Fluoride Etching Research Articles

Formation of Organic Monolayers on KF-Etched Si Surfaces

Silicon is the most commonly used material in the microelectronics industry, due to its inherent advantages of high natural abundance, low cost, and high purity, coupled with the chemical and electrical stability at the interface with its oxide. For molecular electronics applications, oxide-free Si surfaces are widely used because of the relative ease of removing the oxide (SiOx) by chemical means, yielding a surface which forms strong covalent bonds with a wide range of chemical functional groups; another advantage is that these surfaces remain oxide-free in the absence of oxidising agents. Standard procedures require the use of either HF, NH4F, or a mixture of both as the etching solution; however, these two chemicals are highly corrosive and toxic, posing a significant risk to the experimentalist. Here, we report that for silicon wafers etched by using potassium fluoride, a less toxic chemical, the resulting surface is free of oxides and can be functionalized by self-assembled monolayers of 1,8-nonadiyne. To demonstrate this, Si/SiOx wafers were etched by using either KF or NH4F, followed by hydrosilylation with 1,8-nonadiyne and a click reaction of the terminal alkyne with azidomethylferrocene. The surface coverages and electron transfer kinetics of the ferrocene-terminated KF-etched surfaces are comparable to those formed by acidic fluoride etching procedures. This is the first study comparing the differences between surfaces functionalized by self-assembled monolayers of 1,8-nonadiyne which were etched by KF and NH4F. KF could be used as a replacement chemical for etching silicon wafers when a less corrosive and toxic chemical is required.

Fluoride-etched TiO2 microspheres modified with Al2O3 for enhanced photocatalytic conversion of NOx into nitrate ions

Photocatalysis holds promise for the removal of NOx from the air, yet its limited selectivity toward desired products (e.g. nitrate ions) hinders its widespread application. In this study, we synthesized and comprehensively characterized amorphous TiO2 microspheres (ATMS), fluoride-etched TMS (F-TMS), and Al2O3-modified F-TMS (Al2O3/F-TMS) photocatalysts. The photocatalytic NOx abatement tests revealed negligible photoactivity for the ATMS, with significant improvement observed after the fluoride ions etching using the hydrothermal method. However, the selectivity of F-TMS for NOx removal remained lower than that of TiO2 P25. The Al2O3/F-TMS photocatalyst exhibited a substantial increase in selectivity. The analysis of NOx abatement results indicated a 2.7-fold boost in selectivity compared to TiO2 P25. A reasonable correlation between the amount of chemically absorbed water and nitrate selectivity was observed. The electron paramagnetic resonance (EPR) disclosed that the fluoride etching induced the formation of intra-band-gap hole-trapping sites and thus enhanced the photocatalytic activity. Nevertheless, the selectivity improvement was mainly attributed to Al2O3's ability to stabilize the NOx oxidation intermediates at the surface for subsequent oxidation to nitrate ions. This study provides valuable insights for tailoring photocatalysts to enhance both activity and selectivity in photocatalytic NOx abatement.

Humidity sensor based on tapered no-core fiber coated with Ti3C2Tx MXene

An optical fiber relative humidity (RH) sensor based on tapered no-core fiber (TNCF) coated with Ti3C2Tx MXene is proposed. The fluoride etching method is used to prepare Ti3C2Tx, which is used as the humidity sensitive material with a multi-layer structure, large specific surface area and rich hydrophilic groups, and optical deposition method is used to integrate Ti3C2Tx with TNCF. The diameter of TNCF between two sections of SMF is reduced to 11.6 μm to enhance RH sensitivity. The refractive index (RI) of Ti3C2Tx changes as a result of variations in RH, and this ultimately causes a shift in the transmission spectrum. The sensitivity of the device is −77 pm/%RH in the range of 33 %RH to 75 %RH. In the region of 75 %RH to 91 %RH, the sensitivity increases to 685 pm/%RH, the linear correlation coefficients of the two regions are 0.9521 and 0.9940, respectively. On average, the response time is approximately 3.5 s and the recovery time is about 12.7 s. The sensor may be utilized in possible domains, including biological, chemical, and food processing. Its benefits include a simple construction, cheap cost, high reversibility, and stability.

