Visible photocatalytic activity and photoelectrochemical behavior of TiO2 nanoparticles modified with metal porphyrins containing hydroxyl group

Available online at www.sciencedirect.com

CERAMICS INTERNATIONAL

Ceramics International 40 (2014) 7093–7098 www.elsevier.com/locate/ceramint

Visible photocatalytic activity and photoelectrochemical behavior of TiO2 nanoparticles modified with metal porphyrins containing hydroxyl group Cheng Huanga, Ying Lvb, Qin Zhoua, Shizhao Kanga, Xiangqing Lia, Jin Mua,n a

School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 201418, China b Department of Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China Received 29 November 2013; received in revised form 7 December 2013; accepted 8 December 2013 Available online 17 December 2013

Abstract TiO2 nanoparticles modified with 5-(p-hydroxylphenyl)-10,15,20-triphenylporphyrin (HTPP), 5-(p-hydroxylphenyl)-10,15,20-triphenylporphyrin zinc (ZnHTPP) and trans-dichloro-5-(p-hydroxylphenyl)-10,15,20-triphenylporphyrin tin (SnHTPP) were prepared in order to improve the visible photocatalytic activity of TiO2 nanoparticles. The photocatalytic activity of the modified TiO2 nanoparticles was investigated by carrying out the photodegradation of methyl orange in aqueous solution under visible light irradiation. The TiO2 nanoparticles modified with SnHTPP show the highest visible photocatalytic activity with a degradation ratio of 86% of methyl orange after 180 min irradiation among three catalysts. This result indicates that the central metal ions in porphyrins can significantly influence the sensitization efficiency of porphyrins. In addition, the photoelectrochemical behavior of the modified TiO2 nanoparticles was examined and related to their photocatalytic activity. Finally, the photocatalytic mechanism was discussed preliminarily. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: C. Sensitization; D. Metal porphyrin; D. TiO2; E. Photocatalysis

1. Introduction TiO2 is a widely studied photocatalyst due to its nontoxicity, low cost, chemical stability, and high photocatalytic activity. However, with the band gap in the range of 3.0–3.2 eV, TiO2 nanoparticles can only absorb ultraviolet light which only accounts for about 4% of solar irradiance on the surface of earth. Hence, a lot of efforts have been done to explore how to extend its light response to visible range. And a series of methods have been suggested, such as metal ion doping or anion doping [1], sensitization by a semiconductor with narrow band gap [2] and dye sensitization [3], in which dye sensitization is considered to be one of the most efficient methods [1]. The experimental results reported previously confirm that the visible photocatalytic activity of TiO2 can be effectively improved by choosing an appropriate sensitizer, such as phthalocyanine [4], coumarin [5] or porphyrin [6–8]. n

Corresponding author. Tel./fax: þ 86 21 60873061. E-mail address: [email protected] (J. Mu).

Among these sensitizers, porphyrin derivatives are particularly attractive due to the strong absorption in the region of 400– 450 nm (Soret band) and in the region of 500–700 nm (Qbands), excellent photoelectrochemical property and remarkable chemical stability. It is known that the electron transfer efficiency between sensitizer and semiconductor depends on their bonding mode [9–11]. Generally, carboxyl, sulfonyl and phosphate [12] with affinity are the most used anchoring groups of dye molecules to link up with TiO2. But there are some disadvantages need to be overcome. One of them is that the binding between dye molecule and TiO2 is not strong enough so that slow desorption of dye molecule often occurs in aqueous solution [13], which leads to poor long-term stability. Therefore, it is desirable to make dye molecules strongly link to TiO2. Compared with the groups above, phenol hydroxyl possesses higher affinity, which may tightly link up with TiO2 [14]. The photoelectron injection from dye molecule to semiconductor can be efficient when the phenol hydroxyl is used as an anchoring group [15–17]. Some experimental

