Xiao
Wang
a,
Meng
Zhang
*a and
Wei
Cao
*b
aSchool of Physics, East China University of Science and Technology, Shanghai 200237, China. E-mail: mzhang@ecust.edu.cn
bNano and Molecular Systems Research Unit, University of Oulu, FIN-90014, Finland. E-mail: wei.cao@oulu.fi
First published on 19th March 2024
Superatomic clusters have received a lot of attention due to their ability to mimic the electronic configurations of individual atoms. Despite numerous studies of these clusters, their ability to mimic the chemical properties of individual atoms is still unclear. This also applies for Li3O/Li3O+ clusters which simulate the Na atom and its ion, but their capabilities to form a salt or be adsorbed on surfaces remain unexplored. In this work, a density functional theory investigation was performed to study the chemical formation and adsorption behavior of the superatomic Li3O cluster. The results show that Li3O mimics the chemical properties of the sodium element to form Li3O chloride and be adsorbed on graphdiyne and γ-graphyne with similar binding energy as the sodium adsorbate cases. Beyond the isolated cluster individuals, superatoms are demonstrated as elements from the 3D periodic table to construct compounds and attach onto solid surfaces.
Over the past twenty years, many theoretical and experimental studies have been conducted to create superatoms and explore their structures and properties. So far, a variety of superatoms, including superalkalis,9–11 superalkaline-earth-metal atoms,12 superhalogens,13–15 multivalent superatoms,16,17 and magnetic superatoms18–20 have been designed and identified using different electron-counting rules such as the jellium rule, octet rule and Wade–Mingos rule etc. These research studies further interpretate the concept of a superatom and provide a variety of structural building blocks for cluster-assembled materials (CAMs). For example, C60 can form an FCC solid through van der Waals force at room temperature and pressure.21–23 An alkali-like superatom TaSi16 can combine with C60, resulting in a superatomic complex with high thermal and chemical robustness.24,25 Other CAMs, such as ZnO-based CAMs, Al-based CAMs and zintl cluster-based CAMs, have also been synthesized and investigated.26–31 In fact, clusters can be assembled on substrates as well as in confined spaces. A proper substrate is the key to host clusters and preserves their superatomic identities. For instance, weak interactions between the FeCa8 superatom and the h-BN or graphene can maintain the structure and characteristics of the superatomic cluster. On the contrary, the strong interaction between the FeCa8 and a calcium substrate leads to a destruction of the magnetic properties of the cluster.32 Therefore, it is important to understand the interactions between superatomic clusters and their ligands or hosting matrices, and the interactions’ impacts on the structures and properties of bonded or adsorbed systems.
Among the studied superatoms, the structure and properties of superalkali atom Li3O in its neutral or charged form have been systematically investigated.33–36 In our previous work,37 the predicted Li3O+ has been identified in the gas form. Its formation process has been proposed and the possibility of application as a building unit for energy storage has been uncovered. It is further noticed that Li-rich antiperovskite (LiRAP) conductors Li3OA (A = halogen or superhalogens) with lithium superionic conductivity have been created and considered to have great promise for solid-state electrolytes.38–40 Recent first-principles studies of Li3O functionalized graphyne, graphdiyne and h-BN sheets indicated that superalkali clusters can inhibit metal agglomeration. Thanks to facilitated charge transfers, the cluster decorated sheets are found to be suitable for hydrogen storage.41,42 Despite the above advances, the superatomic cluster's bonding schemes with elements and interactions with matrices are far less understood compared to knowledge of the structural and electronic configurations of individual clusters. The chemistry of superatomic clusters (including the Li3O) remains elusive.
In this work, we carried out a density functional theory study to investigate the physical and chemical properties exhibited by Li3O clusters when interacting with ligands or substrates. At the free form level, the Li3O cluster can mimic a single Na atom in electronic configuration. It also mimics the bonding or interaction schemes of the Na when bonding with a halogen atom or being adsorbed on two-dimensional graphynes. A chemical bond is formed between the superatomic Li3O cluster and chlorine, and the adsorption energies of Li3O onto the graphynes are almost the same as the case using Na adsorbate. From the above results, it is hoped that this work will provide a theoretical reference for further application of these clusters in compound formations and surface interactions.
