Synthesis and Characterization of ZnS, CdS, and Composition

Jan 20, 2009 - Yiqing Chen,* Xinhua Zhang, Chong Jia, Yong Su, and Qiang Li. School of Materials Science and Engineering, Hefei UniVersity of Technolo...
0 downloads 0 Views 211KB Size
Download PDF
J. Phys. Chem. C 2009, 113, 2263–2266

2263

Synthesis and Characterization of ZnS, CdS, and Composition-Tunable ZnxCd1-xS Alloyed Nanocrystals via a Mix-Solvothermal Route Yiqing Chen,* Xinhua Zhang, Chong Jia, Yong Su, and Qiang Li School of Materials Science and Engineering, Hefei UniVersity of Technology, Hefei, Anhui 230009, People’s Republic of China ReceiVed: August 28, 2008; ReVised Manuscript ReceiVed: NoVember 6, 2008

The authors developed a simple mix-solvothermal route to synthesize nanocrystals in the mild binary mixed solvent made of diethylenetriamine (DETA) and water. ZnS, CdS, and composition-tunable ZnxCd1-xS alloyed nanocrystals with special architectures have been synthesized by the mix-solvothermal route. The composition of the alloyed nanocrystals was adjusted by controlling the Zn/Cd molar ratios. The products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and photoluminescence (PL). The results show that all of the products have a good crystallinity with the typical hexagonal wurtzite, and the Zn/Cd molar ratios greatly affect final morphologies and properties of the alloyed nanocrystals. The PL peak positions of ZnxCd1-xS shift gradually from pure ZnS to pure CdS. A chelating ligand assisted nucleation-preferential growth process is proposed as the possible formation mechanism of the special architectures. 1. Introduction Zinc sulfide (ZnS) and cadmium sulfide (CdS), as the important wide band gap II-VI group semiconductor materials, are commercially used as the phosphor in thin-film electroluminescent devices, solar cells, and other optical electronic devices.1-4 They are being extensively studied because of their good semiconductor characteristics. However, the applications of ZnS and CdS are restricted due to their settled band gap energies. The good news is that the properties of CdS can be increased by introduction of other materials, such as ZnS. In fact, materials with ternary alloyed nanostructures may offer more unique properties than the corresponding plain and binary compounds,5-11 and their properties can be effectively tuned by adjusting the stoichiometry of the constituent components.7 For example, ZnxCd1-xS nanoribbons, fabricated through a vapor transport process, have demonstrated that their wavelength controlled lasing properties.9 Ternary ZnxCd1-xS nanowire arrays prepared via a noncatalytic and template-free metal-organic chemical vapor deposition process have color tenability.7 It has also been shown that the adjustment of composition in ZnxCd1-xS films can lead to optimization for the photoelectrochemical properties of the films.12 However, controlling the structure and chemical composition of the ZnxCd1-xS nanostructures remains a significant challenge. In recent years, controlling the morphology and the structure of ZnS and CdS nanocrystallites has attracted much attention. Many special structures of ZnS and CdS have obtained by the solvothermal method, such as ZnS nanoplates,13 ZnS microspheres,14-17 ZnS urchinlike nanostructures,18 CdS flowerlike nanostructures,19,20 and CdS multipods.21 The detail analyses demonstrate that most of spherical architectures are hollow or solid, and few spheres can be formed by assembly of nanorods. Herein, we report a simple mix-solvothermal synthetic route to synthesis of ZnS, CdS, and composition-tunable ZnxCd1-xS alloyed nanocrystals with special nanorod-based spherical * To whom correspondence should be addressed. Phone: +86-551-2901365. Fax: +86-551-290-1362. E-mail: [email protected].

