Nanowire-Mesh-Templated Growth of Out-of-Plane ... - ACS Publications

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Nanowire-Mesh-Templated Growth of Out-ofPlane Three-Dimensional Fuzzy Graphene Raghav Garg,‡,⊥ Sahil K. Rastogi,‡,⊥ Michael Lamparski,∥ Sergio C. de la Barrera,§ Gordon T. Pace,‡ Noel T. Nuhfer,† Benjamin M. Hunt,§ Vincent Meunier,∥ and Tzahi Cohen-Karni*,‡,† †

Department of Materials Science and Engineering, ‡Department of Biomedical Engineering, and §Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ∥ Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, United States S Supporting Information *

ABSTRACT: Graphene, a honeycomb sp2 hybridized carbon lattice, is a promising building block for hybrid-nanomaterials due to its electrical, mechanical, and optical properties. Graphene can be readily obtained through mechanical exfoliation, solution-based deposition of reduced graphene oxide (rGO), and chemical vapor deposition (CVD). The resulting graphene films’ topology is two-dimensional (2D) surface. Recently, synthesis of three-dimensional (3D) graphitic networks supported or templated by nanoparticles, foams, and hydrogels was reported. However, the resulting graphene films lay flat on the surface, exposing 2D surface topology. Out-of-plane grown carbon nanostructures, such as vertically aligned graphene sheets (VAGS) and vertical carbon nanowalls (CNWs), are still tethered to 2D surface. 3D morphology of out-of-plane growth of graphene hybrid-nanomaterials which leverages graphene's outstanding surfaceto-volume ratio has not been achieved to date. Here we demonstrate highly controlled synthesis of 3D out-of-plane singleto few-layer fuzzy graphene (3DFG) on a Si nanowire (SiNW) mesh template. By varying graphene growth conditions (CH4 partial pressure and process time), we control the size, density, and electrical properties of the NW templated 3DFG (NT-3DFG). 3DFG growth can be described by a diffusion-limited-aggregation (DLA) model. The porous NT-3DFG meshes exhibited high electrical conductivity of ca. 2350 S m−1. NT-3DFG demonstrated exceptional electrochemical functionality, with calculated specific electrochemical surface area as high as ca. 1017 m2 g−1 for a ca. 7 μm thick mesh. This flexible synthesis will inspire formation of complex hybrid-nanomaterials with tailored optical and electrical properties to be used in future applications such as sensing, and energy conversion and storage. KEYWORDS: 3D graphene, nanowire, hybrid nanomaterials, nanomaterials synthesis, electrical properties

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raphene, a honeycomb sp2-hybridized two-dimensional (2D) carbon lattice, is a promising building block for hybrid nanomaterials due to its chemical stability,1 electrical conductivity (charge carrier mobility up-to 200000 cm2 V−1 s−1),2 mechanical robustness (Young’s modulus of ∼1 TPa),3 high surface-to-volume ratio (theoretical value of ∼2630 m2 g−1),4 and optical transparency (optical transmittance of ∼97.7%).5,6 Graphene can be readily obtained through mechanical exfoliation of highly ordered pyrolithic graphite (HOPG),7 solution-based deposition of reduced graphene oxide (rGO),8 high-temperature epitaxial growth on SiC,9 and chemical vapor deposition (CVD) on transitionmetal catalysts.10 The topology of the resulting graphene films (or flakes) obtained using these techniques is a 2D surface. Recently a three-dimensional (3D) topology of graphene (or rGO) has been demonstrated by various approaches, including, © 2017 American Chemical Society

synthesis of graphene (or assembly of rGO) on nanoparticles followed by their organization in 3D;11−14 synthesis of graphene on Ge nanowires (NWs);15,16 synthesis of graphene on transition-metal foams;17 and synthesis of 3D graphene hydrogels.18 In all of these cases, the graphene (or rGO) flakes or films are lying flat, hence exposing a 2D surface topology. An alternative approach to achieving 3D surface topology is to grow graphene flakes out-of-plane, i.e., vertical growth of graphene. This way, the graphene flakes are exposed and are not completely tethered to the underlying surface. In recent years, growth of out-of-plane carbon nanostructures appeared in numerous reports. 19−23 Large-area vertically aligned Received: April 14, 2017 Accepted: May 26, 2017 Published: May 26, 2017 6301

DOI: 10.1021/acsnano.7b02612 ACS Nano 2017, 11, 6301−6311

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Figure 1. Synthesis process schematics of nanowire templated growth of out-of-plane three-dimensional (3D) fuzzy graphene (NT-3DFG). (a) NWs are synthesized by a AuNP-catalyzed vapor−liquid−solid (VLS) process.(b) NWs are collapsed using liquid N2 and annealed with H2. (c) Out-of-plane 3DFG is synthesized on the NW mesh using a plasma-enhanced chemical vapor deposition (PECVD) process. Inset represents the growth of 3DFG on a collapsed NW.

