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High sensitive and selective formaldehyde sensors based on nanoparticle-assembled ZnO micro-octahedrons synthesized by homogeneous precipitation method
Sensors and Actuators B 160 (2011) 364–370
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
High sensitive and selective formaldehyde sensors based on nanoparticle-assembled ZnO micro-octahedrons synthesized by homogeneous precipitation method Lexi Zhang a,b , Jianghong Zhao a,∗ , Haiqiang Lu a,b , Liming Gong a,b , Li Li a , Jianfeng Zheng a , Hui Li a,b , Zhenping Zhu a,∗ a b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taoyuan South Road 27, Taiyuan 030001, PR China Graduate University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e
i n f o
Article history: Received 31 May 2011 Received in revised form 18 July 2011 Accepted 28 July 2011 Available online 4 August 2011 Keywords: Homogeneous precipitation ZnO micro-octahedron Gas sensor Formaldehyde Defect Oxygen species
a b s t r a c t Nanoparticle-assembled ZnO micro-octahedrons were synthesized by a facile homogeneous precipitation method. The ZnO micro-octahedrons are hexagonal wurtzite with high crystallinity. Abundant structure defects were confirmed on ZnO surface by photoluminescence. Gas sensors based on the ZnO microoctahedrons exhibited high response, selectivity and stability to 1–1000 ppm formaldehyde at 400 ◦ C. Especially, even 1 ppm formaldehyde could be detected with high response (S = 22.7). It is of interest to point out that formaldehyde could be easily distinguished from ethanol or acetaldehyde with a selectivity of about 3. The high formaldehyde response is mainly attributed to the synergistic effect of high contents of electron donor defects (Zni and VO ) and highly active oxygen species (O2− ) on the ZnO surface. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Metal oxide semiconductor (MOS) gas sensors, including SnO2 , ZnO, In2 O3 , WO3 , and so on, have attracted considerable attention owing to their ability of detecting trace gases [1]. As one of the key wide bandgap (∼3.4 eV at 1.2 K) [2] semiconductors, ZnO has been proved to be an excellent gas-sensing material for measuring both oxidative and reductive target gases at ppm (parts per million) level and above [3]. Taking advantages of the small size, large surface-to-volume ratios and high density of surface active sites compared to their bulk counterparts, great interest has been focused on performance-enhanced gas sensors based on ZnO nanostructures, such as nanoparticles [4], nanorods [5], nanobelts [6], nanotubes [7] and nanosheets [8]. Recently, hierarchical structures constructed by low-dimensional nanomaterials, for example, nanoparticle-organized hollow spheres [9], nanorodcombined flower-like structures [10], nanosheet-assembled 3D architectures [11,12], have began to catch much of researchers’ attention, because they exhibited enhanced gas-sensing performances which originated from the improvement in exposing more
∗ Corresponding authors. Tel.: +86 351 4048715; fax: +86 351 4041153. E-mail addresses: zjh [email protected] (J. Zhao), [email protected] (Z. Zhu). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.07.062
available surface, facilitating gas diffusion and transportation, and so forth. However, great efforts are still needed to further develop their synthesis processes, since hard templates, surfactants or relative high temperature is usually necessary for fabricating these hierarchical structures. Accordingly, it is significantly important to develop template-free, facile and low temperature methods to synthesize novel hierarchical nanostructures, and to carry out indepth research on their gas sensing properties. Compared with other methods, homogeneous precipitation is a more economic (no need for special apparatus) and environment-friendly (no need for surfactants or organic solvents) method to prepare metal oxides for sensor applications [13] in large scale at low temperature. In spite of its advantages in preparation, based on this method, it is usually difficult to controlled synthesize metal oxide nanostructures [14] (except for nanoparticles), to say nothing of nano-building block assembled hierarchical architectures. As an important industrial chemical, formaldehyde has been widely used to manufacture plastics, medicine, synthetic fibers and household products. Regrettably, formaldehyde is very harmful to human health because of its volatility, irritability and toxicity, thus is considered as one of the main indoor air pollutants in residential and industrial occupational environments. It is of great practical importance to detect formaldehyde rapidly and accurately in the atmosphere. Till now, significant progress has been
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Fig. 1. (a) Scheme of the gas sensor structure. (b) The measuring electric circuit. Inset: photograph of a sensor.
made to formaldehyde sensors based on ZnO nanostructures; however, their gas-sensing performance is still required to be further improved in sensitivity, especially in selectivity. This is because for the practical MOS formaldehyde gas sensors, acetaldehyde is a common interfering gas [15], and formaldehyde is usually difficult to be distinguished from ethanol especially in indoor air detection [16]. Hence, highly selective formaldehyde gas sensors, in particular, to acetaldehyde and ethanol, are of great meanings in practical application. In this work, ZnO micro-octahedrons assembled by nanoparticles were successfully synthesized through a facile homogeneous precipitation method. Their structure, morphology, surface defects and chemisorbed oxygen were investigated. Gas sensors using ZnO micro-octahedrons were tested and represented excellent formaldehyde sensing properties. Furthermore, the reason of high formaldehyde selectivity and response was also demonstrated.
configuration, excited by the 514 nm line of an argon-ion laser at room temperature. Field emission scanning electron microscope (FE-SEM) images were carried out on a JEOL JSM-6700F microscope operating at 5 kV. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2010 microscope with an accelerating voltage of 200 kV. UV–Vis spectrum was measured on a Shimadzu UV-3600 UV-Vis-NIR spectrophotometer at room temperature. Photoluminescence (PL) was measured on a Hitachi F-7000 FL spectrophotometer by a 325 nm excitation from Xe lamp at room temperature. X-ray photoelectron spectrometry (XPS) was carried out using Al K␣ (h = 1486.6 eV) X-ray beams as the excitation source. Binding energies were calibrated relative to the C1s peak at 284.6 eV.
2. Experimental
The ZnO sample was ground with Triton X-100 in a weight ratio of 1:1 to obtain a fine paste. The paste was coated onto an alumina ceramic tube, on which a pair of gold electrodes was previously installed at each end, followed by sintering at 500 ◦ C for 1 h to remove the organic binder and provide good mechanical strength. Then, a Ni–Cr wire was inserted into the tube and used as a heater. The structural diagram and photograph of a sensor are shown in Fig. 1(a) and inset of Fig. 1(b), respectively. The gas-sensing properties were measured using a HW-30A gas sensitivity instrument (Hanwei Electronics Co. Ltd., PR China). The gas concentration was determined by a stationary state process: a given amount of target gas was injected into a glass chamber and fully mixed with air. In the measuring electric circuit (Fig. 1(b)), a load resistor (RL : 47 k) was connected in series with a gas sensor. The circuit voltage (Vc ) was 5 V, and output
2.1. Preparation and characterization of materials All the chemicals are analytical grade reagents and used as received without further purification. In a typical synthesis, 0.2195 g Zn(CH3 COO)2 ·H2 O was dissolved in 20 mL deionized water under magnetic stirring. Then aqueous ammonia (25–28 wt.%) was added until the pH value was adjusted to 10. After another 2 h stirring, the white precipitation was filtered, washed with deionized water, dried at 80 ◦ C for 12 h and finally calcinated at 500 ◦ C for 1 h. Powder X-ray diffraction (XRD) was recorded on a D8 Advance Bruker X-ray diffractometer with Cu K␣ radiation ( = 0.15406 nm) operating at 40 kV. Raman spectrum was performed by a JY LabRam-HR confocal Raman microscope with a backscattering
2.2. Fabrication and measurement of sensors
Fig. 2. XRD pattern (a) and Raman spectrum (b) of ZnO products.
