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Article

Theoretical Study on the Structures and Stabilities of CunZn3O3 (n = 1–4) Clusters: Sequential Doping of Zn3O3 Cluster with Cu Atoms

by
Zhi-Wei Tao
1,
Han-Yi Zou
1,
Hong-Hui Li
1,
Bin Wang
1,* and
Wen-Jie Chen
2,*
1
College of Chemistry, Fuzhou University, Fuzhou 350108, China
2
Department of Material Chemistry, College of Chemical Engineering and Materials, Quanzhou Normal University, Quanzhou 362000, China
*
Authors to whom correspondence should be addressed.
Inorganics 2024, 12(2), 56; https://doi.org/10.3390/inorganics12020056
Submission received: 25 December 2023 / Revised: 3 February 2024 / Accepted: 7 February 2024 / Published: 9 February 2024
Browse Figures
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Abstract

:
Density functional theory (DFT) and coupled cluster theory (CCSD(T)) calculations are performed to investigate the geometric and electronic structures and chemical bonding of a series of Cu-doped zinc oxide clusters: CunZn3O3 (n = 1–4). The structural evolution of CunZn3O3 (n = 1–4) clusters may reveal the aggregation behavior of Cu atoms on the Zn3O3 cluster. The planar seven-membered ring of the CuZn3O3 cluster plays an important role in the structural evolution; that is, the Cu atom, Cu dimer (Cu2) and Cu trimer (Cu3) anchor on the CuZn3O3 cluster. Additionally, it is found that CunZn3O3 clusters become more stable as the Cu content (n) increases. Bader charge analysis points out that with the doping of Cu atoms, the reducibility of Cu aggregation (Cun−1) on the CuZn3O3 cluster increases. Combined with the d-band centers and the surface electrostatic potential (ESP), the reactivity and the possible reaction sites of CunZn3O3 (n = 1–4) clusters are also illustrated.

Graphical Abstract

1. Introduction

Cu-based catalysts have played important roles in industry, such as in electrocatalytic reduction of CO2, methanol steam reforming and water gas shift reaction [1,2,3,4]. But at the same time, Cu-based catalysts have suffered some restrictions, such as thermal instabilities and low selectivity [1,2]. Sintering and aggregation of the supported Cu nano-particles are considered to be among the reasons for deactivation of Cu-based catalysts [2]. To develop Cu-based catalysts with superior performance, the structure–activity relationship of catalytic active sites and the interaction of the main active component with the additives, including the supports, are the fundamental issues that need to be solved [5]. Cu/ZnO catalysts are among the commonly used Cu-based catalysts, in which Cu usually acts as the main active component and ZnO plays the dual role of promoter and support [6]. Specifically, Cu2+ ions would be reduced to the active Cu0/Cu+ species, and the addition of ZnO would increase copper dispersion and reducibility, which are related to enhanced catalytic activity [3,7]. Despite great progress, the interaction of the active copper component with other additives, as well as the reaction mechanisms on Cu-based catalysts, are still controversial due to the complexity of the surface [5]. In this context, gas-phase clusters with various sizes, charge states and stoichiometries could unravel the structural characteristics, bonding rules and reactivity of the clusters. Furthermore, the clusters which could emulate specific reactive centers have served as a fruitful tool for providing insight into the reaction mechanism of catalytic materials [8,9,10].
Additionally, well-supported single-/dual-atom and cluster catalysts have recently become research hotspots [11,12,13,14,15]. Among these, copper oxide clusters have been suggested to be the catalytic active sites of copper-exchanged zeolites in the methane oxidation to methanol [15]. Herein, copper oxide clusters with different stoichiometries and sizes were embedded in the zeolites channel, and the geometries and stabilities of these clusters and their underlying correlations with the catalytic activities were elaborated. Additionally, a fully exposed Cu7 cluster anchored to the loop-like [6]cycloparaphenylene was found to be highly active and selective in the CO electroreduction [14]. As for the ZnO clusters, it is accepted that (ZnO)n (n < 8) clusters favored the Zn–O alternating ring structure. As the size (n) of the (ZnO)n clusters increased to 8, a ring-to-cage transition occurred [16,17,18]. In these stoichiometric (ZnO)n (n = 1–13) clusters, (ZnO)3, (ZnO)9 and (ZnO)12 were found to possess relatively higher stability [18]. Notably, the discrete Zn3O3 cluster is the planar six-membered ring structure, and the wrinkled Zn3O3 six-membered ring can be found in the larger (ZnO)n (n > 8) clusters and the common exposed surfaces of hexagonal wurtzite ZnO crystals [17,19,20,21,22].
We have constantly strived to explore the novel chemical bonding of gas-phase clusters and to gain further insights into the active sites of complicated catalyst surfaces and the reaction mechanisms of catalytic processes [23,24,25,26]. As mentioned above, the sintering and aggregation of the supported Cu nano-particles are among the deactivation reasons for Cu-based catalyst [2]; whereas small-sized supported Cu clusters possess high activity in some reactions [14]. So, it may be interesting to explore the structural evolution and bonding in the CunZn3O3 (n = 1–4) clusters via the sequential adding of Cu atoms, as well as the interaction of doped Cu atoms with the Zn3O3 cluster. In this work, we make an effort to reveal the evolution rule of the geometric and electronic structures, as well as the chemical bonding in the Cu-doped CunZn3O3 (n = 1–4) clusters. Bader charge analysis, d-band center theory and surface electrostatic potential (ESP) are used to analyze the reactivity and reaction sites of CunZn3O3 (n = 1–4) clusters. This work may prove enlightening for our future studies on the mechanisms of cluster reactions.

