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nanostructures and their optical properties
Applied Surface Science 257 (2011) 2599–2603
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Mixed-solvothermal synthesis of CdS micro/nanostructures and their optical properties Shengliang Zhong ∗ , Linfei Zhang, Zhenzhong Huang, Shangping Wang College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, PR China
a r t i c l e
i n f o
Article history: Received 31 May 2010 Received in revised form 14 September 2010 Accepted 8 October 2010 Available online 15 October 2010 Keywords: CdS Solvothermal Nanostructure
a b s t r a c t Several novel cadmium sulfide (CdS) micro/nanostructures, including cauliflower-like microspheres, football-like microspheres, tower-like microrods, and dendrites were controllably prepared via an oxalic acid-assisted solvothermal route using ethylene glycol (EG) and H2 O as pure and mixed solvents with different S sources. The as-prepared products were characterized by X-ray powder diffraction (XRD), scanning electronic microscope (SEM) and UV–vis spectrophotometer (UV). It was found that CdS micro/nanostructures can be selectively obtained by varying the composition of solvent, concentration of oxalic acid, and sulfur sources. UV–vis absorption spectra reveal that their absorption properties are shape-dependent. The possible formation process of the CdS micro/nanostructures was briefly discussed. This route provides a facile way to tune the morphologies of CdS over a wide range. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Because of their unique physical properties and potential applications, which are unachievable for their bulk counterparts, the preparation and application of nanomaterials have received extensive and intensive study in the past decade [1–3]. It has been well demonstrated that the properties of nanomaterials are remarkably dependent on their morphology (including size, shape and dimension) [1,4]. It is possible to manipulate the properties of nanomaterials for specific application of interest by carefully designing and controlling the parameters that affect their properties [5,6]. Considering these, it is of significant importance to finely control the morphology of the as-obtained products. CdS is one of the most important II–IV group semiconductors, which are extensively applied in optical and electronic fields such as biological labeling, light-emitting diodes, and photoelectric conversion devices, solar cells, photocatalysis and environmental sensors [7–14]. Various methods, such as colloidal method [3,15], hydrothermal/solvothermal method [9–11], chemical vapor deposition process [7,16], template method [17,18], thermal decomposition method [19], and thermal evaporation [20] have been explored to prepare CdS micro/nanostructures with various morphologies. Among them, the chemical vapor deposition method showed its superiorities in synthesizing 1D CdS nanostructures [21–23]. While the hydrothermal/sovlthermal method have been shown to be an effective approach in the preparation of 3D
∗ Corresponding author. Tel.: +86 791 8120380; fax: +86 791 8120380. E-mail address: [email protected] (S. Zhong). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.10.029
CdS complex structures. For example, Zhao et al. prepared netted sphere-like CdS nanostructures by a solvothermal process [10]. 