Acetone sensing characteristics of ZnO hollow spheres prepared by one-pot hydrothermal reaction Peng Song n , Qi Wang, Zhongxi Yang School of Material Science and Engineering, Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China article info Article history: Received 4 June 2012 Accepted 18 July 2012 Available online 27 July 2012 Keywords: ZnO Hollow sphere Microstructure Sensors abstract ZnO hollow spheres were one-pot fabricated by hydrothermal treatment. Zinc nitrate were dissolved together with urea and glucose in water, and the mixtures were heated at 180 1C for 24 h. During the hydrothermal treatment, carbon spheres are formed with zinc ions incorporated into the hydrophilic shell. The removal of carbon via calcinations yields hollow zinc oxide spheres. The as-prepared samples were characterized by X-ray diffraction and the field emission gun scanning electron microscope. The results indicated that the products were pure hexagonal ZnO with the structure of hollow sphere, and the formation mechanism of ZnO hollow spheres was discussed. Consequently, the ZnO hollow spheres exhibited good sensing performance towards acetone gas with rapid response when operated at 300 1C. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Chemical sensors play an important role in the areas of emissions control, environmental protection, public safety, and human health [1]. The general mechanism of the oxide semicon- ductor sensors is based on the changes in electrical properties before and after exposure to the target gases or vapors [2]. Acetone, a common reagent widely used in industries and labs, is harmful to health. Inhalation of acetone causes headache, fatigue and even narcosis and harmfulness to nerve system. Hence it is necessary to monitor acetone concentration in the environment for health and the workplace for safety [3–5]. Zinc Oxide, as an n-type semiconductor material, has been widely investigated as a field-effect transistor [6], optical device [7], dye-sensitized solar cell [8], and solid-state gas sensor [9,10]. Recently, ZnO-based sensors have been investigated for the detection of acetone vapor at various concentration levels [11–14]. For chemical sensor applications, hollow structural features provide enhanced surface activities, high surface-to- volume ratio and fast diffusion, which allow easy gas penetration into the sensing layers. Furthermore, both the outer and inner shells actively interact with target molecules. So, several promis- ing approaches have been developed to produce hollow architec- tures such as spheres, hemispheres and inorganic tubes [15].Up to now, the most important methods for hollow structures rely on the use of sacrificial templates, and the desired hollow interiors are generated upon the removal of templates by calcination or dissolution [16]. Recently, a novel method for the fabrication of metal oxide hollow spheres has been developed. Titirici et al. [17] have reported synthesis of various oxide hollow microspheres (such as Fe 2 O 3 ,Co 2 O 3 , CeO 2 , MgO and CuO) using carbonaceous polysaccharide microspheres prepared from sacharide solution as template. However, preparation of well-crystallized ZnO hollow spheres with controllable surface morphology and high gas response is still a great challenge. In this contribution, ZnO hollow spheres are prepared by the one-pot hydrothermal reaction. The study focuses on the formation mechanism of ZnO hollow spheres and the effect of hollow morphology on the acetone sensing characteristics. 2. Experimental Preparation of ZnO hollow sphere: ZnO hollow spheres were prepared by a hydrothermal approach using zinc nitrate (Zn(NO 3 ) 2 Á 6H 2 O) as a zinc source. In a typical synthesis, glucose monohydrate (C 6 H 12 O 6 Á 6H 2 O, 75 mmol), 5 mmol zinc nitrate and urea (CO(NH 2 ) 2 , 50 mmol) were dissolved in 15 mL of distilled water under stirring, respectively. The above two solutions were mixed immediately before the experiment and placed in a 50 mL capacity Teflon-lined stainless steel autoclave, which was heated in an oven to 180 1C for 24 h. After hydrothermal treatment, the black precipitates were centrifuged, and then with distilled water and absolute alcohol washed several times. The washed precipi- tates were dried in a vacuum oven at 60 1C for 12 h. After synthesis, the zinc-carbon composites were calcined in air at 500 1C (heating rate of 1 1C/min) for 4 h to remove the carbon core, leading to ZnO hollow spheres. