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Highly luminescent ZnO and CdS nanostructures prepared by ionic liquid precursors TAO LeiMing, XU HongTao, AN YanQing, DU ZuLiang & WU SiXin† Key Laboratory for Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475004, China

The ionic liquids containing Cd and Zn, which served as the metal-chalcogenides precursors, were synthesized and reacted with Na2S to synthesize the ionic-liquid-capped semiconductors. The products were detected by XRD and TEM. The results demonstrated that the CdS was composed of 5―6 nm monodispersed nanocrystals. At the same time, the ZnO composed of 1 μm hexagonal-disk nanostructure was prepared under the same experimental condition. The difference of the morphology and structures between Zn and Cd systems was discussed by thermodynamics and crystallography. The fluorescence of as-prepared ZnO and CdS showed the excellent photoluminescence. ionic-liquid, ZnO, CdS

1 Introduction Semiconductor nanocrystals (NCs), due to the quantum confinement effect, show a number of unique optical properties, such as broad excitation spectrum, and narrow tunable emission[1,2]. In addition, the NCs also have strong optical stability and high quantum yield[3]. Therefore, the NCs were widely studied concerning their potential applications in nano-optoelectronic devices[4,5], and biological fluorescence[6,7]. The NCs with different sizes and shapes were synthesized, and their properties were widely studied. At present, high quality NCs are mainly synthesized through the pyrolysis method with organicmetal salt at high temperature[8,9]. The NCs prepared by this method owned the monodispersed, and uniform size, high quality fluorescence. Moreover, this method has been more and more improved[10,11]. However, from the perspective of the synthetic green for avoiding the use of extreme condition, such as high temperature, anaerobic, free-water, and toxic atmosphere[8,12,13], the new methods are still concerned in the field. Ionic liquids[14,15] have received a great deal of attention as green and designer solvents due to their unique properties, such as non-volatile, low melting point,

wide-range solution, strong electric field, wide electrochemical window, good electrical, thermal conductivity, transparency, high refractive index, capacity, stability, ― and selectivity[16 18]. In recent years, ionic liquids was used not only to synthesize special-functional inorganic ― materials[19], but also to prepare nanoparticles[20 25]. These methods changed the previous extreme conditions[8,12,13], and were quickly developed. Rao et al. synthesized nanoparticles of CdS by the reaction of cadmium acetate dihydrate with thioacetamide in imidazolium [BMIM]-based ionic liquids[22]. Nonoguchi et al. synthesized thiocholine bromide (TCB)-capped CdTe NCs in ionic liquid [2 3 ] . However, in the above-mentioned methods, ionic liquids only serve as a medium. Dai et al. synthesized ZnO NCs from Zn-containing ionic liquid precursors[24]. Chen et al. synthesized highly luminescent ZnO NCs by Zn-containing ionic liquids[25]. In these methods[24,25], ionic liquids not only served as a liquid medium, but also acted as a reactant to involve reaction. So far, the preparation of Received November 1, 2008; accepted December 28, 2008; published online July 13, 2009 doi: 10.1007/s11426-009-0104-1 † Corresponding author (email: [email protected]) Supported by the National Natural Science Foundation of China (Grant No. 20871041) and the Key Project of Ministry of Education (Grant No. 208086)

Sci China Ser B-Chem | Dec. 2009 | vol. 52 | no. 12 | 2141-2147

nanomaterials by ionic liquid is still scarce, and there have been few reports about metal- chalcogenides NCs prepared by ionic liquids. Here, we prepare the precursors of Cd and Zn ionic liquid (IL-M, M stands for Zn or Cd, respectively), and their constitutions are shown in Figure 1. Ionic-liquid-capped semiconductors are prepared through these precursors reacting with Na2S. The products are detected by XRD and TEM. The results demonstrate that the CdS is composed of 5―6 nm monodispersed NCs. At the same time, the ZnO is of 1 μm hexagonal-disk nanostructure under the same experimental condition. The difference of the morphology and structures between Zn and Cd systems is discussed by thermodynamics and crystallography. The fluorescence results of as-prepared ZnO and CdS illustrate excellent photoluminescence.

Figure 1

2

IL-M precursor constitution.

