Rapid preparation of highly luminescent CdTe nanocrystals in an ionic liquid
via a microwave-assisted process†
Yu Hayakawa,
aYoshiyuki Nonoguchi,
aHui-Ping Wu,
bEric W.-G. Diau,
bTakuya Nakashima*
aand Tsuyoshi Kawai*
aReceived 11th March 2011, Accepted 14th April 2011 DOI: 10.1039/c1jm11059d
Thiol-capped CdTe nanocrystals are synthesized directly in an ionic liquid by means of microwave irradiation in a similar manner to an aqueous synthetic route. The combination of ionothermal synthesis with microwave heating offers a quick route to highly luminescent CdTe nanocrystals with zincblende crystal structure. The growth of nanocrystals is considered based on a universal growth model under the assumption that the growth rate exponentially converges as the reaction proceeds. The analysis indicates that the ionothermal conditions with high temperature promote the steady growth of nanocrystals after the nucleation, while the microwave irradiation plays a pivotal role in the limiting step of the growth of nanocrystals including the activation of nanocrystal precursor and the subsequent formation of nuclei.
Introduction
Semiconductor nanocrystals (NCs) have attracted considerable interest in recent years as light emitters having remarkable photostability as well as spectral tunability via a single synthetic route by means of bandgap engineering.1–3 These advantages have stimulated various possible applications of NCs including light emitting and photovoltaic devices4,5 and labeling of biomolecules.6Unlike bulk semiconductors, the wet chemically synthesized semiconductor NCs7,8 offer chemical and physical flexibility and thus dimensionally controlled superstructures of a single component or multiple components with extraordinary properties.9,10Recent progress in the wet-chemical synthesis of semiconductor NCs includes a microwave-assisted process for quick preparation of NCs.11 Microwave routes are often per-formed on the synthesis of semiconductor NCs such as CdS,12 CdTe,13CdSe,14and their core/shell NCs15in high polar media. Combined with hydrothermal conditions, the microwave route affords high quality NCs with a high photoluminescence (PL) quantum yield as high as 60% in a short reaction time, whereas it involves safety concern because of the high temperatures required and autogenous pressure.
Recently, room-temperature ionic liquids16are receiving much interest as media for synthesis of inorganic nanomaterials.17,18The importance of ionic liquids in the field of inorganic nanosynthesis
has been realized in terms of their properties such as thermal stability, low vapor pressure, low interfacial tension, and supra-molecular property, while these properties are dependent on the structures and combinations of ionic components. Highly lumi-nescent semiconductor NCs including CdSe,19CdS,20and ZnO21 were synthesized by means of ionothermal synthesis in ionic liquids. Meanwhile we have reported efficient phase transfer of cationic CdTe NCs to hydrophobic ionic liquids from an aqueous solution.22,23In hydrophobic ionic liquids, the CdTe NCs showed great improvements in PL intensity as well as photostability in a wide range of temperature, which enabled us to figure out the specific quantum size effect in the exciton fine structure of zinc-blende CdTe NC.24The improved PL property was attributed to the stabilization of the capping layer of cationic thiols on the NC-surface by the salt effect of ionic components and generally observed for hydrophobic ionic liquids, especially those with bis (trifluoromethanesulfonyl)amide (Tf2N) anion. The
heat-treat-ment of CdTe NCs at 120C in the ionic liquid further improved the PL quantum yield up to more than 50%.21However, the post-treatment by heating in ionic liquids also led to the increase in the size of NCs and the corresponding spectral shift to longer wave-length. In the present study, we synthesized thiol-capped CdTe NCs directly in an ionic liquid at high temperature (150C) with
