Adsorption Configurations and Reactions of Nitric Acid on TiO2 Rutile

Mar 19, 2009 - Center for Interdisciplinary Molecular Science, Institute of Molecular Science, National .... National Center for High-performance Comp...
0 downloads 0 Views 2MB Size
Download PDF
6140

J. Phys. Chem. C 2009, 113, 6140–6149

Adsorption Configurations and Reactions of Nitric Acid on TiO2 Rutile (110) and Anatase (101) surfaces Ching Yi Chang,† Hsin-Tsung Chen,*,‡ and M. C. Lin*,†,§ Center for Interdisciplinary Molecular Science, Institute of Molecular Science, National Chiao Tung UniVersity, Hsinchu 300, Taiwan, National Center for High-performance Computing, No. 28, Nan-Ke Third Road, Hsin-Shi, Tainan, 74147, Taiwan, and Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322 ReceiVed: December 3, 2008; ReVised Manuscript ReceiVed: February 1, 2009

The adsorption and reactions of the monomer and dimer of nitric acid on TiO2 rutile (110) and anatase (101) surfaces have been studied by first-principles density functional theory with ultrasoft pseudopotential approximation. The most stable configuration of HNO3 on the rutile surface is a molecular monodentate adsorbed on the 5-fold coordinated Ti atom with the hydrogen bonded to a neighboring surface bridging oxygen with the adsorption energy of 6.7 kcal/mol. It can dissociate its H atom to a nearest bridged oxygen with almost no barrier to produce NO3(a) + H(a). The rotation of NO3 requires a barrier of 12.2 kcal/mol to form the didentate configuration, Ti5c-ON(O)-Ti5cH-O2c(a), which adsorbs on two 5-fold coordinated Ti atoms with the adsorption energy of 16.5 kcal/mol. In the case of the adsorption of 2HNO3 molecules, the most stable configuration, 2(Ti5c-ON(O)OH...O2c(a)), has a structure similar to two single HNO3 adsorbates on two 5-fold coordinated Ti atoms with the adsorption energy of 12.8 kcal/mol, which is about twice that of the single HNO3 molecule. The result suggests that the interaction of the two planar HNO3 adsorbates is negligible. The dehydration from 2(Ti5c-ON(O)OH...O2c(a)) forming N2O5(a) + H2O(a) requires an energy barrier of 46.2 kcal/mol, indicating that the dimerization of the two HNO3(a) is difficult. Similar adsorption phenomena appear on the anatase (101) surface. In addition, we find that the coadsorption of hydrogen plays a significant role in the adsorption energies of adsorbates, especially for the NO3 radical, which may be employed as a linker between semiconductor quantum dots such as InN and the TiO2 surface. 1. Introduction Because of the rising costs of fossil fuels and the detrimental effect of greenhouse gases, CO2 in particular, produced from fossil fuel combustion, it is vital for us to search for environmentally clean alternative energy sources.1,2 A renewable energy such as solar radiation is one of the more promising possibilities for a future energy source but requires new strategies to harvest incident photons with a higher efficiency, for example, by employing nanostructured semiconductors and molecular assemblies. In 1991, Gra¨tzel et al.3 invented the dye-sensitized solar cells (DSSC) system using TiO2 nanoparticle films. Organic dye molecules were attached onto the porous TiO2 structures as absorbate which provided an alternative photovoltaic device for electricity generation. The efficiency may reach over 10% in a DSSC system.3 However, dyes are relatively expensive, and organic compounds have shorter lifetimes and are less stable than inorganic materials. Many researchers,4-6 therefore, prefer to employ semiconductor quantum dots (QD) instead of dyes for solar cells (by InP, InAs, PbS, and CdS, for example). Further studies of quantum-dot sensitized TiO2 solar cells (QDSSC), by which their optical properties can maximize solar photon adsorption, have been carried out; for example, Wang et al.7 reported that the absorption spectrum of InN/TiO2 films showed a pronounced broadband absorption in the UV-vis * Corresponding author. E-mail: [email protected] (H.-T.C.) and [email protected] (M.C.L.). † National Chiao Tung University. ‡ National Center for High-performance Computing. § Emory University.

range, covering 390-800 nm, similar to the results of Gra¨tzel’s black dye. TiO2 solar cell with InAs8 quantum dots has also been studied. Moreover, TiO2 has been studied broadly for its physical and chemical characteristic and applications, such as in heterogeneous photocatalysis on surfaces,9,10 the nature of photovoltaic action,11,12 and nanomaterial13 properties. All of the previous information provides a rich scientific basis on its potential solar cell development. In order to achieve higher efficiencies, it is essential to find a good linker between quantum dots (InN, InAs,...) and the TiO2 surface. Lin and co-workers have studied the HN3/TMIn14,15 reaction to produce InN quantum dots on TiO2 fabricated films and have analyzed adsorption/reactions of H3BO3,16a BClX,16b NH3,16c H2S,17 and H2O218 experimentally and theoretically. In the present work, we investigate the adsorption and reaction mechanisms of monomer and dimer of nitric acid (HNO3) on TiO2 rutile (110) and anatase (101) surfaces by first-principles calculations. 2. Computational Methods All geometrical structures are calculated by VASP (Vienna ab initio Simulation Package)19 on the basis of density functional theory.20 The exchange-correlation function is treated with the generalized gradient approximation (GGA)21 with spin-polarized Perdew-Wang 1991 (PW91)22 formulation, which has been shown to work well for surfaces. The interaction between ions and electrons is described by ultrasoft Vanderbilt pseudopotentials (US-PP) with a plane basis set. During the calculations, the cut off energy (Ecut) is set at 600 eV, which is bigger than

