Browsing by Subject "Hydrogen"

Now showing 1 - 16 of 16

Open Access
Defect reduction of Ge on Si by selective epitaxy and hydrogen annealing
(2008-10) Yu, H.-Y.; Park, J.-H.; Okyay, Ali Kemal; Saraswat, K. C.
We demonstrate a promising approach for the monolithic integration of Ge-based nanoelectronics and nanophotonics with S-ilicon: the selective deposition of Ge on Si by Multiple Hydrogen Annealing for Heteroepitaxy (MHAH). Very high quality Ge layers can be selectively integrated on Si CMOS platform with this technique. We confirm the reduction of dislocation density in Ge layers using AFM surface morphology study. In addition, in situ doping of Ge layers is achieved and MOS capacitor structures are studied. ©The Electrochemical Society.
Open Access
The effect of iron on the surface graphitization of silicon carbide
(World Scientific, 2020) Mercan, Elif; Cambaz-Büke, G.
In order to decrease the decomposition temperature of SiC, 12nm Fe thin film is applied on SiC substrates as a catalyst layer using electron beam (e-beam) deposition. To investigate the mechanism of Fe-treated SiC decomposition, local Fe regions are formed through dewetting of the catalyst layer by hydrogen annealing. The results show that Fe decreases the decomposition temperature of SiC effectively and increases the kinetics of the graphitization. Studies showed that depending on the amount of Fe, crumpled and ordered graphene films can be synthesized simultaneously on SiC by using this method.
Open Access
Elucidating the barriers on direct water splitting: key role of oxygen vacancy density and coordination over PbTiO3 and TiO2
(American Chemical Society, 2021-01-28) Ersen, M.; Ellialtıoğlu, Ş.; Gülseren, Oğuz; Uner, D.
In this work, using the state-of-the-art first-principles calculations based on density functional theory, we found that the concentration and coordination of surface oxygen vacancies with respect to each other were critical for the direct water-splitting reaction on the (001) surfaces of PbTiO3 and TiO2. For the water-splitting reaction to happen on TiO2-terminated surfaces, it is necessary to have two neighboring O vacancies acting as active sites that host two adsorbing water molecules. However, eventual dissociation of O–H bonds is possible only in the presence of an additional nearest-neighbor O vacancy. Unfortunately, this necessary third vacancy inhibits the formation of molecular hydrogen by trapping the dissociated H atoms on TiO2-terminated surfaces. Formation of up to three O vacancies is energetically less costly on both terminations of PbTiO3(001) surfaces compared with those on TiO2; the presence of Pb leads to weaker O bonds over these surfaces. Molecular hydrogen formation is more favorable on the PbO-terminated surface of PbTiO3, requiring only two neighboring oxygen vacancies. However, the hydrogen molecule is retained near the surface by weak van der Waals forces. Our study indicates two barriers leading to low productivity of direct water-splitting processes. First and foremost, there is an entropic barrier imposed by the requirement of at least two nearest-neighbor O vacancies, sterically hindering the process. Furthermore, there are also enthalpic barriers of formation on TiO2-terminated surfaces or removal of H2 molecules from the PbO-terminated surface. The requirement dictating three nearest-neighbor oxygen vacancies for hydrogen evolution is also consistent with the chemical intuition: The nearest neighbor of the formed hydrogen should be reduced enough to inhibit spontaneous oxidation under ambient conditions.
Open Access
Formation of quantum structures on a single nanotube by modulating hydrogen adsorption
(American Physical Society, 2003) Gülseren, O.; Yildirim, T.; Çıracı, Salim
Using first-principles density functional calculations we showed that quantum structures can be generated on a single carbon nanotube by modulating the adsorption of hydrogen atoms. The band gap of the hydrogen-free zone of the tube widens in the adjacent hydrogen covered zone. The sudden variation of the band gap leads to band offsets at the conduction- and valence-band edges. At the end, the band gap of the whole system is modulated along the axis of the tube, which generates quantum wells or quantum dots. Specific electronic states are confined in these quantum wells. The type and radius of the nanotube and the extent and sequence of hydrogen-free and hydrogen-covered zones can provide several options to design a desired optoelectronic nanodevice.
Open Access
High-capacity hydrogen storage by metallized graphene
(AIP Publishing, 2008) Ataca, C.; Aktürk, E.; Çıracı, Salim; Ustunel H.
