Mesoporous GO-TiO2 nanocomposites for flexible solid-state supercapacitor applications

Fabrication of flexible solid-state supercapacitor is a field of paramount importance because of its potential application in portable devices. In this article, graphene oxide and titanium dioxide hybrid (GO-TiO2) nanocomposites of different compositions are employed for the fabrication of flexible solid-state supercapacitors. An easy, fast, scalable and controlled sol-gel method is devised for GO-TiO2 nanocomposites growth from different amounts of graphene oxide and titanium isopropoxide at atmospheric pressure. Highly monodispersed rutile TiO2 nanospindle is homogenously grafted on GO by a controlled HCl catalyzed reaction. BET N2 adsorption desorption isotherm analysis confirms formation of the mesoporous structure having a large specific surface area favourable for faster reversible adsorption and desorption of ions. An optimum composition of GO-TiO2 nanocomposite (TG25) exhibits a high areal specific capacitance of 73.43 mF cm−2 at a current density of 0.5 mA cm−2 due to formation of electrical double layer in a solid-state supercapacitor. The fabricated device shows high power density (3.5 mW cm−2), a high energy density (0.007 mWh cm−2), good flexibility and excellent cycling stability, 92% specific capacitance retention after 10,000 continuous charge-discharge cycles. Finally, three supercapacitors in a series have illuminated a red LED, indicating the nanocomposite as a potential candidate for energy storage technology.


Introduction
Solid-state supercapacitors having a high power density and high specific capacitance have attracted great attention for their high stability, flexibility, long term use and they are complementary of battery in energy storage applications [1]. The performance of electrical double layer (EDL) supercapacitors, store energy due to reversible charge accumulation and desorption, can be improved by engineering the critical parameters such as increasing the electrical conductivity [2], surface area and interconnectivity of active electrode materials [3]. In this respect, various transition metal oxides such as MnO 2 , Nb 2 O 5 , RuO 2 , Co 3 O 4 , and Ni(OH) 2 have been employed in supercapacitor having different structure and morphology which attributed to high specific surface area. In addition, transition metal oxides shows large pseudocapacitance due to surface redox reactions [4]. RuO 2 is one of the most promising electrode materials due to its ultrahigh pseudocapacitance (700 F g −1 ), low resistivity, high chemical and thermodynamic stability under electrochemical environments. However it's application is restricted due to the high cost [5]. Co 3 O 4 has high theoretical capacitance (up to 3560 F g −1 ), and well-defined redox behavior but delivers poor rate capability and reversibility due to its poor electrical conductivity and large crystallite size [6]. MnO 2 can also be used as electrode material because it is predicted to show a high capacitance and it is inexpensive. But its poor conductivity (10 -5 -10 -6 S cm -1 ) limits the charge/ discharge rate for high-power applications [7]. TiO 2 forms conventional electrical double-layer capacitor (EDLC) and shows rectangular cyclic voltammetry curves which make it a promising electrode material for supercapacitor devices. TiO 2 nanostructures have tunable high surface area, low cost, environmentally benign behavior and long term chemical stability which make it attractive material in bio-implant and other sophisticated energy storage application [8]. However, it has a lower theoretical capacity (1206 F g −1 ) and its high resistivity results in large internal resistance, slow ion diffusion and poor electron transfer [9,10]. On the other hand, supercapacitors of the bare carbon material exhibit high surface area, good conductivity, and restricted ionic accessibility [11]. Therefore, it is interesting to pair transition metal oxides such as TiO 2 with carbonaceous materials such as graphene to form composites to extract the benefits of both materials reducing the limitation of each material [12]. Restacking of graphene sheets can be minimized by using TiO 2 nanoparticles as intercalator, which eventually increases the specific surface area, sustaining better access of ionic electrolyte. Graphene, a sp 2 -hybridized two-dimensional carbon sheet, exhibits extraordinary mechanical flexibility [13], high chemical stability, high electrical conductivity, and high specific surface area (2630 m 2 g −1 ) attractive for the flexible device applications [14,15].
