Article pubs.acs.org/cm
Microscopic View on a Chemical Vapor Deposition Route to BoronDoped Graphene Nanostructures Mattia Cattelan,† Stefano Agnoli,*,† Marco Favaro,† Denis Garoli,‡ Filippo Romanato,‡ Moreno Meneghetti,† Alexei Barinov,§ Pavel Dudin,§ and Gaetano Granozzi† †
Chemical Sciences Department, University of Padua, via Marzolo 1, Padua 35131, Italy LaNN Laboratory for Nanofabrication of Nanodevices, Corso Stati Uniti 4, Padua 35131, Italy § Sincrotrone Trieste S.C.p.A., Area Science Park, Trieste 34012, Italy ‡
S Supporting Information *
ABSTRACT: Single layer boron-doped graphene layers have been grown on polycrystalline copper foils by chemical vapor deposition using methane and diborane as carbon and boron sources, respectively. Any attempt to deposit doped layers in one-step has been fruitless, the reason being the formation of very reactive boron species as a consequence of diborane decomposition on the Cu surface, which leads to disordered nonstoichiometric carbides. However, a two-step procedure has been optimized: as a first step, the surface is seeded with pure graphene islands, while the boron source is activated only in a second stage. In this case, the nonstochiometric boron carbides formed on the bare copper areas between preseeded graphene patches can be exploited to easily release boron, which diffuses from the peripheral areas inward of graphene islands. The effective substitutional doping (of the order of about 1%) has been demonstrated by Raman and photoemission experiments. The electronic properties of doped layers have been characterized by spatially resolved photoemission band mapping carried out on single domain graphene flakes using a photon beam with a spot size of 1 μm. The whole set of experiments allow us to clarify that boron is effective at promoting the anchoring carbon species on the surface. Taking the cue from this basic understanding, it is possible to envisage new strategies for the design of complex 2D graphene nanostructures with a spatially modulated doping. KEYWORDS: chemical vapor deposition, graphene, electronic structure, photoelectron spectroscopy
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INTRODUCTION Recently, the forefront of the research on graphene has moved from the study of the synthesis and properties of pure films, to the investigation of chemically modified systems (i.e., doped graphene).1−7 In fact, the introduction of selected dopants into the honeycomb carbon sp2 lattice allows tuning the physical− chemical properties of the prepared materials. A very complex scenario, where chemistry and physics are closely intertwined, is facing scientists and technologists; nevertheless, the new possibilities offered by the chemical modification of graphene are really huge. Properties like electron conduction (concentration and type of charge carriers), chemical reactivity (catalytic activity, gas sensing), and optical response can be easily modulated, so systems specifically designed for advanced tasks can be envisaged.3,6 More recently, graphene-based materials containing selected defects or heteroatoms, have found application in several catalytic reactions.8,9 Evidence of the high and transverse impact of the chemical functionalization of graphene is represented by nitrogen-doped graphene;10−12 N-doped graphene exhibits n-type doping with a tunable conductivity, it is also a quite efficient catalyst for oxygen reduction reaction13 and a highly specific biosensor.14 Moreover, it is an advanced support that can beneficially modify the © 2013 American Chemical Society
physical chemical properties of supported nanoparticles (NPs).15,16 In the present article, we will report the preparation of boron-doped graphene on Cu foils (hereafter B−G) via a chemical vapor deposition (CVD) route, and we will provide a detailed microscopic view of the growth mechanism. B−G has been intensively studied theoretically17−20 since it is expected to show p-type conductivity representing a crucial material for the development of graphene-based electronics,21 but it is also important for advanced applications as electrode in photovoltaics22 and chemical energy storage.23 Moreover, ab initio calculations indicate that boron doping can dramatically change the gas adsorption properties of graphene, therefore improving hydrogen storage capabilities.24 So far, B−G films have been prepared by wet chemistry methods21,25 or by gas−solid solid− solid reaction,26 starting from graphite or graphene oxide.27,28 However, in these cases, the control of the morphology and purity (especially oxygen contamination) is not optimal: multilayer films with heterogeneous properties and characReceived: September 2, 2012 Revised: January 24, 2013 Published: March 14, 2013 1490
