In2O3 nanowires for gas sensors: morphology and sensing characterisation

Thin Solid Films 515 (2007) 8356 – 8359 www.elsevier.com/locate/tsf

In2O3 nanowires for gas sensors: morphology and sensing characterisation A. Vomiero a,⁎, S. Bianchi a,b , E. Comini a,b , G. Faglia a,b , M. Ferroni a,b , N. Poli b , G. Sberveglieri a,b b

a INFM-CNR SENSOR Lab, Via Valotti 9, 25133 Brescia, Italy Dip. di Chimica e Fisica per l’Ingegneria e i Materiali, Università di Brescia, Via Valotti 9, 25133 Brescia, Italy

Available online 20 March 2007

Abstract Thin and densely packed In2O3 nanowires have been synthesised on alumina substrates via transport and condensation method, starting from nanoparticles of indium or palladium as catalysts for the condensation process. Indium catalyst promoted wires growth according to vapour–solid (VS) mechanism, while palladium catalyst leads to wires formation based on vapour–liquid–solid (VLS) condensation. Electron microscopy and related diffraction analysis demonstrated that the wires are monocrystalline, with atomically sharp termination of the lateral sides, and are free from extended defects. The sensing properties of nanowires bundles have been tested to acetone using the flow through technique in the temperature range between 100 and 500 °C. © 2007 Elsevier B.V. All rights reserved. Keywords: In2O3; Nanowires; Atomic structure; Gas sensors

1. Introduction The synthesis of one-dimensional nanostructures in form of nanobelts, nanorods, and nanowires has stimulated intense research activity due to their novel physical properties and their potential applications in nanotechnology [1–5]. Nanowires of metal oxide semiconductors have been recently synthesised [6]; and their exceptionally high surface-to-volume ratio, monocrystalline assembly, and semiconducting electrical behaviour foresee their implementation in a new class of gas sensors, capable to achieve high gas sensitivities and long term stability [7–9]. Transport and condensation method was demonstrated to be an effective technique for synthesis of metal-oxide nanowires [10]. Two main different growth mechanisms have been observed: vapour–solid (VS) [11–13], and vapour–liquid– solid (VLS) [6,14,15]. In the first case, the vaporised precursor condensates directly along the growing nanowire. In the VLS mechanism, the melted catalyst forms a liquid droplet by itself or by alloying with the growth material, and acts as preferential

⁎ Corresponding author. Tel.: +39 030 371 57 49; fax: +39 030 209 12 71. E-mail address: [email protected] (A. Vomiero). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.03.034

condensation site for the vaporised precursor; when a supersaturated solution is reached, growth material precipitates resulting in one dimensional growth. In2O3 is wide band gap transparent semiconductor (Eg ∼ 3.6 eV). Its application as gas sensing material in form of thick film [16], nanowires [17] and microcubes [18] has been recently investigated. Synthesis of In2O3 in form of nanowires is particularly demanding as decomposition of In2O3 precursor powder occurs in the 1400–1500 °C temperature range. This condition requires extreme chemical stability for deposition environment and substrate. This study reports on the growth of In2O3 nanowires via transport and condensation method through either a VS or VLS process. The sensing properties of In2O3 nanowires bundles at different temperature and at different concentrations of acetone have been investigated. 2. Experimental The experimental apparatus was composed by tubular furnace and gas injection system (Fig. 1). Inert Ar was applied as gas carrier. Nanowires growth occurred at pressure of 200 mbar and Ar flux of 100 sccm. The residual oxygen inside the furnace allowed oxidation of the metallic species during condensation. The precursor (In2O3 powder, purity 99.999%)

A. Vomiero et al. / Thin Solid Films 515 (2007) 8356–8359

8357

Fig. 1. Schematic view of the three-zones furnace for the growth of In2O3 nanowires. The precursors are placed in the central high-temperature zone of the furnace (evaporating temperature 1500 °C). The evaporation products condensate in the cooler region A–C. In region A (T ∼ 750 °C) thin and densely packed nanowires grow; in region B (T ∼ 900°C) precursors condensate in form of micro-cubes and regular polyhedrons; in region C (T ∼ 1000°C) formation of thick and dispersed micro-wires occurs.

