Sensors and Actuators B 173 (2012) 897–902
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Development of microstructure CO sensor based on hierarchically porous ZnO nanosheet thin films Yi Zeng a , Liang Qiao b , Yifei Bing a , Mao Wen a , Bo Zou a , Weitao Zheng a,∗ , Tong Zhang c,1 , Guangtian Zou a a Department of Materials Science, Key Laboratory of Automobile Materials of Ministry of Educations (MOE) and State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China b College of Science, Changchun University, Changchun 130022, PR China c State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China
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Article history: Received 21 January 2012 Received in revised form 30 May 2012 Accepted 31 May 2012 Available online 11 July 2012 Keywords: ZnO Hierarchical porous CO Microstructure sensors
a b s t r a c t A microstructure sensor based on hierarchically porous ZnO nanosheet (NS) thin films has been achieved by using a facile two-step solution phase method. The transparent ZnO seed-layer covered on both the conducting electrodes and the insulating SiO2 spacer regions acts as lattice-matched template, on which the assembly of ZnO NS thin films has been subsequently completed via a facile solvothermal process. The phase purity, morphology, and structure of the as-prepared NSs are investigated, and the results reveal that ZnO NSs, grown vertically on the substrate, exhibit hierarchically porous nanostructure with subunits of nanoparticles. The gas-sensing properties exhibit not only high response to CO with the response time of 25 s, but also low cross response to common interference gases at the operating temperature of 300 ◦ C.
1. Introduction The recognition of the gas sensing capability of semiconducting oxides can date back to the 1960s when it was observed that the adsorption of reducing gas on metal oxides could result in a change in the electrical conductivity of the oxides [1,2], and this phenomenon attracts extensive attention to the development of gas sensors based on various semiconducting oxides [3]. ZnO, a famous functional n-type semiconductor, is one of the most promising candidates for the detection of different sorts of gases [4–6]. With the progress of gas-sensing research, it has been found that nanostructures with the high surface area and surface accessibility can significantly improve the gas-sensing properties, and thus plenty of efforts have been focused on the preparation of ZnO nanostructures [7–9]. On this basis, most of investigations are focused on the fabrication of side-heated gas sensors by coating ZnO pastes on the surface of the ceramic tube with presetted electrodes. However, due to the uncontrolled thickness of the sensing layer and high power consumption, these sensors are not only hard to keep consistency and homogeneity in a large-scale production, but also difficult to integrate with other electronical devices. In comparison
∗ Corresponding author. Tel.: +86 431 85168246; fax: +86 431 85168246. E-mail addresses:
[email protected] (W. Zheng),
[email protected] (T. Zhang). 1 Tel.: +86 431 85168385; fax: +86 431 85168385. 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.05.090
© 2012 Elsevier B.V. All rights reserved.
with the above mentioned sensors, microstructure sensors based on thin films are especially more promising for practical applications due to their merits such as consistency, ease of scale-up, and low power consumption [10–12]. To the best of our best knowledge, so far, there have been no reports on the sensor based on porous ZnO nanosheet (NS) thin films as well as its gas-sensing properties. Herein, a facile and template-free solution method is employed for the preparation of hierarchically porous ZnO NS thin film, on which the sensor realizes the selective detection of CO to other interference gases. 2. Experimental procedures 2.1. Fabrication of microstructure sensor based on ZnO NSs All the chemicals used were analytical grade reagents (Beijing Chemicals Co. Ltd.) and used without further purification. For fabricating the microstructure sensor based on ZnO NS thin film, the typical process is described as follows. (1) Preparation of microstructure sensor. The Si-based sensor was achieved using a conventional photolithography process [13]. The sputtering Pt served as interdigital electrodes and heater was deposited on the silicon substrate with SiO2 and TiO2 layers as insulating and adhesive layers, respectively. The area of the substrate was about 2.0 mm × 1.0 mm. The width of the signal electrodes and heater was 50 m, and the distance between the adjacent Pt strips was also 50 m. The growth of ZnO NS thin films on the patterned SiO2 /Si substrate is related
