Solvent Composition and Resultant Physical Properties

Supplementary Information:

[S1]

Solvent composition and resultant physical properties

An ideal solvent for flame spray pyrolysis has a high solvation capacity for a wide range of metal-organics, large enthalpic heat of combustion, is easy to handle, safe, and is low cost. A high solubility is required to increase the process yield. The flow and spray characteristics of most atomizers are strongly influenced by the liquid properties of density, viscosity and surface tension. This highlights the multiple influences that input parameters can have on the process. For instance, a solvent may be ideal for solvation of a metal-organic but have other properties (i.e., high surface tension or density) that negatively affect atomization.

The solvation capacity of a solvent depends primarily on its structure which results in polarity, electron delocalization, hydrogen bonding, and Van der Waals interactions. Xylene is an aromatic hydrocarbon, acetone is a highly polar C3 ketone, and propane is a gaseous C3 hydrocarbon capable of only Van der Waals interactions [S7]. Propane is generally adverse to solvation of the precursor due to its hydrocarbon structure lacking any functional groups. Therefore, propane is added to the solution, as a compromise, to aid droplet breakup by effervescent atomization. In addition, propane undergoes a liquid to vapor phase change immediately upon exit from the nozzle and this rapidly increases the concentration of gaseous fuel near the burner exit. This effect will accelerate the evaporation and burning rates of the remaining droplets.

Table S1: Formulation of a representative solution used in the RSDT process for synthesis of Pt nanoparticles

[S2]

Oxidant and fuel flow rate: mass, molar, and volumetric flow

Table S2A: O2 flow characteristics calculated for supply channel shown in Fig. 3 with a fuel flow rate of 4 mL/min.

The flow conditions for each component at 4 mL/min are shown in Table 3.

Table S2B: Mass, volume and molar flux for a representative solution used in deposition of Pt nanoparticles in the RSDT process.

[S3]

Oxidant and fuel flow rate velocity calculations

The velocity of the precursor in each tube is calculated as follows:

The Reynolds numbers for solution flow are calculated as

The thresholds between laminar, transitional, and turbulent flows are: Re < 2,300 = laminar; 2300 < Re < 4,000 = transitional flow; and Re > 4,000 are turbulent. Viscocity () and density () were estimated by taking the mass weighted average for each component in the fuel at 20°C (see table S6).

The velocity in tube 1 and tube 2 are calculated from equation [1].

[S4]

Atomization

Atomization of a jet occurs when the pressure drop and decrease in viscosity (due to heating) overcome the surface tension of the fluid. The Weber number, a dimensionless number, expresses the ratio of the fluid's inertia (aerodynamic) and surface tension forces and is useful in describing the breakup of a drop in a flowing stream.

where is the density of the liquid, is the velocity of the liquid, is the characteristic length (droplet diameter) and is the surface tension. A higher Weber number denotes that the deforming external pressure forces are larger than the tendency of the surface tension to maintain the droplet shape. The initial break-up condition occurs when the aerodynamic drag equals the surface tension.

[S5]

Droplet size measurements

The droplet data is displayed in Table S5.

Table S5: Results of droplet size measurements as a function of nozzle temperature at a fixed flow rate of 4 mL/min and O2 flow of 13.6 L/min, with and without Pt-acac.

The ranges are listed to give bounds to the process parameters discovered using RSDT as the synthesis platform for Pt.

D3,2 is the sauter mean diameter (SMD), Dv,50 is the volume mean or mass mean diameter, Cv is the concentration volume, RSF is the relative span number (dimensionless). RSF represents the uniformity of the drop size distribution.

The SMD diameter is calculated based on the volume to surface area ratio. It is equal to the

sum of the cube of all diameters divided by the sum of the square of all diameters. This yields a characteristic droplet diameter that has a volume-to-surface-area ratio equal to the volume-to-surface-area ratio of the entire spray. This metric is important since heat transfer occurs at the interface of the droplets and the surrounding air. Enhanced evaporation occurs when the active surface area is maximized and the internal volume is minimized. Transmission (Trans.) is the percent light transmission through the spray. If this value is below a certain limit, the spray density may be too high and multiple-scattering errors may result. Specific surface area (SSA) is the surface area of the spray, expressed as square meters per cubic centimeters of droplet. The distribution of each solution under different Tcontrol values is plotted in Figure 4.

