1D ZnO nano-assemblies by Plasma-CVD as chemical sensors for flammable and toxic gases

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright

Author's personal copy Sensors and Actuators B 149 (2010) 1–7

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

1D ZnO nano-assemblies by Plasma-CVD as chemical sensors for flammable and toxic gases Davide Barreca a , Daniela Bekermann b , Elisabetta Comini c , Anjana Devi b , Roland A. Fischer b , Alberto Gasparotto d,∗ , Chiara Maccato d , Giorgio Sberveglieri c , Eugenio Tondello d a

CNR-ISTM and INSTM, Department of Chemistry, Padova University, Via Marzolo 1, 35131 Padova, Italy Inorganic Materials Chemistry Group, Lehrstuhl für Anorganische Chemie II, Ruhr-University Bochum, Universitätstrasse 150, 44780 Bochum, Germany CNR-IDASC, SENSOR Lab, Department of Chemistry and Physics, Brescia University, Via Valotti 9, 25133 Brescia, Italy d Department of Chemistry, Padova University and INSTM, Via Marzolo 1, 35131 Padova, Italy b c

a r t i c l e

i n f o

Article history: Received 21 April 2010 Received in revised form 15 June 2010 Accepted 23 June 2010 Available online 30 June 2010 Keywords: ZnO 1D nano-assemblies Plasma enhanced-chemical vapor deposition Gas sensors

a b s t r a c t In this work, 1D ZnO nano-assemblies were prepared on Al2 O3 substrates by plasma enhanced-chemical vapor deposition (PE-CVD), and characterized in their morphology and chemical composition by field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDXS) and X-ray photoelectron spectroscopy (XPS). For the first time, the sensing performances of PE-CVD ZnO nanosystems were tested in the detection of toxic/combustible gases (CO, H2 and CH4 ), revealing very good responses already at moderate working temperatures. In particular, carbon monoxide and hydrogen detection was possible already at 100 ◦ C, whereas methane sensing required a minimum temperature of 200 ◦ C. The performances of the present ZnO nanosystems, that make them attractive candidates for technological applications, are presented and discussed in terms of their unique and controllable morphological organization. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Over the last decade, the development of sensitive and reliable gas sensing devices for the detection of flammable/toxic chemical agents, such as CO, H2 and CH4 , has gained an increasing attention for safety, energy and environmental applications [1–9]. In particular, in spite of various works, continuous efforts are devoted to the development of low-cost, lightweight sensors for H2 detection wherever hydrogen is produced, stored or used, including industrial plants, spacecrafts and fuel cells vehicles [1,10–18]. Furthermore, the detection of leakages of flammable/explosive fuels, like methane, and toxic combustion products, such as CO, is of great importance to control domestic gas boilers, mining environments and combustion processes in automotive engines and industrial plants [5,11,19–22]. It is generally recognized that the performances of solid state gas sensors are directly dependent on the surface and morphology of the active material [1,9,18,23,24]. As a consequence, nanostructured thin films, and, more recently, 1D systems such as nanowires and nanobelts, have attracted the attention of several research groups [16,25–36]. In this context, one of the most promising metal oxides is ZnO, a biosafe II–VI n-type semiconductor with a

∗ Corresponding author. Tel.: +39 0498275192; fax: +39 0498275161. E-mail address: [email protected] (A. Gasparotto). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.06.048

wide direct band-gap (Eg = 3.4 eV), large exciton binding energy, good stability under operating conditions and compatibility with microelectronic processing [6,8,20,22,23,29,37–45]. For gas sensing of explosive/toxic gases in industry, urban and domestic life, ZnO nanosystems have been fabricated in various forms (thin films, nanowires, rings, combs, platelets, flakes, etc.) by means of different techniques, from sputtering, evaporation, pulsed laser deposition and thermal–chemical vapor deposition to thermal oxidation, spray pyrolysis, sol–gel and hydrothermal processes [3,15,19,27,29,39–41,46–50]. An open challenge in the field of gas sensing by ZnO is the use of moderate working temperatures to increase operational safety and reduce power dissipation, yet maintaining a long range stability and high responses to low concentrations of testing gases [8,11,13,20,36,51,52]. In addition, the high complexity of nanostructure-based devices often makes these systems highly expensive and scarcely competitive in view of large-scale utilization [13,43]. These issues can be at least partially addressed by the development of efficient and amenable routes enabling the synthesis of zinc oxide nanostructures with tailored properties. In the present work, we have developed a simple and controllable strategy for the low-temperature growth of 1D ZnO nano-assemblies. In particular, we focus on zinc oxide synthesis on Al2 O3 by plasma enhanced-chemical vapor deposition (PE-CVD), a flexible strategy that takes advantage of high-energy electrons generating reactive radicals and ions. These features, in turn,

