Low‐Temperature Activation of Hematite Nanowires for Photoelectrochemical Water Oxidation

Abstract

Hematite (α-Fe2 O3 ) nanostructures have been extensively studied as photoanode materials for photoelectrochemical (PEC) water oxidation. However, the photoactivity of pristine hematite nanostructures is fairly low and typically requires thermal activation at temperature of 650 °C or above. Here, we report a new method for enhancing the photocurrent of hematite nanowires at a substantially lower temperature of 350 °C by means of a two-step annealing process (activation process). Hydrothermally grown β-FeOOH nanowires were first annealed in a pure N2 environment at 350 °C to form magnetite, followed by partial oxidation in air to convert magnetite to hematite. During this process, Fe(2+) sites (oxygen vacancies) were intentionally created to increase the donor density and therefore the electrical conductivity of hematite. The oxygen-deficient hematite nanowire photoanode created at low temperature (350 °C) show considerably enhanced photoactivity compared to pristine hematite sample that prepared by thermal annealing of β-FeOOH nanowires at 550 °C in air. Moreover, this low-temperature annealing method can be coupled with an element doping method to further increase the photoactivity of hematite nanowire. Sn-doped hematite nanowires prepared by the same low-temperature annealing method show at least three fold enhanced photocurrent compared to the undoped sample. Significantly, the highest temperature in the entire annealing process was 350 °C, which is the lowest activation temperature ever reported for hematite nanowire photoanodes.

Activation of Ultrathin Films of Hematite for Photoelectrochemical Water Splitting via H2 Treatment

“…Figure 3 a is consistent with other hematite photoanodes and materials reported in the literature. [21,41,[44][45][46] Of note is the identical onset of the Fe 2p 3/2 peaks for both samples A and B, which indicates an unlikely change in the crystal phase after H 2 treatment (i.e., no Fe 3 O 4 and no Fe 0 for- mation). We note that although the photoemission spectrum for maghemite (g-Fe 2 O 3 ) is practically indistinguishable from that of hematite, [41,46,47] and although there is one known example of conversion of hematite into maghemite by using H 2 at moderate temperatures, [48] maghemite is known to be photoelectrochemically inactive [49] and was not observed in the Raman spectra for either of the two samples (see Figure S9), nor in the PXRD patterns for thicker hematite films (see Figure S7).…”

Plasma-Induced Oxygen Vacancies in Ultrathin Hematite Nanoflakes Promoting Photoelectrochemical Water Oxidation

“…31,34,35,42 Additional information about the phase of the nanostructures forming the sample top surface is obtained using X-ray diffraction (XRD) patterns performed with a grazing incidence angle of 0.5°. The XRD spectra presented in Figure 2 (a) show that all samples contain hematite, magnetite, and iron, a result consistent with the mixed iron oxides formed during thermal oxidization of iron foils.…”

Templating Sol–Gel Hematite Films with Sacrificial Copper Oxide: Enhancing Photoanode Performance with Nanostructure and Oxygen Vacancies

“…9,10 Therefore, nanorods, nanowire arrays, nanonets and porous colloidal based films are attractive morphologies for a high photon harvesting efficiency. [11][12][13] However, realizing a high surface area does not improve the intrinsic optical and electronic properties of the electrode material. Another strategy to improve the performance of hematite photoanodes is to increase the majority carrier concentration through the use of n-type subsitutional dopants (e.g.…”