Hydrazine-Induced Room-Temperature Transformation of CdTe

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Hydrazine-Induced Room-Temperature Transformation of CdTe Nanoparticles to Nanowires Pratheesh V. Nair and K. George Thomas* Photosciences and Photonics, National Institute for Interdisciplinary Science and Technology (CSIR), Trivandrum 695 019, India, and Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560 064, India

ABSTRACT The effect of hydrazine on the photophysical and morphological properties of water-soluble thioglycolic acid-capped cadmium telluride (CdTe) nanoparticles at room temperature has been investigated. At lower concentrations of hydrazine (0.5 M), a large enhancement in the luminescence of CdTe nanoparticles was observed without any shape change; hydrazine saturates the Cd dangling bonds on the nanoparticles' surface through coordination. Interestingly, highly crystalline CdTe nanowires with hexagonal wurtzite structure were obtained at higher concentrations of hydrazine (2.0 M) through the recrystallization of linearly assembled aggregated CdTe nanoparticles with a zinc blend structure. Strong dipole-dipole interaction between the nanoparticles in the presence of hydrazine assists their linear aggregation, and low activation energy for phasetransition drives their recrystallization to nanowires. Extremely simple methodology presented here opens up novel pathways for the synthesis of one-dimensional semiconductor nanostructures at room temperature and provides valuable information about the growth mechanism of nanowires. SECTION Nanoparticles and Nanostructures

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nanoparticles underwent a large enhancement without any change in the shape, whereas highly crystalline nanowires were obtained at higher concentrations. Two protocols were generally adopted for the synthesis of CdTe nanoparticles, (i) high-temperature organometallic synthesis carried out in high boiling solvents7 and (ii) lowtemperature synthesis in aqueous medium.8 Nanoparticles synthesized by the latter method lack good crystallinity, high luminescence, and monodispersity; however, simplicity in the synthetic procedure and solubility in aqueous medium are the main advantages. We have synthesized CdTe nanoparticles by following a low-temperature method; an aqueous solution of cadmium acetate and freshly prepared sodium hydrogen telluride were refluxed in the presence of thioglycolic acid (TGA) as the stabilizer (Supporting Information). Nanoparticles were purified (by precipitation from methanol followed by centrifugation) and redispersed in argon-saturated distilled water for further studies. To avoid stabilizer depletion during the purification process, precipitation from methanol was carried out only once. TGA-capped CdTe nanoparticles were further characterized by various spectroscopic and microscopic methods (Figures 1A, 2A, and 3A); (i) the UV-vis absorption spectrum showed a broad band with a characteristic first excitonic peak at 460 nm, (ii) the photoluminescence spectrum showed a maximum at 525 nm with a quantum

mong various nanostructured building blocks, semiconductor nanowires have emerged as a promising class of materials for the fabrication of nanoscale devices. The versatility of nanowires is associated with their ability to transport electrons, holes, or photons selectively in one direction, offering exciting possibilities in nanophotonics,1 nanosensors,2 and photovoltaic devices.3 Several protocols have been developed over the years for the synthesis of nanowires, which include solution-based strategies, vapor-liquidsolid (VLS) methods, template-assisted approaches, and lithographic methods.4 The former method is particularly of interest to chemists due to the synthetic flexibility and possibility of controlling the surface chemistry, thereby tuning their solubility and optical properties. However, a limiting aspect of the solution-based methods is that they demand drastic reaction conditions such as high temperatures. An alternative approach is to assemble nanoparticles into nanowires through the destabilization of the ligand shell. Synthesis of highly crystalline CdTe nanowires through the partial removal of stabilizing ligands from nanoparticles was demonstrated earlier by Kotov and co-workers.5 One dimensional growth of nanoparticles can also be achieved by the use of mixed ligands, which can promote the growth along a particular crystal plane.6 Herein, we report a room-temperature method for the synthesis of highly crystalline CdTe nanowires by adding hydrazine hydrate to an aqueous solution of CdTe nanoparticles. By following the optical and microscopic studies, we have investigated the transformation of CdTe nanoparticles to nanowires. At lower concentrations of hydrazine, the luminescence of CdTe

r 2010 American Chemical Society

Received Date: May 4, 2010 Accepted Date: June 16, 2010 Published on Web Date: June 22, 2010

