Chapter 3
Materials and Method
45
3.1. Synthesis Techniques; Solution Process
3.1.1. Synthesis of Cu doped Strontium Titanate (SrTiO 3 ) Nanoparticles
All the chemicals are analytical grade and used without further purification. The
chemicals were used for the synthesis of Cu-doped SrTiO 3 nanoparticles as
follows; the strontium acetate Sr(CH 3 COO) 2 (99.8% Sigma-Aldrich), Copper
acetate (99.8% Sigma-Aldrich), titanium isopropoxide (TTIP, Sigma-Aldrich
98.9%), Citric acid (99.8% Sigma-Aldrich), Ethyl alcohol (99.8% Sigma-Aldrich)
and Ethylene glycol (99.8% Sigma-Aldrich). The Cu doped SrTiO 3 nanoparticles
were synthesized by sol-gel method. In this process, titanium and strontium
precursors solutions were prepared and designated as A and B solution,
respectively. Solution A was prepared by dissolving 6.5 mL of TTIP and 6.7 mL
ethylene glycol into 80 mL ethanol under continuous stirring for 30 min. Then 6.3
g of citric acid was dissolved into 10 mL deionized (DI) water and added slowly
into the solution A. The citric acid acts as a chelating agent in this reaction. In a
separate beaker, 0.275g copper acetate was dissolved in 10 mL DI water under
continuous stirring and get light blue colored solution than this cupper acetate
solution gradually added into the Sr(CH 3 COO) 2 solution (4.12 g dissolved in 25
mL DI water) under continuous stirring to get solution B.
Afterward, the solution B was mixed with solution A and the resultant solution
was then transferred into three-necked round bottom flask and refluxed for 8 h at
80 °C. After completion of the reaction, the pale blue white precipitates were
obtained. It was repeatedly washed with DI water and alcohol and dried in oven at
70 °C. The obtained pale blue white powder was further calcined at 600 °C for 5 h.
46
3.1.2. Synthesis of ZnO nanomaterials
3.1.2.1. ZnO nanoparticles
For the synthesis of ZnO nanoparticles, analytical grade zinc acetate
(Zn(CH 3 COO) 2 ·2H 2 O 99.8%) and oxalic acid (C 2 H 2 O 4 ·2H 2 O, 99.8%) were
procured from Sigma-Aldrich and were used without further purification. In the
typical synthesis, 0.05M zinc acetate solution and 0.1 M oxalic acid solutions were
prepared in deionized water under continuous stirring at ambient temperature.
Oxalic acid solution was gradually added into zinc acetate with constant stirring
and resultant solution was transferred to a three necked round bottom flask. The
resultant solution was heated at 80 °C for 8 hours. After completion of reaction
white colored precipitate was obtained this was washed repeatedly with methanol
and deionized water and air-dried. The growth of ZnO nanoparticles can be
explored by considering the involved chemical reactions in the synthesis process.
Herein, no immediate precipitation or colorless solution was seen by the mixing of
aqueous solutions of zinc acetate and oxalic acid. The sequential reactions are as
follows;
Zn(CH 3 COO) 2 ·2H 2 O + H 2 C 2 O 4 ·2H 2 O → Zn(C 2 O 4 )·2H 2 O + 2CH 3 COOH +
2H 2 O
(3.1)
The formation of zinc oxalate complex, Zn(C 2 O 4 )∙2H 2 O is important primary step
for growth of wurtzite ZnO crystal. In other words, this complex acts as a building
block for synthesis of ZnO nanoparticles. The zinc oxalate complex
Zn(C 2 O 4 )∙2H 2 O has been further calcined at 400°C for 30 minute to convert
Zn(C 2 O 4 )·2H 2 O into pure ZnO crystals.
Zn(C 2 O 4 )·2H 2 O + ½ O 2 → ZnO + 2CO 2 + 2H 2 O
47
(3.2)
Hence, the described reactions explain the proper growth of wurtzite ZnO.
Therefore, it is expected that electrostatic attraction of oxalate ligand also play
pivotal role in the uniform nucleation for ZnO growth.
3.1.2.2. ZnO nanorods
For the facile synthesis of ZnO nanorods assembled into flower shaped
morphology, analytical grade zinc acetate (Zn(CH 3 COO) 2 ·2H2O , 99.8%),
hexamethylenetetramine (HMTA; C 6 H 12 N 4 , 99.8%), sodium hydroxide (NaOH)
and cetyltrimethylammonium bromide (CTAB) were procured from Sigma-Aldrich
and were used without further purification. In a typical procedure, 0.05M zinc
acetate solution, 0.05 M hexamethylenetetramine solution and 1M sodium
hydroxide solutions were prepared in deionized water under continuous stirring at
ambient temperature in separate beakers. First, Hexamethylenetetramine (HMTA)
solution was gradually added into zinc acetate solution with constant stirring
followed by drop-wise addition of sodium hydroxide solution to maintain the pH
~11-12. The above solution was further stirred for 30 minutes. The resulting
solution was gradually added into the cetyltrimethylammonium bromide solution.
