nano8_44_445603

IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 19 (2008) 445603 (5pp) doi:10.1088/0957-4484/19/44/445603 Facile aqueous synthesis and growth mechanism of CdTe nanorods Haibo Gong, Xiaopeng Hao, Chang Gao, Yongzhong Wu, Jie Du, Xiangang Xu and Minhua Jiang Center of Bio & Micro/nano Functional Materials, State Key Lab of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China E-mail: xphao@sdu.edu.cn Received 21 January 2008, in final form 22 February 2008 Published 29 September 2008 Online at stacks.iop.org/Nano/19/445603 Abstract Single-crystal CdTe nanorods with diameters of 50–100 nm were synthesized under a surfactant-assisted hydrothermal condition. The experimental results indicated that with a temporal dependence the morphologies of CdTe nanocrystallites changed from nanoparticles to smooth surface nanorods. The crystal structure, morphology and optical properties of the products were investigated by x-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and fluorescence spectrophotometer. Furthermore, the formation mechanisms of the nanorods were investigated and discussed on the basis of the experimental results. 1. Introduction During the past decades, II–VI semiconductor nanomaterials have been extensively investigated because of their novel optical and electrical properties [1–3], which are intriguing for, for example, photovoltaic devices [4], solar cells [5], photo- and electro-luminescence [6–8], and bio-labeling applications [9–15]. In particular, II–VI quantum dots (QDs) have led to new opportunities for a better understanding and utilization of the size-dependent optical and electrical behaviors [16, 17] since their discovery in the early 1990s by Murray and others through the thermolysis of pyrophoric organometallic reagents [18, 19]. Although high-quality semiconductor nanocrystals are in ever increasing demand, the organometallic approach for the synthesis of high-quality II–IV nanocrystals has only been adopted by a limited number of research groups around the world, because the raw materials, especially the organometallic precursors, are toxic, expensive, unstable, explosive, and/or pyrophoric. Therefore sophisticated equipment, such as a glovebox equipped with a refrigerator, and an inert atmosphere are required [20]. Furthermore, the typical reaction temperature (200–360 ◦C) used in the organometallic approach is too high and the reactions are not easy to control or reproduce. So it is a natural to seek easily adoptable or user-friendly synthetic schemes for semiconductor nanocrystallites. Hydrothermal synthesis of semiconductor nanomaterials has attracted more interest because of its simplicity and reproducibility. There are numerous reports regarding the synthesis of II–VI nanomaterials in the aqueous phase by using inorganic cadmium salts as the cadmium precursor [21–23]. For cadmium telluride, although there exists a few notable reports [24–28], the Te source used in a typical hydrothermal synthesis was hydrogen telluride. Because of the high reductive ability of hydrogen telluride, it is necessary to eliminate oxygen from the reacting media to create a safe environment for highly reactive Te2−. The complexity of degassing makes such an approach potentially problematic. This paper presents a convenient and controllable synthetic method based on a complex reaction in an aqueous medium with a facile and stable Te source, which can produce high- quality CdTe nanorods at relatively low temperature (about 150 ◦C). Furthermore, a model was put forward to demonstrate the formation mechanism of CdTe nanorods based on the Kossel–Stranski theory and our observations. 2. Experimental details 2.1. Materials All chemicals were used as received without further purification. Tellurium powder (Te, AR), cetyltrimethyl ammonium bromide (CTAB, AR) and hydrazine hydrate (N2H4·H2O, 85%) were purchased from Beijing Chemical Reagent Ltd Co. of China. Cadmium nitrate (Cd(NO3)2·4H2O, AR), ammonia (NH3·H2O), and concentrated nitric acid 0957-4484/08/445603+05$30.00 © 2008 IOP Publishing Ltd Printed in the UK1 Nanotechnology 19 (2008) 445603 H Gong et al (HNO3, AR) were from China National Medicines Corporation Ltd. All solutions were prepared with Milli-Q water (18.2 M�) as a solvent. 