CaF2 Surface Research Articles

Molecular Structures of Poly(methyl methacrylate) at Different Buried Interfaces Revealed by Sum Frequency Generation Vibrational Spectroscopy.

Silica or calcium fluoride (CaF2) substrate-supported poly(methyl methacrylate) (PMMA) thin films as insulating layers are commonly used in photoelectric/photovoltaic devices to improve the efficiency or stability of these devices. However, a comparative investigation of molecular structures at buried PMMA/silica and PMMA/CaF2 interfaces under thermal stimuli remains unexplored. In this study, we qualitatively and quantitatively revealed different molecular orderings and orientations of PMMA at two interfaces before and after annealing using sum frequency generation (SFG) vibrational spectroscopy. SFG vibrations were carefully assigned by using various deuterated PMMAs. SFG results indicated that, at the buried PMMA/silica interface, the side OCH3 groups were prone to lie down before annealing and tended to stand up after annealing. In contrast, the case was the opposite at the buried PMMA/CaF2 interface. The relative hydrophobicity/hydrophilicity of the two substrates and the developed hydrogen bonds upon annealing at the buried PMMA/silica interface, which is absent at the CaF2 surface, are believed to be the driving forces for different interfacial molecular structures. This study benefits the molecular-level understanding of the interfacial local structural relaxation of polymers at buried interfaces and the rational design of photoelectric/photovoltaic devices from the molecular level.

Substrate effect on charging of electrified graphene/water interfaces.

Graphene, a transparent two-dimensional (2D) conductive electrode, has brought extensive new perspectives and prospects to electrochemical systems, such as chemical sensors, energy storage, and energy conversion devices. In many of these applications, graphene, supported on a substrate, is in contact with an aqueous solution. An increasing number of studies indicate that the substrate, rather than graphene, determines the organization of water in contact with graphene, i.e., the electric double layer (EDL) structure near the electrified graphene, and the wetting behavior of the graphene: the graphene sheet is transparent in terms of its supporting substrate. By applying surface-specific heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy to the silicon dioxide (SiO2)-supported graphene electrode/aqueous electrolyte interface and comparing the data with those for the calcium fluoride (CaF2)-supported graphene [Y. Wang etal., Angew. Chem., Int. Ed., 2023, 62, e202216604], we discuss the impact of the different substrates on the charging of both the graphene and the substrate upon applying potentials. The SiO2-supported graphene shows pseudocapacitive behavior, consistent with the CaF2-supported graphene case, although the surface charges on SiO2 and CaF2 differ substantially. The SiO2 surface is already negatively charged at +0.57 V (vs. Pd/H2), and the negative surface charge is doubled when negative potentials are applied, in contrast with the CaF2 case, where the positive charge is reduced when negative potentials are applied. Interestingly, the charging of the graphene sheet is almost identical between the negatively charged SiO2 surface and positively charged CaF2 surface, demonstrating that the graphene charging is decoupled from the charging of the substrates.

(Invited) Direct Probe of Electrochemical Pseudocapacitive pH Jump at a Graphene Electrode

Molecular-level insight into interfacial water at buried electrode interface is essential in electrochemistry, but spectroscopic probing of the buried interfaces remains challenging. Here, using surface-specific heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy, we directly access the interfacial water in contact with the graphene electrode supported on calcium fluoride (CaF2). The HD-SFG signal is obtained by overlapping the infrared and visible pulses at the sample position. An SFG signal is enhanced when the infrared frequency is resonant with the molecular vibration. The HD-SFG can provide the information on the orientation of the interfacial molecules. With the molecular specificity, orientational specificity, and surface specificity of the HD-SFG, we clarify the molecular level insight into the structure of the electrified graphene-water interface. At the negative voltage, we found the negative HD-SFG peak at 3630 cm-1, which manifests that the O-H group points to the bulk water, not to the CaF2 supported graphene. This O-H group is the Ca-O-H group, indicating that the chemical reaction occurs at the CaF2 surface. By monitoring the variation of the HD-SFG spectra upon the various applied potential, We find phase transition-like variations of the HD-SFG spectra. We attribute such a variation not to charging/discharging of graphene but to charging/discharging of the CaF2 substrate through the pseudocapacitive process. The potential-dependent spectra are nearly identical to the pH-dependent spectra, evidencing that the pseudocapacitive behavior is associated with a substantial local pH change induced by water dissociation between the CaF2 and graphene. Our work evidences the local molecular-level effects of pseudocapacitive charging at an electrode/aqueous electrolyte interface.

