Youngku Sohn

Synergy of Low-Energy {101} and High-Energy {001} TiO2 Crystal Facets for Enhanced Photocatalysis

Controlled crystal growth determines the shape, size, and exposed facets of a crystal, which usually has different surface physicochemical properties. Herein we report the size and facet control synthesis of anatase TiO2 nanocrystals (NCs). The exposed facets are found to play a crucial role in the photocatalytic activity of TiO2 NCs. This is due to the known preferential flow of photogenerated carriers to the specific facets. Although, in recent years, the main focus has been on increasing the surface area of high-energy exposed facets such as {001} and {100} to improve the photocatalytic activity, here we demonstrate that the presence of both the high-energy {001} oxidative and low-energy {101} reductive facets in an optimum ratio is necessary to reduce the charge recombination and thereby enhance photocatalytic activity of TiO2 NCs.

Nanoscale Shape and Size Control of Cubic, Cuboctahedral, and Octahedral Cu−Cu2O Core−Shell Nanoparticles on Si(100) by One-Step, Templateless, Capping-Agent-Free Electrodeposition

Cu-Cu2O core-shell nanoparticles (NPs) of different shapes over an extended nanosize regime of 5-400 nm have been deposited on a H-terminated Si(100) substrate by using a simple, one-step, templateless, and capping-agent-free electrochemical method. By precisely controlling the electrolyte concentration [CuSO4 x 5H2O] below their respective critical values, we can obtain cubic, cuboctahedral, and octahedral NPs of different average size and number density by varying the deposition time under a few seconds (<6 s). Combined glancing-incidence X-ray diffraction and depth-profiling X-ray photoelectron spectroscopy studies show that these NPs have a crystalline core-shell structure, with a face-centered cubic metallic Cu core and a simple cubic Cu2O shell with a CuO outerlayer. The shape control of Cu-Cu2O core-shell NPs can be understood in terms of a diffusion-limited progressive growth model under different kinetic conditions as dictated by different [CuSO4 x 5H2O] concentration regimes.

Adsorption and UV/Visible photocatalytic performance of BiOI for methyl orange, Rhodamine B and methylene blue: Ag and Ti-loading effects

We synthesized echinoid-like BiOI microspheres with various doped concentrations of Ag (0.1, 1.0, 5.0, 10.0 mol%) and Ti (1.0, 5.0, 10.0, 30.0, 50.0 mol%) in ethylene glycol and then examined their fundamental properties by scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis, UV-visible absorption, FT-IR, Raman, photoluminescence and Brunauer–Emmett–Teller (BET) surface area measurements. We also measured the adsorption and photocatalytic dye-degradation performance of the catalysts using methyl orange (MO), Rhodamine B (RhB) and methylene blue (MB). The adsorption performance was found to be in the order of MO < RhB < MB, and to depend somewhat upon the Ag and Ti-loadings. MO was degraded in the order of BiOI < Ag–BiOI < Ti–BiOI under UV and visible light irradiation, while the degradation of RhB was in the order of Ag–BiOI ≈ Ti–BiOI ≪ BiOI, and Ag–BiOI < BiOI < Ti–BiOI, respectively. MB showed poor photodegradation under UV and visible light. Finally, we used an indirect chemical probe method with active species scavengers and photoluminescence spectroscopy to clarify the dye-sensitized photodegradation mechanism. In the mechanism, ˙O2− and h+ were active species under visible light irradiation. No ˙OH radicals were found by luminescence spectroscopy.

Green Synthesis of Anatase TiO2 Nanocrystals with Diverse Shapes and their Exposed Facets-Dependent Photoredox Activity

The exposed facets of a crystal are known to be one of the key factors to its physical, chemical and electronic properties. Herein, we demonstrate the role of amines on the controlled synthesis of TiO2 nanocrystals (NCs) with diverse shapes and different exposed facets. The chemical, physical and electronic properties of the as-synthesized TiO2 NCs were evaluated and their photoredox activity was tested. It was found that the intrinsic photoredox activity of TiO2 NCs can be enhanced by controlling the chemical environment of the surface, i.e.; through morphology evolution. In particular, the rod shape TiO2 NCs with ∼25% of {101} and ∼75% of {100}/{010} exposed facets show 3.7 and 3.1 times higher photocatalytic activity than that of commercial Degussa P25 TiO2 toward the degradation of methyl orange and methylene blue, respectively. The higher activity of the rod shape TiO2 NCs is ascribed to the facetsphilic nature of the photogenerated carriers within the NCs. The photocatalytic activity of TiO2 NCs are found to be in the order of {101}+{100}/{010} (nanorods) > {101}+{001}+{100}/{010} (nanocuboids and nanocapsules) > {101} (nanoellipsoids) > {001} (nanosheets) providing the direct evidence of exposed facets-depended photocatalytic activity.

