Changhwan Lee

Bio: Changhwan Lee is an academic researcher from KAIST. The author has contributed to research in topics: Nanodiodes & Plasmon. The author has an hindex of 12, co-authored 22 publications receiving 404 citations.

Plasmonic hot carrier-driven oxygen evolution reaction on Au nanoparticles/TiO2 nanotube arrays

Abstract: The use of hot carriers generated from the decay of localized surface plasmon resonance in noble metal nanoparticles is a promising concept for photocatalysis. Here, we report the enhancement of photocatalytic activity by the flow of hot electrons on TiO2 nanotube arrays decorated with 5-30 nm Au nanoparticles as photoanodes for photoelectrochemical water splitting. This enhanced photocatalytic activity is correlated to the size of the Au nanoparticles, where higher oxygen evolution was observed on the smaller nanoparticles. Conductive atomic force microscopy and ultraviolet photoelectron spectroscopy were used to characterize the Schottky barrier between Au and TiO2, which reveals a reduction in the Schottky barrier with the smaller Au nanoparticles and produces an enhanced transfer of photoinduced hot carriers. This study confirms that the higher photocatalytic activity was indeed driven by the hot electron flux generated from the decay of localized surface plasmon resonance.

Boosting hot electron flux and catalytic activity at metal-oxide interfaces of PtCo bimetallic nanoparticles.

Abstract: Despite numerous studies, the origin of the enhanced catalytic performance of bimetallic nanoparticles (NPs) remains elusive because of the ever-changing surface structures, compositions, and oxidation states of NPs under reaction conditions. An effective strategy for obtaining critical clues for the phenomenon is real-time quantitative detection of hot electrons induced by a chemical reaction on the catalysts. Here, we investigate hot electrons excited on PtCo bimetallic NPs during H2 oxidation by measuring the chemicurrent on a catalytic nanodiode while changing the Pt composition of the NPs. We reveal that the presence of a CoO/Pt interface enables efficient transport of electrons and higher catalytic activity for PtCo NPs. These results are consistent with theoretical calculations suggesting that lower activation energy and higher exothermicity are required for the reaction at the CoO/Pt interface.

Chemical-reaction-induced hot electron flows on platinum colloid nanoparticles under hydrogen oxidation: impact of nanoparticle size.

Abstract: Generation of hot electron flows and the catalytic activity of Pt nanoparticles (NPs) with different sizes were investigated using catalytic nanodiodes. We show that smaller Pt NPs lead to higher chemicurrent yield, which is associated with the shorter travel length for the hot electrons, compared with their inelastic mean free path. We also show the impact of capping on charge carrier transfer between Pt NPs and their support.

Hot carrier multiplication on graphene/TiO2 Schottky nanodiodes.

Abstract: Carrier multiplication (i.e. generation of multiple electron–hole pairs from a single high-energy electron, CM) in graphene has been extensively studied both theoretically and experimentally, but direct application of hot carrier multiplication in graphene has not been reported. Here, taking advantage of efficient CM in graphene, we fabricated graphene/TiO2 Schottky nanodiodes and found CM-driven enhancement of quantum efficiency. The unusual photocurrent behavior was observed and directly compared with Fowler’s law for photoemission on metals. The Fowler’s law exponent for the graphene-based nanodiode is almost twice that of a thin gold film based diode; the graphene-based nanodiode also has a weak dependence on light intensity—both are significant evidence for CM in graphene. Furthermore, doping in graphene significantly modifies the quantum efficiency by changing the Schottky barrier. The CM phenomenon observed on the graphene/TiO2 nanodiodes can lead to intriguing applications of viable graphene-based light harvesting.

