First-Principles Study of Formaldehyde Adsorption on TiO2 Rutile (110

Article pubs.acs.org/JPCC

First-Principles Study of Formaldehyde Adsorption on TiO2 Rutile (110) and Anatase (001) Surfaces Huazhong Liu,†,‡,§ Xiao Wang,‡ Chunxu Pan,† and K. M. Liew*,‡ †

School of Physics and Technology and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan 430072, People's Republic of China ‡ Department of Civil and Architectural Engineering, City University of Hong Kong, Kowloon, Hong Kong § Department of Basic Sciences, PLA Military Economics Academy, Wuhan 430035, People's Republic of China ABSTRACT: This study investigated adsorption and reactions of formaldehyde (HCHO) on TiO2 rutile (110) and anatase (001) surfaces by first-principles calculation. The structure, vibrational frequencies, and electronic properties of the interaction system are studied to investigate the adsorption mechanisms of HCHO on TiO2 surfaces. It is found that HCHO can chemically adsorb on all surfaces to form into a dioxymethylene structure with O of HCHO bonding to a coordinatively unsaturated surface Ti atom (Ti4C or Ti5C) and C bonding to a surface O2C. The anatase (001) surface is found to be more active in HCHO adsorption with lower adsorption energy and larger charge transfer. In addition, the (1 × 4) reconstructed anatase (001) surfaces are found to have higher adsorption ability and more stable surface properties than that on (1 × 1) unreconstructed ones. These findings indicate that the (001) surface holds the potential for the improvement of sensitivity to reductive HCHO gas, in which the (1 × 4) reconstructed surface may play an important role for further improving gas-sensing properties of TiO2-based sensors while keeping the stability of them. HCHO at a typical indoor concentration level. Li et al.12 proposed a new way of detecting HCHO based on a TD/CTLbased sensor by using Y2O3-mixed TiO2 as the sensing material and found the gas sensor to exhibit good stability, high sensitivity, and rapid response, implying suitability for continuous monitoring of HCHO in air. Chen et al.13 study UV-light-activated porous TiO2 and ZnO film sensors for detecting HCHO gas and found that, compared with ZnO, TiO2 exhibited a performance superior for detecting HCHO. Recently, the highly ordered vertically grown TiO2 nanotubes for detection of parts per million levels of oxygen, H2, acetone, and humidity at relatively low temperatures have also been studied.14−19 Inspiringly, TiO2 nanotube arrays have also been used successfully for detection of HCHO at room temperature.20 Hence, there is a promising prospect for use of a TiO2based catalyst as an effective gas sensor for detecting toxic gases, such as HCHO, in living environments. As is known, TiO2 has three types of phases: anatase, rutile, and brookite,21 of which only the anatase and rutile are relevant for a variety of technological applications.21,22 In anatase TiO2, most available crystals are dominated by the thermodynamically stable (101) surface (over 94%, according to the Wulff construction23,24). Adsorption of the HCHO molecule on the stoichiometric anatase (101) surface25 and rutile (110) surface26 has been investigated by using density functional theory (DFT) calculation. The HCHO molecule was found to be chemically absorbable on these surfaces, but quite weakly

1. INTRODUCTION Formaldehyde (HCHO), the main indoor air pollutant in modern houses, is a colorless, pungent-smelling gas known to cause several symptoms, such as nausea, watery eyes, burning sensation in the eyes and throat, and difficulty in breathing. High concentrations may cause even serious diseases, such as cardiac problems in people suffering from asthma.1−4 Sources of HCHO in homes include some building materials and lacquer furniture.1−4 Therefore, it is highly desirable to develop sensitive, cheaper gas detectors for homes and work places for monitoring levels of HCHO so that suitable precautions can be taken to guard against this toxic gas. Metal oxide semiconductor gas sensors are quite promising for detection of toxic gas species due to their simple working principle, portability, high sensitivity, and low cost.3,5−10 For HCHO detection, various forms of metal oxides, such as SnO2 or doped SnO2,4,7 WO3,6 NiO thin film,7,8 CdO-mixed In2O3,9 and doped ZnO,10 have been investigated in the past. These sensors have good sensitivity to detect parts per million (ppm) or sub-ppm levels of HCHO and have the potential to be employed in real situations. However, one of the obvious disadvantages is that they work at elevated temperatures, usually around 200−400 °C,3,5−10 and require a heater integrated with the sensor. This makes the sensor design more complicated and expensive, further raising the operating cost. In recent years, photoactivated metal oxides, such as titania-based catalysts, have been found to show high sensitivity, rapid response, good stability, and low cost (require lower temperature), which makes them suitable for use as gas sensors. Yang et al.11 found that TiO2/UV can be technically feasible and economically attractive for reactivation with gaseous © 2012 American Chemical Society

Received: October 31, 2011 Revised: March 9, 2012 Published: March 13, 2012 8044

dx.doi.org/10.1021/jp210465u | J. Phys. Chem. C 2012, 116, 8044−8053

The Journal of Physical Chemistry C

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results,45−47 implying that our calculation methods are reasonable, and the calculated results should be authentic. The rutile and anatase surfaces were modeled as periodically repeated slabs in a supercell. All slabs were separated by a vacuum spacing of about 15 Å, which guaranteed no interactions between the slabs. For the rutile (110) surface, we construct a slab consisting of neutral (O−Ti−O) repeating trilayer units, shown in Figure 1a,b. The (110) surface is

