Design of Highly Sensitive and Selective H2s Gas Sensor Based on CuO-Cu7S4 Microflowers

Abstract

Introduction CuxO (copper oxides, x=1, 2) as a p-type oxide semiconductor is widely used in photocatalysis, electrochemistry, solar energy conversion, gas sensors [1], and photoactivated splitting of water due to its non-toxicity, low cost, and facile controllable synthesis. In order to meet the increasing requirements of the above applications, various morphologies of CuxO such as nanocubes [2], nanowires [3], nanorods, nanoflowers, nanoparticles, nano-octahedrons, nanospheres, have been prepared by different methods. In this work, CuO-Cu7S4 microflowers were successfully synthesized through a simple hydrothermal route and used as sensing materials. In the comparative gas sensing test, the sensor fabricated from CuO-Cu7S4 (S/Cu = 27 mol%) microflowers exhibited the highest response to hydrogen sulfide, and the fast response and recovery time. CuO-Cu7S4 microflowers gas sensor showed not only superior hydrogen sulfide selectivity with other gases, but also ppb-level detection limit at 225 °C. The synergistic catalytic effect between CuO and Cu7S4, optimized structural parameters and marked resistive variation due to the formation of nanoscale p-p heterojunctions were regarded as the main reasons for the ultrasensitive and selective hydrogen sulfide detection. Method For device fabrication, a small amount of CuO-Cu7S4 microflowers were dissolved with a few drops of the deionized water in an agate mortar to form slurry and then coated uniformly on the ceramic tube (4 mm in length, 1.2 mm in external diameter and 0.8 mm internal diameter) with a pair of Au electrodes and Pt wires. The coating was calcined at 90 °C for 1 h in air using a muffle furnace. Then, a Ni-Cr alloy coil heater was inserted through the well-coated ceramic tube, which controlled the working temperature. The sensing measurement of the sensors was carried out on a static system. The resistance of sensing material was continuously recorded by digital multimeter and the sensing response (S) was defined as the percentage of relative variation in resistance (ΔR/Ra*100%), where ΔR represents the variation of resistance before and after contacting with target gases, while Ra represents the resistance of gas sensor in initial state. The response/recovery time was defined as the required time for the resistance to reach 90% of total resistance variation relative to the former equilibrium value upon exposure to/removal from target gases. Results and Conclusions In this work, the Cu7S4-CuO composites were synthesized by hydrothermal method, More importantly, the ppb-level detection limit indicated the potential application on the trace hydrogen sulfide detection. Such behaviors could be attributed to the existence of p-p heterojunctions, co-catalytic effect, the large BET surface areas and the synergistic combination between Cu7S4 and CuO.