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to the key two-step solution reaction process. (2) Preparation of ZnO seed-layer. ZnO seed-layer was preferentially deposited by solgel method on the patterned SiO2 /Si substrate, which has been described in detail elsewhere [14]. (3) Preparation of ZnO NS thin films. ZnO NS thin films were subsequently grown on the ZnO buffer layer by a facile solvothermal process. Briefly, the mixed solution was prepared by adding ammonia (NH4 OH) dropwise into the 0.015 M zinc acetate dehydrate (Zn(CH3 COO)2 ·2H2 O) ethanol solution under vigorous stirring until pH = 10.0. The precursor solution was transferred into a Teflon-lined stainless steel autoclave, in which the pretreated ZnO buffer/SiO2 /Si substrate was fixed vertically. The solvothermal treatments were carried out at 95 ◦ C for 2 h. Subsequently, the autoclave was allowed to cool down naturally and the substrate was washed with absolute ethyl alcohol and deionized water for several times prior to drying in air at 60 ◦ C for further characterization. 2.2. Characterization and gas-sensing measurement The crystalline phase, morphology, and structure of the thin films were characterized by X-ray diffraction (XRD, Bruke D8 tools) with graphite monochromatized and Cu K␣ , = 0.15418 nm, field emission scanning electron microscopy (FESEM, JEOL JSM6700F or -7500F), transmission electron microscopy/selected area electron diffraction (TEM/SAED, Hitachi H-8100, 200 kV), and highresolution TEM (HRTEM, JEOL JEM-2100F, 200 kV), respectively. The gas-sensing properties of ZnO NSs were measured with a gas-sensing characterization system (WS-60A, Weisheng Instruments Co., Ltd., China). The desired concentrations of the testing gases were obtained by the static gas distribution method, which was calculated by the following formula [15]. Q =
V ×ϕ×M 273 + TR × 10−9 × 273 + TB 22.4 × d ×
(1)
where Q (mL) is the liquid volume of the volatile compound, V (mL) is the volume of the testing chamber, ϕ is the required gas volume fraction, M (g mol−1 ) is the molecular weight, d (g cm−3 ) is the specific gravity, and is the purity of the volatile testing liquid, TR and TB (◦ C) are the temperatures at ambient and test chamber, respectively. The response (R) of the sensor was defined as the ratio (Ra /Rg ) of the resistance of the sensor in dry air (Ra ) to that in the testing gas (Rg ). The response and recovery time were defined as the time taken by the sensor to achieve 90% of the total resistance change in the case of adsorption and desorption, respectively. 3. Results and discussion For fabricating the microstructure sensor for ZnO NS thin film, the SiO2 /Si substrate prefabricated interdigital electrodes and heater (Fig. 1a) is firstly achieved using a conventional photolithography process. Fig. 1b shows a typically panoramic surface including different regions (named by A and B in Fig. 1a). The difference of size of the ZnO NSs can clearly be observed, which can be attributed to the different growth habits of ZnO crystallite on different districts, including the insulating SiO2 spacer regions and conducting interdigital electrode strips. According to Xia et al. [16], the noble metal element can effectively promote and accelerate the growth of ZnO. Thus, the ZnO NSs on the region of conducting electrodes show bigger and larger sizes than those on the insulating SiO2 spacer regions (region B). All of these NSs interconnect with each other to form sheet-networks, quasi-vertically and homogenously on the substrates (Fig. 1b–e). Upon measuring carefully, the average thickness for ZnO NSs perpendicular to its two-dimensional (2D) surface is about 30 nm (Fig. 1f). Fig. 1g presents a TEM image for a typical isolated ZnO NS. Interestingly, detailed observations (Fig. 1h and i) clearly confirm that
ZnO NS has a coarse and porous structure with the porous size of about 5–10 nm. The SAED pattern (the inset of Fig. 1h) indicates that the porous ZnO NSs exhibit a hierarchical and quasi-single crystalline structure (Fig. 2), where the subunits are composed of small nanoparticles. Furthermore, the lattice interplanar spacing of a nanoparticle is determined to be 0.28 nm, corresponding to ¯ planes of the wurtzite ZnO (Fig. 1i and j). The result the {0110} indicates that the subunit nanoparticles are single-crystal wurtzite ZnO structure along the 0 1 1¯ 0 crystallographic directions within the {0 0 0 1} planes. In the XRD pattern (Fig. 1k), ZnO NSs have a wurtzite ZnO phase, matching well with the