Structural Characterization of Self-Organized TiO2 Nanoclusters

The self-organization of TiO2 particles in rodlike structures was found to be a .... You Seok Seo , Changhoon Lee , Kee Hag Lee , Kyung Byung Yoon...
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J. Phys. Chem. B 1999, 103, 2172-2177

Structural Characterization of Self-Organized TiO2 Nanoclusters Studied by Small Angle Neutron Scattering T. Rajh,* M. C. Thurnauer, P. Thiyagarajan,† and D. M. Tiede Chemistry DiVision, Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: July 14, 1998; In Final Form: January 12, 1999

Thiolactate-modified 45 Å TiO2 particles were observed to self-assemble into rodlike structures. The rod formation was observed using small-angle neutron scattering (SANS). The diameter of the rods was determined to be 43.2 ( 1.6 Å, the same diameter as unassembled TiO2 particles. The rod diameter was found to be independent of TiO2 concentration or pH, while rod length was sensitive to both. An independent determination of the molecular weight per unit length of a rod using SANS confirms the beadlike structure of the rods with a bead diameter of 40 ( 3 Å. Further confirmation of self-assembly was obtained from transmission electron microscopy where long rods with a diameter of 42.8 ( 3.5 Å were observed. The formation of rodlike structures was not detected when the mercapto group was either blocked by a methyl group or separated from the carboxyl group with an additional methylene group. The self-organization of TiO2 particles in rodlike structures was found to be a consequence of the special conformations of surface modifiers upon bidentate binding of mercapto-carboxylic acids. These special bidentate bindings introduce conformations of surface modifiers that allow a significant increase of a dipole moment at the surface. Single-point ZINDO calculations suggest a change of the dipole moment from 1.5 D for unbound thiolactic acid to 11.2 D for bidentate bound thiolactate surface Ti atoms.

Introduction Self-organized structures of nanostructured materials and selfassembled organic monolayers on metal and metal oxide surfaces are of considerable interest because of their unique collective physical and chemical properties.1-8 Development of these rational designs is focused on their electronic properties because it is expected that self-organized assemblies in three dimensions1-3ssuper crystalsswill exhibit electronic energy transfer9 while assemblies with a preferential conduction axiss nanoparticle wiresswill be capable of anisotropic conductivity by interparticle electron transfer.10 Successful preparation of three-dimensional colloidal super crystals by self-organization of monodispersed nanocrystalline metal oxides1,2 and quantum dots3 has been described. Self-organization of 50 Å TiO2 nanoclusters into closely packed 500 Å mesoaggregates has also been reported.11 The assemblies of monodispersed colloids into patterned structures such as super crystals have been achieved by dispersing the colloids in a solvent and allowing the crystalline regions to form as a consequence of specific particle interactions or entropic effects arising from slow elimination of the solvent. We have reported the formation of a monolayer of R-mercapto-substituted carboxylic acids on the surface of 45 Å TiO2 nanoclusters that results in a red-shift of the optical absorption of TiO2.12 The primary structure (binding of a monolayer of surface derivative) of these systems was determined by FTIR spectroscopy on dried films. In this paper we have investigated secondary structure, represented by self-organization of the colloidal particles, using small-angle neutron scattering (SANS). The primary structure can affect van der Waals forces and the hydrophilic/hydrophobic character of the particle surface, affecting the passive sorption properties of TiO2 particles. The secondary structure, on the other hand, can affect the particle†

Intense Pulsed Neutron Source.