10.1021/jp810635h CCC: $40.75  2009 American Chemical Society Published on Web 03/19/2009

Nitric Acid on TiO2 Rutile (110) and Anatase (101)

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6141

Figure 1. Side and top views for model structures of (a) rutile 1 × 2 × 2 and (b) anatase 2 × 1 × 2 super cell containing of 16 titanium and 32 oxygen atoms.

real kinetic energies h2k2/8π2m in a convergent system. The Monkhorst-Pack k-points based on the Brillouin zone are chosen by 4 × 4 × 1 and 4 × 5 × 1 for rutile and anatase surfaces, respectively. Previously, we have successfully studied and accounted for the adsorption and reactions of HN3 with TMIn14,15 and the adsorption/reaction of H3BO316, H2S17 and H2O218 on the TiO2 surface using the same computational methods as described here. In the following calculations, the slab model is applied, respectively, to study the interaction between nitric acid and the TiO2 rutile (110) surface as well as nitric acid and the anatase (101) surface.23 In order to avoid the interaction of large-size molecules with each other, we use 1 × 2 × 2 and 1 × 4 × 1 supers with Ti16O32 of rutile, for HNO3 monomer/TiO2 and dimer/TiO2 systems shown in Figure 1a and Figure S1a. The surface sizes are 6.495 Å × 5.866 Å and 6.495 Å × 11.732 Å, respectively. For the anatase surface, the models used are the 2 × 1 × 2 super cell shown in Figure 1b for HNO3 monomer/ TiO2 system and 3 × 1 × 2 super cell for dimer HNO3/TiO2 system shown in Figure S1b. In the 2 × 1 × 2 super cell with Ti16O32, the size of the surface is 7.57 Å × 10.239 Å. In the 3 × 1 × 2 super cell with Ti24O48 composition, the surface is 11.355 Å × 10.239 Å. All slabs are separated by a vacuum space of greater than 13.0 Å. This large separation is used to avoid the interaction force between the upper and the lower slabs. The whole layers of the super cell are not fixed in the following calculations in order to simulate with good accuracy.

The adsorption energy, Eads is computed according to the equation:

Eads ) -[Etotal - (Eslab + Emolecule)] where Eslab indicates the energy of the clean surface, Emolecule is the energy of a gas-phase molecule, and Etotal is the total energy of adsorbed species on the surface. For the coadsorption of an adsorbate with H, the adsorption energy is calculated as follows:

Eads ) -[Etotal/H - (Eslab/H + Emolecule)] where Eslab/H indicates the energy of the slab with an H atom adsorbed on the slab, and Etotal/H is the total energy of the slab with the adsorbate and H coadsorbed on it. A positive value of Eads > 0 suggests a stable adsorption. The spin-polarized effect is insignificant, because the difference in the adsorption energy is merely +0.05 kcal/mol with the spin-polarized parameter. The climbing-image nudged-elastic band (CI-NEB) is carried out to search for a transition state.24 Atomic charges of the optimized structures are calculated by utilizing the Bader method with a program designed by Henkelman et al.25 3. Results and Discussion 3.1. Verification. To ensure the reliability of the computational results, we first compared the calculated bulk lattice

6142 J. Phys. Chem. C, Vol. 113, No. 15, 2009

Chang et al.

Figure 2. (a) Optimized possible geometries of adsorbed H2O on (a) TiO2 rutile (110) and (b) TiO2 anatase (101) surfaces.

constants. The predicted lattice constants are a ) 4.593 Å and c ) 2.933 Å for rutile, which are in good agreement with the experimental values26 of a ) 4.594 Å and c ) 2.958 Å, while those for anatase are a ) 3.785 Å and c ) 10.239 Å, which are somewhat larger than the experimental values27 a ) 3.782 Å and c ) 9.502 Å, especially the value of c; but it is close to the theoretical values of a ) 3.785 Å and c ) 10.240 Å.16,28 Figure 1a,b shows different adsorption sites which have been labeled on the TiO2 surface. They are 5-fold coordinated titanium, 6-fold coordinated titanium, 2-fold bridging oxygen, and 3-fold coordinated oxygen, corresponding to Ti5c, Ti6c, O2c, and O3c, respectively. The 2-fold coordinated O atoms and 5-fold coordinated Ti atoms are more active than the 3-fold O atoms and the 6-fold coordinated Ti atom, respectively, because of their unsaturated coordinations. 3.1.1. Adsorption of H2O. The test models have been compared with the adsorption energies of H2O16,29 on the surface. The H2O-rutile and H2O-anatase adsorption structures and their optimized bond lengths are shown in Figure 2 and Table S1, respectively. As shown in Figure 2, the most stable structures are H2O-Ti5c-up/para(a) and H2O-Ti5c-paraleft(a) with one hydrogen bonding to the nearest bridged oxygen for the H2O-TiO2 rutile and anatase systems, respectively, among all adsorption configurations, indicating the adsorption of H2O on the Ti5c with an H atom tilting forward to the bridged surface oxygen. The adsorption energies on the rutile and anatase surfaces are 16.6 and 15.7 kcal/mol, respectively, which are somewhat smaller than other theoretical values of 18.9 kcal/ mol18 on the rutile and 19.2 kcal/mol16 on the anatase surface. Herman et al.30 in an experimental investigation showed the adsorption energies are from 0.74 eV to 0.72 eV (17.1 to 16.6 kcal/mol) for the H2O adsorbed on the anatase surface and from 0.74 to 0.64 eV (17.1 to 14.8 kcal/mol) for the H2O-rutile system due to different water coverages. Our calculated values are in very good with the experimental values.30 Other possible structures (see Supporting Information, Figure S2) with small adsorption energies of 2.3 kcal/mol on the rutile surface and

