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carbon nanostructures by chemical coupling for high performance gas sensors
Sensors and Actuators B 195 (2014) 132–139
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Solution-based synthesis of ZnO/carbon nanostructures by chemical coupling for high performance gas sensors Hailin Tian a , Huiqing Fan a,∗ , Hui Guo b , Na Song a a
State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China Key Laboratory of the Ministry of Education for Wide Band-Gap Semiconductor Materials and Devices, Microelectronics Institute, Xidian University, Xi’an 710071, China b
a r t i c l e
i n f o
Article history: Received 29 September 2013 Received in revised form 6 January 2014 Accepted 7 January 2014 Available online 17 January 2014 Keywords: Chemical coupling Solution-based synthesis ZnO/C nanostructures Gas sensors
a b s t r a c t A hybrid nanostructure of carbon with ZnO nanoparticles (NPs) was synthesized by grafting glucose on the surface of the ZnO NPs as the precursor and heating at 180 ◦ C for 30 min, which morphological feature was characterized as nanoparticles with an average diameter of 20–25 nm. Transmission electron microscopy (TEM) images revealed that the ZnO/C nanostructures preserved good dispersity and uniformity. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), N2 adsorption and gas-sensing measurements were performed to study the structure and the gas-sensing properties of the ZnO/C nanostructures. The gas-sensing properties demonstrated that the sensors based on the ZnO/C nanostructures exhibited a much higher gas response to acetone and ethanol vapor than the sensors based on the pure ZnO nanoparticles and the mechanism was also discussed. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Since nanostructure materials possess uniform morphologies, small sizes, and narrow distributions, they usually exhibit strong quantum confinement effects and unique electrical, optical, magnetic, and catalytic properties [1–5]. Nowadays, the semiconductor nanostructures of metal oxide synthesized by self-assembly technique have been extensively applied for various applications [6–8]. However, the nanostructures are bonded by weak interactions (van der Waals or hydrogen bonds) between building blocks and the adsorption is too weak to keep stable nanostructure [9–11]. Thus, covalent coupling is introduced to obtain a strong interaction between the metal oxide materials and additives [12]. Chemical coupling can effectively modify the surface of the materials and strongly affect the properties of the nanostructures [13]. Zinc oxide (ZnO), is a semiconductor with a direct bandgap of 3.37 eV. It has been widely studied due to its promising electrical, optoelectronic and piezoelectric properties [14,15]. Meanwhile, the carbonaceous materials attract much attention for many important practical applications, including lithium-ion batteries [16], fuel cells [17,18], supercapacitors [19], and adsorption [20,21], as well as catalyst supports [22,23]. The versatility of carbon makes them widely serve as an inter-component to be deposited on the surface of ceramics, polymers, metals, and oxides. The electric
∗ Corresponding author. Tel.: +86 2988494463; fax: +86 2988492642. E-mail address: [email protected] (H. Fan). 0925-4005/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2014.01.016
conductivity, thermal stability, absorbability, or corrosion resistance of these materials can be improved with carbon [24,25]. To date, metal oxide/carbon nanostructures have been widely investigated for photocatalyst and gas sensors. Cao and co-workers [8] reported the accurate deposition of carbon on the surface of ZnO nanorods by a simple, microwave-assisted method and studied the cytotoxicity and photocatalytic activity of the ZnO/C coupling system. Ueda et al. [26] reported the gas sensing properties of single-walled carbon nanotubes/TiO2 hybrid gas sensors to NO molecules at room temperature. Tonezzer et al. [27] investigated ZnO nanowires on carbon microfiber as flexible gas sensor which had a fast and intense response for both oxidizing and reducing gases. In addition, Espinosa et al. [28] reported the responsiveness of micro-sensors based on metal oxides/MWCNT hybrid films was considerably improved for detecting NO2 . According to numerous literatures, the metal oxides and carbonaceous materials with large specific surface area are promising candidates for fabricating gas sensors. Herein, we describe a solution approach was employed to obtain the precursor of the glucose molecules grafted onto the ZnO NPs by 3-aminopropyl triethoxysilane (APTES) as a coupling agent. Through heating the precursor of the glucose grafted ZnO NPs at 180 ◦ C for 30 min, we obtained the ZnO/C nanostructures. The property of the sensors based on the ZnO/C nanostructures and the pure ZnO NPs were systematically investigated for different gases. The as-prepared ZnO/C hybrid nanostructures samples exhibited excellent gas response toward acetone and ethanol vapor with the concentration range from 5 to 500 ppm.
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Fig. 1. Schematic illustrating the formation process of the ZnO/C nanostructures by the chemical coupling.
2. Experimental 2.1. Materials and synthesis 3-Aminopropyl triethoxysilane (APTES) was commercially obtained from Alfa-Aesar. Zinc acetate dihydrate (Zn(CH3 COO)2 ·2H2 O), sodium hydroxide (NaOH), dimethyl sulfoxide (DMSO), glucose, glycerol and ethanol were supplied by China Medicine Co. All the raw materials were analytical reagent (AR) grade and used without further purification. As shown in Fig. 1, schematic illustrating the formation process of the ZnO/C nanostructures is depicted. 2.1.1. Synthesis of ZnO NPs Zn(CH3 COO)2 ·2H2 O (14.75 g, 0.067 mol) and NaOH (5.2 g, 0.13 mol) were dissolved in ethanol of 60 and 30 mL, respectively, under ultrasound. Then, the mixed solutions were stirred at 60 ◦ C for 72 h. The white powder of the ZnO NPs were obtained from the solution by centrifugation, washed several times with deionized water and absolute ethanol and dried at 40 ◦ C under vacuum. 2.1.2. Introduction of amino groups onto the surface of ZnO NPs ZnO NPs (1 g) were dispersed with fresh distilled DMSO of 100 mL to form homogeneous colloidal solution with sonication. After that, APTES (13.5 g, 14 mL) was added to the solution. The solution was stirred at 120 ◦ C for 3 h before finishing the reaction [29]. The amino-functionalized ZnO NPs were separated from the solution by centrifugation and washed repetitively with absolute ethanol. 2.1.3. Preparation of the glucose grafted onto ZnO NPs precursors The amino-functionalized ZnO NPs and 2 g of glucose were dissolved in ethanol of 60 mL. Then the suspension was stirred at 60 ◦ C for 3 h to promote the reaction between surface amino groups of the ZnO NPs and the aldehyde groups of glucose. The precursor of glucose grafted onto the ZnO NPs was obtained after washing the products three times with absolute ethanol. 2.1.4. Preparation of ZnO/C nanostructures A 100 mL quartz flask filled with 50 mL of glycerol was utilized for carbonization of the above ZnO NPs grafted by glucose. Glycerol was chosen as the solvent and thermal medium to promote
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Fig. 2. (a) Photograph of the gas-sensing system (Inset: a fabricated sensor by using ZnO/C nanostructures) and (b) working principle of the measuring electric circuit of the gas sensors.
the ZnO NPs to be well dispersed and prevent the aggregation of the carbonization products. Then, the flask was heated at 180 ◦ C for 30 min in oil bath. The color of the solid product had changed from the initial white to brown, suggesting the existence of carbon components in the products. Finally, the brown solids were separated and washed several times with deionized water and absolute ethanol, and dried at 40 ◦ C under vacuum. 2.2. Characterization and gas sensor measurements The phase structure and compositions of the as-synthesized materials were measured by an X-ray diffraction (XRD; X’pert PRO MPD, Philips, Eindhoven, The Netherlands) with Cu-K␣ radiation ˚ The morphologies of the products were observed ( = 1.5406 A). by a field emission scanning electron microscopy (FE–SEM; JSM6701F, JEOL, Tokyo, Japan), and a high resolution transmission electron microscopy (HRTEM; Tecnai F30G2 , FEI, Hillsboro, OR, USA). Fourier transform infrared (FT-IR) spectra were recorded on an instrument (IRPrestige-21; FTIR-8400S, SHIMADZU, Kyoto, Japan). Bet surface area (BET) results of the products were performed by a full analysis of nitrogen adsorption–desorption tests (3H-2000PS4, Beishide Ltd, Beijing, China). The basic fabricated process of the gas sensors is as follows [30]. The as-prepared ZnO/C nanostructures were mixed and grinded with adhesive in an agate mortar to form a gas-sensing paste. An alumina tube with Au electrodes and platinum wires was used as a substrate which coated the sensitive materials. A Ni–Cr alloy crossed alumina tube was applied as a heating resistor for both substrate heating and temperature controlling. Finally, each element was dried under IR light for several minutes in air, and was sintered at 400 ◦ C for 2 h in the inert atmosphere. The gas-sensing measurements were performed on a gas response instrument (HW-30A, Hanwei Ltd., Zhengzhou, China) (Fig. 2a). The volume of the gas-sensing test chamber is 15 L. First, we will keep the gas sensors to stabilize their base-line in air during a testing, and then the certain volume of the liquid reductive gases is injected into an evaporator with a syringe when we close a cover of the test chamber in 30 s, the centrifugal fan in the test chamber makes the reductive gases to diffuse as soon as possible. Second, the gas sensors will be exposed in air when we open a cover of the test chamber in 110 s to remove the reductive gases. Finally, the whole testing will finish in 180 s. We will repeat the whole testing
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Fig. 3. IR spectra of the ZnO NPs modified by APTES (the blue curve) and glucose (the purple curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Representative XRD patterns of the ZnO NPs and the ZnO/C nanostructures.
