α-Fe2O3/TiO2 stratified photoanodes

doi:10.1016/j.jphotochem.2018.03.015

Journal of Photochemistry and Photobiology A: Chemistry

Volume 366, 1 November 2018, Pages 12-17

https://doi.org/10.1016/j.jphotochem.2018.03.015 Get rights and content

Highlights

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Hematite films were deposited by high power impulse magnetron sputtering.
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Sn diffusion from FTO substrate led to Sn doping.
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An overlayer of TiO₂ reduced the Faradaic efficiency of photocorrosion to 0.1 percent.

Abstract

Bilayer α-Fe₂O₃/TiO₂ thin films were prepared with the aim of minimising photocorrosion of hematite. Conductive fluorine doped tin oxide (FTO)/glass was used as substrate for successive deposition of (i) α-Fe₂O₃ layers by high-power impulse magnetron sputtering (HIPIMS) and (ii) TiO₂ (sol-gel method using dip-coating). The influence of annealing temperature (250–750 °C) and hematite layer thickness on the photoelectrochemical performance was evaluated. Hematite films on FTO substrates, calcined at 650 °C or 750 °C show increased photoelectrochemical response due to doping by Sn diffusion from the substrate. The photoresponse decreases with increasing thickness from 20 to 100 nm due to incomplete doping of the bulk. For bilayer hematite/TiO₂ films, the visible light photoresponse is higher than that for the single hematite film which is ascribed to suppression of surface recombination at the Fe₂O₃/electrolyte junction by capping with TiO₂. The Faradaic efficiency of the photocorrosion reaction was found to be 0.8% for an unprotected hematite electrode and decreased to 0.1% for a hematite electrode covered with a 65 nm thick layer of TiO₂, thus proving the beneficial role of TiO₂ in protecting hematite against photocorrosion.

Graphical abstract

Introduction

Titania (TiO₂) and hematite (α-Fe₂O₃) have potential applications as semiconducting photoanodes for either hydrogen production via photoassisted water electrolysis or photoelectrochemical (PEC) oxidation of water pollutants. The advantages of TiO₂ are high stability, nontoxicity, and low price. However, only a very small part of sunlight (3% of the total power) is absorbed. On the other hand, iron oxide (α-Fe₂O₃) has a more favourable band gap (2.0–2.2 eV). An ideal absorber with a bandgap of 2 eV would capture a substantial fraction of solar light (27% of the total power). However, for a power converter, thermodynamic restrictions apply. Moreover, practical limitations are the actual form of the absorption spectrum, nonideal position of the conduction band, i.e. too large an electron affinity for spontaneous water reduction, low minority carrier diffusion length, surface states that can mediate recombination [1], low stability in acidic media [2], and photocorrosion [3].

To address the main handicaps of hematite several strategies have been introduced. Among them elemental doping is commonly used, e. g Si and Ti [4], Pt [5], Sn [6], etc. doping has been used to improve particularly the low electrical conductivity. Another strategy towards the photoelectrochemical (PEC) activity enhancement has been recently reported also for the TiO₂/α-Fe₂O₃ heterojunction, which showed significantly improved separation efficiency of photo-induced charge carriers and the oxygen evolution kinetics [[7], [8]].

Dense Fe₂O₃ thin films have already been deposited by reactive high power impulse magnetron sputtering (HIPIMS) [[9], [10], [11], [12]]. In the HIPIMS mode of magnetron discharge excitation, high ionization degree of sputtered particles, high ion flux to the substrate during the working pulse and, simultaneously, low heat flux to the substrate are obtained. Due to the back attraction of ionized sputtered particles to the target the deposition rate of HIPIMS magnetron is usually lower than in case of DC magnetron systems [[13], [14]]. Since only a small thickness (20–100 nm) of a hematite film is of use in photoelectrochemistry due to the short diffusion length of carriers in hematite, the HIPIMS technology is suitable for this application.

In several studies, hematite thin films were deposited on fluorine doped tin oxide (FTO)/glass substrates followed by annealing. At temperatures above 650 °C, diffusion of Sn from the substrate into the film was observed, accompanied by increase of conductivity due to doping [[15], [16]]. In the case of HIPIMS hematite films on FTO/glass, films with n-type conductivity exhibiting high photocurrents in a junction with an alkaline electrolyte (up to ≈1 mA/cm² under 100 mW/cm² (AM1.5 spectrum)) were obtained [9]. Sn doping can be also performed by the direct deposition of the Sn containg precursor followed by thermal treatment [[17], [18], [19]].

The aim of the present work was to understand better the doping of HIPIMS hematite films by dopant diffusion from FTO substrates and to optimize the thermal treatment and thickness of the films. Secondly, HIPIMS hematite films, covered with a thin layer of anatase, were studied with respect to photocorrosion suppression.

Section snippets

Film deposition

All films were prepared on FTO/glass (TCO22-15, Solaronix, Switzerland) with a nominal sheet resistance of 15 Ω.

Layers of α-Fe₂O₃ were deposited by HIPIMS as described previously [12]. HIPIMS employed a pure iron target (99.995%, Plasmaterials) with an outer diameter of 50 mm and an Ar-O₂ atmosphere as working gas mixture in an ultra-high vacuum (UHV) reactor (base pressure of 10⁻⁵ Pa). Ar and oxygen were fed to the reactor with a flow rate of 26 sccm (standard cubic centimeters per minute) and

HIPIMS hematite films, influence of annealing temperature and layer thickness

The films were specular and highly reflecting. At 400 nm the reflectivity was 30% which is a major factor contributing to photocurrent loss with these electrodes – an aspect that, however, is not in the scope of the present study. From the interference patterns of the reflectivity spectra the thickness could be estimated and compared well with the profilometric data (results not shown). As-deposited HIPIMS films were not crystalline (Fig. 1). Only lines corresponding to SnO₂ (2ϑ = 26.597°, 33.89

Conclusions

Hematite films on FTO substrates, calcined at 650 °C or 750 °C show increased photoelectrochemical response due to doping of hematite by Sn diffusion from the substrate. The photoresponse decreases with increasing thickness from 20 to 100 nm due to incomplete doping of the bulk. In bilayer hematite/TiO₂ films, only hematite and anatase phases are detected. The visible light photoresponse of a bilayer hematite/TiO₂ film is higher than that of a the single hematite film which is ascribed to

Acknowledgement

The authors acknowledge the financial support from the Grant Agency of the Czech Republic (project number 17-20008S) and from a specific university research grant (MSMT No 20-SVV/2017).

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Citation Excerpt :
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α-Fe2O3/TiO2 stratified photoanodes

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Film deposition

HIPIMS hematite films, influence of annealing temperature and layer thickness

Conclusions

Acknowledgement

Catal. Today

Int. J. Hydrogen Energy

Appl. Catal. B: Environ.

Thin Solid Films

Electrochim. Acta

Catal. Today

Catal. Today

J. Chem. Soc., Faraday Trans. I

Atlas d'Equilibres Électrochimiques

J. Electrochem. Soc.

Appl. Phys. Lett.

Chem. Mater.

α-Fe₂O₃/TiO₂ stratified photoanodes