3D CdS superstructures with flowerlike structure were reported by Chen et al. via a facile hydrothermal treatment [11]. A series of novel 3D CdS micro/nanocrystals were fabricated employing a hydrothermal process [9]. It is well known that the morphology of the fabricated nanocrystals also depends on the solvent agent. In recent years, mixed-solvothermal synthesis of CdS nanostructures has been developed and received increasing interest. For example, CdS nanowires with an average diameter of 25 nm and lengths of 20–40 m have been solvothermally prepared in a mixed solvent of ethylenediamine and dodecanethiol at 180 ◦ C [24]. Highaspect ratio Cd1−x Znx S (x = 0–1) alloy semiconductor nanowires were obtained using an ethylenediamine-assisted solvothermal approach [25]. CdS nanostructures with complex morphologies such as urchin-like CdS nanoflowers, branched nanowires, and fractal nanotrees were produced via a facile solvothermal approach in a mixed solution made of diethylenetriamine and deionized water [26]. Cadmium sulfide nanorods and nanoparticles were successfully produced by a solvothermal reaction at 200 ◦ C for 24 h using ethylenediamine and water as pure and mixed solvents [27]. It has been demonstrated that the Cd and S sources have significant effect on the morphologies of the CdS. However, few reports have been concerned on the preparation of CdS micro/nanostructures employing different Cd and S sources in mixed-solvothermal synthesis [28]. In this work, several novel CdS micro/nanostructures have been fabricated via a simple mixed-solvothermal method in the presence of oxalic acid. The effects of solvent composition, concentration of oxalic acid, and sulfur sources on the synthesis were system-
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Table 1 Summary of the reaction conditions of the as-prepared products. (In all experiments the concentration of CdCl2 and sulfur source was kept at 0.031 M and 0.062 M, respectively. All experiments were carried out at 190 ◦ C for 24 h.). Sample
Solvent composition
Sulfur source
Oxalic acid concentration(M)
A B C D E F G H I J K L
32 mL H2 O 28 mL H2 O + 4 mL EG 16 mL H2 O + 16 mL EG 4 mL H2 O + 28 mL EG 32 mLEG 16 mL H2 O + 16 mL EG 16 mL H2 O + 16 mL EG 16 mL H2 O + 16 mL EG 16 mL H2 O + 16 mL EG 16 mL H2 O + 16 mL EG 16 mL H2 O + 16 mL EG 16 mL H2 O + 16 mL EG
thiourea thiourea thiourea thiourea thiourea thiourea thiourea thiourea thiacetamide cysteine hyposulfite sulfur powder
0.031 0.031 0.031 0.031 0.031 0 0.016 0.062 0.031 0.031 0.031 0.031
atically investigated. The possible formation process of the CdS micro/nanostructures was briefly discussed. UV–vis absorption properties of these products were also investigated. 2. Experimental In a typical procedure, 1 mmol of CdCl2 , 1 mmol of oxalic acid, 16 mL EG and 16 mL H2 O were put into a 45 mL Teflon vessel. Then the mixture was stirred for 15 min. After that, 2 mmol thiourea was added to the solution and 15 min stirring was continued. The autoclave was sealed and got heated at 190 ◦ C for 24 h. After cooling to room temperature naturally, the products were washed with water and alcohol repeatedly. Finally, the products were dried at 80 ◦ C for 6 h. A similar synthetic procedure was employed for the preparation of other samples. The products were denoted as sample A–L, respectively (Table 1). The phase and structure of the products were characterized by a Philips X’Pert Pro Super X-ray diffractormeter equipped with ˚ The graphite monochromatized Cu K␣ radiation ( = 1.54178 A). morphologies and sizes of the products were observed by a fieldemission scanning electron microanalyzer (JEOL JSM-6700F, 15 kV) and an environmental scanning electron microanalyzer (QUANTA 200). UV spectra were recorded on a Lambda 35 UV/vis Spectrometer. 3. Results and discussion The structure and chemical composition of the samples synthesized in this work were confirmed with XRD method. Fig. 1 displays the XRD patterns of sample C and sample G, which were prepared at different concentration of oxalic acid when the solution is composed of 16 mL EG and 16 mL H2 O and thiourea as S source with other conditions unchanged. All peaks can be well indexed to the
Fig. 1. XRD patterns of the as-obtained samples prepared at different concentration of oxalic acid with other conditions unchanged. (a) Sample C (0.031 M); (b) sample G (0.016 M).
