Browsing by Author "Tiwari, Rajanish N."

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A Promising Approach to the Synthesis of 3D Nanoporous Graphitic Carbon as a Unique Electrocatalyst Support for Methanol Oxidation
(2010)
Tiwari, Jitendra N.
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Tiwari, Rajanish N.
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Chang, Yun-Min
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Lin, Kun-Lin
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材料科學與工程學系
A 3D nanoporous graphitic carbon (g-C) material is synthesized by using an adamantane (C(10)H(16)) flame, and utilized to support a Pt(50)-Ru(50) alloy catalyst. The physico-chemical properties of the Pt(50)-Ru(50)/3D nanoporous g-C electrode are examined by a range of spectroscopy techniques as well as Brunauer-Emmett-Teller surface area analysis. Cyclic voltammetry measurements are used for electrochemical characterization of the Pt(50)-Ru(50)/3D nanoporous g-C electrode. The electrochemical investigations show that the supported Pt(50)-Ru(50) has excellent activity and stability towards methanol electro-oxidation. Good CO tolerance is also shown, and considered to be due to the presence of Ru nanoparticles. It is proposed that Ru is able to promote the oxidation of strongly adsorbed CO on Pt by supplying an oxygen source: RU(OH)(ad). Moreover, the presence of 3D nanopores in the g-C support may also contribute to the observed higher current density by virtue of the easy transport of methanol and the oxidation products through these nanopores.
Chemical Precursor for the Synthesis of Diamond Films at Low Temperature
(2010)
Tiwari, Rajanish N.
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Chang, Li
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材料科學與工程學系
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Department of Materials Science and Engineering
In this study, diamond films have been synthesized on adamantane-coated (100) Si substrates at 530 degrees C by microwave plasma chemical vapor deposition from a gaseous mixture of methane and hydrogen. Scanning electron microscopy, Raman spectroscopy, and X-ray diffraction were employed to characterize the carbon chemical species on the Si substrate from adamantane into diamond. These measurements provide definitive evidence for formation of high-crystalline diamond film on Si substrate without any other pretreatments. Moreover, the possible mechanisms for the diamond formation are presented. (C) 2010 The Japan Society of Applied Physics
Controlled synthesis and growth of perfect platinum nanocubes using a pair of low-resistivity fastened silicon wafers and their electrocatalytic properties
(2011-06-01)
Tiwari, Jitendra N.
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Tiwari, Rajanish N.
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Lin, Kunlin
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材料科學與工程學系
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Department of Materials Science and Engineering
Perfect platinum (Pt) nanocubes with high density have been synthesized by controlled reduction of hexachloroplatinic acid in the presence of H(2)SO(4) and HCl, employing a pair of low-resistivity fastened silicon (FS) wafers at room temperature. The presence of the additive charges (induced by prior etching of the silicon surface with HF to remove any SiO(2) layer) between the interfaces of the FS surface results in a high charge density and facilitates fast deposition of Pt nanoparticles via electroless plating. The charge density, stirring time, and homogeneity of the aqueous solution influenced the geometrical shapes of the Pt nanoparticles. The parameters were finely tuned in order to control the nucleation and growth rates and obtain perfect Pt nanocubes. The perfect Pt nanocubes were single crystalline with exposed {100} facets. Per equivalent Pt surface areas, the perfect Pt nanocubes showed enhanced catalytic activity relative to truncated Pt nanocubes or spherical Pt nanoparticles for the electrooxidation of liquid feed fuels such as methanol and ethanol. Moreover, there a strong correlation was observed between the optical, electrical, thermal, magnetic, and catalytic properties of the perfect Pt nanocubes which should lead to a variety of technological applications of these materials.
Diamond plates on dome-like particles: preparation, characterization and field emission properties
(2010-08-01)
Tiwari, Rajanish N.
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Chen, Wei-Chun
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Tiwari, Jitendra N.
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Wang, Wei-Lin
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Chang, Li
Thin diamond microplates have been grown on dome-like/hemispherical carbon particles on titanium carbide by a microwave plasma chemical vapour deposition (MPCVD) method using a gas mixture of methane and hydrogen. The diamond microplates have a thickness of about 200 nm. A thin (300 nm) film of titanium carbide was formed during carburization of sputtered titanium on an Si(100) substrate in MPCVD. The hemispherical carbon particles were covered with diamond microplates. The diamond microplates are isolated electron-emitting spherules and exhibit a low threshold (50 V mu m (1)) and high current density (0.92 mA cm(-2)) in their field emission properties. A possible mechanism for the formation of the diamond microplates and hemispherical carbon particles is presented.
