Yuanyuan Hao, Jiujun Deng, Litao Zhou, Xuhui Sun and Jun Zhong* The reduction of hematite nanostructures was achieved by the pyrolysis of NH3BH3 (AB) solution, which introduced oxygen vacancies in hematite and signifificantly improved the photocurrent for solar water oxidation. Moreover, by covering the sample with a crucible in the annealing process, a signifificant cathodic shift of the onset potential (up to 70 mV) was observed. Data suggested that a higher inside pressure was introduced by the cover, which led to a further reduction of hematite in the near surface region. The depth-reduction resulted in a cathodic shift compared to the sample treated in the surface region. The cathodic shift was also confifirmed at various concentrations of AB and annealing temperatures. Our results suggest that the treating depth could be a key role for the low onset potential of hematite for solar water oxidation.

Received 10th March 2015  

Accepted 17th March 2015  

DOI: 10.1039/c5ra04204f

Introduction

Hematite has emerged as a good photocatalyst for solar water oxidation due to its low cost, abundance, high stability in aqueous solution, and favorable band gap.1–9 However, its practical performance has been limited by various factors such as poor conductivity, short lifetime of the excited-state carrier (10
12 s), poor oxygen evolution reaction (OER) kinetics, short hole diffusion length (2–4 nm), and improper band position for unassisted water splitting.5–16

Because of the improper band position of hematite, a bias of about 0.4 V vs. RHE has to be applied for the oxidation of water. Moreover, the poor OER kinetics, the accumulation of photo-excited holes or many other reasons may lead to additional bias for water oxidation, which will significantly decrease the efficiency.12–14 A low onset potential for the water oxidation process is thus important for the performance of hematite. Many methods have been developed to improve the performance of hematite.3–16 Recently, by introducing oxygen vacancies into hematite its performance was significantly improved with increased conductivity.7,10,11,17,18 Various methods have been used to produce oxygen vacancies such as controlling the oxygen content (oxygen-deficient atmosphere) in the sintering process or the reduction of hematite in a reductive atmosphere such as H2. 10,11,17,18 The first method could significantly improve the photocurrent but typically had a relatively high onset potential (about 1.0 V vs. RHE).10,11,17 This was attributed to the bulk effect of oxygen vacancies, which may act as recombination centers for the electrons and holes.14,18,19 Compared to the first method, the second method recently exhibited advantages by both increasing the photocurrent and decreasing the onset potential to a low value of about 0.87 V vs. RHE.18 This was achieved by the pyrolysis of solid NaBH4 powder in a crucible to release controllable H2. A surface effect of the H2-treatment was revealed to be important for the low onset potential of hematite compared to the bulk effect by controlling the oxygen content.18 Herein, we extend our study of producing oxygen vacancies in hematite by the pyrolysis of NH3BH3 (AB) solution on hematite in a crucible. The photocurrent of AB-treated hematite also shows obvious improvement. In addition, we find that the crucible as a cover may significantly affect the onset potential, which shows an obvious cathodic shift (up to 70 mV) compared to the sample treated without the cover. The two separate processes of increasing the photocurrent and lowering the onset potential allow us to further understand the factors affecting the onset potential shift. Various concentrations of AB and annealing temperatures have been applied to study the cathodic shift and the results suggest that it can be attributed to a depth-reduction of hematite in the near surface region by increased pressure. The present study suggests that the treating depth of hematite is very important for the onset potential, which should be optimized to the near surface region.

Experimental section

Preparation of a-Fe2O3 photoanodes

Hematite nanostructures were prepared on a fuorine-doped SnO2 (FTO, Nippon Sheet Glass, Japan, 14 ohm sq1 ) glass by a modied hydrothermal method.6,17,18 The experimental details to prepare pristine hematite can be seen in ref. 18.

The reduction of the pristine hematite was performed by the pyrolysis of NH3BH3 (AB, Sigma-Aldrich Co.) solution. AB solutions with various concentrations (0.0125 M, 0.025 M, and 0.05 M) were dropped on an FTO–hematite (pristine hematite on FTO glass) slide (cut to 18 mm  12 mm  2 mm). The system was supported on a quartz glass and then annealed in air or with a crucible (15 cm3 ) as a cover at different temperatures (400–600 ℃) for 40 min in a muffle furnace (the experimental setup can be found in ESI Fig. S1†). In this study, the crucible covers the hematite sample with AB solution dropped on the hematite, while for the pyrolysis of solid NaBH4 powder, in ref. 18, the powder is located at the bottom of the crucible and is covered by the FTO with the hematite sample facing the bottom. The different setups allow us to test the role of the crucible, since either with or without the crucible, the hematite will be treated with H2. H2 can be released from AB for the reduction of the pristine hematite when the temperature is higher than 200 ℃. After AB-treatment, the samples were washed with deionized water to remove the residues. The samples treated by AB solution and then annealed in air or in a crucible were labeled as AB-treated hematite and AB-treated hematite with cover, respectively.

