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2008, 112, 1739-1743 Published on Web 01/19/2008
Formation and Ordering of Gold Nanoparticles at the Toluene-Water Interface Milan K. Sanyal,*,†,‡ Ved V. Agrawal,‡ Mrinal K. Bera,†,§ K. P. Kalyanikutty,‡ Jean Daillant,| Christian Blot,| S. Kubowicz,| Oleg Konovalov,⊥ and C. N. R. Rao‡ Surface Physics DiVision, Saha Insititute of Nuclear Physics, 1/AF Bidhannagar, Kolkata-700064, India, Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for AdVanced Scientific Research, Jakkur P.O., Bangalore-560064, India, S. N. Bose National Centre for Basic Sciences, JD-Block, Sector-3, Salt Lake, Kolkata-700098, India, LIONS, CEA Saclay, F-91191 Gif-sur-YVette Cedex, France, and European Synchrotron Radiation Facility, Beamline ID10B, P.O. Box 220, 38043 Grenoble, France ReceiVed: NoVember 6, 2007; In Final Form: January 5, 2008
Microscopic measurements that provide direct information in nanometer length scales are essential to obtain a proper understanding of the interfacial reactions that form nanostructured materials. We present here the results of a synchrotron X-ray scattering study of the formation and ordering of gold nanoparticles at the toluene-water interface through a reduction reaction. The observed X-ray reflectivity and diffuse scattering data show the formation of a monolayer of “magic clusters” at the water-toluene interface. Each cluster consists of 13 nanoparticles with about 12 Å diameter, similar to Au-55 nanoparticles, with about an 11 Å organic layer and an in-plane cluster-cluster separation of 180 Å. The electron density profile of the monolayer of these clusters exhibits three layers of nanoparticles as a function of depth that evolves with time.
Understanding chemical reactivity at the liquid-liquid interface is of fundamental importance in several research areas including drug delivery and diffusion through biological membranes because the effect of the inhomogeneous environment can alter the behavior of the reacting molecules.1-6 The hydrogen bonds in the aqueous surface and the transfer of ions/ charge from one liquid into the other can get altered due to the presence of microscopic roughness, which is related to macroscopic interfacial tension through capillary wave theory.7-9 It has been demonstrated that a toluene-water interface can be exploited to form two-dimensional aggregates of nanoparticles of metals, metal sulfides, and other materials at the interface by clever choice of interfacial chemistry.6 The film of nanoparticles produced at the liquid-liquid interfaces is expected to be very thin because it is known3 that water “fingers” and organic “fingers” protrude into one another only around 10 Å lasting for tens of picoseconds to facilitate interfacial chemical reactions. An in situ study of nanoparticle formation and ordering should provide us with a unique opportunity to probe the inhomogeneous chemical reactivity and associated ion/ charge transfer processes across interfaces. We report here an in situ study of formation and ordering of gold nanoparticles at the water-toluene interface using highenergy synchrotron X-ray scattering techniques.7 The primary motivation of this study is to measure the size of the gold nanoparticles generated through an interfacial chemical reaction and to determine the nature of the ordering of these nanoparticles * Corresponding author. E-mail:
[email protected]. † Surface Physics Division. ‡ Chemistry and Physics of Materials Unit. § S. N. Bose National Centre for Basic Sciences. | LIONS. ⊥ European Synchrotron Radiation Facility.
