Design And Fabrication Of Carbon-based Gas Sensors

Carbon is abundant and widely distributed in the nature.Due to its unique hybrid orbital and electronic structure,there are many kinds of new carbon materials in the world,including amorphous carbon,carbon nanotube,graphene and fullerene,etc.The applications of these new carbon materials in gas sensing have attracted increasing interests.It is very important to design and fabricate new gas sensors based on these carbon materials for safety,environmental monitoring and protection,and so on.In this paper,based on amorphous carbon,carbon nanotube and graphene,we designed and fabricated a new kind of gas sensor: nano carbon film/SiO2/Si heterostructure gas sensor.The hydrogen gas(H2)sensing characteristics of these gas sensors were studied.We also designed and fabricated the reduced graphene oxide-based ammonia gas(NH3)sensor on filter paper(rGO/paper),which is flexible,cheap.The details of our work are as follows:(1)The palladium doped amorphous carbon film and palladium film were deposited on silicon(n type and p type)substrate with native oxide layer in sequence by direct current magnetron sputtering method to fabricate palladium/palladium doped amorphous carbon /SiO2/Si(Pd/a-C:Pd/SiO2/Si)heterostructures.The H2 sensing characteristics of the Pd/a-C:Pd/SiO2/n-Si and Pd/a-C:Pd/SiO2/p-Si heterostructures were studied systematically.The gas sensing mechanism of Pd/a-C:Pd/SiO2/Si heterostructure had been presented based on solid band theory,semiconductor theory.It is found that the Pd/a-C:Pd/SiO2/n-Si and Pd/a-C:Pd/SiO2/p-Si heterostructures have different H2 sensing modes.In H2 ambient,the current of the Pd/a-C:Pd/SiO2/n-Si heterostructure increases,while the current of the Pd/a-C:Pd/Si O2/p-Si heterostructure decreases.The current-voltage(I-V)characteristics of Pd/a-C:Pd/SiO2/Si heterostructure are very sensitive to H2.The response to 0.1%,1% and 1.6% H2 of the Pd/a-C:Pd/SiO2/n-Si(Pd/a-C:Pd/SiO2/p-Si)heterostructure is 210%(185%),5030%(560%)and 13100%(840%),respectively.Furthermore,it is found that the I-V characteristics in air and pure H2 of the Pd/a-C:Pd/SiO2/n-Si and Pd/a-C:Pd/SiO2/p-Si heterostructures are just completely opposite.The Pd/a-C:Pd/SiO2/n-Si heterostructure shows an evident anisotype heterojunction behavior in air,while it shows an evident isotype heterojunction behavior in pure H2.The Pd/a-C:Pd/SiO2/p-Si heterostructure shows an evident isotype heterojunction behavior in air,while it shows an evident anisotype heterojunction behavior in pure H2.(2)Pd nanoparticles-decorated SWNTs(Pd-SWNTs)was prepared by a chemical method.Then we farbricated the Pd-SWNTs film by a filtration method.By transferring the Pd-SWNTs film on silicon(n type and p type)with native oxide layer and glass substrates,we fabricated the Pd-SWNTs/SiO2/Si heterostructure and Pd-SWNTs film resistance-type H2 sensors.The H2 sensing characteristics were studied.The gas sensing mechanism of the Pd-SWNTs/SiO2/Si heterostructure had been presented based on solid band theory,semiconductor theory and thermionic emission theory.It is found that the Pd-SWNTs/Si O2/p-Si heterostructure shows ultrahigh H2 response.The response to 0.02%,0.05% and 0.1% H2 is 125%,320% and 920%,which is tens of times higher than that of Pd-SWNTs film resistance-type H2 sensor and is superior to those of carbon nanotube-based resistance-type H2 sensors reported previously.It is also found that the gas sensing characteristics of the Pd-SWNTs/SiO2/Si heterostructures are closely related to the interface between Pd-SWNT film and Si substrate,and the series resistance of the heterostructure.In addtion,the density of the Pd nanoparticles decorated on SWNTs has also obvious effect on the sensing performance.(3)Two Schottky junctions,Pd nanoparticles and Pd film-decorated reduced graphene oxide/SiO2/p-Si(Pd-rGO/SiO2/p-Si and Pd/rGO/Si O2/p-Si)were fabricated and the H2 sensing characteristics were studied.The gas sensing mechanism of the Pd-r GO/SiO2/p-Si and Pd/rGO/SiO2/p-Si Schottky junctions had been presented based on solid band theory,semiconductor theory and thermionic emission theory.It is found that the I-V characteristics of Pd-rGO/SiO2/p-Si and Pd/r GO/SiO2/p-Si Schottky junctions were very sensitive to H2.In H2 ambient,the currents of the two Schottky junctions decrease.The response of Pd-rGO/SiO2/p-Si(Pd/rGO/SiO2/p-Si)Schottky junction to 0.01%,0.02%,0.04%,0.08% and 0.16% H2 is 63%(30%),103%(55%),195%(112%),410%(202%)and 970%(344%),respectively,which is superior to those of graphene-based resistance-type H2 sensors reported previously.In addition,the capacitance of the Pd/rGO/SiO2/p-Si Schottky junction was also very sensitive to H2.The response to 0.01%,0.02%,0.04%,0.08% and 0.16% H2 is 26%,42%,61%,75% and 81%,respectively.(4)A graphene oxide(GO)film on filter paper was fabricated by a filtration method.Then the GO film was reduced to rGO by sodium borohydride to farbricate rGO-based ammonia(NH3)sensor on filter paper(r GO/paper).The direct current(DC)and alternate current(AC)ammonia sensing characteristics were studied.It is found that both the DC an AC resistances of rGO/paper increase when exposed to NH3.When exposed to air again,both the DC an AC resistances restore to the initial values in air again.The DC(AC)resistance response to 0.01%,0.02%,0.05% and 0.1% NH3 is 14%(12%),24%(15%),41%(23%)and 57%(32%),which is much better than that of rGO film on rigid substrates reported recently.