Modification of Zeolite Morphology via NH4F Etching for Catalytic Bioalcohol Conversion

AbstractVarious commercial zeolites, including FER, MOR, ZSM‐5, BEA, and FAU frameworks, were treated with NH4F aqueous solutions to study the effects of fluoride etching on different zeolite frameworks. NH4F‐treated small‐medium pore FER, MOR, and ZSM‐5 samples showed much higher mesoporosities than the untreated ones without alteration of the structural compositions and acidic properties. On the other hand, the 12‐membered ring zeolites BEA and FAU showed severe dissolution of the framework aluminosilicate structure after NH4F etching due to the high accessibility of fluoride species into the framework structures. The effect of NH4F concentration on the fluoride treatment of H‐ZSM‐5 zeolite was specifically studied. From the results, we observed that structural etching with 20 wt % NH4F was optimal for fabricating open‐pore H‐ZSM‐5 zeolite and resulted in a high mesoporosity with comparable relative crystallinity and acidity with respect to the untreated H‐ZSM‐5. The catalytic activities of the open‐pore H‐ZSM‐5 were evaluated with acid‐catalyzed methanol and bioethanol conversions. Remarkably, the hierarchical open‐pore H‐ZSM‐5 zeolite fabricated via fluoride etching exhibited an enhanced catalytic performance in bioethanol conversion with >85 % conversion over 34 h TOS and a higher catalytic stability in methanol conversion than the parent H‐ZSM‐5 (~50 % of bioethanol conversion at 34 h TOS).

Improving the Nitrogen Reduction Reaction Activity of Ti2n Nitride Mxene through pH and Electrolyte Selection

Electrochemical nitrogen reduction reaction (NRR) is used to convert atmospheric nitrogen (N2) to ammonia (NH3) at ambient temperature and pressure. In general, acidic electrolytes are used to provide protons (H+) for the reduction process. However, this leads to a low NH3 selectivity because of H2 by-product formation via the hydrogen evolution reaction (HER). Recently, we showed that a Ti2N MXene is active and selective for NRR. We demonstrated that this catalyst works through a Mars-van Krevelen (MvK) mechanism rather than the conventional associative/dissociative mechanisms. However, the effect of pH and electrolyte on its catalytic activity and selectivity was not well understood. Here, we investigate these effects by performing experiments in varying electrolytic conditions and develop relations between pH, electrolyte choice, and performance. The Ti2N nitride MXene was synthesized via an oxygen-assisted molten salt fluoride etching technique and characterized through XRD and Raman and FTIR spectroscopies. Our findings showed that changing pH does not affect the NRR onset potential. We found that as pH increases, selectivity towards NH3 also increases presumably due to a lack of free H+ available to participate in HER. We also found that smaller cations with smaller solvation shells suppress HER activity, while smaller anions prevent catalyst poisoning during the NRR process, as evidenced by cyclic voltammetry before and after experimental conditions. We confirm production rates and the method of NH3 formation via NMR analytical techniques, including isotopic exchange. We plan to expand these findings to other materials and systems, and use the knowledge obtained from these studies to design optimal electrolytic conditions to produce NH3 through NRR.

Molecular Lithography on Silicon Wafers Guided by Porous, Extended Arrays of Small DNA Tiles.

Tile-based DNA self-assembly is a cost-effective fabrication method for large-scale nanopatterns. Herein, we report a protocol to directly assemble DNA 2D arrays on silicon wafers and then use the DNA nanostructures as molds to fabricate the corresponding nanostructures on the silicon wafers by hydrogen fluoride (HF) etching. Similar HF etching has been used with robust large DNA origami structures as templates. This work demonstrates that DNA nanostructures assembled from small tiles are sufficiently stable for this process. The resulting feature size (∼8.6 nm) approaches the sizes of e-beam lithography. While the reported method is parallel and inexpensive, e-beam lithography is a serial method and is expensive. We expect that this method will be very useful for preparing fine nanopatterns in large areas.

Exploring the Potential of MBenes Supercapacitors: Fluorine-Free Synthesized MoAl1-xB with Ultrahigh Conductivity and Open Space.

The present study describes the synthesis of multilayered MBenes MoAl1-xB with different degrees of Al deintercalation using a mild, fluorine-free approach of dilute alkali to remove Al from MoAlB. We propose an etching route and compare it to conventional fluoride etching products. Additionally, the study explores the possible application and energy storage mechanism of MBenes in supercapacitors, marking the first investigation of its kind. At room temperature, 1/24-MoAl1-xB with terminal groups -OH exhibits ∼25% Al removal in 1 wt % NaOH for 24 h, outperforming traditional etching technology. Increasing the Al removal exposed more open space, resulting in a higher capacitance. Compared to LiF/HCl-MoAl1-xB (etched by LiF + HCl), 1/24-MoAl1-xB has a higher energy storage capability. The multilayered 1/24-MoAl1-xB film electrode exhibits ultrahigh conductivity with a rapid relaxation time of 0.97 s and high areal capacitance (2006.60 mF cm-2) while maintaining 80.2% capacitance after 5000 cycles. The MoAl1-xB all-solid-state supercapacitor (ASSS) delivers a high capacitance of 741.6 mF cm-2 at 1 mV s-1 for a single electrode and exhibits stable capacitance even at a 90° bending angle, highlighting its potential practical use. Our research represents an important step in the synthesis of MBenes and highlights their potential applications in supercapacitors.

