Fabrication of super-hydrophobic film with dual-size roughness by silica sphere assembly

Two simple approaches were developed for the fabrication of super-hydrophobic film with dual-size roughness by taking advantage of assembling silica micro- and nanospheres. Electrostatic adsorbing technique and template-directed self-assembly were used here. The dual-size surface, which mimics the surface topology of lotus leaves, comprises both the micro-scale and nano-scale roughness. After the roughened surface was chemically modified with a layer of fluoroalkylsilane, super-hydrophobicity with a water contact angle higher than 160° and sliding angle as low as 0.5° can be achieved. The simplicity and cheapness of this procedure may make widespread applications of this super-hydrophobic film possible.     

Fe-containing nanoparticles on the surface of silica microgranules

We have prepared nanomaterials consisting of SiO₂ microgranules and Fe-containing nanoparticles on their surface and determined the size and composition of the nanoparticles. The nanoparticles are shown to have a core-shell structure (α-Fe core and γ-Fe₂O₃ shell). The α-Fe core and Fe₂O₃ shell are responsible, respectively, for the broad and narrow lines in the EMR spectrum of the nanoparticles. The magnetic properties of larger nanoparticles are dominated by the core, while those of smaller nanoparticles are dominated by the oxide shell.     

Increasing surface area of silica nanoparticles with a rough surface

The silica nanoparticles with a rough surface were developed using a silane precursor in a reverse microemulsion followed by a drying treatment. The surface roughness of the nanoparticles was adjustable by changing the amount of the precursor. Within a certain range, the roughness increased as the amount of the silane precursor increased. The rough surface provided a larger surface area than the smooth one. The produced nanoparticles were characterized using the transmission electron microscopy, ultraviolet-visible spectroscopy, energy-dispersive X-ray elemental analysis, and Brunauer-Emmet-Teller analysis technique. Additionally, the amount of surface functional amino groups on the nanoparticles was detected using the traditional acid-base titration and the dissociation constant of this functional group was calculated. On the basis of the experimental results, the mechanism of the formation of the rough surface was proposed. Finally, the produced silica nanoparticles were utilized as a carrier for the chemical binding of a near-infrared dye molecule and the adsorption of the gold nanoparticles. The results demonstrated that the rough surface provide the silica nanoparticles with a high capacity of surface chemical and supramolecular reactions.     

Modifying the surface energy and hydrophobicity of the low-density silica aerogels through the use of combinations of surface-modification agents

Aerogels are lightweight, highly transparent, thermally insulating materials comprising interconnected nanostructured pores. Low surface energy aerogels were prepared from ambient pressure drying of sodium silicate-based gels by modifying the pore surfaces with silylating agents including trimethylchlorosilane (TMCS), hexamethyldisiloxane (HMDSO), and hexamethyldisilazane (HMDZ), in combination with each other. Hydrophobic properties of the resulted aerogels were studied by contact angle measurements. Fourier-transform infrared spectroscopy (FTIR) was used to monitor the changes in chemical bonds within the aerogels due to surface modification. The microstructure was studied by transmission electron microscopy (TEM). Effect of temperature on the hydrophobicity of the aerogels was studied by thermogravimetric analysis/differential thermal analysis (TGA-DTA). Surface modification of silica gels with various mixtures of surface-modification agents showed different behaviors. Aerogels made by HMDZ and HMDSO combination comprised 5 nm pores and particles and showed a high surface energy, whereas aerogels prepared by HMDSO and TMCS combination had lower surface energy with relatively larger particle and pore sizes with a more uniform distribution of both. The properties of the latter sample were attributed to a greater degree of surface modification and negligible condensation of OH groups. This preparation produced silica aerogels with a low density (0.042 g/cc), low surface energy (3.39 N cm⁻¹), low thermal conductivity (0.050 W K⁻¹ m⁻¹), high optical transmission (85% at 700 nm) and hydrophobic (154° contact angle) with high hydrophobic thermal stability (425 °C). Moreover, the contact angle for materials prepared by this method decreased negligibly over 12 months’ storage in ambient conditions.     