1.9 kcal/mol on the anatase surface are physisorbed states without direct interactions with Ti5c atoms. 3.2. Adsorption and Reaction Mechanism of Monomer and Dimer HNO3 on TiO2 Surface. 3.2.1. Adsorption of HNO3 and NO3 on Rutile TiO2 (110). In the present work, the adsorption of nitric acid was found to have four different configurations on different sites of the metal oxide surface (Ti5c, O3c). These adsorption configurations and their fragments on the TiO2 rutile (110) surface are shown in Figure 3, and the associated bond lengths and adsorption energies are listed in Table 1. In these four types of structures, the most stable one is a molecular configuration with an oxygen attached to the surface Ti5c atom, and the hydrogen formed a hydrogen bond with the nearest bridge oxygen on the surface indicated by Ti5c-ON(O)OH...O2c(a). Its adsorption energy is 6.7 kcal/mol and the Ti5c-O bond length is 2.428 Å and O3H1-O2c is 2.288 Å as listed in Table 1. The small adsorption energies of the exhibited four structures indicate that HNO3 interacts weakly with the TiO2 rutile (110) surface, but Ti5c-ON(O)OH...O2c(a) shows clearly that the formation of a hydrogen bond can increase the adsorption energy. For NO3 adsorption, we obtain two different configurations of ON(O)O on the rutile surface as shown in Figure 3; one is Ti5c-ON(O)O-Ti5c(a) with two oxygen atoms bonding with Ti5c atoms on the surface and the other is ON(O)O-Ti5(a) with one oxygen on a Ti5c atom. The adsorption energy of the former is 12.0 kcal/mol, and that of the latter is 4.7 kcal/mol. Thus the formation of an additional O-Ti5c bond can stabilize the structure with a higher adsorption energy. 3.2.2. Adsorption of HNO3 and NO3 on TiO2 (101) Anatase. As shown in Figure 4, three types of HNO3 adsorption configurations on the TiO2 anatase (101) surface were found. Their bond lengths and adsorption energies are shown in Table 2. The most stable structure is similar to that on the rutile (110) surface with the configuration Ti5c-ON(O)OH-O2c(a), having a hydrogen bond involving the H atom and the nearest neighboring oxygen site. Its adsorption energy is 13.3 kcal/mol,

Nitric Acid on TiO2 Rutile (110) and Anatase (101)

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6143

Figure 3. Optimized geometries of adsorbed and dissociatively adsorbed HNO3 monomer on TiO2 rutile (110) surface.

TABLE 1: Optimized Bond Lengths (Å) and Adsorption Energies (kcal/mol) for HNO3 on TiO2 Rutile (110) Surface structure

Ti5c-O1

Ti5c-O3

Ti5c-ON(OH)O-Ti5c(a) Ti5c-ON(OH)O-Ti5c-2(a) Ti5c-O(H)N(O)O-Ti5c(a) Ti5c-ON(O)OH...O2c(a) Ti5c-ON(O)O...H-O2c(a) Ti5c-ON(O)O-Ti5c,H-O2c(a) Ti5c-ON(O)O-Ti5c(a) Ti5c-ON(O)O-Ti5c-2(a)

2.550 2.707 2.520 2.428 2.006 2.164 2.194 2.255

2.554 2.547 2.651

O1H1-O2c

O2H1-O2c

O3H1-O2c

O3-H1O2c

2.393 3.740 2.288

2.153 2.230

which is larger than that on rutile. This is due to the formation of a hydrogen bond. On anatase, the bond length of hydrogen bonding is 1.529 Å in monomer HNO3 adsorption while the bond length is 2.293 Å in monomer HNO3 adsorption on the rutile (see the bond lengths of O3H1-O2c in Tables 1 and 2). The other configurations of HNO3, with two oxygen atoms connecting to Ti5c atoms on the surface, are less stable than the hydrogen-bonded structure, Ti5c-ON(O)OH-O2c(a). Regarding NO3 adsorption, we optimized two configurations on the anatase surface as shown in Figure 4. The adsorption energy of Ti5c-ON(O)O-Ti5c(a) is 6.6 kcal/mol and that of Ti5c-ON(O)O-Ti5c-2(a) is 5.6 kcal/mol. The bond lengths of Ti5c-O1 in Ti5c-ON(O)O-Ti5c(a) and Ti5c-ON(O)O-

1.632 3.485

Ti5c-Ti5c

Ti5c-O3c

Eads

2.934 2.940 2.926 2.932 2.946 2.921 2.922 2.935

1.829 1.823 1.827 1.838 2.002 1.899 1.895 1.867

6.0 5.0 5.7 6.7 8.9 16.5 12.0 4.7

Ti5c-2(a) are 2.259Å and 3.203Å, respectively. In the former, the bond length of Ti5c-O is shorter; the adsorption energy is higher. 3.2.3. Reaction Path for the Adsorption and Dissociation of HNO3 on the Rutile (110) Surface. The potential energy surface of dissociative adsorption reactions of nitric acid on the clean rutile (110) surface is shown in Figure 5. The optimized structures are shown in Figure S3, and related bond lengths are indicated in Table S2. Based on the prior calculation, the most stable adsorption configuration is considered. As seen in Figure 5, nitric acid on the clean rutile (110) surface is a monodentate structure with the hydrogen bond formed with the nearest neighboring bridging oxygen with an adsorption energy of 6.7

6144 J. Phys. Chem. C, Vol. 113, No. 15, 2009

Chang et al.

