Carlo G. Pantano

Structural investigation of silica gel films by infrared spectroscopy

Fourier transform infrared absorption spectroscopy has been utilized to characterize the structure of porous silica gel films, both deposited on c‐Si substrates and free standing. The films were either dried at room temperature or subjected to partial densification at 400–450 °C. The spectra of the gel films are compared to those of thermal SiO2 grown on c‐Si and to Kramers–Kronig analysis of the reflection spectra of bulk SiO2 gels and v‐SiO2. The gel films show small frequency shifts compared to the latter spectra and they also exhibit new bands due to the presence of OH groups, although very little molecular water or residual organic species were found. The results are interpreted in terms of the gel structure. Compared to the thermal oxide, the sharp peak near 1070 cm−1 is narrower for the gels and the spread in intertetrahedral angles is estimated at 24° and 27° for room temperature dried and partially densified gels, respectively, compared to 33° for the thermal oxide. This is in agreement with a st...

Silicon Oxycarbide Glasses

The first attempts to introduce carbon into glass date back to 1951. But up until recently, the use of carbon or carbide raw materials, and the oxidation, volatilization and decomposition that accompany high temperature melting, have limited the synthesis of true silicon oxycarbide glasses. Here, the term silicon-oxycarbide refers specifically to a carbon-containing silicate glass wherein oxygen and carbon atoms share bonds with silicon in the amorphous, network structure. Thus, there is a distinction between black glass, which contains only a second-phase dispersion of elemental carbon, and oxycarbide glasses which usually contain both network carbon and elemental carbon. In addition to exploring the unique properties and applications of these glasses, per se, they are also of interest for developing models of the residual amorphous phases in polymer-derived silicon-carbide and silicon-nitride ceramics.The application of sol/gel techniques to glass synthesis has significantly advanced the development and characterization of silicon oxycarbide glasses. In this approach, alkyl-substituted silicon alkoxides, which are molecular precursors containing oxygen and carbon functionalities on the silicon, can be hydrolyzed and condensed without decomposition or loss of the carbon functional group. A low-temperature (<1000°C) heat-treatment of the gel creates a glassy silicate material whose molecular structure consists of an oxygen/carbon anionic network. In addition, there is always a blackening of the material due to elemental carbon, which forms during pyrolysis and densification of the gel. The nature of the network carbon, and especially the distribution and form of the elemental carbon, are fundamental to the structure and properties of these novel materials. Their chemical and physical characteristics as revealed by NMR, Raman and TEM are discussed in the overview. In addition, the high temperature stability of these glasses (up to 1750°C), and the effect of hot-pressing, are described.It will be shown that the silicon oxycarbide network is stable up to 1000–1200°C. The network carbon is terminated with hydrogen (i.e., CH, =CH2 and –CH3), and with polyaromatic carbon (i.e., nC6Hx) wherein most of the elemental carbon resides. These glasses can be described as molecular composites of polyaromatic graphene-rings dispersed in a silicon oxycarbide network. After heating to temperatures in excess of 1000–1200°C, the oxycarbide network decomposes through the loss of hydrogen, and a two- or three-phase glass-ceramic consisting of nanocrystalline graphite, silicon carbide, and amorphous silica or cristobalite, is created. Some of the properties and applications of these glasses/glass-ceramics for coatings, composites and porous solids are summarized.

Synthesis and characterization of boron-doped carbons

Abstract Two methods have been employed to prepare boron-doped carbon materials. One method utilized ion implantation to dope the surface region of vitreous carbon with up to 12 atomic % boron. The other method used CVD to prepare ~ 1–2 micron thick carbon coatings with various boron contents (0 to 15 atomic %). The quantitative compositional analysis of these materials was performed with the electron microprobe and ion microprobe. Graphitization and redistribution of the boron, due to heat treatment, was examined with SIMS depth profiling and X-ray diffraction. Primarily, X-ray photoelectron spectroscopy (XPS) was used to characterize the bonding state of boron before and after heat treatment and oxidation. It was found that boron could accommodate as many as five different chemical structures in the CVD carbon films. Some of these were identified as mixed BO and BC species (e.g., , and one corresponded to the BC3 type of bonding reported by Kaner. Most interestingly, oxidation caused boron segregation at the surface and shifted the bonding environment of these boron atoms to lower binding energy (less local electron density).

Characterization of silica gels by infrared reflection spectroscopy

Abstract The FTIR specular reflection spectra of porous silica gels, either dried at room temperature or subjected to partial densification at 400°C, are significantly different from the spectrum of bulk vitreous silica. Namely, the high frequency SiOSi stretching vibrational peak near 1100 cm −1 exhibits shifts in the gels of up to 40 cm −1 towards lower frequencies, whereas the relative intensity of the companion high frequency shoulder simultaneously increases. The origin and possible structural meaning of the observed changes were investigated by Kramers-Kronig analysis of the experimental near-normal infrared reflectivity spectra of a series of silica xerogels, porous Vycor and bulk vitreous silica. Major differences were found for the energy loss function Im(−1/ϵ ∗ ), the peaks of which have been associated with the LO components of the dominant vibrational modes, and the largest red shifts corresponded to the highest frequency mode. Simple calculations show that the observed behavior of the infrared spectra could in part be due to a certain degree of strain in the SiOSi bridging bonds at the surface of the gel pores.

