pubs.acs.org/Langmuir © 2009 American Chemical Society
Evaporation Driven Self-Assembly of a Colloidal Dispersion during Spray Drying: Volume Fraction Dependent Morphological Transition D. Sen,*,† S. Mazumder,† J. S. Melo,‡ Arshad Khan,§ S. Bhattyacharya,^ and S. F. D’Souza‡ †
Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India, ‡Nuclear Agriculture and Biotechnology (NA & BT) Division, Bhabha Atomic Research Centre, Mumbai 400085, India, §Radiological Protection and Advisory (RP&A) Division, Bhabha Atomic Research Centre, Mumbai 400085, India, and ^ Technical Physics and Prototype Engineering (TP & PE) Division, Bhabha Atomic Research Centre, Mumbai 400085, India Received January 14, 2009. Revised Manuscript Received February 23, 2009 Morphological transition of droplets during evaporation driven self-assembly of colloidal dispersion of alumina particles has been investigated. It was found that a sphere to doughnut-like transition of the droplet morphology takes place even when the rate of drying remains moderate and is not extremely fast. Further, it has been seen that such transition is strongly dependent on the volume fraction of the colloids in the droplets. The transition proceeds via buckling of the initial spherical droplets, which occurs when the capillary forces driving the deformation overcomes the interparticle electrostatic forces. However, the transition is hindered and the buckling probability is reduced due to the inherent spatial constraint when the colloid volume fraction is increased. Mesoscopic structures of the assembled grains have been investigated by scanning electron microscopy, small-angle neutron scattering, and dynamic light scattering techniques. Interestingly, it has been observed that the functionality of photoluminescence spectrum of the dried nanoporous grains depends somewhat on the grain morphology.
1. Introduction For long time, spray drying has been considered as an indispensable industrial process. This technique is widely used in food, pharmaceutical, ceramic, polymer, chemical, and various other industries to obtain dry particles from solution phase.1,2 Since the past decade, this method has re-embellished itself by finding a special place in science and technology.3-9 This is primarily due to two reasons: first, to understand the evaporation driven selfassembly of colloids at various physicochemical and thermodynamical conditions; second, to synthesize novel nanocomposites, colloidal crystals, ordered mesoporous materials by templating, etc.10-13 It has been observed that organized grains of nanoparticles/colloids prepared by the spray drying technique often lead to various peculiar nonspherical morphologies like doughnut, mushroom, etc. In drying process of a small droplet, containing nanoparticles, evaporation drives the shrinkage of the droplets *Corresponding author. E-mail:
[email protected]. (1) Masters, K. Spray Drying Handbook, 5th ed.; Longman Scientific & Technical: England, 1991. (2) Masters, K. Spray Drying in Practice; Spray Dry Consult International ApS: Charlottenlund, Denmark, 2002. (3) Tsapis, N.; Dufresne, E. R.; Sinha, S. S.; Riera, C. S.; Hutchinson, J. W.; Mahadevan, L.; Weitz, D. A. Phys. Rev. Lett. 2005, 94, 018302. (4) Tsapis, N.; Bennett, D.; Jackson, B.; Weitz, D. A.; Edwards, D. A. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12001. (5) Iskandar, F.; Gradon, L.; Okuyama, K. J. Colloid Interface Sci. 2003, 265, 296–303. (6) Vehring, R.; Foss, W. R.; Lechuga-Ballesteros, D. Aerosol Sci. 2007, 38, 728–746. (7) Messing, G. L.; Zhang, S.-C.; Jayanthi, G. V. J. Am. Ceram. Soc. 1993, 76 (11), 2707–26. (8) Sen, D.; Spalla, O.; Belloni, L.; Charpentier, T.; Thill, A. Langmuir 2006, 22, 3798–3806. (9) Sen, D.; Spalla, O.; Tache, O.; Haltebourg, P.; Thill, A. Langmuir 2007, 23, 4296–4302. (10) Cagnol, F.; Grosso, D.; Soler-Illia, G. J.de A. A.; Crepaldi, E. L.; Babonneau, F.; Amenitsch, H.; Sanchez, C. J. Mater. Chem. 2003, 13, 61–66. (11) Wang, D.; Mohwald, H. J. Mater. Chem. 2004, 14, 459–468. (12) Iskandar, F.; Mikrajuddin; Okuyama, K. Nano Lett. 2001, 1, 231–234. (13) Lyonnard, S.; Bartlett, J. R.; Sizgek, E.; Finnie, K. S.; Zemb, T.; Woolfrey, J. L. Langmuir 2002, 18, 10386–10397.
