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A DFT study on the interaction of Co with an anatase TiO2 (001)-(1×4) surface
Journal of Natural Gas Chemistry 18(2009)78–82
A DFT study on the interaction of Co with an anatase TiO2 (001)-(1×4) surface Zhijun Zuo1 ,
Wei Huang1∗ , Peide Han2 ,
Zhihong Li1 ,
Jian Huang1
1. Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China; 2. College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China [ Manuscript received September 11, 2008; revised October 30, 2008 ]
Abstract The substitution/adsorption structures of Co on an anatase TiO2 (001)-(1×4) surface are investigated using the DFT/local density approximation (LDA) method. Theoretical calculation shows that the Co ion prefers to be adsorbed on the surface of anatase TiO2 . The density of states (DOS) analysis finds that the Co 3d is located mainly in the energy gap region. The Co 3d partial density of states (PDOS) indicates that there is a substantial degree of hybridization between O 2s and Co 3d in valence band (VB) regions in the substitution models. The conclusion is that the mode of substitution is more active when the catalyst is a higher-energy surface. Key words: DFT; anatase (001)-(1×4); Co
1. Introduction TiO2 has a number of technological uses, including catalysis and photocatalysis. However, due to the ready availability of high quality single crystals [1,2], many fundamental studies on the reactivity of TiO2 have concentrated on the rutile polymorph. In fact, most applications of TiO2 use the anatase rather than the rutile form, because the anatase form appears more catalytically active [1,3]. In particular, Takanabe et al. find that Co/TiO2 -anatase catalysts show higher activities than Co/TiO2 -rutile catalysts for CH4 /CO2 reforming [4]. Meanwhile, it has been found that surface (101) is the dominant surface exposed on nanosized anatase crystallites, but that surfaces (001) and (103) are exposed in minority [5]. Investigation of these different crystal facets suggests that these minority surfaces are more reactive than the majority surfaces, and play a key role in the reactivity of anatase nanoparticles [6–8]. The (1×4) reconstruction of the anatase (001) surface has been found to be stable in a wide temperature range (from room temperature up to 850 ◦ C) and under a variety of experimental conditions [5,9–11]. Based on the low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and angle-resolved mass spectroscopy of recoiled ions (ARMSRI), Herman et al. [5] suggested a reconstruction surface ¯ model for TiO2 (001)-(1×4), characterized by (103) and (103)
microfacets (MF). Recently, however, Liang et al. [11] found that the MF model is inconsistent with scanning tunneling microscope (STM) images and proposed an “added-and-missing row” (AMR) model. Michele Lazzeri et al. [12] found that the surface energies calculated by the MF and AMR models are 1.25 and 1.35 J/m2 , respectively; note that both are much larger than the unreconstructed surface energy of 0.90 J/m2 . The authors propose a new reconstruction model named “admolecule” (ADM), which is energetically much more stable than the unreconstructed surface and can be easily adapted to describe other (1×n) periodicities that have been occasionally observed on the (001) surface [12]. Recent STM/AFM measurements are consistent with the ADM model [13]. During the past decades, the anatase-TiO2 supported Co catalysts and ferromagnetic semiconductors have been widely investigated [4,14–17]. However, calculations modeling the system focus almost exclusively on the anatase-TiO2 bulk [18–20], and very few theoretical studies have concentrated on the Co/TiO2 surface at the atomic level. It is obvious that surface properties are more important than bulk ones in catalysis. Studies have found that catalytic behaviors are appreciably affected by preparation method. In our previous work, we have also found that the activity of the catalysts prepared by sol-gel method is higher than that of those prepared by impregnation in the direct synthesis of acetic acid from
∗
Corresponding authors. Fax: +86-351-6018073; E-mail address: [email protected] This work was supported by the National Natural Science Foundation of China (Grant No.20676087) and the National Basic Research Program of China (Grant No 2005CB221204). Copyright ©2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(08)60086-9
Journal of Natural Gas Chemistry Vol. 18 No. 1 2009
