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Influence of annealing temperature on the photoelectric gas sensing of Fe-doped ZnO under visible light irradiation
Sensors and Actuators B 177 (2013) 34–40
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Influence of annealing temperature on the photoelectric gas sensing of Fe-doped ZnO under visible light irradiation Lina Han, DeJun Wang, Yongchun Lu, Tengfei Jiang, Liping Chen, Tengfeng Xie, Yanhong Lin ∗ College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 June 2012 Received in revised form 12 October 2012 Accepted 18 October 2012 Available online 9 November 2012 Keywords: Gas sensor Visible light-illumination Fe-doped ZnO Room temperature Annealing temperature
a b s t r a c t Fe-doped ZnO are fabricated by hydrothermal method. The influence of the annealing temperature on structural and photoelectric property was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–vis diffuse reflectance spectra (UV–vis DRS). Visible light-illumination roomtemperature gas sensing to formaldehyde based on Fe-doped ZnO annealed at different temperatures was subsequently investigated by the surface photocurrent spectra and gas sensor characterization system, respectively. Accompanying with increasing annealing temperature from 400 ◦ C to 600 ◦ C, the response to formaldehyde was enhanced due to the increase in crystal quality and active sites on the surface of Fe-doped ZnO. However, when the annealing temperature goes over 700 ◦ C, the phase of ZnFe2 O4 was observed from XRD pattern. The existence of ZnFe2 O4 hindered the transfer of photo-generated carriers and decreased the amount of surface active sites. As a result, the photoelectric gas response for formaldehyde was weaken to some extent. Our results demonstrated that annealing temperature is a very important parameter in the determination of the sensitivity of photoelectric gas sensing based on Fe-doped ZnO. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Detection of toxic gases such as HCHO, NH3 , CO, toluene, CH3 COCH3 is of great importance due to their detrimental effects on human health and prevalence in various industries [1–5]. At present, various metal oxide semiconductors including ZnO, TiO2 , SnO2 , CdS and MoO3 have been extensively used in the monitoring of toxic pollutants [6–10]. Among them, zinc oxide (ZnO), with a wide band gap, has been considered as a promising photoelectric gas sensing materials due to highly sensitive to environment and excellent photoconductive property, and it has successfully been employed to detect a wide range of gases and vapors under the light irradiation, particularly ethanol, formaldehyde and acetone [11–14]. Nevertheless, the sensitivities of ZnO-based materials are still comparatively low for the photoelectric gas sensor compared to that of the traditional heat-treatment gas sensors [15–17]. Therefore, to improve the reliability, stability and sensitivity of the photoelectric gas sensors, the different factors that may affect the gas sensing of ZnO should be further optimized, such as the morphology, surface-to-volume ratio, surface defects and active center of the material [18,19]. During the preparation of materials, annealing temperature was regarded as one of key factors which significantly affect the structure, optical and electrical
∗ Corresponding author. Tel.: +86 431 85168093. E-mail address: [email protected] (Y. Lin). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.10.096
properties of the sensing materials. Therefore, a systematic study on the effects of thermal annealing on the structural, optical and electrical properties for gas sensing materials is required, and lots of work about it have been reported [20–23]. Song et al. synthesized the perovskite La0.8 Pb0.2 Fe0.8 Cu0.2 O3 nanoparticles and investigated CO gas response properties. The results demonstrated that La0.8 Pb0.2 Fe0.8 Cu0.2 O3 sample calcined at 800 ◦ C exhibited higher response and good selectivity to CO [24]. Koziej et al. made an attempt to understand the influence of the annealing treatment on CO sensing with tin dioxide based sensors. It was concluded that the final annealing temperature influences the concentration of the reactive sites for oxygen ionosorption, which finally determines the main CO reaction route and thus the sensor signal [25]. In conclusion, thermal annealing at elevated temperature may lead to obvious changes of crystallinity, grain size and shape, porosity, active surface area, and even the phase composition, which are known to be the main factors determining the gas-sensing properties of oxide semiconductor-based sensors. However, to the best of our knowledge, the effect of annealing temperature on the photoelectric gas sensing properties of Fe-doped ZnO materials has not been reported. Therefore, it is significant and necessary to comprehensively understand the annealing-induced effects on the photoelectric gas-sensing properties. We have already reported effect of Fe doping on the gas sensing properties of the ZnO [26]. Here, we focused on the impact of annealing temperature on the Fe-doped ZnO by considering 1.0 mol% of Fe. The main goal of the paper is to investigate effect