Figure 4. Optimized geometries of adsorbed and dissociatively adsorbed HNO3 monomer on TiO2 anatase (101) surface.

TABLE 2: Optimized Bond Lengths (Å) and Adsorption Energies (kcal/mol) for HNO3 and Its Fragments on TiO2 Anatase (101) Surface structure

Ti5c-O1

Ti5c-O3

O2H1-O2c

Ti5c-ON(OH)O-Ti5c(a) Ti5c-O(H)N(O)O-Ti5c(a) Ti5c-ON(O)OH-O2c(a) Ti5c-ON(O)O,H-O2c(a) Ti5c-ON(O)O...H-O2c(a) Ti5c-ON(O)O-Ti5c,H-O2c(a) Ti5c-ON(O)O-Ti5c-4(a) Ti5c-ON(O)O-Ti5c(a)

2.575 2.706 2.302 3.060 2.027 2.143 3.203 2.259

2.553 2.620

4.943

d

2.187

O2-H1O2c

O3H1-O2c

O3-H1O2c

3.776 1.529 1.456

2.159 2.281 2.311

1.485 3.133

Ti5c-Ti5c

Ti5c-O3c

O2c-Ti6c

Eads

3.801 3.806 3.808 3.814 3.838 3.764 3.798 3.755

1.809 1.805 1.847 1.813 1.971 1.902 1.809 1.876

1.858 1.854 1.845 1.838 1.845 1.836 1.846 1.834

1.1 2.0 13.3 53.4d 13.7 12.1 5.6 6.6

Dissociate.

kcal/mol. In the dissociation process, the hydrogen atom can migrate to the neighboring O2c forming Ti5c-ON(O) O...H-O2c(a); the reaction is slightly exothermic and occurs without a well-defined transition state. Afterward, the adsorbed NO3(a) can molecularly rotate on the surface. The rotation of the adsorbate Ti5c-ON(O)O...H-O2c(a) resulting in the formation of a covalently bond Ti5c-ON(O)O-Ti5c,H-O2c(a) as shown in Figure S3. This rotation barrier lies 3.3 kcal/mol above the reactants, or 12.2 kcal/mol above the adsorbate. The isomerization process from Ti5c-ON(O)O...H-O2c(a) to Ti5cON(O)O-Ti5c,H-O2c(a) is calculated to be exothermic by 7.6 kcal/mol. The transition state (TS1) of this process shown in Figure S3 and Table S2 corresponds to the formation of the Ti5c-O1 with a bond length of 2.187 Å and the formation of the O2c-H1 with 0.969 Å. According to the previous calculation summarized in Figure 5, eliminating the NO3 gas molecule from Ti5c-ON(O)O-Ti5c,H-O2c(a) to produce H-O2c(a) + NO3(g)

requires 58.7 kcal/mol which implies that the desorption process cannot occur spontaneously. NO3(a) can more easily split into NO2(a) + O(a) by breaking one of the N-O bonds, which is endothermic by about 22.1 kcal/mol. As shown in Figure 5, Ti5c-ON(O)O-Ti5c,H-Ob(a) can also be formed from Ti5c-O(H)N(O)O...Ti5c(a) which lies 5.7 kcal/ mol below the reactants. This path needs to overcome the barrier of 19.3 kcal/mol. Furthermore, Ti5c-ON(O)O-Ti5c,H-Ob(a) is not easy to emit NO(g) from the surface due to the high barrier of approximately 65.5 kcal/mol. The overall reaction of HNO3(g) + TiO2 rutile (110) f H-Ob, Ti5c-O, Ti5c-O(a) + NO(g) is endothermic by 26.6 kcal/mol. 3.2.4. Reaction Path for the Adsorption and Dissociation of HNO3 on the Anatase (101) Surface. The computed potential energy surface for the dissociative adsorption reaction of HNO3 on the clean anatase (101) surface is shown in Figure 6. The optimized structures with the surface model are depicted in

Nitric Acid on TiO2 Rutile (110) and Anatase (101)

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6145

Figure 5. Potential energy surface for the reaction of HNO3 monomer on TiO2 rutile (110) surface.

Figure 6. Potential energy surface for the reaction of HNO3 monomer on TiO2 anatase (101) surface.

Figure S3 and the selected bond lengths are presented in Table S2. Similarly, we consider the most stable adsorption configuration, Ti5c-ON(O)OH-O2c(a). The hydrogen of the coordinating O3H1 group can migrate to the neighboring O2c to form Ti5c-ON(O)O...H-O2c(a) which is slightly exothermic by 0.4 kcal/mol. The phenomenon of hydrogen migrating is same as that on the rutile (110) surface. The bond length of Ti5c-O1 is 2.027 Å. The adsorbed ON(O)O molecule can rotate on the surface to form Ti5c-ON(O)O-Ti5c,H-O2c(a). The rotation requires 1.6 kcal/mol energy and the bond length between Ti-O is increased by 0.12 Å. The rotation of the ON(O)O group in the adsorbate Ti5c-ON(O)O-Ti5c,H-O2c(a) can result in the formation of two Ti-O bonds on two Ti5c sites with the bond lengths of Ti5c-O1 and Ti5c-O3, 2.143 Å and 2.159 Å, respectively. In the isomerization process from Ti5c-ON(O)...H-O2c(a) to Ti5c-ON(O)O-Ti5c,H-O2c(a), it has to overcome an activation barrier of 9.1 kcal/mol at TS3, which lies below the reactants by 4.6 kcal/mol. In the transition state (TS3) shown in Figure S3 and Table S2, the bond lengths of

