SIMPLE SYNTHESIS OF UNIFORM ®-Fe2O3

MICROSPHERES WITH EXCELLENTPHOTOCATALYTIC PERFORMANCE USING

POLY (N-VINYL-2-PYRROLIDONE)

TING-TING ZHANG, TING-TING GUO, ZHI-PING WU, SI-WEI YANG,ZHOU-FENG WANG and FANG LIAO*

School of Chemistry and Chemical IndustryChina West Normal UniversityNanchong 637002, P. R. China

*liaozhang2003@163.com

Received 1 April 2013Accepted 31 May 2013Published 18 July 2013

In this paper, a new simple approach has been developed for the preparation of �-Fe2O3

microspheres by a facile hydrothermal method using PVP as a surfactant. Uniform �-Fe2O3

microspheres could be routinely synthesized through solvothermal approach by controlling thePVP/FeCl3�6H2O ratio. The as-obtained �-Fe2O3 microspheres exhibit high e±ciency on thedecolorization of RhB aqueous solution in the presence of H2O2 at room temperature.

Keywords: �-Fe2O3; PVP; microspheres; RhB; photocatalytic.

1. Introduction

The synthesis of metal microstructures with uni-form morphologies and structures has receivedmuch interest because of their great in°uence ontheir physical/chemical properties. Among thevarious structures, monodisperse metal sphericalstructures have attracted considerable attentionbecause of their unique optical, catalytic, as well asnovel chemical and biological properties.1�17 Forexample, the controllable synthesis of gold nano-spheres can be used for optical studies and arepromising for chemical and biological sensingapplications.18 Therefore, developing a facile one-pot template-free method to prepare new metalspherical structures becomes very signi¯cant.19�22

Hematite (�-Fe2O3Þ is regarded as a promisingmaterial with potential applications in many ¯eldssuch as catalysts, pigments, gas sensors and electrodematerials23�26 owing to its high resistance to cor-rosion, low processing cost, nontoxicity and envir-onmentally friendly properties.27�33 Hence, a simple,low-cost and template-free method for the prep-aration of uniform hematite microspheres is highlydesired. In most cases, Poly (vinylpyrrolidone)(PVP) has often been used as a surfactant or stabil-izer in a reaction.34�37

In this paper, we describe a facile hydrothermalmethod using PVP as a surfactant. By controllingthe reaction conditions, the morphology of �-Fe2O3

could be controlled.

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NANO: Brief Reports and ReviewsVol. 8, No. 5 (2013) 1350053 (7 pages)© World Scienti¯c Publishing CompanyDOI: 10.1142/S1793292013500537

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2. Experiment

All the chemicals were purchased from Kelong(Chengdu, China) and used as received withoutfurther puri¯cation. The water used throughout allexperiments was puri¯ed through a Millipore sys-tem. The phase purity, crystal structure and mor-phologies of the products were characterized using acombination of the following techniques: X-ray dif-fraction (XRD, Rigaku Ultima IV, CuK� radi-ation), scanning electron microscope (SEM, JEOLJSM-6510LV) coupled with an energy-dispersive X-ray spectroscopy (EDS, Oxford instruments X-Max), UV/Vis spectrophotometer (Xinmao UV-7502PC, Shanghai), and Fourier transform infraredspectroscopy (FTIR, Thermo Scienti¯c Nicolet 6700FT-IR Spectrometer).

The �-Fe2O3 spheres were synthesized by a facilehydrothermal approach. In a typical synthesis,0.5mM of FeCl3�6H2O was dissolved in 10mL ofultra-pure water, followed by addition of 1.35 g ofPVP. The mixture was stirred and sonicated untilall the chemicals were well dispersed. Then it wastransferred into a 20mL stainless steel autoclavewith Te°on liner. The autoclave was sealed andmaintained at 160�C for 6 h. After the reaction, theprecipitates were washed with distilled water andethanol several times and then dried at 60�C for 6 hin air for further characterization. Other samples ofiron materials were also prepared with di®erentamounts of PVP and di®erent temperature usingthe same methods under identical conditions.

The photodegradation activity of the as-preparedsample was characterized by the degradation ofrhodamine B (RhB) under ultraviolet light ir-radiation in the presence of H2O2 at room tem-perature. In a typical procedure, 0.05 g catalyst wassuspended in 60mL of RhB (5mg/L) aqueous

solution, followed by addition of 1mL of H2O2

(30 wt.%). Before irradiation, the suspensions werestirred in dark for 1 h to reach an adsorption/deso-rption equilibrium. The ultraviolet light wasprovided by a 70W ultraviolet lamp. The concen-tration of RhB dye was determined by a UV/Vis spectrophotometer (Xinmao UV-7502PC,Shanghai).

3. Results and Discussion

(a) Characterization of �-Fe2O3 microspheresThe morphologies and microstructures of the as-prepared �-Fe2O3 microspheres were investigatedby SEM images. Figures 1(a) and 1(b) show therepresentative SEM images of the as-prepared�-Fe2O3 microspheres with di®erent magni¯cations.The low magni¯cation image [see Fig. 1(a)] indi-cates the large production of uniform and well-dis-persed �-Fe2O3 microspheres. No other �-Fe2O3

morphologies could be found in the whole sample.The diameters of these microspheres were about1�m. The high-magni¯cation image shown inFig. 1(b) clearly reveals the detailed morphologies ofthe products, indicating the formation of uniform,regular microspheres with smooth surfaces.

