J. Phys. Chem. C 2009, 113, 7227–7235
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Copper Deposition and Growth over ZnO Nonpolar (101j0) and (112j0) Surfaces: A Density Functional Theory Study Jia Hu,†,‡ Wen-Ping Guo,‡ Xue-Rong Shi,‡ Bing-Rui Li,† and Jianguo Wang*,†,‡ College of Chemistry and Chemical Engineering, Lanzhou UniVersity, Lanzhou, Gansu 730000, People’s Republic of China, and State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, People’s Republic of China ReceiVed: October 28, 2008; ReVised Manuscript ReceiVed: February 21, 2009
Density functional calculations have been performed on initial copper deposition and its growth over ZnO nonpolar (101j0) and (112j0) surfaces. On (101j0), our results demonstrate that copper atoms first interact with dangling bonds or bonding orbital above ZnO. As the deposition increases, they form a zigzag structure along U-shaped gaps, then the surface layers fluctuate, and finally they develop into three-dimensional clusters on the surface. On (112j0), we show that copper atoms also initially interact with dangling bonds. As the deposition increases, however, they form strips in the U-shaped gaps, then surface layers, and finally twodimensional cages. In either case, our results fit well with experimental observations. Our results also indicate that the bonding of copper with ZnO substrate and that among adsorbed copper atoms are competitive, in the sense that copper (3d104s1) has limited bonding ability. Further, our results indicate that some copper atoms that strongly interact with ZnO become positively charged. 1. Introduction 1
2,3
Widely used in methanol synthesis, higher alcohol synthesis, water-gas shift reaction,4 and steam re-forming of methanol,5 ZnO-supported Cu catalysts are indispensable for modern chemical industry. Since neither ZnO nor bulk metallic Cu alone possesses such high catalytic activity for low-temperature applications, strong synergetic interaction of metal and support has been proposed.6,7 The most widely accepted scheme is that Cu may form monolayer or small clusters over the ZnO surface and that the surface properties of the species thus formed are quite different from those of bulk copper. The atomic nature of copper species, however, remains obscure. There are also conflicting experimental results on whether Cu species are positively charged,8 neutral,9 or in the form of Cu0-Cu+ pairs.10 It is well-known that wurtzite ZnO crystal has four surfaces, i.e., polar surfaces (0001) and (0001j) and nonpolar surfaces (101j0) and (112j0). All of them are low-indexed crystal surfaces as indicated by the X-ray diffraction (XRD) results. Therefore many experiments, such as the copper deposition or substitution, are focused on these surfaces especially on (0001).11-16 Since it has been suggested the nonpolar surfaces (101j0) and (112j0) take up to almost 80% of the total surface area,17 it is thus natural to devote enough emphases on these surfaces. Early study suggests that Cu atoms disperse as a monolayer on the (101j0) surface at low content and form two or three-dimensional clusters as Cu content increases.18 Recent studies by Ozawa et al.19 and Dulub et al.20 using angle-resolved photoemission spectroscopy showed that the formation of three-dimensional clusters is exclusive on this surface. More importantly, they identified Cu+ from copper species at low coverage. Møller et al. investigated the growth of Cu on the (112j0) surface by means of synchrotron radiation photoemission spectroscopy. They * To whom correspondence should be addressed. E-mail: iccjgw@ sxicc.ac.cn. † Lanzhou University. ‡ Chinese Academy of Sciences.
suggested that the growth of Cu occurred via either monolayer simultaneous multilayer (MSM) mode or two-dimensional islands (2DI) mode, but could not further discriminate between these two modes by their experimental method.21,22 For either nonpolar surface, the method of photoemission spectroscopy (PES) employed was sufficient in identifying Cu growth modes, while falling short in detailing the atomic properties of Cu atoms. Other powerful experimental techniques, such as scanning tunneling microscopy (STM), are also ineffective in analyzing atomic properties of Cu atoms due to irregular arrangements of Cu at low coverage, the high mobility of adsorbed Cu under reaction condition,23 and the nonconducting nature of ZnO substrate. Synergistic effect and catalytic activity of the active metallic sites seem to be strongest at very low coverage; for instance, Cu+ is found above the (101j0) surface only with coverage less than 1 monolayer equivalent.19 So, works need to be performed by other means to resolve apparent limitations of the current experimental methods. Theoretically, Rodriguez et al.24 investigated the geometric and electronic structures of various ZnO clusters deposited and substituted with Cu by semiempirical INDO method. However, the calculation on ZnO cluster with strong ionic character is inadequate without embedding Madelung potential. Later studies in this field were focused on polar surfaces. In the works by Bromley et al.25 and a recent paper by French et al.26 in the same group, QM/MM cluster models were utilized to study different charge states of single adsorptions on (0001) and to analyze geometries and relative stabilities of planar and polyhedral clusters. In the works by Meyer et al.,27 periodic density functional theory (DFT) method, along with plane waves, was utilized to study a similar case. Detailed discussions were provided on the impact of defects and coadsorbates over this unstable surface during copper deposition. To our knowledge, computational studies of copper deposition over two nonpolar surfaces were few. On (101j0), Beltra´n et al. investigated the adsorption by putting one Cu atom right on top of one Zn atom or one O atom, but Cu relaxation parallel to this
10.1021/jp809517f CCC: $40.75 2009 American Chemical Society Published on Web 04/03/2009
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Hu et al. TABLE 1: Reconstruction Parameter ω (in degrees) and Relaxation Parameter ∆RB.Zn and ∆RB.O (in percent) for Different Single Copper Deposition Types over ZnO (101j0) and (112j0)a
Figure 1. Schematic side views of relaxation and reconstruction on ZnO surface. Starting from its bulk structure (sketched with gray dashed lines), surface Zn-O bond tilts, marked with parameter ω. Also, bonds linked with surface zinc and oxygen contracts, marked with parameters ∆RB.Zn and ∆RB.O, respectively.
