MICROSTRUCTURAL, MORPHOLOGICAL AND OPTICAL STUDIES …shodhganga.inflibnet.ac.in/bitstream/10603/70233/6/... · 2018. 7. 8. · with space group I4 1 /amd (JCPDS File No. 78-2486)

Chapter 3

MICROSTRUCTURAL, MORPHOLOGICAL AND

OPTICAL STUDIES OF Cr DOPED TiO2

NANOPARTICLES

The present chapter deals with the detailed description of the

effect of Cr substitution in titania nanoparticles with stoichiometric

formula Ti1-xCrxO2 (x = 0.00, 0.03, 0.05 and 0.07) synthesized

through acid modified sol-gel route. The nanoparticles have been

studied with TGA, XRD, FESEM, EDX, HRTEM, FTIR, Raman,

UV-visible and PL spectroscopies.

Ph.D. Thesis Alamgir

Chapter 3 Thermal, Microstructural …. Cr doped … … Page 65

3.1 Introduction

Research investigations on different nanostructures such as nanoparticles

(NPs), nanocrystals, nanolayered thin films, quantum dots, quantum wells, atomic and

molecular clusters etc. have grown dramatically over the last several years. In this

direction TiO2 (titania) nanostructures have emerged as one of the most fascinating

materials in the modern era. It is well known that TiO2 crystallizes in three different

types of polymorphs: rutile, anatase and brookite [1]. It has succeeded in capturing the

attention of physical chemists, physicists, material scientists, and engineers in

exploring unique semiconducting and catalytic properties. A keen interest in titania

has been growing because of incorporation of a suitable dopant element in this host

matrix makes the former an important material for solar cells, sensors, antibacterial

activities, photocatalysis, spintronic devices etc. [2-6]. The wide band gap of titania

makes it a less efficient photocatalytic material in the degradation of pollutants under

visible light. However, doping of a small percent of suitable foreign element enhances

the visible light absorption capability by lowering the effective band gap of the

material, thereby making it a visible light active photocatalyst [7]. Doping creates mid

band gap electronic states that interact with the host band electrons and changes the

electronic structure of the host material [8]. Among these, chromium seems to be one

of the most promising dopant in TiO2. Following the milestone work of Matsumoto et

al. on Co doped TiO2 systems [6], several approaches are made by different groups to

increase the room temperature ferromagnetic response in this otherwise nonmagnetic

TiO2 material [9-11]. The optical response as well as the room temperature

ferromagnetism of anatase Cr doped titania are affected by defect states and oxygen

vacancies in the crystal lattice [12-14]. The analysis of luminescence spectrum is a

cheap and effective way to study the electronic structure, optical and photochemical

properties of semiconductors, through which the information such as oxygen

vacancies and defects, as well as the efficiency of charge carrier trapping and

recombination can be understand [15, 16].

Earlier studies report that the microstructure, morphology and optical

properties of transition metal doped TiO2 appear to be extremely sensitive to the

conditions employed during synthesis. A small percent of Cr doping enhances the



photocatalytic activity of titania. Apart from the existing literature, hardly few

literatures report on structural refinement using Rietveld analysis on TiO2 NPs. In the

present work highly pure and stable anatase Cr (0, 3, 5 and 7 mol %) doped titania

NPs have been synthesized through simple and low cost acid modified sol-gel

method. Structural parameters of the synthesized NPs were refined to confirm the

presence of only anatase phase and hence investigated systematically the

microstructural, morphological and optical responses including fluorescence

properties of the synthesized products.

3.2 Experimental Details

3.2.1 Materials and Synthesis

Chromium doped TiO2 NPs with stoichiometric formula Ti1-xCrxO2 (x = 0.00,

0.03, 0.05, and 0.07) were prepared by acid modified sol-gel route. Details of

materials and synthesis procedure are already discussed in Chapter 2 (section 2.2.1).

Synthesis route is also shown in the flow chart (Fig. 2.3).

