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Sensors and Actuators B 220 (2015) 1152–1160

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

Comparative gas-sensing performance of 1D and 2D ZnO nanostructures Matteo Tonezzer a,∗ , Thi Thanh Le Dang b , Nicola Bazzanella c , Van Hieu Nguyen b , Salvatore Iannotta d a

IMEM-CNR, sede di Trento – FBK, Via alla Cascata 56/C, Povo, Trento, Italy ITIMS, Hanoi University of Science and Technology, Hanoi, Viet Nam c Dipartimento di Fisica, Università degli Studi di Trento, Povo, I-38123 Trento, Italy d IMEM-CNR, Parco Area delle Science 37/a, I-43100 Parma, Italy b

a r t i c l e

i n f o

Article history: Received 6 March 2015 Received in revised form 12 June 2015 Accepted 16 June 2015 Available online 26 June 2015 Keywords: Nanostructure Zinc oxide Gas sensor Liquid petroleum gas Depletion layer

a b s t r a c t In this work we have grown one-dimensional (1D) and two-dimensional (2D) zinc oxide nanostructures. Changing the deposition parameters we were able to obtain ZnO nanowires with an average diameter of 80–250 nm. Nanosheets grown in different conditions show thickness values in the range 70–360 nm. These kinds of nanostructure have been used to fabricate conductometric gas sensors for liquid petroleum gas (LPG) detection. Different sensing parameters are investigated in both cases as a function of the dimensionality and size of the zinc oxide nanostructures. A ﬁrst approximation of the “depletion layer sensing mechanism” is used to explain how the geometrical factors of one- and two-dimensional nanostructures affect their sensing parameters. The depletion layer affects two dimensions of nanowires and only one of nanosheets. This greatly improves the sensor response of 1D-nanostructures. On the other side twodimensional nanostructures have a larger cross-section, which increases the base current, thus lowering the limit of detection. At the same operative conditions, nanowires show a better percentage response when compared to similar thickness nanosheets, but 2D nanosheets demonstrate an improved limit of detection (LoD). © 2015 Elsevier B.V. All rights reserved.

1. Introduction Gas sensors are needed in a wide range of ﬁelds: chemical and petrochemical industries, food and drinks processing, medical diagnosis, automotives, agriculture, environmental monitoring and many more [1]. Amongst the different types of gas sensors, semiconducting metal oxide (SMO) based gas sensors are of particular interest due to enhanced chemical interactions, high catalytic activity, and therefore sensing performance. The response of SMO to a particular gas depends on the speciﬁc gas being analyzed, the interaction between analyte molecules and adsorbed oxygen species on the sensor surface, microstructure of the device and operating temperature [2,3]. Recently, several different nanostructures have been produced and used as effective sensor fundamental building blocks: nanowires [4,5], nanorods [6,7], nanobelts [8,9], nanosprings [10], nanosheets [11,12], ﬂower-like nanostructures [13] etc. The use of nanostructured metal oxide is useful mainly for its huge

∗ Corresponding author. Tel.: +39 0416 314828. E-mail address: [email protected] (M. Tonezzer). http://dx.doi.org/10.1016/j.snb.2015.06.103 0925-4005/© 2015 Elsevier B.V. All rights reserved.

surface-to-volume ratio, which leads to enhanced gas response. Because of their tiny size, comparable to the Debye length, SMO nanostructures’ electronic properties are more strongly affected by the processes that take place at their surface. In other words a smaller size (diameter, thickness) leads to higher gas response. Although the space charge model has been conﬁrmed in many works, there are still few reports on the size-dependence of nanostructured SMO based gas response [14]. Castro-Hurtado et al. [15] noticed that “the sensitivity decreases with the diameter of the nanorods, from S = 0.25 ppm−1 for the nanorods with the smaller diameter (40 nm) to S = 0.0095 ppm−1 for the samples with the diameter of 300 nm”. Lupan et al. [16] found gas response values of 34, 10 and 4% for single ZnO nanowires with diameter of 100, 200 and 600 nm (to 100 ppm H2 ). They also studied photoluminescence from the samples and pointed out that “the concentration of structural defects decreases with the increase in the diameter of the nanowire” and remembered that “structural defects strongly inﬂuence the electrical parameters of the zinc oxide which are extremely important for the gas sensor applications”. Yamazoe et al. [17] experimentally observed that the sensor response to H2 and CO increases very much for SnO2 thin ﬁlms made of smaller