and with a relatively low adsorption energy, due to the high stability/low reactivity of these surfaces.21,23,27,28 In recent years, many studies have found the anatase (001) surface to be more chemically reactive than the (101) surface, and it may affect surface properties of anatase TiO2 significantly by increasing its ratio in the crystal surfaces.23,24,29−32 Gong et al.29,33 found that the dissociative adsorption of formic acid is highly favored on anatase TiO2(001) under different conditions and the (001) surface plays a key role in the reactivity of anatase nanoparticles. Barnard et al.28,32 predicted that proper manipulation of the shape of anatase nanocrystals can enhance the adsorption properties by increasing the effective area of preferred facets, such as the (001) surface. Recently, a large proportion of (001) facets (as large as 47%) has already been successfully synthesized in anatase TiO2 crystals,34 making these facets good potential candidates for detecting HCHO molecules. Interestingly, many gourps have also reported that, under ultra-high-vacuum (UHV) conditions, the atomically clean anatase (001) surface exhibits a (1 × 4) reconstruction.24,31,33,35,36 By an ab initio modeling method, Lazzeri et al.31,35 found that this is driven by the large reduction in surface energy that follows from the decrease of the tensile stress intrinsic to the (1 × 1) surface. In addition, it is found that the formation of HCHO from the dissociation of carboxylic acids on this kind of reconstructed surface mainly occurs on the ridges (a special structural motif of the reconstructed surface; see section 3.2.3).37 Hence, it will be interesting to study the interaction between HCHO and anatase TiO2(001) surfaces. In this work, we performed a first-principles study on the adsorption of HCHO on TiO2(001) unreconstructed and (1 × 4) reconstructed surfaces, and for comparison, we also calculated the interaction on the rutile (110) surface. A detailed knowledge of the gas/surface interaction helps us to understand and elucidate the reactivity and selectivity of HCHO adsorption on titanium-based catalysts.

Figure 1. Optimized structures of clean rutile (110) and anatase (001) surfaces: (a) side view of the rutile (110) surface, (b) top view of the rutile (110) surface, (c) side view of the anatase (001) surface, (d) top view of the anatase (001) surface. Gray and red spheres represent Ti and O atoms, respectively. This notation is used throughout this paper.

2. COMPUTATIONAL DETAILS We performed periodic density functional theory (DFT) calculations using the Quantum Espresso code, PWSCF package,38 within the generalized gradient approximation (GGA).39,40 DFT has already been extensively used to study molecular adsorption on oxide surfaces, and it is generally agreed that results are accurate.41,42 The periodic plane-wave approach, with the Perdew−Burke−Ernzerhof (PBE)43 exchange-correlation function, and ultrasoft pseudopotentials44 was used in this work. Plane-wave basis set cutoffs of 30 and 350 Ry for the smooth part of electronic wave functions and augmented electron density, respectively, were used to expand the valence electronic wave function with valence configurations of Ti-3d24s2, O-2s22p4, C-2s22p2, and H-1s1. The convergence threshold for self-consistency was set to 1.0 × 10−8 Ha, and the total energy tolerance for the electronic selfconsistent field was converged to 1.0 × 10−5 Ha. The groundstate geometries of the bulk and surfaces were obtained by minimizing the forces on each atom until the residual forces are below 1.0 × 10−3 Ha/Å. We obtained bulk lattice parameters with a = 4.629 Å and c = 2.959 Å using a k-point grid of (4 × 4 × 6) for rutile, which are in good agreement with the experimental43 and other theoretical results.44 For anatase, the calculated bulk lattice parameters are a = 3.795 Å, and c = 9.595 Å, with a k-point grid of (4 × 4 × 2). The results also agree well with the previously reported experimental and theoretical

terminated by 2-fold and 3-fold coordinated O atoms (O2C, O3C) and 5-fold coordinated Ti atoms (Ti5C). Bulk O atoms and Ti atoms are 3-fold coordinated (O3C) and 6-fold coordinated (Ti6C), respectively. The molecular adsorption of HCHO with coverage ranging from 1/6 monolayer (ML) to a full monolayer are investigated, and the results indicated that a (3 × 1) supercell (0.33 ML) is large enough to provide adequate separation between adjacent HCHO adsorption sites. Therefore, we chose a (3 × 1) surface unit cell with four trilayers (about 12.4 Å thick), with a surface area of 8.88 × 6.55 Å2 as our model slab of the rutile (110) surface. A (110) slab with four Ti layers was found to give surface properties close to the fully converged results.48−50 In addition, an even number of Ti layers gives better results than an odd number when the entire slab is allowed to relax.49,50 All atomic positions were fully relaxed, except the atoms in the bottom trilayer of the slab, which were kept fixed at the optimized bulk positions. The Brillouin zone was sampled with the chosen Monkhorst−Pack k-points, which also ensures the convergence of the calculation. Different k-point samplings ranging from (1 × 2 × 1) to (3 × 4 × 1) were tested for the bare rutile (110) surface slabs and adsorbed systems. We found the difference between adsorption energies using k-points (2 × 3 × 1) and (3 × 4 × 1) were negligible (less than 0.001 eV). As a result, we used the k-point 8045