0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.12.042

7094

C. Huang et al. / Ceramics International 40 (2014) 7093–7098

results confirmed our speculation that a stable and efficient visible photocatalyst, TiO2 modified porphyrin derivatives through phenol hydroxyl, could be obtained. Lu and coworkers [18] reported that TiO2 nanoparticles modified with CuII porphyrin containing hydroxyl exhibited higher photocatalytic activity than pure TiO2 when they were employed for the degradation of 4-nitrophenol under simulated solar light irradiation. In addition, because SnIV porphyrin is stable against the acid-induced demetallation and the high charge on Sn4 þ ion makes it the most easily ring-reduced among all metalloporphyrins [19], the TiO2 nanoparticles modified with SnIV porphyrin containing hydroxyl should be a high activity photocatalyst. Based on these considerations, two porphyrins containing hydroxyl in circum and different central metals (ZnII, SnIV) are selected to sensitize TiO2 nanoparticles for exploring how the central metal ions influence the sensitization efficiency of porphyrins linked on the surface of TiO2 nanoparticles. The photocatalytic activity is characterized using methyl orange as a model contaminant under visible light irradiation. Furthermore, the photoelectrochemical behavior of the modified TiO2 nanoparticles is also studied to further discuss the corresponding photocatalytic mechanism. 2. Experimental 2.1. Materials TiO2 nanoparticles (P25) were purchased from Degussa AG Company (Germany). HTPP was synthesized according to the procedure reported previously [20] and characterized with UV– vis spectrum (Supplementary Fig. S1) and FT-IR spectrum (Supplementary Fig. S2). From these experimental results, we can conclude that HTPP is successfully synthesized according to the procedure described in Ref. 20 and the purity of HTPP is satisfactory. ZnHTPP and SnHTPP were prepared by the reaction of HTPP with corresponding metal salts. The other chemicals were of analytical grade and used without further purification. Doubly distilled water was used as the solvent. 2.2. Preparation of the TiO2 nanoparticles modified with metal porphyrin The TiO2 nanoparticles modified with metal porphyrin were synthesized as follows: porphyrin (1 mg) was dissolved in 15 mL of CH2Cl2 and then added 100 mg TiO2 nanoparticles to the solution. The resulting suspension was stirred and refluxed at 40 1C for 8 h. In this way, the porphyrin was linked on the surface of TiO2 nanoparticles due to the interaction between the phenol hydroxyls from porphyrin and the hydroxyls on the surface of the TiO2 nanoparticles. The samples obtained were marked as TiO2/HTPP, TiO2/ZnHTPP and TiO2/SnHTPP. 2.3. Characterization Ultraviolet–visible (UV–vis) absorption spectra of solutions were recorded on a Unico UV-2102 UV–vis spectrophotometer

(China). Solid diffuse reflectance UV–vis spectra were measured with a Varian Cary-500 UV–vis NIR spectrophotometer (USA). Fluorescence spectra were taken on a Shimadzu RF5301PC spectrophotometer (Japan). Infrared spectrum was conducted on a Nicolet Magna-IR550 FT-IR spectrometer (USA). I–V characteristic curves of cell were recorded on a Keithly Source Meter 2400 (USA). 2.4. Photocatalytic activity measurements The photocatalytic experiments were performed in a homemade apparatus. A 1000 W iodine–tungsten lamp was used as visible radiation source. The distance between the lamp and the solution was 16 cm. A 400 nm filter was placed between the lamp and the reaction device for cut-off ultraviolet light and then 80.0 mg photocatalyst was added into the reactor containing 80 mL methyl orange aqueous solution (10 mg L  1, pH ¼ 6). The suspension was irradiated after stirred in dark for 30 min. The residual concentration of methyl orange was monitored by measuring its absorbance at 464 nm. The degradation efficiency was calculated according to the following equation: A0  A Degradationð%Þ ¼  100% A0 where A0 is the absorbance of methyl orange solution at 464 nm after the adsorption–desorption equilibrium, A is the absorbance of methyl orange solution after irradiation. 2.5. Photoelectrochemical behavior measurement The photoelectrochemical cells were assembled in a typical sandwich-type cell. The FTO electrodes coated with the TiO2 nanoparticles which were modified with porphyrin (referred to FTO/TiO2/HTPP, FTO/TiO2/ZnHTPP and FTO/TiO2/SnHTPP, respectively) were acted as working electrodes. A FTO/Pt electrode was served as the counter electrode. All the photoelectrochemical experiments were carried out in a mixture of acetonitrile and 3-methoxypropionitrile (volume ratio 7:3) contained 0.5 mol L  1 LiI and 0.05 mol L  1 I2, which was injected into the interspace between the working electrode and the counter electrode. A light-shading mask was used on the cells and the active area was fixed to 0.15 cm2. A 1000 W xenon lamp with a 370 nm cut-off filter was used as the excitation light source. All the experiments were carried out at ambient temperature and pressure. 3. Results and discussion 3.1. Solid diffuse reflectance UV–vis spectra of the TiO2 nanoparticles modified with metal porphyrins The solid diffuse reflectance UV–vis spectra of the samples are shown in Fig. 1. The corresponding UV–vis absorption bands of porphyrin solutions are listed in Table 1. The pure TiO2 nanoparticles possess an optical threshold at 413 nm, which should be ascribed to a charge-transfer process from

