- Email: [email protected]
Effect of Fe doping on the structural and gas sensing properties of ZnO porous microspheres
Journal of Physics and Chemistry of Solids 76 (2015) 51–58
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effect of Fe doping on the structural and gas sensing properties of ZnO porous microspheres Suping Huang a, Tao Wang b, Qi Xiao b,n a b
State key lab of powder metallurgy, Central South University, Changsha 410083, China Department of Inorganic Materials, School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 March 2014 Received in revised form 10 July 2014 Accepted 1 August 2014 Available online 9 August 2014
Fe-doped ZnO porous microspheres composed of nanosheets were prepared by a simple hydrothermal method combined with post-annealing, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Brunauer–Emmett–Teller N2 adsorption–desorption measurements and photoluminescence (PL) spectra. In this paper we report Fe doping induced modifications in the structural, photoluminescence and gas sensing behavior of ZnO porous microspheres. Our results show that the crystallite size decreases and specific surface area increases with the increase of Fe doping concentration. The PL spectra indicate that the 4 mol% Fe-doped ZnO has higher ratio of donor (VO and Zni) to acceptor (VZn) than undoped ZnO. The 4 mol% Fe-doped ZnO sample shows the highest response value to ppblevel n-butanol at 300 1C, and the detected limit of n-butanol is below 10 ppb. In addition, the 4 mol% Fe -doped ZnO sample exhibits good selectivity to n-butanol. The superior sensing properties of the Fe-doped porous ZnO microspheres are contributed to higher donor defects contents combined with larger specific surface area. & 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis C. Electron microscopy D. Electrical properties
1. Introduction N-butanol is a flammable liquid with a characteristic odor. It is often present in industrial waste streams (stationary sources) and can be released into the atmosphere when it was used as a solvent for paints, coatings, varnishes, resins, gums, etc [1]. Recently, n-butanol has been considered as a renewable fuel to replace or complement ethanol for use in internal combustion engines [2,3]. Each year, about 76 million cases are caused in medical and healthcare and loss in productivity due to food-borne illness in the United States. Among the deaths caused by various food-borne pathogens, 31% are due to Sankaran [4]. It has been reported that packaged beef, stored potatoes and dry cured hams can emit n-butanol by subjecting to microbial contamination [5,6]. Especially, n-butanol is one of the VOCs specific to Salmonella contamination in packaged beef [6]. According to the European Committee for Standardization (CEN), the odor threshold of n-butanol is 40 ppb [7]. Therefore, it is highly demanded to monitor the concentration of n-butanol below the critical threshold. Various sensing materials have been used as gas sensors to detect n-butanol, including SnO2 [8–10], ZnO [11–13], WO3 [14,15],
n
Corresponding author. Tel.: þ 86 731 88830543; fax: þ 86 731 88879815. E-mail address: [email protected] (Q. Xiao).
http://dx.doi.org/10.1016/j.jpcs.2014.08.001 0022-3697/& 2014 Elsevier Ltd. All rights reserved.
NiO [16], SnO2/ZnO composite [17] and ZnO-Fe2O3 nanocomposite [5]. For example, SnO2 [8] and SnO2/ZnO composite [17] were capable of showing responses to the sub-ppm level of n-butanol vapor. Huang [8] reported a porous flower-shaped SnO2 nanostructure with a minimum detected concentration of 250 ppb [8]. SnO2/ZnO composite was also investigated as sensing material towards low concentration (0.1–5 ppm) of n-butanol [17]. Therefore, it is necessary to explore gas sensing materials to detect ppb-level n-butanol. Generally, the impurity-doping and varied morphology are regarded as two promising strategies to improve the gas sensing properties of ZnO. On the one hand, doping is an effective way to improve the gas-sensing properties of ZnO gas sensors. For example, the responses to H2S, acetone, ethanol and formaldehyde have been enhanced by introducing Al [18–21], Co , Ce and Sn dopants, respectively. The responses of ZnO materials to ethanol, NO2 and acetone also were improved using Fe dopants [22–24]. Moreover, Fe-doped NiO nanofibers, Fe-doped In2O3 ordered mesoporous material and Fe-doped α-MoO3 micro-structures were reported to improve the responses to ethanol, NO2 and H2S, respectively [25–27]. However, there are few studies on enhancing ZnO sensing properties to n-butanol using dopants. On the other hand, porous ZnO hierarchical nanostructures can enhance gas sensing characteristics due to the channel structure and large surface-to-volume ratio [12]. Based on the above-mentioned
52
S. Huang et al. / Journal of Physics and Chemistry of Solids 76 (2015) 51–58
studies, it is expected that the detection of ppb-level n-butanol can be achieved by combining doping and porous hierarchical nanostructures. In this paper, the method of combining Fe doping with porous ZnO hierarchical microspheres was investigated to improve gas sensing properties. The 4 mol% Fe-doped ZnO sample was found to be the most sensitive sensor for detection of n-butanol below 10 ppb. Furthermore, it also showed a good selectivity and linear dependence of response value on n-butanol concentration.
2. Experimental All chemicals were analytical-grade reagents and were used without further purification. In a typical procedure, 0.015 mol ZnSO4 7H2O, a certain amount of FeSO4 7H2O (the mole ratio of Fe/Zn is 0%, 1%, 2%, 3%, 4% and 5%) and 0.0375 mol urea were dissolved in 150 mL of deionized water under magnetic stirring to form a clear solution. After stirred for 0.5 h, the solution was transferred into a Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 120 1C for 6 h. After cooling to room temperature, the precipitation was filtered and washed with deionized water several times and then dried at 80 1C for 4 h. The obtained ZHC precursors were further calcined in a muffle furnace at 350 1C for 0.5 h. Afterwards, a series of 0, 1, 2, 3, 4, and 5 mol% Fe-doped ZnO powders were obtained. The X-ray diffraction (XRD) patterns of the synthesized samples were obtained by a Brucker D8-advance X-ray powder diffractometer with Cu Kα radiation (λ¼0.15418 nm). The morphologies of the as-prepared samples were investigated by scanning electron microscopy (SEM, JSM-6700F). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained using a JEM3010 transmission electron microscope. Nitrogen adsorption and desorption isotherms were collected at 77 K on a Micromeritic ASAP 2010 instrument. The specific surface areas were calculated using the Brunnauer–Emmett–Teller (BET) equation and the pore size distributions were calculated by applying the Barrett–Joyner– Halenda (BJH) method using the desorption branch of the isotherms. The photoluminescence (PL) spectra of the samples were recorded with a Fluorescence Spectrophotometer F-4500. Gas sensors were fabricated as follows. First, powders of the pure or Fe-doped ZnO were mixed with terpineol saturated with methylcellulose to form diluted slurry. Then the slurry was coated onto an alumina ceramic tube with a diameter of 1 mm and length of 4 mm, positioned with two Au electrodes and four Pt wires at each end. Subsequently the ceramic tube was heated to 400 1C with a heating rate of 1 1C/min, and then kept at this temperature for 2 h. Finally, a small Ni–Cr alloy coil was inserted into the tube as a heater, which provided the operating temperature. The gas sensing test was performed on a WS-30A gas sensing measurement system (Weisheng Electronics Co., Ltd., China). The gas-sensing element was aged at 350 1C for 48 h in air before the measurement in order to improve its stability. The response (S) was defined as Ra/Rg, where Ra and Rg represented the resistance values of the sensor exposed in air and in the reducing atmosphere, respectively. The response and recovery times were defined as the time taken by the sensor to achieve 90% of the total resistance change in the case of adsorption and desorption, respectively.