After 30 minutes of vigorous stirring the obtained solution was transferred into a
250mL Teflon-lined stainless steel autoclave and heated up to 105 ±5 °C for 10
hours. Now, the autoclave was allowed to cool gradually to the room temperature.
White crystalline powder was obtained which was washed repeatedly with
deionized water followed by ethanol and acetone and dried in oven at 50 °C for 30
minutes. The growth of as synthesized ZnO nanorods assembled into flower
shaped morphology can be explored by considering the involved reactions in the
synthesis process. In the typical experiment procedure, the aqueous solution of
hexamethylenetetramine (HMTA) gradually poured into zinc acetate solution, it is
48
important to note that there was no immediate precipitation but the clear zinc
acetate solution changed into turbid solution. Moreover, the pH of resulting
solution was adjusted to ~11-12 by addition of few drops of sodium hydroxide
solution. The gradual increase of temperature in the hydrothermal reactor would
facilitate the thermal degradation of HMTA and subsequently release of hydroxyl
ions which further react with Zn2+ ions to form Zn(OH) 2 .1 The sequential reactions
are as follows;
Zn(CH 3 COO) 2 ∙2H 2 O + 2NaOH
Zn(OH) 2 + 2CH 3 COONa +2H 2 O
(3.3)
(CH 2 ) 6 N 4
+
6H 2 O
6HCHO +
4NH 3
(3.4)
NH 3
+
NH 4 +
H2O
+
OH
‾
(3.5)
Zn2+ + 2OH ‾
Zn(OH) 2
(3.6)
In addition to this, the zinc hydroxide Zn(OH) 2 further reacts with hydroxyl ions to
form Zn(OH) 4 2-,
Zn(OH) 2 + 2OH ‾ (from NaOH)
Zn(OH) 4 2-
(3.7)
As the reaction takes place in the presence of cetyltrimethylammonium bromide
solution at appropriate zinc acetate/CTAB ratio, the CTAB being a cationic
surfactant reduces the surface tension and inhibits the formation of new phase.
CTAB plays two pivotal roles in the synthesis; first CTAB can effectively control
the morphology of building blocks for synthesis of hexagonal ZnO nanorods. It is
well-known that the kinetics for morphological dynamics can be adjusted by using
49
selective adsorption of surfactants.2 Second; CTAB facilitates molecular
aggregation above Critical Micelle Concentration (CMC) producing spherical
micelles at relatively low concentration. The CTAB also facilitates transport of
Zn(OH) 4 2- growth units which come together to form individual rod-like structures.
These rod-like structures further self-assemble into flower like morphology. As the
reaction aged at 105 °C for 10 hours, Zn(OH) 4 2- ions dissociate to form ZnO nuclei
as follow;
Zn(OH) 4 2-
ZnO + H 2 O + 2OH ‾
(3.8)
The formation of flower like assembly of ZnO proceeds by two step mechanism in
aqueous solution, First nucleation followed by growth of nanorods around these
nuclei. The ZnO is polar crystal in which O2- ions are in hexagonal close packing
and Zn2+ ions lie in the tetrahedral hole of four oxygen ions. The Zn and O atoms
are arranged alternatively along the c-axis and top surface-plane is Zn terminated
(0001), which is catalytically active while the bottom surface is O terminated
(000ī), which is chemically inert.3 The morphology of nanostructures is greatly
affected by the growth velocity of ZnO into the different directions. According to
the Laudise et al., the growth velocities of ZnO crystal in the different planes are
[0001] > [00 ī ī] > [01ī 0] > [01 ī 1] > [000 ī ], under the hydrothermal conditions.4
According to the ideal growth rates in different crystal plane, the growth along
(0001) plane is maximum and as-synthesized nanorods contains ± (0001) crystal
plane at their top and bottom and however, enclosed by the six equivalent (01ī0)
crystal planes.5 It is well known that the fastest growing planes generally disappear
earlier and leaving behind the slower growing forms with lower energy. Hence the
(0001) plane will not be in equilibrium with other growing planes. As the specific
surface energy, free energy of the (0001) plane are higher than other growing
50
planes, hence after the synthesis, (01 ī 1) plane would remain with ZnO crystal.