The binding/adsorption energies of the Na atom or Li3O cluster interacted with other Cl/F or GDY/γ-GY systems were computed as follows.
E = Etotal − [E(X) + E(Y)] | (1) |
The molecular orbital (MO) characteristics of the superatom Li3O are illustrated in Fig. 2 and the atomic orbitals (AOs) of the sodium atom are also depicted for comparison. It is shown that the electronic structure of Li3O can be described as 1S21P62S1, which bears resemblance to that of an individual sodium atom. Obviously, it is apt to lose one electron to achieve a closed shell electron configuration of 1S21P6, thus mimicking the chemical behavior of the alkali atoms. We further studied the electronic structure of the cluster in detail. The lowest valence MO of Li3O exhibits 1S character, followed by two degenerate 1Pz orbitals. The degenerate 1Px and 1Py orbitals are pushed 0.37 eV higher in energy owing to the planar structure of the Li3O cluster. This is slightly different from the sodium atom case where three degenerate p orbitals exist. These occupied MOs of Li3O are mainly composed of the AOs of the central atom O. Then, the following highest occupied molecular orbital (HOMO) of the cluster has a half-filled 2S superorbital character. Three 2P orbitals are the next higher energy orbitals. Similar to the case in 1P orbitals, 2P orbitals degenerate into two groups of one (Pz) and two (Px and Py) orbitals, as induced by the two-dimensional planar structure of the cluster. It should be noted that in Fig. 2, the electron cloud pervades the entire cluster. A shape difference stays between the cluster's 2S MO and the Na's 3s AO. The former appears flattened, while the latter is spherical. This discrepancy arises from the planar geometry of the Li3O. Therein, the delocalized electron distribution from each atom is confined around the 2D plane. An energy gap (Egap) of 0.631 eV was found between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO–LUMO). Although the Egap value of the Li3O cluster is smaller than that of the Na atom (1.633 eV), its chemical stability is approximate to that of the Na atom, which will be discussed in the following section.
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Fig. 2 Relative energy levels and orbitals of the Li3O cluster (left) and Na atom (right). EAO represents the empty atomic orbital. |
After careful optimization, the triangle Li3O cluster interacts with a Cl atom through two Li atoms through an edge connection. The binding energy (BE) of Li3O to Cl is −5.718 eV. The bond length (distance between Li and Cl atom) is 2.268 Å, very close to the value of 2.373 Å in the NaCl. Table 1 shows that the natural charge on the Cl is - 0.613 |e|, indicating that almost one electron transfers from the Li3O cluster to the Cl atom to form an ionic bond. Each Li atom of Li3O contributes an average charge of 0.494 |e|, sharing with O and Li atoms. The electrostatic interaction between Li3O+ and Cl− causes a slight variation in the average bond length of Li–O, which is elongated by 0.04 Å compared to the bare Li3O. All these observations indicate that the compound (Li3O)Cl is actually a ‘‘binary salt’’ consisting of Li3O+ and Cl−. The same ionic interaction is also observed in (Li3O)F which has a similar structure of (Li3O)Cl as shown in Fig. 1(c). The BE is −6.275 eV and the distance of the Li and F atom is 1.78 Å. The Li3O subunit carries 0.682 |e| net charge in (Li3O)F. Each Li atom transfers 0.516 |e| to O and F atoms. These characteristic data are also very close to those of NaF molecules. Given the above, it can be seen that Li3O is capable of combining with Cl or F atoms to form stable ionic compounds analogous to the sodium salts.
