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PubMed2006

Amorphous solid water films: transport and guest-host interactions with CO2 and N2O dopants.

Guest-host interactions have been examined experimentally for amorphous solid water (ASW) films doped with CO2 or N2O. The main diagnostics are Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD). ASW films deposited at 90 K are exposed to a dopant, and the first molecules that attach to a film enter its bulk until it is saturated with them. Subsequent dopant adsorption results in crystal growth atop the ASW film. There are distinct spectral signatures for these two cases: LO and TO vibrational modes for the crystal overlayer, and an easily distinguished peak for dopant molecules that reside within the ASW film. Above 105 K, the dopant surface layer desorbs fully. Some dopants residing within the ASW film remain until 155 K, at which point the ASW-to-crystalline-ice transition occurs, expelling essentially all of the dopant. No substantial differences are observed for CO2 versus N2O. It is shown that annealing an ASW film to 130 K lowers the film's capacity to include dopants by a factor of approximately 3, despite the fact that the ASW spectral feature centered at approximately 3250 cm(-1) shows no discernible change. Sandwiches were prepared: ASW-dopant-ASW etc., with the dopant layer displaying crystallinity. Raising these samples past 105 K resulted in the expulsion of essentially all of the crystalline dopant. What remained displayed the same spectral signature as the molecules that entered the bulk following adsorption at the surface. It is concluded that the adsorption sites, though prepared differently, have a lot in common. Dangling OH bonds were observed. When they interacted with a dopant, they underwent a red shift of approximately 50 cm(-1). This is in qualitative agreement with studies that have been carried out with weakly bound binary complexes. As a result of this study, a fairly complete, albeit qualitative, picture is in place for the adsorption, binding, and transport of CO2 and N2O in ASW films.

Journal Article↗
PubMed2006

Synchrotron X-ray scattering study of the mechanism of nanopore generation in nanoporous organosilicate thin films imprinted with a reactive six-armed porogen.

In situ grazing incidence small-angle X-ray scattering analysis was successfully performed during the thermal processing of film blends of polymethylsilsesquioxane (PMSSQ) precursor and triethoxysilyl-terminated six-arm poly(epsilon-caprolactone) (mPCL6) porogen. In addition, thermogravimetric analysis of the films was carried out in a nitrogen atmosphere. These measurements provide important information about the structures of the blend films and of the resulting porous films. In particular, they are used in this paper to establish the mechanism of the formation of imprinted pores within the blend films. During the heating run, the sacrificial thermal degradation of the porogen component commenced at 320 degrees C, generating pores in the resulting cured PMSSQ films. Only very limited porogen aggregation occurred during the blend film formation process (spin-coating and subsequent drying), and these porogen aggregates were of relatively small size and narrow size distribution. The observed restriction of the formation of such porogen aggregates was found to result from the favorable hybridization reaction of the porogen's reactive end groups with the reactive functional groups of the PMSSQ precursor, which competes with aggregation via reaction between the porogen molecules. The average radius (or half-size) of the porogen aggregates was in the range 2.45-3.98 nm, depending on the porogen loading (10-40 wt %). The porogen aggregates retained their size and size distribution until thermal degradation, which resulted in the imprinting of nanopores in the cured PMSSQ films with size and size distribution corresponding to those of the porogen aggregates. The porosities of the resulting nanoporous films were in the range 12.4-41.7%, depending on the initial porogen loading.

Membranes, Artificial↗
PubMed2006

Double-layered TiO2-SiO2 nanostructured films with self-cleaning and antireflective properties.

Dual function of self-cleaning and antireflection can be created in double-layered TiO2-SiO2 nanostructured films. The film were prepared by (1) layer-by-layer deposition of multilayered SiO2 nanoparticles with polydiallyldimethylammonium (PDDA) cations, (2) layer-by-layer deposition of multilayered titanate nanosheets with polications on PDDA/SiO2 multilayer films, and (3) burning out the polymer and converting titanate nanosheets into TiO2 by hearing at 500 degrees C. The as-prepared films, consisting of a porous SiO2 bottom layer and a dense TiO2 top layer, improved the transmittance of glass or quartz substrates, as demonstrated by transmission spectra collected at normal incidence. The photocatalytic properties of the films were studied by the change of the water contact angle together with the decay of the IR absorption of the hydrocarbon chain of octadecylphosphonic-acid-modified films under 2.6 mW cm-2 UV illumination. Both the antireflective and the photocatalytic properties of the films were dependent on the number of PDDA/nanosheet bilayers deposited. however, excellent surface wettability of the films for water was obtained, independent of the preparation conditions. The experimental findings are discussed in terms of the special structure of the double-layered nanostructured film.

