ferroelectric and ferromagnetic oxide based ... - iit delhi

HARNESSING APPLICATIONS
SIMRJIT SINGH
OCTOBER, 2017
FERROELECTRIC AND FERROMAGNETIC
HARNESSING APPLICATIONS
Submitted
in fulfillment of the requirements of the degree of Doctor of Philosophy
to the
NEW DELHI-110016, INDIA
This is to certify that the Thesis entitled “FERROELECTRIC AND
FERROMAGNETIC OXIDE BASED SYSTEMS FOR SOLAR ENERGY
HARNESSING APPLICATIONS” being submitted by Simrjit Singh to the
Department of Physics, Indian Institute of Technology Delhi for the award of the degree
of ‘Doctor of Philosophy’ is a record of bonafide work carried by him. He has worked
under my supervision and guidance and has fulfilled the requirement for the submission
of this Thesis, which in my opinion has reached the requisite standard. The results
contained in this work have not been submitted, in part or full, to any other University or
Institute for the award of any degree or diploma.
Professor Neeraj Khare
Department of Physics
New Delhi-110016, India.
iii
ACKNOWLEDGEMENTS
First of all, I express my deepest gratitude to my supervisor Prof. Neeraj Khare
for his constant support, guidance and encouragement during the course of my research
work. His valuable suggestions and constructive criticism always put me on the right
track in handling many problems. His passion towards research and hardworking nature
always inspired me to carry out my research work with full dedication and sincerity. I am
very much delighted to work under his guidance and learnt fruitful lessons which I am
sure will help me in fulfilling my goals.
I am thankful to Prof. Anurag Sharma, Head, Department of Physics, for providing
basic infrastructural facilities for research. I am also thankful to my SRC members, Prof.
Ratnamala Chatterjee (SRC, Chairman), Dr. Mukesh Chandra Bhatnagar and Prof.
Prashant Mishra (DBEB, IIT Delhi) for their valuable suggestions/comments during the
research tenure that helped me to improve the quality of my research work. I am also
very much thankful to Prof. Vikram Kumar and Prof. Neeraj Khare (Chairman, NRF) for
providing me all the necessary facilities to carry out my research work.
I am also thankful to my host institute IIT Delhi and Director of IIT Delhi for
providing me an opportunity to pursue Ph.D in Physics.
I greatly appreciate the help and support from my lab mates Mohd. Faraz, Mohd.
Zubair Ansari, Ms. Deepti Chaudhary, Mr. Deepanshu Sharma, Mr. Nikhil Aggrawal,
Mr. Sandeep Munjal, Mr. Sunil Kumar, Ms. Surbhi Sharma, Ms. Pratisha Gangwar, Mr.
Huidrom Hemojit, Mr. Dheeraj Kumar and Ms. Mamta. I also acknowledge the help from
project students Dinesh Kumar Patel, Saikat Jana, Sudip Pal, Ashutosh Rathi, Ashok
Khangembam and Punit Gaur.
iv
I cannot explain how thankful I am to all my friends Dr. Mukesh Kumari, Dr. Renu
Tomar, Deepanshu Sharma, Harikrishnan Narayanan, Dr. Aman Jindal, Dr. Sanjay
Sardana, Srikant Madakka, Brijesh Kumar, Raghav Sharma, Samir Kumar, Prabhat
Kumar, Megha Singh who supported me throughout my Ph.D. and made my stay
comfortable at IIT Delhi.
I would like to express my special thanks to my friends Dr. Mukesh Kumari, Dr.
Renu Tomar and Deepanshu Sharma for helping me at every stage in my Ph.D. Their
immense care and affection, never let me felt that I am away from my home.
I also express my heartfelt gratitude to my old buddies Ravish Kumar, Jasvir Singh,
Dr. Gurmeet Singh and Sanjeev Kumar who always stood with me at my tough times.
I pay my sincere regards to my family members: Mr. Pritam Singh (Father), Mrs.
Kuldeep Kaur (Mother), Gurpreet Kaur (Sister), Gagandeep Singh (Brother-in-Law)
Karandeep Singh (Brother) and Gurmanpreet Kaur (Sister-in-Law) for their love,
encouragement, patience and support.
