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J. Phys. Chem. C 2010, 114, 6304–6310
Growth of Cadmium Nanocrystals Laura L. A. Adams,* William R. Sweeney, and Heinrich M. Jaeger The James Franck Institute, The UniVersity of Chicago, 929 East 57th St., Chicago, Illinois 60637 ReceiVed: December 14, 2009; ReVised Manuscript ReceiVed: February 17, 2010
Cadmium nanocrystals were synthesized by reduction of cadmium acetate in three different mediums. Transmission electron microscopy studies of the reaction products revealed a range of different shapes depending on the ratio of capping and reducing agents, the addition time of these agents, and the duration and temperature of the reaction. X-ray results confirmed the crystallinity of these faceted structures with the dominant crystal facet along the (002) direction. Optical absorption spectra showed either one or two plasmon resonances in the ultraviolet (λ ) 280 nm and/or 235 nm) depending on the diameter of the spherical particles in solution. Finally, cadmium nanocrystals were converted to cadmium sulfide by addition of thiourea, TEX. 1. Introduction 1
2
Cadmium sulfide (CdS), cadmium selenide (CdSe), and cadmium telluride (CdTe)3 quantum dots have emerged as one of the most studied nanocrystalline systems in the field of colloidial chemistry. However, little is known about the synthesis of cadmium nanoparticles and their self-assembly. As will be discussed here, chemically produced cadmium nanocrystals can form not only arrays of monodispersed spherical nanocrystals but also a host of other interesting nanostructures depending on growth conditions. The first report of a liquid phase synthesis of cadmium metal particles (to the best of our knowledge) was realized by Hein and Steele in the late 1950s when they made spheres of cadmium with diameters between 40-1200 µm by “whipping molten metal in a bath of hot silicone oil.”4 Alternatively, vapor evaporation of cadmium led to the formation of an assortment of shapes which were predominately spheres and hexagons.5 More recent work on cadmium colloids prepared by highenergy electron irradiation of cadmium perchlorate largely focused on its optical absorption properties in the ultraviolet6 (UV) and exploited this fact in photochemical reactions with dimethylcadmium to transform the shape of cadmium spheres into ellipsoids.7 Kloper et al. found that reducing cadmium oxide powder (in the presence of oleic acid at 310 °C) leads to the formation of three-dimensional hexagonal cadmium nanocrystals.8 In their work, X-ray diffraction results of cadmium confirmed the existence of crystalline Cd with all the Cd diffraction peaks present in the same proportions as expected for bulk Cd with additional CdO diffraction peaks. Sulfidation of Cd nanoparticles has been studied using the Kirkendall effect9 in the work by Cabot et al. In their report, cadmium nanoparticles were synthesized by decomposition of dimethylcadmium in trioctylphosphine before the addition of elemental sulfur. Here we show that Cd nanoparticles can be synthesized by reducing cadmium acetate. This approach mimics the synthetic route used by Murray and co-workers’ in producing monodispersed cobalt nanocrystals.10 In addition to spherical nanocrystals, we find a wide range of other shapes depending on the synthesis conditions. These include hexagons, star-like struc* To whom correspondence should be addressed. E-mail: lladams@ seas.harvard.edu. Current address: The Physics Department and School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138.
tures, and wires. X-ray data obtained from our Cd samples differ from the results reported by Kloper et al.,8 as will be discussed in more detail. Finally, conversion of the Cd nanocrystals into CdS nanocrystals will be discussed as it relates to the Kirkendall effect. 2. Experimental Section 2.1. Materials. With the exception of the precursor, cadmium acetate dihydrate (Fisher Scientific), all chemicals were purchased from Aldrich and used without further purification. Trioctylphosphine (TOP, 90% Aldrich), dioctyl ether (99% Aldrich), and 1-octadecene (90% Aldrich) were stored in a glovebox under N2 gas and used as received. If TOP was exposed to air, then it quickly degraded, and the desired reaction product was not achieved. Oleic acid (90% Aldrich) was stored in a refrigerator. 1,2-Hexanedecanediol (90% Aldrich) was stored in a desiccator and used as received. The solvents (toluene, methanol, and anhydrous acetonitrile) used for precipitating and cleaning the samples were purchased from Aldrich and used as received, with the exception of toluene, which was distilled in the presence of sodium and potassium pellets. Hexanes (spectroscopic grade, Fisher Scientific) were also used as received. 2.2. General Synthesis Technique. In a typical experiment, 0.7 mmol (186 mg) of cadmium acetate dihydrate (Cd acetate) and 1.4 mmol (180 mg) of 1,2-hexandecanediol were added to a 50 mL three-neck flask. The flask was then purged with argon gas for a minimum of two hours. A solution containing 20 mL of octyl ether, 1.4 mmol (1.1 mL) of trioctylphosphine (TOP) and 0.35 mmol (0.11 mL) of oleic acid (OA) was injected into the flask with a syringe. The mixture was stirred at 125 °C for 30 min and degassed under vacuum to remove any residual water vapor. The solution was then heated to 205 °C under argon with constant stirring. As the reaction mixture reached temperatures above 180 °C, the color of the solution turned from colorless to transparent blue and finally to an olive green (Figure 1). During this color transformation, a silvery precipitate was observed which grew as time and temperature progressed. Once the reaction was complete, it was quickly quenched by cooling the outside of the flask with nitrogen gas or submerging the reaction vessel in an ice bath. The reaction product was then centrifuged at room temperature for one hour, followed by decanting the supernatant. A
10.1021/jp911820s 2010 American Chemical Society Published on Web 03/05/2010
Growth of Cadmium Nanocrystals
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Figure 1. Five different solutions indicating the color evolution as time progressed for a typical reaction. The length of time between the first and fourth vial was ∼5 min. An extended reaction (>15 min after the light blue solution) resulted in the deep orange-brown appearance as shown in the last vial.