Determination of the structure of EuTETA and the luminescence properties of EuTETA and EuDOTA (TETA=1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetate and DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate)

Abstract The crystal structure and the luminescence of the complex Na[Eu(TETA)]·2H2O·4NaCl (TETA=1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetate) have been determined. The space group is P 1 , and the lattice parameters are a=9.283(2) A, b=17.794(3) A, c=19.8087(17) A, α=70.733(11)°, β=83.474(12)°, γ=88.478(18)°, V=3068.6(9) A3, ρ=1.890 g cm−3, and Z=2. The luminescence of Na[EuDOTA·H2O]·3H2O (DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) has also been reported. In the TETA macrocycle, the Eu(III) is completely encapsulated via coordination to the four nitrogen atoms and the four carboxylate oxygen atoms of the ligand. The geometry of the eight-coordinate polyhedron is a strongly distorted dodecahedron. The characteristic feature in the geometry is the conformation of the pendant carboxylate arms. The asymmetric unit consists of two independent molecules, differentiated from the helicities of the pendant arms. When the EuTETA and EuDOTA crystals are excited by UV light, they produce very characteristic luminescences responsible for the 5D0→7FJ (J=0, 1, 2, 3, 4) transitions. Unlike the EuDOTA complex, the luminescence structure of the EuTETA complex is significantly affected by the crystalline state. This might be due to the rigidity of the complex. The energy-level schemes of the 7FJ states and detailed assignments for the observed luminescence lines of the EuTETA and EuDOTA complexes have been proposed by phenomenological simulation in the framework of the crystal-field Hamiltonian.

Recyclable magnetic CoFe2O4/BiOX (X = Cl, Br and I) microflowers for photocatalytic treatment of water contaminated with methyl orange, rhodamine B, methylene blue, and a mixed dye

The recycling of photocatalysts and improving their activities by hybridizing two materials are important. Herein, nanosize ferromagnetic (Ms = 62.3 emu g−1) CoFe2O4 nanoparticles (NPs) were embedded into nanosize-assembled BiOX (X = Cl, Br and I) microflowers and examined by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, UV-visible absorption spectroscopy, Fourier-transform infrared spectroscopy, and photoluminescence spectroscopy. The adsorption and photocatalytic performance of CoFe2O4/BiOX for methyl orange (MO), rhodamine B (RhB), methylene blue (MB), and a mixed dye (MO + RhB + MB) were examined under UV and visible light irradiation. The adsorption capacity of CoFe2O4/BiOI for RhB was 160 mg gcat−1, which is significantly larger than the <5 mg gcat−1 obtained for CoFe2O4/BiOCl and CoFe2O4/BiOBr. The photocatalytic activity was observed in the order of CoFe2O4/BiOBr < CoFe2O4/BiOCl < CoFe2O4/BiOI for RhB. Their adsorption and photocatalytic performances were also investigated with pure MO, pure MB and a mixed dye. MO in the mixed dye was the most easily removed by the catalysts under light exposure. Based on the scavenger tests, h+ and ˙O2− play major and minor roles in the photodegradation of the dyes, respectively. Although the ˙OH radical was formed for CoFe2O4/BiOBr and CoFe2O4/BiOCl, it has a much smaller role than the other active species.

Synthesis of In2S3 microspheres using a template-free and surfactant-less hydrothermal process and their visible light photocatalysis

Indium sulfide (In2S3) microspheres of (β-) tetragonal phase were synthesized by varying the indium precursors using a template-free and surfactant-less hydrothermal process at 150 °C. The as-synthesized samples were found to be crystalline and phase pure as confirmed by X-ray diffraction and X-ray photoelectron spectroscopy studies, respectively. Indium precursors play an important role in controlling the shape of the building blocks, i.e. nanoflakes or nanobricks, of In2S3 microspheres. The photocatalytic activity of as-synthesized In2S3 microspheres was tested for the degradation of methylene blue and crystal violet in the presence of visible light produced by an incandescent lamp. The terephthalic acid test using photoluminescence spectroscopy shows hydroxyl radicals as active species for the degradation of organic contaminants. Repeat photocatalysis measurements suggest the high stability of In2S3 microspheres without a change in their morphology and phase.