I am thankful to Ministry of Human Resources and Development (MHRD), India for
providing me the financial assistance to carry out research at IIT Delhi. I am also thankful
to Industrial Research and Development (IRD), IIT Delhi and Nanoscale Research
Facility (NRF), IIT Delhi for providing me financial assistance to attend and present my
work abroad in international conferences.
Last, but not the least, I wish to express my gratitude to the Almighty for giving me
direction, patience, strength and most importantly enthusiasm to complete my Ph.D work.
SIMRJIT SINGH
v
ABSTRACT
Research in the field of solar energy harnessing to meet the increasing demand of
renewable energy and to circumvent the environmental issues has attracted a lot of
attention in the last few decades. Photoelectrochemical (PEC) water splitting, which split
water into hydrogen and oxygen, is a green approach to address the increasing demand
for generation of sustainable energy. For the degradation of organic pollutants from the
industrial waste water, use of semiconductor photocatalysis can be an effective way to
handle the pollution problem. The present thesis focuses on designing of photoanodes
using different nanostructure semiconductor materials and approaches to enhance the
performance of PEC. Also, the design and synthesis of nanostructure photocatalysts for
environmental remediation applications have been attempted.
To carry out work in this direction, semiconducting-ferroelectric NaNbO3
nanostructures have been synthesized and detail study of its structural, morphological,
optical and ferroelectric properties has been carried out. Photoelectrodes of NaNbO3 have
been fabricated by spray coating hydrothermally synthesized NaNbO3 nanoparticles onto
fluorine doped tin oxide (FTO) substrates. Photoelectrochemical properties of NaNbO3
nanostructure ferroelectric films are investigated and it is demonstrated that efficiency of
photoelectrochemical (PEC) water splitting can be tuned (7% to 23%) by electrical
polarization of the NaNbO3 films. Further, due to the piezoelectric properties of NaNbO3
nanostructures, piezopotential endowed enhancement in the photoanodic activity of
NaNbO3 nanostructure films and enhancement in the photocatalytic activity of NaNbO3
particulate suspension for the degradation of methylene blue organic dye is demonstrated.
Flexible PVDF/Cu/PVDF-NaNbO3 photoelectrodes have also been fabricated and tuning
vi
effect is demonstrated.
electrical, optical and magnetic properties of a ferromagnetic-semiconductor material can
be combined to give rise a new phenomenon termed as ‘Magnetophototronic Effect’ For
studying this effect, CoFe2O4 nanostructures have been synthesized and detail study of its
structural, morphological, optical and magnetic properties has been carried out. Effect of
intrinsic strain on the magnetic and optical properties has been investigated. For PEC
activity, CoFe2O4 photoelectrodes have been fabricated using hydrothermally synthesized
CoFe2O4 nanoparticles. Photo-electrochemical properties of CoFe2O4 photoelectrodes in
the presence of external magnetic fields have been investigated and it is demonstrated
that the photoanodic current can be enhanced upto ~59% under low DC magnetic field
~600 Oe.
The formation of nanocomposite systems by combining semiconductor materials
with conducting carbon nanostructures or other semiconductor materials can be a noval
strategy to improve the separation efficiency of the photogenerated charge carriers. In the
present study, CoFe2O4/graphene and CdS/CoFe2O4/graphene nanocomposites have been
synthesized and substantial enhancement in the photoelectrochemical and photocatalytic
activities has been demonstrated. The CdS/CoFe2O4/graphene photocatalyst showed
enhanced stability and magnetic separation which is attributed to the coating of
ferromagnetic CoFe2O4 layer around CdS.