3. Results and discussion Fig. 1(a) shows the XRD pattern of the ZHC precursor. All the diffraction peaks can be well indexed to Zn5(CO3)2(OH)6 (JCPDS Card no. 72-1100). No other different peaks. Fig. 1(b) shows the XRD spectra of the as-synthesized undoped and Fe-doped ZnO
Fig. 1. (a) shows the XRD pattern of the ZHC precursor. (b) XRD patterns of Fe-doped ZnO samples with different ratios of Fe/Zn from 0 to 5 mol%.
samples. All the diffraction peaks can be readily indexed to hexagonal phase of ZnO with a wurtzite structure in good agreement with the reported values (JCPDS file no. 36-1451). No peak corresponding to iron or other iron compound has been observed in the XRD spectra of the doped samples. It is obvious from Fig. 1 that the crystallinity of ZnO decreases with the increasing Fe doping concentration. These results indicate that the assynthesized samples are single phases with a hexagonal wurtzite structure, and Fe ions get incorporated in the lattice of the host materials. The crystallite size (D) was determined according to the Scherrer formula D ¼Kλ/βcosθ [28], where K is a constant (shape factor, about 0.9), λ is the X-ray wavelength (0.15418 nm), β is the full-width at half-maximum and θ is the diffraction angle. The values of β and θ were taken from ZnO (101) diffraction line. According to the Scherrer formula, the crystallite sizes of 0, 1, 2, 3, 4 and 5 mol% Fe-doped ZnO were calculated to be about 12.6, 12.1, 12.0, 11.4, 10.4 and 9.9 nm, respectively. It has been observed that the crystallite size decreases with the increasing Fe doping concentration. Moreover, the peaks are clearly broadened for ZnO samples with Fe dopants. This phenomenon can be explained by reduction of grain size and development of strain in the ZnO lattice with incorporation of Fe-atoms [29]. The morphologies of as-synthesized samples were investigated by SEM. Fig. 2(a) and (b) shows the SEM images of the pure and 4 mol% Fe-doped ZnO samples, respectively. It is shown that the
S. Huang et al. / Journal of Physics and Chemistry of Solids 76 (2015) 51–58
53
Fig. 2. SEM images of (a) the pure and (b) 4 mol% Fe-doped ZnO samples, (c) EDS spectrum of 4 mol% Fe-doped ZnO. The insets in (a) and (b) are the high-magnification SEM images of the corresponding samples and their scale bars equal to 0.5 μm.
microspheres of the pure and 4 mol% Fe-doped ZnO samples are hierarchically assembled by large amount of nanosheets, indicating that Fe ions do not remarkably change the morphology of the microspheres and corresponding nanosheets building blocks. The EDS spectrum of 4 mol% Fe-doped ZnO is shown in Fig. 2(c). 4 mol% Fe-doped ZnO composed of Zn, Fe and O elements only. The mole ratio of Fe/Zn detected by the EDS spectrum was 3.5%. Further detailed structural analysis of the individual porous nanosheet of the 4 mol% Fe-doped ZnO sample was carried out using transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). Fig. 3(a) clearly shows the random porous structure of the 4 mol% Fe-doped ZnO nanosheet. The porous microstructure may result from the thermal decomposition of the precursor and the release of large quantity of gases during the annealing process. Fig. 3(b) shows a typical HRTEM image of the 4 mol% Fe-doped ZnO sample. The fringe spacing marked by arrows are 0.286 and 0.263 nm, corresponding to the (010) and (002) planes of wurtzite-typed ZnO, respectively. In addition, there are no indications of secondary phases or impurities visible in the HRTEM pictures, suggesting that all dopant atoms are homogeneously incorporated into the ZnO nanocrystallines and that no Fe clustering occurred. Fig. 4 shows the nitrogen adsorption–desorption isotherm and BJH pore size distribution curve of the 4 mol% Fe-doped ZnO sample. The isotherms (shown in Fig. 4(a)) are type IV with a distinct hysteresis loop in the range of 0.5–1.0 P/P0, which confirms the presence of mesoporous materials [30]. Using the BJH
method and the desorption branch of the nitrogen isotherm, the calculated pore size distribution curves (shown in Fig. 4(b)) indicate that the 4 mol% Fe-doped ZnO sample exhibits the mesoporous structure, and its pore size falls into 5 33 nm. In addition, based on the Brunauer Emmett Teller (BET) equation, the specific surface areas of the pure and 4 mol% Fe-doped ZnO samples were evaluated to be about 47.1 and 56.2 m2/g, respectively. It can be clearly seen that the specific surface area increases due to doping Fe ions. The PL spectra were also used to study the differences of the defect structures of the samples. As there are primarily two kinds of defects: the donor and the acceptor, therefore we tried to use decomposed PL spectra to distinguish donor-related (DL) and acceptor-related luminescence (AL) and further to investigate the relationship between the defects and the gas sensing property of ZnO. Fig. 5 shows the PL spectra and their deconvolutions by Gaussian distribution of the pure and 4 mol% Fe-doped ZnO excited at 325 and 270 nm. The subpeak position and corresponding origination of the pure and 4 mol% Fe-doped ZnO excited at 325 and 270 nm were shown in Tables 1 and 2, respectively. According to their origination, the peak at about 400 nm is attributed to interstitial zinc (Zni) [31]. The peaks at about 450 and 500 nm are ascribed to zinc vacancy (VZn) and oxygen vacancy (VO), respectively [32]. According to Tables 1 and 2, it can be concluded that 4 mol% Fe-doped ZnO has higher DL contents than that of the pure ZnO at the different excitation wavelengths (325 or 270 nm). The temperature dependence of resistance can be expressed by an Arrhenius equation [33]: R¼ R1exp(ΔE/κT), where R is the
54
S. Huang et al. / Journal of Physics and Chemistry of Solids 76 (2015) 51–58
Fig. 4. (a) N2 adsorption–desorption isotherm and (b) BJH pore size distribution curve of ZnO sample doped with 4 mol% Fe.
Fig. 3. (a) TEM image and (b) HRTEM image of 4 mol% Fe-doped ZnO sample.
resistance of the sensor in air, ΔE is the temperature-independent thermal activation energy of resistance, κ is the Boltzmann constant, and T is the absolute temperature. By plotting the relation between lnR and 1/T (shown in Fig. 6), the thermal activate energy ΔE was estimated from the slope of the plot derived from linear fitting. The thermal activate energies ΔE were about 0.46, 0.44, 0.43, 0.40, 0.36 and 0.28 eV for 0, 1, 2, 3, 4 and 5 mol% Fe-doped ZnO, respectively. Generally, the thermal activate energies ΔE of the Fe-doped ZnO were smaller than that of the pure ZnO. Moreover, the activate energies in our experiment were far less than the band gap of ZnO ( 3.37 eV), indicating that the resistance comes from impurities or defects [33]. In order to determine the optimum operating temperatures, the responses of the pure and Fe-doped ZnO sensors to 100 ppm n-butanol were tested as a function of the operating temperature. Fig. 7 shows the relation between the response values and the operating temperatures of the pure and Fe-doped ZnO sensors. The responses of the Fe-doped ZnO are found to increase with increasing the operating temperature attaining the maximum at
300 1C, and then decrease with a further rise of the operating temperature. In addition, the pure ZnO sensors exhibited the maximum response values at 350 1C. This result indicates that the optimal operating temperatures can be reduced by introducing Fe dopants. Similar observation has been reported by other research groups for Fe-doped ZnO materials [21,34]. Fig. 8(a) shows the response as a function of n-butanol concentration in the range of 1–100 ppm for the porous pure and Fe-doped ZnO operated at their optimal operating temperatures. It is found that Fe doping can enhance the response of the Fe-doped ZnO sensors and the response values are as following: 44 3 45 42 41 40 mol%, which indicates that there is an optimum doping content of Fe ions in ZnO. The 4 mol% Fe-doped ZnO exhibits the highest response value among all of the Fe-doped ZnO sensors, and its response to 100 ppm n-butanol reach 230.4, which is 11.5 times as high as that of the pure ZnO sensor. Furthermore, the 4 mol% Fe-doped ZnO sensor also shows outstanding response towards low concentrations (10–1000 ppb) of n-butanol (shown in Fig. 8(b)). For example, the response to 10 ppb or 50 ppb ethanol exceeds 1.5 or 3, respectively. Therefore, the detection limit of 4 mol% Fe-doped ZnO sensor for n-butanol is 10 ppb when Ra/Rg 41.2 is considered as the criterion for gas detection [35]. Remarkably, the detection limit is lower than the odor threshold of n-butanol (40 ppb) [7]. In previous studies, the minimum detective concentrations of flower-like ZnO [11,12] and Au-nanoparticle-functionalized ZnO nanowires [13] are 1 ppm. SnO2/ZnO composite [17] was capable of detecting n-butanol in the range of 0.1–5 ppm, and the response (Rair/Rn-butanol) to
S. Huang et al. / Journal of Physics and Chemistry of Solids 76 (2015) 51–58
55
Fig. 5. Gaussian deconvolutions of the PL spectra of (a) the pure and (b) 4 mol% Fe-doped ZnO samples excited at 325 nm, (c) the pure and (d) 4 mol% Fe-doped ZnO samples excited at 270 nm.
