Carbon Nanohorns and Their Nanohybrid / Nanocomposites as Sensing Layers for Chemoresistive RH Sensors – a Review

Carbon nanohorns (CNHs), their nanocomposites, and nanohybrids have demonstrated significant potential for relative humidity (RH) monitoring at room temperature (RT) due to their exceptional physicochemical and electronic properties. At the same time, over the last few decades, resistive sensors have been extensively designed and manufactured due to their simple design, small size, low cost, robustness, quick response times, and wide RH measurement ranges. Recently, resistive sensors with CNHs as key sensing elements have been reported for use in RH monitoring. We summarize in this review the recent progress on resistive RT RH sensors based on CNHs. The most effective strategies to synthesize CNHs and approaches for functionalization, as well as the most relevant physicochemical and electronic properties of nanohorns, are presented in the first two sections of this review. The design of various RH resistive sensors, employing carbon nanohorn-based materials as sensing films, and the synthesis and performance of several CNH-based sensing materials in RH monitoring within the context of resistive sensor design are presented in the following two parts of this review. Pristine and functionalized carbon nanohorns, nanocomposites with different hydrophilic polymers, and nanohybrids with several semiconducting metal oxides are compared in terms of sensitivity, response time, and recovery time. The fifth part of this review presents several sensing mechanisms that are involved in RH monitoring. Finally, in the sixth part of this review, the authors aim to explain which are the most important advantages of using CNHs- based sensing layers in resistive RH detection and why these nanocarbonic materials are less used, at least for the moment, than graphene, graphene oxide, reduced graphene oxide, single and double- walled carbon nanotubes.