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Fig. 3. FE-SEM image (a and b), TEM image (c) and the corresponding SAED pattern (inset) of nanoparticle-assembled ZnO micro-octahedrons.
voltage (Vout ) was the terminal voltage of the load resistor. The operating temperature of a sensor was adjusted through varying the heating voltage (Vh ). The sensor resistance in air or a target gas was measured by monitoring Vout . The system measures Vout loaded on the resistor RL , and the resistances (RS ) of the gas sensor can be calculated as: RS = RL (Vc − Vout )/Vout . The gas response (S) is defined as S = Ra /Rg (reductive gas) or S = Rg /Ra (oxidative gas), where Ra and Rg are the resistances of the sensor in air and target gases, respectively. Besides, the selectivity of formaldehyde to the other gases (K) is defined as K = S/Sig , where Sig is the response of the sensor to a certain interfering gas. Here, the response or recovery time was defined as the time taken for the sensor to achieve 90% of its maximum response or decreases to 10% of its maximum response, respectively. Long-term stability of the sensor was measured by repeating the test at 400 ◦ C 8 times in 90 days.
of ZnO products, which is in good accordance with the XRD analysis and provide some useful information about surface defects. The morphology of the ZnO sample was performed by FESEM. Fig. 3(a) exhibits a panoramic image, showing the ZnO are composed of octahedral crystals (their profiles were marked by dashed lines) with very rough surfaces and mean edge length about 4.5 m. Fig. 3(b) displays enlarged image of a representative ZnO micro-octahedron, which is self-assembled of nanoparticles with diameters of 200–300 nm. A large number of interspaces were formed between nanoparticles, which are conducive to enhance the gas response since gas diffusion and transportation can be prominently improved [12]. TEM image (Fig. 3(c)) confirms that the ZnO micro-octahedrons are assembled of nanoparticles. Furthermore, the ZnO octahedron profile, which is partially maintained, suggests that the nanoparticles are loosely interconnected. The corresponding SAED pattern (inset in Fig. 3(c)) is clear and regular concentric rings, indicating that the ZnO octahedrons are polycrystals and well crystallized, which is in good agreement with the XRD and Raman results.
3. Results and discussion 3.2. Optical characterization 3.1. Structure and morphology analysis The structure of the ZnO products was characterized by XRD (Fig. 2(a)). All peaks can be well indexed to hexagonal wurtzite ZnO (JCPDS card no. 36-1451), indicating high purity and crystallinity. In the Raman spectrum (Fig. 2(b)), the peaks at 330 and 436 cm−1 are attributed to the 3E2H –E2L and E2H mode of the ZnO crystal [17], respectively. The peak at 380 cm−1 belongs to the A1 (TO) mode [18]. The peak at 580 cm−1 assigned to the E1 (LO) mode suggests that the ZnO sample are oxygen deficient, which usually leads to electron donor related defects, such as oxygen vacancies (VO ) and zinc interstitials (Zni ) [18]. The Raman results confirm the structure
UV–Vis diffuse reflectance spectroscopy of ZnO microoctahedrons was performed to study the bandgap and electron states (Fig. 4(a)). Extrapolation of the linear part until it intersects the wavelength-axis gives the band edge of 409.74 nm. ZnO is a direct-band semiconductor, so the relation between the absorption coefficient (˛) and incident photon energy (h) can be written as ˛h = C(h − Eg )1/2 , where C is a constant and Eg is the band gap [19]. Through extrapolating the linear portion of the (h) − (˛h)2 plot to (˛h)2 = 0, the bandgap energy of the ZnO micro-octahedrons is 3.14 eV. The steep shape of the UV edge and the strong absorption in the UV region imply that the absorption band of ZnO micro-
Fig. 4. UV–Vis (a) and PL spectra (b) of the nanoparticle-assembled ZnO micro-octahedrons. Inset in (a): plot of (˛h)2 versus h. Inset in (b): sub-peak positions and relative contents separated from the PL spectrum.
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Fig. 5. XPS spectra of the nanoparticle-assembled ZnO micro-octahedrons: Zn2p3/2 (a) and O1s peaks (b). Inset in (b): binding energy positions and percentages of three peaks (OL , OV and OC ) separated from O1s .