3.2.2 Characterizations and Measurements

To investigate the thermal decomposition of undoped and Cr doped TiO2

samples, the dried (at 80 ºC) and grinded gels, obtained during synthesis (Chapter 2)

before annealing, were monitored using a thermogravimetric analysis (TGA), Pyris 1

HT, Perkin Elmer. The crystalline phase, structure and particle size of the Cr (0, 3, 5

and 7 mol %) doped titania NPs were observed with a Rigaku Miniflex-II X-ray

diffractometer at scanning rate of 2°/min in 2θ range from 20° to 80º equipped with

high intense Cu-Kα radiations (λ = 1.5406 Å) operated at a voltage of 30 kV and

current of 15 mA. The surface topography was obtained by field emission scanning

electron microscopy (FESEM) from Bruker (NANO NOVA 450). Elemental

compositions were determined from energy dispersive X-ray spectroscopy (EDX)

equipped with the FESEM. Morphology, crystallinity, shape and size distribution of

the synthesized samples were also observed by high resolution transmission electron

microscopy (HRTEM) along with selected area electron diffraction (SAED) pattern

with a transmission electron microscope (TEM), JEM-2100, JEOL. The presence of

different bondings and vibrational frequencies were determined by a Fourier



transform infrared (FTIR) spectrometer (Spectrum 2, Perkin Elmer) using KBr pellets

as medium. For further confirmation of single phase and to investigate microscopic

structural disorder, Raman spectra of all the samples were recorded at room

temperature on a Raman spectrometer (Renishaw, UK, sensitive for ultra-low signal

detection with spectral resolution 0.2 cm‒1

) with Ar ion laser of wavelength, 514.5 nm

and power, 50 mW. UV-visible absorption spectra were recorded with an UV-visible

spectrometer (Perkin Elmer-Lambda 35) and fluorescent emission spectra by a LS-55

(Perkin Elmer) spectrometer. All the characterization techniques are discussed in

Chapter 2.

3.3 Results and Discussion

3.3.1 Thermogravimetric Analysis (TGA)

TGA patterns of representative undoped and 5 mol % Cr doped TiO2 samples

are shown in Fig. 3.1.

Fig. 3.1 TGA patterns of undoped and 5 mol % Cr doped TiO2 NPs.

The total weight loss in the whole temperature range is ∼ 32.5 % for undoped

and ∼ 42 % for Cr doped (5 mol %) titania. The TGA plot indicates the total weight



loss (%) occurring in three different stages. The initial stage weight loss is in between

room temperature to 150 ºC, which is caused by the loss of physically adsorbed water.

The second stage weight loss is from 150 ºC to 400 ºC and is due to loss of

chemisorbed water, solvent and the removal of residual organic [17]. There is a

negligible weight loss at temperatures above 400 ºC. On further heating above 400 ºC,

the amorphous phase is converted to crystalline form. In TGA plot, Cr doped TiO2

sample shows higher loss in weight as compared to undoped TiO2 sample, therefore,

Cr doped TiO2 has more surface hydroxylation [18]. Taking into consideration of

TGA study, 450 ºC can be used as a suitable annealing temperature and all the Cr

doped samples are calcined at 450 ºC. Therefore, the undoped and Cr doped samples

are utilized for further characterization after annealing.

3.3.2 X-Ray Diffraction (XRD) with Rietveld Analysis

The X-ray diffraction patterns of undoped and Cr doped TiO2 nanoparticles,

calcined at 450 °C are shown in of Fig. 3.2 along with refined patterns by Rietveld

refinement method. The lattice parameters and other detailed structural information

were obtained by using PowderX followed by the Rietveld refinement Fullprof 2000

software. For undoped and Cr doped TiO2 NPs, the diffraction peaks occurring at

25.30º, 37.78º, 48.02º, 53.90º, 54.94º, 62.60º, 68.86º, 70.19º and 75.12º have been

assigned to the lattice planes (101), (004), (200), (105), (211), (204), (116), (220), and

(215) respectively. The indexed lattice planes in the all diffraction patterns correspond

to the signals of pure tetragonal anatase phase of TiO2 with space group I41/amd

(JCPDS File No. 78-2486). The Rietveld refinement parameters, lattice parameters

and unit cell volume for these NPs are tabulated in Table 3.1. Observed patterns are in

a good agreement with the calculated refined profile with χ2 (goodness of fit) ∼1.2. It

also confirms that undoped and Cr doped TiO2 crystallizes in the (100 %) anatase type

tetragonal structure only, without any detectable evidence from rutile or brookite

phases. In Fig. 3.2, the main peak of rutile phase at 2θ = 27.4º (110) plane and

brookite phase at 2θ = 30.8º (121) plane are simultaneously absent. Regardless, there

is no trace of any secondary phases such as Cr-oxides phases and Cr-composites. As

the diffractograms do not show any such extra peak, confirm that the tetragonal

anatase phase is not disturbed upon Cr doping. Tetragonal anatase TiO2 is constructed



with coordination Ti-6 (octahedral) and O-3 (trigonal planer). However, small line

intensity variations with a slight change of diffraction line widths and small shifts of

Bragg peak (101) in the inset of Fig. 3.2 were observed by Cr doping in titania NPs

from 0 to 3, 5 and 7 mol %. These can be attributed to the slight expansion of the unit

cell volume and decrease in the crystallite size (Table 3.1).

