CuO/ZnO nanocomposite gas sensors developed by a plasma-assisted route.

CuO/ZnO nanocomposites were synthesized on Al(2)O(3) substrates by a hybrid plasma-assisted approach, combining the initial growth of ZnO columnar arrays by plasma-enhanced chemical vapor deposition (PE-CVD) and subsequent radio frequency (RF) sputtering of copper, followed by final annealing in air. Chemical, morphological, and structural analyses revealed the formation of high-purity nanosystems, characterized by a controllable dispersion of CuO particles into ZnO matrices. The high surface-to-volume ratio of the obtained materials, along with intimate CuO/ZnO intermixing, resulted in the efficient detection of various oxidizing and reducing gases (such as O(3), CH(3)CH(2)OH, and H(2)). The obtained data are critically discussed and interrelated with the chemical and physical properties of the nanocomposites.

Ag/ZnO nanomaterials as high performance sensors for flammable and toxic gases.

Ag/ZnO nanocomposites supported on polycrystalline Al2O3 were synthesized by an unprecedented approach combining plasma enhanced chemical vapor deposition (PE-CVD) of ZnO matrices and the subsequent deposition of Ag nanoparticles (NPs) by radio frequency (RF) sputtering. The system structure, composition and morphology were investigated by glancing incidence x-ray diffraction (GIXRD), secondary ion mass spectrometry (SIMS), field emission scanning electron microscopy (FE-SEM) and energy dispersive x-ray spectroscopy (EDXS). A tailored dispersion and distribution of silver particles could be obtained under mild conditions by the sole variation of the sputtering time. Gas sensing properties toward flammable and toxic gases, both reducing (CH3CH2OH, CH3COCH3) and oxidizing (O3), were investigated in the temperature range 100-400 °C. Beside the high sensitivity, the developed sensors exhibited a response proportional to Ag content, thanks to catalytic and electronic effects promoted by silver NPs. In addition, discrimination between oxidizing and reducing analytes was enabled by a suitable choice of the adopted working temperature.

F-Doped Co3O4 photocatalysts for sustainable H2 generation from water/ethanol.

p-Type Co(3)O(4) nanostructured films are synthesized by a plasma-assisted process and tested in the photocatalytic production of H(2) from water/ethanol solutions under both near-UV and solar irradiation. It is demonstrated that the introduction of fluorine into p-type Co(3)O(4) results in a remarkable performance improvement with respect to the corresponding undoped oxide, highlighting F-doped Co(3)O(4) films as highly promising systems for hydrogen generation. Notably, the obtained yields were among the best ever reported for similar semiconductor-based photocatalytic processes.

How does Cu(II) convert into Cu(I)? An unexpected ring-mediated single-electron reduction.

Cu(x)O (x=1,2) nanomaterials with tailored composition and properties-a hot topic in sustainable technologies-may be fabricated from molecular sources through bottom-up processes that involve unexpected changes in the metal oxidation state and open intriguing challenges on the copper redox chemistry. How copper(II) sources may lead to copper(I) species in spite of the absence of any explicit reducing agent, and even in the presence of oxygen, is one such question-to date unanswered. Herein, we study copper "reduction without reductants" within one molecule and reveal that the actual reducing agent is abstracted atomic hydrogen. By investigating the fragmentation of a copper(II) precursor for copper oxide nanostructures by combined ESI-MS with multiple collisional experiments (ESI/MS(n)) and theoretical calculations, we highlight a copper-promoted C-H bond activation, leading to reduction of the metal center and formation of a Cu(I)-C-NCCN six-membered ring. Such a novel ring system is the structural motif for a new family of cyclic copper(I) adducts, which show a bonding scheme, herein reported for the first time, that may shed unprecedented light on copper chemistry. Beyond the relevance for the preparation of copper oxide nanostructures, the hydrogen-abstraction/proton-delivery/electron-gain mechanism of copper(II) reduction disclosed herein appears to be a general property of copper and might help to understand its redox reactivity.

Tailored vapor-phase growth of Cu(x)O-TiO2 (x = 1, 2) nanomaterials decorated with Au particles.

