rus
Issue #4/2016
M.Samoilovich, A.Belyanin, V.Odinokov, V.Bovtun, M.Kempa, N.Nuzhnyy, M.Savinov
Structure and physical properties of nanocomposites: opal matrix – titanium oxide
Structure and physical properties of nanocomposites: opal matrix – titanium oxide
The conditions for the formation of nanocomposites based on lattice packings SiO2 nanospheres (opal matrices) with crystallites of titanium oxide in interspherical nanospacing are considered. The composition and structure of the nanocomposites were studied by electron microscopy, X-ray diffraction and Raman spectroscopy. Results of measuring of the frequency dependences of real and imaginary components of the permittivity and microwave conductivity of obtained nanostructures are viewed.
Теги: dielectric properties nanocomposite opal matrix titanium oxide x-ray diffractometry диэлектрические характеристики нанокомпозит оксид титана опаловая матрица рентгеновская дифрактометрия
Nanocomposites based on opal matrices, which interspherical cavities are filled with different substances, are one of the new types of metamaterials. Opal matrix is a 3D three-layer cubic structure based on lattice packing of x-ray amorphous SiO2 nanospheres with a diameter of 200–350 nm [1]. Metamaterials and nanocomposites based on them are promising for use in electronic engineering for creation of solid-state microwave devices (filters, delay lines, phase changers, etc.) and the sources of the x-ray radiation and acoustic waves [2, 3]. It is predicted that on the basis of opal matrixes the devices for phase velocity control in optical, microwave and THz-ranges will be developed [4, 5]. It is supposed that use of metamaterials in the form of 3D nanogrids of clusters of various substances with sizes of 15–40 nm will allow to expand the frequency range of their application (as frequency limits are quite often associated with the dispersion of dielectric permittivity, which is typical for solid materials) and to create the high performance devices controlled in electric fields.
This project is devoted to obtaining of nanocomposites "opal matrix – titanium oxides" (TiO2 and TiO). Titanium dioxide TiO2 has numerous crystalline modifications, but in practice mainly anatase and rutile are used in electronic technique as dielectric, heat-resistant and optical glass (including for fiber transmission systems), the wide band gap semiconductor. Titanium dioxide is one of the most well studied semiconductor photocatalytic materials, photoelectrochemical activity of which is highly dependent on the phase composition (both crystalline and amorphous TiO2 phases) [6].
The synthesis under conditions of limited geometry of interspherical cavities of opal matrices can contribute to the stabilization of phase of TiO.
3D sublattices of titanium oxide were obtained by synthesis with limiting the size of the crystallites in terms of nanoconfinement in which the crystallites do not break in contrast to processes in solid substances [7, 8]. Synthesis in enclosed and confined (nanoscale) volume is of interest in fundamental studies of physical properties and phase stability of nanoscale inorganic systems. Decrease in temperature of formation of synthesized substance and stabilization of high-temperature phases at low temperatures are peculiar to the specified process [9, 10].
This project investigates the effect of preparation conditions on structure and dielectric properties of nanocomposites based on opal matrices, in interspherical cavities of which crystalline phase of titanium oxides are synthesized.
Production of nanocomposites: opal matrix – titanium oxide
Opal matrix were obtained by hydrolysis reaction of tetraethyl orthosilicate Si(OC2H5)4 with a solution of ethanol C2H5OH in the presence of a catalyst of ammonium hydroxide NH4OH [1]. Opal matrix with a volume of 2–3 cm3 and dimensions of single-domain regions ≥0.1 mm3 were used to form nanocomposites. Experimental investigations were carried out with samples of opal matrices with the diameter of SiO2 nanospheres of about 260 nm.
Nanocomposites based on opal matrices, interspherical cavities of which were filled with titanium oxides, have been obtained by an impregnation based on filling due to the capillary effect of these cavities with a solution of a specified chemical composition, followed by heat treatment, during which the necessary substance was formed [1, 8, 11]. In this project, 15% solution of titanium trichloride TiCl3 in a weakly concentrated solution of hydrochloric acid was injected in the interspherical cavities of opal matrices. Filling of cavities with a solution and the preliminary heat treatment at 300–400°C were performed 5–8 times. The preliminary heat treatment leads to the formation of x-ray amorphous and crystalline phases of different TiO2 modifications, and high-temperature heat treatment promotes the crystallization of the phases of a given composition. Crystalline rutile was formed by high temperature heat treatment of the nanocomposites in air at 700–1 000°C, and TiO – by heat treatment in H2. High-temperature heat treatment leads to consistent transformation of x-ray amorphous TiO2 to anatase and rutile. The phase transition "anatase – rutile" in the conditions of limited geometry occurs at lower temperatures compared to polycrystalline massive TiO2.
The formation in the interspherical cavities of opal matrices of the lower titanium oxide TiO was achieved at a controlled high temperature of heat treatment in H2 using a hydrogen generator. OTJIG TM-6 system (Research Institute of Precision Machine Manufacturing, Zelenograd) (Fig.1) provided the defined parameters of the heat treatment. The heating chamber of the installation has a two-section spiral heater with the quartz reactor inside. Two thermoelectric platinum converters allow to adjust the temperature in the range of 20–1 000°C in various sections of the reactor. The gas panel has three flow channels for process gases (Ar, N2, H2) with flow regulators in each channel and provides heat treatment of the samples in the flow of the specified gas. Control unit consists of a analog four-channel system for control of concentrations of gases and of a DC power supply with galvanic isolation. Automatic operation of the heating chamber according to one of 16 preset programs is ensured.
