Percolating SnO2 nanowire network as a stable gas sensor: Direct comparison of long-term performance versus SnO2 nanoparticle films

Sensors and Actuators B 139 (2009) 699–703

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Percolating SnO2 nanowire network as a stable gas sensor: Direct comparison of long-term performance versus SnO2 nanoparticle films V.V. Sysoev a,∗, T. Schneider b, J. Goschnick b,1, I. Kiselev b, W. Habicht b, H. Hahn b, E. Strelcov c, A. Kolmakov c a b c

Saratov State Technical University, Polytechnicheskaya 77, 410054 Saratov, Russia Forschungszentrum Karlsruhe, Postfach 3640, 76021 Karlsruhe, Germany Southern Illinois University, Carbondale, IL 62901-4401, USA

a r t i c l e

i n f o

Article history: Received 6 February 2009 Received in revised form 19 March 2009 Accepted 25 March 2009 Available online 5 April 2009 Keywords: Gas sensor Tin dioxide Nanowire Nanoparticle Stability

a b s t r a c t A comparative study of the long-term gas-sensing performance of chemiresistors made of: (i) mats of randomly oriented single crystal SnO2 nanowires and (ii) thin layers of pristine SnO2 nanoparticles, has been carried out. The sensing elements made of percolating nanowires demonstrate excellent sensitivity and long-term stability toward traces of 2-propanol in air. Different from the nanowire network, the superior initial sensitivity of the nanoparticle layer deteriorates during the first month of the operation and approaches to one observed steadily in the nanowire mats. The better stability of the nanowire mats sensors is explained in framework of reduced propensity of the single crystal nanowires to sinter under real world operation conditions with respect to nanoparticle thin film. At the microscopic level, the letter defines the stability of the percolating paths, analyte delivery and transduction mechanism in nanowire network sensing elements. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The reducible semiconducting oxides are employed since 1960s [1–2] as a major material platform for chemiresistor-type gas sensors [3]. It is widely accepted that in order to have an effective and prompt transduction of surface adsorption–desorption processes into the change of oxide conductance, a size of the sensing element should be in the range of material’s Debye length, Ld , which falls to the 10–100 nm scale for moderately doped oxides [4]. Therefore, thin or thick films composed of percolating nanoparticles became a standard platform for conductometric gas sensors (for example [5,6]). The bottom-up approach developed in the framework of nanotechnology offered completely new morphologies of the sensing elements such as microspheres [7,8], nanowalls [9,10] and nanowires [11–13], which posses at least one characteristic dimension in the size domain of Ld , making them prospective for gas sensing [14,15]. Apparently, the type of the sensing element along with the general morphology of the active layer has a significant effect on analyte delivery and electron transport in the layer and,

∗ Corresponding author. Fax: +7 8452 506740. E-mail addresses: [email protected] (V.V. Sysoev), [email protected] (A. Kolmakov). 1 Deceased. 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.03.065

as a result, on the efficiency of transduction of surface processes into the sensor signal [16]. Among these newly developed sensing layers, ones based on percolating network of nanowires [11,17] are simplest to produce and are closest to commercial realization of the new sensing concepts. In spite of the multiple reports on “pros and cons” in performance of the nanowire-based chemical sensors (see, for example [14,15,18,19]), the direct comparative study of the influence of morphology of these new sensing layers on their major sensing characteristics is still limited [20]. Herein, we experimentally address a long-term stability of gas sensitivity of chemiresistor sensors based on 2-D network of percolating SnO2 nanowires (NWs) and compare it with one of thin films composed out of 3-D porous network of SnO2 nanoparticles (NPs). 2. Experimental The SnO2 nanowires and nanobelts were grown at ca. 950 ◦ C on ceramic substrates using the vapor–solid method in Ar flow according to earlier reported procedure [21]. X-ray diffraction (XRD) and scanning electron microscopy (SEM) data indicated that the NWs were rutile single crystals with a transverse width, DNW , in the range of 200–900 nm and a length of up to 200 ␮m (Fig. 1b). The SnO2 NP layers were prepared via chemical vapor synthesis from an organo-tin precursor which was decomposed in a hot wall reactor at temperatures of ca. 1000 ◦ C [22]. The NPs films were found by XRD and the Brunauer–Emme–Teller method [23] to be crystallized

