In Situ Modification of Three-Dimensional Polyphenylene Dendrimer

Jul 27, 2010 - Nanjing University of Posts and Telecommunications. , ⊥. Katholieke .... P. E. Barry. Chemical Communications 2016 52 (20), 3895-3898...
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J. Phys. Chem. C 2010, 114, 13465–13470

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In Situ Modification of Three-Dimensional Polyphenylene Dendrimer-Templated CuO Rice-Shaped Architectures with Electron Beam Irradiation Xiaoying Qi,† Yizhong Huang,*,† Markus Klapper,§ Freddy Boey,†,‡ Wei Huang,| Steven De Feyter,⊥ Klaus Mu¨llen,§ and Hua Zhang*,†,‡ School of Materials Science and Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore 639798, Singapore, Centre for Biomimetic Sensor Science, Nanyang Technological UniVersity, 50 Nanyang DriVe, Singapore 637553, Singapore, Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany, Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of AdVanced Materials (IAM), Nanjing UniVersity of Posts and Telecommunications, Nanjing 210046, China, and DiVision of Molecular and Nanomaterials, Department of Chemistry and INPAC, Institute of Nanoscale Physics and Chemistry, Katholieke UniVersiteit LeuVen (KULeuVen), Celestijnenlaan 200F, 3001 HeVerlee, Belgium ReceiVed: April 4, 2010; ReVised Manuscript ReceiVed: July 8, 2010

In this study, the high-energy electron beam of the transmission electron microscope (TEM) is utilized as an external force to in situ modify the polyphenylene dendrimer (G2Td(COOH)16) templated CuO rice-shaped architecture (RSA). By virtue of the nanoscale precision of this approach, the electron beam-modified RSA retains its rice shape while the internal primary CuO nanoparticles are converted to the Cu2O nanoparticles with increased size. Detailed investigation using a time-lapse TEM technique reveals that such a modification process is mainly constituted by two stages, involving the arrangement of the primary CuO nanoparticles and the transformation of the primary CuO into Cu2O nanoparticles. Within the modification process, the highenergy electron beam of TEM serves as the external driving force and energy resource to improve the orientation and increase the crystallinity of the single-phase CuO nanoparticles and subsequently transfer the nanoparticle phase from CuO to Cu2O. This study highlights a facile in situ way to finely tune the nanoscale morphology and chemical composition of nanoparticles and nanoparticle-based assembled structures. 1. Introduction Nanostructured materials assembled from small nanoparticles have emerged as one of the key mainstays of modern nanotechnology.1 Recently, the transmission electron microscope (TEM) has been found to be a powerful tool to tailor the assembly and structure of nanoparticles,2 which has the primary advantage of precise control at the nanometer or single nanoparticle level.3 Thus far, TEM has been utilized to induce nanoscale phase/shape transformation,4 modify inorganic nanostructures,5 form particles,6 fabricate nanostructures,7 investigate coalescence of nanoparticles,8 and grind nanomaterials.9 Therefore, the electron beam irradiation of TEM is capable of investigating the morphologic, structural, and chemical transformation of nanoparticles, which is of significance for the development of novel nanomaterials. However, this method has been rarely explored in modification of three-dimensional nanostructures consisting of well-separated and orientated nanoparticles. In addition, the effect of electron beam irradiation on nanoscopic processes, such as oriented attachment, crystallization, and oxidation/reduction of nanoparticles, is still not fully understood.6a,10

Recently, we reported a polyphenylene dendrimer (G2Td(COOH)16) templated self-assembly strategy to construct an ordered architecture of CuO nanoparticles through oriented attachment.11 The resulting hybrid nanomaterial CuOG2Td(COOH)16 possesses a three-dimensional rice-shaped architecture (RSA), within which the dendrimer molecules play an indispensable role in preserving and separating the original primary CuO nanoparticles through Cu2+-COO- coordination bonding. In this contribution, the high-energy electron beam of TEM is used as an external force to in situ modify the CuOG2Td(COOH)16 RSA. The effect of the electron beam on the morphologic, structural, and chemical transformation of the RSA is investigated in detail with a time-lapse TEM technique. Such an electron-beam modification process is found to be comprised of two stages: (1) the electron beam acts as the external force to improve the orientation of CuO primary nanoparticles as well as induce the sintering process to increase the crystallinity of the CuO phase and (2) the phase transformation of primary nanoparticles from CuO to Cu2O, in which CuO is reduced to Cu2O. High-resolution TEM (HRTEM) images and fast Fourier transform (FFT) data are used to analyze this modification process in detail.

