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Author's personal copy Sensors and Actuators B 174 (2012) 594–601
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Design of SnO2 /ZnO hierarchical nanostructures for enhanced ethanol gas-sensing performance Nguyen Duc Khoang, Do Dang Trung, Nguyen Van Duy, Nguyen Duc Hoa, Nguyen Van Hieu ∗ International Training Institute for Materials Science, Hanoi University of Science and Technology, Hanoi, Viet Nam
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Article history: Received 5 May 2012 Received in revised form 27 June 2012 Accepted 30 July 2012 Available online 8 August 2012 Keywords: SnO2 /ZnO hierarchical Ethanol sensors Nanowires
a b s t r a c t Designing nanostructured materials to enhance gas-sensing performance is of important key for the nextgeneration sensor platforms. In this paper, a design of hierarchical SnO2 /ZnO nanostructures for scalable fabrication of high-performance ethanol sensors is developed based on a combination of two simple synthesis pathways. High-quality single crystalline SnO2 nanowire (NW) backbones were first synthesized using the thermal evaporation method, whereas ZnO nanorod (NR) branches were subsequently grown perpendicularly to the axis of SnO2 NWs via the hydrothermal approach. The successful synthesis of SnO2 /ZnO hierarchical nanostructures is confirmed by the results of scanning electron microscope, X-ray diffraction and photoluminescence spectrum. The ethanol-sensing properties of the SnO2 /ZnO hierarchical nanostructures sensors were systematically investigated and compared to those of the bare SnO2 NWs sensor. The effect of growth manipulation of the SnO2 /ZnO hierarchical nanostructures on the ethanol sensing characteristics was also studied. The results revealed that the design of the hierarchical nanostructures enhanced the ethanol gas response and selectivity for interfering gases such as NH3 , CO, H2 , CO2 , and LPG. These enhancements are attributed to the enhancement of homogenous and heterogeneous NW–NW contacts. In addition, the results of this study may serve as a basis for designing various novel hierarchical nanostructures for other applications, including photocatalysis, battery electrode, solar cell, and nanosensors. © 2012 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional (1D) semiconductor metal oxide (SMO) nanostructures have attracted increasing attention in the construction of nanodevices ranging from (opt-) electronic devices to chemical sensors. Nanostructures with high aspect ratio (i.e., size confinement in two coordinates) offer better crystallinity, higher integration density, and lower power consumption [1]. In addition, they demonstrate superior sensitivity to surface chemical processes because of their large surface-to-volume ratio and small diameter comparable with Debye length (a measure of field penetration into the bulk) [1–3]. Designing mutual nanostructures based on SMO have recently merged as a promising issue for the improvement of their potential applications. In particular, hierarchical nanostructures that originated from nanowires (NWs) or nanorods (NRs) provide not only large surface area materials but also multifunctional nanomaterials. Diverse applications have been demonstrated using hierarchical nanostructure
∗ Corresponding author at: International Training Institute for Materials Science, Hanoi University of Science and Technology, No. 1, Dai Co Viet Road, Hanoi, Viet Nam. Tel.: +84 4 38680787; fax: +84 4 38692963. E-mail address:
[email protected] (N. Van Hieu). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.07.118
materials constructed from NWs and NRs, including high-efficiency dye-sensitized solar cell [4], high-performance photocatalysis [5], and gas sensors [6]. To date, numerous hierarchical nanostructures of homo- and/or heterogeneous-nanostructures have been developed, such as ZnO [4,7], WO3 [8], SnO2 [9,10], CdTe [11], Fe2 O3 SnO2 [12], ZnO TiO2 [13], ZnO In2 O3 [14], SnO2 WO3 [15], and SnO2 ZnO [16–18]. Among the semiconducting metal oxides used for constructing hierarchical nanostructures, wide band gap SnO2 (3.6 eV) and ZnO (3.37 eV) are interest because of their advanced physical and chemical properties. In recent years, their low dimensional nanostructures have been extensively investigated for novel gas-sensitive materials [19]. The