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First-Principles Investigation of Adsorption of Ag on Defected and Ce-Doped Graphene

First-Principles Investigation of Adsorption of Ag on Defected and Ce-Doped Graphene

materials

Article First-Principles Investigation of Adsorption of Ag on Defected and Ce-doped

Zhou Fan 1,*, Min Hu 1,*, Jianyi Liu 2, Xia Luo 1, Kun Zhang 1 and Zhengchao Tang 1

1 School of and Engineering, University of Southwest Petroleum, Chengdu 610500, China; [email protected] (X.L.); [email protected] (K.Z.); [email protected] (Z.T.) 2 State Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering, Southwest Petroleum University, Chengdu 610500, China; [email protected] * Correspondence: [email protected] (Z.F.); [email protected] (M.H.)

 Received: 21 January 2019; Accepted: 18 February 2019; Published: 21 February 2019 

Abstract: To enhance the wettability between Ag and graphene of graphene-reinforced silver-based composite filler, the adsorption behavior of Ag atoms on graphene was studied by first-principles calculation. This was based on band structure analysis, both p-type doping and n-type doping form, of the vacancy-defected and Ce-doped graphene. It was verified by the subsequent investigation on the density of states. According to the charge transfer calculation, p-type doping can promote the electron transport ability between Ag atoms and graphene. The adsorption energy and population analysis show that both defect and Ce doping can improve the wettability and stability of the Ag-graphene system. Seen from these theoretical calculations, this study provides useful guidance for the preparation of Ag-graphene composite fillers.

Keywords: first-principles calculation; graphene; Ag adsorption; wettability; defect and Ce doping; Ag-graphene composite filler

1. Introduction Silver-based filler and graphene are the two main materials that will be discussed in this study. Silver-based filler is a very important kind of brazing material, which has the advantages of a high strength, good mechanical properties, a high electrical conductivity, corrosion resistance, and so on [1–3]. It can join carbide, ceramics, glass, and diamond, and therefore plays an important role in the development of brazing material [4–7]. Silver-based filler is widely used in the aerospace, super-hard tools, and other manufacturing fields. Owning to its two-dimensional structure and unique physicochemical properties [8–10], graphene has excellent properties, such as a high surface area, fracture strength, and low density [11,12], making itself an ideal reinforcement for high performance composites [13–16]. At present, although graphene reinforced silver-based composite filler has been extensively studied in various types of metal and ceramic welding [17–22], it still faces the potential problem of poor wettability for graphene [23]. Experiments have shown that the adsorption energy and charge transfer between Ag and graphene are low due to the poor wettability [24–26], whereas doping in graphene can improve the interaction and charge transfer between graphene and metal [27]. Ali Ashraf [28] suggested that the doping-induced modulation of the charge carrier density in graphene influences its wettability and , for the first time. Recently, the results of some experiments showed that the vacancy defect can increase the adsorption energy and increase the Fermi level [29]. In addition, a further study found the formation of p-type doping with vacancy defect graphene, which can promote the electron transport of graphene and metal [30]. The above studies show that doping can significantly improve the adsorption performance between graphene and metal. Rare earth elements are called metal "vitamins", and trace rare earths can significantly increase the properties of metals [31].

Materials 2019, 12, 649; doi:10.3390/ma12040649 www.mdpi.com/journal/materials MaterialsMaterials2019 ,201912, 649, 12, x FOR PEER REVIEW 2 of 88

