J. Phys. Chem. B 1998, 102, 6169-6176
6169
Thiol-Derivatized AgI Nanoparticles: Synthesis, Characterization, and Optical Properties Sihai Chen, Takashi Ida, and Keisaku Kimura* Department of Material Science, Faculty of Science, Himeji Institute of Technology, Harima Science Garden City, Kamigori, Hyogo 678-1297, Japan ReceiVed: February 4, 1998; In Final Form: April 6, 1998
Thiol-passivated AgI nanoparticles of 7-15 nm were synthesized in large quantities by changing the initial molar ratio of iodide to 1-thioglycerol, and thus their competitive reaction rates with silver ions was adjusted, leading to a precise control of the final particle size. Powder X-ray diffraction and transmission electron microscopy showed that these particles crystallized as a mixture of β- and γ-AgI, with the content of the γ-phase increasing from 26% for 7 nm to 60% for 15 nm. Elemental analysis, thermogravimetric analysis, and Fourier transform infrared spectroscopy indicated that thiolate anions are attached to the AgI surface through mercapto groups, and furthermore, the silver thiolate molecular complex exists as a result of the competitive reactions. The percentage of the initial thiolate used for capping AgI particles increased linearly with the increase in [I-]/[1-thioglycerol], showing that almost pure thiol-covered AgI particles of 15 nm can be obtained. Optical spectroscopic studies in diluted solution confirmed the above competitive mechanism of AgI formation. The smallest AgI particles stabilized by 1-thioglycerol and surviving several days in water, observed in a solution of 3.33 × 10-4 M Ag+, 6.67 × 10-4 M I-, and 6.67 × 10-3 M 1-thioglycerol at room temperature, had an absorption peak at 331 nm.
Introduction Solid electrolytes such as AgI, LiI, and CuCl characterized by their high ionic conductivity due to their unique transport properties have attracted much attention because of the possibilities for applications to solid-state battery and chemical sensing systems. Recently, we have found that the electric conductivity of a polycrystalline pellet of AgI particles increases gradually when the particle size decreases from the bulk to 140 nm.1,2 This result suggests the possibility that further decrease in the particle size to several or several tens of nanometers may lead to a much larger increase in the electric conductivity of AgI. Along this line, large-scale synthesis of nanometer-sized powder of AgI which can be used for ionic conductivity measurement is strongly required. Many synthetic routes have been developed for the preparation of AgI nanoparticles. Berry originally prepared AgI particles as small as 15 nm in the presence of gelatin and studied their structure and optical absorption.3 Meisel et al. examined the growth mechanism of AgI from the molecule to larger aggregates through a transient photochemical method, where iodide ions are generated by pulse radiolysis of methylene iodide in aqueous solution and are then used to generate AgI.4 Tanaka et al. have prepared AgI hydrosols which are stabilized by organic polymers such as poly(vinyl alcohol) (PVA), poly(acrylamide) (PAM), poly(ethylene glycol) (PEG), and cationic tetramethylammonium perchlorate (TMA).5 Henglein et al. have studied the optical properties of AgI nanoparticles prepared in the presence of polymers such as PVA, PEG, and poly(Nvinyl)pyrolidone (PVP).6 The photophysical properties of charge carriers trapped in AgI nanoparticles have been studied by pulse radiolysis with AgI particles prepared through controlled precipitation in acetonitrile using polybrene (1,5-dimethyl-1,5-diazaundecamethylene polymethobromide) as a stabilizing agent.7 In all these methods, AgI nanoparticles are
prepared in dilute solution (18.0 MΩ cm) was obtained from Advantec GS-200 aquarius automatic water distillation supplier. Preparation of the AgI Nanoparticles. The standard method has been reported in the communication.19 Briefly, three fresh 0.2 M aqueous stock solutions of AgNO3, KI, and RSH were at first prepared, AgNO3 solution was rapidly added into the iodide-RSH mixture under vigorous stirring, and the final total 40 mL solution was sealed into a cellulose tube (2.4 nm pore size) and dialyzed against flowing water for one day in the dark. The precipitate was finally freeze-dried and stored in the dark. The preparation conditions for samples in this study were shown in Table 1. X-ray powder diffraction was performed on a Rigaku RINT/ DMAX-2000 diffractometer with Cu KR irradiation with the X-ray generator operating at 40 kV and 20 mA. Divergence and scattering slits of 1° and a 0.15 mm receiving slit were used. The scan speed and scan step were set at 2°/min and 0.01°, respectively. The sample was spread in a standard cavity mount. The particle size was estimated by Scherrer’s equation,20 L ) 0.89λ/B(2θ) cos θ, where L is the coherence length or the particle diameter, λ is the wavelength of the X-ray irradiation, B(2θ) is the full width in radians subtended by the halfmaximum width of the powder pattern peak, and θ is the reflection angle of the peak. To correct for the “instrumental broadening” effect, a bulk granular AgI sample (14-24 mesh, 99%, Nacalai tesque) was used as the reference. The structural compositions of some samples were obtained by the simulation of the XRD patterns with a “Rietveld method” using the program RIETAN-94 developed by Izumi;21 it was assumed that the size of β-AgI (hexagonal wurtzite structure) particles was equal to that of γ-AgI (cubic sphalerite structure).