System | Binding energies (eV) | Bond length | Charge (|e|) | |
---|---|---|---|---|
Li3O/Na | Cl/F | |||
(Li3O)Cl | −5.718 | 2.268 | 0.613 | −0.613 |
NaCl | −4.319 | 2.373 | 0.713 | −0.713 |
(Li3O)F | −6.275 | 1.78 | 0.682 | −0.682 |
NaF | −5.251 | 1.94 | 0.74 | −0.74 |
We also constructed the 3Li–O–Cl clusters with all atoms randomly distributed and without a predetermined geometric structure for Li3O. Extensive two dimensional (2D) and three-dimensional (3D) structures were constructed to determine the lowest-energy geometries for the 3Li–O–Cl clusters. The details of the metastable structures are depicted in Fig. S1 in the ESI.† During the geometric optimization process, the O atom and the three Li atoms spontaneously tend to assemble into a superatomic cluster structure before interacting with the Cl atom. In other words, the introduction of the highly reactive chlorine with strong chemical properties does not impact the formation of the Li3O superatomic cluster in its free state. Hence, the superatom Li3O is highly stable and can be assembled easily, consistent with our previous experimental findings based on time-of-flight mass spectrometry.37
Next, the molecular orbitals of (Li3O)Cl and the corresponding NaCl are investigated and shown in Fig. 3. Since the main intention here is to provide a comparison of the orbital shapes between two kinds of compounds, the orbitals in Fig. 3 are arranged in the order of energy without strictly adhering to their exact energy positions. In NaCl, the Na 3s orbital combines with Cl 3pz to form molecular orbitals labeled as 4σ and 4σ* in Fig. 3 (left). They are named as HOMO−2 and LUMO−1. The 4σ orbital is weakly bonded and mostly Cl-like in character. The antibond 4σ* orbital is centered almost entirely on the Na atom. HOMO−1 comes from Cl 3px and 3py which could be labeled as 2π orbitals and displayed as HOMO−1. The electronic configuration of (Li3O)Cl is analogous to that of NaCl. It is obvious that the superatomic S and P orbitals behavior is similar to the s and p AOs of the Na atom. The σ, σ* and π orbitals of (Li3O)Cl are also analogous to the σ, σ* and π orbitals of NaCl, respectively. Two molecular HOMOs are similar to σ* orbitals indicating that the two molecules have some chemical similarities. This finding is in line with previous studies of the halogen-like superatom Al13 and chlorine atom, superatom B6 and Ni atom. Although there are some variations in orbital order that occur in the case of bonding with the Cl atom, the similarity between the Li3O and Na is still exhibited.
The geometrical structures of Li3O cluster adsorption on GDY and γ-GY are systematically studied. First, all possible adsorption sites are considered, including the hollow sites above the center of the carbon rings, the top site right above a carbon atom and the bridge site above the carbon bonds. After careful structural optimization, the most stable configurations of the Li3O decorated GDY and γ-GY are obtained and displayed in Fig. 5. In the complex of Li3O@GDY, the Li3O unit prefers to stay above the center of the triangle pore formed by the –CC–C
C– linkages. An adsorption energy of −3.70 eV is obtained and listed in Table 2, which is a little lower than the previous study.42 The shape of the Li3O cluster changes from a triangle to an umbrella where the plane formed by three Li atoms is parallel to the plane of GDY. The Li–O bond length increases to 1.71 Å after absorption. Meanwhile, the bond angles of Li–O–Li reduced to 113° from 120° in the pristine cluster. Li3O also prefers to bind on the hollow site of the C12-ring due to it having the largest binding energy of −3.56 eV in the Li3O@γ-GY case. The bond length of Li–O is 1.72 Å and the Li–O–Li angle changes to 107°. Additionally, the binding energy between Li3O and GDY is larger than that between Li3O and γ-GY. This is because each Li atom of the Li3O unit binds with four sp hybridized carbon atoms in the Li3O@GDY configuration. Due to the chemical activity of the triple carbon bonds of the GDY, the top site of the acetylenic ring is often the most stable adsorption site for single metal atom adsorption. However, according to the previous works,50–53 the atoms would automatically migrate to the corner when the transition metal (TM) or noble metal atoms were located at the center of the acetylenic ring. While alkali metals such as Li, Na and K would be anchored at the triangle center of GDY.54 These are in line with the adsorption of Na on the GDY and γ-GY, as depicted in Fig. 5(c) and (d). In addition, from Table 2, the adsorption energy of the Na atom on GDY or γ-GY is almost the same as the corresponding one of Li3O on GDY or γ-GY.
