Lijian Xuab,
Wenqian Zhangab,
Ledong Wangc,
Jie Xueab and
Shifeng Hou*ab
aSchool of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong 250100, P. R. China
bNational Engineering Research Center for Colloidal Materials, Shandong University, Jinan, Shandong 250100, P. R. China
cSchool of Physics, Shandong University, Jinan, Shandong 250100, P. R. China
First published on 11th October 2021
In this work, a large-scale preparation of graphene oxide (GO) film is reported, and the structure and the compositional variation was studied after thermal annealing. The electromagnetic interference (EMI) shielding performance of thermally reduced GO films was also investigated. Commercial GO clay was well dispersed by high-speed shearing and formed a stable slurry with a high solid content in water (5%), and this was chosen rather than organic solvent due to its optimal performance in coating procedures and film quality. The optimized thermal annealing procedure resulted in a significant enhancement of electric conductivity and EMI shielding efficiency, which reached 500 S cm−1 and 32–42 dB with the thickness under 0.1 mm. The excellent EMI shielding efficiency of thermally reduced GO film, as well as the easily amplified pilot manufactoring procedure adaptive to commercial equipment, produce graphene for universal EMI shielding materials.
Graphene has been reported as a promising and effective EMI shielding material because of its excellent electrical properties.12 To date, graphene materials with various nano/microstructures or shapes have been built to achieve lightweight and high-performance EMI shielding, including aligned graphene,13 three-dimensional (3D) graphene,14 graphene sponges,15,16 and/or their polymer composites.17–22 Most previous studies have focused on the enhancement of EMI shielding efficiency (SE) on a laboratory scale. However, there have been few works describing large-scale fabrication (pilot scale or above) of graphene materials, which impedes their industrial application.
Herein, a large-scale fabrication of a graphene oxide film coating on polymer matrix by a pilot coating machine is reported using high solid content graphene oxide clay manufactured by a modified Hummer's method. The formed stable GO slurry exhibited a more compact film structure and long storage stability. The engineering parameters were preliminarily optimized, and a satisfying batch product was obtained with a variable thickness of 10–150 μm, which can meet the needs of many scenarios. The thermally reduced GO (trGO-1000) film exhibited high conductivity of 500 S cm−1 and excellent EMI SE of 45–54 dB (8.2–12.4 GHz, the X-band) with a sample thickness under 0.1 mm. Therefore, the high EMI SE was obtained by an easily amplified annealing process, and together with the pilot fabrication of GO film, can produce graphene with broad prospects for industrial applications.
R = |S11|2 | (1) |
T = |S21|2 | (2) |
A = 1 − R − T | (3) |
SET (dB) = −10log(T) | (4) |
SER (dB) = −10log(1 − R) | (5) |
(6) |
The raw graphene oxide material used in this work was produced by LeaderNano Co., Ltd. in batches. It exhibited satisfactory solubility in water and other hydrophilic solvents because of abundant oxygenic groups on the GO sheet, such as hydroxy and carboxy, which were achieved by a modified Hummer's method. The typical size of the GO sheet was approximately 2 μm, and it was semi-transparent at high magnification as a representation of a single-layer material. Slight folding occurred, which was due to the evaporation of water during the TEM sample preparation, and the intrinsic morphology of the graphene sheets should appear as fully stretched after a long period of high-speed shearing.
For the pilot coating procedure, the amount of solids in the slurry is a key to the film formation because of the conservation of mass. The ratio of the GO film thickness to slurry thickness was nearly equal to the solid content of the slurry. From one-pot moulding to final application, a higher solid content should be more optimal. Viscosity is an important process parameter associated with solid content, and it demonstrates the interactions between solvent and solute. The viscosities of a GO slurry made with water, ethanol, or DMF were investigated by a rotating viscosity tester, and the results are shown in Fig. 2a.
The results clearly differed according to solvent, where the viscosity of the water-based slurry was much higher than that of the organic solvent-based slurries with the same amount of solid. The viscosity of the water-based slurry exponentially increased with a coefficient of approximately 1 by increasing the amount of solid. The slurry showed a viscosity of 7300 mPa s@3.5 wt% and 650000 mPa s@4.5 wt%. The organic solvent-based slurries have very low viscosity (under 300 mPa s@5.0 wt%) and little variation among the testing range relative to the water-based slurry. This may be attributed to the high hydrogen bond density in water. These intermolecular interactions enhanced the attraction between GO sheets and work against the shearing deformation.
