Luận văn thạc sĩ synthesis lafeo3 for photodegradation of pollutants in wastewater 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

Moreover, the desired properties of LaFeO; for high photocatalytic activity under visible light, including narrow band gap energy, high surface area as well as high crystallinity, can

MINISTRY OF EDUCATION AND TRALNING

PIIAN CIH NHAN

SYNTHESIS LAFEO; FOR PHOTODEGRADATION OF

POLLUTA

MASTER TIIESIS

HA NOI - 2019

MINISTRY OF EDUCATION AND TRAINING

HA NOI UNIVERSITY OF SCIENCES AND TECHNOLOGY

PHAN CHI NHAN

SYNTHESIS LAFEO; FOR PHOTODEGRADATION OF POLLUTANTS

IN WASTEWATER

CHEMICAL ENGINEERING

MASTER THESIS

SUPERVISOR ASSOCIATE PROFESSOR: PHAM THANH HUYEN

HA NOI - 2019

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

ina good condition and leam 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

DD Optical property sarccscssssivssecessacncscssiarsncservessesvcivvasiniseonvascanvebicessaveedwasiansviwist

2.3 Magnetic property 2220122 gùiioiS0tg16l4ã.030ã8Gưạn 6

3.1 Application in textile wastewater treatment HA ASSN RN NRA

3 Experiment ofphotocatalytic activity Ee:

CHAPTER 3 RESULTS AND DICUSSION .cscssscsssssscsssnsssonessesecesssssesiecnnnsces 33

1: YRRRNETJQHRNINŠ xagiuaeatustitdnhgortoooidisfifildshilediidgaatlegitnsgssgrsnaŸSl

2 XRD analysis

3 Morphological analysis

4 BRT analysis

$ Band Gap energy result .cssscssesietntunsnietseiiieuseaie

6 Calibration curve of dyes and 170-EstradioL s2

7 Photocatalytic activity of LFO nanoparticles

9 Lffect of catalytie concentratian on photodegradation effieiencies

10 Effectof H:Ozconeentration ơn photodearadatiem efficieneies

11 Comparison of catalytic activity for diEferent dyes

LIST OF TABLES Table 1 Optical band gap values of LFO nanomaterials synthesized by various

methods BHGOENHONNGTGNGIDEREHGUWHNGIGESGIdUHgt2ggat su

Table 2 Particle size and morphologies of LFO synthesized by sol-gel method

Table 3 The influences of the hydrothermal temperature on the formation of LFO powders (Adopted ữom (J1, Dai et al., 2013)) - seese 7

Table 4 Photocatalytic performance of LFO for the degradation of dyes .20

Table 5 Degradation rate of dyes solution on different nhàng LFO

nanoparticles ccccccee lu/sxmsStgiSiuyDmsiubishdszssussvgseMsvf B4:

Table 6 Effects of calcination temperature on the BET specific surface area and

pore parameters of LFO nanoparticles đến 634611034488-0d061108/4088i0080.80.a0.0sssg T55

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 eons Liu et al., 2011) (a) and

Figure 5 XRD patterns of different LFO nanostructures (Thirumelairajan, Girija et

Figure 6 Image of LFO ania by HRSEM hinesh XRHinät and Jayavel,

2014) eroreiqseseornseettenrtbvqreorgremmeenceeens “ 16

Figure 1 RhB degradation % by using LFO samples with different particle sizes

(Thirumalairajan, Girija et al., 2012) veosszagetatopsse/JIÐ) Figure 8 SEM images of LFO nanostructures %:8h:6ữ7&rant 'ti60idiölogf66 ()

nanocubes; (b) nanorodes; (c) nanospheres; (d) dendritic nanostructures (e) floral-

like 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., 201 4) 23

Figure 9 (a) XRD pattem at different calcination temperatures; (b and c) degradation of RhB and MB in the presence of LFO calcined at 800°C

Figure 10 (a) XRD pattern of LFO samples saline at t different — (b) Degradation of RhB with the use of different LFO rons and P-25TiO; (Su, Jing

Figure 11 TGA curve of synthesized LaFeO; acne aan:

Figure 13 SEM images of (a) LFO-C600, (b) LFO-C700, (e) LFO- C800, and (đ)

TRO CS ccorseeioeedinniesretrtiolenlge100450010910014018.000060314483.S00 43150086E1800eS1sensax29,

