A Facile Synthesis of PbS-G QDs Nanocomposite as Electrode Material with Enhanced Energy Density for High Performance Supercapattery Application

Bare PbS QDs and PbS-GQDs nanocomposite were prepared by chemical methods for supercapattery application and their physicochemical properties studied by suitable analytical techniques which confirms the formation of PbS-GQDs nanocomposite. The electrochemical studies of the prepared materials showed that the PbS-GQDs nanocomposite exhibited high specific capacity, high energy and power densities of 577.94 C g−1, 166.45 Wh kg−1 and 576.01 W kg−1, respectively at 2 A g−1 compared to that of bare PbS QDs. The enhanced electrochemical performance of PbS-GQDs nanocomposite can be associated with the highly conductive platform and large number of active sites of GQDs which resulted in synergistic effect of the composite. The nonlinearity in charging and discharging curves confirm the supercapattery behaviour of the nanocomposite. Moreover, PbS-GQDs nanocomposite electrode showed highly cyclic stability compared to bare PbS QDs after 5000 cycles. The results emphasize the potential of PbS-G QDs nanocomposite as a stable and active electrode material for energy storage application.


Introduction
The supercapattery (batteries and supercapacitor) combines the high-power capability of usual rechargeable batteries with excellent energy density of energy storage devices [1,2]. In particular, developing new materials and technologybased innovations in design of structures are being increasingly recognised and utilised to develop rechargeable batteries with high energy density and supercapacitors with high power density [3,4]. The energy storage research has seen significant progress in both rechargeable battery and supercapacitor during the last two decades. The supercapacitors and batteries are being observed as the two most promising electrochemical energy storage devices [5][6][7]. On the basis of electrochemical storage process, supercapacitors are generally classified into electric double-layer supercapacitors (EDLCs) and pseudo-capacitors. EDLC process is governed by aggregation of charge (non-Faradaic) at the electrolyte/electrode interface and pseudocapacitive charge by diffusion-controlled Faradaic redox (battery-type) reaction [8][9][10]. The battery operates based on cation based redox reactions at solid state electrode materials whereas supercapacitor operates based on adsorption/desorption of electrolyte ions at electrode/electrolyte interfaces in the electrochemical energy storage process [11]. Both high specific capacity with high power density and energy density characteristics can be achieved in one device named as supercapattery. The behaviour of supercapattery is similar to that of an electrochemical cell with greater performance, high power capability with higher energy density, longer charge-discharge durability and long lifetime [12].
Generally, metal sulphides such as CuS [13], CoS [14], SnS [15], CuS-PbS [16], NiS [17], and ZnS [18] are redox based attractive working electrode materials for achieving high supercapacitor performance by cost effective methods. Nanostructuring the material is a promising way to improve the electrochemical performance due to its enhanced surface area. However, the synthesis and 1 3 characterization of Lead sulphide (PbS) Quantum Dots (QDs) as an electrode material for high performance electrochemical energy storage application are rarely studied. PbS is a chalcogenide group semiconductor having direct band gap (0.41 eV) and Bohr radius of about 18 nm [19]. PbS QDs are promising semiconductors and active material used in different applications such as optical fiber, energy conversion, solar energy harvesting and energy storage devices [20][21][22]. The performance of supercapacitor materials depends on multiple parameters, such as surface morphology, adhesion on to the substrate, reaction temperature, and nanostructure properties of the active electrodes being affected by the particle size distribution, surface area, material thickness, transport/diffusion paths, and accessibility of redox electrolyte/electrode. Carbon based nanocomposite materials have large surface area with high electrical conductivity which makes them as a promising electrode material for energy storage applications. NiO [23], CuO [24,25], CNT/CuS [26], CNT/ CoS [27] and Sn/CNT [28] composite electrodes are found to exhibit high specific capacitance reaching a value of 1120 F/g in the case of CNT/CoS. Recently graphenebased metal oxides such as G-NiO [29], G-MnO 2 [30], G-ZnO [31], G-V 2 O 5 [32], have been reported as an effective electrode material with high specific capacitance compared to their individual performance in energy storage because of the synergistic effect of the composite materials. Apart from transition metal oxides, polymer composites have been tested for electrochemical capacitors and Li-ion batteries due to their good stability [33,34]. However, there is no report on the electrochemical properties of lead sulphide-graphene quantum dots (PbS-G QDs) nanocomposite.
Therefore, in the present work, a cost effective, facile strategy was developed for the design, fabrication and electrochemical testing of PbS-G QDs nanocomposite on a nickel foam substrate using chemical bath (CBD) method for supercapattery applications. The bare PbS QDs, GQDs and PbS-G QDs nanocomposite were synthesized by chemical bath method, ultrasonication method and a simple solvothermal method, respectively using low-cost precursor materials. The structural, morphological, optical properties and specific surface area of the prepared samples were studied and their electrochemical performance was evaluated for energy storage application. The bare PbS QDs and PbS-G QDs nanocomposite electrodes were analyzed by cyclic voltammetry (CV), galvanostatic charge and discharge (GCD) and electrochemical impedance spectroscopy (EIS) at 2 M KOH electrolyte ionic solution. The PbS-G QDs nanocomposite electrode material exhibits a high specific capacity, high energy density with power density compared to bare PbS QDs and exhibited ~ 97.4% stable capacity retention at 5000 cycles.