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terized by a scarce control on the level and type of doping are reported. Very recently,29,30 the CVD synthesis of B−G films has been demonstrated to be a practical synthesis route, which is expected to give a great impulse to the development of new nanostructured materials. Nevertheless, there is still a general gap of knowledge about the growth mechanism and there are quite remarkable differences in the prepared materials even if very similar growth procedures are used (see, for example, the different photoemission spectra reported in refs 28 and 29). After some years of intense work, a detailed microscopic view of graphene growth on Cu has been eventually obtained.31 Nevertheless, on more complex systems, like chemically modified graphene, an interpretative framework is still missing. Our work is intended to start filling this gap by describing the main phenomena taking place during the synthesis of B−G, with particular attention for the effects of different growth parameters on the chemical nature of the material. B−G is already showing great promises as a catalyst for oxygen reduction reaction26,32,33 and as an efficient electrode for solar cells or batteries,34 but the full exploitation of its potential requires the ability to tailor the surface morphology and the atomic structure of heteroatom derived defects with great precision.32
Figure 1. SEM micrographs of the CVD growth of B-doped graphene layers on copper polycrystalline foils and schematic drawings of the growth: (a) first step of the synthesis showing the formation of large graphene domains almost covering about 80% of the copper substrates; (b) second step with the further growth of graphene islands and the nucleation of some 3D clusters of nonstoichiometric boron carbide on bare copper areas (scale bar 3 μm).
Raman spectroscopy (see Figure S1 in Supporting Information) confirms SEM results, i.e., showing the absence of the D band and a very high intensity of the 2D band. After the introduction of B2H6 (10 sccm 1% B2H6 in He) for 30 s, the surface undergoes relevant changes: the graphene patches slightly widen, and on the previously uncovered regions of the Cu substrate, BxCy 3D NPs (with average dimension of 220 ± 15 nm, from the reported SEM micrograph) are formed (light gray areas). The reaction time has been precisely tuned considering that our cold-wall CVD reactor can efficiently decrease the temperature from 1000 to 200 °C in less than 30 s, therefore avoiding the nucleation of further 3D BxCy NPs on top of the B−G film. A typical Raman spectrum of the final material is reported in Figure 2 together with the analogous spectrum of pure
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RESULTS AND DISCUSSION The deposition of B−G layers has been carried out using methane as carbon source and diborane (1% B2H6 in He) as dopant gas. In order to obtain the best morphology and a fully covering layer of graphene, a growth temperature of 1000 °C was selected. The choice of Cu derives from the possibility to grow single layer films, which are the most interesting systems both for basic research and technological applications. Moreover, B−Cu (330 kJ mol−1) and B−B (297 kJ mol−1) bonds are by far weaker than C−C (472 kJ mol−1) and C−B (448 kJ mol−1)35 ones, and boron has a tendency to segregate at high temperature on the Cu surface.36 Therefore, just on the basis of such thermodynamic considerations, it should be possible to use this substrate without possible interferences connected to the competitive growth of metal borides. In order to grow B−G films, a two-step route has been carefully optimized: at first, the Cu surface has been exposed to methane and hydrogen, according to a well-established procedure for nucleating graphene patches on the surface,30 then diborane was introduced providing boron for doping. Despite the great excess of methane with respect to diborane (CH4/B2H6 ≈ 250), under the experimental conditions, nonstoichiometric boron carbide (BxCy)37,38 NPs are formed competitively on the bare Cu areas between graphene patches. These BxCy NPs are not detrimental for the growth of B−G; on the contrary, it can be postulated that they are very effective as a source of boron, which can migrate by diffusion from the peripheral areas into the graphene islands, while we expect that the direct decomposition of B2H6 on the graphene surface would give a minor contribution. SEM micrographs (Figure 1) allow a direct visualization of the actual steps occurring during our preparative procedure: after CH4 dosing for 60 s (25 sccm), wide graphene patches are formed (dark regions), generally covering 85% of the surface and leaving exposed just small areas of bare Cu (light contrast). The graphene patches are a single layer thick, made up by few micrometer wide domains forming a percolation network. A rippled texture of the graphene flakes can be easily observed.
