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Luminescence characteristics of cobalt doped TiO2 nanoparticles
Journal of Luminescence 132 (2012) 178–184
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence characteristics of cobalt doped TiO2 nanoparticles Biswajit Choudhury, Amarjyoti Choudhury n Department of Physics, Tezpur University, Napaam 784028, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 April 2011 Received in revised form 7 August 2011 Accepted 12 August 2011 Available online 22 August 2011
TiO2 nanoparticles doped with two different concentrations of Cobalt, 0.02 and 0.04 mol, are prepared by sol–gel method. The crystalline phase of the doped and undoped nanoparticles and particle sizes are observed with X-ray diffraction and transmission electron microscope. FTIR confirms the bonding interaction of Co2 þ in TiO2 lattice framework. The UV absorption spectra of the doped material shows two absorption peaks in the visible region related to d–d electronic transitions of Co2 þ in TiO2 lattice. Compared to undoped TiO2 nanoparticles, the cobalt doped samples show a red shift in the band gap. Steady state photoluminescence spectra give emission peaks related to oxygen defects. The decrease in the intensity ratio of UV/visible emission peaks confirms distortion of structural regularity and formation of defects after doping. The intensity ratio of different visible emission peaks is nearly same for undoped and 0.02 Co2 þ . However, this ratio decreases profoundly at 0.04 Co2 þ , due to concentration quenching effect. Photoluminescence excitation spectra, recorded at 598 nm emission wavelength, give different excitation peaks associated with oxygen vacancies and Co2 þ . Time resolved photoluminescence spectra give longer decay time for doped samples, indicating longer relaxation of conduction band electrons on the defect and on dopant sites. & 2011 Elsevier B.V. All rights reserved.
Keywords: Doped nanoparticles Defects Luminescence Defect centers Crystal field Decay time
1. Introduction TiO2 exists in three phases, anatase, rutile and brookite, of which anatase and rutile are known to be potentially active materials for many applications; such as in photocatalysis, water splitting, solar cells, sensors, paints etc. [1–3]. But the main drawback of these systems, for many of these applications, is their absorption in the UV region, which corresponds to only 3–5% of solar radiation [4,5]. However, incorporation of a small concentration of impurity ions shifts its absorption onset to the visible region, making it an efficient candidate for photocatalysis applications [6–8]. Both anion and cation doping is performed in TiO2 to tune the electronic, structural properties and likewise to enhance the photocatalytic activity of this material [9,10]. Out of the transition metal ions, Co2 þ is an important dopant because of its optically active nature and also because of the involvement of this metal ion in imparting interesting magnetic properties to TiO2 semiconductor nanostructures, useful for spintronic applications [11,12,17]. Cobalt doped TiO2 materials are also reported to show enhanced photocatalytic response [13,14]. Iwasaki et al. [14] reported that TiO2 doped with 0.03% of Co (II) has high photocatalytic activity under UV–vis light. Bulk TiO2 does not exhibit any photoluminescence at room temperature, but
n
Corresponding author. Tel.: þ913712267120; fax: þ 913712267005. E-mail address: [email protected] (A. Choudhury).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.08.020
nanoparticles of TiO2 show many luminescence peaks in visible region, which are associated with the presence of self trapped excitons, surface trap states, interstitial Ti3 þ , oxygen defects etc. [9,15,21,22]. TiO2 incorporated with cobalt are reported to show weak PL intensity than that of its undoped one, due to inhibition of electron–hole recombination after doping [3]. The dopants introduce new defect levels, which act as a charge carrier trapping site and suppresses the electron–hole recombination, thus increasing their average lifetimes [3,5]. Steady state and time resolved luminescence spectroscopy is a way to get collective information on defect related luminescence in doped TiO2 and also to determine the lifetime of the photogenerated electrons and holes in presence of these defects [16]. The main objective of this article is to analyze the luminescence characteristic of TiO2 nanoparticles both before and after cobalt incorporation. The attempt is to correlate the dopant concentration effect with the intensity of UV and visible emission peaks, and also with luminescence decay behavior related to different trap states.
2. Experimental details 2.1. Preparation method The preparation of cobalt doped TiO2 nanoparticles were carried out with two different cobalt concentrations, 0.02 and
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0.04 mol, using a sol–gel method. In a typical synthesis, 6 ml of titanium isopropoxide was added to 15 ml of 2-propanol under constant stirring. After 15 min of stirring, few drops of water were added to initiate the hydrolysis reaction. Then, cobalt acetate hexahydrate, with the aforementioned concentration, was slowly added to the hydrolyzed mixture under vigorous stirring. When addition of all the cobalt solutions are finished, the mixture was stirred for 6 h. During this time of the reaction, first a sol was formed, which ultimately transformed into gel. After this time period the stirring was stopped, and the gel was left in ageing condition for 6–8 h. The prepared material was dried in air at 80 1C and then heat treated at 430 1C to get the desired cobalt doped TiO2 nanoparticles. 2.2. Characterization details X-ray diffraction (XRD) patterns of undoped and doped TiO2 nanoparticles are recorded with Rigaku Miniflex X-ray diffractometer equipped with high intense CuKa radiation (0.154 nm) at a scanning rate of 11/min and in the scanning range from 10–801. High resolution transmission electron microscope (HRTEM) images of the prepared nanoparticles are observed with JEOL 3010 Transmission electron microscope operating at 300 kV. The presence of different bonding vibrational frequencies is determined by Nicolet Fourier transform infrared spectroscopy (FTIR). Elemental compositions are known from energy dispersive X-ray (EDX) analysis equipped with a JEOL JSM scanning electron microscope (SEM). Diffuse reflectance spectra (DRS) of all samples are taken with Shimadzu-2450 UV–vis spectrophotometer. Steady state photoluminescence spectra (PL) are recorded with Perkin Elmer LS fluorescence spectrophotometer. Time resolved photoluminescence spectra (TRPL) are monitored with life spec II spectrofluorimeter (Edinburgh instrument). The sample was excited by 375 nm laser diode and the decay was measured with a time scale of 0.0048 ns/channel.