Catalysis↗
PubMed2005

Thermal decomposition of generation-4 polyamidoamine dendrimer films: decomposition catalyzed by dendrimer-encapsulated Pt particles.

The thermal decomposition of hydroxyl-terminated generation-4 polyamidoamine dendrimer (G4OH) films deposited on Au surfaces has been compared with decomposition of the same dendrimer encapsulating an approximately 40-atom Pt particle (Pt-G4OH). Infrared absorption reflection spectroscopy studies showed that, when the films were heated in air to various temperatures up to 275 degrees C, the disappearance of the amide vibrational modes occurred at lower temperature for the Pt-G4OH film. Dendrimer decomposition was also investigated by thermogravimetric analysis (TGA) in both air and argon atmospheres. For the G4OH dendrimer, complete decomposition was achieved in air at 500 degrees C, while decomposition of the Pt-G4OH dendrimer was completed at 400 degrees C, leaving only platinum metal behind. In a nonoxidizing argon atmosphere, a greater fraction of the G4OH decomposed below 300 degrees C, but all of the dendrimer fragments were not removed until heating above 550 degrees C. In contrast, Pt-G4OH decomposition in argon was similar to that in air, except that decomposition occurred at temperatures approximately 15 degrees C higher. Thermal decomposition of the dendrimer films on Au surfaces was also studied by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) under ultrahigh vacuum conditions. Heating the G4OH films to 250 degrees C during the TPD experiment induced the desorption of large dendrimer fragments at 55, 72, 84, 97, 127, 146, and 261 amu. For the Pt-G4OH films, mass fragments above 98 amu were not observed at any temperature, but much greater intensities for H(2) desorption were detected compared to that of the G4OH film. XPS studies of the G4OH films demonstrated that significant bond breaking in the dendrimer did not occur until temperatures above 250 degrees C and heating to 450 degrees C caused dissociation of C=O, C-O, and C-N bonds. For the Pt-G4OH dendrimer films, carbon-oxygen and carbon-nitrogen bond scission was observed at room temperature, and further decomposition to atomic species occurred after heating to 450 degrees C. All of these results are consistent with the fact that the Pt particles inside the G4OH dendrimer catalyze thermal decomposition, allowing dendrimer decomposition to occur at lower temperatures. However, the Pt particles also catalyze bond scission within the dendrimer fragments so that decomposition of the dendrimer to gaseous hydrogen is the dominant reaction pathway compared to desorption of the larger dendrimer fragments observed in the absence of Pt particles.

Journal Article↗
PubMed2005

Hygrothermal degradation of (3-glycidoxypropyl)trimethoxysilane films studied by neutron and X-ray reflectivity and attenuated total reflection infrared spectroscopy.

