A comparison of quantum-sized anatase and rutile nanowire thin films: Devising differences in the electronic structure from photoelectrochemical measurements

Nanostructured oxide electrodes are being intensively studied because of their application in energy generation and decontamination. Nanowire (NW) TiO2 electrodes, either rutile or anatase phase, were prepared by chemical bath deposition (CBD) on FTO (F:SnO2) conducting glass substrates using analogous procedures with different precursors. The morphology and crystalline structure of the NWs were characterized using transmission electron microscopy (TEM), atomic force microscopy (AFM) and Raman spectroscopy. The as-synthesized NWs are about 2 nm in diameter, well-crystallized and are gathered in bundles of about 50 nm. The decrease in particle size to the nanometer scale causes a wavenumber shift and broadening of the Raman peaks in comparison to the bulk material as a result of phonon confinement. The cyclic voltammetry of the NW electrodes in combination with spectroelectrochemical measurements and photocurrent action spectra allows us to devise their electronic structure. A significant increase of the band gap energy for NWs with respect to bulk rutile and anatase is revealed. The results also evidence the presence of monoenergetic states in the band gap for both types of NWs whose energy depends significantly on the crystalline phase. Importantly, an exponential trap distribution below the conduction band is evidenced for the anatase NW electrode, while it is absent in the case of rutile NWs. The different electronic structure of these nanomaterials induces changes in their electrochromic properties. More generally, this study illustrates the potentialities of electrochemical measurements in the investigation of the electronic properties of oxides.