First-principles plane wave calculations predict that Li can be adsorbed on graphene forming a uniform and stable coverage on both sides. A significant part of the electronic charge of the Li 2s orbital is donated to graphene and is accommodated by its distorted π* -bands. As a result, semimetallic graphene and semiconducting graphene ribbons change into good metals. It is even more remarkable that Li covered graphene can serve as a high-capacity hydrogen storage medium with each adsorbed Li absorbing up to four H2 molecules amounting to a gravimetric density of 12.8 wt %.
Open Access
Hydrogen storage capacity of titanium met-cars
(IOP Publishing Ltd., 2006) Akman, N.; Durgun, Engin; Yildirim, T.; Çıracı, Salim
The adsorption of hydrogen molecules on the titanium metallocarbohedryne (met-car) cluster has been investigated by using the first-principles plane wave method. We have found that, while a single Ti atom at the corner can bind up to three hydrogen molecules, a single Ti atom on the surface of the cluster can bind only one hydrogen molecule. Accordingly, a Ti8C12 met-car can bind up to 16H2 molecules and hence can be considered as a high-capacity hydrogen storage medium. Strong interaction between two met-car clusters leading to the dimer formation can affect H2 storage capacity slightly. Increasing the storage capacity by directly inserting H 2 into the met-car or by functionalizing it with an Na atom have been explored. It is found that the insertion of neither an H2 molecule nor an Na atom could further promote the H2 storage capacity of a Ti8C12 cluster. We have also tested the stability of the H2-adsorbed Ti8C12 met-car with ab initio molecular dynamics calculations which have been carried out at room temperature.
Open Access
Low-loss as-grown germanosilicate layers for optical waveguides
(A I P Publishing LLC, 2003) Ay, F.; Aydınlı, Atilla; Agan, S.
We report on systematic growth and characterization of low-loss germanosilicate layers for use in optical waveguide technology. The films were deposited by plasma-enhanced chemical vapor deposition technique using silane, germane, and nitrous oxide as precursor gases. Fourier transform infrared spectroscopy was used to monitor the compositional properties of the samples. It was found that addition of germane leads to decreasing of N-H- and O-H-related bonds. The propagation loss values of the planar waveguides were correlated with the decrease in the hydrogen-related bonds of the as-deposited waveguides and resulted in very low values, eliminating the need for high-temperature annealing as is usually done.
Open Access
Low-pressure deuterium storage on palladium-coated titanium nanofilms: a versatile model system for tritium-based betavoltaic battery applications
(American Chemical Society, 2023-08-30) Ghobadi, Türkan Gamze Ulusoy; Koçak, Yusuf; Jalal, Ahsan; Altınkaynak, Yağmur; Çelik, Gülşah; Semiz, Tolga; Çakır, Cihan; Bütün, Bayram; Özbay, Ekmel; Karadaş, Ferdi; Özensoy, Emrah
Deuterium (D2(g)) storage of Pd-coated Ti ultra-thin films at relatively low pressures is fine-tuned by systematically controlling the thicknesses of the catalytic Pd overlayer, underlying Ti ultra-thin film domain, D2(g) pressure (PD2), duration of D2(g) exposure, and the thin film temperature. Structural properties of the Ti/Pd nanofilms are investigated via XRD, XPS, AFM, SEM, and TPD to explore new structure-functionality relationships. Ti/Pd thin film systems are deuterated to obtain a D/Ti ratio of up to 1.53 forming crystallographically ordered titanium deuteride (TiDx) phases with strong Tix+–Dy– electronic interactions and high thermal stability, where >90% of the stored D resides in the Ti component, thermally desorbing at >460 °C in the form of D2(g). Electronic interaction between Pd and D is weak, yielding metallic (Pd0) states where D storage occurs mostly on the Pd film surface (i.e., without forming ordered bulk PdDx phases) leading to the thermal desorption of primarily DOH(g) and D2O(g) at <265 °C. D-storage typically increases with increasing Ti film thickness, PD2, T, and t, whereas D-storage is found to be sensitive to the thickness and the surface roughness of the catalytic Pd overlayer. Optimum Pd film thickness is determined to be 10 nm providing sufficient surface coverage for adequate wetting of the underlying Ti film while offering an appropriate number of surface defects (roughness) for D immobilization and a relatively short transport pathlength for efficient D diffusion from Pd to Ti. The currently used D-storage optimization strategy is also extended to a realistic tritium-based betavoltaic battery (BVB) device producing promising β-particle emission yields of 164 mCi/cm2, an open circuit potential (VOC) of 2.04 V, and a short circuit current (ISC) of 7.2 nA.
