Hydrofluoric Acid Research Articles

Plasma assisted remediation of SiC surfaces

This paper describes a three-step process to remediate surface and sub-surface defects on chemo-mechanically polished SiC surfaces. In this process, a CF4-based inductively coupled plasma with reactive ion etch was used to remove material to a depth, which is unaffected by surface and subsurface polishing damage. This produced a planarized but carbon-rich fluorinated surface. This surface was then exposed to a 2 min rapid thermal oxidation in air at 1000 °C to oxidize and volatilize the excess carbon and fluorinated species, respectively. The resulting surface oxide was then stripped using a dilute hydrofluoric acid in water solution. This process, referred to as plasma assisted remediation, reproducibly yielded planarized, stoichiometric surfaces with low levels of carbon and oxygen contamination suitable for subsequent device fabrication. In the supporting studies described here, 4H- and 6H-SiC(0001) surfaces were remediated and characterized by x-ray photoelectron spectroscopy and atomic force microscopy at each stage of the process. Experimental studies under ion-rich and radical-dominant conditions are also reported which provide greater insight into the underlying chemistry and physics of the process.

Structure, Properties, and Preparation of MXene and the Application of Its Composites in Supercapacitors

Two-dimensional transition metal carbides/nitrides (MXenes) are emerging members of the two-dimensional material family, obtained by removing the A layer of the MAX phase through methods such as liquid-phase etching. This article summarizes the structure and properties of MXenes, as well as several preparation methods, including etching with hydrofluoric acid and fluoride salts, alkali-based etching, electrochemical etching, Lewis acid molten salt etching, and direct synthesis. Due to their unique two-dimensional structure and surface chemistry, MXenes exhibit good metallic conductivity, hydrophilicity, excellent flexibility, and ion intercalation properties, showing great potential in the research and application of supercapacitors and attracting widespread attention. The combination of MXene with other types of materials, including polymers, metal hydroxides, metal oxides, and carbon materials, takes advantage of composites to improve energy storage performance and shows great potential in the research and application of supercapacitors. This article provides a detailed summary of MXene composite materials and capacitor performance and introduces the research progress of MXene materials in the field of supercapacitor energy storage applications, aiming to provide references for the preparation of high-performance MXene supercapacitor electrode materials.

Effect of hydrofluoric acid on Guizhou anthracite structure modification and response to enhanced hydrolysis and permeability of coal

ABSTRACT To investigate the structural modification of Guizhou anthracite through the application of hydrofluoric acid and its corresponding response in terms of enhanced gas desorption and permeability, we utilized Guizhou anthracite from a mining area as the focal point of our study. We conducted a series of experiments encompassing X-ray diffraction (XRD), initial gas release velocity measurements, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR). Our investigation identified hydrofluoric acid as the primary acid solution, with an optimal concentration of 6% and a soaking duration of 12 hours. Furthermore, our theoretical analysis unveiled the physicochemical structural evolution of Guizhou anthracite under the influence of hydrofluoric acid and its subsequent response to enhanced gas desorption and permeability within the coal. Regarding chemical structure, hydrofluoric acid effectively dissolves inorganic minerals in anthracite and reduces the condensation of its regular basic structural units. Peak fitting of functional groups on the surfaces of the experimental coal samples revealed that hydrofluoric acid induces side chain reactions in the molecular structure of anthracite, promoting the gradual ordering of the aromatic structure. Simultaneously decreasing the content of cyclically closely bonded and self-associated hydroxyl hydrogen bonds. These alterations in irregular basic structural units can diminish coal’s adsorption capacity for gas, thereby enhancing gas desorption. Regarding the physical structure, treatment with hydrofluoric acid leads to the dissolution of minerals within specific pores and fissures of the coal samples, generating numerous dissolution pores. These pores and fissures expand and interconnect. The surface morphology quantified by fractal dimension has decreased by about 25.8%, indicating that pore and fracture structures often become relatively simple, which is conducive to gas flow and desorption in coal. Furthermore, the effective porosity and total porosity of the coal samples increase, as does the total movable fluid porosity. This enhances pore connectivity within the coal seam. Calculations reveal that after acidification, the permeability of the experimental coal samples increases by approximately 1.37 times, confirming the permeability-enhancing effect of hydrofluoric acid on anthracite.

