Graphene Ingestion and Regrowth on “Carbon-Starved” Metal

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Graphene Ingestion and Regrowth on “Carbon-Starved” Metal Electrodes

Ming-Sheng Wang,*,†,‡ Yong Cheng,† Longze Zhao,‡ Ujjal K. Gautam,§ and Dmitri Golberg*,∥,⊥ †

Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen, Fujian 361005, China Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, Fujian 361005, China § Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER)-Mohali, Sector 81, Mohali, SAS Nagar, Punjab 140306, India ∥ International Centre for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 3050044, Japan ⊥ School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street, Gardens Point, Brisbane, QLD 4000, Australia ‡

S Supporting Information *

ABSTRACT: The interaction between graphene and various metals plays a central role in future carbon-based device and synthesis technologies. Herein, three different types of metal nanoelectrodes (W, Ni, Au) were employed to in situ study the graphene−metal interfacial kinetic behaviors in a high-resolution transmission electron microscope. The three metals exhibit distinctly different interactions with graphene when driven by a heating current. Tungsten tips, the most carbonstarved ones, can ingest a graphene sheet continuously; nickel tips, less carbon starved, typically “eat” graphene only by taking a “bite” from its edge; gold, however, is nonactive with graphene at all, even in its molten state. The ingested graphene atoms finally precipitate as freshly formed graphitic shells encapsulating the catalytic W and Ni electrodes. Particularly, we propose a periodic extension/thickening graphene growth scenario by atomic-scale observation of this process on W electrodes, where the propagation of the underlying tungsten carbide (WC) dominates the growth dynamics. This work uncovers the complexity of carbon diffusion/segregation processes at different graphene/metal interfaces that would severely degrade the device performance and stability. Besides, it also provides a detailed and insightful understanding of the sp2 carbon catalytic growth, which is vital in developing efficient and practical graphene synthetic routes. KEYWORDS: graphene−metal interaction, sp2 carbon catalytic growth, carbon diffusion, metal carbide, interfacial thermochemistry, in situ TEM

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particularly true for graphene applications in nanoelectronic, nanoelectromechanical, and energy storage devices, etc.1−6 These graphene/metal (abbreviated as “G/M” hereafter) interfaces are also critical for the large-area growth of high-quality

raphene has attracted worldwide attention owing to its appealing properties and great promise for building future carbon-based device architectures.1 Among the diverse graphene-based applications, the interaction between graphene sheets and various metals plays a central role, since the performance of every single graphene-based device is greatly influenced by the electronic/mechanical coupling and chemical/ structural stability at the graphene−electrode interfaces. This is © 2017 American Chemical Society

Received: August 25, 2017 Accepted: September 27, 2017 Published: September 27, 2017 10575

DOI: 10.1021/acsnano.7b06078 ACS Nano 2017, 11, 10575−10582

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Figure 1. Morphology and structure changes of a graphene/W-tip heterostructure under biasing and Joule heating. (a−e) Series of TEM images showing the structural transformation of the G/W interface. In (b) the arrow indicates a wrinkle on the graphene surface, and the inset shows the enlarged image of the contact region with a void formed within the surface layers highlighted by a white dashed line. (f) The W tip finally broke at the carbide region under tensile stress caused by graphene ingestion. The black arrows indicate the positions of the interface between the tungsten carbide and pure tungsten domains. (g) High-resolution TEM image of the WC domain and the corresponding SAED pattern (inset).

and study the real process of graphene layer growth. Instead of gaseous raw materials (e.g., CH4 or C2H2) that are widely used in conventional CVD methods,7,17 in this work, graphene itself served as a solid carbon source for its regrowth on metal surfaces. Different metals were fabricated into nanosized tips, thus naturally offering the cross-section of G/M interfaces required for TEM observations. During a typical G/M kinetic process, a graphene sheet is first “ingested” into a heated liquid-like metal tip and then segregated as new graphitic layers surrounding the tip surface. Three representative metals (tungsten, nickel, and gold) were used in the experiments, and their reactivities with graphene, referred to as “carbon-starved” degrees, were ultimately compared. Our findings would be extremely valuable for the design of efficient graphene−metal contacts with a high thermal stability for device purposes and for in-depth understanding of the general growth mechanism of sp2 carbon structures.

