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Photoluminescence studies on Eu doped TiO2 nanoparticles
Journal of Alloys and Compounds 486 (2009) 864–870
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Photoluminescence studies on Eu doped TiO2 nanoparticles R.S. Ningthoujam a,∗ , V. Sudarsan a , R.K. Vatsa a , R.M. Kadam b , Jagannath c , A. Gupta d a
Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Radio Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India c Technical Physics & Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400085, India d UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 45201, India b
a r t i c l e
i n f o
Article history: Received 3 April 2009 Received in revised form 14 July 2009 Accepted 15 July 2009 Available online 23 July 2009 PACS: 70.20.Eh 71.20.Nr 71.55.Gs 78.55.−m Keywords: Titanium oxide Europium ion X-ray diffraction Photoluminescence
a b s t r a c t Eu3+ doped TiO2 nanoparticles were prepared by urea hydrolysis in ethylene glycol medium at low temperature of 150 ◦ C. X-ray diffraction study showed that anatase phase of tetragonal structure was formed below 500 ◦ C; and above this temperature, additional peaks due to rutile phase were also observed. From luminescence study, it was found that as prepared nanoparticles showed the enhanced luminescence intensity due to energy transfer from host to europium ions. However, photoluminescence from these nanoparticles was found to disappear when the samples were heated above 900 ◦ C. We established the origin of the reduction in the luminescence intensity from Eu3+ when doped in TiO2 and heated at 900 ◦ C. Based on detailed studies at different heat-treatment temperatures using techniques such as Xray diffraction, X-ray photoelectron spectroscopy, electron paramagnetic resonance, Raman spectroscopy, and Mössbauer spectroscopy, it has been established that formation of Eu2 Ti2 O7 phase, wherein Eu3+ ions occupy high symmetric environment (D3d ) and also reduced distance between Eu3+ and Eu3+ ions is responsible for the decrease/loss in the luminescence intensity. © 2009 Elsevier B.V. All rights reserved.
1. Introduction TiO2 is an excellent semiconducting material and is widely used in many applications such as gas sensors, solar cells, protective coatings, biological activities and photocatalyst [1–24]. Eu3+ doping in TiO2 has been carried out by several researchers in order to achieve an improved luminescence of Eu3+ by energy transfer from TiO2 to Eu3+ . Luminescence from nanoparticles and thin films of Eu3+ doped TiO2 showed the enhanced emission for samples heated below 400 ◦ C, whereas for samples heated at higher than 400 ◦ C, luminescence was found to decrease or even completely disappear at 900 ◦ C [17–30]. Different groups have suggested diverse explanations for this behavior. Rocha et al. [26] have suggested that changes in the crystalline structure of TiO2 are responsible for the decrease in PL intensity of Eu3+ in Eu doped TiO2 film with heat-treatment temperature from 400 to 500 ◦ C. In further work, Eu3+ emission disappears when Eu3+ doped TiO2 polycrystalline samples are heated at 1000 ◦ C and they suggested that it may be due to phase transition from anatase to rutile phase of TiO2 [27]. Merino et al. [28] suggested that PL
∗ Corresponding author. Tel.: +91 22 25592321; fax: +91 22 25505151. E-mail address: [email protected] (R.S. Ningthoujam). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.090
intensity becomes weaker after annealing Eu doped TiO2 film at 450 ◦ C due to thickness reduction of TiO2 film and the loss of Eu in the near-surface region. Gallardo et al. [29] suggested that reduction in Eu3+ emission intensity after annealing at 600 ◦ C in Eu doped TiO2 films is due to occupancy of different crystallographic sites in TiO2 amorphous matrix by Eu. Jia et al. [31] reported the decrease of luminescence intensity for 614 nm emission, characteristic of Eu3+ from Eu doped TiO2 film on Si wafer ion subjected to heat-treatment up to 900 ◦ C. They also observed an additional peak around 815 nm, whose intensity increases with increase in heat treatment temperature. This was attributed to the presence of Ti3+ ion in the sample. However this assignment is questionable as silicon nanoparticles/wafers are known to give an emission peak around 800 nm [32]. Moon et al. [33] also pointed out that Eu3+ emission intensity from TiO2 :Eu samples prepared at 500 ◦ C increases with Eu2 O3 content up to 20%. Lifetimes for 5 D0 level are found to vary in the range of 0.45–0.55 ms when Eu3+ concentration increases from 2 to 10 mol%. Very recently, Zhao et al. [34] prepared 2 mol% Eu doped TiO2 samples and incorporated different concentrations of Eu doped TiO2 in SiO2 matrix. Following are the three main observations made from this study. Maximum luminescence is observed from 80 mol% TiO2 :Eu incorporated silica sample prepared at 900 ◦ C. Extent of phase transformation from anatase to rutile increases as TiO2
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concentration increases. For 60 mol% TiO2 :Eu incorporated silica sample, luminescence intensity increases as annealing temperature increases from 600 to 900 ◦ C and then decreases above 900 ◦ C. They suggested that segregation of Eu2 O3 from TiO2 is responsible for the decrease in luminescence intensity above 900 ◦ C. However, no experimental evidence for presence of Eu2 O3 phase was provided. Recently, Li et al. [35] reported that Eu doped TiO2 and Eu2 Ti2 O7 samples prepared by Ar/O2 radio frequency (RF) thermal plasma oxidation show the bright red luminescence (5 D0 → 7 F2 ) at 614 nm. They also noticed that the maximum amount of Eu3+ which can occupy Ti4+ site in TiO2 is limited to 0.5 at.% and above this, Eu2 Ti2 O7 pyrochlore formation starts. Ideally, if Eu3+ ions occupy Ti4+ site (where inversion symmetry occurs) in TiO2 , it should only show the magnetic dipole transition (at 590 nm) and not the electrical dipole transition (at 614 nm). Similarly, for Eu2 Ti2 O7 , which has inversion symmetry at Eu centre with very short Eu3+ –Eu3+ distance of ∼3.6 Å, only magnetic dipole transition should be observed [36]. In view of the above arguments it is necessary to understand “decrease in photoluminescence intensity from Eu doped TiO2 heated at high temperatures of the order 900 ◦ C” [26–36]. In the present study, we established the actual reason for the decrease in luminescent intensity from Eu3+ doped TiO2 nanoparticles heated at high temperatures. 2. Experimental 2.1. Preparation Pure and 3, 5, 7 and 10 at.% Eu3+ doped TiO2 particles were prepared by urea hydrolysis in ethylene glycol medium at low temperature of 150 ◦ C for 4 h. The detailed method for preparation has been reported in our previous publications [25,37,38]. Nanoparticles having size in the range of 5–10 nm can be prepared using this method. As-prepared samples were heated at 500, 700 and 900 ◦ C for 4 h. 2.2. Characterization Room temperature X-ray diffraction (XRD) studies were carried out using a Philips powder X-ray diffractometer (model PW 1071) with Ni filtered Cu K␣ radiation. In situ high-temperature XRD studies were carried out on Philips (X’PERT model) using Tungsten as sample holder. The lattice parameters were calculated from the least square fitting of the diffraction peaks. The average crystallite size was calculated from the diffraction line-width based on Scherrer relation: d = 0.9/ˇ cos , where is the wavelength of X-rays and ˇ is the half maximum line width. All luminescence measurements unless specified were carried out at room temperature with a resolution of 3 nm, using a Hitachi Instrument (F-4500) having a 150 W Xe lamp as the excitation source. Powder samples (∼5 mg) were mixed with methanol, spread over a quartz plate, dried at 100 ◦ C and mounted inside the sample chamber for photoluminescence measurements. Steady state and time resolved luminescence measurements were carried out using Edinburgh Instruments’ luminescence set up (model FLSP 920) with 450 W xenon lamp as the excitation source and red sensitive PMT as the detector. For measuring 5 D0 lifetime of Eu3+ ion, samples were excited at 394 nm using a Nd:YAG laser pumped optical parametric oscillator (OPO) having a pulse width of 10 ns and repetition frequency of 10 Hz. X-ray photoelectron spectroscopy (XPS) was conducted in an electron spectrometer using Mg K␣ ray (h = 1253.6 eV) as the primary source of radiation. The binding energies were calculated with respect to the C1s = 284.6 eV. 151 Eu Mössbauer studies have been carried out at room temperature using a home made Mössbauer spectrometer. Electron paramagnetic resonance (EPR) spectra were recorded on BRUKER ESP 300 spectrometer operated at X band frequency (9.60 GHz). The EPR spectra were recorded at room and liquid nitrogen temperatures. 100 kHz frequency modulation was used for recording the EPR spectra. 1,1-Diphenyl-2-picryl hydrazyl (DPPH) was used for calibration of g values.
