Catalyst-Free Growth of Three-Dimensional Graphene Flakes and Graphene/g‑C3N4 Composite for Hydrocarbon Oxidation Ke Chen,†,§ Zhigang Chai,† Cong Li,†,‡ Liurong Shi,† Mengxi Liu,†,∥ Qin Xie,† Yanfeng Zhang,*,†,‡ Dongsheng Xu,† Ayyakkannu Manivannan,⊥,¶ and Zhongfan Liu*,† †
Center for Nanochemistry (CNC), Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, and ‡Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic of China § Key Laboratory for Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475004, People’s Republic of China ⊥ Department of Aerospace and Mechanical Engineering, West Virginia University, Morgantown, West Virginia 26507, United States ¶ NETL, U.S. Department of Energy, Morgantown, West Virginia 26507, United States ∥ CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology, Beijing 100190, People’s Republic of China S Supporting Information *
ABSTRACT: Mass production of high-quality graphene flakes is important for commercial applications. Graphene microsheets have been produced on an industrial scale by chemical and liquid-phase exfoliation of graphite. However, strong-interaction-induced interlayer aggregation usually leads to the degradation of their intrinsic properties. Moreover, the crystallinity or layer-thickness controllability is not so perfect to fulfill the requirement for advanced technologies. Herein, we report a quartz-powder-derived chemical vapor deposition growth of three-dimensional (3D) high-quality graphene flakes and demonstrate the fabrication and application of graphene/g-C3N4 composites. The graphene flakes obtained after the removal of growth substrates exhibit the 3D curved microstructure, controllable layer thickness, good crystallinity, as well as weak interlayer interactions suitable for preventing the interlayer stacking. Benefiting from this, we achieved the direct synthesis of g-C3N4 on purified graphene flakes to form the uniform graphene/g-C3N4 composite, which provides efficient electron transfer interfaces to boost its catalytic oxidation activity of cycloalkane with relatively high yield, good selectivity, and reliable stability. KEYWORDS: graphene, chemical vapor deposition, three-dimensional, powder, g-C3N4 chemical and liquid-phase exfoliation of graphite.8−11 Due to their strong π−π interlayer interactions, however, those flat graphene sheets aggregate easily in liquid to decrease their exposed surface, which could conceal their extraordinary electronic and thermal properties, as well as catalytic activities.12,13 Numerous efforts have been made for the construction of non-flat-structured graphene sheets for preventing the stacking
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n the past decade, graphene has undoubtedly emerged as the first two-dimensional atomic crystal with exceptional properties, which opened up the field of 2D nanomaterials and their exploitation in various applications.1−3 Large-scale production of high-quality graphene is an important premise for fully realizing its commercial applications. As the most common type of graphene, the microsheets have been scaled up to the level of industrial production. Recent applications have shown a promising future in energy- and environment-related fields, such as catalysts, supercapacitors, and Li-ion batteries.4−7 Currently, many efforts have been made to prepare largequantity graphene sheets and their derived materials by © 2016 American Chemical Society
Received: January 6, 2016 Accepted: February 26, 2016 Published: February 26, 2016 3665
DOI: 10.1021/acsnano.6b00113 ACS Nano 2016, 10, 3665−3673
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Figure 1. Large-quantity graphene flakes grown by CVD on quartz powder. (a) Schematic of the CVD growth of 3D graphene flakes. (b,c) False color SEM images of quartz powder before and after CVD growth of graphene. Inset: Photographs of quartz powder as well as samples before and during wet chemical etching. (d,e) False color SEM images of graphene after separation from quartz powder substrate. Inset in (d): Corresponding photograph of the purified graphene powder. (f) Raman spectroscopy of CVD graphene flakes compared with RGO.
