Joon B Park

Chonbuk National University, Tsiuentcheou, North Jeolla, South Korea

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Publications (13)76.45 Total impact

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    ABSTRACT: Molecular-scale surface structures of self-assembled monolayers (SAMs) prepared by the adsorption of pentafluorobenzenethiols (PFBT) and pentachlorobenzenethiols (PCBT) on Au(111) were investigated by scanning tunneling microscopy (STM). High-resolution STM imaging revealed that PFBT SAMs on Au(111) have long-range ordered domains with a row structure at room temperature, whereas PCBT SAMs have small ordered domains, with disordered domains as the main phase. This may reflect the larger diffusion barriers of PCBT molecules on Au(111) surfaces compared to PFBT molecules during SAM formation. The structural transitions of PCBT SAMs from the mixed phase containing disordered and ordered domains to the uniform ordered domains were observed at 50 degrees C depending on immersion time. The ordered packing structure of PCBT SAMs is an incommensurate (square root of 3 x square root of 10)R45 degrees structure, which differs from that of PFBT SAMs with a (2 x 5 square root of 13)R30 degrees structure. We found that a small modification in the chemical structures of aromatic rings using a halo-substituent strongly affects the self-assembly mechanism and packing structure of aromatic thiol SAMs on Au(111). Moreover, we demonstrated that highly ordered PCBT SAMs can be obtained at a solution temperature of 50 degrees C after immersion for 60 min.
    Journal of Nanoscience and Nanotechnology 07/2014; 14(7):5054-8. · 1.15 Impact Factor
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    ABSTRACT: Graphene has been modified with palladium nanoparticles (Pd NPs) to develop high performance catalysts for the Sonogashira cross coupling reaction. In this research, graphite oxide (GO) sheets exfoliated from graphite were impregnated with Pd(OAc)2 to prepare Pd(2+)/GO. Thermal treatments of the Pd(2+)/GO in H2 flow at 100°C produced Pd/graphene (Pd/G) nanocomposites. TEM images show that Pd NPs were distributed quite uniformly on the graphene sheet without obvious aggregation, and the mean size of Pd NPs was determined to be less than 2nm in diameter. Morphological and chemical structures of the GO, Pd(2+)/GO, and Pd/G were investigated using FT-IR, XRD, XPS, and XAFS. The resulting Pd/G showed excellent catalytic efficiency in the Sonogashira reaction and offers significant advantages over inorganic supported catalysts such as simple recovery and recycling. Finally, deactivation process of the Pd/G in recycling was investigated. We believe that the remarkable reactivity of the Pd/G catalyst toward the Sonogashira reaction is attributed to the high degree of the Pd NP dispersion and thus the increased low coordination numbers of smaller Pd NPs.
    Journal of Colloid and Interface Science 04/2013; · 3.55 Impact Factor
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    ABSTRACT: Three different kinds of hydroxyapatites (HAPs) having different sizes and compositions are prepared by hydrothermal and molten salt syntheses. Using the ion exchange reactions, ruthenium ions are incorporated on the surface of HAPs. The crystallinity, morphology and ruthenium contents are investigated by XRD, SEM, TEM and ICP. We found that smaller size of HAP having large amounts of ruthenium under ion exchange reaction shows higher catalytic activity for aerobic oxidation of alcohols.
    Bulletin- Korean Chemical Society 01/2013; 34(1). · 0.84 Impact Factor
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    ABSTRACT: Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) were used to examine the surface structure and adsorption conditions of hexanethiol (HT) and cyclohexanethiol (CHT) self-assembled monolayers (SAMs) on Au(111) as a function of storage period in ultrahigh vacuum (UHV) conditions of 3×10(-7)Pa at room temperature (RT). STM imaging revealed that after storage for 7days, HT SAMs underwent phase transitions from c(4×2) phase to low coverage 4×√3 phase. This transition is due to a structural rearrangement of hexanethiolates that results from the spontaneous desorption of chemisorbed HT molecules on Au(111) surface. XPS measurements showed approximately 28% reduction in sulfur coverage, which indicates desorption of hexanethiolates from the surfaces. Contrary to HT SAMs, the structural order of CHT SAMs with (5×2√3)R35° phase completely disappeared after storage for 3 or 7days. XPS results show desorption of more than 80% of the cyclohexanethiolates, even after storage for 3days. We found that spontaneous desorption of CHT molecules on Au(111) in UHV at RT occurred quickly, whereas spontaneous desorption of HT molecules was much slower. Thermal desorption spectroscopy (TDS) results suggest CHT SAMs in UHV at RT can desorb more efficiently than HT SAMs due to formation of thiol desorption fragments that result from chemical reactions between surface hydrogen atoms and thiolates on Au(111) surfaces. This study clearly demonstrated that organic thiols chemisorbed on gold surfaces are desorbed spontaneously in UHV at RT and van der Waals interactions play an important role in determining the structural stability of thiolate SAMs in UHV.
