An in situ method of measuring electrolyte solution at the solid–liquid interface with MeV He+ beam in a vacuum

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Abstract

The ion distribution and its dynamic at the interface between solid and liquid play the main role in many applications. Measuring the solution at the solid–liquid interface in situ and noninvasively has been challenging because the electrolyte solution is volatile. Traditional ion beam analysis suffers from the energy straggling caused by the thick window and high-resolution analysis requires in-vacuum measurement. In this work, a cell with a 25 nm thick entrance window was fabricated. Solution of 0.1 mol/l BaCl2 was measured with 2 MeV He+ at the solid-electrolyte interface. It was revealed that a double layer structure was observed from the RBS spectrum. After measuring the cell for 4 h, there was no visible damage to the window. Therefore, this method can be used to measure the electrolyte solution in a vacuum in situ. It can also be expanded to measure gas or other volatile samples.

Introduction

The interfaces with electrolyte solutions containing various ions exist extensively in natural environments, biological systems and industrial products. At the interface between the solid and liquid, due to the nature of the solid surface material or the electric field in the solution, the charged ions in the solution are driven to or are adsorbed at the solid surface. The ion distribution and its dynamic in the interface solution play the main role in the working performance of applications such as nanochannel energy conversion, lithium batteries, supercapacitors, medical implants and drug delivery [1], [2], [3], [4], [5]. Analysis of the liquid electrolyte at the solid–liquid interface gives information on the evolution of electrochemistry processes such as battery charging, surface erosion and implanter degrading [6], [7], [8]. Several methods have been reported on the study of the solid–liquid interface, such as transmission electron microscopy (TEM) and atomic force microscopy (AFM), which provide the evolution process by the solid surface morphology, but the chemical composition of the electrolyte at the interface is not achievable [9], [10]. X-ray diffraction (XRD) can provide physical and chemical information of samples, but the poor resolution limits its application in the microanalysis of the solid–liquid interface [11], [12]. Since the electrolyte solution is volatile and the solid–liquid interface is easily interfered with or even destroyed by the probe, measuring the solution at the solid–liquid interface in situ and noninvasively has been challenging.

Ion beam analysis is a traditional nuclear technique to analyze the composition and depth profiles of elements in different materials, such as semiconductors, polymers, synthetic materials and batteries [13], [14], [15], [16]. As a non-destructive, sensitive, high-resolution analysis technique, ion beam analysis is normally used in vacuum condition due to the interference of the air with the primary and secondary particles. In addition, most aquatic solvents (e.g., water) are not stable in a vacuum and can also destroy the vacuum. Only few reports on ion beam analysis of electrolyte samples using thin isolation window were reported up to now. J.S. Forster et al used 1.6 MeV proton beam to penetrate a 1 μm thick window and analyzed Cu(NO3)2, AgNO3, Pb(NO3)2 solution in 1987 [17]. K. Morita used 9.0 MeV He2+ to pass through a 5.5 μm silicon window and analyzed the solid–liquid interface in 1997 [18]. M. Saito applied external RBS (Rutherford back scattering) to analyze liquid samples with proton in 2017 [19]. These studies suffered from the energy straggling caused by the thick window and the obtained resolution was normally lower than the in-vacuum ion beam analysis [20].

The development of the nanofabrication technology makes it possible to use an ultrathin film as the beam entrance window, which is also strong enough to store the liquid in a vacuum [21]. In the present work, a cell containing aquatic electrolyte with a 15 nm thick Si3N4 window was fabricated. 2 MeV He+ microbeam was applied to penetrate the window and analyze the element in the electrolyte solution. This ultrathin window can minimize the energy struggling and withstand the irradiation and the pressure due to the high vacuum during the entire ion beam analysis experiment. The RBS spectroscopy revealed a double layer structure of the element distribution in the solution. The data also proved that the ion beam had no influence on the distribution of the ions in the solution during the measurement. As a result, this work provides a method to directly investigate the in situ dynamical behavior of the ion distribution and evolution in the solution.

Section snippets

Sample fabrication

A liquid storage cell with an ultra-thin beam entrance window was fabricated. As shown in Fig. 1a, the cell was composed of a PMMA (polymethyl methacrylate)) box with a cavity of 3 × 3 × 3.5 mm3 in the middle. A platinum electrode was pasted at the right bottom of the cavity, and two PEEK (polyetheretherketone) tubes were used to feed liquid into the cell. Then the PMMA box was closed with an ultra-thin Si3N4 window. Fig. 1b shows the fabrication process of the Si3N4 window: a thin film of

Result and discussion

The liquid cell sample filled with 0.1 mol/l solution of BaCl2 was first scanned with PIXE with 2 MeV He+ beam, which was focused to 60 × 60 μm2. The window of the liquid cell was localized by Si mapping of the PIXE measurement, as shown in Fig. 3. Because the induced X-ray fluorescence was absorbed by the concave window frame close to the side of the detector, a blind zone was also observed. Then the He+ microbeam was positioned to the center of the window, and PIXE/RBS measurements were

Conclusion

A cell with a 15 nm thick window was fabricated. It was used to study the interface between the solution of BaCl2 and aluminum electrode with He+ beam in a vacuum. The behavior of the ion distribution and evolution can be observed from the RBS and PIXE spectra directly. The RBS spectrum did not change during the measurement, indicating that the ion beam itself does not affect the distribution of ions in the solution. After a 4-hour measurement, there was no visible damage to the window. In

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.

Acknowledgement

This study was supported by National Natural Science Foundation of China (NSFC grant # 11975283).

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