Synthesis of Colloidal ZnSe Nanospheres by Ultrasonic-Assisted

Dec 10, 2008 - TOPO (2 g)/ODE (10 mL) as a Zn precursor and Se (10 mmol)/TOP (5 mL)/ODE (5 mL) as a Se precursor. Those ZnSe nanoparticles...
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CRYSTAL GROWTH & DESIGN

Synthesis of Colloidal ZnSe Nanospheres by Ultrasonic-Assisted Aerosol Spray Pyrolysis Dae-Jin Kim and Kee-Kahb Koo* Department of Chemical and Biomolecular Engineering and Interdisciplinary Program of Integrated Biotechnology, Sogang UniVersity, Seoul 121-742, Korea

2009 VOL. 9, NO. 2 1153–1157

ReceiVed September 5, 2008; ReVised Manuscript ReceiVed October 14, 2008

ABSTRACT: Colloidal monodisperse ZnSe nanospheres consisting of primary nanocrystals were prepared by aerosol spray pyrolysis with ultrasonic nebulization. The formation mechanism of nanospheres seems to be generation of primary ZnSe nanocrystals and subsequent aggregation of primary nanocrystals in a tube reactor. Monodispersed ZnSe nanospheres were found to be synthesized only from a specific mixture of precursors at a reaction temperature of 525 °C. Typical ratios were ZnO (5 mmol)/OA (10 mL)/ TOPO (2 g)/ODE (10 mL) as a Zn precursor and Se (10 mmol)/TOP (5 mL)/ODE (5 mL) as a Se precursor. Those ZnSe nanoparticles were characterized by TEM, EDX, FT-IR, UV-vis, and PL techniques. The TEM image shows that the average diameter of ZnSe nanospheres is 41 nm, and primary ZnSe nanocrystals have a size of 2-3 nm. The XRD pattern of ZnSe nanoparticles shows that the crystal structure is zinc blende. The ZnSe nanospheres were confirmed to have a quantum size effect in their optical spectra and exhibit near band-edge luminescence.

1. Introduction Semiconductor nanoparticles have important optical, electrical, and magnetic properties which may improve or expand the applications of their bulk-sized materials. The properties and applications of those materials are strongly dependent on their size and size distribution. Therefore, recently much interest has been aroused in the preparation of the monodispersed semiconductor nanoparticles and assessment of the foundation of physicochemical laws at the nanoscale and practical applications.1-8 Among II-VI and III-V compound semiconductor nanoparticles CdSe has been extensively studied due to their high emission efficiency and size-tuned photoluminescence.9-15 However, the band gap of bulk CdSe is 1.74 eV (712 nm), and it is difficult to be tuned to the UV range. Recently, wide bandgap materials such as ZnSe with a band gap of 2.7 eV (460 nm) and ZnSe-based alloys have been investigated to develop UV emission materials16-19 and examined as a leading candidate for fabrication of blue light-emitting diodes (LEDs) and laser diodes.20,21 Both types of devices are technologically important. Blue LEDs especially are building blocks of full-color electroluminescent displays and can be used for generation of white light element.22 The wide band gap of ZnSe nanoparticles also can be used as an inorganic passivation shell for various semiconductor core/shell nanoparticles, which improve the stability and emission properties of semiconductor core nanocrystals.23,24 Colloidal ZnSe nanoparticles were first proposed using organometallic diethylzinc as a precursor by Hines et al.16 Those nanoparticles exhibited strong luminescence and size-tuned color in the UV blue range. Because size-dependent optical properties and other physical properties of ZnSe nanoparticles are strongly affected by a synthetic method, development of novel synthetic methods is necessary to investigate new characteristics of nanoparticles. Therefore, numerous efforts have been given to development of novel methods for production of characteristic ZnSe nanoparticles. To date, monodispersed ZnSe nanocrystals have been successfully prepared through various solution-based * To whom correspondence should be addressed. Phone: +82-2-705-8680. Fax: +82-2-711-0439. E-mail: [email protected].

methods such as the sol-gel process,25 arrested precipitation,26 precipitation in the presence of stabilizers,27,28 reverse-micelles methods,29 hot-injection methods,16,30-33 and heating-up methods.34,35 ZnSe colloidal spheres have been intensively studied because of their unique properties. In particular, hollow nanospheres have potential applications as photonic crystals, delivery vehicle systems, fillers, and catalysts owing to their tailored structural, optical, and surface properties.36-38 However, few works concerning the ZnSe colloidal microspheres39-42 and nanospheres43-45 have appeared. The present article reports the synthesis of colloidal ZnSe nanospheres by ultrasonic-assisted aerosol spray pyrolysis similar to those reported by Didenko and Suslick46 and Skrabalak and Suslick.47 Kim et al.48 also produced nearly monodispersed CdSe and CdTe nanocrystals with an aerosol flow system, and the effect of reaction temperature and reaction media on the crystal structure of colloidal nanocrystals was discussed in detail. However, zinc-based nanoparticles synthesized by aerosol spray pyrolysis have not yet been reported. Here, we introduce and discuss the synthetic method and condition for production of ZnSe nanospheres consisting of primary nanocrystals.

