Interplay of Vanadium States and Oxygen Vacancies in the Structural

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The Interplay of Vanadium States and Oxygen Vacancies in the Structural and Optical Properties of TiO:V Thin Films 2

Awais Ali, Inci Ruzybayev, Emre Yassitepe, S. Ismat Shah, and Arshad Saleem Bhatti J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp406491q • Publication Date (Web): 30 Aug 2013 Downloaded from http://pubs.acs.org on September 14, 2013

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The Journal of Physical Chemistry

The Interplay of Vanadium States and Oxygen Vacancies in the Structural and Optical Properties of TiO2:V Thin Films A. Alia, I. Ruzybayevb, E. Yassitepec, S. Ismat Shahb,c, and A. S. Bhattia* a

Centre for Micro and Nano Devices, Department of Physics, Park Road, COMSATS Institute of Information Technology, Islamabad 44000, Pakistan. b Department of Physics and Astronomy, University of Delaware, Newark DE, 19716, United States. c Department of Material Science and Engineering, University of Delaware, Newark DE, 19716, United States. *Corresponding author: Tel: +92-51-923 5032, Fax: +92-51-924 7006, e-mail: [email protected]

Abstract In this work, we present the customized modifications in the morphology and optical properties of vanadium (V) doped TiO2 thin films sputter deposited on glass substrates a growth rate of ~ 0.6Å/s at 500o C. 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 were anatase. 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 host lattice 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 and confirmed by structural simulation using VESTA code. The confinement of phonon modes was observed in the Raman spectra, which was attributed to the increased nonstoichiometry and enhanced asymmetry in bonding with increased V concentration. XPS spectra confirmed the enhancement in the nonstoichiometry in TiO2 was due to V substitution in the structure. It was suggested that the difference in the valance states of Ti and V resulted in the suppression of (101) planes and augmentation of non-equilibrium (211) planes, which modified the grain morphology of the TiO2 thin films. Photoluminescence (PL) spectroscopy clearly demonstrated the interplay of V defect states and O vacancy states. Pure TiO2 showed mainly green luminescence related to oxygen vacancies, however, addition of V clearly demonstrated orange and red emission bands due to incorporation in V3+ and V5+ states, which increased at a much faster rate than oxygen vacancies on further addition of V. The PL results complimented the XPS findings. 1 ACS Paragon Plus Environment


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Keywords: TiO2, Vanadium, doping, nonstoichiometry, oxygen vacancies

1. Introduction: In recent years, Titanium dioxide (TiO2) has become one of the most studied wide band gap oxide semiconductor due to its application in hydrogen production from water1. TiO2 as in various morphologies, e.g., thin films or nanostructures, has found potential applications in environmental cleaning, e.g., air and water2-4, self cleaning and non-spotting glass coatings5, selfsterilizing coatings2, dye synthesized solar cells6, etc. TiO2 is found naturally in three polymorphs: anatase, rutile and brookite. The band gap of anatse and rutile phases are 3.2 eV and 3.0 eV, respectively. Due to its wide band gap, TiO2 absorbs light in the ultraviolet (UV) region of the spectrum. However, solar spectrum consists of only 3-5 % UV region, and a major portion of 43-47 % is in the visible region. Thus, the efficient utilization of the solar spectrum (mainly a good portion of the visible light) 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 gap7. 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 spectrum8-12. However, the doped ions also contribute in reducing the life time of the photo-induced charge carriers in the doped TiO2. Thus, an increased recombination rate of the exited charge carriers in the doped TiO2 results in a reduced photocatalytic activity. 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 holes10. The degree of success is strongly dependent on the knowledge and deep understanding of the fundamental physical processes ruling the carrier dynamics in such systems. Previous reports on anatase agree on the origin of the visible photoluminescence (PL) in TiO2, attributed to the radiative recombination of selftrapped excitons or surface radiative recombination13-17. But still a lot needs to be understood to control the activity of surface traps, which mediate carrier diffusion if shallow or promote recombination into the electrolyte if deep, The band gap trap states also influence the 2 ACS Paragon Plus Environment

