Nghiên cứu tổng hợp vật liệu quang xúc tác lafeo3 cho phản ứng phân hủy hợp chất hữu cơ gây ô nhiễm trong nước thải

MINISTRY OF EDUCATION AND TRAINING
HA NOI UNIVERSITY OF SCIENCES AND TECHNOLOGY
--------------------------------------PHAN CHI NHAN

PHAN CHI NHAN

CHEMICAL ENGINEERING

SYNTHESIS LAFEO3 FOR PHOTODEGRADATION OF
POLLUTANTS IN WASTEWATER

MASTER THESIS

YEAR : 2017
HA NOI - 2019


MINISTRY OF EDUCATION AND TRAINING
HA NOI UNIVERSITY OF SCIENCES AND TECHNOLOGY
---------------------------------------

PHAN CHI NHAN

SYNTHESIS LAFEO3 FOR PHOTODEGRADATION OF POLLUTANTS
IN WASTEWATER

CHEMICAL ENGINEERING

MASTER THESIS

SUPERVISOR

ASSOCIATE PROFESSOR: PHAM THANH HUYEN

HA NOI - 2019


Declaration statement
I declare that:
This is my own research work, the data and results presented in the thesis are true, are
allowed to use by the authors and have never been published in any works

Ha Noi,

October, 2019

Phan Chi Nhan


ACKNOWLEDGEMENT
This work has been supported by the RoHan Project funded by the German
Academic Exchange Service (DAAD, No. 57315854) and the Federal Ministry for
Economic Cooperation and Development (BMZ) inside the framework “SDG
Bilateral Graduate school programme.

I would like to express my deep gratitude to my principal supervisor, Associate
Professor Pham Thanh Huyen. I thank Assc. Prof. Pham Huyen wholeheartedly for
her great academic guidance, valuable advice, and constant encouragement.

I would like to give a big thank to Professor Malte Brasholz, who is my supervisor
at this time I worked in Germany. Thank you for giving me an opportunity to study
in a good condition and learn more about interesting culture of Germany.

Countless thanks are also dedicated to my Mum, my sister and younger brother, my
other family members and my friends who have always supported and accompanied
me from my MsC. Beginning to end.

Finally, I would like to thank my father, Mr Phan Van Huan (1945 – 2017), who
brought me to science, give me a passion about chemistry and he is the motivation
for me to finish this project. This thesis is a present, which I want to give to my
father. Thank him for all.


TABLE OF CONTENTS

ACKNOWLEDGEMENT ............................................................................................
TABLE OF CONTENT .............................................................................................. I
CHAPTER 1: LITERATURE REVIEW ....................................................................1
1. Introduction .............................................................................................................1
2. Lanthanum Ferrite (LFO) Perovskite ......................................................................3
2.1 Structure .............................................................................................................3
2.2 Optical property .................................................................................................4
2.3 Magnetic property ..............................................................................................5
3. Lanthanum Ferrite as Visible-light Photocatalyst ...................................................7
3.1 Application in textile wastewater treatment ......................................................7
3.2 Photocatalytic mechanism .................................................................................8
4. Synthetic methods of LFO ....................................................................................10
4.1 Sol-gel method .................................................................................................11
4.2 Hydrothermal method .....................................................................................13
4.3 Sol-gel hydrothermal method .........................................................................17
5. Factors affecting photocatalytic activity of LFO material ....................................18
5.1 Surface area......................................................................................................18

5.2 Band gap energy ..............................................................................................21
5.3 Morphology .....................................................................................................22
5.4 Crystallinity .....................................................................................................25
5.5 Dopants ............................................................................................................27
CHAPTER 2: EXPERIMENTAL .............................................................................30
1. Synthesis of LaFeO3 ...........................................................................................30
2. Characterization of LFO .....................................................................................30
3. Experiment of photocatalytic activity ................................................................31
CHAPTER 3. RESULTS AND DICUSSION ..........................................................33
1. Thermal Analysis ...............................................................................................33

i


2. XRD analysis ......................................................................................................34
3. Morphological analysis ......................................................................................35
4. BET analysis.......................................................................................................36
5. Band Gap energy result ......................................................................................37
6. Calibration curve of dyes and 17β-Estradiol ......................................................38
7. Photocatalytic activity of LFO nanoparticles .....................................................40
8. Efect of reaction condition on photodegradation efficiencies ............................42
9. Effect of catalytic concentration on photodegradation efficiencies ...................44
10.

Effect of H2O2 concentration on photodegradation efficiencies .....................45

11.

