Aqueous HF Solution Research Articles

Effect of surface treatments on microstructure and electrochemical properties of La–Ni–Al hydrogen storage alloy

Surface treatment by the fluorination treatment with an aqueous solution of KF and HF with a little addition of KBH 4 followed by electroless Ni–P plating has been carried out to modify the surface structure and electrochemical properties of the La–Ni–Al hydrogen storage alloy. The results of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) show that a Ni-rich and Al-poor layer forms after fluorination treatment. The electroless plating results in the precipitation of the nanometer-scale Ni–P particles with homogeneous distribution on the alloy surface. Both the treatments contribute to an increase in exchange current density, a decrease in charge-transfer resistance and the facility of hydrogen diffusion within the bulk alloy, leading to significant improvements on high rate dischargeability (HRD) of the alloy. The alloy treated by the composite treatment of the fluorination treatment and the following electroless plating possesses best overall electrode performance.

Properties of Na2SiF6:Mn4+ and Na2GeF6:Mn4+ red phosphors synthesized by wet chemical etching

We report a simple method of synthesizing red phosphors, Na2SiF6:Mn4+ and Na2GeF6:Mn4+, by wet chemical etching of Si wafers and Ge shots in aqueous HF solution with the addition of an oxidizing agent NaMnO4. The x-ray diffraction analysis suggests that the red phosphors have the trigonal structure (space group=D32−P321). The optical properties of these phosphors are investigated using photoluminescence, photoluminescence excitation, optical absorption, and diffuse reflectance spectroscopy. The spectral features show no clear difference between Na2SiF6:Mn4+ and Na2GeF6:Mn4+ phosphors. They are well explained by the Tanabe–Sugano energy-level diagram with the crystal-field and Racah parameters of Dq∼1970 cm−1, B∼775 cm−1, and C∼3490 cm−1 with C/B∼4.5.

The standard enthalpy of formation of K2[NiF6](cr)

The enthalpies of interactions of K2[NiF6](cr) with water and aqueous alkali, enthalpies of solution of KF(cr) in dilute aqueous solutions of NiF2 and HF, and enthalpy of mixing of solutions of NiF2 and HF were measured at 298.15 K using isothermic-shell calorimeters. Based on the data obtained and literature values, the standard enthalpy of formation of the compound was determined by two independent methods, \( \Delta _f H_{K_2 [NiF_6 ](cr)}^ \circ \) = −2004 ± 8 kJ/mol.

Toward Understanding the Dissociation of Weak Acids in Water: 1. Using IR Spectroscopy to Identify Proton-Shared Hydrogen-Bonded Ion-Pair Intermediates

Cryogenic conditions favor the formation of ion-pair dissociation intermediates in amorphous mixtures of HF and H(2)O, making possible their characterization by means of infrared spectroscopy. The experimental infrared spectra show a structurally rich "continuous" absorption ranging from 1000 to 3400 cm(-1), which, in principle, contains important information regarding the microscopic structure of the aforementioned dissociation intermediates. Herein, we demonstrate that this microscopic information can be extracted by comparing and contrasting experimental spectra with those obtained by means of carefully designed first-principles molecular dynamics calculations. Very good, systematic agreement between theoretical and experimental spectra can be obtained for HF/H(2)O mixtures of various compositions, revealing the presence of proton-shared, dissociation intermediates F(delta-) * * * H * * * (delta+)OH(2). The existence of similar proton-shared, hydrogen-bonded intermediates of ionization, that are stable in solution, but not in the gas phase, has been previously suggested by other groups, using, among other techniques, low temperature NMR data and aprotic, dipolar, solvents. Our investigation reveals that similar structures are also stable in aqueous solutions of HF. We discuss some of the implications of the present findings as far as the mechanism of dissociation of weak acids is concerned.

