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NaOH Concentration Effect on the Oriented Attachment Growth Kinetics of ZnS Yonghao Wang,†,‡ Jing Zhang,†,‡ Yanlian Yang,§ Feng Huang,† Jinsheng Zheng,†,| Dagui Chen,† Fengbo Yan,† Zhang Lin,*,† and Chen Wang§ State Key Lab of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, People’s Republic of China, and National Center for NanoScience and Technology, Beijing 100080, People’s Republic of China ReceiVed: December 22, 2006; In Final Form: March 6, 2007
In this work, the crystal growth kinetics of ZnS nanoparticles coarsened under 100 °C with NaOH concentration from 2 to 8 M was investigated, aiming to study the role of NaOH concentration on the oriented attachment growth kinetics. It reveals that 2 M NaOH is sufficient to lead to two-stage growth kinetics of ZnS nanoparticles, resulting in pure and multistep oriented attachment growth characteristics in the first stage. When the NaOH concentration increases, the rate of crystal growth by oriented attachment mechanism increases, while the time period for crystal growth at the pure oriented attachment stage was similar. We suggest that the concentration of solute is critical to enhance the oriented attachment growth rate and achieve exclusively oriented attachment growth of nanoparticles at a large size scale.
Introduction Nanometer-scale sulfide semiconductor crystallites have been investigated extensively for their unique size-related properties of optics, electricity, magnetism, and so on.1,2 The preparation of nanoscale materials with controllable size and morphology is significant to the studies on possible novel properties such as quantum size effects, as well as achieving unique physical properties. Studies on the crystal growth kinetics and the microstructure development in nanoparicles play a critical role in controlling the size-dependent properties.3-6 Normally, the kinetic model for coarsening of bulk materials was based on the Ostwald ripening (OR) mechanism,7-9 which involves the dissolution-precipitation processes between particles.10 Once the particle sizes are reduced to nanoscale, this model of crystal growth is not unique. A crystal growth mechanism named “oriented attachment (OA)”,11,12 where two crystallographically oriented nanoparticles combined together to form a larger one, was proved to be another significant mode during nanocrystal growth.13-20 It was found that the OR and OA growth usually occur simultaneously,21 especially when the size of the nanoparticles is relatively small.14,22 In the OR process the growth of larger particles is at the expense of smaller ones, which will lead to a broader size distribution with time theoretically. On the other hand, it is generally believed that in the OA dominating growth stage, the smaller particles grow faster than the larger ones, which possibly facilitates a narrow size distribution once the pure OA period can be achieved for relatively long time periods. It is significant to investigate the crystal growth characteristics at the pure OA stage for nanoparticles. Previously it is found surface adsorption is an effective factor for hindering * To whom correspondence should be addressed. E-mail: zlin@ fjirsm.ac.cn. † Chinese Academy of Sciences. ‡ These two authors contributed equally. § National Center for NanoScience and Technology. | Current address: Department of Chemistry and the Key Laboratory of Analytical Sciences of the Ministry of Education, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, People’s Republic of China.
Figure 1. XRD pattern (left) and HRTEM image (right) of the assynthesized ZnS nanoparticle.
the OR process of ZnS nanoparticles, resulting in multistep OA growth in the NaOH-ZnS system. On the contrary, when the nanocrystals are capped with easily destroyed ligands, such as thiol, the possibility of OA between multilevel particles is relatively low, leading to a short OA dominant period.14,23 The relationship between OA kinetics and temperature was also investigated, revealing that it is the diffusion of the nanoparticles in specific solution but not the attachment that determines the growth rate in most OA systems. This work aims to study the role of NaOH concentration on the OA growth kinetics. The concentration for achieving exclusive OA growth, the relationship of the OA growth rate and NaOH concentration, and the retaining time for the OA period at different NaOH concentrations are investigated. We hope this work benefits the general understanding of the OA growth characteristics under concentrated inorganic solute for proving basic kinetic data for nanosynthesis. Experimental Section The primary ZnS nanoparticles were synthesized in aqueous solution without any surfactant. An aqueous solution of 0.1 M sodium sulfide was dropped into an equimolar, 0.1 M zinc chloride aqueous solution. The mixture was stirred vigorously for a homogeneous reaction, and aged for 30 min. Several washes from distilled water were conducted until the Clconcentration is below detection. The precipitates were dried and ground into a powder for the following kinetic experiments. All of the above experiments were conducted at room temperature.
10.1021/jp0688613 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/24/2007
Oriented Attachment Growth Kinetics of ZnS
J. Phys. Chem. B, Vol. 111, No. 19, 2007 5291
Figure 2. Experimental data and fitting results showing the mean size vs time in NaOH from 2 to 8 M at 100 °C; insets are enlarged plots for the first stage of growth.
