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CdS Hierarchical Nanostructures with Tunable Morphologies: Preparation and Photocatalytic Properties Shenglin Xiong,* Baojuan Xi, and Yitai Qian Department of Chemistry and Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui, 230026, China ReceiVed: May 31, 2010; ReVised Manuscript ReceiVed: July 19, 2010
CdS, an important II-VI semiconductor, has been extensively studied in the areas of structure engineering and photocatalytic properties related to structure and morphology. In this study, by means of a facile L-cysteine and ethanolamine (EA)-synergistically assisted hydrothermal route, CdS with various novel nanostructures has been prepared on a large scale in a water/EA binary solution. With a focus on the regulation of structure, the formation process of nanofans by self-assembly of nanorod bundles was followed by transmission electron microscopy (TEM) and X-ray diffraction (XRD). On the basis of our experimental results, the consecutive processes of L-cysteine and EA-synergistically assisted nucleation growth, oriented assembly, and spherecracking were proposed to explain the growth mechanism. More importantly, CdS nanostructures have been confirmed to possess extraordinary photocatalytic activity for the photodegradation of rhodamine B (RhB) compared to that of TiO2 nanoparticles, which could result from their higher surface area, smaller crystal size, and higher crystallinity of the CdS nanostructures. The present work demonstrates the solvothermal route to be facile, inexpensive, and versatile, which favors scaled-up industrial applications and sheds new light on the synthesis and self-assembly of functional materials. 1. Introduction In recent years, controlling the morphology and size of nanomaterials has been a crucial issue in nanoscience research, due to their fundamental shape- and size-dependent properties and significant applications.1 As an important II-VI semiconductor with a Bohr radius of 2.4 nm2 and direct band gap of 2.40 eV,3 CdS has been used in photovoltaics, light-emitting diodes for flat-panel displays, and other optical devices based on its nonlinear properties.4 In addition to these applications, its unique structure could lead to new properties and applications such as nanoelectronic and photocatalytic materials.5 As we know, the morphology of nanomaterials is one of the key factors that affects their properties. Nanostructures with novel morphologies have been considerably investigated. Therefore, in recent years, much effort has been devoted to fabricating 1D to 3D CdS nanostuctures.6-11 Yu and co-workers10 reported a solvothermal approach in a mixed solution made of diethylenetriamine and deionized water to synthesize urchin-like CdS flowers and branched nanowires. Recently, our group has selectively prepared 1D to 3D CdS nanostructures by changing the solvent composition.11 Although CdS nanocrystals with various shapes have been prepared via different routes, development of synthetic strategies for CdS nanocrystals of various shapes is still very significant to the field of materials science. In the present report, we explore a facile L-cysteine and ethanolamine (EA)-synergistically assisted route for controlled preparation of CdS with various nanostructures including water lily nanostructures, nanorices, nanofans by nanorod bunch selfassembly, urchin microflowers, and porous microparticles via control of reaction conditions, such as the volume ratio of the mixed solvents, reaction temperature, reaction time, etc. Meanwhile, CdS nanostructures have been confirmed to possess * To whom correspondence should be addressed. E-mail: xsl8291@ ustc.edu.cn. Tel: 86-551-3607234. Fax: 86-551-3607402.