To obtain nanocomposites containing TiO, we used the following parameters of the high temperature heat treatment:
•reaction chamber pre purge with use of passive or inert gas (N2 or Ar) during 20 min;
•heating rate in a stream of H2 at a pressure of 5 atmospheres is 600 grad./h;
•treatment temperature in H2 at a pressure of 3–5 atmospheres is 1 000°C (2 hours).
High-temperature heat treatment in H2 leads to the reduction of TiO2 to TiO (balance of reduction of TiO2 to Ti with subsequent interaction of Ti with TiO2, leading to formation of TiO). As a result, the nanocomposites were obtained whose interspherical cavities are filled with the crystallites of rutile or TiO with the degree of fill of 30–40%.
Structure of opal matrices and nanocomposites on their base
The correct packing of SiO2 nanospheres was achieved by their self-organization, and the diameter (d) of the nanospheres was dependent on the formation conditions (the difference of the diameters of the nanospheres of each sample was Δd < 5%). Fig.2 shows the REM surface image of the sample of opal matrix, obtained using Carl Zeiss Leo 1430 VP. The correctness of the cubic packing of SiO2 nanospheres retained during synthesis in their interspherical cavities of the titanium oxides.
Fig.3a shows three layers (1–3) of the dense 3D cubic lattice packing of SiO2 nanospheres (4). Each nanosphere A in a flat dense layer is surrounded by six triangular gaps of different orientations (B and C). The top layer of nanospheres relative to the bottom can be oriented either by positions B or by the positions C (Fig.3a). Under used experimental conditions a three-layer (cubic) structure .../ABC/... was formed [1, 7, 12]. The structure represents the densest packing with the degree of filling of equal space by nanospheres of 74.05% [13]. The densest packing of nanospheres forms a tetrahedral and octahedral interspherical cavities, occupying 25.95% of the volume of opal matrices. The cavities 5 and 7 (Fig.3a) are formed by SiO2 nanospheres of the first and second layers, and the cavities 6, 8 and 9 by nanospheres of the second and third layers of closest packing of nanospheres. Each SiO2 nanosphere is surrounded by eight tetrahedral and six octahedral cavities. Connecting the centres of four and six nanospheres, which form the cavities, we obtain, respectively, octahedrons (Ok) and differently oriented tetrahedrons (T1 and T2) (Fig.3a). The octahedrons and tetrahedrons completely fill the space (Fig.3b). Sections of interspherical space by facets of tetrahedrons and octahedrons are highlighted on their surfaces (Fig.3b).
Octahedral and tetrahedral cavities are conditionally consist of spheres (10 and 11 in Fig.3b), inscribed in the cavity, and the connecting space. The diameter of the spheres inscribed in the tetrahedral and octahedral cavities is equal to about 0.22 d and 0.41 d, respectively. Fig.3b shows a three-dimensional model of the substance, which fills ten interspherical cavities in three-layer package (four octahedral and six tetrahedral).
The structure of the nanocomposites containing crystallites of rutile or TiO was investigated using a transmission electron microscope (TEM) JEM 200C. Samples for TEM were prepared according to the method, allowing to separate the SiO2 nanospheres and particles of titanium oxide, synthesized in interspherical cavities of opal matrices (Fig.4). The particles of the synthesized substances were in the form of crystallites, which is close to equiaxed. The particle size measured by TEM was about 20–30 nm for TiO and about 25–90 nm for rutile. The synthesis temperature in the range from 700°C to 1 000°C does not affect the size and shape of crystallites of titanium oxide.
Phase composition
of nanocomposites:
opal matrix – titanium oxide
The composition of the nanocomposites was monitored using x-ray diffraction and Raman spectroscopy. To identify the crystalline phases of the synthesized compounds the x-ray diffractometer ARL X'tra (Thermo Fisher Scientific) was used in the following modes: CuKα-radiation, energy-dispersive solid-state detector with Peltier cooler, the rotation of the sample with step size of 0.02° in continuous mode (1°/min). X-ray pictures were analyzed using an automated database ICDD PDF-2. Fig.5 shows х-ray diffraction pattern of opal matrices whose interspherical cavities are filled with oxides of titanium. Phase composition and phase structure of the synthesized substances was dependent on the composition and conditions of heat treatment of impregnating solutions. It was determined the presence of the following crystalline phases: rutile (tetragonal system, the space group P42/mnm) (Fig.5, curve 1) and TiO (cubic crystal structure, Fm-3m) (Fig.5, curve 2). Other crystalline phases of titanium oxides (out of 15 known) were not detected.
