Improving room-temperature trace NO2 sensing of black phosphorus nanosheets by incorporating benzyl viologen

Black phosphorus (BP), an appealing two-dimensional (2D) material, has attracted considerable attention in gas sensing by virtue of large specific surface area, high carrier mobility and small out-plane conductivity, but undesirably hindered by incomplete recovery and fragile reproducibility especially toward strongly oxidizing NO2 gas. To overcome these limitations, in this report we attempt to incorporate benzyl viologen (BV) with few-layer BP nanosheets to detect trace NO2 at room temperature (20 degrees C). Compared with BP sensor, BP-BV one showed superior recovery characteristics, repeatability, selectivity and humidity-repelling features. In addition, the sensor response remained quite large and nearly unchanged after BV incorporation (-33 % for BP-BV-1 sensor vs. -32 % for BP counterpart toward 50 ppb NO2, which was among the best cases of BP-involving NO2 detection). Herein, BP layers not only interacted with NO2 gas but also transported electrons. BV mainly played dual roles in the composites during sensing tests. On the one hand, BV offered additional electrons to NO2 gas to compensate for NO2 adsorption loss arising from BV-induced partial covering of some high-energy sorption sites on BP nanosheets. On the other hand, BV effectively passivated BP through occupying these sites, thus maintaining a reversible recovery and a repeatable response. This work revealed an improved NO2 sensing of BP nanosheets by incorporating BV material of optimal content, enriching the alternative strategies for reversible and sensitive operation of novel BP-related sensors.