Interfacial polygonal patterning via surfactant-mediated self-assembly of gold nanoparticles
© Zhang et al.; licensee Springer. 2013
Received: 16 August 2013
Accepted: 9 October 2013
Published: 22 October 2013
In this work, we explored the formation processes of interfacial polygonal patterning via surfactant-mediated self-assembly of gold nanoparticles (AuNPs). We found that a balance between DDT-capped AuNPs and PVP-passivated AuNPs is a key to making these inorganic–organic thin films. The interfacial polygonal patterning possesses many processing advantages and flexibilities, such as controllable interfacial shape and inter-AuNP distance, tuning of particle sizes, thiol population, chain lengths, and other new properties by introducing functional groups to thiol chains. In principle, self-assembly of AuNPs via well-designed interfaces may be useful for fabrications of other complex architectures.
KeywordsNanoparticles Interfaces Thin films Hybrid Self-assembly Polygonal pattern
Gold nanoparticles (AuNPs) are among the most studied nanomaterials in recent years, owing to their outstanding properties in catalytic, electrical, optical, and biomedical applications[1–9]. The controlled fabrication of gold nanoparticles at scales beyond the current limits of characterization techniques is a technological goal of practical and fundamental interest. Important progress has been made over the past few years in the self-assembly and organization of Au nanostructures ranging from one-, two-, and three-dimensional (1D, 2D, and 3D) ordered arrays and superlattices to random aggregates and superstructures[1–14]. While most of this research endeavor relies on the van der Waals interaction of surfactants adsorbed on the surfaces of AuNPs, several types of constructional assemblies have been well developed, in which as-synthesized AuNPs were used as primary building units to grow larger monodisperse particles and to construct continuous 3D networks under heat conditions. One important area remaining to be explored is whether these preassembled AuNPs can be used as structure precursors for fabricating other even more complex Au nanostructures when surface organics are controllably removed[15–25]. Herein, we devise a new synthetic protocol, which combines both surfactant-assisted assembly and heat-activated attachment, to generate interfacial polygonal patterning of self-assembled nanostructures. In particular, we will use small AuNPs (2 to 5 nm in size) as starting units to fabricate several different kinds of complex gold nanostructures in polygonal patterning with a high morphological yield of 100%.
Synthesis of interfacial polygonal patterning via self-assembly of Au nanoparticles
Thiol-capped Au seeds were prepared by Brust's two-phase method with some minor modifications (see Additional file1 for the detailed synthesis procedure)[11, 16, 21, 22]. In a typical experiment, two standard units (denoted as STUs) of Au nanoparticles were redissolved in cyclohexane (2 mL for each STU), followed by the addition of PVP (1.25 mM, 0.5 mL in 2-propanol) and DDT (0.11 M, 22 mL in cyclohexane). The obtained mixture was then placed into a Teflon-lined stainless steel autoclave, and the solvothermal synthesis was conducted at 150°C to 210°C for 2 to 14 h in an electric oven. After the reactions, gold products were harvested by centrifuging and dissolved into ethanol solvent for their stabilization. Detailed preparative parameters adopted in the above experiments can be found in Additional file1: SI-1. The as-prepared gold nanomaterial products were characterized with transmission electron microscopy (TEM; JEM2010F, JEOL Ltd., Akishima-shi, Tokyo, Japan) and field-emission scanning electron microscopy (FESEM; JSM-6700F, JEOL Ltd., Akishima-shi).
Results and discussion
With respect to detailed investigation, two types of patterns such as hexagonal or complex patterns were proposed combined with patterns of foamed construction materials. Indeed, the bubbles continuously evolve toward lower-energy configuration by minimizing the interfacial area, so that we could obviously observe the spherical outlines along PVP cakes standing aside. The PVP cakes inside could compress the surrounding cakes to pursue an equilibrium of interfacial tension, which lies in the size of PVP cakes, exhibiting a perpendicular plane among the cakes. More quantitatively, solid laterals or arc laterals among the patterning could be observed from top and side view. Due to lack of adequate surrounding cakes, the cakes outside could penetrate into the bottom of the ones inside, exhibiting an arc lateral from side view, and/or two crossed arcs from top view.
To further confirm the synergistic effect of PVP and DDT, the effects of stand-alone surfactant-mediated self-assembled nanostructures are carried out first (see Additional file1: SI-2). Besides PVP in-2 propanol solvent (without any addition of fresh DDT), solid PVP powders were also used to tailor self-assembly of AuNPs. Meanwhile, various amounts of freshly prepared DDT were applied to fine tune the gold nanostructures. Nevertheless, the morphology yields for resultant products as gold sponges are extremely high at about 100% instead of interfacial polygonal patterning. If the process time is short enough, or freshly prepared DDT are abundantly added, the worm-like gold nanostructures or closed-packed hexagonal AuNPs (after growth) were obtained, respectively. In light of the above findings, our time-dependent synthesis with combined surfactants was executed to make clear real roles of the surfactants alone. As shown in Additional file1: SI-3a, the contour outlines of PVP cakes with gold nanoparticles were clearly explored, followed by interlinks of PVP cakes (Additional file1: SI-3c) and AuNPs aggregates (Additional file1: SI-3d) on the cakes. Finally, the mixture of soft PVP assemblies and Au sponges was harvested after 5-h heat treatment (Additional file1: SI-3e,f). On the basis of systematical studies, the optimal process time and temperature can be ruled out as 4 h and 180°C. Particularly, from the Additional file1: SI-3, it also proved that higher concentration of PVP in 2-propanol (5 mM, 0.5 mL) went against the formation of interfacial polygonal patterning.
In summary, for the first time, we have developed a self-assembly approach for generation of interfacial polygonal patterning with as-synthesized AuNPs as starting building blocks. It is found that the hydrothermal condition is essential to detach DDT and PVP surfactants and thus trigger the self-assembly of AuNPs. The resultant interfacial polygonal patterning can be further controlled by manipulating surfactant morphology, concentration of metallic nanoparticles, amount of surfactants, process temperature and time, etc. In principle, this self-assembly approach can also be extended to large-scale 3D organizations of other surfactant-capped transition/noble metal nanoparticles.
Transmission electron microscopy
Field-emission scanning electron microscopy
The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (grant no. 51104194), Doctoral Fund of Ministry of Education of China (20110191120014), No.43 Scientific Research Foundation for the Returned Overseas Chinese Scholars, National Key laboratory of Fundamental Science of Micro/Nano-device and System Technology (2013MS06, Chongqing University), and State Education Ministry and Fundamental Research Funds for the Central Universities (project nos. CDJZR12248801, CDJZR12135501, and CDJZR13130035, Chongqing University, People's Republic of China). Dr. Zhang and Chen RD gratefully acknowledge Prof. Zeng Hua Chun for his kind discussions and National University of Singapore for their technical supports.
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