Opening sodalite cages of X zeolite for boosting toluene side-chain alkylation performance

The structural opening of SOD cages in X zeolite is achieved by an easily controllable fluoride etching strategy, resulting in reactant molecules and active sites becoming accessible.

Improving the Selectivity of Nitrogen Reduction Reaction through the Mars-Van Krevelen Mechanism

The electrochemical nitrogen reduction reaction (NRR) process is an attractive alternative to minimizing the energy and greenhouse gas footprint from current ammonia (NH3) production processes. Most NRR catalysts operate through utilizing an associative or dissociative mechanism, during which the NRR competes with the hydrogen evolution reaction (HER), resulting in low selectivity. In this presentation, we report on a new active catalyst for NRR that operates through the Mars-van Krevelen (MvK) mechanism to increase the selectivity of NRR towards NH3. This new catalyst, two-dimensional (2D) Ti2N nitride MXene, was synthesized via an oxygen-assisted molten salt fluoride etching technique. We confirmed its phase purity and stability in aqueous electrolytes using various characterization techniques, including Raman, X-ray diffraction, and UV-Vis. The Ti2N nitride MXene catalyst achieved a high Faradaic efficiency (FE) of 19.85% towards NH3 at an applied potential of –250 mV vs. RHE with a yield of 11.33 μg/cm2/hr in a 0.1 m hydrochloric acid (HCl) N2-saturated electrolyte. Electrocatalytic activity and selectivity obtained in an Ar-saturated electrolyte confirm that the new catalyst operates through an MvK mechanism. These results can be expanded to a broad class of systems enabling the MvK mechanism and constitute the foundation of NRR technology based on MXenes.

(Digital Presentation) Achieving High Selectivity for the Nitrogen Reduction Reaction through the Mars-Van Krevelen Mechanism

Electrochemical nitrogen reduction reaction (NRR) technology is a viable alternative to reducing the energy and greenhouse gas footprint from the current ammonia (NH3) production technology. Most NRR catalysts suffer from low selectivity towards NH3 because they operate by using an associative or dissociative mechanism, during which the NRR competes with the hydrogen evolution reaction (HER). In this presentation, we report on a new catalyst and untapped mechanism for NRR to increase the selectivity towards NH3. This untapped Mars-van Krevelen (MvK) mechanism reduces the competition between NRR and HER by eliminating the sluggish hydrogenation reactions of the dissolved N2 molecule. The new catalyst, two-dimensional (2D) Ti2N nitride MXene, was synthesized via an oxygen-assisted molten salt fluoride etching technique. We confirmed its phase purity and stability in aqueous electrolytes using various characterization techniques, including x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and cyclic voltammetry (CV). Through an MvK mechanism, the Ti2N nitride MXene catalyst achieved a high Faradaic efficiency (FE) of 19.85% towards NH3 at an applied potential of –250 mV vs. RHE with a yield of 11.33 μg/cm2/hr in a 0.1M hydrochloric acid (HCl) N2-saturated electrolyte. These results constitute the foundation of NRR technology based on MXenes and can be expanded to a broad class of systems evoking the MvK mechanism.

Preparation of Hierarchical SSZ-13 with Core-Shell Structure by Post-synthesis Fluoride Etching

Core-shell structured hierarchical SSZ-13 microspheres with mesopore and zeolitic micropore were obtained via combining zeolite crystallization and post-synthesis fluoride etching. Secondary pores in core-shell SSZ-13 crystals were then formed by fluoride-containing chemical etching post-treatment routes. The samples were systematically characterized by XRD, SEM, N2 adsorption/desorption, and NH3-TPD measurements, indicating that the fluoride-treated SSZ-13 samples exhibit core-shell structure, high crystallinity, large specific surface area, and acidity. It is worth mentioning that the core-shell structured hierarchical SSZ-13 catalyst exhibited superior MTO catalytic performance with higher selectivity to ethylene and propylene as well as prolonged catalytic lifetime in comparison to the untreated SSZ-13.

