Observation of Rotated-Oriented Attachment during the Growth of

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Observation of Rotated-Oriented Attachment during the Growth of Ag2S Nanorods under Mediation of Protein Lin Yang,*,† Huayan Yang,† Zongxian Yang,‡ Yanxia Cao,§ Xiaoming Ma,† Zhansheng Lu,‡ and Zhi Zheng| College of Chemistry and EnVironmental Science and Co-constructing Key Laboratory for Cell Differentiation Regulation, Henan Normal UniVersity, Xinxiang 453007, People’s Republic of China, College of Physics & Information Engineering, Henan Normal UniVersity, Xinxiang 453007, People’s Republic of China, College of Materials Science and Engineering, Zhengzhou UniVersity, Zhengzhou 450001, People’s Republic of China, and Institute of Precision Engineering & Department of Physics, The Chinese UniVersity of Hong Kong (CUHK), Shatin, New Territories, Hong Kong ReceiVed: February 27, 2008; ReVised Manuscript ReceiVed: May 12, 2008

In this study, protein-conjugated Ag2S nanorods were prepared in aqueous solution, and high-resolution transmission electron microscopy (HRTEM) was used to track the whole process of the nanorod growth. Our results shew that the final products were formed via two-step oriented attachment, that is, particle-particle and rod-rod oriented. More interestingly, before oriented attachment, the nanoparticles or nanorods attached without sharing the same lattice plane; they could then rotate to a perfect array and fuse together by eliminating the two high energy surfaces. On the basis of the calculation of surface energy, two-step attachments and rotations were brought forward, and the role of protein in the forming process of nanorods was discussed. Introduction Their one-dimensional (1D) nature endows nanorods and nanowires unique electrical, optoelectronic, and mechanical properties with fundamental significance and practical ramifications.1 Thus, one-dimensional nanomaterials have attracted tremendous attention from researchers.2 On the one hand, various preparation methods for nanorods and nanowires have been developed, such as template method,3 gas-solid reaction method,4 and spontaneous self-organization.5 On the other hand, there has been a considerable effort to understand how nucleation, growth, coarsening, and aggregation processes affect these characteristics. Thus, a clear understanding of this relationship will be necessary to realize the preparation of highly controlled nanostructures.6 Important progress has been made in the forming mechanism of the nanocrystal growth. This mechanism, so-called oriented attachment, was first proposed by Penn and Banfield, which described spontaneous selfassembly of adjacent titania nanoparticles to share a common crystallographic orientation under hydrothermal conditions to form 1D nanostructures.7 The oriented attachment mechanism has already been experimentally observed in various systems for several years and considered significant in nanomaterials growth.8 For example, R-FeOOH9 nanorods, ZnO10 nanorods, and CuO11 microspheres were formed free of additives via oriented attachment. It was also found largely in the systems with additives, mainly organic molecules, which could functionalize surfaces of nanoparticles. Thus, the additives become an advantaged tool to oriented attachment of primary nanoparticles.12 Yet the effects of * To whom correspondence should be addressed. Phone: +86-3733328117. Fax: +86-373-3328507. E-mail: [email protected]. † College of Chemistry and Environmental Science and Co-constructing Key Laboratory for Cell Differentiation Regulation, Henan Normal University. ‡ College of Physics & Information Engineering, Henan Normal University. § Zhengzhou University. | The Chinese University of Hong Kong.