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of annealing temperature on the crystal properties, photoelectric gas sensing of the Fe-doped ZnO under visible light irradiation. This work can help us to understand the relationship between the annealing-induced nanostructure change and the gas-sensing properties of the ZnO. 2. Experimental 2.1. Synthesis of Fe-doped ZnO The preparation of the Fe-doped ZnO has been described in detail elsewhere [26]. Briefly, the precipitates consisting of 1.0 mol% Fe-doped ZnO (mole ratio of Fe/Zn is 0.01) were synthesized by hydrothermal method according to the report [27], and then sintered at 400 ◦ C, 500 ◦ C, 600 ◦ C and 700 ◦ C for 2.5 h to get a series of different annealing temperature Fe-doped ZnO, respectively. Meanwhile, the 1.0 mol% Fe-doped ZnO nanopowders without annealed were also synthesized using the identical procedure for comparison. 2.2. Characterization of Fe-doped ZnO The crystalline phase was determined by powder X-ray diffraction (XRD) with a Rigaku D/Max–2550 diffractometer using Cu ˚ at 50 kV and 200 mA in the 2 range K␣ radiation ( = 1.5418 A) 20–70◦ at a scanning rate of 10◦ /min. The micrographs were taken with a scanning electron microscopy (SEM; Shimadzu, SS-550). UV–vis diffuse reflectance spectra (UV–vis DRS) were measured on dispersions using a UV–vis–NIR spectrophotometer (Shimadzu UV-3600) to detect absorption over the range 300–700 nm. Surface photocurrents were measured with a comb-like electrode using light source-monochromator-lock-in detection technique. 2.3. The fabrication of gas sensor devices and gas sensing measurement The Fe-doped ZnO powder was ultrasonically dispersed in ethanol to produce thick slurry. Then the above solution was dripped onto the etched ITO comb-like electrode, and kept at 50 ◦ C for 2 h to vaporize ethanol, which was considered as the prototype device of photoelectric gas sensor. The thickness of sensing film is ∼0.1–0.3 mm. Gas sensing properties were measured using a static test system which included a test chamber and an electrochemistry workstation system (CHI600 made in China) [28]. The monochromatic light (532 nm, 20 mW/cm2 ) was obtained by a light beam (provided by 532 nm laser pointer). The light beam can irradiate the sensor through a quartz window of the test chamber. An electrochemistry workstation (CHI 630b, made in China) was used to record the surface photocurrent intensity across the gas sensor with a bias of 10 V. In gas sensing test, air was used both as a reference gas and a diluting gas to obtain desired concentrations of analyte (formaldehyde). In our experiment, a special volume of formaldehyde aqueous solution was injected into the test chamber with a micro syringe through a rubber plug. Following injection, formaldehyde solution, which is highly volatile at room temperature, will produce formaldehyde vapors. After formaldehyde gas fully mixed with the diluting gas, a series of different formaldehyde gas concentrations (5–50 ppm) were obtained. On a controlled irradiation measurement, the 532 nm laser pointer is immediately turned on after the sensor was put into the test chamber with formaldehyde. After the 532 nm light irradiation lasts ∼300 s, the irradiation is turned off and the sensor was taken out to recover in pure air. The irradiation was turned on–off in different atmospheres, and the surface photocurrent intensity was recorded by the work-station
Fig. 1. X-ray diffraction pattern of Fe-doped ZnO annealed at different temperatures.
system. The maximum surface photocurrent intensity in dry air was measured as reference at the first cycle. The gas response is defined as follows: S=
Ig − Ia × 100% Ia
(1)
where S is the gas response intensity, Ia is the maximum surface photocurrent intensity in dry air, Ig is the maximum surface photocurrent intensity in analyte. In addition, the concentration (C, ppm) of formaldehyde gas in the test chamber could be calculated according to Eq. (2), which is based on the definition of ppm for gas concentration: Cppm =
VL g/mL Mg/mol VmL
× 2.46 × 107
(2)
where Cppm is concentration of the detected gas; VmL is the volume of the diluting gas which is equal to the volume of the test chamber; VL , g/mL and Mg/mol refer to the volume, density and molecular weight of the liquid organic analyte, respectively. All the subscripts are the corresponding units. In addition, all the tests were operated at room temperature (25 ± 1 ◦ C). 3. Results and discussion 3.1. Morphologies structures and UV–vis reflectivity spectra The crystal structures of Fe-doped ZnO annealed at different temperatures were analyzed by X-ray diffraction (XRD). As shown in Fig. 1, all samples have a similar wurtzite phase, and no other impurities were detected when the annealing temperature was less than 600 ◦ C. However, after annealing at 700 ◦ C for 2.5 h, a hump appeared at around 35◦ corresponding to (3 1 1) peak of ZnFe2 O4 , and was marked by star. It was revealed that the annealing treatment changed the positions of Fe ions in ZnO. In addition, the increase of diffraction peak intensity and a shift of the peak positions to higher values of 2 (see inset in Fig. 1) with increasing annealing temperature from 400 ◦ C up to 600 ◦ C indicated that thermal annealing resulted in an increase of crystallinity and was benefit for the incorporation of Fe ions into the ZnO lattice. However, when the annealing temperature arrives at elevated temperature, the dopants will migrate from ordered substitutional sites within the lattice to more disordered regions, most likely the surface of the nanocrystals [29]. Therefore, after thermal annealing at 700 ◦ C, Fe ions which spill over into the surface of ZnO will
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L. Han et al. / Sensors and Actuators B 177 (2013) 34–40
Fig. 2. SEM and UV–vis adsorption spectrums of Fe-doped ZnO with the annealing temperature from 0 ◦ C to 700 ◦ C.
