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Dye-Adsorption-Induced Gelation of Suspensions of Spherical and Rodlike Zinc Oxide Nanoparticles in Organic Solvents† § Cyril Martini,‡ Florian J. Stadler, Aurore Said,‡ Vasile Heresanu,‡ Daniel Ferry,‡ ,§ :: Christian Bailly,* Jorg Ackermann,*,‡ and Frederic Fages*,‡ ‡ Centre Interdisciplinaire de Nanoscience de Marseille (CINaM), UPR 3118, CNRS, Aix-Marseille Universit e, Campus de Luminy, Case 913, F-13288 Marseille Cedex 09, France and § Unit e de Physique et de Chimie des Hauts Polym eres, Universit e Catholique de Louvain, Croix du Sud, 1, B-1348 Louvain-la-Neuve, Belgium
Received December 28, 2008. Revised Manuscript Received February 16, 2009 The adsorption of amphiphilic RuII complex Z907 onto the surface of ZnO nanospheres and nanorods causes the gelation of organic solvents, such as THF and acetone. The gels are thermally stable at very low concentration (nanoparticle volume fraction φ = 0.009) but mechanically fragile, with the behavior being dependent on the nature of the solvent, nanoparticle concentration, and the Z907/ZnO mole/weight ratio. Rheological experiments confirmed that the solid component built up a network to give a viscoelastic gel-phase material with a weak value of storage modulus G0 . However, TEM and SEM experiments did not give evidence that nanoparticle long-range ordering occurred under the experimental conditions investigated. Moreover, time-dependent SAXS measurements pointed to a decrease in the nanoparticle aggregate size upon gelation. All together, the data obtained might be rationalized in terms of the aggregate-to-aggregate transition in solution, with the primitive large aggregates giving rise to smaller ones upon reaction with Z907. The resulting smaller hybrid aggregates could be the active species that act as self-assembling components in the gelation process. Given the interesting electronic and photonic properties of zinc oxide nanoparticles, such hybrid organic-inorganic gels could open new directions in materials science, low-cost electronics, and photovoltaics.
Introduction Inorganic nanoparticles of various shapes, sizes, and chemical composition are key building blocks for the generation of functional nanomaterials.1-6 Critical to their successful embodiment in various electronic, magnetic, and photonic devices is the development of self-assembly strategies that ensure nanoparticle long-range ordering in well-defined arrays.7-9 Recently, molecular gels, based on low-molecularweight gelators,10-16 have been used as fibrillar scaffolds to † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue. *Corresponding authors. E-mail:
[email protected],
[email protected],
[email protected].
(1) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293. (2) Jun, Y.; Choi, J.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 3414. (3) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (4) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834. (5) Lin, Z. Chem.;Eur. J. 2008, 14, 6294. (6) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (7) Jana, N. P. Angew. Chem., Int. Ed. 2004, 43, 1536 and references therein. (8) Tang, Z.; Kotov, N. A. Adv. Mater. 2005, 8, 951. (9) Kovtyukhova, N. I.; T. E.; Mallouk, T. E. Chem.;Eur. J. 2002, 8, 4355. (10) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (11) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237. (12) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201. (13) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263. (14) Sangeetha, N. M.; Maitra, U. Chem. Soc. Rev. 2005, 34, 821. (15) Molecular Gels: Materials with Self-Assembled Fibrillar Networks; Weiss, R. G., Terech, P.; Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2005. (16) Low Molecular Weight Organic Gelators; Smith, D. K.; Ed.; Tetrahedron (Symposium-in-Print), 2007; Vol. 63. (17) van Herrikhuyzen, J.; George, S. J.; Vos, M. R. J.; Sommerdijk, N. A. J. M.; Ajayaghosh, A.; Meskers, S. C. J.; Schenning, A. P. H. J. Angew. Chem., Int. Ed. 2007, 46, 1825.
