TiO2, as a prototypical system for transition-metal oxide, has been intensively studied for decades because of its wide application in photocatalysis, electrocatalysis, and support for metal catalysts.1–5 Due to its high stability, rutile TiO2(1 1 0)-(1 × 1) surface has attracted the most attention. On the stoichiometric rutile TiO2(1 1 0)-(1 × 1) surface, there are twofold-coordinated bridging oxygen (BBO) and fivefold-coordinated Ti (Ti5c). A common type of defects on TiO2 surface is oxygen vacancies (BBOv). The understanding of the interaction between TiO2 and adsorbates is important to obtain the underlying physical mechanism of TiO2 related processes.
The combination of scanning tunneling microscopy (STM) and density functional theory (DFT) provides a powerful tool to watch surface structures and chemical reactions at the atomic scale. Based on the ultimate sensitivity of the tunneling current on the tip height, the STM technique is widely used in surface science.6, 7 Images recorded at positive or negative bias give information on unoccupied and occupied surface states, respectively. DFT can solve electronic structures based on first principles with all approximations packed in the exchange correlation functional, for which good formulations such as Perdew–Wang 91 (PW91),8 Perdew–Burke–Ernzerholf (PBE),9 DFT + U,10 and hybridized Becke 3-parameter Lee–Yang–Parr (B3LYP)11 exist. With the increase of computational ability, DFT offers us an important mean to study the surface physical and chemical properties using atomic models. DFT calculations can provide information on the energy, geometric and electronic structure, such as partial density of states (PDOS) and charge distribution. Based on the Tersoff–Harmann approximation,12 STM images can be directly simulated through DFT calculations as the charges densities between bias and Fermi level. Therefore, by combining STM and DFT, many insights on TiO2 surface chemistry can be obtained.
In this review, we will focus on the atomic understanding of the interaction between small molecules, such as O2, CO, CO2, CH3OH, and CH3CH2OH, and the TiO2(1 1 0) surface and summarize the recent progresses of the corresponding STM observation and DFT explanations.
Defective TiO2(1 1 0) surface
The prepared TiO2(1 1 0) surface is usually defective with oxygen vacancy or hydroxyl group and thus excess electrons are present on surface. The spatial distribution of excess electrons plays a crucial role on the chemical activity of TiO2 surface, affecting properties such as the ability of adsorbing molecules or catalytic nanoparticles and transferring electrons.13–15 Thus, it is quite important to understand the distribution of excess electrons and the related geometric relaxation of the defective surface at atomic scale. The question whether the excess electrons are localized or delocalized on the surface is still currently object of debate.16–18 For this purpose, Minato et al.19 investigated the electronic structure of reduced TiO2(1 1 0) surface by a combined STM and DFT method. A typical empty-state STM image of reduced TiO2(1 1 0)-(1 × 1) surface is obtained by STM at a negative bias (Fig. 1A), in which alternating bright rows, dark rows, and faint and bright protrusions in the dark rows are present.19 High contrast images of occupied states for these two kinds of protrusions are then observed by STM at positive bias (Figs. 1B and 1C). In Figure 1B, a symmetric bright four-lobed contrast is observed, and each lobe extends over three Ti5c sites. In Figure 1C, the excess charge spreads over many surface sites in a shuttle-like shape.
DFT calculations with PW91 functional are performed to obtain the geometric and electronic structures of reduced surfaces with oxygen vacancy or hydroxyl group, which may correspond to the STM characters. (10 × 1) supercell with one missing bridging-oxygen is used to simulate BBOv with 10% coverage. Using the Tersoff–Harmann approximation, the STM images of unoccupied and occupied states of TiO2(1 1 0) with BBOv are simulated as Figures 1D and 1E, respectively. Figure 1D reproduces the faint protrusion in Figures 1A and 1E agree well with Figure 1B. A (6 × 2) supercell with one OH group is used to simulate hydroxyl adsorbed TiO2(1 1 0) surfaces. The STM images obtained at positive bias and negative bias are shown as Figures 1F and 1G, respectively. Figure 1F reproduces well the bright protrusion in Figure 1A as Figure 1F does with the feature in Figure 1C.
