A major environmental threat that humanity faces today is the contamination of water caused by hazardous chemicals released into the environment by industrial processes. Routine daily activities can cause hazardous materials to be introduced into natural water resources, such as aquifers, lakes, oceans, rivers, and groundwater aquifers. Continually introducing such contaminants into natural water can result in an alteration of the characteristic features of natural water as well as polluted water. This is extremely unsuitable for human health. Chemical pollutants like dyes for textiles, halogenated compounds, substituted phenols, insecticides, pesticides, weed killers, and herbicides are known to be carcinogenic and pose major health risks to humans (Anku et al., 2017; Samanta et al., 2019; Brillas and Garcia-Segura, 2020; Kanan et al., 2020; Rafiq et al., 2021). Among toxic organic pollutants- chlorophenols and nitrophenols, because of their stable structure, toxic and carcinogenic effects on the human being, have been a remaining area under concern (Chiou et al., 2008; Adewuyi et al., 2016; Umukoro et al., 2017; Sharma et al., 2019; Barakat et al., 2020). Therefore, it is highly desirable to remove these toxins effectively from the polluted water.

The structural properties of these phenols and their derivatives have made them difficult to remove from the polluted water by classical methods, i.e., activated carbon adsorption, chemical oxidation, and biological treatment. Because activated carbon adsorption leads to phase separation without degradation of injurious pollutants, the chemical oxidation method is unable to remove pollutants efficiently, and biological treatments are slow and pH and temperature dependent, while the advanced oxidation process seems to be better for effective removal of such sort of destructive pollutants using light (Eryılmaz and Genç, 2021; Ren et al., 2021; Saputera et al., 2021).

Advanced oxidation leads to the formation of highly reactive species which ultimately react with organic pollutants to degrade them successfully (Mazivila et al., 2019; Du and Zhou, 2021; Gallo-Cordova et al., 2021; Huang et al., 2021; Luo et al., 2021). Among advanced oxidation processes, photocatalytic degradation and mineralization are efficient methods of converting highly toxic organic pollutants to harmless species under ambient conditions using light and a photocatalyst through homogeneous or heterogenous photocatalysis. In addition, heterogeneous photocatalysis is considered advantageous over homogenous photocatalysis due to an easy retrievability of a catalyst from the reaction mixture and then the reusability of a catalyst (Gisbertz and Pieber, 2020). Following the concept of the advanced oxidation process, the heterogeneous photocatalytic reaction starts with the generation of H₂O₂, OH^∙, O₂ ^∙-, HOO^∙ and H⁺ species by the absorption of a photon having energy equal to or greater than the band gap energy of the photocatalyst which is being used (Loeb et al., 2019; Pandey et al., 2020; Serrà and Philippe, 2020; Danish et al., 2021). In addition, hydroxyl and superoxide anion radicals react with phenols, depending on the position and nature of substituent groups on ph enols, to degrade them into some oxygenate intermediates and consequently, these oxygenates mineralize to their respective harmless species (Qamar et al., 2017a; Qamar et al., 2017b; Alhogbi et al., 2020).

Metal oxide-assisted photocatalytic wastewater treatment is a relatively prospective subject and growing rapidly to remove hazardous pollutants from contaminated water. In this context, metallic oxide semiconductors such as TiO₂, ZnO, Fe₂O₃, SnO₂, WO₃, Bi₂O₃, V₂O₅, Cu₂O, NiO, etc. have been studied extensively both in artificial and natural light sources to acquire pollution free water (Ahmed et al., 2010; Oturan and Aaron, 2014; Wang et al., 2014; Qamar et al., 2015; Zangeneh et al., 2015; Aslam et al., 2018). However, exploration of a potential photocatalyst for the effective removal of toxic pollutants from the wastewater has not over yet and researchers are paying attention to study the photocatalytic properties of CeO₂-based photocatalysts for the abatement of organic pollutants. Although, bare CeO₂ has wide application in electrocatalysis, solar cells, fuel cells and photocatalysis due to its high chemical stability, low toxicity and greater oxygen storage capacity (Ma et al., 2019; Fauzi et al., 2022). However, its performance in photocatalysis is unsatisfactory due to greater photo excitons’ recombination, lower absorption cross-section of light spectrum and higher band gap (2.8–3.1 eV) energy (Ma et al., 2019; Fauzi et al., 2022). Therefore, it is a need to modify CeO₂ in order to increase its utilization as a photocatalyst for removal of organic toxins. The unique thing of this study is the introduction of transition metals into CeO₂ without significantly changing the cubic structure of CeO₂ photocatalyst for the removal of 15 ppm 2-chlorophenol in the illumination of 70 W indoor white light.