Table 1 Defect percentages of the pure and 4 mol% Fe-doped ZnO porous microspheres calculated by their corresponding PL spectra excited at 325 nm. Origination
Peak (nm)
Pure ZnO(%)
4 mol% Fe-doped ZnO(%)
Zni VZn VO DL (Zni þVO) AL (VZn)
400 450 500
20.03 30.13 49.84 69.87 30.13
30.36 14.25 55.39 85.75 14.25
Table 2 Defect percentages of the pure and 4 mol% Fe-doped ZnO porous microspheres calculated by their corresponding PL spectra excited at 270 nm. Origination
Peak (nm)
Pure ZnO(%)
4 mol% Fe-doped ZnO(%)
Zni VZn VO DL (Zni þVO) AL (VZn)
410 460 520
26.73 37.80 35.47 62.20 37.80
32.05 23.42 44.53 76.58 23.42
Fig. 6. The relation between ln(R) and 1/T of Fe-doped ZnO sensors (with the ratios of Fe/Zn from 0 to 5 mol%).
0.5 ppm n-butanol was below 1.5. In our study, 4 mol% Fe-doped ZnO senor is capable of detecting 10 ppb-level n-butanol with a high response and low detection limit, indicating that Fe-doped
56
S. Huang et al. / Journal of Physics and Chemistry of Solids 76 (2015) 51–58
Fig. 7. Responses of Fe-doped ZnO sensors (with the ratios of Fe/Zn from 0 to 5 mol%) to 100 ppm n-butanol as a function of operating temperatures.
Fig. 8. Responses of Fe-doped ZnO sensors (with the ratios of Fe/Zn from 0 to 5 mol%) to different concentrations of n-butanol: (a) 1–100 ppm, (b) 10–1000 ppb.
ZnO senor is suitable as a non-invasive sensing detector for early detection of disease in stored food-stuffs. An empirically equation between the gas response and the gas concentration is as follows: S¼ a(C)b þ1, where S is gas response, C is the gas concentration, a is a prefactor and b is the exponent
Fig. 9. log(S 1) log(C) plots of Fe-doped ZnO sensors with different ratios of Fe/Zn from 0 to 5 mol%.
on C [36]. The value of b is normally between 0.5 and 1.0, which is derived from the surface interaction between chemisorbed oxygen and reductive gas to the n-type semiconductor. In our experiment, the relationship between response and gas concentration satisfies the empirical equation when the concentration of n-butanol is in the range of 1–100 ppm. Fig. 9 shows the corresponding log(S 1) versus log(C) plots. The value of b was estimated from the slope of the plot derived from linear fitting. The value of b was determined to be 0.67 for the pure ZnO sensor, and the values of b were between 0.83 and 0.88 for the Fe-doped ZnO sensors. The operating temperatures of the pure and Fe-doped ZnO sensors are 350 and 300 1C, respectively. It is well known that oxygen molecules adsorbed on the surface of ZnO to form O2 , O , and O2 ions depending on temperature in atmosphere. The stable oxygen ions are O2 below 100 1C, O between 100 and 300 1C, and O2 above 300 1C [37]. According to the values of b and the operating temperatures in this work, the dominated oxygen species may be O2 and O for the pure and Fe-doped ZnO sensors, respectively. Generally, doping can enhance gas-sensing properties by means of changing energy-band structure, regulating the morphology and surface-to-volume ratio, and creating more active centers at the grain boundaries [38]. In this work, the enhancement in gas sensing property on porous Fe-doped ZnO microspheres can be explained by the following reasons. On the one hand, the adsorption capacity for n-butanol molecules and oxygen species is extremely enhanced on the surface of the 4 mol% Fe-doped ZnO porous microspheres due to their larger specific surface area (56.2 m2/g). The specific surface area of 4 mol% Fe-doped ZnO is about 1.2 times as large as that of the pure ZnO. On the other hand, Fe doping can introduce a lot of DL contents (shown in Tables 1 and 2) and thus lead to further enhancement of the response (shown in Figs. 7 and 8). Recent studies demonstrated that the donor defects play the key role in the gas responses of Fe-doped ZnO [23]. Fe-doped ZnO with the more DL contents showed significantly enhanced responses than pure ZnO with the less DL contents [23]. The donor defects can act as active adsorption center in the procedure of gas molecules adsorption. The donor defects promote the reaction of free electron and adsorbed oxygen and make the resistance higher in a more efficient way when the gas-sensing element was exposed to the air [39]. Numerous trapped electrons are released following exposure to a reduction gas. The donor defects increases the released electrons and causes a greater reduction in the resistance and improvement in the sensing properties [40]. Above all, the Fe-doped ZnO sensors have significantly better sensing performance
S. Huang et al. / Journal of Physics and Chemistry of Solids 76 (2015) 51–58
57
4. Conclusion In summary, the crystallite size decreases and specific surface area increases with the increasing Fe doping concentration. The PL spectra indicate that the 4 mol% Fe-doped ZnO has higher ratio of donor (VO and Zni) to acceptor (VZn) than undoped ZnO. The 4 mol% Fe-doped ZnO sample shows the highest response value to ppblevel n-butanol at 300 1C, and the detected limit of n-butanol is below 10 ppb. In addition, the 4 mol% Fe -doped ZnO sample exhibits good selectivity to n-butanol. The superior sensing properties of the Fe-doped porous ZnO microspheres are contributed to higher donor defects contents combined with larger specific surface area.
Acknowledgments
Fig. 10. Dynamic response curves of the pure and 4 mol% Fe-doped ZnO sensors to different concentrations of n-butanol.
The authors are grateful for financial support from the National Natural Science Foundation of China (No. 51102285).
