Evolution of anatase surface active sites probed by in situ sum-frequency phonon spectroscopy

The surface phonon mode and related bonding geometry

In our experiment, the anatase (101) sample was cleaned and placed in a chamber purged with various gases and could be irradiated in situ by UV light (see Materials and Methods for details). For SFG, the incident 800-nm near-infrared (NIR) and the broadband IR beams overlap on the sample surface and generate the SF vibrational spectrum in reflection. The basic theory of SFG is sketched in the Supplementary Materials. Briefly, when the IR frequency is near the phonon resonance, the SF signal (S_SF) is proportional to , where is the nonresonant background, and is the resonant contribution, with , ω_q, and Γ_q being the amplitude, frequency, and damping coefficient of the qth resonance mode, respectively (11). Our SF vibrational spectrum on anatase (101) taken with PPP polarization combination in the spectral range of 700 to 1000 cm⁻¹ is displayed (red curve) in Fig. 1B, in comparison with the Raman spectrum (blue curve) from the same sample. The two bands at 543 and 667 cm⁻¹ are the two highest-frequency Raman-active TO phonon modes of bulk anatase (26–28). In contrast, the SF spectrum exhibits a new resonance at ~860 cm⁻¹ from the (101) surface. On the basis of our ab initio calculation that reveals a surface resonance at ~878 cm⁻¹ on fully relaxed anatase (101) (see calculation details in the Supplementary Materials), we identify the observed SF mode as mainly due to the stretching vibration of the bond between Ti(5c) and the fully coordinated oxygen ion beneath it [O(3c)] (Fig. 1A and fig. S1). Because Ti(5c) exists predominantly on the surface, this is a highly localized surface phonon mode.

To further confirm the assignment, we checked whether symmetry properties of the 860-cm⁻¹ mode agree with that of the assigned surface phonon mode. Anatase (101) belongs to the C_1v point group with a mirror plane perpendicular to the [010] axis (Fig. 2A) (29). Its phonon modes can be characterized by two irreducible representations, A′ and A′′, corresponding to transition dipole moments lying parallel and perpendicular to the mirror plane, respectively. The Ti(5c)–O(3c) bonds lie in the mirror plane, and the bond stretching phonon mode transforms according to the A′ representation. We measured, for different input/output polarization combinations, the SFG spectra of the 860-cm⁻¹ mode versus the azimuthal angle of anatase (101), φ, between the incidence plane and the [10] axis on the surface [see geometries described in Fig. 2 (A to C)]. At φ = 0° and 180° (Fig. 2, A and B), the transition dipole moment of an A′ mode should be parallel to the incident plane, so it cannot be excited by the S-polarized IR input. On the other hand, it can be excited by the P-polarized IR light, but as the transition dipole moment tilts away from the surface normal (which is essential for the C_1v group), the excitation is different for φ = 0° and 180°, and so is the spectral intensity. As seen in Fig. 2 (D to F), these symmetry features are observed in the SPS, SSP, and PPP spectra of the 860-cm⁻¹ mode (see the Supplementary Materials for more details). For φ = 90° (Fig. 2C) and 270°, the geometries for probing an A′ mode are equivalent, and its SF spectra should be the same. This is true within experimental uncertainty for the 860-cm⁻¹ mode (SSP spectra are presented in Fig. 2G as an example). Therefore, the result confirms that the 860-cm⁻¹ mode is an A′ mode expected for the Ti(5c)–O(3c) bond stretching phonon on anatase (101).

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The SFG finding allows us to determine related surface structures on anatase (101). The higher surface phonon frequency, compared to bulk phonons, suggests that the Ti(5c)–O(3c) bond is shorter than bulk counterparts because of surface relaxation. Our calculation finds the former shorter than the latter by ~0.13 Å (30). Next, orientation of the Ti(5c)–O(3c) bonds can be deduced from the spectral anisotropy at φ = 0° and 180° (Fig. 2, E and F). For both SSP and PPP polarization combinations, the intensity ratio of the 860-cm⁻¹ mode is given bywhere μ_a(c),eff is the effective transition dipole moments along the [10] ([101]) axis. We find |μ_a,eff/μ_c,eff| ≈ 1.0 ± 0.2, from which we deduce the dipole moment to tilt by ~25 ± 5° from the [101] axis, close to the theoretical prediction (see the Supplementary Materials for more details). The agreement further validates our assignment of the 860-cm⁻¹ mode and sets the foundation for using it to investigate reactions on anatase (101).

