Research article Received: 14 November 2013,
Revised: 16 January 2014,
Accepted: 30 March 2014
Published online in Wiley Online Library: 16 May 2014
(wileyonlinelibrary.com) DOI 10.1002/bio.2695
Preparation and optical properties of alloyed ZnxCd1-xS/alginate core/shell nanoparticles Liping Wang* and Yujie Sun ABSTRACT: ZnxCd1-xS/alginate core/shell nanoparticles were synthesized via a colloidal route by reacting zinc and cadmium ions with sulfide ions, followed by coating with alginate. The crystal structure, morphology, size and optical properties of the core/shell nanoparticles were characterized by X-ray diffraction, transmission electron microscopy, UV/vis and photoluminescent spectra, respectively. The ZnxCd1-xS nanoparticles are spherical and have a cubic structure with a mean crystalline size of 2–4 nm. The band gap of ZnxCd1-xS/alginate core/shell nanoparticles increases with increasing Zn/Cd molar ratio, and the UV/vis absorption blue-shifts correspondingly. Two emissions related to zinc and sulfide ion vacancies were observed for the ZnxCd1-xS/alginate core/shell nanoparticles due to the surface changes from the alginate coating. A cadmium-related emission was observed for both the uncovered ZnxCd1-xS and ZnxCd1-xS/alginate core/shell nanoparticles, which has a significant blue-shift with increasing Zn/Cd molar ratio. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: ZnxCd1-xS nanoparticles; alloyed semiconductor; core/shell structure; optical properties
Introduction
86
Semiconductor nanomaterials have been extensively investigated over the past two decades aimed at applications in optoelectronics, biodevices, chemical/biological sensors and photocatalysts (1–5). The above applications rely mostly on the special spectroscopic, electronic and chemical properties of nanomaterials, which are unlike bulk materials. Much effort, therefore, has been made to change the properties by varying the structure and composition of semiconductor nanomaterials (6,7). Fortunately, tuning the band gap by combining different semiconductors to form alloyed semiconductors has been verified as a successful way to change optical properties. Alloyed semiconductor nanoparticles have potential application in the design of new electronic devices, light-emitting diodes, biological labels and catalysts (8–10). To prevent the agglomeration of nanoparticles and provide additional properties, organic or inorganic molecules have been used as capping reagents for surface modification or shell structures (11–13). ZnS and CdS, the most important II–VI semiconductors with a wide (3.68 eV) and narrow (2.49 eV) band gap, respectively, have been used in bio-related fields due to their special electronic and optical properties (14,15). For example, CdS nanoparticles have been employed as electroactive tags in electrochemical assays of cancer markers and DNA (16). In this case, the nanoparticles are used as quantitation tags, the electrochemical signal emanating from them can be quantified. Combining wideband-gaped ZnS and narrow-band-gaped CdS to form alloyed semiconductors, ZnxCd1-xS can bring advantages to optical applications due to changes in its structure and band gap compared with bare ZnS and CdS (17,18). Surface modification of alloyed semiconductors has been carried out using an inorganic shell, and some optical properties have been improved (19). However, few reports focus on the fabrication and properties of core/shell structures such as alloyed ZnxCd1-xS covered with biomacromolecules, which may have applications in bio-related systems.
Luminescence 2015; 30: 86–90
Alginate is a polysaccharide extracted from brown algae and is widely used in the food industry as an emulsifier and stabilizer due to its low toxicity and good biocompatibility (20). Alginate is composed of alternating blocks of α-L-guluronic and β-D-mannuronic acid residues linked at the 1,4 positions of each six-membered ring, to which more hydroxyl and carboxylic groups attach as side groups (21). The hydroxyl and carboxylic groups in alginate can bond tightly to the metal ions in nanoparticles. Therefore, its structural properties may make alginate a favorable shell material for covering nanoparticles to form a stable core/shell structured composite. Furthermore, negatively charged alginate as a shell can form polyelectrolyte complexes with positively charged species, which may provide favorable opportunities for the bio-related applications of nanoparticles (22). However, few results have been reported for the study of alloyed semiconductor alginate core/shell structures. In this study, ZnxCd1-xS nanoparticles were prepared by reacting zinc and cadmium ions with sulfide ions in a colloidal solution containing mercaptoacetic acid (MPA) and N,N-dimethylformamide (DMF). ZnxCd1-xS/alginate core/shell nanoparticles were obtained by covering the surface of ZnxCd1-xS nanoparticles with alginate. The crystal structures and size of the ZnxCd1-xS/alginate core/shell nanoparticles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). UV/vis absorption and photoluminescence (PL) emission were investigated at various Zn/Cd molar ratios.
* Correspondence to: L. Wang, Department of Materials Physics and Chemistry, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China. E-mail: [email protected] Department of Materials Physics and Chemistry, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, People’s Republic of China
Copyright © 2014 John Wiley & Sons, Ltd.
