α-Fe2O3 nanospheres: Facile synthesis and highly efficient photo-degradation of organic dyes and surface activation by nano-Pt for enhanced methanol sensing

Article abstract of DOI:10.1039/c6ra13385a

We report a simplified electrochemical route to synthesize thin films of nanosphere α-Fe₂O₃ from a suitable electrolytic solution. X-ray diffraction studies revealed the formation of pure hematite phase (hexagonal structure) α-Fe₂O₃ films. Field emission scanning electron microscopy revealed a highly compact surface morphology with evenly distributed almost spherical grains. Raman, electron paramagnetic resonance and Fourier transform infrared spectroscopic analyses confirmed the presence of α phase Fe₂O₃ (hematite). Optical analysis revealed a band gap energy of 2.15 eV; this is most suitable for visible light driven photocatalysis towards the degradation of Indigo Carmine (IC) and Rhodamine B (Rh B) dyes, which are widely used in the textile industry and were taken as model organic compounds. About 90% photodegradation was achieved at rates of 0.0188 min^-1 for IC and 0.0133 min^-1 for Rh B. The synthesized films were used as modified electrodes, and their catalytic activity towards methanol oxidation was investigated. A comparison was also made between Pt modified FTO/Fe₂O₃ and unmodified FTO/Fe₂O₃ electrodes towards dye degradation and methanol oxidation, and it was found that the Pt modified FTO/Fe₂O₃ electrode yielded superior results.

Full text of DOI:10.1039/c6ra13385a

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αꢀFe2O3 nanospheres: Facile synthesis and highly efficient photoꢀdegradation of
organic dyes and surface activation by nanoꢀPt for enhanced methanol sensing
Bibhutibhushan Showa,c, Nillohit Mukherjee*b, Anup Mondal*c
aDepartment of Chemistry, Jadavpur University, Kolkata 700032, India
bCentre of Excellence for Green Energy and Sensor Systems, Indian Institute of Engineering Science
and Technology, Shibpur, Howrah 711103, Botanic Garden, Howrah 711103, India
cDepartment of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur,
Botanic Garden, Howrah 711103, India
*Corresponding author email: nilsci@yahoo.co.uk (NM), anupmondal2000@yahoo.co.in (AM)
Fax: +91ꢀ33ꢀ2668ꢀ2916
Abstract: We report a simplified electrochemical route to synthesize thin films of nanosphere αꢀ
Fe2O3 from a suitable electrolytic solution. Xꢀ ray diffraction studies revealed the formation of pure
phase of hematite (hexagonal structure) αꢀFe2O3 films. Field emission scanning electron microscopy
revealed a highly compact surface morphology with evenly distributed almost spherical grains.
Raman, Electron Paramagnetic Resonance and Fourier transform infrared spectroscopic analyses
confirmed the presence of α phase of Fe2O3 (hematite). Optical analysis revealed band gap energy of
2.15 eV; most suitable for visible light driven photocatalysis towards Indigo Carmine (IC) and
Rhodamine B (Rh B) dyes, widely used in textile industry and was taken as model organic
compounds. About 90% photodegradation was achieved at a rate of 0.0188 minꢀ1 for IC and 0.0133
minꢀ1 for Rh B. The synthesized films were used as the modified electrodes and their catalytic
activity towards methanol oxidation was investigated. A comparison was also made with Pt modified
FTO/Fe2O3 and unmodified FTO/Fe2O3 electrodes towards dye degradation and methanol oxidation
and found that the Pt modified FTO/Fe2O3 electrode was a better performer.
Keywords: αꢀFe2O3 Nanospheres; Photocatalysis; Electrocatalytic activity; Electroꢀoxidation;
Surface activation; Multifunctional material.
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1. Introduction
Iron oxide in the α phase, i.e. αꢀFe2O3 (hematite) is an attractive semiconductor for many cutting
edge applications, among which photo electrochemical and photocatalytic applications are of major
importance due to its stability, abundance, and environmental compatibility, as well as suitable band
gap and the position of valence band edge.1ꢀ8 In general, hematite exhibits an nꢀtype character, which
can be due to the tendency of αꢀFe2O3 to become oxygen deficient, irrespective of the preparation
method. αꢀFe2O3 is one of the promising candidates for photocatalytic applications due to its narrow
band gap energy of about 2.0ꢀ2.2 eV which leads absorption mainly in visible region. It absorbs light
up to 600 nm, collects up to 40% of the solar spectrum energy. αꢀFe2O3 is cost effective with low
toxicity, high photo/heatꢀstability and has suitable valence band edge position. This makes hematite
a promising anode material for photochemical, photoelectrochemical and electrochemical studies
like methanol electrooxidation9. Nanoparticles of αꢀFe2O3 perform the photocatalytic reactions
through the generation of electronꢀhole pairs upon visible light irradiation which leads to better
charge transfer process between (LUMO)dyes to (HOMO)catalyst. Various techniques have been
investigated for the synthesis of hematite nanocrystalline thin films viz. thermal evaporation,10
chemical vapor deposition,10 aqueous chemical growth10, sprayꢀpyrolysis11, ultrasonic sprayꢀ
pyrolysis11, solꢀgel method12 and the plasma enhanced atomic layer deposition.13 Electrodeposition
is a technique that is wellꢀsuited to the preparation of nanostructures.14 In fact, through proper
parameter control, electrodeposited materials can from nanostructured to even epitaxial films with
better properties.15 Moreover, the electrodeposition technique is now emerging as an important lowꢀ
cost and low temperature method for preparing semiconducting thin films16.
