Photoactive Zinc Ferrites Fabricated via Conventional CVD Approach

Abstract

Owing to its narrow band gap and promising magnetic and photocatalytic properties, thin films of zinc ferrite (ZFO, ZnFe2O4) are appealing for fabrication of devices in magnetic recording media and photoelectrochemical cells. Herein we report for the first time the fabrication of photactive zinc ferrites via a solvent free, conventional CVD approach, and the resulting ZFO layers show promise as a photocatalyst in PEC water-splitting. For large scale applications, chemical vapor deposition (CVD) routes are appealing for thin film deposition; however, very little is known about ZFO synthesis following CVD processes. The challenge in precisely controlling the composition for multicomponent material systems, such as ZFO, via conventional thermal CVD is an issue that is caused mainly by the mismatch in thermal properties of the precursors. The approach of using two different classes of precursors for zinc and iron with a close match in thermal windows led to the formation of polycrystalline spinel type ZFO. Under the optimized process conditions, it was possible to fabricate solely ZFO in the desired phase. This work demonstrates the potential of employing CVD to obtain photoactive ternary material systems in the right composition. For the first time, the application of CVD grown ZFO films for photoelectrochemical applications is being demonstrated, showing a direct band gap of 2.3 eV and exhibiting activity for visible light driven photoelectrochemical water splitting.

Mesoporous ZnFe2O4 Photoanodes with Template‐Tailored Mesopores and Temperature‐Dependent Photocurrents

“…This was attributed to higher crystallinity, which seems to be more important than the pore shape in mesoporous ZnFe 2 O 4 photoelectrodes. The photocurrent at 1.23 V RHE observed for the PLU‐templated thin film calcined for 12 hours at 600 °C is very comparable to those reported for dense thin films produced by expensive synthesis methods such as ALD and CVD . Furthermore, the evaluation of effectivity of a photoelectrochemical system should consider not only the illuminated area as it is the current state of the art.…”

“…Pure PIB‐/PLU‐templated samples calcined at 600 °C for 12 hours show a peak centered at 137.5 eV corresponding to Zn 3 s and a weak signal at 99.8 eV, which originates from the silicon wafer . The O 1s peak is centered at 530.1 eV and the Fe 2p 3/2 peak appears at 711.3 eV matching literature values . In contrast, the samples infiltrated with SiO 2 and consecutively calcined at 600 °C for 12 hours showed an intense Si 2p peak at 154.4 eV typical for SiO 2 indicating the incorporation of SiO 2 into the mesopores.…”

“…Moreover, the same flat band potential can be derived from those measurements, being 0.84 V RHE . These results are in the dimension of values reported for non‐porous ZnFe 2 O 4 thin films prepared by ALD or ZnFe 2 O 4 films prepared by CVD, but are significantly higher, which might be attributed to the very small grain size of the mesostructure increasing the impedance of the thin film. Besides, the observed onset potentials under intermittent irradiation match with the flat band potentials reported elsewhere , .…”

“…These results are in the dimension of values reported for non‐porous ZnFe 2 O 4 thin films prepared by ALD or ZnFe 2 O 4 films prepared by CVD, but are significantly higher, which might be attributed to the very small grain size of the mesostructure increasing the impedance of the thin film. Besides, the observed onset potentials under intermittent irradiation match with the flat band potentials reported elsewhere , . Nevertheless, according to our measured values and the literature values, the valence band of ZnFe 2 O 4 is located at more positive potential than the proton reduction potential, indicating that ZnFe 2 O 4 thin films are suitable to be used as photoanodes for oxygen evolution in PEC water splitting, however not for hydrogen evolution without any bias.…”

“…Very few reports systemically study ZnFe 2 O 4 as a PEC electrode material by itself. The PEC performance of ZnFe 2 O 4 films is affected by various factors such as crystallinity, composition and film thickness, surface defect states, morphology, and surface kinetics . A systematic study of the synergistic effect between several key factors on the PEC performance of ZnFe 2 O 4 films is still lacking while highly desirable to provide useful information to further improve its PEC performance.…”