Therefore, on the basis of DFT (GGA) simulations, the faint and bright protrusions, and bright and dark row in the empty-state STM image (Fig. 1A) can be assigned to BBOv, OH group, and Ti5c row and BBO row, respectively. The excess electrons generated by BBOv and hydroxyl are delocalized over multiple surrounding titanium atoms. This result agrees well with a recent resonant photoelectron diffraction investigation.19 It is known that GGA cannot describe well the energy gap of semiconductors and that methods such as B3LYP and GGA + U give better results.17 Therefore, results based on GGA and B3LYP have been carefully compared with experiments. It is found that B3LYP and PW91 calculations produce asymmetric and symmetric geometry distortions, respectively. However, asymmetric image has never been observed in STM. Recently, ab initio molecular dynamics simulations with PBE + U (U = 4.2 eV) suggest that a dynamic average effect of various localized states contribute to the “effective delocalization.” The understanding of the distribution of excess electrons can help to reveal the interaction of adsorbates with the surface.
Molecular oxygen on TiO2(1 1 0) surface
The interaction of molecular oxygen with TiO2 is important in determining many fundamental processes that take place on TiO2-based materials, such of low temperature oxidation or photocatalytic activity.3–5 However, the adsorption configuration of molecular oxygen has been long debated.20–24 Recently, Tan et al.25 systematically investigated the adsorption of O2 with different coverage on reduced TiO2(1 1 0) surface via STM and DFT calculations.
At low coverage, faint contrasts at BBOv sites (mystery BBOv) (dashed yellow squares in Fig. 2A) or paired protrusions (dashed white rectangles in Fig. 2A) occurring at the neighboring Ti5c sites in opposite rows of preexisting BBOvs in STM images25 or single bright spot on Ti5c row (yellow arrow in Fig. 2A) are observed, which agree well with the observation of Scheiber et al.26 DFT calculations with a (4 × 2) supercell with five OTiO trilayers are adopted to investigate the O2 adsorption at atomic scale. Plane wave basis sets with a cutoff of 460 eV, projector-augmented wave (PAW) potential27 and PBE functional9 are used for these calculations. The most stable configuration of O2 molecular adsorption on BBOv is the flat-lying one (Fig. 2B). The simulated STM image of this structure (Fig. 2C) well reproduces the faint contrast at BBOv in Figure 2A. The OO bond length is elongated to 1.44 Å, which indicates a doubly charged state of O2.21 The two equivalent inclined configurations shown in Figures 2D and 2E are 0.35 eV less stable than the flat-lying one but inelastic tunneling electron during the scanning of STM tip can excite the switching of adsorbed O2 among these three configurations, which leads to the paired protrusions (white rectangle in Fig. 2A). The configuration of dissociative O2 at BBOv site is shown in Figure 2F: one oxygen reveals the BBOv and the other adsorbs at neighboring Ti5c row, corresponding to the single bright spot in Figure 2A.
Increasing the exposure, paired protrusions at Ti5c row (bright arrow in Fig. 2G) are observed at 80 K,25 which are assigned to the dissociative adsorption of O2 at the Ti5c site. The nondissociative adsorption at Ti5c row is directly observed by STM at 50 K with extremely low bias and current,28 and the dissociation is not observed even at 120 K in Wendt's work.20 This discrepancy calls for more experimental and theoretical investigations.
At high oxygen coverage a very stable species at BBOv site is observed under concessive STM scanning. This is also proposed in previous works, see, for example, the species contributing to the desorption peak at 410 K in temperature programmed desorption (TPD) investigation,22 the new species formed at 200–400 K in Kimmel's work23 or the stable “photoblind” species in photosimulated reaction.28 Pillay et al.24 theoretically predict the configuration of oxygen adsorption at high coverage with two molecules at BBOv sites as O42− (Fig. 2H) using the PW91 functional and the vienna ab-initio software package (VASP) code.