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sampling (2 × 3 × 1) in our calculations of the rutile (110) surface systems. We got a surface energy of 0.53 J/m2 for rutile (110). This is consistent with the previously reported DFT values ranging from 0.48 to 0.73 J/m2 using GGA methods.27,49 The anatase (001)-(1 × 1) surface was also represented by a slab in a supercell consisting of neutral (O−Ti−O) repeating trilayer units, shown in Figure 1c,d. Like the rutile (110) surface, the (001)-(1 × 1) surface is also terminated by Ti5C, O2C, and O3C atoms, while the bulk consisted of O3C and Ti6C atoms. The slab was formed from a (2 × 2) supercell (0.25 ML) with a thickness of four trilayers (about 8.0 Å thick) and a surface area of 7.59 × 7.59 Å2, giving a total of 12 atomic layers and 16 TiO2 units per supercell. The (2 × 2) supercell was proved to characterize the anatase (001) surface adsorption system successfully and used in many previous works.30,51−53 The adsorption on the (2 × 2) supercell can provide a similar lateral separation and comparable molecule repulsion between adjacent absorbed HCHO to that on the (1 × 4) reconstructed surface while it is computationally less expensive. Therefore, the (2 × 2) supercell will be adopted in this work. Moreover, a slab with a thickness of four O−Ti−O trilayers is sufficient for convergence of the (001) surface structure23,54 and has been used widely in previous studies of adsorption systems on anatase (001).29,33,35,51,53 In our slab model, the atoms in the bottom two trilayers of the slab were kept fixed at the optimized bulk positions, and the other atoms were fully relaxed. We have tested different k-point samplings from (1 × 1 × 1) to (3 × 3 × 1) for the bare (001)-(1 × 1) surface slabs and adsorbed systems. The (2 × 2 × 1) samplings grid was found to be accurate enough for our work, with less than a 0.01 eV difference in adsorption energies to the (3 × 3 × 1) k-point meshes. Therefore, we used a (2 × 2 × 1) grid for our work involving the anatase (001)-(1 × 1) surface. A surface energy of 0.98 J/m2 was obtained for the (001)-(1 × 1) surface slab, which is consistent with the previously reported values ranging from 0.90 to 0.98 J/m2 using GGA methods.21,23,33 To simulate the reconstructed anatase TiO2(001)-(1 × 4) reconstructed surface, we used the ADM model31 of a p(2 × 4) surface cell, which consists of three trilayers (26 TiO2 per supercell), with a surface area of 7.59 × 15.18 Å2. The model used here is asymmetric, with reconstruction on only one slab surface. The atoms in the bottom layer were fixed to their bulk truncated position, while all the other atoms were allowed to move. We used meshes of (2 × 1 × 1) k-points, which were tested to be accurate enough for the (001)-(1 × 4) surfaces. The computed surface energy of the (001)-(1 × 4) reconstructed surface is 0.52 J/m2, much smaller than that of (001)-(1 × 1) and very close to the value of 0.51 J/m2 obtained by Lazzeri et al. in their ADM model.31 The much lower suface energy indicated that this reconstruction can indeed stabilize the surface to a large degree.

Figure 2. Optimized atomic configurations of the (a) rutile (110) surface and (b) anatase (001)-(1 × 1) surface. The arrows indicate the direction of displacement along the surface normal for the surface atoms during optimizing.

those of their bulks quantitatively, we observe further that the outward displacement of O3C atoms (0.13 Å) is very similar, showing a value of 0.14 Å for Ti6C. The inward movement of surface Ti5C atoms (0.20 Å) is, however, much larger than that of O2C atoms (0.06 Å). On the clean slab, the optimized Ti−O distances (Ti6C−O2C and Ti5C−O3C) are 1.825 and 1.952 Å, respectively. Similar to the rutile (110) surface, the Ti5C atoms on the anatase (001)-(1 × 1) surface are also relaxed inward (0.04 Å for the Ti5C). The major difference in the case of the rutile (110) surface is that O2C atoms are displaced outward and O3C atoms are displaced inward in the (001)-(1 × 1) surface (0.15 Å for the O2C and 0.006 Å for O3C atoms). We found the mirror plane symmetry along the [100] direction to be broken. The two O2C−Ti5C bonds that connect with a 2-fold coordinated bridge atom become strongly inequivalent; the two optimized Ti−O bonds are 1.742 and 2.233 Å. The distance between in-plane O and Ti atoms (Ti5C−O3C) is 1.948 Å. These results agreed very well with available theoretical reports.22,23,52,55 3.2. Adsorption Configurations of HCHO on TiO2. 3.2.1. Adsorption of HCHO on Rutile (110) Surface. We first investigated HCHO adsorption on the rutile (110) surface at 0.33 ML surface coverage. The system consisted of one HCHO molecule over the (3 × 1) surface unit cell. Four possible adsorption configurations of HCHO were examined. Theoretical, as well as spectroscopic, results have indicated that the most favorable configuration for HCHO adsorption over stoichiometric oxides is normal to the surface with the oxygen bound to the metal cation.56−58 Hence, in all adsorption modes, we expected atoms with a negative charge to interact with those with a positive charge, and vice versa. Four adsorption configurations shown in Figure 3, that is, two single O−Ti5C connections, a diagonal-span bonding via C−O2C and O−Ti5C connections, and a bridged bonding via C−O2C and O−Ti5C connections, were found to be stable. In the configuration notation used in this work, the first atom refers to HCHO; the second atom refers to the TiO2 surface. HCHO adsorbs at the bridging site on the rutile (110) surface through O−Ti5C and C−O2C bonds with the CO bond along the orientation of [1̅10] on the rutile (110) surface, denoted as type A1 (Figure 3a,b). This configuration was found to be the most stable molecularly adsorbed structure on the (110) surface with an adsorption energy of 1.10 eV. Interactions between the O atom of HCHO and surface Ti5C, and between the C atom of HCHO and surface O2C contribute to the stabilization of this adsorption structure. The (110) surface is perturbed by the O−Ti5C bond, resulting in the surface Ti5C atom being pulled out of the (110) surface plane by 0.56 Å. Bond lengths of C−O and O−Ti5C are 1.424 and 1.814 Å (Table 1), respectively, comparable to the values of

3. RESULTS AND DISCUSSION 3.1. Relaxation of Rutile (110) and Anatase (001) Surfaces. The displacement direction of atoms on the clean rutile (110) surface and anatase (001)-(1 × 1) surfaces during optimizing are presented in Figure 2. We compared optimized surface structures with their bulk counterparts (Figure 1) and found that atoms in the outermost layers of both surfaces are displaced noticeably. On the optimized rutile (110) surface, Ti6C and O3C atoms are shifted outward along the surface normal, while Ti5C and O2C atoms are displaced inward (see Figure 2a). By comparing the positions of surface atoms with 8046



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Figure 3. Optimized adsorption geometries of a HCHO molecule on the TiO2 rutile (110) surface at 0.33 ML coverage: (a) side view and (b) top view of adsorption type A1, (c) side view and (d) top view of adsorption type A2, (e) side view and (f) top view of adsorption type A3, (g) side view and (h) top view of adsorption type A4. The C atoms are yellow, and H atoms are blue (this notation is used throughout this paper).

