Hierarchically Porous ZnO Architectures for Gas Sensor Application

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Hierarchically Porous ZnO Architectures for Gas Sensor Application Jun Zhang, Shurong Wang, Mijuan Xu, Yan Wang, Baolin Zhu, Shoumin Zhang, Weiping Huang, and Shihua Wu*

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 8 3532–3537

Department of Chemistry, Nankai UniVersity, Tianjin 300071, China ReceiVed March 5, 2009; ReVised Manuscript ReceiVed April 28, 2009

ABSTRACT: Hierarchically three-dimensional (3D) porous ZnO architectures were synthesized by a template-free, economical hydrothermal method combined with subsequent calcination. First, a precursor of hierarchical basic zinc carbonate (BZC) nanostructures self-assembled by sheet-like blocks was prepared. Then calcination of the precursor produced hierarchically 3D porous ZnO architectures composed of interconnected ZnO nanosheets with high porosity resulting from the thermal decomposition of the precursor. The products were characterized by X-ray diffraction, Fourier tranform infrared spectroscopy, thermogravimetric-differential thermalgravimetric analysis, scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller N2 adsorption-desorption analyses. Control experiments with variations in solvent and reaction time respectively revealed that ethanol was responsible for the formation of the BZC precursor, and the self-assembly of BZC nanosheets into hierarchically 3D architectures was highly dependent on the reaction time. Gas sensing tests showed that these hierarchically porous ZnO architectures were highly promising for gas sensor applications, as the gas diffusion and mass transportation in sensing materials were significantly enhanced by their unique structures. Moreover, it is believed that this solution-based approach can be extended to fabricate other porous metal oxide materials with a unique morphology or shape.

1. Introduction Zinc oxide, as a functional n-type semiconductor, has stimulated great research interest owing to its unique optical and electrical properties1-3 that are useful for piezoelectric nanogenerators,4 solar cells,5 nanolasers,6 gas sensors,7-9 and so forth. To date, considerable efforts have been devoted to fabricating various ZnO nanostructures, including nanowires,10,11 nanorods,12 nanobelts,13 nanotubes,14 nanosheets,15,16 and so on, because of their strong size or morphology-dependent properties or device performances.17 Among these nanostructures, onedimensional (1D) ZnO such as nanowires and nanorods have received the most attention, especially those derived from solution-based approaches due to merits such as low reaction temperature, ease of scale-up, and facile and economic synthesis.18,19 This is partly because these 1D nanostructures possess unique electron transport properties,20-22 which are favorable for electronic devices. One the other hand, ZnO is known to readily and intrinsically grow into 1D morphology in a solution environment due to its unique hexagonal crystal structure. It is well-known that elongated ZnO material grown from solution-based approaches has both polar and nonpolar faces.23 Normally, ZnO nuclei tend to aggregate along the polar face direction resulting in a 1D nanostructure (axial growth).24 However, if the polar faces are passivated by growth modifiers, the axial growth would be suppressed and then tabular nanostructures such as sheet- or plate-like ZnO could be obtained (equatorial growth).24 Thus, by choosing the proper growth modifier, one can selectively obtain ZnO crystals with different orientations or morphologies for exploring novel properties. More recently, two-dimensional (2D) porous ZnO nanosheets combining unique sheet-like morphology and porous structure have attracted great research interest because of their significantly enhanced properties in photoluminescence and gas sensor applications. For example, Fu et al.25 have synthesized porous * Corresponding author. Tel: +86 22 2350 5896. E-mail: wushh@ nankai.edu.cn.