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photocatalytic properties of the TiO2 and facet-dependent photo-redox reactions of the shapetailored TiO2 have generated a great deal of interest. 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 activity18. V doped TiO2 has shown to be responsible for increase of superficial hydroxyl groups and electron transfer, resulting in interactions between the adsorbate and the adsorbent on a much shorter time scale19-21. V doped TiO2 has also been used as a catalyst for the selective reduction of NO to NH322. The ionic radius of V is comparable with Ti, which makes it easy to dope TiO2 by replacing Ti. However, difference in the oxidation state of V can change the structure and morphology of TiO2. 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 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 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. Four different targets were prepared by mixing TiO2 powder (99.999% SigmaAldrich) and V2O3 powder (99.6+%, Acros) with 0.0, 1.0, 1.5 and 2.0 at.% V in TiO2. After mixing and proper grinding, powders were statically pressed to make compact and flat targets of 5 cm diameter. Plain microscope glass substrates (Fisher Scientific) were sequentially cleaned by sonication in solutions of detergent, isopropyl alcohol and acetone, then thoroughly rinsed in DI 3 ACS Paragon Plus Environment


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water and then immediately dried in dry nitrogen prior to placing in the sputtering chamber. A base pressure of ~ 2 × 10-6 torr was achieved before back-filling with Ar to the working pressure of 10 mtorr for deposition. The substrate temperature was raised to 500oC. TiO2 films were deposited at an average deposition rate of ~ 0.6 Å/s for four hours at 150 Watt. Structure and phase analyses were performed by X-ray Diffraction (Rigaku D-Max B diffractometer using Cu – Kα radiation (λ = 0.154 nm) equipped with a Ge monochromator crystal). XRD scans were carried out at 30 kV and 30 mA for 2θ in the range of 20° to 80° using a step size of 0.01° at collection time of 2 s/step. Raman spectroscopy was carried out by Bruker’s Micro-Raman spectrometer at room temperature. Raman spectra were taken with an excitation energy of 532 nm from Nd:YAG laser at 20mW incident power and corrected for background. Film thickness and surface morphology was measured by JEOL JSM-7400F field emission scanning electron microscopy (FESEM) equipped with Oxford Instruments’ PentaFET-6900 energy dispersive Xray (EDX) spectrosmetry system. Valance states were investigated by X-ray photoelectron spectroscopy (Omicron EA125). For XPS, an incident beam of non-chromatic Al X-ray (1486.5 eV) was used operating at 10 kV, 10 mA and 100 W for both survey and high resolution scans. Pass energy was set at 50 eV for survey and 25 eV for high resolution scans with a dwell time of 2 s/step for both types of scans. Measured peaks were then charge corrected to C-1s peak position at 284.6 eV.The band gap Photoluminescence (PL) was studied at room temperature by Lab Ram ІІІ from DongWoo Optron. Argon ion laser, with laser line 488 nm (blue) was used. Panorama scan function was used and accumulation time for each scan was 5 minutes, while scanning from 500 to 900 nm. For power dependent PL measurement, incident light was controlled by using neutral density filters of 100, 50, 20, 10, 1 and 0.1 % of incident power of laser.

3. Result and Discussion: 3.1.

Scanning Electron Microscopy: The scanning electron microscopy of the synthesized films revealed interesting surface

morphology and Figures 1(a) and 1(b) shows representative micrographs of the pure and 2 at.% V doped TiO2 thin films. The cross-sectional image showed that the thickness of the grown films was 850 ± 5 nm as seen in the inset of Figure 1(a). The inset of Figure 1(b) shows the EDX spectra of 2 at.% doped film, which confirmed the presence of V along with Ti and O. EDX also 4 ACS Paragon Plus Environment

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confirmed the one to one correspondence of the dopant composition (within acceptable error) in the prepared targets and the grown films. There were two important observations drawn from the micrographs. First, the transformation of morphology from the faceted to elongated grains, and second, the decrease in the grain size from 162 ± 8 nm to 46 ± 3 nm and an increase in the grain density from 990 ± 50 grains/µm2 to 1804 ± 90 grains/µm2 with increase in the doping concentration from 0 to 2 at.% of V. The change in the morphology could easily be ascribed to the dopant incorporation in the host lattice23. The decrease in the grain size was explained on the basis of V replacing Ti in the lattice, which induced stress due to difference in the ionic radii of the two thus hindered the grain growth. The reduction of grain size and its morphology transformation was strongly dependent on the dopant incorporation. It was observed that there were a number of grains with stepped faces; formation of steps is a classical way of releasing the surface tension during the growth of crystals which are subjected to stress caused by the dopant incorporation24. It was also seen that films had uniform columnar structure which rendered crystal growth under saturated regime24. This led to an increased surface area which is always desirable for photocatalysis.

3.2.