experimental studies have shown that doping with rare earth elements can improve the wettability Previousof Ag30CuZn experimental filler [32]. studies Based have on this shown finding, that doping the hypothesis with rare has earth been elements proposed can improvethat doping the wettabilitygraphene with of Ag30CuZn rare earth fillerelement [32]. Ce Based would on enhance this finding, the wettability the hypothesis between has been graphene proposed and thatAg. dopingBesides, graphene it is usually with difficult rare earth to obtain element the Ce data would of adsorption enhance the energy wettability and charge between transfer graphene fromand the Ag.experiments. Besides, itTherefore, is usually a difficult first-principles to obtain quantu the datam-mechanical of adsorption calculation energy and is chargean effective transfer way from to theinvestigate experiments. the interaction Therefore, between a first-principles different types quantum-mechanical of atoms. calculation is an effective way to investigateIn this the study, interaction the adsorption between behavior different typesof Ag of atoms atoms. on vacancy-deficient graphene (VG) and Ce-dopedIn this graphene study, the was adsorption calculated behavior by first-princi of Agples atoms theory. on vacancy-deficient By analyzing the graphene adsorption (VG) energy, and Ce-dopedband structure, graphene and waspopulation, calculated the by effects first-principles of vacancy theory. defects By and analyzing Ce doping the adsorptionon the adsorption energy, bandbehavior structure, of Ag andon graphene population, were the investigated. effects of vacancy The interaction defects and between Ce doping Ag and on thegraphene adsorption and behaviorelectron oftransfer Ag on graphenewere discussed. were investigated. This research The interactionwill help betweento improve Ag andthe graphene wettability and between electron transfergraphene were and discussed. silver-based This composite research willfillers, help th tous improveproviding the theoretical wettability guidance between grapheneand technical and silver-basedsupport for the composite application fillers, of grap thushene-reinforced providing theoretical composite guidance fillers. and technical support for the application of graphene-reinforced composite fillers. 2. Theoretical Method 2. Theoretical Method The calculations were performed using the CASTEP code [33], which is a plane-wave, pseudo potentialThe calculationsprogram werebased performed on density using thefuncti CASTEPonal codetheory [33 ],(DFT) which is[34–36]. a plane-wave, The electron pseudo potentialexchange-correlation program based interactions on density were functional expressed theory with (DFT) a generalized [34–36]. The gradient electron approximation exchange-correlation (GGA) interactions[37,38] in werethe form expressed of the with Perdew–Burke–Ernzerh a generalized gradient approximation (PBE) of functional (GGA) [37 ,38[39].] in theThe form ultrasoft of the Perdew–Burke–Ernzerhpseudopotential was used (PBE) to describe of functional the interaction [39]. The ultrasoftinside and pseudopotential outside the domain was used approximately. to describe theThe interaction plane wave inside kinetic and outsideenergy thecutoff domain was approximately.set to 450 eV, Theand planeBrillouin-zone wave kinetic integration energy cutoff was wasperformed set to 450 with eV, a 3 and × 3 Brillouin-zone× 1 Monkhorst–Pack integration k-point was mesh. performed The setting with of a the 3 × k3 point× 1 Monkhorst–Pack not only ensures k-pointthe accuracy mesh. Theof the setting results, of the but k point also notcontrols only ensures the computational the accuracy ofefficiency. the results, The but alsoconvergence controls thetolerances computational for the geometric efficiency. optimization The convergence were tolerancesset as follows: for the the geometric force on each optimization was were 0.05 eV/Å; set as follows:the max thestress force on each on each atomwas 0.1 was GPa; 0.05 the eV/Å; max thedisplacement max stress on on each each atom cell was was 0.1 0.002 GPa; Å; theand max the −5 displacementconvergence energy on each was atom 2.0 was × 10 0.002−5 eV/atom. Å; and the convergence energy was 2.0 × 10 eV/atom. A 44 ×× 44 supercell supercell with with a a periodic periodic boundary condition along the x-y plane was employed for the infiniteinfinite graphenegraphene sheet sheet model. model. The The vacuum vacuum was was set se witht with 20 20 Å inÅ in the the z direction, z direction, which which could could avoid avoid the interactionthe interaction between between periodic periodic graphene graphene layers layers [40]. By [40]. removing By removing a chosen a C chosen atom in C intrinsic atom in graphene, intrinsic thegraphene, model ofthe VG model was built.of VG The was model built. of The Ce-doped model graphene of Ce-doped was establishedgraphene was by substituting established aby C atomsubstituting in intrinsic a C atom graphene in intrinsic with a graphene Ce atom. Towith create a Ce a atom. model To of create Ag adsorption a model of on Ag VG adsorption or Ce-doped on graphene,VG or Ce-doped a single graphene, Ag atom a wassingle placed Ag atom above was the placed graphene. aboveHere, the graphene. we consider Here, the we adsorption consider the of theadsorption atom on of three the atom sites: on the three hollow sites: (H) the site hollow in the center(H) site of in a hexagonthe center and of hollowa hexagon binding and sitehollow for vacancy-deficientbinding site for vacancy-deficient graphene, the bridge graphene, (B) site the at the bridge midpoint (B) site of aat C–Cthe midpoint bond and of C–Ce a C–C bond, bond the and top (TC–CeC) site bond, directly the top above (TC) a site C atom, directly and above the top a C (T atom,Ce) site and above the top the (T CeCe atom.) site above The models the Ce ofatom. VG andThe Ce-dopedmodels of grapheneVG and Ce-doped are shown graphene in Figure are1. shown in Figure 1.

