Ion transmission spectroscopy of pores filled with Au nanoparticles

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Abstract

Ion track membranes (ITM) are of long-term interest due to the high potential of applications in science and industry. However, the use of the membranes requires a detailed knowledge of (i) their structural parameters, i.e., the density and distribution of the ITM pores, the pore size (radius) and spatial shape, or (ii) the pore filling with other materials (demanding the knowledge of filling efficiency, filler confinement, or filler density). In this work, we studied the nuclear membranes prepared with pores of certain density and size and filled with Au nanoparticles. The analysis was carried out by Ion Transmission Spectroscopy (ITS). ITS is a nondestructive technique to determinate the spatial structure of (sub)micron inhomogeneities (pores, protrusions, etc.) in thin foils from the energy loss of even quasi-monoenergetic alpha particles (e.g., from a thin 241Am source) transmitted through the (empty or filled) pores. The reconstruction of the shape of the pores or pore fillings is performed by simulation of the transmission spectra using the MC code. The nuclear membranes were prepared by the irradiation of a thin polyethylene terephthalate with 157 MeV Xe+26 ions (with the fluence 106 cm−2) and the subsequent chemical etching in 9 M NaOH water solution at 50 °C for 40 min. The pores were filled with the Au nanoparticles (NPs) of a sub-micrometer size using the Pulse Laser Deposition (PLD), and studied by ITS. From the transmission spectra the shape of the pores filled (partially or fully) with Au NPs could be reconstructed. The ITM with pores filled with NPs were also analyzed by scanning electron microscopy (SEM). The results showed good agreement with the MC simulation of the ITS data.

Introduction

The ion track-etched membranes have a wide range of applications in industry and science due to their precise structure characteristics, unique separation properties and ability to acquire versatile functionalities after appropriate modifications [1]. Accurate control of membrane properties (which affect physical processes such as ionic transport [2]) can be accomplished by targeted manipulation of pores shape and with synthesis of NPs on the surface and inside the pores of ITM. The etching process and the pore shape evolution can be investigating indirectly by several techniques, e.g. by the well-known conductometric method [3], [4]. Some information on pore internal structure can also be obtained using no-destructive methods for example using X-ray and neutron diffraction measurements or in a destructive manner by a FIB method combined with the SEM imaging [5] and electrochemical replication [6]. Non-destructive measurement can be performed also to get information about NPs distribution in membrane using fluorescence approach [7]. In this work, the purpose is to investigate the etching process and surface modification by a charge particle transmission technique [8]. Surface modification with nanostructures involves the application of deposition techniques. Various physical vapor deposition techniques (sputtering, e-beam evaporation, ion-beam assisted deposition, or pulsed laser deposition - PLD) offer advantages over laborious chemical production of nanoparticle thin films. In particular, PLD is a suitable technique for production of thin films, where the main advantage is its ability to generate ions and atoms (for further manipulation) with high kinetic energy. PLD is particularly powerful in synthesis of metallic NPs by pulsed laser illumination of targets in ambient gas atmosphere. Because of forceful cooling of the ablation plume by ambient gas, various NPs can be formed and deposited on substrates [9], [10]. By varying the pressure of the working gas, the size and density of NPs can be managed. Doping of nanochannels with nanoparticles changes the charge density of the membrane and affects physico-chemical phenomena [11], [12]. The filling of nanochannels with NPs represents an interesting challenge. In Ref. [13], filling of pores with NPs in liquid phase is classified into four categories (see sketch in Fig. 1): (a) NPs larger than channels; (b) NPs smaller than channels leading to NPs passage; (c) NPs smaller than channels, but adsorptive interactions cause the channel blockage; (d) NPs smaller than channels, but the channels are large enough that even if adsorption causes an initial rejection of NPs, a breakthrough will eventually occur. Additionally, the situation illustrated by Fig. 1(e) should be mentioned. NPs are smaller than the channels but the particles do not enter the pores and are collected on the surface because of electrostatic interaction. This situation is observed in diluted suspensions of nanoparticles bearing electrical charge of opposite sign compared to the charged membrane surface [14]. When the Debye length is comparable to the pore radius, the electric field has a configuration that provides retention of majority of the particles around the pore entrance. This is the case if the particles’ axial velocities are below a certain limit.

The overall study of the above mentioned phenomena prominently concur to the development of a series of environmental friendly applications including microfiltration for water purification, proton-exchange membranes in “green“ fuel cells, ionic transport in cell membranes, track-based biosensors for biomolecules and DNA markers [15], [16], [17], [18], [19], [20]. In order to clarify the process of filling of the pores with NPs without damaging the membrane, it is important to use systematically an analytical technique that does not cause any changes in the sample during the repeated measurements. In this paper, Ion Transmission Spectroscopy using low-fluence MeV ions (alpha particles), was used for nondestructive analysis of pores in a thin polymer foil coated with PLD-deposited Au nanoparticles.

Section snippets

Membrane preparation and deposition

The nuclear membranes were prepared (in JINR Dubna) by the irradiation of polyethylene terephthalate (PET) thin foils (19 μm) with 157 MeV Xe26+ ions (with the fluence 106 cm−2) and the subsequent chemical etching in 9 M NaOH water solution at fixed temperature of 50 °C for 40 min. The different etching conditions allowed to obtain partially developed pits, or empty pores (hollows) with various shapes (cylindrical, conical, etc.). With the selected etching conditions, fully developed pores were

ITS analysis

Fig. 4 shows the energy spectrum of transmitted alpha particles through the etched nuclear membrane. Two peaks are present in the spectrum, the Reduced Energy Peak (REP), due to the particles that pass through the whole thickness of the foils, and the Full Energy Peak (FEP), from the particles transmitted without energy loss through an open pore. The shape of the energy region between the two peaks gives information about the spatial form of the pores. Using the TOM code, the shape of the pores

Conclusions

The PET foils irradiated with the 1.2 MeV/amu Xe26+ ions to the fluence of 106 Xe/cm2 were etched in the 9 M NaOH water solution at 50 °C for 40 min, and inspected by Ion Transmission Spectroscopy to determine the internal shape of the pores. The etched foils were then coated with a thin film of the Au nanoparticles (NPs) using the Pulsed Laser Deposition (PLD) technique. The aim was to test whether it can be possible to effectively fill the pores with Au NPs. The pore filling was examined by

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.

Acknowledgements

The work was supported by the Grant Agency of the Czech Republic (Project No. 18-07619S). The experiments were carried out at the CANAM infrastructure of the Nuclear Physics Institute at Rez supported by the Ministry of Education, Youth and Sports (No LM2015056).

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