Structural characterization

A FEI-quanta 200 scanning electron microscope with an acceleration voltage of 20 kV was used to obtain the scanning electron microscopy (SEM) images. We used a FEI/Philips Techai 12 BioTWIN transmission electron microscope and a CM200 FEG transmission electron microscope to obtain the transmission electron microscopy (TEM) and High-Resolution TEM (HRTEM) images, respectively. X-Ray diffraction (XRD, PANalytical, Empyrean) and X-ray photoelectron spectrometers (XPS, Kratos AXIS UltraDLD) were also used for structure characterization.

PEC measurements

The working area of the hematite photoanodes was about 0.1 cm2 and the remaining parts were covered by nonconductive Hysol epoxy. The white light source was a Xenon High Brightness Cold Light Source (XD-300) coupled with a filter (AM 1.5 G) with a light power density of 100 mW cm
2 (spectrally corrected). A xenon lamp (CEL-HXF300/CEL-HXUV300, 300 W) coupled with a monochromator (Omni l3005) was used to measure the IPCE spectra. A CHI 660D electrochemical workstation in a three-electrode electro-chemical cell was used for the PEC measurement. The electro-lyte was an aqueous solution of NaOH with a pH of about 13.6. The measured potentials were converted to the RHE scale according to the Nernst equation.6 The potential was swept from 0.6 V to 1.8 V vs. RHE at a scan rate of 50 mV s- 1. Capacitance was derived from the electrochemical impedance obtained at each potential with 10 000 Hz frequency in the dark. Mott–Schottky plots were obtained from the capacitance values.

Results and discussion

Fig. 1 shows SEM images of the pristine and AB-treated hematite nanostructures (with and without cover, optimized at an AB concentration of 0.025 M and temperature of 500 ℃). The morphology of the samples is a thin film of nanorods on the FTO substrate. All the samples show similar morphology. Fig. 2a and b show the TEM and HRTEM images of AB-treated hematite with cover at an AB concentration of 0.025 M and temperature of 500 ℃, respectively. TEM dark field images and the corresponding elemental mappings for the AB-treated hematite nanostructures with cover are shown in Fig. 2c–e, and they reveal the uniform distribution of Fe and O in the hematite nanorods. In Fig. S2 (ESI†), the XRD data are also shown, which clearly indicate the existence of the typical hematite structure (JCPDS 33-0664).

In Fig. 3a, we show the J–V scans for the hematite photoanodes before and after the AB-treatment (with and without cover at an AB concentration of 0.025 M and temperature of 500  C). Evidently, the photocurrent of the pristine hematite is relatively low (0.83 mA cm 2 at 1.23 V vs. RHE) compared to that of the AB-treated sample without cover, which is 1.51 mA cm 2 at 1.23 V vs. RHE. The increased photocurrent can be attributed to the introduction of oxygen vacancies by the AB-treatment.10,11,17,18 However, when an additional cover (the crucible) is used in the annealing process, a significant cathodic shift of the onset potential can be observed with an increased photocurrent density of 1.70 mA cm 2 at 1.23 V vs. RHE, which is more than 2 times higher than that of the pristine hematite. The AB-treated sample with cover shows a low onset potential of 0.87 V vs. RHE compared to the onset potential of 0.94 V vs. RHE for the AB-treated sample without cover (herein, we use the potential at the intersection point of the dark current and the tangent at the maximum slope of the photocurrent).12,13 The onset potential shifted cathodically by up to 70 mV. Although enhanced photocurrent and low onset potential caused by the introduction of oxygen vacancies have been observed in a previous report,18 the two separate processes and the lowering of the onset potential just by using an additional cover have not been observed before. The results allow us to further understand the factors affecting the onset potential shift, which are important for practical applications. The incident photon-tocurrent conversion efficiencies (IPCE) data are also shown in Fig. 3b (at 1.23 V vs. RHE). The AB-treated sample shows