Figure 1. Physical models of graphene: (a) VG; (b) Ce-doped graphene.graphene.

The adsorption energy of an Ag atom on graphene (E ) can be computed by The adsorption energy of an Ag atom on graphene (Eadad) can be computed by   = − + EEEad=−+EAg−graphene E EEAg Egraphene ad Ag− graphene( Ag graphene )

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where EAg-graphene, EAg, and Egraphene represent the total energy of the graphene-Ag system, the energy of the Ag atom, and the energy of graphene, respectively.

3. Results and Discussion

3.1. Adsorption Energy and Charge Transfer After structure optimization, it was found that the top-site (T) is energetically favored for Ag atom adsorption in comparison with the adsorption on either hollow (H) or bridge (B) sites in intrinsic graphene and Ce-doped graphene (Table1). This result is similar to that of Granatier, who calculated interaction energies and geometries of the coronene-X2 complexes d at the MP2 and M06-2X levels, and found that the preferred adsorption site corresponds to a double top-top site [41]. We also found that the hollow site was the most stable adsorption position in the vacancy-deficient graphene because of the lowest adsorption energy (−2.241 eV). Based on this, we only studied the top position of the intrinsic graphene, the TCe position of Ce-doped graphene, and the hollow binding site for vacancy-deficient graphene in the subsequent calculation. The distance between C2 and C3 decreases from 2.472 Å of intrinsic graphene to 1.809 Å of VG due to the dangling bonds. In addition, local deformation around Ce takes place in Ce-doped graphene due to the difference in bond lengths between Ce–C and C–C. Table1 shows the final adsorption distance, adsorption energy, and charge transfer between the Ag atom and graphene after Ag adsorption. It was found that due to the formation of the vacancy defect, the adsorption energy between the Ag atom and VG increases from 0.011 eV to 2.439 eV. There is a smaller charge transfer from silver atoms to intrinsic graphene (0.09) in comparison with charge transfer in VG-Ag complexes (0.78). Our results are consistent with Roxana’s finding that the charge is transferred from the Ag atom to graphene sheets in the graphene- silver system [42]. The electronic interaction between Ag and VG is also significantly enhanced, which shows an improvement in wettability. The adsorption height decreases from 1.564 Å of intrinsic graphene to 1.025 Å of VG. For Ce-doped graphene, the charge of Ce lost 1.96e, while the charge of Ag increased by 0.17e. This indicates that electrons are transferred from Ce atoms to graphene, and the adsorption height is also lower than that of intrinsic graphene, which forms , and leads to an increase in stability and wettability of the system. It was also revealed that the deformation increases from 0.055 Å of intrinsic graphene to 0.380 Å of VG and 0.147 Å of Ce-doped graphene, representing a large amount of deformation.

Table 1. The final adsorption distance (D) and adsorption energy (Ead) of three bonding sites: top (T), bridge (B), and hollow (H), and charge transfer (Q) and deformation (∆h = hmax − haverage) after the adsorption of the Ag atom on graphene.

