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Solar & Alternative Energy

Hematite nanoarrays promise water photo-oxidation by solar irradiation

X-rays can be used to investigate quantum confinement effects on the bandgap profiles of semiconductors, allowing better light-harvesting properties to be engineered.
14 August 2007, SPIE Newsroom. DOI: 10.1117/2.1200708.0827

Solar energy can be converted to heat for warming space and water, and to electricity and chemical fuels that provide energy for use and storage.1–4 Sunlight in the near infrared, visible, and near ultraviolet regions has considerable energy (about 0.9–3.2eV per photon) and intensity. Solar energy could therefore provide a significant contribution to our electrical and chemical resources if efficient and inexpensive systems utilizing readily available materials could be devised for the conversion process. However, the high cost and low conversion efficiency of solar energy have hampered its use.

Emerging technologies use semiconductors in light harvesting assemblies and charge transfer processes in solar cells. In such a cell, if the electron-hole pair formation occurs at the interface between the semiconductor and some solution will, upon absorption of light, lead to the oxidation or reduction of species in the solution, which can then be used as fuels. The fabrication of artificial photosynthetic systems for the conversion of H2O and CO2 to fuels such as H2 and CH3OH has garnered significant interest from researchers and has encouraged new fundamental investigations of the interactions between light, electron flow, and chemical reactions.

Figure 1. Energy dependent resonant inelastic soft-x-ray scattering spectra of α-Fe2O3 nanorod-arrays. The upper two insets are electron microscopy images of α-Fe2O3 arrays consisting of oriented (left) and bundled (right) ultrafine nanorods.

The bandgap, band edge positions, and overall band structure in a semiconductor are of crucial importance in photoelectrochemical and photocatalytic applications. The band edges and bandgap can be tailored to achieve specific electronic, optical or photocatalytic properties in nanostructure semiconductors. For example, the energy position of the band edge level can be controlled by the electronegativity of the dopants, by the solution pH (for example, a flatband may have a potential variation of 60mV per pH unit), and by quantum confinement effects.

Synchrotron radiation based soft-x-ray spectroscopy has recently become a useful tool to determine the bandgap properties of semiconductors.5,6 The x-rays originate from an electronic transition between a localized core state and a valence state. Soft-x-ray absorption probes the local unoccupied electronic structure (conduction band), and soft-x-ray emission probes the occupied electronic structure (valence band). In addition, resonant inelastic soft-x-ray scattering (Raman spectroscopy with soft x-rays) can identify the energy levels that determine the chemical and physical properties of the semiconductors. Recently, quantum size effects on the exciton and bandgap energies were observed in semiconductor nanocrystals.7–9 If the observed valence band or conduction band shifts are due to quantum confinement, one would expect the size of the band shifts to increase as the particle size of the nanocrystals decreases.

A very important application of solar energy is the generation of H2 from direct photo-oxidation of water without application of an external bias.10–12 Indeed, to succeed in splitting water via solar irradiation only, the valence band of the semiconductor must be located at a lower energy level than the chemical potential of oxygen evolution (H2O/O2), while the conduction band must be located at a higher energy level than the chemical potential of hydrogen evolution (H2/H+). If these criteria are not met, an external bias must be applied to induce the photocatalytic process, which in turn substantially reduces the overall efficiency.

It has been reported that photo-oxidation of water without an external bias requires an optimal bandgap or 2.46eV.13 Although the bandgap of hematite is reported to be around 1.9–2.2eV (depending on its crystalline status and method of preparation) and its valence band edge is suitable for oxygen evolution, the conduction band edge of hematite is too low to generate hydrogen. Shifting the hematite bandgap to the blue by 0.3–0.6eV would cause a concomitant upward shift of the conduction band edge. This would make hematite an ideal anode material for photocatalytic devices carrying out the photo-oxidation of water in terms of cost, abundance, safety, thermal and structural stability, and photo-corrosion resistance.

Figure 1 presents an investigation of quantum confinement effects on bandgap profiles in nanoarrays of hematite by resonant inelastic soft-x-ray scattering of synchrotron radiation. The 2.5eV excitation,14 which corresponds to the bandgap transition of hematite, appears significantly blue shifted compared to the reported 1.9-2.2eV bandgap of single-crystal and polycrystalline samples. Such findings strongly suggest that nanomaterials designed in this way would meet the bandgap requirement for the photocatalytic oxidation of water without an external bias. Hematite nanoarrays are thus a prime candidate for use in devices that directly photo-oxidize water by solar irradiation.

In conclusion, the direct observation of a 1D quantum confinement effect has been demonstrated in designed oriented bundles of ultrafine hematite nanorods by resonant inelastic x-ray scattering. The bandgap criteria for direct photo-oxidation of water by solar irradiation without an external bias would be satisfied with such purpose-built nanomaterials. Such a result is of great importance for the safe and economical solar production of hydrogen, an environmentally friendly energy source for the future.

Jinghua Guo
Lawrence Berkeley National Laboratory
Berkeley, CA

Jinghua Guo is a staff scientist at the Advanced Light Source at Lawrence Berkeley National Laboratory. He received his PhD in the Department of Physics at Uppsala University, Sweden, in 1995, and was assistant professor there between 1997–2001.