3. Results and discussion 3.1. X-ray diffraction study As-prepared samples of pure and 3, 5, 7 and 10 at.% Eu3+ doped TiO2 particles show a broad peak corresponding to anatase phase.
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Fig. 1. XRD patterns of as prepared, 500 and 900 ◦ C heat treated samples of 5 at.% Eu doped TiO2 .
Fig. 1(a) shows the representative XRD pattern of 5 at.% Eu3+ doped TiO2 particles. It shows a broad peak centered around 2 = 25.31◦ corresponding to anatase phase. Fig. 1(b) shows the representative XRD pattern of 500 ◦ C heated 5 at.% Eu3+ doped TiO2 particles. Similarly, 3, 7 and 10 at.% Eu3+ doped TiO2 particles show the anatase phase of TiO2 (see Supplementary materials S1). It should be noted that in these XRD patterns, peaks corresponding to phase segregation of Eu2 O3 /EuO could not be observed. Lattice parameters of anatase phase are found to be a = 3.780(2) and c = 9.544(2) Å. The reported lattice parameters of anatase phase are a = 3.785 and c = 9.514 Å (JCPDS 18-0507). Fig. 1(c) shows the representative XRD pattern of 900 ◦ C heated 5 at.% Eu3+ doped TiO2 particles. Rutile phase of TiO2 along with cubic phase of Eu2 Ti2 O7 can be clearly seen. Measured lattice parameters of rutile phase (TiO2 ) are a = 4.592(2) and c = 2.959(2) Å which are in excellent agreement with the reported values, a = 4.593 and c = 2.959 Å (JCPDS 21-1276). There is no change in lattice parameters for rutile phase within error limits. Measured lattice parameter of cubic phase Eu2 Ti2 O7 is a = 10.196(2) Å which is in very good agreement with the reported value of a = 10.193 Å (JCPDS 23-1072). From the lattice parameter values, it is inferred that Eu3+ /Eu2+ ions do not occupy Ti4+ sites. This is due to large difference in ionic sizes. Based on co-ordination number (CN = 6), ionic sizes of Eu3+ , Eu2+ and Ti4+ are 0.95, 1.17 and 0.61 Å respectively [39]. However, in the case of 7 and 10 at.% Eu doped TiO2 , anatase phase could also be observed in addition to the rutile phase of TiO2 and Eu2 Ti2 O7 (see Supplementary materials S2). During heat treatment, there could be a possibility of EuO formation. In order to confirm this, the peak positions observed in the standard XRD patterns of EuO (JCPDS 18-0507) and Eu2 Ti2 O7 (JCPDS 23-1072) are shown in the XRD patterns of 5 and 10 at.% Eu doped TiO2 samples annealed at 900 ◦ C (Fig. 2). We can clearly see the presence of Eu2 Ti2 O7 in addition to TiO2 phases (anatase and rutile) in the sample. Significant amount of anatase phase TiO2 can be observed in cases of higher Eu doping in TiO2 . It was reported that anatase phase of TiO2 can be stabilized by addition of foreign materials [40]. Based on the fact that XRD patterns do not show peaks corresponding to EuO, we rule out the formation of EuO in the Eu doped TiO2 . This is further supported by the thermodynamic data which shows that EuO cannot be formed from Eu2 O3 by heating in
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Fig. 2. XRD patterns of 900 ◦ C heat treated samples of 5 and 10 at.% Eu doped TiO2 compared with standard patterns of EuO and Eu2 Ti2 O7 .
presence of air/oxygen up to ∼2000 ◦ C [Ref.: FactSage 5.0 Copyright Thermfact 1976–2001]. We also carried out in situ high temperature XRD studies at different temperatures from 500 to 1200 ◦ C for the 10 at.% Eu doped TiO2 sample (Fig. 3). The starting temperature of 500 ◦ C was chosen to make sure that all the hydrocarbons attached to TiO2 are completely removed. With increasing temperature from 500 to 700 ◦ C, one can see only anatase phase with a slight lattice expansion. At 900 ◦ C, a mixture of anatase and rutile phases of TiO2 along with Eu2 Ti2 O7 is observed. At 1200 ◦ C, a mixture of rutile phase of TiO2 and Eu2 Ti2 O7 has been observed with a slight lattice expansion of
Fig. 3. In situ high temperature XRD patterns of 10 at.% Eu doped TiO2 prepared at 500 ◦ C. The symbol (*) indicates the peak positions of XRD patterns corresponding to Pt, which is used as a sample holder. Only main peaks are assigned. The symbols ‘S’, ‘A’ and ‘R’ indicate Eu2 Ti2 O7 , anatase and rutile phases of TiO2 respectively. It is to be noted that the peaks for (b–e) in situ high temperature samples are shifted towards the lower 2 as compared to those for (a and f) room temperature samples because of thermal expansion.