We report on the fabrication of Cu(x)O-TiO(2) (x = 1, 2) nanomaterials by an unprecedented vapor-phase approach. The adopted strategy involves the growth of porous Cu(x)O matrices by means of chemical vapor deposition (CVD), followed by the controlled dispersion of TiO(2) nanoparticles. The syntheses are performed on Si(100) substrates at temperatures of 400-550 °C under wet oxygen atmospheres, adopting Cu(hfa)(2)·TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N',N'-tetramethylethylenediamine) and Ti(O-(i)Pr)(2)(dpm)(2) (O-(i)Pr = isopropoxy; dpm = 2,2,6,6-tetramethyl-3,5-heptanedionate) as copper and titanium precursors, respectively. Subsequently, finely dispersed gold nanoparticles are introduced in the as-prepared systems via radio frequency (RF)-sputtering under mild conditions. The synthesis process results in the formation of systems with chemical composition and nano-organization strongly dependent on the nature of the initial Cu(x)O matrix and on the deposited TiO(2) amount. The decoration with low-size gold clusters paves the way to the engineering of hierarchically organized nanomaterials.

From thioxo cluster to dithio cluster: exploring the chemistry of polynuclear zirconium complexes with S,O and S,S ligands.

Three different zirconium thio and oxothio clusters, characterized by different coordination modes of dithioacetate and/or monothioacetate ligands, were obtained by the reaction of monothioacetic acid with zirconium n-butoxide, Zr(O(n)Bu)4, in different experimental conditions. In particular, we isolated the three polynuclear Zr3(µ3-SSSCCH3)2(SSCCH3)6·2(n)BuOH (Zr3), Zr4(µ3-O)2(µ-η(1)-SOCCH3)2(SOCCH3)8(O(n)Bu)2 (Zr4), and Zr6(µ3-O)5(µ-SOCCH3)2(µ-OOCCH3)(SOCCH3)11((n)BuOH) (Zr6) derivatives, presenting some peculiar characteristics. Zr6 has an unusual star-shaped structure. Only sulfur-based ligands, viz., chelating dithioacetate monoanions and an unusual ethane-1,1,1-trithiolate group µ3 coordinating the Zr ions, were observed in the case of Zr3. 1D and 2D NMR analyses confirmed the presence of differently coordinated ligands. Raman spectroscopy was further used to characterize the new polynuclear complexes. Time-resolved extended X-ray absorption fine structure measurements, devoted to unraveling the cluster formation mechanisms, evidenced a fast coordination of sulfur ligands and subsequent relatively rapid rearrangements.

Plasma processing of nanomaterials: emerging technologies for sensing and energy applications.

Plasma processing represents an attractive and versatile option for the fabrication of low-dimensional nanomaterials, whose chemical and physical properties can be conveniently tailored for the development of advanced technologies. In particular, Plasma Enhanced-Chemical Vapor Deposition (PE-CVD) is an appealing route to multi-functional oxide nanoarchitectures under relatively mild conditions, owing to the unique features and activation mechanisms of non-equilibrium plasmas. In this context, the potential of plasma-assisted fabrication in advanced nanosystem development is discussed. After a brief introduction on the basic categories of plasma approaches, the perspectives of application to CVD processes are commented, reporting on the growth and characterization of Co3O4 nanomaterials as a case study. Besides examining the interrelations between the material properties and the synthesis conditions, special focus is given to their emerging applications as catalysts for photo-assisted hydrogen production and solid state gas sensors.

Molecular, electronic, and crystal structures of self-assembled hydrothermally synthesized Zn(II)-mercaptonicotinate: a combined spectroscopic and theoretical approach.

A Zn(II) 2-mercaptonicotinate coordination polymer (Zn1), with Zn(II) ions chelated by both sulfur and oxygen in a distorted square pyramidal environment, and a molecular Zn(II) 2-hydroxynicotinate complex (Zn2) were synthesized by the reaction of zinc acetylacetonate with 2-mercaptonicotinic (Zn1) and 2-hydroxynicotinc (Zn2) acid, respectively, under hydrothermal conditions. The crystal structures of Zn1 and Zn2 were determined by single crystal X-ray diffraction measurements. Dispersion-corrected density functional theory (DFT) calculations reproduce very well the experimental structures and show that Zn1 is stable against hydration, whereas Zn2 is stable against dehydration over wide ranges of temperature and pressure, in agreement with thermogravimetric analysis results. The electronic structure of the two compounds is computed with the DFT+U method. The theoretical valence band agrees well with the X-ray photoelectron spectroscopy experiments. Furthermore, the band gap of Zn1 is found to be narrower than that of Zn1 and is characterized by the presence of sulfur lone pairs at the edge of the valence band.