3. Results and discussion The XRD patterns of undoped and doped TiO2 nanoparticles are shown in Fig. 1a. All the peaks correspond to the tetragonal anatase phase of TiO2 with a space group I41/amd (JCPDS 78-2486). An inspection of the diffraction pattern reveals that the doped materials are in pure anatase form without any rutile peak or peaks related to metallic cobalt or cobalt oxide. However, a slight shifting of the peak position towards lower angle side indicates changes in the local structure around Ti4 þ after Co2 þ doping (shown in the inset of Fig. 1a). This shifting confirms well incorporation of Co2 þ on anatase lattice site of TiO2 [16]. The reduction in peak intensity also signifies the structural irregularity after Co2 þ doping [3]. Due to the small size of the nanoparticles, all the added dopants do not undergo host lattice interior; some may sits on the surface or on grain boundary. Because of different ionic charges of Ti (þ 4) and Co (þ2), dopant substitution lead to the creation of oxygen ion vacancy, to balance the charge neutrality. The few dopant atoms on the grain boundary disturb the periodicity of the lattice, and inhibit crystal growth. Since, grain boundary is usually amorphous in nature, so the doping affects the crystallinity of the sample. This can be observed from the widening of the (101) peak with increasing dopant content. But the amount of these dopants on the grain boundary or on surface is negligible, otherwise XRD can possibly detect the presence of CoO or Co3O4 peaks, formed during air annealing of the doped sample. The calculated d-spacing for the two doped samples are higher compared to undoped one (Fig. 1b). The increase in d value after doping confirms elongation of the
Fig. 1. (a) XRD pattern of undoped and cobalt doped TiO2 nanoparticles. Inset shows widening of (101) peak on Cobalt incorporation (b) variation of d-spacing with Co2 þ concentration.
unit cell along a and c-axis when smaller sized Ti4 þ (74 pm) is replaced by larger sized Co2 þ (81 pm) [17]. The transmission electron microscope images of the doped and undoped TiO2 nanoparticles are shown in Fig. 2a–d. Both in undoped and doped TiO2 nanoparticles (Fig. 2a and c), there is no such variation in the distribution of the nanoparticles. In both samples the particles have an average size of 10 nm and somehow agglomerated. However, some larger sized particles with a size of 20 nm are also present in both the samples. The high resolution (HR) image of the undoped sample (Fig. 2b) clearly shows the crystalline nature of the material with the lattice fringes corresponding to (200) crystallographic plane. Presence of amorphous phase in the doped nanoparticle (marked by yellow arrowhead) (Fig. 2d) confirms poor crystallinity of TiO2 nanoparticles on cobalt doping. The EDX pattern for 0.04 Co2 þ doped TiO2 (inset of Fig. 2d) sample shows the presence of the constituent elements in the doped material. The bonding interaction of cobalt in the framework of TiO2 lattice, and formation of oxygen defects are studied with FTIR. Fig. 3a and b depicts the FTIR spectra of undoped and cobalt doped TiO2 nanoparticles. In these spectra, the peaks coming at
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Fig. 2. (a) TEM image of TiO2 nanoparticles, showing distribution of particles, (b) high resolution image of the undoped sample, showing the crystalline planes. (c) TEM image of cobalt doped TiO2 nanoparticles with its (d) high resolution image. The yellow pointer on the image shows the presence of amorphous phase. In the inset of (d) (right side lower image) shows the enlarged view of a part of the doped nanoparticle showing the presence of amorphous phase. The EDX pattern of 0.04 Cobalt doped TiO2 nanoparticles are also shown in the inset of (d) (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
3418.51 cm 1 for all the samples are attributed to O–H stretching frequency of –OH groups of water and the peaks appearing at 1629 cm 1 are attributed to H–O–H bending vibration mode of physisorbed water [8]. Ti–O frequency peaks appear between 600 and 400 cm 1 [16]. As we go on scanning the peaks for both undoped and doped sample, within the above mentioned frequency range, changes in the peak position of Ti–O bond is observed (marked by square in the Fig. 3a). For the undoped TiO2, the Ti–O frequency peak comes at 454 cm 1, while the doped sample shows shifting in the Ti–O frequency to 425 cm 1 and 423 cm 1 for 0.02 and 0.04 Co2 þ doped samples, respectively. This confirms the formation of Co–O bond by the
substitution of Ti4 þ in the TiO2 lattice. This kind of frequency shifting after cobalt doping is also found in different types of TiO2 nanostructures [16]. We can correlate this kind of frequency shifting with the Ti–O or Co–O bond strength using the equation 1 n¼ 2pc
sffiffiffiffi k
m
ð1Þ
where u is the wave number, k is the force constant of the chemical bond, and m is the reduced mass of the bond associating the elements. If Co2 þ has substituted Ti4 þ , then Co–O bond is formed. The reduced mass of Co–O (m ¼12.58) and Ti–O
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Fig. 3. (a) FTIR spectra of undoped and doped TiO2 nanoparticles, (b) shifting in the peak position of Ti–O from 454 cm 1 to lower wave number after Co2 þ incorporation, indicating Co–O bond formation in host TiO2 lattice.
