Double-shell Fe2O3 hollow box-like structure for enhanced photo-Fenton degradation of malachite green dye

Article abstract of DOI:10.1016/j.jpcs.2017.09.033

In this work we demonstrate the synthesis of novel Fe₂O₃ nanosheets with double-shell hollow morphology by replica molding from diatomite framework. The nanostructures of Fe₂O₃ nanosheets were examined by focused-ion-beam scanning electron microscopy (FIB/SEM), X-ray diffraction spectroscopy (XRD), Brunauer-Emmett-Teller (BET) specific surface area measurements and Fourier transform infrared (FT-IR) spectroscopy. The results reveal that (1) Pure Fe₂O₃ nanosheets were successfully obtained; (2) The double-shell Fe₂O₃ hollow structure achieved via the NaOH etching silica method was observed; (3) Fe₂O₃ nanosheets possessed uniformly distributed porous nanosheets. Such structural features enlarged the specific surface area of Fe₂O₃ nanosheets and led to more catalytic active sites. In the heterogeneous photo-Fenton reaction, the double-shell Fe₂O₃ hollow morphology exhibited excellent catalytic capability for the degradation of malachite green (MG) at circumneutral pH condition. Under optimum condition, MG solution was almost completely decolorized in 60 min (99.9%). The Fe₂O₃ nanosheets also showed good stability and recyclability, demonstrating great potential as a promising photo-Fenton catalyst for the effective degradation of MG dye in wastewater.

Full text of DOI:10.1016/j.jpcs.2017.09.033

Journal of Physics and Chemistry of Solids 112 (2018) 209–215
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Double-shell Fe2O3 hollow box-like structure for enhanced photo-Fenton
degradation of malachite green dye
,
,*
De Bin Jiang a, Xiaoying Liu a, Xuan Xu b **, Yu Xin Zhang a
a State Key Laboratory of Mechanical Transmissions, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China
b Key Laboratory of Three Gorges Reservoir Region's Eco-Environment, Ministry of Education, Chongqing University, Chongqing 400045, PR China
A R T I C L E I N F O
A B S T R A C T
In this work we demonstrate the synthesis of novel Fe2O3 nanosheets with double-shell hollow morphology by
replica molding from diatomite framework. The nanostructures of Fe2O3 nanosheets were examined by focused-
ion-beam scanning electron microscopy (FIB/SEM), X-ray diffraction spectroscopy (XRD), Brunauer-Emmett-
Teller (BET) specific surface area measurements and Fourier transform infrared (FT-IR) spectroscopy. The re-
sults reveal that (1) Pure Fe2O3 nanosheets were successfully obtained; (2) The double-shell Fe2O3 hollow
structure achieved via the NaOH etching silica method was observed; (3) Fe2O3 nanosheets possessed uniformly
distributed porous nanosheets. Such structural features enlarged the specific surface area of Fe2O3 nanosheets and
led to more catalytic active sites. In the heterogeneous photo-Fenton reaction, the double-shell Fe2O3 hollow
morphology exhibited excellent catalytic capability for the degradation of malachite green (MG) at circumneutral
pH condition. Under optimum condition, MG solution was almost completely decolorized in 60 min (99.9%). The
Fe2O3 nanosheets also showed good stability and recyclability, demonstrating great potential as a promising
photo-Fenton catalyst for the effective degradation of MG dye in wastewater.
Keywords:
Double-shell hollow Fe2O3
Malachite green
Photo-Fenton
Catalytic degradation
1. Introduction
nanostructured Fe2O3 to overcome the problems of aggregation [11].
However, those carriers are expensive and complicated to synthesize. In
Malachite green (MG), triphenylmethane dye, which is widely used in
textile, paper and leather industries, is a type of non-biodegradable
material and causes potential damage to the human immune and
reproductive systems [1]. Therefore, the removal of malachite green dye
from contaminated water has drawn great attention from the interna-
tional community.
Compared to other removal technologies, such as adsorption [2],
filtration [3], and microbiological treatment [4], advanced oxidation
processes (AOPs) are an innovative and effective approach for decom-
posing malachite green dye directly to an innocuous byproduct [5,6].
