A facile method to synthesize highly reduced graphene oxide in solid phase was developed. The reduced graphene oxide was scarcely prepared in solid phase. Solid substances act as spacers and pillaring agents. Sheets can not be close to each other in reduction process, and sheets agglomeration might not form. After reduction reaction is complete, the spacers and pillaring agents are removed. The average interlayer spacing and surface area of product are bigger than those of reduced graphene oxide. The product has few-layered sheet, and the ratio of carbon to oxygen is high, which might imply that the product is more similar to graphene compared to reduced graphene oxide. The specific capacitance of product is almost three times higher than that of reduced graphene oxide at the same current density.
Graphene has created a revolution in the field of nanotechnology [
Graphene oxide (GO) begins in monolayer form with high area; the extent of surface area losses by restacking during reduction process is very large. In simple reduction process, the surface area of reduced graphene oxide (RGO) decreases quickly as the number of sheets per stack increases [
Pristine graphite was purchased from Qingdao BCSM Co. Ltd. (Qingdao, China), and ammonium chloride (NH4Cl) was supplied by Shanghai Chemical Reagent Company (Shanghai, China). All other chemicals were of analytical grade and were purchased from Beijing Chemical Reagents Company (Beijing, China).
Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy were performed using Hitachi S-4800 field emission and FEI-Quanta 200 scanning electron microscopes, respectively. High-resolution TEM (HRTEM) was performed on JEOL JEM-2011 electron microscope operated at 200 kV, equipped with a Gatan 794 camera. Fourier-transform infrared (FT-IR) spectra were obtained on FTS-40 (Bio-Rad, CA, USA). Raman spectra were recorded on Renishaw InVia multichannel confocal microspectrometer with 532 nm excitation laser. X-ray diffraction (XRD) measurements were obtained on X’pert PRO diffractometer using Co K
A mixture of GO (0.50 g) and water (50 mL) was sonicated for 1 h. Water bath sonication was performed using JL-60 DTH sonicator (100 W). Ammonium chloride (30 g, 0.56 mol) was added to the GO dispersion at 50°C stirring for 2 h, and then it was dried by natural drying method. Then the mixture was calcined at 320°C for 1 h, the mixture was cooled down to ambient temperature, and the mixture was washed with water (20 mL
The RGO was prepared according to that of a previous literature [
The electrode was made by coating 85 wt% of HRGO and 15 wt% of acetylene black onto a working electrode. The working electrode was dried at 100°C for 10 h. The CV and CD of HRGO were measured with CH660D electrochemical work station using a three-electrode configuration. Glassy carbon was used as the working electrode, Pt wire served as the counter electrode, and saturated AgCl/Ag served as the reference electrode [
The morphologies and microstructures are characterized by SEM and HRTEM (Figures
(a) SEM image and EDX spectrum of HRGO. (b) TEM image of HRGO.
The FT-IR of RGO and HRGO are shown in Figure
(a) FT-IR spectra of RGO and HRGO. (b) Raman spectra of RGO and HRGO.
The Raman spectra of HRGO present broadened G band at 1572 cm−1 because of the formation of a conjugated system, in contrast to the G band (1565 cm−1) of RGO (Figure
The XRD patterns of HRGO and RGO are shown in Figure
(a) XRD patterns of the RGO and HRGO. (b) C1s XPS of HRGO.
The XPS is also used to analyze the samples of HRGO. The core-level XPS signals of C1s are shown in Figure
Atomic concentration of HRGO.
Sample | C (%) | O (%) | C/O |
---|---|---|---|
HRGO | 90 | 10 | 9 |
The surface area of HRGO is 468.6 m2 g−1. The common technique to determine the graphene stack mean layer number is the scaling law:
Typical nitrogen adsorption and desorption of HRGO (a) and pore size distribution curves of HRGO (b).
The HRGO degree of reduction with different temperature and HRGO specific capacitances are shown in Table
The HRGO degree of reduction with different temperature and yield and their specific capacitances.
Temperature (°C) | Yield (%) | C/O ratio | Specific capacitance (1 A g−1) |
---|---|---|---|
240 | 85 | 7.6 | 160 |
280 | 81 | 8.2 | 130 |
320 | 67 | 9 | 128 |
360 | 53 | 9.4 | 114 |
To investigate the capacitance behavior of HRGO, the cyclic voltammetry (CV) and the galvanostatic charge-discharge (CD) were carried out as shown in Figures
(a) CV curves of RGO and HRGO at 500 mV s−1; (b) CV curves of HRGO at different scanning rates; (c) the galvanostatic charge/discharge behavior of RGO and HRGO at 1 A g−1; (d) the specific capacitance of RGO and HRGO based supercapacitors at different current densities; and (e) Nyquist plots of RGO and HRGO electrodes using a sinusoidal signal of 25 mV with the frequency range from 10 mHz to 10 kHz.
In all, highly reduced graphene oxide was prepared in practical solid method. The product has few-layered sheet, and the ratio of carbon to oxygen is high. It might be more similar to graphene compared to RGO. The product has higher specific capacitance and has a higher rate capability compared to RGO at high current density. This work shows that HRGO presents unique characteristics, including facile synthesis and full reduction, which render it suitable for high-quality electronic devices, sensors, and supercapacitors, as well as fundamental studies about the chemistry and structure of graphene.
The authors declare that they have no competing interests.
The authors acknowledge the financial support from Natural National Science Foundation of China (no. 51373126), the Science and Technology Department of Henan Province (no. 162300410015), the Education Department of Henan Province (no. 12B210022), the Science and Technology Bureau of Xinxiang (nos. 13SF39 and ZG15022), and Fund of the Xinxiang University (no. 15ZP05).