ZnO nanosheets by a colloidal crystal assisted electrochemical deposition method. Xu et al.26 have prepared, through a selfassembly pathway, hierarchically porous ZnO spherical nanoparticles that could be used for photocatalysts. Reeja-Jayan et al.27 have reported the synthesis and photoluminescence of porous ZnO nanodisks. Almost synchronously, Jing et al.28 and Zhou et al.29 reported the gas sensor and photoluminescence properties of hierarchically three-dimensional (3D) porous ZnO nanosheets mediated by microwave and surfactant cetyltrimethylammonium bromide (CTAB) with a reaction time of more than 8 h, respectively. Compared with low dimensions (1D and 2D), the 3D nanostructures can provide more chances for exploring novel properties and superior device performances. In particular, the solution-based approach for self-assembling low dimensions into 3D nanostructures is recently becoming attractive.30,31 From the viewpoint of gas sensors, hierarchically porous or hollow sphere materials are promising candidates because their special structures can usually provide a large surface-to-volume ratio that can greatly facilitate gas diffusion and mass transport in sensor material, thus improving sensor performance.32-36 The conventional methods for preparing porous or hollow materials usually require the use of poredirecting reagents or templates36 and may suffer from contamination due to the uncompleted removal of the additives either by chemical etching or thermal treatment. Thus, a facile, economical, template-free method for producing porous materials is of great significance from the view of both scientific research and practical application. Herein, a template-free hydrothermal route combined with subsequent thermal treatment is demonstrated for the synthesis of hierarchically porous ZnO architectures. First, a hierarchically 3D basic zinc carbonate (BZC) precursor assembled by 2D nanosheets was prepared. Then, calcination of the BZC precursor produced hierarchically porous ZnO nanostructures without collapse of the 3D morphology. The high porosity of these hierarchical ZnO nanostructures provides a great potential for gas sensing applications. A comparative gas sensing study between the as-prepared

10.1021/cg900269a CCC: $40.75  2009 American Chemical Society Published on Web 05/15/2009

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hierarchically porous ZnO nanostructures and commercial ZnO powders was performed to depict the superior sensing properties of the hierarchically porous ZnO material. Moreover, our work also hints that the present approach is a general and facile method with good potential for scale-up that can be extended to fabricate other porous materials, especially with a defined morphology.

2. Experimental Section 2.1. Materials. All chemicals (A.R. grade) below were purchased from Guangfu Fine Chemical Research Institute (Tianjin, China) and used as received without further purification. Distilled water was used throughout the experiment. 2.2. Synthesis. The basic zinc carbonate (BZC) precursor was prepared as follows: 1.7849 g of Zn(NO3)2 · 6H2O was dissolved into 30 mL of absolute ethanol. Then, 30 mL of 0.1 M hexamethylenetetramine (HMT) aqueous solution was added into the ethanol solution under stirring. After being stirred for 30 min, the mix solution was transferred into a Teflon autoclave with a volume of 75 mL and maintained at 160 °C in an electric oven for 8 h. After naturally being cooled down, the white precipitate was centrifuged and washed with water and absolute ethanol several times, then dried at 80 °C for several hours. The final product of hierarchically porous ZnO nanosheets was obtained by annealing the BZC precursor at 300 °C in a muffle furnace for 2 h. 2.3. Characterizations. The products were characterized by X-ray diffraction analysis (XRD, Rigaku D/max-2500, graphite monochromator, Cu KR, λ ) 1.5418 Å), scanning electron microscope (SEM, Shimadzu SS-550, 15 kV), thermogravimetric analysis (TGA, ZRY2P, 10 °C/min), Fourier transform infrared spectrometer (FTIR, Avatar 380 FT-IR, KBr pellets), transmission electron microscope (TEM, Philips FEI Tecnai 20ST, 200 kV), and Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption (Belsorp Mini). The pore-size distribution was determined from the adsorption branch of the isotherms using the Barett-Joyner-Halenda method. 2.4. Sensor Fabrication and Test. The gas sensor was fabricated by coating aqueous slurry of the prepared porous ZnO material onto an alumina tube with a diameter of 1 mm and length of 4 mm, which is positioned with a pair of Au electrodes and four Pt wires on both ends of the tube. A Ni-Cr alloy coil through the tube was employed as a heater to control the operating temperature. Gas sensing tests were performed on a commercial HW-30A Gas Sensing Measurement System (HanWei Electronics Co., Ltd., Henan, China) at a relative humidity of 20-30%. Target gas such as ethanol was introduced into the testing chamber on HW-30A by a microsyringe. The sensor sensitivity is defined as the ratio S ) Ra/Rg, where Ra and Rg are the electrical resistance of the sensor in air and in test gas, respectively. Photographs of the gas sensor (Figure S1) and working principle of HW-30A system (Figure S2) are shown in the Supporting Information.

