Metal oxide nanomaterials for solar hydrogen generation from photoelectrochemical water splitting
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Introduction Hydrogen is arguably one of the most attractive fuels due to its high energy content per unit mass (120 J/g, about three times that of gasoline) and clean by-product, water, when consumed for energy generation.1–6 However, its generation, storage, and usage on a large scale and at a low cost have been met with significant challenges. Currently, the dominant technology for direct hydrogen production is steam reforming that involves reactions between steam and hydrocarbons mostly based on fossil fuel.7 Storage of hydrogen with high mass or volume density and at ambient temperature and pressure has been extremely difficult.8 Fuel cells are still high in cost in comparison to competitive technologies, such as gasoline internal combustion engines.9 Even given all of these challenges, the attraction of hydrogen as a clean fuel has continued to stimulate interest in research and development in generation, storage, and usage. On the generation front, one key focus has been on developing low-cost materials and strategies. One of the most attractive approaches for hydrogen generation is solar water splitting that uses solar light as the source of energy and water as the source of hydrogen, via processes such as photocatalysis or photoelectrochemistry. Photoelectrochemical (PEC) water splitting is attractive due to
its potential high efficiency, up to >30% in principle, low cost, and environmental friendliness.2 Nanostructured materials offer unique advantages for potential PEC applications.6 First, nanoscale sizes are comparable to carrier scattering lengths, significantly reducing the scattering rate and increasing carrier collection efficiency. Second, nanomaterials have strong absorption coefficients due to an increase of oscillator strength, thereby enabling high conversion efficiency. Third, the bandgap of nanomaterials (e.g., quantum dots [QDs]) can be tuned to absorb in a particular wavelength by varying size and, in principle, cover the whole solar spectrum. Fourth, the electronic band structure can be controlled by doping. Finally, bottom-up growth approaches, which use smaller components to produce larger and more complex structures, allow scalable synthesis of single crystal nanostructures on flexible substrates under mild conditions, leading to light weight and low cost. In this review, we will focus on some recent progress on the study of metal oxide (MO) nanomaterials for PEC hydrogen generation, with emphasis on new strategies developed to enhance PEC efficiency. Efforts have been made to control morphological features, such as nanostructure size, shape, and surface characteristics, to improve PEC
Jin Zhong Zhang, University of California, Santa Cruz, CA 95064, USA, [email protected] DOI: 10.1557/mrs.2010.9
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MRS BULLETIN • VOLUME 36 • JANUARY 2011 • www.mrs.org/bulletin
© 2011 Materials Research Society
METAL OXIDE NANOMATERIALS FOR SOLAR HYDROGEN GENERATION performance. While most research efforts have been on the experimental front, relevant theoretical and computation studies are import
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