Effect of Surface Oxygen Vacancy on Pt Cluster Adsorption and

Muhammad Adnan Saqlain , Akhtar Hussain , Mohammad Siddiq , Ary R. Ferreira , Alexandre A. Leitão. Phys. Chem. Chem. Phys. 2015 17 (38), 25403-25410 ...
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J. Phys. Chem. C 2007, 111, 16397-16404

16397

Effect of Surface Oxygen Vacancy on Pt Cluster Adsorption and Growth on the Defective Anatase TiO2(101) Surface You Han,†,‡ Chang-jun Liu,*,† and Qingfeng Ge*,‡ Key Laboratory of Green Chemical Technology, School of Chemical Engineering, Tianjin UniVersity, Tianjin 30072, China, and Department of Chemistry and Biochemistry, Southern Illinois UniVersity, Carbondale, Illinois 62901 ReceiVed: July 17, 2007

The effect of surface oxygen vacancies on the adsorption and clustering of the Pt adatoms over the defective anatase TiO2(101) surface has been studied using density functional theory slab calculations. The surface oxygen vacancy site was found to be the most active site for a single Pt adatom with an adsorption energy of 4.87 eV. As such, this site may act as a nucleation center for particle growth on the defective anatase TiO2(101) surface. The pathways for forming the two stable Pt2 adsorption configurations from a single Pt adatom in the oxygen vacancy site and another from the neighboring bridging 2cO sites involve the diffusion of the second Pt adatom out of the bridging 2cO site. The transition states for the dimer formation were located at Pt adatom diffusing out of the bridging 2cO site. Furthermore, we found that the bond-breaking step determines the barrier height for the Pt adatom diffusion, which is ∼1 eV. Among five stable Pt3 adsorption structures at the oxygen vacancy site on the defective anatase TiO2(101), the most stable structure is triangular with all three Pt atoms interacting directly with the surface atoms. The possible pathways for Pt adatom diffusion to form the most stable Pt3 configuration were analyzed. The barriers for Pt adatom diffusion in Pt3 formation were predicted to be similar to that of the Pt2 formation due to similar transition state structures. The barrier height indicates that clustering will be kinetically hindered at low temperature.

1. Introduction Surface oxygen vacancies are ubiquitous on the surface of metal oxides, especially on the reducible oxides such as TiO2.1,2 The oxygen vacancies modify the structure of the materials and change the electronic and chemical properties of the surface, possibly resulting in greatly enhanced reactivity.3 As such, many theoretical and experimental studies have been devoted to understanding the role that the oxygen vacancies play in affecting the reactivity of the surface4-24 as well as the interfacial properties of the metal and supporting metal oxides.25-39 Bridge-bonded oxygen vacancies have been shown to be the active sites for water4,5 and methanol6 dissociation on the rutile TiO2(110) surface. The reactivity of the oxygen vacancies depends on their density40 as well as the exposing crystalline surface.41,42 Density functional theory (DFT) studies on the adsorption of Au clusters on both stoichiometric and defective rutile TiO2(110) surface showed that the binding of Au on the defective surface at the oxygen vacancy site is substantially stronger than on the perfect surface.31,32 A gold adatom can diffuse easily on the nondefective area of the surface at low temperatures until it is trapped by an oxygen vacancy site.43 On the other hand, Ag and Cu bind to the defective rutile TiO2(110) surface weaker than the perfect surface.35 A combined DFT and STM study further demonstrated that the oxygen vacancy site on rutile TiO2(110) is indeed the strongest binding site for Au and a single oxygen vacancy can bind three Au atoms on average.33 Upon further growth, a single oxygen vacancy * Corresponding authors. E-mail: (C.-j.L.) [email protected]; (Q.G.) [email protected]. † Tianjin University. ‡ Southern Illinois University.

