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Nanowire (nanorod) arrays-constructed tungsten oxide hierarchical structure and its unique NO2-sensing performances
Journal of Alloys and Compounds 615 (2014) 616–623
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Nanowire (nanorod) arrays-constructed tungsten oxide hierarchical structure and its unique NO2-sensing performances Yuxiang Qin ⇑, Changyu Liu, Mei Liu, Yang Liu School of Electronics and Information Engineering, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 8 June 2014 Accepted 9 July 2014 Available online 15 July 2014 Keywords: Tungsten oxide Gas sensor Hierarchical structure Hydrothermal synthesis
a b s t r a c t Novel WO3 hierarchical structure film demonstrates to be a promising material for building highly sensitive and ultrafast responding gas sensors. The individual hierarchical WO3 structure was constructed hydrothermally through double-sided inductive growth of WO3 nanowire (or nanorod) arrays from the central nanosheet. The nanosheet was performed on the substrate via spin-coating and thermal annealing. Composed of well-aligned ultrathin nanowires (nanorods) as building blocks, the as-synthesized hierarchical WO3 shows high active surface area and loose microstructure favorable for gas adsorption and rapid gas diffusion. The NO2-sensing properties of the hierarchical WO3 film-based sensors were evaluated at room temperature over NO2 concentration ranging from 15 ppb to 5 ppm. At room temperature, the WO3 hierarchical structure behaves as an abnormal p-type semiconductor and exhibits unique gas-sensing performances including excellent sensitivity and excellent response characteristics towards NO2 gas. It is found that the sensors based on hierarchical WO3 responses to NO2 gas as low as 15 ppb with an ultrashort response time of short than 5 s at room temperature, highlighting the capability of the material for rapid detection of dilute NO2 at ppb level. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen dioxide (NO2) is a highly harmful atmospheric pollutant. It contributes to PM 2.5 and it is also the main source of photochemical smog and acid rain. The fast and reliable detection of toxic NO2 gas is therefore important for both environmental protection and human health. Existing studies have shown that oxide semiconductors are promising materials for facile and reliable detection of various toxic and hazardous gases. In particular, tungsten oxide (WO3), a wide band-gap n-type semiconductor, has exhibited remarkable sensing performance to NO2 gas [1,2]. Semiconductor oxide-based gas sensors rely on the modulation of electrical conductivity due to surface oxidation or reduction caused by gas exposure. The sensing response of a solid-state sensor therefore heavily dependents on the surface structure and morphology of its sensing material used [3,4]. Towards this end, various low-dimensional tungsten oxide nanostructures with high surface-to-volume ratio have been widely examined, including nanoparticles [5], nanowires/nanorods [1,2,6], and nanotubes [7]. They achieve much high sensitivity to NO2 gas and gas sensitivity ⇑ Corresponding author. Address: School of Electronics and Information Engineering, Tianjin University, 92 Weijin Road, Nankai District, Tianjin, China. Tel.: +86 22 27402372; fax: +86 22 27401233. E-mail address: [email protected] (Y. Qin). http://dx.doi.org/10.1016/j.jallcom.2014.07.080 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
increases rapidly when the dimensions of the oxide nanostructure become comparable with or smaller than Debye length [8]. However, nano-scale materials easily aggregate to form large and dense secondary aggregates such as much larger particles or relative thicker bundles due to strong and inevitable van der Waals attraction. Such aggregation leads to considerable decrease in active surface area for gas adsorption and difficult diffusion of gas molecules in sensing films. The agglomeration becomes more serious when the sensor operates at high temperature. To date, achieving both high sensitivity and rapid response remains challenging for the gas sensors based on low-dimensional semiconductor oxides. In this aspect, how to keep structure stability of the low-dimensional oxide materials is primarily crucial. Hierarchical structure is a kind of high dimensional micro-/nano-structure composed of many low-dimensional, nanostructured building blocks (particles, rods, wires, or sheets). Hierarchical structure shows a well-developed porous texture as well as a non- or less-agglomerated feature without scarifying high surface area from the nano-constituents; it is therefore very attractive for efficient gas sensor with high sensing performance [9]. Sensing materials using well-designed hierarchical structures are expected to achieve both high gas response and fast response speed. For instance, SnO2 hierarchical microspheres self-assembled from nanosheets show both the ultra-
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fast response (1 s) and high sensitivity to 50 ppm C2H5OH at operating temperature of 400 °C [10]. The sensor based on hierarchical ZnSnO3 hollow microspheres from ultra-thin nanorods presents very short response and recovery times of 0.9 and 2.2 s respectively, to ethanol vapor at 270 °C [11]. High sensitivity and fast response to ethanol are also observed at room temperature and 250 °C from hierarchical vanadium pentoxide with radially oriented ultrathin nanoneedles and nanoribbons as constituents in our group [12]. The excellent response properties exhibited by above hierarchical structures are all because of the good surface accessibility and the high surface area of ultrathin building blocks. As to tungsten oxide, the synthesis and applications of its hierarchical structure in gas sensor has not been well studied. Up to now, successful assembly of hierarchical WO3 microspheres from nanorods or nanosheets has been realized [13,14], and the nanosheets-constructed WO3 hierarchical spheres are found to show enhanced sensitivity to ppm-level NO2. In this work, a novel WO3 hierarchical structure is hydrothermally constructed through double-sided inductive growth of WO3 nanowire (nanorod) arrays on preformed nanosheet. The constituents of the highly ordered arrays of ultrathin nanowire (nanorod) provide favorable microstructure for gas adsorption and rapid gas diffusion. It is found that the developed hierarchical WO3-based sensors are capable of dilute NO2 detection at ppb level (15 ppb) with superfast response about 1–5 s at room temperature. Meanwhile, good selectivity and stability are also achieved.
2. Experiments 2.1. Synthesis and characterization of WO3 hierarchical structure WO3 hierarchical structure films were in-situ prepared on cleaned alumina substrate with a pair of interdigitated Pt electrodes in 100 nm thickness via a hydrothermal method. The electrodes were deposited using RF magnetron sputtering process. For the hydrothermal growth of the hierarchical WO3, a thin WO3 induction layer was firstly preformed on the electrodes-attached substrate by spin coating of precursor solution, followed by thermal annealing. The precursor solution was prepared as follows: 2.5 g sodium tungstate hydrate (Na2WO42H2O) and 1 g potassium chloride (KCl) were dissolved in 15 ml de-ionized water under magnetically stirring. Then 3 ml HCl solution (37 wt%) was added dropwise into the above solution to form a milky suspension. After heating the suspension to 40 °C, 2 ml H2O2 was then added under vigorously stirring to obtain the yellow precursor solution. To prepare the WO3 induction layer, a cycled spin coating procedure was then performed. More specifically, the above as-prepared precursor solution was spincoated onto the cleaned electrodes-attached alumina substrate, and subsequently, the wet substrate was baked for 10 min at 80 °C in a drying oven. During spin coating, a physical mask was used to avoid the presence of the solution at the end of the electrodes. The above spin-coating and baking procedures were repeated for four times to ensure a uniform distribution and adequate coverage of the WO3 particles on the substrate. Finally, the obtained substrate was annealed at 550 °C in ambient atmosphere for 1–3 h to transform the precursor into nanosheet-like WO3 particles to induce the subsequent growth of one-dimensional (1D) WO3 arrays and to guarantee adhesion between the induction layer and the substrate. In the following hydrothermal step, the well-aligned 1D WO3 nanowires (nanorods) were grew from both sides of the tungsten oxide nanosheets simultaneously to form hierarchical structure film on the induction layer-coated substrates. In a typical synthesis procedure, 6 g Na2WO42H2O, 2 g KCl, and 0.4 g P123 surfactant were first co-dissolved in 160 ml deionized water under magnetically stirring. Then the PH value of the mixture solution was adjusted to 2.1–2.5 with a proper volume of HCl (37 wt%). This solution was further aged for about 15 min at room temperature before performing hydrothermal process. After submerging and sealing the induction layer-coated substrates in above resultant solution in a 200 ml Teflonline autoclave, the hydrothermal reaction was conducted at 180 °C for 9 h in an electric oven. After that, the autoclave was cooled naturally to room temperature. The substrates with products were washed several times with deionized water and ethanol, respectively. Finally, the substrates were dried at 80 °C for 5 h at ambient atmosphere. The resultant samples were directly used as sensors to carry out the sensing performance evaluation. The morphology and crystalline structures of hierarchical WO3 were characterized using field emission scanning electron microscope (FESEM, Hitachi S-4800), Xray diffractometer (XRD, RIGAKU D/MAX 2500 V/PC, Cu Ka radiation) and field emission transmission electron microscope (FETEM, TECNAI G2F-20).
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2.2. Evaluation of gas-sensing properties The gas-sensing properties of as-prepared WO3 hierarchical structure were evaluated at room temperature in a home-built static gas-sensing characterization system consisting of a glass test chamber (30 L), a flat heating plate, and two sets of data acquisition systems [15], which permits two samples measured simultaneously. This test equipment is equipped in a humidity-controlled testing room. Appropriate volume of pure NO2 gas was injected into the test chamber directly to get the desired concentration. The resistance change of the sensor during the whole measurement was continuously monitored by an UNI-T UT61E professional digital multimeter with the function of automatic measuring range adjustment. The sampling interval was set to 1 s, and the acquired resistance data were stored in a PC for further analysis. After hydrothermal reaction and subsequent washing, the tungsten oxide film was found to grow only on the area precoated with tungsten oxide induction layer. The bare section of Pt electrodes therefore can be used to realize the electrical connection between the sensor and the digital multimeter. To minimize the effect of humidity fluctuation on the gas-sensing properties, the whole measurement was carried continuously. During the measurement, the ambient relative humidity is about 30–35% and the room temperature is about 20 °C.
