Modifying the electrochemical performance of vertically-oriented few-layered graphene through rotary plasma processing

Jinghuang Lin , Henan Jia , Yifei Cai , Shulin Chen , Haoyan Liang , Xu Wang , Fu Zhang , Junlei Qi *, Jian Cao , Jicai Feng and Wei-dong Fei *
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China. E-mail: jlqi@hit.edu.cn; wdfei@hit.edu.cn; Fax: +86-451-86418146; Tel: +86-451-86418146

Received 24th July 2017 , Accepted 3rd November 2017

First published on 3rd November 2017


Vertically-oriented few-layered graphene (VFG) has a unique three-dimensional morphology and exposed ultrathin edges, and shows great promise for high-performance supercapacitor applications. However, VFG shows limited capacitance owing to poor wettability with electrolytes, which has been a bottleneck for further applications. Herein, we designed and developed an effective strategy, based on rotary plasma etching, to create defects on the side surfaces while simultaneously maintaining the structural integrity of VFG. Rotary plasma etching decreased the contact angle (CA) of VFG from 123° to 34°, compared with conventional vertical etching, which only reduced the CA to 71°. Electrochemical studies demonstrated that VFG samples with a high density of surface defects, introduced by rotary plasma etching, exhibited a high areal capacitance of 1367 μF cm−2 (a volumetric specific capacitance of 137 F cm−3), which was approximately 4 times as large as for pristine VFG-based materials. Our study offers feasible insight into the use of an industrially viable method, rotary plasma processing, for modifying and enhancing the properties of VFG. Our findings may help to accelerate the development of more effective energy storage devices.


1. Introduction

Electrical double-layer capacitors (EDLCs) show great promise as energy storage devices owing to their high-power capabilities, rapid charge–discharge rates, and long lifetimes.1–3 Although EDLCs have high specific power, these devices often suffer from a relatively low energy density, which greatly restricts their practical applications as independent devices.4–6 Since the storage mechanism primarily involves surface storage, the energy densities of EDLCs are highly dependent on the specific surface area of the active material.7–9 Ideal electrode materials for EDLCs should have a high specific surface area to enhance the active area, together with high conductivity to ensure efficient electron and ion transport efficiency for charge storage.10–12 Among various carbonaceous nanomaterials, graphene has been widely studied as a promising electrode material, owing to its high conductivity and high specific surface area.13 However, the practical performance of graphene-based supercapacitors is greatly limited for the following reasons: (i) limited ion-accessible surface area owing to inevitable π–π aggregation and the restacking of graphene sheets; (ii) limited accessible surface area and transport pathways for electrolyte ions owing to the randomly oriented and parallel graphene structure; and (iii) high contact resistance between graphene and the current collectors.13–16

Recently, strategies have been developed to improve the capacitance of graphene-based supercapacitors by assembling three-dimensional (3D) structures.17–19 For example, Hu et al. reported that 3D graphene-like carbon avoided restacking and gave a higher effective graphene surface area, which provided sufficient space for charge storage, leading to excellent supercapacitor performance.19 Furthermore, Yonn et al. reported vertically aligned reduced graphene oxide (rGO) electrodes produced from cutting a rGO sheet roll, which enabled the full use of the graphene surface area.17 Ajayan et al. reported that rGO vertically-oriented to the current collectors displayed a high capacitance of 394 μF cm−2 and fast electrolyte ion diffusion compared with the performance of rGO-based stack devices (140 μF cm−2).18 Especially, Miller et al. demonstrated that vertically-oriented few-layer graphene (VFG) grown on Ni exhibited capacitance values as high as 50–70 μF cm−2 (∼3 F cm−3), ultrafast frequency response, and ultralow contact resistance.20,21 In this respect, VFG is a promising candidate to act as electrode material owing to its good crystallinity, high electronic conductivity, and unique 3D open edge structure with a large surface area.20–24 However, the practical performance of VFG remains far below its potential, which currently prevents practical applications.22–28 Bo et al. reported that the areal capacitance of VFG does not increase linearly as the height of VFG increases, suggesting that the availability of surface area almost remains fixed.25 Miller et al. also demonstrated that the specific capacitance of VFG significantly deceased as its height was increased, suggesting that edges and defect structures in VFG play a dominant role in its capacitance behavior.26 The capacitance of VFG is strongly related to the ion-accessible surface area, according to the energy storage mechanism of EDLCs.1–3 Unfortunately, VFG exhibits very poor wettability with water, because of the hydrophobic nature of pristine graphene and its 3D structure.29–32 Consequently, only the top surface area of VFG is accessible to ionic species in the aqueous electrolyte, resulting in relatively low capacitance. Thus, the full potential of VFG has yet to be fully utilized, and the effective capacitance of VFG-based devices is limited. Therefore, it is necessary to modify the wettability of VFG and enhance the effective surface area in order to unlock the full potential of such devices.