can no longer stabilize the Au cluster and the cluster-vacancy complex will then diffuse. A recent ab initio pseudopotential study of the adsorption of single neutral Pt and Au atoms on the perfect and defective rutile TiO2(110) surface also concluded that both the Pt and Au atoms have higher binding energies at the oxygen vacancy site on the defective surface.36 In contrast to the abundant literature on the defective rutile TiO2 surface, studies on the defective anatase TiO2 surface as well as its interaction with supported metal clusters are scarce. On the other hand, commercial tiania powders that are often used as catalyst support are mixtures of anatase and rutile with anatase being a majority component.2,44 Vittadini and Selloni investigated the interaction of small Aun (n ) 1-3) clusters with the perfect and defective anatase TiO2(101) surface using DFT calculations.30 They concluded Au clusters are bound more strongly on the defective surface at the surface oxygen vacancy site than on the perfect surface. To explain the observed preferential growth of Pt on the rutile surface, Iddir et al. calculated Pt and Pt2 adsorption on the perfect and defective rutile TiO2(110) as well as anatase TiO2(101) surfaces.37 They concluded the higher concentration of oxygen vacancies on the rutile surface is the key factor that controls the growth of Pt particles on TiO2. In fact, surface defects such as oxygen vacancies are generally believed to play important roles in the heterogeneous nucleation mechanism for the growth of transition metal thin film on oxide surfaces.45-47 In this mechanism, the metal adatoms trapped in the defect sites serve as nucleation sites that attract metal adatoms diffusing on the surface. Xu et al. showed that although a single Pd adatom binds to the surface oxygen vacancy site of MgO(100) stronger, a second Pd atom binds to the trapped monomer much weaker.48 These results led them to conclude

10.1021/jp075602k CCC: $37.00 © 2007 American Chemical Society Published on Web 10/12/2007

16398 J. Phys. Chem. C, Vol. 111, No. 44, 2007 Pd dimers at the defect sites dissociate rapidly at room temperature although larger clusters bind to the surface more strongly at the defect sites. Recently, we reported our DFT study on the interaction of Pt clusters with the perfect anatase TiO2(101) surface.49 We identified that Ptn (n ) 1-3) prefers the coordinately unsaturated 2-fold-coordinated oxygen (2cO) sites, that is, the bridging 2cO sites. We suggested that these sites may serve as nucleation centers for the growth of metal clusters on the perfect surface. We also expected the growth mechanism, in particular surface diffusion, would play some important roles but did not explore the diffusion pathways. Furthermore, the presence of surface oxygen vacancy would modify the interfacial property between Pt cluster and the oxide surface. Herein, we examined the roles the surface oxygen vacancies played in Pt-surface interaction. In particular, we analyzed the diffusion pathways of Pt adatom and determined the diffusion barriers. 2. Methodology The same method was used here as in our previous study.49 In brief, we used the Vienna ab intio simulation program (VASP) code50 with ultrasoft pseudopotentials51 and PW91 functional.52 The parameters used in the present study were kept consistent with our previous study49 to compare the results directly. The surface oxygen vacancy was created by removing a surface 2cO atom in the (1 × 3) surface unit cell. Only one 2cO atom was removed in each (1 × 3) unit cell, corresponding to an oxygen vacancy density of 1/6. Similarly, the atoms in the lower half of the slab were fixed at their bulk positions. The atoms in the top half of the slab with the adsorbed Ptn clusters were allowed to relax according to calculated Hellmann-Feynman forces. It has been established on the basis of our previous calculations49 that Γ-point sampling of the surface Brillouin zone is sufficient for the perfect surface system. Test calculations with a 2 × 2 k-point grid for the Pt adatom in the vacancy site on the defective surface resulted an adsorption energy of 4.93 eV, which is very close to 4.87 eV from the Γ-point calculation. We therefore used Γ-point sampling in the present study. Our calculations also showed that spin-polarizations were important to many structures and were therefore included in all structures presented here. In the present paper, we used similar definitions of the cluster ads adsorption energy of Ptn, EPt , and the clustering energy of the n clu adsorbed Ptn clusters, EPtn, so that the values may be compared directly with those on the perfect anatase surface. They are defined as ads EPt ) -(EPtn/TiO2(d) - ETiO2(d) - EPtn) n

and clu EPt ) -(EPtn/TiO2(d) - ETiO2(d) - nEPt)/n n

respectively, with EPtn/TiO2(d), ETiO2(d), EPtn, EPt being the total energies of the defective TiO2 slab with Ptn, the clean defective TiO2 slab, the free Ptn cluster, and a free Pt atom, respectively. We again used the Cambridge Sequential Total Energy Package (CASTEP) code53,54 to analyze the electronic density and structures such as electronic density difference plot, Mulliken charge population,55,56 and projected local density of states (LDOS). These analyses were used to help us understand the nature of bonding and the interaction between Pt clusters and the defective anatase TiO2(101) surface.

Han et al.

Figure 1. A perspective view of the defective anatase TiO2(101) surface with a surface oxygen vacancy, shown as a little black sphere. The O and Ti atoms surrounding the vacancy were labeled.

