Characterization of the prepared BP. (a) TEM image of few-layered BP. (b) HRTEM image of BP. The inset image showed the corresponding SAED pattern. (c) Raman spectrum of BP. (d) AFM image of BP. (e) The height profiles of BP along the white lines in (c). (f) Histogram of BP size distribution. The histograms were developed by counting 200 sheets for each sample, with Gaussian fit curves shown in each histogram.