Synthesis and Application of Nanocomposite Reinforced with Decorated Multi Walled Carbon Nanotube with Luminescence Quantum Dots

Amidst the COVID-19 pandemic, environmental problems such as energy crisis, global warming, and contamination from pathogenic micro-organisms are still prevailed and strongly demanded progress in high-performance energy storing and anti-microbial materials. The nanocomposites are materials that have earned large interest owing to their promising applications for countering global issues related to sustainable energy and a flourishing environment. Here, polypyrrole coated hybrid nanocomposites of multi-walled carbon nanotube and cadmium sulfide quantum dots named MCP were synthesized using facile and low-cost in-situ oxidative polymerization method. Characterization techniques confirmed the synthesis. Electrochemical studies showed that the nanocomposite 1-MCP showed an impressively higher super capacitance behavior in comparison to f-MWCNT, 7-MCP and 5-MCP. The improved performance of the nanocomposites was attributed mainly to the good conductivity of carbon nanotubes and polypyrrole, high surface area, and stability of the carbon nanotubes and the high electrocatalytic activity of the cadmium sulfide quantum dots. Owing to the synergistic effect of MWCNT, CdS, and PPy the synthesized ternary nanocomposite also inhibited the growth and multiplication of tested bacteria such as S. aureus, and E. coli completely within 24 h. On the whole, the assimilated nanocomposite MCP opens promising aspects for the development of upcoming energy storage devices and as an antibacterial agent.


Introduction
Environmental issues, for instance, the energy crisis are leading to the scarcity of major energy resources such as coal, natural gas and petroleum at a very faster rate. In order to fulfill the growing need for feedstock, great efforts have been made by researchers to design and develop unique materials for energy conservation and energy storage [1]. The increasing demand for portable and flexible electronic devices in modern society on one hand and lack of provision of heavy cost wiring, public grids and electricity in rural areas on the other are triggering the development of lightweight, ultrathin, flexible, inexpensive, and sustainable energy storage devices that operate with high performance [2]. Among such energy storage devices, super capacitors (SCs) have mesmerized great attention due to their unique properties like high specific capacitance, high power density (Pd), low maintenance, long cycle life and environment-friendly nature [3] ( Figure 1). These SCs act as an energy-power difference bridge between a traditional capacitor (having high power) and fuel cells/batteries (having high energy storage). To clarify further, the power density (Pd) and energy density (Ed) of various energy storing devices are compared with that of SC in the Ragone plot [3] [4]. This plot explains that the fuel cells are high-energy systems; whereas SCs are high-power. Batteries have intermediary power and energy capabilities.
There exists some overlap in Ed and Pd of fuel cells and SCs with batteries [4].
SCs are used in electronic devices such as power supply stabilizer, flashes deliver power, grid power buffer, energy harvesting, hybrid electric vehicles and energy recovery because of their high power density, long cycle life, stability, fast charge/discharge rates, reversibility, reliability and good operational safety [5] [6]. As the Pd of SCs is high, they work on charge storage mechanism i.e. chargers are stored on the electrodes [7] [8] [9].
The size of QDs is smaller than the Bohr radius, ranging from 2 -6 nm which is similar to the size of biological macromolecules such as nucleic acids and proteins [16]. Cadmium sulfide CdS-QDs falling in group II-VI of the periodic table has a band gap of 2.42 eV. They have a large surface area to volume ratio [14].
Quantum dots-based super capacitors (QDSCs) are considered promising materials, owing to the excellent properties like band gap tunability, high absorption coefficient, hot carrier extraction, multiple exciton generation, solution processibility, and low-cost facile preparation [11]. The conducting polymer (CP), polypyrrole is known to be very useful in optoelectronic devices, storage devices, photocatalysis, and electrochemical property enhancement due to its high specific capacitance, excellent thermal stability, low cost, simple manufacturing process, good environmental stability, excellent electrical conductivity [17]. These properties exist in polypyrrole due to good functionality and substitution pattern of pyrrole monomer. This conducting polymer provides the advantages of chemical diversity: corrosion resistance, flexibility, and low density. Binary composite such as PPy/CdS and CNT/PPy has been reported in the literature [5] [17]- [22]. On blending PPy with CdS-QDs, the electrical conductivity enhances by the electron-hole recombination of hole-enriched PPy and electron enriched CdS. Also, CdS quantum dots, when combined with PPy, are protected from photo corrosion [23]. Poor life cycle stability is a major drawback of PPy based super capacitors which is mastered by the addition of CNTs due to their high surface area, high electric conductivity, chemical and mechanical stability [17]. To the best of our knowledge, the interfacial structure between nanotube and polymer including the morphology and thickness of polymer is critical to tailor their structures and properties in many potential applications [24]. Thus, designing the hybrid nanocomposite structure of the electro capacitive materials is one of the effective ways to achieve a large surface area and high conductivity. This in turn provides more faradaic reaction sites and accelerating the charge transfer, respectively, and therefore enhances the electro capacitive performance of super capacitors [25].
The present study attempts to fill the knowledge gap by investigating the syn- Furthermore, the assimilated nanocomposites also showed momentous anti-bacterial ability with ZOI reported to be 11 nm, 12 nm, and 12 nm for 1-MCP, 5-MCP, and 7-MCP respectively, when screened against E. coli and S. aureus.
On the whole, the assimilated CNT reinforced nanocomposite opens up promising aspects in both the areas viz. designing of energy storage devices as well as bactericidal efficacies.