Comparison of catalytic activity for different dyes ........................................46

12.

Photodegradation of 17β-Estradiol .................................................................47

...............................................................................................................................47
13.

Effect of light intensity on photodegradation efficiencies ..............................48

CONCLUSIONS .......................................................................................................49
Recommendations for future research ......................................................................49
REFERENCES ..........................................................................................................51

ii


LIST OF TABLES
Table 1. Optical band gap values of LFO nanomaterials synthesized by various
methods .....................................................................................................................5
Table 2. Particle size and morphologies of LFO synthesized by sol-gel method
using different templates. ........................................................................................12
Table 3. The influences of the hydrothermal temperature on the formation of LFO
powders (Adopted from (Ji, Dai et al., 2013)) ........................................................17
Table 4. Photocatalytic performance of LFO for the degradation of dyes .............20
Table 5. Degradation rate of dyes solution on different morphologies LFO
nanoparticles ...........................................................................................................24
Table 6. Effects of calcination temperature on the BET specific surface area and
pore parameters of LFO nanoparticles ....................................................................37

iii

LIST OF FIGURES

Figure. 1. Schematic crystalline structure of orthorhombic LFO (Misch, Birkel et
al., 2014). .....................................................................................................................4
Figure. 2. Schematic diagram of LFO antiferromagnetic order (Lee, Yun et al.,
2014)............................................................................................................................6
Figure. 3. Schematic diagram of the reaction mechanism of LFO nanostructures for
organic degradation (Adopted from (Thirumalairajan, Girija et al., 2013)). ..............9
Figure. 4. FESEM images of LFO powders from (Feng, Liu et al., 2011) (a) and
(Liu and Xu, 2011) (b) ..............................................................................................13
Figure. 5. XRD patterns of different LFO nanostructures (Thirumalairajan, Girija et
al., 2013) ....................................................................................................................15
Figure. 6. Image of LFO nanospheres by HRSEM (Dhinesh Kumar and Jayavel,
2014)..........................................................................................................................16
Figure. 7. RhB degradation % by using LFO samples with different particle sizes
(Thirumalairajan, Girija et al., 2012). .......................................................................19
Figure. 8. SEM images of LFO nanostructures with different morphologies: (a)
nanocubes; (b) nanorodes; (c) nanospheres; (d) dendritic nanostructures (e) florallike nanosheets; (f) nanowires; (g) nanofibers (Yang, Huang et al., 2006; Leng, Li et
al., 2010; Thirumalairajan, Girija et al., 2012; Thirumalairajan, Girija et al., 2013;
Dhinesh Kumar and Jayavel, 2014; Thirumalairajan, Girija et al., 2014). ...............23
Figure. 9. (a) XRD pattern at different calcination temperatures; (b and c)
degradation of RhB and MB in the presence of LFO calcined at 800°C
(Thirumalairajan, Girija et al., 2014) ........................................................................26
Figure 10. (a) XRD pattern of LFO samples calcined at different temperatures; (b)
Degradation of RhB with the use of different LFO samples and P-25TiO2 (Su, Jing
et al., 2010). ...............................................................................................................27
Figure 11. TGA curve of synthesized LaFeO3 sample. ............................................33
Figure. 12. XRD patterns of LaFeO3 ........................................................................34

Figure. 13. SEM images of (a) LFO-C600, (b) LFO-C700, (c) LFO-C800, and (d)
LFO-C900. ................................................................................................................35
Figure.14 HRTEM image of the LFO-C800 .............................................................36
Figure.15 Nitrogen adsorption-desorption isotherms of the LFO nanoparticles
calcined at different temperatures. ............................................................................36
Figure 16. (a) LFO solid UV-vis absorption spectrum and (b) Schematic of
determination of band gap energy .............................................................................38
Figure 17. Calibration curve of MB ..........................................................................38
...................................................................................................................................39
iv


