Carbide-Forming Groups IVB-VIB Metals: A New Territory in the

Letter pubs.acs.org/NanoLett

Carbide-Forming Groups IVB-VIB Metals: A New Territory in the Periodic Table for CVD Growth of Graphene Zhiyu Zou,† Lei Fu,† Xiuju Song,† Yanfeng Zhang,*,†,‡ and Zhongfan Liu*,† †

Center for Nanochemistry, Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry for Unstable and Stable Species, College of Chemistry and Molecular Engineering, Academy for Advanced Interdisciplinary Studies and ‡ Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, PR China S Supporting Information *

ABSTRACT: Early transition metals, especially groups IVBVIB metals, can form stable carbides, which are known to exhibit excellent “noble-metal-like” catalytic activities. We demonstrate herein the applications of groups IVB-VIB metals in graphene growth using atmospheric pressure chemical vapor deposition technique. Similar to the extensively studied Cu, Ni, and noble metals, these transition-metal foils facilitate the catalytic growth of single- to few-layer graphene. The most attractive advantage over the existing catalysts is their perfect control of layer thickness and uniformity with highly flexible experimental conditions by in situ converting the dissolved carbons into stable carbides to fully suppress the upward segregation/ precipitation effect. The growth performance of graphene on these transition metals can be well explained by the periodic physicochemical properties of elements. Our work has disclosed a new territory of catalysts in the periodic table for graphene growth and is expected to trigger more interest in graphene research. KEYWORDS: Graphene, chemical vapor deposition, catalyst, early transition metal, metal carbide

A

to monolayer grown on some single-crystalline TMCs under UHV by paying their particular attention to the dispersion relations of valence electrons and phonons.22−24 Here we demonstrate the excellent catalytic performances of polycrystalline groups IVB-VIB early transition metals for the CVD growth of graphene via carbide formation. A precise layer control of graphene, most preferentially monolayer graphene, has been achieved on all groups IVB-VIB TMCs in situ formed from parent metal foils except for Cr, whose carbides have rather complicated structures. A distinct growth mechanism from that on group VIII or IB metals is first proposed, and a periodicity for the CVD growth behaviors of graphene on the eight groups IVB-VIB metals has been observed, attributable to the periodic variation of physicochemical properties of elements. We believe that we have uncovered a new territory in the periodic table for the catalytic CVD growth of graphene, which are comparable to or even surpass some of the wellknown metal catalysts. As schematically illustrated in Figure 1a, commercially available polycrystalline foils of groups IVB-VIB metals (Ti, Zr, Hf, V, Nb, Ta, Mo, and W) were chosen as the CVD growth substrates of graphene. Carbon atoms decomposed from methane precursors at surfaces of these early transition metals

mong the diverse methods for the preparation of graphene, a two-dimensional (2D) honeycomb organization of carbon atoms with excellent electrical and optical properties,1−4 chemical vapor deposition (CVD) has been seen as one of the most promising techniques toward mass production for both high quality and low cost.5,6 Since first utilized in 2009, copper foils have been standing out for the state of the art in CVD synthesis of graphene, especially for its easy access to single-layer and large-area graphene with excellent quality.5−7 Besides, iron series elements (Fe, Co, and Ni), group VIII noble metals (such as Ru, Ir, Pt), and other group IB coinage metals (Au, Ag) have also attracted extensive explorations for their exhibiting varied growth mechanisms and qualities.8−15 Transition-metal carbides (TMCs), especially carbides of groups IVB-VIB early transition metals, have been found to display excellent “noble-metal-like” heterogeneous catalytic activities for 40 years,16 especially for dehydrogenation and aromatization of hydrocarbons,17,18 and have been the subject of many experimental and theoretical investigations.19,20 Unlike the unstable or metastable carbidic compounds of VIII metals such as Ni2C and Fe3C,12,21 groups IVB-VIB TMCs possess high thermal stability (Table S1) and thus can tolerate hightemperature processing. It is therefore not surprising to apply early TMCs to the catalytic growth of graphene. From early 1990s, Aizawa and Nagashima et al. have performed a series of experimental investigations of epitaxial graphite thin films down © 2014 American Chemical Society

Received: March 17, 2014 Revised: May 19, 2014 Published: May 29, 2014 3832

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Figure 1. New territory in the periodic table for catalyzing CVD growth of uniform graphene. (a) Schematic illustration of graphene growth on groups IVB-VIB transition-metal foils facilitated via carbide formation. (b) Optical micrographs of thus-grown graphene transferred onto 285 nm SiO2/Si substrates. The growth was all at 1050 °C, and the parameters can be found in Methods in Supporting Information. The images are arranged following the sequence in the periodic table. Scale bar, 20 μm.

