65

CHAPTER 4

PREPARATION, CHARACTERIZATION OF PEROVSKITE

LaFeO3 NANOSPHERES AND ITS VISIBLE LIGHT

PHOTOCATALYTIC ACTIVITY

4.1 INTRODUCTION

Perovskite-type materials are important functional materials, having

a general formula of ABO3 (where A is a rare-earth element and B is a 3d

transition metal) are very promising due to their innovative use in advanced

technologies. Nowadays perovskite-type oxide RFeO3 (where R stands for

rare-earth element) materials have attracted considerable attention because of

their useful application in many fields (Liu et al 2011, Farhadi et al 2009,

Feng et al 2011). They have attracted considerable attention for various

applications, such as solid oxide fuel cells (Fujii et al 2011, Li et al 2011),

sensors (Farahadi et al 2009, Feng et al 2011), environmental catalysts

(Farahadi et al 2009, Feng et al 2011, Fujii et al 2011, Li et al 2011) and

magnetic material (Abazari et al 2013, Wang et al 2206, Li et al 2007) etc.

LaFeO3 is one of perovskite-type oxide that has an orthorhombic perovskite

structure. LaFeO3 is known to be antiferromagnet with a Neel temperature TN

of 738 K (Fujii et al 2011). Recently, rare-earth perovskites nanoparticles,

such as LaMnO3, LaFeO3, GdFeO3, BiFeO3, TbFeO3 and YFeO3, have

received increasing attention and been applied as photocatalysts (Li et al

2009, Li et al 2014, Li et al 2012, Wang et al 2011, Yang et al 2013, Stevens

et al 2014). When compared with bulk these nanoparticles have higher

66

photocatalytic performance due to their higher catalytic activity and larger

surface area, but their active life is short due to their instability and tendency

to aggregate in to bigger ones.

The LaFeO3 has received considerable attention because of its

strong photocatalytic activity (Tijare et al 2012, Li et al 2010, Gao et al 2012,

Thirumalairajan et al 2012). Recently, LaFeO3 powders were found to exhibit

visible-light photocatalytic activity due to their narrow band gaps (Eg < 3.0

eV). It was found that the structures and properties of the materials were

highly dependent on synthesis process of its precursor powders, so the

improvement of preparation method of these powders is paid much attention

by researchers (Kaiwen et al 2013). The preparation of LaFeO3 have been

achieved by many methods, including solid-state reaction (Kaiwen et al 2013)

sol-gel (Feng et al 2011, Li et al 2007, Rusevova et al 2014), hydrothermal

(Thirumalairajan et al 2012), microwave assisted (Farhadi et al 2009) and

polymerizable complex method (Popa et al 2002, Popa et al 2011) etc.

LaFeO3 are commonly prepared by solid-state reactions at high temperatures

(>900 °C). This technique, which uses metal oxides as starting precursors and

requires several annealing processes at high temperatures during long periods

of time with frequent intermediate grindings, has several problems, e.g., poor

homogeneity and high porosity of the samples, no control on the particle size,

etc. The development of innovative processing methods through chemistry

permits one to lower the preparation temperature and to improve homogeneity

and reproducibility of the ceramic products, with the synthesis of ultrafine and

chemically pure powders of mixed-metal oxides at low temperatures. Among

the various wet chemical methods, hydrothermal method is a facile dominant

tool for the synthesis of nanoscale material. The main advantages of this

method are low temperature growth, controlled size, simple and cost

effective. In general, citric acid is a common and good chelating agent for

metal ions to form coordination complexes. Lanthanum citrate and ion citrate

67

coordination complex can easily be synthesized by hydrothermal reaction.

Therefore, it is attractive to develop a low-cost and facile hydrothermal

approach for the synthesis of LaFeO3.

