Electronic Structures of Hydroxyled Low Index Surfaces of Rutile and Anatase-Type Titanium Dioxide

Different surface planes of various types of TiO2 crystals have diverse catalysis effects on the splitting of H2O and H2 and the electronic structures of the formed hydroxylated titanium dioxide...

Understanding the Role of Rutile TiO2 Surface Orientation on Molecular Hydrogen Activation

Titanium oxide (TiO₂) has been widely used in many fields, such as photocatalysis, photovoltaics, catalysis, and sensors, where its interaction with molecular H₂ with TiO₂ surface plays an important role. However, the activation of hydrogen over rutile TiO₂ surfaces has not been systematically studied regarding the surface termination dependence. In this work, we use density functional theory (PBE+U) to identify the pathways for two processes: the heterolytic dissociation of H₂ as a hydride–proton pair, and the subsequent H transfer from Ti to near O accompanied by reduction of the Ti sites. Four stoichiometric surface orientations were considered: (001), (100), (110), and (101). The lowest activation barriers are found for hydrogen dissociation on (001) and (110), with energies of 0.56 eV and 0.50 eV, respectively. The highest activation barriers are found on (100) and (101), with energies of 1.08 eV and 0.79 eV, respectively. For hydrogen transfer from Ti to near O, the activation barriers are higher (from 1.40 to 1.86 eV). Our results indicate that the dissociation step is kinetically more favorable than the H transfer process, although the latter is thermodynamically more favorable. We discuss the implications in the stability of the hydride–proton pair, and provide structures, electronic structure, vibrational analysis, and temperature effects to characterize the reactivity of the four TiO₂ orientations.

Structures of defects on anatase TiO2(001) surfaces

Defects on oxide surfaces play a crucial role in surface reactivity and thus it is crucial to understand their atomic and electronic structures. The defects on anatase TiO₂(001)-(1 × 4) surfaces are found to be highly reactive; however, due to the surface reconstruction, the defects exhibit a complicated character in different experiments that make it very challenging to determine their atomic structures. Here we present a systematic first-principles investigation of the defects on anatase TiO₂(001)-(1 × 4) surfaces based on a global-search adaptive genetic algorithm (AGA) and density functional theory (DFT). For different Ti-O ratios, we identify the low energy defect structures, investigate their electronic structure using a hybrid functional, and map their regions of stability under realistic conditions. We successfully find novel oxygen vacancy (O_V) and Ti interstitial (Ti_ini) structures that are different from the conventional ones in terms of their charge localization, magnetic state, and their scanning-tunneling-microscopy bright-dark image signature. This provides an insight into the complex geometric and electronic structure of the surface defects, and resolves several experimental discrepancies.

Role of Surface Stress on the Reactivity of Anatase TiO2(001)

In contrast with theoretical predictions in which anatase TiO₂(001) and its (1 × 4) reconstructed surfaces are highly reactive, recent experimental results show this surface to be inert except for the defect sites. In this report, based on a systematic study of anatase TiO₂(001)-(1 × 4) surface using first-principles calculations, the tensile stress is shown to play a crucial role on the surface reactivity. The predicted high reactivity based on add-molecule model is due to the large surface tensile stress, which can be easily suppressed by a stress-release mechanism. We show that various surface defects can induce stress release concomitantly with surface passivation. Thus the synthesis of anatase(001) surface with few defects is essential to improve the reactivity, which can be achieved, for example, via H₂O adsorption. Our study provides a uniform interpretation of controversial experimental observations and theoretical predictions on anatase TiO₂(001) surface and further proposes new insights into the origin of surface reactivity.

The structural and electronic properties of reduced amorphous titania

Reduced amorphous titania has been modeled by removing oxygen atoms to clarify the properties of these materials.

Electric-dipole effect of defects on the energy band alignment of rutile and anatase TiO2

DFT+U calculations determined that the perfect rutile(110) and anatase(101) surfaces have the straddling type band alignment, whereas surfaces with defects have the staggered type.