Synthesis of Bare PbS QDs, GQDs and PbS-G QDs Nanocomposite
In the first half of the synthesis process, bare PbS QDs were synthesized by simple chemical bath method [35]. In a typical synthesis procedure, SDS (0.5 g), CTAB (0.5 g), and EDTA (2 g) were dissolved in 100 mL deionized water (DIW) at 80 °C under constant stirring condition. The pH of the solution was increased to 14 by slowly adding NaOH solution. Next, lead (II) nitrate (2 g) dissolved in 10 mL DIW was injected into the precursor solution under continuous stirring condition. After that, thiourea (0.5 g) dissolved in 15 mL DIW solution was drop wise introduced into the system for a duration of about 30 min with continuous vigorous stirring. The continued vigorous stirring for another 1 h during which the solution turned into brown colour indicating the formation of PbS QDs. The final solution was cooled at room temperature, washed four times with DIW and ethanol by centrifugation. Finally, the sample was dried at vacuum oven and the particles were collected for further analysis.
In the second half, Graphene oxide (GO) was synthesized by modified Hummer's method [36,37]. Next, GQDs were prepared by chemical cutting of graphene oxide with small quantity of strong acid using vertical probe ultrasonication technique [38]. GO (1 g) was dissolved in 200 ml of DIW and ultrasonicated (Sonics Vibracell, 1500 W, 20 kHz, 35° C) for 15 min. Next, this solution was mixed with 50 ml of concentrated sulphuric acid (H 2 SO 4 ) and once again ultrasonicated for 15 min. Finally, the solution was cooled to room temperature, centrifuged at 10,000 rpm and washed with DIW and ethanol several times. After removing the acids, the chocolate brown supernatant was collected and dried in vacuum oven as the final GQDs product.
PbS-Graphene QDs nanocomposite was prepared by solvothermal method [39]. In brief the synthesised PbS QDs (1 g) was dispersed in 60 mL of ethylene glycol by sonication for 1 h to form a homogeneous mixture, 1 3 followed by 0.2 g of synthesised GQDs dissolved separately in ethylene glycol and added to the above PbS QDs dispersed solution under continuous stirring. Then 0.1 M NaOH was diluted in 5 mL of DIW and added to the reaction mixture and continuously stirred for 30 min. After complete dispersion, the solution was transferred into autoclave (100 mL) and then kept at 180 °C for 48 h in an oven. The obtained final solution was washed with DIW and ethanol three times and dried at 80 °C for 10 h. The collected black powder of PbS-G QDs nanocomposite was stored for further analysis.