particle interaction by changing the dielectric dipole interactions and efficiency of light absorption by energy transfer between the assembled particles, e.g., it can allow axial charge separation across the assembled nanoparticle wires. Therefore, both primary and secondary structures will affect the photocatalytic properties of TiO2. Herein we report the assembling of TiO2 nanoclusters into self-assembled rodlike structures after surface modification with thiolactic acid (TLA), which is an R-mercapto-substituted carboxylic acid. Nanoparticle semiconductor colloids are usually characterized by two techniques (microscopy or X-ray diffraction) that involve drying the samples on appropriate supports. In this work, SANS was used in order to characterize TiO2 particles in situ, in colloidal aqueous solutions (0.02 M - 0.3 M), and the results were compared with data obtained from the images obtained with TEM. Rod formation was explained to be a consequence of hydrophobic interactions between methyl groups appendant to the colloid surface and changes of the dipole moment of the modifier upon particle modification. Experimental Section Colloid Preparation. Colloidal TiO2 was prepared by dropwise addition of titanium(IV) chloride to cooled water. The temperature and rate of component mixing of reactants were controlled by a multiport adjustable-temperature liquid nitrogen cooled gas flow system.12 A peristaltic pump with variable size outlet ports was used to control the drop size as well as drop rate of TiCl4. (Ambient moisture was removed by purging the outlet ports with dry nitrogen gas.) Following TiCl4 hydrolysis, the solution was dialyzed against distilled water at 4 °C. Samples in D2O were prepared in the same manner but were dialyzed against D2O. The concentration of TiO2 (0.1-0.6 M) was determined from the concentration of the peroxide complex obtained after dissolving the colloid in concentrated H2SO4.

10.1021/jp9830225 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/09/1999

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SANS data were collected at Intense Pulse Neutron Source (IPNS), Argonne National Laboratory (ANL), using time-offlight SANS instruments, small-angle Diffractometers (SAD and SAND) for TiO2 colloids dispersed in D2O in order to increase the scattering contrast from TiO2 particles.13 These instruments use pulsed neutrons derived from spallation with wavelengths in the range of 0.5-14 Å and fixed sample-to-detector distances of 2 m on SAND and 1.54 m on SAD. The scattered neutrons were measured with position-sensitive, gas-filled, proportional counters with the wavelengths measured by time-of-flight through binning the pulse to 67 constant ∆t/t ) 0.05 time channels. SAD and SAND at IPNS produce data in momentum transfer q (q ) 4π sin(θ)/λ, where θ is one-half of the scattering angle and λ is the wavelength of the probing neutrons), which is in the range 0.005-0.35 and 0.0035-0.6 Å-1, respectively. Samples were held in sealed 3 mm quartz cells under a nitrogen atmosphere. Data were typically collected for 5 h for each sample and corrected for background signals of the instrument, Suprasil cells, and the solvent, as well as for detector nonlinearity.13 The data are presented on an absolute scale by using the known scattering cross-section of a silica-gel sample. TEM. The particle size and shape were determined using transmission electron microscopes from the Electron Microscopy Center for Materials Research at ANL (JEOL 100 CX and highresolution electron microscope Joel 4000). The standard deviations (given in the results) of the particle size determination with both TEM and SANS are the measurement error bars, not the distribution of the particle size. The distribution of the particle sizes in unmodified samples determined using TEM was fairly large (20%), although the distribution determined using the same technique on modified samples was much narrower (10%). This difference in distribution is probably due to coprecipitation of the ionic species present in colloidal solution with unmodified colloidal particles upon drying on microscope grids. Dipole moment calculations were performed with HyperChem software, a product of Hypercube Inc. using the ZINDO/1 semiempirical method for determining the structures of complexes containing transition metals in a single-point calculation. The program was tested with molecules of known dipole moments. The conformations of the modifier that were used for calculations were the ones suggested by IR studies.12 Results and Discussion SANS is a direct technique that allows structural resolution in the size range from 10 to 1000 Å. This technique has been extensively used for studying systems that cannot be readily investigated by crystallography, such as micelles or vesicles, molecular and colloidal solutions, proteins, etc.13 Recently, SANS has been applied to investigate TiO2 nanocrystalline films.11b In this work, we have applied SANS in order to investigate the native colloidal solutions in situ, thus eliminating possible changes of particle size and shape and particle-particle interactions that may occur upon drying. The measured differential scattering cross section, (I(q)), as a function of momentum transfer q (q ) (4π/λ)sin θ/2, θ is the scattering angle, λ is the neutron wavelength), is proportional to the density of particles N, the intraparticle structural factor P(q), the sum of individual scattering lengths of atoms with respect to their atom position vectors within the particle, and the interparticle structural factor S(q):14

I(q) ) NP(q)S(q)

(1)

For dilute systems, where the interparticle interaction is negligible, S(q) approaches 1 and the intraparticle structural

Figure 1. TEM images of unmodified TiO2 colloidal particles used in this work. The arrows point at the particles. The insert presents highresolution electron microscopy of a typical TiO2 particle that contains 12 Ti layers.