2.2. Preparation 1.0 mmol tellurium powder (0.127 g) was added to 5 ml hot and concentrated nitric acid. The gray Te powder was oxidized and formed a milk white precipitate. After adding the above precipitate to a 10 ml NaOH solution (10 M), we obtained a transparent solution. The reaction maybe performed as follows: Te + 4HNO3 → TeO2 + 4NO2(↑) + 2H2O (1) TeO2 + 2NaOH → Na2TeO3 + H2O. (2) 0.5 mmol Cd(NO3)2·4H2O (0.155 g) and 0.1 g surfactant (CTAB) were dissolved in 10 ml deionized water. An appropriate amount of NH3·H2O was added into the above solution until the disappearance of the initially produced white precipitate to convert Cd2+ into Cd(NH3)2+4 . After 5 min, 2.0 ml (1.0 mmol) of Na2TeO3 and 3.0 ml of hydrazine hydrate (N2H4·H2O) were added into the above obtained solution, and finally, the system formed a colorless and transparent solution. After stirring for 10 min, the final solution was transferred into a 25 ml Teflon-lined stainless steel autoclave, which was then filled with deionized water up to 90% of the total volume, sealed, and heated in an oven at 150 ◦C for 24 h. After the heating treatment, the autoclave was allowed to cool to room temperature naturally. A black product was obtained and collected by filtration, washed repeatedly with distilled water and absolute ethanol, and then dried in a vacuum drier at 60 ◦C for 4 h. 2.3. Characterization X-ray diffraction (XRD) analysis was carried out on a Bruker D8 Advance x-ray diffractometer with graphite monochromatized Cu Kα radiation (λ = 1.5406 A˚). The accelerating voltage was set at 40 kV, with a 40 mA current at a scanning rate of 0.1◦ s−1 in the 2θ range from 20◦ to 50◦. TEM images and HRTEM images were taken with the PHILIPS TechnaiU-TWIN electron microscope operating at 200 kV. Samples were prepared by placing a drop of the dilute alcohol suspension of CdTe nanocrystallites, dispersed with a supersonic disperser, onto a carbon-coated copper grid and allowing the alcohol to evaporate in the dark at room temperature. The surface morphologies of the as-grown products were examined through a JSM-6700F field emission scanning electron microscope using a secondary electron mode and with an accelerating voltage of 3.0 kV. The excitation and emission spectra were recorded on an Edinburgh PL S920 fluorescence spectrophotometer at room temperature. 3. Results and discussion 3.1. Crystal structure analysis Figure 1 shows the XRD patterns of the as-prepared CdTe nanocrystallites grown for different times (2, 6, 12, 18 and Figure 1. The XRD pattern of CdTe nanocrystallites synthesized under hydrothermal condition at 150 ◦C for 2 h (a), 6 h (b), 12 h (c), 18 h (d), and 24 h (e). 24 h). All the diffraction peaks of figure 1 can be indexed to phase-pure and zinc-blende CdTe with a lattice constant of a = 6.482 A˚, which is in good agreement with the value of literature (JCPDS No.15-0770, a = 6.481 A˚). It should be pointed out that the samples show similar XRD patterns except for different intensities of the diffraction peaks. Good crystallization of nanocrystallites is an important factor affecting the diffraction peak intensity, so the patterns indicate that the crystallization of nanorods is better than nanoparticles. However no matter how long the reaction time all the samples are cubic zinc-blende structure. The invariability of crystal structure shows that the zinc-blende phase is a stable phase at this reaction temperature (150 ◦C). 3.2. Morphology observation from TEM and HRTEM images A typical TEM image of the uniform CdTe nanorods is displayed in figure 2(e), which makes it clear that the nanorods are well dispersed with diameters of 50–100 nm and lengths up to 400–600 nm. A typical HRTEM image (figure 2(f)) clearly shows the lattice fringe, which also confirms that the nanorod is a well-crystallized single crystal. The calculated interplane distance perpendicular to the axis direction of the nanorod is 0.36 nm, which is close to the (111) lattice spacing of zinc-blende CdTe, 0.3742 nm. The other fringe spacing in the image is 0.23 nm, corresponding to the lattice spacing of (220). The cross angle between (111) and (220) planes is about 34◦, which is consistent with the theoretical angle 35.26◦. Furthermore, we could judge that the nanorods grow along the [111] direction. 