materials

Article Growth of U-Shaped Domains on Copper Foil by Chemical Vapor Deposition

Ming Pan 1, Chen Wang 1, Hua-Fei Li 2, Ning Xie 2, Ping Wu 1, Xiao-Di Wang 1, Zheling Zeng 1, Shuguang Deng 1,3,* and Gui-Ping Dai 1,*

1 Key Laboratory of Poyang Lake Environment and Resource Utilization, Nanchang University, Ministry of Education, School of Resources Environmental & Chemical Engineering, Nanchang University, Nanchang 330031, China; [email protected] (M.P.); [email protected] (C.W.); [email protected] (P.W.); [email protected] (X.-D.W.); [email protected] (Z.Z.) 2 Institute for Advanced Study, Nanchang University, Nanchang 330031, China; hfl[email protected] (H.-F.L.); [email protected] (N.X.) 3 School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ 85287, USA * Correspondence: [email protected] (S.D.); [email protected] (G.-P.D.)

 Received: 14 May 2019; Accepted: 10 June 2019; Published: 12 June 2019 

Abstract: U-shaped graphene domains have been prepared on a copper substrate by chemical vapor deposition (CVD), which can be precisely tuned for the shape of graphene domains by optimizing the growth parameters. The U-shaped graphene is characterized by using scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and Raman. These show that the U-shaped graphene has a smooth edge, which is beneficial to the seamless stitching of adjacent graphene domains. We also studied the morphology evolution of graphene by varying the flow rate of hydrogen. These findings are more conducive to the study of morphology evolution, nucleation, and growth of graphene domains on the copper substrate.

Keywords: graphene; U-shaped; CVD

1. Introduction Graphene, an ideal 2D versatile carbon material with a combination of the honeycomb-like arrangement, has drawn extensive attention in the research field, due to its good properties, such as high carrier mobility [1], mechanical tensile strength [2], and thermal conductivity [3]. Graphene has been prepared in several different ways since Geim et al. first obtained a single-layer polycrystalline graphene by the mechanical stripping of highly oriented pyrolytic [4], such as the exfoliation and chemical reduction of graphene oxide [5–7], epitaxial growth of SiC [8], and by depositing on metal using chemical vapor deposition (CVD) [9,10]. In the method above, graphene growth on metal by CVD is regarded as the most promising and cost-effective way to synthesize graphene for experimental research and industrial development, owing to its convenience, reproducibility, and high controllability. Among the metal substrate used for the synthesis of graphene by CVD, nickel (Ni) [11] and copper (Cu) [9] are the most used. In the meantime, the fabrication of uniform and large-area graphene on copper foil is the most promising synthesis method because of the solubility of the carbon atom in a Cu substrate when compared to a Ni substrate [9,12]. Nowadays, on the one hand, using the CVD method for preparing large-area graphene on a copper foil surface is a synthetic way with great potential for industrial development, but the growth of graphene is accompanied by crystal imperfections and boundaries [13,14]. This has a great influence on its properties and provides an obstacle to the formation of a high-quality single-layer graphene, and this has brought great difficulties to its research and application. Therefore, how to expand and seamlessly

Materials 2019, 12, 1887; doi:10.3390/ma12121887 www.mdpi.com/journal/materials Materials 2019, 12, 1887 2 of 10 merge graphene domains into a large-sized defect-free graphene sheet has become an important research topic. On the other hand, several odd shapes of graphene can be prepared on a copper foil by CVD and by regulating growth parameters, including hexagons [15–20], four-lobed [21,22], triangular [23,24], rectangular [25–28], pentagonal [29], and 12-pointed [30], which contributes to an understanding of the nucleation and expansion mechanisms of graphene grains to some extent. In addition, how to grow high-quality single-crystal graphene by optimizing experiment conditions is still the focus of both the academic field and industrial application. Thus, it can reduce surface defects by either chemical or physical processes, or by prolonging the annealing time to pretreat the metal surface, which is beneficial to the growth of the graphene on the metal surface. Van et al. [31] reported that the surface of a copper foil was pretreated by chemical-mechanical polishing in order to reduce the roughness and defects of the Cu foil, which was later used as a substrate for the growth of a single-crystal graphene by CVD. During that process, they obtained a large-area graphene film by the seamless stitching of the graphene grains to the regular hexagonal graphene domains. Wang et al. [27] found that the decrease of the nucleation density, the reduction of defects, and the contamination on the Cu surface after a prolonged annealing time in a hydrogen environment, obtained a high-quality single-layer rectangular graphene and continuous graphene films on the substrate. Geng et al. [29] investigated whether liquid Cu could eliminate the effects of solid copper grain boundaries, and obtained uniform, single-domain, high-quality hexagonal graphene flakes by CVD. Their works are advantageous for studying the nucleation growth of graphene and forming a continuous graphene film. These studies have also demonstrated the great potential in graphene morphology evolution and the formation of a high-quality large-sized graphene film. Therefore, studying the morphology evolution of graphene is still very significant in order to understand the nucleation growth and expansion mechanism of graphene. In this work, we report for the first time the growth of large-sized “U” shaped graphene domains on a solid-Cu substrate by using ambient pressure CVD. These U-shaped graphene domains also run parallel to each other as well as along the gas flow direction. Evidently, the U-shaped graphene has a smooth edge, which is of importance to seamlessly stitching of the adjacent graphene domains. Moreover, the U-shaped graphene size could change when the methane flow rate was adjusted, and the morphology of the graphene domain could be different when the hydrogen flow rate was adjusted.

