Nvestigation of surfaces of novel iron oxides with grain and grain boundary - Nguyen Viet Long

Vietnam Journal of Science and Technology 56 (1A) (2018) 226-233 INVESTIGATION OF SURFACES OF NOVEL IRON OXIDES WITH GRAIN AND GRAIN BOUNDARY Nguyen Viet Long 1, 2, * , Le Hong Phuc 3, * , Doan Thi Kim Dung 3 , Nguyen Quan Hien 3 , Le Khanh Vinh 2 , Yong Yang 4 , Masayuki Nogami 1, 2, 5 1 Ceramics and Biomaterials Research Group, Ton Duc Thang University, Ho Chi Minh City, Viet Nam 2 Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Viet Nam 3 Ho Chi Minh City Institute of Physics, Viet Nam Academy of Science and Technology, 01 Mac Dinh Chi St, Dist 01, Ho Chi Minh City, Viet Nam 4 Shanghai Institute of Ceramics, Chinese Academy of Science, 1295, Dingxi Road, Shanghai 200050, China 5 Toyota Physical and Chemical Research Institute, Nagakute, Aichi, Japan * Email: nguyenvietlong@tdt.edu.vn Received: 15 August 2017; Accepted for publication: 26 February 2018 ABSTRACT Hierarchical nano/microscale α-Fe2O3 iron oxide particle system was prepared by an improved and modified polyol method with the use of NaBH4 agent with high heat treatment at 900 °C in air. Here, α-Fe2O3 iron oxide particles with different shapes were analyzed. The morphologies of the surfaces of α-Fe2O3 iron oxide particles show the oxide structures with the different nano/microscale ranges of grain sizes. In this research, we have found that grain and grain boundary growth limits can be determined in α-Fe2O3 iron oxide structure. This leads to the possibility of producing new iron oxide structures with distribution of desirable size grain and grain boundary. With α-Fe2O3 structure obtained, the magnetic properties of the α-Fe2O3 iron oxide system are different from those of previously reported studies. Keywords: iron oxides, α-Fe2O3, chemical polyol methods, heat treatment. 1. INTRODUCTION Today, iron (Fe) oxide micro/nanostructures are applied at various research fields [1-4]. According to their particle size ranges, they are used for life, energy, environment, biology and medicine (nanomedicine) [2]. Recently, Fe oxide micro/nanostructures have been used for gas sensors for the detection of common toxic gases [5-7], especially as CO gas in industrial processes. There are so many methods and processes of making Fe oxide particles with the different morphologies, shapes, and sizes [5, 6]. Fe oxide particles with polyhedral and spherical shapes were successfully synthesized by improved and modified chemical polyol methods with Investigation of surfaces of novel iron oxides with grain and grain boundary 227 NaBH4 and heat treatment at high temperature [7-15]. A polyol method with NaBH4 and high heat treatment is a very powerful way for controlled synthesis of α-Fe2O3 oxide [8-15]. In our research, hierarchical nano/microscale α-Fe2O3 iron oxide particle system was prepared by modified polyol method with NaBH4 and heat treatment at 900 °C in air. The grain and grain boundary textures of hierarchical nano/microscale α-Fe2O3 oxide particles have been carefully analyzed for the limitation of grain growth in the specific micro/nano oxide structures. 2. EXPERIMENTAL Synthesis of hierarchical α-Fe2O3 particles by polyol method with PVP, FeCl3 (and/or FeCl2) NaBH4, NaOH, and heat treatment at 900 °C was presented in previous works [8,9]. A polyol mediated synthesis was used for obtaining hierarchical α-Fe2O3 particles (Figure 1). The various α-Fe2O3 particle powder samples were prepared for X-ray diffraction (XRD) and scanning electron microscopy (SEM). The crystal structure was found from the XRD pattern of α-Fe2O3 particles by Rigaku-D/max 2550V (40kV/200mA, CuKα1 at 1.54056Å). The features of size, shape, and morphology of hierarchical α-Fe2O3 particles were investigated by field emission (FE)SEM [8,9]. The prepared surface properties of hierarchical α-Fe2O3 particles with grain and grain boundary textures were done in the step-wise procedure and heat treatment according to controlled synthesis of homogeneous large α-Fe2O3 iron oxide particles. 