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Depositing Ordered Arrays of Metal Sulfide Nanoparticles in Nanostructures Using Supercritical Fluid Carbon Dioxide Joanna S. Wang,*,†,‡ Alexander B. Smetana,†,‡ John J. Boeckl,‡ Gail J. Brown,‡ and Chien M. Wai† †
Department of Chemistry, University of Idaho, Moscow, Idaho 83844 and ‡Air Force Research Lab, Materials and Manufacturing Directorate, WPAFB, Dayton, Ohio 45433-7707 Received June 12, 2009. Revised Manuscript Received August 25, 2009
Silver sulfide and cadmium sulfide nanoparticles of controllable sizes are synthesized using a water-in-hexane microemulsion method and stabilized by dodecanethiol. The stabilized metal sulfide nanoparticles can be deposited homogenously on flat substrates forming ordered 2-D arrays in supercritical fluid carbon dioxide (Sc-CO2). The use of Sc-CO2 leaves the particles unaffected by dewetting effects caused by traditional solvents and produces uniform arrays. The Sc-CO2 deposition technique is capable of filling nanoparticles in nanostructures of silicon wafers which is difficult to accomplish by conventional solvent evaporation methods.
Introduction Fabrication of well-defined two or three-dimensional ordered arrays of nanocrystal metals and semiconductors is a challenge of nanoscience and nanotechnology in scientific communities to produce new types of optical gratings,1,2 optical filters,3,4 antireflective surface coatings,5,6 selective solar absorbers,7 data storage, and microelectronics.8 There have also been increasing activities in the development of bioactive and biocompatible nanomaterials for a variety of applications.9 Metal sulfide quantum dots such as silver sulfide (Ag2S) nanoparticles are also incorporated in sensing applications, including chemical and warfare agent detection as well as environmental monitoring10 and used as a photosensitizer or light detectors for photographic purposes.11-13 Nanoparticles have electric and optical properties that sensitively depend on the size. If the size of the nanoparticles can be controlled, it can be expected that new materials created from these particles will bring about new technical innovations. Recently our research group has developed a method of making 2-D arrays of monodisperse metallic nanoparticles using supercritical carbon dioxide as a medium to deposit gold and platinum nanoparticles uniformly in nanometer-sized trenches on silicon wafers.14 Here we further develop and apply this methodology to the solid state field using silver sulfide and cadmium sulfide (CdS) as model semiconductor nanoparticles. In this study, Ag2S and *To whom correspondence should be addressed. E-mail: jswang@ uidaho.edu. Tel: 937-255-8692. (1) Xia, Y.; Kim, E.; Mrksich, M.; Whitesides, G. M. Chem. Mater. 1996, 8(3), 601. (2) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60. (3) Asher, S. A. U.S. Patent, 4,627,689, 1986; and U.S. Patent 4,632,517, 1986. (4) Xu, X.; Friedman, G.; Humfeld, K. D.; Majetich, S. A.; Asher, S. A. Adv. Mater. 2001, 13, 1681. (5) Yoldas, B. E.; Partlow, D. P. Appl. Opt. 1984, 23, 1418. (6) Hinz, P.; Dislich, H. J. Non. Cryst. Solids 1986, 82, 411. (7) Hahn, R. E.; Seraphin, B. O. Physics of Thin Films; Academic Press: New York, 1978. (8) Kastner, M. A. Phys. Today 1993, 46, 24. (9) Meziani, M. J.; Sun, Y.- P. J. Am. Chem. Soc. 2003, 125, 8015. (10) Lu, X.; Li, L.; Zhang, W.; Wang, C. Nanotechnology 2005, 16, 2233. (11) Motte, L.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1995, 99, 16425. (12) Xiao, J.; Xie, Y.; Tang, R.; Luo, W. J. Mater. Chem. 2002, 12, 1148. (13) Herron, N.; Wang, Y.; Eckert, H. J. Am. Chem. Soc. 1990, 112, 1322. (14) Smetana, A. B.; Wang, J. S.; Boeckl, J. J.; Brown, G. J.; Wai, C. M. J. Phys. Chem. C 2008, 112, 2294.
