Raman and XPS spectra. (a) Raman spectrum of CNWs (original and reduced) showing the presence of D and G bands as well as the overtone and combination mode features taken with 532 nm laser excitation. (b) The C1 s and (c) the O1 s XPS spectra of CNWs before and after thermal annealing. The as-grown CNWs contained many oxygen functional groups, while only a low fraction of hydroxyl groups remained after thermal reduction in H2 for 2 h at 900°C. The peak components (green curves) were analyzed with a Gaussian fit.