Alloyed ZnxCd1–xS/alginate core/shell nanoparticles
Experimental Preparation of ZnxCd1-xS/alginate core/shell nanoparticles Analytical chemicals, zinc acetate, cadmium acetate, sodium sulfide, sodium alginate, sodium hydroxide, MPA and DMF, were used without further purification. A typical preparation of ZnxCd1-xS nanoparticles was carried out as follows. First, known amounts of zinc acetate (1 mmol/mL) and cadmium acetate (1 mmol/mL) solutions were added to a threenecked flask. Then, 22 mL of DMF and 0.6 mL of MPA were added to the flask, followed 5 mol/L of sodium hydroxide solution until the pH reached 10. After that, a known amount of sodium sulfide solution (1 mmol/L) was added and the mixture was stirred for 5 h. Finally, 44 mL of acetone was added to the mixture to precipitate the product. The final product of ZnxCd1-xS nanoparticles was obtained after precipitated under 6500 rpm, washed twice with alcohol and vacuum dried at 40-50°C for 10 h. To obtain the ZnxCd1-xS/alginate core/shell nanoparticles, the following procedures were carried out. An aliquot of 0.04 g ZnxCd1-xS nanoparticles and 0.02 g of alginate sodium were first added to a tube containing 4 mL of distilled water, and then magnetically stirred for 1 h. After precipitation, washing and vacuum drying, ZnxCd1-xS/alginate core/shell nanoparticles were obtained. Characterization XRD patterns were recorded on a D/max-RB X-ray diffractometer (Rigaku Corp.) with CuKα irradiation. TEM images were collected
using a FEI-20 (FEI Co.) electron microscope operating at 200 kV. UV/vis absorption spectra were acquired using a JASCO V570 UV–Vis–NIR spectrophotometer (Japan Spectrophotometric Instruments). PL spectra were obtained using a Hitachi FL-4500 fluorescence spectrophotometer.
Results and discussion XRD and TEM analysis Figure 1 shows the XRD patterns of the ZnxCd1-xS/alginate nanoparticles prepared using various Zn/Cd molar ratios. For the sample with a Zn/Cd molar ratio of 4 : 1, three major diffraction peaks can be indexed to standard cubic ZnS (JCPDS No. 05-0566), shown as the vertical lines against the abscissa axis. For the sample with a Zn/Cd molar ratio of 1 : 4, the diffraction peaks can be indexed to standard cubic CdS (JCPDS No. 89-0440), shown as the vertical lines with asterisk against the abscissa axis. Whereas for the sample with a Zn/Cd molar ratio of 1 : 1, the peaks are in between those of standard ZnS and CdS. The mean crystalline sizes are ~ 2–4 nm, as estimated using the well-known Scherrer equation, D = kλ/βcosθ. The results suggest the formation of cubic ZnxCd1-xS nanoparticles, whose crystal structure varies with the Zn/Cd molar ratio. When Zn2+ is the dominant cation in the preparation, the crystal structure of ZnxCd1-xS nanoparticles is closer to that of cubic ZnS, whereas Cd2+ is the dominant cation, the crystal structure of ZnxCd1-xS nanoparticles is closer to that of cubic CdS. Figure 2 shows TEM images for the ZnxCd1-xS/alginate core/ shell nanoparticles prepared with a Zn/Cd molar ratio of 1 : 1. The ZnxCd1-xS core/shell nanoparticles are uniformly distributed, however, the size is much larger than that estimated from the XRD analysis. This result indicates that ZnxCd1-xS nanoparticles covered with alginate may be composed of many small polycrystalline ZnxCd1-xS particles. UV/vis absorption
Figure 1. XRD patterns of the ZnxCd1-xS/alginate core/shell nanoparticles with a Zn/Cd molar ratio of 1 : 4, 1 : 1 and 4 : 1.
Figure 3 shows the UV/vis absorption spectra of the ZnxCd1-xS nanoparticles and ZnxCd1-xS/alginate core/shell nanoparticles at a Zn/Cd molar ratio of 1 : 1. For the sample of ZnxCd1-xS nanoparticles, the absorption peak is located at ~ 355 nm, whereas for the ZnxCd1-xS/alginate core/shell nanoparticles, the absorption peak locates at ~ 403 nm, which is red-shifted compared with the ZnxCd1-xS nanoparticles. This may be caused
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Figure 2. Typical TEM images for the ZnxCd1-xS/alginate core/shell nanoparticles with a Zn/Cd molar ratio of 1 : 1, (a) at a scale bar of 200 nm and (b) at a scale bar of 50 nm.