Few graphene-TiO 2 nanocomposites have been synthesized at high pressure or using surfactants or additives to minimize the agglomeration of TiO 2 [16,17]. For example, Xiang et al [18] have reported shape and coupling effects of hydrothermally synthesized reduced graphene oxide-TiO 2 composites for supercapacitor applications using expensive nickel disk as the current collector. A bare TiO 2 nanotube array is also employed in supercapacitor to achieve a low aerial capacitances of 2.6 mF cm −2 [19]. Size-tunable anatase TiO 2 nanospindles assembled with titanium oxynitride/titanium nitride graphene nanocomposites show high cycling performance in rechargeable lithium ion batteries. 23 Flexible composite film of rGO/TiO 2 , grown at high temperature (600°C), is employed as supercapacitor electrode in three-electrode cell configuration [20]. However, controlling the size of particles, their morphology and dispersion density of inorganic nanocrystals on graphene sheets, which eventually increase the electrical conductivity and surface area leading to a superior supercapacitor performance in two electrode configuration is still a challenge, and needs more attention.
Here, we report an easy, controlled, in situ growth of mesoporous GO-TiO 2 -nanocomposites of different compositions at atmospheric pressure. The evolutions of HCl catalyzed TiO 2 nanospindles of rutile phase on graphene platform are monitored by SEM, TEM, XPS, XRD and Raman spectroscopy techniques. The nanocomposites are employed in flexible solid-state supercapacitor device using acetylene black conducting filler and a gel PVA-H 3 PO 4 electrolyte on flexible steel plate electrode. The devices show excellent cyclic stability (92% specific capacitance retention after 10 000 charge-discharge cycles), high specific capacitance and power density, indicating the GO-TiO 2 nanocomposite as a potential candidate for energy storage applications. The novelty of the work lies in the easy growth of mesoporous GO-TiO 2 nancomposite and demonstration of flexible solid-state supercapacitor igniting LED bulbs.

Materials and characterizations
Graphite powder, KMnO 4 , Sodium nitrate, Titanium tetraisopropoxide, 30% H 2 O 2 , and conc. H 2 SO 4 (98%), HCl were purchased from Sigma Aldrich and used without further purifications. Ultra pure water was obtained from Sartorius ultra pure water purification system (Model Arium Comfort II) and ultra pure water was used for the whole experiment.

Physical characterization
The morphologies of the prepared samples were characterized by Field emission scanning electron microscopic (FESEM). FESEM Images were taken by SUPRA 40 Field Emission Scanning Electron Microscope (Carl Zeiss SMT AG, Germany). High Resolution Transmission Electron Microscopy images were taken by JEOL 200 keV instrument. The phase analysis and structure were investigated by X-ray diffraction measurements. Raman spectroscopy (Lab RAM HR JovinYvon) is carried to study the crystalline quality and local structural disorder of nanocomposites. The surface chemical composition and chemical states of the elements in the samples were identified by x-ray photoelectron spectroscope (model PHI 5000 Versa Probe II (ULVAC, PHI Inc.). The electrochemical properties and capacitance measurements of the GO-TiO 2 nanocomposites electrodes were characterized using cyclic voltammetry (CV), galvanostatic charge discharge (GCD) and electron impedance spectroscopy (EIS) using an electrochemical station (Gamry, Model 1000 interface) with a two-electrode configuration using H 3 PO 4 /PVA electrolyte, GO-TiO 2 nanocomposite active material deposited on electrodes on steel plate. The EIS measurements were conducted at varying frequency from 0.001 Hz to 10 kHz with amplitude of 5 mV at an open-circuit voltage. The CV and GCD were tested between 0 and 1 V, and the areal specific capacitance (mF cm −2 ) was calculated from discharge curves from the following equation.
where I, Dt, A and DV is discharged current, discharge time, area of electroactive material and potential window, respectively. At a given scan rate the specific energy density (E D; mWh cm −2 ) and Specific power density (P D; mW cm −2 ) of the device was calculated from the following equation Dt is the duration (sec) of the discharge half-cycle in the discharge curve.