Author's personal copy 2

D. Barreca et al. / Sensors and Actuators B 149 (2010) 1–7

enable the preparation of nanosystems with unique features under non-equilibrium conditions, especially at low temperatures. In particular, the occurrence of competing growth and ablation processes may result in system properties hardly attainable by other synthesis techniques, provided that a suitable choice of process parameters is made. In this work, the attention is specifically devoted to the reproducible obtainment of zinc(II) oxide nanoarchitectures endowed with ultrahigh surface-to-volume ratios, in order to make their electrical properties very sensitive to surface-absorbed species, thus providing improved sensing performances. After a preliminary morphological and compositional characterization, efforts have been devoted to the analysis of the system gas sensing responses towards CO, H2 and CH4 . To the best of our knowledge, no similar studies have ever been performed on PE-CVD ZnO nanosystems up to date. 2. Experimental 2.1. Synthesis The Zn precursors, Zn[(R)NC(CH3 ) C(H)C(CH3 ) O]2 , with R = –(CH2 )2 OCH3 (1) or –(CH2 )3 OCH3 (2), were synthesized according to a recently reported procedure [53]. Deposition of ZnO samples was performed starting from 1 and 2 by a custom-built PE-CVD apparatus [54,55]. Polycrystalline alumina slides (3 mm × 3 mm; thickness = 250 ␮m; average surface roughness = 70 nm [56]) were used as substrates and cleaned before each deposition by sonication in dichloromethane and subsequent rinsing in isopropanol. The precursor was placed in an external vessel heated by an oil bath (140–150 ◦ C) and its vapors were transported into the deposition chamber by an Ar flow. Auxiliary O2 and Ar flows were directly introduced in the reactor by means of separate lines. Depositions were performed for a typical duration of 1 h at temperatures between 200 and 300 ◦ C and a total pressure of 0.5–1.0 mbar, using a Radio Frequency power of 20 W and a fixed interelectrode distance of 6 cm. 2.2. Characterization Field emission-scanning electron microscopy (FE-SEM) measurements were carried out at acceleration voltages between 5.0 and 20.0 kV by means of a Zeiss SUPRA 40VP instrument, equipped with an Oxford INCA x-sight X-ray detector for energy dispersive Xray spectroscopy (EDXS) analyses. The mean nanoaggregate sizes were evaluated through the SmartSEMTM software. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Perkin-Elmer 5600ci spectrometer using a standard Mg K␣ radiation (h = 1253.6 eV), at a working pressure lower than 10−9 mbar. The reported binding energies (BEs, standard deviation = ±0.2 eV) were corrected for charging by assigning to the adventitious C1s line a BE of 284.8 eV [57]. The analysis involved Shirley-type background subtraction and spectral deconvolution, which was carried out by nonlinear least-squares curve fitting adopting Gaussian–Lorentzian sum functions. The atomic composition was calculated by peak integration using sensitivity factors provided by V5.4A software. Ar+ sputtering was carried out at 3.5 kV beam voltage, with an Ar partial pressure of 5 × 10−8 mbar. For gas sensing tests, 200-␮m spaced platinum electrodes and a Pt heater were sputtered over ZnO surface and on the backside of the Al2 O3 substrates, respectively. Gas sensing analyses were carried out in a temperature-stabilized sealed chamber using the flow-through technique (atmospheric pressure; relative humidity = 40%; 20 ◦ C; air flow = 0.3 l min−1 ) [58]. The sensor resistance was monitored by means of the volt-amperometric method in the

temperature range 100–400 ◦ C at a constant bias voltage of 1 V, after pre-stabilization at the working temperature for 8 h. As usually performed in the case of n-type semiconductor sensors and reducing gases, for all the analytes considered in this work the sensor response (S) (estimated uncertainty = ±5%) is defined according to Eq. (1) [7,8,14,25,40,41,58]: S =

G Gf − G0 = G0 G

(1)

where G0 and Gf are the baseline conductance value measured in air flow and the steady state value reached upon gas exposure, respectively. The response time was calculated as the time required for sample conductance to reach 90% of the equilibrium value following the test gas injection, whereas the recovery time was the one necessary for the sample to return to 30% of the original conductance in air [30,33,56,59]. Under the adopted measurement conditions, the tested ZnO sensors presented a stable response without appreciable activity losses, the maximum deviation not exceeding 10% upon repeated cycling. As an example, the typical standard deviation on H2 responses at 400 ◦ C for three cycles of measurements, one each 10 days, was calculated to be 7%, a value close to the response uncertainty (see above). These data, in line with our recent report on copper oxide nanosystems [60], indicated a good reproducibility. 3. Results and discussion The morphology of the obtained ZnO systems was analyzed by FE-SEM. For all samples, plane-view micrographs (Fig. 1) evidenced the presence of globular particles typical of the Al2 O3 substrate, whose dimensions ranged from 400 nm to more than 1 ␮m. Higher magnification images (see insets in Fig. 1) clearly displayed smaller ZnO nanostructures uniformly decorating the substrate surface. ZnO deposits were characterized by a homogeneous array of 1D like structures (average diameter = 30 ± 5 nm) grown on the surface of individual Al2 O3 islands, whose surface density depended on the adopted synthesis conditions. Fig. 1a displays uniform array of shorter nano-columns (surface density = 1800 ␮m−2 ), fabricated at 300 ◦ C and by use of precursor 1. At the same temperature and starting from precursor 2, longer 1D nanostructures with a surface density of 1450 ␮m−2 were obtained (Fig. 1b). The development of such systems was specifically enabled by the adopted lowtemperature PE-CVD route, favouring nucleation processes over the subsequent particle growth. Specifically, the observed morphologies, that were directly related to the roughness of the underlying Al2 O3 substrates (see Section 2), resulted in high system active areas, an amenable feature in view of eventual gas sensing utilization. The system chemical composition was preliminarily investigated by EDXS analyses. Irrespective of the preparation conditions, EDX spectra (Fig. 1c) were characterized by the sole lines of oxygen (O K␣ at 0.52 keV) and zinc (Zn L␣ and K␣ peaks located at 1.01 and 8.63 keV), along with the substrate aluminium signal (Al K␣ at 1.49 keV). No C and N peaks, that might arise from the incorporation of undecomposed precursor residuals, were detected, indicating thus a high purity of the obtained systems and a clean decomposition pattern of the used Zn sources under the present PE-CVD conditions. To attain a deeper insight into the surface and in-depth chemical composition, the obtained ZnO specimens were analyzed by XPS. To this regard, Fig. 2 reports the surface wide-scan spectrum of a representative sample, along with the detailed Zn2p and O1s regions. The survey displayed the expected zinc and oxygen signals, along with the C1s and CKVV ones. The complete disappearance of the carbon photopeaks after a mild Ar+ sputtering (data not shown)

