Molecular precursor pathway to Co1−xNixNb2O6 for photocatalytic dye degradation: Optical and charge transport studies

Abstract

A new approach for the formation of binary and ternary mixed metal oxides, namely, CoNb₂O₆, NiNb₂O₆, and Co_0.5Ni_0.5Nb₂O₆, through a modified single-source molecular precursor route was investigated. By optimizing the annealing temperature, heating/cooling rates and holding times, the phase compositions of thermal processing of mixtures of oxalate-based complexes [M(bpy)₃]₂[NbO(C₂O₄)₃]Cl·nH₂O (M = Co²⁺ (1) or/and Ni²⁺ (2); n = 11, 12) and (NH₄)₃[NbO(C₂O₄)₃]·3H₂O (3) were evaluated. Phase-pure columbite oxides Co_1−xNi_xNb₂O₆ (x = 0, 0.5, and 1) were obtained in one step by heating the mixtures of two (1 and 3, or 2 and 3) or three oxalate complexes (1 and 2 and 3) at 1200°C for 10 h with a heating/cooling rate of 10°C min⁻¹. The (micro)structure, morphology, and optical properties of the as-prepared materials were characterized by powder X-ray diffraction (PXRD), field-emission scanning electron microscopy (FE-SEM) and ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS). Impedance spectroscopy was used to investigate the electrical and dielectric properties, providing valuable insights into the charge transport dynamics. The CoNb₂O₆ exhibits the greatest conductivity in the series (2.64 × 10⁻¹¹ Ω⁻¹ cm⁻¹ at 120°C), two orders of magnitude greater than those of NiNb₂O₆ and Co_0.5Ni_0.5Nb₂O₆. The ternary oxide Co_0.5Ni_0.5Nb₂O₆ has a minimum in conductivity and dielectric loss, but it shows a maximum in dielectric permittivity. Estimated band gap energies of CoNb₂O₆, NiNb₂O₆, and Co_0.5Ni_0.5Nb₂O₆ within the visible light excitations prompt us to investigate their photocatalytic activities in the degradation of the methylene blue (MB) dye under visible light irradiation without and with hydrogen peroxide (H₂O₂). It is evident that the Co_1−xNi_xNb₂O₆/H₂O₂ systems exhibit higher photocatalytic activity: the degradation efficiency of MB was found to be between 26% and 38%. In addition, the cyclic stability of the photocatalyst was investigated and a possible proposed photocatalytic degradation mechanism was substantiated by a scavenger analysis study.