3. Results and Discussion 3.1. Surface Oxygen Vacancy on Anatase TiO2(101). Before presenting the results of Pt adsorption and clustering on the defective surface, we first introduce the structural and electronic properties of the clean defective surface. The surface oxygen vacancy on anatase TiO2(101) was created by removing a surface 2cO atom. The slab was kept neutral after the 2cO atom was removed, making the slab electron rich. The original 6cTi and 5cTi atoms bound to the 2cO atom at the vacancy site become five- and four-coordinated, denoted as 5cTi(d) and 4cTi(d), respectively, as shown in Figure 1. We note the notations are used for convenience and the nature of the 5cTi(d) atom may be different from the 5cTi atoms on the perfect surface. As previously stated, the density of the surface oxygen vacancies is 1/6. Creation of the surface oxygen vacancy causes some local relaxation in the vicinity. The 3cO atom directly underneath the vacancy (3cO(d)) relaxed upward by 0.252 Å. From their corresponding initial positions, the 5cTi(d) and 4cTi(d) atoms were displaced away from each other perpendicular to the 2cO step-edge by 0.153 and 0.403 Å, respectively. These relaxations led to a 5cTi(d)-3cO(d)-4cTi(d) angle of 127.22°, considerably larger than 95.89° of the corresponding angle on the perfect surface. The formation energy of an oxygen vacancy was defined with respect to the energy of an oxygen molecule in the triplet state and calculated according to: EVo ) -[Eideal - Edefective 1/2EO2]. Our calculated oxygen vacancy formation energy on anatase TiO2(101) is 4.35 eV. We note that the calculated molecular oxygen-binding energy of 5.89 eV is consistent with previously reported DFT results57,58 but significantly larger than the experimental value of 5.13 eV.59 Furthermore, it has been shown the vacancy formation energy depends strongly on the nature of the oxides.60 The large value of EVo on anatase TiO2(101) indicates the energy cost of forming a surface oxygen vacancy is high. In addition to the expected dependence of oxygen vacancy formation energy on the vacancy density, the reported values for rutile TiO2(110) vary with the number of O-Ti-O trilayers in a calculation as well as the calculation parameters.9,12 In the studies where both rutile TiO2(110) and anatase TiO2(101) were calculated, the oxygen vacancy formation energy on the anatase surface is higher than on the rutile surface.9,37 Consequently, the density of oxygen vacancy is expected to be low on the anatase surface. The scanning tunneling microscopy images revealed that the density of oxygen vacancies is higher on the rutile surface than the anatase surface,61 which is consistent with the lower EVo on the rutile surface than on the anatase surface.

Pt Cluster Adsorption and Growth on TiO2(101) Surface

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Figure 3. LDOS profiles of Pt and relevant surface atoms for a single Pt adatom in (a) Pt1,Vac and (b) Pt1,Bri. The black curve is the sum of LDOSs of 5cTi(d), 4cTi(d), 2cO, 3cO(d), 5cTi, and 6cTi atoms of the clean defective surface after relaxation and shown as a reference. The energy scales for the clean defective surface was aligned to the Fermi level of the atoms at the unrelaxed side of the slab in each system. Figure 2. The adsorption structures for a single Pt adatom on the defective anatase TiO2(101) surface: (a) at the vacancy site, Pt1,Vac; (b) at the bridging 2cO site of the next 2cO step-edge, Pt1,Bri. An empty oxygen vacancy is shown as a little black sphere.

The creation of the surface oxygen vacancy left some unpaired electrons in the slab representing the defective surface as revealed by spin-polarization calculation. A spin-density plot and Mulliken population analysis (shown in Supporting Information) revealed that the excess electrons were largely localized on the 4cTi(d) and 5cTi(d) atoms around the oxygen vacancy. These excess electrons were redistributed to the 3cO(d) atom directly underneath the oxygen vacancy after relaxation, consistent with the previous report.30 3.2. Adsorption of a Single Pt Atom on the Defective Anatase TiO2(101) Surface. A single Pt adatom favors the oxygen vacancy site on the anatase TiO2(101) surface with surface oxygen vacancies, as shown in Figure 2a (Pt1,Vac). The adsorption energy in this configuration, 4.87 eV, is much higher than a single Pt adatom on the bridge site of two 2cO atoms on the step next to the oxygen vacancy site, Pt1,Bri (3.05 eV), as shown in Figure 2b. These results indicate the Pt atom prefers adsorption at the oxygen vacancy site over the bridging 2cO site on the step edge of the surface. The same bridging 2cO site is the most stable for a single Pt adatom on the perfect anatase TiO2(101) surface, although the adsorption energy at the bridging 2cO site on the defective surface is higher than that on the perfect surface (2.84 eV).49 This increase in adsorption energy of Pt in the bridging 2cO site can be attributed to the delocalized effect of surface oxygen vacancy. In Pt1,Vac, the Pt adatom binds directly to the 5cTi(d) and 4cTi(d) atoms formed from the removal of the surface 2cO atom. The filling of the oxygen vacancy site by the Pt adatom causes the two Ti atoms to move toward each other and the 3cO(d) atom to relax back to almost the initial position prior to the creation of the oxygen vacancy. Consequently, the 5cTi(d)3cO(d)-4cTi(d) angle decreases from 127.22° of the relaxed defective surface to 103.46°, closer to that in the clean perfect surface (95.89°). The Mulliken charge of Pt in Pt1,Vac is -0.22 e but is +0.16 e in Pt1,Bri, indicating different bonding environments for Pt adatoms in the two configurations. A significant amount of electron density was transferred from the reduced Ti sites to Pt when the Pt adatom occupies the oxygen

