ScienceDirect Photo-Fenton Degradation of Methyl Orange Using Hematite (α-Fe 2 O 3 ) of Various Morphologies

Abstract

In this work, the effects of type of additives on the morphology and corresponding photocatalytic property of various hematite (α-Fe2O3) nanostructures were investigated. α-Fe2O3 croissant-like structures, urchin-like structures, and textured microspheres were formed by hydrothermal treatment at 120 °C for 6 h in the presence of NaCl, Na2SO4 , and Na2C2O4 as additives, respectively. After heat treatment in air, the photocatalytic activity of the α-Fe2O3 powders was assessed by degrading methyl orange (MO) under UV-C lamp with hydrogen peroxide (H2O2) as activator. The urchin-like α-Fe2O3 hierarchical structures exhibited the best photocatalytic behavior with a 76.5% removal of initial 2.5 ppm MO after 2 h irradiation. This is attributed to the high surface area of the urchin-like morphology which provides more active sites for degradation of MO. The reaction kinetics correspond well to the Langmuir-Hinshelwood model, which indicates that adsorption of MO on the surface of catalyst occurs as a monolayer.

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Materials Today: Proceedings 22 (2020) 248–254
www.materialstoday.com/proceedings
1876-6102 © 2019 Elsevier Ltd. All rights reserved.
Peer-review under responsibility of the scientific committee of the 2018 2nd International Conference on Nanomaterials and Biomaterials.
2018 2nd International Conference on Nanomaterials and Biomaterials, ICNB 2018,
10–12 December 2018, Barcelona, Spain
Photo-Fenton Degradation of Methyl Orange Using Hematite
(α-Fe2O3) of Various Morphologies
Anthea Marika G. Domacena, Christian Laurence E. Aquino,
Mary Donnabelle L. Balela*
Sustainable Electronic Materials Group, University of the Philippines, Diliman, Quezon City 1101, Philippines
Abstract
In this work, the effects of type of additives on the morphology and corresponding photocatalytic property of various hematite (α-
Fe2O3) nanostructures were investigated. α-Fe2O3 croissant-like structures, urchin-like structures, and textured microspheres were
formed by hydrothermal treatment at 120 °C for 6 h in the presence of NaCl, Na2SO4, and Na2C2O4 as additives, respectively.
After heat treatment in air, the photocatalytic activity of the α-Fe2O3 powders was assessed by degrading methyl orange (MO)
under UV-C lamp with hydrogen peroxide (H2O2) as activator. The urchin-like α-Fe2O3 hierarchical structures exhibited the best
photocatalytic behavior with a 76.5% removal of initial 2.5 ppm MO after 2 h irradiation. This is attributed to the high surface
area of the urchin-like morphology which provides more active sites for degradation of MO. The reaction kinetics correspond
well to the Langmuir-Hinshelwood model, which indicates that adsorption of MO on the surface of catalyst occurs as a
monolayer.
© 2019 Elsevier Ltd. All rights reserved.
Peer-review under responsibility of the scientific committee of the 2018 2nd International Conference on Nanomaterials and
Biomaterials.
Keywords: photo-fenton; hematite; photocatalysis, urchin-like nanostructures
* Corresponding author. Tel.: +63-02-9818500 local 3132; fax: +63-02-9818500 local 3164.
E-mail address: mlbalela1@up.edu.ph