extraordinary photocatalytic ability to photodegrade rhodamine B (RhB) compared with that of commercial TiO2 powders under exposure to visible light irradiation. 2. Experimental Section 2.1. Sample Preparation. All chemicals were analytical grade and were used directly without any treatment. In a typical procedure, 1 mmol of cadmium acetate (Cd(Ac)2 · 2H2O) and 2 mmol of L-cysteine (C3H7NO2S) were added to a given amount of distilled water and were gradually dispersed to form a homogeneous solution by vigorous stirring. Then, a given amount of ethanolamine (EA) was added to the above solution at room temperature and continually stirred for 10 min. Finally, the resulting mixture was transferred into a Teflon-lined stainless autoclave (60 mL capacity). The autoclave was sealed and maintained at 120-220 °C for 24 h. After reaction, the system was then cooled to ambient temperature naturally. The resultant product was collected, washed with distilled water and absolute alcohol several times, vacuum-dried, and stored for further characterization. 2.2. Characterization. The products were characterized by X-ray diffraction (XRD) recorded on a Japanese Rigaku D/maxγA rotating anode X-ray diffractometer equipped with the monochromatic high-intensity Cu KR radiation (λ ) 1.54178 Å). SEM images were taken with a field emission scanning electron microscope (FESEM, JEOL-6300F, 15 kV). Microscopy was performed with a Hitachi (Tokyo, Japan) H-800 transmission electron microscope at an accelerating voltage of 200 kV, and a JEOL-2010 high-resolution transmission electron microscope, also at 200 kV. The nitrogen adsorption and desorption isotherms were measured with a Micrometrics ASAP 2000 system after the sample was degassed in vacuum overnight. Raman spectra were recorded on a Jobin Yvon (France) LABRAM-HR confocal laser micro-Raman spectrometer at
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TABLE 1: Summary of Various Morphologies of CdS Nanocrystals Fabricated under Different Experimental Conditions sample
Vwater:VEA
temperature
morphology water lily-like nanorod bunches rice-like nanorods fan-shaped nanorod bunches urchin-like microflowers from nanofibers urchin-like microflowers from nanofibers urchin-like microflowers from nanofibers porous microparticles (PMPs)
CdS CdS CdS CdS
9:1 3:2 1:9 9:1
180 °C 180 °C 180 °C 120 °C
CdS
3:2
120 °C
CdS
1:9
120 °C
CdS
pure water
150 °C
room temperature. UV-vis absorption spectra were recorded on a SolidSpec-3700 spectrophotometer at room temperature. 2.3. Photocatalytic Activity Test. The photocatalytic experiments of the obtained CdS nanostructures for the decomposition of RhB in air were performed in an aqueous solution. A cylindrical Pyrex flask (capacity of ca. 25 mL) was used as the photoreactor vessel. CdS nanoparticles as catalyst (10 mg) were added to the aqueous RhB solution (1.0 × 10-5 M, 20 mL), magnetically stirred in the dark for 10 min to reach the adsorption equilibrium between RhB and the catalyst, and then exposed to visible light (250 W). An appropriate cutoff filter was placed inside the vessel to preclude irradiation below 420 nm, allowing irradiation only in the visible-light wavelength range for the RhB/CdS system. As a comparison, the photocatalytic activity of commercial TiO2 (Degussa P25, Degussa Co.; the surface area is ca. 45 m2/g) was tested under the same experimental conditions. UV-vis absorption spectra were recorded at different intervals using a SolidSpec-3700 UV/vis spectrophotometer to monitor the catalytic reaction. 3. Results and Discussion 3.1. Structure and Morphology. In our experiments, we found that synthesis of CdS nanocrystals with controllable morphologies was conveniently attained by adjusting the experimental conditions such as temperature and the volume
Figure 2. XRD patterns of CdS crystals obtained under different conditions: (a) VH2O/VEA) 3/2; (b) VH2O/VEA ) 9/1; (c) VH2O/VEA ) 1/9. The molar ratio of Cd(Ac)2 and L-cysteine ) 1:2, with reaction for 24 h at 180 °C.
ratio of ethanolamine and water. The obtained various morphologies of CdS nanocrystals are summarized in Table 1. To obtain the morphology and size information of products, the technique of TEM was employed. The TEM images in Figure 1 depict various crystals from a different volume ratio of solvents at 180 °C for 24 h. When the volume ratio is as high as 9:1, as shown in Figure 1a,b, a large area of water lilylike CdS nanocrystals are clearly observed. The inserted panel in Figure 1b is the magnified single nanoflower typically composed of petals with a cone-like tip. By simply decreasing the volume ratio of H2O and EA to 3:2, rice-like nanorods with mean diameter of about 20 nm and length of 90-150 nm are obtained as presented in Figure 1c,d. Interestingly, fan-shaped bunches arranged from longer and thinner nanorods formed only when the volume ratio of H2O and EA was reduced to 1:9 (Figure 1e,f). Displayed in Figure 2 are the X-ray diffraction (XRD) patterns of the as-prepared products with different volume ratios of water
Figure 1. TEM images of CdS crystals obtained under different conditions: (a, b) VH2O/VEA) 9/1; (c, d) VH2O/VEA ) 3/2; (e, f) VH2O/VEA ) 1/9. The molar ratio of Cd(Ac)2 and L-cysteine ) 1:2, with reaction for 24 h at 180 °C.