2. Methods

The initial geometries of CunZn3O3 (n = 1–4) clusters were constructed using the structural searches of the ABCluster program [27,28] in combination with artificial constructions. These initial geometries were optimized using the B3LYP functional [29,30,31] in the Gaussian 09 program [32]. The Stuttgart small-core relativistic effective core potential (RECP) was used for the Cu and Zn atoms, whose corresponding basis sets are Cu: [6s,6p,4d,3f,2g]; Zn: [6s,6p,4d,3f,2g] [33,34,35,36]. As for O atoms, the aug-cc-pVTZ basis set was adopted [37,38]. In structural optimizations, the vibrational frequencies were calculated to ensure that the optimized structures were free of imaginary frequencies. In order to verify the reliability of the above computational methods (denoted as B3LYP/BS), we compared the bond lengths, binding energies, vibrational frequencies and dipole moments of ZnO and CuO molecules from the available experiments with our calculations. As shown in Table S1, our calculations are in agreement with the data from those experiments. The low-lying isomers within 0.50 eV at the B3LYP/BS level were then subjected to more accurate coupled cluster CCSD(T) single-point-energy calculations using the Molpro 2010 software [39]. Multiwfn program [40,41] was employed to analyze the Bader charge, surface electrostatic potential (ESP) and d-band centers of CunZn3O3 (n = 0–4) clusters.

3. Results

It is accepted that the Zn3O3 cluster is a planar six-membered ring structure with D3h symmetry (Figure 1a) [16], in which Zn and O atoms are alternately bonded (dZn-O = 1.817 Å). The distance between two Zn atoms is 2.654 Å.

3.1. Optimized Structures of CuZn3O3

CuZn3O3 clusters are constructed by adding a Cu atom to the most stable Zn3O3 cluster. The most stable structure of CuZn3O3 is shown in Figure 1b, which is consistent with the previous theoretical study [18]. It can be regarded as inserting a Cu atom into the Zn-O bond of the Zn3O3 cluster, leading to the planar seven-membered ring structure. The second low-lying isomer (Figure 1c) is 0.50 eV higher in energy, for which the six-membered Zn3O3 ring is broken. Other CuZn3O3 isomers are higher in energy by at least 0.50 eV. They are collected in the Supporting Information (Figure S1).

3.2. Optimized Structures of Cu2Zn3O3

To search for the ground state of the Cu2Zn3O3 cluster, a Cu atom was added to the most stable CuZn3O3 cluster. The optimized Cu2Zn3O3 clusters with relative energy below 0.50 eV are shown in Figure 2. The ground state of the Cu2Zn3O3 cluster (Figure 2a) can be viewed as inserting a Cu atom into the Zn-Cu bond of the CuZn3O3 cluster, resulting in a planar eight-membered ring structure. It is consistent with the earlier finding of [18] pertaining to the Cu2Zn3O3 cluster. The Cu-Cu bond length is 2.366 Å, slightly longer than the Cu-Zn bond length (2.343 Å). This is in agreement with the covalent radius of the Cu and Zn atoms (dCu = 1.52 Å, dZn =1.45 Å) [42] and suggests the metal–metal bonding of the inserting Cu atom with the neighboring Cu and Zn atoms. The second low-lying isomer (Figure 2b), which can be viewed as adding a bridged Cu to the CuZn3O3 ground state, is 0.39 eV less stable than the ground state. Other isomers are found to be much higher in energy (ΔE > 0.50 eV). They are collected in the Supporting Information (Figure S2).

