Review pubs.acs.org/cm
Simple Reactor for Ultrasonic Spray Synthesis of Nanostructured Materials Jie Fu, Nick N. Daanen, Evan E. Rugen, Dennis P. Chen, and Sara E. Skrabalak* Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States S Supporting Information *
ABSTRACT: Developing a facile and general synthetic strategy toward particles with size, shape, and compositional control is of importance to nanotechnology applications. Ultrasonic spray synthesis (USS) is a continuous route to micro- and nanoscale particles with structural control, which are often difficult to obtain for inorganic solids with complex compositions or of metastable phases. This protocol describes the design and assembling of components for a simple reactor for USS. Components include a nebulizer, a nebulization chamber, a furnace, a furnace tube and the corresponding adapters, and a product collection apparatus. Details of our house-made components are provided as well as insights on material selection based on different synthetic requirements. We exemplify USS with a step-by-step procedure to single-crystalline NaSbO3 nanoplates, and this procedure can be easily modified to accommodate other chemistries. The integration of USS and molten salt synthesis for single-crystalline NaSbO3 nanoplates demonstrates the versatility of USS as a route to materials of different compositions, with shape and size control. With the incorporation of new chemical methods into USS, e.g., molten salt chemistry, topotactic transformations, and combustion chemistry, USS will remain a versatile, continuous flow platform for material syntheses.
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INTRODUCTION The properties of inorganic materials rely on crystallite size and shape as well as composition.1−5 The ability to control these parameters through synthesis is essential to the use of materials and also enables elucidation of structure−property relationships. The multicomponent nature and strong bonding character of complex inorganic solids often require that their syntheses occur at high temperature or pressure as well as for long reaction times.2−8 These conditions often yield a variety of crystallite sizes and morphologies due to limited control over solid-state diffusion processes. Ultrasonic spray synthesis (USS) is recognized as a continuous and potentially scalable route toward inorganic powders or films.9−17 In contrast to traditional solid-state routes, the shorter diffusion lengths imposed by the sizes of the aerosol droplets facilitate rapid and homogeneous growth of the product particles. In laboratory-scale USS, precursor solution is most commonly nebulized into low-velocity aerosol droplets that are micrometersized using low intensity, high frequency (∼2 MHz) ultrasound from a submerged transducer. A solution delivery system (e.g., a syringe pump) can be connected to the nebulization chamber to constantly replenish the starting solution for long working hours. The generated aerosols are carried by a gas into a tube furnace, where processes including solvent evaporation, chemical reactions, crystal growth, and/or annealing take place. Typically, powders composed of polycrystalline microspheres are produced as a result of sintering between several crystallites, which arise from multiple © 2016 American Chemical Society
particles nucleating per droplet (depicted in Figure 1A). Recently, aerosol-assisted molten salt synthesis (AMSS), which integrates USS and molten salt chemistry, was demonstrated as a route to nanoparticles with defined structure and composition (Figure 1B), including for metastable phases and crystal shapes.18−21 Inorganic salt(s) with relatively low melting points are added to the precursor solutions to facilitate flux formation during AMSS. The presence of a molten salt inhibits agglomeration of the individual particles within a droplet in addition to acting as a mineralizer and capping agent for crystal growth. The product is trapped within a salt matrix, and this composite can be collected in deionized (DI) water to dissolve the salt component and release the product. Compared to other synthetic methods, USS and AMSS have three unique features: (1) crystallite size is limited by the micrometer-sized aerosol droplets, (2) a relatively large Laplace pressure is induced inside the micrometer-sized droplets upon molten salt formation, and (3) the droplets are rapidly heated and then cooled within tens of seconds, enabling the formation of kinetically trapped product(s). These features contribute to the capability of USS and AMSS to prepare materials with metastable phases and complex composition. The reactor described in this protocol is suitable for USS and related techniques such as AMSS. Special Issue: Methods and Protocols in Materials Chemistry Received: June 30, 2016 Revised: October 13, 2016 Published: October 24, 2016 62
DOI: 10.1021/acs.chemmater.6b02660 Chem. Mater. 2017, 29, 62−68
Review
Chemistry of Materials
Figure 1. Schematic representation of the physical and chemical processes occurring during (A) traditional USS and (B) AMSS.
Figure 2. (A) An “exploded” representation of the USS reactor, indicating how the components fit together and (B) a photograph of the fully assembled USS reactor with an alumina tube.
