Formation mechanism of nonspherical gold nanoparticles during seeding growth: Roles of anion adsorption and reduction rate

doi:10.1016/j.jcis.2005.06.012

Journal of Colloid and Interface Science

Volume 293, Issue 1, 1 January 2006, Pages 69-76

https://doi.org/10.1016/j.jcis.2005.06.012 Get rights and content

Abstract

A small section of nonspherical particles can be observed in the further growth of spherical gold colloids exposed to a mixture of NH₂OH and HAuCl₄. The concentration ratio of [NH₂OH]:[HAuCl₄] is critical for the formation of nonspherical particles as higher ratios produce lower yields and smaller of such particles. These concentrations also affect the reaction kinetics; the reaction rate increases with [NH₂OH], while independent of [HAuCl₄], which we believe is due to the specific adsorption of AuCl⁻₄ onto gold surface. These nonspherical particles come from the preferential growth of {111} facets as indicated by their TEM images and electron diffraction patterns. We propose this preferential growth is ascribed to the preferential adsorption of AuCl⁻₄ on {111} facets, and some competition which determines the yield of nonspherical particles exists between the AuCl⁻₄ adsorption and the AuCl⁻₄ reduction, faster reduction counteracting the effect of this preferential adsorption and thus suppressing nonspherical particle. This result probably provides some guidance to develop a shape-controlled synthesis of gold particles without any additives.

Graphical abstract

The competition between the adsorption of ${AuCl}_{4}^{-}$ and the reduction of ${AuCl}_{4}^{-}$ at the surface of seed particles determine the shape distribution of resulting gold colloids.

Introduction

Metal nanoparticles with controlled size and shape are of great interest because of their morphology dependent properties [1] and potential applications in a lot of fields [2], [3]. Right now, shape-controlled synthesis of nanoparticles has been achieved either by using geometric templates [4], or by using some additives [5], such as polymers [5], or inorganic anions [6], to regulate the particle growth. However, these methods all require harsh conditions or laborious work in order to remove the unwanted residues from the target products, and thus exploring new strategies for shape-controlled synthesis without any additives attracts much interest [7].

It is reported that some nonspherical nanoparticles, rodlike or polyhedral, could be observed during the further growth of spherical gold particles exposed to a mixture of only mild reducing agent and gold ions although its percentage is low [8]. This fact is quite interesting since no above-mentioned capping additives or templates present and gold particles are supposed to be energetic—favorably spherical in such a system [9]. This result implies that some kind of shape-controlled synthesis method without additives or templates might be developed. Just taking the advantage of seeding-growth method did some groups recently prepare rodlike gold nanoparticles although with the assistance of additives [10], [11], [12], [13]. Natan et al. hypothesized that this anisotropic growth of gold particles might result from different transportation flux of metal ion towards various particle areas or from different properties of some seeds [14]. Jana et al. assumed that it might come from the competition of particle growth with the capping action of ionic reagents [10]. However, these statements are no more than conjectures, and no substantial evidence is given.

Herein, we study the effect of various experimental conditions like reagent ratio on the formation of nonspherical particles as well as the reaction kinetics. All of these experiments turn out that the [NH₂OH]:[HAuCl₄] ratio is very important. TEM and electron diffraction results indicate that nonspherical particles come from the preferential growth of {111} faces. And, it is proposed that the competition between the adsorption and reduction of AuCl⁻₄ determines the formation of nonspherical gold particles.

Section snippets

Reagents

Hydrogen tetrachloroaurate(III) trihydrate (ACS reagent), trisodium citrate dihydrate (99%), hydroxylamine hydrochloride (99%), were obtained from Aldrich. Ultrapure water (>17 MΩ cm) was used throughout the experiments. All reagents were used as received.

Synthesis of gold seeds

Colloidal gold seeds were synthesized by citrate reduction of gold chloride with a recipe proposed by Frens [15]. Briefly, 1.75 ml 1% (wt/v) aqueous sodium citrate was quickly added into 50 ml $2.5 \times 10^{−4}$ M boiling HAuCl₄ solution under rigorous

Influence of [NH₂OH]:[HAuCl₄] ratio on nonspherical gold nanoparticles

Seeding-growth is considered as an ideal approach to prepare monodisperse metal nanoparticles with controlled size [16]. Seed particles may continue growth in the presence of metal ions and mild reducing agents. The reducing agent should be chosen as so that the reduction of metal ions only happens on the surface of the seed particles without new nucleation. NH₂OH is a mild reducing agent, and often used in the continuing growth of gold seed particles; however, some nonspherical particles

Discussion

The structural characterization of these nonspherical gold particles reveals that they come from the preferential growth of {111} faces over {100}, sometimes as well as {110}, which leads to the exposure of more {100} and (110) area onto the particle surface than {111}. In the viewpoint of thermodynamics, this kind of structure is not energetically favorable because surface energies of different crystallographic planes are in a sequence as $γ_{{111}} < γ_{{100}} < γ_{{110}}$ for fcc crystal like gold [21].

The

Conclusion

It is the preferential adsorption of AuCl⁻₄ onto gold {111} faces which causes the preferential growth of the {111} face that accounts for the formation of nonspherical particles. The influence of NH₂OH is to expedite the reduction of AuCl⁻₄ with the concentration of its own increasing, and the rapid reduction may counteract the effect of this preferential AuCl⁻₄ absorption as produces low yield as well as smaller nonspherical particles. Some kind of competition should exist between the AuCl⁻₄

Acknowledgements

We are grateful for the financial supports from National Natural Science Foundation of China (NSFC 29803002) and Ministry of Science and Technology of China (2001CB6105). We also thank Drs. Manhong Liu, Qingwen Li, and Peng Diao for valuable discussions.

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¹: Present address: Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USA.

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Formation mechanism of nonspherical gold nanoparticles during seeding growth: Roles of anion adsorption and reduction rate

Abstract

Graphical abstract

Introduction

Section snippets

Reagents

Synthesis of gold seeds

Influence of [NH2OH]:[HAuCl4] ratio on nonspherical gold nanoparticles

Discussion

Conclusion

Acknowledgements

J. Electroanal. Chem.

Surf. Sci.

J. Colloid Interface Sci.

J. Colloid Sci.

Chem. Rev.

Science

Science

Langmuir

Science

J. Phys. Chem. B

Nano Lett.

Langmuir

Rep. Prog. Phys.

Chem. Mater.

J. Mater. Chem.

Influence of [NH₂OH]:[HAuCl₄] ratio on nonspherical gold nanoparticles