Fig. 4. XPS spectra of Ti for TiO2 and 5 at.% Eu doped TiO2 obtained at 500 and 900 ◦ C.
both the phases. After cooling from 1200 ◦ C to room temperature, a mixture of rutile phase of TiO2 and Eu2 Ti2 O7 remains with a slight lattice contraction. Since EuO or Eu2 O3 could not be observed, these results establish that Eu2 TiO2 phase is formed at higher temperature irrespective of the Eu concentration in TiO2 . 3.2. X-ray photoelectron spectroscopy study Figs. 4–6 show the XPS patterns corresponding to different levels of europium, titanium and oxygen present in 5 at.% Eu doped TiO2 sample heated at 500 and 900 ◦ C. For the purpose of comparison XPS patterns from pure TiO2 sample heated at 500 and 900 ◦ C are also shown in the same figure. Binding energies of Ti: 2p3/2 and 2p1/2 are found to be 457.7 and 463.6 eV, respectively for 500 ◦ C heat-treated pure TiO2 ; while for 900 ◦ C heat-treated pure TiO2 the respective binding energies are shifted to higher energy by 0.5 eV. Binding energy of O: 1s(L) is found to be 529.2 eV for 500 ◦ C heattreated pure TiO2 ; while for 900 ◦ C heat-treated pure TiO2 binding energy is shifted to higher energy by 0.5 eV. This has been attributed to improvement in crystallinity and phase transition from anatase (500 ◦ C) to rutile (900 ◦ C). Reported binding energy values of Ti: 2p3/2 and 2p1/2 for TiO2 obtained at 500 ◦ C for 6 h are 458.3 and 464.0 eV, respectively and O: 1s(L) for same sample is 529.5 eV [41]. The binding energies of Ti: 2p3/2 and 2p1/2 for the 500 ◦ C heattreated TiO2 :Eu (5 at.%) are 458.4 and 464.3 eV, respectively, but these values are higher than the respective values for 500 and 900 ◦ C heat-treated pure TiO2 . Binding energy of O: 1s in the case of 500 ◦ C heat-treated TiO2 :Eu (5 at.%) is 530.1 eV, which is higher than those of 500 and 900 ◦ C heat-treated TiO2 . For 500 ◦ C heat-treated TiO2 :Eu, the presence of Eu3+ and Eu2+ ions can be clearly observed, i.e. binding energies of Eu2+ states: 4d5/2 and 4d3/2 are found to be 128.6 and 134.0 eV, respectively; whereas binding energies of Eu3+ states: 4d5/2 and 4d3/2 are found to be 135.7 and 141.2 eV, respectively. Relative ratio of Eu3+ to Eu2+ is found to be 1.5:1.0 after fitting the data of the individual peaks in the XPS spectrum indicating higher amount of Eu3+ ions present in sample. For 900 ◦ C heated sample, the presence of Eu2+ is significantly reduced. The binding
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Fig. 7. PL spectra of as-prepared, 500 and 900 ◦ C heat-treated samples of 5 at.% Eu doped TiO2 after excitation at 394 nm.
Fig. 5. XPS spectra of O for TiO2 and 5 at.% Eu doped TiO2 obtained at 500 and 900 ◦ C.
energy values of 4d5/2 and 4d3/2 levels of Eu3+ ions have been found to increase for 900 ◦ C heated sample compared to those for 500 ◦ C indicating the formation of Eu–O–Ti in the sample. The details of the binding energy values corresponding to different levels of Eu, Ti and O from these samples along with the standard Ti and Eu compounds are shown in S8 of the Supplementary materials. Chemical bond formation such as Eu–O–Ti was reported in Eu doped TiO2 [41]. It was also reported that binding energies of Ti and O were slightly increased when amount of Eu3+ ions in Ba1−x Eux TiO3 increases [42].
Fig. 6. XPS spectra of Eu for TiO2 and 5 at.% Eu doped TiO2 obtained at 500 and 900 ◦ C.
Based on these results and the binding energy values reported for Eu3+ , Ti and O in different Ti and Eu compounds [41–43], formation of Eu–O–Ti bond in both 500 and 900 ◦ C heated samples has been confirmed. 3.3. Photoluminescence study Fig. 7shows the representative PL spectra of as-prepared, 500 and 900 ◦ C heat-treated samples for 5 at.% Eu doped TiO2 after excitation at 395 nm. As-prepared and 500 ◦ C heat-treated samples show the emission peaks at 590 and 614 nm, which are due to f–f transitions, i.e. magnetic (5 D0 → 7 F1 ) and electrical (5 D0 → 7 F2 ) dipole transitions respectively. However, these emission peaks could not be observed once the sample was heated to 900 ◦ C. Similar observations were made for other samples containing 3, 7 and 10 at.% Eu3+ doped TiO2 (see Supplementary materials S3–S5). In our XRD and XPS studies, there is no peak corresponding to Eu2+ ions in the form of EuO for 900 ◦ C heat-treated sample. From XPS studies, it is found that there is a bond formation such as Eu–O–Ti
Fig. 8. Emission spectra of 3 at.% Eu doped TiO2 prepared at different temperatures 500, 700 and 900 ◦ C for 4 h each and also pure TiO2 prepared at 900 ◦ C for 4 h after correction of grating response. Excitation wavelength is at 394 nm. Inset shows emission spectrum of 3 at.% Eu doped TiO2 prepared at 900 ◦ C after excitation at 394 nm using Nd:YAG pumped OPO (LASER).
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Fig. 10. Decay for 5 D0 level of Eu3+ for 500, 700 and 900 ◦ C heat-treated 3 at.% Eu doped TiO2 nanoparticles along with 900 ◦ C heated Eu2 O3 .
Fig. 9. Emission spectra of 3 at.% Eu doped TiO2 prepared at different temperatures 500, 700 and 900 ◦ C for 4 h each and also pure TiO2 prepared at 900 ◦ C for 4 h after correcting for grating response. Excitation wavelength is at 325 nm.
in case of 500 and 900 ◦ C heat-treated samples. Further, for 900 ◦ C heated sample, Eu2 Ti2 O7 formation is confirmed by XRD studies. In a previous report [36], it was shown that the highly symmetric structure of Eu2 Ti2 O7 does not show the electric dipole transition at 614 nm. In order to see even the magnetic dipole transition at 590 nm, Eu2 Ti2 O7 was excited using laser source, but still luminescence intensity was found to be very week [36]. Thus, because of the formation of highly symmetric Eu2 Ti2 O7 , luminescence intensity is found to disappear completely in our experiments. Distance between Eu3+ centers in Eu2 Ti2 O7 is reported to be 3.6 Å [36]. This short distance could lead to cross relaxation between Eu3+ ions, thereby significantly reducing the luminescence intensity. Additionally, Eu3+ ions occupy site having inversion symmetry in
Eu2 Ti2 O7 which makes the electric dipole transitions forbidden. Thus, the origin for decrease in luminescence intensity is formation of highly symmetric compound (Eu2 Ti2 O7 ) in which Eu3+ ions occupy inversion symmetry as well as the reduced distance between Eu3+ centers. Our finding also explains earlier observation [26–30], where luminescence intensity decreases with heat treatment above 500 ◦ C. Fig. 8 shows emission spectra obtained after the excitation at 394 nm for 3 at.% Eu doped TiO2 prepared at different temperatures namely 500, 700 and 900 ◦ C along with pure TiO2 prepared at 900 ◦ C using 450 W Xe lamp. All the samples were heated for 4 h at the respective temperatures. Emission intensity at 614 nm due to Eu3+ gradually decreases with increase in heat-treatment temperatures. At 900 ◦ C, emission intensity due to Eu3+ is almost negligible. Also, no emission due to Eu3+ is found even after excitation at 394 nm using Nd:YAG pumped optical parametric oscillator (OPO) (inset of Fig. 8). Jia et al. [31] reported that Ti3+ ions are characterized by an emission peak around 815 nm after excitation at 325 nm for Eu doped TiO2 film using Si substrate, however, we fail to see any emission at this wavelength or even in the range 600–850 nm (Fig. 9 and see Supplementary materials S6). It may be that emission peak at 815 nm comes from Si-substrate used to deposit the film [31] as
Fig. 11. Mössbauer spectra of (a) Eu2 O3 and (b) as-prepared, (c) 500 ◦ C and 900 ◦ C heat-treated 5 at% Eu doped TiO2 nanoparticles.