Fig. 3.2 XRD patterns of Ti1-xCrxO2 (x = 0.00, 0.03, 0.05 and 0.07) NPs (experimental

and refined patterns fitted with Rietveld analysis).



Table 3.1 Summary of refined structural data with crystallite size, texture and

optical band gap of various Cr doped anatase titania NPs.

Parameters Cr concentration (mol %) in titania NPs

0 3 5 7

Space group I41/amd I41/amd I41/amd I41/amd

Symmetry Tetragonal Tetragonal Tetragonal Tetragonal

a = b (Å) 3.78460 3.78538 3.78610 3.78741

c (Å) 9.47232 9.47697 9.48759 9.49195

V (Å3) 135.67391 135.79647 136.00036 136.15703

Z 4 4 4 4

Ti (4b) (0, 1/4, 3/8) (0, 1/4, 3/8) (0, 1/4, 3/8) (0, 1/4, 3/8)

O (8e) (0, 1/4,

0.58215)

(0, 1/4,

0.58380)

(0, 1/4,

0.58418)

(0, 1/4,

0.58471)

Rp, Rwp, χ2 8.17, 9.74, 2.21 5.56, 7.16, 1.26 5.59, 7.13, 1.29 5.72, 7.23, 1.36

RB, RF 8.56, 7.61 1.36, 0.852 1.53, 0.946 1.67, 0.991

Crystallite

size (nm)

10.52 9.88 9.70 8.94

Band gap (eV) 3.25 3.58 3.43 3.38

The expansion of the unit cell volume could be recognized due to small

difference between ionic radii of the dopant (Cr) and the host titanium ions. As, Cr3+

(61.5 pm) ions may easily be substituted into Ti4+

(60.5 pm) site of TiO2 with a

coordination number of 6 [19, 20]. The average crystallite size of all the samples,

calculated using Scherrer’s formula [21], is observed in the range from 9 to 11 nm and

are tabulated in Table 3.1. Average crystallite size of Cr doped titania is smaller to

that of undoped titania as shown in Table 3.1. Because of different ionic charges of

Ti4+

and Cr3+

ions, dopant substitutions lead to the creation of crystal defects and

oxygen ion vacancy in order to neutralize the charge imbalance in titania structure.

Due to the small size of the NPs, not all the added dopants undergo to the host lattice

interior (i.e. octahedral position) rather some dopants may sit on the surfaces or some

may be on the grain boundaries, and they go beyond the detection limit of XRD.

These few dopant atoms at the grain boundary disturb the periodicity of the lattice;

inhibit crystal growth and crystallite size decreases. Hence, doped samples show

lowering of peak intensity and an increase in full width at half maximum of XRD

reflections.



3.3.3 FESEM and EDX Analysis

The morphology of synthesized pure and Cr doped (5 mol %) titania NPs was

analyzed by FESEM followed by EDX for the confirmation of presence of expected

elements.

Fig. 3.3 FESEM micrographs of (a) undoped and (b) 5 mol % Cr doped TiO2 NPs.



Fig. 3.4 EDX spectra of (a) undoped and (b) 5 mol % Cr doped TiO2 NPs.

Table 3.2 Compositions of undoped and 5 mol % Cr doped titania NPs by EDX

technique.

Sample Elements Wt %

Undoped TiO2 NPs

Ti 43.94

O 56.06

Total 100

5 mol % Cr doped

TiO2 NPs

Ti 52.50

O 44.99

Cr 2.51

Total 100



Figs. 3.3(a) and 3.3(b) show the FESEM micrographs of undoped and 5 mol

% Cr doped TiO2 NPs respectively, whereas Figs. 3.4(a) and 3.4(b) display chemical

composition present in these two samples through EDX analysis. The FESEM

micrographs show that nanoparticles are somewhat spherical in nature. However,

most of them are in agglomerated form. The compositions obtained from EDX

analysis is presented in Table 3.2 and the data show that the samples are composed of

Cr, Ti and oxygen. EDX analysis shows that there is desired composition of

chromium with respect to titanium centre in the doped sample and Cr doping leads the

oxygen deficiencies in the synthesized doped samples.

3.3.4 TEM with SAED Analysis

The particle size distribution and crystallinity of prepared NPs were

characterized by TEM with the SAED pattern. TEM images in Fig. 3.5(a) and Fig.

