Controlled synthesis of anatase TiO2 octahedra with enhanced photocatalytic activity

Materials Research Bulletin 48 (2013) 4469–4475

Contents lists available at ScienceDirect

Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

Controlled synthesis of anatase TiO2 octahedra with enhanced photocatalytic activity Ligang Gai *, Qinghu Mei, Xuyang Qin, Wenpeng Li, Haihui Jiang, Xiuquan Duan Key Laboratory of Fine Chemicals in Universities of Shandong, School of Chemistry and Pharmaceutical Engineering, Shandong Polytechnic University, Jinan 250353, People’s Republic of China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 20 April 2013 Accepted 24 July 2013 Available online 1 August 2013

Titanium dioxide (TiO2) nanocrystals with specific exposed crystal facet have attracted considerable interest due to their promising applications in the fields of energy and environment. In this paper, we report on a simple solvothermal approach for the synthesis of anatase TiO2 octahedra with high yield, using titanium(IV) sulfate and hydrazine hydrate as the starting materials. The formation mechanism of anatase TiO2 octahedra is suggested. The samples were characterized with XRD, Raman, SEM, TEM, FTIR, XPS, and UV/vis techniques, and further tested as a candidate in photocatalysis to decompose methyl orange in aqueous solution at room temperature. The results show that SO42 ions not only benefit the formation of octahedral nanocrystals, but also inhibit nitrogen doping into TiO2 matrix. More importantly, it is found that the octahedral TiO2 nanocrystals show enhanced photocatalytic activity compared to TiO2 P25 and anatase TiO2 counterparts. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Semiconductors B. Chemical synthesis D. Catalytic properties

1. Introduction The morphology and crystal facet controlled fabrication of semiconductor materials have attracted considerable attention because their photoelectric and photocatalytic properties can be enhanced by tailoring the surface atomic structure [1,2]. Among the semiconductor materials, titanium dioxide (TiO2) nanostructures with specific exposed crystal facet have been subjected to extensive research due to their promising applications in the fields of energy and environment [3–7]. For the application of TiO2 in photocatalysis, it has been demonstrated that decahedral anatase with exposed (0 0 1) and (1 0 1) faces [4,6,7] and dodecahedral rutile with exposed (1 1 0) and (1 0 1) faces [7] exhibit excellent photocatalytic activity, despite the large crystal size and/or small specific surface area. Generally, anatase TiO2 has the highest photoactivity followed by rutile and brookite [8], especially for TiO2 nanoparticles [7]. According to the Wulff construction and surface energy calculation [9], anatase TiO2 tends to assume a bifrustum form as an equilibrium shape, especially for anatase with hydrogenated and/or hydrated surface [10]. In most cases, anatase (1 0 1) facet has the lowest surface free energy among the other crystal facets [3,9,10], and anatase nanocrystals dominated by the (1 0 1) planes have been found to show enhanced photocatalytic activity for decomposing organic compounds [11–13]. The enhanced

photoactivity has been explained that reduction of O2 by photoinduced electrons and oxidation of organic compounds by positive holes are easily promoted on the (1 0 1) facet [11,13]. Also, octahedral anatase TiO2 nanocrystals have been found to show a highly hydrophilic conversion [14], because oxygen bridging sites of the exposed (1 0 1) facets are very active for the photoinduced surface hydrophilic conversion [9,14,15]. Recently, the electrochemical properties of anatase TiO2 octahedra have been examined to exhibit enhanced performance for storing lithium ions as compared to TiO2 P25 and the spherical counterpart [16]. So far, a few papers have demonstrated the synthesis of TiO2 octahedra with well-developed (1 0 1) facets [5,11,12,14,16,17], which can be tentatively categorized into two kinds. Methods of the first kind are based on employing one-dimensional titanate precursor [11,12,14]. The other methods are focused on the surfactant- [5,16,17] and/or functional group-assisting [5] route, where surfactant molecules preferentially binding to the specific facet facilitate the formation of TiO2 octahedra. However, all these studies involve relatively high cost and/or complicated procedures. In this paper, we report on a simple surfactant-free synthesis of TiO2 octahedra as well as their photocatalytic and electrochemical properties. 2. Experimental 2.1. Synthesis of anatase TiO2 octahedra