An autonomous self-healing mechanism to improve the chemical durability vs. moisture of PVDF-based gel electrolytes for lithium-ion batteries

Lithium-ion batteries (LIBs) are at the forefront of electrochemical energy storage both for consumer electronics, and for power/energy more demanding applications, such as in automotive and grid levelling. However, their chemical and electrochemical stability must be further improved. Among the instability causes, a major role is played by residual moisture. As a matter of fact, hydrofluoric acid generated by hydrolysis of LiPF6, the most common lithium salt in LIBs, causes safety issues and performance losses. Here we demonstrate the scavenging properties vs. HF of a barium silicate glass used as a filler in a PVDF-HFP based gel polymer electrolyte. A full LiTi2(PO4)3|electrolyte|LiMn2O4 cell is stable for 200 cycles at 0.1C.

Designing Novel LiDAR-Detectable Plate-Type Materials: Synthesis, Chemistry, and Practical Application for Autonomous Working Environment.

Plate-type hollow black TiO2 (HL/BT) with a high NIR reflectance was fabricated for the first time as a LiDAR-detectable black material. A TiO2 layer was formed on commercial-grade glass by using the sol-gel method to obtain a plate-type structure. The glass template was then etched with hydrofluoric acid to form a hollow structure, and blackness was further achieved through NaBH4 reduction, which altered the oxidation state of TiO2 to black TixO2x-1 or Ti4+ to Ti3+ and Ti2+. The blackness of the HL/BT material was maintained by a novel approach that involved etching prior to reduction. The thickness of the TiO2 layer was controlled to maximize the NIR reflectance when applied as paint. The HL/BT material with a thickness of 140 nm (HL/BT140) showed a blackness (L*) of 13.3 and high NIR reflectance of 23.6% at a wavelength of 905 nm. This is attributed to the effective light reflection at the interface created by the TiO2 layer and the hollow structure. Plate-type HL/BT140 provides excellent spreadability, durability, and thermal stability in practical paint applications compared with sphere-type materials due to the higher contacting area to the applied surface, making it suitable for use as a LiDAR-detectable inorganic black pigment in autonomous environments.

Investigation of the Degradation of LiPF6- in Polar Solvents through Deep Potential Molecular Dynamics.

The nonaqueous electrolyte based on lithium hexafluorophosphate (LiPF6) is the dominant liquid electrolyte in lithium-ion batteries (LIBs). However, trace protic impurities, including H3O+, alcohols, and hydrofluoric acid (HF), can trigger a series of side reactions that lead to rapid capacity fading in high energy density LIBs. It is worth noting that this degradation process is highly dependent on the polarity of the solvents. In this work, a deep potential (DP) model is trained with a certain commercial electrolyte formula through a machine learning method. H3O+ is anchored with polar solvents, making it difficult to approach the PF6-, and suppressing the degradation process quickly at room temperature. Control experiments and simulations at different temperatures or concentrations are also performed to verify it. This work proposes a precise model to describe the solvation structure quantitatively and offers a new perspective on the degradation mechanism of PF6- in polar solvents.

The Influence of the Physicochemical Characteristics of Ores on the Efficiency of Underground Well Leaching of Uranium Deposits in Kazakhstan

The features of uranium mining on Kazakhstan’s enterprises have been examined, and uranium deposits located in the Syrdarya and Shu-Sarysu depressions have been described. Actual and projected data on the development of technological blocks in areas with complex geological structures have been analyzed and compared. Core samples were collected and, using X-ray diffraction analysis, quantitative and qualitative characteristics as well as mineral compositions of ores from various productive horizons of uranium deposits in the Syrdarya and Shu-Sarysu depressions were comparatively analyzed. It was determined that the ores in the Syrdarya depression are relatively homogeneous compared to those in the Shu-Sarysu depression, although in some places, clay minerals and gypsum are present, which hinder the uranium leaching processes. In the ores of the Shu-Sarysu depression, clay minerals that impede the uranium leaching processes are present in certain areas. Microscopic analysis of core material samples using a LEICA DM 2500 P microscope revealed particle sizes and shapes, as well as their distribution within the structure of host rocks in the productive horizon. Using X-ray diffraction analysis, mineral compositions of sediment-forming components during uranium well mining in the considered productive horizons were determined and comparatively analyzed. It was established that in the geotechnological wells of the Syrdarya depression, sediments of predominantly chemical origin, such as gypsum, are formed. However, in the geotechnological wells of the Shu-Sarysu depression, sediments of mechanical origin, consisting predominantly of quartz particles and clay minerals, are formed. Based on the obtained data, a method for intensifying underground uranium leaching in complex geological conditions has been developed, which involves dissolving sediment formations and increasing the oxidative–reductive potential of the leaching solution. The proposed and experimentally substantiated universal methodology for enhancing uranium well production involves the dissolution and prevention of precipitation using hydrofluoric acid solutions, as well as the oxidation of uranium dioxide with hydrogen peroxide.

