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One-Step Fabrication of CdS Nanorod Arrays via Solution Chemistry Fei Chen,†,‡ Renjia Zhou,†,‡ Ligong Yang,†,‡ Minmin Shi,†,‡ Gang Wu,†,‡ Mang Wang,†,‡ and Hongzheng Chen*,†,‡ Department of Polymer Science and Engineering, State Key Laboratory of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, P. R. China and Key Laboratory of Macromolecule Synthesis and Functionalization (Zhejiang UniVersity), Ministry of Education, Hangzhou 310027, P. R. China ReceiVed: March 31, 2008; ReVised Manuscript ReceiVed: June 3, 2008
Well-defined hexangularly faced CdS nanorod arrays were fabricated via a facile one-step and nontemplate hydrothermal approach in large scale by using biomolecules of glutathione as capping agents. Structural and morphological evolutions of CdS nanorod arrays were studied by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. A formation mechanism of CdS nanorod arrays via this one-step synthesis was tentatively studied by investigating the effects of synthesis parameters on the nanorod arrays. The growth process of CdS nanorod arrays was discussed further from the absorption spectra of CdS nanorod arrays obtained at different reaction times. Introduction
Experimental Section
Aligned semiconductor nanowire or nanorod arrays have attracted much attention due to their potential application in the next-generation nanoscale optoelectronics.1 Particularly, highly ordered and dimension-controllable inorganic semiconductor arrays with excellent charge transportation property are considered as effective inorganic candidates to improve the efficiency of solution-processable organic-inorganic hybrid solar cells.2-4 The past few years have witnessed explosive fabrication of many kinds of inorganic nanoarrays, such as ZnO nanowire and nanorod arrays,5 carbon nanotube arrays,6 CdS nanorod arrays,7 etc. These nanoarrays are achieved mainly based on solution chemistry,5 patterned model,6 or templateassisted methods.7 Among the investigated approaches, technique from solution chemistry is much more promising, because it can meet the economic and industrial requirements of mass production. However, to date, all reported cases on fabrication of inorganic semiconductor arrays require to deposit a textured thin layer of inorganic nanoparticles as catalyst or nucleation agent on the nonepitaxial substrate.5,8-10 This significantly increases the tedious synthesis procedures as well as the followed purification procedures to remove the catalysts or templates, thus restricting their popularization in industry due to the increased cost.
Synthesis of CdS Nanorod Arrays. In a typical reaction, the solution used for the preparation of CdS nanorod arrays was composed of 1 mmol of cadmium nitrate Cd(NO3)2 · 4H2O, 3 mmol of thiourea, and 0.6 mmol of glutathione. All the chemicals were reagent grades without further treatments. The etched ITO glass was placed vertically to the bottom of the Teflon-lined stainless-autoclave, which was then sealed and maintained at 200 °C for 3.5 h. After deposition, the film was rinsed with distilled water and dried naturally. Characterization. The products were characterized by X-ray diffraction (XRD) patterns on a Rigaku D/max-2550PC X-ray diffractometer with Cu KR radiation (λ ) 1.5406 Å). Field emission scanning electron microscopy (FESEM) images were taken on a FESEM-4800 scanning electron microscope. Transmission electron microscopy (TEM) images were recorded on a JEM123 electron microscope and a JEOL-2010 high-resolution TEM at 200 kV. For TEM measurements, the synthesized products were scraped from the ITO substrates and were ultrasonically dispersed in ethanol. A drop of the resulted suspension was then placed on a Cu grid coated with carbon film.
In this paper, we demonstrate a facile one-step and nontemplate hydrothermal approach to fabricate large-scale CdS nanorod arrays by using biomolecules of glutathione as capping agents. The advantage of this approach is that it does not need to deposit the seed layer as nucleation agent or catalyst before the growth of nanorod arrays when compared to the other reported methods. The growth mechanism of the CdS nanorod arrays is proposed as well. * To whom correspondence should be addressed. E-mail: hzchen@ zju.edu.cn. † Department of Polymer Science and Engineering, State Key Laboratory of Silicon Materials, Zhejiang University. ‡ Key Laboratory of Macromolecule Synthesis and Functionalization (Zhejiang University), Ministry of Education.
