Langmuir 1992,8, 1475-1478
1475
Colloidal CdS Preparation via Flow Techniques: Ultrasmall Particles and the Effect of a Chromatographic Column Christian-Herbert Fischer’ and Michael Giersig Hahn-Meitner-Imtitut Berlin GmbH, Bereich S, Glienicker Strasse 100, 0-1000 Berlin 39, FRG Received December 27, 1991. In Final Form: February 25,1992
A fast flow technique is described in which two solutions containing Cd2+and SH- ions are mixed, and the absorption spectrum of the resulting CdS sol is rapidly recorded using a diode array detector. Experimente were also performed in which the sol passed through a high-performance liquid chromatography column between the mixing chamber and diode array. The colloidal particles formed with and without the column had practically the same absorption spectrum (very small particles with a diameter down to 1.3 nm, strongly quantized). However, the particles formed using the column grew thermally much more slowly. The effect is attributed to the existence of “active” particles after the precipitation which promote the thermal growth. The active particles are filtered out by the column. With increasing Cd2+/SH- concentration ratio in the mixing chamber, smaller CdS particles are formed. In the most extreme cases studied, new species with a strong absorption maximum at 211 nm were obtained and their size (1.5nm) was determined by size exclusion chromatography, the calibration of which was carried out by transmission electron microscopy. The flow technique is useful for rapid optimization of the precipitation conditions as well as for easy preparation of very small particles with extremely narrow size distribution.
Introduction High-pressure size exclusion chromatography has recently been shown to be useful for the fractionation and determination of the size of CdS sols.’ In the present paper, the influence of a chromatographic column in the preparation of CdS sols is described. A fast technique has been developed to mix Cd(ClO4)eand NaSH solutions, and the mixed solution is directed rapidIy into a column containing standard chromatographic material. The original idea was to push the mixed solution into the column in a time shorter than the time required for complete precipitation, hoping that compartmentation effects in the column would influence the growth of the particles. However, an unexpected phenomenon was observed whereas the column did not change the initial particle size considerably, the subsequent aging of the particles was strongly retarded (as compared to the aging of a sample produced under the same mixing conditions but without subsequent passage through the column). Experimental Section (a) Preqaration of Colloids. Figure 1 shows schematically the experimental arrangement. In most of the cases, only two eolutionewere mixed. Solution1 containedCd(ClOd2and sodium polyphosphate (Riedelde Hien), and solution 2contained NaSH and polyphosphate. The solutionswere bubbled with argon and driven by pumps PI and Pz, respectively (Merck-Hitachi,type L 6OOO), which transported the liquids at a constant flow rate independently of each other and of the flow resistance of the system. By variation of the flow rates, various CdZ+/SH-ratios and total flow rates could be established. The solutions were mixed in the minute volume of the stainlesssteelmixing chamber (Knauer, type A 120;0.1 mm inner diameter of the capillaries). The mixed solution then reached the column within 0.01 s (column: Knauer; Lichroepher Alox T (Merck), 5 pm; 25 cm length; 4 mm i.d.) and the photodiode array detector (Waters, type 990). In someexperimenta,a capillarywas inserted between the mixing chamber and the column in order to prolong the lifetime of the colloidal particles formed before entering the column. The detector allowed the rapid recording of the (1) (a) Fischer, Ch.-H.; Weller, H.; Kataikas, L.; Henglein, A. Langmoir 1989,5,429. (b) Henglein, A. Chem. Reu. 1989,89, 1861.