    Journal of Colloid and Interface Science 12/2012; · 3.55 Impact Factor
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    ABSTRACT: We have carried out a fundamental study of the WGS reaction on model Au/CeOx/TiO2(110) and powder Au/CeOx/TiO2 catalysts paying particular attention to the effect of ceria on the activity of the gold–titania systems. CeOx nanoparticles deposited on TiO2(110) act as anchoring sites for gold improving the dispersion of the admetal on the oxide support. When compared to a typical benchmark system like Cu(111), Au/CeOx/TiO2(110) catalysts exhibit TOFs, which are 10–30 times larger, and a substantial reduction in the apparent activation energy for the WGS, which decreases from 18 kcal/mol on Cu(111) to 7 kcal/mol on Au/CeOx/TiO2(110). Low concentrations of ceria (6 and 15 wt %) were deposited onto a titania powder support via a wetness impregnation process. 1 atom % gold was then deposited on the CeOx/TiO2 mixed-oxide supports via a deposition–precipitation (DP) method. The Au/CeOx/TiO2 powder catalysts were characterized with HRTEM and a combination of in situ time-resolved XRD and XAFS. The XRD measurements indicated that a main effect of ceria was to enhance the concentration of oxygen vacancies in the catalysts and, thus, help with the dissociation of water during the reaction. Results of in situ XAFS showed that the gold oxidation state in the Au/CeOx/TiO2 powder catalysts changed from ionic (Auδ+) to metallic (Au0) with the start of the WGS. The active phase for these powder catalysts contained gold particles with average sizes of 2 nm. This study shows that the phenomena observed in model Au/CeOx/TiO2(110) catalysts do provide useful concepts for the design and preparation of highly active and stable powder catalysts for the WGS reaction.
    The Journal of Physical Chemistry C. 10/2012; 116(44):23547–23555.
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    ABSTRACT: The adsorption and thermal stability of 2-octylthieno[3,4-b]thiophene (OTTP) on the Au(111) surfaces have been studied using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). UHV-STM studies revealed that the vapor-deposited OTTP on Au(111) generated disordered adlayers with monolayer thickness even at saturation coverage. XPS and TPD studies indicated that OTTP molecules on Au(111) are stable up to 450 K and further heating of the sample resulted in thermal decomposition to produce H(2) and H(2)S via C-S bond scission in the thieno-thiophene rings. Dehydrogenation continues to occur above 600 K and the molecules were ultimately transformed to carbon clusters at 900 K. Highly resolved air-STM images showed that OTTP adlayers on Au(111) prepared from solution are composed of a well-ordered and low-coverage phase where the molecules lie flat on the surface, which can be assigned as a (9×2√33)R5° structure. Finally, based on analysis of STM, TPD, and XPS results, we propose a thermal decomposition mechanism of OTTP on Au(111) as a function of annealing temperature.
    Journal of Colloid and Interface Science 06/2012; 384(1):143-8. · 3.55 Impact Factor
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    ABSTRACT: The electronic properties of Pt nanoparticles deposited on CeO(2)(111) and CeO(x)/TiO(2)(110) model catalysts have been examined using valence photoemission experiments and density functional theory (DFT) calculations. The valence photoemission and DFT results point to a new type of "strong metal-support interaction" that produces large electronic perturbations for small Pt particles in contact with ceria and significantly enhances the ability of the admetal to dissociate the O-H bonds in water. When going from Pt(111) to Pt(8)/CeO(2)(111), the dissociation of water becomes a very exothermic process. The ceria-supported Pt(8) appears as a fluxional system that can change geometry and charge distribution to accommodate adsorbates better. In comparison with other water-gas shift (WGS) catalysts [Cu(111), Pt(111), Cu/CeO(2)(111), and Au/CeO(2)(111)], the Pt/CeO(2)(111) surface has the unique property that the admetal is able to dissociate water in an efficient way. Furthermore, for the codeposition of Pt and CeO(x) nanoparticles on TiO(2)(110), we have found a transfer of O from the ceria to Pt that opens new paths for the WGS process and makes the mixed-metal oxide an extremely active catalyst for the production of hydrogen.