The novel LaFeO3 nanosphere has been prepared by hydrothermal

method. The prepared nanospheres were subjected to structural,

morphological, magnetic and optical property studies. The photocatalytic

activity of LaFeO3 nanospheres were investigated by the degradation of MO

under the visible-light irradiation. In addition, the possible reaction

mechanism for the formation of LaFeO3 nanospheres was discussed in detail.

4.2 EXPERIMENTAL DETAILS

A stoichiometric amount of lanthanum (III) nitrate hexahydrate

[La(NO3)3.6H2O] and iron (III) nitrate nanohydrate [Fe(NO3)3.9H2O] were

dissolved in 50 ml of double distilled water under magnetic stirring.

Appropriate amount of citric acid was added to the solution. The molar

amount of citric acid added to the solution was taken in three different ratios

1:1, 1:1.5 and 1:2. The solution was continuously stirred for 5 h at room

temperature. Ammonia solution was slowly added to adjust the pH 9.2 and

also to stabilize the nitrate-citrate solution. After stirring for 3 h at room

temperature, the resulting mixture was -lined

stainless-steel autoclave with a capacity of about 70 mL, the autoclave

was sealed and heated at 180 °C for 20 h and then cooled naturally to

room temperature. The obtained precipitate was collected and washed

with deionized water and anhydrous ethanol several times and then dried

in air at 100 °C, followed by calcination at 800 °C for 6 h to obtain LaFeO3

powder.

68

4.3 CHARACTERIZATION STUDIES

The as-synthesized sample was first subjected to thermal analysis in

order to find the decomposition, stability and phase formation of the material

using SII TG/DTA 6300 thermal analyzer. The TGA curve was recorded in

the range from 27 to 900 °C with a heating rate of 20 °C/min under nitrogen

atmosphere. Powder X-Ray diffraction analysis (XRD) was carried out for the

annealed samples with a GE Inspection Technologies 3003TT model

1

kV, and 30 mA. The X-

from 10º to 70º and was inspected using JCPDS data to identify the

crystallographic phases. The FT-IR measurements have been performed in the

potassium bromide mode (KBR) using the model 6300 FT-IR

spectrophotometer. The vibrational characteristics of the LaFeO3 powders

were investigated using Renishaw invia Raman microscope, Leica DMLM,

RL663 laser. The surface morphology of synthesized samples and elemental

presence has been studied by high resolution scanning electron microscopy

(HRSEM) using FEI Quanta FEG 200 microscope operated at 20 kV. The

particles size and morphology of the synthesized sample has been studied by

high resolution transmission electron microscopy (HRTEM) using Tecnai G2

model T-30 s-twin electron microscope with an accelerating voltage of 300

kV. The atomic force microscopy (AFM) measurements of the sample have

been performed in non-contact mode at room temperature using XE 70 Park

system. X-ray photoelectron spectroscopy (XPS) has been done to confirm

the oxidation state of the elements present in the sample. The XPS data were

collected using Omicron Nanotechnology instrument with a binding energy

resolution of 0.7 eV. The optical absorption study of the synthesized sample

has been carried out in the range of 200-900 nm using Cary 5E high

resolution spectrophotometer. The vibrating sample magnetometer (VSM)

69

measurements were recorded for two different temperatures (20 K and 300 K)

using Lakeshore VSM 7410.

4.4 RESULTS AND DISCUSSION

4.4.1 Thermal Studies

The optimizing annealing temperature of the synthesized powder

was carried out by TG analysis and shown in Figure 4.1. TGA curve shows

four weight loss segments. It can be seen that the first and last segment are

due to the elimination of water and the crystallization process of the material.

The second significant weight loss observed between 200 to 310 °C is mainly

due to the decomposition of organic compounds like C N, C H and C=O.

The third weight loss was up to ~500 °C may be due to the decomposition of

nitrates (NO3-). There is no further weight loss was observed after ~700 °C

which confirms the possible formation of metal oxide phase of perovskite

LaFeO3. With respect to above result the sample was annealed at 800 °C.

