Preparation and characterization of graphene derived from low-temperature and pressure promoted thermal reduction

Abstract

The reduction of graphene oxide was promoted remarkably under pressure via low temperature thermal treatment. Traditionally, graphene oxide is usually reduced in a preheated high temperature environment as a precondition of the thermal reduction. We report a pressure promoted method for low temperature thermal reduction and exfoliation of graphene oxide in large quantity at 260 °C. The physicochemical properties of parent graphite, as well as the microstructure and physicochemical properties of graphene oxide and resultant graphene were investigated by Raman spectrometer, thermograviment analyzer (TGA), transmission electron microscope (TEM), X-ray diffractometer (XRD) and Fourier transform infrared spectroscopy (FT-IR). Results show that graphene oxide was reduced to graphene with less stack via low-temperature pressure promoted thermal treatment, meanwhile, the degree of disorder reduced: the ratio of I_D/I_G in Raman spectrum decreases from 0.64 to 0.56. Moreover, graphene derived from low-temperature pressure promoted treatment exhibit better thermal stability than graphene oxide, and oxygen functional groups were removed with a high level. All of results exhibit improved comprehensive properties than graphene synthesized via traditional thermal reduction at 1000 °C.

Introduction

Graphene has been extensively studied in the last few years even though it was separated for the first time in 2004 [1], [2], [3], [4], [5], [6], [7]. As a burgeoning two-dimensional planar material, graphene has attracted much attention because of unique electronic transportation, superior thermal conductivity, and excellent mechanical properties [8], [9], [10], [11], [12], [13], [14]. Meanwhile, the great potential applications of graphene have also been investigated, such as transparent electronic conduction, and structure materials [11], [15], [16], [17], [18].

Nowadays, various methods are used to prepare of graphene, such as chemical vapor deposition, carbon nanotubes unzipping, graphite mechanical exfoliation, epitaxial growth, microwave plasma induced gas-phase synthesis and graphene oxide reduction methods [8], [19], [20], [21], [22], [23], [24], [25]. Nevertheless, graphene oxide reduction method still is the main preparation method due to its low cost and high yields features, and the most widely used methods are chemical and thermal reduction methods. In chemical reduction method, strong reductants were often used to reduce the oxygen functional groups such as hydrazine hydrate [26], [27], [28], [29], [30], [31], [32], [33]. In the past reports, vitamin C and sodium borohydride were also used to reduce graphene oxide as reductant [26], [27], [28], [29], [30], [31], [32], [33]. And graphene of thermal reduction is reduced via pyrolysis of the oxygen groups in high temperature under the protective gas. Among these methods, thermal reduction is the most promising treatment method for large yield production [19], [34], [35], [36]. McAllister et al. [34] rapid heated graphene oxide to ca. 1000 °C to ensure the reduction process is completed. In his view, the process of exfoliation and reduction occurred at 550 °C which is the critical temperature. But the experimental temperature of exfoliation and reduction was around 1000 °C for completely reduction [34], [35], [37]. The high temperature treatment is hard to be controlled in thermal reduction process, which is a great obstacle for mass production and lowering the cost. Under this precondition, more and more researchers tried to lower the thermal treatment temperature of graphene oxide reduction via various methods. Boehm et al. reduced thermally expanded graphite oxide (TEGO) at low temperature (less than 400 °C) to form graphene, and this temperature is less than the critical graphene production temperature of McAllister [38]. Zangmeister reduced graphite oxide (GO) thin films to form graphene at 220 °C with various analyses [26]. W Lv et al. produced few-layered graphene by thermal reduction method at low temperature (200 °C) under vacuum [8]. The key point of this is the effect of vacuum assisted, which was applied on the surface of expanding graphene layers to pull open two layered graphene as to separate two sheets of paper from stick together. Afterwards, HB Zhang prepared graphene sheets with a high surface area at 135 °C under vacuum in the similar thermal reduction method. Moreover, their resultants have more superior surface area than others’ under the vacuum assisted [19]. Surely, these approaches can reduce graphene oxide at low temperature, but the cost and energy consumption still is a problem to form graphene. In addition, vacuum condition of reduction process is an obstacle for filling composite as nanofillers.

In our works, we treated dry graphene oxide via thermal reduction at low temperature (260 °C) and the treatment process must be accompanied with the nitrogen pressure promoted, which is different with the traditional and vacuum assisted methods. By the pressure promoted, CO₂ propped the graphene layers open under pressure promoted which was from the pyrolysis of the oxygen functional groups. Various characteristics and morphologies of graphene were investigated respectively. During the comparing with the graphene which treated at 1000 °C, the resultant shows the graphene treated at 260 °C has high degree of exfoliation, low degree of disorder and less stacked.