Previously, researchers utilized expensive artificial light sources such as solar simulator, Hg and Xe lamps for photocatalytic degradation of various organic toxins using variety of photocatalysts (Tang et al., 2004; Hao et al., 2015; Aslam et al., 2016; Xu et al., 2017). In addition, a few studies are available for the photocatalytic removal of 2-CP using CeO₂-based photocatalysts such as CeO₂, g-C₃N₄(0.94)/CeO₂(0.05)/Fe₃O₄(0.01), TiO₂-CeO₂-ZrO₂ (Aslam et al., 2016; García-Hernández et al., 2019; Rashid et al., 2019). However, various combinations of CeO₂ with metals, non-metals and photocatalysts such as Ag₂CO₃, rGO, CdS, AgBr, SrFe₁₂O₉, BiOCl, g-C₃N₄, Co₃O₄, etc., are reported for the photocatalytic removal of organic dyes, phenols and pharmaceutical ingredients (Ma et al., 2019; Yao et al., 2021; Fauzi et al., 2022). Moreover, literature reveals that a photocatalysis setup while using an economical 70 W indoor white light for the removal of 2-CP over the proposed composition of CeO₂-based photocatalysts has not been reported before.

In this study, CeO₂ has been synthesized using a co-precipitation methodology and its modification was executed through doping with transition metals, i.e., Fe, Ni and Cu because 3d transition metals are considered capable dopants in tuning the properties of CeO₂ for catalysis applications (Yue and Zhang, 2009; Elias et al., 2014; Qi et al., 2019). The spectral response and bandgap of the synthesized materials were evaluated using UV-visible diffuse reflectance spectroscopy (UV-visible DRS) whereas photoluminescence (PL) fluorometer was used to investigate e ^- –h ⁺ recombination. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to evaluate the structural and chemical characterization of synthesized materials. The photocatalytic activity of the synthesized photocatalysts was studied for the removal of 2-chlorophenol (2-CP) the kinetics for the photodegradation of 2-CP were also investigated.

The solid-state absorption response of the pristine CeO₂ compared to the modified CeO₂ is presented in Figure 1A in UV (200–400 nm) as well as visible (400–800 nm) regions of the spectrum. Wherein, Cu-doped CeO₂ shows higher absorbance in both UV and visible regions than other synthesized materials. Interestingly, Cu-doped CeO₂ and Fe-doped CeO₂ showed higher absorbance of visible light than CeO₂ and Ni-doped CeO₂ whereas in the UV region of the spectrum only Cu-doped CeO₂ presented higher absorbance than other photocatalysts. So, the insertion of Cu and Fe content into the structure of CeO₂ leads to an increase in the spectral response of CeO₂ whereas the same amount of Ni content has a detrimental effect on the spectral response of the pristine CeO₂. Moreover, the response of these photocatalysts in reflecting the light spectrum has also been explored in Figure 1B. Wherein, lower reflectance (%) was noticed for CeO₂ having copper contents followed by Fe and Ni contents in the lower energy region of the spectrum. This higher absorbance and lower reflectance (%) were attributed to the higher absorbance ability of copper content present in the structure of CeO₂ as compared to Fe and Ni.