Ead/eV Type of Graphene D/Å Q/e ∆h/Å TBH Intrinsic graphene 1.564 −0.011 −0.009 −0.010 0.09 0.055 VG 1.025 −2.358 −2.226 −2.439 0.78 0.380 Ce: 1.96 Ce-doped graphene 1.530 TCe: −2.241 TC: −2.230 −2.226 −2.218 0.147 Ag: −0.17

Figure2 shows the final geometric structures, while Figure3 indicates the electron density difference of intrinsic graphene, VG, and Ce-doped graphene after adsorption of the Ag atom. The adsorption distance between Ag and graphene decreases due to the existence of vacancies and Ce atoms, while the local deformation around the Ce atom and defects is larger, in which the amount of deformed VG is the largest. Symmetry appears after VG adsorbs Ag atoms, the distance between C1 and C2 increases from 2.46 to 2.769, and C2 and C3 are symmetric about C1, as shown in Figure2. Like VG, there is symmetry in Ce-doped graphene. C2 and C3 are symmetric about Ce-C1, with the distance between C1 and C2 increasing from 2.471 Å to 3.571 Å of intrinsic graphene. However, the calculated Materials 2019, 12, 649 4 of 8 distortion of the intrinsic graphene upon adsorption of the Ag atom was found to be negligible, in Materials 2019, 12, x FOR PEER REVIEW 4 of 8 agreementMaterials with2019, 12 the, x previousFOR PEER REVIEW results in Amft [43]. 4 of 8

Figure 2. Geometric structures of graphene with the adsorbed Ag atom: (a) front view of intrinsic FigureFigure 2.2. GeometricGeometric structures of graphenegraphene withwith thethe adsorbedadsorbed AgAg atom:atom: (a)) frontfront viewview ofof intrinsicintrinsic graphene; (b) front view of VG; (c) front view of Ce-doped graphene; (d) top view of intrinsic graphene;graphene; (b(b)) front front view view of VG;of VG; (c) front(c) front view view of Ce-doped of Ce-doped graphene; graphene; (d) top view(d) top of intrinsic view of graphene; intrinsic graphene; (e) top view of VG; (f) top view of Ce-doped graphene. (graphene;e) top view (e of) top VG; view (f) top of VG; view (f of) top Ce-doped view of graphene.Ce-doped graphene.