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nanograins. Dan et al. [18] summarized sensing performance of many metal oxide nanowires taking note also of their diameter, noticing that their performance can be signiﬁcantly improved only when the diameter is very small (comparable to the depletion layer). Hernandez-Ramirez et al. [19] found that the response of single nanowires greatly depends on their radius, going from 24% for r = 185 nm to 93% for r = 24 nm. Rahman et al. [20] noticed the same behaviour also for top-down fabricated Si nanowires used as pH sensors. Zinc oxide is one of the most studied metal oxides and among the most promising materials for gas sensing [21,22] also for its stability, safety and biocompatibility [23] that make it suitable for a wide range of applications. First ZnO gas sensors were fabricated in the form of pellets or thick ﬁlms, then improved using thin ﬁlms. Recent research has been devoted towards zinc oxide nanoparticles, since reactions at grain boundaries and complete depletion of carriers in the particles can greatly affect their conductivity. Unfortunately, the high temperature required for the sensors to exploit their high response induces a grain growth by coalescence and avoids the achievement of stable conditions [24]. For this reason the latest research focused on one-dimensional nanostructures that combine tiny dimensions comparable to the depletion layer thickness [14,25], and a much better thermal stability. Such quasi-one-dimensional nanostructures achieve good response values, but usually their low base current does not permit them to have a good limit of detection (LoD). In this work, we report the growth of zinc oxide one- and two-dimensional nanostructures with different characteristic size (diameter for nanowires, thickness for nanosheets) and their use as liqueﬁed petroleum gas (LPG) sensors. Liqueﬁed petroleum gas is a combustible gas widely used for many domestic and industrial applications. Since it is potentially hazardous because explosions may be caused when it leaks from containment vessels, the detection of gas leaks is essential for preventing the occurrence of accidents [26]. In this paper we investigate for the ﬁrst time the sensing performance of 1D- and 2D-nanostructures towards LPG as a function of operating temperature and mainly of dimensionality and characteristic size. We demonstrate that the depletion layer modulation model affects differently the geometries of one- and two-dimensional nanostructures, affecting their sensing performance. Two-dimensional nanostructures show a poorer sensor response, but this is balanced by a better limit of detection, which is important in many applications.

2. Materials and methods 2.1. Nanowires growth Single crystalline ZnO nanowires were synthesized directly on Si/SiO2 substrates (Siltronix) in a horizontal tube furnace at a low temperature by the chemical vapour deposition (CVD) method, where the nanowire growth followed a vapour–liquid–solid mechanism [27,28] driven by gold catalyst. A quartz tube was inserted in a single-zone furnace (Lingdberg Blue M) and an alumina vessel with 0.5 g zinc powder (99.9%, Sigma Aldrich) was placed at the centre of the tube as the source material. The substrate, consisting in a piece of Si/SiO2 wafer (thermally deposited SiO2 was 300 nm thick) deposited with sputters of a thin ﬁlm (nominal thickness of 3–8 nm) of gold acting as catalyst, was placed inside the tube at a distance of 1 cm downstream from the source vessel. The CVD system was ﬁrst pumped down to a base pressure of around 5 × 10−3 mbar using a rotary pump and purged using high purity argon (99.999%). These steps were repeated thrice and then the tube furnace was pumped down to its pressure limit. Thereafter, the furnace was heated up to

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700–850 ◦ C under a ramp rate of 30 ◦ C/min. A carrying gas mixture of N2 (100 sccm) and O2 (5 sccm) was inserted while the temperature was maintained at 700–850 ◦ C for 40 min, after which the system was switched off and allowed to cool down naturally. All the samples showed a white soft ﬁlm made of homogenous ZnO NWs forest. 2.2. Nanosheets growth A two-steps process has been used to grow the two-dimensional nanosheets: a thermal evaporation of pure metallic zinc (≥99.99%, Sigma–Aldrich) onto a Si/SiO2 substrate (Siltronix), followed by a thermal oxidation in a horizontal furnace (Lingdberg Blue M). The substrate was deposited with 70–120 nm of pure zinc and then loaded in an alumina vessel. The vessel was then introduced in a quartz tube inserted in the furnace connected to a vacuum system. The substrates were positioned in the centre of the furnace at its maximum temperature. The whole apparatus was evacuated at its limit pressure (around 5 × 10−3 mbar) and purged with nitrogen (99.9999%, SIAD) three times. After that a mixture of oxygen (5%) in nitrogen was ﬂown inside the quartz pipe and the pressure was maintained at 5 mbar. The thermal oxidation process was then carried out rising the temperature to its maximum (350 ± 0.5 ◦ C) in 15 min, maintaining it for a certain time (40–90 min), and then cooling down slowly. Once taken out of the quartz pipe, the samples showed a very light grey cover, composed of zinc oxide nanosheets. 2.3. Nanostructures characterization The grown nanostructures were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) analysis in order to characterize respectively their morphological, structural and compositional properties. Field emission scanning electron microscopy (FE-SEM) analysis was carried out using a Hitachi S-4800. Fig. 1 shows scanning electron micrograph of six ZnO nanowires samples, grown under different conditions. Fig. 1a–f shows SEM pictures of 1D nanostructures samples obtained with gold layer nominal thickness of 3, 4, 5, 6, 7, 8 nm and growth temperatures of 700, 730, 760, 790, 820 and 850 ◦ C, respectively. In all the cases a homogenous forest of long nanowires with large aspect-ratio is obtained. The bars in all the images are 500 nm long. On the right of each SEM picture a bar graph shows the diameter distribution of nanowires in that sample, evaluated on 5 different pictures and normalized for a better comparison. The average diameter values for the samples are 82, 102, 143, 182, 208 and 236 nm, respectively. Fig. 2a–f shows scanning electron pictures of 2D nanostructures obtained with zinc layers thickness of 70, 80, 90, 100, 110 and 120 nm, and oxidation times of 40, 50, 60, 70, 80 and 90 min. In all the cases a homogenous layer of thin nanosheets was formed. The bars in all the images are 500 nm long. On the right of each SEM picture a bar graph shows the thickness distribution of nanosheets in that sample, again evaluated on 5 different pictures and normalized for a better comparison. The average thickness values for the samples are 68, 120, 157, 218, 267 and 357 nm, respectively. Transmission electron microscopy was performed using a Tecnai G2 SuperTwin, operated at 200 kV. Fig. 3a shows a high resolution image (HR-TEM) of a nanowire, in which the lattice fringes spaced 2.59 A˚ are clearly visible, proving the good crystallinity of the nanostructures. There is no sign of any amorphous layer on the surface of the nanowire. Fig. 3b shows a HR-TEM image of a thin nanosheet. Also in this case the lattice fringes are clearly visible, and there is no amorphous layer on the nanostructure. In both cases the lattice fringes are consistent with the [0 0 1] direction of ZnO lattice. All the samples grown present homogenous one-dimensional