C. Huang et al. / Ceramics International 40 (2014) 7093–7098

the valence band to the conduct band of TiO2 nanoparticles. In the case of the TiO2 nanoparticles linked with HTPP (Fig. 1b), besides the absorption band around 368 nm, we can observe two strong absorption bands at 413 nm and in the range of 500– 650 nm, respectively. The band at 426 nm is assigned to the Soret band of HTPP arising from the transition of a1u (π)–eng (π), and the band in the range of 500–650 nm is attributed to the 1.5

Absorbance

b

1.0

d c

0.5

a

0.0 300

400

500

600

700

800

Wavelength (nm) Fig. 1. Solid diffuse reflectance UV–vis spectra of the pure TiO2 nanoparticles (a), the TiO2 nanoparticles linked with HTPP (b), the TiO2 nanoparticles linked with SnHTPP (c) and the TiO2 nanoparticles linked with ZnHTPP (d).

Table 1 UV–vis absorption bands of porphyrin solutions. Soret band (nm)

Q bands (nm)

HTPP ZnHTPP SnHTPP

417 420 421

515, 551, 590, 648 550, 591 517, 561, 604

PL Intensity (a.u.)

Porphyrin

7095

Q bands of HTPP corresponding to the a2u (π)–eng (π) transition [21,22]. Similarly, the Soret bands and the Q bands of SnHTPP (Fig. 1c) and ZnHTPP (Fig. 1d) can also be found. These phenomena indicate that the porphyrin was successfully linked on the surface of TiO2 nanoparticles. It is interested that we can observe four Q bands again from the UV–vis spectra of the TiO2/SnHTPP (Fig. 1c) and TiO2/ZnHTPP (Fig. 1d), respectively, although the intensities of the first Q band and the fourth Q band decreased obviously in comparison with those of free porphyrins. In contrast, both SnHTPP solution and the ZnHTPP solution only exhibit three or two Q bands in the UV–vis spectra. One possible explanation is that there may exist some interaction between the central metal ions of porphyrins and the hydroxyls on the surface of the TiO2 nanoparticles, which lowers the symmetry of metal porphyrin. As a result, the number of Q band increases. Moreover, compared with the UV–vis absorption bands of porphyrin solutions, the Soret band blue shifts 4 nm for HTPP linked on the TiO2 nanoparticles while the Soret band red shifts 6 nm for SnHTPP linked on the TiO2 nanoparticles. However, in the case of ZnHTPP linked on the TiO2 nanoparticles, we can hardly observe the shift of the Soret band. These phenomena imply that the introduction of metal ions can obviously influence the electron transfer between porphyrin molecules and TiO2 nanoparticles. 3.2. Fluorescence spectra of the TiO2 modified with porphyrins Fig. 2 shows the fluorescence spectra of porphyrins and the TiO2 nanoparticles modified with porphyrins. As can be seen from Fig. 2a, when the pure SnHTPP molecules are excited at 549 nm, there exist two typical emission bands at 652 nm and 717 nm. In contrast, these emission peaks are quenched entirely after the SnHTPP molecules are linked on the surface of TiO2 nanoparticles. Similar phenomena can be observed in

800

800

800

600

600

600

SnHTPP 400

400

400

ZnHTPP HTPP SnHTPP-TiO2 200

200

200

HTPP-TiO2

ZnHTPP-TiO2

0 600

0

0 650

700

750

800

600

650

700

750

800

600

650

700

750

Wavelength (nm) Fig. 2. Fluorescence spectra of porphyrins and the TiO2 nanoparticles linked with porphyrins (λex ¼ 549 nm).