3.3. Optimized Structures of Cu3Zn3O3

As the number of doped Cu atoms increases, more low-lying isomers appear for the Cu3Zn3O3 clusters (Figure 3). The most stable Cu3Zn3O3 cluster (Figure 3a) can be seen as adding a copper dimer (Cu2) between two bridged oxygen atoms of the CuZn3O3 ground state. At this point, the three-fold coordinated bridged oxygen atom (μ3-O) begins to appear in the ground states of CunZn3O3 clusters. The Cu-Cu bond length of Cu dimer (Cu2) is 2.388 Å in the Cu3Zn3O3 cluster, which is much longer than that (2.232 Å) of the isolated Cu2 molecule (D∞h 1Σg+) at the same calculation level. It is inferred that there are relatively strong interactions of the Cu2 moiety with the remaining fragments of Cu3Zn3O3. As shown in Figure 3, there are several isomers that have energies close to the ground state. To further distinguish the stability of these low-lying isomers, single-point CCSD(T) calculations were performed using their B3LYP equilibrium geometries. The single-point CCSD(T) calculations still support the structure contained the Cu2 (Figure 3a) being the most stable Cu3Zn3O3 cluster (Table S2). Other higher-energy isomers (ΔE > 0.50 eV) are shown in the Supporting Information (Figure S3).

3.4. Optimized Structures of Cu4Zn3O3

In our calculations, the most stable Cu4Zn3O3 cluster is shown in Figure 4a. It can be viewed as adding a copper trimer (Cu3) between two bridged oxygen atoms of the CuZn3O3 ground state. Meanwhile, several low-lying isomers within 0.50 eV were found (Figure 4). Among them, there is an isomer (Figure 4b) which contains the Cu4 moiety and is only 0.10 eV higher in energy. The relative energies of these isomers were further refined by CCSD(T) single-point calculations (Table S2). The CCSD(T) results support the structure shown in Figure 4a being the most stable one, and the isomer shown in Figure 4b is 0.28 eV less stable. Other higher-energy isomers (ΔE > 0.50 eV) are displayed in the Supporting Information (Figure S4).

4. Discussion

4.1. Structural Evolution in CunZn3O3 (n = 1–4) Clusters and Their Stability

It has been reported that the supported Cu2 and Cu3 clusters are the multi-atom cluster catalysts in specific reactions, and appropriate supports could improve their stability and dispersibility [43,44]. Zinc oxides are among the most common promoters and supports for Cu-based catalysts, the Zn3O3 six-membered ring is common in the larger (ZnO)n (n > 8) clusters and the wurtzite ZnO [17,19,20,21,22]. Studying the structural evolution of the CunZn3O3 (n = 1–4) clusters via the sequential doping of the Zn3O3 cluster with Cu atoms may help us gain insight into the aggregation behavior of Cu atoms on the Zn3O3 cluster.
For CuZn3O3, the Cu atom is inserted into the Zn-O bond of the Zn3O3 cluster. For Cu2Zn3O3, the Cu atom is inserted into the Zn-Cu bond of the CuZn3O3 cluster. As for Cu3Zn3O3 and Cu4Zn3O3 clusters, the planar seven-membered ring of the CuZn3O3 cluster starts to play an important role in the subsequent aggregation of Cu atoms (Figure 5); that is, the Cu dimer (Cu2) and Cu trimer (Cu3) are attached to the CuZn3O3 cluster by two bridged oxygen atoms (μ3-O). Herein, we found that at low Cu content (n = 1, 2), Cu atoms prefer to insert into the Zn-O bond of Zn3O3 first then aggregate to form the ZnCu2 units. The six-membered ring of Zn3O3 gradually expanded to the eight-membered ring of Cu2Zn3O3. When the Cu content further increases (n = 3, 4), the extra Cu atoms aggregate with each other to form Cun-1 units which are supported on the CuZn3O3 cluster.
The relative stability of CunZn3O3 (n = 1–4) clusters is evaluated via the calculated atomization energy (Eb,1). The atomization energy (Eb,1) of CunZn3O3 clusters was calculated via the following formula:
Eb,1 = nE(Cu) + 3E(Zn) + 3E(O) − E(CunZn3O3).
E(CunZn3O3), E(Cu), E(Zn) and E(O) represent the energy of the CunZn3O3 ground state and Cu, Zn and O atoms, respectively. As seen in Table 1, the Eb,1 increases gradually with the increase in Cu content. This suggests that the CunZn3O3 clusters become more stable with Cu atoms doping (n = 0–4).