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and the constituent particles gets assembled via capillary forces (which appears because of the presence of wetting liquid between solid particles). It has been shown9 that when the rate of drying is slow enough, the droplet shrinks in an isotropic manner and the final grain remains spherical even after drying. However, if the drying speed is fast enough, the deformation forces overcome the electrostatic forces stabilizing the particles and the deformation occurs, which leads to a nonspherical shape of the final grain.3-5 The morphology of the synthesized grain can be tuned by the drying conditions and also by the physicochemical properties of the drying medium. For example, surfactant has been proposed to be favorable in producing doughnut-like grains.14 Further, tuning the interactions between the particles,3 the buckling and vis-a-vis the final grain morphology may be adjusted. It has been indicated that several factors like concentration of the particles inside the initial droplet, hydrodynamics of drying, surface tension, droplet size, etc., influence on the sphericity of the final dried grains.5 In addition to the direct imaging techniques like electron microscopy, scattering techniques like small-angle X-ray (SAXS) and neutron scattering (SANS) are very useful in probing the mesoscopic structures in hierarchically structured and assembled materials or composites, prepared either by top-down or by bottom-up strategies.8,9,15-19 In this paper, we report the evaporation driven self-assembly of colloidal alumina particles during spray drying for varying concentration of alumina in the dispersion. It will be shown that by solely varying the concentration of the colloidal particles in the (14) Velev, O. D.; Lenhoff, A. M.; Kaler, E. W. Science 2000, 287, 2240–2243. (15) Oberdisse, J.; Deme, B. Macromolecules 2002, 35, 4397–4405. (16) Berret, J.-F.; Vigolo, B.; Eng, R.; Herve, P.; Grillo, I.; Yang, L. Macromolecules 2004, 37, 4922–4930. (17) Frankamp, B. L.; Boal, A. K.; Tuominen, M. T.; Rotello, V. M. J. Am. Chem. Soc. 2005, 127, 9731–9735. (18) Berret, J.-F. J. Chem. Phys. 2005, 123, 164703. (19) Fresnais, J.; Berret, J. F.; Qi, L.; Chapel, J.-P.; Castaing, J.-C.; Sandre, O.; Frka-Petesic, B.; Perzynski, R.; Oberdisse, J.; Cousin, F. Phys. Rev. E 2008 78, 040401R.
Published on Web 03/26/2009
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initial dispersion, a transition from spherical shape to doughnut shape of the grains has been observed even at the intermediate rate of drying. A possible mechanism for this transition is also discussed.
2. Experimental Section i. General. Initial colloidal dispersion was 10% alumina (Aldrich). This dispersion was diluted with pure water
Figure 1. Schematic diagram of the spray dryer setup.
(Milli-Q, Millipore) to make 5%, 2%, and 1% dispersions, and finally these diluted dispersions were used for spray drying. ii. Spray Drying. Spray drying of the colloidal dispersions was carried out using the spray drier LU222, Laboratory Ultima, India. The schematic diagram of the spray drier is depicted in the Figure 1. The spray drier consists of cylindrical drying chamber of 10 cm diameter and of 60 cm length. The droplet was generated using compressed air spray nozzle. The dispersion was fed to the nozzle using a peristaltic pump for the atomization of the liquid. The feed was fixed at 2 mL/min. The inlet temperature was kept at 160 C. The aspiration value was kept at 47 m3/h. The dried powders were collected from both the cyclone separators and from the drying chamber itself. The powder, collected from the cyclones, for 5%, 2%, and 1% colloid dispersions are designated as Cy-5, Cy-2, and Cy-1, respectively, while those collected from the drying tube are designated as Tu-5, Tu-2, and Tu-1, respectively (please see the Supporting Information). iii. Characterization of the Powder Grains. The dried grains were characterized using scanning electron microscopy (SEM), small-angle neutron scattering (SANS), dynamic light scattering (DLS), and photoluminescence spectroscopy (PL). SEM micrographs were obtained using VEGA, TeScan instrument. SEM micrographs for 5%, 2%, and 1% samples (CY-5, CY-2, and CY-1), collected in the cyclones, are shown in Figures 2, 3, and 4, respectively. The SEM micrographs for the samples collected in the drying tube (TU) (please see Supporting Information) do not show any regular morphology because
Figure 2. SEM micrograph for spray dried alumina colloids (5%) collected in cyclone.
