Morphology and Phase Evolution of Hierarchical Architectures of

Jun 3, 2009 - A series of hierarchical architectures of cadmium sulfide (CdS) were prepared by the hydrothermal route. The final products were charact...
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J. Phys. Chem. C 2009, 113, 10981–10989

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Morphology and Phase Evolution of Hierarchical Architectures of Cadmium Sulfide Xiaobo He and Lian Gao* State key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China ReceiVed: March 25, 2009; ReVised Manuscript ReceiVed: May 19, 2009

A series of hierarchical architectures of cadmium sulfide (CdS) were prepared by the hydrothermal route. The final products were characterized and investigated by X-ray diffraction, field emission transmission electron microscopy, field emission scanning electron microscopy, and photoluminescence spectrometry. It is found that the complex morphology of the as-synthesized CdS is greatly affected by the used coordination agents such as thiourea and 5-sulfosalicylic acid. Concretely speaking, the hierarchical architectures of CdS can be tuned via facilely adjusting the molar ratio of thiourea to other reactants and/or additives such as cadmium acetate, cadmium nitrate, and 5-sulfosalicylic acid (H3SSA). In some specific conditions, the molar ratio of thiourea to cadmium nitrate or H3SSA can also be used to control the phase composition of the final products. On the basis of the results of various comparative experiments, a possible growth mechanism has been proposed. Furthermore, the hierarchical architectures of CdS exhibit strong yellowish green emission peaking at 565 nm after excitation at 420 nm. This simple route can also be applied to synthesize other inorganic materials. 1. Introduction Hierarchical structures are universal in nature, arising in snowflakes, flowers, the bones of amniotes,1 and so forth. These natural structures have inspired the fabrication of man-made materials with hierarchies, especially nano-/microsized inorganic crystals. In the past few years, some synthetic methods have been developed to synthesize various complexly structured nano-/microcrystals, including mercury(II) sulfide dendrites,2 copper sulfide snowflakes,3 nickel sulfide flowerlike architectures,4 lead sulfide star-shaped multipods,5 WO3 urchinlike spheres,6 and BaMoO4 nestlike spheres.7 Although such hierarchical architectures are similar to their natural archetypes, it is still significant to investigate the growth mechanism of them since it is helpful to controllably fabricate the desired structures and nano-/microdevices.8 Cadmium sulfide (CdS) is a widely investigated semiconductor, owing to its potential applications, such as hydrogen production,9 solar cells,10 and photocatalysis.11 Hence, it is significant to obtain novel nano-/microstructures for improving the properties of CdS in practical applications. Recently, some surfactants, e.g., sodium laurylsulfonate (SDS), peregals, and sodium di(ethyl-2-hexyl)sulfosuccinate, have been used as soft templates to synthesize CdS nanotubes,12 nanoshuttles,13 and triangular nanocrystals,14 respectively. Besides restriction effects of soft templates on the growth of CdS crystals, coordination effects of some organic molecules also have the ability to guide the formation of nano-/microstructures. Xu et al. have synthesized CdS nanorods with the assistance of ethylenediamine, which coordinates with Cd2+ ions.15 Although it is still a challenge to controllably prepare nano-/microstructures with hierarchies, some of hierarchically structured CdS have been successfully obtained by using coordination agents. Han et al. have reported the fabrication of CdS dendrites using thiourea, not only as the sulfur source but also as the complexant.16 Cho et al. have prepared several hyperbranched CdS crystals using * Corresponding author. Telephone: +86 21 52412718. Fax: +86 21 52413122. E-mail: [email protected].

a solvothermal process also with thiourea as sulfur stock.17 Qian et al. have synthesized three-dimensionally structured CdS such as urchinlike spheres in the presence of L-cysteine.18 It is known that the optical properties of CdS nano-/microcrystals are related to their morphologies. The absorption and photoluminescence spectra of CdS nanocrystals can be changed due to different nanostructures such as nanowires and nanospheres.14 The hierarchically urchinlike CdS spheres can show novel red emission, which could be correlated with the structural complexity.18 The photoluminescence peaks of multiarmed CdS nanorods prepared by Zhao et al. can also be tuned by the length of the arms.19 In practical applications, the multiarmed nanorods have been applied in hybrid inorganic-polymer solar cells with a power conversion efficiency of 1.17% due to their distinctive optical properties, while hybrid solar cells fabricated using CdS nanowires have an efficiency of only 0.6%.20 Hence, it is worthwhile to controllably synthesize hierarchically structured CdS nano-/microcrystals. In this paper, we report a simple hydrothermal process to synthesize several hierarchical architectures of CdS such as onedimensional (1D) hierarchical spindles and three-dimensional (3D) flower-shaped multipods. The used coordination agents such as thiourea and 5-sulfosalicylic acid can greatly influence the morphology evolution. Furthermore, the phase composition of the products also can be tuned conveniently. The possible mechanism of the formation of these complex architectures has been also proposed according to the experimental results. 2. Experimental Section In a typical process, the used chemical reagents were all of analytical grade without further purification. Cadmium acetate (0.5 mmol) and thiourea (1 mmol) were dissolved in 50 mL of deionized water in a beaker. After stirring for 30 min, the clear solution was then transferred to an 80-mL Teflon-lined stainless steel autoclave and heated at 180 °C for 15 h. After the autoclave was cooled to room temperature naturally, the precipitates were collected by centrifugation (5500 rpm, 10 min), and washed by deionized water and anhydrous ethanol several times. The final