factor P(q) is the only factor influencing the scattering cross section. It can be used to derive direct informaion on the size, shape, and molecular weight of the colloidal particles. Equation 1 can then be simplified for particles of known geometry. A Guinier approximation15,16 can be used at a small q region (qmaxRg < 1) in order to determine the radius of gyration of the scattering particle, Rg, determined from the slope of ln(I(q)) vs q2 according to the equation

I(q) ) I(0)exp(-q2R2g/3)

(2)

where I(0) ) NV2(∆F)2 is a coherent scattering at zero angle, N is the number of particles per unit volume, V is the volume of the particle, and ∆F is the difference in the scattering length densities of the particle and the solvent. In the case of spherical particles, Rg ) x3/5R and R is the radius of the sphere. This equation also allows determination of the molecular mass of a particle because for a given composition, scattering length densities, and volume are proportional to the molecular weight. Thus, for a given concentration, I(0) is proportional to the molecular weight of the particle. Scattering intensities for concentrated solutions of unmodified 45 Å colloids were found to drop significantly in the low q region (Figure 2). This attenuation is characteristic of particles experiencing strong repulsive interactions.15 This prevents accurate determination of particle size by measurement of Rg and molecular weight. It can be clearly seen from Figure 2 that as the concentration of colloidal solution decreases, the interparticle repulsion also monotonically decreases and the scattering data do not drop in the low q region. Guinier analysis (insert, Figure 2) of the data for TiO2 solution yielded Rg ) 38.1 ( 3.2 Å, suggesting that the particles form aggregates with a diameter of 98.5 ( 8.3 Å. After 2 months of aging at room temperature, the SANS data revealed that the aggregates become larger and the interparticle repulsion decreased substantially due to the decrease in the particle concentration with aging. After derivatization of the surface of 45 Å TiO2 with thiolactic acid (TLA), we obtained a colloid in which surface Ti atoms

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Figure 2. SANS profiles for 45 Å TiO2 colloids in D2O at pD 3.5. Different characters (b, ], and ×) and error bars (s, - - -, and ‚‚‚), respectively, correspond to the different concentrations of TiO2 colloid, as indicated on the figure. Insert: Guinier plots for 0.04 and 0.2 M TiO2 at pD 3.5. Data for the 0.04 M TiO2 colloidal solution has the least interparticle interaction.

Rajh et al.

Figure 4. Modified Guinier rodlike structure analysis for 0.02 M thiolactic acid modified 0.072 M TiO2 at different pDs. The position of the point of inflection shifts toward the low-scattering vector region with the rod length,18 thus relative length lpD3.5 > lpD2.5 > lpD1.5. The diameter of the rods was found to be the same for all pDs and corresponds to the diameter of D ) 43 ( 3 Å.

particles can be confirmed by performing modified Guinier analysis for rodlike particles17 for which

I(q) ) 1/q exp(-q2R2c /2)

Figure 3. SANS profiles for 0.02 M thiolactic acid modified 45 Å TiO2 colloids in D2O at pD 3.5. Insert: Modified Guinier rodlike structure analysis is shown for 0.02 M thiolactic acid modified (b) 0.120 M, (]) 0.072 M, and (×) 0.032 M TiO2 colloids. The slope of the fitted lines indicates the diameter of the rods that was found to be the same for all concentrations and corresponds to the diameter of D ) 43.2 ( 1.6 Å.

are chelated to both the carboxyl and mercapto groups.12 Two binding modes are possible. One is intraparticle binding in which both the thiol and the carboxyl groups chelate the same surface titanium atom. As the mercapto group is in the R position relative to the carboxyl group, this complex is additionally stabilized by the formation of a five-membered ring. The fivemembered ring geometry has a favorable angle configuration for octahedral coordination of Ti surface atoms. Second, a possibility exists for interparticle binding in which the R-mercaptocarboxylic acid bridges two particles with the carboxyl and the mercapto groups binding each particle. We have used SANS to examine the secondary structure of these nanoclusters. After surface modification with thiolactic acid, the repulsion between the particles vanished (see Figure 3) and the measured I(q) exhibits a power-law scattering in q (I(q) ) q-1.2(0.1) in the low q region. One of the possibilities for this type of scattering is that the particles may be rodlike, as rods of infinite length and infinitesimal cross section should exhibit a q-1 power-law scattering (theoretical value). This is manifested as the linear dependence of the logarithm of scattered intensity vs the logarithm of the scattering vector. The existence of rodlike

