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Analytica Chimica Acta 659 (2010) 266–273

Contents lists available at ScienceDirect

Analytica Chimica Acta

journa l homepage: www.e lsev ier .com/ locate /aca

lycine-assisted hydrothermal synthesis of peculiar porous �-Fe2O3 nanospheresith excellent gas-sensing properties

ongmin Chena,b, Yingqiang Zhaoa, Mingqing Yanga, Junhui Hea,∗, Paul K. Chub, Jun Zhangc, Shihua Wuc

Functional Nanomaterials Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academyf Sciences (CAS), Zhongguancun Beiyitiao 2, Haidianqu, Beijing 100190, ChinaPlasma Laboratory, Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, ChinaCollege of Chemistry, Nankai University, Tianjin 300071, China

r t i c l e i n f o

rticle history:eceived 24 October 2009eceived in revised form6 November 2009ccepted 17 November 2009vailable online 24 November 2009

eywords:lycine assistedydrothermal synthesisematiteorous structureas sensor

a b s t r a c t

In this work, peculiar porous �-Fe2O3 nanospheres were fabricated by a glycine-assisted hydrothermalmethod. They have large mesopores (ca. 10 nm) in the core and small mesopores (<4 nm) in the shell. Toour best knowledge, there have been so far no reports on the synthesis of such peculiar porous �-Fe2O3

nanospheres. X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopyand transmission electron microscopy were employed to characterize the obtained Fe2O3 nanospheres.Effects of preparation conditions, such as reactants, reaction temperature and reaction duration, wereinvestigated on the morphology and structure of Fe2O3 nanospheres. It was shown that the morphologyand structure could be readily controlled by the time and temperature of hydrothermal treatment. Theformation mechanism was proposed based on experimental results, which shows that glycine moleculesplay an important role in the formation of the morphology and porous structure of �-Fe2O3. The �-Fe2O3

porous nanospheres were used as gas sensing layer, and exhibited excellent gas-sensing properties to

ethanol in terms of response and selectivity. The sensors showed good reproducibility and stability aswell as short response (9 s) and recovery time (43 s) even at an ethanol concentration as low as 50 ppm.The gas-sensing properties of porous �-Fe2O3 nanospheres are also significantly better than those of pre-viously reported Fe2O3 nanoparticles (ca. 30 nm). The sensitivity of the former is over four times higherthan that of the latter at varied ethanol concentrations. The gas-sensing mechanism was discussed indetails. Both fast response and steady signal make these peculiar nanostructures a promising candidate for ethanol detection.

. Introduction

Nanomaterials of well-defined size and morphology havettracted considerable research efforts because of their size- andorphology-dependent properties. Among them are porous metal

xides. A variety of porous metal oxides including those of TiO21,2], SnO2 [3], ZnO [4–9] and In2O3 [10] have been synthesizednd explored for applications in catalysis, lithium-ion batteries, gasensing, and so on.

Hematite (�-Fe2O3), a stable phase of iron oxide, is an n-

ype semiconductor (Eg = 2.1 eV). It is non-toxic, highly resistant toorrosion, and environment friendly. It has been intensively inves-igated for applications in catalysts [11–13], lithium-ion batteries14,15], water treatment [16,17] and gas sensors [18–24]. Hematite

∗ Corresponding author. Tel.: +86 10 82543535; fax: +86 10 82543535.E-mail address: jhhe@mail.ipc.ac.cn (J. He).

003-2670/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.aca.2009.11.040

© 2009 Elsevier B.V. All rights reserved.

of varied morphologies have been fabricated, including nanocrys-tals [25,26], particles [27–29], cubes [30], rods [31,32], wires [33],tubes [34], flute-like [18], hollow spheres [35] and nanocups [36],and ordered mesoporous Fe2O3 [37].

Glycine-assisted hydrothermal synthesis is frequently usedto prepare specific structures [38,39]. For example, Prevot andco-workers prepared flower-like NiAl-layered double hydroxidesusing Ni (II) glycinate complex as precursor [38]. Zhang andco-workers prepared hollow Ni(OH)2 microspheres using Ni (II)glycinate complex under strong basic conditions. Glycine playedan important role in the formation of the specific morphologiesand architectures [39].