Author's personal copy D. Barreca et al. / Sensors and Actuators B 149 (2010) 1–7

3

Fig. 2. (a) Surface XPS survey spectrum of a ZnO specimen obtained from precursor 1 at 200 ◦ C. Surface photoelectron peaks for Zn2p (b) and O1s (c) photopeaks are displayed as insets, along with the two O1s components resulting from spectral deconvolution.

For all analytes, gas sensing tests on the obtained ZnO nanostructures revealed an appreciable reproducibility of the registered electrical responses upon repeated utilization cycles. Fig. 3a reports a typical response of a ZnO sample to different concentration pulses of carbon monoxide at 200 ◦ C. The measured current increased proportionally to the analyte concentration, without any significant saturation effect, pointing out to a reversible interaction of carbon monoxide with the sensing material. The response and recovery

Fig. 1. Plane-view FE-SEM micrographs for ZnO samples grown at 300 ◦ C from precursor: (a) 1; (b) 2 (c) EDX spectrum for the specimen reported in (a).

confirmed that its presence was limited to the outermost sample layers and that it merely arose from atmospheric contamination, in agreement with EDXS results. As regards the zinc signals, the Zn2p3/2 peak was centred at BE = 1021.2 eV [Full Width at Half Maximum (FWHM) = 2.1 eV]. This value, along with the calculation of the Auger ˛ parameter [˛ = BE(Zn2p3/2 ) + KE(ZnLMM) = 2010.0 eV], confirmed the formation of ZnO free from other zinc-containing phases [13,38,57]. The O1s peak presented a broad spectral profile, that could be fitted by two contributions (see inset of Fig. 2). While the main component at 530.1 eV (FWHM = 1.7 eV) was ascribed to lattice oxygen in the ZnO wurtzite structure, the second one (BE = 531.6 eV; FWHM = 2.0 eV) was mainly related to –OH groups arising from interaction with atmospheric H2 O [22,52,56,61]. The presence of such species, commonly revealed on metal oxides, was reasonably enhanced in the present case due to the morphology of the obtained ZnO nanosystems, resulting in a high surface area (see below), and accounted for the O/Zn ratio (≈1.3) higher than the stoichiometric value.

Fig. 3. (a) Dynamic response of a ZnO sample obtained from precursor 1 at 200 ◦ C to square CO concentration pulses (working temperature = 200 ◦ C). (b) Response to 500 ppm CO as a function of the operating temperature for the same specimen.

Author's personal copy 4

D. Barreca et al. / Sensors and Actuators B 149 (2010) 1–7

times were also quite fast, being both typically of the order of 1 min [30,31] and comparable with the test chamber filling/purging time (≈2 min). Fig. 3b displays the dependence of the response towards 500 ppm of carbon monoxide as a function of the sensor operating temperature (100–400 ◦ C). As can be observed, the measured value was maximum at 200 ◦ C. A similar behaviour in CO detection by ZnO nanosystems has already been reported in literature [7,22,30]. For operating temperatures 1000 ppm [12,37]. This trend suggested that the chemisorption of molecular H2 onto ZnO was the rate-limiting step in the resulting current change [15,16]. Nevertheless, at variance with recent reports [1,3,24,27,46], a very good and rapid (≈1 min) recovery of the pristine air value was observed at the end of each hydrogen pulse, a promising feature for use of the present materials as H2 sensors. Unlike carbon monoxide, the sensing response to hydrogen (Fig. 4b) underwent a progressive increase with the adopted working temperature, reaching the highest measured value at 400 ◦ C. This behaviour agreed to a good extent with that observed

Fig. 4. (a) Dynamic response of a ZnO sample obtained from precursor 1 at 300 ◦ C to square H2 concentration pulses (working temperature = 400 ◦ C). (b) Response to 5000 ppm H2 as a function of the operating temperature for the same specimen.

Fig. 5. (a) Dynamic response of a ZnO sample obtained from precursor 2 at 300 ◦ C to square CH4 concentration pulses (working temperature = 300 ◦ C). (b) Response to 500 ppm CH4 as a function of the operating temperature for the same specimen.