A.M.G. Domacena et al. / Materials Today: Proceedings 22 (2020) 248–254 249
1. Introduction
Dyes are used to provide color to various materials such as papers, textiles, and plastics [1]. In the past, dyes were
made in small scale from natural sources like plants and insects. When synthetic dyes were discovered, their use
became more prevalent due to their low cost and ease of production. However, synthetic dyes are composed of
hazardous compounds, which cannot be easily degraded due to their stability [2-3]. There are currently around
10,000 dyes and pigments in production, yielding more than 7x108 kg per year worldwide [2]. This results to a
significant amount of discharged colored wastewater, containing around 10-20% of the used dyes [2-3]. The textile
industry, by itself, consumes around 80-200 m3 of water per ton of product and produces 1650 m3 of wastewater
each day. Consequently, it discharges around 280,000 tons of dyes in its effluent per year [4].
Conventional methods of treating dye-laden wastewater that are currently being employed include coagulation
and flocculation which require physical separation methods through screening due to the production of sludges.
Photocatalytic degradation is a promising method in treating these contaminated wastewaters [5-7]. The process is
considerably easier to handle and does not produce secondary sludges. Photocatalytic degradation is the
decomposition of a substance using a semiconductor photocatalyst by harnessing light energy to accelerate the
reaction. In the photodegradation of dyes, light energy excites the electrons of the catalyst thereby producing
hydroxyl radicals which can oxidize organic substances, such as dyes, causing degradation [4].
Hematite (α-Fe2O3) is a promising semiconductor photocatalyst that has the advantage of being abundant, cheap,
and environment-friendly [8-12]. It is the most thermodynamically stable iron oxide with a band gap of about 2.1
eV, which theoretically allows it to absorb light energy on a wide range of wavelengths. However, its photocatalytic
efficiency is limited by its very fast photo-excited state lifetime (10-12 s) [13] and short diffusion length (<10 nm)
[14,15]. Morphological engineering and doping have been effective in addressing these issues by providing shorter
paths for the electron-hole pairs to migrate to the surface and inhibiting charge recombination [6, 13, 16].
Solution-based methods offer a simple and economical way of synthesizing α-Fe2O3 nanostructures.
Hydrothermal synthesis, in particular, is also more advantageous as it only involves the use of water as solvent
compared to other wet chemical methods that employ potentially harmful organic substances. α-Fe2O3 with varying
morphologies have been successfully prepared by hydrothermal process [6,11,16]. However, the effect of various
morphologies in the photocatalytic degradation of dyes, specifically methyl orange, has not been investigated to the
best of our knowledge. In this work, the effect of different additives on the morphology and structure of α-Fe2O3 was
investigated. Assessment of the photocatalytic property of the as-synthesized powders was also done through
degradation of methyl orange in the presence of UV light.
2. Methodology
2.1. Materials
Chemical reagents in the study were used without further purification. Anhydrous iron chloride (FeCl3; HiMedia
Laboratories; 98% purity) was used as Fe source. The additives were sodium chloride (NaCl; EMD Millipore; 99.5%
purity), sodium sulfate (Na2SO4; Ajax Finechem; 99% purity), and sodium oxalate (Na2C2O4; B. E. Chemicals;
assumed to be of 99% purity). Absolute ethanol (C2H6O; Univar; Analytical Grade) was used for washing of
precipitates. Meanwhile, methyl orange (C14H14N3NaO3S; Loba Chemie; 95% purity) was used as model pollutant.
Hydrogen peroxide (H2O2; Rhea, 10 volumes) was used as activator for α-Fe2O3 during photocatalysis.
2.2. Synthesis
2 mmol of anhydrous FeCl3 was mixed with 2 mmol of sodium salt (NaCl, Na
2SO4, Na2C2O4) in 40 mL of
deionized (DI) water. The solutions were magnetically stirred at 400 rpm for 30 min until complete dissolution and
then transferred to a Teflon-lined stainless-steel autoclave reactor. The reactor was heated at 120 °C for 6 h. After
cooling naturally, the powders were collected through centrifugation at 5000 rpm for 30 min and washed with DI
water three times followed by absolute ethanol. Drying of powders was done at 80 °C for 1 h before annealing in air