Experimental

was added to precipitate C18-Zn as a white solid, which was isolated by filtration and washed with anhydrous diethyl ether. The white solid was dispersed into 50 mL deionized water, washed and filtered for several times with chloroform. Then the solution was rotationally dried to get C18-Cd, and finally dried at 70℃ in vacuum. (3) N, N-dimethyloctadecylammonium bis((trifluoromethyl)sulfonyl) amide acetate cadmium salt (IL-Cd): An aqueous solution (50 mL) of C18-Cd (1 mmol) and an aqueous solution (50 mL) of lithium bis((trifluoromethyl) sulfonyl)amide (LiN(Tf)2) (2 mmol) were mixed and heated at 70℃ for 20 min while stirring until the solution turned turbid. Chloroform (50 mL) was then added to the hot mixture to extract the reaction product. The extract was concentrated on a rotary evaporator and dried in vacuum at 70℃ to get IL-Cd. (4) IL-CdS: The IL-Cd (1 mmol) was dissolved in 30 mL absolute ethanol. Na2S (20 mmol) was added dropwisely to the above solution. After sufficient reaction, the final product of IL-CdS was washed for several times with the deionized water and evaporated. In the preparation process of ionic liquid ZnO nanoparticles, ZnCl2 served as the raw material, and the other processes were similar to that of CdS NCs.

2.1 Preparation of ionic liquid CdS(IL-CdS) NCs

2.2 Characterization

The preparation of IL-CdS NCs is based on the method in ref. [25] with a little modification. The detailed procedure is as follows: (1) Cadmium bromoacetate: Cd(OH)2 was freshly precipitated from an aqueous solution of Cd(NO3)2 (10 mmol) by addition of NaOH (20 mmol), then mixed with deionized water solution of 10 mmol of bromoacetic acid. The mixture was heated at 70℃ with vigorous stirring for 6―8 h until the pH value of the system reached about 6.0. The residual Cd(OH)2 was removed by filtration and the filtrate was concentrated on a rotary evaporator. The resulting solid was dried in a vacuum oven at 100℃. (2) N, N-dimethyloctadecylammonium bromide acetate cadmium salt (C18-Cd): N, N-dimethyloctadecylam-ine (Aldrich, 10 mmol) and cadmium bromoacetate (5 mmol) were dissolved in absolute ethanol (50 mL) and the vigorously stirred solution was heated under reflux for 24 h. The solution was then concentrated on a rotary evaporator to about 4 mL. This solution was cooled to −6℃ in the refrigerator for 4 to 5 h. Anhydrous diethyl ether (20 mL)

The morphologies and microstructures of the samples were observed using a JEM 100CX-II transmission electron microscope (TEM). The crystal structures of the as-prepared samples were detected by an MPD X-ray diffraction (XRD, Philips X’pert PRO using Cu Kα radiation, λ = 1.5406 Å). A luminescence spectrometer (Perkin-Elmer LS 55) was used to detect the photoluminescence spectra at the wavelength range from 200 to 900 nm. The thermogravimetric and differential thermal analysis (TG-DTA) curves were recorded with a TG-DTA thermal analyzer (Seiko Instruments Inc, EXSTAR6000) at a temperature-increase rate of 20℃·min−1 under nitrogen. 1HNMR spectra (tetramethylsilane was used as reference) were recorded with a Bruker Ultrashield 500 MHz NMR spectrometer and the IR spectra were recorded with a Bruker IFS 66 v/S FTIR spectrometer equipped with a DGTS detector.

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3

Results and discussion

3.1 X-ray diffraction (XRD) analysis Figure 2 demonstrates the XRD patterns of ZnO and

TAO LeiMing et al. Sci China Ser B-Chem | Dec. 2009 | vol. 52 | no. 12 | 2141-2147

CdS, which are marked as the solid and dotted lines, respectively. From Figure 2, we can observe that, for ZnO, the XRD patterns show (100), (002), (101), (102), (110), (103), (112) characteristic planes at 2θ values at 31.65°, 34.48°, 36.25°, 47.72°, 56.57°, 62.70° and 67.91°, respectively, which match those of hexagonal wurtzite ZnO (JCPDS (05-0664)). There are five distinct characteristics of the diffraction peaks at 26.57°, 44.16°, 52.31°, 70.60° and 81.60° in the CdS XRD chart, corresponding with cubic CdS (JCPDS (80-0019)). The five characteristic diffraction peaks are corresponding to the cubic phase (111), (220), (311), (331) and (422) crystal planes.

be expressed as follows: Zn 2 + +S2 − → ZnS

(3)

Zn 2 − + 2OH − → Zn ( OH )2

(4)

Cd 2− + S2 − → CdS

(5)

Cd 2− + 2OH − → Cd ( OH )2

(6)

The values ΔG0 of the reactants and the products of formulae (3) to (6) are showed in Table 1. Table 1 to (6)

The values ΔG0 of the reactants and the products of formulae (3)

Matter

Δf G0 (kJ·mol−1)

Matter

Δf G0 (kJ·mol−1)

Zn2+

−147.176

Cd2+

Zn(OH)2

−559.15

Cd(OH)2

ZnS

−180.249

CdS

−138.490

97.906

OH−

−157.256

S2−

−77.74 −471.662

Data come from “Handbook of Chemistry and Physics” 66-th ED (1985―1986).

Thus, the standard free energy (ΔG0) of formulae (3)―(6) can be calculated: For formula (3) Figure 2

The XRD patterns of as-prepared ZnO and CdS.