Cathodoluminescence evaluation of oxygen vacancy population in nanostructured titania thin films for photocatalytic applications.

Room-temperature results of cathodoluminescence (CL) spectroscopy investigations are presented for nanostructured titanium dioxide (anatase) thin films (500 nm thick) deposited via RF magnetron sputtering on high-purity silica substrates. The collected CL bands of the anatase thin films, as deposited and after different annealing cycles, showed a broad morphology consisting of three Voigtian bands located at 500, 550, and 610 nm that were partially overlapping. The overall CL emission increased with increasing temperature and time of the annealing cycle as a consequence of the increased crystallinity of the thin film. A clear trend was found for the oxygen-vacancy-related band (located at 610 nm), whose relative intensity decreased, as compared with the as-grown sample, after annealing in air; the higher the annealing temperature, the lower the relative intensity. We evaluated the photoactivity of the nanostructured thin film samples by measuring their photocatalytic activity in aqueous solution toward the degradation of phenol. A relationship between the decrease in oxygen vacancy concentration as a consequence of the annealing and the increase in the photoactivity was highlighted.

Vapor phase synthesis, characterization and gas sensing performances of Co3O4 and Au/Co3O4 nanosystems.

Al2O3-supported Co3O4 nanosystems were grown by a Chemical Vapor Deposition route under O2 + H2O atmospheres at 500 degrees C. Subsequently, the preparation of Au/Co3O4 composites was attained by Radio Frequency-Sputtering of gold onto the previous Co3O4 nanodeposits. Important data on the system structure, morphology and chemical composition were obtained by the combined use of complementary techniques, namely Glancing Incidence X-ray Diffraction, Field Emission-Scanning Electron Microscopy, Atomic Force Microscopy, Energy Dispersive X-ray Spectroscopy, X-ray Photoelectron Spectroscopy and Secondary Ion Mass Spectrometry. Finally, the gas sensing properties of the synthesized systems were probed in the detection of ethanol and hydrogen. The obtained results revealed significant responses already at moderate temperatures, which could be further enhanced by Co3O4 functionalization with Au nanoparticles.

Nanostructured copper oxide on silica-zirconia mixed oxides by chemical implantation.

N,N-Dialkylcarbamato complexes of copper(II), [Cu(O(2)CNR(2))(2)] (R = All = allyl, C(3)H(5); iPr, CH(CH(3))(2)) were prepared with the aim of functionalizing silica and nanostructured silica-zirconia matrices. The mixed matrices for the grafting reactions were prepared by copolymerizing MAPTMS (methacryloxypropyltrimethoxysilane), the precursor for the silica matrix, with the zirconium tetranuclear derivative [Zr(4)O(2)(OMc)(12)] (OMc = methacrylate), the precursor for the zirconia nanoparticles. Suspension of the silica and silica-zirconia matrices in a solution of the copper dialkylcarbamate led to the functionalization of the respective substrates. The composition, microstructure, morphology, and physicochemical nature of the copper species grafted on the matrices were investigated by FTIR, X-ray photoelectron spectroscopy (XPS), EPR, X-ray absorption spectroscopy (XAS), XRD, TEM, and dinitrogen adsorption. The effect of selected experimental parameters (the nature of the copper precursor and of the matrix, grafting time, thermal treatment) on the grafting reaction was investigated. The Cu/Si ratio is increased by increasing the grafting time and the ZrO(2)-SiO(2) matrix is more reactive to attack by the carbamato complexes than either prepared or commercial SiO(2). After functionalization of the matrix, thermal treatment yielded nanostructured copper(II) oxide clusters, average diameter 12-15 nm, uniformly supported on the silica and on the silica-zirconia matrices.

Rational design of Ag/TiO2 nanosystems by a combined RF-sputtering/sol-gel approach.

