Loading of alkaline hydroxide nanoparticles on the surface of Fe2O3 for the promotion of photocatalytic activity

Article abstract of DOI:10.1016/j.jphotochem.2019.01.003

This study focused on surface modification of iron oxide (Fe₂O₃) by loading alkaline oxide nanoparticles to investigate the effect of surface interactions on photocatalytic activity of this material. Fe₂O₃ was loaded by NaOH, KOH, and CsOH, with 0.05, 0.1, 0.4, and 1 wt% by impregnation method. The photocatalytic evaluation was carried out in a gas phase for degradation of Iso-Propyl alcohol (IPA) to acetone. The prepared samples were characterized by X-ray diffraction, UV–vis diffuse reflectance spectroscopy, and scanning electron microscopy SEM. The photocatalytic evaluation resulted the photoactivity of Fe₂O₃ increases to an optimum extent by loading of alkaline hydroxides on the surface. The enhancement of photoactivity of Fe₂O₃ after loading can be attributed to the surface chemical reactions and transfer of photogenerated electrons from the conduction band of Fe₂O₃ to alkaline nanoparticles. Also, multi-electron oxygen reduction is another way supposed to facilitate the photocatalytic reaction.

Full text of DOI:10.1016/j.jphotochem.2019.01.003

Accepted Manuscript
Title: Loading of alkaline hydroxide nanoparticles on the
surface of Fe2O3 for the promotion of photocatalytic activity
Authors: M. Khajeh Aminian, S.K. Fatah
PII:
S1010-6030(18)31264-4
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2019.01.003
JPC 11662
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Journal of Photochemistry and Photobiology A: Chemistry
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Revised date:
Accepted date:
31 August 2018
3 January 2019
4 January 2019
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Please cite this article as: Aminian MK-ErrorvCtInIinicfImhlt6c/>, Fatah SK, Loading
of alkaline hydroxide nanoparticles on the surface of Fe2O3 for the promotion
of photocatalytic activity, Journal of Photochemistry and amp; Photobiology, A:
Chemistry (2019), https://doi.org/10.1016/j.jphotochem.2019.01.003
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Loading of alkaline hydroxide nanoparticles on the
surface of Fe2O3 for the promotion of photocatalytic
activity
M. Khajeh Aminian1*, S. K. Fatah1,2
1Department of Physics, Yazd University, Yazd, Iran.
2Department of Physics, University of Garmian, Kalar, KRG, Iraq.
*Corresponding Author: kh.aminian@yazd.ac.ir
Highlights
 Iron oxide (Fe2O3) and iron oxide hybrid based materials have attracted attention as a
photocatalyst.
 A new photocatalyst system is designed based on loading of alkaline hydroxide
nanoparticles on Fe2O3 which is sensitive to visible light.
 This photocatalyst system is based on charge transfer between Fe2O3 as a supporter
material and several alkaline oxide materials as a cocatalyst.
 The photocatalytic evaluation reveals that the new system promotes the photoactivity of
Fe2O3 under visible light irradiation.
Abstract
This study focused on surface modification of iron oxide (Fe2O3) by loading alkaline
oxide nanoparticles to investigate the effect of surface interactions on photocatalytic activity of
this material. Fe2O3 was loaded by NaOH, KOH, and CsOH, with 0.05, 0.1, 0.4, and 1 wt% by
impregnation method. The photocatalytic evaluation was carried out in a gas phase for
degradation of Iso-Propyl alcohol (IPA) to acetone. The prepared samples were characterized
1

by X-ray diffraction, UV-Visible diffuse reflectance spectroscopy, and scanning electron
microscopy SEM. The photocatalytic evaluation resulted the photoactivity of Fe2O3 increases
to an optimum extent by loading of alkaline hydroxides on the surface. The enhancement of
photoactivity of Fe2O3 after loading can be attributed to the surface chemical reactions and
transfer of photogenerated electrons from the conduction band of Fe2O3 to alkaline
nanoparticles. Also, multi-electron oxygen reduction is another way supposed to facilitate the
photocatalytic reaction.
Keywords: Fe2O3, KOH, NaOH, CsOH, Photocatalysis, Photodegradation,
Impregnation.
1. Introduction
Many harmful organic pollutants exist in the air, water and soil environments
extensively [1]. Most of the organic pollutants can be degraded on the surface of photocatalyst
materials under solar illumination in the natural environment [2-5]. The conventional
photocatalysts such as TiO2 [6-9], ZnO [9,10] and WO3 [11-15] have a wide band gap and
therefore they are limited in the absorption of solar energy to less than ~ 4%. Thus, their low
material cost would be overshadowed by the expense of enormous area structures required for
the solar exposure in an environmental application. Among the inexpensive metal oxides
absorb visible-light illumination; iron oxide and iron oxide hybrid based materials have
attracted attention as a potential photocatalyst because of its stability, safety and low cost [4,
15-21]. In addition, Fe2O3 (Hematite) has the advantage that its band gap is ~2 eV allowing for
absorption of 40% of the solar spectrum [9,22,23]. Despite the seemingly favorable
characteristics of Hematite, some factors limit its performance. The maximum solar to
chemical efficiency reported is less than 3% under AM 1.5 illumination [16]. The low
2