Strain Sensing Coatings for Large Composite Structures Based on 2D MXene Nanoparticles.

Real-time strain monitoring of large composite structures such as wind turbine blades requires scalable, easily processable and lightweight sensors. In this study, a new type of strain-sensing coating based on 2D MXene nanoparticles was developed. A Ti3C2Tz MXene was prepared from Ti3AlC2 MAX phase using hydrochloric acid and lithium fluoride etching. Epoxy and glass fibre–reinforced composites were spray-coated using an MXene water solution. The morphology of the MXenes and the roughness of the substrate were characterised using optical microscopy and scanning electron microscopy. MXene coatings were first investigated under various ambient conditions. The coating experienced no significant change in electrical resistance due to temperature variation but was responsive to the 301–365 nm UV spectrum. In addition, the coating adhesion properties, electrical resistance stability over time and sensitivity to roughness were also analysed in this study. The electromechanical response of the MXene coating was investigated under tensile loading and cyclic loading conditions. The gauge factor at a strain of 4% was 10.88. After 21,650 loading cycles, the MXene coating experienced a 16.25% increase in permanent resistance, but the response to loading was more stable. This work provides novel findings on electrical resistance sensitivity to roughness and electromechanical behaviour under cyclic loading, necessary for further development of MXene-based nanocoatings. The advantages of MXene coatings for large composite structures are processability, scalability, lightweight and adhesion properties.

Wettability of MXene and its interfacial adhesion with epoxy resin

The surface energies of MXene nanofillers critically affect the mechanical properties and durability of any polymer-based devices and composites to which these fillers are applied. In this context, this study comprehensively investigates Ti3C2Tz MXenes prepared via the hydrochloric acid/lithium fluoride etching of Ti3AlC2. The surface energy values of 10-layer MXene coatings were evaluated to be between 47.98 and 64.48 mJ/m2 as per contact-angle measurements. The wettability properties were found to depend on the number of coating layers and the liquids used. Additionally, the coating roughness was evaluated by using atomic force microscopy. The effectiveness of MXenes as a reinforcing nanofiller for polymer matrix was investigated by utilising an epoxy resin. The work of adhesion between the MXene and epoxy was calculated to be up to 123.6 mJ/m2. The prediction of a high adhesion was also confirmed with the use of scanning electron microscopy images to examine the fractured surface of the MXene–epoxy specimens.

Porous and ultrahigh surface area SiOC ceramics based on perhydropolysilazane and polysiloxane

Abstract Micro-/meso-porous ceramics with ultrahigh surface areas are highly desired in high-temperature applications. In this work, formation of porous silicon oxycarbide (SiOC) is studied based on perhydropolysilazane (PHPS) and polysiloxane (PSO) precursors. The PHPS can be chemically anchored to the PSO by hydrosilylation reaction, due to the extensive Si–H bonds from the PHPS. The presence of water vapor during pyrolysis not only accelerates the hydrolysis of the PHPS additive, but also facilitates the Si–O–Si bond formation within the SiOC. The resulting SiOC material has the highest specific surface area (SSA) of ~2000 m2/g with an average pore size of 1.72 nm. The effects of the PHPS additive on the phase evolution and the resulting porous SiOC after hydrogen fluoride (HF) etching are investigated. 3D view of pore distributions qualitatively illustrates the PHPS effect on the SiO2 nanocluster formation in the SiOC. The difference between the experimental and calculated SSAs is explained based on the etchability and wall thickness of the SiO2 domains.

Fluoride etching of AlZSM-5 and GaZSM-5 zeolites

The post-synthesis treatment of AlZSM-5 and GaZSM-5 zeolites by etching with buffer solution of ammonium fluoride and 0.25 M HF acid was carried out. The treatment is applied in order to obtain secondary pores in crystals and to provide easier access to the catalytically active zeolite centers. Hydrothermal synthesis of these zeolites was performed from systems containing tetrapropylammonium bromide as a template. The reaction parameters of synthesis conditions for both zeolites were optimized in order to obtain pure crystalline phases. The zeolites obtained were characterized by X-ray diffraction, IR spectroscopy, scanning electron microscopy, physical adsorption–desorption of nitrogen and solid-state NMR spectroscopy. It has been found that the result materials remain with high crystallinity while new mesoporous are created in their structure. The volume due to the presence of mesopores increases by up to 67% from the total volume, which is a drastic increase compared with the parent solids. The investigations with FTIR spectroscopy of low-temperature CO adsorption and solid-state NMR spectroscopy show that the treatment is slightly selective toward dissolution of heteroatom (aluminum or gallium). This phenomenon is observed for the first time for gallium-substituted zeolites. In order to study the influence of the metal atom in zeolite structure and the efficacy of the acid attack on the catalytic activity, the samples obtained were tested in reaction of m-xylene and toluene transformation.