TABLE 1: Compositions of the Reactant Solution and the Parameter of the Products zinc source cadmium source sample No. Zn[mmol] A B C D E F

1.0 0.8 0.6 0.4 0.2

Cd [mmol] 0.2 0.4 0.6 0.8 1.0

crystal lattice molar ratios of Zn/Cd, as determined (a/c)[Å],as by EDS shown by XRD 1 0.917 0.607 0.44 0.154 0

(3.273/6.204) (3.501/6.252) (3.520/6.394) (3.534/6.594) (3.537/6.618) (3.556/6.686)

architectures. Studies found that the morphologies and properties of the products could be well tuned by adjusting the composition of the reactant solution. 2. Experimental Section All chemicals were of analytical grade and were used as received without further purification. The ZnxCd1-xS ternary alloyed nanocrystals were prepared by a mix-solvothermal synthetic route. The typical procedure is as follow: First, Zn (CH3COO)2 · 2H2O and 3CdSO4 · 8H2O with different molar ratios were put into Teflon-lined stainless steel autoclaves together with appropriate amount of (NH4)2S. The compositions of the reactant solution were shown in Table 1. Then the autoclaves were filled with the aqueous solution mixed with 6 mL diethylenetriamine (DETA) and 12 mL H2O up to 80% of the capacity (23 mL). The autoclaves were maintained at 180 °C for 12 h and then cooled down naturally to room temperature. The precipitates were filtered and washed with distilled water and ethanol successively. The final products were dried in vacuum at 60 °C for 4 h. Binary ZnS and CdS nanocrystals were also synthesized in a similar process. The resulting samples were determined by X-ray powder diffractometer (XRD, Rigaku D/MaxrB). The morphologies and microstructures of the products were characterized by fieldemission scanning electron microscopy (FESEM, JEOL JSM-

10.1021/jp8091122 CCC: $40.75  2009 American Chemical Society Published on Web 01/20/2009

2264 J. Phys. Chem. C, Vol. 113, No. 6, 2009

Chen et al.

Figure 1. Powder XRD patterns of the products.

6700F), transmission electron microscope (TEM, Hitachi H-800), and high-resolution transmission electron microscopy (HRTEM, JEOL JEOL-2010). Their components were measured via energy-dispersive X-ray spectroscopy (EDS) attached in the SEM system and the HRTEM system. The photoluminescence (PL) spectra were obtained by a steady-state spectrofluorometer (FLUOROLOG-3-TAU) with a Xe lamp as the excitation light source at room temperature.

Figure 2. FESEM images of the samples (a: ZnS, f: CdS, b-e: ZnxCd1-xS).

3. Results and Discussion To expound more clearly, the samples were marked as A-F. The composition details of samples are arranged in the Table 1. It is worth noting that the Zn/Cd molar ratio is different between precursors and products. In the preparation of ternary alloyed nanocrystals reported in the previous literature,7,9-12 the Zn/Cd molar ratios in the reactants are obviously higher than that found in resulting products because the reaction of Cd with S is faster than that of Zn with S in those reaction systems. In our case, the Zn/Cd molar ratios in the formed ZnxCd1-xS nanocrystals determined by EDS are also not in accord with the sources but it can be seen that this disparity is comparatively less from the Table 1. What can make for this result? As we known, EDTA is a strong chelator, which can react with M2+ (M is Zn or Cd) to form a stable M-EDTA complex. It is obvious that different complexions have the different formation and decomposition rates. Although there is no evident proof to show which one is faster, it is certain that the introduced EDTA greatly affects the composition of the final products. Figure 1 shows the X-ray diffraction patterns of the products, together with the JCPDS cards Nos. 36-1450 and 41-1049 of ZnS and CdS, respectively. The strong and narrow peaks show good crystallinity of the samples. It is clearly seen that the character of sample A and sample F is separately in good agreement with the typical hexagonal wurtzite crystals ZnS (JCPDS 36-1450) and CdS (JCPDS 41-1049). As for the ZnxCd1-xS nanocrystals, the diffraction peaks in the XRD patterns gradually shift from ZnS to CdS with a decrease of Zn content. The continuous peak-shift may rule out phase separation or separated nucleation of CdS and ZnS nanocrystals, and samples (B-E) probably have alloyed structures. FESEM images of the samples are shown in Figure 2, which display that the as-synthesized nanocrystals have been assembled into special architectures. Obviously, the pure ZnS nanocrystals (sample A) have the spherical structure made of fine rods (shown in part a of Figure 2). The pure CdS nanocrystals (sample F) also have similar spherical aggregations but they are more like trees with many arms (shown in part f of Figure 2). As for the ZnxCd1-xS nanocrystals, different Zn contents (x) have different morphologies. The length and the diameter of the ZnxCd1-xS