2. Experimental Section

2.1. Synthesis of Graphene Domains by CVD An atmospheric pressure CVD system was used with a quartz tube which was 6.5 feet in length and has an inner diameter of 2 in. as illustrated in Figure1a. Cu foil with a thickness of 25 µm (Alfa Aesar 13382, 99.8% pure, Alfa Aesar, Haverhill, MA, USA) was used as a growth substrate, which was loaded into a chamber after its surface was pretreated. First, Cu foil was soaked in dilute nitric acid for several seconds after it was immersed in acetic acid and deionized water for 3 min, respectively. The Cu foil was then dried with N2 and the clean Cu foil was placed into the quartz tube. The Cu foil was heated to 900 ◦C under a flow rate of 100 standard cubic centimeters per minute (sccm) for Ar, and 75 sccm for H2. After being annealed for 20 min at 1020 ◦C, CH4 with a flow rate of 20 sccm was introduced to the chamber for 90 s, then the flow of CH4 and H2 are turned off while maintaining a flow rate of 100 sccm for Ar, and the reactor is quickly moved to the other side to bring the temperature of the reaction zone down to room temperature. The whole process is illustrated in Figure1b. Materials 2019, 12, 1887 3 of 10 Materials 2019, 11, x FOR PEER REVIEW 3 of 10

FigureFigure 1.1. (a) Schematic illustration illustration of of the the chemical chemical vapor vapor deposition deposition (CVD (CVD)) synthesis synthesis of ofgraphene. graphene. (b) (Theb) The process process of graphene of graphene growth growth by byCVD CVD (t = (t 90= s).90 s).

2.2.2.2. TransferTransfer ofof GrapheneGraphene TheThe graphene-covered graphene-covered Cu Cu substrate substrate surface surface waswas spin-coatedspin-coated withwith a a polymethylmethacrylatepolymethylmethacrylate (PMMA).(PMMA). AfterAfter 22 h,h, thethe CuCu foilfoil waswas etchedetched byby usingusinga a 0.50.5 M M ammonium ammonium persulfatepersulfate solution.solution. InIn thethe meantime.meantime. The PMMAPMMA/graphene/graphene was transferred to deionized water forfor thethe removal removal of of chemical chemical residues.residues. Then, Then, PMMA PMMA/graphene/graphene was was pasted pasted on to on a targetto a target substrate substrate and then and immersed then immersed into acetone into toacetone remove to theremove PMMA the layer. PMMA Finally, layer. the Finally, substrate the wassubstrate placed was in a placed vacuum in dryinga vacuum oven dryin andg then oven dried and bythen heating. dried by heating. 2.3. Graphene Characterization 2.3. Graphene Characterization Scanning electron microscopy (SEM) (FEI Quanta200F, Waltham, MA, USA), atomic force Scanning electron microscopy (SEM) (FEI Quanta200F, Waltham, MA, USA), atomic force microscopy (AFM) (Agilent 5500, Santa Clara, CA, USA), Raman spectroscopy (Horiba HR Evolution, microscopy (AFM) (Agilent 5500, Santa Clara, CA, USA), Raman spectroscopy (Horiba HR Irvine, CA, USA) with a 100 lens, and transmission electron microscopy (TEM, JEOL 2010F, Peabody, Evolution, Irvine, CA, USA×) with a 100× lens, and transmission electron microscopy (TEM, JEOL MA, USA) were used to characterize the graphene film. 2010F, Peabody, MA, USA) were used to characterize the graphene film. 3. Results and Discussion 3. Results and Discussion As illustrated in Figure2a, we successfully synthesized large-sized U-shaped graphene domains at As illustrated in Figure 2a, we successfully synthesized large-sized U-shaped graphene an appropriate growth time by introducing a higher CH4 flow rate in an ambient pressure CVD on Cu foil,domains which at was an never appropriate reported growth before. Each time U-shaped by introducing graphene a higher domain CH has4 theflow same rate growth in an direction, ambient thepressure domains CVD are on parallel Cu foil, to which each other,was never and thereported domain before. direction Each isU- consistentshaped graphene with the domain reaction’s has gas-flowthe same direction. growth direction The graphene, the domains domains aredisplay parallel differences to each in other, size, indicating and the domain the nucleation direction and is consistent with the reaction’s gas-flow direction. The graphene domains display differences in size, indicating the nucleation and growth of graphene domains are different. Considering the total gas

