Scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (SEM-EDX) analysis was performed to determine the composition of the deposited nanomaterials in the quartz silica tube. In Fig3, Tube 1 exhibited limited structural formation and a less defined morphology, as observed in the SEM analysis. The EDX analysis revealed an atomic composition consisting of 61.08% oxygen (O), 34.30% silicon (Si), and 4.62% carbon (C). The presence of silicon indicates the contribution from the quartz silica tube, while the carbon signifies the incorporation of carbon from the graphene powder in the synthesized structures although it only consists of 2.78% of the entire weight indicating that only a very small percentage of graphene was deposited. In Tube 2 and 3, an agglomeration of different structures was observed, including web-like formations, cloud-like structures, and tiny blocks. The EDX analysis of Tube 2 revealed an atomic composition primarily consisting of nickel (Ni) at 87.52% total weight percentage and a little portion of oxygen (O), and silicon (Si), which aligns with the presence of pure nickel oxide in this tube while Tube 3 showed atomic composition including oxygen (O), silicon (Si), carbon (C), and nickel (Ni). The additional presence of carbon indicates the incorporation of graphene, which was part of the composition in this tube with a combination of graphene and nickel (II) oxide. In Tube 4, there was a lesser formation of structures indicative of high weight percentage of (Si) pertaining to the quartz silica tube. The EDX analysis of Tube 4 displayed a similar atomic composition as Tube 3, with oxygen (O), silicon (Si), carbon (C), and nickel (Ni). However, in Tube 4, the presence of nickel indicates the utilization of pure nickel rather than nickel oxide in the nanomaterial synthesis.
Fig4 shows the gas current responses (IG) of Nickel Oxide (NiO), Graphene Nickel Oxide (GNiO), and Graphene Nickel (GNi) sensors at a voltage of 0.25 volts, focusing on the formaldehyde concentrations ranging between 1.91 ppm and 2.86 ppm. Nickel Oxide Sensor exhibits a decrease in current response from 0.53 nA to 0.39 nA as the formaldehyde concentration increases within the specified range. Similarly, the Graphene Nickel Oxide Sensor shows a decline in current response, dropping from 0.66 nA to 0.47 nA under the same conditions. These observations align with expectations of reduced sensor response with increasing formaldehyde concentrations, indicative of the sensors' sensitivity to the target gas. Unexpectedly, the Graphene Nickel Sensor displays a similar behavior although reaching a negative current measurement with its response decreasing from -0.02 nA to -0.1 nA.
Fig5 shows the sensor response (SR) of Nickel Oxide, Graphene Nickel Oxide, and Graphene Nickel sensors at a voltage of 0.50 volts, focusing on formaldehyde concentrations ranging from 2.86 ppm to 4.77 ppm. Initially, the Nickel Oxide Sensor demonstrates a moderate increase in sensor response, rising from 25.91% to 34.29% as the formaldehyde concentration escalates within the specified range. This observed increment aligns with expectations of heightened sensor response to higher concentrations of the target gas, reflecting the sensor's ability to detect and respond to increasing levels of formaldehyde.
In contrast, the Graphene Nickel Oxide Sensor exhibits a substantial surge in sensor response, escalating from 40.00% to 82.99% under the same conditions. This pronounced increase suggests a heightened sensitivity of the Graphene Nickel Oxide Sensor to formaldehyde, potentially attributed to the unique properties of graphene in enhancing sensing capabilities. Remarkably, the Graphene Nickel Sensor displays an exceptional response, surging from 93.33% to 420.00%. This exponential increase reflects the sensor's remarkable responsiveness to formaldehyde concentrations, surpassing the responses of the other two sensors by a significant margin.
In addition to the previous analysis, the examination of sensor response to formaldehyde gas was extended to include a higher input voltage of 2V, providing further insights into sensor behavior. The presented figure (Fig6) illustrates the Sensor Response (SR) at 2 volts for Nickel Oxide, Graphene Nickel Oxide, and Graphene Nickel Sensors across formaldehyde concentrations from 0.96 ppm to 4.77 ppm. Nickel Oxide Sensor demonstrates a moderate increase in response, from 49.19% to 54.07%, indicative of its responsiveness to formaldehyde within this concentration range. In contrast, the Graphene Nickel Oxide Sensor exhibits a higher increase in response from 7.39% to 37.14%, highlighting its enhanced responsiveness compared to the Nickel Oxide Sensor under similar conditions. Interestingly, the Graphene Nickel Sensor again displays exceptional performance, with its response reaching from 40.63% to 84.64%. This remarkable increase reaffirms the sensor's superior responsiveness to formaldehyde, consistent with previous observations.
Fig7 shows the overview of the sensitivity (S) of both nickel oxide (NiO) and graphene nickel (GNi) sensors at an input voltage of 0.25 volts, highlighting their performance across low and high concentration gaps of formaldehyde. In this context, a low concentration gap pertains to the gas concentration of formaldehyde between 2.86 ppm and 1.91 ppm, while a high concentration gap encompasses gas concentrations between 4.77 ppm and 1.91 ppm.
NiO sensor demonstrates a notable increase in sensitivity from 1 to 3.33 from low to high concentration gap, indicating its enhanced sensitivity to subtle variations in formaldehyde concentration within this range. However, the GNi sensor exhibits a substantially higher sensitivity range, escalating from 52.63 to 131.70 from low to high concentration gap. This significant increase shows the remarkable sensitivity of the GNi sensor to even minor fluctuations in formaldehyde concentration, showcasing its potential for precise and accurate detection within narrow concentration ranges.