Interfacial Electronic Structure of Gold Nanoparticles on Si(100): Alloying versus Quantum Size Effects

Gold nanoparticles (Au NPs) were prepared on a native-oxide-covered Si(100) substrate by sputter-deposition followed by thermal annealing. The size of Au NPs could be controlled in the range of 8-48 nm by varying the sputter-deposition time and post-annealing temperature. The interparticle separation was found to be directly related to the size of Au NPs, with smaller separations for particles of smaller size. The surface morphology, crystal structure, and interfacial composition of the chemical states of these supported Au NPs were studied as a function of their average size by using scanning electron microscopy, glancing-incidence X-ray diffraction, and depth-profiling X-ray photoelectron spectroscopy (XPS), respectively. The new Au 4f7/2 peak found at 1.1-1.2 eV higher in binding energy than that for the metallic Au feature (at 84.0 eV) can be attributed to the formation of Au silicide at the interface between Au NPs and the Si substrate. Depth-profiling XPS experiments revealed no discernible change in the binding energies of the Au silicide and metallic Au 4f features with increasing Ar+ sputtering time, indicating that the Au-to-silicide interface is abrupt. Furthermore, the shift in the Au 5d5/2 valence band to a higher binding energy and the reduction of the Au 5d spin-orbit splitting with increasing Ar+ sputtering time also support the formation of Au silicide. No clear evidence for the quantum size effect was observed for the supported NPs. The finite density of state at the Fermi level and the fixed Au 4f7/2 peak position clearly indicate the metallic nature of the Au silicide at the Au-Si interface.

Metallic Sn spheres and SnO2@C core-shells by anaerobic and aerobic catalytic ethanol and CO oxidation reactions over SnO2 nanoparticles.

SnO2 has been studied intensely for applications to sensors, Li-ion batteries and solar cells. Despite this, comparatively little attention has been paid to the changes in morphology and crystal phase that occur on the metal oxide surface during chemical reactions. This paper reports anaerobic and aerobic ethanol and CO oxidation reactions over SnO2 nanoparticles (NPs), as well as the subsequent changes in the nature of the NPs. Uniform SnO2@C core-shells (10 nm) were formed by an aerobic ethanol oxidation reaction over SnO2 NPs. On the other hand, metallic Sn spheres were produced by an anaerobic ethanol oxidation reaction at 450 °C, which is significantly lower than that (1200 °C) used in industrial Sn production. Anaerobic and aerobic CO oxidation reactions were also examined. The novelty of the methods for the production of metallic Sn and SnO2@C core-shells including other anaerobic and aerobic reactions will contribute significantly to Sn and SnO2-based applications.

Fundamental nature and CO oxidation activities of indium oxide nanostructures: 1D-wires, 2D-plates, and 3D-cubes and donuts

Various indium oxide nanostructures of 1D-wires, 2D-hexagonal plates, 3D-cubes and donuts were synthesized, and their fundamental characteristics and CO oxidation activities were studied in detail. X-ray diffraction and Raman analysis revealed that the as-synthesized wires and cubes are orthorhombic InOOH and cubic In(OH)3, respectively. Upon annealing at 700 °C in air, all the as-prepared samples were recrystallized to cubic In2O3. The direct band gap of various as-synthesized nanostructures was estimated to be ∼2.7 eV from the UV-Vis absorption. Two broad photoluminescence peaks were found at 360 and 450 nm, which are attributed to the oxygen vacancies. The CO oxidation activities were in the order of hexagonal plates ≤ donuts < cubes < wires, tested by temperature-programmed reaction mass spectrometry. The difference in activity is explained on the basis of the surface area and oxygen vacancies of different nanostructures. In particular, the wires showed the CO oxidation onset at around 320 °C, which is 280 °C lower than that of hexagonal plates. The detailed morphology dependent properties and CO oxidation activities of various In2O3 nanostructures presented in this study provide new insights into sensor, energy, and environmental applications.