As society’s quality of life improves and public awareness about the harmful effects of air pollution on humans increases, the importance of monitoring and managing air quality has attracted considerable research attention. Volatile aromatic hydrocarbons (VAHs) such as benzene (B), toluene (T), ethylbenzene (E), p-xylene (X), and styrene (S) are the most representative air pollutants, which are detrimental to human health and safety, even at trace concentrations. Thus, selectively and sensitively detecting traces (on the order of sub-ppm) of VAHs are indispensable for determining the exact VAH source and establishing proper mitigation strategies.
Although various instruments have been used to detect VAHs, these instruments are often cumbersome and expensive and require time-consuming gas-sampling/pretreatment steps, which limit the application of such instruments in instantaneous, portable, and cost-effective gas monitoring. Oxide-based chemiresistors are viable alternatives because of their rapid response, simple structure, good stability, and easy miniaturization. However, using a simple sensing algorithm based on the charge transfer between the analyte gas and sensing surface often leads to a lack of gas selectivity. This issue is even more important for detecting stable low-reactivity gases such as VAHs. To date, although many approaches have been explored for overcoming this obstacle, the available gas sensors exhibiting both high selectivity and response toward VAHs remain limited and insufficient. Furthermore, identifying and quantifying specific chemical species, even in complex gas mixtures, remain major barriers to practical applications.
Therefore, we designed a bilayer sensor with a catalytic CeO2 filter that enables traces of aromatic BTEXS gases to be detected ultraselectively and ultrasensitively with negligible cross-responses to other representative interferants. To the best of our knowledge, the uniquely designed oxide-based bilayer sensor simultaneously exhibits the highest selectivity and response toward VAHs among all previously reported sensors and quantitatively discriminates and analyzes the compositions of aromatic BTEXS in gas mixtures. The key concept of the bilayer sensor is the catalytic oxidation of highly reactive nonaromatic interferants to less- or nonreactive species prior to the gas-sensing reaction. Furthermore, we validated the CeO2-coated bilayer sensor configuration for diverse gas-sensor applications using SnO2, Pt–SnO2, Au–SnO2, In2O3, Rh–In2O3, Au–In2O3, WO3, and ZnO. We believe that the proposed sensor will generate various opportunities for developing highly precise, reliable, and cost-effective personal gas sensors to protect humans from the harmful effects of VAHs.
The highlights of this study are as follows:
- The first demonstration of ultra-selective and sensitive detection of VAHs using oxide semiconductor chemiresistors, even in gas mixtures: Extremely high selectivity and response to VAHs was achieved for the first time using oxide semiconductor gas sensors with a catalytic oxide overlayer. This selectivity and response values to VAHs are higher than those of previously reported gas sensors prepared using oxide-based semiconductors and other emerging materials. In particular, the Rh–SnO2 bilayer sensor with the CeO2 overlayer exhibited nearly the same B, T, E, X, and S responses regardless of the interfering gases in the mixture, enabling exclusive detection of the hazardous indoor pollutants.
- Excellent controllability of VAH selectivity via separate control of gas-sensing and catalytic oxidation reactions: In contrast to conventional single-layered sensors consisting of sensing materials that are uniformly doped/loaded with catalytic materials, the CeO2-coated bilayer sensor can separate the sensing and catalytic reactions into independent processes, which provides excellent controllability for gas selectivity as well as completely new functionality.
- Elucidation of the mechanism underlying ultraselective VAH detection: The catalytic oxidation of gas into non- or less- reactive forms has been suggested as the reasons for the suppression of interfering gases, which are experimentally verified by the ppm-level gas analysis using proton transfer reaction–quadrupole mass spectrometer. Furthermore, we thoroughly investigated the effects of the sensing film, catalytic overlayer materials, and bilayer film configuration on VAH detection to elucidate the sensing mechanism underlying the exclusive detection of aromatic BTEXS gases.
- General strategy for designing high-performance oxide chemiresistors exhibiting negligible cross-responses to interferants: The coating of the CeO2 overlayer on the sensing film effectively reduced the cross-responses to non-aromatic interference for the diverse compositions and morphologies for sensing films using SnO2, Pt–SnO2, Au–SnO2, In2O3, Rh–In2O3, Au–In2O3, WO3, ZnO, and hierarchically porous Rh–SnO2, verifying the general validity of the catalytic filtering CeO2
- Quantitative and discriminative VAH characterization using sensor array: To date, the sensors and sensor arrays have been used to discriminate the aromatic compounds but do not provide the quantification of gas concentration. However, aromatic BTEXS and nonaromatic gases were clearly identified and classified into 5 nonoverlapping clusters, even at various concentrations (e.g., 1, 2.5, and 5 ppm), through principal component analysis using a sensor array exhibiting different partial selectivities and negligible cross-responses to specific aromatic and interfering gases, enabling the discriminative quantification and composition analysis of VAHs under defined atmosphere. Accordingly, the tailoring of gas sensing characteristics using bilayer design provides a facile solution for rational design of high performance gas sensors and electronic noses with new functionalities such as indoor air quality monitoring, leak detection in petroleum industry, the monitoring of air pollution in the congested area and the assessment of air quality at gas station.
Please note that, during the past 5 decades since the discovery of oxide semiconductor gas sensors in 1960s, ultraselective and ultrasensitive detection of VAHs has been a significant challenge despite strong demand from the sensor industry for personalized air solutions. This challenge emanates from a simple sensing algorithm based on the charge transfer between the analyte gas and sensing surface often leads to a lack of gas selectivity even in the presence of oxides or noble-metal catalysts; the issue is even more important for detecting stable low-reactivity gases such as VAHs. Addressing this issue, this work reports the development of a rational and facile strategy for ultraselectively and ultrasensitively detecting traces of aromatic BTEXS gases utilizing a bilayer sensor designed with a catalytic filtering layer.