J. Phys. Chem. C 2008, 112, 13105–13113
13105
Thermospray: A Method for Producing High Quality Semiconductor Nanocrystals Lilac Amirav* and Efrat Lifshitz* Schulich Faculty of Chemistry, Solid State Institute, and the Russell Berrie Nanotechnology Institute, Technion, Haifa 32000, Israel ReceiVed: February 25, 2008; ReVised Manuscript ReceiVed: April 24, 2008
A novel spray-based technique was developed for the production of high quality semiconductor nanocrystals, offering an attractive alternative to conventional production methods, epitaxial growth, and colloidal synthesis. The novel spray technique is simple and low cost and overcomes limitations of the previously introduced methods. According to this new spray-based technique, solutions of semiconductor salts are first sprayed via thermospray nebulizer into monodispersed droplets, which subsequently become solid nanocrystals by solvent evaporation. Each semiconductor nanocrystal is produced from a single spray droplet upon the full vaporization of the liquid. The average diameter and size distribution of the final nanocrystals can be controlled and determined by the concentration of the sprayed solution and the droplet size, hence by spray production parameters. Thus, thermospray experimental parameters such as the control temperature, liquid flow rate, and capillary inner diameter were fully examined and optimized for the production of high-quality semiconductor nanocrystals of the desired size and composition. A model is suggested for estimating the mean droplet diameter of the generated thermospray aerosol droplets, and it indicates a trend similar to the observed experimental results. The solubility levels of various semiconductor salts in different solvents were determined by atomic absorption spectroscopy. The results indicate that the solubility is related to the solvent polarity, salt bonding characteristic (ionic or covalent), and the solution pH (presence of NH3).The method is demonstrated through the production of high quality monodispersed (∼5% size distribution) CdS nanocrystals in the size range of 3-6 nm. The further production of PbS nanocrystals, and preliminary results for ZnS and MoS2 nanocrystals, establish the generalization of the method for different types of semiconductors. 1. Introduction Semiconductor nanocrystals have been the object of intensive scientific and technological interest during the past two decades, due to the pronounced influence of the three-dimensional size confinement on their electronic and optical properties.1,6 Extensive efforts have been devoted to the production of high quality semiconductor nanoparticles. These efforts are motivated by the array of potential uses in a wide variety of applications including the next-generation display devices,7,8 biological markers,9,10 lasers,11 microelectronics12 and spintronics devices,13 photovoltaic cells,14 solid-state devices, and light emitting diodes.15 Currently, there are two main methods for the fabrication of semiconductor nanoparticles, namely epitaxial growth16 and colloidal chemistry techniques.17–20 Epitaxial growth of nanoparticles produces relatively large dots, with weak quantum confinement.21 The nanoparticles produced are not freestanding as they are embedded in a substrate layer. The liberty to choose the substrate is limited, the nanoparticles cannot be assembled, and their incorporation in most of the applications mentioned is unfeasible. Furthermore, this method requires ultrahigh vacuum equipment as well as other complex and expensive equipment. The colloidal synthesis is an old-fashioned chemistry procedure which requires inert and elevated temperature conditions. The method, which is currently considered the dominant one, enables the production of high quality nanoparticles, with narrow size distribution (5-10%), and control over the nanoparticles * To whom correspondence should be addressed. E-mail: lilac@ tx.technion.ac.il (L.A.);
[email protected].