4.2. Chemical Bonding of CunZn3O3 (n = 1–4) Clusters

It is known that zinc has the electronic configuration of 3d104s2. Usually, its 3d electrons do not participate in the bonding with other elements. So, in zinc oxides, there is almost an exclusively +2 oxidation state. But copper, as the neighbor element of zinc, has the 3d104s1 configuration, and its 3d electrons participate in the bonding. So, the oxidation state of Cu is more abundant (+1, +2 and +3) [45]. To better understand the charge transfer in the sequential doping of the Zn3O3 cluster with Cu atoms, we calculated the Bader charges of the CunZn3O3 (n = 1–4) clusters (Table 2). For the Zn3O3 cluster, the Bader charges of Zn and O atom are +1.13 |e| and −1.13 |e|, respectively. Obviously, the oxidation states of Zn and O in the Zn3O3 cluster are +2 and −2, respectively. Thus, the Bader charge of about ±0.5 |e| is indicative of a single-electron transfer, and the Bader charge of about ±1.0 |e| corresponds to a double-electron transfer [46].
With the Cu atoms doping, the Bader charge of the Zn-1 (as labeled in Figure 5) deviates considerably from +1.0 |e|, and its adjacent Cu atom (denoted as Cu-1) has a Bader charge of less than +0.5 |e|. This suggests an electron transfer between the Zn-1 atom and its adjacent Cu atom (denoted as Cu-1), and the valence state of the Cu-1 atom is Cuδ+ (0 < δ ≤ 1). Additionally, the Bader charges of the different elements seem to be related to the electronegativity (Pauling scale): 1.65 (Zn) < 1.90 (Cu) < 3.44 (O). The charges of Zn are always more positive than those of Cu, and the Cu atom which is bonded to oxygen always has a more positive charge than that of the Cu, which only connects to metal atoms. For example, the Bader charge of the Zn-1 atom in CuZn3O3 drops to +0.64 |e| and the charge of the Cu-1 atom is +0.43 |e|. This suggests a charge transfer between the Zn-1 atom and the adjacent Cu-1 atom, and the Cu-1 atom should be Cu+. A metal–metal bond is formed between Zn-1 and Cu-1, as indicated by the singly occupied molecular orbital (SOMO) in Figure 6a. On the other hand, the ZnCu unit in CuZn3O3 transfers 1.07 |e| to the nearby oxygen atoms (denoted as O-1 and O-3) in total. It is inferred that the ZnCu unit in CuZn3O3 transfers total two electrons to their adjacent oxygen atoms (O-1 and O-3), which leads to two metal–oxygen single bonds. With continued adding of a Cu atom to the CuZn3O3 cluster, the added Cu atom (denoted as Cu-2) inserts into the Zn-Cu bond of the CuZn3O3 cluster. As given in Table 2, the Bader charge of Cu-2 in Cu2Zn3O3 is only +0.02 |e|. This could be understood in terms of the electronegativity discussed above, and the Cu-2 atom may be assigned as Cu0. There are metal–metal bonds between Cu-2 and the Zn-1 and Cu-1 atoms, corresponding to the molecular orbital diagrams shown in Figure 6b. Herein, the Bader charge of Zn-1 slightly increases to +0.75 |e|, and the charge of the Cu2 unit is calculated to be +0.39 |e|. This suggests more electron transfers between the Zn-1 atom and its adjacent Cu2 unit. Furthermore, the ZnCu2 unit in the Cu2Zn3O3 cluster transfers 1.14 |e| to the adjacent oxygen atoms (O-1 and O-3) in total, which corresponds to two metal–oxygen single bonds. Compared with the Zn3O3 cluster, the charges of the other atoms in CunZn3O3 (n = 1,2) clusters do not change much.
For CunZn3O3 (n = 3, 4) clusters, they can be viewed as adding the Cu2 (Cu-2 and Cu-3) and Cu3 (Cu-2, Cu-3 and Cu-4) to the CuZn3O3 cluster linked by two three-fold coordinated oxygen atoms (O-1 and O-3). As mentioned above, the Bader charges of the Zn-1 and Cu-1 atom in CuZn3O3 are +0.64 |e| and +0.43 |e|, respectively. But in CunZn3O3 (n = 3, 4), the charge of Zn-1 reduces to +0.37 |e|, and that of Cu-1 also decreases to roughly +0.3 |e|. This suggested fewer charge transfers from the ZnCu diatom of CunZn3O3 (n = 3, 4) to the O-1 and O-3 atoms compared with the charge transfers in the CuZn3O3 cluster. Here, the valence state of Cu-1 atom is predicted to be Cuδ+ (0 < δ < 1). As compensation, the newly added Cu2 and Cu3 units in CunZn3O3 (n = 3, 4) transfer charges of +0.51 |e| and +0.56 |e| to the O-1 and O-3 atoms. As depicted in Figure 6c,d, there are metal–metal bonds in the CuZn unit and in the Cun-1 units of CunZn3O3 (n = 3,4) clusters. To analyze the interaction of Cu aggregation (Cun−1) with the CuZn3O3 cluster, the binding energies (Eb,2) of the isolated Cun−1 clusters with the CuZn3O3 cluster were calculated via the following formula:
Eb,2 = E(CunZn3O3) − E(CuZn3O3) − E(Cun−1).
E(CunZn3O3), E(CuZn3O3) and E(Cun−1) represent the ground-state energy of CunZn3O3, CuZn3O3 and Cun−1 clusters, respectively. The Eb,2 of Cu2 in the Cu3Zn3O3 cluster is calculated to be −1.60 eV, and that of Cu3 in the Cu4Zn3O3 cluster is −3.21 eV. The more negative Eb,2 means a stronger interaction between Cu aggregation (Cun−1) and the CuZn3O3 cluster and a higher stability of Cun−1 on the CuZn3O3 seven-membered ring. Here, the more negative binding energies (Eb,2) coincide with the more transferred charge from Cun−1 to CuZn3O3. For Cu/ZnO catalysts, the addition of ZnO is conducive to increasing the dispersion and reducibility of the active copper component [47]. From the perspective of Bader charge, the Cun−1 in CunZn3O3 (n = 3, 4) is more reducible than the Cun in CunZn3O3 (n = 1, 2). The synergistic interaction between Cu and Zn in CuZn3O3 may enhance the reducibility of Cu species in CunZn3O3 (n = 3, 4).