Ti5c-O1 and Ti5c-O3 are 2.002 Å and 3.027 Å, respectively. The decomposition process producing H-O2c(a) + NO3(g) from Ti5c-ON(O)O-Ti5c,H-O2c(a) requires 56.8 kcal/mol and the final product H-O2c + NO3(g) is 44.7 kcal/mol above the initial reactants. The other possible pathway starts from Ti5c-O(H)N(O)O-Ti5c(a) to Ti5c-ON(O)O-Ti5c, H-Ob(a) via the TS3B with a barrier of 18.9 kcal/mol as shown in Figure 6. Ti5c-O(H)N(O)O-Ti5c(a) has an adsorption energy of 2.0 kcal/ mol. It implies that the H atom cannot easily undergo hydrogen bonding with the nearest bridged oxygen from Ti5c-O(H)N(O)O-Ti5c(a) because of the long separation. The emission of NO(g) from Ti5c-ON(O)O-Ti5c,H-Ob(a) to H-Ob, Ti5c-O,Ti5c-O(a) + NO(g) takes place with a very high barrier of approximately 96.0 kcal/mol at TS3C. This process is more difficult to occur comparing with that on the rutile surface. 3.2.5. Reaction Path for the Adsorption and Dissociation of HNO3 Dimer on the Rutile (110) Surface. The geometrical structures and potential energy surface of dimer HNO3 molecules

6146 J. Phys. Chem. C, Vol. 113, No. 15, 2009

Chang et al.

Figure 7. Potential energy surface for the reaction of the HNO3 dimer on TiO2 rutile (110) surface.

TABLE 3: Optimized Bond Lengths (Å) and Adsorption Energies (kcal/mol) for HNO3 Dimer and Its Fragments on Rutile (110) Surface structure

Ti5c-O1

Ti5c-ON(O)OH-O2c(a) 2(Ti5c-ON(O)OH...O2c(a)) Ti5c-ON(O)ON(O)O-Ti5c(a) Ti5c-ON(O)ON(O)O-Ti5c-2(a) Ti5c-ON(O)ON(O)O-Ti5c,O2c...HOH...O2c(a)

2.426 2.594 2.640 2.878 2.689

c

Ti5c-O4 2.551 2.723 2.791 2.881

O6-H1

O6-H2

O3H1-O2c 2.293 2.266

2.909

2.124

O6H2-O2c 2.163

Ti5c-Ti5c

Ti5c-O3c

Eads

2.937 2.945 2.935 2.930 2.936

1.813 1.810 1.802 1.798 1.810

5.8 12.8c 2.0 1.5 7.8

2(Ti5c-ON(O)OH-O2c(a)) ) 12.31 kcal/mol in 1 × 2 × 2 supercell.

on the TiO2 rutile (110) surface are shown in Figure S3 and Figure 7. The corresponding bond lengths and adsorption energies are listed in Tables 3 and S2. In the HNO3 dimer adsorption on the rutile surface, we use 1 × 4 × 1 super cell as our TiO2 rutile (110) model. The difference between 1 × 2 × 2 and 1 × 4 × 1 that is the latter has 2 times larger width with half of the height of the former. In the reaction path, we only consider the most stable structure of HNO3 dimer as given for the monomer HNO3 adsorption with an adsorption energy of 12.8 kcal/mol, approximately twice that of the monomer. In the most stable conformation, two HNO3 molecules adsorbed parallel to each other on two surface Ti5c atoms, having two hydrogen bonds formed with the closest depicted as neighboring bridged oxygen O2c, 2(Ti5c-ON(O)OH-O2c(a)) in Figure S3. Their bond lengths are Ti5c-O1 ) 2.594 Å, Ti5c-O4 ) 2.551 Å and O3H1 ) 2.266 Å, O3H1 ) 2.163 Å. Starting from the most stable adsorbate 2(Ti5c-ON(O)OHO2c(a)), the H1O3-N1 bond of one HNO3 molecule rotates and forms a hydrogen bond with another HNO3 giving intermediate LM1, whose adsorption energy is higher than that of 2(Ti5c-ON(O)OH...O2c(a)) by 5.1 kcal/mol. In the intermediate LM1, the intermolecular hydrogen bond length is 2.203 Å between the H2 atom and the O3H1 of the first HNO3. This finding is consistent with the results reported by M. J. Gillan31 who have showed the intermolecular hydrogen bonding can stabilize the configuration. From the intermediate, the H atom of the first HNO3 split to form H2O with the OH group of the second HNO3 via TS2, producing a water complex Ti5c-ON(O)ON(O)O-Ti5c,O2c...HOH...O2c(a). The process requires 46.2 kcal/mol for crossing the barrier. From the water complex, it can eliminate the H2O(g) from the surface to form