Transmission Electron Microscopy and Electron Diffraction. The particles of samples I-VI were studied with a Hitachi-8100 transmission electron microscope (TEM), operated at 200 kV. The sample was prepared by dropping water-diluted AgI suspension on an amorphous carbon-coated copper mesh and allowing it to dry in air. The coverage level of colloids was adjusted by varying the initial dispersion concentration or spraying the solution on the copper mesh with a nebulizer. The particle size analysis was conducted by treating the digitized image with a NIH Image 1.55 software package; at least 150 particles were averaged. The high-resolution digitized images were recorded immediately through a CCD camera in order to avoid the possible damage of the AgI particles under strong electron beam irradiation. The images with an atomic resolution were observed with a direct magnification of 1000000 × and a final magnification of 8350000 × calibrated from the highresolution image of a Au particle sample. The lattice spacing values calculated from the electron diffraction pattern were calibrated with reference to the standard electron diffraction pattern of a Au particulate film taken at the same measurement condition. FT-IR spectra of AgI were measured with a HORIBA FT210 infrared spectrophotometer using a 150 mg KBr disc dispersed with the AgI powder at a fixed weight ratio of 0.4%. The spectrum of RSH was measured by placing a drop of RSH liquid on a pure KBr disc. Elemental analyses of C, H, O, S, Ag, and I were conducted commercially at chemical microanalytical lab of the Institute of Physical and Chemical Research, Japan. Elemental analyses of Ag, I, and S for the residues of AgI samples after heating were carried out with a Kevex EDAX analyzer attached to the Hitachi-8100 TEM system. Thermogravimetric analyses were performed on an allcomputerized TG-DTA 2000S system (MAC Sci. Co.) at a heating rate of 10 °C/min in the temperature range 20-420 °C under a nitrogen flow of 40 mL/min. The sampling time was 4 s. Samples of 8-10 mg were analyzed in a standard aluminum pan. An aluminum pan containing 10 mg of exactly weighed Al2O3 powder was set as the reference. UV-vis absorption spectra of all the samples were recorded on a Hitachi U-3210 spectrophotometer. In the case of samples I-VI, an integral sphere (60-mm inner diameter) accessory equipment was used, and the original AgI solutions were diluted 200 times with water before measurement. Cuvettes with 1 cm optical length were used in all cases. Results and Discussion Particle Sizes and Structural Analyses. Six AgI samples (Table 1, samples I-VI) were studied systematically. After preparation the solutions of these samples were turbid and displayed different colors depending on the [I-]/[RSH] ratios. Sample I was white-gray, whereas sample VI showed a yellow color. These solutions were very stable in the dark since no color change was observed even after more than six months. The absorption spectra of these solutions showed typical features of AgI with a peak or shoulder around 422 nm (Figure 1). The exact absorption thresholds, labeled Eg, of the absorption spectra were determined from the intercept of (σhν)2 versus hν plots (where σ is the absorption coefficient; see Figure 1, inset) for the direct band gap AgI and was found to be 2.82-2.86 eV. Applying the relationship between Eg and particle size of AgI calculated by Meisel et al.,4a,b we found that these particles were 6-7 nm. We note that there was only a shoulder in the
Thiol-Derivatized AgI Nanoparticles
Figure 1. Absorption spectra of samples I-VI in Table 1. Inset shows that the band gap of the sample V is 2.86 eV.
