Luminescent Bis-(8-hydroxyquinoline) Cadmium Complex Nanorods
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 564–567
Wei Chen, Qing Peng, and Yadong Li* Department of Chemistry, Tsinghua UniVersity, Beijing, 100084, Peoples Republic of China ReceiVed July 8, 2007; ReVised Manuscript ReceiVed October 24, 2007
ABSTRACT: In this paper, we report a facile solution-based route for the synthesis of bis-(8-hydroxyquinoline) cadmium (CdQ2) complex nanorods and nanoflowers (bundles of nanorods) in an oleic acid-sodium oleate-ethanol-hexane (or not)-H2O system at 55–100 °C. Field emission-scanning electron microscope (FESEM) images indicated that a longer time and a higher temperature would result in nanoflowers, while higer concentrations of the reactants and the surfactant with a lower temperature and a shorter reaction time would be appropriate for the formation of nanorods. Also, the hexane could sabotage the anisotropic crystal growth of the complex, leading to shorter nanorods. The C, H, and N element analysis and thermal gravimetric analysis (TGA) jointly determined the molecular formula of the products, and the Fourier-transform infrared spectrum (FTIR) was utilized to further confirm that the samples were made up of CdQ2. All the samples possessed excellent photoluminescence (PL) properties. This facile methodology could be extended for the controlled large-scale synthesis of nanostructures of other functional complexes, and the obtained CdQ2 nanorods could be introduced as the building blocks for novel optoelectronic devices. Introduction In the past decades, one-dimensional (1D) nanostructures have been the focus of much research owing to their profuse electronic and optic properties1 and potential applications in optoelectronics,2–4 field-effect transistors,5 etc. The most attention, however, has been paid to carbon, chalcogenides, nitrides, fluorides, and so on.1–5 Contrastively, very few efforts were devoted to organics and functional complexes.6–8 As is well-known, complexes gain absolutely different electronic and optic properties due to the noncovalent intermolecular interactions such as the Van der Waals force, hydrogen bonding, and π-π stacking,6 which have allowed them to be widespreadly used for cheap and novel optoelectronic devices.6,7 In addition, complexes have various electronic states not only from inorganic elements but also from ligand molecules with diversity, tailorability, and multifunctionality.9 In these regards, it is desirable and meaningful to synthesize 1D nanostructures of functional complexes. Currently, 8-hydroxyquinoline metal chelates are intensively investigated, due to their excellent electro- and light-emitting properties, such as field emission, photoluminescence (PL) and electroluminescence (EL).10 Herein, we report the facile synthesis of bis-(8-hydroxyquinoline) cadmium (CdQ2) complex nanorods and nanoflowers in an oleic acid (OA)-sodium oleate-ethanol-hexane (or not)-H2O system11 at 55–100 °C. The PL spectrum revealed that all the samples had excellent photoluminescence and the smaller nanorods emitted stronger PL, due to the larger surface area absorbing more UV light. Experimental Section I. Materials. All the reagents in this work, including cadmium nitrate (Cd(NO3)2 · 4H2O), 8-hydroxyquinoline, ethanol (C2H5OH), sodium hydroxide (NaOH), oleic acid (CH3(CH2)7CHdCH(CH2)7COOH), and hexane (C6H14) were of A.R. grade, were bought from the Beijing Chemical Factory, and were used without further purification. Deionized water was used throughout. II. Synthesis of the Samples. In general synthesis, the reaction is mainly on the basis of the precipitation of Cd2+ and 8-hydroxyquinoline * To whom correspondence should be addressed. E-mail: ydli@ mail.tsinghua.edu.cn.