Table 1. Relaxed Structural Parameters and Adsorption Energies for Different HCHO Adsorption Configurations on Rutile TiO2(110) at 0.33 ML Coveragea O−Tisurf (Å) A1 A2 A3 A4

1.814 1.860 2.286 2.298

C−Osurf (Å)

C−O (Å)

C−H (Å)

∠H−C−H (deg)

∠O−C−Osurf (deg)

∠C−O−Tisurf (deg)

Eads (eV)

1.429 1.484

1.424 1.383 1.228 1.216

1.104 1.108 1.108 1.112

111.35 110.13 118.56 117.76

113.06 105.78

126.49 104.13 128.66 177.46

1.10 0.40 0.56 0.35

figure Figure Figure Figure Figure

3a,b 3c,d 3e,f 3g,h

The subscript “surf” indicates that the atom is in the surface, which is bonded to the HCHO molecule; otherwise, it belongs to the HCHO molecule. This notation has the same meaning in the other tables.

a

Figure 4. Optimized geometries of the HCHO molecule adsorption on the TiO2 anatase (001)-(1 × 1) surface at 0.25 ML coverage: (a) side view and (b) top view of adsorption type B1, (c) side view and (d) top view of adsorption type B2, (e) side view and (f) top view of adsorption type B3, (g) side view and (h) top view of adsorption type B4.

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Table 2. Relaxed Structural Parameters and Adsorption Energies for Different HCHO Adsorption Configurations on Clean Anatase TiO2 (001)-(1 × 1) Surface at 0.25 ML Coverage O−Tisurf (Å) B1 B2 B3 B4

1.806 1.863 2.245 2.196

C−Osurf (Å)

C−O (Å)

C−H (Å)

∠H−C−H (deg)



Eads (eV)

1.397 1.450

1.391 1.396 1.224 1.219

1.109 1.106 1.110 1.111

109.69 110.84 118.41 118.14

112.07 115.09

152.62 133.06 128.25 173.17

1.91 0.49 0.43 0.33

1.42 and 1.83 Å obtained by Haubrich et al.26 The slight differences may be due to different lattice constants and pseudo potentials adopted in our work. The distance between the bonding O2C atom and the bonding Ti5C atom is 3.066 Å, which is shrunk by 0.51 Å after adsorption of HCHO. The adsorbed HCHO forms a dioxymethylene (CH2O2) structure59 by combining with a surface oxygen atom. HCHO can also molecularly adsorb to the rutile (110) surface through O−Ti5C and C−O3C bonds with the CO bond along the diagonal orientation of the supercell, denoted as type A2 (Figure 3c,d). This adsorption mode causes a slight perturbation of the (110) surface, with surface O3C and Ti5C atoms having been pulled out of the (110) surface plane by 0.41 and 0.40 Å, respectively. However, this mode has a relatively low adsorption energy (0.40 eV), indicating that it is quite a weak interaction. The reason may be the presence of bridging oxygen electrostatically repulsing the absorbed formaldehyde,58 and the O3C atom, bonding to HCHO, having the saturated coordination. The other two adsorption modes are HCHO molecularly adsorbed to a surface Ti5C atom solely through an O−Ti5C bond, with carbonyl slantwise (type A3) or perpendicularly (type A4) pointing to the Ti5C atom, as shown in Figure 3e−h. The adsorption energies of these two modes are 0.56 and 0.35 eV, respectively, much weaker than the dioxymethylene configuration. The height between the O atom of HCHO and the surface Ti5C atom is 2.286 Å for the slantwise type, and 2.298 Å for the perpendicular type, respectively. The bonded surface Ti5C atom is only 0.12−0.13 Å higher than the other nonbonded Ti5C atoms on the surface. These two adsorption modes cause nearly no perturbation of the rutile (110) surface, and the structure of the HCHO molecules has almost no change, indicating that they are physisorption modes. 3.2.2. Adsorption of HCHO on Anatase (001)-(1 × 1) Surface. Like the rutile (110) surface, four similar adsorption configurations of HCHO on the anatase (001)-(1 × 1) surface were investigated, based on electrostatic matching. Other adsorption modes were also investigated, such as H−O2C, but were found to be unstable. The optimized adsorption structures are shown in Figure 4. All four adsorption configurations, including two bonding via a single O−Ti5C connection, a diagonal-span bonding via the C−O′2C (here, O′2C denotes the next-neighbor 2-fold coordinated O atom on the surface, as shown in Figure 4) and O−Ti5C connections, and the bridged bonding via C−O2C and O−Ti5C connections were found to be stable. The first adsorption mode on anatase (001)-(1 × 1), which is the most stable adsorption configuration of HCHO on this surface, was identified as Type B1 (Figure 4a,b). In this configuration, the O atom of HCHO is bonded to Ti5C, and the C atom is bonded to O2C, with the CO bonded along the orientation of [100] on anatase (001)-(1 × 1). The adsorption energy is 1.91 eV, which is much higher than that on the rutile (110) surface with value of 1.10 eV (Table 1). The surface O2C

