AVS2011 Session EN2+TF-WeA: Thin Films for Solar Fuels

Hematite (a-Fe₂O₃) is a promising material for sustainable generation of H₂ due to its low cost, widespread availability, chemical stability, and ability to absorb a significant fraction of visible light. However numerous challenges remain in order for this material to approach its theoretical potential of 15% solar to hydrogen efficiency. Nanorod geometries are an ideal configuration for this material, decoupling the different length scales required for photon absorption and efficient carrier transport. Unfortunately such structures have historically yielded poor photoelectrochemical performance (<5 mA/cm²).¹ We recently demonstrated that nanorods synthesized by chemical bath deposition (CBD) could be activated through the use of appropriate annealing treatments.² Photocurrents >500 mA/cm² were achieved at 1.23 V versus RHE, and the photoanodes displayed low onset potentials. These changes were correlated with significant amount of tin diffusion from the underlying FTO substrate into the Fe₂O₃ matrix occurs during high temperature annealing process. The benefits of this process may include enhanced conductivity as well as improvement of the FTO/ Fe₂O₃ interface. Despite these advancements, significant room remains for further improvement. In this paper we describe a number of strategies to reach this goal. First, the nanostructure of the hematite can be further improved. This is explored by varying the CBD chemistry, optimizing the post-deposition annealing conditions, and through subtractive processing. A second issue is electron transport at the hematite/FTO interface, and this is explored through the use of novel treatments of the FTO prior to deposition. Finally, the addition of an electrocatalyst can further reduce the onset potential. Each of these three strategies has demonstrated enhanced photocurrent over our previous results. We plan to integrate these advancements in order to maximize performance. Detailed characterization of the structure, composition, and electrochemical changes observed with these processes will be used to provide fundamental insight into the mechanisms underlying the improvements.

References

[1] N. Beermann, L. Vayssieres, S.-E. Lindquist & A. Hagfeldt, "Photoelectrochemical Studies of Oriented Nanorod Thin Films of Hematite", 2456-2461, (2000).

[2] R. Morrish, M. Rahman, J. M. D. MacElroy & C. A. Wolden, "Activation of hematite nanorod arrays for photoelectrochemical water splitting", 474-479, (2011).

Solar energy can be converted to chemical energy stored in hydrogen through water splitting using a photoelectrochemical device. However, decades of research have failed to identify one semiconducting electrode material that can perform the water splitting reaction with adequate efficiency, stability and material cost. Here I present a system that overcomes the limitations of conventional materials to afford direct solar hydrogen production by taking inspiration from natural photosynthesis and using two photosystems in tandem. In addition, to reach the terawatt energy scale needed to have a global impact, I describe the use of abundant and inexpensive semiconducting materials. Specifically, progress developing hematite (Fe₂O₃) photoanodes and cuprous oxide (Cu₂O) photocathodes will be presented. I will show that our water splitting tandem cell, using these materials and inexpensive processing techniques, is capable of solar-to-hydrogen conversion efficiencies of over 5 % at standard conditions. I will further detail how the progress in developing the electrode thin films has been enabled by exploiting recently developed techniques like aerosol–assisted chemical vapor deposition and atomic layer deposition.

Titanium dioxide (TiO₂), thanks to its interesting properties as nontoxicity, low cost and high chemical stability, has been extensively investigated for several application in which, following light absorption, the generated charges can be usefully applied, as for photovoltaic applications or for photocatalytic devices. However, due to the wide intrinsic energy gap of TiO₂ (between 3.0 and 3.2 eV, depending on the crystalline structure), only a small fraction of the solar spectrum can be used to promote the light absorption [6]. In the photocatalytic devices one of the most important goal of the recent research is to be able to prepare photocatalyst which can be active by absorbing the visible light, in order to increase the application of TiO₂-based system in poor regions (where it could be difficult to use UV light sources) or to reduce the application cost . In order to increase the fraction of the solar spectrum that can be absorbed, different approaches have been used, mainly by doping TiO₂ with metals or anions.

One of the most studied doping is using nitrogen. TiO₂ doped with nitrogen atoms can be prepared in different ways, as powders, as thin films by sol-gel or reactive sputtering.

In this paper we will show the photocatalytic results obtained depositing TiO₂ N-doped nanofibers (NF) prepared by means of electrospinning (ES) and near-field electrospinning (NF-ES) techniques for the photo-degradation of methylene blue (MB) in water under visible light. The ES preparation technique allows a quick deposition of fibers on wide surfaces using a cost-effective system. Instead NF-ES method allows the growth of a well ordered NF net, with a spacing lower then few microns.

The NFs were deposited using different deposition and post-deposition parameters (solution composition, annealing temperature and atmosphere) and have been characterized by using X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD) and Secondary Electron Microscopy (SEM). The photocatalytic properties have been studied recording the variation of the optical absorption of MB when the sample is illuminated by an halogen lamp (visible spectrum).

The annealing process determines a partial loss of nitrogen and the formation of the Anatase crystalline phase. The TiO₂ N-doped NFs have shown interesting degradation properties, which are much better than those observed when TiO₂ NFs are used.