Template-free synthesis of Se-nanorods-rGO nanocomposite for application in supercapacitors

Abstract Both selenium and reduced graphene oxide have low specific capacitance due to their chemical nature. Nevertheless, their specific capacitance could be enhanced by hybridizing Se nanomaterials with reduced graphene oxide via formation of electrochemical double layer at their interfacial area. Therefore, novel Se-nanorods/rGO nanocomposite was successfully synthesized by template free hot reflux route starting with graphene oxide and selenium dichloride. The composite of rGO decorated by Se-nanorods is characterized using X-ray diffractometry (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen adsorption– desorption. The unique architecture of the composite exhibits high specific capacitance of 390 F/ g at 5 mV/s scan rate in 1.0 M KOH solution with ~ 90% cyclic stability after 5000 cycles making it very promising electrode material for supercapacitor applications.


Introduction
To overcome some of the limitations of batteries, supercapacitors have been introduced as the next generation en-ergy storage devices [1]. Supercapacitors have various advantages over conventional batteries such as high power density, high charge/discharge rates and long cycle life [2][3][4][5]. Supercapacitors can be categories according to their charge storage mechanism into electrical double layer capacitors (EDLCs) and pseudocapacitor [6]. Various carbonaceous materials such as carbon nanofibers, carbon nanotubes, activated carbon, and graphene have been used for supercapacitor applications. Among these materials, graphene is one of the most promising candidate for supercapacitors, as graphene offer outstanding properties like large surface area (~2700 m 2 /g), high carrier mobility, and high thermal and electric conductivity [7][8][9][10][11][12]. As a result, high theoretical capacitance has been observed when full area of single layer graphene is exploited [13][14][15][16].
However, very low capacitance have been observed when graphene is utilized into the electrode film through different techniques, as the graphene layers stacked and aggregated, resulting in reduced chances of electrolyte access to the full surface [17][18][19][20]. Therefore, several approaches are suggested to improve the capacitive performance of graphene-based electrode including surface and morphology modifications, introducing defects and doping [21][22][23][24][25].
In the search for suitable electrode materials, selenium, an indirect bandgap semiconductor, is promising candidate [26][27][28]. Selenium containing materials such as MoSe2 [29], NiSe2 [30], CuSe2 [31], WSe2 [32] and MnSe2 [33] are widely used in many applications, due to their proven properties and recent studies revealed that selenium with nanomorphology can enhance capacitive performance [33,34]. Hence, the inspiring results of selenium and graphene based nanomaterials motivated this study of the synthesis of selenium incorporated graphene electrode materials.
To date, there are no reported investigations on the synthesis and applications of Se-nanorods/rGO nanocomposite for supercapacitor applications. Herein, we establish template free synthesis of Se-nanorods/rGO nanocomposite. More interestingly, no additional steps i.e. purification or calcination processes were required. The template free synthesis of Se-nanorods/rGO nanocomposite showed high specific capacitance value of 390 F/g at 5 mV/s scan rate with~90 % retention after 5000 cycles.
The electrochemical investigation was carried out in 1.0 M KOH solution, due to its low resistivity, low corrosive and poisoning effect along with high stability, high ionic conductivity and mobility of OH − ions compared to acidic electrolytes [35][36][37].

Synthesis and characterizations of
Se-nanorods / rGO nanocomposite GO was synthesized using modified Hummers' method following previously reported method [2]. Se-nanorods/rGO nanocomposite was prepared with heat reflux technique. Briefly, equal amount (0.2 g) of GO and SeCl 4 were dispersed in 200 ml of distilled water under vigorous stirring and then sonicated for 40 min. The mixture was then transferred to 500 ml round bottom flask. Subsequently, 0.2 ml of hydrazine hydrate was added and refluxed at 150 ∘ C for 10 h. Finally, the Se-nanorods/rGO product was collected by filtration, washing and drying. The Crystallographic structure of the final product was characterized by XRD (Rigaku, Japan) over 2θ range of 10 to 70 ∘ . Raman and FT-IR analysis were performed by using DXR-Raman microscope (Thermo Scientific) and spectrum 400 FT-IR spectrometer (PerkinElmer). The BET surface areas and pore size distributions were analyzed using ASAB 2020 (Micromeritics Instruments). The morphological characterizations were examined by FESEM (Hitachi S-7400) and TEM /HRTEM (JEOL JEM-2200FS) coupled with rapid EDAX.

