Visible-Light-Assisted HCHO Gas Sensing Based on Fe-Doped

Oct 11, 2011 - The room-temperature photoelectric gas sensing of formaldehyde (HCHO) based on the Fe-doped ZnO was also studied under 532 nm light ...
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Visible-Light-Assisted HCHO Gas Sensing Based on Fe-Doped Flowerlike ZnO at Room Temperature Lina Han, DeJun Wang, Yongchun Lu, Tengfei Jiang, Bingkun Liu, and Yanhong Lin* State Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China ABSTRACT: In this work, Fe-doped flowerlike ZnO powders with various doping contents were successfully fabricated by a hydrothermal method. The results of X-ray diffraction and UV vis DRS spectra revealed that the Fe ions have been successfully doped into the crystal lattice of the ZnO host structure, and the optical absorption response of Fe-doped ZnO was extended into the visible region for the incorporation of Fe ions. The room-temperature photoelectric gas sensing of formaldehyde (HCHO) based on the Fe-doped ZnO was also studied under 532 nm light irradiation provided by a green laser pointer. It was found that the as-prepared Fe-doped ZnO samples showed excellent sensitivity, in which the gas response to 5 and 100 ppm formaldehyde can reach to 22% and 287% under 532 nm light irradiation at room temperature, respectively. The sensing mechanism of the obvious visiblelight-induced photoelectric gas sensing was discussed with the help of surface photovoltage measurement. Our results demonstrated that visible light irradiation was a promising approach to achieving a large response for gas sensors at room temperature. This work will pave a way for the development of a low-cost practical gas sensor.

1. INTRODUCTION With the development of technology, the world awareness about environmental problems and human safety is increasing. The requirement for the detection of low concentrations of formaldehyde has been enhanced in furniture and house decoration. Zinc oxide (ZnO), with a wide band gap of 3.4 eV and a large exciton binding energy of 60 meV at room temperature,1,2 is an important functional material in many fields, such as gas sensors,3 solar cells,4 field-effect transistors,5 photodetectors,6 and photocatalysts.7 Among them, gas sensors are one of the most important applications of nanostructured ZnO materials, and ZnO has been successfully employed to detect various gases.8 10 However, for the traditional heat-treatment gas sensor, the high operation temperature restricts the application of gas sensors in many areas, such as an explosive environment and a low-temperature environment. In recent years, there have been several reports of photoelectric gas sensing based on the ZnO nanomaterials.11 13 Peng et al14 synthesized ZnO using a simple hydrothermal method and detected HCHO by ultraviolet (UV) light irradiation. The results demonstrated that the gas response of ZnO nanorods to 110 ppm formaldehyde with UV light irradiation was about 120 times higher than that without UV light irradiation. Jayatissa’s group15 also demonstrated that gas-sensing properties of ZnO were affected by the UV irradiation. The irradiation time of less than 5 min has improved the sensor, whereas the irradiation time of more than 5 min degraded the sensor characteristics. The experimental results show that it is feasible to achieve the room-temperature gas sensing under UV light irradiation. However, UV light, with strong radiation, would hurt the human body and eyes seriously. Especially, the light r 2011 American Chemical Society

source of UV is usually too large in volume to meet the demand for the sensors. Undoubtedly, visible-light-induced photoelectric gas sensing will be a new type of sensor in demand for the practical application. To extend the photoresponse of the large band-gap semiconductor into the visible light region, various methods have been attempted.16 19 Among them, incorporation of transition-metal ions, such as La, Ni, Mn, and Fe, has been demonstrated in an efficient attempt to achieve visible light photocatalytic activity.20 23 However, the corresponding work has scarcely been done in the gas-sensing applications. In this paper, we synthesized Fe-doped flowerlike ZnO with a doping level in the range of 0 5.0 mol % using a facile hydrothermal method. To decrease costs and simplify the detection devices, we use the laser pointer as a light source to detect the gas sensing of formaldehyde (HCHO) based on the Fe-doped ZnO. The excellent sensitivity to HCHO under 532 nm visible light irradiation at room temperature was observed. A possible explanation of the sensing mechanism has been proposed. This work will pave a way for the development of a low-cost practical gas sensor for detection of probable chemical and biological agents.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Fe-Doped Flowerlike ZnO. All chemicals were analytical-grade reagents and used as purchased without further purification. The flowerlike ZnO with various Fe-doping Received: July 6, 2011 Revised: September 30, 2011 Published: October 11, 2011 22939

