Aqueous Hydrofluoric Acid Solution Research Articles

Synthesis of hollow fluorescent polymeric microspheres with movable magnetic cores

AbstractLow‐dispersity hollow poly[(ethyleneglycol dimethacrylate)‐co‐(N‐vinylcarbazole)] (P(EGDMA‐co‐NVCz)) fluorescent microspheres with movable magnetic cores were prepared by quantitative removal of the silica component in hydrofluoric acid aqueous solution from the corresponding magnetite/silica/P(EGDMA‐co‐NVCz) tri‐layer hybrid microspheres. The tri‐layer microspheres with magnetic cores and fluorescent polymer shells were synthesized by distillation precipitation copolymerization of ethyleneglycol dimethacrylate crosslinker and N‐vinylcarbazole fluorescent monomer in acetonitrile in the presence of 3‐(methacryloxy)propyltrimethoxysilane‐modified magnetite/silica particles as seeds, which were afforded by a modified Stöber sol–gel process, to coat the magnetite cores. The corresponding hollow fluorescent polymer microspheres with movable magnetic cores were studied using transmission electron microscopy, Fourier transform infrared spectroscopy, vibrating sample magnetometry, UV‐visible spectroscopy and fluorescence spectroscopy. Copyright © 2010 Society of Chemical Industry

Synthesis of ellipsoidal hematite/polymer/titania hybrid materials and the corresponding hollow ellipsoidal particles

Ellipsoidal tri-layer hematite/poly(N,N′-methylenebisacrylamide-co-methacrylic acid)/titania (Fe2O3/P(MBA-co-MAA)/TiO2) hybrid materials were prepared by a three-stage reaction process, which included the combination of the controlled hydrolysis of inorganic salt to afford ellipsoidal hematite core and titania shell layer and distillation precipitation copolymerization of N,N′-methylenebisacrylamide (MBA) and methacrylic acid (MAA) in acetonitrile for the formation of a P(MBA-co-MAA) sandwiched mid-layer. The corresponding hollow ellipsoidal hematite-sphere-in-titania-spheres were afforded by selective removal of the sandwiched P(MBA-co-MAA) mid-layer viacalcination. Hollow titania ellipsoids were further obtained by selectively etching the hematite core in aqueous hydrofluoric acid (HF) solution from the ellipsoidal hematite-sphere-in-titania-sphere. The resultant ellipsoidal tri-layer Fe2O3/P(MBA-co-MAA)/TiO2 and the corresponding hollow ellipsoidal particles were confirmed by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy, X-ray diffraction (XRD), thermogravimetric (TGA) analysis and vibrating sample magnetometery (VSM).

Crystal structures of crown and aza-crown ether complexes with zirconium, hafnium, niobium, and tantalum fluorometallates

A systematic X-ray diffraction study of the interaction products of Zr(IV), Hf(IV), Nb(V), and Ta(V) oxides (fluorides) with crown-ethers (CEs) in aqueous solutions of hydrofluoric acid is performed. It is shown that oxygen-containing CEs form oxonium complexes with [NbF6]s- and [TaF6]s- hexafluorometallate anions. In two systems, [cis-syn-cis-DCH18C6-H3O][TaF6] and [B18C6·H3O][TaF6], the phenomenon of supramolecular isomerism is found, which is caused by a change in the conformation of the macrocycle or by a partial redistribution of intermolecular hydrogen bonds. The use of aza-crown ethers as extractants made it possible to extract unique hydrolytically unstable anions, the products of incomplete fluorine substitution for oxygen atoms in the starting oxides in the form of crystalline complexes with a composition of [(HA15C5)2][Ta2F10O] and [(HA18C6·H2O)(A18C6·H2O)] [(H2O)Nb2F9O]. In [(18C6)(H7O3)2×(Hf2F10·2H2O)], [(HA18C6)(M2F10·2H2O)·(H3O)·H2O], and [(H2DA18C6) (M2F10·2H2O)·2H2O] (M=Zr, Hf) complexes, the metals are extracted in the form of identical (M2F10·2H2O)2s- anions with a similar topology. The performed study demonstrates that macrocyclic complexones are undoubtedly promising to extract Zr(IV), Hf(IV), Nb(V), and Ta(V) from fluorine-containing aqueous solutions.