3.5(b) respectively reveal that undoped and 5 mol % Cr doped TiO2 NPs were

agglomerated and non-spherical in shapes. The size distributions of them are shown in

histograms in the inset of Fig. 3.5(a) and Fig. 3.5(b) accordingly. Average particle

sizes of undoped and 5 mol % Cr doped titania samples are found to be 12.7 nm and

11.6 nm respectively. It is clear evident from the TEM images that synthesized titania

samples have small size but they are well crystalline in nature. The crystalline nature

is further analyzed from SAED pattern as shown in Fig. 3.6(a) where the ring pattern

indicates polycrystalline anatase nature of 5 mol % Cr doped titania NPs. The SAED

pattern demonstrates that ring consists of well distinct bright spots due to the high

crystallinity of doped TiO2 indicated the anatase phase is enriched with (101) planes.

HRTEM study was further applied to analyze the lattice fringes for the 5 mol % Cr

doped NPs, which is presented in the Fig. 3.6(b). The clear lattice fringes are

obtained, implying that the Cr doped NPs are highly crystalline in nature. The d-

spacing of the crystal plane is calculated as 3.42 Å for 5 mol % Cr doped sample is

also indicating the preferable crystal growth plane is (101) and it is also the highest

intensity peak in the XRD pattern as shown in Fig. 3.2.



Fig. 3.5 TEM micrographs of (a) undoped and (b) 5 mol % Cr doped TiO2 NPs.

Fig. 3.6 (a) SAED pattern and (b) HRTEM image of 5 mol % Cr doped TiO2 NPs.

3.3.5 FTIR Analysis

Generally, the FTIR spectrum gives the information about functional group,

molecular geometry, and inter or intra molecular interactions present in a system.

FTIR spectra of Ti1-xCrxO2 (x = 0.00, 0.03, 0.05 and 0.07) NPs at room temperature

have been recorded in order to study the vibrational bands present in the samples and

are presented in Fig. 3.7. It is a general perception that the band frequencies within



1000 cm‒1

are associated with the bonds between inorganic elements. Therefore in the

low energy region below 1000 cm‒1

, a very strong absorption at 765 cm‒1

has been

observed, which is the characteristic peak of anatase titania due to Ti-O bond [22]. A

little hump can be observed around 500 cm‒1

as the Ti-O-Ti stretching vibration in

titania network. These bonds are important due to their role to the photocatalytic

activity. Shifting of the hump to the higher wavenumbers observed in Fig. 3.7 may be

due to decrease in nanoparticles size or may be due to asymmetric stretching vibration

with the formation of Ti-O-Cr [22]. With Cr doping a peak is appeared at around 946

cm‒1

and is gradually increasing with Cr concentration, which is absent in undoped

TiO2 sample and can be attributed to Cr-O vibrational mode.

Fig. 3.7 FTIR spectra of Ti1-xCrxO2 (x = 0.00, 0.03, 0.05 and 0.07) NPs recorded at room temperature.

Strong broad absorption peak at ~3150 cm‒1

is attributed to the -OH (hydroxyl)

group. These bands characterized the stretching vibration of the O-H of Ti-OH group

on the surface [23]. The small sharp peak around ∼2350 cm‒1

band has been assigned



to stretching mode of CO2 being tapped with the titina nanoparticles [24]. The sharp

band found at 1600 cm‒1

is due to the H-O-H bending mode of absorbed water

indicating the existence of water absorbed on the surface of nanocrystalline titina

powders [25]. There is neither band around ∼1389 cm‒1

due to the bending vibrations

of the C-H bond nor bands assigned to the alkoxy groups [26]. Therefore, addition of

glacial acetic acid during synthesis did not cause any residual impurities on pure and

doped titania NPs surfaces after calcination.

3.3.6 Raman Analysis

In order to investigate the influence of Cr doping on microstructural and

vibrational properties the pure and doped TiO2 NPs were probed using micro-Raman

scattering experiments. Raman spectroscopy is a fast, sensitive and nondestructive

method. It is one of the most effective tools for detecting the phase, crystallinity and

incorporation of dopants and the resulted defects and lattice disorder in the host lattice

[27]. The change of Raman spectrum is related to non-stoichiometry and phonon

confinement effect in nanostructured materials [28-31]. Oxygen deficiency within the

material, however, strongly affects the Raman spectrum by producing frequency shift

and broadening of some spectral peaks. The Raman signal of TiO2 is very sensitive to

the vibrational mode of oxygen ions in the Ti-O bond. The strength of the Raman

signal depends upon the polarizability of oxygen ions surrounding Ti4+

in the basic

TiO6 octahedral [32]. To evaluate the presence of the desired phases in the pure and

Cr doped TiO2 NPs, the Raman spectra of all the samples were recorded at room

temperature as shown in Fig. 3.8 and it gave complimentary phase analysis.

