pubs.acs.org/Langmuir © 2009 American Chemical Society
Structural Phase Behavior and Vibrational Spectroscopic Studies of Biofunctionalized CdS Nanoparticles Priya Thakur,† Satyawati S. Joshi,*,† Sudhir Kapoor,‡ and Tulsi Mukherjee‡ †
Department of Chemistry, University of Pune, Pune-411007, India, and ‡Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai-400085, India Received July 1, 2008. Revised Manuscript Received April 11, 2009
Biomodified CdS nanoparticles were synthesized using L-cysteine as a capping agent in the colloidal state as a function of pH. The role of pH on the size and structure of CdS nanoparticles was investigated in detail. At pH 7.4 and 9.1, X-ray diffraction spectra of as prepared samples showed the presence of a mixture of cubic and hexagonal phases while cubic phase was formed at pH 11.2. A gradual transition to the hexagonal phase was observed for refluxed samples at pH 9.1 and 11.2. Whereas, at pH 7.4, the sample remains in a mixture of cubic and hexagonal phase even after refluxing. The particle size of as prepared samples was about 2 nm, and for refluxed samples the size increased up to 10 nm. The binding of cadmium through thiol group is evidenced by infrared spectra. An intense band due to C-C-N vibration was observed after 24 h of reflux. The formation of a specific molecular cluster determines the growth of a particular phase. Transmission electron microscopy (TEM) studies support the X-ray diffraction (XRD) studies and exhibit well separated spherical particles while refluxed samples show clustering.
Introduction Semiconductor nanocrystallites or quantum dots have been extensively studied due to their novel electronic and optical properties arising from quantum confinement effects.1-5 Electronic properties of these semiconductor nanocrystallites are tremendously size dependent and structure dependent. Semiconductors, especially the group IV, II-VI, and III-V, are more covalent than other compounds and prefer tetrahedral coordination around the ions, giving rise to different phases. The small size of the clusters provides large external and internal surfaces, which can be used to bring structural transformation and to manipulate their catalytic behavior. Recently, conjugation of biological molecules to metals or semiconductors has added a new dimension to nanoparticle research.6-9 These biomodified nanocrystallites represent convolution of biotechnology and nanotechnology and yield hybrid materials. These nanocrystallites have wide applications in biological labeling, disease diagnosis, drug delivery, and molecular electronics. Organisms have developed various energy consuming detoxification strategies in order to remove unwanted organic substances. Cadmium is a soft thiophilic element which is toxic and non-bioessential, and it can be removed by precipitation with a suitable partner, forming CdS. Ion pumps may remove *Corresponding author. Fax: +91-020-25691728. Telephone:91-02025601394-573, 569. E-mail:
[email protected]. (1) (a) Sun, Q. J.; Wang, Y. A.; Li, L. S.; Wang, D.; Zhu, T.; Xu, J.; Yang, C.; Li, Y. Nat. Photonics 2007, 1, 717–722. (b) Brus, L. E. J. Chem. Phys. 1984, 80, 4403– 4409. (2) Lippens, P. E.; Lannoo, M. Phys. Rev. B 1989, 39, 10935–10942. (3) Rama; Krishna, M. V.; Friesner, R. A. J. Chem. Phys. 1991, 95, 8309–8322. (4) Alivisatos, A. P. Science 1996, 271, 933–937. (5) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41–53. (6) Dameron, C. T.; Reese, R. N.; Mehra, R. K.; Kortan, A. R.; Carroll, P. J.; Steigerwald, M. L.; Brus, L. E.; Winge, D. R. Nature 1989, 338, 596–597. (7) Kumar, A.; Mital, S. J. Mol. Catal. A: Chem. 2004, 219, 65–71. (8) Dujardin, E.; Mann, S. Adv. Mater. 2002, 14, 775–788. (9) (a) Schroedter, A.; Weller, H.; Eritja, R.; Ford, W. E.; Wessels, J. M. Nano Lett. 2002, 2, 1363–1367. (b) Hegde, S.; Kapoor, S.; Naumov, S.; Joshi, S.; Mukherjee, T. J. Nanosci. Nanotechnol. 2006, 6, 1–7. (c) Hegde, S.; Kapoor, S.; Joshi, S.; Mukherjee, T. Colloids Surf., A 2006, 280, 116–124.
