Vanadium Doping Induced Structural and Optical Modifications in TiO2 Thin
Films
Arshad S. Bhatti1, Awais Ali1, I Ruzybayev2, Emre Yassitepe2 and S. I. Shah2,3
1
Centre for Micro and Nano Devices, Department of Physics, Park Road, COMSATS Institute of Information
Technology, Islamabad 44000, Pakistan.
2
Department of Physics and Astronomy, University of Delaware, Newark DE, 19716, United States.
3
Department of Material Science and Engineering, University of Delaware, Newark DE, 19716, United States.
*Corresponding author: [email protected]
In this work, we present the customized structure and morphology of vanadium (V)
doped TiO2 thin films synthesized on glass substrates a growth rate of ~ 0.6Å/s at 500oC. The
sputtering targets of pure and V doped TiO2 with three concentrations of V (1.0, 1.5 and 2.0
atomic percentage (at.%)) were prepared from powders. XRD patterns confirmed the grown TiO2
films had the anatase phase. In the doped TiO2 films, the crystallite size reduced by almost half
when V concentration increased from 0 to 2 at.% systematically. Incorporation of V in the TiO2
crystal led to the enhanced growth of (211) planes, which significantly modified the grain
geometry from the faceted to the elongated as observed in the SEM images. This was simulated
by the use of VESTA code. Raman spectroscopy showed phonon confinement, increased
nonstoichiometry and asymmetry in bonding with increased V concentration. The XPS spectra
confirmed an increase in the nonstoichiometry in TiO2 due to V substitution in the structure. It is
suggested that the difference in the valance states of Ti and V produced a clear difference in the
ionic radii of the two, which resulted in the augmentation of non-equilibrium (211) planes and
hence resulted in the emergence of modified morphology of the synthesized TiO2 thin films.
Photoluminescence spectroscopy also revealed the effect of vanadium states on the band gap
states. Pure TiO2 did not show any luminescence in the visible, how ever, incorporation of
vanadium with different percentages clearly demonstrated its incorporation in V3+, V4+ and V5+
states, which were analysed quantitatly and related to structural modifications. The modified
films would be useful for photocatalysis not only structurally but for their improved optical
properties.
Key Words: TiO2, Vanadium, doping, nonstoichiometry, oxygen vacancies
1
1. Introduction:
In recent years, titanium dioxide (TiO2), a wide band gap metal oxide semiconductor,
has become one of the intensely studied material due to its use in hydrogen production from
water [1], environmental cleaning, e.g., air and water [2–4], self cleansing and non-spotting glass
coatings [5], self-sterilizing coatings [2], dye synthesized solar cells [6], etc. TiO2 naturally
exists in three polymorphs: anatase, rutile and brookite. Due to its wide band gap (anatse: 3.2 eV
and rutile: 3.0 eV), TiO2 absorbs light in the ultraviolet (UV) region of the spectrum which is
only 3-5 % of solar spectrum. Thus, the efficient utilization of the solar spectrum (mainly a good
portion of the visible light (43-47 %)) is one of the important subjects for developing the future
generation of TiO2 based photocatalysts. This essentially requires band gap tailoring of the TiO2
which is achieved by modifying its electronic band structure.
Doping of TiO2 is one of the most promising strategies for sensitizing TiO2 to visible
light by forming impurity levels within forbidden gap [7]. The doping of TiO2 with 3d transition
metals (V, Cr, Mn, Fe, Co, and Ni) is particularly considered as one of the best approaches to
narrow the band gap or to define energy levels within the band gap, which significantly enhances
the absorption of the visible part of the spectrum [8–12]. Vanadium (V) is a transition metal with
multiple characteristics, which has shown to improve absorption of light by TiO2 on doping. For
example, V – doped TiO2 with different valance states of V (V0,1+,2+,3+,4+,5+) and Ti (Ti1+,2+,3+,4+)
exhibited a difference in oxidation activity [13]. This difference in the oxidation state of V can
change the structure and morphology of TiO2. Doping with small concentration of transition
metals (around 1-2 at.%) in TiO2 has been demonstrated to successfully reduce the
recombination processes by introducing traps for electrons and/or holes [10]. Previous reports on
anatase agree on the origin of the visible photoluminescence (PL) in TiO2, attributed to the
radiative recombination of self-trapped excitons or surface radiative recombination [14-18]. But
still a lot needs to be understood.