Currently, hydrophobic surfaces of pristine graphene can be made more hydrophilic through various modifications, such as chemical treatment and defect engineering.33–37 For example, Dong et al. achieved the tunable wettability of graphene surfaces through chemical treatment, which resulted in a decrease in the contact angle from 132.9° to 53.1°.33 Although chemical treatment can considerably enhance the wettability of graphene, such treatment may also introduce a large number of oxygen-doped defects or oxygen functional groups, which can impair the electrochemical performance of graphene.35 Furthermore, it is difficult to achieve fine control over chemical treatment processes, which have the potential to destroy the overall graphene structure. On the other hand, the defect engineering of graphene has received considerable attention, owing to its beneficial effects on the fundamental properties of graphene without compromising the excellent conductivity.36,37 Defects on the surface of graphene can improve the wettability, resulting in a transition from hydrophobic to hydrophilic surface characteristics.36,37 An increased defect density in graphene can change the density of states near the Fermi level, which can also improve the measured capacitance and energy density of graphene.38 Plasma etching has shown great potential for use in introducing defects into graphene, because of its scalability potential for mass-production and the shorter reaction time.38–41 Bandaru et al. reported that intentionally introducing charged defects onto the outer surface of planar graphene through plasma etching contributed to an increase in the capacitance from 1.9 to 4.7 μF cm−2.40 However, owing to the unique vertical nanosheet structure of VFG, it remains difficult to form defects on the outer surfaces of VFG using conventional plasma etching. Previous research has indicated that many defects are formed on the top of VFG via normal plasma etching (i.e., vertical plasma etching) because the direction of the plasma etching is parallel to the graphene layers in VFG.39–41 Plasma etching for a sufficiently long time can modify the wettability of VFG, but will also severely etch and damage the VFG, destroying its structure.41 Hence, a more practical approach is needed to introduce defects in a controlled fashion to the bilateral outer surfaces of graphene in VFG to improve the wettability and enhance the effective surface area, while maintaining the structural integrity.

Here, we strategically created defects on the side surfaces of VFG via defect engineering, using rotary plasma etching. The main features of the rotary plasma etching approach are shown in Fig. 1. Rotary plasma etching introduces defects on the side surfaces of VFG, while maintaining structural integrity. This process can simultaneously improve the wettability with aqueous electrolytes and enhance the electrochemical performance. Furthermore, rotary plasma etching also induces more electrochemically active defects on the side surfaces of VFG, which can contribute to the capacitance. The motivation of this work was to improve the wettability of VFG with electrolytes, in order to increase the maximum ion-accessible surface area without degrading the whole structure of VFG.


image file: c7ta06490j-f1.tif
Fig. 1 Schematic illustration of the Ar rotary plasma processing of VFG, which was used to purposefully create surface defects, thus resulting in enhanced wettability.

2. Results and discussion

Fig. 1 schematically illustrates the bilateral outer surface of the graphene nanosheets in VFG treated with Ar plasma etching with rotary processing. The purpose of this treatment was to create surface defects for enhancing wettability with aqueous electrolytes. First, VFG was synthesized on Pt/SiO2 substrates through plasma-enhanced chemical vapor deposition (PECVD). Subsequently, the as-grown VFG (referred to hereafter as “pristine”) was subjected to Ar-based rotary plasma processing by rotating the base substrate to intentionally induce surface defects on the VFG sheet. Further, we also conducted several experiments to optimize the etching angle (5°, 10°, 15°, 20° and 30°) and etching time (15 s, 30 s, 1 min and 3 min) (detailed experimental data and discussion are shown in Tables S1, S2 and Fig. S3). As a result, it can be found that the optimized etching angle is 15°. To investigate the different effects of vertical etching and rotary etching, we conducted the following systematic experiments. We controlled the duration and rotation angle used in the Ar-based rotary plasma process to fabricate five types of VFG sample: (i) pristine VFG; (ii) VFG after normal vertical etching, i.e., where the plasma etching direction was parallel to the graphene layers of VFG, for 1 min (VFG-V1); (iii) VFG after normal vertical etching for 3 min (VFG-V3); (iv) VFG after single-side etching for 0.5 min, rotating the substrate at 15°, i.e., the direction of plasma etching and the graphene layers of VFG formed a 15° angle (VFG-R0.5); and (v) etching one side of VFG for 0.5 min, followed by etching the other side for 0.5 min, rotating the substrate 15° (VFG-R0.5@0.5). Full details of the experiment are outlined in the Experimental section.

