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J. Phys. Chem. C 2008, 112, 2266-2270
Facile Synthesis of Semiconductor and Noble Metal Nanocrystals in High-Boiling Two-Phase Liquid/Liquid Systems Qiyu Yu,†,‡ Chunyan Liu,*,† Zhiying Zhang,† and Yun Liu† Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering, Technical Institute of Physics and Chemistry, the Chinese Academy of Sciences, Beijing 100080, China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, China ReceiVed: August 24, 2007; In Final Form: NoVember 25, 2007
Novel high-boiling liquid/liquid systems were introduced for the synthesis of semiconductor and noble metal nanocrystals. Polyols and long-chain hydrocarbons were utilized to form such two-phase systems. Because of the high-boiling nature, these systems may be conveniently employed for nanocrystal synthesis at relatively high temperatures (under the normal pressure) and thus are useful alternatives to toluene/water. CdS and Ag nanocrystals were successfully prepared in an octadecene (ODE)/glycerol system via interfacial processes, demonstrating the effectiveness of this class of two-phase systems. As-prepared nanocrystals had hydrophobic surfaces and were dispersed in the ODE phase of the system. Furthermore, a congeneric two-phase system of liquid paraffin/glycerol was also found to be effective for the synthesis of CdS and Ag nanocrystals.
1. Introduction Liquid/liquid systems have been playing an important role in the controllable fabrication of colloidal nanomaterials. The synthesis and/or self-assembly of nanocrystals can be excellently achieved with the help of liquid/liquid interfaces.1-7 As to twophase synthesis of colloidal nanocrystals, the most famous method may date back to 1994, when Brust et al.1 reported the preparation of alkanethiol-capped Au nanocrystals in toluene/ water. Ever since then, the Brust method and its variations have been frequently adopted to synthesize noble metal nanocrystals.2 Recently, a two-phase approach and a two-phase thermal approach (conducted in a sealed autoclave at elevated temperatures) were exploited for the synthesis of a series of chalcogenide4,5 and oxide6 nanocrystals. To date, however, colloidal nanocrystals of chalcogenides or oxides are mainly produced via single-phase routes.8 As compared with single-phase approaches, the adopted conditions for two-phase approaches are relatively mild. Significantly, as-prepared nanocrystals can possess excellent properties such as highly luminescent CdS,4 TiO2 with band-edge luminescence,6a and shape-controlled ZrO2.6b Among reported liquid/liquid systems, toluene/water was mostly adopted. Other organic/water systems, such as chloroform/ water,7a cyclohexane/water,7b and benzene/water,7c were also used. In spite of great effectiveness, there exist some problems with the general two-phase synthetic methods. The liquid/liquid systems now often used, e.g., toluene/water and chloroform/ water, are volatile and toxic and thus may do harm to the human body in practice. Almost all reported liquid/liquid systems are water-based and cannot be used for high-temperature (above 100 °C) synthesis under the normal pressure. The thermal twophase approach has proved to be rather effective; 4b,5,6 however, the sealed systems may cause inconvenience in real-time * Corresponding author. E-mail:
[email protected]. † Technical Institute of Physics and Chemistry, the Chinese Academy of Sciences. ‡ Graduate University of the Chinese Academy of Sciences.
monitoring and process control. For these reasons, further developments of the two-phase synthesis in alternative-green systems are desirable. Herein, we made an attempt to introduce novel high-boiling two-phase liquid/liquid systems. Long-chain hydrocarbons and polyols were chosen to construct the two-phase systems such as octadecene (ODE)/glycerol and liquid paraffin/glycol. The novel systems can provide the following advantages: (1) a broad reaction temperature range, from room temperature to well above 100 °C, is allowed to tune under the normal pressure (without using an autoclave), (2) as-used solvents are environmentally benign and almost nontoxic, and (3) the systems may be useful when an anhydrous condition is required. Evidently, this class of two-phase systems could be helpful alternatives for the two-phase synthesis of nanocrystals. This study is focusing on the synthesis of CdS and Ag nanocrystals in ODE/glycerol in order to demonstrate the effectiveness of these novel two-phase systems. 2. Experimental Section 2.1. Chemicals. Cadmium oxide (g99%), thiourea (AR), glycerol (AR), oleic acid (OA, AR), silver acetate (CP), octadecylamine (ODA, CP), L-ascorbic acid (AR), hexane (AR), toluene (AR), and ethanol (AR) were purchased from Beijing Chemical Factory (China). Liquid paraffin (chemical grade, with boiling point higher than 300 °C) was purchased from Shantou Xilong Chemical Factory (China). Octadecene (ODE, 90%) was from Aldrich. Quinine sulfate (g99%) was from Fluka. All the chemicals were used as received. 