Palladium infiltration in high surface area microporous silica nanoparticles

Metallic Pd clusters were embedded into a host matrix of microporous SiO₂ nanoparticles via a solution reduction of Pd(NO₃)₂ by hydrazine hydrate. The infiltration of 33 wt.% Pd leads to a 13% porosity loss of SiO₂ nanoparticles, which demonstrated an initial surface area of 748 m²/g. The presence of Pd in the pores was demonstrated by EDS spectroscopy and by X-ray diffraction. The metallic guest species presumably reside in the accessible micropores with an estimated size about 1.3 nm. Hydrogen uptake was measured for Pd-infiltrated SiO₂ nanoparticles. A possible mechanism for the formation of composite nanoparticles is proposed based on electrostatic interaction between Pd²⁺ and SiO₂ nanoparticles.     

Plasmonic properties of polycrystalline hollow Au nanoparticles: a surface roughness effect

Hollow Au nanoparticles with a 25 nm polycrystalline shell and a 50 nm hollow core were produced in large amounts by using electrochemically evolved hydrogen nanobubbles as templates and reducing agents for electroless deposition from a Na3Au(SO3)2 electrolyte. The surface roughness of these nanoparticles can be tuned by adding NaSO3 into the electrolyte. Different surface roughnesses can be readily obtained for sub-100 nm particles with the same size. As surface roughness increases, surface plasmon resonance (SPR) peaks shift to longer wavelengths. Particles with an 8 nm roughness have a SPR peak centered at 750 nm, which is particularly attractive for in vivo diagnostic and therapeutic applications. A three-dimensional finite difference time domain (FDTD) simulation confirms that the red-shifts of SPR peaks are mainly caused by their surface roughness, and the hollow nature of these particles plays only a minor role. This unique plasmonic property of hollow Au nanoparticles opens up the possibility to maintain the desirable optical properties after loading other substances into the hollow core to form multifunctional core-shell nanoparticles.     

Dispersion of amorphous silica nanoparticles via beads milling process and their particle size analysis,hydrophobicity and anti-bacterial activity

In this present work, amorphous silica is synthesized by simple solution method using sodium silicate (Na₂SiO₃) as raw material. The synthesized silica is dispersed in various dispersing agent obtained from local paint industry and used for painting application. XRD analysis revealed the existence of amorphous silica with a peak at 2θ value of 23° and the SEM analysis exemplified silica nanoparticles demonstrating spherical morphology with agglomeration. Different dispersing agents (as indicated by the codes given by paint industry) were used for the dispersion of SiO₂ by beads milling process and its effects were studied. Among the several dispersing agents used amorphous silica dispersed in SND 504 (Sodium salt of polymeric carboxylic acid with water) dispersing agent exhibit better dispersion compared to the other dispersing agents. Further, 10 wt% of SND 504 dispersing agent was optimized with the particle size to 384 nm and zeta potential value of −24.69 mV. The contact angle measurement of the dispersed silica reveals the superior hydrophobic behaviour of SiO_2, especially with 10 wt% SND 504 dispersing agent. The critical surface tension of SiO₂ with 10 wt% SND 504 dispersing agent reveal low value compared to other concentration of dispersant. Thus, the dispersed silica nanoparticles with enhanced hydrophobicity can be effectively used for painting applications as fillers. Silica dispersed in 10 wt% SND 504 dispersing agent show superior anti-bacterial activity compared to the bare silica which is also reported.     

On the role of surface energy and surface stress in phase-transforming nanoparticles

The role of surface energy and surface stress has been a topic of extensive discussion since the seminal work by Gibbs [Gibbs JW. The scientific papers of J. Willard Gibbs. Vol. I: Thermodynamics. New York and Bombay:Longmans, Green, and Co; 1906; Gibbs JW. Collected works. New Haven:Yale University Press; 1957]. Both quantities have the same value for liquids, but not for solids. The distinction between these terms is of special importance for phase transforming nanoparticles (precipitates, transforming or melting/solidifying single particles), since surface quantities scale as the inverse of the particle size relative to volume quantities.

