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Tailoring luminescence properties of TiO2 nanoparticles by Mn doping
Journal of Luminescence 136 (2013) 339–346
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Tailoring luminescence properties of TiO2 nanoparticles by Mn doping B. Choudhury n, A. Choudhury Department of Physics, Tezpur University, Napaam, Assam-784028, India
a r t i c l e i n f o
abstract
Article history: Received 13 June 2012 Received in revised form 17 November 2012 Accepted 4 December 2012 Available online 13 December 2012
TiO2 nanoparticles are doped with three different concentrations of Mn, 2%, 4% and 6% respectively. Absorption edge of TiO2 is shifted from UV to visible region on amplification of Mn content. Room temperature photoluminescence spectra, excited at 320 nm, exhibit band edge and visible emission peaks associated with self trapped excitons, oxygen defects, etc. Doping of Mn increases the width and decreases the intensity of the UV emission peak. Potential fluctuations of impurities increase the width and auger type non-radiative recombination decreases the intensity of the UV emission peak. The intensity ratio of the UV to defect emission band decreases on doping, indicating degradation of structural quality. Excitation of pure and doped nanoparticles at 390 nm results in Mn2 þ emission peaks at 525 nm and 585 nm respectively. Photoluminescence excitation spectra also indicate the presence of Mn2 þ in the crystalline environment of TiO2. The oxygen defects and Mn related impurities act as efficient trap centers and increases the lifetime of the charge carriers. & 2012 Elsevier B.V. All rights reserved.
Keywords: Band gap Crystal field Oxygen vacancies Auger recombination Concentration quenching
1. Introduction TiO2 is an important wide band gap semiconductor for its applications in photocatalysis, photovoltaics, solar cells, chemical sensors, etc. [1–5]. The band gap of TiO2 is very large ( 3.2 eV for anatase and 3.0 for rutile) for using in visible light photocatalysis [6]. However, incorporation of impurities tailors the band gap of TiO2 and extends its absorption edge to visible region [6–8]. Doping is expected to prolong the lifetime of the charge carriers by acting as charge carrier trapping sites [9,10]. On doping entire dopants may not penetrate inside TiO2, few numbers of dopants may also occupy sites on the surface. Thermodynamically, only a limited number of dopants can enter inside the small nanoparticles, the excess are expelled from the interior due to the self purification nature of the nanocrystals [11]. Doping of metal ions weakens the bonding of neighboring oxygen ions at the surface and releases the oxygen ions forming vacancies [12]. These oxygen vacancies alter the electronic and optical properties of the host TiO2 [13]. Incorporation of dopants produces oxygen vacancies for electron trapping, enhances the visible light absorption property of TiO2 and decreases the electron–hole recombination rate [8,9]. Since Mn-doped TiO2 nanoparticles have these properties, these nanoparticles are used in the photocatalytic degradation of many dyes [14–16]. Theoretical calculations in Mn-doped rutile TiO2 have reported that the effective band gap of TiO2 is reduced due to the creation of dopant d-states in the mid
n
Corresponding author. Tel.: þ91 3712267120; fax: þ91 3712267005. E-mail address: [email protected] (B. Choudhury).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.12.011
band gap [13,17]. Recombination rate and the lifetime of the charge carriers, in pure and doped nanoparticles, can be examined with the help of steady state and time resolved photoluminescence spectroscopy. These spectroscopic techniques are important characterization tools to acquire information on the electron–hole recombination rate, lifetime of the charge carriers, presence of different trap centers, material quality, etc. [9,18,19]. In this article we have examined the doping effect of Mn on the luminescence property of anatase TiO2 nanoparticles. There is hardly any report on the luminescence properties of Mn-doped TiO2 nanoparticles. Mn2 þ related absorption peaks are identified with UV–vis spectroscopy. Room temperature photoluminescence spectra of the samples provide information about the emission peaks associated with excitons, oxygen defects and Mn2 þ . The lifetime of the charge carriers in pure and doped TiO2 is investigated with time resolved photoluminescence spectroscopy.
2. Experimental details 2.1. Preparation of Mn-doped TiO2 nanoparticles Mn-doped TiO2 nanoparticles with nominal Mn (x) concentration of 2%, 4% and 6% was prepared by sol–gel method. The reaction was started with the addition of 7 mL of titanium isopropoxide to 15 mL of 2-propanol. The reaction mixture was stirred for 15 min to get homogenous solution. Required amount of aqueous Mn acetate tetra hydrate solutions were added to the above reaction mixture and stirred. The stirring was continued until the solution was completely transformed to a gel. The gel
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was left for ageing for 10–12 h. The gel was then sonicated in ethanol followed by centrifugation in ethanol and water. The resulting product was kept in an oven at 80 1C to get doped amorphous TiO2 powder. The doped amorphous powder was again annealed at 450 1C for 3 h to get crystalline Mn-doped TiO2 nanoparticles. 2.2. Characterization details The crystalline phases of pure and Mn-doped TiO2 nanoparticles were investigated with Rigaku Miniflex X-ray diffractometer equipped with intense CuKa radiation (l ¼0.154 nm). The scanning rate was 11/min and the step size was 0.051. High resolution transmission electron microscope images (HRTEM) was acquired with JEOL JEM 3100 transmission electron microscope operating at a voltage of 300 kV. Energy dispersive X-ray analyses (EDX) of the samples were studied with JEOL JSM (Model 6390LV) scanning electron microscope (SEM) with an INCAx-Sight (Oxford instruments) EDX detector. Electron spin resonance spectra (ESR) of the doped samples were carried out with a JEOL-JESFA200 electron spin resonance spectrometer with an applied microwave frequency of 9.09 GHz. UV–vis absorption spectra were obtained in diffuse reflectance mode (DRS) in a Shimadzu 2450 UV–vis spectrophotometer. Photoluminescence spectra were examined in a Perkin-Elmer LS- 55 fluorescence spectrometer. Time resolved photoluminescence (TRPL) spectra were obtained with life spec II spectrofluorimeter (Edinburgh instrument). The sample was excited by a laser diode of wavelength 375 nm and the decay was measured with a time scale of 0.0048 ns/channel.
3. Results and discussion 3.1. X-ray diffraction study Fig. 1 depicts the X-ray diffraction pattern of pure and doped TiO2 nanoparticles with different concentrations of Mn. The diffraction spectra of the samples correspond to the tetragonal anatase phase of TiO2 having space group I41/amd (JCPDS 78-2486) [9]. Though XRD shows the diffraction peaks of anatase TiO2, the possibility of the presence of the XRD peaks of metallic Mn and of the different oxide phases of Mn cannot be ruled out. During air annealing of Mn-doped TiO2 nanoparticles, some oxide phases of Mn may have been formed. But, due to the low concentration of the dopants, the intensity of the diffraction peaks of the impurity phases are negligible compared to the intensity of the anatase peaks. Inset of this figure contains enlarged view of the (1 0 1) peak of all the samples. Doping of Mn gradually shifts this peak to lower angle. The d-spacing of this peak is calculated for each of the doping concentration of Mn. The magnitude of the d-spacing is found to increase with the increase in the concentration of Mn (inset of Fig. 1). The positions of (2 0 0) and (0 0 4) diffraction peaks are also shifted to lower angle with the increase in the doping level of Mn. The increase in the magnitude of d-spacing is due to the substitution of large sized Mn2 þ (80 pm) on the Ti4 þ (56 pm) site of anatase TiO2 [20]. 3.2. TEM study Transmission electron microscope images of the pure and 6% Mn-doped TiO2 nanoparticles are shown in the Fig. 2a–d. The histograms of the pure and doped nanoparticles show an average particle size of 6 nm. The crystalline lattice planes of pure and doped nanoparticles are clearly distinguished in their high resolution images (Fig. 2c and d). The unclear part in the HRTEM
Fig. 1. X-ray diffraction spectra of pure and Mn-doped TiO2 nanoparticles. Inset of the figure shows the shifting of the (101) peak position (right side) to lower angle and increase in the d-spacing (left side) of the (101) peak with the increase in Mn concentration.