CH4 and CO2 by two steps [21]. It is widely believed that the substitution mode is produced more easily in the catalysts prepared by sol-gel, and that the adsorption mode is more prominent in those prepared by impregnation. To verify the essential differences between the properties of catalysts prepared by the different methods, we present a density functional study of Co adsorption/substitution on the TiO2 (001)-(1×4) surface. 2. Calculation models and details The general principles of the DFT-pseudopotential method have been described elsewhere [22]. In this paper, we use the local density approximation (LDA) [23,24]. The ionelectron interaction is modeled by ultrasoft local pseudopotentials [25]. A self-consistent field procedure is carried out with a convergence criterion of 10−5 a.u. on energy and electron density, and the geometry is optimized under a symmetry constraint, with convergence criteria of 10−3 a.u. on the gradient and 10−3 a.u. on the displacement. In view of the system size that we wished to consider, we elected to use a plane wave cut-off of 340 eV. 3. Results and discussion 3.1. Surface energies The software package CASTEP [26] has been used in our calculations. For clean anatase TiO2 (001)-(1×4) surfaces, we used the expression [27]: Esurf = (Eslab − nETiO2 )/2A where Eslab is the total energy of an n-layer slab and ETiO2 is the energy of a TiO2 unit in the bulk, n is the number of ETiO2 units in the slab, and A stands for the area of the anatase TiO2 (001)-(1×4) surface. The TiO2 (001)-(1×4) surface is modeled by four layers of oxide, and a p(1×4) super cell is used with the ADM model on both surfaces of the slab (17 TiO2 units in a cell) in which the meshes of 4×1×1 k-points were used [12,28]. The effect of optimization has been analyzed for the (001) surface with ˚ wide. During the structural optimizaa vacuum region 10 A tions, all atoms of the slab except those in the bottom were allowed to move, keeping the volume constant. The surface energy is 0.58 J/m2 , which agrees with the results [12,28]. 3.2. The geometric structures and the electronic properties To further explore the effect of Co adsorption/substitution in TiO2 , a single-Co-adsorption/substitution TiO2 (001)-(1×4) surface is chosen in our calculations. Considering the chemistry of the atomic interaction, we suppose that the TiO2 surface adsorbs Co through O atoms, or that the surface Ti is substituted with Co. According to these hypotheses, some possible doped geometries are proposed as shown in Figure 1. The Figures M1–M3 are three possible doped geometries of Co on a clean TiO2 (001)-(1×4) surface and M4–M7 are possible Co adsorption geometries on stoichiometric surface slabs. The Figure M corresponds to the perfect TiO2 (001)-(1×4) surface.
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Taking M1-M3 together gives the adsorption energy at the surface-substitution module [29]: subs subs Eads = ECo /TiO2 (001) − ETiO2 (001) + ETi − ECo subs where ECo , ETiO2 (001) , ECo atom and ETi repre/TiO2 (001) sent the total energy of the relaxed Co-incorporated TiO2 (001)-(1×4) surface, the perfect TiO2 (001)-(1×4) slab, the isolated-Ti atom and the isolated-Co atom, respectively. The adsorption energy of adsorption modules is defined as the energy required to remove the cobalt from the surface and to create metallic cobalt in the bulk [30,31]: subs Eads = ECo /TiO2 (001) − ETiO2 (001) − ECo subs where ECo and ETiO2 (001) are the total energies of /TiO2 (001) the Co-adsorbed and the stoichiometric surface slabs, respectively, and ECo is the total energy per atom of the metallic cobalt bulk. The structure parameters, adsorption (substitution) ensubs ergy ratios Eads (Eads ) and values of the bond distances of Co substitution(adsorption) on TiO2 (001)-(1×4) surface are subs ) of all seven listed in Table 1. The energy ratios Eads (Eads models are positive, among which the adsorption model M7 subs ), it folis the most favorable. From the values of Eads (Eads lows that the adsorption mode is more stable than the substitution mode, which indicates that adsorption of Co is easier than substitution on the TiO2 (001)-(1×4) surface. There are two kinds of Ti atoms, which are connected respectively to five neighboring oxygen or to four neighboring oxygen on TiO2 (001)-(1×4) surface, so the Co also is connected to same oxygen in the substitution models. Meanwhile, because the two oxygen sites are equivalent, three or four Co–O bonds are shown in Table 1. In the adsorption model, the M7 and M5 structures have three Co–O bonds, but the M4 and M6 only have one Co–O bond. We note that in the M7 and M5 models the Co–O bond is vertical to the (001) surface at the beginning of the calculation and parallel to the (001) at the end; obviously in the M4 and M6 models there is no change. In the M7 model, the lengths of the three Co–O bonds are very ˚ [32] similiar, and close to the bond length of Co–O (1.91 A) in the bulk of CoO, but the bond lengths in M1, M2, M3 and M5 are very different. The Mulliken charges of Co can be seen from Table 1. Co charge of adsorption models is lower than the substitution models, and it indicates that Co oxidation state of substitution models is higher.