800

7096

C. Huang et al. / Ceramics International 40 (2014) 7093–7098

100 90

Degradation （ % ）

2

70 60

3

50 40 30 20 10

4

0

20

40

60

80 100 120 140 160 180 t (min)

Fig. 3. Kinetic curves of photocatalytic degradation of methyl orange in the presence of the TiO2 nanoparticles linked with SnHTPP (1), the TiO2 nanoparticles linked with HTPP (2), the TiO2 nanoparticles linked with ZnHTPP (3), and the pure TiO2 nanoparticles (4).

the cases of the TiO2 nanoparticles linked with ZnHTPP (Fig. 2b) and the TiO2 nanoparticles linked with HTPP (Fig. 2c). From these results, it can be deduced that the photo-produced electrons of the excited porphyrin molecules may transfer effectively to the conduction band of the TiO2 nanoparticles, which implies that these porphyrins may be efficient sensitizers for the TiO2 nanoparticles. 3.3. Photocatalytic activity under visible-light irradiation Fig. 3 shows the relationship of irradiation time and degradation ratio of methyl orange catalyzed by the pure TiO2 nanoparticles, the TiO2 nanoparticles modified with HTPP, the TiO2 nanoparticles modified with SnHTPP and the TiO2 nanoparticles modified with ZnHTPP. From Fig. 3, it can be found that the pure TiO2 nanoparticles hardly exhibit photocatalytic activity under visible irradiation. After irradiated for 180 min, the photodegradation ratio of methyl orange in the presence of the pure TiO2 nanoparticles is only 7.5%. In contrast, when the porphyrin molecules are linked on the surface of TiO2 nanoparticles, the photocatalytic activity of TiO2 nanoparticles is enhanced significantly. Therein, SnHTPP is the most efficient sensitizers for the TiO2 nanoparticles. After irradiated for 180 min, the photodegradation ratio of methyl orange is up to 86%. Compared with the pure TiO2 nanoparticles, the visible photocatalytic activity of TiO2 nanoparticles is enhanced 10 times. Moreover, it can also be observed that the sensitization efficiency of porphyrin is significantly influenced by the metal ions coordinated with porphyrin. The sensitization efficiency of SnHTPP is higher in comparison with that of HTPP while ZnHTPP exhibits relatively lower sensitization efficiency. One possible explanation is that porphyrin SnIV is usually six-coordinated with trans-diaxial ligands. Thus, there may exists some interaction between Sn4 þ ions and the hydroxyls of TiO2 nanoparticles

Degradation(%)

1

80

0

100 80 60 40 20 0 100 80 60 40 20 0 100 80 60 40 20 0

1 2

0

20

40

60

80

100

120

140

160

180

80

100

120

140

160

180

80

100

120

140

160

180

1 2 0

20

40

60

1 2 0

20

40

60

t (min)

Fig. 4. Kinetic curves of photocatalytic degradation of methyl orange in the presence of the TiO2 nanoparticles linked with ZnHTPP (a), the TiO2 nanoparticles linked with HTPP (b), and the TiO2 nanoparticles linked with SnHTPP (c) in air (1) and in N2 (2).

besides the interaction between the phenol hydroxyls of SnHTPP and the hydroxyls of TiO2 nanoparticles, which is favorable to the electron transfer between the porphyrin molecules and the TiO2 nanoparticles. In contrast, there exists no interaction between Zn2 þ ions and the hydroxyls of TiO2 nanoparticles. Moreover, the charge on Zn2 þ may be unfavorable to the electron transfer between the porphyrin molecules and the TiO2 nanoparticles. Therefore, the sensitization efficiency of SnHTPP is the highest while the sensitization efficiency of ZnHTPP is lowest among three porphyrins above. Fig. 4 shows the kinetic curves of photocatalytic degradation of methyl orange under visible-light in various atmospheres. It can be found that when the TiO2 nanoparticles linked with porphyrin are used as a photocatalyst, the reaction atmosphere plays an important role in the photocatalytic process. The degradation rate of methyl orange in N2 atmosphere is obviously slower than that in air. These phenomena indicate that methyl orange is degraded mainly by the reactive species from the reaction of the photogenerated electrons and the adsorbed O2, such as O2  ,  OH, and 1O2. On the basis of the experimental results above, the mechanism for the photocatalytic degradation of methyl orange is suggested as follows. When the TiO2 nanoparticles modified with porphyrin are irradiated by visible light, the SnHTPP molecules can be excited. The excited electrons from SnHTPP transfer to the conduction band of TiO2, and the electrons can react with the adsorbed O2 on the surface of TiO2 to produce the reactive species, such as O2  and  OH, which can degrade methyl orange. 3.4. Photoelectrochemical performance of the products In order to understand deeply the sensitization of porphyrin and the effect of the metal ions coordinated with porphyrin, the photoelectrochemical behavior of the TiO2 nanoparticles linked with porphyrin was explored (Fig. 5). As can be seen from Fig. 5, there exists a close relationship between the photocatalytic activity of the TiO2 nanoparticles modified with

C. Huang et al. / Ceramics International 40 (2014) 7093–7098

Commission (No. 13ZZ135) and the National Natural Science Foundation of China (No. 21301118).

