2.2. Synthesis of Photocatalysts

SnO₂ was prepared by the hydrothermal method [18]. Firstly, 1.4 g SnCl₄·5H₂O, 2.94 g Na₃C₆H₅O₇·2H₂O and 0.16 g NaOH were dissolved in a mixture of 30 mL absolute ethanol and 45 mL deionized (DI) water, and then stirred vigorously for 1 h to form a homogeneous solution. Then, it was transferred to a Teflon-lined stainless-steel autoclave and heated at 180 °C for 12 h. After cooling to room temperature, the precipitate was collected by centrifugation, then washed successively with deionized water and absolute ethanol. Finally, the obtained SnO₂ was dried at 60 °C in air for 12 h.

In₂O₃ was also prepared by a simple hydrothermal route [19]. Firstly, 0.42 g InCl₃·4H₂O and 1.22 g sodium dodecyl sulfate (SDS) were dispersed into 80 mL deionized water and stirred for 30 min. Next, 0.42 g CO(NH₂)₂ was added to the above mixed solution and stirred vigorously for 1 h. The solution was completely dissolved and transferred to a stainless-steel autoclave lined with Teflon, placed in an electric oven and heated at 120 °C for 12 h. The following steps were the same as synthesizing the SnO₂. Finally, the synthesized samples were calcined in a Muffle furnace at 500 °C for 2 h at a heating rate of 2 °C/min. After cooling to room temperature, In₂O₃ was obtained.

In₂O₃/SnO₂ was prepared by the hydrothermal method. Firstly, 30 mL absolute ethanol and 45 mL DI water were mixed and stirred for 30 min to form solution A. Different amounts of In₂O₃ were dispersed into solution A and ultrasonically dispersed for 30 min to form solution B. Next, 1.4 g SnCl₄·5H₂O was added to solution B and fully dissolved. Then, 2.94 g Na₃C₆H₅O₇·2H₂O was added and stirred for 20 min until completely dissolved. Finally, 0.16 g NaOH was added and stirred for 10 min to form solution C. The following steps were the same as the synthesis of SnO₂. The synthesized sample was calcined in Muffle furnace at 700 °C for 1 h. After cooling to room temperature, the composite material In₂O₃/SnO₂ was synthesized. The amounts of In₂O₃ were 0.1, 0.5, 1.0 and 1.5 wt.%.