3. RESULTS AND DISCUSSION 3.1. Characterization Figure 1. (a) XRD pattern of a typical sample of hierarchical α-Fe2O3 particles with grain and grain boundary from SEM image of α-Fe2O3 particles of Figure 4. (b) 64 unit cells of α-Fe2O3 crystal and one unit cell of α-Fe2O3 crystal (VESTA program). Figure 1 shows a typical XRD pattern of hierarchical α-Fe2O3 particles with grain and grain boundary. Typically, Figure 1 also shows the result of XRD of the as-prepared α-Fe2O3 particles (C) Nguyen Viet Long, et al. 228 with particle size in a range of ~10 μm according to a standard pattern, ICDD/JCPDS-PDF#33- 0664, and unit cells of α-Fe2O3, which shows the typical peaks at (012), (104), (110), (113), (024), (116), (122) or (018), (214), (300), (208), (1010), and (220), and other possible (hkl) planes. The XRD data indicate that stable α-Fe2O3 phase was formed with high crystallization, and with the strongest line (104) in its crystal phase [8, 9, 15]. This is a high crystallization of pure α-Fe2O3. 3.2. Morphologies and surfaces of hierarchical α-Fe2O3 particles Figure 2. SEM images of hierarchical α-Fe2O3 particles annealed at 900 o C for 1h by polyol method with NaBH4 and heat treatment. Scale bars: (a)-(d) 1000 nm; (e)-(f) 100 nm. Figures 2 and 3 show that the exciting polyhedral morphologies and shapes of hierarchical α-Fe2O3 particles were clearly identified in SEM images. Here, polyhedral α-Fe2O3 oxide particle contained with the various grain and grain boundary textures. Each grain is α-Fe2O3 oxide with a certain size. A α-Fe2O3 oxide particle contained a certain number of grains and grain boundaries with vartious sizes and shapes. The small α-Fe2O3 grains have the various sizes less than 100 nm at nanoscale, more than 100 nm, 1000 nm, and so on. It is certain that we can Investigation of surfaces of novel iron oxides with grain and grain boundary 229 calculate a number of grains in one face, two faces, four faces, and six faces as an estimation to one particle in Figure 2 (a), (b), and (d). The shapes of the grains are possibly the sharp, unsharp polyhedral, and spherical forms with the right boundaries or curve boundaries. The micro/nanoscale configuration of the one face of such one particle was enlarged in Figure 2 (c)- (f). A large number of about 57 grains were identified just on one face of the particle, which was the S1 face (Table 1). There was the abnormal grain growth on all the surface being investigated, which led that there were both the small grains and large grains formed in high heat treatment. Figure 3. The specific surface of hierarchical α-Fe2O3 particle annealed at 900 o C for 1 h by polyol process with NaBH4 and heat treatment at high temperature. There was a small pore about 100 nm formed on the surface of hierarchical α-Fe2O3 particle. Scale bars: (a)-(c) 1000 nm. Figure 4 typically shows the final formation of various large and small grains. We can calculate a number of the grains on the half surface of one α-Fe2O3 oxide arrow about 5500 nm. Table 2 listed a large number of about 189 α-Fe2O3 grains on the half surface of one particle. Thus, there was the 378 α-Fe2O3 grains on the total surface of this α-Fe2O3 particle. The smallest size of α-Fe2O3 grain was about 38 nm (S1). The biggest size of α-Fe2O3 grain was about 232 nm (Smax). The ratio of Smax/S1 equal to about 6 presented the property of normal and abnormal growth but this ratio is about 13, which is high to lead the abnormal grain growth listed in Table 1. Here, the abnormal growth became favor in all the grain growth mechanisms. The range of grain size of the small α-Fe2O3 grains was less than 100 nm, but that of grain size of intermediate α-Fe2O3 grains more than 100 nm, and less than 200 nm while that of grain size of large α-Fe2O3 grains more than 200 nm. The main roles of the homogeneous α-Fe2O3 grains aim to harden and Nguyen Viet Long, et al. 230 stabilize the structure of prepared materials. This led that hierarchical α-Fe2O3 particles show the weak ferrimagnetism with saturation magnetization under external field (MS > 1 emu/g) but much larger than MS of α-Fe2O3 particles without grain and grain boundary (MS << 1 emu/g) [8, 15]. Figure 4. The surface of one hierarchical α-Fe2O3 particle annealed at 900 o C for 1h by modified polyol process with NaBH4 and heat treatment. There was the clear formation of the certain pores about 100 nm on the whole surface of α-Fe2O3 particle. Scale bar: 1000 nm. Table 1. A large number of about 57 grains on