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undesired substances or render them less harmful with a suitable partner, for example, Cd2+ + S2- f CdS. In the human body, cadmium is concentrated in the liver and kidney, and up to 30% unusually cysteine rich proteins preferentially bind with the soft heavy metal ions of the cadmium. Phase transition is an important issue, since most of the physical properties such as effective masses depend on the crystallographic structure of nanocrystals. These semiconductor nanocrystals have wide applications in many fields such as catalysis, data storage, biotechnology, biomedical, and pharmaceutical industries. However, quantum dot toxicity is an important issue for its applications in biosystems. Cd2+ ions were found to be the primary cause of cytotoxicity because of their ability to bind thiol groups in mitochondria.10a Surface functionalization not only reduces the growth of nanocrystals but also can minimize the quantum dot toxicity of nanocrystals. In addition to this, functionalization helps in the energy stability of the nanoclusters itself10b which has not been yet focused properly. Water-soluble quantum dots are needed for medical applications. Also, site selectivity is important for biochemical applications such as drug carriers. The phase control of quantum dots is necessary to tune the physical properties for appropriate applications. In the present case, due to biofunctionalization, cadmium toxicity can be reduced to certain extent and cysteine also provides biologically active end groups. The most important aspect of this system is that it allows use of aqueous quantum dots with cubic and hexagonal phases instead of conventional solid phase nanocrystals obtained after annealing. These water-soluble quantum dots with different phases may offer selectivity for particular medical applications. CdS is a wide band gap semiconductor (Eg ≈ 2.42 eV). It is wellknown that bulk CdS has a hexagonal wurtzite type crystal structure with a = 0.4160 and c = 0.6756 nm. Phase transition (10) (a) Walling, M. A.; Novak, J. A.; Shepard, J. R. E. Int. J. Mol. Sci. 2009, 10, 441–491. (b) Datta, S.; Kabir, M.; Saha-Dasgupta, T.; Sarma, D. D. J. Phys. Chem. C 2008, 112, 8206–8214. (c) Rapaport, E.; Pistorious, C. W. T. F. Phys. Rev. 1968, 172, 838–847.
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from hexagonal to cubic phase has been observed in the case of CdS nanocrystallites as a function of nanocrystallite size.10c,11-13 Also, phase transformation from cubic to hexagonal has been exhaustively studied for annealed samples.17,27 The crystalline structure of the nanocrystallites is largely influenced by the preparation conditions, nature of the binding agent, and source of sulfur. The formation of a mixed phase or polytype has been reported previously14 by many researchers, as the free energy difference between cubic and hexagonal phases is very low. The mechanism for the formation of a mixed phase of CdS nanoparticles is not yet well established. Phase stability mainly depends on particle size and surface environment. Ketipearachchi et al. observed phase transition from cubic to hexagonal with a decrease in pH for CdS thin films.14f L-Cysteine capped CdS nanoparticles were studied by many researchers.15 Many of the researchers have studied fluorescence properties of cysteine capped CdS nanoparticles. In the present report, CdS nanoparticles were synthesized using L-cysteine as capping agent at different pHs. The study mainly focuses on the effect of pH on the size and structure of CdS nanoparticles. Mixed phase, cubic, and hexagonal CdS nanoparticles were obtained by altering the pH of the system and reflux time. Phase transformations from cubic to hexagonal were observed for refluxed samples at higher pH. Change in the surface environment of nanocrystallites was investigated by infrared spectroscopy. Here, we report the possible mechanism for the formation of different phases of CdS nanoparticles formed as a function of pH.