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DOI: 10.1021/jz100568p |J. Phys. Chem. Lett. 2010, 1, 2094–2098

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Figure 1. (A) Time dependent changes in the absorption and luminescence spectra of TGA-capped CdTe nanoparticles in the (a, a0 ) absence and (d, d0 ) presence of 0.5 M of hydrazine (a0 = 0 h, b0 = 2 h, c0 = 6 h, and d0 = 24 h); (B) variation of the luminescence intensity of TGA-capped CdTe nanoparticles in the presence of hydrazine (0.5 M) as a function of time; (C,D) time-dependent changes in the absorption (C) and photoluminescence spectra (D) of TGA-capped CdTe nanoparticles (a = 0 h, b = 12 h, and c = 24 h) upon treatment with a higher concentration of hydrazine (2.0 M). All the measurements are carried out in argon saturated distilled water.

Figure 3. (A) TEM image of TGA-capped CdTe nanoparticles and HRTEM image of a single CdTe nanoparticle in the inset; (B) TEM image of CdTe nanowires obtained after treating CdTe nanoparticles with hydrazine (2 M); (C,D) HRTEM images of a single CdTe nanowire and the SAED pattern of the same nanowire in the inset of (C).

Figure 1A). Interestingly, the luminescence spectrum showed a dramatic enhancement in intensity as a function of time with a small red shift of ∼3 nm (Figure 1A). The spectral shift is negligible, which clearly indicates that the morphology of the nanoparticles remains unchanged. Time-dependent changes in the luminescence intensity of TGA-capped CdTe nanoparticles, upon addition of hydrazine (0.5 M), are presented in Figure 1B. The photoluminescence quantum yields in the absence and presence (measured after 24 h) of hydrazine were estimated to be 0.07 and 0.36, respectively, using fluorescein isothiocyanate (FITC) as the standard (φf = 0.93).9 The effect of hydrazine on the luminescence lifetime of CdTe nanoparticles was investigated using time-correlated single-photon counting (TCSPC) studies. The luminescence decay curves of TGA-capped CdTe nanoparticles followed triexponential decay, and results are presented as Supporting Information (Figure S1). The average lifetimes of CdTe nanoparticles in the absence and presence (measured after 24 h) of hydrazine were estimated to be 25 and 41 ns, respectively; the enhancement in the average lifetime clearly indicates reduction in the rate of nonradiative charge recombination. Extensive efforts have been carried out to understand the luminescence properties of semiconductor nanoparticles and the exciton recombination process.10-16 The poor luminescence efficiency of semiconductor nanoparticles mainly originates from the (i) dangling bonds associated with the nonpassivated surface atoms and (ii) the presence of the oxidized form of atoms on the surface. The photoluminescence quantum yield of nanoparticles can be improved either by overcoating with a wide band gap material13 or by proper surface treatment.14 In the present case, we have observed a

Figure 2. (A)Te 3d core level photoelectron spectra of TGA-capped CdTe nanoparticles; (B) Cd 3d core level photoelectron spectra of TGA-capped CdTe nanoparticles in the absence (black trace) and presence (red trace) of hydrazine (0.5 M).

