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Sensors and Actuators B 198 (2014) 360–365

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo u r nal homep age: www.elsev ier .com/ locate /snb

olid-state chemical synthesis of mesoporous �-Fe2O3 nanostructuresith enhanced xylene-sensing properties

izhao Li, Yali Cao, Dianzeng Jia ∗, Yang Wang, Jing Xieey Laboratory of Material and Technology for Clean Energy, Ministry of Education, Key Laboratory of Advanced Functional Materials, Autonomous Region,

nstitute of Applied Chemistry, Xinjiang University, Urumqi, Xinjiang 830046, China

r t i c l e i n f o

rticle history:eceived 23 September 2013eceived in revised form1 December 2013ccepted 16 March 2014vailable online 26 March 2014

a b s t r a c t

Uniform mesoporous �-Fe2O3 nanostructures have been handily prepared by a solid-state chemical reac-tion with the features of simple process, mild condition, and high yield. The as-prepared samples with3-dimensional (3D) honeycomb structures consist of a number of small nanosheets. These mesoporous�-Fe2O3 nanostructures have been investigated for application as a sensor to detect various vapors. Theexperiment results have shown that the mesoporous �-Fe2O3 nanostructures exhibited improved per-formances for xylene-sensing in comparison with the �-Fe2O3 nanosheets. The response of mesoporous

eywords:-Fe2O3

esoporous nanostructuresolid-state chemical synthesisensors

nanostructures to 1000 ppm xylene was up to 6 times higher than that of nanosheets. The mesoporous�-Fe2O3 nanostructures-based sensor had also wide detection range of 1–1000 ppm, good selectivity,and short response time to xylene. The enhancement of properties may be attributed to the large specificsurface area and porous nanostructure.

© 2014 Elsevier B.V. All rights reserved.

ylene

. Introduction

Gas sensors have been widely used in gas monitoring and alarmn firefighting, industry, and daily life [1–3]. Nanostructured �-e2O3 has been intensively studied as gas sensing materials owingo its low cost and promising sensing performance associated withanoscale size [4–9]. To date, various �-Fe2O3 structures, suchs nanospheres [10], nanorods [11], nanotubes [12], nanowires13], and nanorings [14], are widely investigated. In particular,

esoporous �-Fe2O3 which possesses unique porous structure hasttracted much attention because of their enhanced properties15,16].

Some efforts have been made to prepare mesoporous �-Fe2O3anostructures for gas sensors. Sun et al. reported the synthesisf mesoporous �-Fe2O3 nanostructures through a soft templateynthesis method using the triblock copolymer surfactant F127s the template [17]. The mesoporous �-Fe2O3 materials showedigh gas sensitivity toward acetic acid and ethanol gas. Haot al. have fabricated flower-like and urchin-like mesoporous

-Fe2O3 nanostructures by annealing the hierarchical �-FeOOHrecursors prepared through a solution-based reaction [18]. Thebtained mesoporous hierarchical �-Fe2O3 architectures exhibited

∗ Corresponding author. Tel.: +86 991 8583083; fax: +86 991 8580032.E-mail addresses: jdz0991@gmail.com, jdz@xju.edu.cn (D. Jia).

ttp://dx.doi.org/10.1016/j.snb.2014.03.056925-4005/© 2014 Elsevier B.V. All rights reserved.

enhanced sensing performances to ethanol. Although the meso-porous �-Fe2O3 nanostructures in the above methods revealedgood gas sensing performance, the preparations of the sampleswere complex, expensive, and time-consuming. Therefore, it is stilla challenge to develop a facile route to synthesize mesoporous �-Fe2O3 nanostructures with improved gas sensing properties.

