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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Pt-functionalized reduced graphene oxide for excellent hydrogen sensing at room temperature

Cost effective and faster detection of H{sub 2} has always remained a challenge. We report synthesis of reduced graphene oxide (RGO)–Pt composite and its application as highly sensitive and selective H{sub 2} sensors at room temperature. Four samples by varying the ratio of RGO and Pt were prepared to test their sensing performance. The tests were carried out in inert (N{sub 2}) ambience as well as air ambience. It was observed that the RGO:Pt (1:3) 1 h reduced sample demonstrated the best H{sub 2} sensing performance in terms of sensitivity, response time, and recovery time at room temperature. Its response varied from ∼19% (200 ppm) to 57% (5000 ppm) against H{sub 2} in air ambience. Also, the response time and recovery time of the RGO:Pt (1:3) sample were found to be as fast as 65 s and 230 s against 5000 ppm, respectively, in air ambience. In N{sub 2} ambience, the RGO:Pt (1:3) sample demonstrated the best response of −97% (500 ppm), but its recovery was found to be poor. The RGO–Pt composite formation was verified by high resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The detailed physics behind the sensing mechanisms have been explained and experimentally verified in this work.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Effect of reducible oxide–metal cluster charge transfer on the structure and reactivity of adsorbed Au and Pt atoms and clusters on anatase TiO 2

We carried out density functional theory calculations to study the influence of oxide–metal charge transfers on the structure, energetics, and reactivity of Au and Pt atoms, dimers, and trimers adsorbed on the (101) surface of reduced anatase TiO 2 . Pt clusters interact much more strongly with the TiO 2 support than Au clusters, and, with the exception of single Pt adatoms, generally behave as electron acceptors on reduced TiO 2 , whereas Au clusters can both accept and donate charge on the reduced surface. The reactivity of the supported clusters was probed by considering their interaction with CO and co-adsorbed O 2 . The effect of surface reduction on the interaction with CO is particularly significant when the CO adsorption site is an interfacial metal atom directly in contact with the TiO 2 surface and/or in the presence of co-adsorbed O 2 . Pt clusters interact strongly with co-adsorbed O 2 and form Pt–O 2 complexes that can easily accept electrons from reduced surfaces. In contrast, Au clusters donate charge to co-adsorbed O 2 even in the presence of excess electrons from a reduced support. In conclusion, the computed differences in the properties of the supported Pt and Au clusters are consistent with several experimental observations and highlight the important role of excess surface electrons in the behavior of supported metal catalysts on reducible oxides

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Communication: Proton exchange in low temperature co-mixed amorphous H 2 O and D 2 O films: The effect of the underlying Pt(111) and graphene substrates

Isotopic exchange reactions in mixed D 2 O and H 2 O amorphous solid water films were investigated using reflection absorption infrared spectroscopy. Nanoscale films composed of 5% D 2 O in H 2 O were deposited on Pt(111) and graphene covered Pt(111) substrates using simultaneous co-dosing using two molecular beams. At 130 K, we discover that the reaction is strongly dependent on the substrate with the H/D exchange occurring more rapidly on the Pt(111) surface than on graphene. At 140 K, the films eventually crystallize with the final products on the two substrates being primarily HOD on Pt(111) and a mixture of HOD and unreacted D 2 O on graphene. We demonstrate by pre-dosing H 2 and O 2 on Pt(111) that the observed differences in reactivity on the two substrates are likely due to the formation of hydrogen ions at the Pt(111) surface that are not formed on graphene. Once formed the mobile protons move through the ASW overlayer to begin the H/D exchange reaction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Strong electrostatic adsorption of Pt onto SiO 2 partially overcoated Al 2 O 3 —Towards single atom catalysts

