Fig. 1.

Synthesis process: from the initial colorless bismuth salt solution (A), it turned to a turbid white color after the addition of NaOH and PVP (B); when BAL was added it changed to a translucent yellow color (C). Finally, when NaBH₄ is added, the solution immediately turned to pitch-black color (D). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)