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Photocatalytic Activity of Novel Tin Porphyrin/TiO2 Based Composites Ming-yue Duan,† Jun Li,*,† Giuseppe Mele,*,‡ Chen Wang,† Xiang-fei Lu¨,† Giuseppe Vasapollo,‡ and Feng-xing Zhang† Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, School of Chemistry & Materials Science, Northwest UniVersity, Xian, Shaanxi 710069, People’s Republic of China, and Dipartimento di Ingegneria dell’InnoVazione, UniVersita` del Salento, Via Arnesano 73100 Lecce, Italy ReceiVed: December 11, 2009; ReVised Manuscript ReceiVed: March 22, 2010
Three dichloro Sn(IV) porphyrins, trans-dichloro[5,10,15,20-tetra-[2 or 3 or 4-(3-phenoxy)-propoxy]phenyl porphyrin]tin(IV) and the corresponding dihydroxo Sn(IV) porphyrins trans-dihydroxo[5,10,15,20-tetra-[2 or 3 or 4-(3-phenoxy)-propoxy]phenyl porphyrin]tin(IV) were synthesized and characterized spectroscopically. The dihydroxo Sn(IV) porphyrin-TiO2 composites were also prepared and characterized, and an interaction between TiO2 and tin porphyrin molecules involving the axial -OH ligand with the TiO2 surface was proposed. The photocatalytic activity was investigated by testing the photodegradation of 4-nitrophenol (4-NP) in aqueous solution under both visible and UV-vis light irradiation. It was found that the distinct space tropisms of peripheral substituents in meso sites of the porphyrin ring lead to different efficiencies in photodegrading the 4-NP. 1. Introduction In recent years the treatment and purification of polluted water by an economical and environment-friendly photocatalysis method1-4 have been topics of growing interest, and the porphyrin-sensitized TiO2 system has been proposed for the oxidative degradation of various kinds of organic pollutants in water.5-7 Given their primary role in photosynthesis, porphyrins and metalloporphyrins are better photosensitizers for extending the light absorption of wide band gap TiO2 into the visible light region than many other dye sensitizers such as erythrosine B8 and rose Bengal,9 due to their small singlet-triplet splitting, high quantum yield for intersystem crossing, the long triplet state lifetimes,10 and good chemical stability. Though the detailed mechanism of the photosensitized catalysis is not fully understood, several factors influence the efficiency of TiO2-porphyrins systems, including the molecule structure of porphyrin, the center metal, axial ligand, and the crystal and specific surface area of TiO2, etc. So it is significant to explore the influential factors of the efficiency of TiO2porphyrins systems by designing and synthesizing special structure porphyrins. Except for major use in photodynamic therapy,11 tin porphyrins have also been studied extensively in catalytic or photocatalytic applications,10,12,13 because of their ease of reduction, single metal oxidation state, and favorable photophysics.14 Besides, different from four-coordinated metolloporphyrins such as Cu(II), Ni(II), and Co(II) porphyrins, Sn(IV) porphyrins are commonly six-coordinated with two trans axial groups. Therefore, we felt that it was reasonable to apply Sn(IV) porphyrins to impregnate polycrystalline TiO2 (anatase) in order to examine their photocatalytic effectiveness, thus evaluating the influence of the Sn(IV) and its axial ligation. * To whom correspondence should be addressed: Jun Li, fax, 00 86 29 88303798, tel, 0086 29 88302604, e-mail,
[email protected]; Giuseppe Mele, fax, 00 39 0832 297279, tel, 00 39 0832 297281, e-mail,
[email protected]. † Northwest University. ‡ Universita ` del Salento.
