*2.4. Photocatalytic Activities*

The photocatalytic activities of the as-synthesized nanomaterials were tested on the degradation of methyl red dye (Scheme 1) as a model pollutant under sunlight as an energy source at ambient temperature. The degradation experiments were performed as follows: 500 L glass vessels containing 60 mg/L solution of methyl red and appropriate amount of nanomaterials (18 mg/L) were stirred under a dark for 1 h to attain absorption–desorption equilibrium before irradiating. All experiments were carried out under direct solar irradiation explicitly sunny days in between 10 AM and 2 PM when the solar intensity variations were insignificant [29]. Thereafter, the mixed solution was exposed to sunlight and 8 mL of the heterogeneous mixture was withdrawn within 20 min intervals of irradiating to sunlight, centrifuged at 3000 rpm for 6 min and filtered using filter paper to separate the catalyst before conducting the absorption measurement. The pure supernatant liquid was evaluated for methyl red dye concentration after measuring the absorbance at 410 nm using an ultraviolet–visible (UV–Vis) spectrophotometer and the percentage degradation of methyl red solution was calculated according to the following equation:

$$\% \text{ degradation} = \frac{\text{A}\_{\text{0}} - \text{A}\_{\text{t}}}{\text{A}\_{\text{0}}} \times 100\% \tag{1}$$

where A0 is initial absorbance of methyl red dye and At is absorbance of methyl red at each time interval "t".

## **3. Results and Discussion**
