The fabrication and characterization of ZnO/NiCO2O4 nanowires synthesized by a hydrothermal method

. The main functional part of the supercapacitors and nanogenerators made up of various types of nanostructured materials. The contemporary devices have been manufactured employing different kinds of nanomaterials based on nanoflake and nanowire structures. These hierarchical materials show high efficiency on engineering electrodes for supercapacitors as well as piezoelectric for nanogenerators. Here, we have successfully synthesized NiCo2O4 nanowires on ZnO nanoflakes by applying two-step hydrothermal method which is absent similar analogous to date. The microscopic analysis has shown the uniform growth of NiCo2O4 nanowires on ZnO nanoflakes. This fabricated material might be used in above mentioned purposes in the future.


Introduction
The global increase in population and rapid industrialization have created a growing demand for energy resources worldwide.However, the limited availability of traditional energy sources has led to a shift towards harnessing energy from alternative sources, such as solar and wind, which have gained significant popularity over the past two decades [1,2].Additionally, the development of devices that operate on low power consumption has become a crucial concern in our modern society.Therefore, there is a pressing need to explore new and innovative approaches to meet the energy demands of the global population [3].To address these challenges, researchers have been actively investigating various renewable energy technologies and exploring novel avenues for energy production and utilization.Efforts are being made to optimize the efficiency of solar and wind energy systems, improve energy storage technologies, and develop sustainable energy conversion methods.Furthermore, the integration of smart grids, energy-efficient appliances, and advanced power management systems are being explored to minimize energy wastage and enhance overall energy sustainability.These endeavors aim to ensure a reliable and environmentally friendly energy supply for future generations.
Mechanical energy is widely recognized as a vast and accessible form of energy in nature.Various options have been proposed to harness this mechanical energy, including the electrostatic effect, electromagnetic fields, triboelectric effect, and piezoelectric effect [4,5].Among these, the piezoelectric effect stands out as it enables the generation of electricity through mechanical deformation of piezoelectric materials [6].Thus, it is essential for piezoelectric materials to exhibit high efficiency in electricity production for their application in self-powered electronics.
Extensive studies have confirmed the viability of ZnO nanoflakes (NFs) as a piezoelectric material for powering portable electronic devices [7,8].These studies have also highlighted the crucial role played by the controlled average crystallite sizes of ZnO nanoparticles in the generation of electricity [6,[8][9][10][11].By precisely manipulating the crystallite sizes, it becomes possible to optimize the performance and enhance the power generation capabilities of the piezoelectric materials.
The advancement and optimization of piezoelectric materials, particularly ZnO NFs, hold great promise for self-powered electronic devices.The ability to efficiently convert mechanical energy into electrical energy opens up new possibilities for wearable electronics, smart sensors, and other portable devices that can harness ambient vibrations and movements to sustain their power requirements.
Particularly, the less than tens of nanometer sized ZnO NFs were synthesized and nanogenerators fabricated using these NFs.Subsequently, the tested results showed that the nanogenerator's power was sufficiently to cover energy consumption of 20 commercial green light-emitting diodes [12].In the literature [13], the hybrid nanogenerators based on ZnO NFs/polydimethylsiloxane nanocomposite material demonstrated nine times higher efficiency in comparison with pure ZnO NFs.Moreover, the molybdenum doped ZnO NFs was used to construct a supercapasitor with negligible loss after 8000 charge-discharge cycles [14].However, further research and development are needed to explore the full potential of piezoelectric materials and optimize their performance for practical applications.This includes investigating novel fabrication techniques, exploring composite materials, and studying the effects of different parameters on the efficiency of electricity production.Through such endeavors, we aim to unlock the full capabilities of piezoelectric materials and contribute to the development of sustainable energy solutions in the field of self-powered electronics.

Materials and methods
In order to synthetize NiCo2O4 nanowires on ZnO NFs, all the necessary reagents such as pure zinc acetate dehydrate [Zn (CH3COO)2•2H2O], NCO, sodium hydroxide (NaOH), and the substrate (sandwiched layers of glass and ITO) were purchased from Sigma-Aldrich, Korea.Initially, 5-mM zinc acetate dihydrate solution was used to obtain the seed layer of the substrate.Further, to acquire the uniform growth of ZnO NFs on the substrate, we have baked the layer by applying of 100 C temperatures for 10 minutes.To achieve the uniform coating the procedure has repeated two times.Next, the prepared mixture of 5 mL of 1 M zinc acetate aqueous solution and 5 mL of 8 M NaOH aqueous solution was stirred 3 min.Then, it was diluted in a 200 mL aqueous solution and followed by stirring for 3 min 20 sec.The substrate was cleaned by means of deionized water and consequently kept on a hot plate for 2 hours at 160 C.As a result, we have successfully synthesized ZnO NFs.We used above mentioned procedures to synthesize ZnO NFs by following the technique given in literature [12].
Subsequently, we have annealed ZnO NFs for 5 min at 80 C after cleaning the latter.In order to prepare a solution, we make use of 0.14 g CoCl2•6H2O, 0.38 g NiCl2•6H2O, 0.108 g urea and 0.17 g NH4F dissolved in 30 mL deionized water.The solution further stirred for 25 min at room temperature.After obtaining a homogeneous solution, it was displaced to autoclave where was heated at 120 C for 10 h together with ZnO NFs.Thus, the hydrothermal method allowed us to grow NiCo2O4 NWs on the ZnO NFs substrate [15].
The synthesized material was cooled until reach to the room temperature.To get rid of the solutions attached to the material, it was heated at 150 C for 2 h on the hot plate in air condition to obtain pure NiCo2O4 NWs.