image of Fig. 2d is due to the incomplete orientation of the nanoparticles along its low index zone axis. When the nanoparticles are completely oriented along the low index zone axis, the Bragg reflected beams interact with the transmitted beam giving phase contrast clear lattice image of the nanoparticles [21]. The EDX pattern of the doped nanoparticles (Fig. 2e) shows the composition of the material with the percentage of each element in the inset of it. 3.3. Electron spin resonance study Fig. 3 shows the room temperature ESR spectrum of 2% Mn-doped TiO2 nanoparticles. From the spectrum, the g-factor is calculated using the equation g ef f ¼
hn mB H
ð1Þ
Where h is the Plank constant, u is the frequency of applied microwave field, mB is the Bohr magneton and H is the applied magnetic field [22]. The spectrum contains two peaks at g 2.0 and g 4.38. Presence of these peaks indicate presence of þ2 state of Mn, since these peaks are the result of the interaction of Mn2 þ with the surrounding O2 ligands of TiO2 [23,24]. The absence of sextet lines and the presence of broad ESR line at g 2 indicates presence of high concentration of paramagnetic centers of Mn2 þ and the weak interaction of the centers with the six O2 ions of TiO2 [23,24]. Apart from ESR, there are many reports where people have carried out X-ray photoelectron spectroscopy (XPS), which is a valuable technique for knowing valence state, to identify the oxidation state of Mn in TiO2 [25,26]. Xu et al. [25] and Sharma et al. [26] have obtained XPS signal corresponding to þ2 state of Mn. However, they also reported that Mn3 þ also displays XPS signal at the same binding energy as of Mn2 þ and therefore they concluded that it is difficult to clearly distinguish the two states with XPS. In our analysis we have obtained ESR line of Mn2 þ only. Since Mn3 þ is EPR silent, we have not detected
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Fig. 2. Low and high resolution transmission electron microscope (TEM) images of pure (a) and (c) and 6% Mn-doped TiO2 nanoparticles, (b) and (d) respectively. (e) EDX pattern shows the presence of Ti, Mn and oxygen in the 6% Mn-doped TiO2 nanoparticles.
Fig. 3. Room temperature ESR curve of 2% Mn-doped TiO2 nanoparticles.
Mn3 þ EPR signal [20]. Out of Mn2 þ and Mn3 þ , substitution of Mn2 þ on Ti4 þ can enlarge the lattice and can shift the peak to lower angle, which we have observed in the XRD. Apart from XRD, our UV–vis and photoluminescence emission spectra also confirm that Mn is in the þ2 state in TiO2. 3.4. UV–vis spectroscopy study The optical properties of pure and doped TiO2 nanoparticles are taken in reflectance mode. The absorption spectra of the samples
are acquired from reflectance curves using Kubelka–Munk plot. The Kubelka–Munk equation is represented by F(R)¼(1 R)2/2R, where F(R) is the absorbance and R is the reflectance [9]. Fig. 4a illustrates the absorption curves (with inset reflectance) of entire samples. Pure TiO2 nanoparticles exhibit its absorption peak at 310 nm. This peak is due to the indirect transition from the upper of the valence band G (VB) to the lower of the conduction band M (CB) in the Brillouin zone (BZ) [27]. Direct consequence of Mn doping on the optical properties of TiO2 is revealed by the shifting of the absorption edge of TiO2 to visible region. The absorption curves of the doped samples have two absorption humps in between 370– 470 nm and in between 470–700 nm. These two broad absorption humps are mainly associated with d–d electronic transition of Mn2 þ in octahedral coordination [28–31]. Pure TiO2 does not exhibit these absorptions. The broad absorption in between 370–470 nm may be assigned to 6A1g(S)-4A1g(G), 4Eg(G) and 6A1g(S)-4T2g(G) transition and the absorption in between 470–700 nm are associated with 6 A1g(S)-4T1g(G) transition of Mn2 þ in octahedral coordination [28–31]. In anatase TiO2, Ti4 þ is surrounded by six oxygen ions forming the basic TiO2 octahedron. On substitution of Mn2 þ on 6 4þ 2þ Ti lattice site, the Mn ions will experience crystal field forces from the surrounding oxygen ions. The strong crystal field interaction of Mn2 þ with the oxygen will split the d-band states of Mn2 þ into ground and various excited energy states. A simplified diagram representing above-mentioned transitions are shown in Fig. 4b. TiO2 exhibits both direct and indirect band gap. However, it is proved that indirect band gap is favoured in anatase TiO2 [27,32]. For bulk TiO2, the indirect band gap is 3.2 eV. The indirect band gap of the samples are calculated by plotting [F(R)hu]1/2 vs. hu. This band gap is due to phonon assisted indirect transition from G to M in the Brillouin zone [27,32]. The band gap values of each sample are represented by Fig. 4c. Pure TiO2 nanoparticles have the band gap of 3.28 eV that is higher than that of bulk TiO2 [33]. The effective band gap of TiO2 is reduced to 2.81 eV, 2.08 eV and
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Fig. 5. Photoluminescence spectra of pure and Mn-doped TiO2 nanoparticles excited at 320 nm. The emission peaks of entire samples are Gaussian-fitted with a correctness, r2 ¼ 0.9989. The dotted line, on the UV emission peak, is drawn to represent the shifting of the emission peak with dopant concentration.
Fig. 4. (a) Kubelka–Munk plot showing the absorbance spectra of pure and Mn-doped TiO2 nanoparticles (inset shows the corresponding reflectance). (b) d–d electronic transition of Mn2 þ representing the absorbance peaks appearing between 370–470 nm and 470–700 nm in the absorbance curves of the doped TiO2 nanoparticles. (c) Determination of the band gap of the samples at [F(R)hu]1/2.