Characterization of the Samples
The X-ray diffraction (XRD) patterns of the synthesized samples were recorded by Rigaku smart X-ray diffractometer equipped with Cu-K α radiation (λ = 1.54178 Å) to determine the crystal structure and lattice parameters. Optical properties and energy gap of the samples were analyzed by UV-vis DRS spectrophotometer. The surface area, pore size and their distribution were analyzed by Brunauer-Emmett-Teller (BET) analysis. Further confirmation of PbS-G QDs nanocomposite was done by Raman analysis. The particle size, crystalline nature and composition of the samples were investigated by TEM and EDS elemental analysis.

Electrochemical Studies
The electrochemical properties were analysed using CHI600C electrochemical workstation with three-electrode system consisting of Ag/AgCl, Nickel foam and platinum wire as reference, working and counter electrodes, respectively. KOH electrolyte was used for the electrochemical analysis of the samples.

Electrode Fabrication
Two different working electrodes one with bare PbS QDs and other with PbS-G QDs nanocomposite were fabricated separately for comparative studies as follows: Mixture of PbS QDs or PbS-G QDs (80 wt %) with carbon black (10 wt %), polyvinylidene fluoride (PVDF) (10 wt %) were mixed with appropriate amount of N-Methylpyrrolidone (NMP) solvent to make a homogeneous mixture. The mixture was coated onto Ni foam (1 × 1 cm 2 ) electrode and dried at 100 °C, for the solvent to evaporate. The active materials on the Ni foam was 1 mg cm −2 in both the cases. The electrochemical performance of bare PbS QDs and PbS-G QDs nanocomposite electrode materials were analysed using GCD, CV and EIS techniques in 2 M KOH electrolyte in the potential window of 0 V to 0.5 V.

X-ray Diffraction (XRD) Analysis
The XRD peaks of the synthesized bare PbS QDs, GQDs are shown in Fig. 1a and that of PbS-G QDs nanocomposite is shown in Fig. 1b result of π-π stacking. In the case of PbS-G QDs nanocomposite (Fig. 1b), the peak at 2θ values of 23°, 31° and 34°, show the presence of both GQDs and PbS QDs. The average crystallite size of PbS-G QDs nanocomposite and bare PbS QDs, are estimated using Debye Scherrer formula [40] and found to be in the range of 1.4 nm and 10.6 nm, respectively.

UV-vis Diffuse Reflectance Spectroscopy (UVvis DRS)
Optical properties of the synthesized bare PbS QDs and PbS-G QDs nanocomposite were analyzed using UV-vis optical absorbance spectra and shown in Fig. 2a The energy gap is determined by plotting a graph between (αhυ) n vs. (hυ), where α is the absorption coefficient of the material, hυ is the photon energy, A is energy independent constant, E g is the band gap, and n is either 2 or ½ for indirect and direct transitions respectively. The intercept of the tangent to the curve on the energy axis gives the energy gap. According to the above equation, based on the indirect and direct transitions, the energy gap of as-obtained bare PbS QDs is 1.25 eV and PbS-G QDs nanocomposite is 1.95 eV which are shown in inset of Fig. 2a, b.

Brunauer-Emmett-Teller (BET)
The surface properties and the results of the porosity analysis of bare PbS QDs and PbS-G QDs nanocomposite are shown in Fig. 3a, b and the corresponding textural parameters are given in Table 1. The nitrogen adsorption/ desorption isotherm results of bare PbS QDs and PbS-G QDs nanocomposite clearly exhibit a similar type II curve. The pore size of bare PbS QDs calculated using the BJH method is about 18.09 nm and the BET surface area of 66.03 m 2 g −1 . According to the hysteresis loop at P/P 0 of about 0.45-0.98, the nitrogen adsorption/desorption isotherms show that the PbS-G QDs nanocomposite exhibit a similar type IV isotherm curve. In other words, the PbS-G QDs nanocomposite exist with a mesoporous structure. Due to the mesoporous structure, the PbS-G QDs nanocomposite show a nanopore size of 11.3 nm and a very large surface area of 107.74 m 2 g −1 as measured from BET analysis. Besides, the total pore volume of PbS-G QDs nanocomposite is 24.753 cm 3 g −1 , which is higher than the total pore volume of bare PbS QDs (15.171 cm 3 g −1 ). The PbS-G QDs nanocomposite can be expected to have a high electrochemical activity with an increase in surface area and decrease in pore size, as well as large total pore volume,

