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Internally Composite Uniform Colloidal Cadmium Sulfide Spheres Sergiy Libert, Dan V. Goia, and Egon Matijevic´* Center for Advanced Material Processing, Clarkson University, Potsdam, New York 13699-5814 Received May 13, 2003. In Final Form: October 7, 2003 Monodispersed spherical polycrystalline cadmium sulfide particles were synthesized by precipitation in acidic aqueous solutions during the thermally controlled reaction between cadmium nitrate and thioacetamide. The effects of the concentration of the components and of the nitric acid on the colloidal growth dynamics and the properties of the final products were investigated. The higher content of nitric acid in reacting solutions and the higher molar ratio [Cd2+]/[S2-] resulted in larger subunit crystallites and bigger final particle sizes. Furthermore, solids precipitated at higher molar ratios were of better crytallinity and indicated partial cubic crystal lattice of the subunits, while those produced at lower [Cd2+]/ [S2-] ratios displayed the X-ray pattern, characteristic of hexagonal crystal lattice. The size of the crystallite, the type of their lattice, and the size of the final particles affected their reactivity at high temperatures.
Introduction It has been experimentally well-established that many monodispersed colloids of different shapes, prepared by precipitation from homogeneous solutions, are built from nanosized subunits.1-3 There is also sufficient evidence available showing that the formation of such uniform particles is due to the aggregation of preformed primary precursors.4-7 As a rule, most of these dispersions exhibit well-defined X-ray patterns, often characteristic of known minerals, which make it possible to assess the size of the internal crystallites. This finding is in contrast to the generally accepted mechanism proposed by LaMer, according to which highly uniform particles are generated when nuclei, formed by a short-lived burst, grow by the attachment of constituent solutes.8,9 Obviously, a different mechanism had to be considered in order to account for the aggregation processes, which yield uniform dispersions. A model was developed and tested on examples of spherical colloidal gold and cadmium sulfide, which qualitatively produced the expected size selection.10,11 However, refinements are required, both in the experimental approach and in the proposed mechanism, to achieve a quantitative agreement between the two. While the theoretical advances are described in the following paper,12 this work focuses on an improved precipitation technique, the results of which can be more readily interpreted by the proposed aggregation model. * To whom correspondence may be addressed. E-mail:
[email protected]. (1) Matijevic´, E. Langmuir 1994, 10, 8. (2) Matijevic´, E. Chem. Mater. 1993, 5, 412. (3) Sugimoto, T., Ed. Fine Particles Synthesis, Characterization, and Mechanisms of Growth; Marcel Dekker: New York, 2000. (4) Hsu, U. P.; Ro¨nnquist, L.; Matijevic´, E. Langmuir 1988, 4, 31. (5) Bailey, J. K.; Brinker, C. J.; Mecartney, M. L. J. Colloid Interface Sci. 1993, 157, 1. (6) Ocan˜a, M.; Matijevic´, E. J. Mater. Res. 1990, 5, 1083. (7) Goia, D. V.; Matijevic´, E. Colloids Surf. 1999, 146, 139. (8) LaMer, V. K. Ind. Eng. Chem. 1952, 44, 1270. (9) LaMer, V. K.; Dinegar, R. J. Am. Chem. Soc. 1950, 72, 4847. (10) Privman, V.; Goia, D. V.; Park, J.; Matijevic´, E. J. Colloid Interface Sci. 1999, 213, 36. (11) Park, J.; Privman, V.; Matijevic´, E. J. Phys. Chem. B 2001, 105, 11630. (12) Libert, S.; Gorshkov, V.; Goia, D. V.; Matijevic´, E.; Privman, V. Langmuir 2003, 19, 10679-10683.
Figure 1. The change in temperature (a, O), thioacetamide concentration (a, 0), and the total mass (b, 4) and size (b, O) of the CdS formed as a function of time in the system with conditions given in Table 1, scheme T5. Table 1. Concentrations of the Solutions Used in the Precipitation of CdS C2H5SN radius of CdS particles figure scheme Cd(NO3)2, (TAA), [Cd2+]/ HNO3, 3 3 23 no. no. mol/dm mol/dm [S ] mol/dm (1200 s), µm T1 T2 T3 T4 T5 T6
0.0270 0.0270 0.0270 0.0270 0.0270 0.0270
0.0135 0.0270 0.0560 0.0135 0.0270 0.0560
2 1 0.5 2 1 0.5
0.1 0.1 0.1 0.3 0.3 0.3
0.65 0.32 0.53 2.99 2.00 1.40
2 3 4
In all these studies the selection of the investigated system is crucial. First, the reaction leading to the solid phase formation needs to be reasonably simple in order to avoid complex chemical processes, difficult to interpret. Next, the precipitation of uniform particles should proceed at a mild temperature and at a measurable rate. The
10.1021/la030205w CCC: $25.00 © 2003 American Chemical Society Published on Web 11/25/2003
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Figure 2. Scanning electron micrographs of the particles prepared according to scheme T1 (Table 1), after heating the reacting solutions for (a) 510, (b) 720, and (c, d) 1200 s.