of graphene.14−16 Typically, the three-dimensional (3D) assembly of graphene sheets into porous monoliths is an efficient way to provide abundant surfaces with the preservation of their intrinsic nature.17 However, chemical oxidation destroys the lattice structures from the basal plane of graphene, and liquid-phase exfoliation is also difficult for realizing the balance between the layer number and lateral size controllabilities of graphene.18−20 Alternatively, the 3D curved and interconnected graphene structures have been explored by chemical vapor deposition (CVD) growth on sacrificial metal templates for resisting the restacking of graphene.21 The crystallinity and thickness controllability of graphene layers can be greatly improved in this way, which is favorable for achieving efficient electron transfer and transport in graphene.22−24 In this case, however, these expensive metal templates (or substrates) are not suitable for low-cost and scalable production of graphene, and metal catalysts could also be inserted into graphene layers to form insoluble impurities in the graphene products.25,26 In the present work, we have demonstrated a direct scalable growth of 3D graphene flakes by using CVD on quartz powder due to the abundant silica resources in the earth. Silica particles possessing abundant oxygen-bearing surfaces can enhance absorption of hydrocarbons at high temperature and provide a great opportunity for carbon−carbon coupling and nucleation without the help of metal catalysts.27 The advantages of the CVD process along with the active surface of the quartz powder substrate for catalytic growth of graphene facilitate the relatively higher crystallinity, layer-thickness controllability, and yield of graphene flakes compared with that of reduced graphene oxide (RGO) and liquid-phase exfoliated graphene (LPEG) sheets. Furthermore, the 3D curved microstructures (not just wrinkles) of graphene flakes easily maintain weak interlayer interactions to prevent the interlayer stacking after removal of the quartz powder substrate. Thus, it provides abundant exposed surfaces/
interfaces for graphene to disperse itself in liquid to yield a uniform graphene-based composite system. For catalytic oxidation of saturated hydrocarbons, graphene has served as an efficient electron donor18 for modifying the polymeric gC3N4 (graphene analogue) to exploit a recyclable, inexpensive, and metal-free catalyst.28,29 Following those works, we utilized the CVD graphene flakes with 3D nonstacking structure and mixed them with g-C3N4 to produce a hybrid composite for selective oxidation of cycloalkanes.
RESULTS AND DISCUSSION The CVD growth process of graphene flakes on quartz powder is shown in Figure 1a. Due to the relatively negative surface adsorption energies of C−H species,27 the quartz powder is employed as a promoter for nucleation and growth of graphene instead of catalytic metal substrates. As templates, the quartz particles also play a role in defining the 3D curved morphology of graphene layers. Field emission scanning electron microscopy (FESEM) is conducted to evaluate the growth and separation of graphene flakes on quartz powder. As shown in Figures 1b,c, the quartz particles (∼400 meshes) showed no apparent change in morphology but became dark gray after CVD at 1050 °C (see insets in Figure 1c). It seems that the quartz particles withstand high temperature and retain their shapes for graphene growth. Due to their hydrophobicity, the graphene flakes can be spontaneously separated from quartz substrates without ultrasonication during the wet-etching process of the graphene/quartz powder product (see insets in Figure 1c). After being washed and dried, the separated graphene layers from quartz powder aggregate with each other to form a foam-like network (Figure 1d). Furthermore, the detailed SEM morphology of locally curved thin-layer graphene is shown in Figure 1e. Notably, the graphene layers replicate the surface morphology of quartz particles following a 2D 3666
DOI: 10.1021/acsnano.6b00113 ACS Nano 2016, 10, 3665−3673
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Figure 2. High-crystallinity graphene flakes with chemically inert surfaces. (a) TEM image and corresponding SAED pattern. (b) HRTEM images. (c,d) STM and corresponding FFT images. (e) Fourier transform infrared spectra and (f,g) X-ray photoelectron spectra of graphene flakes, RGO, and graphite.