* To whom correspondence should be addressed. Phone: +65-6790-5175. Fax: +65-6790-9081. E-mail: [email protected]; [email protected]. † School of Materials Science and Engineering, Nanyang Technological University. § Max-Planck-Institut fu¨r Polymerforschung. ‡ Centre for Biomimetic Sensor Science, Nanyang Technological University. | Nanjing University of Posts and Telecommunications. ⊥ Katholieke Universiteit Leuven.

2. Experimental Section 2.1. Synthesis. The detailed synthesis of CuOG2Td(COOH)16 RSA was demonstrated in our previous report.11 Briefly, the freshly prepared 0.5 mL of NaOH aqueous solution (22 mmol/L) was added into 0.05 mL of a THF solution of G2Td(COOH)16 (0.1 mmol/L) at room temperature with vigor-

10.1021/jp1050468  2010 American Chemical Society Published on Web 07/27/2010

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Figure 1. (A) TEM image of the original CuO-G2Td(COOH)16 RSAs. (Inset) SEM image of CuO-G2Td(COOH)16 RSAs; scale bar ) 200 nm. (B, C, and D) Time-lapse TEM images showing the morphologic evolution of a single CuO-G2Td(COOH)16 RSA under high-energy electron beam irradiation for 0, 35, and 70 min, respectively.

ous stirring. After adjusting the solution pH value to 5-6, the freshly prepared 0.2 mL of NaBH4 aqueous solution (22 mmol/ L) was injected into the solution, followed by addition of 0.01 mL of Cu(NO3)2 solution (20 mmol/L). After 4 h, this solution turned orange yellow, indicating termination of this reaction. The RSAs were collected via centrifugation and washed with Milli-Q water three times. 2.2. Characterization. Nanostructural characterization of the modification process was carried out using the transmission electron microscope (TEM, JEOL JEM-2100F) with an accelerating voltage of 200 kV. The time-lapse method was conducted in the TEM starting with irradiation of RSA at a beam current density of 40.1 pA/cm2 for a short period of time (ca. 10 s). The electron beam was then focused until a current density of 90.8 pA/cm2 was reached. Under illumination of this beam, a series of images was taken every few minutes. TEM samples were prepared by dropping a droplet of the diluted water dispersion of CuO-G2Td(COOH)16 RSA on the surface of a copper grid with the Formvar support, followed by drying under atmosphere. Scanning electron microscopy (SEM) was carried out on a JEOL JSM-6340 field-emission scanning electron microanalyzer at an accelerating voltage of 12 kV. 3. Results and Discussion 3.1. Structural Characterization. Figure 1A shows the morphology of the original CuO-G2Td(COOH)16 RSA, which was obtained from exposing the entire RSA in a weak electron beam with a beam current density of 40.1 pA/cm2 for a very short time (ca. 10 s). The length and width of RSA are 210 ( 40 and 96 ( 30 nm, respectively. A more detailed characterization of the RSA has been reported in our previous report.11 The height of RSA is 78 ( 13 nm, as measured by atomic force microscope (AFM), confirming its three-dimensional nanostructure.11 Moreover, numerous small spherical CuO nanoparticles with a diameter of 6.2 ( 0.4 nm and adjacent particle separation of ∼3 nm within the RSA were measured by TEM and smallangle X-ray scattering (SAXS), respectively.11