Figure 15 Nitrogen adsorption-desorption isotherms of, the LFO nanopertigtis

Figure 16 (a) LFO solid UV-vis absciption 4pevtrti sả ® Scheniatic of

determination of band gap energy - 2222221221211 38

Tigure 18 Cahbrabem curve of MB - 39 igure 19, Calibration curve oŸ MÓ ceoeniirrrree sueeserseroo.3Đ

Fig, 21 The photodegradation elioieneies of RAB under visible light imadation by

different photocatalysts sesseseesuatisunttnssanssseseeeseeessseeeseneee van vvasees 40

Tigure 22, The change in colour of RKB solution during the reaction

Fig.23 Photodegradation efficiencies of Mety! Orange under differences condition

Fig.25 Effect of HạO¿ concentration ơn photodegradation efficiencies 45

Fig.27 ‘the photodegradation efficiencies of 17ƒ-Hstradiol under visible light

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, ineluding 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-Rodriguez, Oller et al., 2013),

photocatalytic oxidation (Zangeneh, Zinatizadeh et al, 2010; Gủmủs and Akbal,

2011; Rodriguez, 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,

Couch, Mevyk of al, 2014), Tn particular, he AOP using pholocatalysts - irradiated semiconductor systems has been suggested as a promising way in environment remediation, because it is efficient, cost-effective and environmental friendly by utihzing solar ight or artificial irradiation, which is abundant wherever (Liu, Niu el

al, 2013; Lai, Juan et al., 2014; Zhao, Tian et al., 2014) ‘This slows 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 semiconduetors have been used as photocatalysts, including TiQ2 (Nakala and Fujishima, 2012), ZnO (Yu, Shi et al, 2013), ZrOs (Basahel, Ali et al, 2015), SnOz (Cheng, Chen et al, 2011), Ceo (Wu, Wang et al., 2015), and InO2 (Li, Zhang et al., 2013) for degradation ol a wide range of envirormncnlal contaminants However, thei applications are limited because these semiconductor catalysts have high activity only under UV illumination, which presents ~ 5% of solar energy spectrum Far this reasơn, many efforts have been made in order to find different altematives harvesting the solar light and afterwards utilizing it in large-scale application Recently, perovskite-based malorials have Iiean reporied as e» olleut visible-light-driven photocatalysis, (Karihere 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), tanlalate perovskites (Li and Zang, 2009: Burtik, Vanck of al., 2013), ferrite perovskites (Sun, Jiang et al, 2010; Soltani and Hntezari, 2013) and complex perovskite materials (Zim, Fu et al, 2008, Clark, Dyer et al, 2010) Among ferrite perovskites, lanthanum ferries, LaFeQs, need to be futher studied due to their interesting physical properties as well as potential applications m photocatalyst, fuel cell, sensors and permeation membranes (Gabal, Ata-Allah et al, 2006; Yoo, Kim et

aL, 3011)

Thus, the study of LaFeOs; materials with properties suitable for photocatalytic

requirements is really necessary So I decided to choose a topic : “ Synthesize LaFeOs

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

LaFeO; 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 LaFeO; 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 LaFeOs is

essential

21 Structure

Lanthanum ferrite, LaFeOs (LFO), belongs to Pbnm space group with the lattice

parameters, a = 5.557 A, b = 5.565 A, c = 7.854 A (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 FeOs octahedral All of the Fe** ions are octahedral surrounded by oxygen ions and the La®* ions are inserted in the interspaces between the FeOs octahedra (Fossdal, Menon et al., 2004, Bellakki,

Manivannan et al, 2009) Fig 1 illustrates the crystalline structure of LFO, in

which the La?" cations are đisplayed as grey spheres and the FeOs 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

22 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

ahw =.A(hv - E¿)!2

where hv is the photon energy, @ is the optical absorption coefficient, A is a

constant and Z; is the band gap energy

It is believed that this optical band gap (Ey) value strongly depends on the

preparation procedure of material as well as particle size (Roduner, 2006,

Koferstein, Jager 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

Microwave- 34 2 236 ŒT: ‘ang, Tong et al., 2 T 1, 2013)

In the orthoferrite family, LFO is an interesting antiferromagnetic material with the

highest value of Neel temperature (Tw~ 740°C) (Scholl, Stohr 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

are in pair, which indicates La** is non-magnetic and subsequently there is no

magnetic interaction between La?" and Fe** 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 Fe** 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 emwg at ambient temperature (Shen, Cheng et al., 2009) which is considerably smaller than those of LFO nanoparticles (~ 21.9 nm) prepared by sol-

gel 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 emwg)

