The nature of this interaction is further elucidated using X-ray photoelectron spectroscopy (XPS, Fig. ). The Ti 2p region in pristine HTO and pillared HTO-PA and HTO-HDA all contain a mixture of Ti oxidation states Ti 4+ and Ti 3+ (Fig. ). The Ti 3+ /Ti 4+ signal intensity ratio increases from 0.196 in pristine HTO to 0.212 in HTO-PA and further to 0.355 in HTO-HDA.