Table 1. Adsorption/substitution energy, O–M bond length and Mulliken charge of Co for the seven models. M stands for Ti or Co
M1 M2 M3 M4 M5 M6 M7
subs ˚ O–M bond length (A) Mulliken charge Eads (Ead ) dM−Ot dM−O2c dM−O3c (q) (eV) 1.653 1.725, 1.723 − 0.97 8.14 − 1.759, 1.775 1.914, 1.869 0.95 8.78 − 1.786, 1.785 1.909, 1.833 0.97 8.44 1.753 − − 0.26 4.06 − 1.894 1.921, 1.965 0.56 2.30 − 1.764 − 0.35 4.53 − 1.940 1.911, 1.912 0.45 1.91
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Figure 1. Possible doped and adsorption models of Co substitution/adsorption on TiO2 (001)-(1×4) surface. M: clear anatase-TiO2 (001)-(1×4) surface; M1M3: Co substitution stoichiometric surface slabs; M4-M7: Co adsorption stoichiometric surface slabs. Oxygen is represented by red spheres whereas white and blue spheres represent titanium and cobalt, respectively
To verify the electronic structure of Co substitution (adsorption) on TiO2 (001)-(1×4) surface, we plot the total DOS. Because the most stable models are M1 and M7, corresponding to the substitution mode and the adsorption mode of Co, respectively, only the M1 and M7 structures are considered. The total density of states (DOS) and the partial DOS for Ti 3d, O 2s and O 2p of the perfect TiO2 (001)-(1×4) surface and the Co substitution and adsorption stoichiometric surface slabs are plotted in Figure 2a-c, and the partial DOS of Co 3d of the Co substitution and adsorption TiO2 (001)-(1×4) sur-
face are shown in Figure 2d. Blurry, low-lying s and p states of Ti and Co are omitted from the Figures. From Figure 2(a), for the clean TiO2 (001)-(1×4) structure, several sets of DOS can be observed. They are well separated, and correspond predominantly to O 2s characters around –17 eV and a little of Ti 3d around –16.5 eV, and O 2p and Ti 3d states below the Fermi level, around –6 eV ∼0 eV, respectively. The conduction band (CB), between 2 and 3.5 eV, corresponds mainly to the Ti 3d states and has the constitution of O 2p states. Therefore, analysis of the DOS suggests that there is a substantial
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Figure 2. The total density of states DOS and the partial DOS for Ti 3d, O 2s and O 2p of the perfect TiO2 (001)-(1×4) surface, Co substitution and adsorption on stoichiometric surface slabs. (a) Total DOS, (b) partial DOS of Ti 3d, (c) PDOS of O 2s and O 2p, dot line and solid line corresponding O 2s and O 2p, respectively; (d) partial DOS of Co 3d. (1), (2), (3) corresponding the clear TiO2 (001)-(1×4) surface, Co-doped and adsorbed stoichiometric surface slabs, respectively. The vertical line indicates the Fermi level
degree of hybridization between O 2p and Ti 3d in both CB and valence band (VB) regions, indicating strong interactions between Ti and O atoms in TiO2 (001)-(1×4). It is clearly shown from Figures 2(b) and 2(c) that when Co is substituted or adsorbed into the surface, the O and Ti states are not much affected by the introduction of Co. The effect of the Co is mainly located in the energy gap region [18,33], and a detailed discussion of the DOS of Co-substitution/adsorption TiO2 anatase in terms of a molecular orbital diagram has been given in Ref [19]. From Figure 2(d), one can see that for both Co substitution and adsorption in TiO2 , the Co states are mainly located in the energy gap region. Generally speaking, the metallic character of adsorption modules is greater than that of the substitution modules. Meanwhile, the Co 3d PDOS suggests that there is a substantial degree of hybridiza-
tion between O 2p and Co 3d in VB regions in doped models. The trend also indicates that the interaction between Co and O atoms in substitution models is bigger than that in the adsorption models. 4. Conclusions In this work, we have systematically studied Co doping on TiO2 (001)-(1×4) surface with LDA. Our results can be summarized as follows: (1) There are two modes of substitution and adsorption in Co doping on TiO2 (001)-(1×4) surface, with adsorption occurring both thermodynamically and by chemiadsorption on the TiO2 (001)-(1×4) surface. (2) Among the four adsorption sites and three substitution sites considered, the most stable modes corresponded to