figure Figure Figure Figure Figure

4a,b 4c,d 4e,f 4g,h

atom has been completely pulled out from the surface and has a displacement of 1.67 Å with respect to its original position. The distance between the bonded O2C and Ti5C atoms is stretched by 1.61 Å after adsorption. The O−Ti5C and C−O2C bond lengths are 1.806 and 1.397 Å (see Table 2), respectively. The Ti5C atom, bonded to the molecule, and the O2C atom have also been repulsed along the opposite orientation of formaldehyde, by 0.12 and 0.26 Å, respectively. The length of the carbonyl bond (CO) of the formaldehyde molecule is 1.391 Å, about 16% longer than in the gas phase (1.214 Å). As a result, the absorbed HCHO molecule is distorted to form a dioxymethylene (CH2O2) structure by combining with a surface oxygen atom. Compared with the adsorption of HCHO on the rutile (110) surface, adsorption geometries of bridged configurations on both surfaces are very similar. Like type A1, it is a chemisorption case. However, HCHO adsorbed on the (001)-(1 × 1) surface is much stronger than that on the (110) surface. This can be attributed to all surface Ti atoms being 5fold coordinated on the anatase (001)-(1 × 1) surface, while only 50% of Ti4+ ions are 5-fold coordinated on the rutile (110) surface. Thus, the anatase (001)-(1 × 1) surface is more favorable to absorb formaldehyde molecule. The second adsorption structure on the anatase (001)-(1 × 1) surface occurs via C−O′2C and O−Ti5C connection interactions (denoted as Type B2, Figure 4). Like the A2 structure, we found this adsorption mode to be relatively weak and having an adsorption energy of only 0.49 eV (Figure 4c,d). This geometry is similar to the A2 geometry (Figure 3a,b), except the C atom of HCHO is not bonded to an O3C, but to an O′2C atom (O′2C denotes the 2-fold coordinated O atom that is next-neighbor to the HCHO). Perturbed by this mode, the surface O′2C atom has been pulled out from its original position by 1.29 Å. We can find that two chemical bonds form in the adsorption; large structural deformations can be observed in both the HCHO molecule and the TiO2 surface. The CO bond of the HCHO is elongated to match the surface lattice; meanwhile, the surface lattice is also distorted to match the HCHO molecule. Though it has bonded with both the under-coordinated Ti atom and O atom, this distortion decreases the stability of the adsorption system and makes the adsorption energy much lower than that of Type B1. HCHO can also weakly adsorb at a surface Ti5C site solely through an O−Ti5C bond, denoted as Types B3 and B4 (Figure 4e−h). In Type B3, HCHO has a tilted configuration with the O atom pointing to a Ti5C atom, with an adsorption energy of 0.43 eV. In Type B4, the carbonyl of HCHO points to the Ti5C atom perpendicularly, with an adsorption energy of 0.33 eV. The bond length of CO in both configurations is ∼1.22 Å, with an elongation of less than 1% with respect to a free HCHO molecule. The height between the O atom of HCHO and the surface Ti5C atom is 2.245 and 2.196 Å, respectively. In both cases, the surfaces and HCHO molecules have nearly no distortion, suggesting that these configurations are in a typical physisorbed state. 8048



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Our calculation results are consistent with previous experimental ones.56,59−61 The TPD experimental results of HCHO adsorption on a fully oxidized TiO2 surface were suggestive of two adsorption modes in which one is an η1 configuration (physisorbed state) and the other an η 2 configuration (chemisorbed structure).59,60 As the strong chemisorbed product, dioxymethylene species have been observed, following HCHO adsorption on TiO2,56,59−62 ZnO,63 MgO,64 and CeO2,65,66 as well as on silica, pure and fluorided alumina, magnesia, thoria, zirconia, iron oxide, and ZnAl2O4.56,62,67 It was suggested that molecular adsorption of HCHO occurs through σ lone-pair donation from the oxygen of carbonyl to Lewis acid sites (in the present case, Ti4+ surface cations).60,68 As a result of this adsorption, C of the carbonyl becomes more electrophilic, favoring an attack from a nucleophilic surface oxygen ion to form dioxymethylene species. Our theoretical calculations confirm that formation of dioxymethylene, after HCHO is adsorbed on the TiO2 surface, is the most stable adsorption mode. 3.2.3. Adsorption of HCHO on Anatase (001)-(1 × 4) Reconstructed Surface. Figure 5 shows the ad-molecule

Figure 6. Optimized adsorption structures of HCHO on different sites of the anatase (001)-(1 × 4) reconstructed surface: (a) chemisorption (type C1) and (b) physisorption configurations (type C2) on the top of a ridge; (c) chemisorption on the side of a ridge (type C3); (d)−(f) are several chemisorption configurations on the terrace (types C4− C6). All the configurations are thermodynamically favorable.

