論文の公開元へPhase and Size Selective Crystal Growth of Nanoparticles under Supercritical Water
概要
Physical properties of materials have a close relation with their atomic arrangement. Size, surface, interface and lattice defects of materials are relating to properties of materials such as electric and/or thermal conductivity. Fine nanoparticles below 10 nm size show interesting physical properties such as a narrower band gap or a lower melting point than bulk materials.
Supercritical hydrothermal synthesis has attracted a lot of attention as a method to synthesize fine metal oxide nanoparticles with uniform size and shape [1]. Nanoparticles are prepared in the water above its critical temperature and pressure of 673 K and 22.1 MPa. Supercritical water is placed between a liquid phase and a gas phase on the phase diagram. Density and dielectric constant of supercritical water are continuously varied with temperature and pressure. Highly functional nanoparticles with large specific surface area can be synthesized in supercritical water [2]. Complex nanoparticles can be synthesized at lower temperature than a conventional method [3]. Mass production of nanoparticles is possible due to its short reaction time. It typically takes a few seconds to one hour.
Supercritical water has superior properties as a reaction field for preparing fine nanoparticles with controllable functionality [4]. Lower solubility can suppress crystal growth through Ostwald ripening in conventional hydrothermal synthesis in a liquid phase. A higher density than that of a gas phase can prevent the collision and aggregation between nucleuses of nanoparticles which are frequently observed in gas phase reaction like a chemical vapor decomposition (CVD) method. Organic solvent can be dispersed in supercritical water due to its low dielectric constants. This makes it possible to synthesize nanoparticles with a highly reactive surface modified by organic molecules. Controlling structure of fine functional nanoparticles is possible by changing properties of a reaction field continuously.
Control of crystalline phase is one of the most important topics of supercritical hydrothermal synthesis of nanoparticle. Functional metal oxide nanoparticles have several crystalline phases. It strongly affects their physical or chemical properties [5], [6]. Phase stability depends on the size of nanoparticles and/or a precursor used in the synthesis [7], [8]. Various crystalline phases were observed for nanoparticles synthesized in supercritical water [3], [9]–[11]. Controllable parameters of synthesis are temperature, pressure, pH of solution, type of solute molecule, ionic composition of solution, and reaction time. It is difficult to carry out complete investigation for all combinations of controllable parameters.
Physical properties of nanoparticles can be changed by local defects in atomic arrangement. Disorder at interface of core-shell structure of nanoparticles attracts attention in terms of controlling optical resonance of quantum dots [12]. Strain induced at interface by lattice mismatch causes upper offset of valence band and red shift of photoluminescence spectra. The improvement of thermoelectric performance was reported for composite nanoparticles [13]. The high density of twin faulting led to higher phonon scattering and lower thermal conductivity. A figure of merit was higher by 50 % than bulk material.
Methods of structure determination in atomic-to-nanoscale are powerful tools, which can help with the development of a new area of materials science which focuses on local structure of nanoparticles. Simultaneous determination of size, shape and local defect of nanoparticles provides important clues to identify structural parameters relating to important properties of nanoparticles. Elucidating local structure in fine nanoparticles enables us to create a guideline for an improvement of nanomaterials properties which depends on local lattice defects.
Monitoring of nanoparticles structure during synthesis can provide clues to phase stability, mechanism of growth or correlation between structure of nanoparticles and physical properties of supercritical water. The critical size between polymorphs of nanoparticles is reported from observation of size and crystalline phase of nanoparticles [8]. Surface stress of nanoparticles was investigated from anomalous lattice expansion or contraction of nanoparticles [14]. The measurement at multi-temperature and pressure conditions can provide an evidence of correlation between microscopic structure and macroscopic properties of a reaction field.
This “in-situ” measurement of nanostructure during synthesis requires intense beam with an angstrom wavelength. An X-ray, a neutron and an electron beam are commonly used for evaluating structure of materials in atomic and nanometer scale [15]. Both X-ray and neutron beams can penetrate the reaction vessel, while electron beam is scattered by electrons on the wall of vessel. A neutron beam is a tool for evaluation of structure of materials composed with light atoms like lithium and oxygen [16]. A neutron experiment requires a sample size of the order of centimeter due to its low intensity per area. Electron diffraction and microscopy can be carried out in milliseconds due to strong interaction between electrons [17]. High vacuum should be kept around samples for preventing the electron from being scattered by air.