diode-array-detector oolvohorohate
grat,ing
U water
‘b
1 /
ohotosensitive
z cuvette
fi
Figure 1. Schematic description of the experimental arrangement. absorption spectrum less than a second after the passage of the solution through the column (depending on the flow rate). The solution was finally collected in a quartz cuvette, in which the spectrum could be measured at various times afterward using a conventionalspectrophotometer. Pumps Ps and PCwere operated occasionally. P3 was used to pump a polyphoephate or NaOH solution into the mixing chamber in order to dilute or vary the pH, respectively,of the mixture of solutions1 and 2. P d was used to pump water througha valve (Rheodyne,Sixport 7010)directly into the diode array spectrometer to obtain the reference spectrum. (b)HPLC Conditione. The equipmentconsistedof a MerckHitachi pump, type L 6000, and a Waters photodiode array detector, type 990, or a Merck Hitachi L4200 W-vis detector. A Knauer column is used, 250 mm long and 4 mm in diameter. The stationary phase was Nucleosil300 C18,7pm (Macherey und Nagel). The mobile phase was an aqueous solution of 1 X M Cd(C10& and 6 X 1O-aMsodium polyphosphate(referring to the formula NaPOs) pumped at a flow rate of 0.5 ml/min, and the injected volume was 10 pL. The chromatograms were recorded at 250 nm, except the ‘211-nm species” at 211 nm. (c) Electron Microscopy. A small drop of sample was adsorbed onto the copper grids coated with a 50 A thick carbon support fib.After 10 s of contact time the fluid was blotted off. The grids were dried under argon and examined in a Phdipe CM 12transmission electron microscope with an accelerationvoltage of 120 kV. The microscope was equipped with a supertwin lens and an EDAX detector. For imaging, axial illumination was used as well as the “nanoprobemode” with a beam spot size of 1.5 nm, to enable the diffraction of the individual clusters. All images were made under conditions of minimum phase contrast artifactsk with magnifications of 120 000 and 430 OOOX.
O743-7463/92/ 2408-1475$03.OO/O 0 1992 American Chemical Society
1476 Langmuir, Vol. 8, No. 5, 1992
Fischer and Giersig
Results and Discussion Figure 2 shows the spectral comparison of solutions, which had passed through the column or had been pumped directly into the array without the column. In both cases the mixing conditions, i.e. the flow rates of solutions 1and 2, were identical: The NaSH solution had a concentration M and flowed at a rate of 1.2 mL/min. The of 1.7 X M Cd(C104)2 solution moved at a rate of 0.75 mL/ min. The polyphosphate concentration in both solutions was 3 X M (the concentration referring to the formula NaP03). The mixed solution finally contained 1.0 X 10" M CdS and 2.8 X 10" M excess Cd2+ions. It can be seen in part 0 that in both the cases very small colloidal particles were obtained, which start to absorb at about 330 nm and have a pronounced exitonic peak a t 275 nm indicating a very narrow size distribution. These particles are strongly quantized.2 Using the most recent relationship between the onset of absorption and particle size3 one finds that the particles had a diameter of about 1.9 nm. Figure 3 shows the results of transmission electron microscopy on this colloid. The average diameter of the narrow size distribution is found to be 1.3 nm. The typical parameters of the cubic crystal structure are seen from the power
spectrum (Figure 3c). The unit cell in such a CdS cluster is 0.049 nm3.4b,c As the volume of a 1.3-nm spherical particle is 1.150 nm3, it should contain about 23 unit cells and therefore 46 atoms, which is in agreement with the theoretical model of Lippens and L a n n 0 0 . ~It~ should be noted that to get reliable TEM results on the very small particles, the method of samplepreparation is very crucial; most important is dilution of the sol by a factor of 5 with methanol which accelerates solvent evaporation. A closer inspection of part 0 in Figure 2 reveals that the spectrum of the solution that had passed through the column is less intense. In fact, one concludes that less CdS had been formed. When the outflowing solution was analyzed (oxidation of the CdS by hydrogen peroxide, then determination of the sulfate formed), 25 % less sulfur was