decrease in the ratio of H/C radical density in the PECVD gas feed.22 An increase in the PECVD process time (under 25.0 mTorr CH4 partial pressure) also leads to an increase in the size of the flakes (79 ± 9, 163 ± 22, 464 ± 25, and 1549 ± 184 nm for 5, 10, 30, and 90 min, respectively) (Figure 2c−f). 3DFG flakes are oriented out of the SiNW mesh template surface and consistent throughout NT-3DFG as observed in Figure 2b−e and Figure S1a.II. The NT-3DFG mesh thickness is 7.2 ± 1.9 μm (Figure S1c). Energy-dispersive spectroscopy (EDS) confirms the elemental composition of the synthesized material as a Si core with a conformal coating of carbon flakes (Figure S2). Details regarding the nature of the carbon flakes were gleaned from Raman spectroscopy (Figure 3). The characteristic peaks in the Raman spectra, i.e., D, G, and 2D peaks, were analyzed to corroborate the presence of graphene (Figure 3a).30 The G peak shows a red-shift with increasing CH4 partial pressure, implying progression of nanocrystalline graphene (Table S2).21 The D and D′ peaks are produced due to a onephonon defect-assisted process, and the D + D′ peak is produced due to a two-phonon defect-assisted process.30 In the case of 3DFG, the emergence of the D peak, at ca. 1335 cm−1, and the D′ peak, as a shoulder to the G peak, is caused by breaks in translational symmetry due to the presence of 3DFG edges, as evident in the SEM images (Figure 2 and Figure S1).22,30 Emergence of such edge defects leads to broader peaks relative to defect free single-layer graphene.30 The observed broad 2D peak can be fitted with a single Lorentzian (Figure 3a and Table S2) and explained by the presence of juxtaposed single- to few-layer graphene flakes in the form of high-density 3DFG.31,32 In the case of NT-3DFG synthesized under 20.0 mTorr CH4 partial pressure for 10 min and 25.0 mTorr CH4 partial pressure for 5 min, the blue shift of ca. 20 cm−1 in the position of the 2D peak and further broadening of the 2D peak, as compared to other PECVD conditions, indicate the presence of folded, misoriented, and turbostratic graphene (Figure 3a and Table S2).30,33 The increase in ID/IG and I2D/IG with increasing CH4 partial pressure (Figure 3b and Table S2) can be attributed to the increase in edge density.30,34 However, NT3DFG synthesized under 25.0 mTorr CH4 partial pressure with increasing PECVD process times (10, 30 and 90 min) do not show change in the position of the G and 2D peaks, ID/IG, I2D/ IG, and 2D peak full width at half-maximum (fwhm(2D)). This can be attributed to the high density of 3DFG flakes when compared to other synthesis conditions. An increase in the density of 3DFG reduces the average distance covered by an electron−hole pair before scattering, which is evident through

graphene sheets (VAGS) have been synthesized by thermal decomposition of SiC.19,20 In addition, by using a plasmaenhanced CVD (PECVD) process, catalyst-free vertical growth of carbon nanowalls (CNWs) was achieved.21−23 The obtained VAGS and CNWs are composed of few to dozens of graphene layers and, therefore, are more similar to graphite than to single- or few-layer graphene nanostructures. Moreover, these VAGS and CNWs are still tethered to a 2D surface.19−23 Outof-plane growth of graphene hybrid nanomaterials that results in 3D morphology that leverages graphene’s outstanding surface-to-volume ratio has not been achieved to date. Here, we demonstrate highly controlled out-of-plane synthesis of single- to few-layer 3D fuzzy graphene (3DFG) on a 3D Si nanowire (SiNW) mesh template. By varying graphene growth conditions (CH4 partial pressure and process time), we can control the size, density, and electrical and electrochemical properties of the NW-templated 3DFG (NT-3DFG). This flexible synthesis will inspire formation of complex hybrid nanomaterials with tailored optical and electrical properties to be used in future applications such as sensing and energy conversion and storage.

RESULTS AND DISCUSSION NT-3DFG hybrid nanomaterial was synthesized using a threestep process, as presented in Figure 1. SiNWs were synthesized by a Au nanoparticle (AuNP) catalyzed vapor−liquid−solid (VLS) process.24 Following this step, SiNWs were collapsed using capillary forces25 by flowing liquid N2 and annealed in H2 to form an interconnected mesh.26 Finally, 3DFG was grown on the 3D SiNWs mesh through inductively coupled PECVD process.27−29 The effect of varying PECVD conditions, i.e., CH4 partial pressure and PECVD process time, on the growth of NT3DFG was explored (Table S1). Scanning electron microscope (SEM) images reveal that varying the CH4 partial pressure affects both the density and size of the 3DFG grown on the SiNWs. At CH4 partial pressure of 20.0 mTorr (Figure 2a) by SEM imaging there are no noticeable 3DFG flakes on individual NWs as compared to pristine SiNW mesh (Figure S1a.I). As the CH4 partial pressure increases (Figure 2b,c), the density of 3DFG flakes on individual NWs increases along with the size of the flakes, as indicated by the increasing average NW diameter (37 ± 6, 38 ± 4, 67 ± 6, and 163 ± 22 nm at 8.3, 20.0, 22.7, and 25.0 mTorr CH4 partial pressure, respectively) (Figure 2f). The notable increase in 3DFG density on the NWs can be attributed to the increase in CH4 partial pressure and 6302


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Figure 2. Effect of varying PECVD conditions on NT-3DFG growth. SEM image of NT-3DFG synthesized under (a) 20.0 mTorr CH4 partial pressure for 10 min (scale bar is 1 μm; inset is the expanded view of the marked red dashed box with a scale bar of 250 nm); (b) 22.7 mTorr CH4 partial pressure for 10 min (scale bar is 1 μm; inset is the expanded view of the marked red dashed box with scale bar of 250 nm; (c) 25.0 mTorr CH4 partial pressure for 10 min (scale bar is 2 μm; inset is the expanded view of the marked red dashed box with scale bar of 250 nm); (d) 25.0 mTorr CH4 partial pressure for 30 min (scale bar is 2 μm; inset is the expanded view of the marked red dashed areas with scale bar of 250 nm); (e) 25.0 mTorr CH4 partial pressure for 90 min (scale bar is 2.5 μm; inset is the expanded view of the marked red dashed area with scale bar of 500 nm); (f) NT-3DFG diameter as a function of CH4 partial pressure with 10 min PECVD process time (represented by open black circles) and PECVD process time under 25.0 mTorr CH4 partial pressure (represented by open red squares). Results are presented as mean ± SD (n = 3).

