Yiseul Park

Role of Interparticle Charge Transfers in Agglomerated Photocatalyst Nanoparticles: Demonstration in Aqueous Suspension of Dye-Sensitized TiO2

The interparticle charge transfer within the agglomerates of TiO2 nanoparticles in slurries markedly enhanced the dye-sensitized production of H2 under visible light. By purposely decoupling the light absorbing part of Dye/TiO2 from the active catalytic center of Pt/TiO2, the role of bare TiO2 nanoparticles working as a mediator that connects the above two parts in the agglomerates was investigated systematically. The presence of mediator in the agglomerate facilitated the charge separation and the electron transfer from Dye/TiO2 to Pt/TiO2 through multiple grain boundaries and subsequently produced more hydrogen. The dye-sensitized reduction of Cr(VI) to Cr(III) was also enhanced when Dye/TiO2 nanoparticles were agglomerated with bare TiO2 nanoparticles. The charge recombination between the oxidized dye and the injected electron was retarded in the presence of bare TiO2 nanoparticles, and this retarded recombination on Dye/TiO2 was confirmed by using transient laser spectroscopy. This phenomenon can be rationalized in terms of an interparticle Fermi level gradient within the agglomerates, which drives the charge separation.

Significance of hydrophilic characters of organic dyes in visible-light hydrogen generation based on TiO2.

A series of dyes were synthesized to examine the roles of the hydrophilic characteristics of R in sensitized hydrogen generation by dye-grafted Pt/TiO(2) under visible light irradiation. The hydrogen-generation efficiencies and optimum amounts of the dyes grafted to Pt/TiO(2) were affected substantially by the hydrophilic and steric effects of R; moderately hydrophilic DEO1 and DEO2 showed higher sensitization activity at a lower loading than hydrophobic D-H.

Synthesis and characterization of Fully rodlike poly(4,4'-biphenylene pyromellitimide)s with various short side groups

Fully rodlike poly(4,4-biphenylene pyromellitimide) (PMDA–BZ) is so brittle in spite of its extremely high modulus. In this study, the brittleness was attempted to be improved without a significant sacrifice of the high modulus by incorporating short side groups. For this, benzidine monomers, which contain methyl, methoxy, fluoro, and trifluoromethyl at the 2,2′-positions, were synthesized and then used for polycondensation reactions with pyromellitic dianhydride in N-methyl-2-pyrrolidone, producing soluble poly(amic acids)s. The synthesized poly(amic acid)s were converted to the fully rodlike polyimides in films by a conventional spin-coating on substrates, soft bake, and thermal imidization. The brittleness of PMDA–BZ was successfully healed with a small portion of sacrifice in the modulus by incorporating methyl, methoxy, and trifluoromethyl groups but could not be healed by the fluoro side group. The improvement in the brittleness might be contributed from the chain mobility and lateral chain packing order enhanced by the incorporation of the side groups, which are evident on the measured structures and properties. The structure and other properties were detected to be influenced by the incorporated side groups. The detailed structures and properties were interpreted by considering roles of side groups and the correlation between structure and properties, respectively.

Effect of film formation process on residual stress of poly(p-phenylene biphenyltetracarboximide) in thin films

Abstract Soluble poly(p-phenylene biphenyltetracarboxamine acid) (BPDA-PDA PAA) precursor, which was synthesized from biphenyltetracarboxylic dianhydride and p-phenylene diamine in N-methyl-2-pyrrolidone (NMP), was spin-cast on silicon substrates, followed by softbake at various conditions over 80–185°C. Softbaked films were converted in nitrogen atmosphere to be the polyimide films of ca. 10 μm thickness through various imidizations over 120–400°C. Residual stress, which is generated at the polymer/substrate interface by volume shrinkage, polymer chain ordering, thermal history, and differences between properties of the polymer film and the substrate, was measured in situ during softbake and subsequent imidization processes. Polymer films imidized were further characterized in the aspect of polymer chain orientation by prism coupling and X-ray diffraction. Residual stress in the polyimide film was very sensitive to all the film formation process parameters, such as softbake temperature and time, imidization temperature, imidization step, heating rate, and film thickness, but insensitive to the cooling process. Softbaked precursor films revealed 9–42 MPa at room temperature, depending on the softbake temperature and time. That is, residual stress in the precursor film was affected by the amount of residual solvent and by partial imidization possibly occurring during softbake above the onset of imidization temperature, ca. 130°C. A lower amount of residual solvent caused higher stress in the precursor film, whereas a higher degree of imidization led to lower stress. Partially imidized precursor films were converted to polyimide films revealing relatively high stresses. After imidization, polyimide films exhibited a wide range of residual stress, 4–43 MPa at room temperature, depending on the histories of softbake and imidization. Relatively high stresses were observed in the polyimide films which were prepared from softbaked films partially imidized and by rapid imidization process with a high heating rate. The residual stress in films is an in-plane characteristic so that it is sensitive to the degree of in-plane chain orientation in addition to the thermal history term. Low stress films exhibited higher degree of in-plane chain orientation. Thus, residual stress in the film would be controlled by the alignment of polyimide chains via the film formation process with varying process parameters. Conclusively, in order to minimize residual stress and to maximize in-plane chain orientation, precursor films should be softbaked for 30 min-2 h below the onset imidization temperature, ca. 130°C, and subsequently imidized over the range of 300–400°C for 1–4 h by a two-step or multi-step process with a heating rate of ⩽ 5.0 K min−1, including a step to cover the boiling point, 202°C, of NMP. In addition, the final thickness of the imidized films should be