additives on the process of nanocrystal formation are still unclear. In the above-mentioned systems, forming processes of the 1D nanomaterials all involved the attachment of multiple nanoparticles as building blocks in pearl-chain-like structures, in which the adjacent nanoparticles sharing a same crystallographic orientation fused with each other. Bottlenecks between adjacent particles are presumably filled by the Ostwald Ripening mechanism volume diffusion.13 The process of the oriented attachment is particularly relevant in the nanocrystalline regime, where bonding between the particles reduces overall energy by removing surface energy associated with unsatisfied bonds. In the oriented attachment, the crystalline lattice planes may be almost perfectly aligned at the contact areas between the adjacent particles, which was called “perfect oriented attachment”.7 The orientation of preformed quasi-spherical ZnO nanoparticles to single-crystalline nanorods may be a good example.14 Yet in the growth process of TiO2, when nanocrystals grow by oriented attachment at crystallographically special surfaces and there is a small misorientation at the interface, dislocations result. It is called “imperfect oriented attachment”.7,15 Recently, the oriented attachment mechanism was modeled by Moldovan et al.16 and investigated by molecular dynamics studies.17 In all of the theoretical works, the authors assumed that the oriented attachment occurred by means of relative rotations between the particles until a thermodynamically favorable interface configuration (i.e., crystallographic alignment) was reached. The rotations played an important role in the oriented attachment, which have attracted much attention from researchers. In addition, Banfields’ group speculated that the rotations might happen in the process of natural biomineralization of iron oxidizing bacteria. Also, they thought that rotations of particles within aggregates might be driven by Brownian motion or by short-range interactions between adjacent surfaces.18 The rotations of nanoparticles in the aggregates/mesocrystals were also described in vitro. Averback et al. showed that Ag nanoparticles initially randomly oriented,

10.1021/jp8017056 CCC: $40.75  2008 American Chemical Society Published on Web 07/23/2008

9796 J. Phys. Chem. B, Vol. 112, No. 32, 2008 and then the particles could rotate under the effect of Cu substrate, resulting in full alignment.19 Chen’s work20 showed that the mesoporous-like structures could be formed through the rotations of the primary nanocrystals capped with long carbon chain alcohols. The rotations of crystallites were also detected in Ollivier’s work,21 which reported the use of the freezing/sublimation technique to probe the detailed growth process of CoC2O4 · 2H2O core-shell structures. According to the micromechanical model developed by Hounslow and coworkers, particles colliding in lower ionic strength solutions may realign themselves into a more favorable energy state before being fixed into place.22 In the present study, protein-conjugated Ag2S nanorods were synthesized in aqueous solution, and two-step attachments and rotations were discovered during formation of the single-crystal nanorods. The difference from the reported mechanism of formation of 1D nanomaterials is two-step oriented attachment with rotations. In other words, instead of sharing the same crystallographic orientation at the beginning of array, the adjacent nanoparticles rotated by two steps to achieve perfect oriented attachment in our system. Thus, we named it “twostep rotated-oriented attachment”. To explore the reason of the rotated-oriented attachment, theory calculations were carried out. The effects of protein on it are also discussed in this Article. These results could benefit the general understanding of crystal growth mechanisms and the preparation of highly controlled nanostructures. Experimental Section Materials. Silver nitrate (g99.8%, Mw ) 169.87 A.R.) and thioacetamide (TAA) (g99.0%, Mw ) 75.13 A.R.) were purchased from Tianjin Chemical Reagent Factory. The BSA (purity g 98%, Mw ) 68 000) was purchased from Xiamen Sanland Chemicals Co. Ltd., China. The activity of the BSA is more than 10 000 units/mg. Fabrication of the Ag2S Nanorods. Nanocrystalline Ag2S was synthesized according to our previous study.5 Briefly, 50 mL of 50 mM silver nitrate aqueous solution and 100 mL of 1 mg/mL BSA aqueous solution were mixed with vigorous stirring for a homogeneous reaction at room temperature. The mixed solution of the BSA-Ag+ emulsion was kept static under nitrogen protection for 6 h. Next, 50 mL of 50 mM TAA aqueous solution was added. Immediately after TAA addition, the solution changed to black. The mixed reaction solution was again kept static under ambient conditions for 72 h. The collected black solid state product was washed with double distilled water and ethanol five times, and then dried in a vacuum at room temperature for 24 h to eliminate the absorbed protein. During the process of nanorods growth, a set of time-tracking experiments was performed for the master solutions. Four replicas of the same experiment were run in parallel. Each replica was terminated at different times (6, 14, 48, and 72 h). To investigate the influence of BSA on the formation of Ag2S nanorods, a control experiment was carried out, silver sulfide was prepared in the aqueous solution without BSA, and other conditions and procedures were the same as in a typical experiment. Characterizations. The final products were separated by centrifugation, washed by ethanol and water three times, and then used for the following characterization analysis. Transmition electron microscopy (TEM) and HRTEM analysis were performed using a FEI Tecnai G220 transmission electron microscope. To prepare the TEM samples, a 5 µL droplet of dilute alcohol solution was dripped onto a holey carbon-coated