3. Results and discussion 3.1. Structure and characterization In this work, we employ a hydrothermal process to directly synthesize three-dimensional (3D) hierarchical tungsten oxide film on sensor substrate with 1D nanowires or nanorods as building blocks. The typical morphological characteristic of the assynthesized tungsten oxide hierarchical structure from 1 hannealed induction layer after hydrothemal reaction of 9 h at 180 °C is illustrated by FESEM observations with different magnifications in Fig. 1. Observe that the well-oriented 1D oxide nanowire arrays grow from both sides of the central nanosheets simultaneously, constructing a kind of 3D hierarchical structure film. Fig. 1(b) and (c) shows the high magnification FESEM images of an individual tungsten oxide hierarchical structure. Clearly, the central nanosheet acts as the architectural core of the whole hierarchical structure, and the constituents of nanowires align perpendicular to its both sides. It can also be observed that each nanowire shows a very small diameter of about 20 nm and the central nanosheet has thickness of about 10 nm. This ultrathin feature of the nanowires and nanosheet is much favorable for achieving high sensitivity when exposure to a detected gas. On the other hand, the ultrathin nanowires trend to assemble together near their tips forming bundles due to much high surface energy [16]. This would affect the alignment of the nanowire arrays in some degree. The growth process of tungsten oxide hierarchical structure can be effectively illustrated by intercepting intermediate product with much shorter growth time. Fig. 2(a) and (b) gives the low magnification and high magnification FESEM images of the product after hydrothermal reaction of 4 h at 180 °C. The inset in Fig. 2(a) shows the SEM image of the tungsten oxide induction layer formed on the substrate after annealing at 550 °C for 1 h, which is composed of stacked ultrathin nanosheets with thickness in 20–40 nm. The maximum size of these nanosheets is less than 2 lm. After hydrothermal reaction of 4 h, many short and ultrathin nanorods grow vertically from both surfaces of the nanosheets in induction layer to transform into an integrated hierarchical structure. Further TEM characterization in Fig. 2(c) gives us a clearer vision that the double-sided nanorods have an average diameter of about 10 nm and meanwhile exhibit high orientation. Extending the reaction time to 9 h results in the bilateral nanowire arrays with longer length and larger diameter (Fig. 1). When the reaction time is shorter than 2 h, no objective nanorods are observed on the surface of nanosheets. The above results unambiguously demonstrate the induction of the preformed nanosheets in constructing tungsten oxide hierarchical structures. They serve as the architectural rachises of tungsten oxide hierarchical structures to support the bilateral
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(a)
2 µm
(b)
300 nm
(c)
100 nm Fig. 1. FESEM images with different magnifications for the tungsten oxide hierarchical structure synthesized after hydrothermal reaction of 9 h at 180 °C.
assembly of aligned nanowire arrays. Fig. 2(d) gives the high magnification SEM image of the nanosheets surface before the hydrothermal process. Observe that each sheet is composed of numerous small nanoparticles with estimated diameter of 10–30 nm. These densely arranged nanoparticles are expected to provide multiple initial nucleation sites to inspire the growth of 1D tungsten oxides. During the hydrothermal process, the tungsten oxide nuclei formed in the hydrothermal solution might preferentially aggregate on these sites, and then the double-sided inductive growth of vertically aligned tungsten oxide nanorods is realized. Meanwhile, the aggregates of these particles, i.e. nanosheets, serve as the architectural center to guide the subsequent self-organized formation of the 3D hierarchical structures. After reaction of 9 h, the thickness of the nanosheets decreases and the size of the double-sided nanowires increases. This seems to be related to a dissolution–recrystallization process followed by Ostwald ripening under hydrothermal environment [11]. The requirement for the inductive nanosheets in constructing hierarchical WO3 was further demonstrated by observing the morphology of the powder centrifuged from the hydrothermal solution. Only unordered nanowire bundles network is obtained, as shown in Fig. 2(e). Note that, the precursor solution and the particle shape of the annealed induction layer are crucial for the formation of this kind of tungsten oxide hierarchical structure. When
WCl6 is employed as precursor to prepare tungsten oxide induction layer, the annealed film exhibits stacked spheres. We note that the subsequent hydrothermal step cannot result in the hierarchical structure presented above. Under this circumstance, only unordered nanowires network is observed instead. Fig. 3(a) shows the morphology of tungsten oxide hierarchical structure film developed from an induction layer with much longer annealing of 3 h. The hydrothermal reaction time remains 9 h. The inset in Fig. 3(a) shows the SEM image of the 3 h-annealed induction layer, which exhibits a morphological characteristic of tungsten oxide particles with much large size and low stacking density due to the possible coalescence of adjacent thin nanosheets during the deep annealing [17]. As the result, the developed tungsten oxide hierarchical structures also show much large size, shown in Fig. 3(a). The high magnification FESEM image in Fig. 3(b) clearly demonstrates the detailed structure of the formed hierarchical oxide, which is constructed from abundant dense nanorods with inconsistent length but well-aligned perpendicular to the double sides of the central nanosheet. In contrast to the sample developed from 1 h-annealed nanosheets in Fig. 1, the bilateral nanorod arrays in Fig. 3(b) show a more ordered morphology. However, the constituents of nanorods and central nanosheets also show much larger sizes in diameter and thickness. The crystalline structures of the tungsten oxide induction layer and the as-developed hierarchical structure films were investigated by XRD technique. Fig. 4(a–c) shows the XRD patterns of the three tungsten oxide powders which were scraped from the substrates to eliminate the diffraction effect of substrate materials. As shown in Fig. 4(a), the coated Na2WO4 precursor was converted to orthorhombic WO3 after thermal annealing. The relative poor intensity of diffraction peaks indicates the low crystalline degree of the WO3 induction layer. This result is consistent with the FESEM observation to the nanosheets presented in Fig. 2(d), i.e., each nanosheet is composed of numerous small grain particles. Fig. 4(b) and (c) respectively show the XRD patterns of the tungsten oxide hierarchical structures grown from the 1 h- and 3 hannealed induction layers. Observe that all typical diffraction peaks of the two samples are well indexed to the profile of hexagonal WO3 (h-WO3) (JCPDS 33-1387). The strong and sharp diffraction peaks indicate the high crystallinity of the as-synthesized hierarchical WO3 with hexagonal structure. The strongest peak of plane (0 0 1) for the two hierarchical samples implies that the main constituents of nanowires (nanorods) grow preferentially along the same c-axis direction. It is well known that h-WO3 is a metastable phase and it can transform into monoclinic WO3 at appropriate temperature. Szilágyi et al. [18] pointed out that the structure of h-WO3 cannot be maintained without some stabilizing ions or molecules in the hexagonal channels. It has also been found that alkaline (Na+, K+, Cs+, etc.) or NH+4 ions can enter the hexagonal channels of crystallites and effectively block the thermodynamically favored hexagonal–monoclinic transformation [19]. In this work, the harvested stable hexagonal crystalline phase of WO3, confirmed by above XRD characterization, is thought to be directly related to the introduced KCl in precursor solution. The inset in Fig. 4 shows the energy dispersive X-ray spectroscopy (EDS) of the as-synthesized hierarchical tungsten oxide in Fig. 1. It illustrates clearly that the constituents of the hierarchical structures are dominated by W and O elements, with a small amount of K element that is believed from the additive of KCl. It is assumed that the introduced K+ plays a crucial role in stabilizing the hexagonal structure of h-WO3. Meanwhile, K+ in the crystal of WO3 is expected to act as dopants to improve the sensitivity of the oxide sensor at low temperature effectively [20]. During hydrothermal process, Cl ions from the additive of KCl may serve as capping agents to adhere to the side crystal faces (parallel to c-axis) of the formed WO3 nuclei [21,22],
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(b)
(a)
1 µm
2 µm
(c)
100 nm
(d)
(e)
200 nm
50 nm
300 nm
Fig. 2. (a) Low magnification and (b) high magnification FESEM images of the tungsten oxide obtained after reaction of 4 h at 180 °C. The inset in (a) shows the morphology of the induction layer on the substrate formed after annealing at 550 °C for 1 h. (c) TEM image of the sample shown in (b). (d) High magnification FESEM image of the nanosheets from 1 h-annealing induction layer. (e) Morphology of the pure tungsten oxide nanowires network formed in the solution.
(a)
(b)
5 µm
5 µm
500 nm
Fig. 3. (a and b) FESEM image and the corresponding high magnification image of the tungsten oxide hierarchical structure developed on the 3 h-annealed induction layer after hydrothermal reaction of 9 h. The inset in (a) shows the morphology of the induction layer formed after annealing at 550 °C for 3 h.
inducing tungsten oxide preferentially growing along [0 0 1] direction and suppressing the growth on other directions. The structural features of the hierarchical tungsten oxide are further studied using HRTEM. Fig. 5 shows the HRTEM image of two nanorods from the hierarchical tungsten oxide developed from 1 h-annealed induction layer. The image exhibits clear lattice fringe with a distance of about 0.39 nm, corresponding to the interspace of (0 0 1) plane of the hexagonal structured WO3 (d001 = 0.3899 nm, JCPDS 33-1387) and then confirming the preferred growth along c-axis. 3.2. Gas-sensing properties The gas-sensing properties were evaluated by measuring the changes of resistance of the sensors before and after the detected gas was introduced. The measurements were taken at room
temperature (20 °C). Fig. 6(a and b) indicates the dynamic responses of the sensors based on the hierarchical WO3 films developed from 1 h- and 3 h-annealed induction layers to varying NO2 gas concentrations at room temperature. It shown that the asdeveloped hierarchical WO3 film is responsive to very small NO2 amounts (15 ppb) at room temperature. This result indicates that this type of WO3 sensor is indeed capable of NO2 detection at ppb level even under room temperature, which remains a big challenge for conventional sensing materials. It can also be observed that, when exposure to different concentrations of NO2 gas, the measured resistances of the sensor based on hierarchical WO3 decrease. In general, tungsten oxide is an n-type semiconductor. It is well known that, for n-type semiconductor oxides, an oxidizing gas such as NO2, chemisorbs on the surface and interacts with the surface to extract electrons from the conduction band of the semiconductor, leading to the increase of resistance. For a p-type
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Fig. 4. XRD patterns of the 1 h-annealed induction layer (a), tungsten oxide hierarchical structures developed from the 1 h-(b) and 3 h-(c) annealed induction layers. The hydrothermal reaction time is 9 h. The inset is the EDS of the hierarchical tungsten oxide shown in Fig. 1.