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Co adsorption on the position where the lengths of three Co– O bonds are similar and very close to that of the Co–O in the bulk of CoO. This site was at least 0.4 eV more stable than the others. (3) There was a substantial degree of hybridization between O 2p and Co 3d in VB regions, especially in the substitution models. Both substitution and adsorption modules are also shown to be half-metals; however, the metallic character of the substitution models is smaller than that of the adsorption models. (4) According our experiment results, it was deduced that the mode of substitution was more active for catalysis because the mode has higher surface energy, so it tends to adsorb reactant molecules to decrease its surface energy. It was a favorable catalytic reaction. Acknowledgements The authors gratefully acknowledge the financial support of this study by the National Natural Science Foundation of China (Grant No.20676087) and the National Basic Research Program of China (Grant No 2005CB221204).
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A DFT study on the interaction of Co with an anatase TiO2 (001)-(1×4) surface Zhijun Zuo1 ,
Wei Huang1∗ , Peide Han2 ,
Zhihong Li1 ,
Jian Huang1
1. Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China; 2. College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China [ Manuscript received September 11, 2008; revised October 30, 2008 ]
Abstract The substitution/adsorption structures of Co on an anatase TiO2 (001)-(1×4) surface are investigated using the DFT/local density approximation (LDA) method. Theoretical calculation shows that the Co ion prefers to be adsorbed on the surface of anatase TiO2 . The density of states (DOS) analysis finds that the Co 3d is located mainly in the energy gap region. The Co 3d partial density of states (PDOS) indicates that there is a substantial degree of hybridization between O 2s and Co 3d in valence band (VB) regions in the substitution models. The conclusion is that the mode of substitution is more active when the catalyst is a higher-energy surface. Key words: DFT; anatase (001)-(1×4); Co
1. Introduction TiO2 has a number of technological uses, including catalysis and photocatalysis. However, due to the ready availability of high quality single crystals [1,2], many fundamental studies on the reactivity of TiO2 have concentrated on the rutile polymorph. In fact, most applications of TiO2 use the anatase rather than the rutile form, because the anatase form appears more catalytically active [1,3]. In particular, Takanabe et al. find that Co/TiO2 -anatase catalysts show higher activities than Co/TiO2 -rutile catalysts for CH4 /CO2 reforming [4]. Meanwhile, it has been found that surface (101) is the dominant surface exposed on nanosized anatase crystallites, but that surfaces (001) and (103) are exposed in minority [5]. Investigation of these different crystal facets suggests that these minority surfaces are more reactive than the majority surfaces, and play a key role in the reactivity of anatase nanoparticles [6–8]. The (1×4) reconstruction of the anatase (001) surface has been found to be stable in a wide temperature range (from room temperature up to 850 ◦ C) and under a variety of experimental conditions [5,9–11]. Based on the low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and angle-resolved mass spectroscopy of recoiled ions (ARMSRI), Herman et al. [5] suggested a reconstruction surface ¯ model for TiO2 (001)-(1×4), characterized by (103) and (103)