Figure 4.1 TGA curve of as-synthesized LaFeO3 sample

70

4.4.2 X-ray Diffraction Studies

Figure 4.2 XRD patterns of LaFeO3 sample synthesized at different metal nitrate to citric acid ratio (1:1, 1:1.5 and 1:2)

Figure 4.2 shows the XRD patterns of the products synthesized at

different concentrations of citric acid. The XRD pattern of as-synthesized

sample shows an amorphous phase while increases the temperature to 800 °C

single phase orthoferrite with high crystallinity was obtained. All the

diffraction peaks of the sample were assigned to orthorhombic phase and are

indexed on the basis of JCPDS file No. 37-1493. The strong and narrow

diffraction peaks indicate that the obtained material has a good crystallinity

and size. No characteristic peaks from other impurities are detected hence the

prepared LaFeO3 sample (metal nitrate to citric acid ratio 1:2) was highly

pure. Using Debye Scherrer formula the average crystallite size was

calculated and it was found to be about 45 nm. It can be seen that 2 ratio of

citric acid is an optimum condition to get the pure perovskite orthorhombic

71

LaFeO3, and less than 2 ratio of citric acid will induce impurities in the final

products. When both 1 and 1.5 ratio of citric acid was used, the products were

almost La(OH)3 and small LaFeO3 was obtained. Hence the purity of the

products was enhanced as the usage of citric acid increases to 2 ratio. This

result is similar to previous report (Thirumalairajan et al 2012).

4.4.3 FT-IR Studies

Figure 4.3 FT-IR spectrum of LaFeO3 sample

FT-IR spectrum of LaFeO3 sample (metal to citric acid ratio 1:2)

recorded in the wavenumber range from 400 to 4000 cm-1 is shown in

Figure 4.3. The band observed at 3443 cm-1 is characteristic of the O H

bending mode of absorbed water or the hydroxyl group. Also, the absorption

band at about 1486 and 1387 cm-1 c 3

asymmetric stretching of metal carbonates. Usually, carbonate species

formation on the surface of perovskite-type oxide is observed. From the

72

spectrum it was observed that the strong absorption band at 555 cm-1 indicates

the formation of LaFeO3. The 555 cm-1 band is attributed to the Fe O

stretching vibration, being characteristics of the octahedral FeO6 groups in

LaFeO3 which is commonly observed in the region 500-700 cm-1 for the

perovskite (ABO3) compounds (Feng et al 2011, Abazari et al 2013).

4.4.4 Raman Studies

Figure 4.4 Raman spectrum of LaFeO3 sample

Raman vibrational spectroscopy is known to be a powerful

technique to determine the structure distortion and oxygen motion of

perovskite-type materials. Therefore, we can use the Raman spectrum of

LaFeO3 to study the order disorder effects in the lattice. Figure 4.4 shows

room temperature Raman spectrum measured for orthorhombic structure of

LaFeO3 nanospheres. Raman spectrum of LaFeO3 presents bands around 434,

628, 960, 1135 and 1320 cm-1 are similar to previous report (Popa et al 2002,

Wang et al 2006). The bands at 434 and 1135 cm-1 were assigned to the one

73

phonon scattering, while 1320 cm-1 were assigned to the two phonon

scattering (Lee et al 2014). The band at 630 cm-1 band in LaFeO3 was

assigned to impurity scattering (Lee et al 2014, Gallego et al 2013,

Chandradass 2010, Koshizuka 1980).

4.4.5 High Resolution Scanning Electron Microscopy Studies

Surface morphology of LaFeO3 samples were characterized by

HRSEM and the micrographs were shown in Figure 4.5. Low and high

magnification of HRSEM images of the prepared LaFeO3 sample shows

narrow size distribution at a large scale. In addition, it was noticed that the

hydrothermal synthesis method produced highly monodispersed LaFeO3

nanospheres with average size of about 45 nm. Further no other impurity

particles and aggregates are detected.

