DOI: 10.1021/cg9007153
Mesoscale Self-Assembly of Highly Luminescent Zinc Tellurite Nanoclusters
2009, Vol. 9 4951–4956
Sudip K. Batabyal,† N. Venkatram,‡ Ji Wei,*,‡ and Jagadese J. Vittal*,† †
Department of Chemistry, National University of Singapore, Singapore 117543, and ‡Department of Physics, National University of Singapore, Singapore 117542 Received June 26, 2009; Revised Manuscript Received August 31, 2009
ABSTRACT: Zinc tellurite (ZnTeO3 or ZTO) nanoclusters synthesized in aqueous medium from zinc acetate and sodium tellurite are amorphous in nature and exhibit intense photoluminescence when excited at the UV region. These nanoclusters assembled in two different compositions with different morphologies by refluxing in water, depending on the pH of the refluxing medium. At pH 5.5, the ZTO transformed to a crystalline phase with rod-like morphology. At pH 4.5, crystalline Zn2Te3O8 resulted and the as-obtained nanoplates of this compound self-assembled in oval-shaped microstructures. On further decreasing the pH to 3.5, the nanoplates self-assembled to rose-like patterns (nanorose). Very big microspheres and a mixture of microcrystals were formed on further reduction of pH to 3. Interestingly, all the crystalline products with different morphologies display luminescence properties. Further, these new materials exhibit three-photon absorption with positive third-order and negative fifth-order nonlinear refraction.
Introduction Colloidal solution of nanoclusters, that is, nanocolloid, has been developed as a new platform for the fabrication of a new type of materials incorporating their functionalities into welldefined assembled structures. Macroscopic assemblies of nanocrystals having a spherical shape from the nanometer to micrometer scale have been reported in the literature.1-5 Nonspherical nanoclusters have also been fabricated from the nanocrystal secondary building blocks.6-8 The term mesocrystals has been used to describe these types of secondary assembled structures of nanoclusters as proposed by Colfen.9 Though there are few reports available on the formation of mesocrystals, the formation mechanism of the mesocrystals is still not well understood. Here we report our study on the secondary structure formation from amorphous nanoclusters of zinc tellurite (ZnTeO3 or ZTO). When the amorphous nanoclusters become more crystalline, depending upon the reaction conditions, two different crystalline compositions are formed, and the nanostructures assemble in different ways to generate the microstructures as rods or as flowers. Tellurite glasses are promising materials for optical communication.10,11 Specifically they are good host materials for wideband optical fiber applications10 and for second harmonic generation.11 Because of the ease of crystallization from the glass matrix, the Zn-Te-O glass system is promising among the nonsilica materials,12,13 and hence the structures and properties of ZnOTeO2 glasses have been well explored.12-14 Most of the studies were focused on the glass formation from the molten mixture of ZnO and TeO2 and the formation of crystals with different compositions during the time of cooling. The optical and electrical properties of ZnTeO3 single crystals were reported by Nawash et al.15 The thin films of Zn-Te-O alloys16-18 were also well studied because of their interesting properties to form isoelectronic traps. As oxygen is a more electronegative
atom among the group VI elements, it attracts electrons from the host materials when it is substituting Te sites in ZnTe, which is responsible for interesting optical properties.17,18 In this paper, we have explored the synthesis and optical properties of ZnTeO3 and Zn2Te3O8 under varied experimental conditions. In addition, the nonlinear optical (NLO) properties of these materials are also studied in detail. The multiexcitonic properties of these materials are expected to give us valuable information about the application of these materials in optical devices and in solar cells. Experimental Section
*To whom correspondence should be addressed. (J.J.V.) E-mail:
[email protected]. Fax: þ65-6779-1691. (J.W.) E-mail: phyjiwei@nus. edu.sg. Fax: þ65-6777-6126.