Facile Synthesis of Hollow Carbon Nanospheres by Using Microwave Radiation

Carbon-based nanomaterials have attracted much research interest in recent years due to their excellent chemical and physiological properties such as chemical stability, low cytotoxicity, and biocompatibility. The traditional methods to prepare hollow carbon nanospheres require complicated instrumentation and harsh chemicals, including high-temperature furnace, gas inlets, and hydrogen fluoride etching. Herein, we propose a new strategy to prepare hollow carbon nanospheres in a simple and fast manner by using microwave radiation. Polypyrrole-coated silica core-shell nanoparticles (SiO2@PPy NPs) were firstly prepared and subsequently processed by microwave radiation and aqueous alkaline solution to obtain the hollow carbon nanospheres. This facile method has potent potential to be utilized in the preparation of different types of hollow carbon nanospheres with various microstructure and elemental composition.

Fluoride etching opens access for bulky molecules to active sites in microporous Ti-beta zeolite

Non-chemically selective fluoride etching opens access for bulky molecules to active sites in microporous Ti-beta zeolite.

Etching-enabled extreme miniaturization of graded-index fiber-based optical coherence tomography probes.

.We introduced and validated a method to miniaturize graded-index (GRIN) fiber-based optical coherence tomography (OCT) probes down to in diameter. The probes consist in an assembly of single-mode (SM), coreless (CL), and graded-index (GRIN) fibers. We opted for a probe design enabling controlled size reduction by hydrogen fluoride etching. The fabrication approach prevents nonuniform etching for both the GRIN and SM fiber components, while it requires no probe polishing postetching. We found that the miniaturized probes present insignificant loss of sensitivity () compared to their thicker () counterparts. We also showed that their focusing capabilities remain tunable and highly predictable. The fabrication process is simple and can be carried out by using inexpensive telecom equipment. Both the fabrication process and the developed probes can benefit the prototyping of minimally invasive endoscopic tools.

Surface Contamination with Si and O Impurities for GaN Single-Crystalline Layers Grown by Ultra-High-Vacuum Sputter Epitaxy

In this study, we demonstrated a GaN surface contamination and cleaning technique focused on O- and Si-based compounds generated by an exposure to the atmosphere. The GaN single-crystalline layers were grown on sapphire (0001) substrates using the ultra-high-vacuum radio-frequency sputter epitaxy technique. During X-ray photoelectron spectroscopy (XPS) measurements, O1s and Si2p XPS signals were detected on the as-grown GaN layer surface. Thermal treatment was performed in order to remove these impurities, and the results of the XPS measurements indicated a significant decrease in the O–Ga bonds on the GaN layer surface. Following the buffered hydrogen fluoride etching treatment, a decrease in the Si compounds on the GaN layer surface was confirmed by a significant decrease in the Si2p/Ga3d XPS signal intensity ratio.

Electrochemical oxygen evolution reaction efficiently catalyzed by a novel porous iron-cobalt-fluoride nanocube easily derived from 3-dimensional Prussian blue analogue

A three-dimensional porous architecture derived from FeCo Prussian blue analogue by low-temperature fluoridation etching is for the first time demonstrated with largely enhanced performances for oxygen evolution reaction. The morphology and structure for FeCo Prussian blue nanocubes are controlled by fluoride etching resulting in the formation of the porous structure with a lot of defects/voids as characterized by a series of spectroscopic techniques. Iron-cobalt-fluoride nanocubes obtained at 400 °C possess the optimal porous structure and morphology to guarantee the channels for mass transportation and ion diffusion. The increased electrochemical surface area, more exposure of the active sites and facile construction of metal oxide layer over the catalyst surface make this novel catalyst very active for the water oxidation reaction. The electrochemical measurements demonstrate this catalyst can drive the benchmark current density of 10 mA cm−2 at a low overpotential of 250 mV with a Tafel slope of 38.3 mV dec−1 and an excellent catalytic stability for 10 h in the long-term water electrolysis; the performance also outperforms most of the advanced analogous nonprecious catalysts and noble commercial IrO2 catalyst. This work provides an effective and convenient approach to design three-dimensional porous architecture catalysts for energy-relevant electro-catalysis reaction.