3. Results and discussion when the base-line recovers. A load resistor is connected in series with a gas sensor in the measuring electric circuit of the gas sensors (Fig. 2b). The circuit voltage Vc is 5 V, and the output voltage (Vout ) is the terminal voltage of the load resistor RL . The heating voltage Vh is adjusted for the working temperature of the sensors. When an amount of tested gas is injected into a chamber, the sensor’s resistance is changed. As a result, the output voltage is changed. The gas response (S) is defined as the ratio of Rair /Rgas , where Rair and Rgas are the resistance values measured in air and reductive gas, respectively. The response and recovery times are defined as the time to reach 90% of the final equilibrium value. For each sample, three sensors are made through the same fabricated process and tested three times at the same conditions to obtain an average value of the gas response.
The IR spectroscopy results provided evidence to APTES and glucose molecules were grafted onto the ZnO NPs via the proposed route. FT-IR spectrum results of the products are recorded in Fig. 3. Several absorption characteristics can be distinguished easily in the spectrum of the ZnO NPs associated with APTES (the blue curve). Both the peak at 2935 cm−1 to purple curve and the peak at 2862 cm−1 to blue curve are due to the stretching vibration of C–H bonds. The strong peak at 1072 cm−1 and the peak at 1004 cm−1 are due to the stretching vibration of C–N and Si–O bonds, respectively. The fingerprint peaks of C–N and Si–O bonds are usually located at 1220–1020 cm−1 and 1080–1000 cm−1 . The peak at 880 cm−1 is from the out-of-plane bending vibration of N–H bonds. After reacting with glucose, the product shows a quite different IR spectrum
Fig. 5. SEM images and EDS spectroscopies of the ZnO NPs (a) and the ZnO/C nanostructures (b).
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Fig. 6. S1 ZnO NPs: (a) TEM image; (b) HRTEM; (c) SAED pattern; (d) Histogram showing particle size distribution.
(the purple curve) relative to that modified by APTES. The major differences are marked by circles. The broadband around 3400 cm−1 is contributed by the hydroxy groups in glucose. The weak absorption band around 1636 cm−1 reveals C=N bond is formed. In addition, the bands at 1072 and 1004 cm−1 are weakened and the characteristic peak of N–H at 880 cm−1 is disappeared, which further confirms the reaction between glucose and APTES and the formation of C=N bond [8,12]. Consequently, according to IR spectroscopy results, it is believed that the glucose molecules are grafted onto the ZnO NPs. The XRD patterns of the ZnO/C nanostructures and the pure ZnO NPs are shown in Fig. 4. The as-prepared pure ZnO NPs can be indexed to ZnO (JCPDS NO. 36-1451) of the hexagonal wurtzite structure and all the diffraction peaks are consistent with the standard values. The average crystal sizes of the ZnO NPs and the ZnO/C nanostructures using the Debye–Scherrer formula from XRD patterns are 25.6 and 22.5 nm. Calculated the crystallite size (D) by the Debye–Scherrer formula can be empirically represented as D = (K × )/(ˇ cos ), where K is the Scherrer constant and has the value of 0.9 for the hexagonal crystal structure, is the wavelength ˚ ˇ is the forward width at half maximum of radiation (1.5406 A), (FWHM) (radian) and is the value of the Bragg diffraction angle in the XRD patterns (degree). The XRD pattern of the ZnO/C nanostructures is similar with the ZnO nanoparticles, except the intensity distinctly decreases, which is reasonable because the amorphous carbon layer can cause the scattering of X-rays [8].
The overall morphology and structural properties of the ZnO NPs and the ZnO/C nanostructures were examined by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The FESEM image in Fig. 5a reveals that the as-synthesized the pure ZnO is composed of nanoparticles aggregations. Fig. 5b shows the SEM image of the ZnO/C nanostructure, the dispersity and connectivity of the ZnO/C nanostructures are better than that of the pure ZnO NPs. The EDS analysis in Fig. 5 suggests the amount of carbon in the ZnO/C nanostructures is higher than the pure ZnO NPs. To further observe the structure, TEM images were used. The samples of the pure ZnO NPs and the ZnO/C hybrid nanostructures were labeled as S1 and S2, respectively. The TEM image of the pure ZnO NPs can be seen in Fig. 6a. A high-resolution TEM image (Fig. 6b) reveals the crystal lattice fringes with d spacing ˚ corresponding to the (1 0 1) plane of the hexagonal ZnO. As of 2.5 A, can be seen in Fig. 6c, the selected-area-electro diffraction (SAED) pattern of the pure ZnO NPs shows rings with d spacing that match perfectly with the hexagonal ZnO, thus corroborating the XRD data. As deduced from histogram (Fig. 6d) which is constructed manually from low magnification TEM image, the average diameter of the nanoparticle is 24.8 nm. Histogram shows the size distribution of the products is quite narrow. For the ZnO/C nanostructures, the TEM image and histogram (Fig. 7a and d) displays the presence of the uniform nanoparticles with the average diameter of 23.1 nm, which matches well with the crystal size from the XRD data by Debye–Scherrer formula. Because of carbon-coated on the surface
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Fig. 7. S2 ZnO/C nanostructures: (a) TEM image; (b) HRTEM; (c) SAED pattern; (d) Histogram showing particle size distribution.
of the ZnO NPs, their edge was rough relative to that of the initial ZnO NPs. The carbon layer can be seen during TEM observation, which suggests that carbon is coated on the surface of the ZnO NPs, as indicated by the arrows in Fig. 7b. As shown in Fig. 7c, the SAED pattern of the ZnO/C nanostructures is observed. Nitrogen adsorption isotherms measured on both the ZnO NPs (S1) and the ZnO/C nanostructures (S2) are presented in Fig. 8. According to the IUPAC classification, the two samples display the similar N2 adsorption isotherms which can be classified as type IV isotherms with the hysteresis loops. The isotherms of the
ZnO/C hybrid nanostructures show N2 adsorption is increased at a relative high pressure. It indicates that the specific surface area of the ZnO/C nanostructures is much higher than that of the ZnO NPs [31]. The amount of N2 adsorbed for S1 and S2 is increased from 95.33 to 210.6 cm3 /g. The surface area of the ZnO NPs and the ZnO/C nanostructures are shown in the inset of Fig. 8. The ZnO NPs and the ZnO/C nanostructures have the specific surface area of 16.2, 33.8 m3 /g, respectively. Considering the gas response of impedance-semiconductor gas sensors was usually dependent on temperature, the parallel experiments were carried out in the range of 260–400 ◦ C to select
Fig. 8. N2 adsorption isotherms for the pure ZnO nanoparticles and the ZnO/C nanostructures; inset shows the BET surface area of two samples.
Fig. 9. Typical response and recovery curves of the sensors based on the ZnO/C nanostructures exposed to 100 ppm ethanol at different working temperatures.