(m ¼12.01) is almost same. In undoped TiO2, the frequency position of Ti–O bond is determined by its strength or by its force constant. If the bond is strong, then its force constant will also be higher. But when Co2 þ substituted Ti4 þ , then for charge neutrality oxygen vacancies are created. As a result, the number of oxygen for forming Co–O or Ti–O bond in the doped system gets diminished. This results in the weakening of the bond and thus lowering in the value of force constant [16]. Reduction in the value of the force constant of the Co–O or Ti–O bond results in the lowering in the frequency of the bond and shifting of Co–O bond to lower wave number. Thus, FTIR spectra can be taken as a tool to understand the formation of Co–O bond and formation of oxygen defects during the substitution of lattice Ti4 þ by Co2 þ . UV–vis absorption spectroscopy is a valuable tool to understand the substituting effect of dopant on host lattice and its co-ordination environment (whether it is present on surface, grain boundary or on the core). The reflectance spectra for all the samples are shown in Fig. 4a. Absorption of the samples (Fig. 4b) is related to reflectance by Kubelka–Munk equation F(R)¼(1 R)2/2R, where R is the reflectance of the sample and F(R) is the corresponding absorbance [18]. The absorption onset for 0rxr0.04 appears in between 320 and 350 nm. For Co2 þ doped TiO2, the absorption tail is shifted towards visible region and exhibits two extra peaks, between 375 and 505 nm and at 609 nm. These peaks are not observed in the undoped TiO2 nanoparticles. These two regions are related to the crystal field splitted d-electronic transition of Co2 þ in octahedral or pseudo octahedral co-ordination [19]. The absorption in between 375 and 505 nm is coming from 4T1g to 4T1g(P) transition and that at 609 nm is coming due to 4T1g to 4A2g. In anatase TiO2, Ti4 þ is surrounded by six oxygen atoms in an octahedral co-ordination,
Fig. 4. (a) Reflectance and the corresponding (b) absorption spectra of doped and undoped TiO2 nanoparticles. Compared to undoped one, doped TiO2 exhibits two absorption peaks in the visible region, related to Co2 þ d–d electronic transition (shown in the inset of (b) and also by red dotted lines region in (b)) (c) shows the lowering in the band gap after doping (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
forming TiO6 octahedra. When Co2 þ substitutes Ti4 þ , it forms bond with six oxygen atoms. Now, according to crystal field theory, the electrons in the d-orbital of Co2 þ will undergo repulsion by the electrons of the six surrounding oxygen atoms. This results in the splitting of d-orbital of Co2 þ , showing the aforementioned d–d electronic transition. These types of electronic transition will be shown by Co2 þ , when it substitutes Ti4 þ and remains in octahedral or pseudo octahedral co-ordination. Thus, absorption spectroscopy can be taken as a tool to understand whether the dopant has undergone the TiO2 lattice or not. Because, depending on the coordination environment (octahedral, tetrahedral etc.) the transition
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probability of these dopants will be different. In our case Co2 þ has gone inside the TiO6 octahedral framework giving these d-electronic transitions. The variation in the band gap of TiO2 with Co2 þ concentration is shown in Fig. 4c. Undoped TiO2 nanoparticle has a larger band gap (3.25 eV) than that of bulk (3.2 eV), which is quite expected due to quantum confinement effect. But, after cobalt doping the band gap is reduced to 2.86 eV and 2.55 eV for 0.02 and 0.04 Co2 þ , respectively. This reduction in the band gap, after doping, is illustrated as a line diagram in the inset of Fig. 4c. The band gap of the material is calculated by extrapolating [F(R)hu]1/2 vs. energy at [F(R)hu]1/2 ¼0. Several approaches are made to explain the reduction of band gap after doping. Some group has reported that doping does not reduce the actual band gap of TiO2, instead introduces some mid-band gap states which results in red shift of band gap. But some other reports specified the narrowing in band gap to the sp–d exchange interactions of sp electrons of host with the d-electrons of dopant [16,20,21]. Fig. 5a and b represents the typical PL spectra of undoped and doped TiO2 samples excited at two different wavelengths, 320 nm and 395 nm, respectively. For clear observation of the position of emission peaks, we have done Gaussian fitting of one of the emission peaks (correctness, r2 ¼0.99983). The emission spectra of the samples usually consists of a UV emission peak at 392 nm, one violet emission peak at 429 nm, two blue emission peaks at 457 nm, 491 nm and one green emission peak at 535 nm (Fig. 5a). The UV emission is considered as the band edge emission of the host TiO2. The appearance of this band is due to the phonon assisted indirect transition from edge (X) to the center (G) of Brillouin zone, X1b to G3 [22]. The 429 nm peak can be ascribed to self trapped excitons (STE) localized on TiO6 octahedra [23]. The STE originates by the interactions of conduction band electrons localized on Ti 3d orbital with holes present in O 2p orbital of TiO2. Whether STE will be localized on TiO6 or not depends on the compactness of the octahedra chain and also on the restriction on excitons transfer from one part of the chain to the other [15]. The peaks at 457 nm and 535 nm are attributed to the defect centers associated with oxygen vacancies. These defect centers are originated by trapping one and two electrons respectively in the oxygen vacancies [21,23]. The most intense blue emission peak at 491 nm is coming as a charge transfer band from Ti3 þ to oxygen anion in a TiO26 complex associated with oxygen vacancies [24]. Changing the excitation wavelength to E nm results in an orange emission peak at 598 nm, in addition to emission peaks at 491 nm and 535 nm, respectively (Fig. 5b). This orange peak can be ascribed to surface related trap states [5]. From the observation of the PL spectra at the two excitations, it can be inferred that doping is neither introducing Co2 þ related emission peak nor is shifting the peak position. Only reduction in PL intensity is found for both the doped samples. The decrease in emission intensity with doping is observed in Ce doped TiO2 samples, although they found a slight shifting of a particular emission peak. They ascribed this shifting due to band tailing effect [5]. While in case of ZnO samples, researchers have found only reduction in emission peak intensity on Mn incorporation, with no appreciable peak shifting [34]. They attributed this lowering in intensity to the formation of large number of nonradiative centers on Mn doping. In our case we have not found any noticeable shifting of emission peaks. Incorporation of Co2 þ in TiO2 only decreases the emission intensity by forming large number of nonradiative centers [34], which act as luminescence quencher. As explained in XRD, the presence of cobalt on the grain boundary affects the crystallinity of the sample. Since photoluminescence is a technique to understand the presence of oxygen defects, trap states etc. Thus, we can conclude the decrease in the emission intensity as a collective contribution from dopants;
Fig. 5. PL spectra at an excitation wavelength of (a) 320 nm (b) 395 nm. Inset of (a) shows the variation of intensity ratio of UV to visible emission peaks with dopant concentration, and (c) represents variation of intensity ratios of visible emission peaks with Co2 þ concentration. (d) PLE spectra monitored at 598 nm (for interpretation of the references to color in this figure, the reader is referred to the web version of this article).
nonradiative centers produced on doping, poor crystallinity of doped samples, and oxygen defects created on Ti4 þ substitution by Co2 þ .
