Photoluminescence (PL) spectra have been used to elucidate the band structure of graphene oxide (GO) reduced in aqueous solution. The GO reduction is measured in situ via the identification of four PL peaks produced from GO solutions with different concentrations. Using corresponding UV-visible and photoluminescence excitation (PLE) spectroscopy, and on progressing from high energy to low energy transitions, the four PL peaks are identified as sigma-sigma* and pi-pi* transitions, a pi band tail due to oxygen localized states, and a pi band tail due to trapped water, respectively. The labeling of the band structure has been used to challenge the prevailing assignation of the low energy transitions, reported in the literature, to molecular sigma-sigma* and pi-pi* transitions alone.
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