2. Experimental Section 2.1. Chemicals. Zinc oxide (ZnO, 99.99%), selenium powder (Se, 99.5+%, 100 mesh), tri-n-octylphosphine oxide (TOPO, tech. 90%), tri-n-octylphosphine (TOP, tech. 90%), oleic acid (OA, tech. 90%), and 1-octadecene (ODE, tech. 90%) were purchased from SigmaAldrich. Ethanol (99.5+%, HPLC grade) and toluene (99.5+%, HPLC grade) were from J. T. Baker. All chemicals were used as received without further purification. 2.2. Preparation of ZnSe nanoparticles. Figure 1 represents the experimental setup for ultrasonic-assisted aerosol spray pyrolysis which consists of an ultrasonic nebulizer (1.7 MHz), a furnace with a quartz tube reactor, and a bubbler for collection of nanoparticles produced. The quartz tube reactor is 80 cm long with a 2.8 cm diameter, and the heating length is 50 cm. The tube reactor was vertically equipped to prevent additional reactions by condensation of reactant solution. For the synthesis of ZnSe nanoparticles a mixture of ZnO (5 mmol)/ OA (10 mL)/TOPO (2 g)/ODE (10 mL) was heated in a three-neck flask to 330 °C under Ar and cooled to 60 °C, forming a clear Zn

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1154 Crystal Growth & Design, Vol. 9, No. 2, 2009

Figure 1. Experimental setup for an ultrasonic-assisted aerosol spray pyrolysis. stock solution. A Se stock solution was prepared by dissolving 10 mmol of Se powder in a mixture of TOP (5 mL)/ODE (5 mL) at 250 °C under Ar and cooled to 60 °C. These stock solutions were diluted with addition of 100 mL of toluene at 60 °C and then ultrasonically nebulized into microdroplets. The dense mist produced was carried by Ar gas (1 L/min) through a quartz tube in a tube furnace kept at 525 °C, where solvent evaporation and precursor reaction occurs and ZnSe nanoparticles are produced. ZnSe nanoparticles formed in the reaction tube were collected in a toluene-filled bubbler. Finally, ZnSe nanoparticles were precipitated with ethanol (50 mL) and isolated by centrifugation and decantation. 2.3. Characterization. The optical characteristics of ZnSe particles were investigated by a JASCO V-550 spectrophotometer and a JASCO FP-6200 spectrofluorometer. The aliquots of samples were diluted with toluene directly and placed in quartz cuvettes (1 cm path length) for characterization without any size sorting. The wavelength of excitation for photoluminescence measurements was 340 nm. Fourier transform infrared (FT-IR) spectra were measured using a Nicolet 380 FT-IR spectrometer. Samples for FT-IR were prepared in the following manner: they were washed with ethanol and then dried under a vacuum. KBr was used as a reference. Powder X-ray diffraction (XRD) patterns were measured using a Rigaku MiniFlex X-ray diffractometer operated at 30 kV and 15 mA with graphite-monochromatized Cu KR radiation (λ ) 1.5418 Å). The scanning rate was 1.0 deg/min. Samples were prepared by washing with ethanol and dried under a vacuum. The resulting powders were crushed with a mortar. A JEOL JEM-2100F field emission transmission electron microscope (TEM) operating at 200 kV was used to obtain images of ZnSe nanoparticles. Samples for TEM were prepared by putting a drop of toluene diluted solution of semiconductor nanocrystals onto an amorphous carbon substrate supported on a copper grid and then allowing the solvent to evaporate at room temperature. The elemental composition of the samples was analyzed using a HITACHI S-4300 scanning electron field emission microscope with X-ray energy-dispersed spectrometry (EDX). Samples for EDX measurements were prepared by washing with ethanol and dried under vacuum. EDX measurements were preformed with powder samples deposited on aluminum substrates.

