This is how the low dimensional material structure relevant to this "sniffer" is made. The device material is essentially just slender graphene oxide (essentially just a carbon paper that is a few atomic layers thick with a few oxygen atoms here and there) along with zinc oxide tiny particles (nanoparticles). Appropriate heat treatment is very essential to get a good device material (in other words; they better be cooked well - like in case of food under or over cooking is problematic). 

At the site of action - methane - an otherwise difficult to detect molecule is oxidized (essentially burnt up); the electronic signatures of which are helpful in sniffing out its presence. ZnO seems to aid and accelerate this oxidation process resulting in higher sensitivity of this device and quicker response.

Ref: Argha Sarkar, Santanu Maity, Aneesh M. Joseph, S Chakraborty, and Tiju Thomas, "Methane-Sensing Performance Enhancement in Graphene Oxide/Mg:ZnO Heterostructure Devices" Journal of Electronic Materials (2017); DOI: 10.1007/s11664-017-5619-1