graphene suitable for commercial purposes, since the graphene growth is strongly linked to the kinetic behavior between epitaxial graphene and metal (or carbide) substrates during metal catalytic syntheses.7−16 Although previous studies based on postsynthesis characterizations and theoretical considerations have been carried out to pursue a detailed understanding of the graphene nucleation and growth mechanisms,12−16 the complexity of the interactions between growing graphene and underlying metals is not completely uncovered. To date, the direct highresolution observations of graphene growth on different metals and study of their interplay at elevated temperatures are still lacking. In this contribution, we present the systematic in situ transmission electron microscopy (TEM) investigation of G/ M interfacial kinetics stimulated by a heating current. The G/M interfacial reaction processes were observed in real time and at atomic-level resolution, which gave a rare opportunity to mimic 10576

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Figure 2. Structural characterization of the W/WC heterostructure encapsulated with the segregated graphite layers. TEM image (a) and the corresponding spatially resolved carbon map (b) of a G/W/WC heterostructure. (c) EEL spectra of the domains indicated in (a). (d) HRTEM image of the indicated region in the inset showing the nucleated graphene layers on the WC substrate. The inset shows the overall image of the G/ W interface. (e−g) Alternating graphene layer lateral extension and vertical thickening growth on the WC substrate. Black arrowheads 1−3 display the advance of graphene termination. (h) Typical case of a W-tip encapsulated with a thick graphite layer with improved structural quality.

RESULTS AND DISCUSSION

which depends on the metal species, tip sizes, G/M contact conditions, etc. We first chose tungsten, a typical refractory metal, as the metal tip. As shown in Figure 1b, when the bias was increased to 1.4 V, with a current of ∼600 μA, the graphene near the G/W contact began to be absorbed into the W tip, resulting in obvious wrinkling of the formerly smooth graphene surface (marked by the arrow in Figure 1b). A closer examination of the enlarged image at the G/W contact reveals that a few surface layers, close to the W tip, were ingested preferentially. The brighter contrast area in the inset of Figure 1b indicates a void formed within these surface layers with its edge highlighted by the white dashed line. In the following process, the W tip worked like an arm to grasp the graphene sheet firmly (Figure 1c) and, later, like a straw to suck the right side of the graphene sheet entirely into its body

The graphene sheets used in our experiments were produced by adopting a supercritical fluid exfoliation method of graphite powder, in which ∼90% of the exfoliated sheets are less than eight layers.18 The electrochemically etched metal tips usually have an initial diameter of less than 50 nm at the tip end (see Method or refs 19−21 for the preparation details of W, Ni, and Au nanotips, respectively). Both graphene sheets and the metal tips were mounted on a TEM-scanning tunneling microscopy (STM) holder,22,23 followed by an in situ manipulation to make contact between a single graphene sheet and a metal tip inside the TEM, as depicted in Figure 1a. The G/M interfacial reaction was initiated when this heterostructure was Joule-heated to high temperature simply by increasing the applied bias up to 1−3 V, 10577