Comparison of photoelectrochemical properties of TiO2-nanotube-array photoanode prepared by anodization in different electrolyte

Self-organized, well-crystallized and high aspect-ratio TiO2 nanotube arrays (TNAs) have been prepared by anodic oxidation in dimethyl sulfoxide (DMSO) containing 5 wt% HF at 40 V (vs. Pt). A 50 h anodization results in a nanotube arrays approximately 19.4 μm in length, referred as long tube. As a comparison, the short titania nanotube arrays, about 500 nm in length, was obtained by anodization in HF aqueous solution, referred as short tube. Different characterization techniques (viz. FESEM, TEM, XRD and DRS) are used to study the nanotubular microstructure. The morphology of the nanotube electrodes shows an evident influence on their photocatalytic (PC) and photoelectrochemical reactivity. The long tube reveals enhanced photocurrent response and PC degradation efficiency of organic compounds. The kinetic constant of PC degradation of methylic orange (MO) for long tube electrode is found 1.55 times as high as the short tube. A significant photoelectrochemcial synergetic effect in MO degradation was observed on the long tube electrode and the photoelectrocatalytic (PEC) degradation of MO on long tube is 27% higher than its PC process.

Preparation and corrosion resistance studies of zirconia coating on fluorinated AZ91D magnesium alloy

Novel anti-corrosion zirconia coating was prepared via the sol–gel method for AZ91D magnesium alloy using zirconium nitrate hydrate as a precursor modified with acetylacetone (AcAc). Magnesium alloy substrates were first fluorinated in 20% HF aqueous solution at room temperature for 20 h, then, the zirconia coating was deposited on the fluorinated sample by dip coating. Basing on the sol–gel process, a chelate complex from the reaction of zirconium coordinating AcAc was formed which was supported by UV–vis spectrum analysis. The result showed that the absorption peak could be seen for the sol at 308 nm, which was red-shifted by 36 nm from that of methanol form of AcAc (272 nm). Moreover, Fourier transform infrared (FT-IR) spectrum analysis was performed to examine the structural differences between the gel and AcAc. The results indicated that the chelate complex with a bidentante structure was formed through the interaction chemically between zirconium nitrate and AcAc. The surface morphology of the zirconia coating was characterized by scanning electron microscope (SEM), an uniform coating can be obtained on the fluorinated sample. The corrosion resistance of the substrate, the fluorinated with and without the zirconia coating in the 3.5 wt.% NaCl solution was studied using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests, respectively. The results demonstrated that the zirconia coating could greatly improve the corrosion resistance of the AZ91D magnesium alloy. Furthermore, the effect of the different heat-treatment temperatures for the zirconia coating on corrosion resistance was also discussed.

Wet chemical etching behavior of β‐Ga2O3 single crystal

AbstractWet chemical etching behavior of β‐Ga2O3 single crystal was investigated to evaluate its chemical stability and to explore etchants for β‐Ga2O3 single crystal. Undoped and Sn‐doped β‐Ga2O3 single crystals were grown by the floating zone method, and (100)‐ and (001)‐oriented samples were chemical‐mechanical‐polished to wafers. The samples were wet chemically etched in solutions such as HCl, H2SO4, HNO3, HF, H2O2:H2SO4:H2O=1:4:1, KOH, and NaOH. The samples were chemically stable against both acids and alkalis except HF and NaOH. Aqueous HF solution was found to etch β‐Ga2O3 uniformly at room temperature. The etch rate increased with increasing immersion time and HF content. Anisotropy of etch rate was observed between the (100) and (001) planes. The etch rate of Sn‐doped β‐Ga2O3 was lower than that of undoped β‐Ga2O3, and the etch rate decreased with increasing Sn doping content. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrochemical Preparation and Characterization of Surface-Fluorinated TiO2 Nanoporous Film and Its Enhanced Photoelectrochemical and Photocatalytic Properties

