All chemicals used in the experiment were analytical reagents and were used as received. In a typical procedure, 0.05 mol Bi(NO3)3 was dissolved by 30 mL 0.05 mol L−1 HNO3 to obtain transparent Bi3+ aqueous solution, to which 20 mL polyethylene glycol (PEG-4000) was added as dispersant. Then 50 mL 4 mol L−1 NaOH aqueous solution was quickly poured into the mixed solution under vigorous stirring, which immediately resulted in the formation of yellowish precipitates in the container. After kept at 90 °C for 2 h under stirring, the as-produced precipitates were filtered, washed with alcohol and deionized water for several times, and then dried at 60 °C in a vacuum drier to obtain the final faint yellow Bi2O3 samples. For comparison, bismuth oxide bulk samples having micrometer-scale size and agglomerated irregular shape have also been prepared via a two-step method. The detail procedure and XRD/TEM results are shown in the supporting informations.