Materials 2019, 11, x FOR PEER REVIEW 4 of 10 flow rate was relatively high in the reactor, it is possible that the reaction gases were in an unstable condition, which is difficult to achieve an even distribution of carbon species in such a short growth time [26]. Therefore, while the simultaneous nucleation of graphene domains, the partial graphene domains growth was limited due to the insufficiency of local carbon species. The magnified SEM image of an individual U-shaped graphene is further shown in Figure 2b, and it shows a perfect U-shaped graphene with a smooth edge, which facilitates seamless stitching between graphene domains. As shown in Figure 2c, it can be seen that many U-shaped graphene domains merge with each other to form a large domain, which shows the different stages of the merging process of the graphene domains with various sizes, and the nature of the U-shaped graphene domain is still obvious. The SEM image shows the U-shaped graphene domains were merged together in two different ways, namely straight-edge merging with the straight one, and straight-edge merging with the curved one. Additionally, it shows that the two ways have no effects for the formation of a larger-sized graphene domain during the merging process. Additionally, the adjacent U-shaped graphene domains were merged to each other if they were close enough. As shown in Figure 2d, the Materials 2019, 12, 1887 4 of 10 SEM image shows the merging process in the early stages for the U-shaped graphene domain. The adjacent U-shape graphene domains were coalesced into a large-area graphene domain as shown in Figuregrowth 2e of graphenewhen the domains growth time are di wasfferent. increased Considering. In general, the total the gas nucleation flow rate and was growthrelatively of highthe graphenein the reactor, domain it is depend possibleed that on the saturation reaction gases concentration were in an of unstable the activated condition, carbon which species is di onffi cultthe copperto achieve surface an even [22]. distribution When the growth of carbon time species was inincreas suched a, short the methane growth timecontinue [26].d Therefore, to decompose, while resultingthe simultaneous in the oversaturation nucleation of grapheneof the carbon domains, species the on partial the copper graphene surface. domains Moreover, growth wasthe limitedcarbon atomsdue to can the insu be continuouslyfficiency of local driven carbon to the species. edge The of the magnified domain, SEM which image then of caused an individual the graphene U-shaped to continuegraphene to is furthergrow. As shown illustrated in Figure in 2Figub, andre 2f, it shows the AFM a perfect image U-shaped shows the graphene merging with process a smooth of several edge, adjacentwhich facilitates graphene seamless domains stitching in the early between stage, graphene which demonstrates domains. the seamless coalescence for the adjacent graphene domains.

Figure 2. ((a)) SEM image of the U-shapedU-shaped graphene domains on the copper foil. ( (bb)) The The magnified magnified SEM image of an individual U U-shape-shape graphene. ( (cc,,dd)) SEM SEM images images of of the the coalescence coalescence of of the the different different graphene domains. domains. (e ()e A) A SEM SEM image image of the of continuous the continuous graphene graphene film. (f) Thefilm. atomic (f) The force atomic microscopy force microscopy(AFM) image (AFM of the) image merging of the graphene merging domains. graphene domains.

TheAs shown formation in Figure of the2c, graphene it can be seen domain that is many closely U-shaped related graphene to the experiment domains mergeparameters with each[17], suchother as to the form methane a large domain,flow rate which. As shown shows theby the diff erentSEM stagesimages of in the Figure merging 3, the process U-shaped of the graphene graphene domains gr withew variousunder different sizes, and flow the rates nature of methane of the U-shaped, while other graphene parameters domain were is still kept obvious. constant. The In SEM image shows the U-shaped graphene domains were merged together in two different ways, namely straight-edge merging with the straight one, and straight-edge merging with the curved one. Additionally, it shows that the two ways have no effects for the formation of a larger-sized graphene domain during the merging process. Additionally, the adjacent U-shaped graphene domains were merged to each other if they were close enough. As shown in Figure2d, the SEM image shows the merging process in the early stages for the U-shaped graphene domain. The adjacent U-shape graphene domains were coalesced into a large-area graphene domain as shown in Figure2e when the growth time was increased. In general, the nucleation and growth of the graphene domain depended on the saturation concentration of the activated carbon species on the copper surface [22]. When the growth time was increased, the methane continued to decompose, resulting in the oversaturation of the carbon species on the copper surface. Moreover, the carbon atoms can be continuously driven to the edge of the domain, which then caused the graphene to continue to grow. As illustrated in Figure2f, the AFM image shows the merging process of several adjacent graphene domains in the early stage, which demonstrates the seamless coalescence for the adjacent graphene domains. Materials 2019, 12, 1887 5 of 10

The formation of the graphene domain is closely related to the experiment parameters [17], such Materialsas the methane2019, 11, x FOR flow PEER rate. REVIEW As shown by the SEM images in Figure3, the U-shaped graphene domains5 of 10 grew under different flow rates of methane, while other parameters were kept constant. In addition, additionthe domain, the size domain decreased size decrease and domaind and densitydomain increased density increase when thed when methane the flowmethane rate flow increased rate increase(Figure3da–d (Figure corresponding 3a–d corresponding to 20, 25, 30to and20, 25, 35 30 sccm, and respectively). 35 sccm, respectively). In general, In when general, the when CH 4 flowthe CHrate4 flow increases, rate increase more methanes, moremolecules methane molecule can be absorbeds can be on absorbed the copper on surfacethe copper and surface are decomposed and are decompoto createsed more to create free carbon more free atoms, carbon and atoms, therefore and the therefore number the of number possible of nucleation possible nucleation sites increases, sites increases,resulting inresulting the shortage in the of shortage carbonspecies of carbon around species the edgesaround of the the edges graphene of the domains, graphene which domains, limited whichthe growth limited in the size growth for an individualin size for an graphene individual domain graphene in our domain case. in our case.