by Qurashi et al. [13], claiming an enhanced reaction between hydrogen and adsorbed oxygen (see reaction (5)) upon increasing the temperature [2,10,14,24]. Working temperatures higher than 400 ◦ C were discarded to avoid undesired alterations of the ZnO nanostructures. Representative data pertaining to methane sensing at an operating temperature of 300 ◦ C are displayed in Fig. 5. Interestingly, the obtained results evidenced the possibility of detecting concentrations as low as 100 ppm, a valuable result considering that methane is a scarcely reducing gas. The dynamical response (Fig. 5a) revealed a rapid increase of the sensor current upon contact with CH4 (response times ≈ 1 min), whereas the recovery of air values at the end of the gas pulses was slower (≈2–3 min). The present kinetic performances compared favourably with previous studies [19]. In addition, it is worthwhile observing that the shape of the response curves was appreciably better than those reported for CH4 sensing by other ZnO systems [5,41,49], indicating a more reversible interaction with the analyte gas in the present case. A significant sensing action for CH4 began at working temperatures higher than 100 ◦ C (Fig. 5b), in agreement with literature data [19]. In addition, the methane response presented a maximumlike behaviour, the best operating temperature being 300 ◦ C, in line with previous papers reporting the best CH4 response by ZnO systems between 200 and 400 ◦ C [7,11,19,41,49]. This indicates that, under the present conditions, a steady equilibrium between CH4 adsorption and desorption was established at 300 ◦ C. Upon further increasing the operating temperature, CH4 desorption phenomena became predominant and the measured response decreased [20,48,51]. The sensing performances of selected ZnO specimens were also investigated as a function of the target gas concentrations, and representative data are plotted in Fig. 6. In contrast to previous works [7,8,21,27,36,39,43,51], the experimental trends yielded a linear

Author's personal copy D. Barreca et al. / Sensors and Actuators B 149 (2010) 1–7

5

arose from the unique electronic properties due to size confinement, i.e. to the mean nanostructure lateral size close to 30 nm, a value of the same order of magnitude of the ZnO Debye length. Under these conditions, the sensing performances were strongly affected by surface interactions. Specifically, a considerable portion of the nanostructure lateral section was depleted and the detection efficiency was directly controlled by the depletion layer width, thus generating enhanced current modulations upon injection of the target chemicals [1,7,24,27,45,52]. 4. Conclusions

Fig. 6. Representative ZnO gas sensing response as a function of concentration for: (a) CO (working temperature = 200 ◦ C); (b) H2 (working temperature = 400 ◦ C); (c) CH4 (working temperature = 300 ◦ C).

dependence in the considered concentration ranges, without evidences of saturation [39,58]. Considering a value of 0.5 as the lowest response value reliably detectable, detection limits extrapolated for CO, H2 and CH4 were 15, 600 and 50 ppm, respectively. For all the investigated gases, the present sensor responses were higher than those reported under similar operating conditions for ZnO systems, both with (e.g. Pd, Ag, Pt [16,19,49,51] and Au [6,20,23]) and without dopants [7,8,10,13,23,24,33,37,39,45–48]. The facilitation in gas molecules recognition can be explained by considering the sensing mechanism of reducing analytes by ZnO, based on the resistance changes occurring upon exposure to the target gases [6,22,46,47]. In particular, the interaction involves analyte adsorption on ZnO surface, where subsequent reactions with oxygen species take place. The processes can be described as follows [1,3,13,31,37,44,50]: O2(g)  O2(ads) −

O2(ads) + e  O2(ads) −

−

(2) −

O2(ads) + e  2O(ads)

(3) −

−

H2 + O(ads)  H2 O + e

−

CH4 + 4O(ads) −  CO2 + 2H2 O + 2e−

Acknowledgements

(4)

When the sensor is exposed to air, oxygen molecules undergo chemisorption on its surface, forming negatively charged species by capturing electrons from the ZnO conduction band [Eqs. (2)–(4)] [8,9,24,32]. The reducing gas adsorbs at the sensor surface and reacts with chemisorbed oxygen species, and, in particular, with O− , the most stable form in the investigated temperature range (Eqs. (5)–(7) [1,7,11,14,21,23,28,36,40,45,47]: CO + O(ads) −  CO2 + e−

In summary, high purity 1D ZnO nano-assemblies were grown on alumina substrates at 200–300 ◦ C by PE-CVD from Ar–O2 plasmas. Electrical tests for the detection of carbon monoxide, hydrogen and methane revealed very good responses, directly dependent on the specific gas and the adopted concentration. For CO and CH4 the maximum sensitivity was detected at 200 and 300 ◦ C, respectively. For H2 sensing, the response increased with the working temperature. The sensing efficiency was directly related to the obtained system morphology, characterized by 1D nanostructures with a high surface-to-volume ratio and reduced lateral dimensions. Overall, the present results are very promising for the integration of high-performance ZnO-based sensors into artificial olfaction systems for a variety of safety-oriented applications. Nevertheless, both the system selectivity and long-term stability upon several utilization cycles represent critical issues in view of practical technological applications. These points will be the object of future research efforts, with particular emphasis on the improvement of the ZnO system selectivity and stability by developing composites with other transition metals and by doping with metal nanoparticles. In addition, the growth of the present ZnO nanostructures may also open up the possibility of exploring their applications in the areas of electronic and photovoltaic devices.

(5)

This work was financially supported by PRIN-COFIN 2008, CNRINSTM PROMO and programs CARIPARO 2006 “Multi-layer optical devices based on inorganic and hybrid materials by innovative synthetic strategies” and PRAT 2008 “Nano-organization of functional molecular architectures on inorganic surfaces for eco-sustainable processes”. The Ruhr-University Bochum Rektorat (support for young female researchers), RD-IFSC, RD-Plasma and NanoSci-ERA, a consortium of national funding organizations within the European Research Area, provided further funding. Mr. A. Ravazzolo (CNR, Padova, Italy) is acknowledged for technical assistance.