3. Results and Discussion 3.1. Basic Zinc Carbonate (BZC) Precursor and Its Thermal Evolution to Hierarchically Porous ZnO Architectures. The hierarchically porous ZnO nanostructures were obtained through a two-step procedure. First, a basic zinc carbonate (BZC) precursor with a hierarchical structure was derived from a hydrothermal process. Then calcination of the precursor yielded hierarchical architectures that were assembled by porous ZnO nanosheets. The crystal phase of the BZC precursor with different reaction times (2, 4, and 8 h) was characterized by XRD, and the data are shown in Figure 1. All the diffraction peaks of the precursors can be indexed as monoclinic hydrozincite Zn5(CO3)2(OH)6 (JCPDS 19-1458), which is in good accordance with the recent report.28 Figure 1d shows the XRD pattern of the calcined ZnO product (300 °C, 2 h), with a perfect indexation as hexagonal wurtzite ZnO (JCPDS 36-1451), from the BZC precursor with a reaction time of 8 h. No peaks for other impurities can be detected, indicating a complete decomposition of the BZC precursor into pure ZnO

Figure 1. XRD patterns of the BZC precursor with different hydrothermal times (a, 2 h; b, 4 h; and c, 8 h) and calcined ZnO product of the BZC precursor with a hydrothermal time of 8 h.

Figure 2. TG-DTA analysis of the BZC precursor with a hydrothermal time of 8 h.

after calcination. The thermal stability of the BZC precursor was examined by TG-DTA analysis (Figure 2). It shows that the thermal decomposition of the precursor is an endothermic process (DTA). The TG curve exhibits a total weight loss of 21.08% attributed to the decomposition of carbonate and hydroxide in the precursor. Figure 3 displays the FTIR spectra of the BZC precursor and ZnO product, showing that the spectra are significantly different from each other. Especially, those peaks at 708, 836, 1388, 1508 cm-1 (Figure 3a) corresponding to the bending vibration of CO32- have disappeared or severely weakened in Figure 3b, indicating the decomposition of carbonate in the precursor. In addition, the peak at 3324 cm-1 (Figure 3a) related to the large amount of -OH in the precursor is substituted by a much weaker peak at 3432 cm-1 (Figure 3b) attributed to surface absorbed water after calcination. The morphology of the precursor before and after calcination is characterized by SEM. Figure 4 displays the SEM images of the BZC precursor with a reaction time of 8 h and the corresponding ZnO product after calnication. It can be seen that both samples have a hierarchically 3D morphology that is assembled by 2D nanosheets with a thickness ranging from tens to hundreds of nanometers. The ZnO product well maintained the hierarchically 3D morphology of the precursor without

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Figure 3. FTIR data of (a) BZC precursor with a hydrothermal time of 8 h and (b) corresponding calcined ZnO product.

Figure 5. TEM images of the calcined ZnO product.

Figure 4. SEM images of (a and b) BZC precursor with a hydrothermal time of 8 h and (c and d) corresponding calcined ZnO product.

