pubs.acs.org/Langmuir © 2009 American Chemical Society
Preparation of Ultrafine Silica Nanowires Using an Iron Based Surfactant Paul Christian* School of Chemistry, University of Manchester, Oxford Road, Manchester, England M13 9PL Received August 13, 2009. Revised Manuscript Received November 9, 2009 Silica nanowires have been shown to have exceptional mechanical properties. However, to date, their preparation with diameters less than 20 nm by an easily implemented colloidal method has been problematic. In this work, we show how a novel iron based surfactant readily forms rodlike micelle structures which may be used to template the formation of silica nanowires with diameters less than 5 nm.
There is significant interest in developing routes to nanoparticles with well-defined shapes.1-3 Silica nanowires are particularly of interest due to their amorphous nature which results in a material with a theoretical modulus in excess of 100 GPa and strength in excess of 25 GPa.4 These properties are not as extreme as those of carbon nanotubes, but silica nanowires are more easily prepared on a large scale and their mechanical properties are not compromised by lattice defects. These properties therefore make silica nanowires possible candidates for reinforcement in high performance nanocomposite materials. A key factor in attaining the excellent properties of silica nanowires is the preparation of very narrow wires. To date, this has been limited to diameters of about 30 nm. In this Letter, we describe a method for preparing ultrafine (1000 >1000
notes TEM A TEM B, D TEM C TEM E 1 M FeCl3 1 M FeCl3 TEM F 10 M FeCl3 10 M FeCl3
Figure 1. TEM images of silica nanoparticles and wires formed in hexane at (A) 0.085 M, (B) 0.168 M, and (C) 0.341 M Fe(AOT)3. High magnification image inset in (C).
Figure 2. TEM images of silica nanowires (tube in the case of F) produced in hexane at iron chloride concentrations of (D) 0 M, (E) 1 M, and (F) 10 M ([Fe(AOT)3] = 0.170 M).
solid (Si/Fe 99:1 atom % EDX). We studied the effect of varying the total concentration of reagents in solution and total aqueous content on the formation of the silica nanowires. We also studied the effect of ionic strength of Fe3þ in the core of the micelle on the morphology of the silica formed. At concentrations of 95 mM, spherical particles with diameters of 2 nm ((0.3) are formed (Figure 1A). Whereas wires were formed at concentrations above 170 mM of AOT surfactant. The transmission electron microscopy (TEM) images (Figure 1B and C) show the nanowires have diameters of approximately 2 nm 1406 DOI: 10.1021/la903011m
((0.5) and aspect ratios of 1:6. This suggests that the spherical micelle phase converts to a rod micelle form at some intermediate concentration. It has previously been shown that the copperAOT system in isooctane shows similar phase behavior14 with spherical micelles being formed at concentrations which are less than 120 mM and rodlike micelles with similar diameters being formed at higher concentrations. The ionic strength of the core of a reverse micelle can have a dramatic effect on the structure of the micelle. Silica nanowires prepared in systems which contained either 1 or 10 M iron Langmuir 2010, 26(3), 1405–1407
Christian
Figure 3. Plot of log(concentration) against intensity for Fe(AOT)3 in hexane collected by dynamic light scattering. The data show the cmc at ∼0.26 mM.
chloride solutions in place of the water phase were also studied. A slight increase in wire diameter (∼2.6 nm, Figure 2E) on addition of 1 M FeCl3 solution was observed. A much larger change was observed when the concentration of iron chloride was increased to 10 M (20-100 nm diameter, Figure 2F). The morphology of the silica wires prepared with a high concentration of FeCl3 also seems to change, resulting in a nanotube with a wall thickness of approximately 10 nm. This is consistent with the results previously obtained by other groups working with iron chloride filled micelles of AOT.16 X-ray diffraction (XRD) patterns of the samples showed no peaks typical of either crystalline silica or iron oxides or hydroxides. Other work in our group has led us to expect to see small iron oxide particles under these conditions. It should be noted however that the assertion that the silica wires are amorphous cannot be applied, as the large unit cell of crystalline silica is almost the size of the wire diameter. We undertook a series of light scattering experiments in order to investigate the phase behavior of the Fe(AOT)3-hexane system. By measuring the intensity of scattered light from solutions at various concentrations of surfactant, it is possible to determine the critical micelle concentration (cmc) of the surfactant and also determine other points where the phase behavior of the system changes. Analysis of the graph shows a sharp change in the intensity of scattered light at ∼0.5 mM (Figure 3); fitting a straight line plot to the data immediately before and after the transition gives and estimated cmc of 0.26 mM. As the concentration of surfactant is increased above the cmc, the intensity of scattered light rises approximately linearly up to concentrations of 5 mM. This observation is consistent with the formation of more micelles of similar size as might be expected. Above 5 mM (Figure 4), the linearity of the curve then begins to deviate, suggesting that there is growth of the micelles, probably via aggregation. The intensity of scatter reaches a maximum at 160 mM after which the intensity of scatter decreases slightly. This would be consistent with a collapse of a spherical micelle
Langmuir 2010, 26(3), 1405–1407
Letter
Figure 4. Plot of concentration against intensity for Fe(AOT)3 in hexane showing concentrations used in the preparation of silica nanowires.
system into another morphology, thereby dramatically decreasing the number of scattering bodies resulting in a decrease in scattering intensity. These changes in scattering are consistent with the morphology of the silica wires prepared at different concentrations. Spherical silica particles formed at 95 mM (Figure 4, point A) are consistent with spherical micelles. Between points A and B (Figure 4), the micelles aggregate to form a rodlike structure which persists to point C. This is consistent with the formation of silica nanowires at surfactant concentrations represented by points B and C. We suggest that the use of micelles such as these offer excellent templating media for the preparation of a range of rodlike structures with small dimensions. Furthermore, given the persistence of the anisotropic structure even at high concentrations of surfactant, they may offer a viable method for preparing anisotropic nanomaterials at reasonably high concentrations.
Experimental Section Fe(AOT)3 was prepared by dissolving NaAOT (10 g) in diethylether (100 mL) and then shaking with a 10 M solution of FeCl3 in water (100 mL). The ether layer was then washed twice with water (2 100 mL) and evaporated to dryness. (analysis: expected Fe(AOT)3 C = 55%, Fe = 4.1%; actual C = 52%, Fe = 4%). Microanalysis for Cl and Na (
