Polarized Luminescence Properties of TiO2:Sm3+ Microfibers and

May 7, 2009 - Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Qinghe Road 390, Jiading. District, Shanghai 201800 ...
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J. Phys. Chem. C 2009, 113, 9595–9600

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Polarized Luminescence Properties of TiO2:Sm3+ Microfibers and Microbelts Prepared by Electrospinning Guoping Dong,*,† Xiudi Xiao,† Yingzhi Chi,‡ Bin Qian,† Xiaofeng Liu,† Zhijun Ma,§ Song Ye,§ E. Wu,‡ Heping Zeng,‡ Danping Chen,† and Jianrong Qiu*,§ Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Qinghe Road 390, Jiading District, Shanghai 201800, People’s Republic of China, Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China, State Key Laboratory of Precision Spectroscopy, East China Normal UniVersity, Shanghai, 200062, People’s Republic of China, and State Key Laboratory of Silicon Materials, Zhejiang UniVersity, Hangzhou, Zhejiang 310027, People’s Republic of China ReceiVed: January 28, 2009; ReVised Manuscript ReceiVed: April 21, 2009

One-dimensional TiO2:xSm3+ microfibers and microbelts were prepared by electrospinning. X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive X-ray spectrometer (EDS), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), electron paramagnetic resonance (EPR), and polarized luminescence measurements were performed to characterize these TiO2:xSm3+ microfibers and microbelts. SEM and HRTEM results indicate that TiO2:xSm3+ microfibers calcined at different temperature are belt-like. The average width and thickness of TiO2:xSm3+ microbelts calcined at 700 °C is about 1.0 and 0.4 µm, respectively. Under ultraviolet excitation (λex ) 330 nm) and commercial 365 nm UV lamp excitation, these TiO2:xSm3+ microbelts show intense 4G5/2 f 6H5/2,7/2,9/2 emissions of Sm3+ ions. Polarized luminescence measurement confirms that the luminescence of single TiO2:xSm3+ microbelt is linear polarization. 1. Introduction Titanium oxide (TiO2) is an important oxide semiconductor that can be applied in the fields of photocatalytics, solar energy cells, self-cleaning, etc.1–4 Due to its outstanding properties, e.g., lower phonon energy, higher refractive index, and chemical stability, a number of investigations have been reported on the photoluminescence (PL) of rare earth (RE) doped TiO2 phosphors and films, which indicates that TiO2 is one of the most favorable host materials for RE elements.5–7 Nanomaterials, due to their unique chemical, physical, and optical properties, have gained great attention recently.8–12 Electrospinning, as one of the most important and convenient techniques to produce superlong nanofibers, has been studied by many groups.10–12 A variety of materials, such as organic, inorganic, and hybrid (organic-inorganic composites), have been electrospun into nanofibers successfully.13–17 By tuning the electrospun parameters (working distance, voltage, flow rate, etc.) and physical-chemical properties of the precursor solution (viscosity, dielectric coefficient, conductivity, etc.), the diameter of electrospun fibers can be changed from tens of nanometers to several micrometers, and the fibers can be electrospun into different shapes, such as solid, hollow, porous, belt-like, and so on.12,18–20 Although a few papers have been reported on the luminescent properties of TiO2:Sm3+ phosphors and films,5–7 the research on TiO2:Sm3+ electrospun fibers is scarce. The light-emitting source based on electrospun nanofibers is confined into onedimensional nanoscale. These luminescent fibers can be applied in the nano/micro devices of lighting, display, sensor, on-chip * To whom correspondence should be addressed. E-mail: [email protected] (J.R.Q.); [email protected] (G.P.D.). † Chinese Academy of Sciences and Graduate School of the Chinese Academy of Sciences. ‡ East China Normal University. § Zhejiang University.