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Figure 1. Schematic illustration of the gas-sensing measurement system. The battery of the gas-sensing measurement is 10 V.

contents was prepared by a hydrothermal method according to ref 24 with slight modification. A 0.878 g (4 mmol) portion of Zn(CH3COO)2 3 2H2O and a certain amount of Fe2(NO3)3 3 9H2O (the mole ratio of Fe/Zn is 0.5, 1.0, 3.0, and 5.0%) were put into 60 mL of water and stirred at room temperature for 5 min, and then 20 mL of 3 M NaOH aqueous solution was introduced into the above solution. After the mixture was magnetically stirred for 10 min, the solution was moved to a 100 mL Teflon-lined stainless steel autoclave, sealed, and maintained at 150 °C for 10 h. When the reactions were completed, the autoclave was cooled to room temperature naturally. The white products were filtered off and washed with deionized water and absolute ethanol several times to remove possible impurities. The precipitates were dried in ambient air at 60 °C for 10 h and then sintered at 600 °C for 2.5 h to get a series of 0.5, 1.0, 3.0, and 5.0 mol % Fe-doped ZnO powders. Meanwhile, the pure ZnO sample was also synthesized using the identical procedure for comparison. 2.2. Characterization of Fe-Doped ZnO. The crystalline phase of Fe-doped ZnO was characterized by X-ray diffraction (XRD) (Rigaku D/Max-2550, Cu K lineα, λ = 1.54056 Å). The morphology of the sample was characterized by scanning electron microscopy (Shimadzu, SS-550). Surface photovoltage was measured with a lock-in based SPV measurement system, which was composed of a source of monochromatic light, a lock-in amplifier (SR830-DSP) with a light chopper (SR540), a sample cell, and a computer. A low chopping frequency of 23 Hz was used. A 500 W xenon lamp (CHFXQ500 W, Global xenon lamp power) and a double-prism monochromator (Hilger and Watts, D 300) provided monochromatic light. The photocurrent signal was detected by a lock-in amplifier (Stanford Research System, SR830-DSP)25 and recorded by a computer with an external bias of 9.6 V on the comblike electrode sides. 2.3. Gas-Sensing Measurement. The schematic diagram of the gas-sensing measurement setup is shown in Figure 1. The sensor was fabricated by putting the sample on a comblike indium doped tin oxide (ITO) transparent electrode and then put into the test chamber. Air was used both as a reference gas and as a diluting gas to obtain desired concentrations of HCHO. HCHO liquid was injected into the test chamber by a syringe through a rubber plug. After HCHO was fully mixed with the diluting gas, the sensor was irradiated with light. The light was blocked when the sample was illuminated by light with a certain period of time. An electrochemistry workstation (CHI 630b, made in China) was used to record the current intensity across the gas sensor with a bias of 10 V. Light (532 nm, 20 mW/cm2) was obtained by a light beam (provided by a 532 nm laser pointer). The 532 nm light can irradiate the sensor through a quartz

Figure 2. X-ray diffraction pattern of Fe-doped flowerlike ZnO with different ratios from 0 to 5 mol %.

window of the test chamber. In this measurement, the vapor concentration of HCHO was calculated according to our groups’ previous research.14 In addition, all the measurements were taken at room temperature.

3. RESULTS AND DISCUSSION 3.1. Morphologies, Structures, and UV vis Reflectivity Spectra. The crystal structure of synthesized samples has been