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Fig. 1. Secondary electron microscopy images of the grown nanowires. The bars in all the images are 500 nm long. On the right of each SEM picture a column graph shows the diameter distribution of the nanowires in that sample.

or two-dimensional ZnO nanostructures with good crystallinity properties. XRD spectra were collected in Bragg-Brentano geometry with a Panalitycal X’Pert Pro diffractometer. A Cu anode with wavelength of 1.5406 A˚ was used. The step size was 0.05◦ (2) and the average time was 60 s/step. Fig. 4 shows typical XRD pattern of 1D and 2D nanostructures. Fig. 4a refers to one-dimensional

Fig. 2. Secondary electron microscopy images of the grown nanosheets. The bars in all the images are 500 nm long. On the right of each SEM picture a column graph shows the thickness distribution of the nanowires in that sample.

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Fig. 3. TEM images of a nanowire (left) and a nanosheet (right) taken from the samples with the lowest average diameter/thickness.

nanowires, Fig. 4c refers to two-dimensional nanosheets and Fig. 4b in the middle (red online) reports the JCPDS reference standard data from 36-1451 card. The well deﬁned peaks conﬁrm the good crystalline nature of both kinds of nanostructures as wurtzite ˚ (hexagonal) ZnO with lattice constants of a = 3.249 A˚ and c = 5.206 A, consistent with the standard values in the reference data (JCPDS 36-1451 card). No extra peaks related to any impurity were observed. This conﬁrms that the as-synthesized ZnO nanostructures are pure wurtzite-type ZnO. EDS spectra (not shown), identify the nanostructures material as ZnO with a good stoichiometry (O: 52 ± 1 at%/Zn: 48 ± 1 at% for nanowires and O: 51 ± 1 at%/Zn: 49 ± 1 at% for nanosheets).

3. Results 3.1. Working temperature We ﬁrst investigated the response of the sensors as a function of the working temperature in the range from 100 ◦ C to 400 ◦ C. The sensor temperature was controlled by a feedback on the thermocouple inside the sensing chamber. The resistance of the sensors in dry air or in partial concentration of LPG could be measured monitoring the output current across the device. Fig. 5 shows the response of the ZnO 1D- and 2D-based gas sensors to 500 ppm of liquid petroleum gas, as a function of working temperature. It shows the inﬂuence that working temperature has on the response of the sensors for 500 ppm of LPG in dry air.

2.4. Gas sensing experiments Gas sensing properties of the ZnO 1D and 2D nanostructures were studied in a home-built apparatus including a test chamber, a sensor holder which can be heated up to 500 ◦ C, mass ﬂow controllers (connected to high purity calibrated bottles), a Keithley 2400 multimeter, a Keithely 6487A electrometer, and a data acquisition system (LabView, National Instruments). In order to achieve a good electrical contact for the sensors measurements, two thin electrodes of pure gold (>99.99%, Sigma Aldrich) were thermally evaporated on the samples through a shadow mask. The gold electrodes, with a gap of 50 ␮m, were then contacted via two passive micromanipulators. The devices were ﬁrst biased with a voltage of 1 V and operated in a temperature-controlled atmospheric pressure of dry air (79% nitrogen, 21% oxygen). All samples showed a good ohmic behaviour (not shown) which is advantageous for the sensing performance, since the response of a gas sensing material can be maximized when the metal–semiconductor junction is ohmic or has a negligible junction resistance. The resistance of the one- and two-dimensional nanostructured samples under dry air atmosphere was in the 0.8–1.5 M and 200–450 k, respectively. Before starting the tests, the gas sensors underwent a thermal conditioning at 350 ◦ C, 1 V in dry air (500 sccm in total, 79% nitrogen, 21% oxygen) for 5 h, in order to stabilize their microstructure and thus improve the repeatability of the measurements. Such a conditioning is needed to balance the presence of oxygen adatoms on the ZnO nanostructures’ surface, thus stabilizing the depletion layer [29]. The sensing performance of the 1D and 2D devices were investigated with an operating voltage of 1 V between the electrodes.

Fig. 4. XRD spectra of the thinnest samples: (a) one-dimensional nanowires and (b) two-dimensional nanosheets.

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Fig. 5. Sensor response of the ZnO nanowires (left) and nanosheets (right) based devices to 500 ppm LPG as a function of working temperature.