10.1021/jp9026833 CCC: $40.75  2009 American Chemical Society Published on Web 06/03/2009

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TABLE 1: Samples and Corresponding Experimental Parameters, Phase Compositions, and Morphologies reaction sample time (min) S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18

50 120 900 50 120 900 50 80 900 120 360 900 900 900 80 120 900 900

Cd source Cd(Ac)2 · 2H2O Cd(Ac)2 · 2H2O Cd(Ac)2 · 2H2O Cd(Ac)2 · 2H2O Cd(Ac)2 · 2H2O Cd(Ac)2 · 2H2O Cd(Ac)2 · 2H2O Cd(Ac)2 · 2H2O Cd(Ac)2 · 2H2O Cd(Ac)2 · 2H2O Cd(Ac)2 · 2H2O Cd(Ac)2 · 2H2O Cd(Ac)2 · 2H2O Cd(Ac)2 · 2H2O Cd(NO3)2 · 4H2O Cd(NO3)2 · 4H2O Cd(NO3)2 · 4H2O Cd(Ac)2 · 2H2O

S source additive thiourea thiourea thiourea thiourea thiourea thiourea thiourea thiourea thiourea thiourea thiourea thiourea thiourea thiourea thiourea thiourea thiourea (NH4)2S

H3SSA H3SSA H3SSA H3SSA H3SSA

Cd/S/additive ratio 1:2 1:2 1:2 1:0.8 1:0.8 1:0.8 1:8 1:8 1:8 1:2:1 1:2:1 1:2:1 1:2:10 1:8:10 1:2 1:2 1:2 1:2

products were dried at 60 °C overnight. To investigate the evolution of hierarchical architectures and phase composition, a series of comparative experiments were carried out. The more detailed experimental conditions are presented in Table 1. It should be noted that the total volumes of reaction solutions were kept at 50 mL and the initial concentration of Cd2+ ion was 0.01 M for all samples. The phase compositions of as-synthesized products were characterized by X-ray diffraction (XRD, Model D/MAX-RB, Rigaku Co., Tokyo, Japan). Field emission scanning electron microscopy (FE-SEM, Model JSM-6700F, JEOL, Tokyo, Japan) and field emission transmission electron microscopy (FE-TEM, Model JEM-2100F, JEOL, Tokyo, Japan) were applied to observe the morphologies and microstructures of products. The emission spectral properties were analyzed by a luminescence spectrometer (LS55, Perkin-Elmer, Shelton, USA) at room temperature. 3. Results and Discussion 3.1. Phases of all Samples. The phases of all samples were characterized by XRD, and the results are listed in Table 1. All samples except S13-S17 are possessed of the hexagonal phase with a wurtzite structure. The detailed analyses will be presented in the following discussions. 3.2. Influence of Thiourea on the Hierarchical Architectures. It is known that thiourea is frequently used as a sulfur source to prepare hierarchical architectures of CdS in the literature.16,17 However, reports on the detailed influence of thiourea on the evolution of hierarchical structures are scarce. In this work, time-dependent and concentration-dependent experiments were carried out, respectively, to investigate the effects of thiourea on the growth of hierarchically structured CdS crystals. The morphology evolution of flower-shaped multipods with time is shown in Figure 1a-c (S1-S3). At the early stage (S1, 50 min), incompletely developed multipods are formed (Figure 1a). The footed architectures are composed of short arms with a mean length of 125 nm. When the reaction time is prolonged (S2, 120 min), a kind of better-developed polypod architectures emerges, as shown in Figure 1b. The arms grow longer and into nanorods which have the average diameter and length of 80 nm and 0.3 µm, respectively. It is found that there exist secondary arms out of the primary ones marked by an arrow. When the reaction time is further increased to 900 min (S3),

phase hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal 67 wt % cubic, 23 wt % cubic, 79 wt % cubic, 82 wt % cubic, 84 wt % cubic, hexagonal

33 77 21 18 16

wt wt wt wt wt

morphology

% % % % %

hexagonal hexagonal hexagonal hexagonal hexagonal

flower-shaped multipods flower-shaped multipods flower-shaped multipods polycrystalline spheres caltrop-shaped spheres caltrop-shaped spheres polycrystalline nanoparticles flower-shaped nanoparticles flower-shaped nanoparticles irregular morphologies incomplete hierarchical spindles hierarchical spindles irregular plates polycrystalline spheres polycrystalline nanonecklaces dendrites tower-shaped dendrites nanoparticles

well-developed flower-shaped multipods can be observed in Figure 1c. The final product contains about 80% such multipods (Figure S1a in the Supporting Information). Compared with the sample S2, the primary arms are evolved from nanorods into tower-shaped rods with a mean length of 0.55 µm. According to the selected area electron diffraction (SAED) pattern shown in Figure 1d, which is collected from the entire primary arm arrowed in Figure 1c, the arm is a single crystal. The highly oriented diffraction spots indicate that the single-crystalline

Figure 1. TEM and SEM images of flower-shaped multipods of CdS at different reaction times by hydrothermal route: (a) TEM image of sample S1 (50 min); (b) TEM images of sample S2 (120 min); (c) TEM image of sample S3 (900 min); (d) corresponding SAED pattern collected from the entire arm shown in the inset of (c); (e) HRTEM image taken from the end of the trunk of the same arm; (f) low-magnification SEM image of flower-shaped multipods (S3; the inset is the high-magnification image of one multipod).