Figure 3. SEM micrograph of spray dried alumina colloids (2%) collected in cyclone. Langmuir 2009, 25(12), 6690–6695
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Figure 4. SEM micrograph of spray dried alumina colloids (1%) collected in cyclone.
Figure 6. XRD pattern of spray-dried powder (CY-5).
Figure 5. SANS profiles of the spray-dried samples in doublelogarithmic scale. The inset below at the left side shows the data in Porod plot, while the inset at the right top shows the fit of the SANS model to the CY-5 data. droplets in this case hit the drying chamber much before the complete drying. Small-angle neutron scattering (SANS) experiments were performed using a double-crystal-based small-angle neutron scattering facility at the Guide Tube Laboratory of Dhruva reactor at Trombay, Mumbai, India.20 The scattered intensities have been recorded as a function of wave vector transfer q [= 4π sin(θ)/λ, where 2θ is the scattering angle and λ (= 0.312 nm) is the incident neutron wavelength]. The specimens under SANS investigations were placed on a sample holder with a circular slit of 1 cm diameter. Measured SANS profiles have been corrected for background, transmission, and instrument resolution.21 SANS data are shown in Figure 5. The same data are also represented as Porod plot in the inset. The XRD pattern of CY-5 sample is shown in Figure 6. DLS experiments have been performed at scattering angle of 90 using a laser of wavelength of 532 nm and of power of 100 mW. The DLS data for the CY samples are depicted in Figure 7. The inset shows the size distributions of the grains as obtained from the DLS measurements. DLS data from CY and TU samples look very different, particularly at large correlation (20) Mazumder, S.; Sen, D.; Saravanan, T.; Vijayaraghavan, P. R. J. Neutron Res. 2001, 9, 39–57. (21) Lake, J. A. Acta Crystallogr. 1967, 23, 191–194.
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Figure 7. Dynamic light scattering (DLS) data for the CY samples in double-logarithmic scale. time and indicate some sort of non-Euclidean morphology22-25 for the TU samples unlike the CY samples (please see Supporting Information). (22) Mandelbrot, B. B. The Fractal Geometry of Nature; W.H. Freeman & Co. Ltd.: New York, 1982. (23) Mazumder, S.; Sen, D.; Patra, A. K.; Khadilkar, S. A.; Cursetji, R. M.; Loidl, R.; Baron, M.; Rauch, H. Phys. Rev. Lett. 2004, 93, 255704. (24) Mazumder, S.; Sen, D.; Patra, A. K.; Khadilkar, S. A.; Cursetji, R. M.; Loidl, R.; Baron, M.; Rauch, H. Phys. Rev. B 2005, 72, 224208. (25) Mazumder, S.; Loidl, R.; Rauch, H. Phys. Rev. B 2007, 76, 064205.
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distribution and correlation of the colloids get somewhat modified with amount of alumina concentration in the droplet. In order to have an idea about the size distribution of the basic constituents, i.e., alumina particles and their correlations inside the grains, the SANS data were analyzed in the light of assembled grain model.8,9,26 It is worthy to mention that this model assumes the fact that the final grains are produced via isotropic shrinkage of the droplet and all the final grains have constant packing fraction. For the two level structures as the present case, the differential scattering cross section per unit of solid volume is given by8,9,26 IðqÞ ¼
Figure 8. Luminescence spectra of the CY samples. Photoluminescence measurements were carried out with an Edinburgh Instruments FLSP 920 setup with a 450 W Xe arc lamp as the excitation source and a red-sensitive Peltier element cooled Hamamatsu R-2658 PMT as the detector. Samples were mixed with methanol and ground well, and the slurry was spread over a glass slide and dried; the same is incorporated into the sample chamber of the luminescence setup. The emission spectra were recorded with a resolution of 3.0 nm by exciting the samples at 220 nm. All the emission patterns were corrected for the detector response. Figure 8 shows the luminescence spectra for the CY samples.
