The ideal plasmonic photocatalysts should simultaneously act as an absorber to capture light as well as a catalytic surface to interact properly with surface intermediates. Despite in some niche applications, good plasmonic metals like gold and silver, are not considered to be good catalysts. Group VIII metals, including rhodium, ruthenium, and platinum, are widely employed as catalysts by virtue of their appropriate positions of d-band centers. We explore the plasmonic properties of group VIII metal nanoparticles and multimetallic nanoparticles containing group VIII metals by tuning their plasmonic properties through the manipulation of shape, size, composition and assembly. These metal nanoparticles possessing both plasmonic and catalytic activities are used as photocatalysts in economically important chemical reactions, such as carbon dioxide hydrogenation. The mechanism of plasmonic photocatalysis, especially processes involving chemical transformations, is investigated to guide the future design of plasmonic photocatalysts.
Presently, we have developed slow-injection polyol methods to synthesize rhodium nanocubes with unprecedentedly large size, wide size tunability (15~60 nm) and narrow size distribution. The slow injection rate of the metal precursor maintains a low supersaturation of reduced rhodium atoms and prevents secondary nucleation during growth, resulting in monodispersed rhodium nanocubes. The large size of these rhodium nanocubes red-shifts the resonant wavelength of localized surface plasmons from deep ultraviolet region to more experimentally accessible near ultraviolet region. The resonant wavelength also red-shifts with increasing size, quantitatively consistent with finite-element simulations by our collaborators (Nanoscale Horizons 1 (1), 75-80). These rhodium nanocubes can serve as a platform to investigate the principles and applications of ultraviolet plasmonics.
Rhodium nanocubes were employed as photocatalysts for carbon dioxide hydrogenation. Photo-enhanced reaction rates and photo-induced product selectivity were observed on rhodium photocatalysts. In contrast to gold photocatalysts, whose exclusively product is carbon monoxide in both thermo- and photo-reactions, methane and carbon monoxide are produced at comparable rates on rhodium catalysts in thermos-reactions. Under light illumination, the methane production rate is significantly and preferentially enhanced on rhodium photocatalysts, while a small increase is observed for carbon monoxide production. The differences in product selectivity from different metals and reaction conditions can be attributed to the different interactions between reaction intermediates, and metal surfaces and hot electrons. The hot-electron-induced reaction offers an additional dimension for the control of product selectivity.