The group-theoretical analysis gives the existence of 15 optical modes or

phonons in the center of Brillouin zone of anatase titania, with the following

irreducible representation of the normal vibrational modes

Γ = A1g + A2u + 2B1g + B2u + 3Eg + 2Eu (3.1)

Among them six modes (A1g + 2B1g + 3Eg) are Raman active, three infrared (IR)

active modes (A2u and 2Eu) and a silent B2u mode [33]. Here the section of this

chapter deals with only Raman active modes. The Raman spectrum of an anatase

single crystal have been investigated by Ohsaka, who concluded that six allowed

modes appear at 144 cm‒1

(Eg), 197 cm‒1

(Eg), 399 cm‒1

(B1g), 513 cm‒1

(A1g), 519



cm‒1

(B1g), and 639 cm‒1

(Eg) respectively [34]. In the Fig. 3.8, Raman bands are

observed at 143 cm‒1

(Eg), 196 cm‒1

(Eg), 396 cm‒1

(B1g), 515 cm‒1

(A1g + B1g), and

638 cm‒1

(Eg) for the synthesized Cr doped samples. It is reported in the literature that

Raman band of anatase TiO2 occurring at 516 cm‒1

at room temperature is split into

two peaks centered at 513 (A1g) and 519 cm‒1

(B1g) [35]. In the present Raman

spectra, modes correspond to the rutile phase, reported around 238 - 240, 447 - 448

(intense line) and 610 - 612 cm‒1

, are simultaneously absent that reconfirms the

presence of only anatase phase of synthesized pure and Cr doped samples [36, 37].

Fig. 3.8 Raman spectra of Ti1-xCrxO2 (x = 0.00, 0.03, 0.05 and 0.07) NPs using

514.5 nm Ar ion laser excitation at room temperature.

Moreover, no bands are observed for oxides of chromium, which also supports

the presence of dopant cations in the substitutional positions of the titania host lattice

for the doped NPs. These substitution of Cr atoms at Ti-sites in Cr doped TiO2 NPs in

the anatase structure causes disorder in the materials, which gives rise to the

relaxation of the q = 0 selection rule in the Raman scattering [38]. The broadening and

small shifts of the Raman scattering modes are due to the microscopic structural



disorder in the periodic titanium atomic sub-lattice and break the translational

symmetry induced by Cr incorporation. Otherwise, there are no apparent impurity

related modes in the Raman spectrum due to Cr in the doped NPs, which is consistent

with the results of XRD. The most intense Raman mode Eg appeared at 143 cm‒1

shows small but consistent blue shift with the increase in Cr doping; this is also

consistent with XRD result. With the increase of doping concentration, the defects

increase and the crystallite size decreases and gradually the Eg peak shifts towards

higher energy. Moreover, for doped samples, all observed peaks are broadened and

intensities are decreased gradually with the proliferation of Cr concentration.

3.3.7 Optical Responses: UV-Visible and Fluorescence Spectroscopies

Absorption and emission spectroscopies are powerful nondestructive

techniques to investigate the optical properties of nanocrystalline semiconductors.

Fig. 3.9 UV-visible absorption spectra of Ti1-xCrxO2 (x = 0.00, 0.03, 0.05 and 0.07)

NPs.



The optical properties of semiconductor are generally determined by its

electronic structure, so the essence of light absorption can be found through

investigating the relationship between electronic structure and optical properties of

TiO2 NPs. The absorption and emission are expected to depend on several factors,

such as band gap, particle size, oxygen deficiency, surface roughness, impurity

centers created by doping, etc. The optical (UV-visible) absorption spectra of

undoped and Cr doped TiO2 NPs are shown in Fig. 3.9. Absorbance spectra exhibit

ultraviolet cut-off around 300 - 375 nm, which can be attributed to the photo-

excitation of electrons from valence band to conduction band. The absorption edge of

different samples varies as the concentration of Cr in the TiO2 NPs varies.

In order to calculate the value of direct band gap (Eg) of the studied samples,

the following Tauc’s relation is used [39]

αhν = A(hν - Eg)n (3.2)

where α is the absorption coefficient given by α = 2.303 log(T/d); T is the

transmission and d is the thickness of the sample; A is a constant and hν is the energy

of photon.

