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A DFT Study of the Adhesion of Pd Clusters on ZnO SWNTs and Adsorption of Gas Molecules on Pd/ZnO SWNTs Yan Su, Qiang-qiang Meng, and Jian-guo Wang* College of Chemical Engineering and Materials Science, State Key Laboratory Breeding Base of Green-Chemical Synthesis Technology, Zhejiang UniVersity of Technology, Hangzhou 310032 China ReceiVed: August 18, 2009; ReVised Manuscript ReceiVed: October 19, 2009
We investigated the adhesion of Pd nanoclusters on ZnO SWNTs and adsorption of probe gas molecules (O2, H2, and CO) on the outside or inside wall of ZnO and Pd1/ZnO SWNTs by means of density functional theory calculations in this study. Our study shows that the binding of Pd clusters on ZnO is mainly via the Pd-O bond interaction. The Pd monomer has the same adhesion ability on both the outside and the inside wall of ZnO SWNTs. However, we found that the adsorption energy of O2 is larger on the inside wall of ZnO and Pd1/ZnO SWNTs than that on the outside one, which is caused by the confinement effect. Introduction 1
Stimulated by the discovery of carbon nanotubes, various metal oxide nanotubular structures have been synthesized.2–8 Among these metal oxide low-dimensional architectures, ZnO is one of the extensively investigated systems for potential applications in several fields, such as gas sensors,9–12 photocatalysis,13 dye-sensitized solar cells,7,14 and photoelectrochemical water-splitting.15 These low-dimensional ZnO architectures (such as nanotubes (NTs), nanorods (NRs), or nanowires (NWs)) have distinct size-12 and shape-dependent3 optical, magnetic, and catalytic properties when compared with the bulk oxide ZnO. It is well-known that ZnO is also one of the catalyst supports, which is widely used in various catalyst systems.16–20 Combining the properties of one-dimensional materials and a wide range of applications of metal oxide on catalyst supports, there are several potential advantages for one-dimensional metal oxide nanotubes serving as catalyst supports. First, the active species can distribute on the surface uniformly due to the high surface-volume ratio of metal oxide NTs. Second, with the hollow structure, the active species can locate inside the NTs, which leads to novel catalytic properties. The experimental studies have indicated that noble metal nanoclusters supported on ZnO or TiO2 NTs show superior catalytic activity and/or stability than those supported on the conventional metal oxide nanoparticles,21–24 which are probably caused by the unique noble metal clusters/metal oxide NTs’ structure. Therefore, it is rather necessary to understand the interaction between metal clusters and metal oxide nanotubes and the interface properties from an electronic level in order to better utilize this kind of nanocomposite. Several theoretical studies have been conducted on electronic, geometrical structures, and various properties (such as optical, dielectric function) of one-dimensional ZnO nanomaterials (such as nanotubes and nanowires).25–28 Zeng et al. further investigated the adsorption of O2, H2, CO, NH3, and NO2 on (6,0) ZnO zigzag single-walled nanotubes with and without oxygen vacancy.25 However, to our knowledge, there are still lacking theoretical studies on the metal clusters/ZnO nanotube systems. * To whom correspondence should be addressed. E-mail: jgw@zjut. edu.cn.