indeed found. However, no loss of sulfur was noted, when a pure NaSH solution was passed through the column (no Cd2+addition). Obviously the column had "filtered out" a certain percentage of the CdS formed. This seems to be the only influence that was exerted by the column, as the particles formed at the moment of passage of the solutions through the diode array detector had a very similar spectrum. Figure 2 also contains the absorption spectra at various times of aging of the solutions after having passed the column. Significant differences in the spectra due to the rate of aging can be recognized. After 1 min, the solution, which had been prepared without using the column, has developed a second peak at 305 nm, whereas the spectrum of the solution prepared with the column has not changed much. After 5 min, the original 275-nm peak of the first solution has completely disappeared, and the spectrum of the second solution still shows this peak and a weak shoulder at 305 nm. Even after 21 h, differences can still be seen: The 275-nm peak of the second solution has now moved to 305 nm, and the first solution has a smeared-out peak at 325 nm and an intense tail at longer wavelengths. It can be concluded that the colloidal particles in the solution which had passed the column are much more stable than the particles in the solution which had not passed through the column. One could be inclined to attribute the difference in thermal stability to the 25 % decrease in the concentration of the particles in the solution which had passed the column. In order to check this, an experiment was performed in which the three pumps PI, Pa, and P3 were in operation, PI and P2 transporting the Cd2+and SHsolutions in the same ratio as before, but P3 driving pure polyphosphate solution to dilute the colloidal solution by 25 % . The column was not used. It turned out that the spectrum of the solution was the same as in Figure 2, Part 0, only decreased in intensity by 25 % ,and the subsequent aging occurred just as before (Figure 2 , l min to 21 h, full line). This indicates that a 25% decrease in particle concentration does not lead to a noticeable change in the rate of thermal growth of the particles. Another trivial reason could be the fact that the column packing binds the protons formed in the precipitation to a small extent (the column eluates had a slightly higher pH than the solutions which were prepared without the column; the pH increased from 6.14 to 7.27). Experiments were carried out in which pump 3 added NaOH to the mixed solution to such an extent that the sol produced had the same pH as that formed with the column. It was
(2) (a) Brus, L. E. J. Chem. Phys. 1983, 79,5566. (b) Brus, L. E. J. Chem. Phys. 1984,80,4403. (c) Fojtik, A.; Weller, H.; Koch, U.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984,88,969. (3) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. SOC. 1987,109, 5649.
(4) (a) Kunath, W.; Zemlin, F.; Weiss, K. Ultramicroscopy 1985,16, 123. (b) Kittel, C. Introduction to Solid State Physics; Wiley & Sons, Inc.: New York, 1966. (c) Unger, K. Verbindungshalbleiter; Akademische VerlagsgesellschaftGeest & Portig K.-G.: Leipzig, 1986. (d) Lippens, P. E.; Lannoo, M. Phys. Rev. B 1989,13,10935.
A lnml
Figure 2. Spectraof CdS solutions: full line, prepared without column; dotted line, column used; 0, spectrum measured with diode arrayimmediately after the column;A, difference spectrum; l', 5', and 21 h, spectrum measured after aging for various time periods using a conventional spectrophotometer.
1
I.
4.2
8.4
12.6
2R
[AI
168
Figure 3. (a) Typical TEM image of the monodisperse small CdS clusters with the strong maximum at 275 nm. (b) One individual particle from part a with a high magnification. (c) Power spectrum of particle (b). (d) Size distribution of the CdS clusters in part a.
Langmuir, Vol. 8, No. 5, 1992 1477
Colloidal CdS Preparation via Flow Techniques
A \ ,
0
10
20
30
200
350
400
450
Cd2’/SH- = 18
500
A [nml Figure 4. Absorption spectra of CdS sols produced without the column (dotted) and with the column (full line). In the latter case different delay times between mixing chamber and column were applied. Inset: Mean particle diameter as a function of the
delay time.
