Photoelectrochemical study of anodized TiO2 Nanotubes prepared using low and high H2O contents. (20th December 2015)

• Record Type: Journal Article
• Authors: Regonini, D.; Groff, A.; Sorarù, G.D.; Clemens, F.J.
• Is Part Of: Electrochimica acta. Volume 186(2015)
• Page Start: 101
• Page End: 111
• Publication Date: 2015-12-20
• Subjects: Electron Transport -- TiO2 Nanotubes -- Anodization -- Water content -- Electrochemical Impedance Spectroscopy -- PhotoElectrochemical Cells (PECs); Electrochemistry -- Periodicals; Electrochemistry, Industrial -- Periodicals; 541.37
• DOI: 10.1016/j.electacta.2015.10.162
• Languages: English
• ISSNs: 0013-4686

Abstract

Graphical abstract: Scheme illustrating the lower density of deep energy states in NTs grown in 12 wt.% H2 O. As a result, such NTs exhibit efficient electron transport properties. In contrast electron transport is difficult in the case of 2 wt.% H2 O NTs as the density of deep states, acting as recombination center, is larger. Abstract: The microstructural and photoelectrochemical properties of anodized TiO2 Nanotubes (NTs) grown in ethylene glycol containing 0.1 M NH4 F and a variable amount of H2 O, either 2 wt.% (low water content) or 12 wt.% (high water content) have been investigated. The study has been conducted at a fixed anodizing potential, as it is known that also the applied potential can affect the electron transport properties of the NTs. The amount of water also has an impact on the microstructure of the NTs; those prepared in 12 wt.% H2 O have a rough wall, whereas those made at 2 wt.% are mostly smooth. In addition the water content affects the dissolution of the forming oxide and consequently has an impact also on the diameter and the length of the NTs. For a fixed potential (30 V), when working at 12 wt.% H2 O content, it is necessary to extend the anodizing time to 60 min to obtain ∼1.4 μm long NTs. A similar length is achieved at 2 wt.% H2 O content after 20 min. The photocurrent density measurements reveal an improved photoactivity for NTs grown in high water content electrolytes (1.1 mA/cm 2 in 12 wt.% H2 O vs 0.75 mA/cm 2 in 2 wt.% H2 O at 1 V vs RHE). Similarly the Incident Photon to Current Efficiency (IPCE) of NTs obtained in 12 wt.% H2 O is consistently higher than for 2 wt.% H2 O over the range 350-300 nm (with values > 40% at 320-300 nm). Electrochemical Impedance Spectroscopy (EIS) analysis shows similar electron transport and charge transfer properties for NTs grown in low and high water contents. EIS is mainly sensitive to the bottom of the NTs, whereas the NTs wall is not active at potentials more positive than the flat band potential. EIS also reveals that the poorer photoelectrochemical properties of NTs grown in 2 wt.% H2 O are due to a larger density of deep intra band-gap energy states in comparison with 12 wt.% H2 O.