hexagonal wurtzite phase of CdS (JCPDS No. 41-1049). In addition, the sharp and narrow peaks indicate that the as-prepared products are well-crystallized. It is well known that the solubility, reactivity, and diffusion behavior of the solvent have great effect on the morphology of the final product. Fig. 2 gives the SEM photos of the as-prepared products synthesized at different volume ratio of EG to H2 O with thiourea as sulfur source when the other conditions kept constant. When pure H2 O was employed as solvent (Sample A), cauliflowerlike microspheres were obtained (Fig. 2a). These microspheres are with an average diameter of ca. 12 m. Fig. 2b shows the high-magnification SEM photo of a single cauliflower-like CdS microsphere, it is interesting that these CdS flower-like microspheres are composed of numerous nanoparticles with diameter of about 100 nm. These primary nanoparticles self-assemble to form bigger particles with diameters of about 1–2 m. And these secondary bigger particles self-assemble to form even bigger microspheres. Once 4 mL EG was introduced into the reaction system (Sample B), microflowers diameter in about 12 m were obtained (Fig. 2c), which clearly demonstrates that the product is composed of spindle-like plates. When the solution was replaced by a mixture of 16 mL EG and 16 mL water (Sample C), CdS microflowers diameter in about 4 m consisted of numerous short conical nanorods were synthesized (Fig. 2d). These conical rods are with widths of about 200–400 nm and lengths of about 0.6–1.5 m. When the solution is composed of 28 mL EG and 4 mL water (Sample D), the product is still follower-like and the flowers are composed of nanoplates (Fig. 2e). The flowers are in the diameter of about 3 m. Another kind of flower-like CdS microstructures diameter in about 2 m composed of numerous nanoflakes with a thickness of about 30 nm were formed when pure EG was employed as solvent (Sample E), as shown in Fig. 2f. It can be seen that the morphology of the product changes with the volume ratio of EG to water. On the basis of above results, the solvent composition has great effects on the morphology of the final products. Fig. 3 displays the SEM photos of the products prepared at different concentration of oxalic acid when the solution is composed of 16 mL EG and 16 mL H2 O using thiourea as sulfur source and other conditions kept unchanged. Fig. 3a is the overview image of the as-obtained CdS samples prepared in the absence of oxalic acid (Sample F), which exhibits that the main products are CdS microrods having the length of 6–10 m. High-magnification SEM image of the entire CdS structures is presented in Fig. 3b, which clearly indicates that the tower-like CdS structures are composed of numerous triangular nanoplates along a certain direction, similar with the results obtained by Chen et al. [9]. Occasionally, symmetric flowers structures composed of dendrite-like petals were found in the products (Fig. 3c). When the oxalic acid concentration is 0.016 M (Sample G), tower-like and flower-like structures are found in the products. Increasing the oxalic acid concentration to 0.062 M
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Fig. 2. SEM images of the as-synthesized products prepared with different solvent composition using thiourea as sulfur source with other conditions unchanged. (a, b) 32 mL H2 O (sample A); (c) 4 mL EG and 28 mL H2 O (sample B); (d) 16 mL EG and 16 mL H2 O (sample C); (e) 28 mL EG and 4 mL H2 O (sample D); (f) 32 mL EG (sample E).
(Sample H), sphere-like product in diverse diameter was obtained and some of the spheres assembled to form even bigger spheres. On the basis of the HSAB (hared/soft acid/base) theory, Cd2+ is a soft acid and C2 O4 2− is a hard base, it can form slightly insoluble CdC2 O4 ·3H2 O . Apparently, a small quantity of Cd2+ is still in the solution. While S2− is a soft base, it can form much more stable CdS , which is much smaller than that of CdC2 O4 ·3H2 O). So, with the mixing of Cd2+ and C2 O4 2− , CdC2 O4 ·3H2 O was obtained firstly. With the elevation of reaction temperature, S2− gradually releases from sulfur sources under the hydrothermal treatment and reacts with the Cd2+ in the solution to form CdS. As a result, the concentration
of Cd2+ and S2− are maintained at a stable level, which is in favor of anisotropic growth of CdS. Clearly, the reaction process is different from the direct reaction between S2− and Cd2+ . At the same time, C2 O4 2− presumably selectively absorbs on crystal planes and thus affects the final crystal shape. However, the exact formation mechanism for the morphology variation is very complex and further work is needed. Fig. 4 presents the SEM images of the as-synthesized CdS samples prepared with different sulfur source when the solution is composed of 16 mL EG and 16 mL H2 O and the concentration of oxalic acid is 0.031 M with other conditions kept the same. Fig. 4a
Fig. 3. SEM images of the samples prepared at different concentration of oxalic acid using thiourea as sulfur source with other conditions unchanged. (a–c) 0 M (sample F); (d) 0.016 M (sample G); (e, f) 0.062 M (sample H).