As shown in Fig. 2a, the dense and uniform structure of VFG, before Ar plasma etching, was observed to grow vertically over the entire Pt/SiO2 substrate. The top edges of VFG are shown in the side-view SEM image in the inset of Fig. 2a. A small number of wrinkles and ripples were observed on the surface of VFG. Such open intersheet channels are beneficial for capacitive properties.22–24 Furthermore, the edges of VFG were sharp and transparent, suggesting that these nanosheets were very thin. We then conducted normal vertical plasma etching of VFG, as illustrated in Fig. 2b. After 1 min of vertical etching, the overall morphology and structure of VFG-V1 showed no obviously change, as shown in Fig. 2c. It can be inferred that conducting a vertical etching process for a short time has a negligible effect on the morphology of VFG. However, after prolonged plasma treatment (3 min), the morphology and structure of VFG-V3 showed more noticeable changes, as shown in Fig. 2d. The height of VFG-V3 was much less than that of VFG, indicating severe etching, which also caused the disappearance of the sharp upright edges. The majority of the edges of VFG-V3 were strongly etched by the Ar plasma (see the inset of Fig. 2d). The extended vertical etching process destroyed the VFG morphology and the vertical structure of VFG, resulting in a dramatic reduction of the ion-accessible surface area. Therefore, vertical etching processes cannot be used for an extended period of time to modify the surface of VFG without compromising the structural integrity.


image file: c7ta06490j-f2.tif
Fig. 2 Schematic illustrations of (b) the vertical etching and (e) rotary etching processes. Top-view SEM images of (a) VFG, (c) VFG-V1 and (d) VFG-V3. (f and g) Top-view SEM images of (f) VFG-R0.5 and (g) VFG-R0.5@0.5. The insets show corresponding side-view SEM images.

To realize the full potential of the graphene surfaces of VFG, we also applied a rotary Ar etching process, as shown in Fig. 2e. Under these conditions, the direction of plasma etching and the graphene layers of VFG formed a 15° angle. The vertical structure of VFG-R0.5 was retained after a 0.5 min rotary plasma etching step (see Fig. 2f), suggesting that the overall morphology and structure of the graphene sheets in VFG sustained minimal etching damage. Further, it can be found that graphene nanosheets were partly etched off and the edges of VFG-R0.5 became sharper, as shown in the inset of Fig. 2f. Fig. 2g shows the morphology of VFG after 0.5 min etching on each side (1 min total), rotating the substrate at 15° (i.e., VFG-R0.5@0.5). As shown in Fig. 2g, the VFG-R0.5@0.5 sample maintained the overall morphology and structure of VFG without obvious damage. Thus, in terms of preserving the overall structure of VFG, rotary plasma etching is a practical approach for surface modification.

Transmission electron microscope (TEM) studies were performed to further characterize the effects of plasma processing on the morphology and structure of VFG. As shown in Fig. 3a1, the average height of the pristine VFG was around 100 nm. VFG with a nano-textured surface possesses well-connected electron transport paths and a high specific area, which are beneficial in terms of energy storage.22–24 The TEM image in Fig. 3a2 shows that the pristine VFG featured smooth and sharp edges. After 1 min of vertical etching, the height of VFG-V1 decreased to approximately 85 nm, as shown in Fig. 3b1. The sharp and folded edges of VFG-V1 were also etched by the Ar plasma to some degree (see Fig. 3b2). This result suggests that vertical plasma etching affected mainly the top surface of VFG. In the case of 3 min of vertical etching, the height of VFG-V3 decreased to approximately 57 nm, as shown in Fig. 3c1. The VFG-V3 sample was severely damaged and the upright sharp edges disappeared, as shown in Fig. 3c2. Thus, a vertical etching process for an excessively long time destroyed the unique vertical-standing structure of VFG, leaving only thick roots remaining.


image file: c7ta06490j-f3.tif
Fig. 3 TEM and HRTEM images of (a) VFG, (b) VFG-V1, (c) VFG-V3, (d) VFG-R0.5, and (e) VFG-R0.5@0.5.

