Article pubs.acs.org/Langmuir
Exploring the Important Role of Nanocrystals Orientation in TiO2 Superstructure on Photocatalytic Performances Feifei Chen,† Fenglei Cao,‡ Hexing Li,† and Zhenfeng Bian*,‡ †
College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai 201300, P. R. China The Education Ministry Key Lab of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234, P. R. China
‡
S Supporting Information *
ABSTRACT: Efficient charge separation has been widely accepted as one of the important factors responsible for the photocatalytic water splitting, organic oxidation, and solar cell, etc. TiO2 mesocrystal is a superstructure which could largely enhance charge separation, where TiO2 nanocrystals with parallel crystallographic alignment assemble in a form of oriented aggregation. Here, the intercrystal misorientation in TiO2 superstructure was first concerned and evaluated on the influence of photocatalytic efficiency. Our results showed that the intercrystal misorientation in TiO2 superstructures had a harmful effect on the charge separation efficiency.
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INTRODUCTION To maximize the photocatalytic efficiency of TiO2, enormous efforts has been made for exploring the most decisive factor including tuning the exposed facets, controlling the nano/ microstructures and the crystal phase ratio (e.g., anatase/rutile), increasing the specific surface area, and tuning the surface/bulk defects, etc.1−3 TiO2 mesocrystal is a novel class of TiO2 material which inherits the advantages of single crystal materials and possesses relatively larger specific surface area than the former.4−18 TiO2 mesocrystals have oriented and porous superstructures assembled by TiO2 nanocrystals, which have attracted much attention in recent years.4−18 Up to now, the studies on TiO2 mesocrystals have been mainly concentrated on the structures/morphologies (e.g., cubic, spherical structures or (001), (101) facets exposing) and porosity control by different synthesis techniques such as hydrothermal/solvothermal, microwave-assisted hydrothermal routes, and one-step annealing, etc.5,8,10,11,19−21 However, the deep understanding regarding the characteristics of oriented attachment fashion of nanocrystals and the relationship with photocatalytic efficiency has been rarely studied. Recently, we have compared the lifetime of charge separation between TiO2 polycrystalline materials and TiO2 mesocrystal superstructure by single molecule fluorescence techniques.11,12 Our results showed that the remarkably more long-lived charges under illumination on TiO2 mesocrystals were observed relative to polycrystalline materials, and thereby it showed greatly enhanced photoconductivity and photocatalytic activities. As shown in Figure 1a,b, physical models of polycrystalline TiO2 and TiO2 mesocrystal in the form of oriented attachment of secondary nanocrystals were presented. © 2015 American Chemical Society
Oriented aggregation of nanocrystals provided the potential to tune material properties by controlling defect concentrations and morphology.22−26 According to our previous research, we found that the misorientation (i.e., crystal lattice mismatch) existed in the boundary of secondary nanocrystals of TiO2 superstructures (Figure 1c).10 Therefore, we envisioned that the presence of misorientations in TiO2 mesocrystals should influence their photocatalytic efficiency. In this work, two typical TiO2 mesocrystalsone with oriented and the other with misoriented alignment of secondary nanocrystalswere obtained by a thermal annealing and a hydrothermal recrystallization process, respectively. We found, for the first time, that the presence of small misorientations had an obvious harmful effect on the charge transfer and thus largely suppressed the photocatalytic efficiencies.
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EXPERIMENTAL SECTION
Preparation of TiO2 Mesocrystals. TiO2 mesocrystals was prepared from a precursor containing TiOSO4 (15 wt % solution in dilute sulfuric acid, purchased from Sigma-Aldrich) and tert-butyl alcohol (molar ratio = 1:165). The above precursors were placed in autoclave at 110 °C for 48 h. The products were filtered, washed with ethanol, and dried at 100 °C. Furthermore, the obtained samples were calcined at different temperature for 2 h, which was designed as MesoTiO2-X (X is the temperature). TiO2 mesocrystal by hydrothermally treating was prepared using a MesoTiO2-350 suspension (1.0 g/L) placed in an autoclave at 110 °C for different times and Received: December 16, 2014 Revised: February 25, 2015 Published: March 4, 2015 3494
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Figure 1. Schematic illustration of disordered aggregation of TiO2 nanoparticles (a), TiO2 mesocrystal with ordered orientation (b), and misorientation (crystal lattice mismatch) of secondary nanocrystals (c). of λ = 365 nm located at 1.0 cm away from the solution surface (0.3 W cm−2). After reaction for 1 h, the amount of hydrogen evolved was determined on a gas chromatograph (GC-2010 shimadzu, N2 carrier) equipped with a thermal conductivity detector (TCD2-C).