Figure 3. Electron density difference. The yellow and cyan regions represent charge depletion Figure 3. Electron density difference. The yellow and cyan regions represent charge depletion and andFigure charge 3. Electron accumulation, density respectively.difference. The (a) yellow Intrinsic and graphene; cyan regions Isosurface represent level: charge 0.015 eÅdepletion−3;(b) VG;and charge accumulation, respectively. (a) Intrinsic graphene; Isosurface level: 0.015 eÅ−3; (b) VG; Isosurfacecharge accumulation, level: 0.05 eÅ respectively.−3;(c) Ce-doped (a) graphene.Intrinsic graphene; Isosurface Isosurface level: 0.05 eÅlevel:−3. 0.015 eÅ−3; (b) VG; Isosurface level: 0.05 eÅ−3; (c) Ce-doped graphene. Isosurface level: 0.05 eÅ−3. Isosurface level: 0.05 eÅ−3; (c) Ce-doped graphene. Isosurface level: 0.05 eÅ−3. The system change of VG adsorption of Ag atoms is the largest, as shown in Figure3. The yellow The system change of VG adsorption of Ag atoms is the largest, as shown in Figure 3. The and cyanThe regionssystem representchange of charge VG adsorption depletion and of Ag charge atom accumulation.s is the largest, The as charge shown transferred in Figure between 3. The yellow and cyan regions represent charge depletion and charge accumulation. The charge theyellow Ag atomand andcyan the regions nearest Crepresent atom is limited,charge anddepletion the Isosurface and charge level is accumulation. just 0.015 eÅ−3 inThe Figure charge3a. transferred between the Ag atom and the nearest C atom is limited, and the Isosurface level is just Thetransferred charge ofbetween the Ag the atom Ag is atom mainly and transferred the nearest to C the atom nearest is limited, C atom and in the VGIsosurface system, level and is there just 0.015 eÅ−3 in Figure 3a. The charge of the Ag atom is mainly transferred to the nearest C atom in the is0.015 a bonding eÅ−3 in pairFigure between 3a. The Ag charge and C,of the forming Ag atom a stable is mainly chemical transferred bond. The to the Ce nearest atom is C in atom the shapein the VG system, and there is a bonding pair between Ag and C, forming a stable chemical bond. The Ce ofVG a system, petal, and and the there bond is a orbital bonding is d-orbital pair between in Ce-doped Ag and C, graphene. forming Thea stable result chemical of charge bond. transfer The Ce is atom is in the shape of a petal, and the bond orbital is d-orbital in Ce-doped graphene. The result of theatom formation is in the ofshape a covalent of a petal, bond and between the bond the orbital Ag atom is d-orbital and the in Ce Ce-doped atom, and graphene. the Isosurface The result level of is charge transfer is the formation of a between the Ag atom and the Ce atom, and the 0.05charge eÅ −transfer3, which is provesthe formation that the of wettability a covalent of bond the system between has the been Ag improved. atom and the Ce atom, and the Isosurface level is 0.05 eÅ−3, which proves that the wettability of the system has been improved. Isosurface level is 0.05 eÅ−3, which proves that the wettability of the system has been improved. 3.2. Density of States 3.2. Density of States 3.2. DensityTo further of States investigate the mechanism and effect of doping, different band structures were To further investigate the mechanism and effect of doping, different band structures were computed,To further including investigate the intrinsic the mechanism graphene, and VG, effect and of Ce-doped doping, graphenedifferent band before structures and after were Ag computed, including the intrinsic graphene, VG, and Ce-doped graphene before and after Ag adsorption.computed, Theincluding Dirac pointthe intrinsic [44,45] formsgraphene, as shown VG, inand Figure Ce-doped4a, which graphene agrees with before the experimentaland after Ag adsorption. The Dirac point [44,45] forms as shown in Figure 4a, which agrees with the experimental dataadsorption. [46]. In The Figure Dirac4b, point the adsorption [44,45] forms of the as Agshown atom in onFigure intrinsic 4a, which graphene agrees resulted with the in experimental little change data [46]. In Figure 4b, the adsorption of the Ag atom on intrinsic graphene resulted in little change indata the [46]. band In structure. Figure 4b, For the VG, adsorption however, of the the dangling Ag atom bonds on intrinsic destroyed graphene the Dirac resulted point in [46 little], resulting change in the band structure. For VG, however, the dangling bonds destroyed the Dirac point [46], resulting inin electronthe band deficiencystructure. For and VG, a slight however, decline the of dangli the Ferming bonds level, destroyed as well asthe movement Dirac point near [46], the resulting top of in electron deficiency and a slight decline of the Fermi level, as well as movement near the top of the thein electron valence deficiency band. In this and case, a slight p-type decline doping of the will Fe form,rmi level, which as promoteswell as movement a transfer near of electrons the top of onto the valence band. In this case, p-type doping will form, which promotes a transfer of electrons onto graphene.valence band. Figure In 4thisc shows case, that p-type after doping the adsorption will form, of thewhich Ag promotes atom on VG, a transfer because of of electrons the existence onto graphene. Figure 4c shows that after the adsorption of the Ag atom on VG, because of the existence ofgraphene. defects, twoFigure new 4c energyshows that bands after are the introduced adsorption between of the Ag the atom bottom on ofVG, the because conduction of the band existence and of defects, two new energy bands are introduced between the bottom of the conduction band and theof defects, top of the two valence new energy band. Thebands bottom are introduced of the conduction between band the bottom moves inof thethe highconduction energy band direction, and the top of the valence band. The bottom of the conduction band moves in the high energy direction, whilethe top the of topthe ofvalence the valence band. The band bottom moves of in the the cond lowuction energy band direction. moves The in the electrons high energy in the direction, Ag atom while the top of the valence band moves in the low energy direction. The electrons in the Ag atom werewhile transferred the top of the to thevalence Fermi band level moves of VG in and the filledlow energy in the direction. electron holes The electrons nearby, causing in the Ag a slightatom were transferred to the Fermi level of VG and filled in the electron holes nearby, causing a slight were transferred to the Fermi level of VG and filled in the electron holes nearby, causing a slight drop of the Fermi level. This result is consistent with the charge transfers in Table 1 (Ag lost 0.78 drop of the Fermi level. This result is consistent with the charge transfers in Table 1 (Ag lost 0.78 electrons). Compared with VG, the Fermi level of Ce-doped graphene in Figure 4d experienced an electrons). Compared with VG, the Fermi level of Ce-doped graphene in Figure 4d experienced an obvious increase, which implied the formation of n-type doping. Since the energy band is relatively obvious increase, which implied the formation of n-type doping. Since the energy band is relatively