As-synthesized nanoparticles of ZnS (0.1 g) and 10 mL of NaOH (2-8 M) were mixed and sealed into a Teflon-lined stainless steel autoclave with 23 mL capacity. The autoclaves were heated at 100 °C. For time series experiments, the autoclave containers were taken out and quenched to room temperature at the appropriate time interval. The precipitates were collected and washed with pure ethanol until the pH was ∼7.0. X-ray diffraction (XRD) was used to identify the crystal structures and average particle sizes of samples. Diffraction data were recorded with a PANalytical X’ Pert PRO diffractometer with Cu KR radiation (45 kV, 40 mA) in the continuous scanning mode. The 2θ scanning range was from 15° to 85° in steps of 0.03° with a collection time of 20 s per step. The average crystallite size was calculated from the peak broadening by using the Scherrer equation. High-resolution transmission electron microscopy (HRTEM) was used to confirm the particle size and to determine the particle morphology. Samples were prepared for HRTEM study by dispersing the ZnS powder onto a holey carbon-coated support. HRTEM analyses were performed with a JEOL JEM2010 HRTEM at 200 kV. Approximately 10 HRTEM images of nanoparticles in zone axis orientations were recorded from the initial sample and each of the hydrothermally treated materials to identify the microstructures. A ZetaPlus Zeta Potential Analyzer produced by Brookhaven Instruments Corporation was used to check the ζ potential of ZnS nanoparticles in NaOH solution. Results Figure 1 shows the typical XRD pattern and HRTEM of assynthesized ZnS nanoparticles. The XRD results reveal that the nanoparticles are in the sphalerite phase, with a calculated average size of ∼2.4 nm in [111], [220], and [311] directions, and they are also confirmed by HRTEM observation.
ZnS nanoparticles coarsened in concentrated NaOH were characterized with the same methods as above. Figure 2a-d shows the calculated sizes of nanoparticles versus coarsening time in NaOH with concentrations from 2 to 8 M at 100 °C. Similar to our previous observation in 4 M NaOH,15 the growth characteristics of ZnS nanoparticles share the same rule: in the first stage, crystal growth fits the asymptotic curve, indicating pure OA-based growth characteristics,14,21,24 while in the second stage, crystal growth fits the parabola curve, in which the growth mode of OA and OR occurs simultaneously. Further analysis reveals that with the concentration of NaOH increased, the growth rate in the pure OA-based growth stage increases. At the end of the first stage, the maximum size of ZnS nanoparticles approaches 5.2, 5.9, 7.2, and 8.5 nm, respectively, according to NaOH concentrations of 2, 3, 4, and 8 M. Figure 3 shows typical HRTEM images of ZnS nanoparticles in the first stage. As illustrated in typical images, the grown crystals display particular characteristics of the OA mechanism, such as small particles as “building blocks” to attach with each other via a common crystallographic orientation, the produced particles having irregular shapes and abrupt edges, and microstructural defects, e.g., twins, stacking faults, and misorientations, incorporating into the products. These observations provide supportive evidence that in the first stage, the crystal growth was controlled by the OA mechanism. When the crystal growth enters into the second stage, it is mainly controlled by the OR mechanism. As proposed before,15 though OR and OA coexist in the second stage, the actual contribution of OA in the second stage should be very small and can be neglected in the fitting. Figure 4 shows typical HRTEM images of the ZnS nanoparticles in this stage. It reveals that crystal morphologies are changed and most of the particles have round shapes and smooth edges, after being hydrothermally treated in 8 M NaOH at 100 °C for 350 h.
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Figure 4. Typical HRTEM images of ZnS nanoparticles hydrothermally treated in 8 M NaOH at 100 °C for 350 h. Most of the particles coarsening by the OR mechanism have a round shape with smooth edges.
TABLE 1: Values Estimated by Fitting the Experimental Data in NaOH at 100 °C for the first stage
Figure 3. Typical HRTEM images of ZnS samples hydrothermally treated in NaOH solution of four concentrations at 100 °C (a, b: in 2 M NaOH for 265 h; c, d: in 3 M NaOH for 250 h; e, f: in 4 M NaOH for 200 h; g, h: in 8 M NaOH for 200 h). Larger crystals are constructed by smaller attached nanoparticles. At the right of each HRTEM image, schematic outlines illustrate one possible attachment scheme when smaller particles share a common crystallographic orientation and the defects forming during the OA growth, such as twins (T), stacking faults (SF), and misorientations (M). Scale bars: 5 nm.
The Fitting of Two-Stage Growth Kinetics. The above analysis reveals that the growth of ZnS nanoparticles in NaOH from 2 to 8 M can be described by similar characteristics. Thus the growth in the first stage can be well fitted by the multistep OA kinetic model developed in previous work.15 The OA behavior of nanoparticles shares some characteristics with the collision reactions of molecules and can be described as follows: Kij
Ai + Aj 98 Ak
(i, j, k ) 1, 2, ...)