In the current work, glycine-assisted hydrothermal synthe-

sis was adopted to prepare �-Fe2O3 nanomaterials. The obtainednanomaterials were analysed by X-ray powder diffraction, scan-ning electron microscopy, energy dispersive X-ray spectroscopy,transmission electron microscopy, and selected area electrondiffraction. The results showed that the obtained nanomaterials

H. Chen et al. / Analytica Chimica

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ig. 1. XRD patterns of porous �-Fe2O3 nanospheres before (a) and after (b) calci-ation.onsist of spherical particles of pure �-Fe2O3 phase. The sphericalarticles have pores of dual sizes in the core and shell, respectively.he gas-sensing properties of the porous �-Fe2O3 nanospheresere evaluated, and better sensing performance was observed

ompared with that using �-Fe2O3 nanoparticles of 30 nm in size40].

. Experimental

.1. Materials and synthesis

All chemicals used were analytic grade, and used as receivedithout further purification. Ultrapure water used in experimentsad a resistivity of 18.2 M� cm, and was obtained from a Milliporeilli-Q system (Academic).In a typical synthesis, 0.9 g of urea, 1 g of FeCl3·6H2O, and 2 g

f glycine were dissolved in 30 mL of pure water, and stirred for0 min to form a purple-black solution. The mixture was trans-erred to a 50 mL Teflon-lined stainless steel autoclave, sealed, and

aintained at 160 ◦C for 10 h. After cooling to room temperature,red product was isolated by centrifugation, and washed several

imes with pure water and absolute ethanol. It was dried in air at0 ◦C for 8 h, and calcined at 450 ◦C for 3 h in air and in nitrogen,espectively.

.2. Materials characterization

The phase structure and purity of the products were exam-ned by X-ray powder diffraction (XRD) using a Bruker D8 Focusiffractometer (Germany) with Cu K� radiation operated at 40 kVnd 40 mA. Scanning electron microscopy (SEM) observations andnergy dispersive X-ray (EDS) analyses were carried out on aitachi S-4300 scanning electron microscope (Japan). Transmis-

ion electron microscope (TEM) images and selected area electroniffraction (SAED) patterns were taken on a JEM 2100F transmissionlectron microscope.

.3. Gas sensor fabrication and characterization

The calcined �-Fe2O3 nanospheres were mixed with water, andhen coated onto an alumina tube which was connected to a pair ofu electrodes and four Pt wires on both ends of the tube. The oper-ting temperature was controlled by adjusting the heating power

sing a Ni–Cr alloy coil placed through the tube. The gas-sensinglement was aged at 300 ◦C for 7 days in order to improve stabilitynd reproducibility.

Gas-sensing properties of the gas-sensing elements were stud-ed using a HW-30A gas sensitivity instrument (China). A given

Acta 659 (2010) 266–273 267

amount of tested gas was injected into a chamber, and mixed withair. The gas sensitivity S is defined as the ratio of Ra/Rg, where Ra

is the resistance measured in air and Rg is that measured in testedgas atmosphere.

3. Results and discussion

3.1. Structure and morphology of porous ˛-Fe2O3 nanospheres

Fig. 1 shows XRD patterns of the Fe2O3 product beforeand after calcination in air. All the diffraction peaks werereadily indexed to a pure rhombohedral phase [space group:R−3c (167)] of �-Fe2O3 (JSPDS card No. 89-0597) [25]. Noother impurities could be detected by XRD. The increases inpeak intensities after calcination indicate that the annealing at450 ◦C for 3 h enhanced the crystallinity of the �-Fe2O3 prod-uct.

The annealed �-Fe2O3 product was observed by SEM and TEM.Fig. 2a shows its typical low-magnification TEM image. It consistsof nearly monodisperse spherical particles of 90 nm in size. Fig. 2bpresents a TEM image, showing that the spherical particles in factcontain randomly distributed pores. A close view further revealsthat there are two types of pore, one is large mesopores (ca. 10 nm)inside the dashed circle, and the other is small mesopores (<4 nm) atthe rim. SEM image (Fig. 2c) also shows that the annealed �-Fe2O3product consists of nearly monodisperse spheres. High-resolutionTEM image (Fig. 2d) discloses clear lattice fringes with a spacing of0.37 nm, which is consistent with the d value of the (0 1 2) plane[25]. SAED pattern (Fig. 2e) indicates a single-crystal diffractionnature of the annealed �-Fe2O3 product. EDS was used to deter-mine the chemical composition of the annealed �-Fe2O3 product(Fig. 2f). The results showed that it contains Fe and O, and the atomicratio of Fe to O is 2:3, in good agreement with the stoichiometry ofFe2O3.