Hence, due to electrostatic interaction between ions and polar surface, there would
be formation of hexagonal ZnO nanorods assembled into flower like morphology.
The as-synthesized product was further calcined at 400 °C for about 30 minutes;
heating will facilitate the decomposition of organic moiety and improve the
crystallinity via eliminating additive from inter-crystal free space.6 Interestingly, it
is observed that the as-synthesized ZnO nanorods follow the same growth pattern
as reported in the literature for ideal growth of hexagonal wurtzite ZnO crystals.7
3.1.3. Synthesis of CuO
All the chemicals are analytical grade and used as without further purification. For
the typical reaction process for the synthesis of CuO microrods, Copper acetate
Cu(CH 3 COO) 2 ·2H 2 O, (99.8% Sigma-Aldrich), Hexamethylenetetramine (HMTA;
C 6 H 12 N 4 , 99.8% Sigma-Aldrich) and NaOH have been used. In the typical
synthesis process, 0.05M copper acetate solution was prepared in distilled water
while in a separate beaker 0.05M hexamethylenetetramine solution was prepared in
deionized water under continuous stirring at ambient temperature. HMTA solution
was added gradually into copper acetate under continuous stirring. Moreover, pH
of solution was adjusted by adding NaOH solution. The resulting blue colour
solution has been transferred into the Teflon flask at 95°C and system was aged for
24 hours. After completion of reaction black coloured precipitate was obtained and
it was further washed repeatedly with methanol and deionized water and dried at
room temperature. The sequential reactions involving are given below:
Cu(CH 3 COO) 2 ·2H 2 O + 2NaOH
Cu(OH) 2 + 2CH 3 COONa + 2H 2 O
(3.9)
The formation of Cu(OH) 2 is very important for growth of CuO crystal which
51
initially serves as building blocks for synthesis of microstructure of pure CuO. The
Cu(OH) 2 has been further calcined at 400°C for 30 min. to convert Cu(OH) 2 to
pure CuO crystals as shown in below reaction;
Cu(OH) 2
∆
CuO + H 2 O
(3.10)
3.2. Characterizations Techniques
3.2.1. Morphological Characterizations
(a) Field Emission Scanning Electron Microscopy (FE- SEM)
The
field
emission
scanning
electron
microscopy
(FE-SEM)
provides
topographical and elemental information at magnification of 10 x to 300,000 x
with virtually unlimited depth of field. As compared to conventional scanning
electron microscopy (SEM), field emission scanning electron microscopy (FESEM) produces clearer, less electrostatically distorted images with spatial
resolution down to nanometer scale. The field emission scanning electron
microscope follows the same operational principle as scanning electron microscope
(SEM), but a field-emission cathode in the electron gun of a scanning electron
microscope provides narrower probing beams at low as well as high electron
energy, resulting in both improved spatial resolution and minimized sample
charging and damage. The Field emission scanning microscope is a type of
electron microscope in which high energy beam of electron passing through the
sample produces verity of signals. These signals that derived from sample reveal
information about general morphology, composition and texture state of sample.
The electron beam is generally scanned in a raster pattern and then beam position
52
is combined with detector which transforms it into an image. Sample generally
mounted rigidly on a holder called specimen stub. To achieve an effective image,
sample should be conductive; in doing so ultrathin coating of electrically
conducting material has been deposited on the sample by low-vacuum sputter
coating method. Gold, platinum, osmium, palladium are frequently used in coating.
Schematic diagram for FE-SEM operation is depicted in Figure 3.1.
Figure 3.1. Schematic diagram for FE-SEM microscope operation. [8]
53
(b) Transmission Electron Microscope (TEM)
Transmission electron microscopy (TEM) is an analytical technique in which high
energy beam of electron passes through the ultrathin sample specimen. An image is
formed as a result of interaction of electron and specimen sample and then image is
magnified and focused onto the fluorescent screen or detected by a sensor such as a
CCD camera. TEM is used to characterize the morphology of sample including
size, shape and arrangement of the particles which make up the specimen as well
as their relationship to each other on the atomic scale. TEM has resolution
significantly higher than light microscope and owing to the small de Broglie
wavelength of electron that directly impact on the resolution of instrument. TEM
magnifies an image ten thousand times as compared to conventional light
microscope. High-resolution transmission electron microscopy images provide
crystallographic structure of a sample at an atomic scale and it is a valuable
method to get information about the crystallanity of a material and their phase.
Sample generally dispersed into the acetone. To get uniform suspension the
resultant suspension is ultrasonicated for 30 minutes. Then, few drops of sample
poured onto the Cu grid. This is further loaded into the specimen part of TEM for
analysis. The schematic lay out for TEM is given in Figure 3.2.