Micro-Raman Scattering Analysis
Raman analysis is performed to examine the formation of PbS-G QDs nanocomposite [42]. Figure 4 shows the Raman spectrum of the PbS-G QDs nanocomposite in the wavenumber range between 100 cm −1 and 3100 cm −1 . In the Raman spectrum of the PbS-G QDs nanocomposite, three intensive Raman peaks observed at 1355.7 cm −1 , 1580.6 cm −1 and 2719.1 cm −1 , corresponds to D, G and 2D bands of GQDs in the composite. The peak related to D band is a disordered band arising from sp 2 hybridized carbon, while the G band shows the graphitic carbon. The broad 2D band of GQDs arises from relaxation in the selection rules caused by phonon scattering at boundaries and defects in GQDs. Based on the Raman spectrum, one can find that the intensity ratio of D band to G band (I D /I G ) which is about 1.15. The intensity ratio I D /I G is found to vary inversely with the cluster size L a according to the following relation (2) [43] where C(λ) is an empirical constant that depends on the excitation laser energy C (λ = 632.8 nm). According to this relation, the average size (L a ) of PbS-G QDs is about 4.3 nm. Further the three peaks observed at 448.6 cm −1 , 650.9 cm −1 and 968.5 cm −1 , are associated with PbS QDs [44]. The Raman mode observed at 448.6 cm −1 corresponds to 1LO vibrational phonon mode of PbS QD. The peaks at 650.9 cm −1 and 968.5 cm −1 correspond to interface phonon mode (2LO) and high phonon frequency mode (3LO) respectively. Also the intensity of the peaks depends on the QD size. The Raman peaks corresponding to PbS QDs and GQDs confirms the formation of PbS-G QDs nanocomposite.

TEM Analysis
The surface morphology of bare PbS QDs and PbS-G QDs nanocomposite obtained from TEM analysis are shown in Figs. 5a, b and d, e. TEM images show distinctly visible and uniformly distributed spherical dot like structures of PbS QDs, which is portrayed in Fig. 5 (a,b). The TEM image of PbS-G QDs nanocomposite under low magnification (Fig. 5d) shows that PbS QDs are decorated onto GQDs, and high magnification image (Fig. 5c) shows dense PbS QDs being uniformly anchored onto GQDs surface. As shown in the figures, the PbS QDs and PbS-G QDs particle size are in the range of 5.0 nm. Figure 5c and 5f shows selected area electron diffraction (SAED) patterns of bare PbS QDs and PbS-G QDs nanocomposites. The circular pattern with clear circles illustrates the polycrystalline nature of both the    Fig. 6a, which depicts the presence of carbon (C) 85.77%, lead (Pb) 12.59% and sulphur (S) 1.64%. The densely agglomerated region of PbS-G QDs nanocomposite from EDS mapping is shown in Fig. 6b, c, d, e, f. The synthesized PbS-G QDs nanocomposite is free from other elemental impurities.