Depending on the synthesis conditions the recrystallization of x-ray amorphous silica is possible with formation of small concentrations of SiO2 crystalline phases of various modifications, for example of SiO2 (quartz (hexagonal crystal structure, P3121) (Fig.5, curve 1). During heat treatment in the H2 crystalline phases of SiO2 are not formed. Polymorphic transformations of SiO2 modifications (quartz, tridymite and cristobalite) are accompanied by volume changes, but at concentrations not exceeding 4%, these changes do not affect the size and shape of interspherical cavities. The transformation of anatase into more dense rutile (junction temperature 400–1 000°C) and of rutile into TiO (density of oxides: 4.05 g/cm3 for anatase; 4.23 g/cm3 for rutile and 4.9 g/cm3 for TiO), which occur irreversibly during heating (in air and in N2), do not affect the toughness of nanocomposites.
Measured using X-ray diffraction pattern period of the unit cell of TiO was a = 0.4156–0.4165 nm (theoretical value is a = 0.4244 nm). Significantly smaller values of the period of the cell compared to the theoretical value for TiO are caused by high concentration of Ti and O vacancies and their compressive effect. The unit cell parameters for rutile were: a = 0.46053–0.46074 nm, c = 0.29568–0.29634 nm, which is close to the theoretical values (a = 0.45929 nm, c = 0,29591 nm).
The synthesized material contains x-ray amorphous phase in addition to the crystal phases. The degree of crystallinity (concentration of crystalline phases in the synthesized substance) depends on the conditions of synthesis and can reach tens of percent. The size of crystallites (areas of coherent scattering of x-rays, LОКР) of crystalline phases of titanium oxides was determined by the widening of diffraction maximums in x-ray diffraction pattern. In our case LОКР = 49.0–59.8 nm for rutile and LОКР = 15.8–18.0 nm for TiO, which is less than the diameters of the spheres inscribed in the tetrahedral and octahedral cavities of opal matrices (57.2 nm or 0.22d and 106.6 nm or 0.41 d, respectively). TEM data (Fig.4) on the particle size of the synthesized substances correspond to the calculations of the x-ray diffraction pattern. According to the results of TEM and x-ray phase analysis it was found that over the whole range of applied temperatures of heat treatment the synthesized oxides of Ti did not interact with SiO2. It is determined that LОКР for crystalline phases does not depend on the degree of crystallinity of the synthesized substance.
X-ray diffractometry is not sensitive to phases with LОКР < 1 nm, which retain the functional properties of the synthesized materials. Raman spectroscopy allows to analyze materials in the х-ray amorphous state, since the composition and structure of substances clearly reflected in their Raman spectrum. In this study, Raman spectroscopy was used to identify and quantify the amorphous and crystalline phases of TiO2. The Raman spectra were recorded using a laser (632.8 nm line of He-Ne laser; laser power <300 mW; spot diameter of the beam is about 4 µm2; depth of analyzed layer is about 3 µm) micro Raman spectrometer LabRAM HR800 (HORIBA Jobin-Yvon). Fig.6 shows Raman spectra of nanocomposites containing crystallites of titanium oxide. These Raman spectra present the most important bands, which characterize the crystal and х-ray amorphous phases of titanium dioxides filling interspherical cavities.
Fig.6 (curve 1) presents the Raman spectra of nanocomposites, where the thermodynamically stable phase of rutile have the main strips in the area of shift Δν = 462 cm–1 and Δν = 622 cm–1 (the broadening of the bands Δν1/2 is about of 36.9 and 39.5 cm–1, respectively) and weak strips in areas of Δν = 235 cm–1, Δν = 295 cm–1 and Δν = 703 cm–1. Strips of crystal phase of the rutile were observed after heat treatment of the х-ray amorphous sample at 400°C and above, becoming more intense with increasing temperature that indicates the enhancement of crystallinity. X-ray amorphous TiO2 transforms mainly into anatase at a temperature of about 400°C. Changes in the position of the strips in the Raman spectrum of nanostructured phases of anatase and rutile are explained by the deviation of composition from stoichiometry [6].
Fig.6 (curve 2) presents a Raman spectrum for a series of nanocomposites containing TiO. In case of samples of opal matrixes, which interspherical cavities TiO are filled with TiO, there are strips relating to rutile and anatase. The broadening of the spectral strips of titanium oxide is caused by the small size of the crystallites of the phases of the synthesized oxides. With increasing synthesis temperature Δν1/2 increases, while the position of the strips show differently directed dependence.
Raman spectroscopy is also applicable to identify x-ray amorphous and polymorphic modifications of crystalline phases of SiO2. The presented spectrum (Fig.6, curve 1) contains strips that are typical for crystalline and amorphous phases of SiO2 that fill interspherical cavities. In the presented Raman spectra the strips for Δν = 246–295 cm–1 and Δν = 1 077–1 170 cm–1 are observed, related to the phases of the various SiO2 modifications: cristobalite, tridymite and α-quartz [14]. X-ray amorphous silica have weak strips in the Raman spectra when Δν = 1 077 cm–1. A weak peak at Δν = 885 cm–1 can be attributed to the quartz.
Dielectric properties
Studies of frequency dependences of real (ε') and imaginary (ε") components of permittivity and conductivity (σ) of opal matrices, interspherical cavities of which are filled with crystallites of rutile and TiO synthesized at 1000°C, have been conducted (Fig.7, 8). The dielectric spectrometer with a coaxial measuring cell Novocontrol BDS 2100 and Agilent 4291B impedance analyzer were used for measurement of real (ε') and imaginary (ε") components of the permittivity in the frequency range 1 ∙ 106–1.8 ∙ 109 Hz. Agilent’s equipment included a component for measurement of the dielectric and magnetic permeability, including losses, in the range from 1 MHz to 110 GHz.