Effect of Stannous Fluoride and Dilute Hydrofluoric Acid on Early Enamel Erosion over Time in vivo

Recent experimental in vivo studies have shown that aqueous solutions of stannous fluoride (SnF₂) and hydrofluoric acid (HF) can reduce enamel solubility after 5 min. The aim of this study was to evaluate the longer-term protective effect of SnF₂ (0.78%, pH 2.9) and HF (0.2%, pH 2.0) (both ∼0.1 mol/l F) using the same experimental model. Labial surfaces of healthy anterior teeth (all four surfaces when possible, otherwise a pair of surfaces) in 103 subjects (n = 399 teeth) were exposed to citric acid (0.01 mol/l, pH 2.7). The acid was applied using a peristaltic pump (5 ml, 6 ml/min) and was collected in coded test tubes (etch I). The test solutions were then applied to the same surfaces of the teeth (1 min, 6 ml/min). After either 1, 7, 14 or 28 days, citric acid was again applied to the same surfaces and subsequently collected (etch II). Enamel solubility was examined by assessment of calcium concentration in etch I and etch II solutions using atom absorption spectroscopy. Median values were calculated for all time periods and statistical analysis was carried out using the Wilcoxon signed-ranks test. Results showed that HF reduced enamel solubility by 54 and 36% after 1 and 7 days, respectively. After 14 and 28 days, there was no longer any effect. SnF₂ showed no protective effect after the first day. Given these results, repeated application of HF and especially SnF₂ may be necessary to improve the protective effect of these fluorides, and this requires further testing.

The interior interfaces of a semiconductor/metal nanocomposite and their influence on its physical properties

AbstractA semiconductor/metal hybrid system with silicon as base material and embedded metal nanostructures is fabricated in two steps by anodizing the silicon substrate in an aqueous hydrofluoric acid solution and a subsequent electrodeposition process. The achieved self‐assembled nanoscopic hybrid system consists of a quasi‐regular 3‐dimensional arrangement of metal nanostructures embedded within the pores of the silicon matrix. The used metals are Ni, Co and their alloys, whereas mainly Ni is considered in the following. In such a hybrid system the interface between metal and silicon nanostructures is of interest in correlation with the properties of the material and can be influenced by different fabrication conditions. If the matrices are aged in ambient air before filling with a metal the inner surface of the porous silicon template offers a native SiOx layer due to the oxidation of the hydrogen terminated surface of the pore‐walls. Using as‐etched samples for deposition of the metal into the pores leads to the formation of a more complex interface during the cathodic deposition procedure which is observed by FTIR‐spectroscopy. Knowledge about the composition and the formation of the interior surface of the porous silicon matrix as well as the interface between semiconductor and metal is necessary on the one hand for interpretation of the gained experimental results and on the other hand to figure out possible applications of the fabricated hybrid system. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of antireflection nanostructure for silicon solar cells using catalysis of single nano-sized silver particle

Antireflection nanostructure was formed by simple wet chemical etching using catalysis of silver (Ag) nanoparticle. Single nano-sized Ag particle dispersion solution was coated onto Si(1 0 0) substrate with polished surface. Then, the samples were soaked in an aqueous etching solution of hydrofluoric acid and hydrogen peroxide. The surface of 9-min-etched Si substrate appeared black, and the reflectivity was reduced to below 5% throughout the entire spectrum from 200 to 1000 nm owing to the formed nanostructure. The absorption was significantly increased after the formation of antireflection structure with 9 min etching, and the conversion efficiency of solar cell with antireflection structure increased from 8.25 to 10.0% owing to the increase of short-circuit current.