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Figure 3. (a and b) HRTEM images of a single petal of CdS water lily shaped nanorod bunches (the FESEM image is shown in c) in VH2O/VEA) 9/1 and a single nanorod of fan-like nanorod bundles (the FESEM image shown in d) in VH2O/VEA ) 1/9, respectively.
and EA in the same initial precursor concentrations. The diffraction patterns distinctly indicate the fine crystallinity of the obtained samples. The reflection peaks of different products can be indexed as wurtzite CdS with lattice contants a ) 4.139 Å and c ) 6.720 Å, which is in good agreement with the literature values (JCPDS Card No. 41-1049). No peaks of impurities were detected, revealing the high purity of the assynthesized products. In order to acquire detailed microstructure information for nanoflowers and fan-like nanorod bundles, the HRTEM technique was employed to examine the samples. Shown in Figure 3a is the HRTEM image of a single petal of nanoflowers, demonstrating the prominent feature of a conical tip and a lattice space of 0.335 nm, which corresponds well with interspacing of the (002) planes of wurtzite CdS. Moreover, for the single nanorods comprising fan-like nanorod bundles (Figure 3b), lattice spacing of 0.67 nm was observed, consistent with the (001) plane interval. The obtained nanoflowers and nanorod bundles both reveal a single crystalline nature and grow along the [001] direction. The panoramic FESEM images of Figure 3c and 3d further represent the overall appearance of CdS nanoflowers and nanorod bundles, demonstrating the facility and versatility of the present route. 3.2. Time-Dependent Experiments. In order to study the growth mechanism of CdS nanofans by nanorod bundles selfassembly, a series of experiments were performed with different reaction times. (a) 50 min at 180 °C. A light-yellow suspension formed after reaction for 50 min, showing the formation of CdS at a fast growth rate. Figure 4a shows the corresponding TEM image of the product obtained after 50 min, which reveals a compostition of near-urchin-like particles with sizes ranging from 200 to 500 nm. The inserted magnified TEM image in Figure 4a reveals a much more legible profile of a typical near-urchin-like particle. As indicated in the periphery, these nanoparticles are assembled by flexible 1D nanostructures, sharing a common center. The
corresponding XRD pattern of product shown in Figure 5a demonstrates the crystallinity of these urchin-like particles. (b) 2 h at 180 °C. When the solvotherml treatment increased to 2 h, nearly monodisperse urchin-like microcrystals presented in Figure 4b formed and grew in size compared to the sample in Figure 4a. A TEM image with higher magnification as shown in Figure 4c clearly displays that these thin nanorods become much straighter than those at 50 min. At the same time, they align densely and protrude outward from the center. (c) 2.5 h at 180 °C. A small amount of fan-shaped rod bundles existed in the company of microspheres as the time was prolonged to 2.5 h, as revealed in Figure 4d. Clearly, these urchin-like microspheres became loose structurally. Noticeably, Figure 4e shows a collapsed loose urchin-like microsphere with the core being visualized (the contrast is lower). Furthermore, some cracks marked by arrows are very obvious, revealing that some fragments of the nanorod bunches are breaking apart from the initial microsphere. (d) 3 h at 180 °C. When the reaction time was prolonged to 3 h, the morphology of the products changed remarkably. As shown in Figure 4f, an abundance of fan-shaped nanorod bundles was formed at the expense of the microspheres. The inserted panel in Figure 4f shows the texture of a characteristic fan-shaped bundle of nanorods, marked by white arrows. In addition, the crystal structure of intermediates intercepted at 50 min, 70 min, 1.5 h, and 2.5 h was further investigated by XRD. As shown in Figure 5, hexagonal CdS was confirmed to form within 50 min and the products have better crystallinity with the elongation of time. Besides, no CdS nanofans selfassembled by nanorod bundles could be produced instead of L-cysteine with thioacetamide (or thiourea) or EA with ethylenediamine (or water), under similar conditions, which confirms that the formation of CdS nanofans by nanorod bundle selfassembly may be relevant to the particular structures of L-cysteine and EA.