form ZnFe2 O4 with ZnO [30], resulting in the shift of (0 0 2) peak toward lower 2 value compared with that of the ZnO annealed at 600 ◦ C. This result also shows that there is an optimal annealing temperature during the synthesis of Fe-doped ZnO materials. The morphologies of the Fe-doped ZnO annealed in air at 400 ◦ C, 500 ◦ C, 600 ◦ C, 700 ◦ C were studied by SEM, respectively. As shown in Fig. 2(a)–(e), the powders are self-assembled by nanorods with the lengths of 3–3.5 m and diameters of 350–450 nm. The original nanoflower is unspoiled after annealed at high temperature. Besides that, grain size of flowerlike ZnO did not change significantly as the increasing annealing temperature, which is in good agreement with the previous report [31]. Fig. 2(f) exhibits the UV–vis absorption spectra of Fe-doped ZnO samples before and after annealing at different temperature. As expected, unannealed ZnO sample showed the characteristic spectrum of ZnO with its sharp fundamental absorption edge rising at 390 nm, indicating that Fe ions did not incorporated in ZnO. With the increasing of annealed temperature, the absorption was extended from UV to UV–vis. This might be ascribed to the formation of a new dopant energy level below the conduction band of ZnO with the incorporation of Fe ions into the host lattice by the annealed treatment [32]. Furthermore, the extend absorbance of Fe-doped ZnO in the visible
region was observed, which can be attributed to the increase in Fe doping concentration with the increasing annealing temperature. The above results indicated that Fe-doped ZnO with annealed treatment has an effective absorption of visible light, which provides a possibility for achieving the visible-light-induced photoelectric gas sensing. 3.2. Gas-sensing performance The surface photocurrent spectra of Fe-doped ZnO are illustrated in Fig. 3. It is clearly seen that the surface photocurrent intensity is enhanced and the photocurrent peak shift to longer wavelengths after annealed treatment at high temperature. Besides that, the response of photocurrent can be extended to 650 nm and the intensity was enhanced dramatically in the visible light region. The photocurrent intensity reaches a maximum after the sample annealed at 600 ◦ C for 2.5 h. As we all known, Fe atoms can rearrange themselves into more suitable positions during the annealing process [33], and generate a new dopant energy below the conduction band of ZnO. Thus, less energy is required by an electron to jump into the conduction band. With the increasing annealing temperature, more Fe atoms were doped into the
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Fig. 3. Surface photocurrent of Fe-doped ZnO annealed at different temperatures.
lattice of ZnO to take part in the formation of dopant energy level. When the Fe-doped ZnO is illuminated by visible light, the photo-generated electrons would be excited from the valance band to the dopant energy level, and then excited to the conducting band of ZnO. On the one hand, thermal annealing improved crystal quality and decreased structure defect in materials so as to facilitate the transfer of photo-generated carriers to the surface. As a result, the photocurrent intensity increased with the increasing annealing temperature. But when the annealing temperature reach to 700 ◦ C, Fe atoms migrated from ordered substitutional sites within the lattice to more disordered regions, most likely the surface of ZnO and finally formed ZnFe2 O4 . With the formation of ZnFe2 O4 at the surface of ZnO, a significant increase in the grain boundary resistance will hinder the transfer of photo-generated carriers to some extent. Consequently, the photocurrent intensity will be decreased. Therefore, the surface photocurrent results confirmed that suitable annealing temperature can improve the transfer property of photo-generated carriers in Fe-doped ZnO. These results are very important for the achievement of visible-light-assisted gas sensor. In addition, according to the results of surface photocurrent spectra, the 532 nm light was selected as irradiation light for sensing measurement because the doped samples exhibit the obvious photocurrent response at this wavelength. Moreover, considering this sensor from a practical, the 532 nm light provide by the laser pointer is benefit for the achievement of micro-photoelectric sensor. The gas sensing properties of Fe-doped ZnO annealed at different temperatures were measured under 532 nm light irradiation at room temperature. The response is an important parameter to reflect the performance of gas sensors. Therefore, the responses of Fe-doped ZnO with different annealing temperature to various concentrations of formaldehyde were calculated in detail according to Eq. (1). The calculating results are shown in Fig. 4. It was found that the response of all Fe-doped ZnO samples increased obviously with the increasing of formaldehyde concentrations. The sample annealed at 600 ◦ C has a much better sensing performance than others in the same detection condition. The response is ∼22%, 50%, 94%, 132%, 165% and 190% corresponding to formaldehyde concentrations of ∼5, 10, 20, 30, 40 and 50 ppm in the chamber, respectively. However, the response of sample annealed at 400 ◦ C is the lowest, which is ∼10%, 32%, 44%, 55%, 63% and 69%, respectively. That is to say, the response to formaldehyde in Fe-doped ZnO sample is very sensitive to the annealing temperature. The main reason is that during the annealing process, on the one hand, Fe atoms can rearrange themselves into more suitable positions and substitute for some of the lattice zinc ions. With the increasing annealing temperature, more Fe atoms were doped into the host of
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Fig. 4. Gas-sensing response cycles of Fe-doped ZnO (annealed at temperature from 400 ◦ C to 700 ◦ C) to different concentrations of formaldehyde under 532 nm light irradiation at the room temperature.