Direct Synthesis of Vertically Interconnected 3-D Graphitic Nanosheets on Hemispherical Carbon Particles by Microwave Plasma CVD
(2011-03-01)
Tiwari, Jitendra N.
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Tiwari, Rajanish N.
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Singh, Gyan
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Lin, Kun-Lin
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材料科學與工程學系
High-quality, free-standing, and vertically interconnected three-dimensional (3-D) graphitic nanosheets (GNSs) were synthesized over the surface of hemispherical carbon particles/GaN at 700 A degrees C by microwave plasma chemical vapor deposition (CVD) in presence of methane gas, whereas the hemispherical carbon particles have been directly deposited on GaN/sapphire template. The GNSs are similar to 1-5 nm in thickness and have a graphitic flake structure on hemispherical carbon particles. The vertically interconnected 3-D GNSs on hemispherical carbon particles have been characterized by scanning electron microscopy, transmission electron microscopy, selective area electron diffraction pattern, X-ray diffraction, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and nitrogen gas adsorption-Brunauer-Emmet-Teller. The present CVD approach is capable of producing large quantities of GNSs with high purity. Moreover, a high-purity free-standing and vertically interconnected 3-D GNSs on hemispherical carbon particles have an enormous potential for applications in electronic devices, biological sensors, gas uptake and storage, fuel cells, lithium ion batteries, and more.
Electrocatalytic activity of Pt nanoparticles electrodeposited on amorphous carbon-coated silicon nanocones
(2010-02-01)
Tiwari, Jitendra N.
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Pan, Fu-Ming
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Chen, Te-Ming
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Tiwari, Rajanish N.
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Lin, Kun-Lin
This study pulse-electrodeposits Pt nanoparticles on amorphous carbon-coated silicon nanocones (ACNCs) and explores them as the electrocatalyst for methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) for direct methanol fuel cell applications. The work prepares silicon nanocones on the Si wafer using porous anodic aluminum oxide as the template and then deposits the amorphous carbon layer on the nanocones by microwave plasma chemical vapor deposition. According to Raman scattering and X-ray photoelectron spectroscopies (XPS), the surface of the ACNC support is composed of a nanocrystalline graphitic structure. and rich in oxygen-containing adspecies. The Pt nanoparticles pulse-electrodeposited on the highly ordered ACNC support disperses well with a large electrocatalytic surface area. The Pt/ACNC electrode exhibits excellent electrocatalytic activity and stability toward both MOR and ORR. This study suggests the abundant oxygen-containing surface species and the nanometer size of the Pt catalyst as the two major factors enhancing electrocatalytic performance of Pt/ACNC electrode. The XPS Study suggests the occurrence of charge transfer from pi-sites of the graphitic Structure to the Pt nanoparticle, thereby improving the electrochemical stability of the electrode. (C) 2009 Elsevier B.V. All rights reserved.
Enhanced Nucleation and Growth of Diamond Film on Si by CVD Using a Chemical Precursor
(2011-08-18)
Tiwari, Rajanish N.
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Tiwari, Jitendra N.
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Chang, Li
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Yoshimura, M.
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材料科學與工程學系
Nucleation of diamond is of great importance for its growth, and a detailed understanding of the nucleation process is, therefore, desired for Many applications. The pretreatment of the substrate surface may impact the initial growth period. This study demonstrates the synthesis of diamond films by microwave plasma chemical vapor deposition on a Pt/SiO(2)/Si substrate. The Pt particles were deposited on the SiO(2)/Si surface at room temperature, Whereas adamantane was seeded on the SiO(2)/Si surface by ultrasonic treatment:The Pt particles on the SiO(2) surface behave as a catalyst, which adsorb hydrocarbons from adamantane contained plasma and give distinct features of carbon in the early stage deposition The adamantane transforms not only in the nanodiamond phase but also in other carbon phases, which can then act as nuclei for diamond growth. These kinds of phases enhanced the diamond density at partially low temperature deposition. The presence of an oxide intermediate layer betweeen Pt and Si prevents silicidation as well as SiC in diamond deposition.
Etching of GaN by microwave plasma of hydrogen
(2010-03-04)
Tiwari, Rajanish N.