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Figure 4. (a-f) Evolution of fan-shaped CdS nanorod bundles upon reaction time in VH2O/VEA ) 1/9: (a) 50 min; (b, c) 2 h; (d, e) 2.5 h; (f) 3 h. (g) XRD patterns of the samples obtained at different formation stages of CdS nanorod bunches: (I) 50 min, (II) 70 min, (III) 1.5 h, (IV) 2.5 h.
Figure 5. (a) Low-magnification TEM image and (b, c) high-magnification TEM images of spherical CdS porous microparticles (PMPs).
On the basis of the experimental results and analyses, the entire formation process of L-cysteine and EA-synergistically assisted nucleation growth, oriented assembly, and subsequent sphere-cracking was proposed to illustrate the growth mechanism. At the initial stage of the solvothermal reaction, free Cd2+ can coordinate with L-cysteine to form an L-cysteine-Cd2+ complex which then decomposes to CdS nuclei at such a fast rate that urchin-like nanoparticles immediately form due to the orientational effect of ethanolamine (see Figure 4a). Although the shapes of inorganic materials often convey their intrinsic crystal structure nature, they can display diversiform morphol-
ogies under the influence of extrinsic environmental factors including the selective adsorption onto crystal facets by capping molecules. This behavior leads to distinct surface energies, directly resulting in different growth rates of the crystal facets. Herein, the morphology of products was subtly predominated by the facet-selective adsorption characteristic of EA, which could serve as a structure-directing coordination template. In fact, among most reports of colloidal synthesis of metal or semiconductor 1D nanocrystals, the anisotropic growth of 1D structures is often driven by using capping ligands that can bind selectively onto a particular facet of the seed particles.12 For wurtzite
CdS Hierarchical Nanostructures CdS, the (010) and (100) planes are equivalent, but the (001) plane is not equivalent. Therefore, it is reasonable to conclude that, in our approach, the reagent ethanolamine maybe adsorbs onto the (010) and (100) planes of incipient CdS nuclei possibly resulting from the match between the special CdS atomic surface structures and the linear molecular structure of ethanolamine, thus endowing the (001) plane with a higher surface energy as compared to EAcovered (010) and (100) planes. Direct evidence for this hypothesis requires further study, which is currently underway in our laboratory. The selective absorption of ethanolamine onto some facets not only prevents particles from agglomeration but also influences the growth of these planes, which strongly favors anisotropic growth of the newborn CdS nuclei along the [001] axis, as further confirmed by HRTEM. These nanorods self-assemble to form ordered 3D structures due to the orientational role of ethanolamine. This may be in accord with the fact that crystal growth is modulated extrinsically by solvent absorption on certain crystallographic facets, which inhibits the growth of some crystal planes and leads to different growth rates during the formation process of particles, thus generating certain novel crystal shapes.13 The oriented attachment of urchin-like particles followed by crystallographic fusion of the (001) faces conforms to that described first by Penn and Banfield for TiO2 nanocrystals.14 However, interestingly, the present urchin-like microspheres are metastable and easily rupture, which can be observed from the typical collapsing loose microsphere shown in Figure 4e. In our experiment, because the reaction system proceeds in a sealed vessel, which provides an unstable and nonequilibrium surrounding, the oscillating phenomenon inevitably occurred. Therefore, the oscillating phenomenon weakens the attraction of neighboring rods, resulting in the collapse of the urchin-like nanospheres and finally the formation of fan-shaped nanorod bundles. More systematic work is now underway to better understand the formation mechanism of CdS fan-shaped nanorod bundles. 