Published: October 11, 2011

r 2011 American Chemical Society 22939 dx.doi.org/10.1021/jp206352u | J. Phys. Chem. C 2011, 115, 22939–22944

ARTICLE

pubs.acs.org/JPCC

Visible-Light-Assisted HCHO Gas Sensing Based on Fe-DopedFlowerlike ZnO at Room TemperatureLina 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

1. INTRODUCTION

With the development of technology, the world awarenessabout environmental problems and human safety is increasing.The requirement for the detection of low concentrations offormaldehyde has been enhanced in furniture and house decora-tion. Zinc oxide (ZnO), with a wide band gap of 3.4 eV and alarge exciton binding energy of 60meV at room temperature,1,2 isan important functional material in many fields, such as gassensors,3 solar cells,4 field-effect transistors,5 photodetectors,6

and photocatalysts.7 Among them, gas sensors are one of themost important applications of nanostructured ZnO materials,and ZnO has been successfully employed to detect variousgases.8�10 However, for the traditional heat-treatment gas sen-sor, the high operation temperature restricts the application ofgas sensors in many areas, such as an explosive environment anda low-temperature environment. In recent years, there have beenseveral reports of photoelectric gas sensing based on the ZnOnanomaterials.11�13 Peng et al14 synthesized ZnO using a simplehydrothermal method and detected HCHO by ultraviolet (UV)light irradiation. The results demonstrated that the gas responseof ZnO nanorods to 110 ppm formaldehyde with UV lightirradiation was about 120 times higher than that without UV lightirradiation. Jayatissa’s group15 also demonstrated that gas-sen-sing properties of ZnO were affected by the UV irradiation. Theirradiation time of less than 5 min has improved the sensor,whereas the irradiation time of more than 5 min degraded thesensor characteristics. The experimental results show that it isfeasible to achieve the room-temperature gas sensing under UVlight irradiation. However, UV light, with strong radiation, wouldhurt the human body and eyes seriously. Especially, the light

source of UV is usually too large in volume to meet the demandfor the sensors. Undoubtedly, visible-light-induced photoelectricgas sensing will be a new type of sensor in demand for thepractical application. To extend the photoresponse of the largeband-gap semiconductor into the visible light region, variousmethods have been attempted.16�19 Among them, incorporationof transition-metal ions, such as La, Ni, Mn, and Fe, has beendemonstrated in an efficient attempt to achieve visible lightphotocatalytic activity.20�23 However, the corresponding workhas scarcely been done in the gas-sensing applications.

In this paper, we synthesized Fe-doped flowerlike ZnO with adoping level in the range of 0�5.0 mol % using a facile hydro-thermal method. To decrease costs and simplify the detectiondevices, we use the laser pointer as a light source to detect the gassensing of formaldehyde (HCHO) based on the Fe-doped ZnO.The excellent sensitivity to HCHO under 532 nm visible lightirradiation at room temperature was observed. A possible explana-tion of the sensing mechanism has been proposed. This work willpave away for the development of a low-cost practical gas sensor fordetection of probable chemical and biological agents.

2. EXPERIMENTAL SECTION

2.1. Synthesis of Fe-Doped Flowerlike ZnO. All chemicalswere analytical-grade reagents and used as purchased withoutfurther purification. The flowerlike ZnO with various Fe-doping

Received: July 6, 2011Revised: September 30, 2011

ABSTRACT: In this work, Fe-doped flowerlike ZnO powders with variousdoping contents were successfully fabricated by a hydrothermal method. Theresults of X-ray diffraction and UV�vis DRS spectra revealed that the Fe ionshave been successfully doped into the crystal lattice of the ZnO host structure,and the optical absorption response of Fe-doped ZnO was extended into thevisible region for the incorporation of Fe ions. The room-temperaturephotoelectric gas sensing of formaldehyde (HCHO) based on the Fe-dopedZnO was also studied under 532 nm light irradiation provided by a green laserpointer. It was found that the as-prepared Fe-doped ZnO samples showedexcellent sensitivity, in which the gas response to 5 and 100 ppm formaldehydecan reach to 22% and 287% under 532 nm light irradiation at roomtemperature, respectively. The sensing mechanism of the obvious visible-light-induced photoelectric gas sensing was discussed with the help of surfacephotovoltage measurement. Our results demonstrated that visible light irradiation was a promising approach to achieving a largeresponse for gas sensors at room temperature. This work will pave a way for the development of a low-cost practical gas sensor.