Results and discussion
The XRD analysis was carried to reveal the crystalline structure of GO, rGO, Se and Se-nanorods/rGO nanocomposite and the diffraction pattern shown in Figure 1(a). As shown in Figure 1(a, inset) the XRD pattern of GO displayed a diffraction peak at 2θ~10.5 ∘ , revealing the intercalation of oxygen containing functional groups on the graphite sheets [38,39]. For rGO, the diffraction peak at 10.5 ∘ completely disappeared after the reduction and replaced by a new diffraction peak at 2θ~22.5 ∘ which confirms the reduction of GO [40].  .945 Å, respectively. Moreover, broad peak appears at 2θ~27.5 ∘ confirms the reduction of GO in a composite [41]. The reduction of GO was further investigated by Raman spectroscopy. Typically, the Raman spectra of carbon based materials displays two characteristic bands positioned at 1342 at 1581 cm −1 assigned to the D (the symmetric A 1g mode) and G bands (E 2g mode of sp 2 carbon atoms), respectively [42]. The reduction of GO leads to a change in the intensity ratio of the D and G band (I D /I G ). As shown in Figure 1(b) the intensity ratio of D and G band of the nanocomposite are increased notably from 0.87 for GO to 0.92 for the nanocomposite, indicating reduction of GO [43][44][45][46]. The increase of I D /I G ratio are very commonly observed in GO chemical reduction and reported frequently [43,44].
The reduction of GO was also confirmed through FT-IR spectroscopy. The GO exhibits number of absorption bands located at 3440, 1631, 1384 and 1118 cm −1 , associated with stretching vibration of hydroxyl groups, aromatic C=C bond, O-H groups and stretching vibrations of C=O groups, respectively [47][48][49]. As shown in Figure 1(c). After the reduction to rGO, the concentration of oxygen functional groups is considerably lower than that of GO, which further confirmed the reduction of GO.
The GO reduction level and elemental composition of Se-nanorods/rGO nanocomposite was also investigated by XPS measurements. The XPS survey spectrum (Figure 2 Figure 3(a  and b), shows the morphology of the Se-nanorods/rGO nanocomposite. As presented in Figure 3(a), the typical crumpled-like features of graphene are faintly visible as densely packed selenium nanorods are anchored on the graphene surface, indicting the strong physical bonding between the selenium nanorods and graphene sheets. Furthermore, the high-resolution FESEM image (Figure 3(b)) reveals the structure of selenium nanorods arrays as having average length and diameter of~1.0 µm and~120 nm with aspect ratio of 8. Figure 4(a) and 4(b) show the characteristic TEM and HR-TEM images of the Senanorods/rGO nanocomposite where Se nanorods are randomly distributed over the 2D rGO sheets (marked by arrows). Furthermore, lattice fringes with 2.9 Å spacing are clearly observed in the HRTEM image (Figure 4(b)), which is consistent with the (101) crystalline plane of trigonal selenium. The EDX line mapping was also carried to confirm the nanocomposite purity and the Se loading, which was found to be 21.42 wt.%. The atomic and weight percentage of carbon, oxygen and selenium are summarized in Figure 4(d). The concentration profile of C, O and Se signals are shown in panel D in Figure 4(d). EDX scanning across the randomly selected line reveals that the profiles of C have a broad spectrum and high intensity than oxygen and Se while the peaks for Se are only noticeable at the nanorods regions suggest the random distribution of selenium nanorods on the graphene sheets. However, the noticeable oxygen could be either from a thin sporadic layer of SeO on the surface of metallic nanorods or the oxygen presence in the rGO.
The surface area, porosity and pore size distribution of the nanocomposite were examined by the nitrogen-adsorption measurements and the characteristic    Figure 5.
The adsorption -desorption isotherm shows a hysteresis loops consistent with type IV isotherm indicating the mesoporous structure of the nanocomposite, Figure 5(a). Furthermore, the nanocomposite has high BET surface areas of 144.10 m 2 /g and average pore size of 5 Åas determined by BJH method, Figure 5(a). All the corresponding parameters of BET and BJH analysis are listed in the inset of Figure 5(b).
The electrochemical investigation of pristine Se, rGO and Se-nanorods/rGO nanocomposite were carried out in 1.0 M KOH solution using three-electrode electrochemical system (VersaSTAT 4, USA). Relative CV measurements of investigated electrodes at various scan rates over the potential window of −0.4 to 0.6 V are shown in Figure 6 (a-c). The specific capacitance (Cs) were calculated as, Here, Cs is the specific capacitance, m is the active mass of nanocomposite, (Vb − Va) is potential window and I (s) is the voltammetric current. Figure 6(a and b) represents the CV curves of the pristine Se and rGO electrodes at different scan rates (5 to 1000 mV/s) in 1.0 M KOH aqueous solution. As shown, the CV curves of the selenium electrode ( Figure 6(a)) exhibit non-rectangular shape, reflecting