3.3. Influence of Reaction Parameters on the Morphology of Products. To study the influence of other reaction conditions on the morphology of CdS, control experiments were designed with variations of parameters including reaction temperature and the composition of solvents. Remarkably, the morphology of CdS nanostructures can be regulated by simply adjusting the reaction temperature. When the reaction temperature was decreased from 180 to 120 °C, the morphology of the final product changed to nearly monodisperse urchin-like microflowers constructed from nanofibers (see Figure S1, Supporting Information), despite the difference in the volume ratio of water and ethanolamine. The results implied that the higher reaction temperature is beneficial to the fine control of shape and size of the as-synthesized CdS nanocrystals in the current reaction system. In addition, the morphology of products varies gradually and spherical CdS porous microparticles (PMPs) formed only when water is the solvent. Figure 5a shows a low-magnification TEM image of a CdS sample, consisting of a large number of nearmonodisperse spherical microparticles with uniform size and shape. From the TEM images, the average size of these microparticles was found to be around 140-180 nm. The highmagnification TEM images in Figure 5b,c exhibit more subtle structural information in that each individual flower microparticle is composed of many smaller nanocrystals with a mean size of 6-15 nm. These small nanocrystals self-assemble into a porous structural configuration, which is demonstrated by the striking contrast within the flower microparticles. The porous properties and surface area of CdS PMPs were further investigated by the nitrogen adsorption-desorption isotherm and Barrett-Joyner-Halenda (BJH) methods. Figure 6 shows the
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Figure 6. Typical nitrogen adsorption/desorption isotherm and Barrett-Joyner-Halenda (BJH) pore-size distribution plot (inset) of CdS PMPs.
Figure 7. The UV-vis absorption spectra of products obtained in a mixed solution with different VH2O/VEA: (a) 3/2; (b) 9/1; (c) 1/9, with reaction for 24 h at 180 °C.
N2 adsorption-desorption isotherm with the evident hysteresis phenomenon and the pore-size distribution (inset) of the CdS PMPs. The isotherm can be ascertained as type IV, which is typically characteristic of mesoporous materials. The hypothesis loop can be defined as H3, exhibiting no limited adsorption under high relative pressure and commonly resulting from the slit-like pores from particle aggregation.15 The pore-size distribution profiles across the mesopore and macropore regions, obtained from the isotherm, show that most pores are less than 7 nm. These pores possibly result from the intraparticle intervals within the CdS PMPs. The minor larger pores of around 26 nm are attributed to interparticle spaces between CdS PMPs. The specific surface area using the Barrett-Joyner-Halenda (BJH) methods for this sample is 62.59 m2/g. The high specific surface area and large total pore volume support the results of excellent photocatalytic properties (indicated later), owing to mesoporous structure and stronger adsorption to the RhB molecules (illustrated later). 3.4. Optical Properties of CdS Nanostructures. The UV-vis absorption spectra for the samples prepared in the current mixed solution with different volume ratios of water and EA are presented in Figure 7. All the curves display a broad absorption peak in the range of 470-490 nm with a central peak positioned at about 479, 485, and 480 nm, respectively, which is obviously
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Figure 8. (a) Absorption spectrum of RhB solution (1.0 × 10-5 M, 20 mL) in the presence of CdS fan-like nanorod bundles (10 mg) under exposure to visible light. (b) Comparison of photocatalytic degradation of RhB in the presence of CdS fan-like nanorod bundles (sample 1), water lily-like nanorod bunches (sample 2), PMPs (sample 3), and P25, respectively, as photocatalysts under exposure to visible light.