In summary, at low oxygen exposure, molecular O2 can adsorb at BBOv site and the interaction of the tip during scanning or the increasing of temperature may induce its dissociation. Instead, the adsorption of O2 at Ti5c site and the new stable species at BBOv site at high coverage still require more experimental and theoretical work to be fully understood.
CO adsorption on reduced TiO2(1 1 0)
Whether CO adsorbs at BBOv site or Ti5c site on TiO2(1 1 0) is a highly controversial issue,29, 30 and thus it is important to understand the mechanism of low-temperature CO oxidation on TiO2 supported metal clusters. Recently, Zhao et al.31 investigated the adsorption of CO on various BBOv concentrations (5, 11, and 22%) by STM at 80 K. Sites for the CO adsorption are illustrated in Figure 3A. The distributions of CO at each site at different BBOv concentrations are shown in Figure 3B. It is found that CO prefers to adsorb at the next-nearest-neighbor Ti5c of BBOv. The DFT calculations were performed with VASP.32, 33 The interaction between ions and electrons is described by PAW potentials.27 Plane wave basis sets with 400 eV energy cutoff and PBE functional9 are adopted. With surface model composed by a (6 × 2) supercell of five-trilayers (OTiO) with bottom two layers fixed, the adsorption of CO at different sites is theoretically studied using DFT. The most stable site for CO adsorption is site 1 labeled in Figure 3A. The relative adsorption energies of CO at different sites are shown in Figure 3C, which can be used to calculate the Boltzmann distribution of the adsorption of CO at different sites (Fig. 3D). The whole picture of CO adsorption can be described as “hot” gas phase in which CO molecules randomly land and eventually reach an equilibrium through thermal diffusion. Adsorbed CO molecules diffuse along Ti5c rows and across the BBO row through the BBOv.
The adsorption of CO on rutile TiO2(1 1 0) surface with preadsorbed oxygen adatoms is also investigated via STM and DFT calculations.34 In STM images two typical complexes with bottle-gourd-like shape and dumbbell shape are observed (Fig. 3E). Using DFT, the geometric structure of COO complex and COOCO complex are optimized as shown in Figures 3F and 3G, respectively. The simulated STM images of two complexes are shown in Figures 3H and 3I and agree well with the experimental observations. Thus, bottle-gourd-like shape and dumbbell shape are assigned to COO and COOCO complex, respectively. The complexes are quite stable under high bias voltage or UV light and no CO oxidation is observed, which indicate such complexes play little role in the photocatalytic oxidation of CO. The reaction barrier is determined using the climbing-image nudged elastic method35 implemented in VASP and the force convergence criterion is set as 0.04 eV/Å. The calculated reaction barrier of COO complex to CO2 is 0.56 eV, which indicates that the probability of CO reacting with O adatom is too low to be observed at 80 K. The reaction of CO and O adatom via Eley–Rideal mechanism is not observed in STM investigation, which disagrees with the barrier-free process predicted by previous theoretical work.36
Therefore, on reduced TiO2(1 1 0) surface, CO prefers to adsorb at the next-neighboring Ti5c sites of BBOv rather than BBOv itself and the adsorption of CO at different Ti5c sites follows the Boltzmann distribution. When atomic oxygen is precovered on TiO2(1 1 0) surface, COO and COOCO are formed and remain stable at low temperature.