Figure 18. Calibration curve of MB ..........................................................................39
Figure 19. Calibration curve of MO ..........................................................................39
Figure 20. Calibration curve of 17β-Estradiol ..........................................................40
Fig. 21. The photodegradation efficiencies of RhB under visible light irradation by
different photocatalysts .............................................................................................40
Figure. 22. The change in colour of RhB solution during the reaction ....................42
Fig.23. Photodegradation efficiencies of Metyl Orange under differences condition
...................................................................................................................................43
Fig. 24. Effect of catalytic concentration on photodegradation of MO ....................44
Fig.25. Effect of H2O2 concentration on photodegradation efficiencies ...................45
Fig.26. Photodegradation efficiencies for bleaching different dyes .........................46
Fig.27. The photodegradation efficiencies of 17β-Estradiol under visible light
irradation ...................................................................................................................47
Fig.28. Photodegradation efficiencies of MB by different lamp ..............................48
Fig.29. Photodegradation efficiencies of 7β-Estradiol by different lamp.................48

v

CHAPTER 1: LITERATURE REVIEW
1. Introduction
In recent years, environmental pollution issue, especially wastewater pollution has
been increasing alarmingly. Due to the rapid development of textile industry and
lack of modern technologies for textile wastewater treatment, a considerable amount
of harmful organic dyes has been discharged into environment (Yi, Chen et al.,
2008). For several dyes with the concentration less than 1 ppm, their presence in
water could easily be observed and undesirable (Robinson, McMullan et al., 2001).
Among these, the most notable ones include Rhodamine B (RhB), methylene blue
(MB) and methyl orange (MO) which have been used as coloured substances for
printing or dyeing cotton, leather, silk, wool (Gupta, Suhas et al., 2004). MB causes
not only permanent injury to eye but also difficulty in breathing to human and
animals (Tan, Ahmad et al., 2008). Meanwhile, experimental research has proven
the negative effects arising from RhB and MO on human well-being and ecological
environment, including carcinogenicity, toxicity, and mutagenicity (Khataee and
Kasiri, 2010). Consequently, wastewater treatment targeting at minimising the
levels of these organic compounds has become essential.

Conventional technologies for treatment of dye-containing water are not sufficiently
effective to achieve the current stringent requirements for discharge. To overcome
the challenges, there has been much attention focusing on advanced oxidation
processes (AOPs) which have been suggested as substitutions to previous treatment
technologies. Until now, there have been a variety of AOPs, including
electrochemical oxidation (Gallios, Violintzis et al.,2010; Pang, Wang et al., 2013;
Wang, Yue et al., 2014), supercritical water oxidation (Williams and Onwudili,
2006; Wang, Lv et al., 2013), ozonation (Prieto-Rodríguez, Oller et al., 2013),
photocatalytic oxidation (Zangeneh, Zinatizadeh et al., 2010; Gümüş and Akbal,
2011; Rodríguez, Gallardo et al., 2012), wet air oxidation (Zhan, Li et al., 2010;
Ovejero, Sotelo et al., 2011) and other combinations (Moreira, Vilar et al., 2012;

1


Couch, Mezyk et al., 2014). In particular, the AOP using photocatalysts - irradiated
semiconductor systems has been suggested as a promising way in environment
remediation, because it is efficient, cost-effective and environmental friendly by
utilizing solar light or artificial irradiation, which is abundant wherever (Liu, Niu et
al., 2013; Lai, Juan et al., 2014; Zhao, Tian et al., 2014). This shows significance in
both of waste treatment and resource conservation.

Photocatalysis is defined as the acceleration of a chemical transformation in the present
of a catalyst with light illumination (Augugliaro, Bellardita et al., 2012). Many
semiconductors have been used as photocatalysts, including TiO2 (Nakata and
Fujishima, 2012), ZnO (Yu, Shi et al., 2013), ZrO3 (Basahel, Ali et al., 2015), SnO2
(Cheng, Chen et al., 2011), CeO2 (Wu, Wang et al., 2015), and InO2 (Li, Zhang et al.,
2013) for degradation of a wide range of environmental contaminants. However, their
applications are limited because these semiconductor catalysts have high activity only
under UV illumination, which presents ~ 5% of solar energy spectrum. For this reason,
many efforts have been made in order to find different alternatives harvesting the solar
light and afterwards utilizing it in large-scale application. Recently, perovskite-based
materials have been reported as excellent visible-light-driven photocatalysts (Kanhere
and Chen, 2014; Wang, Tadé et al., 2015). Several types of perovskite materials have
been studied widely, such as titanate perovskites (Yao, Xu et al., 2004; Zhang, Sun et
al., 2013), tantalate perovskites (Li and Zang, 2009; Buršík, Vaněk et al., 2013), ferrite
perovskites (Sun, Jiang et al., 2010; Soltani and Entezari, 2013) and complex
perovskite materials (Zhu, Fu et al., 2008; Clark, Dyer et al., 2010). Among ferrite
perovskites, lanthanum ferrites, LaFeO3, need to be further studied due to their
interesting physical properties as well as potential applications in photocatalyst, fuel
cell, sensors and permeation membranes (Gabal, Ata-Allah et al., 2006; Yoo, Kim et
al., 2011).