Chemical Reagents and Materials
Pyrrole (PPy) and multi-walled carbon nanotubes (MWCNT) were purchased from Sigma and Aldrich. Cadmium sulfate from Fisher Scientific, thioacetamide (TAA) from Finar chemicals and ferric chloride was taken from CDH.
This acid functionalization changes the character at the ends of the MWCNTs from hydrophobic to hydrophilic by the addition of -COOH group [26]. is dispersed in 10 ml DDW for 3 h (named as, Sol-III) is prepared. Then, dispersion of (Sol-III) to (Sol-I) using sonicator is done for 2 h. Lastly, 2 h ultra-sonication of (Sol II) and (Sol I) together followed by centrifugation at 3200 rpm for 10 min yields MWCNT/CdS (MC) after washed with double distilled ethanol and DDW and overnight drying at 65˚C in oven Scheme 1.

Characterization
High resolution transmission electron microscopy (HRTEM), Scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and mapping analysis were

Electrochemical Measurements
Electrochemical studies were done on a cell having

Test Microorganisms
Some clinical isolates of bacterial strains were selected on the basis of their clinical importance in causing diseases in humans. These were obtained from Jawa-

CV of the Designed Nanocomposite at Different Scan Rate: Effect of Scan Rate on Super Capacitance Behavior of MWCNT/CdS/PPy
It depends on the potential sweep rate, as described in reported literature [40]. It is studied to assess whether the processes in the assimilated nanocomposite is diffusion controlled or adsorption controlled. Figure 6 shows CV area is increasing with the increase in scan rates, this implies the increase in capacitance.
Also, with the increase in scan rate, Epa is shifting towards more positive potential and thus leading to the increase in peak currents, (Ipa) as shown in Table 1.
The peak current for anodic oxidation is proportional to the square root of the scan rates (Figure 7(a)).   where Ipa and Ipc are anodic and cathodic current peaks, respectively, n is the number of electrons, F is the Faraday's constant, Q is the electric quantity, υ is the potential scan rate, and T is the thermodynamic temperature. The peaks of anodic and cathodic current are linearly proportional to the scan rate, based on the linear equation. This indicates that the electrocatalytic behavior of nanocomposite involved surface electron transfer [42].

Electrochemical Impedance Spectroscopy (EIS): Nyquist Plot and Bode Plot
To further confirm the capacitive behavior in synthesized nanocomposites, the electrochemical properties were fully investigated by electrochemical impedance spectroscopy (EIS). The corresponding circuit (Figure 8  Advances in Nanoparticles predict power density of SC, since power density is inversely proportional to (Rs) [43], whereas the second semicircle appearing in the medial frequency region (10 4 to 1 Hz) is proceeded from the charge transfer resistance (Rct) and the corresponding chemical capacitance (Cμ) at the electrode/electrolyte interface [11]. The diameter of EIS semi-circle is directly proportional to the resistance in the electrons mobility at the electrode/electrolytic interface. Larger the diameter, higher will be the charge transfer resistance and lower will be the electron mobility (flow of current) in the cell. Therefore, EIS evaluations justify the CV curves [38].
The linear portion (nearly parallel to the Z imaginary y axis) in the low-frequency region  Figure 8(b). The relationship between the imaginary impedance |Z| and the frequency f can be acquired from EIS measurements. The capacitance (C) can be calculated using Equation (4) using a linear portion of a log |Z| vs. log f curve, which is called the Bode plot.

Conclusion
In summary, we have assimilated a high-performance, flexible, huge storage capacitive super capacitor by incorporating cadmium sulfide and polypyrrole into multi-walled carbon nanotubes using a sol-gel chemical approach. Various cha-