On the other hand, it is important to remark that three different free base porphyrins can be deemed as ortho-, meta-, and parasubstituted TPP (5,10,15,20-tetraphenylporphyrin) with alkyl chain (3-phenoxy)propoxy, which is supposed to affect strongly the porphyrin’s stereochemistry. In this way it is possible to investigate how combined effects such as the spatial tropism of substituents and axial ligation influence the morphology as well as the photoreactivity of porphyrin-TiO2 composites. On the basis of our previous work,15-17 this paper reports the synthesis of three dichloro Sn(IV) porphyrins, transdichloro[5,10,15,20-tetra-[2 or 3 or 4-(3-phenoxy)-propoxy]phenyl porphyrin] tin(IV) (1a-1c) and corresponding dihydroxo Sn(IV) porphyrins trans-dihydroxo[5,10,15,20-tetra-[2 or 3 or 4-(3-phenoxy)-propoxy]phenyl porphyrin] tin(IV) (2a-2c) (Figure 1), as well as the further photocatalytic activities of three SnPp · 2OH(2a-2c)-TiO2 composites by the photodegradation of 4-nitrophenol (4-NP) in aqueous solution under visible light. 2. Experimental Section 2.1. Materials and Measurements. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) was obtained from Aldrich, and other reagents were obtained from Beijing Chemical Reagents Company. They were used without further purification except pyrrole, which was distilled before use. Free porphyrins, 5,10,15,20-tetra[2-(3-phenoxy)propoxy]phenylporphyrin (a), 5,10,15,20-tetra[3-(3-phenoxy)-propoxy]phenylporphyrin (b), and 5,10,15,20-tetra[4-(3-phenoxy)-propoxy]phenylporphyrin (c) were synthesized according to our previous work.15,16 TiO2 was kindly provided by Tioxide Huntsman (anatase phase, BET specific surface area 8 m2/g), using in preparation of loaded samples applied as photocatalysts in photoreactivity experiments. Elemental analyses (C, H, and N) were performed by Vario EL-CHNOS instrument. Diffuse reflectance (DR) spectra were obtained at room temperature in the wavelength range 200-800 nm using a Shimadzu UV-2401PC spectrophotometer with BaSO4 as reference material. UV-vis spectra were obtained with a Shimadzu UV-2550 UV-vis-NIR spectrophotometer.
10.1021/jp911744a 2010 American Chemical Society Published on Web 04/13/2010
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Figure 1. Molecular structures of Cl2SnPp(1a, 1b, 1c) and (OH)2SnPp(2a, 2b, 2c). 1H
NMR spectra were recorded with a Varian Inova 400MHz at room temperature, and chemicals shifts were reported in parts per million with respect to the reference frequency of tetramethylsilane, Me4Si. FT-IR spectra were recorded on a BEQ UZNDX-550 spectrometer on samples embedded in KBr pellets. Raman spectra were recorded at room temperature using a ALMEGA Dispersive Raman spectrometer (Thermo Nicolet, USA) with a 532 nm Ar+ laser as the excitation source in a backscattering geometry. Mass spectrometry (MS) analyses were carried out on a matrix-assisted laser desorption/ionization time-of-flight mass spectrometer (MALDI-TOF MS, Krato Analytical Company of Shimadzu Biotech, Manchester, Britain). One microliter of sample solution and 1 µL of the matrix mixture were spotted into wells of the MALDI sample plate and air-dried. The samples were analyzed in linear ion mode with CHCA as matrix. External calibration was achieved using a standard peptide and protein mix from Sigma. High-resolution transmission electron microscopy (TEM) was carried out using a JEOL JEM-3010 instrument. A study of the composition and properties of the products was performed using X-ray photoelectron spectroscopy (XPS). The XPS spectra were obtained by Axis Ultra, Kratos (U.K.) using monochromatic Al KR radiation (150 W, 15 kV, 1486.6 eV). The vacuum in the spectrometer was 10-9 Torr. Binding energies were calibrated relative to the C 1s peak (284.8 eV) from hydrocarbons adsorbed on the surface of the samples. 