investigated using XRD. As shown in Figure 2, all samples have a similar wurtzite phase. No trace of metal iron, its oxides, or composites can be detected in the samples when the Fe-doping content is 0.5 mol %. However, samples doped with more than 1.0 mol % Fe content display additional diffraction peaks obviously compared with that of the pure ZnO specimen. These additional XRD peaks correspond to ZnFe2O4 and are marked by stars. These marked peaks are (220), (311), and (440), respectively. In addition, for the samples where the Fe-doping content is less than 1.0 mol %, the positions of corresponding diffraction peaks of Fe-doped ZnO are slightly shifted to high angles, as seen from the inset in Figure 2. This strongly suggests that Fe ions were successfully substituted into the ZnO host structure.26 The morphology of Fe-doped ZnO is characterized by scanning electron microscopy. As shown in Figure 3a e, the samples are self-assembled from nanorods with lengths of 3 3.5 μm and diameters of 350 450 nm. It can be clearly seen from the images that the original nanoflowers are unspoiled. However, with the increasing of Fe-doping content, the rods are somewhat deteriorated, which is attributed to the incorporation of Fe ions and the formation of ZnFe2O4. Figure 3f exhibits the UV vis absorption spectra of ZnO and Fe-doped ZnO samples. The absorption of 300 425 nm by ZnO corresponds to its band-to-band transition. Meanwhile, compared with ZnO, Fe-doped ZnO samples exhibit a small red shift and extend their absorption area from UV to UV vis. When the dopant amount of Fe is increased, the intensities between 400 and 600 nm are increased, and a new absorption peak was observed. The red shift may be attributed to the formation of a new dopant energy level below the conduction band for the ZnO by the doping of Fe ions.27 The above results indicated that ZnO incorporated with Fe ions has an effective absorption of visible light, which is a key factor in the visible-lightinduced photoelectric gas sensing. 22940

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Figure 3. SEM (a e) and UV vis (f) adsorption spectrum of Fe-doped flowerlike ZnO with the ratio of Fe/ZnO from 0.1 to 5 mol %.

Figure 4. Photocurrent of Fe-doped flowerlike ZnO with the different ratios of Fe/Zn.

3.2. Gas-Sensing Performance. The surface photocurrent spectra of the Fe-doped ZnO are displayed in Figure 4. It is clearly seen that the photocurrent of pure ZnO is rather weak. With the incorporation of Fe ions, the photocurrent intensity is

increased dramatically, and the response range of the photocurrent is extended to 650 nm. In addition, photocurrent intensity reaches a maximum when the dopant is 1.0 mol % in our experiments. This may be attributed to the formation of ZnFe2O4 when the doping content is more than 1.0 mol %. In previous reports,28 with the increasing of doping concentration, the second phase formed and accumulated on the surface of the host structure, which resulted in the grain and grain boundary resistances increasing. Consequently, the transfer behavior of photogenerated carriers will be hindered to some extent. We all know that the photocurrent spectrum can well reflect the generation and transport processes of the photogenerated charge carriers. Therefore, the surface photocurrent spectra result indicates that adding the appropriate amount of Fe ions can significantly extend the photoresponse range to the visible region. Moreover, the incorporation of Fe ions can also improve the transfer property of photogenerated carriers in Fe-doped ZnO. These results are very important for the achievement of visible-light-assisted gas sensors. In addition, on the basis of the result of the photocurrent spectrum, the 532 nm light was selected as the irradiation light for sensing measurement because the doped samples exhibit the obvious photocurrent response at 22941

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Figure 5. Gas-sensing response cycles of Fe-doped ZnO nanoflowers (with the ratio from 0.5 to 5 mol %) to different concentrations of formaldehyde under 532 nm light irradiation.

this wavelength, and the 532 nm light provided by the laser pointer is very suitable for the practical sensors. The formaldehyde-sensing characteristics of Fe-doped ZnO under 532 nm light irradiation were measured at room temperature. The results are shown in Figure 5. The samples showed the obvious sensing characteristics under the irradiation of visible light, and with the increase of formaldehyde concentrations, the photocurrent responses are significantly enhanced. This may be attributed to the excellent performance of the absorption in visible light, which is aroused by introducing Fe. To observe clearly the gas sensing of samples, the sensor response of Fe-doped ZnO samples is calculated and shown in Figure 6. With Ia being the photocurrent in air and Ib being the photocurrent in the presence of HCHO, the sensor response is defined as [{(Ib Ia)/Ia}  100%]. It was found that, with the increase of HCHO concentrations, the sensor response of all Fe-doped ZnO samples increased apparently. The 1.0 mol % dopant sample has a much better sensing performance than the others, no matter if the test condition is in the low or high concentration of formaldehyde. The response is ∼22, 94, 165, 261, and 287%, corresponding to formaldehyde concentrations of ∼5, 20, 40, 80, and 100 ppm in the chamber, respectively. However, the response of the sample containing 5.0 mol % Fe is the lowest, which is ∼6, 38, 65, 92, and 102%, respectively. The main reason may be the formation of a second phase with the increase of the Fe dopant. On the one hand, the diffusion resistance for the transport of charge carriers increases due to the increase in the amount of potential barriers between grain boundaries as the quantity of ZnFe2O4 on the surface of ZnO increases. On the other hand, with the formation of ZnFe2O4, the active sites on the surface of ZnO will be occupied. Correspondingly, the number of oxygen ions chemisorbing will be reduced. As a result, the response of the higher doping content decreases. This implies that the doped Fe ions have remarkably affected the gas-sensing activity of ZnO for the