Fig. 3.10 Optical band gap estimation of Ti1-xCrxO2 (x = 0.00, 0.03, 0.05 and 0.07)

NPs.



Fig. 3.10 shows the plots of (αhν)2 versus hν for pure and Cr doped TiO2 NPs.

The values of Eg have been estimated by taking the intercept of the extrapolation to

zero absorption with photon energy axis i.e. α = 0 (Table 3.1). This change in the

value of Eg depends on several factors such as grain size, carrier concentration, lattice

strain, size effect etc. The Eg is found to be 3.25 eV for undoped TiO2 sample, which

is slightly higher than the reported value of the bulk TiO2, i.e. 3.23 eV [40]. On

increasing doping concentration of Cr, the band gap energy first goes up then

decreases as shown in Fig. 3.10 and in Table 3.1. The former one can be attributed to

the quantum size effect of the NPs below 10 nm while the latter one is in contrast to

the normal phenomenon of quantum confinement [41]. Latter one can be assigned to

the sufficient interaction of Cr inside the titina host matrix leading effective formation

of dopant level near the valance band (resulting to band gap narrowing) after

overcoming the quantum size effect [42].

Fluorescence emission spectra of Ti1-xCrxO2 (x = 0.00, 0.03, 0.05 and 0.07)

NPs were also recorded at room temperature. Fig. 3.11 illustrates the photo-induced

fluorescence emission spectra of all samples excited at a wavelength, λext = 275 nm

that is higher than the optical band gap of TiO2. After excitation by photon, the

electron-hole pairs recombine in various processes. The recombination rate is closely

related to the electronic structure and crystal structure associated with the materials.

Undoped sample exhibits an emission peak centered within UV region at 375 nm and

at lower wavelength for doped samples. Whereas, the samples exhibit another two

peaks corresponding to blue emissions near 421 nm and green emission near 484 nm.

These two emission peaks lying in between 400 - 800 nm (visible region) are

associated with self-trapped excitons, oxygen vacancies, surface defects, etc. [43].

Generally, oxygen vacancies are known to be the most common defects in

nanocrystaline semiconductor and usually act as radiative center of luminescence in

the visible region. The strong UV emission peaks at 375 nm for the undoped sample

and at lower wavelengths for the Cr doped NPs are due to band gap transition or the

band edge emission of the host titania [44, 45]. Therefore, the band edge emission

peak in UV region gradually shifts towards the lower wavelength with the

proliferation of Cr concentration, whereas the other two in the visible region do not.

These blue shifts of the band emission indicate the widening of optical band gap of



titania NPs and may be associated to the quantum size effects [41]. The blueshift of

photoluminescence (PL) peak due to Cr doping is consistent with the recent reports

[44]. The oxygen defects state in nanocrystaline titania located near the bottom of the

conduction band edge [46, 47]. Therefore, we assign the visible emission peaks to

oxygen vacancies. These oxygen vacancies are mainly F center with trapped

electrons. Depending on the number of trapped electrons these F centers are classified

as F (with two electrons), F+ (one electron), and F

2+ (no electron) [48]. According to

Lei et al. these visible emission peaks present in the spectra are due to the F or F2+

center [43].

Fig. 3.11 Fluorescence emission spectra of Ti1-xCrxO2 (x = 0.00, 0.03, 0.05 and 0.07)

NPs excited at wavelength, λext = 275 nm.

PL spectra also display decreasing intensity with increasing Cr doping though

it is not consistent as shown in Fig. 3.11. The PL intensities are affected by the

localized or delocalized nature of the trapped electrons and by the radiative or non-

radiative pathways of carrier recombination. In general, the quenching of PL intensity

implies an enhanced photocatalytic activity because PL emission is the result of the

combination of excited electrons and holes [49, 50]. In the synthesized NPs, Cr



doping into titania suppresses electron-hole recombination up to 5 mol % doped

samples, then enhances as an excess of Cr leads to the recombination of charge

carriers. A large photocatalytic activity is thus expected for Cr doped titania NPs up to

5 mol % doping.

3.4 Conclusions

Highly stable (0, 3, 5 and 7 mol %) Cr doped anatase TiO2 NPs were

successfully synthesized through acid modified sol-gel method. Thermogravimetric

analysis shows that crystalline formation of undoped and Cr doped titania NPs is

completed at above 400 ºC. The Rietveld refinement has confirmed that whole series

of the titania NPs, Ti1-xCrxO2 (x = 0.00, 0.03, 0.05 and 0.07) belongs to anatase type

tetragonal structure having space group I41/amd. Average crystallite size decreases on

Cr doping as calculated from Scherrer’s formula. Electron microscopies also provide

the morphology and crystallinity of the NPs with particle size and distribution.

