Photoelectrochemical and Corrosion Properties of Fe2 O 3 ‐ TiO2 Artificial Passivation films

Abstract

In order to develop thin coating films having extremely high corrosion resistance, we have synthesized artificial passivation films which simulate the composition and the structure of passive films on stainless steels and valve metals by metallorganic chemical vapor deposition (MOCVD). According to our results, the double-oxide films such as 5 have sufficiently high corrosion resistance in strong acid solutions. Since most of these films have semiconducting nature, applications of these films as semiconductor electrodes as well as corrosion-resistant films are expected. One such application is a photoanode which converts solar energy to electric energy. 6,7 Materials for a photoanode have been required to provide a wide absorption range which meets the wavelength range of sunlight, high chemical stability in aqueous solutions, large photopotential, and low production costs. 6 An Fe 2 O 3 film, which has been studied as a base material for artificial passivation films, 1-3 is an n-type semiconductor and provides the following merits: the wide absorption range of light owing its relatively small bandgap (2.2 eV), ease of production, and the low cost of source material. 8-10 However, at the same time it has some demerits: relatively low chemical stability in acid solutions, low quantum yield, and low energy conversion efficiency owing to its small carrier mobility. 10 It is reported that the carrier mobility of Fe 2 O 3 film can be increased by controlling film thickness. 11 The chemical stability of Fe 2 O 3 in aqueous solutions can be improved by adding TiO 2 , which has high corrosion resistance in strong acid solutions. 3 An n-type semiconductor, TiO 2 , has been known as a suitable photoanode material owing to its good stability in aqueous solutions. Its large bandgap (3.1 eV), 12,13 however, restricts a photoresponse to longwavelength light. In order to improve the photoresponse, doping Al, 14 Cr, 14 Be, 15 or Co 12 into TiO 2 has been studied, and Al and Cr doping are reported to bring about an increase in the photoresponse to visible light. This suggests that the photoresponse of TiO 2 can be improved by adding Fe 2 O 3 .The purpose of the present study is to develop Fe 2 O 3 -TiO 2 artificial passivation films having high energy conversion efficiency and high corrosion resistance. To realize this purpose, the synthesis of Fe 2 O 3 -TiO 2 artificial passivation films with varying composition was performed by MOCVD, and photoelectrochemical and corrosion properties of the films were examined as a function of composition in acidic, neutral, and alkaline solutions. The change in the photoresponse of the films with the crystal structure was also examined. Fe 2 O 3 -TiO 2 artificial passivation films were formed on Pt substrates by low-pressure metallorganic chemical vapor deposition, and their corrosion resistance and photoelectrochemical properties were examined in acidic, neutral, and alkaline solutions. The corrosion resistance was examined by an anodic polarization test and the photocurrent response by a pho...

Cathodic shift of onset potential for water oxidation on a Ti4+doped Fe2O3photoanode by suppressing the back reaction

“…In order to investigate whether there is a similar effect of the surface TiO 2 segregation on the Ti 4+ doped Fe 2 O 3 , we also compared the effect of surface treatment on the photoelectrochemical properties of pure and Ti 4+ doped Fe 2 O 3 samples. Since a pure Fe 2 O 3 sample was dissolved in acid solution more easily than a Ti 4+ doped Fe 2 O 3 sample, 32,33 pure Fe 2 O 3 was corroded for a shorter time (about ve minutes). A similar onset potential shi was observed on the pure Fe 2 O 3 electrode (see Fig.…”

“…S1 †) due to its higher stability than pure Fe 2 O 3 . 32,33 In this study, we employed an acid solution to corrode the surface of Ti 4+ doped Fe 2 O 3 and found that the pretreated Ti 4+ doped Fe 2 O 3 sample exhibited a 100 mV cathodic shi of the photocurrent onset potential. The present hematite photoanodes showed a similar photocurrent onset potential to the lowest values obtained by using electrocatalysts and functional over-layers.…”

Semiconductor-Based Composite Materials: Preparation, Properties, and Performance

“…229 Similarly, a mixed Fe 2 O 3 -TiO 2 composite film exhibited enhanced corrosion resistance relative to a neat Fe 2 O 3 surface. 230 These mixed oxide films were prepared by a cold wall low-pressure MOCVD technique.…”

“…It was found that TiO 2 thin film coatings, applied by a spray pyrolysis technique, cathodically photoprotected the steel surface from corrosion . Similarly, a mixed Fe 2 O 3 −TiO 2 composite film exhibited enhanced corrosion resistance relative to a neat Fe 2 O 3 surface . These mixed oxide films were prepared by a cold wall low-pressure MOCVD technique.…”

Modern Aspects of Electrochemistry, No. 46

“…54) nearly coincident with the mobility gap, m g E , have been 2.95 137 2.80 a 0.15 Fe 0.1 Ti 0.9 O 1. 95 3.15 145 3.00 a 0.15 a Estimated from the corresponding crystalline phases according to Eqs. (60a) and (61).…”