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respectively with corresponding line width 2.7 and 4.2 mm. This establishes that only Eu3+ state exists in 500 and 900 ◦ C annealed samples. This is also supported by the XPS and XRD results discussed above. 3.5. Raman study Fig. 12 shows the Raman spectra of 500 and 900 ◦ C heated 3 at.% Eu doped TiO2 and 900 ◦ C heated 10 at.% Eu doped TiO2 sample. The peaks at 145, 196, 396, 514, 640 cm−1 are obtained from 500 ◦ C heated 3 at.% Eu doped TiO2 sample and correspond to anatase phase of TiO2 [34,45]. This corroborates the XRD study, where anatase phase is detected (Fig. 1). In addition to above peaks, 900 ◦ C heated 3 at.% Eu doped TiO2 sample shows peaks at 445 and 608 cm−1 corresponding to rutile phase of TiO2 [34,45]. An additional peak at 308 cm−1 is also observed and this corresponds to the presence of Eu2 Ti2 O7 phase [34]. The peak intensity at 308 nm increases with increasing Eu concentration. No peaks characteristic of Eu2 O3 or EuO are observed in the Raman spectra of even TiO2 :Eu sample containing highest concentration of Europium (10 at.%) heated at 900 ◦ C. Thus, the Raman studies very clearly establish formation of Eu2 Ti2 O7 phase during heat-treatment of TiO2 :Eu samples rather than the phase segregation of Eu2 O3 or EuO phases. 3.6. Electron paramagnetic resonance study Fig. 12. Raman Spectra of 500 and 900 ◦ C heat-treated 3 at.% Eu doped TiO2 nanoparticles along with 900 ◦ C heated 10 at.% Eu doped TiO2 .
silicon nanoparticles/wafers are known to give an emission peak around 800 nm [32]. While heating, hydrocarbon moiety comes out, and a TiO2 film becomes thinner or form islands resulting in the exposure of the Si surface to the excitation light. This is also supported by IR and XRD studies in Jia et al.’s paper. Decays for 5 D0 level of Eu3+ for 500, 700 and 900 ◦ C heattreated 3 at.% Eu doped TiO2 nanoparticles along with 900 ◦ C heated Eu2 O3 are carried out using a pulsed laser source at 394 nm excitation and emission monitored at 615 nm (Fig. 10). It is found that intensity of 900 ◦ C heated 3 at.% Eu doped TiO2 is almost in based line as compared to Eu2 O3 . If Eu2 O3 phase is segregated during heat-treatment, we should see intensity from 900 ◦ C heated 3 at.% Eu doped TiO2 and its intensity will be comparable with that of Eu2 O3 prepared by the same procedure and heated at 900 ◦ C. But this is not observed. Based on this, it is concluded that no phase segregation of Eu2 O3 occurs in these samples; instead, another phase between Eu2 O3 and TiO2 , i.e. Eu2 Ti2 O7 is formed (supported by XRD and see Supplementary materials S7). Raman studies on these samples which are discussion later further support the above conclusions. 3.4. Mössbauer study Fig. 11 shows the Eu Mössbauer spectra of as-prepared, 500 and 900 ◦ C heat-treated 5 at.% Eu doped TiO2 nanoparticles. For comparison, the Mössbauer spectrum of pure Eu2 O3 is also shown. All the Mössbauer parameters are calculated with respect to Eu2 O3 where isomer shift is kept with 0.0 mm/s. Asprepared sample shows the two peaks corresponding to Eu3+ (isomer shift = −0.62 mm/s, width = 2.2 mm) and Eu2+ (isomer shift = −13.04 mm/s, width = 5.9 mm). Reported values of isomer shift in Eu3+ and Eu2+ are −0.6 and −13.7 mm/s respectively [44]. This is in conformity with the inferences drawn from XPS studies. Sample annealed at 500 and 900 ◦ C shows one peak corresponding to Eu3+ with isomer shift values −0.48 mm/s and −0.64 mm/s
EPR study reveals that Eu doped TiO2 samples shows the presence of Ti3+ in both 500 and 900 ◦ C heated samples. 10 at.% Eu doped TiO2 sample heated at 500 ◦ C shows signals with axial symmetry at g⊥ = 1.990 and g|| = 1.959. The g values are found to be almost independent of the concentration of the dopant ions and temperature of recording (300 and 80 K), however the intensity of perpendicular component (g⊥ = 1.990) is found to decrease (nearly half) with increase in the dopant ion concentration. This g values are related to Ti3+ present in anatase phase of TiO2 [23,46]. However, when these samples are annealed at 900 ◦ C, they show EPR signals at g⊥ = 1.995 and g|| = 1.930. However, the presence of Ti3+ could not be detected in 900 ◦ C heated samples from XPS and XRD studies. It is due to high sensitivity of EPR compared to those of XPS and XRD instruments. Even though EPR data shows the presence of Ti3+ for Eu doped TiO2 samples heated at 500 and 900 ◦ C, we are fail to see luminescence peaks corresponding to Ti3+ . 4. Conclusions TiO2 samples doped with different Eu concentrations are prepared using ethylene glycol method. Up to 500 ◦ C, samples crystallize in with anatase phase and above this temperature, mixture of anatase and rutile phases of TiO2 is formed. Conversion from anatase to rutile phase is completed at 1200 ◦ C. In addition to this, Eu2 Ti2 O7 phase is formed at 900 ◦ C. For Eu2 Ti2 O7 , Eu3+ ions occupy highly symmetry site (D3d ). There is no presence of Eu2+ or EuO for 900 ◦ C heated samples. Interestingly, luminescence intensity decreases with heat-treatment from 500 to 900 ◦ C. Decrease/complete disappearance of photoluminescence of Eu3+ ions in Eu3+ doped TiO2 systems when heated above 500 ◦ C is due to formation of a highly symmetric structure of Eu2 Ti2 O7 . Acknowledgements Authors thank Dr. D. Das, Dr. A.K. Tyagi, Dr. M. Pandey, and Mr. R. Shukla, BARC for their help and encouragement.