octahedrons is ascribed to the intrinsic transition between the valence band (VB) and conduction band (CB). As a useful tool for characterizing intrinsic defects, the room temperature PL spectrum of the ZnO micro-octahedrons is shown in Fig. 4(b), in which both weak UV emission (<400 nm) and strong visible emission (400–800 nm) were observed. The strong visible emission suggests abundant structure defects [20] on the surface of ZnO micro-octahedrons. Through curve deconvolution, the PL spectrum can be well-fitted to the superposition of several Gaussian sub-peaks and were assigned to different defects. Since the band gap of ZnO is 3.1–3.4 eV at room temperature, the UV emission at 366.4 nm is attributed to the recombination of free excitons between the VB and CB [21]. The peak at 406.7 nm belongs to shallow donor-related UV emission [22], on the basis of the band edge (409.74 nm) calculated from the UV–Vis spectrum (Fig. 4 (a)). The peaks at 432.8, 471.1, 520.5 and 623.6 nm are ascribed to interstitial zinc (Zni ) [23], zinc vacancy (VZn ) [24], oxygen antisite (OZn ) [25] and oxygen interstitial (Oi ) [26], respectively. The peaks at 572.8 and 684.5 nm are usually assigned to oxygen vacancies (VO ) [27,28], in spite of long-time controversies about the peak position originated from VO . The gas response can be remarkably enhanced with the increase of electron donor defects (e.g. Zni and VO ) and with the decrease of electron accepter ones (e.g. VZn , Oi and OZn ), because much more oxygen are able to be chemisorbed [20,29,30] and ionized [31] on the surface of MOS. The sub-peak positions and relative contents of the total PL spectrum were listed in Fig. 4(b). The calculated contents of donor and acceptor defects are 48.86% and 51.14%, respectively. Since the content of electron donor defects is higher than that of ZnO nanoparticles (35.5%) [32] and nanorods (47.91%) [31], we speculate that the
ZnO micro-octahedrons should exhibit much higher response to formaldehyde. XPS analysis was also carried out to obtain the surface structural and compositional information of ZnO micro-octahedrons. Fig. 5(a) shows the Zn2p3/2 peak centered at 1020. 8 eV, whose peak pattern is symmetrical, indicating no variable valence of Zn. Correspondingly, the O1s peak is asymmetrical with an obvious shoulder (Fig. 5(b)). The O1s peak can be deconvoluted into three Gaussian components centered at 529.96, 531.09 and 531.95 eV (inset in Fig. 5(b)), which are attributed to O2− ions in ZnO lattice (OL ), O2− ions in oxygen-deficient regions within the ZnO matrix (OV ) and chemisorbed and dissociated oxygen species or OH (OC ) [33], respectively. The gas sensing properties are closely related to the chemisorbed oxygen species [34], thus the high percentage of OC component (14.25%) implies high response of as-synthesized ZnO micro-octahedrons. 3.3. Gas-sensing properties The response of ZnO micro-octahedrons to 200 ppm formaldehyde was tested versus operating temperatures (Fig. 6(a)). Clearly, the response reaches a maximum value at 400 ◦ C, which is determined as the optimum operating temperature (lower than the autoignition temperature of formaldehyde (430 ◦ C)). The maximum response (S = 314) is much higher than some results ever reported for ZnO nanostructure sensors to formaldehyde based on nanoparticles [32], nanorods [35] and nanosheets [8]. Fig. 6(b) illustrates the correlation between the response of the sensor and formaldehyde concentrations (1–1000 ppm) at 400 ◦ C. The response rapidly increases with increasing the formaldehyde
Fig. 6. (a) Responses of the sensor to 200 ppm formaldehyde versus operating temperatures. (b) Responses versus formaldehyde concentrations in the range of 1–1000 ppm at 400 ◦ C and the corresponding log(S − 1) − log(C) plot (inset).
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Fig. 7. Response-recovery curves of the sensor to 1–1000 (a) and 200 ppm (b) formaldehyde at 400 ◦ C.
concentration, indicating that it still has not become saturated at 1000 ppm. When exposed to 1 ppm formaldehyde, the response is as high as 22.7. Usually, the response of MOS gas sensors can be empirically represented as S = a(C)b + 1, where a and b are constants and C is the target gas concentration [36]. The value of b is normally between 0.5 and 1.0, depending on the charge of the surface species and the stoichiometry of the elementary reactions on MOS surface [37]. For b of 0.5, the adsorbed surface oxygen ion is O2− and for b of 1, that is O− [38]. At a certain temperature, the equation can be rewritten as: log(S − 1) = blog(C) + log a. Log(S − 1) has a linear relation with log(C), thus the slope of b value can be calculated from a plot of log(S − 1) − log(C) (inset of Fig. 6(b)). The b value (0.4826) is very close to 0.5, indicating the adsorbed oxygen species is mainly O2− on the surface of ZnO micro-octahedrons, which is well supported by the fact that the dominated oxygen species on ZnO surface is O2− above 300 ◦ C [39]. Furthermore, after extrapolating the linear fitted curve to S = 3, the detection limit (S ≥ 3) of the sensor is calculated to be as low as 0.0028 ppm, which is far lower than the established limits of long-term exposure to formaldehyde by WHO (0.08 ppm), NIOSH (0.016 ppm) and OSHA (0.75 ppm). Fig. 7(a) presents typical response and recovery curve of the sensor to 1–1000 ppm formaldehyde at 400 ◦ C. When exposed to a certain concentration of formaldehyde, the response increases fast and reaches an equilibrium value. Once disengaged from formaldehyde, the response decreases rapidly to the baseline, indicating very good reliability and reproducibility. For 200 ppm formaldehyde, the response and recovery time are 46 and 13 s, respectively (Fig. 7(b)). Actually, in our experiments, in order to provide a certain gas concentration, appropriate volume of formaldehyde aqueous solution was introduced into the test chamber, vaporized and mixed homogeneously with air by two fans, which took about 30 s to fully
vaporize the solution. So, the real response time should be much shorter than 46 s. Formaldehyde and acetaldehyde are usually difficult to be differentiated by MOS gas sensors, because of their quite similar molecular structures and chemical properties [15]. Besides, ethanol is another interfering gas to formaldehyde especially in indoor air detection [16]. It is very interesting to find that the ZnO microoctahedron sensor displays much higher response to formaldehyde than that to acetaldehyde and ethanol in the whole testing range (1–1000 ppm) at 400 ◦ C (Fig. 8(a)). The selectivity is about 5.5–7.5 to ethanol and acetaldehyde in the range of 1–10 ppm. It decreases sharply to approximately 3 when the gas concentration is between 10 and 50 ppm, and then keeps almost unchanged until 1000 ppm (Fig. 8(b)). That is to say, the selective detection of formaldehyde can be achieved in an acetaldehyde or ethanol atmosphere. It is generally accepted that the gas sensing is based on the redox reaction between target gas and chemisorbed oxygen species on the surface of metal oxide semiconductors, which leads to an abrupt resistance change of the metal oxide. Clearly, there are three key factors that determine the redox performances, thus the response and selectivity, including the intrinsic reactivity of the target gas molecules, their adsorption behavior (the adsorption mode and the amount of the adsorbed species) and the diffusion of the gases. Since the ZnO micro-octahedrons are constructed by nanoparticles and only large aggregated pores were observed from the N2 adsorption characterization (data not shown), it is obvious that no big differences exist among the three gases for their diffusion. For their intrinsic reactivity, formaldehyde is more active than acetaldehyde and ethanol. However, in most cases over the same ZnO semiconductors, formaldehyde response is lower or the selectivity between the three gases is poor [40,41]. This means that the
Fig. 8. Responses versus formaldehyde, ethanol and acetaldehyde concentrations (a) and the corresponding selectivity of formaldehyde to ethanol and acetaldehyde (b) in the range of 1–1000 ppm at 400 ◦ C.
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Fig. 9. The cross-response of the sensor to 200 ppm formaldehyde and 15 interfering gases at 400 ◦ C.