Figure 2. Raman spectra of pure and B-doped single layer graphene supported on copper polycrystalline foils.
graphene obtained at 1000 °C with 25 sccm for 10 min (i.e., a fully covering single layer graphene film). It must be noted that the Raman measurements were performed on aged (i.e., few days after deposition) G films. The reason for that is that, when supported on Cu, the Raman spectra of G undergo changes with time because of the progressive electronic decoupling of the metal induced by the formation of an intercalating cuprous oxide or water layer at the interface as a 1491
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actually associated to 3D NPs. Finally, at BE higher than 190 eV, a huge tail comprising different peaks can be observed. This feature is due to different boron−oxygen-carbon compounds (C2−BO at 190.6 eV and C-BO2 at 191.6 eV),27,44,46 which are easily formed when B adopts a low coordinated substitutional site along graphene edges and, more limitedly, to boron oxide (B2O3 at 192.8 eV). During the optimization of the synthesis, it has been observed that oxygen partial pressure has to be kept at the minimum level (better than 5 × 10−2 mbar), otherwise the competitive growth of B2O3 prevents the introduction of substitutional boron into graphene. As a matter of fact, if we compare the photoemission signal of our B−G with similar materials obtained starting from graphene oxide or employing oxygenated precursors like B2O3, we can see that in the latter, the centroid of the B 1s photoemission line is clearly shifted toward higher BEs, indicating the formation of a structurally complex material with oxy-boron-carbide species. These species, however, can be of interest when materials tailored toward more specific chemical applications are targeted. As an example, we mention the enhanced electrochemical activity of B−G in the oxygen reduction reaction;26 however, the most effective functional groups for this reaction have not been identified yet. Therefore, our preparation procedure, if performed in the presence of oxygen, could be a very versatile tool for tuning the surface stoichiometry and functionality of boron graphene composites, providing model systems for a precise investigation of structure/reactivity links. The C 1s photoemission data, reported in Figure 3b, corroborate the previous results. In this case, Doniach−Sunjic functions were used, whose parameters have been optimized on the C1s photoemission spectrum of high quality pure graphene grown on Cu (see Supporting Information Figure S2). The main peak at 284.8 eV is related to the C sp2 component of the honeycomb lattice,47 while the two distinct features at lower BE (283.7 and 282.7 eV) can be ascribed mainly to substitutional B into the honeycomb lattice48 and to atoms associated to BxCy,49 respectively. At higher BE, three further components can be identified: a peak related to sp3 carbon at 285.7 eV and two lower intensity components at 286.7 and 287.3 eV associated to oxidized C species.50,51 The latter species are normally observed also in the case of pure graphene layers grown on Cu and are likely connected to the oxidation of graphene edges and defects. Photoemission allows also an estimate of the level of boron doping (i.e., present as a substitutional defect into graphene), either using the different chemically shifted C 1s components (ratio between C−B and sp2 C 1s) or using the ratio of the B 1s (B−C component) over C 1s (sp2 component) normalized by the corresponding sensitivity factors. The two different evaluations give a doping level of 1.5 at %; however, when also nonsubstitutional boron defects (i.e., C2BO and CBO2) are taken into account, a total of 2.5 at % of boron can be estimated. More advanced photoemission experiments have been performed at the Spectromicroscopy beamline at Elettra synchrotron:52 by measuring angle resolved (AR) valence band (VB) spectra, it is possible to clarify the effect of boron insertion into the electronic structure of graphene. The microscopic photon beam (74 eV with a spot size of 1 μm) allowed measurements on single graphene domains, while the combined degrees of freedom of sample and analyzer made possible to measure selectively the band dispersion along the
consequence of the exposure to ambient oxygen and moisture.39 After a few days, the decoupling process is complete, and stable and reliable spectra analogous to the ones obtained on SiO2 supports can be measured. Several features are immediately visible in the spectra: the decrease of the intensity of the 2D band and, correspondingly, the formation of the D band at 1340 cm−1 in the case of B−G and a significant shift in the positions of the 2D band. More precisely, with respect to pure graphene, in B−G the G band moves from 1583 to 1594 cm−1 and the 2D band from 2656 to 2668 cm−1. These shifts are reproducible and constant on different preparations. Moreover, the fwhm of the G and 2D band passes from 17 and 24 cm−1 in pure graphene to 28 and 51 cm−1, for the B-doped material, respectively. All these experimental findings clearly point to the introduction of new defects into the carbon lattice of graphene (formation of the D band) and to a consequent modification of the electronic structure of the layer.40 In fact, the introduction of B in replacement of C, which has one valence electron more, determines a p-doping of the graphene layers and, consequently, a shift of the Raman bands, as already demonstrated theoretically18 and experimentally.41,42 Quite often the shift of the G band is used as a phenomenological indicator of the number of charge carriers in graphene.43 However, the use of this method to provide an estimation of the amount of boron present into the films would be misleading in the present case. In fact, as discussed in detail in ref 41, the presence of B atoms into the honeycomb lattice of graphene determines a relevant strain that counteracts the shift induced by the modification in the Fermi level position. To get a clearer view about the chemical and electronic structure of the films, a detailed photoemission investigation has been undertaken. Figure 3a reports the B 1s photoemission
Figure 3. B 1s (a) and C 1s (b) photoemission spectra of B−G, together with the multipeak analysis of the single chemically shifted components.
