*3.4. Synthesis of Ternary NiCdTiO<sup>2</sup> Nanocomposites*

Ternary NiCdTiO<sup>2</sup> nanocomposites were prepared via co-precipitation method. Nickel (II) chloride hexahydrate (0.030 mol, 7.13 g) and cadmium (II) chloride hemi(pentahydrate) (0.030 mol, 6.85 g) were dissolved separately in deionized water (250 mL) and stirred for 30 min at ambient temperature. The pH (9–10) of the solutions were adjusted via dropwise addition of 1 M NaOH solution. The precipitates formed during the process were centrifuged and washed with deionized water followed by methanol three times. The obtained precipitates were oven dried at 80 ◦C for 6 h and annealed at 550 ◦C for 4 h. The synthesized Ni (0.001 M) and Cd (0.01 M) NPs were dispersed separately in 5 mL aqueous methanol via sonication for 30 min. The Ni and Cd dispersions were mixed with TiO<sup>2</sup> (1 M) solution made in aqueous methanol. The mixture was sonicated continuously at 80 ◦C for 30 min. The precipitates were centrifuged and the product NiCdTiO<sup>2</sup> was kept overnight and then dried at 100 ◦C for 5 h.

#### *3.5. Characterization*

The morphology and surface analysis were achieved through SEM (Model No. JEOL-5910; company Japan) and TEM (Tecnai F-20 FEI, USA). The elemental analysis was carried out via EDX (model INCA 200/oxford instrument, UK), while the size and phase of crystals were evaluated through XRD measurements (Model JEOL-300). The surface area and the porosity of the materials were evaluated applying a N<sup>2</sup> adsorption instrument (Micrometrics ASAP 2020). The photocatalytic degradation of the dyes was monitored using UV-VIS spectrophotometer (UV-1800, Shimadzu, Japan).

#### *3.6. Dyes Photodegradation*

The photocatalytic activity of the prepared catalysts was evaluated for the photodegradation (PD) of methylene blue (MB) and methyl green (MG) dyes in aqueous media under UV-light. The photocatalysts were individually added to 5 mL deionized water and sonicated constantly for 30 min, followed by an addition of 10 mL dye solutions (30 ppm) to each beaker. The beakers were covered with transparent plastic sheets. The mixtures were again sonicated for 30 min, kept in the dark to attain the adsorption/desorption equilibria and then kept under UV light (UV lamp (254 nm, 15 W)) with constant stirring for specific times. The catalysts were carefully removed through centrifugation (1200 rpm, 10 min). The study was completed by evaluating the effect of irradiation time (20, 40, 60, 80 and 100 min with a 0.03 g of catalyst), photocatalyst dosage (0.010, 0.015, 0.020, 0.025 and 0.03 g at 30 min irradiation time) and medium pH (2, 4, 6, 8 and 10 with 0.03 g catalyst and 30 min

irradiation time) on dyes degradation. The degradation study was followed by UV-Vis spectrophotometer and the %degradation was found by applying the equation:

$$\text{Depradation rate} \left( \% \right) = \left( \frac{A\_o - A}{A\_o} \right) \times 100 \tag{2}$$

where *A<sup>o</sup>* and *A* show dye absorbance before and after UV light irradiation, respectively. While for photocatalytic degradation, a pseudo first-order kinetic model was proposed.

$$-\frac{d\mathbf{C}}{dt} = k\_{ap}\mathbf{C} \tag{3}$$

$$\ln\left(\frac{\mathbb{C}}{\mathbb{C}\_0}\right) = k\_{ap}\mathbf{t} \tag{4}$$

where C0: initial concentration, C: final concentration, *kap*: apparent rate constant and t: time in minutes.

#### **4. Conclusions**

In the present study, neat TiO<sup>2</sup> nanoparticles and its binary CdTiO<sup>2</sup> and ternary NiCdTiO<sup>2</sup> nanocomposites were synthesized by precipitation method. The synthesis of the TiO<sup>2</sup> and their binary and ternary nanocomposites were confirmed through various instrumental techniques. The synthesized materials have shown significant photocatalytic degradation of MB and MG dyes. The photodegradation results demonstrate that the dyes degradation increases with increasing irradiation time, catalyst dosage and pH of the medium. The increase in the degradation of the dyes with increasing photocatalyst dosage is due to more available active sites for dye adsorption followed by photodegradation. Similarly, the rapid photodegradation of the dyes at higher pH values could be a result of more reactive hydroxyl radicals formation. The coupled CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> nanocomposites result in the synergistic effect and have shown better photocatalytic efficiency compared to the neat TiO<sup>2</sup> nanoparticles. The NiCdTiO<sup>2</sup> nanocomposite is the most efficient photocatalyst for the degradation of MG (97.5% degraded in 100 min) as compared to MB (86% degraded in 100 min).

**Author Contributions:** Conceptualization and methodology, S.K., I.K.; Initial manuscript preparation, M.M.; Review and editing of the manuscript, M.S., A.N.; Investigation and supervision, N.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research is supported by the Deanship of Scientific Research, Vice Presidency for Graduate Studies and Scientific Research, King Faisal University, Saudi Arabia (grant no. Grant2058).

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors are grateful to Abdul Wali Khan University Mardan, for supporting the work.

**Conflicts of Interest:** The authors of this work have no conflict of interest to declare.

#### **References**


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