In the case of rotary etching, the heights of the VFG-R0.5 and VFG-R0.5@0.5 samples showed no obvious change compared with pristine VFG (see Fig. a1, d1 and e1). The VFG-R0.5 and VFG-R0.5@0.5 samples maintained sharp and transparent edges, as shown in Fig. 3d2 and e2. Our SEM and TEM analysis indicated that the structural integrity of the VFG-R0.5 and VFG-R0.5@0.5 samples was preserved in the treatment and suggested that rotary Ar plasma etching can maintain the overall morphology and structure of VFG. Since rotary plasma etching, as a surface treating method, carries out an etching role on the surface of graphene nanosheets, it can be used to controllably introduce defects on both sides of VFG, which may enhance the electrochemical performance of the obtained VFG.

Raman spectroscopy studies were conducted to provide additional structural information about the defect density in the obtained VFG samples. Fig. 4a shows typical Raman spectra with the three characteristic peaks of VFG, named D, G and 2D. The D peak at ∼1350 cm−1 indicates the introduction of defects or imperfections into the graphitic domain.42,43 The G-peak at ∼1600 cm−1 indicates that the graphitic domain is well maintained.42,43 The 2D-peak at ∼2690 cm−1 is attributed to three-dimensional interplanar stacking of the hexagonal carbon networks.42,43 In general, the ratio of ID/IG reflects the defect level in the graphitic structures of carbonaceous materials.43 The pristine VFG had an ID/IG ratio as high as 1.39, as shown in Table 1. Plasma etching is an effective method for intentionally introducing surface defects into graphene. To modify the defects in VFG, normal vertical plasma etching was conducted to achieve different defect levels. The ID/IG ratio was 1.92 for VFG-V1, suggesting that vertical plasma etching introduced more defects on the surface of VFG. Vertical plasma etching for a longer time slightly increased the number of defects, giving an ID/IG ratio of 2.10 for VFG-V3 (Table 1); however, the overall morphology and unique vertical structure of VFG were seriously damaged in this sample. In the case of rotary etching, the ID/IG ratio for VFG-R0.5@0.5 was 1.85, which was higher than that for the pristine VFG. This result indicates that rotary plasma etching increased the number of defects on the surface of VFG.


image file: c7ta06490j-f4.tif
Fig. 4 (a) Raman spectra, (b) XPS spectra and (c) photographs of water droplets and corresponding water contact angle measurements for VFG, VFG-V1, VFG-V3, VFG-R0.5 and VFG-R0.5@0.5.
Table 1 The ID/IG ratio, estimated ratio of C sp2 and C sp3, contact angle and capacitance values for VFG, VFG-V1, VFG-V3, VFG-R0.5 and VFG-R0.5@0.5
Sample
VFG VFG-1 VFG-V3 VFG-R0.5 VFG-R0.5@0.5
I D/IG ratio 1.39 1.92 2.10 1.70 1.85
C sp2 86% 68% 52% 75% 69%
C sp3 14% 32% 48% 25% 31%
Contact angle (°) 123 71 28 57 34
Capacitance (μF cm−2) 312 589 414 714 1367


X-ray photoelectron spectroscopy (XPS) measurements can be used to detect defects in graphene through analyzing the C bonding state.44–47 Thus, we conducted XPS tests to better evaluate the degree of defects induced via the different plasma etching modes. As shown in Fig. S1, a clear C 1s peak was observed in all obtained samples. Only a small amount of oxygen (∼2%) was found, which is attributed to the formation of C–O and C[double bond, length as m-dash]O bonds through physical or chemical adsorption when the obtained samples were exposed to ambient conditions.47 The VFG samples were predominantly composed of carbon atoms, with a small amount of oxygen. The small amount of oxygen-doped defects and oxygen functional groups had little effect on the defects in VFG. Furthermore, the C 1 s spectra could be fitted by two peaks, corresponding to C sp2 at ∼284.6 eV, from the sp2-hybridized carbon bonding of graphene, and C sp3 at ∼285.4 eV, from the edge-termination of the graphene lattice or structural defects in graphene.45–47 To analyze the effects of plasma etching on structural changes in VFG, we calculated the relative proportions of C sp2 and C sp3 through determining the ratios of the C 1s peak areas in the XPS spectra, as shown in Table 1. As shown in Fig. 4b and Table 1, the pristine VFG featured a high fraction of C sp2, up to 86%, with a small amount of C sp3 (14%), indicating the high quality of the graphene nanosheets. For the vertically etched samples VFG-V1 and VFG-V3, the estimated proportions of C sp3 were 32% and 48% (see Fig. 4b), suggesting a much higher defect density than in pristine VFG. These results correspond well with the Raman results. In the case of the rotary etched samples, VFG-R0.5 and VFG-R0.5@0.5, the proportions of C sp3 were 25% and 31%, indicating that bilateral rotary plasma etching increased the overall number of defects. Hence, rotary plasma etching can be successfully used to modify the side surfaces without causing obvious damage to the overall morphology and structure of VFG.

