A simple hydrothermal method is utilized to synthesize iron-doped tin oxide nanoparticles (Fe-SnO₂ NPs) at various doping concentrations. The structural characterization using XRD, Raman, and FTIR measurements confirmed the incorporation of Fe ions into the SnO₂ lattice without any deviation in the tetragonal crystal system of SnO₂ nanoparticles. SEM and HRTEM images show the spherical-shaped nanoparticles with agglomeration. The values of interplanar spacing ([ ]-value) calculated from the HRTEM lattice are consistent with the XRD results. Further, optical analysis revealed a red shift in the optical absorption band and a decrease in the band gap energy with an increase in Fe-dopant concentration. The decrease of PL emission peak intensity with Fe doping revealed the generation of singly charged oxygen vacancies. The H₂O₂-assisted photocatalytic degradation efficiency of Fe-SnO₂ NPs investigated against crystal violet dye indicated an efficiency of 98% for 0.05 M Fe-SnO₂ NPs within 30 minutes under visible light illumination. In addition, the effects of pH, scavengers, and reusability of the catalyst are tested. The antibacterial behavior of Fe-SnO₂ NPs against Escherichia coli is examined by using the colony count method, and the inhibition rate was found to be 49, 65, 70, and 78% for pure, 0.01, 0.03, and 0.05 M Fe-SnO₂ NPs, respectively.

Herein, a facile aqueous method was used to prepare multiscale honeycomb-structured activated carbon from mandarin peels using KOH and NaOH as activation agents, and the prepared multiscale honeycomb-structured activated carbon was carbonized under a nitrogen and argon atmosphere. All samples possessed nitrogen due to its natural abundance and exhibited enhanced specific capacitance, whereas metallic sodium (Na)-intercalated samples, which were formed due to NaOH activation, showed large pore size with increased intercalation capacitance although their surface area was reduced. Potassium has been additionally intercalated during cycling in a KOH solution and helped to sustain the micro/mesoporous honeycomb structure as well as increased the number of O–CO bonds with a concomitant reduction in the number of C–C and CC bonds. The honeycomb carbon retains up to 98% of its initial capacitance even after 7000 cycles. The presence of multiscale porosity in the interconnected carbon network structure enhances the reversible adsorption/desorption of ions in the Helmholtz double layer and hence results in high storage capacity and rate capability of the supercapacitors. The highest energy density of 10.92 W h kg⁻¹ at the power density of 240 W kg⁻¹ has been achieved and maintained up to 7.06 W h kg⁻¹ at the higher power density of 1740 W kg⁻¹. A detailed kinetic study has been employed to better understand the specific capacitance contribution in symmetrical supercapacitors.