2


Thus, the study of LaFeO3 materials with properties suitable for photocatalytic
requirements is really necessary. So I decided to choose a topic : “ Synthesize LaFeO3
materials for photodegradation of pollutants in wastewater”.

2. Lanthanum Ferrite (LFO) Perovskite
Perovskites-type oxides present a class of inorganic crystalline solids, having a
general formula of ABO3, where A and B are rare-earth metal and transition metal
cations, respectively (Zhang and Li, 2013). Recently, the perovskite-based material
LaFeO3 has attracted significant interest due to their useful application in biosensors
(Liotta, Puleo et al., 2015), oxygen permeation membrane (Kida, Ninomiya et al.,
2010), fuel cell (Li, Wang et al., 2014) and especially visible-light photocatalytic
reaction (Li, Jing et al., 2007; Thirumalairajan, Girija et al., 2012; Dhinesh Kumar
and Jayavel, 2014). Moreover, the desired properties of LaFeO3 for high
photocatalytic activity under visible light, including narrow band gap energy, high
surface area as well as high crystallinity, can be achieved by adjusting several
parameters during the synthesis. The doping with another rare-earth metal in A-site
and transition metal in B-site of this material may lead to an enhancement in
photocatalytic performance. Therefore, in order to prepare a photocatalyst that meet
above requirements, the in-depth study on the structure and properties of LaFeO3 is
essential.

2.1 Structure
Lanthanum ferrite, LaFeO3 (LFO), belongs to Pbnm space group with the lattice
parameters, a = 5.557 Å, b = 5.565 Å, c = 7.854 Å (Vansutre, Das et al., 2000). It
has an orthorhombic perovskite structure, which is derived from the distortion of
the ideal cubic structure via the tilting of the FeO6 octahedral. All of the Fe3+ ions

are octahedral surrounded by oxygen ions and the La3+ ions are inserted in the
interspaces between the FeO6 octahedra (Fossdal, Menon et al., 2004; Bellakki,
Manivannan et al., 2009). Fig. 1 illustrates the crystalline structure of LFO, in

3


which the La3+ cations are displayed as grey spheres and the FeO6 octahedra are in
blue.

Figure. 1. Schematic crystalline structure of orthorhombic LFO (Misch, Birkel
et al., 2014).
Recently, the preparation and utilization of lanthanum ferrite with novel properties
and morphologies have been reported by several researchers (Ahmed, Selim et al.,
2011; Wang and Gong, 2011; Salah, Rashad et al., 2015). In particular, LFO
nanomaterials are considered to be promising due to theirs large surface area and
interfacial state, which lead to different optical, electrical and magnetic properties
from bulk materials (Thirumalairajan, Girija et al., 2012). For the application as
photocatalysts, the optical and magnetic properties of these materials are
emphasized here.

2.2 Optical property
Optical absorption performance of a semiconductor is related to its electronic structure
as well as band gap. The band gap energy (Eg) is one of important features of the
nanomaterial to evaluate its optical property. Generally speaking, this energy illustrates
the minimum amount of required energy so that an electron could jump from a valence
band to a conduction band. Based on corresponding wavelength of light that material
absorbs, which allows to calculate its band gap value by following formula (Ziegler,
Heinrich et al., 1981):
4

αhν = A(hν - Eg)1/2
where hν is the photon energy, α is the optical absorption coefficient, A is a
constant and Eg is the band gap energy.

It is believed that this optical band gap (Eg) value strongly depends on the
preparation procedure of material as well as particle size (Roduner, 2006;
Köferstein, Jäger et al., 2013). This is one of reasons to interpret for a range of band
gap values of LFO nanoparticles, as summarized in Table 1. Generally, LFO
nanoparticles possess narrow band gap energies, which enable them to be active
under visible light. This will be discussed in detail in the later section.
Table 1. Optical band gap values of LFO nanomaterials synthesized by various
methods
Particle Band gap energy
Methods
References
size (nm)
(eV)
Hydrothermal

(Dhinesh Kumar and Jayavel,

45

2.66

24-104

2.11-2.07

(Parida, Reddy et al., 2010)

Emulsion

32

3.85

(Chandradass and Kim, 2010)

Precipitation

60

2.33

(Tang, Fu et al., 2011)

54

2.36

(Tang, Tong et al., 2013)

31

2.46

(Venkaiah, Rao et al., 2013)