X-ray photoelectron spectroscopy:

Figure 2 shows the high resolution scans of Ti-2p region of pure and “V” doped TiO2 Films. Two characteristic peaks for Ti namely 2p1/2 and 2p3/2 were observed at 464.21 eV and 458.50 eV, respectively, which confirmed that the Ti was in Ti4+ state which is typical for TiO2. A symmetrical shift of 0.37 eV of these peaks to the lower binding energy (LBE) was observed with an increase in the “V” doping concentration from 0 to 2 at.%. LBE shift towards Ti3+ (2p3/2 for Ti4+ peaks at 458.50 eV and for Ti3+ at 453.7 eV) was attributed to the increased nonstoichiometry of TiO2 films. In addition, the peak to peak difference of doublet states of Ti2p (∆=2p1/2 – 2p3/2) states increased from 5.71 eV to 5.78 eV from pure to 2 at.% V doped TiO2. This clearly indicated increased contribution of Tio, and confirmed the enhancement of nonstoichiometry in the doped films (∆=6.05 eV for Tio and ∆=5.72 for Ti4+). In Figure 2, the line shows the trend of the peak shift. Another interesting observation was the difference in the rate of shift of the two peaks. The rate of shift for Ti-2p1/2 was more than that of Ti-2p3/2. This observation revealed that Ti3+ states also emerged along with Ti4+ states with the increase in the

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doping concentration of V, which reflected the presence of oxygen vacancies in the structure. In the following, V states and O states were also analyzed to confirm nonstoichiometry in the films. Figure 3 shows a high resolution scan of V-2p3/2 region. It is well known that V can exist in multiple valance states, thus the peak fitting was performed by using XPSPEAK. The fit confirmed the existence of two valance states (V3+ and V5+) of V and O-1s satellite peak. The O1s satellite peak between V-2p1/2 and V-2p3/2 was a typical signature of V in the oxide state (V1+/2+/3+/4+/5+) as it did not appear for its metallic state (Vo with V2p3/2 @ 512.4 eV) 25. This showed the incorporation of V in TiO2 lattice substituting Ti and bonding with O. With increase in V doping concentration the area of V3+ at 515.3 eV decreased and V5+ at 516.8 eV increased. Another peak corresponding to V5+ at 517.45 eV emerged for 1.5 at.% of V in TiO2 and the area of this peak also increased in the 2 at.% concentration of V TiO2 film. The details of integrated area of each valance states have been summarized in Table I. 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. As discussed in XRD section, V5+ in comparison with Ti4+ has 8.7 % smaller ionic radius, which is food enough to produce stress. This, it was confirmed that an increase in V5+ produced excessive stresses in the TiO2 lattice responsible for reduction in crystallite/grain size, change in morphology and enhanced growth along (211) plane (explained in the XRD section).. As, the LBE shift observed in the Ti-2p spectra, the coexistence of V3+ and V5+ and the presence of satellite peaks of O-1s conformed the nonstoichiometry in TiO2. Thus, the increased nonstoichiometry was mainly due to V substitution in TiO2 lattice.

3.3.

X-Ray Diffraction: XRD patterns as shown in Figure 4 confirmed the growth of TiO2 anatase phase under

the optimized growth conditions. The XRD patterns of (a) pure and (b – d) V doped TiO2 films ((b) V=1.0, (c) 1.5 and (d) 2.0 at.%) are shown in Figure 4. Diffraction peaks were labeled as (101), (004) and (211) at the corresponding 2θ values according to the JCPDS card # 02-0387, which represented the pure anatase phase. The relative intensities of the observed peaks were consistent with the relative intensities of the respective planes given in the JCPDS card. This confirmed the good quality of films with no preferred growth in the pure TiO2 films. On the other hand V doped TiO2 films showed improved preferential growth along (211) plane with increasing at.% of V. The effect of V doping on the XRD patterns was observed in several ways. 6 ACS Paragon Plus Environment