(1)

The standard Gibbs free energy can be showed:

ΔGr0 = ⎡⎣cΔ f G 0 ( C ) + d Δ f G 0 ( D ) ⎤⎦ − ⎡⎣ aΔ f G 0 ( A ) + bΔ f G 0 ( B ) ⎤⎦

(7)

ΔG 0 = −97.462

(8)

ΔG 0 = −158.656

(9)

ΔG 0 = −236.666

(10)

For formula (4)

For Zn and Cd systems, the whole preparation process keeps in the same condition, but the products are significantly different. The final products are CdS and ZnO for Cd and Zn systems, respectively. Here, we explain the composition difference according to the stability of the final products. Since these reactions are thought to be the irreversible reaction, if there are two or more possible final products, we can determine the reaction way through thermodynamic Gibbs free energy. The chemical reaction equation for a general reaction can be expressed as follows:

aA + bB → cC + dD

ΔG 0 = −130.979

(2)

In our experimental condition, there are two possibilities of the final products for the systems of Cd and Zn, respectively and their chemical reaction equations can

For formula (5)

For formula (6)

From the results of (7)―(10), it can be concluded that for Zn system, it is more stable to form Zn(OH)2. Therefore, the final product will be Zn(OH)2. Analogously, as to Cd system, it is more stable to get CdS. The final stable products for Cd and Zn systems are CdS and ZnO, respectively. It is easy to interpret the results under our experimental condition, which confirmed the above expectation. 3.2 Thermal analysis

Figure 3(a) and (b) show the TG-DTA of ionic-liquidcapped ZnO and CdS, respectively. From Figure 3(a), we can observe that there exist three weight-loss peaks from 25 to 600℃. The first weak peak of weight loss


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significant weight loss (about 38%), and DTG shows the peak at 337℃. DTA is an exothermic process at this time. In the range of 384―600℃, there exists obvious weight loss (about 28%), and DTG shows the peak at 439℃ while DTA has an exothermic process. The residue of 10% is kept at 600℃, which corresponds to CdS. 3.3 Transmission electron microscopy (TEM) analysis

Figure 3 The TG-DTA analysis of IL-ZnO (a) and IL-cds (b) under N2 atmosphere

(1%) appears below 100℃, and an endothermic peak is found in DTA. This weight loss is caused by adsorbed water. In the range of 200―300℃, there is a relatively obvious weight loss (about 20%), meanwhile, DTG shows a peak at 231℃. In the range of 300―500℃, there exists significant weight loss (about 70%), and DTG shows a peak round 418℃. The residue of 10% is kept at 600℃, which corresponds to ZnO. From Figure 3(a), we can observe that IL-ZnO is stable before 150℃. In order to maintain the stability of the components of ionic liquid, the reaction temperature is controlled at below 100℃. From Figure 3(b), we can see that there exists four weight-loss peaks from 25 to 600℃. The first weak peak of weight loss (1%) appears at below 100℃, and an endothermic peak is found in DTA. This weight loss is also caused by adsorbed water. In the range of 200―271℃, there is an obvious weight loss (about 23%), meanwhile, DTG shows a peak at 228℃ and DTA has an endothermic process. In the range of 271―384℃, there exists 2144

3.3.1 ZnO TEM images. The TEM photograph of untreated IL-ZnO is showed in Figure 4(a). Obviously, the ZnO surfaces are capped by a large amount of ionic liquids. It can explain the reason of high weight loss (90%) of IL-ZnO in thermal analysis process. In order to investigate the evolution of ZnO, we remove the ionic liquid capped on the ZnO surface before we prepare the samples for the morphology characterization by TEM. The morphologies of ZnO at different reaction time are illuminated in Figure 4(b)―(d). The morphology of ZnO in half an hour is mainly polydispersed hexagonal-disk, with the size ranging between 10 and 100 nm (Figure 4(b)). With further reaction, the shape of ZnO becomes more regular hexagonal-disk in an hour (Figure 4(c)), compared to that in half an hour. The size distribution is more uniform, while the particle size is about 100 nm. When

Figure 4 The TEM photographs of IL-capped ZnO (a), and the morphology evolution of ZnO with different reaction time of 0.5 h (b), 1 h (c) and 5 h (d), respectively. Inset of (d) displays the energy dispersive X-ray spectroscopy of ZnO in 5 h.