FigureFigure 3. 3. (a(a––dd) )SEM SEM images images of of the the growth growth of of U U-shaped-shaped graphene graphene under under different different methane methane flow flow rates rates (20,(20, 25, 25, 30 30 and and 35 35 sccm, sccm, respectiv respectively).ely).

InIn order order to to further further study study the the layer layer number number and and the the defects defects of of graphene, graphene, the the graphene graphene was was transferredtransferred to to a a copper copper grid grid for for TEM TEM observation. observation. As As shown shown in in Figure Figure 4a,4a, the graphene domain was successfulsuccessfullyly transferred transferred to to a a TEM TEM grid grid from from the the copper copper foil. foil. The The structural structural damage damage and and wrinkles wrinkles of of graphenegraphene were observedobserved in in TEM TEM images images because because it was it di ffiwascult difficult to maintain to the maintain original the morphology original morphologyof the film during of the thefilm transfer during process. the transfer Besides, process the dots. Besides, on graphene the dots film on may graphene be PMMA film residuesmay be PMMAthat were residues not completely that were dissolved not completel [26]. Asy dissolved shown in [26]. Figure As4 b,shown the selected-area in Figure 4b, electron the selected di ffraction-area electron(SAED) diffractionpattern shows (SAED a hexagon) pattern di showffraction-patterns a hexagon characteristic, diffraction-pattern indicating characteristic the single-crystalline, indicating thenature single of- graphenecrystalline domain. nature Meanwhile,of graphene the domain. layer number Meanwhile, of graphene the layer exhibited number in Figure of graphene4c–e has exhibitebeen observedd in Figure along 4c the–e edge has regionsbeen observed of the film along under the a high-resolution edge regions magnification, of the film under showing a highthat-resolution the synthesized magnification, graphene showing domains that have the a synthesized single layer graphene and a few domains layers. Inha addition,ve a single we layer also andcharacterized a few layers. the In graphene addition domains, we also by characterized Raman spectra. the Asgraphene shown domains in Figure by4f, weRama learnedn spectra. that theAs 1 shownRaman in spectra Figure show 4f, we the learn twoed prominent that the Raman peaksspectra of show graphene, the two which prominent locate at Raman ~1591 cmpeaks− and of 1 graphene,~2691 cm− which, corresponding locate at 1591 to the cm G-peak−1 and  and2691 the cm 2D−1, corresponding peak, respectively. to the Moreover, G-peak and the the weak 2D D-peak peak, 1 respectively.(~1360 cm− )Moreover, in the Raman the spectraweak D indicate-peak (1360 the high cm−1 quality) in the ofRaman graphene spectr film.a indicate The I2D the/IG highratio quality is about of2.2 graphene for the black film curve,. The which I2D/IG indicatesratio is about a monolayer 2.2 for graphenethe black region. curve, For which the red indicates curve the a monolayer I2D/IG ratio graphene region. For the red curve the I2D/IG ratio is about one, and for the blue curve the I2D/IG intensity ratio is below one, indicating the few layer region of graphene.

Materials 2019, 12, 1887 6 of 10

is about one, and for the blue curve the I2D/IG intensity ratio is below one, indicating the few layer Materialsregion 2019 of graphene., 11, x FOR PEER REVIEW 6 of 10

FigureFigure 4. ((aa)) TEM TEM image image of theof the graphene graphene domain domain on the on TEM the grid. TEM (b grid.) Selected-area (b) Selected electron-area di electronffraction diffraction(SAED) pattern (SAED) showing pattern a showing single-crystalline a single-crystalline nature of thenature U-shaped of the grapheneU-shaped domain.graphene ( cdomain.–e) TEM (cimages–e) TEM of theimages layer of number the layer of number graphene. of (graphene.f) Ramen spectra (f) Ramen of graphene spectra of domain graphene on copper domain foil, on showingcopper foil,monolayer showing and monolayer few-layer and graphene few-layer on graphene copper foil. on copper foil.