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tron depletion layer forms on the ZnO surface, resulting in a high resistance. When exposed to reductive gases, the gases will react with the oxygen species and release the trapped electrons back to the conduction band of ZnO, leading to a distinct decrease in the resistance and thus a response to the detecting gas. Based on the above mechanism, the high formaldehyde response of the nanoparticle-assembled ZnO micro-octahedrons can be explained from two important aspects. On one hand, abundant electron donor defects (Zni and VO ) can adsorb more oxygen and then ionize them on the ZnO micro-octahedron surface (confirmed by XPS analysis). Namely, the amount of oxidant (oxygen species) is increased. This means that much more electrons can be captured from ZnO octahedrons to generate a thicker electron depletion layer and result in a higher resistance of ZnO in air (Ra ). Furthermore, it also means that once formaldehyde is introduced, more formerly tracked electrons can be released back to ZnO through reduction of the large amount of oxygen species with formaldehyde, thus leading to a far lower resistance in testing gas (Rg ). Based on the definition of the gas response (Section 2.2), a larger Ra and a smaller Rg undoubtedly lead to a much higher response value. On the other hand, the dominate oxygen species is O2− at 400 ◦ C in this work, which can seize more electrons from ZnO than the other ones (O2 − , O− ), forming a thicker electron depletion layer and thus a larger Ra . Simultaneously, the oxidation capacity of oxygen species is enhanced, which means that more formaldehyde can be oxidized and a smaller Rg can be achieved. Similarly, the O2− oxygen species also improve the gas response [43]. Actually, the high formaldehyde response of as-synthesized ZnO micro-octahedrons should be attributed to the synergistic effect of the above two items. 4. Conclusions
Fig. 10. Long-term stability of the sensor to 200 ppm formaldehyde at 400 ◦ C.
activity of formaldehyde is not the only factor which determines its high selectivity of the ZnO micro-octahedron sensor. In view of the specific octahedron morphology of ZnO in our work, we speculate that the facet properties of nanoparticle building blocks are unique, which determine the self-assembly of nanoparticles and formation of micro-octahedrons. Thus, the adsorption behaviors of the three gases might vary greatly because facet properties play an important role in gas-sensing performances [34,42]. This would lead to the big differences in response values between formaldehyde and acetaldehyde and ethanol, then present high formaldehyde selectivity. Since the actual sensing process is significantly complicated, further deep investigation are under way in our group. The cross-response was also tested by exposing the sensor to 200 ppm potential interfering gases at 400 ◦ C (Fig. 9). It is obvious that the sensor exhibits considerably higher response to formaldehyde and lower response to the other gases, indicating the excellent selectivity. The long-term stability was also measured by repeating the test several times (Fig. 10). It is clear that the sensor exhibits nearly constant response to 200 ppm formaldehyde in 90 days, confirming the good stability of the sensor. The resistance change of MOS gas sensors is mainly caused by adsorption and desorption of gas molecules on the surface of the sensing materials. In air, oxygen will adsorb on ZnO surface and capture electrons from the conduction band to generate chemisorbed oxygen species (O2 − , O− and O2− ). Thus an elec-
In summary, nanoparticle-assembled ZnO micro-octahedrons were synthesized based on a facile homogeneous precipitation method. Their structure, morphology, surface defects and chemisorbed oxygen species were also investigated. Formaldehyde sensors using ZnO micro-octahedrons exhibited high response, fast response-recovery, good selectivity and stability. Even 1 ppm acetylene could be detected with high response (S = 22.7). Especially, the sensor was able to distinguish formaldehyde from ethanol or acetaldehyde. The high formaldehyde selectivity is speculated to be mainly originated from their big differences in adsorption behavior on the surface of ZnO micro-octahedrons. Moreover, the reason of high formaldehyde response is considered to be the synergistic effect of larger amount of electron donor defects (Zni and VO ) and higher active oxygen species (O2− ). Acknowledgement This research was financially supported by the “BaiRen” program of Chinese Academy of Sciences and autonomic research program (2008BWZ009) of Ministry of Science and Technology, PR China. References [1] R.A. Potyrailo, V.M. Mirsky, Combinatorial and high-throughput development of sensing materials: the first 10 years, Chem. Rev. 108 (2008) 770–813. [2] Y.S. Park, C.W. Litton, T.C. Collins, D.C. Reynolds, Exciton spectrum of ZnO, Phys. Rev. 143 (1966) 512–519. [3] L.S. Mende, J.L.M. Driscoll, ZnO-nanostructures, defects, and devices, Mater. Today 10 (2007) 40–48. [4] C.S. Rout, A.R. Raju, A. Govindaraj, C.N.R. Rao, Hydrogen sensors based on ZnO nanoparticles, Solid State Commun. 138 (2006) 136–138. [5] J. Xu, Y. Chen, D. Chen, J. Shen, Hydrothermal synthesis and gas sensing characters of ZnO nanorods, Sens. Actuators B 113 (2006) 526–531. [6] S. Choopun, N. Hongsith, P. Mangkorntong, N. Mangkorntong, Zinc oxide nanobelts by RF sputtering for ethanol sensor, Physica E 39 (2007) 53–56.
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Biographies Lexi Zhang is currently a candidate for Ph.D. degree in materialogy in the State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences (SKLL, ICCCAS). His research is focused on synthesis and gas sensing properties of low dimensional metal oxide semiconductors. Jianghong Zhao received her Ph.D. degree in catalysis in 2005 from SKLL, ICCCAS and then was appointed as an associate research fellow in 2007 there. Now, she is interested in synthesis of carbon and semiconductor nano- and micro-materials and their applications in the field of energy and environment, especially in photocatalysis and electrocatalysis. Haiqiang Lu received his M.S. degree in chemical engineering in 2005 from Nanjing University of Technology. He is currently a candidate for PhD degree in materialogy in SKLL, ICCCAS. His research is focused on synthesis and photocatalysis of semiconductor nanostructures. Liming Gong received his M.S. degree in industrial catalysis in 2007 from Hunan Normal University. He is currently a candidate for Ph.D. degree in physical chemistry in SKLL, ICCCAS. His research is focused on hydrogen production from water on photocatalysts with solar energy. Li Li received her M.S. degree in 2004 from Tianjin University, majored in synthesis of organic macromolecule polymers. Jianfeng Zhen received his Ph.D. degree in materialogy in 2009 from SKLL, ICCCAS in major of controllable synthesis and mechanism of one dimensional conducting polymers. Recently, his research is focused on synthesis and design of nanostructures, and their TEM characterizations. Hui Li received her Ph.D. degree in materialogy in 2009 from Jilin University. She is now a postdoctoral researcher in SKLL, ICCCAS. Her research is focused on the mechanism of hydrogen evolution driven by visible light through computer simulations. Zhenping Zhu received his M.S. degree in 1992 from Wuhan University and Ph.D. degree in 2000 from SKLL, ICCCAS. He jointed the ICCCAS in 1993 and was appointed as an associated professor in 1997, followed by a full professor in 2000. In 2002, he was admitted a Research Fellow of the Alexander von Humboldt Foundation with Prof. Dr. Robert Schlögl at Fritz-Haber-Institute of MPG, Germany. In 2004, he rejoined ICCCAS and obtained the “Bairen” program support. His recent interests are assembly of nanostructures, their related mechanism and applications in the realm of energy and environment.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