pressure), larger single- to few-layer 3DFG flakes are observed (Figure 4b and c.I). The flakes extend out of the SiNW surface as seen in Figure 4c.II, and a distinguishable border between the Si lattice and graphene flakes is observed. Extension of the process time, under 25.0 mTorr CH4 partial pressure, from 10 to 30 min results in an increase in both graphene edge density and size (Figure 4c.I and d). Selected area electron diffraction (SAED) data indicate that 3DFG is polycrystalline in nature (Figure 4.d (inset)).37 The interplanar distances for the first and second nearest C−C neighbors were experimentally derived to be 0.119 and 0.205 nm (Table S7). These values agree with the expected interplanar spacing for the (112̅0) plane (d112̅0 = 0.123 nm) and the (101̅0) plane (d101̅0 = 0.213 nm). The distance between individual graphene layers (d0002 = 0.350 nm) concurs with the expected value of 0.344 nm (Figure 4b.II,c.II marked blue lines, Figure S3a,b and Table S7) indicating the presence of turbostratic graphene.38−40 Electron energy loss spectroscopy (EELS) C K(1s) analysis yields a sharp peak at 285.5 eV due to 1s to π* transition and a broader peak in the 290−310 eV region due to 1s to σ* transition.41 Extended fine structure analysis of EELS spectra acquired from a NT-3DFG (25.0 mTorr CH4 partial pressure for 30 min) shows the presence of graphite-like material near the center and isolated single-layer graphene near the edge (Figure 5).41 As can be seen in both scanning transmission electron microscope (STEM) and TEM images (Figure 4d and 5a), the center of the NT-3DFG is composed of dense 3DFG flakes compared to the edge, where the incident beam interacts with single-layer 3DFG. Such a closely packed arrangement at

the saturation of ID/IG with increasing 3DFG density as a result of increasing PECVD process time (Figure 3b and Table S2).23,30 The appearance of a strong D peak due to edge effects was further verified by dual-wavelength Raman spectroscopy. An increase in both the position of the G peak as a function of excitation wavelength (Disp(G)) and G peak full width at halfmaximum (fwhm(G)) is observed with an increase in the disorder in the carbon structure.35 Therefore, a higher ID/IG corresponds to higher Disp(G) and fwhm(G) in the case of bulk structural defects, thus facilitating the discrimination between disorder at the edges and in the bulk.31,32 The lack of clear correlation between ID/IG and fwhm(G) as well as ID/IG and Disp(G) (Figure 3c) further corroborates that the major contribution to the D peak is due to edge defects rather than bulk-structural defects.31,32,36 The saturation of Disp(G) at ca. 1600 cm−1 with the change in excitation wavelength, as presented in Tables S3−S5, is another indication of the presence of sp2 hybridization and lack of large structural defects.30,31,35 The structure and growth progression of NT-3DFG were further explored using an aberration-corrected transmission electron microscope (Cs-TEM) (Figure 4). At 20.0 mTorr CH4 partial pressure grown NT-3DFG, a distinct conformal coating of graphene sheath with folds is observed around the SiNW core (Figure 4a). This observation agrees with the obtained Raman spectroscopy data. It is also apparent that there are fewlayer graphene nanoflakes growing from the surface of the SiNW (Figure 4a, yellow arrows). As the carbon content in the PECVD process increases (through increase in CH4 partial 6303


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Figure 3. Raman spectroscopy of NT-3DFG. (a) Representative Raman spectra (n = 3) of NT-3DFG synthesized under (I) 20.0 mTorr (cyan), 22.7 mTorr (green), and 25.0 mTorr (purple) CH4 partial pressure for 10 min, (II) 25.0 mTorr CH4 partial pressure with varying PECVD process times of 5 min (blue), 30 min (yellow) and 90 min (red). (b) Peak intensity ratio of D peak to G peak (ID/IG, represented by open squares) and 2D peak to G peak (I2D/IG, represented by open circles) of NT-3DFG synthesized under (I) 20.0 mTorr (cyan), 22.7 mTorr (green), and 25.0 mTorr (purple) CH4 partial pressure for 10 min, (II) 25.0 mTorr CH4 partial pressure with varying PECVD process times of 5 min (blue), 10 min (purple), 30 min (yellow), and 90 min (red). Results are presented as mean ± SD (n = 3). (c) Peak intensity ratio of D peak to G peak (ID/IG) as a function of (I) fwhm of G peak and (II) dispersion of G peak for samples synthesized under 25.0 mTorr CH4 partial pressure for 10 min (purple), 30 min (yellow), and 90 min (red).

characterized by self-similarity and properties of a fractal nature.42 The computation performed here results primarily in neatly filled planes with few defects. It is notable that the simulation was not explicitly designed to promote the creation of defect-free layers, and in fact, a small but finite amount of (vacancy) defects can be observed during the growth process. A typical result of this simulation is depicted in Figure 6b. The algorithm is found to create a number of larger and smaller graphene flakes featuring a small amount of vacancy site defects (best illustrated in Figure 6c). The presence of large flakes promotes their own growth but stops the growth of smaller flakes, which are no longer accessible to the carbon feedstock. From the simulation and by comparison with the experiments, it seems that the high diffusion of carbon radicals and the propensity toward sp2 structures is captured well by a model based on diffusion limitation. Furthermore, the simulations model the out-of-plane synthesis of graphene in a 3D surface topology. We explored the use of our 3D graphene hybrid nanomaterial as an electrical and an electrochemical platform. We initially measured the electrical properties of the synthesized material by determining the sheet resistance of NT-3DFG through the van der Pauw method.44 The sheet resistance of NT-3DFG decreases with increasing CH4 partial pressure and PECVD process time (Figure 7). This change in the sheet resistance is attributed to the increasing density of single- to few-layer 3DFG flakes, which leads to the ability to sustain large current densities.22 The lowest sheet resistance value measured