InIn CVD, CVD, t thehe evolution evolution of of the the graphene graphene domain domain shape shape is is related related to to many many growth growth conditions, conditions, includinincludingg the the surface surface treatment ofof substrate,substrate, thethe flow flow rate rate of of gas, gas, the the annealing annealing time time and and so on.so on. In thisIn thiswork, work we, found we found that thethat hydrogen the hydrogen flow rate flow can rate significantly can significantly influence influence the final the shape final of shapegraphene of graphenedomains. domains. A series ofA grapheneseries of graphene domains domains with diff erentwith different shapes are shapes formed are onformed a copper on a foil copper at a highfoil atflow a high rate flow of methane rate of methane (20 sccm) (20 and sccm) a short and growth a short time growth (90 s) time by varying (90 s) by the var hydrogenying the flow hydrogen rate as flowshown rate by as theshown typical by the SEM typical images SEM in images Figure5 ina–d. Figure The 5a U-shaped–d. The U graphene-shaped graphene domains domains with smooth with smoothedgescan edges be successfullycan be successfully synthesized synthesiz on theed copperon the copper surface surface at a hydrogen at a hydrogen flow rate flow of 75rate sccm of 75 as sccmshown as inshown Figure in5 a,Figure andthe 5a, U-shapedand the U graphene-shaped graphene domains aredomains highly are reproducible. highly reproducible. When we decreasedWhen we dectherease hydrogend the hydrogen flow rate fromflow rate 75 sccm from to 75 65 sccm sccm, to the65 sccm, morphology the morphology of the graphene of the graphene domain changed.domain cThehange semi-circled. The semi shaped-circlegraphene shaped graphene domains domains (Figure5 (Figureb) formed 5b) onformed the copper on thesurface copper withsurface a smooth with a smoothsemi-circle semi edge.-circle However, edge. However, when the when hydrogen the hydrogen flow rate flow was rate further was adjustedfurther adjusted to 55 or 60to sccm,55 or the60 sccm,heart-shaped the heart graphene-shaped domainsgraphene (Figure domains5c,d) (Figure were synthesized. 5c,d) were Remarkably,synthesized. weRemarkably, could clearly we observe could clearlythat, when observe the that, hydrogen when flow the hydrogen rate varied, flow the rate previous varied relatively, the previous stable relatively structure stable changed, structure which changeresultedd, inwhich the formation resulted in of the the formation different graphene of the different shapes asgraphene shown inshapes Figure as5a–d, shown confirming in Figure that 5a– thed, confirmingmorphology that of graphenethe morphology domains of cangraphene be tuned domains in a controllable can be tuned way. in Besides, a controllable the graphene way. Besides, domains thehave graphene the same domains growth ha directionve the same and growth are paralleled direction to and each are other, parallel whiched to may each be other, beneficial which for may the be beneficial for the growth of a high-quality graphene sheet. In general, the morphology of graphene domains could be predicted from thermodynamics by Wulff construction or kinetic analysis [32]. The morphology and structure of graphene domains have a great influence on graphene properties [17]. The morphology evolution in our case may provide a new way to get different graphene domain shapes by CVD.

Materials 2019, 12, 1887 7 of 10 growth of a high-quality graphene sheet. In general, the morphology of graphene domains could be predicted from thermodynamics by Wulff construction or kinetic analysis [32]. The morphology and structure of graphene domains have a great influence on graphene properties [17]. The morphology Materialsevolution 2019 in, 11 our, x FOR case PEER may REVIEW provide a new way to get different graphene domain shapes by CVD.7 of 10

FigureFigure 5. ((a–d) SEM imagesimages depictingdepicting the the evolution evolution of of the the graphene graphene domain domain growth growth on on the the copper copper foil foilat di atfferent different hydrogen hydrogen flow rates flow by rates atmospheric by atmospheric pressure pressure CVD (75, CVD 65, 60, (75, and 65, 55 sccm, 60, and respectively). 55 sccm, respectively). It is possible that the morphology evolution of the graphene domain is caused by the destruction of theIt dynamic is possible equilibrium that the mechanism morphology [17 ]. evolution Therefore, of understanding the graphene the domain various is graphene caused domain by the destructionshapes in the of initial the dynamic stages is very equilibrium important mechanism for understanding [17]. Therefore, the growth understanding mechanism ofthe graphene. various grapheneGenerally, domain methane shapes molecules in arethe dissociatedinitial stages into is carbon very radicals important and for free understanding carbon atoms on the the growth copper mechanismsurface, and of the graphene. free carbon Generally, atoms aggregate methane into molecules a nucleus are (the dissociated red arrow) into to formcarbon graphene radicals asand shown free carbonin Figure atoms6. The on di theffusion copper of freesurface, carbon and atoms the free on thecarbon copper atoms surface aggregate has two into directions: a nucleus free (the carbon red arrow)atoms ditoff formusion graphene along the as nucleation shown in center Figure (the 6. blackThe diffusion arrow) and of free free carbon carbon atoms atoms surface on the di copperffusion surface(the indigo has two arrow). directions: The competition free carbon between atoms diffusion two diffusions along determinesthe nucleation the center final morphology (the black arrow) of the andgraphene free domain carbon [ atoms17]. A compactsurface structure diffusion is (the formed indigo when arrow) the free. carbonThe competition atom finds a between favorable two site diffusionsalong the nucleationdetermines center the final before morphology other free carbon of the grapheneatoms migrate domain by the [17]. surface A compact diffusion structure to join is it. formedThat is towhen say, the the free carbon atomsatom finds near thea favorable nucleation site center along can the adherenucleation to the center island before edge whenother freethe surfacecarbon diatomsffusion mi rategrate of by free the carbon surface atoms diffusion is not largeto join enough, it. That thus is to forming say, the a free compact carbon graphene atoms neardomain. the nucleation In addition, center hydrogen can adhere is one of to the the key island factors edge in whenthe growth the surface of graphene diffusion [26 ]. rate It not of onlyfree carbonacts as atoms an etchant, is not butlarge it alsoenough, promotes thus forming the dissociation a compact of graphene methane domain. to form freeIn addition carbon, atomshydrogen and isactivate one of thethe copper key factors surface in tothe promote growth theof graphene bonding of [26]. carbon It not atoms. only However,acts as an theetchant, high flowbut it rate also of promotes the dissociation of methane to form free carbon atoms and activate the copper surface to promote the bonding of carbon atoms. However, the high flow rate of hydrogen gas is unfavorable for the dissociation of methane molecules, leaving more undissociated methane molecules on the copper surface, resulting in an energy barrier, which reduces the surface diffusion rate of the free carbon atoms [33], thereby the growth of the graphene domain is suppressed. Simultaneously, during the graphene growth, the high flow rate of hydrogen promotes the etching of the graphene