(3)

where Rc is a cross-sectional radius of gyration. If the particles are indeed rodlike, then a plot of ln(I(q)q) as a function of q2 will be linear in a q region qmaxRc < 1.0. The radius of the rod is related to the cross-sectional radius of gyration as R ) Rc x2. From the plot one can obtain the slope and the intercept which are related to the radius and the mass per unit length of rodlike particles, respectively. We emphasize that if the scattering is due to mass fractals which can also produce a powerlaw scattering in q, the analysis with modified Guinier analysis will not provide physically realistic parameters for the radius and mass per unit length determination. The analysis of the data obtained for different concentrations of thiolactic acid derivatized TiO2 colloids is consistent with the appearance of long rods with a cross-sectional radius of gyration Rc of 15.3 ( 0.6 Å that corresponds to a rod diameter of 43.2 ( 1.6 Å (Figure 3, insert). This diameter of self-assembled rods is identical (within the error) to the diameter of the particles determined by TEM (Figure 1) and is independent of the concentration or pH. However, the length of the rods increases with the concentration of TiO2 and pH (Figure 4). This indicates that upon surface modification of 45 Å TiO2 nanoparticles, a secondary structure, a rodlike, two-dimensional self-assembly of particles occurred. It should be noted that the scattering profile of a rigid rod and a string containing rigid sections would be the same, and our model cannot differentiate between flexible strings and rigid rods. However, as the assembling of the particles is a consequence of the organization of the particles in beadlike structures, it is more likely that they assemble into flexible strings with rigid sections that are larger than several diameters.15 The enhanced larger slope in the very low q region (qRc < 0.14) in Figure 4 can be attributed to two phenomena: 17,20 (1) bundling or contacts between rod-shaped particles due to their long size and (2) bending of the rods into the stringlike structures. Rod-shape analysis of unmodified TiO2 colloids did not produce any linear region. We were able to double-check this structure by independent determination of the molecular mass of unit volume of these beadlike rods. For rodlike particles, the molecular weight per unit length can be determined as the intercept of a modified Guinier plot18

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Figure 5. TEM images of thiolactic acid modified 0.2 M TiO2. The absolute magnification is indicated on the figures. D represents the diameter of the chains and was determined to be 42.8 ( 3.5 Å, presented as the spacing pointed by two arrows. Individual TiO2 colloidal particles that form chains are spherical gray areas within the chains. The low contrast of the image is the consequence of the low atomic number of titanium.

Figure 7. The change of the scattering profile upon surface modification of TiO2 colloids in D2O with different derivatives (4) none, (0) 0.02 M S-methyl-mercapto acetic acid CH3SCH2COOH, (]) mercapto-propionic acid HSCH2CH2COOH, and (b) thiolactic acid HSCH(CH3)COOH. Insert: Modified Guinier rodlike structure analysis for different modifiers.

Figure 6. Absorption spectra of 45 Å TiO2 surface modified with different mercaptocarboxylic acids. The dashed line represents absorption spectrum of thiolactic acid alone in the investigated region.

(

Σb µ - Fsνj M N

(qI(q))(qf0) ) πC N

)

(4)

where µ is the molecular weight per unit length (g mol-1 Å-1), C is the concentration, Σb is the sum of neutron scattering lengths, Fs is the solvent scattering density νj is the partial specific volume of the particle, N is Avogadro’s number, and M is the molecular mass. From eq 4 the mass per unit length of a cylinder formed by assembled particles was determined to be 1.70 × 103 g mol -1Å-1, which is independent of the concentration or pH. The molecular mass of a single TiO2 particle would be

( )

Mp ) µD

Vcyl Vsp

(5)

where Vcyl is the volume of a cylinder containing one TiO2 particle, Vsp is the volume of a single TiO2 particle, D is the diameter of a rod, and µ is the mass per unit length. This correction comes from the fact that in our model we have assumed that particles assemble into beadlike structures. Thus, the volume of a cylinder is filled not only by TiO2 particles, but partially by D2O and thiolactic acid. Therefore, determina-

