Tunable Photocatalytic Selectivity of Hollow TiO 2 Microspheres Composed of Anatase Polyhedra with Exposed {001} Facets

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1 Tunable Photocatalytic Selectivity of Hollow TiO 2 Microspheres Composed of Anatase Polyhedra with Exposed {001} Facets Shengwei Liu, a Jiaguo Yu, a,* and Mietek Jaroniec b,* a State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122#, Wuhan , P. R. China b Department of Chemistry, Kent State University, Kent, Ohio, 44242, USA. *Corresponding authors s: jiaguoyu@yahoo.com, jaroniec@kent.edu Experimental Synthesis of titania samples Hollow titania microspheres (HTS) with exposed {001} facets were synthesized by using a modified fluoride-mediated self-transformation strategy. 1 Ethanol was selected because of its ability to stabilize {001} facets of anatase polyhedra, which are building blocks for the formation of HTS. In typical synthesis, 15 mmol of titanium sulfate, Ti(SO 4 ) 2, and 15 mmol of ammonium fluoride, NH 4 F, were mixed with 100 ml of ethanol and 50 ml of distilled water under vigorous stirring. The mixture was hydrothermally treated at 180 o C in a Teflon-lined stainless steel autoclave for h. After reaction, the product was collected by centrifugation, and then washed 3-4 times with distilled water to remove ionic impurities. Finally, the samples were dried at 80 o C in air. The resulting HTS sample is labeled as E2 (see Table S1); this sample was used to carry out all experiments shown in Figures 1 and 2 of this work. For the purpose of comparison, two additional samples were prepared by using 150 ml of distilled water and no ethanol (E0) or 50 ml of ethanol and 100 ml of distilled water (E1). Table S1 lists the solvent composition and basic adsorption parameters for the E0, E1 and E2 samples. Commercial P25 titania is used for the purpose of comparison too. (1) Yu, J. G.; Liu, S. W.; Yu, H. G. J. Catal. 2007, 249, 59. Surface modification NaOH washing. 0.1 g of as-prepared sample (E0, E1 or E2) was dispersed in 20 ml of 10 M NaOH aqueous solution under vigorous shaking for min in an ultrasonic cleaner. Next, the sample was collected by centrifugation, and then washed 3-4 times with distilled water to reach ph = 7. The samples after this surface modification are labeled as Ex-OH (where x = 0, 1, 2). Calcination. 0.1 g of as-prepared sample (E0, E1 or E2) was calcined at 600 o C for 90 min. The ramping rate is ca. 5 o C/min. Characterization Powder X-ray diffraction (XRD) patterns were obtained on a D/Max-RB X-ray S1

2 diffractometer (Rigaku, Japan) using Cu K α radiation at a scan rate of 5 o 2 s -1. Scanning electron microscopy (SEM) was performed using a Hitachi-4800S microscope (Hitachi, Japan). Transmission electron microscopy (TEM) analysis was conducted using a Tecnai G 2 20S-TWIN microscope (FEI, Czech). The porous structure and Brunauer-Emmett-Teller (BET) surface area of the samples were analyzed by nitrogen adsorption using a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). X-ray photoelectron spectroscopy (XPS) measurements were done on a VG ESCALAB 210 electron spectrometer using Mg K radiation. XPS data were calibrated using the binding energy of C1s ( ev) as the internal standard. Photocatalytic testing The selective photocatalytic decomposition of mixed methyl orange (MO) and methylene blue (MB) dyes was conducted in aqueous solution containing the selected titania sample under UV-light irradiation at ambient temperature. Experimental details were as follows: g of the sample was dispersed in a 20 ml of mixed MO ( mol L -1 ) and MB ( mol L -1 ) in aqueous solution in a 90 mm culture dish. A 15-W 365 nm UV lamp (Cole-Parmer Instrument Co.; 4 cm above the dish) was used as a light source. The concentration of MO and MB was monitored by a UV-2550 UV-visible spectrophotometer (Shimadzu, Japan). Table S1. Physiochemical properties of the E0, E1, E2 and P25 titania samples. a Sample Mixed Solvent ACS (nm) S BET (m 2 /g) V p (cm 3 /g) E0 H 2 O (150mL) E1 H 2 O (100 ml)+ch 3 CH 2 OH (50mL) E2 H 2 O (50 ml) + CH 3 CH 2 OH (100 ml) P APS (nm) a ACS - average crystallite size; S BET - BET surface area; V p - pore volume; APS - average pore size. S2

3 A (101) A: Anatase R: Rutile Intensity (a.u.) R (110) E2 P25 A (103) A (004) A (112) R (101) R (111) R (210) A (200) A (105) A (211) R (211) R (220) Theta (degree) Figure S1. XRD patterns of the E2 and P25 titania samples. In contrast to the P25 titania containing rutile together with anatase phase, all the diffraction peaks of E2 are well indexed according to a single anatase phase. Moreover, the XRD peaks for E2 are much narrower and stronger, suggesting better crystallinity and larger crystallite size of this sample in relation to P25 reference titania. Adsorbed Volume ( cm 3 STP/g) dv/dlogw ( cm 3 /g) 0.4 E P Pore diameter (nm) P25 E P/P0 Figure S2. N 2 adsorption isotherms and the corresponding pore size distribution (inset) for the E2 and P25 titania samples. This figure shows that the adsorption isotherm for E2 is type IV with adsorption branch resembling type II according to the IUPAC classification, 2 indicating the presence of textural mesopores as well as smaller macropores within THS shells. (2) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T.; Pure Appl. Chem. 1985, 57, 603. S3