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An inspection of the UV and visible emission peaks (Fig. 5a) of TiO2 reveals that the emissivity of the material in the UV region is considerably less compared to visible regions, which is even less in the two doped samples. To have an understanding of this, we have plotted IUV/Idefect(Blue) intensity ratio for all the samples, and have found that the intensity ratio decreases as we start from undoped to the two doped samples (inset of Fig. 5a). This indicates that doping distorts the structural regularity and band structure of TiO2 nanoparticles by forming large number of defects [25]. These defects are responsible for enhanced visible emission intensity in the material compared to UV emission. For further understanding on the effect of this dopant concentration on the visible emission peaks (Fig. 5c), we have plotted I491/I535 and I491/I598 emission intensity ratios, of the 395 nm excited emission spectra, against Co2 þ concentration. From the plot it is observed that I491/I535 and I491/I598 emission intensity ratios are nearly same for 0.02 Co2 þ , but decrease profoundly at the Co2 þ doping concentration of 0.04 mol. Many people have reported this observation due to concentration quenching effect [26]. In conformity with this quenching effect, the increase in dopant amount decreases the distance between active ions, and thus increases the interactions between ions forming quenching centers. At lower doping concentration, the interactions between ions are too weak to have an effect on the energy levels of each doped ions. But at higher dopant concentration the distance between ions or between ions and defects decreases, thus allowing energy transfer from one ion to another. In this process, the transferred energy may also be captured by impurity or defects, enhancing nonradiative relaxation process and hence decreasing the visible emission intensity ratio [27–29]. In our case there is not much variation in the intensity ratio at x¼ 0.02 compared to 0.00, since ion centers are not closer enough. But when dopant concentration has a value of 0.04 mol, the intensity ratio is completely quenched, indicating strong coupling among ions or among ions and defects, thus degrading PL intensity. Photoluminescence excitation (PLE) spectra were taken to differentiate the excitation behavior of doped and undoped samples (Fig. 5d). The PLE spectra were recorded monitoring 598 nm emission peak. The two samples show one strong excitation peak at 398 nm. This peak is due to interband transition in host sample. Along with this it also exhibits one long tail in the visible region. Actually the absorptions ranging from 400 to 550 nm are attributed to the surface oxygen vacancies [10]. These two spectra are different only at 459 nm and 441 nm peaks. The 459 nm peak can be associated with d–d transition Co2 þ from 4 T1g to 4T1(P) [19]. The 441 nm band in the excitation spectra was assigned to trapped holes [30]. In Fig. 6 time dependent PL decay of the samples are shown. The decay was measured monitoring 491 nm emission wavelength. The curves were fitted bi-exponentially, using the equation [31] t t y ¼ y0 þ a1 exp þ a2 exp ð2Þ
t1
t2
where t1 and t2 are decay constants and a1 and a2 are preexponential factor. For bare TiO2, the values of t1 and t2 are 1.16 and 0.634 ns and a1 and a2 are 5066.7 and 19,548.06, respectively. The values of t1 and t2 for cobalt doped TiO2 are 1.802 and 0.854 ns, respectively, and values of a1 and a2 are 439.6 and 16,724.85, respectively. The mean lifetimes (tm) are calculated using the following equation [16], and the values are coming out to be 0.90 ns for cobalt doped TiO2 and 0.80 ns for TiO2 Pn a t2 tm ¼ Pin¼ 1 i i ð3Þ i¼1
ai ti
Thus, the lifetime of the doped TiO2 is longer compared to undoped TiO2. Since electrons and holes in the small sized TiO2
183
Fig. 6. Fluorescence decay curves of undoped and cobalt doped TiO2 nanoparticles monitored at the emission wavelength of 491 nm.
nanoparticles are confined within a small region, the extent of overlapping of their wave function is more leading to faster recombination [32]. But inclusion of dopant states in the mid gap states of TiO2 hindered the degree of overlapping of charge carriers thus giving them longer lifetime [16]. In undoped TiO2 the electrons from conduction band relax in the shallow or deep trap states. Usually trapping is a nonradiative process and shallow traps have shorter lifetime than deep traps [33]. In our results the relatively faster component in TiO2 is due to the relaxation of electrons on shallow trap levels such as Ti3 þ or surface trap states. The trapping probability in case of doped one is higher than undoped one because in addition to oxygen related defects, Co2 þ d states are also introduced in the mid-band gap. In undoped TiO2, after relaxation of conduction band electrons in the defect states, it directly transits to the valence band undergoing faster recombination with holes. But Co2 þ doping introduces energy levels of d states, splitted by crystal field, near about oxygen and Ti3 þ related trap states. Therefore, in doped case trap to trap transition of electrons take place from defect to Co d states. It relaxes in the d states of Co2 þ from which it undergoes transition to the ground states. This process results in the relatively longer lifetime of charge carriers in the doped sample.