In recent years, surface fluorination of TiO2 has been demonstrated to be an efficient method for improving its photocatalytic (PC) reactivity toward some pollutants. In this paper, a new and simple method for surface fluorination and porosity-creating of TiO2 is presented. The sample was prepared by anodization of TiO2 in HF aqueous solutions formed by direct thermal oxidization of titanium sheet. The preparation conditions were optimized. The prepared samples were characterized by scanning electron microscopy (SEM), Raman spectroscopy, UV–vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL). The photoelectrochemical (PEC) and photocatalytic (PC) properties of the samples were also investigated. SEM and Raman showed that the anodization made the TiO2 nanoporous, which consequently increased markedly the specific surface area, but did not change the bulk crystal structure under a certain preparation condition. DRS analysis revealed that the light absorption was decreased after electrochemical etching. The band gap energy narrowed from 3.15 to 3.04 eV. The surface fluorination of the etched-TiO2 was evidently supported by XPS. Highly enhanced PEC performance and PC activities for the degradation of target pollutants, phenol, methylene blue, and reactive brilliant red on the etched-TiO2 were observed. The possible reasons for such an improvement were studied in detail by a combination of the above-mentioned methods and (photo) electrochemical techniques. It is mainly attributed to the enhanced specific surface area, negative-shifted appearing energy band edges, decreased surface recombination centers, and/or favorable charge transfer rate. The F-containing TiO2 electrode has an excellent stability against fluoride desorption.

Cylindrical pore arrays in silicon with intermediate nano-sizes: A template for nanofabrication and multilayer applications

An electrochemical technique for the fabrication of intermediate nano-sized pores in silicon (50–200) nm in aqueous HF solution is reported. Attempts to achieve similar nano-sized pores in the presence of organic solvents are also presented. Mesopore formation beginning with pore sizes <50 nm followed by pore widening in alkaline etch solution is also attempted. The desired pore size is obtained with a KMnO4 oxidizing agent in the presence of surfactant. Our findings reveal that the applied current density during the fabrication process is a decisive factor. The concentration of HF and KMnO4 should be controlled within appropriate ranges. A multilayer with two different pore morphologies is fabricated on the same substrate using the same electrolyte simply by changing the applied anodic current. Application of such cylindrical pore arrays as templates for polymer nanofabrication is demonstrated. By selective dissolution of the silicon membranes, we successfully obtained nanowires of polypyrrole (PPy) with ∼100 nm diameter size. The fabrication process, the electrochemical measurements, the formation mechanism and the different pore morphologies are investigated in detail.

Fabrication of nanoporous silicon dioxide/silicon nitride membranes using etched ion track technique

A robust method of fabricating freestanding nanoporous silicon dioxide/silicon nitride membrane on a silicon holder using etched ion track technique was demonstrated in this paper. The basic track etching parameters of silicon dioxide and silicon nitride were studied and appropriate irradiation conditions were obtained. The freestanding membranes were obtained by standard silicon fabrication procedures. The multilayer composite silicon/silicon dioxide/silicon nitride was irradiated by Br ions with fluence of 108–1010cm−2. The latent ion tracks in silicon dioxide were then etched to the silicon nitride layer by aqueous HF solution (4%) to produce conical nanopores. Subsequently, these nanopores in silicon dioxide layer, acting as a mask, were transferred into the silicon nitride layer by reactive ion etching (RIE). Finally, with an extra HF wet etching step, the nanoporous silicon dioxide/silicon nitride membrane can be thinned to nanoporous silicon nitride membrane. The diameter of the nanopores in silicon nitride can be varied by varying the HF wet etching time. Via this method, nanoporous silicon dioxide/silicon nitride membrane with designed pore density and pore diameter of sub 100nm can be achieved.

Ion-induced tracks in amorphous Si3N4 films

Silicon nitride layers of 140nm thickness were deposited on silicon wafers by low pressure chemical vapour deposition (LPCVD). The samples were irradiated in the electronic slowing-down regime, with either Pb ions of 110MeV (Se=19.3keVnm−1) or Xe ions of 710MeV (Se=22.1keVnm−1). Using infrared absorption spectroscopy, the radiation-induced disorder in Si3N4 was analysed as a function of the ion fluence (up to 4×1013cm−2). Some targets irradiated at low fluences (∼109cm−2) were etched at room temperature in aqueous HF solution (10vol.%) for various durations. The processed surfaces were probed using atomic force microscopy (AFM) in order to evidence etched tracks and to measure their mean surface diameter. The non-simultaneous emergences of the nanopores at the Si3N4/Si interface and the limited etching efficiency allow to conclude that the tracks are probably discontinuous in the present irradiation conditions.