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With the increase in V doping concentration, a monotonic downshift in the ”2θ” was observed for all peaks as shown in Figure 4 (b – d). The (211) peak is shown in the inset of Figure 4, which clearly displayed the shift of the peak explicitly plotted in Figure 5 . The downshift was due to the dopant incorporation in the host lattice, which created the stress and resulted in shift of diffraction peaks to lower 2θ value. The shift to lower 2θ value in (211) peak was ~ 1% from pure to 2 at.% V concentration. Another observation was the variation in the FWHM and the area of the diffraction peaks with increasing V concentration which was attributed to the varied crystallite size of TiO2 films due to V doping. The average crystallite size of synthesized films was calculated by using standard Debye Scherrer’s formula, which varied from 84 ± 4 nm to 47 ± 2 nm from pure to 2 at.% V doped TiO2; almost 1.8 times decrease from pure to 2 at.% V doped TiO2 films. This variation in the crystallite size with V concentration is shown in the inset of Figure 5. The crystallite size variation was consistent with the grain size variation observed in the SEM micrographs. Thus the inhibited growth of grain and crystallite sizes was the consequences of V doping. In order to explain the effect of V doping, let’s visualize the TiO bonds in TiO2. It is known that the Ti atom in anatase is surrounded at the equator by four O atoms located at equal distances of 0.194 nm as shown in Figure 6. The distances between the Ti atom and the two axial O atoms are also more or less same, i.e., equal to 0.196 nm. On the other hand, the structure of V5+ in V2O5 is considered to be a strongly distorted octahedron with unequal V-O bond lengths. Thus, on incorporation of V at Ti site, the distances between the V and the six surrounding O would become different. The distances between V and O atoms at the equator, i.e., V-O1, V-O2, V-O3, V-O4 would become equal to 0.178 nm, 0.178 nm, 0.188 nm and 0.202 nm, respectively. The two axial bonds between V atom and O atoms would also be different, i.e., 0.158 nm and 0.279 nm, respectively. The substitution of V with Ti, thus, resulted in varied V-O1-4 distances and V-O5 to the shortening of the bond, which produced a stress and thus shrunk the lattice. The apparently comparable ionic radius of V and Ti (V2+/3+/4+/5+=93/78/72/68 and T2+/3+/4+=100/81/74 pm 26) makes it easy to replace Ti. On the contrary, one to one comparison of the respective oxidation states of Ti and V shows a clear difference in the size of ionic radii. Ti has 1.4 % and 3.7 % bigger ionic radii than V for 4+ and 3+ states, respectively. The difference further increases to 8.7 % when keeping Ti in 4+ and V in 5+ states (most probable states). It was thus concluded that such a difference in the oxidation state of Ti and V produced stress in 7 ACS Paragon Plus Environment


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the host lattice on doping of V at Ti lattice sites due to significant ionic radii difference, which caused the growth of non equilibrium planes e.g., (211). This was the case observed in the trend of XRD patterns with increased V concentration and the ratio of intensities of (101) to (211) as a function of V doping are shown in Figure 5. This showed that during grain growth, there was a competition between growth of equilibrium (101) plane and non-equilibrium (211) plane. The increase of V concentration led to the enhanced growth of non-equilibrium plane (211) with respect to the plane (101). It was also believed that the slow growth rate of the film, i.e., 0.6Å/s at high substrate temperature of 500oC might have played a key role in abnormal growth kinematics along (211) planes. The change in the growth orientation resulted in the change of morphology as observed in the SEM micrographs and XRD results. This was further studied using structural visualization (VESTA software) to confirm the effects of V doping on (211) plane and consequently on morphology.

3.4.

Structure Visualization: Visualization of electrical and structure analysis (VESTA) software uses the periodic

bond chain (PBC) theory and gives 3D visualization of specific materials

27

. According to the

PBC theory, the surface of a crystal structure can be classified in terms of flat, kinked or stepped facets. If the number of PBCs is greater along a face, then its growth will be slow and vice versa. VESTA gives the 3D morphology of grains by using diffractions peaks obtained from the XRD patterns. Figure 7 (a) gives the faceted morphology of anatase with incorporation of (101) plane and (105) plane. Similarly, Figure 7 (b) shows the elongated morphology of anatase phase with the inclusion of (211) plane along with (101) and (105) planes, as observed in the XRD spectra of 2 at.% V doped TiO2 films. The typical value of d-spacing of (101) plane is almost twice the d-spacing of the (211) plane, i.e., 3.51 Å and 1.66 Å, respectively. So, for the pure TiO2, growth of stable planes will be preferred, and the growth rate along (101) would be more than twice the growth rate along (211). However, the doping of “V” led to the enhanced growth of unstable plane, which favored the enhanced growth of (211) planes. Thus with incorporation of (211) plane it was clearly demonstrated that usual large faceted grains of anatase started converting into small elongated grains with reduced grain size as was evident in Figure 4 (a) and (b). Pure TiO2 has symmetrical bond lengths between Ti and O as discussed earlier; however,


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incorporation of V in TiO2 films caused the increase in the asymmetry in lattice, which was further investigated by the Raman spectroscopy.

3.5.

