Concomitant Optimization of Electrodes and Catalytic Surface Dots for 2-D-MoS2 Based Gas Sensors: An Innovative Single Stage Fabrication Approach

The surface modification of gas sensing layer by noble metals is a well-established technique to improve sensor performance, but it is mostly done by chemical methods and the wet process-related hazards viz. non-uniformity in surface modifier dimensions, filling up of nanopores on sensing layer, and compromised safety is bottleneck. The present work introduced an innovative approach to sensor fabrication, where surface modification (in the form of circular-shaped metal dots) and electrode deposition, on 2-D-MoS2 sensing layer, were achieved in a single fabrication stage by physical (e-beam) method. This approach relinquished the requirement of sequential fabrication, which typically consists of wet process-based surface modification followed by electrode deposition by physical means. In this technique, surface modification was achieved through deposition of well-designed equally spaced circular-shaped metal dots (two, four, six, and eight numbers of dots, each having a diameter of 0.5 mm) by e-beam evaporation on the MoS2 sensing layer. Three electrode materials—gold (less catalytic activity), silver (moderate catalytic activity), and palladium (highly promising catalyst)—were chosen to evaluate the effect of electrode material in terms of their catalytic influence on gas sensing performance. Due to the poor sensing performance with silver electrodes, the advanced study was focused on gold and palladium electrodes, exploring the effect of three electrode dimensions [1 mm2 ( $1\times 1$ mm), 4 mm2 ( $2\times 2$ mm), and 9 mm2 ( $3\times 3$ mm)]. Considering alcohols as the target species, sensor study was performed. The results demonstrated that MoS2 sensing layers with palladium electrodes ( $2\times 2$ mm) exhibited the highest response magnitude (RM) (~200%), owing to superior catalytic activity of Pd, followed by gold (~175%) and silver (~150%) at 400 ppm of methanol. With increasing number of surface modification dots, RM increased first, reached the maxima for six dots, and then reduced again for eight dots. With a higher number of dots (eight in the present case), most active sensing sites on the MoS2 surface became covered. As a result, despite the increased catalytic coverage area, the basic sensing matrix reduced, leading to a degraded response toward test species. The excessive coverage of catalytic metal hindered the active gas-sensing layer (2-D MoS2), and an optimal tradeoff between catalytic enhancement and exposed sensing area was found to be critical for achieving the best sensing performance. However, due to the higher coverage by metallic Pd islands, the overall transport kinetics improved. Consequently, with an increasing number of surface modification dots, the response time improved monotonically.