Converting CO2 to useful product such as hydrocarbons is an interesting research topic. Due to its high stability, high energy is required to activate CO2. One of the promising strategies is to reduce CO2 with photogenerated electrons on photocatalysts such as TiO2.37, 38 The understanding of the interaction of CO2 with TiO2 can help to reveal the underlying physical mechanism of CO2 converting, which is important for the development of more efficient photocatalysts. Recently, the adsorption configurations39–41 and tip-induced dissociation probability have been investigated by STM.39, 40 Asymmetric and symmetric CO2/BBOvs are observed as shown in Figure 4A, and the dissociation ratio at different bias is shown in Figure 4B. DFT calculations are used to determine the geometric and electronic structures of adsorbed CO2 on reduced TiO2 surface. Based on the calculation results, CO2 prefers inclined adsorption geometry at BBOv site on reduced TiO2(1 1 0) (Fig. 4C) with a binding energy of 0.44 eV and the vertical configuration (Fig. 4D) is 0.16 eV less stable.39 Thus, the asymmetric and symmetric bright protrusions are assigned to inclined and vertical adsorbed CO2 at BBOv site, respectively.
A threshold voltage of 1.839 (1.740) eV is required to dissociate adsorbed CO2, and the dissociation ratio increases distinctly when the bias voltage is higher than 2.3 eV as shown in Figure 4B. No dissociation events are observed under negative bias voltage. To understand the underlying physical mechanism, the electronic structures of the adsorbed system are analyzed. Calculated PDOS for different adsorption configurations are shown in Figure 4E, and the strong hybridation between the lowest unoccupied molecular orbital (LUMO) of CO2 and Ti 3d orbital is indicated in Figure 4F. Based on these results, it is suggested that hybridized states (HS) spread between 1.2 and 2.0 eV above Fermi level (EF) as shown in the inset of Figure 4E may contribute to the threshold of the CO2 dissociation, and the LUMO lying 2.7 eV above EF contributes to the rapid increasing of dissociation probability. Based on present works,39, 40 it can be proposed that the dissociation of CO2 is a one-electron process induced by the attachment of one tunneling electron. More investigations are needed to indeed verify if it is a one-step process39 or not.40 The schematic map is shown in Figure 4G.
Based on the above investigation, it is suggested that CO2 can adsorb at BBOv site with both inclined and vertical configurations. The interaction between CO2 and TiO2 is carefully investigated suggesting a low probability for photoinduced CO2 dissociation. Thus, under photoexcitation the dissociation of CO2 cannot likely be observed and solvent- or coadsorbate-aided mechanism is needed to improve the efficiency of CO2 conversion based on photoexcitation process.
Methanol and ethanol on TiO2(1 1 0)
Methanol plays an important role in water splitting on TiO2(1 1 0) with photocatalysts42 and the understanding of the interaction of methanol with TiO2 surface at atomic scale could provide important insights to its pohtocatalytic activity. STM can distinguish the intact and dissociative adsorption of methanol on TiO2(1 1 0) by manipulating the bright protrusions.43 TD-2PPE discovers a 5.5 V excited resonance feature, which grows monotomically with exposure time of the probing laser light and is independent by the methanol coverage. DFT based cluster calculations with BLYP functional indicate the puckering caused by the newly formed TiO bond by dissociation of methanol produces a new exited state ∼ 2.5 eV above the LUMO. This can be assigned to the resonance feature observed by TD-2PPE rather than to the wet electrons states suggested in a previous work.44
Recently, the adsorption and dissociation of ethanol on reduced TiO2(1 1 0) surface is also investigated by time-lapsed STM movie combined with DFT calculations with PBE functional.45 Three kinds of adsorbed intact or dissociated ethanol molecules are observed in STM images: BBOv site with apparent height of 2.0 Å (EtObr), Ti5c sites with apparent height of 1.9 Å (EtOTi) and 2.6 Å (EtOHTi). EtOHTi is mobile on the Ti5c row at 180 K ≤ T ≤ 200 K, whereas EtObr and EtOTi are immobile due to the difference of bonding modes. The H adatom at the nearest neighboring of EtObr cannot be identified in STM images. DFT calculations indicate that the dissociative adsorption at BBOv is 0.5 eV more favorable than molecular adsorption, whereas, on Ti5c site, intact adsorption (EtOHTi) is 0.1 eV slightly more stable than dissociative one. The calculated energy barriers of EtOHTi dissociation and diffusion along a Ti5c row and the diffusion of EtOTi are ∼ 0.28, 0.41, and 1.18 eV, respectively, which agree well with the experimental observation.