3.3. Optical properties The excitation and fluorescence spectra of the CdTe nanorods sample were shown in figure 3. It is worth noting that the fluorescence spectrum (λexc = 532 nm) shows a intense, sharp peak centered at 799 nm whose full width at half maximum is 2 Nanotechnology 19 (2008) 445603 H Gong et al Figure 2. TEM and HRTEM images of CdTe nanorods and other morphologies under different magnifications. (The reaction durations of these products are: (a) 2 h, (b) 6 h, (c) 12 h, (d) 18 h, (e) and (f) 24 h, respectively, under hydrothermal condition at 150 ◦C.) Figure 3. Excitation (dashed) and fluorescence (solid) spectra of the typical CdTe nanorods. (The reaction duration of this sample is 24 h under hydrothermal condition at 150 ◦C.) 20 nm. CdTe is a direct bandgap semiconductor with a gap of 1.56 eV at 300 K. According to the equation of λabs(nm) = 1242 Eg , the wavelength of the absorption edge can be calculated as 796 nm. However as reported by Gaponik et al [25], the position of the PL maximum of large (∼6 nm) CdTe QDs is located at 730 nm, whereas small (∼2 nm) CdTe nanocrystals have a PL maximum at 510 nm. This indicates that as the size decreases, the band gap of a material may broaden, due to a quantum confinement effect, with respect to its bulk and the emission peak would shift to shorter wavelengths. However because the particle sizes of CdTe here are much larger than the diameter of the Bohr exciton in bulk CdTe, no strong quantum confinement effect is observed. So the emission peak of CdTe nanorods at 799 nm is a band-to-band emission with a Stokes-shift of 3 nm, since no other emission peaks are observed. All the samples exhibit nearly the same spectra and the measurement results confirm it. 3.4. Synthesis of CdTe nanorods The reaction to form CdTe nanocrystallites can be described as follows: TeO2−3 + N2H4·H2O → Te + N2(↑) + 2H2O + 2OH− (3) 2Te + N2H4·H2O + 4OH− → 2Te2− + N2(↑) + 5H2O (4) 3Te + 6OH− → 2Te2− + TeO2−3 + 3H2O (4′) Cd(NH3)2+4 + Te2− → CdTe(↓) + 4NH3. (5) In these reactions, Na2TeO3 serves as an excellent water- soluble Te source and can be readily reduced to highly reactive amorphous Te by hydrazine hydrate in hydrothermal surroundings. The newly produced Te with high reactivity will be reduced further by hydrazine (equation (4)) or disproportionate (equation (4′)) to lead to Te2−. Te2− would react with Cd(NH3)2+4 to produce cadmium telluride. The complex reaction to control the nucleation and growth of nanocrystallites is a simple but effective way. In our system, NH3·H2O was found to be an appropriate and cheap controlling reagent in the formation of CdTe nanocrystallites, which helped to form a stable, transparent, and colorless solution by forming Cd(NH3)2+4 and avoiding CdTeO3 precipitation before reaction. The uniformity of the solution provides a good environment for the growth of high-quality CdTe nanocrystallites. Second, the formation of Cd(NH3)2+4 greatly decreased the concentration of Cd2+ in the solution due to the high stability constant of Cd(NH3)2+4 . The low concentration 3 Nanotechnology 19 (2008) 445603 H Gong et al Figure 4. SEM images of three selected CdTe nanorods grown for different times (6, 12 and 24 h) at 150 ◦C under hydrothermal condition. They clearly illustrate the evolving process of a single nanorod. Scheme 1. The process of forming a CdTe nanorod. The right part is a partial magnification of coarse rods as marked from the left where there are more kinks and steps. of Cd2+ slowed down the reaction speed of Cd2+ with Te2−, which was the main factor impacting the processes of nucleation and growth. Therefore, the relatively homogeneous nucleation and low supersaturation made it prone to forming fine-sized and nearly monodispersed CdTe nanorods. Moreover, the surfactant (CTAB) does not play a negligible role in the growth of CdTe nanorods. When dissolved in water, CTAB can form micelles, which effectively decrease the aggregation of nanoparticles [29]. The mechanism may be that at the initial stage, CTAB adsorbs to the surfaces of newly nucleated CdTe nanoparticles and prevents other CdTe monomers from freely attaching to the small particles. As a result, at the growth stage some faces are inactive and a special direction is kept as the preferential growth orientation. Therefore, one possible function of CTAB in the present synthetic method is to generate large numbers of rodlike micelles in aqueous solution, which may act as a soft template for the formation of 1D nanostructures as well as stabilizing the 1D nanostructures [30]. 