vacancy site. In contrast, the Pt adatom at the bridging 2cO site donated its electron densities to the undercoordinated 2cO atoms, making it positively charged. The local magnetic moments induced by forming the oxygen vacancy remained when the Pt atom was adsorbed at the bridging 2cO site. Pt is capable of donating and accepting electron densities because of the incompletely filled d-orbitals. Adsorption of the Pt adatom on both perfect and defective MgO(100) was also shown to accept electron densities from the substrate.38 The LDOS plots of the Pt adatom and the relevant surface atoms, shown in Figure 3, were used to gain more insight into the interaction between the Pt adatom and the defective anatase surface. As we demonstrated previously for Pt adsorption on the ideal surface,49 the major contribution to the LDOS originated from p-orbitals of oxygen and from d-orbitals of platinum and titanium in the energy range of interest. We only showed the total projected LDOS for each atom. The sum of LDOS values of surface 5cTi(d), 4cTi(d), 2cO, 3cO(d), 5cTi, and 6cTi atoms, which surround the oxygen vacancy site, is also plotted in Figure 4 and in the subsequent LDOS plots as a reference. The energy scales of the LDOS for the clean defective surface was shifted so as to align the DOS projected onto the atoms of the unrelaxed side of the slab in other LDOS plots involving Pt. We note the Fermi levels of the Pt1,Bri and the clean defective anatase TiO2(101) surface are at the same position. The Fermi level of the clean perfect anatase TiO2(101) surface is at the top of the valence band. Creation of a surface oxygen vacancy shifts the Fermi level to the conduction band, indicating the electrons left behind by removing a neutral oxygen atom fill the antibonding orbitals. The Fermi level of the defective surface is therefore pinned by the oxygen defect. Filling the vacancy site by a Pt adatom redistributes the electrons around the vacancy site and causes the Fermi level to shift with respect to the clean defective surface. On the other hand, the Pt adatom does not interact with the oxygen vacancy directly in the Pt1,Bri configuration. The Fermi level in the latter case is still determined by the oxygen vacancy. In Pt1,Vac, splitting of the Pt d-states close to the Fermi level is observed, indicating the Pt d-states in Pt1,Vac are delocalized. The delocalization is further supported by the electron density difference contour map provided in the Supporting Information.

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Han et al. TABLE 1: Adsorption and Clustering Energies of Ptn Adsorbed on Defective Anatase TiO2(101) Surface system

Eads (eV)

Eclu (eV)

Pt1,Vac Pt1,Bri Pt2,2cO Pt2,TiTi Pt3,f Pt3,v1 Pt3,v2 Pt3,v3 Pt3,line

4.87 3.05 4.91 4.85 5.23 4.90 4.72 4.35 4.52a

4.87 3.05 4.24 4.21 4.14 4.03 3.97 3.85 3.74

a The adsorption energy was calculated with the total energy of linear Pt3 in gas phase as reference.

Figure 4. Structures for Pt2 adsorption on defective anatase TiO2(101) surface: (a) Pt2 parallel to the 2cO step-edge, Pt2,2cO; (b) Pt2 parallel to 5cTi(d)-4cTi(d), Pt2,TiTi.

