Thursday, May 15, 2014

ALD of hematite on Gd2O3 lead to record performance for water splitting electrodes

Omid Zandi and co-workers at the Materials for Energy Conversion Group, Michigan State University are using ALD for the deposition of metal oxide semiconductors, doping and coatings, for solar fuel production purposes. Recent published work in Journal of Physical Letters about solar water oxidation with hematite deposited and modified via ALD allowed them to study and overcome the limitations of this system which led to the record performance reported for hematite based photoanodes recently (J. Phys. Chem. Lett., 2014, 5 (9), pp 1522–1526 below).

Substrate Dependent Water Splitting with Ultrathin α-Fe2O3 Electrodes
Omid Zandi, Joseph A. Beardslee, and Thomas Hamann
J. Phys. Chem. C, Article ASAP

Thin films of hematite (α-Fe2O3) were deposited by atomic layer deposition (ALD), and the effects of metal oxide underlayers on the photocatalytic water oxidation performance were investigated. It was found that a Ga2O3 underlayer dramatically enhances the water oxidation performance of the thinnest hematite films. The performance enhancement is attributed to the increased crystallinity of the ultrathin films induced by the oxide underlayers. The degree of crystallinity was examined by Raman line shape analysis of the characteristic hematite phonon modes. It was found that multiple metal oxide underlayers, including Nb2O5, ITO, and WO3, increase the film crystallinity compared to hematite deposited on bare FTO. The increased crystallite size was also clearly evident from the high resolution SEM images. The degree of crystallinity was found to correlate with absorbance and the photocatalytic water oxidation performance. These findings shed light on the origin of the dead layer at the interface of the FTO substrate and ultrathin hematite films and elucidate strategies at overcoming it. (Picture from graphical abstract)

Enhanced Water Splitting Efficiency Through Selective Surface State Removal
Omid Zandi and Thomas W. Hamann
J. Phys. Chem. Lett., 2014, 5 (9), pp 1522–1526
Hematite (α-Fe2O3) thin film electrodes prepared by atomic layer deposition (ALD) were employed to photocatalytically oxidize water under 1 sun illumination. It was shown that annealing at 800 °C substantially improves the water oxidation efficiency of the ultrathin film hematite electrodes. The effect of high temperature treatment is shown to remove one of two surface states identified, which reduces recombination and Fermi level pinning. Further modification with Co–Pi water oxidation catalyst resulted in unprecedented photocurrent onset potential of 0.6 V versus reversible hydrogen electrode (RHE; slightly positive of the flat band potential). (Picture from graphical abstract).

Growth mechanism and the effect of doping hematite with Ti can be found in this publication: Highly photoactive Ti-doped α-Fe2O3 thin film electrodes: resurrection of the dead layer.

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