Raman Spectroscopy: Figure 8 represents the Raman spectra of the pure grown (a) and V doped TiO2 films ((b)

1.0, (c) 1.5 and (d) 2.0 at.% V). The commonly known six Raman modes of the TiO2 anatase phase, 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 were observed

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and labeled in Figure 8. There were several noticeable

facts in spectra, particularly the behavior of the Eg mode seen at 144 cm-1. A remarkable drop of 67% in intensity was observed of the Eg mode (@144 cm-1) with increased V concentration from 0 to 2 at.%. Also, full width at half maximum (FWHM) of the mode increased from 7.8 to 9.8 cm-1 as V concentration increased. 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 29. 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 as shown in the inset of Figure 8. The shift is plotted in Figure 9 (left vertical). This was a typical indication of the phonon confinement effect as origin of the shift was associated to the reduced crystallite size on doping. It is already established fact that the symmetric stretching vibration of O-Ti-O bonds in TiO2 primarily produces Eg mode, and the symmetric bending vibration of O-Ti-O results in the B1g mode, whereas anti-symmetric bending vibration of O-Ti-O produces the A1g mode

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.

Figure 9 (right vertical) shows the ratio of Eg to A1g mode, which decreased as the doping concentration increased. With incorporation of V in the host lattice, the asymmetry increased and resulted a similar change in the relative intensities of Eg (symmetric starching) and A1g (antisymmetric bending) modes. It was thus confirmed that the asymmetry in the host lattice was produced by both, i.e., the dopant substitution and creation of oxygen vacancies as has already been observed in the XPS measurements. The findings of the Raman spectroscopy clearly demonstrated; (i) phonon confinement effects were due to increased asymmetry of bonds of the



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host material as observed from the increase in the FWHM of Eg mode (@ 144cm-1); and (ii) the enhanced nonstoichiometry resulted the drop in the relative ratio of Eg to A1g modes. 3.6.

Photoluminescence of V doped TiO2 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

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. 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 obtained at the room temperature in the visible region (band gap luminescence) exhibited very interesting features as shown in Figure 9 (a – d). Figure 9 (a) shows two sharp bands centered at 530 nm and 600 nm, and a broad band at 750 nm emerged due to oxygen vacancies (OV), surface oxygen vacancies (SOV) and structural defects (SD), respectively. Here the structural defects are referred to as defects produced by Ti. However, incorporation of the dopant “V” in the host lattice resulted in the appearance of another strong band in the visible and partly in the infra-red (IR) region overlapped with the band due to structural defects. The PL bands were resolved mainly with 5 Gaussian functions also shown in the Figure to determine the contribution of each band to the total PL spectra and was compared for each concentration of the dopant. As has been observed in the XPS spectra that stoichiometry of TiO2 was quite sensitive to the amount of vanadium. In the following paragraph, the origin and variation of observed bands in the PL spectra are discussed. The PL peak observed at ∼ 530 nm was due to the recombination of free electrons with the shallow trapped holes. The shallow traps usually originate from oxygen vacancies (OV) and are considered to be the source of intrinsic n-type nature of TiO2. On the other hand, the PL from the recombination of trapped electrons with the valence band holes gave rise to the orange-red emission with a broader peak between 590 and 650 nm. Such a PL was attributed to the presence of uncoordinated Ti3+ only when carriers occupied it 13. In pure TiO2 film, the major contribution to the PL spectra was due to OV related transitions, however a minute contribution from Ti3+ was also observed as marked in the Figure 9. The PL spectra collected from all doped samples also exhibited the presence of same bands in addition to appearance of some strong side bands 10 ACS Paragon Plus Environment

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with the band due to structural defects (SD). The modified SD band was resolved to determine the contribution of new bands as a function of “V” concentration. Interestingly, the energy position of the new bands remained insensitive with their intensities varied with the change in the “V” concentration. Interestingly, the intensity of the band at 750 nm increased while it dropped for the band at 800 nm with increase in the “V” concentration. Thus, it was ascribed that the new PL bands were due to the existence of V5+ and V3+ states as already confirmed by the XPS as the variation in the PL intensity followed the trend observed in the XPS. Thus, the bands at 750 and 800 nm were reasonably assigned to V5+ and V3+ states, respectively. It has been reported earlier that “V” in GaAs showed John-Teller effect, which gave rise to the intraband transitions of 3d state from 3T2 →3A2 31. It was anticipated that a similar process was responsible for this behavior. The intensity behavior of all PL bands is summarized in Figure 10. It was observed that the surface oxygen vacancies (SOV) were the least and remained almost constant. The PL due to structural defects was the highest but dropped drastically as V was incorporated. The PL due to oxygen vacancies and “V” related defects increased drastically. However, the rate of increase of PL intensity due to “V” defects was higher than the rate of increase of the intensity due to OV. The variation in the PL intensity due to OV and VD was quite consistent with the XPS observation. It is well known that pure TiO2 has almost 90 % of terminating surfaces consist of (101) and one half of Ti atoms on (101) surface are 5-fold coordinated and the other half are 6-fold coordinated13. Since all measurements were performed in ambient (air), so O2 could easily adsorb at the surface, which was an efficient electron scavenging source of PL. The band gap illumination of TiO2 resulted due to trapped electrons which reduced 5-fold coordinated surface Ti from the +4 oxidation state to the +3 oxidation state