Ak is the particle that contains primary particles of number k, and k ) i + j. Kij is the rate constant for the reaction between the particles. So the time evolution of the concentration of Ak is the sum of the formation and loss of the particles. The hypotheses in the growth are (a) the reaction is an irreversible, random, and binary one between Ai and Aj and (b) spatial fluctuations in particle density and particle shape are neglected. The rate matrix Kij is given as15
Kij ) 4πR1D1(i1/3 + j1/3)(iR + jR)
(1)
Here R1 is the radius of the primary particle (1.2 nm for ZnS). D1 is the diffusion coefficient of the primary particle, and R is a constant. According to the definition of the volume-weighted average particle size,25 the average particle size at this certain moment, deq, can be obtained by:
deq )
∑ Nkdk4/∑ Nkdk3
(2)
CNaOH (M)
R1 (nm)
K1 (D1‚N1(0))
2 3 4 8
1.2 1.2 1.2 1.2
4 × 103 5 × 103 7.5 × 103 1 × 104
for the second stage R
t′0 (h)
d′0 (nm)
K2
n
-2.5 -2 -1.5 -1.25
317 300 270 270
5.2 5.9 7.2 8.5
0.06271 0.8596 0.1076 0.1732
2.241 2.282 2.216 2.082
where dk is the size of particle containing k primary particles. Thus, the OA growth curve of size vs time can be obtained by the numerical calculation. The second stage of coarsening kinetics was fitted by the Ostwald ripening equation:
deqn - d0n ) K2(t - t0)
(3)
where deq is the mean particle size at time t, while d0 is the particle size at the starting time t0 in the second stage. K2 is a temperature-dependent material constant, and n is an exponent relevant to the coarsening mechanism. The fitted result shows that n ≈ 2 is the best one, which means that the coarsening kinetics in the second stage are mainly controlled by precipitation/dissolution reactions at the particle/matrix interface.7-9 The fitting growth curves of the two stages are shown in Figure 2. This reveals that the increases of the particle size with time revolution are fitted well with those by experiments. Table 1 shows the fitting parameters. Noting that as proposed before,15 though the crystal growth data of the second stage can be simply fitted by OR theory, OA and OR still coexist in this stage. Discussion 1. The Contribution of Strong Surface Adsorption to the Pure OA Mechanism. As discussed in our previous reports,15 in the first stage, the strong wrapping of NaOH on the nanoparticle surfaces led to an unsaturated condition of the solution, thus prohibiting the OR process thermodynamically. In this work, we found 2 M NaOH is strong enough to lead to pure and multistep OA growth characteristics. On the contrary, in the control experiment, the growth kinetics of ZnS nanoparticles in aqueous solution only behave as the mixture of OA + OR (Figure 5). These results confirmed that the pure and multistep OA growth characteristics can be directly attributed to the strong surface adsorption. This conclusion is also supported by other several experimental results. For example, when the nanoparticles are capped with easily destroyed ligands,
Oriented Attachment Growth Kinetics of ZnS
J. Phys. Chem. B, Vol. 111, No. 19, 2007 5293 ζ potential can be expressed as the following according to the electrostatics:
ζ)
Figure 5. Experimental data and fitting results showing the mean size vs time in H2O at 100 °C. There is no knee point in the growth curve. The growth can be fitted by the mixed growth kinetics model (OA + OR) developed by Huang et al.21
σ r
(4)
where σ is the surface charge density, is the dielectric constant, and r is the radius of particles. According to eq 4 and Figure 6, the surface charge density increases with increasing NaOH concentration. As a result, in NaOH of higher concentration, the diffusion of ZnS nanoparticles becomes easier. This study also reveals that the value of R increases as the NaOH concentration increases. According to the relationship between the diffusion coefficient of the particle containing i primary particles, Di, and that of primary particles, D1,26 we obtain:
Di ) D1‚iR
(5)
Here R means the decrease degree of the diffusion rate of the multilevel particle relevant to the primary one; for this system, R is negative. With R increasing, the decrease degree of diffusion rate slows down. Thus the fact that the R increases as the concentration of NaOH increases, indicating that the diffusion rate of the larger particles in high NaOH concentration is faster than that in low concentration solution, supports the above analysis of the electric double layers to some certainty. Figure 6. The plot of the ζ potential as a function of NaOH concentration.