Experimental Section All the chemicals used were of analytical grade. Cadmium perchlorate hydrate was purchased from Aldrich. L-Cysteine and sodium sulfide (Na2S 3 xH2O) were acquired from SD Fine Chemicals and Central Drug House, respectively. L-Cysteine capped CdS nanoparticles were synthesized by a wet chemistry method. This aqueous method uses cadmium perchlorate, sodium sulfide, and L-cysteine as the main reactants. Cadmium perchlorate (2 mM) and L-cysteine (5 mM) were mixed together with 50 mL of Milli-Q water. The pH of the solution was adjusted with NaOH under vigorous stirring. This was followed by addition of sodium sulfide with stirring in an inert atmosphere to make a final concentration of 1 mM. These CdS nanoparticles were then refluxed for various time intervals. All the as prepared samples and refluxed samples were brought to the powder form by dropwise addition of a nonsolvent 2-propanol and simultaneous stirring. These precipitates were separated by centrifugation and dried under vacuum. (11) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715. (12) Bandaranayake, R. J.; Wen, G. W.; Lin, J. Y.; Jiang, H. X.; Sorensen, C. M. Appl. Phys. Lett. 1995, 67, 831–833. (13) Zelaya-Angel, O.; Lozada-Morales, R. Phys. Rev. B 2000, 62, 13064–13069. (14) (a) Banerjee, R.; Jayakrishnan, R.; Ayyub, P. J. Phys.: Condens. Matter 2000, 12, 10647–10654. (b) Gibson, P. N.; Oezsan, M. E.; Lincot, D.; Cowache, P.; Summa, D Thin Solid Films 2000, 361-362, 34–40. (c) Zhang, H.; Chen, B.; Gilbert, B.; Banfield, J. F. J. Mater. Chem. 2006, 16, 249–254. (d) Lincot, D.; Mokili, B.; Froment, M.; Corteas, R.; Bernard, M. C.; Witz, C.; Lafait, J. J. Phys. Chem. B 1997, 101, 2174–2181. (e) Nanda, J.; Kuruvilla, B. A.; Sarma, D. D. Phys. Rev. B 1999, 59, 7473–7479. (f) Ketipearachchi, U. S.; Lane, D. W.; Rogers, K. D.; Painter, J. D.; Cousins, M. A. Paper Presented at MRS Fall Meeting, Boston, 2004. (g) Bawendi, M. G.; Kortan, A. R.; Steigerwald, M. L.; Brus, L. E. J. Chem. Phys. 1989, 91, 7282. (15) (a) Chen, Y.; Rosenzweig, Z. Anal. Chem. 2002, 74, 5132–5138. (b) BarglikChory, Ch.; Remenyi, Ch.; Strohm, H.; Muller, G. J. Phys. Chem. B 2004, 108, 7637–7640. (c) Chen, J.-L.; Zhu, C.-Q. Anal. Chim. Acta 2005, 546, 147–153. (d) Cai, Z.-X.; Yang, H.; Zhang, Y.; Yan, X.-P. Anal. Chim. Acta 2006, 559, 234–239. (e) Priyam, A.; Chatterjee, A.; BhattacharyaSubhash, C.; Saha Abhijit J. Cryst. Growth 2007, 304, 416–424. (f) Sapra, S.; Nanda, J.; Sarma, D. D.; Abed El-Al, F.; Hodesb, G. Chem. Commun. 2001, 2188–2189. (g) Bae, W.; Rizwana, A.; Mehra, R. K. Chemosphere 1998, 37, 363–385.
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The powder X-ray diffraction (XRD) spectra were recorded on a PW 1840 diffractometer using Cu KR radiation. Particle size was calculated from the full width at half-maximum (fwhm) of the diffracted lines using the Scherrer formula. For determining the quantum dot sizes, diffraction peaks corresponding to hkl values (111) and (110) were used for cubic and hexagonal phases, respectively.12,14a Infrared spectra were recorded on a FTIR 8400 spectrophotometer. Sampling was done using KBr pellets. Transmission electron microscopy (TEM) was carried out on JEOL 1200 EX and Philips CM 200 instruments. The specimens for the TEM measurements were prepared by placing a drop of a dilute, ultrasonicated ethanol dispersion of powder sample on a carbon coated copper grid and drying at room temperature. Microstructure observations were performed on a JEOL JSM 6360A instrument by scanning electron microscopy (SEM). Samples were sputtered with Pt.