ZnO to take part in the formation of dopant energy level between the conduction band and the valence band of ZnO, which resulted in the excellent photoelectric performance in the range of visible light. On the other hand, dislocations and other structural defects will move in the material, and adsorption will occur on the surface easily when the samples were annealed at higher temperature [34,35]. This means that the active sites on the surface of Fe-doped ZnO increases with increasing annealing temperature. As a result, the number of oxygen species adsorbed on the activated surface of Fe-doped ZnO would be increased. Therefore, under irradiated by 532 nm light, the atmospheric oxygen molecules adsorbed on the doped samples’ surface capture a certain amount of free electrons from the Fe-doped ZnO samples and form various oxygen ions. When the Fe-doped ZnO sensors are exposed to formaldehyde gas, the reducing gas and the chemisorbed oxygen ions on the surface of ZnO can give rise to the chemical reaction [36]. The larger the number of oxygen species adsorbed, the faster would be the oxidation reaction of formaldehyde. Correspondingly, a large number of electrons would be re-released, and the conductivity of Fe-doped ZnO is enhanced clearly. Consequently, the response was enhanced with the increasing annealing temperature. But when the annealing temperature reaches to 700 ◦ C, the Fe atoms migrated from ordered substitutional sites within the lattice to more disordered regions, most likely the surface of ZnO and formed ZnFe2 O4 . With the formation of ZnFe2 O4 on the surface of ZnO, the enhanced potential barriers between grain boundaries will impede the transfer of free carriers and improve the probability of recombination. Furthermore, the active sites on the surface of ZnO will be occupied by ZnFe2 O4 nanoparticles, and the number of oxygen ions chemisorbing will be reduced. As a result, the response of higher annealing treatment decreases. Considering the above results, we suggest that the annealing treatment have remarkably affected the gas sensing activity of Fe-doped ZnO for the visible-light-assisted gas sensing, and the optimum annealing temperature is about 600 ◦ C. In addition, in gas sensing test, the formaldehyde aqueous solution was used to prepare the detected gas. Correspondingly, the water vapor will be injected into the test chamber accompanying the formaldehyde gas. Therefore, the important question of whether the above response can really reflect gas sensing to formaldehyde should be solved. To clarify this problem, the response to only water with different volumes was measured. The result is shown in Fig. 5. It can be seen that the response to only water is rather weak compared to formaldehyde. This results show that a trace amount of water vapor injected into the test chamber has little effect on the gas response. Unlike our previous report [28],
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Fig. 5. Response of Fe-doped ZnO annealed at 400 ◦ C to different volumes of formaldehyde aqueous solution and water under the 532 nm light irradiation, respectively.
in our experiment, injection of ∼1.0 l formaldehyde solution into the test chamber can prepare 50 ppm formaldehyde, and the very little water can be injected into the chamber. The injected water is not enough to change humidity of circumstance. Therefore, considering the above analysis and the result of experiment, we conclude that the effect of water vapor on the response can be ignored in our gas sensing test. The change of surface photocurrent intensity was mainly aroused by the introduction of formaldehyde gas, and this method of gas detection is feasible. The perfect sensor would be highly selective and sensitive. Based on this point, selectivity experiment is carried out. Besides formaldehyde, the gas sensing to ethanol, acetone, ammonia and toluene was also measured and the results were recorded in Fig. 6. It can be seen obviously that the responses to ethanol, acetone, ammonia and toluene are much lower than that of formaldehyde. The reason may be that formaldehyde has a higher redox activity compared to the other gases. The results implied that Fedoped ZnO has a well selectivity to formaldehyde at the room temperature.
Fig. 6. Responses of Fe-doped ZnO annealed at 600 ◦ C for 2.5 h to different concentration of formaldehyde, ethanol, acetone, ammonia and toluene under the 532 nm light irradiation, respectively.
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4. Conclusion In this work, we made an attempt to study the influence of annealing treatment on structural and photoelectric property based on Fe-doped ZnO. The photoelectric gas sensing to formaldehyde based on Fe-doped ZnO with different annealing temperature was also investigated. The results demonstrated that the Fe-doped ZnO with annealed at 600 ◦ C for 2.5 h showed excellent response. This could be attributed to the fact that the annealing temperature affected the existing position of Fe atoms in the lattice of ZnO. When annealed at temperatures between 400 ◦ C and 600 ◦ C, Fe atoms were substituted into the ZnO host structure, and existing in the form of ions. Whereas, when the annealing temperature reached 700 ◦ C, parts of Fe atoms can rearrange themselves into more suitable positions during the annealing process, and ZnFe2 O4 was formed. As a result, the response of higher annealing treatment decreases. Besides that, the selectivity of the photoelectric gas sensing based on Fe-doped ZnO to ethanol, acetone, ammonia and toluene was also measured and the results implied that Fe-doped ZnO has a well selectivity to formaldehyde. Our results demonstrated that the annealing temperature is a very important parameter to determining the sensitivity of photoelectric gas sensing based on Fe-doped ZnO.
Acknowledgement This work was supported by the National Natural Science Foundation of China (Nos. 21173103 and 51172090).
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Biographies Lina Han received her bachelor’s degree in chemistry from Jilin University, China in 2012. She is currently studying for degree of PhD at Shanghai Jiaotong University. Her work is studying photocatalysis based on TiO2 mesoporous. Dejun Wang received his PhD degree in 1989 in chemistry from Jilin University. At present, he is a professor in College of Chemistry, Jilin University. His current fields of interest include photoelectric gas sensor, photo-generated charge properties at the surface and interface of semiconductor materials, and dye-sensitized solar cell. Yongchun Lu received her bachelor’s degree in chemistry at Mudanjiang Normal University, China in 2008. She is currently studying for degree of PhD at Jilin University. Her work is studying visible-light-driven photogenerated charge transfer properties and photocatalytic activity of metal doped ZnO nanoparticles.
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Tengfei Jiang received his bachelor’s degree in 2009 at Jilin University, China. He is currently studying for degree of PhD at Jilin University. His work is studying the behaviors of photoinduced charge carriers in photoelectrochemical cells.
Tengfeng Xie received his PhD degree in 2001 in chemistry from Jilin University. At present, he is a professor in College of Chemistry, Jilin University. His current fields of interest include photoelectric gas sensor, photo-generated charge properties at the surface and interface of semiconductor materials, and dye-sensitized solar cell.
Liping Chen received his bachelor’s degree in chemistry at Jilin University in 2009. He is currently studying for degree of PhD at Jilin University. His work is studying property of photo-induced charge carriers and their relations with photocatalytic reaction.