In this paper, we report the effect of the varied concentration of the doped V and the
growth conditions on the structure, morphology and optical properties of TiO2 thin films. The
pure and doped TiO2 films were RF sputter deposited on ITO coated glass substrates at a very
slow growth rate and at high substrate temperature. It was demonstrated that incorporation of V
resulted in the reduction of crystallite size and enhanced growth of a certain plane at the
2
employed growth conditions. Furthermore, the films showed phonon confinement of certain
modes along with red shift due to anti-symmetry of the bonding. The photoluminescence
spectroscopy also confirmed creation of “V” defect states in the band gap, which showed strong
relation with the content of V and the oxygen vacancies in TiO2. The coexistence of multiple
valance states of V with reduced Ti confirmed the substitution of V in TiO2, which altered the
stoichiometry, structure, morphology and optical properties of the grown films.
2. Experiment:
TiO2 thin films were deposited by RF magnetron sputtering using 5 cm diameter custom
prepared targets prepared by mixing TiO2 powder (5N) and V2O3 powder (2N) with 0.0, 1.0, 1.5
and 2.0 at.% V in TiO2 by means of standard solid state reaction technique. ITO coated (
~100nm thick, 4-8 Ω-cm conductivity) microscope glass substrates were cleaned by sonication in
various solutions, such as detergent, isopropyl alcohol and acetone in a proper sequence and then
thoroughly rinsed in DI water. The ITO coated glass slides were then dried in dry nitrogen
before placing in the chamber. After achieving the base pressure of ~ 2 × 10-6 torr, the chamber
was back filled with Ar and brought to the working pressure of 10 mtorr. All depositions were
done at this pressure and the substrate temperature was kept at 500o C. The films were deposited
at a very slow deposition rate of ~ 0.6 Å/s for four hours at 150 Watt. Structure and phase
analyses were performed by Rigaku D-Max B X-ray Diffractometer using Cu – Kα radiation
(λ = 0.154 nm). The 2θ scans were made in the range from 20° to 80°. Room temperature
Raman spectroscopy was carried out by Bruker’s Micro-Raman spectrometer and spectra were
collected with an excitation energy of 532 nm from Nd:YAG laser at an incident power of
20mW. The surface and cross-sectional microscopy was performed by JEOL JSM-7400F field
emission scanning electron microscopy (FESEM) and the EDX spectra was taken by Oxford
Instruments’ PentaFET-6900 energy dispersive X-ray (EDX) spectrometry system installed in
the FESEM. XPS was performed by using an incident beam of non-chromatic Al X-ray (1486.5
eV) operating at 10 kV, 10 mA for all scans (survey and high resolution). Measured peaks were
then charge corrected to C-1s peak position at 284.6 eV. Photoluminescence (PL) spectroscopy
in the band gap of TiO2 was studied at room temperature by Lab Ram ІІІ from DongWoo Optron
with Ar+ laser emitting at 488 nm.
3. Result and Discussion:
3
The scanning electron microscopy of the synthesized films revealed interesting surface
morphology of the pure and 2 at.% V doped TiO2 thin films as shown in Figures 1(a) and 1(b),
respectively. The thickness of the deposited thin film was 850 ± 5 nm as seen in Figure 1 (c),
which shows the cross-sectional image of the sample showing all three layers, glass substrate,
ITO, and TiO2. The presence of V was confirmed using EDX as shown in Figure 1 (d) of the 2
at.% V doped TiO2 film. It was also confirmed that the prepared thin films had the one to one
correspondence of the dopant composition with the prepared targets. Two clear observations
were drawn from the micrographs, first, the transformation of morphology, i.e., from the faceted
in the pure TiO2 film to the elongated grains in the TiO2:V (2 % at.) doped film, and second, the
decrease in the grain size in the doped TiO2 film. The average grain size dropped to almost one
third of the average grain size of the pure TiO2 film, i.e., from 160 ± 10 nm to 45 ± 5 nm and the
density increased by almost two times. With the increase in the concentration from 0 to 2 at.%
of vanadium. The modification in the morphology was ascribed to the dopant incorporation in
the host lattice [19] and the decrease in the grain size was due to the replacement of Ti by V in
the host lattice. As a consequence of doping, the induced stress due to difference in the two ionic
radii hindered the grain growth.