L. Wang and Y. Sun the number of Zn ions is smaller than the number of Cd ions. Zn ions dope into the CdS lattices, causing slight changes in the lattice constants and in absorption, owing to the small size of Zn ions. When the Zn/Cd molar ratio is > 1 : 1, Zn ions are dominant in ZnxCd1-xS nanoparticles. Cd ions dope into the ZnS lattices, causing dramatic changes in the lattice constants and significant changes in the absorption owing to the large size of the Cd ions. Photoluminescence analysis
by a decrease in the quantum size effect due to an increase in the size of the particles with an alginate cover. The estimated particle size is ~ 2–4 nm from UV spectra, which is consistent with the result obtained from XRD. Similar phenomena were observed for other samples at various Zn/Cd molar ratios, as shown in Table 1. It is obvious that the absorption peak for each sample shows a significant redshift before and after covering with alginate. This might be due to formation of a complex between ZnxCd1-xS and the carboxylic group in the alginate molecule (23). In addition, the absorption peaks blue-shift gradually and the band gaps increase gradually with the increasing Zn/Cd molar ratio for uncovered ZnxCd1-xS nanoparticles. The results may be caused by a decrease in the particle size and changes in the lattices. As the Zn/Cd molar ratio increases, more Zn ions enter the lattice of ZnxCd1-xS nanoparticles, which may cause the decrease in particle size and the blue-shift in absorption. It is well known that the band gap of ZnS is higher than that of CdS. The band gap of the ZnxCd1-xS nanoparticles is in between that of ZnS and CdS. As the Zn/Cd molar ratio increases, the band gap of ZnxCd1-xS nanoparticles increases, changing from the CdS side to the ZnS side. Furthermore, it is worth noticing that the blue-shift in absorption is smaller for the samples with a Zn/Cd molar ratio between 1 : 4 and 1 : 1. However, the blue-shift in absorption is larger for the samples with a Zn/Cd molar ratio between 1 : 1 and 4 : 1. This may be because of the difference in radius between Zn and Cd ions. When the Zn/Cd molar ratio is < 1 : 1,
Table 2 shows the PL emission peaks for ZnxCd1-xS and ZnxCd1core/shell nanoparticles at various Zn/Cd molar ratios when excited at a wavelength of 310 nm. For the samples of ZnxCd1-xS nanoparticles, there is one emission peak and it is gradually blue-shifted with increasing Zn/Cd molar ratios, which suggests that the composition of the sample is alloyed ZnxCd1-xS nanoparticles rather than pure ZnS or CdS. This can also be deduced from the theoretical estimation that emission will blue-shift with the increasing band gap as the Zn/Cd molar ratio increases. For the ZnxCd1-xS/alginate core/shell nanoparticles, two new emission peaks were observed, with the exception of the peak in green light region, although it is obviously red-shifted compared with that of uncovered ZnxCd1-xS nanoparticles. Figure 4 shows the PL emission spectra of the ZnxCd1-xS/ alginate core/shell nanoparticles at various Zn/Cd molar ratios and exited at a wavelength of 310 nm. As shown in Fig. 4(a), two broad emissions were observed at around 430 and 560–620 nm and, as shown in Fig. 4(b), there is an insignificant emission peak at around 340 nm. The emission peak around 340 nm can be ascribed to the ZnS-related luminescence of zinc vacancies (24). The emission peak around 430 nm can be attributed to the recombination of electrons from the energy level of sulfur vacancies with the holes from the energy level of zinc vacancies (25). The two emissions around 340 and 430 nm may result from the combination of zinc ions and alginate through carboxylic groups. In this case, there might be more zinc vacancies left in the nanoparticle core, which will increase the opportunity for emissions from zinc vacancies and recombination. The emission peak around 560 - 620 nm is due to CdS-related luminescence in ZnxCd1-xS nanoparticles. By contrast, the two broad emission peaks imply that the alginate coverage might be insufficient for some of the nanoparticles.
Table 1. UV/vis absorption and band gap value for the ZnxCd1 xS and ZnxCd1 xS/alginate nanoparticles with various Zn/Cd molar ratios
Table 2. PL emission peaks for the ZnxCd1 xS and ZnxCd1 xS/alginate core/shell nanoparticles with various Zn/Cd molar ratios
xS/alginate
Figure 3. UV/vis absorption spectra of the ZnxCd1-xS nanoparticles (labeled Before) and ZnxCd1-xS/alginate core/shell nanoparticles (labeled After) with a Zn/Cd molar ratio of 1 : 1.