Qi et al. 3.2. Identification of the Electron-Beam Modification Process. The effect of electron beam irradiation on the morphology of a single CuO-G2Td(COOH)16 RSA is studied. Figure 1B, 1C, and 1D shows the morphological evolution of a single RSA under electron beam irradiation with a constant beam current density (90.8 pA/cm2) for 0, 35, and 70 min, respectively. The primary nanoparticles were found to be sintered, leading to the recrystallization and internal morphology change of the RSA (Figure 1C). After a long-time electron beam exposure (up to 70 min), all primary CuO nanoparticles disappeared and integrated into large nanoparticles (Figure 1D). More importantly, the RSA retains its rice shape, which could be attributed to the intrinsic tendency for CuO nanoparticles to form the ellipsoidal architecture.11 To gain insight into the modification process, a specific irradiation procedure in combination with a time-lapse study was set up. TEM images were taken from RSA in the region marked as a square in Figure 2A before (t ) 0 min) and after (t ) 70 min) electron beam irradiation, as shown in Figure 2B and 2C, respectively. Their corresponding FFT patterns are shown in Figure 2B′ and 2C′, respectively. Obviously, after sufficient long-time electron beam irradiation (t ) 70 min), the primary CuO nanoparticles coalesce to form a single-crystal structure (Figure 2C) imaged along the [001] axis. The resulting structure is highly crystalline with clearly resolved lattice fringes of the (200) lattice plane (d spacing is measured to be 0.21 nm).12 This structure is quite different from that of the original RSA (t ) 0), which is made of primary CuO nanoparticles with a lattice distance at the (200) plane of 0.23 nm (Figure 2B).11,13 To understand the structural evolution and phase transformation of the modification process mentioned above, one selected area diffraction (SAD) pattern of a single original RSA (Figure 2A′) and two FFT patterns of the square areas in the RSA (Figure 2B′ and 2C′) are analyzed. Figure 2A′, 2B, and 2C′ correspond to the phases of the original RSA (Figure 2A), the square-marked area of the original RSA (Figure 2B, t ) 0 min), and the finally modified RSA (Figure 2C, t ) 70 min), respectively. The indexes are implemented on the basis of the following structural information. The diffraction spots in Figure 2A′ can be assigned to diffraction of CuO material along the [001] zone axis. The slight curvature of those diffraction spots suggests a small misorientation among the primary CuO nanoparticles in the original RSAs.11 The FFT pattern of the original RSA matches well with the standard CuO material diffraction pattern along the [001] zone axis (Figure 2B′), suggesting that CuO is a monoclinic oxide with space group C2/c (15) and has lattice parameters a ) 0.468 nm, b ) 0.342 nm, c ) 0.513 nm, and β ) 99.55°. However, after electron beam irradiation for 70 min, the SAD from the finally modified RSA, as shown in Figure 2C′, apparently arises from Cu2O. It has a cubic structure with space group Pn3m (224) and lattice constant a ) 0.427 nm. Those unmarked spots are generated from double diffractions. These patterns will be used as the standards to monitor the structural evolution processes during in situ electron beam modification. Thus, it is believed that the modification process involves sintering of the primary CuO nanoparticles (stage 1) and structural transformation between CuO and Cu2O (stage 2). 3.3. Changes in Orientation and Crystallinity. Changes in the orientation arrangement and crystallinity and of CuO nanoparticles in RSA (stage 1) under electron beam radiation are analyzed as follows. Prior to the discussion, it is worthwhile to mention that within the original RSA, Cu2+-COO- coordination bonds exist to bridge, disperse, and fix the primary CuO

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Figure 2. (A) TEM image and (A′) SAD pattern of an original CuO-G2Td(COOH)16 RSA. (B) HRTEM image of the white square in the original CuO-G2Td(COOH)16 in image A RSA. (B′) The corresponding FFT of image B with identifiable CuO SAD pattern, designated by the white circles. (C) HRTEM image of image B under electron beam irradiation for 70 min. (C′) The corresponding FFT of image C with identifiable Cu2O SAD pattern, designated by white triangles (note that those extra spots are generated from the double diffraction).