In the last couple of years, several reports have appeared concerning the electrodeposition of iron
oxide thin films onto foreign substrates like Fe3O4 (magnetite), αꢀFeOOH (goethite), and γꢀFeOOH
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(lepidocrocite)17. Among the various oxides of iron, hematite (αꢀFe2O3), maghemite (γꢀFe2O3) and
wustite (Fe1ꢀxO), αꢀFe2O3 is of special interest due to its unique catalytic properties, however, γꢀ
Fe2O3 nanoparticles are excellent candidate for nanoscale magnetism18. Owing to its suitable
electronic properties like flatband potential, surface current density and doping density, αꢀFe2O3 is a
very important material for electrooxidation of various toxic chemicals like methanol9,19. It also
serves as a good photocatalyst for aniline20 and photo polymerization reaction promoter for
acrylamide through radical mechanism21. αꢀFe2O3 with different nanostructures can act as an
efficient photocatalyst for the degradation of phenolic compounds and other dyes like methylene
blue (MB)22, Rose Bengal (RB)23 and Methyl Orange (MO)24. Xu et al.25 and Tang et al.26 have also
investigated the uvꢀlight driven photocatalytic performance of hollow spindle25 and flower like26 αꢀ
Fe2O3 towards phenol degradation. αꢀFe2O3 is also used for the improvement of photocatalytic
efficiency of other semiconductors like TiO2 by making a composite, owing to facts that Fe3+can act
as a better electron scavenger than O2 and significant matching of band structures between Fe2O3
27
and TiO2 . Hematite is also very important for the synthesis of composite heterostructures like
Fe2O3/SnO2 , Fe2O3/CdS29 etc. for better photocatalytic performances through effective charge
28
carrier separation.
Over the last few decades, our society has become increasingly sensitive towards the
protection of the environment against the adverse effects of the industries. The effluents of dyeing,
textile, pulp and paper industries contains various dye pigments and methanol which should be
removed before discharging to the environment to avoid health hazards and protect the ecosystem.
Due to low biodegradability of dyes, the conventional biological wastewater treatment pathways are
not very efficient. To solve this problem, scientists have developed semiconductor photocatalysis.
The main advantage of this technology is the environmental compatibility, as its main reagent, the
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electrons, is clean reagent. As well as, the semiconductor photocatalysis has faster degradation rate
over biodegradation. In this regard, efficient, stable, lowꢀcost and environment friendly
photocatalysts must be developed. These are the motivations behind this work. In this work, the
deposition of nanocrystalline thin films of αꢀFe2O3 on fluorineꢀdoped tin oxide (FTO) coated glass
was carried out by a simple electrochemical method using the chemicals FeCl2.4H2O, KCl and H2O2.
The method reported here is cheaper, simpler and less time consuming for large scale production. In
electrochemical synthesis of iron oxides, an accurate control of growth potential and pH is necessary
which have been taken care of in this work. The catalytic behavior of the deposited αꢀFe2O3 films for
the photoꢀdegradation of hazardous organic dyes like Indigo Carmine (IC) and Rhodamine B (Rh B)
has been explored. A comparison was also made between the Pt modified FTO/Fe2O3 and
unmodified FTO/Fe2O3 electrodes towards methanol electrooxidation, and the latter was found to be
better performer.
2. Experimental Procedure
2.1. Materials
All chemicals and solvents were of analytical reagent grade and used as received, without any
further purification. Commercially available FTO substrates with ~ 10 ohm/sq surface resistivity
were cleaned properly and used as the cathode for the deposition.
2.2. Deposition of αꢀFe2O3 thin films
The working solution was prepared by mixing thoroughly 1 mL of 0.1(M) FeCl2, 4H2O and 1 mL
0.1(M) KCl in 100 mL double distilled water. KCl was taken as the supporting electrolyte for
stabilizing the redox system. Next, 0.5 mL 0.02 (M) H2O2 was added drop wise with stirring at 500
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rpm for 15 minutes. The pH of the working solution was maintained around 2.5 by adding dilute
hydrochloric acid (1:30) drop wise. The working solution was brownish yellow in color.
The electrochemical cell was constructed by immersing the FTO glass and a Zn rod (99.8%
pure) in the solution, which were shortꢀcircuited externally through a copper wire. The Zn rod served
as the selfꢀdecaying anode to produce Zn2+ ions and the FTO glass as the cathode (schematic is
reported elsewhere30). As soon as the two electrodes are shortꢀcircuited, the deposition starts very
slowly , which was continued for around 24 hours at room temperature without stirring, to obtain
almost pinhole free compact films with good adherence. After deposition, the films were washed
thoroughly with double distilled water and dried in a hot air oven at 60˚C for 15 minutes.
The mechanism for αꢀFe2O3 deposition involves two reactions that take place simultaneously on the
FTO cathode surface, when the electrodes are short circuited externally.
FeCl2. 4H2O dissolves in water at low pH (c.a. 2.5) and dissociates as:
FeCl2.4H2O → Fe2+ + Clꢀ + 4H2O....................... (i)
Now, as we have used Zn rod as the sacrificial anode and it was dipped in to a low pH (c.a. 2.5)
medium, metallic Zn will dissociate in the following manner:
Zn → Zn2+ + 2eꢀ
E0 = + 0.76 V....................... (ii)
The Zn2+ ions will come to the solution and the two electrons that were released on the Zn surface,
will flow to the FTO cathode through the external path, which in turn, will reduce the Fe2+ ions
present in the vicinity of the cathode as the E0 value for Fe2+/Fe0 is much lower (ꢀ0.41 V) than that of
Zn2+/Zn0 (ꢀ0.76 V).