efficiency may be a result of these factors: (1) low electron mobility (~0.1 cm2V-1S-1) [18, 24,
25], which reduces the electron collection efficiency, (2) high electron-hole recombination
rates [9, 21,22,23,26, 27], (3) high conduction band energy potential (~0.2 V vs NHE), and (4)
anisotropic conductivity, which is 4 orders of magnitude higher within the [001] plane than
parallel to [001] [25, 28-30].
To enhance the photoactivity of Fe2O3, it needs to think about the mentioned challenges
above. Recombination in low mobility materials can be minimized by decreasing the particle
size to less than the minority carrier diffusion length [31]; for Fe2O3-Hematite, the hole
diffusion length was reported less than 20 nm [32-34]. Therefore, fabrication of Fe2O3-
Hematite nanostructures is a way to enhance the photoactivity of it [23]. Grätzel and his
colleagues have reported a high water photooxidation property by dendritic Fe2O3
nanostructures having a feature size of 10 to 20 nm at the surface [35]. The surface of this
material was treated with Co(NO3)2 solution to load a monolayer of Co atoms on the surface.
Water photooxidation efficiency of this material was promoted by loading of Co atoms on the
surface. Recently water photooxidation was investigated by Fe2O3 nanoparticles embedded in
clay compound (Na+ treated Montmorillonite) [36]. The efficiency of photooxidation increased
with the loaded photocatalyst until the loading amount reached ca. 3 wt%, and decreased in the
larger loading amount.
High recombination rate and high conduction band energy potential of Fe2O3 can be
influenced simultaneously by loading of a relevant material on the surface of Fe2O3. Loading
of a relevant material on the surface of Fe2O3 may cause transfer of photogenerated electrons
from conduction band (CB) of Fe2O3 to energy levels of this new material as it has been
already reported for different materials [37-39]. This phenomenon reduces the electron-hole
recombination rate in Fe2O3. In addition, the transferred photoelectrons can be excited to a
lower energy potential by absorption of new energy photons.
3

On the basis of this background, we designed a new photocatalyst system sensitive to
visible light. This photocatalyst system is based on charge transfer between Fe2O3 as a
supporter material and several alkaline oxide materials as cocatalysts. The photocatalytic
evaluation shows that the new system promotes the photoactivity under visible light irradiation.
2. Experimental Methods
Three different photocatalyst systems of alkaline oxide/Fe2O3 were prepared by
impregnation method. 100 mg of NaOH, KOH, and CsOH from (Wako corp.) were dissolved
in 100 g of distilled water separately. Four samples of 1.0 g commercial Fe2O3-Hematite
(Wako corp., BET surface area 5.2 m2 g−1) were mixed with 0.3, 0.5, 0.7 and 1 ml NaOH
solution separately. The mixtures were dried on a hot plate and heated at 450 °C for 2h.
Loading of KOH and CsOH on Fe2O3 were performed in the same procedure.
The photocatalytic evaluation was carried out in a cylindrical air-filled static pyrex
glass vessel (500 ml of total volume) for degradation of isopropyl alcohol (IPA) to acetone.
The catalyst powders (~0.6 g) were evenly dispersed on the bottom of a circular glass dish to
have a uniform area of ~8 cm2 and the dish was mounted in the vessel. The vessel was filled
with artificial pure air to replace the CO2-containing natural air. After injection of IPA and
prior to light irradiation, the vessel was kept in the dark for one hour to get adsorption
saturation of IPA on the sample surface. Upon light irradiation, gaseous sample (0.5 ml) was
periodically extracted from the reaction vessel and measured on a gas chromatograph (GC-
14B, Shimadzu) equipped with a flame ionization detector (FID) for analysis of IPA/acetone.
The light source was a 300 W xenon arc lamp (ILC Technology, CERMAX LX-300). A cutoff
filter (L44, Hoya Optics) was used to obtain visible light (λ > 440 nm). UV–visible diffuse
reflectance spectrum was measured on a UV–visible spectrometer (UV-2500, Shimadzu) at
room temperature. A field emission scanning electron microscope (FE-SEM, JEOL-JSM
6500F) was used for morphology observation and X-ray diffractometer (XRD, X’pert Pro;
4