Figure 2. Large-area and highly uniform graphene grown on different groups IVB-VIB transition-metal foils. The growth temperature in (a−d) was 1050 °C, and the growth processes in (a−f) were all as depicted in Methods in Supporting Information. (a) Photographs of graphene CVD-grown on tungsten foil after transferred onto 285 nm SiO2/Si. (b) AFM images of molybdenum-grown graphene on SiO2/Si substrates. Scale bar, 500 nm. (c) TEM images taken at the folded edges of graphene grown on Mo, W, and V foils, respectively. Scale bar, 5 nm. (d) Atomic resolution TEM image and SAED pattern (inset) of graphene obtained on Mo foil. (e,f) Raman spectra of graphene grown on VIB and VB metals, respectively.

in the periodic table. In all cases, highly uniform and large-area graphene, mostly monolayer graphene has been grown using atmospheric pressure chemical vapor deposition (APCVD), which could be easily transferred to arbitrary substrates using peeling-off techniques such as solution etching or bubbling26,27 (CVD conditions and transfer methods can be found in Supporting Information). The uniformity and layer-thickness of graphene were further characterized via various methods. Macroscopic observation reveals that no recognizable color difference can be seen with bare eyes throughout the whole graphene-covered regions (Figures 2a and S1). The ultraviolet−visible spectra of

were dissolved into the bulk metals, succeeded by carbide formation.25 Highly stable stoichiometric or substoichiometric interstitial carbides (MC, M2C, or MC1−x, where M refers to metal atom and 0 < x < 1) result19 instead of forming a solid solution8−10 or metastable carbides (such as Ni2C or Fe3C).12,21 Thus-formed early TMCs on the parent metal foils actually served as catalysts for graphene growth16,19 as discussed later. In fact, some of them such as carbides of Mo, W and V have manifested their activities in the nonoxidative dehydroaromatization of methane.17,18 Listed in Figure 1b are the optical microscope (OM) images of growth results corresponding to the eight elements arranged by the sequence 3833

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Figure 3. Evidence for in situ carbide formation during graphene growth. (a,b) Optical micrographs of graphene grown on Mo (a) and W foils (b) with high carbon source concentration and slow cooling rate (200 sccm CH4/100 sccm H2, 23 °C/min) after transferred to 285 nm SiO2/Si substrates. Scale bar, 20 μm. (c) XPS depth profiles of the atomic ratio (metal/carbon) for Ti, V, Mo, and W foils after CVD growth. The surface layers were removed by argon ion bombardment, and the etching rate is assumed to be equal to that on tantalum oxide. (d) XPS C 1s peaks from surface and bulk on a Mo foil after CVD growth. (e) Changes of C 1s core-level signal with depth on a W foil after CVD growth without normalization of core-level peak heights. (f) Raman spectra of graphene grown on a 300 nm sputtered Mo film at 1050 °C under the same condition as that on Mo foil.

1050 °C has notable D bands at 1350 cm−1 as well as D′ peaks at ∼1620 cm−1, indicative of existence of defects, which is also evidenced by electrical measurements (Figure S7). According to Raman spectra and device performances, the quality of graphene is considered to be comparable with or better than those grown on Au, Ag, and some insulating substrates.15,32−34 The quality of graphene can be significantly improved by the elevation of growth temperatures. As the blue curve shows, ID/ IG of molybdenum-grown graphene decreases remarkably at the growth temperature of 1300 °C. For groups IVB-VB metals, a prominent Raman feature is the intense G and D peaks as compared with the small 2D bands (Figures 2f, S8, and S9), typical for nanocrystalline graphene.35 Since both IG/I2D and 2D bandwidth get increased together with the increase of defects,36 it becomes difficult to extract layer thickness information from Raman spectroscopy. The existence of defects for graphene grown on groups IVB-VIB metals can be attributed to the polycrystalline nature of substrates and their high melting temperatures (Figure S10 and Table S1), for which it is difficult to obtain single-crystalline surface even applying long duration of preannealing treatment. Faceting during carburization and lattice mismatch between substrates and graphene would also be disadvantageous for obtaining large-domain graphene. Since the interactions between early TMCs and graphene can be rather intense,24 the influence of polycrystalline nature of substrates on the orientation of graphene domains would be significant. This can also explain the much weaker D-band of graphene grown at 1300 °C (Figure 2e). The elevated growth temperature accelerates surface carbon migration and provides more energy for graphene expansion over surface defects (such as grain