designed as MesoTiO2-350(T) (T is the time of hydrothermal treatment). Characterization of Materials. The samples were characterized using X-ray diffraction (XRD, Rigaku D/MAX-2000, Cu Kα source), scanning electron microscopy (SEM, HITACHI S4800), transmission electron microscopy (TEM, JEOL JEM-2010, operated at 200 kV), and nitrogen sorption (Quantachrome NOVA 4000 instrument at 77 K). The Brunauer−Emmett−Teller (BET) method was utilized to calculate the specific surface area. The pore volume and pore diameter distribution were derived from the adsorption isotherms by the Barrett−Joyner−Halenda (BJH) model. TGA were carried out on a DTG-60H thermogravimetric analyzer with a heating speed of 10 K/min. Meanwhile, •OH trapping photoluminescence spectra of terephthalic acid solution (PL-TA, λex = 312 nm, λem = 426 nm) were recorded on a photoluminescence spectrum (PL, Varian Cary-Eclipse 500). Photoelectrochemical measurements were carried out in a conventional three-electrode quartz cell by an electrochemical station (CHI 660D). TiO2 was coated on FTO glass as a photoanode, the Pt sheet as cathode, and the SCE as the reference electrode (0.5 mol L−1 Na2SO4 aqueous solution as the electrolyte, Xe lamp (100 mW cm−2) as the light source). Electrochemical impedance spectroscopy (EIS) was carried out on an electrochemical workstation (CHI 660D) under open-circuit potential conditions (Xe lamp (100 mW cm−2)). Transient absorption spectroscopy was measured using a YAG laser (355 nm, 6 ns full width at half-maximum, 10 mJ/pulse). The reflected analyzing light was from a 500 W Xe lamp. The transient signals were recorded by a digitizer (HP54510B, 300 MHz). All experiments were carried out at room temperature. Photocatalytic Activity Tests. For typical photocatalytic runs, 50 mL of TiO2 dispersion (1.0 g/L) containing aqueous solution (phenol, 1.0 × 10−4 M) was sonicated for 20 min and then transferred into a self-designed reactor. The percentages of preadsorbed substrates in aqueous suspensions of TiO2 before UV irradiation were below 5% of total amount under equilibrium conditions. The photocatalytic reaction was initiated by a xenon lamp (CEL-HXUV300, 100 mW cm−2). After stopping the light illumination, the sample was centrifuged at 10 000 rpm to remove the particles. The concentration of unreacted molecules was analyzed by a UV spectrophotometer (UV 7504/PC) at its characteristic wavelength of 270 nm, from which the degradation yield was calculated. H2 evolution under UV light irradiation was carried out at room temperature in a photocatalytic hydrogen evolution reactor containing 50 mg of catalyst coated with 1.0 wt % Pt and 100 mL of aqueous solution (25 wt % methanol). The mixture was stirred for 30 min in dark in order to reach the adsorption−desorption equilibrium. The photocatalysis was initiated by irradiating the reaction system by using four 3.0 W UV-LEDs lamps with characteristic wavelength
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RESULTS AND DISCUSSION As-prepared TiO2 mesocrystals were prepared by an alcoholysis route according to our previous work (Figure S1).10 TiO2 mesocrystal with ordered stacking superstructure of secondary nanocrystals was prepared by annealing as-prepared TiO2 mesocrystal at 700 °C, which was designed as MesoTiO2-700 (Figure 2a). As-prepared TiO2 mesocrystals contains a small amount of organic species and/or amorphous TiO2, which can be removed and/or well-crystallized by thermal annealing at 350 °C (Figure S2). TiO 2 mesocrystal with stacking misorientations of secondary nanocrystals was prepared by hydrothermally treating of MesoTiO2-350 (as-prepared TiO2 mesocrystals annealed at 350 °C) at 110 °C for 36 h (designed as MesoTiO2 -350(36)) (Figure 2b). Wide-angle X-ray diffraction (XRD) analysis reveals that both MesoTiO2-700 and MesoTiO2-350(36) have anatase structure (JCPDS-211272) and comparable