Results and Discussion The general synthesis of CdS nanorod arrays is described as below. The etched ITO glass is immersed vertically into a Teflon-lined stainless reaction kettle containing Cd(NO3)2 and thiourea as precursors, glutathione as capping agent, and water as solvent, followed by heating to 200 °C and keeping this temperature for 3.5 h. Figure 1 shows the typical scanning electron microscopy (SEM) images of the as-prepared CdS nanorod arrays by this one-step solution approach. The SEM images of the arrays at different locations and magnifications reveal that the whole ITO glass is covered with highly uniform CdS nanorods. SEM image with larger magnification uncovers that these nanorods are hexangularly faced (inset in Figure 1B), which is rare in nanocrystals grown on substrates. The crosssectional SEM image of the CdS arrays shows that the CdS arrays are composed of uniform rods about 100 nm in diameter and 300 ∼ 400 nm in length (Figure 1C). Figure 1D shows the
10.1021/jp802745b CCC: $40.75 2008 American Chemical Society Published on Web 08/09/2008
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Figure 1. Morphologies of CdS nanorod arrays fabricated by one-step approach. (A, B): FESEM images of CdS nanorod arrays at different locations and magnifications. Inset in part B is the high-magnification image. Scale bars: A, 1 µm; B, 200 nm. (C) Cross-sectional SEM image of CdS nanorod arrays grown on ITO substrate, the average length of the nanorods is 400 nm. (D) Photograph of the as-prepared CdS nanorod arrays (yellow region) in an etched ITO substrate.
Figure 2. Structure orientation of CdS nanorod arrays. (A) XRD pattern of CdS nanorod arrays. (B) TEM image of an individual CdS nanorod. Inset is its SAED pattern. (C) HRTEM image of the single CdS nanorod (area marked with a rectangle in part B)). The fringe spacing of 0.334 and 0.353 nm observed in the image corresponds to the separation of the (002) and (100) lattice plane, respectively. (D) Energy-dispersive X-ray spectroscopy (EDX) spectrum of the corresponding CdS nanorods scraped from the ITO glass (sample was deposited onto carbon-coated copper grids for the test).
photograph of the obtained CdS nanorod arrays (yellow region) on an etched ITO substrate. It can be seen that the uniform film containing CdS nanorod arrays is realized in the area of 2.1 cm × 0.2 cm. The structure of the CdS nanorod arrays is further characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The XRD pattern of the as-prepared CdS nanorod arrays (Figure 2 A) shows that all the diffraction peaks can be assigned to wurtzite CdS with the
exception of some peaks attributed to ITO glass. The [002] direction is obviously much more intensified than other directions, which suggests that the nanorods grow preferentially oriented along the [002] direction. Parts B and C of Figure 2 show the TEM image of the single CdS nanorod and its HRTEM image, respectively. The fringe spacing of 0.334 and 0.353 nm observed in the HRTEM image corresponds to the separation of the (002) and (100) lattice plane, respectively. Furthermore, the HRTEM image and the selected area electron diffraction
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Figure 3. Kinetic study of the formation of CdS nanorod arrays by one-step approach. A-D): SEM images of samples prepared by varying reaction time (A, 2.5 h; B, 3 h; C, 3.5 h; D, 4 h); scale bar ) 200 nm. (E) Proposed mechanism of the growth of CdS nanorod arrays by one-step approach and the crystal habit of wurtzite hexagonal rod.
