PDF Publication Title:
Text from PDF Page: 002
144 CARBON 54 (2013) 143–148 and chemical properties. It should be noted that so far the reduction of oxygen content in reduced graphene oxide (RGO) is still very difficult [17]. After thermal or chemical treatment, the C@O and O@CAOH could be partially removed or converted to a new chemical species (CAOH), whereas the remaining CAOH or CAOAC in RGO could not be reduced easily, leading to the degradation of electrical properties of the graphene sheets. Scalable conversion of chemically modified graphenes to C-pure graphene sheets still remains a central challenge. Herein, we report a simple and effective route to convert graphene oxide (GO) sheets to graphene sheets using hot pressing. The product materials were assessed by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) as well as Raman spectroscopy. Significantly, no appreciable oxygen content was observed from XPS, and in the Raman spectrum, no D peak was detected, while the G and 2D peaks characteristic of highly crystalline graphene were readily ob- served. Moreover, the product graphene sheets had much higher electron mobility (1000 cm2 V1 S1) than other chem- ically modified graphenes [18–20]. We believe that, highly crystalline graphene sheets from hot pressing treatment will provide an opportunity and a possibility to radically overcome the barrier for further application of graphene. 2. Experimental 2.1. Materials Graphite powder, natural, 325 mesh, 99.95% was purchased from Alfa Aesar. P2O5, K2S2O4, and KMnO4 with analytical grade and 98% H2SO4, 30% H2O2 aqueous solution were pur- chased from Shanghai Chemical Reagents Company, and were used directly without further purification. 2.2. Preparation and thermal reduction of GO Graphite oxide was synthesized from natural graphite by a modified Hummers method [21]. The product (graphite oxide) was then purified by dialysis to completely remove residual salts and acids. After ultrasonication for 1 h and drying at 60 °C, the graphite oxide was calcined at 700 °C for 2 h under argon atmosphere with a heating rate 100 °C min1 [22]. 2.3. Hot pressing treatment of RGO The product graphene was then treated in a Spark Plasma Sintering (SPS) system (SPS-3.20MK-II, Sumitomo Heavy Industries) under vacuum at 500, 1000, and 1500°C for 5 min with 0, 5, 10, 20, 30, 40 MPa uniaxial pressures, respec- tively. The heating rate was 100 °C min1, and vacuum level was less than 30 Pa. A graphite mold was chosen as the elec- trode and heating stage. 2.4. Characterization of the samples The surface morphology of the pristine RGO and the hot pressed graphene sheets were examined by SEM (FEI SIRION) operated at 15 kV. Two kinds of the samples were sonicated in ethanol for 2 h and the suspensions were dropped onto the micro-grid for HRTEM observation (JEOL JEM 2010FEF) at 200 kV. The Atomic Force Microscope (AFM) measurements were performed with a Nanoscope multimode instrument in the air at ambient temperature and pressure. Ethanol suspen- sions of the graphene sheets were spin-coated onto a SiO2 (300 nm) substrate, and the solvent was removed by anneal- ing under Ar gas at 600 °C for 1 h. The IR spectrum was mea- sured with a FT-IR spectrometer (Nicolet iS10, Thermo) for detecting the surface functional groups on the samples. In order to verify the improvement of quality and structural integrity in the hot pressed graphene, chemical compositions and chemical environment of the carbon atoms were mea- sured by using XPS (AXIS-Ultra instrument, Kratos Analytical), with monochromatic Al Ka radiation (225 w, 15 Ma, 15 kV) and low-energy electron flooding for charge compensation. Raman measurement was carried out using Raman spec- troscopy (HORIBA Jobin Yvon LabRAM HR). The power of laser was 15 mW, and the laser excitation was 488 nm. Scans were taken on an extended range (1000–3000 cm1) and the expo- sure time was 5 s. Samples were sonicated in ethanol and drop-casted onto a SiO2 (300nm) substrate for optic observation. The electrical transport properties were measured by a Lakeshore probe station (Lakeshore TTPX probe station with Agilent analyser) with home built data acquisition system in ambient condition at room temperature. Toward the electrical characterization, the pristine and hot pressed graphene sheets were first deposited on a highly doped p-type silicon substrate with a 300 nm thick thermal silicon oxide layer, and then contacted by e-beam lithography (JEOL 6510 with NPGS) and metallization process to define the external and drain electrodes. About 20 different sets of samples were tried to verify the reproducibility of the electrical test. 2.5. Theoretical calculations Molecular dynamics simulation was carried out to under- stand the structural change of RGO treated by hot pressing. A 0.5 ns step size, constant temperature and constant uniax- ial pressures were applied to the molecular dynamics simula- tion with periodic boundary conditions. The CAC bond was treated with the Tersoff potential while the CAO and OAO were described by a constructed Morse potential fitted from first-principle total energy calculations [23,24]. 3. Results and discussion The conversion process consists of four steps, shown in Fig. 1a. Our novel contribution is that the RGO sheets were then treated by hot pressing at 1500 °C and 40 MPa uniaxial pressures for 5 min in vacuum. The RGO sheets were thereby converted from a pile of powder into a compacted lamellar material with tight combination (see Fig. S1). From the characterizations studies (see Fig. S2), the hot pressing did not change the original mor- phologies of the RGO sheets (such as size and number of lay- ers), i.e., the RGO sheets did not transform into graphite; this high T, moderate pressure treatment resulted in highly crystal- line graphene with a gram-scale output.PDF Image | graphene sheets from graphene oxide by hot-pressing
PDF Search Title:
graphene sheets from graphene oxide by hot-pressingOriginal File Name Searched:
345.pdfDIY PDF Search: Google It | Yahoo | Bing
Salgenx Redox Flow Battery Technology: Power up your energy storage game with Salgenx Salt Water Battery. With its advanced technology, the flow battery provides reliable, scalable, and sustainable energy storage for utility-scale projects. Upgrade to a Salgenx flow battery today and take control of your energy future.
| CONTACT TEL: 608-238-6001 Email: greg@infinityturbine.com | RSS | AMP |