Thin films of organosilanes have great technological importance in the areas of adhesion promotion, durability, and corrosion resistance. However, it is well-known that water can degrade organosilane films, particularly at elevated temperatures. In this work, X-ray and neutron reflectivity (XR and NR) were combined with attenuated total reflection infrared (ATR-IR) spectroscopy to study the chemical and structural changes within thin films of (3-glycidoxypropyl)trimethoxysilane (GPS) after exposure for various periods of time to air saturated with either D2O or H2O at 80 degrees C. For NR and XR, ultrathin (approximately 100 A) films were prepared by spin-coating. Both D2O and H2O provide neutron scattering contrast with GPS. Variations in the neutron scattering length density (SLD) profiles (a function of mass density and atomic composition) with conditioning time were measured after drying the samples out and also swelled with H2O or D2O vapor at room temperature. For samples that were dried out prior to measurement, little or no change was observed for H2O conditioning up to 3.5 days, but large changes were observed after 30 days of conditioning. The range of conditioning time for this structural change was narrowed to between 4 and 10 days with XR. The SLD profiles indicated that the top portion of the GPS film was transformed into a thick low-density layer after conditioning, but the bottom portion showed little structural change. A previous NR study of as-prepared GPS films involving swelling with deuterated nitrobenzene showed that the central portion of the film has much lower cross-link density than the region nearest the substrate. The present data show that the central portion also swells to a much greater extent with water and hydrolyzes more rapidly. The chemical degradation mechanism was identified by IR as hydrolysis of siloxane bonds. For ATR-IR, GPS films were prepared by dip-coating, which resulted in a greater and more variable thickness than for the spin-coated samples. The IR spectra revealed an increase in vicinal silanol generation over the first 3 days of conditioning followed by geminal silanol generation. Thus, the structural change detected by NR and XR roughly coincided with the onset of geminal silanol generation. Finally, little change in the reflectivity data was observed for films conditioned with D2O at 80 degrees C for 1 month. This indicates that hydrolysis of Si-O-Si is much slower with D2O than with H2O.

Journal Article↗
PubMed2005

Adhesive transfer of thin viscoelastic films.

Micellar suspensions of acrylic diblock copolymers are excellent model materials for studying the adhesive transfer of viscoelastic solids. The micellar structure is maintained in films with a variety of thicknesses, giving films with a well-defined structure and viscoelastic character. Thin films were cast onto elastomeric silicone substrates from micellar suspensions in butanol, and the adhesive interactions between these coated elastomeric substrates and a rigid indenter were quantified. By controlling the adhesive properties of the film/indenter and film/substrate interfaces we were able to obtain very clean transfer of the film from the substrate to the portion of the glass indenter with which the film was in contact. Adhesive failure at the film/substrate interface occurs when the film/indenter interface is able to support an applied energy release rate that is sufficient to result in cavity nucleation at the film/substrate interface. Cavity formation is rapidly followed by delamination of the entire region under the indenter. The final stage in the transfer process involves the failure of the film that bridges the indenter and the elastomeric substrate. This film is remarkably robust and is extended to three times its original width prior to failure. Failure of this film occurs at the periphery of the indenter, giving a transferred film that conforms to the original contact area between the indenter and the coated substrate.

Journal Article↗
PubMed2004

Release of a dye from hydrogen-bonded and electrostatically assembled polymer films triggered by adsorption of a polyelectrolyte.

The absorption of dyes within hydrogen-bonded and electrostatically assembled multilayers and subsequent release of the dyes from the films were studied in situ using FTIR-ATR. Multilayers were composed of poly(methacrylic acid), PMAA, and poly(ethylene oxide), PEO (hydrogen-bonded multilayers), or of PMAA and 22% quarternized copolymer of N-ethyl-4-vinylpyridium bromide and 4-vinylpyridine, Q22 (electrostatically stabilized multilayers). After multilayer deposition, the solution pH was changed to produce excess charge within the films. Dyes with charge opposite to the excess charge of the film (Rhodamine 6G for hydrogen-bonded films or Bromophenol Blue for electrostatically assembled multilayers) were then allowed to absorb within multilayers. In both systems, the dyes were uniformly included within the films. The top layers largely affected the loading capacity of the multilayers, suggesting weaker binding of the dyes with the top layers. Dye release into a 0.01 M phosphate buffer was significantly smaller as compared to release in the presence of 0.05-0.5 mg/mL solutions of adsorbing polymers whose charge was the same as the excess charge within the films. We found that with the PMAA/PEO films, dye release did not depend on the concentration of polymer in solution, but was largely controlled by the amount of charge accumulated within the adsorbing polymer layer on the top of the film. For electrostatically stabilized PMAA/Q22 systems, dye release increased with increasing concentration of Q22 in solution, suggesting a significant contribution of the competition of solution species in the release mechanism. Our findings contribute to the understanding of interactions of small molecules with polymer multilayers and might have ramifications for novel applications of multilayer films as new materials for the controlled delivery of chemicals.