deposition type j (1010):1a (101j0):1b (101j0):1c clean (101j0) PBE, clean (101j0), ref 39 LEED, clean (101j0), ref 38 (112j0):1a (112j0):1b clean (112j0) PBE, clean (112j0), ref 39
ω
∆RB.Zn
∆RB.O
0.24 2.03 2.57 8.37 10.1 12 ( 5 -0.18 3.11 6.33 7.4
0.65 0.50 -2.01 -3.02 -3.1
1.16 2.52 -1.06 -3.52 -3.5
1.15 -0.15 -1.61 -1.5
4.87 4.42 -2.00 -1.8
The reference state is the bulk truncated surface of (101j0). Values for the relaxed clean surface have also been listed. The results correlate well with LEED results in ref 38 and PBE calculations in ref 39. a
surface was completely neglected.28 On another nonpolar surface (112j0) with larger unit slab, computational study is yet to be known. Our studies intend to fill some of the void in this area of research. By means of periodic slabs and DFT, we calculated structures and adsorption energies of copper species at various deposition quantities on perfect (101j0) and (112j0) ZnO surfaces. In fact, calculation of copper deposition on defected ZnO, such as over O vacancy, is now being undertaken. On the basis of the geometric shapes formed by copper atoms in various stages of deposition, growth modes were identified. The charge states of the deposited coppers were also provided. 2. Methods and Models 2.1. Methods. All calculations were carried out by using the Cambridge Sequential Total Energy Package (CASTEP).29-31 The exchange and correlation energies were calculated using the Perdew, Burke, and Ernzerhof functional (PBE)32 within generalized gradient approximation (GGA).33 Ionic cores were described by ultrasoft pseudopotential in reciprocal space,34 and Kohn-Sham one-electron states were expanded on a plane wave basis, set up to 340 eV. A Fermi smearing of 0.1 eV was utilized. Monkhorst-Pack meshes35 of k-point sampling were set to (3 × 4 × 1) and (2 × 4 × 1) in the first Brillouin zone for (101j0) and (112j0), respectively. In general, all systems with odd number of electrons were treated with spin-polarization. Also, it was found that as the number of deposited coppers increases, their energy bands around the Fermi level gradually broadens and metallicity develops. As a result, unrestricted PBE seems more proper and thus is utilized for all Cu-containing systems, regardless of the pairity of electrons. The convergence criterion for maximum force was set to 5 × 10-3 eV/Å to allow for possible geometrical relaxation, since Cu/ZnO locates on the borderline between ionic, covalent, and metallic bonds and is a structure-sensitive catalyst. Convergence criteria for selfconsistent field (SCF), energy change, and maximum displacement were set to 2.0 × 10-6 eV/atom, 2.0 × 10-5 eV/atom, and 2.0 × 10-3 Å, respectively. 2.2. Models. For bulk, the calculated lattice constants are 3.266 and 5.247 Å for wurtzite ZnO and 3.624 Å for fcc Cu crystal. These values are in reasonable agreement with corresponding experimental values of 3.250 and 5.207 Å for ZnO36 and 3.61 Å for Cu.37 Slab calculation was implemented for the two nonpolar surfaces with 10 Å vacuum regions to avoid interactions among periodic images. The calculated tilt angles ω of the surface Zn-O bond (Figure 1), an indicator of surface reconstruction, are 8.37° for (101j0) and 6.33° for (112j0), respectively. The other two parameters are the in-plane bond length contraction parameters ∆RB.Zn and ∆RB.O (also Figure
Figure 2. Top view (center) and two side views (bottom and right, surrounded by dashed lines) of three different single copper deposition sites over ZnO (101j0) (shown with top two layers), along with surface lattice vectors. U-shaped gaps and V-shaped gaps could be clearly seen via two side views. Different kinds of gray-colored blocks are also used on the top view to strengthen its stereo perception. The calculation model is a (2 × 1) slab extended along the [12j10] direction. Copper in all three deposition types is saliently positively charged, marked with δ+ on their right side (if it is impossible, it sites on their top), which is caused by bonds formed between it and ZnO substrate, schematically shown. (Silver, zinc; red, oxygen; cyan, copper (same in figures hereafter)).