standard XRD values (JCPDS No. 36-1451). In addition, the XRD pattern also confirms the formation of ZnO seed-layer on the substrate after the first reaction step (Fig. 3). To further understand the formation mechanism of ZnO NSs, some controlled experiments have been completed. Firstly, if there is no substrate in the second step of the reaction process, flowerlike aggregates consisted of hexagonal NSs can be obtained (Fig. 4). To understand the effect of ethanol on ZnO nanostructures, a controlled experiment by using water instead of ethanol as the solvent is carried out. In this case, ZnO nanorod arrays, rather than ZnO NSs are obtained [17], indicating that the growth of ZnO crystallite along c-axis is restrained and ethanol is responsible for the formation of 2D ZnO NSs. Furthermore, a controlled experiment with the only change in reaction time of 1 h is carried out, indicating that the reaction time has a great effect on the products. ZnO NSs with different shapes in different regions are obviously observed (Fig. 5a). In region A, there is not any obvious difference in shape and size for ZnO NSs, compared with the product obtained with reaction time of 2 h. In region B, besides ZnO NSs, a large quantity of ZnO sprouts appear (Fig. 5b and c), which actually consist of aggregated nanoparticles (NPs) with the size of 40–100 nm (Fig. 5d). Generally, based on the surface energy minimization in the solution system, the fastest growth of ZnO crystallite, determined by the intrinsic structure, spurs it to grow along the c-axis, and thus, the elongated rod-like shape is much more favorable in the thermodynamic equilibrium state [18]. On the other hand, the growth of ZnO crystallite is also affected by the external conditions such as solvent, pH value, and temperature. In this work, it is found that solvent plays a key role in determining the growth of ZnO. A possible mechanism for the present ZnO NSs is schematically plotted in Fig. 5e. In the case of the growth of ZnO NSs, ZnO seed-layer is firstly formed through the restrained nucleation and growth process, which serves as the lattice-matched template for the growth of ZnO NSs. In the second-step process, a large quantity of ZnO nuclei are formed and aggregated when the degree of supersaturation exceeds its critical value. In the pure ethanol solution, Pan et al. [19] have proposed that less hydroxyl groups on the surface of ZnO nuclei can increase the packing probability of ZnO clusters along the direction including c-axis, making only one direction with OH− restricted during growth. As a result, the separated clusters finally grow into 2D NSs through the solvothermal process. In the growth process of 2D ZnO NSs [18], ethanol leads to a non-equilibrium growth of the ZnO crystalline, and the growth rate changes to V0 1 1¯ 0 > V0 0 0 1 . Hence the sample finally shows sheet-like morphology with basal {0 0 0 1} planes. Recently, semiconductor oxides with hierarchically hollow or porous architectures are considered to be ideal structure for the gas-sensing applications due to their efficiently high surface area and surface accessibility. Because of the special porous architecture of our ZnO NSs, which are not available from bulk or solid materials, sensor based on these ZnO NSs is expected to exhibit excellent gas-sensing properties. The CO-sensing behaviors of ZnO NSs are firstly performed for the optimal operating temperature, which retains the equilibration between the diffusion of gas molecules compensated to the sensing surface and the reaction of the testing
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Fig. 1. (a) Schematic diagrams of the different views for the microstructure sensor. Morphologies and crystal structures of ZnO nanosheets (NSs): (b–f) FESEM images of ZnO NSs taken from different regions in the microstructure sensor; (g–j) TEM and HRTEM images and (k) XRD pattern of ZnO NSs, in which the inset of (h) is the SAED pattern of ZnO NS.
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Fig. 2. (a) HRTEM image and (b) its corresponding Fourier transform (FFT) pattern of ZnO NSs.
Fig. 3. XRD pattern of ZnO buffer layer after the first reaction step.
Fig. 4. FESEM image of the products obtained in the absence of the microstructure substrate.
Fig. 5. (a–d) FESEM images of ZnO nanostructures from the sensor obtained for the reaction time of 1 h, and (e) schematic illustration of the possible formation mechanism for ZnO NSs.
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Fig. 6. Gas response of ZnO NSs and buffer layer to 100 ppm CO as the function of the operating temperatures, respectively.