2.2. Preparation of CdS Nanocrystals. A cadmium stock solution was prepared as follows. CdO (1.28 g, 10 mmol) and 20 mL of oleic acid (OA) were loaded into a three-neck flask. Under N2 flow, the mixture was vigorously stirred, quickly heated to 140 °C, and kept at this temperature for about 10 min, resulting in an optically clear solution. Into this solution, 30 mL of ODE was added to dilute and cool the solution. Typically, 38 mg of thiourea (0.5 mmol) was dissolved in 20 mL of glycerol by stirring and moderately heating at about
10.1021/jp076783t CCC: $40.75 © 2008 American Chemical Society Published on Web 01/29/2008
Two-Phase Synthesis of Nanocrystals 40 °C. Then 5 mL of the cadmium stock solution was diluted to 20 mL with ODE. The two solutions were then transferred into a 100 mL reaction flask. Under N2 flow, the mixture was vigorously stirred, quickly heated to 120 °C (in less than 10 min), and maintained for a given time. To follow the reaction, aliquots were taken at different time intervals for optical measurements. 2.3. Preparation of Ag Nanocrytals. Ascorbic acid (35 mg, 0.2 mmol) was dissolved into glycerol by stirring and heating to form a clear solution. The solution was allowed to cool to room temperature. Then 108 mg of octadecylamine (ODA, 0.4 mmol), 19 mg of silver acetate (0.1 mmol), 2 mL of OA, and 18 mL of ODE were loaded into a 100 mL flask. Then the mixture was vigorously stirred, heated to 90 °C, and kept for 15 min to form a clear solution. Into this solution, the ascorbic acid/glycerol solution was added quickly. The two-phase system was then aged at 90 °C for 10 min. The formation of Ag nanoparticles could be followed visually by the disappearance of the faint yellow coloration and the concomitant appearance of a brown color of the mixture. 2.4. Isolation and Purification of Nanocrystals. When the reaction finished, the two-phase mixture was cooled to room temperature. The ODE phase (the upper layer) was collected, and an aliquot of the phase was taken. Addition of enough ethanol to the aliquot resulted in the flocculation of the nanocrystals. The suspension was separated by centrifugation, and the resulting precipitate was dispersed in a small amount of hexane. The dissolution-precipitation step was repeated for several times using the hexane/ethanol pair. This purification process was applicable to CdS as well as Ag nanocrystals. 2.5. Characterization. Each CdS or Ag nanocrystal aliquot (made up of two phases) was diluted with hexane. The upper colloidal solution in hexane was used for optical measurements without any other posttreatment. Ultraviolet-visible (UV-vis) absorptions were recorded on a Shimadzu UV-1601 PC spectrophotometer. The photoluminescence (PL) spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer with the excitation wavelength of 350 nm. The absorption onset used the minimum of the second derivative.9 The band gap PL quantum yields (QYs) were assessed by using quinine sulfate (PL QY 55% in 1 N sulfuric acid) as a reference.10 Optical densities of all solutions at the excitation wavelength were kept small (below 0.1) to avoid obvious reabsorption. The TEM images were taken on a JEOL JEM-200CX transmission electron microscope, using an accelerating voltage of 160 kV. Samples for TEM investigations were prepared by dropping dilute solutions of properly washed nanocrystals in toluene onto carbon-coated copper grids, allowing the solvent to slowly evaporate. The XRD pattern of CdS nanocrystals was obtained with a Rigaku D/Max-2500 diffractometer under Cu KR radiation (λ ) 1.54056 Å) and employing a scanning speed of 4°/min in the 2θ range 10-60°. The XRD sample was prepared by evaporating drops of concentrated nanocrystal solution in hexane on a glass plate. 3. Results and Discussion 3.1. CdS Nanocrystal Synthesis and Characterization. 3.1.1. Synthetic Mechanism. Pan et al. 4a previously reported the synthesis of CdS nanocrystals in toluene/water, using cadmium myristate and thiourea as precursors. They proposed a synthetic mechanism that H2S was generated from thiourea upon heating at 100 °C and reacted in situ with the cadmium precursor at the interface. In our case, cadmium oleate (in ODE) and thiourea (in glycerol) were used as cadmium source and
J. Phys. Chem. C, Vol. 112, No. 7, 2008 2267
Figure 1. Temporal evolution of the UV-vis absorption (solid) and PL (dash) spectra of CdS nanocrystals during the growth in the ODE/ glycerol system. The labeled positions in the absorption spectra (R, 312 nm; β, 323 nm) are due to the magic-size clusters of CdS.
