Electron traps and their effect on the surface chemistry of TiO2(110)

Abstract

Oxygen vacancies on metal oxide surfaces have long been thought to play a key role in the surface chemistry. Such processes have been directly visualized in the case of the model photocatalyst surface TiO(2)(110) in reactions with water and molecular oxygen. These vacancies have been assumed to be neutral in calculations of the surface properties. However, by comparing experimental and simulated scanning tunneling microscopy images and spectra, we show that oxygen vacancies act as trapping centers and are negatively charged. We demonstrate that charging the defect significantly affects the reactivity by following the reaction of molecular oxygen with surface hydroxyl formed by water dissociation at the vacancies. Calculations with electronically charged hydroxyl favor a condensation reaction forming water and surface oxygen adatoms, in line with experimental observations. This contrasts with simulations using neutral hydroxyl where hydrogen peroxide is found to be the most stable product.

Fig. 1.

Reaction of O₂ with TiO₂(110). (A) Ball model of TiO₂(110). Red and blue spheres denote O and Ti, respectively. The pink spheres are bridging O atoms, which lie in the [001] azimuth of the substrate. Parallel Ti rows that lie between the bridging-O rows are fivefold coordinated Ti atoms. Green spheres indicate H atoms (from OH_b). (B) 130 × 170 Å ² STM image (V = 1.5 V, I = 0.25 nA) of an as-prepared TiO₂(110) surface that contains O_b-vac and OH_b. OH_b forms from dissociation of water from the residual vacuum at O_b-vac. An O_b-vac, an OH_b, and an OH_b pair are indicated. (C) The surface in B after exposure to ∼90 L O₂ at 300 K. One of the bright spots assigned to O_ad is circled. B and C have been smoothed using Image SXM (12) v.1.75. (D) A histogram showing the height distribution of 276 bright spots found on the Ti_5c rows fitted to two Gaussian curves. The data are taken from an unsmoothed, larger version of the image in C. The histogram indicates that the reaction products are almost entirely from one species.

Fig. 2.

Calculated electronic structure of O_b-vac and OH_b. (A) The total density of states for the optimized layers in the presence of different amounts of extra electronic charge. (B) The TiO₂(110) surface shown as a stick model where the blue intersections indicate Ti sites and the red intersections indicate O sites. The arrow points at the O_b-vac and each Ti atom is labeled. The global charge density of the BGS is shown in yellow (10^-6 e Å ^-3) for O_b-vac(0), O_b-vac(1 - ), and O_b-vac(2 - ) in C, D, and E, respectively.

Fig. 3.

Electronic charge density distribution (10^-5 e Å ^-3) for the BGS of the considered 8 trilayer systems. (A) O_b-vac, (B) one Ti_int between the fourth and fifth trilayers, (C) O_b-vac modeled together with one Ti_int between the fourth and fifth TiO₂(110) trilayers (O_b-vac + Ti_int). Blue intersections indicate Ti atoms and red intersections O atoms. Ti_int is shown as a blue sphere. The positions of O_b-vac and Ti_int are marked by the black arrows. Different colors have been used to distinguish between Ti_int-donated (black, gray, green, orange), and surface O_b-vac induced (purple, blue) BGS. (D) Single-state energy level diagram with respect to the CB onset (E-E _CB^∗ = 0) for the BGS in A–C after vacuum level electrostatic alignment. Up and down arrows refer to the modeled spin of the specific state. The same BGS color labelling has been used for all the displayed A–F panels. (E and F) For clarity, the hybridized BGS of O_b-vac + Ti_int (orange and purple) circled in C, are redisplayed with the omission of the orange in E and purple in F BGS.

Fig. 4.

Experimental STM and STS data. (A) (44 Å)² STM image recorded simultaneously with the STS. (B) (44 Å)² CITS current map at -2 V. The squares in A and B show the positions of O_b-vac (green), OH_b (purple), and some bright features associated with O_b-vac (black). One impurity is also present and marked with a white square. Using a larger-scale image, the positions of O_b-vac, OH_b, and other impurities outside the area imaged in A and B are also indicated. A and B have been smoothed using Image SXM (12) v.1.75. (C) A correlation map between O_b-vac and bright features in B. The center of the map represents the position of an O_b-vac shown as a green square. The black rectangles represent unit cells centered on Ti_5c atoms that surround O_b-vac. The results are averaged between the four quadrants with the numbers shown only in one quadrant. The results are expressed as percentages that add to 100% when the numbers in all four quadrants are summed. The darker the shading, the greater the probability of finding a bright feature at the separation indicated by the map. (D) STS spectra represented as LDOS plots by plotting (dI/dV) × (V/I) vs V (26). The LDOS plots are taken from the bright features associated with O_b-vac (black), O_b-vac (green), OH_b (purple), Ti_5c (yellow), and O_b (red). Each curve is averaged from 180 individual spectra taken from the CITS set shown in this work and another 180 individual spectra taken from an equivalent CITS set recorded in an identical area of the surface. The black squares in A and B indicate which bright features contribute to the curves for bright features associated with O_b-vac; none were counted when they were also diagonally adjacent to OH_b or in close vicinity to impurities.

Fig. 5.

Modeled STM and CITS appearance for O_b-vac. Simulated STM images (+2 V, 10^-7 e Å ^-3) and current maps (-2 V, same height above the surface as from the corresponding left-side topography) are shown for O_b-vac(0) in A and B, for O_b-vac(1 - ) in C and D, and for O_b-vac(2 - ) in E and F. Ti_5c rows are indicated by black lines, and an X marks the O_b-vac. STM simulations were performed with a tip-surface distance of ∼5 Å [the detailed procedure can be found in ref. (28)].

Fig. 6.

Plan view of surface species together with their grand-canonical formation free energies (T = 300 K, P _O2 = 1.3 × 10^-8 mbar, P _H2O = 1 × 10^-11 mbar). Ti is shown blue, lattice O red, O from adsorbates orange, and H green. Bridging oxygen (O_b) atoms are shown larger to highlight them. The energies are in eV and the brackets after the energies indicate the electronic charge of the surface species. Geometries are optimized (3 × 2 supercell) for the neutral state. The total density of states for the optimized layers (filled red) of O_ad(2 - )/TiO₂(110) is displayed together with the O_ad(2 - )-projected density of states (PDOS, filled orange) in the bottom-right panel.