Figure 9. (a-d) TEM images of CdS PMPs irradiated for (a) 60 min, (b) 2 h, (c) 3 h, and (d) 5 h by visible light. (e) XRD patterns of CdS PMPs (1) before irradiation, (2) irradiated for 60 min, (3) irradiated for 3 h, and (4) irradiated for 5 h by visible light.
different from that of the bulk CdS (at ∼513 nm).16 Obviously, the presently obtained CdS samples are so small in size compared with that of bulk wurtzite CdS that the energy levels near the Fermi level are not consecutive anymore and become discrete, which is called a quantum-size effect. Thus, the band gap of CdS nanostructures will become larger and the absorption peaks of CdS nanostructures notably blue-shifted. 3.5. Photocatalytic Activity of CdS Nanostructures. To demonstrate the potential applicability of the as-obtained CdS nanostructures in photocatalysis, we investigated the photocatalytic activity and compared it with that of commercial TiO2 powders, choosing the photocatalytic degradation of rhodamine B (RhB) as the reference and selecting the characteristic absorption at about 553 nm for monitoring the adsorption and photocatalytic degradation process. Figure 8a shows the absorption spectra of RhB aqueous solution (initial concentration: 1.0 × 10-4 M, 20 mL) in the presence of 10 mg of CdS fan-like nanorod bundles under a visible-light lamp (250 W) for various irradiation durations. After irradiation for 1 h, the absorption peak corresponding to RhB at 553 nm diminished sharply with a concomitant wavelength-shift to the blue region gradually. When the illumination time was extended to 4.5 h, the ab-
sorption band shifted to 502 nm, similar to the results previously reported in the RhB/TiO2 system.17 Under visible illumination, the dye is de-ethylated in a stepwise manner with the color of the solution changing from an initial red color to a very light yellow-green. That is, ethyl groups are removed one by one, as demonstrated by the gradual peak wavelength shift toward the shorter wavelength. The color of the solution vanished after 5 h irradiation, revealing that at least the chromophoric structure of the dye was destroyed. In comparison to the initial spectrum, the solution after 3 h irradiation shows that about 54% of the RhB formed rhodamine after being completely demethylated and the rest was degraded through the destruction of the conjugated structure. For further comparison, Figure 8b shows the curves of the concentration of residual RhB with irradiation time over the as-prepared CdS nanparticles and commercial TiO2 powders. Within the allotted time, the photodegradation rate by commercial TiO2 powders was rather low. On the contrary, the photocatalytic degradation velocity by CdS nanparticles was very high. Clearly, the CdS samples possessed photocatalytic activity higher than that of commercial TiO2 powder. The order of photocatalytic activities was sample 3 > sample 2 > sample
CdS Hierarchical Nanostructures 1 > P25. The superior photodegradation properties of the CdS nanparticles relative to commercial TiO2 powders is attributed to their higher surface area, smaller crystal sizes, and higher crystallinity of the obtained samples. Specifically, a large surface can furnish more active adsorption/desorption sites for photocatalytic reaction, a smaller crystal size can lead to powerful redox ability due to the quantum-size effect, and the high crystallinity implies few defects in the as-prepared nanoparticles. As we know, defects may act as the recombination centers for photoexcited electron-hole pairs during photocatalysis, which would reduce the photocatalytic activity. The stability of a photocatalyst always influences its application. Figure 9 reveals TEM images and XRD patterns of the as-prepared samples subjected to different irradiation times, indicating that the morphologies and crystal structure of the CdS PMPs did not change after the photocatalytic reaction. This result demonstrates that CdS nanocrystals are stable and do not decompose (or do not photocorrode) during the photocatalytic process. 4. Conclusions In summary, various self-assembled CdS nanostructures, including water lily-like nanocrystals, rice-like nanorods, nanofans by nanorod bunches, urchin microflowers, and porous microparticles, have been prepared by means of a facile L-cysteine and EA-synergistically assisted solvothermal route. The effects of synthetic conditions such reaction time, temperature, and the volume ratio of mixed solvent on the nanocrystals have been investigated in detail. We proposed a possible mechanism of formation of CdS nanofans by nanorod bundle self-assembly through systematic investigation of the timedependent reaction process. All of the CdS nanocrystals showed photocatalytic activity higher than that of commercial TiO2 powders, possibly originating from the larger surface area, smaller crystal size, and higher crystallinity of the CdS nanocrystals. Acknowledgment. The financial support of this work, by National Natural Science Foundation of China (No. 20901072), the China Postdoctoral Science Foundation (No. 20090460723), and the 973 Project of China (No. 2005CB623601), is gratefully acknowledged. Supporting Information Available: Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
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