gas-sensing properties of hierarchical metal oxide nanostructures have been comprehensively reviewed and reported in Ref. [6]. The gas-sensing properties of the homo-hierarchical nanostructures of SnO2 [9,10,20–23] and ZnO [7,24–27] are the most frequently mentioned topics in the literature. Little attention has been given to the design and use of heterogeneous hierarchical SnO2 /ZnO nanostructures for gas-sensing applications. ZnO and SnO2 materials have been combined with special nanostructures, such as nanocomposite thin film [28], nanocomposite nanofiber [29], and core–shell [30]. ZnO-doped SnO2 [31] have been confirmed to have outstanding ethanol gas-sensing properties. However, developing an effective route for the controllable fabrication of scalable 1D
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hierarchical SnO2 /ZnO nanostructures remains a great challenge. In addition, the combination of 1D ZnO and SnO2 nanostructures to form a heterojunction may enhance the surface-depletion effect more easily and improve the gas-sensing performance accordingly. Thus, designing SnO2 /ZnO hierarchical nanostructures from NWs or NRs is expected to result in an excellent ethanol gas-sensing performance. In the present study, we report a controllable and scalable route for preparing SnO2 /ZnO hierarchical nanostructures with SnO2 NW backbones and ZnO NR branches by combining thermal evaporation (for SnO2 NWs) and hydrothermal methods (for ZnO NRs). The comparative gas-sensing properties of the bare SnO2 NWs and SnO2 /ZnO hierarchical nanostructures are investigated to demonstrate the potential application of hierarchical nanostructures for gas-sensing applications, in which the density and length of ZnO NRs branches are adjusted for the best ethanolsensing performance. In addition, the gas-sensing mechanism of heterogeneous-hierarchical SnO2 /ZnO nanostructures is also discussed in the light of NW–NW contact enhancement. 2. Experimental 2.1. Material synthesis SnO2 /ZnO hierarchical nanostructures were prepared through thermal evaporation and hydrothermal processes (Fig. 1). In the first step, SnO2 NWs were synthesized according to our previous works [10]. In brief, SnO2 NWs were synthesized on Au-coated Si substrates through a simple thermal evaporation of Sn powders (99.9%). The source material was loaded in an alumina boat placed at the center of a quartz tube located in a horizontal-type furnace, which was heated to 800 ◦ C and kept for 30 min during the synthesis of the NWs. The pressure in the quartz tube was adjusted from Torr to 10 Torr using O2 gas with a flow rate of 0.4–0.5 sccm. The as-synthesized SnO2 NWs were coated with ZnO nanoparticles by spray-coating of 0.01 M Zn(CH3 COO)2 solution and subsequent heat-treatment at 300 ◦ C. In the second step, the SnO2 NW substrate coated with ZnO nanoparticles was immersed in an aqueous solution of Zn(NO3 )2 (0.01 M) and C6 H12 N4 (0.01 M) to allow the growth of the ZnO NR branches. The hydrothermal process was conducted at 90 ◦ C for different periods (i.e., 1, 2, and 4 h) to control the length of the NRs. After the reactions, the substrates were removed from the solution, rinsed with deionized water, and then blow dried with Ar. The as-obtained SnO2 NWs and SnO2 /ZnO hierarchical nanostructures were analyzed via field emission scanning electron microscopy (FE-SEM, 4800, Hitachi, Japan) and X-ray diffraction (XRD, Philips Xpert Pro) with Cu K␣ radiation generated at a voltage of 40 kV as source. The photoluminescence (PL) spectrum at room temperature was acquired from 360 nm to 910 nm using a 325 nm He Cd laser. 2.2. Gas sensor fabrication and characterization For gas-sensing characterization, the as-obtained SnO2 NWs and SnO2 /ZnO hierarchical nanostructures were detached from the Si substrate through dispersion in isopropanol under ultrasonic treatment and then dried at 70 ◦ C for 24 h. The as-obtained materials were mixed with organic binders and pasted on Pt-interdigitated electrodes with an area of 800 m × 1600 m. A Pt-interdigitated electrode was fabricated using a conventional photolithographic method with a finger width of 20 m and a gap size of 20 m. The interdigitated electrodes were fabricated by sputtering 10 nm Cr and 200 nm Pt on a layer of silicon dioxide (SiO2 ) with a thickness of approximately 300 nm thermally grown on top of the silicon wafer.
