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Adsorption of Ag on M-doped graphene: First principle calculations

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  • Corresponding author:

    Zhou Fan    E-mail: fanzhou505@163.com

  • Received: 22 November 2019Revised: 5 January 2020Accepted: 20 January 2020Available online: 11 February 2020
  • Graphene is an ideal reinforcing phase for a high-performance composite filler, which is of great theoretical and practical significance for improving the wettability and reliability of the filler. However, the poor adsorption characteristics between graphene and the silver base filler significantly affect the application of graphene filler in the brazing field. It is a great challenge to improve the adsorption characteristics between a graphene and silver base filler. To solve this issue, the adsorption characteristic between graphene and silver was studied with first principle calculation. The effects of Ga, Mo, and W on the adsorption properties of graphene were explored. There are three possible adsorbed sites, the hollow site (H), the bridge site (B), and the top site (T). Based on this research, the top site is the most preferentially adsorbed site for Ag atoms, and there is a strong interaction between graphene and Ag atoms. Metal element doping enhances local hybridization between C or metal atoms and Ag. Furthermore, compared with other doped structures (Ga and Mo), W atom doping is the most stable adsorption structure and can also improve effective adsorption characteristic performance between graphene and Ag.
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Adsorption of Ag on M-doped graphene: First principle calculations

  • Corresponding author:

    Zhou Fan    E-mail: fanzhou505@163.com

  • 1. School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China
  • 2. State Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering, Southwest Petroleum University, Chengdu 610500, China
  • 3. Journal Center, Southwest Petroleum University, Chengdu 610500, China

Abstract: Graphene is an ideal reinforcing phase for a high-performance composite filler, which is of great theoretical and practical significance for improving the wettability and reliability of the filler. However, the poor adsorption characteristics between graphene and the silver base filler significantly affect the application of graphene filler in the brazing field. It is a great challenge to improve the adsorption characteristics between a graphene and silver base filler. To solve this issue, the adsorption characteristic between graphene and silver was studied with first principle calculation. The effects of Ga, Mo, and W on the adsorption properties of graphene were explored. There are three possible adsorbed sites, the hollow site (H), the bridge site (B), and the top site (T). Based on this research, the top site is the most preferentially adsorbed site for Ag atoms, and there is a strong interaction between graphene and Ag atoms. Metal element doping enhances local hybridization between C or metal atoms and Ag. Furthermore, compared with other doped structures (Ga and Mo), W atom doping is the most stable adsorption structure and can also improve effective adsorption characteristic performance between graphene and Ag.

    • In recent years, the research of graphene-reinforced Ag-based filler metals has been given significant attention in various fields, such as aviation, equipment, and tools, particularly in electronic packaging, and the manufacturing of appliances that join different stainless steels (SS), glass, ceramics, and diamond [16]. Graphene form bonds with sp2 hybridization [78], which also behaves like a zero-gap semiconductor [8].

      Graphene, a carbon monolayer, has unique electronic properties in which the behavior of the electrons is similar to that of the massless Dirac fermions with high mobility [9]. Therefore, graphene is an ideal reinforcement for high-performance composites [1013] given its outstanding crack strength, high special electronic effects, and mechanical properties [1415]. However, graphene still has a problem of poor adsorption with fillers, which can lead to uneven distribution of graphene in the filler and even agglomeration [16]. Relevant experimental studies show that the main reason for the poor adsorption characteristic between silver and graphene is the low adsorption energy and charge transfer between them [1719]. Various solutions have been proposed to improve the adsorption property, including doping and introducing defects on graphene [2024]. As a result, the charge transfer and interaction between metal elements and graphene can be improved in doped graphene [21]. Ashraf et al. [22] indicated that doping can modulate charge carriers in graphene, thus affecting the adsorption characteristics. Recently, studies have shown that introducing defects can improve the Fermi level and adsorption performance [23]. In addition, the introduction of defects leads to the formation of a P-type semiconductor, which can increase the electrons transfer between systems [24]. He et al. [25] demonstrated that doping can increase the charge transfer between the adatom and graphene. Xu et al. [26] demonstrated that Ni doping of graphene avoids the problem of clustering. Therefore, doping can promote the electronic transfer between graphene and Ag atoms.