On the basis of the above results, it can be obviously discovered that the surface structure and bonding state of the vertical graphene nanosheets in VFG can be modified via the two modes of plasma etching. To reveal the effects of the two modes of plasma etching on the wettability of VFG, we conducted contact angle measurements by placing a droplet of deionized water (2 μL) on the surface of the samples, as shown in Fig. 4c. In general, graphene with a planar structure shows a CA of 80–90°.29–32 VFG can be regarded as a large number of graphene sheet arrays arranged vertically to a substrate, which leads to an enhanced hydrophobic state. The pristine VFG had a contact angle (CA) of 123°. The natural hydrophobicity of the pristine VFG limits the full potential of VFG, particularly regarding the side surface area of VFG, which shows the disadvantages of a 3D structure in the formation of EDLCs. All the VFG samples changed from hydrophobic to hydrophilic after plasma etching. For VFG-V1, the CA decreased to 71°, which may be attributed to increased defects in VFG-V1. Owing to the disappearance of the 3D structure and the large amount of defects induced by the longer plasma etching time, the CA of VFG-V3 markedly reduced to 28°. Although 3 min of vertical etching achieved better wettability, the intense plasma etching also destroyed the morphology and structure of VFG-V3. We can infer that this serious damage would adversely affect the electrochemical performance of VFG-V3. As a result, when using conventional plasma etching, that is vertical plasma etching, it is difficult to keep the structural integrity and improved wettability of VFG at the same time. For rotary plasma etching, the CA of VFG-R0.5 decreased to 57°. According to Raman and XPS analysis, more defects were formed in VFG-V1 than in VFG-R0.5. However, VFG-R0.5 showed better wettability than VFG-V1. This result suggests that defects on the side surfaces of VFG play a more important role in modifying the wettability of VFG. Furthermore, the CA of VFG-R0.5@R0.5 was 34°. Thus, rotary plasma etching can achieve enhanced wettability while maintaining the structural integrity of VFG. The CAs of VFG-V1 and VFG-R0.5@R0.5 were 71° and 34°, indicating that defects on the side surfaces of VFG, rather than the top surface, are more favorable for improving the wettability. Objectively, it is more vital to form defects on the side surfaces of VFG in order to promote wettability. The markedly enhanced hydrophilicity can be attributed to the greater number of defects introduced on the VFG surfaces after rotary plasma etching. Therefore, we believe that rotary plasma etching can be used to effectively modify the wettability of VFG while maintaining its structural integrity.

The supercapacitive performances of the pristine VFG and VFG after plasma processing as binder-free electrodes were evaluated using a three-electrode cell configuration (see the Experimental section). Fig. 5a shows cyclic voltammetry (CV) curves for VFG, VFG-V1, VFG-V3, VFG-R0.5, and VFG-R0.5@R0.5 at a scan rate of 50 mV s−1 in 6 M KOH aqueous electrolyte. Fig. S2 shows CV curves for the obtained samples at scan rates from 50 mV s−1 to 2 mV s−1. The CV curves gradually enlarged as the scan rate was increased, while maintaining nearly rectangular shapes. These measurements confirmed the predominant EDLC behavior of the electrodes.15,24 Notably, the CV curves showed a delay in the current saturating, especially at high scan rates, which can be mainly attributed to the distributed capacitance of the 3D-structured electrodes.48,49 The VFG-R0.5@R0.5 electrode yielded the greatest current and showed much higher capacitance than the other VFG samples, as shown in Fig. 5a. A plot summarizing specific capacitance versus scan rate is shown in Fig. 5b. Because volumetric and areal specific capacitances are more important than mass-based values for real applications, here we calculated specific capacitance on the basis of volume and area for comparison (see Table 1 and Fig. 5b). For the vertically etched samples, the specific capacitance of VFG-V1 increased from 312 to 589 μF cm−2 at a scan rate of 2 mV s−1, which could be attributed to the improved wettability. Although the wettability of VFG-V3 was greatly enhanced, the specific capacitance of VFG-V3 at 2 mV s−1 was only slightly larger (414 μF cm−2) than that of pristine VFG (312 μF cm−2). This result can be attributed to the damage to the morphology and structure of VFG induced by the longer etching time. The damage to VFG not only impeded charge transport, but also reduced the specific surface area of the active material. Thus, only a small portion of graphene at the top of the pristine VFG participated in energy storage. In the pristine VFG, the low performance may be attributed to the poor wettability of the pristine VFG with the electrolyte. Enhanced wetting induced by defect formation enhanced the electrochemical performance of VFG, while guaranteeing the structural integrity of the graphene nanosheets in VFG.