Dissolution of nepheline, jadeite and albite glasses: toward better models for aluminosilicate dissolution

Abstract SLB acknowledges many educational and entertaining conversations with Hal Helgeson (ranging from kinetics to bent head morphologies) over the last 17 years. To investigate the effects of changing the Al/Si ratio on plagioclase dissolution without complications of varying Na/Ca content or exsolution, three glasses with varying Al/Si ratios (albite, jadeite, and nepheline glasses) were synthesized and dissolved. Many similarities in dissolution behavior between plagioclase crystals and this suite of glasses were observed: 1) dissolution was slowest at near-neutral pH and increased under acid and basic conditions; 2) dissolution rate at all pH values increased with increasing Al/Si ratio; 3) the pH dependence of dissolution was higher for the phase with Al/Si = 1 than the phase with Al/Si = 0.3; 4) after acid leaching, the extent of Al depletion of the altered surface increased with increasing bulk Al/Si ratio from Al/Si = 0.3 (albite glass) to 0.5 (jadeite glass), but then decreased in nepheline glass (Al/Si = 1.0), which dissolved stoichiometrically with respect to Al; and 5) little to no Al depletion of the surface of any glass occurred at pH > 7. In contrast with some observations for plagioclase dissolution, however, log (rate) increased linearly with Al content, and n, the slope of the log (rate) − pH curve at low pH, varied smoothly from albite glass to jadeite glass to nepheline glass (n = −0.3, −0.6, and −1.0, respectively). These results, plus the observation that the slope calculated at high pH, m, did not differ between glasses (m = 0.4 ± 0.1), may be consistent with an identical mechanism controlling dissolution of albite, jadeite, and nepheline glasses, although no Si-rich layer can develop on nepheline because of the lack of SiOSi linkages. Such a conclusion is consistent with a transition state for these aluminosilicates at high pH consisting of a deprotonated Q3Si hydroxyl group (where Qvx refers to an x atom in a tetrahedral site with v bridging oxygens) or a five-coordinate Si site after nucleophilic attack by OH−. At low pH, bridging oxygens between Q4Si and Q4Al may be rate limiting if they are slower to hydrolyze than QvSiQwSi linkages (v,w≤ 3). According to this mechanism, dissolution rate increases from albite to jadeite to nepheline glass because hydrolysis of AlOSi bonds become more energetically favorable as the number of Al atoms per Si tetrahedron increases, a phenomenon documented here by geometry optimizations by use of ab initio methods. A model wherein Q4AlQ4Si linkages are faster to hydrolyze than lower connectivity linkages between Si atoms (QvSiQwSi, v,w≤ 3) may also explain aspects of this data. Further computational and experimental measurements are needed to distinguish the models.

X-ray photoelectron evidence for bacteria-enhanced dissolution of hornblende

An Arthrobacter species capable of extracting Fe from hornblende was isolated from a soil from the Adirondacks, NY (USA). This bacteria isolate, used in batch experiments with hornblende, accelerated the release of Fe from hornblende without measurably affecting Al release. The isolate produces both low molecular weight organic acids (LMWOA) and a catecholate siderophore. Polished hornblende (glass and crystal) discs were analyzed with X-ray photoelectron spectroscopy (XPS) before and after incubation with growing Arthrobacter sp. to investigate whether the bacteria caused a distinguishable chemical signature on the upper 100 A of mineral surface. After removal of the arthrobacter grown on hornblende crystal or glass substrates using lysozyme, XPS revealed surface depletion of Fe for samples grown for several days in buffered (crystal) and unbuffered (crystal and glass) media. Fe/Si ratios of hornblende surfaces dissolved under biotic conditions are significantly lower than Fe/Si ratios on surfaces dissolved under abiotic conditions for similar amounts of time. Enhanced Fe release and the formation of Fe-depleted surfaces is inferred to be caused by catechol complexation at the mineral surface. Because natural siderophore was not isolated in sufficient quantities to run bacteria-free leaching experi- ments, parallel investigations were run with a commercially available siderophore (desferrioxamine B). Desferrioxamine B was observed to enhance release of Fe, Si, and Al from hornblende both with and without added bacteria. Formation of desferrioxamine-Fe surface complexes were probed by studying the multiple splitting and shift in intensities of the N 1s line analyzed by XPS on siderophore 6 Fe on gold surfaces and siderophore 1 hornblende crystal surfaces. Based upon the observed formation of an hydroxamate (desfer- rioxamine) surface complex on hornblende, we infer that catecholate siderophores, such as those produced by the arthrobacter, also complex on the hornblende surface. Surface complexation is favored because of the extremely high association constants for siderophore 1 Fe(III). X-ray photoelectron spectroscopic data is therefore consistent with a model wherein enhanced Fe release by these bacteria or desferrioxamine B is caused by Fe-siderophore complexation at the silicate surface. Such complexation presumably weakens bonds between the Fe and the oxide lattice, causing enhanced Fe leaching and an Fe-depleted surface. Some leaching may also be due to LMWOA, although this is interpreted to be of secondary importance. Copyright