Among the existing AOPs, nanostructured Fe2O3 used as a heterogeneous
photo-Fenton catalyst has the merits of being a simple technology, having
a wide reaction pH range and being an inexpensive material. Despite the
advantages of the nanostructured Fe2O3 as a heterogeneous photo--
Fenton catalyst, the treatment efficiency is not always satisfactory [7]. To
improve the degradation efficiency, carriers such as CNTs [8], zeolite [9]
and graphene oxide [10] have been used to hybridize with the
recent years, diatomite has been widely used as a catalyst carrier [12,13]
owing to its abundance, chemical inertness, low cost and nontoxicity.
Due to its box-like hollow shells and inner voids built by amorphous
silica, diatomite offers low density and a large surface area which greatly
improves degradation efficiency.
So far, several studies have evaluated the use of Fe2O3/diatomite
composite in heterogeneous photo-Fenton catalysis for the degradation
of dye [14,15]. The Fe2O3 nanoparticles have been supported on diato-
mite by the chemical precipitation method [16,17]. However, the Fe2O3
nanoparticles on the diatomite are not uniformly distributed. The non-
uniform morphology hinders full use of the sophisticated hollow struc-
ture of diatomite, whose interior voids can afford free volume to enlarge
the specific surface area and shorten the ways of diffusion of hydroxyl
radicals [18]. Recently, we hydrothermally synthesized a novel box-like
hollow diatomite@MnO2 composite [19]. For the first time, to our best
knowledge, the MnO2 thin layers consisted of nanosheets were uniformly
coated on the outside and inside of the diatomite shell by replica molding
* Corresponding author.
** Corresponding author.
E-mail addresses: xuxuan@cqu.edu.cn (X. Xu), zhangyuxin@cqu.edu.cn (Y.X. Zhang).
https://doi.org/10.1016/j.jpcs.2017.09.033
Received 17 August 2017; Received in revised form 21 September 2017; Accepted 25 September 2017
Available online 28 September 2017
0022-3697/© 2017 Elsevier Ltd. All rights reserved.

D.B. Jiang et al.
Journal of Physics and Chemistry of Solids 112 (2018) 209–215
from the diatomite box-like hollow structures [20]. Meanwhile, the
diatomite shell owns large inner space with micro- and nano-scale
porosity. The new hollow structure in the diatomite shell could be ob-
tained via the NaOH etching silica method. As a result, this synthetic
method enabled the construction of the double-shell hollow structure via
the NaOH etching silica method to increase the surface area and afford
more free volume during redox reaction.
2.1.3. Synthesis of double-shell hollow Fe2O3
60 mg of diatomite@FeOOH was dispersed in 50 mL NaOH solution
(20 wt %) and then heated in an 80 ꢀC oven for 18 h. The final products
were washed with ethanol and water three times and were subsequently
dried at 60 ꢀC in an oven to obtain double-shell hollow Fe2O3.
2.2. Materials characterization
In the present work, we report a facile synthesis of the double-shell
Fe2O3 hollow structure achieved via the NaOH etching silica method.
The double-shell Fe2O3 hollow morphology replicates the box-like hol-
low diatomite@FeOOH that was obtained from diatomite@MnO2 com-
posite by a simple and economical template-engaged route. It doubles the
surface area of the nanostructures, resulting in more catalytic active sites
and inner voids of the nanostructured Fe2O3 nanosheets in the hetero-
geneous photo-Fenton catalytic process.
The crystallographic information and chemical composition of as-
prepared products were established by powder X-ray diffraction (XRD,
D/max 2500, Cu Kα). The morphological investigations of the as-
prepared products were carried out with a focused ion beam (Zeiss
Auriga FIB/SEM). The BET specific surface area was defined by nitrogen
adsorption-desorption isotherms (77 K) using a micromeritics instrument
(ASAP 2020), and the pore size distributions were calculated from the
adsorption curve by the Barrett–Joyner–Halenda (BJH) method. Fourier
transform infrared spectra were obtained using FT-IR Nicolet Nexus
(Nicolet, USA) with the KBr method. The FT-IR spectra in the range of
4000 cmꢁ1– 400 cmꢁ1 were recorded.
