ZnOâPDMS Nanohybrids: A Novel Optical Sensing Platform for

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ZnO-PDMS Nanohybrids: A Novel Optical Sensing Platform for Ethanol Vapor Detection at Room Temperature Argyro Klini, Stavros Pissadakis, Rabindra N. Das, Emmanuel P. Giannelis, Spiros H Anastasiadis, and Demetrios Anglos J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 09 Dec 2014 Downloaded from http://pubs.acs.org on December 9, 2014

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The Journal of Physical Chemistry

ZnO-PDMS Nanohybrids: A Novel Optical Sensing Platform for Ethanol Vapor Detection at Room Temperature Argiro Klini,†,‡ Stavros Pissadakis,† Rabindra N. Das,ơ,§ Emmanuel P. Giannelis,ơ, Spiros H. Anastasiadis,†,‡ and Demetrios Anglos*,†,‡ †

Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas,

100 N. Plastira St., GR 70013 Heraklion, Crete, Greece ‡

Department of Chemistry, University of Crete, GR 71003 Heraklion, Crete, Greece

ơDepartment

of Materials Science and Engineering, Cornell University, Ithaca, New York 14853,

USA §

Present address: MIT, Lincoln Lab. 244 Wood Street, Lexington, MA, USA 02421, USA

Corresponding Author: *(D.A.) E-mail: [email protected], Tel.:+30 2810 391154, Fax: +30 2810 391305

1 ACS Paragon Plus Environment


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ABSTRACT A new optical gas sensor platform based on highly luminescent ZnO-polymer nanohybrids is demonstrated. The nanohybrids consist of ZnO nanoparticles, typically 125 (± 25) nm in size, dispersed in an inert cross-linked polydimethylsiloxane (PDMS) matrix. Upon exposure to ethanol enriched air at room temperature, the nanocomposites exhibit a clear increase in their photoluminescence (PL) emission, which shows a nearly Langmuir dependence on the alcohol vapor pressure. The response time is on the order of 50 s particularly at low ethanol concentrations. The limit of ethanol vapor detection (LOD) is as low as 0.4 Torr, while the sensor remains unaffected by the presence of water vapor, demonstrating the potential of the ZnO-PDMS system as optical gas sensing device. The interaction of the ZnO nanoparticles with molecular oxygen plays an essential role on the overall performance of the sensor, as shown in comparative experiments performed in the presence and absence of atmospheric air. Notably, O2 was found be quite effective in accelerating the sensor recovery process, compared to N2 or vacuum.

KEYWORDS Photoluminescence, Zinc oxide, Ethanol sensing, Oxygen sensing.


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1. INTRODUCTION Semiconducting metal oxides, in various structural forms, bulk, thin film or nanostructured, are known to undergo changes in their properties (electrical, optical or mechanical) when molecular species are physically or chemically bound onto their surface; this behavior makes oxide semiconductors highly promising materials for a number of chemical sensor applications and sensing schemes.1,2 In recent years, research studies have increasingly investigated the use of nanostructured metal oxides, such as nanorods, nanowires or nanoparticles, as the active material of the sensor. These novel structural motifs offer enhanced performance arising from their high surface-to-volume ratio, while their reduced dimensionality induces specific and controllable modifications to the oxide electronic structure that, in effect, determine the sensing response of the semiconductor. In this context, zinc oxide (ZnO), a widely studied metal oxide semiconductor, has been extensively investigated with regards to its potential use as a solid state chemical sensor for the detection of different substances, ranging from simple molecular gases and volatile liquids, such as O3, NO2, aromatic hydrocarbons, gasoline and ethanol, in air3-5 to bio-molecules (e.g. DNA, glucose or cholesterol) in aqueous solutions.6-8 With respect to gas sensing, studies have shown that gas molecules or organic vapors, chemisorbed on nanostructured ZnO surface, modify the electronic properties of the semiconductor via a number of reduction or oxidation reactions. These modifications manifest themselves as changes in electrical conductivity, which can be correlated to the amount of gas bound on the nanostructured surface and, thus, enable sensing. For example, ZnO nanostructures grown on ceramic substrates have shown sensitive response to trace levels of volatile organic compounds, such as benzene and toluene, at concentrations as low as 0.01 and 1 ppm, respectively9,4 based on electrical resistance measurements performed at 150 and 290 °C.