* Corresponding author. Tel.: +86 531 89631208; fax: +86 531 89631207. E-mail address: [email protected] (L. Gai). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.07.057

Analytical grade titanium(IV) sulfate (Ti(SO4)2) and hydrazine hydrate (N2H4
H2O, 80%) were directly used without further

4470

L. Gai et al. / Materials Research Bulletin 48 (2013) 4469–4475

purification. In a typical synthesis of anatase TiO2 octahedra, 2 mmol Ti(SO4)2 was dissolved in 5 mL of distilled water to form a clear solution, followed by dropwise addition of 35 mL of hydrazine hydrate with vigorous stirring for 1 h. Then, the mixture was transferred into a Teflon-lined autoclave (50 mL capacity). The autoclave was heated at 200 8C for 10–24 h, and then allowed to cool to room temperature. The precipitate was collected by filtration and washed thoroughly with distilled water and anhydrous ethanol, and finally dried to obtain anatase TiO2 octahedra with productivity over 80%. 2.2. Characterizations X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer using Cu Ka radiation (l = 1.5406 A˚) with a step size of 0.38 2u s 1, operating at 40 kV and 40 mA. Raman spectra were collected at room temperature on a Jobin-Yvon HR800 spectrometer with a 488 nm Ar+ ion laser. Scanning electron microscopy (SEM) images were taken on a Hitachi S-4800 fieldemission scanning electron microscope. Transmission electron microscopy (TEM) images were taken on a Philips Tecnai Twin-20U high-resolution transmission electron microscope, operating at an accelerating voltage of 200 kV. Fourier transform infrared (FTIR) spectra were collected on a Nicolet Avatar 370 infrared spectrometer using pressed KBr discs. X-ray photoelectron spectroscopy (XPS) analyses were performed on a Thermo Fisher Scientific ESCALAB 250 X-ray photoelectron spectrometer using monochromatic Al Ka radiation. A Shimadzu UV-2550 spectrometer equipped with an ISR-2200 diffuse reflection integrating sphere was employed to acquire the diffuse-reflection absorbance ultraviolet–visible (UV–vis) spectra over a range of 200–800 nm, using BaSO4 as a reflection reference. Nitrogen adsorption and desorption isotherms were collected at liquid nitrogen temperature using a Micromeritics Gemini VII surface area and pore size analyzer. All samples were degassed at 200 8C for 6 h under vacuum before analysis. The specific surface area was calculated by the linear portion of Brunauer–Emmett– Teller (BET) model (P/P0 = 0.06–0.3). 2.3. Photocatalysis experiments The photocatalysis experiments were carried out in a commercial photoreactor (BL–GHX–V, Shanghai, China) equipped with a 300 W mercury lamp for ultraviolet light illumination. The photoactivity of the catalysts was evaluated by decomposing methyl orange (MO) at neutral pH without any external oxidative or reductive radical-generating sources. The photocatalysis procedures have been published in reference [18], and were provided in detail in the Supplementary data.

Fig. 1. XRD patterns of the samples prepared with: (a) 70% hydrazine hydrate; (b) 35% hydrazine hydrate; and (c) distilled water.

