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Near-Room-Temperature Ethanol Detection Using Ag-Loaded Mesoporous Carbon Nitrides Vijay K. Tomer,† Ritu Malik,‡ and Kamalakannan Kailasam*,† †
Institute of Nano Science and Technology (INST), Mohali, Punjab 160062, India Department of Applied Physics, Mahavir Swami Institute of Technology, Sonepat, Haryana 131001, India
‡
S Supporting Information *
ABSTRACT: Development of room-temperature gas sensors is a much sought-after aspect that has fostered research in realizing new two-dimensional materials with high surface area for rapid response and low-ppm detection of volatile organic compounds (VOCs). Herein, a fast-response and low-ppm ethanol gas sensor operating at near room temperature has been fabricated successfully by utilizing cubic mesoporous graphitic carbon nitride (g-CN, commonly known as g-C3N4), synthesized through template inversion of mesoporous silica, KIT-6. Upon exposure to 50 ppm ethanol at 250 °C, the optimized Ag/g-CN showed a significantly higher response (Ra/Rg = 49.2), fast response (11.5 s), and full recovery within 7 s in air. Results of sensing tests conducted at 40 °C show that the sensor exhibits not only a highly selective response to 50 ppm (Ra/Rg = 1.3) and 100 ppm (Ra/Rg = 3.2) of ethanol gas but also highly reversible and rapid response and recovery along with long-term stability. This outstanding response is due to its easily accessible three-dimensional mesoporous structure with higher surface area and unique planar morphology of Ag/g-CN. This study could provide new avenues for the design of next-generation roomtemperature VOC sensors for effective and efficient monitoring of alarming concern over indoor environment.
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INTRODUCTION The quench of realizing highly sensitive, fast response/recovery, and selective gas sensors that can operate at room temperature for detecting trace concentrations of volatile organic compounds (VOCs) in indoor climate has always fostered the development of novel synthesis routes and new materials.1,2 Ethanol is one of the kinds of beverage VOC present in alcoholic drinks (beer, wine, whiskey, liquors, spirits, etc.) in diluted form.3,4 The detection of the concentration of ethanol in breath, blood, or urine facilitates prosecuting drunk drivers.5−7 In addition, the quantitative detection of ethanol in human breath can be utilized as breath marker for specific diseases.3−5 Ethanol is also used as an industrial solvent, and its vapors are released in air during the production of chemical compounds such as lacquers, aerosols, cleaning/mouthwash products, polishes, surface coatings, dyes, inks, pesticides, preservatives, explosives, and petrol additives.8,9 Ethanol is a central nervous system depressant and affects brain, brainstem, and spinal cord when inhaled or consumed. Exposure to ethanol leads to stupor, nausea, mental excitement or depression, vomiting, loss of coordination (ataxia), sleepiness, narcosis, impaired perception, lack of coordination, and death.10,11 Considering the harmful effects of the potential exposure of ethanol gas on human, much efforts have been invested in the development of low-cost, highly sensitive, selective, and durable ethanol sensors, which can precisely © 2017 American Chemical Society
recognize and detect ethanol gas when operating at room temperature as well as result in significantly fast response and recovery.12−14 Recently, explosive growth of interest in generating novel two-dimensional (2D) materials because of their high surfaceto-volume ratio, quantum hall effect at room temperature, high electron transfer rate, and excellent thermal stability has stimulated their use in a variety of applications, such as energy-storage devices, catalysis, and chemical sensors.15,16 Graphitic carbon nitrides (g-CNs, commonly known as gC3N4) are a metal-free polymeric semiconductor consisting of tri-s-triazine building blocks with structure analogous to that of 2D layered graphite.17−20 Owing to its exciting physicochemical properties, appropriate band gap, and excellent thermal/ chemical stability, g-CN has been extensively explored in photocatalysis, electrocatalysis, fuel cell, bioimaging, and sensing applications.15−19 However, in conventional g-CN, the high degree of polycondensation of monomeric heptazine units results in a very low surface area (3 times) toward detecting
Hence, the ethanol gas removes the adsorbed oxygen, frees up electrons, and increases the electrical conductivity of the Ag/ meso-CN sensor. Role of Ag NPs. Ag NPs because of their excellent electrical conductivity have been known to increase the response of ntype semiconductors. The presence of Ag NPs accelerates the process of catalytic oxidation and chemical sensitization, which increases active oxygen species on the surface of meso-CN. Chemical sensitization (spillover effect) of Ag NPs greatly enhances the adsorption−desorption rate of molecular oxygen over the sensor surface and converts them to oxygen ions (O−) 3665
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of the samples was observed using HRTEM instrument (JEOL) at an acceleration voltage of 200 kV. Quantitative information on the elemental composition was obtained using SEM instrument (JSM-IT300; JEOL). Gas-Sensing Measurement. Sensing measurements were performed on Ag−Pd interdigitated substrates with five pairs of electrodes. The procedure for sensor fabrication and gassensing experiments was similar to that described in our previous study.12,24 To measure the sensor response under highly humid conditions, the humidity level was maintained at 90% RH in the testing sensor chamber. The resulting response (R = Ra/Rg) is the mean value obtained from repeated experiments at a particular temperature.