size and shape. However, the nanoparticles are coated with an organic ligands shell which is an inherent feature of the growth procedure. This insulating capping prevents the formation of highly packed structures and direct contact between nanoparticles, weakens both mechanically and chemically any available assemblies, and limits all scientific research and technological applications involving electron transport or energy transfer between nanoparticles. Furthermore, doping of colloidal nanoparticles is inhibited due to the large diffusivity of the dopant to the nanocrystalline surface at the elevated temperatures required. In light of the problems and limitations exhibited by the conventional production methods, there is an increasing requirement for a novel technique. We present a novel spray-based method for the production of high quality semiconductor nanocrystals, characterized by several new and unique features and which offers an attractive alternative to the conventional production methods. According to this spray-based method, aqueous or organic solutions of semiconductor salts are first sprayed via a thermospray nebulizer into monodispersed droplets, which subsequently become solid nanocrystals by solvent evaporation. While the spray-produced droplets are moving forward, at a certain point in time, due to their evaporation, the semiconductor salt concentration reaches the point of over saturation and salt condensation spontaneously occurs. Such spontaneous condensation occurs within isolated droplets during their flight as unsupported single droplets, so that each semiconductor nanocrystal is produced from a single spray droplet, upon the full vaporization of the solvent. Thus, the nascent spray droplets become a stream of freestanding, unsupported, and uncoated
10.1021/jp801651g CCC: $40.75 2008 American Chemical Society Published on Web 08/01/2008
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semiconductor nanocrystals. Those features are unique to the spray method, and of great fundamental scientific significance and practical importance as they enable the production of the densest possible assemblies (mechanically robust) due to the lack of organic capping. Electron transport properties as well as energy transfer mechanisms, which drastically change with the semiconductor nanocrystals organization, can then be measured within a new and more interesting range of interparticle distances. In addition, the lack of capping on the semiconductor surface is important for catalytic applications where direct contact between the material to be catalyzed and the catalyzing agent is desired. Furthermore, the effect of the organic capping on the nanocrystals physical properties could be obtained and explored for the first time. The potential of the novel spray method further includes the following: (1) production of core-shell or core-alloyed shell structures due to separate precipitation rates for different components within the droplet; (2) dry collection on any selected supporting surface substrate. This opens exciting opportunities for the incorporation of semiconductor nanoparticles in a variety of new applications; and (3) the introduction of extra carriers (i.e., electrons or holes) or spins into the nanocrystals by means of doping since kinetic processes govern the nanocrystals’ generation. The average diameter and size distribution of the final nanocrystals can be controlled and determined by the concentration of the sprayed solution and by the droplet size; hence, by the spray production parameters, following the relationship:22
droplet size 3
√dilution factor
) nanocrystal diameter
(1)
For example, a droplet of 1 µm diameter with solution concentration of 1 ppm would result in a 10 nm diameter nanocrystal. The successful demonstration of the method through the production of CdS nanocrystals was reported recently.23 However, minimum consideration was devoted to the available means of control over the nanocrystals’ size. This manuscript thoroughly describes the principles of thermospray nebulization and the experimental parameters governing the spray mean droplet diameters and size distribution. The paper then discusses optimization of these principles and parameters to produce high quality semiconductor nanocrystals of specified sizes. Furthermore, a model is suggested for estimating the mean droplet diameter of the thermospray aerosol droplets generated. Since the nanocrystals size distribution depends also on the solution concentration, information on the solubility levels of various semiconductor salts in different solvents, as measured by atomic absorption spectroscopy, is included. Finally, results of different semiconductor nanocrystals produced via thermospray establish the generalization of the method. 2. Experimental Section 2.1. Semiconductor Compound Solution. Solution Preparation. The procedure is initiated by directly dissolving the selected semiconductor compound(s) in an appropriate solvent, forming the solution from which the monodispersed spray droplets are to be generated. This solution-based procedure is simple, cheap and requires only a small amount of starting material. It also avoids prolonged handling of hazardous precursors as required in known colloidal procedures. The semiconductor compounds used during the course of this research include II-VI semiconductor salts such as cadmium sulfide (CdS), zinc sulfide (ZnS), and manganese sulfide (MnS),
Figure 1. The apparatus of the pneumatic assisted thermospray utilized for the production of semiconductor nanocrystals.