(SAED) pattern reveal that the nanorod is single crystalline in nature and grows preferentially along the [001] direction, which is consistent with the XRD pattern. The corresponding energydispersive X-ray spectroscopy (EDX) spectrum (see Figure 2D) indicates the presence of Cd and S elements with a ratio of 1:1.05, in agreement with the stoichiometric composition of CdS. The observed signal of Cu element is from the Cu grid for TEM measurement. To better understand the formation process of these one-step CdS nanorod arrays, the influence of reaction parameters, including the reaction time and temperature and the concentration of capping agent, on the growth of nanorod arrays is investigated systematically. Parts A-D of Figure 3 present the morphology evolution of the as-prepared CdS films from nanoparticles to nanorod arrays for different reaction times. By observing the cross-sectional SEM image of CdS nanorod arrays grown on ITO glass (see Figure 1C), we find that a layer of CdS nanoparticles is deposited on the ITO glass. Therefore, it is suggested that the formation of CdS nanorod arrays can be divided into two steps,
which is illustrated in Figure 3E. In the beginning, CdS nanoparticles are selectively deposited on the ITO substrate. Then CdS nanorod arrays grow upon those nanoparticles. Figure 4 shows the FE-SEM images of CdS samples prepared at different temperatures with otherwise identical conditions. No CdS nanorod arrays are formed when the reaction happens at 160 °C. CdS nanorod arrays are observed accompanied by some rough appearance when the reaction temperature increases to 180 °C. If the temperature increases to 200 °C, well-defined hexangularly faced CdS nanorod arrays are grown. However, increasing the temperature to 220 °C, CdS film is formed with irregular particle-like morphology rather than anisotropic nanorods. These observations indicate that the temperature of 200 °C is the optimal reaction condition for CdS nanorod arrays growth. Besides the thermodynamic and kinetic factors controlling the synthesis of inorganic nanorod arrays, organic molecules serving as capping agents also play a critical role. It is wellknown that both the size and the shape of many inorganic nanocrystals can be state-of-the-art tuned by employing various
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Figure 4. Thermodynamic study of the formation of CdS nanorod arrays by one-step approach at different reaction temperatures: (A) 180 °C; (B) 200 °C; (C) 220 °C. If the temperature was 160 °C or even lower, no film of nanorod arrays was observed on the ITO glass. Scale bars: 200 nm.
organic molecules as capping agents during the synthesis process.11 More importantly, biomolecules with naturally defined chemical compositions and structures are widely employed to synthesize inorganic species by biomineralization.12-15 In this study, glutathione, a kind of peptides with several active binding groups of mercapto, amino and carboxyl, is used to guide the growth of CdS nanocrystals in an oriented approach as capping agent. It is found that the morphology and the quality of the CdS nanorod arrays are significantly determined by the amount of glutathione in the starting reaction solution. SEM images in Figure 5A-G vividly depict this dependence. As the concentration of glutathione increases from 0 to 0.6 mmol/L, the quality of the as-prepared CdS nanorod arrays eventually reaches a critical point. Further increase of the glutathione concentration over 0.6 mmol/L results in the breaking or etching of CdS nanorods. Although the original molecular conformation of glutathione may be greatly changed at a high reaction temperature, its chemical composition and structure can still be perfect. Therefore, the active groups like mercapto and amino can be selectively adsorbed on some faces of CdS nanocrystals, particularly favoring the low-index faces [parallel to the c axis, like (010), (100), (110) etc.]. It is known that the high-energy facets grow more quickly than the low-energy facets in a kinetic regime.11 Consequently, the introduction of glutathione molecules, which are selectively adsorbed to those low-energy facets, can be used to slow the growth along that side relative to others, leading to the formation of CdS nanorods with oriented structure. However, if the glutathione concentration is less than 0.4 mmol/L, the number of glutathione molecules is insufficient to adsorb on these particular crystal facets, resulting in many defects of nanorod arrays. On the other hand, if the glutathione concentration is over 0.8 mmol/L, the number of glutathione molecules is surplus compared to the maximum amount of the adsorption of the specific crystal faces. These excessive glutathione molecules can be adsorbed on other crystal facets, which prevent CdS nanocrystals from growing with oriented structures. To our interest, some CdS nanorods with sharp tips are observed. From Figure 5C-F, it is found that the number of CdS nanorods with the extent of sharp tips increases with increasing the amount of the biomolecule capping agents. It is likely that the sharp tips of the CdS nanorods might be resulted from the etching effect of biomolecules. The exact mechanism of the tips on the nanorods is not very clear at this moment. The illustration of the possible nucleation and growth processes for CdS nanorod arrays is presented in Figure 5H. In order to know whether the ITO substrate contributes to the oriented growth of CdS nanorods, we conducted some control experiments on the glasses without ITO coating. Figure 6 shows the morphology of the as-prepared CdS nanocrystals on glasses without changing the reaction conditions. It is seen that many CdS nanorods pointing to different directions along with some particles are formed. The oriented growth of CdS