High sensitive and selective formaldehyde sensors based on nanoparticle-assembled ZnO micro-octahedrons synthesized by homogeneous precipitation method Lexi Zhang a,b , Jianghong Zhao a,∗ , Haiqiang Lu a,b , Liming Gong a,b , Li Li a , Jianfeng Zheng a , Hui Li a,b , Zhenping Zhu a,∗ a b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taoyuan South Road 27, Taiyuan 030001, PR China Graduate University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e
i n f o
Article history: Received 31 May 2011 Received in revised form 18 July 2011 Accepted 28 July 2011 Available online 4 August 2011 Keywords: Homogeneous precipitation ZnO micro-octahedron Gas sensor Formaldehyde Defect Oxygen species
a b s t r a c t Nanoparticle-assembled ZnO micro-octahedrons were synthesized by a facile homogeneous precipitation method. The ZnO micro-octahedrons are hexagonal wurtzite with high crystallinity. Abundant structure defects were confirmed on ZnO surface by photoluminescence. Gas sensors based on the ZnO microoctahedrons exhibited high response, selectivity and stability to 1–1000 ppm formaldehyde at 400 ◦ C. Especially, even 1 ppm formaldehyde could be detected with high response (S = 22.7). It is of interest to point out that formaldehyde could be easily distinguished from ethanol or acetaldehyde with a selectivity of about 3. The high formaldehyde response is mainly attributed to the synergistic effect of high contents of electron donor defects (Zni and VO ) and highly active oxygen species (O2− ) on the ZnO surface. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Metal oxide semiconductor (MOS) gas sensors, including SnO2 , ZnO, In2 O3 , WO3 , and so on, have attracted considerable attention owing to their ability of detecting trace gases [1]. As one of the key wide bandgap (∼3.4 eV at 1.2 K) [2] semiconductors, ZnO has been proved to be an excellent gas-sensing material for measuring both oxidative and reductive target gases at ppm (parts per million) level and above [3]. Taking advantages of the small size, large surface-to-volume ratios and high density of surface active sites compared to their bulk counterparts, great interest has been focused on performance-enhanced gas sensors based on ZnO nanostructures, such as nanoparticles [4], nanorods [5], nanobelts [6], nanotubes [7] and nanosheets [8]. Recently, hierarchical structures constructed by low-dimensional nanomaterials, for example, nanoparticle-organized hollow spheres [9], nanorodcombined flower-like structures [10], nanosheet-assembled 3D architectures [11,12], have began to catch much of researchers’ attention, because they exhibited enhanced gas-sensing performances which originated from the improvement in exposing more
∗ Corresponding authors. Tel.: +86 351 4048715; fax: +86 351 4041153. E-mail addresses: zjh [email protected] (J. Zhao), [email protected] (Z. Zhu). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.07.062
available surface, facilitating gas diffusion and transportation, and so forth. However, great efforts are still needed to further develop their synthesis processes, since hard templates, surfactants or relative high temperature is usually necessary for fabricating these hierarchical structures. Accordingly, it is significantly important to develop template-free, facile and low temperature methods to synthesize novel hierarchical nanostructures, and to carry out indepth research on their gas sensing properties. Compared with other methods, homogeneous precipitation is a more economic (no need for special apparatus) and environment-friendly (no need for surfactants or organic solvents) method to prepare metal oxides for sensor applications [13] in large scale at low temperature. In spite of its advantages in preparation, based on this method, it is usually difficult to controlled synthesize metal oxide nanostructures [14] (except for nanoparticles), to say nothing of nano-building block assembled hierarchical architectures. As an important industrial chemical, formaldehyde has been widely used to manufacture plastics, medicine, synthetic fibers and household products. Regrettably, formaldehyde is very harmful to human health because of its volatility, irritability and toxicity, thus is considered as one of the main indoor air pollutants in residential and industrial occupational environments. It is of great practical importance to detect formaldehyde rapidly and accurately in the atmosphere. Till now, significant progress has been
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Fig. 1. (a) Scheme of the gas sensor structure. (b) The measuring electric circuit. Inset: photograph of a sensor.
made to formaldehyde sensors based on ZnO nanostructures; however, their gas-sensing performance is still required to be further improved in sensitivity, especially in selectivity. This is because for the practical MOS formaldehyde gas sensors, acetaldehyde is a common interfering gas [15], and formaldehyde is usually difficult to be distinguished from ethanol especially in indoor air detection [16]. Hence, highly selective formaldehyde gas sensors, in particular, to acetaldehyde and ethanol, are of great meanings in practical application. In this work, ZnO micro-octahedrons assembled by nanoparticles were successfully synthesized through a facile homogeneous precipitation method. Their structure, morphology, surface defects and chemisorbed oxygen were investigated. Gas sensors using ZnO micro-octahedrons were tested and represented excellent formaldehyde sensing properties. Furthermore, the reason of high formaldehyde selectivity and response was also demonstrated.
configuration, excited by the 514 nm line of an argon-ion laser at room temperature. Field emission scanning electron microscope (FE-SEM) images were carried out on a JEOL JSM-6700F microscope operating at 5 kV. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2010 microscope with an accelerating voltage of 200 kV. UV–Vis spectrum was measured on a Shimadzu UV-3600 UV-Vis-NIR spectrophotometer at room temperature. Photoluminescence (PL) was measured on a Hitachi F-7000 FL spectrophotometer by a 325 nm excitation from Xe lamp at room temperature. X-ray photoelectron spectrometry (XPS) was carried out using Al K␣ (h = 1486.6 eV) X-ray beams as the excitation source. Binding energies were calibrated relative to the C1s peak at 284.6 eV.
2. Experimental
The ZnO sample was ground with Triton X-100 in a weight ratio of 1:1 to obtain a fine paste. The paste was coated onto an alumina ceramic tube, on which a pair of gold electrodes was previously installed at each end, followed by sintering at 500 ◦ C for 1 h to remove the organic binder and provide good mechanical strength. Then, a Ni–Cr wire was inserted into the tube and used as a heater. The structural diagram and photograph of a sensor are shown in Fig. 1(a) and inset of Fig. 1(b), respectively. The gas-sensing properties were measured using a HW-30A gas sensitivity instrument (Hanwei Electronics Co. Ltd., PR China). The gas concentration was determined by a stationary state process: a given amount of target gas was injected into a glass chamber and fully mixed with air. In the measuring electric circuit (Fig. 1(b)), a load resistor (RL : 47 k) was connected in series with a gas sensor. The circuit voltage (Vc ) was 5 V, and output
2.1. Preparation and characterization of materials All the chemicals are analytical grade reagents and used as received without further purification. In a typical synthesis, 0.2195 g Zn(CH3 COO)2 ·H2 O was dissolved in 20 mL deionized water under magnetic stirring. Then aqueous ammonia (25–28 wt.%) was added until the pH value was adjusted to 10. After another 2 h stirring, the white precipitation was filtered, washed with deionized water, dried at 80 ◦ C for 12 h and finally calcinated at 500 ◦ C for 1 h. Powder X-ray diffraction (XRD) was recorded on a D8 Advance Bruker X-ray diffractometer with Cu K␣ radiation ( = 0.15406 nm) operating at 40 kV. Raman spectrum was performed by a JY LabRam-HR confocal Raman microscope with a backscattering
2.2. Fabrication and measurement of sensors
Fig. 2. XRD pattern (a) and Raman spectrum (b) of ZnO products.