Agilent Е4991А network analyzer was used for investigation of ε' and ε" components of dielectric and magnetic permeability in the range from 200 MHz to 3 GHz, and Agilent PNA E8361 network analyzer was used in the range from 75 GHz to 110 GHz. In the microwave region (2 ∙ 108–2 ∙ 1010 Hz) the measurements were conducted using a coaxial probe with an open end (Agilent 8507E) with Agilent E8364B network analyzer. In the terahertz region the terahertz transmission spectroscopy was used with femtosecond Ti:sapphire laser system.
Coaxial measurements (1 ∙ 106–
–1.8 ∙ 109 Hz) were conducted using samples in the form of cylinders with a diameter of 3 mm and a height of 4–5 mm and all other measurements were carried out using samples in the form of plates 10×10 mm with a thickness of 1–3 mm. The parameters of the microwave conductivity and ε' and ε" components of the permittivity were evaluated using superlattice0000_04_my.m subroutine. All measurements were performed on samples without the application of electrodes. Spectra of samples were measured at room temperature. Fig.7 shows the results of measurements of a sample, which according to x-ray phase analysis contains crystallites of rutile. According to the Raman spectroscopy, this sample also contains x-ray amorphous phase of titanium oxides.
Terahertz and microwave data indicate a higher loss, absorption with the center in the range of 30–100 GHz, where measurement is difficult. The curves of the frequency dependence of the dielectric permittivity and conductivity of nanocomposites that contain titanium monoxide (TiO), are close to shown in Fig.7 results for the nanocomposites containing rutile. The introduction of the crystallites of titanium oxide leads to increase in ε' of opal matrices for 40–200%, but does not affect the dielectric loss, which is low (ε" < 0,1) in all used frequency range. There is a small increase in dielectric losses at low frequencies (about 106 Hz) and their growth in the range of high frequencies (1010–1012 Hz). Growth losses in the terahertz region are obviously caused by low-frequency wing of the phonon spectrum of the input compounds.
Samples of opal matrices, interspherical cavities of which are filled with crystallites of titanium oxide, have a low conduction current at a direct current. This is evidenced by the low-frequency plateau of σ(f), which is most likely associated with surface leakage currents, and also with the presence of x-ray amorphous phases. Dielectric permittivity ε' of the investigated opal matrices, the cavities of which are filled with crystallites of TiO2 and TiO, is higher than the values of ε' for the unfilled opal matrix. Dielectric dispersion, which is typical for composite materials, is evident throughout the given frequency range, and the values of ε' slightly decreases with frequency. Simultaneously, the dielectric loss increases towards low frequencies (f < 10MHz) and towards terahertz frequencies.
The study of the effects of electric field on the specified dependencies in the frequency range 1–106 Hz were conducted for the samples of nanocomposites containing TiO (Fig.8). The measured dependencies on the electric field displacement (at a temperature of 300 K) for this sample showed low conductivity at a direct current and a weak dependence on the displacement field at low frequencies. The investigated materials are "bad" dielectrics with high losses and advanced relaxation polarization, whose loss and conductivity at high frequencies differ little from the parameters of the unfilled opal matrix. At 1 kHz the linearity is almost negligible, and at a frequency of 1 Hz there is a small decrease of ε', ε" and σ with increasing field strength (Fig.8). Weak nonlinearity indicates that large low-frequency reduction of ε', ε" and σ are not associated with near-electrode processes, and are caused by the relaxation polarization (or hopping conduction) of nanocomposite, with the main contribution from the titanium oxides synthesized in interspherical cavities of opal matrices. The enclosed field mainly affects the opal matrix (SiO2), which is electrically inactive and does not have any significant conductivity or a dielectric nonlinearity. The formation of crystalline phases of SiO2 occurs from the surface of the nanospheres, at the same time, the structure and concentration of crystalline phases of SiO2 depended mainly on heat treatment conditions.
All materials behave as composites of ceramic type with a concentration of conductive filler slightly above the percolation threshold. The specified properties for all investigated nanocomposites are close to each other. Low level of low frequency conductivity, hence of the direct current conductivity, also confirm a weak percolation. The measured spectra of the frequency dependence of conductivity and permittivity parameters show that the studied samples belong to the materials passed the percolation threshold for the input components. We can also assume that some characteristics are influenced by the real structure of the samples, leading to the absence of the mirror plane between the layers and to the chirality [15].
Conclusion
Samples of opal matrixes (3D-lattice packings of x-ray amorphous SiO2 nanospheres with diameters of d = 250–280 nm) with a volume more of 2 cm3 with sizes of single-domain regions up to 0.1 mm3 which interspherical cavities are filled with crystallites of titanium oxides (TiO2 and TiO) have been obtained. The synthesis of titanium oxides was conducted under conditions of limited geometry in interspherical cavities of opal matrices. The formation in interspherical cavities of opal matrices of the lower titanium oxide (TiO) was achieved at a controlled high temperature heat treatment in hydrogen, which was performed using a specially designed OTJIG TM-6 system.