Yanhong Lin received her PhD degree in 2006 in chemistry from Jilin University. At present, she is a professor in College of Chemistry, Jilin University. Her current fields of interest include photoelectric gas sensor, photo-generated charge properties at the surface and interface of semiconductor materials, and dye-sensitized solar cell.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Influence of annealing temperature on the photoelectric gas sensing of Fe-doped ZnO under visible light irradiation Lina Han, DeJun Wang, Yongchun Lu, Tengfei Jiang, Liping Chen, Tengfeng Xie, Yanhong Lin ∗ College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 June 2012 Received in revised form 12 October 2012 Accepted 18 October 2012 Available online 9 November 2012 Keywords: Gas sensor Visible light-illumination Fe-doped ZnO Room temperature Annealing temperature
a b s t r a c t Fe-doped ZnO are fabricated by hydrothermal method. The influence of the annealing temperature on structural and photoelectric property was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–vis diffuse reflectance spectra (UV–vis DRS). Visible light-illumination roomtemperature gas sensing to formaldehyde based on Fe-doped ZnO annealed at different temperatures was subsequently investigated by the surface photocurrent spectra and gas sensor characterization system, respectively. Accompanying with increasing annealing temperature from 400 ◦ C to 600 ◦ C, the response to formaldehyde was enhanced due to the increase in crystal quality and active sites on the surface of Fe-doped ZnO. However, when the annealing temperature goes over 700 ◦ C, the phase of ZnFe2 O4 was observed from XRD pattern. The existence of ZnFe2 O4 hindered the transfer of photo-generated carriers and decreased the amount of surface active sites. As a result, the photoelectric gas response for formaldehyde was weaken to some extent. Our results demonstrated that annealing temperature is a very important parameter in the determination of the sensitivity of photoelectric gas sensing based on Fe-doped ZnO. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Detection of toxic gases such as HCHO, NH3 , CO, toluene, CH3 COCH3 is of great importance due to their detrimental effects on human health and prevalence in various industries [1–5]. At present, various metal oxide semiconductors including ZnO, TiO2 , SnO2 , CdS and MoO3 have been extensively used in the monitoring of toxic pollutants [6–10]. Among them, zinc oxide (ZnO), with a wide band gap, has been considered as a promising photoelectric gas sensing materials due to highly sensitive to environment and excellent photoconductive property, and it has successfully been employed to detect a wide range of gases and vapors under the light irradiation, particularly ethanol, formaldehyde and acetone [11–14]. Nevertheless, the sensitivities of ZnO-based materials are still comparatively low for the photoelectric gas sensor compared to that of the traditional heat-treatment gas sensors [15–17]. Therefore, to improve the reliability, stability and sensitivity of the photoelectric gas sensors, the different factors that may affect the gas sensing of ZnO should be further optimized, such as the morphology, surface-to-volume ratio, surface defects and active center of the material [18,19]. During the preparation of materials, annealing temperature was regarded as one of key factors which significantly affect the structure, optical and electrical
∗ Corresponding author. Tel.: +86 431 85168093. E-mail address: [email protected] (Y. Lin). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.10.096
properties of the sensing materials. Therefore, a systematic study on the effects of thermal annealing on the structural, optical and electrical properties for gas sensing materials is required, and lots of work about it have been reported [20–23]. Song et al. synthesized the perovskite La0.8 Pb0.2 Fe0.8 Cu0.2 O3 nanoparticles and investigated CO gas response properties. The results demonstrated that La0.8 Pb0.2 Fe0.8 Cu0.2 O3 sample calcined at 800 ◦ C exhibited higher response and good selectivity to CO [24]. Koziej et al. made an attempt to understand the influence of the annealing treatment on CO sensing with tin dioxide based sensors. It was concluded that the final annealing temperature influences the concentration of the reactive sites for oxygen ionosorption, which finally determines the main CO reaction route and thus the sensor signal [25]. In conclusion, thermal annealing at elevated temperature may lead to obvious changes of crystallinity, grain size and shape, porosity, active surface area, and even the phase composition, which are known to be the main factors determining the gas-sensing properties of oxide semiconductor-based sensors. However, to the best of our knowledge, the effect of annealing temperature on the photoelectric gas sensing properties of Fe-doped ZnO materials has not been reported. Therefore, it is significant and necessary to comprehensively understand the annealing-induced effects on the photoelectric gas-sensing properties. We have already reported effect of Fe doping on the gas sensing properties of the ZnO [26]. Here, we focused on the impact of annealing temperature on the Fe-doped ZnO by considering 1.0 mol% of Fe. The main goal of the paper is to investigate effect