Average particle size for undoped and 5 mol % Cr doped samples are found to be 12.7

and 11.6 nm respectively from TEM analysis. EDS result confirms the presence of the

Cr in the doped TiO2 NPs. Raman analysis gives further compliment to the phase

analysis. Spectra reveal that the characteristic intense band of anatase TiO2 at 143

cm‒1

decreases in intensity and the peak position shifts gradually with increasing Cr

doping concentration. It implies that the Cr incorporation in TiO2 does not change the

anatase structure but microscopic structural disorder is taking place in the periodic

titanium atomic sublattices. The optical responses have been analyzed using optical

absorption and fluorescence spectroscopies. The band gap of the doped NPs shows a

widening effect and then it decreases with increasing doping concentration as

estimated from Tauc’s relation. The fluorescence spectra show defect related visible

blue and green emissions near 421 near 484 nm respectively. Incorporation of Cr ion

increases the number of non-radiative oxygen vacancies and decreases the emission

intensity. So, a large photocatalytic activity is expected for small amount of Cr doping

in to anatase TiO2. The results, presented in this chapter, may come up with useful

guidelines for the synthesization of tunable photonic devices material, as there is an

indication of simultaneous effect from quantum size confinement and creation of

doner level.



References

[1] D. R. Coronado, G. R. Gattorno, M. E. Pesqueira, C. Cab, R. Coss, and R. G.

Oskam, Nanotechnology 19, 145605 (2008).

[2] X. Lu, X. Mou, J. Wu, D. Zhang, L. Zhang, F. Huang, F. Xu, and S. Huang,

Adv. Func. Mat. 20, 509 (2010).

[3] S. Klosek and D. Raftery, J. Phy. Chem. B 105, 2815 (2001).

[4] H. Zhang and G. Chen, Environ. Sci. Technol. 43, 2905 (2009).

[5] B. Choudhury, B. Borah and A. Choudhury, Photochem. Photobiol. 88, 257

(2012).

[6] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M.

Kawasaki, P. Ahmet, T. Chikyow, S. Y. Koshihara, and H. Koinuma, Science

291, 854 (2001).

[7] S. In, A. Orlov, F. Garcia, M. Tikhov, D. S. Wright, and R. M. Lambert,

Chem. Commun. 40, 4236 (2006).

[8] R. Long and N. J. English, Phys. Rev. B 83, 155209 (2011).

[9] J. D. Bryan, S. A. Santangelo, S. C. Keveren, and D. R. Gamelin, J. Amer.

Chem. Soc. 127, 15568 (2005).

[10] B. Choudhury, A. Choudhury, P. Alagarsamy, A. Islam, and M. Mukherjee,

J. Magn. Magn. Mater. 323, 440 (2011).

[11] L. H. Ye and A. J. Freeman, Phys. Rev. B 73, 081304 (2006).

[12] B. Choudhury and A. Choudhury, Mater. Sci. Eng. B 178, 794 (2013).

[13] F. D. Pieve, S. D. Matteo, T. Rangel, M. Giantomassi, D. Lamoen, G. M.

Rignanese, and X. Gonze, Phys. Rev. Lett. 110, 136402 (2013).

[14] S. M. Koohpayeh, A. J. Williams, J. S. Abell, J. Lim, and E. Blackburn, J.

Appl. Phys. 108, 073919 (2010).

[15] J. C. Yu, J. G. Yu, K. W. Ho, Z. T. Jiang, and L. Z. Zhang, Chem. Mater. 14,

3808 (2002).

[16] M. Anpo, N. Aikawa, Y. Kubokawa, M. Che, C. Louis, and E. Giamello, J.

Phys. Chem. 89, 5017 (1985).

[17] K. Nagaveni, M. S. Hegde and G. Madras, J. Phys. Chem. B 108, 20204

(2004).



[18] J. T. Carneiro, T. J. Savenije, J. A. Moulijn, and G. Mul, J. Phys. Chem. C

115, 2211 (2011).

[19] K. Das, S. N. Sharma, M. Kumar, and S. K. De, J. Phys. Chem. C 113, 14783

(2009).

[20] B. Santara, B. Pal and P. K. Giri, J. Appl. Phys. 110, 114322 (2011).

[21] K. Ishikawa, K. Yoshikawa and N. Okada, Phys. Rev. B 37, 5852 (1988).

[22] P. Goswami and J. N. Ganguli, Dalton. Trans. 42, 14480 (2013).