Electrochemical Studies of Bare PbS QDs and PbS-G QDs Nanocomposite
The cyclic voltammetry (CV) curves of bare PbS QDs and PbS-G QDs nanocomposite for various scan rates (5-100 mV s −1 ) in the potential window of 0.0 to 0.5 V are shown in Fig. 7a, b. The electrochemical properties of the electrode materials were studied in 2 M KOH electrolyte solution at room temperature. It is clearly seen from Fig. 7, that both the materials exhibit redox CV curves, indicating that the charge storage mechanism is due to pseudocapacitance behaviour [45]. The fast and irreversible Faradic redox (oxidation/reduction) reaction of Pb present in both the materials in responsible for this behaviour [46]. While making a comparison between the CV curves of bare PbS QDs and PbS-G QDs nanocomposite, later possesses maximum area under the curve, revealing its improved capacitance. The K + ions present in the electrolyte also involve in the Faradaic reaction. Moreover, it is known that PbS material hold the battery type behaviour [47], and hence these two materials can be used for supercapattery application. The CV curves almost remain same in shape even at high scan rates, indicating high-rate capability of the prepared electrode materials. The proposed possible oxidation-reduction process concerning the intercalation/ deintercalation of alkaline medium (K + ) into PbS QDs/ PbS-G QDs could be given via the following reactions [48,49] The introduction of GQDs had significantly reduced the particle aggregation leading to exposure of more active sites in the electrode for the redox reactions. In addition, the porous structure and large surface area of GQDs provide rapid charge transfer kinetics, which aid Faradaic reactions [50,51]. In the case of PbS-G QDs nanocomposite (Fig. 7b), the distortion in the CV curve and shift in redox peaks are significant compared to bare PbS QDs. This reveals the improved electrochemical reversible behaviour of the nanocomposite. The observed significant increase in redox peak is due to the synergistic effect of GQDs which provides extended channels for electron transfer during redox process. Hence the decoration of PbS QD on GQDs increases the electronic conductivity and the electrochemical performance of the PbS-G QDs nanocomposite. The specific capacity (Qs) of the electrode materials is calculated from the CV plot at different scan rates using the following Eq. (2) [52] where, v is the scan rate (mV s −1 ), m is the mass of the active electrode materials (g) and the anodic peak area under the CV curve is equivalent to the integral term. In the case of PbS-G QDs nanocomposite, a maximum specific capacity of 934.6 C g −1 is achieved at a scan rate of 5 mV s −1 which is significantly higher compared to the value of 681.5 C g −1 of bare PbS QDs at the same scan rate.

Galvanostatic Charge Discharge (GCD)
The GCD plots are analyzed for the fabricated electrodes at various current densities (2 to 12 A g −1 ) in the potential window range of 0 V to 0.39 V and are shown in Fig. 8a, b. Nonlinearity of the discharge curves as can be seen from Fig. 8a, b reveals that bare PbS QDs and PbS-G QDs nanocomposite exhibit battery type behaviour. However, PbS-G QDs nanocomposite exhibits longer discharge time compared to bare PbS QDs (Fig. 8b), which may be due to the presence of GQDs. The large interfacial area accessible for the electrons for Faradaic reactions extends the duration of energy discharging time [53]. The specific capacity (Qs) of the materials is calculated from GCD plots using the following relation (3) [54] where, I is the discharge current (A), ∆t is the discharge time (s) and m is the mass (g) of the active material. The calculated specific capacity and specific capacitance of bare PbS QD and PbS-G QDs nanocomposite at various current densities are shown in Fig. 8c. From Fig. 8c, it can be seen that at a current density of 2 A g −1 , the specific capacities are 261.4 C g −1 and 577.9 C g −1 for bare PbS QD and PbS-G QDs nanocomposite, respectively. Also, the specific capacitance and specific capacity decreased with current density in both the cases. At lower current density, the electrolyte ions have sufficient time to reach the active sites of the electrode surface which resulted in high specific capacity, whereas the accessibility of the electrolyte ions towards electrode surface is restricted at high current densities resulting in decreased specific capacities. It is clear that PbS-G QDs nanocomposite exhibit high performance compared to PbS QDs. The introduction of GQDs into PbS QDs increased the rate capability which may be due to high dispersion and the intercalation of PbS QDs onto the GQDs networks which boosted the electrochemical performance of PbS-G QDs nanocomposite.