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The gas-sensing characteristics of the SnO2 NWs and SnO2 /ZnO sensors were measured under identical experimental conditions. The gas concentration was controlled by changing the mixing ratio of dry parent gases and dry synthetic air. A flow-through technique with a constant flow rate of 200 sccm was used, employing a previously described homemade system [10].
3. Results and discussion 3.1. Material characterization Fig. 2 shows the FE-SEM images of the pristine SnO2 NWs and hierarchical SnO2 /ZnO nanostructures. The as-synthesized SnO2 NWs had smooth surface of single crystal with an average diameter of approximately 100 nm and lengths of several tens micrometers (Fig. 2a). The growth mechanism of the SnO2 NWs in the present work was explained based on the vapor–liquid–solid mechanism [32]. More details are expounded in our previous works [33,34]. Fig. 2b–d shows the morphologies of the SnO2 /ZnO hierarchical nanostructures after 1, 2, and 4 h growth of the ZnO NR branches, respectively. After the hydrothermal growth of ZnO NRs, the ZnO NRs branched out from the smooth SnO2 NWs, forming hierarchical structures with SnO2 NWs as a backbone and ZnO NRs as branches (Fig. 2b). The average diameter of the ZnO NRs was approximately 50 nm, which was controlled by the size of the ZnO seeds. The length of ZnO NRs increased with increasing hydrothermal growth time. The average length of the ZnO NRs was approximately 150, 300, and 600 nm after 1, 2, and 4 h of growth, respectively. The growth mechanism of the ZnO NRs on the surface of SnO2 NWs was as follows. When a layer of Zn(CH3 COO)2 solution was sprayed on the surface of the SnO2 NWs and heat-treated at 300 ◦ C, Zn(CH3 COO)2 was oxidized and crystallized to form ZnO nanoparticles. The ZnO nanoparticles coated on the surface of the SnO2 NW backbone played as the seeds for the growth of the ZnO NR branches during the hydrothermal process. In the early state of the hydrothermal process, the ZnO nucleated and grew out on the ZnO seeds, in which the solid ZnO nuclei were formed through the dehydration of Zn(OH)4 2− (aq) and Zn(NH3 )4 2+ (aq) [35]. The ZnO crystal was supposed to grow continuously by the condensation of the surface hydroxyl groups with the zinc-hydroxyl complexes [35]. XRD analysis was performed to investigate the crystal structures of the SnO2 NWs and hierarchical SnO2 /ZnO. The results are depicted in Fig. 3. The XRD pattern (Fig. 3a) of bare SnO2 NWs exhibited very sharp diffraction peaks because of their high crystallinity. The typical diffraction peaks at 2 of 27.02◦ , 34.44◦ , 38.52◦ , and 52.10◦ were indexed as the (1 1 0), (1 0 1), (2 0 0), and (2 1 1) planes of tetragonal rutile SnO2 , respectively. All typical diffraction peaks measured in the 2 range correspond to the tetragonal structure of ˚ These peaks are SnO2 with lattice constants a = 4.73 A˚ and c = 3.18 A. in good agreement with those on the standard card (JCPDS, card no. 41-1445). Fig. 3b and c illustrates the XRD patterns of the SnO2 /ZnO hierarchical nanostructures at different hydrothermal growth periods (1, 2, and 4 h). The diffraction peak of the ZnO phase was hardly found in the XRD patterns of SnO2 /ZnO samples grown for 1 and 2 h because of the relatively low amount of ZnO compared with SnO2 . However, the diffraction peaks of the SnO2 and ZnO phases were found to coexist in the XRD pattern of the SnO2 /ZnO hierarchical nanostructures grown for 4 h. The typical peaks at 2 of 31.73◦ and 36.59◦ were well indexed as the (0 0 2) and (1 0 1) planes of ZnO, respectively. All these peaks measured in the 2 range correspond to the tetragonal structure of ZnO with lattice constants of a = 3.25 A˚ ˚ These peaks are in good agreement with those on and c = 5.21 A. the standard card (JCPDS card no. 36-1451). No significant shift was observed in the diffraction peaks. This result indicates that no
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Fig. 1. Experimental steps to prepare the SnO2 /ZnO hierarchical nanostructures: (a) the deposition of Au catalytic layer; (b) the growth of the bare SnO2 nanowires; (c) the decoration of ZnO nanoparticles on the SnO2 nanowires surface; (d) the hydrothermal growth of ZnO nanorods; the actual SEM images of the SnO2 nanowires (e), the nanoparticles-decorated SnO2 nanowires (f), and the SnO2 /ZnO hierarchical nanostructures (h).