(4.39) as compared to Fig. 1b (3.88) and Fig. 1c (3.30). This result indicates that crystal grains of the sample prepared with 70% hydrazine have a more surface area belonging to exposed (1 0 1) facet. To further examine the phase purity of the sample prepared with 70% hydrazine hydrate, Raman spectrum was recorded as shown in Fig. 2. All the peaks centered at 145, 199, 397, 516, and 639 cm 1 can be separately assigned to Eg(v6), Eg(v5), B1g(v4), A1g(v3)–B1g(v2), and Eg(v1) modes of anatase TiO2 [19]. There is no other peak related to brookite and/or rutile phase. In short, both the XRD and Raman spectra confirm a high-purity anatase phase for the sample prepared with 70% hydrazine hydrate. The size and morphology of the sample prepared with 70% hydrazine hydrate were examined by the SEM and TEM techniques (Fig. 3). The low-magnification SEM image (Fig. 3a) exhibits octahedral nanoparticles in high yield. The high-magnification SEM image (Fig. 3b) shows the octahedra have clean surface, edge width in the range of 70–160 nm, and length between the two pointed ends ranging from 100 to 300 nm, apart from adhesion of smaller nanoparticles to the surface in some cases. The octahedral characteristic is also evidenced by the TEM image (Fig. 3c). Welldefined lattice spacing of ca. 0.35 nm (Fig. 3d), corresponding to the d101 plane of anatase TiO2, reveals high crystallinity of the TiO2 crystals. On the basis of the SEM and TEM images, the octahedral anatase TiO2 in this research has a near-equilibrium shape, similar to the case for synthesizing anatase octahedra through a surfactant-assisted approach [16]. 3.2. Formation mechanism of anatase TiO2 octahedra In order to understand the formation mechanism of anatase TiO2 octahedra, control experiments were performed. The phase

3. Results and discussion 3.1. Structure, composition, and morphology Fig. 1 shows the XRD patterns of the samples prepared with 70% hydrazine hydrate (Fig. 1a), 35% hydrazine hydrate (Fig. 1b), and distilled water (Fig. 1c), using Ti(SO4)2 as the titanium source. It is apparent that the crystallinity of the sample increases as the hydrazine concentration increases, likely due to the relatively higher self-elevated pressure arising from decomposition of hydrazine at the reaction temperature (200 8C) to form N2, H2, and NH3 gases. All the diffraction peaks marked in Fig. 1 can be indexed to (1 0 1), (1 0 3), (0 0 4), (1 1 2), (2 0 0), (1 0 5), (2 1 1), (2 1 3), (2 0 4), (1 1 6), (2 2 0), (2 1 5) and (3 0 1) lattice planes of anatase TiO2 (JCPDS no. 78-2486). A noteworthy point is that Fig. 1a displays a higher (1 0 1)/(0 0 4) diffraction intensity ratio

Fig. 2. Raman spectrum of the sample prepared with 70% hydrazine hydrate.

L. Gai et al. / Materials Research Bulletin 48 (2013) 4469–4475

4471

Fig. 3. SEM (a and b) and TEM images (c and d) of the TiO2 octahedra: (a) low-magnification; (b) high-magnification; (c) low-resolution; and (d) high-resolution TEM image corresponding to the squared area in (c). Inset is the schematic drawing of a bifrustum.

and composition of the samples prepared by control experiments are presented in Table S1 and Fig. S1 (see Supplementary data). Using Ti(SO4)2 or TiF4 as the titanium source, anatase TiO2 octahedra can be easily obtained in solutions with hydrazine concentration in the range of 45–70% (Fig. 4a and b and Fig. S1a and b). However, irregular titania nanoparticles in anatase/brookite/ rutile ternary structure with size smaller than 50 nm dominate the sample prepared with tetra-n-butyl titanate (Ti(On-C4H9)4, TBT) (Fig. 4c and Fig. S1c). Using titanium tetrachloride (TiCl4) as the titanium source, TiO2 nanowires in anatase/brookite/rutile ternary structure occur (Fig. 4d and Fig. S1d). The individual wire with an average diameter of ca. 130 nm is an assembly of nanowires with diameter of ca. 30 nm. Apart from the dramatic influence of titanate anions, it is also found the reaction medium plays an important role in the formation of TiO2 octahedra. As expected, the sample prepared with distilled water is composed of aggregated nanoparticles with an average size of ca. 40 nm (Fig. 4e). Also, the hydrazine concentration has an important effect on the size and morphology of the product. When the hydrazine concentration is decreased to 35%, spindle-like anatase nanocrystals occur with edge width ranging from 45 to 90 nm and length between the two pointed ends ranging from 70 to 220 nm (Fig. 4f). By fixing the hydrazine concentration at 70%, intermediate product with a flower-like surface constructed by nano-plates can be obtained at the reaction time of 3 h (Fig. 4g); and anatase octahedral rudiments come to occur at 7 h (Fig. 4h). According to the thermodynamic model based on surface free energy and surface tension obtained from first principles calculations [10], alkaline environment not only facilitates the formation of elongated TiO2 nanoparticles, but also favors phase transformation from anatase to rutile. In the cases of using TBT, TiCl4, and/or hydrazine with concentration less than 45%, the experimental results mainly conform to the theoretical predict. However, octahedral TiO2 nanocrystals of pure anatase phase can be

