Technique of obtaining synthetic opals and nanostructured materials based on them

1. Technique of obtaining synthetic opals and nanostructured materials based on them

1.1. Technique of synthesis of monodisperse colloidal silica particles and production of opal-like structures (matrices)

SYNTHESIS OF THE PARTICLES

We have developed the technique of synthesizing nano- and submicron spherical silica particles with a diameter from 20 nm to 2 µm and a high degree of monodispersity (the deviation from the average size is < 5% for particles less than 100 nm in diameter and < 3% for particles more than 100 nm in diameter).

Figure 1. SEM images of monodisperse spherical silica particles

The technique allows obtaining colloidal SiO₂ particles in the form of stable neutral aqueous suspensions with the particle concentration up to 30 wt % and a high purity degree. The main advantages of the developed technique are an effective control over the shape, sizes and dispersity of the particles under obtaining.

The particles are obtained by the sol-gel method based on homogeneous tetraethoxysilane hydrolysis in the presence of ammonium, heterogeneous hydrolysis with the use of amino acids as catalysts, and combination of the aforementioned methods.

There is a possibility of controllable obtaining spherical particles with both smooth and developed “rough” surface.

Figure 2. SEM images of SiO₂ particles with a rough (a) and smooth (b) surface

The colloidal spherical particles of amorphous silica (SiO₂) have a complex internal fractal structure. Moreover, they are a promising material for creation of novel materials to be used in various industries, medicine, micro- and optoelectronics, and as soft abrasive materials for final polishing of metal and semiconductor products. The particle density varies between 1.43-1.58 g/cm³, the porosity is 29-36% depending on the particle size.

Particle nanopores are accessible for water (kinetic diameter is 0.264 nm) and inaccessible for liquids with a large molecule size. The apparent density of opal matrices is in the range of 1.19 – 1.06 g/cm³. The open porosity (measured by water method) is 47.5 ÷ 52.5%. The open porosity for largemolecular liquids is 35-36%.

The technique does not require the use of complex and expensive equipment for its realization. The initial materials and reagents are cheap and available.

STRUCTURES

The Institute of Solid State Physics RAS has developed the technique of obtaining opal matrices in the form of mono- and multilayered films, as well as in the form of bulk structures. Opal matrices represent close-packed structures of monodisperse spheres of amorphous silica with the size of 0.1-1.0 µm. These structures are the basis for creation of photonic crystals which have a photonic band gap (PBG) for electromagnetic radiation with wavelengths comparable with periodic structure parameters.

The opal matrices of submicron silica particles have a good combination of physical-mechanical characteristics, chemical inertness, and high heat resistance (up to 1000 °С). The aforementioned properties allow wide using these structures in nanotechnologies. Filling voids between structural units of an opal matrix with different materials, one can create nanoperiodic structures of optically active, magnetic, semiconductor, and other materials for optoelectronics, magnetic recording systems, semiconductor and other technical fields. The highly developed nanoporous structure of an opal matrix allows it being used in liquid chromatography, catalysis, etc.

Figure 3. General view of an opal-like matrix with reflection at different angles and the angular dependence of reflection spectra

Derivative products:

1.2 Photonic crystals in the form of monolithic transparent silica

We have created a novel type of photonic crystals, where a three-dimensional lattice of nanocrystals periodically located in a transparent silica matrix is formed. Also we have determined the conditions for synthesis and obtained SiO₂-ZrO₂ nanocomposites in the form of a monolithic (without pores) transparent sample. We have implemented an idea consisting in retaining the periodic structure of the opal matrix upon heat treatment by means of the introduction of a stabilizing phase, which does not react with silica during annealing, into pores of the opal. Electron microscopy has revealed that the arrangement of ZrO₂ clusters represents a replica of the system of voids between the initial SiO₂ spheres (cell structure). The lattice parameters are determined by the diameter of the initial silica spheres with due regard for an approximately 15% shrinkage of the matrix during high-temperature annealing. The size of zirconia particles located at the corners of the hexagonal cells varies in the range from 10 to 50 nm, and the cell faces contain ZrO₂ particles with a considerably smaller size. The transmission spectrum of the composite exhibits a minimum of transmission, which indicates the presence of a photonic band gap in the material. The angular dependence of the reflection spectrum reveals a shift of the maximum of the reflection line toward the short-wavelength range with an increase in the angle of detection in accordance with the Bragg law. In contrast to globulin photonic crystals based on opal matrices, the photonic crystal synthesized is transparent in the visible spectral range and can be used as a homogeneous optical element in different optical devices. In particular, this crystal is of interest as an alternative to known notch filters. A plate of such photonic crystal can be used as a selective filter in the recording of Raman spectra, which reflects exciting radiation and transmits both the Stokes signal and the anti-Stokes signal. The proposed approach to retaining a periodic structure in a transparent material can be promising in the design of effective glass lasers.