Yang et al. Formvar support. TEM equipped with an X-ray energy dispersive spectroscopy (EDS) and accompanied by selected-area electron diffraction (SAED) was used to confirm the particles size and determine the form of attachment. X-ray powder diffraction (XRD) measurements were performed on a Bruker D&Advance X-ray powder diffractometer with graphite monochromatized Cu/KR (γ ) 0.15406 nm). The thermogravimetrydifferential thermal analyses (TG-DTA) were performed on an EXSTAR TG/DTA6300 instrument. The FT-IR spectra were recorded on a Bio-Rad FTS-40 Fourier transform infrared spectrograph in the wavenumber range of 4000-400 cm-1. The spectra were collected at 2 cm-1 resolution with 128 scans by preparing KBr pellets with a 3:100 “sample-to-KBr” ratio. Calculational Method for the Free Energy of Crystal Surfaces. The systematic studies on the energetics and surface relaxation were based on the first-principles density functional theory (DFT). The calculations were performed using the frozencore all-electron projector-augmented-wave (PAW) method,23 as implemented in the ab initio total-energy and molecular dynamics program VASP (Vienna ab initio simulation program).24 The calculational parameters were set according to Yangs’ report,25 but the Kohn-Sham orbitals were expanded in plane waves with a kinetic energy cutoff of 350 eV in the current calculation. The calculated lattice parameters are a ) 4.300 Å, b ) 7.049 Å, c ) 7.997 Å, β ) 99.61°, respectively, which are in agreement with P21/n (14) space group (a ) 4.229 Å, b ) 6.931 Å, c ) 7.863 Å, β ) 99.61°).26 Based on the relaxed bulk Ag2S cell, the three Ag2S surfaces, that is, (111), (-101), and (-121), were molded as slab mode with a vacuum layer of 10 Å to separate the films. As a measure of surface stability, the surface energies were evaluated by:

Esurf )

1 (E - Ebulk) 2A slab

(1)

where Eslab is the total energy of the slab, Ebulk is the energy of the bulk unit cell containing the same number of atoms as in the slab, and A is the surface area. The lower is the surface energy, the more stable is the surface.27 Results and Discussion Time-Tracking of the Forming Process of Ag2S Nanorods. The synthesis of silver sulfide nanorods was performed by a two-step procedure. The first step was the generation of the silver(I)-BSA complex by mixing of the AgNO3 and BSA solutions. The second step was the formation of Ag2S nanorods by adding TAA into the above mixing solution at ambient temperature. TAA was comparatively unstable and slowly hydrolyzed to release sulfide ions into the reaction solution. The slow release of S2- was essential for the formation of the nanorods. To understand the nanocrystals growth mechanism, timedependent TEM studies were performed at 6, 14, 48, and 72 h, respectively. Figure 1a is the TEM image of the Ag2S nanoparticles aged for 6 h in the BSA solution. The spheral colloid Ag2S nanoparticles were well-dispersed. The corresponding crystallography was examined by SAED (Figure 1b). The appearance of diffuse rings in any area supported the amorphous nature of the particles. The result was in agreement with the attitude brought forward recently, which pointed out that amorphous phase was a precursor phase and potential intermediate for crystalline biomaterials in biomineralization.28 The EDS spectrum (Figure 1c) of the Ag2S nanorods showed the significant presence of Ag and S with an atomic ratio (Ag/S)

Rotated-Oriented Attachment in the Growth of Ag2S Nanorods

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Figure 1. (a) Representative TEM image of the Ag2S particles grown in BSA aged for 6 h. (b) SAED of the Ag2S particles. (c) EDS of Ag2S nanoparticles.