(Hui, Jiayue et al, 2010) Obviously, the magnetization values of LFO nanomaterials in those studies are considerably different This also suggests that the

LFO particle sizes leads to an increase of magnetization values (Phokha,

Pinitsoontor 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,

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 RKB, MB, MO, LFO nanomaterials were also used as photocatalysts for

degradation of several other dyes For example, Abazari ef 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 ef 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 170-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

Visible light irradiation

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 O3 presenting in

water to form superoxide anion radicals O;~ (the reducion of Oz), which

subsequently combine with H” to fom the hydroperoxy radieals HO; 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 ef al (Chong, Jin et al., 2010)

LaFeOs + hv > LaFeOs + e’cs + h’vp

OH'+ dye ->CO2 + M0

h'yp + dye -> CO¿ + HạO

ecg + h'vp > Av + (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

produced by the conventional solid state reaction exhibit slow kinetics, nonuniform

particle size and low surface area (Popa and Moreno, 2011) Meanwhile, the co-

precipitation 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

41 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 60-70°C under magnetic stirring, resulting in a gel state with high viscosity Then, the prepared sample is dried at 70°C overnight Finally, LFO is obtained by calcining the dried sample at

appropriate temperature (600-700°C) 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 ef 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

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

were obtained at 700°C 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 ef al., successfully synthesized nanosized LFO with a

diameter of about 30 nm at lower synthetic temperature (500°C) 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

42 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).

‘Typically, hydrothermal process for synthesizing LO nanoparticles is based on a

reaction of aqueous solution of lanthanum salts and ferric salts with or without

structure-direcling agent The reaction takes place al a temperature of >100°C and a pressure of > latm by using a sealed system, called autoclave, to produce LKQ Afterwards, the obtained product is centrifuged, washed with water and ethanol for several tume 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, T.FO nanoparticles were produced from hydrothermal method by

complexing lanthanum nitrate, ferric sall and struclue-direcling agenl Interestingly, the size of final product could be controlled by changing starting

Tnalerials According to the experimental results of Kumar and Jayavel, 45 na FO

nanospheres were formed by hydrothamal process with La(NOs):.6H20, Fe(NO3)s.9b0 and citric acid as the starting materials (Dhinesh Kumar and

Tayavel, 2014) On the other side, by using another ferric source - K3[Fe(CN)e] lo

replace Ke(NOs)s.9H20 acting with La(NOs)s.6H20 and citric acid, Thirumalairajan and co-workers successfully fabricated LFO nanospheres with the average

crystallite size of 52 nm (Thirumalairajan, Girija of al., 2013)

So far, different structure-directing agents have been studied, inchding citric acid (Dhinesh Kumar and Jayavel, 2014), cetylirimethyl ammonium bromide (CTAB) (Yao, Wang et al., 2013), sodium carbonate (Zheng, Lin et al., 2000), etc It is reported that synthesis of LFO nanoparticles with or without structure-directing agent produged three different morphologies via the hydrothermal method (Thirumalairajan, Girija et al., 2013) In this work, the hydrothermal synthesis was carried out by selecting La(N'O,)3.6H20 and Ka[Fe(CN)6] as starting materials in the presence or absence of siructuro-directing agent, (NH22CO or CeHeO7.Hi0, al

180°C 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 pattem of LFO nanorods and

nanospheres, formed with the use of urea or citric acid (Fig, Sb-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

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

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); 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

Table 3 The influences of the hydrothermal temperature on the formation of

LFO powders (Adopted from (Ji, Dai et al., 2013))

Catalyst XRD results BET surface | Average crystallite

area (m?/g) size (nm)

and trace hexagonal La(OH)s

Consequently, there is no doubt that hydrothermal method enables the fabrication of highly crystallized and well-defined powders at relatively low temperature (Lu,

Yuan et al., 2013) Moreover, this method have allowed to produce LFO with

various morphologies, including spheres (Dhinesh Kumar and Jayavel, 2014),

floral-like nanostructure (Thirumalairajan, Girija et al., 2014), nanowires

(Thirumalairajan, Girija et al., 2012), cubes and nanorods (Thirumalairajan, Girija

et al., 2013) It appears that such differences in size, crystallinity and morphology of

photocatalyst could be better controlled during hydrothermal process

43 Sol-gel hydrothermal method

Taking the double merits of both hydrothermal and sol-gel process, sol-gel-

hydrothermal method offers high purity, high crystallinity, well-controlled morphology and narrow particle size distribution of the product while carrying out the reaction under moderate temperature (Li, Hou et al., 2005; Wang, Cao et al., 2013) In perovskite family, various materials have been synthesized by sol-gel-

hydrothermal method for photocatalyst purpose, including NaTaO; (Hu, Tsai et al.,

2009), SrTiO; (Yu, Ouyang et al., 2011), Biz2WOs (Liu, Tang et al., 2014), BiFeOs

(Liu and Zuo, 2012), however, to the best of our knowledge, synthesizing LFO nanoparticles by using sol-gel-hydrothermal method has not been reported in open

literature.

