Asian Journal of Chemistry; Vol. 25, Supplementary Issue (2013), S22-S26
Synthesis, Characterization and Photocatalytic Application of Ultra-Fine α-Fe2O3 Nanofiber† JYOTI P. DHAL, B.G. MISHRA and G. HOTA* Department of Chemistry, National Institute of Technology, Rourkela-769 008, India *Corresponding author: E-mail:
[email protected] AJC-12782
In this study, we have synthesized ferrous oxalate nanofibers by electrospinning methods using poly(vinyl pyrrolidone)/ethanol polymer solution and ferrous oxalate powder. The ferrous oxalate powder was prepared by chemical precipitation method. The obtained as-spun nanofiber was calcined at 500 ºC to form α-Fe2O3 nanofiber. The formation, phase analysis, surface morphology of the nanofibers were characterized using SEM, EDAX, XRD and FTIR analytical techniques. XRD studies confirmed the formation of α-Fe2O3. The SEM and EDAX analysis indicates the formation of fiber like nanomaterials and the presence of Fe and O elements. The obtained nanofibers were used as catalyst for the photochemical decolourization of toxic organic dye from aqueous solution. Key Words: Electrospinning, Nanofibers, Photochemical decolourization, Organic dye.
INTRODUCTION One-dimensional nanostructure such as nanorods, nanotubes, nanowires and nanofiber have attracted much interest over the past decade due to their combination of superior properties like small dimension structure, high aspect ratio and unique device function that lead to a large range of promising applications in catalysis, adsorption, electronics, photonics, chemical sensors, field emission devices, solar cells, lithium ion battery, hydrogen storages and drug deliveries1-3. The superiority of one dimensional nanostructure over its two dimensional and three dimensional counterparts is that: one dimensional nanostructure possesses two quantum confined directions with one unconfined direction for electrical conduction, which allows these materials for electrical conduction rather than tunneling transport. Further in the limit of small diameter, the unique density of electronic state in case of one dimensional nanostructures, causes significantly different optical, electrical and magnetic properties from their bulk 3D counterparts. Again, incorporation of functional heterostructures is a potential application of one-dimensional material which is very difficult or impossible in case two-dimensional systems4-6. The unique properties of nano-sized fiber as compared to bulk fiber are as: they possess high surface to volume ratio, fiber interconnectivity, micro scale interstitial space and also high porosity7. Among the various methods of synthesis of nanofibers, electrospinning is a simple and effective method
for fabricating ultrathin nanofibers and is used to prepare a wide variety of polymer, ceramic and composite nanofibers8. In the present investigation, we have prepared α-Fe2O3 nanofibers, by electrospinning ferrous oxalate (FeC2O4.2H2O) mixed with poly(vinyl pyrrolidone)/ethanol solution followed by sintering at higher temperature. Then the prepared α-Fe2O3 nanofiber was used as a catalyst for solar light degradation of methylene blue, a carcinogenic dye, from aqueous solution. Methylene blue is a heterocyclic aromatic chemical compound with molecular formula: C16H18N3SCl. The structural formula of methylene blue is given in Fig. 19,10. N
CH3
H3C N
S
N
CH3
CH3
Fig. 1. Structural formula of methylene blue
EXPERIMENTAL Ferrous sulphate heptahydrate, oxalic acid dihydrate, poly(vinyl pyrrolidone), ethanol and methylene blue were of analytical grade and were used without further purification. The chemicals were obtained from Merck (India). Synthesis of α-Fe2O3 nanofiber: To synthesize α-Fe2O3 nanofibers, we first prepared FeC2O4.2H2O powder by a simple chemical precipitation method. In which, 2.78 g of FeSO4·7H2O
†International Conference on Nanoscience & Nanotechnology, (ICONN 2013), 18-20 March 2013, SRM University, Kattankulathur, Chennai, India
Synthesis, Characterization and Photocatalytic Application of Ultra-Fine α-Fe2O3 Nanofiber S23
C0 − Ct C0
Decolourization rate = 100 ×
A − At % = 100 × 0 A0
RESULTS AND DISCUSSION X-ray diffraction analysis: The formation and phase analysis of the prepared nanofiber was analyzed by XRD using CuKα radiation. Fig. 2 shows the XRD pattern of the calcined α-Fe2O3 nanofiber. The pattern contains the characteristics peaks of α-Fe2O3 (hematite) and contains rhombohedral crystal structure according to JCPDS No: 079-0007. The X-ray diffractograms reveal the well crystalline nature of the compounds. The broadening of the peaks also indicates the decrease in the diameter and an increase in the surface to volume ratio of the compounds.