Figure 3. HRTEM images, SAED pattern, and EDS of the product.

nanorods with decrease of x (shown in parts b-d of Figure 2) increased obviously, comparing to that of pure ZnS (part a of Figure 2).When Zn content is tiny, the ZnxCd1-xS nanocrystals have the multiarms structures instead of the spherical structures (part e of Figure 2). This structure in detail is more similar to that of pure CdS (part f of Figure 2). Therefore, the Zn/Cd ratios greatly affect morphologies of the alloyed nanocrystals, and there is a gradual changing process with decrease of Zn content. Detailed microstructures and composition information of asgrown ternary ZnxCd1-xS nanostructures were further characterized by HRTEM. Here, HRTEM images of the sample E are shown in the Figure 3. The rodlike structure is found instead of agminate architecture, which can be attributing to long time ultrasonic decentralization. EDS exhibits that there are Zn, Cd, and S in the sample, and the ratio of them is accord with stoichiometric composition of ZnxCd1-xS. Little signal of O may come from exterior oxidation, and that of Cu is generated from

Composition-Tunable ZnxCd1-xS Alloyed Nanocrystals

J. Phys. Chem. C, Vol. 113, No. 6, 2009 2265

Figure 4. TEM images of the products synthesize by different heat treatment (A, 1 h; B, 2 h; C, 4 h; D, 12 h). Figure 5. PL spectra of the products (b-e, ZnxCd1-xS).

the copper grid. The selected area electron diffraction (SAED) pattern and the HRTEM image confirm that the typical sample E have hexagonal single-crystal structure. The displayed clearcut lattice spacing of 0.33 nm (part A of Figure 3) is close to the lattice spacing of the wurtzite CdS (001) plane. All of the evidence demonstrate that the growth direction of the nanorod is , a preferential growth direction. It is noteworthy that there are some structure deformations due to elements default or asymmetric doped, such as vacancies (part B of Figure 3) and dislocations (part C of Figure 3) in the local of the nanorod. The HRTEM results of ZnS and CdS provided in the Supporting Information demonstrate the undoped nanorods have a good crystallinity and also grow along direction. It has been previously reported that solvent affects on the shape, size, and phase formation of various semiconductor nanocrystals under solvothermal conditions.22 To understand the effect of different reaction conditions, other reaction sources were adopted in the synthesis. In our experiment, the content of DETA in solvent plays a critical role in the formation of the alloyed structures. Separate ZnS or CdS particles will appear in our experiments when the ratio of VDETA/VH20 is lower than 1:2 or the pure water is used as solvent. When the ratio of VDETA/ VH20 is higher than 1:2, ZnS · DETA or CdS · DETA are detected, which is the same as what Yao reported.23 In additions, sulfur sources are also important. Using Na2S instead of (NH4)2S, no pure ternary ZnxCd1-xS nanocrystals are obtained, partly because that Na2S can easily dissolve in the solution, which results in rapid reaction speed. Obviously, the DETA and (NH4)2S play key roles in such a process. The assembly of the nanocrystals possibly connects to the using of DETA. Unfortunately, up to now, there is no a reasonable growth mechanism can explain the form of these special architectures. To obtain a better understanding of the formation mechanism and evolution of the special architectures, time-dependent experiments where the Zn/Cd molar ratio is 6:4 were carried out. The morphology evolutions of the time-dependent products are shown in Figure 4. From part A of Figure 4, it is obvious that some smooth microspheres are present at the early reaction stage (1 h). Some wool-like structures appeared on the outer surface of the spheres, and as time passes they are observed in the low magnification TEM images (parts B and C of Figure 4). After the reaction finished in 12 h, the nanorod-based spherical architectures are in our sight (part D of Figure 4). The observations suggest that the nanorods gradually branch out from the microspheres with the passage of time. On the

basis of the chelating and capping effects of EDTA and the above morphology evolution evidence with the changes of the reaction time, the formation of the special architectures can be attributed to a chelating ligand assisted nucleation-preferential growth process. First, the complexions (Zn-EDTA and CdEDTA) are formed as follow:

M2+ + EDTA f M-EDTA (M ) Zn or Cd)