2.2. General Procedure for the Synthesis of SnPp · 2Cl (1a-1c). A 0.12 g (0.1 mmol) portion of H2Pp and 0.45 g (2 mmol) of SnCl2 · 2H2O were added to 50 mL of pyridine and heated with stirring at 115 °C on an oil bath for 5 h. The unreacted solid salt was filtered and the solvent was removed under vacuum. The residual was dissolved in 100 mL of CH2Cl2 and washed several times to neutral with 0.5 M HCl followed by distilled water. The water was removed and the oil layer was dried with Na2SO4. After concentration, appropriate amount of hexane was added to yield crystal product and the crude product was recrystallized, or the concentrated solution was purified by chromatography using a silica gel column with CH2Cl2/EtOH (9/1 v/v) as eluent. The same method was employed to obtain 1a, 1b, and 1c using free porphyrins a, b, and c, respectively, as starting materials. SnPp · 2Cl (1a). Yield: 76%. Mp: >250 °C. Anal. Calcd (found) for SnC80H68N4O8Cl2 (mol wt 1403.03): C, 68.56 (68.48); H, 4.95 (4.89); N, 3.90 (3.99). MS: m/z 1367.35 ([M - Cl]+) amu. UV-vis (CHCl3): λmax, nm, 430, (Soret band), 562, 600, 627 (Q bands). FT-IR: ν, cm-1, 2925, 2362, 1595, 1536, 1493, 1290, 1243, 1166, 1117, 1024, 796, 752, 690. SnPp · 2Cl (1b). Yield: 79%. Mp >250 °C. Anal. Calcd (found) for SnC80H68N4O8Cl2 (mol wt 1403.03): C, 68.59
(68.48); H, 4.96 (4.89); N, 3.84 (3.99). MS: m/z 1367.64 ([M - Cl]+) amu. UV-vis (CHCl3): λmax, nm, 428, (Soret band), 559, 611, 627 (Q bands). FT-IR: ν, cm-1, 3424, 2923, 1599, 1534, 1494, 1470, 1287, 1244, 1171, 1061, 1026, 754, 692. SnPp · 2Cl (1c). Yield: 78%. Mp >250 °C. Anal. Calcd (found) for SnC80H68N4O8Cl2 (mol wt 1403.03), %: C, 68.54 (68.48); H, 4.96 (4.89); N, 3.85 (3.99). MS: m/z 1367.81 ([M - Cl]+) amu. UV-vis (CHCl3): λmax, nm, 434, (Soret band), 566, 608, 628 (Q bands). FT-IR: ν, cm-1, 3442, 2925, 2855, 2363, 1600, 1498, 1465, 1242, 1172, 1056, 1028, 806, 753, 689. 2.3. General Procedure for the Synthesis of (OH)2SnPp (2a-2c). A 0.14 g (0.1 mmol) portion of SnPp · 2Cl was added into mixed solution of 40 mL of THF and 10 mL of H2O, then 0.28 g (2 mmol) of K2CO3 was added. The mixture was heated with stirring at 65 °C for 3 h, then THF and H2O were distilled under vacuum. The residual was dissolved in 30 mL of CH2Cl2 and washed several times with distilled water. After drying by Na2SO4, most CH2Cl2 was removed and the remaining product was purified by chromatography using a silica gel column with CH2Cl2/EtOH (9/1 v/v) as eluent. The same method was employed to obtain (2a), (2b), and (2c) using (1a), (1b), and (1c), respectively, as starting materials. (OH)2SnPp (2a). Yield: 75%. Mp: >200 °C. Anal. Calcd (found) for SnC80H70N4O10 (mol wt 1366.14): C, 70.40 (70.33); H, 5.09 (5.16); N, 4.04 (4.10). MS: m/z 1367.02 ([M + H]+) amu. UV-vis (CHCl3): λmax, nm, 432, (Soret band), 561, 602, 627 (Q bands). FT-IR: ν, cm-1, 3440, 2925, 2854, 1743, 1596, 1462, 1284, 1240, 1175, 1028, 752, 690. (OH)2SnPp (2b). Yield: 78%. Mp: >200 °C. Anal. Calcd (found) for SnC80H70N4O10 (mol wt 1366.14): C, 70.43 (70.33); H, 5.21 (5.16); N, 4.14 (4.10). MS: m/z 1366.98 ([M + H]+) amu. UV-vis (CHCl3): λmax, nm, 430, (Soret band), 561, 602, 627 (Q bands). FT-IR: ν, cm-1, 3441, 2924, 2854, 1743, 1595, 1467, 1288, 1241, 1174, 1027, 752, 692. (OH)2SnPp (2c). Yield: 78%. Mp: >200 °C. Anal. Calcd (found) for SnC80H70N4O10 (mol wt 1366.14): C, 70.39 (70.33); H, 5.05 (5.16); N, 4.15 (4.10). MS: m/z 1366.98 ([M + H]+) amu. UV-vis (CHCl3): λmax, nm, 429, (Soret band), 563, 600, 628 (Q bands). FT-IR: ν, cm-1, 3442, 2923, 2855, 1742, 1595, 1462, 1290, 1238, 1176, 1025, 750, 691. 2.4. Preparation of the Photocatalyts: (2a-TiO2), (2b-TiO2), and (2c-TiO2). The loaded samples used as photocatalysts for the photodegradation experiments were prepared in the following way: 6 µmol amounts of SnPp · 2OH (2a-2c) (2a-2c of CHCl3 10-3 mol/L solution) were dissolved in 30 mL of CHCl3, and then 1 g of finely ground TiO2 was added. The resulting suspension was magnetically stirred at room temperature for 5 h. Then the solvent was removed under vacuum and the catalyst was collected.