Figure 2. X-ray diffraction patterns of AgI powder of samples I-VI in Table 1. g and h show the position and relative intensity of the standard XRD pattern of β- and γ-AgI, respectively.
absorption spectra for samples I-III; Berry has reported a similar observation for AgI crystallites with a mean diameter of 15 nm.3 Since “quantum confinement effect” is not apparent for AgI particles whose sizes are larger than 5 nm,4a XRD analysis was further applied to determine the particle sizes. Figure 2 shows the X-ray diffraction patterns for samples I-VI. Compared to that of the bulk AgI, three broadened diffraction peaks at 2θ ) 23.7°, 39.3°, and 46.4°, which can be ascribed to either β- or γ-AgI (see Figure 2g,h), were observed. As the application of Scherrer’s equation required an isolated Bragg peak and a welldefined baseline,20 the strong peak at 2θ ) 23.7° was not chosen because it may comprise two (γ-) or three (β-) Bragg peaks.
J. Phys. Chem. B, Vol. 102, No. 32, 1998 6171 Alternatively, the peak at 39.3° [β-(110)] was selected, and the diameters of these samples were calculated to be 7-15 nm (Table 1). The relative composition of β- and γ-phases in the AgI powder was obtained by studying the relative intensities of the three peaks at 2θ ) 22.4°, 23.7°, and 25.5°. In the case of pure wurtzite structure, the intensity of the center line of the above triplet is weaker than that of the other two outside lines (Figure 2g). However, the inverse trend was observed in all the measured AgI samples, indicating that these samples should be the mixture of β- and γ-AgI. The stronger the intensity of the middle line is, the more the content of γ-AgI. According to the theoretical dependence of the X-ray diffraction peak intensity ratio, R, on the content of hexagonal and cubic AgI (Figure 1 in ref 3, where R is the relative ratio of intensity of the center line of the diffraction triplet to the sum of intensities of the two outside lines, [I(111)γ + I(002)β]/[I(100)β + I(101)β]), the relative percentage of β- and γ-AgI in the powder samples was deduced (Table 1). The content of the γ-phase was found to increase when the [I-]/[RSH] ratio (or the particle size) increased. This result was also confirmed by the data obtained from the simulation of the XRD pattern with the Rietveld method (see Table 1). Since γ-phase is metastable at room temperature,22 its appearance should be encouraged under nonequilibrium conditions such as the rapid mixing in the present study (kinetical control). This was shown in the case of sample VII in Table 1: when Ag+ was mixed with I- ions with 1:1 ratio in the absence of RSH, the highest content of the γ-phase of 68% was obtained. At the same time, increase in the relative amount of RSH from sample VII to I results in an increase in the relative ratio of the more stable hexagonal component. This strong role of thiolates for controlling the structure of semiconductor nanoparticles has been reported for CdS;13 hexagonal structured particles are obtained when the synthesis is conducted in the presence of thiolates, whereas sodium polyphosphate results in particles of cubic structure. This is similar to the present results showing that thiolates facilitate the formation of thermodynamically stable hexagonal nanoparticles. TEM was further used to determine the size and structure of AgI. For example, the size of sample VI in Table 1 was found to be 16 ( 9 nm, which is consistent with that of the XRD measurements (15 nm). The particles were normally spherical (see Figure 3); this morphology is different from that of AgI prepared by just mixing the AgNO3 and KI in diluted solutions.5b The latter often shows the shape of regular triangles with rounded corners or irregular polygons whose formation can be explained by the coalescence and recrystallization of smaller particles through “Ostwald ripening” processes. This difference can be ascribed to strong adsorption of RSH on the AgI particle surface, which inhibited the anisotropic growth of the special crystal plane of the particles. In spite of the wide size distribution caused by aggregation and coalescence during mixing, the mean particle size was still controlled by the [I-]/[RSH] ratio even when the total concentration of Ag+ or I- reached 0.1 M. This demonstrated the strong ability of the thiolates to control the particle size of AgI. High-resolution images showed that the most frequently observed lattice fringes in samples I-VI were 0.375 ((5) nm, which can be indexed to the lattice spacing of the β-(002) or γ-(111) plane of AgI. The lattice spacings at 0.231 ((3) and 0.397 ((5) nm were also detected, the former belonging to the β-(110) or γ-(220) plane of AgI, the latter to the β-(100) plane of AgI. Figure 3A shows two particles from sample II with
6172 J. Phys. Chem. B, Vol. 102, No. 32, 1998
Figure 3. Electron micrograph and electron diffraction pattern of (A) sample II; the mean diameter, lattice fringe of particles X and Y were 7.7, 0.374 and 7.1, 0.397 nm, respectively; (B) sample V; the 15.4 nm particle was viewed along the [001] zone axes with the {100} lattice spacing of 0.397 nm.