Figure 6. Gas response of Fe-doped ZnO nanoflowers (with the ratio from 0.5 to 5 mol %) to various concentrations of HCHO with 532 nm light irradiation.

visible-light-assisted sensors, and there is an optimal dopant concentration of Fe ions in ZnO. As we all know, one of the important features for the ideal photoelectrical gas sensor is stability for irradiation light. For this reason, the five-cycle experiment with and without illumination of a 532 nm laser pointer was carried out. With an applied electric filed, photogenerated electrons under the irradiation light of 532 nm move toward a certain direction and the photocurrent appears. Therefore, as seen from Figure 7, the photocurrent increases rapidly under the irradiation light of 532 nm. However, in the absence of light, due to electron hole recombination at the surface, the number of free electrons decreases. Accordingly, the photocurrent intensity decreases obviously. Importantly, the photocurrent response intensity did not change during the cycle experiment. In addition, all Fe-doped ZnO samples also have 22942

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various oxygen ions, namely, O2 , O , and O2 , on the surface of ZnO. 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.30 Thus, the electrons captured by oxygen will be rereleased, and the conducstivity of Fe-doped ZnO is enhanced clearly, realizing the visible light detection for formaldehyde gas at room temperature.

Figure 7. Surface photocurrent of Fe-doped ZnO (1 mol %) nanoflowers exposed to five cycles of light on off with the illumination of 532 nm.

4. CONCLUSION This work reports a simple and rapid approach for the synthesis of Fe-doped flowerlike ZnO nanomaterials. The obtained Fe-doped ZnO samples exhibit a significant performance of visible-light-induced photoelectric gas sensing of formaldehyde, and the gas response to 5 and 100 ppm formaldehyde can arrive to 22% and 287% under the irradiation of 532 nm at room temperature, respectively. As evidenced by continuous testing, the Fe-doped ZnO as a gas-sensing element shows excellent stability for irradiation light and formaldehyde. The present work provides useful insight for the development of a low-cost practical gas sensor. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +86 431 85168093.

’ ACKNOWLEDGMENT We are grateful to the National Natural Science Foundation of China (Nos. 21173103 and 51172090) and the Scientific Forefront and Interdisciplinary Innovation Project, Jilin University, China (421031401412), for financial support. ’ REFERENCES Figure 8. Photovoltage of Fe-doped flowerlike ZnO with the different ratios of Fe/Zn.

the same current change for different irradiation cycles. It is demonstrated that ZnO doped with Fe ions is not prone to light poisoning, light corrosion, and photodegration itself under the irradiation light of 532 nm, and it is a suitable candidate material for photoelectric gas sensors. To better understand the gas-sensing mechanism, the separation and transfer direction of photoinduced charges carriers are needed to be known. For this reason, the surface photovoltage measurement was carried out. As shown in Figure 8, compared to the pure ZnO, the surface photovoltage response band (300 400 nm) related to the band-to-band transition of ZnO is enhanced clearly, and the photoresponse range was extended to the visible light area in Fe-doped ZnO samples. Importantly, it can be seen from the phase spectrum that the phase of Fe-doped ZnO is in the range of 90 100°, indicating that photogenerated electrons migrate to the surface of Fe-doped ZnO29 totally. This result will benefit the understanding of sensing behaviors. When Fe-doped ZnO sensors are irradiated by 532 nm light, the photogenerated electrons would be excited from the valence band of ZnO to the dopant energy level first. The electrons then migrate to the surface of ZnO. The atmospheric oxygen molecules adsorbed on the doped samples' surface capture a certain amount of free electrons from the doped ZnO samples and form

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