Hydrogen bonds are a weak interaction, and they are easily broken by strong external shearing force and are rebuilt when the situation returns to stability. All types of slurries showed non-Newtonian fluid characteristics similar to those of polymer. The viscosity curves of organic solvent-based slurry varied with the rotation speed, and are shown as Fig. 2b. The organic solvent-based slurries both contained 5.0% solid content, while their apparent viscosities were approximately 100 mPa s@60 rpm. When the rotation speed was reduced to 3 rpm, the apparent viscosities increased to approximately 2000 mPa s. This rheological feature of a GO slurry limits the coating process because the heading speed of the matrix was in a fixed interval. In fact, PET was chosen as a matrix because of its robust mechanical variation, and Al foil and Cu foil failed in a pre-coating test as a preparation for a high-viscosity slurry. As a compromise to the pilot equipment, the requirement of the slurry viscosity was set as 10000 mPa s, which resulted in a stable thick slurry coating on the PET matrix.
The film-forming ability is the main indicator used to test the adequacy of a coating slurry. All the different types of slurries had finished the pre-coating test conducted on a small auto-coating machine (MRX-TMH250, Mingruixiang Automation Equipment), and the thickness of the slurry coating was set as 1 mm on the PET matrix. The amount of solid in the organic solvent-based slurry was adjusted to a proper viscosity that would form a stable slurry coating. The coated samples were transferred to an oven maintained at 40 °C until fully dried.
The appearances of the slurries and films are shown in Fig. 2c–e. The gross view of the GO films reveals no significant difference in surface flatness or roughness. The static appearances of different fresh GO slurries were nearly the same, but for long-term storage, the precipitation of the GO sheets became obvious in the organic solvent-based slurries. However, this did not occur with the water-based slurry.
SEM images show that the film composed of a water-based slurry was approximately 50 μm in thickness, which is similar to the theoretical value, and exhibited a compact sectional structure without cavities or defects. There were a few defects observed on the film surface, as shown in Fig. 3, and the magnified image (Fig. 3a and d) shows a smooth GO sheet with some small wrinkles. The GO sheets were well spread in water by their satisfactory hydrophilicity, and a tight arrangement was obtained due to the tension caused by evaporation of interlayer water. For films made of an organic solvent-based slurry, additional defects such as holes and upwarp were observed on the film surface (Fig. 3b and c) and section (Fig. 3e and f), which may be due to the incomplete extension or curling of the GO sheets. Additionally, the lower surface tension and rapid evaporation of the organic solvent may also contribute.
Fig. 3 (a, b, and c) Top view and (d, e and f) section view of GO films made by water slurry (a and d), ethanol slurry (b and d), and DMF slurry (c and f). |
Using water as a solvent is suitable for both the slurry-coating process and material storage, and also does not contribute to environmental pollution caused by organic solvent vapor. Therefore, the GO films used below are all made from a water-based slurry (1 cm GO slurry coating with 5% solid content) with a thickness of 50 μm and without any other modifications.
The sectional morphology of a preheated sample is shown as Fig. 4b and c. The total film structure was relatively intact, with small interlayer cracking that occurred near the film surface, and the surface sheets curled off the films. A magnified image of the cracking zone shows that most stacked sheets were not totally separated, but formed small tunnels with a diameter of nanometers that can act as an exit for interlayer water molecules. The separated sheets in Fig. 4c exhibit small wrinkles, which indicates that the preheating temperature was near the critical zone, and the film would expand under higher treatment temperature. The optimal length and temperature of this period are open to debate, but from the results of subsequent experiments, our scheme is feasible. After the pre-heating, the GO film samples were cut into small sizes pieces for further annealing processes.
The TGA curve shows the variation in the weight of samples during the preparation of trGO-800 made with fresh pilot product, as illustrated in Fig. 4d. The first stage for 1 hour stabilization at 125 °C was set to smooth any possible tension caused by the thermal history (during the drying section) and remove the excess interlayer water in accordance with the previously mentioned procedure. The sample's weight clearly decreased during this stage and showed a weight loss of 13%, which approximated the results shown in Fig. 4a. During the second stage, the temperature was increased to 250 °C by 5 °C per minute, and was maintained for 2 hours. The sample lost more than 30% of its initial weight during this stage, and showed a dramatic decrease above 200 °C, which was mostly due to the complete removal of the water contained as interlayer water and bound water.