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Photoluminescence studies on Eu doped TiO2 nanoparticles R.S. Ningthoujam a,∗ , V. Sudarsan a , R.K. Vatsa a , R.M. Kadam b , Jagannath c , A. Gupta d a
Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Radio Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India c Technical Physics & Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400085, India d UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 45201, India b
a r t i c l e
i n f o
Article history: Received 3 April 2009 Received in revised form 14 July 2009 Accepted 15 July 2009 Available online 23 July 2009 PACS: 70.20.Eh 71.20.Nr 71.55.Gs 78.55.−m Keywords: Titanium oxide Europium ion X-ray diffraction Photoluminescence
a b s t r a c t Eu3+ doped TiO2 nanoparticles were prepared by urea hydrolysis in ethylene glycol medium at low temperature of 150 ◦ C. X-ray diffraction study showed that anatase phase of tetragonal structure was formed below 500 ◦ C; and above this temperature, additional peaks due to rutile phase were also observed. From luminescence study, it was found that as prepared nanoparticles showed the enhanced luminescence intensity due to energy transfer from host to europium ions. However, photoluminescence from these nanoparticles was found to disappear when the samples were heated above 900 ◦ C. We established the origin of the reduction in the luminescence intensity from Eu3+ when doped in TiO2 and heated at 900 ◦ C. Based on detailed studies at different heat-treatment temperatures using techniques such as Xray diffraction, X-ray photoelectron spectroscopy, electron paramagnetic resonance, Raman spectroscopy, and Mössbauer spectroscopy, it has been established that formation of Eu2 Ti2 O7 phase, wherein Eu3+ ions occupy high symmetric environment (D3d ) and also reduced distance between Eu3+ and Eu3+ ions is responsible for the decrease/loss in the luminescence intensity. © 2009 Elsevier B.V. All rights reserved.
1. Introduction TiO2 is an excellent semiconducting material and is widely used in many applications such as gas sensors, solar cells, protective coatings, biological activities and photocatalyst [1–24]. Eu3+ doping in TiO2 has been carried out by several researchers in order to achieve an improved luminescence of Eu3+ by energy transfer from TiO2 to Eu3+ . Luminescence from nanoparticles and thin films of Eu3+ doped TiO2 showed the enhanced emission for samples heated below 400 ◦ C, whereas for samples heated at higher than 400 ◦ C, luminescence was found to decrease or even completely disappear at 900 ◦ C [17–30]. Different groups have suggested diverse explanations for this behavior. Rocha et al. [26] have suggested that changes in the crystalline structure of TiO2 are responsible for the decrease in PL intensity of Eu3+ in Eu doped TiO2 film with heat-treatment temperature from 400 to 500 ◦ C. In further work, Eu3+ emission disappears when Eu3+ doped TiO2 polycrystalline samples are heated at 1000 ◦ C and they suggested that it may be due to phase transition from anatase to rutile phase of TiO2 [27]. Merino et al. [28] suggested that PL
∗ Corresponding author. Tel.: +91 22 25592321; fax: +91 22 25505151. E-mail address: [email protected] (R.S. Ningthoujam). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.090
intensity becomes weaker after annealing Eu doped TiO2 film at 450 ◦ C due to thickness reduction of TiO2 film and the loss of Eu in the near-surface region. Gallardo et al. [29] suggested that reduction in Eu3+ emission intensity after annealing at 600 ◦ C in Eu doped TiO2 films is due to occupancy of different crystallographic sites in TiO2 amorphous matrix by Eu. Jia et al. [31] reported the decrease of luminescence intensity for 614 nm emission, characteristic of Eu3+ from Eu doped TiO2 film on Si wafer ion subjected to heat-treatment up to 900 ◦ C. They also observed an additional peak around 815 nm, whose intensity increases with increase in heat treatment temperature. This was attributed to the presence of Ti3+ ion in the sample. However this assignment is questionable as silicon nanoparticles/wafers are known to give an emission peak around 800 nm [32]. Moon et al. [33] also pointed out that Eu3+ emission intensity from TiO2 :Eu samples prepared at 500 ◦ C increases with Eu2 O3 content up to 20%. Lifetimes for 5 D0 level are found to vary in the range of 0.45–0.55 ms when Eu3+ concentration increases from 2 to 10 mol%. Very recently, Zhao et al. [34] prepared 2 mol% Eu doped TiO2 samples and incorporated different concentrations of Eu doped TiO2 in SiO2 matrix. Following are the three main observations made from this study. Maximum luminescence is observed from 80 mol% TiO2 :Eu incorporated silica sample prepared at 900 ◦ C. Extent of phase transformation from anatase to rutile increases as TiO2
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concentration increases. For 60 mol% TiO2 :Eu incorporated silica sample, luminescence intensity increases as annealing temperature increases from 600 to 900 ◦ C and then decreases above 900 ◦ C. They suggested that segregation of Eu2 O3 from TiO2 is responsible for the decrease in luminescence intensity above 900 ◦ C. However, no experimental evidence for presence of Eu2 O3 phase was provided. Recently, Li et al. [35] reported that Eu doped TiO2 and Eu2 Ti2 O7 samples prepared by Ar/O2 radio frequency (RF) thermal plasma oxidation show the bright red luminescence (5 D0 → 7 F2 ) at 614 nm. They also noticed that the maximum amount of Eu3+ which can occupy Ti4+ site in TiO2 is limited to 0.5 at.% and above this, Eu2 Ti2 O7 pyrochlore formation starts. Ideally, if Eu3+ ions occupy Ti4+ site (where inversion symmetry occurs) in TiO2 , it should only show the magnetic dipole transition (at 590 nm) and not the electrical dipole transition (at 614 nm). Similarly, for Eu2 Ti2 O7 , which has inversion symmetry at Eu centre with very short Eu3+ –Eu3+ distance of ∼3.6 Å, only magnetic dipole transition should be observed [36]. In view of the above arguments it is necessary to understand “decrease in photoluminescence intensity from Eu doped TiO2 heated at high temperatures of the order 900 ◦ C” [26–36]. In the present study, we established the actual reason for the decrease in luminescent intensity from Eu3+ doped TiO2 nanoparticles heated at high temperatures. 2. Experimental 2.1. Preparation Pure and 3, 5, 7 and 10 at.% Eu3+ doped TiO2 particles were prepared by urea hydrolysis in ethylene glycol medium at low temperature of 150 ◦ C for 4 h. The detailed method for preparation has been reported in our previous publications [25,37,38]. Nanoparticles having size in the range of 5–10 nm can be prepared using this method. As-prepared samples were heated at 500, 700 and 900 ◦ C for 4 h. 2.2. Characterization Room temperature X-ray diffraction (XRD) studies were carried out using a Philips powder X-ray diffractometer (model PW 1071) with Ni filtered Cu K␣ radiation. In situ high-temperature XRD studies were carried out on Philips (X’PERT model) using Tungsten as sample holder. The lattice parameters were calculated from the least square fitting of the diffraction peaks. The average crystallite size was calculated from the diffraction line-width based on Scherrer relation: d = 0.9/ˇ cos , where is the wavelength of X-rays and ˇ is the half maximum line width. All luminescence measurements unless specified were carried out at room temperature with a resolution of 3 nm, using a Hitachi Instrument (F-4500) having a 150 W Xe lamp as the excitation source. Powder samples (∼5 mg) were mixed with methanol, spread over a quartz plate, dried at 100 ◦ C and mounted inside the sample chamber for photoluminescence measurements. Steady state and time resolved luminescence measurements were carried out using Edinburgh Instruments’ luminescence set up (model FLSP 920) with 450 W xenon lamp as the excitation source and red sensitive PMT as the detector. For measuring 5 D0 lifetime of Eu3+ ion, samples were excited at 394 nm using a Nd:YAG laser pumped optical parametric oscillator (OPO) having a pulse width of 10 ns and repetition frequency of 10 Hz. X-ray photoelectron spectroscopy (XPS) was conducted in an electron spectrometer using Mg K␣ ray (h = 1253.6 eV) as the primary source of radiation. The binding energies were calculated with respect to the C1s = 284.6 eV. 151 Eu Mössbauer studies have been carried out at room temperature using a home made Mössbauer spectrometer. Electron paramagnetic resonance (EPR) spectra were recorded on BRUKER ESP 300 spectrometer operated at X band frequency (9.60 GHz). The EPR spectra were recorded at room and liquid nitrogen temperatures. 100 kHz frequency modulation was used for recording the EPR spectra. 1,1-Diphenyl-2-picryl hydrazyl (DPPH) was used for calibration of g values.