2.03 eV on incorporation of 2%, 4% and 6% Mn respectively. Doping induced reduction in the fundamental band gap of TiO2 nanoparticle is associated with band gap renormalization (BGR) effect [34,35]. In conformity with this effect, addition of extra electrons in the form of Mn ions results in the hybridization of the d-states of Mn with the conduction band edge of the host. This leads to an uplift of the valence band maximum and downward shift of the conduction band minimum [34,35] with an effective reduction in band gap. The increase in the concentration of the dopants increases the interaction of the electrons in d-states of Mn with the host conduction electron, resulting in the lowering in the effective band gap of TiO2. Shao with the help of DFT calculated the band gap of Mn-doped TiO2 and explained that doping of Mn creates Mn d-states in the band gap and interaction of these d-states with the host electrons result in the effective narrowing of band gap [17]. 3.5. Photoluminescence (PL) study Fig. 5 shows the photoluminescence spectra of pure and Mn-doped TiO2 nanoparticles excited at 320 nm. The emission peaks are Gaussian fitted with a correctness r2 ¼0.9989. The UV emission peak at 380 nm is owing to phonon assisted indirect transition from edge (M) to the center (G) of Brillouin zone [27,32,33]. The emission peak at 429 nm is assigned to self trapped excitons (STE) localized on TiO6 octahedra [36]. After photoexcitation at 320 nm, the electrons in conduction band
move through the ionic lattice, interact with the lattice ions and then localizes on a lattice site. The localized electron captures a hole and generates self trapped excitons [37]. The localization of the STE depends on the length and compactness of the TiO6 octahedra chain [38]. The emission peak at 457 nm and 537 nm are associated with oxygen related defect states [36]. The emission peak at 491 nm is ascribed to charge transfer transition from Ti3 þ to TiO26 octahedra [39–41]. The UV emission peak of TiO2 is gradually shifted to higher wavelength (dotted line) with increasing doping concentration of Mn (Fig. 5). The shifting in the position of UV emission peak with dopant concentration is illustrated in Fig. 6a. The red-shifting of the UV emission peak is due to band gap renormalization effect [34,35,41]. The interaction of dopant electrons with the conduction band electrons result in the lowering in the conduction band edge, shrinkage of the band gap and effective shifting of the band edge emission peak to lower energy [34]. Since the conduction band edge is shifted downward on doping, the emission is shifted gradually to low energy side as compared to that of pure TiO2. Therefore, the red-shifting in the band gap and the shifting of UV emission peak has direct correlation. Another important feature, observed in the band edge emission, is the increase in the width of this band with the increase in Mn concentration (Fig. 6b). This is due to potential fluctuations caused by random distribution of dopant impurities [19,41,42]. Randomly distributed dopant impurities result in unavoidable fluctuations at the microscopic scale. These microscopic fluctuations result in potential fluctuations and ultimately lead to band tailing and broadening of the emission line [19,42]. As it is seen in the spectra, the intensity of the band edge, self-trapped excitonic peak and defect related emission peaks are quenched on Mn doping. The quenching of UV and STE emission peak can be correlated with Auger type non-radiative recombination process [43,44]. In wide band gap semiconductors, the possibility of Auger type recombination depends on the concentration of dopants and amount of defects in the lattice [43].
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Fig. 6. Changes in (a) the position of band edge position, (b) full width at half maximum (FWHM), (c) IUV/I457 intensity ratio and (d) I457/I537 intensity ratio with the changes in the concentration of Mn2 þ in TiO2 nanoparticles.
Doping introduces additional free electrons in the lattice of TiO2. These free electrons decrease emission efficiency by producing a non-radiative channel associated with auger like recombination [44]. In Auger process, the energy released on recombination is absorbed by one electron and then by another and ultimately the energy is dissipated as phonons [43,44]. In this way the Auger type non-radiative recombination process quenches the intensity of the UV emission peaks. The quenching of the self trapped excitonic peak is also resulted due to the interaction of the excitons with the lattice defects, dopants, etc. These interactions collapse the STE to free electron and hole and reduce the emission efficiency. We have plotted the intensity ratio of IUV/I457 emission peak vs. Mn2 þ concentration and found a reduction in the intensity ratio with increase in Mn doping (Fig. 6c). Decrease in the intensity ratio indicates degradation of material quality on doping [9]. This degradation of quality occurs on account of the creation of large number defects on Mn incorporation. A plot of the intensity ratio of defect emission band reveals the dopant concentration effect on the intensity of the visible luminescence of the nanoparticles. Both 457 nm and 537 nm emission peaks are attributed to oxygen defect centers [9]. The variation of the intensity ratio of I457/I537 is plotted against Mn concentration. In Fig. 6d it is seen that the intensity ratio is decreasing with the increase of Mn2 þ concentration. We can correlate this reduction in intensity to the increase of non-radiative relaxation of conduction band electrons, linked with dopants and defects [45,46]. A non-radiative energy transfer process is regulating this quenching of emission. After photoexcitation, the electrons in the conduction band can transit to the oxygen defect levels or to the Mn d-states. Increase in the concentration of dopants decreases the distance between dopants and between dopants and defects. Decrease in the distance will increase the interactions among them, increasing the energy transfer between the dopants and between defects and dopants. In this way the emission energy is dissipated non radiatively decreasing the overall luminescence intensity [9,45].
Fig. 7. Photoluminescence spectra of pure and 2%, 4% and 6% Mn-doped TiO2 nanoparticles at the excitation wavelength of 390 nm.
The normalized photoluminescence spectra of the samples, excited at 390 nm, are illustrated in Fig. 7. The emission characteristics of pure and doped TiO2 nanoparticles are differentiated by the position of their emission peaks. Both undoped and Mn-doped TiO2 nanoparticles exhibit an emission peak at 491 nm. Doped TiO2 nanoparticles show two intense emission peaks at 525 nm and 585 nm that are otherwise absent in pure TiO2. These two peaks are the characteristic peaks of Mn2 þ in the crystalline environment of TiO2. The first peak is due to 4T2g(G) to 6 A1(S) transition and the second peak is due to 4T1g(G) to 6 A1g(S) transition [29,47]. Individual Mn2 þ does not show this spin forbidden transition. When Mn2 þ is in association with another Mn2 þ ion or its d-orbital is undergoing interaction with the host ligand ions, the spin selection rule is relaxed, electric dipole interaction occurs and these transitions become permissible [48]. These emissions may have been occurring by the following
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pathways. Photoexcited electrons in the conduction band may jump to the oxygen defects and then jump to the 4T1g(G) state, provided the energy states of the oxygen defects should be in higher position than that of 4T1g(G) state. The electrons may also jump from the conduction band to the higher energy state of the Mn. From the higher energy state the electron release energy nonradiatively to the 4T1g(G) state and then undergoes transition to the 6 A1(S) state giving 585 nm peak. The electrons may jump in a similar way to the Mn 4T2g(G) state and then transit to the 6 A1(S) state giving 525 nm peak. Intensity of Mn2 þ emission increases at 2% and 4% and then again decreases at 6%. The reduction in the emission intensity at 6% can be explained by concentration quenching effect [46,49]. At 6% of Mn doping concentration, many of the Mn2 þ ions are closely associated and form Mn2 þ –Mn2 þ ion pairs. After photoexcitation, the energy is transferred from one Mn2 þ to the neighboring Mn2 þ ion and then finally dissipated the energy to a non-radiative quenching centers [46,49].
