## 1 Introduction  
Methane gas is extensively employed as fuel in houses and industries, and as a propellant for automotive vehicles [1; 2; 3]. As a chemical raw material, it also finds application in the production of carbon black, acetylene, carbon disulfide, and methanol [4], among others. CH4 is the main gas component in coal mines, and it is extremely dangerous, since it easily explodes when it reaches a volume concentration between 4.9% and 15.4% in air. Because of the odorless and flammable nature of this asphyxiant gas, the development of sensitive detectors for real-time detection at concentrations lower than its explosive limit is in pressing need. Furthermore, low-cost fabrication of CH4 sensors is an asset in order to make such devices easily accessible to the general public. In this framework, the ideal combination of sensors properties includes sensitivity, quick response, low-cost sensing material and sensor components. Platforms such as the electrochemical ones, which often proved to be effective toward multiple targets [5; 6; 7; 8; 9; 10; 11; 12], were also applied to CH4 sensing [13; 14; 15], which required the gas being trapped in liquids. It must be added that, for other gases such as CO2, potentiometric sensors were developed, without a need for trapping in liquid, albeit working at high temperatures and requiring a BaCO3 layer for limiting the humidity interference [16]. Under these premises, we designed and probed a low-cost material to insert in a home-assembled sensor, built with an easily available set of components, for high-sensitivity CH4 detection. The rationale in the approach for defining the sensing material is the best match of substrate and additional modifying material to enhance the sensing properties. We opted for NiO as a substrate, because of its good sensingproperties toward CH4[17], and because it can be synthesized in different morphologies and various surface areas and porosity [18; 19; 20], with the aid of several synthetic techniques [21; 22; 23], whereby the morphological properties reflect the gas-sensing performance [24; 25; 26; 27]. Furthermore, it was shown it can be successfully modified to further improve its sensing capabilities [28]. In general, several variants of NiO-based materials have been proposed, as sensing elements of gases, with different morphologies, doping, and p-n junction constructions conditioning the sensing properties. Recently, nanocrystalline NiO nanoplates [29], porous NiO nanosheets [30], and Pt@NiO core-shell nanostructure [31] were employed in hydrogen sensing. Selective sensitivity toward NO2 in a humid environment was confirmed by the limited response for different reducing gases, such as CO, CH4, NH3, and SO2, using ultrathin NiO sensors [32]. Acetone could be detected using Ru-doped NiO [33] or NiFe2O4 nanoparticle-decorated NiO nanosheet sensors [34]. Detectors for toluene [35], formaldehyde [36], triethylamine [37], xylene [38], methanol [39], and ethanol [24; 40] were implemented, using NiO heterojunctions, porous NiO, NiO nanoparticles, nanoplatelets, and decorated NiO. The modifications put in place for methane detection so far are largely based on the improvement of the response properties by creating nanocomposites such as NiO-rGO (reduced graphene oxide) [28] and hetero-junctions [41] with p-type NiO. As far as methane detection is concerned, sensors are often based on n-type semiconductors, and SnO2 is typically used. Recently, Pd-SnO2[42], Pt-SnO2[43], and r-GO/SnO2[44] were investigated as materials for methane sensors. On the other hand, SnO2 was also used by decorating NiO for the same sensing purpose [45]. In terms of performance, these sensor materials display optimum working temperatures in the range 150-340 \({}^{\circ}\)C for detection of 100 to 1000 ppm CH4.