Citation

Fei Li, Wen-Hao Huang, Xue-Qing Gong. Unique adsorption behaviors of NO and O₂ at hydrogenated anatase TiO₂(101)[J]. Chin. Chem. Lett., 2018, 29(6): 765-768  

Fei Li, Wen-Hao Huang, Xue-Qing Gong    

Titanium dioxide (TiO₂) has various practical applications in many important fields, such as semiconductor electrodes in photochemical cells, coating materials used for UV protection, photocatalysts for water splitting, sensing and photochromic materials, etc. [1, 2]. It has been intensively studied by wide range of experimental and theoretical methods [3-5]. However, for its different polymorphs, anatase TiO₂ is relatively less investigated compared with rutile [10-13], though it usually exhibits higher activities in catalysis. As the most stable and abundant facets exposed at these two TiO₂ nanomaterials, anatase TiO₂(101) and rutile TiO₂(110) have been carefully studied. In particular, several probe molecules, such as NO, O₂, CO, HCOOH, have been chosen to study the surface properties of these facets. In fact, such probe molecules are also important in practice, as O₂ is one of the most common gases in the air and the main product in photocatalytic water splitting, and NO is also one of the main components in automotive exhaust [6-9].

A number of theoretical studies on the adsorptions of NO and O₂ at rutile TiO₂(110) are available [10-13], while those on anatase TiO₂(101) are rather limited. Nevertheless, it has been found that defect sites on these surfaces, including step edges, surface hydroxyls, oxygen vacancies and interstitial titanium [14-17], can modify their structural and electronic properties and act as the active sites for various applications. In particular, the surface hydroxyls attracted great attention since they are popular surface species at TiO₂ in real use and related to surface processes in photocatalysis. Wendt et al. [18] and Liu et al. [19] found that the hydroxylated rutile TiO₂(110) can significantly affect the diffusion of H₂O and the adsorptions of O₂. The presence of surface hydroxyl is also essential for the covalent coupling of aryl halide monomers on the rutile TiO₂(011) [20]. In addition, Liu et al. [21] showed that the hydroxyls on rutile surfaces can greatly promote the transformation of NO₂ into HNO₃.

In order to further explore the catalytic behaviors of surface hydroxyls at anatase TiO₂, we calculated the adsorption properties of NO and O₂ at hydrogenated anatase TiO₂(101). Various surface adsorption sites were considered in the calculations, and it has been determined that the hydrogenation can affect the surface electronic structures and increase the adsorption strengths of the NO and O₂ molecules through charge transfer. Interestingly, we also found that, different from the hydrogenated rutile TiO₂(110), the hydrogenated anatase TiO₂(101) can favor the adsorptions at restricted positions beside the hydroxyl, and further analyses indicate that the local bonding configurations mainly contribute to such unique properties.

In this work, all the calculations were conducted using the Vienna ab initio simulation package (VASP) [22, 23], and the electron exchange-correlation was described by the PBE-GGA functional [24]. The core-valence electron interaction was represented by the project-augmented wave (PAW) [25, 26] method. The calculations also included the on-site Coulomb corrections [27, 28] (DFT + U, U = 4.2 eV for Ti 3d states as suggested by Deskins et al. [29] and Yoon et al. [30]) and long-range dispersion interactions [31, 32] (DFT-D). The kinetic energy cut-off of 400 eV was used for the plane wave basis-set expansion. The titanium (3s, 3p, 3d, 4s), the hydrogen (1s), and the nitrogen and oxygen (2s, 2p) electrons were treated as valence electrons.

The anatase TiO₂(101) surface was modeled as a periodic slab with four TiO₂ layers of oxide, and the vacuum between slabs was ~10 Å. The 2 × 3 surface cell and corresponding 2 × 2 ×1 k-point mesh were used in the calculations. The adsorption was modeled on one side of the slab, and during structural optimizations, all of the atoms except those in the bottom TiO₂ layer of the slab were allowed to relax until the atomic forces reached below 0.05 eV/Å.

To estimate the adsorption energies (E_ads) of the molecules (NO and O₂), the following expression was considered:

where E_mol/TiO2 is the total energy of the system with one adsorbed molecule and TiO₂ surface slab; E_TiO2 is the total energy of the surface slab; and E_mol is the total energy of a single molecule in gasphase.

