Figure 9

a and b XPS spectra of OH-signified titanium oxide nanoparticles (sample 2 in Table 1) and microtubes (sample 4 in Table 1), respectively, showing O 1 s and Ti 2p_3/2 signals with Gaussian fits showing a predominant Ti³⁺ as opposed to Ti⁴⁺ and Ti²⁺ in the presence of OH species (cf. text). Note the Ti²⁺ content is significantly higher in sample 2 than sample 4 in accord with a lower minimum band gap for the former