Hierarchical Architectures of CdS

Figure 2. XRD patterns of as-prepared CdS with different reactant ratios of cadmium acetate to thiourea: (a) 1:2 (S3); (b) 1:0.8 (S6); (c) 1:8 (S9).

primary arm grows along the [0001] direction. However, the whole multipod is not single crystalline according to the corresponding SAED pattern (not shown here). The prepared CdS crystals are of faults. The bright streaks (arrowed) imply a number of defects in the lattices. The high-resolution TEM (HRTEM) images collected from the end of the trunk of the primary arm (Figure 1e) and one of the nearby secondary arms (Figure S1b in the Supporting Information) show that both of the interplanar spacings are 0.334 nm indexed as (0002) of wurtzite CdS, which demonstrates that both the trunk and secondary arm preferentially evolve along the [0001] direction. Furthermore, there are no boundaries found between them as viewed from the TEM images. Consequently, the secondary arms vertically extend outward from the trunks as a whole and grow along the trunks. It is implied that the evolution of the secondary arms is closely related to that of the trunks, which is different from the self-growth of nuclei adhered to the trunks.16 The 3D image of such flower-shaped multipods is shown in Figure 1f. The secondary arms are arranged zigzaggedly in one line along the trunk, while the wholly primary arm presents the 6-fold symmetry, as shown in the high-magnification SEM image (inset of Figure 1f). Figure 2a shows the XRD pattern of sample S3. The strong diffraction peaks demonstrate that well-crystallized products are prepared. All the diffraction peaks can be indexed to wurtzitestructure CdS (space group P63mc, a ) 0.414 nm, c ) 0.672 nm), which are consistent with the standard diffraction card (JCPDS No. 41-1049). Obviously, the intensity of the (002) peak is more significant compared to that of the standard diffraction spectrum, which is indicated by the vertical lines coupled with patterns in Figure 2, implying the preferred growth of flowershaped multipods along the [0001] direction. Morphology evolution emerges with the change of the reactant ratio of cadmium acetate to thiourea. When the ratio was increased from 0.5 (as for S3) to 1.25, the caltrop-shaped spheres (S6) were prepared for 900 min with a yield of about 95% (Figure S2 in the Supporting Information), while the flower-shaped nanoparticles (S9) were the final products if the ratio was tuned to 0.125. Although the hierarchical architectures differ from that of sample S3, the diffraction peaks of samples S6 and S9 (Figure 2b,c) can also be indexed to hexagonal structured CdS. One can observe the morphology evolution of caltrop-shaped spheres with time in Figure 3a-c. At an early stage (S4, 50 min), the polycrystalline spheres are obtained through the aggregation of crystallites (Figure 3a). When the reaction continues, small caltrop-shaped spheres emerge and some short cone-shaped spines grow out of the central spheres

J. Phys. Chem. C, Vol. 113, No. 25, 2009 10983 (S5, 120 min, Figure 3b). The final product (S6, 900 min) can be viewed in Figure 3c. The representative caltrop-shaped spheres have mean diameters of 1 µm, and spines with an average length of 0.2 µm are located on the surface of the central sphere. It is noteworthy that there are no secondary spines found (inset of Figure 3d). The HRTEM image (Figure 3d) shows that the spines grow along the [0001] direction. The SAED patterns collected from the different spines of one caltrop-shaped sphere (not shown) indicate the same growth direction along the c-axis. If these spines were grown on the same plane which is similar to that of the tetrapods whose arms grow out of a cubic zinc-blende seed,8 there would exist much cubic phase in the corresponding XRD patterns and the arrangement of the spines would be symmetric. However, according to the actual XRD patterns, sample S6 is of pure hexagonal phase. The spines are asymmetrically located on the surface of the centrally spherical seed. Hence, the spines and the central sphere are independent and the caltrop-shaped sphere is not a single crystal. The spines are merely adhered to the central sphere. Furthermore, it is interesting to notice that the low reactant ratio of cadmium acetate to thiourea (0.125) induces a totally different complex architecture, namely flower-shaped nanoparticles. Only polycrystalline nanoparticles are obtained at the early stage of reaction (S7, 50 min). Some of the crystallites aggregate into nanoparticles (Figure 3e). When the reaction time is prolonged to 80 min, the flower-shaped nanoparticles (S8) are achieved and the petals are not well developed (Figure 3f). Finally, almost all such flower-shaped nanoparticles constitute sample S9, shown in Figure 3g. It is noteworthy that the preferential growth of petals of sample S9 are more significant than those of sample S8 (Figure 3f, h). The bilayered structures of these nanoparticles can be observed in Figure 3g, and each layer is composed of several individual nanocrystals. The polycrystalline characteristics of these flower-shaped nanoparticles are confirmed by the electron diffraction pattern (inset of Figure 3h). In fact, the additives that have coordination effects with Cd2+ can further influence not only complex architectures but also phase compositions. 3.3. Influence of 5-Sulfosalicylic Acid (H3SSA) on the Hierarchical Architectures. 5-Sulfosalicylic acid is a widely used ligand for formation of 3D coordination polymers with metal ions.21 The oxygen atoms of groups such as -OH, -COOH, and -SO3H can be significantly coordinated by Cd2+ ions and participate in hydrogen bonding. Hence, the coordination environment surrounding Cd2+ cations could be greatly changed due to adding H3SSA to the reactant solution, which could further influence the growth of CdS crystals. The evolution of wurtzite CdS totally different from that of the samples synthesized without H3SSA is viewed by TEM. Figure 4a shows the TEM image of sample S10. Although an irregular morphology is obtained, some of crystallites are assembled into onedimensional aggregates with a spindle-shaped contour. When the reaction time was increased to 360 min, the integrated spindles emerged (S11), as shown in Figure 4b. It is noticeable that several nanorods are adhered to the trunk with the visible boundaries between them. Finally, the well-developed hierarchical spindles are the main products with a yield of about 90% (Figure S3a in the Supporting Information) after hydrothermal treatment for 900 min (S12). Representative complex spindles with rodlike branches (Figure 4c) have an average length of 2-4 µm along the trunks. The three-dimensional structures can be clearly observed in the SEM images (Figure S3a in the Supporting Information), especially in the magnified one (inset of Figure S3a in the Supporting Information). To investigate