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Chang, Li
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材料科學與工程學系
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Department of Materials Science and Engineering
An etching process for GaN on a sapphire substrate using a microwave plasma of hydrogen has been studied. Scanning electron microscopy observations of the surface morphology show that the etching of GaN with H(2) plasma can lead to the formation of etch pits in hexagonal shape. The average size of hexagonal pits is greater than 200 nm. The effects of processing pressure and etching time are demonstrated.
Facile synthesis of continuous Pt island networks and their electrochemical properties for methanol electrooxidation
(2008)
Tiwari, Jitendra N.
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Pan, Fu-Ming
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Tiwari, Rajanish N.
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Nandi, S. K.
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材料科學與工程學系
A two-dimensional continuous Pt island network was successfully synthesized by pulse-potentiostatic electrodeposition on a flat silicon substrate, which showed markedly enhanced catalytic activity toward methanol electrooxidation and high CO tolerance, probably due to the synergistic effect of the Pt island catalyst and surrounding SiO(2) surface layer.
Growth and characterization of diamond films on TiN/Si(100) by microwave plasma chemical vapor deposition
(2009-02-01)
Chen, Wei-Chun
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Wang, Wei-Lin
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Tiwari, Rajanish N.
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Chang, Li
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材料科學與工程學系
Polycrystalline diamond films were deposited on silicon (100) substrate by microwave plasma chemical vapor disposition (MPCVD) using similar to 300 nm thick < 001 > textured titanium nitride (TiN) films as buffer layer which were prepared by radio-frequency reactive sputtering. The diamond/TiN films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. The results show that no apparent change can be observed for the < 100 > oriented TiN buffer layers after MPCVD even with a negative bias voltage applied onto the substrates. (C) 2008 Published by Elsevier B.V.
Growth, microstructure, and field-emission properties of synthesized diamond film on adamantane-coated silicon substrate by microwave plasma chemical vapor deposition
(2010-05-15)
Tiwari, Rajanish N.
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Chang, Li
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材料科學與工程學系
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Department of Materials Science and Engineering
Diamond nucleation on unscratched Si surface is great importance for its growth, and detailed understanding of this process is therefore desired for many applications. The pretreatment of the substrate surface may influence the initial growth period. In this study, diamond films have been synthesized on adamantane-coated crystalline silicon {100} substrate by microwave plasma chemical vapor deposition from a gaseous mixture of methane and hydrogen gases without the application of a bias voltage to the substrates. Prior to adamantane coating, the Si substrates were not pretreated such as abraded/scratched. The substrate temperature was similar to 530 degrees C during diamond deposition. The deposited films are characterized by scanning electron microscopy, Raman spectrometry, x-ray diffraction, and x-ray photoelectron spectroscopy. These measurements provide definitive evidence for high-crystalline quality diamond film, which is synthesized on a SiC rather than clean Si substrate. Characterization through atomic force microscope allows establishing fine quality criteria of the film according to the grain size of nanodiamond along with SiC. The diamond films exhibit a low-threshold (55 V/mu m) and high current-density (1.6 mA/cm(2)) field-emission (FE) display. The possible mechanism of formation of diamond films and their FE properties have been demonstrated. (C) 2010 American Institute of Physics. [doi:10.1063/1.3427436]
Recent progress in the development of anode and cathode catalysts for direct methanol fuel cells
(2013-09-01)
Tiwari, Jitendra N.
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Tiwari, Rajanish N.
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Singh, Gyan
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Kim, Kwang S.
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生物科技學系
Continuous growth in global energy demand has sparked concerns about energy security and environmental sustainability. In the past two decades, attempts have been made in the development of innovative energy technologies. The direct methanol fuel cell (DMFC) is among the most promising alternative energy sources for the near future. Simple construction, compact design, high energy density and relatively high energy-conversion efficiency give the DMFC an advantage over other promising power sources in terms of portability. However, the translation of DMFCs into commercially successful products is precluded due to poor performance. In addition, low activity, poor durability and reliability and an expensive anode and cathode further discourage the application of DMFCs. In this regard, the present review article focuses on recent progress in the development of anode and cathode catalysts for DMFCs. The first part of the review discusses the recent developments in the synthesis of single-, double-, and multiple-component catalysts and new catalyst supports for anode electrodes. The section is followed by the chemical approaches employed to make alloys and composite catalysts, aiming to enhance their activity, reliability and durability for the methanol oxidation reaction. Finally, exciting new research that pushes the development of single-, double-, and multiple-component catalysts and new catalyst supports for cathode electrodes is introduced. In addition, size-, shape- and composition-dependent electrocatalysts that are advocated for methanol oxidation at the anode and oxygen reduction at the cathode are highlighted to illustrate the potential of the newly developed electrocatalysts for DMFC applications. Moreover, this article provides a comprehensive review of the experimental work that is devoted to understanding the fundamental problems and recent progress in the development of anode and cathode catalysts for DMFCs. (C) 2013 Elsevier Ltd. All rights reserved.