It is frequently desired to synthesize supported metal catalysts that consist of very small clusters or single atoms. In this work, we combine strong electrostatic adsorption (SEA) of H 2 PtCl 6 and engineered oxide supports to ultimately produce very small Pt clusters, including a large fraction of single Pt atoms. The supports are synthesized by depositing controlled amounts of SiO 2 onto Al 2 O 3 (SiO 2 @Al 2 O 3 ) that has been previously grafted with bulky organic templates. After the templates are removed, the oxide supports are largely negatively charged, like SiO 2 , but have small patches of positively charged Al 2 O 3 , derived from the regions previously covered by the template. The overall point of zero charge of these materials decreases from pH 6.4 for 1 cycle of SiO 2 deposition to a SiO 2 -like <2 for materials with more than 5 cycles of SiO 2 deposition. SEA at pH 4 on templated SiO 2 @Al 2 O 3 deposits from 1 wt. % to 0.05 wt. % Pt as the amount of SiO2 increases. Pt loadings drop to near zero in the absence of a template. The resulting Pt nanoparticles are generally <1 nm and have dispersion near 100% by CO chemisorption. Finally, CO DRIFTS shows that the CO nanoparticles become increasingly well defined and have a higher percentage of Pt single atoms as the amount of SiO 2 increases on the SiO 2 @Al 2 O 3 particles. Overall, this method of synthesizing patches of charge on a carrier particle appears to be a viable route to creating extremely highly dispersed supported metal catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Degrees and signatures of broken PT symmetry in nonuniform lattices

We investigate the robustness of the parity- and time-reversal (PT) symmetric phase in an N-site lattice with a position-dependent, parity-symmetric hopping function and a pair of imaginary, PT-symmetric impurities. We find that the ''fragile''PT-symmetric phase in these lattices is stronger than its counterpart in a lattice with constant hopping. With an open system in mind, we explore the degrees of broken PT symmetry and their signatures in single-particle wave-packet evolution. We predict that, when the PT-symmetric impurities are closest to each other, the time evolution of a wave packet in an even-N lattice is remarkably different from that in an odd-N lattice. Our results suggest that PT symmetry breaking in such lattices is accompanied by rich, hitherto unanticipated, phenomena.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

SU-E-T-419: Workflow and FMEA in a New Proton Therapy (PT) Facility

Purpose: Workflow is an important component in the operational planning of a new proton facility. By integrating the concept of failure mode and effect analysis (FMEA) and traditional QA requirements, a workflow for a proton therapy treatment course is set up. This workflow serves as the blue print for the planning of computer hardware/software requirements and network flow. A slight modification of the workflow generates a process map(PM) for FMEA and the planning of QA program in PT. Methods: A flowchart is first developed outlining the sequence of processes involved in a PT treatment course. Each process consists of a number of sub-processes to encompass a broad scope of treatment and QA procedures. For each subprocess, the personnel involved, the equipment needed and the computer hardware/software as well as network requirements are defined by a team of clinical staff, administrators and IT personnel. Results: Eleven intermediate processes with a total of 70 sub-processes involved in a PT treatment course are identified. The number of sub-processes varies, ranging from 2-12. The sub-processes within each process are used for the operational planning. For example, in the CT-Sim process, there are 12 sub-processes: three involve data entry/retrieval from a record-and-verify system, two controlled by the CT computer, two require department/hospital network, and the other five are setup procedures. IT then decides the number of computers needed and the software and network requirement. By removing the traditional QA procedures from the workflow, a PM is generated for FMEA analysis to design a QA program for PT. Conclusion: Significant efforts are involved in the development of the workflow in a PT treatment course. Our hybrid model of combining FMEA and traditional QA program serves a duo purpose of efficient operational planning and designing of a QA program in PT.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2017

Thermal stability of Pt nanoclusters interacting to carbon sublattice

The catalytic activity of Pt clusters is dependent not only on the nanoparticle size and its composition, but also on its internal structure. To determine the real structure of the nanoparticles used in catalysis, the boundaries of the thermal structure stability of Pt clusters to 8.0 nm in diameter interacting with carbon substrates of two types: a fixed α-graphite plane and a mobile substrate with the diamond structure. The effect of a substrate on the processes melting of Pt nanoclusters is estimated. The role of the cooling rate in the formation of the internal structure of Pt clusters during crystallization is studied. The regularities obtained in the case of “free” Pt clusters and Pt clusters on a substrate are compared. It is concluded that platinum nanoparticles with diameter D ≤ 4.0 nm disposed on a carbon substrate conserve the initial fcc structure during cooling.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2012