Sol-gel autocombustion

Microwaveassisted method
Solution
combustion

2014)

2.3 Magnetic property
In the orthoferrite family, LFO is an interesting antiferromagnetic material with the
highest value of Neel temperature (TN ~ 740oC) (Scholl, Stöhr et al., 2000). In terms
of magnetic property of LFO, it originates from the interactions between the
magnetic moments of atom La and Fe. It is reported that all the electrons of La atom

5


are in pair, which indicates La3+ is non-magnetic and subsequently there is no
magnetic interaction between La3+ and Fe3+. Therefore, the magnetization pattern of
LFO nanoparticles is governed by the Fe sub-lattices (Wei, Wang et al., 2012). The
authors have reported that in the Fe sub-lattices, the magnetic moments of Fe3+ are
slightly canted, as the source of the ferromagnetic character at room temperature of
LFO nanoparticles, which were prepared by an auto-combustion process. Moreover,
LFO is well known as an antiferromagnetic material, which is caused by the
collinear arrangement of the sub-lattices, as shown in Fig. 2 (Lee, Yun et al., 2014).
These antiferromagnetic particles often display increasing magnetization because of
the presence of uncompensated surface spins (Kodama and Berkowitz, 1999; Lee,
Pakhomov et al., 2010).

Figure. 2. Schematic diagram of LFO antiferromagnetic order (Lee, Yun et al.,
2014).
Generally speaking, the magnetization of LFO nanoparticles is different from the
bulk crystals. For example, the spontaneous magnetization value of LFO bulk
crystals is 0.1 emu/g at ambient temperature (Shen, Cheng et al., 2009) which is
considerably smaller than those of LFO nanoparticles (~ 21.9 nm) prepared by solgel method (0.38 emu/g) (Saad, Khan et al., 2013), LFO nanofibers (~ 20 nm)
fabricated by electrospinning (0.9 emu/g) (Lee, Yun et al., 2014), and LFO
nanoparticles (~25-50 nm) synthesized via sol-gel combustion process (0.35 emu/g)
(Hui, Jiayue et al., 2010). Obviously, the magnetization values of LFO
nanomaterials in those studies are considerably different. This also suggests that the
preparation procedures, particle sizes and surfaces indeed play key roles in the
properties of product (Kumar, Raja et al., 2014). It can be seen that a reduction in
6


LFO particle sizes leads to an increase of magnetization values (Phokha,
Pinitsoontorn et al., 2014; Qiu, Luo et al., 2014). LFO exhibits a ferromagnetism,
which should be retained or even enhanced, facilitating the recyclability of material
after photocatalytic degradation of dyes with the use of magnetic field.

In conclusion, LFO has attracted great research interest for various applications as
mentioned before; because of its unique structure, optical and magnetic properties.
Among these applications, LFO has shown great potential for use in the field of
photocatalysis by virtue of narrow band gap energy, which makes it active in visible
region. Moreover, if its inherent ferromagnetic behaviour is enhanced, LFO would
be a photocatalyst which is capable to be recycled and reused.

3. Lanthanum Ferrite as Visible-light Photocatalyst
Photocatalysis employing irradiated semiconductors has been proven to be very
effective in decomposition of a wide range of organic pollutants (Li, Chen et al.,

2010; Sayama, Hayashi et al., 2010). Numerous efforts have been made in
synthesizing the semiconducting materials, such as TiO2, ZnO, CdS; however, they
are only active under ultraviolet irradiation because of their large band gap energy.
Therefore, the selection of alternative, which has an appropriate band gap, with
enhanced photocatalytic activity in visible region is necessary (Reddy, Martha et al.,
2012). Taken account of narrow band gap energy as aforementioned, LFO
nanostructures exhibit promising photocatalytic activities under visible light
illumination (Li, He et al., 2014; Thirumalairajan, Girija et al., 2014) and in turn has
been investigated as potential photocatalyst candidates to degrade organic dyes in
wastewater. This section reviewed the literature on the applications of LFO in
wastewater treatment as well as discussed photocatalytic mechanism.

