Different morphology at different reactant molar ratios: synthesis of silver halide low-dimensional nanomaterials in microemulsions

Abstract

A simple method is introduced to prepare silver halide low-dimensional nanomaterials in water/oil (W/O) microemulsions. AgCl, AgBr nanoparticles and AgI nanoparticles/nanorods/nanowires are respectively synthesized. The reactant molar ratio plays an important role on the morphology of AgI nanomaterials. The possible reasons are tentatively discussed here.

---

Introduction

Silver halides have been used in photography for one and a half centuries and their light sensitiveness forms the basis of modern photography. They are also utilized in photochromic glass¹ and the photochromic phenomenon has close relationships with the shape of silver halides.² In addition, silver halide fibers are flexible, water insoluble, non-toxic, and have good transmission in the mid-infrared region (3–25 µm), therefore they can be used in many applications such as fiber-optic infrared spectroscopy, radiometry, heterodyne detection, transmission of CO₂ laser power for medical purposes and for construction of mid-infrared lasing fibers.³ Recently nanomaterials have attracted attention because they have special characteristics different from those of bulk materials. Therefore synthesis of silver halide nanomaterials will offer opportunities to study the characteristics of low-dimensional nanostructured silver halide and explore their new uses. Here we introduce a simple method to prepare silver halide nanomaterials in water/oil (W/O) microemulsions.

It is known that surfactant molecules aggregate to form reverse micelles in W/O microemulsions and the water core is surrounded by hydrophilic head group of these surfactant molecules. The diameter of the water core is in the range of 1–100 nm, so it's favorable for preparing nanomaterials. Since Boutonnet first reported that Pt, Pd, Rh, Ir nanoparticles were synthesized in W/O microemulsions,⁴ many kinds of nanoparticles have been successfully synthesized in W/O microemulsions.^5,6 However, one-dimensional nanomaterials prepared in W/O microemulsions limit to a few kinds of compounds, such as BaCO₃,⁷ BaSO₄,⁸ CaSO₄.⁹ To our knowledge, the preparation of silver halide one-dimensional nanomaterials has not been reported in the literature, though silver halide nanoparticles have been synthesized in microemulsions.⁶ In this paper we use W/O microemulsions to prepare silver halide low-dimensional nanomaterials. AgCl, AgBr nanoparticles and AgI nanoparticles/nanorods/nanowires were successfully synthesized. Interestingly, it was found that the reactant molar ratio plays an important role in determining the morphology of the final products, especially of the AgI nanomaterials.

Experimental

The nonionic surfactant, Triton X-100 [polyoxyethylene(9) 4-(1,1,3,3-tetramethylbutyl) phenyl ether], cyclohexane, n-pentanol, ethanol, AgNO₃, NaCl, NaBr and KI were all of A. R. grade and used without further purification. The water used in this work was deionized.

The microemulsions were prepared as follows. Aqueous solution containing silver nitrate or halide was added to cyclohexane/Triton X-100/n-pentanol system. After 15 min vigorous agitation, the equivalent volume of the two microemulsions containing silver ions or halide ions respectively were mixed rapidly to give a final molar ratio of water to surfactants, w = [H₂O]∶[Triton X-100] ≈ 10. The mixed microemulsions were stirred slightly, then laid aside in the dark at room temperature for several days.

After the aging time, one drop of the mixed microemulsions was dispersed in 1–2 ml ethanol via ultrasonication. The ethanol solution containing products was dropped on the copper grid and dried in air. An Hitachi Model-800 transmission electron microscope (TEM) at an accelerating voltage of 200 kV was used to characterize the morphology of the products and electron diffraction was performed to probe the crystalline structure.

Results and discussion

AgCl spherical nanoparticles and AgBr tadpole-like nanoparticles

In the cyclohexane–Triton X-100–n-pentanol–aqueous solution system, AgCl and AgBr nanoparticles were respectively synthesized. Their TEM micrographs and electron diffraction patterns are given in Fig. 1. It is shown that AgCl nanocrystals are spherical with average diameters of 20–30 nm (Fig. 1a) while AgBr nanoparticles are tadpole-like, whose size ranges from 50 nm to 80 nm (Fig. 1b). In the respect that many particles stay together, the electron diffraction pattern represents the total diffraction results.