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Fig. 10. Typical transient response curves of the sensors based on (I) the pure ZnO nanoparticles and (II) the ZnO/C nanostructures exposed to (a) ethanol, (b) acetone at the concentration range from 5 to 500 ppm at 400 ◦ C. (Inset: the gas response of the sensors based on the ZnO/C nanostructures exposed to ethanol (a) and acetone (b) at the concentration range from 0 to 5 ppm at 400 ◦ C.) (c) Typical response and recovery curves of the sensors with the ZnO/C nanostructures exposed to 300 ppm of the different gases at 400 ◦ C. (d) The response/recovery times for the ZnO/C nanostructures sensors exposed to 100 ppm acetone at 400 ◦ C.
the proper working temperature. As shown in Fig. 9, the sensors based on the ZnO/C nanostructures show the highest response to ethanol vapor of 100 ppm at 400 ◦ C. When the operating temperature increases further, the response value is slightly promoted. According to the reported papers [32,33], the oxidation temperature ranges for amorphous and graphitic carbons are 450–600 ◦ C and 600–750 ◦ C, respectively. Therefore, we select 400 ◦ C as the proper operating temperature to process the subsequent experiments. When the sensors are exposed to air, oxygen can adsorb on the surface of the materials and form four kinds of oxygen species (as illustrated in Eqs. (1)–(4)), which are O2 (80 ◦ C), O2 − (150 ◦ C), O− (300–400 ◦ C) and O2− (560 ◦ C), respectively [34]. Thereby, the semiconductor gas sensors are operated at working temperature about 400 ◦ C, O− is the main reactive species with the reductive gas such as ethanol and acetone vapor (Eqs. (5)–(6)). On the basis of the above discussion, the mechanism of gas sensing can be explained as following reactions [35]: O2(gas) → O2(ads)
(1)
O2(ads) + e− → O2 − (ads)
(2)
O2 − (ads) + e− → 2 O− (ads)
(3)
O− (ads) + e− → O2− (ads)
(4)
CH3 CH2 OH(ads) + 6 O− (ads) → 2 CO2(gas) + 3 H2 O(gas) + 6 e−
(5)
CH3 COCH3(ads) + 8 O− (ads) → 3 CO2(gas) + 3 H2 O(gas) + 8 e−
(6)
Fig. 10a and b illustrates the typical response/recovery curves of the sensors based on the ZnO nanoparticles and the ZnO/C hybrid nanostructures are exposed to ethanol and acetone vapor with the concentration of 5, 10, 30, 50, 100, 200, 300 and 500 ppm at 400 ◦ C. The gas response of the sensors with the ZnO/C nanostructures is much higher than that of the pure ZnO nanoparticles under the same conditions and the response is improved dramatically with increasing the concentrations of the reductive gases. Besides, it can be seen from the inserts that the detection limit of the sensors for ethanol and acetone is about 0.12 and 0.19 ppm, respectively. Fig. 10c shows the selectivity results of the sensors based on the ZnO/C nanostructures are exposed to different organic gases of 300 ppm at 400 ◦ C. It indicates that the sensors of the ZnO/C nanostructures display the best selectivity for ethanol and acetone vapor. As can be seen in Fig. 10d, the response/recovery times of the ZnO/C nanostructures toward 100 ppm acetone are 7 and 10 s, respectively. Except for the 100 ppm case, other concentrations show the same behavior. The patterns of the gas response versus the vapor concentrations are shown in Fig. 11a and b when the sensors based on the ZnO/C nanostructures and the pure ZnO nanoparticles are exposed to ethanol and acetone vapor. The result shows that the response
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Fig. 11. Gas response curves of the sensors based on (I) the ZnO nanoparticles and (II) the ZnO/C nanostructures exposed to (a) ethanol, (b) acetone at the concentration range from 5 to 500 ppm at 400 ◦ C (Inset: dilogarithm fitting curves of the gas response of the sensors based on the ZnO/C nanostructures to the concentration of ethanol and acetone). (c) The gas response comparison with two kinds of the sensors exposed to 300 ppm of the different gases at 400 ◦ C. (d) Stability of the ZnO/C nanostructures sensors to 300 ppm of acetone at 400 ◦ C.
of the sensors based on the ZnO/C nanostructures can reach almost three times as large as the gas response of the pure ZnO nanoparticles at the same concentration. In our previous paper [10], the gas response of the semiconductor oxide gas sensor can usually be empirically represented as S = 1 + Ag (Pg )ˇ , where Pg is the target gas partial pressure, which is directly proportional to the gas concentration. Ag is a pre-factor, and ˇ is the response exponent on Pg [36], which is derived from the surface interaction between chemisorbed oxygen and reductive gases to the n-type semiconductor. As a result, logarithm of the gas response can be liner with logarithm of the gas concentration. The insets of Fig. 11a and b displays the dilogarithm fitting curves of gas response (S) of ZnO/C nanostructures versus the concentration (C) of ethanol and acetone. The data can be fitted linearly and the correlation coefficient R of the fitting curves are 0.98429, 0.96755, respectively. To compare the gas response of two kinds of the sensors, including ethanol, acetone, methanol, toluene and ammonia are tested to 300 ppm at 400 ◦ C. As shown in Fig. 11c, the sensor response of the ZnO/C nanostructures is much higher than that of the pure ZnO nanoparticles for all the gas tested. However, the selectivity of the ZnO/C nanostructures devices is actually decreased between acetone and ethanol. Further studies are planned to improve the selectivity of the sensors by modifying the surface of the sensing material with the different functional molecules. Fig. 11d displays the stability of the sensors based on the ZnO/C nanostructures are exposed to acetone vapor with the concentration of 300 ppm at 400 ◦ C. The result indicates that the gas response gradually declines 15% in 3
months, but the gas response is still higher than that of the pure ZnO nanoparticles. Based on the above results, the gas-sensing measurements demonstrate that the sensors based on the ZnO/C nanostructures exhibit a much higher gas response than the sensors based on the pure ZnO nanoparticles. The enhanced sensing property of the ZnO/C nanostructures can be explained as following reasons. First, the specific surface area of the ZnO/C nanostructures is 33.8 m2 /g while the ZnO nanoparticles samples just have a specific surface area of 16.2 m2 /g. The carbon layer on the surface of the ZnO nanoparticles can increase the adsorptivity of the materials. Both oxygen and reductive gases can easily absorb on the surface of the sensors, and the changed resistance of the sensors becomes more notable, which enhance its sensing performance. In addition, compared with the pure ZnO NPs, the N2 desorption branch (Fig. 7) of the ZnO/C nanostructures shift toward high relative pressures reflects some increase in the interconnection between spherical pores [31]. The connectivity and dispersity of the ZnO/C nanostructures will be improved by carbon matrix and the gases will be easily transported in materials, which are in favor of their applications in fields such as catalyst and sensor. 4. Conclusions In summary, the samples of the ZnO/C nanostructures were synthesized by grafting the glucose on the surface of the ZnO NPs with a coupling agent and followed by heating the glucose at
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180 ◦ C for 30 min. The chemical coupling was feasible for the stable and uniform nanostructures between the ZnO NPs and carbon due to a strong interaction of the covalent bonds. The as-prepared ZnO/C nanostructures possessed largely specific surface area and increased randomly interconnected pores. The gas-sensing testing revealed that the sensors based on the ZnO/C nanostructures exhibited a much higher gas response to ethanol and acetone vapor than the sensors based on the pure ZnO nanoparticles. Furthermore, the proposed method is an effective way to synthesis the metal oxide/carbon materials with high surface area by the chemical coupling method for the gas sensing field. Acknowledgments This work was supported by the National Natural Science Foundation (51172187), the SPDRF (20116102130002, 20116102120016) and 111 Program (B08040) of MOE, and Xi’an Science and Technology Foundation (CX12174, XBCL-1-08), and Shaanxi Province Science Foundation (2013KW12-02), and NPU Fundamental Research Foundation (NPU-FRF-JC201232) of China.