TEM and STM measurements confirm high crystallinity of graphene flakes, which are consistent with the Raman analyses. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopic (XPS) techniques were also used to characterize graphene flakes. The micro-FTIR spectrum of graphene flakes (Figure 2e) displays several peaks from 1400 to 1600 cm−1, which should be ascribed to the vibration bands of sp2 C−C. Two other peaks at ∼2850 and 2920 cm−1 are mainly attributed to the stretching vibration modes of sp3 C−H, which are associated with defects in the edge of the resultant graphene.32 No characteristic peak for the silicon−oxygen bond is observed. This suggests the complete separation of silica powder substrate. In the case of RGO flakes, several peaks representing hydroxyl (C−OH), epoxide (C−O−C), carbonyl (CO), and carboxyl (CO−OH) groups are presented in the FTIR spectrum.33 For the FTIR spectrum of graphene flakes, the absence of the peaks corresponding to those oxygen-containing surface groups reveals the improved quality of graphene obtained in the present method. Further evidence for pristine graphene flakes is also shown by the XPS measurements (Figure 2f,g and Supporting Information Figure S2). The XPS spectrum of graphene flakes shows that there is no corresponding binding energy peak for Si, confirming the complete removal of silica particles. From the semiquantitative analyses of the samples, the oxygen percentage for graphene flakes (∼4 wt %) is close to the value for graphite (∼2 wt %) but much lower than that of RGO (∼15 wt %) (Table S1, Supporting Information), suggesting almost oxygenfree surface groups. The deconvolution of the C 1s peak of graphene flakes reveals three components centered at 284.5, 285.2, and 289.0 eV, corresponding to sp2 carbon atoms, C− OH, and OC−OH groups, respectively.34 These peaks could be ascribed to the adsorbed water and oxygen in graphene, similar to graphite powder. The graphene layers grown at 1050 °C under 0.9% CH4 concentration are separated from silica particles and then
surface growth feature, thus preserving 3D architectures along the single-particle surface after removal of the SiO2 substrate. Based on the free-standing curved structures, the purified graphene products are defined as 3D graphene flakes. Besides, the quartz powder can also be reused for graphene growth after etching and extraction (Figure S1, Supporting Information). Intriguingly, the Raman spectrum (Figure 1f) of purified graphene flakes shows a sharp 2D peak, suggesting its good crystallinity with a long-range conjugation structure of sp2 carbon.30 In contrast, the Raman spectrum of RGO flakes obtained from the modified Hummer’s method18 shows no obvious 2D peak. In addition, the graphene layers also show a D peak near 1350 cm−1 (ID/IG = ∼0.5), which could be attributed to the abundant domain boundaries and the small domain sizes. However, the D peak is relatively weak and sharp compared to that of RGOs showing broadened D and G peaks (ID/IG = ∼1.1). The Raman data indicate the relatively good crystallinity of the metal-free catalyzed CVD graphene flakes grown on quartz powder. The microstructure of graphene flakes has been further investigated by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). The graphene flakes exhibit a thin-layered feature with wrinkles and corrugations, as shown in the TEM image (Figure 2a). The selected area electron diffraction (SAED) pattern (inset Figure 2a) of graphene sheets shows more than one set of six-fold symmetric spots (the in-plane hkil reflections of graphene31), which could arise from the small domains and non-AB-stacked structures of graphene. High-resolution TEM images (Figure 2b) of graphene layers show the folded edges of graphene flakes, which confirm the presence of 1L, 2L and >2L graphene. The STM image (Figure 2c) shows the atomic resolution of the hexagonal lattice (lattice constant ∼0.25 nm) of graphene layers, corroborated by its corresponding fast Fourier transform (FFT) image (Figure 2d). The applied bias and the tunneling current were Vtip = 0.07 V and I = 1300 pA, respectively. The 3667
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Figure 3. Layer thickness and uniformity controlled growth of graphene flakes via CVD. (a,b) False color SEM and OM images of graphene transferred onto SiO2/Si substrates. (c) Two-dimensional Raman mapping of the marked square area in (b). (d) AFM image and (e) layerthickness distribution of graphene flakes grown at 1050 °C under a methane concentration of 0.9%. (f) Intensity ratios of I2D/IG and ID/IG as a function of growth temperature and methane concentration.
