In this article, nanowire thin films (NTFs) and the feasibility of their application for ultra-sensitive chemical detection are discussed.
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What Are Nanowire Thin Films?
NTFs are composed of thin layers of nanostructured objects, such as nanowires, nanotubes, nanorods, and nanoporous networks. NTFs, such as nanowire thin films, have received significant attention for several advanced applications owing to their exceptional physical and chemical properties.
NTFs are synthesized using different methods, including template-based synthesis techniques, electrospinning methods, spin coating methods, electrochemical etching/deposition methods, catalyst-assisted fabrication methods, diblock copolymer methods, and chemical/physical vapor deposition methods.
The thermal vapor transport method is employed for directly depositing the nanowire thin films from the vapor of the materials under supersaturating conditions. Thin film deposition can be a physical process through nucleation. Chemical reactions such as oxidation can also involve during the deposition process.
Application of Nanowire Thin Films for Chemical Detection
Several studies have demonstrated that nanowire thin films can be used effectively for chemical detection owing to their good stability, selectivity, sensitivity, and fast response.
In a study published in the Applied Physics Letters, researchers investigated the sensing characteristics of drop-cast thin ﬁlms of poly(3-hexylthiophene) (P3HT), zinc oxide (ZnO) nanowires, and P3HT:ZnO-nanowire hybrids with different compositions for several gases, including nitrogen dioxide (NO2), hydrogen sulfide (H2S), ammonia (NH3), methane (CH4), and carbon monoxide (CO) to determine the most sensitive thin film for NO2 detection.
In urban environments, NO2 is a common air pollutant that adversely affects human health, necessitating the development of sensors that detect extremely small NO2 concentrations.
Metal oxide semiconductors, such as tin oxide, tungsten trioxide, titanium dioxide, and ZnO, have been studied extensively for NO2 detection owing to their good stability, sensitivity, and convenience of operation.
Specifically, nanostructured semiconductors in the form of belts, particles, and wires demonstrate better sensitivity to different target chemicals/gases due to their large surface-to-volume ratio. However, metal oxide semiconductors are unsuitable for commercial sensors as they lack selectivity and require high working temperatures.
Although conducting polymers possess several advantages for sensing applications, including room temperature operation, low cost, and easy processing, these polymers lack specificity and display a sluggish recovery and response.
In this study, pure P3HT and ZnO-nanowire thin ﬁlms demonstrated higher sensitivity to H2S and NO2, and moderate sensitivity to CO. These films did not display any sensitivity to CH4 and NH3.
However, the drop-cast hybrid P3HT:ZnO-nanowire thin films showed a gradual increase in NO2 sensitivity and a reduction in H2S sensitivity with the increasing ZnO nanowire content. Hybrid thin films composed of P3HT:ZnO-nanowire in a 1:1 ratio by weight demonstrated the highest NO2 selectivity.
The increase in P3HT resistance after the addition of ZnO nanowires in hybrid P3HT:ZnO-nanowire thin ﬁlms indicated the role of ZnO nanowires in reducing P3HT by donating electrons. Thus, the highly reduced P3HT displays very high sensitivity towards oxidizing NO2, and a sluggish response and weak sensitivity towards the reducing H2S.
The study displayed that drop-cast hybrid thin films with P3HT:ZnO-nanowire in a 1:1weight ratio can be used effectively for rapid NO2 detection with high sensitivity and selectivity at room temperature. These hybrid films can be used feasibly to fabricate ultra-sensitive NO2 gas sensors in the 0–10 ppm range operating at room temperature.
In another study published in the journal Advanced Optical Materials, researchers demonstrated a conductometric NO2 gas sensor based on uniform cadmium sulfide (CdS)/ZnO core-shell nanowires. The core-shell nanowires were synthesized hydrothermally using a two-step process and assembled into a NO2 optoelectronic sensor and a photodetector.
The CdS/ZnO nanowire thin films fabricated on alumina substrates are composed of nanowires with 20–60 nm diameters and lengths of several μm. The CdS/ZnO nanowire films generated significantly larger photocurrents compared to CdS nanowires at room temperature under visible light irradiation at 468 nm wavelength.
Additionally, the CdS/ZnO nanowire thin films showed a reversible electrical resistivity change at 5–1000 ppb NO2 concentrations in the air. The response to NO2 was dependent on the 468 nm light irradiation intensity, with the maximum response of 6.7% for five ppm NO2 and 337% for 1000 ppm NO2 observed at 0.68 mW/cm2 light intensity.
Fast response and recovery times of 26 ms and 2.1 ms, respectively, were also observed, and the sensor characteristics remained unchanged for five months, indicating exceptional long-term stability.
The fast photoresponse and excellent photosensitivity of these films were attributed to the heterojunction structure formation between ZnO and CdS, which inhibited the recombination of photoinduced holes and electrons. Thus, the CdS/ZnO nanowire thin films can be used effectively to realize optically controlled, instant, ultrasensitive detection of ppb-level NO2.
To summarize, nanowire thin films will be used more extensively in the future for ultra-sensitive chemical detection as they can effectively address the drawbacks of metal oxide semiconductors and conducting polymers and rapidly detect the target chemical with extremely high sensitivity at room temperatures.
References and Further Reading
Mabe, T. L., Ryan, J. G., Wei, J. (2018). Functional thin films and nanostructures for sensors. Fundamentals of Nanoparticles, pp. 169-213. https://www.sciencedirect.com/science/article/abs/pii/B9780323512558000070?via%3Dihub
Saxena, V., Aswal, D.K., Kaur, M., Koiry, S.P., Gupta, S.K., Yakhmi, J.V., Kshirsagar, R.J., Deshpande, S.K. (2007). Enhanced NO2 selectivity of hybrid poly(3-hexylthiophene): ZnO-nanowire thin films. Applied Physics Letters, 90 (4), p. 043516. https://pubs.aip.org/aip/apl/article-abstract/90/4/043516/333680/Enhanced-NO2-selectivity-of-hybrid-poly-3?redirectedFrom=fulltext
Yang, Z., Guo, L., Zu, B., Guo, Y., Xu, T., Dou, X. (2014). CdS/ZnO Core/Shell Nanowire-Built Films for Enhanced Photodetecting and Optoelectronic Gas-Sensing Applications. Advanced Optical Materials, 2(8), pp. 738-745. https://onlinelibrary.wiley.com/doi/10.1002/adom.201400086
Ando, M., Kawasaki, H., Tamura, S., Haramoto, Y., Shigeri, Y. (2022) Recent Advances in Gas Sensing Technology Using Non-Oxide II-VI Semiconductors CdS, CdSe, and CdTe. Chemosensors, 10, p. 482. https://www.mdpi.com/2227-9040/10/11/482