2. Experimental section
2.1. Synthesis of double-shell Fe2O3 hollow box-like structure
All the reagents were of analytical-reagent grade, and used without
further purification. Double-shell Fe2O3 was successfully prepared by
three steps as shown in Scheme 1.
2.3. Photo-Fenton degradation experiments
In the present study, the degradation efficiency of MG using the
double-shell hollow Fe2O3 catalyst under different conditions was eval-
uated by a batch of experiments. In all the experiments, a stock solution
of 100 mL MG was prepared in a 250 mL glass beaker with the initial
concentration (C0) of 20 mg/L at circumneutral pH condition. A 500 W
Xe lamp (NbeT, China) was used as a light source. Visible light was
allowed to pass through a λ > 400 nm cut-off filter covering the window
of the beaker; this filter absorbed UV light and only allowed visible light
of λ > 400 nm to pass through. In the first experiment, only the double-
shell hollow Fe2O3 or H2O2 was added into the MG solution under visible
light irradiation with constant magnetic stirring (around 550 rpm). Then
in the Fenton experiment, the double-shell hollow Fe2O3 was added into
the MG solution with constant magnetic stirring (around 550 rpm) for
30 min to reach absorption–desorption equilibrium in the dark condi-
tions. Subsequently, H2O2 was added with constant magnetical stirring to
obtain the suspension. Lastly, in the photo-Fenton process, the visible
light irradiated immediately on the suspension obtained in the Fenton
experiment. In every experiment, a 4 mL suspension was withdrawn at
given time intervals and centrifuged to determine the MG concentration
on a UV–vis spectrophotometer (Shimadzu, UV-2450) at the maximum
2.1.1. Synthesis of diatomite@MnO2 [19,20]
Briefly, 30 mg of purified diatomite was dispersed into 30 mL of
KMnO4 solution (0.05 M) by stirring to form a homogeneous suspension.
The mixture was placed into a Teflon-lined stainless steel autoclave
which was subsequently maintained at 160 ꢀC for 24 h. Finally, the
product was washed and calcined at 200 ꢀC for 2 h to obtain the dia-
tomite@MnO2 composites.
2.1.2. Synthesis of diatomite@FeOOH
45 mg of FeSO4⋅7H2O was dissolved in 64 mL of mixed ethylene
glycol (EG) and deionized water (v/v ¼ 1/7). After stirring for about
10 min, 60 mg of this as-obtained diatomite@MnO2 was added to the
solution, stirring for another 10 min. The mixture was transferred into a
100 mL Teflon-lined stainless steel autoclave which was then maintained
at 120 ꢀC for 12 h. After heating, the autoclave was cooled naturally to
room temperature. Finally, the product was washed with distilled water
and ethanol and dried at 60 ꢀC to yield the diatomite@FeOOH
composites.
Scheme 1. The schematic illustration of the synthesis of the double-shell hollow Fe2O3.
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Journal of Physics and Chemistry of Solids 112 (2018) 209–215
absorption wavelength of 617 nm.
3. Results and discussion
3.1. Structure and morphology of double-shell Fe2O3
The typical X-ray diffraction (XRD) patterns of the diatomite@FeOOH
and double-shell hollow Fe2O3 are shown in Fig. 1. It can be seen from
Fig. 1a that the diffraction peaks at 2θ of 21.9ꢀ, 28.4ꢀ, 31.4ꢀ, and 36.0ꢀ
are in good agreement with the standard pattern of the SiO2 phase
(JCPDS card no. 39-1425), corresponding to (101), (111), (102), and
(200) planes. The other diffraction peaks at 26.7ꢀ, 33.1ꢀ and 53.9ꢀ are in
clear accordance with the standard pattern of the goethite phase (JCPDS
card no. 29-0713), corresponding to (120), (130) and (221) planes. In
Fig. 1b, diffraction peaks of (012), (104), (110), (113), (024), (116),
(214) and (300) are well indexed to the hematite phase (JCPDS card no.