      In this paper, M (M = Ga, Mo, W) element doping of graphene is proposed. The manufacturing process of graphene-reinforced silver-based filler is mechanical stirring, and Ma Chaoli’s research shows that adding Ga to a silver-based filler can significantly reduce the oxidation resistance of the filler, improve its spreading performance and the mechanical properties of the brazing seam [27]. Therefore, Ga-doped graphene can increase the wettability of the graphene system as well as improve the brazing characteristics of the Ag filler. Some studies have shown that adding Mo particles to the Ag filler can reduce the mismatch of the expansion coefficient (CTE). In order to reduce the CTE mismatch between porous Si3N4 and brazing filler, Liu et al. [28] introduced Mo particles into an Ag−Cu−Ti filler and the strength of the weld joint was improved. Therefore, doped Mo in graphene may improve the adsorption between graphene and Ag and reduce the residual stress caused by the mismatch of expansion coefficients of Ag-based fillers at different interfaces [29]. In order to reduce the mismatch of thermal expansion coefficient, Yang et al. [30] added W particles when brazing ceramic and TiAl with silver-based solder, thereby reducing the residual stress and improving the bonding strength. Therefore, W was doped in graphene to improve the adsorption between graphene and Ag and reduce the mismatch of the thermal expansion of Ag-based filler metals in the brazing process.

      To resolve and improve the adsorption characteristic of graphene, the adsorption mechanism of graphene using a first principle calculation was investigated. Considering the adsorbed site, three adsorption sites were designed depending on their structural characteristics. Notably, the study of the effect of doping metal atoms on the adsorption characteristics of graphene was investigated. In addition, it was necessary to investigate the stability of the metal elements used in the doping. According to the stability of the different adsorption sites, graphene was doped with three metal elements: Ga, Mo, and W.

    2.   Model and method
    • The adsorption characteristics of graphene is largely related to the adsorption distance, doped elements, the interaction between adatoms and graphene, charge transfer, and the adsorption sites of the adsorption atom. Therefore, it is necessary to study the structure and morphology of graphene. Graphene is a sp2 hybrid and presents as a honeycomb-like plane. Because of its strong conductivity, thermal stability, and reliable mechanical properties, graphene is considered an ideal reinforcement for high-performance composite materials [3133].

      The optimized lattice constant of graphene is 2.46 Å, which is consistent with experimental and theoretical values [34]. According to structural characteristics, there are three typical highly symmetrical adsorption sites as presented in Fig. 1: hollow (H), bridge (B), and top site (T). The most stable adsorption site was obtained by calculation. Then, the effect of metal elements (M) on the adsorption characteristic of graphene was investigated. The adsorption structure of M-doped graphene is shown in Fig. 1.

      Figure 1.  Three adsorption sites on graphene.

      The first principle calculations of the parent graphene and M-doped graphene were calculated with the density functional theory (DFT) method and CASTEP code in Material studio software [3538]. The generalized gradient approximation (GGA) [3940] and Perdew–Burke–Ernzerh (PBE) [4142] were adopted to calculate the correlation effect and electron exchange-correlation interactions. The ultrasoft pseudopotential was employed in the interactions between the ions and electrons [43]. To eliminate the influence of the interaction between different cells on the calculation results, the supercell of 4 × 4 × 1 was established as the periodic boundary condition. Specifically, the vacuum in the system was 20 Å to ensure that periodic graphene layers did not react with each other, the cutoff energy was 450 eV and the k-point grid was 3 × 3 × 1. The self-consistent convergence condition was as follows: the total energy was less than 2.0 × 10−5 eV/atom, the force on each atom was less than 0.05 eV/Å, the tolerance offset was less than 0.002 Å, and the stress deviation was less than 0.1 GPa.

