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Crystallinity control on photocatalysis and photoluminescence of TiO2-based nanoparticles
Journal of Alloys and Compounds 496 (2010) 234–240
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Crystallinity control on photocatalysis and photoluminescence of TiO2 -based nanoparticles Jianjun Wu, Xujie Lü, Linlin Zhang, Yujuan Xia, Fuqiang Huang ∗ , Fangfang Xu ∗ CAS Key Laboratory of Materials for Energy Conversion, Inorganic Materials Analysis and Testing Center, Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), Shanghai, PR China
a r t i c l e
i n f o
Article history: Received 20 August 2009 Received in revised form 6 November 2009 Accepted 10 November 2009 Available online 13 November 2009 Keywords: NanoTiO2 Photocatalysis Photoluminescence Crystallinity Hydrothermal
a b s t r a c t Single anatase TiO2 nanocrystals with controllable crystallinity are prepared by a simple and readily available hydrothermal route. The crystallinity has been successfully manipulated via varying the temperature. TiO2 -240 prepared at 240 ◦ C holds the highest crystallinity, i.e., the largest crystallite size of 14.2 nm and the lowest lattice strain of 12.56 × 10−3 , and the highest photocatalytic efficiency. The feasible method is also successfully applied in the synthesis of Eu3+ -doped TiO2 (TiO2 :Eu3+ ) nanoparticles. Both the photocatalytic activity of TiO2 and the photoluminescence (PL) properties of the TiO2 :Eu3+ intensifies obviously as the crystallinity increases. The photocatalytic activity of nanoTiO2 and PL properties of the TiO2 :Eu3+ are discussed in detail from the viewpoints of crystallinity based on various characterizations. This method can be developed for the deliberate synthesis of other functional nanomaterials. © 2009 Elsevier B.V. All rights reserved.
1. Introduction As a low-cost, widely available, nontoxic and physicochemically stable substance, titanium dioxide (TiO2 ) has attracted great attentions during the past decade due to its scientific and technological importances. TiO2 is widely used in various areas because of its versatile properties [1–12]: such as catalytic activity, photocatalytic activity for pollutant removal, good stability toward adverse environment, photoelectrochemical conversion, sensitivity to humidity and gas, and nonlinear optics. Being a potential solution to the recent severe problems of energy shortages and environment crises, photocatalysis has been studied extensively since Fujishima and Honda announced a TiO2 photochemical electrode for splitting water in 1972 [13]. The general photocatalytic process of a semiconductor involves the formation of photoinduced electrons at the conduction band and holes at the valence band and subsequent chemical reactions with the surrounding media after photostimulated charges move to powder surface. A great number of researches show that the photocatalytic activity of TiO2 depends to a large extent on its crystal structure, morphology, particle sizedistribution, specific surface area, etc. [14–17]. The crystallinity of nanoTiO2 lattice plays an important role on the excited car-
∗ Corresponding author at: CAS Key Laboratory of Materials for Energy Conversion, Inorganic Materials Analysis and Testing Center, Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), 1295 Ding Xi Road, Shanghai 200050, PR China. Fax: +86 21 52416360. E-mail addresses: [email protected] (F. Huang), [email protected] (F. Xu). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.074
rier (electron or hole) transport to affect its photocatalytic activity. However, the systematic experimental photocatalytic studies have made some progress on this nanomaterial, but not sufficient and satisfactory. Despite these extensive research interests in photocatalysis, investigations and applications for the PL properties of TiO2 have not been simultaneously satisfied. As an indirect band gap semiconductor, the band-edge luminescence of TiO2 is very difficult to be observed, and especially for bulk TiO2 the photoluminescence was observed only at 77 K with the intensity maximum at 500 nm [18]. However, recent studies [19,20] on the intense PL properties of TiO2 :Eu3+ have rekindled people’s growing interests. TiO2 was proved as an efficient candidate host in some papers [19–22]. Other researchers suggested that TiO2 :Eu3+ exhibits much higher emission intensity in anatase host than in rutile [23]. For the two charge transfer-related processes, photocatalysis includes generation of charges, separation, transport to the surface, and recombination through the chemical reaction on the photocatalyst surface; PL process of TiO2 :Eu3+ comprises the intrinsic excitation resulted from the f–f inner-shell transitions and the host excitation ascribed to the charge transfer band (CTB) from O–Ti to the Eu3+ ions. Both photocatalysis and PL require a perfect lattice of TiO2 for charges transfer, in order to avoid space charge regions and e–h recombination. So the crystallinity of the TiO2 lattice is to have a pronounced effect on the two processes. However, the correlation between the crystallinity of the host nanoTiO2 and the photoluminescence properties of the TiO2 :Eu3+ has been rarely documented to our best knowledge. The particular interest of our research is to find a simple and feasible hydrothermal route suitable for anatase
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nanoTiO2 with controllable crystallinity as well as TiO2 :Eu3+ , which enable us to make further research on how the photocatalytic activities and PL properties vary with crystallinity of the as-prepared samples. Here, we developed a one-step route to synthesize phase-pure anatase TiO2 with controllable crystallinity by a facile, surfactantfree hydrothermal method, which turns out to be a suitable doping process for Eu3+ . The dependence of photocatalytic activity and PL properties on the crystallinity of TiO2 was fully discussed based on the information obtained via X-ray powder diffraction (XRD), Fast-Fourier Transform (FFT), UV–vis diffuse reflectance spectra (UV–vis), transmission electron microscope (TEM) associated with high-resolution TEM (HRTEM), energy-dispersive X-ray spectra (EDS), Brunauer–Emmett–Teller (BET) specific surface areas, Fourier transform infrared spectroscopy (FT-IR) and photoluminescence (PL) spectra.
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the intercept of the extrapolation of this linear relation to the ordinate (i.e., sin = 0) and ε was determined from the slope of this linear relation. For poorly crystallized samples, the linear relationship between  cos and sin was not very clear. While for highly crystallized samples, however, a good linearity was observed, and so the reliable values of D and ε were obtained [25]. The band gap energy of the powders was determined via UV–vis diffuse reflectance spectrum on a spectrophotometer (Hitachi U3010) equipped with an integrating sphere and with BaSO4 as a reference in the wavelength range of 250–800 nm. Field-emission TEM (JEM 2100F) as well as HRTEM was used to study the morphology, crystallinity and dimensions of the TiO2 -based powders. The excitation and emission spectra were recorded in a spectrofluorophotometer (Shimadzu RF-5301 PC) at room temperature to investigate the PL properties of TiO2 :Eu3+ , and the composition ratio was semi-quantitatively analyzed by EDS (Oxford INCA Model 6498). The FT-IR information is recorded using Shimadzu IR Prestige-21 instrument in the range of 400–4000 cm−1 . The Brunauer–Emmett–Teller (BET) specific surface areas were determined through nitrogen sorption isotherms at 77 K using a Micromeritics ASAP2010 instrument and calculated from the linear part of the BET plot. Photocatalytic activity test was conducted as shown in our previous papers [26–28]. Methyl orange (MO) was adopted as a representative organic pollutant to evaluate the photocatalytic activity of as-prepared nanoTiO2 under UV irradiation.