The present work is devoted to the preparation of Ag/TiO(2) nanosystems by an original synthetic strategy, based on the radio-frequency (RF) sputtering of silver particles on titania-based xerogels prepared by the sol-gel (SG) route. This approach takes advantage of the synergy between the microporous xerogel structure and the infiltration power characterizing RF-sputtering, whose combination enables the obtainment of a tailored dispersion of Ag-containing particles into the titania matrix. In addition, the system's chemico-physical features can be tuned further through proper ex situ thermal treatments in air at 400 and 600 degrees C. The synthesized composites are extensively characterized by the joint use of complementary techniques, that is, X-ray photoelectron and X-ray excited Auger electron spectroscopies (XPS, XE-AES), secondary ion mass spectrometry (SIMS), glancing incidence X-ray diffraction (GIXRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), electron diffraction (ED), high-angle annular dark field scanning TEM (HAADF-STEM), energy-filtered TEM (EF-TEM) and optical absorption spectroscopy. Finally, the photocatalytic performances of selected samples in the decomposition of the azo-dye Plasmocorinth B are preliminarily investigated. The obtained results highlight the possibility of tailoring the system characteristics over a broad range, directly influencing their eventual functional properties.

A cobalt(II) hexafluoroacetylacetonate ethylenediamine complex as a CVD molecular source of cobalt oxide nanostructures.

An adduct of Co(II) 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate with N,N,N',N'-tetramethylethylenediamine is synthesized by a simple procedure and, for the first time, thoroughly characterized by several analytical methods in order to elucidate its structure (single-crystal X-ray diffraction), chemical composition (elemental analyses, FT-IR), optical properties (UV-vis absorption spectroscopy), thermal behavior (thermogravimetric analysis and differential scanning calorimetry), and fragmentation pathways (electrospray ionization mass spectrometry and tandem mass spectrometry). The target complex is monomeric with a pseudo-octahedral Co(II) core and presents a clean decomposition pathway and a high volatility at moderate temperatures. Preliminary chemical vapor deposition (CVD) experiments highlight its very promising features as a CVD/atomic layer deposition molecular source for cobalt oxide nanosystems.

Gas sensing properties of columnar CeO2 nanostructures prepared by chemical vapor deposition.

Columnar CeO2 nanostructures are grown on alumina substrates by a template- and catalyst-free Chemical Vapor Deposition (CVD) approach and subsequently tested as resistive gas sensors of CH3COCH3, H2, NO2. The sensor response is stable and reproducible throughout the whole working temperature range (200-500 degrees C) and directly dependent on the analyte gas and the adopted operating conditions. The higher sensitivity with respect to that displayed by continuous CeO2 thin films demonstrates the potential of fabricating nanostructured sensing devices characterized by improved functional performances.

Structural investigations on the hydrolysis and condensation behavior of pure and chemically modified alkoxides. 2. Germanium alkoxides.

Structural investigations on the hydrolysis and condensation behavior of germanium alkoxides were for the first time performed by means of X-ray absorption fine structure and Raman spectroscopy. The studies reveal that germanium alkoxides are monomeric in nature and undergo very fast hydrolysis and condensation reactions upon water addition. However, the chelation of germanium alkoxides by acetylacetone does not take place even 48 h after mixing, and any change in hydrolysis and condensation behavior is not observed after acetylacetone addition. When mixed with prehydrolyzed silicon alkoxide, the structures of germanium alkoxides are not modified. Both Si and Ge precursors are insensitive to the presence of each other in the reaction solution even after 48 h of aging. The addition of water to this mixture catalyzes the hydrolysis and condensation reactions very fast and leads to the formation of Ge-O-Ge (and consequently Si-O-Si) homocondensation products.

Structural investigations on the hydrolysis and condensation behavior of pure and chemically modified alkoxides. 1. Transition metal (Hf and Ta) alkoxides.

Structural investigations on the hydrolysis and condensation behavior of hafnium and tantalum alkoxides were for the first time performed by means of X-ray absorption fine structure and Raman spectroscopy. The studies reveal that both of the alkoxides are dimeric in nature and instantaneously undergo hydrolysis and condensation reactions upon the addition of water. The results indicate that the chemical reaction of the alkoxides with acetylacetone occurs immediately with an increase from 6- to 8-fold coordination around the metal. As a consequence of the coordination by a bidentate ligand, hydrolysis and condensation reactions are hindered in solutions of the chemically modified Hf(OnBu)4 and Ta(OEt)5. Furthermore, the investigations demonstrate that the structure of metal alkoxides is not altered after mixing with prehydrolyzed silicon tetraethoxide, and even after 48 h, both of the species remain as separate entities in the mixture. The addition of water to this mixture starts the hydrolysis and condensation reactions instantaneously and leads to the formation of a M-O-M homocondensation product due to the different reactivity of the two alkoxides.