Fig. 5a reports the sensor response of ZnO nanowires, exhibiting a typical bell-shape with a maximum at 250–300 ◦ C. In this paper we use the deﬁnition of sensor percentage response, as SR% = (Rair − RLPG )/Rair × 100 where Rair and RLPG are, respectively, the resistance of the device exposed to dry air without and with liquid petroleum gas. As can be seen in Fig. 5a, the ZnO nanowires respond even at 100 ◦ C, but their response greatly improves in the 150–250 ◦ C range. Devices fabricated with different average diameter nanowires show different best working temperature: 300 ◦ C for the smallest diameter nanowires (82 nm) with a percentage response of 56%, 250 ◦ C for the largest diameter nanowires (236 nm) with a response of 35% for 500 ppm LPG in dry air. Fig. 5b shows the sensor response of ZnO nanosheets, with a bell-shape similar to that of Fig. 5a, with best sensor response at 250–350 ◦ C. Devices with different average thickness show different best working temperature values: 250 ◦ C for the thinnest nanosheets (68 nm) with a response of 33% and 350 ◦ C for the nanosheets with an average thickness of 267 nm, with a response of 24%. Fig. 5 qualitatively illustrates that thinner nanostructures (both one- and two-dimensional) show higher sensor response. This is due to the higher relative importance of the depletion layer compared with the nanostructure cross section (diameter or thickness). In order to have the best balance between sensors response and power consumption, 250 ◦ C has been chosen as the working temperature for the next investigations. The optimal working temperature of ZnO nanostructures sensors (250 ◦ C) fully matches with the transition temperature at which some kinds of surface states are replaced with others. This bell shaped dependence of sensor response on working temperature is likely due to two competing processes: (a) the reaction of LPG molecules with oxygen atoms on the zinc oxide surface, leading to the increase of conductivity, and (b) gas desorption from its surface. As the reaction rate constant rises along with working temperature, the equilibrium concentration of gas on the surface of the metal oxide nanostructures decreases due to an increase in the desorption rate [30,31]. 3.2. Liqueﬁed petroleum gas concentration All the experiments have been carried out at 250 ◦ C, as it has been found the ideal working temperature for both 1D- and 2Dbased sensing devices (efﬁcient balance between sensor response and low temperature, leading to low power consumption and wider

usability). The devices were exposed to different concentrations of liqueﬁed petroleum gas in dry air, ranging from 50 to 1000 ppm. Fig. 6 shows the dynamic resistance response and recovery of ZnO nanowires (82 nm average diameter) and nanosheets (68 nm average thickness) upon exposure to 1000, 500, 300, 200, 100 and 50 ppm LPG at a work temperature of 250 ◦ C. In the absence of LPG ﬂow in the testing chamber, the resistance measured was high and stable. However, when LPG was injected in the chamber, the resistance of the sensing devices decreased abruptly. It is also observed that the response increases as a function of LPG concentration in the measured range. The initial resistance is steady, and the response and recovery is rapid (response and recovery times will be discussed later in a subsequent section). Semiconducting oxide gas sensors operate on the basis of the electrical properties variations of the active element (ZnO nanostructures in this case) triggered by the adsorption of the analyte (LPG in this case) on the surface of the sensor. When a ZnO nanostructured sensor is exposed to air, a number of oxygen molecules and atoms adsorb on its surface and form ions by draining electrons from its conduction band. Thus, ZnO nanostructures show a high resistance in air. When the ZnO nanostructures are exposed to a reductive gas like LPG at moderate temperature, the gas reacts with the absorbed oxygen species, decreasing the surface concentration of oxygen ions thus increasing the electron concentration inside the nanostructures. This in the end enlarges the conductivity of the ZnO nanostructures. In the case of thick ﬁlms, the electrical alteration only affects the grain boundary or porous surface. In the case of oneor two-dimensional nanostructures where two or one dimension is comparable with the depletion layer thickness, the electronic transport properties of most of the nanostructure cross section will be affected. The large fraction of the nanostructure which is depleted of charge carriers explains why ZnO nanostructures always show higher sensing response compared to thin ﬁlm or bulk counterparts. As can be seen in Fig. 7, the sensor response of nanosheetsbased device is lower than that of nanowires-based device along the whole concentration range measured. The ratio between the response values of 1D- and 2D-nanostructures goes from 1.3 to 2.4 along the 50–1000 ppm range. Experimental data conﬁrm that 1D-nanostructures are more strongly affected by the same concentration of gas than 2D-nanostructures. This is due to their higher surface-to-volume ratio, that makes them more sensitive to the same amount of gas molecules.

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Fig. 6. Dynamic response of ZnO nanowires (left) and nanosheets (right) based sensors working at 250 ◦ C while different concentration of liqueﬁed petroleum gas are injected and evacuated.