3.1 Application in textile wastewater treatment
Many researchers have studied photocatalytic degradation of dye solutions by using
LFO nanoparticles (Tang, Tong et al., 2013; Dhinesh Kumar and Jayavel, 2014; Li,
7


Wang et al., 2014; Gaikwad, Sheikh et al., 2015). Most of studies have focused on
degradation of Rhodamine B (RhB), methylene blue (MB) and methyl orange (MO)
(Su, Jing et al., 2010; Gao and Li, 2012; Sacco, Stoller et al., 2012; Wei, Wang et
al., 2012; Tang, Tong et al., 2013; Xiao, Hong et al., 2013) because they are one of
the common dyes which is widely used in a variety of industrial applications. This
will be discussed in detail in later section. Apart from the decomposition of dyes
such as RhB, MB, MO, LFO nanomaterials were also used as photocatalysts for
degradation of several other dyes. For example, Abazari et al., carried out the
photodegradation of toluidine blue O (TBO) under solar light condition using LFO
nanoparticles which were fabricated in emulsion nanoreactors in the presence of
cetyltrimethyl ammonium bromide (CTAB) (Abazari, Sanati et al., 2014). They
concluded that the TBO dye was decomposed completely (99.98%) after 90 min

exposure to the solar light. Yang et al., conducted investigations on degradation of
Acid Red 18 in the presence of visible light using LFO as nanocatalysts. They
proposed that the nanoparticles displayed a relatively high activity; that 60% of
Acid Red 18 was decomposed after 60 min (Yang, Zhong et al., 2009). Meanwhile,
68.2% of dye Acid Red 3B was decomposed after 2 hour illumination by LFO
prepared via citric-based sol-gel method (Wang, Shen et al., 2012).

In my study, some of the most notable dyes will be selected, including RhB, MB,
MO and 17β-Estradiol, which have been widely used as coloured substances in
industries. In order to enhance degradation of these dyes by LFO, photoreaction
mechanism should be well understood.

3.2 Photocatalytic mechanism
Photodegradation of organic contaminants is based on the use of light irradiation to
initiate photoreaction. Fig. 3 illustrates the process of a LFO nanoparticle absorbing
light energy to generate electron-hole pairs.

8


Figure. 3. Schematic diagram of the reaction mechanism of LFO
nanostructures for organic degradation (Adopted from (Thirumalairajan,
Girija et al., 2013)).
Particularly, when the LFO semiconductor oxide absorbs visible light, which has
energy equal to or greater than its band gap, an electron-hole pair is generated due
to the excitation of an electron from the valence band (VB) and then transferred to
the conduction band (CB) of the semiconductor photocatalyst, subsequently leaving
a hole in the VB. The photo-induced charge carriers (electrons and holes) then
migrate to the surface of the photocatalyst and carry out the redox reactions, while
some of them recombine.

When photocatalytic processes carry out in aqueous solution, the photo-induced
holes (h+) react with hydroxide ions and water to yield hydroxyl radicals (OH•)
which are responsible for the photocatalytic oxidation of the dye into non-toxic
products (Cheng, Huang et al., 2010; Zhang, Zhou et al., 2011). It is reported that
repeated attack of organic dyes by OH• result in complete oxidation (William IV,
Kostedt et al., 2005). The process of producing OH• can take place by two
pathways. The first route is that the electrons react with dissolved O2 presenting in
9


water to form superoxide anion radicals O2•− (the reduction of O2), which
subsequently combine with H+ to form the hydroperoxy radicals HO2• and rapidly
decompose to OH• (Hou, Jiao et al., 2011). The second one is the oxidation of OH-.
The reaction mechanism of LFO by light could take place in the following steps, as
suggested by Chong et al. (Chong, Jin et al., 2010)
LaFeO3 + hν -> LaFeO3 + e-CB + h+VB
e-CB + O2 -> O2•−
O2•− + H2O -> HO2• + OH
HO2• + H2O -> H2O2 + OH•
H2O2• ->2OH•
h+VB + OH− -> OH•
OH•+ dye ->CO2 + H2O
h+VB + dye -> CO2 + H2O
e-CB + h+VB -> hν + (or heat) recombination
Because of the complexity of photocatalytic mechanism, some researchers assumed
that photocatalytic performance has been mainly determined by the light absorption
capability of photocatalyst and the charge separation as well as light utilization
yield to carry out reduction and oxidation reactions (Wang, Huang et al., 2010).

The research on the application of LFO in photocatalytic degradation of organic
dyes has found its properties and then activity is strongly affected by the synthetic
method selected, each of which usually involves specific preparation conditions.
Therefore, different synthesis methods have been employed to prepare LFO.

