Ultrafast Microwave Hydrothermal Synthesis of BiFeO 3 Nanoplates Shun Li, ‡ Riad Nechache, § Ivan Alejandro Velasco Davalos, ‡ Gregory Goupil, ‡ Liliya Nikolova, ‡ Mischa Nicklaus, ‡ Jonathan Laverdiere, ‡ Andreas Ruediger, ‡ and Federico Rosei ‡,¶,† ‡ Centre  Energie, Mat eriaux et T el  ecommunications, Institut national de la recherche scientifique, INRS, 1650, boulevard Lionel-Boulet, Varennes, Qu ebec J3X 1S2, Canada § NAST Centre & Department of Chemical Science and Technology, University of Rome Tor Vergata Via della Ricerca Scientifica 1, Rome 00133, Italy ¶ Center for Self-Assembled Chemical Structures, McGill University, H3A 2K6 Montr eal, Quebec, Canada We report the synthesis of {100} c facets exposed single-crystalline BiFeO 3 (BFO) nanoplates, with thickness ranging from 20 to 160 nm and lateral size of submicrometers, via a simple and very rapid (1–2 min) microwave-assisted hydrothermal approach. We show that the microwave treatment gives comparable improve- ment in crystallinity of BFO nanocrystals with respect to tradi- tional hydrothermal processes while requiring significantly less time and energy. In addition, we show that microwave radiation power, reaction time, and alkali concentration play important roles in the crystallinity and morphology of the products. We dis- cuss a possible formation mechanism of the nanoplates based on our experimental results. Additionally, the BFO nanoplates exhi- bit weak ferromagnetic properties at room temperature, which we attribute to the size-confinement effect on magnetic ordering. The present microwave hydrothermal method has great potential in large-scale fabrication of BFO nanomaterials as well as other composite functional materials due to significantly reduced time and energy. I. Introduction M ULTIFERROIC materials that exhibit a coupling of the electrical and magnetic order parameters in the same phase, have attracted increasing interest because of their potential applications in data storage, spintronics, sensors, quantum electromagnets, photonics, and electronics. 1 Typical multiferroics belong to the group of the perovskite transi- tion-metal oxides (e.g., BiFeO 3 ), and include rare-earth man- ganites and ferrites (e.g., TbMnO 3 , HoMn 2 O 5 ). 2 The recent emergence of new types of multiferroics such as Bi 2 FeCrO 6 and Bi 2 CoMnO 6 , with large polarization and magnetization at room temperature (RT), create opportunities for practical applications of multiferroics. 3,4 Among them, BiFeO 3 (BFO) is one of the most widely studied multiferroic materials, pri- marily because both its electrical and magnetic ordering occur above RT. In particular, multiferroic BFO nanostruc- tures exhibit interesting magnetic and optical properties because of nanoscale size effects. 5–8 So far, BFO nanomateri- als with various sizes and shapes such as nanotubes, 9 nano- wires, 10,11 nano/microcubes, 12,13 nanospindles, 14 and nanorods 15 have been reported and exhibit different proper- ties compared to the bulk form. Therefore, the design of multiferroic BFO nanostructures with novel and well-defined morphologies is important for both fundamental research and relevant for designing new multifunctional materials combining magnetic, ferroelectric, and optoelectronic properties. Two- dimensional (2D) nanomaterials such as nanosheets and nano- plates have been studied extensively because their anisotropic shape is advantageous with respect to irregular-shaped nano- crystals for constructing nanodevices. 16,17 The design and mor- phological control of crystal facets is a commonly employed strategy to optimize the physical and chemical properties of var- ious crystalline semiconductors. Recent developments in the synthesis of 2D crystalline nanosheets/plates show promising properties for developing a new generation of optoelectronic devices and high-performance catalysts. 18–20 Recently, Lu et al. reported the synthesis of 2D BFO plates using the surfactant ce- tyltrimethylammonium bromide. 21 However, synthesizing sin- gle-crystalline planar BFO nanosheets or nanoplates with controllable crystallographic facets by template- or surfactant- free solution routes is still a major challenge. Numerous reports have described the synthesis of BFO materials by various routes that involve solid-state reactions, 22 rapid molten salt sintering, 23 mechanochemical synthesis, 24 sol–gel method, 25 or wet chemistry. 26 The majority of these processes require long reaction times and high temperatures, therefore involving high-energy consumption and cost. The most commonly used conventional solid-state synthesis of BFO crystals requires a prolonged treatment at considerably high calcination temperatures (800°C or higher), 22,27 causing a loss of bismuth and severe impurity contaminations. The hydrothermal technique is becoming one of the most impor- tant tools for advanced materials processing, as it is a simple and low-cost route with and low reaction temperature, and also because it is very useful for producing various new types of nanohybrid and nanocomposite materials. 28,29 Recently, hydrothermal processing methods have been shown to yield highly crystalline BFO products at low temperatures (~200°C). 13,14,30 Although this method can be efficient in the synthesis of materials using relatively low temperatures (100– 250°C), its main disadvantage is related to the long processing times due to slow reaction kinetics at relevant temperatures. Microwave-assisted hydrothermal (M-H) methods are becoming widespread for the synthesis of nanomaterials as they offer a simple, low-cost approach to obtain highly crys- talline nanocrystals within a very short reaction time with high yields and good reproducibility. 31 Microwave heating [Fig. 1(b)] provides energy to the reactants by means of molecular interaction with high-frequency electromagnetic radiation, which is different from that of conventional ther- mal treatment by convection current [Fig. 1(a)]. In M-H processing, the precipitate can be rapidly dissolved in aque- ous solution to provide a saturated solution, resulting in enhancement of the reaction kinetics by one to two orders of magnitude by high-frequency electromagnetic radiation (2.45 GHz). In addition to the extensively studied BaTiO 3 , 32 D. Damjanovic—contributing editor Manuscript No. 32554. Received January 8, 2013; approved May 28, 2013. † Author to whom correspondence should be addressed. e-mail: rosei@emt.inrs.ca 3155 J. Am. Ceram. Soc., 96 [10] 3155–3162 (2013) DOI: 10.1111/jace.12473 © 2013 The American Ceramic Society J ournal