image file: c7ta06490j-f5.tif
Fig. 5 (a) CV comparison of VFG, VFG-V1, VFG-V3, VFG-R0.5 and VFG-R0.5@0.5 in 6 M KOH at a voltage scan rate of 50 mV s−1. (b) Specific capacitances of VFG, VFG-V1, VFG-V3, VFG-R0.5 and VFG-R0.5@0.5 supercapacitors at various voltage scan rates. (c) Cycling performance of VFG-R0.5@0.5 during charge–discharge tests at 0.1 mA cm−2 over 5000 cycles. The inset shows the corresponding charge–discharge curves. (d) Nyquist plots for VFG, VFG-V1, VFG-V3, VFG-R0.5 and VFG-R0.5@0.5. The inset shows magnified portions of the Nyquist plots near the origin.

Following rotary etching, the specific capacitance of VFG-R0.5@R0.5 was as high as 1367 μF cm−2 at 2 mV s−1, while the values for the pristine VFG, VFG-V1, and VFG-R0.5 were 312, 589, and 714 μF cm−2, respectively. The specific capacitance of VFG-R0.5 was clearly higher than that of VFG-V1. The VFG-R0.5@R0.5 sample had the highest capacitance among the samples, approximately 2 and 4 times as large as the VFG-V1 and pristine VFG-based electrode systems, respectively. Notably, the highest specific capacitance of the VFG-based supercapacitor was calculated to be 137 F cm−3, since the height of the VFG was ∼100 nm, which was the high performance value in the current work.20,25,50–52 Table S3 compares the capacitance values of various carbonaceous nanomaterials using different techniques. It can be concluded that the capacitance of VFG-R0.5@R0.5 is competitive among these carbonaceous nanomaterials. These findings suggest that Ar rotary plasma processing can enhance the electrochemical performance of VFG electrodes without compromising their physical properties. Comparing the specific capacitance values of VFG-V1 and VFG-R0.5@R0.5, we found that defects on the side surfaces of VFG contributed more to the enhancement of specific capacitance. The introduction of defects onto the side surfaces of VFG is a useful approach to modifying its wettability and enhancing the effective specific surface area, resulting in higher specific capacitance. Hence, rotary plasma etching shows clear advantages for enhancing the electrochemical performance of VFG.

Cycling performance is another key parameter for supercapacitors used in practical applications, and we performed charge–discharge tests at a current density of 0.1 mA cm−2. Fig. 5c shows that the VFG-R0.5@R0.5 based electrode retained more than 90% of its initial capacitance after 5000 cycles, demonstrating that the VFG-R0.5@R0.5 sample has reasonable electrochemical stability and cycle reversibility. As shown in the inset of Fig. 5c, for every charge–discharge cycle the curve remained close to linear, indicating good EDLC performance.26 Electrochemical impedance spectroscopy (EIS) studies of the obtained samples over a frequency range from 100 kHz to 0.1 Hz were conducted to further confirm the influence of the plasma etching mode on supercapacitor performance. As shown in Fig. 5d, all the obtained samples exhibited a near-vertical Nyquist line in the low frequency region, indicating the easy diffusion of ions and good double-layer capacitance behavior.53 The equivalent series resistance (ESR), which is related to both the electrical resistance of the electrodes and the ion diffusion resistance in the electrodes, can be calculated from the intercept of the corresponding Nyquist plots with the real Z-axis.54,55 All the supercapacitors exhibited an ESR lower than 2 Ω, suggesting that Pt is an appropriate current collector for these active materials. Furthermore, all the VFG samples were synthesized in situ on Pt without organic binders. Thus, the VFG samples had good electrical contact with the current collector, leading to rapid charge transport from VFG to the current collector and low contact resistance.