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SCHEME 1: Synthesis of Tin Porphyrins
2.5. Photodegradation Setup and Photodegradation Experiments. The photodegradation setup, placed in a black box, consisted of a 200 mL beaker irradiated by a 500 W, 24 V xenon lamp-house system (CHF-XM35-500W, Beijing Trusttech Co., Ltd., China) providing UV-vis light. The solution was 110 cm in distance below the lamp, where the light intensity was 4.061 mW/cm2 (New Port Dual-Channel Power Meter, model 2832C, Irvine, CA). For visible-light degradation studies, a shortwave cutoff filter of 400 nm wavelength was employed between the lamp and the breaker to absorb the UV light. The temperature inside the reactor was maintained at ca. 25 °C by means of a continuous circulation of water in a jacket surrounding the reactor. The reacting suspension consisting of 100 mL of 10-4 mol/L 4-NP and 20.0 mg of catalyst was magnetically stirred and had air bubbled into the suspension for 30 min before switching on the lamp. The initial pH value of the suspension was 6.40 units. The photoreactivity runs lasted 500 min, and samples of 3 mL were withdrawn from the suspension every 50 min during the irradiation. The photocatalysts were separated from the solution by centrifugation, and the quantitative determination of 4-NP was determined by measuring its absorption at 316 nm with a Shimadzu UV2550 UV-vis-NIR spectrophotometer.
TABLE 1: UV-vis Data of the Tin Porphyrins and Diffuse Reflectance Spectral Data of Photocatalysts compounds a12 2a 2a-TiO2 b11 2b 2b-TiO2 c12 2c 3c-TiO2
λmax (nm) 419 428 434 420 429 435 423 433 438
514 520 521 516 523 523 519 524 528
548 559 562 550 561 562 556 563 570
589 598 600 589 602 602 593 606 614
644 644 627 630 649
3. Results and Discussion 3.1. Synthesis and Characterization of Tin Porphyrins. The synthetic route is illustrated in Scheme 1. The insertion of Sn(IV) into porphyrin rings occurs by heating the free base porphyrin with excess hydrated SnCl2 in pyridine,18,19 and dihydroxo Sn(IV) porphyrins are obtained by hydrolysis of dichloro Sn(IV) porphyrins. Figure 2 shows the UV-vis spectra of (OH)2SnPp (2a, 2b, 2c) in CHCl3 (10-4 M). The insertion of Sn(IV) into the porphyrin ring caused red shifts of 7-11 nm of the corresponding Soret bands for UV-vis spectra as well as a decreasing
number of Q bands (Table 1). Mass spectroscopy data perfectly correspond to the expected m/z values for the individual tin porphyrins. The main change of the IR spectra of tin porphyrins compared with free base porphyrins is the disappearance of stretching vibration of N-H. Besides, it should be noticed that for the IR spectra of SnPp · 2Cl (1b) and SnPp · 2Cl (1c), broad peaks at 3424 and 3442 cm-1, respectively, were observed which were deemed as streching vibrations of -OH, while no peak around 3400 was found for SnPp · 2Cl (1a). This lies in the different methods adopted to obtain chloro tin complexes: SnPp · 2Cl (1a) was obtained and purified by crystallization, whereas the others were by silica gel column. It was probably due to silica gel causing chloro complex to hydrolyze forming undesired hydroxo complex, which was reported previously to occur under the use of alumina.14 So recrystallization is the favorable method if chloro tin porphyrin is the desired product, but chromatography is feasible if hydroxo complex is the target product. 3.2. Characterization of Photocatalyts 2a-TiO2, 2bTiO2, and 2c-TiO2. 3.2.1. The Diffuse Reflectance Spectra. Figure 3 shows the diffuse reflectance spectra of the bare TiO2, 2a-TiO2, 2b-TiO2, and 2c-TiO2 photocatalysts recorded in the range of 300-750 nm.