Fig. 1 TEM micrographs and electron diffraction patterns of silver halide nanoparticles prepared in Triton X-100 microemulsions: (a) AgCl nanoparticles: [Ag⁺] = [Cl⁻] = 0.5 M; (b) AgBr tadpole nanoparticles: [Ag⁺] = 0.1 M, [Br⁻] = 0.3 M.

The formation mechanism of AgCl or AgBr nanoparticles is proposed as follows. Silver ions and halide ions are respectively solubilized in reverse micelles. After the mixing of two microemulsions, the reverse micelles collide and exchange reactant ions. Silver ions and halide ions meet and nucleate. The water cores inside of the reverse micelles restrict the growth of the nuclei, therefore the nuclei only grow into nanostructures.

AgI nanoparticles, nanorods and nanowires

In the cyclohexane–Triton X-100–n-pentanol–aqueous solution system, AgI nanomaterials such as nanoparticles, nanorods, and nanowires are respectively synthesized under certain conditions. It is found that the reactant molar ratio plays an important role on the morphology of AgI nanomaterials.

When the molar ratio of iodine ions to silver ions is equal to 1, only AgI nanoparticles are obtained in microemulsions, as is shown in Fig. 2a. The diameters of the particles are 30–40 nm. During observation by TEM, AgI nanoparticles are prone to assemble and become larger particles. Increasing the amount of iodine ions to [I⁻]∶[Ag⁺] = 2, AgI nanorods are predominately prepared in microemulsions. Fig. 2b shows the lengths of AgI nanorods ranging from 0.5 µm to 0.9 µm and the diameters are 30–60 nm. The dark Ag dots emerge in the rods due to the decomposability of light sensitive AgI. At molar ratios of [I⁻]∶[Ag⁺] ≈ 3, AgI nanowires are synthesized with lengths up to 6 µm and diameters of 40–100 nm, as seen in Fig. 2c. Some AgI nanowires array parallel to each other (Fig. 2d) and some even aggregate into nanobundles (Fig. 2e). This self-assembly phenomenon may be induced by interactions of surfactants absorbed onto the AgI nanowires. At molar ratios of [I⁻]∶[Ag⁺] > 3, much wider rods with 500 nm wide and 2–4 µm long appear. When [I⁻]∶[Ag⁺] = 10, pairs of AgI nanorods appear, as shown in Fig. 2f.

Fig. 2 TEM micrographs of AgI nanomaterials prepared in Triton X-100 microemulsions: (a) AgI nanoparticles, [I⁻]∶[Ag⁺] = 1; (b) AgI nanorods, [I⁻]∶[Ag⁺] = 2; (c) AgI nanowires, [I⁻]∶[Ag⁺] = 3; (d) AgI nanowires array parallel to each other, [I⁻]∶[Ag⁺] = 3; (e) AgI bundles, [I⁻]∶[Ag⁺] = 3; (f) AgI wider rods, [I⁻]∶[Ag⁺] = 10.

In conclusion, at different reactant molar ratios AgI nanomaterials have different morphologies. A one-dimensional AgI nanostructure is favored to form in microemulsions when the amount of iodine ions exceed that of silver ions. The molar ratio of [I⁻]∶[Ag⁺] ≈ 3 may be the optimal condition for preparing AgI nanowires.

In experiments, it has been found that the existence of silicate in microemulsions can promote AgI nanowires to form. In the preparation of microemulsions, iodine ions and silicate ions are solubilized in the same microemulsions, then are mixed with other microemulsions containing silver ions. As a result, more nanowires are synthesized (Fig. 3a) and energy diffraction X-ray analysis (EDXA) performed on them shows that only silver and iodine elements exist in these nanowires, which proves that these nanowires are AgI nanowires (Fig. 4). While mixing the two microemulsions containing silver ions and silicate ions respectively, only nanoparticles are obtained. It is assumed that silicate ions hydrolyze and form silica gel, which may be favorable to make AgI grow directionally into wires. The detailed reason is not clear yet.

Fig. 3 TEM micrographs of AgI nanowires prepared in Triton X-100 microemulsions: (a) AgI nanowires: [Ag⁺] = 0.05 M, [SiO₃²⁻] = 0.1 M, [I⁻] = 0.15 M; (b) decomposed AgI nanowires after a few seconds illumination; (c) a selected area of (b) with high magnification.