Structure and optical properties of silica-supported Ag-Au nanoparticles.

Bimetallic Ag-Au nanoparticles are synthesized by sequential deposition of Au and Ag on amorphous silica by Radio Frequency (RF)-sputtering under mild conditions. Specimens are thoroughly characterized by a multi-technique approach, aimed at investigating the system properties as a function of the Ag/Au content, as well as the evolution induced by ex-situ annealing under inert (N2) or reducing (4% H2/N2) atmospheres. The obtained results demonstrate the possibility to obtain Ag-Au alloyed nanoparticles with controllable size, shape, structure, and dispersion under mild conditions, so that the optical properties can be finely tuned as a function of the synthesis and thermal treatment conditions.

Structural characterization of nanocrystalline lanthanum oxyfluoride films obtained by chemical vapor deposition.

This study is devoted to a thorough structural and microstructural characterization of nanophasic LaOF-based thin films. The coatings were synthesized by Chemical Vapor Deposition (CVD) onto Si(100) substrates at growth temperatures between 250 and 500 degrees C, using La(hfa)3.diglyme (Hhfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; diglyme = bis(2-methoxyethyl)ether) as both La and F molecular source under nitrogen + wet oxygen atmospheres. The system structure and microstructure were investigated by Glancing Incidence X-ray Diffraction (GIXRD) and X-ray microdiffraction. The most relevant sample features, with particular attention to the phase composition as a function of the synthesis conditions, are critically discussed.

Synthesis of copper sulphide nanoparticles in carboxylic acids as solvent.

A novel method for the preparation of CuS nanoparticles based on the fast nucleation of the sulphide has been developed. The particles have been synthesized by reaction of thioacetic acid with water and copper carboxylates (acetate, propionate) in the corresponding carboxylic acid (acetic, propionic) as a solvent. The use of carboxylic acids presents several advantages: (i) the hydrolysis of the C-S bond is favoured thus producing a fast CuS supersaturation and a high nucleation rate; (ii) the mobility of the precursor molecules is limited so that nucleation events are favoured with respect to particle growth; (iii) the low dielectric constant of the medium stabilises the nanoparticles dispersion by reducing the critical coagulation concentration. The prepared nanoparticles were investigated by UV-Vis spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy and dynamic light scattering. The nanoparticle suspensions are clear and characterized by a blue-shifted adsorption edge with respect to bulk CuS. Light scattering measurements performed on acetic acid suspensions evidence the formation of monodispersed nanoparticles with an average diameter of about 5 nm.

Structural and magnetic properties of pure and Ca-doped LaCoO3 nanopowders obtained by a sol-gel route.

Pure and Ca-doped LaCoO3 nanopowders were prepared by a non-alkoxidic sol-gel route using cobalt(II) acetate, lanthanum(III) nitrate and calcium(II) acetate as oxide precursors. The structural evolution and magnetic properties of the samples were studied as a function of thermal treatments in air up to 1273 K. In particular, the microstructure and composition of the systems were analyzed by X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and X-ray Photoelectron Spectroscopy (XPS). Both pure and calcium-doped samples annealing at 973 K resulted in the formation of cubic LaCoO3 (average crystallite size <30 nm). This phase was fully retained in the calcium-doped materials even after annealing at higher temperatures, whereas a transition to the rhomboedral polymorph was detected in the pure samples at 1073 K. The magnetic behavior of the nanopowders was investigated as a function of temperature and applied field using both dynamic and static susceptibility measurements. Pure lanthanum cobaltite samples underwent a transition to an ordered state at 88 K, and their magnetic properties changed as a function of thermal treatments. As concerns calcium-doped samples, they ordered ferromagnetically at 171 and 185 K depending on the annealing temperature and displayed open hysteresis loops with coercive fields as large as 1.75 T at low temperatures.