was placed in the central zone of the furnace, and condensation products were collected on alumina substrates in the A–C zones (see Fig. 1). The precursor evaporates at temperature T = 1500 °C, and condensates in the 700–1000 °C temperature range. Heating and cooling process of the furnace before and after wire growth lasted about 1.5 and 3 h, respectively. During these transients, a reverse Ar flux was operated to avoid uncontrolled deposition of volatile species. Two different catalytic methods have been applied to favour wires nucleation and growth: thin layers (nominal thickness lower than 2 nm) of either indium (self-catalysis) or palladium (hetero-catalysis) were sputtered over alumina substrates before condensation. Morphological investigation of the wires was carried out by LEO 1525 FEG-SEM operated at 3 keV. TEM investigation was carried out with a FEI Tecnai F20 microscope equipped with field emission source, super-twin objective lens with 0.19 nm resolution limit and 0.14 nm information limit for high resolution imaging. Convergent beam electron diffraction and analysis of zero-order and higher-order Laue-zone diffraction was carried out for precise determination of unit cell, lattice parameters and space group. Metal contacts and heater circuit were sputtered over the nanowires bundle and on the rear side of the substrate for electrical measurement. The gas sensing properties of the nanowires were tested towards acetone using the flow-through technique [19]. The reference atmosphere of synthetic air was maintained at the constant condition of 0.3 l/min flow, 20 °C temperature, and 40% relative humidity. Acetone was mixed to the synthetic air flow in controlled concentration. The operating

temperature of the sensors was maintained through a feedback circuit. The sensors were biased by 1 V direct voltage. 3. Results and discussion 3.1. Morphology and structure Nanowires growth is very complex process, which occurs under unusual conditions of pressure, temperature, carrier flow. Nice energy landscape of nanowires formation was described by Tu and co-workers [20] in case of ZnO, in which the condensation products are regarded as a mesophase. Their transformation path towards bulk polycrystals, micro- or nanowires shape is driven by the thermodynamical conditions of the process. In case of In2O3, under the same conditions of flux and pressure, condensation at different temperatures on substrates seeded by indium leads to different products: Fig. 1 clearly shows the formation of micro-wires in the hightemperature region C (T ∼ 1000 °C) of the substrate. Lower temperature (region B, T ∼ 900 °C) causes formation of microcubes, parallelepipeds, and regular polyhedrons. In region A (T ∼ 750 °C) thin and densely packed nanowires grow, up to tens of microns in length. Presence of catalyst affects the growth mechanism of the wires. Indium-based seeds during furnace heating are formed on the substrate, preliminary to evaporation of precursor (Fig. 2A). The residual oxygen allows formation of oxidized seeds, which act effectively as nucleation centers for the nanowires. This process leads to the growth of nanowires with sharp lateral facets and pyramidal termination (Fig. 2B). Lateral dimension

8358

A. Vomiero et al. / Thin Solid Films 515 (2007) 8356–8359

of the thinnest nanowires is about 20 nm. Lateral dimension averages 110 ± 20 nm. The presence of pyramidal tip indicates that nanowires growth was driven by VS growth mechanism, without any external catalytic mechanism [10,13]. Pd seeding resulted in growth of nanowires, terminated by a metallic droplet each, and with micro-faceted lateral sides (Fig. 2B). The thinnest nanowires are as narrow as 20 nm. The distribution of lateral dimension for the nanowires averages 210 nm. In this case, the presence of the catalytic tip clearly indicates a dominating VLS mechanism [21,22]. Micro-faceting is attributed to the so-called roughening transition process [10,23]: at temperature above the “roughening temperature”, the thermal motion of the surface atoms overcomes the interfacial energy, and causes the roughening of a faceted crystal. As far as the characterization of nanowires produced by VS process, TEM investigation proved their monocrystalline assembly (see Fig. 3). The wires exhibit atomically-sharp termination of the lateral sides, constant thickness, and absence of extended defects. Convergent beam electron diffraction (not reported in figure) indicates that the crystalline structure of the nanowire is Ia-3 body-centered cubic In2O3 (space group 206). The growth direction of the nanowire resulted the [100] vector

Fig. 2. (A) Oxidised In seeds after transient furnace heating, just before nanowires growth in case of In seeding of the substrate. (B–C) SEM images of the terminations of two nanowires synthesized by templating the substrate with either In (B) or Pd (C). Pyramidal termination of the tip in (B) suggests that In templating leads to nanowires growth according to VS mechanism; the catalytic Pd tip in (C) clearly calls for VLS growth mechanism in case of Pd templating.

Fig. 3. (A) Bright field TEM image of a single In2O3 nanowire obtained via selfcatalysis of In-seeded substrate. (B) High resolution detail of the nanowire reported in (A); constant thickness across the belt and atomically sharp lateral termination are visible; the nanowire grows along the [100] vector of the cubic In2O3 crystalline cell.

Fig. 4. (A) Dynamic electric response of nanowires bundle grown by selfcatalytic In-seeding method towards 25, 50, and 100 ppm of acetone in synthetic air, at 40% relative humidity. The operating temperature of the sensor was 400 °C. (B) Conductance of the sensor as a function of the operating temperature in case of bare synthetic air at 40% of relative humidity (full circles) and after introduction of 100 ppm of acetone (open circles).

A. Vomiero et al. / Thin Solid Films 515 (2007) 8356–8359

of the cubic In2O3 crystalline cell. Complementary TEM-Energy dispersive X-ray microanalysis did not detect the presence of impurities in the nanowires within the sensitivity of the technique. The same results can be drawn for Pd-seeded nanowires. Investigation is ongoing to obtain exhaustive comprehension of the different growth direction for In- and Pd-seeded nanowires. The monocrystalline assembly of the nanowires is very promising to obtain highly stable nanowires-based sensors; in fact re-crystallisation and grain coalescence processes during long term operation are the main contributors of degradation of sensor stability in polycrystalline thin and thick films.

