71 American Academic Scientific Research Journal for Engineering, Technology, and Sciences ISSN (Print) 2313-4410, ISSN (Online) 2313-4402 http://asrjetsjournal.org/ Acetone Adsorption to Co3O4（111）Surface: A Density Functional Theory (DFT) Study Yanyun Feng a* , Haiming Zhang b , Peng Lu c , Xiaomin Jia d a,b,c,d School of physical science and technology, Tiangong University, Tianjin 300387, China a Email: 1784023659@qq.com, b Email: zhmtjwl@163.com, c Email: lupeng@tiangong.edu.cn d Email: 1767306505@qq.com Abstract Acetone, as one of VOCs, is not only polluting to the environment, but also harmful to humans. Therefore, detecting acetone is an important topic in the field of gas sensing. The carbonyl functional group determines the chemical properties of acetone. Aldehydes also contain carbonyl functional group. In this paper, we have calculated adsorption energy, adsorption distance and transfer charge by DFT. The results showed that the top of Co 3+ on Co3O4 (111) surface has the best selectivity for sensing acetone. Our study contributes to the further study of the sensing properties of p-type metal oxide semiconductor sensors. Keywords: Acetone; Carbonyl functional group; Co3O4(111) surface; DFT methods; Sensing mechanism. 1. Introduction Volatile organic compounds (VOCs) can pollute the environment and harm human health. Acetone is one of VOCs and one of the gas in human respiration. It is used as a respiratory marker for non-invasive diagnosis of diabetes mellitus[‎[1]]. Therefore, the detection of acetone is important for environmental safety and human health. In recent decades, many metal oxides (ZnO, In2O3, Fe2O3, SnO2, MnO2, WO3) have been used in sensitive studies of acetone[2, 5]. Among them, Co3O4 crystal, as a typical p-semiconductor, has the advantages of high cost-effectiveness, low pollution and good stability. Many experimental and theoretical results prove that Co3O4 has great potential to detect VOCs in the field of gas sensing. At the experimental level, Yao and colleagues synthesized four Co3O4 nanocatalysts with main exposure (100), (111), (110) and (112) surfaces by hydrothermal method[‎[6]]. At the theoretical level, Liu and colleagues studied and reported on the ability of Co3O4 to improve acetone response and gas sensitivity by doping W and Mn atoms[‎[7]]. Some researchers also found that the preferential growth of Co3O4 (111) surface favors the formation of Co3O4 octahedra[8, 11]. In summary, we can guess that (111) surface is special for Co3O4 and according to previous reports on acetone adsorption, the top of Co 3+ has the best selectivity for sensing acetone[‎[12]]. ------------------------------------------------------------------------ * Corresponding author. American Academic Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2023) Volume 92, No 1, pp 71-76 72 To our knowledge, the functional group of acetone molecule is the carbonyl group (O=C). Its chemical properties are expressed in the carbonyl group. Inspired by this, we have studied the selectivity of Co3O4(111) surface to formaldehyde and butanone gas molecules that also contain carbonyl group, to better understand the mechanism of gas sensitivity of Co3O4 special surface to acetone. In this paper, the adsorption properties of acetone on Co3O4 (111) surface were studied based on density functional theory (DFT). The adsorption energy, adsorption distance and transfer charge were calculated to study the effect of carbonyl group on adsorption of acetone from the theoretical level. The research method is universal and can be used as an effective research route. 2. Computational methods and modeling All density functional theory (DFT) calculations are implemented with Cambridge Serial Total Energy Package (CASTEP) contained in the Materials Studio (MS) software[13]. The generalized-gradient approximation (GGA) of Perdew, Burke, and Ernzerh (PBE) of exchange-correlation functional is used for geometry optimization and adsorption properties[13]. For Co3O4 bulk geometry optimizations, a Monkhorst-Pack grid of 5 × 5 × 5 was used to sample the first Brillouin zone of k space for better accuracy. For the slabs geometry optimizations, the Monkhorst−Pack grid was 3 × 3 × 1. The cutoff energy of 400 eV was applied, and the convergence threshold parameters for the optimization were 1 × 10 -5 Ha (energy), 0.03 eV/ Å (force), and 1 × 10 -3 Å (displacement), respectively[14]. When the structure of Co3O4 bulk was optimized, fixed the bottom three atoms and relaxed the top three atoms. In this study, the Co3O4 (111) surface structure was derived from a 2 × 2 × 1 supercell block, with 56 atoms (Co24O32). Then, a 15 Å vacuum was added above layers to simulate the surface[15]. The fully relaxed bulk structure parameters of Co3O4 (111) surface are a = b = 11.58 Å, c = 18.70 Å and α = β = γ = 90°. Adsorption energy and transfer charges are utilized to analyze the stability of acetone gas adsorbed on the Co3O4 (111) surface. The