3.3. Response reversibility The initial thermal annealing at a high temperature in dry air renders the ZnO nanostructures very stable [32]. Their response values are therefore reversible during different gas cycles, as shown in Fig. 6. Response reversibility is an important but under-evaluated parameter, since a low reversibility implies a high drift in the sensor response and this renders hard its usage in real applications. In order to quantitatively calculate the reversible degree of the sensors, the percentage recovery degree %R will be used in this work, where %R = (R − I)/I × 100 and I is the response intensity. Nanowires-based and nanosheets-based sensors showed a maximum %R of 2.5% and 3.4% in the worst condition (50 ppm response), respectively. Such a small value attests the good performance of the sensors over time. All the sensors were tested for 2 months in order to check their stability. Reproducibility was observed during different measurement sessions, with pulses of 50 and 500 ppm LPG introduced into the sensor chamber consecutively. There is no evidence that any

Fig. 7. Sensor response of the thinnest nanostructures-based devices (nanowires: black squares; nanosheets: red triangles) as a function of the LPG concentration. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

of the sensors suffer from long-term drift of resistance or a performance degrading in our measurements. It was found that both ZnO nanowires and nanosheets produce repeatable responses of the same magnitude with good baseline stability. Such good reversibility and stability values are very important for the gas sensors to be used in realistic applications. 3.4. Response and recovery times It is well known that response and recovery performance is an important feature while evaluating gas sensors. In this paper the response time has been deﬁned as the time required for the response signal to get to 90% of the equilibrium value after LPG was injected, and the recovery time as the time necessary for a sensor to reach a response 10% above its fully recovered value in dry air. In order to investigate the behaviour of response and recovery times, multiple testing of the thinnest nanostructures-based devices have been made: 82 nm diameter nanowires and 68 nm thick nanosheets. Both response and recovery time values have been averaged on 3 values for each sample at different working temperatures along the whole LPG concentration range. As can be seen in Fig. 8a, the response times for the device based on nanowires ranged from 22 to 37 s at a working temperature of 200 ◦ C. At a working temperature of 400 ◦ C both times decrease to 9 and 17 s, respectively. Recovery times for this kind of sensors were comparable, ranging from 24 s to 34 s at a working temperature of 200 ◦ C and from 12 to 15 s at 400 ◦ C. In the case of two-dimensional nanostructures the situation is similar for response time values, while recovery times are slightly longer. Response times of nanosheets-based-devices are similar to those of their counterpart: they range from 26 to 38 s at a working temperature of 200 ◦ C and decrease to 11–19 s at 400 ◦ C. Recovery times of two-dimensional nanostructures range from 37 to 51 s at a working temperature of 200 ◦ C and decrease to 21–29 s at 400 ◦ C. These times are clearly the result of a convolution of the sensor response (and recovery) time and the test chamber ﬁlling (and evacuating) time. In our case, considering that the test chamber volume measures 50 cm3 and the inlet gas ﬂow is 500 sccm, a ﬁlling time of 6 s can be considered. This time is shorter than the response times measured for the whole system (sensor plus test chamber), thus meaning that the sensors response and recovery times would be even shorter. As can be noticed in Fig. 8, both response and recovery times of all devices show the same behaviour, decreasing with increasing working temperature. It is known that metal oxides gas sensing is based on two opposing processes: during the response

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Fig. 8. Response and recovery times as a function of working temperature, measured at 500 ppm of LPG. (a) Response times and (b) recovery times of nanowires-based sensors; (c) response times and (d) recovery times of nanosheets-based sensors.

phase the previously adsorbed oxygen is desorbed while testing gas is adsorbed, and during the sensor recovery phase the analyte is desorbed while oxygen is adsorbed again. Both adsorption and desorption processes are affected by several parameters like the surface material and crystalline interface, gas particles and their molecular, atomic or ionic state. Both of them are thermally activated reactions, very sensitive to temperature. This explains why the working temperature has such a large effect on both response and recovery times. Their fast response and recovery times make ZnO 1D- and 2D-nanostructures a very good choice for real-time sensing in many ﬁelds and applications. When reducing gases like LPG are sensed using n-type oxide semiconductor materials like ZnO, the recovery times are typically longer than the response time values [33,34]. This can be explained by the slower series of surface reactions to form O− adatoms during the recovery, compared to the oxidation reaction of reducing gas by O− ads. In pure ZnO, a series of reactions are necessary for the recovery, as following: (1) desorption of H2 O, (2) diffusion of oxygen to the sensing surface, (3) adsorption of oxygen gas, (4) dissociation into atomic oxygen on the surface, and (5) ionization to negative charge of surface oxygen (O− ads). Such sequence of processes, necessarily in this order, makes the sensor recovery slower than its response.

3.5. Sensitivity As can be noticed in Fig. 7, both sensors response increase linearly in the 0–300 ppm range and then start to saturate. The slope of the curves in Fig. 7 gives the sensitivity of the sensors. In our case, the nanowires-based sensor sensitivity starts at 0.099 ppm−1 then decreases to 0.024 ppm−1 at 500 ppm. The nanosheets-based sensor sensitivity starts at 0.090 ppm−1 then decreases to 0.031 ppm−1 at 500 ppm. This indicator is very important because it indicates how precise can be the sensor to distinguish an analyte concentration from another one. 3.6. Limit of detection The limit of detection (LoD) of each sensor has been extrapolated from the experimental data. The sensor noise has been calculated using the variation in the gas response at baseline using the root-mean-square deviation (rms). The rms deviation was calculated on more than 50 points at baseline (without analyte gas) as noiserms =

i

(Si − S)2 /N, where Si are the experimental data

points and S is the average value. According to the IUPAC deﬁnition, the LoD is calculated as 3(noiserms /slope), where slope is the