Journal Article↗
PubMed2004

Surface modification of an ethylene-acrylic acid copolymer film: grafting amine-terminated linear and branched architectures.

Polymer films can be tailored for a specific application by modifying their surface properties. In this study, linear and branched architectures were grafted to ethylene-acrylic acid (EAA) copolymer films using the so-called grafting from approach. Dicyclohexylcarbodiimide was used to activate the carboxylic acid functionality on the surface of the EAA copolymer film before reacting it with selected di- and tri-amine compounds. The carboxylic acid functionality was subsequently regenerated by reacting the amine-grafted film with succinic anhydride. These reaction steps were then repeated to create the linear and branched architectures on the EAA film surface. The film surface resulting from each reaction step was analyzed using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and contact angle measurements. A systematic analysis of the ATR-FTIR results was performed to estimate the average conversion of the reaction schemes and to explain the observed contact angle results. A significant reduction in water contact angle for the EAA film grafted with a branched architecture was observed. The EAA film grafted with a linear architecture showed a marginal reduction in water contact angle when ethanol was used as a solvent for ethylenediamine. When the solvent for ethylenediamine was changed to water, the contact angle decreased noticeably. However, analysis of control films showed that the reduction in the contact angles was due to the solvent treatment. In the case of branched architectures, such reduction in contact angle due to the solvent treatment was not observed. Several control experiments were performed to ensure that the reduction in the contact angles was in fact due to the grafted species and not due to exposure to various solvents used in the reaction scheme.

Journal Article↗
PubMed2004

Walljet electrochemistry: quantifying molecular transport through metallopolymeric and zirconium phosphonate assembled porphyrin square thin films.

By employing redox-active probes, condensed-phase molecular transport through nanoporous thin films can often be measured electrochemically. Certain kinds of electrode materials (e.g. conductive glass) are difficult to fabricate as rotatable disks or as ultramicroelectrodes-the configurations most often used for electrochemical permeation measurements. These limitations point to the need for a more materials-general measurement method. Herein, we report the application of walljet electrochemistry to the study of molecular transport through model metallopolymeric films on indium tin oxide electrodes. A quantitative expression is presented that describes the transport-limited current at the walljet electrode in terms of mass transport through solution and permeation through the film phase. A comparison of the film permeabilities for a series of redox probes measured using the walljet electrode and a rotating disk electrode establishes the accuracy of the walljet method, while also demonstrating similar precision for the two methods. We apply this technique to a system consisting of zirconium phosphonate assembled films of a porphyrinic molecular square. Transport through films comprising three or more layers is free from significant contributions from pinhole defects. Surprisingly, transport through films of this kind is 2-3 orders of magnitude slower than through films constructed via interfacial polymerization of nearly identical supramolecular square building blocks (Keefe; et al. Adv. Mater. 2003, 15, 1936). The zirconium phosphate assembled films show good size exclusion behavior. The details of the observed dependence of permeation rates on probe molecule size can be rationalized with a model that assumes that the walls of the squares are slightly tilted from a strictly vertical geometry, consistent with atomic force microscopy measurements, and assumes that the individual wall geometries are locked by rigid interlayer linkages.

Journal Article↗
PubMed2005

Perturbation of nanoscale structure of polypeptide multilayer thin films.

Multilayer thin films formed by sequential deposition of oppositely charged polypeptides on a charged surface are known from previous studies to comprise a mixture of types of secondary structure. Here, study of the perturbation of polypeptide film structure by deposition of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate) (PSS) on the film surface has revealed differences in behavior attributable to physical properties of the peptides. The methods of analysis were circular dichroism spectroscopy (CD), ultraviolet spectroscopy (UVS), and quartz crystal microbalance (QCM). Films made of poly(L-lysine) (PLL) and poly(L-glutamic acid) (PLGA) with an average charge per monomer of about 1 were substantially more susceptible to perturbation of structure than films made of designed polypeptides with an average charge per monomer of about 0.5, despite preparation under identical conditions. PLL-PLGA films showed loss or gain of material and change in secondary structure content on perturbation, whether made of high molecular mass (ca. 90 kDa) or low molecular mass (ca. 14 kDa) polymers. By contrast, films made of very low molecular mass (ca. 3.5 kDa) designed polypeptides showed little change in secondary structure content. The data suggest that the penetrability of PSS or PAH into a film and therefore film density can depend substantially on the polypeptides of which it is made and the character of intermolecular interactions.