[19] [20] [21]
[22]
[23]
[24]
[25]
[26]
[27] [28]
References [1] C.B. Murray, C.R. Kagan, M.G. Bawendi, Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies, Annu. Rev. Mater. Sci. 30 (2000) 545–610. [2] S.A. McDonald, G. Konstantatos, S.G. Zhang, P.W. Cyr, E.J.D. Klem, L. Levina, E.H. Sargent, Solution-processed PbS quantum dot infrared photodetectors and photovoltaics, Nat. Mater. 4 (2005) 138–142. [3] S.H. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices, Science 287 (2000) 1989–1992. [4] H.C. Kim, S.M. Park, W.D. Hinsberg, Block copolymer based nanostructures: materials, processes, and application to electronics, Chem. Rev. 110 (2010) 146–177. [5] L.S. Zhong, J.S. Hu, H.P. Liang, A.M. Cao, W.G. Song, L.J. Wan, Self-assembled 3D flowerlike iron oxide nanostructure and their application in water treatment, Adv. Mater. 18 (2006) 2426–2431. [6] J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators, B 140 (2009) 319–336. [7] J. Li, H.Q. Fan, X.H. Jia, Multilayered ZnO nanosheets with 3D porous architectures: synthesis and gas sensing application, J. Phys. Chem. C 114 (2010) 14684–14691. [8] Y. Guo, H.S. Wang, C.L. He, L.J. Qiu, X.B. Cao, Uniform carbon-coated ZnO nanorods: microwave-assisted preparation, cytotoxicity, and photocatalytic activity, Langmuir 25 (2009) 4678–4684. [9] K. Miyasaka, K. Watanabe, E. Jojima, H. Aida, M. Sumita, K. Ishikawa, Electrical conductivity of carbon-polymer composites as a function of carbon content, J. Mater. Sci. 17 (1982) 1610–1616. [10] W. Yan, H.Q. Fan, Y.C. Zhai, C. Yang, P.R. Ren, L.M. Huang, Low temperature solution-based synthesis of porous flower-like ␣-Fe2 O3 superstructure and their excellent gas-sensing properties, Sens. Actuators, B 160 (2011) 1372–1379. [11] J.W. Liu, X.J. Li, L.M. Dai, Water-assisted growth of aligned carbon nanotube–ZnO heterojunction arrays, Adv. Mater. 18 (2006) 1740–1744. [12] Y. Yang, T.X. Liu, Fabrication and characterization of graphene oxide/zinc oxide nanorods hybrid, Appl. Surf. Sci. 257 (2011) 8950–8954. [13] C.S. Lao, Y. Li, C.P. Wong, Z.L. Wang, Enhancing the electrical and optoelectronic performance of nanobelt devices by molecular surface functionalization, Nano Lett. 7 (2007) 1323–1328. [14] W.T. Zheng, Y.M. Ho, H.W. Tian, M. Wen, J.L. Qi, Y.A. Li, Field emission from a composite of graphene sheets and ZnO nanowires, J. Phys. Chem. C 113 (2009) 9164–9168. [15] Y. Zeng, L. Qiao, Y.F. Bing, M. Wen, B. Zou, W.T. Zheng, T. Zhang, G.T. Zou, Development of microstructure CO sensor based on hierarchically porous ZnO nanosheet thin films, Sens. Actuators, B 173 (2012) 897–902. [16] S.H. Ng, J.Z. Wang, D. Wexler, K. Konstantinov, Z.P. Guo, H.K. Liu, Highly reversible lithium storage in spheroidal carbon-coated silicon nanocomposites as anodes for lithium-ion batteries, Angew. Chem. Int. Ed. 45 (2006) 6895–6899. [17] A.L. Dicks, The role of carbon in fuel cells, J. Power Sources 156 (2006) 128–141. [18] A.A. Nechitailov, T.K. Zvonareva, A.D. Remenyuk, V.A. Tolmachev, D.N. Goryachev, O.S. Eltsina, L.V. Belyakov, O.M. Sreseli, Catalytic properties of composite
[29]
[30]
[31]
[32]
[33]
[34] [35] [36]
139
amorphous carbon–platinum layers in fuel cells, Semiconductors 42 (2008) 1249–1254. A. Janes, H.S. Kurig, E. Lust, Characterization of activated nanoporous carbon for supercapacitor electrode materials, Carbon 45 (2007) 1226–1233. B. Panella, M. Hirscher, S. Roth, Hydrogen adsorption in different carbon nanostructures, Carbon 43 (2005) 2209–2214. B.H. Hameed, A.T.M. Din, A.L. Ahmad, Adsorption of methylene blue onto bamboo-based activated carbon: kinetics and equilibrium studies, J. Hazard. Mater. 141 (2007) 819–825. G.S. Chai, S.B. Yoon, J.S. Yu, J.H. Choi, Y.E. Sung, Ordered porous carbons with tunable pore sizes as catalyst supports in direct methanol fuel cell, J. Phys. Chem. B 108 (2004) 7074–7079. H. Yoon, S. Ko, J. Jang, Nitrogen-doped magnetic carbon nanoparticles as catalyst supports for efficient recovery and recycling, Chem. Commun. 43 (2007) 1468–1470. Z.D. Wei, H.T. Guo, Z.Y. Tang, Heat treatment of carbon-based powders carrying platinum alloy catalysts for oxygen reduction: influence on corrosion resistance and particles size, J. Power Sources 62 (1996) 233–236. E.K. Athanassiou, R.N. Grass, W.J. Stark, Large-scale production of carboncoated copper nanoparticles for sensor applications, Nanotechnology 17 (2006) 1668–1673. T. Ueda, K. Takahashi, F. Mitsugi, T. Ikegami, Preparation of single-walled carbon nanotube/TiO2 hybrid atmospheric gas sensor operated at ambient temperature, Diamond Relat. Mater. 18 (2009) 493–496. M. Tonezzer, R.G. Lacerda, Zinc oxide nanowires on carbon microfiber as flexible gas sensor, Physica E 44 (2012) 1098–1102. E.H. Espinosa, R. Ionescu, B. Chambon, G. Bedis, E. Sotter, C. Bittencourt, A. Felten, J.J. Pireaux, X. Correig, E. Llobet, Hybrid metal oxide and multiwall carbon nanotube films for low temperature gas sensing, Sens. Actuators, B 127 (2007) 137–142. J.Y. Kim, F.E. Osterloh, ZnO–CdSe nanoparticle clusters as directional photoemitters with tunable wavelength, J. Am. Chem. Soc. 127 (2005) 10152–10153. J. Li, H.Q. Fan, X.H. Jia, W.W. Yang, P.Y. Fang, Enhanced blue–green emission and ethanol sensing of Co-doped ZnO nanocrystals prepared by a solvothermal route, Appl. Phys. A 98 (2010) 537–542. S.B. Yoon, G.S. Chai, S.K. Kang, J.S. Yu, K.P. Gierszal, M. Jaroniec, Graphitized pitch-based carbons with ordered nanopores synthesized by using colloidal crystals as templates, J. Am. Chem. Soc. 127 (2005) 4188–4189. W.J. Gao, Y. Wan, Y.Q. Dou, D.Y. Zhao, Synthesis of partially graphitic ordered mesoporous carbons with high surface areas, Adv. Energy Mater. 1 (2011) 115–123. N.P. Wickramaratne, V.S. Perera, B.W. Park, M. Gao, G.W. McGimpsey, S.P. Huang, M. Jaroniec, Graphitic mesoporous carbons with embedded Prussian Blue-derived iron oxide nanoparticles synthesized by soft templating and lowtemperature graphitization, Chem. Mater. 25 (2013) 2803–2811. N. Yamazoe, J. Fuchigami, M. Kishikawa, T. Seiyama, Interaction of tin oxide surface with O2 , H2 O and H2 , Surf. Sci. 86 (1979) 335–344. L.B. Kong, Y.S. Shen, Gas-sensing property and mechanism of Cax La1 –xFeO3 ceramics, Sens. Actuators, B 30 (1996) 217–221. R.W.J. Scott, S.M. Yang, G. Chabanis, N. Coombs, D.E. Willams, G.A. Ozin, Tin dioxide opals and inverted opals: near-ideal microstructure for gas sensors, Adv. Mater. 13 (2001) 1468–1472.