3. Results and discussion 3.1. X-ray diffraction study As-prepared samples of pure and 3, 5, 7 and 10 at.% Eu3+ doped TiO2 particles show a broad peak corresponding to anatase phase.
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Fig. 1. XRD patterns of as prepared, 500 and 900 ◦ C heat treated samples of 5 at.% Eu doped TiO2 .
Fig. 1(a) shows the representative XRD pattern of 5 at.% Eu3+ doped TiO2 particles. It shows a broad peak centered around 2 = 25.31◦ corresponding to anatase phase. Fig. 1(b) shows the representative XRD pattern of 500 ◦ C heated 5 at.% Eu3+ doped TiO2 particles. Similarly, 3, 7 and 10 at.% Eu3+ doped TiO2 particles show the anatase phase of TiO2 (see Supplementary materials S1). It should be noted that in these XRD patterns, peaks corresponding to phase segregation of Eu2 O3 /EuO could not be observed. Lattice parameters of anatase phase are found to be a = 3.780(2) and c = 9.544(2) Å. The reported lattice parameters of anatase phase are a = 3.785 and c = 9.514 Å (JCPDS 18-0507). Fig. 1(c) shows the representative XRD pattern of 900 ◦ C heated 5 at.% Eu3+ doped TiO2 particles. Rutile phase of TiO2 along with cubic phase of Eu2 Ti2 O7 can be clearly seen. Measured lattice parameters of rutile phase (TiO2 ) are a = 4.592(2) and c = 2.959(2) Å which are in excellent agreement with the reported values, a = 4.593 and c = 2.959 Å (JCPDS 21-1276). There is no change in lattice parameters for rutile phase within error limits. Measured lattice parameter of cubic phase Eu2 Ti2 O7 is a = 10.196(2) Å which is in very good agreement with the reported value of a = 10.193 Å (JCPDS 23-1072). From the lattice parameter values, it is inferred that Eu3+ /Eu2+ ions do not occupy Ti4+ sites. This is due to large difference in ionic sizes. Based on co-ordination number (CN = 6), ionic sizes of Eu3+ , Eu2+ and Ti4+ are 0.95, 1.17 and 0.61 Å respectively [39]. However, in the case of 7 and 10 at.% Eu doped TiO2 , anatase phase could also be observed in addition to the rutile phase of TiO2 and Eu2 Ti2 O7 (see Supplementary materials S2). During heat treatment, there could be a possibility of EuO formation. In order to confirm this, the peak positions observed in the standard XRD patterns of EuO (JCPDS 18-0507) and Eu2 Ti2 O7 (JCPDS 23-1072) are shown in the XRD patterns of 5 and 10 at.% Eu doped TiO2 samples annealed at 900 ◦ C (Fig. 2). We can clearly see the presence of Eu2 Ti2 O7 in addition to TiO2 phases (anatase and rutile) in the sample. Significant amount of anatase phase TiO2 can be observed in cases of higher Eu doping in TiO2 . It was reported that anatase phase of TiO2 can be stabilized by addition of foreign materials [40]. Based on the fact that XRD patterns do not show peaks corresponding to EuO, we rule out the formation of EuO in the Eu doped TiO2 . This is further supported by the thermodynamic data which shows that EuO cannot be formed from Eu2 O3 by heating in
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Fig. 2. XRD patterns of 900 ◦ C heat treated samples of 5 and 10 at.% Eu doped TiO2 compared with standard patterns of EuO and Eu2 Ti2 O7 .
presence of air/oxygen up to ∼2000 ◦ C [Ref.: FactSage 5.0 Copyright Thermfact 1976–2001]. We also carried out in situ high temperature XRD studies at different temperatures from 500 to 1200 ◦ C for the 10 at.% Eu doped TiO2 sample (Fig. 3). The starting temperature of 500 ◦ C was chosen to make sure that all the hydrocarbons attached to TiO2 are completely removed. With increasing temperature from 500 to 700 ◦ C, one can see only anatase phase with a slight lattice expansion. At 900 ◦ C, a mixture of anatase and rutile phases of TiO2 along with Eu2 Ti2 O7 is observed. At 1200 ◦ C, a mixture of rutile phase of TiO2 and Eu2 Ti2 O7 has been observed with a slight lattice expansion of
Fig. 3. In situ high temperature XRD patterns of 10 at.% Eu doped TiO2 prepared at 500 ◦ C. The symbol (*) indicates the peak positions of XRD patterns corresponding to Pt, which is used as a sample holder. Only main peaks are assigned. The symbols ‘S’, ‘A’ and ‘R’ indicate Eu2 Ti2 O7 , anatase and rutile phases of TiO2 respectively. It is to be noted that the peaks for (b–e) in situ high temperature samples are shifted towards the lower 2 as compared to those for (a and f) room temperature samples because of thermal expansion.