the one face of one particle (Figure 2d). Grain size (nm) Grain size (nm) Grain size (nm) Grain size (nm) Grain size (nm) Grain size (nm) 875.000 750.000 875.000 875.000 300.000 625.000 610.000 750.000 500.000 690.000 200.000 450.000 565.000 800.000 445.000 500.000 940.000 500.000 500.000 1065.000 375.000 375.000 690.000 500.000 440.000 565.000 315.000 500.000 750.000 750.000 440.000 190.000 625.000 1125.000 300.000 200.000 375.000 440.000 625.000 375.000 875.000 98.000 815.000 1000.000 1315.000 550.000 875.000 1000.000 1000.000 400.000 440.000 565.000 315.000 1250.000 350.000 625.000 850.000 Investigation of surfaces of novel iron oxides with grain and grain boundary 231 Table 2. A large number of about 189 grains on the half surface of one rod particle (Figure 4). Grain size (nm) Grain size (nm) Grain size (nm) Grain size (nm) Grain size (nm) Grain size (nm) Grain size (nm) Grain size (nm) 214.000 116.000 170.000 232.000 54.000 155.000 124.000 78.000 59.000 194.000 124.000 139.000 139.000 54.000 97.000 108.000 60.000 136.000 136.000 105.000 186.000 155.000 108.000 209.000 97.000 93.000 93.000 93.000 108.000 139.000 186.000 108.000 77.000 38.000 93.000 116.000 155.000 139.000 116.000 201.000 155.000 155.000 78.000 136.000 46.000 78.000 116.000 124.000 116.000 155.000 214.000 76.000 139.000 155.000 139.000 46.000 62.000 136.000 194.000 62.000 62.000 132.000 116.000 46.000 110.000 97.000 186.000 116.000 155.000 124.000 108.000 124.000 155.000 139.000 124.000 155.000 186.000 155.000 108.000 124.000 108.000 194.000 136.000 136.000 175.000 116.000 155.000 136.000 175.000 108.000 97.000 76.000 139.000 124.000 78.000 139.000 124.000 139.000 124.000 170.000 76.000 139.000 93.000 124.000 163.000 93.000 136.000 194.000 108.000 93.000 139.000 155.000 105.000 186.000 97.000 155.000 116.000 108.000 124.000 136.000 136.000 194.000 186.000 93.000 120.000 108.000 147.000 38.000 194.000 194.000 93.000 186.000 186.000 170.000 93.000 116.000 108.000 108.000 155.000 194.000 155.000 139.000 85.000 186.000 93.000 155.000 186.000 194.000 178.000 97.000 136.000 90.000 170.000 59.000 175.000 155.000 186.000 124.000 108.000 93.000 97.000 93.000 194.000 97.000 97.000 78.000 93.000 62.000 194.000 214.000 108.000 97.000 71.000 155.000 175.000 124.000 116.000 136.000 108.000 116.000 59.000 108.000 139.000 139.000 232.000 193.000 93.000 186.000 139.000 Figure 5. The corresponding grain-size distribution histograms of two hierarchical α-Fe2O3 particles annealed at 900 o C for 1 h. Figure 5 shows the distribution histograms of grain size of two hierarchical α-Fe2O3 particles that are illustrated in Figures 3 and 4 corresponding to the grain sizes that are listed in Tables 1 and 2. This shows that average grain sizes (d) of the two prepared particles with their different standard deviations (std), and grain numbers (n) are clearly different. In comparison to the distribution of grain size in Table 1, that of grain size in Table 2 is significantly narrower in scope. Recently, researchers have shown the important role of grain growth, grain size, and grain Nguyen Viet Long, et al. 232 boundary as the core in the creation of new structures for use in theory and experiment [16-18], particularly in the field of magnetic materials and simulation. 4. CONCLUSION Iron oxide particles (α-Fe2O3) with grain and grain boundary textures were prepared by improved and modified polyol methods with NaBH4 and heat treatment at high temperature about 900-1000 °C according to the experiences of the author. The oxide grains were analyzed for the limitation of grain growth in the micro/nanoscale ranges. The limit of small grain size was obtained in a nanoscale range less than 100 nm, and large grain size in a microscale range of around 1000 nm. After our high heat treatment at 900 °C, the limit of grain size of large grain size was obtained in a microscale range more than 1000 nm. Therefore, this can lead that we can control grain and grain boundary textures of various magnetic micro/nanoscale structures. Acknowledgement. Our research was financially supported by the National Foundation for Science and Technology Development of Vietnam (NAFOSTED) through Grant number 103.02-2016.92. REFERENCES 1. Hemery G., Keyes Jr. A. 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