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(iv). Perovskite Nanostructures:
1. Mild, low-temperature, hydrothermal synthesis of BaTiO3 and SrTiO3 nanotubes using TiO2 nanotubes as templates. Outer diameter range of the BaTiO3 and SrTiO3 nanotubes produced was from 8-15 nm, inner diameter range was from 4-7 nm, and lengths varied from 50 to >500 nm. Ref.: Chem. Commun., (3), 408 (2003).
2. Large-scale, environmentally friendly synthesis of BaTiO3 nanorods and SrTiO3 nanocubes. Single-crystalline perovskite nanostructures of reproducible shape were prepared using a simple, readily scaleable solid-state reaction in the presence of molten NaCl and a nonionic surfactant. Pristine BaTiO3 nanowires had diameters ranging from 50 to 80 nm with an aspect ratio larger than 25. Single-crystalline SrTiO3 nanocubes with a mean edge of 80 nm were also produced. Ref.: J. Am. Chem. Soc., v.125, 15718 (2003).
3. The ability to conveniently synthesize single-crystalline nanomaterials with controllable size and shape is essential for rationally exploiting their nanoscale physical and optical properties. A series of single-crystalline Ca1-xSrxTiO3 (0 ≤ x ≤ 1) perovskite nanoparticle samples of reproducible, tunable composition were prepared using a simple, readily scaleable solid-state reaction between metal oxalates and anatase TiO2 in the presence of NaCl and a nonionic surfactant. Shapes of the generated Ca1-xSrxTiO3 nanoparticles altered from cubes to quasi-spheres with decreasing ‘x’ values. Nanoparticles had sizes ranging between 70 and 110 nm, irrespective of Sr or Ca content. Refs.: Adv. Mater., v.17, 2194 (2005) and Appl. Phys. Lett., v.89, 223130/1 (2006).
4. Synthesis, shape control, characterization, and spectroscopy of BaZrO3 nanoparticles (including cubes and spheres) using a simple, readily scaleable solid-state reaction in the presence of salts such as either (i) NaCl, (ii) NaOH / KOH, or (iii) NaCl / KCl. Rational control over shape is of great importance due to their strongly structure-dependent properties. For instance, zirconate cubes are useful in piezoelectric applications whereas zirconate spheres may find use in phosphors with high-luminescence efficiency. The effects of different parameters, such as salt, surfactant, reaction temperature, reaction time, precursor type, amount of salt, heating rates, and precursor ratios, on the resultant product purity, size, shape, and morphology were specifically analyzed. Among these various parameters, we found the selection of salt to be the most important one, because solubility and reactivity effects associated with the salt can alter the synthesis process as well as the resultant particle size and shape. In general, the production of relatively high-quality barium zirconate samples was favored by high annealing temperatures, slow cooling rates, and overall long reaction times. Shorter annealing times coupled with higher cooling rates resulted in the production of smaller-sized cubic particles. By contrast, longer annealing times and/or slower cooling rates induced particle conversion from cubes to spheres and usually brought forth a mixture of cubic and spherical morphological motifs. For example, either increasing the annealing time or slowing the cooling rates led to the formation of larger-sized spherical particles. Refs.: J. Mater. Chem., v.17, 1707 (2007) and Chem. Mater., v.19, 5238 (2007).
5. A large-scale synthesis of single-crystalline LiNbO3 nanowires with diameters of 300-400 nm, containing only minor amounts of impurities, has been reported using a modified molten salt procedure. The isolated product is composed of rhombohedral-phase LiNbO3 nanowires with the c-axis oriented along its length. Raman investigations further confirm the purity and ferroelectric order of the nanowires and are consistent with electron microscopy data. Ref.: CrystEngComm., v.12, 2675 (2010).
6. SrRuO3 (SRO) has been studied extensively at the bulk and thin film scales for a variety of applications, such as multilayer devices, field-effect devices, multiferroics, and even ferroelectric random access memory (FeRAM). Conversely, the exploration of SRO nanoscale and submicrometer scale structures in terms of both synthesis and applications has been significantly limited. Herein, we are the first to report on the synthesis of SRO submicrometer-sized particles via a molten-salt method. We have accomplished this by systematically probing experimental parameters such as precursors, type of salt, annealing time, annealing temperature, surfactant, cooling rate, and reaction atmosphere in an effort to predictably control the resulting size, shape, and morphology of the SRO product particles. In particular, by quenching the reaction at a cooling rate of 100 °C/min, we can produce rounded SRO particles averaging 149 ± 100 nm in size. Moreover, with the addition of a mixture of mineral oil in Triton X-100, the final SRO particles are highly faceted, single-crystalline octahedra, averaging 126 ± 45 nm in size. Apart from the choice of molten salt which primarily controls chemical composition of SRO, we have determined the most important experimental parameter for shape and aggregation control is the surfactant, because of a combination of its hydrophobic and hydrophilic characteristics. We have found that the decomposition of the surfactant promotes the formation of SrCO3, which is consistent with the generation of SRO through the reaction of SrCO3 with RuO2. Our successful syntheses have allowed us to explore physical properties, that is, magnetic and electronic, of these submicrometer-sized SRO particles, which are in good agreement with bulk SRO. Furthermore, we have also explored the potential of our as-synthesized particles as effective methanol oxidation reaction (MOR) catalysts for direct methanol fuel cells (DMFCs). Not only are our as-synthesized SRO particles MOR active but also our well-defined faceted octahedra exhibit a 4-fold increase in mass-activity and a 4-fold increase in surface area by comparison with the rounded particles, thereby emphasizing a clear advantage of faceted submicrometer SRO for electrochemical applications. Ref.: Chem. Mater., v.23, 3277 (2011).
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