illumination, etc.21,22 In addition, compared with the bulk TiO2: Sm3+ phosphors and films, these one-dimensional electrospun fibers may possess some peculiar optical properties (such as polarized luminescence etc.). Therefore, it is meaningful to study the luminescent properties of TiO2:Sm3+ electrospun fibers/belts. In this paper, we first prepare Sm3+ doped TiO2 microfibers and microbelts by electrospinning with calcination at different temperatures. PL properties of TiO2:xSm3+ electrospun samples are also investigated systematically with a variety of doping concentrations and calcined temperatures. What is more important, the polarized luminescence property of a single in situ TiO2:xSm3+ electrospun microfiber is studied, which indicates that the PL from the microfiber is partly polarized. This TiO2: xSm3+ electrospun microfiber with intense polarized PL can be potentially used as a linearly polarized light resource in nano/ micro photoelectric devices. 2. Experimental Section 2.1. Preparation of Microfibers and Microbelts. The TiO2: xSm3+ microfibers and microbelts were prepared by electrospinning and successive calcination. The doping concentrations of Sm3+ ions are changed from 1 to 10 mol % of TiO2. First, the stoichiometric amount of Sm(NO3)3 was added to 10 mL of ethanol under magnetic stirring. Then 3.4 mL of tetrabutyl titanate (Ti(OC4H9)4) was dropped slowly under magnetic stirring for 2 h. Lastly, 1 g of poly(vinyl pyrrolidone) (Mw ) 1 300 000) was dissolved in the above solution with thorough stirring at room temperature. This viscous solution was used for electrospinning. The electrospinning equipment was similar to that used in our previous work.17 Grounded aluminum foil was used as the collector plate. The parameters of electrospinning were optimized as follows: the working distance was 12 cm, the diameter of the needle was 1.2 mm, a voltage of 20 kV was applied to the solution, and the feeding rate of the solution was maintained as 2 mL/h. After electrospinning for 5 h, the

10.1021/jp900819w CCC: $40.75  2009 American Chemical Society Published on Web 05/07/2009

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Figure 1. Experimental setup for characterization of the polarized luminescence property of a single microfiber.

as-prepared nonwoven mats were taken off and calcined at 500, 600, 650, 700, 750, and 800 °C in air for 2 h with a heating rate of 100 deg/h. The samples (TiO2:2 mol % Sm3+) after calcination at 500, 600, 650, 700, 750, and 800 °C were labeled samples a, b, c, d, e, and f, respectively. And the samples with doping concentrations of Sm3+ ions (x value) at 1, 3, 4, 6, and 10 mol % after calcination at 700 °C were labeled samples g, h, i, j, and k, respectively. 2.2. Characterizations. X-ray diffraction (XRD) was performed to determine the structure of the samples, using a Rigaku D/MAX X-ray diffractometer with Cu KR radiation. Scanning electron microscope (SEM, JSM-6535) equipped with an energy-dispersive X-ray spectrometer (EDS) was used to study the size, morphology, and element distribution of the electrospun nanofibers. The high-resolution transmission electron microscopy (HRTEM) images were obtained with a JSM-2100F transmission electron microscope. The photoluminescence (PL) spectra and decay curve of electrospun samples were measured with a FLS920 fluorescence spectrophotometer. The electron paramagnetic resonance (EPR) spectrum was recorded with a Bruker A300 EPR spectrometer at the frequency of X-band (9.866 GHz). All the measurements were performed at room temperature. The homemade experimental setup for characterization of the polarized luminescence property of single microfiber is illustrated in Figure 1. The single microfiber is addressed by scanning confocal microscopy. By focusing a 532-nm excitation laser source (CW, 0.6 mW) to a 1-µm spot in the center of a microfiber with a high-resolution numerical aperture microscopy objective (NA ) 0.90, ×100), PL from the TiO2:xSm3+ microfiber was collected and transmitted to the silicon avalanched photodiode (APD) after spatial and spectral filtering. The PL signal will be analyzed and recorded by spectrograph when the flip mirror was on. The area of the raster scan was maintained at 20 × 20 µm2 by driving the mobile mirror. The TiO2:xSm3+ microfiber was excited with a linearly polarized laser parallel to the long axis of the microfiber. By rotating the waveplate before the polarizer, polarized PL was checked dot by dot as a function of the polarization angle. The polarization ratio (P) was calculated as follows: P ) (Imax - Imin)/(Imax + Imin), where Imax and Imin were the maximum and minimum intensity of PL, respectively. 3. Results and Discussion 3.1. Structure and Morphology. The XRD patterns of TiO2: xSm3+ microfibers calcined at various temperatures are shown in Figure 2. For the sample TiO2: 2 mol % Sm3+ calcined at 500 °C for 2 h (sample a), single anatase TiO2 has been detected. The diffraction peaks at 2θ ) 25.3°, 37.8°, 48.1°, 54°, 62.8°,

Figure 2. XRD patterns of the TiO2:xSm3+ fibers calcined at different temperatures. The asterisk denotes the rutile TiO2 phase.