Closed-loop cleaning treatment system — resource recovery of coal gangue

ABSTRACT Coal gangue (CG) is one of the main solid wastes emitted by coal mining. It causes severe damage to the environment. CG contains high levels of valuable elements (Al, Si) and has the potential for utilization and clean disposal. Here, an environment-friendly process for separating valuable components from CG was proposed: the closed-loop system of activation-leach-separation. The results show that the decarbonization rate was close to 100% under the optimum conditions of CG particle size of 0.075 mm to 0.15 mm and activated at 800°C for 120 min. Subsequently, it was transformed into a mixed-acid system with a hydrochloric acid (HCl) concentration of 4.5 mol/L, hydrofluoric acid (HF) volume fraction of 15%, and the liquid-solid ratio of 10:1, which achieved excellent separation efficiency. The residual rate of Al in the solution was only 0.7%, and the extraction rate of Si can reach 71.6%. Further, industrial-grade AlF3 products were also recovered. By adding saturated KCl solution and reacting at 33°C for 15 min, Si was further recovered in K2SiF6, and the yield was 85.83%. Particularly, the leaching solution maintained good extraction efficiency in the subsequent three cycles, the yield of K2SiF6 was 88.29%, confirming the feasibility of the closed-loop system. This treatment embodied the environmental-friendly properties of low acid consumption and low effluent discharge, providing a feasible method for clean treatment and resource utilization of CG.

Stabilizing high-nickel cathodes via interfacial hydrogen bonding effects using a hydrofluoric acid-scavenging separator

Nickel-rich layered Li transition metal oxides are the most promising cathode materials for high-energy-density Li-ion batteries. However, they exhibit rapid capacity degradation induced by transition metal dissolution and structural reconstruction, which are associated with hydrofluoric acid (HF) generation from lithium hexafluorophosphate decomposition. The potential for thermal runaway during the working process poses another challenge. Separators are promising components to alleviate the aforementioned obstacles. Herein, an ultrathin double-layered separator with a 10 μm polyimide (PI) basement and a 2 μm polyvinylidene difluoride (PVDF) coating layer is designed and fabricated by combining a non-solvent induced phase inversion process and coating method. The PI skeleton provides good stability against potential thermal shrinkage, and the strong PI-PVDF bonding endows the composite separator with robust structural integrity; these characteristics jointly contribute to the extraordinary mechanical tolerance of the separator at elevated temperatures. Additionally, unique HF-scavenging effects are achieved with the formation of -CO···H-F hydrogen bonds for the abundant HF coordination sites provided by the imide ring; hence, the layered Ni-rich cathodes are protected from HF attack, which ultimately reduces transition metal dissolution and facilitates long-term cyclability of the Ni-rich cathodes. Li || NCM811 batteries (where “NCM” indicates LiNixCoyMn1-x-yO2) with the proposed composite separator exhibit a 90.6% capacity retention after 400 cycles at room temperature and remain sustainable at 60 °C with a 91.4% capacity retention after 200 cycles. By adopting a new perspective on separators, this study presents a feasible and promising strategy for suppressing capacity degradation and enabling the safe operation of Ni-rich cathode materials.

How the repair bonding strength of hybrid ceramic CAD/CAM blocks is influenced by the use of surface treatments and universal adhesives.