The cross-sectional image of the synthesized films also
revealed that films had uniform columnar structure which rendered crystal growth under
saturated regime [20]. The increased surface area due to morphological modification is always
desirable for better efficiency in the photocatalysis.
XRD patterns as shown in Figure 2 (a) confirmed the growth of TiO2 anatase phase. The
XRD patterns of the grown ITO (bottom), pure TiO2 (middle) and TiO2:V (2 at %) doped film
(top) are shown. The unidentified peaks in the TiO2 films emerged from the ITO substrate. . The
diffraction peaks were due to anatase (101), (004) and (211) according to the JCPDS card # 020387 as labeled in the Figure 2 (a). This also confirmed the good quality of the synthesized films
with no preferred oriented growth in the pure TiO2 films. On the other hand, when doped with V,
TiO2 films showed preferential growth along (211) plane as the at.% of V was increased. This is
shown in the top XRD pattern and is marked by a dotted circle to show the difference in the
intensities of (211) diffraction plane in the two films. A downshift in the diffraction angle was
also observed with the increase in V doping and this was considered a signature of dopant
incorporation in the host lattice, which created the stress. The shift to lower 2θ value in (211)
peak was ~ 1% from pure to 2 at.% V concentration.
4
Figure 1: SEM micrographs of (a) the pure TiO2, (b) 2% at. V doped TiO2 thin films. (c) Crosssectional ciew of the film showing the thickness of the grown film and various layers.
(d) EDX spectrum of the 2% at. V doped TiO2 film.
The average crystallite size of synthesized films decreased by an almost 1.8 times from
the pure to 2 at.% V doped TiO2 films as determined by Debye Scherrer’s formula. This
variation was consistent with the SEM findings. This explained the inhibited growth of grain
and crystallite sizes was the result of V doping. This was ascribed to the difference in the
oxidation state of Ti and V, which had different ionic radii and produced stress in the host lattice,
which caused the growth of non equilibrium (211) planes.
It was also assumed that during the
grain growth, there was a competition between growth of equilibrium (101) plane and nonequilibrium (211) plane and the increase of V concentration led to the pronounced growth of
non-equilibrium plane (211). The change in the growth orientation caused the change of
morphology as observed in the SEM micrographs and XRD results. This was further confirmed
by using structural visualization (VESTA software). The substitution of Ti by V caused the
change in the bond lengths as well, which was studied and confirmed by Raman spectroscopy.
5
The substitution of V also caused introduction of structural defects and dopant related defects,
which was further studied by photoluminescence spectroscopy.
Figure 2: (a) X-ray diffraction patterns of the ITO film (bottom), pure TiO2 film (centre) and
TiO2:V (2 % at.) film (top). ITO diffraction pattern was included to distinguish TiO2
films from the substrate film diffraction peaks. (b) Room temperature Raman spectra
as obtained from ITO film (bottom), pure TiO2 film (center) and TiO2:V (2 % at.)
film (top). No Raman modes from the ITO substrate are observed.
Figure 2 (b) represents the room temperature Raman spectra of the ITO film (bottom),
pure TiO2 film (middle) and TiO2:V 2.0 at.% V (top). ITO showed no Raman mode in the
region of interest, however, the commonly known six Raman modes of the TiO2 anatase phase
were observed, i.e., three Eg modes at 144, 197, and 639 cm-1, one B1g mode at 399 cm-1, and
one A1g or B1g mode at 519 cm-1 [21] and labeled in Figure 2 (b). The Raman spectra showed a
remarkable drop of 67% in intensity of the Eg mode (@144 cm-1) with the increased V
6
concentration from 0 to 2 at.%. In addition, FWHM of the mode increased from 7.8 to 9.8 cm-1
as V concentration increased in the same range. This was ascribed to the relaxation of Raman
selection rules near the centre of the Brillouin zone and a range of q (i.e., q+∆q) wavevectors
became accessible, where ∆q~1/L, and L is the crystallite size [22]. So, the systematic drop in
intensity and increase in FWHM of Eg mode (@ 144 cm-1) with the addition of V in the TiO2
lattice was a clear indication of the change in the bond lengths and nonstoichiometry in the
grown films. Another observation was the red shift of Eg mode (@144 cm-1) by 3.32 cm-1.
