Fig. 6: Spectral characterization.

a XPS Survey spectra of N-G-Ca sample. b, c High-resolution XPS spectra of N-G-Ca and Ca 2p1/2 and Ca 2p2/3, respectively. d XRD patterns of samples including O-G-Ca, P-G-Ca, N-G-Ca and S-N-G-Ca. e Nitrogen adsorption isotherms and BET surface area for N-G-Ca samples.