In this paper, we report a solid-state chemical reaction to syn-thesize mesoporous �-Fe2O3 nanostructures constructed by smallnanosheets. The fabrication shows the features of simple process,mild condition, high yield, and low cost. The gas sensing prop-erties of the resulting mesoporous nanostructures and �-Fe2O3nanosheets were investigated. The mesoporous nanostructures-based gas sensor showed improved xylene sensing performancesin comparison with the compact nanosheets-based gas sensor. Theresults are promising for further application of mesoporous �-Fe2O3 nanostructures as xylene sensor.

2. Experimental

2.1. Synthesis

All the reagents were analytically pure from commercial sources

and used without further purification. The mesoporous �-Fe2O3nanostructures were synthesized through a solid-state chemi-cal reaction method. In a typical experiment, FeCl2·4H2O (0.60 g,3 mmol) and NaBH4 (0.23 g, 6 mmol) were mixed with sodium

Y. Li et al. / Sensors and Actuato

dtshApsf

sors were aged at 300 ◦C for 5 days in air prior to use. The test

Fig. 1. XRD patterns of the mesoporous nanostructures and the nanosheets.

odecyl sulfate (SDS) (0.87 g, 3 mmol) by grinding in an agate mor-ar at room temperature. Subsequently, several drops of water werelowly added into the mixtures. Accompanied with the release ofeat and vapor, the color of mixtures changed from green to black.fter the mixtures were ground for about 30 min, the resulting solid

roducts were washed with distilled water and absolute ethanol foreveral times. The products were then dried at room temperatureor 24 h followed by annealing at 600 ◦C for 1 h in air atmosphere.

Fig. 2. Typical TEM image (a) and SEM image (b) of the mesoporous �-Fe2O3 nanostr

rs B 198 (2014) 360–365 361

The �-Fe2O3 nanosheets were synthesized by a similar process asdescribed above, except for without the addition of SDS.

2.2. Characterization

Powder X-ray diffraction (XRD) patterns were recordedon a Bruker D8 X-ray diffractometer with Cu-K� radiation(� = 1.54056 A) and a scanning speed of 2◦ min−1 ranging from20◦ to 80◦. Transmission electron microscope (TEM) images wereobtained on a Hitachi H-600 transmission electron microscopewith accelerating voltage of 100 kV. Scanning electron micro-scope (SEM) images were obtained on a LEO 1430VP scanningelectron microscope with an accelerating voltage of 20 kV. TheBrunauer–Emmett–Teller (BET) and Barret–Joyner–Halender (BJH)results were measured on a Micromeritics ASAP 2020 surface areaand porosity analyzer. UV–vis spectrum was obtained by a HitachiU-3900H spectrophotometer.

2.3. Gas sensing test

Gas sensors were made in a conventional way [19,20]. Briefly,the as-prepared products were dispersed in terpineol, which wasused as the binder to form pastes. The alumina ceramic tube, assem-bled with platinum wire electrodes for electrical contacts, wasdipped into the paste several times to form the sensing film. Thena Ni–Cr alloy wire as a resistance heater was passed through theceramic tube. To improve the stability and repeatability, the sen-

was carried out in a commercial gas sensing measurement sys-tem of WS-30A (Zhengzhou Winsen Electronic Technology Co.,Ltd.).

uctures; typical TEM image (c) and SEM image (d) of the �-Fe2O3 nanosheets.

362 Y. Li et al. / Sensors and Actuators B 198 (2014) 360–365

3 nan

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Fig. 3. Typical TEM images of (a) the �-Fe2O

Response of a sensor was defined as follows:

esponse = Rair

Rgas(1)

air is the resistance of the sensor in air, and Rgas is that in a mix-ure of testing gases and air. Response time is defined as the timeequired for the conductance to reach 90% of the equilibrium valuefter a test gas is injected, which recovery time is the time nec-ssary for a sensor to attain a conductance 10% above its originalalue in air [21].