The composition and structure of obtained nanocomposites that contain synthesized in the interspherical cavities crystalline and x-ray amorphous phases of titanium oxide have been investigated by scanning and transmission electron microscopy, x-ray diffractometry and Raman spectroscopy. Synthesized in the interspherical cavities crystallites of titanium oxide had a size of 49.0–59.8 nm for TiO2 and 15.8–18.0 nm for TiO with a shape close to the equiaxed. By varying the synthesis conditions it is possible to change the phase composition and structure of substances formed in interspherical cavities of opal matrices.
Frequency dependences of the real and imaginary components of the dielectric permittivity of nanocomposites, as well as conductivity have been measured in the range from 1 MHz to 110 GHz. Researches have allowed to establish the relationship between microwave properties and phase composition of titanium oxides, and to obtain data necessary for the application of such non-crystalline inhomogeneous materials with spatial modulation (dispersion) of the electric and dielectric properties at the nanoscale. ■
The project is supported by RFBR (grant No.15-07-00529).
This project is devoted to obtaining of nanocomposites "opal matrix – titanium oxides" (TiO2 and TiO). Titanium dioxide TiO2 has numerous crystalline modifications, but in practice mainly anatase and rutile are used in electronic technique as dielectric, heat-resistant and optical glass (including for fiber transmission systems), the wide band gap semiconductor. Titanium dioxide is one of the most well studied semiconductor photocatalytic materials, photoelectrochemical activity of which is highly dependent on the phase composition (both crystalline and amorphous TiO2 phases) [6].
The synthesis under conditions of limited geometry of interspherical cavities of opal matrices can contribute to the stabilization of phase of TiO.
3D sublattices of titanium oxide were obtained by synthesis with limiting the size of the crystallites in terms of nanoconfinement in which the crystallites do not break in contrast to processes in solid substances [7, 8]. Synthesis in enclosed and confined (nanoscale) volume is of interest in fundamental studies of physical properties and phase stability of nanoscale inorganic systems. Decrease in temperature of formation of synthesized substance and stabilization of high-temperature phases at low temperatures are peculiar to the specified process [9, 10].
This project investigates the effect of preparation conditions on structure and dielectric properties of nanocomposites based on opal matrices, in interspherical cavities of which crystalline phase of titanium oxides are synthesized.
Production of nanocomposites: opal matrix – titanium oxide
Opal matrix were obtained by hydrolysis reaction of tetraethyl orthosilicate Si(OC2H5)4 with a solution of ethanol C2H5OH in the presence of a catalyst of ammonium hydroxide NH4OH [1]. Opal matrix with a volume of 2–3 cm3 and dimensions of single-domain regions ≥0.1 mm3 were used to form nanocomposites. Experimental investigations were carried out with samples of opal matrices with the diameter of SiO2 nanospheres of about 260 nm.
Nanocomposites based on opal matrices, interspherical cavities of which were filled with titanium oxides, have been obtained by an impregnation based on filling due to the capillary effect of these cavities with a solution of a specified chemical composition, followed by heat treatment, during which the necessary substance was formed [1, 8, 11]. In this project, 15% solution of titanium trichloride TiCl3 in a weakly concentrated solution of hydrochloric acid was injected in the interspherical cavities of opal matrices. Filling of cavities with a solution and the preliminary heat treatment at 300–400°C were performed 5–8 times. The preliminary heat treatment leads to the formation of x-ray amorphous and crystalline phases of different TiO2 modifications, and high-temperature heat treatment promotes the crystallization of the phases of a given composition. Crystalline rutile was formed by high temperature heat treatment of the nanocomposites in air at 700–1 000°C, and TiO – by heat treatment in H2. High-temperature heat treatment leads to consistent transformation of x-ray amorphous TiO2 to anatase and rutile. The phase transition "anatase – rutile" in the conditions of limited geometry occurs at lower temperatures compared to polycrystalline massive TiO2.
The formation in the interspherical cavities of opal matrices of the lower titanium oxide TiO was achieved at a controlled high temperature of heat treatment in H2 using a hydrogen generator. OTJIG TM-6 system (Research Institute of Precision Machine Manufacturing, Zelenograd) (Fig.1) provided the defined parameters of the heat treatment. The heating chamber of the installation has a two-section spiral heater with the quartz reactor inside. Two thermoelectric platinum converters allow to adjust the temperature in the range of 20–1 000°C in various sections of the reactor. The gas panel has three flow channels for process gases (Ar, N2, H2) with flow regulators in each channel and provides heat treatment of the samples in the flow of the specified gas. Control unit consists of a analog four-channel system for control of concentrations of gases and of a DC power supply with galvanic isolation. Automatic operation of the heating chamber according to one of 16 preset programs is ensured.
To obtain nanocomposites containing TiO, we used the following parameters of the high temperature heat treatment:
•reaction chamber pre purge with use of passive or inert gas (N2 or Ar) during 20 min;
•heating rate in a stream of H2 at a pressure of 5 atmospheres is 600 grad./h;
•treatment temperature in H2 at a pressure of 3–5 atmospheres is 1 000°C (2 hours).
High-temperature heat treatment in H2 leads to the reduction of TiO2 to TiO (balance of reduction of TiO2 to Ti with subsequent interaction of Ti with TiO2, leading to formation of TiO). As a result, the nanocomposites were obtained whose interspherical cavities are filled with the crystallites of rutile or TiO with the degree of fill of 30–40%.