Therefore, methanol and ethanol can adsorb both intact and dissociative on reduced TiO2(1 1 0) surface and the newly formed O–Ti5c bond apparently modifies the electronic structure of the adsorbed system.
Conclusion and Outlook
Recently, adsorption and reaction of small molecules on rutile TiO2(1 1 0) surface have been studied from both STM and DFT perspective. Some conclusions can be drawn out:
1.At the current experimental condition, delocalized distribution of excess electrons generated by point defect such as BBOv or OH is observed by STM and GGA calculations.
2.At low coverage, molecular oxygen can adsorb at BBOv site molecularly or dissociatively according to the experimental condition. With the increasing of oxygen exposure, the dissociation of O2 at Ti5c and also a quite stable species at BBOv site may be present.
3.On reduced TiO2(1 1 0) surface, CO prefers to adsorb at the next-nearest Ti5c site rather than BBOv site or its nearest neighboring Ti5c site; when CO coadsorbs with O adatom, complexes of COO or COOCO forms on Ti5c, and the oxidation of CO is not easy to be observed.
4.The adsorption of CO2 on BBOv sites with two configurations are observed at 80 K. Adsorbed CO2 can be dissociated by the electron injection from tip. The energy threshold for CO2 dissociation may be contributed by the HS between CO2 and TiO2 surface.
5.The mixed adsorption of methanol and ethanol are observed directly by STM measurements. The formation of new TiO bond from the dissociation of methanol produces new states in the conductive bands.
Although big progresses have been made by combined STM and DFT investigations, there are still some important open questions to be answered, such as the exact stable oxygen species at BBOv sites at high oxygen exposure, the effect of interstitial Ti on surface chemistry, and the adsorption of CO2 on Ti5c site. To study tip or temperature-induced dissociation or desorption effect, it is critical to well control the experimental conditions. On the theoretical side, results can be sensitive to the thickness of the TiO2 slab model or the functional used in DFT calculations. Therefore, systematic benchmark studies are very critical.
Wenhua Zhang, received her Ph.D. degree in chemistry from a joint program of the University of Science and Technology of China (USTC) and the Royal Institute of Technology (KTH) in 2010 under the supervising of Prof. Jinlong Yang and Prof. Yi Luo. Currently, she is an associate professor at the department of materials science and technology, USTC. Her research interests include computational catalysts and theoretical spectroscopy. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Zhenyu Li, received his PhD in chemistry from USTC in 2004 under the supervision of Prof. Jinlong Yang. Since then, he has been working as a postdoctoral researcher at University of Maryland, College Park and University of California, Irvine. Currently, he is an associate professor at USTC. His current research interests include computational characterization of nanostructures and mechanisms of graphene growth. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Bing Wang, obtained his Ph. D degree at University of Science and Technology of China (USTC) in 1995. After two years as post-doctoral fellow at Hong Kong University of Science and Technology, he became an Associate Professor at USTC in 2000 and became a Full Professor in 2004. His research interests include experimental studies on the electronic and transport properties of single molecules and nanostructures, and fundamental issues related to the photocatalytic processes on the surface of oxide semiconductors. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Jinlong Yang, currently a Changjiang professor of chemistry, dean of the School of Chemistry and Material Sciences, University of Science and Technology of China (USTC), received his Ph.D. degree in condensed matter physics from USTC in 1991. He was selected as a fellow of the American Physical Society in 2011. His research interests focus on developing first principles methods and their application on clusters, nanostructures, solid materials, surfaces, and interfaces. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]