Chemical, energetic, and geometric heterogeneity of device-quality (1 0 0) surfaces of single crystalline silicon after HF aq etching

An analysis, based on angle-resolved X-ray photoelectron spectroscopy, multiple-internal-reflection infrared spectroscopy, and atomic force microscopy, of device-quality (1 0 0)silicon surfaces after etching in dilute aqueous solution of HF is presented. The analysis shows that the surface is mainly formed by a heterogeneous distribution of SiH, SiH 2and SiH 3 terminations, but contains sub-stoichiometric oxidized silicon. The analysis shows moreover the existence of a form of reduced silicon, not consistent with the currently accepted picture of the native HF aq-etched surface.

Covalent Attachment of Organic Monolayers to Silicon Carbide Surfaces

This work presents the first alkyl monolayers covalently bound on HF-treated silicon carbide surfaces (SiC) through thermal reaction with 1-alkenes. Treatment of SiC with diluted aqueous HF solutions removes the native oxide layer (SiO2) and provides a reactive hydroxyl-covered surface. Very hydrophobic methyl-terminated surfaces (water contact angle theta = 107 degrees ) are obtained on flat SiC, whereas attachment of omega-functionalized 1-alkenes also yields well-defined functionalized surfaces. Infrared reflection absorption spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy measurements are used to characterize the monolayers and show their covalent attachment. The resulting surfaces are shown to be extremely stable under harsh acidic conditions (e.g., no change in theta after 4 h in 2 M HCl at 90 degrees C), while their stability in alkaline conditions (pH = 11, 60 degrees C) also supersedes that of analogous monolayers such as those on Au, Si, and SiO2. These results are very promising for applications involving functionalized silicon carbide.

Detection of G-Type Nerve Agent Simulants Based on a Double Reflection DBR Porous Silicon Interferometer

Distributed Bragg re ector (DBR) porous silicon (PSi) chips with two re ection peaks were fabricated by electrochemical etching in an aqueous ethanolic HF solution and were used to detect of chemical nerve agent simulants. Dimethyl methylphosphonate (DMMP) is a simulant for G-type nerve agents. The manufactured DBR PSi chips exhibit two sharp photonic bands, 560 and 717 nm, with narrow full widths at half maximum (FWHM) of 15 and 20 nm, respectively. The detection method involved the shift of the DBR re ection peaks in the re ectivity spectra under exposure to vapors of the analyte. Rapid detection was achieved in few seconds in situ and the observed red shift of the DBR peaks resulted from an increase in the refractive indices in DBR PSi. When the DBR PSi chips were exposed to vapor of DMMP, the two re ection peaks from DBR PSi were red-shifted by 22 and 32 nm, respectively. Real-time detection for the stimulant indicated that the detection measurements were reversible. Detection of other analytes, such as triethyl phosphate (TEP) and diethyl ethylphosphonate (DEEP), has been also achieved.

Etching of Ion Irradiated LiNbO3 in Aqueous Hydrofluoric Solutions

Homogeneously damaged surface layers of a thickness of were generated in x-cut single crystals using multiple-energy -irradiation to study the etching behavior in aqueous HF solutions. Rutherford backscattering/channeling analysis was applied to investigate the damage formation. Different acid temperatures and concentrations were used, showing that the etching rate can be increased by increasing the temperature from maintaining the high contrast of the technique. The dissociation of HF in aqueous solution is discussed, and reaction kinetics well established for the HF etching of are then applied to obtain information on the etching mechanism. Therefore, the concentration dependence of the etching rate is analyzed and it is found that the etching process can be described as an attack of either HF or , which is catalytically supported by the presence of ions.