Materials 2019, 12, 649 5 of 8 drop of the Fermi level. This result is consistent with the charge transfers in Table1 (Ag lost 0.78 electrons).Materials Compared2019, 12, x FOR with PEER VG, REVIEW the Fermi level of Ce-doped graphene in Figure4d experienced5 anof 8 Materials 2019, 12, x FOR PEER REVIEW 5 of 8 obvious increase, which implied the formation of n-type doping. Since the energy band is relatively flat, the locality is very strong, which infers that the Ce-doping slightly hinders the electron transfer flat, the locality is very strong, which infers that the Ce-doping slightly hinders the electron transfer flat,onto the the locality graphene. is very strong, which infers that the Ce-doping slightly hinders the electron transfer onto the graphene.

Figure 4. Band structure: (a) Intrinsic graphene; (b) intrinsic graphene with adsorbed Ag atom; (c) FigureVG; (d ) 4.4. Ce-doped BandBand structure:structure: graphene ( a(a) ) Intrinsicwith Intrinsic adsorbed graphene; graphene; Ag atom. (b ()b intrinsic) intrinsic graphene graphene with with adsorbed adsorbed Ag atom;Ag atom; (c) VG; (c) (VG;d) Ce-doped (d) Ce-doped graphene graphene with with adsorbed adsorbed Ag atom. Ag atom. The partial density of states (DOS) in Figure 5a shows that, for intrinsic graphene, the d orbital of theThe adsorbed partialpartial densitydensityAg atom ofof would statesstates (DOS) not(DOS) interact in in Figure Figure effectively5a 5a shows shows with that, that, the for forp intrinsic orbital intrinsic of graphene, itsgraphene, nearest the theC d atom orbital d orbital near of theofthe the adsorbedFermi adsorbed level, Ag Agconfirming atom atom would would that not notthe interact interacttype effectivelyof effectivelyadsorption with with is the physical the p orbital p orbital adsorption, of its of nearestits nearest which C atom Cis atomconsistent near near the Fermithewith Fermi the level, adsorption level, confirming confirming energy that theresultsthat type the in oftype Table adsorption of 1.adsorp LaraCastells istion physical is physical [47] adsorption, found adsorption, that which the is Ag2which consistent dimer is consistent withbinds the to adsorptionwiththe surface the adsorption energymostly resultsdue energy to invan results Table der1 . Waals-typein LaraCastells Table 1. LaraCastellscontri [ 47]butions, found [47] that which found the Ag2confirms that dimer the our Ag2 binds results. dimer to the In binds surfaceFigure to mostlythe5b, bothsurface due the tomostly 4d van orbital der due Waals-type andto van 2p orbitalder contributions, Waals-type of the Ag contri whichatombutions, matched confirms which well our results. withconfirms the In 2p Figureour orbital results.5b, bothof Inthe theFigure three 4d orbital5b,dangling both and the C 2p atoms4d orbital orbital in VG, of and the resulting 2p Ag orbital atom in matched ofa strongthe Ag well coupli atom with ngmatched the[48]. 2p This orbitalwell verifies with of the the the three 2p formation orbital dangling of of the Ca atoms stablethree indanglingchemical VG, resulting adsorptionC atoms in in a strongbetweenVG, resulting coupling VG and in [48 athe ].strong ThisAg atom. verifiescoupli Inng the Figure [48]. formation This 5c, although verifies of a stable the the formation chemical4d orbital adsorptionof and a stable the p betweenchemicalorbital of VG adsorptionC atom and thein betweengraphene Ag atom. VG display In and Figure the little5 c,Ag althoughcoupling atom. In the withFigure 4d the orbital 5c, 4d although orbital and the of the pthe orbital4d Ag orbital atom of C and atomnear the the in p grapheneorbitalFermi level,of displayC atomthe Ce littlein atom graphene coupling matches display with well the littlewith 4d orbital couplinggraphe ofne, thewith suggesting Ag the atom 4d near orbitalthat the the of Fermi adsorption the Ag level, atom thestructure Cenear atom the is matchesFermistable level,in well Ce-doping the with Ce graphene, atom graphene. matches suggesting The well results thatwith thecome graphe adsorption fromne, suggestingthe structure analysis that is of stable Figurethe adsorption in Ce-doping5 conformed structure graphene. to the is Thestablededuction results in Ce-doping from come Figure from graphene. the4. analysis The of results Figure 5come conformed from the to the analysis deduction of Figure from Figure5 conformed4. to the deduction from Figure 4.

