COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Nanosized anatase TiO2 single crystals for enhanced photocatalytic activityw Gang Liu,ab Chenghua Sun,bc Hua Gui Yang,d Sean C. Smith,c Lianzhou Wang,c Gao Qing (Max) Lu*b and Hui-Ming Cheng*a Received (in Cambridge, UK) 24th September 2009, Accepted 16th November 2009 First published as an Advance Article on the web 27th November 2009 DOI: 10.1039/b919895d Nanosized anatase TiO2 single crystals with 18% {001} facets have a raised conduction band minimum by 0.1 eV, and exhibit photocatalytic activity both for generating OH radicals and for splitting water into hydrogen that is markedly superior—by factors of 5.6 and 8.2, respectively—to reference ca. 3 lm anatase TiO2 with 72% {001} facets. The efficiency of heterogeneous reactions intrinsically depends on both the surface atomic structure and the surface area of catalysts.1–3 The effective integration of favorable surface atomic structure with a large surface area is expected to be very effective in promoting the separation and transfer of photo-induced charge carriers and thus enhancing photocatalytic efficiency. Since the successful synthesis of singlecrystal anatase titanium dioxide (TiO2) sheets with the particle size of ca. 1 mm and 47% {001} facets by Yang et al.,4 increasing attention5 has focused on maximizing the percentage of the reactive {001} facets in order to accomplish high photocatalysis efficiency of anatase TiO2.6 Although the relative reactivity of anatase TiO2 with dominant {001} facets has been demonstrated to be higher than that of the normalized or chosen reference anatase without preferential {001} facets,5a,f the practical photocatalytic activity of such materials has hitherto been intrinsically restricted by the relatively large particle size and correspondingly very small surface area. Clearly, it is highly desirable to accomplish small sized anatase TiO2 with preferential {001} facets. In this important aspect, unfortunately, only few cases have been reported so far.5c,d Here, we report a powerful new method for one-pot synthesis of nanosized anatase TiO2 single crystals a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China. E-mail:
[email protected]; Fax: +86 24 23903126; Tel: +86 24 23971611 b ARC Centre of Excellence for Functional Nanomaterials, School of Engineering and Australian Institute of Bioengineering and Nanotechnology, The University of Queensland, Qld. 4072, Australia. E-mail:
[email protected]; Fax: +61 7 33656074; Tel: +61 7 33653735 c Centre for Computational Molecular Science, Australia Institute for Bioengineering and Nanotechnology, The University of Queensland, Qld 4072, Australia d Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science & Technology, Shanghai 200237, China w Electronic supplementary information (ESI) available: Experimental details, XRD, UV-visible absorption spectra, XPS, surface energy and electronic structure calculation results, valence band spectra, fluorescence spectra, Raman spectra and emission spectrum of the lamp used. See DOI: 10.1039/b919895d
This journal is
c
The Royal Society of Chemistry 2010
with 18% {001} facets on a large scale. This new generation of TiO2 nanoparticles exhibits much superior photocatalytic activity to reference micron sized anatase with 72% {001} facets. In a typical synthesis, 100 mM titanium sulfate (Ti(SO4)2) in 40 mL 0.2 M hydrofluoric acid (HF) was hydrothermally treated at 180 1C in a Teflon-lined stainless steel autoclave. After hydrothermal treatment for 22 h, the product of nanosized anatase TiO2 was obtained. As shown in Fig. S1 of ESI,w all X-ray diffraction (XRD) peaks of the TiO2 synthesized can be indexed to anatase phase TiO2. Scanning electron microscopy (SEM, Fig. 1A) shows that most nanosized anatase TiO2 particles ranging from 30–85 nm are truncated octahedral in shape, even though a small portion of irregular particles can be observed. Low-magnification transmission electron microscopy (TEM) images, Fig. 1B–D, demonstrate the typical profiles of these particles: the laterally-viewed hexagonal shape and top-viewed square shape of truncated octahedra. The interfacial angle between the top two parallel surfaces and the lateral surfaces is determined to be ca. 68.51 as shown in Fig. 1C, which matches well with the theoretical value of the angle between the {001} and {101} facets of anatase.6a According to the symmetries of anatase TiO2, these two flat square surfaces in the crystal structure of anatase TiO2 can be therefore ascribed to (001), and the eight isosceles trapezoidal surfaces are {101} facets of the anatase TiO2 crystal.4,5a The percentage of {001} facets in the presently synthesized nanocrystals was estimated to be ca. 18%. The X-ray photoelectron spectrum (XPS) of F 1s with a binding energy of 684.1 eV for the nanosized anatase particles (Fig. S2, ESIw) confirms the existence of typical surface Ti–F species, which results in the formation of preferential {001} facets as evidenced in Fig. 1 by lowering surface energy of {001} facets.4 It is worth noting that there are some cavities of size of several nanometres on the particle surface as a result of the substantial F etching.7 Similar cavities were also observed in the reported boron doped anatase TiO2 nanoparticles, which were prepared by the hydrothermal hydrolysis of TiF4 precursor in boric acid solution.8 Ultrathin anatase sheets with the percentage of {001} facets up to 72% can be easily obtained by simply lowering the concentration of titanium precursor in the reaction solution. Fig. S3 (ESIw) shows the sharper XRD diffraction peaks of the ultrathin anatase sheets in contrast with the nanosized anatase (see Fig. S1, ESIw), indicating the larger particle size of the former. A typical SEM image in Fig. 2A demonstrates the morphology of ultrathin sheets with ca. 3 mm lateral Chem. Commun., 2010, 46, 755–757 | 755
Fig. 1 SEM (A) and low-magnification TEM (B) images of nanosized anatase TiO2 single crystals, and lateral view (C) and top-view (D) TEM images of a truncated octahedron.
