Article pubs.acs.org/JPCC
Identifying the Site-Dependent Photoactivity of Anatase TiO2(001)(1×4) Surface Zhengwang Cheng,† Haoqi Tang,† Xuefeng Cui,* Shihui Dong, Xiaochuan Ma, Bin Luo, Shijing Tan, and Bing Wang* Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information & Quantum Physics,Key Laboratory of Strong-Coupled Quantum Matter Physics (CAS), University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *
ABSTRACT: We here report our investigation on the photocatalytic reactions of methanol on the anatase TiO2(001)-(1×4) surface using scanning tunneling microscopy and temperature-programmed desorption. Our results show that in the reduced surface the Ti-pair defect sites are photoinactive, but in the reoxidized surface the partially oxidized defect sites are photoactive to dissociate methanol and methoxy to formaldehyde. The perfect (1×4) lattice sites are photoactive, even though they are inactive in thermally driven reaction. The photocatalytic reaction of methanol (CD3OH) at the perfect lattice sites is evidenced by the conversion of CD3OH to various other deuterated methanol (CD3OD and CD2HOH) and a small amount of formaldehyde (CD2O) after ultraviolet light irradiation, in contrast with no such conversion reaction in the nonirradiated sample. The production of CD3OD and CD2HOH can be attributed to the reverse reaction between the produced CD2O and the H or D adatoms during or after ultraviolet light irradiation, which also leads to the small amount of formaldehyde in final products. The identification of the site-dependent photocatalytic reactions of methanol, in addition to our recently characterized site-dependent thermally driven reactions, may provide an insightful understanding about the activity and photoactivity of the anatase TiO2(001)-(1×4) surface. (1×4) ridges are five-fold coordinated (Ti5c), can well explain the low activity in thermally driven reactions at the perfect ridge sites. Instead, we have identified that two types of defects are responsible for the thermally drive reactions.49,50 However, the calculations showed that the AOM is energetically less favored than the ADM.45,46 The discrepancy between the experimental and the theoretical results is still an unsettled question. The understanding of the performance of the defects in the photocatalytic reactions can be an important step for understanding the atomic geometry of the (1×4) reconstruction and its (photo)activity of the surface. In this work, we investigate the site-dependent behaviors of methanol on anatase TiO2(001)-(1×4) surfaces under ultraviolet (UV) light irradiation, characterized by scanning tunneling microscopy (STM) and temperature-programmed desorption (TPD). We first examine the photocatalytic reaction of methoxy (dissociatively adsorbed methanol) adsorbed at two types of defects, that is, the defect sites of Ti pairs (dR‑Ti) in the reduced (001) surface and the defect sites of O-bridged Ti pairs (dO−Ti) in the reoxidized (001) surface.49 We find that the dR‑Ti sites are not active for photocatalytic reaction of methoxy, but the dO−Ti sites are active for photocatalytic reaction of methoxy. For the molecular methanol (CD3OH) adsorbed at the perfect
1. INTRODUCTION As a model system, TiO2 has been widely studied for its potential applications in solar energy conversion and pollutant degradation.1−10 One of the efforts is to understand the catalytic and photocatalytic activities of various TiO2 facets from its different polymorphs, for seeking highly active TiO2based (photo)catalysts. The existing defects, especially the defects at the surfaces, are considered to play a key role in the (photo)catalytic properties.11,12 In comparison with the mostly studied rutile TiO2(110) and anatase TiO2(101) surfaces,6−8,13−33 the photoactivity of the anatase TiO2(001) surface is less studied; in particular, there is no report about the photocatalytic properties of the surface defects on an atomic scale, to our best knowledge. As for the (1×4) reconstruction, because of its relatively high surface energy in the anatase TiO2(001) surface,34−44 the structural model of the surface and its activity and the photoactivity are still under debate.45−55 In the conventionally believed model, so-called “ad-molecule” model (ADM),37 the Ti atoms in the (1×4) ridges are suggested to be four-fold coordinated (Ti4c). The calculations based on the ADM predicted a high reactivity of the surface for many molecules. 