Shape evolution of 3D flower-like gold microstructures from gold nanosheets via oriented attachment
Graphical abstract
The shape evolution of flower-like gold microstructures from gold nanosheets via oriented attachment induced by H2O2 with the presence of starch.
Introduction
In the past decade, synthesis and fabrication of structural-controlled metal nanostructures have been extensively studied [1], [2] due to their potential applications as catalysts [3], sensors [4], and photovoltaic devices [5]. The applications take advantages of the unique size- and shape-dependent properties of the nanostructures which are not existing in the bulk materials or spherical nanostructures [6].
Gold nanostructures have continuously been the centre of attention as it can be engineered into various morphologies, such as spheres [7], [8], rods [9], [10], wires [11], sheets [12], [13], polyhedrons [14], stars [15], and dendrites [16], with distinctively high chemical stability. Among numerous gold nanostructures, flower-like gold structures (FLGSs) with sub-micrometre size consist of various sub-structures such as nanogrooves, sharp edges and tips. The sub-structures provide not only high surface area, but also numbers of nanometre-scale junctions and interconnections, which can serve as hotspots for surface-enhanced Raman spectroscopy (SERS) [17]. The FLGSs have been used as substrates for SERS [18], [19], [20] and electrochemical catalysts [21], [22]. Furthermore, FLGSs exhibited potential usages as nanocarriers of DNA for cellular uptake, drug or gene delivers and contrasting agents as they provide acceptable cytotoxicity toward cells in toxicological investigations [23], [24].
Various techniques for the fabrication of FLGSs have been developed, particularly electrochemical [20], [21], [25] and wet chemical procedures [18], [19], [23], [24], [26], [27]. The wet chemical approaches seem to be the most practical choice as the complicated instruments are not required and the process can be easily scaled up. A development of simple, effective, fast and green protocol using environmentally benign method and low cost chemicals is still a challenge. In general, additives (i.e., capping agents, shape-directing agents, and stabilizers) play a pivotal role as it directs the formation of anisotropic nanostructures. The specific absorption of the additive molecules on the particular crystal facets of nanoparticles leads to the modification of the surface energy and consequently alters their growth rate [17] to generate particles with shape selectivity. Various additives (i.e., 2-[4-(2-hydroxyethly)-1-piperazinyl] ethanesulfonic acid (HEPES) [28], dopamine [19], poly(vinyl pyrrolidone) (PVP), sodium dodecyl sulfate (SDS) [27], and gum Arabic [24]) have been employed. Although there were several successful techniques for FLGSs fabrications, only a few demonstrated a basic understanding on the growth mechanism of the FLGSs structure. As discussed in our previous work on the synthesis of gold nanosheets using H2O2 as a reducing agent and starch as a green stabilizer [12], a very low concentration of H2O2 leads to a selective preservation and growth of high anisotropic gold nanosheets as starch preferentially adsorbed and then prevented the growth on the {111} basal planes. Tridib Kumar Sarma et al. [29]. reported the change in shape from spherical to triangular and to hexagonal particles by increasing initial concentration of HAuCl4 in the presence of H2O2 and starch with assisted ultrasonic waves. From the work, it suggested that the ratio of [H2O2]/[HAuCl4] might be an important factor for the shape selectivity. Decreasing of the ratio by increasing concentration of HAuCl4 induced the formation of plate shapes. Interestingly, we found that a complex structure (flower-like structure) could be generated instead of the nanosheets when high concentration H2O2 was involved. This indicates that the concentration of H2O2 and the high ratio of [H2O2]/[HAuCl4] plays a pivotal role in the complex structure formation. However, in this system, the shape evolution roles of this complex structure are still unclear.
In this study, we report the shape evolution pathways of 3D flower-like gold microstructures (3D-FLGMSs) from the nanosheets. The influences of the concentration of H2O2, starch and molar ratio of [H2O2]/[HAuCl4] were systematically investigated in detail. The developed method for the fabrication of 3D-FLGMSs is simple, effective, fast, green and efficient with industrial-scale production capability. To our knowledge, this is the first for the preparation of such a complex microstructure of 3D-FLGMSs using H2O2 as a green reducing agent and starch as stabilizer. It could serve as a challenge for the fabrication of microstructures using a simple chemical approach. Furthermore, a potential application of 3D-FLGMSs as SERS substrate was demonstrated.
Section snippets
Chemicals
Nitric acid (HNO3, 65% w/v), hydrochloric acid (HCl, 37% w/v), sodium hydroxide (NaOH), soluble starch and hydrogen peroxide (H2O2, 30% w/w) were purchased from Merck (Thailand). All chemicals were analytical grade and were used as received. A solution of tetrachloroauric (III) acid (HAuCl4, 0.5 M) employed as gold metal precursors, was prepared from a stock solution of concentrated HAuCl4 solution. The stock solution was prepared using a method described elsewhere [12]. Prior to use, all
H2O2 induced 3D-FLGMS formation
H2O2 is well known as a strong oxidizing agent. However, its mild reducing capability is suitable for the formation of anisotropic nanostructures, particularly silver nanoplates [30], [31] and gold nanosheets [12], [13]. The utilization of low concentration H2O2 is the key parameter as it enables a slow reduction allowing the nucleation and growth of gold nanosheets under a kinetically controlled environment [12]. This phenomenon provides the formation of plate-like seeds that will further grow
Conclusions
In summary, we have successfully developed a simple and efficient synthetic method for the fabrication of 3D-FLGMSs using H2O2 as the green reducing agent and starch as the stabilizer. The influence of the H2O2 and starch on the size, morphology and structural evolution of 3D-FLGMSs were systematically investigated. Shape evolution of 3D-FLGMSs using the green chemicals was revealed. By performing the synthesis at high concentration of H2O2, the formation of 3D-FLGMSs with the size as large as
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
Acknowledgements
This research was supported by National Research University Project, Office of Higher Education Commission and Chulalongkorn University (WCU-58-008-FW). S. Nootchanat is a postdoctoral fellow supported by Rachadapisek Sompote Endowment Fund, Chulalongkorn University. K. Wongravee thanks Grant for International Research Integration: Chula Research scholar, Ratchadaphiseksomphot Endowment Fund and Thailand Research Fund (TRG 5880238). Finally, authors would like to delicate the manuscript for “In
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