Colloidal Silica Particles Research Articles

Surface modification of colloidal silica particles using cationic surfactant and the resulting adsorption of dyes

Surfactant interactions, self-association, and self-organizing properties can play an important role in customizing colloidal particles surface. Colloidal silica (CS) is an essential biocompatible material, which makes it a suitable candidate for various studies related to surface modifications. In this work, colloidal silica (SiO2) was synthesized using a modified Stober's process. Point of zero charge (PZC) was measured by standard pH titrations. Field emission scanning electron microscopy (FE-SEM) was employed to examine the particle's morphology while phase integrity was confirmed using powder X-rays diffraction (XRD) technique. CS interactions with a cationic dye “methylene blue” (MB) and anionic dye “congo red” (CR) were investigated spectrophotometrically. Later on, CS was modified using cetyltrimethylammonium bromide (CTAB) and observed the changes in PZC and surface morphology, zeta potential and hydrodynamic diameter. The interactions of CTAB modified particles with MB and CR were reinvestigated. For both modified colloidal SiO2 (MCS) and unmodified colloidal SiO2 (CS), different models of adsorption and adsorption kinetics were applied to the UV–Vis spectroscopic data to quantify binding interactions, adsorption efficiency, changes in enthalpy (ΔH), entropy (ΔS), and free energy (ΔG) of the adsorption processes. CS has shown efficient adsorption of MB while MCS was found as a good adsorbent for CR. CTAB enabled the modification of an “anionic surface” of CS to an efficient “cationic surface” and this was demonstrated by significant absorption of an anionic dye CR on the CS surface as well as by the changes in zeta potential and hydrodynamic radius of particles before and after modifications by CTAB.

Comparative Study of Simultaneous Removal Performance of Silica and Solid Colloidal Particles from Chalcopyrite Bioleachate Solution by Washing and Coagulation Methods

Most of the surface reserves of copper minerals are oxidized while underground mining leads to rich silica–chalcopyrite–pyrite ore. Based on the challenges of the twenty-first century, the (bio)-hydrometallurgy processing is suggested as an alternative route to replace conventional techniques. However, the search for a sustainable metallurgy and/or more effective method to decrease impurities for efficient solvent extraction led to a comparative study of the removal performances of silica and solid colloidal particles from bioleachate solution by comparing the techniques using Shellsol set #2325 and Magnafloc set #1597, respectively. Three ratios (thinner/aqueous) or/and (coagulant /aqueous) of 1/2, 1, and 3/2 were assessed, along with the disengagement time and the required absorbed dose of Magnafloc 1597. The search for the alternative route also sought to ensure that the requirements in terms of concentration values at solvent extraction stage did not exceed 500 and 75 ppm, respectively. Findings show that the diluent/wash method is less efficient in terms of the removal performances of silica and solid colloidal particles from the bioleachate solution, but the phase disengagement time was similar, being comparatively around 150 s. The results of Magnafloc 1597 coagulation tests show removal performances of silica and colloidal particles from the bioleachate solution to be greater than 300 ppm, with a volume ratio of 1/1 for recovery rates of 68 and 58%, respectively, for SiO2 and colloidal solids in suspension. In comparison, the employment of Shellsol 2325 achieved silica removal efficiencies of 20.92 and 40%, respectively, of SiO2 and colloidal fine particles in suspension. A decreased phase separation time in aqueous continuity from 245 to 148 s has been recorded with a ratio of 1/1 (thinner or coagulant/aqueous). On the other hand, coagulant concentration of 300 ppm was retained. Hence, it could be assumed that the coagulation showed better results than the washing method.

Preparation of Dimpled Polystyrene-Silica Colloidal Nanocomposite Particles.