Figure 8. Change of the scattering profile upon surface modification of TiO2 colloids in D2O with different derivatives (O) none, (0) 0.02 M R-mercapto-butyric acid HSCH(CH2CH3)COOH, (/) Mercaptoacetic acid HSCH2COOH, and (b) thiolactic acid HSCH(CH3)COOH. Insert: Modified Guinier rodlike structure analysis for different modifiers. The radius obtained for mercapto-acetic acid does not present a physically realistic solution, implying that aggregation does not result in the formation of rodlike structures.

tion of the molecular weight of a single TiO2 particle (bead) should be corrected for the volume occupied by the solvent. Using eq 5, the molecular mass of a particle was calculated to be (7.63 ( 0.4) × 104 g mol-1, which corresponds to an agglomeration number of n ) 955 and consequently to a particle diameter of 40 ( 2 Å. Although there is a slight underestimation of the particle diameter, this can be explained by difficulties in determining the partial specific volume (eq 4) of TiO2 particles which are covered with the layer of surface modifier. This result confirms our conclusion that the formation of rods is a consequence of the assembling of 43 Å particles into beadlike structures. TEM images confirm the existence of rodlike structures

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Figure 9. Dependence of dipole moment (D, obtained from singlepoint semiempirical ZINDO calculations), stability constants (K), and dipole moment per surface area (µ) of surface modified TiO2 particles on the modifier used in this work.

with a diameter of 42.8 ( 3.2 Å. Although investigation of the samples by TEM involves a change in the sample environment by solvent exclusion, we were able to record rodlike particles assembled on grid areas with low material loadings (Figure 5). The areas with high loading were reminiscent of nanoparticulate films,11b but residual rod-shape history could be resolved with high magnification. To understand the structural components that are responsible for the formation of self-assembled rods, we altered the functional groups of the surface modifier, their relative positions, as well as their hydrophobic character. The change of the optical absorption upon binding of several modifiers is shown in Figure 6. The colloidal solutions were optically clear. The optical properties of TiO2 colloids were found to change when an R-mercapto carboxylic acid was used as a surface modifier. The role of the mercapto group of thiolactic acid in assembling the nanoparticles was probed by using S-methyl mercapto acetic acid as the modifier in which the mercapto group is blocked with a methyl group. In this way the surface modifier could

Rajh et al. bind surface Ti atoms only through the carboxyl group. Modified Guinier analysis of the SANS data of TiO2 colloids in the presence of thiolactic acid (TLA), S-methyl mercapto acetic acid (S-MAA), and mercaptopropionic acid (MPA) are shown in Figure 7. While all systems indicate the presence of rodlike particles with the diameter 43 ( 2 Å, the length of the rods is smaller in the case of TiO2 colloidal solutions modified with MPA and S-MAA than in the case of TLA-modified TiO2 colloidal solution. This is deduced from the bend over of the curves in the low q2 region of data of both S-MAA- and MPAmodified TiO2 particles. Surface modification with a modifier that has an alkane unit between the carboxyl and mercapto groups, β-mercaptocarboxylic acid, did not lead to the assembling of TiO2 particles into long rods. In this case, bridging of the particles with surface modifiers would occur with the same probability as in TLAmodified TiO2 colloids, while bidentate square-planar octahedral chelation of surface titanium atoms does not occur.19 These results indicate that rod formation is not the consequence of bridging two particles by mercaptocarboxylic acid. However, the results suggest that rod formation is the consequence of bidentate binding of R-mercapto-substituted thiolactic acid and subsequent assembling of the particles through the appending alkyl group. To understand the nonspecific binding of these nanoparticles and the mechanism of assembling in the presence of thiolactic acid, we have examined two additional compounds from the series of R-mercapto-substituted alkyl carboxylic acids: mercapto acetic and mercapto butyric acids. The scattering profiles of mercapto butyric acid (CH2CH3 group in the R position) modified nanoparticles results in the formation of shorter rods compared to thiolactic acid modified colloids with the diameter of the TiO2 particle, while surface modification with mercapto acetic acid (absence of alkane group in R position) did not lead to the formation of rodlike particles (Figure 8). Consequently, the self-organization of TiO2 particles in rodlike structures cannot be explained by hydrophobic interactions only, because the assembly is not improved with a prolonged alkyl chain. Bidentate binding of R-mercapto-substituted alkyl carboxylic acids

SCHEME 1. Topographic Presentation of Isopotential Lines in Thiolactic Acid Modified 40 Å TiO2 Colloidsa

a The potential was obtained by electrostatic potential calculations assuming a dipole moment obtained from single-point semiempirical ZINDO calculations for TiO4H4/TLA complex. (a) Potential redistribution when an additional particle approaches the dimer on a side. Two nonjoining particles experience repulsion due to the existence of positive charges at the surface modifier. This repulsion shifts the end particles and straightens the Angle to the linear configuration of three particles. (b) Potential distribution when an additional particle is placed in the linear stringlike structure.