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stabilize and organize surface-functionalized quantum dots into 1D structures.17-25 Colloidal gels,26 which are used in a wide variety of applications such as the processing of ceramics, coatings, and inks, may also represent valuable intermediates in synthesis schemes based on self-assembly. Attractive interactions between nanoparticles can impart solidlike behavior to a wide variety of complex fluids at often low particle volume fraction (φ).27,28 The combination of unusual rheological properties and nanoparticle shape-dependent aggregation features7 could render colloidal gels especially attractive in low-cost, large-area processes for electronics and photovoltaics. In that connection, we report here the finding that grafting the tris(2,20 -bipyridyl)ruthenium(II) complex, known as the Z907 dye (Figure 1),29 onto the surface of zinc oxide nanospheres and nanorods can induce the gelation of a range of organic solvents. ZnO is a versatile, environmentally friendly (18) Bhattacharya, S.; Srivastava, A.; Pal, A. Angew. Chem., Int. Ed. 2006, 45, 2934. (19) Kimura, M.; Kobashashi, S.; Kuroda, T.; Hanabusa, K.; Shirai, H. Adv. Mater. 2004, 16, 335. (20) Bhat, S.; Maitra, U. Chem. Mater. 2006, 45, 4224. (21) Miljanic, S.; Frkanec, L.; Biljan, T.; Meic, Z.; Zinic, M. Langmuir 2006, 22, 9079. (22) Simmons, B.; Li, S.; John, V. T.; McPherson, G. L.; Taylor, C.; Schwartz, D. K.; Mastos, K. Nano Lett. 2002, 2, 1037. (23) Ray, S.; Das, A. K.; Banerjee, A. Chem. Commun. 2006, 2816. (24) Love, C. S.; Chechik, V.; Smith, D. K.; Wilson, K.; Ashworth, I.; Brennan, C. Chem. Commun. 2005, 1971. (25) Vemula, P. K.; John, G. Chem. Commun. 2006, 2218. (26) Lu, P. J.; Zaccarelli, E.; Ciulla, F.; Schofield, A. B.; Sciortino, F.; Weitz, D. A. Nature 2008, 453, 499. (27) Varadan, P.; Solomon, M. J. Langmuir 2001, 17, 2918. (28) Wierenga, A.; Philipse, A. P.; Lekkerkerker, H. N. W.; Boger, D. V. Langmuir 1998, 14, 55. (29) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; :: Sekiguchi, T.; Gratzel, M. Nat. Mater. 2003, 2, 402.
Published on Web 03/20/2009
DOI: 10.1021/la804280m
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In this article, we study the influence of molecular surface coverage, volume fraction, and particle shape on gelation ability in different solvents at room temperature. We used zinc oxide nanospheres (NSs) and nanorods (NRs). We observed the gels to be thermally stable but mechanically fragile, with the behavior being dependent on the nature of the solvent, the concentration, and the Z907/ZnO ratio. Transmission and scanning electron microscopy (TEM and SEM, respectively), small-angle X-ray scattering (SAXS), and rheological experiments were performed as a first attempt to provide insight into the gelation mechanism and gel structure.
Experimental Section
Figure 1. Structure formula of the Z907 coordination complex. Photograph of a THF gel of zinc oxide nanorods surface-functionalized with Z907. semiconducting and photoactive material that can be easily produced as nanoparticles with a variety of shapes, such as spheres,30 rods,30 wires,30,31 belts,32 and tetrapods,33 using chemical processes in solution. As a metal oxide with a high isoelectric point, ZnO undergoes facile surface functionalization with carboxylic acid derivatives,34 such as Z907. The :: latter compound has been developed by Gratzel and coworkers for photosensitization of TiO2 in solar cells.29 Suspensions of rodlike particles, with diverse chemical compositions (boehmite, rutile TiO2, β-FeOOH, tobacco mosaic virus,...), represent an important class of colloidal systems that are known to give rise to liquid-crystalline ordering and gelation.35-39 The rheological behavior of gels of nanorod suspensions remains a relatively unexplored field with respect to other colloidal systems. It was found that the birefringence of hairy nanorod solutions increases significantly under shear,40 which is not surprising considering that such species can be oriented quite well under shear. Interestingly, some of these orientations also were shown to persist after shearing. In the case of boehmite nanorod solutions, significant interactions between nanoparticles were found at a volume fraction of only 0.35%.28 However, such systems do not behave as real gel systems because their storage modulus G0 (ω) distinctly increases its slope toward the lowest angular frequencies ω. In other words, such colloidal suspensions having a terminal regime behave rather like viscoelastic fluids than gels. In a later paper by the same group,41 it was shown that the viscosity of such solutions can be lowered by up to five decades as a result of heavy preshear. (30) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188. (31) Huang, M. H.; Wu, Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. D. Adv. Mater. 