Electrokinetic comparison of CaF2 and AgCl from a MDS perspective

CaF2 has limited electrokinetic sensitivity to fluoride ions in solution and the positive CaF2 surface charge is not reversed at high fluoride ion concentration. Similar insensitivity of surface charge to high fluoride ion concentration has been observed for other alkali and alkaline earth fluoride salts. On the contrary, chloride ions act as potential determining ions for AgCl and reverse the surface charge, as expected. FTIR-IRS results indicate that the AgCl/water interface has a water structure-breaking tendency, whereas the CaF2/water interface has a structure-making tendency. It is speculated that the strong hydrogen bonding between fluoride ions and water prevents the accommodation of excess fluoride ions at surface lattice sites of fluorite, and therefore explains the low sensitivity of the fluorite surface charge sign and magnitude to fluoride ion concentration. However, this speculation has not been examined at the molecular level and is evaluated by molecular dynamics simulations (MDS) in the research presented in this paper. Higher hydrogen bond density at the CaF2 mineral/water interface confirms that the water structure has stronger hydrogen bonding at both the CaF2 mineral surface and with the F− ion in solution. In this way, the interaction of the F− ion with the CaF2 surface is inhibited.

High Optical Harmonics Generation on Solid Surfaces Irradiated by Mid-IR Femtosecond Laser Pulses

We obtained the spectra of high optical harmonics produced by subrelativistic femtosecond pulses on the surface of polystyrene, CaF2, BK7, and Al solid targets. High harmonics of up to the 51st order of radiation with central 3.85 µm wavelength were observed. The highest order harmonics were generated from the polystyrene target surface. The harmonics energy versus their numbers is shown to fit well a decaying power law with the exponent ranging from 4 to 8/3.

Nano-effect enhanced cooperative luminescence of Yb3+ clusters in bulk materials

In order to improve the multi-ion cooperative transition, we proposed and exploited a novel nanoscale effect, namely the nanoshell effect in bulk materials. Based on the effect, an optimal material structure was designed by coating the surfaces of CaF2:Yb3+ micron size particles with ZrO2. An about 2 times higher intensity of cooperative luminescence is observed upon laser excitation at 980 nm. Dynamical analysis exhibits that the novel effect plays a key role in improving the performance of cooperative transitions. Our results also suggest that the nanoshell effect in bulk materials is likely to be significant in some special cases, which have not been reported yet in the literature.

Influence of fluoride ion on the performance of PCE in hemihydrate gypsum pastes

Impurities in phosphogypsum have a significant impact on its properties and the performance of admixture. This study aims to reveal the influence of fluoride ion (F−) on the performance of polycarboxylate superplasticizer (PCE) in hemihydrate gypsum pastes. By introducing F− into the natural hemihydrate gypsum - PCE system, workability, mechanical properties, and hydration process were systematically studied. The results showed that F− had a negative effect on the fluidity of PCE plasticized hemihydrate gypsum pastes. Moreover, F− significantly reduced the setting time regardless of the presence or absence of PCE. It was found that CaF2 with low solubility was preferentially formed to CaSO4⋅2H2O and served as nuclei to promote the hydration of hemihydrate gypsum. The adsorption behavior of PCE on the surface of CaF2 consumed PCE, resulting in the decline of dispersion ability.

Tendencies in ABO3 Perovskite and SrF2, BaF2 and CaF2 Bulk and Surface F-Center Ab Initio Computations at High Symmetry Cubic Structure

We computed the atomic shift sizes of the closest adjacent atoms adjoining the (001) surface F-center at ABO3 perovskites. They are significantly larger than the atomic shift sizes of the closest adjacent atoms adjoining the bulk F-center. In the ABO3 perovskite matrixes, the electron charge is significantly stronger confined in the interior of the bulk oxygen vacancy than in the interior of the (001) surface oxygen vacancy. The formation energy of the oxygen vacancy on the (001) surface is smaller than in the bulk. This microscopic energy distinction stimulates the oxygen vacancy segregation from the perovskite bulk to their (001) surfaces. The (001) surface F-center created defect level is nearer to the (001) surface conduction band (CB) bottom as the bulk F-center created defect level. On the contrary, the SrF2, BaF2 and CaF2 bulk and surface F-center charge is almost perfectly confined to the interior of the fluorine vacancy. The shift sizes of atoms adjoining the bulk and surface F-centers in SrF2, CaF2 and BaF2 matrixes are microscopic as compared to the case of ABO3 perovskites.

Prospective nanostructured coatings for modification of calcium fluoride surfaces

The basic properties of materials can be changed by structuring their volume with various nanoparticles as well as by surface modification. In this study, calcium fluoride (CaF2) is selected as a model matrix for investigating the characteristic surface properties, and its spectral and mechanical properties, as well as the surface wetting of a CaF2 surface modified with carbon nanotubes, are compared with the properties of pure crystals. Laser-oriented deposition and an additional electric field varied in the range of 100–600 V/cm are applied for surface structuring. The experimental results are confirmed by quantum-chemical modeling.