the center of the NT-3DFG will generate EELS spectra resembling graphite-like material. The nucleation and growth processes of out-of-plane 3DFG on a SiNW can be rationalized by specific computer algorithms (Figure 6). The algorithm is based on the method of diffusionlimited aggregation (DLA) to simulate a process of structural growth as a series of single-particle nucleation events.42 The event begins with the introduction of a single particle at a random point A. Point A is chosen with uniform probability along a surface of radius Rintro, which exceeds the maximum currently occupied radius Roccu (Figure 6a). The particle performs random walk to simulate Brownian motion until it approaches the existing structure (or nucleation particle) close enough to adhere to the nucleus. Two examples are illustrated in Figure 6a. In an unsuccessful event, the particle (which started at A) strays so far from the nucleus that it has almost no chance of returning. When it hits the surface at Rkill (point B), the event is restarted by replacing the particle at a new location A′. The event starting at A′ is successful when the particle trajectory approaches close enough to meet a defined criterion for nucleation (point C) (detailed information regarding the nucleation conditions is available in the Methods), and the event is concluded by permanently fixing the position of the particle to become part of the nucleus for the next event. This method has been used in the modeling of surface roughness evolution and on-surface nanofilaments growth.43 One of its key features is that no explicit use of chemical information is made during bonding. Diffusion-limited aggregation is typically 6304


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Figure 4. HR-TEM of NT-3DFG synthesized with varying PECVD conditions. NT-3DFG synthesized under (a) 20.0 mTorr CH4 partial pressure (scale bar is 30 nm; yellow arrows denote few-layer graphene nanoflakes); (b) (I) 22.7 mTorr CH4 partial pressure (scale bar is 30 nm; yellow arrows denote few-layer graphene nanoflakes), (II) expanded view of the marked red dashed area (scale bar is 10 nm; black arrow denotes single-layer 3DFG flakes; marked blue line denotes fringe intensity cross-section profile) (see Supplementary Figure 3); (c) (I) 25.0 mTorr CH4 partial pressure (scale bar is 100 nm; black arrows denote single layer 3DFG flakes), (II) expanded view of the marked red dashed area (scale bar is 10 nm; dashed black line and marked blue line denote the boundary between SiNW lattice (left) and 3DFG flakes (right) and fringe intensity cross-section profile, respectively) (see Supplementary Figure 3); process time for a−c is 10 min; (d) 25.0 mTorr CH4 partial pressure for 30 min (scale bar is 200 nm; inset is a representative SAED pattern). Polycrystalline diffraction rings of 3DFG are numbered as 1 (d0002 ≈ 0.350 nm), 2 (d101̅0 ≈ 0.205 nm), and 3 (d112̅0 ≈ 0.119 nm). Scale bar is 5 nm−1.

correction will further reduce the observed sheet resistance values (and thus increase conductivity).52 NT-3DFG mesh was further used as an electrode in a threeelectrode electrochemical cell. Prior to these experiments, the surface wettability was evaluated by measuring the contact angle, θ, of different synthesized materials (Figure 8a). Compared to both low pressure CVD (LPCVD) synthesized single-layer graphene film transferred to Si/600 nm SiO2 (θ ≈ 90°) and pristine SiNW mesh (θ ≈ 0°, since the mesh absorbed the water droplet), NT-3DFG is a superhydrophobic material (θ ≈ 155°) (Figure 8a and Table S8). Although a single-layer graphene film does not exhibit superhydrophobicity,53 the combination of graphene and nanoscale edges makes the surface superhydrophobic.54 The superhydrophobicity of NT3DFG can be explained by the Cassie−Baxter model of porous surface wettability.55 Briefly, the presence of air pockets between the 3DFG flakes allows for the deionized water droplet to be suspended on 3DFG edges.56 The faradaic redox peak currents increase for NT-3DFG compared to planar Au working electrode (Figure 8b). This is attributed to the increase in the electrochemically active surface area due to the presence of 3DFG (Figure 2).57 Treating NT3DFG meshes with HNO3 further increases the peak currents due to change in the surface wettability from superhydrophobic to hydrophilic (Figure 8a.IV). SEM imaging and Raman spectroscopy analysis reveal that HNO3 treatment does not alter the physical characteristics of NT-3DFG (Figure S4). Both anodic and cathodic faradaic peak currents increase linearly with increasing square-root of scan rate and increasing

Figure 5. EELS spectroscopy of NT-3DFG. (a) STEM image of NT3DFG synthesized under 25.0 mTorr CH4 partial pressure for 30 min. Scale bar is 250 nm. (b) Corresponding EELS spectra acquired from the edge (red cross) and center (blue cross) of the NT-3DFG in panel a.

is for the 90 min PECVD process (84 ± 6 Ω □−1, conductivity of 1655 ± 450 S m−1). Interestingly, this value is much lower than the published sheet resistance of polycrystalline graphene films.45 Furthermore, HNO3 treatment, adapted from previous work,46,47 reduces the sheet resistance of NT-3DFG to 59 ± 12 Ω □−1 (conductivity of 2355 ± 785 S m−1) by increasing the carrier concentration.47 The determined electrical conductivity of NT-3DFG exceeds the literature-reported values for 3D graphene nanostructures and 3D graphene composites (Table 1).4,17,48−51 We note that in these measurements NT-3DFG is assumed to be a continuous surface without any pores. Porosity 6305


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Figure 6. DLA-based growth model simulation of NT-3DFG. (a) Illustration of a single nucleation event in a simulation of diffusion-limited aggregation. O, Rintro, and Roccu are the origin, surface radius of dimer introduction and maximal surface radius of currently occupied site, respectively. A and B (or Rkill), and A′ and C correspond to unsuccessful and successful nucleation events, respectively. (b) Perspective view of rendered model of a typical result from the DLA based growth model simulations after 290000 nucleation events. Scale bar is 20 nm. (c) Expanded view of the marked red dashed area in b. Scale bar is 5 nm. Yellow cylinder and cyan flakes denote SiNW and 3DFG flakes, respectively.