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Figure 3. TEM and SEM images of as-prepared CdS at different reaction times with different ratios of cadmium acetate to thiourea: (a) TEM image of sample S4; (b) TEM image of sample S5; (c) SEM image of sample S6; (d) HRTEM image of sample S6 (the inset shows the corresponding TEM image of the caltrop-shaped sphere); (e) TEM image of sample S7; (f) TEM image of sample S8; (g) SEM image of sample S9; (h) TEM image of sample S9 (the inset is the SAED pattern collected from the entire flower-shaped nanoparticles).

Figure 4. TEM images of products synthesized in the presence of H3SSA: (a) S10; (b) S11; (c) S12; (d) HRTEM image of the juncture between the trunk and the branch collected from the sample S12 (the inset shows the corresponding SAED pattern); (e) S13; (f) S14.

the detailed conjunctive pattern between the trunks and the branches, the HRTEM image (Figure 4d) of the juncture was acquired. It is distinct that there is a boundary between them, resembling the grain boundary of ceramics, which is different from that of sample S3. The whole view is shown in Figure S3b in the Supporting Information. The boundary is marked by the black lines. This configuration implies that the self-growth of branch nuclei is independent of that of the trunks. The

interplanar spacing of both the trunks and the branches is about 0.678 nm, in good agreement with (002) of hexagonal CdS, which indicates that both of them grow along the [0001] direction. The SAED pattern obtained from the entire area exhibits two sets of highly oriented diffraction spots. Compared with the inset of Figure S3b in the Supporting Information, actually, the diffraction spots of the entire region are linearly overlapped by those collected from the trunk and the branch, which also agrees with the connecting pattern as mentioned above. It is still found that there are bright streaks between diffraction spots, probably caused by defects. The corresponding HRTEM images could also provide the possible clues that some of the defects exist by treating images (Figure S3b-d in the Supporting Information). When the ratio of H3SSA to thiourea was changed to 10:2 (S13), the hierarchical architectures degenerated to two-dimensional irregular plates (Figure 4e). However, a kind of complex structure (S14) different from sample S12 is achieved by only increasing the dosage of thiourea, keeping the concentration of H3SSA constant. The products of sample S14 are composed of about 90% such complex spheres (Figure S4 in the Supporting Information) which are polycrystalline (Figure 4f), confirmed by the ring pattern of the corresponding electron diffraction (inset of Figure 4f). Interestingly, the phase compositions of the samples are also altered with the change of the ratio between H3SSA and thiourea. Figure 5a shows the XRD pattern of sample S12. All diffraction peaks can be indexed to wurtzite CdS, which is in good agreement with the standard card (JCPDS No. 41-1049). However, sample S13 is composed of about 67 wt % (weight proportion) cubic zinc-blende and about 33 wt % hexagonal wurtzite CdS (Figure 5b).22 When more thiourea was added, the phase composition was inversed. About 77 wt % hexagonal CdS is obtained, while the amount of cubic CdS is decreased to about 23 wt % (Figure 5c). Hence, there probably exists competition between H3SSA and thiourea which further induces the change of morphologies and phase compositions of the products. 3.4. Influence of Other Cadmium Sources and Sulfur Sources. Samples S15-S17 were prepared by replacing cadmium acetate with cadmium nitrate at different reaction stages,

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Figure 5. XRD patterns of as-prepared products: (a) S12; (b) S13; (c) S14; (d) S17; (e) S18. Figure 7. Schematic diagram of crystal structures of CdS: (a) crystal cell of wurtzite CdS; (b) style of stacking of CdS4 tetrahedrons along the [0001] direction; (c) preferred form of wurtzite CdS crystal (the p and m facets represent {101j1} and {101j0}, respectively); (d) crystal cell of zinc-blende CdS; (e) style of stacking of CdS4 tetrahedrons along the [111] direction.

Figure 6. TEM and SEM images of as-synthesized products: (a) TEM image of sample S15; (b) TEM image of sample S16; (c) TEM image of sample S17; (d) HRTEM image of one branch of sample S17 (the inset is the corresponding SAED pattern of the entire tower-shaped dendrite); (e) SEM image of sample S17; (f) TEM image of sample S18.

keeping other experimental parameters constant. The XRD pattern of the final product (S17) is presented in Figure 5d. The main diffraction peaks can be indexed to cubic zinc-blende CdS. The cell parameters (a ) 0.582 nm) and space group (F4j3m) agree well with the standard diffraction data (JCPDS No. 100454). However, the peaks arising from the second phase, namely hexagonal wurtzite CdS, are also detected. When (NH4)2S is substituted for thiourea, pure hexagonal CdS constitutes the final product (S18), which is confirmed by the corresponding XRD pattern (Figure 5e). Actually, the growth processes of CdS crystals are also altered due to the substitution of cadmium sources and sulfur sources. Figure 6a shows the TEM image of sample S15 (80 min). It is obvious that the polycrystalline nanospheres are orientedly attached to each other in one-dimensional nanonecklaces with several divaricators.