Figure 2. (a) TEM image of quasi-spherical Ag2S crystals grown in BSA aged for 14 h. (b) HRTEM image of a single primary crystallite.

of nearly 2, in good agreement with the stoichiometric molar ratio of silver sulfide. Figure 2a shows TEM images after aging for 14 h. The monodispersed particles with a good uniformity in size (approximately 25 nm) were conjugated by the protein (shown by arrows). Figure 2b is the HRTEM image of an individual nanoparticle. Facetted crystalline surfaces were already present, representing the transition from amorphous Ag2S to crystalline Ag2S phase. Therefore, we could presume that the nanometersized amorphous precursors slowly nucleated to form the crystalline phase. Figure 3 displays the typical morphologies of the products after aging for 24 h. From these TEM images, we could observe a series of changes during the growth of nanorods, which could result from the different growth velocity. Figure 3a showed that peanut-like, pearl-necklace-like assemblies and shorter nanorods were generated. The assemblies were investigated further by HRTEM. As shown in Figure 3b and c, the adjacent nanoparticles attached without sharing a same crystallographic orientation, and the angles were 75° and 67°, respectively. Figure 3d records representative HRTEM images of another peanut-like nanostructure. The connected particles partly shared a common crystallographic orientation (shown by the arrows), which illuminated that the connected particles were rotating to find the common crystallographic orientation. After the rotations of adjacent particles completed, they fused to form almost a perfect nanorod (Figure 3e).

We thought that the rotations played an important role in the growth process of Ag2S nanorods. The lattice spacings of 0.298 nm corresponded to that of the (111) plane of the Lt-Ag2S phase as shown in Figure 3e; it was suggested that the direction of crystal growth was (-121) direction, which was perpendicular to the (111) plane. Bottlenecks between the adjacent particles were still visible in Figure 3e. The correlative fast Fourier transform (FFT) (Figure 3f) further confirmed the oriented attachment along the (-121) direction. As described above, the growth from individual nanoparticles to shorter nanorods underwent the following procedures: attaching, rotating to array perfectly, and then fusing to single crystals. Figure 4 shows TEM images of Ag2S nanostructures after aging for 48 h. The “zigzag” structures (seen in the rectangle) and nanorods coexisted in Figure 4a. It could be another stage of the growth of Ag2S nanorods. A close look to the connections of the “zigzag” structure (Figure 4b and c) showed that angles between the two planes of adjacent nanoparticles were 120° and 13°, revealing that the connected plane did not share the same crystallographic orientation. The “zigzag” structures were intermediate nanorods species. Figure 4d showed the lattice planes of the particles were almost perfectly aligned. It might be the further growth stage of the nanorods as compared to that in Figure 4b and 4c, indicating that the adjacent nanoparticles might rotate to find the same oriented direction. When the

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Figure 3. TEM images showing oriented attachment of Ag2S nanocrystals grown in BSA solution for 24 h. (a) Low-magnification TEM image of sample. (b and c) HRTEM images of two or four primary crystallites forming “peanut” or “chain” via oriented attachment. (d) HRTEM images showing the progression of the “rotation” in two particles with magnified attachment interfaces. (e) HRTEM image and (f) FFT showing a single nanorod after being fused together.

nanostructure accomplished rotations and fused together, the lattice planes of them would go straight through the contact areas. Figure 5a is a representative TEM image of the as-prepared Ag2S nanorods in the BSA aqueous solution after aging for 72 h. Each nanorod had a uniform diameter along its entire length, which indicated that the growth anisotropy in the C axis was strictly maintained and the rotated-oriented attachment was

brought to an end. These nanorods were obviously well dispersed with 30 nm in diameter and 90-240 nm in lengths. Furthermore, the corresponding selected area electron diffraction (SAED) pattern (Figure 5b) revealed that the nanorods were crystalline and could be indexed to amonoclinic Lt-Ag2S. The high-magnification TEM image (Figure 5c) showed the nanorods were enwrapped by a shell (shown by arrows), which could be confirmed to be the protein by FT-IR and TG-DTA. Further-

Rotated-Oriented Attachment in the Growth of Ag2S Nanorods

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Figure 4. (a) Overview of TEM image of Ag2S samples in BSA solution for 48 h. (b and c) HRTEM images of two nanorods with magnified attachment interfaces. (d) HRTEM image of a single nanocrystal after two shorter nanorods were fused.