4.3. Reactivity of CunZn3O3 (n = 1–4) Clusters

The model of the d-band center was developed by Nørskov and co-workers [48] and was used as an important descriptor to determine the reactivity of surfaces and clusters [49,50,51,52,53]. The partial density of states (PDOS) for the d-orbitals of metal atoms in CunZn3O3 (n = 0–4) clusters are depicted in Figure 7, and the d-band centers (εd) are denoted by the red solid line. For the open-shell systems, the spin up (α) and spin down (β) d-band centers (εd) were calculated separately (Table S3), and the spin down ones were always higher in energy. So, we uniformly use the spin down d-band centers (εd) for the subsequent comparison. The energy levels of the highest occupied molecular orbital (HOMO-β) are marked by the blue dashed line. For comparison, all HOMO energy levels in Figure 7 are shifted to zero. As shown in Figure 7f, the εd moves toward HOMO-β as the Cu content (n) increases. This suggests that the interactions between nucleophilic molecules and the metal atoms become stronger as the Cu content (n) increases [49,52] and also indicates that the reactivity of CunZn3O3 (n = 0–4) clusters increase as the Cu content (n) increases.
The electrostatic potential (ESP) provides a means of identifying the active sites [49,54]. The surface ESP for CunZn3O3 (n = 1–4) clusters are shown in Figure 8. Obviously, the red-colored (positive ESP) regions are positioned at the metal atoms, and the ESP of CunZn3O3 clusters are less localized compared to the Zn3O3 clusters. Additionally, the cyan and yellow tiny spheres in Figure 8 point out the locations of the extreme points of the surface ESP, and the arrows indicate the extreme points with the maximum absolute values. The sites with the most positive values of molecular ESP are associated with the ideal adsorption positions for nucleophilic reagents, whereas the most negative ESP are related to that of electrophilic reagents. In this series of CunZn3O3 (n = 1–4) clusters, the most-positive regions of ESP are always nearby the Zn-2 atom, except for Cu2Zn3O3. Except for Cu2Zn3O3, the other CunZn3O3 clusters can be viewed as adding the Cu2 and Cu3 units to the CuZn3O3 cluster linked by two three-fold coordinated oxygen atoms (O-1 and O-3). For Cu2Zn3O3, the newly added Cu atom (Cu-2) expands the seven-membered ring of CuZn3O3 to the eight-membered ring. The most-positive region of ESP of Cu2Zn3O3 is nearby the newly added Cu-2 atom. In CunZn3O3 (n = 1–4) clusters, the ESP of the three-fold coordinated oxygen atoms is more negative than that of the two-fold coordinated oxygen atoms. For CuZn3O3 and Cu2Zn3O3, the most-negative regions are located near the O-1 or O-3 atom. For Cu3Zn3O3 and Cu4Zn3O3, the most-negative regions are located near the O-2 atoms. They indicate the sensitivity of reactivity to the structures.

