OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 Electrode Influence on Sensor Performance The influence of various electrode materials (FTO, silver, and copper) on sensor performance was assessed. FTO electrodes gave stable and reproducible responses, probably because of their chemical stability and strong adhesion to the sensing material. Silver electrodes improved sensitivity, probably because of their higher conductivity, which supports efficient charge transfer. Nevertheless, copper electrodes showed minor performance degradation with time, possibly because of oxidation effects, which can increase the contact resistance and lower the sensor sensitivity [32]. Comparison with existing sensors Comparative studies between various humidity sensors emphasize the relative merits of ZnO–NGM sensors compared to multiple carbon-based and metal oxide-based materials. As an example, gram carbon quantum dot-based sensors have good sensitivity (178.6–254.86 pF/% RH) but less desirable response and recovery times (7.3–14.1 s) and therefore experience difficult detection at lower relative humidity (RH) concentrations. PAA-MWCNT composite sensors also register a notable resistance change (930 Ω) with humidity fluctuation but have very high response (680 s) and recovery (380 s) times, restricting their applicability. Graphene oxide (GO)-modified PEDOT sensors, though highly responsive to humidity (4.97% sensitivity at 97% RH), are characterized by sluggish response (31 s) and recovery (72 s). Other graphene-based sensors, including Fe-GO and GO/WS₂ composites, offer modest performance but tend to have low sensitivity or high response/recovery times. In order to put the performance of the ZnO–NGM humidity sensor prepared in the present research into perspective, its performance was compared to a vast array of recently published nanomaterial-based humidity sensors. Table 2 gives an overview of sensitivity, response time, recovery time, and some observations from past research work along with data from this research work. The study shows that although some materials like gram carbon quantum dots have quite high sensitivity (178.6–254.86 pF/% RH), they have slow response and recovery times of up to 14.1 seconds, which restricts their use in fast-switching environments. Some designs, like the PAA-MWCNT composites, possess large resistance changes (930 Ω) but are weighed down by very long response (680 s) and recovery (380 s) times, making them unsuitable for real-time sensing. Likewise, GO-modified PEDOT sensors exhibit a humidity sensitivity of 4.97% at 97% RH but with slower response (31 s) and recovery (72 s) cycles. Fe–GO and GO/WS₂ sensors possess better speed but relatively lower sensitivity, particularly at lower RH values. Even GO-based sensors with abnormally high sensitivity (e.g., 37,800%) suffer from slow recovery times (~41 s) or are limited due to poor applicability in real-world situations because they are unstable at lower RH. Conversely, ZnO–NGM sensors prepared in this work show a balanced and superior performance profile. The 5% NGM-doped ZnO sensor recorded a sensitivity of 53.9 pF/% RH, a response time of 4.2 s, and a recovery time of 6.6 s, outperforming many of the reported sensors with a significantly better dynamic response while retaining good sensitivity. At lower doping levels (e.g., 2% NGM), the sensor retained fast response (4.5 s) and recovery (6.9 s) times with a nominally lower sensitivity of 38.7 pF/% RH. In addition, in contrast to most carbon-based sensors that exhibit scattered results across different RH levels, the ZnO–NGM sensors exhibited stable and reproducible performance within a broad RH range (10%–95%), rendering them feasible for application in real-time environmental or industrial monitoring systems. The fact that such sensors can also be made from inexpensive materials and using a facile doctorblade method on FTO substrates further contributes to their practical benefits. Structural and morphological analyses confirmed the successful incorporation of nanographite material (NGM) into the ZnO matrix. X-ray diffraction (XRD) patterns showed distinct peak shifts with increasing NGM content, indicating lattice strain and structural modification. Scanning electron microscopy (SEM) images revealed a uniform distribution of NGM within the ZnO matrix, resulting in increased surface roughness and the formation of additional active sites for water molecule adsorption. Fourier transform infrared (FTIR) spectroscopy further confirmed the presence of functional groups associated with humidity sensing, such as O–H stretching and C=C vibrations.
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