line and its deconvolution into single peak components: six different peaks can be clearly identified. For the analysis, we used Voigt functions with a fwhm between 1.5 and 1.6 eV, as determined by suitable standards (B2O3 and boron carbides) and in agreement with other previous works.44 The peak having a binding energy (BE) of 189.6 eV can be associated to sp2 C−B bonds present in graphene and, limitedly, into the CxBy NPs,45 and it is indicative of the formation of substitutional bonds. The components centered at 188.0 eV and 188.8 can relate to the nonstoichiometric BxCy, and in particular to B4C and boron doped carbon NPs, respectively, as observed in the SEM micrographs (Figure 1b) in between graphene islands. XPS measurements taken at different takeoff angles indicate that these components are 1492
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Γ−K direction of the first Brillouin zone (Figure 4). For these experiments, the samples were introduced into the ultra high
relatively high level of doping, the band structure of B−G is essentially similar to pure graphene with a linear dispersion of the π-band close to the Dirac point, which is the typical fingerprint of massless fermions. This is the experimental confirmation of theoretical calculations predicting that the intrinsic superb conduction properties of graphene are maintained even if a heteroatom is inserted into the carbon lattice.18 Moreover, the fact that the π-band is not split into two sub-bands indicates the formation of single layers,49 corroborating SEM and Raman spectroscopy (very intense 2D band) data and well in agreement with the typical self-limiting growth of graphene on Cu.31 On the other hand, one notable difference with respect to pure graphene is the altered position of the Dirac point, which for B-doped layer coincides with the Fermi level, within the experimental error, (less than 50 meV). Theoretically, B−G should be a p-type semiconductor with the Fermi level cutting the Dirac cones below their intersection. The reason why this phenomenon is not observed experimentally can be attributed to the interaction with the metal substrate that, most probably, determines a tunnelling of the electrons from Cu into the empty states introduced by the presence of B. Therefore, the doping operated by the metal contact, which determines ndoping on pure graphene, counterbalances the intrinsic pdoping induced by the presence of boron in the graphene lattice, leading to the formation of an almost perfect semimetal. This is a quite interesting result showing how the intrinsic properties of the material (i.e., p-doping) can be modified by the interaction with the substrate. Further investigations are in progress to understand how strong the Fermi level pinning can be on graphene as a function of B concentration. However, the present outcome is of paramount importance because it suggests that it would be possible to use this same phenomenon of electronic exchange between metals and doped graphene in order to modulate the electronic structure of supported metal NPs. As a matter of fact, the electron density/charge of metal NPs could be tailored by the amount of dopant in the support,56 therefore allowing the design of new systems with easily tunable reactivity that could be exploited in catalysis and sensing applications.5 This can be the interpretative key for explaining the high chemical activity shown by some metal NPs when supported on doped graphene.14 Theoretical investigations of the interaction of Pd nanoparticles with B−G indicate that the interaction with boron substitutional edge defects can alter the position of the 4d band centroid enhancing the chemical activity in the oxygen reduction reaction.57 In the present case, we can speculate that metal copper NPs supported on B−G should experience a relevant charge withdrawal effect and therefore exhibit peculiar catalytic properties. It is worth to mention that any attempt to deposit B−G layers in one step has been unsuccessful. The reason for that can be traced back to the formation of very reactive boron species as a consequence of diborane decomposition on the Cu surface. In fact, it has been observed that the amount of the total carbon fixed on the surface (either as graphene or CxBy) is at least 5 times higher (as determined by photoemission experiments) in the presence of the boron precursors and that the growth of graphene itself is much faster. However, in these experimental conditions, graphene is not the most prevalent structure: nonstoichiometric carbide, B4C, or B-doped carbon can grow quite fast (see Supporting Information Figure S3
Figure 4. Dispersion of the valence band spectra of pure and B-doped graphene layers on polycrystalline Cu: (a) dispersion along the Γ−K direction of the first Brillouin zone of B-doped graphene; (b,c) dispersion around the K point, obtained as a snapshot of the analyzer 2D detector, for pure and B-doped graphene, respectively.