References
Fig. 11. Responses of 4 mol% Fe-doped ZnO sensor to different gases at a fixed concentration of 250 ppb.
than the pure ZnO sensor due to higher specific surface area and the larger quantity of donor defects. Fig. 10 shows the dynamic response curves of the pure and 4 mol% Fe-doped ZnO sensors. The response and recovery times of 4 mol% Fe-doped ZnO sensor to 250 ppb n-butanol are 38 and 110 s, respectively. The selectivity of the gas sensor is very important in gas sensing applications. In the real atmospheric environment, the presence of possible interfering gases may cause undesirable problems. Hence, we examined the responses of 4 mol% Fe-doped ZnO sensor to different volatile target gases, including n-butanol, ethanol, formaldehyde, methanol, acetone, toluene and ammonia. As shown in Fig. 11, the sensor based on 4 mol% Fe-doped ZnO shows the highest response to n-butanol. The responses to 250 ppb interfering gases are all below 1.5, which are even lower than the response to 10 ppb n-butanol. Therefore, this demonstrates that the sensor has good selectivity towards n-butanol at low ppb level. Moreover, it can be seen that the length of alkyl-chain of alcohols plays an important role in the sensing characteristics. From n-butanol, ethanol to methanol, the response of the sensor decreased with reducing the length of alkyl-chain, which agrees well with other studies [9,12,41]. Vijaya et al. [41] explained this phenomenon in terms of ease of oxidation. Alcohols with increased –CH2 groups were easily decomposed and oxidized. Therefore, n-butanol was most easily adsorbed on the surface of the sensing material, followed by ethanol and methanol, respectively.
[1] M.F. Finol, J. Rooke, B.-L. Su, M. Trentesaux, J.-M. Giraudon, J.-F. Lamonier, Catal. Today 192 (2012) 154–159. [2] S.S. Vasu, D.F. Davidson, R.K. Hanson, D.M. Golden, Chem. Phys. Lett. 497 (2010) 26–29. [3] C. Jin, M. Yao, H. Liu, C.F. Lee, J. Ji, Renew. Sust. Energy Rev. 15 (2011) 4080–4106. [4] S. Sankaran, S. Panigrahi, S. Mallik, Sens. Actuators B 155 (2011) 8–18. [5] S. Sankaran, S. Panigrahi, J. Nanosci. Nanotechnol. 12 (2012) 2346–2352. [6] B.P.J. de Lacy Costello, R.J. Ewen, P.R.H. Jones, N.M. Ratcliffe, R.K.M. Wat, Sens. Actuators B 61 (1999) 199–207. [7] L. Lvova, M. Mastroianni, G. Pomarico, M. Santonico, G. Pennazza, C.D. Natale, R. Paolesse, A. D’Amico, Sens. Actuators B 170 (2012) 163–171. [8] J. Huang, K. Yu, C. Gu, M. Zhai, Y. Wu, M. Yang, J. Liu, Sens. Actuators B 147 (2010) 467–474. [9] X. Liu, J. Zhang, X. Guo, S. Wu, S. Wang, Sens. Actuators B 152 (2011) 162–167. [10] H. Zhang, Q. He, X. Zhu, D. Pan, X. Deng, Z. Jiao, CrystEngComm 14 (2012) 3169–3176. [11] J. Huang, Y. Wu, C. Gu, M. Zhai, K. Yu, M. Yang, J. Liu, Sens. Actuators B 146 (2010) 206–212. [12] H. Zhang, R. Wu, Z. Chen, G. Liu, Z. Zhang, Z. Jiao, CrystEngComm 14 (2012) 1775–1782. [13] C. Gu, S. Li, J. Huang, C. Shi, J. Liu, Sens. Actuators B 177 (2013) 453–459. [14] X. Shen, G. Wang, D. Wexler, Sens. Actuators B 143 (2009) 325–332. [15] D. Chen, X. Hou, H. Wen, Y. Wang, H. Wang, X. Li, R. Zhang, H. Lu, H. Xu, S. Guan, J. Sun, L. Gao, Nanotechnology 21 (035501) (2010) 12 pp. [16] G. Zhu, C. Xi, H. Xu, D. Zheng, Y. Liu, X. Xu, X. Shen, RSC Adv. 2 (2012) 4236–4241. [17] B.P.J. de Lacy Costello, R.J. Ewen, N.M. Ratcliffe, P.S. Sivanand, Sens. Actuators B 92 (2003) 159–166. [18] S.S. Badadhe, I.S. Mulla, Sens. Actuators B 156 (2011) 943–948. [19] L. Liu, S. Li, J. Zhuang, L. Wang, J. Zhang, H. Li, Z. Liu, Y. Han, X. Jiang, P. Zhang, Sens. Actuators B 155 (2011) 782–788. [20] C. Ge, C. Xie, S. Cai, Mater. Sci. Eng. B 137 (2007) 53–58. [21] N. Han, X. Wu, D. Zhang, G. Shen, H. Liu, Y. Chen, Sens. Actuators B 152 (2011) 324–329. [22] Y. Cai, H. Fan, M. Xu, Q. Li, C. Long, CrystEngComm 15 (2013) 7339–7345. [23] S. Bai, T. Guo, Y. Zhao, J. Sun, D. Li, A. Chen, C.C. Liu, Sens. Actuators B 195 (2014) 657–666. [24] A.P. Rambu, C. Doroftei, L. Ursu, F. Iacomi, J. Mater. Sci. 48 (2013) 4305–4312. [25] J.-W. Yoon, H.-J. Kim, I.-D. Kim, J.-H. Lee, Nanotechnology 24 (444005) (2013) 8 pp. [26] J. Zhao, T. Yang, Y. Liu, Z. Wang, X. Li, Y. Sun, Y.u. Du, Y. Li, G. Lu, Sens. Actuators B 191 (2014) 806–812. [27] Q.-Y. Ouyang, L. Li, Q.-S. Wang, Y. Zhang, T.-S. Wang, F.-N. Meng, Y.-J. Chen, P. Gao, Sens. Actuators B 169 (2012) 17–25. [28] H.P. Klong, L.F. Alexander, X-Ray Diffraction Procedures for Crystalline and Amorphous Materials, Wiley, New York, 1954. [29] S. Kumar, S. Mukherjee, R.K. Singh, S. Chatterjee, A.K. Ghosh, J. Appl. Phys. 110 (103508) (2011) 7 pp. [30] Z. Chen, M. Cao, C. Hu, J. Phys. Chem. C 115 (2011) 5522–5529. [31] M. Chen, Z. Wang, D. Han, F. Gu, G. Guo, Sens. Actuators B 157 (2011) 565–574. [32] M. Chen, Z. Wang, D. Han, F. Gu, G. Guo, J. Phys. Chem. C 115 (2011) 12763–12773. [33] L.C. Bourne, P.Y. Yu, Z. Zettl, M.L. Cohen, Phys. Rev. B 40 (1989) 10973–10976. [34] N. Han, L. Chai, Q. Wang, Y. Tian, P. Deng, Y. Chen, Sens. Actuators B 147 (2010) 525–530.
58
S. Huang et al. / Journal of Physics and Chemistry of Solids 76 (2015) 51–58
[35] K.-M. Kim, H.-R. Kim, K.-I. Choi, H.-J. Kim, J.-H. Lee., Sens. Actuators B 155 (2011) 745–751. [36] L. Zhang, J. Zhao, H. Lu, L. Gong, L. Li, J. Zheng, H. Li, Z. Zhu, Sens. Actuators B 160 (2011) 364–370. [37] V. Kruefu, C. Liewhiran, A. Wisitsoraat, S. Phanichphant, Sens. Actuators B 156 (2011) 360–367.