with a molar ratio 1:2 in a mixed ethanol–water (1:1) solvent at designed temperatures. In detail, NaOH, oleic acid (OA) and hexane (or not) were orderly added into the mixed solvent with continuous stirring to produce a homogeneous normal emulsion. Then, the aqueous solution of Cd(NO3)2 was relaxedly poured into the system. Subsequently, the ethanol solution of 8-hydroxyquinoline was rapidly introduced into to start the main reaction and the total volume of the mixture was kept at a certain value 43 mL. After stirring for about 5 min, the mixture was transferred into a 45 mL Teflon vessel, which was treated at a designated temperature for appropriate reaction time. Then, the system was cooled to room temperature naturally. The bottom precipitates were thoroughly washed with ethanol twice and then redispersed in ethanol for future use and characterization. The detailed experimental conditions for each sample are listed in Table 1. III. Characterization. The sizes and morphologies of the samples were determined by a LEO-1530 field-emission scanning electron microscope (FESEM). Samples were prepared by placing a drop of a dilute ethanol dispersion of the product on the surface of a silicon grid. The C, H, and N element analysis was carried out on a CE-440 elemental analyzer. The thermal behavior of the products was studied by thermogravimetric analysis (TGA) with a TGA2050 thermal analysis device (American TA Corporation). TGA determination was carried out in air at a heating rate of 10 °C/min in a range of room temperature to 600 °C. FTIR spectra were conducted with a Nicolet 560 Fouriertransform infrared spectrophotometer. Fluorescent spectra were recorded with a Hitachi F-4500 fluorescence spectrophotometer, and the color PL image was taken on a OLYMPUS IX71 epifluorescence microscope with the sample dip-coated on a glass substrate.
Results and Discussion In our synthesis process, shown in Figure 1, as the consequence of the addition of NaOH, sodium oleate (NaOA) would form and then coordinate with Cd2+ to produce cadmium oleate, which released Cd2+ slowly on the basis of the equilibrium equation to control the nucleation and growth process accurately. The other function of sodium oleate was to produce a buffer media jointly with the superfluous oleic acid, and the pH value of the media was about 6.0–6.5, which was lucrative for the precipitation between Cd2+and 8-hydroxyquinoline.12 On the other hand, the interactions such as van der Waals and the hydrophobic force between the alkyl chain and quinoline ring would have made it easier for 8-hydroxyquinoline molecules to collide with Cd2+ capped on the surface of sodium oleate to start the main reaction. At last, due to the low solubility
10.1021/cg0706316 CCC: $40.75 2008 American Chemical Society Published on Web 12/21/2007
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Table 1. As-obtained CdQ2 with Different Morphologies and Sizes on Different Conditions in the Oleic Acid (Oa)-Sodium Oleate-Ethanol-Hexane (Or Not)-H2O Systema sample
Cd2+ (mmol)
OA (mL)
NaOH (g)
Hexane (mL)
T (°C)
t (h)
morphology
lengthb (µm)
diameterb (nm)
I II III IV V VI VII
0.3 0.3 0.3 0.3 0.6 0.6 0.6
3.7 3.7 3.7 3.7 3.7 7.5 3.7
0.2 0.2 0.2 0.2 0.2 0.4 0.2
0 0 0 0 0 0 4
55 55 75 100 55 55 55
2.5 5.0 2.5 2.5 2.5 2.5 2.5
nanorods nanoflowers nanoflowers nanoflowers nanorods nanorods nanorods
∼1.2 ∼5.0 ∼2.0 0.9–1.5 10–20 1.5–2.6 ∼4.2
90–100 120–450 80–150 ∼80 300–500 200–300 300–500
a
The molar ratio between Cd2+ and 8-hydroxyquinoline was 1:2. those of the nanorods of which they are composed.
b
When it comes to samples of nanoflowers, the length and diameter values are
Figure 1. Schematic diagram showing the formation of CdQ2 nanostructures.
