Fig. 1. (Color online) (a) NiO ink, (b) Nickel oxide-graphene ink, (c) Nickel oxide-silver nanowire ink.

Fig. 2. (Color online) (a) DIW printing of IDE. (b) Schematic of IDE pattern. (c) IDE pattern after drying. (d) Printed nickel oxide-graphene sensor. (e) Printed nickel oxide-silver nanowire sensor.

Fig. 3. Schematic of gas sensor setup containing gas chamber, mass flow controller, source meter, and data acquisition system.

Fig. 4. (a) PXRD patterns of synthesized NiO nanoparticles. (b) PXRD patterns of as-purchased graphene powder. (c) PXRD patterns of synthesized silver nanowires. All peaks can be indexed to the respective materials. No impurity peaks are seen in the XRD plots. (d) SEM micrograph of the NiO nanoparticles. (e) SEM micrograph of silver nanowires with a larger magnification image in the inset. These are analyzed to obtain the average particle size, nanowire length, and diameter.

Fig. 5. (a) XRD pattern of nickel oxide-graphene nano-composite thin film, "X" represents the NiO peaks, and "O" represents the graphene peaks. (b) XRD of nickel oxide-silver nanowires nano-composite thin film, "X" represents the NiO peaks, and "O" represents the silver peaks. (c) Raman spectrum of nickel oxide-graphene containing NiO (2LO) and graphene (G and 2D bands). (d) Raman spectrum of nickel oxide-silver nanowires corresponding to NiO (1P-LO) and Ag (LO and 2M).

Fig. 6. (Color online) (a) HRSEM images of nickel oxide-graphene morphology showing the graphene flakes decorated with NiO. (b) HRSEM images of nickel oxide-silver nanowires morphology. (c) EDS data for nickel oxide-graphene. (d) EDS data for nickel oxide-silver nanowires.

Fig. 7. (Color online) (a) XPS of nickel oxide-graphene (NG) and nickel oxide-silver nanowires (NA) composites showing C_1s. (b) XPS of nickel oxide-silver nanowires (NA) composite showing Ag-3d peaks. (c) XPS of nickel oxide-graphene (NG) and nickel oxide-silver nanowires (NA) composite showing O_1s. (d) XPS of nickel oxide-graphene (NG) and nickel oxide-silver nanowires (NA) composite showing Ni-2p.

Fig. 8. (a) FTIR of nickel oxide-graphene representing the associated functional groups. (b) FTIR of nickel oxide-silver nanowires representing the bond formation. (c) UV−Vis absorption spectrum of nickel oxide-graphene, inset representing the band gap information. (d) UV−Vis absorption spectrum of nickel oxide-silver nanowires, with an inset showing the band gap information obtained using the Tauc plot method.

Fig. 9. (Color online) I−V plots of (a) Nickel oxide-graphene in the presence of air and NO₂. (b) Nickel oxide-silver nanowires in the presence of air and NO₂. (c) Nickel oxide graphene in the presence of air and CO₂. (d) Nickel oxide-silver nanowires in the presence of air and CO₂. The I−V plots were measured before (in the air) and after 5 SCCM of NO₂ and 30 SCCM of CO₂ gas exposure. The current has slightly increased in the presence of gas. These devices exhibit superior gas sensing at room temperature and low applied voltage thereby enabling the development of low-power sensors.

Fig. 10. (Color online) R−t plot curves for (a) Nickel oxide-graphene in the presence of NO₂. (b) Nickel oxide-silver nanowires in the presence of NO₂. (c) Nickel oxide-graphene in the presence of CO₂. (d) Nickel oxide-silver nanowires in the presence of CO₂.

Fig. 11. (Color online) (a) and (b) Stability of the sensors over 100 days, with measurements once every 7−10 days. The device was tested at 5 SCCM for NO₂ and 30 SCCM for CO₂ from −1 to 1 V and stored in an ambient atmosphere between tests. (c) Histogram representing nickel oxide-graphene is more stable for NO₂ and nickel oxide-silver nanowires are more stable for CO₂.