Amino Acid Sequence↗
PubMed2005

Stimuli-sensitive thin films prepared by a layer-by-layer deposition of 2-iminobiotin-labeled poly(ethyleneimine) and avidin.

Layered thin films composed of avidin and 2-iminobiotin-labeled poly(ethyleneimine) (ib-PEI) were prepared by a layer-by-layer deposition of avidin and ib-PEI on a solid surface, and the disintegration induced by changing environmental pH and adding biotin in the solution was studied. The avidin/ib-PEI layered film could be deposited only from the solutions of pH 10-12. The film did not form in pH 9 or more acidic media because of a low affinity of protonated 2-iminobiotin residues in ib-PEI to avidin. The avidin/ib-PEI layered films were stable in pH 8-12 solutions, while in pH 5-7 media the film decomposed spontaneously as a result of the protonation to 2-iminobiotin residues in ib-PEI. The avidin/ib-PEI films were disintegrated also upon addition of biotin and analogues in the solution owing to the preferential binding of biotin or analogues to the binding site of avidin. The decomposition rate was arbitrarily controlled by changing the type of stimulant (biotin or analogues) and its concentration. The avidin/ib-PEI films were disintegrated rapidly by addition of 10(-)(5) M of biotin or desthiobiotin, while the rate was slower upon adding the same concentration of lipoic acid or 2-(4'-hydroxyphenylazo)benzoic acid. On the other hand, the film was fully decomposed within 1 min in the 10(-)(3) M lipoic acid or 2-(4'-hydroxyphenylazo)benzoic acid solution. Thus, the decomposition rate is highly dependent on the concentration of the stimulants. It was observed that the stimuli-induced decomposition of the films is slow at pH 12, in contrast to a rapid decomposition in pH 8 medium due to a low affinity of the protonated 2-iminobiotin to avidin. The present system may be useful for constructing stimuli-sensitive devices that can release drug or other functional molecules.

Avidin↗
PubMed2005

Covalently networked monolayer-protected nanoparticle films.

Covalently networked films of nanoparticles can be assembled on various substrates from functionalized monolayer-protected clusters (MPCs) via ester coupling reactions. Exposure of a specifically modified substrate to alternating solutions of 11-mercaptoundecanoic acid exchanged and 11-mercaptoundecanol exchanged MPCs, in the presence of ester coupling reagents, 1,3-dicyclohexylcarbodiimide and 4-(dimethylamino)pyridine, results in the formation of a multilayer film with ester bridges between individual nanoparticles. These films can be grown in a controlled manner to various thicknesses and exhibit certain properties that are consistent with films having other types of interparticle connectivity, including chemical vapor response behavior and quantized double layer charging. Ester coupling of MPCs into assembled films is a straightforward and highly versatile approach that results in robust films that can endure harsher chemical environments than other types of films. The stability of these covalent films is assessed and compared to other more traditional MPC film assemblies.

Journal Article↗
PubMed2006

Interaction of alcohols and ethers with a-CF(x) films.

The surfaces of the magnetic data storage hard disks used in computers are coated with a thin film of amorphous carbon and a layer of perfluoropolyalkyl ether (PFPE) lubricant. Both protect the surface of the magnetic layer from contact with the read-write head flying over the disk surface. Although the most commonly used carbon films are amorphous hydrogenated carbon, a-CH(x), it has been suggested that the thermal properties of amorphous fluorinated carbon films, a-CF(x), might be superior. This work has probed the interaction of small fluorinated ethers and alcohols with the surfaces of a-CF(x) films to understand the effects of carbon film fluorination on the interaction of the lubricant with its surface. Temperature-programmed desorption was used to measure the desorption energies of small fluorocarbons from the a-CF(x) surface and to compare their desorption energies with those from the surfaces of a-CH(x) films. These measurements reveal that, similarly to a-CH(x) films, a-CF(x) films expose a heterogeneous surface on which fluorocarbons adsorb at sites with a range of binding energies. The fluorocarbon ethers all have lower heats of adsorption than their hydrocarbon counterparts, suggesting that the ethers adsorb by donation of electron density from the oxygen lone-pair electrons to sites on the surface. Fluorinated alcohols have roughly the same heats of adsorption as their hydrocarbon counterparts. There is little significant difference between the interactions of fluorinated ethers (or alcohols) with the surfaces of either a-CF(x) or a-CH(x) films.