3.6. Photoluminescence excitation (PLE) analysis Photoluminescence excitation spectra of the samples are carried out monitoring the 585 nm emission wavelength (Fig. 8). Pure TiO2 exhibits the excitation band at 389 nm, which is the band to band excitation peak. This peak is shifted to longer wavelength as Mn2 þ concentration increases. The red-shifting is due to BGR effect. The excitation peaks of Mn-doped TiO2 nanoparticles are different from that of pure one. The excitation peaks of pure TiO2 nanoparticles, in the range from 420–500 nm, are associated with oxygen related defect centers [50,51]. The excitation peaks of doped nanoparticles at 443 nm, 471 nm and 489 nm are ascribed to d–d excitation of Mn2 þ from ground 6A1 state to different excited states. The peak at 443 nm, 471 nm and 489 nm can be ascribed to 6A1g(S)-4A1g(G), 4Eg(G), 6A1g(S)- 4T2g(G) and 6A1g(S)-4T1g(G) transition respectively [28–31] (the corresponding d–d transition is shown in Fig. 4b). From the analysis of the photoluminescence, at 320 nm and 390 nm excitation, and from absorption and excitation spectroscopy it is clearly understood that absorption shown by the samples is directly with their emissions. The UV absorption and its corresponding emission spectra of the doped samples show clearly shifting to high wavelength. The oxygen defect related visible emission peaks are also the result of the absorption peaks
Fig. 8. Photoluminescence excitation (PLE) spectra of pure and Mn-doped TiO2 nanoparticles. The excitation spectra of the samples are obtained monitoring the emission wavelength at 585 nm.
of oxygen vacancies. Absorption spectra of pure and codoped TiO2 has not shown any oxygen defect related peaks. PLE is a much more sensitive technique to identify defect related excitation peaks than UV–vis absorption [52]. PLE clearly shows the oxygen defect related absorption peaks in the range from 400–500 nm. In TiO2 the oxygen vacancy levels are distributed up to 1 eV from the lower of CB [53]. Therefore, the visible emission peaks are due to the energy released by the electrons trapped in these oxygen vacancies. We have found that the energy states of Mn2 þ are also present at 2.8 eV, 2.63 eV and 2.53 eV, which are in the same range as that of oxygen vacancy levels. Therefore, the visible emission peaks are due to the energy transfer from the oxygen defects to Mn2 þ . The Mn2 þ emissions are also occurring at higher wavelength than the band gap of 2.81 eV observed in 2% Mn doping. As explained the band gap narrowing is due to the interaction of the dopant d-states with the host band electrons. Since, increase in electron concentration will increase the interaction and hence will narrows the band gap effectively. Therefore, the characteristic emission of Mn2 þ at 525 nm and 585 nm are at higher wavelength than its corresponding excitation and above band gap also. A schematic representation of the different emission peaks occurring at excitation of 320 nm and 390 nm are shown in Fig. 9. 3.7. Time resolved photoluminescence (TRPL) study The lifetime of the charge carriers for pure and doped TiO2 nanoparticles are studied. Fig. 10 depicts the lifetime of the charge carriers in pure and 6% Mn-doped TiO2 nanoparticles. A bi-exponential fitting is performed for the TRPL curves. The fitting follows the equation t t y ¼ y0 þ a1 exp þ a2 exp ð1Þ
t1
t2
Where y0 is the baseline correction (y-offset), t1 and t2 are the excited state emission decay time and a1 and a2 are the pre exponential factors [9,54]. After fitting (r2 ¼0.9994) the values come out to be 0.2803 (a1), 0.8107 (a2) and 1.2631 ns (t1) and 0.682 ns (t2) for pure TiO2 nanoparticles. For 6% Mn-doped TiO2 these values are 0.27627 (a1), 0.8646 (a2) and 2.295 ns (t1) and 0.3737 ns (t2) respectively. The average lifetime is calculated
Fig. 9. Schematic diagram showing emissions at an excitation of 320 (3.87 eV) nm and 390 nm (3.18 eV). (1) Excitation at 320 nm excites the electrons to the upper of the valence band. (a) Radiationless decay and transition of electrons to the bottom of conduction band giving UV emission. (b) The electrons jump to the defect states giving defect emissions. (c) Few electrons also jump to the nearby Mn d-states and from these states the electron recombines with the VB holes. (2) Excitation at 390 nm (3.18 eV) excites the electrons to the defect states and from these states few electrons give defect emission and others jump to Mn d-state giving Mn2 þ emissions.
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As dopant d-states interact with the conduction band edge, therefore relative shifting of the CB with respect to VB will be more and therefore their band edges will move further away from zone centers. Hence, the electrons and holes will have to take a longer time for undergoing recombination. From the analysis, and on doping these CB and VB edges are shifted relative to each other.
4. Conclusion
Fig. 10. Time resolved photoluminescence (TRPL) curves of TiO2 and 6% Mn-doped TiO2 nanoparticles. The TRPL curves of both samples are fitted bi-exponentially with a correctness r2 ¼ 0.9994.
using the equation [9,54].
tav ¼
a1 t21 þ a2 t22 a1 t1 þ a2 t2
ð2Þ
In this equation, the meaning of the parametersa1, t1 and a2, t2 are already explained in Eq. (1). The values of these parameters, obtained from Eq. (1), are inserted in Eq. (2) and obtained an average lifetime of 0.904 ns for pure and 1.61 ns for 6% Mn-doped TiO2 nanoparticles. Doped TiO2 has prolonged life period of the charge carriers compared to pure one. The long lifetime of the charge carriers can be explained considering different factors such as structural defects, mobility of the carriers, band structure, etc. In literature reports of pure TiO2, people have attributed the slow or longer lifetime component to be associated with recombination of excitons via oxygen defects [52,55]. Defects, either on surface or on bulk, act as active trap center or suppress the carrier recombination [56]. Due to the presence of these oxygen vacancies, the electrons do not directly jump to the valence band to recombine with holes. Instead, the electrons, below the bottom of CB, first captured by non-radiative oxygen vacancies and then the electrons undergo hopping from one defect to another until they find an emission center [52]. On doping the number of oxygen vacancies increases, and therefore more number of carriers are trapped in these defect centers. Therefore, as compared to pure one the electrons in the doped system now transit from one defect to another defect of Mn d-state until it finds an emission centers. Das et al., in Co doped TiO2 nanostructures, observed a longer lifetime of the carriers and attributed the long lifetime to the trapping of the carriers in the deep oxygen defect center for longer period [54]. There are reports where people have doped TiO2 with metal ions such as Nd, Pd, Pt, Fe, Cr, etc. They observed that d-states of these ions act as active trap centers and also generate electron acceptor centers and separate the charge carriers for longer period [57,58]. The oxygen vacancies, with trapped electrons, act as charged defects. Therefore, these charged oxygen vacancies interact with the mobile electrons and scatter them, thereby decreasing their mobility [52]. Decrease in mobility will separate the electrons-holes for longer time and hence increase the lifetime. The band structure of anatase TiO2 also influence in the increase of carrier lifetime. Since anatase has an indirect band gap, the conduction band electron and valence band hole will be in different position in the Brillouin zone [59].