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Fig. 4. SEM images of samples prepared with different sulfur sources with other conditions kept the same. (a and b) thioacetamide (sample I); (c and d) cysteine (sampleJ); (e and f) hyposulfite (sampleK); (g and h) sulfur powder (sampleL).
shows the low-magnification SEM image of the as-obtained products when thiacetamide was used (Sample I), indicating that the uniform football-like CdS microspheres were obtained for the first time and the average diameter of the microsphere is about 3 m. Fig. 4b displays a detailed microstructure of a single CdS microsphere, which clearly exhibits that the CdS microspheres were constructed by many irregular nanopolyhedrons in diverse size. And these irregular polyhedrons connected tightly each other to form the football-like microspheres. When cysteine was used (Sample J), microspheres diameter in 1–5 m with smooth surface were obtained (Fig. 4c and d). Microspheres were obtained when hyposulfite was employed as sulfur source (Sample K). Interestingly, many dumbbell-like products constructed by two semispheres were found in the product. Microflowers diameter in about 9 m were prepared when sulfur powder was used as sulfur source (Sample L). From an enlarged SEM photo it can be seen that these microspheres are constructed by numerous smaller microspheres built from many irregular polyhedral particles. It turns out that the sulfur source also has great effect on the morphology of the product. The optical absorption of the II–IV semiconductor is well known to be closely related with their shape and size. Several samples were selected and their UV–vis absorption properties were investigated. Fig. 5 shows the UV–vis spectra of the as-obtained CdS samples with different morphologies. All the samples have a steeper absorption edge, which is similar with the results obtained by Ray [29].
A practical method is to equate Eg with the wavelength at which the absorption is 50% of that at the excitonic peak (or shoulder), called 1/2 [30]. The 1/2 for sample A, sample C, sample E, sample F and sample I are at 550 nm, 520 nm, 510 nm, 513 nm and 545 nm, respectively. Compared with bulk CdS with a 1/2 of about 515 nm [31], sample A and sample I have obvious red shift. While the1/2 of the other three samples are close to the bulk value. It has been well demonstrated that CdS basically has blue shift when they are at nanoscale owing to the quantum confinement of electrons and holes. Absorption spectroscopy is a good way to study any change in particle size with small change of the chemical nature of the surface of particles [32]. We can see that the above mentioned samples are all built from subunits at nanoscale. However, no blue shift was observed in the five samples. The reason may be ascribed to that these building blocks have self-assembled to form the microstructures. On the contrary, red shift is observed in the cauliflower-like (sample A) and football-like CdS (sample I) microstructures. This may be due to their subunits are tightly stacked and have small surface area, as verified by SEM results (Figs. 2a and 4a). While the subunits of the other three microstructures are not so compactly stacked and have larger surface area, and their UV absorption property is familiar with the bulk CdS. It turns out that the UV absorption properties of the as-prepared products are strongly shape-dependent.
4. Conclusion Several novel CdS micro/nanostructures have been successfully synthesized via a simple mixed-solvothermal process. Results demonstrate that the solvent composition, concentration of oxalic acid, and sulfur source have great effect on the morphologies of the products. The possible formation mechanism of these microstructures was briefly discussed. The UV–vis absorption results show that their optical properties are strongly morphology-dependent, which may have potential application in photonic devices.
Acknowledgements
Fig. 5. UV–vis spectra of some of the as-obtained samples. (a) sample A; (b) sample E; (c) sample I; (d) sample C; (e) sample F.