Synthesis of GO
Graphite oxide was synthesized by the modified Hummer and Offeman's method with graphite powder [21]. Graphite powder was oxidized using KMnO 4 in presence of conc H 2 SO 4 . Graphite oxide slurry was filtered and exfoliated in DI water by ultrasonication in a bath sonicator as described in our previous work [22]. The graphene oxide was purified sequentially by centrifugation and dispersion of the precipitations in DI water. The GO was collected after removing small and big graphene oxide particle at a high and low centrifugation speed, respectively.

Synthesis of GO-TiO 2
The schematic of GO-TiO 2 nanocomposite synthesis is shown in figure 1. Concisely, 0.937 μL of titanium tetraisopropoxide (TTIP) was dissolved in 40 ml ultrapure DI containing 5 ml HCl under vigorous stirring with a magnetic stirrer for 30 min. Different amounts of as-synthesized GO (10, 25, 50 and 100 mg) are added to the solution and stirred for 30 min, separately. After that, the temperature of the growth solution was maintained at 120°C for 2 h for each sample. A gray precipitation was obtained. The resultant precipitation was washed with distilled water for three times by centrifugation at 10,000 rpm after dispersing and followed by discarding the supernatant to remove remaining ions in the by-product. The precipitation was collected and dried in an oven at 100°C for 12 h for further characterization and device fabrications. Few control TiO 2 samples were also prepared without addition of GO at similar condition using different acid concentrations (0.5 M HCl, 0.5 M HNO 3 , 1 M HCl). The resultant GO-TiO 2 nanocomposites were termed as TG10, TG25, TG50, and TG100 prepared using 10, 25, 50, and 100 mg of GO, respectively.

Supercapacitor device fabrication
Electrodes comprised of GO-TiO 2 nanocomposite as active material was prepared by mixing the 85:15 mixture of GO-TiO 2 nanocomposite and acetylene black in N, N-dimethylformamide (DMF). The solution of the active material was prepared by stirring the mixture of GO-TiO 2 nanocomposites and acetylene black for overnight at 60°C in DMF. The black color solution was drop casted on to stainless steel plates (0.01 mm width) and dried at 80°C for 30 min in the oven. The active layer film was soaked in gel electrolyte aq. PVA-H 3 PO 4 gel electrolyte for 10 min. The gel electrolyte was prepared by dissolving 1 g PVA in 10 ml DI water at 60°C for 12 h, then 1 ml H 3 PO 4 was added into the solution and stirred for 2 h. After drying in an oven for 2 h at 60°C, the electrodes were kept for 24 h at room temperature. Next, the two symmetric fabricated electrodes were sandwiched carefully. PVA plays a dual role here as a binder to hold the active layer as well as a separator between two electrodes preventing shorting. To evaporate extra amount of water, the device was further dried at 60°C in an oven for 2 h before the measurement was performed.  figure S1 is available online at stacks.iop.org/MRX/6/125546/mmedia (ESI). Compared with homogeneous growth in the solution, the nanospindles prefer to nucleate heterogeneously at the defect sites (oxygen containing moieties) on the GO sheets so free unattached single nanospindles can't be found in the product. There is no effect seen on morphology, including the size and shape by varying the reaction time after two hours, while keeping other conditions constant. Nanospindles are grafted uniformly onto the graphene oxide sheets after induction of GO to the TTIP precursor, keeping the nanospindle-shape of TiO 2 unchanged. However, the size of TiO 2 nanospindles changes from 154 to 311 nm in length and 46 to 118 nm in diameter, as shown in table S1 (ESI) with the varying proportion of GO from 1:0.1 to 1:1.

Results and discussions
Typical FESEM image of TG25 is shown in figure 2(c) where ∼268 nm long nanospindle of ∼78 nm diameter is homogenously grafted on GO. Figure 2(d) shows the TEM images of TiO 2 nanospindle grafted on graphene oxide in TG25. Presence of TiO 2 prevents the restacking of graphene oxide layers and expected to exhibit a higher specific surface area which is essential for supercapacitor applications [23]. It may be noted that after a prolonged sonication and washing TiO 2 nanospindles are remain attached to GO sheets, indicating a strong interaction between GO sheets and TiO 2 nanospindles. This intimate hybridization endows easy and fast charge transport between GO and TiO 2 nanospindles beneficial for improved supercapacitor performance. The lattice fringe spacing of TiO 2 nanoparticle in HRTEM image (figure 2(e)) is found to be 0.32 assigned to the (110) inter planner distance of the rutile phase [24,25]. Selected area electron diffraction (SAED) patterns confirm good crystallinity and the lattice planes of the pure tetragonal-rutile TiO 2 phase.