We examined how different methods of surface treatment and different universal adhesives with or without extra silane affected the repair bonding strength of hybrid ceramic CAD/CAM restorations. Cerasmart specimens (n=320) were subjected to thermocycling and assigned to the following surface pretreatment protocols: control, diamond bur (DB), hydrofluoric acid (HF), and tribochemical silica coating (TSC). Half the specimens received a coating of silane, followed by application of the universal adhesives Futurabond M+ (FMU), Tokuyama Universal Bond (TUB), Single Bond Universal (SBU), or Clearfil Universal Bond Quick (CUQ) (n=10). A hybrid composite resin was used to simulate repair; then the specimens underwent further thermocycling. Shear bond strength (SBS) was determined and modes of failure were examined. The TSC-CUQ silane (-) group showed the highest SBS values. The best repairs were obtained when the surface was treated with TSC, with the exception of the DB-TUB silane (-) group. TUB increased SBS more than the other adhesives. Additional silane decreased SBS in the HF-TUB and TSC-CUQ groups, while increasing it in the TSC-TUB and DB-FMU groups (p<0.05).

The Study on Corrosion Resistance of Ti-6Al-4V ELI Alloy with Varying Surface Roughness in Hydrofluoric Acid Solution

The corrosion resistance of titanium alloy poses a crucial challenge, significantly affecting its prospect for service and application. The present study aimed to investigate the corrosion resistance of Ti-6Al-4V ELI alloys with varying surface roughness in hydrofluoric acid solution, in order to assess the influence of roughness on their corrosion resistance performance. The weight loss percentage, surface morphology evolution, and roughness variation of Ti-6Al-4V ELI alloys before and after exposure to hydrofluoric acid corrosion were characterized. While the weight loss and weight loss percentage of the Ti-6Al-4V ELI alloy increased with prolonged corrosion, the overall weight loss rate decreased. The accumulation of TiF3 phases and depletion of the Ti-6Al-4V ELI matrix mutually led to the alterations of the surface roughness. Due to the inability to prevent fluoride ions from contacting with the Ti-6Al-4V ELI alloy, continuous corrosion occurred in hydrofluoric acid. Based on these experimental results and analysis, the corrosion mechanism of the Ti-6Al-4V ELI alloy corroded by hydrofluoric acid solution was elucidated. Furthermore, an analysis was conducted to explore the influence of corrosion time on mechanical properties by analyzing the decay in compressive properties of the Ti-6Al-4V ELI titanium alloy after hydrofluoric acid corrosion treatment. The bearing capacity of the Ti-6Al-4V ELI alloy deteriorated over the corrosion time.

The influence of sodium fluoride on the dissolution kinetics of metallic titanium in citric acid solution using the rotating disc method

This study reports the effect of sodium fluoride on the dissolution kinetics of pure metallic titanium in a citric acid solution. The effects of sodium fluoride concentration, citric acid concentration, temperature, stirring speed, and disc surface area were examined. The dissolution rate of titanium increases strongly with increasing citric acid and sodium fluoride concentrations. Fluoride ions react with hydrogen ions to form hydrofluoric acid, which removes the passive layer of titanium dioxide. An increase in the concentration of NaF and acid causes the formation of a brown layer on the surface of titanium. X-ray diffraction and SEM-EDX analyses showed that the layer composition is mostly of titanium fluoride (TiF3) and titanium fluoride oxide (TiOF2). A mixed kinetic model with an activation energy of 26.4 kJ/mol can be used to explain the reaction kinetics.

Comprehensive synthesis of Ti3C2Tx from MAX phase to MXene.