a Ti5c-ON(O)ON(O)O-Ti5c(a) + H2O(g) with 5.8 kcal/mol endothermicity. The adsorption energy of Ti5cON(O)ON(O)O-Ti5c(a) is not big enough to stabilize the structure on TiO2. Furthermore, the N2O5 molecule adsorbed on the rutile surface has two different structures shown in Figure S3. The adsorption energies of Ti5c-ON(O)ON(O)O-Ti5c(a) and Ti5cON(O)ON(O)O-Ti5c-2(a) are 2.0 and 1.5 kcal/mol, respectively. These structures are not stable enough to form N2O5(a). Ti5c-ON(O)ON(O)O-Ti5c(a) easily splits into NO3 and NO2 molecules without a well-defined transition state. The final product of NO3 and NO2 on the surface lies 3 kcal/mol below Ti5c-ON(O)ON(O)O-Ti5c(a). 3.2.6. Reaction Path for the Adsorption and Dissociation of HNO3 Dimer on the Anatase (101) Surface. Similarly, Figure 8 shows the potential energy diagrams, and Figure S3 shows the related geometrical structures of HNO3 dimer on the TiO2 anatase (110) surface. For this process, we utilize a bigger 3 × 1 × 2 super cell with the 11.355Å × 12.289 Å area on the TiO2 (101) anatase surface. For the HNO3 dimer adsorbed on a smaller 2 × 1 × 2 super cell, there is an unavoidable interaction between the cells. The related bond lengths and the adsorption energies are presented in Tables 4 and S2. We consider the most stable configuration Ti5c-ON(O)OH...O2c(a) given in the Figure S3. which has an adsorption energy of 13.3 kcal/mol. Thus, the most stable HNO3 dimer is 2(Ti5c-ON(O)OH...O2c(a)) with the adsorption energy 28.3 kcal/mol, which is approximately twice that of the monomer Ti5c-ON(O)OH-O2c(a) and is also more than twice that of the HNO3 dimer on rutile (12.7 kcal/ mol). In the most stable configuration, 2 HNO3 molecules with oxygen atoms attach to 2 Ti5c atoms on the surface with the 2 hydrogen atoms forming hydrogen bonds with the nearest two

Nitric Acid on TiO2 Rutile (110) and Anatase (101)

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6147

Figure 8. Potential energy surface for the reaction of HNO3 dimer on TiO2 anatase (101) surface.

TABLE 4: Optimized Bond Lengths (Å) and Adsorption Energies (kcal/mol) for HNO3 Dimer and Its Fragments on Anatase (101) Surface structure 2(Ti5c-ON(O)OH...O2c(a)) Ti5c-ON(O)ON(O)O-Ti5c, O2c-HOH-O2c(a) Ti5c-ON(O)ON(O)O-Ti5c(a) Ti5c-ON(O)ON(O)O-Ti5c-2

Ti5c-O1 Ti5c-O4 O3-H1 O6-H2 O3H1-O2c O6H1-O2c O6H2-O2c Ti5c-Ti5c Ti5c-O3c O2c-Ti6c 2.309 2.778

2.332 2.513

2.778 3.313

2.513 2.857

1.050

1.053 0.960

bridged oxygen atoms. Their bond lengths are Ti5c-O1 2.309 Å, Ti5c-O4 2.332 Å, O3H1-O2c 1.498 Å and O6H2-O2c 1.484 Å, respectively. From the most stable dimer adsorbate 2(Ti5c-ON(O)OH-O2c(a)), one of the H atoms can react with the OH group of the second HNO3 to form H2O giving Ti5c-ON(O)O-Ti5c,O2c...HOH...O2c(a) via TS4 as shown in Figure S3. The transition state (TS4) is 40.0 kcal/mol higher than the initial reactants. The activation barrier for breaking the hydrogen bond requires 68.2 kcal/mol. The bond lengths of O3-H1 and O6-H1 are 1.873 Å and 3.120 Å. The Ti5c-ON(O)O-Ti5c,O2c...HOH...O2c(a) is 3.1 kcal/mol above the initial reactants. From Ti5c-ON(O)O-Ti5c,O2c ...HOH...O2c(a), it can eliminate H2O(g) to produce Ti5c-ON(O)ON(O)O-Ti5c(a) + H2O (g). The dehydration reaction requires 4.8 kcal/mol. Thus, Ti5c-ON(O)O-Ti5c,O2c...HOH...O2c(a) is not stable and can not retain H2O on the surface. Similarly, the N2O5 molecule also adsorbs weakly on the anatase surface with two different structures as shown in Figure S3. The adsorption energies of Ti5c-ON(O)ON(O)O-Ti5c(a) and Ti5c-ON(O)ON(O)O-Ti5c-2(a) are 2.1 and 0.2 kcal/mol, respectively; they are too small for the N2O5 molecule to adsorb on the anatase surface. 3.3. Hydrogen Effect on Adsorbate Structures and Adsorption Energies. As mentioned above, HNO3(a) can easily dissociate H atom to the surface bridging oxygen. Thus in order to understand the OH group effect, we also study the HNO3 and NO3 adsorption on the TiO2 surface in the presence of OH.

1.498 4.139

1.484 1.810

3.807 3.822

1.858 1.795

1.849 1.863

3.822 3.806

1.795 1.794

1.862 1.859

Eads 28.3 -3.1 2.1 0.2

TABLE 5: Adsorption Energies (kcal/mol) for Some Species Calculated at the PW91 Level species Rutile/Ti5c-ON(O)O-Ti5c, H-O2c (a) Anatase/Ti5cON(O)ON(O)O-Ti5c(a) H2S-Ti5c(a)f HS-Ti5c(a) H2O-Ti5c(a) HO-Ti5c(a) Ti5c-OB(OH)O-Ti5c(a)g

without hydrogen effect

with hydrogen effecte

12.0

58.7

6.6

56.7

12.9 12.2 19.9 40.7 53.8

9.3 42.4 16.8 77.3 134.6

68.9h

e

The adsorption surface is considered with one hydrogen adsorbed on the O2c site. Eads ) -[Etotal/H - (Eslab/H + Emolecule)]. f Reference 17. g Reference 16. h The adsorption energy is considered with two hydrogen adsorbed on the O2c site.