Transition metals specifically electrodeposited into porous silicon

AbstractPorous silicon (PS) is fabricated by anodization in aqueous hydrofluoric acid solution in the transition region between meso and macro pores. In the employed process the pore‐diameters are adjustable from 30 nm up to 100 nm using the same electrolyte as well as the same doping density of the wafer. Furthermore the self‐assembled pore‐arrangements show a quasi‐regular three‐dimensional nano‐architecture. These achieved PS‐membranes offering oriented pores with an aspect ratio around 1000 are filled with a transition metal in a galvanic deposition process. The use of different metals (Ni, Co, NiCo, Cu) together with the proper variation of the deposition parameters (e.g. current density and pulse duration) as well as the employment of different morphologies of the templates allow to tune the shape of the metal precipitations as well as their spatial distribution within the channels and therefore the possibility arises to influence the physical properties of the specimens. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of laser microfabrication on silicon electrochemical behavior in HF solution

Influence of direct laser writing with femtosecond pulses on electrochemical etching of n-type low conductivity (>1,000 Ωcm) silicon is demonstrated. It has been shown that thermal 1-µm-thick SiO2 layer on silicon surface can be used as a protective layer in the electrochemical etching process. It has been found that laser ablation changes not only the surface morphology and structure of silicon samples but also the character of their anodic etching in aqueous solution of hydrofluoric acid. Formation of microvoids and caverns of irregular shape has been observed at the laser-ablated sites. It is proposed that the change of silicon conductivity from n- to p-type takes place at the laser fabricated regions. Processes of Si anodic oxidation and electrochemical etching are discussed.

STICKY AND SLIPPY: TWO HYDROPHOBIC STATES TRANSITION OF ZnO HIERARCHICAL FILMS BY FLUOROALKYLSILANE MODIFICATION

ZnO films with hierarchical structures were prepared in an aqueous solution of zinc nitrate, hexamethylenetetramine and hydrofluoric acid. They demonstrated superhydrophobicity and two superhydrophobic states conversion from sticky to slippy after they were modified by fluoroalkylsilane (FAS) self-assemble layers. Before FAS modification, ZnO hierarchical structures, having sphere-like microstructures with nano-petals, showed water contact angle of 151° and a sticky behavior: water droplets rest on such surface did not slide when the surface was tilted to any angle or even upside down, indicating a strong adhesion between water and the surface. However, after FAS modification, ZnO films showed an increase of water contact angle up to 164° and a large difference in dynamic wetting properties: water droplets moved spontaneously and hardly came to rest even on a horizontal surface. The transition from sticky ("Wenzel's state") to slippy ("Cassie's state") resulted from surface free energy change from high to low. The results also confirm that the low enough surface energy, cooperating with surface roughness, is crucial in making Cassie's state superhydrophobicity. The realization of conversion from sticky to slippy is of importance for further understanding wetting properties and applications of ZnO micronanostructures.

Fabrication and chemical surface modification of nanoporous silicon for biosensing applications

In this work, porous silicon (PS) films with varied porosity (68–82%) were formed on the p-type, boron-doped silicon wafer (100) by the electrochemical anodisation in an aqueous hydrofluoric acid and isopropyl alcohol solution at different current densities (I d) ranging from 20–70 mA cm−2, respectively. Biofunctionalisation of the PS surface was carried out by chemically modifying the surface of PS by the deposition of 3-aminopropyltriethoxysilane thermally leading to high density of amine groups covering the PS surface. This further promotes the immobilisation of immunoglobulin (human IgG and goat anti-human IgG binding) on to the PS surface. Formation of nanostructured PS and the attachment of antibody–antigen to its surface were characterised using photoluminescence (PL), Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy techniques, respectively. The possibility of using these structures as biosensors has been explored based on the significant changes in the PL spectra before and after exposing the PS optical structures to biomolecules. These experimental results open the possibility of developing optical biosensors based on the variation of the PL position of the PL spectra of PS-based devices.