3. Results and Discussion From the SEM micrographs (Figures 2-4), it is evident that the powder grains for the samples CY-5, -2, and -1 possess some definite Euclidean shape (spherical for CY-5 and doughnut like for CY-2 and CY-1). Further, when compared carefully the morphology of the grains for the CY samples, it is observed that the majority of the assembled grains are almost spherical in shape in the case of CY-5 (where the concentration of alumina colloids is more (5%)), while most of the grains for CY-2 and CY-1 (where the concentration of alumina colloids is 2% and 1%, respectively) samples are doughnut-like and far away from the sphericity. Further, in the case of CY-1, a few multifaced doughnuts are also observed. These observations clearly indicate that the concentration of the colloidal particles in the initial liquid droplet (which is spherical in shape for obvious reason) plays an important role in deciding the sphere to doughnut-like morphological transition of the grains during the drying process. We will discuss this issue in more detail in the later part of this section. Here, we will concentrate on the mesoscopic characterization of these grains. From Figure 5, it is discernible that for all the samples the SANS profiles within the accessible q range (0.003-0.173 nm-1) can be subdivided, for clarity, into two distinct zones, namely, zone I and zone II. As the real space and the reciprocal space are connected by Fourier transform, zone I (for q range ∼0.003-0.04 nm-1) primarily bears the information about larger structure (the overall size of assembled grains) while zone II (for q range ∼0.04-0.173 nm-1) mainly contains the information about the smaller density fluctuations, i.e., the morphology and the correlations of the alumina colloids. Although the scattering profiles for the samples are nearly identical in nature, particularly in the intermediate zone, some slight variations in functionality of the profiles in zone II and sharpness of the profiles in the very low q regime of zone I cannot be totally ignored. This indicates that although the size and of alumina particles remain mostly unaffected, the overall grain size or size Langmuir 2009, 25(12), 6690–6695
ðφA ΔFA 2 IA ðqÞ þ vg ΔFg 2 Pg ðqÞÞ φA
ð1Þ
where φA is the volume fraction of alumina in a grain of volume vg. If FA is the scattering length density of alumina, then ΔFA = FA - Fint (Fint being the scattering length density of interstices between alumina colloids). As interstices between alumina colloids is air Fint = 0, and hence ΔFA can be taken equal to FA. ΔF2g represents the average scattering contrast of the grains; hence ΔFg= φAΔFA + ΔFint. As the medium in which the grains are embedded and the medium of the interstices are both air in this case, ΔFint = 0 and thus ΔFg = φAΔFA. Pg(q) is the normalized form factor for the grains. R¥ PA ðq, rÞv2 ðrÞSðq, rÞ dr ð2Þ IA ðqÞ ¼ 0 R ¥ 0 vðrÞDðrÞ dr S(q,r) is the interparticle structure factor. D(r) represents the size distribution of the colloids. PA(q,r) represents the normalized form factor for colloids with radius r. The parameters obtained from SANS analysis are tabulated in Table 1. The fit of the model to the data is shown in the inset of Figure 5. In order to avoid the overlapping of various data, the fit is shown only for one sample as the functionalities of profiles for the other two samples are not very different. The average size of the crystallites inside the alumina particles was also calculated from the width of the XRD profile (Figure 6) using the Scherer formula27 and was found to be ∼7 nm. From SANS and XRD results, it is obvious that each of the alumina colloidal particle consists of several smaller crystallites. Further, scattering results suggest that the jammed grains are nanoporous in nature. As the grains consist of alumina particles of ∼32 nm in radius (results from SANS give a radius of ∼32 nm which also corroborates with that obtained from the image analysis of SEM micrograph, which gives a radius of ∼35 nm) with a packing fraction of ∼0.48, this indicates that the radius of the interparticle pores remains nearly 25 nm. Dynamic light scattering (DLS) experiments were performed in order to access further low q (which becomes difficult in case of SANS because of instrument resolution) vis-a-vis the higher size of the grains. It is worthy to mention here that DLS measures the temporal fluctuations of the scattered intensity in a time regime of ∼ 0.5 ns to few milliseconds. The scattered electric field time-autocorrelation function, g(1)(t), measured in the DLS experiment is proportional to the time autocorrelation function of the fluctuations in the refractive index, i.e. gð1Þ ðtÞ ¼ Æδnðq, 0Þδnðq, tÞæ where δn(q,t) is a Fourier transform of refractive index fluctuation ∂n(r,t) at position r in time t. g(1)(t) can be written as (26) Thill, A.; Spalla, O. J. Colloid Interface Sci. 2005, 291, 477–488. (27) Patterson, A. L. Phys. Rev. 1939, 56, 978–982.
