Graphene-Nickel Oxide Composites For Photocatalysts And Supercapacitors

NiO is widely used for supercapacitor electrode materials owing to its super high theoretical specific capacitance(2584 F g^-1),lower price,and excellent chemical/thermal stability.Due to the poor conductivity of NiO and low material utilization,therestill have a large gap with its theoretical specific capacitance ofNiO-based supercapacitors.At the same time,NiO,as a p-type semiconductor,has an absorption edge at 3.5 eV and a strong absorption tail in the visible spectrum,and is usually compounded with other semiconductors to adjust the energy band position,reduce the band gap,and improve stability,and then improves the catalytic properties of the material.Graphene has a specific surface area of 2600 m²g^-1,a carrier mobility of 15000 cm²V^-ss and a tensile strength of 130 GPa,applied in enhancement materials,hydrogen storage,energy storage,and catalysiswidely.Especially in the field of energy storage and catalysis,graphene can effectively improve the conductivity andenhance the mobility of carriers,and composite it with other materials（especially in combination with NiO）is a hot spot in the current research.However,there is still much room for improvement in the synergy between different materials and the efficient use of active materials.In this paper,the NiONiG for supercapacitors and RGO-NiO/ZnO for photocatalysis were prepared by hydrothermal method combined with heat treatment process.XRD,XPS,Raman,BET,FESEM and TEM were used to characterize,analyze and observe the composition and microstructure of the prepared precursors（RGO-Ni）,electrode materials and photocatalytic materials.The electrochemical performances of electrode material were characterized by electrochemical workstation,and the photocatalytic performance of photocatalyst was measured using an ultraviolet spectrophotometer.In addition,the Ni²⁺concentration of the catalyzed solution was determined.Their structure and the electrochemical performance of NiONiG as a supercapacitor and the photocatalytic degradation of contaminants of RGO-NiO/ZnO were studied.The main research contents and results of this paper are as follows:1.Hydrothermal method was used to prepare precursors of uniform high-density nickel nanoparticles grown on GO surface,and its micro-morphology was characterized to determine the optimal ratio of nickel and GO is 8:1.2.The precursors were heat treated at 0,250,400,and 600°C,respectively,used in supercapacitors and investigated the effect of oxidation time on electrochemical performance.RGO-NiO/ZnO were successfully prepared by thermal stirring using zinc acetate and precursor that heated with 600°C,and RGO-NiO and RGO-ZnO were used as contrast to investigate the effect ofthe combination on the catalytic performance.3.Electrochemical test results show that the NiONiG-400-2 with semi-coated core-shell structure has the highest specific capacitance(2048.3 F g^-1 at a current density of 1 A g^-1,about 80%of the theoretical capacity of NiO)and excellent cycle stability(the capacitance retention is 77.8%after the 10,000 cycle at the current density of50 A g^-1).The results of the analog circuit indicate that NiONiG-400-2 has the minimum equivalent series resistance（R_s）and interface charge transfer resistance(R_ct),which are 0.363 and 0.569,respectively.According to the BET test,the specific surface area of NiONiG-400-2 is 52.8m²g^-1,and the pore size is mainly distributed at the range of 2-20 nm.4.In the photocatalytic test,the band gaps of the RGO-NiO/ZnO,RGO-ZnO,and RGO-NiOare 3.03 eV,3.23 eV,and 3.11 eV,respectively,obtainedfrom（Ahν）² vs hνcurve,indicating that the band gap of materials can be effectively reduced by compounding two kinds of semiconductor.At the same time,the time of three materials（10 mg）degraded completely 20 mL（20 mg/L）methyl orange solution were 40,140,and 280 minutes,respectively.The concentration of Ni²⁺in the solution after complete degradation byRGO-Ni O and RGO-NiO/ZnO were 2.160 mg/L and 0.0645 mg/L according to ICP test,demonstrating the cover of ZnO on RGO-NiO can effectively reduce Ni²⁺into solution.RGO-NiO/ZnO were tested to degrade the same mass of methyl orange solution in five times,the results showed that the efficiency was maintained at more than 90%.