10 nm
0.39 nm
[001]
Fig. 5. HRTEM image of two nanorods from the hierarchical tungsten oxide developed from 1 h-annealed induction layer.
semiconductor, the resistance decreases because of the increasing holes concentration induced by electron-extracting. Therefore, the described hierarchical WO3-based sensors show typical p-type semiconductor behavior at room temperature. In fact, this kind of abnormal conductive behavior has been reported for gas sensor based on WO2.72 nanorod [23], WO3 nanoparticles [24], functionalized WO3 nanoneedles [25], and other metal oxide semiconductors exposed to various gases [26,27]. Similar behavior has also been observed on the flower-like V2O5 hierarchical structure by our group recently [12]. Until now, it is still not clear for the real origin of p–n transition. Several explanations which can be pursued in the literatures include different reactions occurring under different conditions [24], oxygen adsorption-induced inversion layer [25,26], and different motilities of carriers [28]. Based on the surface-controlled gas-sensing mechanism of tungsten oxide, it is assumed that the abnormal p-type response characteristic exhibited by the n-type WO3 with hierarchical structure might be related to the surface
of the oxide. That is, the anomalous p-type conductive behavior seems to be able to be explained by the formation of an inversion layer at the surface of n-type hierarchical WO3. It was reported that higher surface-to-volume ratio for low-dimensional nanomaterials favors a larger quantity of surface oxygen vacancy [29]. Moreover, the foreign ions in WO3 crystal can introduce additional surface states, as evidence of a band of surface states energetically appearing inside the semiconductor band gap [30]. Thus the surface of the hierarchical WO3 constructed with 1D ultrathin nanowires or nanorods is much active to motivate strong absorption of oxygen even at low temperature due to the large density of surface states. The strong oxygen adsorption significantly increases the depletion region inside the oxide surface and upward band bending, causing an inversion layer (the Fermi level EF lies below the intrinsic level Ei) instead of a depletion layer created by normal surface adsorption, as shown in Fig. 7. In the surface inversion layer, holes are indeed the major charge carriers, which indicate the p-type feature. The strong ability of surface adsorption of the hierarchical structure induced by its ultrathin constituents and additive of K+ also explains its remarkably sensitive performance exhibiting at room temperature. It is also worthy noting from Fig. 6 that the sensor resistances could recover to its initial value after gas removal, exhibiting very good dynamic response–recovery characteristics at room temperature. Even after four response–recovery cycles to different NO2 concentrations (0.015–5 ppm), there is almost no obvious change between recovered resistance and baseline resistance, indicating excellent reversibility and stability of the sensor. It seems that the perfect stability and reversibility can be ascribed greatly not only to the good interface performance between the electrodes and the oxide films caused by the direct bottom-up growth of WO3 from the substrate [31], but also to the high autologous structure stability of the hierarchical oxide originated from its unique 3D architecture. In particular, the characteristic architecture of double-sided assembly of well-aligned nanowire (nanorod) arrays is thought to provide a distinct structure promotion in achieving fast sensor response. It is observed from Fig. 6 that, at room temperature, the sensor resistances show a rapid change upon exposure to NO2 gas with concentrations from ppb to ppm level, indicating an ultrafast response characteristic for this kind of hierarchical WO3-based sensors. The right images in Fig. 6(a) and (b) respectively show the enlarged response curves of both hierarchical WO3-based sensors to 1 ppm NO2 gas. The response times (tres, the time required for the sensor resistance to reach 90% of the equilibrium value after the test gas is injected) are 4 s and 3 s respectively for the sensors developed from 1 h- and 3 h-annealed induction nanosheets. Fig. 8 gives the response time values of the two hierarchical WO3 sensors to different NO2 concentrations at room temperature. It can be seen that the response times of both sensors are no more than 5 s upon exposure to 0.015–5 ppm NO2, exhibiting a unique ultrafast response characteristic. Especially, the immediate response is achieved with observable response time as short as 1 s (the sampling interval in our measurement) for the WO3 hierarchical structure developed from 3 h-annealed induction layer. Above ultrafast response characteristic could hardly be observed from single 1D nanostructured oxidebased sensors. For the sensors based on WO3 nanorod, nanowire or Au functionalized nanowires, the reported response time is usually above one hundred seconds to 0.5–2 ppm NO2 at their optical working temperature of 150–200 °C [32–34]. Thus a comparison of the response performance further highlights the advantage of 3D hierarchical structure describe here in achieving fast response. It is thought that the efficient utilization of the surface area and surface accessibility are crucial to maintain the high sensitivity and fast response characteristic of a gas sensor [9]. At a given temper-
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Fig. 6. Dynamic responses of the hierarchical WO3 films-based sensors developed from 1 h-(a) and 3 h-(b) annealed induction layers to varying NO2 gas concentrations at room temperature. The right figures show the corresponding enlarged response time to 1 ppm NO2 gas.
(a)
WO3 nanowires
(b) O2(ads)
O2(ads)
EC EF
O2-(ads)
Ei
O2(ads)
EV
O-+O2-(ads)
Depletion layer n-type semiconductor
Depletion layer n-type semiconductor
O-+O2-(ads)
Inversion layer n-type semiconductor
Fig. 7. Schematic energy-level diagrams of 1D WO3 with normal (a) and strong (b) surface adsorption of oxygen. Normal oxygen adsorption leads to a depletion layer. Strong surface adsorption of oxygen increases the upward band bending and hence causes an inversion layer (the Fermi level EF lies below the intrinsic level Ei) with a p-type surface conductivity.
ature, gas diffusion can be considered a primary factor that determines the gas response kinetics [35]. Thus, the excellent response capability of the hierarchical WO3 sensor at room temperature could be understood under the framework of gas diffusion towards the oxide surface. For the WO3 hierarchical structure developed here, the ultrathin feature of the 1D nanowires and nanorods constituents assures the large active area interacting with the gas molecules. Meanwhile, the well-aligned arrays of nanowires (nanorods) make NO2 molecules diffuse agilely in the sensing layer. These features are expected and assumed to result in the ultrafast response as well as the high response value for the hierarchical WO3 sensors. The relatively faster response rate for the hierarchical WO3 developed from 3 h-annealed nanosheets, as shown in Fig. 8, is thought to be directly related to the more
ordered microstructure of the thicker nanorod arrays beneficial to much easier gas diffusion. However, the much thicker constituents of nanorods and central nanosheets apparently decrease the specific surface area of the hierarchical structure, hence degrade the response value of the sensor in some degree. The response values of the hierarchical WO3 sensor developed from 1 h-annealed nanosheets to 0.015–5 ppm NO2 range from 1.6 to 59.9, while the values for that induced by 3 h-annealed nanosheets are 1.3– 17.0. Here, the sensor response is defined as Ra/Rg, where Rg and Ra are the resistances of the sensitive film in the measuring gas and that in clean air, respectively. It should be noted that, in our experiments, two sensor samples of each type were fabricated under similar process conditions, and the sensing results for these two samples are reproducible within an accuracy of ±5%. The
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Fig. 8. Response times of the hierarchical WO3 structures developed from the 1 hand 3 h-annealed seed layers to different NO2 concentrations at room temperature.
on the substrate. The as-developed hierarchical WO3 shows very effective microstructure for achieving perfect gas-sensing performances. Large active surface area of the ultrathin constituents of nanowires (nanorods) and well-aligned structure of microarrays assure high performances in gas adsorption and diffusion. As a result, the hierarchical WO3-based sensors exhibit high sensitivity, ultrafast response, good stability and selectivity towards NO2 gas at room temperature. Especially, the sensor based on the WO3 hierarchical structure is capable of NO2 detection at ppb level with ultrafast response characteristic at room temperature. The measured response time values to 0.015–5 ppm NO2 are shorter than 5 s. The WO3 hierarchical structures are also observed to show an anomalous p-type conductive behavior at room temperature. This behavior is supposed to be due to the strong surface adsorption ability of the ultrathin structural constituents and then be related to its high sensitivity. The presented results demonstrate that the well-developed WO3 hierarchical structure in this work is very promising in NO2 sensors with low power consumption, high sensitivity and rapid response characteristics.
Acknowledgements This work was financially supported by the National Natural Science Foundation (Nos. 61274074 and 61271070) and Tianjin Natural Science Foundation (No. 11JCZDJC15300) of China.
References
Fig. 9. Responses of the sensor based on hierarchical WO3 developed from 1 hannealed nanosheets to different gases at room temperature.
results from two continual tests to the same sensor sample show a much lower fluctuation with an accuracy of ±3%. After retaining the hierarchical WO3 sensor developed from 1 h-annealed nanosheets in the atmosphere for 10 days (240 h), its response value to 5 ppm NO2 decreased from 59.9 to 53.2. To evaluate the selectivity of the hierarchical WO3 sensor, the gas responses of the sensor developed from 1 h-annealed nanosheets to other five reductive gases (NH3, acetone, ethanol, 1-propanol, isopropanol) were also examined and compared with that to 5 ppm NO2. Fig. 9 shows the sensor responses measured at room temperature. The gas concentrations for NO2 and for each reductive gas are 5 ppm and 50 ppm respectively, and the sensor response is calculated by Rg/Ra or Ra/Rg to ensure the response values are always not smaller than 1. The results show that the hierarchical WO3 sensor is insensitive to NH3 and other four reductive organic gases with evidence of very low response values of 1.04– 1.24, which are significantly lower than that to NO2 (59.9). The large discrepancy in response value is ascribed to the interaction discrepancy between the sensing surface and different sorts of gas molecules [23]. It confirms that the hierarchical WO3 sensor presented here has a satisfying selectivity to NO2 against other testing gases at room temperature.