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of WC make less than half of what is actually needed to build the graphite as observed in Figure 2a. This apparently means that more C atoms in the grown graphite come directly from the ingested source graphene via carbon diffusion through the bulk WC and segregation at the graphite/WC interface.22 In spite of the complexity of the carbon supply mode in our experiments, the graphite growth on WC and SiC also share some common features. First, these two types of carbides are both hexagonal crystals with C and W/Si atomic planes alternately stacked along the c-axis. Second, the graphite growth on WC is found to proceed in a bottom-up manner, as proposed for the epitaxial graphene growth on SiC.13 As indicated in Figure 2d, the top layer of a four-layer graphene has extended beyond the right-hand side concave step, whereas the layers below terminate into the concave step, forming the growth front for their further extension. Here, the extent of layer formation is inversely related to the distance from the top layer of graphene, which is consistent with the bottom-up growth where each subsequent graphene layer is formed between the above layer and the underlying carbide substrate. The G/W reaction was then controlled to proceed slowly by adjusting the applied bias, which facilitates the tracking of graphene growth fronts and the study of its growth dynamics. As shown in Figure 2e−g, the bottom-up graphene thickening is found to alternate with its lateral extension during the graphene growth on WC. In the lateral extension step, as indicated by the rightward arrow in Figure 2e, single or bilayer graphene propagates first from the site of its former termination (black arrowhead 1). In the following thickening step, the top layer extension stops at the new graphene termination (arrowhead 2 in Figure 2f), and the layers below extend successively, resulting in a vertical growth of graphene, as indicated by the upward arrow. In the next growth cycle, the top layer(s) propagates once again; its new termination is marked by arrowhead 3 in Figure 2g. This periodic extension/thickening growth scenario can be repeated until the G/W reaction is halted. Notably, the graphene extension never surpasses the propagation of the underlying carbide region, highlighting the dominant role of carbide substrate for graphene lateral growth. Besides, the discontinuous top layer, due to the formation of a large void (as pointed out by the white arrowhead in Figure 2e), has been repaired during the subsequent graphene thickening. This suggests that the segregated carbon atoms not only incorporate into the carbon network to form graphene at the G/WC interface but can also diffuse along the graphene plane and participate in its reconstruction at elevated temperature. It is worth noting that during the above-discussed graphene growth process, the WC substrate changes marginally without notable W desorption from its surface. Actually, graphene growth and the associated WC morphology are highly influenced by the growth conditions. During the controlled growth with a relatively low heating current, the graphene growth proceeds via alternating extension and thickening, and the WC can largely maintain its morphology. By contrast, by increasing the applied heating current, the growth can be remarkably accelerated, resulting in the graphene formation up to tens of layers within a few seconds. Such instantly formed graphene usually displays an improved structural quality due to the significantly increased growth temperature (see the case in Figure 2h and Supporting Information Figure S3). Meanwhile, the increase in temperature also accelerates the process of W desorption (via its sublimation or migration), leading to a drastic morphological change of

(Figure 1d,e and Supporting Information, Movie S1). It should be noted that the W tip actually remains crystalline throughout the whole process, though it behaves somewhat like a liquid. The W tip finally broke under tensile stress caused by the graphene absorption (Figure 1f), indicating a strong adhesive force at the G/W interface. The ingested carbon transformed the W tip into tungsten carbide (α-WC), which was confirmed by highresolution TEM (HRTEM) imaging and selected area electron diffraction (SAED) patterns taken from the newly formed tip end (Figure 1g). This carbide phase usually exhibits a distinctly different optical contrast from pure tungsten. As indicated by the arrowheads in Figure 1f, tungsten carbide has expanded about 500 nm deep into the W tip body. Interestingly, the carbide region can even extend up to micrometers if the W tip is continuously pushed forward to keep contact with a large graphene sheet that could provide enough carbon feedstock. The continuous graphene ingestion leads to carbide extension within the W tip body, which subsequently causes the precipitation of new graphitic layers surrounding the tip surface where C concentration exceeds the supersaturation level. Figure 2a shows a typical as-obtained W electrode with a part of its surface covered by segregated graphitic layers (see Supporting Information Figure S4 and Video S2 for the graphite segregation process). The carbon distribution within this heterostructure was analyzed under elemental mapping using spatially resolved electron energy loss spectroscopy (EELS) (Figure 2b). The sharp boundary between the brighter and darker regions marked by the arrowheads in the C map agrees well with the carbide/ metal interface marked by arrowheads in Figure 2a. The brightest region in the C map corresponds to the graphitic layers surrounding the carbide region. This clearly shows that the intermediate tungsten carbide, instead of pure tungsten, plays the actual catalytic role during the present graphitic layer growth. Figure 2c presents EEL spectra showing the C K-edge obtained from three typical regions marked with circles A, B, and C in Figure 2a. The well-known π* peak at ∼284.5 eV indicating the sp2 carbon bonding, which shows up only in the spectrum from region A, confirms the formation of graphite structures, whereas the carbon concentration in tungsten, as expected, is negligible, as shown in the spectrum from region C. In some cases, in order to reveal the details of graphene nucleation, the G/W reaction was terminated at the early stage of graphene segregation by switching off the bias. As documented by the HRTEM image in Figure 2d, graphene nucleation had been found to be associated with the formation of morphological defects on the WC surface. The four-layer graphene seems to nucleate at the surface step (indicated by the arrowhead 1), extend along the surface plane, and terminate at a slightly concave step that is more distant from the W tip end (indicated by the arrowhead 2). This strongly suggests that the graphene growth on WC would be accompanied by W atom desorption from the WC substrate, leaving behind concave steps on the surface. It is more evident in Figure 2a, compared to the pure WC region (region B), that the graphene growth has led the underlying WC (region A) to shrink drastically in the radial direction due to W atom desorption. The desorbed W atoms could migrate toward the tip end or, more likely, be evaporated into the vacuum at a high local temperature, quite similar to the process of graphene fabrication by vacuum annealing of silicon carbide via Si sublimation.8 Differently, the graphite growth during our graphene−metal reaction process can not solely be attributed to the W desorption from the WC substrate. A simple calculation reveals that the C atoms derived from the missing part 10578