the aid of microwave irradiation. Since microwaves directly excite polar molecules and ions, ionic liquids are essentially ideal media absorbing microwave.25Back-and-forth oscillation of ions with friction induced by the electromagnetic field of microwaves raises the temperature rapidly, which substantially shortens the reaction times compared to the convective heating method. Furthermore, in contrast to the hydrothermal synthesis, the ionothermal one has no requirement for pressure conditions due to high thermal stability and low vapor pressure of ionic liquids. The combination
aGraduate School of Materials Science, Nara Institute of Science and
Technology, NAIST, Ikoma, Nara, 630-0192, Japan. E-mail: ntaku@ms. naist.jp; [email protected]
bDepartment of Applied Chemistry, National Chiao Tung University,
Hsinchu, 30010, Taiwan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c1jm11059d
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 8849
www.rsc.org/materials
PAPER
luminescent CdTe NCs capped with cationic thiol in a similar manner to an aqueous synthetic route.8 While syntheses of luminescent NCs in ionic liquids have been already reported,19–21 they are based on the decomposition of organometallic precur-sors or metal salts in the presence of elemental chalcogenides, which follows the organometallic route as demonstrated in highly boiling organic solvents.7 The colloidal approach where the thiolate–metal interaction controls the growth of nano-particles based on ionic association between metal and chalco-genide ions has been first demonstrated in an ionic liquid. The microwave irradiation of an ionic liquid solution of the NC-precursor gave highly luminescent CdTe NCs passivated with a thiol ligand. The CdTe NCs prepared in the ionic liquid at 150C possessed a zincblende crystal structure as well as NCs produced via the aqueous route, whereas the PL quantum yield was much higher than NCs obtained in an aqueous solution.
Experimental
Materials
CdCl2$2.5H2O (99.9%), 2-(dimethylamino)ethanethiol
hydro-chloride (DMAET, 95%), and Al2Te3 were purchased from
Wako, Aldrich, and Hayashi Chemical, respectively. A hydro-phobic ionic liquid, 1-methoxymethyl-1-methylpyrrolidinium bis (trifluoromethanesulfonyl)amide (MOMPyrTf2N), was used as
a solvent. All chemicals were used without further purification. Synthesis of CdTe NCs
CdTe NCs were synthesized following the reported procedure32 with some modifications. Briefly, 29 mg (1.3 104 mol) of CdCl2$2.5H2O and 43 mg (3.0 104mol) of DMAET were
dissolved in 10 mL of deionized water, and the pH of the aqueous solution was adjusted to 5.5 by 1 N NaOH. The aqueous solution of cadmium salt and DMAET was mixed with the same amount (10 mL) of MOMPyrTf2N and stirred vigorously to extract
cadmium–DMAET complex from the aqueous solution to the hydrophobic ionic liquid. After being stirred for more than 10 h, the upper aqueous solution was removed and the lower ionic liquid solution was dried in vacuo. The ionic liquid solution of cadmium–DMAET complex was then subjected to the injection of H2Te gas, which was generated by the reaction of 20 mg of
Al2Te3with 5 mL of 0.5 M H2SO4, under N2atmosphere to give
a CdTe NC-precursor solution as a brownish suspension. The CdTe NCs were prepared by heating the precursor suspension at 150C by microwave irradiation and also by convective heating (mantle heater) for various periods. The microwave irradiation was conducted by IMCR-25003 Green-Motif I (iDX corp.), which operates at 2450 MHz frequency and 0–300 W power range.
measurement. For TEM observation, a drop of the ionic liquid solution of CdTe NCs was cast on a carbon-coated copper grid and the excess ionic liquid was washed with methanol. X-Ray powder diffraction (XRD) patterns were recorded using a Rigaku RINT-TTR III/NM X-ray diffractometer. CdTe NCs in the ionic liquid were precipitated by methanol and centrifuged for the XRD measurement.1H NMR spectra were recorded on a JEOL AL-300 spectrometer (300 MHz).