Zn/Cd molar ratio
1 3 2 1 3 7 4
: : : : : : :
4 7 3 1 2 3 1
ZnxCd1-xS nanoparticles
ZnxCd1-xS/alginate nanoparticles
Absorption (nm)
Band gap (eV)
Absorption (nm)
Band gap (eV)
363 363 358 355 346 321 306
3.41 3.41 3.47 3.49 3.58 3.86 4.06
410 409 401 403 384 367 327
3.02 3.03 3.09 3.08 3.23 3.38 3.79
Zn/Cd molar ratio
1 3 2 1 3 7 4
: : : : : : :
ZnxCd1-xS nanoparticles peak (nm)
4 7 3 1 2 3 1
556 549 542 536 514 501 496
ZnxCd1-xS/alginate nanoparticles Peak 1 (nm)
Peak 2 (nm)
Peak 3 (nm)
347 344 340 344 343 344 344
450 435 425 427 426 414 426
618 609 570 562 559 545 529
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Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 86–90
Alloyed ZnxCd1–xS/alginate core/shell nanoparticles Figure 5 shows the variation in the emission peaks as a function of the Zn/Cd molar ratio for ZnxCd1-xS and ZnxCd1-xS/alginate core/ shell nanoparticles. As shown in Fig. 5(a), for the ZnxCd1-xS/alginate core/shell nanoparticles, the position of the two emissions around 340 and 430 nm varies a little, which indicates that the emissions are independent of the variation in composition. This suggests that the two emissions arise from the covering effect of alginate in the core/shell nanoparticles. By contrast, the emission around 560–620 nm varies with the Zn/Cd molar ratio for the ZnxCd1-xS/ alginate core/shell nanoparticles, indicating that it corresponds to the alloy structured nanoparticles for the uncovered ZnxCd1-xS nanoparticles. Moreover, the emission around 560–620 nm shows a significant blue-shift with increasing Zn/Cd molar ratio for the ZnxCd1-xS/alginate core/shell nanoparticles, just as the emission does for the uncovered ZnxCd1-xS nanoparticles. However, as shown in Fig. 5(b), compared with the emission of uncovered ZnxCd1-xS nanoparticles at the same Zn/Cd molar ratio, that of the ZnxCd1-xS/alginate core/shell nanoparticles is red-shifted, which suggests formation of the core/shell structure (26). The reason for the red-shift might be due to the change in energy level as a result of combination of the ZnxCd1-xS core with the alginate shell.
Conclusions
Figure 4. Emission spectra of the ZnxCd1-xS/alginate core/shell nanoparticles with a Zn/Cd molar ratio of (a) 1 : 4, 3 : 7, 2 : 3 and (b) 3 : 2, 7 : 3, 4 : 1.
ZnxCd1-xS/alginate core/shell nanoparticles were prepared via a two-step colloid-related synthesis. The spherical ZnxCd1-xS nanoparticle core has a cubic structure, and its band gap can be tuned with variations in the composition. The band gap of ZnxCd1-xS nanoparticles increases gradually with increasing Zn/Cd molar ratio, changing from closer to the CdS side to closer to the ZnS side. As a result, the UV/vis absorption blue shifts gradually. Three emission peaks were observed for the ZnxCd1-xS/alginate core/shell nanoparticles, two of which may arise from the increase in zinc and sulfide vacancies due to changes in the surface covering. The Cd-related emission in ZnxCd1-xS nanoparticles has a significant blue-shift for both the uncovered ZnxCd1-xS and ZnxCd1-xS/alginate core/shell nanoparticles. Acknowledgement The authors gratefully acknowledge financial support from the National Science Foundation of China (Grant No. 21073012 and 20773012).
References
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Figure 5. Emission peaks as a function of Zn/Cd molar ratio for (a) ZnxCd1-xS/ alginate core/shell nanoparticles, and (b) comparison of ZnxCd1-xS (labeled Before) and ZnxCd1-xS/alginate core/shell (labeled After) nanoparticles.
1. Bruchez M Jr, Moronne M, Gin P, Weiss S, Alivisatos AP. Semiconductor nanocrystals as fluorescent biological labels. Science 1998;281:2013–6. 2. Rosenthal SJ, Chang JC, Kovtun O, McBride JR, Tomlinson ID. Biocompatible quantum dots for biological applications. Chem Biol 2011;18:10–24. 3. Medintz IL, Uyeda HT, Goldman ER, Mattoussi H. Quantum dot bioconjugates for imaging, labeling and sensing. Nat Mater 2005;4:435–46. 4. Koneswaran M, Narayanaswamy R. L-Cyteine-capped ZnS quantum 2+ dots based fluorescence sensor for Cu ion. Sensors Actuat B 2009;139:104–9. 5. Hoffmann MR, Martin ST, Choi WY, Bahenemann DW. Environmental applications of semiconductor photocatalysis. Chem Rev 1995;95:69–96. 6. Wu JC, Zheng J, Zacher CL, Wu P, Liu ZK, Xu R. Hybrid functional study of band bowing, band edges and electronic structures of Cd (1-x)Zn(x)S solid solution. J Phys Chem C 2011;115:19741–8. 7. Zan F, Ren J. Significant improvement in photoluminescence of ZnSe(S) alloyed QDs prepared in high pH solution. Luminescence 2010;25:378–83.
