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Ind. Eng. Chem. Res. 2007, 46, 4358-4362
APPLIED CHEMISTRY Hydrothermal Synthesis of CdSe Nanoparticles Juandria V. Williams, Claire N. Adams, Nicholas A. Kotov, and Phillip E. Savage* UniVersity of Michigan, Department of Chemical Engineering, Ann Arbor, Michigan 48109
We report herein the feasibility of CdSe nanoparticle synthesis in high-temperature water. The base-case experimental conditions (T ) 200 °C, Cd/Se molar ratio ) 8:1, and a reaction time, t, of 1.5 min) produced nanoparticles that exhibited quantum confinement behavior. The quantum yield was 1.5%, but it was easily increased to approximately 7% by adding a CdS shell. The mean particle size can be manipulated by changing the process variables. The particle size increased with increasing reaction time, temperature, stabilizer concentration, and Cd/Se ratio. The mean size decreased with increasing pH. This effort is a step in the development of alternative syntheses that can reduce limitations posed by current approaches. Introduction The optical properties of CdSe nanocrystals can be tuned by varying their size, a result of the phenomenon known as quantum confinement. Owing to this unique property, they have attracted attention as a promising electronic material for applications such as transistors,1 light-emitting diodes2,3 and biological labeling.4,5 Many synthetic methods for CdSe nanocrystals have been developed, including photochemical,6 γ-irradiation,7 sonochemical,8 and solvothermal9 techniques. While these strategies yield particles with properties unique to their fabrication strategy, colloidal chemistry using molecular precursors is the best preparation technique to obtain particles with strong quantumconfinement effects. The traditional synthesis of CdSe semiconductor nanocrystals, based on the pyrolysis of an organometallic cadmium reagent in a hot coordinating solvent, has realized high-quality CdSe nanocrystals for over 10 years.10-14 Although this organic solvent-based method produces high-quality crystals, it possesses some disadvantages. For example, additional processing is needed to modify the nanocrystals’ optical properties or to convert them into a water-soluble product. Also, toxicity of the solvents used in the process is quite high. An ideal synthetic route to high-quality nanocrystals would include the use of less expensive, less toxic, and more environmentally benign reagents and solvents. It would also use simple procedures. Some researchers have sought to incorporate “greener” approaches into their synthetic approach. Peng and Peng15,16 investigated the formation of CdSe nanocrystals using alternative, less toxic precursors. Asokan et al. prepared CdSe nanocrystals by replacing the typical expensive solvents with inexpensive heat transfer fluids.17 Using water as the reaction medium, however, could move the process closer to the ideal. Indeed, some early work in this field used water as the continuous phase, but the synthesis was performed using an inverse micelle technique.18 Subsequent studies have demonstrated aqueous-based synthetic routes at relatively low temperatures (200 °C). This medium is of interest because high-temperature processes (250-350 °C), such as those in organic-based methods, generally yield higher-quality product.10,23,24 Additionally, by varying temperature, some of the properties of high-temperature water (HTW) can be tuned to mimic those of common organic solvents. Different organic solvents are used to synthesize, store, and exchange capping groups on nanocrystals. If HTW can be tuned to adequately mimic the necessary properties of these different solvents, then HTW could be a suitable replacement. Finally, high-temperature water has a lower surface tension than water at cooler temperatures. A lower surface tension increases wettability and, when coupled with the high temperature, could anneal the surface of the crystal and reduce the number of surface defects. These attributes make high-temperature water an intriguing reaction medium for the synthesis and processing of nanomaterials. Indeed, numerous studies have demonstrated the synthesis of metallic and ceramic nanomaterials using hightemperature water.25-29 In this paper, we demonstrate, for the first time, the hydrothermal synthesis of citrate-stabilized CdSe nanoparticles and identify the effects of the important process variables. Experimental Methods Cadmium perchlorate (Aldrich, 99%), N,N-dimethylselenourea (Sigma-Aldrich, 97%), sodium citrate tribasic dihydrate (Sigma-Aldrich, 99%), and thioacetamide (Sigma-Aldrich, 99+%) were used as received. Deionized water served as the solvent. All experiments were carried out in 1.54 mL stainless steel batch reactors comprising Swagelok caps and port connectors. Prior to the experiments, the reactors were loaded with deionized water and heated for 1 h at 300 °C to remove any residual material from the surface. Citrate-stabilized CdSe nanocrystals were prepared as follows (based on a procedure by others30): using a pipet, 5 mL of 8 ×
10.1021/ie061413x CCC: $37.00 © 2007 American Chemical Society Published on Web 05/23/2007
Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4359
Figure 1. Reactor temperature profile at a sand-bath temperature of 200 °C.
Figure 2. Emission spectrum for CdSe nanocrystals produced under the base-case conditions.
10-2 M cadmium perchlorate dihydrate was added to 45 mL of deionized water containing 0.1 g of sodium citrate. Different Cd/stabilizer molar ratios were obtained by adding different amounts of sodium citrate. Enough 0.1 M NaOH was added to increase the pH to 9.0. Then, 5 mL of this mixture was added to a glass vial. Immediately prior to running an experiment, a carefully measured amount of 1 × 10-2 M N,N-dimethylselenourea was added to the cadmium/sodium citrate solution in the vial. This resulting solution (1.5 mL) was then transferred to the reactor, which was tightly sealed. The selenium reagent was added immediately before the reaction to minimize the fast oxidative loss of the selenourea. Adding 0.5 mL of the selenourea solution yielded a Cd/Se molar ratio of 8:1. Different Cd/Se molar ratios were obtained by adding different volumes of the N,N-dimethylselenourea. We chose to run experiments with excess cadmium, primarily due to successes with a similar CdSe synthesis by Rogach et al.30 The sealed reactor was immersed in a preheated Techne SBL-2 fluidized sand bath fitted with a Techne temperature controller. After the appointed time, the reactors were withdrawn from the sand bath, immediately placed in a water bath at room temperature, and allowed to cool for about 1 min. They were then opened, and their contents were retained in glass vials for subsequent analysis. It is important to note that the temperature inside the reactor was not the same as the sand-bath temperature in these experiments. The synthesis is taking place under non-isothermal conditions, as the batch holding times we explored (
