2.1. Sample preparation
2.1.1. Synthesis of X-shaped hollow α-FeOOH nanostructures
X-shaped hollow α-FeOOH nanostructures were synthesized by uk101 modified hydrothermal method . In a typical synthesis route, 1 mmol of FeCl3·6H2O, 0.2 mmol of Na2CO3 and 1.5 mmol of NaF were dissolved in 60 mL of deionized water. The mixture was then stirred for 20 min followed by transferring into a Teflon-lined stainless steel autoclave of 100 mL capacity. The autoclave was sealed and maintained at 200 °C for 24 h. After the autoclave was cooled to room temperature naturally, the obtained product was collected and washed several times with deionized water and ethanol, and finally dried at 60 °C for 12 h.
2.1.2. Synthesis of rod-like hollow α-FeOOH nanostructures
The procedure was the same as immovable joint for the synthesis of X-shaped hollow α-FeOOH nanostructures mentioned above except for 1 mmol of Na2CO3 was used and the reaction time was decreased to 12 h.