    Journal of the American Chemical Society 05/2012; 134(21):8968-74. · 10.68 Impact Factor
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    ABSTRACT: The surface structures, adsorption conditions, and thermal desorption behaviors of cyclopentanethiol (CPT) self-assembled monolayers (SAMs) on Au(111) were investigated by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS). STM imaging revealed that although the adsorption of CPT on Au(111) at room temperature generates disordered SAMs, CPT molecules at 50 °C formed well-ordered SAMs with a (2√3 × √5)R41 o packing structure. XPS measurements showed that CPT SAMs at room temperature were formed via chemical reactions between the sulfur atoms and gold surfaces. TDS measurements showed two dominant TD peaks for the decomposed fragments (C 5 H 9 + , m/e = 69) generated via C-S bond cleavage and the parent molecular species (C 5 H 9 SH + , m/e = 102) derived from a recombination of the chemisorbed thiolates and hydrogen atoms near 440 K. Interestingly, dimerization of sulfur atoms in n-alkanethiol SAMs usually occurs during thermal desorption and the same reaction did not happen for CPT SAMs, which may be due to the steric hindrance of cyclic rings of the CPT molecules. In this study, we demonstrated that the alicyclic ring of organic thiols strongly affected the surface structure and thermal desorption behavior of SAMs, thus providing a good method for controlling chemical and physical properties of organic thiol SAMs.
    Bulletin- Korean Chemical Society 01/2011; 32. · 0.84 Impact Factor
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    ABSTRACT: At small coverages of ceria on TiO(2)(110), the CeO(x) nanoparticles have an unusual coordination mode. Scanning tunneling microscopy and density-functional calculations point to the presence of Ce(2)O(3) dimers, which form diagonal arrays that have specific orientations of 0, 24, and 42 degrees with respect to the [1 -1 0] direction of the titania substrate. At high coverages of ceria on TiO(2)(110), the surface exhibits two types of terraces. In one type, the morphology is not very different from that observed at low ceria coverage. However, in the second type of terrace, there is a compact array of ceria particles with structures that do not match the structures of CeO(2)(111) or CeO(2)(110). The titania substrate imposes on the ceria nanoparticles nontypical coordination modes, enhancing their chemical reactivity. This phenomenon leads to a larger dispersion of supported metal nanoparticles (M = Au, Cu, Pt) and makes possible the direct participation of the oxide in catalytic reactions. The M/CeO(x)/TiO(2)(110) surfaces display an extremely high catalytic activity for the water-gas shift reaction that follows the sequence Au/CeO(x)/TiO(2)(110) < Cu/CeO(x)/TiO(2)(110) < Pt/CeO(x)/TiO(2)(110). For low coverages of Cu and CeO(x), Cu/CeO(x)/TiO(2)(110) is 8-12 times more active than Cu(111) or Cu/ZnO industrial catalysts. In the M/CeO(x)/TiO(2)(110) systems, there is a strong coupling of the chemical properties of the admetal and the mixed-metal oxide: The adsorption and dissociation of water probably take place on the oxide, CO adsorbs on the admetal nanoparticles, and all subsequent reaction steps occur at the oxide-admetal interface. The high catalytic activity of the M/CeO(x)/TiO(2)(110) surfaces reflects the unique properties of the mixed-metal oxide at the nanometer level.
    Journal of the American Chemical Society 12/2009; 132(1):356-63. · 10.68 Impact Factor
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    Angewandte Chemie International Edition 09/2009; 48(43):8047-50. · 11.34 Impact Factor
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    ABSTRACT: Mixed-metal oxides play a very important role in many areas of chemistry, physics, materials science, and geochemistry. Recently, there has been a strong interest in understanding phenomena associated with the deposition of oxide nanoparticles on the surface of a second (host) oxide. Here, scanning tunneling microscopy, photoemission, and density-functional calculations are used to study the behavior of ceria nanoparticles deposited on a TiO(2)(110) surface. The titania substrate imposes nontypical coordination modes on the ceria nanoparticles. In the CeO(x)/TiO(2)(110) systems, the Ce cations adopt an structural geometry and an oxidation state (+3) that are quite different from those seen in bulk ceria or for ceria nanoparticles deposited on metal substrates. The increase in the stability of the Ce(3+) oxidation state leads to an enhancement in the chemical and catalytic activity of the ceria nanoparticles. The codeposition of ceria and gold nanoparticles on a TiO(2)(110) substrate generates catalysts with an extremely high activity for the production of hydrogen through the water-gas shift reaction (H(2)O + CO --> H(2) + CO(2)) or for the oxidation of carbon monoxide (2CO + O(2) --> 2CO(2)). The enhanced stability of the Ce(3+) state is an example of structural promotion in catalysis described here on the atomic level. The exploration of mixed-metal oxides at the nanometer level may open avenues for optimizing catalysts through stabilization of unconventional surface structures with special chemical activity.