4. Conclusion In summary, the optical property of TiO2 nanoparticles changes on cobalt doping. XRD and FTIR confirm Co2 þ substitution on Ti4 þ site in TiO2 and also formation of oxygen defects on doping. Doping shifts the absorption onset to visible region and lowers the band gap. The decrease in emission intensity on doping indicates lower recombination of charge carriers due to the presence of nonradiative centers and poor crystallinity of doped samples. The lowering in UV to visible intensity ratio, after each doping, confirms degradation of structural quality. This was also confirmed by X-ray diffraction. The decrease in intensity ratio of visible emission peaks after doping is due to concentration quenching effect. Doping of cobalt increases the lifetime of photogenerated carriers by introducing trap states associated with d states of cobalt, where increase in relaxation time of conduction electrons is observed before it undergoes recombination. The increased lifetime indicates the availability of charge carriers for photocatalysis.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence characteristics of cobalt doped TiO2 nanoparticles Biswajit Choudhury, Amarjyoti Choudhury n Department of Physics, Tezpur University, Napaam 784028, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 April 2011 Received in revised form 7 August 2011 Accepted 12 August 2011 Available online 22 August 2011
TiO2 nanoparticles doped with two different concentrations of Cobalt, 0.02 and 0.04 mol, are prepared by sol–gel method. The crystalline phase of the doped and undoped nanoparticles and particle sizes are observed with X-ray diffraction and transmission electron microscope. FTIR confirms the bonding interaction of Co2 þ in TiO2 lattice framework. The UV absorption spectra of the doped material shows two absorption peaks in the visible region related to d–d electronic transitions of Co2 þ in TiO2 lattice. Compared to undoped TiO2 nanoparticles, the cobalt doped samples show a red shift in the band gap. Steady state photoluminescence spectra give emission peaks related to oxygen defects. The decrease in the intensity ratio of UV/visible emission peaks confirms distortion of structural regularity and formation of defects after doping. The intensity ratio of different visible emission peaks is nearly same for undoped and 0.02 Co2 þ . However, this ratio decreases profoundly at 0.04 Co2 þ , due to concentration quenching effect. Photoluminescence excitation spectra, recorded at 598 nm emission wavelength, give different excitation peaks associated with oxygen vacancies and Co2 þ . Time resolved photoluminescence spectra give longer decay time for doped samples, indicating longer relaxation of conduction band electrons on the defect and on dopant sites. & 2011 Elsevier B.V. All rights reserved.
Keywords: Doped nanoparticles Defects Luminescence Defect centers Crystal field Decay time
1. Introduction TiO2 exists in three phases, anatase, rutile and brookite, of which anatase and rutile are known to be potentially active materials for many applications; such as in photocatalysis, water splitting, solar cells, sensors, paints etc. [1–3]. But the main drawback of these systems, for many of these applications, is their absorption in the UV region, which corresponds to only 3–5% of solar radiation [4,5]. However, incorporation of a small concentration of impurity ions shifts its absorption onset to the visible region, making it an efficient candidate for photocatalysis applications [6–8]. Both anion and cation doping is performed in TiO2 to tune the electronic, structural properties and likewise to enhance the photocatalytic activity of this material [9,10]. Out of the transition metal ions, Co2 þ is an important dopant because of its optically active nature and also because of the involvement of this metal ion in imparting interesting magnetic properties to TiO2 semiconductor nanostructures, useful for spintronic applications [11,12,17]. Cobalt doped TiO2 materials are also reported to show enhanced photocatalytic response [13,14]. Iwasaki et al. [14] reported that TiO2 doped with 0.03% of Co (II) has high photocatalytic activity under UV–vis light. Bulk TiO2 does not exhibit any photoluminescence at room temperature, but
n
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0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.08.020
nanoparticles of TiO2 show many luminescence peaks in visible region, which are associated with the presence of self trapped excitons, surface trap states, interstitial Ti3 þ , oxygen defects etc. [9,15,21,22]. TiO2 incorporated with cobalt are reported to show weak PL intensity than that of its undoped one, due to inhibition of electron–hole recombination after doping [3]. The dopants introduce new defect levels, which act as a charge carrier trapping site and suppresses the electron–hole recombination, thus increasing their average lifetimes [3,5]. Steady state and time resolved luminescence spectroscopy is a way to get collective information on defect related luminescence in doped TiO2 and also to determine the lifetime of the photogenerated electrons and holes in presence of these defects [16]. The main objective of this article is to analyze the luminescence characteristic of TiO2 nanoparticles both before and after cobalt incorporation. The attempt is to correlate the dopant concentration effect with the intensity of UV and visible emission peaks, and also with luminescence decay behavior related to different trap states.
2. Experimental details 2.1. Preparation method The preparation of cobalt doped TiO2 nanoparticles were carried out with two different cobalt concentrations, 0.02 and
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0.04 mol, using a sol–gel method. In a typical synthesis, 6 ml of titanium isopropoxide was added to 15 ml of 2-propanol under constant stirring. After 15 min of stirring, few drops of water were added to initiate the hydrolysis reaction. Then, cobalt acetate hexahydrate, with the aforementioned concentration, was slowly added to the hydrolyzed mixture under vigorous stirring. When addition of all the cobalt solutions are finished, the mixture was stirred for 6 h. During this time of the reaction, first a sol was formed, which ultimately transformed into gel. After this time period the stirring was stopped, and the gel was left in ageing condition for 6–8 h. The prepared material was dried in air at 80 1C and then heat treated at 430 1C to get the desired cobalt doped TiO2 nanoparticles. 2.2. Characterization details X-ray diffraction (XRD) patterns of undoped and doped TiO2 nanoparticles are recorded with Rigaku Miniflex X-ray diffractometer equipped with high intense CuKa radiation (0.154 nm) at a scanning rate of 11/min and in the scanning range from 10–801. High resolution transmission electron microscope (HRTEM) images of the prepared nanoparticles are observed with JEOL 3010 Transmission electron microscope operating at 300 kV. The presence of different bonding vibrational frequencies is determined by Nicolet Fourier transform infrared spectroscopy (FTIR). Elemental compositions are known from energy dispersive X-ray (EDX) analysis equipped with a JEOL JSM scanning electron microscope (SEM). Diffuse reflectance spectra (DRS) of all samples are taken with Shimadzu-2450 UV–vis spectrophotometer. Steady state photoluminescence spectra (PL) are recorded with Perkin Elmer LS fluorescence spectrophotometer. Time resolved photoluminescence spectra (TRPL) are monitored with life spec II spectrofluorimeter (Edinburgh instrument). The sample was excited by 375 nm laser diode and the decay was measured with a time scale of 0.0048 ns/channel.