the ridge and one of the H atoms pointing to the adjacent O2C atom, which has an adsorption energy of 0.48 eV (type C2, Figure 6b). The strongest adsorption structure for adsorbed HCHO on the reconstructed (1 × 4) surface (denoted as type C1; see Figure 6a), which, similar to the results of on the (1 × 1) surface, exhibits the breaking of a Ti4C−O2C−Ti4C bridge and the formation of a dioxymethylene structure, in which the O atom of HCHO bonding to a Ti4C site, and the C atom bonding to a ridge O2C site. As expected, the ridges of the (001)-(1 × 4) reconstructed surface are found to be much more reactive to absorb the HCHO molecule than the terraces. The adsorption energy at a Ti4C ridge site is 2.18 eV, which is significantly higher than at a (1 × 1) Ti5C site, and well in agreement with previous works.33,35 It was reported that the adsorption abilities of the adsorption of some other molecules on the ridge are all much higher than that on the terrace. For example, water and formic acid dissociate spontaneously on the (1 × 4) reconstructed surface, which occurs predominantly on the ridge,33,69 which is supported by the experimental work.37 The dissociation of dimethyl methylphosphonate on the (1 × 4) reconstructed surface and formation of a stable TiO titanyl group have also been reported35 to occur at Ti4C ridge sites, but not at Ti5C terrace sites, with a much larger adsorption energy than that on the terrace. The origin of this phenomemon may be due to the highly coordinatively unsaturated Ti atoms (Ti4C) that only bind to four oxygen atoms on the bridging TiO3 chain (ridge). Despite that the ridges have high reactivity, this reconstruction can still stabilize the surface obviously. As we mentioned before, the computed surface energy of the reconstructed (001)-(1 × 4) surface is 0.52 J/m2, much smaller than that of the (001)-(1 × 1) unreconstructed one. Gong et al.33 attribute this stability to the fact that it leads to a strong relief of the surface stress after the surface reconstruction. Indeed, the calculated longest Ti5C−O2C bonds on the original (1 × 1) surface are 2.233 Å, while those on the terrace of the

Figure 5. Relaxed structure of the anatase (001)-(1 × 4) reconstructed surface: (a) view along the [010] direction, i.e., parallel to the ridge; (b) view along the [100] direction, i.e., vertical to the ridge.

(ADM) model31 for anatase (001)-(1 × 4) reconstructed surfaces, which is energetically much more stable than the unreconstructed one. In this model, one of four O2C rows in the [010] direction is replaced with TiO3 units to form three O2C and one Ti4C site per (1 × 4) unit cell. The “ridges”, which consist of the original bridging atoms and added TiO2 units (“ad-molecules”), are separated by a “terrace” with widths of three lattice constants. On top of the ridge, rows of bridging atoms emerge, which are vertical to the rows on the perfect surface. On the ridge, each Ti atom (Ti4C) then binds to four oxygen atoms only and is, therefore, highly coordinatively unsaturated. As a result, a Ti4C ridge site was proved to be more reactive than a Ti5C terrace site in previous work.33,35,37,69 Relaxed adsorption structures of HCHO on the TiO2 anatase (001)-(1 × 4) reconstructed surface are shown in Figure 6, which we denoted as types C1−C6. Table 3 lists the structural parameters of the configurations and indicates that the HCHO can be adsorbed on either the ridge sites or the terrace sites as chemical adsorption modes. On the terrace sites, the HCHO molecule can adsorb at the middle part of the terrace (type C5, Figure 6e) with an adsorption energy of 0.48 eV, stronger than other sites (types C4 and C6, Figure 6d,f). It is much less stable than that on the top of ridge sites (type C1, Figure 6a), which is the strongest adsorption site of HCHO on reconstructed (1 × 4) surface. The adsorption of HCHO on the side of a ridge is quite weak (type C3, Figure 6c), with an adsorption energy of only 0.04 eV, implying that this configuration can be hardly formed. HCHO can also absorb on a ridge in a physical mode, with the carbonyl group aslant standing on the top of Ti4C in 8049



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Table 3. Relaxed Structural Parameters and Adsorption Energies for Different HCHO Adsorption Configurations on Clean Anatase TiO2 (001)-(1 × 4) Reconstructed Surface in a p(2 × 4) Supercell C1 C2 C3 C4 C5 C6

O−Tisurf (Å)

C−Osurf (Å)

C−O (Å)

C−H (Å)

∠H−C−H (deg)



Eads(eV)

1.798 2.289 1.900 1.981 1.892 1.895

1.395

1.394 1.229 1.374 1.399 1.384 1.377

1.107 1.109 1.106 1.106 1.108 1.106

109.96 121.31 111.88 110.92 110.78 111.04

111.10

153.89 130.00 98.02 106.43 104.72 103.43

2.18 0.48 0.04 0.20 0.48 0.35

1.501 1.438 1.444 1.462

102.96 105.42 103.70 102.60

figure Figure Figure Figure Figure Figure Figure

6a 6b 6c 6d 6e 6f

Table 4. Calculated and Experimental Vibrational Frequencies (cm−1) for Gas-Phase and Adsorbed HCHO in Different Adsorption Modes νas(CH2)

modes dioxymethylene species chemisorption

gas phase physisorption

ref 56 refs 60, 73, 74 A1 B1 A2 B2 C1 modes νas(CH2) refs 71, 72 this work A3 B3 A4 B4 C2

2843 2833 2979 2946 2915 2929 2977

2950 2945 2978 2907 3004 2937 2923

νs(CH2) 2868 2882 2920 2873 2815 2892 2884 νs(CH2) 2782 2782 2876 2847 2836 2846 2854

ν(CO)

ρ(CH2)

1172, 1156, 1113, 1070 1086 1186 1034 1064 1028 1062 1086 1057 1061 1044 1053 1071 ν(CO) δ(CH2) 1746 1758 1693 1708 1763 1740 1681

1500 1477 1460 1463 1479 1472 1410

δ(CH2) 1482, 1464 1473 1435 1437 1445 1442 1448 ω(CH2) 1249 1216 1212 1217 1212 1208 1209

τ(CH2) 1300 1302 1352 1359 1330 1349 1363 γ(CH2) 1167 1145 1149 1144 1144 1123 1175