33-0664), and the characteristic peaks of the SiO2 disappear completely.
This suggests that SiO2 was etched completely and the FeOOH was
transformed to Fe2O3.
Fig. 2a confirms the presence of FeOOH on the surface of the diato-
mite. The spectrum of diatomite@FeOOH exhibits a broad stretch at
Fig. 2. FT-IR spectra of (a) diatomite@FeOOH and (b) double-shell hollow Fe2O3.
3453 cmꢁ1 and 1644 cmꢁ1 that corresponds to O H stretching and
–
bending vibrations, respectively. The characteristic sharp peak at
hollow structure of the pure Fe2O3 was obtained by etching the hollow
shell of diatomite while replicating its morphology. Furthermore, the
collapse of the broken structure in Figs. 3c and 1S confirms that the
hollow structure has a double-shell. The detailed elemental composition
mapping of double-shell hollow Fe2O3, as shown in Fig. 3e and f, reveal
that the Fe and O signals have similar shapes, further confirming the
uniform distribution of the double-shell hollow Fe2O3 nanosheets. In
addition, it can be observed that the double-shell hollow Fe2O3 has the
same nanosheet morphology as the FeOOH, suggesting that double-shell
hollow Fe2O3 has high specific area and porosity, which is beneficial for
their application in wastewater treatments.
1094 cmꢁ1 is assigned to the asymmetric stretching vibration of
– – – –
O Si [21]. The bending vibration of O Si O group appears at
477 cmꢁ1 [22].
Si
As shown in Fig. 2b, after the etching process, the peaks of the
– – – –
bration disappear. Meanwhile, the new bands at 551 cmꢁ1 and 470 cmꢁ1
Si
O Si asymmetric stretching vibration and O Si O bending vi-
–
appear, which corresponds to the vibration of Fe O group in Fe2O3
[23,24]. This result suggests that the synthesized material is pure Fe2O3
after the etching process, which is also consistent with the XRD
characterization.
Fig. 3 illustrates the morphology and structure of the diatom-
ite@FeOOH and double-shell hollow Fe2O3 examined by FIB/SEM. As
shown in Fig. 3a, the detailed surface image of the FeOOH nanosheets,
template-engaged redox etched from the MnO2 nanosheets, shows good
replication of the structure of diatomite@ MnO2 [25], and it can be
observed from Fig. 3b that the abundant ultrathin FeOOH nanosheets are
well coated on the diatomite and interconnected to each other, forming a
highly uniform surface morphology. Fig. 3c and d shows the SEM images
of double-shell hollow Fe2O3, indicating that double-shell hollow Fe2O3
sustains the framework of the diatomite@FeOOH. The double-shell
The nitrogen adsorptionꢁdesorption isotherm and Bar-
rettꢁJoynerꢁHalenda (BJH) were employed to determine the pore-size
distribution of diatomite@FeOOH and double-shell hollow Fe2O3.
Fig. 4a shows that diatomite@FeOOH and double-shell hollow Fe2O3
both exhibit an isotherm of type IV. The isotherm of double-shell hollow
Fe2O3 has
a sharp inflection at a relative pressure around P/
P0 ¼ 0.50–1.0, which suggests the double-shell hollow Fe2O3 is a typical
mesoporous material [26]. The hysteresis loop (Type H3) in the isotherm
of double-shell hollow Fe2O3 was caused by the slit-shaped pores
resulting from aggregates of plate-like particles, suggesting the existence
of nanosheet-like morphology. After etching the diatomite, the observed
BET surface area and pore volume of double-shell hollow Fe2O3 increased
from 22.12 m2/g and 0.077 cm3/g to 47.64 m2/g and 0.215 cm3/g,
respectively. The corresponding pore size distribution curves, shown in
Fig. 4b, display the same broad pore size distribution of diatom-
ite@FeOOH and double-shell hollow Fe2O3, which proves that
double-shell hollow Fe2O3 replicated the FeOOH nanosheet structure
[27]. The hollow and mesoporous structures favour the promotion of the
catalytic degradation efficiency.