MXenes are a large family of two-dimensional materials that have attracted attention across many fields due to their desirable optoelectronic, biological, mechanical and chemical properties. There currently exist many synthesis procedures that lead to differences in flake size, defects and surface chemistry, which in turn affect their properties. Herein, we describe the steps to synthesize Ti3C2Tx-the most important and widely used MXene, from a Ti3AlC2 MAX phase precursor. The procedure contains three main sections: synthesis of Ti3AlC2 MAX, wet chemical etching of the MAX in hydrofluoric acid/HCl solution to yield multilayer Ti3C2Tx and its delamination into single-layer flakes. Three delamination options are described; these use LiCl, tertiary amines (tetramethyl ammonium hydroxide/ tetrabutyl ammonium hydroxide) and dimethylsulfoxide respectively. These procedures can be adapted for the synthesis of MXenes beyond Ti3C2Tx. The MAX phase synthesis takes about 1 week, with the etching and delamination each requiring 2 d. This protocol requires users to have experience working with hydrofluoric acid, and it is recommended that users have experience with wet chemistry and centrifugation; characterization techniques such as X-ray diffraction and particle size analysis are also essential for the success of the protocol. While alternative synthesis methods, such as minimally intensive layer delamination, are desirable for certain MXenes (such as Ti2CTx) or specific applications, this protocol aims to standardize the more commonly used hydrofluoric acid/HCl etching method, which produces Ti3C2Tx with minimal concentration of defects and the highest conductivity and serves as a guideline for those working with MXenes for the first time.

Delayed cardiac arrest after hydrofluoric acid ingestion

Delayed cardiac arrest after hydrofluoric acid ingestion

PDA@UiO-66-NH2-derived nitrogen and oxygen-doped hierarchical porous carbon for efficient adsorption of BPA and dyes

MOF-derived carbon materials have garnered significant attention in the field of adsorption due to their abundant porosity, excellent stability, and structural diversity. In this study, nitrogen and oxygen-doped hierarchical porous carbon materials were obtained by carbonizing PDA@UiO-66-NH2 at various temperatures and subsequently etching with hydrofluoric acid for different times. The adsorption capacity of these materials towards bisphenol a (BPA), methyl orange (MO), and methylene blue (MB) was evaluated. The results suggest excessive carbonization temperature and etching time may cause the collapse of the pore structure. In addition, the introduction of a PDA carbon layer can reduce the nitrogen–oxygen ratio and modify the nitrogen species in the materials. Among these materials, UPCH800, which was carbonized at 800 °C followed by etching for 6 h, exhibits the best adsorption performance towards BPA and MO, with a maximum adsorbed amount of 350.51 and 417.34 mg/g respectively. For MB, the maximum adsorption amount is 227.53 mg/g. All adsorptions reach equilibrium within 1 h. Thermodynamic studies reveal that the adsorption of MO by UPCH800 is an entropy-driven adsorption process, while the adsorption of BPA and MB is an enthalpy-driven exothermic process. Based on experimental and characterization results, the primary adsorption mechanisms involve π-π interactions, n-π interactions, pore-filling, and hydrogen bonding between the pollutants and the material. Moreover, UPCH800 exhibits high removal efficiency after 5 adsorption cycles, making it a promising material for the adsorption of BPA and MO.

Influence of electrical polarization on thrombogenicity of titania nanotubes

AbstractThe development of hemocompatible biomaterials with antithrombogenic surface coatings remains a challenge in cardiovascular applications. There is interest in negatively charged surfaces that inhibit thrombus formation through electrostatic repulsion between the biomaterial surface and negatively charged platelets. Hence, the present study investigated the influence of electrical polarization on the thrombogenicity of titania nanotubes (TNT), which are promising candidates for inhibiting thrombogenicity via surface modification. TNTs were formed on commercially pure titanium plate by the electrochemical anodization technique using platinum as a counter electrode at 60 V for 24 h with two kinds of electrolytes (hydrofluoric acid diluted with dimethyl sulfoxide [D‐TNT] or ethylene glycol [E‐TNT]) followed by an annealing at 540°C for 3 h in air. Both TNTs were mixture of anatase and rutile, and the D‐TNT had a diameter of 108.76 ± 2.55 nm and the E‐TNT, 53.833 ± 2.42 nm. The TNTs were electrically polarized at 100 V of DC field and 400°C for 1 h. Water contact angle measurements showed that the non‐polarized (0‐) TNT surface was hydrophilic whereas the positively (P‐) or negatively (N‐) polarized TNT surfaces showed high‐hydrophilicity. Antithrombogenicity was evaluated using the thrombus coverage area ratio (TCAR) after soaking the TNTs in bovine whole blood. The TCARs for 0‐polarized E‐ and D‐TNTs were 5.30 ± 4.34% and 36.3 ± 5.8% and for P‐polarized E‐TNT and D‐TNT were 1.50 ± 0.77% and 2.76 ± 1.07%, whereas no thrombus formation (0 ± 0%) for N‐polarized E‐TNT and very few thrombus formation (0.12 ± 0.22%) for N‐polarized D‐TNT. The electrostatic repulsion between the N‐polarized E‐TNTs and platelets completely inhibits thrombus formation, which cannot be achieved by the nanomorphology and high‐hydrophilicity of other TNTs. Hence, N‐TNTs formed by electrical polarization are potential candidates for cardiovascular devices, such as artificial heart valves with long‐term hemocompatibility.

