NO₂ gas sensing based on graphene synthesized via chemical reduction process of exfoliated graphene oxide

Khai, Tran Van,Prachuporn, Maneeratanasarn,Shim, Kwang-Bo The Korea Association of Crystal Growth 2012 韓國結晶成長學會誌 Vol.22 No.2

Single and few-layer graphene nanosheets (GNs) have successfully synthesized by a modified Hummer's method followed by chemical reduction of exfoliated graphene oxide (GO) in the presence of hydrazine monohydrate. GO and GNs were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), X-ray diffractions (XRD), Raman spectroscopy, Transmission electron microscopy (TEM), Atomic force microscopy (AFM), Optical microscopy (OM) and by electrical conductivity measurements. The result showed that electrical conductivity of GNs was significantly improved, from $4.2{\times}10^{-4}$ S/m for GO to 12 S/m for GNs, possibly due to the removal of oxygen-containing functional group during chemical reduction. In addition, the $NO_2$ gas sensing characteristics of GNs are also discussed.

Synthesis and characterization of single- and few-layer mica nanosheets by the microwave-assisted solvothermal approach

Van Khai, Tran,Gil Na, Han,Sub Kwak, Dong,Jung Kwon, Yong,Ham, Heon,Bo Shim, Kwang,Woo Kim, Hyoun IOP Pub 2013 Nanotechnology Vol.24 No.14

<P>We have successfully fabricated single- and few-layer mica nanosheets, by means of using a solvothermal method in conjunction with a microwave irradiated expansion process. In the solvothermal process, dissolved potassium ions were intercalated onto the interlayer space of the mica. Following this, microwave irradiation facilitates the exfoliation of individual nanosheets. The synthesized products were characterized by field emission scanning electron microscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM), x-ray diffraction, x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy measurements. AFM and TEM studies claimed the existence of single-layer mica. High-resolution TEM (HR-TEM) investigations revealed that the exfoliated product corresponded to a crystalline mica structure, being comprised of Si, Al, O, and K elements. XPS spectra exhibited the major constituent peaks, including O 1s, Si 2p, Al 2p, and K 2p. In addition, C atomic concentration has been slightly increased by the contamination during exfoliation, presumably due to the increase of the exposed mica surface. The C 1s XPS spectra revealed that the C–C bonding in organic surface contaminants was broken, whereas the Si–C bonding was enhanced, by the exfoliation process. The O 1s XPS spectra revealed that the Si–O bonding in mica was broken, generating the O–Si–C bonding. This study paves the way towards the fabrication of single- or few-layer inorganic nanosheets of desired materials, via a convenient and efficient route.</P>

Catalyst-free growth of ZnO nanowires on Si (100) substrates by thermal evaporation

Tran Van Khai,Sin Young Bang,Dong Keun Oh,Bong Geun Choi,심광보,황동목 한양대학교 세라믹연구소 2011 Journal of Ceramic Processing Research Vol.12 No.1

Vertically well-aligned ZnO nanowires with a high density were successfully grown on Si (100) substrates without a catalyst by conventional thermal evaporation. The grown ZnO nanowires are relatively uniform in diameter of about 180 nm and length of about 4 μm. They exhibit the morphological features of faceted planes at the tips of hexagonal columns and are preferentially oriented along the [001] direction. The optimum growth condition was obtained from the experimental result that the degree of vertical alignment of the ZnO nanowires depends sensitively on the distance between the substrate and the Zn source. The PL characteristics of the grown ZnO nanowires reveal a strong and sharp UV emission at 380 nm and a broad green emission in the region of 500 nm, indicating excellent optical properties.