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Sen et al. Table 1. Parameters Obtained from Fitting of Model to the SANS Data
sample CY-5 CY-2 CY-1
effective packing fraction of the alumina particles in the grains
mean of radius distribution for alumina particles (nm)
standard deviation of size distribution for alumina particles (nm)
average grain size (nm)
0.49 0.48 0.49
32 32 32
8 8 8
>1000 >1000 >1000
the Laplace transform of the distribution (G(Γ)) of the relaxation rates Γ Z ¥ GðΓÞ expð -ΓtÞ dΓ gð1Þ ðtÞ ¼ 0
For relaxation time τ, g (t) is expressed as Z ¥ ð1Þ g ðtÞ ¼ AðτÞ expð -t=τÞ dτ (1)
0
where τA(τ) ΓG(Γ) and D = Γav/q0 2 = KBT/3πηdh (q0 is the wave vector transfer, KB is the Boltzmann constant, and η is the viscosity of the solvent at the absolute temperature T. dh is the hydrodynamic diameter). To obtain τA(τ), DLS data were analyzed using CONTIN method.28 It is seen from the inset of the Figure 7 that the grain size distributions, obtained from DLS, for the CY-1, CY-2, and CY-5 samples do not follow a normal trend of continuous droplet shrinkage with decrease in the volume fraction of alumina in the droplet. It is noteworthy to mention at this stage that for an isotropic shrinkage of a droplet radius (R) is proportional to the cube root of the volume fraction (Φin) of colloidal particles in the initial droplet (i.e., R Φ1/3 in ) when all the droplets are of same size and constant packing fraction of the final grains. However, the situation becomes more complex when initial droplets are polydisperse in nature and final droplets do not have the constant packing fraction. The peak positions of the size distributions of dried and assembled grains for CY-1, CY-2, and CY-5 are found to be 450, 1612, and 780 nm, respectively. If the shrinkage of the droplets during drying would have been isotropic in nature for all the CY samples, as normally observed for slow drying process,8,9,29 the trend in final size distribution should have shown a unidirectional increasing trend with increase in colloid concentration. However, this is not observed from the DLS results. This can be attributed to the fact that the shrinkage process, during drying, significantly differs with colloid concentration and may not be exactly isotropic in nature. At this juncture, it is important to mention two of the following points. First, DLS data for each sample were collected for nearly 10 s, and five repeated DLS measurements were performed with care on each sample in order to avoid the doubt regarding the settling effect of the grains. It was found that the results from all measurement were consistent within 3% error level. Second, although the DLS assumes spherical grains for the quantitative analysis, the drag force, in reality, remains morphology dependent; i.e., the diffusion of a spherical grain will be different than that of a doughnut-like grain of identical volume at a certain temperature. Now, we will discuss the sphere to doughnut-like morphological transition in detail that has been observed (Figures 2-4) in the present case by solely varying the concentration of the alumina particles in the droplets. It has been conjectured3,5 that the deformation during the spray drying process may occur due to various factors like thermodynamical instabilities, hydrodynami(28) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 229. (29) Eslamian, M.; Ahmed, M.; Ashgriz, N. Nanotechnology 2006, 17, 1674– 1685.
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cal instabilities, or particle-particle interactions in the drying droplet. Kinetics of drying drives suspensions far from equilibrium, leading to dramatic changes in morphology as solvent evaporates. The rate of drying is a key feature in this regard. The quantitative measure of the strength of drying is represented by Peclet number (Pe). This is defined as the ratio of R2 and Dτ, where R is the radius of the droplets, D is the diffusion coefficient of the colloidal particles in the droplet, and τ is the time of drying. If Pe , 1, the drying process is regarded as slow process and the droplets shrink isotropically throughout the drying process;8 however, for Pe . 1, the drying is fast enough, and there is a possibility of formation of hollow particles during drying depending on the other physical parameters. For Pe ∼ 1 the drying is in the intermediate stage. D can be calculated from the EinsteinStokes relation, D = KBT/6πηr, where KB is Boltzmann constant, η is the viscosity, and r represents the radius of the colloids. For T = 40 C, r = 35 nm (for the alumina colloids as observed from SANS and SEM), and η = 6.53 10-4 Pa s (assuming viscosity of pure water at 313 K) D comes out to be 1.003 10-11 m2/s. The drying time (τ) can be calculated from the tube geometry (cross section and length) and the gas flow rate. In the present case, the velocity of the droplets was calculated to be ∼1.666 m/s (from the ratio of the volume of gas entering in the tube per second and the cross section of the drying tube), and hence the drying time in a 60 cm long drying chamber is ∼0.36 s. From these data, the Peclet number, Pe, becomes equal to 6.9 (assuming ∼10 μm size of the initial liquid droplets), which is of course more than unity but is below 10. This suggests that in the present case the rate of drying lies is in intermediate stage and is