0.7

Photocurrent (mA·cm-2)

0.6

1

Appendix A. Supporting information

0.5 0.4 0.3

7097

2

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ceramint. 2013.12.042.

3

0.2

References

0.1 0.0 0.0

0.1

0.2 0.3 Voltage(V)

0.4

0.5

Fig. 5. I–V curves of the TiO2 nanoparticles linked with SnHTPP (1), the TiO2 nanoparticles linked with HTPP (2) and the TiO2 nanoparticles linked with ZnHTPP (3).

porphyrin and their photoelectrochemical behavior. Likewise, the short-circuit currents of the three kinds of photocatalysts are in the order of the TiO2 nanoparticles linked with SnHTPP4the TiO2 nanoparticles linked with HTPP4the TiO2 nanoparticles linked with ZnHTPP. The bigger short-circuit currents may imply that the electron transfer from the TiO2 nanoparticles modified with porphyrin to the surface of FTO is more efficient. Therefore, we can deduce that the photogenerated electron injection from SnHTPP to the conduction band of TiO2 is the most efficient among three photocatalysts, which may be a main reason for the highest photocatalytic activity. The effect of the metal ions coordinated with porphyrin on the sensitization of porphyrin may originate from the difference in the photogenerated electron injection from the excited porphyrin molecules to the conduction band of TiO2 induced by the metal ions.

4. Conclusions The TiO2 nanoparticles modified with porphyrin can be prepared in the process described above. The TiO2 nanoparticles linked with SnHTPP is an efficient visible photocatalyst for the degradation of organic dye. The HTPP is an efficient sensitizer for TiO2. The metal ions coordinated with porphyrin strongly influence the sensitization of porphyrin. This effect may be ascribed to the difference in the photogenerated electron injection from the excited porphyrin molecules to the conduction band of TiO2 induced by the metal ions. Moreover, there exists a close relationship between the photocatalytic activity of photocatalyst and its photoelectrochemical behavior, which may give us an efficient tool to explore the electron transfer in the photocatalytic system.

Acknowledgments This work was financially supported by the key project of Science and Technology Innovation of Shanghai Education

