Hyung Ju Kim

Plasmon-Enhanced Photoelectrochemical Water Splitting with Size-Controllable Gold Nanodot Arrays

Size-controllable Au nanodot arrays (50, 63, and 83 nm dot size) with a narrow size distribution (± 5%) were prepared by a direct contact printing method on an indium tin oxide (ITO) substrate. Titania was added to the Au nanodots using TiO(2) sols of 2-3 nm in size. This created a precisely controlled Au nanodot with 110 nm of TiO(2) overcoats. Using these precisely controlled nanodot arrays, the effects of Au nanodot size and TiO(2) overcoats were investigated for photoelectrochemical water splitting using a three-electrode system with a fiber-optic visible light source. From UV-vis measurement, the localized surface plasmon resonance (LSPR) peak energy (ELSPR) increased and the LSPR line width (Γ) decreased with decreasing Au nanodot size. The generated plasmonic enhancement for the photoelectrochemical water splitting reaction increased with decreasing Au particle size. The measured plasmonic enhancement for light on/off experiments was 25 times for the 50 nm Au size and 10 times for the 83 nm Au nanodot size. The activity of each catalyst increased by a factor of 6 when TiO2 was added to the Au nanodots for all the samples. The activity of the catalyst was proportional to the quality factor (defined as Q = E(LSPR)/Γ) of the plasmonic metal nanostructure. The enhanced water splitting performance with the decreased Au nanodot size is probably due to more generated charge carriers (electron/hole pair) by local field enhancement as the quality factor increases.

Gold Nanoclusters Promote Electrocatalytic Water Oxidation at the Nanocluster/CoSe2 Interface

Electrocatalytic water splitting to produce hydrogen comprises the hydrogen and oxygen evolution half reactions (HER and OER), with the latter as the bottleneck process. Thus, enhancing the OER performance and understanding the mechanism are critically important. Herein, we report a strategy for OER enhancement by utilizing gold nanoclusters to form cluster/CoSe2 composites; the latter exhibit largely enhanced OER activity in alkaline solutions. The Au25/CoSe2 composite affords a current density of 10 mA cm-2 at small overpotential of ∼0.43 V (cf. CoSe2: ∼0.52 V). The ligand and gold cluster size can also tune the catalytic performance of the composites. Based upon XPS analysis and DFT simulations, we attribute the activity enhancement to electronic interactions between nanocluster and CoSe2, which favors the formation of the important intermediate (OOH) as well as the desorption of oxygen molecules over Aun/CoSe2 composites in the process of water oxidation. Such an atomic level understanding may provide some guidelines for design of OER catalysts.

Silicon Nanoparticles with Surface Nitrogen: 90% Quantum Yield with Narrow Luminescence Bandwidth and the Ligand Structure Based Energy Law

Silicon nanoparticles (NPs) have been widely accepted as an alternative material for typical quantum dots and commercial organic dyes in light-emitting and bioimaging applications owing to silicons intrinsic merits of least toxicity, low cost, and high abundance. However, to date, how to improve Si nanoparticle photoluminescence (PL) performance (such as ultrahigh quantum yield, sharp emission peak, high stability) is still a major issue. Herein, we report surface nitrogen-capped Si NPs with PL quantum yield up to 90% and narrow PL bandwidth (full width at half-maximum (fwhm) ≈ 40 nm), which can compete with commercial dyes and typical quantum dots. Comprehensive studies have been conducted to unveil the influence of particle size, structure, and amount of surface ligand on the PL of Si NPs. Especially, a general ligand-structure-based PL energy law for surface nitrogen-capped Si NPs is identified in both experimental and theoretical analyses, and the underlying PL mechanisms are further discussed.