the IV-VI semiconductor salt lead sulfide (PbS), and the metal sulfide MoS2. The powders, which are commercially available (purchased from Aldrich), were kept under standard inert conditions in a glovebox. Various solvents were tested during the course of this research, including ionized water, methanol (HPLC or LC-MS grade g99.9%), ethanol, isopropyl alcohol, acetone, and more. The solvent is selected according to the semiconductor compound used, based on the solubility upper limit it dictates. However, the solvent characteristics, in particular its surface tension, would affect the spray droplets size as well. Strong preference was given to highly volatile solvents, since droplet size distributions shift to smaller diameters as solvent volatility increased.24 Attempts to improve the solubility included heating, using an ultrasonic bath, and changing the solution pH with ammonia. Two ammonia solutions were examined: ammonia solution 2.0 M in methanol (Sigma-Aldrich) and ammonia solution 2.0 M in ethyl alcohol (Aldrich). Aging of the sample and storage preferences were examined as well. Determination of the Solution Concentration. The semiconductors salt solubility was determined through the cation concentration in the solution. This was measured with a Spectra AA atomic absorption spectrometer (Varian Inc., Palo Alto, CA USA) equipped with a standard air-acetylene burner system. A multielement hollow-cathode lamp, operated according to the manufacturer’s recommended conditions, was used at the primary cation resonance line. The burner height and lamp position were adjusted for optimum sensitivity, and the nebulizer uptake rate was regulated to provide the optimum absorbance signal for a conventional sample. All samples were diluted with ionized water for safety precautions and filtered with a 0.02 µm porous filter prior to the measurement. 2.2. Thermospray System. Droplets of the semiconductor salts solutions were generated using pneumatic assisted thermospray (apparatus shown in Figure 1). The thermospray system used in this laboratory consists of direct electrical heating, in which the power required to vaporize the liquid is supplied by passing an electrical current through the capillary tube itself.25 The heating process can be precisely controlled and optimized for different solvents and various flow rates.26 The sample can be introduced directly through a stainless steel capillary (127 µm ID, 510 µm OD, 20 cm long, Upchurch Scientific) or a fused silica capillary inserted into a stainless steel (silco-steel) capillary for indirect heating. With fused silica capillaries, the heating process is less direct; however, the capillary has the advantage of greater chemical inertness (thus less vulnerable to clogging than the stainless steel capillary) and can provide lower contamination from previously sprayed solutions. Fused
Thermospray Production of Semiconductor Nanocrystals silica capillaries are commercially available in a wide variety of internal and external diameters, parameters which are expected to significantly affect the diameter of the spray droplets. The various capillary diameters that were used through the course of this research include 360 µm OD with 150, 100, and 75 µm ID. The solution is pumped through the capillary by an HPLC pump or a syringe-pump and through a polyetheretherketone (PEEK) tubing (1.6 mm (1/16”) OD and 65 µm ID for the HPLC pump, or 1 mm ID for the syringe pump). The HPLC pump (model PU-1585, JASCO, Japan) enables stable work with relatively low liquid flow rates. The syringe pump (infusion, model 100, KD Scientific) enables processing highly concentrated solutions without contamination of the pump (and consequently the followed solution to be sprayed). The PEEK tubing is connected to the stainless steel or fused silica capillary by a PEEK union with a 0.010′′ thru hole (P-742, Upchurch Scientific). A molded delrin plug (U-467, Upchurch scientific) which was drilled into a 0.020′′ hole is used to hold the stainless steel capillary. A PEEK microtight fitting sleeve with a standard head plug is used to hold the fused silica capillary. A practical advantage of this design is that interchange of capillaries is a rapid, simple process. A heating power supply is connected to two points on the stainless steel capillary (or silco-steel tube), by specially designed clamps. The positive point located near the inlet side and the negative point located about 8-9 cm further downstream is illustrated in Figure 1. The length between these connections points could be easily varied and served as an experimental parameter for varying the droplets’ diameter. Optimal conditions are monitored using thermocouples attached directly to the nebulization apparatus itself. The capillary is then placed into a PEEK T-shaped structure (P-727, with original thru hole of 0.020′′ that was slightly expanded) in order to supply a nitrogen gas flow for pneumatic assisted spray formation. The gas flows out of the T-shaped structure through a quartz tube, surrounding the capillary. A Kanthal wire is looped on the quartz tube for heating the nitrogen nebulizing gas. A small, axially open glass oven was used to heat the air through which the spray passes on its way to the substrate target. 2.3. Spray Characterization. Droplets Size Measurement Wia Low Angle Laser Light Scattering. Droplet-size distribution and mean droplet diameters are the most important determinants in dictating the quality of the spray as they will directly control the nanocrystals size and size distribution. Droplet size distributions for the various spray operation conditions used in the course of this research were determined and compared. Due to the crucial influence of the droplet size on the final nanocrystals produced, it was monitored online with the aid of a commercial low angle laser light scattering (laser diffraction) system (Mastersizer S, Malvern). This approach allowed rapid, unperturbed measurement of nearly complete particle size distributions as a function of parameters such as thermospray control temperature, liquid flow rate or capillary ID. The Mastersizer S is equipped with a low-power He-Ne laser (2 mW power, 633 nm wavelength) and a multielement photodetector. This nonintrusive particle sizer is based on the diffraction of a parallel beam of monochromatic light by a moving droplet. The analysis relies on the fact that scattering angle is inversely proportional to droplet size, and thus the scattering profile can be translated into a droplet size distribution using Mie theory.27 The application of the non linear Mie theory is required due to the properties