Figure 2. UV-vis spectra of (OH)2SnPp(2a, 2b, 2c) in CHCl3 (10-4 M).
Figure 3. The diffuse reflectance spectra of bare TiO2 and (OH)2SnPp(2a, 2b, 2c)-TiO2 composites.
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Figure 4. TEM images of bare TiO2 (a), 2a-TiO2 (b), 2b-TiO2 (c), and 2c-TiO2 (d).
Obviously, there is no absorption above 400 nm for bare TiO2, while SnPp(2a, 2b, 2c)-TiO2 exhibits the feature peaks of 2a, 2b, 2c observed in CHCl3 solution (Table 1), indicating that metalloporphyrins successfully loaded onto the TiO2 surface with maintaining the porphyrin framework. Consequently, SnPp · 2OH(2a, 2b, 2c)-TiO2 composite exhibits a broader absorption range for the solar spectrum than bare TiO2. It was noticed that the DR spectra of SnPp(2a, 2b, 2c)-TiO2 have a significant red shift compared with the spectra of 2a, 2b, 2c in CHCl3 solution, implying that there exists an interaction between TiO2 and tin porphyrin molecules involving the axial -OH ligand with the TiO2 surface. 3.2.2. TEM Images. Figure 4 shows typical TEM images of bare TiO2 and 2a, 2b, 2c-TiO2 composites. It can be clearly seen that the surface of 2a, 2b, 2c-TiO2 composites (Figure 4b-d) were covered respectively with a thin film of tin porphyrins uniformly distributed onto the TiO2 surface. 3.2.3. FTIR Spectra. The bonding characteristics of functional groups in bare TiO2 and 2a-, 2b-, 2c-TiO2 composites were identified by FT-IR spectroscopy (Figure 5). The absorption peak at 3425 cm-1 is associated with the stretching vibrations of surface hydroxyl groups, whereas the band around 1632 cm-1 is attributed to the bending vibration of H-OH groups and Ti-OH bond for bare TiO2 and 2b-TiO2, respectively. The stretching vibrations of C-C, C-H bonds can be observed in 2a-, 2b-, 2c-TiO2 composites, indicating that there contains tin porphyrins on the surface of TiO2. Further observation shows that the peaks corresponding to the stretching vibrations of hydroxyl groups are broader and stronger in the bare TiO2 than that of 2b-TiO2, indicating that the decrease of hydroxyl groups after TiO2 is modified by tin porphyrin. In fact, during the preparation of tin porphyrin-TiO2 photocatalyst, part of the hydroxyl groups on the TiO2 surface can react with (OH)2SnPp giving, after dehydratation, covalently bonded species like Ti-O-SnPp(OH). However, the physisorption of species, like Ti-O · · · H · · · O-SnPp(OH), weakly bonded onto the TiO2 surface cannot be excluded.
Figure 5. FTIR spectra of bare TiO2 and (OH)2SnPp(2a, 2b, 2c)-TiO2 composites.