Figure 4. A novel type of photonic crystal in the form of monolithic transparent silica with an ordered distribution of ZrO₂ nanocrystals throughout the sample, its transmission spectrum and angular dependence of the reflection spectrum

1.3. Photonic crystals in the form of spherical microparticles

We have synthesized spherical microparticles with the size from 5 to 20 µm, formed by monodisperse colloidal SiO₂ particles by their close-packing into a face-centered cubic lattice analogously to opal-like structures. The synthesis was conducted by spray drying of the aqueous suspension of colloidal silica particles in the air at room temperature not using surfactants. Such particles having controllable sizes and given pore system, special structure and morphology are promising for application in photonics, biological and chemical sensors, catalysis, pharmacology, etc.

Figure 5. STM image of the microparticle with the diameter of 30 µm formed of colloidal SiO₂ particles with the diameter of 430 nm and reflection spectra of the particles formed by colloidal particles with the diameter of 200 nm (λ₂) and 430 nm (λ₄₁)

1.4. Development of a standard sample for ensuring uniformity of optical measurements of zeta potential

Zeta potential is the main stability index of colloidal systems in liquid media. Colloidal systems are used in a lot of fields of science and engineering such as, for instance, medicine, pharmaceutics, chemical industry, mineral dressing, water purification, soil cleansing, and many others. Certified standard materials are necessary to increase the accuracy and correctness of measurements. At present, European and American standard samples are used for these purposes because of the absence of domestic materials. For their import substitution, the Institute of Solid State Physics RAS and the All-Russian Research Institute for Optical and Physical Measurements have developed domestic standard samples (SS) based on aqueous suspension of silica nanoparticles synthesized by heterogeneous tetraethyl orthosilicate (TEOS) hydrolysis using the environmentally friendly catalyst (L-arginine). We have obtained silica particles with controlled electrokinetic potential, which exhibit the values of zeta potential (ZP) in the range -30 mV ÷ - 50 mV (Fig. 6).

Figure 6. Distribution of the zeta potential values of samples No. 18 (-30.2 mV) и No. 509 (-57.4 mV)

1.5. Inverse opal based on a polymer filler

Opal-like photonic crystals based on inverse opal with a polymer skeleton, which exhibit shifts of selective reflection bands toward both the long-wavelength and short-wavelength ranges with respect to the diffraction band of the initial opal consisting of SiO₂ spheres, have been synthesized. The contributions from the skeletons forming three-dimensional periodic structures and from the fillers to the spectral position of diffraction bands have been determined.

Figure 7. SEM images of (a) the initial opal matrix formed by silica spheres of diameter D = 255 ± 10 nm ((111) surface orientation), (b) polymer ED-20–opal composite with (100) orientation, and (c) inverse opal of ED-20 polymer ((111) surface orientation)