Materials. Zinc acetate, sodium tellurite, and acetic acid were purchased from Aldrich, and all the chemicals were used as received without further purification. Deionized water from Milli-Q water purification system was used for all the experiments. Synthesis of Amorphous Zinc Tellurite. The aqueous solutions of zinc acetate (219 mg, 1 mmol) and Na2TeO3 (221 mg, 1 mmol) were mixed together in a molar ratio of 1:1 to obtain an instant white precipitate of zinc tellurite which was filtered, washed with water and ethanol, and finally dried in a vacuum. Synthesis of ZnTeO3 and Zn2Te3O8 Mesocrystals. In a typical experiment, 120 mg of zinc tellurite and 50 mL of water were taken in a round bottomed flask equipped with a condenser. After dispersion of zinc tellurite via mild sonication, the content was refluxed for 24 h. Then it was cooled to room temperature naturally, and the white precipitate found in the bottom of the flask was filtered, washed several times with water, and finally dried in a vacuum for further characterization. To study the assembly at different pH values, appropriate amounts of acetic acid were added to the aqueous dispersion of zinc tellurite and then same experimental condition was followed as described above. Characterization. The X-ray powder diffraction (XRPD) experiments were carried out using a D5005 Bruker AXS X-ray diffractometer. The instrument was operated at a 40 kV voltage and a 40 mA current (λ = 1.5406 A˚) and was calibrated with a standard silicon sample. The samples were prepared by making thin films on glass side by drop cast from ethanol dispersion and scanned from 2θ = 20 to 70° at the step scan mode (step size 0.01, preset time 1 s). The morphology of the product was studied using a transmission electron microscope (TEM, JEOL, 2010EX) operated at an
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accelerated voltage of 200 kV. A drop of dilute solution of the product in ethanol dispersed by sonication on the carbon-coated copper grid was dried finally in a vacuum and was directly used for the TEM experiments. For the field emission scanning electron microscopy (FESEM), the samples were made into paste and mounted on a double-sided carbon tape and were platinum coated by a fine coater, and then the samples were observed using JEOL JSM-6700F operated at 5 kV. Photophysical measurements were made by dispersing the products in water. The UV-vis absorption spectra were obtained from Shimadzu UV-2401PC. Fluorescence spectra were obtained from a Perkin-Elmer LS 55 luminescence spectrometer. Measurement of NLO Properties. The NLO properties of the zinc tullerite nanomaterials dispersed in water were investigated by femtosecond (fs) Z-scans19 at a wavelength of 780 nm, with a laser pulse duration of 300 fs and a repetition rate of 1 kHz. The laser pulses were generated by a mode-locked Ti: Sapphire laser (Quantronix, IMRA), which seeded a Ti: Sapphire regenerative amplifier (Quantronix, Titan). The laser pulses were focused with a 25 cm biconvex lens onto a 1-mm-thick quartz cuvette which contained the solution of zinc tullerite nanomaterials in water with a minimum beam waist of 30 μm. The linear transmittance of all the different nanometerial solutions was adjusted to be 90% at 780 nm. The incident and transmitted laser powers were monitored as the cuvette was moved (or Z-scanned) along the propagation direction of the laser pulses. An aperture of 1.5-mm diameter was introduced to collect closed-aperture data.
Results and Discussion Zinc tellurite is obtained as a precipitate by mixing the aqueous solutions of zinc acetate and sodium tellurite and its amorphous nature was suggested by XRPD (Figure S1, Supporting Information) patterns. The FTIR spectrum of ZTO (Figure S2, Supporting Information) showed a broad hump at 698 cm-1, but it has been reported20 that there is splitting of the TeO3 vibration at 670 cm-1, 700 cm-1, and 770 cm-1. Here, the observed broadening of the band may arise from the amorphous nature of the product. The morphology of the majority of ZTO as revealed by FESEM and TEM is found to be spherical with an approximate diameter of 50 nm (Figure S3, Supporting Information) with a small percentage of irregularly shaped nanoclusters. These nanoclusters could be well dispersed in water, methanol, or ethanol, which were used for their optical characterizations. The UV-vis absorption spectrum of ZTO dispersed in water has a small hump at ∼250 nm. The band gap of ZTO is in the range of the band gap of wide-band gap semiconductors, and the optical band gap of ZTO as calculated from the band edge is ∼3.1 eV. As-prepared ZTO exhibits intense emission in the high energy region on excitation by UV light as shown in Figure 1. It shows a sharp peak at 365 nm when excited at 300 nm. As this emission band is very close to the onset of absorption, probably the emission comes from the radiative annihilation of the excitons. However, there seems to be no report on the optical properties of colloidal Zn-Te-O compounds, but the optical properties of oxygen-doped ZnTe or ZnTeO alloys have been well studied due to their interesting isoelectronic trap formation. Substituting Te by O in ZnTe allows O to attract electrons in the host materials to form the isoelectronic trap state which forms the localized state in the band gap of ZnTe.21 The main feature of the PL spectrum of these O doped ZnTe (ZnTe:O) is the appearance of an oxygen emission band around 1.98 eV.21 Similar emission properties also reported by others and the intensity of the emission band due to oxygen increases with the increase of O doping concentration,16 but the emission energy does not depend
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Figure 1. Absorption spectrum and photoluminescence spectrum of ZTO1.