faradic capacitance behavior. On the other hand, the CV curve of pristine rGO electrode (Figure 6(b)) at different scan rates exhibits a quasi-rectangular shape without any redox peaks, which confirmed the electrical double layer behavior of rGO. Further, as shown in Figure 6(a  and b), the integrated area/current density increases with the scan rate; however, the shape of the CV curve was little distorted at high scan rate (1000 mV/s), indicating poor reversibility. Moreover, the current response from the pristine selenium and rGO electrode is very low, showed negligible specific capacitance. Interestingly, the CV curve of the nanocomposite (Figure 6(c)) shows a relatively rectangular without any obvious redox peaks, indicating that the electrical double-layer capacitance is dominated by non- Figure 5: (a) N 2 adsorption-desorption isotherm and (b) BJH pore size distribution of the synthesized Se-nanorods/rGO nanocomposite faradic mechanism, instead of a pure pseudo capacitance, also the profile of the CV curve is still well retained even at high scan rate (1000 mV/s).
Further, the integrated area/current density is higher than pristine Se and rGO electrodes, indicating the better electrochemical performance. However, the corresponding specific capacitance of Se, rGO and Senanorods/rGO nanocomposite were 8.9, 5.7 and 390 Fg −1 , respectively. The relationship between the specific capacitance and the scan rate of pristine Se, rGO and Se-rGO nanocomposite shown in Figure 7(a&b). As shown the value significant decrease in capacitance with increase of scanning rate. This phenomenon is expected as at lower scan rate more time is available for diffusion of H + ions from the electrolyte to the inner active sites of the electrode leading to high capacitance [2]. Therefore, the CV results confirms that Se-nanorods/rGO nanocomposite have high capacitance with slight dependence on scan rate, while pristine Se and rGO have very low specific capacitance with good rate capability.
The interface charge transport properties of the nanocomposite were characterized by electrochemical impedance spectroscopy. Figure 7(c) shows the representative Nyquist plot for the nanocomposite in 1.0 M KOH solution. It can be seen that, the EIS curve consists of a single semi-circle in higher frequency region, implying there is only one type of interface and indicates good charge transfer performance of the electrode. Based on the representative equivalent circuit (inset Figure 7(c)) the value of Rp (the charge transfer resistance) were 198.6Ω. The constant phase element (CPE) is one of the most common circuit elements, used to describe the capacitive performance of the electrode. When the value of CPE is equal to 1, the CPE element resembles an ideal capacitor without defects and grain boundaries [50][51][52]. Interestingly, the fitting data shows that the CPE value of the nanocomposite is close to 1. Therefore, the synthesized nanocomposite can be used as supercapacitors electrode material.
The electrochemical stability is an important characteristic for the application of electrode materials. Therefore, electrochemical stability test was performed over 5000 consecutive cycles and the results are shown in Figure 7(d). As shown, the electrochemical stability of the nanocomposite electrode is quite good with~90% capacitance retention after 5000 cycles. It is obvious that the surface of electrode was turned inactive due to surface morphology change and compact structure after 5000 cycles. Moreover, the corresponding CV curves reveal no noticeable distortion as shown in inset of Figure 7(d). The high electrochemical performance of Se-nanorods/rGO nanocomposite is attributed to the following: 1. The unique nanoroad morphology of selenium is beneficial for the electron transfer reaction. 2. The 2D conductive structure of graphene reduces the overall resistance, improves the electrical conductivity, and provides high surface area and active sites for the electron transfer. 3. The nanocomposite of selenium nanorods with the conductive graphene allows the effective intercalation/deintercalation of H + ions from the electrolyte. 4. Furthermore, the synergetic effect of the selenium nanorods and graphene enhance the charge storage capacity of the nanocomposite through both faradaic and non-faradaic energy-storage processes.

Conclusions
In conclusion, Se-nanorods decorated rGO nanocompositebased electrode material was synthesized by facile heat reflux technique without any template. The CV profile of the synthesized nanocomposite-based electrode confirming that the electrical double-layer capacitance was dominated by non-faradic mechanism, instead of a pure pseudo capacitance. Furthermore, compared to pristine Se and rGO, the synthesized nanocomposite-based electrode exhibited high specific capacitance of 390 Fg −1 with 90% capacitance retention after 5000 cycles. These results demonstrate that the synthesized nanocomposite is very promising candidate for energy storage devices.