13

. Results of the XRD and VESTA

suggested that with the increase in the doping concentration, the number of (101) terminating planes decreased. On the other hand, XPS showed the pure films were stoichiometric (with Ti4+ state) and increased the nonstoichiometry (Ti3+) when dopant atoms were added. With addition of dopant, the number of Ti3+ atoms reduced at the surface (with decrease in (101) terminating surface) but their number in the bulk TiO2 increased (as the LBE shift in Ti-2p (XPS spectra)), thus the red PL existed for the doped samples along with pure one but with a different and enhanced characteristic signature of the PL.



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The incident power dependent PL confirmed the intrinsic nature of emission and no carrier confinement effects in the pure and doped TiO2 films were observed

16

. As have already

demonstrated 19-21 that the substitution of V at Ti sites in TiO2 results in interactions between the adsorbate and the adsorbent on much shorter time scales. Thus in the present work, successful substitution of V in TiO2 was achieved along with the enhanced surface area due to reduction of grain size and increase in grain density, which is very important in applications related to photo activity, e.g., photocatalysis.

4. Conclusion: V doped TiO2 thin films were synthesized by RF sputtering. V concentration was varied from 0.0 to 2.0 at.%, which modified the structure, morphology and optical properties of the grown films. XRD confirmed the existence of anatase phase, reduction in crystallite size and emergence of preferred growth along the plane (211). The substitution of V in place of Ti made the V – O bonds asymmetrical, which created stress to modify the crystallite and grain sizes as the ionic radii of V was considerably different than Ti. Furthermore, the oxidation state of V was also found to be different.

The morphology was transformed from faceted to small

elongated grains as the concentration of V increased. VESTA verified the transformation of morphology was due to enhanced growth of (211) planes. Furthermore, Raman spectroscopy confirmed the increase in nonstoichiometry, phonon confinement, and asymmetry in the bonding with the increase in V concentration. XPS showed L.B.E. shift and an increase in the B.E. difference of Ti-2p doublet peaks, which also confirmed the increase in the nonstoichiometry. This was attributed to the substitution of V at Ti sites in the V3+ and V5+ states. This was also evident in the band gap PL spectra, where strong contribution came from the states associated with V5+ and V3+ and dominated the spectra on OV and defect associated PL. It is concluded here that substitution of V in an oxidation state different than Ti oxidation state resulted in the morphology, deviation from the stoichiometry and introduction of additional bands in the photoluminescence of TiO2, which could improve its photocatalytic properties. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (A.S.B.)


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Notes The authors declare no competing financial interest.

5. Acknowledgments: The research work was funded by Higher Education Commission (HEC) of Pakistan though IRSIP program and Indigenous scholarship to A. Ali for his M.S. leading to Ph.D. studies (under pin No. 074-3560Ps4-134). The work was supported by HEC NRPU Grant # 1770.

6. References: (1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. (2) Fujishima, A.; Zhang, X.; Tryk, D. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Scie. Rep. 2008, 63, 515-582. (3) Carp, O.; Huisman, C. L.; Reller, A. Photoinduced Reactivity of Titanium Dioxide. Prog. Sol. Stat. Chem. 2004, 32, 33-177. (4) Linsebigler, A. L.; Lu, G.; Yates, J. T.; Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95, 735-758. (5) Mills, A.; Wang, J.; Crow, M. Photocatalytic Oxidation of Soot by P25 TiO2 Films. Chemosphere 2006, 64, 1032-1035. (6) Regan, B. O; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737-740. (7) Dholam, R.; Patel, N.; Adami, M.; Miotello, A. Hydrogen Production by Photocatalytic Water-Splitting Using Cr- or Fe-Doped TiO2 Composite Thin Films Photocatalyst. Int. J. Hyd. Ener. 2009, 34, 5337-5346. (8) Ni, M.; Leung, M. K. H.; Leung, D. Y. C.; Sumathy, K. A Review and Recent Developments in Photocatalytic Water-Splitting Using TiO2 for Hydrogen Production. Rene. Sust. Ener. Rev. 2007, 11, 401-425. (9) Choi, W.; Termin, A.; Hoffmann, M. R. The Role of Metal Ion Dopants in Quantum-Sized TiO2: Correlation Between Photoreactivity and Charge Carrier Recombination Dynamics. J. Phys. Chem. 1994, 98, 13669-13679.