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released back to the conduction band and contribute to current increase through the semiconductor metal oxide. For n-type crystals like ZnO, the intrinsic carrier concentration is mainly determined by deviations from stoichiometry, usually in the form of interstitial zinc and oxygen vacancies, which act as electron donors [36]. The conduction electrons coming from the point defects play a crucial role in gas sensing of metal oxides; this is even more important in ZnO nanostructures because their electrical conductivity strongly depends on the surface states created by molecular adsorption that result in depletion layer and band modulation [37]. The charge carriers depletion affects the surface the metal oxide structure, therefore metal oxide nanostructures are much more responsive than thin ﬁlms. As for the case of nanocrystallites size [38], also for nanowires and nanosheets some geometrical aspects are important. The sensing properties of one-dimensional and two-dimensional nanostructures are affected differently because of their different cross-section. If we assume a nanowire as cylindrical, its response goes as Fig. 9. Limit of detection of nanowires-based (black squares) and nanosheets-based (red circles) sensors, measured at 250 ◦ C. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

ﬁrst derivative of the graphs in Fig. 7, at their beginning. Doing a linear ﬁt of the initial part of the curves in Fig. 7, and taking into account the current noise in Fig. 6, the LPG limit of detection (LoD) is obtained. In order to evaluate how limit of detection is affected by the dimensionality and size of different nanostructures, we calculated it for all the different sensors at the working temperature of 250 ◦ C. As can be seen in Fig. 9, all the devices based on nanowires show a higher LoD compared to that of nanosheetsbased sensors. The thickest nanowires give a limit of detection of 38 ppm and this value decreases down to 16 ppm for the thinnest nanowires. All the nanosheets-based sensors show LoD values that are two orders or magnitude lower. The sensor based on the thickest nanosheets gives a LoD of 400 ppb while the one based on the thinnest nanosheets gives a value of 140 ppb. In both cases of one- and two-dimensional ZnO nanostructures the limit of detection of the devices slightly increases while increasing the average size of the nanostructures composing them. This is due to the larger effect that the same depletion layer depth has on different size nanostructures [14]. Considering that the nanostructures are made of the same material and have similar characteristic dimensions (diameter and thickness), such a large difference in their LoD is only due to their different dimensionality [35]. Depending on the speciﬁc application, a lower limit of detection can be very important in order to detect a leak at its very beginning. 4. Discussion The basic mechanism that causes metal oxide gas response is known: adsorbing molecules withdraw and trap electrons from the bulk material causing a band bending and changing its conductivity. Typically, oxygen particles are adsorbed on the surface of the ZnO sensing structure in air. The adsorbed oxygen species drain electrons from the core of the ZnO nanostructure. These oxygen species entrap charge carriers causing a depletion layer thus decreasing the conductivity of the material. When the n-type semiconductor material is exposed to a reducing gas like LPG, the electrons trapped by the oxygen adsorbates will be released to the ZnO structure core, leading to an increase in conductivity. When the ZnO nanostructure is exposed to a reducing test gas such as liqueﬁed petroleum gas, its atoms react with the chemisorbed oxygen ions on the surface consuming it from the metal oxide surface and releasing electrons. Thus electrons will be

Response proportional to

r2 (r − l)

(1)

2

where r is the nanowire radius and l is the depletion layer depth. The response of a two-dimensional nanostructure goes instead as Response proportional to

r r−l

L L − 2l

(2)

where r is the sheet thickness and L its width. Eq. (2) simpliﬁes in Response proportional to

r r−l

(3)

when L l (which is true in the case of nanosheets investigated in this paper). This means that when r has the same order of magnitude of l, the response of a one-dimensional nanostructure raises faster than that of a two-dimensional one. On the other hand, the larger cross-section of the nanosheets leads to a higher and more stable base current, which provides a lower limit of detection. This characteristic make two-dimensional nanostructures better candidate to gas sensing than mono-dimensional nanostructures for applications in which LoD is more important than just sensor response. 5. Conclusions In summary, wide layers of ZnO nanostructures (both one- and two-dimensional) have been grown controlling their size (diameter for nanowires, thickness for nanosheets) and have been used as sensors for liqueﬁed petroleum gas. The large surface-to-volume ratio of both types of nanostructures gives all the devices good sensing performance, optimized for a working temperature of 250 ◦ C (or 300 ◦ C in few cases). The sensor response varies with dimensionality and average size of the nanostructures composing the active material, and is higher for one-dimensional nanowires, increasing with smaller diameters. The limit of detection of the sensors based on two-dimensional nanostructures is found to be very higher than those of nanowires-based devices. This can be explained by the different cross-section, which permits the nanosheets to have a more stable base current. Acknowledgement This research is funded by the bilateral project HyMN between Italy and Vietnam with the support of the Italian Ministry of Foreign Affairs, Directorate General for the Promotion of the Country System.