Structure of opal matrices and nanocomposites on their base
The correct packing of SiO2 nanospheres was achieved by their self-organization, and the diameter (d) of the nanospheres was dependent on the formation conditions (the difference of the diameters of the nanospheres of each sample was Δd < 5%). Fig.2 shows the REM surface image of the sample of opal matrix, obtained using Carl Zeiss Leo 1430 VP. The correctness of the cubic packing of SiO2 nanospheres retained during synthesis in their interspherical cavities of the titanium oxides.
Fig.3a shows three layers (1–3) of the dense 3D cubic lattice packing of SiO2 nanospheres (4). Each nanosphere A in a flat dense layer is surrounded by six triangular gaps of different orientations (B and C). The top layer of nanospheres relative to the bottom can be oriented either by positions B or by the positions C (Fig.3a). Under used experimental conditions a three-layer (cubic) structure .../ABC/... was formed [1, 7, 12]. The structure represents the densest packing with the degree of filling of equal space by nanospheres of 74.05% [13]. The densest packing of nanospheres forms a tetrahedral and octahedral interspherical cavities, occupying 25.95% of the volume of opal matrices. The cavities 5 and 7 (Fig.3a) are formed by SiO2 nanospheres of the first and second layers, and the cavities 6, 8 and 9 by nanospheres of the second and third layers of closest packing of nanospheres. Each SiO2 nanosphere is surrounded by eight tetrahedral and six octahedral cavities. Connecting the centres of four and six nanospheres, which form the cavities, we obtain, respectively, octahedrons (Ok) and differently oriented tetrahedrons (T1 and T2) (Fig.3a). The octahedrons and tetrahedrons completely fill the space (Fig.3b). Sections of interspherical space by facets of tetrahedrons and octahedrons are highlighted on their surfaces (Fig.3b).
Octahedral and tetrahedral cavities are conditionally consist of spheres (10 and 11 in Fig.3b), inscribed in the cavity, and the connecting space. The diameter of the spheres inscribed in the tetrahedral and octahedral cavities is equal to about 0.22 d and 0.41 d, respectively. Fig.3b shows a three-dimensional model of the substance, which fills ten interspherical cavities in three-layer package (four octahedral and six tetrahedral).
The structure of the nanocomposites containing crystallites of rutile or TiO was investigated using a transmission electron microscope (TEM) JEM 200C. Samples for TEM were prepared according to the method, allowing to separate the SiO2 nanospheres and particles of titanium oxide, synthesized in interspherical cavities of opal matrices (Fig.4). The particles of the synthesized substances were in the form of crystallites, which is close to equiaxed. The particle size measured by TEM was about 20–30 nm for TiO and about 25–90 nm for rutile. The synthesis temperature in the range from 700°C to 1 000°C does not affect the size and shape of crystallites of titanium oxide.
Phase composition
of nanocomposites:
opal matrix – titanium oxide
The composition of the nanocomposites was monitored using x-ray diffraction and Raman spectroscopy. To identify the crystalline phases of the synthesized compounds the x-ray diffractometer ARL X'tra (Thermo Fisher Scientific) was used in the following modes: CuKα-radiation, energy-dispersive solid-state detector with Peltier cooler, the rotation of the sample with step size of 0.02° in continuous mode (1°/min). X-ray pictures were analyzed using an automated database ICDD PDF-2. Fig.5 shows х-ray diffraction pattern of opal matrices whose interspherical cavities are filled with oxides of titanium. Phase composition and phase structure of the synthesized substances was dependent on the composition and conditions of heat treatment of impregnating solutions. It was determined the presence of the following crystalline phases: rutile (tetragonal system, the space group P42/mnm) (Fig.5, curve 1) and TiO (cubic crystal structure, Fm-3m) (Fig.5, curve 2). Other crystalline phases of titanium oxides (out of 15 known) were not detected.
Depending on the synthesis conditions the recrystallization of x-ray amorphous silica is possible with formation of small concentrations of SiO2 crystalline phases of various modifications, for example of SiO2 (quartz (hexagonal crystal structure, P3121) (Fig.5, curve 1). During heat treatment in the H2 crystalline phases of SiO2 are not formed. Polymorphic transformations of SiO2 modifications (quartz, tridymite and cristobalite) are accompanied by volume changes, but at concentrations not exceeding 4%, these changes do not affect the size and shape of interspherical cavities. The transformation of anatase into more dense rutile (junction temperature 400–1 000°C) and of rutile into TiO (density of oxides: 4.05 g/cm3 for anatase; 4.23 g/cm3 for rutile and 4.9 g/cm3 for TiO), which occur irreversibly during heating (in air and in N2), do not affect the toughness of nanocomposites.
Measured using X-ray diffraction pattern period of the unit cell of TiO was a = 0.4156–0.4165 nm (theoretical value is a = 0.4244 nm). Significantly smaller values of the period of the cell compared to the theoretical value for TiO are caused by high concentration of Ti and O vacancies and their compressive effect. The unit cell parameters for rutile were: a = 0.46053–0.46074 nm, c = 0.29568–0.29634 nm, which is close to the theoretical values (a = 0.45929 nm, c = 0,29591 nm).