In summary we can write that doping of Mn has a pronounced effect in the tailoring of the luminescence properties of TiO2 nanoparticles. Both Kubelka–Munk plot and photoexcitation spectroscopy confirms the presence of the absorption peaks of Mn2 þ . The intensity of the UV emission peak of TiO2 is decreased with the simultaneous increase in the peak width. Auger type non-radiative recombination and potential fluctuations of impurities result in the reduction and broadening of UV emission peak. Decrease in the intensity ratio of UV to defect emission band indicates degradation of structural quality of the material on doping. Doping also reduces the defect emission intensity ratio by increasing the non-radiative centers. The characteristic emission peak of Mn2 þ appears at 585 nm, associated with 4T1g(G) to 6 A1g(S) emission. Lifetime of the charge carriers increases in doped TiO2 due to the relaxation of the conduction band electrons in the oxygen defects and on the d-band states.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Tailoring luminescence properties of TiO2 nanoparticles by Mn doping B. Choudhury n, A. Choudhury Department of Physics, Tezpur University, Napaam, Assam-784028, India
a r t i c l e i n f o
abstract
Article history: Received 13 June 2012 Received in revised form 17 November 2012 Accepted 4 December 2012 Available online 13 December 2012
TiO2 nanoparticles are doped with three different concentrations of Mn, 2%, 4% and 6% respectively. Absorption edge of TiO2 is shifted from UV to visible region on amplification of Mn content. Room temperature photoluminescence spectra, excited at 320 nm, exhibit band edge and visible emission peaks associated with self trapped excitons, oxygen defects, etc. Doping of Mn increases the width and decreases the intensity of the UV emission peak. Potential fluctuations of impurities increase the width and auger type non-radiative recombination decreases the intensity of the UV emission peak. The intensity ratio of the UV to defect emission band decreases on doping, indicating degradation of structural quality. Excitation of pure and doped nanoparticles at 390 nm results in Mn2 þ emission peaks at 525 nm and 585 nm respectively. Photoluminescence excitation spectra also indicate the presence of Mn2 þ in the crystalline environment of TiO2. The oxygen defects and Mn related impurities act as efficient trap centers and increases the lifetime of the charge carriers. & 2012 Elsevier B.V. All rights reserved.
Keywords: Band gap Crystal field Oxygen vacancies Auger recombination Concentration quenching
1. Introduction TiO2 is an important wide band gap semiconductor for its applications in photocatalysis, photovoltaics, solar cells, chemical sensors, etc. [1–5]. The band gap of TiO2 is very large ( 3.2 eV for anatase and 3.0 for rutile) for using in visible light photocatalysis [6]. However, incorporation of impurities tailors the band gap of TiO2 and extends its absorption edge to visible region [6–8]. Doping is expected to prolong the lifetime of the charge carriers by acting as charge carrier trapping sites [9,10]. On doping entire dopants may not penetrate inside TiO2, few numbers of dopants may also occupy sites on the surface. Thermodynamically, only a limited number of dopants can enter inside the small nanoparticles, the excess are expelled from the interior due to the self purification nature of the nanocrystals [11]. Doping of metal ions weakens the bonding of neighboring oxygen ions at the surface and releases the oxygen ions forming vacancies [12]. These oxygen vacancies alter the electronic and optical properties of the host TiO2 [13]. Incorporation of dopants produces oxygen vacancies for electron trapping, enhances the visible light absorption property of TiO2 and decreases the electron–hole recombination rate [8,9]. Since Mn-doped TiO2 nanoparticles have these properties, these nanoparticles are used in the photocatalytic degradation of many dyes [14–16]. Theoretical calculations in Mn-doped rutile TiO2 have reported that the effective band gap of TiO2 is reduced due to the creation of dopant d-states in the mid
n
Corresponding author. Tel.: þ91 3712267120; fax: þ91 3712267005. E-mail address: [email protected] (B. Choudhury).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.12.011
band gap [13,17]. Recombination rate and the lifetime of the charge carriers, in pure and doped nanoparticles, can be examined with the help of steady state and time resolved photoluminescence spectroscopy. These spectroscopic techniques are important characterization tools to acquire information on the electron–hole recombination rate, lifetime of the charge carriers, presence of different trap centers, material quality, etc. [9,18,19]. In this article we have examined the doping effect of Mn on the luminescence property of anatase TiO2 nanoparticles. There is hardly any report on the luminescence properties of Mn-doped TiO2 nanoparticles. Mn2 þ related absorption peaks are identified with UV–vis spectroscopy. Room temperature photoluminescence spectra of the samples provide information about the emission peaks associated with excitons, oxygen defects and Mn2 þ . The lifetime of the charge carriers in pure and doped TiO2 is investigated with time resolved photoluminescence spectroscopy.
2. Experimental details 2.1. Preparation of Mn-doped TiO2 nanoparticles Mn-doped TiO2 nanoparticles with nominal Mn (x) concentration of 2%, 4% and 6% was prepared by sol–gel method. The reaction was started with the addition of 7 mL of titanium isopropoxide to 15 mL of 2-propanol. The reaction mixture was stirred for 15 min to get homogenous solution. Required amount of aqueous Mn acetate tetra hydrate solutions were added to the above reaction mixture and stirred. The stirring was continued until the solution was completely transformed to a gel. The gel
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was left for ageing for 10–12 h. The gel was then sonicated in ethanol followed by centrifugation in ethanol and water. The resulting product was kept in an oven at 80 1C to get doped amorphous TiO2 powder. The doped amorphous powder was again annealed at 450 1C for 3 h to get crystalline Mn-doped TiO2 nanoparticles. 2.2. Characterization details The crystalline phases of pure and Mn-doped TiO2 nanoparticles were investigated with Rigaku Miniflex X-ray diffractometer equipped with intense CuKa radiation (l ¼0.154 nm). The scanning rate was 11/min and the step size was 0.051. High resolution transmission electron microscope images (HRTEM) was acquired with JEOL JEM 3100 transmission electron microscope operating at a voltage of 300 kV. Energy dispersive X-ray analyses (EDX) of the samples were studied with JEOL JSM (Model 6390LV) scanning electron microscope (SEM) with an INCAx-Sight (Oxford instruments) EDX detector. Electron spin resonance spectra (ESR) of the doped samples were carried out with a JEOL-JESFA200 electron spin resonance spectrometer with an applied microwave frequency of 9.09 GHz. UV–vis absorption spectra were obtained in diffuse reflectance mode (DRS) in a Shimadzu 2450 UV–vis spectrophotometer. Photoluminescence spectra were examined in a Perkin-Elmer LS- 55 fluorescence spectrometer. Time resolved photoluminescence (TRPL) spectra were obtained with life spec II spectrofluorimeter (Edinburgh instrument). The sample was excited by a laser diode of wavelength 375 nm and the decay was measured with a time scale of 0.0048 ns/channel.
3. Results and discussion 3.1. X-ray diffraction study Fig. 1 depicts the X-ray diffraction pattern of pure and doped TiO2 nanoparticles with different concentrations of Mn. The diffraction spectra of the samples correspond to the tetragonal anatase phase of TiO2 having space group I41/amd (JCPDS 78-2486) [9]. Though XRD shows the diffraction peaks of anatase TiO2, the possibility of the presence of the XRD peaks of metallic Mn and of the different oxide phases of Mn cannot be ruled out. During air annealing of Mn-doped TiO2 nanoparticles, some oxide phases of Mn may have been formed. But, due to the low concentration of the dopants, the intensity of the diffraction peaks of the impurity phases are negligible compared to the intensity of the anatase peaks. Inset of this figure contains enlarged view of the (1 0 1) peak of all the samples. Doping of Mn gradually shifts this peak to lower angle. The d-spacing of this peak is calculated for each of the doping concentration of Mn. The magnitude of the d-spacing is found to increase with the increase in the concentration of Mn (inset of Fig. 1). The positions of (2 0 0) and (0 0 4) diffraction peaks are also shifted to lower angle with the increase in the doping level of Mn. The increase in the magnitude of d-spacing is due to the substitution of large sized Mn2 þ (80 pm) on the Ti4 þ (56 pm) site of anatase TiO2 [20]. 3.2. TEM study Transmission electron microscope images of the pure and 6% Mn-doped TiO2 nanoparticles are shown in the Fig. 2a–d. The histograms of the pure and doped nanoparticles show an average particle size of 6 nm. The crystalline lattice planes of pure and doped nanoparticles are clearly distinguished in their high resolution images (Fig. 2c and d). The unclear part in the HRTEM
Fig. 1. X-ray diffraction spectra of pure and Mn-doped TiO2 nanoparticles. Inset of the figure shows the shifting of the (101) peak position (right side) to lower angle and increase in the d-spacing (left side) of the (101) peak with the increase in Mn concentration.