[38] K. Zheng, L. Gu, D. Sun, X. Mo, G. Chen, Mater. Sci. Eng. B 166 (2010) 104–107. [39] D. Zhang, X. Wu, N. Han, Y. Chen, J. Nanopart. Res. 15 (1580) (2013) 10pp. [40] A. Khayatian, M.A. Kashi, R. Azimirad., S. Safa, J. Phys. D: Appl. Phys. 47 (075003) (2014) 10 pp. [41] J.J. Vijaya, L.J. Kennedy, G. Sekaran, A. Meenakshisundaram, R.T. Kumar, P. Amalthi, K.S. Nagaraja, Sens. Actuators B 129 (2008) 741–749.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effect of Fe doping on the structural and gas sensing properties of ZnO porous microspheres Suping Huang a, Tao Wang b, Qi Xiao b,n a b
State key lab of powder metallurgy, Central South University, Changsha 410083, China Department of Inorganic Materials, School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 March 2014 Received in revised form 10 July 2014 Accepted 1 August 2014 Available online 9 August 2014
Fe-doped ZnO porous microspheres composed of nanosheets were prepared by a simple hydrothermal method combined with post-annealing, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Brunauer–Emmett–Teller N2 adsorption–desorption measurements and photoluminescence (PL) spectra. In this paper we report Fe doping induced modifications in the structural, photoluminescence and gas sensing behavior of ZnO porous microspheres. Our results show that the crystallite size decreases and specific surface area increases with the increase of Fe doping concentration. The PL spectra indicate that the 4 mol% Fe-doped ZnO has higher ratio of donor (VO and Zni) to acceptor (VZn) than undoped ZnO. The 4 mol% Fe-doped ZnO sample shows the highest response value to ppblevel n-butanol at 300 1C, and the detected limit of n-butanol is below 10 ppb. In addition, the 4 mol% Fe -doped ZnO sample exhibits good selectivity to n-butanol. The superior sensing properties of the Fe-doped porous ZnO microspheres are contributed to higher donor defects contents combined with larger specific surface area. & 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis C. Electron microscopy D. Electrical properties
1. Introduction N-butanol is a flammable liquid with a characteristic odor. It is often present in industrial waste streams (stationary sources) and can be released into the atmosphere when it was used as a solvent for paints, coatings, varnishes, resins, gums, etc [1]. Recently, n-butanol has been considered as a renewable fuel to replace or complement ethanol for use in internal combustion engines [2,3]. Each year, about 76 million cases are caused in medical and healthcare and loss in productivity due to food-borne illness in the United States. Among the deaths caused by various food-borne pathogens, 31% are due to Sankaran [4]. It has been reported that packaged beef, stored potatoes and dry cured hams can emit n-butanol by subjecting to microbial contamination [5,6]. Especially, n-butanol is one of the VOCs specific to Salmonella contamination in packaged beef [6]. According to the European Committee for Standardization (CEN), the odor threshold of n-butanol is 40 ppb [7]. Therefore, it is highly demanded to monitor the concentration of n-butanol below the critical threshold. Various sensing materials have been used as gas sensors to detect n-butanol, including SnO2 [8–10], ZnO [11–13], WO3 [14,15],
n
Corresponding author. Tel.: þ 86 731 88830543; fax: þ 86 731 88879815. E-mail address: [email protected] (Q. Xiao).
http://dx.doi.org/10.1016/j.jpcs.2014.08.001 0022-3697/& 2014 Elsevier Ltd. All rights reserved.
NiO [16], SnO2/ZnO composite [17] and ZnO-Fe2O3 nanocomposite [5]. For example, SnO2 [8] and SnO2/ZnO composite [17] were capable of showing responses to the sub-ppm level of n-butanol vapor. Huang [8] reported a porous flower-shaped SnO2 nanostructure with a minimum detected concentration of 250 ppb [8]. SnO2/ZnO composite was also investigated as sensing material towards low concentration (0.1–5 ppm) of n-butanol [17]. Therefore, it is necessary to explore gas sensing materials to detect ppb-level n-butanol. Generally, the impurity-doping and varied morphology are regarded as two promising strategies to improve the gas sensing properties of ZnO. On the one hand, doping is an effective way to improve the gas-sensing properties of ZnO gas sensors. For example, the responses to H2S, acetone, ethanol and formaldehyde have been enhanced by introducing Al [18–21], Co , Ce and Sn dopants, respectively. The responses of ZnO materials to ethanol, NO2 and acetone also were improved using Fe dopants [22–24]. Moreover, Fe-doped NiO nanofibers, Fe-doped In2O3 ordered mesoporous material and Fe-doped α-MoO3 micro-structures were reported to improve the responses to ethanol, NO2 and H2S, respectively [25–27]. However, there are few studies on enhancing ZnO sensing properties to n-butanol using dopants. On the other hand, porous ZnO hierarchical nanostructures can enhance gas sensing characteristics due to the channel structure and large surface-to-volume ratio [12]. Based on the above-mentioned
52
S. Huang et al. / Journal of Physics and Chemistry of Solids 76 (2015) 51–58
studies, it is expected that the detection of ppb-level n-butanol can be achieved by combining doping and porous hierarchical nanostructures. In this paper, the method of combining Fe doping with porous ZnO hierarchical microspheres was investigated to improve gas sensing properties. The 4 mol% Fe-doped ZnO sample was found to be the most sensitive sensor for detection of n-butanol below 10 ppb. Furthermore, it also showed a good selectivity and linear dependence of response value on n-butanol concentration.
2. Experimental All chemicals were analytical-grade reagents and were used without further purification. In a typical procedure, 0.015 mol ZnSO4 7H2O, a certain amount of FeSO4 7H2O (the mole ratio of Fe/Zn is 0%, 1%, 2%, 3%, 4% and 5%) and 0.0375 mol urea were dissolved in 150 mL of deionized water under magnetic stirring to form a clear solution. After stirred for 0.5 h, the solution was transferred into a Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 120 1C for 6 h. After cooling to room temperature, the precipitation was filtered and washed with deionized water several times and then dried at 80 1C for 4 h. The obtained ZHC precursors were further calcined in a muffle furnace at 350 1C for 0.5 h. Afterwards, a series of 0, 1, 2, 3, 4, and 5 mol% Fe-doped ZnO powders were obtained. The X-ray diffraction (XRD) patterns of the synthesized samples were obtained by a Brucker D8-advance X-ray powder diffractometer with Cu Kα radiation (λ¼0.15418 nm). The morphologies of the as-prepared samples were investigated by scanning electron microscopy (SEM, JSM-6700F). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained using a JEM3010 transmission electron microscope. Nitrogen adsorption and desorption isotherms were collected at 77 K on a Micromeritic ASAP 2010 instrument. The specific surface areas were calculated using the Brunnauer–Emmett–Teller (BET) equation and the pore size distributions were calculated by applying the Barrett–Joyner– Halenda (BJH) method using the desorption branch of the isotherms. The photoluminescence (PL) spectra of the samples were recorded with a Fluorescence Spectrophotometer F-4500. Gas sensors were fabricated as follows. First, powders of the pure or Fe-doped ZnO were mixed with terpineol saturated with methylcellulose to form diluted slurry. Then the slurry was coated onto an alumina ceramic tube with a diameter of 1 mm and length of 4 mm, positioned with two Au electrodes and four Pt wires at each end. Subsequently the ceramic tube was heated to 400 1C with a heating rate of 1 1C/min, and then kept at this temperature for 2 h. Finally, a small Ni–Cr alloy coil was inserted into the tube as a heater, which provided the operating temperature. The gas sensing test was performed on a WS-30A gas sensing measurement system (Weisheng Electronics Co., Ltd., China). The gas-sensing element was aged at 350 1C for 48 h in air before the measurement in order to improve its stability. The response (S) was defined as Ra/Rg, where Ra and Rg represented the resistance values of the sensor exposed in air and in the reducing atmosphere, respectively. The response and recovery times were defined as the time taken by the sensor to achieve 90% of the total resistance change in the case of adsorption and desorption, respectively.