Residual stress and optical properties of fully rod‐like poly(p‐phenylene pyromellitimide) in thin films: Effects of soft‐bake and thermal imidization history

A soluble poly(amic acid) precursor solution of fully rod-like poly(p-phenylene pyromellitimide) (PMDA-PDA) was spin cast on silicon substrates, followed by soft bake at 80–185°C and subsequent thermal imidization at various conditions over 185–400°C in nitrogen atmosphere to be converted to the polyimide in films. Residual stress generated at the interface was measured in situ during imidization. In addition, the imidized films were characterized in the aspect of polymer chain orientation and ordering by prism coupling and X-ray diffraction. The soft-baked precursor film revealed a residual stress of 16–28 MPa at room temperature, depending on the soft bake condition: higher temperature and longer time in the soft bake gave higher residual stress. The stress variation in the soft-baked precursor film was not significantly reflected in the final stress in the resultant polyimide film. However, the residual stress in the polyimide film varied sensitively with variations in imidization process parameters, such as imidization temperature, imidization steps, heating rate, and film thickness. The polyimide film exhibited a wide range of residual stress, −7 MPa to 8 MPa at room temperature, depending on the imidization condition. Both rapid imidization and low-temperature imidization generated high stress in the tension mode in the polyimide film, whereas slow imidization as well as high temperature imidization gave high stress in the compression mode. Thus, a moderate imidization condition, a single- or two-step imidization at 300°C for 2 h with a heating rate of < 10 K/min was proposed to give a relatively low stress in the polyimide film of < 10 μm thickness. However, once a precursor film was thermally imidized at a chosen process condition, the residual stress–temperature profile was insensitive to variations in the cooling process. All the films imidized were optically anisotropic, regardless of the imidization history, indicating that rod-like PMDA-PDA polyimide chains were preferentially aligned in the film plane. However, its degree of in-plane chain orientation varied on the imidization history. It is directly correlated to the residual stress in the film, which is an in-plane characteristic. For films with residual stress in the tension mode, higher stress films exhibited lower out-of-plane birefringence, that is, lower in-plane chain orienta-tion. In contrast, in the compression mode, higher stress films showed higher in-plane chain orientation.

Hybrid CuxO–TiO2 Heterostructured Composites for Photocatalytic CO2 Reduction into Methane Using Solar Irradiation: Sunlight into Fuel

Photocatalytic CO2 conversion to fuel offers an exciting prospect for solar energy storage and transportation thereof. Several photocatalysts have been employed for CO2 photoreduction; the challenge of realizing a low-cost, readily synthesized photocorrosion-stable photocatalytic material that absorbs and successfully utilizes a broad portion of the solar spectrum energy is as yet unmet. Herein, a mesoporous p-type/n-type heterojunction material, CuxO–TiO2 (x = 1, 2), is synthesized via annealing of Cu/Cu2O nanocomposites mixed with a TiO2 precursor (TiCl4). Such an experimental approach in which two materials of diverse bandgaps are coupled provides a simultaneous opportunity for greater light absorption and rapid charge separation because of the intrinsic p–n heterojunction nature of the material. As detailed herein, this heterostructured photocatalyst demonstrates an improved photocatalytic activity. With the CO2 reduction of our optimal sample (augmented light absorption, efficacious charge separation, and mesoporosity) that utilizes no metal cocatalysts, a remarkable methane yield of 221.63 ppm·g−1·h−1 is achieved.

