Methylbenzene sensors using Ti-doped NiO multiroom spheres: Versatile tunability on selectivity, response, sensitivity, and detection limit

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Highlights

  • Ultrasensitive and selective methylbenzene sensor using Ti-NiO multiroom micro-reactors.

  • Versatile tunability on gas response, selectivity, sensitivity, and detection limit.

  • Unprecedentedly high response (resistance ratio) of 337.8 to 1 ppm p-xylene.

  • Elucidation of gas reforming/oxidation mechanism for selective gas detection.

Abstract

Pure and Ti-doped NiO multiroom spheres were prepared via ultrasonic spray pyrolysis, and their gas sensing characteristics were investigated. The sensor using 10 at% Ti-doped NiO multiroom spheres exhibited an unprecedented high response (resistance ratio = 337.8) to 1 ppm p-xylene at 350 ℃, whereas the sensor using pure NiO multiroom spheres exhibited a negligibly low response (1.3). Moreover, the control of the Ti doping and film thickness provided intriguing strategies for tuning the xylene and methylbenzene sensing characteristics, such as the selectivity, response, sensitivity (slope between response and gas concentration), and detection limit. The versatile tunability on gas sensing characteristics was explained by the Ti-doping-induced variation of the oxygen adsorption, mesoporosity, specific surface area, and charge-carrier concentration, as well as the control over the reforming and oxidation of the analyte gases using the multiroom-structured micro-reactors with high catalytic activity.

Introduction

Xylene and toluene, two of most important aromatic volatile organic compounds (VOCs), have been widely used in paint, varnishes, ink, rubber, adhesives, and cleaning agents, as well as in the petroleum industry [1,2]. Even low concentrations of methylbenzenes are known to induce headaches, irritability, depression, insomnia, agitation, tiredness, tremors, hearing loss, and nausea [[1], [2], [3], [4]]. Accordingly, methylbenzene sensors are indispensable for assessing indoor/outdoor air quality, detecting leaks in the petroleum industry, and monitoring harmful aromatic pollutants near gas stations [[3], [4], [5], [6]]. It should be noted that the health hazards posed by xylene and toluene depend on the total amount of exposed chemicals; thus, the exposure limit depends on the duration of exposure. For instance, the minimal risk levels of xylene for an exposure duration of 8 h (time-weighted average) set by the Occupational Safety and Health Administration (OSHA), ≤14 days set by the Agency for Toxic Substances and Disease Registry (ATSDR), and ≥1 year set by the ATSDR are 100 ppm, 2 ppm, and 50 ppb, respectively [1,7], and those of toluene are similar [2,8]. Moreover, the gas sensing characteristics, such as the response, selectivity, sensitivity (slope between response and concentration), detectable concentration range, and detection limit, vary depending on the intended applications with different requirements and ambient environmental conditions.

Although fluorescence spectroscopy and gas chromatography have been used for analyzing VOCs [9,10], the bulky and expensive instruments and the long sampling time limit their widespread applications. Oxide semiconductor gas sensors with high response, fast responding speed, and reliability [[11], [12], [13], [14], [15], [16]] can be viable and cost-effective alternatives for monitoring low concentrations of harmful methylbenzenes. Moreover, their facile integration into highly miniaturized devices offers new opportunities for diverse environmental monitoring applications using wireless sensor networks assisted by the Internet of Things, as well as artificial olfaction via pattern recognition. To date, many sensing materials have been investigated for detecting methylbenzenes, such as NiO nanostructures doped/added with Cr, Sn, W, and Mo [[17], [18], [19], [20]]; SnO2 yolk–shell spheres loaded with Pd [21]; and Co3O4 nanostructures loaded with Pd and Cr [[22], [23], [24], [25], [26]]. However, most studies reported the enhancement of only one or two sensing characteristics among the various required properties, including the response, selectivity, sensitivity, detectable concentration range, and detection limit. This suggests that further enhancement and versatile tuning of the gas sensing characteristics using new sensing materials are highly necessary to satisfy the demand-based requirements of methylbenzene sensors for practical applications.