Materials
The crystal phase characterizations of synthesized samples were analyzed by means of Xray Diffractometer (XRD, advanced D8).The surface morphology was investigated using scanning electron microscope (SEM, JEOL, JSM-7600F).

Results and discussion
In order to do the detailed analysis, we have obtained the Scanning Electron Microscope (SEM) image and X-ray diffraction (XRD) pattern for ZnO NFs which was grown on the substrate.The morphology of our sample was studied by SEM (see Fig. 1b).It can be seen that the shape of grown ZnO NFs was uniform and the architecture was almost perpendicular to the substrate.Moreover, the individual NFs were also attached one to another (see Fig. 1c).As is clear from the Fig. 1c, XRD patterns of ZnO NFs show the similar peaks which was obtained in previous literatures [18].According to the ZnO NF's XRD data, these were not identified any other additional peaks (see Fig. 1c) which confirms purity of fabricated ZnO NFs.The sizes of NF thickness were around ~8-12 nm and height vary from 80 to 250 nm.
Further, we used NiCo2O4 to grow NWs on ZnO NFs by applying hydrothermal method for 10 hours (see the section Experimental procedures).Again, we performed SEM analysis together with Energy Dispersive X-Ray Analysis (EDX) with fabricated material (see Fig. 1d, e and f).As is obvious, NiCo2O4 NWs have grown on ZnO NFs surface (see Fig. 1 d  and e).The grown NWs pattern again corresponds to the perpendicular shape to the surface of ZnO NFs (see Fig. 1e).In addition, EDX pattern also evidences the presence of the signal that corresponds to Zn, Ni, Co and O (See Fig. 1f).The flexibility factor is essential in manufacturing of flexible supercapacitors.These kinds of electrodes based on ZnO microrod arrays and chip-like 0D interconnected ZnCo2O4 nanoparticles were fabricated by solvothermal method [19,20].Chen et al, were synthesized the nanocactus-shaped NiCo2O4 on NiCo2O4 NFs by applying hydrothermal and cyclic voltammetry deposition techniques [21].This material was employed to fabricate a supercapacitor with high power density which can be used for energy storage applications.In other study, nickel nanowires and porous NiCo2O4 nanorods arrays was grown on the surface of nickel foam which showed high-pseudocapacitance with efficient cycling stability [22].Zhang et al, were obtained MnO2 NFs on NiCo2O4 nanoneedles which was grow on the roughened surface of the Ni wire [23].Further, the synthesized material was used to synthesized NiCo2O4 NWs on ZnO NF structure by applying two-step hydrothermal method.ZnO construct fiber-shaped supercapacitor by twisting two electrodes.The results of interconnected ZnO/NiCO2O4 nanowires show outstanding longterm stability, as confirmed by morphology, crystal structure, and chemical state.This indicates it can withstand harsh chemical environments, making it an excellent option for various electrochemical applications.

Conclusions
In this study, we conducted a comprehensive investigation of the synthesized NiCo2O4 nanostructures using XRD, SEM, and EDX techniques.The characterization results provided valuable insights into the morphology and composition of the ZnO NFs and NiCo2O4 NWs.These nanostructures hold significant potential for the development of novel supercapacitors and piezoelectric materials.
The unique properties exhibited by these nanostructures make them promising candidates for energy storage applications, particularly as supercapacitors and nanogenerators for self-powered electronic devices.The high surface area and tailored nanoarchitecture of our fabricated nanomaterial offer favorable conditions for efficient energy storage and conversion.
However, further investigations are necessary to fully explore and validate the performance of these nanostructures in energy storage systems.In-depth studies are required to evaluate their specific capacitance, power density, cycling stability, and longterm performance under various operating conditions.Such investigations will provide crucial information for optimizing and fine-tuning these materials for practical energy storage applications.
Overall, our findings demonstrate the potential of the NiCo2O4 nanostructures as promising materials for energy storage purposes.Through further research and development, we aim to unlock the full capabilities of these nanostructures and contribute to the advancement of sustainable energy technologies.