Fig. 4 EDXA spectrum of AgI nanowires shown in Fig. 3.

It is noteworthy that many discrete Ag particles could be found along AgI wires under the electron beam for 1–2 s, as shown in Fig. 3b and 3c (with a larger magnification). Therefore electron diffraction (ED) analysis for individual AgI wire is difficult to perform. Nevertheless, the decomposition of light sensitive AgI suggests the possibilities of getting one-dimensional silver nanostructure from AgI nanowires.

The formation mechanism of AgI nanowires is proposed that surfactant molecules assemble to rod-like aggregates, which can serve as template and restrict the growth of products.¹⁰ In consideration of different shapes of AgI nanomaterials at different molar ratios, the silver complex ions are assumed to exert significant influence on the formation of one-dimensional AgI nanostructure. In aqueous solution, when the amount of I⁻ ions exceeds that of Ag⁺ ions, the complex ions can be formed,^11,12 such as AgI₂⁻, AgI₃²⁻, Ag₂I₆⁴⁻, Ag₃I₈⁵⁻…etc. At molar ratios of [I⁻]∶[Ag⁺] = 1, most of the reactant ions form AgI precipitate and no complex ions appear, therefore only AgI nanoparticles are obtained. While [I⁻]∶[Ag⁺] = 2, the complex ions form, which may slow the growth of nuclei and is favorable to make the nuclei grow along the template into one-dimensional structure. As a result, AgI nanorods emerge predominatingly in microemulsions. At molar ratios of [I⁻]∶[Ag⁺] = 3, AgI nanowires are synthesized due to more complexation. As for molar ratios of [I⁻]∶[Ag⁺] > 3 AgI wider rods are prepared, this may be because there exists the trade-off between preferential growth rate of AgI nanowires and complexation. [I⁻]∶[Ag⁺] = 3 may be an optimal condition for the synthesis of long AgI nanowires.

Different reactant molar ratios were also selected to prepare AgCl and AgBr products, but only nanoparticles were formed, one-dimensional nanostructures such as nanowires didn’t appear. This may be because the AgI crystal lattice is similar with ZnS, while AgCl and AgBr have the same crystal lattice as NaCl. The different crystal lattice causes different preferential growth, therefore AgI has different morphology from AgCl or AgBr. Further study is under way.

Conclusions

AgCl nanoparticles, AgBr tadpole-like nanoparticles and AgI nanoparticles/nanorods/nanowires were correspondingly synthesized in Triton X-100 microemulsions. The reactant molar ratio plays a crucial role on the morphology of AgI nanomaterials. At different reactant molar ratios AgI has different morphologies. The molar ratio of [I⁻]∶[Ag⁺] ≈ 3 may be the optimal condition for preparing AgI nanowires. Synthesis of silver halide low-dimensional nanomaterials, especially AgI nanowires, will advance their application and explore new uses.

Acknowledgements

This work was supported by NSFC (20025102, 50028201, 20151001), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China, and the state key project of fundamental research for nanomaterials and nanostructures.

References

1. W. H. Armisteread, Science, 1964, 144, 150.
2. Y. Kimura, R. Kikuchi and T. Kawamoto, Phys. Chem. Glasses, 1977, 18, 96.
3. O. Eyal, D. Shemesh and A. Katzir, Appl. Opt., 1997, 36, 1185.
4. M. Boutonnet, J. Kizling, P. Stennis and G. Maire, Colloids Surf., 1982, 5, 209.
5. M. P. Pileni, Catal. Today, 2000, 58, 151.
6. L. Jeunieau, F. Debuigne and J. B. Nagy, in Surfactant Science Series, ed. J. Texter, Marcel Dekker Inc., New York, Basel, 2001, vol. 100, p. 609.
7. L. M. Qi, J. M. Ma, H. M. Cheng and Z. G. Zhao, J. Phys. Chem. B, 1997, 101, 3460.
8. M. Li and S. Mann, Langmuir, 2000, 16, 7088.
9. G. D. Rees, R. Evans-Gowing, S. J. Hammnond and B. H. Robinson, Langmuir, 1999, 15, 1993.
10. S. Xu, H. C. Zhou, J. Xu and Y. D. Li, Langmuir, submitted.
11. I. Leden, Acta Chem. Scand., 1956, 10, 812.
12. I. Leden, Acta Chem. Scand., 1956, 10, 540.

---