crystallinity and crystal size (35 nm calculated by the Scherrer equation) (Figure S3). The textural data show that MesoTiO2-700 (18 m2/g, 22 nm) and MesoTiO2-350(36) (17 m2/g, 23 nm) have a comparable specific surface area and pore diameter (Table 1). FESEM images of MesoTiO2-700 and MesoTiO2-350(36) show similar spheroid morphologies, and the measured average sizes of secondary nanocrystal measured are around 36 nm, consistent with the XRD results (Figure 2a,b and Table 1). The spheroid morphologies of MesoTiO2-700 and MesoTiO2350(36) are further confirmed by TEM images (insets of Figure 2a,b). Selected-area electron diffraction (SAED) patterns recorded on mesocrystal particles show typical single crystal anatase along the [001] zone axis (Figure 2c,d). Combining with the measured lattice spacing, the diffraction spots can be indexed as anatase (200) and (020) planes. In the case of MesoTiO2-700, the diffraction spot is a single dot, while a line of diffused spots appears for MesoTiO2-350(36) (insets of Figure 2c,d). A similar tendency from the TEM and SAED images was observed for more than six individual particles examination (Figure S4). The result indicates that the presence of a large amount stacking faults (i.e., misorientations between 3495
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Figure 2. SEM image of (a) MesoTiO2-700 and (b) MesoTiO2-350(36) (insets are the TEM images), SAED pattern recorded on the entire particles of (c) MesoTiO2-700 and (d) MesoTiO2-350(36) (insets are the amplification of diffraction spot), and HRTEM image of the junction of several TiO2 nanocrystals from (e) MesoTiO2-700 and (f) MesoTiO2-350(36) (insets are the cross-sectional TEM images of TiO2 mesocrystal).
Table 1. Structural Parameters of TiO2 Mesocrystals samples MesoTiO2-350 MesoTiO2-550 MesoTiO2-700 MesoTiO2-750 MesoTiO2-350(0.5) MesoTiO2-350(6) MesoTiO2-350(24) MesoTiO2-350(36)
inside the TiO2 superstructure, reflecting the porous property. The stacking of nanocrystals in TiO2 mesocrystal was further investigated by high-resolution transmission electron microscopy (HRTEM). Figure 2e recorded on two attached secondary nanocrystals (MesoTiO2-700) shows the same crystallographic orientations of lattice fringes. Figure 2f recorded on MesoTiO2-350(36) shows three sets of lattice fringes with a small deflection angle with each other. MesoTiO2-700 with clear spotty diffraction pattern shows a mutual crystal lattice orientation throughout the grain boundary, which suggests an oriented superstructure of TiO2 nanocrystals. MesoTiO2350(36) with diffused diffraction spots displays different crystal lattice domains (misorientation) outlined by white lines due to the stacking faults, and the lattice orientation deviation between two areas was about 2°−5°, consistent with the Laue diffraction results (Figure 2f). Two illustrative models are presented in Figure S5. Furthermore, the lattice parameters (a = b and c) of TiO2 samples were obtained by using Bragg’s law equations (Table S1). The values of a and b are slightly decreased by the reducing the misorientation in mesocrystal TiO2 samples. Upon annealing/hydrothermal treatment, the crystallinity (crystal size) of TiO2 mesocrystals was obviously increased as the annealing temperature increased and/or the hydrothermal treatment was prolonged (Table 1 and Figure S6). In the above
surf. area pore vol pore sizeb nanoparticles SBETa (m2/g) Vpb (cm3/g) (nm) sizec (nm) 129 37 18 14 92 30 18 17
0.10 0.13 0.11 0.11 0.10 0.14 0.10 0.11
3 9 22 27 4 16 20 23
14 20 35 46 16 23 33 35
a
The specific surface area calculated by the Brunauer−Emmett−Teller (BET) method. bTotal pore volume and pore diameter distribution derived from the adsorption isotherms by the Barrett−Joyner− Halenda (BJH) model. cAverage secondaty nanocrystal size of TiO2 mesocrystals estimated from the XRD results by the Scherrer equation.