From above discussion, it can be concluded that LFO nanoparticles with different

morphologies as well as particle sizes can be fabricated by either employing

different synthetic methods or altering processing parameters The effect of particle

characteristics on the photocatalytic performance of LFO will be discussed in detail

ina following section

5 Factors affecting photocatalytic activity of LFO material

Perovskite-based photocatalyst prove to be promising materials with good

photostability and high photocatalytic activity due to unique structure and optical

property (Wu and Zhang, 2004; Xiong, Tian et al., 2015) Moreover, the vacant

metal or oxygen sites in the structure of perovskite are able to promote oxygen

adsorption onto the surface of photocatalyst, which boosts the reaction (Pena and Fierro, 2001; Worayingyong, Kangvansura et al, 2008), Generally, the photocatalytic performance could be affected by particle size, band gap energy,

morphology and crystalline structure From a technological point of view, the

properties of photocatalyst, including morphology, surface area, and crystallinity are strongly dependent on the preparation procedures

5.1 Surface area

From an aspect of photocatalytic application, the specific surface area of semiconductors considerably affects the activity of photocatalyst It is believed that surface area is closely related to particle size of material In particular, during the

photoreaction, an undesirable step is that the recombination of electron-hole pair

before they migrate to the photocatalyst surface (Parida, Reddy et al., 2010) The small size of photocatalyst reduces the distance from the place of electron-hole pair generation to photocatalyst surface; thereby photogenerated charge carriers have more chance to migrate to surface before recombination taking place These captured carriers are able to initiate photoreaction, causing the enhancement in photocatalytic activity This has been demonstrated by the degradation efficiency of

RhB when using LFO nanoparticles with different particle sizes as photocatalysts in

Figure 7 (Thmumalairajan, Gimija et al., 2012) Nano-sized LFO with an average particle size of approximately 90 nm was found to decompose more than 76% of

RbB after 180 min (Thirumalairajan, Girija et al., 2013) The efficiency could be

further improved to 93% and 96% if the particles possessed an average size of 85

nm and 70 nm, respectively (Thirumalairajan, Girja et al., 2012) On the other side, the activity of photocatalyst decreased with increasing particle size

Figure 7 RhB degradation % by using LFO samples with different particle

sizes (Thirumalairajan, Girija et al., 2012)

Moreover, the smaller average particle size leads to an increase in specific surface

area of photocatalyst and in tum enhances dye decomposition The large specific

surface area creates favourable conditions for photocatalytic reactions In terms of

nanometer regime, many studies had reported the inverse relation between specific

surface area and average particle size of photocatalyst (Abazari, Sanati et al., 2014);

that is the larger particles of LFO exhibits smaller surface area Therefore, specific surface area of LFO nanoparticles is believed as a key factor in evaluating their

photocatalytic activities Table 4 summarizes some of recent studies on the fabrication of nano-sized LFO by using different methods and the resulting

photocatalytic performances

19

As seen in Table 4, a catalyst with high surface area is more desirable because it can

achieve much more active sites where pollutant molecules can be absorbed to

undergo its degradation by reacting with charge carriers coming from excitation of

semiconductor For example, the nanosized LFO having a specific surface area of

278 m?/g displayed excellent photocatalytic performance for RhB decomposition as

the yield was found to be 80% (Xiao, Hong et al., 2013) These results were found

to be relatively higher compared to the reported value of synthesized LFO/SAB-16,

with 85 m?/g of surface area and decomposing efficiency of RhB was 35% (Su,

Jing et al, 2010) By contrast, LFO with a specific area of 6.4m?/g evidently has

showed low photocatalytic activity The degradation rate of phenol just achieved 30%

(Wu, Hu et al., 2015) Therefore, the higher the surface area is, the higher the

photocatalytic activity of LFO nanoparticle is Consequently, the high surface area of LFO nanomaterial could benefit the degradation of dyes in aqueous solution

Table 4 Photocatalytic performance of LFO for the degradation of dyes

Method Particle | Surface | Dye | Degradati | Illuminat | References

d Jayavel,

2014)