Activation of fluoranhydrite with various chemical additives for the production of gypsum fiberboards

Preliminary studies of fluoranhydrite as a binder showed that on its own it almost does not harden, intervention in the technological process of basic production is almost impossible, so in order to obtain materials it is necessary to develop ways of modifying it to initiate the binder properties or use “acidic” fluoranhydrite before the neutralization stage. In this work the influence of various additives (sodium sulfate and sodium sulfite, potassium sulfate, copper sulfate, iron sulfate crystallohydrate, alumina aluminate, sodium carbonate) on the properties of fluorine hydrite binders produced by neutralization of sour waste from hydrofluoric acid production with an excess of limestone was studied. In this work to obtain dependences of anhydrite binder technological properties on the number of introduced additives and determine the optimal composition of the binder, as well as create mathematical models of the processes under study and their statistical analysis used mathematical planning of the experiment. As the conducted studies showed the speed of setting of products based on anhydrite binder and their strength mainly depend on the temperature of water and its amount for mixing at the optimum dispersity of the binder. The strength of the samples made from the neutralized waste was found to be in the range of 0.5‒1.2 MPa, and the strength of the samples based on the activated anhydrite binder ― in the range of 5.3‒10.7 MPa that corresponds to the parameters of the material suitable for the production of boards.

Bonding Pretreatment of Aesthetic Dental CAD-CAM Materials through Surface Etching with a Mixed Aqueous Solution of Ammonium Fluoride and Ammonium Hydrogen Sulfate.

Hydrofluoric acid (HF) is commonly used as an etchant for the pretreatment of dental computer-aided design/computer-aided manufacturing (CAD-CAM) materials, such as glass-ceramics and resin composites. Despite its effectiveness, the harmful and hazardous nature of HF has raised significant safety concerns. In contrast, ammonium fluoride (AF) is known for its relatively low toxicity but has limited etching capability. This study explored the potential of ammonium hydrogen sulfate (AHS), a low-toxicity and weak acid, to enhance the etching ability of aqueous AF solutions for the bonding pretreatment of CAD-CAM materials. This study investigated five types of aesthetic CAD-CAM materials: lithium disilicate glass, feldspathic porcelain, polymer-infiltrated ceramic networks, resin composites, and zirconia. Seven experimental etchants were prepared by varying the amount of AHS added to aqueous AF solutions, with each etchant used to etch the surfaces of the respective CAD-CAM materials. The treated surfaces were analyzed using scanning electron microscopy and confocal laser scanning microscopy. Additionally, the shear bond strength (SBS) of the CAD-CAM materials treated with a luting agent (resin cement) was evaluated. The results indicated that the AF1/AHS3 (weight ratio AF:AHS = 1:3) etchant had the most substantial etching effect on the surfaces of silica-containing materials (lithium disilicate glass, feldspathic porcelain, polymer-infiltrated ceramic networks, and resin composites) but not on zirconia. The SBS of the materials treated with the AF1/AHS3 etchant was comparable to that of the commercial HF etchant. Hence, an AF/AHS mixed solution could effectively etch silica-containing CAD-CAM materials, thereby enhancing their bonding capabilities.

Simple and Safe Synthesis of Yolk-Shell-Structured Silicon/Carbon Composites with Enhanced Electrochemical Properties.