Tailoring the Electronic and Catalytic Properties of Au25 Nanoclusters via Ligand Engineering

To explore the electronic and catalytic properties of nanoclusters, here we report an aromatic-thiolate-protected gold nanocluster, [Au25(SNap)18](-) [TOA](+), where SNap = 1-naphthalenethiolate and TOA = tetraoctylammonium. It exhibits distinct differences in electronic and catalytic properties in comparison with the previously reported [Au25(SCH2CH2Ph)18](-), albeit their skeletons (i.e., Au25S18 framework) are similar. A red shift by ∼10 nm in the HOMO-LUMO electronic absorption peak wavelength is observed for the aromatic-thiolate-protected nanocluster, which is attributed to its dilated Au13 kernel. The unsupported [Au25(SNap)18](-) nanoclusters show high thermal and antioxidation stabilities (e.g., at 80 °C in the present of O2, excess H2O2, or TBHP) due to the effects of aromatic ligands on stabilization of the nanoclusters frontier orbitals (HOMO and LUMO). Furthermore, the catalytic activity of the supported Au25(SR)18/CeO2 (R = Nap, Ph, CH2CH2Ph, and n-C6H13) is examined in the Ullmann heterocoupling reaction between 4-methyl-iodobenzene and 4-nitro-iodobenzene. Results show that the activity and selectivity of the catalysts are largely influenced by the chemical nature of the protecting thiolate ligands. This study highlights that the aromatic ligands not only lead to a higher conversion in catalytic reaction but also markedly increase the yield of the heterocoupling product (4-methyl-4-nitro-1,1-biphenyl). Through a combined approach of experiment and theory, this study sheds light on the structure-activity relationships of the Au25 nanoclusters and also offers guidelines for tailoring nanocluster properties by ligand engineering for specific applications.

Molecular Dynamics Study of Water Flow across Multiple Layers of Pristine, Oxidized, and Mixed Regions of Graphene Oxide

A molecular dynamics graphene oxide model is used to shed light on commonly overlooked features of graphene oxide membranes. The model features both perpendicular and parallel water flow across multiple sheets of pristine and/or oxidized graphene to simulate brick-and-mortar microstructures. Additionally, regions of pristine/oxidized graphene overlap that have thus far been overlooked in the literature are explored. Differences in orientational and hydrogen-bonding features between adjacent layers of water in this mixed region are found to be even more prominent than differences between pristine and oxidized channels. This region also shows lateral water flow in equilibrium simulations and orthogonal flow in non-equilibrium simulations significantly greater than those in the oxidized region, suggesting it may play a non-negligible role in the mechanism of water flow across graphene oxide membranes.

Highly selective transformation of glycerol to dihydroxyacetone without using oxidants by a PtSb/C-catalyzed electrooxidation process

We demonstrate an electrocatalytic reactor system for the partial oxidation of glycerol in an acidic solution to produce value-added chemicals, such as dihydroxyacetone (DHA), glyceraldehyde (GAD), glyceric acid (GLA), and glycolic acid (GCA). Under optimized conditions, the carbon-supported bimetallic PtSb (PtSb/C) catalyst was identified as a highly active catalyst for the selective oxidation of glycerol in the electrocatalytic reactor. The product selectivity can be strongly controlled as a function of the applied electrode potential and the catalyst surface composition. The main product from the electrocatalytic oxidation of glycerol was DHA, with a yield of 61.4% of DHA at a glycerol conversion of 90.3%, which can be achieved even without using any oxidants over the PtSb/C catalyst at 0.797 V (vs. SHE, standard hydrogen electrode). The electrocatalytic oxidation of biomass-derived glycerol represents a promising method of chemical transformation to produce value-added molecules.

Computer Simulation Study of Graphene Oxide Supercapacitors: Charge Screening Mechanism

Graphene oxide supercapacitors in the parallel plate configuration are studied via molecular dynamics (MD) simulations. The full range of electrode oxidation from 0 to 100% is examined by oxidizing the graphene surface with hydroxyl groups. Two different electrolytes, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI(+)BF4(-)) as an ionic liquid and its 1.3 M solution in acetonitrile as an organic electrolyte, are considered. While the area-specific capacitance tends to decrease with increasing electrode oxidation for both electrolytes, its details show interesting differences between the organic electrolyte and ionic liquid, including the extent of decrease. For detailed insight into these differences, the screening mechanisms of electrode charges by electrolytes and their variations with electrode oxidation are analyzed with special attention paid to the aspects shared by and the contrasts between the organic electrolyte and ionic liquid.

