Effect of reaction pH on the microstructural, optical and electrical behaviour of ZnO/CuO heterostructured nanocomposite films

CuO films and ZnO/CuO heterostructured nanocomposite films were synthesized by a series of chemical reactions at different pH values based on the hydrothermal and SILAR (successive ionic layer adsorption and reaction) methods. The ultimate purpose of this work is to explore the impacts of solution pH value and ZnO-based heterostructure on the structural, optical and electrical features of CuO films using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), transmittance spectrum, current-voltage (I-V) measurements and capacitance-conductance (C-G) measurements in 300-600 K temperature range and 20 Hz-1.5 MHz frequency range. XRD analysis revealed that the crystallite size and strain lattice parameters decrease with the increase in pH value without affecting the crystalline structure of the synthesized films. SEM images revealed the regular growth of ZnO structure without any deterioration in the CuO substrate layer, where an increase in the thickness of the formed rods was observed with the increase in reaction pH value. Transmittance dropped greatly with the formation of heterojunction from about 40% (for CuO) to about 1% (for ZnO/CuO) in the visible range followed by a crystallite size-induced change in bandgap from 2.29 to 2.32 eV (for CuO) and from 2.03 to 2.11 eV (for ZnO/CuO). The increase in pH from 11.0 to 11.2 increased the room-temperature resistance of ZnO/CuO composites from 377 to 474 M omega, the ideality factor from 4.41 to 4.68 and the barrier height from 0.74 to 0.83 eV. It can be deduced that reaction pH value is an important synthesis parameter that can tune the structural, morphological, optical and electrical properties of ZnO/CuO nanocomposite films for advanced industrial or technological implementations by controlling the nucleation and crystal growth rates during chemical synthesis processes.