vacuum system (∼10−10 mbar) and mildly annealed at 673 K for a few hours. This procedure is necessary to remove adsorbed gas, which could introduce spurious peaks in the spectra, and to restore an intimate interface between graphene layers and the Cu metal, (differently from Raman measurements where the graphene layers were decoupled from the substrate). The VB spectra were uniform and showed the same band structure maps [E(k)] on different regions of the B−G sample. Figure 4a shows the evolution of VB spectra of a B−G single domain along the Γ−K direction. It is possible to identify clearly a band connected to graphene π states (red dashed line) starting from a minimum in Γ at 8 eV, and cutting K in correspondence of the Fermi level. In Figure 4c, we report a better-resolved image of the π-band near the K-point with the dispersion direction perpendicular to the Γ−K direction, showing a clear linear dispersion as expected for massless Dirac fermions characteristic of monolayer graphene.53 As a comparison, analogous measurements carried out on pure graphene give results similar to what was recently reported in literature,54 i.e., that graphene layers supported on Cu foils show n-doping because of the interaction with the metal substrate55 (the so-called metal contact doping). In fact, for pure graphene on copper, we observed that the π-band intersects the K point at 0.3 eV below the Fermi level (Figure 4b). Therefore, our experimental data indicate that, despite the 1493
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confirming some recent theoretical speculations,16−18 but also providing important information about the coupling between chemically doped graphene and metallic systems.
reporting Raman and photoemission data of the single step preparation). Another important point to outline is that the incorporation rate of the dopant (defined as the ratio between the overall film stoichiometry and the gas phase concentration relative to carbon and dopant) is in our case rather different from the literature data on N-doped graphene. Actually, when comparing such rate for NH3/CH412 and B2H6/CH4 CVD growths, we have ca. 0.2 and 40, respectively. In other cases, such as the growth using precursors with a relatively high content of heteroatom (e.g., s-triazine11,58), the incorporation rate of N is even lower. The data reported above could be explained by assuming the formation of stable and very mobile CBx clusters that are very effective to increase the uptake of C from the gas phase. As a final point, we want to discuss the possibility to have grown 2D core−shell graphene nanostructures by using the reported two-step CVD procedure. A careful scrutiny of the Raman G band of the B−G sample (see Figure 2) seems to indicate the presence of two different components: one characterized by a large fwhm (connected to a highly doped material) and a sharper peak (connected to almost pure graphene). Similarly, the relatively high intensity of the 2D band can be ascribed to the presence of a region with a doping lower than an average homogeneous doping of 2.5 at %.41 As a matter of fact, the formation of spatially heterogeneous structures, using a sequential dosing of graphene precursors, has been widely demonstrated by Ruoff’s group:59,60 in this case, very sharp alternating domains of 13C and 12C graphene, with micrometric dimension, were produced. The preparation of core−shell graphene based nanostructures is still an emerging field, but with great potential since it would allow a fine-tuning of the functional properties.61 For example, it has been demonstrated that graphene@h-BN nanostructures have very peculiar electronic properties62 that can be exploited for a wide gamut of applications.63
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