3.3. Diffusion of Pt Adatom and the Formation of Pt2. When the oxygen vacancy site is already occupied by a Pt adatom, a second Pt atom may be adsorbed at three possible sites: (a) another available oxygen vacancy site; (b) next to the first Pt adatom and forming Pt2 in the vacancy site; and (c) a bridging 2cO site away from the oxygen vacancy. In Section 3.2, we showed the Pt adatom prefers the oxygen vacancy site over the bridging 2cO site. In this section, we will focus on the Pt2 formation in the oxygen vacancy site and compare it with the other adsorption modes. Two stable Pt2 adsorption configurations on the defective anatase TiO2(101) surface were obtained after relaxing five different initial configurations, as shown in Figure 4. The adsorption and clustering energies for both structures were summarized in Table 1. Clearly, in both stable adsorption structures, Pt2 prefers a lying-down configuration with the PtPt bond axis being almost parallel to the surface plane at the surface oxygen vacancy site but aligned differently from each other. The two configurations are almost energetically degenerate with an energy difference of only 0.06 eV. In the slightly more stable structure, denoted as Pt2,2cO (Figure 4a), the Pt-Pt bond axis is almost parallel to the edge of the 2cO steps of the anatase TiO2(101) surface with the Pt-Pt bond length of 2.26 Å. The bond length in this case is much shorter than any of the Pt2 adsorbed on the perfect surface49 and even shorter than a free Pt dimer.62 On the other hand, the Pt-Pt bond axis is aligned along the direction of 4cTi(d)-5cTi(d) in Pt2,TiTi (Figure 4b), perpendicular to the 2cO step-edge. The Pt-Pt bond distance in this configuration is 2.51 Å. In the Pt2,2cO configuration, Pt1 occupies primarily the oxygen vacancy site and Pt2

bridges Pt1 and the next 2cO atom. This configuration was also reported by Iddir et al. for Pt2 on the defective anatase TiO2(101) surface.37 In Pt2,TiTi, the two Pt atoms are in the oxygen vacancy site with Pt1 and Pt2 sitting almost atop of 5cTi(d) and 4cTi(d) atoms, respectively. Although Pt2,2cO and Pt2,TiTi have similar stability, the Pt atoms in adsorbed Pt2 interact with the surface differently. The differences are originated from the different local bonding environments and reflected in the projected LDOS plots shown in Figure 5. In Pt2,2cO, Pt1 of the adsorbed Pt2 interacts primarily with the vacancy 5cTi(d) and 4cTi(d) while Pt2 bridges Pt1 and the 2cO atom. The interaction of Pt1 with Pt2 in the adsorbed Pt dimer stabilizes Pt1, as reflected in downward shift of the center of its d-states with respect to the LDOS of Pt in Pt1,Vac. The major contribution to the states close to the Fermi level and in the gap came from the d-states of the Pt2 atom. Direct interaction of Pt2 with the 2cO atom is manifested by the Pt2 d-states in the energy range of -5.5 to -2 eV. In Pt2,TiTi, both Pt atoms occupy the oxygen vacancy site with Pt1 interacting with 5cTi(d) and the lower step 3cO(L) atom while Pt2 interacts primarily with the 4cTi(d) atom. The two Pt atoms have similar contributions to the states in the gap and close to the Fermi level. The small peak at ∼ -3.8 eV of the Pt1 LDOS can be attributed to the mixing of platinum d-states with the 3cO(L) p-orbitals. In both Pt2,2cO and Pt2,TiTi, the band gap almost disappeared, indicating metallization of the system. Comparing the clustering energies of both Pt2 adsorption configurations and the adsorption energy of a single Pt adatom in Pt1,Vac and Pt1,Bri, we find that the dimers are more stable than the sum of one Pt1,Vac and one Pt1,Bri but less stable than two Pt1,Vac. This comparison confirms if the density of the oxygen vacancy on the surface is higher than the coverage of the Pt adatoms, these adatoms will preferably occupy the oxygen vacancy sites until all the vacancies are filled. Further increasing the Pt adatom coverage beyond the density of oxygen vacancy, the additional Pt adatoms will have to either adsorb next to the existing Pt adatom in the vacancy site to form an adsorbed Pt dimer or occupy the bridging 2cO sites. Our results show forming a dimer by combining the existing Pt adatom in Pt1,Vac and an incoming Pt atom from the gas phase will lead to an energy gain of either 3.61 eV in Pt2,2cO or 3.55 eV in Pt2,TiTi. These energy gains are significantly greater than the adsorption energy of Pt in the bridging 2cO site and will drive the Pt adatoms to form dimers when Pt coverage is increased beyond the density of the oxygen vacancy. As such, the adsorbed Pt2 in either Pt2,2cO or Pt2,TiTi will start to form at the Pt coverage beyond the oxygen vacancy density. This is consistent with a recent report by Iddir et al.37 who showed the concentration of the oxygen vacancies is the key controlling mechanism for the preferential growth of Pt. Similar results have been reported for Au on defective anatase TiO2 surface.30,31

