Article pubs.acs.org/Langmuir
Two-Dimensional Nanoparticle Cluster Formation in Supercritical Fluid CO2 Joanna S. Wang,*,† Chien M. Wai,‡ Gail J. Brown,† and Scott D. Apt† †
Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433-7707, United States ‡ Department of Chemistry, University of Idaho, Renfrew Hall, Moscow, Idaho 83844, United States S Supporting Information *
ABSTRACT: Supercritical fluid carbon dioxide (sc-CO2) is capable of depositing nanoparticles in small structures of silicon substrates because of its gas-like penetration, liquid-like solvation abilities, and near-zero surface tension. In nanometersized shallow wells on silicon surface, formation of twodimensional (2D) monolayer metal nanoparticle (NP) clusters can be achieved using the sc-CO2 deposition method. Nanoparticles tend to fill nanostructured holes first, and then, if sufficient nanoparticles are available, they will continue to cover the flat areas nearby, unless defects or other surface imperfections are available. In addition, SEM images of twodimensional gold (Au) nanoparticle clusters formed on a flat silicon surface with two to a dozen or more of the nanoparticles are provided to illustrate the patterns of nanoparticle cluster formation in sc-CO2.
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not the case for both copper and gold.6 Stable 2D assemblies of Au NP assemblies on different substrates under ambient conditions exhibit electrocatalytic activity toward methanol oxidation reaction, which is relevant for direct methanol fuel cells.7 Collier et al.8 reported an insulator-to-metal transition by tuning the distance between functionalized Ag quantum dots (QDs). As the distance between metal surfaces was decreased from 1.2 to 0.5 nm, both quantum and classical effects were observed in the optical signals. Andres and co-workers9 fabricated a superlattice of a 3.7 nm diameter Au cluster deposited on a SiO2 substrate, which exhibited nonlinear Coulomb charging behavior. The possible applications of a Au chemical sensor was explored by Willner and co-workers.10 The Au NPs were immobilized as an organized monolayer and multilayer via a coupling agent that has suitable functional groups at both ends. Vogel et al.11 demonstrated the behavior of hybrid Au@PNIPAM (poly-N-isopropylacrylamide) core− shell particles at the air/water interface of Langmuir trough which allowed for an adjustment of the interparticle spacing in the array. In their process, the hybrid Au@PNIPAM colloids were added to the air/water interface, and afterward the NPs at the interface were compressed into a close-packed monolayer, which was then relocated on a solid substrate using a surface lowering transfer. After that, the organic shell of the hybrid Au@PNIPAM was thermally treated. As a result, arrays of purely inorganic Au NPs were obtained. The change in 2D
INTRODUCTION Fabrication of nanostructured materials has drawn great attention in recent years for developing highly efficient nanoelectronics and electrochemical devices such as fuel cells, sensors, nanostructured templates, supercapacitors, lithium-ion batteries, etc.1−3 Self-assembled nanoparticles (NPs) on a solid surface can create novel chemical, electronic, and optical properties that are promising for electronic nanodevice, photonic detection, and biomedical sensors. Two-dimensional (2D) nanomaterials are one of the widely studied areas in nanoscience because of their unique physical properties.4 For example, fabrication of 2D metallic photonic crystals (MPCs) based on colloidal gold NPs was demonstrated by Pang and coworkers.5 The laser interference ablation combining subsequent high temperature annealing is employed for the construction of 2D gold nanodot arrays in square lattices. The strong coupling between the waveguide resonance mode and the particle plasmon resonance of the MPCs indicates the success of the fabrication method, which shows potential applications in optoelectronic devices and sensors.5 Calculations using the discrete dipole approximation method have been performed for 2D close-packed array of three noble metals (silver, copper, and gold).6 Among metallic spherical nanoparticles assembled in close proximity to each other in 2D arrays, the optical aspects of plasmon coupling occurring through the near-field interactions have been investigated. For the silver 2D array system, it is evident that the longitudinal and transverse modes of the plasmon resonance phenomena observed are wellseparated when p-polarized incident light is applied, but it is © 2016 American Chemical Society
Received: March 14, 2016 Published: April 18, 2016 4635
DOI: 10.1021/acs.langmuir.6b01011 Langmuir 2016, 32, 4635−4642
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Langmuir
ions suspended in AOT water-in-hexane microemulsions. Separate metal ion and reducing agent solutions were prepared by dissolving 0.0178 g of AOT in 2 mL of hexane and adding an aqueous solution of 7.2 μL (water/surfactant molar ratio, W = 10) of either the 1.2 M NaCNBH3 reducing agent or the 0.4 M Au3+ gold ion. The W value was manipulated by the amount of aqueous solution added. These micellar solutions were stirred for 1 h before reduction to equilibrate the reagents to the reaction temperature. The reaction was at an ambient temperature. The microemulsion containing the reducing agent was added dropwise over a time span of 60 s to the gold ioncontaining microemulsion under vigorous stirring. Dodecanethiol (70 μL) was added to the reaction immediately after all of the reducing agent had been added to the solution. This solution was then allowed to stir for another hour. After this time, the gold particles were precipitated by adding a mixture of 6 mL of ethanol and 4 mL of methanol, followed by centrifugation. The supernatant was discarded, and the remaining particles were washed two more times with 6 mL of ethanol to remove AOT, spectator ions, and excess dodecanethiol. The particles were then resuspended in 0.5 mL of toluene. All procedures were conducted on the benchtop without the need for inert environments. The sizes of Au nanoparticles synthesized are in the range of 6.2 ± 0.7 nm. Nanoparticle Deposition in Supercritical Fluid CO2. The sc-CO2 deposition process was carried out using a 14 mL high-pressure stainless steel chamber. Si substrates (sizes: 4.5 × 4.5 mm2 in dimension) with premilled wells were placed next to a mini glass vial containing Au colloidal solution (∼250 μL, or depending on the concentration). The Si substrates and the glass vial were placed inside the high-pressure chamber. The chamber was slowly charged with liquid CO2 (60 atm) at room temperature over a period of 10 min, and the pressure was raised to 70 atm. During initial pressurization to 70 atm, the volume of the toluene solvent containing the dissolved nanoparticles increased. The system was then slowly heated from room temperature to 40 °C to convert the liquid carbon dioxide to the supercritical fluid state. At this time the pressure inside the chamber was about 140 atm. The ISCO pump then slowly raised the pressure up to 160 atm in the chamber. The high-pressure apparatus was left at this condition (40 °C and 160 atm) for 30 min to reach an equilibrium state. The thiol-stabilized Au nanoparticles will precipitate evenly and self-assemble to form a uniform 2D clusters on the nanostructures or on substrates in the sc-CO2 phase with a small fraction of dissolved toluene (