Which was a typical indication of the phonon confinement effect associated to the reduction of
the crystallite size on V doping.
It is already established fact that the Eg mode is produced due to the symmetric stretching
vibration of O-Ti-O bonds in TiO2 and the B1g mode is produced due symmetric bending
vibration of O-Ti-O results in the B1g mode, whereas the anti-symmetric bending vibration of OTi-O produces the A1g mode [23]. It was observed that the ratio of Eg to A1g mode also
decreased as the V doping concentration increased. It further confirmed that the asymmetry in
the host lattice was produced by both, i.e., the dopant substitution and creation of oxygen
vacancies as was observed in the XPS measurements.
Visualization of electrical and structure analysis (VESTA) software [24] was employed
to determine the crystal structure of the pure and doped TiO2. VESTA employes the periodic
bond chain (PBC) theory to visualize materials in 3D. VESTA confirmed the faceted
morphology of the anatase phase with the incorporation of (101) plane and (105) plane.
However, elongated morphology of the anatase phase was observed with the inclusion of (211)
plane in addition to (101) and (105) planes for the 2 at.% V doped TiO2 films. It must be noted
that the d-spacing of (211) plane (1.66 Å) is almost half the d-spacing of the (101) plane (3.51
Å). Thus, for the pure TiO2, growth of stable planes was preferred and the growth rate along
(101) was more than twice the growth rate along (211). However, the doping of “V” led to the
enhanced growth rate of unstable (211) plane compared to (101) plane and this resulted into
small elongated grains with reduced grain size as observed in the SEM micrographs.
Figure 3 shows a high resolution scan of V-2p3/2 region, which confirmed the substitution
of V in multiple valance states in TiO2:V (1 % at. and 2 % at.) doped films. The fit confirmed the
existence of valance states of V and O-1s satellite peak between V-2p1/2 and V-2p3/2. This was
7
a typical signature of V in the oxide state (V1+/2+/3+/4+/5+) [25]. This confirmed the incorporation
of V in TiO2 lattice substituting Ti and bonding with O. With the increase in V doping
concentration the area of V3+ at 515.3 eV decreased and V5+ at 516.8 eV increased. The details of
integrated area of each valance states were determined. For 1.0 at% V doping, only 20 % of V
was in 5+ states but this number increased to almost 54 % for 2.0 at.% V concentration. it is
interesting to note that V5+ in comparison with Ti4+ has 8.7 % smaller ionic radius, which was
good enough to produce stress in the TiO2 lattice responsible for reduction in crystallite/grain
size, change in morphology and enhanced growth along (211) plane. The coexistence of V3+ and
V5+ and the presence of satellite peaks of O-1s also confirmed the nonstoichiometry in TiO2.
Figure 3: High resolution XPS spectra of TiO2:V (1 % at. (bottom) and 2 % at. (top)) to
demonstrate the contribution of various ionized V substitution in TiO2 host lattice. The
spectra have been fitted with Gaussian functions to determine the contribution of V+3,
V+4, and V+5 states.
TiO2 in both phases, i.e., anatase and rutile have a band gap in the ultraviolet (UV) region
with no visible emission expected from it. However, a few papers have recently highlighted the
influence of defects, particularly structural, on the luminescence characteristics in the visible and
IR from the pure and modified TiO2 [14-18]. It was anticipated that three types of defects would
emanate in the TiO2 films; due to oxygen vacancies, structural defects (e.g., Ti3+ or grain
boundaries) and defects introduced by the dopant “V”. The PL spectra of the pure TiO2 and
TiO2:V (2 % at.) doped films obtained at the room temperature in the visible region (band gap
luminescence) exhibited very interesting features as shown in Figure 4 (a, b). Figure 4 (a) shows
8
two sharp bands centered one at 530 nm due to oxygen vacancies (SV), other at 600 nm due to
surface oxygen vacancies (SOV), and a broad band at 750 nm due to structural defects (SD).