SCHEME 1: Two-phase Synthetic Route for OA-Capped CdS Nanocrystals (NCs)
sulfur source, respectively. Oleic acid (OA) was used as a capping agent for the generated CdS nanocrystals. To test if H2S could be produced during the synthesis, the glycerol phase (thiourea/glycerol) was heated separately to 120 °C under N2 bubbling and maintained for a corresponding time. However, no released H2S was detected. Thus, it seems more likely that, in our system, thiourea reacted with cadmium oleate directly at the ODE/glycerol interface. The possible synthetic process can be illustrated in Scheme 1. 3.1.2. Properties. Figure 1 shows the temporal evolution of UV-vis absorption and photoluminescence (PL) spectra of CdS nanocrystals during the preparation at 120 °C. After 4 h of the reaction, the maximum achievable absorption onset located at about 400 nm, corresponding to an estimated particle size of about 3 nm.11 The sharp absorption peaks at 312 and 323 nm indicate the coexistence of magic-size clusters4,11,12 of CdS at early stages of the preparation. The typical band gap PL spectral width (full width at half-maximum, fwhm) was 20-30 nm, representing nanocrystals prepared with a narrow size distribution. A broad tail due to surface-trap emission was inevitable for each PL spectrum just as many reported results.8c, 13 The calculated band gap PL quantum yield (QY) was typically over 10%. The growth of CdS nanocrystals was quite slow, and the achievable particle sizes were very small. Figure 2A shows the TEM image of the CdS nanocrystals with an average diameter less than 3 nm. It is worth noting that, during the preparation, no precursor injection technique8a,14 was utilized to achieve a separation between nucleation and growth stages; however, asprepared CdS nanocrystals were of fairly good monodispersity (Figure 2A,B). This may be due to a very slow particle growth rate, which permits a prolonged focusing of the size distribution without Ostwald ripening.14 The formation of the magic-size
2268 J. Phys. Chem. C, Vol. 112, No. 7, 2008
Figure 2. TEM image (A), the corresponding size-distribution histogram (B), and XRD pattern (C) of as-prepared CdS nanocrystals on a glass substrate. The shoulder indicated by an arrow was from the glass substrate.
clusters at early stages was also believed to have an effect on the particle size distribution. They acted as uniform and stable nuclei at the beginning and then grew into significantly larger particles.12 As-prepared CdS nanocrystals had a cubic structure (Figure 2C), which is common for the semiconductor nanocrystals synthesized at low temperatures. 3.1.3. Effect of O2. Above results and discussion are based on the synthesis under N2 flow. To see if O2 has an effect to the synthesis, we also conducted the reaction in air. Figure 3A shows that the initially generated CdS nanocrystals began to shrink from 80 min or so, as indicated by the blue-shift of the absorption onset. Figure 3B (derived from Figure 3A and Figure 1) further compares the syntheses under N2 flow and in air, from which one can find that the results obtained in the initial 50 min, however, remains to be nearly the same. Probably, the particles would undergo some kind of oxidative etching process in the ambient air, which was in competition with particle growth. At the early stages, the reactant concentrations in the system were high enough to maintain continuous growth of
Yu et al. particles. Thus, it appears that O2 had little effect on the nanocrystal growth in this period. Because of particle growth, the reactants became depleted at the late stages, and thus the etching process dominated the subsequent size evolution until the particles disappeared eventually. Generally, extremely small particles with broadened size distributions were obtained at late stages of the reaction (see Supporting Information). Therefore, an inert atmosphere is necessary for the synthesis of CdS nanocrystals. 3.2. Ag Nanocrystal Synthesis and Characterization. 3.2.1. Synthetic Mechanism. Ag nanocrystals were synthesized by mixing a silver (I) solution in ODE and an ascorbic acid/glycerol solution at about 90 °C (see Experimental Section). During this process, the color of the reaction mixture gradually changed to yellow and to dark brown, indicating the formation of Ag nanocrystals. The formation of silver nanocrystals can also be followed by the UV-vis absorption spectra. Figure 4 (curve a) shows the absorption of as-prepared Ag nanocrystal solution. Ascorbic acid in the glycerol phase was supposed to be the reducing agent to silver (I) as previously reported.15 Octadecylamine (ODA) and oleic acid (OA) in the ODE phase were used as stabilizing agents for as-prepared Ag nanocrystals. Besides ascorbic acid, the amine (ODA) or the polyol (glycerol) in the current system could also be a possible reductant to silver (I).16,17 We conducted two control experiments to test the possibility. In one experiment, glycerol alone (without ascorbic acid) was used; in the other, the glycerol phase (ascorbic acid/ glycerol) was not used. In these two cases, no characteristic absorption of silver was observed (curves b and c in Figure 4). This means that silver (I) could not be reduced by glycerol, neither by ODA within the ODE phase. The control experiments provide strong evidence that the reaction happened at the interface of ODE and glycerol and silver (I) was reduced exclusively by ascorbic acid. The interfacial redox reaction can be summarized by eq 1.
m C H O (glycerol) f 2 6 8 6 m (Agm)(ODE) + C6H6O6(glycerol) + mH+ (1) 2
mAg+(ODE) +
In the Brust method, before a reducing agent was added, silver (I) ions had to be transferred from water to toluene with the help of a phase transfer reagent. In our case, silver (I) organic
Figure 3. (A) Temporal evolution of the absorption (solid) and PL (dashed) spectra of CdS nanocrystals during the preparation in air. (B) Temporal evolution of the absorption onsets of CdS nanocrystals prepared under N2 and in air. All conditions were the same except the atmospheres.