secondary nanocrystals) in MesoTiO2-350(36). To investigate the atomic structure on subnanometer length scale in the TiO2 mesocrystal, the mesocrystals were sectioned by an ultramicrotome to reveal their cross sections. Insets of Figure 2e,f give the cross-sectional TEM images. There are many voids 3496
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MesoTiO2-700, the photogenerated charge carriers migrate from bulk to surface through a smoother pathway in the oriented single-crystal-like superstructure. Meanwhile, •OH trapping photoluminescence spectra of terephthalic acid solution (PL-TA, λex = 312 nm, λem = 426 nm) were measured (PL, Varian Cary-Eclipse 500). The t-plots of peak intensity of • OH-trapping PL-TA spectra (Figure 3b) revealed that the generation of •OH radicals is more efficient on thermally annealed MesoTiO2-700 relative to the hydrothermal treated TiO2 mesocrystal MesoTiO2-350(36).28−30 This confirms furthermore that the rapid migration of photogenerated charges in TiO2 mesocrystal can effectively suppress the recombination of photoelectrons and holes. In order to clarify the influence of superstructure misorientations on the photocatalytic activity, the photocatalytic performances of phenol degradation and H2 production were evaluated on MesoTiO2-700, MesoTiO2-350(36), and P25 (commercial TiO2, 55 m2/g) (Figure S9). As demonstrated in Figure 4a, the phenol degradation efficiency of MesoTiO2-350(36)
treatment conditions, primary nanocrystals with low crystallinity and small crystal size underwent crystallographic reorganization and fusion to form larger secondary nanocrystals with high crystallinity. An obvious decrease tendency of SBET and increase tendency of pore diameters were observed from MesoTiO2-350 to MesoTiO2-750 and MesoTiO2-350(0.5) to MesoTiO2-350(36) (Table 1 and Figure S7). Based on the above characterizations, MesoTiO2-700 and MesoTiO2-350(36) possess comparable surface areas and crystallinity but different degrees of stacking ordering of secondary nanocrystals. It has been proved that the separation efficiency of photoexited charges (e/h pair) is highly responsible for the photocatalytic activity, which can be reflected by the photocurrent under light irradiation.11,27 The photocurrents of MesoTiO2-700 and MesoTiO2-350(36) were measured under UV irradiation (Figure 3a). The photocurrent
Figure 4. Liquid-phase photocatalytic degradation of phenol in the presence of different TiO2 (red column) and the activity normalized by specific surface area (blue column) (a); H2 production rates of different TiO2 mesocrystal samples (red column) and the AQE (blue column) for hydrogen evolution (b), which is calculated by using the expression AQE = (2 × number of evolved H2/number of incident photons ×100%).13,31
Figure 3. (a) Photocurrent responses of different TiO2 samples under UV light irradiation and (b) t-plots of peak intensity (λ = 426 nm) of • OH-trapping PL-TA spectra.
generated on MesoTiO2-700 under irradiation is 2 times higher than that on MesoTiO2-350(36). The result reflects that the much easier migration of photogenerated charge carriers in MesoTiO2-700 relative to MesoTiO2-350(36) mesocrystal due to the large amount of intercrystal misorientations. With the annealing temperature increased, the photocurrent increased significantly, while the extension of hydrothermal time on the improvement of photocurrent is limited. The reason is that the orderly arrangement of TiO2 nanocrystals can be more effective for the migration of photogenerated charges to have a higher photocurrent. Both the crystallinity and intercrystal orientations of TiO2 mesocrystal are improved during the annealing process (Figure S8), while only the crystallinity of TiO2 mesocrystal is improved during the hydrothermal treatment. In the case of
is 3 times lower than that of MesoTiO2-700 and P25. As the differences of specific surface areas for varied TiO2 materials, the specific phenol degradation efficiency of MesoTiO2-700 is about 2 times higher than that of MesoTiO2-350(36) and P25. The specific photocatalytic activities of TiO2 mesocrystals annealed at 350 and 550 °C were only 3.7% and 23.7% relative to that of MesoTiO2-700, respectively. This suggests that the harness of intercrystal orientations by the annealing process can effectively increase the photocatalytic performance of TiO2 mesocrystals. On the other hand, the H2 production rate of MesoTiO2-700 and P25 is about 2 times higher than that 3497