2. Experimental
3. Results and discussion
2.1. Synthesis All the chemicals were used without further purification. In all cases, the molar ratio was fixed to be H2 O:TNB = 2:1, and Eu3+ :TiO2 = 2%. In a typical synthesis for TiO2 :Eu3+ , 5.0 mL titanium n-butoxide (TNB, Ti(OBu)4 ) was diluted in 18.5 mL of anhydrous ethanol, stirred for 10 min to obtain a homogeneous pale yellowish solution simultaneously with Eu2 O3 dissolved in 5.0 mL glacial acetic acid (HAc). After the two solutions were mixed via vigorous stirring for 30 min, 1.0 mL deionized water was added dropwise to the mixture. Then, the uniform translucent sol was transferred into a Teflon-lined autoclave (50 mL capacity, 60% filling). Further, it was heated to a desired temperature with a rate of 3 ◦ C/min and maintained for definite reaction duration (6 h) without stirring. Followed by cooling gradually to room temperature in air, the as-prepared powders were collected by centrifuging (3000 r/min, 15 min), washing with anhydrous ethanol for 3 times and drying at 80 ◦ C about 4 h. The undoped anatase TiO2 nanocrystals were synthesized in the same procedure without Eu2 O3 and HAc added. Hereafter, Eu3+ /TiO2 -240 and TiO2 240 are denoted for Eu3+ -doped (undoped) TiO2 samples respectively, which have been hydrothermally treated at 240 ◦ C for 6 h. 2.2. Characterization The crystal structure and phase identification of the samples were carried out by FFT and XRD (Rigaku DMax-2200) with a monochromatized source of Cu K␣1 radiation ( = 0.154056 nm) at 1.6 kW (40 kV,40 mA). Since the peak-broadenings generally depend on the two predominat factors: lattice strain (ε) and crystallite size (D), ε and D were seperately determined via Williams and Hall equation [24]: ˇ cos = (K/D) + 2ε sin where ˇ was the full width at half maximum intensity (FWHM) observed, shape factor K was assumed to be 0.90 and was the wavelength of Cu K␣ radiation (0.154056 nm). FWHM of each diffraction line was determined from the profile measured with a scanning rate of 0.25◦ 2/min−1 , which was calibrated by referring to standard silicon powder for instrumental broadening. The plots of cos against sin for different samples were approximated to be linear. D was calculated from
3.1. XRD information The XRD patterns of the nanoTiO2 and TiO2 :Eu3+ samples treated at different temperatures are shown in Fig. 1. All of the powders except for sample TiO2 -120 belong to the anatase type of TiO2 (JCPDS 21-1272). No diffraction peaks observed in sample TiO2 120 indicates that the assembly units are composed of amorphous particles and crystallization only occurs above 120 ◦ C in this condition. But for sample Eu3+ /TiO2 -120, HAc used to dissolve Eu2 O3 (see Section 2.1) also serves as an inhibitor for hydrolysis. Thus, the hydrolysis rate is slowed and it is easier to form anatase. In other words, HAc can promote crystallization [29]. Moreover, from XRD peak broadening analysis, the diffraction peaks become broader with the decrease of hydrothermal-treated temperature, accompanied with greater background noise near the peaks. The crystallinity has been reported to be evaluated by the crystallite size combined with the lattice strain [25]. Crystallite size (D) and lattice strain (ε), calculated separately via the Williams and Hall equation [24] based on the XRD patterns (Fig. 1), are shown in Fig. 2 (the amorphous TiO2 -120 is excluded). Via increasing the hydrothermal temperature, D increases from 9.6 nm to 14.2 nm and ε decreases from 25.08 × 10−3 to 12.56 × 10−3 for the undoped nanoTiO2 , while from 7.3 nm to 11.8 nm, and from 38.25 × 10−3 to 14.82 × 10−3 for the TiO2 :Eu3+ samples. The growth of crystallite and the decrease in lattice strain, (this will be also confirmed with the coming TEM observations), indicate that the crystallinity of the nanoparticles has been enhanced, and that various structural defects, such as small displacement of neighboring atoms, non-
Fig. 1. The XRD patterns of (a) the nanoTiO2 samples, and (b) the TiO2 :Eu3+ samples.
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Fig. 2. The dependence of the crystallite size D and lattice strain ε for (a) the nanoTiO2, and (b) the TiO2 :Eu3+ nanoparticles on the hydrothermal temperature.
following equation using the data of optical absorption vs. wavelength near the band-edge: ˛h = A(h − Eg)
Fig. 3. The UV–vis spectra of the nanoTiO2 smaples, and the plot of (˛h)1/2 vs. the absorption energy (inset).
uniform strain and residual stress of the lattice, have been gradually eliminated. These defects were reasonably supposed to act as scattering centers for electrons and holes, as a consequent, to promote the recombination of these e− /h+ couples and influence the photocatalytic and PL performance. This will be fully discussed later. 3.2. UV–vis analysis From Figs. 3 and 4, which revealed the UV–vis spectra of nanoTiO2 and TiO2 :Eu3+ respectively, an uniform red shift of absorbance edge is observed. The band gap was determined by the
Fig. 4. The UV–vis spectra of the TiO2 :Eu3+ samples and their optical band gaps in the inset.
n/2
where a, n, A and Eg are absorption coefficient, light frequency, proportionality constant and band gap, respectively. In the equation, n reflects the characteristics of the transition in a semiconductor, i.e., n = 1 for direct transition and n = 4 for indirect transition. So plots of (˛h)1/2 vs. the energy of absorbed is used to obtain the band gaps of the nanoTiO2 because of its indirect transition nature [30]. The extrapolation in the inset of Fig. 3 shows that optical energy gap displays a decrease with increase of the hydrothermal temperature from 120 ◦ C to 240 ◦ C, corresponding to that the increasing crystallinity with a larger “band width” narrows the band gap of the sample [31–33]. A similar case occurs in TiO2 :Eu3+ samples with band gap energy dereased from 3.34 eV to 3.09 eV in the increasing order of the hydrothermal temperature (Fig. 4). 3.3. TEM observations The TEM images of TiO2 -120, TiO2 -180 and TiO2 -240 are shown in Fig. 5a–c. The TEM images for TiO2 -150 and TiO2 -210 are shown in Supporting Information (SI) SI-1 and SI-2, respectively. As the TEM images show, the shape and crystallinity of TiO2 particles are strongly dependent on the synthetic temperatures. The nanoparticles are severely agglomerated and poorly crystallized at a low temperature (Fig. 5a). It should be noted that higher temperatures can tailor the crystallinity and the shape tend to exhibit equiaxed geometry bounded by crystallographic facets. The value of the crystallite size is about 11 nm for TiO2 -180, 14 nm for TiO2 240 in agreement with former XRD analyses. HRTEM observation confirms the anatase structure of the as-obtained TiO2 products. Fig. 5d shows the lattice image of a TiO2 grain and its Fast-Fourier Transformed (FFT) diffractogram which is consistent to a [1 0 0]projected diffraction pattern of the anatase TiO2 . Further, the crystallinity of TiO2 improves owing to the increased hydrothermal temperature, seeing from the gradually transparent and clear HRTEM (actually, we almost cannot get the HRTEM for the worstcrystallized TiO2 -120). The enhanced crystallinity for the TiO2 :Eu3+ samples obtains the same order as the nanoTiO2 when tuning the temperature from 120 ◦ C to 240 ◦ C, in accordance with the XRD peak broadening analysis. But the morphology of the nanoparticles changes from polyhedron to rod-like with Eu3+ doping, shown in Fig. 6, which implies that the Eu3+ doping plays an important effect on the crystallographic orientation of TiO2 nanocrystal. As the recent report shows, Eu3+ hinders the growth of specific facets of anatase TiO2 based on the “oriented attachment” mechanism [23]. The similar
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Fig. 5. The TEM images of (a) TiO2 -120, (b) TiO2 -180, (c) TiO2 -240, (d) HRTEM of TiO2 -240, and Fast-Fourier Transformed diffractogram of TiO2 -240 (inset).