MD Study of Stokes Shifts in Ionic Liquids: Temperature Dependence

Effects of temperature on Stokes shifts, solvation structure, and dynamics in ionic liquids EMI(+)Tf2N(-), EMI(+)PF6(-), and BMI(+)PF6(-) (EMI(+) = 1-ethyl-3-methylimidazolium, BMI(+) = 1-butyl-3-methylimidazolium, Tf2N(-) = bis(trifluoromethylsulfonyl)imide, and PF6(-) = hexafluorophosphate) are investigated via molecular dynamics (MD) computer simulations in the temperature range 350 K ≤ T ≤ 500 K. Two different types of solutes are considered: a simple model diatomic solute and realistic coumarin 153, both of which are characterized by more polar S1 and less polar S0 states. In all three ionic liquids studied, the Stokes shift tends to decrease with increasing temperature. For coumarin 153, as T increases, the Franck-Condon energy for steady-state absorption decreases, whereas that for steady-state emission increases. Our findings indicate that the effective polarity of ionic liquids decreases as T increases. Their solvation dynamics are characterized by an ultrafast initial decay in the subpicosecond time scale, followed by slow dissipative relaxation, regardless of temperature. For both solutes, the solvent frequency that quantifies initial ultrafast dynamics shows little temperature dependence. By contrast, the long-time dissipative dynamics become significantly faster with rising T. Variations of solvation structure with temperature and their connection to Stokes shift and solvation dynamics are briefly examined.

Spectroscopic and MD Study of Dynamic and Structural Heterogeneities in Ionic Liquids

The structure of ionic liquids (ILs) surrounding solute dyes and the effects of solvent structure on solute diffusion have been investigated using molecular dynamics (MD) andu2009the experimental tools of confocal and fluorescence correlation spectroscopies. Although confocal microscopy and simulations show that the local environment around solutes in ILs is heterogeneous and that the structural heterogeneity is rather long-lived, the local polarity andu2009the diffusion constant were found to be uncorrelated. Moreover, the complex diffusion observed experimentally is not due to the structural heterogeneity of the IL but rather due to the dynamic heterogeneity arising from the viscous glassy nature of the IL environment. MD simulations show that the degree of dynamic heterogeneity depends on the first nonvanishing electric multipole moment of the solute. The dynamics of a cationic solute are the least heterogeneous, whereas those of a solute without an electric multipole moment are the most heterogeneous. This indicates that the length scale over which the solute-solvent interactions occur, and thus the number of solvent degrees of freedom that couple to the solute, are the key factors governing the dynamic heterogeneity of the solute.

The Role of Ruthenium on Carbon-Supported PtRu Catalysts for Electrocatalytic Glycerol Oxidation under Acidic Conditions

A series of binary PtRu catalysts with different Pt/Ru atomic ratios (from 7:3 to 3:7) were synthesized on a carbon support using the colloidal method; they were then used for electrooxidation of glycerol in acid media. X‐ray diffraction, transmission electron microscopy, X‐ray photoelectron spectroscopy, and X‐ray absorption spectroscopy analyses were used to investigate particle size, size distribution, and structural and electronic properties of the prepared catalysts. Ru added to the Pt‐based catalysts caused structural and electronic modifications over the PtRu alloy catalyst formation. The electrocatalytic activities of PtRu/C series catalysts were investigated using cyclic voltammetry. The Pt5Ru5/C catalyst shows enhanced catalytic activity at least 40u2009% higher than that of the Pt/C catalyst, with improved stability for glycerol electrooxidation; these improvements are attributed to structural and electronic modifications of the Pt catalysts. Using an electrocatalytic batch reactor, product analysis after the oxidation reaction was performed by high‐performance liquid chromatography to determine and compare the reaction pathways on the Pt/C and PtRu/C catalysts. To understand different catalytic activities of glycerol oxidation on the PtRu alloy surfaces, density functional calculations were performed.