    3.   Results and discussion
    • The adsorption characteristics of graphene system are related to the adsorption energy ($ {E}_{\rm{ad}} $). The formula for calculating the adsorption energy is defined as

      where ${E_{{\rm{Ag - grapene}}}}$ and ${E_{{\rm{graphene}}}}$ are the total energy of Ag−graphene and the energy of the parent graphene, respectively; $ {\mu }_{\rm{Ag}} $ is the chemical potential of Ag.

      The adsorption characteristics of M-doped graphene are determined by the adsorbed energy ($ {E}_{\rm{M}}^{\rm{Ag}} $), and it is denoted as

      where $E_{{\rm{graphene/dop}}}^{{\rm{Ag}}}$ and ${E_{{\rm{graphene/dop}}}}$ are the total energy of the M-doped graphene adsorbed with Ag and the energy of the M-doped graphene, respectively [44]. Eq. (2) was used to study the effect of metal elements on the adsorption characteristics of graphene.

      To reveal the adsorption characteristic mechanism, the calculated lattice parameters, volume, and adsorbed energy of Ag−graphene were compared with the parent graphene in Table 1. There Ag-adsorbed types were considered: T site model, B site model, and H site model. The Ag atom was easily adsorbed on graphene because the adsorbed energy of all models was less than zero.

      The adsorption characteristics of graphene depend on the formation of a C−Ag bond according to the analysis of the electronic and adsorption structure. Notably, the thermodynamic stability of Ag adsorption at T site (−0.0585 eV) was more stable than that at B site (−0.0430 eV) and H site (−0.0202 eV), because the calculated value of the adsorbed energy of the Ag was the smallest. Therefore, the Ag atom is more likely to be adsorbed on the T site than other sites. This is similar to Granatier’s research [45], in which in the study of the interaction and electronic structure of the complexes at MP2 and M06-2X levels, the T site was determined to be the best site.

      Therefore, only the T adsorbed sites of the M-doped graphene are studied in the subsequent calculations. In particular, the adsorbed height of T site (3.554 Å) was the smallest than the other two (3.676 Å and 3.858 Å for B and H, respectively). Note that the calculated deformations of these three adsorption sites were small (0.0188, 0.0530 and 0.0711 Å) enough to be ignored, which is consistent with the results of Amft et al. [46]. At the same time, the number of electrons transferred from graphene to Ag (0.08 e) was greater than that at B and H sites (0.06 e).

      Typea / ÅD / ÅQ / eh / ÅEad / eV
      Graphene9.840
      T9.8593.5540.080.0188−0.0585
      B9.8983.6760.060.0530−0.0430
      H9.8523.8580.060.0711−0.0202
      Note: ∆h = hmax - haverage, where hmax represents the maximum atomic deformation height of the graphene layer and haverage represents the average atomic height of the graphene layer.

      Table 1.  Calculated lattice parameter (a-axis), the final adsorption distance (D), charge transfer (Q), deformation height (∆h) and adsorbed energy (Ead) of graphene

      The adsorption characteristic mechanism of graphene depends on its atomic configuration, which impacts the electrons transfer between graphene and the adsorbed atom [47]. Therefore, the electronic structure information of the Ag-adsorbed graphene system was further studied. As shown in Table 1, the adsorbed energy of the graphene ranges from −0.0202 to −0.0585 eV, which is in good agreement with the results of Pham et al. [48]. Obviously, the stability of Ag adsorption depends on the atomic structure of graphene, and the structure is determined by different lattice parameters [49]. The calculation results show that the Ag adsorbed on the T site of graphene causes the graphene supercell to expand along the a and b axes.