Fe2++ 2eꢀ → Fe0....................... (iii)
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In presence of H2O2, the Fe⁰ formed on the FTO surface will undergo the following series of
reactions to finally produce Fe2O3 as described earlier by SchreblerꢀGuzman et al. and Kelly et al.31ꢀ
32
According to them, H2O2 first reacts with the dissolved electrons (PhotoꢀFenton reaction) in the
following manner to produce the active species like OHꢀ and OH*:
H2O2 + eꢀ(aq) → OH* + OHꢀ ....................... (iv)
Fe⁰ + OHꢀ
[FeOH] + eꢀ ....................... (v)
[FeOH]+ + eꢀ ....................... (vi)
[FeOH]
[FeOH]+ + OH−
Fe(OH)2 ....................... (vii)
Fe(OH)2 + OH− → [FeOOH] + H2O+ eꢀ ....................... (viii)
2[FeOOH]
Fe2O3.H2O ....................... (ix)
The Clꢀ ions released by Fe2+ ions during reduction, captures the free Zn2+ ions present in the
solution, and thus restricting their deposition along with Fe and enhancing the purity of the deposited
films.
To avoid ‘poisoning’ of the catalyst, i.e. the deposited Fe2O3 films in this case, Pt nanoparticles were
deposited on the film surface by dipping the film in a beaker containing 1% H2PtCl6.H2O solution
for 2 minutes. After taking out, the films were washed thoroughly with distilled water and dried at
200⁰C.
2.3. Characterization techniques
The crystalline structure and phase of the deposited films were determined by Xꢀray diffraction
(XRD) technique using Bruker D8 advance Xꢀray diffractometer with BraggꢀBrentano goniometer
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geometry and CuꢀKα Xꢀradiation source (λ=1.5418 Å). Morphological analyses of the films were
carried out by both field emissions scanning electron microscopy (FESEM) (JEOL, JSMꢀ6700F) and
atomic force microscopy (AFM) (NTꢀMDT Solver Next). AFM measurements were done in
semicontact mode, and a silicon probe (length = 95 ꢁm, width = 30 ꢁm, thickness = 2 ꢁm, resonance
frequency = 240 kHz, force constant = 18 N/m2) was used for the scanning purpose. Scans were
made with a scanning speed of 0.5 Hz. Electron Paramagnetic Resonance (EPR) spectra were
recorded using an EPR spectrometer (JEOLꢀJES FA200) with XꢀBand frequency: 8.75ꢀ9.65 GHz,
Sensitivity: 7x109 spins/0.1 mT, Resolution: 2.35 ꢁT. UVꢀVIS spectroscopic measurements were
carried out using a PerkinꢀElmer Lambda 25 UVꢀVis spectrophotometer to evaluate the optical
properties of the deposited films. Infrared spectrum was recorded using a PerkinꢀElmer Spectrum
Two with ATR Mode. Raman (NXR FTꢀRAMAN MODULE, Thermo electron Corporation, USA,
operating with 1064 nm Nd: YVO4 excitation laser source at 0.1W power) analyses was also carried
out to establish the phase of the deposited material. Perkin Elmer, LSꢀ55 fluorescence
•
spectrophotometer was used to study the OH radical trapping phenomenon. The electrochemical
measurements were carried out using a CHI 620D, USA made electrochemical analyzer.
2.4. Photocatalytic measurement
Photocatalytic behavior of the deposited thin films was measured by noting the gradual decrease of
the characteristic absorption peak of the test dye in presence of light. Standard aqueous solution of
Indigo Carmine (IC) (100 ppm, pH c.a. 6) and Rhodamine B (RhB) (100 ppm, pH c.a. 5.5) were
prepared and taken after attaining the equilibrium in dark for 30 minutes, to study the photocatalytic
performance of the films. Two prototype films, each with 2 cm × 3 cm area, were placed at the
bottom of two 100 ml beakers containing 50 ml of 100 ppm IC and RhB dye solutions, respectively.
The same technique was also applied for the Pt activated films. As Fe2O3 is a semiconductor with
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main electronic transitions in the visible region, a 200W tungsten filament lamp (Philips) was used
as the source of visible light, which was placed 2.0 cm above the beaker so that it can produce 1 Sun
intensity. The entire experimental set up was kept in a water bath at 30⁰C with constant flow of air to
the beaker to eliminate the effect of any probable heating that might be caused due to the light
source. At certain time intervals, specific amount of the solution were withdrawn and the changes in
concentration of the dye were observed using UVꢀVis spectrophotometer by noting the lowering in
intensity of the characteristics absorption peak of the respective dye.
2.5. Electrocatalytic activity measurement
The electrochemical oxidation and chronoamperometric detection of CH3OH by αꢀFe2O3 and Pt
modified αꢀFe2O3 thin films (FTO/αꢀFe2O3 and FTO/PtꢀαꢀFe2O3) with 1.0 cm2 active areas each,
were monitored with the help of an electrochemical analyzer using conventional three electrode
system with Ag/AgCl as reference electrode. Cyclic voltamograms (CV) were recorded in a cell
containing 10 mL of 0.1 M phosphate buffer solution (PBS) (pH c.a. 7.4, 32⁰C) with a scan rate of
1
0.1 V s− . For chronoamperometric measurements, similar cell setup was used with an applied
potential of ꢀ1.0 V (vs. Ag/AgCl).