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Thi Thanh Le Dang acknowledges the ﬁnancial support of National Fund for Science and Technology Development (NAFOSTED) for the project’s code 103.02-2013.23. References [1] J. Chou, Hazardous Gas Monitors: A Practical Guide to Selection, Operation, and Application, McGraw-Hill, New York, 1999. [2] J.F. McAleer, P.T. Moseley, J.0.W. Norris, D.E. Williams, Tin dioxide gas sensors. Part 1. Aspects of the surface chemistry revealed by electrical conductance variations, J. Chem. Soc. Faraday Trans. I 83 (1987) 1323–1346. [3] J.K. Srivastava, P. Pandey, V.N. Mishra, R. Dwivedi, Sensing mechanism of Pd-doped SnO2 sensor for LPG detection, Solid State Sci. 11 (2009) 1602–1605. [4] C. Jin, K. Changhyun, H. Kim, S. Choi, S.S. Kim, C. Lee, Synthesis, structure, and gas-sensing properties of Pt-functionalized TiO2 nanowire sensors, J. Nanosci. Nanotechnol. 14 (2014) 5833–5838. [5] N.M. Kiasari, S. Soltanian, B. Gholamkhass, P. Servati, Environmental gas and light sensing using ZnO nanowires, Trans. Nanotechnol. 13 (2014) 368–374. [6] C.P.T. Nguyen, P.P.H. La, T.T. Trinh, T.A.H. Le, S. Bong, K. Jang, S. Ahn, J. Yi, Fabrication of ZnO nanorods for gas sensing applications using hydrothermal method, J. Nanosci. Nanotechnol. 14 (2014) 6261–6265. [7] Z. Zhang, Y.V. Kaneti, X. Jiang, A. Yu, Hydrothermal synthesis of sodium vanadium oxide nanorods for gas sensing application, Sens. Actuators B 202 (2014) 803–809. [8] Y. Cheng, R. Yang, J. Zheng, Z.L. Wang, P. Xiong, Characterizing individual SnO2 nanobelt ﬁeld-effect transistors and their intrinsic responses to hydrogen and ambient gases, Mater. Chem. Phys. 174 (2012) 495–499. [9] E. Comini, G. Faglia, G. Sberveglieri, Z. Pan, Z.L. Wang, Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts, Appl. Phys. Lett. 81 (2002) 1869–1871. [10] V. Dobrokhotov, L. Oakes, D. Sowell, A. Larin, J. Hall, A. Kengne, P. Bakharev, G. Corti, T. Cantrell, T. Prakash, J. Williams, D.N. McIlroy, Toward the nanospring-based artiﬁcial olfactory system for trace-detection of ﬂammable and explosive vapors, Sens. Actuators B 168 (2012) 138–148. [11] W. Guo, M. Fu, C. Zhai, Z. Wang, Hydrothermal synthesis and gas-sensing properties of ultrathin hexagonal ZnO nanosheets, Ceram. Int. 40 (2014) 2295–2298. [12] Y. Zeng, Liang Qiao, Y. Bing, M. Wen, B. Zou, W. Zheng, T. Zhang, G. Zou, Development of microstructure CO sensor based on hierarchically porous ZnO nanosheet thin ﬁlms, Sens. Actuators B 173 (2012) 897–902. [13] Y.-B. Zhang, J. Yin, L. Li, L.-X. Zhang, L.-J. Bie, Enhanced ethanol gas-sensing properties of ﬂower-like p-CuO/n-ZnO heterojunction nanorods, Sens. Actuators B 202 (2014) 500–507. [14] M. Tonezzer, N.V. Hieu, Size-dependent response of single-nanowire gas sensors, Sens. Actuators B 163 (2012) 146–152. [15] I. Castro-Hurtado, G.G. Mandayo, E. Castano, Conductometric formaldehyde gas sensors. A review: from conventional ﬁlms to nanostructured materials, Thin Solid Films 548 (2013) 665–676. [16] 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 144 (2010) 56–66. [17] N. Yamazoe, G. Sakai, K. Shimanoe, Oxide semiconductor gas sensors, Catal. Surv. Asia 7 (2003) 63–75. [18] Dan, S. Evoy, A.T. Johnson, Chemical Gas Sensors Based on Nanowires arXiv:0804.4828. [19] F. Hernández-Ramírez, A. Tarancón, O. Casals, J. Arbiol, A. Romano-Rodríguez, J.R. Morante, High response and stability in CO and humidity measures using a single SnO2 nanowire, Sens. Actuators B 121 (2007) 3–17. [20] S.F.A. Rahman, N.A. Yusof, U. Hashim, M.N.M. Nor, Design and fabrication of silicon nanowire based sensor, Int. J. Electrochem. Sci. 8 (2013) 10946–10960. [21] A. Wei, L. Pan, W. Huang, Recent progress in the ZnO nanostructure-based sensors, Mater. Sci. Eng. B: Adv. 176 (2011) 1409–1421. [22] M. Tonezzer, R.G. Lacerda, Zinc oxide nanowires on carbon microﬁber as ﬂexible gas sensor, Physica E 44 (2012) 1098–1102. [23] D. Panda, T. Tseng, One-dimensional ZnO nanostructures: fabrication, optoelectronic properties, and device applications, J. Mater. Sci. 48 (2013) 6849–6877. [24] G. Korotcenkov, B.K. Cho, The role of grain size on the thermal instability of nanostructured metal oxides used in gas sensor applications and approaches for grain-size stabilization, Prog. Crystal Growth 58 (2012) 167–208. [25] N. Ramgir, N. Datta, M. Kaur, S. Kailasaganapathi, A.K. Debnath, D.K. Aswal, S.K. Gupta, Metal oxide nanowires for chemiresistive gas sensors: issues, challenges and prospects, Colloid Surf. A 439 (2013) 101–116. [26] L.D. Feng, X.J. Huang, Y.K. Choi, Dynamic determination of domestic liqueﬁed petroleum gas down to several ppm levels using a Sr-doped SnO2 thick ﬁlm gas sensor, Microchim. Acta 156 (2007) 245–251.