Preparation of polymer-silica colloidal nanocomposite particles with concave shape is challenging and seldom reported. This paper presents a novel and facile method to prepare dimpled polymer-silica nanocomposite particles with a thin silica shell through the judicious combination of alcoholic dispersion polymerization and the decane evaporation method. Submicrometer-sized polystyrene-silica (PS-SiO2) nanocomposite particles were first prepared by dispersion polymerization of styrene in methanol in the presence of a methanolic silica sol, and then dimpled PS-SiO2 particles were prepared by heating the near-spherical PS-SiO2 particles dispersed in methanol/water media in the presence of decane and subsequent cooling. The effects of different heat treatment parameters, such as methanol/water ratio, stirring temperature, and stirring rate on the formation of the nanocomposite particles were investigated. Optimization of the heating conditions allowed ∼100% of dimpled particles to be achieved with one dimple on each particle. Moreover, calcination of the dimpled PS-SiO2 nanocomposite particles led to the formation of hollow dimpled particles with a thin silica shell. This method is expected to enrich the shapes of polymer-silica nanocomposite particles.

Removal Performance of Silica and Solid Colloidal Particles from Chalcopyrite Bioleaching Solution: Effect of Coagulant (Magnafloc Set #1597) for Predicting an Effective Solvent Extraction

Removal Performance of Silica and Solid Colloidal Particles from Chalcopyrite Bioleaching Solution: Effect of Coagulant (Magnafloc Set #1597) for Predicting an Effective Solvent Extraction

Control of degradation rate of Mg alloys using silica sol–gel coatings for biodegradable implant materials

Silica sol–gel coatings were deposited on AZ31B and AZ91D magnesium substrates with the aim of controlling the degradation rate, corrosion attack and further dissolution of magnesium alloys for temporal implants. Two different silica sols were prepared with and without colloidal silica particles (MTL and TG sols) and deposited by dip-coating on both alloys. The coatings were characterised in SBF using three different in vitro tests; hydrogen evolution, pH variation and potentiodynamic polarization curves. The results show that the corrosion behaviour depends on the alloy and on the coating composition. Coatings on AZ91D-MTL were able to block the degradation of the alloy during 8 days of immersion in SBF, whereas coatings AZ91D-TG only maintain their stability during 3 days. Mg(OH)2 and hydroxyapatite (HAp) were identified as the compounds precipitated on AZ31B samples, not appearing on AZ91D samples. The electrochemical tests confirm the promising corrosion results obtained for coated AZ91D samples.

Forming nanostructured surfaces through Janus colloidal silica particles with nanowrinkles: A new strategy to superhydrophobicity

Proper nano-structuring coupled with low surface energy properties is exploited to obtain super-hydrophobic surfaces. A biomimicking hierarchically structured coating upon a material surface is obtained by depositing Janus nano-structured wrinkled colloidal particles. The morphology of the surface is thus ruled by different scales: the size of the wrinkles (few tens of nanometer), particle radius (several hundreds of nanometer) and interparticle distance. Janus wrinkled silica particles have one side exposing hydroxyl groups while the other side is partially silanized with dichlorodimethylsilane. To obtain these Janus particles, wax colloidosomes with surface covered by starting rough particles were successfully prepared. The exposed surface of rough wrinkled silica particles was then silanized, thus obtaining a Janus structure. The Janus character was prooved in different manners: the simple so called “visual test”, through Energy Filtered Transmission Electron Microscopy (was proven in different EFTEM) and by further functionalization with polypropylene grafted maleic anhydride (PPgMA).A glass microscope slide was covered by drop casting the particles obtaining super-hydrophobic features of the surface characterized by a high water contact angle (149°) and, more interestingly, by a very low water contact angle hysteresis (CAH = 2°) and roll-off angle (ROA = 1.8°). Superhydrophobicity is confirmed by the very low values of apparent surface free energy estimated with the OWRK (Owens-Wendt-Rabel e Kaelble) method. SEM and AFM analyses prove the multiple scale hierarchical roughness, from nano to micro size, that resembles the one present at the surface of the lotus leaves.