1), where ∆RB.Zn stands for the change of bond length R linking with surface zinc and ∆RB.O stands for change of bond length R linking with surface O. The changes are caused by relaxation, for (101j0); they are -3.02 and -3.52%, where the minus signs imply downward relaxation and shortening. For (112j0), they are -1.61 and -2.00%. Detailed values are also listed in Table 1. All these are in good agreement with LEED data38 and the foundational calculation by Meyer and Marx.39 To treat with copper deposition, because the ZnO surface is anisotropic and thus copper deposition is also reported to behave anisotropically,19 slabs were deliberately extended to (2 × 1) supercells in a certain way to get more meaningful results (see Figure 2 and Figure 3). For (101j0), the extension was along its U-shaped gap, or the [12j10] direction, while, for (112j0), it was along the [11j00] direction, perpendicular to its U-shaped gap. A total of eight layers with the lower four fixed were utilized for calculations with (101j0), which has been carefully calibrated for copper deposition and H2 adsorption.40 A similar model was
Copper over ZnO (101j0) and (112j0) Surfaces
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ECu-Cu /Cu ) ECu-Cu /N
(3)
In the second step, ECu-Cu was subtracted from Eads. The result is ECu-ZnO, an indicator of interaction between coppers and their substrate. For superslab, it was normalized to value per unit cell:
ECu-ZnO ) Eads - ECu-Cu
Figure 3. Top view (center) and two side views (bottom and right, surrounded by dashed lines) of three different single copper deposition sites over ZnO (112j0) (shown with top two layers), along with surface lattice vectors. Only U-shaped gaps along [0001] could be seen via the right side views, but no gap in another direction. Different kinds of gray-colored blocks are also used on the top view to strengthen its stereo perception. The calculation model is a (2 × 1) slab extended along the [11j00] direction. Copper in all three deposition types is saliently positively charged, marked with δ+ on their right side (if it is impossible, it sites on their top), which is caused by bonds formed between it and ZnO substrate, schematically shown.
also adopted to study oxygen adsorption on this surface.41 In addition, our calculated energy for single copper deposition was also close to the value reported by Beltra´n et al.28 For (112j0), four-layer slabs with the lower one fixed were utilized, because of exceptional computational costs (this surface has twice the amount of atoms per unit cell as its (101j0) counterpart). For comparison, eight-layer slabs were used for all (1 × 1) models and some (2 × 1) models. It was shown that the total energy of deposition differs less than 0.1 eV between the four-layer and eight-layer models. In addition, we calculated the binding energy of some small neutral copper clusters, and the results also fit well with those of previous experimental and theoretical investigations.42,43 2.3. Energetics. The adsorption energy of n Cu atoms on ZnO, Eads, was calculated using eq 1. EZnO and ECu/ZnO stand for calculated energies for copper-free slab and copper-deposited slab, respectively. ECu stands for the energy of an isolated copper atom, placed in the center of a 10 × 10 × 10 Å periodic box.
Eads ) ECu/ZnO - EZnO - nECu
(1)
To quantify the synergistic effect, the interactions of Cu and ZnO were calculated by removing binding energies among surface Cu from Eads. This was reached in two steps: in the first step, ECu-Cu, the binding energies among N numbers of surface Cu, were calculated. It represents the energy difference between N isolated copper atoms and copper frameworks formed of layers or clusters or zigzag lines. These frameworks are congruent to copper frameworks in depositions (placed in the same slab) but without ZnO substrate.
ECu-Cu)ECu layers or clusters - NECu
(2)
ECu-Cu could also be used as an indication of metallicity developed among depositions. To be more clear and persuasive, its average was obtained using
(4)
2.4. Thicknesses of Deposited Cu. Experimentally, the thickness of Cu was expressed in the form of monolayer equivalent (MLE), in multiples of Cu interlayer thickness of 2.56 Å.19,37 However, in a microscopic point of view, an overlayer thickness of 2.56 Å is reached only if Cu grows along the [110] direction, not applicable for either surface in our study (see Figure 4 for details). In this analysis, Cu groups like the (111) surface of the fcc crystal. As a result, MLE was not employed throughout this paper. 3. Cu Deposition over ZnO (101j0) 3.1. Single Cu Atom Deposition. There are three types of single copper depositions over ZnO (101j0), denoted by (101j0): 1a, (101j0):1b, and (101j0):1c. The digit 1 denotes there is 1 Cu per unit slab, and the letters a-c denote the locations of the atom, as shown in Figure 2. The top view as well as two side views has been depicted, because gaps and ridges of the ZnO surface could only be clearly seen via side views. Changes of ZnO surface relaxation and reconstruction after deposition have been listed in Table 1, in terms of ∆RB.Zn, ∆RB.O, and ω. In the most stable configuration (101j0):1a, a single copper atom interacts with dangling bonds on the ZnO surface. This Cu sits in octahedron vacancy on the surface and is placed over the U-shaped gap along the [12j10] direction. Tilt angle ω of the surface Zn-O bond is reduced from 8.37° in the original surface to 0.24°, indicating an ease-off of reconstruction. Interestingly, the surface bond contraction caused by relaxation is overcorrected, since both ∆RB.Zn and ∆RB.O