4. Conclusions Gas sensor based on hierarchical WO3 nanostructures were formed by an inductive hydrothermal assembly of ordered nanowire or nanorod arrays on both sides of nanosheets preformed
[1] B. Cao, J. Chen, X. Tang, W. Zhou, Growth of monoclinic WO3 nanowire array for highly sensitive NO2 detection, J. Mater. Chem. 19 (2009) 2323–2327. [2] Y.X. Qin, X. Li, F. Wang, M. Hu, Solvothermally synthesized tungsten oxide nanowires/nanorods for NO2 gas sensor applications, J. Alloys Comp. 509 (2011) 8401–8406. [3] Y.B. Shen, T. Yamazaki, Z.F. Liu, D. Meng, T. Kikuta, N. Nakatani, Influence of effective surface area on gas sensing properties of WO3 sputtered thin films, Thin Solid Films 517 (2009) 2069–2072. [4] A. Ponzoni, E. Comini, G. Sberveglieri, J. Zhou, S.Z. Deng, N.S. Xu, Y. Ding, Z.L. Wang, Ultrasensitive and highly selective gas sensors using three-dimensional tungsten oxide nanowire networks, Appl. Phys. Lett. 88 (2006) 203101– 203103. [5] F. Fang, J. Kennedy, J. Futter, T. Hopf, A.M. arkwitz, E. Manikandan, G. Henshaw, Size-controlled synthesis and gas sensing application of tungsten oxide nanostructures produced by arc discharge, Nanotechnology 22 (2011) 335702. [6] M.E. Achhab, H. Shanak, K. Schierbaum, NO2 sensing properties of WO3 nanorods grown on mica, Phys. Status Solid A 208 (2011) 1229–1234. [7] R.A. Gerlitz, K.D. Benkstein, D.L. Lahr, J.L. Hertz, C.B. Montgomery, J.E. Bonevich, S. Semancik, M.J. Tarlov, Fabrication and gas sensing performance of parallel assemblies of metal oxide nanotubes supported by porous aluminum oxide membranes, Sens. Actuators B 136 (2009) 257–264. [8] Z.W. Pan, Z.R. Dai, Z.L. Wang, Nanobelts of semiconducting oxides, Science 291 (2001) 1947–1949. [9] J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators B 140 (2009) 319–336. [10] H.R. Kim, K.I. Choi, J.H. Lee, S.A. Akbar, Highly sensitive and ultra-fast responding gas sensors using self-assembled hierarchical SnO2 spheres, Sens. Actuators B 136 (2009) 138–143. [11] Y. Bing, Y. Zeng, C. Liu, L. Qiao, Y. Sui, B. Zou, W. Zheng, G. Zou, Assembly of hierarchical ZnSnO3 hollow microspheres from ultra-thin nanorods and the enhanced ethanol-sensing performances, Sens. Actuators B 190 (2014) 370– 377. [12] Y. Qin, G.T. Fan, K. Liu, M. Hu, Vanadium pentoxide hierarchical structure networks for high performance ethanol gas sensor with dual working temperature characteristic, Sens. Actuators B (2013) 141–148. [13] Z. Gu, T. Zhai, B. Gao, X. Sheng, Y. Wang, H. Fu, Y. Ma, J. Yao, Controllable assembly of WO3 nanorods/nanowires into hierarchical nanostructures, J. Phys. Chem. B 110 (2006) 23829–23836. [14] S. Bai, K. Zhang, R. Luo, D. Li, A. Chen, C.C. Liu, Sonochemical synthesis of hierarchically assembled tungsten oxides with excellent NO2-sensing properties, Mater. Lett. 11 (2013) 32–34. [15] Y.X. Qin, M. Hu, J. Zhang, Microstructure characterization and NO2-sensing properties of tungsten oxide nanostructures, Sens. Actuators B 150 (2010) 339–345. [16] R.F. Mo, G.Q. Jin, X.Y. Guo, Morphology evolution of tungsten trioxide nanorods prepared by an additive-free hydrothermal route, Mater. Lett. 61 (2007) 3787– 3790.
Y. Qin et al. / Journal of Alloys and Compounds 615 (2014) 616–623 [17] S.B. Sun, Y.M. Zhao, Y.D. Xia, Z.D. Zou, G.H. Min, Y.Q. Zhu, Bundled tungsten oxide nanowires under thermal processing, Nanotechnology 19 (2008) 305709. [18] I.M. Szilágyi, J. Madarász, G. Pokol, P. Király, G. Tárkányi, S. Saukko, J. Mizsei, A.L. Tóth, A. Szabó, Varga-Josepovits, Stability and controlled composition of hexagonal WO3, Chem. Mater. 20 (2008) 4116–4125. [19] X.P. Shen, G.X. Wang, D. Wexlera, Large-scale synthesis and gas sensing application of vertically aligned and double-sided tungsten oxide nanorod arrays, Sens. Actuators B 143 (2009) 325–332. [20] S. Bai, D. Li, D. Han, R. Luo, A. Chen, C. Chung, Preparation, characterization of WO3–SnO2 nanocomposites and their sensing properties for NO2, Sens. Actuators B 150 (2010) 749–755. [21] J. Su, X. Feng, J.D. Sloppy, L. Guo, C.A. Grimes, Vertically aligned WO3 nanowire arrays grown directly on transparent conducting oxide coated glass: synthesis and photoelectrochemical properties, Nano Lett. 11 (2011) 203–208. [22] J. Wang, E. Khoo, P.S. Lee, J. Ma, Controlled synthesis of WO3 nanorods and their electrochromic properties in H2SO4 electrolyte, J. Phys. Chem. C 113 (2009) 9655–9658. [23] Y.S. Kim, S.C. Ha, K. Kim, H. Yang, J.T. Park, C.H. Lee, J. Choi, J. Paek, K. Lee, Room temperature semiconductor gas sensor based on nonstoichiometric tungsten oxide nanorod film, Appl. Phys. Lett. 86 (2005) 213105. [24] C. Zhang, M. Debliquy, A. Boudiba, H. Liao, C. Coddet, Sensing properties of atmospheric plasma-sprayed WO3 coating for sub-ppm NO2 detection, Sens. Actuators B 144 (2010) 280–288. [25] S. Vallejos, T. Stoycheva, P. Umek, C. Navio, R. Snyders, C. Bittencourt, E. Llobet, C. Blackman, S. Moniz, X. Correig, Au nanoparticle-functionalised WO3 nanoneedles and their application in high sensitivity gas sensor devices, Chem. Commun. 47 (2011) 565–567. [26] T. Siciliano, A. Tepore, G. Micocci, A. Genga, M. Siciliano, E. Filippo, Transition from n- to p-type electrical conductivity induced by ethanol adsorption on atellurium dioxide nanowires, Sens. Actuators B 138 (2009) 207–213.