observed that the process of aging occurred at the same rate as in Figure 2,full line. The “filter” effect of the column must, therefore, be the reason for the higher stability of the remaining particles. The filtered particles are believed to be active accelerators for aging. T w o properties of the particles could bring about this promoting effect: Either the large size of the filteredout particles and/or their surface reactivity. In the previous HPLC experiments,l it was observed that larger particles (>20nm diameter) are filtered out. Such large particles were not produced in the present experiments. In fact the column was passable for particles in the 5-6 nm range as is described below (Figure 4)and the difference spectrum in Figure 2,part 0, due to the removed particles showed a strong peak at 211 nm, also suggesting very small particles. One can therefore postulate that the ultrasmall species (with the 211-nm band) have a stronger surface reactivity than larger ones. Such particles with active surfaces are responsible for the fast growth via association; they behave as an adhesive for the combination of bigger particles; the growth of small CdS particles has been shown to occur mainly through association of smaller particles.la However, the particles with “active”surfaces also interact strongly with the column packing and are therefore retained. The property that makes the surface active is not known;it could be the roughness (edges, apexes) of the crystallites or the incomplete cover by polyphosphate or even the presence of CdS with a different crystal structure (it is known that depending on kinetic factors the aqueous precipitation of CdS can yield the wurtzite or zinc-blende structure5). In the experiments of Figure 4,a M Cd2+and a M SH- solution were mixed, both flowing at a rate of 0.75 mL/min and having a polyphosphate concentration of 1 X M. The final sol contained a higher concentration of CdS and had less excess Cd2+ions than the solsof Figure 2,the result being that larger particles were formed. The spectra shown in Figure 4 were taken with a conventional spectrophotometer. In the absence of the column, the dotted spectrum was obtained. It has an absorption threshold close to 520 nm, i.e. the wavelength at which macrocrystalline CdS starts to absorb. The other spectra in the figure were obtained by using the column. However, the lifetime after which the colloidal particles entered the (5) (a) Saw, R. Nature 1959,184, 2005. (b) Milligan, W.0.J. Phys. Chem. 1943,47, 537.
250
300
350”
A [nml Figure 5. Absorption spectra of CdS colloids as measured by the diode array detector at different concentrations ratios Cd2+/SH-. (Inset) Zero lines of the spectra.
column could be increased by inserting 0.75” capillaries of different lengths between the mixing chamber and the column. The spectra obtained by using those different delay times show different onsets of absorption. It is shifted to longer wavelengths with increasing delay time, which indicates that the particles became larger. The effect is again explained in terms of “active” particles promoting particle growth. Most of these active particles die out in the second to minute time regime. If they are rapidly removed by the filtering column (short delaytimes) the final sol contains rather small particles. As the delay time increases, the active particles get more time to promote particle growth. - The flow technique is quite useful for finding optimum conditions for the preparation of colloids. The concentrations and concentration ratio of the reactants can easily be varied via the pumps and the effects produced can be immediatelyrecognized in the absorption spectrum of the sol produced. In Figure 5, an example is shown where the change in the absorption spectrum of new ultrasmall CdS species were investigated. In this experiment, a 2 X 10-4 M NaSH solution (containing 1X M polyphosphate) was moved by pump PI at a constant rate of 1 mL/min. M Cd(C104)Z solution Pump Pa transported a 1 X (containing the same concentration of polyphosphate) a t different flow rates, pump P3 transported a pure polyphosphate solution, the sum of the flow rates of pumps P2 and P3 being kept constant at 4 mL/min. Under these conditions, sols containing cadmium sulfide a t a concentration of 2 X M were formed, although the Cd2+/SHconcentration ratio changed over a wide range. The column was not used in these experiments. The absorption spectra a t different Cd2+/SH-ratios show that the onset of absorption always occurs in the UV region, suggesting very small particles which are strongly quantized. In the case of a ratio of 4:l the mean particle size was calculated to be roughly 2.0 nm. The development of an extremely strong absorption band at 211 nm occurs at higher concentration ratios. At the highest ratio used (18:1),only this band is present and practically no absorption occurs above 250 nm. The Xthre,,ho1d-sizerelation in Figure 15 of ref 3 thus can no longer be used to calculate the size. Unfortunately TEM fails to give the correct size for these smallest known CdS species. The images showed the diametersto be between 3 and 4 nm. Probably evaporation of the solvent during sample preparation and/or high electron energy led here to a dramatic growth. Therefore the size exclusion chromatography (SEC) seems to be a more gentle method for size determination. However, using the standard conditionslawith a silica phase modified by butyl groups the spectrum of the eluate showed now
Fischer and Giersig
1478 Langmuir, Vol. 8, No. 5, 1992
. ( . . . . , . . . . , . l . . , . . . . , . . l . . , . . . . ,
35
40
4 5
5 0
55
60
6 5
Retention Time [min] Retention Time [min] Figure 6. Calibration of a Nucleosil300 C18 (7 pm) column for CdS sols with four sols of known size and determination of the mean size of ultrasmall CdS particles with A, = 211 nm: right, overlay plot of chromatograms; left, calibration curve.