104
JCPDS:079-0007
20
30
40
50
60
70
220 036
018
208 1 0 10
113
214 300
116
024
110 012
dissolved in 100 mL of water with stirring to form solution-A and 2.52 g of H2C2O4.2H2O dissolved in 100 mL of water to form solution-B. Then solution-A was added drop wise to solution-B with continuous stirring to form yellow precipitate of FeC2O4.2H2O. After addition the stirring was continued for another 5 h. Then the solution was washed, centrifuged and dried to form FeC2O4.2H2O powder. To electrospin precursor fiber, we have prepared a solution containing 0.6 g of poly(vinyl pyrrolidone), 0.3 g of prepared ferrous oxalate powder in 4.1 g of ethanol, followed by magnetic stirring for 12 h. The resulting poly(vinyl pyrrolidone)-ferrous oxalate solution was loaded into a plastic syringe of 3 mL in volume with a stainless steel needle. The syringe was placed vertically so that small drop of solutions appeared on the tip of the needle when the piston of the syringe was pushed towards the solution by the help of a syringe pump. The flow rate was maintained to be 1 mL/h. A grounded aluminum foil served as fiber collector during electrospinning. A high voltage power supply of 13 kV was applied with the positive terminal connected to the needle and the negative terminal to the grounded aluminum foil. After electrospinning the as spun poly(vinyl pyrrolidone)/ferrous oxalate composite fibers were calcined at 500 ºC for 2 h with heating rate 10 ºC/min to obtained α-Fe2O3 nanofiber. In order to know the formation the X-ray diffraction patterns of fibers were recorded on a PAN analytical diffractometer using CuKα (λ = 1.541 Å) radiation at a scanning rate of 2º/min in 2θ ranging from 20º to 80º. The surface morphology of the as spun and calcined nanofibers was characterized using JEOL JSM-5300 scanning electron microscope operated at 20 kV. FTIR spectra were recorded using Perkin-Elmer FTIR (Spectrum RX-I) spectrophotometer. The UV-visible absorbance spectra of the sample were recorded using Shimadzu spectrometer (model 2450) with BaSO4 coated integration sphere in the range of 200-800 nm. Photocatalytic activity measurement: Methylene blue photodecomposition experiments were carried out using solar light under following procedure: 0.1 g photocatalyst powder was added into 100 mL of methylene blue solution with an initial concentration of 10 mg/L; prior to photoreaction the aqueous mixture was magnetically stirred in dark for 2 h to reach adsorption-desorption equilibrium. Then the reaction mixture was irradiated in the solar light with continuous stirring. During photoreaction the samples were drawn from the reaction suspension at 30 min time interval and the collected samples were centrifuged and filtered to remove the particles. After that the methylene blue concentration of the filtrate were analyzed by UV-VIS spectrometer (Shimadzu spectrometer, model-2450) at its maximum adsorption wavelength of 664 nm. The decolourization rate of methylene blue was calculated by the following equation11:
Intensity (a.u.)
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2θ (º) Fig. 2. XRD pattern of α-Fe2O3 nanofiber obtained after sintering at 500 ºC
Surface morphology: Fig. 3 indicates the SEM micrographs of a) as spun poly(vinyl pyrrolidone)/ferrous oxalate nanofiber before calcination and b) α-Fe2O3 nanofiber after calcined the as spun nanofiber at 500 ºC and c) EDAX of calcined α-Fe2O3 nanofiber. Fig. 3a) micrographs suggests that the formation of ultra-fine continuous smooth fibers with diameter around 300-600 nm. When the fiber calcined at 500 ºC in air, the poly(vinyl pyrrolidone)/ferrous oxalate nanofiber converted to α-Fe2O3, retaining its fiber like morphology. Fig. 3b) suggests the ultra-fine fiber like morphology of α-Fe2O3. The EDAX analysis of α-Fe2O3 nanofiber clearly indicates the presence of iron and oxygen elements (Fig. 3c).
a
b
c
% (1)
where C0 and Ct were the concentration of methylene blue when reaction time was 0 and t and A0 and At were the absorbance of methylene blue when reaction time was 0 and t, respectively.
Fig. 3. SEM micrographs of a) PVP/ferrous oxalate nanofiber, b) α-Fe2O3 nanofiber, after calcination at 500 ºC and c) EDAX analysis of α-Fe2O3 nanofiber
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FTIR analysis: For further confirmation of formation and transformation to α-Fe2O3, FTIR analysis was employed. Fig. 4a) shows the FTIR spectrum of poly(vinyl pyrrolidone)/ FeC2O4.2H2O nanofiber (as spun). This figure contains the peaks at 3427, 1654, 1294 and 540 cm-1 and these peaks are corresponding to O-H, C=O, C-O and Fe-O functional groups, respectively, indicating the formation of FeC2O4.2H2O. Fig. 4b) contains peaks at 543 and 458 cm-1 and these are due to presence of Fe-O vibrational mode, indicating the formation of α-Fe2O3 (hematite)12.
Eg can be experimentally obtained from absorption coefficient measurements using Tauc's formula15: (αhν) = A(hν - Eg)n (3) where α is the absorption coefficient, A is a constant and n is equal to 1/2 for allowed direct transitions and 2 for allowed indirect transitions. The band gap value Eg, of the prepared nanofibers was evaluated by extrapolating the linear portion 0.38
(a)
PVP/FeC2O4.2H2O
0.36
Absorbance (a.u)
0.34 0.32 0.30 0.28 0.26 0.24 0.22 0.20
458
200
543
300
400
500
600
700
800
Wavelength (nm)
a 0.9
540
1294 1654
3427 4000
3500
3000
2500
2000
1500
1000
500
Absorbance (a.u.)