The elemental composition of these microsphereswas also analyzed by EDX, and characteristic ironand oxygen absorption peaks were observed (seeFig. 2). The XRD pattern of the Fe2O3 microspheresis shown in Fig. 3. All the di®raction peaks of theproducts can be indexed as spherical iron oxidestructure (JCPDS ¯le no. 33�664, �-Fe2O3Þ. Nocharacteristic di®raction peaks from other phases orimpurities were detected. The products were alsocharacterized by FTIR spectra. Figure 4 gives thetypical FTIR spectra of the Fe2O3 microspheres.

Fig. 1. SEM images of microspheres. Note that the low-magni¯cation image (a) and the higher magni¯cation image (b).

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The characteristic peak of iron oxide was appearedat near 479 cm�1.

(b) Shape control of the �-Fe2O3 microspheresThe amounts of PVP and the reaction temperaturesigni¯cantly a®ect the morphology of the product.For example, when the amount of PVP was under1.35 g and maintained the reaction temperature at160�C, as shown in Figs. 5(a) and 5(b), the particlesobtained are seen to be somewhat irregular andagglomerated at some places. When the amount ofPVP was increased to 1.35 g, large numbers of uni-form, regular microspheres with smooth surfaceswere formed. Thus, PVP is considered to act as asurfactant in this synthesis.

To investigate the in°uence of reaction tem-perature on the morphology of the product, weincreased the temperature from 140�C to 180�Cwhile keeping the concentration of FeCl3�6H2O

constant at 0.05 M and the 1.35 g amounts of PVP.The morphology of the sample changed continu-ously. As shown in Fig. 6(a), a large number ofrough spherical microspheres were obtained. A closeview of a single microparticle reveals that it is anaggregate of very small spherical nanoparticles andthe average diameter of every single sphere is about0.730�m [see inset of Fig. 6(a)]. When the tem-perature was increased to 160�C, as shown inFig. 6(b), the microspheres we obtained were uni-form, and with smooth surfaces. We also ¯nd thatthe average diameter of every sphere increased asthe same time [see inset of Fig. 6(b)]. As the tem-perature rises to 180�C, the diameter increased to1.222�m [see inset of Fig. 6(c)]. This indicates thatthe diameter of the particle increased with thetemperature rising. Therefore, we propose a sche-matic process possible for these smooth micro-spheres in Fig. 7. These results suggest that it ispossible to control and tune the size of �-Fe2O3

sphere microstructures by controlling the kineticparameters of the reaction process, that is, tem-perature and concentration.

K3 [Fe (CN)6] was used as another source of ironto make a comparison. The concentration of K3 [Fe(CN)6] is constant at 0.01M, and the amounts ofPVP were changed from 0 g to 0.5 g, while otherexperimental conditions were kept unchanged. Thelow-magni¯cation image in Fig. 8(a) shows that theproduct consists almost entirely of such dendriticstructures which show a sixfold-symmetric struc-ture, like a snow°ake without the addition of PVP.However, with increasing the amounts of PVP, acontinuous transition from thin to thick nanorod-branched dendrites that ¯nally turn to be sphereswere observed. Figures 8(a)�8(d) show SEM images

1 2 3 4 5 6 70

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FeFe

Fe

Fig. 2. EDX spectrum of the as-prepared microstructures.

20 30 40 50 60 70

2 θ /degree

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Fig. 3. XRD patterns of (a) standard �-Fe2O3 (JCPDS ¯leno. 33-664) and (b) As-prepared �-Fe2O3 samples.

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Fig. 4. Typical FTIR spectra of the �-Fe2O3.

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of samples that were synthesized at the amounts ofPVP of 0 g, 0.1 g, 0.3 g and 0.5 g, respectively. Thehigher the amounts of PVP, the thicker the nanorodbranches of the dendrites. The mechanism of thechange of morphology may be explained by themodel proposed in Fig. 9. From the crystal struc-ture, [10�10] are six equivalent directions. The ad-dition of PVP is the reason of spatial con¯nement

structure of hematite. When there is no PVP, thegrowth is symmetric along the six crystal-lographically equivalent directions [10�10]; a sixfold-symmetric \snow°ake" structure would be formed[see Fig. 8(a)]. While increasing the amounts ofPVP, all of the branches become thicker [Fig. 8(b)],

Fig. 6. SEM images of �-Fe2O3 micromaterials obtained witha concentration of FeCl3�6H2O of 0.05M, PVP 1.35 g andreaction temperatures of 140�C (a), 160�C (b) and 180�C (c),respectively.

Fig. 5. SEM image of �-Fe2O3 micromaterials were obtainedby di®erent amount of PVP (a) 0 g, (b) 1.00 g and (c) was 1.35 g.