68.8°, 75.1°, and 82.8° can be ascribed to the diffraction of (101), (004), (200), (105), (204), (116), (215), and (224) crystal faces of anatase TiO2. No additional diffraction peak can be observed. When the calcined temperature increases from 500 to 600 and 700 °C (samples b and d), the XRD patterns are similar to that of sample a except for the sharpness of diffraction peaks. The anatase TiO2 formed at 700 °C is well-crystalline compared with that obtained at lower temperature. When the calcined temperature reaches 800 °C (sample f), several new diffraction peaks at 2θ ) 28°, 35.7°, 41.1°, 45°, 54.7°, 57.8°, 65.4°, and 69.8° appear, which are assigned to the diffraction of (110), (101), (111), (210), (211), (220), (310), and (301) crystal faces of rutile TiO2. The anatase TiO2 was transformed into rutile TiO2 at 800 °C and sample f consisted of mixed anatase and rutile phases. The ratio between anatase and rutile can be calculated by using XRD patterns according to the empirical relationship reported by Depero et al.23

R(T) ) 0.679

(

IR IR + 0.312 IR + IA IR + IA

)

2

where R(T) is the percentage content of rutile, and IA and IR are the intensity of the main anatase (101) (2θ ) 25.3°) and rutile (110) (2θ ) 28°) diffraction peaks, respectively. Then the percentage content of rutile in sample (f) is about 45%, which indicates that the main phase is anatase TiO2. XRD patterns of the TiO2:xSm3+ fibers with different Sm3+ concentration are shown in Figure S1 in the Supporting Information. All the samples are calcined at 700 °C. With the increase of Sm3+ ions from 2 mol % to 6 mol %, almost no obvious difference can be observed. The main anatase diffraction peak of (101) crystal face maintains at 2θ ) 25.3° (d ) 3.51 Å). When the concentration of Sm3+ ions reaches 10 mol % (sample k), the diffraction peak ascribed to the (101) crystal face of anatase TiO2 shifts slightly to high 2θ ) 25.5° (d ) 3.49 Å). Several new diffraction peaks at 2θ ) 31.3° and 36.3° appear, which are ascribed to (111) and (200) crystal faces of faced-centered cubic (fcc) Sm2O3 crystals (JCPDF: 33-1146). The cell parameters of FCC Sm2O3 crystals are as follows: a ) b ) c ) 4.93 Å, which is larger than those of body-centered anatase TiO2 (a ) b ) 3.782 Å, c ) 9.502 Å, JCPDF: 841286). Therefore, the crystal lattice of anatase TiO2 will be compressed and the diffraction peaks shift to high 2θ slightly. In addition, with the continuous substitution of Ti4+ ions by

Luminescence Properties of TiO2:Sm3+ Microfibers

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Figure 3. SEM images of TiO2:2 mol % Sm3+ microfibers calcined at (a) 0, (b) 500 (sample a), (c) 600 (sample b), (d) 700 (sample d), and (e) 800 °C (sample f). The scale bar in panels a-e is 10 µm. (f) The EDS spectrum of sample d. The presence of the C peak results from the conducted C films coated on the sample in the course of SEM measurement.