Fig. 6. The TEM images of (a) Eu3+ /TiO2 -120, (b) Eu3+ /TiO2 -180, (c) Eu3+ /TiO2 -240, (d) HRTEM of Eu3+ /TiO2 -240, and Fast-Fourier Transformed diffractogram of Eu3+ /TiO2 -240 (inset).
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Fig. 7. (a) MO photodegradation over the nanoTiO2 samples under UV-light irradiation, and (b) photocatalytic reaction kinetics of MO decomposition.
case was also observed in Er3+ -doped TiO2 [34]. Meanwhile, HRTEM of a representative rod also shows its anatase structure, and the corresponding FFT diffractogram demonstrate its single crystal nature (Fig. 6d). Moreover, we synthesized an undoped sample with acetic acid added under the same condition (240 ◦ C, 6 h) as a reference (SI3). The shape of nanoparticle is close to the previous results [29]. No obvious variation in shape can be observed between Fig. 5c and SI-3, which excludes the possibility that the addtion of acetic acid should be responsible for the changes of nanoparticle shape. 3.4. Photocatalytic activity investigations Fig. 7a shows the results of the photocatalytic degradation of MO for nanoTiO2 . The photocatalytic efficiency decreased gradually in the order of the hydrothermal temperature from 240 ◦ C to 150 ◦ C and drastically at 120 ◦ C. To obtain a quantitative understanding on the reaction kinetics of the MO degradation, we applied the pseudo-first order model as expressed by following equation, which is generally used for photocatalytic degradation process if the initial concentration of pollutant is low [35]: ln
C o
C
= kt
where Co and C are the concentrations of dye in solution at time 0 and t, respectively, and k is the pseudo-first-order rate constant. Fig. 7b depicts the photocatalytic reaction kinetics of MO degradation in solution based on the data plotted in Fig. 7a. A rather good correlation to the pseudo-first-order reaction kinetics (R > 0.98) was found from these results. The reaction constant k increased in the order: TiO2 -120 < TiO2 -150 < TiO2 -180 < TiO2 -210 < TiO2 240, from 0.0070 min−1 to 0.3985 min−1 simultaneously with an enhancement of the crystallinity, i.e., the increase in crystallite size and the decrease in lattice strain. Generally, large surface area is likely to exhibit better photocatalytic activity, because a large surface area provides more active sites for adsorbing methyl orange molecules. However, large surface area does not always give better photocatalytic activity. Although TiO2 -120 (118.5 m2 /g), as well as TiO2 -180 (101.3 m2 /g), has a larger surface area than TiO2 -240 (88.2 m2 /g), its photocatalytic activity is lower (Fig. 7b). Considering the degree of crystallinity of TiO2 -240 is higher than that of TiO2 -120 and TiO2 -180 (Figs. 1, 2 and 5), the gradually promoted crystallinity should be mainly responsible for the photocatalysis improvement in the present system. As mentioned above, the photocatalysis improvement is mainly attributed to the gradually enhanced crystallinity. Crystallinity was proved to have an indispensible effect on the two most important processes of the photocatalysis: separation and transport of
charges, as follows [14,15,25,33,36]: (1) the highly crystallized anatase can promote the charge transfer from particle center to surface. The non-uniform strain or residual stress of the poorcrystallized TiO2 lattice leads to disorder and distortion of the TiO2 matrix, which have a severe scattering effect on the transport of charges. Furthermore, an electron and a hole can migrate a longer distance in a well-crystallized crystal than in a poorcrystallized one, separating more of the reducing and oxidizing sites on the surface of the crystal. So the volume recombination may occur less frequently; (2) it eliminates the crystal defects, i.e., impurities, dangling bonds, and microvoids, which behave as recombination centers for the e− /h+ pairs, thus the surface recombination is greatly suppressed. Therefore, it is no wonder that TiO2 -240 of which the crystallite size is about 14.2 nm and lattice strain about 12.56 × 10−3 holds the maximum in the reaction constant k of MO decomposition, i.e., about 57 times of that for TiO2 -120. Further investigation of how the surface area affects the photocatalytic activity is necessary, when the products obtain the similar degree of crystallinity.
4. Photoluminescence studies Fig. 8a depicts the typical excitation spectra of the Eu3+ /TiO2 240. By monitoring the emission line of 612 nm, excitation lines appear at 394 nm, 416 nm, 464 nm, and 534 nm are ascribed to the f–f inner-shell transitions within the Eu3+ 4f6 configuration [37,38]. Besides, a new band appears in the range from 320 nm to 380 nm, although it is not obvious. Based on the previous papers, the new wide band can be attributed to the host excitation and assigned to the charge transfer band (CTB) from O−Ti to the Eu3+ ions [37]. Similar broad band has also been observed and attributed to the CTB from O−Ti to Eu3+ ions in the previous works [21,37]. It is widely accepted that, by distributing the doping Eu3+ ions in the amorphous or glassy TiO2 region, the CTB plays a dominant role in the energy transfer from TiO2 nanocrystallite to Eu3+ ions, and the photoluminescence excitation of Eu3+ though the energy transfer from the anatase nanocrystals is much more efficient than the direct excitation of the Eu3+ crystal field states [19,20]. But here we found that once the Eu3+ ions were localized in the well-crystallized TiO2 region, the characteristic CTB were not obvious and hard to be detected, indicating that the direct excitation of the Eu3+ crystal field states is superior over the host excitation in the case of the photoluminescence excitation of Eu3+ . Additionally, the intrinsic transition 7 F0 → 5 D4 (366 nm) and 7 F0 → 5 G2 (386 nm) of the direct excitation [38] can also decrease (or overlap) the signals detected from the host excitation to a extent.
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Fig. 8. (a) The excitation spectrum of Eu3+ /TiO2 -240 (em = 612 nm), (b) the emission spectra (ex = 394 nm) of the TiO2 :Eu3+ samples, where their maximum emission (em = 612 nm) intensities at 612 nm in the inset.