H2O/D2O Contrast Variation for Ultra-Small-Angle Neutron Scattering to Minimize Multiple Scattering Effects of Colloidal Particle Suspensions

This study investigated the use of solvent contrast (H2O/D2O ratio) as a means to optimize the ultra-small-angle neutron scattering (USANS) signal. By optimizing the signal, it was possible to reduce the undesirable effects of coherent multiple scattering while still maintaining a measurable scattered intensity. This result will further enable the use of USANS as a probe of the interactions between colloidal particles and their structures within concentrated suspensions as well as particle dispersion/aggregation. As a model system, we prepared silica colloidal particle suspensions at different solid concentrations. USANS curves were measured using the classical Bonse–Hart double crystal diffractometer while varying the scattering length density of the aqueous phase, thus varying the contrast to the silica particles. As a means of assessing the impact of multiple scattering effects on different q-values, we analyzed the scattered intensity at different contrasts at three different q values. The data were then used to determine the match point of the silica particle suspensions from the expected square root dependence of the scattered intensity with solvent composition, to analyze any differences associated with the solid concentration change, and to determine the optimum H2O/D2O ratio in terms of high transmission (TSAS > 80%) and high enough scattering intensity associated with the contrast of the system. Through this investigation series, we confirmed that adjusting the contrast of the solvent (H2O/D2O) is a good methodology to reduce multiple scattering while maintaining a strong enough scattering signal from a concentrated suspension of silica particles for both USANS and rheometric USANS (rheo-USANS) experiments.

Investigating the effective interaction between silica colloidal particles near the critical point of a binary solvent by small angle neutron scattering.

An effective attractive potential can be introduced between colloidal particles dispersed in a binary solvent when the solvent condition approaches its demixing temperatures. Despite the debate of the physical origins of this effective attraction, it is widely termed as the critical Casimir force and is believed to be responsible for the colloidal stability in a wide range of particle concentration at both critical and near-critical solvent concentrations. Here, we study the effective attraction and equilibrium phase transition of charged spherical silica particles in the binary solvent of 2,6-lutidine and water as a function of the particle volume fraction and temperature at the critical solvent concentration. By analyzing our small angle neutron scattering (SANS) data, we found that at a relatively small particle volume fraction, the density fluctuation introduced attraction between silica particles can be satisfactorily explained by the function form commonly used for the critical Casimir interaction. However, at large silica particle volume fractions, an additional long range attraction has to be introduced to satisfactorily fit our SANS data and explain the large shift of the phase transition temperature. Therefore, while at relatively low volume fractions, the solvent introduced attraction may be dominated by the critical Casimir force, the physical mechanism of the effective attraction at large particle volume fractions seems to be different from the critical Casimir force. Furthermore, the range of this long range attraction is consistent with a recently proposed new theory, where the attraction can be introduced by the solvent capillary condensation between particles. We also demonstrate that the reduced second virial coefficient close to the particle phase transition is similar to the values of the binodal transition of the sticky hard sphere system.

Effects of mixed ultrafine colloidal silica particles on chemical mechanical polishing of sapphire

The effects of ultrafine colloidal silica particles adsorbed on the surfaces of large silica particles in slurries used for the chemical mechanical polishing of sapphire were studied. Sapphire wafers were polished using hybrid silica particles that were composed of a mixture of ultrafine (4 nm) colloidal silica particles and large (20, 55, and 105 nm) silica particles. Dynamic light scattering results showed that the hybrid particles became larger after mixing. Transmission electron microscopy images revealed that the ultrafine particles adhered to the surfaces of the large particles and that the surface shape of the hybrid particles were changed by the mixing process. Atomic force microscopy and polishing results showed that the surface roughnesses and material removal rates of sapphire substrates were improved by using the hybrid particles. The coefficient of friction between the surface of the sapphire substrate and the hybrid particles was higher than that obtained using single-sized particles. These results confirm that the hybrid silica particles enhanced the performance of sapphire polishing.