yield of 0.07 (including contributions from band edge and trap state emission), and (iii) the full width at half-maximum (fwhm) of the band edge emission was found to be 0.21 eV, indicating a broad size distribution, which is evident from TEM images. TGA-capped CdTe nanoparticles possess cubic zinc blende structure, which is evident from the XRD analysis (Supporting Information). These results are in good agreement with earlier reports.8 In order to study the effect of hydrazine on the photophysical properties and morphology of TGA-capped CdTe nanoparticles, two sets of experiments were carried out at lower and higher concentrations of hydrazine. The absorption and photoluminescence spectral changes of CdTe nanoparticles in the presence of hydrazine hydrate (0.5 M) were monitored as a function of time, and these results are presented in Figure 1A. The position of the first excitonic peak at 460 nm in the UV-vis absorption spectrum turned sharp after 24 h; however, no shift in peak position was observed (trace d in


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Figure 4. (A) Optical and (B) emission photographs of CdTe nanoparticles in the absence and presence of hydrazine: (a) absence of hydrazine, (b) presence of 0.5 M of hydrazine recorded after 24 h, and (c) presence of 2.0 M of hydrazine recorded after 36 h.

large enhancement in the photoluminescence quantum yield upon addition of 0.5 M hydrazine. To have a better understanding of this aspect, we have investigated the surface chemical bonding of TGA-capped CdTe nanoparticles using X-ray photoelectron spectroscopic (XPS) studies. Figure 2 represents the Te and Cd 3d photoelectron spectra of CdTe nanoparticles. The binding energy values of Te 3d5/2 and 3d3/2 core levels were observed at 572.5 and 582.9 eV, respectively, and no peaks corresponding to the oxidized form of Te atoms were observed (Figure 2A). Similarly, the binding energy values of Cd 3d5/2 and 3d3/2 core levels were observed at 405.5 and 412.1 eV, respectively (Figure 2B). It was reported earlier that TGA-capped CdTe nanoparticles, prepared slowly in a dynamic equilibrium of growth and dissolution, contain a Cd-rich surface.15 This type of Cd-rich surface structure of nanoparticles reduces the possibility of Te oxidation. From these studies, it is clear that the poor luminescence of semiconductor nanoparticles mainly originates from the surface dangling bonds. It was reported earlier that the TOPOcapped CdSe nanoparticles upon ligand exchange with primary amines resulted in the enhancement of photoluminescence through the passivation of dangling bonds.16 Hydrazine, being a strong Lewis base with lone pairs of electrons, can saturate the Cd dangling bonds, and these aspects were further investigated using XPS studies (Figure 2B and Supporting Information). The photoelectron spectrum measured for the Cd 3d core level in the presence of hydrazine (0.5 M) showed the peak at 400.8 eV, corresponding to the N 1s level, which clearly indicates that the hydrazine molecules are bound on to the nanoparticles' surface. Further, we have investigated the photophysical properties and morphology of TGA-capped CdTe nanoparticles by increasing the concentration of hydrazine hydrate (Figure1C,D). In contrast to that of lower concentrations, the absorption spectrum underwent a bathochromic shift, as a function of time, upon addition of 2.0 M hydrazine. It was observed that the solution turned bright orange (λmax of the first excitonic peak at 520 nm) after 24 h, which subsequently turned to dark blue after 36 h (Figure 4). The photoluminescence maximum also underwent a bathochromic shift, along with a large enhancement in intensity. The red shift in both the absorption and photoluminescence spectra indicates an increase in the overall size of nanoparticles. Interestingly, the HRTEM images of nanoparticles in the presence of hydrazine (2.0 M) after 36 h showed the formation of nanowires with a diameter of 12(2 nm (Figure 3). On the basis of the elemental analysis using the energy-dispersive X-ray (EDX)