microfacets (MF). Recently, however, Liang et al. [11] found that the MF model is inconsistent with scanning tunneling microscope (STM) images and proposed an “added-and-missing row” (AMR) model. Michele Lazzeri et al. [12] found that the surface energies calculated by the MF and AMR models are 1.25 and 1.35 J/m2 , respectively; note that both are much larger than the unreconstructed surface energy of 0.90 J/m2 . The authors propose a new reconstruction model named “admolecule” (ADM), which is energetically much more stable than the unreconstructed surface and can be easily adapted to describe other (1×n) periodicities that have been occasionally observed on the (001) surface [12]. Recent STM/AFM measurements are consistent with the ADM model [13]. During the past decades, the anatase-TiO2 supported Co catalysts and ferromagnetic semiconductors have been widely investigated [4,14–17]. However, calculations modeling the system focus almost exclusively on the anatase-TiO2 bulk [18–20], and very few theoretical studies have concentrated on the Co/TiO2 surface at the atomic level. It is obvious that surface properties are more important than bulk ones in catalysis. Studies have found that catalytic behaviors are appreciably affected by preparation method. In our previous work, we have also found that the activity of the catalysts prepared by sol-gel method is higher than that of those prepared by impregnation in the direct synthesis of acetic acid from
∗
Corresponding authors. Fax: +86-351-6018073; E-mail address: [email protected] This work was supported by the National Natural Science Foundation of China (Grant No.20676087) and the National Basic Research Program of China (Grant No 2005CB221204). Copyright ©2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(08)60086-9
Journal of Natural Gas Chemistry Vol. 18 No. 1 2009
CH4 and CO2 by two steps [21]. It is widely believed that the substitution mode is produced more easily in the catalysts prepared by sol-gel, and that the adsorption mode is more prominent in those prepared by impregnation. To verify the essential differences between the properties of catalysts prepared by the different methods, we present a density functional study of Co adsorption/substitution on the TiO2 (001)-(1×4) surface. 2. Calculation models and details The general principles of the DFT-pseudopotential method have been described elsewhere [22]. In this paper, we use the local density approximation (LDA) [23,24]. The ionelectron interaction is modeled by ultrasoft local pseudopotentials [25]. A self-consistent field procedure is carried out with a convergence criterion of 10−5 a.u. on energy and electron density, and the geometry is optimized under a symmetry constraint, with convergence criteria of 10−3 a.u. on the gradient and 10−3 a.u. on the displacement. In view of the system size that we wished to consider, we elected to use a plane wave cut-off of 340 eV. 3. Results and discussion 3.1. Surface energies The software package CASTEP [26] has been used in our calculations. For clean anatase TiO2 (001)-(1×4) surfaces, we used the expression [27]: Esurf = (Eslab − nETiO2 )/2A where Eslab is the total energy of an n-layer slab and ETiO2 is the energy of a TiO2 unit in the bulk, n is the number of ETiO2 units in the slab, and A stands for the area of the anatase TiO2 (001)-(1×4) surface. The TiO2 (001)-(1×4) surface is modeled by four layers of oxide, and a p(1×4) super cell is used with the ADM model on both surfaces of the slab (17 TiO2 units in a cell) in which the meshes of 4×1×1 k-points were used [12,28]. The effect of optimization has been analyzed for the (001) surface with ˚ wide. During the structural optimizaa vacuum region 10 A tions, all atoms of the slab except those in the bottom were allowed to move, keeping the volume constant. The surface energy is 0.58 J/m2 , which agrees with the results [12,28]. 3.2. The geometric structures and the electronic properties To further explore the effect of Co adsorption/substitution in TiO2 , a single-Co-adsorption/substitution TiO2 (001)-(1×4) surface is chosen in our calculations. Considering the chemistry of the atomic interaction, we suppose that the TiO2 surface adsorbs Co through O atoms, or that the surface Ti is substituted with Co. According to these hypotheses, some possible doped geometries are proposed as shown in Figure 1. The Figures M1–M3 are three possible doped geometries of Co on a clean TiO2 (001)-(1×4) surface and M4–M7 are possible Co adsorption geometries on stoichiometric surface slabs. The Figure M corresponds to the perfect TiO2 (001)-(1×4) surface.