Sm3+ ions, a large number of oxygen vacancies may be formed to compensate charge balance (this can be confirmed by the EPR result in Figure 8). This can also compress the crystal lattice of anatase TiO2 and result in the shift to high 2θ. Figure 3a-e shows the SEM images of TiO2:2 mol %Sm3+ microfibers calcined at various temperatures. For the as-prepared microfibers in Figure 3a and Figure S2 in the Supporting Information, it can be seen that most of the microfibers are beltlike, which may be due to incomplete drying before the fiber reaches the grounded collector. When a partially drying jet collides on the surface of the grounded collector, the jet can spread out on the surface, and then both edges of the jets can rebound slightly until the jet is dry enough. Then electrospun belts with a shallow groove in the middle are formed.24,25 From the SEM images in Figure 3a and Figure S2 in the Supporting Information, it can be estimated that the width and thickness of the as-prepared microbelts is about 2-2.5 and 1-1.2 µm, respectively. After calcinatioin at 500 °C (sample a), the size of the microbelts shrinks significantly due to the decomposition of the organic species. The width and thickness of the microbelts decreases to 1.5-1.8 and 0.6-0.8 µm, respectively. When the calcined temperature increases to 600 °C (sample b), the size of the microbelts shrinks significantly due to the decomposition of the residual organic species. The width and thickness of the microbelts further decreases to ∼1.0 and ∼0.5 µm, while the surface of the microbelts is still smooth. When the calcined temperature reaches 700 and 800 °C (samples d and f), almost no change of size and morphology can be observed of microbelts. This indicates that the decomposition of organic species has been accomplished at 600 °C. Figure 3f shows the EDS spectrum of sample d, which confirms the presence of Ti, Sm, and O elements. The composition of microbelts is about 99.07TiO2:0.93Sm2O3, although an unavoidable error may be involved in the EDS measurement. This is close to the as-designed composition (TiO2:2 mol % Sm). The HRTEM images of TiO2:2 mol % Sm3+ microbelts calcined at 700 °C (sample d) are shown in Figure 4. Figure 4a displays the TEM image of a twisted microbelt, and the insert is the HRTEM image of the end of this twisted microbelt. From these images, it can be seen that this fiber is belt-like, and the

Figure 4. (a-c) The HRTEM images of TiO2:2 mol % Sm3+ microbelt calcined at 700 °C (sample d). Insert: The end of the microbelt in panel a. (d) The corresponding selected area electron diffraction pattern. The dotted frame in panel c shows a single anatase TiO2 particle.

width and thickness of the microbelt is about 1.0 and 0.4 µm, which is consistent with the results of SEM in Figure 3. To distinguish the size of anatase TiO2 crystals, we prepare the amplified TEM image of the microbelt in Figure 4b, which indicates that the average size of anatase TiO2 crystals is about 15 nm. This result can be seen more clearly from Figure 4c and Figure S3 in the Supporting Information. Figure 4c shows the HRTEM image of a single anatase TiO2 particle from the microbelt. The crystal lattice fringe with a spacing d value of ∼0.35 nm can be observed directly, which corresponds to the (101) crystal face of anatase TiO2. This result, which is consistent with the results of XRD, further confirms the formation of anatase TiO2. The selected area electron diffraction pattern (SEAD) of the corresponding microbelt is illustrated in Figure 4d, which illustrates the polycrystalline rings and can

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Figure 5. Excitation (λem ) 610 nm) and PL (λex ) 330 nm) spectra of TiO2:2 mol % Sm3+ microfibers calcined at 700 °C (sample d). The inset shows the luminescence photograph of TiO2:2 mol % Sm3+ microfibers ultrasonically dispersed in the ethanol solution under the excitation of a commercial 365 nm UV lamp.