The five characteristic peaks at 579 nm, 592 nm, 612 nm, 651 nm, 699 nm corresponding to 5 D0 → 7 F0 , 5 D0 → 7 F1 , 5 D0 → 7 F2 , 5 D → 7 F , 5 D → 7 F transitions of Eu3+ ion, respectively, are 0 3 0 4 observed for all the Eu3+ doped samples at the excitation wavelength of 394 nm in Fig. 8b. It can be seen that the 5 D0 emission is intensified with the increment in hydrothermal temperature accompanied with gradually enhanced crystallnity. For 5 D → 7 F transition, the PL intensity was quantitatively anal0 2 ysed and tabulated in the inset of Fig. 6b. A similar case is also observed in the emission spectra excited at 464 nm (shown in SI-4). 5 D0 → 7 F1 (592 nm) is a magnetic-dipole transition while 5 D → 7 F (612 nm) is a hypersensitive forced electric-dipole tran0 2 sition being allowed only at low symmetry with no inversion centre. The intensity ratio (R) of 5 D0 → 7 F2 to 5 D0 → 7 F1 increases as the degree of Eu–O covalence increases [37], so R is widely used to investigate the bonding environment of the Eu3+ ions. The integrated intensity ratio (R) of the samples obtained at different temperatures is shown in Table 1. Note that R increases with hydrothermal temperature, accompanied with the promoted crystallinity, indicating that the covalence degree of the Eu3+ ions increases. The increase of the covalence degree implies the formation of Eu3+ –O2− –Ti4+ bonding. This was also confirmed by the further investigation using FT-IR (Fig. 9). The undoped TiO2 -240 has a strong and broad band in the range of 500–1000 cm−1 , due to Ti–O stretching vibration modes [39], while this band is observed to shift towards lower wavenumber for Eu3+ /TiO2 -240. This behavior is related to the formation of Ti–O–Eu bonding [40]. Moreover, we can also attribute this new band (894 cm−1 ) to the Ti–O stretching model involving the interaction (or chemical contact) with the distributed Eu3+ ions [41,42] through the Eu–O–Ti bonding. On the other hand, the great mismatch of ionic radius between Eu3+ (0.95 Å) and Ti4+ (0.68 Å) makes the doping Eu3+ hardly enter into the TiO2 lattice [20,43], but inclined to distribute in the crystallite surface or interstitials of TiO2 nanocrystals [44]. For the poor-crystallized TiO2 matrix, the Eu3+ has a tendency to form clusters due to the reduction of Eu3+ –Eu3+ distances [45]. The clusters Table 1 The integrated intensity ratio of 5 D0 → 7 F2 /5 D0 → 7 F1 of the samples. Sample 3+
Eu /TiO2 -120 Eu3+ /TiO2 -150 Eu3+ /TiO2 -180 Eu3+ /TiO2 -210 Eu3+ /TiO2 -240 a
I [5 D0 → 7 F2 ] (a.u.)
I [5 D0 → 7 F1 ] (a.u.)
Ra
2.324 2.793 3.228 3.415 3.822
0.901 1.054 1.117 1.149 1.258
2.58 2.65 2.89 2.97 3.05
Integrated intensity ratio of 5 D0 → 7 F2 and 5 D0 → 7 F1.
Fig. 9. FT-IR information of the nanoTiO2 and TiO2 :Eu3+ samples.
are undesirable which lead to an enhanced interparticle contact of the Eu–Eu pairs, thus quench its luminescence through cross relaxation [19,37]. As the crystallinity enhances, the gradual formation of Eu3+ –O2− –Ti4+ bonding leads to reducing the extent of the Eu3+ clusters [37], suppressing the cross relaxation and intensifying the luminescence effectively. Furthermore, the great elimination of the crystal defects, as quenching centers for luminescence, can diminish the undesired nonradiative recombination routes for electrons and holes [46], contributing to the enhanced luminescence. The typical EDS for Eu3+ /TiO2 -120 is shown in SI-5, from which the presence of Eu3+ in the rod-like crystals is confirmed with the atomic percentage about 1.72%, and the value for Eu3+ /TiO2 -240 is 1.68% (SI-6). The “oriented attachment” induced by the Eu3+ is under research. 5. Conclusions Phase-pure anatase TiO2 and TiO2 :Eu3+ nanoparticles with tunable crystallinity were prepared by a one-pot hydrothermal method with no aid of any addictive. The crystallinity was successfully controlled via tailoring the hydrothermal temperature from 120 ◦ C to 240 ◦ C. TiO2 -240 prepared at 240 ◦ C holds the highest crystallinity, i.e., the largest crystallite size of 14.2 nm and the lowest lattice strain of 12.56 × 10−3 . The high crystallinity provides great improvement for the transport and separation of charges, and the elimination of crystal defects, thus contributes to the enhanced photocatalytic activity, e.g., the reaction constant k of MO decomposition for TiO2 -240 is almost 57 times of that for TiO2 -120. Similar
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to the case of photocatalysis, the PL emission intensity of TiO2 :Eu3+ was also promoted as the crystallinity increased. The gradual formation of Eu3+ (O2− (Ti4+ bonding leads to reducing the extent of the Eu3+ clusters, suppressing the cross relaxation. Furthermore, the great elimination of the crystal defects, as quenching centers for luminescence, can diminish the undesired nonradiative recombination, resulting in the enhanced luminescence. These full analyses enrich us a closer understanding on the photocatalysis and photoluminescence, especially in light of crystallinity and this method provides flexibility, selectivity and efficiency for the deliberate synthesis of other functional nanomaterials. Acknowledgements Financial support from National 973 Program of China Grant 2007CB936704 & 2009CB939903, National Science Foundation of China Grant 50772123, 20901083 & 50902143 and Science and Technology Commission of Shanghai Grant 08JC1420200 & 0952nm06500 are gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jallcom.2009.11.074. References [1] M. Okumura, S. Nakamura, S. Tsubota, M. Azuma, M. Haruta, Catal. Lett. 51 (1998) 53–58. [2] P.J. Senogles, J.A. Scott, G. Shaw, H. Stratton, Water Res. 35 (2001) 1245–1255. [3] E. Traversa, G. Gnappi, A. Montenero, G. Gusmano, Sens. Actuators B 31 (1996) 59–70. [4] Y.F. Tu, S.Y. Huang, J.P. Sang, X.W. Zou, J. Alloys Compd. 482 (2009) 382–387. [5] K.S. Raja, M. Misra, V.K. Mahajan, T. Gandhi, P. Pillai, S.K. Mohapatra, J. Power Sources 161 (2006) 1450–1457. [6] Y.Y. Zhang, X.Y. Ma, P.L. Chen, D.R. Yang, J. Alloys Compd. 480 (2009) 938–941. [7] M. Li, S.F. Zhou, Y.W. Zhang, Z.L. Hong, Appl. Surf. Sci. 254 (2008) 3762–3766. [8] M. Li, Z.L. Hong, Y.N. Fang, F.Q. Huang, Mater. Res. Bull. 43 (2008) 2179–2186.
