Enhanced stability and tunable bandgap of Zn- and Cu-doped cobalt aluminate

Abstract

Pristine and metal-doped cobalt aluminate samples were well synthesized using the sol–gel auto-combustion method. The physical properties of the synthesized samples were investigated employing X-ray diffraction and Raman spectroscopy. To validate structure stability, the influence of lattice strain was carried out by comparing Debye–Scherrer’s and Williamson-Hall plot method. Furthermore, Rietveld analysis was performed for precise structural parameters. Four Raman active modes (T_2g⁽²⁾ + T_2g⁽³⁾ + E_g + A_1g) were examined within the range of 400 to 900 cm⁻¹, confirming the formation of spinel structure. Here, an anomaly of the quadruple splitting of A_1g mode in pristine and Zn²⁺-doped cobalt aluminate was observed. The FTIR spectroscopy shows metallic bonds confirming the absence of other functional groups. In UV–Vis spectroscopy, doping of Cu²⁺ (0.72 Å) and Zn²⁺ (0.74 Å) with nearly comparable ionic radii as Co²⁺ (0.72 Å), optical bandgap increases ranging from 3.1 to 4.29 eV. Further, Urbach’s energy was plotted for all samples. Results show that doping of Cu²⁺ and Zn²⁺ supports reducing local lattice distortion. These features improved stability and wide optical bandgap ensure a vital role in state-of-the-art applications.