. Results and discussion

.1. Structure of the products

The phases of the mesoporous nanostructures and nanosheetsere investigated by the XRD, as shown in Fig. 1. All peaks of the

RD patterns can be indexed to pure �-Fe2O3 (JCPDS No. 33-0664).o obvious peaks from impurity are observed, indicating the highurity of the products synthesized by the solid-state chemical reac-ion. The broader diffraction peaks suggest the small crystallite sizef the products.

ig. 4. (a) Relationship of the working temperature versus response of the sensors to 1ylene based on the mesoporous nanostructures at 340 ◦C and the nanosheets at 400 ◦C; ianostructures in range of 1–5 ppm xylene.

oplates and (b) the �-Fe2O3 nanoparticles.

3.2. Morphology of the products

Typical morphology of the as-prepared mesoporous �-Fe2O3nanostructures and �-Fe2O3 nanosheets is shown in Fig. 2. TheTEM image (Fig. 2a) of the mesoporous nanostructures shows thatsmall nanosheets are assembled together. The inset clearly showsthe thickness of the small nanosheets is 5–10 nm. The SEM image(Fig. 2b) of the mesoporous �-Fe2O3 nanostructures displays thesample with 3-dimensional (3D) honeycomb structures are com-posed of abundant randomly assembled nanosheets. In contrast, asshown in Fig. 2c and d, the �-Fe2O3 nanosheets with a large lateralsize distribution of 30–200 nm are obtained in the absence of SDS.The irregular sheets are stacked together forming large aggregates.

Taking the advantages including simple process, high yielding,and mild reaction condition, the low-heating solid-state chemicalreaction method has been widely applied in the synthesis of variousnanomaterials [22,23]. In this paper, the �-Fe2O3 with mesoporousnanostructure was prepared by the solid-state chemical method.The yield can reach to 93.7% in the typical synthesis. It can be easily

scaled to produce large quantities of products, which is beneficialto the application. In addition, when SDS was not added under thesimilar reaction conditions, the mesoporous nanostructures werenot observed in our experiment. Herein, the role of SDS which

00 ppm xylene. (b) Relationship between the response and the concentration ofnset: the concentration dependence of response based on the mesoporous �-Fe2O3

Y. Li et al. / Sensors and Actuators B 198 (2014) 360–365 363

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ig. 5. Nitrogen adsorption–desorption isotherms of (a) the mesoporous �-Fe2O3 nistributions.

s often used as an oriented agent is inferred to be very criticalo the formation of mesoporous architectures. The effect of thedditions on the morphology and structure of the products wasurther investigated. As shown in Fig. 3, �-Fe2O3 nanoplates andanoparticles were prepared with the additions of cetyl trimethylmmonium bromide (CTAB) and polyethylene glycol 400 (PEG-00), respectively. In the solid-state chemical reaction, the Feucleus was formed through reduction of FeCl2. The anionic SDSas more easily adsorbed on the surface of the Fe nucleus. Then, the

e nuclei further grew into nanocrystals with mesoporous struc-ure through oriented aggregation because of the presence of SDS.fter the drying and calcinations, the Fe is gradually oxidized into-Fe2O3.

.3. Gas-sensitive properties of the products

Xylene is hazardous to the environment and human becausef its flammable and carcinogenic. Thus, it is significant to

Fig. 6. Responses of the sensors to 100 ppm various vapors at 340 ◦C.

uctures and (b) the �-Fe2O3 nanosheets; the insets are the corresponding pore size