formation of CdS in homogeneous solutions satisfies both these conditions. It was demonstrated before13,14 that spheres of a rather narrow size distribution can be obtained e90 °C by decomposition of thioacetamide (TAA) in acidified aqueous solutions of cadmium salts and that their particles are composed of nanosized subunits.13 In a previous study,15 the controlled double jet precipitation (CDJP) technique was employed to produce colloidal CdS by supplying the reagents at given rates. However, theoretical model simulation showed that the initial stages of nucleation and aggregation are rather important in determining the growth dynamics. The peristaltic pumps, used in the CDJP, coupled with conventional mixing, could not provide the necessary homogeneity in the reactor during the exceedingly short initial times, an essential condition for accurate numerical modeling. As a result, (13) Matijevic´, E.; Murphy-Wilhelmy, D. J. Colloid Interface Sci. 1982, 86, 476. (14) Murphy-Wilhelmy, D.; Matijevic´, E. J. Chem. Soc., Faraday Trans. 1984, 80, 563. (15) Libert, S.; Gorshkov, V.; Privman, V.; Goia, D.; Matijevic´, E. Adv. Colloid Interface Sci. 2003, 100-102, 169.
the agreement between the theoretically calculated and experimentally determined width of the size distributions was poor, even though the calculated mean diameter was in good agreement. The technique, described below eliminates the shortcomings of the previously used technique. The precipitation of CdS was achieved by continued slow heating of an aqueous solution of cadmium nitrate and thioacetamide (TAA); the latter releases sulfide ions in an acidic environment in an irreversible manner. In doing so, the uneven concentration distribution of reactants, due to external flow of continuously introduced solutions was avoided, while the volume of the system remained constant. The so obtained dispersions were of much narrower size distributions as compared to those obtained by the CDJP process. It should be recognized that in employing the described technique one has to account for the change in the release rate of sulfide ions, thus affecting the degree of supersaturation. However, these conditions can be taken into consideration in the application of the theoretical model. Indeed, it was possible
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Figure 3. The same as Figure 2 for the scheme T4 (Table 1).
to show that the experimental results obtained by the described experimental technique can be theoretically well accounted for.12 Experimental Section Precipitation Procedure. In this study 0.1 dm3 of aqueous solutions containing Cd(NO3)2, thioacetamide (TAA), and nitric acid (HNO3), in predetermined concentrations (as given in Table 1), was slowly heated according to the graph in Figure 1a, whose profile was shown to be reproducible. Samples were then withdrawn with a microsyringe at different times (510, 720, and 1200 s) and rapidly filtered using membranes, having pore size of 0.22 µm. The separation process under vacuum took less than 3 s. The membrane with the colloidal solids was then pasted on the aluminum holders for scanning electron microscopy. Characterization. The morphology, the structure, and the composition of the particles were elucidated by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), energy dispersive spectroscopy (EDS), and thermal gravimetric analysis (TGA). The crystallite sizes were calculated from XRD data by means of the Scherrer equation using the peak with Miller index (103), i.e., 2Θ ≈ 47.5°. The line broadening due to crystallite size was determined and corrected for the instrumental peak broadening
by the Pythagorean function with aluminum as standard. For the TGA measurements the carrier gas was a N2/air mixture (1:1 volume ratio) and the heating rate was 10 °C‚min-1. The amount of cadmium sulfide precipitated at any given time was obtained indirectly by determining spectrophotometrically the concentration of the TAA in the reactor. For this purpose, every 60 s 1 cm3 of the reacting system was withdrawn with a syringe and diluted 200 times with deionized (DI) water. The absorption of the diluted solution was measured at λ ) 261 nm, within approximately 8 s after collection. The mean size of the particles at given times was determined by the scanning electron microscopy.
Results Figure 1 demonstrates the dynamics of key parameters in the synthesis of colloidal CdS particles according to the scheme T5 (Table 1). The mass of CdS generated in the system (Figure 1b) can be calculated from the TAA concentration (Figure 1a) left in the reactor, taking into consideration the low solubility of CdS and assuming that the amount of sulfide ions, which possibly escaped the reactor as H2S, to be negligible. Figures 2, 3, and 4 display SEM images of the CdS particles obtained in schemes T1, T4, and T5, respectively,
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Figure 4. The same as Figure 2 for the scheme T5 (Table 1).
at different precipitation times (a, 510; b, 720; c, 1200 s). In all SEM micrographs (panel d) a higher magnification of the final particles is shown, which offers a clearer view of the surface morphology. Figures 2 and 3 demonstrate that increasing the concentration of HNO3 from 0.1 to 0.3 mol‚dm-3 yields much larger CdS spheres with a significant difference in the appearance of the surface. The preparation of dispersions in Figure 3 (system T4) and Figure 4 (system T5) differs in terms of reactant concentrations at the same HNO3 content, which shows that the molar ratio [Cd2+]/[S2-] affects both the size and the surface roughness of the particles. The change in the particle radius with time for the five selected samples, listed in Table 1, is shown in Figure 5. The increase in the diameter of the particles is more pronounced at higher acidity, while a higher [Cd2+]/[S2-] molar ratio translates in a larger final particle size. The size histograms of the precipitated particles of sample T5, collected at different times, as evaluated from electron micrographs, are displayed in Figure 6; although the mean radius increases, the width of the distribution remains essentially the same.
