Fluctuation enhanced gas sensing with WO3-based nanoparticle gas sensors modulated by UV light at selected wavelengths
Introduction
Metal-oxide-based gas sensors are of much current interest as a consequence of their facile production and simple applications to a wide range of applications. These sensors are promising for the detection of low concentrations of various gases since they are compatible with various fabrication technologies (even nanotechnology) and also because there is a large variety of metal oxides with different gas sensing characteristics [1], [2], [3]. Unfortunately, the sensitivity and selectivity of metal-oxide-based sensors are still too low for a number of emerging applications. This problem can be partially solved by using an array of gas sensors with different sensing properties, but this solution is expensive and requires increased amounts of energy for heating because resistive metal-oxide-based gas sensors normally operate at an elevated temperature. Consequently, new methods for enhancing the gas detection are a driving force in the field of gas sensing.
Noise spectroscopy, with or without complementary resistance recordings, was proposed more than a decade ago and proved to be a powerful tool for boosting the selectivity and sensitivity of resistive gas sensors [4], [5], [6]. The conductivity of these sensors is altered upon exposure to ambient gases and depends on the reducing or oxidizing ability of these gases. The conductivity change depends on the gas concentration, which hence can be monitored. The power spectral density (PSD) of the gas sensors’ resistance fluctuations can change concurrently, and these changes can be utilized to provide more efficient gas detection [7], [8], [9]. This latter method is referred to as fluctuation-enhanced sensing (FES) [7].
Recently, there have been reports of resistive gas sensors demonstrating a photocatalytic effect when irradiated by ultraviolet (UV) light [10], [11], [12], [13]. The pertinent materials (e.g., WO3 and TiO2) have been applied to gas sensing for more than two decades [14]. Irradiating sensors of these materials with UV light is an interesting option for activating chemical reactions at the metal oxide surface and could to some extent replace the necessity of running the sensors at high operating temperatures. In the present work we apply UV light of different wavelengths and demonstrate that the FES method then provides even more information about the sensor’s ambient atmosphere.
Section snippets
Fluctuation enhanced sensing and UV light
Noise at low-frequency f, especially 1/f noise, is often employed to assess the quality of materials and devices [15], [16]. Such noise is utilized in the FES method to determine the presence of various gases in the ambient atmosphere where the resistive gas sensors are placed. In the case of WO3-based gas sensors, the noise sources appear to be similar to those in devices incorporating intrinsic semiconductors [17]. Some common types of noise are thermal noise, shot noise, burst noise, 1/f
Experimental procedures
Measurements were performed on a prototype WO3-based nanosensor (see below) irradiated by either of two UV light emitting diodes (LEDs) with different spectral characteristics (Fig. 1). The measurements were performed in the presence of ethanol (C2H5OH), methane (CH4) or formaldehyde (CH2O) at several concentrations up to a maximum value between 15 and 75 ppm for the various gases. To enable comparisons of the sensors’ response at different irradiations, the maximum optical power of the emitted
Results
The sensor was first stabilized in an ambient atmosphere of synthetic air (80% N2 and 20% O2, denoted SA), and the flow of a chosen calibration gas was then introduced to obtain a gas mixture of selected concentration. Low and similar flow rates were used for pure SA and for SA with added gases (below 150 ml/min, see Table 2) in order to avoid gas turbulence and allow direct comparisons between consecutive measurements. Changes in the DC resistance RS were monitored in the presence of the
Discussion
We found that UV light is able to modulate the DC resistance and resistance noise of sensors based on AuNP-decorated WO3-NWs, and that this effect depends on the gas and the wavelength of the UV light. The optical power emitted by the UV-LEDs is inversely proportional to the emitted wavelength, and therefore the energy of the photons from the UV diode LED1 (362 nm) is greater than the energy from LED2 (394 nm) (Fig. 2). But the influence of the UV irradiation depends not only on its energy but
Conclusion
The influence of UV light on AuNP-decorated WO3-NW gas sensing layers was experimentally investigated by measuring DC resistance and using fluctuation-enhanced sensing. We found that the sensor response is related to photochemical reactions on the surface of the gas sensing material. Thus its properties can be modulated by irradiation, and the observed effect depends critically on the wavelength of the UV light. Such changes were observed both in DC resistance and in resistance noise. Moreover,
Acknowledgements
This research was partially financed by the National Science Center, Poland, project No. UMO-2012/06/M/ST7/00444 “Detection of gases by means of nanotechnological resistance sensors”. R.I. acknowledges a ‘Ramón y Cajal’ fellowship from the Ministry of Economy and Competiveness (MINECO), Spain. C.G.G. received support from the European Research Council under the European Community’s Seventh Framework Program (FP/2007–2013)/ERC Grant Agreement No. 267234 (“GRINDOOR”). J.S. was partially financed
Maciej Paweł Trawka (M’14) was born in Poland, 1988. He graduated in Electronics and Telecommunications from the Poznan University of Technology, Poland. He was employed as an Intern Student in Mentor Graphics Corporation, where developed the fastest serial interface for VLSI system-on-chip testing. He is currently a Ph.D. student at the Metrology and Optoelectronics Department, Gdansk University of Technology, Poland. His thesis is associated with nanoparticle gas sensors and new approaches of
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Maciej Paweł Trawka (M’14) was born in Poland, 1988. He graduated in Electronics and Telecommunications from the Poznan University of Technology, Poland. He was employed as an Intern Student in Mentor Graphics Corporation, where developed the fastest serial interface for VLSI system-on-chip testing. He is currently a Ph.D. student at the Metrology and Optoelectronics Department, Gdansk University of Technology, Poland. His thesis is associated with nanoparticle gas sensors and new approaches of gas detection.