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Crystallinity control on photocatalysis and photoluminescence of TiO2 -based nanoparticles Jianjun Wu, Xujie Lü, Linlin Zhang, Yujuan Xia, Fuqiang Huang ∗ , Fangfang Xu ∗ CAS Key Laboratory of Materials for Energy Conversion, Inorganic Materials Analysis and Testing Center, Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), Shanghai, PR China
a r t i c l e
i n f o
Article history: Received 20 August 2009 Received in revised form 6 November 2009 Accepted 10 November 2009 Available online 13 November 2009 Keywords: NanoTiO2 Photocatalysis Photoluminescence Crystallinity Hydrothermal
a b s t r a c t Single anatase TiO2 nanocrystals with controllable crystallinity are prepared by a simple and readily available hydrothermal route. The crystallinity has been successfully manipulated via varying the temperature. TiO2 -240 prepared at 240 ◦ C holds the highest crystallinity, i.e., the largest crystallite size of 14.2 nm and the lowest lattice strain of 12.56 × 10−3 , and the highest photocatalytic efficiency. The feasible method is also successfully applied in the synthesis of Eu3+ -doped TiO2 (TiO2 :Eu3+ ) nanoparticles. Both the photocatalytic activity of TiO2 and the photoluminescence (PL) properties of the TiO2 :Eu3+ intensifies obviously as the crystallinity increases. The photocatalytic activity of nanoTiO2 and PL properties of the TiO2 :Eu3+ are discussed in detail from the viewpoints of crystallinity based on various characterizations. This method can be developed for the deliberate synthesis of other functional nanomaterials. © 2009 Elsevier B.V. All rights reserved.
1. Introduction As a low-cost, widely available, nontoxic and physicochemically stable substance, titanium dioxide (TiO2 ) has attracted great attentions during the past decade due to its scientific and technological importances. TiO2 is widely used in various areas because of its versatile properties [1–12]: such as catalytic activity, photocatalytic activity for pollutant removal, good stability toward adverse environment, photoelectrochemical conversion, sensitivity to humidity and gas, and nonlinear optics. Being a potential solution to the recent severe problems of energy shortages and environment crises, photocatalysis has been studied extensively since Fujishima and Honda announced a TiO2 photochemical electrode for splitting water in 1972 [13]. The general photocatalytic process of a semiconductor involves the formation of photoinduced electrons at the conduction band and holes at the valence band and subsequent chemical reactions with the surrounding media after photostimulated charges move to powder surface. A great number of researches show that the photocatalytic activity of TiO2 depends to a large extent on its crystal structure, morphology, particle sizedistribution, specific surface area, etc. [14–17]. The crystallinity of nanoTiO2 lattice plays an important role on the excited car-
∗ Corresponding author at: CAS Key Laboratory of Materials for Energy Conversion, Inorganic Materials Analysis and Testing Center, Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), 1295 Ding Xi Road, Shanghai 200050, PR China. Fax: +86 21 52416360. E-mail addresses: [email protected] (F. Huang), [email protected] (F. Xu). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.074
rier (electron or hole) transport to affect its photocatalytic activity. However, the systematic experimental photocatalytic studies have made some progress on this nanomaterial, but not sufficient and satisfactory. Despite these extensive research interests in photocatalysis, investigations and applications for the PL properties of TiO2 have not been simultaneously satisfied. As an indirect band gap semiconductor, the band-edge luminescence of TiO2 is very difficult to be observed, and especially for bulk TiO2 the photoluminescence was observed only at 77 K with the intensity maximum at 500 nm [18]. However, recent studies [19,20] on the intense PL properties of TiO2 :Eu3+ have rekindled people’s growing interests. TiO2 was proved as an efficient candidate host in some papers [19–22]. Other researchers suggested that TiO2 :Eu3+ exhibits much higher emission intensity in anatase host than in rutile [23]. For the two charge transfer-related processes, photocatalysis includes generation of charges, separation, transport to the surface, and recombination through the chemical reaction on the photocatalyst surface; PL process of TiO2 :Eu3+ comprises the intrinsic excitation resulted from the f–f inner-shell transitions and the host excitation ascribed to the charge transfer band (CTB) from O–Ti to the Eu3+ ions. Both photocatalysis and PL require a perfect lattice of TiO2 for charges transfer, in order to avoid space charge regions and e–h recombination. So the crystallinity of the TiO2 lattice is to have a pronounced effect on the two processes. However, the correlation between the crystallinity of the host nanoTiO2 and the photoluminescence properties of the TiO2 :Eu3+ has been rarely documented to our best knowledge. The particular interest of our research is to find a simple and feasible hydrothermal route suitable for anatase
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nanoTiO2 with controllable crystallinity as well as TiO2 :Eu3+ , which enable us to make further research on how the photocatalytic activities and PL properties vary with crystallinity of the as-prepared samples. Here, we developed a one-step route to synthesize phase-pure anatase TiO2 with controllable crystallinity by a facile, surfactantfree hydrothermal method, which turns out to be a suitable doping process for Eu3+ . The dependence of photocatalytic activity and PL properties on the crystallinity of TiO2 was fully discussed based on the information obtained via X-ray powder diffraction (XRD), Fast-Fourier Transform (FFT), UV–vis diffuse reflectance spectra (UV–vis), transmission electron microscope (TEM) associated with high-resolution TEM (HRTEM), energy-dispersive X-ray spectra (EDS), Brunauer–Emmett–Teller (BET) specific surface areas, Fourier transform infrared spectroscopy (FT-IR) and photoluminescence (PL) spectra.
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the intercept of the extrapolation of this linear relation to the ordinate (i.e., sin = 0) and ε was determined from the slope of this linear relation. For poorly crystallized samples, the linear relationship between  cos and sin was not very clear. While for highly crystallized samples, however, a good linearity was observed, and so the reliable values of D and ε were obtained [25]. The band gap energy of the powders was determined via UV–vis diffuse reflectance spectrum on a spectrophotometer (Hitachi U3010) equipped with an integrating sphere and with BaSO4 as a reference in the wavelength range of 250–800 nm. Field-emission TEM (JEM 2100F) as well as HRTEM was used to study the morphology, crystallinity and dimensions of the TiO2 -based powders. The excitation and emission spectra were recorded in a spectrofluorophotometer (Shimadzu RF-5301 PC) at room temperature to investigate the PL properties of TiO2 :Eu3+ , and the composition ratio was semi-quantitatively analyzed by EDS (Oxford INCA Model 6498). The FT-IR information is recorded using Shimadzu IR Prestige-21 instrument in the range of 400–4000 cm−1 . The Brunauer–Emmett–Teller (BET) specific surface areas were determined through nitrogen sorption isotherms at 77 K using a Micromeritics ASAP2010 instrument and calculated from the linear part of the BET plot. Photocatalytic activity test was conducted as shown in our previous papers [26–28]. Methyl orange (MO) was adopted as a representative organic pollutant to evaluate the photocatalytic activity of as-prepared nanoTiO2 under UV irradiation.