Figure 5. The change of the radius of CdS particles with time of samples described in Table 1.
The energy-dispersive X-ray spectroscopy (Figure 7) indicated no presence of impurities in the CdS particles, while the X-ray diffraction patterns (Figure 8) of all samples were characteristic of minerals greenockite and hawleyite.16-18 The calculated crystallite sizes using the (16) Natl. Bur. Stand. (U.S.) Circ. 1995, No. 539, 4, 15. (17) Dana’s System of Mineralogy, 7th ed.; Wiley: New York, 1.
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Figure 9. Thermogravimetric analysis for particles prepared according to schemes T2 and T5 (Table 1). Figure 6. The size distribution histograms for particles prepared according to scheme T5, collected at 510, 720, and 1200 s, as determined by SEM.
Figure 7. Energy dispersive spectroscopy of CdS particles, which was typical for all samples.
Figure 10. SEM image of cadmium oxide particles after decomposition at high temperature during the TGA analysis (T5). Table 2. Estimated Crystallite Sizes for Six Samples, Described in Table 1 crystallite size, Å
Figure 8. The X-ray diffraction patterns of the final particles, prepared under conditions described in Table 1.
Scherrer equation are summarized in Table 2. Obviously, the subunits become smaller as the molar ratio [Cd2+]/ [S2-] and the concentration of nitric acid decrease. This correlation is similar to the one observed for the overall size of the colloids. The thermogravimetric analysis for samples T1-T6 (Figure 9) yields significantly different profiles. It appears (18) Trail, R.; Boyle, R. Am. Mineral. 1955, 40, 555.
T1
T2
T3
T4
T5
T6
370
160
50
440
310
145
that, when heated, larger CdS particles (T4-T6) do not gain weight before the solids are decomposed >700 °C. In contrast, the smaller particles (T1-T3) start to gain weight >500 °C before the outset of decomposition at higher temperature. In all cases, however, when the decomposition process is concluded (∼1100 °C) the original CdS spheres are converted into highly crystalline, partially sintered particles (Figure 10), the EDS and XRD analyses of which (Figure 11) show them to be cadmium oxide (CdO) of isometric (hexoctahedral) class. Discussion The preparation method described in this study yielded spherical CdS particles of rather narrow size distribution, all clearly exhibiting composite substructures. At the same reaction times, the increase in the concentration of nitric acid favors the formation of larger substructural crys-
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Figure 11. The XRD pattern of the particles shown in the Figure 10, which is characteristic for natural cadmium oxide (CdO) mineral monteponite.
tallites and of final CdS spheres. This is a rather unexpected result, since at such conditions one would expect a more rapid decomposition of the TAA, resulting in a faster release of S2- ions and, therefore, smaller crystallites and final particles. However, the increased acidity of the system also causes a decrease in the concentration of free sulfide ions and, therefore, a lower degree of supersaturation, due to the higher conversion rate of S2- to H2S and HS-. The molar ratio of [Cd2+]/[S2-] affects both the size of the crystallite subunits and the size of the final particles. In this case one must consider the sulfide ions release rate, which depends on the concentration of TAA in the reacting solution. At a higher concentration of thioacetamide, or the lower [Cd2+]/[S2-] ratio, the ion release rate is also higher, resulting in the formation of greater number of condensation centers, which then grow to smaller primary particles, as seen in Table 2. The effect on the size of the final spheres (Figure 5) is again due to
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a similar effect. The higher concentration of primary particles yields a greater number of aggregation centers, which then grow to smaller final spheres, as demonstrated by the micrographs of Figures 3 and 4. The molar ratio [Cd2+]/[S2-] also influences the internal structure of the particles, as evident from the X-ray diffractograms in Figure 8. There are two natural CdS minerals, greenockite and hawleyite; the former has hexagonal (dihexagonal pyramidal class) crystal structure, while the second one is isometric (hextetrahedral class). In general, the precipitation of CdS yields preferentially solids with a hexagonal structure. However, it appears that with an increase in the [Cd2+]/[S2-] ratio the solids of mixed crystal lattices are formed. The size and the internal structure of the precipitated CdS have a pronounced impact on the dynamics of their interaction with oxygen at elevated temperatures. As Figure 9 shows, a weight gain is observed when smaller particles, consisting of smaller crystallites, are heated to >500 °C. In contrast, during the heating of larger particles no such weight gain occurs, and the dynamics of the process is consistent with the straight conversion of CdS to CdO at 700 °C. Regardless of the nature of the transformation at intermediate temperatures, in all cases the solids are converted at high temperature (>1100 °C) into highly crystalline CdO particles. The details and development of theoretical model as well as results of its simulation are described in the paper that follows.12 Acknowledgment. This research has been supported by the National Science Foundation (Grant DMR-0102644) and by the donors of the Petroleum Research Fund, administered by the American Chemical Society (Grant 37013-AC5,9). LA030205W