3. Results and discussion Fig. 1(a) shows the XRD pattern of the ZHC precursor. All the diffraction peaks can be well indexed to Zn5(CO3)2(OH)6 (JCPDS Card no. 72-1100). No other different peaks. Fig. 1(b) shows the XRD spectra of the as-synthesized undoped and Fe-doped ZnO
Fig. 1. (a) shows the XRD pattern of the ZHC precursor. (b) XRD patterns of Fe-doped ZnO samples with different ratios of Fe/Zn from 0 to 5 mol%.
samples. All the diffraction peaks can be readily indexed to hexagonal phase of ZnO with a wurtzite structure in good agreement with the reported values (JCPDS file no. 36-1451). No peak corresponding to iron or other iron compound has been observed in the XRD spectra of the doped samples. It is obvious from Fig. 1 that the crystallinity of ZnO decreases with the increasing Fe doping concentration. These results indicate that the assynthesized samples are single phases with a hexagonal wurtzite structure, and Fe ions get incorporated in the lattice of the host materials. The crystallite size (D) was determined according to the Scherrer formula D ¼Kλ/βcosθ [28], where K is a constant (shape factor, about 0.9), λ is the X-ray wavelength (0.15418 nm), β is the full-width at half-maximum and θ is the diffraction angle. The values of β and θ were taken from ZnO (101) diffraction line. According to the Scherrer formula, the crystallite sizes of 0, 1, 2, 3, 4 and 5 mol% Fe-doped ZnO were calculated to be about 12.6, 12.1, 12.0, 11.4, 10.4 and 9.9 nm, respectively. It has been observed that the crystallite size decreases with the increasing Fe doping concentration. Moreover, the peaks are clearly broadened for ZnO samples with Fe dopants. This phenomenon can be explained by reduction of grain size and development of strain in the ZnO lattice with incorporation of Fe-atoms [29]. The morphologies of as-synthesized samples were investigated by SEM. Fig. 2(a) and (b) shows the SEM images of the pure and 4 mol% Fe-doped ZnO samples, respectively. It is shown that the
S. Huang et al. / Journal of Physics and Chemistry of Solids 76 (2015) 51–58
53
Fig. 2. SEM images of (a) the pure and (b) 4 mol% Fe-doped ZnO samples, (c) EDS spectrum of 4 mol% Fe-doped ZnO. The insets in (a) and (b) are the high-magnification SEM images of the corresponding samples and their scale bars equal to 0.5 μm.
microspheres of the pure and 4 mol% Fe-doped ZnO samples are hierarchically assembled by large amount of nanosheets, indicating that Fe ions do not remarkably change the morphology of the microspheres and corresponding nanosheets building blocks. The EDS spectrum of 4 mol% Fe-doped ZnO is shown in Fig. 2(c). 4 mol% Fe-doped ZnO composed of Zn, Fe and O elements only. The mole ratio of Fe/Zn detected by the EDS spectrum was 3.5%. Further detailed structural analysis of the individual porous nanosheet of the 4 mol% Fe-doped ZnO sample was carried out using transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). Fig. 3(a) clearly shows the random porous structure of the 4 mol% Fe-doped ZnO nanosheet. The porous microstructure may result from the thermal decomposition of the precursor and the release of large quantity of gases during the annealing process. Fig. 3(b) shows a typical HRTEM image of the 4 mol% Fe-doped ZnO sample. The fringe spacing marked by arrows are 0.286 and 0.263 nm, corresponding to the (010) and (002) planes of wurtzite-typed ZnO, respectively. In addition, there are no indications of secondary phases or impurities visible in the HRTEM pictures, suggesting that all dopant atoms are homogeneously incorporated into the ZnO nanocrystallines and that no Fe clustering occurred. Fig. 4 shows the nitrogen adsorption–desorption isotherm and BJH pore size distribution curve of the 4 mol% Fe-doped ZnO sample. The isotherms (shown in Fig. 4(a)) are type IV with a distinct hysteresis loop in the range of 0.5–1.0 P/P0, which confirms the presence of mesoporous materials [30]. Using the BJH
method and the desorption branch of the nitrogen isotherm, the calculated pore size distribution curves (shown in Fig. 4(b)) indicate that the 4 mol% Fe-doped ZnO sample exhibits the mesoporous structure, and its pore size falls into 5 33 nm. In addition, based on the Brunauer Emmett Teller (BET) equation, the specific surface areas of the pure and 4 mol% Fe-doped ZnO samples were evaluated to be about 47.1 and 56.2 m2/g, respectively. It can be clearly seen that the specific surface area increases due to doping Fe ions. The PL spectra were also used to study the differences of the defect structures of the samples. As there are primarily two kinds of defects: the donor and the acceptor, therefore we tried to use decomposed PL spectra to distinguish donor-related (DL) and acceptor-related luminescence (AL) and further to investigate the relationship between the defects and the gas sensing property of ZnO. Fig. 5 shows the PL spectra and their deconvolutions by Gaussian distribution of the pure and 4 mol% Fe-doped ZnO excited at 325 and 270 nm. The subpeak position and corresponding origination of the pure and 4 mol% Fe-doped ZnO excited at 325 and 270 nm were shown in Tables 1 and 2, respectively. According to their origination, the peak at about 400 nm is attributed to interstitial zinc (Zni) [31]. The peaks at about 450 and 500 nm are ascribed to zinc vacancy (VZn) and oxygen vacancy (VO), respectively [32]. According to Tables 1 and 2, it can be concluded that 4 mol% Fe-doped ZnO has higher DL contents than that of the pure ZnO at the different excitation wavelengths (325 or 270 nm). The temperature dependence of resistance can be expressed by an Arrhenius equation [33]: R¼ R1exp(ΔE/κT), where R is the
54
S. Huang et al. / Journal of Physics and Chemistry of Solids 76 (2015) 51–58
Fig. 4. (a) N2 adsorption–desorption isotherm and (b) BJH pore size distribution curve of ZnO sample doped with 4 mol% Fe.
Fig. 3. (a) TEM image and (b) HRTEM image of 4 mol% Fe-doped ZnO sample.
resistance of the sensor in air, ΔE is the temperature-independent thermal activation energy of resistance, κ is the Boltzmann constant, and T is the absolute temperature. By plotting the relation between lnR and 1/T (shown in Fig. 6), the thermal activate energy ΔE was estimated from the slope of the plot derived from linear fitting. The thermal activate energies ΔE were about 0.46, 0.44, 0.43, 0.40, 0.36 and 0.28 eV for 0, 1, 2, 3, 4 and 5 mol% Fe-doped ZnO, respectively. Generally, the thermal activate energies ΔE of the Fe-doped ZnO were smaller than that of the pure ZnO. Moreover, the activate energies in our experiment were far less than the band gap of ZnO ( 3.37 eV), indicating that the resistance comes from impurities or defects [33]. In order to determine the optimum operating temperatures, the responses of the pure and Fe-doped ZnO sensors to 100 ppm n-butanol were tested as a function of the operating temperature. Fig. 7 shows the relation between the response values and the operating temperatures of the pure and Fe-doped ZnO sensors. The responses of the Fe-doped ZnO are found to increase with increasing the operating temperature attaining the maximum at
300 1C, and then decrease with a further rise of the operating temperature. In addition, the pure ZnO sensors exhibited the maximum response values at 350 1C. This result indicates that the optimal operating temperatures can be reduced by introducing Fe dopants. Similar observation has been reported by other research groups for Fe-doped ZnO materials [21,34]. Fig. 8(a) shows the response as a function of n-butanol concentration in the range of 1–100 ppm for the porous pure and Fe-doped ZnO operated at their optimal operating temperatures. It is found that Fe doping can enhance the response of the Fe-doped ZnO sensors and the response values are as following: 44 3 45 42 41 40 mol%, which indicates that there is an optimum doping content of Fe ions in ZnO. The 4 mol% Fe-doped ZnO exhibits the highest response value among all of the Fe-doped ZnO sensors, and its response to 100 ppm n-butanol reach 230.4, which is 11.5 times as high as that of the pure ZnO sensor. Furthermore, the 4 mol% Fe-doped ZnO sensor also shows outstanding response towards low concentrations (10–1000 ppb) of n-butanol (shown in Fig. 8(b)). For example, the response to 10 ppb or 50 ppb ethanol exceeds 1.5 or 3, respectively. Therefore, the detection limit of 4 mol% Fe-doped ZnO sensor for n-butanol is 10 ppb when Ra/Rg 41.2 is considered as the criterion for gas detection [35]. Remarkably, the detection limit is lower than the odor threshold of n-butanol (40 ppb) [7]. In previous studies, the minimum detective concentrations of flower-like ZnO [11,12] and Au-nanoparticle-functionalized ZnO nanowires [13] are 1 ppm. SnO2/ZnO composite [17] was capable of detecting n-butanol in the range of 0.1–5 ppm, and the response (Rair/Rn-butanol) to
S. Huang et al. / Journal of Physics and Chemistry of Solids 76 (2015) 51–58
55
Fig. 5. Gaussian deconvolutions of the PL spectra of (a) the pure and (b) 4 mol% Fe-doped ZnO samples excited at 325 nm, (c) the pure and (d) 4 mol% Fe-doped ZnO samples excited at 270 nm.
