Hollow silica sphere colloidal crystals: insights into calcination dependent thermal transport

Heat: A powerful tool for colloidal particle shaping

Colloidal particles of spherical shape are important building blocks for nanotechnological applications. Materials with tailored physical properties can be directly synthesized from self-assembled particles, as is the case for colloidal photonic crystals. In addition, colloidal monolayers and multilayers can be exploited as a mask for the fabrication of complex nanostructures via a colloidal lithography process for applications ranging from optoelectronics to sensing. Several techniques have been adopted to modify the shape of both individual colloidal particles and colloidal masks. Thermal treatment of colloidal particles is an effective route to introduce colloidal particle deformation or to manipulate colloidal masks (i.e. to tune the size of the interstices between colloidal particles) by heating them at elevated temperatures above a certain critical temperature for the particle material. In particular, this type of morphological manipulation based on thermal treatments has been extensively applied to polymer particles. Nonetheless, interesting shaping effects have been observed also in inorganic materials, in particular silica particles. Due to their much less complex implementation and distinctive shaping effects in comparison to dry etching or high energy ion beam irradiation, thermal treatments turn out to be a powerful and competitive tool to induce colloidal particle deformation. In this review, we examine the physicochemical principles and mechanisms of heat-induced shaping as well as its experimental implementation. We also explore its applications, going from tailored masks for colloidal lithography to the fabrication of colloidal assemblies directly useful for their intrinsic optical, thermal and mechanical properties (e.g. thermal switches) and even to the synthesis of supraparticles and anisotropic particles, such as doublets.

Hollow Silica Nano and Micro Spheres with Polystyrene Templating: A Mini-Review

Synthesis of monodisperse hollow silica nanospheres, especially using a hard template route, has been shown to be successful, but a high yield is needed for this strategy to be used on an industrial scale. On the other hand, there is a research gap in the synthesis of hollow silica microspheres due to the popularity and easiness of the synthesis of silica nanospheres despite the larger spheres being beneficial in some fields. In this review, current trends in producing hollow silica nanospheres using hard templates, especially polystyrene, are briefly presented. Soft templates have also been used to make highly polydisperse hollow silica spheres, and complex designs have improved polydispersity. The effect of the main parameters on the coating is presented here to provide a basic understanding of the interactions between the silica and template surface in the absence or presence of surfactants. Surface charge, surface modification, parameters in the sol-gel method and interaction between the silica and templates need to be further improved to have a uniform coating and better control over the size, dispersity, wall thickness and porosity. As larger organic templates will have lower surface energy, the efficiency of the micro sphere synthesis needs to be improved. Control over the physical structure of hollow silica spheres will open up many opportunities for them to be extensively used in fields ranging from waste removal to energy storage.

Iridescent Colloidal Crystals Composed of SiO2 Porous Hollow Sphere for SERS Application

In recent years, the application of low-refractive-index materials in the optical field has attracted considerable attention due to it high transmittance and high optical sensitivity. In this study, we synthesized SiO₂ porous hollow spheres (SPHS) with an ultralow refractive index (n = 1.05) by using a templating method. Their refractive indices could be easily controlled from 1.05 to 1.08 by tuning the thickness of shell. In addition, a droplet coatings method is proposed for SPHS colloidal crystal (CC) by controlling the temperature and humidity. The SPHS CCs displayed distinct structural colors when the incident angle was adjusted and demonstrated high angular resolution. Moreover, the iridescent color changes could be observed with the naked eye. For surface-enhanced Raman scattering application, more analyte could be absorbed by the porous shells, and metal nanoparticles were coated on the SPHSs surface to increase the hot spot density for improving the SERS intensity.