[27] R.S. Wagner, W.C. Ellis, Vapor–liquid–solid mechanism of single crystal growth, Appl. Phys. Lett. 4 (1964) 89. [28] S. Barth, F. Hernandez-Ramirez, J.D. Holmes, A. Romano-Rodriguez, Synthesis and applications of one-dimensional semiconductors, Prog. Mater. Sci. 55 (2010) 563–627. [29] A. Kolmakov, Y. Zhang, G. Cheng, M. Moskovits, Detection of CO and O2 using tin oxide nanowire sensors, Adv. Mater. 15 (2003) 997–1000. [30] S. Ahlers, G. Muller, T. Doll, A rate equation approach to the gas sensitivity of thin ﬁlm metal oxide materials, Sens. Actuators B 107 (2005) 587–599. [31] G. Sakai, N. Matsunaga, K. Shimanoe, N. Yamazoe, Theory of gas-diffusion controlled sensitivity for thin ﬁlm semiconductor gas sensor, Sens. Actuators B 80 (2001) 125–131. [32] I. Mora-Seró, F. Fabregat-Santiago, J. Denier, B. Bisquert, R. Tena-Zaera, J. Elias, C. Lévy-Clémen, Determination of carrier density of ZnO nanowires by electrochemical techniques, Appl. Phys. Lett. 89 (2006) 203117. [33] G. Neri, A. Bonavita, G. Micali, G. Rizzo, E. Callone, G. Carturan, Resistive CO gas sensors based on In2 O3 and InSnOx nanopowders synthesized via starch-aided sol–gel process for automotive applications, Sens. Actuators B 132 (2008) 224–233. [34] J.H. Park, J.H. Lee, A facile fabrication of semiconductor nanowires gas sensor using PDMS patterning and solution deposition, Sens. Actuators B 136 (2009) 224–229. [35] M. Tonezzer, S. Iannotta, H2 sensing properties of two-dimensional zinc oxide nanostructures, Talanta 122 (2014) 201–208. [36] Z.M. Jarzebski, Oxide Semiconductors, vol. 4, Pergaman Press, 1973. [37] H. Windischmann, P. Mark, A model for the operation of a thin-ﬁlm SnOx conductance-modulation carbon monoxide sensor, J. Electrochem. Soc. 126 (1979) 627–633. [38] S. Seal, S. Shukla, Nanocrystalline SnO gas sensors in view of surface reactions and modiﬁcations, JOM 54 (2002) 35–38.

Biographies Matteo Tonezzer graduated “cum laude” in Physics of the Matter and received his PhD degree “with honor” from the Faculty of Physics at the University of Trento, Italy, in 2011. His thesis was the optimization of inorganic and organic nanostructured materials towards gas sensing. In 2011, he won the Young Scientist Award from the European Materials Research Society (EMRS). He worked in research centres in France (ESRF), Brazil (UFMG), Vietnam (HUST), and USA (GaTech). He is currently working for IMEM at the Italian National Research Council. Major research interest is synthesis and characterization of nanostructured materials. Thi Thanh Le Dang obtained her MSc and PhD degrees in Materials Science from International Training Institute for Materials Science (ITIMS) – Hanoi University of Science and Technology (HUST), Hanoi, Vietnam in 2001 and 2011. She worked as a visiting postdoc at The Angstrom Laboratory – Uppsala University, Sweden in the academic year of 2011–2012. She is working as a researcher/lecturer at ITIMS. Her current interests include synthesis, characterization and application of nanomaterials for gas-sensing and bio-sensing. Nicola Bazzanella received the PhD in Physics (Hydrogen kinetics and thermodynamic in magnesium-based metal hydrides) from the University of Trento, Italy, in 2005. His main research activity is in the area of Materials Physics related with hydrogen technologies (production, puriﬁcation, storage, etc.). From 2008 his work concerns electron microscopy (SEM-EDXS) and PVD techniques (DC and RF sputtering, Pulsed Laser Deposition). He is presently as technician with the IdEA Laboratory at the Department of Physics, University of Trento. Van Hieu Nguyen joined the International Training Institute for Material Science (ITIMS) at Hanoi University of Science and Technology (HUST) in 2004, where he is currently associate professor. He received his PhD degree from the Faculty of Electrical Engineering at University of Twente, The Netherlands in 2004. In 2007, he worked as a post-doctoral fellow at the Korea University. Currently, he is the vice director of ITIMS and chairs the research group of gas sensors. His current research interests include nanomaterials, nanofabrications, characterizations and applications to electronic devices, gas sensors and biosensors. Salvatore Iannotta graduated in Physics at the University of Bologna, then in August 1984 the PhD in Chemistry at the (GWC) (Guelph Waterloo Centre for Graduate Work in Chemistry – Ontario, Canada). He is presently Director of CNR-IMEM of materials for electronic and magnetism. Major research interests are: synthesis and characterization of nanostructured and molecular materials; supersonic beams of metallic and semiconductor clusters as well as of molecular aggregates, organic electronics, gas a VOC sensors, memristive materials devices and systems, active bio-sensors and bio-electronics. He is author of more than 130 papers and invited speaker to a wide number of international conferences.