Fig. 4. XPS spectra of Ti for TiO2 and 5 at.% Eu doped TiO2 obtained at 500 and 900 ◦ C.
both the phases. After cooling from 1200 ◦ C to room temperature, a mixture of rutile phase of TiO2 and Eu2 Ti2 O7 remains with a slight lattice contraction. Since EuO or Eu2 O3 could not be observed, these results establish that Eu2 TiO2 phase is formed at higher temperature irrespective of the Eu concentration in TiO2 . 3.2. X-ray photoelectron spectroscopy study Figs. 4–6 show the XPS patterns corresponding to different levels of europium, titanium and oxygen present in 5 at.% Eu doped TiO2 sample heated at 500 and 900 ◦ C. For the purpose of comparison XPS patterns from pure TiO2 sample heated at 500 and 900 ◦ C are also shown in the same figure. Binding energies of Ti: 2p3/2 and 2p1/2 are found to be 457.7 and 463.6 eV, respectively for 500 ◦ C heat-treated pure TiO2 ; while for 900 ◦ C heat-treated pure TiO2 the respective binding energies are shifted to higher energy by 0.5 eV. Binding energy of O: 1s(L) is found to be 529.2 eV for 500 ◦ C heattreated pure TiO2 ; while for 900 ◦ C heat-treated pure TiO2 binding energy is shifted to higher energy by 0.5 eV. This has been attributed to improvement in crystallinity and phase transition from anatase (500 ◦ C) to rutile (900 ◦ C). Reported binding energy values of Ti: 2p3/2 and 2p1/2 for TiO2 obtained at 500 ◦ C for 6 h are 458.3 and 464.0 eV, respectively and O: 1s(L) for same sample is 529.5 eV [41]. The binding energies of Ti: 2p3/2 and 2p1/2 for the 500 ◦ C heattreated TiO2 :Eu (5 at.%) are 458.4 and 464.3 eV, respectively, but these values are higher than the respective values for 500 and 900 ◦ C heat-treated pure TiO2 . Binding energy of O: 1s in the case of 500 ◦ C heat-treated TiO2 :Eu (5 at.%) is 530.1 eV, which is higher than those of 500 and 900 ◦ C heat-treated TiO2 . For 500 ◦ C heat-treated TiO2 :Eu, the presence of Eu3+ and Eu2+ ions can be clearly observed, i.e. binding energies of Eu2+ states: 4d5/2 and 4d3/2 are found to be 128.6 and 134.0 eV, respectively; whereas binding energies of Eu3+ states: 4d5/2 and 4d3/2 are found to be 135.7 and 141.2 eV, respectively. Relative ratio of Eu3+ to Eu2+ is found to be 1.5:1.0 after fitting the data of the individual peaks in the XPS spectrum indicating higher amount of Eu3+ ions present in sample. For 900 ◦ C heated sample, the presence of Eu2+ is significantly reduced. The binding
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Fig. 7. PL spectra of as-prepared, 500 and 900 ◦ C heat-treated samples of 5 at.% Eu doped TiO2 after excitation at 394 nm.
Fig. 5. XPS spectra of O for TiO2 and 5 at.% Eu doped TiO2 obtained at 500 and 900 ◦ C.
energy values of 4d5/2 and 4d3/2 levels of Eu3+ ions have been found to increase for 900 ◦ C heated sample compared to those for 500 ◦ C indicating the formation of Eu–O–Ti in the sample. The details of the binding energy values corresponding to different levels of Eu, Ti and O from these samples along with the standard Ti and Eu compounds are shown in S8 of the Supplementary materials. Chemical bond formation such as Eu–O–Ti was reported in Eu doped TiO2 [41]. It was also reported that binding energies of Ti and O were slightly increased when amount of Eu3+ ions in Ba1−x Eux TiO3 increases [42].
Fig. 6. XPS spectra of Eu for TiO2 and 5 at.% Eu doped TiO2 obtained at 500 and 900 ◦ C.
Based on these results and the binding energy values reported for Eu3+ , Ti and O in different Ti and Eu compounds [41–43], formation of Eu–O–Ti bond in both 500 and 900 ◦ C heated samples has been confirmed. 3.3. Photoluminescence study Fig. 7shows the representative PL spectra of as-prepared, 500 and 900 ◦ C heat-treated samples for 5 at.% Eu doped TiO2 after excitation at 395 nm. As-prepared and 500 ◦ C heat-treated samples show the emission peaks at 590 and 614 nm, which are due to f–f transitions, i.e. magnetic (5 D0 → 7 F1 ) and electrical (5 D0 → 7 F2 ) dipole transitions respectively. However, these emission peaks could not be observed once the sample was heated to 900 ◦ C. Similar observations were made for other samples containing 3, 7 and 10 at.% Eu3+ doped TiO2 (see Supplementary materials S3–S5). In our XRD and XPS studies, there is no peak corresponding to Eu2+ ions in the form of EuO for 900 ◦ C heat-treated sample. From XPS studies, it is found that there is a bond formation such as Eu–O–Ti
Fig. 8. Emission spectra of 3 at.% Eu doped TiO2 prepared at different temperatures 500, 700 and 900 ◦ C for 4 h each and also pure TiO2 prepared at 900 ◦ C for 4 h after correction of grating response. Excitation wavelength is at 394 nm. Inset shows emission spectrum of 3 at.% Eu doped TiO2 prepared at 900 ◦ C after excitation at 394 nm using Nd:YAG pumped OPO (LASER).
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Fig. 10. Decay for 5 D0 level of Eu3+ for 500, 700 and 900 ◦ C heat-treated 3 at.% Eu doped TiO2 nanoparticles along with 900 ◦ C heated Eu2 O3 .
Fig. 9. Emission spectra of 3 at.% Eu doped TiO2 prepared at different temperatures 500, 700 and 900 ◦ C for 4 h each and also pure TiO2 prepared at 900 ◦ C for 4 h after correcting for grating response. Excitation wavelength is at 325 nm.
in case of 500 and 900 ◦ C heat-treated samples. Further, for 900 ◦ C heated sample, Eu2 Ti2 O7 formation is confirmed by XRD studies. In a previous report [36], it was shown that the highly symmetric structure of Eu2 Ti2 O7 does not show the electric dipole transition at 614 nm. In order to see even the magnetic dipole transition at 590 nm, Eu2 Ti2 O7 was excited using laser source, but still luminescence intensity was found to be very week [36]. Thus, because of the formation of highly symmetric Eu2 Ti2 O7 , luminescence intensity is found to disappear completely in our experiments. Distance between Eu3+ centers in Eu2 Ti2 O7 is reported to be 3.6 Å [36]. This short distance could lead to cross relaxation between Eu3+ ions, thereby significantly reducing the luminescence intensity. Additionally, Eu3+ ions occupy site having inversion symmetry in
Eu2 Ti2 O7 which makes the electric dipole transitions forbidden. Thus, the origin for decrease in luminescence intensity is formation of highly symmetric compound (Eu2 Ti2 O7 ) in which Eu3+ ions occupy inversion symmetry as well as the reduced distance between Eu3+ centers. Our finding also explains earlier observation [26–30], where luminescence intensity decreases with heat treatment above 500 ◦ C. Fig. 8 shows emission spectra obtained after the excitation at 394 nm for 3 at.% Eu doped TiO2 prepared at different temperatures namely 500, 700 and 900 ◦ C along with pure TiO2 prepared at 900 ◦ C using 450 W Xe lamp. All the samples were heated for 4 h at the respective temperatures. Emission intensity at 614 nm due to Eu3+ gradually decreases with increase in heat-treatment temperatures. At 900 ◦ C, emission intensity due to Eu3+ is almost negligible. Also, no emission due to Eu3+ is found even after excitation at 394 nm using Nd:YAG pumped optical parametric oscillator (OPO) (inset of Fig. 8). Jia et al. [31] reported that Ti3+ ions are characterized by an emission peak around 815 nm after excitation at 325 nm for Eu doped TiO2 film using Si substrate, however, we fail to see any emission at this wavelength or even in the range 600–850 nm (Fig. 9 and see Supplementary materials S6). It may be that emission peak at 815 nm comes from Si-substrate used to deposit the film [31] as
Fig. 11. Mössbauer spectra of (a) Eu2 O3 and (b) as-prepared, (c) 500 ◦ C and 900 ◦ C heat-treated 5 at% Eu doped TiO2 nanoparticles.