Role of Pt Nanoparticles in Photoreactions on TiO 2 Photoelectrodes

Highly efficient Pt-TiO 2 composite photoelectrodes were synthesized by combining two novel deposition methods: ACVD and a room temperature RF (radio frequency) magnetron sputtering method. A room temperature RF magnetron sputtering method allowed uniform deposition of Pt nanoparticles (NPs) onto the as-synthesized nanostructured columnar TiO 2 films by ACVD. Pt NP sizes from 0.5 to 3 nm demonstrating a high particle density (>10 12 cm -2 ) could be achieved by varying deposition time with constant pressure and power intensity. As-synthesized Pt-TiO2 films were used as photoanodes for water photolysis. Pt nanoparticles deposited onto the TiO 2 film for 20s produced the highest photocurrent (7.92 mA/cm 2 to 9.49 mA/cm 2 ) and maximized the energy conversion efficiency (16.2 % to 21.2 %) under UV illumination. However, as the size of Pt particles increased, more trapping sites for photogenerated electron-hole pairs decreased photoreaction.

14 SOLAR ENERGY↗
DOE OSTI2014

IC W13_auptphase Highlight: Phase Diagram of Pt from Z Methodology

Pt phase diagram from combining the Z methodology with the fcc-rhcp (face-centered cubic -- random hexagonal closepacked) solid-solid phase boundary. The experimental melting curve of Pt is shown as dashed blue line. Solid-solid phase boundaries in Figs. 1 and 2 are consistent with each other. Phase diagram of Pt from the Z methodology: fcc-Pf melting curve (green line), liquid Pt solidified into solid fcc (green bullets), 9R-Pt melting curve (blue line), liquid Pt solidified into solid 9R (blue bullets), and the (tentative) fcc-9R solid-solid phase boundary (violet).

36 MATERIALS SCIENCE↗
NASA NTRS2007

Bulk Crystal Growth of Piezoelectric PMN-PT Crystals Using Gradient Freeze Technique for Improved SHM Sensors

There has been a growing interest in recent years in lead based perovskite ferroelectric and relaxor ferroelectric solid solutions because of their excellent dielectric, piezoelectric and electrostrictive properties that make them very attractive for various sensing, actuating and structural health monitoring (SHM) applications. We are interested in the development of highly sensitive and efficient PMN-PT sensors based on large single crystals for the structural health monitoring of composite materials that may be used in future spacecrafts. Highly sensitive sensors are needed for detection of defects in these materials because they often tend to fail by distributed and interacting damage modes and much of the damage occurs beneath the top surface of the laminate and not detectable by visual inspection. Research is being carried out for various combinations of solid solutions for PMN-PT piezoelectric materials and bigger size crystals are being sought for improved sensor applications. Single crystals of this material are of interest for sensor applications because of their high piezoelectric coefficient (d33 greater than 1700 pC/N) and electromechanical coefficients (k33 greater than 0.90). For comparison, the commonly used piezoelectric ceramic lead zirconate titanate (PZT) has a d33 of about 600 pC/N and electromechanical coefficients k33 of about 0.75. At the present time, these piezoelectric relaxor crystals are grown by high temperature flux growth method and the size of these crystals are rather small (~3x4x5 mm(exp 3). In the present paper, we have attempted to grow bulk single crystals of PMN-PT in a 2 inch diameter platinum crucible and successfully grown a large size crystal of 67%PMN-33%PT using the vertical gradient freeze technique with no flux. Piezoelectric properties of the grown crystals are investigated. PMN-PT plates show excellent piezoelectric properties. Samples were poled under an applied electric field of 5 kV/cm. Dielectric properties at a frequency of 1 kHz are examined. The grown PMN-PT crystals show typical relaxor dielectric properties. Additionally, the thermal properties of the sample are tested. The results are in good agreement with those found in the literature and some are reported for the first time.

Aggarwal, Mohan D.↗
NASA NTRS2010

Low Pt content direct methanol fuel cell anode catalyst: nanophase PtRuNiZr

A method for the preparation of a metallic material having catalytic activity that includes synthesizing a material composition comprising a metal content with a lower Pt content than a binary alloy containing Pt but that displays at least a comparable catalytic activity on a per mole Pt basis as the binary alloy containing Pt; and evaluating a representative sample of the material composition to ensure that the material composition displays a property of at least a comparable catalytic activity on a per mole Pt basis as a representative binary alloy containing Pt. Furthermore, metallic compositions are disclosed that possess substantial resistance to corrosive acids.