250 A.M.G. Domacena et al. / Materials Today: Proceedings 22 (2020) 248–254
at 400 °C for 2 h, with a ramp rate of 2 °C/min. The samples were labelled as Samples A, B, and C for the powders
synthesized with NaCl, Na2SO4, and Na2C2O4, respectively.
2.3. Photocatalytic Degradation
The photocatalytic properties of the α-Fe2O3 microstructures were investigated through the photocatalytic
degradation of methyl orange. 30 mg of the α-Fe2O3 samples was added to 30 mL of 5 ppm methyl orange. The
mixtures were magnetically stirred for 30 min in the dark to allow for adsorption equilibrium before the addition of
30 μL of H2O2. Photocatalytic degradation was done by irradiation of the mixtures for 2 h under two 10 W UV-C
lamps (Sankyo Denki G10T8, λ=253.7 nm) inside a blackbox set-up. Filtration and centrifugation were used to
separate solids from the solution. The sample which exhibited the greatest degradation capability was used for
kinetic study. To investigate the kinetics of the degradation, 30 mg of the chosen sample was added to different
concentrations (2.5, 5.0, and 7.5 ppm) of methyl orange solution. The mixtures were similarly stirred in the dark for
30 min. Then, 30 μL H2O2 was added to each mixture before irradiation. Aliquots of about 2 mL each were obtained
at set time intervals (0, 15, 30, 45, 60, 75, 90, 105, 120 min.) from each mixture.
2.4 Characterization
The synthesized α-Fe2O3 powders were observed in a scanning electron microscope (SEM, Jeol JSM-5310). The
phase composition and crystal structure of the products were characterized by X-ray diffraction (XRD, Shimadzu
7000) using Cu Kα radiation. The concentration of methyl orange was determined using a UV-Visible
spectrophotometer (Ocean Optics). The absorbance of the MO solutions was taken at a peak wavelength of 465 nm.
3. Results and Discussion
3.1. Structural and Morphological Characterization
Fig. 1 shows the XRD patterns of α-Fe2O3 structures prepared with different additives after annealing at 400 °C.
All patterns show major peaks at 2θ = 33.2, 35.7, 49.5, 54.1, 62.5, 64.1° which correspond to the (104), (110), (024),
(116), (214), and (300) peaks of α-Fe2O3. The diffraction peaks of all samples were found to correspond to
rhombohedral α-Fe2O3 (PDF No. 84-0307). Difference in peak intensities can be observed from the XRD patterns,
indicating the influence of additives on the crystallinity of the α-Fe2O3 structures. The XRD peaks of sample A
(NaCl) appear sharper compared to samples B (Na2SO4) and C (Na2C2O4) as seen in Fig. 1a. Additionally, the
maximum peak is indexed to the (104) plane, which corresponds well with the standard diffraction pattern of α-
Fe2O3. On the other hand, the (110) plane was the strongest peak in the diffraction pattern of the sample with
Na2SO4 [11,16]. For sample C, the (104) and (110) planes exhibit almost similar intensity. The deviation of the
intensity ratios from the standard diffraction pattern may be attributed to the apparent anisotropic growth of the
structures as shown in Fig 2 (b-c). The formation of nanorods as seen in Fig. 2 (b) suggests a preferential growth in a
particular direction. Peak broadening was observed for samples B and C. This indicates smaller crystallite sizes
compared to sample A. Using the Scherrer equation, the crystallite size for samples A, B, and C were calculated to
be about 33.95, 21.30, and 14.60 nm respectively. In addition, the XRD pattern of sample A show an extra minor
peak at 2θ=44.0°. This is attributed to NaCl (PDF 5-628), which indicates that some of the additive in the synthesis
might also have crystallized during the process.
Fig. 2 shows SEM images of the annealed α-Fe2O3 samples. α-Fe2O3 croissant-like structures with a mean length
of about 419 nm and an average width of 264 nm were formed for sample A, as seen in Fig. 2 (a). The presence of
cleavages at the surface of the grown particles are strongly suggestive of agglomeration. After the nucleation of the
individual nanoparticles, their aggregation was facilitated by the Cl- ions in the solution. It is known that Cl- has a
very strong coordinative property with Fe3+ which enables the aggregation of fine primary particles [17]. Meanwhile,
the urchinlike structures formed in sample B were similar to those shown in previous researches [11, 16]. These
structures have a spherical body from which nanorods point outward like needles, creating a morphology resembling
an urchin. The hydrolysis of Fe3+ ions results to the formation of nanoparticles which interconnect into spheres. An