3. Results and discussion The XRD patterns of undoped and doped TiO2 nanoparticles are shown in Fig. 1a. All the peaks correspond to the tetragonal anatase phase of TiO2 with a space group I41/amd (JCPDS 78-2486). An inspection of the diffraction pattern reveals that the doped materials are in pure anatase form without any rutile peak or peaks related to metallic cobalt or cobalt oxide. However, a slight shifting of the peak position towards lower angle side indicates changes in the local structure around Ti4 þ after Co2 þ doping (shown in the inset of Fig. 1a). This shifting confirms well incorporation of Co2 þ on anatase lattice site of TiO2 [16]. The reduction in peak intensity also signifies the structural irregularity after Co2 þ doping [3]. Due to the small size of the nanoparticles, all the added dopants do not undergo host lattice interior; some may sits on the surface or on grain boundary. Because of different ionic charges of Ti (þ 4) and Co (þ2), dopant substitution lead to the creation of oxygen ion vacancy, to balance the charge neutrality. The few dopant atoms on the grain boundary disturb the periodicity of the lattice, and inhibit crystal growth. Since, grain boundary is usually amorphous in nature, so the doping affects the crystallinity of the sample. This can be observed from the widening of the (101) peak with increasing dopant content. But the amount of these dopants on the grain boundary or on surface is negligible, otherwise XRD can possibly detect the presence of CoO or Co3O4 peaks, formed during air annealing of the doped sample. The calculated d-spacing for the two doped samples are higher compared to undoped one (Fig. 1b). The increase in d value after doping confirms elongation of the
Fig. 1. (a) XRD pattern of undoped and cobalt doped TiO2 nanoparticles. Inset shows widening of (101) peak on Cobalt incorporation (b) variation of d-spacing with Co2 þ concentration.
unit cell along a and c-axis when smaller sized Ti4 þ (74 pm) is replaced by larger sized Co2 þ (81 pm) [17]. The transmission electron microscope images of the doped and undoped TiO2 nanoparticles are shown in Fig. 2a–d. Both in undoped and doped TiO2 nanoparticles (Fig. 2a and c), there is no such variation in the distribution of the nanoparticles. In both samples the particles have an average size of 10 nm and somehow agglomerated. However, some larger sized particles with a size of 20 nm are also present in both the samples. The high resolution (HR) image of the undoped sample (Fig. 2b) clearly shows the crystalline nature of the material with the lattice fringes corresponding to (200) crystallographic plane. Presence of amorphous phase in the doped nanoparticle (marked by yellow arrowhead) (Fig. 2d) confirms poor crystallinity of TiO2 nanoparticles on cobalt doping. The EDX pattern for 0.04 Co2 þ doped TiO2 (inset of Fig. 2d) sample shows the presence of the constituent elements in the doped material. The bonding interaction of cobalt in the framework of TiO2 lattice, and formation of oxygen defects are studied with FTIR. Fig. 3a and b depicts the FTIR spectra of undoped and cobalt doped TiO2 nanoparticles. In these spectra, the peaks coming at
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Fig. 2. (a) TEM image of TiO2 nanoparticles, showing distribution of particles, (b) high resolution image of the undoped sample, showing the crystalline planes. (c) TEM image of cobalt doped TiO2 nanoparticles with its (d) high resolution image. The yellow pointer on the image shows the presence of amorphous phase. In the inset of (d) (right side lower image) shows the enlarged view of a part of the doped nanoparticle showing the presence of amorphous phase. The EDX pattern of 0.04 Cobalt doped TiO2 nanoparticles are also shown in the inset of (d) (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
3418.51 cm 1 for all the samples are attributed to O–H stretching frequency of –OH groups of water and the peaks appearing at 1629 cm 1 are attributed to H–O–H bending vibration mode of physisorbed water [8]. Ti–O frequency peaks appear between 600 and 400 cm 1 [16]. As we go on scanning the peaks for both undoped and doped sample, within the above mentioned frequency range, changes in the peak position of Ti–O bond is observed (marked by square in the Fig. 3a). For the undoped TiO2, the Ti–O frequency peak comes at 454 cm 1, while the doped sample shows shifting in the Ti–O frequency to 425 cm 1 and 423 cm 1 for 0.02 and 0.04 Co2 þ doped samples, respectively. This confirms the formation of Co–O bond by the
substitution of Ti4 þ in the TiO2 lattice. This kind of frequency shifting after cobalt doping is also found in different types of TiO2 nanostructures [16]. We can correlate this kind of frequency shifting with the Ti–O or Co–O bond strength using the equation 1 n¼ 2pc
sffiffiffiffi k
m
ð1Þ
where u is the wave number, k is the force constant of the chemical bond, and m is the reduced mass of the bond associating the elements. If Co2 þ has substituted Ti4 þ , then Co–O bond is formed. The reduced mass of Co–O (m ¼12.58) and Ti–O
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Fig. 3. (a) FTIR spectra of undoped and doped TiO2 nanoparticles, (b) shifting in the peak position of Ti–O from 454 cm 1 to lower wave number after Co2 þ incorporation, indicating Co–O bond formation in host TiO2 lattice.