Materials 2019, 12, 1887 8 of 10 hydrogen gas is unfavorable for the dissociation of methane molecules, leaving more undissociated methane molecules on the copper surface, resulting in an energy barrier, which reduces the surface diffusion rate of the free carbon atoms [33], thereby the growth of the graphene domain is suppressed. Simultaneously,Materials 2019, 11, xduring FOR PEER the REVIEW graphene growth, the high flow rate of hydrogen promotes the etching8 of of 10 the graphene domain. Thus, the U-shaped graphene domain is formed at a high hydrogen flow rate (75domain. sccm) inThus, our case.the U With-shaped the hydrogengraphene flowdomain rate is decreasing, formed at the a high energy hydroge barriern alsoflow reduces, rate (75 resulting sccm) in inour the case. increasing With the of the hydrogen surface diflowffusion rate rate decreasing, of free carbon the energy atoms, barrier and thus also making reduces, it unstable resulting from in itsthe originalincreasing equilibrium of the surface state, diffusion so some uniquerate of graphenefree carbon morphologies atoms, and thus [18,26 making] are formed, it unstable for example from its Heart-shapedoriginal equilibrium characterization state, so some in our unique case. Itgraphene should be morphologies mentioned that [18,26 the] processare formed, of synthesizing for example grapheneHeart-shaped domains characterization by CVD involves in ourcomplex case. It should physical be andmentioned chemical that reactions. the process Thus, of despitesynthesiz thising investigationgraphene domains is conducted by CVD on involves the morphology complex control physical and and evolution chemical of reactions. the graphene Thus, domain, despite thethis detailedinvestigation accurate is conducted mechanism on of nucleationthe morphology and growth control of grapheneand evolution remains of the unclear, graphene which domain, requires the to bedetailed further accurate studied. mechanism of nucleation and growth of graphene remains unclear, which requires to be further studied.

FigureFigure 6. 6.Schematic Schematic illustrationillustration showsshows thethe morphologymorphology ofof thethegraphene graphene domainsdomains onon coppercopper foilfoil inin didifferentfferent hydrogen hydrogen flow flow rates rates during during CVD. CVD.

4.4. ConclusionsConclusions InIn summary,summary, wewe havehave controlled the growth growth of of U U-shape-shape graphene graphene domains domains on on a copper a copper foil foil by byusing using CVD CVD at atatmospheric atmospheric pressure. pressure. The The change change in methane in methane flow flow rate rate has has a great a great influence influence on onthe thenucleation nucleation density density and and the the size size of of the the U U-shaped-shaped graphene domain. domain. Besides, di differentfferent domain domain morphologiesmorphologies of of graphene graphene have ha beenve formed,been formed, such as semi-circle-shaped such as semi-circle graphene-shaped and graphene heart-shaped and graphene,heart-shaped through graphene, varying through the flow varying rate of hydrogen,the flow rate which of hydrogen, indicates thewhich important indicates role the of hydrogenimportant inrole the of morphology hydrogen in evolution the morphology of graphene evolution domain. of graphene This study domain. may provide This study a new may way provide to fabricate a new theway novel to morphologyfabricate the of novel graphene morphology domains and of graphene may contribute domains to an and understanding may contribute the growth to an mechanismunderstand ofing graphene. the growth mechanism of graphene.

AuthorAuthor Contributions: Contributions:Conceptualization Conceptualization and and methodology, methodology M.P.;, M. formalP.; formal analysis, analysis, M.P., M C.W.,.P., H.F.L., H.F.L., N.X., P.W., X.D.W.,X.D.W., Z.Z., Z.Z., S.D.; S.D.;writing—original writing—original draft draft preparation, preparation, M.P.;M.P.; writing—reviewwriting—review and and editing, editing, S.D.S.D. and and G.-P.D.; G.-P.D.; fundingfunding acquisition, acquisition, G.-P.D. G.-P.D. Funding: G.-P.D acknowledges the National Natural Science Foundation of China (Grants 51762032 and 51462022) Funding: G.-P.D acknowledges the National Natural Science Foundation of China (Grants 51762032 and and the Natural Science Foundation Major Project of Jiangxi Province of China (Grant 20152ACB20012) for the financial51462022) support and of the this Natural research. Science Foundation Major Project of Jiangxi Province of China (Grant 20152ACB20012) for the financial support of this research.