3. Results and discussions
3.2. Structural and morphological properties of αꢀFe2O3 and Pt activated αꢀFe2O3 thin films
The composition and purity of the deposited films were examined by Xꢀray diffraction (XRD)
technique and the resulting diffractrograms are displayed in Figure 1. The diffraction pattern of the
as deposited film (Figure 1a) showed characteristic peaks from (104), (202), (116), (214), (300),
(119) and (223) planes which can be indexed to the pure hematite phase (hexagonal structure), i.e. αꢀ
Fe2O3 (JCPDS ID: 33ꢀ0664). The sharpness of the peak indicates the crystallinity of the deposited
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material. The diffraction peaks marked “*” correspond to the SnO2 of the bottom FTO coated glass
substrate (JCPDS ID 21ꢀ1250). However, two diffraction peaks of αꢀFe2O3 viz. (104) and (116) were
found to overlap with the diffractions from SnO2. The crystallite size corresponding to the highest
intense diffraction was estimated using the DebyeꢀScherrer equation32: D = 0.9λ/β Cos θ, where D is
the crystallite size, λ is the wavelength of the CuꢀKα radiation used (λ = 1.5418 Å), β is the
experimentally observed diffraction peak width at half maximum (FWHM), θ is the Bragg’s angle;
the average crystallite size was estimated to be around 10 nm. The dislocation density (δ), which is
the length of dislocation lines per unit volume, was also estimated using the equation33: δ = 1/D2 and
found to be significantly low (0.01/nm2) indicating lesser amount of crystal defects and good
crystalline nature of the deposited films. The high value (3.83×10ꢀ2) of microstrain (ε) as obtained
using the relation ε = (β cos θ)/4, again supports the formation of crystallites with low dimension.
After activating the surface of the deposited Fe2O3 films with Pt, the samples were also subjected to
the XRD analysis and the result is given as Figure 1b, which reveals the presence of diffractions
from the (111), (200) and (220) planes of metallic Pt (JCPDS ID: 04ꢀ0802) corresponding to the 2θ
values 39.76, 46.24 and 67.45 degrees, respectively. The deposition of metallic Pt leads to the
suppression of some peaks corresponding to αꢀFe2O3. The standard diffraction ID indicates the
presence of quasi spherical metallic platinum on the surface of the deposited αꢀFe2O3 film. The small
intensity of the diffraction peaks indicates the presence of metallic platinum in small amount.
The 2D and 3D surface morphology and the average particle size of the as deposited αꢀFe2O3 films
were investigated using field emission scanning electron microscopy (FESEM) and atomic force
microscopy (AFM), which are shown in Figure 2. One can see from the FESEM image (Figure 2a)
that the FTO surface is fully covered with uniformly distributed spherical grains of αꢀFe2O3 with
diameter in the range 40 to 60 nm. The grains are distinct and the grain boundaries are prominent. To
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study the nature of an individual grain, high magnification image (Figure 2a inset) was captured and
it has been found that, such a grain is composed of crystallites with average 15 nm diameter. This
value of crystallite diameter agrees well with what we have obtained from the XRD analysis.
However, the average grain size as obtained from low magnification FESEM is somewhat larger
than that was calculated from XRD analysis, which is attributed to the formation of larger grains
from smaller crystallites, essential for spontaneous lowering of Gibb’s free energy in the system to
gain stability. The total free energy of a system containing nanoparticles (NPs) is given by:
G
total = ꢁNP×NNP + γ×S
Where, ꢁNP is the chemical potential of the nanoparticles and NNP is the total number of
nanoparticles. γ is the interfacial tension (or the interfacial energy per unit area) and S is the surface
area. Thus γ×S denotes the total surface energy of the system. As we know, NPs have higher
surfaceꢀtoꢀvolume ratio, making the total surface energy of the system significantly high. As every
system has a tendency to lower down the free energy spontaneously, this γ×S part of free energy for
a system consisting of nanoparticles also has a tendency to get lowered down by means of forming
larger grains (which reduces the value of S) from smaller crystallites by agglomeration. For this
reason, from FESEM image analysis we see the grains to have somewhat larger size then the
crystals.
AFM studies were also carried out to find the salient topographical and surface features of the as
deposited films and the typical 3D AFM image is shown in Figure 2b. From this image also, we can
see the average grain size to be about 50 nm, which is in good agreement with the value obtained
from the FESEM analysis. AFM revealed a significantly undulated topography along with the
presence of numerous finer crystallites in the grains of the as deposited αꢀFe2O3 films.
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On the other hand, the FESEM image of the Pt activated αꢀFe2O3 film (Figure 2c) clearly reveals the
presence of numerous almost spherical Pt nanoparticles with an average diameter of 50 nm. This is
in good agreement with the xꢀray diffraction from quasiꢀspherical Pt nanoparticles (JCPDS ID: 04ꢀ
0802), as explained before. The spikes in the 3D AFM image of the Pt activated αꢀFe2O3 sample
(Figure 2d) also indicate the growth of distinct nanoparticles on the surface of the deposited
semiconductor. Various parameters as obtained from XRD, FESEM and AFM analyses, relating to
the structure of the deposited αꢀFe2O3 thin films before and after Pt activation are summarized in
Table 1. It has been observed from the AFM measurements and can be seen from Table 1 that both
Root Mean Square (RMS) and average (Ra) roughness increased significantly after Pt activation.
The increase in surface roughness indicates that, Pt activated αꢀFe2O3 thin films may serve as a
better candidate towards photochemical and electrochemical activities.
3.3. Electron paramagnetic resonance studies (EPR)
The room temperature electron paramagnetic resonance (EPR) spectrum of αꢀFe2O3 is shown in
Figure 3 which exhibits an intense resonance signal at g ≈ 2.30 within 300 to 325 mT. This confirms
the oxidation state of +3 in the deposited material as the Fe3+ ions belong to d5 configuration with 6S
as ground state, and there is no spinꢀorbit coupling. The theoretical g value for the free Fe3+ ion is
2.0023. Our experimentally observed value is in good agreement with the theoretical value. The
slightly high experimentally obtained g value might be attributed to the formation of clusters of Fe3+
ions which causes exchange interactions between the coupled ions34. Previous report34 indicates, if
the Fe2O3 crystallites stay within few nanometers of domain, as the case here (Section 3.2), a broad
spectrum of g (≈ 2.3) may be observed. Therefore, the observed resonance signal at g ≈ 2.3 in this
study justifies that the oxidation state of Fe in the deposited iron oxide is +3, which is in good
agreement with the XRD analysis confirming the presence of αꢀFe2O3. The broad nature and slightly
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higher value of g also indicate the formation of small clusters of Fe2O3 with nanoꢀdimension, which
in turn supports the findings from FESEM and AFM studies.