(m ¼12.01) is almost same. In undoped TiO2, the frequency position of Ti–O bond is determined by its strength or by its force constant. If the bond is strong, then its force constant will also be higher. But when Co2 þ substituted Ti4 þ , then for charge neutrality oxygen vacancies are created. As a result, the number of oxygen for forming Co–O or Ti–O bond in the doped system gets diminished. This results in the weakening of the bond and thus lowering in the value of force constant [16]. Reduction in the value of the force constant of the Co–O or Ti–O bond results in the lowering in the frequency of the bond and shifting of Co–O bond to lower wave number. Thus, FTIR spectra can be taken as a tool to understand the formation of Co–O bond and formation of oxygen defects during the substitution of lattice Ti4 þ by Co2 þ . UV–vis absorption spectroscopy is a valuable tool to understand the substituting effect of dopant on host lattice and its co-ordination environment (whether it is present on surface, grain boundary or on the core). The reflectance spectra for all the samples are shown in Fig. 4a. Absorption of the samples (Fig. 4b) is related to reflectance by Kubelka–Munk equation F(R)¼(1 R)2/2R, where R is the reflectance of the sample and F(R) is the corresponding absorbance [18]. The absorption onset for 0rxr0.04 appears in between 320 and 350 nm. For Co2 þ doped TiO2, the absorption tail is shifted towards visible region and exhibits two extra peaks, between 375 and 505 nm and at 609 nm. These peaks are not observed in the undoped TiO2 nanoparticles. These two regions are related to the crystal field splitted d-electronic transition of Co2 þ in octahedral or pseudo octahedral co-ordination [19]. The absorption in between 375 and 505 nm is coming from 4T1g to 4T1g(P) transition and that at 609 nm is coming due to 4T1g to 4A2g. In anatase TiO2, Ti4 þ is surrounded by six oxygen atoms in an octahedral co-ordination,
Fig. 4. (a) Reflectance and the corresponding (b) absorption spectra of doped and undoped TiO2 nanoparticles. Compared to undoped one, doped TiO2 exhibits two absorption peaks in the visible region, related to Co2 þ d–d electronic transition (shown in the inset of (b) and also by red dotted lines region in (b)) (c) shows the lowering in the band gap after doping (for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
forming TiO6 octahedra. When Co2 þ substitutes Ti4 þ , it forms bond with six oxygen atoms. Now, according to crystal field theory, the electrons in the d-orbital of Co2 þ will undergo repulsion by the electrons of the six surrounding oxygen atoms. This results in the splitting of d-orbital of Co2 þ , showing the aforementioned d–d electronic transition. These types of electronic transition will be shown by Co2 þ , when it substitutes Ti4 þ and remains in octahedral or pseudo octahedral co-ordination. Thus, absorption spectroscopy can be taken as a tool to understand whether the dopant has undergone the TiO2 lattice or not. Because, depending on the coordination environment (octahedral, tetrahedral etc.) the transition
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probability of these dopants will be different. In our case Co2 þ has gone inside the TiO6 octahedral framework giving these d-electronic transitions. The variation in the band gap of TiO2 with Co2 þ concentration is shown in Fig. 4c. Undoped TiO2 nanoparticle has a larger band gap (3.25 eV) than that of bulk (3.2 eV), which is quite expected due to quantum confinement effect. But, after cobalt doping the band gap is reduced to 2.86 eV and 2.55 eV for 0.02 and 0.04 Co2 þ , respectively. This reduction in the band gap, after doping, is illustrated as a line diagram in the inset of Fig. 4c. The band gap of the material is calculated by extrapolating [F(R)hu]1/2 vs. energy at [F(R)hu]1/2 ¼0. Several approaches are made to explain the reduction of band gap after doping. Some group has reported that doping does not reduce the actual band gap of TiO2, instead introduces some mid-band gap states which results in red shift of band gap. But some other reports specified the narrowing in band gap to the sp–d exchange interactions of sp electrons of host with the d-electrons of dopant [16,20,21]. Fig. 5a and b represents the typical PL spectra of undoped and doped TiO2 samples excited at two different wavelengths, 320 nm and 395 nm, respectively. For clear observation of the position of emission peaks, we have done Gaussian fitting of one of the emission peaks (correctness, r2 ¼0.99983). The emission spectra of the samples usually consists of a UV emission peak at 392 nm, one violet emission peak at 429 nm, two blue emission peaks at 457 nm, 491 nm and one green emission peak at 535 nm (Fig. 5a). The UV emission is considered as the band edge emission of the host TiO2. The appearance of this band is due to the phonon assisted indirect transition from edge (X) to the center (G) of Brillouin zone, X1b to G3 [22]. The 429 nm peak can be ascribed to self trapped excitons (STE) localized on TiO6 octahedra [23]. The STE originates by the interactions of conduction band electrons localized on Ti 3d orbital with holes present in O 2p orbital of TiO2. Whether STE will be localized on TiO6 or not depends on the compactness of the octahedra chain and also on the restriction on excitons transfer from one part of the chain to the other [15]. The peaks at 457 nm and 535 nm are attributed to the defect centers associated with oxygen vacancies. These defect centers are originated by trapping one and two electrons respectively in the oxygen vacancies [21,23]. The most intense blue emission peak at 491 nm is coming as a charge transfer band from Ti3 þ to oxygen anion in a TiO26 complex associated with oxygen vacancies [24]. Changing the excitation wavelength to E nm results in an orange emission peak at 598 nm, in addition to emission peaks at 491 nm and 535 nm, respectively (Fig. 5b). This orange peak can be ascribed to surface related trap states [5]. From the observation of the PL spectra at the two excitations, it can be inferred that doping is neither introducing Co2 þ related emission peak nor is shifting the peak position. Only reduction in PL intensity is found for both the doped samples. The decrease in emission intensity with doping is observed in Ce doped TiO2 samples, although they found a slight shifting of a particular emission peak. They ascribed this shifting due to band tailing effect [5]. While in case of ZnO samples, researchers have found only reduction in emission peak intensity on Mn incorporation, with no appreciable peak shifting [34]. They attributed this lowering in intensity to the formation of large number of nonradiative centers on Mn doping. In our case we have not found any noticeable shifting of emission peaks. Incorporation of Co2 þ in TiO2 only decreases the emission intensity by forming large number of nonradiative centers [34], which act as luminescence quencher. As explained in XRD, the presence of cobalt on the grain boundary affects the crystallinity of the sample. Since photoluminescence is a technique to understand the presence of oxygen defects, trap states etc. Thus, we can conclude the decrease in the emission intensity as a collective contribution from dopants;
Fig. 5. PL spectra at an excitation wavelength of (a) 320 nm (b) 395 nm. Inset of (a) shows the variation of intensity ratio of UV to visible emission peaks with dopant concentration, and (c) represents variation of intensity ratios of visible emission peaks with Co2 þ concentration. (d) PLE spectra monitored at 598 nm (for interpretation of the references to color in this figure, the reader is referred to the web version of this article).
nonradiative centers produced on doping, poor crystallinity of doped samples, and oxygen defects created on Ti4 þ substitution by Co2 þ .