spectroscopy, major elements present in the nanowires were identified as Cd and Te (Supporting Information). The crystal structure of the nanowires was further analyzed using selected area electron diffraction (SAED) and fast Fourier transform (FFT) patterns. The SAED pattern of the CdTe nanowire, presented in the inset of Figure 3C, showed a hexagonal wurtzite structure. Spacings of the lattice planes in the HRTEM image of a single CdTe nanowire were estimated to be 0.391 and 0.328 nm (Figure 3D), consistent with the (100) and (101) planes of wurtzite CdTe.17 These results indicate that the growth of the nanowire is along the (001) wurtzite direction, perpendicular to the (100) plane. The FFT pattern of a single CdTe nanowire further confirms a hexagonal wurtzite structure (Supporting Information). However, TGA-capped CdTe nanoparticles possess a cubic zinc blende structure, which is evident from the XRD analysis (Supporting Information). The obvious question is that how the transformation of cubic zinc blende CdTe nanoparticles to hexagonal wurtzite nanowires occur upon addition of hydrazine. Surface charge and colloidal stability play a major role in the transformation of nanoparticles to other shapes, and zeta potential measurements can provide valuable information. These aspects are presented below. The surface charge density of TGA-capped CdTe nanoparticles in the presence of hydrazine was investigated by following the variation in the zeta potential (ζ) as a function of time (Supporting Information). It was observed that the TGA-capped CdTe nanoparticles possess a large negative ζ value (-48 mV) due to the presence of carboxylic acid groups on the nanoparticles' surface. The strong repulsive force between the nanoparticles prevents them from aggregation in solution. Interestingly, ζ become less negative upon addition of hydrazine; at lower concentrations of hydrazine (0.5 M), ζ changes to ∼-30 mV in 3 h and remains constant, while a large change in ζ was observed upon addition of 2 M of hydrazine (∼-15 mV after 3 h and remains constant). These results indicate that a partial removal of stabilizing ligands occurs through the interaction of hydrazine molecules with the surface of CdTe nanoparticles. At lower concentrations of hydrazine, the ζ value of ∼-30 mV is sufficiently large to prevent nanoparticles from aggregation. However, addition of a higher concentration of hydrazine reduces the interparticle repulsion and promotes aggregation. According to the classical Derjaguin-LandauVerwey-Overbeek (DLVO) theory, the total interaction potential plays a decisive role in controlling the stability of nanoparticles against aggregation.18 This is expressed as the sum of the electrostatic repulsion potential, the van der Waals

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Scheme 1. Schematic Representation of Hydrazine Induced-Luminescence Enhancement and One-Dimensional Growth of TGA-Capped CdTe Nanoparticles

attraction potential, and the dipolar attraction potential. Nanoparticles are aggregated if the total repulsion is weaker than the attraction, and they remain dispersed in the reverse situation. Lowering of the ζ value upon addition of hydrazine indicates that the total repulsion potential is weaker than the attraction potential which results in the aggregation of nanoparticles. It is well established that CdTe nanoparticles exist in two different crystal structures, cubic zinc blende (ABCABC stacking) and hexagonal wurtzite (ABAB stacking). In general, the transformation of one crystalline form of a material to another depends mainly on their energy and structure. In the case of CdTe, transformation from zinc blende to wurtzite structure does not involve significant changes in the immediate coordination. In both of these crystal forms, the immediate environment of Cd and Te are identical, and the difference between the structures will appear only in the next to nearest layer. Since the interconversion involves rupture and reformation of interatomic bonds, the activation energy of the process plays a crucial role in the transformation. The activation energy for phase transition from zinc blende to wurtzite for CdTe is low enough to allow the process to occur at room temperature.19 An overall increase in the diameter was observed when TGA-capped CdTe nanoparticles were transformed to nanowires; the diameter of the nanoparticles was found to be ∼4 nm, and that of the nanowires was ∼12 nm. It may be interesting to compare these results with some of the earlier reports on the transformation of CdTe nanoparticles to nanowires. Kotov and co-workers have reported that the overall diameter of nanoparticles and nanowires remains more or less the same during the selforganization of stabilizer-depleted CdTe nanoparticles.5 Partial removal of the stabilizing ligand reduces the interparticle repulsion, which brings the nanoparticles closer through dipole-dipole interaction. In another report, formation of nanochains of CdTe nanoparticles with an overall increase in diameter was observed upon addition of phosphate buffer through dipole-dipole interaction of nanoparticles as well as van der Waals and hydrogen bonding of ligand molecules.20 In the present case, it is proposed that the nanoparticles having a diameter of 4 nm aggregates to form nanochains