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Figure 3. Interfacial reaction between Ni tips and graphene sheets with different contact conditions. (a, b) Local melting of a nickel tip on the surface of a graphene sheet at a large heating current. (c−h) The series of TEM images showing the etching of the graphene edge by a Ni tip with its end locally melted. The black arrows indicate a fullerene-like carbon particle that maintains its integrity during two reactions (e,f and g,h) with the Ni tip.

tungsten carbide encapsulated in the grown graphite, as seen in Figure 2a and h. Nickel, a typical transition metal for catalytic growth of graphene, was also employed to study its interaction with graphene using the present in situ technique. A Ni tip was brought into contact with the surface of a graphene sheet to initiate the G/M reaction (Figure 3a). In contrast to W, the increasing current did not cause an interfacial reaction on the graphene surface, although the Ni tip was locally melted and swelled at the end due to Joule heating (Figure 3b). However, in the case when a Ni tip was made to contact the edge of a graphene sheet, the applied current heating could readily initiate graphene edge etching, as seen in Figure 3c−h or Supplementary Video S3. The G/Ni interfacial reaction usually leads to the formation of the irregularly notched edge on graphene as if gnawed by the Ni tip. Unlike the continuous graphene ingestion

by the W tip, the G/Ni reaction is often interrupted due to the relatively weak adhesion between the molten Ni tip end and the etched graphene edge. The local temperature at the G/M interface upon graphene “eating” is likely around the melting point of bulk Ni (1450 °C). At such high temperature, the Ni tip end always slightly changes its morphology after each “bite” of graphene, thus frequently detaching from the graphene sheet (Figure 3f). The Ni tip, therefore, needs to be made in contact with the graphene sheet again to continue the G/Ni reaction (Figure 3g,h). Actually, the Ni tip can also be strongly bonded to a graphene edge so as to pull the graphene sheet in case the tip is cooled to form covalent bonds at the G/Ni interface (see Video S4 in the Supporting Information).24 The above experiments unambiguously demonstrate that the graphene edge is more reactive with Ni than its surface. This can be primarily attributed to the different bonding coordination of C 10579

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Figure 4. Structural characterization of Ni tips encapsulated with a layer of segregated graphite obtained by G/Ni reaction. (a−c) Low- and highmagnification TEM images showing the graphene/Ni heterostructure and the corresponding SAED pattern. (d) Graphitic tube encapsulating a thin Ni core wire during G/Ni reaction. (e) Magnified image of the boxed region in (d). (f) The carbon tube with the Ni core being largely removed upon further G/Ni reaction.