    Proceedings of the National Academy of Sciences 03/2009; 106(13):4975-80. · 9.81 Impact Factor
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    ABSTRACT: Using STM, infrared absorption reflection spectroscopy experiments and density functional calculations we show that low temperature adsorption of CO on gold surfaces modified by vacancy islands leads to morphological changes and the formation of nanosized Au particles. These results demonstrate a dynamic response of a surface during adsorption with consequences for the surface reactivity.
    Journal of the American Chemical Society 01/2009; 130(51):17272-3. · 10.68 Impact Factor
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    ABSTRACT: Mixed-metal oxides play a very important role in many areas of chemistry, physics, materials science, and geochemistry. Recently, there has been a strong interest in understanding phenomena associated with the deposition of oxide nanoparticles on the surface of a second (host) oxide. Here, scanning tunneling micros-copy, photoemission, and density-functional calculations are used to study the behavior of ceria nanoparticles deposited on a TiO2(110) surface. The titania substrate imposes nontypical coor-dination modes on the ceria nanoparticles. In the CeOx/TiO2(110) systems, the Ce cations adopt an structural geometry and an oxidation state (3) that are quite different from those seen in bulk ceria or for ceria nanoparticles deposited on metal substrates. The increase in the stability of the Ce 3 oxidation state leads to an enhancement in the chemical and catalytic activity of the ceria nanoparticles. The codeposition of ceria and gold nanoparticles on a TiO2(110) substrate generates catalysts with an extremely high activity for the production of hydrogen through the water– gas shift reaction (H2O CO 3 H2 CO2) or for the oxidation of carbon monoxide (2CO O2 3 2CO2). The enhanced stability of the Ce 3 state is an example of structural promotion in catalysis described here on the atomic level. The exploration of mixed-metal oxides at the nanometer level may open avenues for optimizing catalysts through stabilization of unconventional surface structures with special chemical activity. heterogeneous catalysis imaging structural properties surface reactivity M ixed-metal oxides play a very important role in many areas of chemistry, physics, materials science, and geochemistry (1–6). In technological applications, they are used in the fabri-cation of microelectronic circuits, piezoelectric devices, and sensors and as catalysts. Over the years, there has been a strong interest in understanding the behavior of mixed-metal oxides at a fundamental level (1–3). What happens when nanoparticles (NPs) of a given metal oxide are deposited on the surface of a second (host) oxide (3, 7)? In principle, the combination of 2 metals in an oxide matrix can produce materials with novel structural and/or electronic properties. At a structural level, a dopant can introduce stress into the lattice of an oxide host, inducing in this way the formation of defects. On the other hand, the lattice of the oxide host can impose on the dopant element nontypical coordination modes. Finally, metal 7 metal or metal 7 oxygen 7 metal interactions in mixed-metal oxides can give electronic states not seen in single-metal oxides. In this article, we use photoemission, scanning tunneling micros-copy (STM), and calculations based on density-functional theory (DFT) to study the behavior of ceria NPs in contact with TiO 2 (110). Ceria and titania are among the most widely used oxides in catalysis (1, 4–6, 8–12). These oxides are important components in catalysts used for the production of clean hydrogen through the water–gas shift reaction (H 2 O CO 3 H 2 CO 2) and for the oxidation of carbon monoxide (2CO O 2 3 2CO 2). Ceria and titania adopt different crystal lattices in their most stable bulk phases, fluorite and rutile, respectively (2, 13). Within the fluorite structure each Ce atom is bonded to 8 O atoms, whereas 6 O atoms surround the Ti atoms in the rutile structure. One of the most interesting properties of ceria is its ability to undergo a conversion between ''4'' and ''3'' formal oxidation states (13). The surface chemistry and catalytic properties of CeO 2 depend on the formation of Ce 3 ions (13), and different approaches are followed to maximize their concentration (4, 5, 8). In the CeO x /TiO 2 (110) systems, the titania substrate imposes on the ceria NPs nontypical coordination modes with a subsequent change in the relative stability of the Ce 3 /Ce 4 oxidation states that leads to a significant enhancement in chemical activity. Furthermore, the deposition of gold NPs on CeO x / TiO 2 (110) produces surfaces with an extremely high catalytic activity for the water–gas shift reaction and the oxidation of CO. This is quite remarkable because neither Au/TiO 2 (110) nor Au/ CeO 2 (111) come close to matching the catalytic activity of Au/ CeO x /TiO 2 (110).
    Proceedings of the National Academy of Sciences 01/2009; · 9.81 Impact Factor