When the reaction time is increased to 120 min, dendrites are developed from such nanonecklaces (S16, Figure 6b). Finally, tower-shaped dendrites with a mean length of 4.3 µm are obtained (S17) as shown in Figure 6c. The percentage of such dendrites is up to about 90% (Figure S5a in the Supporting Information). The HRTEM image of one branch can be viewed in Figure 6d. The interplanar distance is 0.335 nm, corresponding to (111) of cubic zinc-blende CdS. The SAED pattern (inset of Figure 6d) is collected from the entire area, including the trunk and branches. Only one set of diffraction spots is acquired, demonstrating that the whole dendrite is a single crystal. According to the HRTEM image and the SAED pattern, both the branches and the trunk grow along the [111] direction. The growth of the branches is dependent on that of the trunk, which is similar to that of sample S3. The three-dimensional view can be observed in Figure 6e. Although the tower-shaped dendrite is single crystalline, it is not single phase. Fortunately, the lattice fringes related to hexagonal wurtzite CdS can be observed, which are marked by the white rectangle in Figure S5b in the Supporting Information. The coexistence of cubic and hexagonal fringes in one line implies the emergence of stacking faults. When (NH4)2S is used as the sulfur source, the final products (S18) are composed of only nanoparticles without hierarchical structures (Figure 6f). 3.5. Growth Mechanism of Hierarchical CdS. 3.5.1. Crystal Structures of CdS. It is well-known that the final morphologies of crystals depend on the intrinsic crystal structures and the external environments. First, the crystal structures of CdS will be discussed. Generally, there are two types of polymorphic forms for CdS, namely wurtzite and zinc blende, respectively. In our experiments, both of them were obtained under different conditions. Figure 7a shows the typical crystal cell of wurtzite structure. According to the view of crystalline chemistry, the S2- anions are arranged with a hexagonal-close-packed (hcp) pattern due to their large radii of about 0.138 nm, while the Cd2+ cations with an average radii of 0.078 nm are filled in the tetrahedral interstitials. Actually, from the angle of the polyhedron, the central Cd2+ cation and the ligand S2- anions constitute the anionic coordination tetrahedral unit CdS4 which is labeled as dashed lines in Figure 7a. These tetrahedrons are connected with the mutual vertexes and each vertex is shared

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by four tetrahedrons. They also closely stack along the [0001] direction (c-axis) in the alternate arrangement of A and B sites (Figure 7b), resembling that of the individual Cd2+ or S2- ions. It is noteworthy that the Cd2+ cation in each CdS4 tetrahedron deviates from (0001j) and approaches (0001). Although the entire CdS4 tetrahedron is electrically neutral, the distribution of changes along the 3-fold axis which is parallel to the c-axis is asymmetric. The positive charge is concentrated on the vicinity of (0001), which can be considered as the positively polar face. Contrarily, (0001j) acts as the negatively polar one. The whole CdS4 tetrahedron can be equivalent to a dipole, and the polar axis is along the [0001] direction. Hence, on the whole, the CdS crystal is composed of such dipoles, resulting in macroscopic polarity. Similarly, the wurtzite ZnO is also a polar crystal. The polar surfaces of ZnO may induce the formation of nanorings.23 Therefore, the polarity of CdS crystals may also influence the growth process. According to the coordination polyhedron growth mechanism mode,24 the anionic coordination tetrahedrons (CdS4) as the growth units can be easily formed in the reaction solution compared with the cationic coordination tetrahedrons (SCd4), which is due to the size differences of Cd2+ and S2- ions. Generally, the CdS4 tetrahedrons are facilely combined with the positively polar faces, but difficultly combined with the negatively polar ones. Consequently, the positively polar faces grow faster and gradually disappear, while the negatively polar ones are maintained. As a result, the preferred form of wurtzite CdS is formed, which is similar to that of natural greenockite CdS,25 as shown in Figure 7c. The bottom pedion is indicated by the (0001j) facet, while the hexagonal prism and pyramid are enveloped by m {101j0} and p {101j1}, respectively. At the initial stage, crystal nuclei may be preferentially developed into seeds with the preferred form, especially in the nonequilibrium system. During the further stage, the external conditions may influence the combining ability of tetrahedral growth units with different facets of the seeds. Although the growth rates of various facets may differ, the final morphology of the crystals still shows the characteristics of the preferred form of wurtzite CdS, as shown in the above TEM and SEM images. The detailed effects of external environments on the growth of hierarchical architectures of CdS will be further discussed. Moreover, zinc-blende CdS was also synthesized in our experiments. Figure 7d presents its typical crystal cell. According to the XRD pattern (e.g., Figure 5d) and the standard card (JCPDS No. 10-0454), the space group of the products is F4j3m. The CdS4 tetrahedrons are closely packed in (111) (the closepacked plane of such a structure), stacked along the [111] direction and periodically positioned in A, B, and C sites, respectively, as shown in Figure 7e. According to the above analyses of the wurtzite structure, similarly, (111) is the positively polar face, while the negatively polar one is (1j1j1j), which is usually maintained due to the slow growth rate. The polar axis parallels the [111] direction. Hence, the preferred form of zinc-blende CdS is a tetrahedron enveloped by {1j1j1j} facets (not shown here).24 3.5.2. Phase EWolution of Hierarchical Architectures. The final phase of the as-prepared CdS mainly depends on the type of crystal cells of the stable nuclei which are formed at the stage of nucleation. As analyzed above, one wurtzite cell contains at least two layers of CdS4 tetrahedrons stacking along the [0001] direction, while at least four layers of tetrahedrons along the [111] direction can constitute one zinc-blende cell. Moreover, the binding energy of zinc-blende CdS is lower than that of wurtzite one.26 Hence, generally speaking, the cubic zinc-blende