Journal Article↗
PubMed2006

Thin film drainage: hydrodynamic and disjoining pressures determined from experimental measurements of the shape of a fluid drop approaching a solid wall.

Accurate measurements of the shape of a mercury drop separated from a smooth flat solid surface by a thin aqueous film reported recently by Connor and Horn (Faraday Discuss. 2003, 123, 193-206) have been analyzed to calculate the excess pressure in the film. The analysis is based on calculating the local curvature of the mercury/aqueous interface, and relating it via the Young-Laplace equation to the pressure drop across the interface, which is the difference between the aqueous film pressure and the known internal pressure of the mercury drop. For drop shapes measured under quiescent conditions, the only contribution to film pressure is the disjoining pressure arising from double-layer forces acting between the mercury and mica surfaces. Under dynamic conditions, hydrodynamic pressure is also present, and this is calculated by subtracting the disjoining pressure from the total film pressure. The data, which were measured to investigate the thin film drainage during approach of a fluid drop to a solid wall, show a classical dimpling of the mercury drop when it approaches the mica surface. Four data sets are available, corresponding to different magnitudes and signs of disjoining pressure, obtained by controlling the surface potential of the mercury. The analysis shows that total film pressure does not vary greatly during the evolution of the dimple formed during the thin film drainage process, nor between the different data sets. The hydrodynamic pressure appears to adjust to the different disjoining pressures in such a way that the total film pressure is maintained approximately constant within the dimpled region.

Journal Article↗
PubMed2006

Stimuli-responsive properties of N-isopropylacrylamide-based ultrathin hydrogel films prepared by photo-cross-linking.

To develop stimuli-responsive ultrathin polymer films on a solid substrate, a novel photo-cross-linkable polymer with both temperature- and pH-responsive properties was prepared. The photoreactive 4-aminobenzophenone (BP) was introduced onto the side groups of poly(N-isopropylaclylamide-co-2-carboxyisopropylaclylamide) [poly(NIPAAm-co-CIPAAm)]. This copolymer was designed for highly random sequences of comonomers. After the formation of spin-coated polymer films on a solid substrate, UV-light irradiation started the cross-linking reaction. The spin-coating processes and stability of the polymer films were quantitatively monitored by a quartz crystal microbalance (QCM), and the thickness was estimated using an atomic force microscope (AFM). These measurements confirmed the formation of a very plain polymer film, and the film thickness was precisely controlled by the concentration of the polymer solution used for spin coating. Moreover, the obtained films showed a high stability due to the cross-liking reaction and UV irradiation. Cyclic voltammetry using potassium ferricyanide revealed that the ions could permeate the photo-cross-linked ultrathin polymer films. The permeability of the ultrathin hydrogel films was dramatically changed by varying the pH and temperature of the aqueous media. These observations suggest that the preparation of isopropylacrylamide-based stimuli-responsive ultrathin hydrogel films is possible.

Journal Article↗
PubMed2006

Electrochemical preparation and structural characterization of Co thin films and their anomalous IR properties.