Soft materials based on colloidal self-assembly in ionic liquids

Ionic liquids (ILs) have attracted much attention as dispersion media for colloidal systems as alternatives to organic solvents and electrolyte solutions. Although colloidal stability is an essential factor for determining the properties and performance of colloidal systems containing ILs, detailed mechanisms for colloidal stabilization have not yet been studied. In the first part of this paper, we highlight our fundamental studies on colloidal stability. Three different repulsive forces, electrostatic, solvation, and steric interactions, are examined for their effectiveness in stabilizing colloidal particles in ILs. In the second part of this report, we provide an overview of our recent studies on colloidal soft materials in the presence of ILs. On the basis of the suspended state of the silica colloid particles, two different soft materials, a colloidal gel and a colloidal glass, were prepared in ILs. Their functional properties, including ionic transport, rheological, and optical properties, are discussed in relation to the microstructures of the colloidal materials. Soft materials based on colloidal self-assembly in ionic liquids: fundamental studies on the colloidal stability in ionic liquids (ILs) are highlighted. Three different repulsive forces—electrostatic, solvation, and steric—are examined for their effectiveness to stabilize colloidal particles in ILs. An overview of recent studies on colloidal soft materials in the presence of ILs is also provided. Based on suspended states of colloid particles, two different soft materials—colloidal gel and colloidal glass—were prepared in ILs. Their functional properties, including ionic transport, rheological and optical properties, are discussed in relation to the microstructures of the colloidal materials.

Cd2+-Doped Amorphous TiO2 Hollow Spheres for Robust and Ultrasensitive Photoelectrochemical Sensing of Hydrogen Sulfide.

Hydrogen sulfide is a highly toxic molecule to human health, but high-performance detection of it remains a challenge. Herein, we report an ultrasensitive photoelectrochemical (PEC) sensor for H2S by modifying indium tin oxide (ITO) electrodes with Cd2+-doped amorphous TiO2 hollow spheres, which are prepared by templating against colloidal silica particles followed by a cadmium-sodium cation exchange reaction. The amorphous TiO2 hollow spheres act as both the probing cation carrier and the photoelectric beacon. Upon exposure to sulfide ions, the photocurrent of the functionalized photoanode proportionately decreases in response to the formation of CdS nanoparticles. The decreased photocurrent could be attributed to the mismatching bandgap between the amorphous TiO2 and CdS nanoparticles: the photoexcited electrons and holes from amorphous TiO2 are transferred to the conduction band and valence band of CdS, respectively, and then recombined. The decrease in photocurrent is linear with the concentration of sulfide ions in the range from 1 to 10 000 pmol L-1 with a detection limit of 0.36 pmol L-1. Enabled by a unique sensitization mechanism, this PEC sensor features excellent performance in a wide linear range, high selectivity and sensitivity, high stability, and low fabrication cost.

Bottom-Up Assembly of Silica and Bioactive Glass Supraparticles with Tunable Hierarchical Porosity.

We investigate the formation of spherical supraparticles with controlled and tunable porosity on the nanometer and micrometer scales using the self-organization of a binary mixture of small (nanometer scale) oxidic particles with large (micrometer scale) polystyrene particles in the confinement of an emulsion droplet. The external confinement determines the final, spherical structure of the hybrid assembly, while the small particles form the matrix material. The large particles act as templating porogens to create micropores after combustion at elevated temperatures. We control the pore sizes on the micrometer scale by varying the size of the coassembled polystyrene microspheres and produce supraparticles from both silica- and calcium-containing CaO/SiO2 particles. Although porous supraparticles are obtained in both cases, we found that the presence of calcium ions substantially complicated the fabrication process since the increased ionic strength of the dispersion compromises the colloidal stability during the assembly process. We minimized these stability issues via the addition of a steric stabilizing agent and by mixing bioactive and silica colloidal particles. We investigated the interaction of the porous particles with bone marrow stromal cells and found an increase in cell attachment with increasing pore size of the self-assembled supraparticles.