more, the HRTEM image (Figure 5d) indicated that the interplanar distance along the growth axis was 0.268 nm, which was consistent with the interplanar distance of the (-121) plane of the Lt-Ag2S. It confirmed that the preferred growth direction was (-121). Figure 5e shows powder X-ray diffraction (XRD) patterns. All diffraction peaks could be well-indicated to LtAg2S (JCPDS Card File 14-0072), which further confirmed the SAED and HRTEM results. We also found that some Ag2S nanoparticles and nanorods adhesively contacted with nanodots from the TEM image, which displayed a particular characteristic of the Ostwald ripening mechanism. Ostwald ripening volume diffusion smoothed the irregular surface to produce nanorods with smooth surface. As a result, more nanorods with large diameters were generated. As mentioned above, FT-IR and TG-DTA were carried out to study the conjugation of Ag2S nanocrystals with protein. The IR peaks of pure BSA at 3308, 3068, 1656, and 1539 cm-1 were assigned to the stretching vibration of -OH, amide A′ (mainly -NH stretching vibration), amide I, and amide II bands, respectively. Comparing the IR spectrum of BSA-Ag2S products with that of pure BSA, the characteristic peaks of -OH groups and amide A′ bands shifted to a high wavenumber of about 100 and 80 cm-1, respectively. The results showed that there might be conjugate bonds between the Ag2S nanorod surfaces and -OH and -NH groups in BSA.29 To understand the role of protein, the IR spectrum of BSA-Ag+ was also mensurated. Comparing the IR spectrum of BSA-Ag+ with that

of pure BSA, there were negligible variations in the characteristic peaks of -OH groups, amide I, and amide II bands, but the characteristic peak of amide A′ bands shifted to a high wavenumber of about 20 cm-1, which suggested that there might be coordination interaction between silver ions and -NH groups of BSA. TG and DTA curves of pure BSA and the as-prepared nanorods both showed there were two stages of weight loss. From the TG curve of the nanorods, it was observed that the total weight loss was about 11% from 200 to 650 °C. As compared to that of pure BSA, they had similar weight loss curves. From the DTA curve of the nanorods, there was a wide endothermic peak from 50 to 650 °C, due to the transformation from R-Ag2S to Lt-Ag2S at 178 °C and the breakup and combustion of BSA molecules. According to the high degree of crystallinity of the nanorod (Figure 5) and the FT-IR and TG-DTA results, it could be presumed that the protein (11%) was conjugated on the surface of the nanorods. By setting a control experiment, silver sulfide was prepared in the aqueous solution without BSA, in which the Ag2S crystals were randomly aggregated with elliptical particles with different size. The results showed that the presence of BSA was a key factor in controlling and regulating the aggregation of the Ag2S nanocrystals. Theory Model of Two-Step Rotated-Oriented Attachment. To understand the reasons for the rotated-oriented attachment, we performed the calculations of surface energy to check the

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Figure 5. (a) Typical TEM image of Ag2S nanorods aged in the BSA solution for 72 h. (b) SAED of some nanorods showing a single crystal nature. (c) Magnification image of a single Ag2S nanorod. (d) HRTEM image of the Ag2S nanorod. (e) X-ray diffraction pattern of the products.

surface stabilities of the Ag2S nanocrystals. According to the HRTEM results, three vicinal crystal faces, (111), (-101), and (-121), were selected for the calculations. The following calculations could be made on the basis of the current first principle and yielded some important results. The (-121) plane had the lowest surface energy among the considered surfaces, 0.122 J/m2. The (-101) surface came next with a surface energy of 0.220 J/m2. The (111) surface had the highest surface energy, 0.268 J/m2. In brief, the calculational results showed that the relative stability decreased in the order (-121) < (-101) < (111), suggesting that the (-121) plane was the most thermodynamically favorable surface among the considered surfaces. However, it should be admitted that the differences of surface energy were not big enough to form one-dimensional structures in the absence of protein. Therefore, the protein might play an important role in the formation of nanorods in the rotatedoriented attachment. The role could be presumed as follows. First, the chelating reaction between the protein and the Ag+ was very crucial to the formation of nanorods. Protein bound Ag+ through functional groups (such as -H). So the formation of Ag+-protein complex could slow down the nucleation speed when the S2- was added. After the hybrid amorphous Ag2S/ protein was formed, it further transformed to protein-conjugated Ag2S crystals. Second, the protein might be selectively adsorbed on the (-121) facet of Ag2S nanocrystals and minimized its surface energy. Similar phenomena of selectively adsorption could be observed in many other organic moleculars, which