vii

-
() ,
,
, -

,

, - (NaNbO3)
, , ,
NaNbO3
- NaNbO3
(FTO) - NaNbO3
- -
NaNbO3
7% 23% , NaNbO3
, NaNbO3
NaNbO3
viii
PVDF/Cu/PVDF-
NaNbO3
-
, -
,
, CoFe2O4
, ,

- , CoFe2O4
- CoFe2O4
CoFe2O4 -

(~ 600 Oe) ~ 59%

, CoFe2O4/graphene CdS/CoFe2O4/graphene
-
CdS/CoFe2O4/graphene
CdS - CoFe2O4

1.3.1 Semiconductor-Electrolyte Interface 6
1.4 Basic Requirements of a Semiconductor to be used 7
as Photoelectrode/Photocatalyst
1.6 Composite Systems 10
1.7 Some New Phenomena using Ferroelectric 13
and Piezoelectric Materials
1.8 Background of the Choice of Problem 16
x
1.9 Magnetophototronic Effect 18
1.9.1 Background of the Work on Magnetic Field Effect 18
1.9.2 Cobalt Ferrite (CoFe2O4) 19
1.10 Thesis Problem 20
1.12 Organization of the Thesis 22
References
Techniques
2.2 Characterization Techniques 34
2.2.1 X-ray Diffraction 35
2.2.5 Ferroelectric Hysteresis (P-E) and Electrical Poling 40
2.2.6 UV-Vis-NIR Spectrophotometer 41
2.2.7 Raman Spectroscopy 43
xi
2.3.2 Mott-Schottky Analysis 49
2.4 Photocatalytic Measurements 50
Ferroelectric, Photoelectrochemical and Photocatalytic Properties
3.1 Introduction 54
3.3 Synthesis of NaNbO3 Nanostructure Films 56
3.4 Structural and Morphological Studies 56
3.4.1 X-ray Diffraction Study 56
3.4.2 Transmission Electron Microscopy Study 57
3.4.3 Scanning Electron Microscopy Study 58
3.4.4 X-ray Photoelectron Spectroscopy Study 59
3.5 Optical Absorption Study 60
3.6 Ferroelectric Properties 61
3.8 Photoelectrochemical Solar Cell: Effect of 63
Electrical Poling of NaNbO3 Films
3.9 Piezophototronic Effect 70
xii
Ferroelectric and Photoelectrochemical Properties
4.2.1 Fabrication of PVDF/Cu Flexible Substrate 84
4.2.2 Fabrication of PVDF/Cu/PVDF-NaNbO3 Photoelectrode 84
4.3 Scanning Electron Microscopy Study 85
4.4 X-ray Diffraction Study 86
4.5 Ferroelectric Properties 87
4.6.1 Effect of Electrical Poling of PVDF-NaNbO3 Films 87
4.6.2 Study of Piezophototronic Effect on PVDF-NaNbO3 Films 94
4.7 Conclusions 96
Magnetic, Optical and Photoelectrochemical Properties
5.1 Introduction 99
Growth Temperatures
xiii
5.2.6 Magnetic Study 107
5.3 Study on the Effect of Intrinsic Strain on Uniform Size 112
CoFe2O4 Nanoparticles
5.3.4 Optical Bandgap Study 114
5.3.5 Raman Spectroscopy Study 116
Films Under External Magnetic Field
5.4.5 Seebeck Measurement 124
5.4.6 Photoelectrochemical Measurements 124
5.4.6.2 Capacitance and EIS Study 128
5.5 Conclusions 129
Characterization, Photoelectrochemical and Photocatalytic Properties
6.1 Introduction 133
6.2.7 Optical Bandgap Study 140
6.3 Study of CdS-CoFe2O4-Graphene Nanocomposite: 149
Application to Photoelectrochemical Solar Cell and Photocatalytic
Activity
6.3.1.2 Synthesis of CdS/CoFe2O4 Core/Shell Nanostructures 149
6.3.1.3 Synthesis of CdS/CoFe2O4/Graphene Nanocomposite 150
xv
6.3.8 Photocatalytic Study 158
6.3.10 Photoluminescence Study 162
6.3.11 Reaction Mechanism 163
6.3.12 Stability Test 167
7.1 Summary of the Present Study 172
7.1.1 NaNbO3 Nanostructures for Photoelectrochemical and 172
Photocatalytic Properties
Tunable Photoelectrochemical Properties
Photoelectrochemical and Photocatalytic Properties
LIST OF PUBLICATIONS 179
AUTHORS’S BIODATA 185
semiconductor photocatalytic process, (b) reaction mechanism
involved in the photoctalytic degradation of organic pollutants.