Biographies Hailin Tian was born in 1987, and he is a Ph.D. candidate at the school of materials science and engineering, Northwestern Polytechnical University, under the supervision of Prof. Huiqing Fan. His research interests include the synthesis of functional materials and their applications in gas sensors and photo catalysts. Huiqing Fan received his Ph.D. degree in electronic material science in 1998 from Xi’an Jiaotong University, China. He did research in Seoul National University, Korea, for two and a half years from 2000 to 2002. He is now a professor at the school of materials science and engineering, Northwestern Polytechnical University, China. He has published over 250 peer-reviewed papers and holds 19 Chinese patents. He is interested in the ceramic materials for dielectric and piezoelectric applications as well as nanocrystalline materials for gas sensors, photo catalysts and thin-layer devices. Hui Guo received his Ph.D. degree in microelectronics and solid state electronics in 2007 from Xidian University, China. He is now an associate professor at the microelectronics institute, Xidian University, China. He did research in Australian Institute for Bioengineering and Nanotechnology as a visiting scholar from 2010 to 2011. He is interested in the devices for wide band-gap semiconductor materials and graphene materials. Na Song was born in 1990, and she is now a master student at school of materials science and engineering, Northwestern Polytechnical University.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Solution-based synthesis of ZnO/carbon nanostructures by chemical coupling for high performance gas sensors Hailin Tian a , Huiqing Fan a,∗ , Hui Guo b , Na Song a a
State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China Key Laboratory of the Ministry of Education for Wide Band-Gap Semiconductor Materials and Devices, Microelectronics Institute, Xidian University, Xi’an 710071, China b
a r t i c l e
i n f o
Article history: Received 29 September 2013 Received in revised form 6 January 2014 Accepted 7 January 2014 Available online 17 January 2014 Keywords: Chemical coupling Solution-based synthesis ZnO/C nanostructures Gas sensors
a b s t r a c t A hybrid nanostructure of carbon with ZnO nanoparticles (NPs) was synthesized by grafting glucose on the surface of the ZnO NPs as the precursor and heating at 180 ◦ C for 30 min, which morphological feature was characterized as nanoparticles with an average diameter of 20–25 nm. Transmission electron microscopy (TEM) images revealed that the ZnO/C nanostructures preserved good dispersity and uniformity. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), N2 adsorption and gas-sensing measurements were performed to study the structure and the gas-sensing properties of the ZnO/C nanostructures. The gas-sensing properties demonstrated that the sensors based on the ZnO/C nanostructures exhibited a much higher gas response to acetone and ethanol vapor than the sensors based on the pure ZnO nanoparticles and the mechanism was also discussed. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Since nanostructure materials possess uniform morphologies, small sizes, and narrow distributions, they usually exhibit strong quantum confinement effects and unique electrical, optical, magnetic, and catalytic properties [1–5]. Nowadays, the semiconductor nanostructures of metal oxide synthesized by self-assembly technique have been extensively applied for various applications [6–8]. However, the nanostructures are bonded by weak interactions (van der Waals or hydrogen bonds) between building blocks and the adsorption is too weak to keep stable nanostructure [9–11]. Thus, covalent coupling is introduced to obtain a strong interaction between the metal oxide materials and additives [12]. Chemical coupling can effectively modify the surface of the materials and strongly affect the properties of the nanostructures [13]. Zinc oxide (ZnO), is a semiconductor with a direct bandgap of 3.37 eV. It has been widely studied due to its promising electrical, optoelectronic and piezoelectric properties [14,15]. Meanwhile, the carbonaceous materials attract much attention for many important practical applications, including lithium-ion batteries [16], fuel cells [17,18], supercapacitors [19], and adsorption [20,21], as well as catalyst supports [22,23]. The versatility of carbon makes them widely serve as an inter-component to be deposited on the surface of ceramics, polymers, metals, and oxides. The electric
∗ Corresponding author. Tel.: +86 2988494463; fax: +86 2988492642. E-mail address: [email protected] (H. Fan). 0925-4005/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2014.01.016
conductivity, thermal stability, absorbability, or corrosion resistance of these materials can be improved with carbon [24,25]. To date, metal oxide/carbon nanostructures have been widely investigated for photocatalyst and gas sensors. Cao and co-workers [8] reported the accurate deposition of carbon on the surface of ZnO nanorods by a simple, microwave-assisted method and studied the cytotoxicity and photocatalytic activity of the ZnO/C coupling system. Ueda et al. [26] reported the gas sensing properties of single-walled carbon nanotubes/TiO2 hybrid gas sensors to NO molecules at room temperature. Tonezzer et al. [27] investigated ZnO nanowires on carbon microfiber as flexible gas sensor which had a fast and intense response for both oxidizing and reducing gases. In addition, Espinosa et al. [28] reported the responsiveness of micro-sensors based on metal oxides/MWCNT hybrid films was considerably improved for detecting NO2 . According to numerous literatures, the metal oxides and carbonaceous materials with large specific surface area are promising candidates for fabricating gas sensors. Herein, we describe a solution approach was employed to obtain the precursor of the glucose molecules grafted onto the ZnO NPs by 3-aminopropyl triethoxysilane (APTES) as a coupling agent. Through heating the precursor of the glucose grafted ZnO NPs at 180 ◦ C for 30 min, we obtained the ZnO/C nanostructures. The property of the sensors based on the ZnO/C nanostructures and the pure ZnO NPs were systematically investigated for different gases. The as-prepared ZnO/C hybrid nanostructures samples exhibited excellent gas response toward acetone and ethanol vapor with the concentration range from 5 to 500 ppm.
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Fig. 1. Schematic illustrating the formation process of the ZnO/C nanostructures by the chemical coupling.
2. Experimental 2.1. Materials and synthesis 3-Aminopropyl triethoxysilane (APTES) was commercially obtained from Alfa-Aesar. Zinc acetate dihydrate (Zn(CH3 COO)2 ·2H2 O), sodium hydroxide (NaOH), dimethyl sulfoxide (DMSO), glucose, glycerol and ethanol were supplied by China Medicine Co. All the raw materials were analytical reagent (AR) grade and used without further purification. As shown in Fig. 1, schematic illustrating the formation process of the ZnO/C nanostructures is depicted. 2.1.1. Synthesis of ZnO NPs Zn(CH3 COO)2 ·2H2 O (14.75 g, 0.067 mol) and NaOH (5.2 g, 0.13 mol) were dissolved in ethanol of 60 and 30 mL, respectively, under ultrasound. Then, the mixed solutions were stirred at 60 ◦ C for 72 h. The white powder of the ZnO NPs were obtained from the solution by centrifugation, washed several times with deionized water and absolute ethanol and dried at 40 ◦ C under vacuum. 2.1.2. Introduction of amino groups onto the surface of ZnO NPs ZnO NPs (1 g) were dispersed with fresh distilled DMSO of 100 mL to form homogeneous colloidal solution with sonication. After that, APTES (13.5 g, 14 mL) was added to the solution. The solution was stirred at 120 ◦ C for 3 h before finishing the reaction [29]. The amino-functionalized ZnO NPs were separated from the solution by centrifugation and washed repetitively with absolute ethanol. 2.1.3. Preparation of the glucose grafted onto ZnO NPs precursors The amino-functionalized ZnO NPs and 2 g of glucose were dissolved in ethanol of 60 mL. Then the suspension was stirred at 60 ◦ C for 3 h to promote the reaction between surface amino groups of the ZnO NPs and the aldehyde groups of glucose. The precursor of glucose grafted onto the ZnO NPs was obtained after washing the products three times with absolute ethanol. 2.1.4. Preparation of ZnO/C nanostructures A 100 mL quartz flask filled with 50 mL of glycerol was utilized for carbonization of the above ZnO NPs grafted by glucose. Glycerol was chosen as the solvent and thermal medium to promote
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Fig. 2. (a) Photograph of the gas-sensing system (Inset: a fabricated sensor by using ZnO/C nanostructures) and (b) working principle of the measuring electric circuit of the gas sensors.
the ZnO NPs to be well dispersed and prevent the aggregation of the carbonization products. Then, the flask was heated at 180 ◦ C for 30 min in oil bath. The color of the solid product had changed from the initial white to brown, suggesting the existence of carbon components in the products. Finally, the brown solids were separated and washed several times with deionized water and absolute ethanol, and dried at 40 ◦ C under vacuum. 2.2. Characterization and gas sensor measurements The phase structure and compositions of the as-synthesized materials were measured by an X-ray diffraction (XRD; X’pert PRO MPD, Philips, Eindhoven, The Netherlands) with Cu-K␣ radiation ˚ The morphologies of the products were observed ( = 1.5406 A). by a field emission scanning electron microscopy (FE–SEM; JSM6701F, JEOL, Tokyo, Japan), and a high resolution transmission electron microscopy (HRTEM; Tecnai F30G2 , FEI, Hillsboro, OR, USA). Fourier transform infrared (FT-IR) spectra were recorded on an instrument (IRPrestige-21; FTIR-8400S, SHIMADZU, Kyoto, Japan). Bet surface area (BET) results of the products were performed by a full analysis of nitrogen adsorption–desorption tests (3H-2000PS4, Beishide Ltd, Beijing, China). The basic fabricated process of the gas sensors is as follows [30]. The as-prepared ZnO/C nanostructures were mixed and grinded with adhesive in an agate mortar to form a gas-sensing paste. An alumina tube with Au electrodes and platinum wires was used as a substrate which coated the sensitive materials. A Ni–Cr alloy crossed alumina tube was applied as a heating resistor for both substrate heating and temperature controlling. Finally, each element was dried under IR light for several minutes in air, and was sintered at 400 ◦ C for 2 h in the inert atmosphere. The gas-sensing measurements were performed on a gas response instrument (HW-30A, Hanwei Ltd., Zhengzhou, China) (Fig. 2a). The volume of the gas-sensing test chamber is 15 L. First, we will keep the gas sensors to stabilize their base-line in air during a testing, and then the certain volume of the liquid reductive gases is injected into an evaporator with a syringe when we close a cover of the test chamber in 30 s, the centrifugal fan in the test chamber makes the reductive gases to diffuse as soon as possible. Second, the gas sensors will be exposed in air when we open a cover of the test chamber in 110 s to remove the reductive gases. Finally, the whole testing will finish in 180 s. We will repeat the whole testing
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Fig. 3. IR spectra of the ZnO NPs modified by APTES (the blue curve) and glucose (the purple curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Representative XRD patterns of the ZnO NPs and the ZnO/C nanostructures.
