Quaternary Oxidized Carbon Nanohorns—Based Nanohybrid as Sensing Coating for Room Temperature Resistive Humidity Monitoring

Abstract

We report the relative humidity (RH) sensing response of a resistive sensor, employing sensing layers, based on a quaternary organic–inorganic hybrid nanocomposite comprising oxidized carbon nanohorns (CNHox), graphene oxide (GO), tin dioxide, and polyvinylpyrrolidone (PVP), at 1/1/1/1 and 0.75/0.75/1/1/1 mass ratios. The sensing structure comprises a silicon substrate, a SiO2 layer, and interdigitated transducer (IDT) electrodes. The sensing film was deposited via the drop-casting method on the sensing structure. The morphology and the composition of the sensing layers were investigated through Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and RAMAN spectroscopy. The organic–inorganic quaternary hybrid-based thin film’s resistance increased when the sensors were exposed to relative humidity ranging from 0 to 100%. The manufactured devices show a room temperature response comparable to that of a commercial capacitive humidity sensor and characterized by excellent linearity, rapid response and recovery times, and good sensitivity. While the sensor with CNHox/GO/SnO2/PVP at 0.75/0.75/1/1 as the sensing layer has the best performance in terms of linearity and recovery time, the structures employing the CNHox/GO/SnO2/PVP at 1/1/1/1 (mass ratio) have a better performance in terms of relative sensitivity. We explained each constituent of the quaternary hybrid nanocomposites’ sensing role based on their chemical and physical properties, and mutual interactions. Different alternative mechanisms were taken into consideration and discussed. Based on the sensing results, we presume that the effect of the p-type semiconductor behavior of CNHox and GO, correlated with swelling of PVP, dominates and leads to the overall increasing resistance of the sensing layer. The hard–soft acid–base (HSAB) principle also supports this mechanism.

Ternary Holey Carbon Nanohorns/TiO2/PVP Nanohybrids as Sensing Films for Resistive Humidity Sensors

“…For instance, Figure 7b shows Raman spectra recorded in four different positions of the CNHox/TiO 2 /PVP = 2/1/1 (w/w/w) film plotted in red, grey, green, and blue colors. It can be observed that three active Raman bands (D, G, and 2D) were recorded at the wavenumbers of 1318.3, 1591.9, and 2627.8 cm −1 , which confirms the presence of the nanocarbonic material (CNHox) [27,30,34]. One can also identify specific anatase TiO 2 bands as follows: E g1 mode at 149.1 cm −1 (very sharp and intense in all the samples, except for 1/1/1), B 1g at 397.6 cm −1 , A 1g at 513.9 cm −1 , and E g3 at 634.7 cm −1 [47][48][49].…”

“…Each X-ray diffraction pattern shows a broad feature at 21.08 • , which becomes more intense at greater CHN concentrations (Figure 8). This can be attributed to the (002) reflection of CNHox nanoparticles [34,50] The relative humidity monitoring capability of each carbonaceous nanohybrid-based thin film was explored by applying a constant current between the two electrodes and measuring the voltage difference when varying the RH from 0% to 100%. For the convenience of our analysis, we use the following abbreviations:…”

“…RH sensing capabilities of the ternary nanohybrid, such as CNHox/ZnO/PVP, CNHox/SnO 2 /PVP, and the quaternary nanohybrid from the type CNHox/GO/SnO 2 /PVP, were investigated within the design of the resistive humidity sensor using an interdigitated metal structure (IDT) deposited on the Si/SiO 2 structure [32][33][34]. For all the tested sensing layers, the resistance linearly increased when the sensors were exposed to a relative humidity ranging from 0 to 100%.…”

Ternary Oxidized Carbon Nanohorns/TiO2/PVP Nanohybrid as Sensitive Layer for Chemoresistive Humidity Sensor

“…The IDT's metal stripes were made by successive deposition of 10 nm chrome (Cr) and 100 nm gold (Au). The sensor has dimensions of 20 × 11 mm 2 [6,7,10].…”

“…At the same time, a lot of recently reported work focused on using carbon-based nanomaterials as sensitive layers within the design of humidity sensors [5]. Among these, in the last years, oxidized carbon nanohorns-single-graphene tubules with oxygen functionalities, mostly carboxylic groups (CNHox) and their nanocomposites, have been extensively explored and have proven to be an attractive option [6][7][8][9][10]. Interestingly, the oxidized carbon nanohorns-TiO 2 nanohybrid was recently used for enhanced photocatalytic hydrogen production [11].…”