produced in high yield by using titanium(IV) sulfate and hydrazine with concentration more than 45%. This result shows both SO42 ions and N2H4 molecules have an important effect on the controlled synthesis of TiO2 nanocrystals. As documented in literature, tetragonal SO42 ions strongly bind to anatase (0 0 1) plane which contains edge-sharing TiO6 octahedra with the highest density of vertexes [5]. Similarly, F ions prefer to bind to anatase (0 0 1) and (1 0 1) planes, yielding not only the lowest surface energy for the both planes, but also more stable (0 0 1) plane compared to (1 0 1) plane [3]. For SO42 and F ions, the strong affinity to titanium cations inhibits the structural rearrangement and, subsequently, the phase transformation [20]. Compared to SO42 and F , C4H9O and Cl ions show weaker affinity to titanium cations [21]. This can explain why pure anatase nanocrystals can be obtained with Ti(SO4)2 and/or TiF4, whereas mixed TiO2 polymorphs occur in the cases of using TBT and/or TiCl4 as the titanium source under otherwise identical conditions. On the other hand, first principles calculations have demonstrated that oxygen atom binding to anatase surface causes a smaller increase in surface energy for both the (0 0 1) and (1 0 1) planes of anatase as compared to that for nitrogen atom [3]. Also, the surface energy for the (0 0 1) plane is higher than that for the (1 0 1) plane with respect to anatase with hydrogen-poor and/or oxygenated surface [10]. In view of the same number of coordination site for an individual SO42 anion and N2H4 molecule, it is reasonable to deduce that it is SO42 rather than N2H4 that binds to the (0 0 1) plane of anatase in a hydrazine-mediated alkaline environment. At the same time, N2H4 molecules tend to bind to the (1 0 1) plane of anatase due to the higher surface energy for the (1 0 1) plane than that for the (1 0 0) plane [10]. On the basis of the above analysis, a dissolve-recrystallization formation mechanism of TiO2 octahedra prepared with titanium sulfate and hydrazine is proposed as illustrated in Scheme 1. At the room temperature, titanium(IV) sulfate reacts with hydrazine

4472

L. Gai et al. / Materials Research Bulletin 48 (2013) 4469–4475

Fig. 4. SEM images of the TiO2 samples prepared in control experiments: (a) Ti(SO4)2, N2H4
H2O (45%); (b) TiF4, N2H4
H2O (70%); (c) Ti(On-C4H9)4, N2H4
H2O (70%); (d) TiCl4, N2H4
H2O (70%); (e) Ti(SO4)2, H2O; (f) Ti(SO4)2, N2H4
H2O (35%); (g) Ti(SO4)2, N2H4
H2O (70%); and (h) Ti(SO4)2, N2H4
H2O (70%). The reaction temperature is 200 8C. The reaction time for (a–f), (g), and (h) is 12 h, 3 h, and 7 h, respectively.

hydrate to form titanate precursor (Scheme 1a). At the elevated temperature, the titanate precursor tends to dissolve in the form of TiO6 octahedra due to the alkaline environment created by hydrazine (Scheme 1b). Then, the TiO6 octahedra recrystallize to form anatase TiO2 nanocrystals surrounded by SO42 and N2H4

(Scheme 1c). The bonding between SO42 and Ti4+ ions on the (0 0 1) plane together with the combination between N2H4 and the (1 0 1) plane enable the growth rate along [0 0 1] direction to be delicately faster than that along [1 0 1] direction, a key factor to produce (1 0 1)-plane-dominated octahedral bifrustums (Scheme 1d).