Role of Penetrating Agent on Colloidal Silica Particle Removal during Post Cu CMP Cleaning

The cleaning of copper surface after chemical mechanical planarization (CMP) is a critical step in integrated circuits (ICs) fabrication. Because the copper surface would be contaminated by particles, such as silica abrasives, which may cause device failure. The purpose of this work is to propose a kind of penetrating agent for particle removal. The cleaning solution consisted of FA/O II as a chelating agent and primary alcobol ethoxylate (PAE) as a penetrating agent. The removal of particles was characterized using scanning electron microscope (SEM). The wettability of the cleaning solutions as a function of PAE concentration was quantified using contact angle measurements. The effect of PAE on surface tension was also studied. The surface quality of copper after cleaning was monitored using atomic force microscope (AFM) measurements. The test results showed that PAE can reduce the surface tension of cleaning solutions and contact angle on surface. SEM results showed that PAE could effectively remove the silica particles and a lower surface roughness was obtained after cleaning. The cleaning performance on copper patterned wafers also showed that PAE could reduce defect count effectively.

Particle-Resolved Phase Identification in Two-Dimensional Condensable Systems

A method for phase identification (MPI) in two-dimensional (2D) condensable systems is proposed on the basis of the analysis of the Voronoi cells’ characteristics. A simple algorithm is developed for determination of particles belonging to a condensate, a gaseous state, and an interface between them (“surface”). The efficiency and stability of the developed method is studied using molecular dynamics simulations and calculation of binodals in 2D systems with both short- and long-range attraction between particles. To illustrate prospectives of the method for experimental studies, 2D clusters of 2.12 μm diameter colloidal silica particles assembled in a rotating external electric field have been analyzed using the MPI. The results prove that the MPI possesses high accuracy and can be applied to analyze in detail both simulation data (obtained by molecular dynamics or Monte Carlo methods) and results of particle-resolved experimental studies (with colloidal suspensions and complex (dusty) plasmas) of solids ...

Modifying the Morphology of Silicon Surfaces by Laser Induced Liquid Assisted Colloidal Lithography.

Single, or isolated small arrays of, spherical silica colloidal particles (with refractive index ncolloid = 1.47 and radius R = 350 nm or 1.5 μm) were placed on a silicon substrate and immersed in carbon tetrachloride (nliquid = 1.48) or toluene (nliquid = 1.52). Areas of the sample were then exposed to a single laser pulse (8 ps duration, wavelength λ = 355 nm), and the spatial intensity modulation of the near field in the vicinity of the particles revealed via the resulting patterning of the substrate surface. In this regime, ncolloid < nliquid and the near-field optical intensification is concentrated at and beyond the edge of the particle. Detailed experimental characterization of the irradiated Si surface using atomic force microscopy reveals contrasting topographies. The same optical behavior is observed with both liquids, i.e., the incident laser light diverges on interaction with the colloidal particle, but the resulting interaction with the substrate is liquid dependent. Topographic analysis indicates localized ablation and patterning of the Si substrate when using toluene, whereas the patterning induced under carbon tetrachloride is on a larger scale and extends well below the original substrate surface—hinting at a laser induced photochemical contribution to the surface patterning.

Effect of mixing ratio of non-spherical particles in colloidal silica slurry on oxide CMP

Colloidal silica and fumed silica are common slurry materials for oxide chemical mechanical polishing (CMP). Non-spherical colloidal silica particles are manufactured via a multi-step feeding method to compensate for the material removal rate of colloidal silica slurry, which is lower than that of fumed silica slurry. Additionally, mixed abrasive slurry has been used by combining non-spherical and spherical colloidal silica particles. Experiments were conducted on a 4-inch wafer with silicon dioxide film (SiO 2 , 1-μm thick) and KOH-based colloidal silica slurry. The slurry had spherical colloidal silica particles with the size of 30 nm and 70 nm and non-spherical colloidal silica particles with the size of 70 nm. Effects of different colloidal particles in the oxide CMP were observed under the same pH condition in order to achieve high material removal rate. The mixing ratios were 2:1, 1:1, and 1:2 respectively. The analysis of the particle combinations showed that the mixing of particles with different sizes caused agglomeration and increased the material removal rate. The relatively smaller spherical particles improved the surface roughness and overall performance, especially when mixed with non-spherical particles. Such mixtures of particles with different sizes and shapes mixed at an appropriate ratio can outperform the fumed silica slurry.