The cracking of the oxygenic group on the graphene sheets also contributes, as the elemental analysis results show in Table 1. After stabilization at 250 °C, a fixed period (4 hours) elapsed with the goal of reaching the peak temperature. This was maintained for 2 hours so that the samples would be fully annealed, and this exhibited a smooth weight loss with a linear relationship with time as opposed to the decomposition of carboxyl on the interior GO sheets.
The gross view of the samples before and after thermal treatment is shown as Fig. 4g. The as-prepared GO film (1st from right) was pre-heated for 24 hours@125 °C (2nd from right), and exhibits an approximate appearance with no obvious bending or curling. The trGO-250 (3rd from right) exhibits a grey color and graphite shine, which indicates that graphitization of the surface sheets of the film had occurred. Some bending at the ribbon edges and some small humps were found on the surfaces, and were possibly caused by the asymmetrical internal stress that arose from group decomposition. The trGO-800 (4th from right) exhibited additional humps and bending among the ribbon, and the shine and texture was the same as that of trGO-250, which demonstrated that the completeness of graphitization cannot be determined by the appearance of the annealed films.
SEM images of trGO-250 and trGO-800 are shown in Fig. 4. The total section views of the sample show obvious differences around the interlayer gap. The section of trGO-250 (Fig. 4e and f) exhibits the impact stacking structure and shows a more parallel gap uniformly distributed among the layers. The magnified image of section part (Fig. 4f) shows more wrinkles on the sheets as compared to the preheated sample (Fig. 4c), and the trace of a tunnel could be observed and connected to form the interlayer gap. For the trGO-800 (Fig. 4h and i), the stacking structure was nearly destroyed by the decomposition of the sheet, and showed a foam-like appearance with irregular holes inside the film. The surface layers of trGO-800 (Fig. 4h) show small fluctuations spread all over the plane, and crevices between single sheets were also found on the surface, which depicts the separation of the stacked sheets. The magnified image of trGO-800 (Fig. 4i) shows additional wrinkles on sheets and larger gaps between the layers, which is coincident with the thermal expansion of graphene material. The expansion of the film resulted in an increase in film thickness (50 μm to 65 μm by trGO-800), which is larger than that of the sample treated at lower temperature. The samples treated at higher temperature exhibited the same micro-morphology as that of trGO-800. For large-scale preparation of the trGO film, the irregular transformation of trGO film relative to flat GO film would hamper its application. A few trials were performed to solve this problem, but there has not been much progress.
FTIR was employed to study the variation in the chemical groups on the GO sheets during the annealing process, and the results are shown as Fig. 5a. The spectra of unannealed GO film exhibited several peaks of typical oxygenic groups on the GO sheets, such as –OH (3446 cm−1), –COOH (1630 cm−1), and –C–O–C– (1104 cm−1). After the thermal annealing process, all the trGO samples underwent considerable weight loss by the decomposition of oxygenic group, which actually was the carboxy, because the peak located at wavenumber 1630 cm−1 obviously decreased, while there was little change in the broad peak at 3440 cm−1. As a consequence of decarboxylation, C–H bending was observed at 1385 cm−1 on the spectra of trGO samples, and the peak at 2925 cm−1 represents the formation of a methylene structure.24 With the increase in annealing temperature, the –OH became decomposed, and the –C–O–C was generated as another type of deoxidization. For trGO-500, the C–H content was obviously higher than that of other trGO samples, which was supposed to be intermediate amorphous carbon unstably attached to the graphene sheets. This peak decreased with the increase in annealing temperature due to the instability of non-aromatic carbon. It was concluded that the annealing process in our work removed most oxygenic groups but could not reduce the oxygen content to zero, and these data are also compatible with the results of the element analysis.