nanoparticles, and thus, without external driving force, the further orientation of the primary nanoparticles is unlikely.11 However, it is assumed that under the strong electron beam irradiation, the organic dendrimer molecules gradually decompose, resulting in the decreased amount of Cu2+-COO- coordination bonds in some crystal planes. As such, the partially exposed primary nanoparticles gain thermodynamic freedom to orient into a more stable form in order to reduce the surface area of the unstable plane. The stability of the plane without ligand protection decreases with increased surface density of copper atoms; in contrast, the stability of the plane with ligand protection is dependent on the ligand coverage and increases with increased surface density of copper atoms.14 Since decomposition of the dendrimer molecules and reorientation of primary nanoparticles occur simultaneously and both last a certain time, the ligand-covered and ligand-free planes coexist in the RSA. Thus, it is hard to predict the reorientation rule for the CuO nanoparticles in the RSA. As mentioned in section 3.2, the SAD pattern of an original RSA depicts a slightly curved single-crystal diffraction pattern instead of the sharp diffraction spots along the [001] zone axis (Figure 2A′). This difference suggests the presence of a small misorientation among the primary CuO nanoparticles in the RSAs. It is interesting to find out that the electron beam acts as a driving force in the earlier stage of the modification process to rearrange the primary nanoparticles by orienting them along the [010] direction. This event is found to occur very rapidly. As a result, recording of the structural changes at this stage by TEM is very difficult. As shown in Figure S1 in the Supporting Information, the crystal face lines contributed by the (010) planes change from curves to straight lines and the FFT pattern of (010) planes changes from curves to spots with increased irradiation time. Because the copper atomic density on the (001), (100),

and (010) planes of monoclinic CuO is 12.5, 11.5, and 8.5 atoms/nm2, respectively, the most unstable planes for ligandcovered and ligand-free nanoparticles are (010) and (001), respectively.14 Thus, observation of the orientation along [010] for the CuO nanoparticles in the RSA under electron beam irradiation indicates that despite the complexity of our system, the reorientation is predominantly determined by the rule that is the same as the ligand-covered nanoparticles. Because the sintering of the primary nanoparticles is achieved without melting, the nanoparticles have to rotate over large angles and their interfaces have to be resolved.15 Hence, it is reasonable to hypothesize that with the rapid improvement of orientation along [010], fusion along the [100] and [001] directions occurs. According to the FFT patterns in Figure 3, it can be seen that all products are CuO phase. Moreover, the FFT patterns become sharper and the number of spots in the patterns increases with prolonged irradiation time, indicating the increased crystallinity of the CuO nanoparticles. On the basis of these results, it can be concluded that in stage 1 of the electron beam modification process, the electron beam irradiation acts as a driving force to improve the orientation and increase the crystallinity of the CuO nanoparticles within the RSA. 3.4. Phase Transformation. The phase transformation of the RSA under electron beam irradiation (stage 2) is discussed based on the HRTEM and their associated FFT and further confirmed by the observation of the evolution of the moire´ pattern. The time-lapse TEM images and FFT patterns are shown in Figure 4 for analysis of the specific phase transformation process. The phases (CuO and Cu2O) are identified by indexing and comparing these observed FFT patterns with the typical patterns shown in Figure 2B′ and 2C′. As the phase transformation from CuO to Cu2O under electron beam irradiation occurs gradually, the coexistence of CuO and Cu2O phases is observed during stage

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Figure 3. Series of in situ TEM images (A, B, and C) and the corresponding [001] FFT patterns (A′, B′, and C′, designated by the white rings) taken from the same area of the CuO-G2Td(COOH)16 RSA, designated by white squares in (A, B, and C), under electron beam irradiation at 90.8 pA/cm2 for 10, 20, and 30 min, respectively.

2 (Figure 4A and 4A′, t ) 44 min). With increasing irradiation time at intervals of 2 min, the FFT patterns arising from the CuO phase gradually disappear from Figure 4A′ to 4F′. Finally, the phase is completely converted into the pure Cu2O phase (Figure 4F′, t ) 54 min). Since Cu2O possesses a more symmetrical and thermally stable cubic structure while CuO has a monoclinic structure,11 the atom rearrangement and lattice/unit cell reconstruction are required for this transformation. The electron beam in the highvacuum environment is considered to be the only source which can provide sufficient energy to make the structure transformation. However, since absorption and accumulation of sufficient energy take time, the transformation process happens after a period of electron beam exposure. On one hand, the previous study revealed that the thermal decomposition temperature of CuO to form Cu2O is decreased from 850 °C in atmosphere to about 50-200 °C in a vacuum environment.16 This indicates the high-vacuum environment is beneficial to the structural transformation of the nanoparticles under electron beam irradia-