L. Gai et al. / Materials Research Bulletin 48 (2013) 4469–4475

Scheme 1. Proposed formation mechanism of anatase TiO2 octahedra: (a) formation of titanate precursor; (b) titanate precursor dissolves in the form of TiO6; (c) TiO6 octahedra recrystallize to form anatase TiO2 with SO42 and N2H4 binding to the (0 0 1) and (1 0 1) planes; and (d) formation of anatase octahedral. The squares, red spheres, and green spheres separately denote TiO6 octahedra, SO42 ions, and N2H4 molecules. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

3.3. Surface chemistry In our previous work [18], nitrogen species doping into TiO2 matrix tends to occur in a hydrazine-mediated solvothermal process, using TiO2 colloids as the starting material. Although hydrazine molecules prefer to bind to the (1 0 1) plane of anatase during the formation of octahedral TiO2, nitrogen species do not incorporate into TiO2 lattice. This result is based on the fact that there is no peak corresponding to the NOx species in the FTIR spectrum (Fig. 5) [18]. In Fig. 5, the strong peak ranging from 567 to 800 cm 1 corresponds to the normal stretching vibration of Ti–O [22], and the other weak peaks around 3250 and 1660 cm 1 are due to surface-adsorbed water [22]. In addition, there is no peak for nitrogen element in the survey XPS spectrum (data not shown). More importantly, there is no discernable shift in binding energy of Ti 2p region between the two samples prepared with hydrazine and distilled water (Fig. 6). The peaks centered at 459.0 and 464.6 eV correspond separately to the binding energy of Ti 2p3/2 and Ti 2p1/2 for prinstine TiO2 [23]. As mentioned above, SO42 ions strongly bind to the (0 0 1) plane of TiO2 to produce anatase octahedra. The strong interaction between SO42 and Ti4+ ions during the growth of TiO2 embryos likely inhibits nitrogen doping into TiO2 matrix. 3.4. Optical properties Fig. 7a shows the UV/vis diffuse-reflection absorbance spectra of the samples. It is apparent that the absorbance of the spherical, spindle-like, and octahedral nanocrystals sequentially increases in

Fig. 5. FTIR spectrum of the TiO2 octahedra prepared with 70% hydrazine hydrate.

4473

Fig. 6. High-resolution XPS spectra for the Ti2p region of the samples prepared with: (a) 70% hydrazine hydrate and (b) distilled water.

the UV region ranging from 200 to 400 nm. In correlation with the XRD result (Fig. 1), the sequential increase in UV light absorbance is assigned to the sequential enhancement in crystallinity of the three samples. The very weak absorbance in the wavelength region over 500 nm, a region where absorbance is related to oxygen vacancies [24], reveals good crystalline perfection of the octahedral nanocrystals [24,25], in concert with the high-resolution TEM result. The strong absorbance of octahedra in the UV region enables the photocatalyst to be highly efficient under UV light irradiation. This result will be confirmed by the photocatalysis experiments (Fig. 8). Fig. 7b shows the plots of (ahv)2 versus hv from the spectral data in Fig. 7a. In comparison with those of (ahv)1/2 versus hv (Fig. S2), the plots in Fig. 7b exhibit good linearity, suggesting that the light absorption of the samples conforms to allowed direct band transitions [26]. The intercepts of the plots in Fig. 7b place Franck– Condon type transitions [24] at ca. 3.3 eV for the spherical, spindlelike, and octahedral nanocrystals, close to the band gap of 3.2 eV for bulk anatase TiO2. The octahedral TiO2 nanocrystals possess pure anatase phase, good crystallinity, and high optical properties. These characteristics benefit the application in photocatalysis and facilitate to understand the size and shape-dependent electrochemical properties with respect to anatase TiO2. 3.5. Photocatalytic activity In order to evaluate the shape-dependent photoactivity of the catalysts, the spherical, spindle-like, and octahedral TiO2 nanoparticles were examined by decomposing methyl orange in aqueous solution with neutral pH at room temperature. TiO2 P25 was also examined for comparison. Although methyl orange can be almost completely decomposed by the four catalysts under UV light illumination for 60 min, the photocatalytic efficiency of the samples differs from each other (Fig. 8a). Under UV light illumination for 30 min, the photocatalytic efficiency of octahedral TiO2 is 1.38, 2.12, and 2.64 times that of P25, spindle-like, and spherical nanoparticles, respectively. Theoretical calculations have demonstrated that the exposed (1 0 1) facet yields an enhanced reactivity with molecular O2 [13]. This result not only facilitates generation of superoxide radical, a well-recognized species that involves in the oxidation of organics [27], but also enhances charge separation due to trapping of photogenerated electrons by chemisorbed molecular O2 [13]. Also, experimental results have evidenced that oxidation and reduction reaction predominantly proceed on the (0 0 1) and (1 0 1) planes of decahedral anatase TiO2, respectively [7]. In view of the specific surface areas of 18.5, 26.4, and 63.7 m2/g belonging, respectively, to the octahedral,