nanorods on the glass is obviously not as good as that grown on the ITO substrates. There exist some holes among the bundles of CdS nanorods, which reveal that the seed layer does not cover the glass completely. It is also found that the adhesion between the CdS nanocrystal layer and the glass is very weak, and the CdS nanocrystals on the glass can be swept away by deionized water easily. These observations are different from those grown on ITO substrate, indicating that the ITO substrate might contribute to the oriented growth of CdS nanorods. It is thought that the ITO layer is rougher than the bare glass, this may facilitate the growth of CdS nanoparticles, which serve as seeds for the later growth of nanorods. Besides, this seed layer may enhance the affinity between ITO substrate and CdS nanocrystals. The exact role of ITO layer during the growth of nanorods is still not very clear at this moment, yet we believe that the ITO layer, at some extent, assisted the vertical growth of CdS nanorod arrays. Figure 7 shows the optical property of the as-prepared CdS nanorod arrays from different reaction times of 2.5, 3, 3.5, and 4 h, whose morphologies correspond to those in Figure 3A-D. It is found that the absolute absorption intensity increases with increasing the reaction time, implying that the effective thickness of CdS film varies correspondingly. The inset in Figure 7 plots the normalized absorption spectra. It can be seen that the spectra of early reaction stages of A and B have no sharp band edge and show structureless continuum absorption at low photon energies. The broad absorption plateau at about 2.6 eV is resulted from the interband transition within grains of nanoparticle layer, whereas the absorption near 2.2-2.3 eV should be attributed to the transitions of gap states caused by a large number of defects distributed at grain boundaries. As prolonging the reaction time over 3.5 h, an evident absorption edge appears in curve “C” or “D”. The absorption edge becomes sharper and the absorption tail at low photon energies reduces with nanorods growing. These observations suggest that a thin layer of CdS polycrystalline nanoparticles is formed at the early stage so that the absorption intensity is low and no apparent absorption edge can be identified. With increasing the reaction time, CdS nanorods are formed and their length increases rapidly, leading to a stronger absorption intensity. Besides, a slight red shift of the absorption edge can be observed in the inset. This phenomenon verifies that both the size and the density of CdS crystalline nanorods increase with increasing the reaction time. Conclusions In summary, a one-step method rising from solution chemistry has been developed for the first time to prepare CdS nanorod arrays on ITO substrates in large scale. The formation mechanism of these CdS nanorod arrays is proposed based on both the thermodynamic and kinetic perspectives and the role of biomolecules glutathione as a capping agent. The glutathione
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Figure 5. Glutathione concentration dependent evolution of CdS nanorod arrays. Glutathione concentration: (A) 0; (B) 0.2 mmol/L; (C) 0.4 mmol/ L; (D) 0.6 mmol/L; (E) 0.8 mmol/L; (F) 1 mmol/L; (G) 1.2 mmol/L. (H) Schematic illustration of the possible function of glutathione in the nucleation and growth processes for CdS nanorod arrays. Scale bars: 200 nm.
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Figure 6. SEM image of CdS nanorods grown on the glass without ITO coating while keeping the reaction conditions as: 1 mmol of cadmium nitrate Cd(NO3)2 · 4H2O, 3 mmol of thiourea, and 0.6 mmol of glutathione, 200 °C for 3.5 h.
partly supported by the developing program of Changjiang Scholar and Innovation Team from the Ministry of Education of China under Grant No IRT0651. References and Notes
Figure 7. Absorption spectra of CdS nanorod arrays prepared from different reaction time of 2.5 h (A), 3 h (B), 3.5 h (C), and 4 h (D). Inset: normalized absorption spectra.
concentration plays a key role in the one-step synthesis of CdS nanorod arrays. The as-prepared high-quality CdS nanorod arrays are promising in many applications like as building blocks for optoelectronics. It is believed that this approach can be expanded to fabricate other inorganic semiconductor materials with nanostructured arrays on ITO substrates. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Nos. 50433020, 50520150165, and 50673083). The work was also
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