2001, 13, 113. (32) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (33) Li, J.; Peng, H.; Liu, J.; Everitt, H. O. Eur. J. Inorg. Chem. 2008, 3172. (34) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813. (35) van Herrikhuyzen, J.; Janssen, R. A. J.; Meijer, E. W.; Meskers, S. C. J.; Schenning, A. P. H. J. J. Am. Chem. Soc. 2006, 128, 686. (36) Cao, Y. C. J. Am. Chem. Soc. 2004, 126, 7456. (37) Dessombz, A.; Chiche, D.; Davidson, P.; Panine, P.; Chaneac, C.; Jolivet, J.-P. J. Am. Chem. Soc. 2007, 129, 5904. (38) van Bruggen, M. P. B.; Lekkerkerker, H. N. W. Langmuir 2002, 18, 7141. (39) Li, L.; Walda, J.; Manna, L.; Alivisatos, A. P. Nano Lett. 2002, 2, 557. (40) Hilliou, L.; Vlassopoulos, D.; Rehahn, M. Rheol. Acta 1999, 38, 514. (41) ten Brinke, A. J. W.; Bailey, L.; Lekkerkerker, H. N. W.; Maitland, G. C. Soft Matter 2008, 4, 337.
8474 DOI: 10.1021/la804280m
Synthesis of Zinc Oxide Nanospheres and Nanorods. Both types of ZnO nanoparticles were obtained upon treatment of a methanol solution of zinc(II) diacetate with NaOH as a base, following an experimental procedure similar to that reported in the literature.30 Zinc acetate (99.99%) and sodium hydroxide (99.998%) were purchased from Aldrich and used without further purification. Methanol was distilled over CaO prior to use. A solution of sodium hydroxide (288.8 mg, 7.22 mmol) and methanol (23 mL) was sonicated until a fine dispersion was obtained. The latter was then added dropwise over 15 min to a stirred solution of zinc acetate (818.2 mg, 4.46 mmol), methanol (42 mL), and water (318 mL) at 60 °C under an argon atmosphere. Stirring and heating at 60 °C were maintained for 2 h 15 min, which produced ZnO NSs with a diameter of 4 ( 1 nm. For functionalization purposes, methanol (50 mL) was added to the suspension of the so-obtained NSs, and the mixture was stirred for 5 min. After centrifugation, the supernatant was removed, and the solid residue was washed several times with methanol until residual water was withdrawn, which was indicated by a clear increase in solubility. Alternatively, in order to produce NRs, the growth solution containing the NSs was condensed to 10 mL under reduced pressure and heated at 60 °C for 48 h. The NRs were washed as described above for NSs. Gelation Tests. Z907 (cis-bis(isothiocyanato)(2,20 -bipyridyl4,40 -dicarboxylato)(2,2’-bipyridyl-4,4’-dinonyl) ruthenium(II)) was purchased from Solaronix and used as received.29 It included two water molecules of crystallization (C42H54O6N6S2Ru, formula weight 903 g/mol). A titrated suspension of ZnO nanoparticles in the solvent to be investigated was poured into a test tube containing a weighed solid sample of Z907. The mixture was stirred and sonicated, and then left to stand in the dark until gelation occurred, which was checked visually by gently inverting the test tube. The solvents used in this study were toluene, tetrahydrofuran (THF), chloroform, dichloromethane, 1,2,3-trichloropropane (TCP), acetone, acetonitrile, ethyl acetate (EtOAc), chlorobenzene, dioxane, diethylene glycol dimethyl ether (DEGME), dimethylformamide (DMF), N-methyl-pyrrolidone (NMP), and 1,2-propanediol monomethyl ether acetate (PGMEA). TEM. TEM characterizations were performed with a JEOL 3010 (300 kV). A piece of the gel material was deposited onto a carbon grid. SEM. Scanning electron microscopy characterizations were performed with a FEGSEM JEOL 6320 F equipped with a cooled cathode. Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra were obtained with a Bruker Equinox 55 in transmission mode (100 scans, resolution 2 cm-1). SAXS. SAXS measurements were made using a Kratky camera produced by Hecus.42 A PSD 50 m linear detector produced by Hecus is used, and pertinent data can be collected from an angular range of 0.07-7°. The solution was gelated (42) Kratky, O.; Stabinger, H. Colloid Polym. Sci. 1984, 262, 345.