Pt Cluster Adsorption and Growth on TiO2(101) Surface

Figure 5. LDOS profiles of Pt and relevant surface atoms for Pt2 in (a) Pt2,2cO and (b) Pt2,TiTi. The black curve is the sum of LDOSs of 5cTi(d), 4cTi(d), 2cO, 3cO(d), 5cTi, and 6cTi atoms of the clean defective surface after relaxation and shown as a reference. The energy scales for the clean defective surface was aligned to the Fermi level of the atoms at the unrelaxed side of the slab in each system.

On anatase TiO2(101), dimer formation is clearly favorable when the coverage of Pt adatom is greater than the surface oxygen vacancy density. On the other hand, the dimer and cluster formation also depends on mobility of the adatoms. If a Pt adatom has already been adsorbed in Pt1,Bri, is it easy to diffuse out of the bridging 2cO site? On the basis that one Pt adatom occupies the oxygen vacancy site and forms the same structure as Pt1,Vac, the second Pt adatom in the bridging 2cO site of either the same 2cO step-edge or the next 2cO stepedge without oxygen vacancy will have to diffuse out of the site to form the Pt2 adsorption structures. To quantify the barrier for Pt adatom escaping the bridging 2cO site, we started with two initial structures: (1) one Pt adatom occupies the oxygen vacancy site and forms the same structure as Pt1,Vac and the second Pt adatom in the bridging 2cO site of the same 2cO step-edge (I.S. in Figure 6a), and (2) one Pt adatom occupies the oxygen vacancy site and forms the same structure as Pt1,Vac and the second Pt adatom in the bridging 2cO site of the next 2cO step-edge (I.S. in Figure 6b). We then determined the energy barriers for dimer formation using nudged elastic band method as implemented in VASP. The combined adsorption energies of these two initial structures are 4.11 and 4.03 eV, respectively, which are smaller by ∼0.8 eV than those of Pt2,2cO and Pt2,TiTi. Assuming that these two initial structures lead to the formation of Pt2,2cO and Pt2,TiTi, respectively, we analyzed the potential energy profile for Pt adatom diffusing out of the

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Figure 6. Potential energy profiles for a second Pt adatom diffusing out of the bridging 2cO site in different 2cO step-edges to form a Pt dimer at the oxygen vacancy site. (a) The second Pt adatom diffuses from the same 2cO step-edge as oxygen vacancy to form Pt2,2cO; (b) the second Pt adatom diffuses from the next 2cO step-edge without oxygen vacancy to form Pt2,TiTi. Key distances were labeled in the corresponding structures.

bridging 2cO sites and forming the corresponding adsorbed dimers. The potential energy profiles and transition state structures for both cases were shown in Figure 6. In the transition state for forming Pt2,2cO, the distance of Pt2-2cO(2) is highly stretched with respect to the distance in the initial structure but Pt1 and Pt2 adatom are on different sides of the 2cO(1) atom. For Pt2 to diffuse over the 2cO(1) atom, the Pt2 adatom was rotated out of the Pt1-2cO(1)-2cO(2) plane. The transition state is reactantlike, and the barrier lies at the Pt22cO(2) bond-breaking. The relaxation beyond the transition state showed that the potential energy profile includes a fairly flat and long shoulder before Pt1 and Pt2 finally form a bond as in Pt2,2cO. The activation energy along this pathway is 1.01 eV. In the transition state of forming Pt2,TiTi by displacing the Pt adatom in the bridging 2cO site on the upper step edge, both Pt-2cO bonds were stretched to ∼2.6 Å while the Pt1-Pt2 distance was shortened from 5.09 to 3.40 Å. Again, the barrier lies at the breaking of Pt2-2cO bond, and in this case, both bonds. The height of the activation barrier is 1.09 eV. These results demonstrated that if a Pt adatom is already adsorbed at a bridging 2cO site, it needs to break at least one of the Pt-O bonds for it to move to the oxygen vacancy site. Both pathways analyzed here have similar activation barriers; therefore, the substrate temperature at which the deposition was performed

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Han et al.

Figure 8. Two possible reaction pathways for a third Pt adatom diffusing out of the bridging 2cO site in different 2cO step edges to form Pt3,f: (a) from the next 2cO step-edge without oxygen vacancy; (b) from the same 2cO step edge as oxygen vacancy.