Reaction temperature usually plays an important role inhydrothermal synthesis. Fig. 3 shows TEM images of uncalcined �-Fe2O3 products obtained at different hydrothermal temperatures.When the hydrothermal temperature is 120 ◦C, the product consistsof nearly monodisperse solid spherical particles of rough surface.Pores are not clearly noticeable inside the spherical particles. Theproduct obtained at 160 ◦C (Fig. 3c), however, consists of spheri-cal particles with smooth surface. Pores are clearly seen inside thespherical particles (Fig. 3d). Apparently, the porous structure hadformed before calcination though calcination might further expandthe pore size. Therefore, hydrothermal temperature of 160 ◦C isappropriate for forming the porous structure.

The addition of urea and glycine may also play an important rolein obtaining the porous �-Fe2O3 spheres [38,39]. When FeCl3 andglycine were hydrothermally treated without addition of urea at160 ◦C, spherical particles were obtained, which have smooth sur-face and are uniform in size (Fig. 4a). Mesopores are not clearlynoticeable, as shown in magnified TEM image (Fig. 4b) thoughglycine molecules could form small micelles in solution [38,39].Without addition of glycine, particles of irregular shapes wereobtained (Fig. 4c), and they are nonporous, as revealed in Fig. 4d. Thecorresponding XRD patterns (Fig. 4e and f) show that both productshave a pure rhombohedral phase of �-Fe2O3.

3.2. Plausible formation mechanism of porous ˛-Fe2O3nanospheres

Before the hydrothermal treatment, i.e., during the mixing ofprecursors, several reactions may have occurred, such as precipi-tation, hydrolysis, dissolution and metal ion complexation. FeCl3dissolved in water, and formed a yellow-brown transparent solu-

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68 H. Chen et al. / Analytica Ch

ion. When glycine was added into the solution, the color of theixture changed to purple-black, and the solution was opaque.

e3+ ion and glycine might form a coordination compound. At thearly stage of the hydrothermal treatment, urea decomposed toH3 and CO2 [41,42]. CO2 could form stable bubbles due to glycine,nd NH3 dissolved in water and formed a basic solution (after theydrothermal treatment and the autoclave was opened, NH3 couldmell). Under such basic conditions, glycine (pKa1 = 2.34, pKa2 = 9.60nd pHi = 5.97) was present in the Gly− (H2N–CH2–COO−) form39]. Fe3+ ions hydrolyzed, and iron oxide nanocrystals formednder the basic conditions. The –COO− terminal of glycine con-

ected with iron oxide nanocrystal. Under the conditions of highemperature and pressure in the autoclave, the nanocrystal andubble could connect by free glycine molecules in the solution.o understand the formation of the porous �-Fe2O3 nanospheres,ime-dependent experiments were carried out, and resultant prod-

Fig. 2. Typical TEM (a and b), SEM (c), HRTEM (d) images and SAED (e), EDS (f) patt

Acta 659 (2010) 266–273

ucts were analysed by TEM. When the reaction time was ≤30 min,small Fe2O3 nanocrystals (shown in Fig. 5a by black arrows) andstable bubbles would form in the mixture [43,44]. By increasingthe reaction time to 1 h, nanocrystals assembled together with theassistance of glycine, forming nearly monodisperse raspberry-likeporous aggregates (Fig. 5a). With increase of the reaction time, ironoxide nanocrystals further deposited on the surface of raspberry-like aggregates, which continuously transformed to nanosphereshaving a core with large mesopores (Fig. 5b). Finally, a porouscore/shell structure gradually formed (Fig. 5c and d) resulting inthe peculiar porous structures. This process is similar to the hol-

lowing process described in a previous report in which glycinewas also used as an additive. Solid spheres formed first, and aninterior cavity gradually formed via a core evacuation process[38,39]. XRD patterns of the products (Fig. 6) show that peakintensities gradually increase with the reaction time, indicating

erns of porous �-Fe2O3 nanospheres after calcination at 450 ◦C for 3 h in air.