3. Results and discussion when the base-line recovers. A load resistor is connected in series with a gas sensor in the measuring electric circuit of the gas sensors (Fig. 2b). The circuit voltage Vc is 5 V, and the output voltage (Vout ) is the terminal voltage of the load resistor RL . The heating voltage Vh is adjusted for the working temperature of the sensors. When an amount of tested gas is injected into a chamber, the sensor’s resistance is changed. As a result, the output voltage is changed. The gas response (S) is defined as the ratio of Rair /Rgas , where Rair and Rgas are the resistance values measured in air and reductive gas, respectively. The response and recovery times are defined as the time to reach 90% of the final equilibrium value. For each sample, three sensors are made through the same fabricated process and tested three times at the same conditions to obtain an average value of the gas response.
The IR spectroscopy results provided evidence to APTES and glucose molecules were grafted onto the ZnO NPs via the proposed route. FT-IR spectrum results of the products are recorded in Fig. 3. Several absorption characteristics can be distinguished easily in the spectrum of the ZnO NPs associated with APTES (the blue curve). Both the peak at 2935 cm−1 to purple curve and the peak at 2862 cm−1 to blue curve are due to the stretching vibration of C–H bonds. The strong peak at 1072 cm−1 and the peak at 1004 cm−1 are due to the stretching vibration of C–N and Si–O bonds, respectively. The fingerprint peaks of C–N and Si–O bonds are usually located at 1220–1020 cm−1 and 1080–1000 cm−1 . The peak at 880 cm−1 is from the out-of-plane bending vibration of N–H bonds. After reacting with glucose, the product shows a quite different IR spectrum
Fig. 5. SEM images and EDS spectroscopies of the ZnO NPs (a) and the ZnO/C nanostructures (b).
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Fig. 6. S1 ZnO NPs: (a) TEM image; (b) HRTEM; (c) SAED pattern; (d) Histogram showing particle size distribution.
(the purple curve) relative to that modified by APTES. The major differences are marked by circles. The broadband around 3400 cm−1 is contributed by the hydroxy groups in glucose. The weak absorption band around 1636 cm−1 reveals C=N bond is formed. In addition, the bands at 1072 and 1004 cm−1 are weakened and the characteristic peak of N–H at 880 cm−1 is disappeared, which further confirms the reaction between glucose and APTES and the formation of C=N bond [8,12]. Consequently, according to IR spectroscopy results, it is believed that the glucose molecules are grafted onto the ZnO NPs. The XRD patterns of the ZnO/C nanostructures and the pure ZnO NPs are shown in Fig. 4. The as-prepared pure ZnO NPs can be indexed to ZnO (JCPDS NO. 36-1451) of the hexagonal wurtzite structure and all the diffraction peaks are consistent with the standard values. The average crystal sizes of the ZnO NPs and the ZnO/C nanostructures using the Debye–Scherrer formula from XRD patterns are 25.6 and 22.5 nm. Calculated the crystallite size (D) by the Debye–Scherrer formula can be empirically represented as D = (K × )/(ˇ cos ), where K is the Scherrer constant and has the value of 0.9 for the hexagonal crystal structure, is the wavelength ˚ ˇ is the forward width at half maximum of radiation (1.5406 A), (FWHM) (radian) and is the value of the Bragg diffraction angle in the XRD patterns (degree). The XRD pattern of the ZnO/C nanostructures is similar with the ZnO nanoparticles, except the intensity distinctly decreases, which is reasonable because the amorphous carbon layer can cause the scattering of X-rays [8].
The overall morphology and structural properties of the ZnO NPs and the ZnO/C nanostructures were examined by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The FESEM image in Fig. 5a reveals that the as-synthesized the pure ZnO is composed of nanoparticles aggregations. Fig. 5b shows the SEM image of the ZnO/C nanostructure, the dispersity and connectivity of the ZnO/C nanostructures are better than that of the pure ZnO NPs. The EDS analysis in Fig. 5 suggests the amount of carbon in the ZnO/C nanostructures is higher than the pure ZnO NPs. To further observe the structure, TEM images were used. The samples of the pure ZnO NPs and the ZnO/C hybrid nanostructures were labeled as S1 and S2, respectively. The TEM image of the pure ZnO NPs can be seen in Fig. 6a. A high-resolution TEM image (Fig. 6b) reveals the crystal lattice fringes with d spacing ˚ corresponding to the (1 0 1) plane of the hexagonal ZnO. As of 2.5 A, can be seen in Fig. 6c, the selected-area-electro diffraction (SAED) pattern of the pure ZnO NPs shows rings with d spacing that match perfectly with the hexagonal ZnO, thus corroborating the XRD data. As deduced from histogram (Fig. 6d) which is constructed manually from low magnification TEM image, the average diameter of the nanoparticle is 24.8 nm. Histogram shows the size distribution of the products is quite narrow. For the ZnO/C nanostructures, the TEM image and histogram (Fig. 7a and d) displays the presence of the uniform nanoparticles with the average diameter of 23.1 nm, which matches well with the crystal size from the XRD data by Debye–Scherrer formula. Because of carbon-coated on the surface
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Fig. 7. S2 ZnO/C nanostructures: (a) TEM image; (b) HRTEM; (c) SAED pattern; (d) Histogram showing particle size distribution.
of the ZnO NPs, their edge was rough relative to that of the initial ZnO NPs. The carbon layer can be seen during TEM observation, which suggests that carbon is coated on the surface of the ZnO NPs, as indicated by the arrows in Fig. 7b. As shown in Fig. 7c, the SAED pattern of the ZnO/C nanostructures is observed. Nitrogen adsorption isotherms measured on both the ZnO NPs (S1) and the ZnO/C nanostructures (S2) are presented in Fig. 8. According to the IUPAC classification, the two samples display the similar N2 adsorption isotherms which can be classified as type IV isotherms with the hysteresis loops. The isotherms of the
ZnO/C hybrid nanostructures show N2 adsorption is increased at a relative high pressure. It indicates that the specific surface area of the ZnO/C nanostructures is much higher than that of the ZnO NPs [31]. The amount of N2 adsorbed for S1 and S2 is increased from 95.33 to 210.6 cm3 /g. The surface area of the ZnO NPs and the ZnO/C nanostructures are shown in the inset of Fig. 8. The ZnO NPs and the ZnO/C nanostructures have the specific surface area of 16.2, 33.8 m3 /g, respectively. Considering the gas response of impedance-semiconductor gas sensors was usually dependent on temperature, the parallel experiments were carried out in the range of 260–400 ◦ C to select
Fig. 8. N2 adsorption isotherms for the pure ZnO nanoparticles and the ZnO/C nanostructures; inset shows the BET surface area of two samples.
Fig. 9. Typical response and recovery curves of the sensors based on the ZnO/C nanostructures exposed to 100 ppm ethanol at different working temperatures.