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Fig. 5 The optimized structures of Li3O adsorption on (a) GDY and (b) γ-GY. The optimized structures of Na atom adsorption on (c) GDY and (d) γ-GY. |
System | Adsorption energies (eV) | Charge (|e|) | |||
---|---|---|---|---|---|
Li atoms | O atom | Li3O/Na | Carbon sheet | ||
Li3O@GDY | −3.70 | 1.35 | −0.84 | 0.51 | −0.51 |
Na@GDY | −3.71 | 0.84 | −0.84 | ||
Li3O@GY | −3.56 | 1.50 | −0.87 | 0.63 | −0.63 |
Na@GY | −3.18 | 0.80 | −0.80 |
To better understand and compare the binding mechanism of these complexes, we studied the electronic structures of Na@GDY and Li3O@GDY as typical cases. The Na atom donates a single valence electron in the 3s orbital to the LUMO of GDY. Consequentially, a singly occupied molecular orbital (SOMO) is formed in Na@GDY. It is composed of p orbitals perpendicular to the GDY plane as shown in Fig. 6. Li3O@GDY has a similar intramolecular charge transfer mechanism. Due to the low ionization potential, the Li3O cluster donates its electrons to the acetylenic linkages of the GDY sheets and these electrons occupy the empty pz orbitals of the carbon frame. It is worth noting that Fig. 6 shows inconsistent electron clouds around the adsorbed Na on Na@GDY and Li3O on Li3O@GDY. While the Na atom does not exhibit obvious electron density in the center of the triangle, the adsorbed superatom behaves differently. The electron clouds are centered next to the oxygen and keep a similar P-type distribution as in the free cluster (Fig. 2) or in ionic bonds (Fig. 3). However, almost no electron cloud could be detected on the Li, denoting the charge transfer from the Li to neighboring atoms. A zoomed-in image of the adsorbed structure in Fig. 6(c) further denotes slight increase of the electron densities on the C atoms next to the Li atoms. This proves that the charge transfer happens from Li3O to the GDY after absorption. In other words, the inner shell electronic structure is kept within the cluster to keep the superatomic feature, but the valence electron from 2S is involved in charge transfer in the adsorption.
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Fig. 6 SOMO orbital diagram of the (a) Na@GDY, (b) Li3O@GDY and (c) zoomed-in region next to Li3O on (b). |
Furthermore, the bandgap energy of the adsorbed system is investigated. The pristine GDY is a semiconductor with a bandgap of 0.483 eV. However, both Na@GDY and Li3O@GDY exhibit metal behavior after decorations of the Na or Li3O. The bandgap energy decreases due to the redistribution of electrons following intramolecular charge transfers. Similar phenomena have also been reported in TM adsorbed GDY/GY due to electron transfers between the TM adatoms and the GDY/GY.50,51
The partial density of states (PDOS) of the Na@GDY and Li3O@GDY are further investigated and illustrated in Fig. 7. It can be observed that the PDOS shapes of both systems are quite similar. The s and p orbitals of Na atoms, as well as s orbitals of Li in the Li3O clusters, exhibit pronounced hybridization with C-2p orbitals of the GDY system around 2–6 eV. This suggests that both Na atoms and Li3O clusters can interact with GDY stably. The Li3O cluster adsorbed on GDY is comparable to that of a single Na atom adsorbed on GDY. However, in Li3O@GDY, besides the significant hybridization between the C-2p orbitals of GDY and the s orbitals of Li in the 2.5–6 eV range, the PDOS of Li below the Fermi level overlaps with O-2p. The overlaps indicate strong bonding between Li and O and denote high stability of the Li3O superatom after absorption. This is different from the case in Na@GDY where no overlap is observed at a similar energy range.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cp05478k |
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