3.5. Growth mechanism Figures 2(a)–(e) show the evolving process of CdTe nanocrystallites. When the reaction duration was 2 h, the CdTe product were uniformly fractals. Although there were a few hexagonal particles, most were without any regular shape. At this stage the surfactant, CTAB, may not satisfactorily perform its role as a soft template. With the processing of the reaction, a small amount of nanorods were formed, but most parts of the product were also fractals. With careful observation, we can note that the first formed nanorods were coarse and likely to be aligned by a series of nanoparticles formed at the initial stage. This phenomenon has been found by Zhang et al [31] in the investigation of synthesizing CdS nanorods via a hydrothermal microemulsion method. Thereafter, the amount of unshaped fractals or particles decreased with a temporal dependence while the proportion of rods increased. When the reaction time exceeded 24 h, the nanorods underwent an obvious change of morphology into smooth rods. In addition, the smooth rods were well dispersed and this may be the final effect of CTAB. The growth of CdTe nanorods can be interpreted with the Kossel–Stranski theory of crystal growth. The Kossel–Stranski theory [32] envisages that an apparently flat crystal surface is in fact made up of moving layers (steps) of monatomic height, which may contain one or more kinks. In addition, there will be loosely adsorbed growth units (atoms, molecules or ions) on the crystal surfaces and vacancies in the surfaces and steps. Growth units are most easily incorporated into the crystal at a kink and then a step. In our experimental process the rough surfaces of coarse nanorods contained more kinks and steps. With a longer time, the growth of the nanocrystallites preferentially performed on the kinks and the steps followed the concept of the Kossel–Stranski theory. And finally, the vacancies on the surfaces were filled with growth units and smooth rods were formed. The whole process is shown in scheme 1. The SEM images (figure 4) reveal the same mechanism as demonstrated above. At first, the small plates arranged into rods and the boundaries among plates were clear cut, as depicted in figure 4(a). Then the rods evolved gradually with the coarse particles turning into smooth surface ones. We can see from figure 4(b) that the evolving process has not finished and the rod has two obvious different parts: one ordered section and the other with ragged facets. Finally, as shown in figure 4(c), the single rod becomes smoother and smoother with time. So, our model of the growth of CdTe nanorods is proved to be in good agreement with both TEM and SEM observations. 4 Nanotechnology 19 (2008) 445603 H Gong et al 4. Conclusions In summary, using an air-stable and water-soluble Te source, we have synthesized single-crystal CdTe nanorods with diameters of 50–100 nm under surfactant-assisted hydrothermal condition. The experimental results indicated that the morphology of the formed nanocrystallites changed from rough rods to smooth ones and the reaction time plays a vital role in the evolution of the nanorods. Due to the simple apparatus and relatively facile conditions, the step-by- step advancing hydrothermal method will have more and more wide-spread applications. The formation mechanism of CdTe nanorods here may be useful for other materials analogous to cadmium telluride, such as CdSe, HgTe, and even CdMnTe, HgCdTe semiconductor nanocrystallites. Acknowledgments This work was supported by the NSFC (Contract No. 50721002), the Key Program of Ministry of Education (305010), the Program for New Century Excellent Talents in University, the Fund for the Natural Science of Shandong Province (Y2005F25), and the Scientific Research Award for the Excellent Middle-Aged and Young Scientists of Shandong Province (2005BS04006, 2007BS04013). References [1] Grieve K, Mulvaney P and Grieser F 2000 Curr. Opin. Colloid In. 5 168 [2] Koberling F, Mews A and Basche T 2001 Adv. Mater. 13 672 [3] Cordero S R, Carson P J, Estabrook R A, Strouse G F and Buratto S K 2000 J. Phys. Chem. B 104 12137 [4] Huynh W, Peng X and Alivisatos A P 1999 Adv. Mater. 11 923 [5] Huynh W U, Dittmer J J and Alivisatos A P 2002 Science 295 2425 [6] Mattoussi H, Radzilowski L H, Dabbousi B O, Thomas E L, Bawendi M G and Rubner M F 1998 J. Appl. Phys. 83 7965 [7] Qu L and Peng X 2002 J. Am. Chem. 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Growth Des. 6 2567 [31] Zhang P and Gao L 2003 Langmuir 19 208 [32] Mullin J W 1997 Crystallization 3rd edn (London: Butterworth–Heinemann) p 206 5 1. Introduction 2. Experimental details 2.1. Materials 2.2. Preparation 2.3. Characterization 3. Results and discussion 3.1. Crystal structure analysis 3.2. Morphology obser