3.2. Photo-Fenton degradation of MG by double-shell hollow Fe2O3
Fig. 5 reports the degradation efficiency of MG by using the double-
shell hollow Fe2O3 catalyst. As shown in Fig. 5a, under visible light
irradiation, when only double-shell hollow Fe2O3 or only H2O2 was used
as a catalyst, the degradation efficiency of MG is less than 10% in 60 min.
When both the double-shell hollow Fe2O3 and H2O2, were used in the
dark, the degradation efficiency of MG increased approximately to
43.7%, indicating that the Fenton system has a certain degradation ca-
pacity for MG. However, under the double-shell hollow Fe2O3/H2O2/vis
system, the degradation rate of MG remarkably increased to 99.9% in
60 min, which demonstrates the remarkable synergetic effect of the
Fig. 1. XRD patterns of (a) diatomite@FeOOH and (b) double-shell hollow Fe2O3.
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Journal of Physics and Chemistry of Solids 112 (2018) 209–215
Fig. 3. SEM and EDX elemental mapping images of (a, b) diatomite@FeOOH and (c, d, e) double-shell hollow Fe2O3.
Fig. 4. (a) The nitrogen adsorptionꢁdesorption isotherms and (b) the corresponding pore-size distribution curves of diatomite@FeOOH and double-shell hollow Fe2O3.
Fenton system constituted of double-shell hollow Fe2O3 catalyst and
H2O2, and the photo system – the visible irradiation– for the discolor-
ation of MG.
To understand and clarify the changes in molecular structure of MG in
the presence of the double-shell hollow Fe2O3/H2O2/vis system, UV–vis
spectra changes in the MG dye solution over various time intervals are
shown in Fig. 5b. Clearly, the peak intensities located at 617 nm and
424 nm decrease with increasing reaction time, indicating the effective
degradation of MG dyes. After 60 min of reaction, the almost invisible
absorbance peak at 617 nm points to the completion of MG degradation.
However, a new absorption peak at 334 nm was observed at the same
time, suggesting the generation of intermediate products. This result is
also consistent with the gradual color change of the reaction solution
from green to almost colorless during the different reaction times in the
inset of Fig. 5b.
It has been observed that the increased dosages of double-shell hol-
low Fe2O3 and H2O2 significantly promoted the degradation efficiency of
MG as presented in Fig. 5c. At the given dosage of H2O2 (90 mM), the
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Journal of Physics and Chemistry of Solids 112 (2018) 209–215
Fig. 5. (a) MG degradation under different condition. (b) UV–vis spectral changes for MG degradation with double-shell hollow Fe2O3/H2O2/Vis system. (Reaction condition:
[MG] ¼ 20 mg/L, [catalysts] ¼ 0.1 g/L, [H2O2] ¼ 90 mM); (c) Effect of catalysts content on the photo-Fenton degradation of MG (Reaction condition: [MG] ¼ 20 mg/L, [H2O2] ¼ 90 mM);
(d) Effect of H2O2 content on the photo-Fenton degradation of MG. (Reaction condition: [MG] ¼ 20 mg/L, [catalysts] ¼ 0.1 g/L). (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
degradation efficiency of MG increased from 5% to 99% with the
increasing of double-shell hollow Fe2O3 dosage from 0 to 0.1 g/L. The
high dosage of double-shell hollow Fe2O3 could increase adsorption sites
and provide more catalytic active sites for the activation of H2O2 to
generate more ⋅OH radicals, resulting in a significant enhancement of the
degradation rate. Similarly, Fig. 5d shows that the increase of H2O2
dosage could also enhance the decomposition of MG in the double-shell
hollow Fe2O3/H2O2/vis system due to the improvement of ⋅OH radical
formation. Under optimum conditions, MG solution can be almost
completely decolorized in 60 min (99.9%).