He and Gao phase is metastable from the view of thermodynamics. The formation of stable cubic-phase nuclei may be hindered by the relatively high nucleation barrier and disturbed by the fluctuation of the reaction environment. It is also important to note that the metastable cubic-phase nuclei may be easily redissolved into the reaction solution due to the high pressure during the hydrothermal process. Therefore, in the reported research where CdS has been synthesized, whether by liquid-phase methods16-18 or by physical evaporation process,27 the hexagonal phase is usually obtained. However, the cubic phase can also be achieved by epitaxial growth technique,28 or in some specific experimental environments.29-31 Although the detailed factors influencing the formation of a certain phase is unclear, it can be still speculated that some appropriate stabilizers are necessary in the reaction solution through investigating the experimental conditions of the above relevant literature. The stabilizers, such as some coordination agents (functional molecules30,31 and coordination polymers32), may have the polar cubic-phase nuclei stable through the complexation interaction between Cd2+ cations and them, and also lower the nucleation barrier to a certain extent, to promote further growth. In this work, 5-sulfosalicylic acid (H3SSA) has the ability to facilitate the formation of cubic CdS, which probably arises from the coordination effects on Cd2+ cations. However, the situation is complex. The phase composition of final products depends on the molar ratio of H3SSA to thiourea, keeping the concentration of cadmium acetate constant. More specifically, when the molar ratio is set to 10:2 (S13), the final products are a mixture of about 67 wt % cubic-phase and about 33 wt % hexagonal-phase CdS (Figure 5b and Table 1). Subsequently, if the molar ratio is decreased to 10:8 (S14), more hexagonal phase (∼77 wt %) is obtained (Figure 5c and Table 1). Finally, pure hexagonal wurtzite CdS (sample S12) constitutes the products with the ratio down to 1:2 (Figure 5a and Table 1). On the basis of the above results, it can be speculated that there exists competition between thiourea and H3SSA, which further significantly influences the composition of final products. Concretely speaking, thiourea can promote the formation of hexagonal-phase CdS, while 5-sulfosalicylic acid can favor the emergence of the cubic-phase one. Furthermore, interestingly, nitrate anions arising from cadmium nitrate could also act as the stabilizers probably due to the strongly selective adsorption on the positively polar faces of the cubicphase nuclei, which is different from the results of the ref 33, probably due to different experimental conditions. Similarly, the phase composition also relies on the competition of nitrate anions and thiourea. The higher molar ratio of nitrate anions to thiourea, the more cubic-phase CdS is obtained. When the ratio is set as 1:8, 1:2, and 1:0.5, the weight percentage of cubic phase in the final products is gradually increased from about 62 wt %, to 84 wt %, and to 89 wt % (Figure S6a,b in the Supporting Information and Figure 5d). Moreover, the amount of cubic phase increases with time during the reaction (S15-S17, Table 1). It is also pointed out that sample S17 is composed of hierarchically dendritic architectures with relatively large size, which quite differs from the nanostructures with the cubic or mixed phase reported in the literature.34,35 However, it seems that the acetate anions cannot induce the formation of cubicphase CdS, which is quite different from that of 5-sulfosalicylic acid and nitrate anions. Only the hexagonal phase can be obtained whatever the ratio of acetate anions to thiourea is (Figure 2). According to the above analyses, it seems that thiourea may serve as the troublemaker for the formation of stable cubic-phase nuclei. In this reaction system, thiourea acts as not only a sulfur source but also the ligands of Cd-thiourea

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SCHEME 1: Proposed Growth Process of Hierarchical Architectures of Cadmium Sulfide