Figure 7. Perspective (left) and top views (right) of the stable structures for Pt3 adsorption on defective anatase TiO2(101) surface: (a) Pt3,f; (b) Pt3,v1; (c) Pt3,v2; (d) Pt3,v3; and (e) Pt3,line.

will be a key factor in determining the morphology and size of the Pt particles grown. In this section, we established dimer formation at the oxygen vacancy site is favorable when the Pt adatom coverage is greater than the surface oxygen vacancy density on the defective anatase TiO2(101) surface. What will happen when the Pt adatom coverage is further increased? We will analyze the formation and adsorption of Pt3 in detail and discuss some preliminary results of bigger clusters in Section 3.4. 3.4. The Formation and Adsorption of Pt3. Five Pt3 stable adsorption structures were determined on the defective anatase TiO2(101) surface and were shown in Figure 7. All five structures were confirmed to be minima by frequency analyses. The adsorption and clustering energies for all these structures were summarized in Table 1. We would like to point out that

the adsorption energy in Table 1 for the open structure, Pt3,line in Figure 7e, was computed with respect to the isolated linear Pt3. In fact, Pt3,line is the least among all five structures. If the energy of the isolated triangular Pt3 was used as the reference, the adsorption energy of Pt3,line becomes 4.04 eV and is comparable with the other Pt3 adsorption structures. The most stable adsorption configuration corresponds to a triangular Pt3 occupying the oxygen vacancy site with the triangle plane almost lying flat on the surface (Figure 7a, Pt3,f). In this configuration, the Pt1-Pt2 bond axis is almost parallel to 5cTi(d)-4cTi(d) direction, similar to that of Pt2,TiTi, and the Pt3 atom bridges Pt1-Pt2 and the neighboring 2cO atom. In this configuration, all three Pt atoms interact with the surface directly. The Pt-Pt bond lengths are in the range of 2.55-2.70 Å. The two less stable configurations, Pt3,v1 and Pt3,v3 in Figure 7b,c, are the results of relaxing initial structures constructed from adding a third Pt atom to Pt2,TiTi and Pt2,2cO. On the perfect surface, we showed Pt3 adsorbed in triangular structures are generally favorable over the open structures.49 Here, we found the same trends for Pt3 adsorbed on the defective surface. The only open structure, Pt3,line is the least stable among all five stable structures shown here. In fact, Pt3,v2 is a result of relaxing a linear Pt3 initially placed on the surface. The relaxation

Pt Cluster Adsorption and Growth on TiO2(101) Surface

Figure 9. Clustering energy of Pt on the perfect and defective anatase TiO2(101) surfaces at different cluster sizes.

transformed the linear structure into the triangular Pt3,v2. This configuration is only 0.19 eV less stable than Pt3,v1. The main difference between the two structures is Pt1 in Pt3,v1 is bonded to 3cO(L) whereas Pt1 in Pt3,v2 is bonded to 3cO(F). The difference in Pt1 bonding environment makes the Pt1-Pt2 bond axis of Pt3,v2 cross 5cTi(d)-4cTi(d) of the vacancy site, as shown in Figure 7c. There is a small energy barrier of 0.34 eV for the transformation from Pt3,v2 to Pt3,v1. In Pt3,v2, the distances of Pt1-3cO(F) and Pt1-3cO(L) are 2.14 and 2.89 Å, respectively. The corresponding distances in Pt3,v1 are 2.97 and 2.08 Å, respectively. At the transition state, both distances became 2.44 Å, indicating that the balance between Pt1-3cO(F) breaking and Pt1-3cO(L) forming controls the critical point in the transformation from Pt3,v2 to Pt3,v1. The clustering energy of Pt3,f, 4.14 eV, is less than the adsorption energy of a single Pt adatom in Pt1,Vac but more than that of Pt1,Bri. This again indicates Pt adatoms favor forming a cluster at the vacancy site over remaining as individual adatoms in the bridging 2cO sites. This is especially true when all the vacancy sites were occupied. These results further showed that the ratio of the density of surface oxygen vacancy and the coverage of Pt adatoms is the key factor controlling whether the Pt adatoms cluster or fill the surface oxygen sites. The Pt3,f configuration can be considered as a product of adding a third Pt atom to either Pt2,2cO or Pt2,TiTi. To form Pt3,f by adding a third Pt atom from the gas phase to Pt2,2cO and Pt2,TiTi resulted in different energy gains of 3.93 and 3.99 eV, respectively. These energy gains are not only greater than the Pt adsorption energy in the bridging 2cO site but also larger than the energy gain when adsorbed Pt2 is formed. Two possible initial configurations, one with Pt3 in the same 2cO step-edge as the oxygen vacancy and the other with Pt3 in the next 2cO step-edge without oxygen vacancy, were constructed in Figure 8 to schematically show the dimer formation process. In Figure 8a, the Pt3 adatom needs to diffuse out of the bridging 2cO site and Pt1-Pt2 relax from a Pt2,2cO like configuration before the final Pt3,f structure is formed. This pathway is similar to the pathway of forming Pt2,TiTi and, therefore, is expected to have a similar barrier height, that is, ∼1 eV. Figure 8b depicted the second pathway of forming the same Pt3,f structure from the Pt2,TiTi and a Pt adatom in the neighboring bridging 2cO site. In this case, Pt3 will diffuse out of the bridging 2cO site and attach on the side of Pt2,TiTi to form Pt3,f. This pathway involves activation of the Pt adatom from the bridging 2cO site, which is similar to the formation of Pt2,2cO, as shown in Figure