of the complex, the final products deposited at the bottom of the container, giving convenience for collection and purification. In order to control the morphology and size of the product and understand the crystal growth behaviors of the intentional molecule, we investigated the effects of the following factors, reaction time and temperature, reactant concentration, surfactant concentration and the addition of hexane. As shown in Table 1 and Figure 1, a longer time and a higher temperature would result in nanoflowers, while higher concentrations of the reactants and the surfactant (OA) with a lower temperature and a shorter reaction time would be appropriate for the formation of nanorods. Figure 2 parts a and b are the FESEM images of sample I obtained at 55 °C for 2.5 h, which indicated that the product
was composed of abundant nanorods, with about 1.2 µm in length and 90–100 nm in diameter. When the reaction time increased from 2.5 to 5.0 h with other conditions fixed, the monomer of nanorods became longer and thicker and then aggregated together to form rod-bundles (nanoflowers), which are clearly shown in Figure 2c and d. The rods were about 5.0 µm long and 120 nm wide at one end and 450 nm at the other end. Under the irradiation of the electron beam, the thinner end of a few nanorods bent over, due to their inherent flexibility and the different size along the entire rod, leading to different resistibility under the bombardment of the electron beam. Inspired by the formation of sample II, we considered that if the temperature increased, the monomers of nanorods would move faster and collide vigorously with each other to aggregate into nanoflowers before they grew longer and thicker. This viewpoint was verified by the SEM images of sample III and IV. As shown in Figure 2e for sample III (75 °C) and Figure S1 for sample IV (100 °C), the sizes of the nanorod monomer were about 2.0 µm × 80–150 nm and 0.9–1.5 µm × 80 nm, respectively.
Figure 2. FESEM images of (a and b) sample I, (c and d) sample II, (e) sample III, and (f) sample VII. The detailed experimental conditions are listed in Table 1.
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Figure 3. Typical TGA curve of the product obtained in air. Figure 4. Typical FTIR spetrum of the product.
Keeping the short time (2.5 h) and low temperature (55 °C) constant, a double reactant concentration led to longer and thicker rods with sizes of 10–20 µm × 300–500 nm (Figure S2, sample V) instead of bundles of nanorods (nanoflowers). Comparing sample VI to V, we can find that the more OA that was introduced into the system, the smaller the rods became. As shown in Table 1 and Figure S3, we can see that the size decreased from 10–20 µm × 300–500 nm to 1.5–2.6 µm × 200–300 nm. In addition, the hexane was also an important factor in the morphology of the product. As clearly presented in Table 1 and Figure 2f, after 4 mL hexane was added into the system on the basis of the conditions of sample V, the nanorods grew shorter, from 10–20 µm to about 4.2 µm, with the diameter nearly unchanged. Generally, in a higher polarity system, the stronger dipole–dipole interactions would induce the assembly and stacking of CdQ2 molecules in a particular direction, for easy formation of rod- or wirelike structures.13 In our system, water and ethanol are high polar solvents, and hexane is a kind of nonpolar solvent which would lessen the polarity of the media, which subsequently sabotaged the anisotropic crystal growth of the complex, leading to shorter nanorods. The C, H, and N element analysis combined with thermal gravimetric analysis (TGA) and Fourier-transform infrared spectra (FTIR) confirmed that the samples were made up of bis-(8-hydroxyquinoline) cadmium (CdQ2). From the C, H, and N element analysis we found C, 53.75; H, 2.74; N, 7.02% and calculated Cd(C9H6NO)2(CdQ2) C, 54.00; H, 3.00; N, 7.00%. It was obvious that the experimental results were in good agreement with the calculated data. The result of the thermal gravimetric analysis (TGA) taken in air (Figure 3) indicated that the main weight loss occurred in the temperature range from 380 to 505 °C, and the final remaining weight was about 31.90%, which agreed well with the theoretical weight surplusage of CdO (32.00%) produced from the reaction between CdQ2 and O2 in air. Therefore, it was unambiguous that the products were composed of bis-(8-hydroxyquinoline) cadmium. Additionally, the component of the nanostructures was further identified with an FTIR spectrum. As indicated in Figure 4, similar to the congeneric compounds,14 the two bands of 1600 and 1569 cm-1 should correspond to a CdC stretching vibration in the quinoline group. The bands at 1489 and 1459 cm-1 are assigned to a CC/CN stretching and CH bending vibration of the pyridyl and phenyl groups in CdQ2. The vibrations centered at 1389, 1370, and 1320 cm-1 are CC/CN stretching and CH bending of the quinoline fragments of CdQ2. The bands observed in the spectrum with peak positions at 1279, 1232, and 1110 cm-1 are attributed to a CH/CCN bending and CsN/CsO stretching vibrations. The intense absorptions at 821, 802, 788, 748, and 729 cm-1 should
Figure 5. Room-temperature PL spectrum of CdQ2 with different shapes and sizes dispersed in ethanol: (a) nanorods; (b) nanoflowers. The excitation wavelength was 355 nm.