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contents was prepared by a hydrothermal method according toref 24 with slight modification. A 0.878 g (4 mmol) portion ofZn(CH3COO)2 3 2H2O and a certain amount of Fe2(NO3)3 39H2O (the mole ratio of Fe/Zn is 0.5, 1.0, 3.0, and 5.0%) wereput into 60 mL of water and stirred at room temperature for5 min, and then 20 mL of 3 M NaOH aqueous solution wasintroduced into the above solution. After the mixture was mag-netically stirred for 10 min, the solution was moved to a 100 mLTeflon-lined stainless steel autoclave, sealed, and maintained at150 �C for 10 h.When the reactions were completed, the autoclavewas cooled to room temperature naturally. The white productswere filtered off and washed with deionized water and absoluteethanol several times to remove possible impurities. The precipi-tates were dried in ambient air at 60 �C for 10 h and then sinteredat 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 wasalso 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). Themorphology of the sample was characterized by scanning elec-tron microscopy (Shimadzu, SS-550). Surface photovoltage wasmeasured with a lock-in based SPV measurement system, whichwas composed of a source of monochromatic light, a lock-inamplifier (SR830-DSP) with a light chopper (SR540), a samplecell, and a computer. A low chopping frequency of 23 Hz wasused. A 500 W xenon lamp (CHFXQ500 W, Global xenon lamppower) and a double-prism monochromator (Hilger and Watts,D 300) provided monochromatic light. The photocurrent signalwas detected by a lock-in amplifier (Stanford Research System,SR830-DSP)25 and recorded by a computer with an external biasof 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. Thesensor was fabricated by putting the sample on a comblikeindium doped tin oxide (ITO) transparent electrode and thenput into the test chamber. Air was used both as a reference gasand as a diluting gas to obtain desired concentrations of HCHO.HCHO liquid was injected into the test chamber by a syringethrough a rubber plug. After HCHO was fully mixed with thediluting gas, the sensor was irradiated with light. The light wasblocked when the sample was illuminated by light with a certainperiod of time. An electrochemistry workstation (CHI 630b,made in China) was used to record the current intensity across thegas sensor with a bias of 10 V. Light (532 nm, 20 mW/cm2) wasobtained by a light beam (provided by a 532 nm laser pointer).The 532 nm light can irradiate the sensor through a quartz

window of the test chamber. In this measurement, the vaporconcentration of HCHO was calculated according to our groups’previous research.14 In addition, all themeasurements were takenat room temperature.

3. RESULTS AND DISCUSSION

3.1. Morphologies, Structures, and UV�vis ReflectivitySpectra. The crystal structure of synthesized samples has beeninvestigated using XRD. As shown in Figure 2, all samples have asimilar wurtzite phase. No trace of metal iron, its oxides, orcomposites can be detected in the samples when the Fe-dopingcontent is 0.5 mol %. However, samples doped with more than1.0 mol % Fe content display additional diffraction peaksobviously compared with that of the pure ZnO specimen. Theseadditional XRD peaks correspond to ZnFe2O4 and are markedby stars. These marked peaks are (220), (311), and (440),respectively. In addition, for the samples where the Fe-dopingcontent is less than 1.0 mol %, the positions of correspondingdiffraction peaks of Fe-doped ZnO are slightly shifted to highangles, as seen from the inset in Figure 2. This strongly suggeststhat Fe ions were successfully substituted into the ZnO hoststructure.26

The morphology of Fe-doped ZnO is characterized by scan-ning electron microscopy. As shown in Figure 3a�e, the samplesare self-assembled from nanorods with lengths of 3�3.5 μm anddiameters of 350�450 nm. It can be clearly seen from the imagesthat the original nanoflowers are unspoiled. However, with theincreasing of Fe-doping content, the rods are somewhat deterio-rated, which is attributed to the incorporation of Fe ions and theformation of ZnFe2O4. Figure 3f exhibits the UV�vis absorptionspectra of ZnO and Fe-doped ZnO samples. The absorption of300�425 nm by ZnO corresponds to its band-to-band transi-tion. Meanwhile, compared with ZnO, Fe-doped ZnO samplesexhibit a small red shift and extend their absorption area fromUVto UV�vis. When the dopant amount of Fe is increased, theintensities between 400 and 600 nm are increased, and a newabsorption peak was observed. The red shift may be attributed tothe formation of a new dopant energy level below the conductionband for the ZnO by the doping of Fe ions.27 The above resultsindicated that ZnO incorporated with Fe ions has an effectiveabsorption of visible light, which is a key factor in the visible-light-induced photoelectric gas sensing.

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

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

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3.2. Gas-Sensing Performance. The surface photocurrentspectra of the Fe-doped ZnO are displayed in Figure 4. It isclearly 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 photo-current is extended to 650 nm. In addition, photocurrentintensity reaches a maximum when the dopant is 1.0 mol % inour experiments. This may be attributed to the formation ofZnFe2O4 when the doping content is more than 1.0 mol %. Inprevious reports,28 with the increasing of doping concentration,the second phase formed and accumulated on the surface of thehost structure, which resulted in the grain and grain boundaryresistances increasing. Consequently, the transfer behavior ofphotogenerated carriers will be hindered to some extent. We allknow that the photocurrent spectrum can well reflect thegeneration and transport processes of the photogenerated chargecarriers. Therefore, the surface photocurrent spectra resultindicates that adding the appropriate amount of Fe ions cansignificantly extend the photoresponse range to the visibleregion. Moreover, the incorporation of Fe ions can also improvethe transfer property of photogenerated carriers in Fe-dopedZnO. These results are very important for the achievement ofvisible-light-assisted gas sensors. In addition, on the basis of theresult of the photocurrent spectrum, the 532 nm light wasselected as the irradiation light for sensing measurement becausethe doped samples exhibit the obvious photocurrent response at

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 differentratios of Fe/Zn.

