*3.2. Synthesis of Cu2O*

The octahedron Cu2O (denoted as od-Cu2O) was prepared based on a previously reported method [31] with some modifications. In a typical process, 100 mL of 0.01 mol L−<sup>1</sup> CuCl<sup>2</sup> aqueous solution and 10 mL of 2 mol L−<sup>1</sup> NaOH aqueous solution and 0.6 mol L−<sup>1</sup> of ascorbic acid aqueous solution was first prepared. A total of 5 g of PVP (molecular weight: 58,000) was added into 100 mL of 0.01 mol L−<sup>1</sup> CuCl<sup>2</sup> aqueous solution under constant stirring over two hours. Then, the prepared 2M NaOH aqueous solution was slowly dropped into the above mixed solution until the color of the solution gradually changed from mild milk blue to sky-blue and then to dark brown. After the above mixed solution was immersed in a constant temperature bath for 30 min, the 0.6 mol L−<sup>1</sup> ascorbic acid aqueous solution was slowly dropped into the above solution until the color of the solution was changed to brick-red. The turbid solution was obtained by continuing the bath at 55 ◦C for 2.5 h. The resulting product was washed with deionized water and ethanol several times, collected by centrifugation, and finally dried at room temperature under vacuum condition overnight.

The cuboctahedron Cu2O (denoted as cod-Cu2O): The preparation process of the cod-Cu2O was the same as that of the od-Cu2O, except that the PVP with molecular weight of 58,000 was replaced by the PVP with molecular weight of 130,000.