lattice spacings of 0.374 and 0.397 nm, respectively. Another example showing one particle from sample V with the {100} spacing of 0.397 nm of hexagonal AgI is shown in Figure 3B. Comparing the electron diffraction pattern of sample V (Figure 3B) with that of sample II (Figure 3A), the former contained much stronger rings and more distinct spots arranged in the rings, showing that sample V consisted of larger microcrystals. All the rings could be indexed to the known βand γ-AgI forms. The images of atomic resolution of the crystallite frequently showed the standard hexagonal pattern of the β-(001) plane of AgI, especially for samples V and VI. In some cases, some deviations of the interplanar angles and lattice spacings from the bulk values were also noticed. An example was shown for sample VI in Figure 4A. The particle, which was viewed along the [001] axis, had lattice spacings of d100 ) 0.396 nm, d010 ) 0.403 nm, and d1-10 ) 0.377 nm. The angles between the identified planes were 60°, 65°, and 55°. A similar situation was also met from a particle of sample II (not shown), whose parameters were d100 ) 0.408 nm, d010 ) 0.389 nm, and d1-10 ) 0.410 nm. The angles between the planes were 64°, 59°, and 57°; they also showed some distortions. There is a possibility that distortion may be observed when we viewed the samples not from the exact normal direction of the (001) plane of AgI samples. But this is not the case here because the lattice spaces such as d010 in the formal case and d100 and d1-10 in the latter case are larger than the standard value of 0.396 nm. More likely, these distortions belong to the intrinsic structural distortions of AgI. An example of the face-centered
Chen et al.
Figure 4. High-resolution electron micrographs and power spectra of sample VI: (A) hexagonal structure in the [001] orientation with distortions d100, d010, d1-10 ) 0.396, 0.377, 0.403 nm; (B) cubic structure close to the [110] orientation with distortions d111 ) 0.407 nm, d-111 ) 0.394 nm, and d200 ) 0.332 nm.
cubic structure in the [110] orientation was shown in Figure 4B. The corresponding lattice spacings were d111 ) 0.407 nm, d-111 ) 0.394 nm, and d200 ) 0.332 nm. The angles between these planes were 73°, 54°, and 53°. The lattice constants calculated from the above lattice spacing values were 0.664, 0.643, and 0.665 nm, respectively. These constants were close to the standard constant of cubic AgI (0.649 nm) but still showed a certain amount of distortions. The reason for the above distortion may be due to the strong interaction between Ag+ and thiolate, as well as the rapid crystallization under vigorous mixing. Interestingly, in a recent study, distortions of the structural factors from the bulk values were also found for thiolstabilized CdS nanoparticles.23 Composition Analyses. In addition to the size and structure analysis of AgI particles, the composition of the nanoparticles was studied by elemental analysis, and the result is shown in Table 2. The summation of all the detected elements for samples I-VI was near 100%, indicating that the other elements did not exist in a significant amount. Transformation of the mass ratio into atomic ratio gave an empirical formula that was approximately equal to the apparent formula calculated from the initial addition of AgNO3, KI, and RSH within experimental error. The general formula of the particle was found to be AgIx(SC3H7O2)1-x (x ) 0.4-0.9 for samples I-VI). This showed that the yields of these products were practically quantitative with respect to the reactants. Note that the formula of the organic phase is SC3H7O2, not SC3H8O2 of RSH; that is, one hydrogen atom in the RSH molecule was lost during the reaction. FT-IR spectra of all the samples were similar to that of RSH except for the absence of the S-H vibration peak at 2558 cm-1. The broad O-H vibration peak at 3370 cm-1
Thiol-Derivatized AgI Nanoparticles
J. Phys. Chem. B, Vol. 102, No. 32, 1998 6173
TABLE 2: Composition and Empirical Formula of AgI Powder, as Well as Theoretical Formula for a Particle of Structure A or B in Figure 5 elemental analysis mass % sample I II III IV V VI
structure A
structure B
Ag/I/C/H/O/S
∑
empirical formula
theoretical formulaa