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of annealing temperature on the crystal properties, photoelectric gas sensing of the Fe-doped ZnO under visible light irradiation. This work can help us to understand the relationship between the annealing-induced nanostructure change and the gas-sensing properties of the ZnO. 2. Experimental 2.1. Synthesis of Fe-doped ZnO The preparation of the Fe-doped ZnO has been described in detail elsewhere [26]. Briefly, the precipitates consisting of 1.0 mol% Fe-doped ZnO (mole ratio of Fe/Zn is 0.01) were synthesized by hydrothermal method according to the report [27], and then sintered at 400 ◦ C, 500 ◦ C, 600 ◦ C and 700 ◦ C for 2.5 h to get a series of different annealing temperature Fe-doped ZnO, respectively. Meanwhile, the 1.0 mol% Fe-doped ZnO nanopowders without annealed were also synthesized using the identical procedure for comparison. 2.2. Characterization of Fe-doped ZnO The crystalline phase was determined by powder X-ray diffraction (XRD) with a Rigaku D/Max–2550 diffractometer using Cu ˚ at 50 kV and 200 mA in the 2 range K␣ radiation ( = 1.5418 A) 20–70◦ at a scanning rate of 10◦ /min. The micrographs were taken with a scanning electron microscopy (SEM; Shimadzu, SS-550). UV–vis diffuse reflectance spectra (UV–vis DRS) were measured on dispersions using a UV–vis–NIR spectrophotometer (Shimadzu UV-3600) to detect absorption over the range 300–700 nm. Surface photocurrents were measured with a comb-like electrode using light source-monochromator-lock-in detection technique. 2.3. The fabrication of gas sensor devices and gas sensing measurement The Fe-doped ZnO powder was ultrasonically dispersed in ethanol to produce thick slurry. Then the above solution was dripped onto the etched ITO comb-like electrode, and kept at 50 ◦ C for 2 h to vaporize ethanol, which was considered as the prototype device of photoelectric gas sensor. The thickness of sensing film is ∼0.1–0.3 mm. Gas sensing properties were measured using a static test system which included a test chamber and an electrochemistry workstation system (CHI600 made in China) [28]. The monochromatic light (532 nm, 20 mW/cm2 ) was obtained by a light beam (provided by 532 nm laser pointer). The light beam can irradiate the sensor through a quartz window of the test chamber. An electrochemistry workstation (CHI 630b, made in China) was used to record the surface photocurrent intensity across the gas sensor with a bias of 10 V. In gas sensing test, air was used both as a reference gas and a diluting gas to obtain desired concentrations of analyte (formaldehyde). In our experiment, a special volume of formaldehyde aqueous solution was injected into the test chamber with a micro syringe through a rubber plug. Following injection, formaldehyde solution, which is highly volatile at room temperature, will produce formaldehyde vapors. After formaldehyde gas fully mixed with the diluting gas, a series of different formaldehyde gas concentrations (5–50 ppm) were obtained. On a controlled irradiation measurement, the 532 nm laser pointer is immediately turned on after the sensor was put into the test chamber with formaldehyde. After the 532 nm light irradiation lasts ∼300 s, the irradiation is turned off and the sensor was taken out to recover in pure air. The irradiation was turned on–off in different atmospheres, and the surface photocurrent intensity was recorded by the work-station
Fig. 1. X-ray diffraction pattern of Fe-doped ZnO annealed at different temperatures.
system. The maximum surface photocurrent intensity in dry air was measured as reference at the first cycle. The gas response is defined as follows: S=
Ig − Ia × 100% Ia
(1)
where S is the gas response intensity, Ia is the maximum surface photocurrent intensity in dry air, Ig is the maximum surface photocurrent intensity in analyte. In addition, the concentration (C, ppm) of formaldehyde gas in the test chamber could be calculated according to Eq. (2), which is based on the definition of ppm for gas concentration: Cppm =
VL g/mL Mg/mol VmL
× 2.46 × 107
(2)
where Cppm is concentration of the detected gas; VmL is the volume of the diluting gas which is equal to the volume of the test chamber; VL , g/mL and Mg/mol refer to the volume, density and molecular weight of the liquid organic analyte, respectively. All the subscripts are the corresponding units. In addition, all the tests were operated at room temperature (25 ± 1 ◦ C). 3. Results and discussion 3.1. Morphologies structures and UV–vis reflectivity spectra The crystal structures of Fe-doped ZnO annealed at different temperatures were analyzed by X-ray diffraction (XRD). As shown in Fig. 1, all samples have a similar wurtzite phase, and no other impurities were detected when the annealing temperature was less than 600 ◦ C. However, after annealing at 700 ◦ C for 2.5 h, a hump appeared at around 35◦ corresponding to (3 1 1) peak of ZnFe2 O4 , and was marked by star. It was revealed that the annealing treatment changed the positions of Fe ions in ZnO. In addition, the increase of diffraction peak intensity and a shift of the peak positions to higher values of 2 (see inset in Fig. 1) with increasing annealing temperature from 400 ◦ C up to 600 ◦ C indicated that thermal annealing resulted in an increase of crystallinity and was benefit for the incorporation of Fe ions into the ZnO lattice. However, when the annealing temperature arrives at elevated temperature, the dopants will migrate from ordered substitutional sites within the lattice to more disordered regions, most likely the surface of the nanocrystals [29]. Therefore, after thermal annealing at 700 ◦ C, Fe ions which spill over into the surface of ZnO will
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Fig. 2. SEM and UV–vis adsorption spectrums of Fe-doped ZnO with the annealing temperature from 0 ◦ C to 700 ◦ C.