5. Conclusions

We report a systematic theoretical study of a series of copper-doped zinc oxide clusters: CunZn3O3 (n = 1–4). The geometric and electronic structures and the chemical bonding of CunZn3O3 (n = 1–4) clusters are investigated via extensive density functional theory (DFT) and coupled cluster theory (CCSD(T)) calculations. The structural evolutions of CunZn3O3 (n = 1–4) clusters are found in our work. At the low Cu content (n = 1, 2), Cu atoms prefer to insert into the Zn-O bond of Zn3O3 first; then, they aggregate to form the ZnCu2 units. The six-membered ring of Zn3O3 gradually expands to the eight-membered ring of Cu2Zn3O3. When the Cu content further increases (n = 3, 4), the extra Cu atoms aggregate with each other to form Cun−1 units on the CuZn3O3 cluster. Additionally, the relative stability of CunZn3O3 (n = 1–4) clusters is evaluated. The CunZn3O3 clusters become more stable with the doping of Cu atoms (n = 1–4). Bader charge analysis suggests that as the Cu content (n) increases, the reducibility of Cu aggregation (Cun−1) on the CuZn3O3 cluster increases. The studies on the d-band centers of CunZn3O3 (n = 0–4) clusters indicate that the reactivity also increase as the Cu content (n) increases. Information on the possible reaction site of CunZn3O3 (n = 1–4) clusters are predicted by surface electrostatic potential (ESP) calculations. This work may inspire future studies on the reactions of related clusters.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/inorganics12020056/s1. Table S1: Calculated results at the B3LYP/BS level for the bond lengths, binding energies and other properties of ZnO and CuO along with the corresponding available experimental data. Table S2: Relative energies of CunZn3O3 (n = 1–4) clusters which were further refined by the CCSD(T) single-point calculations. Table S3: The calculated d-band centers for the spin up (α), spin down (β) and both spin modes of CunZn3O3 (n = 0–4) clusters. Figures S1–S4: Alternative optimized structures for CunZn3O3 (n = 1–4) clusters at the B3LYP/BS level. Table S4: Cartesian coordinates for the optimized CunZn3O3 (n = 0–4) clusters. References [55,56,57,58] are cited in the Supplementary Materials.

Author Contributions

Investigation, Z.-W.T.; visualization, H.-Y.Z. and H.-H.L.; writing—original draft preparation, Z.-W.T. and H.-Y.Z.; writing—review and editing, B.W. and W.-J.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (NSFC Grant Nos. 21301030 and 21603117).

Data Availability Statement

The data presented in this study are available from the corresponding authors upon reasonable request.