Figure 5.5. Partial DOS ofof absorbedabsorbed AgAg atomatom andand itsits nearestnearest C/CeC/Ce atom: ( a)) Intrinsic Intrinsic graphene; graphene; ( b) VG; (Figure(cc)) Ce-dopedCe-doped 5. Partial graphene.graphene. DOS of absorbed Ag atom and its nearest C/Ce atom: (a) Intrinsic graphene; (b) VG; (c) Ce-doped graphene. 3.3. Population Analysis 3.3. Population Analysis To further study the interaction between graphene and the adsorbed Ag atom, population analysisTo furtherwas completed study the for interaction intrinsic graphene, between VG,grap andhene Ce-doped and the grapheneadsorbed afterAg atom,adsorption population of the analysis was completed for intrinsic graphene, VG, and Ce-doped graphene after adsorption of the

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3.3. Population Analysis To further study the interaction between graphene and the adsorbed Ag atom, population analysis was completed for intrinsic graphene, VG, and Ce-doped graphene after adsorption of the Ag atom. The results are summarized in Table2. In the intrinsic graphene–Ag system, there is no population between the Ag atom and the nearest C atom, which shows that the interaction is physical adsorption. In the VG–Ag system, the population between the Ag atom and its nearest C atom is 0.540, and its number of population per unit bond length is 2.561 charge·nm−1, indicating that the defect causes chemical adsorption between Ag atoms and graphene, and there is a stable chemical bond, which is consistent with the previous adsorption energy, adsorption height, and DOS analysis. Compared with intrinsic graphene, the population of the Ce-doped graphene-Ag system between the Ce and Ag atoms is 0.28, and its number of population per unit bond length is 0.933 charge·nm−1, implying the formation of a stable chemical bond between them.

Table 2. The population between the Ag and its nearest C/Ce atom in graphene.

Adsorption System Bond Population Length/10−1 nm P/charge·nm−1 Ag–C1 0.54 2.14657 2.516 VG Ag–C2 −0.06 2.51845 −0.238 Ag–C2 −0.06 2.52454 −0.238 Ce-G Ag–Ce 0.28 2.99968 0.933

4. Conclusions Vacancy defects in graphene trigger electron deficiency and p-type doping, which could significantly improve the adsorption energy, charge transfer, and wettability between graphene and the adsorbed Ag atom. Conversely, for Ce-doped graphene, n-type doping formed and thus increased the Fermi level of graphene, which might hinder the electron transfer between Ag and graphene. However, the adsorption energy and population analysis shows that Ce doping could enhance the stability of the Ag–graphene system and wettability. The adopted research methods, mechanism cognitions, and obtained conclusion in this study might contribute to the future development of efficient and stable brazing materials, and provide useful guidance for the preparation of graphene-reinforced Ag filler with fine properties.

Author Contributions: Conceptualization, Z.F. and M.H.; Data curation, Z.F. and M.H.; Formal analysis, Z.F. and M.H.; Funding acquisition, J.L.; Investigation, K.Z. and Z.T.; Methodology, M.H.; Project administration, X.L.; Resources, Z.F.; Software, M.H. and K.Z.; Supervision, Z.F. and J.L.; Validation, Z.F., X.L., and K.Z.; Writing—original draft, M.H.; Writing—review & editing, J.L. Acknowledgments: Support by the Key Project of National Natural Science Foundation of China (No.51474181) and the Extracurricular Open Experiment of Southwest Petroleum University (No.KSZ18513) is gratefully acknowledged. Conflicts of Interest: The authors declare no conflict of interest.

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