enlarge to positive values, implying a stronger interaction between Cu and ZnO than that between Zn and O. Similar cases were found in (101j0):1b and (101j0):1c. But, in (101j0):1b, Cu atom interacts with dangling bonds of two adjacent ZnO dimers. It sits in octahedron vacancy on the surface and is stabilized in a V-shaped gap. The V-shaped gap is narrower than the above-mentioned U-shaped gap and, along the [0001] direction, perpendicular to U-shaped gaps. In (101j0): 1c, the Cu atom actually interacts with the bonding orbital of the Zn-O bond, as Cu sits right above the surface Zn-O dimers, beyond the reach of the dangling bond. Involvement of the Zn-O antibond could be excluded, because its ∆RB.Zn and ∆RB.O are even smaller than those in (101j0):1b. Migration barriers among these single deposition sites were also calculated. The forward transition barrier from (101j0):1a to (101j0):1c is 0.81 eV, and that from (101j0):1c to (101j0):1b is 0.08 eV, while reverse barriers are 0.50 and 0.08 eV, respectively. All of these values are low and uncommon for pure sp3 substances (both Zn and O are hybridized in sp3), indicating some metallic nature of the bond between Cu and ZnO. Anisotropy is so remarkable that copper migration along [12j10] is much easier than that along [0001], correlating well with the findings by Ozawa et al.19 3.2. Higher Deposition. All structures are depicted in Figure 5. In addition, a diagram of Cu-Cu bond length versus the number of deposited coppers is provided in Scheme 1:
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Figure 4. Minimal distances between two parallel low-index planes in an fcc piled metal, such as copper. d is the distance, and a is the most contact interval between two adjacent atoms in that fcc piled metal; for copper, it equals 2.56 Å. The lattice constant is2a, which is namely 3.62 Å.
SCHEME 1: Range of Bond Length of Cu-Cu for the Most Stable Species at Each Deposition Amount above ZnO (101j0), Filled with Greena
a Average values are marked with a red line. The black dashed line refers to the bulk value, all in units of angstroms.
Figure 5. More copper deposition on ZnO (101j0), top view, along with surface lattice vectors. Coppers with positive Mulliken charges are labeled with δ+ on their right side. Energetic data are provided in the order of Eads (left, bold), ECu/ZnO (middle, plain), ECu-Cu/Cu (right, italic), all in units of minus eV/slab, which is defined through eqs 1-4, and are similar for following figures.
In the case of two copper atoms per unit slab, two types of zigzag structures along the [12j10] direction are formed. These two are actually the combination of (101j0):1a + (101j0):1b and (101j0):1b + (101j0):1c, respectively, denoted as (101j0):2a and (101j0):2b, with the former being more stable. The combination of (101j0):1a + (101j0):1c also results in (101j0):2a. For up to 2 coppers per slab, copper structures are basically the same for (1 × 1) and (2 × 1) slabs. However, in the case of three copper atoms, the situation starts to change. The deposition type, as a combination of (101j0):2a + (101j0):2b, is denoted as (101j0):3a. This type is available only for (1 × 1) (the deposition amount is only 3 because one Cu overlaps another; see Figure 5). This rhombus-like structure rearranges when extending the slab to (2 × 1) along the [12j10] direction, resulting in (101j0):3b, a new zigzag strip with its periodicity being twice that in (101j0):2a or (101j0):2b. Arrangement of Cu from now on is topologically similar to the (111) surface of metallic Cu, but its Cu-Cu bond length is still shorter by about
0.1 Å. Actually, for larger slabs, the structure of (101j0):3a seems to be a transition state, since it can rearrange to a different structure from (101j0):3b, if the initial structure is slightly perturbed. This resulted structure is in fact a translation of (101j0):3b by a half-phase position along the [12j10] direction, with the same energy. Extending the unit slab along the [0001] direction produces no rearrangement. Thus it does not reflect the experimental reality as the whole crystal is actually relaxed. We therefore excluded this from our consideration. The arranging process appears to be a Peierls type distortion,44 but is not, because the calculated density of states (DOS) is almost unchanged in this process. Further discussions are given in section 3.3. Through this rearrangement process, the bond among Cu atoms is lengthened somewhat. It is also noted that copper atoms have already taken up most of the surface area of ZnO (101j0) after this rearrangement. Therefore subsequent Cu adsorptions will be forced to protrude out, foreclosing the flat monolayer. This rearrangement actually rectifies our choice of utilizing a (2 × 1) superslab, and further Cu deposition is calculated on the basis of this supercell. Thus, adding one new copper will increase the deposition amount by only 0.5. When one more Cu is deposited, jigsaw frameworks marked by regularly arranged convex and concave pieces along the [0001] direction emerge and are denoted as (101j0):3.5. Copper arrangement along the [12j10] direction is still clearly seen. The coppers formation in (101j0):3.5 can also be regard as a surface layer, but with significant fluctuation (see Figure 5). One additional Cu will likely deposit in the large concaved region of (101j0):
Copper over ZnO (101j0) and (112j0) Surfaces SCHEME 2: Change of ECu-Cu/Cu (Red Line) and ECu-ZnO (Blue Line) versus the Number of Deposited Copper above ZnO (101j0)a
a The black dashed line denotes the bulk value of ECu-Cu/Cu; all in units of electronvolts.
