Short Communication
Residual- and linker-free metal/polymer nanofluids prepared by direct deposition of magnetron-sputtered Cu nanoparticles into liquid PEG

https://doi.org/10.1016/j.molliq.2021.116319Get rights and content

Highlights

  • A novel method for the production of nanofluids is developed.

  • The method deploys the synthesis of Cu nanoparticles by magnetron sputtering-driven gas condensation.

  • Cu NPs are deposited into vacuum-compatible liquid poly(ethylene) glycol.

  • Direct loading allows for the synthesis of plasmonic and photoluminescent nanofluids without residuals and chemical linkers.

Abstract

Colloidal solutions of metal nanoparticles (NPs) are typically produced by multi-step methods using a variety of chemicals. Resultant nanofluids require extra purification steps to remove the reactions by-products and modification steps to introduce anticoagulants for the solution stabilization. Here, we suggest a single-step method to produce nanofluid in which only two components are present: 22 nm-sized Cu NPs as a filler and polyethylene glycol (PEG, 400 g/mol) as a liquid base. The method employs magnetron sputtering to synthesize Cu NPs in a gas aggregation cluster source and their subsequent loading into vacuum-compatible PEG. The resultant nanofluid demonstrates strong plasmonic and photoluminescent activity, and shows enhanced stability over time, in contrast to conventional colloidal solutions. The approach offers an alternative for the production of nanofluids in which the processes of the NP formation are decoupled from the processes of their mixing with the liquid base.

Introduction

A nanofluid is defined as a colloidal mixture consisting of nanometer-sized particulates dispersed in a liquid base. The nanoobjects may be very diverse, including metal and metal oxide nanoparticles (NPs), carbon nanotubes, graphene sheets, etc. Typical liquid matrices involve water-based solutions, ethylene glycol, ionic liquids, and oils. Conventional approaches to produce nanofluids involve mechanical mixing of base liquids with commercially available nano-powders or various chemical routes in which specific reactants undergo chemical transformations with the formation of nano-sized particulates [1]. The wet chemical routes often employ the multi-step synthesis, sometimes complicated by unwanted side reactions. Furthermore, they lead to the generation of by-products that may require additional purification steps for their elimination.

The use of low-pressure gas-phase techniques for the production of nanofluids is seemingly counter-intuitive because liquids typically vaporize if introduced under vacuum. Nevertheless, Yatsuya probably was the first to show in 1974 that vacuum evaporation of silver onto silicon oil leads to the formation of Ag NPs [2], [3]. Later in 1996, Ye showed that sputtering of silver can also be employed and that patterned metal films grow at the silicon oil-vacuum interface [4]. A prerequisite for using liquid substrates under this approach is that the liquid withstands vacuum, i.e., it is characterized by sufficiently low vapor pressure.

The Ye's work was followed by other researchers who reported on sputtering of metals onto the surface of synthetic and vegetable oils [5], [6], [7], [8], [9], [10], alcohol [11], and ionic liquids [12], [13], [14], [15], [16]. Markedly, the formation of metal NPs has often been reported, and it opened vigorous discussions about the feasibility of this method for the “green” synthesis of nanofluids loaded with high-purity metal NPs. For the enhancement of the colloidal stability, the addition of polymers has been considered that enabled steric repulsion between the polymer layers adsorbed on the NPs [15]. It is not surprising that liquid polymers themselves have also been considered as host liquids. Low-molar mass polyethylene glycol (PEG) and pentaerythritol ethoxylate (PEEL) were the macromolecules of choice to synthesize Au-, Ag- and Cu NP-loaded nanofluids by magnetron sputtering [9], [17], [18], [19], [20], [21], [22], [23]. Plasmonic and photoluminescent single-metal NPs with stable emission in solution were synthesized; furthermore, bi-metal Cu-Au NPs [24] and Ag-Au NPs [25] were produced, the latter showing tunable emission within the UV-NIR range depending on the component ratio. Thus, polymer-based nanofluids can be highly attractive in optical applications. They are also thought to be promising in other fields, including heat transfer, catalysis, fuel cells, drug delivery, and many others.