Laser processing of sapphire by strongly focused femtosecond pulses

The high-temperature annealing (1500°C for 15 min) of the shock-amorphized and compressed sapphire around the voids rendered the amorphous sapphire insoluble in aqueous solution of hydrofluoric acid. There were no observable back relaxation and reflow of the compressed amorphous phase. The overlapping of femtosecond pulses by >70% of the focal diameter also made the photomodified regions insoluble in aqueous solution of hydrofluoric acid. Femtosecond laser cutting of hard dielectric materials is discussed.

Intense photoluminescence of thin films of porous hydrogenated microcrystalline silicon

We report the preparation of highly photoluminescent porous silicon in the form of thin films with thicknesses of around 1 μm. These films are obtained by stain etching thin films of microcrystalline silicon deposited by plasma-enhanced chemical vapor deposition with a flow of silane gas (SiH4) that is highly diluted in hydrogen. Stain etching is performed with aqueous solutions of hydrofluoric acid and Fe(III) in the form of ferric chloride (FeCl3). The porous silicon obtained shows a strong photoluminescence with a quantum efficiency estimated in the 1%–10% range. It could be used for the design of inexpensive large-surface electroluminescent devices.

Antireflection subwavelength structure of silicon surface formed by wet process using catalysis of single nano-sized gold particle

Subwavelength structure (SWS) was formed by simple wet chemical etching using catalysis of gold (Au) nanoparticle. Single nano-sized Au particle dispersion solution was coated onto silicon (Si)(1 1 1) substrate with polished surface. Then, the samples were soaked in an aqueous etching solution of hydrofluoric acid and hydrogen peroxide. The surface of 15-min-etched Si substrate appeared black. The reflectivity of the Si substrate was reduced to below 5% throughout the entire spectrum from 300 to 800 nm owing to SWS. The fractional area occupied by Si as a function of the depth across the textured layer showed a smooth increase of density up to a depth. The observed optical effects were explained by the formation of a subwavelength scale taper structure, representing an effective medium with a smooth transition of the refractive index from air to Si.

Representation of Excess Properties and Liquid Composition of Aqueous Solutions of the HF+Water System

The electrolyte-NRTL model was used to describe the thermodynamic excess properties and the liquid composition of aqueous hydrofluoric acid solutions for overall HF concentrations ranging from dilute to a molality of 20 mol⋅kg−1. Within the framework of this model, three different mechanisms were tested. The first one considers only the dissociation of HF into H+ and F−. The second mechanism takes into account the dissociation of HF and the chemical reaction for formation of HF 2 − . The third mechanism considers, in addition to these two chemical reactions, the reaction that leads to formation of the dimer H2F 3 − . The experimental osmotic coefficients of the HF + H2O system were used to estimate the interaction parameters of this model. The resulting parameters yield an accurate description of the Gibbs-Duhem relation for the three mechanisms. The estimated liquid phase speciation for fluoride in the HF+H2O system is comparable to the experimental values of Braddy et al.

SiO2–Si under swift heavy ion irradiation: A comparison between normal and grazing incidence features

A comparison between experimental results on SiO2–Si under swift heavy ion irradiation at normal and grazing incidence is dealt with. After irradiation, the samples were gradually etched in an aqueous hydrofluoric acid solution. Atomic force microscopy has been used to study the evolution of the tracks regarding their extension and shape. At normal incidence, the latent tracks are revealed by chemical etching. Increasing the etching time, nano-hillocks appeared in the depths of each of the conical holes formed at the beginning of the chemical process. When the SiO2 layer is completely removed, nano-bumps are clearly visible at the interface between the two materials. Under grazing incidence (1° of beam inclination), elongated discontinuous tracks are directly observable after irradiation on the SiO2 surface. While with a sequential chemical etching, the intermittent tracks leave room to lengthy craters of decreasing depth, which disappear when the oxide layer is completely etched-away.

