Synthesis and the photoelectrochemical performance of Fe2O3 photoanode through pretreatment

The photo-electrochemical performance of hematite (α-Fe2O3) photoanode was evaluated and the preparation conditions were optimized in terms of maximizing the photoelectrochemical performances of the hematitephotoanodes. The photoelctro chemicalper for mances of hematite photoanode can be improved by the electrochemical surface pretreatment, and a possible mechanism was proposed to explain the reason for the improvement of photoelctro chemicalper for mances.


Introduction
Since the production of hydrogen through the photo-electrolysis of water could be realized by Fujishima and Honda, to solve environmental andenergy issues, photocatalysis technique has received more attention and been used as apromising,renewable and clean strategy. [1][2][3] TiO2 as a classical semiconductor photocatalyst has been widely studied, however, only a small fraction of the solar spectrum (< 420nm) can be utilized by TiO2 for its wide band gap (3.2 eV). Thus, the shift of the light response range of photoanode materials to visible light and the improvement of the quantum efficiency are important forthe wide application of a photoelectrode. [4]Due to the visible lightresponse,themetal oxide materials with narrower bandgaps have been studied intensively as photoanodes. [5,6]Hematite (α-Fe2O3) with a narrow band gap of 2.0~2.2 eV and favorable band position has gained significant attractionand been a promising material, also forits non-toxicity, low cost, naturalabundance and electrochemical stability.
There are also somemethodsto improve thephotoelectrochemical performance, such assurfacemodification [18] and morphology modulation [19]. In recent years, by removing the surface recombination center, the surface pretreatment by electrochemicalcyclic voltammetry (CV) in the dark is expected as a universal way to increase thephotoelectrochemical performances of the photoelectrode [20,21]. In the present study,the photoelectrochemical performances of the hematite photoanodes were analyzed with changing preparation conditions and the preparation conditionsfor the calcination temperatures were optimized. After theelectrochemical surface pretreatment,thehematite photoanode exhibits improved photoelectrochemical performance and the possible mechanism was also discussed.

Materials
The starting materials utilized are FeCl3·6H2O, Fe(NO3)3·9H2O and ethanol (Sinopharm Chemical Reagent Co. Ltd.).All reagents were of analytical purity grade. The surfactantwas purchased from Aldrichand used as received without further purification.

Synthesis of α-Fe2O3 photoanode
In the typical process, the F-doped tin oxide (FTO) covered glass substrates which were dipped in the ferric nitride ethanol solution and dried for three times and then calcinedat 500 °C for 4h, were put on the bottom of a 50 mL Teflon-lined stainless steel autoclave. The reaction solution with FeCl3·6H2O (1.6217 g) and surfactant in deionized water was transferred into the Teflon-lined stainless steel autoclave. The autoclave was sealed, heated and kept for different time.

Photoelectrochemical (PEC) measurements
Photoelectrochemical properties were characterized by using a threeelectrode configuration (PCI4/300™ potentiostat with PHE200™ software, Gamry Electronic Instruments, Inc.) in a standard three-electrode configurationcoupled with the as-prepared sample films as the working electrode, anAg/AgCl electrode as the reference electrode and a highpurity Pt foil as the counter electrode. Thephotocurrents of water oxidation were measured in 1 M KOH aqueoussolution with a scan rate of 20 mV·s -1 . The electrochemicalpretreatment was carried out as follows before the photoelectrochemical properties of the samples weremeasured. [20] The as-prepared sample was scanned by cyclicvoltammetry for 30 cycles in 1 M KOH in the dark. The cyclic voltammetryscans were performed at the scan speed of 20 mV·s -1 and withthreshold reduction potential.A solar simulator was used as light source for photoelectrochemical measurement.

Results and Discussion
The influence of reactiontime on the photo-electrochemical property of hematite samples through hydrothermal route after surface pretreatment is also investigated. Photocurrent curves of the hematitesamples prepared at 180 °C for different time through hydrothermal route are shown in Fig. 1. When the hydrothermalreactiontime is increased from 2 h min to 3 h, the photocurrent obviously decreases. The crystal size of the hematite sample grows with the increase of hydrothermalreactiontime, which may be due to the easier recombination of photo-generated carriers. Thus, proper hydrothermalreactiontemperature and time are crucial to obtain high photo-electrochemical performance. The photocurrent curves of hematitefilms before and after surface pretreatment are shown in Fig. 2. Compared with the hematitesample without electrochemical pretreatment, it is shown from Fig. 2 that the sample after the pretreatment shows the obvious enhancement of photocurrent. The electrochemical pretreatment make the photocurrent of the hematitesample 1.3 times more than that of the sample without surface pretreatment at 0.4 V vs. Ag/AgCl. It has been previously reported that associatedphysically with the partial transformation of hematite, electrochemical reductionpretreatment can cause a favorable photohole transfer and a significant decrease of charge recombination [20] From the above results, it is proven that the electrochemical pretreatment is also a efficient method to improve the photocurrent of the as-prepared hematitesample prepared by hydrothermal method.  As shown in Fig. 3, Fig. 4 and Fig. 5, low and high-resolution SEM images of the as-prepared hematite film prepared by the hydrothermal method for different time are measured. From the SEM images, it can be observed that the planar surface structure of the as-prepared hematite film prepared by the hydrothermal method for 2 h is compact and no obvious cracks appear. Thus, through hydrothermal method, the compact hematite films can be prepared. And with the increasing hydrothermal reaction time, the particles on the surface of the as-prepared hematite films which can be observed by SEM images. The bigger particle size may influence the photo-electrochemical efficiency of the as-prepared hematite film prepared by the hydrothermal method.

Summary
In summary, to improve the photoelctrochemical performances of the hematitephotoanode, the preparation conditions were optimized. Theelectrochemical surface pretreatmentcan improve thephotoelctrochemical response of the hematite thin films, which is attributed tothe favorable photohole transfer andthe reduced electron-hole recombination.