The bottleneck of the sputter-based synthesis of nanofluids is that the NP nucleation processes occur from atomic metal fluxes arriving at the vacuum/liquid interface [26]. Incoming atoms may be reflected from the surface back to the gas phase; they can also diffuse over the surface or penetrate the bulk. It has been argued that the complex interplay between the thermodynamic (the interaction energy between the metal atoms and the polymer) and kinetic (the deposition flux) aspects defines whether thin films or NPs are created, and determines the size distribution of the latter.

Herein, we propose a new concept in which the NP formation is decoupled from the processes of their embedding into a liquid polymer. We take advantage of magnetron sputtering with subsequent gas-phase condensation of atomic metal vapors in a cold, inert gas (Ar) as it was first implemented in 1991 by Haberland [27]. In this approach, dc magnetron discharge is maintained within a separate vacuum chamber (gas aggregation cluster source, GAS) to sputter atoms of the target material (Fig. 1) [28], [29], [30], [31], [32], [33], [34]. Ar is cooled, and its pressure is chosen to induce the processes of spontaneous nucleation in atomic metal vapors, followed by the formation and growth of metal NPs. The GAS is mounted onto a deposition vacuum chamber from which it is separated by an orifice. The differential pressure is created between the two chambers, and NPs are dragged by the Ar flow from the GAS through the orifice into the deposition chamber. Typically, NPs are collected on solid supports. Instead, a Petri dish with a vacuum-compatible liquid can be placed beneath the orifice to collect the NPs on the liquid's surface. In this approach, the formation of NPs does not depend on either the supply of metal atoms from the gas phase or on the atomic diffusion over the liquid/vacuum interface, but the NPs themselves are supplied onto the liquid surface as ready-made entities.

Section snippets

Experimental

Herein, we choose to produce beams of Cu NPs by the magnetron-based GAS similar to our recent works [35], [36]. The GAS consisted of a cylindrical chamber of 10 cm in diameter with water-cooled walls. A planar magnetron with a diameter of 81 mm was utilized for dc sputtering of a Cu target (3 mm thick, 99.99% purity). The magnetron was powered by a DC01 BP power supply (Kurt J. Lesker). The discharge current was maintained constant at 150 mA. The sputtering was performed in Ar (Linde, purity

Results and discussion

The deposition of Cu NPs onto PEG was performed, and it resulted in the transformation of the initially clear and colorless liquid into a highly turbid and greyish solution. An example photo of such a solution is shown in Fig. 1. The solution gradually lightens over time and acquires a green color as is shown by the photo of the same sample taken after 120 h of storage (Fig. 1). We attribute this effect to the initial formation of an unstable colloidal suspension during the deposition, in which

Conclusions

We developed a single-step plasma-based method for the production of Cu NPs/PEG nanofluids in which the NPs are synthesized independently by magnetron sputtering, and their beams are subsequently deposited into vacuum-compatible liquid PEG. The resultant nanofluid consists of only two components and is free of chemical residuals. The use of stabilizing additives is also avoided.

Immediately after the preparation, the solutions are highly polydisperse and appear grey; however, larger agglomerates

CRediT authorship contribution statement

Andrei Choukourov: Conceptualization, Methodology, Validation, Resources, Supervision, Funding acquisition, Writing - original draft. Daniil Nikitin: Investigation, Validation, Formal analysis. Pavel Pleskunov: Investigation, Software, Resources, Data curation. Renata Tafiichuk: Investigation, Formal analysis. Kateryna Biliak: Visualization, Formal analysis. Mariia Protsak: Visualization, Formal analysis. Ksenia Kishenina: Visualization. Jan Hanuš: Resources, Validation. Milan Dopita:

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The work was supported by the Czech Science Foundation via the grant GACR 21-12828S. R.T., K.B. and M.P. also appreciate the support from the Charles University via a student grant SVV 260 579-2021. The authors thank Ivan Gordeev for the video editing.

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