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Fig. 3. FE-SEM image (a and b), TEM image (c) and the corresponding SAED pattern (inset) of nanoparticle-assembled ZnO micro-octahedrons.
voltage (Vout ) was the terminal voltage of the load resistor. The operating temperature of a sensor was adjusted through varying the heating voltage (Vh ). The sensor resistance in air or a target gas was measured by monitoring Vout . The system measures Vout loaded on the resistor RL , and the resistances (RS ) of the gas sensor can be calculated as: RS = RL (Vc − Vout )/Vout . The gas response (S) is defined as S = Ra /Rg (reductive gas) or S = Rg /Ra (oxidative gas), where Ra and Rg are the resistances of the sensor in air and target gases, respectively. Besides, the selectivity of formaldehyde to the other gases (K) is defined as K = S/Sig , where Sig is the response of the sensor to a certain interfering gas. Here, the response or recovery time was defined as the time taken for the sensor to achieve 90% of its maximum response or decreases to 10% of its maximum response, respectively. Long-term stability of the sensor was measured by repeating the test at 400 ◦ C 8 times in 90 days.
of ZnO products, which is in good accordance with the XRD analysis and provide some useful information about surface defects. The morphology of the ZnO sample was performed by FESEM. Fig. 3(a) exhibits a panoramic image, showing the ZnO are composed of octahedral crystals (their profiles were marked by dashed lines) with very rough surfaces and mean edge length about 4.5 m. Fig. 3(b) displays enlarged image of a representative ZnO micro-octahedron, which is self-assembled of nanoparticles with diameters of 200–300 nm. A large number of interspaces were formed between nanoparticles, which are conducive to enhance the gas response since gas diffusion and transportation can be prominently improved [12]. TEM image (Fig. 3(c)) confirms that the ZnO micro-octahedrons are assembled of nanoparticles. Furthermore, the ZnO octahedron profile, which is partially maintained, suggests that the nanoparticles are loosely interconnected. The corresponding SAED pattern (inset in Fig. 3(c)) is clear and regular concentric rings, indicating that the ZnO octahedrons are polycrystals and well crystallized, which is in good agreement with the XRD and Raman results.
3. Results and discussion 3.2. Optical characterization 3.1. Structure and morphology analysis The structure of the ZnO products was characterized by XRD (Fig. 2(a)). All peaks can be well indexed to hexagonal wurtzite ZnO (JCPDS card no. 36-1451), indicating high purity and crystallinity. In the Raman spectrum (Fig. 2(b)), the peaks at 330 and 436 cm−1 are attributed to the 3E2H –E2L and E2H mode of the ZnO crystal [17], respectively. The peak at 380 cm−1 belongs to the A1 (TO) mode [18]. The peak at 580 cm−1 assigned to the E1 (LO) mode suggests that the ZnO sample are oxygen deficient, which usually leads to electron donor related defects, such as oxygen vacancies (VO ) and zinc interstitials (Zni ) [18]. The Raman results confirm the structure
UV–Vis diffuse reflectance spectroscopy of ZnO microoctahedrons was performed to study the bandgap and electron states (Fig. 4(a)). Extrapolation of the linear part until it intersects the wavelength-axis gives the band edge of 409.74 nm. ZnO is a direct-band semiconductor, so the relation between the absorption coefficient (˛) and incident photon energy (h) can be written as ˛h = C(h − Eg )1/2 , where C is a constant and Eg is the band gap [19]. Through extrapolating the linear portion of the (h) − (˛h)2 plot to (˛h)2 = 0, the bandgap energy of the ZnO micro-octahedrons is 3.14 eV. The steep shape of the UV edge and the strong absorption in the UV region imply that the absorption band of ZnO micro-
Fig. 4. UV–Vis (a) and PL spectra (b) of the nanoparticle-assembled ZnO micro-octahedrons. Inset in (a): plot of (˛h)2 versus h. Inset in (b): sub-peak positions and relative contents separated from the PL spectrum.
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Fig. 5. XPS spectra of the nanoparticle-assembled ZnO micro-octahedrons: Zn2p3/2 (a) and O1s peaks (b). Inset in (b): binding energy positions and percentages of three peaks (OL , OV and OC ) separated from O1s .