Continuum mechanics and, especially, the concept of configurational forces (stresses) provide a convenient framework for distinguishing between “surface energy”, “surface tension” and “surface stress”. Therefore, this progress report gives a rather detailed introduction into the continuum mechanics and thermodynamics of a moving surface.

The transformation conditions for the cases where an entire nanoparticle transforms suddenly and when the transformation is interface-driven are discussed. A global transformation condition for a sudden phase-transforming nanoparticle is explained. For the interface-driven transformation, the concept of configurational forces is applied to derive a local transformation condition in a material point at the phase interface.

Four examples of nanoparticles (growing precipitate, growing solid nucleus in liquid, melting particle, solidifying particle) are studied in detail. The surface energy and surface stress are shown to contribute to the thermodynamic driving force on the interface in different ways. These contributions are quantified and discussed with respect to the case of a sudden transformation of the nanoparticle.     

Superhydrophobic films on glass surface derived from trimethylsilanized silica gel nanoparticles

The paper deals with the fabrication of sol-gel-derived superhydrophobic films on glass based on the macroscopic silica network with surface modification. The fabricated transparent films were composed of a hybrid -Si(CH(3))(3)-functionalized SiO(2) nanospheres exhibiting the desired micro/nanostructure, water repellency, and antireflection (AR) property. The wavelength selective AR property can be tuned by controlling the physical thickness of the films. Small-angle X-ray scattering (SAXS) studies revealed the existence of SiO(2) nanoparticles of average size ～9.4 nm in the sols. TEM studies showed presence of interconnected SiO(2) NPs of ～10 nm in size. The films were formed with uniformly packed SiO(2) aggregates as observed by FESEM of film surface. FTIR of the films confirmed presence of glasslike Si-O-Si bonding and methyl functionalization. The hydrophobicity of the surface was depended on the thickness of the deposited films. A critical film thickness (>115 nm) was necessary to obtain the air push effect for superhydrophobicity. Trimethylsilyl functionalization of SiO(2) and the surface roughness (rms ≈30 nm as observed by AFM) of the films were also contributed toward the high water contact angle (WCA). The coated glass surface showed WCA value of the droplet as high as 168 ± 3° with 6 μL of water. These superhydrophobic films were found to be stable up to about 230-240 °C as confirmed by TG/DTA studies, and WCA measurements of the films with respect to the heat-treatment temperatures. These high water repellant films can be deposited on relatively large glass surfaces to remove water droplets immediately without any mechanical assistance.     

On the connection between microstructure and surface roughness of brass sheets and their formability

The aim of this work was the analysis of the relationships between material properties and forming limits of a sheet, caused by the strain localization in the groove. In particular, we aim at the replacement of a nonmeasurable inhomogeneity coefficient of the material, the value of which has to be taken a priori, by measurable coefficients. In the proposed model, it is assumed that the material heterogeneity is a result of surface roughness and presence of internal defects (voids). The value of inhomogeneity coefficients changes with increasing strain. The experiments were carried out for M85 brass sheets, annealed to produce different microstructures. The measured values of some material parameters: strain-hardening constants n and C, normal anisotropy factors, inhomogeneity coefficients (surface roughness and volume fraction of voids) as well as forming limit curve demonstrated their dependence on the grain size. As a result of the experimental work, original equations were proposed, describing the relationship between inhomogeneity coefficients and effective strain and grain size. These equations were used in the theoretical calculation of the forming limit diagram. The theoretical calculations of the strain limits of the tested sheets were based on the associated flow rules, assuming strain-hardening and strain-softening process (Shima–Oyane equation and equations based on the Gurson theory). The analysis of the influence of different material parameters on the forming limit diagram has shown that in the case of the material tested, the value of limit strains depends decisively on the values of the inhomogeneity coefficients and grain size. The postulates showing proportional relationship between the value of a strain-hardening exponent and limit strains were not confirmed.     