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[27] A. Gurlo, N. Barsan, A. Oprea, M. Sahm, T. Sahm, U. Weimar, An n- to p-type conductivity transition induced by oxygen adsorption on a-Fe2O3, Appl. Phys. Lett. 85 (2004) 2280–2282. [28] Y.S. Kim, I.S. Hwang, S.J. Kim, C.Y. Lee, J.H. Lee, CuO nanowire gas sensors for air quality control in automotive cabin, Sens. Actuators B 135 (2008) 298–303. [29] M.H. Huang, Y.Y. Wu, H. Feick, N. Tran, E. Weber, P.D. Yang, Catalytic growth of zinc oxide nanowires by vapor transport, Adv. Mater. 13 (2001) 113–116. [30] A. Cabot, A. Diéguez, A. Romano-Rodriguez, J.R. Morante, N. Bârsan, Influence of the catalytic introduction procedure on the nano-SnO2 gas sensor performances-where and how stay the catalytic atoms?, Sens Actuators B 79 (2001) 98–106. [31] Y.J. Choi, I.S. Hwang, J.G. Park, K.J. Choi, J.H. Park, J.H. Lee, Novel fabrication of an SnO2 nanowire gas sensor with high sensitivity, Nanotechnology 19 (2008) 095508. [32] A. Ponzonia, V. Russo, A. Bailini, C.S. Casari, M. Ferroni, A. Li Bassi, A. Migliori, V. Morandi, L. Ortolani, G. Sberveglieri, C.E. Bottani, Structural and gas-sensing characterization of tungsten oxide nanorods and nanoparticles, Sens. Actuators B 153 (2011) 340–346. [33] T. Stoycheva, S. Vallejos, C. Blackman, S.J.A. Moniz, J. Calderer, X. Correig, Important considerations for effective gas sensors based on metal oxide nanoneedles films, Sens. Actuators B 161 (2012) 406–413. [34] N.V. Hieu, H.V. Vuong, N.V. Duy, N.D. Hoa, A morphological control of tungsten oxide nanowires by thermal evaporation method for sub-ppm NO2 gas sensor application, Sens. Actuators B 171–172 (2012) 760–768. [35] E. Rossinyol, A. Prim, E. Pellicer, J. Arbiol, F. Hernández-Ramírez, F. Peiró, A. Cornet, J.R. Morante, L.A. Solovyov, B. Tian, T. Bo, D. Zhao, Synthesis and characterization of chromium-doped mesoporous tungsten oxide for gassensing applications, Adv. Funct. Mater. 17 (2007) 1801–1806.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Nanowire (nanorod) arrays-constructed tungsten oxide hierarchical structure and its unique NO2-sensing performances Yuxiang Qin ⇑, Changyu Liu, Mei Liu, Yang Liu School of Electronics and Information Engineering, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 8 June 2014 Accepted 9 July 2014 Available online 15 July 2014 Keywords: Tungsten oxide Gas sensor Hierarchical structure Hydrothermal synthesis
a b s t r a c t Novel WO3 hierarchical structure film demonstrates to be a promising material for building highly sensitive and ultrafast responding gas sensors. The individual hierarchical WO3 structure was constructed hydrothermally through double-sided inductive growth of WO3 nanowire (or nanorod) arrays from the central nanosheet. The nanosheet was performed on the substrate via spin-coating and thermal annealing. Composed of well-aligned ultrathin nanowires (nanorods) as building blocks, the as-synthesized hierarchical WO3 shows high active surface area and loose microstructure favorable for gas adsorption and rapid gas diffusion. The NO2-sensing properties of the hierarchical WO3 film-based sensors were evaluated at room temperature over NO2 concentration ranging from 15 ppb to 5 ppm. At room temperature, the WO3 hierarchical structure behaves as an abnormal p-type semiconductor and exhibits unique gas-sensing performances including excellent sensitivity and excellent response characteristics towards NO2 gas. It is found that the sensors based on hierarchical WO3 responses to NO2 gas as low as 15 ppb with an ultrashort response time of short than 5 s at room temperature, highlighting the capability of the material for rapid detection of dilute NO2 at ppb level. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen dioxide (NO2) is a highly harmful atmospheric pollutant. It contributes to PM 2.5 and it is also the main source of photochemical smog and acid rain. The fast and reliable detection of toxic NO2 gas is therefore important for both environmental protection and human health. Existing studies have shown that oxide semiconductors are promising materials for facile and reliable detection of various toxic and hazardous gases. In particular, tungsten oxide (WO3), a wide band-gap n-type semiconductor, has exhibited remarkable sensing performance to NO2 gas [1,2]. Semiconductor oxide-based gas sensors rely on the modulation of electrical conductivity due to surface oxidation or reduction caused by gas exposure. The sensing response of a solid-state sensor therefore heavily dependents on the surface structure and morphology of its sensing material used [3,4]. Towards this end, various low-dimensional tungsten oxide nanostructures with high surface-to-volume ratio have been widely examined, including nanoparticles [5], nanowires/nanorods [1,2,6], and nanotubes [7]. They achieve much high sensitivity to NO2 gas and gas sensitivity ⇑ Corresponding author. Address: School of Electronics and Information Engineering, Tianjin University, 92 Weijin Road, Nankai District, Tianjin, China. Tel.: +86 22 27402372; fax: +86 22 27401233. E-mail address: [email protected] (Y. Qin). http://dx.doi.org/10.1016/j.jallcom.2014.07.080 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
increases rapidly when the dimensions of the oxide nanostructure become comparable with or smaller than Debye length [8]. However, nano-scale materials easily aggregate to form large and dense secondary aggregates such as much larger particles or relative thicker bundles due to strong and inevitable van der Waals attraction. Such aggregation leads to considerable decrease in active surface area for gas adsorption and difficult diffusion of gas molecules in sensing films. The agglomeration becomes more serious when the sensor operates at high temperature. To date, achieving both high sensitivity and rapid response remains challenging for the gas sensors based on low-dimensional semiconductor oxides. In this aspect, how to keep structure stability of the low-dimensional oxide materials is primarily crucial. Hierarchical structure is a kind of high dimensional micro-/nano-structure composed of many low-dimensional, nanostructured building blocks (particles, rods, wires, or sheets). Hierarchical structure shows a well-developed porous texture as well as a non- or less-agglomerated feature without scarifying high surface area from the nano-constituents; it is therefore very attractive for efficient gas sensor with high sensing performance [9]. Sensing materials using well-designed hierarchical structures are expected to achieve both high gas response and fast response speed. For instance, SnO2 hierarchical microspheres self-assembled from nanosheets show both the ultra-
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fast response (1 s) and high sensitivity to 50 ppm C2H5OH at operating temperature of 400 °C [10]. The sensor based on hierarchical ZnSnO3 hollow microspheres from ultra-thin nanorods presents very short response and recovery times of 0.9 and 2.2 s respectively, to ethanol vapor at 270 °C [11]. High sensitivity and fast response to ethanol are also observed at room temperature and 250 °C from hierarchical vanadium pentoxide with radially oriented ultrathin nanoneedles and nanoribbons as constituents in our group [12]. The excellent response properties exhibited by above hierarchical structures are all because of the good surface accessibility and the high surface area of ultrathin building blocks. As to tungsten oxide, the synthesis and applications of its hierarchical structure in gas sensor has not been well studied. Up to now, successful assembly of hierarchical WO3 microspheres from nanorods or nanosheets has been realized [13,14], and the nanosheets-constructed WO3 hierarchical spheres are found to show enhanced sensitivity to ppm-level NO2. In this work, a novel WO3 hierarchical structure is hydrothermally constructed through double-sided inductive growth of WO3 nanowire (nanorod) arrays on preformed nanosheet. The constituents of the highly ordered arrays of ultrathin nanowire (nanorod) provide favorable microstructure for gas adsorption and rapid gas diffusion. It is found that the developed hierarchical WO3-based sensors are capable of dilute NO2 detection at ppb level (15 ppb) with superfast response about 1–5 s at room temperature. Meanwhile, good selectivity and stability are also achieved.
2. Experiments 2.1. Synthesis and characterization of WO3 hierarchical structure WO3 hierarchical structure films were in-situ prepared on cleaned alumina substrate with a pair of interdigitated Pt electrodes in 100 nm thickness via a hydrothermal method. The electrodes were deposited using RF magnetron sputtering process. For the hydrothermal growth of the hierarchical WO3, a thin WO3 induction layer was firstly preformed on the electrodes-attached substrate by spin coating of precursor solution, followed by thermal annealing. The precursor solution was prepared as follows: 2.5 g sodium tungstate hydrate (Na2WO42H2O) and 1 g potassium chloride (KCl) were dissolved in 15 ml de-ionized water under magnetically stirring. Then 3 ml HCl solution (37 wt%) was added dropwise into the above solution to form a milky suspension. After heating the suspension to 40 °C, 2 ml H2O2 was then added under vigorously stirring to obtain the yellow precursor solution. To prepare the WO3 induction layer, a cycled spin coating procedure was then performed. More specifically, the above as-prepared precursor solution was spincoated onto the cleaned electrodes-attached alumina substrate, and subsequently, the wet substrate was baked for 10 min at 80 °C in a drying oven. During spin coating, a physical mask was used to avoid the presence of the solution at the end of the electrodes. The above spin-coating and baking procedures were repeated for four times to ensure a uniform distribution and adequate coverage of the WO3 particles on the substrate. Finally, the obtained substrate was annealed at 550 °C in ambient atmosphere for 1–3 h to transform the precursor into nanosheet-like WO3 particles to induce the subsequent growth of one-dimensional (1D) WO3 arrays and to guarantee adhesion between the induction layer and the substrate. In the following hydrothermal step, the well-aligned 1D WO3 nanowires (nanorods) were grew from both sides of the tungsten oxide nanosheets simultaneously to form hierarchical structure film on the induction layer-coated substrates. In a typical synthesis procedure, 6 g Na2WO42H2O, 2 g KCl, and 0.4 g P123 surfactant were first co-dissolved in 160 ml deionized water under magnetically stirring. Then the PH value of the mixture solution was adjusted to 2.1–2.5 with a proper volume of HCl (37 wt%). This solution was further aged for about 15 min at room temperature before performing hydrothermal process. After submerging and sealing the induction layer-coated substrates in above resultant solution in a 200 ml Teflonline autoclave, the hydrothermal reaction was conducted at 180 °C for 9 h in an electric oven. After that, the autoclave was cooled naturally to room temperature. The substrates with products were washed several times with deionized water and ethanol, respectively. Finally, the substrates were dried at 80 °C for 5 h at ambient atmosphere. The resultant samples were directly used as sensors to carry out the sensing performance evaluation. The morphology and crystalline structures of hierarchical WO3 were characterized using field emission scanning electron microscope (FESEM, Hitachi S-4800), Xray diffractometer (XRD, RIGAKU D/MAX 2500 V/PC, Cu Ka radiation) and field emission transmission electron microscope (FETEM, TECNAI G2F-20).
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2.2. Evaluation of gas-sensing properties The gas-sensing properties of as-prepared WO3 hierarchical structure were evaluated at room temperature in a home-built static gas-sensing characterization system consisting of a glass test chamber (30 L), a flat heating plate, and two sets of data acquisition systems [15], which permits two samples measured simultaneously. This test equipment is equipped in a humidity-controlled testing room. Appropriate volume of pure NO2 gas was injected into the test chamber directly to get the desired concentration. The resistance change of the sensor during the whole measurement was continuously monitored by an UNI-T UT61E professional digital multimeter with the function of automatic measuring range adjustment. The sampling interval was set to 1 s, and the acquired resistance data were stored in a PC for further analysis. After hydrothermal reaction and subsequent washing, the tungsten oxide film was found to grow only on the area precoated with tungsten oxide induction layer. The bare section of Pt electrodes therefore can be used to realize the electrical connection between the sensor and the digital multimeter. To minimize the effect of humidity fluctuation on the gas-sensing properties, the whole measurement was carried continuously. During the measurement, the ambient relative humidity is about 30–35% and the room temperature is about 20 °C.