(6) (7)

Electrons released in this process cause a decrease of the system resistance [6,11,32,42]. This mechanism implies that the gas sensing behavior is stronly related to the system surface properties [8,30,36], and, in particular, to its morphological organization, which, in turn, is appreciably influenced by the synthesis conditions. Specifically, the superior performances of 1D-like zinc oxide nanosystems (see Figs. 3–6) with respect to literature ones could be ascribed to their ultrahigh surface-to-volume ratio and to the strong synergistic coupling of their electronic and chemical properties [22,27,30]. In the present case, the former parameter, resulting from the controlled morphology of ZnO nano-assemblies, was likely responsible for an increased content of oxygen vacancies and, as a consequence, for an enhanced chemical reactivity and a more efficient gas uptake with respect to conventional systems [8,12–14,30,33,36,37,46,47]. An additional contributing effect

References [1] O. Lupan, V.V. Ursaki, G. Chai, L. Chow, G.A. Emelchenko, I.M. Tiginyanu, A.N. Gruzintsev, A.N. Redkin, Selective hydrogen gas nanosensor using individual ZnO nanowire with fast response at room temperature, Sens. Actuators B: Chem. 144 (2010) 56–66. [2] S.K. Hazra, S. Basu, Hydrogen sensitivity of ZnO p–n homojunctions, Sens. Actuators B: Chem. 117 (2006) 177–182. [3] A.P. Chatterjee, P. Mitra, A.K. Mukhopadhyay, Chemically deposited zinc oxide thin film gas sensor, J. Mater. Sci. 34 (1999) 4225–4231. [4] B.S. Kang, H.-T. Wang, L.-C. Tien, F. Ren, B.P. Gila, D.P. Norton, C.R. Abernathy, J. Lin, S.J. Pearton, Wide bandgap semiconductor nanorod and thin film gas sensors, Sensors 6 (2006) 643–666. [5] P. Bhattacharyya, P.K. Basu, H. Saha, S. Basu, Fast response methane sensor based on Pd(Ag)ZnO/Zn MIM structure, Sens. Lett. 4 (2006) 371–376. [6] S.-J. Chang, T.-J. Hsueh, I.-C. Chen, B.-R. Huang, Highly sensitive ZnO nanowire CO sensors with the adsorption of Au nanoparticles, Nanotechnology 19 (2008), 175502/1–175502/5. [7] N. Jayadev Dayan, S.R. Sainkar, A.A. Belkehar, R.N. Karekar, R.C. Aiyer, On the highly selective ZnO:Al2 O3 based thick film hydrogen sensors, J. Mater. Sci. Lett. 16 (1997) 1952–1954.