To investigate the effect on the integrity of the sheets during the annealing process, a Raman spectrometer was applied to detect the defects in the trGO sheets (Fig. 5b). The unannealed GO film exhibited a typical feature consisting of a double-peak spectra with a D band at 1350 cm−1 and a G band at 1590 cm−1. The oxidation of graphite caused structural defects and a disordered configuration expressed as the D band, and the G band represented the entire graphitic structure. The annealing process in our work resulted in the removal of additional oxygenic groups, but it was incapable of rebuilding the graphitic structure, which should be performed above 2800 °C. The intensity ratio of the D band to G band was similar (ID:IG = 1.19 by raw GO, and 1.13 by trGO-800), which showcased the amount of ordered sp2 and disordered sp3 carbon domain changes that occurred as a result of the decomposition of carboxyl groups added to the graphene sheets.
Fig. 5c displays the XRD pattern of unannealed GO film and trGO films treated under different peak temperatures. A single strong peak located at 2θ = 12.2° of the unannealed GO and no peak at 2θ = 26.5° (002 of graphite) proved that the sheets in the raw graphene oxide clay were fully separated by the oxidation of graphite. The gap between the graphene oxide sheets did not decrease during the slurry preparation and coating process. After thermal annealing, the XRD pattens of the trGO samples showed a significant shift near 2θ = 26.5°, which was caused by the decrease in the interlayer space following the decomposition of the oxygenic groups on the graphene sheets. The trGO-500 exhibited a peak located at 2θ = 25.2° and for trGO-600, at 2θ = 26.0° while the peaks of trGO-700 and trGO-800 were both located at 2θ = 26.5°. These data also indicate that the additional group obtained by graphite oxidation could be mostly removed by the annealing process above 700 °C, which is coincident with the results above.
In the XPS analysis of unannealed GO film and trGO-800, shown in Fig. 5d, which was used to ensure that chemical composition changes occurred during the annealing process, the relative intensity of O 1s (approximately 532 eV) to C 1s (approximately 285 eV) decreased after the annealing process. This clearly implies that thermal reduction occurred during the annealing process, and some oxygenic groups were removed. According to the FTIR results and previous studies, it was confirmed that there are different carbon components, including C–C/CC (approximately 285.0 eV), C–OR (approximately 285.4 eV), CO (approximately 287.4 eV), O–CO (approximately 288.9 eV), and π–π* (291.7 eV). The XPS spectra of the C 1s region of unannealed GO film (Fig. 5e) and trGO-800 (Fig. 5f) illustrate this more intuitively. The spectra show that the CO content of trGO-800 was greatly decreased compared to raw GO, which indicates that decarboxylation occurred.
The EMI shielding performance was investigated by testing the samples with a vector network analyzer at a frequency range of 8.2–12.4 GHz, as represented in Fig. 6. All samples exhibited a mild floating electromagnetic shielding performance among the X-band, and the appearance of the EMI SE curves was the same, with several peaks above the satisfying baseline. The maximum EMI SE of each sample appears with frequencies of 9.3 GHz and 9.8 GHz. With the increase in the annealing temperature, the EMI SE is dramatically increased, which illustrates the significant relativity between conductivity and EMI SE.
Fig. 6 (a) The total EMI shielding performance of trGO samples and raw GO film; (b) SET, SEA and SER of trGO samples. |
As the treatment temperature increased above 500 °C, the EMI SE of the thermally reduced graphene films exceeded the commercial requirement standard value of 20 dB. The maximum EMI SE of trGO-1000 is as high as 54 dB@9.8 GHz, which is superior to values in the reported literature. Further investigation revealed an obvious tendency of variation in each SE component, as shown in Fig. 6b. As observed in Fig. 4, the trGO sample did not undergo a large increase in thickness from the initial GO film, but many cavities were formed among the layered structures that could increase the SEA by internal multiple reflections.
The sheets of trGO-800 were fully separated by the thermal treatment, which indicates that additional interfaces were created to enlarge the means by which multiple reflections occur. Additionally, the increased conductivity resulted in a greater mismatch of microwave impedance between material and air, and also resulted in interface reflection, which may explain the enhancement of SER for the trGO samples.25,26
The most optimal annealing temperature for samples with 50 μm thickness was 1000 °C in our work. The trGO-1000 exhibited electrical conductivity as 500 S cm−1 EMI shielding efficiency above 45 dB among the X-band and a maximum of 54.3 dB@9.8 GHz. For the excellent performances and operability of the batch product, the trGO film reveals great potential in high efficiency EMI shield applications.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra06070h |
This journal is © The Royal Society of Chemistry 2021 |