J. Phys. Chem. C, Vol. 112, No. 34, 2008 13107 of the spray droplets: high transparency, spherical shape, and droplet size in the range of the laser wavelength. The laser diffraction technique measures droplets within a volume defined by the intersection of the beam and the aerosol, and therefore provides line-of-sight averaged results. For micron sized droplets, vaporization occurs rapidly, and so the relatively large measuring volume results in a wider distribution. Care should be taken with respect to the measurement position relative to the nebulizer tip. Since smaller droplets vaporize faster, the size distribution is distorted and shifts to a larger mean diameter. Multiple scattering events of the light by more than one droplet distort the diffraction pattern and result in a smaller mean diameter and a broader distribution. This measurement approach is independent of the position of the droplet in the light beam or its velocity. The applicable range (according to ISO13320) is 0.1-3000 µm, depending upon the focal length of the lenses. For Malvern Mastersizer S, the measurable droplet size range is from 0.5-900 µm. Terms Statistics. Analytically, the mass distribution is more appropriate to characterize an analytical aerosol, because the mass distribution reflects the distribution of analyte on the basis of droplet size. Thus, most of the spray characteristics found in the scientific literature refer to mass or volume distributions. However, for proper correlation of the droplet size distribution with the resulting nanocrystal size distribution, a number distribution is required. The Mie theory, and hence the commercial laser diffraction technique, assume the volume of the particle (as opposed to Fraunhofer which is a projected area prediction) and generates a volume distribution, which can be converted to any number or length diameter. However, care should be taken when interconversion is used. Most aerosols exhibit a skewed, nominally log-normal, size distribution with a long tail to the large droplet side, which results in mass-related average diameters that are larger than those of count. Note that the calculation of “average” diameters requires immense care, especially when these quantities are located on the tail end of the distribution. Smaller aerosol droplets might be completely immeasurable by the laser diffraction system (10 µm).32–35 This was confirmed by the droplets size distribution obtained with laser diffraction, as seen in Figure 2. Solvent vaporization is enhanced for smaller droplets and higher temperatures, with both aspects leading to faster droplets desolvation and increasing levels of the unvolatile semiconductor salt concentration. Veber and36 coauthors postulate that tuning the aerosol properties primarily effects the particle size distribution of the thermospray aerosol. The primary droplet size distributions for thermospray-generated aerosols are dependent on several variables,30 such as the percent of vaporization (thermospray control temperature), capillary internal diameter, solvent characteristics (mostly volatile organics) and liquid flow rate. These points are discussed below. Note that heat transfer characteristics are specific to both the materials employed and the design of the nebulizer, with the only serious requirement being that sufficient energy can be input through whatever design is employed to generate an aerosol. This means that measurements detailing optimum conditions are not necessarily comparable, even for different vaporizers of the same design. The trends in optimization, however, are universally followed with all designs. The ability of thermospray to produce relatively small droplets with narrow droplet size distribution, along with the capability to easily tune these characteristics,24,30,37,38 have made it a promising technique for the production of nanocrystals. 3.3. Thermospray Heating and Solvent Vaporization Degree. As discussed, the thermospray technique employed is based on direct electrical heating of the solution. The current provided (power supplied) to the capillary has a major impact on the solvent temperature, the percent of intratube solvent vaporization, and the quality of the spray. The fraction of solvent vaporized determines the liquid to gas flow ratio at the exit of
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TABLE 2: Produced Particles vs Thermospray Heating Power comments coarse pneumatic spray in obtained stable thermospray is achieved end of the vaporization process
size distribution nanocrystals size T [°C] I [A] 16%
5.6 ( 0.9 nm
64 °C 2.00
8.7%
5.1 ( 0.5 nm
66 °C 2.60
tenth of the micron
66 °C 2.70
