficiencies. TiO2 is a commonly used semi-conductor for solar water splitting [1, 2]. Whilst the large bandgap of TiO2 is somewhat prohibitive for widespread use, it has been widely adopted as a model material for experimental and theoretical study. In practice, nanostructured, predominantely anatase TiO2 is most commonly used [3–5]. These systems are difficult to study at an atomic scale experimentally as well as computationally. However, pristeen clean surfaces of TiO2 can be investigated with a number of experimental and theoretical techniques. These are useful as model systems, where the surface structure can be analysed, as well as the interaction of surfaces with adsorbates[6, 7]. Scanning Tunneling Microscopy (STM) is commonly used to study surfaces, as it probes the atomic and electronic structure of the surface layer. The rutile (110) TiO2 surface has been studied using STM previously [8, 9]. Experimentally, the observed bright spots are attributed to the surface undercoordinated Ti atoms. Theoretical studies played an important role in predicting/confirming this observation. The LDA approximation to DFT and the plane wave (pseudopotential) approach was used to reproduce these experimental results computationally by relaxing the surface structure and then simulating STM images [9]. It has been previously shown that the use of hybrid exchange functionals (where a proportion of Fock exchange is included in the exchange functi
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