Nanometer scale cobalt thin films of different structures and thicknesses supported on glassy carbon were prepared by electrochemical deposition under cyclic voltammetric conditions (denoted nm-Co/GC(n)). The thickness of Co thin films was altered systematically by varying the number (n) of potential cycling within a defined potential range in electrodeposition. Electrochemical in situ scanning tunneling microscopy (STM) and ex situ scanning electron microscopy (SEM) were employed to characterize the surface structure of Co thin films. It has been illustrated that the Co thin films were uniformly composed of Co nanoparticles, whose structure and size varied with increasing n. The structure of nanoparticles inside the Co thin films underwent a transition from bearded nanoparticles to multiform nanoparticles and finally to hexagonal nanosheets, accompanying with an increase of average size. In situ FTIR reflection spectroscopic studies employing CO adsorption as probe reaction revealed that the Co thin films all exhibited anomalous IR properties; that is, along with their different nanostructures they presented abnormal IR effects, Fano-like IR effects, and surface-enhanced IR absorption effects. CO adsorbed on Co thin films dominated by bearded nanoparticles yielded abnormal IR absorption bands; that is, the direction of the bands is inverted completely, with enhanced intensity in comparison with those of CO adsorbed on a bulk Co electrode. The enhancement of abnormal IR absorption has reached a maximal value of 26.2 on the nm-Co/GC(2) electrode. Fano-like IR features, which describe the bipolar IR bands with their positive-going peak on the low wavenumbers side, were observed in cases of CO adsorbed on Co thin films composed mainly of multiform nanoparticles, typically on the nm-Co/GC(8) electrode. IR features were finally changed into surface-enhanced IR absorption as CO adsorbed on the nm-Co/GC(30) electrode, on which the Co thin film is dominated by Co hexagonal nanosheets.

Cobalt↗
PubMed2006

Ozone- and thermally activated films of palladium monolayer-protected clusters for chemiresistive hydrogen sensing.

Here we describe the chemiresistive H2-sensing properties of drop-cast films comprised of 3.0 nm average diameter hexanethiolate-coated Pd monolayer-protected clusters (C6 Pd MPCs) bridging a pair of electrodes separated by a 23 microm gap. The gas-sensing properties were measured for 9.6-0.11% H2 in a H2/N2 mixture. The sensing mechanism is based on changes in the resistance of the film upon reaction of Pd with H2 to form PdH(x), which is known to be larger in volume and more resistive than pure Pd. As-prepared Pd MPC films are highly insensitive to H2, requiring O3 and thermal treatment to enhance changes in film resistance in the presence of H2. Exposure to O3 for 15 min followed by activation in 100% H2 leads to an increase in film conductivity in the presence of H2, with a detection limit of 0.11% H2. When exposed to temperatures of 180-200 degrees C, the conductivity of the film increases and a decrease in conductivity occurs in the presence of H2 with a detection limit of 0.21%. The sensing behavior reverses after further heating to 260 degrees C, exhibiting an increase in conductivity in the presence of H2 as in O3-treated films and a detection limit of 0.11%. The sensitivity of the variously treated films follows the order O3 > high temp > low temp, and the response times at 1.0% H2 range from 10 to 50s, depending on the treatment. FTIR spectroscopy, Raman spectroscopy, and atomic force microscopy provide information about the C6 monolayer, Pd metal, and film morphology, respectively, as a function of O3 and heat treatment to aid in understanding the observed sensing behavior. This work demonstrates a simple chemical approach toward fabricating a fast, reversible sensor capable of detecting low concentrations of H2.

Journal Article↗
PubMed2002

A functionally graded titanium/hydroxyapatite film obtained by sputtering.

A functionally graded film of titanium/hydroxyapatite (HA) was prepared on a titanium substrate using a radio frequency magnetron sputtering. The ratio of titanium to HA was controlled by moving the target shutter. The film was composed of five layers, with overall film thickness of 1 microm. The HA was concentrated close to the surface, while the titanium concentration increased with proximity to the substrate. The bonding strength between the film and the substrate was 15.2 MPa in a pull-out test and the critical load from a scratch test was 58.85 mN. The corresponding values of a pure HA sputtered film were 8.0 MPa and 38.47 mN, respectively. The bonding strength of a pure HA plasma spray coating was 10.4 MPa in the pull-out test. The graded film and the pure HA film were sputter-coated to a thickness of 1 microm on titanium columns (10 mm in length and 4 mm in diameter). These columns were implanted in diaphyses of the femora of six adult dogs and a push-out test was carried out after 2, 4, and 12 weeks. After 12 weeks, the push-out strengths of the graded film, the pure HA film and the non-coated columns were 3.7, 3.5, and 1.0 MPa.

Journal Article↗