54
Figure 3.2. Schematic layout for TEM operation. [9]
55
3.2.2. Structural Characterizations
(a) Powder X- ray Diffraction (PXRD) method
X-ray diffraction is a versatile, non-destructive analytical method for identification
and quantitative determination of various crystalline forms, known as ‘phases’ of
compound present in powder and solid samples. Diffraction occurs as waves
interact with a regular structure whose repeat distance is about the same as the
wavelength. The phenomenon is common in the natural world, and occurs across a
broad range of scales. For example, light can be diffracted by a grating having
scribed lines spaced on the order of a few thousand angstroms, about the
wavelength of light. It happens that X-rays have wavelengths of the order of a few
angstroms, the same as typical inter-atomic distances in crystalline solids. That
means X-rays can be diffracted from minerals which, by definition, are crystalline
and have regularly repeating atomic structures. When certain geometric
requirements are met, X-rays scattered from a crystalline solid can constructively
interfere, producing a diffracted beam. In 1912, W. L. Bragg recognized a
predictable relationship among several factors. The distance between similar
atomic planes in a mineral (interatomic spacing) which we call the d-spacing and
measure in angstroms. The angle of diffraction as the theta angle is measured in
degrees. For practical reasons, the diffractometer measures an angle twice that of
56
the theta angle. Not surprisingly, we call the measured angle '2θ'. The wavelength
of the incident X-ray, symbolized by the Greek letter lambda and, in our case,
equal to 1.54 Å.
n λ = 2 d sinθ
(3.11)
Where, λ-wavelength of X-ray, d-interplaner spacing, θ-diffraction angle and n-1,
2, 3….The powder sample generally mounted on the holder which is further put
into the diffractometer as shown in Figure 3.3 and 3.4 respectively.
After X-ray measurement, the sample has been collected. On the basis of angle of
diffraction X-ray diffraction further classified into two types;
(i) Small angle X-ray scattering in which scattering angle 2θ close to 0°.
(ii) Wide angle X-ray scattering in which scattering angle 2θ greater than 5°.
Figure 3.3: Basic mechanism of X-ray diffraction. [10]
57
Figure 3.4: Layout diagram of XRD machine operation. [10]
58
(b) BET Theory
59
It is a destructive analytical technique. Surface and textural properties have been
studied by adsorption and desorption plot. Surface area and pore diameter were
calculated from m plot and BJH plot successfully.
3.2.3. Compositional Characterizations
(a) Fourier Transform Infrared (FTIR) Spectroscopy
Fourier transform infrared (FTIR) spectroscopy is used to execute qualitative and
quantitative analysis of organic compounds and to determine chemical structure of
inorganic materials. It is a destructive analytical technique which requires samples
as in few microns in diameter. The chemical bonds absorb infrared (IR) energy at
specific frequency; the compound exhibits different types of bonds which mean
they will absorb different frequency. The plot of a compound’s IR transmittance vs
frequency is known as fingerprint, which when compared to reference spectra
identifies the material. The infrared portion of the electromagnetic spectrum is
divided into three zone i.e. the near, mid and far- infrared zone. The near IR zone
is usually high in energy and belongs to approximately 1400-400 cm-1 can excite
overtone or harmonic vibration. The mid-infrared approximately extends 4000-400
cm-1 used to study the fundamental vibration and associated rotational-vibrational
structure. The far-infrared, usually low energy 400-10 cm-1 which belongs to
microwave region of electromagnetic spectrum and used for rotational
spectroscopy as depicted in Figure 3.5.
60
Figure 3.5: Layout diagram of Fourier Transform infrared (FTIR) spectroscopy.
[11]
61
(b) Energy dispersive X-ray spectroscopy
Energy dispersive X-ray spectroscopy (EDS, EDX or EDXRF) is an analytical
technique used to determine the elemental analysis or chemical characterization of
a sample through interaction in between sample and source of X-ray excitation. At
rest, every atom has definite arrangement of electron in discrete energy levels
around the nucleus. When high energy beam of radiation hit the sample (matter), it
may excite an electron from inner shell and create a hole over there. To
compensate this vacancy an electrons from outer shell migrate to inner shell, while
the difference in energy between outer and inner shell may be generated in the
form of X-ray. As, every element has a definite electronic arrangement, when the
sample is being hit with high energy electromagnetic radiations it will generate
characteristic X-ray this is like fingerprint for every element. The number and
energy of X-rays emitted from sample can be measured by a detector. The Energy
dispersive spectroscopy follows the same principle which has been discussed in the
FE-SEM. Energy dispersive spectroscopy indentifies the elemental composition in
the sample for all the elements with an atomic number greater than boron and their
concentration less than 0.1 percent.