A.M.G. Domacena et al. / Materials Today: Proceedings 22 (2020) 248–254 251
electrical double layer around the spheres is formed by the acidic solution and the SO4
2- ions [16]. These anions
attract the other Fe3+ ions in the solution and allow for the epitaxial growth of nanorods onto the surface of the
microsphere [16]. The microspheres have a mean diameter of 4.13 μm, while the needles have an average length of
321 nm. Sample C show textured microspheres with a mean diameter of 1.63 μm as seen in Fig. 2 (c). The textured
surface is due to short and thick rods growing on the surface of the spheres. The C2O4
2- ions possibly coordinate with
Fe3+ similar to the effect of SO4
2-. However, it is apparent that coordination between C2O4
2- and Fe3+ ions are not
strong, leading to thick and short rods compared to that in Fig. 2 (b).
Fig. 1. XRD Patterns of α-Fe2O3 structures prepared with different additives: (a) A [NaCl], (b) B [Na2SO4,] and (c) C [Na2C2O4.].
Fig. 2. SEM images of α-Fe2O3 structures prepared with different additives: (a) A [NaCl], (b) B [Na2SO4,] and (c) C [Na2C2O4.].
3.2. Photocatalytic Property of α-Fe2O3 structures
The absorbance spectra of MO with and without H2O2, and with α-Fe2O3/H2O2 samples are shown in Fig. 3. The
H2O2 was used as an activator for the α-Fe2O3 catalysts. α-Fe2O3 cannot degrade MO by itself efficiently because it
degrades through the photo-Fenton process. The mechanism for the initiation of degradation is as follows: [11, 16].
𝐹𝑒𝑂 → 𝐹𝑒𝑂 𝑒,ℎ (1)
𝐹𝑒  𝑒 → 𝐹𝑒 (2)
𝐹𝑒 𝐻𝑂→𝐹𝑒 𝑂𝐻•𝑂𝐻 (3)

252 A.M.G. Domacena et al. / Materials Today: Proceedings 22 (2020) 248–254
Upon irradiation of the catalyst, photoexcitation happens which produces electron hole pairs (Eq.1). The excited
electron then reduces Fe3+ to Fe2+ which consequently reacts with H2O2 to produce the Fenton reagent, OH• (Eq. 2
and 3). The produced radical species is responsible for the degradation of the dye [11, 16].
It can be observed from Fig. 3 that Sample B was able to substantially degrade the MO dye after 2 h of exposure
to UV-C light. This is possibly due to its high surface area [19]. The nanorods radiating out of the spheres could
have provided more sites for adsorption of H2O2, thus producing more OH•. Meanwhile, samples A and C were able
to degrade the dye by only a small degree. It is interesting to note that H
2O2 alone actually registered a higher
degradation of MO than with the presence of samples A and C. H2O2 in the presence of UV light can also dissociate
and produce OH• radicals. Intuitively, samples A and C in the presence of H2O2 would most likely degrade more
MO than with H2O2 alone does. However, after irradiation, it was observed that only sample B settled at the bottom
of the container of MO while samples A and C were left dispersed. The dispersed particles may have occluded the
whole mixture thereby impeding efficient penetration of light throughout the vessel. Only those at the top most
accessible to UV irradiation may have successfully absorbed photon energy and thus contribute to the degradation of
MO. Having different morphologies and sizes, the samples exhibited different properties when mixed in a solution.
An excess dose of catalyst promotes aggregation, light scattering, and turbidity, thereby decreasing light penetration
in the solution [7].
Fig. 3. Absorbance spectra of 5 ppm MO containing the three samples and the control set-ups after 2 h irradiation in UV light
3.3. Degradation Kinetics
The time-dependent degradation of 2.5, 5.0, and 7.5 ppm MO solutions containing α-Fe2O3 Sample B was
investigated. Shown in Fig. 4 and 5(a) are the absorption spectra and percent removal vs. time for the three
concentrations. The degraded fraction was computed using Eq. 4. C0 was taken to be the concentration of the system
at time zero, while C was taken to be the concentration at the actual time of measurement.
Percent removal 100   
∗100 (4)
It can be observed from the figures that all solutions were degraded over time. The solution concentration
decreased as irradiation time is increased. Kinetic investigation of the degradation of the different systems was also
done to determine the fit of the reaction to the Langmuir-Hinshelwood model. A good fit was obtained for all
systems, evidenced by the R2 values ranging from 0.93 to 0.97 as seen in Fig. 5(b). This strongly suggests that the
adsorption of OH• and MO molecules occurs as a monolayer on the α-Fe
2O3 surface. A monolayer adsorption
highlights the advantage of the large surface area of the urchin-like structure. However, the Langmuir-Hinshelwood