Portable Hyperpolarized Xe-129 Apparatus with Long-Time Stable Polarization Mediated by Adaptable Rb Vapor Density

The extraordinary sensitivity of ¹²⁹Xe, hyperpolarized by spin-exchange optical pumping, is essential for magnetic resonance imaging and spectroscopy in life and materials sciences. However, fluctuations of the polarization over time still limit the reproducibility and quantification with which the interconnectivity of pore spaces can be analyzed. Here, we present a polarizer that not only produces a continuous stream of hyperpolarized ¹²⁹Xe but also maintains stable polarization levels on the order of hours, independent of gas flow rates. The polarizer features excellent magnetization production rates of about 70 mL/h and ¹²⁹Xe polarization values on the order of 40% at moderate system pressures. Key design features include a vertically oriented, large-capacity two-bodied pumping cell and a separate Rb presaturation chamber having its own temperature control, independent of the main pumping cell oven. The separate presaturation chamber allows for precise control of the Rb vapor density by restricting the Rb load and varying the temperature. The polarizer is both compact and transportable─making it easily storable─and adaptable for use in various sample environments. Time-evolved two-dimensional (2D) exchange spectra of ¹²⁹Xe absorbed in the microporous metal-organic framework CAU-1-AmMe are presented to highlight the quantitative nature of the device.

Macroporous Silica Foams Fabricated via Soft Colloid Templating

Macroporous materials with controlled pore sizes are of high scientific and technological interest, due to their low specific weight, as well as unique acoustic, thermal, or optical properties. Solid foams made of titania, silica, or silicon, as representative materials, have been previously obtained with several hundred nanometer pore sizes, by using sacrificial templates such as spherical emulsion droplets or colloidal particles. Macroporous structures in particular are excellent candidates as photonic materials with applications in structural coloration and photonic bandgap devices. However, whereas using spherical building blocks as templates may provide tight control over pore shape and size, it results in materials with an often unfavorable local topology. Templating dry-foam or compressed-emulsion structures appear as attractive alternatives, but have not been demonstrated so far for submicron pore sizes. Herein, the use of soft, flexible microgel colloids decorated with silica nanoparticles as templates of macroporous foams is reported. These purposely synthesized core-shell colloids are assembled at ultra-high effective volume fractions by centrifuging and thermal swelling, thereby resulting in uniform disordered materials with facetted pores, mimicking dry foams. After removal of the polymer component via calcination, lightweight pure silica structures are obtained with a well-defined cellular or network topology.

Investigation of the Thermal Conductivity of Resin-Based Lightweight Composites Filled with Hollow Glass Microspheres

The design and development of thermal insulation materials is very important for the treatment of offshore oil pipelines. Understanding thermal energy transport in thermal insulation materials and predicting their thermal conductivities have important theoretical and practical value for the design of thermal insulation materials. In this work, lightweight and thermally insulated (LWTI) composites with the desired mechanical strength for offshore oil pipelines applications were prepared using epoxy resin (EP) as the matrix and hollow glass microspheres (HGMs) as the filler. The morphology, density, and mechanical properties of HGM/EP LWTI composites were studied first. The flexural strength and the flexural modulus of HGM/EP LWTI composites could still be as high as 22.34 ± 2.75 Mpa and 1.34 ± 0.03 GPa, respectively, while the density was only 0.591 g/cm³. The relationship between the effective thermal conductivity of HGM/EP LWTI composites and material parameters (sizes and contents together) has been studied systematically. A three-phase prediction model was built using the self-consistent approximation method to predict the effective thermal conductivity of HGM/EP LWTI composites, and the resin matrix, the wall thickness, the HGM particle size, and other parameters (such as air) were fully considered during the derivation of this three-phase thermal conductivity model. Finally, the insulation mechanism of HGM/EP LWTI composites was systematically analyzed. The thermal conductivities of HGM/EP LWTI composites with different diameters and HGM contents calculated by the three-phase prediction model agreed well with the experimental test results, with a minimum error of only 0.69%. Thus, this three-phase thermal conductivity model can be used to theoretically simulate the thermal conductivity of epoxy resin-based LWTI composites and can be the theoretical basis for the design and prediction of the thermal conductivity of other similar hollow spheres filled materials.