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16403 8b. We predict that this pathway will have a similar energy barrier to the that of forming Pt2,2cO. We also compared adsorption of bigger clusters, up to 6 Pt atoms, on perfect and defective anatase TiO2(101) surfaces. In Figure 9, we plotted the clustering energy against the cluster size for the most stable adsorption configurations. On a defective surface, the clustering energy decreases rapidly from that of a single Pt adatom in the oxygen vacancy to almost a constant of the dimer and bigger clusters, indicating that the contribution to the overall stability by adding a Pt atom to the existing adatom/cluster is dominated by Pt-Pt interactions. In contrast, the cluster formation leads to an increase in the clustering energy on the perfect surface, and the clustering energy approaches a constant at about 4 Pt atoms. The fact that the clustering energy on the defective surface is greater indicates the oxygen vacancy site has some advantages over the bridging 2cO site as an anchoring site for the adsorbed Pt particle. As the stability of the adsorbed Pt cluster is determined by the Pt-Pt interactions when the size of the cluster is increased, the advantage of the oxygen vacancy is expected to diminish eventually. For small clusters of up to 6 Pt atoms, the advantage of an oxygen vacancy is obvious. In summary, it is clear that once all of the surface oxygen vacancies are occupied by the Pt adatoms, these Pt adatoms in the oxygen vacancy sites become nucleation sites for forming Pt dimers and trimers. If the Pt adatom was initially adsorbed in a bridging 2cO site, it needs to diffuse out of the site to bind with the existing Pt adatom/cluster in the oxygen vacancy site. The barrier for such a diffusion process was located at the breaking of the Pt-2cO bond(s) and was determined to be ∼1 eV. On the basis of our results, we speculate the alignment of Pt particles with the substrate after plasma treatment observed experimentally63 may be related to the oxygen vacancy organization under the influence of the plasma. The mobility of Pt adatoms on the surface will play important roles in controlling the final morphology as well as enhancing the reactivity. 4. Conclusions The effect of the surface oxygen vacancy on the adsorption and cluster formation of Pt adatoms over the defective anatase TiO2(101) surface has been studied using first principles DFT calculations. We analyzed the adsorption structure and energetics of Pt1-3 as well as the diffusional energy profile of a Pt adatom. We showed that removing a surface 2cO atom to form a surface oxygen vacancy causes the surrounding atoms to relax significantly. Pt adatom in the oxygen vacancy site with an adsorption energy of 4.87 eV was energetically favored over adsorption at the regular bridging 2cO sites. Both Pt2 and Pt3 prefer the surface oxygen vacancy site with the Pt-Pt bond axes being nearly parallel to the surface plane. The fact the clustering energies of the adsorbed Pt2 and Pt3 in the corresponding stable configurations are higher than the adsorption energy of Pt adatoms in Pt1,Bri indicates cluster formation will be energetically favorable once all the surface oxygen vacancy sites have been completely filled by Pt adatoms. For a Pt adatom initially adsorbed at the bridging 2cO site, it has to diffuse out of the site and combine with the existing Pt adatom/dimer at the vacancy site to grow the cluster. The activation barriers for the Pt adatom diffusing out of the bridging 2cO site were found at the breaking of the Pt-2cO bond(s) with a barrier height of ∼1 eV. The barrier height indicates clustering will be kinetically hindered at low temperature. Increasing temperature improves the mobility of the Pt adatoms, and therefore favors formation of clusters at the surface oxygen vacancy sites.

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