3
semiconductor photoanode, Pt cathode electrode and Ag/AgCl
reference electrode immersed in an electrolyte solution.
4
photoelectrochemical water splitting for generation of H2 and O2.
5
Fig.1.4 (a) Flat band situation of a semiconductor before equilibrium
with electrolyte, (b) Electrochemical equilibrium situation
between the Fermi level of semiconductor with redox potential
of electrolyte.
Fig.1.5 Band edge positions of various semiconductor materials used for
photocatalytic and photoelectrochemical water splitting
applications.
9
band edge positions.
Fig.1.7 Schematic diagram showing the band alignment in the formation
of type-II heterostructure system.
14
Fig.1.10 CoFe2O4 (AB2O4) inverse spinel unit cell structure. 20
xvii
Figure caption
Page No.
Fig. 2.1 Hydrothermal setup used for the synthesis of nanostructures. 32
Fig.2.2 Experimental setup of the chemical solution method. 33
Fig.2.3 Experimental setup of spray coating technique used for
depositing thin films.
34
Fig.2.4 (a) Schematic of periodic arrangement of atoms in a crystal
following Bragg’s condition (b) Geometry of an X-ray
diffractometer.
35
Fig.2.5 (a) Schematic representation of the working of scanning electron
microscope, (b) Zeiss EVO 50 SEM machine used for
measurements.
37
Fig.2.6 (a) Schematic representation of the working of TEM, (b) FEI
Tecnai G220 S-TWIN TEM machine used for measurements.
38
Fig.2.7 Princeton Micro Mag 2900 AGM system used for magnetic
measurements.
40
Fig. 2.8 (a) Ferroelectric loop tester system used for P-E measurements
(b) Schematic of modified Sawyer-Tower circuit.
41
Fig. 2.9 (a) Schematic diagram showing working principle of UV-Vis-
NIR spectrophotometer, (b) Lambda-1050 spectrophotometer
used for the measurements.
43
Fig. 2.10 Schematic of the Raman set-up, (b) Horiba Jobin Yvon Raman
instrument used for the measurements.
44
Fig. 2.11 (a) Schematic diagram of XPS, (b) XPS (SPECS) machine used
for the measurements.
46
Fig. 2.12 (a) Schematic representation of working principle of PFM, (b)
Bruker, Dimension Icon machine used for the measurements.
47
for the measurements.
48
Fig. 2.14 Photocatalytic setup used for degrading the organic dyes. 50
xviii
peaks confirm orthorhombic crystal structure of NaNbO3.
57
Fig.3.2 (a) TEM and (b) HRTEM image of NaNbO3 nanorods. 58
Fig.3.3 (a) Top view SEM image of NaNbO3 film and cross-sectional
SEM image (inset of figure) of NaNbO3 film.
58
Fig.3.4 Wide range XPS scan of NaNbO3 film and core level XPS scans
for Na (1s), Nb (3d) and O (1s) states.
59
Fig.3.6 Optical bandgap of NaNbO3 nanostructure film 61
Fig.3.7 Polarization vs. Electric field loop of NaNbO3. 61
Fig.3.8 (a) PFM amplitude butterfly loop and (b) PFM phase hysteresis
loop of NaNbO3 nanostructure film coated onto ITO/PET
substrate.
62
Fig.3.9 (a) Schematic of electric field poling of NaNbO3 films (b)
schematic representation of NaNbO3 films showing negative
polarization and (c) positive polarization.
64
Current-potential curves of photoanodes with different
polarization conditions (b) Incident photon to current
conversion efficiency of differently polarized NaNbO3 films, (c)
Nyquist plots for differently polarized NaNbO3 electrodes
measured in the frequency range of 100 mHz and 1 kHz.
67
interface for (a) Negative polarized and (b) Positive polarized
NaNbO3 photoanodes.
Fig.3.12 Mott-Schottky plots for NaNbO3 films measured at a fixed
frequency of 1kHz under dark conditions.
