Fabrication of nano-sized particles of metallic copper and copper sulfide in Langmuir–Blodgett films

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Abstract

Langmuir monolayers of arachidic acid on a subphase of 0.3 mM CuSO4/17 mM NH3 were transferred to hydrophobic glass and mica substrates to give Langmuir–Blodgett films of cupric arachidate (CuAr). Transfer ratios of close to 1 were observed and regular “Y”-type deposition was demonstrated with hydrophobic glass. With mica as the substrate, “Z”-type deposition occurred. The films were exposed to H2S and hydrazine, and evidence for the formation of copper sulfide and metallic copper particles, respectively, was obtained by ultraviolet/visible spectrophotometry, X-ray photoelectron spectroscopy and atomic force microscopy.

Introduction

The deposition of ordered transition metal ion/fatty acid films on substrates with the Langmuir–Blodgett (LB) technique is well known 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16. The technique involves spreading a solution of a long-chain fatty acid, such as arachidic acid, on a subphase containing metal ions. At the appropriate pH, ion exchange occurs between the protons of the carboxyl group of the fatty acid and the metal ions in solution. For a divalent metal ion, the stoichiometry of the reaction with arachidic acid is given by:M2+(X−)2+2HOOC(CH2)18CH3→M2+{−OOC(CH2)18CH3}2+2HX

Centrosymmetric bilayers, with metal ions sandwiched between the polar head groups in the film, can be deposited on vertically mounted substrates which are dipped down and then up through the monolayer at the air/water interface. A diverse range of metal ions has been incorporated into LB films by means of this technique. Some of these include Pb2+ from the main block elements [17], Ti4+ [18], Fe3+ [9], M2+ (M=manganese [8], ruthenium [19], nickel, palladium, platinum [20], cadmium [6], mercury [21]) and Ag+ from the transition metal series [16], and Ca2+ and Ba2+ from the Group IIa metals [22]. It has been demonstrated that such LB films act as matrices for the formation of size-quantised or “Q-state” metallic 16, 23, 24and metal chalcogenide 2, 3, 4, 6, 7, 10, 11, 21semiconductor particles by simple exposure of the films to reducing agents and dihydrogen chalcogenides, respectively. The interesting properties of Q-state particles and the molecular control offered by LB film technology could provide a means of fabricating films for micro-electronics or optical applications.

Previous attempts at preparing ordered multilayers with “Y”-type transfer from fatty acid monolayers on Cu2+-containing subphases have not been successful 8, 25, 26. Electron spin resonance spectroscopic measurements of films prepared from Cu2+ subphases 8, 5have shown that a doublet from an isolated paramagnetic Cu2+ appears only after exposure to NH3. This has been attributed to the formation of a mononuclear Cu(NH3)2(OOCR)2 complex from the diamagnetic spin-paired Cu2(OOCR)4 type complex. It is postulated that the dinuclear species, present in the Langmuir monolayer, gives rise to the expanded isotherms and poor transfer characteristics. In the current study, the success in preparing multilayer Y-type LB films containing Cu2+ ions has been attributed to the addition of NH3 to the Cu2+ subphase. The films have been reacted with H2S and hydrazine as part of an ongoing investigation of LB films as a medium for the preparation of Q-state particles.

Section snippets

Materials

Ammonia (approximately 25% w/w, analytical reagent, BDH), anhydrous cupric sulfate (BDH, 98%), arachidic acid (eicosanoic acid) (Sigma, 99%), chloroform (Ajax, spectroscopic grade), dichlorodimethylsilane (Aldrich, 99%), dichloromethane (BDH, HPLC grade), dodecanol (Aldrich, 98%), hexane (BDH, HPLC grade), hydrazine monohydrate (Aldrich, 98%), hydrochloric acid (Rhone-Poluenc, 36%), hydrogen sulfide (CIG), methanol (Ajax, HPLC grade) and sodium hydroxide (BDH, 98%) were used as supplied. Glass

LB film preparation and properties

The surface pressure–area per molecule isotherms for arachidic acid (ArH) spread on 0.3 mM CuSO4/17 mM NH3 and 0.3 mM CuSO4 subphases are shown in Fig. 1. The π–A isotherm of ArH on CuSO4 was expanded relative to that of ArH on the CuSO4/NH3 subphase, consistent with previous measurements of fatty acid monolayers on an uncomplexed Cu2+ subphase [8]. All of the following discussion concerns films deposited from CuSO4/NH3 subphases. For the CuSO4/NH3 subphase a steep rise in surface pressure occurs

Conclusions

Y-type LB films of cupric arachidate could be prepared on hydrophobic substrates and a transfer ratio of close to unity was maintained over several dip cycles. This has been accomplished by the addition of NH3 to a Cu2+ subphase which has presumably suppressed formation of the Cu2{OOC(CH2)18CH3}4 species at the air/water interface that leads to expanded isotherms, unstable films and poor transfer characteristics. Exposure of the CuAr films to H2S and hydrazine leads to the formation of a copper

Acknowledgements

This work was supported in part by the Australian Research Council. The authors would also like to thank Thomas Gengenbach for providing the XPS analysis.

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