Favored local structures in amorphous colloidal packings measured by microbeam X-ray diffraction

Local structure and symmetry are keys to understanding how a material is formed and the properties it subsequently exhibits. This applies to both crystals and amorphous and glassy materials. In the case of amorphous materials, strong links between processing and history, structure and properties have yet to be made because measuring amorphous structure remains a significant challenge. Here, we demonstrate a method to quantify proportions of the bond-orientational order of nearest neighbor clusters [Steinhardt, et al. (1983) Phys Rev B 28:784-805] in colloidal packings by statistically analyzing the angular correlations in an ensemble of scanning transmission microbeam small-angle X-ray scattering (μSAXS) patterns. We show that local order can be modulated by tuning the potential between monodisperse, spherical colloidal silica particles using salt and surfactant additives and that more pronounced order is obtained by centrifugation than sedimentation. The order in the centrifuged glasses reflects the ground state order in the dispersion at lower packing fractions. This diffraction-based method can be applied to amorphous systems across decades in length scale to connect structure to behavior in disordered systems with a range of particle interactions.

Study on effect of the surface variation of colloidal silica abrasive during chemical mechanical polishing of sapphire

The surface and diameter size variations of colloidal silica particles during the chemical mechanical polishing (CMP) of sapphire substrates were investigated using different particle diameters of 20 and 55 nm. Dynamic light scattering (DLS) results show that the silica particles became larger after CMP under both conditions. The increase in particle size in the slurry was proportional to the material removal amount (MRA) as a function of the removed volume of sapphire substrates by CMP and affected the material removal rate (MRR). Transmission electron microscopy (TEM) images revealed an increase in the size of the fine particles and a change in their surface shape in the slurry. The colloidal silica was coated with the material removed from the substrate during CMP. In this case, the increase in the size of 55 nm diameter particles is larger than that of 20 nm diameter particles. X-ray fluorescence spectrometry (XRF) results indicate that the aluminum element from polished sapphire substrates adhered to the surfaces of silica particles. Therefore, MRR decreases with increasing of polishing time owing to the degradation of particles in the slurry.

Bio-Inspired Bright Structurally Colored Colloidal Amorphous Array Enhanced by Controlling Thickness and Black Background.

Inspired by Steller's jay, which displays angle-independent structural colors, angle-independent structurally colored materials are created, which are composed of amorphous arrays of submicrometer-sized fine spherical silica colloidal particles. When the colloidal amorphous arrays are thick, they do not appear colorful but almost white. However, the saturation of the structural color can be increased by (i) appropriately controlling the thickness of the array and (ii) placing the black background substrate. This is similar in the case of the blue feather of Steller's jay. Based on the knowledge gained through the biomimicry of structural colored materials, colloidal amorphous arrays on the surface of a black particle as the core particle are also prepared as colorful photonic pigments. Moreover, a structural color on-off system is successfully built by controlling the background brightness of the colloidal amorphous arrays.

Confined Assemblies of Colloidal Particles with Soft Repulsive Interactions.

We investigate the microconfinement of charged silica nanoparticles dispersed in refractive index matching monomers in poly(dimethylsiloxane) (PDMS) porous membrane. Here, the silica colloidal particles interact with each other and the pore wall via electrostatic double layer forces. Different from the hard sphere systems where the assembled morphologies are prescribed by the diameter ratio between the cylindrical confinement and the nanoparticles, here we observe a much richer variety of assemblies that are highly sensitive to both bulk and local nanoparticle concentration with fixed particle size and channel size. The experimentally observed assembly morphologies are consistent with theoretical predictions from the literature, based on Yukawa potential in the low packing density regime. Also, most of the configurations found in the experiment are well described by computer simulations using pairwise additive long-range repulsive interactions, demonstrating the ability to control the system to obtain a desired structure.