adsorption energy (Eads) and transfer charges (Qt) were calculated as follow: Eads = ECo3O4 + Eacetone - Eacetone/Co3O4 (1) Qt = Qa - Qb (2) Where Eacetone/Co3O4 is the total energy of acetone gas molecule adsorbed on the Co3O4 (111) surface, Eacetone and ECo3O4 are the energy of acetone gas molecule in a big box and the Co3O4 slab, respectively. A positive value of Eads indicates that the adsorption process is exothermic, while a negative value of Eads indicates that the adsorption process is endothermic[‎[15]]. In formula (2), Qa and Qb represent the amount of charge before and after adsorption of gas molecules, respectively[‎[16]]. If Qt > 0, it represents that electrons transfer from acetone gas molecule to Co3O4 surface. If Qt < 0, it represents that electrons transfer from the Co3O4 surface to acetone gas molecule. In addition, we have mapped the charge density difference (CDD) to better clarify electron interactions and charge transfer between gas molecules and adsorbents[17]. American Academic Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2023) Volume 92, No 1, pp 71-76 73 3. Results and discussion 3.1. Construction of references In order to investigate configurations of acetone adsorbed on Co3O4 (111) surface, We firstly analyzed the characteristics of Co3O4 crystal structure. The space group of Co3O4 crystal is Fd-3m (227). As is shown in Figure 1, there are two kinds of cobalt atoms in Co3O4 crystal, namely Co 2+ (Co fills the tetrahedral void) and Co 3+ (Co fills the octahedral void). As is shown in Figure 2 (a), the terminating surface of Co3O4 (111) surface contains Co 3+ and Co 2+ . Referring to previous reports, the O atom of acetone usually attaches to metal atom vertically downward[18, 19]. Therefore, as is shown in Figure 2 (b), the top of Co 3+ atom are selected as the adsorption sites of gas molecules. Figure 1: Side view of the structure of Co3O4 (2×2) supercell. (a) (b) Figure 2: Optimal configuration of Co3O4(111) surface: (a) side view; (b) Top view. 3.2. Adsorption properties As is shown in Figure 3, these are adsorption configurations of gas on Co3O4 (111) surface. The adsorption energy, transfer charge and Co-O bond length parameters calculated by each adsorption configuration are listed in Table 1. From table 1, it can be found that all three adsorption energy are positive. It indicates that the adsorption processes are exothermic, and each adsorption system is spontaneously adsorbed. By comparison (Table 1), it can be found that the adsorption energy of acetone is relatively larger than the remaining two. It illustrates that acetone molecules is more easily adsorbed to Co3O4. In addition, the Co-O bond lengths (2.14 Å, American Academic Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2023) Volume 92, No 1, pp 71-76 74 2.418 Å, 2.036 Å) are all less than the Van der Waals radius (3.0-5.0 Å) [‎[20]] . It illustrates that a chemical bond is formed between the Co atom of Co3O4 and the O atom of gas. Table 1: Parameters of adsorption properties of gas molecular on the surface of Co3O4 (111). Model Adsorption energy (eV) Distance(Å) Transferred electrons(e ) Acetone 1.86 2.140 0.16 Formaldehyde 1.10 2.418 0.11 Butanone 1.28 2.036 0.09 Figure 3: Adsorption configuration of gas molecular on Co3O4(111) surface: (a) Acetone; (b) Formaldehyde; (c) Butanone. 3.3. Hirshfeld charge analysis Hirshfeld charge refers to the description of the affinity strength of an atom or intramolecular electron in a system. The hirshfeld charge analysis of gas molecule on Co3O4 (111) surface are reported in the Figure 4. Here, the green region denotes the neutral zone, the bule region denotes trapped electrons, and the red region denotes the loss of the electrons [21] . It is clear that the O atoms of acetone molecule and the Co atom regions of the Co3O4 are more blue and red than the others. It indicates that the number of electrons transferred during this adsorption process is higher, indicating that acetone molecules are more easily adsorbed to Co3O4 (111). Figure 4: Differential charge diagram of gas adsorption to Co3O4(111) surface: (a) Acetone; (b) Formaldehyde; (c) Butanone. (a) (b) (c) (a) (b) (c) American Academic Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2023) Volume 92, No 1, pp 71-76 75 4. Conclusion Acetone, formaldehyde and butanone all contain carbonyl functional groups. The influence of adsorption properties of Co3O4 (111) toward them have been studied by DFT. The most stable adsorption structures and adsorption characteristic parameter (the adsorption energy, transfer charges and adsorption distances ) have been calculated to analyze the gas sensing performance of gas molecular adsorbed Co3O4 (111). The results showed that Co3O4 (111) surface had the best selectivity for acetone. This report contributes to the further study of the associated sensing property of p-type metal oxide semiconductor gas sensors. 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