Ultra-Early Diagnosis of Acute Myocardial Infarction in Rats Using Ultrasound Imaging of Hollow Double-Layer Silica Nanospheres

Timely diagnosis of acute myocardial infarction (AMI) strongly impacts the survival rate of patients. The authors report the development of a two-shell hollow silica contrast agent useful for ultrasound (US) imaging, which is able to provide ultra-early diagnosis of AMI. To target the characterization of fast blood flow and high blood pressure in the heart, two shells of hollow silica are adopted with opposite polarities, which assemble based on amino and perfluorodecyl silanes. The external amino silane facilitates the attachment of disease-targeted groups, while the internal perfluorodecyl silane provides great US imaging contrast. The material also possesses superior water dispersity, controllable morphology, low toxicity, and biodegradability both in vitro and in vivo, thus promoting its applications in the ultra-early diagnosis of AMI in rats, and is particularly useful for delineation of myocardial necrosis sites.

Ultrasonication-assisted synthesis of novel strontium based mixed phase structures for supercapattery devices

An upsurge in sustainable energy demands has ultimately made supercapattery one of the important choice for energy storage, owing to highly advantageous energy density and long life span. In this work, novel strontium based mixed phased nanostructures were synthesized by using probe sonicator with sonication power 500 W at frequency of 20 kHz. The synthesized material was subsequently calcined at different temperature ranging from 200 to 800 °C. Structural and morphological analysis of the synthesized materials reveals the formation of mixed particle and rod like nanostructures with multiple crystal phases of strontium oxides and carbonates. Crystallinity, grain size and morphology of grown nanomaterials significantly improved with the increase of calcination temperature due to sufficient particle growth and low agglomeration. The electrochemical performance analysis confirms the redox activeness of the Sr-based electrode materials. Material calcined at 600 °C show high specific capacitance of 350 F g^-1 and specific capacity of 175 C g^-1 at current density of 0.3 A g^-1 due to less particle agglomeration, good charge transfer and more contribution of electrochemical active sites for redox reactions. In addition, the developed supercapattery of Sr-based nanomaterials//activated carbon demonstrated high performance with maximum energy density of 21.8 Wh kg^-1 and an excellent power density of 2400 W kg^-1 for the lower and higher current densities. Furthermore, the supercapattery retain 87% of its capacity after continuous 3000 charge/discharge cycles. The device characteristics were further investigated by analyzing the capacitive and diffusion controlled contributions. The versatile strategy of developing mixed phased nanomaterials pave the way to synthesize other transition metal based nanomaterials with superior electrochemical performance for hybrid energy storage devices.

Hollow-Structured Materials for Thermal Insulation

Heating and cooling represent a significant portion of overall energy consumption of our society. Due to the diffusive nature of thermal energy, thermal insulation is critical for energy management to reduce energy waste and improve energy efficiency. Thermal insulation relies on the reduction of thermal conductivity of appropriate materials that are engineerable in compositions and structures. Hollow-structured materials (HSMs) show a great promise in thermal insulation since the existence of high-density gaseous voids breaks the continuity of heat-transport pathways in the HSMs to lower their thermal conductivities efficiently. Herein, a timely overview of the recent progress in developing HSMs for thermal insulation is presented, with the focus on summarizing the strategies for creating gaseous voids in solid materials and thus synthesizing various HSMs. Systematic analysis of the documented results reveals the relationship of thermal conductivities of the HSMs and the size and density of voids, i.e., reducing the void size below ≈350 nm is more favorable to decrease the thermal conductivity of the HSMs because of the possible confinement effect originated from the nanometer-sized voids. The challenges and promises of the HSMs faced in future research are also discussed.

Synthesis of Polydopamine Hollow Capsules via a Polydopamine Mediated Silica Water Dissolution Process and Its Application for Enzyme Encapsulation

Herein, we present a systematic study on the preparation of polydopamine (PDA) hollow capsules by templating silica particles which were subsequently removed by a PDA mediated water dissolution process without using any harsh chemical treatment. It was found that the time required for silica removal varied depending on the PDA coating and dissolution conditions. Factors that could influence the core removal process including the PDA thickness and coating temperature, silica calcination duration and the availability of water were then examined in detail. Additionally, catalase was used as a model enzyme to be encapsulated into PDA hollow capsules and its bio-functionality was found to remain active. The bioactivity test results also indicated that the as-synthesized PDA capsules possessed a porous structure, which allows the penetration of small molecules such as H₂O₂. This study offers a better insight into silica dissolution process that mediated by PDA and contributes to the development of an eco-friendly approach for the fabrication of hollow capsules that have promising applications in drug delivery systems.