Electropolishing of Cu Foils. Cu foils (Goodfellow, 99.9%, 0.125 mm thickness) were cleaned in an ultrasonic bath of isopropyl alcohol for 10 min then dipped in the electropolishing solution, composed of distilled water and orthophosphoric acid 85% (1:4 volume ratio). The counter electrode was a stainless steel cylinder surrounding the sample; the contacts were ensured by stainless steel tweezers. The voltage was set to the plateau after rapidly checking the current trend as a function of the voltage (usually set to 2.4 V). The electropolishing was 5 min long. After electropolishing, the foils were rinsed in distilled water and sonicated in isopropyl alcohol for 10 min. Low Pressure Chemical Vapor Deposition. Pure and B−G films were deposited on electropolished Cu foils in a homemade lowpressure cold-wall CVD system. The growth temperature was reached using a resistive heating system directly in contact with the Cu foils. The Cu foil surface was activated according to the following procedure: the system was evacuated by a scroll pump and a liquid nitrogen cold trap obtaining a base pressure of 5× 10−2 mbar, and 200 sccm of Ar were introduced into the chamber for 20 min to replace air. The argon flux was then replaced by a 25 sccm flux of hydrogen during the annealing stage (950 °C) for 25 min. During graphene growth, the hydrogen flux was substituted by 25 sccm of methane. For pure graphene, the growth time was 10 min at 1000 °C; for B−G, the growth was divided in two steps: 1 min of 25 sccm of methane dosing and 30 s of codosing of 25 sccm of methane and 10 sccm of diborane (1% in helium). Immediately after the growth, the resistive heating was shut down to rapidly decrease the substrate temperature (from 1000 to 200 °C in less than 30 s), and all the fluxes were stopped. Raman Measurements. A Renishaw inVia Raman microscope was used for the Raman measurements. Spectra were recorded using the He−Ne laser line at 632.8 nm (8.5 mW), which was focused on the sample with a 50× objective (Olympus). Photoemission Measurements. Core level photoemission spectra were taken on a VG ESCALAB MKII spectrometer using the Mg anode of a conventional unmonochromatized X-ray source (1253.6 eV) and with the analyzer pass energy set to 20 eV. All measurements were taken at room temperature with an emission angle of 10° off with respect to the surface normal. For AR-VB measurement at Spectromicroscopy beamline,51 we used 74 eV photons focused to less than 1 μm by Schwarzschild objective. A total energy resolution of 110 meV was used in the experiment. Samples were at room temperature. The Dirac cone spectra were obtained from snapshots of a 2D detector. SEM Measurements. The SEM instrument used in this work is a FEI Nova 600 NanoLab with a maximal resolution of 1.1 nm at 15 kV. Micrographs have been taken by using an acceleration voltage of 5 kV.
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CONCLUSIONS In conclusion, we reported a detailed description of the CVD growth of B-doped graphene on Cu polycrystalline foils that can open the way to the development of effective strategies for the synthesis of single layer B−G samples. Chemical routes using GO as a starting materials suffer of modest quality of the layers (very intense D band and conversely quite low 2D band of Raman spectra)26 and of a poor control of the chemical states of B defects21,28 (many different oxidized species), while routes using B-doped graphite are limited by the complexity required for obtaining single layer films.41 With our work, we demonstrated that, if performed in controlled conditions, a CVD approach using Cu as a catalytic substrate can lead to the formation of single layer graphene films with an excellent control of the dopant chemical nature and therefore of the electronic properties. In particular, we optimized a two-step procedure based on pure graphene preseeding and B diffusion from nonstochiometric boron carbides. The reported experimental data allowed us to unravel the role played by boron in offering a good anchoring for carbon species due to its strong affinity with carbon. Taking the cue from this basic understanding, it is possible to envisage new strategies to design more complex 2D graphene architectures.3 We also report for the first time spatially localized (∼μm) angle resolved valence band measurements that shed light on the electronic structure of boron-doped single layer graphene,
S Supporting Information *
XPS and Raman spectra of pure graphene and B-doped carbon \B4C. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(S.A.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.F. acknowledges Fondazione Cariparo for a Ph.D. grant. 1494
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