3.2.4. Raman Spectra. Figure 6 represents the Raman spectra of bare TiO2 and 2b-TiO2 composite. In the spectra, absorption bands were observed at 145 cm-1 (Eg), 197 cm-1 (Eg, weak), 395 cm-1 (B1g), 516 cm-1 (A1g), and 638 cm-1 (Eg), which can be attributed to the characteristic peaks of the anatase phase.20 Although the Raman bands of 2b-TiO2 composite are similar with bare TiO2, the Raman peak intensity decreased. These observations support the interpretation that there is a break in isotropic symmetry at the TiO2 surface.21 On the basis of this observation, it can be inferred that tin porphyrin is bonded to the surface of TiO2 through -O-Sn bond by means of two different kinds of adsorption modes, that is, by a simple molecular physisorption mode or by a dissociative chemisorptions mode. 3.2.5. X-ray Photoelectron Spectroscopy. XPS is a highly surface-selective technique, and different forms of surface or
Tin Porphyrin/TiO2 Based Composites
Figure 6. Raman spectra of bare TiO2 and (OH)2SnPp(2b)-TiO2.
Figure 7. XPS spectra of bare TiO2 and (OH)2SnPp(2b)-TiO2.
bulk material could be distinguished. Figure 7 compares the XPS spectra of bare TiO2 and 2b-TiO2 composite in the O(1s), Ti(2p), and Sn(3d) bands. No obvious shifts for the O(1s) and Ti(2p) peaks of TiO2 upon the modification of tin porphyrin (2b) have been observed probably due to the different interactions ascribable at the diverse kinds of adsorption modes of the tin porphyrin on the TiO2 surface. The peaks located at 486.9 eV can be attributed to the Sn(II, IV) (3d5/2) in 2b-TiO2 composite according to the reported
J. Phys. Chem. C, Vol. 114, No. 17, 2010 7861 binding energy of Sn(II, IV) at 486.87 eV,22,23 Sn(II) and Sn(IV) species have a slight difference in the binding energy, but Sn(II) porphyrin is highly air sensitive and unstable in the ambient conditions. 3.3. Photoreactivity Experiments. The photocatalytic activities for the degradation of 4-nitrophenol (4-NP) in water under both visible light and UV-vis light irradiation using prepared photocatalysts were tested (Figure 8). Under visible light conditions, all three composites, (2a)-TiO2, (2b)-TiO2, and (2c)-TiO2, showed higher photoactivity than bare TiO2, and in particular (2c)-TiO2 exhibited the best effect of all samples. Under UV-vis light irradiation, the photocatalytic efficiencies of all samples were largely enhanced compared with those observed under visible light conditions. The bare TiO2 displayed the most significant improvement on photoactivity, and three composites still exhibited higher photoactivity than bare TiO2, in the same decreasing order (2c)-TiO2 > (2b)-TiO2 > (2a)-TiO2. It is worth noting that the distinction of photoactivity between three composites, as well as between three composites and bare TiO2, was evidently reduced. This is due to that in the presence of UV light TiO2 itself is excited producing excitedstate electrons and valence band holes essential to generate powerful oxidant species. Table 2 reports the photodegradation relevant results of three catalysts under visible and UV-vis light irradiation. The result of a typical experiment carried out by using the most active photocatalyst, i.e., SnPp · 2OH (2c), indicated that 98.1% of 4-NP disappeared after 500 min of irradiation under visible light. These results could be explained by different molecular structures and space tropisms of peripheral substituents. It is obvious that the structure of (OH)2SnPp (2a) represents a more stressed and stereochemically complicated molecular structure. The macrocycles were somewhat wrinkled with four substituents almost perpendicular to porphyrin planes to overcome the strong steric interactions, while substituents of (OH)2SnPp (2c) are well stretched out with the least deviation from perfect planarity, making porphyrin molecules easily parallel to the TiO2 surface. Hence, it can be assumed that the molecules of (OH)2SnPp (2c) easily approach the TiO2 surface, which is beneficial to the process of initial electron injection from the porphyrins excited singlet state to the CB of the TiO2, leading to effective photoactivity. (OH)2SnPp (2a) is in the contrary situation, with
Figure 8. 4-NP concentration vs irradiation time using different (OH)2SnPp-TiO2 photocatalysts.
