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Herein we have demonstrated facile synthesis approach to grow K + and Na + ions preinserted MnO 2 (K x MnO 2 and Na x MnO 2 ) nanostructured thin films with the help of successive ionic layer adsorption and reaction (SILAR) method. K x MnO 2 and Na x MnO 2 thin films were grown directly on the stainless-steel substrate at room temperature using different reducing agents like manganese sulfate (MnSO 4 ) and sodium borohydride (NaBH 4 ). Reducing agents control reduction of MnO 4− ions, morphological evolution, and preinsertion of Na + ions in MnO 2 matrix. The plausible growth mechanism was proposed behind the occurrence of different morphologies. Due to preinsertion of alkali ions, nonstoichiometric MnO 2 phase was formed and confirmed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy studies. The electrochemical measurements of Na x MnO 2 thin films demonstrated and 1.45 times superior specific capacitance at a higher scan rate (100 mV/s) and current density (32 A/g) than those of K x MnO 2 thin films. Enhanced surface capacitance of Na x MnO 2 thin films corresponds to superior electric double layer capacitance (EDLC) and surface redox reactions between Mn 3+ /Mn 4+ ions. On the contrary, K x MnO 2 thin films showed higher diffusion controlled reactions, which were supported by electrochemical impedance spectroscopy (EIS) data. Insertion of Na + ions was found to be advantageous over K + ions, because it provides higher specific capacitance values, specific morphology, improved capacitive retention, accessibility of more electroactive area, stable salt concentration and electrical conductivity of the electrolyte, fast and dynamic utilization of electrode material and higher conservation of energy.

Novel hierarchical nanostructures have huge potential in different applications. Herein, we report a facile way of engineering different morphologies of zinc oxide (ZnO) thin films by using successive ionic layer adsorption and reaction (SILAR) method. By changing the degree of supersaturation, significant variation in the crystal orientation and the morphology were obtained for ZnO thin films. X-ray diffraction (XRD) study revealed wurtzite crystal structure of zinc oxide nanostructures, which was further supported by Raman spectra. Surface morphology unveils nanostructures such as nanoflakes, nanoflowers, and branched nanorods. To our best knowledge, this is the first report on branched nanorod nanostructures by SILAR method. Nucleation and crystal growth are driven by the amount of Zn(OH) 4 2− species in the solution through which new structures can be obtained. Photoluminescence spectra revealed ultraviolet and visible emission (green and yellow) from all of the morphologies. Wettability test shows hydrophobic surfaces of all of the morphologies. This one-step, wet-chemical mild approach is controllable and reproducible for obtaining different morphologies which can be conveniently transferred to applications such as photocatalysis and gas sensing.

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