4. Synthetic methods of LFO
To date, several methods have been studied for the preparation of perovskite LFO,
such as co-precipitation (Nakayama, 2001), solid state reaction (Chu, Zhou et al.,
2009), sol-gel process (Qi, Zhou et al., 2003), sonochemical method (Sivakumar,
Gedanken et al., 2004), hydrothermal synthesis (Zheng, Liu et al., 2000) and
microemulsion method (Giannakas, Ladavos et al., 2004). However, the particles
10


produced by the conventional solid state reaction exhibit slow kinetics, nonuniform
particle size and low surface area (Popa and Moreno, 2011). Meanwhile, the coprecipitation method needs to use more chemicals as well as longer time to obtain
LFO powders (Nakayama, 2001). By contrast, the hydrothermal method and sol-gel
process have attracted increasing attention in the synthesis of nanostructured
photocatalysts, because the produced LFO exhibits high crystallinity, good purity,
controllable morphology and narrow particle size distribution at a relatively low
synthetic temperature.

4.1 Sol-gel method
Sol-gel method has been suggested as a potential route to synthesize LFO particles
with a uniform composition and good crystallinity at a relatively low synthetic
temperature (Tien, Mittova et al., 2014). Generally, sol-gel process includes four
main steps from a precursor to final product: precursor solution to form a gel, aging
of a gel, drying and calcination to obtain the end product. In the typical synthesizing
of LFO by sol-gel method, one starts with the certain amounts of lanthanum nitrate
and iron nitrate, which is dissolved in deionized water. The citric acid is added in

the nitrates solutions and then gently evaporated at 60-70oC under magnetic stirring,
resulting in a gel state with high viscosity. Then, the prepared sample is dried at
70oC overnight. Finally, LFO is obtained by calcining the dried sample at
appropriate temperature (600-700oC) for several hours.
In this process, several parameters could affect the properties of final product,
including composition and concentration of precursor (starting material), type of
solvent, concentration, temperature, heating rate, etc.

For example, the composition of starting materials has found to affect particle sizes
and shapes of LFO nanoparticles (Ita, Murugavel et al., 2003). Ita et al. revealed
that nano-sized particles of LFO could be slightly tailored by changing the ferric
source. The particles were spherical in shape with an average size of 41.4 nm and

11


42.5 nm when synthesized with the use of iron nitrate and iron acetylacetonate,
respectively.

The selection of an appropriate “chelating agent” or “structure-directing agent” or
“template” could considerably improve the quality of the product. The morphology
or pore size of the catalyst strongly depends on the type of templates selected. This
could be explained by each of structure-directing agent will hold different volumes
in the gel network after complexing with cations in the synthetic solution (Wang,
Xiang et al., 2013). Table 2 shows the different morphologies of LFO produced
after sol-gel method by using different templates.
Table 2. Particle size and morphologies of LFO synthesized by sol-gel method
using different templates.
Formation
Particle size

Templates
Morphology
References
temperature
(nm)
(oC)
Citric acid
Polyvinyl
alcohol
Ethylene
glycol

Glucose

Citric acid

680

600

600

500

800

Not
mentioned
70-90

40

30

Not
mentioned

Nanowires

(Yang, Huang
et al., 2006)

Irregular

(Feng, Liu et

shape

al., 2011)

Not
mentioned

(Aono,
Tomida et al.,
2009)

Agglomerated

(Liu and Xu,

particles

2011)
(Gosavi and

Highly porous

Biniwale,
2010)

For example, by a sol-gel method using La2O3, Fe(NO3)3.9H2O and polyvinyl
alcohol (PVA) as raw materials, single-phase and well-crystallized LFO particles

12


were obtained at 700oC with an average particle size of approximately 50 nm (Feng,
Liu et al., 2011), as shown in Fig. 4a. In comparison, by adding glucose as a
structure-directing agent, Liu et al., successfully synthesized nanosized LFO with a
diameter of about 30 nm at lower synthetic temperature (500oC) with the mole
ratios of glucose to metal ions 3:5 and 3:10 via sol-gel route (Fig. 4b) (Liu and Xu,
2011)

Figure. 4. FESEM images of LFO powders from (Feng, Liu et al., 2011) (a) and
(Liu and Xu, 2011) (b)
To conclude, sol-gel method has been widely used to synthesize LFO particles
because it offers a facile way to control the size and morphology of the particles.
However, depending on target applications, the synthetic parameters should be
carefully controlled to obtain the desired morphology as well as particle size.