image of Fig. 2d is due to the incomplete orientation of the nanoparticles along its low index zone axis. When the nanoparticles are completely oriented along the low index zone axis, the Bragg reflected beams interact with the transmitted beam giving phase contrast clear lattice image of the nanoparticles [21]. The EDX pattern of the doped nanoparticles (Fig. 2e) shows the composition of the material with the percentage of each element in the inset of it. 3.3. Electron spin resonance study Fig. 3 shows the room temperature ESR spectrum of 2% Mn-doped TiO2 nanoparticles. From the spectrum, the g-factor is calculated using the equation g ef f ¼
hn mB H
ð1Þ
Where h is the Plank constant, u is the frequency of applied microwave field, mB is the Bohr magneton and H is the applied magnetic field [22]. The spectrum contains two peaks at g 2.0 and g 4.38. Presence of these peaks indicate presence of þ2 state of Mn, since these peaks are the result of the interaction of Mn2 þ with the surrounding O2 ligands of TiO2 [23,24]. The absence of sextet lines and the presence of broad ESR line at g 2 indicates presence of high concentration of paramagnetic centers of Mn2 þ and the weak interaction of the centers with the six O2 ions of TiO2 [23,24]. Apart from ESR, there are many reports where people have carried out X-ray photoelectron spectroscopy (XPS), which is a valuable technique for knowing valence state, to identify the oxidation state of Mn in TiO2 [25,26]. Xu et al. [25] and Sharma et al. [26] have obtained XPS signal corresponding to þ2 state of Mn. However, they also reported that Mn3 þ also displays XPS signal at the same binding energy as of Mn2 þ and therefore they concluded that it is difficult to clearly distinguish the two states with XPS. In our analysis we have obtained ESR line of Mn2 þ only. Since Mn3 þ is EPR silent, we have not detected
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Fig. 2. Low and high resolution transmission electron microscope (TEM) images of pure (a) and (c) and 6% Mn-doped TiO2 nanoparticles, (b) and (d) respectively. (e) EDX pattern shows the presence of Ti, Mn and oxygen in the 6% Mn-doped TiO2 nanoparticles.
Fig. 3. Room temperature ESR curve of 2% Mn-doped TiO2 nanoparticles.
Mn3 þ EPR signal [20]. Out of Mn2 þ and Mn3 þ , substitution of Mn2 þ on Ti4 þ can enlarge the lattice and can shift the peak to lower angle, which we have observed in the XRD. Apart from XRD, our UV–vis and photoluminescence emission spectra also confirm that Mn is in the þ2 state in TiO2. 3.4. UV–vis spectroscopy study The optical properties of pure and doped TiO2 nanoparticles are taken in reflectance mode. The absorption spectra of the samples
are acquired from reflectance curves using Kubelka–Munk plot. The Kubelka–Munk equation is represented by F(R)¼(1 R)2/2R, where F(R) is the absorbance and R is the reflectance [9]. Fig. 4a illustrates the absorption curves (with inset reflectance) of entire samples. Pure TiO2 nanoparticles exhibit its absorption peak at 310 nm. This peak is due to the indirect transition from the upper of the valence band G (VB) to the lower of the conduction band M (CB) in the Brillouin zone (BZ) [27]. Direct consequence of Mn doping on the optical properties of TiO2 is revealed by the shifting of the absorption edge of TiO2 to visible region. The absorption curves of the doped samples have two absorption humps in between 370– 470 nm and in between 470–700 nm. These two broad absorption humps are mainly associated with d–d electronic transition of Mn2 þ in octahedral coordination [28–31]. Pure TiO2 does not exhibit these absorptions. The broad absorption in between 370–470 nm may be assigned to 6A1g(S)-4A1g(G), 4Eg(G) and 6A1g(S)-4T2g(G) transition and the absorption in between 470–700 nm are associated with 6 A1g(S)-4T1g(G) transition of Mn2 þ in octahedral coordination [28–31]. In anatase TiO2, Ti4 þ is surrounded by six oxygen ions forming the basic TiO2 octahedron. On substitution of Mn2 þ on 6 4þ 2þ Ti lattice site, the Mn ions will experience crystal field forces from the surrounding oxygen ions. The strong crystal field interaction of Mn2 þ with the oxygen will split the d-band states of Mn2 þ into ground and various excited energy states. A simplified diagram representing above-mentioned transitions are shown in Fig. 4b. TiO2 exhibits both direct and indirect band gap. However, it is proved that indirect band gap is favoured in anatase TiO2 [27,32]. For bulk TiO2, the indirect band gap is 3.2 eV. The indirect band gap of the samples are calculated by plotting [F(R)hu]1/2 vs. hu. This band gap is due to phonon assisted indirect transition from G to M in the Brillouin zone [27,32]. The band gap values of each sample are represented by Fig. 4c. Pure TiO2 nanoparticles have the band gap of 3.28 eV that is higher than that of bulk TiO2 [33]. The effective band gap of TiO2 is reduced to 2.81 eV, 2.08 eV and
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Fig. 5. Photoluminescence spectra of pure and Mn-doped TiO2 nanoparticles excited at 320 nm. The emission peaks of entire samples are Gaussian-fitted with a correctness, r2 ¼ 0.9989. The dotted line, on the UV emission peak, is drawn to represent the shifting of the emission peak with dopant concentration.
Fig. 4. (a) Kubelka–Munk plot showing the absorbance spectra of pure and Mn-doped TiO2 nanoparticles (inset shows the corresponding reflectance). (b) d–d electronic transition of Mn2 þ representing the absorbance peaks appearing between 370–470 nm and 470–700 nm in the absorbance curves of the doped TiO2 nanoparticles. (c) Determination of the band gap of the samples at [F(R)hu]1/2.