Synchrotron radiation (SR) is an intense X-ray source. A highly intense and parallelized X-ray are emitted by accelerated relativistic electrons in a magnetic field with GeV order energy. The brilliance at the third generation SR facility is more than 1010 times higher than that of an X-ray tube. The high brilliance decreases the time for collecting powder X-ray diffraction (PXRD) data which can be used for structure analysis. A large space can be assured around samples in the setup of SR-PXRD measurement, since a highly parallelized X-ray does not require optics elements such as soller slit. The various measurement systems can be installed for conducting measurement at the high temperature, the high pressure or the vacuum conditions.
SR is a powerful tool for evaluation of structure of nanoparticles. Diffraction data can be collected from samples with weight of microgram. It is possible to observe the weak scattering from nanoparticles, discriminating it with scattering from a pressure vessel. The composition of crystalline phase of nanoparticles can be identified from a set of positions of diffraction lines. The average diameter of nanoparticles can be estimated from width of diffraction profiles.
In-situ SR-PXRD measurement has been carried out for observation of nucleation and crystal growth of nanoparticles during synthesis [18], [19]. A reaction vessel or equipment for synthesis was installed at a sample position. PXRD data were collected from nanoparticles during synthesis with a second time scale. Reaction processes of various synthesis were investigated. An anatase phase of TiO2 nanoparticle was monitored in reaction of a sol-gel method [20]. The formation of Ru nanoparticle with fcc and hcp structure was observed in solvothermal reaction with supercritical ethanol [21]. Phase transition during the mechanochemical reaction was reported for ball- milling process [22].
High quality PXRD data are required for determination of structure of nanoparticles from in-situ experiment. Diffraction data should be collected simultaneously from a wide range of diffraction angles. In the case of 10 nm ZrO2, data with resolution d > 0.6 Å were required for determining lattice constants with 0.001 Å accuracy. The broadening of diffraction profiles due to energy dispersion should be minimized for estimating a size of nanoparticles. Crystal growth was investigated at the beamline with the 10-4 scale of the energy dispersion [23].
Evaluation of size and lattice constants can be complicated due to local lattice defects. For example, a shift and broadening of diffraction lines can be caused by planar defects such as twin and deformation faulting in the stacking structure [24]. Lattice defects of nanoparticles cause a greater change to PXRD data than those of bulk material, since larger fraction of atoms is related to defect structures. We found the height of 200 reflection of Ag nanosphere was almost half of the calculated data of Rietveld refinement. Several reflections of ZrO2 nanoparticle prepared by supercritical hydrothermal synthesis were not fitted by Rietveld refinement.
Issues about reproducibility of parameters obtained from analysis of diffraction data of in-situ SR-PXRD experiment were reported by Iversen et al. [25]. In work presented by this group, diffraction data were collected 10 times for in-situ experiments of hydrothermal synthesis in identical conditions. In Rietveld refinement, the absolute value of an intensity scaling factor of diffraction data were not reproduced. It was partly due to occasional movement of nanoparticles in initial heating process, since it made difficult to control amount of samples exposed by an X-ray. The diameter of nanoparticles estimated from diffraction line width showed uncertainty of about 3 nm. Iversen et al. found that diffraction lines with relatively narrow width corresponding to 15 nm diameter required a careful treatment than those corresponding to the diameter of less than 10 nm. The time dependence of an intensity scaling factor and that of the diameter were qualitatively reproduced for 10 times of experiments.
The methods have been developed for analyzing powder diffraction data of nanoparticles. Leonardi et al. reported effect of dislocation to PXRD data for the rod-shaped nanoparticle of iridium and palladium [26]. Bertolotti et al. investigated atomic arrangement of colloidal nanoplatelet [27]. Parakh et al. reported high pressure study about gold nanoparticles structure [28]. Atomic arrangements were modeled by using a combination of diffraction data calculation and a molecular dynamics simulation. Some of these studies were performed on the large-scale parallel computing platform.
In the present study, we carried out in-situ SR-PXRD experiment to investigate the crystal growth of nanoparticles in supercritical water. The measurement system was developed for in-situ SR-PXRD experiment at the beamline BL02B2 of the third generation SR facility SPring-8. Powder diffraction data were collected at thirteen combinations from the synthesis of ZrO2 nanoparticles. The analysis method was developed for analyzing powder diffraction data of nanoparticles in the reaction vessel. Debye Scattering Equation (DSE) was used for the calculation of powder profiles of nanoparticles. DSE was used for the analysis of a series of the powder diffraction data of nanoparticles by Ozawa [29]. The calculation was performed on the large-scale parallel computing platform with GPU.
The crystal growth of ZrO2 nanoparticles in supercritical water was investigated by in-situ SR-PXRD experiment. The crystalline phases and the size of nanoparticles were determined from the analysis of the powder diffraction data. The relation between the crystalline phases, size of nanoparticles and synthesis conditions were investigated by the analysis.
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