3.4. Fourier transform infrared (FTIR) and Raman spectroscopy
For further detection of phase and purity of the deposited iron oxide films, FTIR analysis was carried
1
out within the range 500ꢀ4000 cm− (Figure 4). The absorption band at ~3340 cmꢀ1 is assigned to the
presence of adsorbed water moiety.35 The band around ~1626 cmꢀ1 might be attributed to the OꢀH
bending vibrations of the adsorbed H2O molecules (moisture) and the band around 1100 cmꢀ1 is
attributed to the FeꢀOH vibrational mode of iron oxide in presence of moisture. The bands ~540 and
1
~462 cm− correspond to the characteristic metalꢀoxygen (FeꢀO) vibrational modes of spinel iron
oxide. The small intensity of the FeꢀO bands agrees well with the previous report.36 The observed
shift is gradual and depends on the particle size. The main band has been attributed to the FeꢀO or
FeꢀOꢀFe bindings of hematite. This confirms the formation of Fe2O3 films with α phase.
Raman spectra of various phases of iron oxides (viz. αꢀFe2O3, Fe3O4 and γꢀFe2O3) are
distinctly different from each other and this technique can be used effectively to determine the phase
of deposited iron oxide.37 Iron oxide belongs to D3d6 space group and typically seven phonon lines
are expected in the Raman spectrum, viz. two A1g and five Eg phonon modes.38 The spectrum is
normally dominated by the overtones of the longitudinal optical (LO) phonons. First of all, a
background scan within 200ꢀ700 cmꢀ1 was performed, taking a blank FTO glass and then for the iron
oxide sample deposited on FTO glass. The spectrum (Figure 5) reveals the presence of five strong
resonant Raman peaks at 220, 285, 408, 491 and 608 cmꢀ1. These values are typical for αꢀFe2O3 and
are in well agreement with the previous reports. The Raman lines are broader and are found to be
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shifted to low wavenumbers with respect to the values obtained for commercial hematite (αꢀFe2O3).
The broader line width and the shifting of wavenumbers of the Raman peaks are due to phonon
confinement effect in the nanocrystals, as was described previously. Bersani et al.39 proved that the
peak that appears at 660 cmꢀ1 is typical for the hematite iron (αꢀFe2O3) and seems to be related to the
disorder effects and/or to the presence of nanocrystals. Xu et al.40 recorded Raman spectrum of αꢀ
Fe2O3 nano leaves synthesized by oxygenating pure iron and observed Ag at 225 and 498 cmꢀ1 and
Eg at 252, 293, 411, 612 cmꢀ1. So, in our case, Raman analysis confirms the presence of αꢀFe2O3 in
the deposited films.
3.5. UVꢀVis analysis
As displayed in Figure 6, the absorption spectrum showed a moderately sharp onset in absorbance
around 580 nm which represents the typical band to band transition for αꢀFe2O3. The band gap
energy for the αꢀFe2O3 thin film was calculated using the Tauc’s relation: [(αhν)1/n = A(hνꢀEg)],
where A is a constant related to the effective masses of charge carriers, h is the Planck constant, Eg is
the band gap energy, hν is the incident photon energy and 1/n is the exponent that depends on the
nature of the optical transition (n = 0.5 and 2 for direct and indirect transition, respectively). The
Tauc’s plot (Figure 6 inset) indicates the direct band gap energy of 2.15 eV for the deposited αꢀ
Fe2O3 films. The higher band gap energy compared to that of the bulk material (~1.8 eV) is due to
the presence of smaller particles (~50 nm) in our case. The band structure of semiconductors
changes from continuous to discrete pattern with the reduction in crystallite size and the lowest
transition energy (El) is blue shifted with respect to the original band gap energy (Eg). The quantum
confined or blue shifted energy (∆E) can be expressed by the standard equation as:
∆E = El – Eg = (ћ2π2/2ꢁR2) ꢀ (1.8e2/4πεR)
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where, El is the lowest transition level, Eg is the original band gap energy, ћ is the reduced Planck’s
constant, R is the radius of the nanocrystal, ꢁ is the effective mass of electron, ε is the permittivity of
the material and e is the electronic charge. Since (∆E) is inversely proportional to R, the reduction of
crystallite size leads to an increase in the band gap energy, what we have also observed in this case.
3.6. Photochemical activities toward toxic organic dyes
The photocatalytic activity of αꢀFe2O3 thin films was evaluated by observing the photodegradation
of Indigo Carmine (IC) and Rhodamine B (RhB) dyes under simulated irradiation as mentioned in
Section 2.4. The photocatalytic degradations were evaluated by measuring the successive lowering
of characteristic absorbance peaks for IC and RhB at different time intervals in presence of light and
the catalyst. Significant decrease in the absorption intensity with increasing irradiation time was
observed, which are shown in Figure 7a and 8a respectively for the IC and RhB. Two sets of blank
experiments by taking the dye solution in absence of αꢀFe2O3 films but presence of light and dye
solution in presence of αꢀFe2O3 but absence of light were also carried out for each dye to confirm the
role of both catalyst and light in the degradation process.