interface reaction exists between ZnO and SnO2 for the formation of the Zn2 SnO4 phase. The optical characteristics of the SnO2 NWs and SnO2 /ZnO hierarchical nanostructures were also studied through PL at room temperature (Fig. 4). The PL spectrum of the bare SnO2 NWs (curve 1) exhibited a broad emission peak at a visible region of 620 nm
(2.0 eV), which was smaller than the band gap width of the SnO2 NWs (3.6 eV). Hence, the visible emission peaks cannot be ascribed to the direct recombination of a conduction electron in the Sn4d band and a hole in the O2p valence band. The semiconducting behavior of SnO2 is attributed to the oxygen vacancies in the crystal structure, which is also crucial to their optical properties [36].
Fig. 2. FE-SEM images of the bare SnO2 nanowires (a), the SnO2 /ZnO hierarchical nanostructures grown at 1 h (b), 2 h (c), and 4 h (d).
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Therefore, the emission peak at approximately 620 nm is believed to originate from the luminescence centers formed by tin interstitials or dangling bonds in the SnO2 NWs. The oxygen vacancies with high density interact with interfacial tin and from a considerable amount of trapped states within the band gap, giving rise to a high PL intensity at room temperature [6]. The PL spectra of the SnO2 /ZnO hierarchical nanostructures are also presented in Fig. 4, in which curves (2), (3), and (4) correspond to the spectra of the ZnO NRs after 1, 2, and 4 h of growth, respectively. Some differences were found in the PL spectrum of the bare SnO2 NWs. Aside from the emission peak at 620 nm, the PL spectra of the SnO2 /ZnO hierarchical nanostructures showed a weak emission peak at 385 nm (approximately 3.2 eV). This peak could be attributed to the attached ZnO NRs because the PL spectrum of pure ZnO NRs shows emission peaks at approximately 380 and 520 nm. These peaks correspond to the near band-edge emission and deep-level/trap-state emission, respectively [37]. In addition, the intensity of these emission peaks at 385 and 620 nm increased with increasing length of the ZnO NR branches. 3.2. Gas-sensing properties
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Fig. 5. The gas response of the bare SnO2 nanowires sensor and the SnO2 /ZnO (grown for 2 h) sensors to C2 H5 OH, NH3 , CO, H2 , CO2 , and LPG gases.
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Wavelength (nm) Fig. 4. The PL spectrum of the bare SnO2 nanowires, hierarchical nanostructures grown at 1 h, 2 h (c), and 4 h (a) and its magnification at emission peak at 385 nm (b).