be the out-of-plane CH wagging vibrations of the quinoline groups. Peaks at 652, 600, and 573 cm-1 should correspond to CdsO stretching vibrations, and bands at 490 cm-1 are attributed to the CdsN stretching vibrations. Photoluminescence is a very important characteristic for the 8-hydroxyquinoline metal chelates. Figure 5 is the room-temperature PL spectrum of CdQ2 nanostructures dispersed in ethanol irradiated by 355 nm of UV light, from which we can see that all the emissions are ranging from 425 to 675 nm and the maximum peaks are all located around 510 nm. There is no obvious shift as the size of the nanostructure changes, due to the weak interaction in the molecular crystal CdQ2. For rational comparison, the concentration of each sample was fixed as 0.075 mmol/L. To get
Luminescent CdQ2 Nanorods
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Figure 6. Color PL image of sample I.
nearly the same disperse concentration, three steps were needed. First of all, the precipitate was washed by ethanol four times and then dried at 50 °C overnight. Second, 0.3000 g (0.75 mmol) of CdQ2 nanorods was quantified and dispersed in 100 mL ethanol by ultrasound (5 min) to get a 7.5 mmol/L suspension. At last, 20 µL of the suspension was taken out and diluted to 2 mL. Therefore, the sample for PL measurement was obtained and the concentration of it was 0.075 mmol/L. The PL spectra (Figure 5a) reveal that the nanorods with smaller size gained stronger emission, and maybe the reason is that the larger surface area of the nanorods with smaller size would increase the absorption of UV light and then emit stronger PL.15 But unfortunately, the variation of PL (Figure 5b) intensity of the nanoflowers with different sizes cannot be explained with this reasoning, and the exact law is still under investigation. As an example, Figure 6 is the typical color PL image of sample I, which presented strong photoluminescence.
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Conclusion In summary, bis-(8-hydroxyquinoline) cadmium (CdQ2) complex nanorods and nanoflowers were first obtained easily based on the precipitation of Cd2+ and 8-hydroxyquinoline in an oleic acid-sodium oleate-ethanol-hexane (or not)-H2O system at 55–100 °C. Reaction time and temperature, reactant and surfactant concentration, and the addition of hexane were the main influential factors to the size and morphology of the product. The C, H, and N element analysis and thermal gravimetric analysis (TGA) jointly determined the molecular formula of the products, and FTIR was utilized to further confirm the that samples were made up of CdQ2. All the products emitted strong photoluminescence. For the nanorods, the smaller they were, the stronger PL they emitted if irradiated by the same UV light, but this law was not applicable in the case of nanoflower samples. This facile methodology could be extended for the controlled synthesis of other functional coordination compound nanostructures, and the obtained CdQ2 nanorods could be introduced as the building block for novel optoelectronic devices. Acknowledgment. This work was supported by NSFC (90606006) and the State Key Project of Fundamental Research (2006CB932300). Supporting Information Available: FESEM images of sample IV, V, and VI and X-ray diffraction (XRD) patterns. This material is available free of charge via the Internet at http://pubs.acs.org.
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