The structural defects were produced by Ti. However, substitution of the dopant “V” in the host
lattice resulted in the appearance of another strong band in the visible and in the infra-red (IR)
region overlapped with the SD band. The PL bands were resolved mainly with Gaussian
functions, which enabled determination of the contribution of each component. XPS spectra has
already confirmed that the stoichiometry of TiO2 was quite sensitive to the amount of V. In the
following paragraph, the origin and variation of observed bands in the PL spectra are discussed.
Figure 4: Room temperature band gap photoluminescence spectra of (a) the pure TiO2 film, and
(b) TiO2:V (2 % at.) doped film. The contribution from various responsible defects is
marked in (b) as discussed in the text. Structural defect and Vanadium defects
contribution is pronounced in the doped film.
The PL peak observed at around 530 nm was attributed to the recombination of free
electrons with the shallow trapped holes usually originated from oxygen vacancies (OV). The
orange – red emission broader peak between 590 and 650 nm was from the recombination of
trapped electrons with the valence band free holes. Such a PL was attributed to the presence of
uncoordinated Ti3+ only when electrons occupied it [14]. In pure TiO2 film, the major
contribution to the PL spectra was due to oxygen vacancies related transitions; however a minute
contribution from Ti3+ was also observed. The PL spectra collected from all doped samples also
9
exhibited the presence of same bands but with appearance of strong side bands of the band due to
structural defects (SD). The contribution of new bands in the modified SD band was determined
as a function of “V” concentration and summarized in Table I. Interestingly, energy position of
new bands remained insensitive to the V concentration, however their intensities varied
drastically.. Interestingly, the intensity of the band at 750 nm increased while intensity dropped
for the band at 800 nm when “V” concentration was increased. Thus, new PL bands emerged at
750 nm and 800 nm were due to the presence of V5+ and V3+ states as previously confirmed by
the XPS.
The intensity behavior of all PL bands is summarized in Table I. It was observed that
the PL due to oxygen vacancies and “V” defects increased drastically, although it was greater for
“V” defects. The variation in the PL intensity due to OV and VD was quite consistent with the
XPS observation.
Table I: Table of contribution of various defects in the band gap luminescence of the pure TiO2 and V doped TiO2
with varying V concentration.
Integrated PL Intensity
V at. % in TiO2
Oxygen Vacancies
Vanadium Defects
(OV)
Surface Oxygen
Vacancies (SOV)
Titanium Structural
Defects (TSD)
0.0
15.25
20.18
09.81
00.00
1.0
10.16
00.63
07.57
37.56
1.5
14.90
01.20
00.47
41.53
2.0
16.82
00.52
09.05
57.20
(VD)
4. Conclusions
In summary, V doped TiO2 thin films doped with varied V concentration from 0.0 to 2.0
% at. were synthesized on ITO coated glass slides by RF sputtering. The incorporation of V
systematically modified the structure, morphology and optical properties of the grown TiO2
films. The growth orientation changed from (101) to (211) in pure to 2 % at. doped TiO2 films as
confirmed by XRD patterns. The substitution of V in place of Ti created stress and modified the
crystallite and grain sizes as the ionic radii of V was considerably different than Ti. In addition,
the oxidation state of V was also found to be different. Raman spectroscopy confirmed the
10
increase in phonon confinement, and Ti – O bond asymmetry with the increase in V
concentration. XPS showed also confirmed the increase in the nonstoichiometry. This was
attributed to the substitution of V in the V3+ and V5+ states. The band gap PL spectra showed
strong contribution from the states associated with V5+ and V3+ and dominated the spectra on the
OV and defect associated PL. It was therefore concluded that the substitution of V in an
oxidation state different than Ti oxidation state was the main cause of the change in the
morphology, non-stoichiometry and appearance of additional emission bands in the
photoluminescence of TiO2.
The research work was funded by Higher Education Commission (HEC) of Pakistan
though the NRPU grant 1770. One of the author AA is also thankful to HEC for its IRSIP
program and indigenous PhD scholarship.
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