upon the O concentration.17 Oxygen doped ZnTe films grown by molecular beam epitaxy shows a broad weak emission in the region of 3.0-3.4 eV along with the oxygen emission band at 1.8 eV.18 The emission in the higher energy region (3.03.4 eV) has been assigned as the emission coming from the ZnTeO alloy.18 To study the mesoscale assembly of these nanoclusters, the ZTO sample was refluxed in aqueous medium at different pHs. For this purpose, different amounts of acetic acid were added to ZTO dispersed in water and refluxed for several hours. The morphology as well as the crystallinity largely depends on the pH of the dispersion medium. It is observed that two different crystal compositions, namely, ZnTeO3 and Zn2Te3O8, were formed and Table 1 summarizes the details of the reaction conditions and the products obtained. Generally, there are several phases reported for the Zn-Te-O system and different phases were formed when the ZnO-TeO2 glasses were heated above the crystallization temperature or slowly cooled after melting the mixture.12-14,22 The amorphous ZTO (sample ZTF1) became highly crystalline after refluxing in water for 24 h (orthorhombic phase, JCPDS No. 00-044-0240) as shown in Figure 2. When the pH of the medium is 4.5, ZTO changed to the monoclinic phase of Zn2Te3O8 (JCPDS No. 00-044-0241), as revealed by the XRPD patterns shown for the sample ZTF2.
Figure 2. XRPD patterns of samples refluxed at different pH values. The vertical lines at the bottom represent standard patterns of ZnTeO3 (green line) and Zn2Te3O8 (red line) from the JCPDF data sheet. The peaks marked with an X indicates the tetragonal phase of TeO2.
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Table 1. Details Description of the Reaction Condition and the Details of the Product Obtained sample code ZTF1 ZTF2 ZTF3 ZTF4
reactants 120 mg of ZTO in 50 mL of H2O refluxed for 24 h 120 mg of ZTO in 50 mL of H2O refluxed for 24 h 120 mg of ZTO in 50 mL of H2O refluxed for 24 h 120 mg of ZTO in 50 mL of H2O refluxed for 24 h
amount of acetic acid
pH of the medium
composition of the product
0
5.5
rod-like (diameter 100-200 nm, length 2-5 μm)
ZnTeO3
0.05 mL
4.5
Zn2Te3O8
0.1 mL
3.5
0.5 mL
2.5-3
nanoplates assembled in elliptical structure (thickness of plates ∼500 nm and width ∼10 μm). rose-like assembly of nanoplates (thickness of plates ∼500 nm and width ∼10 μm) microcrystals and big spherically assembled-cluster
The temporal evolution of this crystalline phase from the amorphous ZTO was investigated. The sample ZTF1 after refluxing for 12 h (pH ∼ 5.5) shows a broad amorphous hump along with the crystalline peak for ZnTeO3 (Figure S4, Supporting Information) suggesting that the amorphous ZTO is slowly converted to the crystalline ZnTeO3 phase with time and 100% conversion occurs only after refluxing for 24 h. Similar results were obtained for other pH values as well. At pH 4.5, the sample ZTF2 contains a mixture of ZnTeO3 and Zn2Te3O8 as indicated by the XRPD patterns of the sample refluxed for 12 h, but after refluxing for 24 h ZnTeO3 was completely converted to Zn2Te3O8 (Figure S5, Supporting Information). It was reported that the ZnTeO3 crystallites were grown during the time of annealing the Zn-Te-O glass at relatively low temperature; on the other hand, the Zn2Te3O8 phase was formed comparatively at higher temperature.13 In the ZnO-TeO2 glass system, ZnTeO3 is known to exist in the temperature range from 683 to 743 K and after which Zn2Te3O8 started to form. We observed that it is possible to stabilize both compositions by controlling the pH of the medium during synthesis. Here the crystallization is promoted in acidic conditions, while lower pH or prolonged refluxing removes one-third of Zn from ZnTeO3 as soluble Zn(CO2CH3)2 species to furnish Zn2Te3O8 as shown in eq 1 below. 3ZnTeO3 þ 2CH3 CO2 H f Zn2 Te3 O8 þ ZnðCO2 CH3 Þ2 þ H2 O
comments on size and shape
ð1Þ