UV-induced surface structural change

Ti(5c) is the major adsorption site on TiO₂ for key reactants, such as methanol and water, but how they adsorb is still a subject of debate (31, 32). If adsorbed molecules bind strongly to Ti(5c), the corresponding Ti(5c)–O(3c) stretching frequency should exhibit a shift (see the Supplementary Materials). In our experiment, the 860-cm⁻¹ mode barely shifted when the clean anatase (101) surface was exposed to saturated methanol vapor. This indicates that methanol was not very tightly bonded to Ti(5c). If adsorbed methanol molecules were dissociated, we would expect that with methoxy as the dissociated fragment bonded to Ti(5c), the corresponding Ti–O stretching mode should have a significant red shift, and the C–H stretching spectrum of methoxy should appear. Neither was observed in our SFG measurement (fig. S2). So, at least at room temperature and high coverage, methanol must have adsorbed on anatase (101) in the molecular form [similar to the case of methanol adsorption on rutile (110) (31, 32)]. Methanol dissociation is either not favored, or dissociated fragments would recombine rapidly (33). For water, its dissociation upon adsorption is predicted to be even less favorable (31, 34).

Upon UV irradiation, methoxy is still not detectable (fig. S2). However, the 860-cm⁻¹ phonon intensity dropped sharply (Fig. 3A) not only in methanol and water vapors but also in other ambient gases, such as nitrogen and hydrogen. It is known that UV can generate oxygen vacancies on TiO₂ (14). For anatase (101), the surface oxygen vacancy, , is preferentially formed at the O(2c) site (Fig. 3B), lowering the coordination of the conjoint Ti(5c) to Ti(4c) (19, 21). Accordingly, the number density of Ti(5c) decreases, and so does the intensity of the 860-cm⁻¹ mode. To establish that the intensity drop is due to creation of , we compared the UV-induced spectral change of anatase (101) in pure oxygen with that in other gases. As shown in Fig. 3C, the UV-induced intensity change of the 860-cm⁻¹ mode in oxygen differs markedly from the others: When UV is on, it drops to below 25% in pure nitrogen and methanol vapor but remains as high as 60% in oxygen; when UV is off, the signal recovers to less than 50% in nitrogen and methanol but almost fully in oxygen. In other gases, the spectral changes are similar to those in nitrogen and methanol, but in clear contrast to that in oxygen. Obviously, oxygen is unique because it can directly replenish surface oxygen vacancies. Hence, we conclude that the UV-induced intensity drop of the 860-cm⁻¹ mode is predominantly due to the increment of ’s.

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Stabilization of surface oxygen vacancies by ambient gases

We can thus use the 860-cm⁻¹ mode to monitor the amount of directly under ambient conditions. In UHV, is unstable and tends to migrate down to form subsurface vacancies () (Fig. 3B) (18, 19, 22). Such migration is also evident under ambient conditions. Under UV irradiation, the mode intensity drops upon generation of but levels off at a nonzero value (Fig. 3C). This suggests that while is being created, it also tends to disappear from the surface by forming . The two processes balance each other at a finite number of in dynamic equilibrium. When UV is off, creation of stops, but the conversion to does not, so that the number of decays until a new equilibrium is reached between and (Fig. 3C), depending on their free energy difference. We found this balance differs for different ambient gases. Recently, the enhanced stability of on anatase (101) by ambient water vapor was proposed by Li and Gao (25). Here, we extend the calculation to cases of methanol, nitrogen, and hydrogen with results shown in Fig. 3D (circles) (see also table S1). The calculated energy difference (ΔE) between and is found to be as high as +0.46 eV in vacuum, but becomes +0.30 eV in hydrogen, and is only +0.09 eV in methanol. If we assume that the entropy difference between and does not depend on the ambient gases, the ranking order of the free energy difference will be the same as that of ΔE. Then, for smaller ΔE, the stability of is higher, or the final density of is larger. Experimentally, the density of can be monitored by the 860-cm⁻¹ mode intensity. We plot in Fig. 3D the final percentage drop of the 860-cm⁻¹ mode because of persistent on anatase (101) in the four ambient gases after the same UV irradiance. It shows that the persistent follows the ranking order of methanol→water→nitrogen→hydrogen, correlating nicely with the opposite ranking order of the calculated ΔE.

To understand the underlying physics, we resort to adsorption configurations of gas molecules on anatase (101) (see details in fig. S3). Nitrogen and hydrogen only interact weakly through van der Waals interaction with the anatase surface, but they can still adsorb to ’s (Fig. 3E) with a low surface coverage. Methanol and water bind more strongly to anatase (101); they can yield a high surface coverage (Fig. 3F) (31, 34) and slightly dope the surface conduction band with their oxygen lone pair electrons. As discussed by Li and Gao (25), the surface with has more electronic density of states near the bottom of the conduction band so that the stability of can be enhanced by electron doping. Methanol, in particular, is most efficient in stabilizing ’s. Because ’s are regarded as reaction centers on titanium dioxides (25, 35), enhancing the stability of may be the reason why methanol appears as a promotional agent in some reactions on anatase (23, 36). As all molecules we have studied are widely present in applications of TiO₂ as catalysts, one could anticipate the reactivity in ambient conditions to be very different from that in vacuum. The same is likely to be true on other reactive surfaces of oxides (25).