Tungsten Disulfide Nanotube-Modified Conductive Paper-Based Chemiresistive Sensor for the Application in Volatile Organic Compounds’ Detection

Toxic and nontoxic volatile organic compound (VOC) gases are emitted into the atmosphere from certain solids and liquids as a consequence of wastage and some common daily activities. Inhalation of toxic VOCs has an adverse effect on human health, so it is necessary to monitor their concentration in the atmosphere. In this work, we report on the fabrication of inorganic nanotube (INT)-tungsten disulfide, paper-based graphene–PEDOT:PSS sheet and WS2 nanotube-modified conductive paper-based chemiresistors for VOC gas sensing. The WS2 nanotubes were fabricated by a two-step reaction, that is oxide reduction and sulfurization, carried out at 900 °C. The synthesized nanotubes were characterized by FE-SEM, EDS, XRD, Raman spectroscopy, and TEM. The synthesized nanotubes were 206–267 nm in diameter. The FE-SEM results show the length of the nanotubes to be 4.5–8 µm. The graphene–PEDOT:PSS hybrid conductive paper sheet was fabricated by a continuous coating process. Then, WS2 nanotubes were drop-cast onto conductive paper for fabrication of the chemiresistors. The feasibility and sensitivity of the WS2 nanotube-modified paper-based chemiresistor were tested in four VOC gases at different concentrations at room temperature (RT). Experimental results show the proposed sensor to be more sensitive to butanol gas when the concentration ranges from 50 to 1000 ppm. The limit of detection (LOD) of this chemiresistor for butanol gas was 44.92 ppm. The WS2 nanotube-modified paper-based chemiresistor exhibits good potential as a VOC sensor with the advantages of flexibility, easy fabrication, and low fabrication cost.

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

In this paper, we present the relative humidity (RH) sensing response of a chemiresistive sensor, employing sensing layers based on a ternary nanohybrids comprised of holey carbon nanohorns (CNHox), titanium (IV) oxide, and polyvinylpyrrolidone (PVP) at 1/1/1/(T1), 2/1/1/(T2), and with 3/1/1 (T3) mass ratios. The sensing device is comprised of a silicon-based substrate, a SiO2 layer, and interdigitated transducer (IDT) electrodes. The sensitive layer was deposited via the drop-casting method on the sensing structure, followed by a two-step annealing process. The structure and composition of the sensing films were investigated through scanning electron microscopy (SEM), Raman spectroscopy, and X-ray diffraction (XRD). The resistance of the ternary nanohybrid-based sensing layer increases when H increases between 0% and 80%. A different behavior of the sensitive layers is registered when the humidity increases from 80% to 100%. Thus, the resistance of the T1 sensor slightly decreases with increasing humidity, while the resistance of sensors T2 and T3 register an increase in resistance with increasing humidity. The T2 and T3 sensors demonstrate a good linearity for the entire (0–100%) RH range, while for T1, the linear behavior is limited to the 0–80% range. Their overall room temperature response is comparable to a commercial humidity sensor, characterized by a good sensitivity, a rapid response, and fast recovery times. The functional role for each of the components of the ternary CNHox/TiO2/PVP nanohybrid is explained by considering issues such as their electronic properties, affinity for water molecules, and internal pore accessibility. The decreasing number of holes in the carbonaceous component at the interaction with water molecules, with the protonic conduction (Grotthus mechanism), and with swelling were analyzed to evaluate the sensing mechanism. The hard–soft acid-base (HSAB) theory also has proven to be a valuable tool for understanding the complex interaction of the ternary nanohybrid with moisture.

Exclusive and ultrasensitive detection of formaldehyde at room temperature using a flexible and monolithic chemiresistive sensor

Formaldehyde, a probable carcinogen, is a ubiquitous indoor pollutant, but its highly selective detection has been a long-standing challenge. Herein, a chemiresistive sensor that can detect ppb-level formaldehyde in an exclusive manner at room temperature is designed. The TiO2 sensor exhibits under UV illumination highly selective detection of formaldehyde and ethanol with negligible cross-responses to other indoor pollutants. The coating of a mixed matrix membrane (MMM) composed of zeolitic imidazole framework (ZIF-7) nanoparticles and polymers on TiO2 sensing films removed ethanol interference completely by molecular sieving, enabling an ultrahigh selectivity (response ratio > 50) and response (resistance ratio > 1,100) to 5 ppm formaldehyde at room temperature. Furthermore, a monolithic and flexible sensor is fabricated successfully using a TiO2 film sandwiched between a flexible polyethylene terephthalate substrate and MMM overlayer. Our work provides a strategy to achieve exclusive selectivity and high response to formaldehyde, demonstrating the promising potential of flexible gas sensors for indoor air monitoring.

Fluorination vs. Chlorination: Effect on the Sensor Response of Tetrasubstituted Zinc Phthalocyanine Films to Ammonia

In this work, the effect of fluorine and chlorine substituents in tetrasubstituted zinc phthalocyanines, introduced into the non-peripheral (ZnPcR4-np, R = F, Cl) and peripheral (ZnPcR4-p, R = F, Cl) positions of macrocycle, on their structure and chemiresistive sensor response to low concentration of ammonia is studied. The structure and morphology of the zinc phthalocyanines films (ZnPcR4) were investigated by X-ray diffraction and atomic force microscopy methods. To understand different effects of chlorine and fluorine substituents, the strength and nature of the bonding of ammonia and ZnPcHal4 molecules were studied by quantum chemical simulation. It was shown on the basis of comparative analysis that the sensor response to ammonia was found to increase in the order ZnPcCl4-np < ZnPcF4-np < ZnPcF4-p < ZnPcCl4-p, which is in good agreement with the values of bonding energy between hydrogen atoms of NH3 and halogen substituents in the phthalocyanine rings. ZnPcCl4-p films demonstrate the maximal sensor response to ammonia with the calculated detection limit of 0.01 ppm; however, they are more sensitive to humidity than ZnPcF4-p films. It was shown that both ZnPcF4-p and ZnPcCl4-p and can be used for the selective detection of NH3 in the presence of carbon dioxide, dichloromethane, acetone, toluene, and ethanol.