4.2 Hydrothermal method
Hydrothermal method is a dominant method to prepare nanosized materials. The
advantages of this method include easy control of size and morphology, facile
fabrication process and cost-effectiveness. In particular, the hydrothermal method
can directly synthesize LFO at low temperature, as compared to the sol-gel method.
So far, many researchers have used this method to synthesize LFO nanoparticles (Ji,
Dai et al., 2013; Thirumalairajan, Girija et al., 2013; Dhinesh Kumar and Jayavel,
2014).
13


Typically, hydrothermal process for synthesizing LFO nanoparticles is based on a
reaction of aqueous solution of lanthanum salts and ferric salts with or without
structure-directing agent. The reaction takes place at a temperature of >100oC and a
pressure of > 1atm by using a sealed system, called autoclave, to produce LFO.
Afterwards, the obtained product is centrifuged, washed with water and ethanol for
several time and dried. Finally, the LFO powder is obtained after calcination.
Different parameters in the hydrothermal synthesis can be varied to control the sizes
and morphologies of product, including precursor material, structure-directing
agent, and temperature.

In general, LFO nanoparticles were produced from hydrothermal method by
complexing

lanthanum

nitrate,

ferric

salt

and

structure-directing

agent.

Interestingly, the size of final product could be controlled by changing starting
materials. According to the experimental results of Kumar and Jayavel, 45 nm LFO
nanospheres were formed by hydrothermal process with La(NO3)3.6H2O,
Fe(NO3)3.9H2O and citric acid as the starting materials (Dhinesh Kumar and
Jayavel, 2014). On the other side, by using another ferric source - K3[Fe(CN)6] to
replace Fe(NO3)3.9H2O acting with La(NO3)3.6H2O and citric acid, Thirumalairajan
and co-workers successfully fabricated LFO nanospheres with the average
crystallite size of 52 nm (Thirumalairajan, Girija et al., 2013).

So far, different structure-directing agents have been studied, including citric acid
(Dhinesh Kumar and Jayavel, 2014), cetyltrimethyl ammonium bromide (CTAB)
(Yao, Wang et al., 2013), sodium carbonate (Zheng, Liu et al., 2000), etc. It is
reported that synthesis of LFO nanoparticles with or without structure-directing
agent produced three different morphologies via the hydrothermal method
(Thirumalairajan, Girija et al., 2013). In this work, the hydrothermal synthesis was
carried out by selecting La(NO3)3.6H2O and K3[Fe(CN)6] as starting materials in the
presence or absence of structure-directing agent, (NH2)2CO or C6H8O7.H2O, at
14


180oC for 12 h. Figure 7 shows the change of product morphology synthesized

under different conditions. The LFO nanocubes which are produced in the absence
of structure-directing agents were indexed according to the perovskite phase with a
cubic structure (Fig. 5a); whereas the XRD pattern of LFO nanorods and
nanospheres, formed with the use of urea or citric acid (Fig. 5b-c), suggested the
samples as a perovskite phase of orthorhombic structure. By using Scherrer’s
fomular, the average crystallite size was found to be 52, 64, 85 nm for nanospheres,
nanorods and nanocubes, respectively. Therefore, the selection of an appropriate
structure-directing agent plays an important role in producing the desire sizes and
morphologies of products.

Figure. 5. XRD patterns of different LFO nanostructures (Thirumalairajan,
Girija et al., 2013)
Moreover, the changes in the ratio of metal ions to structure-directing agent should
be carefully control to improve the purity of the end products. Dhinesh Kumar and
Jayavel suggested the optimum condition to obtain the pure orthorhombic LFO be
1:2 molar ratios between metal ions and citric acid. The HRSEM image (Fig. 6)
shows the produced LFO nanoparticles with a sphere-like morphology and uniform

15


size distribution of approximately 45 nm. However, the use of metal and citric acid
with a molar ratio of less than 2 formed impurities in the product; whilst most of the
final particles were La(OH)3 when the ratio between metal and citric acid reached 1
- 1.5 (Dhinesh Kumar and Jayavel, 2014)

Figure. 6. Image of LFO nanospheres by HRSEM (Dhinesh Kumar and
Jayavel, 2014)
Ji et al., studied the effect of hydrothermal temperature on the crystallinity and
particle size of final products (Ji, Dai et al., 2013). The hydrothermal process took

place at 110, 140, 170 and 200 °C for 14 h, respectively. These samples were
denoted as LFO-110, LFO-140, LFO-170 and LFO-200. The authors realized that
the hydrothermal temperature had a significant influence on the obtained products.
In particularly, the average crystallite size of the LFO-110 was much smaller than
those of LFO-200 sample, but much bigger than that of LFO-140 and LFO-170
samples. The detailed results of processing parameters on the formation of LFO are
listed in Table 3.

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