What Determines the Electrochemical Properties of Nitrogenated Amorphous Carbon Thin Films?

Abstract

Linking structural and compositional features with the observed electrochemical performance is often ambiguous and sensitive to known and unknown impurities. Here an extensive experimental investigation augmented by computational analyses is linked to the electrochemical characterization of in situ nitrogen-doped tetrahedral amorphous carbon thin films (ta-C:N). Raman spectroscopy combined with X-ray reflectivity shows nitrogen disrupting the sp3 C–C structure of the reference ta-C, supported by the observations of graphitic nitrogen substitution in X-ray absorption spectroscopy. The surface roughness also increases, as observed in atomic force microscopy and atomic-level computational analyses. These changes are linked to significant increases in the hydrogen and oxygen content of the films by utilizing time-of-flight elastic recoil detection analysis. The conductivity of the films increases as a function of the nitrogen content, which is seen as a facile reversible outer-sphere redox reaction on ta-C:N electrodes. However, for the surface-sensitive inner-sphere redox (ISR) analytes, it is shown that the electrochemical response instead follows the oxygen and hydrogen content. We argue that the passivation of the required surface adsorption sites by hydrogen decreases the rates of all of the chemically different ISR probes investigated on nitrogenated surfaces significantly below that of the nitrogen-free reference sample. This hypothesis can be used to readily rationalize many of the contradictory electrochemical results reported in the literature.

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“…The surface and bulk elemental composition, as well as surface roughness, of same pristine ta‐C films studied in this work were also recently thoroughly characterized by Etula et al. with XAS, XPS, AFM and TOF‐ERDA [9]. Additionally, previous work on 30 nm thick ta‐C films from the same deposition equipment has shown them to be featureless in scanning electron microscopy (SEM) as well as in transmission electron microscopy (TEM) [4].…”

“…Substrates were placed 220 mm from the end of the filter coil on a 17 rpm rotating substrate holder. The deposition process of pristine ta-C films is detailed in literature [4,9]. Silicon wafers with ta-C films were cleaved into pieces and prepared into electrodes by masking them with PTFE tape (Irpola, Finland) onto FR-4 copper laminate sheets (MG Chemicals, USA).…”

Machine learning based modeling of disordered elemental semiconductors: understanding the atomic structure of a-Si and a-C

“…Residual elements present in the reactor chamber and sample setup lead to unintentional doping of a-C films with a wide range of chemical species, the most significant of which are H, O, N and Si (see, e.g. time-of-flight elastic recoil detection analysis [122] results of the elemental makeup of a-C:N [13]). Besides unintentional doping, it is possible to incorporate impurities in order to achieve a desired effect.…”

“…Current uses of a-C and a-C thin films include biocompatible and bioimplantable devices (such as hip replacement implants) [9], electrochemical sensors for in vivo analysis [10] and hard coatings for tribological applications [11]. Furthermore, modified a-C such as oxygen-rich a-C (a-COx) [12], nitrogen-doped a-C (a-C:N) [13], different carbon hybrid materials [10], nanocarbons modified under extreme conditions [14][15][16] and the wider family of disordered carbons are starting or expected to make their way to emerging applications in energy storage [17]. More generally speaking, carbonbased materials are envisioned to be key in the transition to renewable raw materials utilization and the bioeconomy [18].…”

Carbon nanotube membranes for EUV photolithography: a versatile material platform

“…As quantified in Table 1, in addition to metal contaminants being removed, the remaining surface oxygen loading is also remarkably low when compared to typical carbon nanomaterials. [1][2][3][4] This further increases EUV transmission, since oxygen absorbs EUV more than carbon. 5…”