3.5, forming (101j0):4. From this point on, arrangement along [0001] is gradually blurred, but coppers still structure as a surface layer, though fluctuation increases. Three-dimensional clusters are observed with addition of just one new copper on (101j0):4; this encaged deposition structure is named (101j0): 4.5. This phenomenon correlates well with experimental findings.19,20 This is a three-dimensional copper cluster because there is a Cu located on fcc-hollow (or hcp-hollow) of the copper layer underneath. Thus, the arrangement of coppers resembles that of bulk Cu, although the top copper in the cluster is also stabilized by neighboring protruding coppers. The surface of (101j0):4.5 is still rough, indicating more clusters will form with more copper depositions. The bond length between coppers as depicted in Scheme 1 is generally elongated as more copper atoms deposit. When copper forms clusters, most bond lengths are around 2.50 Å, which is shorter than 2.563 Å for bulk coppers. Measurements of angle-resolved photoemission spectroscopy by Ozawa et al.19 also suggest that the bond difference may last until the cluster is large enough. 3.3. Energy and Charge. As defined in section 2.3, the adsorption energy Eads is divided into two parts: ECu-Cu and ECu-ZnO. Changes of ECu-Cu/Cu and ECu-ZnO versus the number of deposited coppers were depicted in Scheme 2. Detailed values are given in Table 2 and Figure 5. For single adsorption, due to the small size of the unit cell, copper atom interacts with its periodic images for all three adsorption types, with interaction energy of -0.71 eV. A larger value of ECu-ZnO in (101j0):1a is the main reason for more stable adsorption than the other two. To lessen ECu-Cu, one copper atom in an adsorption type similar to (101j0):1a was put into a (2 × 1) superslab extended along the [12j10] direction, producing (101j0):0.5. Doing so, ECu-Cu is extenuated to be less than -0.01 eV and ECu-ZnO enlarged from -1.29 to -1.80 eV. Thus, it seems that for bonding among Cu and that between Cu and ZnO, there exists a competitive mechanism. This phenomenon reflects the limited bonding ability of this 3d104s1 metal and is essential for interpreting subsequent copper deposition. Compared with reported Eads values of polar surfaces (0001) and (0001j),27 some of our computed values on (101j0) are even
J. Phys. Chem. C, Vol. 113, No. 17, 2009 7231 larger (e.g., compared with Eads of relatively unstable sites over (0001) and (0001j), in which copper deposits over the perfect site and near a vacancy). The reason can be that there is no on-top adsorption in our study. In contrast, on-top adsorption exists over polar surfaces. In all three adsorption types, copper interacts with zinc and oxygen simultaneously through dangling bond ((101j0):1a, (101j0):1b), or bonding orbital ((101j0):1c). We found that both Cu-Zn and Cu-O bonds contribute significantly to ECu-ZnO, with the latter being larger. With more copper depositions, Eads increases by -2 to -4 eV for each additional copper. As mentioned earlier, this increase can be considered a compromise between ECu-Cu and ECu-ZnO. The value of ECu-Cu will increase steadily, so does ECu-Cu/Cu, which will follow an asymptote to approach the calculated bulk value of -3.65 eV (see Scheme 2; in (101j0): 4.5, it is already -2.88 eV.). In contrast, ECu-ZnO hits a maximum point as copper is added, occurring at (101j0):2a. This turning point appears because electrons are less shared between Cu and ZnO as coppers are added beyond (101j0):2a. From calculated ECu-Cu and ECu-ZnO, it is easy to learn that the driving force of the aforementioned rearrangement from (101j0):3a to (101j0):3b is in fact ECu-ZnO, which rises from -0.92 to -1.77 eV. (For consistency, the structure of (101j0):3a was extended to a (2 × 1) superslab, with no optimization.) Interestingly, ECu-Cu actually decreases in the arrangement, although the total adsorption energy Eads increases. From this point, this arrangement can be an outcome of lattice mismatch between surface copper and ZnO, similar to the findings by Dmitriev et al. who investigated copper structure above R-Al2O3 using molecular dynamics.45 To study copper charge states, Mulliken46 analysis was used throughout this paper, and coppers with positive charges are labeled in Figure 5. All calculated charges are smaller than 1. These charges are the result of filling of the sp3 dangling bond (or bonding orbital), so coppers closer to the ZnO substrate have higher positive charges. The largest value, 0.142, indeed comes from (101j0):1b, as copper interacts with two adjacent Zn-O dimers. Considering the charge states of neighboring Zn or O ions, we find that only Zn2+ is significantly reduced, indicating the 4s electron on copper is now shared by zinc ions. This further supports the idea of alloy sites.6,7 When more coppers are deposited, charges for each copper decreases, while the total charge of copper reaches its maximum in the case of two Cu ((101j0):2a). It then decreases, following the same trend as ECu-ZnO. Some coppers in (101j0):2a, (101j0):2b, and beyond are negative in charge. This can be a consequence of the induction effect. We calculated the DOS for all these species. We found that there is a clearly band gap in (101j0):2a or (101j0):2b. This gap continues to shrink as the copper deposition amount increases, qualitatively in agreement with observation by Ozawa et al.19 Unfortunately, classical DFT is known to underestimate the band gap,47 and our current versions of CASTEP29-31 do not have correction methods such as LDA + U. So it is improper to determine the size of the gap by this research. 