Investigation of the controllable growth of the TiO2 nanotube arrays fabricated by anodic oxidation method

The highly ordered and uniform TiO2 nanotube arrays were fabricated by anodic oxidation of titanium foil in HF aqueous solution or ethylene glycol containing NH4F and deionized water. The diameter，density，length and wall thickness of nanotube arrays can be controlled effectively by varying the anodization parameters including electrolyte composition，voltage，and time. The microstructures of the titania nanotube arrays were characterized by X-ray diffraction (XRD). The results showed that the as-anodized titania nanotubes were amorphous. And when annealed in vacuum or in oxygen ambient at 500℃，the samples prepared in HF aqueous solution showed only anatase phase，and the samples prepared in ethylene glycol containing NH4F and deionized water showed both anatase and rutile phases. And the former and the latter batch of samples became better crystallized when annealed in oxygen and in vacuum，respectively. Furthermore，the controllable growth mechanism of nanotube arrays is discussed.

Electrochemically driven intrusion of silver particles into silicon under polarization

The pore formation with the diameter of around 100 nm into a lightly doped p-type Si was achieved with the intrusion of silver particles by electrolysis in HF aqueous solution. The route resembles the metal-catalyzed electroless pore formation, but the present method uses anodic polarization instead of chemical etching in the presence of oxidizing agent. A microporous layer was observed around the pores formed as the tracks of silver particles. Thickness of the microporous layer around the track increased with the increase in current density. The thickness was varied in accordance with the time-programmed variation of current density. Conversely, the intrusion of silver particles seldom occurred in heavily doped p-type silicon, while micropores were formed independently of the location of the particles. The concentration of dopant affects the silver-particle-assisted porosification.

Anodic electrode reaction of p-type silicon in 1-ethyl-3-methylimidazolium fluorohydrogenate room-temperature ionic liquid

The anodic electrode behavior for a p-type silicon single crystal electrode ((1 0 0), ρ = 0.01–0.02 Ω cm, boron doped) was examined in the 1-ethyl-3-methylimidazolium fluorohydrogenate, EtMeIm(FH) 2.3F, room-temperature ionic liquid (RTIL). The electrochemical behavior was very similar to that in conventional HF aqueous solution. After the anodic electrode reaction, the Si electrode was uniformly covered with a mesoporous Si layer having a pore size of ∼25 nm. The mesoporous layer did not exhibit a photoluminescence spectrum in the visible region due to the lack of Si–H termination. However, after chemical treatment with an ethanolic HF solution, a subset of the porous Si samples showed a very weak photoluminescence.

Coverage properties of SiN x films prepared by catalytic chemical vapor deposition on trenched substrates below 80 °C

Coverage properties of SiN x films prepared by catalytic chemical vapor deposition on trenched substrates below 80 °C were investigated by using SiH 4, NH 3 and H 2 as source gases. The aspect ratio was changed between 0.35 and 3.4. Conformal step coverage was obtained when the aspect ratio was less than unity, but the coverage property was degraded with the increase in the aspect ratio. SiN x films on the side walls of the trenches were found to have low etch-resistance by an aqueous solution of HF compared with those on the top and bottom of the trenches. Thermal radiation from the heated catalyzer should be the cause of this difference. Coverage properties at the concave corners of the trenches were improved by increasing the H 2 flow rate. This improvement may be ascribed to the local heating of the substrate surfaces by H atoms.

Synthesis of Zeolites in the Presence of Diquaternary Alkylammonium Ions as Structure-Directing Agents

This paper deals with the synthesis results obtained mostly from pure silica systems in the presence of (CH3 )3 N+ (CH2 )n N+ (CH3 )3 ions (n = 5 , 6, 9 or 10) in fluoride media, but also in the presence of (CH3 )3 N+ (CH2 )n O(CH2 )n N+ (CH3 )3 ions (n = 2 or 4) in hydroxide or fluoride media. A variety of zeolites form, namely ZSM-48, MTW-type, BEA-type, MFI-type, ITH-type zeolites, but also, in case of pronounced degradation of the template into N(CH3 )4 + or N(CH3 )3 , AST-type, MTN-type and RUT-type materials. The samples are characterized by XRD, SEM, thermal and elementary analyses, 1 H liquid NMR (after dissolution of the sample in HF aqueous solution) and solid-state NMR (19 F, 13 C, 27 Al) spectroscopy.