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Fig. 10. Typical transient response curves of the sensors based on (I) the pure ZnO nanoparticles and (II) the ZnO/C nanostructures exposed to (a) ethanol, (b) acetone at the concentration range from 5 to 500 ppm at 400 ◦ C. (Inset: the gas response of the sensors based on the ZnO/C nanostructures exposed to ethanol (a) and acetone (b) at the concentration range from 0 to 5 ppm at 400 ◦ C.) (c) Typical response and recovery curves of the sensors with the ZnO/C nanostructures exposed to 300 ppm of the different gases at 400 ◦ C. (d) The response/recovery times for the ZnO/C nanostructures sensors exposed to 100 ppm acetone at 400 ◦ C.
the proper working temperature. As shown in Fig. 9, the sensors based on the ZnO/C nanostructures show the highest response to ethanol vapor of 100 ppm at 400 ◦ C. When the operating temperature increases further, the response value is slightly promoted. According to the reported papers [32,33], the oxidation temperature ranges for amorphous and graphitic carbons are 450–600 ◦ C and 600–750 ◦ C, respectively. Therefore, we select 400 ◦ C as the proper operating temperature to process the subsequent experiments. When the sensors are exposed to air, oxygen can adsorb on the surface of the materials and form four kinds of oxygen species (as illustrated in Eqs. (1)–(4)), which are O2 (80 ◦ C), O2 − (150 ◦ C), O− (300–400 ◦ C) and O2− (560 ◦ C), respectively [34]. Thereby, the semiconductor gas sensors are operated at working temperature about 400 ◦ C, O− is the main reactive species with the reductive gas such as ethanol and acetone vapor (Eqs. (5)–(6)). On the basis of the above discussion, the mechanism of gas sensing can be explained as following reactions [35]: O2(gas) → O2(ads)
(1)
O2(ads) + e− → O2 − (ads)
(2)
O2 − (ads) + e− → 2 O− (ads)
(3)
O− (ads) + e− → O2− (ads)
(4)
CH3 CH2 OH(ads) + 6 O− (ads) → 2 CO2(gas) + 3 H2 O(gas) + 6 e−
(5)
CH3 COCH3(ads) + 8 O− (ads) → 3 CO2(gas) + 3 H2 O(gas) + 8 e−
(6)
Fig. 10a and b illustrates the typical response/recovery curves of the sensors based on the ZnO nanoparticles and the ZnO/C hybrid nanostructures are exposed to ethanol and acetone vapor with the concentration of 5, 10, 30, 50, 100, 200, 300 and 500 ppm at 400 ◦ C. The gas response of the sensors with the ZnO/C nanostructures is much higher than that of the pure ZnO nanoparticles under the same conditions and the response is improved dramatically with increasing the concentrations of the reductive gases. Besides, it can be seen from the inserts that the detection limit of the sensors for ethanol and acetone is about 0.12 and 0.19 ppm, respectively. Fig. 10c shows the selectivity results of the sensors based on the ZnO/C nanostructures are exposed to different organic gases of 300 ppm at 400 ◦ C. It indicates that the sensors of the ZnO/C nanostructures display the best selectivity for ethanol and acetone vapor. As can be seen in Fig. 10d, the response/recovery times of the ZnO/C nanostructures toward 100 ppm acetone are 7 and 10 s, respectively. Except for the 100 ppm case, other concentrations show the same behavior. The patterns of the gas response versus the vapor concentrations are shown in Fig. 11a and b when the sensors based on the ZnO/C nanostructures and the pure ZnO nanoparticles are exposed to ethanol and acetone vapor. The result shows that the response
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Fig. 11. Gas response curves of the sensors based on (I) the ZnO nanoparticles and (II) the ZnO/C nanostructures exposed to (a) ethanol, (b) acetone at the concentration range from 5 to 500 ppm at 400 ◦ C (Inset: dilogarithm fitting curves of the gas response of the sensors based on the ZnO/C nanostructures to the concentration of ethanol and acetone). (c) The gas response comparison with two kinds of the sensors exposed to 300 ppm of the different gases at 400 ◦ C. (d) Stability of the ZnO/C nanostructures sensors to 300 ppm of acetone at 400 ◦ C.
of the sensors based on the ZnO/C nanostructures can reach almost three times as large as the gas response of the pure ZnO nanoparticles at the same concentration. In our previous paper [10], the gas response of the semiconductor oxide gas sensor can usually be empirically represented as S = 1 + Ag (Pg )ˇ , where Pg is the target gas partial pressure, which is directly proportional to the gas concentration. Ag is a pre-factor, and ˇ is the response exponent on Pg [36], which is derived from the surface interaction between chemisorbed oxygen and reductive gases to the n-type semiconductor. As a result, logarithm of the gas response can be liner with logarithm of the gas concentration. The insets of Fig. 11a and b displays the dilogarithm fitting curves of gas response (S) of ZnO/C nanostructures versus the concentration (C) of ethanol and acetone. The data can be fitted linearly and the correlation coefficient R of the fitting curves are 0.98429, 0.96755, respectively. To compare the gas response of two kinds of the sensors, including ethanol, acetone, methanol, toluene and ammonia are tested to 300 ppm at 400 ◦ C. As shown in Fig. 11c, the sensor response of the ZnO/C nanostructures is much higher than that of the pure ZnO nanoparticles for all the gas tested. However, the selectivity of the ZnO/C nanostructures devices is actually decreased between acetone and ethanol. Further studies are planned to improve the selectivity of the sensors by modifying the surface of the sensing material with the different functional molecules. Fig. 11d displays the stability of the sensors based on the ZnO/C nanostructures are exposed to acetone vapor with the concentration of 300 ppm at 400 ◦ C. The result indicates that the gas response gradually declines 15% in 3
months, but the gas response is still higher than that of the pure ZnO nanoparticles. Based on the above results, the gas-sensing measurements demonstrate that the sensors based on the ZnO/C nanostructures exhibit a much higher gas response than the sensors based on the pure ZnO nanoparticles. The enhanced sensing property of the ZnO/C nanostructures can be explained as following reasons. First, the specific surface area of the ZnO/C nanostructures is 33.8 m2 /g while the ZnO nanoparticles samples just have a specific surface area of 16.2 m2 /g. The carbon layer on the surface of the ZnO nanoparticles can increase the adsorptivity of the materials. Both oxygen and reductive gases can easily absorb on the surface of the sensors, and the changed resistance of the sensors becomes more notable, which enhance its sensing performance. In addition, compared with the pure ZnO NPs, the N2 desorption branch (Fig. 7) of the ZnO/C nanostructures shift toward high relative pressures reflects some increase in the interconnection between spherical pores [31]. The connectivity and dispersity of the ZnO/C nanostructures will be improved by carbon matrix and the gases will be easily transported in materials, which are in favor of their applications in fields such as catalyst and sensor. 4. Conclusions In summary, the samples of the ZnO/C nanostructures were synthesized by grafting the glucose on the surface of the ZnO NPs with a coupling agent and followed by heating the glucose at
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180 ◦ C for 30 min. The chemical coupling was feasible for the stable and uniform nanostructures between the ZnO NPs and carbon due to a strong interaction of the covalent bonds. The as-prepared ZnO/C nanostructures possessed largely specific surface area and increased randomly interconnected pores. The gas-sensing testing revealed that the sensors based on the ZnO/C nanostructures exhibited a much higher gas response to ethanol and acetone vapor than the sensors based on the pure ZnO nanoparticles. Furthermore, the proposed method is an effective way to synthesis the metal oxide/carbon materials with high surface area by the chemical coupling method for the gas sensing field. Acknowledgments This work was supported by the National Natural Science Foundation (51172187), the SPDRF (20116102130002, 20116102120016) and 111 Program (B08040) of MOE, and Xi’an Science and Technology Foundation (CX12174, XBCL-1-08), and Shaanxi Province Science Foundation (2013KW12-02), and NPU Fundamental Research Foundation (NPU-FRF-JC201232) of China.
