Nano-photocatalysis

Can we achieve a 100% quantum efficiency on photocatalytic hydrogen generation reactions?

Recently, hydrogen evolution reactions (HERs) have become the most popular process in sustainable energy production. At the current stage, hydrogen gas is gathered during petroleum processes at a cheap price (grey hydrogen). Electrolysis of water seems practical for this purpose, but the total efficiency is still limited, giving an unaffordable cost of green hydrogen.

As a chemist, we are interested in figuring out an ultimate solution of hydrogen production directly from sunlight. Semiconductors can absorb light and generate charge-separated states, and the resulting electrons and holes lead to water-splitting reactions. The addition of metal cocatalysts paramountly increases the lifetime of charge-separated states by facile electron transfer. The ideal combination of the semiconductor and metal domains may maximize quantum efficiency,1,2 defined as the total amount of hydrogen production times two divided by the total number of absorbed photons.

We designed a model catalyst, semiconductor nanorods with metal tips, and investigated various factors critical for total efficiency, including the number of metal tips,3 tip components,4 the choice of semiconductor alloys,5 the nanorod length,6 and geometrical factors of semiconductors and metal tips. When we changed CdSe to CdSe-seeded CdS nanorods, we could yield 100% quantum efficiency at a specific wavelength. Our observations enable us to understand the basic principle of photocatalyst design with high performance. We will extend our model catalyst design into two- and three-dimensional structures and pursue making practical catalysts with direct sunlight irradiation.

Publications

[1] "Research Update: Regulation of electron-hole recombination kinetics on uniform metal-semiconductor nanostructures for photocatalytic hydrogen evolution", W. Choi, J. Y. Choi, H. Song, APL Mater. 7, 100702 (2019), as an invited review. 

[2] "Metal Hybrid Nanoparticles for Catalytic Organic and Photochemical Transformations", H. Song, Acc. Chem. Res. 48, 491-499 (2015). 

[3] "Geometric effect of single or double metal-tipped CdSe nanorods on photocatalytic H2 generation", J. U. Bang, S. J. Lee, J. S. Jang, W. Choi, H. Song, J. Phys. Chem. Lett. 3, 3781-3785 (2012). 

[4] "Engineering Reaction Kinetics by Tailoring the Metal Tips of Metal-Semiconductor Nanodumbbells", J. Y. Choi, D. Jeong, S. J. Lee, D.-g. Kang, S. K. Kim, K. M. Nam, H. Song, Nano Lett. 17, 5688-5694 (2017).

[5] "Composition Effect of Alloy Semiconductors on Pt-tipped Zn1-xCdxSe Nanorods for Enhanced Photocatalytic Hydrogen Generation", J. Y. Choi, K. M. Nam, H. Song, J. Mater. Chem. A 6, 16316-16321 (2018).

[6] "Optimal Length of Hybrid Metal-Semiconductor Nanorods for Photocatalytic Hydrogen Generation”, J. Y. Choi, W.-W. Park, B. Park, S. Sul, O.-H. Kwon, H. Song, ACS Catal. 11, 13303-13311 (2021). 

Can we make photocatalytic carbon dioxide conversion a practically good process?

Photocatalysts for converting carbon dioxide have been intensively studied as cutting-edge environmental technology for producing renewable energy. Based on our synthetic experience on nanomaterials, we focus on heterogeneous semiconductor photocatalysts for converting carbon dioxide into value-added compounds such as methane or carbon monoxide. To improve catalytic efficiency, we planned to design an "ideal photocatalyst," consisting of a semiconducting light absorber and a metal oxide cocatalyst forming a well-defined ohmic junction.1 This structure can facilitate a Z-scheme-type reaction pathway to supply high overpotentials for cathodic and anodic reactions and maintain stable charge-separated states with long half-lives. 

In our group, we already synthesized colloidal ZnO-Cu2O hybrid catalysts, exhibiting a 1.5% quantum efficiency to produce methane without any sacrificial reagents in aqueous media.2 This structure is also suitable for acetone sensing, relying on its uniform p-n junctions formed between the hetero-domains.3 Besides, we developed a simple surface treatment of the stable metal oxide nanoparticles to remarkably enhance photochemical activities.4 Now, we focus on visible-active photocatalysts by bandgap engineering, which helps to utilize full wavelengths of direct sunlight more efficiently. We also try to develop flow-type gas phase reactors to approach mass-scale carbon dioxide conversion processes.

Publications

[1] "Strategies for Designing Nanoparticles for Electro- and Photocatalytic CO2 Reduction", J. Y. Choi, W. Choi, J. W. Park, C. K. Lim, H. Song, Chem. Asian J. 15, 253-265 (2020).

[2] "Colloidal zinc oxide-copper(I) oxide nanocatalysts for selective aqueous photocatalytic carbon dioxide conversion into methane", K-L. Bae, J. Kim, C. K. Lim, K. M. Nam, H. Song, Nat. Commun. 8, 1156 (2017).

[3] "ZnO-CuO Core-Hollow Cube Nanostructures for Highly Sensitive Acetone Gas Sensors at a ppb Level", J. E. Lee, C. K. Lim, H. J. Park, H. Song, S.-Y. Choi, D.-S. Lee, ACS Appl. Mater. Interfaces 12, 35688-35697 (2020).

[4] "Surface Activation of Cobalt Oxide Nanoparticles for Photocatalytic Carbon Dioxide Reduction to Methane", J. Y. Choi, C. K. Lim, B. Park, M. Kim, A. Jamal, H. Song, J. Mater. Chem. A 7, 15068-15072 (2019).