S.L. Zhong acknowledges the supporting projects from The China Postdoctoral Science Foundation (No. 20100470841) and the Natural Science Foundation of Jiangxi Province (No. 2009GQH0057)
S. Zhong et al. / Applied Surface Science 257 (2011) 2599–2603
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Mixed-solvothermal synthesis of CdS micro/nanostructures and their optical properties Shengliang Zhong ∗ , Linfei Zhang, Zhenzhong Huang, Shangping Wang College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, PR China
a r t i c l e
i n f o
Article history: Received 31 May 2010 Received in revised form 14 September 2010 Accepted 8 October 2010 Available online 15 October 2010 Keywords: CdS Solvothermal Nanostructure
a b s t r a c t Several novel cadmium sulfide (CdS) micro/nanostructures, including cauliflower-like microspheres, football-like microspheres, tower-like microrods, and dendrites were controllably prepared via an oxalic acid-assisted solvothermal route using ethylene glycol (EG) and H2 O as pure and mixed solvents with different S sources. The as-prepared products were characterized by X-ray powder diffraction (XRD), scanning electronic microscope (SEM) and UV–vis spectrophotometer (UV). It was found that CdS micro/nanostructures can be selectively obtained by varying the composition of solvent, concentration of oxalic acid, and sulfur sources. UV–vis absorption spectra reveal that their absorption properties are shape-dependent. The possible formation process of the CdS micro/nanostructures was briefly discussed. This route provides a facile way to tune the morphologies of CdS over a wide range. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Because of their unique physical properties and potential applications, which are unachievable for their bulk counterparts, the preparation and application of nanomaterials have received extensive and intensive study in the past decade [1–3]. It has been well demonstrated that the properties of nanomaterials are remarkably dependent on their morphology (including size, shape and dimension) [1,4]. It is possible to manipulate the properties of nanomaterials for specific application of interest by carefully designing and controlling the parameters that affect their properties [5,6]. Considering these, it is of significant importance to finely control the morphology of the as-obtained products. CdS is one of the most important II–IV group semiconductors, which are extensively applied in optical and electronic fields such as biological labeling, light-emitting diodes, and photoelectric conversion devices, solar cells, photocatalysis and environmental sensors [7–14]. Various methods, such as colloidal method [3,15], hydrothermal/solvothermal method [9–11], chemical vapor deposition process [7,16], template method [17,18], thermal decomposition method [19], and thermal evaporation [20] have been explored to prepare CdS micro/nanostructures with various morphologies. Among them, the chemical vapor deposition method showed its superiorities in synthesizing 1D CdS nanostructures [21–23]. While the hydrothermal/sovlthermal method have been shown to be an effective approach in the preparation of 3D
∗ Corresponding author. Tel.: +86 791 8120380; fax: +86 791 8120380. E-mail address: [email protected] (S. Zhong). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.10.029
CdS complex structures. For example, Zhao et al. prepared netted sphere-like CdS nanostructures by a solvothermal process [10]. 