1.6. Nanostructured and nanoporous carbon-based materials obtained by globular SiO₂ structure invertion (С-IOP)

Nanostructured carbon-based materials are attractive from both fundamental and practical points of view. They are widely used in many technologies including electrode materials for supercapacitors, batteries and fuel elements, sorbents of various application and materials for catalysis. The most actively developed prospects are linked to portable power supplies in microelectronics, energy storage systems, components for force pulsing devices and other instruments that require a high-speed energy source. The key parameters of carbon-based materials used as electrodes in electrochemical energy sources are their specific surface area and the size and topology of their pores. The high specific surface area of carbon increases its capacity to store electric charge on its surface. Micropores of at most 2 nm in diameter are the main component of its specific surface area. For quicker ion transport it is crucial to have mesopores (their diameter being from 2 to 50 nm) in the volume of the electrode material. The interconnected micro- and mesopore system combined with high electrode surface area raise the output characteristics of the devices. Micro- and mesoporic carbon-based materials with specific surface area values close to their possible limit (2500 m²/g) and pore volume up to 2 cm³/g have got synthesized by the template method at the Institute of Solid State Physics RAS. An opal matrix represented by a three-dimensional close-packed system of monodisperse globular particles (globules) of silica has been used as a template. The interconnected micro- and mesoporic system combined with the high surface area of an inverse opal improves both sorption properties and electrochemical output characteristics of the material. Figure 1 shows the synthesis scheme of carbon structures with an inverse opal lattice, and a fragment of the structure that demonstrates the interconnected pore system.

It has been shown that nanostructures have richer variety of properties including new functional properties when compared to the previously investigated materials. For example, spherical carbon particles containing concentric graphite-like (onion-like) shells have been discovered in the composite structures. The first investigation results allow an assumption that a diamond-like phase exists in an inverse opal SiC/C composite.

Figure 8. Synthesis scheme of carbon structures with an inverse opal lattice; fragments of the structure that demonstrates the interconnected pore system; onion-like particles; isotherms (N2, 77K) of nitrogen adsorption – desorption by inverse opal nanostructures

Novel sorbents for recovery and separation of actinides and lanthanides from highly active wastes of nuclear fuel cycles and electrode materials for supercapacitors have been synthesized on the basis of carbon inverse opal structures.

- C-IOP modified with tetraphenylmethylenediphospine dioxide have demonstrated high sorption ability towards Th (IV), U (VI) ions and lanthanides (III) in nitric acid solutions as compared to the known sorbents. Table 1 shows the distribution ratio of Eu and La/Lu separation factor for our C-IOP sorbent and other carbon-based sorbents.

Table 1

- C-IOP sorbent modified with tetraoctyldiglycolamide (TODGA) (C-IOP – TOGDA) has shown high sorption ability towards lanthanides (III) ions in nitric acid solutions as compared to the known sorbents (Fig.).

- Electrochemical cells with C-IOP/NiO(Ni7S6) electrodes using 6M KOH electrolyte have demonstrated high values of specific capacitance at room temperature: the ED-20 polymer-based samples showed the gravitation capacity of 220 F/g at current density of 0.5 A/g (Fig.), the sucrose-based sample has shown the gravitation capacity of 600 F/g at current density during discharge of 5 mA/cm² (0.625 A/g).

Figure 9. Electrochemical characteristics of (C-IOP – NiO(Ni7S6)) composite fabricated using ED-20 epoxy resin: (a) cyclic voltammetry, (b) galvanostatic charge–discharge measurements, (c) specific capacitance at different scanning rates, (d) impedance plot

Nickel and cobalt sulfates bicrystals of K₂Ni(SO₄)₂·6H₂O/K₂Co(SO₄)_2*6H₂O composition and K₂Ni_xCo_(1-x)(SO₄)₂·6H₂ mixed crystals for solar-blind UV filters

The method of growth has been developed and nickel and cobalt sulfates bicrystals of K₂Ni(SO₄)₂·6H₂O/K₂Co(SO₄)_2*6H₂O composition and K₂Ni_xCo_(1-x)(SO₄)₂·6H₂ mixed crystals have been obtained for the first time. The crystals demonstrate high transmittance (80%) in the solar-blind spectral region of 200-300 nm UV wavelength range and opacity in other wavelength ranges and allow devices recording radiation in this spectrum range to work in sunlight. Band filtering permits to maintain a high signal/noise ratio and reach giant (up to 10⁸ times) gain coefficients in the UV range providing unique equipment sensitivity. UV filters based on the obtained crystals are used for remote inspection of power lines, environmental monitoring of terrestrial and water space, tracking of paths of rocket and missile motion.

Figure 1. General view of K₂Ni(SO₄)₂·6H₂O/K₂Co(SO₄)_2*6H₂O bicrystal and transmission spectrum of K₂Ni_xCo_(1-x)(SO₄)₂·6H₂mixed crystal