supply from the source graphene. As in the case shown in Figure 4d, the Ni tip portion has been encapsulated by a thick layer of graphite. The quick segregation of graphite has expelled a part of the melted Ni into the tip end (or partly evaporated), leaving behind a much thinner Ni core wire. The segregated graphite is estimated to be of 200 layers, and its morphology is highly dependent on the deformation of the underlying Ni catalyst (Figure 4e). When the Ni core wire was largely removed upon further reaction, this segment of graphite actually turned into a carbon nanotube (CNT) or nanofiber, as seen in Figure 4f. Finally, gold, a typical noble metal, was tried to study its interfacial reaction with graphene. Gold was found to have a very weak reactivity with graphene. Except for local melting of a gold nanoelectrode at the G/M contact at a high current, no dissolution of graphene into gold or formation of graphitic layers on the gold surface was detected, irrespective of whether the gold tip contacts the graphene surface or its edge (see Supporting Information Figure S7 and Video S5). These results are consistent with the previous reports that gold has very low catalytic activity toward the decomposition of a C feedstock.28 The experimental results of the interfacial reactions of three types of metals with graphene are summarized in Table 1. Obviously, these three metals can be lined up with respect to

atoms located at the graphene edge and those within the basal plane. Each C atom within the honeycomb network is bound to three neighboring C atoms by strong covalent sp2 bonds, which keeps them nonactive with Ni, even in the case of an extremely curved sp2 carbon network. This can be verified by a fullerenelike particle that maintains its integrity during its two reactions with the Ni tip, as indicated by the arrows in Figure 3e−h. In contrast, at the open edge of graphene, the reduced bonding coordination of C atoms and their dangling bonds make them more susceptible to the call from Ni metal: they dissolve into the molten Ni or form strong bonding with the crystallized Ni. The etched graphene should be primarily dissolved into the Ni tip, since we can not fully rule out the possibility that a small quantity of lost carbon may be oxidized (by residual oxygen in the TEM column) into carbon monoxide or carbon dioxide at the G/Ni interface.25,26 (Another graphene etching mechanism through Ni-nanoparticle catalytic hydrogenation of carbon should not be involved due to the absence of H2.27) When a certain amount of graphene was “eaten” by the Ni tip, as seen in the case in Figure 4a, it became saturated with carbon. Once the Ni tip detached from graphene, it became cooled instantly to the ambient temperature, leading to the multilayer graphene segregated on the Ni surface, as indicated by the arrow in Figure 4b. No existence of metastable nickel carbide phase (Ni3C) was detected, as revealed by the corresponding SAED pattern in Figure 4c (or see the HRTEM image of another Ni tip after reaction in Figure S6). This is in line with the binary phase diagram of Ni−C in which C has a limited solubility in Ni metal without the presence of a carbide compound.7,9 In a particular situation, the molten Ni tip end can also be bonded to the graphene edge due to the larger contact area, thus offering the opportunity of the growth of thicker graphite on the Ni tip due to a constant heating current and a continuous carbon

Table 1. Summarized Experimental Results and Their Comparison with Respect to the Interfacial Reaction of Three Types of Metals with Graphene