Table 1 Defect percentages of the pure and 4 mol% Fe-doped ZnO porous microspheres calculated by their corresponding PL spectra excited at 325 nm. Origination
Peak (nm)
Pure ZnO(%)
4 mol% Fe-doped ZnO(%)
Zni VZn VO DL (Zni þVO) AL (VZn)
400 450 500
20.03 30.13 49.84 69.87 30.13
30.36 14.25 55.39 85.75 14.25
Table 2 Defect percentages of the pure and 4 mol% Fe-doped ZnO porous microspheres calculated by their corresponding PL spectra excited at 270 nm. Origination
Peak (nm)
Pure ZnO(%)
4 mol% Fe-doped ZnO(%)
Zni VZn VO DL (Zni þVO) AL (VZn)
410 460 520
26.73 37.80 35.47 62.20 37.80
32.05 23.42 44.53 76.58 23.42
Fig. 6. The relation between ln(R) and 1/T of Fe-doped ZnO sensors (with the ratios of Fe/Zn from 0 to 5 mol%).
0.5 ppm n-butanol was below 1.5. In our study, 4 mol% Fe-doped ZnO senor is capable of detecting 10 ppb-level n-butanol with a high response and low detection limit, indicating that Fe-doped
56
S. Huang et al. / Journal of Physics and Chemistry of Solids 76 (2015) 51–58
Fig. 7. Responses of Fe-doped ZnO sensors (with the ratios of Fe/Zn from 0 to 5 mol%) to 100 ppm n-butanol as a function of operating temperatures.
Fig. 8. Responses of Fe-doped ZnO sensors (with the ratios of Fe/Zn from 0 to 5 mol%) to different concentrations of n-butanol: (a) 1–100 ppm, (b) 10–1000 ppb.
ZnO senor is suitable as a non-invasive sensing detector for early detection of disease in stored food-stuffs. An empirically equation between the gas response and the gas concentration is as follows: S¼ a(C)b þ1, where S is gas response, C is the gas concentration, a is a prefactor and b is the exponent
Fig. 9. log(S 1) log(C) plots of Fe-doped ZnO sensors with different ratios of Fe/Zn from 0 to 5 mol%.
on C [36]. The value of b is normally between 0.5 and 1.0, which is derived from the surface interaction between chemisorbed oxygen and reductive gas to the n-type semiconductor. In our experiment, the relationship between response and gas concentration satisfies the empirical equation when the concentration of n-butanol is in the range of 1–100 ppm. Fig. 9 shows the corresponding log(S 1) versus log(C) plots. The value of b was estimated from the slope of the plot derived from linear fitting. The value of b was determined to be 0.67 for the pure ZnO sensor, and the values of b were between 0.83 and 0.88 for the Fe-doped ZnO sensors. The operating temperatures of the pure and Fe-doped ZnO sensors are 350 and 300 1C, respectively. It is well known that oxygen molecules adsorbed on the surface of ZnO to form O2 , O , and O2 ions depending on temperature in atmosphere. The stable oxygen ions are O2 below 100 1C, O between 100 and 300 1C, and O2 above 300 1C [37]. According to the values of b and the operating temperatures in this work, the dominated oxygen species may be O2 and O for the pure and Fe-doped ZnO sensors, respectively. Generally, doping can enhance gas-sensing properties by means of changing energy-band structure, regulating the morphology and surface-to-volume ratio, and creating more active centers at the grain boundaries [38]. In this work, the enhancement in gas sensing property on porous Fe-doped ZnO microspheres can be explained by the following reasons. On the one hand, the adsorption capacity for n-butanol molecules and oxygen species is extremely enhanced on the surface of the 4 mol% Fe-doped ZnO porous microspheres due to their larger specific surface area (56.2 m2/g). The specific surface area of 4 mol% Fe-doped ZnO is about 1.2 times as large as that of the pure ZnO. On the other hand, Fe doping can introduce a lot of DL contents (shown in Tables 1 and 2) and thus lead to further enhancement of the response (shown in Figs. 7 and 8). Recent studies demonstrated that the donor defects play the key role in the gas responses of Fe-doped ZnO [23]. Fe-doped ZnO with the more DL contents showed significantly enhanced responses than pure ZnO with the less DL contents [23]. The donor defects can act as active adsorption center in the procedure of gas molecules adsorption. The donor defects promote the reaction of free electron and adsorbed oxygen and make the resistance higher in a more efficient way when the gas-sensing element was exposed to the air [39]. Numerous trapped electrons are released following exposure to a reduction gas. The donor defects increases the released electrons and causes a greater reduction in the resistance and improvement in the sensing properties [40]. Above all, the Fe-doped ZnO sensors have significantly better sensing performance
S. Huang et al. / Journal of Physics and Chemistry of Solids 76 (2015) 51–58
57
4. Conclusion In summary, the crystallite size decreases and specific surface area increases with the increasing Fe doping concentration. The PL spectra indicate that the 4 mol% Fe-doped ZnO has higher ratio of donor (VO and Zni) to acceptor (VZn) than undoped ZnO. The 4 mol% Fe-doped ZnO sample shows the highest response value to ppblevel n-butanol at 300 1C, and the detected limit of n-butanol is below 10 ppb. In addition, the 4 mol% Fe -doped ZnO sample exhibits good selectivity to n-butanol. The superior sensing properties of the Fe-doped porous ZnO microspheres are contributed to higher donor defects contents combined with larger specific surface area.
Acknowledgments
Fig. 10. Dynamic response curves of the pure and 4 mol% Fe-doped ZnO sensors to different concentrations of n-butanol.
The authors are grateful for financial support from the National Natural Science Foundation of China (No. 51102285).