3. Results and discussion 3.1. Structure and characterization In this work, we employ a hydrothermal process to directly synthesize three-dimensional (3D) hierarchical tungsten oxide film on sensor substrate with 1D nanowires or nanorods as building blocks. The typical morphological characteristic of the assynthesized tungsten oxide hierarchical structure from 1 hannealed induction layer after hydrothemal reaction of 9 h at 180 °C is illustrated by FESEM observations with different magnifications in Fig. 1. Observe that the well-oriented 1D oxide nanowire arrays grow from both sides of the central nanosheets simultaneously, constructing a kind of 3D hierarchical structure film. Fig. 1(b) and (c) shows the high magnification FESEM images of an individual tungsten oxide hierarchical structure. Clearly, the central nanosheet acts as the architectural core of the whole hierarchical structure, and the constituents of nanowires align perpendicular to its both sides. It can also be observed that each nanowire shows a very small diameter of about 20 nm and the central nanosheet has thickness of about 10 nm. This ultrathin feature of the nanowires and nanosheet is much favorable for achieving high sensitivity when exposure to a detected gas. On the other hand, the ultrathin nanowires trend to assemble together near their tips forming bundles due to much high surface energy [16]. This would affect the alignment of the nanowire arrays in some degree. The growth process of tungsten oxide hierarchical structure can be effectively illustrated by intercepting intermediate product with much shorter growth time. Fig. 2(a) and (b) gives the low magnification and high magnification FESEM images of the product after hydrothermal reaction of 4 h at 180 °C. The inset in Fig. 2(a) shows the SEM image of the tungsten oxide induction layer formed on the substrate after annealing at 550 °C for 1 h, which is composed of stacked ultrathin nanosheets with thickness in 20–40 nm. The maximum size of these nanosheets is less than 2 lm. After hydrothermal reaction of 4 h, many short and ultrathin nanorods grow vertically from both surfaces of the nanosheets in induction layer to transform into an integrated hierarchical structure. Further TEM characterization in Fig. 2(c) gives us a clearer vision that the double-sided nanorods have an average diameter of about 10 nm and meanwhile exhibit high orientation. Extending the reaction time to 9 h results in the bilateral nanowire arrays with longer length and larger diameter (Fig. 1). When the reaction time is shorter than 2 h, no objective nanorods are observed on the surface of nanosheets. The above results unambiguously demonstrate the induction of the preformed nanosheets in constructing tungsten oxide hierarchical structures. They serve as the architectural rachises of tungsten oxide hierarchical structures to support the bilateral
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(a)
2 µm
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300 nm
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100 nm Fig. 1. FESEM images with different magnifications for the tungsten oxide hierarchical structure synthesized after hydrothermal reaction of 9 h at 180 °C.
assembly of aligned nanowire arrays. Fig. 2(d) gives the high magnification SEM image of the nanosheets surface before the hydrothermal process. Observe that each sheet is composed of numerous small nanoparticles with estimated diameter of 10–30 nm. These densely arranged nanoparticles are expected to provide multiple initial nucleation sites to inspire the growth of 1D tungsten oxides. During the hydrothermal process, the tungsten oxide nuclei formed in the hydrothermal solution might preferentially aggregate on these sites, and then the double-sided inductive growth of vertically aligned tungsten oxide nanorods is realized. Meanwhile, the aggregates of these particles, i.e. nanosheets, serve as the architectural center to guide the subsequent self-organized formation of the 3D hierarchical structures. After reaction of 9 h, the thickness of the nanosheets decreases and the size of the double-sided nanowires increases. This seems to be related to a dissolution–recrystallization process followed by Ostwald ripening under hydrothermal environment [11]. The requirement for the inductive nanosheets in constructing hierarchical WO3 was further demonstrated by observing the morphology of the powder centrifuged from the hydrothermal solution. Only unordered nanowire bundles network is obtained, as shown in Fig. 2(e). Note that, the precursor solution and the particle shape of the annealed induction layer are crucial for the formation of this kind of tungsten oxide hierarchical structure. When
WCl6 is employed as precursor to prepare tungsten oxide induction layer, the annealed film exhibits stacked spheres. We note that the subsequent hydrothermal step cannot result in the hierarchical structure presented above. Under this circumstance, only unordered nanowires network is observed instead. Fig. 3(a) shows the morphology of tungsten oxide hierarchical structure film developed from an induction layer with much longer annealing of 3 h. The hydrothermal reaction time remains 9 h. The inset in Fig. 3(a) shows the SEM image of the 3 h-annealed induction layer, which exhibits a morphological characteristic of tungsten oxide particles with much large size and low stacking density due to the possible coalescence of adjacent thin nanosheets during the deep annealing [17]. As the result, the developed tungsten oxide hierarchical structures also show much large size, shown in Fig. 3(a). The high magnification FESEM image in Fig. 3(b) clearly demonstrates the detailed structure of the formed hierarchical oxide, which is constructed from abundant dense nanorods with inconsistent length but well-aligned perpendicular to the double sides of the central nanosheet. In contrast to the sample developed from 1 h-annealed nanosheets in Fig. 1, the bilateral nanorod arrays in Fig. 3(b) show a more ordered morphology. However, the constituents of nanorods and central nanosheets also show much larger sizes in diameter and thickness. The crystalline structures of the tungsten oxide induction layer and the as-developed hierarchical structure films were investigated by XRD technique. Fig. 4(a–c) shows the XRD patterns of the three tungsten oxide powders which were scraped from the substrates to eliminate the diffraction effect of substrate materials. As shown in Fig. 4(a), the coated Na2WO4 precursor was converted to orthorhombic WO3 after thermal annealing. The relative poor intensity of diffraction peaks indicates the low crystalline degree of the WO3 induction layer. This result is consistent with the FESEM observation to the nanosheets presented in Fig. 2(d), i.e., each nanosheet is composed of numerous small grain particles. Fig. 4(b) and (c) respectively show the XRD patterns of the tungsten oxide hierarchical structures grown from the 1 h- and 3 hannealed induction layers. Observe that all typical diffraction peaks of the two samples are well indexed to the profile of hexagonal WO3 (h-WO3) (JCPDS 33-1387). The strong and sharp diffraction peaks indicate the high crystallinity of the as-synthesized hierarchical WO3 with hexagonal structure. The strongest peak of plane (0 0 1) for the two hierarchical samples implies that the main constituents of nanowires (nanorods) grow preferentially along the same c-axis direction. It is well known that h-WO3 is a metastable phase and it can transform into monoclinic WO3 at appropriate temperature. Szilágyi et al. [18] pointed out that the structure of h-WO3 cannot be maintained without some stabilizing ions or molecules in the hexagonal channels. It has also been found that alkaline (Na+, K+, Cs+, etc.) or NH+4 ions can enter the hexagonal channels of crystallites and effectively block the thermodynamically favored hexagonal–monoclinic transformation [19]. In this work, the harvested stable hexagonal crystalline phase of WO3, confirmed by above XRD characterization, is thought to be directly related to the introduced KCl in precursor solution. The inset in Fig. 4 shows the energy dispersive X-ray spectroscopy (EDS) of the as-synthesized hierarchical tungsten oxide in Fig. 1. It illustrates clearly that the constituents of the hierarchical structures are dominated by W and O elements, with a small amount of K element that is believed from the additive of KCl. It is assumed that the introduced K+ plays a crucial role in stabilizing the hexagonal structure of h-WO3. Meanwhile, K+ in the crystal of WO3 is expected to act as dopants to improve the sensitivity of the oxide sensor at low temperature effectively [20]. During hydrothermal process, Cl ions from the additive of KCl may serve as capping agents to adhere to the side crystal faces (parallel to c-axis) of the formed WO3 nuclei [21,22],
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(b)
(a)
1 µm
2 µm
(c)
100 nm
(d)
(e)
200 nm
50 nm
300 nm
Fig. 2. (a) Low magnification and (b) high magnification FESEM images of the tungsten oxide obtained after reaction of 4 h at 180 °C. The inset in (a) shows the morphology of the induction layer on the substrate formed after annealing at 550 °C for 1 h. (c) TEM image of the sample shown in (b). (d) High magnification FESEM image of the nanosheets from 1 h-annealing induction layer. (e) Morphology of the pure tungsten oxide nanowires network formed in the solution.
(a)
(b)
5 µm
5 µm
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Fig. 3. (a and b) FESEM image and the corresponding high magnification image of the tungsten oxide hierarchical structure developed on the 3 h-annealed induction layer after hydrothermal reaction of 9 h. The inset in (a) shows the morphology of the induction layer formed after annealing at 550 °C for 3 h.
inducing tungsten oxide preferentially growing along [0 0 1] direction and suppressing the growth on other directions. The structural features of the hierarchical tungsten oxide are further studied using HRTEM. Fig. 5 shows the HRTEM image of two nanorods from the hierarchical tungsten oxide developed from 1 h-annealed induction layer. The image exhibits clear lattice fringe with a distance of about 0.39 nm, corresponding to the interspace of (0 0 1) plane of the hexagonal structured WO3 (d001 = 0.3899 nm, JCPDS 33-1387) and then confirming the preferred growth along c-axis. 3.2. Gas-sensing properties The gas-sensing properties were evaluated by measuring the changes of resistance of the sensors before and after the detected gas was introduced. The measurements were taken at room
temperature (20 °C). Fig. 6(a and b) indicates the dynamic responses of the sensors based on the hierarchical WO3 films developed from 1 h- and 3 h-annealed induction layers to varying NO2 gas concentrations at room temperature. It shown that the asdeveloped hierarchical WO3 film is responsive to very small NO2 amounts (15 ppb) at room temperature. This result indicates that this type of WO3 sensor is indeed capable of NO2 detection at ppb level even under room temperature, which remains a big challenge for conventional sensing materials. It can also be observed that, when exposure to different concentrations of NO2 gas, the measured resistances of the sensor based on hierarchical WO3 decrease. In general, tungsten oxide is an n-type semiconductor. It is well known that, for n-type semiconductor oxides, an oxidizing gas such as NO2, chemisorbs on the surface and interacts with the surface to extract electrons from the conduction band of the semiconductor, leading to the increase of resistance. For a p-type
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Fig. 4. XRD patterns of the 1 h-annealed induction layer (a), tungsten oxide hierarchical structures developed from the 1 h-(b) and 3 h-(c) annealed induction layers. The hydrothermal reaction time is 9 h. The inset is the EDS of the hierarchical tungsten oxide shown in Fig. 1.