∆Ag:∆RSb
theoretical formulaa
∆Ag:∆RSb
45.0/23.0/9.4/1.8/9.2/8.9 45.0/27.9/7.7/1.5/7.8/5.9 45.6/33.2/6.3/1.3/6.2/5.8 45.6/38.9/4.5/0.9/4.5/3.4 46.0/43.6/3.1/0.5/3.1/2.5 46.7/48.5/1.5/0.3/1.5/0.9
97.3 95.8 98.4 97.8 98.8 99.4
AgI0.43(S1.0C3.0H7.0O2.2)0.62 AgI0.52(S0.9C3.0H7.1O2.3)0.51 AgI0.62(S1.0C3.0H7.3O2.2)0.42 AgI0.72(S0.8C3.0H7.1O2.2)0.30 AgI0.81(S1.0C3.0H6.2O2.2)0.20 AgI0.88(S0.7C3.0H7.2O2.2)0.10
Ag0.50I0.43(RS)0.07 Ag0.58I0.52(RS)0.07 Ag0.68I0.62(RS)0.07 Ag0.78I0.72(RS)0.07 Ag0.87I0.81(RS)0.07 Ag0.94I0.88(RS)0.07
50:55 42:45 32:36 22:24 13:14 6:4
Ag0.49I0.43(RS)0.13 Ag0.58I0.52(RS)0.11 Ag0.68I0.62(RS)0.12 Ag0.78I0.72(RS)0.12 Ag0.87I0.81(RS)0.12 Ag0.95I0.88(RS)0.12
51:49 42:40 32:30 22:18 13:8 5:-2
a Iodide content was kept the same in the empirical and theoretical formulas. b ∆Ag and ∆RS represent the residual amount (in %) of Ag and RS after subtraction of the theoretical formula from the impirical one, respectively.
k1
Ag+ + RSH 98 RSAg + H+
Figure 5. Models for the surface structures of thiol-modified AgI particles. The shadow represents the inner part of particles. The solid and dashed lines show the surface and adsorbed ions, respectively.
remains. This showed that the thiolate anion was part of the nanocomposites and combined with AgI particles through the mercapto group, not the O-H group. The content of organics in the powder obtained from elemental analysis seems too high if supposing that the powder solely comprises particles covered with a monolayer of organics, especially in the case of smaller particles. In the following we will give a detailed analysis on this point. For samples I-VI, since they are all synthesized with Ag+ in excess, two different surface structures of thiol-modified AgI particles are assumed and are shown in Figure 5. Structure A supposes that all the Ag+ sites on the AgI particle surface are occupied by adsorbed thiolates and all the surface I- sites are covered by Ag+ ions to compensate the excess negative charge caused by the adsorbed RS- ions. Structure B shows the largest coverage of thiolates on the AgI surface by the fact that all the Ag+ sites in structure A are bound to RS- ions. The theoretical composition ratio of Ag+, I-, and RS- ions is calculated for structures A and B by assuming that the Ag+ and I- ions are evenly spread on the AgI surface. With the known ionic radius of Ag+ (1.15 Å) and I- (2.20 Å) and the density of bulk AgI (5.68 g/cm3), the number ratios of Ag+/ I-/RS- for structures A and B are calculated to be (0.61R+6.5)/ 0.61R/6.5 and (0.61R+6.5)/0.61R/13, respectively (R is the radius (Å) of the AgI particle). As shown in Table 2 for samples I-VI, the relative amounts of Ag+ and RS- in the empirical formula were both higher than that in the theoretical formula of thiol-covered AgI particles with structures A and B. The molar ratio of the excess amount of Ag+ (defined as ∆Ag) to that of RS- ions (defined as ∆RS) are close to 1 in all the samples; this suggests that the RSAg complex which resulted from the reaction
(1)
should exist in addition to the thiol-covered AgI particles in the powder. The RSAg complex was a side product and was found to be amorphous by XRD measurement. It cannot be removed in the present procedure because of its small Ksp value in water.24 In the method to synthesize CdS nanoparticles, cadmium thiolate complex has been successfully separated from CdS nanoparticles since it is soluble in high pH ()11) solution.13 Here, RSAg was also found to be soluble in high pH solution, but another problem appeared because Ag2O was irreversibly formed. Nevertheless, the pH of the solutions must be considered in further improving the preparation method and removing the undesired RSAg complex. The growth process of ionic sparingly soluble salts is commonly believed to be a combination of four stages: formation of molecules and complexes, formation and growth of nuclei, aggregation and, ripening.4b In the case of AgI, the reaction can be written as k2
Ag+ + I- 98 AgI k3
hAgI 98 {AgI}h (h ) i, j) k4
{AgI}i + {AgI}j 98 {AgI}i+j
(2) (3) (4)
The overall reaction is k5
mAg+ + mI- 98 {AgI}m
(5)
The formation of thiol-modified AgI particles can be regarded as a consecutive reaction: the first step is reaction 5; the second step is k6