69
xix
photoelectrochemical water splitting activity.
PEC three electrode assembly showing overall water splitting
reaction mechanism, (b) Current-potential curves of the
photoanodes, (c) Electrochemical impedance spectra of the
photoanodes under different experimental conditions, (d)
Incident photon to current conversion efficiency of the
photoanodes.
73
Fig.3.15 Schematic representation of the PEC process. (a) Band structure
of NaNbO3 nanorods without piezopotential, (b) Band structure
of NaNbO3 nanorods with piezopotential.
74
suspension (a) under dark with physical mixing, (b) under dark
with ultrasonic vibrations, (c) under light with physical mixing,
(d) under light with ultrasonic vibrations, (e) comparison of the
photocatalytic degradation efficiency performed under different
experimental conditions (solid lines connecting the dots are
guide to the eye only), (f) comparison of the rate constant
values for the degradation of MB dye.
76
strained NaNbO3 nanorods.
photoelectrode and (b) photograph of the fabricated flexible
PVDF/Cu/PVDF-NaNbO3 photoelectrode.
85
xx
Fig.4.2 (a) SEM image and (b-f) elemental mapping images of PVDF-
NaNbO3 nanocomposite film coated onto PVDF/Cu substrate.
85
substrate.
86
of PVDF-NaNbO3 film coated onto PVDF/Cu substrate as
photoanode, Pt wire as counter electrode and Ag/AgCl as
reference electrode, (b) Current-potential curves of the as
prepared PVDF-NaNbO3 film coated onto PVDF/Cu substrate.
Current-potentials curves of PVDF-NaNbO3 films coated onto
PVDF/Cu substrates after (c) positive polarization and (d)
negative polarization.
polarized under different conditions (measured at a fixed
frequency of 1kHz under dark conditions).
91
polarization condition and (c) under positive polarization
condition.
93
polarization conditions measured in the frequency range of 100
mHz and 1 kHz. Experimental data points are fitted with Z-View
software (solid line).
Fig.4.10 (a) Experimental set-up used for investigating piezo-
photoelectrochemical water splitting activity, (b) Current-
potential curves of negatively polarized PVDF-NaNbO3 film
coated onto PVDF/Cu substrate measured under ultrasonic
vibrations and without ultrasonic vibrations.
96
xxi
Fig.5.1 (a) XRD patterns of the CFO nanoparticles synthesized at
different growth temperatures, (b) shows the variation of
crystallite size and strain with the growth temperature.
102
Fig.5.2 TEM images of the CFO nanoparticles synthesized at (a) 80 C,
(b) 100 C, (c) 120 C and (d) 140 C growth temperatures
respectively (insets of the (a), (b), (c) and (d) show the particle
size distribution histograms). High resolution TEM images of
CFO nanoparticles synthesized at (e) 80 C, (f) 100 C, (g) 120
C and (h) 140 C (insets of the (e), (f), (g) and (h) show the
magnified HRTEM images of the same samples). Variation of
the particle size with the growth temperature (i) and the plot
between the particle size and crystallite size (j) of the CFO
nanoparticles grown at four different temperatures.
103
Fig.5.3 (a) XPS scans of O 1s for CFO nanoparticles synthesized at
different growth temperatures. Deconvoluted Gaussian
distribution with peaks centered at 529.5 eV, 531.1 eV and 532.4
eV are shown by dotted lines. (b) Variation of oxygen vacancy
defects and intrinsic strains with the size of the CFO
nanoparticles synthesized at different growth temperatures.
105
Fig. 5.4 (a) Raman spectra of the CFO nanoparticles of different particle
sizes synthesized at different growth temperatures. (b) Variation
of relative intensity ratio (Iv) of Raman peaks at 624 cm-1 and
468 cm-1 with size of the nanoparticles. Inset of (b) shows the
plot of the relative intensity ratio with the intrinsic strain present
in the CFO nanoparticles.
107
Fig.5.5 M-H loops of the CFO nanoparticles of different particle sizes
synthesized at different growth temperatures. Insets show the
variation of coercivity and magnetization with particle size.