graphene grown on Mo and W foils both manifest the characteristic optical transmittance of monolayer graphene (Figure S2). Moreover, atomic force microscopy (AFM) images of graphene grown on Mo and W foils exhibit uniform film thickness around 1.1 and 1.2 nm, respectively, as well as wrinkles with a height of 0.7−8 nm (Figures 2b, S3, and S4), probably attributable to corrugated substrate morphology and mismatch of thermal expansion coefficients between metal foils and graphene.5,28 To further identify the layer number, transmission electron microscope (TEM) images were taken at the folded edges of graphene lying on lacey carbon-coated grids. Only monolayer edges were observed on molybdenum-, tungsten- and vanadium-grown graphene (Figure 2c). The atomic resolution TEM with a “one-layer deep” hole pointed by a buff arrow and selected-area electron diffraction (SAED) of molybdenum-grown graphene with only one set of hexagonal pattern reveals the feature of monolayer graphene with domain size of hundreds of nanometers as well as good crystallinity29,30 (Figure 2d). In contrast, the SAEDs of tungsten- and vanadium-grown graphene reveal several sets of hexagonal patterns or diffraction rings (Figures S5 and S6), the nature of smaller domain sizes. More comprehensive information on graphene was obtained from Raman spectroscopy. As seen in Figure 2e, molybdenumgrown graphene (red curve) exhibits a G-to-2D intensity ratio (IG/I2D) of 5 °C/s) carbon precipitation from solid solution carbide phase and crystal structures of major carbide phase

Table 1. Periodic Behaviors of Groups IVB-VIB Metals for APCVD Growth of Graphenea

uniform G grown with slow cooling (23 °C/min)

intense hydrogen intake

B.E. of bulk carbidic C (eV)

B.E. of graphitic C (eV)

fwhm of graphitic C peak (eV)

At last, we will discuss the highly periodic performance of groups IVB-VIB metals for CVD growth of graphene. As extensively researched, with very small carbon solubility copper is suitable to prepare single-layer graphene, abiding by surface catalytic mechanism.5 The similar scenario can also be found on gold, another group IB metal in the periodic table.15 On the contrary, for polycrystalline group VIII elements such as Ni, Co, and Fe with higher carbon solubility, carbon dissolution− precipitation always induce inhomogeneity in graphene.8,9,11,12 Apparently, for metal catalysts located at different positions in the periodic table, graphene has divergent growth modes. When we move to another location where groups IVB-VIB metals reside, as discussed above, a brand new carbide-participating (both as catalyst and carbon-trapping reservoir) growth mode takes effect. It is known that the bond strength between carbon and transition-metal atoms (carbon affinity) depends on the number of metal d-electrons and decreases from left to right in the same period.49−51 The formation enthalpy of 3d TMCs becomes positive for Fe3C, Co2C, and Ni3C,50 indicating that they are thermodynamically unfavorable and cannot exist or will decompose at the growth temperature of graphene. This well explains the distinct growth modes between groups IVBVIB and VIII-IB metals, the latter forming either unstable or even no carbides. The carbon affinity also influences the diffusion of carbon atoms in metal bulks. The formation of extremely stable carbides and lack of interstitial voids make it difficult for carbon diffusion in the lattices of groups IVB-VIB metals because metal−carbon (M-C) bond breaking and neighboring hollow sites are both demanded (Table S2). Therefore, although bulk carbons can be released from group IVB metals during cooling according to binary phase diagram,38 uniform graphene can be obtained by trapping free carbons with fast cooling. However, due to much larger carbon diffusion coefficient in Ni52 we only obtained inhomogeneous graphene and graphite on polycrystalline Ni films and foils, respectively, using fast cooling (Figures S21 and S22). Finally, it is also necessary to take into consideration of the graphene−TMC interaction, which should be distinguished from M-C interaction in TMCs but can as well affect graphene growth. Generally speaking, the graphene−substrate interaction is stronger for group VIII metals (with open ground-state valence shell configuration) than for group IB metals (with close-like ground-state valence shell configuration), and among VIII group the interaction increases as it moves from 5d to 4d metals or from right to left along the same row in the periodic table.53 We evaluated the interactions between graphene and substrates originating from their p−d orbital hybridization by fwhm of graphitic C 1s peak in XPS (Table 1), which has been utilized when graphene lies on single-crystalline TMCs or noble metals.23,53 To strengthen the reliability of comparison B.E. of Au 4f7/2 bands was adopted for calibration and shouldered sp3 peaks at 285.3−285.5 eV were ruled out on Shirley backgrounds. It is notable that there exists broadening of graphitic C 1s bands on groups IVB-VIB TMC substrates compared to that on nickel film (Table 1), indicative of strong interactions between graphene and the in situ generated TMCs. This conclusion consists with that previously reported by literatures,22,24 suggesting the quality of graphene is intensely influenced by substrate and can be improved via obtaining smoother surface with less lattice mismatch to graphene, which is under research. The periodic behaviors can also be found in between groups IVB-VIB metals (Table 1) though they share many similarities.

