Sensing of organic vapors by flame-made TiO2 nanoparticles
Introduction
Titanium dioxide (TiO2) based sensors have been applied for measuring many gases including oxygen [1], [2], carbon monoxide [3], [4], hydrogen [5], nitrous/nitric oxide [6], [7], water vapor [8], [9] and hydrocarbon gases [10], [11]. Titania sensors are particularly attractive as reducing gas sensors since their response is affected to a lesser extent by humidity than that of the common SnO2 sensors [12].
Larger surface area materials provide high sensitivity at low gas concentrations [13]. For example, Gao et al. [14] found that nanoscale titania films exhibited better oxygen sensing performance than micron-sized ones. Thermal pre-treatment of sensing devices is often required to ensure sensor stability. This causes grain growth of the material, resulting in a lower surface area and poor sensor response. Moreover, in the case of TiO2, temperatures over 600 °C lead to a crystallographic phase transition from anatase to rutile [15]. Dopants are typically added to titania either to increase its thermal stability (Si [16]; Nb [13]; Ta [12]; La [17]) or sensor sensitivity and selectivity (CuO [18]).
Large and easily accessible surface area, high crystallinity and the ability to include noble metal doping are all requirements for TiO2 sensor material synthesis routes. Nano- and micro-meter TiO2 particles for gas-sensing have been produced by sol–gel [8], oxidation of metallic titanium foil [3], laser pyrolysis [13], magnetron sputtering [5], supersonic cluster beam deposition [19] and ball milling of commercial powders [20].
An alternative and highly attractive synthesis route for nanostructured TiO2 is flame technology which is used largely for manufacture of about 2 million tonnes/year pigmentary titania [21]. Size, crystallinity and morphology of flame-made TiO2 can be controlled by changing the high temperature residence time of the particles in the flame [22], [23]. Doped-TiO2 can be readily made by co-oxidation of the precursors in the flame. For example, silica stabilizes the anatase phase, while alumina or tin oxide promote rutile formation [24], [25]. Addition of low-volatility dopant precursors (e.g. for platinum) can be facilitated by flame spray pyrolysis [26] that have resulted in highly sensitive Pt/SnO2 sensors [27]. Tin dioxide nanoparticles made by flame spray pyrolysis (FSP) have already shown high and fast response to propanal and NO2 [28]. Furthermore, flame technology allows for direct deposition of nanostructured SnO2 particles on sensing substrates avoiding the rather demanding deposition of slurries and pastes on electrodes [29].
Here, the sensing of volatile organic compounds and CO is explored using flame-made TiO2 anatase nanoparticles. For this purpose ethanol, isoprene (2-methyl-1,3-butadiene) and acetone are selected; representing alcohols, alkenes and ketones, respectively. Acetone is a common airborne contaminant [30]. Isoprene has the highest concentration among the hydrocarbon metabolites in human breath and originates from the decomposition of organic phosphate in cholesterol [31]. It can also be found over forested environments, where it is biosynthesized by the carboxylation process in the leaf chloroplast [32]. Ethanol detection is required for applications such as breath analyzers, monitoring devices for food-quality and fruit ripening [33].
Section snippets
Particle and sensing film synthesis
TiO2 nanoparticles were produced in a flame spray pyrolysis (FSP) reactor described in detail elsewhere [34], [35]. Precursor solutions (0.5 or 0.67 M) were prepared from titanium-tetra-isopropoxide (TTIP, Aldrich, purity > 97%) diluted in an 11:5 (v/v) mixture of xylene (Fluka, >98.5%) and acetonitrile (Fluka, >99.5%) and fed at 5 ml/min through the inner reactor capillary. Through the surrounding annulus, 5 l/min of oxygen (Pan Gas, purity > 99%) were fed dispersing the precursor solution into a
Particle and sensing film properties
Fig. 2 shows TEM images of as-prepared TiO2 nanoparticles. Sample P1 was produced from a 0.5 M TTIP solution, while sample P2 was produced from a 0.67 M solution with the glass tube enclosing the flame. The particles in sample P1 (Fig. 2a) are spherical and non-agglomerated of 15 nm in BET diameter consistent with Schulz et al. [35] for FSP-made TiO2 at similar conditions. The particles in sample P2 (Fig. 2b) are larger, 43 nm BET diameter, and polyhedral as they were made at higher temperature (in
Conclusions
Anatase TiO2 nanoparticles were made by flame spray pyrolysis of titanium-tetra-isopropoxide. Films about 30 μm thick were deposited by drop-coating a suspension of the particles onto alumina substrates interdigitated with gold electrodes. The gas sensing properties of the films were mainly investigated for ppm levels of ethanol, acetone and isoprene vapors at 500 °C. The sensors had n-type response to these vapors with response and recovery times within a few seconds or minutes, respectively. A
Acknowledgements
The authors thank Dr. Frank Krumeich at the Laboratory of Inorganic Chemistry, ETH Zurich (Switzerland) for preparation of SEM and TEM images. Financial support by the Swiss National Science Foundation (No. 200021-100325) and the U.S. National Science Foundation DMR NIRT grant is acknowledged.
Alexandra Teleki received her Master of Science in Chemical Engineering from the Royal Institute of Technology in Stockholm (Sweden) in 2003. She is currently a PhD student at the Swiss Federal Institute of Technology in Zurich (Switzerland). Her research interests are in nanoscale science and engineering.
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Alexandra Teleki received her Master of Science in Chemical Engineering from the Royal Institute of Technology in Stockholm (Sweden) in 2003. She is currently a PhD student at the Swiss Federal Institute of Technology in Zurich (Switzerland). Her research interests are in nanoscale science and engineering.
Sotiris E. Pratsinis has a Diploma in Chemical Engineering from the Aristotle University of Thessaloniki, Greece (1977) and a PhD from University of California, Los Angeles (1985). He was Professor (1985–2000) of Chemical Engineering at the University of Cincinnati, Ohio, USA until he was elected Professor at the Swiss Federal Institute of Technology (ETH Zurich). There he established the Particle Technology Laboratory and teaches Mass Transfer, Particle Technology, Nanoscale Engineering and Combustion Synthesis of Materials since 1998. His research program focuses on the fundamentals of aerosol synthesis of metal–ceramic nanoparticles and their applications in catalysis, sensors and nanocomposites.
Krithika Kalyanasundaram received her Bachelor of Engineering in Metallurgy from the National Institute of Technology (formerly Regional Engineering College), Tiruchirappalli, India. She is currently pursuing her doctoral studies at the Center for Nanomaterials and Sensor Development at the State University of New York at Stony Brook (USA). Her current research interest is in the development of novel nanomaterials for selective gas sensing.
Perena I. Gouma has a Bachelor of Science in Applied Physics from the Aristotle University of Thessaloniki (1990, Greece), a Master of Science in Materials Science and Engineering (1992) as well as a Master of Philosophy in Organizational Management (1993) from the University of Liverpool (UK) and a PhD in Materials Science and Engineering from the University of Birmingham (1996, UK). She was a post-doctoral researcher (1996–2000) at the Ohio State University, Columbus (USA) until she was elected Assistant Professor at the State University of New York at Stony Brook (USA). Her research program at the Center of Nanomaterials and Sensor Development focuses on the processing and characterization of advanced materials, such as nanostructured metal oxides and bio-composites for selective chemosensors and electronic noses.