The direct and indirect band gap evaluations (Figures 2A, B) reveal a lowering in the band gap energy of Cu and Fe-doped CeO₂ photocatalysts as compared to pristine CeO₂ was noticed which may be attributed to the shifting of conduction band edges to lower energy due to the addition of Cu and Fe contents in the structure of CeO₂ (Yue and Zhang, 2009; Qi et al., 2019). Whereas a significant increase in the band gap energy of Ni-doped CeO₂ was observed. The direct and indirect band gaps of the pristine CeO₂ were noticed at ∼3.0 and ∼2.9 eV, respectively which have good agreement with the literature values (Aslam et al., 2016). Moreover, the direct band gap energies found for Cu, Fe, and Ni-doped CeO₂ photocatalysts are ∼2.85, ∼2.8, and ∼3.2 eV whereas evaluated indirect band gap energies are ∼2.78, ∼2.7, and ∼3.0 eV, respectively as shown in Figures 2A, B. A greater decrease in the band gap energy was noticed for Fe doped CeO₂ as compared to other photocatalysts which reveal that this photocatalyst requires lower energy as compared to pristine and other modified photocatalysts to make good use of excitons for the removal of organic toxins under the illumination of light.

Photoluminescence analysis is a promising technique to study the emission intensity of the excited electron under the illumination of light which ultimately leads to the study of e ^- –h ⁺ recombination process. Figure 3 depicts the comparison of photoluminescence spectra of pristine and doped CeO₂ photocatalysts, wherein a significantly lower emission intensity was noticed for Cu and Ni-doped CeO₂ as compared to unmodified CeO₂. The relative decrease in emission intensities of the Cu and Fe doped CeO₂ as compared to CeO₂ is strong evidence for lowering e ^- –h ⁺ recombination rate than pristine CeO₂ which favors the photocatalytic degradation of organic toxins. Moreover, the decrease in strong emission bands around 470 nm is attributed to the trapping of photoexcitons by the surface defects generated due to the insertion of Cu or Fe into the structure of CeO₂ (George et al., 2020).

The comparison of x-ray diffraction patterns of the modified CeO₂ with pristine CeO₂ is presented in Figure 4 and the cubic phase of the synthesized CeO₂ was confirmed by matching diffractions planes (111), (200), (220), (311), (222), (400), (331), and (420) with literature and JCPDS 34-0394 (Aslam et al., 2016). The absence of additional diffraction peaks related to the dopant entities in the respective diffraction patterns verifies the successful doping of CeO₂ with Cu, Ni, and Fe. Moreover, few changes such as reduction in few peaks’ height, minor peak shift at 2θ (28.525^°) and peaks’ broadening in XRD patterns of modified CeO₂ as compared to pristine CeO₂ also favour the introduction of dopants in CeO₂ (Kumar et al., 2010). Moreover, the successful insertion of dopants into CeO₂ without significantly altering its structure were also evident from the micrographs as shown in Figure 5. The average crystallite size of the photocatalytic materials was calculated using a high-intensity diffraction peak at 2θ (28.525^°) with the help of the Debye-Scherrer equation. The calculated crystallite sizes were 9.76, 3.46, 4.97, and 8.28 nm for pristine CeO₂, Fe doped CeO₂, Cu doped CeO_2, and Ni doped CeO₂, respectively. Moreover, a significant difference in doped CeO₂ as compared to unmodified CeO₂ were also reported in literature which support the changes in crystallite sizes of Fe and Cu doped CeO₂ (Yue and Zhang, 2009; Kumar et al., 2010; Qi et al., 2019).