2. Experimental
3. Results and discussion
2.1. Synthesis All the chemicals were used without further purification. In all cases, the molar ratio was fixed to be H2 O:TNB = 2:1, and Eu3+ :TiO2 = 2%. In a typical synthesis for TiO2 :Eu3+ , 5.0 mL titanium n-butoxide (TNB, Ti(OBu)4 ) was diluted in 18.5 mL of anhydrous ethanol, stirred for 10 min to obtain a homogeneous pale yellowish solution simultaneously with Eu2 O3 dissolved in 5.0 mL glacial acetic acid (HAc). After the two solutions were mixed via vigorous stirring for 30 min, 1.0 mL deionized water was added dropwise to the mixture. Then, the uniform translucent sol was transferred into a Teflon-lined autoclave (50 mL capacity, 60% filling). Further, it was heated to a desired temperature with a rate of 3 ◦ C/min and maintained for definite reaction duration (6 h) without stirring. Followed by cooling gradually to room temperature in air, the as-prepared powders were collected by centrifuging (3000 r/min, 15 min), washing with anhydrous ethanol for 3 times and drying at 80 ◦ C about 4 h. The undoped anatase TiO2 nanocrystals were synthesized in the same procedure without Eu2 O3 and HAc added. Hereafter, Eu3+ /TiO2 -240 and TiO2 240 are denoted for Eu3+ -doped (undoped) TiO2 samples respectively, which have been hydrothermally treated at 240 ◦ C for 6 h. 2.2. Characterization The crystal structure and phase identification of the samples were carried out by FFT and XRD (Rigaku DMax-2200) with a monochromatized source of Cu K␣1 radiation ( = 0.154056 nm) at 1.6 kW (40 kV,40 mA). Since the peak-broadenings generally depend on the two predominat factors: lattice strain (ε) and crystallite size (D), ε and D were seperately determined via Williams and Hall equation [24]: ˇ cos = (K/D) + 2ε sin where ˇ was the full width at half maximum intensity (FWHM) observed, shape factor K was assumed to be 0.90 and was the wavelength of Cu K␣ radiation (0.154056 nm). FWHM of each diffraction line was determined from the profile measured with a scanning rate of 0.25◦ 2/min−1 , which was calibrated by referring to standard silicon powder for instrumental broadening. The plots of cos against sin for different samples were approximated to be linear. D was calculated from
3.1. XRD information The XRD patterns of the nanoTiO2 and TiO2 :Eu3+ samples treated at different temperatures are shown in Fig. 1. All of the powders except for sample TiO2 -120 belong to the anatase type of TiO2 (JCPDS 21-1272). No diffraction peaks observed in sample TiO2 120 indicates that the assembly units are composed of amorphous particles and crystallization only occurs above 120 ◦ C in this condition. But for sample Eu3+ /TiO2 -120, HAc used to dissolve Eu2 O3 (see Section 2.1) also serves as an inhibitor for hydrolysis. Thus, the hydrolysis rate is slowed and it is easier to form anatase. In other words, HAc can promote crystallization [29]. Moreover, from XRD peak broadening analysis, the diffraction peaks become broader with the decrease of hydrothermal-treated temperature, accompanied with greater background noise near the peaks. The crystallinity has been reported to be evaluated by the crystallite size combined with the lattice strain [25]. Crystallite size (D) and lattice strain (ε), calculated separately via the Williams and Hall equation [24] based on the XRD patterns (Fig. 1), are shown in Fig. 2 (the amorphous TiO2 -120 is excluded). Via increasing the hydrothermal temperature, D increases from 9.6 nm to 14.2 nm and ε decreases from 25.08 × 10−3 to 12.56 × 10−3 for the undoped nanoTiO2 , while from 7.3 nm to 11.8 nm, and from 38.25 × 10−3 to 14.82 × 10−3 for the TiO2 :Eu3+ samples. The growth of crystallite and the decrease in lattice strain, (this will be also confirmed with the coming TEM observations), indicate that the crystallinity of the nanoparticles has been enhanced, and that various structural defects, such as small displacement of neighboring atoms, non-
Fig. 1. The XRD patterns of (a) the nanoTiO2 samples, and (b) the TiO2 :Eu3+ samples.
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Fig. 2. The dependence of the crystallite size D and lattice strain ε for (a) the nanoTiO2, and (b) the TiO2 :Eu3+ nanoparticles on the hydrothermal temperature.
following equation using the data of optical absorption vs. wavelength near the band-edge: ˛h = A(h − Eg)
Fig. 3. The UV–vis spectra of the nanoTiO2 smaples, and the plot of (˛h)1/2 vs. the absorption energy (inset).
uniform strain and residual stress of the lattice, have been gradually eliminated. These defects were reasonably supposed to act as scattering centers for electrons and holes, as a consequent, to promote the recombination of these e− /h+ couples and influence the photocatalytic and PL performance. This will be fully discussed later. 3.2. UV–vis analysis From Figs. 3 and 4, which revealed the UV–vis spectra of nanoTiO2 and TiO2 :Eu3+ respectively, an uniform red shift of absorbance edge is observed. The band gap was determined by the
Fig. 4. The UV–vis spectra of the TiO2 :Eu3+ samples and their optical band gaps in the inset.
n/2
where a, n, A and Eg are absorption coefficient, light frequency, proportionality constant and band gap, respectively. In the equation, n reflects the characteristics of the transition in a semiconductor, i.e., n = 1 for direct transition and n = 4 for indirect transition. So plots of (˛h)1/2 vs. the energy of absorbed is used to obtain the band gaps of the nanoTiO2 because of its indirect transition nature [30]. The extrapolation in the inset of Fig. 3 shows that optical energy gap displays a decrease with increase of the hydrothermal temperature from 120 ◦ C to 240 ◦ C, corresponding to that the increasing crystallinity with a larger “band width” narrows the band gap of the sample [31–33]. A similar case occurs in TiO2 :Eu3+ samples with band gap energy dereased from 3.34 eV to 3.09 eV in the increasing order of the hydrothermal temperature (Fig. 4). 3.3. TEM observations The TEM images of TiO2 -120, TiO2 -180 and TiO2 -240 are shown in Fig. 5a–c. The TEM images for TiO2 -150 and TiO2 -210 are shown in Supporting Information (SI) SI-1 and SI-2, respectively. As the TEM images show, the shape and crystallinity of TiO2 particles are strongly dependent on the synthetic temperatures. The nanoparticles are severely agglomerated and poorly crystallized at a low temperature (Fig. 5a). It should be noted that higher temperatures can tailor the crystallinity and the shape tend to exhibit equiaxed geometry bounded by crystallographic facets. The value of the crystallite size is about 11 nm for TiO2 -180, 14 nm for TiO2 240 in agreement with former XRD analyses. HRTEM observation confirms the anatase structure of the as-obtained TiO2 products. Fig. 5d shows the lattice image of a TiO2 grain and its Fast-Fourier Transformed (FFT) diffractogram which is consistent to a [1 0 0]projected diffraction pattern of the anatase TiO2 . Further, the crystallinity of TiO2 improves owing to the increased hydrothermal temperature, seeing from the gradually transparent and clear HRTEM (actually, we almost cannot get the HRTEM for the worstcrystallized TiO2 -120). The enhanced crystallinity for the TiO2 :Eu3+ samples obtains the same order as the nanoTiO2 when tuning the temperature from 120 ◦ C to 240 ◦ C, in accordance with the XRD peak broadening analysis. But the morphology of the nanoparticles changes from polyhedron to rod-like with Eu3+ doping, shown in Fig. 6, which implies that the Eu3+ doping plays an important effect on the crystallographic orientation of TiO2 nanocrystal. As the recent report shows, Eu3+ hinders the growth of specific facets of anatase TiO2 based on the “oriented attachment” mechanism [23]. The similar
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Fig. 5. The TEM images of (a) TiO2 -120, (b) TiO2 -180, (c) TiO2 -240, (d) HRTEM of TiO2 -240, and Fast-Fourier Transformed diffractogram of TiO2 -240 (inset).