be indexed for anatase TiO2. This indicates that the anatase TiO2:2 mol % Sm3+ microbelt is polycrystalline. 3.2. Luminescence Properties. Figure 5 shows the excitation (λem ) 610 nm) and PL (λex ) 330 nm) spectra of TiO2:2 mol % Sm3+ microbelts calcined at 700 °C (sample d). Three obvious emission peaks centered at 580, 610, and 660 nm were observed in the PL spectrum of sample d, which can be ascribed to the transitions from the excited 4G5/2 state to the 6H5/2, 6H7/2, and 6 H9/2 states, respectively. These results are consistent with those of TiO2:Sm3+ phosphors and films.5–7 Due to the split of 6HJ levels into 2J + 1 sublevels in the crystal field, the fine structure can be observed clearly in the PL spectrum. For the excitation spectrum of sample d, one intense broad excitation band centered at 330 nm is observed, which corresponds to the anatase TiO2 host absorption. This confirms the effective energy transfer from TiO2 host to Sm3+ ions. Under the excitation of a commercial 365 nm UV lamp, these microbelts also show the intense red emission (the insert in Figure 5), which will remarkably broaden their potential applications. To compare the relative PL intensity of different samples, the integrated intensity of the strongest emission peak at 610 nm (4G5/2 f 6H7/2 emission of Sm3+) in PL spectra is used as the standard. The calcined temperature dependence of the PL intensity of TiO2:2 mol % Sm3+ microbelts under 330 nm excitation is shown in Figure 6a. With the increase of calcined temperature from 500 to 700 °C, the PL intensity of the sample enhances gradually. When the calcined temperature reaches 800 °C, the PL quenches quickly. This is probably due to the phase transformation from anatase into rutile TiO2 under the calcination at 800 °C. To obtain the optimal PL parameters of TiO2: xSm3+ microbelts, we prepare the TiO2:xSm3+ microbelts calcined at 700 °C with different doping concentrations of Sm3+ ions. The doping concentration dependence of the PL intensity is displayed in Figure 6b, which indicates the optimal doping concentration of Sm3+ ions is 2 mol %. When the doping concentration of Sm3+ ions exceeds 2 mol %, the PL of TiO2: xSm3+ microfibers quenches quickly. From the results discussed above, it can be deduced that the optimal parameters for PL is TiO2:2 mol % Sm3+ microbelts calcined at 700 °C. The decay kinetics for the 4G5/2 f 6H7/2 emission (λem ) 610 nm) of Sm3+ from TiO2:2 mol % Sm3+ microbelts calcined at

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Figure 6. (a) Integrated PL intensity (λem ) 610 nm) of TiO2:2 mol % Sm3+ microfibers collected as a function of the calcined temperature; (b) Integrated PL intensity (λem ) 610 nm) of TiO2:xSm3+ microfibers calcined at 700 °C collected as a function of the doping concentrations of Sm3+ ions.

Figure 7. Decay curve for the 4G5/2 f 6H7/2 emission of Sm3+ (λem ) 610 nm) from TiO2:2 mol % Sm3+ microfibers calcined at 700 °C (sample d).

700 °C (sample d) was measured at room temperature (Figure 7). The decay curve for the emission was a non-exponential process, which is usually observed when the PL is contributed by different origins or energy transfer occurs. This decay curve could be fitted by a bi-exponential curve: I ) I1 exp(-t/τ1) + I2 exp(-t/τ2). τ1 and τ2 are determined to be 141.8 and 517.2 µs, respectively. The fast component (τ1 ) 141.8 µs) can be associated with luminescence from defect states in the TiO2,26 while the slow component (τ2 ) 517.2 µs) is ascribed to the 4 G5/2 f 6H7/2 transition of Sm3+ ion. The decay curves for the 4 G5/2 f 6H5/2,9/2 emissions (λem ) 580, 660 nm) of sample d and 4G5/2 f 6H7/2 emission (λem ) 610 nm) of sample b are also displayed in Figure S4 in the Supporting Information, which are almost the same as those in Figure 7. It is well-known that nanostructural TiO2 is good at trapping electrons for their high defect states.27,28 To investigate the energy transfer mechanism from TiO2 host to Sm3+ ions in TiO2: xSm3+ microbelts, an EPR spectrum of sample d is measured

Luminescence Properties of TiO2:Sm3+ Microfibers

Figure 8. EPR spectrum of TiO2:2 mol % Sm3+ microfibers calcined at 700 °C (sample d).