Author's personal copy 6

D. Barreca et al. / Sensors and Actuators B 149 (2010) 1–7

[8] S.N. Das, J.P. Kar, J.-H. Choi, T. Il Lee, K.-J. Moon, J.-M. Myoung, Fabrication and characterization of ZnO single nanowire-based hydrogen sensor, J. Phys. Chem. C 114 (2010) 1689–1693. [9] O. Lupan, G. Chai, L. Chow, Novel hydrogen gas sensor based on single ZnO nanorod, Microelectron. Eng. 85 (2008) 2220–2225. [10] M. Stamataki, I. Fasaki, G. Tsonos, D. Tsamakis, M. Kompitsas, Annealing effects on the structural, electrical and H2 sensing properties of transparent ZnO thin films, grown by pulsed laser deposition, Thin Solid Films 518 (2009) 1326–1331. [11] P.K Basu, P. Bhattacharyya, N. Saha, H. Saha, S. Basu, Methane sensing properties of platinum catalysed nano porous zinc oxide thin films derived by electrochemical anodization, Sens. Lett. 6 (2008) 219–225. [12] L.C. Tien, P.W. Sadik, D.P. Norton, L.F. Voss, S.J. Pearton, H.T. Wang, B.S. Kang, F. Ren, J. Jun, J. Lin, Hydrogen sensing at room temperature with Pt-coated ZnO thin films and nanorods, Appl. Phys. Lett. 87 (2005), 222106/1–22106/3. [13] A. Qurashi, N. Tabet, M. Faiz, T. Yamzaki, Ultra-fast microwave synthesis of ZnO nanowires and their dynamic response toward hydrogen gas, Nanoscale Res. Lett. 4 (2009) 948–954. [14] N. Le Hung, E. Ahn, S. Park, H. Jung, H. Kim, S.K. Hong, D. Kim, C. Hwang, Synthesis and hydrogen gas sensing properties of ZnO wirelike thin films, J. Vac. Sci. Technol. A 27 (2009) 1347–1351. [15] H.T. Wang, B.S. Kang, F. Ren, L.C. Tien, P.W. Sadik, D.P. Norton, S.J. Pearton, J. Lin, Detection of hydrogen at room temperature with catalyst-coated multiple ZnO nanorods, Appl. Phys. A 81 (2005) 1117–1119. [16] J.S. Wright, W. Lim, D.P. Norton, S.J. Pearton, F. Ren, J.L. Johnson, A. Ural, Nitride and oxide semiconductor nanostructured hydrogen gas sensors, Semicond. Sci. Technol. 25 (2010), 024002/1–024002/8. [17] H.T. Wang, B.S. Kang, F. Ren, L.C. Tien, P.W. Sadik, D.P. Norton, S.J. Pearton, J. Lin, Hydrogen-selective sensing at room temperature with ZnO nanorods, Appl. Phys. Lett. 86 (2005), 243503/1–243503/3. [18] P.K. Basu, S.K. Jana, M.K. Mitra, H. Saha, S. Basu, Hydrogen gas sensors using anodically prepared and surface modified nanoporous ZnO thin films, Sens. Lett. 6 (2008) 699–704. [19] P. Mitra, A.K. Mukhopadhyay, ZnO thin film as methane sensor, Bull. Pol. Acad. Sci. 55 (2007) 281–285. [20] P. Bhattacharyya, P.K. Basu, H. Saha, S. Basu, Fast response methane sensor using nanocrystalline zinc oxide thin films derived by sol–gel method, Sens. Actuators B: Chem. 124 (2007) 62–67. [21] H. Gong, J.Q. Hu, J.H. Wang, C.H. Ong, F.R. Zhu, Nano-crystalline Cu-doped ZnO thin film gas sensor for CO, Sens. Actuators B: Chem. 115 (2006) 247–251. [22] C. Liangyuan, L. Zhiyong, B. Shouli, Z. Kewei, L. Dianqing, C. Aifan, C.C. Liu, Synthesis of 1-dimensional ZnO and its sensing property for CO, Sens. Actuators B: Chem. 143 (2010) 620–628. [23] D.-S. Kang, S.K. Han, J.-H. Kim, S.M. Yang, J.G. Kim, S.-K. Hong, D. Kim, H. Kim, ZnO nanowires prepared by hydrothermal growth followed by chemical vapor deposition for gas sensors, J. Vac. Sci. Technol. B 27 (2009) 1667–1672. [24] A.Z. Sadek, S. Choopun, W. Wlodarski, S.J. Ippolito, K. Kalantar-Zadeh, Characterization of ZnO nanobelt-based gas sensor for H2 , NO2 and hydrocarbon sensing, IEEE Sens. J. 7 (2007) 919–924. [25] E. Comini, C. Baratto, G. Faglia, M. Ferroni, A. Vomiero, G. Sberveglieri, Quasione dimensional metal oxide semiconductors: preparation, characterization and application as chemical sensors, Prog. Mater. Sci. 54 (2009) 1–67. [26] J. Huang, Q. Wan, Gas sensors based on semiconducting metal oxide onedimensional nanostructures, Sensors 9 (2009) 9903–9924. [27] J.X. Wang, X.W. Sun, Y. Yang, H. Huang, Y.C. Lee, O.K. Tan, L. Vayssiries, Hydrothermally grown oriented ZnO nanorod arrays for gas sensing applications, Nanotechnology 17 (2006) 4995–4998. [28] P.K. Basu, S.K. Jana, H. Saha, S. Basu, Low temperature methane sensing by electrochemically grown and surface modified ZnO thin films, Sens. Actuators B: Chem. 135 (2008) 81–88. [29] X. Jiaqiang, C. Yuping, L. Yadong, S. Jianian, Gas sensing properties of ZnO nanorods prepared by hydrothermal method, J. Mater. Sci. 40 (2005) 2919–2921. [30] T. Krishnakumar, R. Jayaprakash, N. Pinna, N. Donato, A. Bonavita, G. Micali, G. Neri, CO gas sensing of ZnO nanostructures synthesized by an assisted microwave wet chemical route, Sens. Actuators B: Chem. 143 (2009) 198–204. [31] T.-J. Hsueh, Y.-W. Chen, S.-J. Chang, S.-F. Wang, C.-L. Hsu, Y.-R. Lin, T.-S. Lin, I.-C. Chen, ZnO nanowire-based CO sensors prepared at various temperatures, J. Electrochem. Soc. 154 (2007) J393–J396. [32] X. Jiaqiang, C. Yuping, S. Jianian, Solvothermal preparation and gas sensing properties of ZnO whiskers, J. Nanosci. Nanotechnol. 6 (2006) 248–253. [33] Y. Cao, D. Jia, Y. Huang, R. Wang, Y. Sun, Facile one-step solid-state chemical synthesis and gas-sensing property of ZnO nanorods, Sens. Lett. 7 (2009) 110–113. [34] J. Yang, G. Liu, J. Lu, Y. Qiu, S. Yang, Electrochemical route to the synthesis of ultrathin ZnO nanorod/nanobelt arrays on zinc substrate, Appl. Phys. Lett. 90 (2007), 103109/1–103109/3. [35] L.C. Tien, H.T. Wang, B.S. Kang, F. Ren, P.W. Sadik, D.P. Norton, S.J. Pearton, J. Lin, Room-temperature hydrogen-selective sensing using single Pt-coated ZnO nanowires at microwatt power levels, Electrochem. Solid-State Lett. 8 (2005) G230–G232. [36] L.-J. Bie, X.-N. Yan, J. Yin, Y.-Q. Duan, Z.-H. Yuan, Nanopillar ZnO gas sensor for hydrogen and ethanol, Sens. Actuators B: Chem. 126 (2007) 604–608. [37] O. Lupan, L. Chow, G. Chai, A single ZnO tetrapod-based sensor, Sens. Actuators B: Chem. 141 (2009) 511–517.