Hybrid mesoporous Cu2ZnSnS4 (CZTS)–TiO2 photocatalyst for efficient photocatalytic conversion of CO2 into CH4 under solar irradiation

A major concern facing global society is the ongoing excessive release of CO2 into the atmosphere where it acts as a heat-trapping greenhouse gas. One approach to helping control atmospheric CO2 concentrations is to use solar energy to convert CO2 into useful products, namely hydrocarbons, by use of specifically designed photocatalytic materials. While numerous photocatalysts have been investigated for use in CO2 reduction, the field remains in its infancy with, overall, relatively poor photoconversion efficiencies and product selectivity. This study reports the synthesis and design of a mesoporous noble metal free p-type Cu2ZnSnS4 (CZTS)/n-type TiO2 heterojunction photocatalyst for broad spectrum light absorption, enhanced charge separation and transfer that, in turn, enhances photocatalytic CO2 conversion. A maximum methane production rate of 118.75 ppm g−1 h−1 is observed, which represents a methane evolution rate approximately 12 times greater than that of pure TiO2. The key factors contributing to the enhanced photocatalytic performance seen in the mesoporous CZTS–TiO2 samples include improved light absorption, high surface area, and effective charge separation.

Optical resonance and charge transfer behavior of patterned WO3 microdisc arrays

One- to three-dimensional alignments of semiconductors on the micro- or nanoscale have been achieved to tailor their opto-physicochemical properties and improve their photoelectrochemical (PEC) performance. Here, to the best of our knowledge, we report for the first time the fabrication of vertically aligned, well-ordered WO3 microdisc arrays via an electrodeposition process on lithographically patterned indium tin oxide (ITO) substrates as well as their geometry-specific photoelectrochemical properties. The as-fabricated WO3 microdisc arrays exhibit enhanced light absorption as well as facilitated charge separation, leading to significantly higher PEC performance than WO3 films. A finite-difference time-domain simulation of a single WO3 microdisc indicates that strong optical resonances occur particularly in the central part of the microdisc, leading to enhanced optical absorption. A time-resolved photoluminescence study further reveals that the average lifetime of charge carriers (τ) in a microdisc array is shorter than that in a film by ∼60%. The reductively deposited Au particles are localized on the side of the microdisc and ITO substrate, which suggests that the photogenerated electrons are transferred to the same location. In addition, the oxidative deposition of FeOOH particles on the top surface and side of a microdisc indicates hole transfer pathways at the same location. This downward transfer of electrons and upward transfer of holes lead to efficient charge separation, and the radial direction appears to be the most preferred shortcut for the carriers inside the bulk of a microdisc.

Solar-rechargeable battery based on photoelectrochemical water oxidation: Solar water battery

As an alternative to the photoelectrochemical water splitting for use in the fuel cells used to generate electrical power, this study set out to develop a solar energy rechargeable battery system based on photoelectrochemical water oxidation. We refer to this design as a “solar water battery”. The solar water battery integrates a photoelectrochemical cell and battery into a single device. It uses a water oxidation reaction to simultaneously convert and store solar energy. With the solar water battery, light striking the photoelectrode causes the water to be photo-oxidized, thus charging the battery. During the discharge process, the solar water battery reduces oxygen to water with a high coulombic efficiency (>90%) and a high average output voltage (0.6 V). Because the reduction potential of oxygen is more positive [E0 (O2/H2O) = 1.23 V vs. NHE] than common catholytes (e.g., iodide, sulfur), a high discharge voltage is produced. The solar water battery also exhibits a superior storage ability, maintaining 99% of its specific discharge capacitance after 10 h of storage, without any evidence of self-discharge. The optimization of the cell design and configuration, taking the presence of oxygen in the cell into account, was critical to achieving an efficient photocharge/discharge.

Solar Hydrogen Production Coupled with the Degradation of a Dye Pollutant Using TiO2 Modified with Platinum and Nafion

The simultaneous production of molecular hydrogen (H2) and degradation of rhodamine B (RhB) was successfully achieved using TiO2 modified with platinum and nafion (Pt/TiO2/Nf) under visible light  nm). Pt/TiO2/Nf exhibited high activity for H2 production in the presence of RhB and EDTA as a photosensitizer (also an organic dye pollutant) and an electron donor, respectively. However, the activity of TiO2 modified with either platinum or nafion for H2 production was negligible under the same experimental conditions. The negatively charged nafion layer enhances the adsorption of cationic RhB and pulls protons, a source of hydrogen, to the surface of TiO2 through electrostatic attraction. On the other hand, platinum deposits on TiO2 can act as an electron sink and a temporary electron reservoir for the reduction of protons. With the production of H2, RhB was gradually degraded through -deethylation, which was confirmed by the spectral blue shift of the maximum absorption wavelength from 556 to 499 nm (corresponding to the of rhodamine 110). With Pt/TiO2/Nf employed at  M (0.6 mol), approximately 70 mol of H2 was produced and RhB and its intermediates were completely removed over a 12 h period. A detailed reaction mechanism was discussed.