References
Fig. 11. Responses of 4 mol% Fe-doped ZnO sensor to different gases at a fixed concentration of 250 ppb.
than the pure ZnO sensor due to higher specific surface area and the larger quantity of donor defects. Fig. 10 shows the dynamic response curves of the pure and 4 mol% Fe-doped ZnO sensors. The response and recovery times of 4 mol% Fe-doped ZnO sensor to 250 ppb n-butanol are 38 and 110 s, respectively. The selectivity of the gas sensor is very important in gas sensing applications. In the real atmospheric environment, the presence of possible interfering gases may cause undesirable problems. Hence, we examined the responses of 4 mol% Fe-doped ZnO sensor to different volatile target gases, including n-butanol, ethanol, formaldehyde, methanol, acetone, toluene and ammonia. As shown in Fig. 11, the sensor based on 4 mol% Fe-doped ZnO shows the highest response to n-butanol. The responses to 250 ppb interfering gases are all below 1.5, which are even lower than the response to 10 ppb n-butanol. Therefore, this demonstrates that the sensor has good selectivity towards n-butanol at low ppb level. Moreover, it can be seen that the length of alkyl-chain of alcohols plays an important role in the sensing characteristics. From n-butanol, ethanol to methanol, the response of the sensor decreased with reducing the length of alkyl-chain, which agrees well with other studies [9,12,41]. Vijaya et al. [41] explained this phenomenon in terms of ease of oxidation. Alcohols with increased –CH2 groups were easily decomposed and oxidized. Therefore, n-butanol was most easily adsorbed on the surface of the sensing material, followed by ethanol and methanol, respectively.
[1] M.F. Finol, J. Rooke, B.-L. Su, M. Trentesaux, J.-M. Giraudon, J.-F. Lamonier, Catal. Today 192 (2012) 154–159. [2] S.S. Vasu, D.F. Davidson, R.K. Hanson, D.M. Golden, Chem. Phys. Lett. 497 (2010) 26–29. [3] C. Jin, M. Yao, H. Liu, C.F. Lee, J. Ji, Renew. Sust. Energy Rev. 15 (2011) 4080–4106. [4] S. Sankaran, S. Panigrahi, S. Mallik, Sens. Actuators B 155 (2011) 8–18. [5] S. Sankaran, S. Panigrahi, J. Nanosci. Nanotechnol. 12 (2012) 2346–2352. [6] B.P.J. de Lacy Costello, R.J. Ewen, P.R.H. Jones, N.M. Ratcliffe, R.K.M. Wat, Sens. Actuators B 61 (1999) 199–207. [7] L. Lvova, M. Mastroianni, G. Pomarico, M. Santonico, G. Pennazza, C.D. Natale, R. Paolesse, A. D’Amico, Sens. Actuators B 170 (2012) 163–171. [8] J. Huang, K. Yu, C. Gu, M. Zhai, Y. Wu, M. Yang, J. Liu, Sens. Actuators B 147 (2010) 467–474. [9] X. Liu, J. Zhang, X. Guo, S. Wu, S. Wang, Sens. Actuators B 152 (2011) 162–167. [10] H. Zhang, Q. He, X. Zhu, D. Pan, X. Deng, Z. Jiao, CrystEngComm 14 (2012) 3169–3176. [11] J. Huang, Y. Wu, C. Gu, M. Zhai, K. Yu, M. Yang, J. Liu, Sens. Actuators B 146 (2010) 206–212. [12] H. Zhang, R. Wu, Z. Chen, G. Liu, Z. Zhang, Z. Jiao, CrystEngComm 14 (2012) 1775–1782. [13] C. Gu, S. Li, J. Huang, C. Shi, J. Liu, Sens. Actuators B 177 (2013) 453–459. [14] X. Shen, G. Wang, D. Wexler, Sens. Actuators B 143 (2009) 325–332. [15] D. Chen, X. Hou, H. Wen, Y. Wang, H. Wang, X. Li, R. Zhang, H. Lu, H. Xu, S. Guan, J. Sun, L. Gao, Nanotechnology 21 (035501) (2010) 12 pp. [16] G. Zhu, C. Xi, H. Xu, D. Zheng, Y. Liu, X. Xu, X. Shen, RSC Adv. 2 (2012) 4236–4241. [17] B.P.J. de Lacy Costello, R.J. Ewen, N.M. Ratcliffe, P.S. Sivanand, Sens. Actuators B 92 (2003) 159–166. [18] S.S. Badadhe, I.S. Mulla, Sens. Actuators B 156 (2011) 943–948. [19] L. Liu, S. Li, J. Zhuang, L. Wang, J. Zhang, H. Li, Z. Liu, Y. Han, X. Jiang, P. Zhang, Sens. Actuators B 155 (2011) 782–788. [20] C. Ge, C. Xie, S. Cai, Mater. Sci. Eng. B 137 (2007) 53–58. [21] N. Han, X. Wu, D. Zhang, G. Shen, H. Liu, Y. Chen, Sens. Actuators B 152 (2011) 324–329. [22] Y. Cai, H. Fan, M. Xu, Q. Li, C. Long, CrystEngComm 15 (2013) 7339–7345. [23] S. Bai, T. Guo, Y. Zhao, J. Sun, D. Li, A. Chen, C.C. Liu, Sens. Actuators B 195 (2014) 657–666. [24] A.P. Rambu, C. Doroftei, L. Ursu, F. Iacomi, J. Mater. Sci. 48 (2013) 4305–4312. [25] J.-W. Yoon, H.-J. Kim, I.-D. Kim, J.-H. Lee, Nanotechnology 24 (444005) (2013) 8 pp. [26] J. Zhao, T. Yang, Y. Liu, Z. Wang, X. Li, Y. Sun, Y.u. Du, Y. Li, G. Lu, Sens. Actuators B 191 (2014) 806–812. [27] Q.-Y. Ouyang, L. Li, Q.-S. Wang, Y. Zhang, T.-S. Wang, F.-N. Meng, Y.-J. Chen, P. Gao, Sens. Actuators B 169 (2012) 17–25. [28] H.P. Klong, L.F. Alexander, X-Ray Diffraction Procedures for Crystalline and Amorphous Materials, Wiley, New York, 1954. [29] S. Kumar, S. Mukherjee, R.K. Singh, S. Chatterjee, A.K. Ghosh, J. Appl. Phys. 110 (103508) (2011) 7 pp. [30] Z. Chen, M. Cao, C. Hu, J. Phys. Chem. C 115 (2011) 5522–5529. [31] M. Chen, Z. Wang, D. Han, F. Gu, G. Guo, Sens. Actuators B 157 (2011) 565–574. [32] M. Chen, Z. Wang, D. Han, F. Gu, G. Guo, J. Phys. Chem. C 115 (2011) 12763–12773. [33] L.C. Bourne, P.Y. Yu, Z. Zettl, M.L. Cohen, Phys. Rev. B 40 (1989) 10973–10976. [34] N. Han, L. Chai, Q. Wang, Y. Tian, P. Deng, Y. Chen, Sens. Actuators B 147 (2010) 525–530.
58
S. Huang et al. / Journal of Physics and Chemistry of Solids 76 (2015) 51–58
[35] K.-M. Kim, H.-R. Kim, K.-I. Choi, H.-J. Kim, J.-H. Lee., Sens. Actuators B 155 (2011) 745–751. [36] L. Zhang, J. Zhao, H. Lu, L. Gong, L. Li, J. Zheng, H. Li, Z. Zhu, Sens. Actuators B 160 (2011) 364–370. [37] V. Kruefu, C. Liewhiran, A. Wisitsoraat, S. Phanichphant, Sens. Actuators B 156 (2011) 360–367.
[38] K. Zheng, L. Gu, D. Sun, X. Mo, G. Chen, Mater. Sci. Eng. B 166 (2010) 104–107. [39] D. Zhang, X. Wu, N. Han, Y. Chen, J. Nanopart. Res. 15 (1580) (2013) 10pp. [40] A. Khayatian, M.A. Kashi, R. Azimirad., S. Safa, J. Phys. D: Appl. Phys. 47 (075003) (2014) 10 pp. [41] J.J. Vijaya, L.J. Kennedy, G. Sekaran, A. Meenakshisundaram, R.T. Kumar, P. Amalthi, K.S. Nagaraja, Sens. Actuators B 129 (2008) 741–749.