(1)

Then M-EDTA will react with H2S which was produced from the decomposition of (NH4)2S, and some MS nuclei are formed. Finally, the rods branch out from the nucleation site along the preferential growth direction of the hexagonal wurtzite crystal. In our mixed solvent system, EDTA can gather together as one or more mini spaces, and these mini spaces usually present as microspheres because of surface tensions. It is the reason why the as-synthesized products seem to be spherical aggregations. As for the different morphology of CdS, it may be attributed to the discrepancy between the different formation and decomposition rates of the complexions (Zn-EDTA and Cd-EDTA) and the affects of inherent crystallographic features of ZnS and CdS. The PL was used to further investigate the optical properties of the as-synthesized products. The room-temperature PL spectra of samples were measured using a He-Cd laser line at 285 nm as the excitation source and a 399 nm filter wavelength. All of the spectra were shown in the Figure 5 to contrast the gradual change. From the PL spectrum of ZnS, it can be found the assynthesized ZnS nanocrystals have a strong broad emission at 465 nm and a weak emission at 556 nm. Similarly, the CdS nanocrystals also have a strong emission at 709 nm and a weak emission at 549 nm. As shown in lines b-e of Figure 5, all of the ZnxCd1-xS nanocrystals also have two central emission peaks. It was reported that the photoluminescence of II-VI semiconductor materials usually consists of two emission bands at room temperature: a near-band-edge (UV) light emission corresponded to the intrinsic near band edge emission (NBE), and a broad, deep-level (visible) emission corresponded to the extrinsic deep-level emission (DLE) in the lower energy region.24-26 In addition, the visible emission is usually considered to be related to various intrinsic defects. For CdS (Eg ) 2.58 eV) and ZnS (Eg ) 3.83 eV), it is obvious that these relative strong peaks would not come from the intrinsic near band edge emission but from the extrinsic deep-level emission. It might be caused by vacancy states or interstitial states of a particular nanostructure. The result is consistent with our

2266 J. Phys. Chem. C, Vol. 113, No. 6, 2009 previous analysis that there are some vacancies in the composition as Zn or Cd default. Undoubtedly, more work must be performed before the origins and the nature related to the DLE of these ZnxCd1-xS nanocrystals could be identified, which needs to be confirmed by further studies. It is noteworthy that the PL peaks position of ZnxCd1-xS nanocrystals change gradually with increasing concentration of Cd, a progressive red shift from pure ZnS to pure CdS. Besides, the profile of the PL curves of ZnxCd1-xS cannot be simulated by a superposition of the corresponding PL spectra of ZnS and CdS, indicating that the ZnxCd1-xS products are complex compounds rather than simple mixtures of ZnS and CdS. These results not only prove indirectly that the synthesized samples are ternary alloyed nanostructures but also show that their properties can be effectively tuned. 4. Conclusions In summary, ZnS, CdS, and composition-tunable ZnxCd1-xS alloyed nanocrystals with special architectures have been synthesized by the mix-solvothermal route. The composition of the alloyed nanocrystals can be adjusted by controlling the Zn/Cd molar ratios. More importantly, these compositiontunable ZnxCd1-xS nanocrystals with special architectures exhibit tunable properties. The semiconductor nanocrystals with special architectures are promising in the optoelectronics research field and have broad prospects in photocatalysis field. The roomtemperature PL spectra show that the ZnxCd1-xS nanocrystals have a progressive red shift from pure ZnS to pure CdS. We propose a chelating ligand assisted nucleation-preferential growth process to explain the formation of the special architectures. The mix-solvothermal synthetic route is hopeful to be extended to design and prepare other ternary alloyed nanostructures in the future. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (NSFC, No.20671027). Supporting Information Available: TEM image, SAED pattern, and HRTEM image of the ZnS, which display that the as-synthesized nanorod is single-crystal with about 60 nm width, growing along direction. SAED pattern and HRTEM image of the multiarms CdS confirm that the typical sample F also have hexagonal single-crystal structure and grow along the