4. Copper Deposit over ZnO (112j0) 4.1. Single Deposition. Over (112j0), single copper all sits in the octahedron vacancy. There are two types of single copper depositions observed. They are denoted as (112j0):1a and (112j0): 1b, shown in Figure 3. Side views were also provided to view the gaps and ridges of the ZnO surfaces clearly. Unexpectedly, a deposition type similar to (101j0):1c does not exist. Copper could only stay in the U-shaped gap along
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TABLE 2: Calculated Energetics (in electronvolts, Defined through Equations 1-4) for the Most Stable Conformation of Each Copper Deposition Amount above ZnO (101j0)a deposition type (101j0):1a (101j0):2a (101j0):3b (101j0):3.5 (101j0):4 (101j0):4.5 a
Eads
ECu-ZnO
ECu-Cu
ECu-Cu/Cu
total charge
range of charge
-2.00 -5.51 -8.87 -10.77 -12.59 -14.25
-1.29 -1.74 -1.77 -1.35 -1.46 -1.30
-0.71 -3.76 -7.11 -9.42 -11.12 -12.95
-0.71 -1.88 -2.37 -2.69 -2.78 -2.88
0.094 0.102 0.063 0.043 0.059 0.069
0.094 -0.025-0.127 -0.039-0.126 -0.069-0.131 -0.039-0.136 -0.056-0.157
The total copper Mulliken charge and range of charge (in atomic units) are also provided.
the [0001] direction for both types, but could not locate on the ridge above the Zn-O bond as that in (101j0). An explanation is that the lack of inverse symmetry in (112j0) precludes identical interaction with copper from two images. Similarly, the absence of screw axis C6 could be the main reason for the disappearance of on-top adsorption above two nonpolar surfaces. This phenomenon has a profound impact on subsequent copper depositions. Changes of ω, ∆RB.Zn, and ∆RB.O are quite similar to those single deposition types above (101j0). Copper is bonded with dangling bonds on (112j0) for both (112j0):1a and (112j0):1b. The tilt angle ω of the surface Zn-O bond also diminishes and is even overcorrected, because, in (112j0):1a, ω reduces to a negative value, indicating that Zn is now higher above the surface than O. Values of ∆RB.Zn and ∆RB.O for relaxation are also overcorrected, indicating interaction between copper and zinc oxide at (112j0):1a is quite strong. (Here copper lies nearly at the middle of the trench.) In (112j0):1b, ω is larger and ∆RB.Zn and ∆RB.O are smaller, indicating that copper sitting at the edge of the trench interacts less with ZnO than in (112j0):1a. The geometric details are provided in Table 1. Migration barriers between two single adsorption sites were calculated. The forward barrier from (112j0):1a to (112j0):1b (the left (112j0):1b in Figure 3, top view) is 0.58 eV, while the reverse is 0.51 eV (transition barrier from (112j0):1a to the right (112j0): 1b is about 0.04 eV higher for both forward and reverse reactions). These barriers are generally larger than most values found in (101j0). Anisotropy is also observable; the transition barrier along the [0001] direction, i.e., from (112j0):1a in one U-shaped gap to another (112j0):1a in a neighboring gap is higher, to 0.80 eV. 4.2. More Deposition. Cu species located in the trench as well as three subsequent depositions on the ridge were depicted in Figure 6. In addition, a diagram of bond length between Cu-Cu versus the number of deposited copper was provided in Scheme 3: Since (112j0):1a seems quite stable, one more copper can be accommodated near the middle of the trench above ZnO, resulting in (112j0):2a. For this deposition type, copper linking zinc is higher above ZnO, because of the longer bond length of Cu-Zn than Cu-O. There are other two types, denoted as (112j0):2b and (112j0):2c. Both are essentially (112j0):1a + (112j0):1b, for the reason that there exist two identical sites for (112j0):1b per unit slab. Further combinations are found when more coppers deposit. The combinations are (112j0):2a + (112j0):1b (denoted as (112j0): 3a), (112j0):1b + (112j0):1a + (112j0):1b (denoted as (112j0): 3b), and (112j0):1a + (112j0):1b + (112j0):1a (denoted as (112j0): 3c). The first one shapes like a two-dimensional triangle island located in the U-shaped gap, correlating well with the findings by Møller et al.22 The other two are zigzag structures also laying in the trench along the [0001] direction but with less regularity than those in (101j0):2a or (101j0):2b.