5.2 Band gap energy

The narrow band gap of the semiconductor can absorb more photons to generate electrons in the CB and holes in the VB, which then carry out redox reactions

(Dong, Xu et al., 2009), For this reason, semiconductor photocatalyst with low band

gap energy would be more desirable for efficient light harvesting, thus improving photocatalytic activity

It is reported that LFO is a high visible-light-responsive photocatalyst as a result of

its narrow band gap (Yang, Zhong et al., 2009; Ding, Lit et al., 2010) The E, value

of LFO nanoparticles have been reported in the range of 2.1 eV - 3.85 eV (Bellakki,

Kelly et al., 2010; Popa and Moreno, 2011; Sorescu, Xu et al., 2011; Tang, Fu et al., 2011), which is suitable for visible region, accounting for ~ 43% of the total energy from the sun, with a wavelength ranging from 400 nm to 800 nm This absorption

range is interesting because LFO nanoparticles could be developed to be a new

visible-light-driven photocatalyst, which has potentially useful for photocatalytic as well as other photoactive applications

In particular, Parida et al., successfully synthesized LFO nanoparticles by a sol-gel auto-combustion method having a band gap energy of 2.1 eV (Parida, Reddy et al.,

2010) Phokha ef al., have reported the direct band gaps in the range of 2.15 eV —

2.23 eV for all samples of LFO nanoparticles prepared by polymerized complex

method, which was synthesized on polyester network of ethylene glycol and citric

21

acid solution to better distribution of ions (Phokha, Pinitsoontom et al., 2014) Meanwhile, LFO which were synthesized by calcining an emulsion method had the energy band gap level of 3.85 eV (Chandradass and Kim, 2010) UV-Vis NIR spectrum of this sample indicated that the LFO nanocrystals synthesized by this method could be a type of photocatalyst

5.3 Morphology

Particle morphologies play an important role in determining photocatalytic performance Figure 8 shows LFO with diverse morphologies prepared by various methods; and Table 5 summarizes the photocatalytic performances of LFO with different morphologies on dyes Seen from Figure 8 and Table 5, the photocatalytic activities of LFO nanoparticles under visible light vary considerably according to the morphologies of samples The LFO nanospheres displayed the relatively high

photocatalytic performance, which could be ascribed to the small crystallite size,

large surface area and uniform pore size distribution

Figure 8 SEM images of LFO nanostructures with different morphologies: (a)

nanocubes; (b) nanorodes; (c) nanospheres; (d) dendritic nanostructures (¢)

floral-like 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)

Three different morphologies of LFO nanostructure, including nanorods, nanocubes

and nanospheres were reported by Thirumalairajan and co-workers

(Thirumalairajan, Girija et al., 2013), The efficiency of RhB decolorization under

visible light was found highest, 90.80%, as compared with those of the samples

with other morphologies, 88.36% for nanorods and 76.81% for nanocubes,

respectively, In the same photocatalytic activity test condition such as visible light

source, irradiation time (180 min), the MO degradation rate with the use of LFO nanospheres was about 83% (Dhinesh Kumar and Jayavel, 2014) Other recent

studies have also explored and studied the influence of different morphologies on the photo-decomposition of organic contaminants For example, Thirumalairajan et

el found the floral-like LFO nanostructure comprised of nanosheets exhibited better

photocatalytic efficiency (92.85%) than LFO nanofibers (80%) in photodegradation

of MB (Li, He et al., 2014; Thirumalairajan, Girija et al., 2014)

Table 5 Degradation rate of dyes solution on different morphologies LEO

nanoparticles

Catalyst | Morphology | Dye type and | Catalyst | Degradation | Reference

concentration | loading rate (%) §

Gia set al., 2014) LFO Nanofibers | MB, 10mg/l 100 75 (Li, He et

aL, 2014)

5.4 Crystallinity

Crystallinity, as another factor that can effect on the photocatalytic performance of

LFO photocatalyst, strongly depends on the nature of the starting material as well as

the synthesis method (Bessekhouad, Robert et al., 2003) It is commonly known that

the crystal structure is one of essential properties of the particles, thus the

investigation on it is needed (Guo, Quan et al., 2007)