The well-known structures of stoichiometric anatase TiO₂(101) and rutile TiO₂(110) are illustrated in Fig. 1. Both fully saturated 3 (6)-fold coordinated O(Ti) (O_3c and Ti_6c) atoms and coordinatively unsaturated 2(5)-fold coordinated O(Ti) (O_2c and Ti_5c) are exposed on these surfaces.

Figure 1

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Fig. 1. Calculated structures (side view) of the stoichiometric (a) anatase TiO2(101) and (b) rutile TiO2(110). Ti and O atoms are in light grey and red, respectively.

It is generally believed that the stoichiometric terminations of these most stable facets of anatase and rutile TiO₂ are relatively unreactive. At the same time, it has been found that the occurrence of defective species would increase their activities. Yu et al. [33] reported that on the stoichiometric rutile TiO₂(110), the adsorption energy of NO is as low as 0.53 eV, and it can be significantly increased to 0.82 eV when the surface is hydrogenated. Hu and coworkers [19] determined that the adsorption energy of O₂ would increase by 0.5 eV on the hydrogenated rutile TiO₂(110). It also needs to be mentioned that, according to these studies, the various Ti_5c sites beside the surface O_2cH hydroxyl (Fig. 1b) give almost no obvious difference regarding the adsorption energies of NO and O₂.

In the current work, we also chose the clean and hydrogenated anatase TiO₂(101) to calculate and compare their reactivities. As one can see from Fig. 2a, the exposed Ti_5c binds with four O_3c and one O_2c; the Ti_5c-O_3c bonds are slightly above 2.0 Å, while the Ti_5cO_2c bond is as short as 1.86 Å. Following hydrogenation, the O_2cH hydroxyl occurs and the original Ti_5c-O_2c bond is significantly stretched to as long as 2.07 Å though the other bonds are only slightly stretched.

Figure 2

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Fig. 2. Calculated structures (top view) of the (a) stoichiometric and (b) hydrogenated anatase TiO2(101), and (c) NO and (d) O2 being adsorbed on stoichiometric anatase TiO2(101) (the insets are corresponding side views). Several important calculated bond lengths are also given. Three sites (Site 1, Site 2 and Site 3) on the hydrogenated surface where NO and O2 adsorptions were calculated are specified in (b). O of adsorbed molecules are in pink, and N in blue.

Then, we calculated the adsorptions of NO and O₂ on the clean anatase TiO₂(101) surface firstly. The most favorable adsorption site for the molecules is again Ti_5c (Fig. 2), and the calculated E_ads are 0.49 eV and 0.20 eV for NO and O₂, respectively. On the hydrogenated anatase TiO₂(101), we then considered the adsorption Ti_5c sites at different positions with respect to the hydroxyl (Fig. 2b). In Table 1, we list the calculated adsorption energies of these two molecules at the different Ti_5c sites. As one can see from the table, the calculated adsorptions of NO and O₂ at the three different positions exhibit very different performance compared to those at rutile TiO₂(110). The NO and O₂ molecules gave very different adsorption energies at these three different sites, and in particular, for NO, its adsorption strength is drastically increased only at Site 1, which is just beside the hydroxyl, while it is barely affected at other sites further away from the hydroxyl. Similarly, O₂ adsorption is also heavily enhanced at such nearest Site 1, while the enhancement effect is less significant at Site 2 and further decreases at the Site 3.

Table 1

Table 1 Calculated adsorption energies of NO and O2 at clean and different sites (Site 1–3) of hydrogenated anatase TiO2(101), with comparison of those of NO at clean and various Ti5c of hydrogenated rutile TiO2(110) (in parentheses) [33].

Table 1 Calculated adsorption energies of NO and O2 at clean and different sites (Site 1–3) of hydrogenated anatase TiO2(101), with comparison of those of NO at clean and various Ti5c of hydrogenated rutile TiO2(110) (in parentheses) [33].

In order to better understand the different performances of NO and O₂ adsorptions at rutile TiO₂(110) and anatase TiO₂(101), we conducted analyses from electronic and structural points of view. Yu et al. [33] reported that on the hydrogenated rutile TiO₂(110), the single electron transferred from the adsorbed H could be localized at different nearby surface Ti_5c (nearest, second-nearest and third-nearest to the surface hydroxyl) with similar stabilities. On the contrary, our calculations in the current work determined that the localized electron (Fig. 3a) at the hydrogenated anatase TiO₂(101) can occur only at the nearest Ti_5c (Fig. 2b, Site 1). It is well known that the localized electron at TiO₂ surfaces can turn the Ti cation where it is localized from one Ti⁴⁺ into a Ti³⁺, which will then have a significantly larger ion radius. Accordingly, the local bonding structures will also be affected by the occurrence of Ti³⁺, i.e., the Ti-O bonds should be stretched due to the large Ti³⁺ ion radius. At clean and hydrogenated rutile TiO₂(110), each Ti_5c still binds with five O_3c and therefore they each have similar capacity to take the localized electron, and such relatively weak bonds with O_3c can be also readily stretched.

