Theoretical Studies On The Healing Mechanism Of Defects In Graphene

most exciting materials. Graphene possesses a single sheet of hexagonal honeycombcarbon network, which make it the building block of other significant carbonallotropes. For instance, it can be staked into three-dimensional graphite, rolled intoone-dimensional carbon nanotubes, and wapped into zero-dimensional fullerenes.These structural status and characteristics of graphene ensure its many extraordinaryproperties. Compared with other materials, graphene has the following advantages.Such as high electron mobility, high mechanical strength, high thermal and electricalconductivities. Due to above mentioned advantages, graphene is considered as a newgeneration promising materials to replace silicon.In recent yeas, many synthetic methods of graphene have been proposed.However, defects always inevitably exist in graphene. These defects greatly degradethe quality of the graphene and reduce the performance of the graphene. For example,it destroy many vital properties of graphene, i.e., reducing thermal and electricalconductivities, mechanical strength, carrier mobility, etc. Consequently, these defectsof graphene hinder application and development of graphene. How to heal thesedefects and restore the intrinsic properties of perfect graphene, which will be anurgent issue for carbon nanomaterials field. Herein, using density functional theory(DFT) calculations, we disclose the healing mechanism of several defects in graphene.The obtained results are summarized as follows:(1) Many outstanding properties of graphene are blocked by the existence of structural defects. Hence, the healing of defective graphene become extremelynecessary. Herein, we propose an important healing mechanism for the growth ofgraphene, which is produced via the plasma enhanced chemical vapor decomposition(PECVD) of CH4on metal nanoparticles, i.e. the healing of graphene with singlevacancy by decomposed CH4(hydrocarbon radical CHx, x=1,2,3). The healingprocesses undergo three evolutionary steps:1) the chemisorption of the hydrocarbonradicals,2) the incorporation of the C atom of the hydrocarbon radicals into thedefective graphene, accompanied by the adsorption of the leaving H atom on thegraphene surface,3) the removal of the adsorbed H atom and H2molecule to generatethe perfect graphene. The overall healing processes are barrierless with the releasedhuge heat of530.8,290.7, and159.0kcal/mol, respectively, indicative of the easyhealing of graphene with single vacancy by hydrocarbon radicals. Therefore, wepropose that among the available graphene synthesis procedures, the good performanceof the PECVD of CH4on metal nanoparticles might be ascribed to the dual role of CH4,i.e., CH4both provides carbon-source and defect-healer.(2) Recent experiments have shown that using ethylene (acetylene) as carbonsource to obtain graphene can dramatically decrease the defects in prepared graphene.However, the inherent mechanism with regard to reduction of defects is quite unclear.Herein, using density functional theory (DFT) calculations, we disclose the healingmechanism of the divacancy defect in graphene by ethylene/acetylene, i.e.,1) thechemisorption of the ethylene/acetylene,2) the incorporation of the C atoms of theethylene/acetylene into the defective graphene, accompanied by the adsorption of theleaving H atoms on the graphene surface, and3) the removal of the adsorbed H atoms togenerate the perfect graphene. The entrance adsorption step of ethylene/acetylene has abarrier of28.8/25.3kcal mol-1and the eventual formation of graphene is stronglyexothermal by189.1/243.2kcal mol-1. Considering that the graphene growth usuallytakes place at high temperature conditions, the healing of divacancy defects could beeffectively realized in the presence of ethylene or acetylene molecule. Therefore, wepropose that the good performance of the ethylene/acetylene-based graphene synthesismethods might be ascribed to the dual role of ethylene/acetylene, i.e., they can act as both the carbon source and as the defect healer.(3) Stone-Wales (SW) defect is one typical topological structure in the carbonnanomaterials. Unfortunately, the SW defect in graphene has to overcome a very highrestoration barrier (ca.6eV). Very recent theoretical work showed the promise toreduce the restoration barrier by the adsorbed transition metal atoms down to2.86eV(for W)(yet still too high). In the present density functional theory (DFT) study, we findthat through a mechanically different process, adsorption of carbon atoms candramatically reduce the restoration barrier to hitherto the lowest value, i.e.,20.0kcalmol-1(0.87eV), which could make the SW-healing experimentally accessible.Subsequently, with a very low barrier (9.4kcal mol-1), the C-adatom can migrate veryeasily on the graphene surface. As a result, one carbon adatom could principallycatalyze the healing of all the SW defects in a cascade mode if no termination stepsexist. During the graphene growth, the presently proposed carbon-adatom catalyticmechanism could have played a role in healing the SW defect. Moreover, we proposethat in the post-treatment of graphene, adsorption of the carbon adatom could be used asan effective catalyst for the SW-healing. The catalytic role of carbon atom on SW defectshould be include in the modeling of graphene growth.(4) Recently, some defect-healing mechanismes of graphene have been proposed,However, these suggested mechanismes are still provided with the high energybarrieres or inconveniently manipulated. Consequently, it is unfavorable for healingdefect of graphene under a mild condition within previous research results. Herein,using density functional theory (DFT) calculations, we have systematically exploredthe divalent carbon(0) adsorption on the graphene surface with a single vacancy (SV)defect. It is found that for C(PH3)2, C(PMe3)2, C(CO)2, the overall healing processes arebarrierless, with a huge release heat of257.3,227.7,181.7kcal mol-1, respectively.Fortunately, the C(PMe3)2, C(CO)2have been synthesized, as a result, the divalentcarbon(0) could make the defect-healing experimentally accessible. Using divalentcarbon(0) as the defect healer shall open a high-efficiency avenue to heal defects ofcarbon nanomaterials.(5) With the prediction and synthetic of increasing divalent carbon(0) compounds, this novel compound attract more and more attention of scientists. As an importantligands, N-heterocyclic carbene (NHC) have attained a special status inorganometallic chemistry. The transition metal-NHC complexes (TM-NHC) havebeen widely used in catalysis and hydrogenation fields, etc. As a ligand, the NHC isused to constitute a new class of compounds, first theoretically predicted in2007andshortly after synthesized and characterized. Herein, the divalent carbon(0)compound-C(NHC)2is selected as defect healer of graphene, as a result, the C(NHC)2can successfully repaire single-vacancy defects in graphene.