The effects of heterogeneous hierarchical structure on the gas-sensing performance of the materials were determined by measuring the bare SnO2 NWs and SnO2 ZnO (2 h) sensors with different test gases (C2 H5 OH, NH3 , CO, H2 , CO2 , and LPG) at a fixed concentration of 100 ppm and an operating temperature of 400 ◦ C. As shown in Fig. 5, the responses (i.e., Ra /Rg , where Ra is the resistance in air, and Rg is the resistance in ethanol gas) of the bare SnO2 NW sensors to C2 H5 OH, NH3 , CO, H2 , CO2 , and LPG were not much different. They were proximately in the range of 1.2–2.2. Meanwhile, the responses of the SnO2 ZnO hierarchical sensors to those gases were larger (i.e., 1.5–6.2). The highest enhancement in response was observed for ethanol gas. This result indicates the potential application of the sensors for screening inebriated drivers. The gas-sensing performances of the SnO2 /ZnO hierarchical structures are dependent on the length of the ZnO NR branches. Therefore, to obtain the best ethanol-sensing performance of SnO2 /ZnO hierarchical sensors, we measured the response of the bare SnO2 NWs and SnO2 /ZnO sensors. The branched ZnO NRs were grown at different times (i.e., 1, 2, and 4 h) with ethanol gas (25–500 ppm) at an operating temperature of 400 ◦ C. The ethanolsensing transient results are shown in Fig. 6, in which the graphs were plotted with the same scale for quickly comparing the sensor response. Apparently, the SnO2 /ZnO hierarchical sensors exhibited better ethanol response than bare SnO2 NWs. All the SnO2 /ZnO hierarchical sensors showed very stable sensing and recovery characteristics. The responses to 25–500 ppm ethanol gas of SnO2 /ZnO
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Fig. 6. Dynamic sensing response to ethanol gas (25–500 ppm) of the bare SnO2 nanowires (a) the SnO2 /ZnO hierarchical nanostructures grown at 1 h (b), 2 h (c), and 4 h (d).
hierarchical sensor prepared from the branched ZnO NRs grown for 1, 2, and 4 h were in the range of 2.3–13.1, 3.0–16.2, and 1.7–8.1, respectively. This result suggests that 2 h of ZnO branch growth is optimal in designing SnO2 /ZnO hierarchical nanostructures for the best gas-sensing applications. The highest response to 25 ppm ethanol of the SnO2 /ZnO hierarchical sensor was approximately 3. This sensor has the capacity to detect ethanol even at lower concentrations down to sub-ppm level. In practical applications of ethanol sensors to screen intoxicated drivers, the sensor should be able to detect an ethanol concentration of approximately 200 ppm, which corresponds to approximately 0.5 g of C2 H5 OH per liter of blood [38]. Therefore, these results suggest that SnO2 /ZnO hierarchical nanostructure sensors are effective for the enhanced detection of low ethanol gas limits. The sensor response plotted as a function of ethanol gas concentration is shown in Fig. 7. As shown in the figure, the response increased with increasing ethanol gas concentration. In addition, the SnO2 /ZnO hierarchical nanostructures with ZnO NRs grown for 2 h clearly exhibited the best response to ethanol gas. Its response to 25–500 ppm ethanol was approximately threefold and fivefold higher than that of bare SnO2 NWs sensors, respectively. This result suggests that the enhancement of the response becomes evident
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for low and high ethanol gas concentrations. The response of oxide semiconductor gas sensors is usually depicted as [39]: Rg = Ra (1 + K[C])−ˇ
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where Rg and Ra are the sensor’s resistance in ethanol and air, respectively, C is the gas concentration in ppm, and ˇ and K are constants. The data of sensor response (S = Ra /Rg ) versus ethanol gas concentration (C) can be expressed as follows: S = (1 + K[C])ˇ
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The fitted parameters ˇ and K are particularly useful because they provide meaningful information for comparing sensor performances. The sensor’s response switches from zero order to first order when [C] = K−1 . Therefore, the inverse of K is called the sensitivity threshold [39]. The power-law exponent ˇ is related to the slope of the log–log plot of the sensor response versus ethanol concentration. Thus, it can be understood as the ability of the sensor to distinguish similar concentrations [39]. Table 1 shows the fitting parameters for the observed data presented in Fig. 7. Evidently, the SnO2 /ZnO sensor can detect ethanol gas down to a concentration of