[S6]

Thermophysical properties of precursor solution

Table S6: Thermophysical properties of the solvent system used in RSDT synthesis of Pt

[S7]

Heat transferred to nozzle from induction coil

Heat was calculated from equation 6

 Heat (W) = T tube 1 *stainless steel* Ltube * dtube 2 [6]

where stainless steel is the thermal heat transfer coefficient of stainless steel (~80 ) and length is 10 cm and the diameter is 0.0254 cm. T tube 1 was calculated by measuring the inlet temperature of the fuel in tube 1 (20°C) and subtracting the outlet temperature (45°C) measured by thermocouple. Assuming a specific heat capacity, of 1720 J/kg*K (the specific heat capacity of xylene) and a mass flow rate of 5.02E-05 kg/s then the heat exchanged between the by the tube with the fuel is 2.2 W according to equation 7.

[7]

[S8]

Evaporation

The heat and mass transfer processes in the gas phase near the droplet surface determine the instantaneous vaporization rate, and the amount of heat penetrating into the droplet interior, . Convective heat transfer occurs since a gas surrounds the droplets (i.e. the matrix gas). For larger droplet the evaporation stage includes the following steps: evaporation of the solvent from the surface of the droplet, diffusion of the solvent vapors into the gas phase, droplet shrinkage, a change in droplet temperature and solute diffusion toward the center of the droplet. The extent of solute diffusion will be a function of its diffusivity and this can affect whether a core, shell or shattered shell of solid precursor results after all fuel is evaporated. If droplet formation begins with a uniform temperature distribution then the change in temperature with time is

Where Td is the droplet temperature, t is the time, [J/kg-K] is the heat capacity of the droplet (i.e., fuel), heat-transfer rate [J/s] and [kg] the droplet mass. The droplet loses heat as the latent heat required for evaporation and gains heat when the ambient temperature is greater than the droplet temperature. From a process perspective, choosing a fuel with a low , heating the surrounding air, and creating ultra-fine droplets will accelerate the change in temperature thus creating conditions whereby droplet evaporation and hence precursor decomposition occur closer to the exit orifice. The changes in the droplet heat transport are

Where is the Spalding heat transfer number (indicative of the strength of evaporation) and as the latent heat of vaporization [J/kg] under conditions present at the droplet surface

represents the heat transferred into the droplet [J/s]. The change in droplet diameter as used by Abramson and Sirignano (Ref 1) and extended to account for the droplet’s density as a dependence on temperature is given as

The changes in the droplet mass due to evaporation are:

Where , is the diffusion coefficient of the gas, is the modified Sherwood number representing the ratio of convective to diffusive mass transfer coefficients. is the Spalding mass transfer number, a non-dimensional parameter measuring the ratio of the heat of combustion (along with the sensible enthalpy difference between the ambient environment and the droplet surface) to the heat of vaporization. This number represents the drive toward vaporization.

[S9]

Equivalance ratio

Oxidation or reduction occurs according to the relative oxidizing strength of the flame, the activity of the metal species, and the partial pressures of CO2 and H2O. The oxidizing strength can be controlled by the O2 partial pressure in the combustion atmosphere (Ref 2, 3), and adjustments in the equivalence ratio (Ref 4). The equivalence ratio is the relationship between the stoichiometric oxidant and fuel molar flow rates divided by the actual process ratio; the relationship is expressed in equation 6.

[6]

Equivalence ratios greater than 1 indicate a fuel rich mixture while ratios equal to 1 indicate a stoichiometric mixture of oxidant to fuel. When the equivalence ratio is <1 the combustion mixture is O2 rich. The O2 flow has profound implications on the velocity (and hence residence time) and temperature profiles. Velocity and temperature in-turn will influence the particle crystallinity, morphology and deposition efficiency.

References

1. B. Abramzon, W. A. Sirignano, Droplet vaporization model for spray combustion calculations, Int.J.Heat Mass Transfer, 1989, 32(9), p 1605-1618.

2. R. N. Grass, W. J. Stark, Flame spray synthesis under a non-oxidizing atmosphere: Preparation of metallic bismuth nanoparticles and nanocrystalline bulk bismuth metal, 2006, 8(5), p 729-36.

3. E K Athanassiou and R N Grass and,W.J.Stark, Large-scale production of carbon-coated copper nanoparticles for sensor applications, Nanotechnology, 2006, 17(6), p 1668.

4. L. Mädler, H. K. Kammler, R. Mueller, S. E. Pratsinis, Controlled synthesis of nanostructured particles by flame spray pyrolysis, J.Aerosol Sci., 2002, 33(2), p 369-389.