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TABLE 2: The Initial Photoreaction Rates and the Conversion (%) of 4-NP after 250 and 500 min of Irradiation Time
vis UV-vis
a
photocatalysts
109 r0 (mol L-1 s-1)a
109r0′ (mol L-1 s-1 m2)b
4-NP (%) converted, 250 min
4-NP (%) converted, 500 min
(OH)2SnPp(2a)-TiO2 (OH)2SnPp(2b)-TiO2 (OH)2SnPp(2c)-TiO2 (OH)2SnPp(2a)-TiO2 (OH)2SnPp(2b)-TiO2 (OH)2SnPp(2c)-TiO2
1.09 1.79 2.96 7.37 8.18 9.15
6.81 11.19 18.5 46.06 51.13 57.19
38.7 45.9 57.0 83.2 85.5 88.3
89.7 92.7 98.1
r0, The initial photoreaction rates per used mass. b r0′, Initial photoreaction rates per used mass and per unit surface area of the catalysts.
inaccessibility to TiO2 surface impeding the involvement of porphyrins in the photocatalytic process. It is noteworthy that the presence of the axial ligands can effectively prevent intermolecular π-π interactions of porphyrins molecules, especially for the most planar (OH)2SnPp (2c), and consequently the decrease of aggregation, which is considered crucial to ensure higher activity of the composite catalyst. Moreover, according to the diffuse reflectance spectra, the 2c-TiO2 has more absorption compared with 2a-TiO2 and 2b-TiO2, especially in the longer wavelength, which is also responsible for the high efficiency of 2c-TiO2. In the presence of visible light, the process of SnPp-TiO2 photocatalytic reaction can be intrigued by the excitation of the ground state of the sensitizer [SnPp] via one photon transition (hν) to its singlet excited state 1[Pp]*. As the lifetime of a singlet state (nanoseconds) is shorter than that of the triplet state (milliseconds), a process of intersystem crossing generates the sensitizer’s triplet state 3[Pp]* (eq 1). Both of the excited states under visible light irradiation can yield electrons, some of which are transferred into the conduction band of the TiO2 and then can be trapped by adsorbed O2 on the TiO2 surface producing reactive species •O2(eq 2). In addition, the formation of singlet oxygen during the photocatalytic process may not be excluded (eq 3).6 hν
intersystem crossing (isc)
TiO2[SnPp] 98 TiO21[SnPp]* 98 TiO23[SnPp]* (1) O2
TiO21 or 3[SnPp]* f TiO2(e-CB)[SnPp]+ 98 TiO2[SnPp]+ + •O2- (2) TiO2[SnPp]* + 3O2 f TiO2[SnPp] + 1O2(1∆)
(3)
While under UV-vis light irradation, as TiO2 can be readily excited by UV light and participates more actively in the photocatalytic process, the beneficial effect on the photoreactivity is due to a cooperative mechanism. A series of reactions produce reactive intermediates (e.g., •O2-, 1O2 (1∆), •OH, HO2 · ) effective for the photodegradation process. 4. Conclusion The synthesis and the spectroscopic characterization of three chloro-tin and three hyroxo-tin porphyrins have been described and corresponding (OH)2SnPp(2a, 2b, 2c)-TiO2 composites exhibited high photoactivities compared with the bare TiO2 when they were employed for the photodegradation of 4-nitrophenol under visible light irradiation, with the following decreasing photocatalytic activity order: SnPp(2c)-TiO2 > SnPp(2b)-TiO2 > SnPp(2a)-TiO2 . TiO2(bare). It was found that the distinct space tropisms of peripheral substituents in meso sites of porphyrin rings lead to different efficiency in photodegrading