3. Results and Discussion In the present experiments ZnSe nanospheres were produced from a mixture of ZnO/OA/TOPO/ODE and Se/TOP/ODE as

Kim and Koo

precursors. In an ultrasonic-assisted aerosol spray pyrolysis system the selection of precursors is very important since the precursor solutions should move to the reactor as a mist under 60 °C or room temperature. Therefore, preparation of the liquid state of the precursor solutions is an overriding work. Various Zn precursors such as ZnO, diethylzinc ((C2H5)2Zn), zinc chloride (ZnCl2), zinc stearate (Zn(C18H35O2)2), and zinc acetate (Zn(O2CCH3)2) have been examined in the present experiments. Among them, ZnO was found to be the most suitable precursor in the present method. (C2H5)2Zn is very expensive and toxic and needs sophisticated control. Precursor solutions of ZnCl2, Zn(C18H35O2)2, and Zn(O2CCH3)2 were solidified when the temperature of reactant mixture went down to 60 °C after reaction with organic surfactants at higher temperature. In general, the characteristics of as-synthesized nanocrystals such as size, shape, and crystallinity are affected by chemical species used as reaction media or surfactant ligands. The coordinating surfactants and their chain length affect both the nucleation and growth of nanocrystals. Therefore, the coordinating solvents with suitable chain lengths should be considered. The stability and boiling temperature of the solvent or cosolvent are also crucial because an unstable state and a low reaction temperature will lead to imperfection of nanocrystals. As shown in our previous work48 on the synthesis of CdSe and CdTe nanocrystals by an aerosol method, a combination of CdO/OA/TOPO/ODE as a Cd precursor was used. The size and shape of the nanocrystals prepared were shown to be strongly dependent on the experimental conditions such as the molar ratio of Cd to Se or Te, surfactants, and reaction temperature. For preparation of CdSe, CdO is a good candidate as a Cd precursor and combination of TOP/TOPO surfactants is extremely successful, where TOP binds preferentially to the Se and TOPO binds to Cd. Synthesis of ZnSe should not differ much from that of CdSe on the basis of the similarity of the organometallic precursors. However, monodispersed ZnSe nanoparticles have been rarely synthesized in the present aerosol system. The TOP/TOPO coordination solvent system was found not to be appropriate for synthesis of ZnSe. The ZnSe nanocrystals synthesized from various TOP/TOPO systems were so small that they could not be isolated by a standard precipitation method or they precipitated as large aggregates from the growth solution. The difficulties are due to the fact that TOPO binds too strongly to Zn, and thus, the Zn-Se bond is harder to form than the Cd-Se bond.16,30,31,49 ZnO is amphoteric and dissolves in both acids and alkalis. Since the Zn ion is small, short chain length ligands are usually considered to be suitable for ZnSe synthesis. Chen et al.33 successfully produced ZnSe nanoparticles with a smaller coordinating ligand, lauric acid (LA), for dissolution of ZnO. However, short chain length ligands such as LA could not be used in the present aerosol system because a mixture of precursor solution was solidified as the reaction temperature decreased under 60 °C. Therefore, a long chain length surfactant combination of coordinating solution and a noncoordinating solution, ODE and OA, respectively, was obliged to be used to dissolve ZnO in the present study. The composition of precursor solutions was ZnO (5 mmol), OA (10 mL), TOPO (2 g), and ODE (10 mL) as Zn precursor and Se (10 mmol), TOP (5 mL), and ODE (5 mL) as Se precursors. It was found that the monodispersed ZnSe nanoparticles were not produced under a reaction temperature of 500 °C and the ideal reaction temperature was 525 °C, which were monitored by UV-vis absorption spectroscopy.

Synthesis of Colloidal ZnSe Nanospheres

Crystal Growth & Design, Vol. 9, No. 2, 2009 1155

Figure 2. TEM images and size distribution of as-prepared ZnSe nanoparticles.

Figure 3. Powder XRD pattern of as-prepared ZnSe nanoparticles.