Acknowledgments: The assistance of Zhi-Qun Tian (HRTEM measurements) at Guangxi University is also greatly appreciated.

Conflicts of Interest: The authors declare no conflict of interest.

References

Materials 2019, 12, 1887 9 of 10

Acknowledgments: The assistance of Zhi-Qun Tian (HRTEM measurements) at Guangxi University is also greatly appreciated. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Huang, P.Y.; Ruiz-Vargas, C.S.; Van der Zande, A.M.; Whitney, W.S.; Levendorf, M.P.; Kevek, J.W.; Garg, S.; Alden, J.S.; Hustedt, C.J.; Zhu, Y.; et al. Grains and grain boundaries in single-layer graphene atomic patchwork quilts. Nature 2011, 463, 389–392. [CrossRef][PubMed] 2. Lee, C.; Wei, X.D.; Kysar, J.W.; Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008, 321, 385–388. [CrossRef][PubMed] 3. Bagri, A.; Kim, S.P.; Ruoff, R.S.; Shenoy, V.B. Thermal transport across twin grain boundaries in polycrystalline graphene from nonequilibrium molecular dynamics simulations. Nano Lett. 2011, 11, 3917–3921. [CrossRef] [PubMed] 4. Novoselov, K.S.; Geim, A.K.; Morozov, S.V.; Jiang, D.; Zhang, Y.; Dubonos, S.V.; Grigorieva, I.V.; Firsov, A.A. Electric field effect in atomically thin carbon films. Science 2004, 306, 666–669. [CrossRef][PubMed] 5. Schniepp, H.C.; Li, J.L.; McAllister, M.J.; Sai, H.; Herrera-Alonso, M.; Adamson, D.H.; Prud’homme, P.K.; Car, R.; Saville, D.A.; Aksay, I.A. Functionalized single graphene sheets derived from splitting graphite oxide. J. Phys. Chem. B 2006, 110, 8535–8539. [CrossRef][PubMed] 6. Eda, G.; Fanchini, G.; Chhowalla, M. Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material. Nat. Nanotechnol. 2008, 3, 270–274. [CrossRef] 7. Yan, Z.; Lin, J.; Peng, Z.W.; Sun, Z.Z.; Li, L.; Xiang, C.S.; Samuel, E.L.; Kittrell, C.; Tour, J.M. Toward the synthesis of wafer-scale single-crystal graphene on copper foils. ACS Nano 2013, 7, 2872. [CrossRef] 8. Berger, C.; Song, Z.M.; Li, T.B.; Ogbazghi, A.Y.; Feng, R.; Dai, Z.T.; Marchenkov, A.N.; Conrad, E.H.; First, P.N.; et al. Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based . J. Phys. Chem. B 2004, 108, 19912–19916. [CrossRef] 9. Li, X.S.; Cai, W.W.; An, J.H.; Kim, S.; Nah, J.; Yang, D.X.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; et al. Large-area synthesis of high-quality and uniform graphene films on copper foils. Science 2009, 324, 1312–1314. [CrossRef] 10. Reina, A.; Jia, X.T.; Ho, J.; Nezich, D.; Son, H.B.; Bulovic, V.; Dresselhaus, M.S.; Kong, J. Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition. Nano Lett. 2009, 9, 30–35. [CrossRef] 11. Li, X.S.; Cai, W.W.; Colombo, L.; Ruoff, R.S. Evolution of graphene growth on Ni and Cu by carbon isotope labeling. Nano Lett. 2009, 9, 4268–4272. [CrossRef] 12. Yu, Q.K.; Lian, J.; Siriponglert, S.; Li, H.; Chen, Y.P.; Pei, S.S. Graphene segregated on Ni surfaces and transferred to insulators. Appl. Phys. Lett. 2008, 93, 113103. [CrossRef] 13. Grantab, R.; Shenoy, V.B.; Ruoff, R.S. Anomalous strength characteristics of tilt grain boundaries in graphene. Science 2010, 330, 946–948. [CrossRef][PubMed] 14. Yu, Q.K.; Jauregui, L.A.; Wu, W.; Colby, R.; Tian, J.F.; Su, Z.H.; Cao, H.L.; Liu, Z.H.; Pandey, D.; Wei, D.; et al. Control and characterization of individual grains and grain boundaries in graphene grown by chemical vapor deposition. Nat. Mater. 2011, 10, 443–449. [CrossRef] 15. Robertson, A.W.; Warner, J.H. Hexagonal single crystal domains of few-layer graphene on copper foils. Nano Lett. 2011, 11, 1182–1189. [CrossRef] 16. Wu, B.; Geng, D.C.; Guo, Y.L.; Huang, L.P.; Xue, Y.Z.; Zheng, J.; Chen, J.Y.; Yu, G.; Liu, Y.Q.; Jiang, L.; et al. Equiangular hexagon-shape-controlled synthesis of graphene on copper surface. Adv. Mater. 2011, 23, 3522. [CrossRef][PubMed] 17. Wu, B.; Geng, D.C.; Xu, Z.P.; Guo, Y.P.; Huang, L.P.; Xue, Y.Z.; Chen, J.Y.; Yu, G.; Liu, Y.Q. Self-organized graphene crystal patterns. NPG Asia Mater. 2013, 5, e36. [CrossRef] 18. Vlassiouk, I.; Regmi, M.; Fulvio, P.F.; Dai, S.; Datskos, P.; Eres, G.; Smirnov, S. Role of hydrogen in chemical vapor deposition growth of large single-crystal graphene. ACS Nano 2011, 5, 6069–6076. [CrossRef][PubMed] 19. Geng, D.C.; Wu, B.; Guo, Y.L.; Huang, L.P.; Xue, Y.Z.; Chen, J.Y.; Yu, G.; Jiang, L.; Hu, W.P.; Liu, Y. Uniform hexagonal graphene flakes and films grown on liquid copper surface. Proc. Natl. Acad. Sci. USA 2012, 109, 7992–7996. [CrossRef][PubMed] Materials 2019, 12, 1887 10 of 10