      According to Fig. 2, for the three doped models, the adsorption distance between Ag and graphene is shortened due to the existence of M atom, while the local deformation around the M atom is larger than parent graphene. Among them, W-doped graphene has the smallest deformation, but the deformation difference caused by the three kinds of doping was only about 0.04 Å, and can be considered negligible and ignored. The calculated adsorption distance of the W-doped graphene is 2.523 Å, which is shorter than those of graphene systems doped with other metal elements. Additionally, as shown in Table 2, the doped formation energy of the W-doped graphene system is −6.465 eV, which is almost twice that of the Ga-doped graphene system (−2.931 eV), and is 0.488 eV greater than that of the Mo-doped graphene system. Generally, the interaction between Ag and W is the strongest.

      Figure 2.  Front view of the (a) parent graphene, (b) Ga-doped graphene, (c) Mo-doped graphene, and (d) W-doped graphene.

      Compared with the charge transfer in an M-doped graphene/Ag system, the charge transfer from the Ag atom of the parent graphene (0.01) to a C atom is smaller. The charge transfer between Ag and Ga, Mo, and W atoms is 0.85, 1.49, and 1.96 e respectively, respectively. It indicates that electrons transfer from M atom to Ag atom via chemisorption, which greatly improves the stability and adsorption characteristics of the system. Compared with the other two doped atoms, the W atom can improve the adsorption characteristics more effectively. The results demonstrate that the local hybridization of Ag−W is stronger than that of graphene doped with other metal elements.

      Typea / ÅD / ÅQ / eh / Å$ {{{E}}}_{\bf{M}}^{\bf{Ag}} $ / eV
      Graphene/Ag9.8400.35540.010.0188−0.0585
      Ga-doped/Ag9.8980.34370.851.451−2.931
      Mo-doped/Ag9.8920.25911.491.489−5.977
      W-doped/Ag9.9040.25231.961.441−6.465

      Table 2.  Calculated lattice parameter (a-axis), the final adsorption distance (D), charge transfer (Q), deformation height (∆h), and adsorbed energy ($ {E}_{\rm{M}}^{\rm{Ag}} $) of Ga-doped, Mo-doped, and W-doped graphene at the T site

      To further observe the charge transfer between Ag and graphene, the electron density difference of the M-doped graphene/Ag system with the parent/Ag system is shown in Fig. 3. The yellow region represents the depletion of the charge and the blue region represents the accumulation of the charge. For parent graphene/Ag system in Fig. 3(a), there is almost no charge transfer among the Ag atom and the nearest C atom because the isosurface level is just 0.001 e·Å−3 The electron transfer from Ga to Ag in the adsorption system indicates that there is a bond pair among the Ag and Ga atom, which forms stable chemical adsorption. The different charge densities of the other two atoms (Mo and W) are intuitively the same; only the isosurface level is different, at 0.04 and 0.05 e·Å−3, respectively. The calculated outcome corroborated the calculated Q in Table 2 (only a slight difference in charge transfer). The differential charge densities of Mo and W atoms are petal-shaped near the Mo and W atoms, and the bonding orbitals are d orbitals in the Mo- or W-doped graphene. The charge transfer indicates that covalent bonds are formed between Mo or W and Ag, and the isosurface level value of the W-doped graphene/Ag is greater than that of the other doped elements, indicating that W better improves the adsorption characteristics of the system.

      Figure 3.  Three-dimensional electron density difference: (a) parent grapheme, isosurface level: 0.001 e·Å−3; (b) Ga-doped grapheme, isosurface level: 0.03 e·Å−3; (c) Mo-doped grapheme, isosurface level: 0.04 e·Å−3; (d) W-doped grapheme, isosurface level: 0.05 e·Å−3. The yellow region represents the depletion of the charge and the blue region represents the accumulation of the charge.