Fluorescence imaging of antibiotic clofazimine encapsulated within mesoporous silica particle carriers: relevance to drug delivery and the effect on its release kinetics

We report on the encapsulation of the antibiotic clofazimine (CLZ) within the pores of mesoporous silica particles having hydrophilic (CBET value of 137) and more hydrophobic (CBET value of 94 after calcination at 600 °C) surfaces. We studied the effect of pH on the released amount of CLZ in aqueous solutions and observed a maximum at pH 4.1 in correlation with the solubility of the drug. Less release of the drug was observed from the more hydrophobic particles which was attributed to a difference in the affinity of the drug to the carrier particles. Fluorescence lifetime imaging microscopy, emission spectra, and fluorescence lifetimes of single drug loaded particles provided detailed understanding and new knowledge of the physical form of the encapsulated drug and the distribution within the particles. The distribution of CLZ within the particles was independent of the surface chemistry of the particles. The confirmation of CLZ molecules as monomers or aggregates was revealed by controlled removal of the drug with solvent. Additionally, the observed optical "halo effect" in the fluorescent images was interpreted in terms of specific quenching of high concentration of molecules. The emission lifetime experiments suggest stronger interaction of CLZ with the more hydrophobic particles, which is relevant to its release. The results reported in this work demonstrate that tuning the hydrophilicity/hydrophobicity of mesoporous silica particles can be used as a tool to control the release without impacting their loading ability.

Low Thermal Conductivity through Dense Particle Packings with Optimum Disorder

Heat transport plays a critical role in modern batteries, electrodes, and capacitors. This is caused by the ongoing miniaturization of such nanotechnological devices, which increases the local power density and hence temperature. Even worse, the introduction of heterostructures and interfaces is often accompanied by a reduction in thermal conductivity, which can ultimately lead to the failure of the entire device. Surprisingly, a fundamental understanding of the governing heat transport processes even in simple systems, such as binary particle mixtures is still missing. This contribution closes this gap and elucidates how strongly the polydispersity of a model particulate system influences the effective thermal conductivity across such a heterogeneous system. In a combined experimental and modeling approach, well-defined mixtures of monodisperse particles with varying size ratios are investigated. The transition from order to disorder can reduce the effective thermal conductivity by as much as ≈50%. This is caused by an increase in the thermal transport path length and is governed by the number of interparticle contact points. These results are of general importance for many particulate and heterostructured materials and will help to conceive improved device layouts with more reliable heat dissipation or conservation properties in the future.

Interfacial and volumetric sensitivity of the dry sintering process of polymer colloidal crystals: a thermal transport and photonic bandgap study

We introduce the in situ characterization of the dry sintering process of face-centred cubic colloidal crystals by two complementary techniques: thermal transport and photonic stopband characterization. Therefore, we employed time-dependent, isothermal laser flash analysis and specular reflectivity experiments close to the glass transition temperature of the colloidal crystal. Both methods yield distinctly different time constants of the film formation process. This discrepancy can be attributed to a volume- (photonic stopband) and interface-driven (thermal transport) sensitivity of the respective characterization method. Nevertheless, both methods yield comparable apparent activation energies. Finally, we extended the sintering process characterization to further polymer compositions, with vastly different glass transition temperatures. We could show that the film formation rate is governed by the viscoelastic properties of the polymers at the respective annealing temperature.

Thermal transport in binary colloidal glasses: Composition dependence and percolation assessment

The combination of various types of materials is often used to create superior composites that outperform the pure phase components. For any rational design, the thermal conductivity of the composite as a function of the volume fraction of the filler component needs to be known. When approaching the nanoscale, the homogeneous mixture of various components poses an additional challenge. Here, we investigate binary nanocomposite materials based on polymer latex beads and hollow silica nanoparticles. These form randomly mixed colloidal glasses on a sub-μm scale. We focus on the heat transport properties through such binary assembly structures. The thermal conductivity can be well described by the effective medium theory. However, film formation of the soft polymer component leads to phase segregation and a mismatch between existing mixing models. We confirm our experimental data by finite element modeling. This additionally allowed us to assess the onset of thermal transport percolation in such random particulate structures. Our study contributes to a better understanding of thermal transport through heterostructured particulate assemblies.