Nebulizer. A variety of nebulizers has been applied in aerosol syntheses and includes pressure nebulizers, two-fluid nebulizers, electrostatic nebulizers, and ultrasonic nebulizers.10,13,23 The average size and size distribution of the product particles depends on the properties of the generated aerosol droplets and the concentration of the starting solution.24 Ultrasonic nebulizers are available in both submerged and nozzle form. Submerged transducers provide smaller aerosol droplets (1−10 μm) and a narrower size distribution, operating in a low-velocity regime.25 The size of the droplets is determined by the frequency of the transducer and properties of the solution being nebulized in
Each component of the reactor is reviewed and includes comments for reactor modification, followed by a description of an AMSS of NaSbO3 nanoplates as a representative procedure.22 Significantly, the experimental designs presented here can be modified for other systems.
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EXPERIMENTAL DESIGN Overview of USS Equipment and Reactor Setup. The reactor consists of a nebulizer, a nebulization chamber, a furnace, a furnace tube and corresponding adapters, and a product collection apparatus (a gas washing bottle in our case) (Figure 2). 63
DOI: 10.1021/acs.chemmater.6b02660 Chem. Mater. 2017, 29, 62−68
Review
Chemistry of Materials accordance with the Lang Equation.25 In the reactor reported here, the ultrasonic transducer is submerged and operates at 1.7 MHz, ∼5 W/cm2. A custom acrylic base houses the transducer and the controlling electronic components taken from a commercially available ultrasonic humidifier (Vicks Ultrasonic Humidifier V5100N, Walmart, S1 for construction details). We note that most household ultrasonic humidifiers operate near these frequency and power conditions and can be used to provide an inexpensive ultrasonic platform. Alternatively, one can be constructed by selecting a nebulizing board, e.g., 1.65 MHz Nebulizer Board − RoHS (cat. 50-1011.1, APC International, Ltd.). When using components from an ultrasonic humidifier, the dimensions of the housing are 6.5″ (H) × 6″ (W) × 5.8″ (D) in total, with a 2.5″ (H) × 6″ (W) × 5.8″ (D) compartment intended to hold water. The housing allows the transducer to be in direct contact with DI water, which transmits the ultrasonic waves to the precursor solution through a thin polymer film which is used as the base to the nebulization chamber. A chilling coil (photograph in S2) is typically passed through the coupling DI water to prevent overheating of the precursor solution and the nebulizer. The main design consideration, besides operating frequency and power, is to build the nebulizer so that the nebulization chamber can be easily positioned ∼2.5 cm above the ultrasonic transducer. Nebulization Chamber. A starting precursor solution of 5−20 mL is typically used to generate aerosols; however, continuous flow for longer operating hours is possible by using a syringe pump to feed and replenish the precursor solution. We note that larger volumes of precursor solution positioned above the transducer can hamper efficient aerosol generation. The nebulization chamber consists of a glass 57 mm diameter O-ring groove opening (5.2 cm I.D., 5.7 cm O.D., ChemGlass CG-138-02) with a chamber height of ∼15.2 cm. Two 14/20 female ground glass joints are fused at the sides, and a 24/40 female ground glass joint terminates the top of the chamber, as shown in Figure 2A and 3A. One of the 14/20 joints serves as an inlet for the carrier gas through a gas sparging tube. The other 14/20 joint is the inlet to refill the precursor solution via syringe. The 24/40 joint is the outlet for the aerosols toward the furnace tube. The O-ring opening is covered by a 15 cm × 15 cm piece of plastic wrap (7.5−12.5 μm in thickness, polyethylene) and sealed by clamping the flange to an O-ring (Size 331) attached to a Teflon base with a corresponding O-ring groove (Figure 3B). This plastic wrap allows the ultrasonic waves to transmit to the precursor solution in the chamber, nebulizing it into a mist that is then carried by a gas into the furnace tube. (Note: in practice, we have found that Glad cling wrap works best for the generation of a water-based aerosol, but other thin membranes are also suitable). The carrier gas can be selected to react with the droplets (oxidant or reductant) or can be inert depending on the synthesis (see ●SAFETY NOTES in the Procedure section). The clamp is designed to apply equal pressure around the membrane and minimize leakage, unlike a horseshoe clamp, and consists of a stainless steel ring with six evenly distributed holes (0.635 cm O.D.) for bolts (0.63 cm O.D., 4.5 cm in height) and wingnuts. See the video in the Supporting Information for a demonstration of the clamp assembly around the nebulization chamber. Furnace. The residence time and heating rate of a droplet in the heating zone, which are determined by the furnace temperature profile and flow rate of the carrier gas, will affect the morphology of the product.25 A single-zone furnace (heated zone
Figure 3. (A) Photograph of the assembled nebulization chamber connected with a gas sparger and a syringe pump and (B) schematic showing the connection of each component in a nebulization chamber.