The synthesized material contains x-ray amorphous phase in addition to the crystal phases. The degree of crystallinity (concentration of crystalline phases in the synthesized substance) depends on the conditions of synthesis and can reach tens of percent. The size of crystallites (areas of coherent scattering of x-rays, LОКР) of crystalline phases of titanium oxides was determined by the widening of diffraction maximums in x-ray diffraction pattern. In our case LОКР = 49.0–59.8 nm for rutile and LОКР = 15.8–18.0 nm for TiO, which is less than the diameters of the spheres inscribed in the tetrahedral and octahedral cavities of opal matrices (57.2 nm or 0.22d and 106.6 nm or 0.41 d, respectively). TEM data (Fig.4) on the particle size of the synthesized substances correspond to the calculations of the x-ray diffraction pattern. According to the results of TEM and x-ray phase analysis it was found that over the whole range of applied temperatures of heat treatment the synthesized oxides of Ti did not interact with SiO2. It is determined that LОКР for crystalline phases does not depend on the degree of crystallinity of the synthesized substance.
X-ray diffractometry is not sensitive to phases with LОКР < 1 nm, which retain the functional properties of the synthesized materials. Raman spectroscopy allows to analyze materials in the х-ray amorphous state, since the composition and structure of substances clearly reflected in their Raman spectrum. In this study, Raman spectroscopy was used to identify and quantify the amorphous and crystalline phases of TiO2. The Raman spectra were recorded using a laser (632.8 nm line of He-Ne laser; laser power <300 mW; spot diameter of the beam is about 4 µm2; depth of analyzed layer is about 3 µm) micro Raman spectrometer LabRAM HR800 (HORIBA Jobin-Yvon). Fig.6 shows Raman spectra of nanocomposites containing crystallites of titanium oxide. These Raman spectra present the most important bands, which characterize the crystal and х-ray amorphous phases of titanium dioxides filling interspherical cavities.
Fig.6 (curve 1) presents the Raman spectra of nanocomposites, where the thermodynamically stable phase of rutile have the main strips in the area of shift Δν = 462 cm–1 and Δν = 622 cm–1 (the broadening of the bands Δν1/2 is about of 36.9 and 39.5 cm–1, respectively) and weak strips in areas of Δν = 235 cm–1, Δν = 295 cm–1 and Δν = 703 cm–1. Strips of crystal phase of the rutile were observed after heat treatment of the х-ray amorphous sample at 400°C and above, becoming more intense with increasing temperature that indicates the enhancement of crystallinity. X-ray amorphous TiO2 transforms mainly into anatase at a temperature of about 400°C. Changes in the position of the strips in the Raman spectrum of nanostructured phases of anatase and rutile are explained by the deviation of composition from stoichiometry [6].
Fig.6 (curve 2) presents a Raman spectrum for a series of nanocomposites containing TiO. In case of samples of opal matrixes, which interspherical cavities TiO are filled with TiO, there are strips relating to rutile and anatase. The broadening of the spectral strips of titanium oxide is caused by the small size of the crystallites of the phases of the synthesized oxides. With increasing synthesis temperature Δν1/2 increases, while the position of the strips show differently directed dependence.
Raman spectroscopy is also applicable to identify x-ray amorphous and polymorphic modifications of crystalline phases of SiO2. The presented spectrum (Fig.6, curve 1) contains strips that are typical for crystalline and amorphous phases of SiO2 that fill interspherical cavities. In the presented Raman spectra the strips for Δν = 246–295 cm–1 and Δν = 1 077–1 170 cm–1 are observed, related to the phases of the various SiO2 modifications: cristobalite, tridymite and α-quartz [14]. X-ray amorphous silica have weak strips in the Raman spectra when Δν = 1 077 cm–1. A weak peak at Δν = 885 cm–1 can be attributed to the quartz.
Dielectric properties
Studies of frequency dependences of real (ε') and imaginary (ε") components of permittivity and conductivity (σ) of opal matrices, interspherical cavities of which are filled with crystallites of rutile and TiO synthesized at 1000°C, have been conducted (Fig.7, 8). The dielectric spectrometer with a coaxial measuring cell Novocontrol BDS 2100 and Agilent 4291B impedance analyzer were used for measurement of real (ε') and imaginary (ε") components of the permittivity in the frequency range 1 ∙ 106–1.8 ∙ 109 Hz. Agilent’s equipment included a component for measurement of the dielectric and magnetic permeability, including losses, in the range from 1 MHz to 110 GHz.
Agilent Е4991А network analyzer was used for investigation of ε' and ε" components of dielectric and magnetic permeability in the range from 200 MHz to 3 GHz, and Agilent PNA E8361 network analyzer was used in the range from 75 GHz to 110 GHz. In the microwave region (2 ∙ 108–2 ∙ 1010 Hz) the measurements were conducted using a coaxial probe with an open end (Agilent 8507E) with Agilent E8364B network analyzer. In the terahertz region the terahertz transmission spectroscopy was used with femtosecond Ti:sapphire laser system.