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$ -see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.07.058 n Corresponding author. Tel.: þ 86 531 82765473; fax: þ 86 531 87974453. E-mail address: mse_songp@ujn.edu.cn (P. Song). Materials Letters 86 (2012) 168–170 Characterization: The crystalline phase in the samples were characterized by an X-ray diffraction (XRD, Bruker D8 Advance) using CuK a radiation ( l ¼0.15406 nm) at 30 kV and 40 mA at a scanning rate of 21 at 2 y min À1 ranging from 201 to 701. The FESEM micrographs were obtained on a FEI Sirion 200 field emission gun scanning electron microscope (FESEM, Hitachi S4800). FESEM measurements were mounted on aluminum studs using adhesive graphite tape and sputter coated with gold before analysis. 3. Results and discussion Phase and morphology of the products:TypicalXRDpatternofthe as-prepared ZnO is shown in Fig. 1, where one can see that all the diffraction peaks are in good agreement with t hose of t he hexagonal structure of ZnO (JCPDS card 36-1451). No other diffraction peaks are found, indicating that the products are pure ZnO and t he carbon microsphere t empla tes w ere completely removed. In addition, it can be found that several diffraction peaks are strong and sharp, w hich indicates that the prepared ZnO are hi ghly cry stallized yet p olycrys- talline. The average crystallite size of ZnO samples was estimated according to the li ne width analysis o f the diffraction peaks based on the Scherrer formu la, D¼0.89 l / b cos y , which was calculated to be about 1 9.8 nm. Fig. 2(a) shows FESEM image of the as-prepared zinc-carbon composite m icrospheres obtained b y the hyd rothermal treatment before calcinations. The diameter of spheres is about 4–7 m m. Many spheres are aggregated and linked to each other and their surf a ces are smooth. After calcinations, we obtained ZnO hollow spheres with diameters of about 1–2 m m, as shown in Fig. 2(b). More details can be found in Fig. 2(c) and (d), some small o penings in the spheres can be seen clearly, implying the hollow structure of t he spheres. And, this porous network is believed to be favorable for gas sensor, which can facilitate the inward and outward ga s d iffusion. Furthermore, FESEM images of the spheres before and after calcinations reveal a considerable shrinkage ( from approximately 4–7 m m to 1–2 m m in diameter) of the structures during calcina- tions tr eatment, showing a transition from loosely adsorbed zinc ions to a dense zinc oxide network in the hollow spheres. The formation mechanism o f ZnO hollow spheres is discussed, as shown in Fig. 3. Firstly, the formation of the carbon spheres involves the dehydration of the carbohydrate and subsequent carbonization of the organic compounds. The surface of carbon spheres is hydrophilic and has a distribution of OH and C¼O g roups, which are f ormed from non- or just partially dehydrated carbohydrates. The secondary step is the embedding of zinc precursors (Zn(OH) 4 2À ) into the hydrophilic shell of as-prepared carbon spheres due to the fact that the functional groups in the surface layer can bind Zn cations through coordination or electrostatic interactions. Finally, the removal of carbon core and densification of incorporati ng Zn cationic ions in the layer via calcinations results in the formation of hollow zinc oxide s pheres. As we all kn own, ZnO nuclei form f rom dehydration of Zn(OH) 4 2À ions in alk a line environment [18 ,19], and the reactions are as follows: Zn 2 þ þ4OH À -ZnðOHÞ 4 2À ð1Þ ZnðOHÞ 4 2À -ZnðOHÞ 2 þ2OH À ð2Þ ZnðOHÞ 2 -ZnOþH 2 O ð3Þ Acetone sensing properties: Fig. 4 displays the concentration dependent sensitivity of the sensor based on ZnO hollow spheres for acetone detection a t a n operating tempera ture o f 3 00 1C. It can be seen that the sensitivity rapidl y increases with increasing acetone concentration. Furthermore, a quick response/recovery time was observed with this sensor. T he t ypical dynamic response curve of ZnO hollow spheres sensor toward 500 ppm acetone at 300 1Cis shown in t he inset of Fig. 4. We can find that the response of the sensor increased abruptly on the injection of acetone, and