3D CdS superstructures with flowerlike structure were reported by Chen et al. via a facile hydrothermal treatment [11]. A series of novel 3D CdS micro/nanocrystals were fabricated employing a hydrothermal process [9]. It is well known that the morphology of the fabricated nanocrystals also depends on the solvent agent. In recent years, mixed-solvothermal synthesis of CdS nanostructures has been developed and received increasing interest. For example, CdS nanowires with an average diameter of 25 nm and lengths of 20–40 m have been solvothermally prepared in a mixed solvent of ethylenediamine and dodecanethiol at 180 ◦ C [24]. Highaspect ratio Cd1−x Znx S (x = 0–1) alloy semiconductor nanowires were obtained using an ethylenediamine-assisted solvothermal approach [25]. CdS nanostructures with complex morphologies such as urchin-like CdS nanoflowers, branched nanowires, and fractal nanotrees were produced via a facile solvothermal approach in a mixed solution made of diethylenetriamine and deionized water [26]. Cadmium sulfide nanorods and nanoparticles were successfully produced by a solvothermal reaction at 200 ◦ C for 24 h using ethylenediamine and water as pure and mixed solvents [27]. It has been demonstrated that the Cd and S sources have significant effect on the morphologies of the CdS. However, few reports have been concerned on the preparation of CdS micro/nanostructures employing different Cd and S sources in mixed-solvothermal synthesis [28]. In this work, several novel CdS micro/nanostructures have been fabricated via a simple mixed-solvothermal method in the presence of oxalic acid. The effects of solvent composition, concentration of oxalic acid, and sulfur sources on the synthesis were system-
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S. Zhong et al. / Applied Surface Science 257 (2011) 2599–2603
Table 1 Summary of the reaction conditions of the as-prepared products. (In all experiments the concentration of CdCl2 and sulfur source was kept at 0.031 M and 0.062 M, respectively. All experiments were carried out at 190 ◦ C for 24 h.). Sample
Solvent composition
Sulfur source
Oxalic acid concentration(M)
A B C D E F G H I J K L
32 mL H2 O 28 mL H2 O + 4 mL EG 16 mL H2 O + 16 mL EG 4 mL H2 O + 28 mL EG 32 mLEG 16 mL H2 O + 16 mL EG 16 mL H2 O + 16 mL EG 16 mL H2 O + 16 mL EG 16 mL H2 O + 16 mL EG 16 mL H2 O + 16 mL EG 16 mL H2 O + 16 mL EG 16 mL H2 O + 16 mL EG
thiourea thiourea thiourea thiourea thiourea thiourea thiourea thiourea thiacetamide cysteine hyposulfite sulfur powder
0.031 0.031 0.031 0.031 0.031 0 0.016 0.062 0.031 0.031 0.031 0.031
atically investigated. The possible formation process of the CdS micro/nanostructures was briefly discussed. UV–vis absorption properties of these products were also investigated. 2. Experimental In a typical procedure, 1 mmol of CdCl2 , 1 mmol of oxalic acid, 16 mL EG and 16 mL H2 O were put into a 45 mL Teflon vessel. Then the mixture was stirred for 15 min. After that, 2 mmol thiourea was added to the solution and 15 min stirring was continued. The autoclave was sealed and got heated at 190 ◦ C for 24 h. After cooling to room temperature naturally, the products were washed with water and alcohol repeatedly. Finally, the products were dried at 80 ◦ C for 6 h. A similar synthetic procedure was employed for the preparation of other samples. The products were denoted as sample A–L, respectively (Table 1). The phase and structure of the products were characterized by a Philips X’Pert Pro Super X-ray diffractormeter equipped with ˚ The graphite monochromatized Cu K␣ radiation ( = 1.54178 A). morphologies and sizes of the products were observed by a fieldemission scanning electron microanalyzer (JEOL JSM-6700F, 15 kV) and an environmental scanning electron microanalyzer (QUANTA 200). UV spectra were recorded on a Lambda 35 UV/vis Spectrometer. 3. Results and discussion The structure and chemical composition of the samples synthesized in this work were confirmed with XRD method. Fig. 1 displays the XRD patterns of sample C and sample G, which were prepared at different concentration of oxalic acid when the solution is composed of 16 mL EG and 16 mL H2 O and thiourea as S source with other conditions unchanged. All peaks can be well indexed to the
Fig. 1. XRD patterns of the as-obtained samples prepared at different concentration of oxalic acid with other conditions unchanged. (a) Sample C (0.031 M); (b) sample G (0.016 M).