Results and discussion
Since inorganic salts are not well soluble in hydrophobic Tf2
N-based ionic liquids due to the relatively low dielectric constants of ionic liquids, the cadmium–DMAET complex was dissolved in MOMPyrTf2N via a simple extraction process from the aqueous
solution. The phase transfer from water to the ionic liquid was confirmed by1H NMR (see the ESI†). The1H NMR signals of the cadmium–DMAET complex before the extraction dis-appeared and the new signals assigned to the pyrrolidium cation of MOMPyrTf2N appeared in D2O after the extraction,
indi-cating the cation-exchange mechanism.23b Upon injection of gaseous H2Te with N2flow to the ionic liquid solution, the CdTe
NC-precursor formed to give a brownish suspension. The precursor suspension was then subjected to microwave irradia-tion at 150 C under ambient conditions (open air and atmo-spheric pressure). The brownish suspension turned to a clear yellowish solution within two minutes by microwave irradiation, indicating the dissolution of precursor aggregates and the formation of very small CdTe NCs. This change was also observed for the aqueous synthesis of CdTe NCs capped with cationic DMAET, while it took about 10 min via the convective heating method.32Further microwave irradiation promoted the growth of CdTe NCs.
Fig. 1 shows the representative TEM images of CdTe NCs prepared via microwave irradiation. The average diameter of CdTe NCs obtained by microwave irradiation for 80 min at 150C was measured to be 4.1 nm with a standard deviation of 14%, which was determined by measuring more than 150 indi-vidual NCs. The lattice fringes observed in the high-resolution TEM image (Fig. 1b) indicate that the CdTe NCs are composed of a high crystalline single crystal. XRD peaks of CdTe NCs prepared by the microwave-assisted method can be indexed as (111), (220) and (311) crystallographic planes and match well with the zincblende crystal structure of CdTe as indicated by lines (Fig. 2). The crystal size of CdTe calculated using the Scherrer’s equation from the diffraction peak (111) was found to be 2.5 nm for NCs grown for 20 min. The zincblende crystal structure is consistent with the dominant crystal phase of bulk CdTe and also with that of CdTe NCs prepared in the aqueous conditions (see
the ESI†).8 There was no marked difference between the XRD profiles of CdTe NCs prepared by microwave irradiation and convective heating (also see the ESI†). Unlike CdTe NCs prepared in the aqueous conditions employing thioglycolic acid (TGA) as a capping ligand,8,13bformation of CdS shell due to the partial hydrolysis of TGA was not suggested from XRD patterns. Thus, TEM and XRD results showed no substantial difference in the size, morphology and crystal structure between CdTe NCs prepared in the ionic liquid and in water. Some recent studies have reported that ionic liquids have an effect on the regulation of size and shape of NCs due to their low interfacial tension and the adsorption of ionic components to the polar facets of the NC-surface.18–20In the present case, the thiol ligand, DMAET, which sticks to the surface of CdTe NCs more strongly than the ionic liquid components, would predominantly control the growth of NCs in a similar manner to the aqueous synthetic route employing thiol ligands.8
The absorption and PL spectra showed red-shift as a function of reaction period for both CdTe NCs prepared via microwave irradiation and convective heating methods (Fig. 3), suggesting the continuous growth of NC size. Broadening of PL spectra as the increase of reaction time was observed for both CdTe NCs (Fig.3a and b), which was generally found for the aqueous synthesis of CdTe NCs8and attributed to the growth via Ostwald ripening at the later stage of NC formation. Fig. 4 summarizes the temporal evolution of PL peak position, size and PL quantum yield of CdTe NCs. For comparison, evolution plots for CdTe NCs prepared in water by microwave and convective
heating methods at 100C are also shown. Continuous shift of PL peak positions (Fig. 4A) represents the growth of NC size since the PL peak position clearly correlates the degree of quantum confinement effect. The microwave-assisted method in the ionic liquid gave CdTe NCs with strong red emission by 80 min irradiation, while only green emissive CdTe NCs with PL peak at 540 nm were obtained by the conventional aqueous synthesis for the reaction period as long as 6 h. The conventional aqueous synthesis gave DMAET-capped CdTe NCs whose PL peak position appeared at 600 nm with broad size-distribution even for a 24 h reaction period.22Given the linear progression of PL peaks, whereas the formation of NCs indeed involves multiple and complex growth kinetics, the rate of PL peak shift
Fig. 1 Representative TEM (a) and high-resolution TEM (b) images of CdTe NCs prepared in the ionic liquid via microwave irradiation at 150C. Scale bars: 20 nm for (a); 5 nm for (b).