then decreased rapidly and recovered to its initial value after the test gas was released. T he response time and recovery time of the sensor were less t han 10 s. ZnO is well-known as an n-type semiconductor, characterized by its high free carrier concentration. When the ZnO hollow spheres were exposed to air, oxygen molecules are firstly adsorbed on the inner and outer surface of ZnO hollow spheres and capture free electrons from the conduction band to produce chemisorbed oxygen species (O À ,O 2À or O 2 À ). When ZnO hollow spheres are exposed to acetone gas at higher temperature (300 1C), acetone gas molecules can react with adsorbed oxygen species on the inner and outer surface. This liberates free electrons in the conduction band, leading to an increase in the resistance of an n-type semiconductor. The final reaction takes place as C 3 H 6 Oþ 8OÀðadsÞ-3H 2 Oþ 3CO 2 þ8e À ð4Þ Therefore, the specific surface area of the sensors plays an important role in the contact and subsequent reaction of oxygen species with the tested gas. In our case, the high response of the ZnO hollow spheres sensor can be ascribed to the larger specific surface area (not only the outer but also the inner surface) of the sensing materials. Furthermore, the porous structure and open framework of ZnO hollow spheres may also contribute to the improved sensor response. 4. Conclusions In summary, ZnO hollow spheres were prep ared by the glucose- mediated, one-pot hydrothermal synthesis of Z n-coated carb on spheres and then calcined at 500 1C, the hollow ZnO microspheres with diameters of 1–2 m m were gradually transformed into solid microspheres. The ZnO hollow spheres sensor shows high response, 10 20 30 40 50 60 70 80 2 Theta / degree Intensity (a.u.) (100) (002) (101) (102) (110) (103) (200) (112) (201) Fig. 1. XRD pattern of as-prepared ZnO hollow spheres. P. Song et al. / Materials Letters 86 (2012) 168–170 169 low d etection a nd fast response and r ecovery towards acetone gas. The excellent sensing performances are attributed to the hollow and porous microstructu re. Acknowledgment This work was financially supported by National Natural Science Foundation of China (No. 61102006), Natural Science Foundation of Shandong Province, China (No. ZR2010EQ009), Shandong Distinguished Middle-aged and Young Scientist Encou- rage and Reward Foundation (No. BS2009CL056). References [1] Polleux J, Gurlo A, Barsan N, Weimar U, Antonietti M, Niederberger MM. Angew Chem Int Ed 2006;45:261. [2] Chen DL, Hou XX, Li T, Yin L, Fan BB, Wang HL, et al. Sensor Actuators B 2011;153:373. [3] Yang M, Huo LH, Zhao H, Gao S, Rong ZM. 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[16] Caruso F. Chem Eur J 2000;6:413. [17] Titirici MM, Antonietti M, Thoma A. Chem Mater 2006;18:3808. [18] Cho S, Jung SH, Lee KHJ. Phys Chem C 2008;112:12769. [19] Krishnan D, Pradeep T. J Cryst Growth 2009;311:3889. 5 µm 1 µm 100 µm 500 nm Fig. 2. FESEM images of (a) zinc-carbon composite microspheres via hydrothermal synthesis, (b) low magnification image of ZnO hollow spheres after calcinations, (c) and (d) high magnification images of ZnO hollow spheres. Glucose Urea Zinc nitrate Hydrothermal treatment Zinc precursor Carbon sphere Calcination ZnO hollow sphere Fig. 3. Synthetic scheme of ZnO hollow spheres fabricated by hydrothermal treatment. 0 200 400 600 800 1000 1200 2 4 6 8 10 12 14 16 Sensitivity (Ra/Rg) Acetone concentration (ppm) T = 300°C 0 102030405060 0 2 4 6 8 10 12 Time (s) 500 ppm Sensitivity Fig. 4. Sensitivity of ZnO hollow spheres versus acetone concentration. The inset shows the response and recovery charact eristic s of the sensor to 500 ppm acetone at 300 1C. P. Song et al. / Materials Letters 86 (2012) 168–170170 . mechanism of ZnO hollow spheres and the effect of hollow morphology on the acetone sensing characteristics. 2. Experimental Preparation of ZnO hollow sphere: ZnO hollow spheres were prepared by a. Acetone sensing characteristics of ZnO hollow spheres prepared by one-pot hydrothermal reaction Peng Song n , Qi Wang, Zhongxi Yang School of Material Science and Engineering,. of (a) zinc-carbon composite microspheres via hydrothermal synthesis, (b) low magnification image of ZnO hollow spheres after calcinations, (c) and (d) high magnification images of ZnO hollow spheres. Glucose Urea Zinc