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18. Poormohammadi-Ahandani Z, Habibi-Yangjeh A. Fast, green and template-free method for preparation of Zn1-xCdxS nanoparticles using microwave irradiation and their photocatalytic activities. Physica E 2010;43:216–23. 19. Augst AD, Kong HJ, Mooney DJ. Alginate hydrogels as biomaterials. Macromol Biosci 2006;6:623–33. 20. George M, Abraham TE. Polyionic hydrocolloids for the intestinal delivery of protein drugs: algiante and chitosan – a review. J Control Release 2012;114:1–14. 21. Bardajee GR, Hooshyar Z, Rostami I. Hydrophilic alginate based multidentate biopolymers for surface modification of CdS quantum dots. Colloid Surface B 2011;88:202–7. 22. Paul W, Sharma CP. Synthesis and characterization of alginate coated zinc calcium phosphate nanoparticles for intestinal delivery of insulin. Proc Biochem 2012;47:882–6. 23. Adhyapak PV, Singh N, Vijayan A, Aiyer RC, Khanna PK. Single mode waveguide properties of m-NA doped Au/PVA nano-composites: synthesis, characterization and studies. Mater Lett 2007;61:3456–61. 24. Garaje SN, Apte SK, Naik SD, Ambekar JD, Sonawane RS, Kulkarni MV, et al. Template-free synthesis of nanostructured CdxZn1-xS with tunable band structure for H2 production and organic dye degradation using solar light. Environ Sci Technol 2013;47:6664–72. 25. Apte SK, Garaje SN, Arbuj SS, Kale BB, Baeg JO, Mulik UP, et al. A novel template free, one pot large scale synthesis of cubic zinc sulfide nanotriangles and its functionality as an efficient photocatalyst for hydrogen production and dye degradation. J Mater Chem 2011;21:19241–8. 26. Tripathi SK, Sharma M. Synthesis and optical study of green light emitting polymer coated CdSe/ZnSe core/shell nanocrystals. Mater Res Bull 2013;48:1837–44.
90 wileyonlinelibrary.com/journal/luminescence
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 86–90
Revised: 16 January 2014,
Accepted: 30 March 2014
Published online in Wiley Online Library: 16 May 2014
(wileyonlinelibrary.com) DOI 10.1002/bio.2695
Preparation and optical properties of alloyed ZnxCd1-xS/alginate core/shell nanoparticles Liping Wang* and Yujie Sun ABSTRACT: ZnxCd1-xS/alginate core/shell nanoparticles were synthesized via a colloidal route by reacting zinc and cadmium ions with sulfide ions, followed by coating with alginate. The crystal structure, morphology, size and optical properties of the core/shell nanoparticles were characterized by X-ray diffraction, transmission electron microscopy, UV/vis and photoluminescent spectra, respectively. The ZnxCd1-xS nanoparticles are spherical and have a cubic structure with a mean crystalline size of 2–4 nm. The band gap of ZnxCd1-xS/alginate core/shell nanoparticles increases with increasing Zn/Cd molar ratio, and the UV/vis absorption blue-shifts correspondingly. Two emissions related to zinc and sulfide ion vacancies were observed for the ZnxCd1-xS/alginate core/shell nanoparticles due to the surface changes from the alginate coating. A cadmium-related emission was observed for both the uncovered ZnxCd1-xS and ZnxCd1-xS/alginate core/shell nanoparticles, which has a significant blue-shift with increasing Zn/Cd molar ratio. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: ZnxCd1-xS nanoparticles; alloyed semiconductor; core/shell structure; optical properties
Introduction
86
Semiconductor nanomaterials have been extensively investigated over the past two decades aimed at applications in optoelectronics, biodevices, chemical/biological sensors and photocatalysts (1–5). The above applications rely mostly on the special spectroscopic, electronic and chemical properties of nanomaterials, which are unlike bulk materials. Much effort, therefore, has been made to change the properties by varying the structure and composition of semiconductor nanomaterials (6,7). Fortunately, tuning the band gap by combining different semiconductors to form alloyed semiconductors has been verified as a successful way to change optical properties. Alloyed semiconductor nanoparticles have potential application in the design of new electronic devices, light-emitting diodes, biological labels and catalysts (8–10). To prevent the agglomeration of nanoparticles and provide additional properties, organic or inorganic molecules have been used as capping reagents for surface modification or shell structures (11–13). ZnS and CdS, the most important II–VI semiconductors with a wide (3.68 eV) and narrow (2.49 eV) band gap, respectively, have been used in bio-related fields due to their special electronic and optical properties (14,15). For example, CdS nanoparticles have been employed as electroactive tags in electrochemical assays of cancer markers and DNA (16). In this case, the nanoparticles are used as quantitation tags, the electrochemical signal emanating from them can be quantified. Combining wideband-gaped ZnS and narrow-band-gaped CdS to form alloyed semiconductors, ZnxCd1-xS can bring advantages to optical applications due to changes in its structure and band gap compared with bare ZnS and CdS (17,18). Surface modification of alloyed semiconductors has been carried out using an inorganic shell, and some optical properties have been improved (19). However, few reports focus on the fabrication and properties of core/shell structures such as alloyed ZnxCd1-xS covered with biomacromolecules, which may have applications in bio-related systems.