The amorphous ZTO nanoclusters assembled to different shaped microstructures when annealed at different pH values. At pH 5.5 (sample ZTF1), the crystalline ZnTeO3 are stacked together in a particular direction to form rod-like morphology and Figure 3 depicts the FESEM micrographs of such morphology. In some higher magnified images, it is observed that the surfaces of the rod-like structures are not smooth, implying that the rods are in fact meso-assembled structures formed by stacking of nanoclusters. These types of secondary structures generated from the primary nanoparticles are of recent research interest. Mo et al.23 observed the meso-assembly of ZnO nanorods on annealing in aqueous medium. We observe that the rods having smaller lengths persist along with the amorphous clusters initially during 12 h of refluxing (Figure S7, Supporting Information) and completely converted to rod-like microstructures after 24 h of refluxing. The TEM investigation of ZTF1 corroborates the results obtained in SEM studies and Figure 3b shows a typical TEM image of ZTF1 which exhibits nanoclusters assembled in rod-like morphology. For the sample ZTF2 prepared at pH 4.5 the morphology is no longer rods but nanoplates. These nanoplates are assembled in elliptical patterns. The FESEM images in
Zn2Te3O8 Zn2Te3O8
Figure 4a show that the elongated spherical clusters are composed of hierarchical assembly of nanoplates. As the pH decreased from 5.5 to 4.5 the morphological changes are accompanied by a change in composition to Zn2Te3O8. On decreasing the pH of the refluxing medium further to pH 3.5, the morphology of the product (ZTF3) changes insignificantly and the assemblies of these plates are totally different from ZTF2, although the composition remained the same as Zn2Te3O8. The FESEM images of ZTF3 show rose flower morphology (hereafter nanorose). The primary building block, that is, the nanoplate, is slightly curved and the stacked plates interdigitate to form nanoroses. Figure 4b shows the nanoroses at different magnifications. The dimension of the nanorose is ∼50 μm. Further decrease in the pH of the refluxing medium (pH ∼ 2.5) leads the products (ZTF4) to form very big microcrystals along with some spherical stacking of nanoplates (Figure S9, Supporting Information). It is obvious that the nanoplates stack together and the junction of the plates is fused to form microsized crystals. The optical properties of the assembled structures of ZTO have been investigated. These nanorods and nanoroses are highly luminescent just like the ZTO. But the absorption curves are different. Figure 5a shows the absorption spectra for these assembled ZTO. The spectrum for ZTF1 is similar to ZTO (shown in Figure 1). As the size of these rods and plates is in the micrometer region, the absorption is weakened by the scattering. On the other hand, the PL spectra for all the samples are identical and on excitation at 300 nm of the samples they show strong emission at 365 nm as shown in Figure 5b. The nonlinear optical properties of the nanoclusters (ZTO), nanorods (ZTF1), and nanoroses (ZTF3) have been investigated and Figure 6 displays typical open- and closed-aperture Z-scan curves for these samples. All the three samples show three-photon absorption and positive nonlinear refraction properties. This may be attributed to this positive nonlinear refractive index as a self-focusing effect. The intensity independence of the open-aperture Z-scans shows pure threephoton absorption processes and closed-aperture Z-scans show pure third-order nonlinear processes at lower laser intensities and a combination of third- and fifth-order nonlinear processes at higher laser intensities for these nanomaterials. If we consider changes in the absorption (ΔR) and refraction (Δn) are due to the nonlinear processes at higher intensities, the nonlinear absorption and refraction can be described by ΔR = R3I2, Δn = n2I (at lower intensities), and Δn = n2I þ n4I2 (at higher intensities), where R3, n2, and n4 are the three-photon absorption coefficient, third- and fifth-order nonlinear refractive indices of the sample (nanocrystals in water), respectively, and I is the light intensity. Both R3, n2, and n4 values can be extracted from the best fitting between the Z-scan theory and the data.24,25
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Figure 3. (a) FESEM micrographs of the sample ZTF1 at two different magnifications, and (b) TEM micrographs of the sample ZTF1 at two different magnifications.