Figure 6. Copper deposition in the trench of ZnO (112j0) (N(Cu) e 4), and copper deposition over the ridge (N(Cu) > 5), top view, along with surface lattice vectors.
Saturation of adsorption on the trench is reached when four coppers are deposited per unit slab. This type, denoted as (112j0): 4, can be regarded as the combination of (112j0):1a + (112j0): 2a + (112j0):1a. It is featured by linear strips of coppers stabilized in the U-shaped gap along the [0001] direction. These strips could be regarded as two-dimensional, or even quasi onedimensional, strips. But (112j0):4 is not structured as metastable phase, as it will be seen in the next section that there is no discontinuity for ECu-ZnO, ECu-Cu, or ECu-Cu/Cu around (112j0): 4. The striplike structure is totally governed by the dangling bond direction, rooted in (112j0):1a and (112j0):1b. Up to now, copper structures are virtually the same for (1 × 1) and (2 × 1) and (1 × 2), but, from (112j0):4 on, deformations were observed when adopting larger slabs. Generally, the newly coming coppers will first be stabilized above the ridges of ZnO
Copper over ZnO (101j0) and (112j0) Surfaces
J. Phys. Chem. C, Vol. 113, No. 17, 2009 7233
SCHEME 3: Range of Bond Length of Cu-Cu for the Most Stable Species at Each Deposition Amount above ZnO (112j0), Filled with Greena
a Average values are marked with a red line. The black dashed line refers to the bulk value, all in units of angstroms.
(112j0). Up to 2 more coppers per unit slab could be accommodated on one ridge. These coppers have no direct linkage to the ZnO substrate and are stabilized only by neighboring Cu atoms. In (1 × 1), adsorption types were denoted as (112j0):5a and (112j0):5b for one more copper deposition on the ridge and (112j0):6a for two more coppers. In (112j0):6a, deposited coppers form a surface layer, which is topologically similar to the (111) surface of metallic copper. This, in principle, agrees with experimental findings.22 It is also noted that, after filling two coppers on one ridge, subsequent coppers are not limited to being stabilized in another empty ridge. The two coppers on the ridge can act as a backbone (such as (112j0):5c, see below), stabilizing the copper framework. As a result, the newly added coppers could locate over the entire surface layer, producing periodicity twice as large as its precursors. In addition, transmogrification is observed when extending (112j0):6a along the [11j00] direction in a (2 × 1) supercell. The driving force is stronger ECu-Cu. This actually affirms our decision to utilize a (2 × 1) superslab, extended along the [11j00] direction. In this case adding one more copper will result in a 0.5 increase of deposition amount. From (112j0):4, adding two more coppers on one ridge results in five coppers per unit slab, denoted as (112j0):5c. Similarly, adding one more copper on (112j0):5c will result in five more minimum. All five are two-dimensional islands: First, copper can locate at the concaved regions of (112j0):5c, forming new surface layers with more fluctuation. These structures are respectively denoted as (112j0):5.5a, (112j0): 5.5b, and (112j0):5.5c. Second, copper may also be located on another ridge, forming (112j0):5.5d, which is a precursor of (112j0):6a. Interestingly, we found a quite different structure (112j0):5.5e. In this structure, deposited coppers adjust themselves to the (112j0) surface, curving into an icosahedron-like structure in concaved regions. From (112j0):5.5a to (112j0):5.5e, Eads are similar, indicating copper might be mobile over the (112j0) surface of ZnO under reaction conditions. All these structures have been depicted in Figure 7. When one new copper is deposited on concaved regions of (112j0):5.5a, (112j0):5.5b, or (112j0):5.5c, agglomerated coppers in forms of very flat disk- or lens-like cages were obtained, correlating well with experimental findings.21,22 One of them
Figure 7. Copper deposition pattern over ZnO (112j0) when one ridge is filled, along with surface lattice vectors.
SCHEME 4: Change of ECu-Cu/Cu (Red Line) and ECu-ZnO (Blue Line) versus the Number of Deposited Copper above ZnO (112j0)a
a
The black dashed line refers to bulk value of ECu-Cu/Cu, all in units of electronvolts.
was denoted as (112j0):6b, depicted in Figure 7, where the encaged part was highlighted. This could also be called a twodimensional island rather than a three-dimensional one, because no Cu is located on the fcc-hollow (or hcp-hollow) of the copper layer underneath. Thus, nearly no Cu-Cu bonding character perpendicular to the (112j0) surface exists. In fact, structures similar to (101j0):4.5 were not observed even when coppers were added to our computational limit, (112j0):7.5. So, we believe that copper growth actually follows the mode of twodimensional islands over the (112j0) surface of ZnO, as there exists several 2DIs (such as (112j0):3a, (112j0):5.5a, etc.) before the formation of the second layer.