Luminescence 2015; 30: 86–90
Alginate is a polysaccharide extracted from brown algae and is widely used in the food industry as an emulsifier and stabilizer due to its low toxicity and good biocompatibility (20). Alginate is composed of alternating blocks of α-L-guluronic and β-D-mannuronic acid residues linked at the 1,4 positions of each six-membered ring, to which more hydroxyl and carboxylic groups attach as side groups (21). The hydroxyl and carboxylic groups in alginate can bond tightly to the metal ions in nanoparticles. Therefore, its structural properties may make alginate a favorable shell material for covering nanoparticles to form a stable core/shell structured composite. Furthermore, negatively charged alginate as a shell can form polyelectrolyte complexes with positively charged species, which may provide favorable opportunities for the bio-related applications of nanoparticles (22). However, few results have been reported for the study of alloyed semiconductor alginate core/shell structures. In this study, ZnxCd1-xS nanoparticles were prepared by reacting zinc and cadmium ions with sulfide ions in a colloidal solution containing mercaptoacetic acid (MPA) and N,N-dimethylformamide (DMF). ZnxCd1-xS/alginate core/shell nanoparticles were obtained by covering the surface of ZnxCd1-xS nanoparticles with alginate. The crystal structures and size of the ZnxCd1-xS/alginate core/shell nanoparticles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). UV/vis absorption and photoluminescence (PL) emission were investigated at various Zn/Cd molar ratios.
* Correspondence to: L. Wang, Department of Materials Physics and Chemistry, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China. E-mail: [email protected] Department of Materials Physics and Chemistry, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, People’s Republic of China
Copyright © 2014 John Wiley & Sons, Ltd.
Alloyed ZnxCd1–xS/alginate core/shell nanoparticles
Experimental Preparation of ZnxCd1-xS/alginate core/shell nanoparticles Analytical chemicals, zinc acetate, cadmium acetate, sodium sulfide, sodium alginate, sodium hydroxide, MPA and DMF, were used without further purification. A typical preparation of ZnxCd1-xS nanoparticles was carried out as follows. First, known amounts of zinc acetate (1 mmol/mL) and cadmium acetate (1 mmol/mL) solutions were added to a threenecked flask. Then, 22 mL of DMF and 0.6 mL of MPA were added to the flask, followed 5 mol/L of sodium hydroxide solution until the pH reached 10. After that, a known amount of sodium sulfide solution (1 mmol/L) was added and the mixture was stirred for 5 h. Finally, 44 mL of acetone was added to the mixture to precipitate the product. The final product of ZnxCd1-xS nanoparticles was obtained after precipitated under 6500 rpm, washed twice with alcohol and vacuum dried at 40-50°C for 10 h. To obtain the ZnxCd1-xS/alginate core/shell nanoparticles, the following procedures were carried out. An aliquot of 0.04 g ZnxCd1-xS nanoparticles and 0.02 g of alginate sodium were first added to a tube containing 4 mL of distilled water, and then magnetically stirred for 1 h. After precipitation, washing and vacuum drying, ZnxCd1-xS/alginate core/shell nanoparticles were obtained. Characterization XRD patterns were recorded on a D/max-RB X-ray diffractometer (Rigaku Corp.) with CuKα irradiation. TEM images were collected
using a FEI-20 (FEI Co.) electron microscope operating at 200 kV. UV/vis absorption spectra were acquired using a JASCO V570 UV–Vis–NIR spectrophotometer (Japan Spectrophotometric Instruments). PL spectra were obtained using a Hitachi FL-4500 fluorescence spectrophotometer.
Results and discussion XRD and TEM analysis Figure 1 shows the XRD patterns of the ZnxCd1-xS/alginate nanoparticles prepared using various Zn/Cd molar ratios. For the sample with a Zn/Cd molar ratio of 4 : 1, three major diffraction peaks can be indexed to standard cubic ZnS (JCPDS No. 05-0566), shown as the vertical lines against the abscissa axis. For the sample with a Zn/Cd molar ratio of 1 : 4, the diffraction peaks can be indexed to standard cubic CdS (JCPDS No. 89-0440), shown as the vertical lines with asterisk against the abscissa axis. Whereas for the sample with a Zn/Cd molar ratio of 1 : 1, the peaks are in between those of standard ZnS and CdS. The mean crystalline sizes are ~ 2–4 nm, as estimated using the well-known Scherrer equation, D = kλ/βcosθ. The results suggest the formation of cubic ZnxCd1-xS nanoparticles, whose crystal structure varies with the Zn/Cd molar ratio. When Zn2+ is the dominant cation in the preparation, the crystal structure of ZnxCd1-xS nanoparticles is closer to that of cubic ZnS, whereas Cd2+ is the dominant cation, the crystal structure of ZnxCd1-xS nanoparticles is closer to that of cubic CdS. Figure 2 shows TEM images for the ZnxCd1-xS/alginate core/ shell nanoparticles prepared with a Zn/Cd molar ratio of 1 : 1. The ZnxCd1-xS core/shell nanoparticles are uniformly distributed, however, the size is much larger than that estimated from the XRD analysis. This result indicates that ZnxCd1-xS nanoparticles covered with alginate may be composed of many small polycrystalline ZnxCd1-xS particles. UV/vis absorption
Figure 1. XRD patterns of the ZnxCd1-xS/alginate core/shell nanoparticles with a Zn/Cd molar ratio of 1 : 4, 1 : 1 and 4 : 1.
