Elsevier

Electrochimica Acta

Volume 280, 1 August 2018, Pages 238-247
Electrochimica Acta

Influence of the atomic nitrogen content in amorphous carbon nitride thin films on the modulation of their polarizable interfaces properties

https://doi.org/10.1016/j.electacta.2018.05.116Get rights and content

Abstract

Amorphous carbon nitride (a-CNX) thin films possessing different nitrogen contents (0.12 ≤ x ≤ 0.30) have been synthetized by the DC cathodic reactive magnetron sputtering technique on transparent glass/indium tin oxide (ITO) substrates. They were characterized ex-situ by using two microscopy techniques: scanning electron microscopy (SEM) and atomic force microscopy (AFM), a spectroscopic technique: X-ray photoelectron spectroscopy (XPS), and in solution by using electrochemical methods: cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results obtained with these methods have permitted to highlight why a-CN0.30, the a-CNx sample with the highest nitrogen content among those investigated in this work, behaves both as a dielectrical material (low electronic conductivity) and as an ideally polarizable interface (high polarization resistance). For this purpose, the role of both the ionic strength and the overpotential were investigated in blocking electrode conditions, i.e. only in the presence of the supporting electrolyte. It was observed that the interfacial capacitance can be strongly electrically modulated with the ionic charge concentration and with the applied overpotential. The understanding of the behaviour of a-CNx materials presents a great challenge for their integration in new microfluidic transistor type devices in view of the elaboration of polarizable interface flow-field effect transistors (PI-FFETs).

Introduction

In the last years, amorphous carbon nitride (a-CNx) thin films were abundantly investigated. In particular, electrochemically pre-treatments were developed to improve their surface electrochemical reactivity [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. Most of the time, a low atomic nitrogen content was selected for a-CNx thin film elaboration and cathodic and/or anodic electrochemical pre-treatments were empirically optimized in an acidic or alkaline medium, respectively. For this purpose, correlations between electrochemical properties and chemical composition, and between conductivity measurements and electrochemical reactivity of these films were investigated. In these studies, the atomic nitrogen and carbon contents in a-CNx thin films were determined by XPS [4,[11], [12], [13]]. Indeed, it has been previously reported that local variation of the sp2 and sp3 type bonding arrangements of nitrogen and carbon atoms induced local conductivity and electrochemical reactivity variations on the a-CNx surface [7,14,15], which played a key role on the a-CNx electrochemical properties. These results were in agreement with the literature where it was reported that sp3 Csingle bondN groups act as insulating sites while sp2 Cdouble bondC groups favor high conductivity areas [11,[16], [17], [18]]. Additionally, an increase of Csp2/Csp3 Raman band ratio reveals a transition towards a more graphitic microstructure in the a-CNx layer [[19], [20], [21]]. These studies have permitted the control and the exploitation of the surface chemistry of a-CNx [7] for the development of electroanalytical procedures allowing the detection and the separation of biomolecules with biological interest such as ascorbic acid and dopamine [8,22], or a drug called furosemide [10].

In 2010, Plecis et al. [23] were the first ones to successfully exploit a-CNx layers to produce a polarizable interface (PI) in microfluidic transistors using the Flow-Field Effect Transistors (FFETs) configuration [24,25]. It was thus an alternative to the conventional metal-insulator-electrolyte systems (MIE-FFETS) for controlling electro-osmotic flows (EOF) in microfluidics [[26], [27], [28]]. Indeed, in such microdevices, no charge transfer reaction (case of MIE-FFETS) from or towards the conducting layer takes place in the potential window or must be negligible (case of PI-FFETS) whatever the overpotential applied [23,29,30]. As a matter of fact, in the MIE configuration, the control of the EOF is strongly narrowed by the low value of the serial capacitance of the dielectrical film covering the electrodes. This limitation obviously does not exist with the PI configuration. However, in this later development [23], the authors often observed a deterioration of the a-CNx PI whose origin was not entirely understood. We put forward two reasons: (i) the a-CNX composition on glass was not optimized although the FFET was evidenced, and (ii) the absence of potential regulation system because neither a reference electrode nor an adapted apparatus have been used and connected to the first PI-FFET prototype to precisely measure the potential difference between the liquid at microchannel entrance and the lateral electrode. If the applied potential difference is larger than the potential window, it could entail degradation either on the cathodic side by disruption of the film due to hydrogen evolution or on the anodic side by erosion and formation of carbon dioxide.

In this work, we provide an answer to the first problematics. For this purpose, we optimized amorphous carbon nitride (a-CNx) deposition on transparent and conductive glass/ITO slides so that it fully complies only the PI requirements mentioned above. Electrochemical measurements were performed in a conventional electrochemical cell to precisely control the PI potential. As mentioned previously, these polarizable glass slides were not electrochemically pre-treated as traditionally investigated in the literature in order to enhance or catalyze its electroanalytical performances. In this work, we focused our attention on the moderately high nitrogen contents (P(N2)/Ptot reaching up to 30%) and their detrimental effects on the kinetics of Faradaic reactions. These results are important for a better knowledge of the interfacial phenomena occurring at a-CNx electrode/aqueous solution interfaces mainly when a high atomic nitrogen content is present in the a-CNx material.

Section snippets

Elaboration of glass/ITO/a-CNx samples

Glass/ITO samples possessing a square resistance of 30 Ω were purchased from SOLEMS (Palaiseau, France). They were first cleaned with ethanol, acetone, and bi-distilled water, and then submitted to a radiofrequency ion etching (13.56 MHz) in the reactor (model MP 300S, PLASSYS S.A., France). a-CNx thin films were deposited on glass/ITO substrates using the DC reactive magnetron sputtering technique, a graphite target and a Ar/N2 plasma with a total pressure of 0.4 Pa. The different N2 partial

X-ray photoelectron spectroscopy

The obtained XPS spectra have been analyzed to determine the bulk atomic nitrogen content of the various a-CNx thin layers deposited from different Ar/N2 plasma compositions (P(N2)/Ptot = 3%, 7%, 15%, or 30%). For this, each peak was isolated, and the baseline was eliminated using the Shirley method [31]. Spectra of O 1s for a-CNx layer are located around 536 eV whereas those for N 1s and C 1s are located at 400 eV and 282 eV, respectively. The chemical surface analysis of the layer was

Conclusion

Synthesis of amorphous carbon nitride (a-CNx) thin films was carried out on glass/ITO substrates so as to study the role of moderately high atomic nitrogen content on the polarizability properties of these materials in blocking electrode experiments i.e. with only KCl as supporting electrolyte in presence of residual dissolved O2. Chemical composition, using XPS characterization, nanoscale morphology and local conductivity using AFM measurements were investigated to confirm that high nitrogen

Acknowledgments

This research was supported by the French ANR (Agence Nationale de la Recherche) in the context of the P2N “DIMIPOLE” project. The authors would like to thank Christophe Methiviers (CNRS-LRS engineer) for XPS measurements, and also grateful to A. Pallandre, and I. Le Potier for fruitful discussions.

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