H. Chen et al. / Analytica Chimica Acta 659 (2010) 266–273 269

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Fig. 3. TEM images of porous �-Fe2O3 nanospheres obtained at differen

hat the pure �-Fe2O3 crystal structure already formed in thearly stage of precipitation. The increase in crystallinity confirmshat Ostwald ripening (crystallites grow at the expense of smallernes) is the underlying mechanism in this pore-forming process38,39,45–47].

.3. Gas-sensing properties of porous ˛-Fe2O3 nanospheres

Development of gas sensors with improved performance hasecently become a hot topic because of increasing concerns overnvironmental pollution and public safety [48,49]. Porous metalxides are favorable to the diffusion of target gases due to their largeurface-to-volume ratio and well-defined pore structure [49]. As an-type semiconductor, one of the most important applications of-Fe2O3 is gas sensors. For example, hematite nanostructures con-

aining hierarchical mesopores, such as colloidal crystal clustersnd monodisperse porous nanospheres, were used for gas sen-ors, and showed better sensing performance as compared withommercial �-Fe2O3 particles [50,51]. Therefore, a gas sensor wasabricated using porous �-Fe2O3 nanospheres, and its sensing prop-rties were investigated.

It is well known that the operation temperature largely affects

he sensitivity of a semiconductor sensor [48–55]. In order toetermine the optimal operation temperature, the response ofhe porous �-Fe2O3 nanosphere gas sensor to ethanol of variedoncentrations in air was evaluated as a function of the opera-ion temperature. As shown in Fig. 7, the sensor response varies

rothermal temperatures: 120 ◦C (a and b) and 160 ◦C (c and d) for 10 h.

with the operation temperature. At three different ethanol con-centrations, all the sensitivities increase with increase of theoperation temperature from 180 to 260 ◦C. When the operationtemperature is over 260 ◦C, however, the sensitivity gradu-ally drops with further increase of temperature. The optimaloperation temperature of 260 ◦C was selected for following stud-ies.

Fig. 8 shows a response curve of porous �-Fe2O3 nanospheresensor to ethanol of varied concentrations at 260 ◦C. Clearly, thevoltage response is proportional to the ethanol concentrationbetween 50 and 1000 ppm. The response and recovery times, whichare defined as the time to reach 90% of the final resistance, areimportant parameters for gas sensors. For 50 ppm ethanol, theresponse and recovery times are 9 and 43 s, respectively. The sensorstill has fast response and recovery even exposed to ethanol vaporof high concentration, indicating good response/recovery capabili-ties.

Practically, the sensor selectivity is extremely important. Wethus measured the response of the sensor to several other sub-stances such as CHCl3, ethyl ether, and CO at identical concentration(50 ppm) and operation temperature (260 ◦C). As shown in Fig. 9,the porous �-Fe2O3 nanosphere sensor has higher sensitivity to

ethanol than to the other substances. Therefore, the porous �-Fe2O3nanosphere sensor could selectively detect ethanol in the presenceof other potential interfering substances.

The sensing performance of the porous �-Fe2O3 nanosphereswas compared with that of �-Fe2O3 nanoparticles of ca. 30 nm

270 H. Chen et al. / Analytica Chimica Acta 659 (2010) 266–273

F at 16g

aFscc

P

ig. 4. TEM images and XRD patterns of porous �-Fe2O3 nanospheres obtainedlycine.

s reported in a previous work [40]. The results are shown inig. 10. Clearly, the response profiles are similar in shape, and theensitivity increases almost linearly with increase of the ethanoloncentration in both cases. From Fig. 10, two linear equations

ould be derived as follows:

orous �-Fe2O3 nanospheres : S = 0.062C + 5.59, r = 0.992,

(1)

0 ◦C for 10 h. (a, b, and e) Without using urea and (c, d, and f) without using

�-Fe2O3 nanoparticles of ca. 30 nm :

S = 0.0072C + 1.61, r = 0.952, (2)

where S is the sensitivity, C is the concentration of ethanol. The

sensitivity of the porous �-Fe2O3 nanosphere sensor increasesdramatically with increase of ethanol concentration, and is muchhigher than that of the �-Fe2O3 nanoparticles, especially athigher ethanol concentrations [40]. The sensitivities of the for-mer are almost several times greater than those of the latter for

H. Chen et al. / Analytica Chimica Acta 659 (2010) 266–273 271

sing v

asbc

maagtswt

Fr

Fig. 5. TEM images of porous �-Fe2O3 nanospheres obtained at 160 ◦C u

ll the ethanol vapors of varied concentrations. Thus, the gas-ensing performance of the porous �-Fe2O3 nanospheres is muchetter than that of the previously reported �-Fe2O3 nanoparti-les.