In this study, for the first time, we presented the results of adhesion properties of small Pd clusters on the ZnO nanotube and the adsorption of three probe molecules (CO, H2, O2) on both the outside and the inside wall of ZnO and Pd1/ZnO SWNTs, which is also an elementary step for both catalysis and gas sensors. The interface properties of Pd1/ZnO nanotubes are characterized by density of states, frontier molecular orbitals, and population analysis. We found that the adsorption of O2 on the inside of Pd1/ZnO SWNTs is stronger than that one on the outside wall. Calculations The first-principles DFT calculations were carried out by using the DMol3 package.29,30 The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional31 was used to describe the exchange-correlation (XC) effects. The double numerical plus d-functions (DND) basis set was used in the expanded electronic wave functional. The Brillouin zone is sampled by 1 × 1 × 2 k points with 2K points along SWNTs’ axis using the Monkhorst-Pack scheme. For these systems containing O2 molecules, the calculations were performed by using spin-restricted methods. For other systems in our study, spin unrestricted methods were used in the calculations. For all of the calculations, the convergence in energy and force was set to 10-5 eV and 2 × 10-3 eV/Å. The adhesion energy of Pdn clusters on ZnO SWNTs is calculated as follows:
Eadh ) E(Pdn/ZnO) - E(Pdn) - E(ZnO)
(1)
where E(Pdn/ZnO), E(Pdn), and E(ZnO) are the total energies of the Pdn/ZnO SWNTs, the ZnO SWNTs, and the most stable Pdn cluster, respectively. The adsorption energy of CO, H2, and O2 is defined as
Eads ) E(Gas Molecule+Pdn/ZnO) - E(Gas Molecule) - E(Pdn/ZnO) (2) where E(Gas Molecule+Pdn/ZnO), E(Gas Molecule), and E(Pdn/ZnO) are the total energies of the combined system, the detected gas molecule (O2,
10.1021/jp907977q CCC: $40.75 2009 American Chemical Society Published on Web 11/11/2009
DFT Study of ZnO SWNTs and Pd/ZnO SWNTs
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H2, and CO), and Pdn/ZnO SWNTs, respectively. According to the definition of adhesion energy of Pdn and the adsorption energy of gas molecules, negative (positive) Eadh and Eads denote an exothermic (endothermic) adsorption process, respectively. In this study, an armchair ZnO (5,5) single-walled nanotube (SWNT) was used, which consists of 40 Zn and 40 O atoms. The periodic hexagonal supercell was adopted with the lengths of a, b, and c lattices of 25, 25, and 13.67 Å. The length of c is 3 times of that in the peridodicity of ZnO (5,5) SWNTs. The minimum distance between opposing sidewalls of neighboring ZnO SWNTs is bigger than 15.79 Å, which can render the interactions among repeating slabs negligible. Results and Discussion Pd Clusters on ZnO Nanotubes. The diameter of optimized ZnO (5,5) SWNTs is 8.96 Å, which makes the adsorption of small Pd clusters and gas molecules inside the tube feasible. The bond distance of Zn-O and the angle of Zn-O-Zn and O-Zn-O is 1.93 Å, 113.91°, and 117.27°, respectively. The calculated band gap of ZnO SWNTs is 2.05 eV, which is typical of a semiconductor by taking into account the underestimating of this value by DFT calculations. These results are nicely in agreement with the several recent theoretical studies on ZnO nanotubes.25–28 Although there are several theoretical studies on ZnO nanotubes,25–28 no theoretical studies have been reported on the geometric and electronic properties of metal nanoclusters/ZnO nanotube systems. The interaction between metal clusters and metal oxide substrates plays a very important role in heterogeneous catalysis, surface science, and other research fields. Metal oxide nanotubes have large surface areas and large volumes, which have potential applications in catalyst supports. It is wellknown that carbon nanotubes have been confirmed as a superior catalyst support among a series of reactions.32–38 As catalyst supports, metal oxide NTs are more promising candidates than CNTs due to their geometric structures of NTs and electronic structures of metal oxides. Several recent experimental studies have shown this evidence.21–24 In this study, we selected Pd as the model metal active component to investigate the interaction between Pd nanoclusters and ZnO nanotubes. The optimized Pdn (n ) 1-6) on the outside wall of ZnO (5,5) nanotubes are shown in Figure 1. The binding energies of these small Pd clusters range from 1.50 to 2.50 eV. The most favorable binding site for Pd is atop the site of oxygen in ZnO. We found that the most stable structure of these clusters Pdn (n ) 1-4) is located in one hexagonal ZnO ring. The Pd monomer is bounded on the atop site of oxygen of ZnO, with the adsorption energy and Pd-O bond distance being 1.50 eV and 2.07 Å. The inside of the tube is the unique site for the metal clusters and gas molecules. We also investigated the binding of the Pd monomer on the inside of the ZnO SWNTs, which are also shown in Figure 1. The adhesion energy of the Pd monomer is 1.55 eV, which is slightly larger than that on outside wall. The Pd dimer, bonded with two oxygens, is much more stable than that with one oxygen (2.02 vs 1.76 eV). The Pd-Pd distance is 2.74 Å, which is smaller than the oxygen-oxygen distance of ZnO. The triangular Pd3 cluster with an upright position is more stable than that one parallel with ZnO. Only two of the Pd in Pd3 are coordinated with oxygen in ZnO. The tetrahdedron Pd4 cluster in one hexagonal is more stable by about 0.34 eV than that one across two rings due to the repulsion between Pd clusters and Zn of ZnO. The most stable structure of the Pd5 cluster occupies two hexagonal rings with four Pd in the first layer. Only three of them are bonded with oxygen in
Figure 1. Optimized geometry of Pd clusters on ZnO (5,5) SWNTs.