10.1021/jp710635e CCC: $40.75
at the interface. The other aim of this work is to study the toluene-water interfacial tension as the nanoparticles form at the interface by measuring the X-ray diffuse scattering intensity8 as a function of the in-plane wave vector transfer (qy). Although liquid surfaces have been studied by diffuse scattering measurements of capillary wave fluctuations,8-11 it is only recently that the buried liquid interfaces are receiving attention.7,11-13 We have performed high-energy X-ray scattering measurements at the ID10B beamline of the European Synchrotron Radiation Facility (ESRF) to investigate the formation of ultrathin films of gold nanoparticles at the toluene-water interface. We have used a large trough (7 cm wide and 30 cm long) for this reaction to keep the in-plane pressure low and an antivibration table to stabilize the interface for carrying out the X-ray measurements. The energy of the monochromatic beam was set to 21.9 keV to allow the X-ray beam to pass through the upper liquid (toluene here), and reflectivity and diffuse scattering data were collected from the toluene-water interface. Two thin silicon wafers of equal heights were used near the entry and exit X-ray windows of the Langmuir trough to anchor the toluene-water interface. In this setup, the X-ray intensity reduces by a factor of 0.142 and 0.0195 as the 21.9 keV beam pass through 7 cm of toluene and water, respectively. We also collected scattering data in the same geometry from the toluene bulk by moving down the interface 0.2 mm to subtract the background arising from bulk scattering. We neglected the contribution of bulk water scattering in the analysis because the incident angle of the beam was low and the absorption coefficient of water is much higher than that of toluene. The scattered intensity profiles measured as function of horizontal and vertical components of the wave-vector transfers qz and qy, being related to the out-of-plane and inplane angles θf and φ,11,14 provide information regarding the © 2008 American Chemical Society
1740 J. Phys. Chem. C, Vol. 112, No. 6, 2008
Letters
out-of-plane and in-plane ordering of nanoparticles formed at the liquid-liquid interface. The position of the interface was adjusted to minimize the meniscus with controlled addition and removal of water and by monitoring the sharpness of the reflected beam at small θi. Reflectivity data of the toluenewater interface were collected using a point detector by changing the incident and reflected angles and keeping these two angles equal (θi ) θf). For diffuse scattering measurements, a positionsensitive detector (PSD) was used to collect the data as a function of the in-plane angle (φ), keeping the incident grazing angle (θi) fixed at 80 millidegrees. All of the measurements were performed at room temperature (23 °C) with the incident beam size of 0.017 × 1.0 mm2 (V × H) defined using conventional slits. Details of the experimental arrangement are available in the literature.9,10,12 We carried out the reaction at the interface between a metalorganic compound triphenylphosphine gold chloride, Au(PPh3)Cl (P ≡ phosphorus, Ph ≡ phenyl), in the toluene layer and the reducing agent, tetrakishydroxymethylphosphonium chloride (THPC), in the aqueous layer.6 The process was repeated several times to confirm reproducibility of the scattering data. It was shown earlier5 using X-ray diffraction data of the transferred films on solid substrates that the reduction reaction with THPC may produce Au-55 particles with a magic number of atoms having a diameter of around 12 Å, though transmission electron microscopy (TEM) indicated much larger sizes due to electron beam damage. A TEM study of gold nanoparticles transferred on to a grid from the toluene-water interface6 also indicated a particle size of about 80 Å diameter though the reaction zone at liquid-liquid interface is expected to be around 10 Å due to protrusion of liquid “fingers”. The results of synchrotron X-ray scattering and atomic force microscopy (AFM) presented here, however, indicate formation of an Au-55 “magic particle” having a diameter of around 12 Å. The use of an antivibration table that minimizes macroscopic interdiffusion across the toluene-water interface is of crucial importance to obtain the results presented here. In Figure 1a, we have shown six reflectivity curves collected after the initiation of the reaction and indicated the time of data collection. We have also presented in the insets of Figure 1a and b, the reflectivity data and electron density profile of the bare toluene-water interface, respectively. We required around 30 min time to collect a reflectivity profile and have not presented the data collected within 194 min of initiation of reaction here because the profiles changed during scans. The oscillations in the measured reflectivity curves indicate the presence of a thin film at the toluene-water interface. These curves were fitted using an iterative inversion technique based on the Born approximation.15 The inversion scheme is based on recursive Fourier transform (FT) and inverse Fourier transform (FT-1) of the equation relating the model electron density profile (Fm(z)) with actual electron density profile (Fe(z)) as
Fe(z) ) FT-1
[x
Re Rm
∫F′m(z) exp(iqzz) dz
]
(1)
Here Re is the experimental reflectivity curve and Rm is the model reflectivity curve calculated from the model profile Fm(z) by the standard slicing technique. In each iteration, Fm(z) is replaced by Fe(z) obtained from eq 1 and the process is continued until Re and Rm become indistinguishable. We used the water-toluene profile (with a small increase in toluene density due to presence of Au(PPh3)Cl to match the critical
Figure 1. (a) Variation of reflectivity and fits as function of qz after initiation of the reaction (pink, 194 min; light blue, 224 min; grey, 253 min; green, 283 min; red, 312 min; orange, 364 min) and the fits (solid lines). The reflectivity (dashed line) generated from the simple box model is also shown. Inset: The reflectivity data (