Ultra low-dielectric-constant methylated mesoporous silica films with high hydrophobicity and stability

Vapor phase treatment with tetraethyl orthosilicate (TEOS) is used to improve the performance of methylated mesoporous silica films spin-coated on silicon wafers. Subsequent calcination leads to formation of ultra low dielectric-constant (k) films with high hydrophobicity and structural stability. The k value of the films is about 1.75, and remains as low as 1.82 in an 80%-relative-humidity environment over seven days. Mechanical strength (elastic modulus and hardness) is high enough to withstand the stresses that occur during the chemical mechanical polishing and wire bonding process (E = 10.9 GPa). Effects of the methyl group and TEOS vapor treatment on the structural stability and hydrophobicity are systematically studied.     

Resonance energy transfer-amplifying fluorescence quenching at the surface of silica nanoparticles toward ultrasensitive detection of TNT

This paper reports a resonance energy transfer-amplifying fluorescence quenching at the surface of silica nanoparticles for the ultrasensitive detection of 2,4,6-trinitrotoluene (TNT) in solution and vapor environments. Fluorescence dye and organic amine were covalently modified onto the surface of silica nanoparticles to form a hybrid monolayer of dye fluorophores and amine ligands. The fluorescent silica particles can specifically bind TNT species by the charge-transfer complexing interaction between electron-rich amine ligands and electron-deficient aromatic rings. The resultant TNT-amine complexes bound at the silica surface can strongly suppress the fluorescence emission of the chosen dye by the fluorescence resonance energy transfer (FRET) from dye donor to the irradiative TNT-amine acceptor through intermolecular polar-polar interactions at spatial proximity. The quenching efficiency of the hybrid nanoparticles with TNT is greatly amplified by at least 10-fold that of the corresponding pure dye. The nanoparticle-assembled arrays on silicon wafer can sensitively detect down to approximately 1 nM TNT with the use of only 10 microL of solution (approximately 2 pg TNT) and several ppb of TNT vapor in air. The simple FRET-based nanoparticle sensors reported here exhibit a high and stable fluorescence brightness, strong analyte affinity, and good assembly flexibility and can thus find many applications in the detection of ultratrace analytes.     

Modeling and measurements of atomic surface roughness

We present a geometrical model of atomic topography with which to obtain a quantitative assessment of surface roughness. A series of two- and three-dimensional atomic surface roughness equations with sufficiently realistic parameters is developed to permit quantitative comparison with scanning-tunneling microscope and atomic-force microscope (AFM) experimental results. The model is sufficiently simple that one can easily use it to interpret experimental data. Tables are provided with estimated values for two- and three-dimensional rms atomic surface roughness in pure metal crystals and ionic crystals based on the atomic surface roughness equations. We use these roughness equations to determine the roughness of cleaved muscovite mica [essentially, KAl(2)(OH)(2)Si(3)AlO(10)]; the calculated values for both two- and three-dimensional roughness are consistent with those obtained in our AFM measurements. In addition, we demonstrate both theoretically and experimentally that atomic surface roughness is never zero.     

Synthesis and properties of hollow silica nanoparticles

Hollow nanoparticles of silicon dioxide (SiO₂) have been obtained using Cu/SiO₂ core-shell nanoparticles as precursors. An original technique based on heating the precursor nanoparticles to T = 400°C followed by a nanochemical reaction of copper oxide separation from hollow silica particles has been proposed and implemented for the first time. The obtained hollow SiO₂ nanoparticles have been studied by transmission electron microscopy. Mechanisms involved in the formation of hollow silica nanoparticles are discussed.     

Improvement of thermal stability of UV curable pressure sensitive adhesive by surface modified silica nanoparticles

Pressure sensitive adhesives (PSAs) with higher thermal stability were successfully prepared by forming composite with the silica nanoparticles modified via reaction with 3-methacryloxypropyltrimethoxysilane. The acrylic copolymer was synthesized as a base resin for PSAs by solution polymerization of 2-EHA, EA, and AA with AIBN as an initiator. The acrylic copolymer was further modified with GMA to have the vinyl groups available for UV curing. The peel strength decreased with the increase of gel content which was dependent on both silica content and UV dose. Thermal stability of the composite PSAs was improved noticeably with increasing silica content and UV dose mainly due to the strong and extensive interfacial bonding between the organic polymer matrix and silica.