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inside a quartz capillary of 1 mm diameter and 50 mm length. The exposed volume of solution is around 10 1 0.4 mm3. XRD. The diffraction powder measurements were realized with an INEL diffractometer equipped with an INEL CPS 120 linear detector. The powder was put into a glass capillary of 0.7 mm diameter. Rheometry. The rheological characterizations were carried out on a Malvern Bohlin Gemini HR nano setup using coneand-plate geometry (cone, 25 mm/deg; plate, 20 mm). The temperature was kept constant at 25 °C ( 0.1 °C using a Peltier heated plate. DEGME, with a high boiling point, was used as a solvent for these experiments. However, to avoid the loss of solvent, a solvent trap whose atmosphere was saturated with DEGME was used. The samples were prepared as indicated above, but instead of leaving them in the tube, they were injected into the rheometer after sonication to follow the gelation process online. The samples proved to be too fragile to be inserted into the rheometer after gelation.
Results and Discussion Preparation of the Gels. We used ZnO NSs with a diameter of 4 ( 1 nm and NRs having an aspect ratio of around 6 with an average diameter of 8 ( 1 nm and length of 50 ( 5 nm, as determined by TEM (Figure 2). In the powder X-ray diffraction (XRD) patterns of THF gel samples of ZnO NRs and NSs (Supporting Information Figure S1), all :: reflections could be indexed to Wurtzite with (100), (002), and (101) as the strongest signals. They were all observed to be size-broadened except for the (002) reflection that was sharper for NRs than for NSs, confirming the anisotropy along the [002] direction expected for rodlike species. The average particle sizes determined by the Scherrer formula confirmed the TEM data. Gelation tests were performed using 25 mg of ZnO nanoparticles (rods or spheres) in 0.5 mL of a solvent and adding this suspension to 6 mg of a powdered sample of Z907. Under these conditions, the nanoparticle concentration C was 50 mg/mL, and the Z907/ZnO ratio, denoted as R, equaled 2.65 10-7 mol of Z907/mg of ZnO. This value gives a packing density of about 45 A˚2 per molecule in the case of nanorods. After mixing the two components, the mixture immediately became more soluble and gelation then occurred at different times, depending on the solvent nature, concentration, and Z907/ZnO ratio. At the concentration C, given the value of 11 100 M-1 cm-1 for the molar absorptivity of the complex at 525 nm,43 which is the wavelength of the maximum of the metal-to-ligand charge-transfer transition band, the absorbance of the solution exceeded 150, which resulted in the gels being dark purple in color and completely opaque (Figure 1). Gelation Tests. The results of the gelation tests on ZnO NRs are collected in Table 1. In all of the solvents investigated, bare ZnO nanoparticles did not give gels. Toluene, TCP, acetonitrile, chlorobenzene, and ethyl acetate did not solubilize the ruthenium complex at all; these solvents were not tested for reaction with ZnO nanoparticles. It can be seen that gelation is particularly effective in solvents of medium polarity, such as acetone or THF. FTIR spectra of the adsorbed Z907complex on ZnO NRs were obtained for a THF xerogel (C, 2R) and for a solid precipitate obtained from the same solvent under nongelating conditions (43) Zakeeruddin, S. M.; Nazeeruddin, M. K.; Humphry-Baker, R.; P.; :: Pechy, P.; Quagliotto, P.; Barolo, C.; Viscardi, G.; Gratzel, M. Langmuir 2002, 18, 952.
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Figure 2. TEM micrographs of bare (a, b) and Z907-functionalized (c, d) zinc oxide nanorods (xerogel). Scale bars: 70 nm (a, c) and 40 nm (b, d). Table 1. Organic Solvents Tested for the Gelation of ZnO Nanorods Grafted with Z907 at Different Concentrations and Ratios at Room Temperaturea solvent toluene chlorobenzene CH2Cl2 CHCl3 TCP acetone THF
dioxane EtOAc acetonitrile DEGME
concentrationb Z907/ZnO ratioc
timed
observation e e
C C
R R
C C C C/2 C/5 C/10 C C C
R 2R R R R R R/2 2R 2R
P S e