In the present study, methylbenzene sensors with versatile controllability over the gas response, selectivity, sensitivity, and detection limit were designed using Ti-doped NiO multiroom-structured micro-reactor spheres. The doping of 10 at% Ti into NiO multiroom spheres unprecedentedly increased the responses and selectivity to p-xylene and toluene, and the thickness control of the sensing film facilitated the tuning of the detectable gas concentration range, sensitivity, and detection limit. The main focus of this study was elucidating the mechanisms underlying the multi-tunability of the gas sensing characteristics in relation to the micro-reactor role of the multiroom-structured spheres with catalytic activity, the reforming/oxidation of analyte gases within the sensing film, and the Ti-doping-induced changes in the charge-carrier concentration, oxygen adsorption, and mesoporosity.

Section snippets

Preparation of gas sensing materials

Ti-doped NiO multiroom spheres were synthesized via ultrasonic spray pyrolysis. Nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O, 99.999 %, Sigma–Aldrich, USA), Titanium (IV) bis(ammonium lactato) dihydroxide solution ((CH3CH(O-)CO2NH4]2Ti(OH)2, 50 wt% in H2O, Sigma–Aldrich, USA), and 3 g of dextrin ((C6H10O5)x·nH2O, Samchun, Korea) were dissolved in 200 mL of distilled water. The total metal-ion concentration ([Ti] + [Ni]) was fixed at 0.1 M and three spray solutions with different molar ratios

Results and discussion

Multiroom-structured pure NiO, 0.05Ti-NiO, 0.1Ti-NiO, and 0.2Ti-NiO spheres were prepared via ultrasonic spray pyrolysis and subsequent heat treatment. A schematic of the formation mechanism of the multiroom-structured pure and Ti-doped NiO spheres is presented in Fig. 1a. The following reactions are suggested to occur during the spray pyrolysis: 1) the formation of droplets containing the metal salts and dextrin by ultrasonic nebulization and their transfer to the heated zone by N2; 2) the

Conclusion

Methylbenzene sensors with versatile tunability over gas response, selectivity, sensitivity, and detection limit were designed using Ti-doped NiO multiroom spheres. The doping of 5–20 at% Ti to NiO significantly enhanced the responses to methylbenzenes, providing an unprecedented high response and good selectivity to xylene and methylbenzenes. Moreover, the sensitivity, the slope between the gas response and the concentration, and the detection limit could be tailored by manipulating the

Acknowledgements

This work was supported by a grant from the Samsung Research Funding & Incubation Center for Future Technology (SRFC), Grant No. SRFC-TA1803-04.

Ki Beom Kim studied Materials Science and Engineering and received his B.S. from Korea University, Korea, in 2019. He is currently studying for an M.S./Ph.D. integrated degree at Korea University. His research interest oxide semiconductor gas sensors using aerosol-derived powders.

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  • Cited by (0)

    Ki Beom Kim studied Materials Science and Engineering and received his B.S. from Korea University, Korea, in 2019. He is currently studying for an M.S./Ph.D. integrated degree at Korea University. His research interest oxide semiconductor gas sensors using aerosol-derived powders.

    Seong-Yong Jeong studied Materials Science and Engineering and received his B.S. from Chonbuk National University, Korea, in 2015. He is currently studying for an M.S./Ph.D. integrated degree at Korea University. His research interest is oxide semiconductor gas sensors using multilayer structure.

    Tae-Hyung Kim studied Materials Science and Engineering and received his B.S. from Korea University, Korea, in 2014. He is currently studying for an M.S./Ph.D. integrated degree at Korea University. His research interest is p-type oxide semiconductor gas sensors.

    Yun Chan Kang joined the Department of Materials Science and Engineering at Korea University as a full professor in 2014. He received his M.S. and Ph.D. degrees from the Korea Advanced Institute of Science and Technology in 1995 and 1997, respectively. Between 2000 and 2004, he developed phosphor materials at the Korea Research Institute of Chemical Technology. He was assistant professor and associate professor at Konkuk University from 2004 to 2014. His current research interests include materials for flat panel displays, batteries, and solar cells.

    Jong-Heun Lee joined the Department of Materials Science and Engineering at Korea University as an associate professor in 2003, where he is currently a professor. He received his B.S., M.S., and Ph.D. degrees from Seoul National University in 1987, 1989, and 1993, respectively. Between 1993 and 1999, he developed automotive air–fuel ratio sensors at the Samsung Advanced Institute of Technology. He was a Science and Technology Agency of Japan (STA) fellow at the National Institute for Research in Inorganic Materials (currently NIMS, Japan) from 1999 to 2000 and a research professor at Seoul National University from 2000 to 2003. His current research interests include chemical sensors, functional nanostructures, and photoelectrochemical water splitting.

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