Figure 4. FESEM micrographs of (a) ZTF2 and (b) ZTF3 (nanorose) at different magnifications.
Closed-aperture Z-scan curves were collected at different input intensities. Values of third-order nonlinear refractive indices (n2) of these three materials are obtained from the theoretical fits for lower intensity closed-aperture Z-scan curves, and these values are used to obtain fifth-order nonlinear refractive indices (n4) of the materials. The R3NM, n2NM, and n4NM values are calculated from R3NM = R3/(|f|6Vf), n2NM = (n2 (1 - Vf)n2 sol)/(|f|4Vf), and n4NM = (n4 - (1 - Vf)n4 sol)/(|f|6Vf)
where R3NM is the three-photon absorption coefficient per nanocrystal, n2NM is the third-order nonlinear refractive index per nanocrystal, n4NM is the fifth-order nonlinear refractive index per nanocrystal, n2 sol is the third-order nonlinear refractive index of water, n4 sol is the third-order nonlinear refractive index of water, f is the local field factor [ f = 3n20sol/(n20NM þ 2n20sol), where n0sol is the linear refractive index of water and n0NM is the linear refractive
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Figure 5. (a) UV-vis absorption spectra and (b) PL spectra of refluxed ZTO samples.
Figure 6. Open- and closed-aperture Z-scans curves for the ZTO (a, b), ZTF1 (c, d), and ZTF3 (e, f), respectively. Table 2. Third-Order NLO Properties of ZTO Samples three-photon absorption coefficient per nanocrystal, R3NM (cm3 GW-2) third-order nonlinear refractive indices per nanocrystal, n2NM (cm2 GW-1) fifth-order nonlinear refractive indices per nanocrystal, n4NM (cm4 GW-2)
index of ZTO], and Vf is the volume fraction of materials relative to water. In our case, f is ∼0.7, where n0sol = 1.33 and n0NM ∼ 226 and Vf ∼ 0.002, 0.003, and 0.005 for ZTO, ZTF1, and ZTF3, respectively. The obtained values of three-photon absorption coefficient per nanocrystal (R3NM), third-order nonlinear refractive
nanocluster ZTO 6.37 6.02 10-3 -5.95 10-5
nanorods ZTF1 0.59 2.08 10-3 -3.03 10-5
nanorose ZTF3 0.17 8.32 10-4 -6.62 10-6
indices per nanocrystal (n2NM) and fifth-order nonlinear refractive indices per nanocrystal (n4NM) for these materials are given in Table 2 by considering the values of water R3sol= 8.57 10-5 cm3 GW-2, n2 sol = 5.1 10-7 cm2 GW-1, and n4 sol = -1.1 10-8 cm4 GW-2. The ZTO possesses the best nonlinear optical properties among the three samples.
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Conclusions Hierarchical 3D meso-assemblies of ZnTeO3 and Zn2Te3O8 with different morphologies such as nanorose were obtained by refluxing amorphous ZnTeO3 at different pHs. They also exhibit very strong luminescence at 365 nm for the excitation at 300 nm. The meso-assemblies made up of nanorods or thin nanoplates can be dispersed in water and alcohols and paved the way to study the third-order nonlinear optical properties. All these materials possess three-photon absorption with positive third-order and negative fifth-order nonlinear refraction. Acknowledgment. We thank the Ministry of Education, Singapore, for financial support through a NUS FRC grant, R-143-000-371-112. Supporting Information Available: XRPD patterns; FTIR spectra; FESEM micrographs; XRPD patterns. This material is available free of charge via the Internet at http://pubs.acs.org.
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