Langmuir 2010, 26(2), 1117–1123
CdS nanoparticles are synthesized by chemical reactions of metal cations with freshly prepared sulfide anion solutions dissolved in the water core of water-in-oil microemulsions. Sodium bis(2-ethylhexyl)sulfosuccinate (AOT) is used as an anionic surfactant with hexane as an organic solvent. Dodecanethiol is then added to the microemulsion solution to stabilize the synthesized metal sulfide nanoparticles. The alkanethiol-coated metal sulfide nanoparticles are easily separated from the reaction medium and dispersed in a nonpolar solvent. The protected nanoparticles dispersed in an organic solution can be precipitated onto carbon-coated copper grids or silicon wafers to generate selfassembled 2-D arrays in Sc-CO2. A unique feature of the Sc-CO2 evaporation technique is that the nanoparticles can be deposited into small trenches on Si wafers which can not be achieved by traditional solvent deposition methods.
Experimental Section Chemicals. Silver nitrate, cadmium nitrate, hexanes, toluene, ethanol, methanol, sodium sulfide, and sodium bis(2-ethylhexyl)sulfosuccinate (98%) were purchased from Aldrich and used as received. Metal sulfide nanoparticles were prepared by a microemulsion method described in the literature.15 Synthesis and Preparation. For the metal sulfide nanoparticle synthesis, solutions of silver ions (AgNO3, 0.8 M) and sulfide ions (Na2S, 0.8 M) or Cd(NO3)2 (0.8 M) and Na2S (1.2 M) in AOT water-in-hexane microemulsions were prepared. The microemulsion solutions were prepared individually by dissolving 0.0178 g of AOT in 2 mL of hexane and adding an aqueous solution of 7.2 μL (water/surfactant ratio, W=10) of metal ions or freshly prepared sulfide ions. The W value was manipulated between 6 and 15 by the amount of solution added. These micelle solutions were stirred for 1 h before mixing the two microemulsion solutions together. The reaction temperature was controlled by placing the reaction vials in a NaCl salt/ice bath (-15 °C), an ice bath (0 °C), room temperature (20 °C), or a mineral oil bath (40 °C). The reaction temperature was monitored using a thermocouple placed in the liquid solution before and after reaction. The microemulsion containing Na2S was added dropwise over 60 s to the Agþ or Cd2þ ion-containing microemulsion under vigorous stirring. Dodecanethiol (65 μL) was added to the reaction (15) Smetana, A. B.; Wang, J. S.; Boeckl, J. J.; Brown, G. J.; Wai, C. M. Langmuir 2007, 23, 10429.
Published on Web 12/11/2009
DOI: 10.1021/la902108s
1117
Article
Wang et al.
Figure 1. TEM images of Ag2S nanoparticle deposited from a toluene solution in air (top) and in Sc-CO2 (bottom). immediately after the fresh S2- ions had been added to solution. This solution was then allowed to stir at the specific reaction temperature for 1 h. After chemical reaction, the metal sulfide clusters would aggregate in the water core of the microemulsion to form nanoparticles. The microemulsion thus serves as a nanoreactor and a template for controlling the size of the nanoparticles. After that the dedecanethiol stabilized metal sulfide particles were isolated by adding a mixture of 6 mL of ethanol and 4 mL of methanol and then centrifuged. The supernatant was decanted, and the remaining nanoparticles were washed two more times with the same ratio of ethanol/methanol to remove AOT and excess dodecanethiol. The nanoparticles were then dried in air and resuspended in 250 μL of toluene or hexane. All of the procedures described above were conducted on the benchtop without the need for inert environments.
Deposition of Metal Sulfide Nanoparticles in Sc-CO2. One-Vial Method. In this method, deposition of the nanoparticles was carried out in a 14 mL high-pressure closed chamber. Carbon coated copper grids or pieces of Si wafer were immersed in a metal sulfide nanoparticle/toluene solution placed in a small vial. The chamber was slowly charged with liquid CO2 (60 atm) at room temperature over a period of 10 min and then the pressure was raised to 70 atm. The system was then slowly heated to 40 °C to convert the liquid carbon dioxide to the supercritical fluid. 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 reason to increase pressure from 140 to 160 atm is to ensure the system pressure is consistent and reproducible. The thiol-stabilized Ag2S nanoparticles would precipitate evenly and self-assemble to form a uniform 2-D array on the TEM copper grid in the Sc-CO2 phase with a small fraction of dissolved toluene (