Narayanan, Sekharipuram R.↗
DOE OSTI2011

TUNING OF SIZE AND SHAPE OF AU-PT NANOCATALYST FOR DIRECT METHANOL FUEL CELLS

In this paper, we report the precise control of the size, shape and surface morphology of Au-Pt nanocatalysts (cubes, blocks, octahedrons and dogbones) synthesized via a seed-mediated approach. Gold 'seeds' of different aspect ratios (1 to 4.2), grown by a silver-assisted approach, were used as templates for high-yield production of novel Au-Pt nanocatalysts at a low temperature (40 C). Characterization by electron microscopy (SEM, TEM, HRTEM), energy dispersive X-ray analysis (EDX), UV-Vis spectroscopy, zeta-potential (surface charge), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma mass spectrometry (ICP-MS) were used to better understand their physico-chemical properties, preferred reactivities and underlying nanoparticle growth mechanism. A rotating disk electrode was used to evaluate the Au-Pt nanocatalysts electrochemical performance in the oxygen reduction reaction (ORR) and the methanol oxidation reaction (MOR) of direct methanol fuel cells. The results indicate the Au-Pt dogbones are partially and in some cases completely unaffected by methanol poisoning during the evaluation of the ORR. The ORR performance of the octahedron particles in the absence of MeOH is superior to that of the Au-Pt dogbones and Pt-black, however its performance is affected by the presence of MeOH.

10 SYNTHETIC FUELS↗
DOE OSTI2017

Functionalized graphene-Pt composites for fuel cells and photoelectrochemical cells

A method of growing crystals on two-dimensional layered material is provided that includes reversibly hydrogenating a two-dimensional layered material, using a controlled radio-frequency hydrogen plasma, depositing Pt atoms on the reversibly hydrogenated two-dimensional layered material, using Atomic Layer Deposition (ALD), where the reversibly hydrogenated two-dimensional layered material promotes loss of methyl groups in an ALD Pt precursor, and forming Pt-O on the reversibly hydrogenated two-dimensional layered material, using combustion by O.sub.2, where the Pt-O is used for subsequent Pt half-cycles of the ALD process, where growth of Pt crystals occurs.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Pt(NCl)2 by Materials Project

Pt(NCl)2 crystallizes in the tetragonal P4/m space group. The structure is one-dimensional and consists of one Pt(NCl)2 ribbon oriented in the (0, 0, 1) direction. there are two inequivalent Pt2+ sites. In the first Pt2+ site, Pt2+ is bonded in a square co-planar geometry to two equivalent Pt2+ and four equivalent Cl1- atoms. Both Pt–Pt bond lengths are 2.58 Å. All Pt–Cl bond lengths are 2.47 Å. In the second Pt2+ site, Pt2+ is bonded in a linear geometry to two equivalent Pt2+ atoms. N is bonded in a single-bond geometry to one Cl1- atom. The N–Cl bond length is 1.58 Å. Cl1- is bonded in a bent 120 degrees geometry to one Pt2+ and one N atom.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Materials Data on Pt(NCl)2 by Materials Project

Pt(NCl)2 crystallizes in the triclinic P-1 space group. The structure is two-dimensional and consists of one Pt(NCl)2 sheet oriented in the (-1, 1, 0) direction. there are two inequivalent Pt2+ sites. In the first Pt2+ site, Pt2+ is bonded in a distorted linear geometry to two equivalent N and four Cl1- atoms. Both Pt–N bond lengths are 1.75 Å. There are two shorter (2.47 Å) and two longer (2.75 Å) Pt–Cl bond lengths. In the second Pt2+ site, Pt2+ is bonded in a distorted rectangular see-saw-like geometry to four N atoms. There is two shorter (1.91 Å) and two longer (1.96 Å) Pt–N bond length. There are two inequivalent N sites. In the first N site, N is bonded in a distorted single-bond geometry to one Pt2+ and two equivalent Cl1- atoms. There are one shorter (2.43 Å) and one longer (2.74 Å) N–Cl bond lengths. In the second N site, N is bonded in a distorted linear geometry to two Pt2+ and one Cl1- atom. The N–Cl bond length is 2.18 Å. There are two inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a 4-coordinate geometry to one Pt2+, one N, and one Cl1- atom. The Cl–Cl bond length is 2.26 Å. In the second Cl1- site, Cl1- is bonded in a 4-coordinate geometry to one Pt2+, two equivalent N, and one Cl1- atom.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Ultrafine Pt Nanoparticle-Decorated Pyrite-Type CoS 2 Nanosheet Arrays Coated on Carbon Cloth as a Bifunctional Electrode for Overall Water Splitting