The detailed size and shape of the ZnSe nanoparticles taken by high-magnification TEM are given in Figure 2. The average diameter of the resulting ZnSe nanospheres was 41 nm, and its standard deviation was 0.13, as can be seen from Figure 2a and 2c. The TEM image (Figure 2b) shows that the nanospheres are aggregates of small uniform nanocrystals with a size of 2-3 nm. Figure 3 shows the XRD pattern performed to confirm the structure of ZnSe nanoparticles. The positions of all diffraction peaks in the pattern match well with the standard powder diffraction data (Joint Committee for Powder Diffraction Studies (JCPDS) No. 37-1463). The diffraction peaks at 27.0°, 45.5°, and 53.2° of ZnSe nanocrystals could be indexed to the (111), (220), and (311) planes of the cubic-phase ZnSe with a crystal constant of a ) 5.67 Å. The broad diffraction peaks indicate that the colloidal nanospheres are formed by small nanocrystals. This agrees with the high-magnification TEM results in Figure 2b. The absence of crystalline peaks of ZnO, Se, and others in the XRD pattern indicates clearly that ZnO and Se dissolved completely in a reactant solution and the as-synthesized ZnSe is pure zinc blende structure with high purity. On the basis of the fwhm (full width at half-maximum) of ZnSe (111) and (220) diffraction peaks, the average diameter of the nanoparticles composing the nanospheres was calculated using Scherrer’s formula: D ) (0.9λ)/(B cos θ), where D is the average diameter of nanoparticle, λ is the wavelength of the X-ray used, B is the full width in radians at half-maximum of the peak, and θ is the Bragg angle of the X-ray diffraction peak. The calculated values for the average diameter of the primary ZnSe nanocrystals with the (111) peak at 2θ ) 27.0° and the (220) peak at 2θ ) 45.5° are 2.56 and 2.62 nm, respectively. Those values calculated from Scherrer’s formula agree with the TEM results as shown in Figure 2b.

The monodispersed or aggregated semiconductor nanocrystals may be controlled by selection of appropriate surfactant ligands in the synthetic routes. Recently, simple hot-injection methods have been successfully applied to production of semiconductor nanoparticles with controlled size and shape. Those systems typically involve initial nucleation by hot injection and then growth with appropriate surfactant ligands which finally lead to formation of monodispersed nanoparticles50,51 or secondary aggregated nanoparticles by attachment of primary nanoparticles.42,52 Formation of those colloidal nanospheres is explained by a two-stage aggregation process:53-56 The initial stage is nucleation, and then the nuclei grow to form the primary nanocrystals. The sequential second stage is aggregation of the primary nanocrystals to form the secondary particles. In the synthesis of the monodispersed nanocrystals by the hot-injection method50,51 strong stabilizing surfactants such as OA and TOPO were always used to stabilize the nanoparticles formed. The strong surfactants can prevent the nanocrystals from aggregation into large ones and keep the monodisperse state. On the contrary, if the stabilizing ability of the ligands is weak the primary particles are aggregated.52 The surface free energy of the primary particles should be the driving force for aggregation. The weak ligand results in higher surface free energy of the primary ZnSe nanoparticles, which provides the driving force for aggregation of the primary nanoparticles. For instance, Zhong et al.42 reported that ZnSe microspheres consisting of primary nanocrystals were synthesized using a weak-ligating trioctylamine. On the other hand, in the present experiments ZnSe nanospheres were found to be formed by aggregation of primary nanocrystals even though strong-ligating surfactants like OA and TOPO were used. It may be explained that the primary ZnSe nanocrystals are formed from microdroplets and subsequently aggregation of primary nanocrystals occur in the tube reactor. Therefore, it seems that the strong surfactants of OA and TOPO do not play a role in stabilizing the present system for two reasons. First, because the reaction temperature is too high, the liquid surfactants in microdroplets will be rapidly vaporized in the tube reactor. Therefore, the surface of nanocrystals cannot be effectively passivated by the surfactants, which would be the main reason for aggregation of the nanocrystals. Second, the reaction time in the tube reactor is very short. The heating volume of the tube reactor used is 1.23 L, and the flow rate of the aerosol reactant is 1 L/min. Therefore, the short resident time of the aerosol in the tube reactor may suppress growth of nanocrystals and reaction between nanocrystals and surfactants. The composition of the ZnSe nanospheres was investigated by EDX analysis as shown in Figure 4. The EDX pattern clearly confirms the presence of zinc and selenium, and the atomic ratio

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Figure 4. EDX spectra of as-prepared ZnSe nanoparticles. Figure 6. FT-IR spectra of pure liquid oleic acid and as-prepared ZnSe nanoparticles.