Figure 3 shows the UV/vis absorption spectra of the ZnxCd1-xS nanoparticles and ZnxCd1-xS/alginate core/shell nanoparticles at a Zn/Cd molar ratio of 1 : 1. For the sample of ZnxCd1-xS nanoparticles, the absorption peak is located at ~ 355 nm, whereas for the ZnxCd1-xS/alginate core/shell nanoparticles, the absorption peak locates at ~ 403 nm, which is red-shifted compared with the ZnxCd1-xS nanoparticles. This may be caused
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Figure 2. Typical TEM images for the ZnxCd1-xS/alginate core/shell nanoparticles with a Zn/Cd molar ratio of 1 : 1, (a) at a scale bar of 200 nm and (b) at a scale bar of 50 nm.
L. Wang and Y. Sun the number of Zn ions is smaller than the number of Cd ions. Zn ions dope into the CdS lattices, causing slight changes in the lattice constants and in absorption, owing to the small size of Zn ions. When the Zn/Cd molar ratio is > 1 : 1, Zn ions are dominant in ZnxCd1-xS nanoparticles. Cd ions dope into the ZnS lattices, causing dramatic changes in the lattice constants and significant changes in the absorption owing to the large size of the Cd ions. Photoluminescence analysis
by a decrease in the quantum size effect due to an increase in the size of the particles with an alginate cover. The estimated particle size is ~ 2–4 nm from UV spectra, which is consistent with the result obtained from XRD. Similar phenomena were observed for other samples at various Zn/Cd molar ratios, as shown in Table 1. It is obvious that the absorption peak for each sample shows a significant redshift before and after covering with alginate. This might be due to formation of a complex between ZnxCd1-xS and the carboxylic group in the alginate molecule (23). In addition, the absorption peaks blue-shift gradually and the band gaps increase gradually with the increasing Zn/Cd molar ratio for uncovered ZnxCd1-xS nanoparticles. The results may be caused by a decrease in the particle size and changes in the lattices. As the Zn/Cd molar ratio increases, more Zn ions enter the lattice of ZnxCd1-xS nanoparticles, which may cause the decrease in particle size and the blue-shift in absorption. It is well known that the band gap of ZnS is higher than that of CdS. The band gap of the ZnxCd1-xS nanoparticles is in between that of ZnS and CdS. As the Zn/Cd molar ratio increases, the band gap of ZnxCd1-xS nanoparticles increases, changing from the CdS side to the ZnS side. Furthermore, it is worth noticing that the blue-shift in absorption is smaller for the samples with a Zn/Cd molar ratio between 1 : 4 and 1 : 1. However, the blue-shift in absorption is larger for the samples with a Zn/Cd molar ratio between 1 : 1 and 4 : 1. This may be because of the difference in radius between Zn and Cd ions. When the Zn/Cd molar ratio is < 1 : 1,
Table 2 shows the PL emission peaks for ZnxCd1-xS and ZnxCd1core/shell nanoparticles at various Zn/Cd molar ratios when excited at a wavelength of 310 nm. For the samples of ZnxCd1-xS nanoparticles, there is one emission peak and it is gradually blue-shifted with increasing Zn/Cd molar ratios, which suggests that the composition of the sample is alloyed ZnxCd1-xS nanoparticles rather than pure ZnS or CdS. This can also be deduced from the theoretical estimation that emission will blue-shift with the increasing band gap as the Zn/Cd molar ratio increases. For the ZnxCd1-xS/alginate core/shell nanoparticles, two new emission peaks were observed, with the exception of the peak in green light region, although it is obviously red-shifted compared with that of uncovered ZnxCd1-xS nanoparticles. Figure 4 shows the PL emission spectra of the ZnxCd1-xS/ alginate core/shell nanoparticles at various Zn/Cd molar ratios and exited at a wavelength of 310 nm. As shown in Fig. 4(a), two broad emissions were observed at around 430 and 560–620 nm and, as shown in Fig. 4(b), there is an insignificant emission peak at around 340 nm. The emission peak around 340 nm can be ascribed to the ZnS-related luminescence of zinc vacancies (24). The emission peak around 430 nm can be attributed to the recombination of electrons from the energy level of sulfur vacancies with the holes from the energy level of zinc vacancies (25). The two emissions around 340 and 430 nm may result from the combination of zinc ions and alginate through carboxylic groups. In this case, there might be more zinc vacancies left in the nanoparticle core, which will increase the opportunity for emissions from zinc vacancies and recombination. The emission peak around 560 - 620 nm is due to CdS-related luminescence in ZnxCd1-xS nanoparticles. By contrast, the two broad emission peaks imply that the alginate coverage might be insufficient for some of the nanoparticles.