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Taking M1-M3 together gives the adsorption energy at the surface-substitution module [29]: subs subs Eads = ECo /TiO2 (001) − ETiO2 (001) + ETi − ECo subs where ECo , ETiO2 (001) , ECo atom and ETi repre/TiO2 (001) sent the total energy of the relaxed Co-incorporated TiO2 (001)-(1×4) surface, the perfect TiO2 (001)-(1×4) slab, the isolated-Ti atom and the isolated-Co atom, respectively. The adsorption energy of adsorption modules is defined as the energy required to remove the cobalt from the surface and to create metallic cobalt in the bulk [30,31]: subs Eads = ECo /TiO2 (001) − ETiO2 (001) − ECo subs where ECo and ETiO2 (001) are the total energies of /TiO2 (001) the Co-adsorbed and the stoichiometric surface slabs, respectively, and ECo is the total energy per atom of the metallic cobalt bulk. The structure parameters, adsorption (substitution) ensubs ergy ratios Eads (Eads ) and values of the bond distances of Co substitution(adsorption) on TiO2 (001)-(1×4) surface are subs ) of all seven listed in Table 1. The energy ratios Eads (Eads models are positive, among which the adsorption model M7 subs ), it folis the most favorable. From the values of Eads (Eads lows that the adsorption mode is more stable than the substitution mode, which indicates that adsorption of Co is easier than substitution on the TiO2 (001)-(1×4) surface. There are two kinds of Ti atoms, which are connected respectively to five neighboring oxygen or to four neighboring oxygen on TiO2 (001)-(1×4) surface, so the Co also is connected to same oxygen in the substitution models. Meanwhile, because the two oxygen sites are equivalent, three or four Co–O bonds are shown in Table 1. In the adsorption model, the M7 and M5 structures have three Co–O bonds, but the M4 and M6 only have one Co–O bond. We note that in the M7 and M5 models the Co–O bond is vertical to the (001) surface at the beginning of the calculation and parallel to the (001) at the end; obviously in the M4 and M6 models there is no change. In the M7 model, the lengths of the three Co–O bonds are very ˚ [32] similiar, and close to the bond length of Co–O (1.91 A) in the bulk of CoO, but the bond lengths in M1, M2, M3 and M5 are very different. The Mulliken charges of Co can be seen from Table 1. Co charge of adsorption models is lower than the substitution models, and it indicates that Co oxidation state of substitution models is higher.