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catalysis to optoelectronics. Rapid electron migration can effectively separate the photoexcited electrons and holes, thus preventing charge recombination.32 Mesocrystal materials possess not only relative high specific surface area but also single-crystal-like properties. The most attractive characteristic of single crystal materials is the less internal defects relative to polycrystalline material, which greatly facilitates the migration of photoexcited electrons, i.e., small internal resistance.32 The most obvious factors affecting the photocatalytic performances were the specific surface areas, exposed facets, and micro/ nanostructures of mesocrystal materials,4−18 while the influence from the stacking behaviors (orientations) of adjacent nanocrystals was disregarded. Fortunately, in our present study, two comparable samples with ordered intercrystal orientations (MesoTiO2-700) and misorientations (MesoTiO2-350(36)) in TiO2 superstructures were obtained. At the conditions of comparable specific surface areas, exposed facets, crystallinity, and microstructures, the large difference in photocatalytic efficiencies for the above mesocrystals was obviously found, which clearly discloses the important role of intercrystal orientations on the efficient charge migration and thus photocatalytic performance. Thermal annealing with the increasing the temperatures (350−750 °C) was an effective way for enhancing the specific photocatalytic degradation efficiency (Figure 4). It should be mentioned that the specific photocatalytic activity of MesoTiO2-750 having comparable intercrystal orientations and crystallinity with MesoTiO2-700 is well dependent on the surface areas (Figure S11 and Table 1). This phenomenon reflects that the intrinsic TiO2 mesocrystal materials with less intercrystal misorientations should be a very attractive material, whose specific photocatalytic efficiency is higher than P25. Accordingly, a definite target for designing efficient TiO2 mesocrystal materials should be the increase of specific surface areas on the premise of harnessing the intercrystal misorientations. In summary, our study confirmed that the small misorientation of TiO2 nanocrystals in TiO2 mesocrystals is very harmful to the photocatalytic activity. These small misorientations could act as charge trapping sites that hindered the charge transport across the superstructure of TiO2 mesocrystals. High temperature annealing can effectively reduce the small misorientation and highly improve the photocatalytic performance. Our results disclose that the presence of misorientations in the TiO2 superstructure bring seriously negative effects on the photocatalytic efficiency, which should be taken into account and avoided during the design and synthesis process of semiconductor mesocrystal materials.
of MesoTiO2-350(36) (Figure 4b). Furthermore, the H2 production rate of per unit surface area of MesoTiO2-700 is about 2 times higher than that of MesoTiO2-350(36) and P25 (Figure S10). The apparent quantum efficiencies (AQE) of the above two reactions were calculated and are shown in Figure 4b. The AQE of MesoTiO2-700 reached 50% at 365 nm, which is similar to P25 (55%) and much higher than those of MesoTiO2-350(36) (27%). Transient absorption spectroscopy was employed to measure the lifetime of a charge-separated state. TiO2 exhibits a broad transient absorption peak in the visible to near-infrared range under 355 nm laser excitation, which represents the overlapping of the trapped holes (about 440−600 nm) and trapped electrons (about 660−900 nm).11 The absorption data of the 550 nm absorption should be used to determine the rate of charge recombination in TiO2. As shown in Figure 5, the
Figure 5. Differential time traces of %Abs at 550 nm obtained from different TiO2 samples.
lifetime of photogenerated charges of MesoTiO2-700 is much longer than that of MesoTiO2-350(36), MesoTiO2-350, and MesoTiO2-550. Furthermore, the lifetime of photogenerated charges of MesoTiO2-550 is longer than MesoTiO2-350(36) and MesoTiO2-350. The results are consistent with the tendency of photocatalytic activity. Moreover, Figure 6 shows
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ASSOCIATED CONTENT
S Supporting Information *
Additional results of structural characterizations and photocatalytic activity tests of the samples. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 6. Electrochemical impedance spectroscopy (EIS) Nyquist plots obtained from different TiO2 samples.
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the electrochemical impedance spectroscopy (EIS) spectra of TiO2 samples. The diameter of the arc radius on the EIS Nyquist plot of the MesoTiO2-700 is obviously smaller than other TiO2 samples. Such small arc radius of the EIS Nyquist plot results from the high efficiency of charge separation. The low charge recombination rate of semiconductor materials is highly desired for applications ranging from
AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (Z.B.). Notes
The authors declare no competing financial interest. 3498
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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (21407106, 21237003, 21261140333), Shanghai Government (14ZR1430800, 13SG44), and Program for Changjiang Scholars and Innovative Research Team in University (IRT1269) and International Joint Laboratory on Resource Chemistry (IJLRC). Research is also supported by The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.
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