Fig. 2 XRD profile of CdTe NCs prepared by microwave irradiation for 20 min in the ionic liquid. The calculated peaks of the CdTe zincblende structure are indicated by lines (ICSD 31844).
Fig. 3 Absorption (broken lines) and the corresponding PL (lex¼ 400 nm,
solid lines) spectral changes of CdTe NCs prepared by (a) microwave irradiation and (b) convective heating in the ionic liquid at 150C.
Fig. 4 Evolutions of the PL peak position (A), size (B) and PL quantum yield (C) of CdTe NCs prepared by four-different methods: (a) micro-wave irradiation in the ionic liquid at 150C, (b) convective heating in the ionic liquid at 150C, (c) microwave irradiation in water at 100C, and (d) convective heating in water at 100 C. The broken lines in (B) represent the fitting curves following eqn (1).
parameter. Integrating the linear relation, the growth eqn (1) was provided as follows:
D(t)¼ D0+as(1 exp (t/s)), (1)
where D(t) is the size of NC at time t, D0is the critical size of NC
at the early growth stage including nucleation, anda is the initial growth rate after the nucleation. We also assume that the fitting parameters is the time constant of growth which depends on precursor concentrations. The equation was applied to the growth plots in Fig. 4B, that were derived from the first excitonic peak positions of NCs according to the empirical equation reported by Peng and co-workers,34to estimate parameters for the growth of NCs. The fitting curves are depicted as broken lines in Fig. 4B, well tracing the evolution plots of NC-size with the correlation coefficients of 0.999. As summarized in Table 1, the microwave irradiation has little effect on the growth ratea. The same conditions of temperature and solvent gave the similar values ofa (ca. 0.03 nm min1in the ionic liquid at 150C and ca.
0.005 nm min1in water at 100C). High temperature (150C) and ionic liquid solvent conditions provided the faster growth by more than 5 times than the aqueous conditions (100C). Since both the ionic liquid and water are good microwave absorbers, the selective activation35of NCs was not prominent in the steady growth of NCs after nucleation. On the other hand, a noticeable effect of microwave irradiation was found in the values of D0.
The microwave irradiation conditions gave larger D0(a and c; ca.
2.3 nm) than those obtained by convective heating (b and d; ca. 1.7 nm), which implies that the faster consumption of NC-precursor in the microwave conditions at the early growth stage including the nuclei formation shifts the critical size (D0) of NCs
to a larger value.13c,36There are two plausible effects of micro-wave irradiation on the fast consumption of NC-precursor. One is that the rapid temperature elevation of the system including media due to the high utilization factor of microwave energy accelerates the nucleation. The higher convection heating rate induced by microwave irradiation might result in the faster consumption of all precursors nearby before the diffusion-limited growth of NC starts, affording large D0values. The other
dissolution of agglomerates and the subsequent consumption of NC-precursor. Thus both the rapid elevation of temperature and the selective heating by microwave might be effective for the limiting step of NC growth which includes the first two events (activation of the precursor and nucleation).35
The CdTe NCs prepared in the ionic liquid showed a much higher PL quantum yield than those obtained via aqueous routes (Fig. 4C, a, b vs. c, d). The higher reaction temperature and the stabilization effect of the ionic liquid would be responsible for the high PL efficiency. As for the heating method, given the same solvent and temperature conditions, the microwave irradiation afforded a higher PL quantum yield than convective heating for the same reaction periods (a vs. b, c vs. d). The higher PL quantum yields indicate the selective microwave effect on the surface quality of CdTe NCs, which has been generally observed for the microwave synthesis of semiconductor NCs.12–15The PL quantum yield of CdTe NCs prepared by the microwave irradi-ation in the ionic liquid reached up to 61% with PL peak at 640 nm. The systematic study for the reaction conditions such as the structure of ionic liquids, precursor concentration, temper-ature, programmed heating, and the ratio of thiol ligand to cadmium ion would offer the optimum conditions for CdTe NCs with a desirable PL peak and high PL quantum yield.13