Two-Phase Synthesis of Nanocrystals
Figure 4. UV-vis spectra of the samples prepared (a) in the presence of ascorbic acid/glycerol, (b) in the presence of glycerol alone, and (c) in the absence of ascorbic acid/glycerol, respectively. The insert shows the UV-vis spectrum of purified Ag nanocrystals in hexane.
solution was prepared directly and thus a phase transfer reagent was not needed. This could eliminate some issues posed by the use of phase transfer reagents, including cytotoxicity and persistent contamination.18 3.2.2. Properties. The absorption of a silver colloid was due to the excitation of surface plasma of silver.19 It is generally sensitive to the size distribution of the colloid, to the shape of the colloidal core, to its state of aggregation, as well as to the refractive index of the continuous medium. As presented in Figure 4 (curve a), the absorption peak of as-prepared Ag nanocrystals appeared at about 420 nm. The inset in Figure 4 shows the absorption spectrum of Ag nanocrystal solution after purification. Figure 5A presents typical TEM image of as-prepared silver nanoparticles, which were roughly spherical in appearance. Some very large particles formed by aggregation were also observed. The particles were of a wide size distribution, frequently ranging from 5-17 nm (Figure 5B). An average particle size of about 11 nm is consistent with the observed absorption peak around 420 nm.20 Presumably, the polydispersity of the nanocrystals was due to the strong binding of the surfactants with silver ions, which resulted in a low reactivity of the silver precursor.21 The crystalline nature of the Ag nanoparticles was revealed by the corresponding electron diffraction pattern (Figure 5C). The observed diffraction rings can be indexed as (111), (200), (220), and (311) reflections, respectively, according to the face-centered cubic silver. 3.2.3. Synthesis in ODE/Water. Because the synthetic temperature was below 100 °C (90 °C), it seemed plausible that ODE/glycerol might be replaced by ODE/water, which is
J. Phys. Chem. C, Vol. 112, No. 7, 2008 2269 cheaper and easy to process. However, quite different results were obtained in this case. At 90 °C, a dark color appeared immediately after the addition of the ascorbic acid/water solution. Micrometer crystals were produced and were not dispersible in the ODE phase (see Supporting Information). This was the case even when the reaction was conducted at room temperature. Perhaps, ascorbic acid possessed much higher reactivity in water than in glycerol, and thus the crystal growth could not be arrested in nanometer scale. The failure to synthesize Ag nanocrystals in ODE/water further demonstrated the special advantage of the ODE/glycerol system in nanocrystal synthesis. 3.3. Synthesis of CdS and Ag Nanocrystals in Paraffin/ Glycerol. In addition to ODE/glycerol, CdS and Ag nanocrystals can also be prepared in another high-boiling two-phase system of paraffin/glycerol (see Supporting Information). The synthetic procedures were similar to that described in Experimental Section except that ODE was replaced by paraffin. This result highlights two points. First, paraffin/glycerol is more natural and cheaper. Second, the introduction of these long-chain hydrocarbon/polyol systems is successful. 4. Conclusions In summary, high-boiling liquid/liquid systems were developed for the facile synthesis of semiconductor and noble metal nanocrystals. These systems were made up of polyols and longchain hydrocarbon, which are almost nontoxic. In the ODE/ glycerol system, fairly monodisperse, luminescent, and very small CdS nanocrystals could be prepared. This two-phase synthesis of CdS nanocrystals, without precursor injection, may be suitable for industrial production. Ag nanocrystals were also prepared in this system. In addition to ODE/glycerol, paraffin/ glycerol also proved to be available for the synthesis of CdS and Ag nanocrystals. The introduction of these systems is an effort toward green nanosynthesis.18,22 Moreover, this highboiling two-phase approach may be extended to the synthesis of other semiconductor and noble metal nanocrystals. Acknowledgment. This work was financially supported by the National Natural Foundation of China (20573126) and the Chinese Academy of Sciences. Supporting Information Available: Experimental results concerning the synthesis of CdS and Ag nanocrystals in paraffin/ glycerol, TEM images of the CdS nanocrystals prepared in air and the Ag sample prepared in ODE/water, and more supporting results. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 5. TEM image (A), the corresponding size-distribution histogram (B), and selected area electron diffraction pattern (C) of the Ag nanocrystals.
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