8359

tested at various operating temperatures for different acetone concentrations. Acknowledgements Financial support from European Union and MIUR is gratefully acknowledged: “Nanostructured solid-state gas sensors with superior performance-NANOS4” STREP project no. 001528. “Nanostructured semiconductors for chemical sensing” PRIN project 2004. “Quasi mono dimensional nanosensors for label free ultra sensitive biological detection” PRIN project 2005.

3.2. Sensing properties References The sensing properties of densely packed nanowires were tested towards acetone at different temperatures and analyte concentrations. The dynamic electric response to 25, 50 and 100 ppm of acetone at operating temperature of 400 °C is collected in Fig. 4A. Introduction of reducing gas causes enhancement of electric current, proving the n-type behaviour of the In2O3 nanowires. The conductance of the nanowires is shown in Fig. 4B at temperature ranging between RT and 500 °C. At operating temperatures below 100 °C, introduction of 100 ppm of acetone causes negligible fluctuations of the electric signal. Electric conductance as a function of temperature in synthetic air exhibits a minimum at the operating temperature of 400 °C, corresponding to maximum oxygen chemisorption by In2O3 nanowires bundle. Top sensitivity occurred at 400 °C, accordingly. The response of the sensor, defined as the relative variation of the conductance, is about 7 when introducing 25 ppm of acetone at operating temperature of 400 °C.

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

4. Conclusions [16]

The effect of In and Pd substrate templating on the growth of In2O3 nanowires via transport and condensation method was investigated. In case of In, oxidation of the catalyst and formation of nanometre seeds occurred during furnace heating, leading to condensation of In2O3 nanowires according to VS mechanism. Pd catalyst led to nanowires growth via VLS mechanism. In both the cases nanowires exhibit monocrystalline habit, atomically sharp termination of the lateral sides, and no evidence of extended line defects, which is promising result to obtain highly stable sensors for long term operations. Sensing properties of sensors based on nanowires bundle were

[17] [18] [19] [20] [21] [22] [23]

X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001) 66. Y. Cui, C.M. Lieber, Science 291 (2001) 851. F. Leonard, A.A. Talin, Phys. Rev. Lett. 97 (2006) 026804. J. Hu, T.W. Odom, C.M. Lieber, Amer. Chem. Res. 32 (1999) 435. Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353. Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. E. Comini, Anal. Chim. Acta 568 (2006) 28. E. Comini, G. Faglia, G. Sberveglieri, Zhengwei Pan, Zhong L. Wang, Appl. Phys. Lett. 81 (2002) 1869. I.D. Kim, A. Rothschild, B.H. Lee, D.Y. Kim, S.M. Jo, H.L. Tuller, Nano Lett. 6 (2006) 2009. G. Cao, Nanostructures & Nanomaterials, Imperial Collage Press, London, 2004. Z. Cui, G.W. Meng, W.D. Huang, G.Z. Wang, L.D. Zhang, Mater. Res. Bull. 35 (2000) 1653. W.K. Burton, N. Cabrera, F.C. Frank, Philos. Trans. R. Soc. 243 (1951) 299. H.Z. Zhang, Y.C. Kong, Y.Z. Wang, X. Du, Z.G. Bai, J.J. Wang, D.P. Yu, Y. Ding, Q.L. Hang, S.Q. Feng, Solid State Commun. 109 (1999) 677. J. Lao, J. Huang, D. Wang, Z. Ren, Adv. Mater. 16 (2004) 65. P. Nguyen, H.T. Ng, T. Yamada, M.K. Smith, J. Li, J. Han, M. Meyyappan, Nano Lett. 4 (2004) 651. G. Korotcenkov, V. Brinzari, A. Cerneavshi, M. Ivanov, V. Golovanov, A. Cornet, J. Morante, A. Cabot, J. Arbiol, Thin Solid Films 460 (2004) 315. S. Bianchi, E. Comini, M. Ferroni, G. Faglia, A. Vomiero, G. Sberveglieri, Sens. Actuators, B, Chem. 118 (2006) 204. A. Gurlo, N. Barsan, U. Weimar, M. Ivanovskaya, A. Taurino, P. Siciliano, Chem. Mater. 15 (2003) 4377. G. Sberveglieri, L.E. Depero, S. Groppelli, P. Nelli, Sens. Actuators, B, Chem. 26–27 (1995) 89. Z.C. Tu, Q.X. Li, X. Hu, Phys. Rev., B 73 (2006) 115402. R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 3396. E.I. Givargizov, J. Crys. Growth 31 (1975) 20. H.N.V. Temperley, Proc.Camb. Philol. Soc. 48 (1952) 683.