By coupling Ti-doping and oxygen vacancies in hematite nanostructures, an efficient photoelectrode for solar water oxidation was prepared which showed a high photocurrent of 2.25 mA cm^-2 at 1.23 V vs. RHE and a remarkable maximum value of 4.56 mA cm^-2 at 1.6 V vs. RHE at a relatively low activation temperature of 550 °C. In addition, the partial oxygen pressure range suitable to produce oxygen vacancies in Ti-doped hematite could be expanded to a wide region compared to that in undoped hematite, which was critical to the photoelectrode production in practical applications. The facile way by coupling independently developed methods with the cumulative effect stands for an effective strategy for efficient solar water oxidation

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 1.5G) 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, 300 W) coupled with a monochromator (Omni-l3005). Capacitance was derived from the electrochemical impedance obtained at each potential with 10000 Hz frequency in the dark. Mott–Schottky plots were generated from the capacitance values 

Fig. 1 (a) and (b): SEM images of Ti-doped hematite nanostructures sintered in a partial oxygen pressure of 2.4  10
2 Torr. (c) Dark field image of TEM and the corresponding TEM elemental mappings of Tidoped hematite nanostructures (rectangle box marked area): O (red), Ti (yellow), and Fe (green) distribution in the selected area. The bottom panel shows the EDX spectrum of the selected area

J–V scans for various hematite samples sintered at 550 ℃. Black curves: undoped hematite nanostructures with oxygen vacancies (sintered in a partial oxygen pressure of 2.1 × 10^-2 Torr with the best performance for undoped samples); red curves: Ti-doped hematite nanostructures without oxygen vacancies (sintered in ambient air); green curves: Ti-doped hematite nanostructures with oxygen vacancies (sintered in a partial oxygen pressure of 2.4 × 10^-2Torr with the best performance for Ti-doped samples).

(a) J–V scans for Ti-doped hematite sintered in various partial oxygen pressures at 550℃. (b) Photocurrent density of undoped and Ti-doped hematite at 1.23 V vs. RHE as a function of partial oxygen pressure. (c) Photocurrent density of undoped and Ti-doped hematite at 1.6 V vs. RHE as a function of partial oxygen pressure. (d) IPCE spectra for Ti-doped hematite sintered in a partial oxygen pressure of 2.4 × 10^-2 Torr at 1.23 and 1.6 V vs. RHE.

Conclusions 

We presented the preparation of Ti-doped hematite nanostructures with various oxygen vacancies. The Ti-doped hematite nanostructures with optimized oxygen vacancies achieved a photocurrent of 2.25 mA cm-2 at 1.23 V vs. RHE and a remarkable maximum value of 4.56 mA cm-2 at 1.6 V vs. RHE for solar water oxidation. Moreover, Ti-doping can expand the applicative partial oxygen pressure to a wide range compared to that for undoped hematite. The expansion of partial oxygen pressure range might be useful for the practical application. Data suggest that oxygen vacancies in hematite mainly affect the performance by improving the donor density with lower oxidation state of Fe (such as Fe2+), while Ti-doping might affect the performance of hematite by surface catalytic effects or more active sites for water oxidation. The coupling of extrinsic Ti-doping and intrinsic oxygen vacancies stands as an effective strategy to design oxide-based photoanodes for efficient solar water oxidation.