hexagonal wurtzite phase of CdS (JCPDS No. 41-1049). In addition, the sharp and narrow peaks indicate that the as-prepared products are well-crystallized. It is well known that the solubility, reactivity, and diffusion behavior of the solvent have great effect on the morphology of the final product. Fig. 2 gives the SEM photos of the as-prepared products synthesized at different volume ratio of EG to H2 O with thiourea as sulfur source when the other conditions kept constant. When pure H2 O was employed as solvent (Sample A), cauliflowerlike microspheres were obtained (Fig. 2a). These microspheres are with an average diameter of ca. 12 m. Fig. 2b shows the high-magnification SEM photo of a single cauliflower-like CdS microsphere, it is interesting that these CdS flower-like microspheres are composed of numerous nanoparticles with diameter of about 100 nm. These primary nanoparticles self-assemble to form bigger particles with diameters of about 1–2 m. And these secondary bigger particles self-assemble to form even bigger microspheres. Once 4 mL EG was introduced into the reaction system (Sample B), microflowers diameter in about 12 m were obtained (Fig. 2c), which clearly demonstrates that the product is composed of spindle-like plates. When the solution was replaced by a mixture of 16 mL EG and 16 mL water (Sample C), CdS microflowers diameter in about 4 m consisted of numerous short conical nanorods were synthesized (Fig. 2d). These conical rods are with widths of about 200–400 nm and lengths of about 0.6–1.5 m. When the solution is composed of 28 mL EG and 4 mL water (Sample D), the product is still follower-like and the flowers are composed of nanoplates (Fig. 2e). The flowers are in the diameter of about 3 m. Another kind of flower-like CdS microstructures diameter in about 2 m composed of numerous nanoflakes with a thickness of about 30 nm were formed when pure EG was employed as solvent (Sample E), as shown in Fig. 2f. It can be seen that the morphology of the product changes with the volume ratio of EG to water. On the basis of above results, the solvent composition has great effects on the morphology of the final products. Fig. 3 displays the SEM photos of the products prepared at different concentration of oxalic acid when the solution is composed of 16 mL EG and 16 mL H2 O using thiourea as sulfur source and other conditions kept unchanged. Fig. 3a is the overview image of the as-obtained CdS samples prepared in the absence of oxalic acid (Sample F), which exhibits that the main products are CdS microrods having the length of 6–10 m. High-magnification SEM image of the entire CdS structures is presented in Fig. 3b, which clearly indicates that the tower-like CdS structures are composed of numerous triangular nanoplates along a certain direction, similar with the results obtained by Chen et al. [9]. Occasionally, symmetric flowers structures composed of dendrite-like petals were found in the products (Fig. 3c). When the oxalic acid concentration is 0.016 M (Sample G), tower-like and flower-like structures are found in the products. Increasing the oxalic acid concentration to 0.062 M
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Fig. 2. SEM images of the as-synthesized products prepared with different solvent composition using thiourea as sulfur source with other conditions unchanged. (a, b) 32 mL H2 O (sample A); (c) 4 mL EG and 28 mL H2 O (sample B); (d) 16 mL EG and 16 mL H2 O (sample C); (e) 28 mL EG and 4 mL H2 O (sample D); (f) 32 mL EG (sample E).
(Sample H), sphere-like product in diverse diameter was obtained and some of the spheres assembled to form even bigger spheres. On the basis of the HSAB (hared/soft acid/base) theory, Cd2+ is a soft acid and C2 O4 2− is a hard base, it can form slightly insoluble CdC2 O4 ·3H2 O . Apparently, a small quantity of Cd2+ is still in the solution. While S2− is a soft base, it can form much more stable CdS , which is much smaller than that of CdC2 O4 ·3H2 O). So, with the mixing of Cd2+ and C2 O4 2− , CdC2 O4 ·3H2 O was obtained firstly. With the elevation of reaction temperature, S2− gradually releases from sulfur sources under the hydrothermal treatment and reacts with the Cd2+ in the solution to form CdS. As a result, the concentration
of Cd2+ and S2− are maintained at a stable level, which is in favor of anisotropic growth of CdS. Clearly, the reaction process is different from the direct reaction between S2− and Cd2+ . At the same time, C2 O4 2− presumably selectively absorbs on crystal planes and thus affects the final crystal shape. However, the exact formation mechanism for the morphology variation is very complex and further work is needed. Fig. 4 presents the SEM images of the as-synthesized CdS samples prepared with different sulfur source when the solution is composed of 16 mL EG and 16 mL H2 O and the concentration of oxalic acid is 0.031 M with other conditions kept the same. Fig. 4a
Fig. 3. SEM images of the samples prepared at different concentration of oxalic acid using thiourea as sulfur source with other conditions unchanged. (a–c) 0 M (sample F); (d) 0.016 M (sample G); (e, f) 0.062 M (sample H).