Figure 4.5 HRSEM images of LaFeO3 nanospheres

74

4.4.6 High Resolution Transmission Electron Microscopy Studies

Figure 4.6 shows the TEM images and corresponding HRTEM

images of the LaFeO3 nanospheres. It was observed for the TEM image that

the LaFeO3 sample shows sphere-like morphology with uniform size

distribution. From the HRTEM image the interference fringe was clearly

observed with d-spacing of 0.393 nm corresponds to the (101) plane. The

SAED pattern (inset of Figure 4.6b) clearly indicates the crystalline nature of

the LaFeO3. From the image J software it was found that the average diameter

of nanosphere was about 45 nm. The size of the particle agrees well with both

HRSEM and XRD results.

Figure 4.6 TEM and HRTEM images of LaFeO3 nanospheres

4.4.7 Atomic Force Microscopy Studies

AFM analysis has been implemented as a structural

characterization technique for the examination of nanopowders materials in

non-contact mode. AFM has proved to be very helpful for the determination

and verification of various morphological features and parameters. Figure 4.7

shows two-dimensional (2D) and three-dimensional (3D) AFM surface height

morphologies of LaFeO3 sample with different scanning areas 2.5 X 2.5 µm,

75

1000 X 1000 nm and 500 X 500 nm. The 2D images confirm that the

nanospheres were uniformly distributed in surface of the sample and hence

the average size of the nanospheres was about 45 nm which was consistent

with HRSEM result.

Figure 4.7 2D and 3D AFM images of LaFeO3 nanospheres

76

4.4.8 Formation Mechanism of LaFeO3 Nanospheres

The formation of LaFeO3 nanospheres can be obtained by the

following reaction mechanism,

[La(NO3)3.6H2O] + [Fe(NO3)3.9H2O] + [C6H8O7] + 3O2 LaFeO3 + 6NO2

+ 6CO2 + 19H2O

La(NO3)3. 6H2O and Fe(NO3)3 .9H2O dissolved in deionized water

with 1:1 ratio followed by that 2 ratio of citric acid was added to the solution.

They readily react with each other and finally produce LaFeO3 after heating it

with 800 °C with the elimination of nitrogen, carbon dioxide and water.

4.4.9 Elemental Studies

Figure 4.8 EDS spectrum of LaFeO3 nanospheres

Figure 4.8 shows EDS spectrum of LaFeO3 nanospheres which

confirm the presence of the elements in the samples. From the EDS spectra, it

77

is further confirmed that no elements other than La, Fe and O were present

hence it shows that the prepared samples were highly pure.

4.4.10 Magnetic Property Studies

The magnetic behaviour of the prepared LaFeO3 nanospheres was

investigated. Figure 4.9 shows the M-H hysteresis loop of LaFeO3 nanosphere

at (300 K) room temperature and 20 K. The saturation magnetization Ms value

is found to be about 1.152 emu/g for 300 K and 1.305 emu/g for 20 K, which

is higher than previous report (Tang et al 2013), indicating that LaFeO3 has a

weak magnetic behaviour at 300 K and shows strong magnetic bahaviour at

20 K. Further from the M-H hysteresis loop it was found that the coercivity

(HC) for 300 K was about 230 Oe and for 20 K it was about 950 Oe.

Figure 4.9 M-H hysteresis loops of LaFeO3 sample at 300 K (room temperature) and 20 K

78

4.4.11 X-Ray Photoelectron Spectroscopy Studies

Figure 4.10 XPS spectra of LaFeO3 sample (a) wide scan survey (b) core level of La 3d region (c) core level of Fe 2p region (f,g) core level of O 1s region

