Optimized Mn doped ZnO@rGO nanocomposites: a breakthrough for advanced energy storage and PEC systems

Abstract

Pristine ZnO and Mn doped ZnO (Mn-ZnO) were synthesized by sol–gel auto-combustion method. Three different concentrations of Mn-ZnO (1%, 5%, and 10%) were treated with rGO in hydrothermal reactor to obtained (1%, 5%, and 10%) Mn-ZnO@rGO nanocomposites. X-ray diffraction (XRD) pattern confirmed the successful incorporation of Mn into the ZnO lattice and the formation of Mn-ZnO@rGO nanocomposites. The peak at 2θ = 25.16° signifies the presence of reduced graphene oxide (rGO) thus confirmed formation of Mn-ZnO@rGO nanocomposite. Rietveld refined pattern showed that all the samples are pure with wurtzite structure. The Fourier-transform infrared (FTIR) spectroscopy revealed the presence of metal–oxygen bonds and functional groups within the composites. The stretching vibration of ZnO at 464 cm⁻¹ confirmed the wurtzite structure. The peak at 600 cm⁻¹ and 880 cm⁻¹ assigned for vibrational, antisymmetric stretching mode of MnO and Mn–O respectively. Significant peaks at 1080 cm⁻¹ and 1392 cm⁻¹ are due to C-O stretching vibrations from C–O–C bonds and C–OH bending vibrations, respectively. The peak at 1432 cm⁻¹ indicates the -C = O group stretching vibration from inorganic carbonate species. The peak at 2850 cm⁻¹ corresponds to the symmetrical stretching vibration of the C-H group. FTIR analysis confirmed formation of Mn-ZnO@rGO nanocomposite. Field emission scanning electron microscopy (FESEM) images demonstrate a high density of irregularly sized nanoparticles, confirming the effective deposition of Mn-ZnO nanoparticles on to rGO sheets and the robust binding of these nanoparticles, resulting in Mn-ZnO@rGO nanocomposites. X-ray photoelectron spectroscopy (XPS) provided detailed insights into the oxidation states of the elements with a focus on 5% Mn-ZnO@rGO nano composite. The survey spectrum for the 5% Mn-ZnO@rGO nanocomposite confirmed the presence of Zn, Mn, O, and C. The lack of contaminants peaks in the XPS analysis supports the successful synthesis of Mn-ZnO@rGO nanocomposite. Raman spectroscopy detected vibrational modes between 1300 cm⁻¹ and 1600 cm⁻¹, which are characteristic of rGO and Mn-ZnO@rGO nanocomposites. There is increase in the D band and G band intensity ratio (ID/IG), and this confirms the disorder in the carbon components. It further confirmed that during the Mn-ZnO@rGO composite's formation, GO was converted to rGO. Electrochemical performance, assessed through Electrochemical Impedance Spectroscopy (EIS), Galvanostatic Charge–Discharge (GCD), and Cyclic Voltammetry (CV). Long-term cycling stability over 5000 cycles indicated that 5% Mn-ZnO@rGO nanocomposite exhibited superior performance compared to 1% and 10% counterparts. Additionally, photoelectrochemical cell (PEC) measurements further validated the exceptional performance of the 5% Mn-ZnO@rGO nanocomposite. These findings demonstrate that the 5% Mn-ZnO@rGO nanocomposite is a highly promising material for supercapacitor and PEC applications.