20. Zhou, H.L.; Yu, W.J.; Liu, L.X.; Cheng, R.; Chen, Y.; Huang, X.Q.; Liu, Y.; Wang, Y.; Huang, Y.; Duan, X. Chemical vapour deposition growth of large single crystals of monolayer and . Nat. Commun. 2013, 4, 2096. [CrossRef][PubMed] 21. Wofford, J.M.; Nie, S.; McCarty, K.F.; Bartelt, N.; Dubon, O.D. Graphene islands on Cu foils: The interplay between shape, orientation, and defects. Nano Lett. 2010, 10, 4890–4896. [CrossRef] 22. Li, X.S.; Magnuson, C.W.; Venugopal, A.; Venugopal, A.; An, J.H.; Suk, J.W.; Han, B.Y.; Borysiak, M.; Cai, W.W.; Velamakanni, A.; et al. Graphene films with large domain size by a two-step chemical vapor deposition process. Nano Lett. 2010, 10, 4328–4334. [CrossRef][PubMed] 23. Geng, D.C.; Gao, E.L.; Wang, H.P.; Xu, J.; Xu, Z.P.; Yu, G. Large-area growth of five-lobed and triangular graphene grains on textured Cu substrate. Adv. Mater. Interfaces 2016, 3, 1600347. [CrossRef] 24. Liu, J.W.; Wu, J.; Edwards, C.M.; Berrie, C.L.; Moore, D.; Chen, Z.J.; Maroni, V.A.; Paranthaman, M.P.; Goyal, A. Triangular graphene grain growth on cube-textured Cu substrates. Adv. Funct. Mater. 2011, 21, 3868–3874. [CrossRef] 25. Wu, Y.A.; Robertson, A.W.; Schaffel, F.; Speller, S.C.; Warner, J.H. Aligned rectangular few-layer graphene domains on copper surfaces. Chem. Mater. 2011, 23, 4543–4547. [CrossRef] 26. Dai, G.P.; Wu, M.H.; Taylor, D.K.; Vinodgopal, K. Square-shaped, single-crystal, monolayer graphene domains by low-pressure chemical vapor deposition. Mater. Res. Lett. 2013, 1, 67–76. [CrossRef] 27. Wang, H.; Wang, G.Z.; Bao, P.F.; Yang, S.L.; Zhu, W.; Xie, X.; Zhang, W.J. Controllable synthesis of submillimeter single-crystal monolayer graphene domains on copper foils by suppressing nucleation. J. Am. Chem. Soc. 2012, 134, 3627–3630. [CrossRef][PubMed] 28. Liu, W.; Li, H.; Xu, C.; Khatami, Y.; Banerjee, K. Synthesis of high-quality monolayer and bilayer graphene on copper using chemical vapor deposition. Carbon 2011, 49, 4122–4130. [CrossRef] 29. Jung, D.H.; Kang, C.; Yoon, D.; Cheong, H.; Lee, J.S. Anisotropic behavior of hydrogen in the formation of pentagonal graphene domains. Carbon 2015, 89, 242–248. [CrossRef] 30. Geng, D.C.; Meng, L.; Chen, B.Y.; Gao, E.L.; Yan, W.; Yan, H.; Luo, B.R.; Xu, J.; Wang, H.P.; Mao, Z.; et al. Controlled growth of single-crystal twelve-pointed graphene grains on a liquid Cu surface. Adv. Mater. 2014, 26, 6423–6429. [CrossRef] 31. Nguyen, V.L.; Shin, B.G.; Duong, D.L.; Kim, S.T.; Perello, D.; Lim, Y.J.; Yuan, Q.H.; Ding, F.; Jeong, H.Y.; Shin, H.S.; et al. Seamless stitching of graphene domains on polished copper. Adv. Mater. 2015, 27, 1376–1382. [CrossRef][PubMed] 32. Xue, R.W.; Abidi, I.H.; Luo, Z.T. Domain size, layer number and morphology control for graphene grown by chemical vapor deposition. Funct. Mater. Lett. 2017, 10, 1730003. [CrossRef] 33. Zhang, Z.Y.; Lagally, M.G. Atomistic processes in the early stages of thin-film growth. Science 1997, 276, 377–383. [CrossRef][PubMed]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).