Fig. 2 (A) SEM and, (B) TEM image of micron sized anatase TiO2 sheet with preferential {001} facets, (C) SAED patterns and (D) highresolution TEM image recorded from a single TiO2 sheet.
dimension. Selected area electron diffraction (SAED) patterns in Fig. 2C and a high-resolution TEM image in Fig. 2D of a single sheet of anatase TiO2 (Fig. 2B) show the diffraction spots of the [001] zone and the (200) atomic lattice spacing of 1.9 A˚.4 Further XPS analysis shows that the amount of surface F species detected on these anatase TiO2 sheets is much higher than that on the nanosized particles (8.58 vs. 3.54 at%), though the chemical state of the F species—namely as Ti–F bonds—in both samples is similar. This implies that more extensive surface Ti–F bonding can play a key role in obtaining higher percentages of {001} facets in anatase sheets. To further confirm this point, four different surface coverage fractions, y = 0, 0.25, 0.50 and 1 ML, of fluorine atoms over (001), have been modeled with ab initio density functional theory (DFT) calculations. The calculated surface energies decrease monotonously from 0.96 to 0.64 J m 2 with the coverage increment, as shown in Fig. S4 (ESIw), suggesting an increased surface stability. This result is not surprising because both Ti3d and O2p electrons can interact strongly with F2p electrons, which stabilizes Ti and O atoms on the surfaces due to the formation of strong F–Ti bonds. Fig. 3 shows the UV-visible absorption spectra of the nanosized and micron-sized anatase TiO2 synthesized. Surprisingly, in contrast to the micron sized anatase with 72% {001} facets, the intrinsic absorption edge of nanosized anatase with 18% {001} facets has an obvious blue-shift by 9 nm. The corresponding bandgap is widened from 3.2 to 3.3 eV. Furthermore, this blue-shift is still evidenced when surface fluorine species on TiO2 was removed by thermal oxidation of surface fluorine species into volatile products such as gaseous F2 (Fig. S5, ESIw), indicating that the blue-shift is not a function of the surface Ti–F bonds. Although the particle size of TiO2 has decreased from 3 mm to 30–85 nm, it has been demonstrated previously both experimentally and theoretically that no optical blue-shift as a result of the quantummediated size effect can occur in TiO2 for particle sizes larger than 5 nm.9 XPS valence band (VB) spectra (Fig. S6, ESIw) reveal that the VB maximums of both TiO2 samples are at
2.18 eV, in contrast to anatase TiO2 without preferential {001} facets, which has its VB maximum at 2.01 eV.10 The unchanged VB maximum indicates therefore that the conduction band (CB) minimum of the nanosized anatase TiO2 is raised by 0.1 eV in contrast to the micron-sized anatase. The DFT electronic structure calculations of anatase (101) and (001) surfaces (see Fig. S7, ESIw) reveal two significant points: (i) the (001) face has a smaller bandgap than (101) face; (ii) the VB maximum of (101) is the same as that of (001), but the CB minimum of (101) is higher. Such differences can be originated from different atomic configurations on each surface. Based on the above results, the difference in the percentage of {001} facets between the micron and nanosized samples is thus concluded as the reason for the optical blue-shift of nanosized anatase with 18% {001} facets. The photo-oxidation and photo-reduction activities of the synthesized anatase TiO2 samples were estimated by detecting the amount of OH radicals generated using methods described previously8,10 and the hydrogen evolution rate from the photochemical reduction of water in the presence of methanol as a scavenger, respectively. Table 1 compares the
756 | Chem. Commun., 2010, 46, 755–757
Fig. 3 UV-visible absorption spectra of the surface fluorine terminated nanosized (a) and micron-sized (b) anatase TiO2 with preferential {001} facets, and the inset is their corresponding plots of transformed Kubelka–Munk function vs. the energy of light.