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(10) Litter, M. I.; Navfo, J. A. Photocatalytic Properties of Iron-Doped Titania Semiconductors. J. Photochem. Photobio. A 1996, 98, 171-181. (11) Khan, M. A; Woo, S. I.; Yang, O. B. Hydrothermally Stabilized Fe(III) Doped Titania Active Under Visible Light for Water Splitting Reaction. Inter. J. Hyd. Ener. 2008, 33, 53455351. (12) Zhu, J.; Deng, Z.; Chen, F.;

Zhang, J.; Chen, H.; Anpo, M.; Huang, J.; Zhang, L.

Hydrothermal Doping Method for Preparation of Cr3+-TiO2 Photo-Catalysts with Concentration Gradient Distribution of Cr3+. Appl. Cata. B 2006, 62, 329-335. (13) Mercado, C. C.; Knorr, F. J.; McHale, J. L.; Usmani, S. M.; Ichimura, A. S.; Saraf, L. V. Location of Hole and Electron Traps on Nanocrystalline Anatase TiO2. J. Phys. Chem. C 2012, 116, 10796-10804. (14) Rex, R. E.; Knorr, F. J.; McHale, J. L. Comment on Characterization of Oxygen Vacancy Associates within Hydrogenated TiO2: A Positron Annihilation Study. J. Phys. Chem. C 2013, 117, 7949-7951. (15) Stevanovic, A.; Buttner, M.; Zhang Z.; Yates Jr, J. T. Photoluminescence of TiO2: Effect of UV Light and Adsorbed Molecules on Surface Band Structure. J. Amer. Chem. Soc. 2012, 134, 324-332. (16) Wang, X.; Feng, Z.; Shi, J.; Jia, G.; Shen, S.; Zhouab, J.; Li,C. Trap States and Carrier Dynamics of TiO2 Studied by Photoluminescence Spectroscopy Under Weak Excitation Condition. Phys. Chem. Chem. Phys. 2010, 12, 7083-7090. (17) Cavigli, L.; Bogani, F.; Vinattieri, A.; Faso, V.; Baldi, G. Volume Versus Surface-Mediated Recombination in Anatase TiO2 Nanoparticles. J. Appl. Phys. 2009, 106, 053516-053523. (18) Nguyen-Phan, T. D.; Song, M. B.; Yun, H.; Kim, E. J.; Oh, E. S.; Shin, E. W.; Characterization of Vanadium-Doped Mesoporous Titania and its Adsorption of Gaseous Benzene. Appl. Surf. Sci. 257 (2011) 2024. (19) Chen, H. Y.; Zahraa, O.; Bouchy, M.; Thomas, F.; Bottero, J. Y. Adsorption Properties of TiO2 Related to the Photocatalytic Degradation of Organic Contaminants in Water. J. Photochem. Photobio. A 1995, 85, 179. (20) Liang, X.; Zhu, S.; Zhong, Y.; Zhu, J.; Yuan, P.; He, H.; Zhang, J. The Remarkable Effect of Vanadium Doping on the Adsorption and Catalytic Activity of Magnetite in the Decolorization of Methylene Blue. Appl. Cata. B 2010, 97, 151. 14 ACS Paragon Plus Environment