Appearance of no new absorption peak during the whole process indicates the complete photolysis in
presence of the proposed catalyst. No degradation of these dyes was observed in dark and very slow
degradations were observed in absence of the catalyst, i.e. αꢀFe2O3 thin films. About 90%
degradation of the dye IC was observed after 150 minutes and about 95% degradation of RhB was
achieved after 225 minutes, which is evident from Figure 7b and 8b. Figure 7c and 8c show the
extent of photocatalytic degradation of IC and RhB respectively, by the αꢀFe2O3 thin films in the
form of relative concentration (Ct/C0) change with irradiation time under three different conditions
viz. (a) in absence of both light and catalyst), (b) in presence of light but in absence of catalyst and
(c) in presence of both light and catalyst. Here C0 is the initial concentration of the dye and Ct is the
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concentration of the dye at time ‘t’. To determine the nature of the degradation kinetics, Ln (C0/Ct)
was plotted against irradiation time (Figure 7d and 8d) and the negative slopes in each case indicate
pseudoꢀfirst order kinetics. The rate constant ‘k’ for IC was found to be about 0.0188 minꢀ1 and the
same for RhB was about 0.0133 minꢀ1. These two values are notably higher than many other
previous reports (Table 2) on the degradation of IC and RhB. Significant enhancement of
degradation rate in presence of both catalyst and light, as seen from the Figures 7c and 8c, indicate
that αꢀFe2O3 thin films can act as an excellent photocatalyst towards the degradation of Indigo
Carmine and Rhodamine B dyes under visible light irradiation.
Figure 9 depicts the plausible schematic band structure and charge transfer process for the generation
of active species leading to the photodegradation of organic dye molecules on the surface of αꢀFe2O3
nanoparticles. Firstly, electronꢀhole pairs are generated in αꢀFe2O3 nanoparticles on light
irradiation49,50 and the electrons occupy the conduction band (CB) and holes the valence band (VB).
The photogenerated electrons (e−) thus produced in the conduction band of αꢀFe2O3 is scavenged by
O2 to produce O2• − anion radical which on protonation yields HOO• in presence of water. On the
other hand, the holes (h+) which are produced in the valence band of αꢀFe2O3 reacts with H2O or
−OH ions coming from H2O to produce highly active species •OH radical. The hydroxyl radicals thus
produced are responsible for the complete mineralization of dye thereby producing products like
CO2 and H2O. The degradation processes normally follow both oxidative (using holes) and reductive
(using electrons) pathways which may be summarized as follows:51ꢀ53
αꢀFe2O3 + hν → e−CB + h+
VB
• −
e−CB + O2 → O2
O2• − + H+ → HOO•
HOO• → H2O2 + O2
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H2O2 + e− → •OH + −OH
h+VB + OH−/H2O → •OH
•OH + dyeꢀintermediates → CO2 + H2O
3.6.1. Evaluation of active oxidative species:
In the studies photocatalysis, there are two common ways for the degradation of organic pollutants,
through direct oxidation reaction by the photogenerated holes and the other one is through oxidative
radicals. It can also be concluded that the activity of a particular catalyst will be considered as higher
•−
•
if it generates reactive species like O2 and OH at higher rate54,55. Here, the hydroxyl radicals
perform the key role for the decomposition reactions mentioned above. So, it is necessary to
investigate the presence of hydroxyl radicals in the medium produced by the catalyst, to support the
•
above mechanism. We have used fuorescence probe technique to establish the formation of OH
radicals by the deposited αꢀFe2O3 films, where terephthalic acid (TPA) was taken as the fuorescence
probe. TPA was illuminated in the same light source used for photocatalysis in slightly alkaline
medium (pH ≈ 8). The fuorescence intensity was measured by a fluorescence spectrophotometer and
the emission spectrum upon excitation at 320 nm was measured every 10 min. interval during
irradiation (Figure 10). TPA is directly attacked by the •OH radicals to form 2ꢀhydroxyꢀterephthalic
acid (TPAOH), giving the fluorescence signal at ~425 nm. The fluorescent intensity is directly
related to the number of hydroxyl radicals formed by the photocatalyst, i.e. αꢀFe2O3 in this case. This
means, higher the rate of hydroxyl radical formation, more is the yield of TPAOH and more intense
will be the fluorescence peak. Intense fluorescence signal on the other hand indicates enrichment in
degradation efficiency.
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3.6.2. Reactive Oxygen Species (ROS) scavenging studies:
In order to get an insight of the mechanism of photocatalytic dye degradation by αꢀFe2O3 thin films,
specific ROS scavengers e.g., tertiary butyl alcohol (TBA) as hydroxyl radical scavenger was added
in the reaction medium. As the ROS generation study was based on scavenging action, it is
necessary to confirm that the evidence for the photocatalytic mechanism, so, TBA has been chosen
•
for the detection and quantification of OH radicals. The effect of different concentrations of TBA
(0.01M, 0.1M and 1M) on dye degradation was observed and the results showed quenching of
degradation efficiency of both IC and Rh B with increase in the concentration of the scavenger
(Figure 11). Therefore, it can be concluded that the drastic inhibition of dye degradation is due to the
scavenging of hydroxyl species (•OH) which were produced in the reaction medium by the αꢀFe2O3
films. The consequences of mimicking hydroxyl radical generation towards dye degradation with
radical scavenger in various concentrations affect the rate of the reactions. These are the evidences
behind proposed photocatalytic mechanism.