the 4-NP, and the axial group also affects the efficiency of the photodegradation to a certain extent. The photoactivity of (OH)2SnPp(2c)-TiO2 is higher than other metalloporphyrin-TiO2 compounds investigated such as Co(II)Pp-TiO2 and Zn(II)Pp, but still lower than Cu(II)PpTiO2.11,12 Therefore it is the metal ion among various factors which plays the dominant role in the photocatalytic process, as the excited potential of metalloporphyrin should best match with TiO2 conduction band potential. Acknowledgment. We are very grateful for the financial support from National Nature Science Funds (20971103) and The International Cooperation Project of Shaanxi Province (2008KW-33). References and Notes (1) Debabrata, C.; Shimanti, D. J. Photochem. Photobiol., C 2005, 6, 186–205. (2) Zhao, W.; Sun, Y.; Castellano, F. N. J. Am. Chem. Soc. 2008, 130, 12566–12567. (3) Wang, C.; Wang, X.-M.; Xu, B.-Q.; Zhao, J.-C.; Mai, B.-X.; Peng, P.-A.; Sheng, G.-Y.; Fu, J.-M. J. Photochem. Photobiol., A 2004, 168, 47– 52. (4) Wang, C.; Xu, B.-Q.; Wang, X.-M.; Zha, J.-C. J. Solid State Chem. 2005, 178, 3500–3506. (5) Granados-Oliveros, G.; Pa´ez-Mozo, E. A. F.; Ortega, F. M.; Ferronato, C.; Chovelon, J. M. Appl. Catal., B 2009, 89, 448–454. (6) Mele, G.; Sole, R. D.; Vasapollo, G.; Garcı´a-Lo´pez, E.; Palmisano, L.; Schiavello, M. J. Catal. 2003, 217, 334–342. (7) Cho, Y.; Choi, W. EnViron. Sci. Technol. 2001, 35, 966–970. (8) Kamat, P. V.; Fox, M. A. Chem. Phys. Lett. 1983, 102, 379–384. (9) Ross, H.; Bendig, J.; Hecht, S. Sol. Energy Mater. Sol. Cells 1994, 33, 475–481. (10) Kim, W.; Park, J.; Jo, H. J.; Kim, H.-J.; Choi, W. J. Phys. Chem. C 2008, 112, 491–499. (11) Pogue, B. W.; Ortel, B.; Chen, N.; Redmond, R. W.; Hasan, T. Cancer Res. 2001, 61, 717–724. (12) Wang, S.; Tabata, I.; Hisada, K.; Hori, T. Dyes Pigm. 2002, 55, 27–33. (13) Song, X.-Z.; Jia, S.-L.; Miura, M.; Ma, J.-G.; Shelnutt, J. A. J. Photochem. Photobiol., A 1998, 113, 233–241. (14) Arnold, D. P.; Blok, Janet Coord. Chem. ReV. 2004, 248, 299– 319. (15) Wang, C.; Li, J.; Mele, G.; Duan, M.-Y.; Lu¨, X.-F.; Palmisano, L.; Vasapollo, G.; Zhang, F.-X. Dyes Pigm. 2010, 84, 183–189. (16) Wang, C.; Yang, G.-M.; Li, J.; Mele, G.; S1ota, R.; Broda, M. A.; Duan, M.-Y.; Vasapollo, G.; Zhang, X.-F.; Zhang, F.-X. Dyes Pigm. 2009, 80, 321–328. (17) Wang, C.; Li, J.; Mele, G.; Yang, G.-M.; Zhang, F.-X.; Palmisano, L.; Vasapollo, G. Appl.Catal., B 2007, 76, 218–226. (18) Rothemund, P.; Menotti, A. R. J. Am. Chem. Soc. 1948, 70, 1808– 1812. (19) Dorough, G. D.; Miller, J. R.; Huennekens, F. M. J. Am. Chem. Soc. 1951, 73, 4315–4320. (20) Dong, F.; Wang, H.-Q.; Wu, Z.-B. J. Phys. Chem. C 2009, 113, 16717–16723. (21) Bhattacharyya, K.; Varma, S.; Tripathi, A. K.; Bharadwaj, S. R.; Tyagi, A. K. J. Phys. Chem. C 2008, 112, 19102–19112. (22) Lin, A. W. C.; Armstrong, N. R.; Kuwana, T. Anal. Chem. 1977, 49, 1228–1235. (23) Baronetti, G. T.; De Miguel, S. R.; Scelza, O. A.; Castro, A. A. Appl. Catal. 1986, 24, 109–116.
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