Figure 5. UV-vis absorption and PL emission spectra of as-prepared ZnSe nanoparticles.

of Zn:Se calculated from EDX analysis is 1:1.25. The strong peaks for oxygen, carbon, and phosphorus in the EDX spectrum are due to OA and TOPO, which is capping the surface of ZnSe particles. Figure 5 shows the UV-vis absorption and photoluminescence spectra of the ZnSe nanoparticles obtained in the present work. The first excitonic peak and absorption band edge of the as-prepared ZnSe nanoparticles can be estimated to be 367 and 401 nm, respectively. The band-gap energy of the as-prepared ZnSe nanoparticles, 3.1 eV, is blue shifted to higher energies from the band-gap energy of bulk ZnSe, 2.7 eV. This is consistent with previous work that colloidal ZnSe nanoparticles exhibit size-dependent absorption and photoluminescence in the wavelength range of 350-450 nm.16,29-34 The photoluminescence (PL) spectrum shows a broad band edge emission centered at 386 nm (for excitation at 340 nm) without any evidence of trap emission, and it is blue shifted in comparison with that of bulk ZnSe. It is explained that those results are due to the high growth temperature of 525 °C, resulting in minimal crystalline defects and efficient capping by organic ligands.30 However, the fwhm of the PL peak is 41 nm, and this value is a little larger than that of the narrowest emission peak of ZnSe nanocrystals obtained from a hotinjection method.32 The broad emission peak was the result of the small primary ZnSe crystallites. In addition, the difference between the maximum emission peak and the first excitonic peak (Stoke’s shift) was measured to be 0.015 eV. The smaller overlap area between the absorption and the emission spectra would be useful for optoelectronic applications.57 To understand quantitatively absorption of organic surfactants on the surface of ZnSe nanoparticles FT-IR measurements on pure oleic acid and as-prepared ZnSe nanoparticles were carried out as shown in Figure 6. In FT-IR on pure oleic acid a weak band at 3003 cm-1 is assigned to the vinyl C-H stretch. The

very broad feature from 3500 to 2500 cm-1 is due to a very broad O-H stretch of the carboxylic acid. The sharp bands at 2924 and 2854 cm-1 are assigned to the asymmetric and symmetric CH2 stretch, respectively. The intense carbonyl stretch at 1713 cm-1 is derived from the CdO of oleic acid carbonyl. The stretch at 1285 cm-1 is assigned to a C-O stretch. The O-H in-plane and out-of-plane bands appear at 1466 and 939 cm-1, respectively. In the FT-IR on as-prepared ZnSe nanoparticles the weak vinyl C-H stretch and intense carbonyl CdO stretch are shifted to 2959 and 1632 cm-1, respectively. Although we could not find any noticeable carboxylate (COO-) stretch, the absence of the carbonyl peaks at 1713 cm-1 suggests that there is no free oleic acid in the ZnSe nanoparticles. The asymmetric and symmetric CH2 stretches are shown to be shifted to 2922 and 2851 cm-1, respectively. The surfactant molecules in the adsorbed state were subject to the field of the solid surface. As a result, the characteristic bands are shown to be shifted to a lower frequency region.58,59 The results for FT-IR measurements indicate that the OA chains from the zinc oleate precursor in the preparation solution could be attached on the surface of the ZnSe nanoparticles and the surface of the ZnSe nanospheres was partially covered with the organic ligands.

4. Conclusions Uniform and monodispersed ZnSe nanospheres consisting of primary nanoparticles were synthesized by an ultrasonic-assisted aerosol spray pyrolysis method. The combination of OA, TOPO, and ODE was examined to dissolve ZnO by forming a complex and react with Se precursor, and it was found that monodispersed ZnSe nanospheres are produced only from a specific mixture of precursors and a reaction temperature of higher than 500 °C. The typical specific composition of precursor solutions was ZnO (5 mmol), OA (10 mL), TOPO (2 g), and ODE (10 mL) as Zn precursor and Se (10 mmol), TOP (5 mL), and ODE (5 mL) as Se precursor. The average size of ZnSe nanospheres by TEM was 41 nm, and the crystal structure of primary ZnSe nanocrystals by XRD was a zinc blende lattice with a size of 2-3 nm. The ZnSe nanospheres were shown to have a quantum size effect in their optical spectra and exhibit near band-edge luminescence. The formation mechanism of ZnSe nanospheres may be explained by generation of the primary ZnSe nanocrystals and the subsequent aggregation of primary nanocrystals in the tube reactor. Those results suggest that the aerosol spray pyrolysis method with ultrasound could be applied to other II-VI or III-V semiconductor materials for production of nanocrystals or nanospheres. Acknowledgment. This work was supported by the Nano/ Bio Science & Technology Program (M10536090001-05N3609-

Synthesis of Colloidal ZnSe Nanospheres

00110) of the Ministry of Education, Science and Technology (MEST) and a Special Research Grant of Sogang University.

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