Nickel-Decorated Black Phosphorus for Room Temperature NO2 detection

Abstract

We study the electrical characterization at room temperature (RT) of black phosphorus nanoflakes decorated with nickel nanoparticles (Ni/bP). The latter has been exposed to different concentrations of NO2 at RT in a dry air environment highlighting fast and stable response over the time. This latter characteristic is particularly relevant since environmental instability of exfoliated BP has been an issue so far, under ambient conditions few-layer BP degrades completely in less than a week, preventing its applications in several fields. Introduction In the rapidly emerging field of layered two-dimensional (2D) functional materials, bP, the all P-counterpart of graphene, is a potential candidate for various applications e.g., nanoscale optoelectronics, rechargeable ion batteries, electrocatalysts, thermoelectrics, solar cells, and sensors [1]. bP has been reported to exhibit superior chemical sensing performance. In particular, bP is selective for the detection of paramagnetic molecules, e.g., NO2, in addition to high sensitivity at a limit of detection (LOD) of ppb levels.[2] In this work, by applying a multiscale characterization approach we demonstrated a stability and functionality improvement of Nickel-decorated bP (Ni/bP) films for gas sensing prepared by a simple, reproducible and affordable deposition technique. Furthermore, we studied possible electrical activity of these films for their employment as functional layers in gas sensors by exposing them to different gaseous compounds (NO2, CO2, H2, NH3, CO, Benzene, etha-nol, ethylene, Formaldehyde) in different relative humidity (RH%) conditions. Moreover, the influence in sensing perfor-mance of nickel nanoparticle (NP) dimensions in related to the decoration technique and the film thickness were investigated. Materials and Methods Black phosphorus microcrystals were suspended in dimethylsulfoxide and kept under ultrasonication for several hours. The exfoliated bP nanosheets were decorated with nickel nanoparticles following two different procedures: 1) preformed Ni NPs were immobilized on bP, 2) Ni NPs were grown directly on bP flakes. Electrical conductance measurements have been performed on Ni/bP flakes deposited via spin coating on Al2O3 substrates with built-in interdigitated gold electrodes, in order to better control the film thickness. Electrical conductance measurements have been performed at RT (25±2°C) by exposing the sensing film to certified mixtures of NO2 diluted in dry air (20% O2 and 80% N2) with different NO2 concentration ranging from 200 ppb to 1 ppm. Results and Conclusions Exfoliated black phosphorus nanoflakes decorated with nickel nanoparticles (Ni/bP) have been prepared and characterized with particular attention to the role of Ni nanoparticle dimension in the sensing mechanism. Electrical activity towards different concentrations of NO2 at room temperature in a dry air environment highlighted fast and stable response over the time. This latter characteristic is particularly relevant since environmental instability of exfoliated BP has been an issue so far, under ambient conditions few-layer bP degrades completely in less than a week, preventing its applications in several fields. In situ grown Ni/bP (2) shows stable response during all the four weeks measurements period with no dependence of the response on the film thickness. Ni decoration effectively suppress ambient degradation, for at least 1 week in Ni/bP (1) and over 4 weeks in Ni/bP (2) in ambient conditions. This result highlighted that Ni/bP (2) sensor can be practically used under ambient conditions for a reasonable period without performance degradation of the devices.The obtained results pave the way for the use of chemically exfoliated bP flakes for gas sensing applications, avoiding the use of a time-consuming technique for the exfoliation and for the fabrication of the metallic contacts, like electron beam lithography, focused ion beam lithography, etc.