tion. Kaito et al. also found that the ex situ decomposition from CuO to Cu2O occurs at a higher temperature of about 300-400 °C in a vacuum environment.17 On the other hand, it was demonstrated that Cu2O is quite stable, even after being exposed at 300 °C for over 12 h.16a Accordingly, the structural transformation in the modification process occurs from CuO to Cu2O rather than to Cu mainly due to the relative stability of Cu2O. The structural transformation is further confirmed by the TEM observation of the evolution of a moire´ pattern with the electron irradiation time. One of the examples is shown in Figure 4A-G, where the moire´ patterns are marked as dotted circles. The magnified moire´ pattern in Figure 4A is shown in Figure 4G, which matches well with the simulated pattern (Figure 4G′) obtained from superposition of both the CuO (001) lattice structure (Figure 4H) and the Cu2O (001) lattice structure (Figure 4I).16a Moreover, from Figure 4B-F, the moire´ pattern gradually disappears with increasing irradiation time, as proven by the corresponding FFT data, suggesting that the CuO phase is almost completely transformed to Cu2O. These findings are helpful for

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Figure 4. Series of in situ TEM images (A-F) and corresponding [001] FFT patterns (A′-F′) taken from the same area of CuO-G2Td(COOH)16 RSA (note that those extra spots are generated from the double diffraction). (A-F) TEM images demonstrate the phase transformation of nanoparticles within RSA, as revealed by the appearance and disappearance of the moire´ pattern in the dotted line circles. For A, B, C, D, E, and F, the electron beam irradiation times are 44, 46, 48, 50, 52, and 54 min, respectively. (A′-F′) The spots designated by white circles and triangles in the FFT patterns correspond to the phase of CuO and Cu2O, respectively. (G) Magnified moire´ pattern from Figure 4A. (G′) Simulated moire´ pattern along the [001] axis, obtained from superposition of the CuO (001) lattice structure (H) and Cu2O (001) plane lattice structure (I).

understanding the growth and transformation process of nanocrystals under electron beam irradiation. 4. Conclusion We demonstrated an electron beam-mediated in situ modification process of three-dimensional CuO-G2Td(COOH)16 RSAs. Due to the nanoscale precision of this approach, the modified RSA retains its rice shape while the internal primary CuO nanoparticles are transferred to the Cu2O nanoparticles with

increased size. During the modification process, the high-energy electron beam of TEM serves as the external driving force and energy resource to improve the orientation and increase the crystallinity of the single-phase CuO nanoparticles and subsequently transfer the crystal phase from CuO to Cu2O. This investigation reveals that electron beam-mediated modification has a great potential in finely tuning the nanoscale morphology and changing the chemical composition of nanoparticles and nanoparticle-based assembled structures. In view of the visual

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tracking capability and high outputting energy used, this method provides new opportunities for the facile real-time manipulation of nanomaterials with nanoscale accuracy and rapid in situ creation of nanomaterials. Acknowledgment. This work was supported by AcRF Tier 1 (RG 20/07) from MOE and the Center for Biomimetic Sensor Science at NTU in Singapore. W.H. is thankful for financial support from the National Basic Research Program of China (973 Program, grant no. 2009CB930601), National Natural Science Foundation of China (grant nos. 20774043, 20804019, and 50803028), and Cultivation Fund of the Key Project, Ministry of Education of China (grant no. 707032). S.D.F. is thankful for support from the Fund of Scientific ResearchFlanders (FWO) and K. U. Leuven (GOA). S.D.F. and K.M. are thankful for support from the Belgian Federal Science Policy Office through IAP-6/27. Supporting Information Available: TEM images of a selected area of CuO-G2Td(COOH)16 RSA with increasing electron beam irradiation time. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ruffino, F.; Grimaldi, M. G.; Giannazzo, F.; Roccaforte, F.; Raineri, V.; Bongiorno, C.; Spinella, C. J. Phys. D: Appl. Phys. 2009, 42, 075304. (2) Chen, Y.; Palmer, R. E.; Wilcoxon, J. P. Langmuir 2006, 22, 2851. (3) (a) Weeks, B. L.; Vollmer, A.; Welland, M. E.; Rayment, T. Nanotechnology 2002, 13, 38. (b) Lee, L. J. BioMEMS and Biomedical