0.12 V) is smaller than that observed for 30 min NT-3DFG (ca. 0.30 V). This is attributed to faster electron transfer rates in 90 min NT-3DFG when compared to 30 min NT-3DFG.58 The double-layer capacitance of the working electrode was calculated as the change in current density with respect to the scan rate (Figure 8c).59 The double-layer capacitance of NT3DFG (0.56 ± 0.01 and 1.85 ± 0.02 mF cm−2 for 30 and 90 min NT-3DFG, respectively) is higher than that of the Au working electrode (0.009 ± 0.001 mF cm−2) due to the remarkably high surface area of NT-3DFG (calculated specific electrochemical surface area of 117 ± 13 and 340 ± 42 m2 g−1 for 30 and 90 min NT-3DFG, respectively). HNO3 treatment significantly increases the double-layer capacitance of NT3DFG (2.25 ± 0.07 and 6.50 ± 0.10 mF cm−2 for 30 and 90 min NT-3DFG, respectively; calculated specific electrochemical surface area of 472 ± 53 and 1017 ± 127 m2 g−1 for 30 and 90 min NT-3DFG, respectively). This is attributed to enhanced wettability and exceptional pseudocapacitance of 3DFG due to introduction of oxide-containing species through redox reactions.60 The electrochemical surface area for NT-3DFG electrodes was determined by computing the capacitance ratios of the electrodes with respect to the Au working electrode.59,61,62 We note that our calculated electrochemical surface area represents a lower value range compared to nitrogen adsorption experiments. Nonetheless, the determined electrochemical surface area values exceed the literature reported surface area values for 3D carbon based electrode materials such as graphene foam,17 3D macroporous chemically modified graphene (CMG) electrodes,48 graphene aerogel,49 and carbon nanotube (CNT) based platforms (such as composites, graphene-SWCNT gels, films and electrodes) (Table 1).51,63−65 NT-3DFG electrodes maintain their electrochemical performance for over 1 month (Figure 8d), implying stable electrochemical and corrosion-resistive properties of 3DFG.66

Figure 7. Electrical characterization of NT-3DFG. (a) Sheet resistance of NT-3DFG synthesized under varying CH4 partial pressure for 10 min. (b) Sheet resistance of NT-3DFG synthesized under 25.0 mTorr CH4 partial pressure with varying PECVD process times. Inset presents the use of NT-3DFG as a conductor to complete a simple LED circuit. Scale bar is 2 cm. Results are presented as mean ± SD (n = 3).

Table 1. Surface Area and Electrical Conductivity of Various Carbon-Based Materials material NT-3DFG graphene foams CMG agglomerates 3D macroporous CMG electrodes graphene aerogels 3D porous rGO films graphene−SWCNT cogels graphene-coated SWCNT gels CNT films and electrodes CNT−MnO2 composites

electrical conductivity (S m−1)

ref(s)

1017 850 705 194.2

2400 1000 200 1204

this work 17 4 48

584

100 1905 20

49 50 51

surface area (m2 g−1)

800 686 120−500 234

63 64, 65

CONCLUSION We demonstrate here the synthesis of hybrid-nanomaterial composed of out-of-plane single- to few-layer 3DFG on a SiNW mesh template. The density and size of out-of-plane graphene flakes is closely controlled by varying CH4 partial pressure and PECVD process time. Through Raman spectroscopy, electron microscopy (SEM and TEM), and EELS, the flakes were characterized and found to consist of single- to fewlayer graphene with a high density of exposed graphene edges. DLA-based growth models accurately simulate the growth of the 3DFG on a NW core. The out-of-plane structure of 3DFG

65

[Fe(CN)6]3− concentration (Figure 8b.II and Figure S5). These results are in good agreement with the Randles−Sevčik model and establish that diffusion is the sole means of mass transport for NT-3DFG electrodes.57 Increase in the slope of the peak current versus square root of scan rate curve (Au < NT-3DFG < HNO3 treated NT-3DFG) further supports the increase in electrochemically active surface area (Figure 8b.II).57 Faradaic peak separation for 90 min NT-3DFG (ca. 6306