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Fig. 4. SEM images of samples prepared with different sulfur sources with other conditions kept the same. (a and b) thioacetamide (sample I); (c and d) cysteine (sampleJ); (e and f) hyposulfite (sampleK); (g and h) sulfur powder (sampleL).
shows the low-magnification SEM image of the as-obtained products when thiacetamide was used (Sample I), indicating that the uniform football-like CdS microspheres were obtained for the first time and the average diameter of the microsphere is about 3 m. Fig. 4b displays a detailed microstructure of a single CdS microsphere, which clearly exhibits that the CdS microspheres were constructed by many irregular nanopolyhedrons in diverse size. And these irregular polyhedrons connected tightly each other to form the football-like microspheres. When cysteine was used (Sample J), microspheres diameter in 1–5 m with smooth surface were obtained (Fig. 4c and d). Microspheres were obtained when hyposulfite was employed as sulfur source (Sample K). Interestingly, many dumbbell-like products constructed by two semispheres were found in the product. Microflowers diameter in about 9 m were prepared when sulfur powder was used as sulfur source (Sample L). From an enlarged SEM photo it can be seen that these microspheres are constructed by numerous smaller microspheres built from many irregular polyhedral particles. It turns out that the sulfur source also has great effect on the morphology of the product. The optical absorption of the II–IV semiconductor is well known to be closely related with their shape and size. Several samples were selected and their UV–vis absorption properties were investigated. Fig. 5 shows the UV–vis spectra of the as-obtained CdS samples with different morphologies. All the samples have a steeper absorption edge, which is similar with the results obtained by Ray [29].
A practical method is to equate Eg with the wavelength at which the absorption is 50% of that at the excitonic peak (or shoulder), called 1/2 [30]. The 1/2 for sample A, sample C, sample E, sample F and sample I are at 550 nm, 520 nm, 510 nm, 513 nm and 545 nm, respectively. Compared with bulk CdS with a 1/2 of about 515 nm [31], sample A and sample I have obvious red shift. While the1/2 of the other three samples are close to the bulk value. It has been well demonstrated that CdS basically has blue shift when they are at nanoscale owing to the quantum confinement of electrons and holes. Absorption spectroscopy is a good way to study any change in particle size with small change of the chemical nature of the surface of particles [32]. We can see that the above mentioned samples are all built from subunits at nanoscale. However, no blue shift was observed in the five samples. The reason may be ascribed to that these building blocks have self-assembled to form the microstructures. On the contrary, red shift is observed in the cauliflower-like (sample A) and football-like CdS (sample I) microstructures. This may be due to their subunits are tightly stacked and have small surface area, as verified by SEM results (Figs. 2a and 4a). While the subunits of the other three microstructures are not so compactly stacked and have larger surface area, and their UV absorption property is familiar with the bulk CdS. It turns out that the UV absorption properties of the as-prepared products are strongly shape-dependent.
4. Conclusion Several novel CdS micro/nanostructures have been successfully synthesized via a simple mixed-solvothermal process. Results demonstrate that the solvent composition, concentration of oxalic acid, and sulfur source have great effect on the morphologies of the products. The possible formation mechanism of these microstructures was briefly discussed. The UV–vis absorption results show that their optical properties are strongly morphology-dependent, which may have potential application in photonic devices.
Acknowledgements
Fig. 5. UV–vis spectra of some of the as-obtained samples. (a) sample A; (b) sample E; (c) sample I; (d) sample C; (e) sample F.
S.L. Zhong acknowledges the supporting projects from The China Postdoctoral Science Foundation (No. 20100470841) and the Natural Science Foundation of Jiangxi Province (No. 2009GQH0057)
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