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An inspection of the UV and visible emission peaks (Fig. 5a) of TiO2 reveals that the emissivity of the material in the UV region is considerably less compared to visible regions, which is even less in the two doped samples. To have an understanding of this, we have plotted IUV/Idefect(Blue) intensity ratio for all the samples, and have found that the intensity ratio decreases as we start from undoped to the two doped samples (inset of Fig. 5a). This indicates that doping distorts the structural regularity and band structure of TiO2 nanoparticles by forming large number of defects [25]. These defects are responsible for enhanced visible emission intensity in the material compared to UV emission. For further understanding on the effect of this dopant concentration on the visible emission peaks (Fig. 5c), we have plotted I491/I535 and I491/I598 emission intensity ratios, of the 395 nm excited emission spectra, against Co2 þ concentration. From the plot it is observed that I491/I535 and I491/I598 emission intensity ratios are nearly same for 0.02 Co2 þ , but decrease profoundly at the Co2 þ doping concentration of 0.04 mol. Many people have reported this observation due to concentration quenching effect [26]. In conformity with this quenching effect, the increase in dopant amount decreases the distance between active ions, and thus increases the interactions between ions forming quenching centers. At lower doping concentration, the interactions between ions are too weak to have an effect on the energy levels of each doped ions. But at higher dopant concentration the distance between ions or between ions and defects decreases, thus allowing energy transfer from one ion to another. In this process, the transferred energy may also be captured by impurity or defects, enhancing nonradiative relaxation process and hence decreasing the visible emission intensity ratio [27–29]. In our case there is not much variation in the intensity ratio at x¼ 0.02 compared to 0.00, since ion centers are not closer enough. But when dopant concentration has a value of 0.04 mol, the intensity ratio is completely quenched, indicating strong coupling among ions or among ions and defects, thus degrading PL intensity. Photoluminescence excitation (PLE) spectra were taken to differentiate the excitation behavior of doped and undoped samples (Fig. 5d). The PLE spectra were recorded monitoring 598 nm emission peak. The two samples show one strong excitation peak at 398 nm. This peak is due to interband transition in host sample. Along with this it also exhibits one long tail in the visible region. Actually the absorptions ranging from 400 to 550 nm are attributed to the surface oxygen vacancies [10]. These two spectra are different only at 459 nm and 441 nm peaks. The 459 nm peak can be associated with d–d transition Co2 þ from 4 T1g to 4T1(P) [19]. The 441 nm band in the excitation spectra was assigned to trapped holes [30]. In Fig. 6 time dependent PL decay of the samples are shown. The decay was measured monitoring 491 nm emission wavelength. The curves were fitted bi-exponentially, using the equation [31] t t y ¼ y0 þ a1 exp þ a2 exp ð2Þ
t1
t2
where t1 and t2 are decay constants and a1 and a2 are preexponential factor. For bare TiO2, the values of t1 and t2 are 1.16 and 0.634 ns and a1 and a2 are 5066.7 and 19,548.06, respectively. The values of t1 and t2 for cobalt doped TiO2 are 1.802 and 0.854 ns, respectively, and values of a1 and a2 are 439.6 and 16,724.85, respectively. The mean lifetimes (tm) are calculated using the following equation [16], and the values are coming out to be 0.90 ns for cobalt doped TiO2 and 0.80 ns for TiO2 Pn a t2 tm ¼ Pin¼ 1 i i ð3Þ i¼1
ai ti
Thus, the lifetime of the doped TiO2 is longer compared to undoped TiO2. Since electrons and holes in the small sized TiO2
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Fig. 6. Fluorescence decay curves of undoped and cobalt doped TiO2 nanoparticles monitored at the emission wavelength of 491 nm.
nanoparticles are confined within a small region, the extent of overlapping of their wave function is more leading to faster recombination [32]. But inclusion of dopant states in the mid gap states of TiO2 hindered the degree of overlapping of charge carriers thus giving them longer lifetime [16]. In undoped TiO2 the electrons from conduction band relax in the shallow or deep trap states. Usually trapping is a nonradiative process and shallow traps have shorter lifetime than deep traps [33]. In our results the relatively faster component in TiO2 is due to the relaxation of electrons on shallow trap levels such as Ti3 þ or surface trap states. The trapping probability in case of doped one is higher than undoped one because in addition to oxygen related defects, Co2 þ d states are also introduced in the mid-band gap. In undoped TiO2, after relaxation of conduction band electrons in the defect states, it directly transits to the valence band undergoing faster recombination with holes. But Co2 þ doping introduces energy levels of d states, splitted by crystal field, near about oxygen and Ti3 þ related trap states. Therefore, in doped case trap to trap transition of electrons take place from defect to Co d states. It relaxes in the d states of Co2 þ from which it undergoes transition to the ground states. This process results in the relatively longer lifetime of charge carriers in the doped sample.
4. Conclusion In summary, the optical property of TiO2 nanoparticles changes on cobalt doping. XRD and FTIR confirm Co2 þ substitution on Ti4 þ site in TiO2 and also formation of oxygen defects on doping. Doping shifts the absorption onset to visible region and lowers the band gap. The decrease in emission intensity on doping indicates lower recombination of charge carriers due to the presence of nonradiative centers and poor crystallinity of doped samples. The lowering in UV to visible intensity ratio, after each doping, confirms degradation of structural quality. This was also confirmed by X-ray diffraction. The decrease in intensity ratio of visible emission peaks after doping is due to concentration quenching effect. Doping of cobalt increases the lifetime of photogenerated carriers by introducing trap states associated with d states of cobalt, where increase in relaxation time of conduction electrons is observed before it undergoes recombination. The increased lifetime indicates the availability of charge carriers for photocatalysis.
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