Fig. 6. The TEM images of (a) Eu3+ /TiO2 -120, (b) Eu3+ /TiO2 -180, (c) Eu3+ /TiO2 -240, (d) HRTEM of Eu3+ /TiO2 -240, and Fast-Fourier Transformed diffractogram of Eu3+ /TiO2 -240 (inset).
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Fig. 7. (a) MO photodegradation over the nanoTiO2 samples under UV-light irradiation, and (b) photocatalytic reaction kinetics of MO decomposition.
case was also observed in Er3+ -doped TiO2 [34]. Meanwhile, HRTEM of a representative rod also shows its anatase structure, and the corresponding FFT diffractogram demonstrate its single crystal nature (Fig. 6d). Moreover, we synthesized an undoped sample with acetic acid added under the same condition (240 ◦ C, 6 h) as a reference (SI3). The shape of nanoparticle is close to the previous results [29]. No obvious variation in shape can be observed between Fig. 5c and SI-3, which excludes the possibility that the addtion of acetic acid should be responsible for the changes of nanoparticle shape. 3.4. Photocatalytic activity investigations Fig. 7a shows the results of the photocatalytic degradation of MO for nanoTiO2 . The photocatalytic efficiency decreased gradually in the order of the hydrothermal temperature from 240 ◦ C to 150 ◦ C and drastically at 120 ◦ C. To obtain a quantitative understanding on the reaction kinetics of the MO degradation, we applied the pseudo-first order model as expressed by following equation, which is generally used for photocatalytic degradation process if the initial concentration of pollutant is low [35]: ln
C o
C
= kt
where Co and C are the concentrations of dye in solution at time 0 and t, respectively, and k is the pseudo-first-order rate constant. Fig. 7b depicts the photocatalytic reaction kinetics of MO degradation in solution based on the data plotted in Fig. 7a. A rather good correlation to the pseudo-first-order reaction kinetics (R > 0.98) was found from these results. The reaction constant k increased in the order: TiO2 -120 < TiO2 -150 < TiO2 -180 < TiO2 -210 < TiO2 240, from 0.0070 min−1 to 0.3985 min−1 simultaneously with an enhancement of the crystallinity, i.e., the increase in crystallite size and the decrease in lattice strain. Generally, large surface area is likely to exhibit better photocatalytic activity, because a large surface area provides more active sites for adsorbing methyl orange molecules. However, large surface area does not always give better photocatalytic activity. Although TiO2 -120 (118.5 m2 /g), as well as TiO2 -180 (101.3 m2 /g), has a larger surface area than TiO2 -240 (88.2 m2 /g), its photocatalytic activity is lower (Fig. 7b). Considering the degree of crystallinity of TiO2 -240 is higher than that of TiO2 -120 and TiO2 -180 (Figs. 1, 2 and 5), the gradually promoted crystallinity should be mainly responsible for the photocatalysis improvement in the present system. As mentioned above, the photocatalysis improvement is mainly attributed to the gradually enhanced crystallinity. Crystallinity was proved to have an indispensible effect on the two most important processes of the photocatalysis: separation and transport of
charges, as follows [14,15,25,33,36]: (1) the highly crystallized anatase can promote the charge transfer from particle center to surface. The non-uniform strain or residual stress of the poorcrystallized TiO2 lattice leads to disorder and distortion of the TiO2 matrix, which have a severe scattering effect on the transport of charges. Furthermore, an electron and a hole can migrate a longer distance in a well-crystallized crystal than in a poorcrystallized one, separating more of the reducing and oxidizing sites on the surface of the crystal. So the volume recombination may occur less frequently; (2) it eliminates the crystal defects, i.e., impurities, dangling bonds, and microvoids, which behave as recombination centers for the e− /h+ pairs, thus the surface recombination is greatly suppressed. Therefore, it is no wonder that TiO2 -240 of which the crystallite size is about 14.2 nm and lattice strain about 12.56 × 10−3 holds the maximum in the reaction constant k of MO decomposition, i.e., about 57 times of that for TiO2 -120. Further investigation of how the surface area affects the photocatalytic activity is necessary, when the products obtain the similar degree of crystallinity.
4. Photoluminescence studies Fig. 8a depicts the typical excitation spectra of the Eu3+ /TiO2 240. By monitoring the emission line of 612 nm, excitation lines appear at 394 nm, 416 nm, 464 nm, and 534 nm are ascribed to the f–f inner-shell transitions within the Eu3+ 4f6 configuration [37,38]. Besides, a new band appears in the range from 320 nm to 380 nm, although it is not obvious. Based on the previous papers, the new wide band can be attributed to the host excitation and assigned to the charge transfer band (CTB) from O−Ti to the Eu3+ ions [37]. Similar broad band has also been observed and attributed to the CTB from O−Ti to Eu3+ ions in the previous works [21,37]. It is widely accepted that, by distributing the doping Eu3+ ions in the amorphous or glassy TiO2 region, the CTB plays a dominant role in the energy transfer from TiO2 nanocrystallite to Eu3+ ions, and the photoluminescence excitation of Eu3+ though the energy transfer from the anatase nanocrystals is much more efficient than the direct excitation of the Eu3+ crystal field states [19,20]. But here we found that once the Eu3+ ions were localized in the well-crystallized TiO2 region, the characteristic CTB were not obvious and hard to be detected, indicating that the direct excitation of the Eu3+ crystal field states is superior over the host excitation in the case of the photoluminescence excitation of Eu3+ . Additionally, the intrinsic transition 7 F0 → 5 D4 (366 nm) and 7 F0 → 5 G2 (386 nm) of the direct excitation [38] can also decrease (or overlap) the signals detected from the host excitation to a extent.
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Fig. 8. (a) The excitation spectrum of Eu3+ /TiO2 -240 (em = 612 nm), (b) the emission spectra (ex = 394 nm) of the TiO2 :Eu3+ samples, where their maximum emission (em = 612 nm) intensities at 612 nm in the inset.