significant collapse after calcination, as revealed in Figure 4c,d. Unlike the work reported by Jing et al.,28 the pore structures in our ZnO nanosheets are not visible from SEM images (Figure 4d). This is probably due to the much higher thickness (tens to hundreds of nanometers) of our nanosheets compared with their only ∼19 nm thickness. However, detailed observation from the TEM images (Figure 5) clearly confirms the high porosity of the ZnO nanosheets, which resulted from the thermal decomposition of carbonate and hydroxide of the precursor. As can be seen, a large amount of irregular pores of tens of nanometers are randomly distributed in the nanosheets. The SAED pattern (Figure 5d) confirms that these porous ZnO nanosheets are well crystallined after calcination. To get further information of the pores in the nanosheet, BET N2 adsorptiondesorption analysis was performed. Figure 6 displays the adsorption-desorption isotherm and BJH pore size distribution as the inset. The porous ZnO nanosheets material has a surface area of 37.47 cm3/g, which is not very high because the pores embedded in the nanosheets have a small inner surface and low pore volume. According to the BJH pore size distribution curve (inset of Figure 6) calculated from the adsorption isotherm, the ZnO nanosheets exhibit a mesoporous structure with pore diameter centered at 6.96 nm.

Figure 6. N2 adsorption-desorption isotherm and BJH pore size distribution of the calcined ZnO product.

3.2. Formation Mechanism of the BZC Precursor. The basic zinc carbonate (BZC) precursor was prepared by hydrothermally treating zinc nitrate in the presence of hexamethylenetetramine (HMT). This hydrothermal method has been extensively employed to produce nanostructured ZnO, in particular, 1D ZnO nanorods.37-42 As previously mentioned, introducing a special additive or growth modifier into the reaction system enables us to control the crystal morphology of the ZnO product. In this work, ethanol was selected as the solvent for zinc nitrate to react with HMT aqueous solution forming an ethanol-water hydrothermal system. We suppose that ethanol is responsible for the formation of the hierarchical BZC precursor. A control experiment using water instead of ethanol as the solvent with a reaction time of 2 h was done to verify our assumption. The XRD pattern is shown in Figure 7, implying that the obtained product is hexagonal wurtzite ZnO (JCPDS 36-1451). The SEM images of the product are exhibited in Figure 8a,b, showing that only bundles consisting of ZnO microrods with a diameter on the order of micrometers were

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Figure 7. XRD pattern of the ZnO product from control experiment. Figure 10. Dynamic response-recovery curves of the two sensors based on the prepared porous ZnO nanosheets and commercial ZnO powder to methanol. Inset shows the comparison of their gas concentrationdependent sensitivities.

Figure 8. SEM images of the ZnO product from control experiment.

Figure 9. SEM images of the BZC precursor with a hydrothermal time of (a and b) 2 h and (c and d) 4 h.

produced, which agrees well with the previous report.10 These results strongly proves our assumption that ethanol plays an essential role in the formation of BZC nanosheets precursor. Reaction time is also known to have a great effect on experimental result. Thus another two control experiments with the only change in reaction time, that is, 2 h and 4 h, were performed to examine the time-dependent effect on products. The results are shown in Figure 9. For a reaction time of 2 h, a large amount of irregular BZC nanosheets were obtained (Figure 9a,b). However, as shown in Figure 9a, a spot of hierarchical BZC architecture has come forth within 2 h of reaction time. When the reaction time was increased to 4 h, a

mass of hierarchical BZC architectures self-assembled by BZC nanosheets was obtained (Figure 9c,d). When the reaction time was further increased to 8 h, no significant change in the morphology of the product can be observed except the increased thickness of nanosheets (Figure 4). In most of the HMT-assisted hydrothermal systems, it has been well-accepted that HMT can hydrolyze to generate OHand serve as the nucleation-control reagent for Zn(OH)42- or Zn(OH)2 nucleus.39-42 In a previous work related to ZnO nanodiscs, Fu et al.43 have demonstrated that ethanol could form carboxlate species that acted as a surfactant favoring the formation of ZnO nanodiscs. In another study, Zhou et al.29 have prepared hierarchical hydroxide carbonate using CTAB as a shape-directing agent. They claimed that CTAB controlled agglomeration of Zn(OH)2 nucleus by adsorbing onto their surface. Combining with these reports, we suggest that ethanol used in our experiments should serve as both a surfactant and shape-directing agent, which is vitally important for generating the hierarchical BZC precursor. 3.3. Gas Sensing Property of the Hierarchically Porous ZnO Nanosheets. Hierarchical or porous semiconductor metal oxides are promising materials for gas sensor. Their special structures can usually provide a large surface-to-volume ratio, which is most favorable for the diffusion of target gases in sensor materials. Many studies have proven that these special structures could significantly enhance the sensor performance. In addition, the high porosity of the as-prepared hierarchically porous architectures provides excellent channels and “surface accessibility” for the mass transportation of target gases. Because of these advantages that are not available from bulk or solid materials, the as-prepared hierarchically porous nanostructures are expected to exhibit excellent properties for gas sensor applications. Two gas sensors were fabricated from the as-prepared porous ZnO nanosheets and commercial ZnO powders for comparison. The methanol and ethanol sensing performances of the sensors were examined and compared for discussion. Different gas concentrations were tested at an operating temperature of 280 °C in the sequence of 10, 100, 200, 500, and 1000 ppm. Figures 10 and 11, respectively, show the dynamic response-recovery curves of the two sensors to methanol and ethanol. In both