3.7. Comparative study on the methanol oxidation by as deposited and Ptꢀactivated αꢀFe2O3 thin
films:
Electrocatalytic activities of bare FTO/αꢀFe2O3 and Ptꢀactivated αꢀFe2O3/FTO electrodes (will be
designated as FTO/αꢀFe2O3ꢀPt hereafter) towards methanol were also studied. These measurements
were carried out using conventional three electrode systems where a platinum wire acted as the
counter and an Ag/AgCl electrode was acted as the reference electrode. The FTO/αꢀFe2O3 and
FTO/PtꢀαꢀFe2O3 electrodes were taken as the working electrodes, for each case. The reaction bath
was prepared by taking 10 mL 0.1 M phosphate buffer solution (PBS, pH= 7.4) and 10 mL of 1 M
CH3OH in a 25 mL beaker, in which the electrodes were dipped and the solution was degassed by
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purging pure N2 for 15 mins. Figures 12a, 12b and 12c represent the cyclic voltammograms (CV) of
blank FTO, FTO/αꢀFe2O3 and FTO/αꢀFe2O3ꢀPt electrodes in this working solution. The
characteristics CV curve for the FTOꢀPBS system shows maximum current in nanoꢀampere range
(Figure 12a), whereas, when the FTO electrode was modified with αꢀFe2O3, it was found to show
moderate enhancement in current to microꢀampere (Figure 12b). Notable increase in peak current
was observed (Figure 12c) when the FTO/αꢀFe2O3 electrode was further modified with nanoꢀPt to
produce FTO/αꢀFe2O3ꢀPt electrode. From the CV studies, it is clear that the catalytic activity of
FTO/Pt–αꢀFe2O3 electrode towards methanol is highest among the three. The less reactivity of
FTO/αꢀFe2O3 electrode towards methanol might be attributed to the restricted movement of electrons
from the surface to the bulk and also due to the poisoning of the electrode in presence of the
oxidized byproducts.
Figure 12d represents the cyclic voltammograms of FTO/αꢀFe2O3ꢀPt in the same working solution
with gradual addition of 10, 30, 50, 100, 200, 300, 400, 500, 1000, 1500 and 2000 micromolar
methanol solutions respectively. Steady increase in current occurs with increasing methanol
concentration which indicates stability of the FTO/αꢀFe2O3ꢀPt electrode even with gradual addition
of methanol. In this process, the anodic current (ipc) increases sharply due to the fast electroꢀ
oxidation of methanol on the electrode surface and reaches the anodic current height (ipa) at the peak
potential around ꢀ1.0 V (Vs. Ag/AgCl). The electroꢀoxidation of methanol was significantly fast
within the potential range ꢀ0.85 to ꢀ1.0 V (Vs. Ag/AgCl). The CV curves also indicate the quasiꢀ
reversibility of the electroꢀoxidation process using FTO/αꢀFe2O3ꢀPt electrodes with progressive
increase in analyte concentration where, the anodic peak current (ipa) increases with concomitant
increase in cathodic peak current (ipc) to achieve the quasiꢀreversibility. Thus, the Pt nanostructures
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on the αꢀFe2O3 serve as the foreign domains for modifying the physicochemical properties and
consequently enhance the methanol electroꢀoxidation reactions.
3.7.1. Explanation for enhanced electroꢀoxidation by Ptꢀactivation:
The activity of FTO/αꢀFe2O3 electrode may be enhanced by modifying the electrode surface with
noble metal nanoparticles like Pt, which helps in the chemisorption of oxidized byproducts like
carbon monoxide (CO) on the metal surface by the following reactions9,19,56, thus minimizing
catalyst (αꢀFe2O3) poisoning.
Pt + CH3OH → PtꢀCOads + 4H+ + 4eꢀ
αꢀFe2O3 + H2O → αꢀFe2O3ꢀOH + H+ + eꢀ
Coupling Pt with αꢀFe2O3 increases the rate of methanol oxidation by decreasing the cathodic peak
current and increasing the anodic peak current. Continuous removal of adsorbed CO from the Pt
active sites also favors the oxidation. During the electrochemical process, reactions between the
chemisorbed CO and OHꢀ take place to produce CO2, which is removed from the electrode surface
according to the following scheme and the Pt active sites roll back to their initial positions:
PtꢀCO + αꢀFe2O3ꢀOH → Pt + αꢀFe2O3 + CO2 + H+ + eꢀ
From AFM analyses (Section 3.2), it has been observed that the activation of αꢀFe2O3 film surface
by Pt nanoparticles increases both RMS and average roughness significantly. Enhanced surface
roughness in turn helps in better adsorption of the analyte molecules on the electrode surface,
leading to better electrochemical performances.
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On the other hand, the decoration of αꢀFe2O3 surface by noble metal, in this case Pt nanoparticles,
can enhance the electroꢀoxidation performance due to the difference in Fermi levels (Ef) of αꢀFe2O3
and the metal nanoparticles, where the energy difference at the semiconductor/metal interface drives
the electrons from the CB of the αꢀFe2O3 into the metal nanoparticles (Figure 13). In other words, the
metal acts as an electron trap, promoting interfacial charge transfer and therefore minimizing
recombination of the eꢀ/h+ pairs, as shown in Figure 13. The effective band alignment between the
semiconductor (Fe2O3) and metal (Pt) in presence of the redox system helps in better electrocatalytic
activity towards methanol oxidation.
4. Conclusion
Polycrystalline αꢀFe2O3 thin films with hematite (hexagonal) phase were deposited on FTO coated
glass substrates by a modified electrochemical technique. The morphology of the deposited films
was highly dense with a regular distribution of the spherical nanoparticles. The material showed
direct band gap energy of about 2.2 eV. Such films were found to degrade efficiently to toxic dyes,
viz. Indigo Carmine and Rhodamine B in presence of visible light. The degradation followed pseudo
first order kinetics and the measured rate coefficient has been found to be 0.0188 minꢀ1 for Indigo
Carmine and 0.0133 minꢀ1 for Rhodamine B respectively. After activating the surface of the FTO/αꢀ
Fe2O3 electrodes with Pt nanoparticles, significant enhancement in methanol oxidation has been
observed. This indicates, the deposited thin films can act as a good photocatalyst for the degradation
of Indigo Carmine and Rhodamine B, as well as it is a good candidate for the methanol detection and
electroꢀoxidation.