10 nm
0.39 nm
[001]
Fig. 5. HRTEM image of two nanorods from the hierarchical tungsten oxide developed from 1 h-annealed induction layer.
semiconductor, the resistance decreases because of the increasing holes concentration induced by electron-extracting. Therefore, the described hierarchical WO3-based sensors show typical p-type semiconductor behavior at room temperature. In fact, this kind of abnormal conductive behavior has been reported for gas sensor based on WO2.72 nanorod [23], WO3 nanoparticles [24], functionalized WO3 nanoneedles [25], and other metal oxide semiconductors exposed to various gases [26,27]. Similar behavior has also been observed on the flower-like V2O5 hierarchical structure by our group recently [12]. Until now, it is still not clear for the real origin of p–n transition. Several explanations which can be pursued in the literatures include different reactions occurring under different conditions [24], oxygen adsorption-induced inversion layer [25,26], and different motilities of carriers [28]. Based on the surface-controlled gas-sensing mechanism of tungsten oxide, it is assumed that the abnormal p-type response characteristic exhibited by the n-type WO3 with hierarchical structure might be related to the surface
of the oxide. That is, the anomalous p-type conductive behavior seems to be able to be explained by the formation of an inversion layer at the surface of n-type hierarchical WO3. It was reported that higher surface-to-volume ratio for low-dimensional nanomaterials favors a larger quantity of surface oxygen vacancy [29]. Moreover, the foreign ions in WO3 crystal can introduce additional surface states, as evidence of a band of surface states energetically appearing inside the semiconductor band gap [30]. Thus the surface of the hierarchical WO3 constructed with 1D ultrathin nanowires or nanorods is much active to motivate strong absorption of oxygen even at low temperature due to the large density of surface states. The strong oxygen adsorption significantly increases the depletion region inside the oxide surface and upward band bending, causing an inversion layer (the Fermi level EF lies below the intrinsic level Ei) instead of a depletion layer created by normal surface adsorption, as shown in Fig. 7. In the surface inversion layer, holes are indeed the major charge carriers, which indicate the p-type feature. The strong ability of surface adsorption of the hierarchical structure induced by its ultrathin constituents and additive of K+ also explains its remarkably sensitive performance exhibiting at room temperature. It is also worthy noting from Fig. 6 that the sensor resistances could recover to its initial value after gas removal, exhibiting very good dynamic response–recovery characteristics at room temperature. Even after four response–recovery cycles to different NO2 concentrations (0.015–5 ppm), there is almost no obvious change between recovered resistance and baseline resistance, indicating excellent reversibility and stability of the sensor. It seems that the perfect stability and reversibility can be ascribed greatly not only to the good interface performance between the electrodes and the oxide films caused by the direct bottom-up growth of WO3 from the substrate [31], but also to the high autologous structure stability of the hierarchical oxide originated from its unique 3D architecture. In particular, the characteristic architecture of double-sided assembly of well-aligned nanowire (nanorod) arrays is thought to provide a distinct structure promotion in achieving fast sensor response. It is observed from Fig. 6 that, at room temperature, the sensor resistances show a rapid change upon exposure to NO2 gas with concentrations from ppb to ppm level, indicating an ultrafast response characteristic for this kind of hierarchical WO3-based sensors. The right images in Fig. 6(a) and (b) respectively show the enlarged response curves of both hierarchical WO3-based sensors to 1 ppm NO2 gas. The response times (tres, the time required for the sensor resistance to reach 90% of the equilibrium value after the test gas is injected) are 4 s and 3 s respectively for the sensors developed from 1 h- and 3 h-annealed induction nanosheets. Fig. 8 gives the response time values of the two hierarchical WO3 sensors to different NO2 concentrations at room temperature. It can be seen that the response times of both sensors are no more than 5 s upon exposure to 0.015–5 ppm NO2, exhibiting a unique ultrafast response characteristic. Especially, the immediate response is achieved with observable response time as short as 1 s (the sampling interval in our measurement) for the WO3 hierarchical structure developed from 3 h-annealed induction layer. Above ultrafast response characteristic could hardly be observed from single 1D nanostructured oxidebased sensors. For the sensors based on WO3 nanorod, nanowire or Au functionalized nanowires, the reported response time is usually above one hundred seconds to 0.5–2 ppm NO2 at their optical working temperature of 150–200 °C [32–34]. Thus a comparison of the response performance further highlights the advantage of 3D hierarchical structure describe here in achieving fast response. It is thought that the efficient utilization of the surface area and surface accessibility are crucial to maintain the high sensitivity and fast response characteristic of a gas sensor [9]. At a given temper-
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Fig. 6. Dynamic responses of the hierarchical WO3 films-based sensors developed from 1 h-(a) and 3 h-(b) annealed induction layers to varying NO2 gas concentrations at room temperature. The right figures show the corresponding enlarged response time to 1 ppm NO2 gas.
(a)
WO3 nanowires
(b) O2(ads)
O2(ads)
EC EF
O2-(ads)
Ei
O2(ads)
EV
O-+O2-(ads)
Depletion layer n-type semiconductor
Depletion layer n-type semiconductor
O-+O2-(ads)
Inversion layer n-type semiconductor
Fig. 7. Schematic energy-level diagrams of 1D WO3 with normal (a) and strong (b) surface adsorption of oxygen. Normal oxygen adsorption leads to a depletion layer. Strong surface adsorption of oxygen increases the upward band bending and hence causes an inversion layer (the Fermi level EF lies below the intrinsic level Ei) with a p-type surface conductivity.
ature, gas diffusion can be considered a primary factor that determines the gas response kinetics [35]. Thus, the excellent response capability of the hierarchical WO3 sensor at room temperature could be understood under the framework of gas diffusion towards the oxide surface. For the WO3 hierarchical structure developed here, the ultrathin feature of the 1D nanowires and nanorods constituents assures the large active area interacting with the gas molecules. Meanwhile, the well-aligned arrays of nanowires (nanorods) make NO2 molecules diffuse agilely in the sensing layer. These features are expected and assumed to result in the ultrafast response as well as the high response value for the hierarchical WO3 sensors. The relatively faster response rate for the hierarchical WO3 developed from 3 h-annealed nanosheets, as shown in Fig. 8, is thought to be directly related to the more
ordered microstructure of the thicker nanorod arrays beneficial to much easier gas diffusion. However, the much thicker constituents of nanorods and central nanosheets apparently decrease the specific surface area of the hierarchical structure, hence degrade the response value of the sensor in some degree. The response values of the hierarchical WO3 sensor developed from 1 h-annealed nanosheets to 0.015–5 ppm NO2 range from 1.6 to 59.9, while the values for that induced by 3 h-annealed nanosheets are 1.3– 17.0. Here, the sensor response is defined as Ra/Rg, where Rg and Ra are the resistances of the sensitive film in the measuring gas and that in clean air, respectively. It should be noted that, in our experiments, two sensor samples of each type were fabricated under similar process conditions, and the sensing results for these two samples are reproducible within an accuracy of ±5%. The
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Fig. 8. Response times of the hierarchical WO3 structures developed from the 1 hand 3 h-annealed seed layers to different NO2 concentrations at room temperature.
on the substrate. The as-developed hierarchical WO3 shows very effective microstructure for achieving perfect gas-sensing performances. Large active surface area of the ultrathin constituents of nanowires (nanorods) and well-aligned structure of microarrays assure high performances in gas adsorption and diffusion. As a result, the hierarchical WO3-based sensors exhibit high sensitivity, ultrafast response, good stability and selectivity towards NO2 gas at room temperature. Especially, the sensor based on the WO3 hierarchical structure is capable of NO2 detection at ppb level with ultrafast response characteristic at room temperature. The measured response time values to 0.015–5 ppm NO2 are shorter than 5 s. The WO3 hierarchical structures are also observed to show an anomalous p-type conductive behavior at room temperature. This behavior is supposed to be due to the strong surface adsorption ability of the ultrathin structural constituents and then be related to its high sensitivity. The presented results demonstrate that the well-developed WO3 hierarchical structure in this work is very promising in NO2 sensors with low power consumption, high sensitivity and rapid response characteristics.
Acknowledgements This work was financially supported by the National Natural Science Foundation (Nos. 61274074 and 61271070) and Tianjin Natural Science Foundation (No. 11JCZDJC15300) of China.
References
Fig. 9. Responses of the sensor based on hierarchical WO3 developed from 1 hannealed nanosheets to different gases at room temperature.
results from two continual tests to the same sensor sample show a much lower fluctuation with an accuracy of ±3%. After retaining the hierarchical WO3 sensor developed from 1 h-annealed nanosheets in the atmosphere for 10 days (240 h), its response value to 5 ppm NO2 decreased from 59.9 to 53.2. To evaluate the selectivity of the hierarchical WO3 sensor, the gas responses of the sensor developed from 1 h-annealed nanosheets to other five reductive gases (NH3, acetone, ethanol, 1-propanol, isopropanol) were also examined and compared with that to 5 ppm NO2. Fig. 9 shows the sensor responses measured at room temperature. The gas concentrations for NO2 and for each reductive gas are 5 ppm and 50 ppm respectively, and the sensor response is calculated by Rg/Ra or Ra/Rg to ensure the response values are always not smaller than 1. The results show that the hierarchical WO3 sensor is insensitive to NH3 and other four reductive organic gases with evidence of very low response values of 1.04– 1.24, which are significantly lower than that to NO2 (59.9). The large discrepancy in response value is ascribed to the interaction discrepancy between the sensing surface and different sorts of gas molecules [23]. It confirms that the hierarchical WO3 sensor presented here has a satisfying selectivity to NO2 against other testing gases at room temperature.