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to surface Ti atoms, however, is accompanied with conformation adjustments of the mercapto-carboxylic acids that produce significant dipole moment changes. Single-point ZINDO calculations suggest a change of the dipole moment from 1.5 D for unbound thiolactic acid to 11.2 D for bidentate bound thiolactate to surface Ti atoms. Using computational methods, the most extensive change of the dipole moment in the investigated series was found for mercapto-acetic acid (Figure 9). The dipole moment per surface area (µ) was determined from the ratio

µ)

Nad D NTi S

(6)

where Nad is the number of adsorbed molecules of surface modifier and is equal to Nad ) (CMCACparK)/(Cpar + CMCAK), CMCA is the concentration of adsorbed mercapto carboxylic acid, Cpar is particulate concentration of TiO2 particles, K is the stability constant of the complex, NTi is the number of surface titanium atoms (NTi ) R/12.5, R is radius of the particle), D is the dipole moment of the complex, and S is the average surface area of each TiO2 molecule (S ) 15.7 Å2). While the dipole moment decreases in the series, the stability constants of the surface modifiers with surface Ti atoms was found to increase (the stability constants were determined from Benesi-Hilldebrandt plots12). Thus, the dipole moment per surface area is the largest for bidentate bound thiolactic acid (µ ) 0.486 D/Å2). The effects associated with dipole moment of the surface complex should be most pronounced in the case of surface modification with thiolactic acid. This is consistent with our measurements. Because of the fact that the dielectric constant of TiO2 is only 2 times larger than the dielectric constant of water, we were able to test the proposed mechanism by using a simple electrostatic calculation based on Coulomb interactions in TLAmodified TiO2 (Scheme 1). For simplicity, only four perpendicular vectors were taken into consideration, and the results of the calculation should be taken as qualitative. Scheme 1(a) presents our simple modeling approach. A particle of TiO2 was modified with four molecules of thiolactic acid, and each molecule generates the electrical field due to the presence of a significant dipole moment. The positive charge (δ+) is placed on a donating surface modifier, while the negative charge (δ-) is placed at surface Ti atoms. Collision of two particles results in the formation of dimers because of the attraction of surface appendant CH3 groups in order to minimize their exposure to the polar water molecules. Once the dimers are formed, there is an asymmetry of the electrostatic field on the elongated particles and points with the smallest density of positive charges are established at the end poles of a particle. As a consequence, these poles appear different from the rest of the particle and become more probable points for additional assembling of particles. If an additional particle approaches the side of the dimer, the repulsive forces are established between the nonadjoining particles. This repulsion will push the particles to the point of the weakest repulsive interaction at the poles of the assembled particles. The effect is repetitive, and the particles assemble into the chain of beadlike (rod-shaped) structures. Surface complexation of colloidal titanium dioxide nanoparticles with these R-mercapto-substituted alkyl carboxylic acids also results in a red-shifted optical absorption (Figure 6). However, the threshold of absorption differs between surface modifiers, 460 nm for mercapto-acetic and mercapto-butyric acid and 520 nm for thiolactic acid. This trend of the change of the absorption spectra follows the change of the dipole moment