Results and Discussion In order to understand the exact phase and particle size of CdS, the X-ray diffraction studies were carried out for as prepared samples synthesized at different pHs, namely, 7.4, 9.1, and 11.2, and for the samples refluxed at different time intervals (Figure 1). XRD patterns show broadened peaks as compared to bulk CdS. The XRD spectrum of CdS at pH 7.4 (Figure 1A-a) exhibits an asymmetric broad peak at ∼28 ° corresponding to the (111) plane in the cubic phase (rock salt) corresponding to JCPDS file no. 21-829. However, asymmetry of the peaks also reveals that the broad peak at ∼28 ° may be resulting from overlap of (100), (002), and (101) planes of the hexagonal structure. Therefore, formation of the hexagonal phase also cannot be ruled out. Banerjee et al. have discussed the transformation of cubic to hexagonal by a simple mechanism based on the periodic insertion of stacking faults.14a Estimation of particle size is not possible due to the presence of a mixture of cubic and hexagonal phases. The XRD spectrum of CdS at pH 9.1 (Figure 1B-a) also exhibits a broad peak but with marked symmetry. This implies that the 28° peak is not an overlap but it is a single plane corresponding to (111) plane of cubic phase. However, planes in the region 44-52° show signatures of the hexagonal phase of nanocrystallites. Thus, it is possible that at pH 9.1 CdS nanoparticles exist as a mixture of cubic and hexagonal phases.14g Nanda et al. also suggested a possible mixture of two phases, since they cannot conclusively identify the structure.14e At pH 11.2 (Figure 1C-a), CdS nanocrystallites show two broad peaks at ∼28° and ∼48° indicating cubic phase. The particle size calculated from the fwhm of the (111) peak is about 2 nm at pH 11.2. In order to check the effect on the particle size of CdS nanoparticles, the as prepared samples were refluxed at different time intervals in air. As shown in Figure 1, all the planes become sharper and narrower after refluxing, indicating an increase in the particle size. The XRD pattern of the sample prepared at pH 7.4 indicates cubic phase (zinc blende) corresponding to JCPDS no. 10-454 (Figure 1A-b and c). However, the asymmetry of the peaks maintained even after 24 h of reflux indicates the existence of a mixture of cubic and hexagonal phases. At pH 9.1 (Figure 1B-b and c) and 11.2 (Figure 1C-b-e), the XRD pattern of samples can be indexed to hexagonal phase (JCPDS no. 6-314). Growth and resolution of (110), (103), and (112) planes was observed after 1 h. Well resolved (100), (002), and (101) planes of the hexagonal phase can be seen only at pH 11.2 (Figure 1C-e). No prominent changes were observed at 6 h (Figure 1C-c) and 12 h (Figure 1C-d). The particle size calculated from the fwhm of the (110) plane at pH 9.1 is 5.7 and 6.6 nm at 1 and 24 h, respectively. At pH 11.2, the particle size varies in the range 7.1-10.7 nm from 1-24 h. It can be easily observed from Figure 1 that as prepared DOI: 10.1021/la900437x
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Figure 1. (A) XRD patterns of L-cysteine capped CdS nanoparticles synthesized at pH 7.4 (a) as prepared and after refluxing at (b) 1 h and (c) 24 h. (B) XRD patterns of L-cysteine capped CdS nanoparticles synthesized at pH 9.1 (a) as prepared and after refluxing at (b) 1 h and (c) 24 h. (C) XRD patterns of L-cysteine capped CdS nanoparticles synthesized at pH 11.2 (a) as prepared and after refluxing at (b) 1 h, (c) 6 h, (d) 12 h, and (e) 24 h.
samples show amorphous nature, while with increasing particle size crystallinity seems to increase and long-range order of the planes starts appearing. The incomplete phase transition observed may be due to the presence of sulfur vacancies in the CdS lattice.16,17 IR studies were done to understand the structure-spectral relationship induced as a function of pH and the capping effect of L-cysteine on the phase behavior. L-Cysteine has three potential binding sites: sulfihydryl, amino, and carboxylate groups. It can easily form five membered rings with N and S or N and O and six membered rings with S and O chelates with metal ions.18 Figure 2A shows infrared spectra of L-cysteine and L-cysteine capped CdS nanoparticles at various pHs. The main frequencies of L-cysteine (Figure 2A-a) include -SH stretching at about 2552 and 941 cm-1 corresponding to the stretching and bending mode, respectively. Intense bands at about 1595 and 1400 cm-1 due to NH3+ asymmetric bending and COO- symmetric stretching modes, respectively, confirm the zwitterionic nature of L-cysteine. The S-C mode occurs in the range 657-746 cm-1. The C-N stretching vibrations can be found at about 1064 cm-1. A band at 540 cm-1 was assigned to the COO- deformation mode.24a A band at 1141 cm-1 can be assigned to the NH3+ rocking mode. It can be pointed out from the spectra (Figure 2A-b-d) of CdS nanoparticles that all the spectra are devoid of -SH absorption. It shows that Cd2+ is coordinated to cysteine through S linkage by cleavage of S-H bonds and formation of S-Cd bonds. In acidic media, formation of thiolate ions and protonation (16) Lozada-Morales, R.; Zelaya-Angel, O. Cryst. Res. Technol. 2004, 39, 1115– 1119. (17) Zelaya-Angel, O.; Esparza-Garcia, A. E.; Falcony, C.; Lozada-Morales, R.; Ramirez-Bon, R. Solid State Commun. 1995, 94, 81–85. (18) Chandrasekharan, M.; Udupa, M. R.; Aravamudan, G. Inorg. Chim. Acta 1973, 7, 88–90.