a pronounced peak at 305 nm. This indicated the growth of "211-nm species" during the passage through the analytical column. Finally successful separation of the unchanged ultrasmall species was performed by using a silica phase modified by octadecyl groups. These longer alkyl chains protect the analyte more effectively from the active surface silica. From the calibration curve of four CdS sols of different particle size (determined by TEM), a diameter of 1.5 nm is found (Figure 6). Assuming spherical particles, the number of elemental cells in such a ultrasmall particle is calculated to be about 36 with 72 Cd atoms. A slight discrepancy to the TEM for the CdS sol with the 275-nm peak, where a diameter of 1.3 nm was found, should be discussed. According to the general spectral properties of Q-particles, one would expect a value below 1.3 nm for the "211-nm CdS". Possible reasons could be the following: Dealing with the smallest known polyphosphate-stabilized CdS species, extrapolation instead of interpolation had to be carried out for the evaluation by size exclusion chromatography. Unfortunately Dance CdS species6cannot be used on a C18 column. The organic surface of them would cause reversed phase retention, and pure size exclusion mechanism would no longer be applicable. The next larger particles "275-nm CdS" still grow even on the C18 column, so that a direct comparison is not possible. The linear range of the calibration curve for CdS is also not known (indeed the value for Cd2+ions fits well with those of the particles, but ions are different and may be not compatible species). The role of polyphosphate in the SEC separation mechanism is not well understood. In the case of very small particles the thickness of the polyphosphate layer compared to the CdS diameter is much larger. This could cause deviation from the straight calibration line. Another possibility is the loose aggregationof much smaller species,e.g. kept together by polyphosphate chains. In this case it would be interesting that the same narrow peak at the same retention time is always found, indicating the same aggregation number. (6) Banda, R. M. H.; Dance, I. G.; Bailey, T. D.; Craig, D. C.; Scudder, M. L.Znog. Chem. 1989,28, 1862.
0.8
8.. e .
.:
0 '
24 h
:: ae .
0.6
i
[nml
Figure 7. Spectra of CdS solutions containing ultrasmall particles: full line, column used; dotted line, prepared without column; 0, spectrum measured with diode array immediately after the column; 24 h, spectrum measured after aging for 1day using a conventional spectrophotometer.
Also in the preparation of these ultrasmall particles an alumina column just after the mixing point has a strong effect on the stability (Figure 7). The concentration of CdS after the column was about one-third less than without the column, so again the particles with the highest surface reactivity must have been retained. Even after 24 h the 211-nm peak still showed the same extinction, whereas it has completely disappeared in the solution formed without the column and particles grew considerably as indicated by the onset of absorbance already at wavelengths above 450 nm. Sols containing particles in the 6 nm range also grew much more slowly, after they had passed through the column. This demonstrates the general effect of the Alox T column to produce thermally more stable CdS sols. Acknowledgment. The authors thank Professor Dr. A. Henglein for his fruitful suggestions, Mr. T. Siebrandta for his excellent work on size exclusion chromatography, and Ms. U. Michalczik for helpful assistance with the laboratory work. Registry No. CdS, 1306-23-6.
