1,721,218 research outputs found
Amorphous WO3 Films via Chemical Vapor Deposition from Metallorganic Precursors Containing Phosphorus Dopant.
No full textAmorphous WO3 films, doped with phosphorus, have been synthesized by chemical vapor deposition of volatile, low-melting P-substituted tungsten carbonyls. The presence of a small quantity of dopant released by the precursor during its decomposition is sufficient to inhibit the crystallization of the tungsten oxide on the matrix (P/W 2-4 atom % on Si(100) and ca. 10 atom % on KGlass). The nature of the film is scarcely affected by the experimental conditions of deposition (namely p(O2) partial pressure) and the quantity of the P dopant is properly tuned by an appropriate choice of the molecular precursor, being in the order W(CO)(4)[P(OEt)(3)](2) (P/W 1/1) > W-2(mu-PR2)(2)(CO)(8) (P/W 1/4) > W(CO)(4)(PEt3)(2) (P/W 1/10 on KGlass, 1/30 on Si). The films on KGlass exhibit interesting electrochromic properties with a maximum efficiency of 66 cm(2)/C
A Molecular Cluster Approach to the Study of the Bonding of CO and NH3 to a d10 ion on ZnO(0001) and CuCl(111)
No full textThe local-density-functional approximation coupled to the molecular cluster approach is used here to compare the electronic structure of CO and NH3 molecules chemisorbed on the ZnO(0001) and CuCl(111) polar surfaces. For both substrates the interaction with the adsorbate is strongly dependent on the charge carried by the atom representative of the Lewis acid site. In particular, a realistic description of the surface-adsorbate bonding scheme is only obtained by forcing the partially occupied dangling bonds on ZnO(0001)/CuCl(111) to be empty. The bonding of CO to CuCl(111) looks similar to that present in metal-carbonyl complexes, with a donation from the CO 5σ HOMO into the empty levels of the coordinatively unsaturated Cu surface ions assisted by a significant backdonation from the fully occupied Cu 3d orbitals into the CO 2π* LUMO. At variance to that, the bonding of CO to ZnO(0001) is limited to a donation from the CO HOMO into the empty levels of the surface Zn2+ ions. The surface-molecule electrostatic interaction, negligible for CuCl(111), plays for ZnO(0001) an important role in determining the relative energy position of CO based MOs. As far as the bonding of NH3 to CuCl(111) and ZnO(0001) is concerned, it has been found to be characterized in both cases by a donation from the NH3 3a1 HOMO into the empty levels of the unsaturated metal sites. Any backbonding from the 3d orbitals of the Lewis acid sites is prevented by the high energy of the NH3 2e LUMO. Finally, for NH3 on ZnO(0001), the electrostatic interaction between the permanent NH3 dipole moment and the high value of the Lewis acid site effective charge plays a leading role in determining the binding energy
Gold nanotubes by template-directed synthesis
No full textGold nanotubes were prepared by radiofrequency-sputtering through a template-directed synthesis
in porous alumina substrates. The resulting composite material was subsequently treated in acidic
or alkaline aqueous solutions in order to selectively remove the membrane, thus resulting in the
obtainment of self-supporting Au nanotubules. The adopted strategy allows the preparation of both
composites and free-standing metal nanostructures with an aspect ratio tunable as a function of
the synthesis conditions and the membrane pore size
Poly[(oligoethylene glycol) dihydroxytitanate] as organic-inorganic polymer-electrolytes
No full textTwo new poly[(oligoethylene glycol) dihydroxytitanate] electrolytic systems were prepared by a polycondensation reaction between: Ti(OEt)4 and polyethylene glycol 400 (PEG 400) (I), and Ti(OEt)4 and PEG 400/LiCl complex (II). The polymers thus obtained show a solid rubbery consistency and are very stable under inert atmosphere. The structure, morphology and conductivity of these materials were investigated. On the basis of analytical data and vibrational studies it was concluded that polymers (I) and (II) are inorganic-organic materials with titanium atoms bonded together by PEG bridges. Particularly, inter-chain interactions in polymer (I) occur by means of hydrogen bonds between titanium hydroxyl groups and ethereal oxygens of PEG chains, while polymer (II) contains both titanium-PEG interactions and a second type of inter-chain interaction due to the presence of hydrogen bonding clusters around Cl- ions. Scanning electron microscopy (SEM) revealed two types of morphologies in polymers (I) and (II). Conductivity measurements made at different temperatures indicated that both polymers conduct ionically. Two conductivity mechanisms are present in these materials, which are strongly influenced by segmental motions of the polymer chains. At 25 degrees C polymers (I) and (II) showed conductivities of ca. 3 x 10-6 and 4 x 10-6 S/cm-1 respectively