2.03 eV on incorporation of 2%, 4% and 6% Mn respectively. Doping induced reduction in the fundamental band gap of TiO2 nanoparticle is associated with band gap renormalization (BGR) effect [34,35]. In conformity with this effect, addition of extra electrons in the form of Mn ions results in the hybridization of the d-states of Mn with the conduction band edge of the host. This leads to an uplift of the valence band maximum and downward shift of the conduction band minimum [34,35] with an effective reduction in band gap. The increase in the concentration of the dopants increases the interaction of the electrons in d-states of Mn with the host conduction electron, resulting in the lowering in the effective band gap of TiO2. Shao with the help of DFT calculated the band gap of Mn-doped TiO2 and explained that doping of Mn creates Mn d-states in the band gap and interaction of these d-states with the host electrons result in the effective narrowing of band gap [17]. 3.5. Photoluminescence (PL) study Fig. 5 shows the photoluminescence spectra of pure and Mn-doped TiO2 nanoparticles excited at 320 nm. The emission peaks are Gaussian fitted with a correctness r2 ¼0.9989. The UV emission peak at 380 nm is owing to phonon assisted indirect transition from edge (M) to the center (G) of Brillouin zone [27,32,33]. The emission peak at 429 nm is assigned to self trapped excitons (STE) localized on TiO6 octahedra [36]. After photoexcitation at 320 nm, the electrons in conduction band
move through the ionic lattice, interact with the lattice ions and then localizes on a lattice site. The localized electron captures a hole and generates self trapped excitons [37]. The localization of the STE depends on the length and compactness of the TiO6 octahedra chain [38]. The emission peak at 457 nm and 537 nm are associated with oxygen related defect states [36]. The emission peak at 491 nm is ascribed to charge transfer transition from Ti3 þ to TiO26 octahedra [39–41]. The UV emission peak of TiO2 is gradually shifted to higher wavelength (dotted line) with increasing doping concentration of Mn (Fig. 5). The shifting in the position of UV emission peak with dopant concentration is illustrated in Fig. 6a. The red-shifting of the UV emission peak is due to band gap renormalization effect [34,35,41]. The interaction of dopant electrons with the conduction band electrons result in the lowering in the conduction band edge, shrinkage of the band gap and effective shifting of the band edge emission peak to lower energy [34]. Since the conduction band edge is shifted downward on doping, the emission is shifted gradually to low energy side as compared to that of pure TiO2. Therefore, the red-shifting in the band gap and the shifting of UV emission peak has direct correlation. Another important feature, observed in the band edge emission, is the increase in the width of this band with the increase in Mn concentration (Fig. 6b). This is due to potential fluctuations caused by random distribution of dopant impurities [19,41,42]. Randomly distributed dopant impurities result in unavoidable fluctuations at the microscopic scale. These microscopic fluctuations result in potential fluctuations and ultimately lead to band tailing and broadening of the emission line [19,42]. As it is seen in the spectra, the intensity of the band edge, self-trapped excitonic peak and defect related emission peaks are quenched on Mn doping. The quenching of UV and STE emission peak can be correlated with Auger type non-radiative recombination process [43,44]. In wide band gap semiconductors, the possibility of Auger type recombination depends on the concentration of dopants and amount of defects in the lattice [43].
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Fig. 6. Changes in (a) the position of band edge position, (b) full width at half maximum (FWHM), (c) IUV/I457 intensity ratio and (d) I457/I537 intensity ratio with the changes in the concentration of Mn2 þ in TiO2 nanoparticles.
Doping introduces additional free electrons in the lattice of TiO2. These free electrons decrease emission efficiency by producing a non-radiative channel associated with auger like recombination [44]. In Auger process, the energy released on recombination is absorbed by one electron and then by another and ultimately the energy is dissipated as phonons [43,44]. In this way the Auger type non-radiative recombination process quenches the intensity of the UV emission peaks. The quenching of the self trapped excitonic peak is also resulted due to the interaction of the excitons with the lattice defects, dopants, etc. These interactions collapse the STE to free electron and hole and reduce the emission efficiency. We have plotted the intensity ratio of IUV/I457 emission peak vs. Mn2 þ concentration and found a reduction in the intensity ratio with increase in Mn doping (Fig. 6c). Decrease in the intensity ratio indicates degradation of material quality on doping [9]. This degradation of quality occurs on account of the creation of large number defects on Mn incorporation. A plot of the intensity ratio of defect emission band reveals the dopant concentration effect on the intensity of the visible luminescence of the nanoparticles. Both 457 nm and 537 nm emission peaks are attributed to oxygen defect centers [9]. The variation of the intensity ratio of I457/I537 is plotted against Mn concentration. In Fig. 6d it is seen that the intensity ratio is decreasing with the increase of Mn2 þ concentration. We can correlate this reduction in intensity to the increase of non-radiative relaxation of conduction band electrons, linked with dopants and defects [45,46]. A non-radiative energy transfer process is regulating this quenching of emission. After photoexcitation, the electrons in the conduction band can transit to the oxygen defect levels or to the Mn d-states. Increase in the concentration of dopants decreases the distance between dopants and between dopants and defects. Decrease in the distance will increase the interactions among them, increasing the energy transfer between the dopants and between defects and dopants. In this way the emission energy is dissipated non radiatively decreasing the overall luminescence intensity [9,45].
Fig. 7. Photoluminescence spectra of pure and 2%, 4% and 6% Mn-doped TiO2 nanoparticles at the excitation wavelength of 390 nm.
The normalized photoluminescence spectra of the samples, excited at 390 nm, are illustrated in Fig. 7. The emission characteristics of pure and doped TiO2 nanoparticles are differentiated by the position of their emission peaks. Both undoped and Mn-doped TiO2 nanoparticles exhibit an emission peak at 491 nm. Doped TiO2 nanoparticles show two intense emission peaks at 525 nm and 585 nm that are otherwise absent in pure TiO2. These two peaks are the characteristic peaks of Mn2 þ in the crystalline environment of TiO2. The first peak is due to 4T2g(G) to 6 A1(S) transition and the second peak is due to 4T1g(G) to 6 A1g(S) transition [29,47]. Individual Mn2 þ does not show this spin forbidden transition. When Mn2 þ is in association with another Mn2 þ ion or its d-orbital is undergoing interaction with the host ligand ions, the spin selection rule is relaxed, electric dipole interaction occurs and these transitions become permissible [48]. These emissions may have been occurring by the following
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pathways. Photoexcited electrons in the conduction band may jump to the oxygen defects and then jump to the 4T1g(G) state, provided the energy states of the oxygen defects should be in higher position than that of 4T1g(G) state. The electrons may also jump from the conduction band to the higher energy state of the Mn. From the higher energy state the electron release energy nonradiatively to the 4T1g(G) state and then undergoes transition to the 6 A1(S) state giving 585 nm peak. The electrons may jump in a similar way to the Mn 4T2g(G) state and then transit to the 6 A1(S) state giving 525 nm peak. Intensity of Mn2 þ emission increases at 2% and 4% and then again decreases at 6%. The reduction in the emission intensity at 6% can be explained by concentration quenching effect [46,49]. At 6% of Mn doping concentration, many of the Mn2 þ ions are closely associated and form Mn2 þ –Mn2 þ ion pairs. After photoexcitation, the energy is transferred from one Mn2 þ to the neighboring Mn2 þ ion and then finally dissipated the energy to a non-radiative quenching centers [46,49].