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competes with binding of Cd2+ ions,19 leading to the formation of larger particles. Furthermore, the bands at 1595 and 1402 cm-1 due to -NH3+ asymmetric bending and COO- symmetric stretching modes, respectively, are present in all three spectra.20 This indicates the presence of free NH3+ and COO- groups. At pH 7.4, a band at 676 cm-1 due to C-S linkage was observed.24a The intensity of this band diminishes in spectra (c) and (d) due to the decomposition of cysteine at higher pH.21 Bands at 32003400 cm-1 due to the -NH asymmetric stretching mode and at 2850 and 2925 cm-1 corresponding to CH vibrations are present. The -NH band becomes broader at pH 9.1, and pH 11.2 may be due to the hydrogen bonding (N-H 3 3 3 O) at higher pH.23 Figure 2-B-D shows IR spectra of CdS nanoparticles for the refluxed samples. At pH 11.2 (Figure 2-B), a band at 1047 cm-1 due to C-N stretching can be seen in Figure 2B-a-d. In addition to this, a shoulder at 1082 cm-1 corresponding to the C-C stretching mode is observed in (c) and (d). After 24 h (Figure 2B-e), an increase in intensity of the band at 1082 cm-1 corresponding to the C-C-N asymmetric stretching mode22,23 with a shoulder at 1211 cm-1 associated to the -CH2 wagging vibration24b was observed. However, the peak intensity of these two bands goes on decreasing with time. There may be a possibility of resonance formation between carbon attached to the thiol group and nitrogen of the amino group, due to disappearance of C-H vibrations and enhanced C-C-N vibrations. (19) Lawless, D.; Kapoor, S.; Meisel, D. J. Phys. Chem. 1995, 99, 10329–10335. (20) Shindo, H.; Brown, T. L. J. Am. Chem. Soc. 1965, 87, 1904–1909. (21) (a) Haobo, B.; Wei, H.; Mingyuan, G. Chin. Sci. Bull. 2006, 51, 2576–2580. (b) Nicolet, B L. J. Am. Chem. Soc. 1931, 53, 3066–3072. (22) Silva, B. L.; Freire, P. T. C.; Melo, F. E. A.; Guedes, I.; Araujo Silva, M. A.; Filho, J. M.; Moreno, A. J. D. Braz. J. Phys. 1998, 28, 19–24. (23) Pandiarajan, S.; Umadevi, M.; Rajaram, R. K.; Ramakrishnan, V. Spectrochim. Acta, Part A 2005, 62, 630–636. (24) (a) Wolpert, M.; Hellwig, P. Spectrochim. Acta, Part A 2006, 64, 987–1001. (b) Ramachandran, E.; Natarajan, S. Cryst. Res. Technol. 2004, 39, 308–312.
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Figure 2. (A) IR spectra of (a) L-cysteine and (b) L-cysteine capped CdS nanoparticles at pH 7.4, (c) at pH 9.1, and (d) at pH 11.2. (B) IR spectra of CdS at pH 11.2 (a) as prepared and after refluxing at (b) 1 h, (c) 6 h, (d) 12 h, and (e) 24 h. (C) IR spectra of CdS at pH 9.1 (a) as prepared and after refluxing at (b) 1 h and (c) 24 h. (D) IR spectra of CdS at pH 7.4 (a) as prepared and after refluxing at (b) 1 h and (c) 24 h.
Furthermore, the C-S band disappears after refluxing. Also, in all the spectra, bands due to -NH3+ asymmetric bending and COOsymmetric stretching modes are present. This indicates that the decomposition rate of cysteine increases with increasing reflux time. At pH 9.1 (Figure 2C), similar to pH 11.2, C-C-N Langmuir 2009, 25(11), 6334–6340
asymmetric stretching (1090 cm-1) becomes prominent after 24 h (Figure 2C-c). A slight decrease in intensity of the band due to the NH3+ mode was observed, but there is no change in intensity due to the -NH mode, whereas a significant decrease in intensity of the band due to the COO- mode was observed at 24 h. DOI: 10.1021/la900437x
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Figure 3. TEM images of L-cysteine capped CdS nanoparticles synthesized at pH 7.4.