TABLE 1: Typical Reaction Parameters for the Synthesis of Monodispersed Spherical Cd Nanocrystals with an Average Diameter of 6 nma diol
OLA
TOP
temp
time
(mmol)
(mmol)
(mmol)
solvent
(°C)
(min)
0.7 s s
0.35 0.35 s
2.1 2.1 2.3
octyl ether octadecene diphen. ether
194 205 242
8 15 20
a The time indicated is the time taken from when the temperature was 125 °C to the given temperature listed.
small amount of hexanes (5 mL) was added to the precipitate followed by twice as much methanol. After shaking the vial, the precipitate would cling to the walls of the vial. The solvents were discarded, and the process of adding hexanes/methanol was repeated. Afterward, the precipitate was redispersed in either toluene (for TEM imaging) or hexanes. It is important to note the addition time of the ligands. If oleic acid and TOP were added to cadmium acetate before heating (and before adding the solvent), then the reaction product consisted of very small seed particles with a few small rods and triangles, instead of larger hexagons. However, injecting oleic acid and TOP after pumping on the bath (with temperatures greater than 125 °C) resulted in a larger yield of monodisperse colloids and hexagons. Also it should be emphasized that cadmium is a carcinogen and highly toxic. Special precautions should be considered during the synthesis, handling, and disposal of the reaction product. 2.3. Preparation of Spherical Cd Colloids and Their Self-Assembly. When preparing monodispersed (in both size and shape) spherical colloids, it was preferable to stop the reaction before the formation of silvery particles emerged. However, in cases when a silverly precipitate developed, it was possible to separate the precipitate from the collodial solution by centrifuging the reaction product after the reaction was complete. Table 1 showcases the best ratio for producing colloids using three different solvents. The process of forming self-assembled cadmium colloids began by centrifuging either the light blue or olive green samples (after precipitating the sample with methanol) in toluene or hexanes and separating the resulting supernatant. This supernatant was placed in a separate wide opening vial which was vacuum-dried in a desiccator for several days by continuously pumping on it through a cold trap in series with a mechanical pump. The sample was then reconstituted with a very small amount of distilled toluene and placed on either a TEM grid or a silicon nitride membrane (in Supporting Information) and allowed to
Figure 2. (a.) Transmission electron microscope (TEM) images of monodispersed spherical Cd colloids produced using octadecene as the solvent. The average diameter is ∼6.1 nm, and the average spacing between dots is ∼2.5 nm. (b.) TEM image of cadmium nanowires several micrometers in length. (c.) Scanning electron microscope image of asymmetric and symmetric Cd hexagons. (d.) TEM image of starlike structures of cadmium were obtained by removing the diol and using only oleic acid and TOP in a prolonged reaction as described in the text.
Figure 3. Doubling the concentration of oleic acid led to the formation of these less compact cadmium structures. Solvent used was octadecene with no hexadecanediol.
dry (∼1 h). A TEM image of self-assembled Cd nanocrystals (using octadecene) on a TEM grid is shown in Figure 2a. 2.4. Preparation of Cd Nanowires. The formation of crystalline nanowires was achieved by increasing the pumping time from 30 min to over one hour on the reaction at 125°, thereby reducing the volume of solvent and removing water. The reaction color evolved from a light blue to a deep royal blue with a large amount of silverly precipitate. In many cases, the wires had a large rounded triangular head at their ends in maintaining their hexagonal structure. In Figure 2b, a TEM image of nanowires is shown. 2.5. Preparation of Cd Hexagons and Star-like Structures. Following the recipe outlined in the general synthesis, large yields of hexagonal shapes (Figure 2c) resulted as a silverly precipitate. Extended reaction times only increase the overall width of the hexagons. Star-like structures (Figure 2d) were obtained by removing the hexadecanediol from the reaction and using only oleic acid and TOP and carrying out the reaction time for 40 min. 2.6. Preparation of Faceted Non-Hexagonal Structures. Faceted structures (Figure 3) which were not hexagons were synthesized in octadecene without hexadecanediol but twice the molar amount of oleic acid used in the general synthesis recipe. 2.7. Preparation of Hollow CdS Nanocrystals. Hollowed CdS nanocrystals (Figure 4) were synthesized by the addition
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Adams et al. taken with a Philips wide angle locally automated powder diffractometer (using a Cu KR λ ) 1.5418 Å radiation source).
Figure 4. Conversion from Cd nanocrystals to hollowed CdS nanocrystals by addition of thiorea. The mechanism which produces hollowed nanocrystals is known as the Kirkendall effect.23 The two left panels are TEM images taken from an aliquot of Cd before adding thiourea. The right panel is a TEM image of CdS after thiourea is added to the reaction.
3. Results and Discussion 3.1. Evolution of Synthesis: Color Changes. The most prevalent indicator of sizes and shapes of the resulting reaction product were the distinct color changes as the synthesis evolved in time. In Figure 1, different stages of the reaction (from five different reactions using the same procedure) are shown. These typical color changes evolved in time (∼8 min) from the first to the fourth vial. The initial light blue colored solution had the smallest particles which contained not only spherically shaped crystals, but also a few rods and triangularly shaped crystals. The olive green solutions (vials 3 and 4 of Figure 1) contained large yields of 2D hexagons and spherical nanocrystals (