Transmittance (a.u.)
b
α-Fe2O3
(b)
0.8
0.7
0.6
0.5
Wavenubmer (cm-1) Fig. 4. FTIR spectra of a) PVP/Fe2O4.2H2O nanofiber and b) α-Fe2O3 nanofiber
0.4
200
UV-VIS diffuse reflectance spectra: Electronic transition and band gap calculation of the prepared nanofibers were done by UV-visible absorbance spectra. The spectral absorption coefficient α, is deifned as:
400
500
600
700
800
Wavelength (nm) 0.30
(c)
PVP/FeC2O4 .2H2O
0.28
Absorbance (nm)
4πκ(λ) (2) λ where (λ) is the spectral extinction coefficient obtained from the absorption curve and λ is the wavelength. Fig. 5a) shows the UV-visible DRS spectra of poly(vinyl pyrrolidone)/ FeC2O4.2H2O nanofiber. From the figure it is observed that there is an intense band around 210 nm and an asymmetric band at 400 nm. The intense band corresponds to the charge transfer band and the less intense band arises due to 5T2g→5Eg transition13. From the spectrum of α-Fe2O3 (Fig. 5b), it is observed the peak at 532 nm corresponds to finger print region of the band edge of hematite14. This experiment further confirms the formation of pure α-Fe2O3. The optical band gap α( λ ) =
300
0.26
0.24
0.22
Egap=2.85 eV
0.20
2
Photon energy (nm)
3
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(d)
α-Fe2O3
0.8
0.7
0.6
E gap=1.97 eV
0.5
(a)
100
Percentage Decolorization
Absorbance (a.u.)
0.9
Synthesis, Characterization and Photocatalytic Application of Ultra-Fine α-Fe2O3 Nanofiber S25
80
60
40
20
0
2
Photon energy (nm) Fig. 5. Visible and near-ultraviolet absorption spectra of a) PVP/ FeC2O4·2H2O, b) α-Fe2O3 and Tauc plots of c) PVP/FeC2O4·2H2O and d) α-Fe2O3
Absorbance (a.u.)
2.0
1.5
0.5
0.0
500
20
40
60
80
100
120
140
160
180 200
8
120
140
160
180 200
(b)
7 6 5 4 3 2 1 0 -1 -20
0
20
40
60
80
100
Time (min) Fig. 7. a) Photocatalytic degradation of methylene blue on α-Fe2 O3 nanofiber under solar light irradiation; (b) Kinetics of different irradiation time
Conclusion We have synthesized α-Fe2O3 nanofiber by electrospinning method using poly(vinyl pyrrolidone)/ethanol polymer solution and ferrous oxalate powder followed by calcination at higher temperature. The SEM images indicates fiber like morphology with diameter around 300-600 nm. The formation of α-Fe2O3 (hematite) phase was confirmed by XRD and FTIR analysis. The UV-VIS-DRS spectra shows that the prepared α-Fe2O3 nanofiber exhibits semiconductor nature and hence can be used as an efficient visible light photocatalyst. The methylene blue decomposition kinetics was studied. It is observed that the methylene blue was absolutely decomposed by increasing the irradiation time up to 180 min.
0 min 30 min 60 min 90 min 120 min 150 min 180 min
1.0
400
0
Time (min)
ln(C0 /Ct)
of the curve. It is found that the energy band gap of poly(vinyl pyrrolidone)/FeC2O4.2H2O and α-Fe2O3 are 2.85 eV, 1.97 eV, respectively. It is found that the prepared α-Fe2O3 nanofiber possesses semiconducting properties with narrow band gap and hence it is a very efficient solar light photocatalyst. Photocatalytic degradation of methylene blue: The photocatalytic activities of the prepared α-Fe2O3 nanofiber is evaluated by photocatalytic degradation of aqueous solution of methylene blue under natural sunlight irradiation by monitoring the intensity of the characteristics absorption peak at 664 nm of methylene blue. Fig. 6 shows the UV-VIS spectra of methylene blue through the photocatalytic degradation on α-Fe2O3 nanofiber. The degradation of methylene blue was recorded as a function of time in presence of solar light as shown in Fig. 7 (a). It is observed that the proportion of methylene blue degradation increased by increasing solar light exposure time and almost all the methylene blue molecules were decomposed within 180 min. The experimental data were found to fit approximately a pseudo-first-order kinetic model by linear transforms ln(C0/Ct) = f(t) = kt (k is rate constant), as shown in Fig. 7(b). Furthermore the used α-Fe2O3 nanofiber photocatalyst can be recycled by a simple washing with distilled water.
-20
600
700
800
Wavelength (nm)
Fig. 6. UV-VIS spectral changes of methylene blue as a function of reaction time
ACKNOWLEDGEMENTS The authors acknowledged NIT Rourkela for providing the research facilities & funding.
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