As this study was designed to investigate the effect of transition metal dopants on the photocatalytic performance of CeO₂ under the illumination of indoor white light for the degradation of 15 ppm 2−CP. Before the exposure of suspension containing photocatalyst and 2-CP to light as mentioned in the experimental section, the suspension was kept in dark for 30 min to establish an equilibrium between pollutant and catalyst. The photolysis of 2-CP was also evaluated by recording the absorption spectrum of the substrate after 240 min of light exposure without the presence of a photocatalyst. The amount of photocatalyst was also optimized (100 mg of photocatalyst) while studying the photocatalytic removal of 15 ppm 2-CP with varying doses of CeO₂ catalyst under the illumination of indoor white light. The comparison of absorption spectra of photocatalytic removal of 2-CP over pristine CeO₂, Fe doped CeO₂, Cu doped CeO₂, and Ni-doped CeO₂ under the illumination of indoor white light (800 × 102 lx) is provided in Figure 6, respectively at different exposure time. Whereas the photocatalytic degradation (%) of 2-CP at different exposure over the pristine and modified CeO₂ photocatalysts is given in Figure 7, respectively and highest photodegradation (∼ 65%) was noticed for Fe doped CeO₂ followed by Cu doped CeO₂ (∼ 60%), pure CeO₂ (∼59%) and Ni doped CeO₂ (∼45%) under the exposure of white light after 240 min of exposure. The decreasing trend of removal efficiency (%) of the photocatalysts is given below. F e   d o p e d   C e O 2   >   C u   d o p e d   C e O 2   >   C e O 2   >   N i   d o p e d   C e O 2

Moreover, the rate of photodegradation of 15 ppm 2-CP over synthesized photocatalysts was also investigated and higher photocatalytic removal efficiency with rate constant (k = 3.9 × 10⁻³ min^-1) was observed by Fe doped CeO₂ than others as shown in Figure 8. The calculated bandgap energy values as shown in Figure 2 also support the possible higher photodegradation efficiency of Fe-doped CeO₂ due to its lower bandgap energy than CeO₂. Moreover, a lower photo-excitons’ recombination in PL spectra presented by Fe-doped CeO₂ also arguments its higher removal efficiency due to the possible charge transferability for the generation of ROS. The decreasing trend of rate of removal of 2-CP by different photocatalysts is provided below. F e   d o p e d   C e O 2   >   C e O 2   >   C u   d o p e d   C e O 2   >   N i   d o p e d   C e O 2

The kinetic study also reveals that the photocatalytic removal of 2-CP by Fe doped CeO₂, pure CeO_2, Cu doped CeO₂ followed Langmuir Hinshelwood (L-H) kinetic model whereas the photocatalytic removal of 2-CP by Ni doped CeO₂ did not follow Langmuir Hinshelwood (L-H) kinetic model. Previously, Zhang et al. (2022) reported a decrease in degradation rate of 2-CP removal in the illumination of 5W LED white light as compared to 300 W Xe arc lamp. In another study, BiFeO₃/Bi₂Fe₄O₉ heterojunctions were able to remove 95% of the 2-CP in the exposure of 150W LED white light (Wang et al., 2022).

In this study, x-ray photoelectron spectroscopy (XPS) was also carried out to investigate the chemical and electronic states of the elements in modified materials as shown in Figure 9. The presence of Ce3d_3/2, Ce3d_5/2, Fe2p_1/2, Fe2p_3/2, Cu2p_1/2, Cu2p_3/2, Ni2p_1/2, Ni2p_3/2 core levels and O1s levels in the synthesized photocatalysts can be seen in Figure 9, which confirm the existence of dopants (Fe³⁺, Cu²⁺ and Ni²⁺) in doped CeO₂ as shown in inset figures. Moreover, the effect of dopants was also verified by the appearance of peak correspond to O1s level at different binding energies which ultimately confirm the different environment of oxygen.

In conclusion, cubic CeO₂ was doped successfully with Fe, Cu and Ni through co-precipitation method followed by calcination at 400°C for 4 h. The crystallite sizes were reduced from 9.76 to 3.46 nm by incorporating 3d transition metals into CeO₂. Solid-state absorption analysis revealed a better spectral response and lower bandgap energy of Fe and Cu-doped CeO₂ than unmodified CeO₂ whereas increase in bandgap energy was noticed for Ni-doped CeO₂. Moreover, the insertion of Cu and Fe into CeO₂ suppressed its e ^- –h ⁺ recombination process by generating trapping sites for photo-excitons. Among the synthesized photocatalysts in this study, Fe-doped CeO₂ is excellent for the removal of 2-CP in the illumination of 70 W indoor LED white light.