Preparation of Large-Area Uniform Silicon Nanowires Arrays through Metal-Assisted Chemical Etching

A facile fabricating method has been established for large-area uniform silicon nanowires arrays. All silicon nanowires obtained were single crystals and epitaxial on the substrate. Six kinds of silicon wafers with different types, surface orientations, and doping levels were utilized as starting materials. With the catalysis of silver nanoparticles, room-temperature mild chemical etching was conducted in aqueous solution of hydrofluoric acid (HF) and hydrogen peroxide (H2O2). The corresponding silicon nanowires arrays with different morphologies were obtained. The silicon nanowires possess the same type and same doping level of the starting wafer. All nanowires on the substrate have the same orientation. For instance, both (100)- and (111)-oriented p-type wafers produced silicon nanowires in the (100) direction. For every kind of silicon wafer, the effect of etching conditions, such as components of etchant, temperature, and time, were systemically investigated. This is an appropriate method to produce a large amount of silicon nanowires with defined type, doping level, and growth direction for industrial applications.

Binding of fluoro-containing anions to hexaazamacrocyclic ligand: Competitive interactions of fluoride and tetrafluoroborate anions with hexaprotonated [18]aneN6

Reaction of hexaazamacrocycle, 1,4,7,10,13,16-hexaazacyclooctadecane (L=[18]aneN6) with boron trifluoride etherate in an aqueous solution of hydrofluoric acid yields the proton-transfer complex of the composition [H6L(F) (BF4)5]·3H2O (1). X-ray crystallographic analysis shows that the macrocyclic cation adopts an unusual conformation to accommodate the fluoride anion and to keep the tetrafluoroborate anions in a close proximity. The tetrafluoroborate anions and water molecules participate in NH⋯F, NH⋯O and OH⋯F hydrogen bonding responsible for the 3D supramolecular architecture.

Bis(1-adamantylammonium) hexafluoridogermanate

The title compound, (C10H18N)2[GeF6], was obtained hydro­thermally from an aqueous solution of GeO2, H3BO3, NiCl2, adamantylammonium chloride, butanol and hydro­fluoric acid. The structure consists of discrete bis(1-adamantylammonium) cations lying on crystallographic mirror planes and hexa­fluoridogermanate anions disordered about sites of 2/m point symmetry. In the latter, the Ge atom lies on the site of 2/m symmetry, one F atom lies on the mirror plane and two further F atoms are included in general positions with 50% site occupancy. The cations and anions lie in layers with N—H⋯F hydrogen bonds formed between them.

Stabilizing Light Emission of Porous Silicon by In-situ Treatment

The photoluminescence of porous silicon prepared by etching in an aqueous solution of hydrofluoric acid mixed with phosphoric and that with nitric acid were red and orange, respectively. The optical characteristics were stable under laser illumination or when stored in air for the aging test. For etching in an aqueous hydrofluoric acid/phosphoric acid electrolyte, the stability is attributed to the existence of Si–P and P–O–Si bonds formed on the porous silicon surface. For etching in an aqueous hydrofluoric acid/nitric acid electrolyte, it is attributed to more Si–O bonds formed on the porous silicon surface.

Novel Electrochemical Patterning Method of Silicon Carbide

Silicon Carbide (SiC) is the most promising material for the fabrication of a new category of sensors and devices, to be used in very hostile environments (height temperature, corrosive ambient, presence of radiation ...). The fabrication of Silicon Carbide sensors requires new processes able to realize microstructures on bulk material or on the silicon carbide surface. A very promising method to create microstructures in SiC is the electrochemical etch in aqueous solution of hydrofluoric acid. In this paper the bulk micromachining of SiC substrate and the structural and electrical characterization of porous silicon carbide formation are reported. These results were used to create a novel method for the bidimensional pattering of SiC substrate. In particular a novel Si-based hard mask to transfer the design onto the SiC substrate was used. The hard mask is formed by a sandwich structures made by subsequent deposition of VAPOX, POLY Si, SiO2.