Plasmonic photocatalysis


The chemical industry heavily relies on heterogeneous catalytic processes to manufacture products, such as fuels and fertilizers, which are critical to sustaining human life and development. Current catalytic reactions are almost exclusively driven by thermal energy, thus requiring a high operating temperature to achieve practical reaction rates. High operating temperatures in industrial reactions demand a considerable amount of thermal energy input and deteriorate the catalyst lifetime due to the sintering of catalyst nanoparticles. The advancements toward sustainable and environmentally benign chemical processes necessitate heterogeneous catalytic reactions with low operating temperature and preferential formation of targeted products. Introducing other forms of energy, such as light, is a promising strategy to enhance the reaction rate and mitigate the energy cost. Plasmonic metal nanoparticles with strong light absorption capability have been demonstrated as a new family of photocatalysts that offer distinctly different characteristics compared to conventional semiconductor photocatalysts. The collective oscillations of surface electron density in metal nanoparticles excited by photons with certain energy, so-called localized surface plasmon resonance (LSPR), decay into energetic "hot" electrons with energies not accessible by heating. The hot electrons can transfer to the unoccupied antibonding orbitals of surface intermediates to weaken the chemical bonds and accelerate reactions. This unique electron-mediated reaction mechanism exhibits a favorable super-linear dependence of reaction rates on light intensity, in contrast to the sub-linear dependence observed on semiconductor photocatalysts.



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.





 

Contact Information

Dr. Jie Liu
Department of Chemistry
Duke University
2105 French Family Science Center
Durham, NC, 27708-0354
Tel: (919) 660-1549
Fax: (919)660-1605
Email: j.liu@duke.edu

Available Positions:

For available positions, please contact Dr. Jie Liu at j.liu@duke.edu for more information

 

 

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