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in ambient atmosphere for 3 h. The thickness DA of the NP layers checked by imaging its cross-section in high-resolution transmission electron microscopy (HRTEM) was found to be ca. 150 nm. The conductance of SnO2 NW and NP structures was studied for 25 days in dry synthetic air, followed by a sequence of 20 days in humidified, 50 rel. %, synthetic air of constant flow rate, ca. 0.3 l/min. The 2-propanol vapor, as a test reducing gas, was introduced into the air flow feeding the chips for 60 min at each concentration of 0.5–50 ppm range with 90 min break flushing by pure air. The operating temperature was maintained to be at the level of about 300 ◦ C, same for both sensors. The gas response S = ( gas −  air )/ air of the SnO2 nanostructure samples under the investigation was evaluated as S = S0 C␣ , where  gas and  air are the conductance of sensor segment at the test gas (2-propanol) mixture with air and at pure air, respectively; C the test gas concentration in the air mixture; S0 the gas sensitivity or the response to 1 ppm of the test gas; ˛ the power law exponent. The latter two empirical parameters are conventional ones to evaluate the performance of semiconductor chemiresistors [28]. 3. Results and discussion Fig. 2 shows the gas response measured for as-prepared NP-based (a) and NW-based (c) chips to 2-propanol vapors of stepchanged concentrations, 0.5–50 ppm, in air. The response of the same samples as measured after 46 days of continuous operation is plotted in the panels (b and d), correspondingly. As can be seen, the

Fig. 1. HRTEM and SEM images of (a) SnO2 3-D mesoporous nanoparticle layer and (b) SnO2 2-D nanowire mat used in this study. See the main text for details on fabrication.

in cassiterite structure with a mean particle diameter, DNP , of 4 nm and a specific surface area of 123 m2 /g. Subsequently, the particles were dispersed in an alkaline aqueous solution containing a nonionic surfactant. The obtained colloidal dispersion was investigated with photon correlation spectroscopy showing agglomerates in the size of ca. 140 nm [24]. To gain better statistics for gas-sensing data, both NP film and NW mats were deposited onto thermally oxidised SiO2 /Si KAMINA-type substrates, 8 mm × 10 mm, equipped with 39 Pt strip electrodes, 4 Pt meander heaters maintaining the operation temperature profile and 2 meander-shaped thermoresistors [25]. The electrode architecture allowed us simultaneous and comparative conductometric measuring the 38 sensor segments in the microarray of each type. SnO2 NWs grown on the alumina target were deposited onto the SiO2 /Si substrates by gentle dry pressing of the ceramic target with nanowires against the substrate [26]. As a result of this procedure, clean one-two monolayer network of percolating nanowires was formed. The density of the network can be adjusted by repeating this protocol. In this study, the network density was kept in the proximity to the percolation threshold ∼LNW −2 ∼1 × 10−3 ␮m−2 (where LNW is an average length of the nanowire) [27]. The electrostatic affinity between the NWs and the substrate was found to be sufficient against mechanical distortions. At the same time, the SnO2 NP layers were deposited onto the substrates by spin-coating from an aqueous colloidal dispersion of the particles (Fig. 1a). A viscosity trimmer has been added to adjust the layer thickness. These NP deposits were dried at room temperature for 3 days until the photoresist has been removed with subsequent annealing at 400 ◦ C

Fig. 2. The change of the SnO2 nanostructures resistance of median sensor segment relative to the maximum value under the exposure to 2-propanol vapors of step changed concentration at 1st day (a and c) and 46th day (b and d): (a and b)—NP 3-D layer; (c and d)—NW 2-D mat.

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Fig. 3. The change of sensing characteristics of median sensor segment of SnO2 3-D mesoporous NP layer and 2-D NW mat versus measurement day: (a) background resistance in air; (b) sensitivity or response to 1 ppm of 2-propanol vapors; (c) exponent of response power law function S(C). Open and filled circles correspond to NP and NW samples; the error bars at all the curves indicate a standard deviation of the data over the sensor segments in the chip microarray. The dotted lines are given to navigate an eye. (d) A scheme of the electrical transport in the studied SnO2 nanostructures layers: the left and right panels correspond to NW and NP samples. Line (A–C) shows the as-deposited state (A), state after a long-term operation (B) and the band diagram including potential barriers (C).

SnO2 NW 2-D mats have a stable gas-sensing performance over the whole measuring time period. The Fig. 3(a–c) depicts the experimental data on a long-term evolution of the characteristics of the SnO2 nanostructures toward the 2-propanol vapor. The response of 2-D NW mat to 1 ppm of 2-propanol is about 6.4 ± 1.1% and the exponent of S(C) power law is 0.57 ± 0.04. On the contrary, the as-prepared NP 3-D mesoporous layer exhibits a rapid drop of the response from approximately 4 × 103 % value down to approximately 102 % during the first 10 days. Following the introduction of 50 rel. % humidity to the gas mixture on 26th day, the NP response magnitude drops again by approximately one order of magnitude with a further stabilization at the level of about 16 ± 5%, close to one shown by the NW 2-D mats. Over the whole measuring period, the exponent of S(C) power law of NP response has fluctuated at the level 0.72 ± 0.05 which is consistent with the data observed earlier in SnO2 NP 3-D mesoporous layers exposed to CO [29]. The evolution of gas sensitivity of the SnO2 nanostructures of interest under the long-term operation at elevated temperatures could be explained by considering the electrical transport through their networks. There are two distinct electron transport regimes [30] which depend on the ratio between the characteristic transverse size of the nanostructures, D (here, DNW or DNP ) and the space charge region width



W = Ld

eVs kT

(1)

where e is the elementary charge, Vs the surface potential, k the Bolzmann’s constant, T the operating temperature in K) [31].