Janusz M. Smulko received his M.Sc., Ph.D. and D.Sc. degrees in electronics from Gdansk University of Technology, Poland, in 1989, 1996 and 2007, respectively. Since 2013 the Head of the Metrology and Optoelectronics Department of the Gdansk University of Technology. He had also conducted scientific research in short-term positions in Texas A&M University (2003), Uppsala University (2006/07), Massachusetts Institute of Technology (2011, 2013). As a researcher he focuses on applications of 1/f noise for gas sensing and reliability assessment of electronic components and structures, and influence of noise on the detection efficiency in Raman spectroscopy systems. He is a member of the Committee on Metrology and Scientific Instrumentation of the Polish Academy of Science and Editor-in-Chief of Metrology and Measurement Systems Journal. He is the author of more than 60 papers in international journals and the holder of 3 patents.
Lech Zbigniew Hasse (M’99) received his M.Sc. and Ph.D. degrees in electronics from Gdansk University of Technology, Poland, in 1969 and 1979, respectively. Since 1979 he was Asst. Prof. at the Department of the Metrology and Optoelectronics, Gdansk University of Technology. As a researcher he is involved in theoretical (modeling and simulation) and practical applications of random signals. He is the author and co-author of more than 100 papers in journals, two books and the holder of 4 patents.
Claes G. Granqvist is Senior Professor of Solid State Physics at the Ångström Laboratory, Uppsala University, Sweden. Much of Granqvist’s research is concerned with materials for energy efficiency and solar energy utilization in buildings. Electrochromic and thermochromic “smart windows” with variable transmittance of visible light and solar energy are two examples. Other recent research topics include nanomaterials for gas sensing and for photo-catalytic purification of gases and fluids. Granqvist has published more than 500 scientific articles (ISI) and about 30 books and conference proceedings. He has given more than 300 invited talks and he has organized about 40 international conferences. Granqvist is a member of the Royal Swedish Academy of Sciences and the Royal Swedish Academy of Engineering Sciences. He has received several prizes and awards, most recently the 2015Czochralski award given by the European Materials Society, the Polish Academy of Sciences, and other organizations. Granqvist is one of the founders of ChromoGenics AB, a company with some 20 employees, for producing electrochromic foils.
Fatima Ezahra Annanouch was born in Morocco, 1986. She graduated in Physics from the Moulay Ismail University in Meknes, Morocco and is currently a Ph.D. student at the Electronic Engineering Department, University Rovira i Virgili, Tarragona, Spain. Her thesis is oriented towards the bottom-up integration of nanofibers in sensor microsystems.
Radu Ionescu is ‘Ramon y Cajal' Senior Researcher in the Department of Electronics, Electrical and Automatic Engineering from Rovira i Virgili University, Tarragona, Spain. He received his degree in Electrical Power Engineering from the Polytechnic University of Bucharest, Romania, in 1998, and his Ph.D. degree in Electronics Engineering from the Polytechnic University of Catalonia, Barcelona, Spain, in 2003. He was postdoctoral researcher at Rovira i Virgili University (4 years), experienced researcher (FP6 Marie Curie Tranfer of Knowledge Actions) at Mediterranean University of Reggio Calabria, Italy (2 years), and EC Senior Researcher in the Department of Chemical Engineering from the Technion − Israel Institute of Technology, Haifa, Israel (2 years).