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Figure 8. Electrochemical characterization of NT-3DFG. (a) Representative contact angle measurement images (n = 3) of (I) flat single-layer graphene on Si/600 nm SiO2, (II) pristine SiNW mesh on Si/600 nm SiO2, (III) NT-3DFG synthesized under 25.0 mTorr CH4 partial pressure for 30 min, and (IV) NT-3DFG synthesized under 25.0 mTorr CH4 partial pressure for 30 min after 2 h of HNO3 treatment (scale bars are 1 mm). (b) Cyclic voltammetry characterization: (I) representative cyclic voltammograms (n = 3) acquired with 5.00 mM [Fe(CN)6]3− in 1 M KCl at a scan rate of 50 mV s−1. Blue, yellow, and red colors denote Au working electrode, NT-3DFG synthesized under 25.0 mTorr CH4 partial pressure for 30 and 90 min, respectively. Dashed lines denote NT-3DFG after 2 h of HNO3 treatment. Inset presents the cyclic voltammetry setup with representative NT-3DFG working electrode (WE), Ag/AgCl reference electrode (RE) and Pt wire counter electrode (CE). (II) Peak current as a function of square root of scan rate (anodic and cathodic peak currents are denoted by circles and triangles, respectively). Closed and open circles and triangles denote NT-3DFG before and after 2 h of HNO3 treatment, respectively. (c) Double-layer capacitance characterization. (I) Representative cyclic voltammograms (n = 3) acquired with 1 M KCl at scan rate of 500 mV s−1. Blue, yellow, and red colors denote Au working electrode, NT-3DFG synthesized under 25.0 mTorr CH4 partial pressure for 30 and 90 min, respectively. Dashed lines denote NT-3DFG after 2 h of HNO3 treatment. (II) Current density as a function of scan rate. Blue, yellow, and red colors denote Au working electrode, NT-3DFG synthesized under 25.0 mTorr CH4 partial pressure for 30 and 90 min, respectively. Closed and open squares denote NT-3DFG before and after 2 h of HNO3 treatment, respectively. (d) Normalized anodic peak current acquired with 5.00 mM [Fe(CN)6]3− in 1 M KCl at 50 mV s−1 as a function of number of days after synthesis for NT-3DFG synthesized under 25.0 mTorr CH4 partial pressure for 30 min. Nova Electronic Materials Ltd., Catalog No. CP02 11208-OX) or 1.5 cm × 1.5 or 1.5 cm × 2.0 cm fused silica substrate (University Wafer, Catalog No. 1013, fused silica was used for electrical and electrochemical measurements) was cleaned with acetone and isopropyl alcohol (IPA) in an ultrasonic bath for 5 min each and N2 blow-dried. The substrate was placed in a UV−ozone system (PSD Pro series digital UV-Ozone, Novascan) for 10 min at 150 °C. The substrate was then functionalized with 450 μL (400 μL for 1.5 cm by 1.5 cm substrate) of 4:1 deionized (DI) water/poly-L-lysine (PLL) (0.1% w/v, Sigma-Aldrich, Catalog No. P8920) for 8 min. Following this step, the substrate was gently washed three times in DI-water and N2 blowdried. A 450 μL (400 μL for 1.5 cm by 1.5 cm substrate) portion of 30 nm AuNP solution (Ted Pella, Inc., Catalog No. 15706-1) was dispersed onto the PLL-coated substrate for 8 min. The substrate was gently washed three times in DI-water, N2 blow-dried, and introduced into a custom-built CVD setup. Once a baseline pressure of 1 × 10−5 Torr was reached, the temperature was ramped up to 450 °C in 8 min, followed by a 5 min stabilization step. Nucleation was conducted at 450 °C for 15 min with 80 standard cubic centimeters per minute (sccm) H2 (Matheson Gas) and 20 sccm SiH4 (10% in H2, Matheson Gas) at 40 Torr. This was followed by a growth step of 100 min with 60 sccm H2, 20 sccm SiH4, and 20 sccm PH3 (1000 ppm in H2, Matheson Gas) at 40 Torr. The sample was then rapidly cooled to room temperature at base pressure.

confers superhydrophobic properties to the material. Assynthesized NT-3DFG demonstrate exceptional electrical conductivity of 1655 ± 450 S m−1 (84 ± 6 Ω □−1). Treatment with HNO3 renders the superhydrophobic surface as hydrophilic and further increases the electrical conductivity to 2355 ± 785 S m−1 (59 ± 12 Ω □−1). NT-3DFG electrodes demonstrate functionality in an electrochemical cell model wherein the material exhibits enhanced faradaic peak currents, capacitance, and electrochemical surface area up to 1017 ± 127 m2 g−1 upon HNO3 treatment. Furthermore, NT-3DFG electrodes show electrochemical stability for more than a month. Synthesis of graphene with 3D topology will enable formation of complex hybrid nanomaterials. By controlling the nature of the NW core and modifying graphene’s electrical properties, it is possible to synthesize hybrid materials with finely tailored electrical properties67 for use in both sensing and energy-related research.

METHODS Nanowire Synthesis. SiNWs were synthesized by a AuNPcatalyzed VLS growth process. Briefly, either a 1.5 cm × 2.0 cm (100) Si substrate with a 600 nm wet thermal oxide (p-type, ≤0.005 Ω cm, 6307


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ACS Nano Mesh Formation. The synthesized NWs were collapsed by flowing liquid N2 into the CVD quartz tube under 200 sccm Ar flow. The system was evacuated to base pressure followed by a 10 min mesh annealing step at 800 °C under 200 sccm H2 flow at 1.6 Torr. Finally, the system was rapidly cooled to room temperature. Out-of-Plane 3D Fuzzy Graphene Synthesis. 3DFG was synthesized by PECVD process. A SiNWs mesh sample was taken out of the NWs CVD and immediately introduced into a custom-built PECVD setup. The synthesis process was carried out at 800 °C and at a total pressure of 0.5 Torr. The sample was placed onto a carrier wafer to position it at the center of the tube and was placed 4.0 cm from the edge of the RF coil. The temperature was ramped up to 800 °C in 13 min, followed by stabilization at 800 °C for 5 min, under a flow of 100 sccm Ar (Matheson Gas). Inductively coupled plasma was generated using a 13.56 MHz RF power supply (AG 0313 Generator and AIT-600 RF, power supply and auto tuner, respectively, T&C Power Conversion, Inc.). The plasma power was kept constant at 50 W. The furnace was moved over the sample following plasma ignition. The synthesis step was conducted by varying either the flow ratios of CH4 precursor (5% CH4 in Ar, Airgas) and H2 (Matheson Gas) or the process time. Table S1 summarizes the conditions of the synthesis processes (three independently synthesized samples, n = 3, were performed for each reported condition). The plasma was shut down after the synthesis step and the sample was rapidly cooled from growth temperature to 80 °C in 30 min under 100 sccm Ar flow. Scanning Electron Microscopy Imaging. SEM imaging was carried out using a FEI Quanta 600 field emission gun (FEG) SEM. The samples were imaged at acceleration voltages of 5−20 kV and a working distance of 5 mm. The samples were not coated with a conductive coating prior to imaging. NT-3DFG diameter and mesh thickness evaluations were done with the use of ImageJ. For the analysis, high-resolution (2048 × 1768 pixels) SEM images were used. Data presented in Figure 2f is the mean ± SD of 80 NWs counted per synthesis condition (three independently synthesized samples per synthesis condition). Transmission Electron Microscope Imaging. TEM imaging was carried out using a FEI Titan G2 80−300 Cs-corrected TEM/ STEM fitted with a high resolution Gatan Imaging Filter (GIF) Tridiem energy-filter. High-resolution TEM (HR-TEM) and selected area electron diffraction (SAED) images were acquired at 300 kV accelerating voltage with 2048 × 2048 slow-scan CCD cameras. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was performed at 300 kV accelerating voltage. Energy-dispersive spectroscopy (EDS) was performed at 300 kV accelerating voltage with the EDAX energy dispersive X-ray detector. Electron energy loss spectroscopy (EELS) was performed with a step voltage of 0.2 eV with a high-resolution GIF Tridiem energy-filter. Fringe intensity cross-section profiles (Figure S3) were generated from the corresponding HR-TEM images with the use of ImageJ. The interlayer distance was evaluated by dividing the total length by the number of fringes. Determination of diffraction rings and their dimensions from the corresponding SAED image (Figure 4d (inset)) was done with the use of ImageJ. High-resolution (2048 × 2048) SAED images were used for this purpose. Raman Spectroscopy. Raman spectroscopy was performed by NT-MDT NTEGRA spectra (100× objective) using 532 nm excitation. A laser power of 2.38 mW was used, and the spectra were recorded with an acquisition time of 30 s. For G dispersion ((Disp(G)) calculations, Raman spectra of each point was acquired using dual lasers 532 and 633 nm (2.38 mW for both wavelengths). Raman spectra (Figure 3 and Figure S4) were acquired from 10 randomly distributed points across each 1.5 cm by 2.0 cm NT-3DFG samples with three independently synthesized samples (n = 3) per NT-3DFG synthesis condition. Contact Angle Measurements. Contact angle measurements were performed with VCA optima (AST Products, Inc.) with a 1.5 μL of DI-water droplet injected onto the sample. The images were taken 5 s after the water droplet was dropped onto the sample surface. Contact