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Similarly, recent studies have demonstrated efficient sensing of ethanol by various types of ZnO nanostructures on the basis of electrical resistance measurements. Highly sensitive and fast response (30-90% increase in electrical conductivity within 16 s) to ethanol concentrations ranging from 10 to 600 ppm has been demonstrated for ZnO nanowires fabricated through an electrospinning method.(5) Moreover, ZnO nanorods, deposited on the walls of an alumina tubing, forming a layer with thickness in the range of a few tens of micrometers, were tested and found capable of sensing ethanol, in concentrations as low as 1 ppm, with a typical response time of less than 60 s.10 However, in all these cases, measurements require the presence of electrical circuitry and associated contacts with the sensor. Additionally, optimal sensor performance is achieved at elevated temperatures, typically in the range of 200-400 °C.11-13 Reliable determination and monitoring of alcohols (and ethanol, in particular) using miniaturized and low cost sensing probes could be of commercial importance, since it applies directly to the automotive and bio-fuel industry,14(14) the wine and spirits industry15 as well as in law enforcement for monitoring alcohol consumption by drivers.16 Obviously, development of sensing devices, which require no heating elements and are capable of probing ethanol in a broad dynamic range of concentrations, is clearly of great interest in several of the above mentioned application fields. An alternative to electrical measurements relies on sensing based on the optical properties of ZnO,17,18 which to date has received much less attention. In a recent study, the green photoluminescence (PL) emission of ZnO nanowires (excited by a He-Cd laser at 325 nm or by a UV light-emitting-diode at 330 nm) was shown to be a good probe for monitoring the presence of NO2, ethanol vapor and humidity in air.19 The nanowires showed a 1.5% relative change in their PL intensity, when exposed to 1000 ppm of ethanol vapor in air. A novel sensor concept, introduced recently by some of the authors, exploits an optical fiber long period grating (LPG) covered with a thin layer of ZnO nanorods. Sensitive ethanol detection (partial vapor pressures


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as low as 50 Torr) was demonstrated by monitoring changes either in the LPG transmission or in the PL emission of the ZnO nanorods.20 In the present study, a new type of optical gas sensor based on the strong UV photoluminescence of ZnO nanoparticles is introduced and its capacity as regards its ethanol sensing is investigated. The sensor consists of highly luminescent ZnO nanoparticles dispersed in a soft, flexible cross-linked polydimethylsiloxane (PDMS) matrix.21 Cross-linked PDMS, a soft elastomer widely used in the microfabrication of various lab-on-a-chip devices,22 offers several advantages in device fabrication. It is inexpensive, biocompatible, self-sealable, easily processable and highly elastic. Moreover, on the basis of its permeability properties PDMS may be able to selectively enhance or retard the diffusion of specific gases, potentially enabling controlled selectivity and sensitivity.23,24 Concerning its optical properties, PDMS is transparent in the UV (down to 300 nm) permitting efficient excitation of the embedded ZnO nanoparticles and detection of their emission. It also acts as refractive index matching material ensuring efficient coupling of the excitation beam to the hybrids and reducing losses in the emission collection. The performance of this ZnO-PDMS nanocomposite sensor, in particular its sensitivity, speed and reversibility, was investigated by monitoring the UV-photoluminescence emission of ZnO, which was found to respond sensitively to changes of ethanol vapor pressure, at room temperature. Additionally, the effect of oxygen, present in the atmospheric air, on the PL response of the nanohybrids was studied, illustrating the critical role of molecular oxygen on the overall sensor function.



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2. EXPERIMENTAL METHODS Silanol-terminated poly(dimethyl siloxane), PDMS, homopolymers with average molecular weight values of 150,000 (code: PS 348.7), and 49,000 (code: PS 343.5) were obtained from United Chemical Technologies. Tetraethyl orthosilicate cross-linking agent (TEOS) was obtained from Gelest, Inc., whereas tin(II) 2-ethylhexaonate and ZnO particles were purchased from Aldrich Chemical Co. The commercially available ZnO nanoparticles have an average particle size 125 ± 25 nm and 99.9% purity (Aldrich 20, 553-2). In a typical procedure the nanocomposites were prepared by mixing the appropriate amount of ZnO nanoparticles in powder form with silanol-terminated PDMS (PS348.7:PS343.5 = 5:1) and TEOS at 3000 rpm for 2 min. Then a stoichiometric amount of tin(II) 2-ethylhexanoate catalyst (TEOS:Sn = 5:1) was added to the mixture, to effect cross-linking. The mixture was speed mixed for an additional 30 seconds, was transferred to a Teflon mold and allowed to cure at room temperature for a minimum of 48 hours. A series of ZnO-PDMS nanocomposites containing 10, 40, 70% w/w ZnO (1.5, 6, 12% v/v) were prepared using this procedure. Samples used for testing were typically 1×1×0.2 cm3 in size. The ethanol sensing behavior of the ZnO-PDMS hybrids was monitored by recording their laser-induced photoluminescence emission at room temperature (RT, Τ=293 Κ) inside a stainless steel optical chamber, which enabled control and variability of ambient conditions. The chamber (500 ml internal volume) was equipped with appropriate gas inlets and outlets for introducing ethanol vapors or flushing with air. A commercial ethanol probe (PS-2194, Pasco) was attached via a special port and used for monitoring changes in ethanol vapor pressure during measurements. A quartz window on the front side of the chamber permitted optical excitation of the sample and collection of the PL emission. For introducing ethanol vapors in the chamber, air at a variable flow rate, in the range of 0.1-1.0 L/min was enriched in ethanol vapor, while


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passing through a bubbler containing liquid ethanol (99.9%) and flowed through the chamber. To refresh the chamber atmosphere, air was flushed at 50 L/min. In order to achieve low ethanol concentration in air (pEtOH

ZnOâPDMS Nanohybrids: A Novel Optical Sensing Platform for

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