The drawback of the ionic liquids as media for NCs is the less accessibility to NCs due to the very stable solvation and poor volatility of ionic liquids. The use of ionic liquids therefore limits the application of ionic liquid–NC composites to photo-electrochemical devices utilizing the photo-electrochemical property of ionic liquids and luminescent polymer composites23 via in situ polymerization of ionic liquid monomers. We therefore investi-gated the extraction of NCs from the ionic liquid solution to organic media and water by ligand-exchange. The capping ligand DMAET in the ionic liquid was substituted with 1-dodecanethiol so that the NCs are soluble in chloroform.37The both absorption and PL peak positions shifted to a shorter wavelength from 600 nm to 580 nm and from 570 nm and 550 nm, respectively, by the ligand-exchange from DMAET to 1-dodecanethiol (Fig. 5). The blue-shift in both spectra might indicate the etching of Te during the ligand exchange with 1-dodecanethiol as reported previ-ously.38The PL quantum yield after the extraction was measured to be 52% in chloroform solution, which is almost identical to that in MOMPyrTf2N before the extraction. Although DMAET is
readily soluble in water, the DMAET-capped CdTe NCs trans-ferred to ionic liquids were hardly back extracted to water due to the stable solvation with ionic components.22,23 We therefore extracted the DMAET-capped CdTe NCs by ligand-exchange with anionic TGA. In contrast to cationic DMAET-capped CdTe NCs, the NCs with anionic TGA ligand were not transferred from water to ionic liquids.23The CdTe NCs in the ionic liquid were
Table 1 Fitting parameters for eqn (1) Conditionsa D
0/nm a/nm min1 s/min R2
a 2.28 0.043 0.036 0.0033 54 7.1 0.999 b 1.67 0.189 0.035 0.0091 61 16 0.999 c 2.39 0.038 0.0050 0.00065 177 23 0.999 d 1.72 0.037 0.0066 0.00067 161 16 0.999
aConditions a–d correspond to those in Fig. 4.
precipitated by the addition of a 0.5 M TGA methanol solution. The orange precipitate was re-dissolved in weakly alkaline water (pH¼ 12) to give a stable dispersion of TGA-capped CdTe NCs in water. The PL quantum yield decreased but was still 14% even after the extraction to water. The optimization of the ligand-exchange conditions might afford the aqueous dispersion of NCs with PL property as high as those in ionic liquids.
Conclusion
We have demonstrated the preparation of thiol-capped CdTe NCs directly in the ionic liquid in a similar manner to the aqueous synthetic route. The combination of microwave irradiation with ionothermal conditions at high temperature offered the quick route to CdTe NCs with a high PL intensity. The specific microwave effect has been found in the activation of NC-precursor both in the ionic liquid and water, which plays a pivotal role in the nucleation of CdTe NCs at the early stage of NC formation. The CdTe NCs prepared in the ionic liquid were transferred in water or organic solvents via ligand-exchange. The optimization of conditions would provide the strongly luminescent CdTe NCs in a wide range of media.
Acknowledgements
The authors thank Mr S. Katao for his assistance in XRD measurements. This work was partly supported by Grant-in-Aids for Scientific Research on Priority Area ‘‘Ionic liquids’’ from the Ministry of Education, Culture, Sports, Scientific and Technology (MEXT) of Japan.
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Fig. 5 Absorption (broken lines) and PL spectra (solid lines) of CdTe NCs capped with (a) DMAET in MOMPyrTf2N and (b) 1-dodecanethiol
in chloroform.