angle values were determined with the use of AutoFAST Imaging Software (AST Products, Inc.). Contact angle measurements were performed at three randomly distributed points across each 1.5 cm by 2.0 cm NT-3DFG sample with three independently synthesized samples per NT-3DFG synthesis conditions. Data are presented as mean ± SD in Table S8. Sheet Resistance Measurements (van der Pauw Measurements). A custom-made van der Pauw apparatus was used with 1.2 mm wide flat Au-plated pogo pins placed at the four corners of a square pattern with edge length of 1.0 cm. Current sweep was performed from −100 to +100 μA, and potential change was recorded with a source meter unit (SMU, Keithley 2401). The NT-3DFG sample is assumed to be a flat surface with no pores (i.e., porosity of NT-3DFG samples was not taken into consideration when the sheet resistance was determined). Sheet resistance measurements were performed three times for each NT-3DFG sample with three independently synthesized samples per NT-3DFG synthesis condition. Data is presented as mean ± SD in Figure 7. Cyclic Voltammetry. Cyclic voltammetry (CV) experiments were performed using PalmSens3 (Palmsens BV). All experiments were carried out following a three-electrode system with NT-3DFG samples (synthesized under 25.0 mTorr CH4 partial pressure for 30 min, before and after 2 h of HNO3 treatment, and under 25.0 mTorr CH4 partial pressure for 90 min, before and after 2 h of HNO3 treatment) as the working electrode, a Pt wire (CH Instruments, Inc., Catalog No. CHI115) as the counter electrode, and Ag/AgCl electrode (CH Instruments, Inc., Catalog No. CHI111) as the reference electrode. Au working electrode (CH Instruments, Inc., Catalog No. CHI101) was used as a control. CV experiments were performed in an electrolyte solution containing 1 M KCl in DI-water. Analyte solutions were prepared with varying concentrations (1.25, 2.50, and 5.00 mM) of K3Fe(CN)6 (≥99.0%, Sigma-Aldrich, Catalog No. 244023) in 1 M KCl (≥99.0%, Sigma-Aldrich, Catalog No. P5405) solution. Prior to conducting CV experiments, contacts composed of 5 nm Cr (99.99%, The R.D. Mathis Co.) followed by 150 nm Au (99.999%, Praxair) were evaporated at the edge of the samples using a thermal evaporator (Angstrom Thermal Evaporator), and a polystyrene well was sealed to the sample using 10:1 base:curing agent poly dimethyl-siloxane (PDMS) (Sylgard 184 Silicone Elastomer, Dow Corning). Geometric area of the NT-3DFG sample exposed to the analyte after PDMS application was determined by imaging the sample surface at 1× magnification with a stereo microscope (AmScope) and analyzing the images with the use of ImageJ. IPA infiltration was used to increase the interaction of the aqueous analyte with the NT-3DFG sample. 2 mL IPA was introduced into the well, and the sample was placed in the desiccator under vacuum for 5 min. A 1.8 mL portion of IPA was aspirated, and 1.8 mL of DI-water was introduced, followed by 5 min in the desiccator under vacuum. This process was repeated three times to exchange IPA with DI-water. A similar procedure was followed to introduce the analyte solutions in place of DI-water. Pt wire counter electrode and Ag/AgCl reference electrode were then introduced into the well. To determine the effect of varying scan rates on the faradaic anodic and cathodic peak currents, the CV measurement were recorded within a potential range from −0.2 to 0.7 V versus Ag/AgCl at increasing scan rates of 5, 30, 50, 80, and 100 mV s−1 in the presence of 5.00 mM [Fe(CN)6]3− in 1 M KCl solution. To determine the effect of varying analyte concentration on the peak currents, the CV measurement were recorded within a potential range from −0.2 to 0.7 V versus Ag/AgCl with 1.25, 2.50, and 5.00 mM [Fe(CN)6]3− in 1 M KCl solution at a scan rate of 50 mV s−1. The anodic and cathodic peak currents were determined with the use of PSTrace software (PalmSens BV). Double-layer capacitance was determined by conducting CV with 1 M KCl electrolyte solution within a potential range from −0.2 to +0.3 V versus Ag/AgCl at increasing scan rates of 5, 30, 50, 80, and 100 mV s−1. Geometric current density at 0.05 V (where no Faradaic process took place) was plotted against the scan rate and the slope of the linear regression was computed to determine the double-layer capacitance. 6308