This journal is
c
The Royal Society of Chemistry 2010
Table 1 Fluorescence signal intensity of 2-hydroxyterephthalic acid (TAOH) at 426 nm and hydrogen evolution rate from water splitting for different samples of anatase TiO2 with preferential {001} facets under the irradiation of UV-visible light: A, micron-sized with surface fluorine termination; B, micron-sized fluorine-free; C, nanosized with surface fluorine termination; and D, nanosized fluorine-free Sample TAOH/a.u. H2a/m mol h
1
A
B
C
D
17 444 3
30 600 25
67 370 70
170 560 205
a
For hydrogen evolution, 1 wt% Pt species was loaded on each sample.
intensity of the fluorescence signal associated with TAOH from the reaction of terephthalic acid (TA) with OH radicals generated from the different anatase TiO2 samples under the irradiation of UV-visible light for 25 min. Importantly, nanosized TiO2 shows a 5.6 times (170 560 vs. 30 600) stronger photo-oxidation capability than micron-sized TiO2, even though the latter has a much higher percentage of {001} facets (cf. columns B and D). A related and significant observation is that the nanosized TiO2 (surface area ca. 21 m2 g 1) also has a much superior photoactivity to mesoporous TiO2 with a large surface area of 150 m2 g 1 (170 560 vs. 80 388 from Fig. S8, ESIw).11 It is apparent from the data in Table 1 that removal of the surface fluorine species results in a substantially enhanced capacity for generating OH radicals for both the nanosized and micron-sized anatase TiO2. This is also true of the hydrogen evolution rates from the samples loaded by 1 wt% Pt species co-catalyst (Pt is well known as efficient reactive sites of hydrogen12), with the effect being greatest for the micron-sized particles, which as mentioned above, have higher surface coverage fraction of fluorine species. This trend may be rationalized considering the fact that surface Ti–F bonding can change surface five-coordinated Ti atoms in exposed (001) and (101) facets to six-coordinated ones, which will have greatly lowered surface reactivity.13 The modified surface structure resulting from terminating fluorine can be clearly indicated by comparing the Raman spectra of TiO2 with and without surface fluorine species as shown in Fig. S9 and S10 (ESIw). In contrast to surface fluorine-free anatase, two features are apparently caused by surface terminating fluorine: (i) shifting of the Eg mode at 144 cm 1 towards higher frequency; (ii) weakening of the B1g mode at 397 cm 1. The higher the fraction of surface Ti–F bonds, the more obvious these effects become (see Fig. S9, ESIw). These changes may plausibly be regarded as the result of modifications to the force constants for the surface Ti–O–Ti network8,14,15 imposed by the presence of the surface terminating Ti–F bonds. As indicated in Table 1, the hydrogen evolution rate of nanosized TiO2 without surface fluorine is 8.2 times higher than that of micron-sized TiO2 without surface fluorine (columns B and D). This substantial enhancement is attributed to the synergistic effects of the increased surface area to volume ratio associated with the nanosized crystals and the raised CB minimum, which promotes photo-reduction reactions due to the higher potential of the excited electrons. On the other hand, it should be pointed out that an This journal is
c
The Royal Society of Chemistry 2010
indispensable condition in photo-oxidation reactions is the efficient consumption of photoexcited electrons by electron acceptors. The above promoted photo-reduction reactions can therefore simultaneously improve the photo-oxidation reactions as evidenced in Table 1 by decreasing the recombination probability of electrons and holes. Financial support from Major Basic Research Program, Ministry of Science and Technology of China (No. 2009CB220001), NSFC (No. 50921004), the Solar Energy Program of Chinese Academy of Sciences, the IMR SYNLT.S. Keˆ Research Fellowship, Australian Research Council through its Centre’s grant and DP0666345) is gratefully acknowledged.