[23] H. Y. Chuang and D. H. Chen, Nanotechnology 20, 105704 (2009).

[24] D. Bianchi, T. Chafik, M. Khalfallah, and S. J. Teichner, Appl. Catal. A 105,

223 (1993).

[25] S. Musić, M. Gotić, M. Ivanda, S. Popović, A. Turković, R. Trojko, A.

Sekulić, and K. Furić, Mater. Sci. Eng. B 47, 33 (1997).

[26] J. A. Wang, R. L. Ballesteros, T. Lopez, A. Moreno, R. Gomez, O. Novaro,

and X. Bokhimi, J. Phys. Chem. B 105, 9692 (2001).

[27] O. Pages, M. Ajjoun, D. Bormann, C. Chauvet, E. Tournie, J. P. Faurie, and

O. Gorochov, J. Appl. Phys. 91, 9187 (2002).

[28] J. C. Parker and R. W. Seigel, Appl. Phys. Lett. 57, 943 (1990).

[29] D. Bersani, P. P. Lottici and X. Z. Ding, Appl. Phys. Lett. 72, 73 (1998).

[30] W. Ma, Z. Lu and M. Zhang, Appl. Phys. A 66, 621 (1998).

[31] W. F. Zhang, Y. L. He, M. S. Zhang, Z. Yin, and Q. Chen, J. Phys. D: Appl.

Phys. 33, 912 (2000).

[32] B. Choudhury and A. Choudhury, J. Appl. Phys. 114, 203906 (2013).

[33] M. Giarola, A. Sanson, F. Monti, G. Mariotto, M. Bettinelli, A. Speghini, and

G. Salviulo, Phys. Rev. B 81, 174305 (2010).

[34] T. Ohsaka, F. Izumi and Y. Fujiki, J. Raman Spectrosc. 7, 321 (1978).

[35] S. Sharma, S. Chaudhary, S. C. Kashyap, and S. K. Sharma, J. Appl. Phys.

109, 083905 (2011).

[36] M. Gotic, M. Ivanda, S. Popovic, S. Music, A. Sekulic, A. Turkovic, and K.

Furic, J. Raman Spectrosc. 28, 555 (1997).

[37] H. Chang and P. J. Huang, J. Raman Spectrosc. 29, 97 (1998).

[38] P. Parayanthal and F. H. Pollak, Phys. Rev. Lett. 52, 1822 (1984).



[39] S. Suwanboon, P. Amornpitoksuk, A. Haidoux, and J. C. Tedenac, J. Alloys.

Compd. 462, 335 (2008).

[40] M. Litter, Appl. Catal. B: Environ. 23, 89 (1999).

[41] K. M. Reddy, S. V. Manorama and A. R. Reddy, Mater. Chem. Phys. 78, 239

(2002).

[42] S. D. Delekar, H. M. Yadav, S. N. Achary, S. S. Meena, and S. H. Pawar,

Appl. Surf. Sci. 263, 536 (2012).

[43] Y. Lei, L. D. Zhang, G. W. Meng, G. H. Li, X. Y. Zhang, C. H. Liang, W.

Chen, and S. X. Wang, Appl. Phys. Lett. 78, 1125 (2001).

[44] J. Xu, S. Shi, L. Li, X. Zhang, Y. Wang, X. Chen, J. Wang, L. Lv, F. Zhang,

and W. Zhong, J. Appl. Phys. 107, 053910 (2010).

[45] B. Choudhury and A. Choudhury, J. Lumin. 132, 178 (2012).

[46] G. Lucovsky, J. Mol. Struct. 838, 187 (2007).

[47] D. Z. Li, Y. Zheng and X. Z. Fu, Chin. J. Mater. Res. 14, 639 (2000).

[48] N. Serpone, D. Lawless and R. Khairutdinov, J. Phys. Chem. 99, 16646

(1995).

[49] B. S. Kwak, J. Chae, J. Kim, and M. Kang, Bull. Korean Chem. Soc. 30,

1047 (2009).

[50] T. C. Jagadale, S. P. Takale, R. S. Sonawane, H. M. Joshi, S. I. Patil, B. B.

Kale, and S. B. Ogale, J. Phys. Chem. C 112, 14595 (2008).

*************

Documents

MICROSTRUCTURAL, MORPHOLOGICAL AND OPTICAL STUDIES …shodhganga.inflibnet.ac.in/bitstream/10603/70233/6/... · 2018. 7. 8. · with space group I4 1 /amd (JCPDS File No. 78-2486)