37−43 In contrast with the theoretical predictions, we found that the perfect lattice sites in the (1×4) ridges are quite inert for the thermally driven reaction of water, methanol, and formaldehyde, using epitaxially grown anatase (001) films in our previous works.48−50 Our proposed “ad-oxygen” model (AOM),48 in which the Ti atoms in the © XXXX American Chemical Society
Received: July 23, 2017 Revised: August 25, 2017 Published: August 28, 2017 A
DOI: 10.1021/acs.jpcc.7b07256 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
grown anatase TiO2(001)-(1×4) surface is quite inert for thermally driven reaction of methanol and formaldehyde.49,50 After Ar+ ion sputtering and vacuum annealing, the surface can be reduced, resulting in the formation of reduced defect sites (dR‑Ti) at the (1×4) ridges, showing as bright protrusions, as shown in Figure 1a. After the sample surface was exposed to
lattice sites, our results show that the originally adsorbed CD3OH can be converted to CD3OD, CD2HOH, and CD2O. The observation of CD3OD and CD2HOH species can be understood by considering the dissociation of CD3OH to CD2O and hydrogen adatoms (Had or Dad) during UV light irradiation, while the reverse reaction happens between the intermediate species of CD2O and Had or Dad during or after the UV light irradiation. The results indicate that the perfect lattice sites are active for the photocatalytic reaction of molecular methanol.
2. EXPERIMENTAL SECTION The anatase TiO2(001) thin films were epitaxially grown on 0.7 wt % Nb-doped SrTiO3(001) substrate at 920 K and at O2 pressure of 1.5 × 10−3 Pa using pulsed laser deposition (PLD) method. A KrF excimer laser (Coherent, 248 nm) was operated at a repetition rate of 4 Hz and a pulse duration of 20 ns with an output power of ∼200 mJ/pulse. The typical thickness of the films was ∼30 nm, grown at a deposition rate of 11 Å/h. After deposition, the samples were slowly cooled to room temperature by keeping the O2 pressure at 1.5 × 10−3 Pa. The reduced sample was prepared by Ar+ (1 keV), sputtering for 5 min using a cold cathode ion sputter source (Omicron, ISE 5), and then annealing at 900 K for 20 min in UHV. The reoxidized sample was prepared by annealing at 700 K for 20 min in O2 (1.5 × 10−3 Pa) from a reduced sample. The TPD measurements were performed using a quadrupole mass spectrometer (QMS) from Extrel (MAX 120) in a chamber with a base pressure of ∼3 × 10−11 mbar, equipped with a low-energy electron diffraction and Auger electron spectrometer (LEED/AES, SPECTALEED, Omicron). The cleanness and the quality of the sample surface were examined by LEED/AES. The sample temperature was measured by a Ktype thermocouple that was glued on the sample surface by high-temperature ceramic adhesive (552, Aremco). The sample with dimensions of 10 × 4 × 0.5 mm3 was resistively heated by a Ta foil (thickness 0.02 mm). CD3OH and CD3OD (>99.5%, Sigma-Aldrich), and CH3OH (>99.9%, Riedel-de Haën) were purified by several freeze−pump−thaw cycles. An extended tube with a length of ∼5 mm and an inner diameter of 3 mm was mounted in front of the QMS. During TPD measurements, the tube was perpendicular to the sample surface, and the tube inlet was away from the surface by ∼1 mm. In TPD measurements, the sample temperature was programmed with a constant rate of 1 K/s. The details has been described elsewhere.49 In the UV-light irradiation, a high-pressure Xe lamp (E7536, Hamamatsu) was used. A filter (FGUV5, Thorlabs) was used to remove infrared and visible light. At the sample surface, the intensity of the UV (