Electrochemical Impedance Spectroscopy (EIS)
To understand the intrinsic mechanism occurring at the electrodes, EIS studies have been performed by measuring the resistance of charge transfer (R ct ) in the frequency range of 0.1 to 100 kHz at an amplitude of 10 mV. In the high frequency region of the complex plane impedance plot (Nyquist plot) the R ct value is acquired from the diameter of the semicircle describing the charge transfer resistance occurring at the interface of electrode/electrolyte [55,56]. From Fig. 9, bare PbS QDs shows large semicircle with R ct value of 7.74 Ω. Large semicircle demonstrates that PbS QDs has high interfacial resistance with poor behaviour of charge propagation. The diameter of the semicircle reduces significantly due to the introduction of GQDs into PbS QDs, with R ct value of 1.14 Ω, indicating that PbS-G QDs nanocomposites has better charge transport behaviour and low interfacial resistance in comparison to bare PbS QDs. Equivalence series resistance (ESR) value is obtained from the intercept of the curve on the real axis in the high frequency region of the Nyquist plot which is related to the internal resistance of the electrode and the total solution resistance. The ESR value of 1.41 Ω for PbS-G QDs nanocomposite is lower in comparison to 1.59 Ω for bare PbS QDs. The energy (E) and power (P) densities are the prime parameters to evaluate the functioning of energy storage devices. For evaluation of both the parameters, following relations have been exploited [57,58] The fabricated supercapattery with PbS-G QDs nanocomposite stored relatively higher energy and power densities of 166 Wh kg −1 and 576 W kg −1 respectively at a current density of 2 A g −1 compared to bare PbS QDs, which exhibited energy and power densities of 110 Wh kg −1 and 250 W kg −1 at the same current density. However, the value of energy density gradually decreased with a significant increase in the power density of bare PbS QDs and PbS-G QDs nanocomposite at higher current densities as shown in Fig. 10a. The cyclic stability evaluation of supercapattery is an important parameter for the galvanostatic charging/discharging process at a current density of 2 A g −1 . The high cyclic stability is observed for PbS-G QDs nanocomposite based supercapattery at a potential window range of 0 to 0.39 V for 5000 cycles as shown in Fig. 10b. Furthermore, PbS-G QDs nanocomposite showed high-capacity retention of 97.4% after 5000 cycling operation compared to bare PbS QDs (94.1%) due to the excellent reversibility of the carbon materials (graphene) in PbS-G QDs. The experimental results demonstrated that PbS-G QDs nanocomposite exhibits high specific capacity, high energy density with power density and excellent cyclic stability which makes it suitable for device applications. The accompaniment of carbon material (GQDs) with these pseudocapacitive materials further improves the capacity, energy density and also the electrochemical stability of PbS-G QDs nanocomposite.

Conclusions
Bare PbS QDs, GQD's and PbS-G QDs nanocomposite were prepared by simple processes of chemical bath method, probe ultrasonication method and solvothermal process respectively. The several analytical techniques used to characterize the prepared samples confirmed the successful formation of PbS-G QDs nanocomposite and uniform distribution of PbS QDs onto the GQDs matrix. The superior performance of PbS-G QDs nanocomposite in terms of its specific capacity was revealed from electrochemical studies.
The improvement was particularly due to the conducting platform provided by GQDs in the nanocomposite. Furthermore, supercapattery assembled with PbS-G QDs nanocomposite electrode material achieved high energy density and power density of 166.45 Wh kg −1 and 576.01 W kg −1 respectively compared to bare PbS QDs electrode with energy density and power density of 48.76 Wh kg −1 and 373.24 W kg −1 respectively. The cyclic stability evaluation of PbS-G QDs nanocomposite showed its superior long-term stability with high-capacity retention of 97.4% compared to bare PbS (94.1%) after 5000 cycles. All these features make PbS-G QDs nanocomposite a superior material for supercapattery application.
Funding The authors have not disclosed any funding.