The stability of SACs largely depends on the adsorption location and surface defects. The oxygen vacancies on the surface of TiO2 not only provide active sites for fixing metal atoms, but also affect the catalytic performance. Single metal atom is introduced on perfect and oxygen vacancies surface of Anatase(001), (101) and Rutile(100), (011), (101) in supporting information Fig.S1 and Fig.S2. We calculated the adsorption energy of possible adsorption sites to find the most stable energy in supporting information Table S1.We calculated the most stable adsorption energy (E(ads)) and d-band center energy of different oxygen vacancies and different perfect surfaces for comparison in Fig. 1 and Fig. 2. We found that the adsorption energy we calculated always shows the same trends on these surfaces: the adsorption energy of the surface containing oxygen vacancies is significantly higher than tthat of the perfect surface. It is speculated that the oxygen vacancies change the electronic distribution state of the surrounding Ti atoms, which makes the binding force of Pt and Ti atoms stronger.
The d-band center energy of (001) and (101) surface of Anatase TiO2 and (100), (011), (110) surface of Rutile TiO2 have been calculated in supporting information Table S2. We plot the d-band center energy with the most stable adsorption energy on surface in Fig. 3. We found from Fig. 3 that for the five types of TiO2 surfaces, the movement of the center energy of the d-band also shows the same trend: for a surface containing oxygen vacancies, the d-band center energy of Pt atom will be closer to the Fermi level. This shows that the surface containing oxygen vacancies may be more suitable for the substrate of Pt atom than the perfect surface for SACs.
The partial Density of States of perfect surface and oxygen vacancies surface of Anatase (001),(101) and Rutile (100), (011), (101) are given in Fig. 4 and Fig. 5. We can clearly see the change of the d-band energy of Pt atom on different surface. For the five types of surfaces, the d-band energy of Pt after introducing oxygen vacancies has increased significantly. At the same time, the hybridization between O atoms and Pt atom becomes weak, and the hybridization between Pt atom and Ti atoms increases.The interaction between Pt atom and Ti atoms may be the reason for increasing the d-band energy of Pt atoms.
Otherwise, We calculated the d-band center energy of the Pt atom on the Pt metal bulk, which is -2.22eV, consistent with -2.23 eV which calculated by Tianyi Wang et al..We found that the d-band centers of Pt atom on selected TiO2 surfaces are closer to the Fermi level than the d-band centers of Pt atom on the metal bulk. According to the d-band center theory of transition metals, the Pt atom on the surface containing oxygen vacancies are more conducive to adsorbing other atoms than the Pt atom on the metal bulk. Fig. 6. shows the relationship between the adsorption energy and the center of the d-band of Pt atom on different surfaces. under the line is the perfect surface, above the line is oxygen vacancies surface. Compared with the perfect surface, it is much easy to obtain a larger adsorption energy or a higher d-band center for the containing oxygen vacancy surface.
Generally, the adsorption energy and cohesive energy was compared to evaluate the thermodynamic stability of SACs. The cohesive energy of Pt is -5.48 eV calculated in our work in supporting information Table S3, consistent with 5.53 eV which calculated by Philipsen P H T . Fig. 7 shows the relationship between adsorption energy and cohesive energy of different TiO2 surfaces. We found that on the surface of Rutile(110) and Rutile(011) with oxygen vacancies, the adsorption energy of Pt atom is higher than the cohesive energy of Pt metal. This means that its thermodynamic stability is much better than the that of those perfect surface whose adsorption energy is much smaller than their cohesive energy which may not be stable enough as SACs except Rutile(110).