The five characteristic peaks at 579 nm, 592 nm, 612 nm, 651 nm, 699 nm corresponding to 5 D0 → 7 F0 , 5 D0 → 7 F1 , 5 D0 → 7 F2 , 5 D → 7 F , 5 D → 7 F transitions of Eu3+ ion, respectively, are 0 3 0 4 observed for all the Eu3+ doped samples at the excitation wavelength of 394 nm in Fig. 8b. It can be seen that the 5 D0 emission is intensified with the increment in hydrothermal temperature accompanied with gradually enhanced crystallnity. For 5 D → 7 F transition, the PL intensity was quantitatively anal0 2 ysed and tabulated in the inset of Fig. 6b. A similar case is also observed in the emission spectra excited at 464 nm (shown in SI-4). 5 D0 → 7 F1 (592 nm) is a magnetic-dipole transition while 5 D → 7 F (612 nm) is a hypersensitive forced electric-dipole tran0 2 sition being allowed only at low symmetry with no inversion centre. The intensity ratio (R) of 5 D0 → 7 F2 to 5 D0 → 7 F1 increases as the degree of Eu–O covalence increases [37], so R is widely used to investigate the bonding environment of the Eu3+ ions. The integrated intensity ratio (R) of the samples obtained at different temperatures is shown in Table 1. Note that R increases with hydrothermal temperature, accompanied with the promoted crystallinity, indicating that the covalence degree of the Eu3+ ions increases. The increase of the covalence degree implies the formation of Eu3+ –O2− –Ti4+ bonding. This was also confirmed by the further investigation using FT-IR (Fig. 9). The undoped TiO2 -240 has a strong and broad band in the range of 500–1000 cm−1 , due to Ti–O stretching vibration modes [39], while this band is observed to shift towards lower wavenumber for Eu3+ /TiO2 -240. This behavior is related to the formation of Ti–O–Eu bonding [40]. Moreover, we can also attribute this new band (894 cm−1 ) to the Ti–O stretching model involving the interaction (or chemical contact) with the distributed Eu3+ ions [41,42] through the Eu–O–Ti bonding. On the other hand, the great mismatch of ionic radius between Eu3+ (0.95 Å) and Ti4+ (0.68 Å) makes the doping Eu3+ hardly enter into the TiO2 lattice [20,43], but inclined to distribute in the crystallite surface or interstitials of TiO2 nanocrystals [44]. For the poor-crystallized TiO2 matrix, the Eu3+ has a tendency to form clusters due to the reduction of Eu3+ –Eu3+ distances [45]. The clusters Table 1 The integrated intensity ratio of 5 D0 → 7 F2 /5 D0 → 7 F1 of the samples. Sample 3+
Eu /TiO2 -120 Eu3+ /TiO2 -150 Eu3+ /TiO2 -180 Eu3+ /TiO2 -210 Eu3+ /TiO2 -240 a
I [5 D0 → 7 F2 ] (a.u.)
I [5 D0 → 7 F1 ] (a.u.)
Ra
2.324 2.793 3.228 3.415 3.822
0.901 1.054 1.117 1.149 1.258
2.58 2.65 2.89 2.97 3.05
Integrated intensity ratio of 5 D0 → 7 F2 and 5 D0 → 7 F1.
Fig. 9. FT-IR information of the nanoTiO2 and TiO2 :Eu3+ samples.
are undesirable which lead to an enhanced interparticle contact of the Eu–Eu pairs, thus quench its luminescence through cross relaxation [19,37]. As the crystallinity enhances, the gradual formation of Eu3+ –O2− –Ti4+ bonding leads to reducing the extent of the Eu3+ clusters [37], suppressing the cross relaxation and intensifying the luminescence effectively. Furthermore, the great elimination of the crystal defects, as quenching centers for luminescence, can diminish the undesired nonradiative recombination routes for electrons and holes [46], contributing to the enhanced luminescence. The typical EDS for Eu3+ /TiO2 -120 is shown in SI-5, from which the presence of Eu3+ in the rod-like crystals is confirmed with the atomic percentage about 1.72%, and the value for Eu3+ /TiO2 -240 is 1.68% (SI-6). The “oriented attachment” induced by the Eu3+ is under research. 5. Conclusions Phase-pure anatase TiO2 and TiO2 :Eu3+ nanoparticles with tunable crystallinity were prepared by a one-pot hydrothermal method with no aid of any addictive. The crystallinity was successfully controlled via tailoring the hydrothermal temperature from 120 ◦ C to 240 ◦ C. TiO2 -240 prepared at 240 ◦ C holds the highest crystallinity, i.e., the largest crystallite size of 14.2 nm and the lowest lattice strain of 12.56 × 10−3 . The high crystallinity provides great improvement for the transport and separation of charges, and the elimination of crystal defects, thus contributes to the enhanced photocatalytic activity, e.g., the reaction constant k of MO decomposition for TiO2 -240 is almost 57 times of that for TiO2 -120. Similar
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to the case of photocatalysis, the PL emission intensity of TiO2 :Eu3+ was also promoted as the crystallinity increased. The gradual formation of Eu3+ (O2− (Ti4+ bonding leads to reducing the extent of the Eu3+ clusters, suppressing the cross relaxation. Furthermore, the great elimination of the crystal defects, as quenching centers for luminescence, can diminish the undesired nonradiative recombination, resulting in the enhanced luminescence. These full analyses enrich us a closer understanding on the photocatalysis and photoluminescence, especially in light of crystallinity and this method provides flexibility, selectivity and efficiency for the deliberate synthesis of other functional nanomaterials. Acknowledgements Financial support from National 973 Program of China Grant 2007CB936704 & 2009CB939903, National Science Foundation of China Grant 50772123, 20901083 & 50902143 and Science and Technology Commission of Shanghai Grant 08JC1420200 & 0952nm06500 are gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jallcom.2009.11.074. References [1] M. Okumura, S. Nakamura, S. Tsubota, M. Azuma, M. Haruta, Catal. Lett. 51 (1998) 53–58. [2] P.J. Senogles, J.A. Scott, G. Shaw, H. Stratton, Water Res. 35 (2001) 1245–1255. [3] E. Traversa, G. Gnappi, A. Montenero, G. Gusmano, Sens. Actuators B 31 (1996) 59–70. [4] Y.F. Tu, S.Y. Huang, J.P. Sang, X.W. Zou, J. Alloys Compd. 482 (2009) 382–387. [5] K.S. Raja, M. Misra, V.K. Mahajan, T. Gandhi, P. Pillai, S.K. Mohapatra, J. Power Sources 161 (2006) 1450–1457. [6] Y.Y. Zhang, X.Y. Ma, P.L. Chen, D.R. Yang, J. Alloys Compd. 480 (2009) 938–941. [7] M. Li, S.F. Zhou, Y.W. Zhang, Z.L. Hong, Appl. Surf. Sci. 254 (2008) 3762–3766. [8] M. Li, Z.L. Hong, Y.N. Fang, F.Q. Huang, Mater. Res. Bull. 43 (2008) 2179–2186.
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