We take the most stable structure, Ti5c-ON(O)OH...O2c(a), as an example. The spin-polarization effect is negligible due to negligible changes in geometries, and the difference in adsorption energies of nitric acid is no more than (0.1 kcal/mol. The adsorption energies raises to 9.9 kcal/mol with 1 H atom coadsorbed and 11.7 kcal/mol with 2 H atoms coadsorbed on (110) rutile surface from 6.7 kcal/mol (without the OH group). The Ti6c-O2c bond lengths of Ti5c-ON(O)OH...O2c(a) are 1.855 Å and 1.838 Å, 2.070 Å and 1.838 Å, and 2.087 Å and 2.008 Å for no H, 1 H, and 2 H atoms coadsorbed on the surface. Our results show that the adsorption energy is enhanced with

6148 J. Phys. Chem. C, Vol. 113, No. 15, 2009

Chang et al.

Figure 9. Bader atomic charges (e) on the adsorbed NO3 with and without H coadsorption on TiO2 rutile (A and B) and TiO2 anatase (C and D) surfaces.

the increase of the OH coverage, which are consistent with the observation of Hu and co-worker.32 In addition, we compare hydrogen effects on Ti5c-ON(O)O-Ti5c(a) and Ti5c-ON(O)O-Ti5c,H-O2c(a) on the rutile and anatase surfaces. As shown in Table 5, the adsorption energy with hydrogen attaching to the neighboring bridged oxygen on the rutile surface is 58.7 kcal/mol, and that without hydrogen is 12.0 kcal/mol. On the anatase surface, the adsorption energy with and without hydrogen on the bridged oxygen are 56.7 and 6.6 kcal/mol, respectively. The enhancement is stronger than that of HNO3 molecule. Compared with other species shown in Table 5, it is evident that the hydrogen enhancement effect is distinct for those which have unpaired electrons such as HO, HS, OB(OH)O, on the contrary the enhancement is small for H2O and H2S. In order to explain the hydrogen effect on the adsorption energy, we analyze the Bader charges of Ti5c-ON(O)O-Ti5c(a) and Ti5c-ON(O)O-Ti5c, H-O2c(a) on both surfaces. Shown in Figure 9, H atom coadsorption increases the charge of the ON(O)O adsorbate by 0.21 and 0.22 e in the rutile (110) and anatase (101) surfaces, respectively, where e is the magnitude of the charge of an electron. The charges of the bridged surface oxygen with an H atom are -1.45 (1.00 e for H atom) and -1.35 e (0.86 e for H). Comparing with to the bridged surface oxygen without H, they are -0.82 (rutile) and -0.84 e (anatase). This phenomenon suggests that an H atom on the bridged oxygen will distribute the atomic charge into the adsorbate and stabilize the configuration with a higher adsorption energy as listed in Table 5, in which the difference in the adsorption energy of NO3 with and without a coadsorbed H atom is 46.7 kcal/mol on the TiO2 rutile (110) surface, and 50.1 kcal/mol in the TiO2 anatase (101) surface. The hydrogen effect on the NO3 adsorbate is significant and the difference in the adsorption energy can be attributed to the charge transfer from surface to NO3.

4. Conclusions In summary, we perform a systematic investigation on the HNO3 adsorbed on the TiO2 rutile (110) and anatase (101) surfaces by DFT calculations. The key conclusions are summarized as follows: (1) The most stable configuration for nitric acid on the clean rutile (110) surface is a monodentate structure with one hydrogen bonded to a neighboring bridging oxygen; its adsorption energy is 6.7 kcal/mol (13.3 kcal/mol for anatase). The resulting Ti5c-ON(O)OH...O2c(a) can go over a small barrier for H atom migration to the bonding bridged oxygen forming a more stable Ti5c-ON(O)O...H-O2c(a). The monodentate ON(O)O adsorbate in Ti5c-ON(O)O...H-O2c(a) can isomerize to a didentate confinguration in Ti5c-ON(O)O-Ti5c,H-O2c(a). The elimination of NO3(g) and NO(g) from Ti5c-ON(O)O-Ti5c,H-O2c(a) is negligible due to its high endothermicity and barrier. (2) In the case of HNO3 dimer, two equivalent monodentate structures nitric acid 2(Ti5c-ON(O)OH...O2c(a)) absorbed on two 5-fold coordinated Ti atoms of the rutile surface is the most stable configuration with an adsorption energy of 12.8 kcal/ mol (28.2 kcal/mol for anatase). According to the PESs shown in Figures 7 and 8, the dehydration from the two HNO3(a) is difficult because of the high barrier required. (3) The effect of H coadsorption has been found to play an important role in the adsorption energies of the adsorbates on the TiO2 surface. According to our calculated results, the adsorption energies of NO3(g) on the rutile and anatase surface increase by 46.7 and 50.1 kcal/mol, respectively, with a H atom coadsorbed on the nearest bridged oxygen atom. By analyzing the Bader charges, the charge of NO3 is -0.63 and -0.65 e on the rutile and anatase, respectively, which is more negative than that of NO3 on the clean surface (-0.42 and -0.43 e on the rutile and anatase, respectively). Thus, there is a charge transfer

