Defects boost graphitization for highly conductive graphene films

ABSTRACT Fabricating highly crystalline macroscopic films with extraordinary electrical and thermal conductivities from graphene sheets is essential for applications in electronics, telecommunications and thermal management. High-temperature graphitization is the only method known to date for the crystallization of all types of carbon materials, where defects are gradually removed with increasing temperature. However, when using graphene materials as precursors, including graphene oxide, reduced graphene oxide and pristine graphene, even lengthy graphitization at 3000°C can only produce graphene films with small grain sizes and abundant structural disorders, which limit their conductivities. Here, we show that high-temperature defects substantially accelerate the grain growth and ordering of graphene films during graphitization, enabling ideal AB stacking as well as a 100-fold, 64-fold and 28-fold improvement in grain size, electrical conductivity and thermal conductivity, respectively, between 2000°C and 3000°C. This process is realized by nitrogen doping, which retards the lattice restoration of defective graphene, retaining abundant defects such as vacancies, dislocations and grain boundaries in graphene films at a high temperature. With this approach, a highly ordered crystalline graphene film similar to highly oriented pyrolytic graphite is fabricated, with electrical and thermal conductivities (∼2.0 × 104 S cm−1; ∼1.7 × 103 W m−1 K−1) that are improved by about 6- and 2-fold, respectively, compared to those of the graphene films fabricated by graphene oxide. Such graphene film also exhibits a superhigh electromagnetic interference shielding effectiveness of ∼90 dB at a thickness of 10 μm, outperforming all the synthetic materials of comparable thickness including MXene films. This work not only paves the way for the technological application of highly conductive graphene films but also provides a general strategy to efficiently improve the synthesis and properties of other carbon materials such as graphene fibers, carbon nanotube fibers, carbon fibers, polymer-derived graphite and highly oriented pyrolytic graphite.

added to adjust pH within a certain range of [8][9], then heated at 150 °C (120 °C) for 16 h (8 h) to obtain the dispersions of 10.3 wt% N-rGO (9.3 wt% N-rGO) sheets. The dispersion of rGO sheets was synthesized by hydrothermal treatment of GO sheets in water without the addition of ammonia at 150 °C for 8 h.
Synthesis of graphene films. As-synthesized N-rGO or rGO dispersion (4.5 L) was assembled into a film by compression filtration at 6 bar on polyether sulfone (PES) microfiltration membrane. For comparison, GO dispersion containing 1 g GO sheets was also assembled into a film with the same method. The diameter of the films was ~14 cm which was determined by the diameter of the pressure-driven filtration tank.
The as-assembled wet-state films were sandwiched by two pieces of sponges, and dried on a hot plate at 50 °C with mild pressing. After drying, the films were peeled off from PES membrane and kept in oven at 100 °C to remove unbound water and NH 3 for further thermal treatment and structural characterizations. To fabricate graphene films, the N-rGO, GO and rGO films were heated to 3000 °C in argon flow (8000 sccm) in a graphitization furnace (Zhuzhou Chenxin Induction Equipment Co., Ltd, CX-GF20/30VT) within 420 min, then kept graphitization for 10 minutes, and finally naturally cooled down to room temperature in Ar flow (6000 sccm). After that, a cold-pressing at 30 MPa was carried out for 5 minutes to remove the voids and obtain highly compact graphene films, i.e., N-rGO-, GO-and rGO-derived graphene films. In our experiments, all the graphene films obtained after 3000 °C annealing were measured at a similar thickness of ~10 m by a micrometer caliper. It should be pointed out that the GO films were preheated at 200 °C for 5 h in air oven before the above annealing process, in order to avoid the explosion caused by violent gas release from the rapid decomposition of oxygen functional groups during the following high-temperature annealing.
To reveal the structure and properties evolution during graphitization process, in addition to the above films obtained at 3000 °C, the N-rGO and GO films were heated to three different temperatures (2000, 2500 or 2800 °C ) in argon flow and kept for 0.5 h for each temperature, and then natually cooled down to room temperature followed by cold-pressing at 30 MPa for 5 min. To investigate the expansion occured during the whole annealing process, the N-rGO and GO films annealed at seven different temperatures (100, 1000, 1400, 2000, 2500, 2800 and 3000 °C ) were synthesized and no further cold-pressing was used before characterizations for such samples. Thermal annealing at no higher than 1400 °C was carried out in a tube furnace (Lindberg Blue MTF55030C), and a graphitization furnace (Zhuzhou Chenxin Induction Equipment Co., Ltd, CX-GF20/30-VT) was used for annealing at higher than 1400 °C.
Chemical composition analyses. The elemental composition was characterized by elemental analyzer (vario MACRO cube of Elementar), which worked with high-temperature combustion and gas separation method to achieve precise quantification of elemental composition. The chemical states of C, O and N species were characterized by XPS on ESCALAB 250 using monochromatic Al Kα radiation, and all spectra were calibrated to the binding energy of C=C bonds (284.6 eV).
XRD measurements and analyses. XRD patterns of the graphene films were collected by a XRD system (Rigaku, SmartLab D/teX Ultra 250 using Cu Kα radiation).
The d-spacing (d) of graphene films was estimated by the Bragg's equation based on the XRD (002) peak: where λ is the X-ray wavelength (0.15406 nm), and (rad) is the scattering angle.
To quantify the degree of ordering of graphene films, P disorder (p), was estimated using Bacon's equation [42]: To make a comparison with the L c derived from ADF-STEM observations, Scherrer equation was also used to estimte L c [34,35]: where is a constant (here, 0.89), (nm) is the X-ray wavelength, (rad) is the scattering angle, and is the full width at half maximum (FWHM) of the (002) peak without eliminating the influence of instrumental broadening.

Raman measurements for defects and stacking order analyses. Raman spectra
were acquired on LabRAM HR800 (laser wavelength  L = 532 nm). The laser spot was ~1 μm and the laser power on the sample surface was kept below 15 mW to avoid heating effect and structural damages. Raman mapping was performed with a step of 1 μm.
We performed Lorentzian fitting on the 2D peaks to evaluate the fraction of AB-stacking based on the intensity ratio of 2D 2 /(2D 2 + 2D T ) [28,29]. The proportion of turbostratic-stacked regions in large-area GO-and N-rGO-derived graphene films was analyzed from the Raman mappings of intensity ratio of 2D 2 /(2D 2 + 2D T ), which contain 5751 spectra for each sample. The intensity ratio of 2D 2 /(2D 2 + 2D T ) below 0.99 was considered as turbostratic-stacked regions.
Thermal stability analyses. Thermogravimetric-differential thermal analysis    * refers to the graphene films made by N-rGO sheets with 10.3 wt% nitrogen. # refers to the graphene films made by N-rGO sheets with 9.3 wt% nitrogen. § L c was estimated by Scherrer equation for comparison. L c , L a and the fraction of AB stacking were determined by ADF-STEM, SEM-ECC and Raman spectroscopy, respectively, as discussed in the manuscript.