Open Access
Magnetization of graphane by dehydrogenation
(AIP Publishing, 2009) Şahin, H.; Ataca, C.; Çıracı, Salim
Using first principles calculations, we show that each hydrogen vacancy created at graphane surface results in a local unpaired spin. For domains of hydrogen vacancies the situation is, however, complex and depends on the size and geometry of domains, as well as whether the domains are single or double sided. In single-sided domains, hydrogen atoms at the other side are relocated to pair the spins of adjacent carbon atoms by forming ππ-bonds. Owing to the different characters of exchange coupling in different ranges and interplay between unpaired spin and the binding geometry of hydrogen, vacancy domains can attain sizable net magnetic moments.
Open Access
Photosensitization of PVC dehydrochlorination by hydroquinone for improved optical and electrical properties
(Elsevier, 2004) Balci, S.; Birer, O.; Süzer, Şefik
Hydroquinone (HQ) is incorporated into the PVC films containing methyl violet or polyaniline (emeraldine base) for sensitizing the UV induced optical or electrical changes, respectively. It is observed that introduction of a small amount (less than 10% by weight) of HQ not only brings the dehydrochlorination onset down to 310 nm but also sensitizes the process by more than one order of magnitude as well as leading to strong polyene formation. UV-Vis-NIR spectroscopy is used to characterize the changes and investigate the mechanism. Accordingly, it is postulated that this HQ assisted photo-dehydrochlorination involves predominantly the formation of an excited triplet via an efficient intersystem crossing in HQ followed by abstraction of hydrogen from the poly(vinyl chloride) to initiate a zipping reaction in the PVC matrix. © 2004 Elsevier Ltd. All rights reserved.
Open Access
Secondary structure prediction of beta-hairpin peptide tryptophan zipper-I
(2008) Gökoǧlu G.; Çelik, T.
We have investigated the folding properties of tryptophan zipper-I molecule which folds into a stable β-hairpin motif in aqueous solution as suggested by nuclear magnetic resonance (NMR) experiments. An all-atom presentation, including hydrogen, was used with an implicit solvent. As a simulation technique, simulated tempering algorithm was used to obtain equilibrium conformations of the molecule at ten distinct temperatures. Our minimum energy configuration obtained from simulated tempering algorithm is a β-hairpin motif with 1.30 Å backbone root-mean-square deviation from the reference PDB structure (1le0.pdb). Several quantities and exhaustive folding free energy landscapes were determined and discussed in order to clarify the folding behavior. © 2008 Elsevier Ltd. All rights reserved.
Open Access
Titanium-decorated carbon nanotubes as a potential high-capacity hydrogen storage medium
(American Physical Society, 2005) Yildirim, T.; Çıracı, Salim
We report a first-principles study, which demonstrates that a single Ti atom coated on a single-walled nanotube (SWNT) binds up to four hydrogen molecules. The first H2 adsorption is dissociative with no energy barrier while the other three adsorptions are molecular with significantly elongated H-H bonds. At high Ti coverage we show that a SWNT can strongly adsorb up to 8 wt% hydrogen. These results advance our fundamental understanding of dissociative adsorption of hydrogen in nanostructures and suggest new routes to better storage and catalyst materials.
Open Access
Transition-metal-ethylene complexes as high-capacity hydrogen-storage media
(American Physical Society, 2006) Durgun, Engin; Çıracı, Salim; Zhou, W.; Yildirim, T.
From first-principles calculations, we predict that a single ethylene molecule can form a stable complex with two transition metals (TM) such as Ti. The resulting TM-ethylene complex then absorbs up to ten hydrogen molecules, reaching to gravimetric storage capacity of ∼14wt%. Dimerization, polymerizations, and incorporation of the TM-ethylene complexes in nanoporous carbon materials are also discussed. Our results are quite remarkable and open a new approach to high-capacity hydrogen-storage materials discovery.
