Joo Hwan Koh

Structure control of organized mesoporous TiO2 films templated by graft copolymers for dye-sensitized solar cells

Randomly microphase-separated graft copolymers have been self-reorganized so as to exhibit a micellar structure with excellent connectivity upon tuning the solvent affinity. These copolymers are used as a structure-directing agent for organized mesoporous TiO(2) films with no grain boundaries, leading to enhanced solar conversion efficiency of dye-sensitized solar cells.

Formation of mesoporous TiO2 with large surface areas, interconnectivity and hierarchical pores for dye-sensitized solar cells

TiO2 nanoparticles with anatase/rutile mixed phase and <100 nm in size were surface-modified using hydroxyethyl methacrylate (HEMA) and sulfosuccinic acid (SA), which can coordinate to the TiO2 precursor, titanium(IV) isopropoxide (TTIP). The HEMA in TiO2-HEMA nanoparticles underwent a graft/crosslink polymerization to poly(hydroxyethyl methacrylate) (PHEMA), i.e.TiO2-PHEMA. Following the application of a sol–gel process with TTIP, 3-dimensional (3D) nanostructured TiO2 photoelectrodes with interconnectivity, large surface area and bimodal pores were successfully obtained. The energy conversion efficiency of a polymer electrolyte dye-sensitized solar cell (DSSC) fabricated with TiO2-PHEMA/TTIP photoelectrode reached 3.5% at 100 mW cm−2, which was much higher than those of pristine TiO2 (1.4%), TiO2/TTIP (1.6%) and TiO2-HEMA/TTIP (2.0%) photoelectrodes. The higher cell performance of TiO2-PHEMA/TTIP is due to enhanced light harvesting, reduced charge recombination and excellent penetration of polymer electrolytes into the TiO2 pores.

Synthesis and characterization of AgBr nanocomposites by templated amphiphilic comb polymer

A novel amphiphilic graft copolymer, poly(vinylidene fluoride-co-chlorotrifluoroethylene)-g-poly(4-vinyl pyridine) (P(VDF-co-CTFE)-g-P4VP) at 65:35 wt.%, respectively, was synthesized via atom transfer radical polymerization (ATRP), as confirmed by nuclear magnetic resonance (1H NMR) and transmission electron microscopy (TEM). Silver bromide (AgBr) nanoparticles were in situ generated within the self-assembled P(VDF-co-CTFE)-g-P4VP graft copolymer. TEM, UV-visible spectroscopy and X-ray diffraction (XRD) analyses support the successful formation of P(VDF-co-CTFE)-g-P4VP nanocomposites consisting of stabilized AgBr nanoparticles mostly 20-40 nm in size, which is presumably due to the capping action of the coordinating pyridine groups of the graft copolymer. The wavenumber of pyridine nitrogen in FT-IR spectra and the glass transition temperature (Tg) of the graft polymer measured by DSC shifted upon the formation of AgBr nanoparticles, indicating specific interactions between the nanoparticles and the graft copolymer matrix.

Synthesis and Characterization of Nanocomposite Films Consisting of Vanadium Oxide and Microphase-separated Graft Copolymer

Nanocomposite films were prepared by sol-gel synthesis from vanadium triisopropoxide with poly((oxyethylene)9 methacrylate)-graft-poly(dimethyl siloxane), POEM-g-PDMS, producing in situ growth of vanadium oxide within the continuous ion-conducting POEM domains of microphase-separated graft copolymer. The formation of vanadium oxide was confirmed by wide angle x-ray scattering (WAXS) and Fourier transform infrared (FTIR) spectroscopy. Small angle x-ray scattering (SAXS) revealed the spatially-selective incorporation of vanadium oxide in the POEM domains. Upon the incorporation of vanadium oxide, the domain periodicity of the graft copolymer monotonously increased from 17.2 to 21.0 nm at a vanadium content 14 v%, above which it remained almost invariant. The selective interaction of vanadium oxide with POEM was further verified by differential scanning calorimetry (DSC) and FT-IR spectroscopy. The nanocomposite films exhibited excellent mechanical properties (10−5-10−7 dyne/cm2), mostly due to the confinement of vanadium oxide in the POEM chains as well as the interfaces created by the microphase separation of the graft copolymer.