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(21) Cusumano, J. A.; Low, M. J. D. Interactions Between Surface Hydroxyl Groups and Adsorbed Molecules. I. The Thermodynamics of Benzene Adsorption. J. Phys. Chem. 1970, 74, 792. (22) Asaduzzaman, A. M.; Kruger, P. Adsorption and Cluster Growth of Vanadium on TiO2(110) Studied by Density Functional Theory. J. Phys. Chem. C 2008, 112 4622-4625. (23) Ali, A.; Yassitepe, E.; Ruzybayev, I.; Shah, S. Ismat.; Bhatti, A. S. Improvement of (004) Texturing by Slow Growth of Nd Doped TiO2 Films. J. Appl. Phys. 2012, 112, 113505113511. (24) Borras, A.; Sanchez-valencia, J. R.; Widmer, R.; Rico, V. J.; Justo, A.; Gonzalez-elipe, A. R. Growth of Crystalline TiO2 by Plasma Enhanced Chemical Vapor Deposition. Cryst. Grow. Desi. 2009, 9, 2868-2876. (25) Silversmit, G.; Depla, D.; Poelman, H.; Marin, G. B.; De Gryse, R. Determination of the V2p XPS Binding Energies for Different Vanadium Oxidation States (V5+ to V0+). J. Elec. Spect. Rel. Phen. 2004, 135, 167-175. (26) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystal. A 1976, 32, 751-767. (27) Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystal. 2011, 44, 1272-1276. (28) Rossella, F.; Galinetto, P.; Mozzati, M. C.; Malavasi, L.; Fernandez, Y. D.; Drera, G.; Sangaletti, L. TiO2 Thin Films for Spintronics Application: A Raman Study. J. Raman Spect. 2009, 41, 558-565. (29) Campbell, I. H.; Fauchet, P.M. The Effects of Microcrystal Size and Shape on the One Phonon Raman Spectra of Crystalline Semiconductors. Sol. Stat. Commun. 1986, 58, 739. (30) Tian, F.; Zhang, Y.; Zhang, J.; Pan, C. Raman Spectroscopy: A New Approach to Measure the Percentage of Anatase TiO2 Exposed (001) Facets. J. Phys. Chem. C 2012, 16, 75157519. (31) Kao, Y. J.; Haegel, N. M. Jahn-Teller Effects in the Photoluminescence Excitation Spectrum of Vanadium-Doped GaAs. Phys. Rev. B 1993, 48, 4433.



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Table I: Table of variation in the V oxidation states (V3+ and V5+) at various doping concentration of V in TiO2 films.

V at.% in TiO2

Oxidation States of V (Area under the curve) V3+ @ 515.3 eV

V5+ @ 516.8 eV

V5+ @ 517.45 eV

1.0

10197 (80%)

2587 (20%)

-

1.5

5390 (48%)

3012 (27%)

2795 (25%)

2.0

4950 (46%)

2175 (20%)

3633 (34%)


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Figure Captions: Figure 1: Scanning electron micrographs showing the morphology of (a) pure, and (b) 2 at.% V doped TiO2 films. The insets in (a) shows cross-sectional image and (b) shows the EDX spectra of 2 at.% V doped TiO2 film. Figure 2: High resolution XPS spectra of the Ti-2p region obtained from the pure and V doped (1.0, 1.5 and 2.0 at.% concentration) TiO2. The low and high energy labeled peaks are due to Ti-2p3/2 and Ti-2p1/2, respectively. Figure 3: High resolution XPS spectra in the V-2p3/2 region of the TiO2 films doped with (a) 1.0 at.%, (b) 1.5 at.% and (c) 2 at.% V. The Gaussian fits also plotted represent different oxidation states labeled as V3+, V5+ and O-1s satellite peaks. Figure 4: XRD spectra of TiO2 films; (a) pure, (b) 1.0 at.%, (c) 1.5at.% and (d) 2.0 at.% V doping concentrations. The inset shows the (211) peak and the dashed line is a guide for eye to show the shift in the peak. Figure 5: Left vertical shows the shift in peak position of (211) and right vertical shows the variation in the ratio of (101) to (211) peaks as determined from the XRD patterns plotted as a function of V concentration. The inset shows the change in the crystallite size calculated using the Scherrer’s formula as a function of V concentration in TiO2. Figure 6: Visulization of unit cell of anatase and it’s one octahedral along with distorted octahedral of Vanadium oxide (V5+ state). Bond legths are shown along equatorial and axial planes. (Note: figure is not to the scale). Figure 7: The representation of the TiO2 anatase crystal obtained from the VESTA code; (a) in the absence, and (b) due to the presence of (211) planes. Blue/big and red/small balls represent the O and Ti atoms, respectively. Figure 8: Stokes Raman spectra of (a) 0.0 at.%, (b) 1.0 at.%, (c) 1.5 at.% and (d) 2.0 at.% V doped TiO2 films. The inset shows the shift in Eg mode (@144 cm-1) at respective V doping. The dashed line in the inset is a guide to show the shift in the peak. Figure 9: The plot of variation in the Raman shift (left) and the ratio of intensities of Eg to A1g mode (right) as a function of V doping in TiO2. 17 ACS Paragon Plus Environment


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Figure 10: Room temperature band gap photoluminescence spectra of the pure and V doped TiO2 films. The origin of various bands is also labeled as oxygen vacancies (OV), surface oxygen vacancies (SOV), vanadium defects (VD) and structural defects (SD). Figure 11: Plots of the integrated intensity of each band as a function of “V” concentration.


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Figure 10 23 ACS Paragon Plus Environment


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Figure 11


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Table of Contents – Graphic


Interplay of Vanadium States and Oxygen Vacancies in the Structural

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