Open Access
Trimetalic heterogeneous catalyst for dehydrogenation of formic acid with enhanced CO tolerance
(2017-09) Perşembe, Elif
Hydrogen energy is considered to be a promising alternative for the sustainable and environmentally friendly solution of the global energy problem. One of the major obstacles of hydrogen energy applications is to maintain safe and efficient storage of hydrogen which can also be achieved chemically using suitable carrier materials. Formic acid (HCOOH, FA) can be utilized as a hydrogen carrier due to its low molecular weight (46 g/mol) and high hydrogen density (%4.4 weight). FA is a stable, non-flammable, and non-toxic biomass side-product rendering it a perfect candidate for an alternative hydrogen vector. Design of novel heterogeneous catalysts which can substitute the existing homogeneous catalytic systems may allow overcoming catalyst isolation and recovery costs and associated logistical problems hindering their applications in on-board operations. FA can be catalytically decomposed via dehydrogenation and dehydration reactions. Selective dehydrogenation of FA is crucial because, the production of CO from dehydration mechanism can suppress the activity of the catalyst by blocking/poisoning the precious metal sites. Consequently, development of CO-resistant, selective, catalytically active, and reusable heterogeneous catalysts has a great significance. In the current work, a new material that can produce H2(g) from FA under ambient conditions in the absence of additives with high CO-poisoning tolerance will be introduced, which is comprised of Pd-based trimetallic active centers functionalized with Ag and Cr in addition to amine-functionalized MnOx promoters dispersed on a SiO2 support surface. A novel trimetallic FA dehydrogenation catalyst was prepared and studied using analytical, ex-situ and in-situ spectroscopic techniques and compared to the results obtained for monometallic, bimetallic and active site-free counterparts. Trimetallic catalysts were found to reveal superior catalytic activity and stability compared to all of the currently investigated catalysts. Structural and catalytic properties of the trimetallic catalysts were investigated as a function of metal loadings. Structural characterization of the synthesized materials was carried out by Raman spectroscopy, Inductively-Coupled Plasma Optical Emission Spectroscopy (ICP-OES), X-ray Diffraction (XRD), Brunauer, Emmett and Teller (BET) Specific Surface Area Analysis, Transmission Electron Microscopy (TEM), High Resolution TEM (HRTEM), Scanning Transmission Electron Microscopy (STEM), and STEM/Energy Dispersive X-Ray (EDX), High-Angle Annular Dark Field (HAADF)/STEM. In addition, interaction of the catalyst surfaces with reactants and products were also monitored via in-situ FTIR spectroscopy for functional characterization. Detailed in-situ FTIR spectroscopic experiments were also performed using HCOOD, DCOOH and DCOOD in order to understand the nature of the adsorbed species, products and catalytic inhibitors.
Open Access
Two-and one-dimensional honeycomb structures of silicon and germanium
(American Physical Society, 2009) Cahangirov, S.; Topsakal, M.; Aktürk, E.; Şahin, H.; Çıracı, Salim
First-principles calculations of structure optimization, phonon modes, and finite temperature molecular dynamics predict that silicon and germanium can have stable, two-dimensional, low-buckled, honeycomb structures. Similar to graphene, these puckered structures are ambipolar and their charge carriers can behave like a massless Dirac fermion due to their π and π* bands which are crossed linearly at the Fermi level. In addition to these fundamental properties, bare and hydrogen passivated nanoribbons of Si and Ge show remarkable electronic and magnetic properties, which are size and orientation dependent. These properties offer interesting alternatives for the engineering of diverse nanodevices.
Open Access
Visualization of one-dimensional diffusion and spontaneous segregation of hydrogen in single crystals of VO2
(Institute of Physics Publishing, 2016-07) Kasirga, T. S.; Coy, J. M.; Park, J. H.; Cobden, D. H.
Hydrogen intercalation in solids is common, complicated, and very difficult to monitor. In a new approach to the problem, we have studied the profile of hydrogen diffusion in single-crystal nanobeams and plates of VO2, exploiting the fact that hydrogen doping in this material leads to visible darkening near room temperature connected with the metal-insulator transition at 65 ï¿½C. We observe hydrogen diffusion along the rutile c-axis but not perpendicular to it, making this a highly one-dimensional diffusion system. We obtain an activated diffusion coefficient ∼ 0.01 e-0.6eV/kBT cm2s-1, applicable in metallic phase. In addition, we observe dramatic supercooling of the hydrogen-induced metallic phase and spontaneous segregation of the hydrogen into stripes implying that the diffusion process is highly nonlinear, even in the absence of defects. Similar complications may occur in hydrogen motion in other materials but are not revealed by conventional measurement techniques.