Chen et al. direction. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Duan, X. F.; Huang, Y.; Argarawal, R.; Liber, C. M. Nature 2003, 421, 241. (2) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (3) Wu, Y. Y.; Yan, H. Q.; Huang, M.; Messer, B.; Song, J. H.; Yang, P. D. Chem.sEur. J. 2002, 8, 1261. (4) Zhang, H.; Zhang, S. Y.; Zuo, M.; Li, G. P.; Hou, J. G. Eur. J. Inorg. Chem. 2005, 1, 47. (5) Wang, A.; Dai, J.; Cheng, J.; Chudzik, M. P.; Marks, T. J.; Chang, R. P. H.; Kannewurf, C. R. Appl. Phys. Lett. 1998, 73, 327. (6) Young, D. L.; Williamson, D. L.; Coutts, T. J. J. Appl. Phys. 2002, 91, 1464. (7) Lin, Y. F.; Hsu, Y. J.; Lu, S. Y.; Chen, K. T.; Tseng, T. Y. J. Phys. Chem. C 2007, 111, 13418. (8) Pan, A.; Yang, H.; Liu, R.; Yu, R.; Zou, B.; Wang, Z. J. Am. Chem. Soc. 2005, 127, 15692. (9) Liu, Y.; Zapien, J. A.; Shan, Y. Y.; Geng, C. Y.; Lee, C. S.; Lee, S. T. AdV. Mater. 2005, 17, 1372. (10) Wang, M.; Fei, G. T.; Zhang, Y. G.; Kong, M. G.; Zhang, L. D. AdV. Mater. 2007, 19, 4491. (11) Kim, Y.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Paladugu, M.; Zou, J.; Suvorova, A. A. Nano Lett. 2006, 6, 599. (12) Rincon, M. E.; Martinez, M. W.; Miranda-Hernandez, M. Sol. Energy Mater. Sol. Cells 2003, 77, 25. (13) Zhou, G. T.; Wang, X. C.; Yu, J. C. Cryst. Growth Des. 2005, 5, 1761. (14) Cao, X. B.; Gu, L.; Zhuge, L. J.; Gao, W. J.; Wang, W. C.; Wu, S. F. AdV. Funct. Mater. 2006, 7, 896. (15) Jiang, C. L.; Zhang, W. Q.; Zou, G. F.; Yu, W. C.; Qian, Y. T. Mater. Chem. Phys. 2007, 103, 24. (16) Tong, H.; Zhu, Y. J.; Yang, L. X.; Li, L.; Zhang, L.; Chang, J.; An, L. Q.; Wang, S. W. J. Phys. Chem. C 2007, 111, 3893. (17) Liu, H. J.; Ni, Y. H.; Han, M.; Liu, Q.; Xu, Z.; Hong, J. N.; Ma, X. Nanotechnology 2005, 16, 1908. (18) Xiong, S.; Xi, B.; Wang, C.; Xu, D.; Feng, X.; Zhu, Z.; Qian, Y. AdV. Funct. Mater. 2007, 17, 2728. (19) Wang, L. C.; Chen, L. Y.; Luo, T.; Qian, Y. T. Mater. Lett. 2006, 60, 3627. (20) Gao, F.; Lu, Q. Y.; Meng, X. K.; Komarneni, S. J. Phys. Chem. C 2008, 112, 13359. (21) Chen, M.; Kim, Y. N.; Li, C.; Cho, S. O. Cryst. Growth Des. 2008, 8, 629. (22) Yu, S. H.; Yang, J.; Han, Z. H.; Zhou, Y.; Yang, R. Y.; Xie, Y.; Qian, Y. T.; Zhang, Y. H. J. Mater. Chem. 1999, 9, 1283. (23) Yao, W. T.; Yu, S. H.; Pan, L.; Li, J.; Wu, Q. S.; Zhang, L.; Jiang, J. Small 2005, 3, 320. (24) Xi, Y. Y.; Cheung, T. L. Y.; Dickon, H. L. Ng. Mater. Lett. 2008, 62, 128. (25) Cheng, B.; Shi, W. S.; Russell-Tanner, J. M.; Zhang, L.; Samulski, E. T. Inorg. Chem. 2006, 45, 1213. (26) Fonoberov, V. A.; Balandin, A. A. Appl. Phys. Lett. 2004, 85, 5971.

JP8091122