the capillary and is an important determinant for the properties of the aerosols that result, in particular the droplet size, and consequently, the nanocrystal size. Thus, the droplet size characteristic of thermospray aerosols may be varied electrically.30 A measure of spray characteristics with respect to the heating current was reported and discussed previously,23 and only key points are mentioned here. As the heating power is raised, the temperature increases to a relatively high temperature, above the solvent boiling point. The temperature then decrease sharply to the solvent boiling point and remains nearly constant until the vaporization is complete, since the heat flux is used to provide the latent heat to the solvent.25 At the point corresponding to complete vaporization, the temperature again rises rapidly since only the heat capacity of the vapor is available to absorb the input energy. The extent to which the temperature initially rises was at first attributed to the high internal pressure in the capillary, which results in a rise of the solvent boiling point causing the vaporization to begin at relatively high temperatures. With the spray formation, a relief of the pressure is followed by a sharp decrease of the temperature. However, the extent to which the temperature rose above the solvent temperature altered from one experiment to another and was not found to be related to the capillary’s internal diameter. Koropchak39 reported a similar rise in temperature and suggested that the jump at the infection point is due to a superheated region where liquids are heated above their boiling temperatures due to the lack of stirring. If the temperature of the heater is higher than the minimum required for steady vaporization, then the necessary heat can be supplied in a length shorter than the length of the heater and vaporization will tend to occur inside the capillary; on the other hand, if insufficient heat is supplied, superheated liquid will emerge and begin to vaporize only after exiting the capillary. Since heat is supplied to the liquid primarily by conduction from the walls, the vaporization of the liquid near the axis of the capillary will tend to lag behind the vaporization near the walls. If the nominal liquid-vapor interface is near the exit of the vaporizer, this portion of the liquid emerges as a visible mist entrained in the vapor jet. At slightly higher vaporizer temperatures, the mist disappears and a hot, dry, very intense vapor jet emerges from the capillary, presumably with sonic velocity at the exit.29 Correlating visual observation of the vaporizer jet in the laboratory with the semiconductor nanocrystals produced by the spray, it appears that the best performance probably corresponds to partial vaporization occurring at the capillary tip. The products of a spray achieved by varied heating currents and measured temperatures were collected and compared. These results are summarized in Table 2. The first sample was collected prior to the formation of a stable thermospray, when mainly a coarse pneumatic spray was obtained. The resultant CdS nanocrystals had a wide size distribution of about 16%. The
Figure 3. Variations of the droplet number-median diameter, as measured with laser diffraction, versus the thermospray heating power supplied to a stainless steel capillary.
second sample was collected at moderate heating with partial solvent vaporization and stable thermospray formation. The resultant CdS nanocrystals were found to have an average size of 5.1 ( 0.5 nm and a size distribution of less than 9%. The third sample was collected for spray produced with higher heating current, near the end of the vaporization process. The results were CdS crystals in a size range of some tenth of microns. At a certain level of solvent vaporization, the CdS crystallizes prior to exiting the capillary and before spray droplet formation. These conditions favor micron-sized particles as was obtained by the aforementioned spray. Moreover, high degrees of sample stream vaporization are thought to cause deposition within the nebulizer and plug the capillary. Increasing the degree of vaporization would increase the amount of solvent vapor available for aerosol formation and the gas velocity, and as a result decrease the droplet diameter. Thus, as the exit temperature is increased, the droplet size distribution shifts to smaller particle diameters due to the enhanced solvent evaporation.35,37 However, as was demonstrated previously, if the vaporization degree is too high, precipitation occurs prior to droplet formation, resulting with micron sized particles. Direct measurements of the thermospray droplet number-median diameter versus the thermospray heating power were made using laser diffraction, and the results are displayed in Figure 3. A sharp decrease of about 20% in droplet number-median diameter is observed when stable thermospray is formed, followed by slower linear decrease as the heating power was increased. The last point on the plot is less reliable as it was measured for a high degree of vaporization with less solvent available to scatter the laser beam. 3.4. Drop Size Distribution: Influence of the Liquid Flow. Figure 4 shows the variations of the droplet number-median diameter versus the liquid flow rate (Ql), indicating that droplets shift to smaller droplet diameters as the liquid flow rate is increased. The behavior is similar to that shown by for thermospray in the literature32,35,37,40 and opposite to that of the concentric pneumatic nebulizers.41–43 This behavior was explained in the literature by taking into account that the absolute amount of liquid vaporized increases with the liquid flow rate.40 Thus, at low degrees of vaporization, a decrease in droplet diameter is expected with increasing liquid flow rate due to increases in the velocity difference between the gas and liquid flows. From the experimental results it can be seen that the decrease in droplet diameter is strong only for increasing the liquid of
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Figure 4. Variations of the droplets number-median diameter, as measured with laser diffraction, versus the liquid flow rate.