Figure 7. Local density of states (LDOS) for the adsorption of HCHO on TiO2 surfaces. DOS projected on the surface atoms for the TiO2 surfaces, and the C and O atoms of HCHO. LDOS for (a) the gas-phase HCHO molecule, clean rutile (110) and anatase (001)-(1 × 1) surfaces, and (1 × 4) reconstructed surface. (b) HCHO adsorption on the rutile (110) surface at 0.33 ML coverage. (c) HCHO adsorption on the anatase (001)-(1 × 1) surface at 0.25 ML coverage. (d) HCHO adsorption on the anatase (001)-(1 × 4) reconstructed surface in a p(2 × 4) supercell. The Fermi level is set to 0 eV.

reconstructed surface have lengths of 1.81−1.85 Å. The shortened bonds indicate that the strength of those terrace bonds has increased. As a consequence of the stronger surface bonds, the Ti5C and O2C atoms on the terrace of the reconstructed surface become much less reactive and make the reconstructed surface more stable. Since the reactivity of anatase (001)-(1 × 4) is mainly determined by ridges, the later discussion of properties of the (1 × 4) surface is just concerned with adsorptions of HCHO on the top of ridges (C1 and C2) in the subsequent part of this work. 3.3. Vibrational Frequency Analysis. We also performed vibrational frequency calculations for different adsorption modes of HCHO on both rutile (110) and anatase (001)

surfaces. The calculated vibrational frequencies of adsorbed and free HCHO are given in Table 4, together with experimental values for free HCHO. It is found that the asymmetric and symmetric CH2 stretching vibrational frequencies of both chemisorption and physisorption modes are calculated to be blue shifted by 30−170 cm−1 compared with those of free HCHO molecules. Meanwhile, upon the chemisorption modes, the coupling of the HCHO molecule and the surface leads to distinct red shifts for the stretch frequency of the carbonyl bond and rocking frequency of the CH2 group by 670−730 and 150−170 cm−1, respectively. The wagging frequency of the CH2 group showed an obvious blue shift of 180−210 cm−1, while that of physisorption modes showed nearly no shift. It 8050



Article

degree of electron sharing between HCHO and the surface. These peaks are contributed by both HCHO and TiO2 surface atoms. When the HCHO molecule is adsorbed on the Ti5C site, a strong hybridization between HCHO 2p and the Ti 3d at the bottom of the valence band orbital have taken place, which results in an observable change of electronic properties of the surface. The LDOS for HCHO adsorption on the anatase (001)(1 × 4) reconstructed surface are given in Figure 7d. The higher peaks of Ti4C and broader orbital overlap of O2C show a much stronger interaction than that on the (001)-(1 × 1) unconstructed surface. The bands of the O atom in HCHO are nearly the same to that of the ridge O2C, which, bonded to the C atom, is derived from the formation of the quite symmetrical structure of dioxymethylene. In fact, the two Ti−O bonds that bond to dioxymethylene are all 1.80 Å. Considering the presence of such an active O2C and of the highly coordinatively unsaturated Ti4C as well, we expect that the ridges of the reconstructed anatase (1 × 4) surface may play an important role in improving sensing abilities to act as a HCHO gas sensor. The LDOS for the physical adsorption on the anatase (001) unreconstructed and reconstructed surfaces are also very similar, showing the very weak interactions between the HCHO and surfaces. In these cases, only the O atoms of HCHO contribute to a weak covalent interaction with the outof-plane Ti states. CDD plots shown in Figure 8 indicate how the charges redistribute upon adsorption. The CDD is defined as the

was reported that the bands shifted to the region of 1200− 1500 cm−1 were assigned to characteristic frequencies of adsorbed HCHO on the TiO2 surface,25,56,70 and the bands at 2865− 2870, 2755−2766, 1454−1459, 1301−1303, 1115−1115, and 1035−1062 cm−1 were attributed to dioxymethylene species on the surface.60 For the physical adsorption mode, the calculated frequencies are also close to the corresponding experimental values.71,72 Our results are in accord with experimental results and confirm the formation of dioxymethylene species for the strong adsorption modes on the TiO2 surfaces. 3.4. Electronic Structures. To further understand adsorption modes of HCHO on rutile (110) and anatase (001) surfaces, we analyzed the local density of states (LDOS), charge density difference (CDD) induced by adsorption, and Löwdin charge distributions of the energetically most favorable adsorption configurations of chemisorption and physisorption. Figure 7a shows the LDOS for gas-phase HCHO, clean rutile (110) and anatase (001)-(1 × 1), and the (1 × 4) reconstructed surface. The LDOS were taken for the toplayer O/Ti atoms of surfaces, and C/O atoms of HCHO. The HCHO gas-phase peaks are very sharp, typical for a molecule. There are some similarities between the LDOS for rutile (110) and anatase (001)-(1 × 1) surfaces. From the DOS further projected onto different O species of the slab, we can see that the higher-energy edge of the valence band is mainly constituted by states from O2C atoms on the surfaces, while electronic states from middle-layer (bulk) O contribute to the lower part of the band. Therefore, we can conclude that reactive activity of these surfaces is actually formed mainly by the surface O2C. In the case of the clean (1 × 4) reconstructed surface, the states of terrace O2C atoms are shifted to lower part, indicating that terrace atoms are stabilized by surface reconstruction. The higher-energy end of the valence band is occupied by the states of ridge O2C. This feature confirms that the ridges contributed the high reactivity of the anatase (1 × 4) reconstructed surface and agrees with both previous experimental21 and theoretical29 studies of the TiO2 surface properties. In Figure 7b, we show the LDOS for HCHO adsorption on the rutile (110) surface. For the chemisorption bonding case, we can see the obvious covalent character. There is some broadening of HCHO O bands, as the atom interacts with the surface Ti5C atom. An even larger change can be seen with HCHO C bands. The overlap between C bands and O2C is considerable and a large reorganization of C bands shows strong bonding between C and O2C atoms taking place. In contrast, a primarily electrostatic character can be seen in physisorption modes. In this case, O bands of HCHO and Ti5C bands are nearly unperturbed by adsorption of HCHO. Figure 7c shows the LDOS for adsorption on the anatase (001) unreconstructed surface. The significant changes after adsorption indicate that the interaction between the HCHO molecule and the surface is strong. After adsorption, the states of HCHO are broadly dispersed. Dispersion of this kind is generally associated with electron delocalization, suggesting that a number of HCHO molecular orbitals have overlapped with TiO2 valence bands. DOS of the adsorbed HCHO move to the lower-energy state, and the movement is very big. Meanwhile the changes of states near the Fermi level are very obvious (Figure 4c). For LDOS of the slab, we can see that, after HCHO adsorption, two new local energy levels occur; one locates in the middle of the band gap and the other locates near the bottom of the valence band, suggesting a