There are many oxygen vacancies on the surface of the sensingaterial [9,19,45,49]. When the sensor is in air, oxygen molecules

re adsorbed at these oxygen vacancies, forming active oxygendsorbates, such as O2−, O−, and O2

−. The formation of the oxy-en adsorbate layer would lead to the transfer of electrons fromhe sensor surface to the adsorbate layer, and so the electron den-

ity would decrease on the sensor surface. The oxygen ions reactith ethanol molecules on the surface when the sensor is exposed

o ethanol vapor, and thus release the trapped electrons to the sen-

ig. 6. XRD patterns of porous �-Fe2O3 nanospheres obtained at 160 ◦C using variedeaction times.

aried reaction times: 1 h (a), 2 h (b), 5 h (c) and 8 h (d). Scale bar: 20 nm.

sor. This leads to the change in resistance of the �-Fe2O3 sensor.The amount of oxygen and test gas on the surface of materials isstrongly dependent on the microstructure of the materials, suchas particle size, morphology and porosity. Conventional �-Fe2O3nanoparticle sensors have poor porosity and relatively large parti-cle size, whereas porous �-Fe2O3 spheres have abundant pores intheir structures, which can facilitate the diffusion of gas moleculesand provide more adsorption–desorption sites. Very interestingly,if the as-synthesized Fe2O3 nanospheres were calcined in nitro-gen (named as Fe2O3 (N2)) instead of air, the sensor based onFe2O3 (N2) showed no sensing properties even in ethanol vapor

of higher concentration. The mechanism is still under investiga-tion.

Fig. 7. Response versus operation temperature of porous �-Fe2O3 nanosphere sen-sor to ethanol of varied concentrations.

272 H. Chen et al. / Analytica Chimica

Fig. 8. Response curve of porous �-Fe2O3 nanosphere sensor to ethanol of variedconcentrations.

Fig. 9. Sensitivities of porous �-Fe2O3 nanosphere sensor to different chemicals of50 ppm at an operation temperature of 260 ◦C.

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ig. 10. Comparison of sensitivities of porous �-Fe2O3 nanospheres (�) and previ-usly reported �-Fe2O3 nanoparticles (�) to ethanol vapors of varied concentrations.

. Conclusions

In summary, peculiar porous �-Fe2O3 nanospheres werebtained by glycine-assisted synthesis. They have large mesopores

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Acta 659 (2010) 266–273

(ca. 10 nm) in the core and small mesopores (<4 nm) in the shell.To our best knowledge, there have been so far no reports on thesynthesis of such peculiar porous �-Fe2O3 nanospheres. The timeand temperature of hydrothermal treatment influence the struc-ture and morphology of product. The �-Fe2O3 crystal phase formsat the early stage, and extended reaction time results in enhancedcrystallinity of product. �-Fe2O3 nanostructures obtained at dif-ferent reaction times are shown to follow a nucleation and growthprocess resembling Ostwald ripening with the assistance of glycinemolecules. The porous �-Fe2O3 nanospheres were used as a gassensing layer, and exhibited excellent gas-sensing properties toethanol in terms of response and selectivity. The gas-sensing prop-erties of porous �-Fe2O3 nanospheres are also significantly betterthan those of previously reported Fe2O3 nanoparticles (ca. 30 nm).Both fast response and steady signal make these peculiar nanos-tructures a promising candidate for ethanol detection.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (Grants No. 10776034), and the KnowledgeInnovation Program of the Chinese Academy of Sciences (Grant Nos.KGCX2-YW-111-5 and KSCX2-YW-G-059), National Basic ResearchProgram of China (No. 2006CB933000), and Hong Kong ResearchGrants Council (RGC) General Research Funds (GRF) No. CityU112307.

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