A broad class of materials with distinctive physicochemical characteristics, metal oxides are extremely beneficial in a wide range of industrial and scientific applications [1], [2]. Metal oxides including CuO, ZnO, and TiO₂ have strong photocatalytic, antibacterial, and anticancer properties due to their strong redox potential, large surface area, and adjustable band gaps [3], [4], [5], [6]. Doping (e.g., Ag, Ba, or N) promotes charge separation, changes the electronic structure, and boosts stability. Metal oxides have been extensively studied as photocatalysts due to their chemical stability, non-toxicity, and high oxidative ability [7]. The material is extensively utilized in various environmental and energy applications [8], including air purification, self-cleaning surfaces, and, most notably, wastewater treatment [9]. Among its polymorphic phases, anatase TiO₂ is generally recognized for superior photocatalytic activity due to its higher surface area and lower recombination rate of charge carriers compared to rutile [10], [11], [12], [13]. However, the wide band gap of TiO₂ (∼3.2 eV for anatase and ∼ 3.0 eV for rutile) limits its absorption to the ultraviolet (UV) region, restricting its efficiency under visible light irradiation. To overcome this limitation, various strategies such as metal doping, non-metal doping, and heterojunction formation have been explored to enhance the visible light response of TiO₂. Among these approaches, doping with transition and rare-earth metals has shown promising results in modifying the electronic structure of TiO₂, reducing the band gap energy, and minimizing charge carrier recombination.

Doped metal oxides are efficient photodegradation catalysts that break down organic pollutants when exposed to light, offering ecologically friendly wastewater treatment solutions [14], [15]. They are efficient against bacteria and fungus because of their antibacterial activity, which is derived from the production of reactive oxygen species (ROS), membrane rupture, and metal ion release [16]. Furthermore, by causing oxidative stress, DNA damage, and apoptosis in cancer cells, doped metal oxides have encouraging anticancer capabilities, underscoring their potential for use in biomedical applications [17], [18]. One such dopant that has gained attention in recent years is zirconium (Zr), owing to its ability to improve thermal stability, increase surface acidity, and reduce recombination losses in TiO₂-based photocatalysts [9], [10], [11]. The sol-gel method has been widely employed for the synthesis of TiO₂ and Zr-doped TiO₂ nanoparticles due to its ability to produce homogeneous, nanostructured materials with controlled morphology and phase composition [19], [20]. The synthesis conditions, including precursor selection, pH, calcination temperature, and doping concentration, play a crucial role in tailoring the structural and optical properties of the resulting photocatalysts.

Several studies have reported that lower pH conditions promote rutile phase formation, whereas higher pH favors the stabilization of anatase, highlighting the significance of synthetic parameters in material design [21], [22], [23], [24]. The optical properties of TiO₂ and Zr-TiO₂ nanostructures have been extensively studied using diffuse reflectance spectroscopy (DRS) and UV–vis., absorption analysis. The introduction of zirconium ions has been observed to induce a redshift in absorption spectra, extending the photocatalytic response into the visible region. This shift is attributed to localized states introduced within the band gap, which facilitate sub-bandgap absorption, thereby improving light-harvesting efficiency under uv irradiation. Recent findings indicate that compositional engineering significantly improves catalytic performance. Ikram et al. [48] and Moeen et al. [49] reported advanced nanostructured materials exhibiting enhanced catalytic efficiency, underscoring the promise of customized nanomaterials for energy and environmental applications.

Several experimental investigations have demonstrated the superior photocatalytic performance of Zr-TiO₂ compared to undoped TiO₂ in degrading organic dyes such as methylene blue, rhodamine B, and Brilliant Green [35], [36], [37]. The improved activity has been linked to a combination of factors, including enhanced charge carrier separation, increased light absorption, and surface modifications due to zirconium incorporation. The effectiveness of Zr-TiO₂ photocatalysts is further influenced by operational parameters such as pH, catalyst loading, and irradiation time, necessitating optimization for practical applications. Various metal oxides, including zinc oxide (ZnO), tungsten oxide (WO₃), and iron oxide (Fe₂O₃), have also been investigated for their photocatalytic capabilities. ZnO, with a band gap of ∼3.3 eV, shares similar properties with TiO₂ but suffers from photo-corrosion issues under prolonged irradiation. WO₃, on the other hand, possesses a lower band gap (∼2.8 eV) and excellent stability, making it an effective visible-light-driven photocatalyst. Fe₂O₃ has been explored for its ability to utilize a broader solar spectrum, though its rapid recombination of charge carriers limits its performance [25], [33].

Despite the promising photocatalytic attributes of Zr-TiO₂, challenges such as agglomeration, recyclability, and long-term stability remain critical concerns for real-world applications [25], [26]. Recent advancements in surface engineering, co-doping strategies, and heterojunction formation have been explored to further enhance the efficiency and durability of various metal oxides-based photocatalysts [27], [28], [29], [30]. Future research should focus on scalable synthesis approaches, stability assessments under prolonged exposure conditions, and practical implementation in wastewater treatment systems [31], [32], [33], [34]. In this context, this study generated nanocrystalline Zr-TiO₂ nanoparticles using a controlled sol-gel technique, focusing on pH-regulated phase evolution and zirconium-induced structural change. Unlike prior publications, this study systematically links phase composition, band gap tuning, and charge carrier behavior to multifunctional performance. The photocatalytic effectiveness of Brilliant Green degradation under UV irradiation was studied, as well as the precise adjustment of reaction parameters. The work goes beyond traditional photocatalysis to evaluate the material's photobiological (antibacterial, antioxidant, anti-inflammatory, and anticancer assessments) potential. This provides a full structure-property-performance link for advanced TiO₂-based systems.