could decrease the surface energy of certain faces by facet selective adsorption.12a,30 In the present study, the efficiency of attachment and fusion could be enhanced by eliminating the (111) plane; thus the building blocks rotated to find the (111) planes and attached directly in both steps. These results demonstrated that the rotated-oriented attachment would occur if the total surface energy of the fused building blocks was favorable for it. The oriented attachment system won a substantial amount of energy by eliminating two high energy surfaces to reach the thermodynamically favorable interface configuration. So, the enhancement of surface energy difference could enhance the tendency of rotation and fusion of adjcent nanoparticles. This should be the underlying reason for rotatedoriented attachment. A similar report could be found in Hyeon’s work,12a in which the enhancement of surface energy differences by facet selective amine ligand adsorption would make the oriented attachment of ZnS nanocrystals in one direction. Finally, as in Chen’s work,20 protein, like a lubricant, made the rotations of adjacent particles more free, and the perfect attachment could be accomplished easily. Furthermore, when the eventual products were formed, the conjugated protein would stabilize the nanostructures. In summary, according to the above statement, the formation of protein-conjugated Ag2S nanorods could be illustrated in Scheme 1. The nanorods grew via two-step rotated-oriented attachment. First, protein bound to free Ag+ strongly with the functional groups. Also, with increasing sulfate ions, the

Rotated-Oriented Attachment in the Growth of Ag2S Nanorods SCHEME 1: Proposed Formation Mechanism of the Oriented Attachment of the Nanoparticles into Nanorods

amorphous Ag2S was formed. Second, the amorphous Ag2S slowly crystallized into nanoparticles. Third, with increased aging time, the nanoparticles attached without sharing the same lattice plane, and then they could rotate to perfectly arrayed and fuse together by eliminating the two high energy surfaces. Finally, the two building blocks fused to form a single crystal. At the same time, Ostwald ripening volume diffusion smoothed the necklaces. Similar phenomena occurred in rod-rod rotatedoriented attachment. Eventually, the two short nanorods fused together. Conclusion Hybrid protein-conjugated Ag2S nanorods were formed via two-step rotated-oriented attachment in the presence of BSA. The main reason for the rotations and attachment was the selective adsorption of protein, which could decrease the surface energy of the crystals. The results showed that 1D nanostuctures could be formed via rotated-oriented attachment under the control of some additives. Therefore, the whole system obtained thermodynamic stability. These results were important to understand the growth mechanism of 1D nanomaterials. Acknowledgment. This study was supported by the National Basic Research Program of China (Grant No. 2005CB724306), the National Science Foundation of China (Grant No. 20771036), and the National Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20070476001). References and Notes (1) Wen, X. G.; Wang, S. H.; Xie, Y. T.; Li, X. Y.; Yang, S. H. J. Phys. Chem. B 2005, 109, 10100. (2) (a) Schmid, G. Nanoparticles: From Theory to Application; WileyVCH: Weinheim, Germany, 2004. (b) Pramod, P.; Shibu Joseph, S. T.; Thomas, K. G. J. Am. Chem. Soc. 2007, 129, 6712. (c) Klimov, V. I.; Mihkailovsky, A.; Xu, S.; Malko, A.; Hollingsworth, J.; Leatherdale, C.; Bawendi, M. G. Science 2000, 290, 314. (d) Dumestre, F.; Chaudret, B.; Amiens, C.; Renaud, P.; Fejes, P. Science 2004, 303, 821. (3) (a) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (b) Gates, B.; Wu, Y. Y.; Yin, Y. D.; Yang, P. D.; Xia, Y. N. J. Am. Chem. Soc. 2001, 123, 11500. (c) Sun, Y. G.; Xia, Y. N. AdV. Mater. 2002, 14, 833. (d) Goldberger, J.; He, R. R.; Zhang, Y. F.; Lee, S. K.; Yan, H. Q.;

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