Dissolution of albite glass and crystal

Abstract When normalized by initial surface area, crystalline and amorphous albite release Si and Al at the same rate within error (±40%) as measured at pH 2, 5.6, and 8.4 at 25°C. Differences in density and tetrahedral ring structure between the glass and crystal structures, however, lead to more extensive Na and Al depletion from the glass surface, especially in acid. X-ray photoelectron spectroscopy (XPS) indicates that the chemistry of the altered layers on glass and crystal must be significantly different at a depth of ∼17A–87A. Nevertheless, angle-resolved XPS (ARXPS) indicates that the outermost 17A of the glass and crystal surface are compositionally similar. In neutral and weakly basic conditions, XPS indicates less extensive depletion of Na and Al from reacted glass and crystal surfaces than in acidic conditions. Al enrichment was not observed at any pH on either the crystal or glass surface. At steady state, Al release was stoichiometric for all phases and all pH values, but Na release was always faster than release of Si, especially for the glass. These results are consistent with a model where only the outer surface controls dissolution and the deeper layers of the altered surface do not significantly affect dissolution rate. The similarity in dissolution rate between glass and mineral, if consistent for other phases, may also indicate that some future studies of mineral dissolution could be completed more efficiently by investigation of glass because such studies could reveal the chemical effects in dissolution independent of the microstructure and defects that populate natural mineral samples.

Biomimetization of butterfly wings by the conformal-evaporated-film-by-rotation technique for photonics

Mimetization of biological structures aims to take advantage of their spatial features for the development of devices of tailored functionality. In this work, we replicated the wing of a butterfly at the micro- and nanoscales by implementing the conformal-evaporated-film-by-rotation (CEFR) technique. Chalcogenide glasses were used due to their good optical and mechanical properties. Morphological characterization and optical measurements indicate high-fidelity replication of the original biotemplate; furthermore, the optical properties of the butterfly wings have a structural origin. The CEFR technique might be useful for the fabrication of highly efficient, biomimetic optical devices.

Computer modeling of water adsorption on silica and silicate glass fracture surfaces

Molecular dynamics (MD) has been used to simulate water adsorption behavior on glass fracture surfaces. Energy minimization techniques were developed to obtain energy distributions of surface adsorption sites as well as a breakdown of site energies by site type. MD physisorption simulations were used to illustrate the role of the high-energy sites in the initial adsorption of water molecules onto the surface. It is shown that the strongest adsorption sites are associated with defects in the network, and not with modifier species. The modifier species also introduce water adsorption sites, but they are weaker than those associated with the network defects. Finally, surface hydroxylation is shown to greatly reduce the strength of fracture surface defect sites with respect to physisorption of water.

Formation and behavior of surface layers on electron emission glasses

Abstract The thermochemical reduction of thin surface layers on multicomponent lead-silicate glasses is fundamental to their use in electron multiplier and microchannel plate devices. These surface layers can exhibit a specific conductivity as high as 10 −2 (Ω cm) −1 and secondary electron yields up to 3.5. However, due to the complex processing used in the fabrication of the devices, a basic understanding of the chemical and structural surface characteristics responsible for these properties has not been established. Moreover, the effects of prolonged electron bombardment upon the chemical characteristics of the surface have not been extensively investigated, nor related to any associated degradation of the electron emission properties. In this study, the clean fracture surfaces of these glasses were investigated. The effects of hydrogen reduction, chemical etching, and prolonged electron bombardment were determined. Ion-scattering spectroscopy (ISS) was used for its monolayer sensitivity, especially to alkali species, while secondary ion mass spectroscopy (SIMS) provided depth profiles. The hydrogen profile created by the reduction could also be obtained with SIMS. X-ray photoelectron spectroscopy (XPS) was employed selectively to examine changes in the oxidation state of the surface species. It was found that the hydrogen reduction of these glasses creates a thin 20–50 nm silica-rich surface layer. The layer of reduced lead atoms is beneath this zone, and is visible to depths of the order 5 μm, but the hydrogen profiles which are found in these surfaces extend only 0.5 μm in depth. The electron bombardment of these surfaces leads to a decrease in concentration of alkali and lead in the surface monolayer, and to a change in the hydrogen profile. The cross-section for this bombardment-induced change in the surface composition correlates with the reported gain degradation in microchannel plate devices.