Figure 8. CDD for HCHO adsorption on the TiO2 surface: CCD for (a) chemisorption (type A1) and (b) physisorption (type A3) on rutile (110), (c) chemisorption (type B1) and (d) physisorption (type B3) on clean anatase (001)-(1 × 1), (e) chemisorption (type C1) and (f) physisorption (type C2) on ridge of the anatase (001)-(1 × 4) reconstructed surface. Orange and blue colors represent positive (gaining electrons) and negative (losing electrons) values, respectively. The isosurface value of each case is 0.01 e/bohr.3.

difference between charge on the clean surface, gas-phase HCHO, and surface with adsorbed HCHO. Clearly, a large amount of charge transfer can be seen between the HCHO molecule and the TiO2 surface in chemisorption modes. The charge transfer mainly occurs between C and surface O2C atoms and O and Ti5C atoms, which is in agreement with the LDOS 8051



Article

Between the two experimentally observed surfaces of rutile (110) and anatase (001), the latter is found to have a stronger interaction with HCHO with a larger charge transfer, which changes the surface conductivity of TiO2 but ultimately results in a sensitivity enhancement. Among the cases of anatase (001) surfaces, the (1 × 4) reconstructed surface shows a much higher adsorption reactivity than that on the (1 × 1) unreconstructed surface. It has a surface energy of only 0.52 J/m2, implying that it is much more stable than the unreconstructed (001) surface. Therefore, improving the proportion of the (1 × 4) reconstructed surface may improve not only the reactivity but also the stability. These results indicate that a careful preparation of novel anatase TiO2 crystals with a large amount of (001) facets may be important for further improvement of sensing properties of titania-based gas sensors.

analysis. However, very little charge rearrangement occurs between HCHO and the surface in physisorption modes. Table 5 lists the changes of the Löwdin charge population for a typical HCHO molecule during the adsorption. From this Table 5. Changes of Löwdin Charge for HCHO Molecule on Both Rutile (110) and Anatase (001) Surfacesa configuration

q(CHCHO)

q(OHCHO)

q(HCHO)

q(Ti−O)b

q(O−C)b

A1 A3 B1 B3 C1 C2

0.035 −0.022 0.003 −0.018 −0.002 −0.022

0.180 0.017 0.022 −0.012 0.247 0.020

0.111 −0.080 0.139 −0.091 0.154 −0.088

−0.003 −0.016 0.017 −0.014 0.037 0.004

−0.042 −0.225 −0.289

a

Positive values mean gaining electrons, while negative values mean losing electrons. bTi−O means the surface Ti atom bonding with the O atom of HCHO, the same meaning for O−C.

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AUTHOR INFORMATION

Corresponding Author

table, we can see that the HCHO molecule gains electrons from surface TiO2 atoms in chemisorption modes, whereas it loses electrons in physisorption modes. The numbers of obtained electrons in the two rutile (110) cases are 0.111 e (A1) and −0.080 e (A3), respectively, whereas those in the two anatase (001)-(1 × 1) cases are 0.139 e (B1) and −0.091 e (B2), respectively. This clearly shows that electron transfer on the (001) surface is larger than that on the (110), which is in accord with the above results that adsorption of HCHO on the anatase (001) surface is energetically more favorable. In the case of chemisorption on the ridge of anatase (001)-(1 × 4), whether on the HCHO molecule or on the surface, we can see an obviously larger electron transfer than that on the (001)(1 × 1) surface, confirming the fact of a more reactive ability for the ridges. Further, the large charge transfer may cause depletion of charge surrounding the TiO2 surface, thereby impeding possible current flow across the surface. These electrons produced by the surface reactions may change the conductivity of the surface, thereby resulting in a change of output voltage, as generally observed at the working stage of a sensor.

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work described in this paper was fully supported by Grants from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 9041674, CityU 118411), and the China National Natural Science Foundation (Grant No. 11172253).

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4. CONCLUSIONS Using the DFT method, we have systematically studied HCHO adsorption on rutile (110) and anatase (001) surfaces aimed at determining stable configurations, estimating adsorption energetics, and providing insights into electronic structures of adsorbed surfaces. On both rutile (110) and anatase (001)-(1 × 1) as well as (1 × 4) reconstructed surfaces, we have found that the most stable adsorption structure of HCHO on these surfaces leads to breaking of a Ti−O−Ti bridge on the surfaces and formation of a dioxymethylene structure, in which the O atom of HCHO is bonding to a coordinatively unsaturated surface Ti atom and C bonding to a coordinatively unsaturated surface O atom. These configurations have the highest adsorption energy on all surfaces, suggesting that the dioxymethylene structure is an important product of HCHO adsorption, which has been widely observed in previous experimental work. The carbonyl of dioxymethylene is longer by 14−17% than that of the free HCHO molecule, indicating that the interaction between C and O atoms has been weakened and it is easy to be decomposed, which may be an important process in the degradation of HCHO. 8052



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