complexes where Cd2+ cations are coordinated with sulfur atoms. It is also well-known that thiourea and thiourea derivatives are generally used as organocatalysts and building blocks for constructing organic crystals, owing to their properties as hydrogen-bond donors.36,37 Similarly, there exists the hydrogen bonding interaction between thiourea and hydrogen-bond receptors such as H3SSA and nitrate anions in the reaction solution, which probably weakens the stabilization effects of nitrate anions and H3SSA on the cubic-phase nuclei to some extent. Hence, it is inevitable to obtain hexagonal-phase CdS in all samples. It can be concluded that nitrate anions and 5-sulfosalicylic acid are the promoters for the formation of the cubic phase, while thiourea stimulates the emergence of the hexagonal one through hydrothermal treatments. Moreover, it seems that acetate anions have little effect on the phase composition of the final products. Hence, it is facile to control the phase structure of CdS nano-/ microcrystals through tuning the molar ratio of nitrate anions or 5-sulfosalicylic acid to thiourea under hydrothermal conditions. 3.5.3. Morphology EWolution of Hierarchical Architectures. Generally speaking, the hierarchical architectures of CdS are generated in nonequilibrium environments, which can be provided by hydrothermal processes.2,16 Although it is difficult to figure out the exact growth mechanisms of these hierarchical architectures, a possible one can be proposed according to the above results of experiments. It is known that the whole growth process can be divided into two main stages: the nucleation stage and the further growth stage. The subsequent growth stage could also include three substages: (1) aggregation, (2) Ostwald ripening, and (3) preferential growth, as shown in Scheme 1. The detailed discussion will follow. In the initial stage, thiourea, as the water-soluble sulfur source whose CdS bonds are easily attacked by oxygen atoms of H2O, can release S2- ions slowly. The relatively free S2- ions can react with the vicinal Cd2+ ions, which are coordinated by thiourea, to form CdS clusters slowly. The above reactions can be described as follows:

CS(NH2)2 + H2O f CO(NH2)2 + 2H+ + S2-

(1)

Cd2+ + S2- f CdS

(2)

When the mean sizes of CdS clusters are increased to the critical size due to the fluctuation of the hot solution, stable nuclei emerge. Actually, the networks where thiourea and other additives such as H3SSA and nitrate anions are linked by

hydrogen bonds could probably serve as heterogeneous nucleation sites. Hence, CdS is always surrounded by them in each stage of growth. As analyzed above, nitrate anions and H3SSA favor the formation of stable cubic-phase nuclei, while thiourea facilitates the construction of stable hexagonal-phase ones. The stable nuclei with different crystal structures further evolve into seeds with their own preferred forms. These small seeds may tend to aggregate in order to reduce the high surface energy. On the other hand, the dipole-dipole interaction between the seeds, which could be owing to the polarity of CdS crystals as mentioned above, may also drive them together. Naturally, the aggregates differ in the different reaction solutions. According to the results of the time-dependent experiments, there are two main types of aggregates: 3D spherical aggregates and 1D rodshaped aggregates. It is found that the seeds are habitually assembled into spheres in the presence of thiourea, while the 1D polycrystalline rods are obtained by means of adding H3SSA to the reaction solution or substituting acetate anions for nitrate anions (Figures 4a and 6a). However, sample S14 may be an exception. When the concentration of H3SSA and thiourea is high enough, the seeds tend to aggregate into spherical aggregates, not rod-shaped ones. It also should be pointed out that the 1D rod-shaped aggregates as for sample S15 (the intermediate of sample S17) synthesized in the existence of nitrate anions are composed of several polycrystalline nanospheres, which are connected with each other into nanonecklaces (Figure 6a). As the growth proceeds, the Ostwald ripening process may occur. Consequently, the relatively large particles grow, while the smaller ones dissolve and reprecipitate on the surfaces of the larger ones. However, as for sample S14, it seems that the Ostwald ripening effects are invalid and the polycrystalline spheres are maintained in the final products without further preferential growth (Figure 4f). That may be the result of the excessively strong protection effects of these complexants. When the growth process continues, some of the smaller particles adhering to the surfaces of larger ones further grow preferentially with the guidance of the coordination agents. As mentioned above, the polar CdS4 and SCd4 tetrahedrons in the reaction solution may be filling the role of growth units for CdS crystals according to the coordination polyhedron growth mechanism mode.24 The subsequently anisotropic growth can be considered as the process that the growth units preferentially bind onto the specific surfaces of the small particles. As analyzed above, it is relatively easy for the polar growth units to combine with the polar surfaces from the angle of thermodynamics, namely the anionic CdS4 tetrahedrons with the positive surfaces and the cationic SCd4 tetrahedrons with the negative ones. However, it is more difficult for them to bind on the other faces due to the higher energetic barrier. Consequently, the growth rate along the polar axis is higher than along other directions, which may result in the formation of 1D structure.17,18 Therefore, in this work, the [0001] direction as the polar axis of hexagonal CdS is the preferred growth orientation, while the [111] direction for the cubic CdS is preferred. Furthermore, noticeably, the relative quantities of the CdS4 and SCd4 tetrahedrons depend on the concentration of thiourea. As for sample S3, the CdS4 tetrahedrons are the dominant growth units, while the SCd4 tetrahedral units are dominant for sample S6.On the other hand, it is more advantageous for CdS4 tetrahedrons to form than for SCd4 tetrahedrons in the reaction solution due to the size differences between Cd2+ and S2- ions. Kinetically, the anisotropic growth rate of sample S3 along the [0001] direction is higher than that of sample S6. Consequently, the primary arms of sample S3 grow longer than the cone-shaped spines of sample

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He and Gao hierarchical architectures at room temperature. The typical two emission peaks can be observed, as for all samples (S3, S12, and S17). The peak at 533 nm assigned to the green emission corresponds to the near-band emission, while the peak at 565 nm indicated the yellowish green emission commonly arises from the deep-level or trap-state emission. It is distinct to find that the intensities of the yellowish green emission are stronger than those of green emission for all the as-detected samples. It is speculated that there exist many defects such as sulfur vacancies in these hierarchical architectures after the formation out of the reaction solution,18 which results in the strong yellowish green emission.

Figure 8. Photoluminescence spectra of some of the synthesized hierarchical architectures of CdS at room temperature excitated at 420 nm: (a) S3; (b) S17; (c) S12.