Figure 4.10 displays the XPS spectra of La 3d, Fe 2p and O 1s core

levels for LaFeO3 nanospheres. The resulting binding energy values were

corrected using the C1s peak at 284.6 eV. Figure 4.10a shows a wide scan

spectrum of the sample, where peaks of La, Fe, O, and C were detected. In

Figure 4.10b the peaks of La 3d5/2 and La 3d3/2 were situated at 834.4 eV and

838.4 eV and at 851.2 eV and 855.4 eV, respectively. The spin-orbit splitting

of La 3d level is 16.8 eV and it is similar to the previous report (Zhang et al

2014). In Figure 4.10c, the peaks at 710.6 and 724.4 eV correspond to the

binding energies of Fe 2p3/2 and 2p1/2, respectively (Parida et al 2010). No

79

noticeable shoulder peaks are found in the Fe 2p XPS spectrum, indicating

that Fe mainly exhibits +3 oxidation state. The broad and asymmetric O1s

XPS spectra (Figure 4.10d) correspond to two kinds of oxygen chemical

states according to the binding energy range (527.0 533.0 eV), including

crystal lattice oxygen (OL) and hydroxyl oxygen (OH) with increasing binding

energy.

4.4.12 UV-Vis Absorption Studies

Figure 4.11 UV-Visible absorbance spectrum of LaFeO3 sample

Figure 4.11 shows the UV-Vis absorption spectrum of LaFeO3

nanospheres with absorption at about 466 nm. Generally for perovskite type

oxides the strong absorption edge can mainly be attributed to the electronic

transition from the valence band to conduction band (O 2p Fe 3d) (Farhadi

et al 2009). The UV-Vis spectrum of LaFeO3 allows us to calculate the optical

band gap (Eg) of 2.66 eV based on the absorption. The result indicates that

LaFeO3 nanosphere prepared by hydrothermal method can absorb

80

considerable amounts of visible light, implying their potential applications as

visible light-driven photocatalysts.

4.4.13 Photocatalytic Property Studies

Figure 4.12 UV-Vis time dependent absorption spectra during photocatalytic reaction of MO for LaFeO3 sample. Degradation percentage of MO as the function of irradiation time (insert) optical photograph showing gradual Color change of MO at different time intervals during photodegradation process

The photocatalytic activity of the prepared LaFeO3 nanosphere was

tested for the degradation of MO in aqueous suspensions under visible light

irradiation. Figure 4.12a shows the UV-Vis time dependent absorbance

spectrum during photocatalytic reaction of MO for LaFeO3 nanospheres after

visible light irradiation. For MO a strong absorption was observed at 465 nm.

Initially we have tried without any photocatalyst, the result shows only 3 % of

degradation within 180 min visible light radiation. Then we have tried with

LaFeO3 nanospheres as a catalyst here as the irradiation time increases from 0

to 180 min, the position of the absorption peak slowly shifts to shorter

wavelength and hence the strength of the peak was reduced. After 180 min,

the absorption peak was very low and the intense orange colour of the starting

MO solution faded (insert Figure 4.12b). The degradation rate for LaFeO3

81

nanosphere under visible light irradiation is about 83% hence the result

suggests that the LaFeO3 nanospheres are effective catalysts for MO

degradation.

4.5 CONCLUSION

LaFeO3 nanospheres have been successfully prepared by

hydrothermal method followed by calcination. The citric acid concentration

plays a major role in the formation of LaFeO3 nanospheres. XRD result shows

that the prepared LaFeO3 sample possessed orthorhombic perovskite

structure, with high crystallinity. From the results of Raman spectroscopy,

bands of crystalline LaFeO3 are observed. HRSEM and AFM image shows

that the prepared LaFeO3 sample exhibits uniform distribution of spherical-

like morphology with average size of about 45 nm. The UV-Vis study shows

strong absorption at 466 nm which has excellent visible light absorption

ability. VSM measurement indicates the products shows weak magnetic

behaviour at 300 K and strong magnetic behavior at 20 K. XPS analysis

confirms the oxidation state of all the elements. Further we proved LaFeO3

nanospheres exhibit good photocatalytic activity for the degradation of MO

under visible-light illumination within 180 min.