Notes and references 1 M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann, Chem. Rev., 1995, 95, 69. 2 A. L. Linsebigler, G. Q. Lu and J. T. Yates Jr, Chem. Rev., 1995, 95, 735. 3 (a) P. D. Yang, D. Y. Zhao, D. I. Margolese, B. F. Chmelka and G. D. Stucky, Nature, 1998, 396, 152; (b) D. Grosso, C. Boissiere, B. Smarsly, T. Brezesinski, N. Pinna, P. A. Albouy, H. Amenitsch, M. Antonietti and C. Sanchez, Nat. Mater., 2004, 3, 787; (c) Y. Sakatani, D. Grosso, L. Nicole, C. Boissiere, G. J. D. A. Soler-Illia and C. Sanchez, J. Mater. Chem., 2006, 16, 77; (d) E. Martı´ nez-Ferrero, Y. Sakatani, C. Boissiere, D. Grosso, A. Fuertes, J. Fraxedas and C. Sanchez, Adv. Funct. Mater., 2007, 17, 3348; (e) X. C. Wang, K. Maeda, X. F. Chen, K. Takanabe, K. Domen, Y. D. Hou, X. Z. Fu and M. Antonietti, J. Am. Chem. Soc., 2009, 131, 1680. 4 H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H. M. Cheng and G. Q. Lu, Nature, 2008, 453, 638. 5 For example: (a) H. G. Yang, G. Liu, S. Z. Qiao, C. H. Sun, Y. G. Jin, S. C. Smith, J. Zou, H. M. Cheng and G. Q. Lu, J. Am. Chem. Soc., 2009, 131, 4978; (b) B. Wu, C. Guo, N. Zheng, Z. Xie and G. D. Stucky, J. Am. Chem. Soc., 2008, 130, 17563; (c) X. Han, Q. Kuang, M. Jin, Z. Xie and L. Zheng, J. Am. Chem. Soc., 2009, 131, 3152; (d) Y. Q. Dai, C. M. Cobley, J. Zeng, Y. M. Sun and Y. N. Xia, Nano Lett., 2009, 9, 2455; (e) F. Amano, T. Yasumoto, O. O. Prieto-Mahaney, S. Uchida, T. Shibayama and B. Ohtani, Chem. Commun., 2009, 2311; (f) D. Q. Zhang, G. S. Li, X. F. Yang and J. C. Yu, Chem. Commun., 2009, 4381; (g) F. Amano, O. O. Prieto-Mahaney, Y. Terada, T. Yasumoto, T. Shibayama and B. Ohtani, Chem. Mater., 2009, 21, 2601; (h) Y. Alivov and Z. Y. Fan, J. Phys. Chem. C, 2009, 113, 12954; (i) G. Liu, H. G. Yang, X. W. Wang, L. N. Cheng, J. Pan, G. Q. Lu and H. M. Cheng, J. Am. Chem. Soc., 2009, 131, 12868. 6 (a) U. Diebold, Surf. Sci. Rep., 2003, 48, 53; (b) X. Q. Gong and A. Selloni, J. Phys. Chem. B, 2005, 109, 19560; (c) A. Vittadini, A. Selloni, F. P. Rotzinger and M. Gratzel, Phys. Rev. Lett., 1998, 81, 2954. 7 J. K. Zhou, L. Lv, J. Q. Yu, H. L. Li, P. Z. Guo, H. Sun and X. S. Zhao, J. Phys. Chem. C, 2008, 112, 5316. 8 G. Liu, C. H. Sun, L. N. Cheng, Y. G. Jin, H. F. Lu, L. Z. Wang, S. C. Smith, G. Q. Lu and H. M. Cheng, J. Phys. Chem. C, 2009, 113, 12317. 9 (a) N. Serpone, D. Lawless and R. Khairutdinov, J. Phys. Chem., 1995, 99, 16646; (b) N. Satoh, T. Nakashima, K. Kamikura and K. Yamamoto, Nat. Mater., 2008, 3, 106. 10 G. Liu, L. Z. Wang, C. H. Sun, X. X. Yan, X. W. Wang, Z. G. Chen, S. C. Smith, H. M. Cheng and G. Q. Lu, Chem. Mater., 2009, 21, 1266. 11 G. Liu, X. W. Wang, L. Z. Wang, Z. G. Chen, F. Li, G. Q. Lu and H. M. Cheng, J. Colloid Interface Sci., 2009, 334, 171. 12 F. E. Osterloh, Chem. Mater., 2008, 20, 35. 13 A. Selloni, Nat. Mater., 2008, 7, 613. 14 G. Liu, Z. G. Chen, C. L. Dong, Y. N. Zhao, F. Li, G. Q. Lu and H. M. Cheng, J. Phys. Chem. B, 2006, 110, 20823. 15 G. Liu, L. Z. Wang, H. G. Yang, H. M. Cheng and G. Q. Lu, J. Mater. Chem., 2010, DOI: 10.1039/b909930a.
Chem. Commun., 2010, 46, 755–757 | 757