the reaction time further extends to 5 h, the size of ZnO particles (Figure 4(d)) sharply increases to about average 1 μm, and the morphology illuminates perfect hexagonal-disk structure. The energy dispersive X-ray spectroscopy of ZnO demonstrates a hexagonal single crystal (inset of Figure 4(d)), which is corresponding to that observed by the wide-angle XRD. From the above result (Figure 4(b)―(d)), it is concluded that with increasing the reaction time, the morphology of ZnO almost keeps hexagonal-disk. At the same time, the particle size obviously increases from about 10 nm to 1000 nm. After 5 h, the size and morphology of ZnO will have no significant change with time. 3.3.2 CdS TEM images. Figure 5 shows the TEM photographs of IL-CdS. From Figure 5(a), we can find the CdS is composed of microspheres of 50―100 nm, and each of the microspheres is composed of the smaller CdS NCs, which are linked by ionic liquid. The similar result has been reported for ZnO NCs[25]. From the partial enlargement of microsphere (Figure 5(b)), it is clearly observed that the average size of the smaller CdS NCs in microspheres is about 5―6 nm, which is consistent with the result by Scherrer formula from XRD. Every CdS NC shows single crystal structure. From the higher magnification of TEM, it becomes clear that the

lattice spacing (Figure 5(c)) of the planes parallel to the growth direction is 0.325 nm. According to the formula calculation: 2d sin θ = λ Here,

(11)

λ = 0.1549 nm

We obtain:

θ = 13.7° which corresponds to the (111) plane. As we have known, during the preparation of ZnO and CdS nanoparticles, the morphologies are also totally different in Zn and Cd systems. Figures 4 and 5 show that ZnO nanoparticles have hexagonal-disk structure. At the same time, CdS NCs are composed of the spherical particles. According to the previous reports[25], the morphology of IL-ZnO NC is spherical when prepared through Zn ionic liquid precursors reacting with LiOH. So it has to be clarified that why the morphology of the ZnO is totally different when Zn ionic liquid precursors react with Na2S. Here, we try to explain the above difference according to the growth model of anion coordinate tetrahedron[26]. This model asserts that the morphology of ZnO is significantly influenced by pH value[27]. In the weak basic solution, the positive axis of ZnO grows fast. Finally, this axis disappears while the negative axis remains due to the slow rate. Therefore, the ZnO forms a hexagonal structure. In the strong basic solution, the growth rate keeps the same for every crystal plane, and thus the final shape is spherical. In our reaction system, the pH value of Na2S solution displays a weak base. Under such conditions[28], the order of the growth rate of the crystal face for ZnO is as follows: V{0001}＞V{ 01 1 1 }＞V{ 01 10 }＞V{ 01 11} ＞ V{ 000 1 }. Therefore, ZnO positive axis direction

[0001] and the cone grow much faster along { 01 1 1 }, and these axes are easy to disappear. However, the negative axis grows relatively slow and roughly keeps its position. Therefore, the hexagonal structure of ZnO can be formed. As reported in ref. [25], since LiOH is a strong base, the IL-ZnO NCs prepared through Zn ionic liquid precursors reacting with LiOH are spherical particles. 3.4 Fluorescence Figure 5 The TEM photographs of IL-CdS NCs with the different magnification. (a) TEM image of IL-CdS; (b) HRTEM of IL-CdS; (c) HRTEM image of the pane region of (b).

Figure 6 shows the fluorescence spectra of the as-prepared ZnO and CdS. For the fluorescence measurement,


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the ZnO and CdS NCs are dispersed in chloroform. From Figure 6, we can observe that the as-prepared ZnO and CdS NCs both display strong fluorescence emission. For ZnO, the fluorescence spectrum is located at 375 nm under excitation of 280 nm, the half-width of the fluorescence spectrum of ZnO is about 60 nm. This fluorescence spectrum corresponds to the intrinsic fluorescence emission of ZnO. Meanwhile, we can see that there is a weak shoulder peak at 468 nm, which corresponds to the defect emitting of ZnO. The fluorescent emission peak of CdS NCs is located at about 412 nm with the excitation wavelength of 370 nm, corresponding to the intrinsic emission peak. In order to evaluate the fluorescence quantum yields (QYs) of the IL-ZnO and IL-CdS, quinine sulfate is used as the reference. The fluorescence QYs of the as-prepared IL-ZnO and IL-CdS dispersed in solvent are about 25% and 37%, respectively. For the conventional ZnO nanoparticles, the fluorescence QYs ― are about 1%―16%[29 31]. Therefore, IL-ZnO NCs by our method are greatly improved. For the IL-CdS NCs, compared to the previous results[32], the fluorescence QYs range from 18% to 31%, and the fluorescence QY also has been improved obviously.

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Figure 6 The fluorescence spectra of the ionic-liquid-capped ZnO and CdS NCs.

4 Conclusions In this paper, the hexagonal-disk structural ZnO and the spherical CdS NCs have been prepared through the ionic liquids. The result of fluorescence measurement of the products shows that both ZnO and CdS NCs display higher QYs compared to some previous methods. The difference of the morphology and structures between Zn and Cd systems is investigated in detail based on the anion coordinate tetrahedron model of growth and Gibbs free energy.

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