1. W < D, Ns ≈ Nd W,where Nd is the donor concentration within the oxide nanocrystal; Ns the surface density of electrons captured at the oxide local states induced by adsorbed species. In this case, the number of free electrons is diminished by the charge captured at the oxide surface states induced by adsorbed gas and the electron transport is modulated by potential barriers between crystals Vs =

e2 Ns εε0 Nd

(2)

The conductance of crystal network is G∝



e0 Nd (D − W )2 eVs exp − l kT

 (3)

where 0 is the crystal electron mobility, l the distance between electrodes. Thus, the gas-sensing mechanism is dependent on both the width and height of the contact potential barriers. We expect that such a mechanism exists in the SnO2 NW 2-D mats (Fig. 3d, left panel). 2. W ≥ D, Ns ≈ Nd D.This is a case of fully depleted crystals when the Fermi level is totally controlled by surface states under nearly flat energy bands [32,33] and the gas-sensing mechanism depends mostly on the release of electrons from the surface states to the conduction band due to surface reactions. Such a situation is characteristic for as-deposited SnO2 NP 3-D layer whose initial high porosity allows gas diffusion into the NP agglomerates rather fast so that a significant number of the NPs within the agglomerates are exposed to the incoming gas. Therefore, the

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influence of potential barriers which are most significant at the contacts between agglomerates [28] can be neglected in asdeposited NP 3-D layer (Fig. 3d, right panel). Nevertheless, the long-term exposure to 2-propanol vapors at elevated operating temperature seems to facilitate sintering of NPs, what leads to aggregations and encapsulation of NPs into larger agglomerates (see Fig. 3d(A and B)) [34,35]. The important role in these aging processes is presumably played by the water presence [36] what is evidenced, on one hand, by experimentally observed significant drop of gas response of the NP layer following the humidity introduction, 26–46 day interval (Fig. 3b). On the other hand, even in dry air conditions the oxidation of 2-propanol at the heated SnO2 crystal surface supplies water to the oxide surface as [37] − X(CH3 )2 CH − OH + 9O− x → 3XCO2 + 4XH2 O + 9e ,

(4)

where x = 1, 2 [38]. As a result of the morphology changes due to aggregation of NPs, the diffusion of gas molecules through the pores inside the NP 3-D networks becomes hampered [39]. Thus, in addition to the particle size increase, even the un-sintered NPs which may exist within the agglomerates become excluded from the gas exchange with environment (and become insensitive to analyte) compared to those at the outer part of the agglomerates. The number of possible percolation paths [40] in the sensing layer is dramatically decreased proportionally to the ratio of the number of NPs composing the whole agglomerate to the number of NPs in its outer layer which corresponds to DA /DNP . Moreover, the sintering of particles leads to surface area reduction and, hence, to lesser number of available adsorption sites which further limits the resistance modulation by the adsorbed gas. Finally, under a long operation time, the sensing mechanism in the SnO2 NP 3-D layer switches to that observed in the SnO2 NW 2-D mats, i.e. to be controlled mostly by  = W/D ratio and potential barriers at the contacts between large NP agglomerates. At the same time, the encapsulation of pores within the NP agglomerates supports the conservation of oxygen chemisorbed at the surface of internal NPs whose coverage is not altered by the reducing gas. Therefore, the resistance of NP layer under pure air conditions has only a small drift (Fig. 3a) which presumably is caused, as well as that in the case of NWs, by the diffusion of oxygen vacancies in the oxide crystals, which are n-type shallow donors in SnO2 [41], toward their surface [18,42]. It is worth noting that the exponent of S(C) power law, ˛, is conventionally considered to follow the Freundlich isotherm and is described in the framework of Wolkenstein’s electronic adsorption theory [43] by means of  value, operating temperature and energy of the surface states associated with the adsorbed gas [44]. In particular, higher values of ˛ characterizing the same gas exposure at certain temperature indicate larger  magnitude in the semiconductor structure. Thus, the differences between ˛ exponents observed for gas response of the NP layer and NW mat (Fig. 3c) are believed to be influenced by the differences in the  values for these two classes of nanostructures. 4. Conclusion It has been shown that oxide NW mats combine a number of positive features, as facile fabrication, open surface, high gas sensitivity and long-term stability, which make them prospective material platform for the next generation of durable conductometric gas sensors. Further enhancement of the gas sensitivity of such mat monolayers might be achieved by employing oxide NWs with smaller diameters. As-prepared SnO2 NP 3-D network layers have superior gas sensitivity which, however, degrades in the long-term down to that observed in percolating SnO2 NW 2-D mats. The latter