per surface area determined for the investigated R-mercaptosubstituted alkyl carboxylic acids. However, the optical properties do not change significantly when the pH is changed from 3.5 to 2 while the length of the chain decreases noticeably. These results indicate that formation of the rods and the change of the optical properties are both consequences of the change of the dipole moment upon binding at the surface, and the change of the optical properties are not a consequence of rod formation. To conclude, we have observed the self-organization of TiO2 nanoparticles into rodlike assemblies when they are modified with thiolactic acid. Comparison of the diameters of the particles and the rods indicate that the particles form beadlike structures with only one particle per cross section. Further experiments are needed to determine possible extended coupling along the rodlike structures. Acknowledgment. This work benefited from the use of the Intense Pulsed Neutron Source and Electron Microscopy Center for Materials Research and was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences under Contract No. W-31-109-Eng-38. We acknowledge help provided by Denis G. Wozniak and ChwenYuan Ku at IPNS and Jim Brener for obtaining initial HREM images. The authors thank Joseph Gregar for his inventive design of glassware. References and Notes (1) Tholen, A. R. Phylos. Magn. A 1986, 53, 259. (2) Bentzon, M. D.; van Wonterghem, J.; Morup, S.; Tholen, A.; Koch, C. J.; Phylos. Magn. B. 1989, 60, 169. (3) Murrey, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. Nozik A. J.; Micic, O. I. MRS Bull. 1998, 23, 24. Micic, O. I.; Jones, K. M.; Cahill, A.; Nozik, A. J. J. Phys. Chem. B 1998. Trau, M.; Saville, D. A.; Akasay I. A. Science 1996, 272, 706. Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezimar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904. (4) Siegal, R. W. Phys. Today 1994, 64, 64. (5) Arias, F.; Godinez, L. A.; Wilson, S. R.; Kaifer, A. E.; Echegoyen, L. J. Am. Chem. Soc. 1996, 118, 6086. (6) Golden, J. H.; DiSalvo, F. J.; Frechet, J. M. J.; Silcox, J.; Thomas, M.; Elman, J. Science 1996, 273, 782. (7) Chan, K. C.; Kim, T.; Schoer, J. K.; Crooks, R. M. J. Am. Chem. Soc. 1995, 117, 5875. (8) Dhirani, A. A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319. (9) Kagan, C. R.; Murray, C. B.; Nirmal, M.; Bawendi, M. G. Phys. ReV. Lett. 1996, 76, 1517. (10) Kemp, M.; Roitberg, A.; Mujica, V.; Wanta, T.; Ratner, M. A. J. Phys. Chem. 1996, 100, 8349. (11) (a) Zhu, Z.; Tsung, L. Y.; Tomkiewicz, M. J. Phys. Chem. 1995, 99, 15945. (b) Zhu, Z.; Lin, M.; Dagan, G.; Tomkiewicz, M. J. Phys. Chem. 1995, 99, 15950. (12) (a) Rajh, T.; Tiede, D. M.; Thurnauer, M. C. J. Noncryst. Solids 1996, 207, 815. (b) Rajh, T.; Tiede, D. M.; Thurnauer, M. C. Acta Scand. 1997, 51, 610. (13) (A)Thiyagarajan, P.; Epperson, J. E.; Crawford, R. K.; Carpenter, J. M.; Klippert, T. E.; Wozniak, D. G. J. Appl. Crystallogr. 1997, 30, 280. (b) Thiyagarajan, P.; Urban, V.; Littrekk, K.; Ku, C.; Wozniak, D.; Hammonds, J.; Carpenter, J. M.; Crawford, R. K. Proc. of the 14th International Collaboration on AdVanced Neutron Sources, Utica, Illinois, June 14-19, 1998. (14) Tiede, D. M.; Thiyagarajan, P. In Biophysical Techniques in Photosynthesis; Kluwer Academic Publishers: Berlin, 1995; p 375. (15) Guinier, A.; Fournet, G. Small Angle Scattering; Wiley: New York, 1955. (16) Thiyagarajan, P.; Tiede, D. M. J. Phys. Chem. 1994, 98, 10343. (17) (a) Kratky, O.; Pilz, I. Q. ReV. Biophys. 1978, 11, 39. (b) Hjelm, R. P. J. Appl. Crystallogr. 1985, 18, 452. (18) (a) Jacrot, B.; Zaccai, G. Biopolymers 1981, 20, 2413. (b) Jacrot, B.; Zaccai, G. Mol. Biol. 1981, 151, 329. (c) Zaccai, G.; Jacrot, B. Annu. ReV. Biophys. Bioenerg. 1983, 12, 139. (d) Worcester, D. L.; Michalski, T. J.; Katz, J. J. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 3791. (19) Rajh, T.; Thurnauer, M. C. to be published. (20) Pilz, O. In Small-Angle X-ray Scattering; Glatter, O., Kratky, O., Eds.; Associated Press: 1982; p 264.