CdS synthesized at neutral pH shows different behavior (Figure 2D). After 24 h of refluxing, no change in the intensity of the band due to the -NH mode was observed, but peaks were shifted from 3024 to 3465 cm-1. The band due to the NH3+ mode was shifted from 1585.4 to 1606.6 cm-1 with no change in the intensity, and the band due to the COO- mode showed a decrease in intensity at 24 h. The most prominent feature is that, unlike the cases of pH 9.1 and 11.2, both the bands due to C-N stretching and the intense band due to the C-C stretching mode were observed at 1047 and 1120 cm-1, respectively (Figure 2D-c). This observation showed that it remains in the metastable state even after 24 h of refluxing. This is in accordance with the XRD studies where no phase transition was observed at 24 h. Moreover, the band due to the C-S mode was observed in all the spectra at 24 h. Thus, at 24 h, the decomposition rate of cysteine is faster at higher pH. This leads to a change of surface properties of CdS nanoparticles. The increase in particle size after refluxing can be explained on the basis of “Ostwald ripening”.11,26 Ostwald ripening is characterized by the simultaneous growth and dissolution of the particles. Nanoparticles are thermodynamically less stable, and the free energy driving force is also higher. Due to the transfer of smaller particles to the larger through the solution, there is a tendency to minimize surface free energy by dissolving small particles and growing large particles after nucleation for a significant length of time. At the time of nucleation, the energy level increases momentarily to a higher value. Energy distribution depends on the statistical distribution of molecular velocities. The final size is determined by stabilization of the system via reduction in free energy at that instant. It has been observed that the thermodynamics of the system was affected by the surface properties of nanoparticles. As discussed earlier, Fourier transform infrared (FTIR) studies reflect the change in behavior of cysteine at various experimental conditions. These variations can affect the final structure of CdS nanoparticles. As per a previous report by Haobo et al.,21a cysteine is easily enolized in which OH- acts as a catalyst and this reaction can be accelerated by reflux. Thus, we can propose that, at pH 7.4, most of the cysteine molecules are in the form of Cd2+-cysteine complex. Whereas, at pH 9.1 and 11.2 cysteine molecules could slightly decompose. Further, the decomposition rate is increased with an increase in refluxing time. This was supported by FTIR spectra where decreased intensity at 24 h (25) Li, Y.; Li, X.; Yang, C.; Li, Y. J. Mater. Chem. 2003, 13, 2641–2648. (26) Eberl, D. D.; Srodon, J. Am. Mineral. 1988, 73, 1335–1345.
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indicates decomposition of the capping molecule (Figure 2B-e, C-c, and D-c). This reduces the relative amount of capping molecule at the particle surface at higher pH with an increase in refluxing time. Also, IR studies have demonstrated that the chelating behavior at 24 h of refluxing is different at pH 9.1 and 11.2 compared to pH 7.4. Thus, modification of the surface after refluxing changes the surface energy of CdS nanoparticles. Consequently, change in surface energy induces the phase transition from cubic to hexagonal at higher pH. Also, change of structure with size is mostly attributed to the different nucleation kinetics. In addition to this, reaction kinetics of the system determines the structure of CdS nanoparticles. At pH 7.4, reaction of cadmium ions with sulfur ions is slow and may be due to the stable complex form of Cd-cysteine (observed by optical spectra, not shown here); this leads to the formation of mixed phase CdS nanoparticles. At higher pH, reaction rate is high, forming cubic phase CdS nanoparticles.14c It was observed that phase transitions generally occur at an annealing temperature between 300 and 400 °C.27 In our case, just the variation of pH allows the transformation of the structure to a lower energy hexagonal phase as a consequence of lattice relaxation. Moreover, at pH 7.4, mixed cubic and hexagonal phase was observed for as prepared as well as refluxed samples, which may be due to the lattice mismatch induced strain.28 Many authors have reported that material showed mixed structure with many stacking faults.14 Gibson et al. have proposed that this type of structure is stabilized by a surface energy-volume energy competition.14b For ZnS nanoparticles, Luther et al.29 and Zhang et al.14c have shown that the formation of specific molecular cluster determines the growth of particular phases. Also, they have investigated that the pH of the solution influences the structures of ZnS nanoparticles.14c Consistent with this model, it can be proposed that two different clusters, β-cluster, Cd3S9 [3d-3s chair], and R-cluster, Cd4S11 [4d4s], can be formed in the initial stage.30a Structural studies of the mammalian form have been carried out using X-ray diffraction, EXAFS, UV, CD, and 113Cd NMR spectroscopy. It was found that a total of seven metal centers, each of them 4-coordinate, can (27) Bandaranayake, R. J.; Wen, G. W.; Lin, J. Y.; Jiang, H. X.; Sorensen, C. M. Appl. Phys. Lett. 1995, 67, 831–833. (28) Cockayne, B.; Wright, P. J. Growth and Optical Properties of Wide-Gap IIVI Low-Dimensional Semiconductors; Gebhardt Plenum: New York, 1989; p 75. (29) Luther, G. W.; Theberge, S. M.; Rickard, D. T. Geochim. Cosmochim. Acta 1999, 63, 3159. (30) (a) Kaim, W.; Schweerski, B. Bioinorganic Chemistry: Inorganic Elements in the Chemistry of Life; John Wiley and Sons: Chichester, England, 1996; Chapter 17. (b) Furey, W. F.; Robbins, A. H.; Clancy, L. L.; Winge, D. R.; Wang, B. C.; Stout, C. D. Science 1986, 704, 213.
