Nanostructured Metal Oxides vs. Single Crystal Semiconductors: Affordable and Efficient Photoelectrochemical Conversion to Fuels

Solar energy is among one of the few energy sources capable of meeting the global energy demands; unfortunately, the transient nature of sunlight necessitates the development of technologies to store that energy for on-demand use to supplement photovoltaics. Photoelectrochemical (PEC) solar cells use sunlight to electrolyze water and produce hydrogen as a storable fuel, which can then be converted to electricity by fuel cells.

TiO₂ nanotube arrays (NTs) exhibit significant potential as inexpensive photoanodes in low cost PEC cells for solar water oxidation. The wide bandgap and low mean free path of charge carriers of these materials, however, limits performance. Sub-stoichiometric phases or suitable defects have been proven to enhance photoresponse, and thermal treatments under reducing atmosphere result in oxygen deficient TiO₂-xmaterials, forming some of the Magneli phases, characterized by higher conductivity and lower band gaps. Nitrogen modification on the other hand may decrease the band gap, enabling increased light absorption. Such modifications result in significant performance increases, but so far these materials cannot generate photocurrents above few mA/cm² .

Single crystal semiconductors (Si, GaAs) in contrast provide suitable band gaps and high charge carrier mobility, allowing high performance conversion. n-GaAs in particular is a promising photoanode as it provides a suitable bandgap (1.43 eV) to absorb energy from the solar spectrum (<867 nm) and a more negative photocurrent onset compared to Si. Bare GaAs, however, suffers from photocorrosion under water oxidation conditions, leading to insulating oxide layer formation or dissolution. Efforts to overcome the instability of GaAs include the addition of protective layers, such as noble metal films, polymer coatings, or atomic layer deposition of TiO₂. In this talk, self-terminated electrodeposition is carried out to form very thin Ni or CoNi hydroxide layer onto GaAs substrates. Self-limiting growth is evidenced by XPS, showing that the deposition time is independent of the layer thickness. Short-time deposition (t ≤ 1.0s) is investigated to achieve the optimum compromise between the formation of a sufficiently protective layer while simultaneously minimizing the reduction of incident light absorption by the metallic layer

Biography: Dr. Giovanni Zangari is currently a professor of Materials Science at the University of Virginia. He received his M.Sc. in Nuclear Engineering from Politecnico di Milano and his Ph.D. in Metallurgical Engineering from the Politecnico di Torino (Italy). Before coming to Virginia, he was a post-doc at Carnegie Mellon University and an assistant, then associate professor at the U. of Alabama, Tuscaloosa. His research activities lie at the intersection of materials science and electrochemistry; in particular, his group is focused on developing the fundamental science and processing knowledge to tailor materials for micro/nano-electronics, magnetics, and most recently energy conversion applications. Current interests encompass the fundamentals of alloy electrodeposition, the synthesis of semiconductor materials for energy applications, the tailoring of electron transport properties in dielectric oxides, and the design of novel magnetic storage devices. Prof. Zangari has published more than 200 articles and has coauthored a book on electrodeposition science and technology. He is a Fellow of the Electrochemical Society.