ethanol gas compared to that of measured test VOCs. Interestingly, the response at near room temperature (40 °C) shows that the sensor was also able to selectively detect 50 ppm (R = 1.3) and 100 ppm (R = 3.2) ethanol gas among a variety of test VOCs. The excellent sensing response results from the high surface area and the unique planar morphology of Ag/gCN, which facilitates the adsorption and diffusion and fastens the reaction kinetics of ethanol molecules on its surface. We believe that the demonstration of outstanding room-temperature sensing results in this study holds promise for the accurate identification of harmful VOCs present in indoor climate at room temperature and thus provides a positive glimpse for futuristic miniaturized, handheld sensing devices with superior gas-sensing attributes.
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EXPERIMENTAL SECTION Synthesis of Mesoporous Silica, KIT-6. Typically, Pluronic P123 (2.0 g; Sigma-Aldrich) was dissolved in 70 mL of distilled water and vigorously stirred (1200 rpm) at 40 °C for 1 h. HCl (2.75 mL, 35%; Fisher Scientific) was added to the above solution, and the mixture was again stirred for 45 min. Thereafter, 1-butanol (2.5 mL; Fisher Scientific) was added and stirred to obtain a clear transparent solution. To this solution, tetraethoxy orthosilicate (4.6 mL; Sigma-Aldrich) was added and further stirred for 24 h. The white gel product thus obtained was autoclaved in a hydrothermal reactor at 100 °C for 24 h. After cooling to room temperature, the products were recovered, washed, and dried at 80 °C. Finally, the recovered products were calcined at 550 °C (heating rate, 2 °C/min) for 4 h to produce KIT-6. Mesoporous g-CN (meso-CN). The mesoporous g-CN was synthesized using as-prepared KIT-6 silica as hard template. Initially, CH2N2 (cyanamide, 1 g; Sigma-Aldrich) was dissolved in 1 mL of distilled water and then poured dropwise in KIT-6 (0.5 g). The mixture was stirred for 1 h at 40 °C, followed by drying in air at 80 °C for 3 h. The white powder was then moved to a covered crucible and heated at 550 °C (heating rate, 2 °C/min) for 4 h. The obtained products were mixed with NH4HF2 (ammonium hydrogen difluoride, 2M, 50 mL; Sigma-Aldrich) and stirred for 24 h. The bright yellow filtrate was washed with distilled water and dried at 100 °C for 6 h to obtain ordered meso-CN nanostructures. Ordered mesoporous Ag-doped g-CN nanostructures (Ag/ meso-CN) were obtained by sequential addition of an adequate amount of AgNO3 (silver nitrate; Fisher Scientific) aqueous solution to the solution of cyanamide, and further the same route was followed as utilized for the synthesis of meso-CN nanostructures. A series of optimizing experiments were carried out with different weight ratios of Ag in Ag/meso-CN nanocomposite, and the prepared products at above-mentioned ratios were denoted as Ag(X)/meso-CN, where X represents 1, 3, and 5 wt % ratios of Ag NPs in meso-CN. Conventional gCN was synthesized by directly heating cyanamide according to the reported procedure.20 Characterization. The crystalline phases of the materials were evaluated by XRD using advance Bruker D8 diffractometer with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA in the 2θ range of 10−70°. SAXS data were recorded at 2θ = 0.5−3° on SAXSess mc2 instrument (Anton Paar) equipped with a high-performance charge-coupled device detector. Nitrogen-sorption isotherms were collected using Autosorb iQ2 instrument (Quantachrome) at 77 K. The samples were degassed overnight under vacuum at 200 °C. The morphology
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00479. Low-angle X-ray diffraction spectra and Barrett−Joyner− Halenda pore size distribution for all of the samples; response/recovery time of the Ag(3)/meso-CN sensor toward detection of 1 ppm ethanol gas at 250 °C and varying concentrations (50−500 ppm) of ethanol gas at 40 °C; response of as-prepared materials toward varying concentrations (10−1500 ppm) of ethanol gas and 100 ppm test gases at 40 °C; repeated response/recovery of the Ag(3)/meso-CN sensor to 50 ppm (250 °C) ethanol; the response and recovery times of all of the materials to 1 and 50 ppm (250 °C) ethanol gas; the response and recovery times of the Ag(3)/meso-CN sensor to 50 ppm (250 °C) test gases; stability in response of the Ag(3)/ meso-CN sensor to ethanol for 5 weeks (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected],
[email protected]. ORCID
Vijay K. Tomer: 0000-0002-4368-9149 Kamalakannan Kailasam: 0000-0002-5931-7649 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS V.K.T. acknowledges INST and DST, Government of India, for postdoctoral fellowship. K.K. thanks DST, Government of India, for financial support to INST, Mohali, Punjab, India. The authors gratefully acknowledge Professor Surender Duhan, DCR University of Science & Technology (Haryana, India) for assistance in gas-sensing measurements.
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