{AgI}m + nAg+ + lRSH 98 (RS)l(Ag+)n{AgI}m + lH+ (6) where RSH acts as a terminating agent and is adsorbed on the particle surface. Since RSH at the same time reacts with Ag+ to form the RSAg complex through reaction 1, reaction 6 is also a parallel reaction with reactions 1. Thus, the formation of the RSAg complex cannot be avoided in the present reaction mode. A way to decrease the content of RSAg complex is to increase the rate of reactions 5 and 6 while decreasing that of the reaction 1. This can be achieved by increasing [I-]/[RSH]. As shown in Figure 6, a linear relationship was found between the percentage of RSH used for AgI capping. Almost pure thiolcovered AgI nanoparticles with a particle size of 15 nm were formed when the [I-]/[RSH] ratio was 9 (see Table 1). Although the increase in the initial concentration of RSH will lower the purity of particles, it is important for controlling the
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Chen et al. TABLE 3: Comparison of Mass Loss from TG of AgI Samples with Total Organics Content and That of the Sum of C + H + O from the Elemental Analysis mass loss (%) total organics (%) C + H + O (%)
I
II
III
IV
V
VI
21.78 29.30 20.40
18.41 22.90 17.00
14.63 19.60 13.80
11.33 13.30 9.90
7.30 9.20 6.70
4.23 4.20 3.30
Figure 6. Relationship between the percentage of initial RSH used for AgI capping (P) and [I-]/[RSH]. P is calculated from Table 2.
Figure 7. TGA and DTA result of sample I: (a) mass loss curve; (b) derivative curve of (a); (c) differential thermal analysis curve.
particle size. Compare the particles prepared as samples VIIIXI in Table 1 to that as samples III-VI, respectively; the sizes of the former were smaller than the corresponding samples in the latter just by increasing the initial RSH concentration while keeping [I-]/[Ag+] unchanged. To confirm the stoichiometry derived from the elemental analysis and study the thermostability of the thiol-stabilized AgI particles, thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed. Figure 7 shows the data for sample I. A mass loss of 22% was found after heating the sample from 210 °C to 350 °C (curve a). The derivative curve of mass loss (curve b) suggests that at least four steps were included during the decomposition. This conclusion was further confirmed by the DTA data shown in Figure 7, curve c, where four endothermic peaks were observed. For all the AgI samples the TGA and DTA curves were similar. It is noted that the relative ratios of mass loss in these four steps were almost identical for different samples, suggesting that the four decomposition steps result from the bond break in the organics. Otherwise, if the bond cleavage for Ag-SR or AgI-SR occurred, owing to the changed relative amount of Ag-SR and AgI-SR in different samples as shown in Table 2, the relative ratio of mass loss in the four steps should change. After heating, the sample turned black and bulklike. EDAX showed that the residues contained S in addition to a large amount of Ag and I. X-ray diffraction analysis gave the main diffraction peaks of AgI, indicating that the residues were mainly AgI. Total mass losses of samples I-VI obtained from TGA lie between the total organics content and total mass amount of C + H + O in the samples from the elemental analysis (Table 3). This result supported that heating mainly resulted in the decomposition of organics in the powders. Optical Spectra in Diluted Solution. To clearly understand the formation mechanism of AgI, the reactions conducted in
Figure 8. (A) Absorption spectra of AgI nanoparticles prepared after one day of aging with 3.33 × 10-4 M Ag+ and 6.67 × 10-4 M I- in the presence of RSH. The concentrations of RSH were (a) 0 M, (b) 1.67 × 10-3 M, (c) 3.33 × 10-3 M, (d) 6.67 × 10-3 M, (e) 1.00 × 10-2 M, (f) 1.33 × 10-2 M, and (g) 1.67 × 10-2 M. For comparison, curve i represents the absorption spectrum of a freshly prepared solution containing 3.33 × 10-4 M Ag+ and 1.67 × 10-2 M RSH. The inset shows a double logarithmic plot of mean diameter of AgI particles vs concentration of RSH for curves b, c, and d. (B) Time course of the absorption spectra of condition (d).