Amirav and Lifshitz refers to fused silica capillaries. This might account for the shift to slightly higher droplet diameter. 3.6. Model for Thermospray Droplets Size Prediction. A model for the thermospray droplet production process will be helpful in estimating the mean droplet diameter of the generated aerosol. Vestal28 has qualitatively predicted an aerosol formation process for thermospray involving the shattering of bulk liquid by a high velocity blast of solvent vapor. Conceptually, this process is similar to aerosol formation via concentric pneumatic nebulization. A temperature gradient across the internal cross section of the heated capillary, decreasing toward the center, could result in localization of unvaporized solvent in the center. Solvent vapor would be emphasized along the hot inner walls of the capillary and would be of significantly higher velocity than the liquid. Sheering forces resulting from the interaction of this gas with the liquid should be sufficient to fragment the liquid stream and ultimately result in aerosol formation. Because of the conceptual similarities between thermospray and concentric pneumatic nebulizers, models generated for pneumatic nebulizers would also be applicable to thermospray aerosol generation. The most common means of predicting the effect of operating conditions of pneumatic nebulizers on droplet sizes is the Nukiyama-Tanasawa empirical equation48,49
D[3,2] )
Figure 5. Variations of the droplet number-median diameter, as measured with laser diffraction, versus the capillary inner diameter.
flow rate up to 250 µL/min. It is suggested here that above this flow rate the heat transfer, and thus the degree of solvent vaporization, decrease, resulting in the change of the slope to a more modest decrease in droplets diameter. Combined with smaller capillary inner diameter effects, shifts in droplets sizes with flow rate are more important due to the mutual effect on the capillary internal pressure. 3.5. Drop Size Distribution: Influence of the Capillary Inner Diameter. More than one study has documented the significant effect of capillary diameter on droplet size distribution. The capillary (or aperture) diameter was found to affect the spray performance, with smaller diameters providing smaller sized droplets.24,30,35,37,38,44,45 As shown in Figure 5, the droplet number-median diameter shifts to lower diameters with decrease in the capillary inner diameter. It is consistent with previously published data obtained with thermospray nebulizers. This behavior could be explained by taking into account that, under the same set of experimental conditions, the fraction of solvent that is vaporized and consequently the nebulizing gas flow are about the same for both diameters, since they depend mainly on the power supplied to the liquid, which is the same in both cases. As the pressure required to reach a given gas flow rate is much higher for the narrowest outlet capillary, the energy available for surface generation in this case will also be higher and, hence, the mean drop size smaller.37,42,43,46,47 The experimentally observed decrease in the droplet numbermedian diameter with decrease in the capillary inner diameter was linear. The point on the plot measured for 125 µm corresponds to stainless steel capillary, while all other point
585 σ V F
0.5
()
(
+ 597
η (σF)0.5
) ( ) 0.45
103
Ql Qg
1.5
(2)
where D[3,2] is the Sauter median (volume to surface area ratio) diameter (µm), ν is the velocity difference between the gas and liquid flows (m/s), σ is the surface tension of the liquid (dyn/ cm), F is the liquid density (g/cm3), η is the liquid viscosity (poise), and Ql and Qg are the volume flow rate of the liquid and gas (cm3/s), respectively. The relationships can be found in many monographs on atomic spectrometry.50–52 This empirical equation relates the physical parameters of the nebulizer with the nebulizer solution and the mean droplet diameter of the aerosol, and is often useful for predicting trends for the effects on d of operating parameters or physical characteristics, when the particles are larger than 5-10 µm. However, this model is not typically an accurate indicator of smaller particle sizes which are produced by most thermospray nebulizers. On the basis of this treatment, a modified version of the Nukiyama-Tanasawa equation for thermospray was suggested by Koropchak and Winn35