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this wavelength, and the 532 nm light provided by the laserpointer is very suitable for the practical sensors.The formaldehyde-sensing characteristics of Fe-doped ZnO

under 532 nm light irradiation were measured at room tempera-ture. The results are shown in Figure 5. The samples showedthe obvious sensing characteristics under the irradiation of visiblelight, and with the increase of formaldehyde concentrations,the photocurrent responses are significantly enhanced. This maybe attributed to the excellent performance of the absorption invisible light, which is aroused by introducing Fe. To observeclearly the gas sensing of samples, the sensor response ofFe-doped ZnO samples is calculated and shown in Figure 6. WithIa being the photocurrent in air and Ib being the photocurrentin the presence of HCHO, the sensor response is defined as[{(Ib� Ia)/Ia}� 100%]. It was found that, with the increase ofHCHO concentrations, the sensor response of all Fe-dopedZnO samples increased apparently. The 1.0 mol % dopantsample has a much better sensing performance than the others,no matter if the test condition is in the low or high concentra-tion of formaldehyde. The response is ∼22, 94, 165, 261, and287%, 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 thelowest, which is ∼6, 38, 65, 92, and 102%, respectively. Themain reason may be the formation of a second phase with theincrease of the Fe dopant. On the one hand, the diffusionresistance for the transport of charge carriers increases due tothe increase in the amount of potential barriers between grainboundaries as the quantity of ZnFe2O4 on the surface of ZnOincreases. 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 willbe reduced. As a result, the response of the higher dopingcontent decreases. This implies that the doped Fe ions haveremarkably affected the gas-sensing activity of ZnO for the

visible-light-assisted sensors, and there is an optimal dopantconcentration of Fe ions in ZnO. As we all know, one of theimportant features for the ideal photoelectrical gas sensor isstability for irradiation light. For this reason, the five-cycleexperiment with and without illumination of a 532 nm laserpointer was carried out. With an applied electric filed, photo-generated electrons under the irradiation light of 532 nm movetoward a certain direction and the photocurrent appears. There-fore, as seen from Figure 7, the photocurrent increases rapidlyunder the irradiation light of 532 nm. However, in the absenceof light, due to electron�hole recombination at the surface, thenumber of free electrons decreases. Accordingly, the photo-current intensity decreases obviously. Importantly, the photo-current response intensity did not change during the cycleexperiment. In addition, all Fe-doped ZnO samples also have

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

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

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the same current change for different irradiation cycles. It isdemonstrated that ZnO doped with Fe ions is not prone to lightpoisoning, light corrosion, and photodegration itself under theirradiation light of 532 nm, and it is a suitable candidate materialfor photoelectric gas sensors.To better understand the gas-sensing mechanism, the separa-

tion and transfer direction of photoinduced charges carriers areneeded to be known. For this reason, the surface photovoltagemeasurement was carried out. As shown in Figure 8, comparedto the pure ZnO, the surface photovoltage response band(300�400 nm) related to the band-to-band transition of ZnO isenhanced clearly, and the photoresponse range was extended tothe visible light area in Fe-doped ZnO samples. Importantly, itcan be seen from the phase spectrum that the phase of Fe-dopedZnO is in the range of 90�100�, indicating that photogeneratedelectrons migrate to the surface of Fe-doped ZnO29 totally. Thisresult will benefit the understanding of sensing behaviors. WhenFe-doped ZnO sensors are irradiated by 532 nm light, thephotogenerated electrons would be excited from the valenceband of ZnO to the dopant energy level first. The electrons thenmigrate to the surface of ZnO. The atmospheric oxygen mol-ecules adsorbed on the doped samples' surface capture a certainamount of free electrons from the doped ZnO samples and form

various oxygen ions, namely, O2�, O�, and O2�, on the surface

of ZnO. When the Fe-doped ZnO sensors are exposed toformaldehyde gas, the reducing gas and the chemisorbed oxygenions on the surface of ZnO can give rise to the chemical reac-tion.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 roomtemperature.

4. CONCLUSION

This work reports a simple and rapid approach for thesynthesis of Fe-doped flowerlike ZnO nanomaterials. The ob-tained Fe-doped ZnO samples exhibit a significant performanceof visible-light-induced photoelectric gas sensing of formalde-hyde, and the gas response to 5 and 100 ppm formaldehyde canarrive to 22% and 287% under the irradiation of 532 nm at roomtemperature, respectively. As evidenced by continuous testing,the Fe-doped ZnO as a gas-sensing element shows excellentstability for irradiation light and formaldehyde. The presentwork provides useful insight for the development of a low-costpractical gas sensor.

’AUTHOR INFORMATION

Corresponding Author*E-mail: linyh@jlu.edu.cn. Tel: +86 431 85168093.

’ACKNOWLEDGMENT

We are grateful to the National Natural Science Foundation ofChina (Nos. 21173103 and 51172090) and the Scientific Fore-front and Interdisciplinary Innovation Project, Jilin University,China (421031401412), for financial support.

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