2D/G and D/G intensity ratios for the graphene layers grown at different temperatures and methane concentrations (Figures S6 and S7, Supporting Information) are presented in Figure 3f. Notably, the intensity ratio of D/G peaks decreases with either the increase of growth temperature from 1000 to 1050 °C (at a constant methane flow of 0.9%) or the increase of methane concentration from 0.6 to 1.5% (at a constant growth temperature of 1050 °C), while the ratio of 2D/G peaks does not change much. This suggests that moderate increases of temperature and methane concentration are reliable for decreasing the defect density and improving the crystallinity of graphene. However, with increasing growth temperature above 1050 °C, the D/G ratio increases along with the decrease of the 2D/G ratio. The increased D/G ratio should be understandable. Higher growth temperature could accelerate the pyrolysis of carbon species and increase the nucleation density of graphene, leading to reduced domain sizes and increased sp3 defects. In addition, the weakening and broadening of 2D peaks should indicate the variation of layer thicknesses of graphene flakes from monolayer to few layers. This was further confirmed by corresponding AFM analyses (Figure S8, Supporting Information). With increasing temperature from 1000 to 1100 °C, the average thicknesses of graphene layers increase from ∼0.7 to ∼2.8 nm, respectively. In the same way, an excessive increase of methane concentration was not favorable for improving the quality of graphene, resulting in the increased thickness of graphene layers, as evidenced by the AFM analyses (Figure S9, Supporting Information). Briefly, it seems that suitable growth temperature and methane flow rate should play more important roles in the metal-free catalyzed growth of high-quality graphene, compared with that of the coppercatalyzed growth of graphene.22
transferred onto the SiO2/Si substrates (300 nm thick oxide layer) by a poly(methyl methacrylate) (PMMA)-assisted transfer process in order to evaluate their thicknesses. SEM and optical microscopy (OM) images (Figure 3a,b) show the relatively continuous and uniform graphene layers that are successfully exfoliated. Based on color contrast in parallel with layer numbers, the thickness of the graphene layers can be tentatively distinguished at different regions. The successful synthesis of mainly monolayer graphene is confirmed by the Raman analysis based on region 1 (Figure S3, Supporting Information). Its 2D/G intensity ratio is about 1.3, and the full width at half-maximum of the 2D band is identified as 49 cm−1, in accordance with the previous data for mono- to bilayer graphene on planar substrates.24,35 As evidenced by the Raman mappings (Figure 3c and Supporting Information Figure S4) of graphene flakes in the marked square area in Figure 3b, most graphene flakes exhibit a uniform one-layer thickness, but there are a number of graphene islands with more than one layer thickness (see region 2 and the dark-spot distribution in Figure 3b). The brightness in contrast (Figure 3c) corresponds to the layer thickness, which could be caused by the non-AB-stacked few-layer growth and interlayer stacking of the quasi-3D graphene flakes. Atomic force microscopy (AFM) image of the transferred graphene layers on SiO2/Si substrates (Figure 3d) shows an average height of ∼1.6 nm, which is consistent with that of bilayer graphene.24 Based on the data collected on various isolated graphene flakes using AFM (Figure 3e and Supporting Information Figure S5), it is estimated that ∼80% of the graphene flakes comprise one and two layers of graphene. Effective control of the layer thickness of graphene flakes can be achieved by varying the growth conditions, such as growth temperature and methane concentration. Raman analyses of 3668
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Figure 4. Direct synthesis of polymeric g-C3N4 on graphene. (a) Schematic of graphene and g-C3N4 molecular structures. (b,c) False color SEM and TEM images of GCN composites. Insets: SEM and TEM images of the synthesized graphene flakes. (d,e) C 1s and N 1s XPS spectra of GCN composites. (f) UV−vis spectroscopy of GCN composites. Insets: Photographs of the powder samples.
It is worth mentioning that the SiOx-catalyzed growth of single-walled carbon nanotubes (SWNTs) was previously achieved by a similar CVD process.27 In this regard, it is necessary to compare the different growth mechanisms of the 3D graphene and SWNTs. Metal nanoparticles are often used to catalyze the growth of SWNTs through a thermal CVD method, the sizes of which can define the diameters of as-grown SWNTs.36,37 Intriguingly, monodispersed SiOx nanoparticles can also been prepared to grow SWNTs with similar diameters.27 However, only nanoparticles with suitable sizes (e.g., 50%) (or feeding rate) of methane and a relatively short growth time (several minutes). However, the CVD growth of graphene on quartz powder is required to operate at a very low concentration (