[19] [20] [21]
[22]
[23]
[24]
[25]
[26]
[27] [28]
References [1] C.B. Murray, C.R. Kagan, M.G. Bawendi, Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies, Annu. Rev. Mater. Sci. 30 (2000) 545–610. [2] S.A. McDonald, G. Konstantatos, S.G. Zhang, P.W. Cyr, E.J.D. Klem, L. Levina, E.H. Sargent, Solution-processed PbS quantum dot infrared photodetectors and photovoltaics, Nat. Mater. 4 (2005) 138–142. [3] S.H. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices, Science 287 (2000) 1989–1992. [4] H.C. Kim, S.M. Park, W.D. Hinsberg, Block copolymer based nanostructures: materials, processes, and application to electronics, Chem. Rev. 110 (2010) 146–177. [5] L.S. Zhong, J.S. Hu, H.P. Liang, A.M. Cao, W.G. Song, L.J. Wan, Self-assembled 3D flowerlike iron oxide nanostructure and their application in water treatment, Adv. Mater. 18 (2006) 2426–2431. [6] J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators, B 140 (2009) 319–336. [7] J. Li, H.Q. Fan, X.H. Jia, Multilayered ZnO nanosheets with 3D porous architectures: synthesis and gas sensing application, J. Phys. Chem. C 114 (2010) 14684–14691. [8] Y. Guo, H.S. Wang, C.L. He, L.J. Qiu, X.B. Cao, Uniform carbon-coated ZnO nanorods: microwave-assisted preparation, cytotoxicity, and photocatalytic activity, Langmuir 25 (2009) 4678–4684. [9] K. Miyasaka, K. Watanabe, E. Jojima, H. Aida, M. Sumita, K. Ishikawa, Electrical conductivity of carbon-polymer composites as a function of carbon content, J. Mater. Sci. 17 (1982) 1610–1616. [10] W. Yan, H.Q. Fan, Y.C. Zhai, C. Yang, P.R. Ren, L.M. Huang, Low temperature solution-based synthesis of porous flower-like ␣-Fe2 O3 superstructure and their excellent gas-sensing properties, Sens. Actuators, B 160 (2011) 1372–1379. [11] J.W. Liu, X.J. Li, L.M. Dai, Water-assisted growth of aligned carbon nanotube–ZnO heterojunction arrays, Adv. Mater. 18 (2006) 1740–1744. [12] Y. Yang, T.X. Liu, Fabrication and characterization of graphene oxide/zinc oxide nanorods hybrid, Appl. Surf. Sci. 257 (2011) 8950–8954. [13] C.S. Lao, Y. Li, C.P. Wong, Z.L. Wang, Enhancing the electrical and optoelectronic performance of nanobelt devices by molecular surface functionalization, Nano Lett. 7 (2007) 1323–1328. [14] W.T. Zheng, Y.M. Ho, H.W. Tian, M. Wen, J.L. Qi, Y.A. Li, Field emission from a composite of graphene sheets and ZnO nanowires, J. Phys. Chem. C 113 (2009) 9164–9168. [15] Y. Zeng, L. Qiao, Y.F. Bing, M. Wen, B. Zou, W.T. Zheng, T. Zhang, G.T. Zou, Development of microstructure CO sensor based on hierarchically porous ZnO nanosheet thin films, Sens. Actuators, B 173 (2012) 897–902. [16] S.H. Ng, J.Z. Wang, D. Wexler, K. Konstantinov, Z.P. Guo, H.K. Liu, Highly reversible lithium storage in spheroidal carbon-coated silicon nanocomposites as anodes for lithium-ion batteries, Angew. Chem. Int. Ed. 45 (2006) 6895–6899. [17] A.L. Dicks, The role of carbon in fuel cells, J. Power Sources 156 (2006) 128–141. [18] A.A. Nechitailov, T.K. Zvonareva, A.D. Remenyuk, V.A. Tolmachev, D.N. Goryachev, O.S. Eltsina, L.V. Belyakov, O.M. Sreseli, Catalytic properties of composite
[29]
[30]
[31]
[32]
[33]
[34] [35] [36]
139
amorphous carbon–platinum layers in fuel cells, Semiconductors 42 (2008) 1249–1254. A. Janes, H.S. Kurig, E. Lust, Characterization of activated nanoporous carbon for supercapacitor electrode materials, Carbon 45 (2007) 1226–1233. B. Panella, M. Hirscher, S. Roth, Hydrogen adsorption in different carbon nanostructures, Carbon 43 (2005) 2209–2214. B.H. Hameed, A.T.M. Din, A.L. Ahmad, Adsorption of methylene blue onto bamboo-based activated carbon: kinetics and equilibrium studies, J. Hazard. Mater. 141 (2007) 819–825. G.S. Chai, S.B. Yoon, J.S. Yu, J.H. Choi, Y.E. Sung, Ordered porous carbons with tunable pore sizes as catalyst supports in direct methanol fuel cell, J. Phys. Chem. B 108 (2004) 7074–7079. H. Yoon, S. Ko, J. Jang, Nitrogen-doped magnetic carbon nanoparticles as catalyst supports for efficient recovery and recycling, Chem. Commun. 43 (2007) 1468–1470. Z.D. Wei, H.T. Guo, Z.Y. Tang, Heat treatment of carbon-based powders carrying platinum alloy catalysts for oxygen reduction: influence on corrosion resistance and particles size, J. Power Sources 62 (1996) 233–236. E.K. Athanassiou, R.N. Grass, W.J. Stark, Large-scale production of carboncoated copper nanoparticles for sensor applications, Nanotechnology 17 (2006) 1668–1673. T. Ueda, K. Takahashi, F. Mitsugi, T. Ikegami, Preparation of single-walled carbon nanotube/TiO2 hybrid atmospheric gas sensor operated at ambient temperature, Diamond Relat. Mater. 18 (2009) 493–496. M. Tonezzer, R.G. Lacerda, Zinc oxide nanowires on carbon microfiber as flexible gas sensor, Physica E 44 (2012) 1098–1102. E.H. Espinosa, R. Ionescu, B. Chambon, G. Bedis, E. Sotter, C. Bittencourt, A. Felten, J.J. Pireaux, X. Correig, E. Llobet, Hybrid metal oxide and multiwall carbon nanotube films for low temperature gas sensing, Sens. Actuators, B 127 (2007) 137–142. J.Y. Kim, F.E. Osterloh, ZnO–CdSe nanoparticle clusters as directional photoemitters with tunable wavelength, J. Am. Chem. Soc. 127 (2005) 10152–10153. J. Li, H.Q. Fan, X.H. Jia, W.W. Yang, P.Y. Fang, Enhanced blue–green emission and ethanol sensing of Co-doped ZnO nanocrystals prepared by a solvothermal route, Appl. Phys. A 98 (2010) 537–542. S.B. Yoon, G.S. Chai, S.K. Kang, J.S. Yu, K.P. Gierszal, M. Jaroniec, Graphitized pitch-based carbons with ordered nanopores synthesized by using colloidal crystals as templates, J. Am. Chem. Soc. 127 (2005) 4188–4189. W.J. Gao, Y. Wan, Y.Q. Dou, D.Y. Zhao, Synthesis of partially graphitic ordered mesoporous carbons with high surface areas, Adv. Energy Mater. 1 (2011) 115–123. N.P. Wickramaratne, V.S. Perera, B.W. Park, M. Gao, G.W. McGimpsey, S.P. Huang, M. Jaroniec, Graphitic mesoporous carbons with embedded Prussian Blue-derived iron oxide nanoparticles synthesized by soft templating and lowtemperature graphitization, Chem. Mater. 25 (2013) 2803–2811. N. Yamazoe, J. Fuchigami, M. Kishikawa, T. Seiyama, Interaction of tin oxide surface with O2 , H2 O and H2 , Surf. Sci. 86 (1979) 335–344. L.B. Kong, Y.S. Shen, Gas-sensing property and mechanism of Cax La1 –xFeO3 ceramics, Sens. Actuators, B 30 (1996) 217–221. R.W.J. Scott, S.M. Yang, G. Chabanis, N. Coombs, D.E. Willams, G.A. Ozin, Tin dioxide opals and inverted opals: near-ideal microstructure for gas sensors, Adv. Mater. 13 (2001) 1468–1472.
Biographies Hailin Tian was born in 1987, and he is a Ph.D. candidate at the school of materials science and engineering, Northwestern Polytechnical University, under the supervision of Prof. Huiqing Fan. His research interests include the synthesis of functional materials and their applications in gas sensors and photo catalysts. Huiqing Fan received his Ph.D. degree in electronic material science in 1998 from Xi’an Jiaotong University, China. He did research in Seoul National University, Korea, for two and a half years from 2000 to 2002. He is now a professor at the school of materials science and engineering, Northwestern Polytechnical University, China. He has published over 250 peer-reviewed papers and holds 19 Chinese patents. He is interested in the ceramic materials for dielectric and piezoelectric applications as well as nanocrystalline materials for gas sensors, photo catalysts and thin-layer devices. Hui Guo received his Ph.D. degree in microelectronics and solid state electronics in 2007 from Xidian University, China. He is now an associate professor at the microelectronics institute, Xidian University, China. He did research in Australian Institute for Bioengineering and Nanotechnology as a visiting scholar from 2010 to 2011. He is interested in the devices for wide band-gap semiconductor materials and graphene materials. Na Song was born in 1990, and she is now a master student at school of materials science and engineering, Northwestern Polytechnical University.