gas molecules with the adsorbed oxygen species [20]. As shown in Fig. 6, the response reaches the maximum value of 11.2 at the optimal temperature of 300 ◦ C, which is applied for the CO-sensing experiments in the following. Responses of sensors based on ZnO NSs, ZnO NP and NSs, or ZnO seed-layer (named as sensors A, B, and C, respectively) as a function of CO concentration from 5 to 500 ppm are investigated, respectively (Fig. 7a). For all of the sensors, response does not exhibit significant difference when CO concentration is low. As the CO concentration increases, the difference among the sensors based on different ZnO structures gradually increases, and the response of sensor A shows the fastest increase. It indicates that sensor based on ZnO seed-layer is almost insensitive to the variety of CO concentration and the interconnected ZnO NSs on the seed-layer would play a crucial role in the improvement of CO sensing performances. The selective capacity of the sensor based on ZnO NSs is tested to various testing gases at 300 ◦ C (Fig. 7b). It can be observed that the sensor shows the largest response to CO, compared with other gases, including SO2 , C7 H8 , and so on, which seems to depend on the interaction discrepancy between the sensing layer and different testing
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gas [21]. The sensor based on ZnO NSs has a satisfying selectivity to CO against other testing gases. The inset shows the responses of different sensors to 100 ppm CO and other two typical interfering gases (SO2 and C7 H8 ) at 300 ◦ C, respectively. Although the sensor B exhibits a partially selective detection to CO, the response to 100 ppm CO (6.2) is much smaller than that of sensor A. In contrast, sensor C nearly has no response and selectivity to all the testing gases. Among them, sensor A based on ZnO NSs has a capability to detect CO in a selective manner with a high response. Generally, the surface-controlled sensing mechanism of n-type metal oxides involves the adsorption and desorption processes occurring at the surface of sensing materials [21,22]. The adsorption/desorption reactions of oxygen molecules lead to the trapping/release of electrons, and thus a decrease/increase in electrical conductivity, that is, a high/low resistance. Therefore, ZnO is an intrinsic n-type semiconductor, and its response and recovery process can be established by comparing the resistance of ZnO NSs in different ambiences. The response transient of ZnO NSs to 100 ppm CO is shown in Fig. 7c. The response and recovery time of ZnO NSs are about within 25 and 36 s, respectively. The four sequential cycles of response transients to 100 ppm CO show a reversible, repeatable, yet stable characteristic of microstructure sensor based on ZnO NSs for CO detection (inset of Fig. 7c). Fig. 7d shows the response and recovery behaviors of the sensor upon being orderly exposed to 5–100 ppm CO. In the measurements, the responses are 1.7, 3.3, 5.4, 8.5, and 11.2 to 5, 10, 20, 50, and 100 ppm CO, respectively. Compared with the CO-sensing properties of other ZnO-based nanostructures [12,23–26], sensor in this work exhibits comparable or higher response and selectivity but has a faster response and recovery time, likely due to the hierarchically porous structure of these ZnO NSs. It is well known that the response of sensor based on sensing oxides is mainly determined by the interactions between the target gas and the sensing surface. Thus it can be sure that the surface area of the sensing materials is greater, and the stronger interaction and the higher response can be expected [24]. Our ZnO NSs with the interlaced-connected network maintain the high surface area and efficiently avoid the agglomerated configuration, resulting in an improved response. The recovery time, as another important characteristic to evaluate a sensor, is determined by
Fig. 7. (a) Response–concentration and (b) selectivity curves of sensors based on different nanostructures, and dynamic response of ZnO NSs: (c) single-cycle for 100 ppm CO; (d) response transients of the sensor to different concentrations of CO. The inset of (c) displaying four periods for 100 ppm CO.
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the surface accessibility of the sensing framework, which includes the gas diffusion toward the sensing surface, the reaction with chemisorbed oxygen ions, and the subsequent re-oxidation process of the sensing surface to yield oxygen species. According to Lee [27], ideal nanostructures (hierarchically porous, or hollow) can provide well-defined and well-aligned micro-, meso-, and nanoporosities for effective gas diffusion, which greatly increases the response and recovery speed. Here, the less agglomerated of hierarchically porous ZnO NSs can provide various micro- and nanoporosities for effective surface area and surface accessibility, resulting in the improved CO-sensing properties. 