octahedrons is ascribed to the intrinsic transition between the valence band (VB) and conduction band (CB). As a useful tool for characterizing intrinsic defects, the room temperature PL spectrum of the ZnO micro-octahedrons is shown in Fig. 4(b), in which both weak UV emission (<400 nm) and strong visible emission (400–800 nm) were observed. The strong visible emission suggests abundant structure defects [20] on the surface of ZnO micro-octahedrons. Through curve deconvolution, the PL spectrum can be well-fitted to the superposition of several Gaussian sub-peaks and were assigned to different defects. Since the band gap of ZnO is 3.1–3.4 eV at room temperature, the UV emission at 366.4 nm is attributed to the recombination of free excitons between the VB and CB [21]. The peak at 406.7 nm belongs to shallow donor-related UV emission [22], on the basis of the band edge (409.74 nm) calculated from the UV–Vis spectrum (Fig. 4 (a)). The peaks at 432.8, 471.1, 520.5 and 623.6 nm are ascribed to interstitial zinc (Zni ) [23], zinc vacancy (VZn ) [24], oxygen antisite (OZn ) [25] and oxygen interstitial (Oi ) [26], respectively. The peaks at 572.8 and 684.5 nm are usually assigned to oxygen vacancies (VO ) [27,28], in spite of long-time controversies about the peak position originated from VO . The gas response can be remarkably enhanced with the increase of electron donor defects (e.g. Zni and VO ) and with the decrease of electron accepter ones (e.g. VZn , Oi and OZn ), because much more oxygen are able to be chemisorbed [20,29,30] and ionized [31] on the surface of MOS. The sub-peak positions and relative contents of the total PL spectrum were listed in Fig. 4(b). The calculated contents of donor and acceptor defects are 48.86% and 51.14%, respectively. Since the content of electron donor defects is higher than that of ZnO nanoparticles (35.5%) [32] and nanorods (47.91%) [31], we speculate that the
ZnO micro-octahedrons should exhibit much higher response to formaldehyde. XPS analysis was also carried out to obtain the surface structural and compositional information of ZnO micro-octahedrons. Fig. 5(a) shows the Zn2p3/2 peak centered at 1020. 8 eV, whose peak pattern is symmetrical, indicating no variable valence of Zn. Correspondingly, the O1s peak is asymmetrical with an obvious shoulder (Fig. 5(b)). The O1s peak can be deconvoluted into three Gaussian components centered at 529.96, 531.09 and 531.95 eV (inset in Fig. 5(b)), which are attributed to O2− ions in ZnO lattice (OL ), O2− ions in oxygen-deficient regions within the ZnO matrix (OV ) and chemisorbed and dissociated oxygen species or OH (OC ) [33], respectively. The gas sensing properties are closely related to the chemisorbed oxygen species [34], thus the high percentage of OC component (14.25%) implies high response of as-synthesized ZnO micro-octahedrons. 3.3. Gas-sensing properties The response of ZnO micro-octahedrons to 200 ppm formaldehyde was tested versus operating temperatures (Fig. 6(a)). Clearly, the response reaches a maximum value at 400 ◦ C, which is determined as the optimum operating temperature (lower than the autoignition temperature of formaldehyde (430 ◦ C)). The maximum response (S = 314) is much higher than some results ever reported for ZnO nanostructure sensors to formaldehyde based on nanoparticles [32], nanorods [35] and nanosheets [8]. Fig. 6(b) illustrates the correlation between the response of the sensor and formaldehyde concentrations (1–1000 ppm) at 400 ◦ C. The response rapidly increases with increasing the formaldehyde
Fig. 6. (a) Responses of the sensor to 200 ppm formaldehyde versus operating temperatures. (b) Responses versus formaldehyde concentrations in the range of 1–1000 ppm at 400 ◦ C and the corresponding log(S − 1) − log(C) plot (inset).
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Fig. 7. Response-recovery curves of the sensor to 1–1000 (a) and 200 ppm (b) formaldehyde at 400 ◦ C.
concentration, indicating that it still has not become saturated at 1000 ppm. When exposed to 1 ppm formaldehyde, the response is as high as 22.7. Usually, the response of MOS gas sensors can be empirically represented as S = a(C)b + 1, where a and b are constants and C is the target gas concentration [36]. The value of b is normally between 0.5 and 1.0, depending on the charge of the surface species and the stoichiometry of the elementary reactions on MOS surface [37]. For b of 0.5, the adsorbed surface oxygen ion is O2− and for b of 1, that is O− [38]. At a certain temperature, the equation can be rewritten as: log(S − 1) = blog(C) + log a. Log(S − 1) has a linear relation with log(C), thus the slope of b value can be calculated from a plot of log(S − 1) − log(C) (inset of Fig. 6(b)). The b value (0.4826) is very close to 0.5, indicating the adsorbed oxygen species is mainly O2− on the surface of ZnO micro-octahedrons, which is well supported by the fact that the dominated oxygen species on ZnO surface is O2− above 300 ◦ C [39]. Furthermore, after extrapolating the linear fitted curve to S = 3, the detection limit (S ≥ 3) of the sensor is calculated to be as low as 0.0028 ppm, which is far lower than the established limits of long-term exposure to formaldehyde by WHO (0.08 ppm), NIOSH (0.016 ppm) and OSHA (0.75 ppm). Fig. 7(a) presents typical response and recovery curve of the sensor to 1–1000 ppm formaldehyde at 400 ◦ C. When exposed to a certain concentration of formaldehyde, the response increases fast and reaches an equilibrium value. Once disengaged from formaldehyde, the response decreases rapidly to the baseline, indicating very good reliability and reproducibility. For 200 ppm formaldehyde, the response and recovery time are 46 and 13 s, respectively (Fig. 7(b)). Actually, in our experiments, in order to provide a certain gas concentration, appropriate volume of formaldehyde aqueous solution was introduced into the test chamber, vaporized and mixed homogeneously with air by two fans, which took about 30 s to fully
vaporize the solution. So, the real response time should be much shorter than 46 s. Formaldehyde and acetaldehyde are usually difficult to be differentiated by MOS gas sensors, because of their quite similar molecular structures and chemical properties [15]. Besides, ethanol is another interfering gas to formaldehyde especially in indoor air detection [16]. It is very interesting to find that the ZnO microoctahedron sensor displays much higher response to formaldehyde than that to acetaldehyde and ethanol in the whole testing range (1–1000 ppm) at 400 ◦ C (Fig. 8(a)). The selectivity is about 5.5–7.5 to ethanol and acetaldehyde in the range of 1–10 ppm. It decreases sharply to approximately 3 when the gas concentration is between 10 and 50 ppm, and then keeps almost unchanged until 1000 ppm (Fig. 8(b)). That is to say, the selective detection of formaldehyde can be achieved in an acetaldehyde or ethanol atmosphere. It is generally accepted that the gas sensing is based on the redox reaction between target gas and chemisorbed oxygen species on the surface of metal oxide semiconductors, which leads to an abrupt resistance change of the metal oxide. Clearly, there are three key factors that determine the redox performances, thus the response and selectivity, including the intrinsic reactivity of the target gas molecules, their adsorption behavior (the adsorption mode and the amount of the adsorbed species) and the diffusion of the gases. Since the ZnO micro-octahedrons are constructed by nanoparticles and only large aggregated pores were observed from the N2 adsorption characterization (data not shown), it is obvious that no big differences exist among the three gases for their diffusion. For their intrinsic reactivity, formaldehyde is more active than acetaldehyde and ethanol. However, in most cases over the same ZnO semiconductors, formaldehyde response is lower or the selectivity between the three gases is poor [40,41]. This means that the
Fig. 8. Responses versus formaldehyde, ethanol and acetaldehyde concentrations (a) and the corresponding selectivity of formaldehyde to ethanol and acetaldehyde (b) in the range of 1–1000 ppm at 400 ◦ C.
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Fig. 9. The cross-response of the sensor to 200 ppm formaldehyde and 15 interfering gases at 400 ◦ C.