3.2.4. Optical Characterizations
(a) UV- Visible absorption spectroscopy
UV-Visible absorption spectroscopy is a quantitative analytical technique that is
being used to determine different analyte such as transition metal ions, highly
conjugated organic compounds and biological macromolecules present in the
sample solution. The UV-Visible absorption spectroscopy is working out in the
ultraviolet-visible spectral region i.e. absorption of sample directly depends upon
the colour of sample. The molecules containing π electron or non-boning electron
62
when irradiate under ultraviolet or visible radiation, the electron can absorb
definite amount of energy and moved from ground state to excited state of energy
(i.e. excitation of electron from HOMO to LUMO) and exhibited a definite
spectrum. The UV-Visible absorption spectroscopy is following the Beer-Lambert
law, which states that the absorbance of a solution is directly proportional to the
concentration of absorbing species in the solution and path length.
A = log 10 (I 0 /I) = €. c. L
(3.12)
Where, A is the absorbance in absorbance units (A.U.), I 0 is the intensity of
incident radiation while I is intensity of transmitted radiation, L is the path length,
c is the concentration of absorbing species and € is the molar absorptivity constant.
(b) UV-Visible diffused reflectance spectroscopy (UV-Vis DRS)
UV-Visible diffused reflectance spectroscopy is a kind of infrared spectroscopy in
which spectrum of powder samples is recorded with no preparation. The sample is
simply mounted on the cup holder and data is collected in the bulk sample. Upon
illumination of infrared irradiation on the sample holder, ray is reflected and
transmitted at different amounts depending on the bulk properties of the material.
The diffused reflection is produced by the sample surface, when light from a
surface is reflected at many angles rather than a single angle as in the case of
specular reflection and is collected by use of an ellipsoid mirror. Shape,
compactness, refractive index and absorption of particles are all characteristic of
the material being analyzed. If the sample possesses high absorptivity then, it can
be diluted with mixing the sample with nonabsorbent materials like potassium
bromide, potassium chloride etc. The particle size of the materials should be less
63
than the wavelength of incident radiation i.e. size should be less than 5 microns for
mid-range infrared spectroscopy. The plot between log reflectance (log1/R) verses
wavelength gives diffused reflectance spectrum. The band gap energy of material
can be calculated from the optical absorption edge onset (λ) of reflectance
spectrum by equation:
E (eV) = 1240 / λ (nm)
(3.13)
Where λ is absorption edge and E is band gap energy of synthesized sample. While
an alternative plots of Kubelka-Munk can be used to relate reflectance to
concentration factor.
3.2.5. Photocatalytic Activity Measurement
The photocatalytic activities of as-synthesized photocatalyst that is either Cu doped
SrTiO 3 nanoparticles or ZnO nanomaterials were measured by monitoring the
decompositions of organic dyes such as Methylene Blue (MB) and Rodamine B
(RhB). The photocatalytic reaction was executed in Pyrex flask type reactor under
the illumination by using xenon arc lamp (Thoshiba, SHLS-1002) at ambient
temperature to about 28 ºC. The efficiency of photocatalytic activities were studied
by monitoring the changes in absorbance spectra of the basic organic dyes; MB
and RhB in an aqueous solution containing either Cu doped SrTiO 3 or ZnO
nanomaterials (i.e. nanoparticles and nanorods) photocatalyst under constant
stirring and also provided with continuous exposure of
visible and UV-light
irradiation respectively.
For the effective degradation of organic dyes such as MB and RhB, an appropriate
amount of dyes were dissolved in 100 mL water to get the concentration of 10 ppm
and disperse 0.15g of as-synthesized photocatalyst under constant stirring. Prior to
illumination, suspension was continuously stirred for about 1 hour to develop
64
adsorption–desorption equilibrium between dye and photocatalyst in the dark.
After that oxygen is commonly provided to stable aqueous suspension in order to
scavenge electron from the catalyst surface. Then, the stable aqueous dye
suspension was exposed to either visible or UV light illumination under constant
stirring. The sample was periodically and successively taken out from Pyrex
reactor after every 10 minutes of time interval and subjected to centrifuge at
12,000 rpm to filter out ZnO powder, and then an absorption spectrum of
decomposed dye solution was measured using UV-visible spectrophotometer.
Moreover, the photo-catalytic degradation of dye followed the pseudo-first order
kinetics and rate constant was determined by following relation;
ln(C o /C) = kt
(3.14)
The k was calculated from graph between ln(C o /C) vs time interval, where C o and
C denote the dye concentration at time, t=0 and t = t respectively.
65
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