[38] D. Barreca, A.P. Ferrucci, A. Gasparotto, C. Maccato, C. Maragno, E. Tondello, Temperature-controlled synthesis and photocatalytic performance of ZnO nanoplatelets, Chem. Vap. Deposition 13 (2007) 618–625. [39] Y.-J. Chen, C.-L. Zhu, G. Xiao, Ethanol sensing characteristics of ambient temperature sonochemically synthesized ZnO nanotubes, Sens. Actuators B: Chem. 129 (2008) 639–642. [40] P.K. Basu, P. Bhattacharyya, N. Saha, H. Saha, S. Basu, The superior performance of the electrochemically grown ZnO thin films as methane sensor, Sens. Actuators B: Chem. 133 (2008) 357–363. [41] P. Bhattacharyya, P.K. Basu, N. Mukherjee, A. Mondal, H. Saha, S. Basu, Deposition of nanocrystalline ZnO thin films on p-Si by novel galvanic method and application of the heterojunction as methane sensor, J. Mater. Sci.: Mater. Electron. 18 (2007) 823–829. [42] S. Kar, B.N. Pal, S. Chaudhuri, D. Chakravorty, One-dimensional ZnO nanostructure arrays: synthesis and characterization, J. Phys. Chem. B 110 (2006) 4605–4611. [43] D. Calestani, M. Zha, R. Mosca, A. Zappettini, M.C. Carotta, V. Di Natale, L. Zanotti, Growth of ZnO tetrapods for nanostructure-based sensors, Sens. Actuators B: Chem 144 (2010) 472–478. [44] V. Kobrinsky, A. Rotschild, V. Lumelsky, Y. Komem, Y. Lifshitz, Tailoring the gas sensing properties of ZnO thin films through oxygen nonstoichiometry, Appl. Phys. Lett. 93 (2008), 113502/1–113502/3. [45] H.-W. Ra, R. Khan, J.T. Kim, B.R. Kang, Y.H. Im, The effect of grain boundaries inside the individual ZnO nanowires in gas sensing, Nanotechnology 21 (2010), 085502/1–085502/5. [46] H.W. Ra, K.-S. Choi, J.-H. Kim, Y.-B. Hahn, Y.-H. Im, Fabrication of ZnO nanowires using nanoscale spacer litography for gas sensors, Small 4 (2008) 1105–1109. [47] Y. Zhang, J. Xu, Q. Xiang, H. Li, Q. Pan, P. Xu, Brush-like hierarchical ZnO nanostructures: synthesis, photoluminescence and gas sensor properties, J. Phys. Chem. C 113 (2009) 3430–3435. [48] N. Mukherjee, S.F. Ahmed, K.K. Chattopadhyay, A. Mondal, Role of solute and solvent on the deposition of ZnO thin films, Electrochim. Acta 54 (2009) 4015–4024. [49] P. Bhattacharyya, P.K. Basu, C. Lang, H. Saha, S. Basu, Noble metal catalytic contacts to sol–gel nanocrystalline zinc oxide thin films for sensing methane, Sens. Actuators B: Chem. 129 (2008) 551–557. [50] H.-W. Ryu, B.-S. Park, S.A. Akbar, W.-S. Lee, K.-J. Hong, Y.-J. Seo, D.-C. Shin, J.S. Park, G.-P. Choi, ZnO sol–gel derived porous film for CO gas sensing, Sens. Actuators B: Chem. 96 (2003) 717–722. [51] P. Bhattacharyya, P.K. Basu, B. Mondal, H. Saha, A low power MEMS gas sensor based on nanocrystalline ZnO thin films for sensing methane, Microelectron. Reliab. 48 (2008) 1772–1779. [52] C.C. Li, Z.F. Du, L.M. Li, H.C. Yu, Q. Wan, T.H. Wang, Surface-depletion controlled gas sensing of ZnO nanorods grown at room temperature, Appl. Phys. Lett. 91 (2007), 032101/1–032101/3. [53] D. Bekermann, D. Rogalla, H.-W. Becker, M. Winter, R.A. Fischer, A. Devi, Volatile, monomeric, and fluorine-free precursors for the metal organic chemical vapor deposition of zinc oxide, Eur. J. Inorg. Chem. 9 (2010) 1366–1372. [54] D. Barreca, A. Gasparotto, E. Tondello, C. Sada, S. Polizzi, A. Benedetti, Nucleation and growth of nanophasic CeO2 thin films by plasma-enhanced CVD, Chem. Vap. Deposition 9 (2003) 199–206. [55] D. Bekermann, A. Gasparotto, D. Barreca, L. Bovo, A. Devi, R.A. Fischer, O.I. Lebedev, C. Maccato, E. Tondello, G. Van Tendeloo, Highly oriented ZnO nanorod arrays by a novel plasma chemical vapor deposition process, Cryst. Growth Des. 10 (2010) 2011–2018. [56] D. Barreca, E. Comini, A.P. Ferrucci, A. Gasparotto, C. Maccato, C. Maragno, G. Sberveglieri, E. Tondello, First example of ZnO−TiO2 nanocomposites by chemical vapor deposition: structure, morphology, composition, and gas sensing performances, Chem. Mater. 19 (2007) 5642–5649. [57] D. Briggs, M.P. Seah, Practical Surface Analysis, vol. 1, Wiley, New York, 1990. [58] D. Barreca, A. Gasparotto, C. Maccato, C. Maragno, E. Tondello, E. Comini, G. Sberveglieri, Columnar CeO2 nanostructures for sensor application, Nanotechnology 18 (2007), 125502/1–125502/6. [59] D. Barreca, A. Gasparotto, C. Maccato, E. Tondello, E. Comini, G. Sberveglieri, Innovative metal oxide nanosystems for gas sensing: from design to application, Nuovo Cimento Soc. Ital. Fis., B 123 (2008) 1369–1380. [60] D. Barreca, E. Comini, A. Gasparotto, C. Maccato, C. Sada, G. Sberveglieri, E. Tondello, Chemical vapor deposition of copper oxide films and entangled quasi1D nanoarchitectures as innovative gas sensors, Sens. Actuators B: Chem. 141 (2009) 270–275. [61] D. Barreca, A. Gasparotto, C. Maccato, C. Maragno, E. Tondello, ZnO nanoplatelets obtained by chemical vapor deposition, studied by XPS, Surf. Sci. Spectra 14 (2007) 19–26.