4474

L. Gai et al. / Materials Research Bulletin 48 (2013) 4469–4475

Fig. 7. (a) Diffuse-reflection UV/vis absorbance spectra and (b) plots of (ahv)2 versus hv from the spectral data in (a).

Fig. 8. (a) Photocatalytic decomposition of methyl orange in aqueous solution at room temperature under UV light irradiation and (b) recycling test for the anatase octahedra. P25 is employed for comparison in photocatalysis.

spindle-like, and spherical nanoparticles, it suggests that the high crystallinity and the predominantly exposed (1 0 1) plane of anatase octahedra are responsible for the enhanced photoactivity. In correlation with the high (1 0 1)/(0 0 4) diffraction intensity ratio displayed in Fig. 1a, it is concluded that octahedral anatase with a large surface area of (1 0 1) and small surface area of (0 0 1) is suitable for photocatalytic decomposition of organic dyes. As demonstrated in Ref. [6], anatase nanocrystals with specific exposed facet can inhibit recombination of photogenerated electron–hole pairs by spatial separation of redox sites in the nanocrystals, a situation beneficial for efficient decomposition of organic compounds. For practical applications of a photocatalyst, an important factor should be considered, i.e., the stability in resisting photocorrosion and in keeping high activity after long-term usage [28]. In this research, the anatase octahedra were examined for reuse five times to test its stability (Fig. 8b). After five catalysis cycles for decomposing methyl orange, the sample does not show observable deactivation. This result is indicative of high photostability of the catalyst. 4. Conclusions In summary, well-defined octahedral anatase TiO2 nanocrystals have been prepared by a simple solvothermal approach, using titanium(IV) sulfate and hydrazine hydrate as the starting materials. Both the SO42 ions and the N2H4 molecules play an important role in the formation of octahedral anatase nanocrystals. Also, SO42 ions can inhibit nitrogen doping in the TiO2 matrix. The as-prepared TiO2 octahedra possess high-purity anatase phase, good crystallinity, and enhanced optical and photocatalytic properties as compared to the spindle-like and spherical TiO2