effectively detect and monitor it using suitable gas sensor. How-ever, there are few reports about the detecting of xylene [21,24,25].Therefore it is important to develop a novel sensing materials forxylene. The gas-sensing properties of the as-prepared mesoporous�-Fe2O3 nanostructures and �-Fe2O3 nanosheets were investi-gated in this study. Fig. 4a shows the response values to 100 ppmxylene of the sensors based on mesoporous nanostructures andnanosheets at different operating temperatures, respectively. Themaximum sensitivity of the mesoporous �-Fe2O3 nanostructuresis 6.45 at 340 ◦C. For the �-Fe2O3 nanosheets, the highest responseis only 1.90 at 400 ◦C. It can be seen that the mesoporous �-Fe2O3nanostructures has a greater sensitivity and a lower workingtemperature than the �-Fe2O3 nanosheets. Fig. 4b shows thatthe correlation between the concentrations and the responsesto xylene vapor based on the mesoporous nanostructures andnanosheets. These measurements were manipulated by injectingspecific amounts of the testing liquid into the testing chamberat the optimal operating temperatures of the two sensors. Asshown in Fig. 4b, the responses of the mesoporous nanostruc-tures and the nanosheets to xylene rise obviously with theincrease of concentration, and the responses of the two samplesto 1000 ppm xylene reach 17.54 and 2.91, respectively. It is foundthat the response of the mesoporous �-Fe2O3 nanostructuresto 1000 ppm xylene is up to 6 times higher than that of the�-Fe2O3 nanosheets. Furthermore, the response of the meso-porous �-Fe2O3 nanostructures increases rapidly as the xyleneconcentration is increased and remains unsaturated at 1000 ppm,displaying a very wide detection range. Importantly, a low xyleneconcentration of 1–5 ppm can be detected by the mesoporousnanostructures-based sensor (inset of Fig. 4b). The response of themesoporous �-Fe2O3 nanostructures to 1 ppm xlyene is 1.27. Itimplies that the as-prepared mesoporous architectures sensor hasa lower detection limit of xylene, which could meet the demandof practical application.

In the case of metal oxide-based sensor, the gas sensingmechanism involves an adsorption–oxidation–desorption process[7,26,27]. The adsorbed O2 on the surface of the sensor, which trapelectrons from the bulk, will transform to the O2

−, O−, and O2− ions.

When a reducing gas such as xylene is introduced, the interactionof oxygen ions with xylene will happen. The electrons produced inthe oxidation reaction then go back to the bulk. The behavior of sen-sor is based on changes of electric resistance in the whole process.

364 Y. Li et al. / Sensors and Actuators B 198 (2014) 360–365

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ig. 7. Response transients of the sensors to switching on and off the 1000 ppmylene at 340 ◦C.

ere, xylene may undergo the following reaction on the surface ofhe �-Fe2O3 sensors:

6H4(CH3)2 + 21O− → 8CO2 + 5H2O(g) + 21e− (2)

The improvement of the sensing performance of theesoporous �-Fe2O3 nanostructures may be attributed to

he large surface area and porous nanostructure. Nitrogendsorption–desorption isotherm measurements were carried outo examine the specific surface area and the pore size distribution,s shown in Fig. 5. The as-obtained �-Fe2O3 hierarchical nano-tructures manifest a high specific surface area of 200.7 m2 g−1,hile is higher than that of the �-Fe2O3 nanosheets (116.7 m2 g−1).

he pore size distributions show that the average pore sizes aren the mesoporous range with around 6 nm for the mesoporous-Fe2O3 nanostructures and 11 nm for the �-Fe2O3 nanosheets.urthermore, the total pore volume of the mesoporous �-Fe2O3

anostructures is 0.304 cm3/g. The high specific surface areand large pore volume of the mesoporous nanostructures aref great benefit to gas diffusion and mass transport, which willmprove the reaction of the test gas with surface-adsorbed oxygen

ig. 8. (a) UV–vis spectrum of the mesoporous �-Fe2O3 nanostructures and the �-Fe2O3

-Fe2O3 nanosheets.

Fig. 9. Stability of the mesoporous �-Fe2O3 nanostructures-based sensor to100 ppm xylene at 340 ◦C.

[28,29]. Therefore, the sensor based on the mesoporous �-Fe2O3nanostructures exhibits improvement sensitivity in detectingxylene.