3.6. Photoluminescence excitation (PLE) analysis Photoluminescence excitation spectra of the samples are carried out monitoring the 585 nm emission wavelength (Fig. 8). Pure TiO2 exhibits the excitation band at 389 nm, which is the band to band excitation peak. This peak is shifted to longer wavelength as Mn2 þ concentration increases. The red-shifting is due to BGR effect. The excitation peaks of Mn-doped TiO2 nanoparticles are different from that of pure one. The excitation peaks of pure TiO2 nanoparticles, in the range from 420–500 nm, are associated with oxygen related defect centers [50,51]. The excitation peaks of doped nanoparticles at 443 nm, 471 nm and 489 nm are ascribed to d–d excitation of Mn2 þ from ground 6A1 state to different excited states. The peak at 443 nm, 471 nm and 489 nm can be ascribed to 6A1g(S)-4A1g(G), 4Eg(G), 6A1g(S)- 4T2g(G) and 6A1g(S)-4T1g(G) transition respectively [28–31] (the corresponding d–d transition is shown in Fig. 4b). From the analysis of the photoluminescence, at 320 nm and 390 nm excitation, and from absorption and excitation spectroscopy it is clearly understood that absorption shown by the samples is directly with their emissions. The UV absorption and its corresponding emission spectra of the doped samples show clearly shifting to high wavelength. The oxygen defect related visible emission peaks are also the result of the absorption peaks
Fig. 8. Photoluminescence excitation (PLE) spectra of pure and Mn-doped TiO2 nanoparticles. The excitation spectra of the samples are obtained monitoring the emission wavelength at 585 nm.
of oxygen vacancies. Absorption spectra of pure and codoped TiO2 has not shown any oxygen defect related peaks. PLE is a much more sensitive technique to identify defect related excitation peaks than UV–vis absorption [52]. PLE clearly shows the oxygen defect related absorption peaks in the range from 400–500 nm. In TiO2 the oxygen vacancy levels are distributed up to 1 eV from the lower of CB [53]. Therefore, the visible emission peaks are due to the energy released by the electrons trapped in these oxygen vacancies. We have found that the energy states of Mn2 þ are also present at 2.8 eV, 2.63 eV and 2.53 eV, which are in the same range as that of oxygen vacancy levels. Therefore, the visible emission peaks are due to the energy transfer from the oxygen defects to Mn2 þ . The Mn2 þ emissions are also occurring at higher wavelength than the band gap of 2.81 eV observed in 2% Mn doping. As explained the band gap narrowing is due to the interaction of the dopant d-states with the host band electrons. Since, increase in electron concentration will increase the interaction and hence will narrows the band gap effectively. Therefore, the characteristic emission of Mn2 þ at 525 nm and 585 nm are at higher wavelength than its corresponding excitation and above band gap also. A schematic representation of the different emission peaks occurring at excitation of 320 nm and 390 nm are shown in Fig. 9. 3.7. Time resolved photoluminescence (TRPL) study The lifetime of the charge carriers for pure and doped TiO2 nanoparticles are studied. Fig. 10 depicts the lifetime of the charge carriers in pure and 6% Mn-doped TiO2 nanoparticles. A bi-exponential fitting is performed for the TRPL curves. The fitting follows the equation t t y ¼ y0 þ a1 exp þ a2 exp ð1Þ
t1
t2
Where y0 is the baseline correction (y-offset), t1 and t2 are the excited state emission decay time and a1 and a2 are the pre exponential factors [9,54]. After fitting (r2 ¼0.9994) the values come out to be 0.2803 (a1), 0.8107 (a2) and 1.2631 ns (t1) and 0.682 ns (t2) for pure TiO2 nanoparticles. For 6% Mn-doped TiO2 these values are 0.27627 (a1), 0.8646 (a2) and 2.295 ns (t1) and 0.3737 ns (t2) respectively. The average lifetime is calculated
Fig. 9. Schematic diagram showing emissions at an excitation of 320 (3.87 eV) nm and 390 nm (3.18 eV). (1) Excitation at 320 nm excites the electrons to the upper of the valence band. (a) Radiationless decay and transition of electrons to the bottom of conduction band giving UV emission. (b) The electrons jump to the defect states giving defect emissions. (c) Few electrons also jump to the nearby Mn d-states and from these states the electron recombines with the VB holes. (2) Excitation at 390 nm (3.18 eV) excites the electrons to the defect states and from these states few electrons give defect emission and others jump to Mn d-state giving Mn2 þ emissions.
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As dopant d-states interact with the conduction band edge, therefore relative shifting of the CB with respect to VB will be more and therefore their band edges will move further away from zone centers. Hence, the electrons and holes will have to take a longer time for undergoing recombination. From the analysis, and on doping these CB and VB edges are shifted relative to each other.
4. Conclusion
Fig. 10. Time resolved photoluminescence (TRPL) curves of TiO2 and 6% Mn-doped TiO2 nanoparticles. The TRPL curves of both samples are fitted bi-exponentially with a correctness r2 ¼ 0.9994.
using the equation [9,54].
tav ¼
a1 t21 þ a2 t22 a1 t1 þ a2 t2
ð2Þ
In this equation, the meaning of the parametersa1, t1 and a2, t2 are already explained in Eq. (1). The values of these parameters, obtained from Eq. (1), are inserted in Eq. (2) and obtained an average lifetime of 0.904 ns for pure and 1.61 ns for 6% Mn-doped TiO2 nanoparticles. Doped TiO2 has prolonged life period of the charge carriers compared to pure one. The long lifetime of the charge carriers can be explained considering different factors such as structural defects, mobility of the carriers, band structure, etc. In literature reports of pure TiO2, people have attributed the slow or longer lifetime component to be associated with recombination of excitons via oxygen defects [52,55]. Defects, either on surface or on bulk, act as active trap center or suppress the carrier recombination [56]. Due to the presence of these oxygen vacancies, the electrons do not directly jump to the valence band to recombine with holes. Instead, the electrons, below the bottom of CB, first captured by non-radiative oxygen vacancies and then the electrons undergo hopping from one defect to another until they find an emission center [52]. On doping the number of oxygen vacancies increases, and therefore more number of carriers are trapped in these defect centers. Therefore, as compared to pure one the electrons in the doped system now transit from one defect to another defect of Mn d-state until it finds an emission centers. Das et al., in Co doped TiO2 nanostructures, observed a longer lifetime of the carriers and attributed the long lifetime to the trapping of the carriers in the deep oxygen defect center for longer period [54]. There are reports where people have doped TiO2 with metal ions such as Nd, Pd, Pt, Fe, Cr, etc. They observed that d-states of these ions act as active trap centers and also generate electron acceptor centers and separate the charge carriers for longer period [57,58]. The oxygen vacancies, with trapped electrons, act as charged defects. Therefore, these charged oxygen vacancies interact with the mobile electrons and scatter them, thereby decreasing their mobility [52]. Decrease in mobility will separate the electrons-holes for longer time and hence increase the lifetime. The band structure of anatase TiO2 also influence in the increase of carrier lifetime. Since anatase has an indirect band gap, the conduction band electron and valence band hole will be in different position in the Brillouin zone [59].
In summary we can write that doping of Mn has a pronounced effect in the tailoring of the luminescence properties of TiO2 nanoparticles. Both Kubelka–Munk plot and photoexcitation spectroscopy confirms the presence of the absorption peaks of Mn2 þ . The intensity of the UV emission peak of TiO2 is decreased with the simultaneous increase in the peak width. Auger type non-radiative recombination and potential fluctuations of impurities result in the reduction and broadening of UV emission peak. Decrease in the intensity ratio of UV to defect emission band indicates degradation of structural quality of the material on doping. Doping also reduces the defect emission intensity ratio by increasing the non-radiative centers. The characteristic emission peak of Mn2 þ appears at 585 nm, associated with 4T1g(G) to 6 A1g(S) emission. Lifetime of the charge carriers increases in doped TiO2 due to the relaxation of the conduction band electrons in the oxygen defects and on the d-band states.
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