nanocrystals. The anatase TiO2 octahedra presented here have great potentials for applications in photocatalysis, solar cells, and lithium ion batteries. Acknowledgment This research was partially supported by NSFC (no. 51272143).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull.2013.07.057. References [1] Y. Bi, S. Ouyang, N. Umezawa, J. Cao, J. Ye, J. Am. Chem. Soc. 133 (2011) 6490–6492. [2] G. Liu, J.C. Yu, G.Q. Lu, H.M. Cheng, Chem. Commun. 47 (2011) 6763–6783. [3] H.G. Yang, C.H. Sun, S.Z. Qiao, J. Zou, G. Liu, S.C. Smith, H.M. Cheng, G.Q. Lu, Nature 453 (2008) 638–641. [4] F. Amano, O.O. Prieto-Mahaney, Y. Terada, T. Yasumoto, T. Shibayama, B. Ohtani, Chem. Mater. 21 (2009) 2601–2603. [5] D. Wang, J. Liu, Q. Huo, Z. Nie, W. Lu, R.E. Williford, Y.B. Jiang, J. Am. Chem. Soc. 128 (2006) 13670–13671. [6] N. Murakami, Y. Kurihara, T. Tsubota, T. Ohno, J. Phys. Chem. C 113 (2009) 3062– 3069. [7] T. Ohno, K. Sarukawa, M. Matsumura, New J. Chem. 26 (2002) 1167–1170. [8] V. Etacheri, M.K. Seery, S.J. Hinder, S.C. Pillai, Chem. Mater. 22 (2010) 3843–3853. [9] M. Lazzeri, A. Vittadini, A. Selloni, Phys. Rev. B 63 (2001) 155409. [10] A.S. Barnard, L.A. Curtiss, Nano Lett. 5 (2005) 1261–1266. [11] F. Amano, T. Yasumoto, O.O. Prieto-Mahaney, S. Uchida, T. Shibayama, B. Ohtani, Chem. Commun. (2006) 2311–2313. [12] J. Li, Y. Yu, Q. Chen, J. Li, D. Xu, Cryst. Growth Des. 10 (2010) 2111–2115. [13] N. Wu, J. Wang, D.N. Tafen, H. Wang, J.G. Zheng, J.P. Lewis, X. Liu, S.S. Leonard, A. Manivannan, J. Am. Chem. Soc. 132 (2010) 6679–6685.

L. Gai et al. / Materials Research Bulletin 48 (2013) 4469–4475 [14] M. Miyauchi, J. Mater. Chem. 18 (2008) 1858–1864. [15] R. Wang, N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B 103 (1999) 2188–2194. [16] J. Du, J. Zhang, D.J. Kang, CrystEngComm 13 (2011) 4270–4275. [17] E. Hosono, S. Fujihara, H. Imai, I. Honma, I. Masaki, H. Zhou, ACS Nano 1 (2007) 273–278. [18] L. Gai, X. Duan, H. Jiang, Q. Mei, G. Zhou, Y. Tian, H. Liu, CrystEngComm 14 (2012) 7662–7671. [19] T. Ohsaka, F. Izumi, Y. Fujiki, J. Raman Spectrosc. 7 (1978) 321–324. [20] X. Bokhimi, A. Morales, E. Ortı´z, T. Lo´pez, R. Go´mez, J. Navarrete, J. Sol–Gel Sci. Technol. 29 (2004) 31–40. [21] M. Wu, G. Lin, D. Chen, G. Wang, D. He, S. Feng, R. Xu, Chem. Mater. 14 (2002) 1974–1980.

4475

[22] A.I. Kontos, I.M. Arabatzis, D.S. Tsoukleris, A.G. Kontos, M.C. Bernar, D.E. Petrakis, P. Falaras, Catal. Today 101 (2005) 275–281. [23] S. Hoang, S.P. Berglund, N.T. Hahn, A.J. Bard, C.B. Mullins, J. Am. Chem. Soc. 134 (2012) 3659–3662. [24] N. Serpone, D. Lawless, R. Khairutdinov, J. Phys. Chem. 99 (1995) 16646– 16654. [25] Z. Lin, A. Orlov, R.M. Lambert, M.C. Payne, J. Phys. Chem. B 109 (2005) 20948– 20952. [26] X.H. Wang, J.G. Li, H. Kamiyama, M. Katada, N. Ohashi, Y. Moriyoshi, T. Ishigaki, J. Am. Chem. Soc. 127 (2005) 10982–10990. [27] B.G. Kwon, J. Photochem. Photobiol. A 119 (2008) 112–118. [28] T. Cao, Y. Li, C. Wang, Z. Zhang, M. Zhang, C. Shao, Y. Liu, J. Mater. Chem. 21 (2011) 6922–6927.