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Biographies Victor V. Sysoev is docent of Physics Department in Saratov State Technical University (Russia). After studying in Leningrad Mechanical Institute and Saratov State University (SSU) he graduated from SSU in 1994 with Diploma of engineer-physicist in microelectronics and semiconductor devices. In 1999 he was awarded by SSU Council with Candidate of Phys. & Math. Sc. degree in physics of semiconductors and dielectrics. He worked many times as a visiting researcher in such institutions as National Microelectronics Research Center (Cork, Ireland), Karlsruhe Research Center (Karlsruhe, Germany) and Southern Illinois University (Carbondale, USA). His scientific interests are chemical sensors and multisensor systems. Thomas Schneider studied chemistry at the University of Karlsruhe, Germany. In 1996, he joined Dr. Goschnick’s group for development of gas analytical micro sensors at the Karlsruhe Research Center (Germany) and received his PhD degree from the University of Karlsruhe in 1999. He is responsible for the development of colloidal dispersions and the development, fabrication, gas-analytical testing and surfaceanalytical characterization of thin nanogranular layers for gas sensor microarrays.

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Joachim Goschnick studied chemistry at the Free University of Berlin. He accomplished his PhD thesis on metal surface based heterogeneous reactions at the Fritz-Haber-Institute of the Max-Planck-Association in 1987, in Berlin. In 1988 he set up a laboratory for surface and depth-resolved analysis of environmental materials in the Karlsruhe Research Centre. With launching the Microsystem Technology Project of the Karlsruhe Research Center in 1993, his R&D field was expanded to include the development of gas analytical microsystems aiming at inexpensive, small and robust Electronic Noses (EN) based on micro/nano-technology. Dr. Goschnick died in 2007. Ilia Kiselev studied physics at Moscow State University (Russia) and received his PhD degree in physics and mathematics from Kyrgyz Science Academy, where he was specialized in the area of mathematical physics. In 2000 he joined Dr. Goschick’s group at the Karlsruhe Research Center (Germany) to work toward development of gas analytical microsensors based on metal oxide films. He is responsible for modeling of processes at the metal oxide structures and signal analysis. Wilhelm Habicht received his diploma of engineer in technical physics from University of Applied Sciences (Wiesbaden, Germany) in 1976. Since then he works in Institute for Technical Chemistry at the Karlsruhe Research Centre specializing in analysis of micro- and nanostructures through different imaging techniques as SEM, SFM, etc. Prof. Horst Hahn is Managing Director of the Institute for Nanotechnology at the Karlsruhe Research Centre and Director of the Research Laboratory of Nanomaterials at the Technische Universität Darmstadt (Germany). He studied Materials Science at the Universität des Saarlandes and received his PhD from the Technische Universität Berlin. He was a post-doctoral fellow at the Universität des Saarlandes working in the area of interfaces and nanocrystalline metals. From 1985 to 1987 he was a research associate in the Materials Science Division at Argonne National Laboratory. Subsequently, he was research assistant professor in the Materials Research Laboratory at the University of Illinois at Urbana-Champaign for two years. In 1992, he became associate professor of Materials Science at Rutgers-The State University of New Jersey. From 1992 to 2004 Dr. Hahn was full professor in the Department of Materials Science at Technische Universität Darmstadt and Head of the Thin Films Division. For three years he served as Chairman of the Department. He is a member of the DFG funded Center for Functional Nanostructures and of the Landeskompetenznetzwerk “Funktionelle Nanostrukturen” at the Universität Karlsruhe. His main research interests are in the areas of synthesis, characterization and physical and chemical properties of nanostructured materials in the form of thin films, nanoparticles and bulk materials. Evgheni Strelcov, a PhD student of the Physics Department at Southern Illinois University at Carbondale, works with Prof. A. Kolmakov since 2006. He graduated from Moldova State University in 2003 with a specialization in inorganic chemistry and received his MS degree in chemistry from the same University a year later. Fields of interest include coordination chemistry of d- and f-elements and application of nanoscience to gas sensing. Evgheni has authored and co-authored several papers in these areas. Prof. Andrei Kolmakov specializes in surface science, transport properties and imaging techniques of nano-objects relevant to gas sensing and catalysis. He has authored or co-authored over 70 technical papers, including 2 book chapters and 5 review articles. He received his MS in physics from Moscow Physical Technical Institute (Russia) in 1986. He started his research work as a staff member at the Research Center Kurchatov Institute (Moscow, Russia), where he completed his PhD in solid-state physics in 1996. He currently holds an appointment in the Physics Department at Southern Illinois University at Carbondale (USA).