Synthesis of Pt Nanopetals on Highly Ordered Silicon Nanocones for Enhanced Methanol Electrooxidation Activity
(2010-08-01)
Tiwari, Jitendra N.
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Tiwari, Rajanish N.
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Lin, Kun-Lin
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材料科學與工程學系
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Department of Materials Science and Engineering
Platinum (Pt) nanopetals were electrodeposited on highly ordered silicon nanocones (SiNCs) and explored as the electrocatalyst for methanol oxidation reaction (MOR) for direct methanol fuel cells applications. Highly ordered SiNCs array fabricated using the porous anodic aluminum oxide as the template had a high surface area. Well-dispersed Pt nanopetals possessing high electrocatalytic surface area was synthesized by pulse-electrodeposition on the SiNCs. Pt nanopetals loaded on highly ordered SiNC support exhibited very good catalytic activity for MOR and a high tolerance against CO poisoning, as compared to Pt nanoflowers/flat Si, Pt nanoparticles/flat Si, and many previously reported works. The abundance of a large surface area for facile transport of methanol, SiO2 sites in the vicinity of the SiNCs, as well as less contact area between the Pt nanopetals catalyst and SiNCs are suggested to be the major factors enhancing the electrocatalytic performance of the Pt nanopetal/SiNC electrode. Moreover, we believe this new nanostructure (Pt nanopetals/SiNCs) will enable many new advances in nanotechnology.
The synthesis of diamond films on adamantane-coated Si substrate at low temperature
(2010-04-15)
Tiwari, Rajanish N.
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Tiwari, Jitendra N.
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Chang, Li
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材料科學與工程學系
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Department of Materials Science and Engineering
Diamond films have been synthesized on the adamantane-coated Si (1 00) substrate by microwave plasma chemical vapor deposition from a gaseous mixture of methane and hydrogen gases with a growth rate of similar to 6.7 nm/min. The substrate temperature was similar to 475 degrees C during diamond deposition. The films obtained have good crystallinity that is characterized by scanning electron microscopy and Raman spectrometry. X-ray photoelectron spectroscopy analysis has confirmed that diamond nucleation and growth are on SiC rather than clean Si. The possible mechanism of high rate growth diamond films has been demonstrated. (C) 2010 Elsevier B.V. All rights reserved.
Thin-Film Composite Materials as a Dielectric Layer for Flexible Metal-Insulator-Metal Capacitors
(2010)
Tiwari, Jitendra N.
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Meena, Jagan Singh
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Wu, Chung-Shu
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Tiwari, Rajanish N.
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Chu, Min-Ching
A new organic-organic nanoscale composite thin-film (NCTF) dielectric has been synthesized by solution deposition of 1-bromoadamantane and triblock copolymer (Pluronic P123, BASF, EO(20)-PO(70)-EO(20)), in which the precursor solution has been achieved with organic additives. We have used a sol-gel process to make a metal-insulator-metal capacitor (MIM) comprising a nanoscale (10 nm-thick) thin-film on a flexible polyimide (PI) substrate at room temperature. Scanning electron microscope and atomic force microscope revealed that the deposited NCTFs were crack-free, uniform, highly resistant to moisture absorption, and well adhered on the Au-Cr/PI. The electrical properties of 1-bromoadamantane-P123 NCTF were characterized by dielectric constant, capacitance, and leakage current measurements. The 1-bromoadamantane-P123 NCTF on the PI substrate showed a low leakage current density of 5.5 x 10(-11) A cm(-2) and good capacitance of 2.4 fF at 1 MHz. In addition, the calculated dielectric constant of 1-bromoadamantane-P123 NCTF was 1.9, making them suitable candidates for use in future flexible electronic devices as a stable intermetal dielectric. The electrical insulating properties of 1-bromoadamantane-P123 NCTF have been improved due to the optimized dipole moments of the van der Waals interactions.