Hot electrons generated by intraband and interband transition detected using a plasmonic Cu/TiO2 nanodiode

Abstract: The use of metal nanoparticles, including Au, Ag, Cu, and Al, can increase the efficiency of photovoltaic devices by electromagnetic field enhancement, which is driven by the excitation of localized surface plasmon resonance. Direct energy conversion from light into electricity via the decay of surface plasmons causing the excitation of hot electron–hole pairs is also a feasible channel. The generation of hot electrons in metal nanostructures can occur through intraband excitation within the conduction band or through interband transition, which is caused by transitions from other bands to the unoccupied conduction band states. Here, we show the distinction between hot electron generation induced by intraband excitation and interband transition on a plasmonic Cu/TiO2 nanodiode by measuring the current conversion efficiency with a monochromator system. We also show the dependence of the production of photocurrent on the thickness of the Cu layer and the effect of an aluminum oxide protection layer on the hot electron flux versus oxidation of the Cu layer. Our results can provide a better understanding for copper-based hot electron photovoltaics, which could lead to more efficient plasmonic energy conversion.

Screening in crystalline liquids protects energetic carriers in hybrid perovskites

Abstract: Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient scattering with charged defects and optical phonons is unknown. Here, we reveal the carrier protection mechanism by comparing three single-crystal lead bromide perovskites: CH3NH3PbBr3, CH(NH2)2PbBr3, and CsPbBr3. We observed hot fluorescence emission from energetic carriers with ~102-picosecond lifetimes in CH3NH3PbBr3 or CH(NH2)2PbBr3, but not in CsPbBr3. The hot fluorescence is correlated with liquid-like molecular reorientational motions, suggesting that dynamic screening protects energetic carriers via solvation or large polaron formation on time scales competitive with that of ultrafast cooling. Similar protections likely exist for band-edge carriers. The long-lived energetic carriers may enable hot-carrier solar cells with efficiencies exceeding the Shockley-Queisser limit.

Applications of Plasmon-Enhanced Nanocatalysis to Organic Transformations.

Abstract: Localized surface plasmon resonance (LSPR) is a physical phenomenon exhibited by nanoparticles of metals including coinage metals, alkali metals, aluminum, and some semiconductors which translates into electromagnetic, thermal, and chemical properties. In the past decade, LSPR has been taken advantage of in the context of catalysis. While plasmonic nanoparticles (PNPs) have been successfully applied toward enhancing catalysis of inorganic reactions such as water splitting, they have also demonstrated exciting performance in the catalysis of organic transformations with potential applications in synthesis of molecules from commodity to pharmaceutical compounds. The advantages of this approach include improved selectivity, enhanced reaction rates, and milder reaction conditions. This review provides the basics of LSPR theory, details the mechanisms at play in plasmon-enhanced nanocatalysis, sheds light onto such nanocatalyst design, and finally systematically presents the breadth of organic reactions hence catalyzed.

Role of hot electrons and metal-oxide interfaces in surface chemistry and catalytic reactions.

Abstract: Chemistry and Catalytic Reactions Jeong Young Park,*,†,‡ L. Robert Baker, and Gabor A. Somorjai* †Center for Nanomaterials and Chemical Reactions, Institute for Basic Science, Daejeon 305-701, South Korea ‡Graduate School of EEWS, KAIST, Daejeon 305-701, South Korea Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States Materials Sciences and Chemical Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, Berkeley, California 94720, United States

Hydrogen peroxide electrochemistry on platinum: Towards understanding the oxygen reduction reaction mechanism

Abstract: Understanding the hydrogen peroxide electrochemistry on platinum can provide information about the oxygen reduction reaction mechanism, whether H(2)O(2) participates as an intermediate or not. The H(2)O(2) oxidation and reduction reaction on polycrystalline platinum is a diffusion-limited reaction in 0.1 M HClO(4). The applied potential determines the Pt surface state, which is then decisive for the direction of the reaction: when H(2)O(2) interacts with reduced surface sites it decomposes producing adsorbed OH species; when it interacts with oxidized Pt sites then H(2)O(2) is oxidized to O(2) by reducing the surface. Electronic structure calculations indicate that the activation energies of both processes are low at room temperature. The H(2)O(2) reduction and oxidation reactions can therefore be utilized for monitoring the potential-dependent oxidation of the platinum surface. In particular, the potential at which the hydrogen peroxide reduction and oxidation reactions are equally likely to occur reflects the intrinsic affinity of the platinum surface for oxygenated species. This potential can be experimentally determined as the crossing-point of linear potential sweeps in the positive direction for different rotation rates, hereby defined as the "ORR-corrected mixed potential" (c-MP).