PANalytical Co., Netherlands) was used for determining the crystallization properties of
samples.
3. Results and Discussion
IPA decomposition is a standard reaction for photocatalytic activity evaluation [27]. In the
beginning, the IPA decomposition should be evaluated into acetone by photogenerated holes at
the initial reaction stage [27,40]. In figure 1-a the results of the generated acetone in the
photodegradation process of IPA on the surface of iron oxide (Fe2O3) by loading nanoparticles
of sodium hydroxide (NaOH) has been demonstrated. The figure detailed on the different
amount of NaOH has been used on the surface of Fe2O3 with the amounts of 0.06 wt%, 0.1
wt%, 0.4 wt% and pure Fe2O3. All the samples were heated for 2 h at 450 0C. It can be noticed
from the figure, the samples with the 0.06 wt% and 0.1 wt% of NaOH can improve the
photoactivity of Fe2O3 surface more than samples of 0.4 wt% by increasing acetone
concentration in the reaction. The highest value of acetone concentration was obtained by
sample 0.06 wt%, which is more than 2 times higher than that of the pure Fe2O3 sample.
In order to understand more about surface modification of Fe2O3 with the loading of
alkaline hydroxides, various samples were loaded by KOH and CsOH with the same method of
NaOH samples. Figure 1-b indicates the acetone generation during the process of
photodegradation of IPA reaction on the surface of Fe2O3 with different amounts of 0.05, 0.1
and 0.4 wt% KOH loading. Figure 1-c indicates the same plots with amounts of 0.05, 0.1, 0.3
and 1 wt% of CsOH loading. It can be observed from both figures, the samples with 0.05 and
0.1 wt have the highest photoactivity and they enhanced the photoactivity of Fe2O3 3 times
than that of the pure one. While, 0.4 wt% of both alkaline hydroxides have decreased the
degradation of acetone concentration compared to the pure one, due to the photocatalytic
activity of iron oxide declined. Notably speaking from figure 1-a, 1-b and 1-c loading of
alkaline hydroxides NaOH, KOH, and CsOH have a similar effect on the photoactivity of
5