diluted solution were studied through UV-vis spectroscopy. In the following experiments, silver nitrate solution was injected into the iodide-RSH mixture under vigorous stirring. Figure 8A shows the absorption spectra of the one day aged solution containing 3.33 × 10-4 M Ag+, 6.67 × 10-4 M I-, and RSH. When the concentration of RSH increased from 0 to 6.67 × 10-3 M (curves a-d), the absorption spectra of AgI particles blue-shifted because of the “quantum confinement effect”. The relationship between the particle diameter, d (Å), which was obtained from the onset of the absorption spectra,4a and the concentration of RSH, c (M), was plotted in the inset; the double logarithmic plot of d versus c showed a straight line which obeyed the relationship
log d ) log a - b log c
(7)
with a ) 4.7 and b ) 0.36. The particle size decreased with
Thiol-Derivatized AgI Nanoparticles the 0.36th power of the RSH concentration. The aggregation number is proportional to the cube of the size; hence, the number of AgI ion pairs decreased with the 1.08th power of RSH concentration. This shows that RSH is more efficient in terminating the particle growth of AgI than that in the case of CdS. In the latter, when CdS was stabilized with thiols in THF solvent, the aggregation number decreased with the 0.36-0.42th power of the concentration of a size controller.14c This may be due to the stronger interaction between Ag+ and thiol in water than that between Cd2+ and thiol in THF. The same relationship (eq 7) in the above two cases may be due to the same ionic nature of the above two kinds of semiconductor and same reaction modules of thiols with them. An extension of this finding assumes that thiolates may be used for the synthesis of other I-VII ionic nanoparticles. Interestingly, when the concentration of RSH was increased from 6.67 × 10-3 M to 1.67 × 10-2 M (Figure 8A, curves d-g), the onsets of the absorption spectra did not blue-shift but did red-shift somewhat. This is a significant phenomenon which has not been observed before. In the case of curve g, since the RSAg complex should exist as we have discussed above, the true absorption spectrum of AgI can be expressed as a difference spectrum between curve g and i. This difference spectrum (not shown) can be separated as two Gaussian-shaped curves: one had an absorption peak at 331 nm, the other at 374 nm which are attributed to the larger aggregates of AgI. The absorption band at 320-330 nm, which has been observed during the kinetic study of AgI particle formation from aqueous ions with pulse radiolysis techniques,4 was attributed to initial aggregates of silver iodide molecules, i.e., nuclei developed from the polyiodoargentate complex species. This unstable species are only detected in a time 10-700 µs after the start of the reaction. In the present research, these species were successfully stabilized with RSH and can survive for several days before further aggregation. Their size was estimated to be 20-30 Å, as judged from the onset of 405 nm.4a The stabilization of small AgI particles was further illustrated in the kinetic experiments shown under the condition of curve d in Figure 8A (Figure 8B). The intensity of the shoulder peak of the spectrum increased after 3 days of aging but without redshift of the onset, showing that the particles actually do not grow. Since under the condition that particle size is much smaller than the optical wavelength, the absorption of the colloidal solution is simply proportional to the total mass of the colloid.25 It is concluded that new particles should form; that is, nucleation of AgI should occur. When the concentration of RSH is smaller than 6.67 × 10-3 M, the onset of the AgI spectrum shifted to red after aging, showing that the amount of RSH is not enough to completely cover the AgI surface. In the competitive mechanism represented by reaction 1 and reactions 5 and 6, not only RSH but also I- are critical in controlling the size of AgI particles. If the experiments of Figure 8A are conducted in ethanol instead of water, since ethanol is a less polar solvent than water, the dissociation of RSH in ethanol should be weaker than in water; therefore, RSH should show less ability in inhibiting the growth of AgI. In water, under the condition of Figure 8A, curve g, about 30 nm blue-shift of the onset of the absorption spectrum referenced from the bulk onset was observed. In contrast, in ethanol, only 2 nm blue-shift was observed under the same condition (Figure 9, curve d). However, if we decreased the concentration of iodide, i.e., decreased the reaction rate of reaction 5, the absorption spectra of AgI blue-shifted drastically. In Figure 9, we show these shifts as a function of the iodide concentration.