Coaxial measurements (1 ∙ 106–
–1.8 ∙ 109 Hz) were conducted using samples in the form of cylinders with a diameter of 3 mm and a height of 4–5 mm and all other measurements were carried out using samples in the form of plates 10×10 mm with a thickness of 1–3 mm. The parameters of the microwave conductivity and ε' and ε" components of the permittivity were evaluated using superlattice0000_04_my.m subroutine. All measurements were performed on samples without the application of electrodes. Spectra of samples were measured at room temperature. Fig.7 shows the results of measurements of a sample, which according to x-ray phase analysis contains crystallites of rutile. According to the Raman spectroscopy, this sample also contains x-ray amorphous phase of titanium oxides.
Terahertz and microwave data indicate a higher loss, absorption with the center in the range of 30–100 GHz, where measurement is difficult. The curves of the frequency dependence of the dielectric permittivity and conductivity of nanocomposites that contain titanium monoxide (TiO), are close to shown in Fig.7 results for the nanocomposites containing rutile. The introduction of the crystallites of titanium oxide leads to increase in ε' of opal matrices for 40–200%, but does not affect the dielectric loss, which is low (ε" < 0,1) in all used frequency range. There is a small increase in dielectric losses at low frequencies (about 106 Hz) and their growth in the range of high frequencies (1010–1012 Hz). Growth losses in the terahertz region are obviously caused by low-frequency wing of the phonon spectrum of the input compounds.
Samples of opal matrices, interspherical cavities of which are filled with crystallites of titanium oxide, have a low conduction current at a direct current. This is evidenced by the low-frequency plateau of σ(f), which is most likely associated with surface leakage currents, and also with the presence of x-ray amorphous phases. Dielectric permittivity ε' of the investigated opal matrices, the cavities of which are filled with crystallites of TiO2 and TiO, is higher than the values of ε' for the unfilled opal matrix. Dielectric dispersion, which is typical for composite materials, is evident throughout the given frequency range, and the values of ε' slightly decreases with frequency. Simultaneously, the dielectric loss increases towards low frequencies (f < 10MHz) and towards terahertz frequencies.
The study of the effects of electric field on the specified dependencies in the frequency range 1–106 Hz were conducted for the samples of nanocomposites containing TiO (Fig.8). The measured dependencies on the electric field displacement (at a temperature of 300 K) for this sample showed low conductivity at a direct current and a weak dependence on the displacement field at low frequencies. The investigated materials are "bad" dielectrics with high losses and advanced relaxation polarization, whose loss and conductivity at high frequencies differ little from the parameters of the unfilled opal matrix. At 1 kHz the linearity is almost negligible, and at a frequency of 1 Hz there is a small decrease of ε', ε" and σ with increasing field strength (Fig.8). Weak nonlinearity indicates that large low-frequency reduction of ε', ε" and σ are not associated with near-electrode processes, and are caused by the relaxation polarization (or hopping conduction) of nanocomposite, with the main contribution from the titanium oxides synthesized in interspherical cavities of opal matrices. The enclosed field mainly affects the opal matrix (SiO2), which is electrically inactive and does not have any significant conductivity or a dielectric nonlinearity. The formation of crystalline phases of SiO2 occurs from the surface of the nanospheres, at the same time, the structure and concentration of crystalline phases of SiO2 depended mainly on heat treatment conditions.
All materials behave as composites of ceramic type with a concentration of conductive filler slightly above the percolation threshold. The specified properties for all investigated nanocomposites are close to each other. Low level of low frequency conductivity, hence of the direct current conductivity, also confirm a weak percolation. The measured spectra of the frequency dependence of conductivity and permittivity parameters show that the studied samples belong to the materials passed the percolation threshold for the input components. We can also assume that some characteristics are influenced by the real structure of the samples, leading to the absence of the mirror plane between the layers and to the chirality [15].
Conclusion
Samples of opal matrixes (3D-lattice packings of x-ray amorphous SiO2 nanospheres with diameters of d = 250–280 nm) with a volume more of 2 cm3 with sizes of single-domain regions up to 0.1 mm3 which interspherical cavities are filled with crystallites of titanium oxides (TiO2 and TiO) have been obtained. The synthesis of titanium oxides was conducted under conditions of limited geometry in interspherical cavities of opal matrices. The formation in interspherical cavities of opal matrices of the lower titanium oxide (TiO) was achieved at a controlled high temperature heat treatment in hydrogen, which was performed using a specially designed OTJIG TM-6 system.
The composition and structure of obtained nanocomposites that contain synthesized in the interspherical cavities crystalline and x-ray amorphous phases of titanium oxide have been investigated by scanning and transmission electron microscopy, x-ray diffractometry and Raman spectroscopy. Synthesized in the interspherical cavities crystallites of titanium oxide had a size of 49.0–59.8 nm for TiO2 and 15.8–18.0 nm for TiO with a shape close to the equiaxed. By varying the synthesis conditions it is possible to change the phase composition and structure of substances formed in interspherical cavities of opal matrices.
Frequency dependences of the real and imaginary components of the dielectric permittivity of nanocomposites, as well as conductivity have been measured in the range from 1 MHz to 110 GHz. Researches have allowed to establish the relationship between microwave properties and phase composition of titanium oxides, and to obtain data necessary for the application of such non-crystalline inhomogeneous materials with spatial modulation (dispersion) of the electric and dielectric properties at the nanoscale. ■
The project is supported by RFBR (grant No.15-07-00529).
Readers feedback