The stability of a catalyst is a key factor for its application, and if it can
be reused, it will save resources and cost less. Therefore, it is necessary to
investigate the stability and reusability of a catalyst from the economic
and environmental perspective. In this study, after each cycle experi-
ment, the catalyst was separated by a simple precipitation and reused
without further treatment. As shown in Fig. 6, the double-shell hollow
Fe2O3 catalyst shows good repeatability in the first two rounds. It is
suggested that the double-shell hollow Fe2O3 catalyst shows good sta-
bility and recyclability by well-maintained the double-shell hollow
structure. Afterwards, the double-shell hollow Fe2O3 catalyst exhibits
little loss of catalytic activity in the last three rounds [28]. This drop
could be attributed to the deterioration of the double-shell hollow
structure, resulting in the loss of Fe2O3. Another possible reason is the
less activated site due to the coverage of area by intermediate products
[29]. In all, the double-shell hollow Fe2O3 catalyst shows good stability
and recyclability.
To demonstrate the effectiveness of double-shell hollow Fe2O3 as
catalyst, it has been compared with previously reported and published
procedures which shows that double-shell hollow Fe2O3 is a good cata-
lyst for degradation of MG with visible light at circumneutral pH con-
dition. (Table S1). The results undoubtedly illustrate that this procedure
using double-shell hollow Fe2O3 catalyst is indeed environmentally
Fig. 6. Recycled performance of double-shell hollow Fe2O3 catalysts for photo degrada-
tion of MG dye under visible light irradiation. (Reaction condition: [MG] ¼ 20 mg/L,
[catalysts] ¼ 0.1 g/L, [H2O2] ¼ 90 mM).
friendly than other in terms of discoloration efficiency.
In terms of the results above, we propose a possible mechanism for
the double-shell hollow Fe2O3 as a heterogeneous catalyst for the
discoloration of MG dye (Scheme 2). In general, it is believed that the
photo-Fenton processes can generate ⋅OH radicals in the solution, which
can accelerate the degradation of the organic compounds in water. In this
study, the degradation of MG with double-shell hollow Fe2O3 catalysts or
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Journal of Physics and Chemistry of Solids 112 (2018) 209–215
Scheme 2. The schematic illustration of the mechanism of MG degradation by the double-shell hollow Fe2O3.
H2O2 did not occur in visible light irradiation. However, when both
double-shell hollow Fe2O3 catalysts and H2O2 were used with visible
light irradiation, the degradation of MG reached 99.99%, suggesting this
process follows the heterogeneous photo Fenton reaction with a redox
cycle between Fe2þ and Fe3þ on the catalyst surface [30,31]. When the
MG dye is exposed to visible light, the dye is excited, and in this state it is
adsorbed on the surface of the double-shell hollow Fe2O3 nanosheets
where electrons are transferred from the excited dye to Fe3þ, generating
Fe2þ to oxidize the dye. In this process, the visible light source enhanced
the very regeneration of Fe2þ in the Fenton reaction [32]. The generated
Fe2þ species then reacted with H2O2, producing highly oxidative ⋅OH
radicals that initiated the degradation of the MG dye. In addition,
because the porous Fe2O3 nanostructure can provide a large surface for
electron transfer, the double-shell hollow Fe2O3 can effectively increase
the liquid-solid interfacial reaction area or the catalytic active sites,
providing a fast path for the oxidation reduction cycle of iron. Subse-
quent degradation of the MG takes place via the double-shell hollow
Fe2O3 nanosheets.
106112016CDJZR135506 and 106112017CDJSK04XK11). The authors
would like to extend special thanks to Ms. Qian Chen of Chongqing
University for language polishing. The authors also thank the Electron
Microscopy Center of Chongqing University for materials
characterizations.
Appendix A. Supplementary data
Supplementary data related to this article can be found at https://doi.
org/10.1016/j.jpcs.2017.09.033.
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The double-shell hollow Fe2O3 nanosheets were prepared by dia-
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precursor by the template-engaged method. The analysis of the structure
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Acknowledgements
The authors gratefully acknowledge the financial support provided by
the National Natural Science Foundation of China (Grant no. 21576034),
Chongqing University Postgraduates' Innovation Project (No.
CYB16014), the Innovative Research Team of Chongqing
(CXTDG201602014) and the State Education Ministry and Fundamental
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