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respectively with corresponding line width 2.7 and 4.2 mm. This establishes that only Eu3+ state exists in 500 and 900 ◦ C annealed samples. This is also supported by the XPS and XRD results discussed above. 3.5. Raman study Fig. 12 shows the Raman spectra of 500 and 900 ◦ C heated 3 at.% Eu doped TiO2 and 900 ◦ C heated 10 at.% Eu doped TiO2 sample. The peaks at 145, 196, 396, 514, 640 cm−1 are obtained from 500 ◦ C heated 3 at.% Eu doped TiO2 sample and correspond to anatase phase of TiO2 [34,45]. This corroborates the XRD study, where anatase phase is detected (Fig. 1). In addition to above peaks, 900 ◦ C heated 3 at.% Eu doped TiO2 sample shows peaks at 445 and 608 cm−1 corresponding to rutile phase of TiO2 [34,45]. An additional peak at 308 cm−1 is also observed and this corresponds to the presence of Eu2 Ti2 O7 phase [34]. The peak intensity at 308 nm increases with increasing Eu concentration. No peaks characteristic of Eu2 O3 or EuO are observed in the Raman spectra of even TiO2 :Eu sample containing highest concentration of Europium (10 at.%) heated at 900 ◦ C. Thus, the Raman studies very clearly establish formation of Eu2 Ti2 O7 phase during heat-treatment of TiO2 :Eu samples rather than the phase segregation of Eu2 O3 or EuO phases. 3.6. Electron paramagnetic resonance study Fig. 12. Raman Spectra of 500 and 900 ◦ C heat-treated 3 at.% Eu doped TiO2 nanoparticles along with 900 ◦ C heated 10 at.% Eu doped TiO2 .
silicon nanoparticles/wafers are known to give an emission peak around 800 nm [32]. While heating, hydrocarbon moiety comes out, and a TiO2 film becomes thinner or form islands resulting in the exposure of the Si surface to the excitation light. This is also supported by IR and XRD studies in Jia et al.’s paper. Decays for 5 D0 level of Eu3+ for 500, 700 and 900 ◦ C heattreated 3 at.% Eu doped TiO2 nanoparticles along with 900 ◦ C heated Eu2 O3 are carried out using a pulsed laser source at 394 nm excitation and emission monitored at 615 nm (Fig. 10). It is found that intensity of 900 ◦ C heated 3 at.% Eu doped TiO2 is almost in based line as compared to Eu2 O3 . If Eu2 O3 phase is segregated during heat-treatment, we should see intensity from 900 ◦ C heated 3 at.% Eu doped TiO2 and its intensity will be comparable with that of Eu2 O3 prepared by the same procedure and heated at 900 ◦ C. But this is not observed. Based on this, it is concluded that no phase segregation of Eu2 O3 occurs in these samples; instead, another phase between Eu2 O3 and TiO2 , i.e. Eu2 Ti2 O7 is formed (supported by XRD and see Supplementary materials S7). Raman studies on these samples which are discussion later further support the above conclusions. 3.4. Mössbauer study Fig. 11 shows the Eu Mössbauer spectra of as-prepared, 500 and 900 ◦ C heat-treated 5 at.% Eu doped TiO2 nanoparticles. For comparison, the Mössbauer spectrum of pure Eu2 O3 is also shown. All the Mössbauer parameters are calculated with respect to Eu2 O3 where isomer shift is kept with 0.0 mm/s. Asprepared sample shows the two peaks corresponding to Eu3+ (isomer shift = −0.62 mm/s, width = 2.2 mm) and Eu2+ (isomer shift = −13.04 mm/s, width = 5.9 mm). Reported values of isomer shift in Eu3+ and Eu2+ are −0.6 and −13.7 mm/s respectively [44]. This is in conformity with the inferences drawn from XPS studies. Sample annealed at 500 and 900 ◦ C shows one peak corresponding to Eu3+ with isomer shift values −0.48 mm/s and −0.64 mm/s
EPR study reveals that Eu doped TiO2 samples shows the presence of Ti3+ in both 500 and 900 ◦ C heated samples. 10 at.% Eu doped TiO2 sample heated at 500 ◦ C shows signals with axial symmetry at g⊥ = 1.990 and g|| = 1.959. The g values are found to be almost independent of the concentration of the dopant ions and temperature of recording (300 and 80 K), however the intensity of perpendicular component (g⊥ = 1.990) is found to decrease (nearly half) with increase in the dopant ion concentration. This g values are related to Ti3+ present in anatase phase of TiO2 [23,46]. However, when these samples are annealed at 900 ◦ C, they show EPR signals at g⊥ = 1.995 and g|| = 1.930. However, the presence of Ti3+ could not be detected in 900 ◦ C heated samples from XPS and XRD studies. It is due to high sensitivity of EPR compared to those of XPS and XRD instruments. Even though EPR data shows the presence of Ti3+ for Eu doped TiO2 samples heated at 500 and 900 ◦ C, we are fail to see luminescence peaks corresponding to Ti3+ . 4. Conclusions TiO2 samples doped with different Eu concentrations are prepared using ethylene glycol method. Up to 500 ◦ C, samples crystallize in with anatase phase and above this temperature, mixture of anatase and rutile phases of TiO2 is formed. Conversion from anatase to rutile phase is completed at 1200 ◦ C. In addition to this, Eu2 Ti2 O7 phase is formed at 900 ◦ C. For Eu2 Ti2 O7 , Eu3+ ions occupy highly symmetry site (D3d ). There is no presence of Eu2+ or EuO for 900 ◦ C heated samples. Interestingly, luminescence intensity decreases with heat-treatment from 500 to 900 ◦ C. Decrease/complete disappearance of photoluminescence of Eu3+ ions in Eu3+ doped TiO2 systems when heated above 500 ◦ C is due to formation of a highly symmetric structure of Eu2 Ti2 O7 . Acknowledgements Authors thank Dr. D. Das, Dr. A.K. Tyagi, Dr. M. Pandey, and Mr. R. Shukla, BARC for their help and encouragement.
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