D[3,2] )
585 σ V′ F
0.5
()
(
+ 597
η (σF)0.5
) ( 0.45
103
(1 - f)Ql fQg
)
1.5
(3)
where f is the fraction of liquid vaporized and Qg is the gaseous flow rate for complete vaporization. This model assumed that the gas flow rate at any operating temperature is the product of the fraction of liquid vaporized (f) at that temperature and the flow of gas that would result for 100% vaporization (Qg), and the liquid flow rate at the outlet to be the product of (1 - f) and the input liquid flow. ν′ is the velocity difference between the gas and liquid flows, which was calculated by an intuitive version
ν′ )
Ql (F∆VV - 1) A
(4)
where ∆VV is the specific volume of vaporization and A is the capillary cross sectional area. This mathematical model for thermospray vaporization relates to the capillary cross sectional area effect on the velocity
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Figure 6. TEM images of thermospray produced CdS nanocrystals, with average diameter that varied between 3.1 to 5.6 nm (A and C) and a representative size distribution histogram indicating standard deviation of as low as 5%.
thus, the validity of the Koropchak and Winn modified version for thermospray should be questioned as well. An alternative relationship for droplet size prediction for pneumatic nebulizers was proposed by Groom et al.,53 based on significant dimensionless characteristic numbers, and it includes the capillary inner diameter dC as a linear size parameter. The relationship contains the gas Laplace number, the liquid to gas mass flow ratio, and Ohnesorge number. The nondimensional Sauter mean diameter D[3,2]/dC can be predicted by the semiempirical equation valid for given density ratios and adiabatic exponents
[
D[3,2] ∆p/g ) C1 dC (1 + µ)2
Figure 7. HR-TEM images of thermospray produced PbS nanocrystals. Three individual nanocrystals (A-C), a low magnification image indicating the relatively large size distribution (D), and a representative FFT indicating a cubic rock salt structure.
(1 + C2On)
(5)
For each nebulizer design, the parameters C1 and C2 are different and have to be determined by drop size measurements. It is suggested here to modify the Groom equation for thermospray in a similar manner to the Nukiyama-Tanasawa equation modification by Koropchak and Winn. The modified version would then be
(
D[3,2] ) dCC1∆p/g 1 + 103 difference between the gas and liquid flows, and predicts a reduction in particle size for decreasing capillary diameters. The prediction in general has been proven by experimental results.24,35,37,44 However, Koropchak, and Winn’s modified version of the Nukiyama-Tanasawa equation for thermospray,35 suggests that the droplet diameter is proportional to the square of the capillary inner diameter. Yet, according to the Nukiyama-Tanasawa equation, the dimension of the pneumatic nebulizer has no effect on the mean diameter, while the influence of the nebulizer outlet diameter on the droplet size has been supported with experimental results. The validity of this equation for the prediction of the mean droplet diameter for pneumatic nebulizers was questioned, and
]
-0.75
(1 - f)Ql fQg
)
1.5
(1 + C2On)
(6)
where D[3,2] is the Sauter median (volume to surface area ratio) diameter, dC is the capillary inner diameter, ∆p*g is Laplace number, f is the fraction of liquid vaporized, Ql and Qg are the volume flow rate of the liquid and gas, respectively, and On is the Ohnesorge number. This model predicts linear decrease of the thermospray droplets with decreasing the capillary inner diameter, and provides a similar trend to the observed experimental results. Furthermore, this model predicts other observed trends for the effect of the vaporization degree of liquid flow rate on the spray droplets diameter. For any solvent system, with increasing fraction of vaporization, the gas velocity and differential gas
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Figure 8. HRTEM images of thermospray produced MoS2 and ZnS nanocrystals.