Significant enhancement of blue emission and electrical conductivity of N-doped graphene

Van Khai, Tran,Na, Han Gil,Kwak, Dong Sub,Kwon, Yong Jung,Ham, Heon,Shim, Kwang Bo,Kim, Hyoun Woo The Royal Society of Chemistry 2012 Journal of materials chemistry Vol.22 No.34

<P>Nitrogen (N)-doped graphene with different atomic percentages (2.3–4.7 at%) of N has been synthesized by thermal annealing of reduced graphene oxide (RGO) in ammonia gas for different times. The effects of annealing time on the structure, electrical and optical properties of N-doped graphene have been systematically investigated by using various analytical techniques. XPS, FTIR, Raman, and XRD studies show that there is a gradual structural change in N-doped graphene sheets with increasing annealing time, resulting from the increase of carbon and simultaneous decrease of oxygen and N contents. High resolution N1s spectra reveal that the pyridine-N and pyrrolic-N contents decrease with increasing annealing time, whereas the amount of quaternary-N increases. Importantly, it has been found that the annealing time caused significant changes in both the electrical and the optical properties of N-doped graphene. The electrical resistance of N-doped graphene is greatly reduced compared to that of GO and RGO, and found to further decrease with increasing annealing time, possibly due to the increase of sp<SUP>2</SUP> carbon networks and decrease of oxygen content as well as defects associated with the incorporation of N. The room-temperature photoluminescence (PL) properties of graphene oxide (GO), RGO and N-doped graphene were systematically studied with regard to the annealing time. The results showed that the PL spectrum of GO exhibits a peak emission maximum at around 700 nm, while that of RGO is found to be strongly blue-shifted with two distinct emission peaks: green emission at 485–500 nm and blue emission at 420–428 nm. For N-doped graphene samples, the blue emission intensity could significantly be enhanced by controlling the annealing time, which leads to a promising blue and green light-emitting material with controllable optical properties.</P> <P>Graphic Abstract</P><P>N-doped graphene was fabricated by annealing reduced graphene oxide in ammonia, by which electrical conductivity and blue emission were drastically enhanced. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2jm33194b'> </P>

High-quality ZnO nanowire arrays directly synthesized from Zn vapor deposition without catalyst

Khai, Tran Van,Prachuporn, Maneeratanasarn,Choi, Bong-Geun,Kim, Hyoun-Woo,So, Dae-Sup,Lee, Joon-Woo,Park, No-Hyung,Huh, Hoon,Tung, Ngo Trinh,Ham, Heon,Shim, Kwang-Bo The Korea Association of Crystal Growth 2011 韓國結晶成長學會誌 Vol.21 No.4

Vertically well-aligned ZnO nanowire (NW) arrays were synthesized directly on GaN/sapphire and Si substrate from Zn vapor deposition without catalysts. Experimental results showed that the number density, diameter, crystallinity and degree of the alignment of ZnO NWs depended strongly on both the substrate position and kind of the substrates used for the growth. The photoluminescence (PL) characteristics of the grown ZnO NW arrays exhibit a strong and sharp ultraviolet (UV) emission at 379 nm and a broad weak emission in the visible range, indicating that the obtained ZnO NWs have a high crystal quality with excellent optical properties. The as-grown ZnO NWs were characterized by using scanning electron microscopy (SEM), high resolution transmission electronic microscopy (HR-TEM), and X-ray diffraction (XRD).

Synthesis of SnO₂ nanowires on one-dimensional carbonization cotton fabric

Khai, Tran Van,Shim, Kwang-Bo The Korea Association of Crystal Growth 2012 韓國結晶成長學會誌 Vol.22 No.1

Tin-oxide ($SnO_2$) nanowires have been synthesized on one-dimensional (1D) carbonization cotton fabric using chemical vapour deposition method. One-dimensional (1D) carbonization cotton fabric has been synthesized from cotton fabric using annealing process in nitrogen gas at $1000^{\circ}C$. The $SnO_2$ nanowires are single-crystalline rutile structures with 20 nm in diameter and 10 ${\mu}m$ in length. Scanning electron microscopy (SEM), x-ray photoelectron spectroscopy, Raman spectroscopy, transmission electron microscopy and photoluminescence (PL) spectroscopy were utilized to characterize the as-synthesized products.

High-quality graphene thin films synthesized by H2 ambient-annealing of reduced graphene oxide sheets

Tran Van Khai 한양대학교 세라믹연구소 2013 Journal of Ceramic Processing Research Vol.14 No.3

Highly conductive low-oxygen graphene thin films were produced by thermal annealing of chemically reduced graphene oxide (RGO) sheets under H2 ambient. X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy measurements showed that the thermal annealing efficiently removed residual oxygen-containing functional groups on the surface of the chemically reduced RGO sheets and simultaneously recovered sp2 carbon networks in the graphene sheets. Consequently, the electrical conductivity of the graphene films was greatly improved, from 24 S/cm for RGO films to 200 S/cm after the thermal annealing process. In addition, we have studied the NO2 gas sensing characteristics of the prepared graphene films.