A photocatalyst with a good degree of crystallinity, which is presented by narrow and strong diffraction peaks in XRD spectrum, is necessary because it helps suppress recombination between photogenerated charge carriers, which improves

photocatalytic efficiency (Li, Liu et al., 2013, Martin, 2014) In contrast, the

amorphous characteristics of compounds induce a large amount of defects and each one acts as a recombination centre when electrons and holes are generated under

illuminations (Bessekhouad, Robert et al., 2003) For example, Thirumalairajan et

al reported that when the sample of LFO with a floral structure is calcined at 600°C, the amorphous phase disappeared and characteristic peaks of LFO appeared

with low intensity at 650, 700 and 750°C Well crystalline powders were obtained

after increasing the calcination temperature to 800°C, which possessed a high

degradation efficiency to RhB (90.66%) and MB (92.85%) under illumination of visible light (Thirumalairajan, Girija et al., 2014), as shown in Figure 9

Irradiation time (min) Irradiation time (min)

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)

Su and co-workers have also proven that the increase of calcination temperature led

to an enhanced photocatalytic activity; the LFO nanoparticles calcined at 900 °C

exhibited the best photocatalytic performance, with more than 50% of RhB degraded (Su, Jing et al., 2010) It is worth noted that all the LFO samples exhibited higher photocatalytic activity in visible light region than P25 TiO; Figure 10

displayed the X-ray diffraction patterns of LFO calcined at different temperatures

and photocatalytic performances of as-prepared LFO compared to P25 TiO: in visible light for degradation of RhB solution

26

Adsorption in the dark 900°C

fa User ihe radon

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-25TiOz

(Su, Jing et al., 2010)

To conclude, one of factors affecting photocatalytic performance is attributed to the

high crystallinity of sample This implied that the high crystallinity of LFO

nanoparticles which caused by high calcination temperature could promote the

photoinduced charge separation, thus enhancing the photocatalytic activity (Jin, Wu

et al., 2006)

5.5 Dopants

Previous studies have reported that the enhanced photocatalytic activity and

superparamagnetic property of perovskite-based photocatalysts could be achieved

by several methods, including doping or adding a desirable component to the A or

B site One of the purposes of doping strategy is to supress the recombination of electron — hole pairs by reducing the particle size (Wang, Wang et al., 2012; Wei, Wang et al., 2012) For example, among the various doping strategies, Ca and Zn were used to dope LFO nanoparticles at A and B site, respectively Typically, after

the substitution of A site with Ca, 10 mol% of Ca-doped LFO (LaixCaxFeOs)

photocatalyst exhibited a larger light absorption capability at 4 > 420 nm than that

of pure LFO, thus improving the photodecomposition efficiency of MB from 48.9%

to 77.5% after 1 h photoirradiation (Li, Liu et al., 2010) The results revealed that

the substitution of Ca** for La** led to a decrease of crystallite size and improve

visible light adsorption capability of the photocatalyst as well as photocatalytic

performance The other reason is related to enhancement of photocatalytic performance could be discussed as follow Photocatalytic performance is associated with the formation of © vacancies and Car.’donor centres due to displacement of La® by Ca ion, When loading of dopant was low, the number of Cars’donor centres increased, resulting in an increase of photogenerated electrons-holes and thus favouring the photocatalytic performance Ilowever, when the doping concentration was too high, the dopant acted as an photoinduced electron-hole recombination centre, causing the deactivation of photocatalyst (Asilttirk, Sayilkan

et al, 2009)

In comparison with B-site doping process, Dang et al, evaluated photodegradation

yate of MB by using nano-sized LaFer.7n.O; synthesized by a sol-gel auto-

combustion method Based on (heir resulls, we cam sce thal dhe photocatalytic activity of LaFe).,Zn,(; nanoparticles with Zn doping at B site was higher than that

of undoped LFO, which reached to 75% degradalion rale as compared wilh 45%

degradation rate of undoped LFO This was because the partial substitution of Fe"

in LFO with Zr" Jed to the increase of lattice constant of Zn-doped LF'O, whilst its

crystal size decreased and in lum specific surface arca increased (Dong, Xu et al, 2009)

So far, different metals, e.g Li Al, Ag have been utilized as dopants to vary the

inherent properties of perovskite-based photocatalysts (Hou, Sun et al, 2006; Acharya and Chakrabarti, 2010; Desai and Athawale, 2013); whilst non-metal doping is

considered as a new roule lo improve the photocalalytic performance of 1.FO for dye

decomposition ‘Ihe photocatalytic performance of 3.5% 13-doped LI and undoped

‘LFO were studied by decomposing of phenol under the condition of simulated sunlight

irradiation Afler 300 min exposed to light souree, Ihe degradation rate of 3.5% Be doped LFO photocatalyst reached 87.9%, which was higher than that of undoped LFO