Browsing by Author "Tiwari, Jitendra 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.
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.
Facile approach to the synthesis of 3D platinum nanoflowers and their electrochemical characteristics
(2009)
Tiwari, Jitendra N.
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Pan, Fu-Ming
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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
Three-dimensional (3D) platinum nanoflowers have been successfully synthesized by potentiostatic pulse plating method on a silicon substrate, and electrochemical study shows that the nanostructured Pt catalyst has an excellent electrocatalytic activity toward methanol and CO oxidation due to preferential (100) and (110) surface orientations on the Pt nanoflowers.
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.
Flexible metal-insulator-metal capacitor using plasma enhanced binary hafnium-zirconium-oxide as gate dielectric layer
(2010-05-01)
Meena, Jagan Singh
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Chu, Min-Ching
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Tiwari, Jitendra N.
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You, Hsin-Chiang
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Wu, Chung-Hsin
We have used a sol-gel spin-coating process to fabricate a new metal-insulator-metal capacitor comprising 10-nm thick binary hafnium-zirconium-oxide (Hf(x)Zr(1-x)O(2)) film on a flexible polyimide (PI) substrate. The surface morphology of this Hf(x)Zr(1-x)O(2) film was investigated using atomic force microscopy and scanning electron microscopy, which confirmed that continuous and crack-free film growth had occurred on the PI. After oxygen plasma pre-treatment and subsequent annealing at 250 degrees C, the film on the PI substrate exhibited a low leakage current density of 3.22 x 10(-8) A/cm(2) at -10 V and maximum capacitance densities of 10.36 fF/mu m(2) at 10 kHz and 9.42 fF/mu m(2) at 1 MHz. The as-deposited sol-gel film was oxidized when employing oxygen plasma at a relatively low temperature (similar to 250 degrees C), thereby enhancing the electrical performance. (C) 2010 Elsevier Ltd. All rights reserved.
High temperature electrical properties of ultra thin Ta2O5 films on ZnO/n-Si heterolayrs
(2007)
Nandi, S. K.
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Tiwari, Jitendra N.
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材料科學與工程學系
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Department of Materials Science and Engineering
Ultrathin Ta2O5 films have been deposited oil ZnO/n-Si substrate by microwave plasma enhanced chemical vapor deposition technique. Using metal insulator semiconductor (MIS) capacitor structures, the capacitance-voltage and the leakage Current characteristics of Ta2O5 films have been investi-ated in the temperature range of 27-200 degrees C. Poole-Frenkel current conduction mechanism was found to be dominated at high temperature.
Ordered silicon nanocones as a highly efficient platinum catalyst support for direct methanol fuel cells
(2008-08-01)
Tiwari, Jitendra N.
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Chen, Te-Ming
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Pan, Fu-Ming
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Lin, Kun-Lin
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材料科學與工程學系
Platinum nanoparticles were electrodeposited on ordered silicon nanocones (SNCs) and used as the catalyst for methanol electro-oxidation in direct methanol fuel cells (DMFCs). Because of uniform dispersion of Pt nanoparticles and the high surface area, the Pt-SNC electrode exhibited superior electrocatalytic properties toward the methanol electro-oxidation, with the onset potential of 0.08 V (vs. SCE). According to chronoamperometric analysis and CO stripping cyclic voltammetric (CV) study, the Pt/SNC electrode had a stable electro-oxidation activity with a very good CO tolerance. The Si surface oxide surrounding the Pt nanoparticles on the SNCs was suggested to play a key role in improving the CO tolerance via the bifunctional mechanism. (c) 2008 Elsevier B.V. All rights reserved.
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.
TiO2/GeOxNy stacked gate dielectrics for Ge-MOSFETs
(2007-12-01)
Bera, M. K.
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Mahata, C.
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Chakraborty, A. K.
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Nandi, S. K.
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Tiwari, Jitendra N.
In this work, we present the results of physical and electrical characterization of Ti-based high-k gate dielectrics on Ge substrates. Titanium tetrakis isopropoxide (TTIP) was used as the organometallic source for the deposition of ultra-thin TiO2 films on p-Ge (100) at low temperature (< 200 degrees C) by plasma enhanced chemical vapor deposition (PECVD) technique in a microwave (700 W, 2.45 GHz) plasma cavity discharge system at a pressure of similar to 65 Pa. The presence of an ultra-thin lossy GeO2 interfacial layer between the deposited high-k film and the substrate, results in frequency-dependent capacitance-voltage (C-V) characteristics in strong accumulation and a high interface state density (similar to 10(13) cm(-2) eV(-1)). To improve the electrical properties, nitrogen engineering has been employed to convert the lossy GeO2 interfacial layer to its oxynitride, thus forming TiO2/GeOxNy/Ge stacked-gate structure with improved interface/electrical properties. Different N sources, such as NO, NH3 and NO/NH3, have been used for nitrogen engineering. XPS and Raman spectroscopy analyses have been used for surface morphological study. Electrical properties, such as gate leakage current density, interface state density, charge trapping, flatband voltage shift, etc, have been studied in detail for TiO2/GeOxNy/Ge MIS capacitors using the current-voltage (I-V), capacitance-voltage (C-V), conductance-voltage (G-V) and stress (both constant voltage and current) measurements. Although a significant improvement in electrical characteristics has been observed after nitridation in general, the formation of the interfacial GeOxNy layer, obtained from NO-plasma nitridation, is found to provide the maximum improvement among all the nitridation techniques used in this study. It is shown that the insertion of an ultra-thin oxynitride (GeOxNy) interfacial layer is advantageous for producing gate-quality TiO2 high-k dielectric stacks on Ge substrates.