CVD Cu2O and CuO nanosystems characterized by XPS
No full textIn the present investigation, X-ray photoelectron and X-ray excited Auger electron spectroscopy
analyses of the principal core levels (O 1s, Cu 2p, and Cu LMM) of Cu2O and CuO nanosystems
are proposed. The samples were obtained by chemical vapor deposition starting from a novel
second-generation copper(II) precursor, Cu(hfa)2·TMEDA (hfa1,1,1,5,5,5-hexafluoro-
2,4-pentanedionate; TMEDA=N,N,N’,N’- tetramethylethylenediamine), under a dry O2
atmosphere. The obtained route led to pure, homogeneous and single-phase Cu(I) and Cu(II) oxide
nanosystems at temperatures of 300 and 500 °C, respectively, whose chemical nature could be
conveniently distinguished by analyzing the Cu 2p band shape and position, as well as by evaluating
the Auger parameters. The samples were characterized by O/Cu atomic ratios greater than the
expected stoichiometric values, due to marked interactions with the outer atmosphere attributed to
their high surface-to-volume ratio
A LCAO-LDF Study of CO and NH3 Chemisorption on ZnO(0001)
No full textThe coordination of CO and NH3 to ZnO(0001) has been studied by the LCAO-LDF molecular-cluster approach. A realistic description of the electronic structure of the Lewis acid site has been found to be the most important point of the chemisorption simulation. An excellent agreement between experiment and theory has been obtained by considering the dangling bonds of ZnO(0001) to be empty. The bonding of CO to the surface is dominated by a covalent interaction involving a donation from the CO HOMO into the empty AOs of the Zn surface ions. The same kind of mechanism is active for the NH3 chemisorption, even if the electrostatic interaction between the NH3 dipole moment and the Lewis acid site is at least as important as the covalent one
Coordination Chemistry of CO and NH3 on ZnO(0001): a Molecular Cluster Study of the CO and NH3 Bonding Interaction with a d10 Ion
No full textA detailed investigation of the electronic structure of CO chemisorbed on the ZnO(0001) polar surface has been carried out by using the local-density-functional molecular-cluster approach. A Zn22O22/CO model cluster is used to evaluate internuclear distances, adsorption energies and vibrational frequencies of the surface-CO and C-O vibrational modes. The chaining to the rest of the solid has been simulated with pseudo-atoms carrying a not-integer nuclear charge. A realistic description of the electronic structure of the Lewis acid site is shown to be crucial to obtain a correct chemisorptive interaction. In particular, it appears that removing half an electron from each surface Zn allows an excellent agreement between experiment and theory as a consequence of the absence of occupied dangling bonds on ZnO(0001). Preliminary results relative to NH3 chemisorption over the same surface are also presented and discussed. The bonding mechanism of CO to ZnO(0001) is dominated by a covalent interaction involving a donation from the highest occupied molecular orbital of CO into the empty levels of the coordinatively unsaturated Zn surface ions. The same kind of mechanism is active in NH3 chemisorption, even though the electrostatic interaction between the nh3 dipole moment and the field at ZnO(0001) is at least as important as the covalent one. Slight variations of the electronic structure of the surface greatly affect the bonding of CO to the surface itself as a consequence of a consistent donation into the CO lowest unoccupied molecular orbital from the partially occupied dangling bonds of the Zn unsaturated ions nearest to the Lewis acid site. Transition state calculations seem to indicate that the assignments of the ultraviolet photoelectron spectra of ZnO(0001) after exposure to CO or NH3 need to be slightly modified
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