4. Conclusions In summary, ZnO NSs with hierarchical structure, exhibiting porous, easy penetrability, and accessible surface, can be gained through a facile two-step solution method. In our synthesis strategy, ZnO seed-layer firstly covered on the substrate acts as the lattice-matched template, on which the interlaced-connected ZnO NSs grow vertically via the second synthesis process. The less agglomerated and porous network structures maintain not only the high surface area but also the accessible surface for the rapid and effective diffusion of CO gas. Therefore, the microstructure sensor based on ZnO NSs shows improved sensing properties with high response and short recovery time. Acknowledgments We thank the National Natural Science Foundation of China (Nos. 50832001, 51002014, and 60971012), the Postdoctoral Science Foundation of China (No. 20110491319), Program for Changjiang Scholars and Innovative Research Team in University, the “211” and “985” project of Jilin University, the special Ph.D. program (No. 200801830025) from MOE, and the Fundamental Research Funds for Jilin University (No. 450060323432) for the financial support of this research. References [1] T. Seiyama, A. Kato, K. Fujushi, M. Nagatani, A new detector for gaseous components using semiconductive thin films, Analytical Chemistry 34 (1962) 1502f. [2] N. Yamazoe, New approaches for improving semiconductive gas sensors, Sensors and Actuators B 5 (1991) 7–19. [3] N. Yamazoe, Toward innovations of gas sensor technology, Sensors and Actuators B 108 (2005) 2–14. [4] L. Schmidt-Mende, J.L. MacManus-Driscoll, ZnO-nanostructures, defects, and devices, Materials Today 10 (2007) 40–48. [5] L. Vayssieres, K. Keis, A. Hagfeldt, S. Lindquist, Three-dimensional array of highly oriented crystalline ZnO microtubes, Chemistry of Materials 13 (2001) 4395–4398. [6] E. Comini, C. Bratto, G. Faglia, M. Ferroni, A. Vomiero, G. Sberveglieri, Quasi-one dimensional metal oxide semiconductors: preparation and characterization and application as chemical sensors, Progress in Materials Science 54 (2009) 1–67. [7] Z.H. Jing, J.H. Zhan, Fabrication and gas-sensing properties of porous ZnO nanoplates, Advanced Materials 20 (2008) 4547–4551. [8] J. Zhang, S.R. Wang, M.J. Xu, Y. Wang, B.L. Zhu, S.M. Zhang, W.P. Huang, S.H. Wu, Hierarchically porous ZnO architectures for gas sensor applications, Crystal Growth and Design 9 (2009) 3532–3537. [9] K.M. Kim, H.R. Kim, K.I. Choi, H.J. Kim, J.H. Lee, ZnO hierarchical nanostructures grown at room temperature and their C2 H5 OH sensor applications, Sensors and Actuators B 155 (2011) 745–751. [10] Q. Wan, T.H. Wang, Single-crystalline Sb-doped SnO2 nanowires: synthesis and gas sensor application, Chemical Communications (2005) 3841–3843. [11] C.W. Na, H.S. Woo, J.H. Lee, Design of highly sensitive volatile organic compound sensors by controlling NiO loading on ZnO nanowire networks, RSC Advances 2 (2012) 414–417. [12] J.X. Wang, X.W. Sun, Y. Yang, H. Huang, Y.C. Lee, O.K. Tan, L. Vayssieres, Hydrothermally grown oriented ZnO nanorod arrays for gas sensing applications, Nanotechnology 17 (2006) 4995–4998. [13] T. Zhang, L. Liu, Q. Qi, S.C. Li, G.Y. Lu, Development of microstructure In/Pddoped SnO2 sensor for low-level CO detection, Sensors and Actuators B 139 (2009) 287–291.
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Biographies Yi Zeng received his MS degree from State Key Laboratory of Superhard Materials, Jilin University, China in 2007. He received his PhD degree from College of Electronic Science and Engineering, Jilin University, China in 2010 majored in microelectronics and solid state electronics. He was appointed the lecturer in Department of Materials Science, Jilin University in July, 2010. Now, he is engaged in the synthesis and characterization of the semiconducting functional materials, nanocomposites, and gas sensors. Liang Qiao is an associate professor at the College of Science, Changchun University, China. She received her PhD degree from the Department of Materials Science, Jilin University, China in 2007. Now, she is engaged in the simulation of the gas sensing properties of semiconducting functional materials, and graphene nanocomposites. Yifei Bing received his BE degree from the Department of Materials Science, Jilin University, China in 2010. Now he is working for his MS degree, majoring in Materials Physics and Chemistry, in Jilin University. He is interested in the synthesis and applications of graphene composites, functional nanomaterials, and gas sensors. Mao Wen received his PhD degree from Department of Materials Science, Jilin University, China in 2010. He was appointed the lecturer in Department of Materials Science, Jilin University in December, 2010. Now, he is engaged in the synthesis and characterization of the functional thin films, and nanocomposites. Bo Zou is a professor of State Key Laboratory of Superhard Materials, Jilin University, Changchun, China. He earned his PhD degree (2002) in the field of polymer chemistry and physics, Jilin University. In 2003–2005, he started his postdoctoral studies at Dortmund University, Germany. His research interests are focused on high pressure chemistry, synthesis and application of semiconductor nanocrystals. Weitao Zheng is professor and dean at the School of Materials Science and Engineering, Jilin University, China. He obtained his PhD degree from Jilin University in the field of condensed matter physics in 1990. His research interests concentrate on superhard thin film materials and carbon related nanomaterials and he has published more than 200 papers in peer-refereeing international journals. Tong Zhang completed her MS degree in semiconductor materials in 1992 and her PhD in the field of microelectronics and solid-state electronics in 2001 from Jilin University. She was appointed as a full-time professor in the College of Electronics Science and Engineering, Jilin University in 2001. Her research interests are sensing functional materials, gas sensors, and humidity sensors. Guangtian Zou is a professor of State Key Laboratory of Superhard Materials, Jilin University, Changchun, China. He obtained his postgraduate degree from Jilin University in the field of solid state physics in 1965. He was elected the academician of the Chinese Academy of Sciences in 2001, and his research interests concentrate on high pressure physics and superhard materials.