Table 1. UV/vis absorption and band gap value for the ZnxCd1 xS and ZnxCd1 xS/alginate nanoparticles with various Zn/Cd molar ratios
Table 2. PL emission peaks for the ZnxCd1 xS and ZnxCd1 xS/alginate core/shell nanoparticles with various Zn/Cd molar ratios
xS/alginate
Figure 3. UV/vis absorption spectra of the ZnxCd1-xS nanoparticles (labeled Before) and ZnxCd1-xS/alginate core/shell nanoparticles (labeled After) with a Zn/Cd molar ratio of 1 : 1.
Zn/Cd molar ratio
1 3 2 1 3 7 4
: : : : : : :
4 7 3 1 2 3 1
ZnxCd1-xS nanoparticles
ZnxCd1-xS/alginate nanoparticles
Absorption (nm)
Band gap (eV)
Absorption (nm)
Band gap (eV)
363 363 358 355 346 321 306
3.41 3.41 3.47 3.49 3.58 3.86 4.06
410 409 401 403 384 367 327
3.02 3.03 3.09 3.08 3.23 3.38 3.79
Zn/Cd molar ratio
1 3 2 1 3 7 4
: : : : : : :
ZnxCd1-xS nanoparticles peak (nm)
4 7 3 1 2 3 1
556 549 542 536 514 501 496
ZnxCd1-xS/alginate nanoparticles Peak 1 (nm)
Peak 2 (nm)
Peak 3 (nm)
347 344 340 344 343 344 344
450 435 425 427 426 414 426
618 609 570 562 559 545 529
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Alloyed ZnxCd1–xS/alginate core/shell nanoparticles Figure 5 shows the variation in the emission peaks as a function of the Zn/Cd molar ratio for ZnxCd1-xS and ZnxCd1-xS/alginate core/ shell nanoparticles. As shown in Fig. 5(a), for the ZnxCd1-xS/alginate core/shell nanoparticles, the position of the two emissions around 340 and 430 nm varies a little, which indicates that the emissions are independent of the variation in composition. This suggests that the two emissions arise from the covering effect of alginate in the core/shell nanoparticles. By contrast, the emission around 560–620 nm varies with the Zn/Cd molar ratio for the ZnxCd1-xS/ alginate core/shell nanoparticles, indicating that it corresponds to the alloy structured nanoparticles for the uncovered ZnxCd1-xS nanoparticles. Moreover, the emission around 560–620 nm shows a significant blue-shift with increasing Zn/Cd molar ratio for the ZnxCd1-xS/alginate core/shell nanoparticles, just as the emission does for the uncovered ZnxCd1-xS nanoparticles. However, as shown in Fig. 5(b), compared with the emission of uncovered ZnxCd1-xS nanoparticles at the same Zn/Cd molar ratio, that of the ZnxCd1-xS/alginate core/shell nanoparticles is red-shifted, which suggests formation of the core/shell structure (26). The reason for the red-shift might be due to the change in energy level as a result of combination of the ZnxCd1-xS core with the alginate shell.
Conclusions
Figure 4. Emission spectra of the ZnxCd1-xS/alginate core/shell nanoparticles with a Zn/Cd molar ratio of (a) 1 : 4, 3 : 7, 2 : 3 and (b) 3 : 2, 7 : 3, 4 : 1.
ZnxCd1-xS/alginate core/shell nanoparticles were prepared via a two-step colloid-related synthesis. The spherical ZnxCd1-xS nanoparticle core has a cubic structure, and its band gap can be tuned with variations in the composition. The band gap of ZnxCd1-xS nanoparticles increases gradually with increasing Zn/Cd molar ratio, changing from closer to the CdS side to closer to the ZnS side. As a result, the UV/vis absorption blue shifts gradually. Three emission peaks were observed for the ZnxCd1-xS/alginate core/shell nanoparticles, two of which may arise from the increase in zinc and sulfide vacancies due to changes in the surface covering. The Cd-related emission in ZnxCd1-xS nanoparticles has a significant blue-shift for both the uncovered ZnxCd1-xS and ZnxCd1-xS/alginate core/shell nanoparticles. Acknowledgement The authors gratefully acknowledge financial support from the National Science Foundation of China (Grant No. 21073012 and 20773012).
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Figure 5. Emission peaks as a function of Zn/Cd molar ratio for (a) ZnxCd1-xS/ alginate core/shell nanoparticles, and (b) comparison of ZnxCd1-xS (labeled Before) and ZnxCd1-xS/alginate core/shell (labeled After) nanoparticles.
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