On the basis of these results, the superior supercapacitor performance of the VFG-R0.5@R0.5 sample can be attributed to the following reasons. First, VFG-R0.5@R0.5 was synthesized directly on Pt/Si without organic binders. Thus, the VFG has good electrical contact with the current collector, leading to rapid charge transport from VFG to the current collector. Second, the favorable wettability of VFG-R0.5@R0.5 increased electrode–electrolyte interactions and allowed full use of the VFG surfaces, resulting in a considerable enhancement of the specific capacitance. Third, the unique vertical-standing structure of VFG-R0.5@R0.5, with more electroactive sites induced via rotary plasma etching, provided a large contact area with the electrolyte, resulting in fast and easy access for electrolyte ions and low diffusion resistance for the electrolyte. Consequently, VFG subjected to rotary plasma etching featured better wettability and an integrated structure, which makes it an attractive candidate material for use in ultrathin supercapacitor electrodes.

In order to better analyze the correlation between wettability and capacitance, we have conducted XPS analysis of the C 1s peaks from VFG-R0.5@0.5 immersed in water and after 5000 cycles, as shown in Fig. 6. The C 1s peak can be decomposed into five peaks – C sp2 at 284.6 eV, C sp3 at 285.4 eV, C–OH at 286.7 eV, C[double bond, length as m-dash]O at 287.9 eV and O[double bond, length as m-dash]C–OH at 289.8 eV.38,45,47 After the samples were immersed in water, the low content C–OH (8.0%), C[double bond, length as m-dash]O (6.8%), and O[double bond, length as m-dash]C–OH (4.2%) peaks mainly come from physical or chemical adsorption at the surface of the VFG samples.47 After cycling, it can be found that the ratio of C sp3 and C sp2 reduces and the oxygen-bonded carbon content is increased. The fraction of C–OH, C[double bond, length as m-dash]O and O[double bond, length as m-dash]C–OH is increased to 11.6%, 9.3% and 12.7%, respectively. Based on these XPS results, it can be inferred that defective carbon atoms are prone to form oxides with carbon atoms (C–OH, C[double bond, length as m-dash]O and O[double bond, length as m-dash]C–OH) when defective VFG electrodes are exposed to aqueous conditions, as illustrated in Fig. 7a. In other words, it can be inferred that those oxides of carbon atoms (C–OH, C[double bond, length as m-dash]O and O[double bond, length as m-dash]C–OH) are the main reason for the improved wettability and enhanced capacitance.


image file: c7ta06490j-f6.tif
Fig. 6 C 1s spectra of VFG-R0.5@0.5 (a) when immersed in water and (b) after 5000 cycles.

image file: c7ta06490j-f7.tif
Fig. 7 (a) Ar rotary plasma was used to purposefully create defects of armchair or zigzag varieties in VFG structures. Schematic diagrams of (b) pristine VFG and (c) VFG after plasma etching, with electrolyte, and the corresponding HRTEM images.

Defects are generally perceived as material performance limiters. Contrary to this established notion, we demonstrate that appropriate defect induction using rotary plasma etching can be used to tune the wettability and enhance the specific capacitance of VFG-based supercapacitors without altering the vertical structure. On the one hand, defects formed in VFG after plasma etching break the crystal symmetry (see Fig. 7a) and thereby enhance the electronic density of states at the Fermi level, which modifies the performance of an EDLC.38–40 According to previous research,56–60 electrically active defects in graphene itself are an effective way to improve quantum capacitance (CQ), thus leading to higher capacitance. On the other hand, the improved wettability of VFG with the electrolyte plays a key role in enhancing the capacitance. On the basis of the mechanism of EDLCs, the capacitance of VFG is mainly derived from the effective contact area of the electrode with the electrolyte. The side surfaces of VFG have better crystallinity and fewer defects; thus, the greater number of defects induced via rotary plasma etching to the side surfaces. Further, the defective carbon atoms are prone to form oxides of carbon atoms (C–OH, C[double bond, length as m-dash]O and O[double bond, length as m-dash]C–OH) when defective VFG electrodes are exposed to aqueous conditions, which can be the main cause of wettability and ion accommodation. As shown in the HRTEM images in Fig. 7b, pristine VFG clearly shows higher crystallinity and fewer defects. The VFG sample treated via rotary plasma etching featured many nanoholes on the side surfaces (see Fig. 7c), while the overall structure was maintained. This suggests that rotary plasma etching can introduce defects into the side surfaces while maintaining the overall structure of VFG. Defects on the outer layers of graphene in VFG can provide electroactive sites, while maintaining the good electrical conductivity of the internal graphene in VFG, which is beneficial for electrochemical performance. Further, improving the wettability of VFG can help more effectively utilize the full potential of VFG, particularly the side surfaces, without altering the VFG structure, as shown in Fig. 7b and c. Thus, the rotary plasma etched VFG featured high capacitance during electrochemical tests. In this work, we designed an effective rotary plasma etching process to induce defects on the surface of graphene in VFG structures and experimentally showed an increase in the specific capacitance. The VFG samples showed excellent cycling stability and retained their electrical conductivity and structural integrity.