ZnO. For the Pd6 cluster, the binding energy of Pd6a and Pd6b is nearly the same (2.39 vs 2.37 eV), although the first layer of the Pd6 cluster is three or four Pd atoms, respectively. To understand the bonding mechanism of Pd clusters on both the outside and the inside wall of ZnO SWNTs, the projected density of states of Pd and oxygen directly bonded with Pd and the HOMO and LUMO orbitals of Pd1/ZnO are shown in Figure 2. It can be seen that the PDOS of Pd and oxygen is very similar, which is independent of the Pd1 adhesion site. The HOMO and LUMO of Pd1/ZnO SWNTs consists of 4dz2 of Pd and 2pz, which is in bonding and antibonding states, respectively. Adsorption of Gas Molecules (O2, H2, and CO) on ZnO and Pd1/ZnO SWNTs. The adsorption of O2, H2, and CO is an elementary step in gas sensors and catalysis processes. The inside of the tube is a unique adsorption site of ZnO SWNTs compared with other forms of ZnO. In this study, we investigate the adsorption of O2, H2, and CO on the outside and inside wall of ZnO and Pd1/ZnO systems (Figure 3). Three kinds of molecules (O2, H2, and CO) weakly adsorb on both the inside and the outside wall of pristine ZnO SWNTs. The adsorption energies of the molecules are nearly the same on two different adsorption sites. The distances between O2, H2, and CO and ZnO is 3.50, 3.89, and 3.75 Å on the inside wall of ZnO SWNTs, which are larger than those on the outside one (2.73, 2.83, and 2.38 Å). The adsorption of three molecules on Pd1/ZnO SWNTs was investigated in this study. For H2 and CO adsorption, the adsorption energy is nearly the same on both the outside and the inside wall of ZnO SWNTs. For example, the adsorption energy of CO is -2.59 and -2.64 eV on the outside and inside of Pd/ZnO SWNTs, which is in agreement with the value (-2.63) in the similar system of Pd1/BaO39 but slightly different with that of CO adsorption (2.06 eV) on the Pd (111).40 For O2 adsorption, the adsorption energy is -1.06 eV on the outside of Pd/ZnO SWNTs, and it is 1.10 eV on the atop state in the O2/Pd system41and 0.95 eV on the pure Pd (111).42 However, when confined inside of Pd/ZnO SWNTs, the adsorption energy is -1.22 eV. To provide insights of the enhanced adsorption of O2 by the inside wall of ZnO SWNTs, the PDOS of O2, Zn, and Pd of
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Figure 2. Projected density of states of Pd and oxygen (a, d). HOMO (b, e) and LUMO (c, f) of Pd1/ZnO (5,5) SWNTs. (a-c) Pd1 on the outside and (d-f) Pd1 on the inside wall of ZnO SWNTs.
Figure 3. Adsorption of O2, H2, and CO on ZnO (5,5) and Pd1/ZnO (5,5) SWNTs.