Acknowledgments

The authors gratefully acknowledge supports from the National Natural Science Foundation of China.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Optimized structures (ΔE ≤ 0.50 eV) for Zn3O3 and CuZn3O3. The bond lengths are in angstroms and the relative energies (ΔE) in eV are in the parentheses.
Figure 1. Optimized structures (ΔE ≤ 0.50 eV) for Zn3O3 and CuZn3O3. The bond lengths are in angstroms and the relative energies (ΔE) in eV are in the parentheses.
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Figure 2. Optimized structures (ΔE < 0.50 eV) for Cu2Zn3O3. The bond lengths are in angstroms and the relative energies (ΔE) in eV are in the parentheses.
Figure 2. Optimized structures (ΔE < 0.50 eV) for Cu2Zn3O3. The bond lengths are in angstroms and the relative energies (ΔE) in eV are in the parentheses.
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Figure 3. Optimized structures (ΔE < 0.50 eV) for Cu3Zn3O3. The bond lengths are in angstroms and the relative energies (ΔE) in eV are in the parentheses.
Figure 3. Optimized structures (ΔE < 0.50 eV) for Cu3Zn3O3. The bond lengths are in angstroms and the relative energies (ΔE) in eV are in the parentheses.
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Figure 4. Optimized structures (ΔE < 0.50 eV) for Cu4Zn3O3. The bond lengths are in angstroms and the relative energies (ΔE) in eV are in the parentheses.
Figure 4. Optimized structures (ΔE < 0.50 eV) for Cu4Zn3O3. The bond lengths are in angstroms and the relative energies (ΔE) in eV are in the parentheses.
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Figure 5. Structural evolution of CunZn3O3 (n = 1–4) clusters.
Figure 5. Structural evolution of CunZn3O3 (n = 1–4) clusters.
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Figure 6. Selected molecular orbitals for the ground-state CunZn3O3 (n = 1–4) clusters. The red and blue colors stand for different phases of the wave functions.
Figure 6. Selected molecular orbitals for the ground-state CunZn3O3 (n = 1–4) clusters. The red and blue colors stand for different phases of the wave functions.
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Figure 7. (a–e) The d-band density of states for the lowest-energy CunZn3O3 (n = 0–4) clusters. The inset is the enlarged drawing of the d-band center. (f) The d-band center (εd) as a function of Cu content (n) in CunZn3O3 (n = 0–4) clusters.
Figure 7. (a–e) The d-band density of states for the lowest-energy CunZn3O3 (n = 0–4) clusters. The inset is the enlarged drawing of the d-band center. (f) The d-band center (εd) as a function of Cu content (n) in CunZn3O3 (n = 0–4) clusters.
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Figure 8. The electrostatic potential (ESP) map on the van der Waals surface for the lowest-energy CunZn3O3 (n = 0–4) clusters.
Figure 8. The electrostatic potential (ESP) map on the van der Waals surface for the lowest-energy CunZn3O3 (n = 0–4) clusters.
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Table 1. Atomization energy (Eb,1) of the CunZn3O3 cluster. The energies are in eV.
Table 1. Atomization energy (Eb,1) of the CunZn3O3 cluster. The energies are in eV.
ClusterZn3O3CuZn3O3Cu2Zn3O3Cu3Zn3O3Cu4Zn3O3
Eb,122.0724.0326.6828.2831.27
Table 2. Bader charges (|e|) analysis of CunZn3O3 (n = 0–4).
Table 2. Bader charges (|e|) analysis of CunZn3O3 (n = 0–4).
ClusterZn-1Zn-2Zn-3O-1O-2O-3Cu-1Cu-2Cu-3Cu-4
Zn3O31.131.131.13−1.13−1.13−1.13
CuZn3O30.641.121.15−1.15−1.14−1.060.43
Cu2Zn3O30.751.141.12−1.15−1.15−1.100.370.02
Cu3Zn3O30.371.111.12−1.16−1.14−1.090.280.250.26
Cu4Zn3O30.371.121.11−1.17−1.14−1.110.260.310.31−0.06
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Tao, Z.-W.; Zou, H.-Y.; Li, H.-H.; Wang, B.; Chen, W.-J. Theoretical Study on the Structures and Stabilities of CunZn3O3 (n = 1–4) Clusters: Sequential Doping of Zn3O3 Cluster with Cu Atoms. Inorganics 2024, 12, 56. https://doi.org/10.3390/inorganics12020056

AMA Style

Tao Z-W, Zou H-Y, Li H-H, Wang B, Chen W-J. Theoretical Study on the Structures and Stabilities of CunZn3O3 (n = 1–4) Clusters: Sequential Doping of Zn3O3 Cluster with Cu Atoms. Inorganics. 2024; 12(2):56. https://doi.org/10.3390/inorganics12020056

Chicago/Turabian Style

Tao, Zhi-Wei, Han-Yi Zou, Hong-Hui Li, Bin Wang, and Wen-Jie Chen. 2024. "Theoretical Study on the Structures and Stabilities of CunZn3O3 (n = 1–4) Clusters: Sequential Doping of Zn3O3 Cluster with Cu Atoms" Inorganics 12, no. 2: 56. https://doi.org/10.3390/inorganics12020056

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