form ZnFe2 O4 with ZnO [30], resulting in the shift of (0 0 2) peak toward lower 2 value compared with that of the ZnO annealed at 600 ◦ C. This result also shows that there is an optimal annealing temperature during the synthesis of Fe-doped ZnO materials. The morphologies of the Fe-doped ZnO annealed in air at 400 ◦ C, 500 ◦ C, 600 ◦ C, 700 ◦ C were studied by SEM, respectively. As shown in Fig. 2(a)–(e), the powders are self-assembled by nanorods with the lengths of 3–3.5 m and diameters of 350–450 nm. The original nanoflower is unspoiled after annealed at high temperature. Besides that, grain size of flowerlike ZnO did not change significantly as the increasing annealing temperature, which is in good agreement with the previous report [31]. Fig. 2(f) exhibits the UV–vis absorption spectra of Fe-doped ZnO samples before and after annealing at different temperature. As expected, unannealed ZnO sample showed the characteristic spectrum of ZnO with its sharp fundamental absorption edge rising at 390 nm, indicating that Fe ions did not incorporated in ZnO. With the increasing of annealed temperature, the absorption was extended from UV to UV–vis. This might be ascribed to the formation of a new dopant energy level below the conduction band of ZnO with the incorporation of Fe ions into the host lattice by the annealed treatment [32]. Furthermore, the extend absorbance of Fe-doped ZnO in the visible
region was observed, which can be attributed to the increase in Fe doping concentration with the increasing annealing temperature. The above results indicated that Fe-doped ZnO with annealed treatment has an effective absorption of visible light, which provides a possibility for achieving the visible-light-induced photoelectric gas sensing. 3.2. Gas-sensing performance The surface photocurrent spectra of Fe-doped ZnO are illustrated in Fig. 3. It is clearly seen that the surface photocurrent intensity is enhanced and the photocurrent peak shift to longer wavelengths after annealed treatment at high temperature. Besides that, the response of photocurrent can be extended to 650 nm and the intensity was enhanced dramatically in the visible light region. The photocurrent intensity reaches a maximum after the sample annealed at 600 ◦ C for 2.5 h. As we all known, Fe atoms can rearrange themselves into more suitable positions during the annealing process [33], and generate a new dopant energy below the conduction band of ZnO. Thus, less energy is required by an electron to jump into the conduction band. With the increasing annealing temperature, more Fe atoms were doped into the
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Fig. 3. Surface photocurrent of Fe-doped ZnO annealed at different temperatures.
lattice of ZnO to take part in the formation of dopant energy level. When the Fe-doped ZnO is illuminated by visible light, the photo-generated electrons would be excited from the valance band to the dopant energy level, and then excited to the conducting band of ZnO. On the one hand, thermal annealing improved crystal quality and decreased structure defect in materials so as to facilitate the transfer of photo-generated carriers to the surface. As a result, the photocurrent intensity increased with the increasing annealing temperature. But when the annealing temperature reach to 700 ◦ C, Fe atoms migrated from ordered substitutional sites within the lattice to more disordered regions, most likely the surface of ZnO and finally formed ZnFe2 O4 . With the formation of ZnFe2 O4 at the surface of ZnO, a significant increase in the grain boundary resistance will hinder the transfer of photo-generated carriers to some extent. Consequently, the photocurrent intensity will be decreased. Therefore, the surface photocurrent results confirmed that suitable annealing temperature can improve the transfer property of photo-generated carriers in Fe-doped ZnO. These results are very important for the achievement of visible-light-assisted gas sensor. In addition, according to the results of surface photocurrent spectra, the 532 nm light was selected as irradiation light for sensing measurement because the doped samples exhibit the obvious photocurrent response at this wavelength. Moreover, considering this sensor from a practical, the 532 nm light provide by the laser pointer is benefit for the achievement of micro-photoelectric sensor. The gas sensing properties of Fe-doped ZnO annealed at different temperatures were measured under 532 nm light irradiation at room temperature. The response is an important parameter to reflect the performance of gas sensors. Therefore, the responses of Fe-doped ZnO with different annealing temperature to various concentrations of formaldehyde were calculated in detail according to Eq. (1). The calculating results are shown in Fig. 4. It was found that the response of all Fe-doped ZnO samples increased obviously with the increasing of formaldehyde concentrations. The sample annealed at 600 ◦ C has a much better sensing performance than others in the same detection condition. The response is ∼22%, 50%, 94%, 132%, 165% and 190% corresponding to formaldehyde concentrations of ∼5, 10, 20, 30, 40 and 50 ppm in the chamber, respectively. However, the response of sample annealed at 400 ◦ C is the lowest, which is ∼10%, 32%, 44%, 55%, 63% and 69%, respectively. That is to say, the response to formaldehyde in Fe-doped ZnO sample is very sensitive to the annealing temperature. The main reason is that during the annealing process, on the one hand, Fe atoms can rearrange themselves into more suitable positions and substitute for some of the lattice zinc ions. With the increasing annealing temperature, more Fe atoms were doped into the host of
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Fig. 4. Gas-sensing response cycles of Fe-doped ZnO (annealed at temperature from 400 ◦ C to 700 ◦ C) to different concentrations of formaldehyde under 532 nm light irradiation at the room temperature.
ZnO to take part in the formation of dopant energy level between the conduction band and the valence band of ZnO, which resulted in the excellent photoelectric performance in the range of visible light. On the other hand, dislocations and other structural defects will move in the material, and adsorption will occur on the surface easily when the samples were annealed at higher temperature [34,35]. This means that the active sites on the surface of Fe-doped ZnO increases with increasing annealing temperature. As a result, the number of oxygen species adsorbed on the activated surface of Fe-doped ZnO would be increased. Therefore, under irradiated by 532 nm light, the atmospheric oxygen molecules adsorbed on the doped samples’ surface capture a certain amount of free electrons from the Fe-doped ZnO samples and form various oxygen ions. When the Fe-doped ZnO sensors are exposed to formaldehyde gas, the reducing gas and the chemisorbed oxygen ions on the surface of ZnO can give rise to the chemical reaction [36]. The larger the number of oxygen species adsorbed, the faster would be the oxidation reaction of formaldehyde. Correspondingly, a large number of electrons would be re-released, and the conductivity of Fe-doped ZnO is enhanced clearly. Consequently, the response was enhanced with the increasing annealing temperature. But when the annealing temperature reaches to 700 ◦ C, the Fe atoms migrated from ordered substitutional sites within the lattice to more disordered regions, most likely the surface of ZnO and formed ZnFe2 O4 . With the formation of ZnFe2 O4 on the surface of ZnO, the enhanced potential barriers between grain boundaries will impede the transfer of free carriers and improve the probability of recombination. Furthermore, the active sites on the surface of ZnO will be occupied by ZnFe2 O4 nanoparticles, and the number of oxygen ions chemisorbing will be reduced. As a result, the response of higher annealing treatment decreases. Considering the above results, we suggest that the annealing treatment have remarkably affected the gas sensing activity of Fe-doped ZnO for the visible-light-assisted gas sensing, and the optimum annealing temperature is about 600 ◦ C. In addition, in gas sensing test, the formaldehyde aqueous solution was used to prepare the detected gas. Correspondingly, the water vapor will be injected into the test chamber accompanying the formaldehyde gas. Therefore, the important question of whether the above response can really reflect gas sensing to formaldehyde should be solved. To clarify this problem, the response to only water with different volumes was measured. The result is shown in Fig. 5. It can be seen that the response to only water is rather weak compared to formaldehyde. This results show that a trace amount of water vapor injected into the test chamber has little effect on the gas response. Unlike our previous report [28],
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Fig. 5. Response of Fe-doped ZnO annealed at 400 ◦ C to different volumes of formaldehyde aqueous solution and water under the 532 nm light irradiation, respectively.