Figure 4. PDOS of oxygen, adsorbed on the (a) outside and (b) inside, and Zn of ZnO. PDOS of O2, adsorbed on the (c) outside and (d) inside, and Pd of Pd1/ZnO.
ZnO and Pd1/ZnO are shown in Figure 4. As expected, the PDOS of O2 and Zn are nearly the same (Figure 4a,b) when O2 adsorbs on the outside and inside wall of ZnO SWNTs. However, the PDOS of O2 and Pd have different characters when Pd1 is located on the inside or outside wall of ZnO SWNTs. The PDOS of O2 significantly increased and some additional peaks of O2 appear in the conduction band when O2
is confined inside of the Pd1/ZnO. All of them are responsible for the enhanced adsorption of O2. Further, the charges of Pd and O2 in the Pd1/ZnO, O2/ZnO, and O2/Pd1/ZnO systems by means of Hirshfeld analysis are summarized in Table 1. We found that very few charge transfer from O2 to ZnO nanotubes with 0.02 and 0.08 when O2 adsorbed on the outside or inside of pure ZnO nanotubes. When adsorbed
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TABLE 1: Charges of the Pd Atom and O2 on Pd/ZnO, O2/ZnO, and O2/Pd/ZnO Pd1/ZnO or O2/ZnO
O2/Pd1/ZnO
outside
inside
outside
inside
0.14 -0.02
0.14 -0.08
0.20 -0.21
0.31 -0.31
Pd O2
on Pd1/ZnO nanotube systems, the charge of O2 increases at least 4 times than that on pure ZnO nanotubes. The charge of O2 increases significantly when confined inside of Pd1/ZnO than that on the outside of Pd1/ZnO nanotubes. Conclusions The adhesion properties of small Pd nanoclusters on ZnO SWNTs and the adsorption of O2, H2, and CO on ZnO and Pd1/ ZnO have been investigated by means of DFT calculations. Our study shows that the binding of Pd clusters on ZnO is mainly via the Pd-O bond interaction. The Pd monomer has the same adhesion ability on both the outside and the inside wall of ZnO SWNTs. The adsorption energies of the three molecules are nearly the same on both the outside and the inside wall of ZnO SWNTs. While on Pd1/ZnO, not only the adsorption energy of O2 is enhanced by 0.16 eV but also the adsorption site changes from the atop to bridge site when O2 adsorbed on the outside and inside of ZnO SWNTs. Our finding will be helpful for future experimental study on metal nanoparticles/metal oxide nanotube systems for both catalysis and gas sensors. Acknowledgment. This work was supported by the “Qianjiang Scholars” program of Zhejiang Province, People’s Republic of China, the Key Projects of Science and Technology Research of the Ministry of Education of China (No. 209055), and the Scientific Research Foundation for Returned Scholars, Ministry of Human Resources and Social Security of China. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Pan, H.; Feng, Y. P. ACS Nano 2008, 2, 2410. (3) Zhou, H. J.; Wong, S. S. ACS Nano 2008, 2, 944. (4) Cheng, B. C.; Xiao, Y. H.; Wu, G. S.; Zhang, L. D. AdV. Funct. Mater. 2004, 14, 913. (5) Liu, J. W.; Li, X. J.; Dai, L. M. AdV. Mater. 2006, 18, 1740. (6) Yuan, G. D.; Zhang, W. J.; Jie, J. S.; Fan, X.; Zapien, J. A.; Leung, Y. H.; Luo, L. B.; Wang, P. F.; Lee, C. S.; Lee, S. T. Nano Lett. 2008, 8, 2591. (7) Martinson, A. B. F.; Elam, J. W.; Hupp, J. T.; Pellin, M. J. Nano Lett. 2007, 7, 2183. (8) Zhang, X. H.; Xie, S. Y.; Jiang, Z. Y.; Zhang, X.; Tian, Z. Q.; Xie, Z. X.; Huang, R. B.; Zheng, L. S. J. Phys. Chem. B 2003, 107, 10114.
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