in Figure 8. The EPR signal with a g value of 2.000-2.004 corresponds to a single electron trapped in oxygen vacancies, and the broad EPR signals on the left side of the sharp peak are most likely due to those of surface oxygenated active species.29,30 On the basis of the results discussed above and previous papers,31 a model for the energy transfer from TiO2 host to Sm3+ ions is proposed (Figure S5, Supporting Information). The excited electron on the conduction band (CB) of TiO2 relaxes to defect states in TiO2, followed by energy transfer to the 4G5/2 excited state of the Sm3+ ion, resulting in luminescence when the electron transmits to the 6HJ ground state. Bashouti et al. have studied the polarized luminescence of aligned CdS nanowires embedded in polymer nanofibers by electrospinning and found the effectively polarized emission of these aligned CdS nanowires.32 However, no investigation of the polarized luminescence property of single in situ electrospun microfiber is reported up to the present. By rotating the waveplate before the polarizer (see Figure 1), the polarized angle dependence of the PL intensity of TiO2:2 mol % Sm3+ microbelts calcined at 700 °C (sample d) is measured and shown in Figure 9. The difference of the polarized angle corresponding to Imax and Imin is about 90°. The dotted line in Figure 9 shows the fitted sinusoid (I ) 0.65 sin(2θ - 2π/3) + 7.37). It can be found that the experimental curve is consistent with this fitted sinusoid. By using the equation P ) (Imax - Imin)/(Imax + Imin), the polarization ratio (P) of sample c is calculated to be 0.09. In the previous paper,33 Ma et al. have observed polarizationdependent PL from silicon nanowire fibers, and they ascribe it to the combined effects of the one-dimensional shape of the silicon nanowire and the large dielectric contrast between the silicon nanowire and the ambient. Therefore, it can be deduced that the one-dimensional structure of TiO2: Sm3+ fibers may induce the linear polarization PL. In addition, from the TEM images in Figure S3 in the Supporting Information, it can be observed that the TiO2 crystal grains have the tendency to arrange parallel to the surface of fibers, which may be due to the confine effect of the one-dimensional structure. This slightly ordered arrangement of the TiO2 crystal may also partially contribute to the linear polarization PL of TiO2: Sm3+ microsized fibers. From the course of polarized luminescence measurement, it can be anticipated that the polarization ratio can be improved by the size and shape control of electrospun fibers. This will be further studied in detail.

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Figure 9. PL intensity of TiO2:2 mol % Sm3+ microbelts calcined at 700 °C (sample d) collected as a function of the polarization angle. The black squares are the experimental results, and the red dotted line is the fitted sinusoid (I ) 0.65 sin(2θ - 2π/3) + 7.37).

4. Conclusions TiO2:xSm3+ microfibers and microbelts were prepared by electrospinning. XRD patterns confirm the formation of anatase TiO2 in TiO2: xSm3+ microfibers calcined at 500 to 600 and 700 °C, and the anatase partially transforms into rutile TiO2 after calcination at 800 °C. SEM and HRTEM results indicate that TiO2:xSm3+ microfibers are belt-like. The width and thickness of the as-prepared TiO2:xSm3+ microbelts is 2-2.5 and 1-1.2 µm, and those of the microfibers calcined at 700 °C are about 1.0 and 0.4 µm, respectively. PL spectra indicate that these TiO2:xSm3+ microbelts show intense 4G5/2 f 6H5/2,7/2,9/2 emissions of Sm3+ ions, and the energy transfer mechanism from TiO2 host to Sm3+ ions is also discussed. From the results of polarized luminescence measurement, it can be deduced that the luminescence of single TiO2:xSm3+ microbelt is linear polarization. This TiO2:xSm3+ electrospun microfiber with intense polarized PL can be potentially used as a biological marker, linearly polarized light resource, polarization sensor, etc. in nano/micro photoelectric devices. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 50872123, 50802083, and 60807027), the National Basic Research Program of China (2006CB806000), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT0651). The authors appreciate Dr. Yixi Zhuang of Zhejiang University, Dr. Jian Ruan, Dr. Qiang Zhang, and Dr. Geng Lin of Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences for their help with measurements and helpful discussion. Supporting Information Available: Figure S1, XRD patterns of the TiO2:xSm3+ fibers with different Sm3+ concentrations; Figure S2, SEM images of the as-prepared TiO2:2 mol % Sm3+ microfibers; Figure S3, TEM image of a single TiO2:2 mol % Sm3+ microbelt calcined at 700 °C and the HRTEM image of this microbelt; Figure S4, decay curves for the emissions of Sm3+ from TiO2:2 mol % Sm3+ microfibers calcined at different temperatures; and Figure S5, Schematic diagram of energy transfer from TiO2 host to Sm3+ ions in TiO2: xSm3+ microbelts. This material is available free of charge via the Internet at http://pubs.acs.org.

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