such as thiols, the coarsening of PbS or ZnS nanoparticles usually holds a very short OA dominating stage. This process is terminated when the surface capping ligand is totally desorbed.23 2. The NaOH Concentration Effect. As shown in Figure 2 and Table 1, when the concentration of NaOH increases from 2 to 8 M, the time required for arriving at the second growth stage is almost the same, around 270-320 h. This means that the time needed for the ZnS to achieve dissolution-precipitation equilibrium is almost the same, no matter the solution concentration of NaOH. Furthermore, we speculate that the surface adsorption situations of ZnS nanoparticles are almost the same in NaOH solution from 2 to 8 M. Theoretically, when the concentration of NaOH increases, the viscosity (η) of the liquid phase increases, thus the diffusion rate (D) of nanoparticles should decrease, according to the Stoke-Einstein equation, D ∝ constant/η. Interestingly, we found the OA growth rate K increases as the NaOH concentration increases. According to eq 1, observation reveals that the diffusion rate (D) of nanoparticles increases with the NaOH concentration goes up. From the microscopic view, we hypothesize the apparent inconsistency may relate to the electric double layers surrounding the nanoparticles. It is possible that electric double layers are built on the surface of nanoparticles via the strong Na+ and OH- adsorption, thus the electric attraction and repulsion interaction between surface-capped nanoparticles and surrounding ions facilitates the suspension of nanoparticles in the solution. Therefore the higher the NaOH concentration, the higher the suspension degree of the nanoparticles. The migration of these suspending nanoparticles can be facilitated. Such a hypothesis can be verified by measuring the zeta potential (ζ) of as-synthetic raw ZnS (2.4 nm) as a function of NaOH concentration at room temperature. As shown in Figure 6, the ζ potential (negative) increases as the NaOH concentration goes up.
Conclusions The concentration effect of NaOH on the growth characteristics of ZnS nanoparticles was investigated. It reveals that 2 M NaOH is concentrated enough to lead to pure and multistep OA growth characteristics. We suggest that strong surface adsorption is a key point for achieving pure OA growth characteristics, while the concentration of solute is critical to increase the OA growth rate and achieve exclusive OA growth of nanoparticles at a large size scale. Acknowledgment. We thank Feng Bao at Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences for helping with the TEM. Financial support for this study was provided by the Foundation for Overseas Scholar Fellowship and the Special Project on Science and Technology of Fujian Province (2005YZ1026). F. Huang acknowledges the financial support of the Outstanding Youth Fund (50625205), One Hundred Talent Program in Chinese Academy of Sciences, the National Natural Science Foundation of China (Grant 20501021). References and Notes (1) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (2) Kim, S.; Fisher, B. R.; Eisler, H.-J.; Bawendi, M. G. J. Am. Chem. Soc. 2003, 125, 11466. (3) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (4) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798. (5) Huang, F.; Banfield, J. F. J. Am. Chem. Soc. 2005, 127, 4523. (6) Nanoparticles and the Environment; Banfield, J. F., Navrotsky, A., Eds.; Reviews in Mineralogy & Geochemistry, Vol. 44; Geochemical Society and Mineralogical Society of America: Washington, DC, 2001. (7) Wagner, C. Z. Elektrochem. 1961, 65, 581. (8) Speight, M. V. Acta Metall. 1968, 16, 133. (9) Kirchner, H. O. K. Metall. Trans. 1971, 2, 2861. (10) Campbell, C. T.; Parker, S. C.; Starr, D. E. Science 2002, 298, 811. (11) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (12) Penn, R. L.; Banfield, J. F. Am. Mineral. 1998, 83, 1077. (13) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343. (14) Huang, F.; Zhang, H.; Banfield, J. F. Nano Lett. 2003, 3, 373.
5294 J. Phys. Chem. B, Vol. 111, No. 19, 2007 (15) Zhang, J.; Lin, Z.; Lan, Y.; Ren, G.; Chen, D.; Huang, F.; Hong, M. J. Am. Chem. Soc. 2006, 128, 12981. (16) Korgel, B. A.; Fitzmaurice, D. AdV. Mater. 1998, 10, 661. (17) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (18) Tang, Z. Y.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (19) Sampanthar, T.; Zeng, H. C. J. Am. Chem. Soc. 2002, 124, 6668. (20) Lou, X. W.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 2697. (21) Huang, F.; Zhang, H.; Banfield, J. F. J. Phys. Chem. B 2003, 107, 10470.
Wang et al. (22) Zhang, H.; Banfield, J. F. Am. Mineral. 1999, 84, 528. (23) Zhang, J.; Wang, Y.; Zheng, J.; Huang, F.; Chen, D.; Lan, Y.; Ren, G.; Lin, Z.; Wang, C. J. Phys. Chem. B 2007, 111, 1449. (24) Ribeiro, C.; Lee, E. J. H.; Longo, E.; Leite, E. R. Chem. Phys. Chem. 2005, 6, 690. (25) Kaelble, E. Handbook of X-Rays; McGraw-Hill: New York, 1967; pp 17-18. (26) Meakin, P. AdV. Colloid Interface Sci. 1988, 28, 249.