Biographies D. Barreca got his degree in chemistry at Padua University (Italy) in 1996 and the PhD degree in chemical sciences at the same University in 2000. Since 2002 he is a senior research scientist at the ISTM-CNR Institute in Padua. He is the European editor of Nanoscience and Nanotechnology Letters and associate editor of Journal of Nanoscience and Nanotechnology and of Surface Science Spectra. His actual research interests concern the development of innovative inorganic nanosystems for applications in photocatalysis and gas sensing. During his career, he published more than 150 papers on international journals and presented several oral communications, even as invited speaker, at international conferences.

Author's personal copy D. Barreca et al. / Sensors and Actuators B 149 (2010) 1–7 D. Bekermann received her M.Sc. degree in chemistry (functional materials) at the Ruhr-University of Bochum (Germany) in 2008. Since then she is doing her PhD studies at the Ruhr-University of Bochum and at the University of Padua (Department of Chemistry/ISTM-CNR, Italy), which is supported by a fellowship from the SFB 558. Her main research interest is devoted to the synthesis of ZnO-based nanostructures by MOCVD and PE-CVD for optical, gas sensing and photocatalytic applications. E. Comini received her degree in physics at the University of Pisa in 1996. Then she started her PhD on chemical sensors, focusing on the deposition of thin films by PVD technique and electrical characterisation of MOX thin films. She received her PhD degree in material science at the University of Brescia. In 2001 she has been appointed assistant professor of physics of matter at Brescia University. She has been the organizer of several symposia in the sensing field for MRS and EMRS. She has filed four Italian patents. She has published more than 135 papers on international journals and given several invited presentations, including the plenary talk at the International Symposium on Advanced Materials and Processing. She is responsible of the research line on “Metal oxide nanocrystalline quasi 1D structure preparation” at the SENSOR laboratory (CNR-IDASC). She is a reviewer for several international journals such as NanoLetters or Advanced Materials and she is member of the editorial board of Sensor Letters. Her h-index is 27. A. Devi received her PhD in materials science from the Indian Institute of Science, Bangalore, India in 1998. She moved to Germany as a AvH fellow to carry out post doctoral studies at the Ruhr-University Bochum (1998). Since 2002 she is a junior professor (inorganic materials chemistry) at the Faculty of Chemistry and Biochemistry at the same university. Her research area covers precursor chemistry and development of nanostructured thin films of functional materials using MOCVD and ALD techniques. R.A. Fischer received his Dr. rer. nat and Dr. rer. nat habil in chemistry at the Technical University of Munich (Germany) in 1989 and 1995, respectively. He was appointed associate professor at Heidelberg University (1996) and was promoted full professor in 1998 at the Ruhr-University Bochum, Faculty of Chemistry and Biochemistry. He has been active in various academic functions including vice rector of the Ruhr-University from 2000–2002, dean of the Faculty of Chemistry and Biochemistry (2005–2008) and of the Ruhr-University Research School (2006–2009), and member of the central selection committee of the Alexander von Humboldt Foundation (2000–2009). His research interests are in metal–organic and organometallic molecular and coordination chemistry, with special focus on transition metal compounds and precursor development for nanomaterials. More recently, he is active in the field of metal–organic frameworks (MOFs).

7

A. Gasparotto graduated in chemistry at Padova University in 2002. He received his PhD in chemical sciences in 2006 at the same university. His main research activity is devoted to the chemico–physical and functional investigation of inorganic nanosystems synthesised by CVD, both thermal- either plasma-assisted, RF-sputtering and sol–gel. From 2007 he is a research scientist at the University of Padova, Department of Chemistry. Up to date, he has published more than 80 papers on international journals. C. Maccato took her degree in chemistry at Padova University in 1995. In 1999, she got the PhD in chemical sciences at the same university. Since 2000 she is research scientist and her main interest is the chemico–physical characterization of innovative inorganic nanoarchitectures (such as powders, films, clusters) for applications in the fields of optics, gas sensing, energetics. She has co-authored more than 50 papers on international journals and she is currently referee for many of them, like ChemComm, Materials Chemistry and Physics, Thin Solid Films, Journal of Nanoscience and Nanotechnology. G. Sberveglieri is the director of the Sensor Laboratory. He received his degree in physics from Parma University. In 1988, he established the Gas Sensor Lab, mainly devoted to the preparation and characterization of thin film chemical sensors based on metal oxide semiconductors and, since the mid-1990s, to the area of electronic noses. In 1994, he was appointed full professor in physics. He is referee of many international journals and has acted as chairman in several conferences on materials science and on sensors. During 30 years of scientific activity he published more than 250 papers on international journals; he presented more than 110 oral communications to international congresses. He is also an evaluator of IST and Growth Projects of EU and the coordinator of the Applied Network of INFM since April 2001. E. Tondello got his degree in chemistry at Padova University in 1965. From 1965 to 1975 he has been head of research for the National Council of Research (CNR). In 1975 he became full professor at the Faculty of Science, Padova University. From 1992 to 2001 he has been director of the CNR Center for the Study of the Stability and Reactivity of Coordination Compounds, actually Institute of Sciences and Molecular Technologies. In 2004 he has been head of the Department of Chemistry, Padova University. The research activity has been mainly devoted to the study of structure–property relationships of molecular systems, nanomaterials and surfaces. His interests have also been focused on the development of advanced characterization techniques such as UPS, XPS and XE-AES, XRD, AFM, FE-SEM and STM.