Synthesis and characterisation of robust emulsion-templated silica microcapsules

Robust silica microcapsules were synthesised using an emulsion template via a seeded growth strategy. Multiple additions of the silica precursor tetraethyl orthosilicate (TEOS) were observed to result in a number of physical and property changes of the capsule shells as compared to a single coating. Scanning electron microscopy indicated a morphological transition from a smooth to a roughened surface. Improved cargo retention and consolidation of the pore structure of the silica shells were observed using dye release experiments and nitrogen porosimetry respectively. In comparison to a typical hollow silica shell synthesis procedure, this one-pot loading and synthesis allows the simple production of robust capsules that are capable of sustained release, using mild conditions and reagents.

Hydrophobization of Silica Nanoparticles in Water: Nanostructure and Response to Drying Stress

We report on the impact of surface hydrophobization on the structure of aqueous silica dispersions and how this structure resists drying stress. Hydrophilic silica particles were hydrophobized directly in water using a range of organosilane precursors, with a precise control of the grafting density. The resulting nanostructure was precisely analyzed by a combination of small-angle X-ray scattering (SAXS) and cryo-microscopy (cryo-TEM). Then, the dispersion was progressively concentrated by drying, and the evolution of the nanostructures as a function of the grafting density was followed by SAXS. At the fundamental level, because the hydrophobic character of the silica surfaces could be varied continuously through a precise control of the grafting density, we were able to observe how the hydrophobic interactions change particles interactions and aggregates structures. Practically, this opened a new route to tailor the final structure, the residual porosity, and the damp-proof properties of the fully dried silica. For example, regardless of the nature of the hydrophobic precursor, a grafting density of 1 grafter per nm² optimized the interparticle interactions in solution in view to maximize the residual porosity in the dried material (0.9 cm³/g) and reduced the water uptake to less than 4% in weight compared to the typical value of 13% for hydrophilic particles (at T = 25 °C and relative humidity = 80%).

Hollow porous Cu particles from silica-encapsulated Cu2O nanoparticle aggregates effectively catalyze 4-nitrophenol reduction

A hollow metal micro/nanomaterial with a porous wall is one of the most attractive structures for catalysts. The synthesis of hollow porous Cu particles remains a challenge due to their air-sensitive characteristics. In this study, we report a facile and scalable method for the preparation of high-quality hollow porous Cu particles in the range of 500 nm-1.5 μm with a well-defined structure from Cu₂O nanoparticle aggregates (NPAs). The synthetic procedure involves the silica-encapsulation and depth-controlled reduction of Cu₂O NPAs followed by heat-treatment in air and selective removal of the encapsulating layer. The catalytic performance of the hollow porous Cu particles was evaluated through the catalytic reduction of 4-nitrophenol with NaBH₄ as a model reaction. The hollow porous Cu particles exhibited a high activity factor, K = 186 s^-1 g^-1, which is the highest K value obtained among the unsupported Cu catalysts to date. And the K value is better than that of some noble metal catalysts, such as Au, Ag, and Pd. In addition, the catalyst could be easily separated from the reaction system and still possessed high activity as well as stability in recycled reactions.

Ultrahigh capacitance of amorphous nickel phosphate for asymmetric supercapacitor applications

This article presents the effect of different calcination temperatures on the structural, morphological and capacitance of nickel phosphate (Ni₃(PO₄)₂) as an electrode material for supercapacitor applications.

Synthesis and characterization of geometrically tunable nano-size hollow silicate particles and their dip-coating prepared films for thermal management applications

The thermal insulating properties of NHSPs were demonstrated by coating glass with NHSPs. A reduction of approximately 29.9% in thermal conductivity was observed, which was evaluated by a contactless approach using a xenon flash lamp set-up.