Kyung Hee Park

Two-step process for programmable removal of oxygen functionalities of graphene oxide: functional, structural and electrical characteristics

Here we report a two-step programmable reduction of graphene oxide (GO) which was synthesized by oxidation of graphite. X-Ray photoelectron spectroscopic (XPS) analysis confirmed the synthesis of exfoliated graphene oxide (GO) by introduction of oxygen as carboxylic (–COOH), epoxy (C–O–C) and hydroxyl (–OH) groups. The first step of GO reduction was achieved separately by (i) hydrazine (rGO11) and (ii) sodium borohydride (rGO21). Soda lime was used in the second-stage reduction of (a) hydrazine reduced GO (rGO12) and (b) sodium borohydride reduced GO (rGO22) to remove most of the remaining carboxylic functionalities from the rGO11 and rGO21 surface. XPS spectra of rGO21 showed a decrease (38 to 30%) in the oxygen whereas the further reduction of rGO21 with soda lime can further reduce the oxygen content. Quantitative analysis of C(O)OX in GO shows about 43% of carbon atoms (Cu20061s signal) as carboxylic functionalities whereas the reduction of the GO with sodium borohydride reduced this signal to about 10%. The use of soda lime for both rGO11 and rGO21 further reduced the amount of carboxylic functionalities. An increase in the proportion of carbon atoms as sp2 and decrease in the oxygen functionalities were controlled in the two-step reduction. A good correlation in the conductivity of reduced GO with the percentage proportion of sp2 carbon was observed.

Dye-sensitized solar cells based on electrospun poly(vinylidenefluoride-co-hexafluoropropylene) nanofibers

Poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) nanofibers were prepared by the electrospinning method and used as polymer electrolytes in dye-sensitized solar cells (DSSCs). The electrolyte uptake and ionic conductivity of electrospun PVDF-HFP nanofibers with different diameters changed significantly, regardless of the nanofiber thickness. The PVDF-HFP nanofibers prepared from a 15xa0wt% spinning solution showed high ionic conductivity (1.295xa0S/cm) and electrolyte uptake (947xa0%). DSSCs based on the 15xa0wt% PVDF-HFP nanofiber electrolyte showed an electron transit time of 6.34xa0×xa010−3xa0s, electronic recombination time of 5.88xa0×xa010−2xa0s, and conversion efficiency of 3.13xa0%. Thus, we concluded that the electrospun PVDF-HFP nanofibers can be used as polymer electrolytes in flexible DSSCs as well.

Characteristics of ultrasonication assisted assembly of gold nanoparticles in hydrazine reduced graphene oxide

Here we report a new ultrasonication assisted method for increased diffusion of a gold salt in hydrazine reduced graphene oxide (hrGO) sheets. Gold nanoparticles (AuNP) were formed through in situ reduction of diffused gold chloride within the hrGO sheets by sodium borohydride. Transmission electron microscopic (TEM) analysis confirmed uniform distribution of ∼5–10 nm AuNP in hrGO sheets. Raman spectra of hrGO–AuNP showed an increase in the ratio of D-band to G-band intensity as well as the absence of a 2D band. This confirmed distortion of the multilayer assembly into much thin layers by the process of AuNP nucleation in the composite material. X-ray diffraction (XRD) spectra of hrGO–AuNP confirmed the presence of crystallite carbonic materials and AuNP by observing strong diffraction peaks of Au (111), Au (200), Au (220) and Au (311). UV-visible spectra of the oxide hrGO–AuNP showed a spectral shift of 21 nm in reduced graphene oxide which confirmed the binding of AuNP with hrGO. X-ray photo electron spectroscopy (XPS) analysis revealed 13.4, 69.3, 13 and 2.3% mass proportions for gold, carbon, oxygen and nitrogen, respectively in hrGo–AuNP. XPS analysis also showed an increase in sp3 carbons as compared to sp2 carbons in C1s after gold nucleation in hrGO. The I–V response of hrGO remained unaffected by the nucleation of AuNP in the hrGO composite material. This method may be useful to address the challenges associated with the incorporation of metals into reduced graphene oxide without chemical functionalization of inert surfaces.

Pulse electrodeposition of hydroxyapatite/chitosan coatings on titanium substrate for dental implant

In this work, calcium phosphate/chitosan coatings were produced by the pulse electrodeposition method with variation of the deposition cycle and contents of chitosan solution. To investigate the morphology of the crystals and the electrochemical properties of hydroxyapatite/chitosan coating, a uniform hydroxyapatite coating was applied on the Ti-6Al-4xa0V substrate using the pulse voltage electrodeposition method. The coating formed in the as-deposited condition was identified as dicalcium phosphate dihydrate (DCPD) (brushite), which was converted to hydroxyapatite (HA, Ca10(PO4)6(OH)2) after immersion in 1xa0M NaOH at 80xa0°C for 2xa0h. The XRD patterns confirmed the formation of DCPD and HA. During electrodeposition, the H2PO4− ion was reduced and the reaction between Ca2+ ions and the reduced phosphate ions induced the formation of DCPD, which was converted to HA following treatment in NaOH. The coatings were composed of 5 to 20% chitosan by volume. Homogeneous nano-size deposits were formed under the controlled deposition period of 15xa0cycles and chitosan content of 15 v%. The composite coatings showed significant improvement in corrosion resistance compared to the bare Ti-6Al-4xa0V substrate.

Fabrication and biological properties of calcium phosphate/chitosan composite coating on titanium in modified SBF

In order to increase the biocompatibility and bioactivity of chitosan, hydroxyapatite was in situ combined into the spin-coated chitosan layer on the titanium substrate by incubating in modified simulated body fluid (m-SBF). The calcium phosphate/chitosan (CaP/CS) composite prepared in m-SBF showed a homogeneous distribution of spherical nano-clusters. The hydrophilicity of the coatings was increased by performing NaOH post-treatment of CaP/CS composites, which also affected apatite formation. Biocompatibility of the coatings was assessed by investigating the cellular response of human osteoblast-like MG-63 cells with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Cell adhesion and osteogenic properties of the mesoporous CaP/CS composite were evaluated by SEM and ALPase assay, respectively. This in vitro study showed improved cell adhesion and differentiation on nanostructured CaP/CS composites. These results indicate that this CaP/CS composite could be a promising candidate for bone tissue engineering.

Phenanthroline-based ruthenium complexes for enhanced charge transportation in solvent-free ionic liquid electrolyte

In the present work, we synthesized a series of 1,10-phenanthroline-based ruthenium complexes (PRCs). The PRCs were characterized by 1H and 13C NMR, UV–Vis and fluorescence spectroscopic analyses. Further, we used these PRCs complexes as effective additives in ionic liquid (IL) electrolyte to enhance current conduction in electrochemical cells. Anodic peak current in cyclic voltammetry response of PRCs showed functional group dependence on the nature of ruthenium complex. Mono-propenyl-functionalized PRC additive in IL electrolyte exhibited relatively higher current density, lower charge transfer resistance and decreased impedance compared to the tri-propenyl-functionalized PRC as well as without functional groups on 1,10-phenanthroline ligand in Ru complex. Finally, the PRCs as additives in IL electrolyte were tested in dye-sensitized solar cell which showed an increase of 65% in the efficiency when mono-propenyl-functionalized PRC complex was used as an additive.