with an overall increase in diameter. Due to the increase in the diameter of the nanowires, the possibility of individual nanoparticles organizing as nanowires can be ruled out. These results indicate that CdTe nanowires are formed through the organization of aggregated nanoparticles, and details are presented below. It may be noted that the intrinsic unit cell structure of zinc blende is isotropic in nature, whereas wurtzite is anisotropic with a unique c-axis.21 In the coalescence process of the aggregated nanoparticles, the intrinsic anisotropic structure of the hexagonal wurtzite unit cell can act as a natural template for directing the one-directional growth of nanoparticles. In other words, the lattice structure of TGA-capped CdTe nanoparticles undergoes reorganization from cubic zinc blende to hexagonal wurtzite to achieve symmetry matching between uniaxial geometries of nanowires and wurtzite unit cells.5 The low activation energy of this process facilitates the transformation.19 The driving force for the one-dimensional assembly of nanoparticles is the dipole-dipole interaction between the nanoparticles, which is long-range and substantially strong. The magnitude of the dipole-dipole attraction energy between CdTe nanoparticles can be as high as 10 kJ/ mol, in comparison with the energy of regular molecular dipole-dipole attractions (∼1.5 kJ/mol).5 Formation of uniform nanowires can be explained through the Ostwald ripening process; Cd2þ and Te2- ions dissociate from smaller nanoparticles within or surrounding agglomerates, diffuse into the linear assembly of nanoparticles, and fill in the gaps between them. This process results in the formation of uniform nanowires which are highly crystalline. In summary, we have developed an elegant and simple room-temperature methodology for improving the luminescent properties of water-soluble CdTe nanoparticles and for synthesizing highly crystalline CdTe nanowires (Scheme1). We have demonstrated that at lower concentrations, the hydrazine molecule saturates the Cd dangling bonds on the nanoparticles' surface, resulting in an enhanced luminescence. At higher concentrations of hydrazine, highly crystalline CdTe nanowires were formed through the recrystallization of linearly assembled aggregated CdTe nanoparticles.

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Semiconductor nanowires with high crystallinity and aspect ratio are proposed as potential candidates for many practical applications such as photovoltaics, circuit design, and fabrication of functional architectures. The procedure presented here for the synthesis of water-soluble crystalline nanowires is extremely simple and can be carried out even in an undergraduate laboratory. Studies are in progress to extend this methodology for the design of various semiconductor nanowires.

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SUPPORTING INFORMATION AVAILABLE Details on the

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synthesis of CdTe nanoparticles, XRD pattern of CdTe nanoparticles, EDAX spectrum, and FFT pattern of CdTe nanowires. This material is available free of charge via the Internet at http://pubs.acs.org. (14)

AUTHOR INFORMATION Corresponding Author:

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*To whom correspondence should be addressed. E-mail: georgetk@ md3.vsnl.net.in.

Present Addresses:

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†

School of Chemistry, Indian Institute of Science Education and Research, Thiruvananthapuram, 695016, India.

ACKNOWLEDGMENT The authors thank CSIR (NWP 23) and

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JNCASR, Bangalore (P.V.N.), for financial support. We thank Professor D. D. Sarma and Mr. Sumanta Mukharjee of IISc, Bangalore, for XPS analysis. This is contribution no. NIIST-PPG-297 from NIIST, Trivandrum, India. This Letter is dedicated to Professor T. J. Abraham of St. Berchmans College, Changanassery (Kerala), who has motivated both of the authors during their undergraduate studies.

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Hydrazine-Induced Room-Temperature Transformation of CdTe

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