Effect of hydrothermal time on the structure and property of graphene oxide membrane

Tran Van Khai,Pham Thuy Trang,Le Ngoc Long,Le Van Thang,Tran Duc Chau,Vuong Vinh Dat,Mai Thanh Phong 한양대학교 세라믹연구소 2021 Journal of Ceramic Processing Research Vol.22 No.4

Two dimensional graphene oxide (GO) has potential application in membrane separation owing to its unique structure andphysicochemical properties. In this study, the reduced graphene oxide (rGO) was synthesized from GO via hydrothermaltreatment at 160 oC for 1, 2, 3 and 4 h, and the rGO membranes were prepared on cellulose nitrate supporting membranesby vacuum filtration. The structural change and chemical composition of GO were investigated using X-ray diffraction (XRD),Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM),atomic force microscopy (AFM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) andcontact angle measurements. The result shows that uniformly intact rGO membranes with good hydrophobicity could beachieved by adjusting the reduction degree of GO through changing the hydrothermal reaction time. The huge improvementof the hydrophobic property of rGO could be attributed to the removal of the most the hydrophilic oxygen-containingfunctional groups on the surface of GO. Additionally, the structure, chemical composition, and d-spacing of the GO can alsobe controlled by adjusting the reduction time. This method holds great potential because it can be prepared in large quantitiesat low cost, and suitable for applications in membrane technologies.

Catalyst‐free thermally‐evaporated growth and optical properties of ZnO nanowires on Si, GaN and sapphire substrates

Khai, Tran Van,Sub Kwak, Dong,Jung Kwon, Yong,Bo Shim, Kwang,Woo Kim, Hyoun WILEY‐VCH Verlag 2013 Crystal research and technology Vol.48 No.2

<P><B>Abstract</B></P><P>Vertically well‐aligned zinc oxide nanowires (NWs) with high density were successfully synthesized on Si, sapphire and GaN/sapphire substrates by thermal evaporation of zinc powders without catalysts or additives. The growth behavior of ZnO NWs was strongly dependent on the substrate materials. The effects of the substrate position on the structures and properties of ZnO NWs were primarily discussed. The morphology and crystallinity of the resultant NWs were studied by scanning electron microscope, transmission electronic microscope and X‐ray diffraction. The photoluminescence (PL) characteristics of the ZnO NWs on the different substrates were studied. The results showed that the as‐grown ZnO NWs exhibit a sharp and strong ultraviolet emission at 3.27 eV and a very weak green emission at around 2.48 eV, indicating that the a‐synthesized NWs have excellent PL properties with good crystalline quality and can be an ideal candidate for making luminescent devices. By comparison of PL spectra, we revealed that the green‐to‐UV emission intensity ratios were considerably dependent on the substrate materials, which was explained by the difference in the structural morphology of the produced nanowires.</P>

Cascaded Single-Phase Boost Inverter Using High Frequency Transformer

( Van Thuan Tran ),( Tan Tai Tran ),( Minh Khai Nguyen ),( Hong Woo Lim ),( Won Bae Kim ) 조선대학교 공학기술연구원 2016 공학기술논문지 Vol.9 No.1

Quasi-switched boost inverter was investigated to replace Z-source inverter and switched-boost inverter in the field of low power applications because it uses one-LC-pair less than Z-source inverter. In this paper, we suggest the configuration of H-bridge quasi-switched boost inverter based on high frequency transformer (H-qSBI) to improve voltage gain factor. The proposed topology contains the same devices like switched boost inverter, where the active switch is replaced by an H-bridge inverter. This paper focuses on analyzing quasi-switched boost inverter (qSBI) implementation and simulating the performing results of cascaded single-phase dual-inverter using high frequency transformer by PSIM simulation software. The results are tested by an input DC voltage of 36 V to generate an AC voltage of 220Vrms/50Hz.