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metal type

reactivity with G

reaction site on G

carbide formation

C shell formation

W Ni Au

high medium low

everywhere edge none

yes no no

yes yes no

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ACS Nano their carbon-starved degree as follows: W > Ni > Au. Compared to Ni and Au, the higher reactivity of tungsten with graphene may stem from a chemical driving force due to the negative Gibbs free energy of the carbide formation.29 Although the transformation from a flat graphene sheet to a curved or tubular structure causes an increase in bending energy, what governs this transformation is the driving force to form bonding of the curved graphene surface onto the catalyst metal substrate and to reduce dangling bonds on the graphene edges, which can significantly reduce the energy of the whole system. Besides, the polarity of an applied bias was found to have little influence on the reaction behaviors, suggesting a thermal or thermochemical activated process rather than an electrically driven one. All three metals exhibit liquid-like behaviors, and their morphologies change strikingly at G/M interfaces at the elevated temperatures estimated to be over ∼1000 °C (for W, the formation of WC was observed at a temperature of more than 1200 K30). However, the degree of damage brought to graphene during the reactions varies a lot for different metals. As seen in Table 1, a heated W tip can cause severe graphene damage and carbon loss wherever it touches the graphene; for Ni, graphene etching only starts for its edge, whereas in the case of Au, no real damage is found on graphene, although its shape could be altered slightly due to gold melting. These results highly suggest that the nonactive noble metals can retain the maximum integrity of graphene, whereas carbide-forming metals may cause the maximum damage to graphene, which would drastically influence the performance of G/M contacts and even the entire device. We believe that using this in situ technique, it would be of great interest to further analyze the interface dynamics between graphene and other metals, such as Ti, Pd, and Sc, which have demonstrated great promise as high-performance electrode materials in the recently developed graphene and CNT device technologies.31−33 The present in situ experiments also demonstrate an entirely condensed phase transformation between different sp2 carbon allotropes, from graphene to nanotube (i.e., tubular graphene), with the assistance of a heated W and Ni catalyst (interestingly, the reverse process, i.e., from nanotube to graphene, has been achieved using several other approaches34,35). In recent years, the graphene growth from solid carbon sources through a hightemperature process has attracted increasing research interest. In addition to the above-mentioned SiC annealing approach, graphite, amorphous carbon, and polymer films, for example, can also be transformed into graphene on Ni, Co, or Cu substrates in this way.9−12 Our in situ observation provides many valuable details of the graphene growth dynamics that may represent the actual situation during any ex situ graphene growth process on a metal or carbide surface.

be not carbon-starved at all, even in its molten state. These findings provide detailed and valuable information with respect to the degraded structural stability of G/M contacts at a high working current, as well as the growth dynamics of sp2 carbon structures with a metal catalyst involved, which is vital for graphene-based device and synthesis technologies.

CONCLUSIONS To sum up, three different metal nanotips (W, Ni, Au) were employed to study the graphene−metal interfacial kinetic behaviors by using the in situ TEM technique. Driven by a heating current, a graphene sheet is “ingested” into a liquid-like tungsten tip at the G/W interface, followed by the segregation of new graphitic layers surrounding the tip surface. The periodic extension/thickening scenario is proposed to describe the graphene growth on W tip surface, where the formation of WC and its propagation were observed to dominate the growth process. Nickel tips, showing a lower reactivity, can “eat” graphene only from its edge, leading to graphene segregation on the Ni surface without carbide formation. Au, however, seems to

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 61471307), the National Key Research and Development Program of China (Grant No. 2016YFA0202602), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130121120009), the Fundamental Research Funds for the Central Universities, and National Program for Thousand Young Talents of China. It was also supported by the International Center for Materials Nanoarchitectonics (MANA) of the National Institute for Materials Science (NIMS), Tsukuba, Japan. D.G. is also grateful to the Australian Research Council for granting a Laureate Fellowship (Grant No. FL 160100089).

METHODS Tungsten, nickel, and gold nanotips were prepared by dc electrochemical etching of the corresponding metal wires (0.2 mm in diameter) with 2 M NaOH, 2 M KCl, and HCl/ethanol (1:1) solutions as the electrolytes, respectively (see refs 1−3 for more technical details). The graphene sheets were assembled onto a freshly cut gold wire (0.25 mm in diameter) before each experiment by rubbing the gold wire in the graphene powder. This gold wire was then transferred and fixed by inserting the wire into a small-diameter pipe welded to the TEM−STM sample holder frame. An electrochemically etched metal wire was inserted into the three-dimensional movable part of the piezo-driven holder. The in situ TEM manipulation, electrical probing, EELS analysis, and structural observations were performed in a JEM-3100FEF HRTEM (Omega Filter) equipped with a “Nanofactory” TEM−STM holder; a part of the in situ experiments was conducted in a JEM-2100 TEM by using a “PicoFemto” TEM−STM holder as well. Experimental videos were obtained by recording sequential TEM images using a CCD camera with a rate of 2 frames/s.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06078. In situ TEM videos showing the structural evolution at the metal−graphene interfaces (Videos S1−S5) (MOV) (MOV) (MOV) (MOV) (MOV) Details of experimental procedures; additional data for metal−graphene interactions (Figures S1−S7) (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected], [email protected]. jp. ORCID

Ming-Sheng Wang: 0000-0003-3754-2850 Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acsnano.7b06078 ACS Nano 2017, 11, 10575−10582