Jiujun Deng, Xiaoxin Lv, Jing Gao, Aiwu Pu, Ming Li, Xuhui Sun* and Jun Zhong*

Carbon-coated hematite nanostructures for solar water splitting were prepared by a simple pyrolysis of ferrocene which showed a remarkable photocurrent of 2.1 mA cm2 at 1.23 V vs. RHE, compared to a value of 0.5 mA cm
2 for hematite without the carbon layer. The carbon layer is a few nm thick covering the surface of hematite nanostructures. X-Ray photoelectron spectroscopy and X-ray absorption spectroscopy revealed that the electronic structure of hematite was significantly modified with the existence of oxygen vacancy, which was responsible for the remarkable photocurrent. The carbon layer plays an important role for the appearance of oxygen vacancy. The simple and cheap method could be scaled up easily which may pave the way for the practical application for efficient solar water splitting.

Introduction 

Hematite has emerged as a good photocatalyst for efficient solar water splitting due to its favorable optical band gap (2.1–2.2 eV), extraordinary chemical stability in oxidative environment, abundance, and low cost.1–10 According to theoretical prediction, the solar-to-hydrogen efficiency of hematite can be 16.8% and the water splitting photocurrent can be 12.6 mA cm
2 . 4 However, the practical performance of hematite for solar water splitting is far from the ideal case which has been limited by several 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 Enormous efforts have been focused on improving the performance of hematite photoelectrode.3–16 Different methods such as morphology control,6,9,12 elemental doping,16,17 or improvement of the charge transport of hematite have been reported to be effective way for better performance.15 In all these methods, the synthesis of hematite plays a key role for the performance of hematite. Up to now, various synthesis methods have been used to prepare hematite nanostructures such as spray pyrolysis,18 sol–gel,19 hydrothermal6 and atmosphere pressure chemical vapor deposition (APCVD).6,9 However, a facile and cheap synthesis method of hematite nanostructures with high efficiency is still on the way for future large scale application.20,21 Here we report a facile synthesis of hematite nanostructures for solar water splitting via a simple pyrolysis of ferrocene under ambient pressure. The photocurrent of the hematite nanostructures can be 2.1 mA cm
2 at 1.23 V vs. RHE. In literature, ferrocene was also used as the synthesis source of hematite in APCVD method.22 However, the obtained hematite showed a very poor performance with a maximal photocurrent density less than 1 mA cm
2 . Here our synthesis method is much easier and the photocurrent is relatively high. The facile synthesis of hematite nanostructures could be a start for further treatment such as elemental doping to achieve high performance in practical applications.

Experimental section

PEC measurements Hematite photoanodes on FTO substrate were covered by nonconductive hysol epoxy except for a working area of 0.1 cm2 . All PEC measurements were carried out using CHI 660D electrochemical workstation in a three-electrode electrochemical cell with a Pt wire as a counter electrode and an Ag/AgCl electrode as a reference. The electrolyte was an aqueous solution of NaOH with a pH of 13.6, bubbled with N2 for 20 min before measurement. The measured voltage was converted into the potential vs. reversible hydrogen electrode (RHE). In a typical experiment, the potential was swept from 0.7 V to 1.8 V vs. RHE at a scan rate of 50 mV s
1 . Xenon High Brightness Cold Light Sources (XD-300) coupled with a lter (AM 1.5 G) were used as the white light source and the light power density of 100 mW cm
2 (spectrally corrected) was measured with a power meter (Newport, 842 PE). IPCE were measured using a xenon lamp (CEL-HXF300/CEL-HXBF300, 300W) coupled with a monochromator (Omni-l3005). Capacitance was derived from the electrochemical impedance obtained at each potential with 1000 Hz frequency in the dark. Mott–Schottky plots were generated from the capacitance values. Electrochemical impedance spectroscopy (EIS) measurement was carried out in the same workstation with a frequency range of 1–100 kHz at the potential of 1.23 V vs. RHE.

HRTEM images of hematite nanostructures scratched from CH (a) and EH (b) sintered at 550 ℃ for 2 h and then annealed at 750 ℃ for 10 min

Elemental mapping of hematite nanostructures scratched from CH: C (red), O (orange), and Fe (yellow and green) distribution in the selected area. The bottom panel shows the EDX spectrum of the selected area.

Results and discussion

Hematite nanostructures were synthesized by the pyrolysis of ferrocene in a crucible covered by a 40 × 40  2 mm uorinedoped SnO2 glass. The detailed experiments are described in Experimental Section. Illustration of the experimental set up can be found in the bottom of Fig. 1. The melting and boiling point of ferrocene is 172.5 ℃ and 249  ℃, respectively. When the temperature is higher than 400  ℃, ferrocene will sublime and then decompose to form hematite with the oxidation of air. A layer of red color lm can be observed on the FTO glass aer sintering. In the bottom panel of Fig. 1, a photo of the covered FTO glass aer the pyrolysis of ferrocene at 550  ℃ for 2 h has been shown at the right side. It is clear that the FTO glass is covered by a red layer. However, the center part shows a light red color (more than 80% of the circle area) while the edge part (around the circle) shows a dark red color. We labeled the center part hematite nanostructures on FTO glass with CH while the edge part with EH in the image.