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ACS Nano The mass of carbon in NT-3DFG electrodes was calculated through gravimetric analysis with Excellence XS105 Analytical Balance (Mettler Toledo), before and after UV-ozone treatment for 90 min at 150 °C (PSD Pro series digital UV-Ozone, Novascan). The average mass of carbon in NT-3DFG synthesized under 25.0 mTorr CH4 partial pressure for 30 and 90 min was measured to be 53 ± 3 and 71 ± 6 μg cm−2 (n = 3). NT-3DFG with Au contacts and polystyrene cuvette well was infiltrated with IPA and then by DI-water as described in the process above. A 1.9 mL portion of DI-water was pipetted out, and 1.9 mL of HNO3 (70% w/w) was introduced into the wells for 2 h. HNO3 was then pipetted out, and the sample was cleaned three times with DIwater. CV experiments were then performed as described above. Stability of the NT-3DFG electrode surface was determined by plotting the anodic peak current (with 5.00 mM [Fe(CN)6]3− in 1 M KCl solution at a scan rate of 50 mV s−1) against the number of days (1, 3, 5, 7, 14, 21, 28, 35, 42 and 49). CV data was acquired with five scans for every experimental setup with every NT-3DFG samples with three independently synthesized samples for every NT-3DFG condition. Representative voltammograms are presented in Figure 8. Acquired CV current values were normalized with respect to the geometric area of respective samples. Diffusion-Limited Aggregation Based Growth Model Simulations. The simulations performed are not a substitute for a full atomistic description that would consider thermodynamic variables, and local and global chemistry. However, since such simulation is currently out of reach of any modeling method, the proposed approach has several well-controlled features that enable us to shed light on the experimentally observed growth process. In the calculation performed, each nucleating particle consists of a single carbon dimer, and its movement is constrained to discrete locations on a hexagonal lattice. Notice that these rules explicitly constrain the structure to form honeycomb shapes in regions which are filled, and if one were to fill all sites in space, the resulting structure would be AA-stacked graphite. However, instead of modeling optimal packing the algorithm used explored the effect of diffusion-limited growth on the generation of large-scale nanostructures. To seed the process, we start with a fixed cylindrical core to mimic the SiNW used in the experimental conditions. The core is oriented along the direction of stacking in the lattice and is periodic in this direction. Dimers are introduced at the boundary of a large periodic cell centered at the core, and each step of Brownian motion moves the dimer to a randomly chosen neighboring lattice point, defined as the six nearest triangular lattice neighbors in the xy-plane and the two immediate neighbors along ±z. Lattice parameters do not enter explicitly into the formalism; the particle’s destination each step is chosen randomly and uniformly from those neighbors which are vacant. This means that movement may be anisotropic when interpreted spatially, as there is generally a 75% chance of movement within the xy-plane versus a 25% chance of movement along z, regardless of the relative physical lengths of these motions. The nucleation event ends when two adjacent neighboring sites in the xyplane are occupied, forming a complete triangle with the new dimer. This rule specifically promotes the creation of layers, as nucleation in the z-direction is thus impossible. Together with the hexagonal lattice structure, this strongly promotes local sp2 chemistry. The simulated results are of 290000 nucleation events, the NW diameter is 30 nm, with a length of 27 nm.

data summary for Raman analysis of NT-3DFG synthesized with varying PECVD conditions, data summary for Raman analysis of NT-3DFG before and after HNO3 treatment, data summary for dual laser Raman analysis of NT-3DFG synthesized under varying PECVD conditions, data summary of interplanar spacing determined from SAED patterns); associated references (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Tzahi Cohen-Karni: 0000-0001-5742-1007 Author Contributions ⊥

R.G. and S.K.R. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS T.C.K. thanks the National Science Foundation (CBET1552833) and the Charles E. Kaufman Foundation of the Pittsburgh Foundation (KA2014-73917). V.M. thanks the National Science Foundation (EFRI EFMA-1542707) and acknowledges support provided by a NY State NYSTAR grant. We thank J. Whitacre, M. Fantin, and A. Mohamed for their assistance with electrochemistry experiments. We thank E. Mataev for his assistance with nanowire synthesis. We also acknowledge support from the Carnegie Mellon University Nanofabrication Facility, and the Department of Materials Science and Engineering Materials Characterization Facility (MCF). REFERENCES (1) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530−1534. (2) Bolotin, K. I.; Sikes, K.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. Ultrahigh Electron Mobility in Suspended Graphene. Solid State Commun. 2008, 146, 351−355. (3) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385−388. (4) Stoller, M. D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R. S. GrapheneBased Ultracapacitors. Nano Lett. 2008, 8, 3498−3502. (5) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M.; Geim, A. K. Fine Structure Constant Defines Visual Transparency of Graphene. Science 2008, 320, 1308− 1308. (6) Huang, X.; Qi, X.; Boey, F.; Zhang, H. Graphene-Based Composites. Chem. Soc. Rev. 2012, 41, 666−686. (7) 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. (8) Stankovich, S.; Dikin, D. A.; Dommett, G. H.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-Based Composite Materials. Nature 2006, 442, 282−286. (9) Berger, C.; Song, Z.; Li, T.; Li, X.; Ogbazghi, A. Y.; Feng, R.; Dai, Z.; Marchenkov, A. N.; Conrad, E. H.; First, P. N. Ultrathin Epitaxial Graphite: 2D Electron Gas Properties and a Route Toward GrapheneBased Nanoelectronics. J. Phys. Chem. B 2004, 108, 19912−19916. (10) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E. Large-Area Synthesis of HighQuality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312−1314.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02612. Figures S1−S5 (SEM characterization, EDS elemental mapping, interlayer distance characterization, effect of HNO3 treatment on NT-3DFG, dependence of peak current on analyte concentration); Tables S1−S8 (Summary of various NT-3DFG synthesis conditions, 6309


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