The crystal structure of TiO 2 , GO-TiO 2 nanocomposites, and graphene oxide were characterized using x-ray diffraction (XRD) analysis and the XRD patterns are shown in figure 3(a). For TiO 2 and GO-TiO 2 nanocomposites, all of the peaks can be indexed as the tetragonal rutile phase with primitive lattice, space group-P42/mnm (136), and lattice constant a=b=4.601 c=2.956 from (JCPDS 89-0552). Interestingly, it is observed that the concentration of hydrochloric acid is critical to obtain Rutile phase TiO 2 . Below 1.0 M concentration of HCl produces a mixture of rutile and anatase. Whereas above 0.5 M concentration of nitric acid produces spherical particles of anatase phase as shown in figure S2 (ESI). When the concentration of hydrochloric acid in the solution is higher than 1.0 M, the product is nanospindle of pure rutile phase, (figure S3, ESI). Even at the same concentration, hydrochloric acid is more effective for the formation of the rutile phase than nitric acid [26]. From above observations, we can conclude that HCl plays major role in the reaction: (i) due to an acidic medium, hydrolysis rate of TTIP got suppressed and slowed down and (ii) HCl is the major factor for the formation of the nanospindle shape of TiO 2 .
The bonding structures of the GO-TiO 2 nanocomposites were characterized by Raman spectroscopy shown in figure 3(b). The characteristic Raman peaks for TiO 2 and TG25 nanocomposites are observed at ∼441 and 606 cm −1 attributed to the E g and A 1g modes of rutile TiO 2 , respectively. Two characteristic peaks are observed for bare GO at ∼1349 and 1592 cm −1 , and for TG25 at ∼1352 and 1597 cm −1 , corresponding to the well-defined D and G bands for the defect and graphitized structures, respectively. The D-band corresponds to the disorder with sp 3 structural imperfections. The intensity ratio (I D /I G ) for GO is found to be 0.83, signifying the presence of the large amount of defect (sp 3 C) in the sample. The change of I D /I G from 0.83 to 1.30 with the increasing proportion of TiO 2 indicates a strong hybridization of TiO 2 and graphene oxide. Raman spectra of GO and different GO-TiO 2 nanocomposites are shown in figure S4.
The nanocomposite formation is also investigated by x-ray photoelectron spectroscopy (XPS). Figure 3(c) shows the C1s electron binding energy peaks, with a binding energy of 285 eV attributed to the C-C and C=C To get an insight into the surface property and supercapacitor performance of the nanocomposites, the specific surface area, a pore diameter of bare TiO 2 , TG 25, TG50 and bare GO are investigated by BET nitrogen adsorption-desorption isotherm measurements using Beckman Coulter SA3100 surface area analyzer. Specific   GO-TiO 2 Solvothermal at atmospheric pressure. 73.43mF cm −2 0-1 V 92% capacity retention after 10 000 cycles This Work surface area and pore diameter of nanocomposites are tabulated in (figure 4(a)). TG 25 has shown higher surface area than TG50, bare TiO 2 and bare GO. The high specific surface area of TG25 nanocomposite may attribute to a superior specific capacitance performance in a supercapacitor. From BET measurement, the average pore diameter of TG25 is found to be 12.48 nm, indicating mesoporous nature of the electrode material which allows easy adsorption and desorption of smaller H + (1.2 Å) and PO 4 3− (2.38 Å) ions and hence contributing to the capacitance of the supercapacitor. The TG25 shows type IV isotherm (figure 4(b)) which is a characteristics of mesoporous structures. Electrochemical charge storage properties of the fabricated solid-state supercapacitor devices were evaluated by cyclic voltammetry (CV). Figure 5(a) presents the steady state CV curves of GO-TiO 2 based solid-state supercapacitors of different compositions at a scan rate 200 mV s −1 . The nearly rectangular shape and enclosed area under the CV curves of all compositions indicate that the supercapacitor performance is composition dependent. The TG25 based supercapacitor shows the highest specific capacitance value because of the optimum composition of GO and TiO 2 provides good electrical contact and maximum surface area (see table S2 ESI). Figure 5(b) represents the CV curve of TG25 based supercapacitor at various scan rates (from 2 to 200 mV s −1 ). The CV curves of the TG25 based supercapacitor maintain nearly rectangular shapes without having a faradic current at all the scan rates, indicating its excellent capacitive behavior originating from the electrical double layer (EDL) formation. It may be noted that the bare TiO 2 has a very low electrochemical capacitance (<40 μF cm -2 ) because of its low conductivity and negligible faradic capacitance [27].