With its substantial theoretical capacity, silicon (Si) is a prospective anode material for high-energy-density lithium-ion batteries (LIBs). However, the challenges of a substantial volume expansion and inferior conductivity in Si-based anodes restrict the electrochemical stability. To address this, a yolk-shell-structured Si-carbon composite, featuring adjustable void sizes, was synthesized using tin (Sn) as a template. A uniform coating of tin oxide (SnO2) on the surface of nano-Si particles was achieved through a simple annealing process. This approach enables the removal of the template with concentrated hydrochloric acid (HCl) instead of hydrofluoric acid (HF), thereby reducing toxicity and corrosiveness. The conductivity of Si@void@Carbon (Si@void@C) was further enhanced by using a high-conductivity carbon layer derived from pitch. By incorporating an internal void, this yolk-shell structure effectively enhanced the low Li+/electron conductivity and accommodated the large volume change of Si. Si@void@C demonstrated an excellent electrochemical performance, retaining a discharge capacity of 735.3 mAh g-1 after 100 cycles at 1.0 A g-1. Even at a high current density of 2.0 A g-1, Si@void@C still maintained a discharge capacity of 1238.5 mAh g-1.

Trace Silicon Determination in Biological Samples by Inductively Coupled Plasma Mass Spectrometry (ICP-MS): Insight into the Volatility of Silicon Species in Hydrofluoric Acid Digests for Optimal Sample Preparation and Introduction to ICP-MS

A method for the determination of trace levels of silicon from biological materials by inductively coupled plasma mass spectrometry (ICP-MS) has been developed. The volatility of water-soluble silicon species, hexafluorosilicic acid (H2SiF6), and sodium metasilicate (Na2SiO3) was investigated by evaporating respective solutions (50 µg/mL silicon) in HNO3), nitric acid + hydrochloric acid (HNO3 + HCl), and nitric acid + hydrochloric acid + hydrofluoric acid (HNO3 + HCl + HF) at 120 °C on a hot-block to near dryness. The loss of silicon from H2SiF6 solutions was substantial (&gt;99%) regardless of the digestion medium. Losses were also substantial (&gt;98%) for metasilicate solutions heated in HNO3 + HCl + HF, while no significant loss occurred in HNO3 or HNO3 + HCl. These results show that H2SiF6 species were highly volatile and potential losses could confound accuracy at trace level determinations by ICP-MS if digestates prepared in HF are heated to eliminate HF. Among the various matrices comprising major elements, sodium appeared to be effective in reducing silicon loss from H2SiF6 solutions. Excess sodium chloride (NaCl) matrix provided better stability, improving silicon recoveries by up to about 80% in evaporated HF digestates of soil and mine waste samples, but losses could not be fully prevented. To safely remove excess acids and circumvent the adverse effects of excess HF (e.g., risk of high Si background signals), a two-step digestion scheme was adopted for the preparation of biological samples containing trace silicon levels. A closed-vessel digestion was performed either in 4 mL of concentrated HNO3 and 1 mL of concentrated HCl or 4 mL of concentrated HNO3, 1 mL of concentrated HCl and 1 mL of concentrated HClO4 on a hot plate at 140 °C. Digestates were then evaporated to incipient dryness at 120 °C to remove the acids. A second closed-vessel digestion was carried out to dissolve silicates in 0.5 mL of concentrated HNO3 and 0.5 mL of concentrated HF at 130 °C. After digestion, digestates were diluted to 10 mL. The solution containing about 5% HNO3 and 5% HF was directly analyzed by ICP-MS equipped with an HF-inert sample introduction system. The limit of detection was about 110 µg/L for 28Si when using the Kinetic Energy Discrimination (KED) mode. The method was used to determine silicon in various plant and tissue certified reference materials. Data were acquired for 28Si using KED and standard (STD) modes, and 74Ge and 103Rh as internal standard elements. There was not any significant difference between the accuracy and precision of the results obtained with 74Ge and 103Rh within the same measurement mode. Precision, calculated as relative standard deviation for four replicate analyses, varied from 5.3 (tomato leaves) to 21% (peach leaves) for plant and from 2.2 (oyster tissue) to 33% (bovine liver) for tissue SRM/CRMs. Poor precision was attributed to material heterogeneity and the large particle size distribution. An analysis of lung tissue samples from those with occupational exposure to silica dust revealed that tissues possessed substantial levels of water-soluble silicates, but the most silicon was present in the particulate matter fraction.