in our experiment, injection of ∼1.0 l formaldehyde solution into the test chamber can prepare 50 ppm formaldehyde, and the very little water can be injected into the chamber. The injected water is not enough to change humidity of circumstance. Therefore, considering the above analysis and the result of experiment, we conclude that the effect of water vapor on the response can be ignored in our gas sensing test. The change of surface photocurrent intensity was mainly aroused by the introduction of formaldehyde gas, and this method of gas detection is feasible. The perfect sensor would be highly selective and sensitive. Based on this point, selectivity experiment is carried out. Besides formaldehyde, the gas sensing to ethanol, acetone, ammonia and toluene was also measured and the results were recorded in Fig. 6. It can be seen obviously that the responses to ethanol, acetone, ammonia and toluene are much lower than that of formaldehyde. The reason may be that formaldehyde has a higher redox activity compared to the other gases. The results implied that Fedoped ZnO has a well selectivity to formaldehyde at the room temperature.
Fig. 6. Responses of Fe-doped ZnO annealed at 600 ◦ C for 2.5 h to different concentration of formaldehyde, ethanol, acetone, ammonia and toluene under the 532 nm light irradiation, respectively.
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4. Conclusion In this work, we made an attempt to study the influence of annealing treatment on structural and photoelectric property based on Fe-doped ZnO. The photoelectric gas sensing to formaldehyde based on Fe-doped ZnO with different annealing temperature was also investigated. The results demonstrated that the Fe-doped ZnO with annealed at 600 ◦ C for 2.5 h showed excellent response. This could be attributed to the fact that the annealing temperature affected the existing position of Fe atoms in the lattice of ZnO. When annealed at temperatures between 400 ◦ C and 600 ◦ C, Fe atoms were substituted into the ZnO host structure, and existing in the form of ions. Whereas, when the annealing temperature reached 700 ◦ C, parts of Fe atoms can rearrange themselves into more suitable positions during the annealing process, and ZnFe2 O4 was formed. As a result, the response of higher annealing treatment decreases. Besides that, the selectivity of the photoelectric gas sensing based on Fe-doped ZnO to ethanol, acetone, ammonia and toluene was also measured and the results implied that Fe-doped ZnO has a well selectivity to formaldehyde. Our results demonstrated that the annealing temperature is a very important parameter to determining the sensitivity of photoelectric gas sensing based on Fe-doped ZnO.
Acknowledgement This work was supported by the National Natural Science Foundation of China (Nos. 21173103 and 51172090).
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Biographies Lina Han received her bachelor’s degree in chemistry from Jilin University, China in 2012. She is currently studying for degree of PhD at Shanghai Jiaotong University. Her work is studying photocatalysis based on TiO2 mesoporous. Dejun Wang received his PhD degree in 1989 in chemistry from Jilin University. At present, he is a professor in College of Chemistry, Jilin University. His current fields of interest include photoelectric gas sensor, photo-generated charge properties at the surface and interface of semiconductor materials, and dye-sensitized solar cell. Yongchun Lu received her bachelor’s degree in chemistry at Mudanjiang Normal University, China in 2008. She is currently studying for degree of PhD at Jilin University. Her work is studying visible-light-driven photogenerated charge transfer properties and photocatalytic activity of metal doped ZnO nanoparticles.
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Tengfei Jiang received his bachelor’s degree in 2009 at Jilin University, China. He is currently studying for degree of PhD at Jilin University. His work is studying the behaviors of photoinduced charge carriers in photoelectrochemical cells.
Tengfeng Xie received his PhD degree in 2001 in chemistry from Jilin University. At present, he is a professor in College of Chemistry, Jilin University. His current fields of interest include photoelectric gas sensor, photo-generated charge properties at the surface and interface of semiconductor materials, and dye-sensitized solar cell.
Liping Chen received his bachelor’s degree in chemistry at Jilin University in 2009. He is currently studying for degree of PhD at Jilin University. His work is studying property of photo-induced charge carriers and their relations with photocatalytic reaction.
Yanhong Lin received her PhD degree in 2006 in chemistry from Jilin University. At present, she is a professor in College of Chemistry, Jilin University. Her current fields of interest include photoelectric gas sensor, photo-generated charge properties at the surface and interface of semiconductor materials, and dye-sensitized solar cell.