Figure 3

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Fig. 3. Calculated spin-polarized charge densities (in yellow) of (a) the hydrogenated anatase TiO2(101) and the surfaces with the adsorbed NO at (b–d) Site 1, 2 and 3. The corresponding adsorption energies are also listed, as well as the calculated Bader charges of the NO (see insert, side view).

However, the situation is completely different at anatase TiO₂(101). Besides the four O_3c, the Ti_5c at the clean surface now also binds with one O_2c which is much more reactive than the O_3c, and as we have illustrated in the above, the Ti_5c-O_2c bond then becomes much shorter (Fig. 2a). Consequently, the Ti_5c at hydrogenated anatase TiO₂(101) is less feasible to increase its radius compared to that at rutile TiO₂(110), and only the one directly binding with the hydroxyl can be readily reduced since such hydrogenated O_2c is now 3-fold coordinated and binding less strongly with the Ti_5c (Fig. 2b).

The combined electronic and structural effects discussed in the above can not only explain the different performances of the localized electron at anatase TiO₂(101) and rutile TiO₂(110), and they may also help understand the adsorptions of NO and O₂. In Figs. 3b-d, we plot the calculated spin-polarized charge densities of the systems with adsorbed NO at different Ti_5c sites of the hydrogenated anatase TiO₂(101). As one can see, for the NO at Site 1 and Site 2, the originally localized electron (Fig. 3a) is now partially transferred to the NO (see the calculated Bader charges of NO in the insert) and largely shared by it and the Ti_5c below. Therefore, the adsorption at Site 2 is less favorable since the Ti_5c cation is binding with a clean O_2c and less likely to take the partial electron compared to the one of Site 1. For the NO adsorbed at Site 3, it gains no extra electron and the localized electron is still at the Ti_5c of Site 1 as a whole. Therefore, the adsorption energy is nearly identical to that at stoichiometric anatase TiO₂(101).

Similar electronic analyses were also conducted on the anatase TiO₂(101) surfaces with adsorbed O₂ at different sites (Figs. 4a-c). Different from the case with adsorbed NO, the O₂ at all the three sites can take partial electron and become negatively charged, which could be due to the relatively lower 2π* orbital of O₂ compared to that of NO where the partial electron stays. Again, the different stabilities of the adsorption systems with O₂ can be explained by the positions of the Ti_5c with the localized (partial) electron. In addition, the existence of H-bond between the hydroxyl and the adsorbed O₂ may also contribute to the stronger adsorption of O₂ at Site 2 compared to that at Site 3.

Figure 4

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Fig. 4. Calculated spin-polarized charge densities (in yellow) of the hydrogenated anatase TiO2(101) with the adsorbed O2 at (a–c) Site 1, 2 and 3. The corresponding adsorption energies are also listed, as well as the calculated Bader charges of the O2 (see insert, side view).

In this work, we calculated the hydrogenated anatase TiO₂(101) by focusing on its electronic and structural properties and activities toward adsorptions of NO and O₂. We found that, different from the hydrogenated rutile TiO₂(110), the hydrogenated anatase TiO₂(101) can generate the localized electron at only one specific surface Ti cation that directly binds with the hydroxyl, and favor the NO and O₂ adsorptions at such position. Detailed analyses indicate that the local bonding configurations mainly contribute to these unique properties. The results obtained in this work also suggest that in photocatalytic processes, anatase TiO₂(101) with the excited electron at a surface Ti_5c may give rise to the specifically activated O_2c directly binding with such Ti³⁺, where unique activities such as proton adsorption and water dissociation may occur since these processes can efficiently stabilize the enlarged Ti³⁺.

We are grateful for financial support from the National Natural Science Foundation of China (Nos. 21421004, 21573067, 91545103) and Program of Shanghai Academic Research Leader (No. 17XD1401400). The authors also thank the National Super Computing Center in Jinan for computing time.