neither very fast nor very slow. It is noteworthy to mention that for a very fast drying process Pe may be as large4 as 2000. However, the present morphological transition indicates that even at this intermediate rate of drying the buckling process takes place. From these above observations, it becomes evident that although initially the droplets behave like pure liquids and shrink isotropically, eventually the formation of a viscoelastic shell of densely packed particles takes place at its surface. This occurs due to a thermophoretic force, which originates from the temperature gradient at the surface of the droplet and moves the colloids at the air-water interface. Initially, such shell is produced and gets thicker as the droplet shrinks. However, at a certain instant, the capillary forces driving the deformation of the shell overcome the electrostatic forces stabilizing the colloidal particles. The shell becomes elastic and undergoes a sol-gel type transition and then buckles. This gives rise to doughnut-like particles with a central hole. Moreover, this process depends significantly on the volume fraction (Φ) of the alumina particles in the droplets. If the number density of the particles in the droplets is less, the above process becomes more favorable than the situation where the number density of the colloidal particles is more. In the second case, the buckling probability is reduced because of the fact that the total solid concentration in the droplet hinders significantly the buckling process because of the inherent constraints of availability of space. The situation has been explained more clearly by a schematic representation as depicted in Figure 9. Further, from Figure 8, it is interesting to note that the shape of the photoluminescence (PL) spectra from CY-5, CY-2, and CY-1 Langmuir 2009, 25(12), 6690–6695
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Figure 9. Schematic diagram of drying process for small and large volume fractions of particles.
samples are also different. In particular, the spectrum is somewhat broader in case of the CY-5 sample in comparison to that of the CY-2 and the CY-1 samples. This may be attributed to effect of the morphology-dependent strain which is present in the powder grains. However, such incomprehensible observation on PL is yet to be understood in detail from the theoretical point of view. Here, we would like to mention two important aspects. First, the solvent, in which the colloids are dispersed, also may determine the rate of drying depending on its evaporation rate at a particular temperature and hence may be an important factor in tuning such morphological transition. In our present case the solvent has been water. However, in the near future, the effect of variation of nature of solvent on such kind of morphological transitions will be attempted within experimental limitations. Second, the size of the initial droplets may also play a role in determining such transitions. However, one of the important challenges in this direction is to generate exactly monodisperse droplets with reasonable generation efficiency. In the next step, the effect of droplet size on such transitions will also be attempted within some technical limitations.
4. Conclusions Volume fraction dependent morphological transition from spherical to doughnut-like shape of self-assembled grains of colloidal alumina was observed. Even at the moderate rate of drying, a buckling process occurs via the formation of a viscoelastic shell of densely packed particles and eventually leads to a morphological transition when the volume fraction of the colloids remains small. However, for more volume fraction of the colloids,
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the buckling probability is reduced due to the inherent constraints on space availability and the morphological transition is hindered. The shape of the photoluminescence spectrum depends significantly on the morphology of the nanoporous grains and may be attributed to the morphology dependent strain present in the powder grains. The present study indicates that the sphere to doughnut-like morphological transition during evaporation driven self-assembly of nanoparticles should be brought into closer scrutiny for various colloidal systems in the future in order to approach a unique evolution model for the self-assembly kinetics during spray drying. In the near future, the effects of solvent and initial droplet size/size distribution on such morphological transitions will be attempted within experimental limitations. Further, the present study also calls for a theoretical model to explain the shape dependence of photoluminescence spectrum for nanoporous grains of assembled colloids. Acknowledgment. The authors thank Dr. V. Sudarsan of ChD, BARC, and Dr. G. Ghosh of CSR-UGC-DAEF, Mumbai, for their kind help in PL and DLS measurements, respectively. D.S. thanks Dr. Antoine Thill and Dr. Olivier Spalla of LIONS, SCM, DRECAM, CEA Saclay, France, for many fruitful discussions on spray drying during his visit at CEA Saclay, France. Supporting Information Available: SEM micrographs of the TU samples and comparison of the DLS data from CY-5 and TU-5 samples. This material is available free of charge via the Internet at http://pubs.acs.org.
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