A new preparation method of CaF2@SiO2 nano solid lubricant and analysis of its coating mechanism

The microstructure of materials is the basic factor affecting their properties. In order to reduce the influence of adding solid lubricant on the performance of ceramic cutting tools, a new preparation method was adopted to prepare CaF2@SiO2 solid lubricant. Firstly, uniformly dispersed CaF2 nanoparticles were obtained by precipitation and azeotropic distillation, and then SiO2 was coated on the surface of CaF2 by using the hydrolytic condensation effect and heterogeneous nucleation effect of TEOS, so as to prepare CaF2@SiO2 solid lubricant with core-shell structure. The effects of process parameters on the microstructure and surface morphology of coated nanoparticles were analyzed. The results show that TEOS titration speed, pH value, temperature and azeotropic distillation have great influence on the quality of nano-silica powder. The optimum preparation parameters are: pH 8.5, TEOS dropping rate 0.4 mL/min, reaction temperature 40 °C and azeotropic distillation. The formation process and surface coating modification mechanism of CaF2@SiO2 nanoparticles were analyzed. Owing to heterogeneous nucleation theory, SiO2 can be adsorbed on the surface of CaF2 stably, and the interaction between SiO2 and CaF2 can form a stable adsorption interface. And the driving force of nucleation changes with the process parameters. By changing different process parameters, the controllable preparation of coating can be realized, and the optimal thickness of coating is about 10 nanometers.

Plumbing the depths of the graphene wetting controversy

Plumbing the depths of the graphene wetting controversy

Double sample holder for efficient high-resolution studies of an insulator and a metal surface.

A double sample holder supporting both a metal sample and an insulator crystal for high-resolution scanning probe microscopy experiments is described. The metal sample serves as a substrate for tip preparation and tip functionalization to efficiently and reliably enable high-resolution studies of the adjacent insulator surface. Imaging of Ag(111)/mica, Au(111)/mica, CaF2(111), and calcite(104) surfaces is demonstrated at 5K, including images on calcite(104) produced with a CO terminated tip, which was prepared on the adjacent metal sample.

Protruding hydrogen atoms as markers for the molecular orientation of a metallocene.

A distinct dumbbell shape is observed as the dominant contrast feature in the experimental data when imaging 1,1’-ferrocene dicarboxylic acid (FDCA) molecules on bulk and thin film CaF2(111) surfaces with non-contact atomic force microscopy (NC-AFM). We use NC-AFM image calculations with the probe particle model to interpret this distinct shape by repulsive interactions between the NC-AFM tip and the top hydrogen atoms of the cyclopentadienyl (Cp) rings. Simulated NC-AFM images show an excellent agreement with experimental constant-height NC-AFM data of FDCA molecules at several tip–sample distances. By measuring this distinct dumbbell shape together with the molecular orientation, a strategy is proposed to determine the conformation of the ferrocene moiety, herein on CaF2(111) surfaces, by using the protruding hydrogen atoms as markers.

High-precision atomic force microscopy with atomically-characterized tips

Traditionally, atomic force microscopy (AFM) experiments are conducted at tip–sample distances where the tip strongly interacts with the surface. This increases the signal-to-noise ratio, but poses the problem of relaxations in both tip and sample that hamper the theoretical description of experimental data. Here, we employ AFM at relatively large tip–sample distances where forces are only on the piconewton and subpiconewton scale to prevent tip and sample distortions. Acquiring data relatively far from the surface requires low noise measurements. We probed the CaF2(111) surface with an atomically-characterized metal tip and show that the experimental data can be reproduced with an electrostatic model. By experimentally characterizing the second layer of tip atoms, we were able to reproduce the data with 99.5% accuracy. Our work links the capabilities of non-invasive imaging at large tip–sample distances and controlling the tip apex at the atomic scale.

Adsorption of Water Molecule on Calcium Fluoride and Magnesium Fluoride Surfaces: A Combined Theoretical and Experimental Study

The adsorption and dissociation of H2O molecule on various surfaces of MgF2 and CaF2 have been investigated theoretically by using a dispersion-corrected density functional theory based method. The...