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Figure 9. Absorption spectra of AgI colloids prepared immediately in ethanol solution with 3.33 × 10-4 M Ag+ and 1.67 × 10-2 M RSH in the presence of KI: (a) 0, (b) 1.33 × 10-4 M, (c) 4.00 × 10-4 M, (d) 6.67 × 10-4 M. Curve e represents the spectrum of a freshly prepared solution containing 3.33 × 10-4 M Ag+ and 6.67 × 10-4 M I-.
This set of experiments clearly demonstrated the competitive nature between RSH and I- in AgI formation. Conclusion The competitive reaction of iodide and 1-thioglycerol with silver ions was found effective for the synthesis of I-VII nanoparticles of AgI of size 7-15 nm. These particles were the mixture of β- and γ-AgI, and the structural factors showed some distortions from that of standard constants due to the strong interaction of thiolate with AgI particles. Amorphous silver thiolate complex was also found in the solid powder of AgI as a result of competitive reactions. However, this complex can be reduced by increasing the [I-]/[RSH] ratio. Almost pure thiol-capped AgI nanoparticles were formed with a particle size of 15 nm. An optical spectroscopic study in diluted solution confirmed the competitive mechanism of AgI formation. The smallest AgI particles stabilized by 1-thioglycerol and surviving for several days in water, observed in a solution of 3.33 × 10-4 M Ag+, 6.67 × 10-4 M I-, and 6.67 × 10-3 M 1-thioglycerol at room temperature, had an absorption peak at 331 nm. Acknowledgment. This work was supported in part by Grants-in-Aid for Scientific Research on Basic Research (A: 09304068) and on Priority Areas (No. 260: 09215234), and a Grant-in-Aid for JSPS fellow from the Ministry of Education, Science, Sports and Culture, Japan. S.C. thanks the Japan Society for Promotion of Science for the postdoctoral fellowship. We thank Prof. S. Nakatsuji, Prof. K. Toriumi, and Dr. Y. Ozawa for assistance with the elemental analysis and thermogravimetric analysis, respectively. References and Notes (1) Ida, T.; Saeki, H.; Hamada, H.; Kimura, K. Surf. ReV. Lett. 1996, 3, 41. (2) Ida, T.; Kimura, K. Solid State Ionics 1998, 107, 313. (3) Berry, C. R. Phys. ReV. 1967, 161, 848. (4) (a) Schmidt, K. H.; Patel, R.; Meisel, D. J. Am. Chem. Soc. 1988, 110, 4882. (b) See Figure 3 of ref 4a, in which Meisel et al. correlated the shift of Eg of band-to-band transition to the size of the particle. (c) Hayes, G.; Schmidt, K. H.; Meisel, D. J. Phys. Chem. 1989, 93, 6100. (5) (a) Tanaka, T.; Saijo, H.; Matsubara, T. J. Photogr. Sci. 1979, 27, 60. (b) Saijo, H.; Iwasaki, M.; Tanaka, T.; Matsubara, T. Photogr. Sci. Eng. 1982, 26, 92. (6) Henglein, A.; Gutierrez, M.; Weller, H.; Fojtik, A.; Jirkovsky, J. Ber. Bunsen-Ges. Phys. Chem. 1989, 93, 593. (7) (a) Vucemilovic, M. I.; Micic, O. I. Radiat. Phys. Chem. 1988, 32, 79. (b) Micic, O. I.; Meglic, M.; Lawless, D.; Sharma, D. K.; Serpone, N. Langmuir 1990, 6, 487. (c) Gopidas, K. R.; Bohorquez, M.; Kamat, P. V. J. Phys. Chem. 1990, 94, 6435.
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