1.06 0.94 0.88 1.33 1.00 0.91 0.93 0.94 PC film: 0.79 (our data)

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Z.Y.Z. wrote the manuscript, and Z.F.L. revised the manuscript. All authors contributed to data analysis and scientific discussions.

For instance, best growth results were obtained on group VIB metals, and monolayer graphene could be grown even with high methane concentration and slow cooling (Figures 3a,b). For group VB metals, however, typically uniform few- to singlelayer graphene was obtained with fast cooling rate (>5 °C/s), while slow cooling with high methane concentration would lead to multilayer graphene or graphite. Considering that carbon precipitation is negligible, and the chemical diffusion coefficients of carbon in group VB metals are smaller than in group VIB metals (Table S2), decomposed carbon species accumulate on VB metal surfaces more easily. The difference of carbon diffusion rates between group VB and VIB metals also partly originates from different carbon affinity as aforementioned. Here we used charge transfer between metal and carbon atoms to partly represent the M-C interaction, although it has a rather complex nature.47 Since the charge transfer in groups IVB-VIB TMCs is considered to be from metal to carbon,19 lower B.E. of carbidic C 1s peaks in XPS implies more negative charge received by bulk carbon atoms. From Table 1, it can be seen that from group VIB to IVB the M-C ionic interaction increases. The variance of carbon diffusion coefficients can also be proofed by an empirical rule that the lower melting point of TMCs, the lower activation energy for carbon diffusion.54 For VC, WC, and Mo2C having the lowest melting points among groups IVB-VIB TMCs (Table S1), they also demonstrated the largest growth windows for uniform monolayer graphene. When it comes to group IVB metals, besides slower carbon diffusion, the situation is also complicated by hydrogen storage ability55 and carbon solubility of bulk metals38 (Figure 3c). In fact the experimental growth window to obtain uniform thinlayer graphene for IVB metals was found to be the narrowest in groups IVB-VIB. Fast temperature ramping and short growth time were employed together with a low dosage of H2 and CH4 to suppress either the break of bulk metals from overdose H2 intake or carbon segregation/precipitation (see Methods section and Figure S23 in Supporting Information). In conclusion, we have succeeded in growing highly uniform single- to few-layer graphene on groups IVB-VIB transition metals with APCVD via carbide formation. A brand new growth mechanism different from that on groups VIII and IB metals has been proposed. The growth performance of graphene on different metals and TMCs can be well explained by the periodicity of elements in the periodic table. Our work greatly extended the territory of catalysts for graphene growth, which is expected to trigger more interest in graphene research and to contribute to the needs for high-quality graphene.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by the National Natural Science Foundation of China (Grants 51290272, 51121091) and the Ministry of Science and Technology of China (Grants 2013CB932603,2012CB933404, 2012CB921404, 2011CB933003, 2011CB921903).

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ASSOCIATED CONTENT

* Supporting Information S

Description of experimental details, properties of groups IVBVIB metals/TMCs, and carbon diffusion coefficients in transition metals both listed in tables, electrical measurements, XRD, and other supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: zfl[email protected] *E-mail: [email protected] Author Contributions

Z.F.L. developed the concept and conceived the experiments. Z.Y.Z., Y.F.Z., L.F., and X.J.S. carried out the experiments. 3838

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NOTE ADDED AFTER ASAP PUBLICATION The reference citations in Table 1 have been updated. The revised version was re-posted on June 3, 2014. 3839

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