Development of ammonia gas sensor based on Ni-doped reduce graphene oxide Thanva Tubthong, Anurat Wisitsoraat, Chookiat Tansarawiput, Pakorn Opaprakasit and Paiboon Sreearunothai MATEC Web Conf., 192 (2018) 02048 Published online: 14 August 2018 DOI: 10.1051/matecconf/201819202048