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tron depletion layer forms on the ZnO surface, resulting in a high resistance. When exposed to reductive gases, the gases will react with the oxygen species and release the trapped electrons back to the conduction band of ZnO, leading to a distinct decrease in the resistance and thus a response to the detecting gas. Based on the above mechanism, the high formaldehyde response of the nanoparticle-assembled ZnO micro-octahedrons can be explained from two important aspects. On one hand, abundant electron donor defects (Zni and VO ) can adsorb more oxygen and then ionize them on the ZnO micro-octahedron surface (confirmed by XPS analysis). Namely, the amount of oxidant (oxygen species) is increased. This means that much more electrons can be captured from ZnO octahedrons to generate a thicker electron depletion layer and result in a higher resistance of ZnO in air (Ra ). Furthermore, it also means that once formaldehyde is introduced, more formerly tracked electrons can be released back to ZnO through reduction of the large amount of oxygen species with formaldehyde, thus leading to a far lower resistance in testing gas (Rg ). Based on the definition of the gas response (Section 2.2), a larger Ra and a smaller Rg undoubtedly lead to a much higher response value. On the other hand, the dominate oxygen species is O2− at 400 ◦ C in this work, which can seize more electrons from ZnO than the other ones (O2 − , O− ), forming a thicker electron depletion layer and thus a larger Ra . Simultaneously, the oxidation capacity of oxygen species is enhanced, which means that more formaldehyde can be oxidized and a smaller Rg can be achieved. Similarly, the O2− oxygen species also improve the gas response [43]. Actually, the high formaldehyde response of as-synthesized ZnO micro-octahedrons should be attributed to the synergistic effect of the above two items. 4. Conclusions
Fig. 10. Long-term stability of the sensor to 200 ppm formaldehyde at 400 ◦ C.
activity of formaldehyde is not the only factor which determines its high selectivity of the ZnO micro-octahedron sensor. In view of the specific octahedron morphology of ZnO in our work, we speculate that the facet properties of nanoparticle building blocks are unique, which determine the self-assembly of nanoparticles and formation of micro-octahedrons. Thus, the adsorption behaviors of the three gases might vary greatly because facet properties play an important role in gas-sensing performances [34,42]. This would lead to the big differences in response values between formaldehyde and acetaldehyde and ethanol, then present high formaldehyde selectivity. Since the actual sensing process is significantly complicated, further deep investigation are under way in our group. The cross-response was also tested by exposing the sensor to 200 ppm potential interfering gases at 400 ◦ C (Fig. 9). It is obvious that the sensor exhibits considerably higher response to formaldehyde and lower response to the other gases, indicating the excellent selectivity. The long-term stability was also measured by repeating the test several times (Fig. 10). It is clear that the sensor exhibits nearly constant response to 200 ppm formaldehyde in 90 days, confirming the good stability of the sensor. The resistance change of MOS gas sensors is mainly caused by adsorption and desorption of gas molecules on the surface of the sensing materials. In air, oxygen will adsorb on ZnO surface and capture electrons from the conduction band to generate chemisorbed oxygen species (O2 − , O− and O2− ). Thus an elec-
In summary, nanoparticle-assembled ZnO micro-octahedrons were synthesized based on a facile homogeneous precipitation method. Their structure, morphology, surface defects and chemisorbed oxygen species were also investigated. Formaldehyde sensors using ZnO micro-octahedrons exhibited high response, fast response-recovery, good selectivity and stability. Even 1 ppm acetylene could be detected with high response (S = 22.7). Especially, the sensor was able to distinguish formaldehyde from ethanol or acetaldehyde. The high formaldehyde selectivity is speculated to be mainly originated from their big differences in adsorption behavior on the surface of ZnO micro-octahedrons. Moreover, the reason of high formaldehyde response is considered to be the synergistic effect of larger amount of electron donor defects (Zni and VO ) and higher active oxygen species (O2− ). Acknowledgement This research was financially supported by the “BaiRen” program of Chinese Academy of Sciences and autonomic research program (2008BWZ009) of Ministry of Science and Technology, PR China. References [1] R.A. Potyrailo, V.M. Mirsky, Combinatorial and high-throughput development of sensing materials: the first 10 years, Chem. Rev. 108 (2008) 770–813. [2] Y.S. Park, C.W. Litton, T.C. Collins, D.C. Reynolds, Exciton spectrum of ZnO, Phys. Rev. 143 (1966) 512–519. [3] L.S. Mende, J.L.M. Driscoll, ZnO-nanostructures, defects, and devices, Mater. Today 10 (2007) 40–48. [4] C.S. Rout, A.R. Raju, A. Govindaraj, C.N.R. Rao, Hydrogen sensors based on ZnO nanoparticles, Solid State Commun. 138 (2006) 136–138. [5] J. Xu, Y. Chen, D. Chen, J. Shen, Hydrothermal synthesis and gas sensing characters of ZnO nanorods, Sens. Actuators B 113 (2006) 526–531. [6] S. Choopun, N. Hongsith, P. Mangkorntong, N. Mangkorntong, Zinc oxide nanobelts by RF sputtering for ethanol sensor, Physica E 39 (2007) 53–56.
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Biographies Lexi Zhang is currently a candidate for Ph.D. degree in materialogy in the State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences (SKLL, ICCCAS). His research is focused on synthesis and gas sensing properties of low dimensional metal oxide semiconductors. Jianghong Zhao received her Ph.D. degree in catalysis in 2005 from SKLL, ICCCAS and then was appointed as an associate research fellow in 2007 there. Now, she is interested in synthesis of carbon and semiconductor nano- and micro-materials and their applications in the field of energy and environment, especially in photocatalysis and electrocatalysis. Haiqiang Lu received his M.S. degree in chemical engineering in 2005 from Nanjing University of Technology. He is currently a candidate for PhD degree in materialogy in SKLL, ICCCAS. His research is focused on synthesis and photocatalysis of semiconductor nanostructures. Liming Gong received his M.S. degree in industrial catalysis in 2007 from Hunan Normal University. He is currently a candidate for Ph.D. degree in physical chemistry in SKLL, ICCCAS. His research is focused on hydrogen production from water on photocatalysts with solar energy. Li Li received her M.S. degree in 2004 from Tianjin University, majored in synthesis of organic macromolecule polymers. Jianfeng Zhen received his Ph.D. degree in materialogy in 2009 from SKLL, ICCCAS in major of controllable synthesis and mechanism of one dimensional conducting polymers. Recently, his research is focused on synthesis and design of nanostructures, and their TEM characterizations. Hui Li received her Ph.D. degree in materialogy in 2009 from Jilin University. She is now a postdoctoral researcher in SKLL, ICCCAS. Her research is focused on the mechanism of hydrogen evolution driven by visible light through computer simulations. Zhenping Zhu received his M.S. degree in 1992 from Wuhan University and Ph.D. degree in 2000 from SKLL, ICCCAS. He jointed the ICCCAS in 1993 and was appointed as an associated professor in 1997, followed by a full professor in 2000. In 2002, he was admitted a Research Fellow of the Alexander von Humboldt Foundation with Prof. Dr. Robert Schlögl at Fritz-Haber-Institute of MPG, Germany. In 2004, he rejoined ICCCAS and obtained the “Bairen” program support. His recent interests are assembly of nanostructures, their related mechanism and applications in the realm of energy and environment.