Galvanostatic charge-discharge (GCD) curves are shown in figures 5(c) and (d) for different compositions at 0.5 mA cm −2 current density and different current densities of TG25 based device, respectively. The chargedischarge curves are nearly triangle shape having a negligible internal resistance (IR) drop, indicating easy adsorption and desorption of ions. According to the GCD curves, the specific capacitances of the composites are calculated in the potential windows ranging from 0 to 1 V. It is found that the TG25 based supercapacitor shows a highest specific capacitance of ∼73.43 mF cm −2 . The specific capacitance value of the TG25 based supercapacitor is compared to that of other GO/RGO-metal oxide based solid-state supercapacitor and displayed in table 1. The fabricated TG25 based device outperforms other devices, indicating the GO-TiO 2 as a potential candidate for energy storage applications. To investigate the origin of high specific capacitance, electrochemical impedance measurements were carried out over a frequency range from 0.001 Hz to 10 kHz. Figure 6 shows the Nyquist plots of the all compositions of GO-TiO 2 nanocomposite based supercapacitors. Nyquist plot for each composition shows a single semicircle and straight line slope in the high-frequency region and low frequency regions, respectively. The sloping straight line in the low frequency region is attributed to the ion diffusion/transportation in the electrode, and the single semicircle corresponds to the interfacial charge-transfer resistance between electrodes and electrolyte. Charge transfer resistance (R ct ) values of TG10, TG25, TG50, and TG100 based supercapacitors are calculated to be 113.5, 2.3, 3.1, 9.3 ohm, respectively after fitting the Nyquist plot in the equivalent circuit shown in the inset of figures 6(a)-(d). It is found that R ct value for TG25 based supercapacitor is the lowest and that's the reason for high specific capacitance value of TG25 based supercapacitors [32]. The equivalent series resistance (ESR) values are found to be 1.64, 0.82, 2.30 and 3.17 Ω, deduced from the point of the curve intersection with a lateral axis for TG10, TG25, TG50 and TG100, respectively. ESR is the sum of electrolyte resistance, intrinsic resistance of the active materials and the contact resistance between electrode electrolyte interfaces. The low value of ESR confirms a good contact between electrode materials and current collector (steel) and a small resistance between the electrode and electrolyte interfaces.
Ragone plots shown in figure 7(a) describe the corresponding relation between the specific energy density (E) and the specific power density (P), which are the main parameters that reflects the electrochemical performances of the supercapacitors. The energy densities of the TG25 supercapacitor were dramatically higher compared with that of other composites based supercapacitor, especially in cases with lower power densities (table S3, ESI), indicating it as an ideal composition to get superior performances.
The flexibility of the devices is also tested by measuring the CV curve at different bending angles. The unaltered CV curves ( figure 7(b)) at different bending angles confirm the nanocomposites as a potential candidate for flexible energy storage technology. Interestingly, the TG25 based supercapacitor device shows high (92%) specific capacitance retention after continuous 10,000 charge discharge cycles (7 c). Finally, to provide a clear evidence of the electrical charge storage property, we have illuminated a red LED, as shown in figure 7(d) and video S1 in the ESI. A series combination of three supercapacitors is charged by applying 2 V for 3 min. After disconnecting it from the voltage sources, it glows the red LED for 5 min and then it starts discharging and fully discharged within next 5 min.