It is well known that selectivity is the important parameter ofgas sensors for the practical application. Fig. 6 shows the responsesof the as-prepared two sensors to the various organic vapors, suchas xylene, toluene, benzene, ethanol, acetone, methanal, ammonia,with the concentration of 100 ppm at 340 ◦C. Clearly, the responsesof the mesoporous nanostructures-based sensor to seven vaporsare all improved compared with the nanosheets-based one. Inaddition, the mesoporous nanostructures-based sensor shows rela-tively lower responses to the interferential gases than to the xylene.It suggests that the mesoporous �-Fe2O3 nanostructures have agood selectivity to the target vapor.

The response time and recovery time are also the main param-eters of gas sensors. Fig. 7 shows the transient response of the twosensors to 1000 ppm xylene with the gas turning-on and turning-off

at operating temperature of 340 ◦C. The response time and recoverytime are about 1 and 13 s for the mesoporous nanostructures-basedsensor, while are about 4 and 13 s for the nanosheets-based sensor.

nanosheets; (b) plots of (˛h�)2 versus h� for the �-Fe2O3 nanostructures and the

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his result reveals that the mesoporous �-Fe2O3 nanostructuresave the good response and recovery characteristics. Based on theV–vis spectrum, the direct band gap energies of the as-obtained

wo samples were determined according to the reported equation30,31]. As shown in Fig. 8, the band gap energy of the mesoporous-Fe2O3 nanostructures is evaluated to be 2.90 eV, which is big-er than that of the �-Fe2O3 nanosheets (2.77 eV). The broadeningand gap leads to the enhancement of redox potential promotinghe gas adsorption–desorption [32].

Furthermore, the long-time stability of the mesoporous �-Fe2O3anostructures-based sensor has been detected by repeating theest within a thirty-day period. As shown in Fig. 9, no obvious vari-tions are found in the gas-sensing performance, indicating theensor’s good reproducibility and stability.

. Conclusion

In summary, a solid-state chemical reaction process is reportedor the synthesis of mesoporous �-Fe2O3 nanostructures. TheseD honeycomb architectures are assembled by abundant smallanosheets, which have thickness of 5–10 nm. It was found thathe SDS, as an assistant agent, plays an important role in the for-

ation of mesoporous nanostructures. The mesoporous �-Fe2O3anostructures exhibited a higher sensitivity and a lower detec-ion limit to xylene than the �-Fe2O3 nanosheets. The sensor basedn the mesoporous nanostructures also showed a good selectivitys well as a short response time to xylene. Thus, the mesoporous �-e2O3 nanostructures prepared by the solid-state chemical reactionoute can be an ideal candidate for xylene sensor.

cknowledgements

This work was financially supported by the Doctoral Innovationrogram of Xinjiang University (No. XJUBSCX-2012019), Gradu-te Research Innovation Project of Xinjiang (No. XJGRI2013020),ational Natural Science Foundation of China (Nos. 21101132,1361024 and 21271151) and Program for Changjiang Scholars andnnovative Research Team in University of Ministry of Education ofhina (IRT1081).

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Biographies

Yizhao Li is a doctoral student at Institute of Applied Chemistry, Xinjiang Universityof China. His current research work is concentrated on the synthesis and applicationsfor nanometer functional materials by solid-state reaction technique.

Yali Cao is a professor at Institute of Applied Chemistry, Xinjiang University of China.Her current research interests are in the area of gas sensor and nanostructuredmaterials.

Dianzeng Jia is a professor at Institute of Applied Chemistry, Xinjiang Universityof China. His research field is the studies on nanometer functional materials andphotochromic materials.

Yang Wang is a master student at Institute of Applied Chemistry, Xinjiang Universityof China. His current research work is concentrated on the synthesis for nanometer

functional materials by solid-state reaction technique.

Jing Xie is a master student at Institute of Applied Chemistry, Xinjiang Universityof China. Her current research work is concentrated on the synthesis for nanometerfunctional materials by solid-state reaction technique.