XAFS study for formation mechanism of nanoparticles in supercritical water

Supercritical hydrothermal synthesis is expected as a methodology for fabricating new nanomaterials. High super-saturation degree in supercritical water enables rapid formation of nanostructures, and rapid crystallization can produce materials of characteristic composition and structure. Formation of composite oxide in supercritical water has been studied with ex situ and in situ methods. In the previous study, we observed the existence of the highly defective structure during the crystallization in the case of perovskite-type composite oxide. In this study, perovskite-type composite oxides nanoparticles such as BaZrO3 and SrZrO3 were synthesized in supercritical water using a continuous flow reactor, and the nanoparticle formation at highly supersaturated environments was studied. Local structure of nanoparticles was focused, and characterized mainly by X-ray absorption fine structure (XAFS). In addition to the measurements of nanoparticles, a theoretical calculation was also conducted to study surface structure of nanoparticles. Formation of highly defective structure which was observed in the previous studies was intensively investigated in terms of local structure. Increase in coordination number and decrease in the Debye-Waller factor with an increase in residence time was observed by the XAFS measurements, which suggest the formation of highly defective structure and following uptake reaction filling the deficiency in supercritical water. INTRODUCTION Supercritical hydrothermal synthesis method [1] is a methodology for fabricating various kinds of nanoparticles with small size and narrow size distribution. One of the characteristic points of the technique is a rapid formation of nanostructures owing to the high super-saturation degree in supercritical water. Rapid crystallization can produce materials of characteristic composition and structure under non-equilibrium state [2,3], and supercritical hydrothermal method is promising for finding new nanomaterials. Also, fine-control of nanoparticles enables us to study nanostructures which are different from the bulk structure. Not only single-component metal oxides but also composite oxides composed of multiple metal ions can be synthesized by the supercritical method. Non-equilibrium state and non-stoichiometric composition is an important aspect in the synthesis of composite oxides. We have studied perovskite-type composite oxides formation in supercritical water with ex situ [4,5] and in situ [6] methods and showed the existence of the highly defective structure of zirconate during the crystallization. Single particle analysis with TEM-EDX revealed that A-site ion, for example, Ba, is taken up into defective perovskite structure in supercritical water. Also, lattice expansion of the order of milliseconds caused by the ion uptake was detected by in situ synchrotron X-ray diffraction. To elucidate such a characteristic deficient structure, X-ray absorption fine structure (XAFS) analysis is suitable. By use of XAFS technique, the local structure such as coordination number and coordination distance can be analyzed. In addition, the atomic selective analysis is possible with XAFS, which is impossible by other measurements such as diffraction. Moreover, non-crystalline or disordered structures frequently observed in nanoparticles are also measurable by XAFS. In this study, ex situ study using XAFS was conducted for composite oxides formation of the order of seconds via time-resolved experiments with a continuous flow reactor. Perovskite-type composite oxide nanoparticles such as BaZrO3 and SrZrO3 were synthesized in supercritical water using a continuous flow reactor, and the nanoparticle structures formed at highly supersaturated environments as supercritical conditions were investigated. Local structure of nanoparticles formed in supercritical water was focused, and the XAFS measurements were mainly conducted for detailed analysis of the local structure. In addition to the XAFS measurements of nanoparticles, theoretical calculation for nanostructure formation [7] was also conducted to study disordered structure of nanoparticles. MATERIALS AND METHODS Barium nitrate [Ba(NO3)2 (purchased from Kanto Chemical Co., Inc.)] and oxyzirconium nitrate [ZrO(NO3)2·2H2O (purchased from Kanto Chemical Co., Inc.)] were used as starting materials. Potassium hydroxide [KOH (purchased from Wako Pure Chemical Industries Ltd.)] was used as a base solution for tuning the pH. Nitric acid [HNO3 (purchased from Wako Pure Chemical Industries Ltd.)] was used to neutralize the recovered solution. Distilled water (produced from RFD240HA; Advantec MFS, Inc.) was used in the preparation of the feed solution. An excess of Ba, relative to the amount of Zr (4 times), was used in the starting solutions to obtain mono-phase products following the previous study of BaZrO3 formation [4]; otherwise, the ZrO2 phase is also contained in products. For the XAFS measurements, micrometer order of ZrO2 and BaZrO3 (purchased from Sigma Aldorich) were used as references of the bulk crystal. All experiments were performed using the continuous flow reactor. Starting solutions of barium and zirconium were simultaneously mixed with the base solution and heated distilled water, using a cross-shaped part (SUS316; inner diameter: 1.3 mm). The starting solution and base solution were introduced into the reactor at a flow rate of 10 g min, and heated distilled water was introduced at 80 g min using a non-pulsation pump (NP-KX-500; Nihon Seimitsu Co., Ltd.). The starting solution and the base solution were cooled just before the mixing point, to prevent warming of the starting solutions via conductive heat transfer before mixing. The flow rates set a Reynolds number (Re) of 4 × 10 at the reactor, just after mixing. Kawasaki et al. investigated the relationship between particle size and Re, and suggested that sufficient mixing was achieved at values of Re ≥ 4 × 10; at these values, the mixing conditions did not affect the particle size [8]. The temperature of the water was set at 400 °C at the point just before cooling. The pressure was set at 30 MPa using a backpressure regulator (BP66-112865; Go Inc.). The residence time was set from 0.1-10.7 s by changing the reactor volume from the mixing point to the point just before the cooling. After cooling, an HNO3 solution was introduced using a flow rate of 5 g min , to reconcile the pH and prevent the formation of carbonate in the recovered dispersion. The solid products were recovered via pressurized filtering using a nitrocellulose filter (VSWP14250; Merck Millipore Corporation, pore size: 0.025 μm), and were dried in a vacuum oven at room temperature (ADP-31; Yamato Corporation). The recovered particles were analyzed as described as follows. XAFS was conducted at BL01B1 of SPring-8, JASRI. For the Zr K edge spectra, Si (111) double-crystal monochromator was used, and for Ba K edge spectra, Si (311) doublecrystal monochromator was used. Transmission method was applied, and pellet sample with BN binder was used for the measurements. X-rays intensity was measured by ionization chambers before (I0) and after (I) the sample (70%-N2 and 30%-Ar for I0, 90%-Ar, and 10%-Kr for I). EXAFS data were analyzed by FEFF code. For the initial structures for the fitting, crystallographic information obtained by diffraction was used. As for the analysis, EXAFS frequency in k space, χ(k) written as follows was used. χ(k) = S0 ∑ Nj|Fj(k)| exp(−2k σj ) krj sin (2krj + ∅j(k)) j Here, coordination number, Nj, coordination distance, rj, and Deby-Waller factor, σj , are the fitting parameters, and backscatter factor, |Fj(k)| and phase shift, ∅j(k) were theoretically calculated by FEFF code. As can be seen in the equation, amplitude of χ(k) increase with an increase in coordination number or decrease in Debye-Waller factor, and frequency of χ(k) increase with an increase in coordination distance. As an index of the accuracy of the fitting, R-factor, as follows, was checked after fittings. Smaller R-factors indicate the high accuracy of the fittings, and R-factor < 0.1 was confirmed for all the samples in this study. R − factor = √ ∑ (χi(k) − χi(k, [a])) 2 i