[1] M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production, Renew. Sustain. Energy Rev. 11 (2007) 401–425. [2] M. Hodos, E. Horvath, H. Haspel, A. Kukovecz, Z. Konya, I. Kiricsi, Photosensitization of ion-exchangeable titanate nanotubes by CdS nanoparticles, Chem. Phys. Lett. 399 (2004) 512–515. [3] J. Zhang, X.Q. Li, S.Z. Kang, J. Mu, Visible light photocatalytic activity of porphyrin tin (IV) sensitized TiO2 nanoparticles for the degradation of 4-nitrophenol and methyl orange, J. Dispersion Sci. Technol. 32 (2011) 943–947. [4] Z. Guo, B. Chen, J. Mu, M. Zhang, P. Zhang, Z. Zhang, J. Wang, X. Zhang, Y. Sun, C. Shao, Y. Liu, Iron phthalocyanine/TiO2 nanofiber heterostructures with enhanced visible photocatalytic activity assisted with H2O2, J. Hazard. Mater. 219-220 (2012) 156–163. [5] S. Palmas, A.D. Pozzo, M. Mascia, A. Vacca, P.C. Ricci, Sensitization of TiO2 nanostructures with Coumarin 343, Chem. Eng. J. 211 and 212 (2012) 285–292. [6] S. Murphy, C. Saurel, A. Morrissey, J. Tobin, M. Oelgemoller, K. Nolan, Photocatalytic activity of a porphyrin/TiO2 composite in the degradation of pharmaceuticals, Appl. Catal. B 119 and 120 (2012) 156–165. [7] M. Duan, J. Li, G. Mele, C. Wang, X. Lu, G. Vasapollo, F. Zhang, Photocatalytic activity of novel tin porphyrin/TiO2 based composites, J. Phys. Chem. C 114 (2010) 7857–7862. [8] M.-Y. Chang, Y.-H. Hsieh, T.-C. Cheng, K.-S. Yao, M.-C. Wei, C.-Y. Chang, Photocatalytic degradation of 2,4-dichlorophenol wastewater using porphyrin/TiO2 complexes activated by visible light, Thin Solid Films 517 (2009) 3888–3891. [9] G. Benko, M. Hilgendorff, A.P. Yartsev, V. Sundstrom, Electron injection and recombination in fluorescein 27-sensitized TiO2 thin films, J. Phys. Chem. B 105 (2001) 967–974. [10] Y.-X. Weng, Y.-Q. Wang, J.B. Asbury, H.N. Ghosh, T. Lian, Back electron transfer from TiO2 nanoparticles to FeIII(CN)36  : origin of nonsingle-exponential and particle size independent dynamics, J. Phys. Chem. B 104 (2000) 93–104. [11] Y. Tachibana, J.E. Moser, M. Gratzel, D.R. Klug, J.R. Durrant, Subpicosecond interfacial charge separation in dye-sensitized nanocrystalline titanium dioxide films, J. Phys. Chem. 100 (1996) 20056–20062. [12] W.M. Campbell, A.K. Burrell, D.L. Officer, K.W. Jolley, Porphyrins as light harvesters in the dye-sensitised TiO2 solar cell, Coord. Chem. Rev. 248 (2004) 1363–1379. [13] M.K. Nazeeruddin, S.M. Zakeeruddin, R. Humphry-Baker, M. Jirousek, P. Liska, N. Vlachopoulos, V. Shklover, C.-H. Fischer, M. Gratzel, Acid– base equilibria of (2,20 -bipyridyl-4,40 -dicarboxylic acid) ruthenium(II) complexes and the effect of protonation on charge-transfer sensitization of nanocrystalline titania, Inorg. Chem. 38 (1999) 6298–6305. [14] H.N. Ghosh, Effect of strong coupling on interfacial electron transfer dynamics in dye-sensitized TiO2 semiconductor nanoparticles, J. Chem. Sci. 119 (2007) 205–215. [15] L.G.C. Rego, V.S. Batista, Quantum dynamics simulations of interfacial electron transfer in sensitized TiO2 semiconductors, J. Am. Chem. Soc. 125 (2003) 7989–7997. [16] G. Ramakrishna, H.N. Ghosh, A.K. Singh, D.K. Palit, J.P. Mittal, Dynamics of back-electron transfer processes of strongly coupled triphenyl methane dyes adsorbed on TiO2 nanoparticle surface as studied

7098

C. Huang et al. / Ceramics International 40 (2014) 7093–7098

by fast and ultrafast visible spectroscopy, J. Phys. Chem. B 105 (2001) 12786–12796. [17] G. Ramakrishna, A.K. Singh, D.K. Palit, H.N. Ghosh, Slow back electron transfer in surface-modified TiO2 nanoparticles sensitized by alizarin, J. Phys. Chem. B 108 (2004) 1701–1707. [18] X. Lu, W. Sun, J. Li, W. Xu, F. Zhang, Spectroscopic investigations on the simulated solar light induced photodegradation of 4-nitrophenol by using three novel copper(II) porphyrin–TiO2 photocatalysts, Spectrochim. Acta Part A 111 (2013) 161–168. [19] D.P. Arnold, J. Blok, The coordination chemistry of tin porphyrin complexes, Coord. Chem. Rev. 248 (2004) 299–319.

[20] H. Guo, J. Jiang, Y. Shi, Y. Wang, J. Liu, S. Dong, UV–vis spectrophotometric titrations and vibrational spectroscopic characterization of meso-(p-hydroxyphenyl)porphyrins, J. Phys. Chem. B. 108 (2004) 10185–10191. [21] W. Zheng, N. Shan, L. Yu, X. Wang, UV–visible, fluorescence and EPR properties of porphyrins and metalloporphyrins, Dyes Pigments 77 (2008) 153–157. [22] D.F. Marsh, L.M. Mink, Microscale synthesis and electronic absorption spectroscopy of tetraphenylporphyrin H2(TPP) and metalloporphyrins ZnII(TPP) and NiII(TPP), J. Chem. Educ. 73 (1996) 1188–1190.