To improve the utilization efficiency of precious metals, metal-supported materials provide a direction for fabricating highly active and stable heterogeneous catalysts. Here, carbon cloth (CC)-supported Earth-abundant CoS 2 nanosheet arrays (CoS 2 /CC) are presented as ideal substrates for ultrafine Pt deposition (Pt-CoS 2 /CC) to achieve remarkable performance toward the hydrogen and oxygen evolution reactions (HER/OER) in alkaline solutions. Notably, the Pt-CoS 2 /CC hybrid delivers an overpotential of 24 mV at 10 mA cm –2 and a mass activity of 3.89 A Ptmg –1 , which is 4.7 times higher than that of commercial Pt/C, at an overpotential of 130 mV for catalyzing the HER. An alkali-electrolyzer using Pt-CoS 2 /CC as a bifunctional electrode enables a water-splitting current density of 10 mA cm –2 at a low voltage of 1.55 V and can sustain for more than 20 h, which is superior to that of the state-of-the-art Pt/C+RuO 2 catalyst. Further experimental and theoretical simulation studies demonstrate that strong electronic interaction between Pt and CoS 2 synergistically optimize hydrogen adsorption/desorption behaviors and facilitate the in situ generation of OER active species, enhancing the overall water-splitting performance. This research highlights the regulation of interfacial and electronic synergy in pursuit of highly efficient and durable supported catalysts for hydrogen and oxygen electrocatalytic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Synthesis and stabilization of small Pt nanoparticles on TiO 2 partially masked by SiO 2

Controlling metal nanoparticle size and preserving metal dispersion at elevated temperature remain key challenges in designing new supported metal catalysts. Many methods have been proposed to stabilize metal nanoparticles for catalysis, but the use of specialized equipment or metal precursors can limit the application of these methods for scalable production. Here, we demonstrate a synthesis strategy to improve the dispersion and thermal stability of Pt nanoparticles on an oxide support. A thin SiO 2 coat (<2 nm) was deposited on TiO2 through repeated condensation cycles of tetraethyl orthosilicate (TEOS) with or without an organic template on the surface. H 2 PtCl 6 was deposited using wetness impregnation, and the samples were dried, calcined, and reduced. The as-synthesized Pt nanoparticles are 1–2 nm by TEM and maintain dispersion >45% by CO chemisorption even after prolonged heating at 500 °C, whereas Pt nanoparticles on unmodified TiO 2 are less dispersed (~33%) and their dispersion falls further upon prolonged heating. Ethylene hydrogenation demonstrates that the Pt nanoparticles on modified TiO 2 preserve the catalytic activities of Pt on unmodified TiO 2 . The use of wet chemistry-based oxide modification and wetness impregnation makes this strategy a scalable and generalizable synthesis method to prepare other supported metal nanoparticles for catalysis applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Effect of single atom Platinum (Pt) doping and facet dependent on the electronic structure and light absorption of Lanthanum Titanium Oxide (La 2 Ti 2 O 7 ): A Density Functional Theory study

Charge generation and separation are two key processes for semiconductor photocatalysis. Here, we use Pt as a single atom catalyst to systematically examine the facet-dependent electronic band structure and light absorption of the layered perovskite-type wide-gap semiconductor, lanthanum titanium oxide (La 2 Ti 2 O 7 , LTO) by means of density functional theory simulations. In this study, it is found that single Pt atom doping of different LTO surfaces (here, (100), (101) and (001)) can not only create states in the bandgap that would promote the formation of recombination centers, but also shift the optical absorption edge to the visible region. Interestingly, the Pt doping forms a heterojunction, with the valence band maximum consisting of Pt states and the conduction band minimum consisting of LTO states, respectively. To study the facet dependent surface activity, acetic acid (CH 3 COOH) was used as a model molecule to investigate the adsorption and charge transfer on the (101), (100) and (001) Pt-LTO surface facets. The results show that the (101) facet could enable stronger adsorption of CH 3 COOH by promoting more electron transfer during the interfacial interaction. Our theoretical findings aim to promote the design and optimization of the single atom catalysts (SACs) for photocatalytic applications and other broad catalysis systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