Table 1. Adsorption/substitution energy, O–M bond length and Mulliken charge of Co for the seven models. M stands for Ti or Co
M1 M2 M3 M4 M5 M6 M7
subs ˚ O–M bond length (A) Mulliken charge Eads (Ead ) dM−Ot dM−O2c dM−O3c (q) (eV) 1.653 1.725, 1.723 − 0.97 8.14 − 1.759, 1.775 1.914, 1.869 0.95 8.78 − 1.786, 1.785 1.909, 1.833 0.97 8.44 1.753 − − 0.26 4.06 − 1.894 1.921, 1.965 0.56 2.30 − 1.764 − 0.35 4.53 − 1.940 1.911, 1.912 0.45 1.91
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Figure 1. Possible doped and adsorption models of Co substitution/adsorption on TiO2 (001)-(1×4) surface. M: clear anatase-TiO2 (001)-(1×4) surface; M1M3: Co substitution stoichiometric surface slabs; M4-M7: Co adsorption stoichiometric surface slabs. Oxygen is represented by red spheres whereas white and blue spheres represent titanium and cobalt, respectively
To verify the electronic structure of Co substitution (adsorption) on TiO2 (001)-(1×4) surface, we plot the total DOS. Because the most stable models are M1 and M7, corresponding to the substitution mode and the adsorption mode of Co, respectively, only the M1 and M7 structures are considered. The total density of states (DOS) and the partial DOS for Ti 3d, O 2s and O 2p of the perfect TiO2 (001)-(1×4) surface and the Co substitution and adsorption stoichiometric surface slabs are plotted in Figure 2a-c, and the partial DOS of Co 3d of the Co substitution and adsorption TiO2 (001)-(1×4) sur-
face are shown in Figure 2d. Blurry, low-lying s and p states of Ti and Co are omitted from the Figures. From Figure 2(a), for the clean TiO2 (001)-(1×4) structure, several sets of DOS can be observed. They are well separated, and correspond predominantly to O 2s characters around –17 eV and a little of Ti 3d around –16.5 eV, and O 2p and Ti 3d states below the Fermi level, around –6 eV ∼0 eV, respectively. The conduction band (CB), between 2 and 3.5 eV, corresponds mainly to the Ti 3d states and has the constitution of O 2p states. Therefore, analysis of the DOS suggests that there is a substantial
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Figure 2. The total density of states DOS and the partial DOS for Ti 3d, O 2s and O 2p of the perfect TiO2 (001)-(1×4) surface, Co substitution and adsorption on stoichiometric surface slabs. (a) Total DOS, (b) partial DOS of Ti 3d, (c) PDOS of O 2s and O 2p, dot line and solid line corresponding O 2s and O 2p, respectively; (d) partial DOS of Co 3d. (1), (2), (3) corresponding the clear TiO2 (001)-(1×4) surface, Co-doped and adsorbed stoichiometric surface slabs, respectively. The vertical line indicates the Fermi level
degree of hybridization between O 2p and Ti 3d in both CB and valence band (VB) regions, indicating strong interactions between Ti and O atoms in TiO2 (001)-(1×4). It is clearly shown from Figures 2(b) and 2(c) that when Co is substituted or adsorbed into the surface, the O and Ti states are not much affected by the introduction of Co. The effect of the Co is mainly located in the energy gap region [18,33], and a detailed discussion of the DOS of Co-substitution/adsorption TiO2 anatase in terms of a molecular orbital diagram has been given in Ref [19]. From Figure 2(d), one can see that for both Co substitution and adsorption in TiO2 , the Co states are mainly located in the energy gap region. Generally speaking, the metallic character of adsorption modules is greater than that of the substitution modules. Meanwhile, the Co 3d PDOS suggests that there is a substantial degree of hybridiza-
tion between O 2p and Co 3d in VB regions in doped models. The trend also indicates that the interaction between Co and O atoms in substitution models is bigger than that in the adsorption models. 4. Conclusions In this work, we have systematically studied Co doping on TiO2 (001)-(1×4) surface with LDA. Our results can be summarized as follows: (1) There are two modes of substitution and adsorption in Co doping on TiO2 (001)-(1×4) surface, with adsorption occurring both thermodynamically and by chemiadsorption on the TiO2 (001)-(1×4) surface. (2) Among the four adsorption sites and three substitution sites considered, the most stable modes corresponded to
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Co adsorption on the position where the lengths of three Co– O bonds are similar and very close to that of the Co–O in the bulk of CoO. This site was at least 0.4 eV more stable than the others. (3) There was a substantial degree of hybridization between O 2p and Co 3d in VB regions, especially in the substitution models. Both substitution and adsorption modules are also shown to be half-metals; however, the metallic character of the substitution models is smaller than that of the adsorption models. (4) According our experiment results, it was deduced that the mode of substitution was more active for catalysis because the mode has higher surface energy, so it tends to adsorb reactant molecules to decrease its surface energy. It was a favorable catalytic reaction. Acknowledgements The authors gratefully acknowledge the financial support of this study by the National Natural Science Foundation of China (Grant No.20676087) and the National Basic Research Program of China (Grant No 2005CB221204).
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