      To further understand the properties of chemical bond, the band structure and density of states was calculated as shown in Figs. 4 and 5, respectively, including M-doped graphene/Ag and parent graphene. It is clear that the Dirac point [5051] was formed at Fermi level in Fig. 4(a), which is consistent with previous experimental data [52]. In Figs. 4(b)4(d), the M element doping destroys the Dirac point, leading to a slight decrease in the Fermi level, and moving near the top of the valence band (VB) [53]. The Fermi levels pass through the VB and a band gap appears. Electrons transfer from the Ag atom to the Fermi level and fill the hole near the Fermi level, which results in a slight decrease in the Fermi level. This conclusion verifies the electron transfer data in Table 2. After the M doping in the graphene/Ag system, many new energy bands were introduced near the Fermi level and the number of energy bands increased the most in the W-doped system. Since the energy band of Ga or Mo was relatively flat, the locality was strong, indicating that the Ga- or Mo-doped slightly impeded the charge transfer on graphene. Alternatively, the energy band of the W-doped system was sharp, the locality was very weak, which indicates the W doping advances the charge transfer on the graphene to a great extent. The results are consistent with previous research.

      Figure 4.  Band structure: (a) parent graphene with adsorbed Ag atoms; (b) Ga-doped graphene with adsorbed Ag atoms; (c) Mo-doped graphene with adsorbed Ag atoms; (d) W-doped graphene with adsorbed Ag atoms.

      Figure 5.  PDOS of M-doped graphene adsorbed Ag: (a) parent graphene/Ag; (b) Ga-doped graphene/Ag; (c) Mo-doped graphene/Ag; (d) W-doped graphene/Ag.

      The partial density of states (PDOS) configuration file is provided by Mo-4p and Mo-4d orbital, Ag-4d and Ag-4p orbital, Ga-4p and Ga-3d orbital, Mo-4p and Mo-4d orbital, and W-5d and W-4p orbital. It is noteworthy that the Fermi level is at the vertex of the intersection of the conduction band (CB) and VB, and the band gap is equal to zero, which indicates that the graphene system is a semiconductor. However, due to the M doping, electrons migrate from a low-energy area to the Fermi level, which improves the electron overlay between CB and VB adjacent to the Fermi level, and further alters the electronic characteristics of graphene. As shown in Figs. 5(b)5(d), the violent local hybridization between the M and Ag atoms results in the formation of M−Ag bonds, which is consistent with the charge density difference in this study. The curves of the PDOS of the W-doped graphene/Ag system are especially different from those of the Ga- or Mo-doped graphene/Ag systems. For the Ga- and Mo-doped graphene/Ag system, the highest peak adjacent to the Fermi level is on the VB region (see Figs. 5(b) and 5(c)). However, for the W-doped graphene/Ag system, the highest peak adjacent to the Fermi level is situated on the CB region (see Fig. 5(d)), indicating there is violent localized hybridization among Ag and W. The calculated value of the W-doped system at the Fermi level is larger than that of the other doped system, which proves that the W-doped system is more stable than that of the Mo-doped system. In Fig. 5(d), because the 4d state and 3p state of Ag match the 5d state of W very well, the strong coupling confirms the existence of a steady chemical bond between W and Ag, which is consistent with the deduction in Fig. 4.

    4.   Conclusions
    • The adsorption mechanism of graphene using first principle calculation was studied. To explore the adsorption characteristic, three possible Ag-adsorbed sites were considered, hollow (H), bridge (B), and top (T). Importantly, the influence of metal elements among the adsorption characteristics of graphene was explored. The doped formation energy of the T site was smaller than the other two sites (B and H site) due to the graphene structure because the adsorbed energy of all models was less than zero. The stability of the M-doped graphene system was due to the formation of the M−Ag bond. Specifically, the doped formation energy and adsorption distance of the W-doped graphene was less than that of Ga- or Mo-doped graphene. The addition of metallic elements effectively enhanced the local hybridization of Ag and the M atom. The adsorption distance of Ag−W for W-doped graphene was smaller than that of Ga- or Mo-doped graphene. Consequently, W is a valuable element to improve the adsorption characteristics of graphene. The W-doped graphene-reinforced phase has strong adsorption characteristics with silver-based filler, which can prevent element diffusion and the formation of brittle compounds and improve the reliability of brazing.

    Acknowledgments
    • This work was financially supported by the Extracurricular Open Experiment of Southwest Petroleum University (No. KSZ18513) and the State Key Program of National Natural Science Foundation of China (No. 51474181).

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