A.M.G. Domacena et al. / Materials Today: Proceedings 22 (2020) 248–254 253
model is limited to the description of the surface reaction on the catalyst. It does not consider interactions involving
free molecules, i.e. the degradation of adsorbed MO by free OH• [20].
Fig. 4. Absorbance spectra of the photodegradation of MO using sample B at different initial concentrations.
The rate constants for the 2.5, 5.0, and 7.5 ppm MO systems were calculated to be 0.0118, 0.0071, and 0.0058
min-1 respectively. This shows that the α-Fe2O3 urchin-like structures are able to degrade MO faster at lower dye
concentrations. This is attributed to fewer organic molecules to be oxidized and degraded by OH• radicals produced
from the photocatalytic reaction. Conversely, a higher amount of organic molecules in solution also impedes the
efficient reaction between H2O2 and Fe2+ resulting to less OH• available for the degradation of MO. In addition, this
also decreases photon absorption by the photocatalyst [7]. Nevertheless, in all systems, the dye was degraded
substantially by the urchin-like α-Fe2O3 photocatalysts. This demonstrates the promising capability of such α-Fe2O3
structures in the degradation of organic pollutants in water.
Fig. 5. (a) Percent removal of MO at various initial concentrations and (b) Langmuir-Hinshelwood Model for the three systems.
4. Conclusion
This work presents the synthesis of α-Fe2O3 structures with different morphologies by varying the type of additive
during hydrothermal synthesis. Croissant-like microstructures, urchin-like structures, and textured microspheres
were formed using NaCl, Na2SO4, and Na2C2O4 as additives, respectively. It was shown that photocatalytic property
is greatly affected by the morphology of the samples with urchin-like hierarchical structures exhibiting the best
photocatalytic activity for the degradation of MO. The urchin-like structures provide higher surface areas than the
(a) (b)

254 A.M.G. Domacena et al. / Materials Today: Proceedings 22 (2020) 248–254
croissant-like structures and textured microspheres. Kinetic studies show that photocatalytic degradation of MO is
more enhanced at lower concentrations due to greater photon absorption of the photocatalyst and efficient interaction
between H2O2 and Fe2+.
Acknowledgements
The authors would like to acknowledge the Engineering Research and Development for Technology (ERDT), the
UP Diliman College of Engineering, and the Department of Science and Technology (DOST) through the National
Academy of Science and Technology (NAST) for funding this project through its Grants for Outstanding
Achievements in Science entitled “Hydrothermal Synthesis of Hierarchical Hematite (α-Fe2O3) Nanostructures for
Environmental Cleaning”.
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