S6 (Figures 1c and 3c). Although the CdS4 tetrahedrons are mainly formed in the reaction solution for both samples S3 and S9, the concentration of excess thiourea and the formed urea may greatly influence the dynamics of anisotropic growth. It is known that the coordination interaction between Cd2+ and thiourea is destroyed and thiourea is transformed into urea during the nucleation and growth processes. According to the results of the experiments and the preferred form of hexagonal CdS, the excess thiourea and urea could selectively largely adsorb on the {101j0} surfaces of CdS particles. However, when the concentration of these organic molecules is high, the adsorption on {101j0} may approach saturation, which could result in the increase of quantity of adsorption on other surfaces. The difference of the growth rates of various surfaces may be decreased due to the protection of excess thiourea and urea. Hence, the degree of anisotropic growth of sample S9 is lower than that of sample S3 (Figures 1c and 3g), and the flowershaped nanoparticles are achieved. One can observe that the arrangement of these anisotropic arms (S3) and cone-shaped spines (S6) on the surfaces of the primary particles is asymmetric. However, as for sample S3, the arrangement of secondary structures is possessed of 6-fold symmetry, probably owing to the single-crystalline characteristics of the primary arms and the nature of hexagonal CdS. When introducing H3SSA into the reaction solution, the kinetics of the growth of CdS crystals is changed. The hierarchical spindles (S12) are obtained and well developed, which is probably due to the moderate concentrations of H3SSA and thiourea. The anisotropic growth of the trunks and branches is mutually independent. However, the complex architecture collapses for sample S13 due to the existence of overmuch H3SSA compared to the dosage of thiourea. Moreover, although nitrate anions induce the formation of cubic-phase CdS, the tower-shaped dendritic architectures with the mixed phases are still achieved after Ostwald ripening and preferential growth, whatever the molar ratios of nitrate anions and thiourea are (not shown here), which is different from that of H3SSA. Only the sizes of these tower-shaped dendrites differ from each other. The average length of the trunks of such dendrites is decreased with the increase of the molar ratio of thiourea to nitrate anions. According to the above analyses, it could be speculated that the influence of nitrate anions on the phase evolution is more remarkable than the influence on the morphology, while H3SSA and thiourea strongly coordinated with Cd2+ ions have great impact on both morphology and phase evolutions. 3.6. Optical Properties of Hierarchical Architectures. Figure 8 shows the photoluminescence spectra of some of the

4. Conclusion In summary, a series of well-developed hierarchical architectures of CdS crystals, such as flower-shaped multipods, caltrop-shaped spheres, flower-shaped nanoparticles, hierarchical spindles, polycrystalline spheres, and tower-shaped dendrites, were prepared via a simple hydrothermal method. Not only morphology but also phase composition can be facilely controlled. In our experiments, the as-synthesized complex architectures all preferentially grow along the polar axis, whether they are of hexagonal or cubic structures, which could be due to the growth habits of the polar CdS crystals and the controlling effects of the complexants. After excitation at 420 nm, a strong yellowish green emission probably related to the sulfur vacancies can be observed obviously. These architectures, especially the hierarchical spindles, may have potential applications in piezoelectric nanogenerators.38,39 This simple synthetic method could be allowed to fabricate the hierarchical structures of other functionally inorganic materials. Acknowledgment. This work was supported by the National Key Project of Fundamental Research (Grant 2005CB6236-05) and the Shanghai Nanotechnology Promotion Center (Grant 0852 nm01900), respectively. Special thanks are given to Dr. S. W. Yang for his guidance in our research work. Supporting Information Available: Large-scale SEM image and HRTEM image of one secondary arm of the sample S3; large-scale SEM image of the sample S6; large-scale SEM, TEM images and FFT, inverse FFT patterns of the sample S12; SEM image of the sample S14; large-scale SEM image and partially amplified HRTEM image of the sample S17; XRD patterns of the products synthesized with different molar ratios of cadmium nitrate and thiourea. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Rho, J. Y.; Kuhn-Spearing, L.; Zioupos, P. Med. Eng. Phys. 1998, 20, 92. (2) Chen, X. Y.; Wang, X.; Wang, Z. H.; Yang, X. G.; Qian, Y. T. Cryst. Growth Des. 2005, 5, 347. (3) Lim, W. P.; Low, H. Y.; Chin, W. S. Cryst. Growth Des. 2007, 7, 2429. (4) Li, H. B.; Chai, L. L.; Wang, X. Q.; Wu, X. Y.; Xi, G. C.; Liu, Y. K.; Qian, Y. T. Cryst. Growth Des. 2007, 7, 1918. (5) Quan, Z. W.; Li, C. X.; Zhang, X. M.; Yang, J.; Yang, P. P.; Zhang, C. M.; Lin, J. Cryst. Growth Des. 2008, 8, 2384. (6) Gu, Z. J.; Zhai, T. Y.; Gao, B. F.; Sheng, X. H.; Wang, Y. B.; Fu, H. B.; Ma, Y.; Yao, J. N. J. Phys. Chem. B 2006, 110, 23829. (7) Luo, Z. J.; Li, H. M.; Shu, H. M.; Wang, K.; Xia, J. X.; Yan, Y. S. Cryst. Growth Des. 2008, 8, 2275. (8) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382. (9) Bao, N. Z.; Shen, L. M.; Takata, T.; Domen, K. Chem. Mater. 2008, 20, 110.

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