Fabrication of 3D interconnected porous TiO2 nanotubes templated by poly(vinyl chloride-g-4-vinyl pyridine) for dye-sensitized solar cells

Porous TiO(2) nanotube arrays with three-dimensional (3D) interconnectivity were prepared using a sol-gel process assisted by poly(vinyl chloride-graft-4-vinyl pyridine), PVC-g-P4VP graft copolymer and a ZnO nanorod template. A 7 µm long ZnO nanorod array was grown from the fluorine-doped tin oxide (FTO) glass via a liquid phase deposition method. The TiO(2) sol-gel solution templated by the PVC-g-P4VP graft copolymer produced a random 3D interconnection between the adjacent ZnO nanorods during spin coating. Upon etching of ZnO, TiO(2) nanotubes consisting of 10-15 nm nanoparticles were generated, as confirmed by wide-angle x-ray scattering (WAXS), energy-filtering transmission electron microscopy (EF-TEM) and field-emission scanning electron microscopy (FE-SEM). The ordered and interconnected nanotube architecture showed an enhanced light scattering effect and increased penetration of polymer electrolytes in dye-sensitized solar cells (DSSC). The energy conversion efficiency reached 1.82% for liquid electrolyte, and 1.46% for low molecular weight (M(w)) and 0.74% for high M(w) polymer electrolytes.

Templated formation of silver nanoparticles using amphiphilic poly(epichlorohydrine-g-styrene) film

This work has demonstrated that a novel amphiphilic poly(epichlorohydrine)-graft-polystyrene (PECH-g-PS) copolymer at 34:66 wt% was synthesized via atom transfer radical polymerization (ATRP) of styrene using PECH as a macroinitiator. The structure of the graft copolymer was characterized by nuclear magnetic resonance (1H NMR) and FTIR spectroscopy, demonstrating that the “grafting from” method using ATRP was successful. The self-assembled graft copolymer was used as a template film for thein-situ growth of silver nanoparticles from AgCF3SO3 precursor under UV irradiation. Thein situ formation of silver nanoparticles with 6-8 nm in average size in thesolid state template film was confirmed by transmission electron microscopy (TEM), UV-visible spectroscopy and wide angle X-ray scattering (WAXS). Differential scanning calorimetry (DSC) also displayed the selective incorporation and thein situ formation of silver nanoparticles within the hydrophilic PECH domains, probably due to stronger interaction of the silvers with the ether oxygens of PECH backbone than that with hydrophobic PS side chains.

Preparation of nanoporous films from self-assembled poly(vinyl chloride-g-methyl methacrylate) graft copolymer

We report on the preparation of nanoporous films based on an amphiphilic graft copolymer of poly(vinyl chloride-graft-methyl methacrylate), i.e., PVC-g-PMMA. The PVC-g-PMMA graft copolymer was synthesized via atom transfer radical polymerization (ATRP), as confirmed by nuclear magnetic resonance spectroscopy (1H NMR), Fourier transform-infrared (FT-IR) spectroscopy, and gel permeation chromatography (GPC) analysis. The PVC-g-PMMA graft copolymer molecularly self-assembled into nanophase domains of PVC main chains and PMMA side chains, as revealed by wide angle X-ray scattering (WAXS) and transmission electron microscopy (TEM). The graft copolymer film prepared from tetrahydrofuran (THF), a good solvent for both chains, had a random microphase-separated morphology. However, when prepared from dimethyl sulfoxide (DMSO), a solvent selectively good for PVC, the film exhibited a micellar morphology consisting of a PMMA core and a PVC corona. Nanoporous films with different pore sizes and shapes were prepared through the selective etching of PMMA chains using a combined process of UV irradiation and acetic acid treatment.

Use of graft copolymer for preparation of organized mesoporous TiO 2 films

Randomly microphase-separared graft copolymers have been self-reorganized so as to exhibit a micellar structure with excellent connectivity upon tuning the solvent affinity. These copolymers are used as a structure-directing agent for organized mesoporous TiO2 films with non grain boundaries, leading to enhanced solar conversion efficiency of dye-sensitized solar cells.