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and ¯nally interconnect to form a sphere [seeFig. 8(d)]. This indicates that PVP can inhibit thegrowth of [1�100] plane.

(c) Photocatalytic Activity of �-Fe2O3 spheresTo evaluate the photodegradation properties of as-prepared �-Fe2O3 microspheres (the iron source isFeCl3�6H2O), a photodegradation of rhodamine B(RhB) aqueous solution experiment was carried outunder ultraviolet light irradiation in the presence ofH2O2 at room temperature. Before UV irradiation,

the suspensions were stirred in dark for 1 h to reachan adsorption/desorption equilibrium. The color ofthe RhB solution with addition of �-Fe2O3 micro-spheres was rapidly degraded with time, and thecorresponding digital photographs were recordedand shown in Fig. 10(a). It can be seen that the RhBsolution was totally colorless after irradiation for270min, indicating the RhB molecules had beencompletely photodecomposed by �-Fe2O3 micro-spheres. According to the Beer�Lambert law, theconcentration of RhB is linearly proportional to theintensity of the absorption peak at 553 nm, and thusthe decomposition e±ciency of RhB can be calcu-lated using the following expression:

RhB decomposition ð%Þ ¼ 100%� C0 � Cn

C0

C0 and Cn are the equilibrium concentrations ofRhB before and after UV irradiation, respectively.In Fig. 10(b), the self-degradation of RhB aqueoussolution without the addition of catalysts or H2O2 isreally negligible, only about 10% of RhB molecules

Fig. 7. Schematic illustration for the growth mechanism of�-Fe2O3 smooth microspheres.

Fig. 8. SEM images of �-Fe2O3 micromaterials obtained with a concentration of K3[Fe(CN)6] of 0.01M, reaction temperature of160�C, and the amounts of PVP of 0 g (a), 0.2 g (b), 0.3 g (c) and 0.5 g (d), respectively.

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were degraded in 250min. When just adding H2O2

to the RhB aqueous solution, the degradation rateincreased a little, which could be attributed to thepresence of highly oxidative OH� formed by thephotodecomposition of H2O2 in the reaction. How-ever, when �-Fe2O3 sample was added to the RhBaqueous solution, the photodegradation rate wasincreased obviously. It indicates that the as-pre-pared �-Fe2O3 sample can greatly promote thephotodegradation of RhB in the presence of H2O2

under ultraviolet light irradiation at room tem-perature. After 250min, about 90% of RhB mol-ecules were degraded when adding the preparedcatalyst in the presence of H2O2, while only about30% RhB molecules were degraded when addingH2O2 only for the same irradiation time. Thus, thecontrolled experiment showed that RhB moleculeswere very di±cult to be decomposed under UVirradiation by itself or adding H2O2 only withoutaddition of �-Fe2O3 samples. According to reportsin the literature,38 the electrons may be consumedby the following ways. They are trapped by H2O2 toform OH radicals, which could degrade RhB dyese±ciently, or the electrons are trapped by Fe3þ toform Fe2þ. The Fe2þ/H2O2 is the famous Fenton'sreagent39 and the Fenton reaction is an e®ectivemethod to oxidize contaminants.

Fe2O3 ! Fe2O3ðe�cb; h�vbÞ

H2O2 þ e�cb ! OH � þ �OH

Fe3þ þ e�cb ! Fe2þ

Fe2þ þH2O2 ! Fe3þ þ �OHþOH �

Figure 10(c) shows the decomposition e±ciency ofRhB over �-Fe2O3 microspheres. In our experiments,the �-Fe2O3 catalyst was separated via centrifu-gation each time, and reused for 5 cycles. From thequantitative measurements as shown in Fig. 10(c),

one can see that the decomposition e±ciency of RhBover the �-Fe2O3 microspheres after 5 cycles was67.18%. The observed catalyst inactivation mightbe partially caused by photocorrosion or photo-dissolution of the �-Fe2O3 catalyst.

Fig. 9. The six crystallographically quivalent directions of themicro-pine dendrite structure.

(a)

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Fig. 10. (a) Photographs of RhB upon photodegradation cat-alyzed by �-Fe2O3 microspheres, showing decolorization of thedye with time, (b) Photodegradation of RhB under ultravioletlight irradiation (� ¼ 70 nm). C and C0 denote the reaction andinitial concentration of RhB and (c) Decomposition e±ciency of�-Fe2O3 samples for the degradationof RhB (color online).

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4. Conclusion

In summary, uniform �-Fe2O3 microspheres havebeen successfully prepared by a facile hydrothermalmethod using PVP as a surfactant. The structures,morphologies and properties of the �-Fe2O3 micro-spheres were systematically investigated. Uniform�-Fe2O3 microspheres with an average diameterof 1�m could be routinely synthesized throughthis solvothermal approach by controlling PVP/FeCl3�6H2O ratio. The as-obtained �-Fe2O3 micro-spheres exhibit high e±ciency on the decolorizationof RhB aqueous solution in the presence of H2O2 atroom temperature. Considering the uniform struc-ture, the prepared �-Fe2O3 microspheres may ¯ndpromising applications in photodegradation andother ¯elds such as sensors, biomedical engineeringand magnetics.

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