pressure would increase, followed by an increase of the Laplace number. This would result in reduced droplet sizes. 3.7. Influence of Pneumatic Assistance Gas Flow on the Spray. The aerosol produced is normally carried by an independent nitrogen flow that theoretically can be optimized without affecting the aerosol production, unlike the case of pneumatic nebulizer. The makeup gas-based pneumatic assistance was found to be important for spreading the droplets and preventing impaction, turbulent, gravitational effects, etc. Furthermore, the added makeup gas can be heated to improve the solvent vaporization efficiency. Thermospray aerosol formation occurs at temperatures and pressures far above ambient, and the emerging jet must equilibrate with the new conditions found in the laboratory. This occurs by cooling through adiabatic expansion, producing supersaturation of the gas phase with solvent vapor. These conditions favor condensation of the vapor. Since the heated section of the stainless steel capillary is located a few cm from the capillary tip, condensation often occurs on the capillary tip itself. This condensation produces a liquid droplet that suffocates the aerosol formation. Sufficient nitrogen gas flow prevents such a scenario. 3.8. Thermospray Produced CdS and PbS Nanocrystals. The aforementioned spray production parameters were optimized for the production of high quality CdS nanocrystals. The TEM analysis, for which two representative micrographs are shown in Figure 6, reveals spherical CdS nanocrystals, with average diameter that varied between 3.1 to 5.6 nm and a standard deviation of as low as 5% (representative size distribution histogram shown in Figure 6B). The CdS nanocrystals exhibit well resolved cubic lattice fringes. Selected area electron diffraction and fast Fourier transform (FFT) preformed on the lattice images confirmed the formation of nanocrystals with a zinc blende cubic structure and a high degree of crystallinity. These results demonstrate the usefulness of this novel technique. Each semiconductor salt should be examined separately for the optimization of its specific required production conditions. The spray production of PbS nanocrystals appears to be less straightforward than that of CdS nanocrystals, and the experimental parameters are not yet fully optimized for controlled production; however, lead sulfide nanocrystals were successfully produced by thermospray. Three individual spray-produced PbS nanocrystals are shown in Figure 7A-C, along with a low magnification image indicating the still relatively large size
distribution (D), and a representative FFT indicating a cubic rock salt structure (E). 3.9. Other Semiconductor Nanocrystals Produced via Thermospray. Preliminary results of MoS2 and ZnS nanocrystals, produced by thermospray are demonstrated in Figure 8. These nanocrystals are not of high-quality yet and the conditions required for their production were not optimized. However, these preliminary results demonstrate the potential of the method for producing such semiconductor nanocrystals. 4. Conclusions A novel thermospray technique for the production of high quality semiconductor nanocrystals was described. This simple, low cost method offers an attractive alternative to conventional production methods, epitaxial growth and colloidal synthesis. Accordingly, solutions of semiconductor salts are first sprayed via thermospray nebulizer into monodispersed droplets, which subsequently become solid nanocrystals by solvent evaporation. Each semiconductor nanocrystal is produced from a single spray droplet upon the full vaporization of the liquid, and the nanocrystals are free-standing, unsupported, and uncoated. The average diameter and size distribution of the final nanocrystals can be controlled and determined by the concentration of the sprayed solution and the droplet size, hence by spray production parameters. Thus, the principles of thermospray nebulization and the experimental parameters governing the spray mean droplet diameters and size distribution were thoroughly described, and their optimization for the production of high-quality semiconductor nanocrystals of the desired size is discussed. The spray droplet number-median diameter was found to shift to smaller particle diameters when increasing the thermospray control temperature and the degree of solvent vaporization, or the liquid flow rate, and when decreasing the capillary inner diameter. A model was suggested for estimating the mean droplet diameter of the determined generated thermospray aerosol droplets, which provides a similar trend to the observed experimental results. Since the nanocrystals size distribution depends also on the solution concentration, information on the solubility levels of various semiconductor salts in different solvents, as measured by atomic absorption spectroscopy, was included. The results indicate that the solubility is related to the solvent polarity, salt bonding characteristic (ionic or covalent), and the solution pH (presence of NH3).
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