4. Conclusions Gas sensor based on hierarchical WO3 nanostructures were formed by an inductive hydrothermal assembly of ordered nanowire or nanorod arrays on both sides of nanosheets preformed
[1] B. Cao, J. Chen, X. Tang, W. Zhou, Growth of monoclinic WO3 nanowire array for highly sensitive NO2 detection, J. Mater. Chem. 19 (2009) 2323–2327. [2] Y.X. Qin, X. Li, F. Wang, M. Hu, Solvothermally synthesized tungsten oxide nanowires/nanorods for NO2 gas sensor applications, J. Alloys Comp. 509 (2011) 8401–8406. [3] Y.B. Shen, T. Yamazaki, Z.F. Liu, D. Meng, T. Kikuta, N. Nakatani, Influence of effective surface area on gas sensing properties of WO3 sputtered thin films, Thin Solid Films 517 (2009) 2069–2072. [4] A. Ponzoni, E. Comini, G. Sberveglieri, J. Zhou, S.Z. Deng, N.S. Xu, Y. Ding, Z.L. Wang, Ultrasensitive and highly selective gas sensors using three-dimensional tungsten oxide nanowire networks, Appl. Phys. Lett. 88 (2006) 203101– 203103. [5] F. Fang, J. Kennedy, J. Futter, T. Hopf, A.M. arkwitz, E. Manikandan, G. Henshaw, Size-controlled synthesis and gas sensing application of tungsten oxide nanostructures produced by arc discharge, Nanotechnology 22 (2011) 335702. [6] M.E. Achhab, H. Shanak, K. Schierbaum, NO2 sensing properties of WO3 nanorods grown on mica, Phys. Status Solid A 208 (2011) 1229–1234. [7] R.A. Gerlitz, K.D. Benkstein, D.L. Lahr, J.L. Hertz, C.B. Montgomery, J.E. Bonevich, S. Semancik, M.J. Tarlov, Fabrication and gas sensing performance of parallel assemblies of metal oxide nanotubes supported by porous aluminum oxide membranes, Sens. Actuators B 136 (2009) 257–264. [8] Z.W. Pan, Z.R. Dai, Z.L. Wang, Nanobelts of semiconducting oxides, Science 291 (2001) 1947–1949. [9] J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators B 140 (2009) 319–336. [10] H.R. Kim, K.I. Choi, J.H. Lee, S.A. Akbar, Highly sensitive and ultra-fast responding gas sensors using self-assembled hierarchical SnO2 spheres, Sens. Actuators B 136 (2009) 138–143. [11] Y. Bing, Y. Zeng, C. Liu, L. Qiao, Y. Sui, B. Zou, W. Zheng, G. Zou, Assembly of hierarchical ZnSnO3 hollow microspheres from ultra-thin nanorods and the enhanced ethanol-sensing performances, Sens. Actuators B 190 (2014) 370– 377. [12] Y. Qin, G.T. Fan, K. Liu, M. Hu, Vanadium pentoxide hierarchical structure networks for high performance ethanol gas sensor with dual working temperature characteristic, Sens. Actuators B (2013) 141–148. [13] Z. Gu, T. Zhai, B. Gao, X. Sheng, Y. Wang, H. Fu, Y. Ma, J. Yao, Controllable assembly of WO3 nanorods/nanowires into hierarchical nanostructures, J. Phys. Chem. B 110 (2006) 23829–23836. [14] S. Bai, K. Zhang, R. Luo, D. Li, A. Chen, C.C. Liu, Sonochemical synthesis of hierarchically assembled tungsten oxides with excellent NO2-sensing properties, Mater. Lett. 11 (2013) 32–34. [15] Y.X. Qin, M. Hu, J. Zhang, Microstructure characterization and NO2-sensing properties of tungsten oxide nanostructures, Sens. Actuators B 150 (2010) 339–345. [16] R.F. Mo, G.Q. Jin, X.Y. Guo, Morphology evolution of tungsten trioxide nanorods prepared by an additive-free hydrothermal route, Mater. Lett. 61 (2007) 3787– 3790.
Y. Qin et al. / Journal of Alloys and Compounds 615 (2014) 616–623 [17] S.B. Sun, Y.M. Zhao, Y.D. Xia, Z.D. Zou, G.H. Min, Y.Q. Zhu, Bundled tungsten oxide nanowires under thermal processing, Nanotechnology 19 (2008) 305709. [18] I.M. Szilágyi, J. Madarász, G. Pokol, P. Király, G. Tárkányi, S. Saukko, J. Mizsei, A.L. Tóth, A. Szabó, Varga-Josepovits, Stability and controlled composition of hexagonal WO3, Chem. Mater. 20 (2008) 4116–4125. [19] X.P. Shen, G.X. Wang, D. Wexlera, Large-scale synthesis and gas sensing application of vertically aligned and double-sided tungsten oxide nanorod arrays, Sens. Actuators B 143 (2009) 325–332. [20] S. Bai, D. Li, D. Han, R. Luo, A. Chen, C. Chung, Preparation, characterization of WO3–SnO2 nanocomposites and their sensing properties for NO2, Sens. Actuators B 150 (2010) 749–755. [21] J. Su, X. Feng, J.D. Sloppy, L. Guo, C.A. Grimes, Vertically aligned WO3 nanowire arrays grown directly on transparent conducting oxide coated glass: synthesis and photoelectrochemical properties, Nano Lett. 11 (2011) 203–208. [22] J. Wang, E. Khoo, P.S. Lee, J. Ma, Controlled synthesis of WO3 nanorods and their electrochromic properties in H2SO4 electrolyte, J. Phys. Chem. C 113 (2009) 9655–9658. [23] Y.S. Kim, S.C. Ha, K. Kim, H. Yang, J.T. Park, C.H. Lee, J. Choi, J. Paek, K. Lee, Room temperature semiconductor gas sensor based on nonstoichiometric tungsten oxide nanorod film, Appl. Phys. Lett. 86 (2005) 213105. [24] C. Zhang, M. Debliquy, A. Boudiba, H. Liao, C. Coddet, Sensing properties of atmospheric plasma-sprayed WO3 coating for sub-ppm NO2 detection, Sens. Actuators B 144 (2010) 280–288. [25] S. Vallejos, T. Stoycheva, P. Umek, C. Navio, R. Snyders, C. Bittencourt, E. Llobet, C. Blackman, S. Moniz, X. Correig, Au nanoparticle-functionalised WO3 nanoneedles and their application in high sensitivity gas sensor devices, Chem. Commun. 47 (2011) 565–567. [26] T. Siciliano, A. Tepore, G. Micocci, A. Genga, M. Siciliano, E. Filippo, Transition from n- to p-type electrical conductivity induced by ethanol adsorption on atellurium dioxide nanowires, Sens. Actuators B 138 (2009) 207–213.
623
[27] A. Gurlo, N. Barsan, A. Oprea, M. Sahm, T. Sahm, U. Weimar, An n- to p-type conductivity transition induced by oxygen adsorption on a-Fe2O3, Appl. Phys. Lett. 85 (2004) 2280–2282. [28] Y.S. Kim, I.S. Hwang, S.J. Kim, C.Y. Lee, J.H. Lee, CuO nanowire gas sensors for air quality control in automotive cabin, Sens. Actuators B 135 (2008) 298–303. [29] M.H. Huang, Y.Y. Wu, H. Feick, N. Tran, E. Weber, P.D. Yang, Catalytic growth of zinc oxide nanowires by vapor transport, Adv. Mater. 13 (2001) 113–116. [30] A. Cabot, A. Diéguez, A. Romano-Rodriguez, J.R. Morante, N. Bârsan, Influence of the catalytic introduction procedure on the nano-SnO2 gas sensor performances-where and how stay the catalytic atoms?, Sens Actuators B 79 (2001) 98–106. [31] Y.J. Choi, I.S. Hwang, J.G. Park, K.J. Choi, J.H. Park, J.H. Lee, Novel fabrication of an SnO2 nanowire gas sensor with high sensitivity, Nanotechnology 19 (2008) 095508. [32] A. Ponzonia, V. Russo, A. Bailini, C.S. Casari, M. Ferroni, A. Li Bassi, A. Migliori, V. Morandi, L. Ortolani, G. Sberveglieri, C.E. Bottani, Structural and gas-sensing characterization of tungsten oxide nanorods and nanoparticles, Sens. Actuators B 153 (2011) 340–346. [33] T. Stoycheva, S. Vallejos, C. Blackman, S.J.A. Moniz, J. Calderer, X. Correig, Important considerations for effective gas sensors based on metal oxide nanoneedles films, Sens. Actuators B 161 (2012) 406–413. [34] N.V. Hieu, H.V. Vuong, N.V. Duy, N.D. Hoa, A morphological control of tungsten oxide nanowires by thermal evaporation method for sub-ppm NO2 gas sensor application, Sens. Actuators B 171–172 (2012) 760–768. [35] E. Rossinyol, A. Prim, E. Pellicer, J. Arbiol, F. Hernández-Ramírez, F. Peiró, A. Cornet, J.R. Morante, L.A. Solovyov, B. Tian, T. Bo, D. Zhao, Synthesis and characterization of chromium-doped mesoporous tungsten oxide for gassensing applications, Adv. Funct. Mater. 17 (2007) 1801–1806.