Photoluminescence properties of CuO/ZnO composite powders as a function of their composition

Nanocrystalline CuO/ZnO composite powders with CuO:ZnO molar ratios of 100:0, 98:2, 92:8, 60:40, 40:60, 8:92, 2:98, and 0:100 were synthesized by the sol–gel auto combustion method using diethanolamine (DEA) as fuel and subsequently calcined at 1000 °C for 4 h. The structural, morphological, compositional, local atomic, and photoluminescence properties were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), synchrotron-based X-ray absorption near-edge structure (XANES), and photoluminescence (PL) spectroscopy. XRD analysis confirmed the formation of phase-pure composites consisting solely of monoclinic CuO and hexagonal wurtzite ZnO, without detectable impurity phases. The average lattice parameters of the ZnO phase were a = b = 3.246 Å and c = 5.201 Å (c/a = 1.602), whereas the CuO phase exhibited average lattice parameters of a = 4.688 Å, b = 3.420 Å, and c = 5.129 Å. SEM observations revealed a morphology evolution from polyhedral CuO particles of 3–5 μm and hexagonal ZnO particles of 200–300 nm to mixed morphologies with particle sizes ranging from 500 nm to 2 μm in the composites. XANES measurements demonstrated that both Cu and Zn ions retained four-fold oxygen coordination throughout the entire composition range. PL spectra exhibited emission bands at 387, 393, 409, 422, 428, 442, 453, and 459 nm, corresponding to ultraviolet, violet, and blue luminescence that may be associated with defect-related electronic states, including oxygen-vacancy-, metal-interstitial-, and cation-vacancy-related centers reported in the literature. Increasing ZnO content significantly enhanced the PL intensity and induced a blue shift of the emission bands. These findings demonstrate that tuning the CuO/ZnO ratio effectively modifies the structural and luminescence characteristics of the composites, making them promising candidates for photocatalytic and fluorescence-related applications.