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enhanced sensing performances are attributed to the high porosity and 3D morphology, which can significantly facilitate gas diffusion and mass transportation in sensing materials. The as-synthesized hierarchically porous ZnO nanostructures are also expected to be useful for other applications such as photoluminescence and photocatalysts. Moreover, this work further hints that this facile and economical approach can be extended to synthesize other porous metal oxide materials with a unique morphology or shape. Acknowledgment. The authors would like to thank Prof. Jun Chen for nitrogen adsorption-desorption analysis. This work was supported by the National Natural Science Foundation of China (Nos. 20771061 and 20871071), 973 program (2005CB623607), and the Applied Basic Research Programs of Science and Technology Commission Foundation of Tianjin (Nos. 08JCYBJC00100 and 09JCYBJC03600).

Figure 11. Dynamic response-recovery curves of the two sensors based on the prepared porous ZnO nanosheets and commercial ZnO powder to ethanol. Inset shows the comparison of their gas concentrationdependent sensitivities.

figures, it shows clearly that the response amplitudes of the two sensors are increased on increasing gas concentration. As expected, the sensor based on porous nanosheets exhibits enhanced responses for each concentration compared with that based on commercial powder. Their corresponding sensor sensitivities are displayed in the insets. Obviously, the sensorbased porous nanosheets exhibit higher sensitivities than those of a commercial powder sensor. These results strongly prove that the as-prepared hierarchically porous ZnO nanosheets are promising candidates for gas sensing applications. Considering their high porosity, these hierarchically porous ZnO nanostructures are also expected to be attractive for other surface-related applications such as photocatalysts and photoluminescence. More importantly, we think that the synthesis involved in this work denotes a general, economical, and template-free method that can be used for large-scale production of other porous materials without using any templates or pore-directing agents, especially with a defined morphology. Through such a method, a precursor of hydroxide, carbonate, or basic carbonate with a unique morphology is first synthesized, and then calcination of the precursor will yield porous material with the preset morphology. For example, some other porous metal oxides with unique morphologies have been recently produced via this strategy, including porous Co3O4 cubes44 and nanosheets45 porous NiO nanosheets,46 R-Fe2O3 nanorods,47-49 and porous ZnO nanobelts,50 which effectively prove the validity of this procedure.

4. Conclusions A template-free hydrothermal method combined with a subsequent annealing process was demonstrated for the synthesis of hierarchically 3D porous ZnO nanostructures, which were composed of interconnected ZnO nanosheets with high porosity. A gas sensor was fabricated from the as-synthesized hierarchically porous ZnO nanostructures and applied to detecting ethanol and methanol. Comparative gas sensing tests between gas sensors based on hierarchically porous ZnO nanostructures and commercial ZnO powder clearly show that the former exhibits more excellent sensing performances, implying a good potential of the porous ZnO nanostructures for sensor applications. The

Supporting Information Available: Photograph of the gas sensor (Figure S1) and working principle of HW-30A gas sensing measurement system (Figure S2). This information is available free of charge via the Internet at http://pubs.acs.org.

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