dimensions: length 12″, diameter 1″; e.g., Lindberg Blue/M Mini-Mite) is used in our reactor, where a constant temperature is set and preserved throughout most of the tube length, deviating slightly at the ends of the tube due to cooling from the ambient external conditions. Both ends of the tube extend ∼4.5″ beyond the furnace housing for connections to the nebulization chamber and the gas washing bottle. This furnace operates with a maximum temperature of 1100 °C. Multiplezone furnaces or single-zone furnaces in sequence can be used to finely tune the temperature profile and droplet residence time. Such designs can enable sequential reactions to be integrated into aerosol syntheses.26,27 Furnace Tube. The reactions within the individual aerosol droplets occur in the furnace tube, which needs to be chemically compatible with the droplets at all stages of the reaction to avoid any undesired side reactions or safety issues (e.g., tube breakage during operation). The choice of tubing material depends on a variety of reaction conditions, such as the furnace temperature, the carrier gas, the pH of the droplets, and the chemical composition and reactions of the droplets. Commonly used tube materials are listed in Table 1 with their recommended conditions. For example, an amorphous glass tube starts to deform at ∼600 °C while alumina and fused silica begin to soften at temperatures around 1700 °C. Even though the melting temperature of a stainless steel tube (316L used in our syntheses) is 1400 °C, the recommended maximum temperature for a long duration of heating is 870 °C due to its susceptibility to precipitation of chromium carbides in grain boundaries at high temperature. Adapters. The furnace tube is connected to the nebulization chamber and collection apparatus through adapters, which are different for each type of furnace tube material (Figure 4). 64
DOI: 10.1021/acs.chemmater.6b02660 Chem. Mater. 2017, 29, 62−68
Review HF, strong bases or acids. Oxidation reactions at temperatures of 1000 °C and higher 1″ I.D. × 1.3″ O.D. × 21.5″ L High concentrated sulfuric acid. More resistant to chloride compared to 316L 0.88″ I.D. × 1.06″ O.D. × 20.5″ L High concentration acids or bases, chlorides 0.88″ I.D. × 1.06″ O.D. × 20.5″ L HF, species of sodium, phosphorus and vanadium above 1100 °C 1″ I.D. × 1.1″ O.D. × 21″ L HF, concentrated alkali metal hydroxide, sodium oxide, potassium oxide 1″ I.D. × 1.1″ O.D. × 21″ L
3−10 5−11 5−11 4−9 4−9
∼1500 °C ∼1200 °C
Glass adapters are commonly used as they are widely available, easy to modify, and chemically inert. The fused silica furnace tube ends are comprised of ground glass joints that can be connected to the nebulizer chamber and the gas washing bottle connector through the use of glass adapters. The stainless steel tubing can be flared into an O-ring type joint so a glass O-ring joint can be used with a Buna-N/Viton O-ring to form an airtight seal. The O-rings have a working temperature from −30 to 200 °C, which is suitable, as the metal tube cools substantially once out of the heated zone of the furnace. Alumina is difficult to modify into an O-ring type joint. As a result, a more complicated adapter is required that includes sets of triclamps, fittings, and gaskets to seal the end of the tube with an airtight Swagelok seal, which connects the tube to the nebulization chamber and collection apparatus (see video in the SI for the assembly of adaptors and the overall reactor). The stainless steel Swagelok adapter and end plates can be replaced with polytetrafluoroethylene (PTFE) materials for more chemically resistant purposes. Product Collection Apparatus. In our reactor, a gas washing bottle is used to collect the product on account of its simplicity; however, sampling filters, electrostatic precipitators, and thermophoretic samplers could potentially be integrated into the reactor.10,28,29 Typically, a 500 mL bottle is filled with 100−200 mL of DI water. After completion of the USS, the product can be isolated from the collection water and dissolved species by centrifugation. Multiple collection bottles can be connected in sequence to increase the collection yield. We note that the product particles are typically smaller in size in bottles positioned later in sequence.
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EXAMPLE OF AN AMSS OF NASBO3 NANOPLATES Synthesis of NaSbO3 nanoplates through AMSS is selected as a representative procedure. Sodium antimonyl L-tartrate is chosen as the single source precursor, while cesium nitrate is used as the molten salt.22
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Melting temperature Maximum working temperature Recommend pH range Possible corrosion sources Tube dimensions
S: 0.03% Misc. traces: 0.1%
∼870 °C ∼1100 °C ∼560 °C
1750 °C 1665 °C (softening temperature)
R2O: 99.6% SiO2:
