Exploring the impact of Sm3+ doping on the structural, optical, and photocatalytic properties of ZnAl2O4 spinels

The development of novel nano-photocatalysts to address the ongoing water purification and organic dye degradation issues is a major concern to the research community today. In view of it, in the present study, Sm3+-doped ZnAl2O4, (ZnAl2-2xSm2xO4; x = 0, 0.005, 0.01, 0.02 & 0.03) nano-catalysts were prepared through solution combustion method using Isoleucine as a fuel. Systematic studies were conducted to evaluate their structural, microstructural and band structure properties to ascertain the feasibility of their use as photocatalysts against acid orange 8 dye under visible light irradiation. It was observed that the substitution of Sm3+ at the Al3+ site has a greater impact on the formation of single-phase spherical nanoparticles of the sample crystallized in the cubic crystal structure. With the increase in the concentration of Sm3+, the average crystallite size was found to decrease towards the quantum limit. Additionally, BET analysis reveals an increase in specific surface area and pore size, accompanied by a reduction in pore volume with higher Sm3+ concentrations. SEM micrographs reveal that the prepared samples are highly porous in nature, supporting the XRD and BET analysis. Also, the band gap was found to increase with Sm3+ concentration. It is worth noting that ZnAl2-2xSmxO4 nanoparticles demonstrate significant photocatalytic activity against Acid Orange 8 dye under visible light, although the band gap increases with Sm3+ concentration. Notably, 3 mol% Sm3+-doped ZnAl2O4 achieves 92 % degradation efficiency of Acid Orange 8 under visible light due to its high surface area, increased active sites, and reduced photogenerated charge carrier recombination. The photocatalytic dye degradation mechanism highlights the role of dye sensitization in enabling visible light photocatalytic activity for Sm3+-doped ZnAl2O4, which primarily absorbs in the UV region. The process relies on the efficient transfer of electrons from the excited dye to the photocatalyst, followed by the formation of reactive species that drive the degradation of pollutants. The corresponding mechanism for this is proposed in detail.