Nitric Acid on TiO2 Rutile (110) and Anatase (101) from TiO2 to NO3 in the presence of H atom coadsorbed on a bridged oxygen atom and enhances the NO3 adsorption energy. Such strongly adsorbed NO3 may potentially be used as an interface linker between the semiconductor quantum dots, such as InN, and the TiO2 surface for solar cell applications. Acknowledgment. The authors are grateful to the Ministry of Education for the support by the ATU fund and to Taiwan’s National Center for High-performance Computing for the CPU’s facility and to the Institute of Nuclear Energy Research, Atomic Energy Council, Taiwan and National Science Council, Republic of China, under Grant NSC 97-2113-M-492-001-MY2 for the research support. M.C.L. also wants to acknowledge Taiwan Semiconductor Manufacturing Co. for the TSMC Distinguished Professorship and Taiwan National Science Council for the Distinguished Visiting Professorship at the Center for Interdisciplinary Molecular Science, National Chiao Tung University, Hsinchu, Taiwan. Supporting Information Available: Bond lengths, absorption energies, model structures, optimized geometries, and geometrical illustrations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Crabtree, G. W.; Dresselhaus, M. S.; Buchanan, M. V. Phys. Today 2004, 57, 39. (2) Dresselhaus, M. S.; Thomas, I. L. Nature 2001, 414, 332. (3) Gra¨tzel, M. J. Photochem. Photobiol. C: Photochem. ReV. 2003, 4, 145. (4) Blackburn, J. L.; Selmarten, D. C.; Nozik, A. J. J. Phys. Chem. B 2003, 107, 14154. (5) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994, 98, 3183. (6) Yu, P.; Zhu, K.; Norman, A. G.; Ferrere, S.; Frank, A. J.; Nozik, A. J. J. Phys. Chem. B 2006, 110, 25451. (7) Wang, J. H.; Lin, M. C. ChemPhysChem 2004, 5, 1615. (8) Yu, P.; Zhu, K.; Norman, A. G.; Ferrere, S.; Frank, A. J.; Nozik, A. J. J. Phys. Chem. B 2006, 110, 25451.

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6149 (9) Linsebigler, A. L.; Lu, G.; Yates Jr, J. T. Chem. ReV. 1995, 95, 735. (10) Mills, A.; LeHunte, S. J. Photochem. Photobiol. A 1997, 108, 1. (11) Cahen, D.; Hodes, G.; Grazel, M.; Guillemoles, J. F.; Riess, I. J. Phys. Chem. B 2000, 104, 2053. (12) Durrant, J. R.; Haque, S. A.; Palomares, E. Chem. Commun. 2006, 2006, 3279. (13) Chen, X.; Mao, S. S. Chem. ReV. 2007, 107, 2891. (14) (a) Wang, J. H.; Lin, M. C. J. Phys. Chem. B 2006, 110, 2263. (b) Chang, J.-G.; Ju, S. P.; Chang, C. S. J. Phys. Chem. C 2008, 112, 18017. (15) Tzeng, Y. R.; Raghunath, P.; Chen, S. C.; Lin, M. C. J. Phys. Chem. A 2007, 111, 6781. (16) (a) Raghunath, P.; Lin, M. C. J. Phys. Chem. C 2008, 112, 8276. (b) Chang, J.-G.; Wang, J. H.; Lin, M. C. J. Phys. Chem. A 2007, 111, 6746. (c) Chang, J.-G.; Ju, S. P.; Chang, C. S.; Chen, H. T. J. Phys. Chem. C 2009, in press. (17) Huang, W. F.; Chen, H. T.; Lin, M. C. Chem. Phys. Lett. 2008, . communicated. (18) Huang, W. F.; Raghunath, P.; Lin, M. C. J. Comput. Chem. 2008, . communicated. (19) Kresse, G.; Furthmuller, J. Vienna Ab-initio Simulation Package VASP the guide 2005, . (20) Hohenberg, P.; Kohn, W. Phys. ReV. B 1964, 136, 864. (21) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (22) Perdrw, J. P.; Yang, Y. Phys. ReV. B 1992, 45, 244. (23) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (24) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. J. Chem. Phys. 2000, 113, 9901. (25) Henkelman, G.; Arnaldsson, A.; Jo´nsson, H. Comput. Mater. Sci. 2006, 36, 354. (26) Vinet, P.; Ferrante, J.; Smith, J.; Rose, J. J. Phys. C 1986, 19, L467. (27) Burdett, J. K.; Hughbanks, T.; Miller, G. J.; Richardson Jr, J. W.; Smith, J. V. J. Am. Chem. Soc. 1987, 109, 3639. (28) Tilocca, A.; Selloni, A. Langmuir 2004, 20, 8379. (29) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gra¨zel, M. Phys. ReV. Lett. 1998, 81, 2954. (30) Herman, G. S.; Dohnalek, Z.; Ruzycki, N.; Diebold, U. J. Phys. Chem. B 2003, 107, 2788. (31) Lindan, P. J. D.; Harrison, N. M.; Gillan, M. J. Phys. ReV. Lett. 1998, 80, 762. (32) Liu, L. M.; McAllister, B.; Ye, H. Q.; Hu, P. J. Am. Chem. Soc. 2006, 128, 4017.

JP810635H