Fe2O3. However, the samples loaded with KOH are more reactive than that of NaOH and
CsOH, means KOH can enhance the photoactivity the most. It can be observed in figure 2.
This result has corresponded to the same effect which published earlier about WO3 surface
modification [39]. A chemical property is not the only reason for the enhancement of
photoactivity, but also the photoreduction and photooxidation is the reason in the process.
After the acetone generation under the irradiation of visible light, the next process was CO2
generation due to further visible light irradiation. This step of CO2 generation can be described
as the final process of photocatalytic reaction on the surface of alkaline loaded Fe2O3 [27]. In
figure 3 CO2 generated under visible light irradiation for different samples with amounts of
0.05 wt%, 0.1 wt%, 0.4 wt% of KOH and the pure sample of Fe2O3 can be seen. According to
the figure, all samples loaded with KOH have enhanced the photocatalytic properties of Fe2O3
more than that of the pure sample.
The surface area of 0.05 wt% KOH/Fe2O3 was measured by BET analysis about 5.5 m2 g−1.
It can be observed that it is not so different from the origin pure sample (5.2 m2 g−1).
The scanning electron microscopy (SEM) images of Fe2O3 samples loaded with alkaline
nanoparticles can be seen in figure 4. Figure 4-a shows SEM of NaOH/Fe2O3 sample. Some
small particles of NaOH or maybe Na2O, having a size less than 20 nm, have covered the
surface of Fe2O3. Similarly, this case can be observed in figure 4-b for the sample of
KOH/Fe2O3 with some nanoparticles with the size less than 20 nm on the surface. However,
CsOH on the surface of Fe2O3 has formed a bit differently, in figure 4-c can be noticed that the
small sphere-like particles formed on the surface of Fe2O3 which they agglomerated in some
places and made particles with the size between 10-50 nm. The morphology of pure Fe2O3
sample is observed in figure 4-d. It shows the same structure as 4-a, 4-b, and 4-c except for the
number of small particles on the surface. It can be concluded that the small particles do not
belong to the Fe2O3 structure. The histogram of particle size for pure Fe2O3 and 0.05 wt%
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KOH/Fe2O3 was plotted in fig. 5. The average particle size for pure Fe2O3 and 0.05 wt%
KOH/Fe2O3 are 106 and 116 nm respectively. It shows the average particle size does not
change so much after KOH modification. This might be due to the loss of water during reaction
between KOH and Fe2O3 surfaces, this reaction has been explained in reaction (2) in the next
sections.
After photocatalytic process, the SEM images of Fe2O3 loaded alkaline nanoparticles have
been shown in fig. 6. Comparing to fig. 4, it can be observed that the morphology is
approximately same as that of the samples before photocatalytic reaction.
FT-IR spectra has been used to clarify whether alkaline hydroxides are existing on the
surface of Fe2O3. FTIR spectrum of pure Fe2O3 and KOH/Fe2O3 can be seen in fig. 7. The
spectra of samples before and after surface modification are approximately similar to each
other. The range (3000-3500 cm-1) is mainly related to OH groups [41]. In this range the
intensity of the peak has not changed after surface modification.
To investigate the optical properties of alkaline loaded nanoparticles and the pure sample of
Fe2O3, the UV-Visible diffuse reflectance spectroscopy has been used to demonstrate the
reflectance and absorbance of samples. In figure 8-a and 8-b reflectance and the absorbance of
the used samples of 0.05 wt% KOH/Fe2O3 and Fe2O3 loaded by 1wt% KOH have been
illustrated respectively. For comparison, the spectrum of pure Fe2O3 has been indicated. The
reflectance of both samples are similar to each other, except for sample with KOH has less
value of reflectance than the pure one, this may due to the small change attributed in surface
functional groups [27]. Furthermore, the absorbance of samples shows that the edge of
absorbance is very similar to each other and it is approximately at the same wavelength.
Furthermore, the optical band gap of the pure Fe2O3 and KOH loaded Fe2O3 was calculated
from the Tauc plot in (figure 9) which was derived from a reference [42], and the perfect band
7

gap found to be the direct band gap with values 2.2 eV and 2.25 eV respectively. The observed
optical band gap is approximately agreed with published works [16, 18, 33, 43].
To investigate the change in the crystal structure of pure Fe2O3 after loaded with KOH, the
X-ray diffraction was used. In figure 10, the spectra of pure Fe2O3, Fe2O3 loaded with KOH
and after photoreaction have been shown. One can observe the structure of Fe2O3 was
crystallized with a monoclinic structure and this result is in a good agreement with the work
published [16, 32]. Moreover, other phases of iron oxide are not found in the diffraction peaks
and this showed that the impurities are not appeared in the material [32,43]. Then compared
spectra of pure one with loaded KOH showed no difference appears in any peaks. That means
the crystal structure of Fe2O3 was not changed after KOH loading or the quantity of new
composition is very low.
According to the results obtained for the photocatalytic activity of samples loaded with
alkali hydroxide materials, the reason for the increase of photocatalytic activity should be
found in the surface interaction between two materials. We know that metal oxide surfaces are
usually covered with hydroxide groups. Two different reactions were proposed for KOH on
Fe2O3 as below:
Fe2O3 + 6 KOH
Fe2O3 + 2 KOH
2 Fe(OH)3 + 3K2O
2 KFeO2 + H2O(g)
(1)
(2)
According to FTIR spectra in fig. 7 the peaks related to OH groups (3000-3500 cm-1) were
not intensified after surface modification. The FTIR spectra shows less OH groups on the
surface of Fe2O3 due to the modification of KOH on the surface. It shows the first reaction can
not be carried out on the surface. The second reaction (2) is mainly happened on the surface
and some new nanoparticles are existed on the surface. This is in agreement with the report of
L. Khanna et al [41].
8