108
xxii
Fig.5.6 Variation of saturation magnetization with the size of the CFO
nanoparticles. Curve (A) is the simulated curve using Eqn. (5.3)
which presents a variation of Ms with size when only the effect
of intrinsic strain is taken into account, curve (B) is the
simulated curve using Eqn. (5.4) which presents the variation of
Ms with size when the presence of dead layer is taken into
account and –•– are the experimentally observed points.
110
Fig.5.7 Plots of log Hc vs log D of CFO nanoparticles. The dotted line is
a variation of Hc with D3/2. Inset shows the variation of Hc with
the strain of the nanoparticles.
111
Fig.5.8 (a) XRD patterns of the CFO nanoparticles synthesized at
different growth temperatures (b) Variation of crystallite size
and intrinsic strain (ε) with the growth temperature.
113
Fig.5.9 TEM images of (a) CFO120 (b) CFO140 and (c) CFO160
nanoparticles.
114
Fig. 5.10 (a) Optical bandgap of CFO nanoparticles (b) the variation of
energy gap with the intrinsic strain.
115
Fig.5.11 (a) Raman spectra of the CFO nanoparticles (b) Variation of
relative intensity ratio (Iv) with intrinsic strain.
116
Fig.5.12 Variation of the band gap (Eg) of the CFO nanoparticles with the
intrinsic strain (ε). Solid line is the simulated curve for the
variation of Eg with ε [Eqn. 5.6].
118
Fig.5.13 M-H loops of the CFO nanoparticles synthesized at different
growth temperatures. Inset shows the variation of magnetization
(Ms) with intrinsic strain (ε) present in the nanoparticles.
118
Fig 5.14 Variation of the saturation magnetization of similar size CFO
nanoparticles with intrinsic strain present in the nanoparticles.
Solid line is the simulated curve for the variation of
magnetization of CFO nanoparticles with intrinsic strain.
120
xxiii
Fig.5.16 Top view SEM image of CoFe2O4 nanostructure film with the
inset showing cross-sectional view of the SEM image.
122
film.
123
Fig.5.18 Optical absorbance of CoFe2O4 film with inset showing Tauc
plot of the film.
Fig.5.20 (a) Schematic representation of three electrode cell assembly for
photoelectrochemical measurements, (b) current-potential curves
of CoFe2O4 photoanode measured with and without magnetic
field (under 100 mW/cm2 UV-Vis illumination).
125
Fig.5.21 (a) Raman spectroscopy results of CoFe2O4 film under different
magnetic field strengths, (b) variation of intensity ratio (Iv) with
magnetic field.
measured with and without magnetic field at a fixed frequency
of 1kHz, (b) Nyquist plots of CoFe2O4 photoelectrodes measured
under different magnetic field conditions in the frequency range
of 100 mHz to 1 kHz.
129
Fig.6.1 X-ray diffraction patterns of (a) graphene oxide (GO) and
reduced graphene oxide (RGO), (b) CoFe2O4 and
CoFe2O4/RGO nanostructures.
CoFe2O4/RGO nanostructures.
136
Fig.6.3 X-ray photoelectron spectroscopy scans of C1s peak of (a) GO
(b) RGO (c) CoFe2O4/RGO nanostructures and core level X-
ray photoelectron spectroscopy scans of (d) Co2p, (e) Fe2p and
138
xxiv
Fig.6.4 Transmission electron microscopy images of (a) RGO sheets,
(b) CoFe2O4 nanoparticles and (c) CoFe2O4/RGO
nanocomposite.
139
Fig.6.5 Scanning electron microscope images of (a) CoFe2O4 film and
(b) CoFe2O4/RGO film.
of figure shows Tauc plot of RGO film.
141
Tafel plots for CoFe2O4 and CoFe2O4/RGO photoanodes.
142
films.
143
Fig.6.9 Schematic diagrams describing the (a) flat band situation of
semiconductor before equilibrium, (b) band alignments under
equilibrium condition with electrolyte, (c) surface states
mediated charge transfer mechanism at
semiconductor/electrolyte interface.