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Figure 4. TEM images of L-cysteine capped CdS nanoparticles synthesized at pH 11.2 (a) as prepared and (b) after refluxing at 24 h.
Figure 5. SEM images of CdS nanoparticles after refluxing at (a) 1 h and (b) 24 h at pH 11.2.
be bound in two clusters by 9 and 11 cysteinate residues as stated above 30. The mixed phase observed can be a result of the aggregation of these two clusters. At higher pH, β-clusters may grow to form cubic phase CdS nanoparticles. No phase change was observed after refluxing of mixed structure nanoparticles at physiological pH, whereas for cubic nanoparticles phase transition to stable hexagonal phase was observed at pH 11.2. Thus, phase stability of CdS nanoparticles was determined by the pH of the solution where we have to take into account related factors such as reaction time, nature of binding of cysteine, and nucleation and growth kinetics. Langmuir 2009, 25(11), 6334–6340
A TEM study was undertaken to highlight the shape, size, size distribution, and crystallinity of the particles. At pH 7.4 (Figure 3), TEM images showed spherical particles linked to each other due to the cross-linking of cysteine. At pH 11.2, TEM micrograph shows well separated spherical particles (Figure 4a), whereas after 24 h of refluxing the sample exhibited spherical particles linked to each other, forming clusters (Figure 4b). The diffraction pattern showed diffused rings showing an amorphous nature (Figure 3). The sharpness of the planes goes on increasing with size (Figure 4a,b). The cubic structure can be easily observed in Figure 4b. DOI: 10.1021/la900437x
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SEM micrographs of L-cysteine capped CdS exhibited a regular, cross-linked network of particles. The particles are elongated, joined to each other in a definite manner with uniform morphology at 24 h (Figure 5b), while at 1 h particles are agglomerated together (Figure 5a). Our reaction system is kinetically driven. With the reaction time, anisotropy increases and the growth of a particular crystallographic surface of the nanoparticles seems to be controlled kinetically. At low temperature, thermal energy is low and slow growth of the hexagonal phase (Figure 1C-e) is observed by a kinetically controlled process leading to elongated morphology.25
Conclusions Being an amino acid, the pH of the solution plays a crucial role in deciding the X-ray structure and morphology. The phase of the CdS nanoparticles was influenced by the pH of the solution, reaction kinetics, and refluxing time. Mixed phase
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CdS nanoparticles were formed at pH 7.4 and at pH 9.1, and no phase change was observed after refluxing for the former. At higher pH, the hexagonal phase was obtained for refluxed samples, since the pH of the solution affects the decomposition rate of cysteine at different refluxing times as investigated by FTIR spectroscopy. In addition to this, a model was proposed where the formation of different molecular clusters decides the final structure of nanocrystallites. The above studies will be definitely useful as a role model in detection of toxic elements and can be used as a diagnostic tool in detoxification using amino acids. This study further requires more investigation and is in progress. Acknowledgment. The authors acknowledge BARC, India for the financial support. We are grateful to Mr. Gholap, Centre for Material and Characterization, NCL, Pune and SAIF, IIT, Mumbai for TEM analysis.
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