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Acknowlwdgements:
The author B.S would like to acknowledge the Department of Chemistry, IIEST, Shibpur and the
Department of Chemistry, Jadavpur University, for infrastructural support.
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Figure Captions:
Figure 1: Xꢀray diffraction patterns of (a) as deposited αꢀFe2O3 thin film and (b) Ptꢀactivated αꢀ
Fe2O3 thin film.
Figure 2: Microstructural analyses. (a) FESEM and (b) AFM images of as deposited αꢀFe2O3 thin
films. (c) FESEM and (d) AFM images of Ptꢀactivated αꢀFe2O3 thin films.
Figure 3: EPR analyses. Plots for (a) Lande gꢀfactor vs. intensity and (b) magnetic field strength vs.
intensity.
Figure 4: FTIR spectrum of as deposited αꢀFe2O3 thin film.
Figure 5: Raman Spectrum of as deposited αꢀFe2O3 thin film.
Figure 6: Absorption spectrum of as deposited αꢀFe2O3 thin film (inset: Tauc’s plot for band gap
determination).
Figure 7: For Indigo Carmine (a) Spectrophotometric degradation curves in presence of light and
catalyst (b) degradation efficacy with respect to time (c) relative change in concentration with time
and (d) rate kinetics curves.
Figure 8: For Rhodamine B (a) Spectrophotometric degradation curves in presence of light and
catalyst (b) degradation efficacy with respect to time (c) relative change in concentration with time
and (d) rate kinetics curves.
Figure 9: Schematic of active species formation by the absorption of light in αꢀFe2O3 nanoparticles.
•
Figure 10: Fluorescence spectrum upon excitation at 320 nm showing OH radical trapping in
2×10−5 M alkaline solution of terephthalic acid.
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Figure 11: Relative concentration profile of photocatalytic degradation of (a) IC and (b) Rh B as a
function of irradiation time in presence of αꢀFe2O3 thin films and different concentration of the
scavenger.
Figure 12: Cyclic Voltammograms in 10 mL 0.1 M PBS and 10 mL 1.0 M CH3OH for (a) blank
FTO (b) FTO/αꢀFe2O3 (c) FTO/αꢀFe2O3ꢀPt electrodes and (d) Cyclic Voltammograms of FTO/αꢀ
Fe2O3ꢀPt electrode with successive addition of CH3OH with different concentrations.
Figure 13: Schematic for energy level alignment and electron trapping by Pt nanoparticles towards
enhanced methanol electroꢀoxidation.
Table Caption:
Table 1: Comparative discussion on the parameters obtained from XRD, FESEM and AFM
analyses.
Table 2: Comparison of photocatalytic performances of different systems towards RhB and IC.
Table 3: Comparison of methanol electrooxidation properties.
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Table 1:
Root mean
square (RMS)
roughness
(nm) obtained
from AFM
Crystallite size
(nm)
calculated
from XRD
Average (Ra)
Grain size
(nm) obtained (nm) obtained
from FESEM
40 – 60
Grain size
Name of the
sample
roughness
(nm) obtained
from AFM
from AFM
αꢀFe2O3 thin
film
10
50
8.055
6.208
9.148
Pt activated αꢀ
Fe2O3 thin
film
20 for Pt
50 for Pt
70 for Pt
14.648
Table 2:
Sl. No.
Type of Catalysts
Dyes Degradation
time (min)
Degradation
References
Rate (min-1)
0.01719
1
TitaniaꢀSilica Nanosol
SiO2@TiO2 (590 nm)
SiO2@TiO2 (470 nm)
SiO2@TiO2 (220 nm)
Peptide I Xerogel
RhB
240
720
480
480
1440
150
180
41
0.00348
0.00465
0.00473
2
3
4
5
RhB
Not Mentioned
Not Mentioned
0.01237
42
43
44
45
50 At% Bi2WO6/BiOBr RhB
LaFeO3
Bi2WO6
RhB
RhB
1×10ꢀ4 MLꢀ1
5×10ꢀ5 MLꢀ1
1×10ꢀ5 MLꢀ1
5×10ꢀ6 MLꢀ1
1×10ꢀ6 MLꢀ1
TiO2ꢀ Zeolite (TZ)
Composite
Nb2O5
60
60
60
60
60
0.0146
0.0218
0.0517
0.0592
0.0714
0.0155
6
RhB
360
46
7
8
9
IC
IC
IC
90
90
150
Not Mentioned
0.33
47
48
Ti/TiO2
0.0188
This Work
αꢀFe2O3
10
RhB
225
0.0133
This Work
αꢀFe2O3
28

Page 29 of 36
RSC Advances
View Article Online
DOI: 10.1039/C6RA13385A
Table 3:
Sl. No.
Composition of the electrode material
Sensitivity
References
1
PtꢀCeO2
0.45 mA/cm2
57
10% Pt/TiO2ꢀNFs
16% Pt/TiO2ꢀNFs
21% Pt/TiO2ꢀNFs
27% Pt/TiO2ꢀNFs
Pt/C
0.110 mA/cm2
0.161 mA/cm2
0.336 mA/cm2
0.277 mA/cm2
0.259 mA/cm2
0.1 mA/cm2
2
58
3
5
PtꢀCePO4
59
PtꢀFe2O3
0.748 mA/cm2
This Work
29