By investigating the surface reaction between two materials, it can be supposed that in the
single layer of the interface there is an exchange of electrons between the Fe2O3 surface and
the loaded nanoparticles. Since the alkali nanoparticles are mostly smaller than 20 nm, they can
act as cocatalyst material for electron transfer. Actually, it is supposed the connection between
iron oxide and alkali nanoparticles causes the photogenerated electrons are transferred from the
conduction band of iron oxide to the alkali nanoparticles (fig. 11). It is a routine reported
before for different materials [37,38,44].
On the other hand, the photoactivity of iron oxide is low mainly due to the high energy
potential of the conduction band in iron oxide (0.2 V vs NHE~) and the low tendency of
-
conduction band to provide electron for one-electron oxygen reduction (O2 + H+ + e
HO2,
-0.046 V). Therefore, one reason may consider for the enhancement of photodegradation is a
multi-electron reduction. Two-electron oxygen reduction (O2 + 2H+ + 2e-
and O2 + 4H+ + 4 e-
H2O2, +0.68 V
2H2O, +1.23 V vs. NHE pH 0) proceeded over Pt/WO3 and
alkaline/WO3 has been reported before [39, 45, 46]. It seems reasonable to consider such multi-
electron O2 reduction more readily proceed on the surface of alkaline/Fe2O3, compared to the
bare surface of Fe2O3. Unfortunately, there is not any apparatus to check which mechanism is
more probable for this experiment.
4. Conclusion
The loading of very small amounts of alkali metal oxide, such as sodium, potassium, and
cesium on the surface of iron (III) oxide particles, increases the photocatalytic activity of iron
oxide in the reaction of isopropanol to acetone under visible light irradiation. The results
showed that the loaded KOH on Fe2O3 surface has improved the most photocatalytic activity
compared to other alkaline NaOH and CsOH. Besides, among KOH loaded samples, 0.05 wt%
was the desired choice of generated acetone in the photodegradation process. Moreover, for the
CO2 generation the 0.05 wt% loaded KOH sample was the perfect candidate among other
9

samples. XRD spectra showed that the surface of Fe2O3 before and after KOH loading had
similar crystalized monoclinic structure. Due to the high energy potential of the conductive
band in iron oxide and the low tendency of the conduction band to allow the electron to carry
out photocatalytic reactions, it can be assumed that the superfacial interaction between the two
materials leads to the transfer of photogenerated electrons from iron oxide to alkali
nanoparticles on the surface. It results in alkali nanoparticles on the surface act as cocatalyst
material (fig. 11). Multi-electron oxygen reduction is another way supposed to facilitate the
photocatalytic reaction.
5. Acknowledgment
We would like to express our sincere gratitude and thanks to National Institute for
Materials Science (NIMS) specially professor Jinhua Ye who have provided a nice situation for
the experiments and study of photocatalytic materials.
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Figures
Fig. 1. Acetone generation of Fe2O3 under Xe Lamp with 440 nm filter
loaded by (a) NaOH (b) KOH (c) CsOH.
Fig.2.ComparisonofthehighestacetonegenerationonthesurfaceofFe2O3thatloa
dedbyNaOH,KOH,CsOH.
Fig. 3. CO2 generation under the irradiation of visible light on the process of
photodegradation
13

Fig. 4. SEM images of Fe2O3loaded by alkaline nanoparticles (a) NaOH(b)
KOH (c) CsOH(d) Pure Fe2O3.
Average size 106 nm
Fig. 5. Particle size histogram of samples from SEM images (a) Fe2O3,
average size 106 nm (b) KOH/ Fe2O3, average size 116 nm
Fig. 6. SEM images of Fe2O3loaded by alkaline nanoparticles after
photocatalyticreaction (a) NaOH (b) KOH (c) CsOH (d) Pure Fe2O3.
14

Fig. 7. FTIR spectra shows less OH groups on the surface of
Fe2O3 due to surface modification by KOH molecules.
Fig. 8. UV-visible spectroscopy of Fe2O3 loaded by KOH. (a) Reflectance and
(b) Absorbance
15

Fig. 9. Bandgap energy of (a) pure Fe2O3 2.2 eV and (b) KOH loaded Fe2O3
2.25 eV
Fig. 10. XRD spectra of pure Fe2O3, Fe2O3loaded with 1 wt% KOH and the
sample used for photocatalyticreaction.
Fig. 11. A Scheme for photocatalyticreaction of IPA on the surface of Fe2O3
modified by alkaline nanoparticles.
16