3. Conclusions

In summary, rotary plasma etching is conducted to modify the wettability of VFG, which leads to substantial performance improvements when using the material in supercapacitors. The rotary plasma etching introduced defects onto the side surfaces of VFG, while maintaining the structural integrity of VFG. Furthermore, defects on the side surfaces of VFG induced via rotary plasma etching contributed to the improved wettability and provided electroactive sites, improving the electrochemical performance. The resulting VFG samples showed capacitance values as high as 1367 μF cm−2 (approximately 4 times as high as the pristine VFG) and showed excellent cycling stability after 5000 cycles. Our results represent a new approach for introducing defects onto the side surfaces of VFG in a controlled fashion, leading to enhanced performance for energy storage device applications.

4. Experimental section

VFG synthesis and Ar rotary plasma etching process

The VFG samples were deposited via radio-frequency (RF) PECVD. 200 nm thick Pt film deposited on Si was used as the substrate without any catalyst. The whole process mainly consists of a synthesis section and a plasma etching section. In the first section (synthesis section), the Pt/Si substrate was firstly transferred to a PECVD chamber, and the PECVD chamber was evacuated to a pressure below 2 × 10−3 Pa. Then the system was heated to 750 °C under an Ar/H2 mixture flow (flow rate: Ar[thin space (1/6-em)]:[thin space (1/6-em)]H2 = 40[thin space (1/6-em)]:[thin space (1/6-em)]20 sccm) at a pressure of 200 Pa. After that, an Ar/CH4 mixture (flow rate: Ar[thin space (1/6-em)]:[thin space (1/6-em)]CH4 = 95[thin space (1/6-em)]:[thin space (1/6-em)]5 sccm) was then introduced into the reactor for VFG growth. During the entire 60 min growth process, the pressure was maintained at 600 Pa and the power of the RF plasma was 200 W. Then, VFG samples were successfully deposited on Pt/Si substrates.

The plasma etching process was conducted following the synthesis process without taking the samples out of the PECVD chamber. For the second process (plasma etching section), the power of the RF plasma was turned off. We also halted the CH4 flow, and the Ar flow was adjusted to 0 sccm. The PECVD chamber was evacuated to a pressure below 2 × 10−3 Pa for the purging process. The plasma etching section was conducted under an Ar gas atmosphere (the Ar gas flow was maintained at 95 sccm) under a pressure of 200 Pa. Then we adjusted the base substrate angle. The power of the RF plasma was 200 W.

Material characterization

The morphologies and structures of the obtained samples were characterized using scanning electron microscopy (SEM; JEOL JSM-6700F), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM; JEOL TEM-2010 operated at 200 kV), and Raman spectroscopy (Renishaw-InVia, with an excitation wavelength of 532 nm). X-ray photoelectron spectroscopy (XPS) measurements were conducted using an ESCALAB 250Xi spectrometer (Al Kα X-rays, 1486.7 eV) with a spot size of 650 μm and an energy step size of 0.1 eV. The TEM samples were prepared by scraping off VFG from the Pt substrates, then transferring these to TEM copper grids for TEM tests.

Electrochemical performance measurements

Cyclic voltammetry (CV), galvanostatic charge/discharge (CD), and self-discharge tests were conducted using an electrochemical workstation (CHI-760E and PARSTAT 2273) with a three-electrode system with an aqueous electrolyte solution of 6 M KOH within a potential window from −0.2 to 0.4 V vs. a saturated calomel electrode. All samples were wetted with electrolyte before all electrochemical measurements. EIS testing was performed at a DC bias of 0 V over a frequency range of 100 kHz to 100 MHz using the same three-electrode system.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This project is supported by the National Natural Science Foundation of China (Grant No. 51575135 and U1537206).

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta06490j
These authors contributed equally.

This journal is © The Royal Society of Chemistry 2018