Qi et al. Nanotechnology: Nanoscale Polymer Fabrication for Biomedical Applications; Springer: New York, 2006; Chapter 3, p 51. (4) Latham, A. H.; Eilson, M. J.; Schiffer, P.; Williams, M. E. J. Am. Chem. Soc. 2006, 128, 12632. (5) Warner, J. H. AdV. Mater. 2008, 20, 784. (6) (a) Sepulveda-Guzman, S.; Elizondo-Villarreal, N.; Ferrer, D.; Torres-Castro, A.; Gao, X.; Zhou, J. P. Jose´-Yacama´n, M. Nanotechnology 2007, 18, 335604. (b) Kim, J. U.; Cha, S. H.; Shin, K.; Jho, J. Y.; Lee, J. C. J. Am. Chem. Soc. 2005, 127, 9962. (7) Yang, Y.; Scholz, R.; Berger, A.; Kim, D. S.; Knez, M.; Hesse, D.; Go¨sele, U.; Zacharias, M. Small 2008, 12, 2112. (8) Yacama´n-Jose´, M.; Wing-Gutierrez, C.; Miki, M.; Yang, D.-Q.; Piyakis, K. N.; Sacher, E. J. Phys. Chem. B 2005, 109, 9703. (9) Wang, M. S.; Chen, Q.; Peng, L. M. AdV. Mater. 2008, 20, 724. (10) Wang, Y.; Xia, Y. Nano Lett. 2004, 4, 2047. (11) Qi, X. Y.; Xue, C.; Huang, X.; Huang, Y. Z.; Zhou, X. Z.; Li, H.; Liu, D. J.; Boey, F.; Yan, Q. Y.; Huang, W.; De Feyter, S.; Mu¨llen, K.; Zhang, H. AdV. Funct. Mater. 2010, 20, 43. (12) (a) Golberg, D.; Mitome, M.; Yin, L. W.; Bando, Y. Chem. Phys. Lett. 2005, 416, 321. (b) Chen, Y.; Palmer, R. E.; Wilcoxon, J. P. Langmuir 2006, 22, 2851. (c) Xu, Y.; Jiao, X.; Chen, D. J. Phys. Chem. C 2008, 112, 16769. (d) Ram, S.; Mitra, C. Mater. Sci. Eng., A 2001, A304-306, 805. (e) Kuo, C. H.; Huang, M. H. J. Am. Chem. Soc. 2008, 130, 12815. (13) Liu, J. P.; Huang, X. T.; Li, Y. Y.; Sulieman, K. M.; He, X.; Sun, F. L. Cryst. Growth Des. 2006, 6, 1690. (14) (a) Liu, X.; Geng, B.; Du, Q.; Ma, J.; Liu, X. Mater. Sci. Eng., A 2007, 448, 7. (b) Zhang, Z. P.; Sun, H. P.; Shao, X. Q.; Li, D. F.; Yu, H. D.; Han, M. Y. AdV. Mater. 2005, 17, 42. (15) van Huis, M. A.; Kunneman, L. T.; Overgaag, K.; Xu, Q.; Pandraud, G.; Zandbergen, H. W.; Vanmaekelbergh, D. Nano Lett. 2008, 8, 3959. (16) (a) Wang, Z. L.; Kong, X. Y. J. Phys. Chem. B 2003, 107, 8275. (b) Shah, I. D.; Russi, P. L.; Schluter, R. B. U. S. Patent, 4,080,430, 1978. (17) Kaito, C.; Nakata, Y.; Saito, Y.; Naiki, T.; Fujita, K. J. J. Cryst. Growth 1986, 74, 469.

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