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TABLE 3: Calculated Energetics (in electronvolts, Defined through Equations 1-4) for the Most Stable Conformation of Each Copper Deposition Amount above ZnO (112j0)a deposition type (112j0):1a (112j0):2c (112j0):3a (112j0):4 (112j0):5c (112j0):5.5c (112j0):6a a
Eads
ECu-ZnO
ECu-Cu
ECu-Cu/Cu
total charge
range of charge
-1.81 -5.41 -7.93 -11.32 -14.64 -16.47 -18.11
-1.80 -2.91 -3.11 -2.89 -2.52 -2.70 -2.61
-0.01 -2.50 -4.81 -8.44 -12.12 -13.77 -15.50
-0.01 -1.25 -1.60 -2.11 -2.42 -2.50 -2.58
0.124 0.148 0.189 0.176 0.143 0.149 0.045
0.124 0.061-0.087 0.023-0.102 -0.092-0.130 -0.062-0.143 -0.081-0.144 -0.051-0.101
Total copper Mulliken charge and range of charge (in atomic units) are also provided.
4.3. Energy and Charge. Our results indicate that the tendency of ECu-ZnO, ECu-Cu/Cu, and copper charge above this surface is qualitatively similar to its (101j0) counterpart. The changes of ECu-Cu/Cu and ECu-ZnO versus the number of coppers are depicted in Scheme 4. Detailed energy values along with charge are given in Table 3 and Figures 6 and 7. Comparing with corresponding values for (101j0), we found that the largest difference resides in ECu-ZnO. Not only is ECu-ZnO larger, but its turning point versus the number of coppers is postponed from 2 to 3 as well (Scheme 4). The total charge behaves similarly. This is also the consequence of lower symmetry and thus the larger unit slab of the (112j0) surface. The reason is that coppers can only interact with ZnO since their periodic images are separated. Interestingly, in the case of (101j0), zigzag structures appear when 2 copper atoms are deposited per unit slab, while, in the case of (112j0), they appear only from 3 depositions per unit slab. Because of the aforementioned competitive mechanism for bonding among Cu and that between Cu and ZnO, ECu-Cu/ Cu is smaller than that in (101j0), but it still approaches asymptotically to the bulk value of 3.65 eV. One more point should be mentioned, that although coppers added beyond that (112j0):4 stage have no direct contact with the ZnO substrate, they could force neighboring coppers to move closer to Zn or O and thus to interact more strongly with the substrate. This could explain the abnormal rise of ECu-ZnO in Scheme 4 when N ) 5.5.
and recombination of several single adsorption types. Coppers in these two types could be identified as quasi one-dimensional copper rows. It could virtually be regarded as the maximum number of coppers that one ZnO substrate per unit slab can directly link with. Additional coppers would adsorb on their concaved regions to maximize ECu-Cu. For (101j0), the adsorption types are exclusive for a given deposition copper number greater than (101j0):3b, while for (112j0), various structures, for instance, (112j0):5.5a to (112j0):5.5e, can coexist for a given copper number. Adsorption shapes can hop among these types of structures, indicating dynamic exchanges. For both surfaces, ECu-ZnO reaches its maximum at a very early stage of deposition, even before the exclusive structures mentioned above. But influence of the ZnO substrate remains significant after many more depositions, rendering copper frameworks different from that of Cu (111), with only some topological similarity. Among surface coppers, Cu-Cu distances are generally smaller than Cu (111), and many coppers, especially those interacting strongly with ZnO, are slightly positively charged with Mulliken values smaller than Cu2O, between the values suggested by Klier6 (charged by +1), and by Fleisch9 (neutral). Preliminary results of our ongoing analyses40 suggest that these copper species are able to activate H2 and CO very effectively. We venture to say that our research may shed some light on finding the active center of the Cu/ ZnO catalyst.
5. Conclusion
Acknowledgment. The authors are grateful for the financial support of the State Key Fundamental Research Project (2007CB216401) and the Natural Science Foundation of China (Grant 20590363) and Shanxi Province (Grant 200603019).
In this paper, we investigated copper deposition and subsequence growth on the (101j0) and (112j0) surface of ZnO by means of density functional theory. It is shown that the adsorption pattern of copper is dictated by corresponding surface structure: for single adsorption on (101j0), copper atom could locate in its gap or over a Zn-O bond. Subsequent adsorption patterns will follow the order of various zigzag structures, more and more fluctuating surface layers, and finally curved into 3D clusters. This is because of strong interaction among copper atoms induced by a small surface area of the unit cell and thus of relatively low interaction between Cu and ZnO. In the case of (112j0), in contrast, the lacking of inverse symmetry forces singly adsorbed copper in its trench. A larger unit slab leads to stronger interaction between coppers and their substrate. This renders subsequent adsorption shapes follow the order of strips, surface layers, and finally two-dimensional cages. Lattice mismatch between adsorbed Cu atoms and ZnO substrate also plays an indispensable role in differentiating these two deposition types, because coppers above the larger (112j0) surface have more space and a chance to relax and extenuate. Considering the geometric detail associated with the number of deposited coppers, it is interesting to note that there exist some exclusive structures at certain deposition amount, such as (101j0):3b and (112j0):4, which are formed by combination
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