Fig.6.10 (a) Equivalent circuit model for fitting EIS experimental data,
(b) Experimental and fitted EIS Nyquist curves of CoFe2O4 and
CoFe2O4/RGO photoanodes.
mechanism and overall water splitting mechanism in
CoFe2O4/RGO photoanode.
Fig.6.12 (a) XRD patterns of the CFO, CdS, CdS/CFO and
CdS/CFO/RGO nanostructures. In the core/shell
nanostructures, peaks marked with (*) correspond to CdS and
peaks marked with (^) correspond to CFO. (b) Shift in the peak
position of the (110) peak of CdS in the CdS/CFO and
CdS/CFO/RGO nanostructures.
151
Fig.6.13 Raman spectra of the CdS, CdS/CFO, CdS/CFO/RGO and GO 152
xxv
nanostructures at room temperature.
Fig.6.14 XPS scans of (a) Cd 3d, (b) S 2p, (c) Co 2p, (d) Fe 2p, (e) O 1s
and (f) C 1s in the CdS/CFO/RGO nanostructure.
154
Fig.6.15 (a) TEM image of the CdS nanorods, (b) high resolution TEM
image of the CdS nanorods, (c) TEM image of the CdS/CFO
core/shell nanorods, (d) high resolution TEM image of the
CdS/CFO nanorods, (e) TEM image of the GO, (f) TEM image
of the CdS/CFO/RGO nanostructure.
Fig.6.16 Tauc plots for the CFO, CdS, CdS/CFO and CdS/CFO/RGO
nanostructures.
156
at room temperature. The inset shows a photograph of the
separation of the CdS/CFO/RGO nanostructures from the
aqueous solution using an external magnetic field.
157
Fig.6.18 (a) UV-visible absorption spectra of MB in aqueous solution in
the presence of the CdS/CFO/RGO nanostructures as a
function of irradiation time (inset shows a photograph of the
MB solution after different durations of visible light exposure),
(b) comparison of the photocatalytic efficiency of the CFO,
CdS, CdS/CFO and CdS/CFO/RGO nanostructures for the
photodegradation of MB, (c) comparison of the values of the
rate constants for the degradation of MB, (d) degradation of
MB dye using the CdS/CFO and CdS/CFO/RGO
nanostructures for five recycles.
CdS/CFO/RGO nanostructures in the dark (D; dotted line
plots) and under visible light irradiation (L; solid line plots)
using 0.5 M Na2SO4 as an electrolyte, (b) electrochemical
impedance spectroscopy Nyquist plots of the CdS, CdS/CFO
and CdS/CFO/RGO nanostructures. The inset of the figure
shows the equivalent circuit model and magnified view of the
161
xxvi
CdS/CFO/RGO nanostructures at room temperature.
162
Fig.6.21 Schematic band diagrams of the reaction mechanism of charge
transfer in the CdS/CFO (a and b) and CdS/CFO/RGO (c and
d) nanostructures.
Fig.6.22 Control experiments for the photodegradation of MB using the
CdS/CFO/RGO nanostructure in the presence of benzoquinone
(BQ, scavenger for superoxide anion radicals), tert-butyl
alcohol (TBA, scavenger for hydroxyl radicals) and
ammonium oxalate (AO, scavenger for holes) under visible
light irradiation.
nanostructures before photocatalytic reaction and after
photocatalytic reaction. The peaks marked with (*) correspond
to CdS and peaks marked with (^) correspond to CFO.
168
xxvii
LIST OF TABLES
Table 3.1 Comparison of the enhanced IPC efficiency of NaNbO3 sample to that of
other materials reported in the literature.
Table 6.1 Fitting parameters of the elements used in the equivalent circuit model
calculated using Z-View software.
Table 6.2 Conduction band and valence band positions of the CdS, CFO
nanostructures.
11. CHAPTER-I (1-29)
12. CHAPTER-II (30-53)
13. CHAPTER-III (54-81)
14. CHAPTER-IV (82-98)
15. CHAPTER-V (99-132)
16. CHAPTER-VI (133-171)
17. Chapter-VII (172-178)
19.Biodata (185)

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