Synthesizing Nanoparticles Using Reactions Occurring in Aerosol Phases ()
1. Introduction
1.1. Concept
We propose a novel, versatile method of synthesizing nanoparticles and outline it here. Figure 1 is a block diagram illustrating the overall concept. Our approach is based on using the two nebulizers and electrical circuitry to produce aerosols having oppositely charged monodisperse drops. Each aerosol will originate from two solutions containing at least one of the nanoparticles’ soluble precursors with concentration ratios required by the stoichiometry of the reaction forming the target nanoparticles. The two aerosols will be mixed in a Reaction Phase and only the oppositely charged aerosol drops will merge. Their contents will react to produce the nanoparticles in a larger uncharged drop containing the soluble products of reaction. A carrier gas will transport the mixture of drops through an electrostatic filter that allows only uncharged drops to pass. The uncharged drops will then be introduced into the Processing-Phase to remove volatile reaction products. Evaporation of the uncharged drops in the Processing Phase will increase the internal drop pressure applied to its contents (via the inverse drop radius surface arising from the surface tension effect). This will increase the rates of any 2nd and higher order reactions. The resultant droplets containing only the nanoparticles are then transported to the Collecting Phase. Details of the processes occurring in the various Phases are discussed below.
Our proposed method differs from the approach of Salata [1] who used a single electrospray to create an aerosol of drops having several charges of the same sign that contained one component of a nanoparticle. These drops were then exposed to an excess of a reactive gas that reacted with the species in the charged drops to form nanoparticles. In our approach the masses of the nanoparticles is determined by the mass of the species that react
Figure 1. Synthesis scheme for nanoparticles production.
in the two oppositely charged drops when they merge together.
In Section 2, we give our initial approach using two uncharged aerosols and the results we have obtained by merging them. In Section 3, we describe our future plans and their rationale.
1.2. Background
The production of nanoparticles starting with a solution of species in a nanodroplet has developed over the past two decades [2,3]. These original studies used several methods to produce nanodroplets, e.g. by ultrasonification [4,5], electro spraying [6,7] and various thermal methods [8,9]. Evaporation of solution droplets created in these different ways produced a solid whose size was determined by the precursor masses in a droplet [10,11]. Frequently, during the latter step Chemical Vapor Deposition (CVD) was used to produce the final nanoparticle. CVD involves thermal heating to evaporate the liquid in a droplet in order to pyrolyze (sinter) intermediates in the final nanoparticle [12]. We have cited only a selected number of the fairly large literature papers using the just described approaches and their many possible combinations. Our concept described above in Section 1.1 differs from all the prior approaches in that of the final nanoparticle requires the merger of two nanodroplets as the first step in its formation. This two drop approach allows the direct formation of nanoparticle composition that is not possible using a classical single drop technique.
2. Experimental Results
2.1. Preliminary Experiments
We carried out experiments designed to validate the concept of producing nanoparticles by merging two oppositely charged drops to produce a product that would convert to nanoparticles.
These initial experiments were unsophisticated and used inexpensive commercial ultrasonic humidifiers to generated uncharged aerosols. Also, QCM equipment in out laboratories designed for other purposes was used. The objective of these experiments was to demonstrate that reactions do occur rapidly on mixing two uncharged aerosols produced from dilute solutions. The experiments we report in Sections 2.1.1 and 2.1.2 involve visual detection of a reaction product produced by the merger of aerosol drops.
2.1.1. Luminol
This experiment involved the oxidation of Luminol by potassium ferricyanide to produce light as shown in Figure 2. Initially, a solution containing Luminol and one containing potassium ferricyanide were nebulized and their aerosols were not mixed. No visible light was seen in each case in the Reaction Phase chamber. Next, the two solutions were nebulized and merged together in the Reaction Phase chamber to determine if the amino dicaboxylate anion of Luminol formed in the merged drops. A bright yellow light was observed where the aerosols droplets merged. This is the expected result that accompanies the formation of the amino dicaboxylate anion.
2.1.2. Umbelliferone
This compound is a weak monoprotic Brönsted acid that is not fluorescent in acid solutions below pH = 8 when a solution containing it is exposed the UV radiation it is a singly charged, green fluorescent anion. First, we illuminated the aerosol produced from a slightly acid umbelliferone, HB, solution with UV and there was no visible green fluorescence emerging from the HB solution (Figure 3).
Second, we merged aerosols from this slightly acid solution of umbelliferone with one from a basic (pH = ~12) sodium carbonate solution. This produced the conjugate base anion of umbelliferon and a green fluorescence typical of the umbelliferone anion was seen when UV light illuminated the region where the two aerosols merged (Figure 4). Both these above experiments show that uncharged droplets did collide, merged and made a product with the fluorescent properties we expected.
2.2. Aerosol Droplet Size Change during Nitrogen Transport from an Aerosol Generator
The purpose of these experiments was to determine if
Figure 2. Chemiluminescent reaction on oxidation of Luminol.
Figure 3. Umbelliferone in acid solutions.
Figure 4. Umbelliferone solution pH > 8.
drop sizes within an aerosol decrease as drops are transported by the carrier gas from the point of their generation to elsewhere in the apparatus. With this goal in mind the QCM was applied to determining whether aerosol drops would evaporate in carrier gas that was not humidified.
Figure 5 is a block diagram of the experimental procedure in which first we measured the weight of solution collected for specified times by a QCM from an aerosol made from pure water and then from a sodium chloride solution that impinged on the QCM. The drops were transported through our system using ambient air flow created from the pressure difference produced with a vacuum pump.