Ti-doped hematite nanostructures have been synthesized for effificient solar water splitting by adding TiCN as the Ti precursor in a hydrothermal method. Ti-doped hematite nanostructures show an urchin-like morphology with nano feature size, which increases the effective surface area compared to undoped nanostructures. A remarkable plateau photocurrent density value of 3.76 mA/cm2 has been observed for Ti-doped nanostructures under standard illumination conditions in 1 M NaOH electrolyte, which is 2.5 times higher than that for undoped nanostructures (1.48 mA/cm2 ). The photocurrent at 1.23 V vs. RHE (1.91 mA/cm2 ) is also enhanced to be over 2 times higher than that for undoped nanostructures (0.87 mA/cm2 ). X-ray photoelectron spectroscopy and x-ray absorption spectroscopy have been used to investigate the electronic structure of Ti-doped hematite, which suggest the increased donor density of hematite by Ti doping. The remarkable plateau current density in Ti-doped hematite nanostructures can be attributed to both the favorable urchin-like morphology and the Ti doping. VC 2012 American Institute of Physics.

[http://dx.doi.org/10.1063/1.4759278]

INTRODUCTION

Effificient solar water splitting into usable hydrogen and oxygen by the photoelectrochemical approach has been considered to be a promising method for harnessing and storing solar energy.1–3 To accomplish the process, different semiconductor materials have been developed as photocatalysts for water splitting. Hematite has emerged as a good candidate due to its favorable optical band gap (2.1–2.2 eV), extraordinary chemical stability in oxidative environment, abundance, and low cost.3–9 The theoretical solar-to-hydrogen effificiency for a semiconductor material with this band gap can be 16.8% and a total 12.6 mA/cm2 of water splitting photocurrent, as has been predicted for hematite.4 However, the practical performance of hematite for solar water splitting 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 positions for unassisted water splitting.5–9 Typically, the performance of hematite can be described by the photocurrent curve in which the two most important metrics are the plateau current and the onset potential.8–10 For the ideal case, the onset potential is about þ0.4 V versus reversible hydrogen electrode (RHE) and the plateau current reaches 12.6 mA/cm2 (under AM1.5 G 100 mW/cm2 simulated sunlight conditions), which corresponds to an incident photon to current ef- fificiency (IPCE) of unity.4,9 However, the performance of hematite as a water-oxidizing photoanode is still far from the ideal values due to the above limitations.Enormous efforts have been focused on improving the performance of hematite photoelectrode.3–9,11–15 The development of favorable hematite nanostructures and the modififi- cation of their electronic structure by elemental doping have been suggested to be effective methods. The morphology control of various hematite nanostructures can effectively reduce the feature size and increase the relative volume of the space-charge layer with respect to that of the bulk, thereby reducing exciton recombination and increasing the plateau current.6,9,11 A signifificant increase in water oxidation photocurrent has been observed in a recent report by decoupling the feature size and functionality of hematite nanostructures.11 Recently, a water splitting photocurrent of over 3 mA/cm2 at 1.23 V versus RHE and a plateau photocurrent of 3.75 mA/cm2 have been achieved by a cauliflflower-type hematite nanostructure with IrO2 catalyst.9 It is a signifificant advance in the performance of hematite indicating the effective way of the continued parallel optimization of both the nanostructure and the catalysis for realizing the full potential of solar water splitting. Elemental doping has also been shown to play important roles in improving the performance of hematite.6,15,16 Si-doped hematite nanocrystalline fifilm has been reported to show higher photocurrent density than that of undoped hematite.16 Sn-doped hematite nanostructure has also been reported to improve the photoactivity due to increase of electrical conductivity and surface area.6 The Tidoped hematite prepared by a deposition-annealing method can enhance the photocurrent at a relatively low bias by improved donor density and reduced electron-hole recombination at the time scale beyond a few picoseconds,15 with a plateau photocurrent density less than 3.5 mA/cm2 . Here via a facile hydrothermal method, we report the preparation of Ti-doped hematite nanostructures with an urchin-like morphology, which perform a high plateau photocurrent of 3.76 mA/cm2 . The photocurrent at 1.23 V vs. RHE (1.91 mA/cm2) is also enhanced to be over 2 times higher than that for undoped nanostructures (0.87 mA/cm2 ).

EXPERIMENTAL

Hematite nanostructures were fashioned into photoanodes by securing a copper wire using conductive silver epoxy onto a bare portion of FTO substrate. Afterward, the substrate was covered by non-conductive hysol epoxy except for a working area of 0.1–0.2 cm2 . All PEC measurements were carried out using CHI 660Delectrochemical 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. 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. Xenon high brightness cold light sources (XD-300) coupled with a fifilter (AM 1.5G) were used as the white light source and the light power density of 100 mW/cm2 (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 (Omnik3005).

RESULTS AND DISCUSSION

Hematite nanostructures were synthesized on a flfluorinedoped SnO2 (FTO) glass substrate using a hydrothermal method, followed by high-temperature sintering in air. SEM images for undoped and Ti-doped hematite nanostructures sintered at 750  C are shown in Fig. 1. Undoped hematite nanostructures in Figs. 1(a) and 1(c) show a uniform morphology like short nanorods network covering on the FTO substrate. However, Ti-doped hematites in Figs. 1(b) and 1(d) show new urchin-like nanostrucutres consisting of longer nanowires which stand on the short nanorods network fifilm on the FTO substrate. The urchin-like nanostructures have more surface area and smaller feature size, which may increase the effective interface of hematite and water favorable for the performance of water splitting. Elemental analysis by SEM reveals that the urchin-like nanostructures also show a Ti content of 3.62 wt. % confifirming Ti doping, while the region for the background nanorods in the same sample shows similar compositions as that in undoped hematite and no Ti signal. The combination of Ti-doped urchin-like structures and undoped background nanorods may help for the performance of solar water splitting. X-ray diffraction (XRD) data for undoped and Ti-doped nanostructures sintered at 750  C are shown in Fig. 2, which can be indexed to the characteristic peaks of typical hematite (JCPDS 33-0664) after subtracting the SnO2 peaks from the FTO substrate. These data confifirm the successful preparation of the hematite phase. Dominant (110) diffraction peak for undoped hematite has been observed indicating the highly oriented growth in the [110] direction on the substrate. For Ti-doped hematite, the intensity for (110) peak is still strong, but other diffraction peaks such as (104) also increase a lot, suggesting less oriented growth of the urchin-like hematite. An increased peak width (about 20%) of the (110) peak for Ti-doped hematite also suggests decreased feature size.

SUMMARY

We presented the preparation of Ti-doped hematite nanostructures with an urchin-like morphology, which enhanced the plateau photocurrent to a remarkable value of 3.76 mA/cm2 . The highest IPCE data measured for Ti-doped sample is about 60% (at a wavelength of 350 nm), which is comparable to the high values in existing reports. The high plateau photocurrent density and IPCE can be attributed to both the favorable urchin-like morphology and the Ti doping, which increase the effective surface area, reduce the electron-hole recombination, and increase the donor density in hematite. XPS and XAS have been used to clarify the Ti doping effect in hematite nanostructures. Future studies will be focused on improving the onset potential with different catalysts.