4 Figure S3. TEM and HRTEM images of the E2 sample: (a) TEM image showing overall view of hollow titania microspheres; (b) enlarged TEM image of a portion of the HTS shell consisting of anatase polyhedra with exposed {001} facets; (c) enlarged TEM images of truncated octahedra; (d) HRTEM image of a single truncated octahedron. The TEM image (Figure S3a) confirms the hollow nature of titania microspheres. The typical diameter of these microspheres is 1-2 m and the shell thickness is ca. 200 nm. High magnification TEM image (Figure S3b) reveals that the shells are composed of anatase nanocrystals ( nm), resulting in a hollow spherical shape. Figure S3c indicates that these nanocrystals obtained under ultrasonication of HTS exhibit more or less square shapes, a typical top-view of anatase TiO 2 truncated octahedra with exposed {001} facets. The exposure of {001} facets is also confirmed by high-resolution TEM image (Figure S3d) of an individual anatase truncated octahedron. S4

5 Figure S4. Low (a; scale bar - 5μm) and high (b; scale bar -1 μm) magnification SEM images of the typical sphere-in-shell intermediates recorded during the time-dependent morphological evolution of; Inset in (a) is a typical TEM image (scale bar nm) for the sphere-in-shell structure. The external shells are composed of larger nanocrystals, while the interior sphere consists of smaller ill-shaped nanocrystals. Figure S5. High magnification SEM images of the external surface of the E0 (a), E1 (b) and E2 (c) HTS samples. The roughness of the external surface of the HTS shells appears to increase with increasing volume of ethanol in the mixed solvent. The primary nanocrystals not only increase in size but also simultaneously evolve into faceted polyhedra. S5

6 F 1s E2 (F/Ti = 10.11%) Intensity (a.u.) E1 (F/Ti = 8.98%) E0 (F/Ti = 8.17%) Binding Energy (ev) Figure S6. High-resolution XPS spectra of F 1s for the E0, E1 and E2 titania samples. Intensity (a.u.) F 1s E2 E2_NaOH washing E2_600 o C-calcination < 0.5 at.% < 0.3 at.% Binding energy (ev) Figure S7. High-resolution XPS spectra of F 1s for the E2 samples before and after NaOH surface modification, and after calcination at 600 o C. S6

7 ln( C0/C) ln( C0/C) (a) E0-F: MO, kmo = 1032 E0-F: MB, kmb = 0629 r = kmo/kmb = 1.64 E0-OH: MO, kmb = 0492 E0-OH: MB, kmb = 0945 r = kmb/kmo = 1.92 E2-F:MO, kmo = 1573 E2-F:MB, kmb = 1276 r = kmo/kmb = 1.23 E2-OH:MO, kmo = 0509 E2-OH:MB, kmb = 2553 r = kmb/kmo = 5.02 (c) ln ( C0/C) ln( C0/C) E1-F : MO, kmo = 1357 E1-F: MB, kmb = 1059 r = kmo/kmb = 1.28 E1-OH: MO, kmo = 06 E1-OH: MB, kmb = 1323 r = kmb/kmo = (b) P25: r = kmo/kmb = 3.07 P25: MO, kmo = P25:MB: kmb = 0464 (d) Figure S8. Tunable photocatalytic selectivity of the E0, E1 and E2 titania samples toward photodecomposition of MO and MB before (E0-F, E1-F, E2-F) and after (E0-OH, E1-OH, E2-OH) NaOH washing. For the purpose of comparison the last panel shows photocatalytic selectivity of P25 toward photodecomposition of MO and MB. The catalytic selectivity is defined as the ratio (r) of the apparent rate constants (k). S7

8 A/A <5% 22% 53% MO: E2-OH MO: E2-F MO: E2-c MB: E2-F MB: E2-c MB: E2-OH Figure S9. A comparison of adsorption of methyl orange (MO) and methyl blue (MB) on hollow TiO 2 microspheres (HTS) before and after surface modification. Adsorption curves were measured for as-prepared fluorinated HTS (E2-F), E2-F modified by NaOH washing (E2-OH), and E2-F calcined at 600 o C (E2-c). Before surface modification, the as-prepared fluorinated HTS (E2-F) shows negligible dye adsorption for either MB or MO (below 5% of initial concentration). After surface modification of HTS by either NaOH washing (E2-OH) or calcination (E2-c), adsorption of MO is still low (below 5%), while the calcined and NaOH washed HTS samples showed significantly enhanced adsorption of MB; about 22% and 53% of initial concentration, respectively. Note that the A/A o values for two MO adsorption curves exceed unity, which is caused by experimental error (~5%) as well as by the difficulty in measuring MO concentration due to the observed slight shift in the peak position of UV-vis spectra and a small fluctuation in the peak intensity; these variations are possibly caused by interactions between MO and hydrogen ions or residual fluorine ions desorbed from the surface of the HTS samples (Ti-F + H 2 O Ti-OH + H + + F - ). These variations are not observed on UV-vis spectra of the MB-containing systems. S8

9 Enlarged Figure 1. SEM images of the fluoride-mediated TiO 2 samples: (a) overall view of TiO 2 microspheres; (b) image of a few microspheres showing their unique structure consisting of primary TiO 2 nanoparticles; (c) a single microsphere showing its hollow nature; (d) a portion of the microsphere shell composed of nanosized polyhedra with exposed {001} facets. S9