Preparation of nano - coating powder CaF2@Al(OH)3 and its application in Al2O3/Ti(C,N) self-lubricating ceramic tool materials

Nano CaF2 particles of different sizes were prepared by direct precipitation. The diameters of nano CaF2 particles prepared in mixed solvent can reach 5–7 nm, and can be effectively dispersed. The surface of nano CaF2 was modified and coated by heterogeneous nucleation method. A shell layer of Al(OH)3 was coated on the surface of nano CaF2, and the structure and coating mechanism of the coated powder were analyzed. Under varying preparation conditions, the surface morphology of CaF2@Al(OH)3 was analyzed using TEM and SEM. The results showed that the coating powder showed good dispersion in mixed solvents, and the particle size of the composite powder was about 20 nm. Self-lubricating ceramic tool materials were prepared by adding coating particles to the Al2O3/Ti(C,N) matrix. The coating powder shell and the matrix material melt during sintering, so that CaF2 forms nanostructures in the particles. thereby improving the mechanical properties of the material. Cutting experiments show that the addition of coating particles can effectively reduce the temperature, cutting force and friction coefficient in the cutting process of the tool, thus improving the cutting performance of the tool material.

Synthesis and Simulation of CaF2@Al(OH)3 Core-Shell Coated Solid Lubricant Composite Powder

In self-lubricating ceramic tools, adding CaF2 will significantly reduce the mechanical properties of ceramic tools. Based on heterogeneous nucleation theory, we have recently prepared aluminum hydroxide (Al(OH)3) coating on calcium fluoride (CaF2) through a liquid-phase heterogeneous nucleation method. By adding CaF2@Al(OH)3 coated powder to replace CaF2 powder, the self-lubricating ceramic tools maintain higher lubricity while also having better mechanical properties. The coating process was further confirmed by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In addition, we used the molecular simulation software to simulate the suspension system of CaF2, Al(NO3)3·9H2O, and Al(OH)3 to study the process of Al(OH)3 coating on the surface of CaF2 particle to form CaF2@Al(OH)3 powders with core-shell structure. Further, the formation and evolution of Al(OH)3 molecules on the surface of CaF2 were analyzed.

Synthesis of CaF2 Nanoparticles Coated by SiO2 for Improved Al2O3/TiC Self-Lubricating Ceramic Composites.

In order to reduce the influence of CaF2 addition on the mechanical properties of self-lubricating ceramic tools, we applied a silicon dioxide (SiO2) coating on calcium fluoride (CaF2) nanoparticles through hydrolysis and condensation reactions using the tetraethoxysilane (TEOS) method. The powder was dried by the azeotropic method, so that it acquired a better dispersibility. The resulting composite powders were characterized using XRD (X-ray diffraction) and TEM (transmission electron microscopy), showing that the surface of CaF2 was coated with a layer of uniform and compact SiO2. SiO2 shells with different thicknesses could be obtained by changing the amount of TEOS added, and the thickness of the SiO2 shells could be controlled between 1.5 and 15 nm. At the same time, a ceramic material containing CaF2 nanoparticles and CaF2@SiO2-coated nanoparticles was prepared. It had the best mechanical properties when CaF2@SiO2-coated nanoparticles were added; its flexural strength, fracture toughness, and hardness were 562 ± 28 MPa, 5.51 ± 0.26 MPa·m1/2, and 15.26 ± 0.16 GPa, respectively. Compared with the ceramic tool containing CaF2 nanoparticles, these mechanical properties were increased by 17.57%, 12.67%, and 4.88%, respectively. The addition of CaF2@SiO2-coated nanoparticles greatly improved the antifriction and wear resistance of the ceramic material, and the antifriction and wear resistance were balanced.

The Morphology and Electronic Properties of Si Nanoscale Structures on a CaF2 Surface

The surface morphology, crystal structures, and band-energy parameters have been studied for nanofilms and regularly arranged nanoscale Si phases with a thickness of 1–2 nm. The bandgap thickness of nanocrystalline Si phases with 2–3 single layers is found to be ~1.4 eV.

Mechanical properties and microstructure of Al2O3/Ti(C,N)/CaF2@Al2O3 self-lubricating ceramic tool

Due to the poor mechanical properties of solid lubricants, direct addition to the ceramic matrix will reduce the mechanical properties of the material. In this study, Al(OH)3 was coated on the surface of nano CaF2 by non-uniform nucleation method and added to Al2O3/Ti(C,N) ceramic matrix. Al2O3/Ti(C,N)/CaF2@Al2O3 was prepared by vacuum hot pressing sintering. Compared with the material directly added with nano-solid lubricant, the mechanical properties of ceramic tools added with coated nano-solid lubricant have been significantly improved. Among them, when the content of coated nano-solid lubricant is 10 vol%, the ceramic material has better comprehensive mechanical properties. SEM observation shows that the cross-sectional particle distribution of nano-coated powder ceramic material is relatively uniform and has good compactness. The fracture mode of ceramic materials is a mixed fracture mode of intergranular fracture and transgranular fracture.