Photocatalytic degradation of methylene blue under visible light irradiation by non-peripherally tetra substituted phthalocyanine-TiO2 nanocomposites

Photocatalytic degradation of methylene blue under visible light irradiation by non-peripherally tetra substituted phthalocyanine-TiO2 nanocomposites

Accepted Manuscript Research paper Photocatalytic degradation of methylene blue under visible light irradiation by non-peripherally tetra substituted ...

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Accepted Manuscript Research paper Photocatalytic degradation of methylene blue under visible light irradiation by non-peripherally tetra substituted phthalocyanine-TiO2 nanocomposites Semih Gorduk, Oguzhan Avciata, Ulvi Avciata PII: DOI: Reference:

S0020-1693(17)31317-8 https://doi.org/10.1016/j.ica.2017.11.004 ICA 17974

To appear in:

Inorganica Chimica Acta

Received Date: Revised Date: Accepted Date:

19 August 2017 1 November 2017 3 November 2017

Please cite this article as: S. Gorduk, O. Avciata, U. Avciata, Photocatalytic degradation of methylene blue under visible light irradiation by non-peripherally tetra substituted phthalocyanine-TiO2 nanocomposites, Inorganica Chimica Acta (2017), doi: https://doi.org/10.1016/j.ica.2017.11.004

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Photocatalytic degradation of methylene blue under visible light irradiation by non-peripherally tetra substituted phthalocyanineTiO2 nanocomposites Semih Gorduk1,*, Oguzhan Avciata2, Ulvi Avciata1,*

1

Yildiz Technical University, Faculty of Arts and Science, Department of Chemistry, Esenler34210, Istanbul 2

Yildiz Technical University, Faculty of Chemistry- Metallurgical, Department of Metallurgical and Materials Engineering, Esenler-34210, Istanbul, Turkey

*

Corresponding Authors:

Semih Gorduk; E-mail: [email protected] TEL: 0212 383 42 19 Ulvi Avciata; E-mail: [email protected] TEL: 0 212 383 41 31

1

Abstract In this study, novel 2-hydroxymethyl-1,4-benzodioxane substituted phthalonitrile ligand (1) and its non-peripherally tetra-substituted phthalocyanine derivatives with metal-free (2), zinc(II) (3), cobalt(II) (4), nickel(II) (5) and copper(II) (6) were synthesized. These compounds were characterized by several techniques including elemental analysis, FT-IR, UV–vis, 1H-NMR,

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C-NMR, GC-MS and MALDI-TOF MS. The TiO2 nanocomposites of

these phthalocyanines (Pcs) were synthesized via the hydrothermal method in the presence of titanium(IV) isopropoxide. These nanocomposites were also characterized by FT-IR, XRD, FEG-SEM, EDX, UV-DRS and BET techniques. By monitoring the UV-vis absorption spectra, we found all the nanocomposites to have significant photocatalytic activities on methylene blue (MB), a common organic pollutant, under visible light irradiation (250 W). All the phthalocyanine-TiO2 nanocomposites have similar activities within the range of 100140 minutes for full degradation (100%). Furthermore, the reusability studies showed the maintaining of more than 76% of the activity even after 5 recycles.

Keywords: Phthalocyanine;

TiO2;

photocatalytic

activity;

2

nanocomposite;

degradation

1

Introduction

The industrial use of hazardous organic dyes has increased by bringing environmental issues such as pollution, toxicity and carcinogenic effect all over the world[1]. This leads to increasing of scientific efforts to reduce the risks of these pollutants. Some of promising approaches are adsorption[2], chemical precipitation[3], sedimentation[4], ion-exchange[5], electrochemical[6] and photocatalytic[7] methods. Among these approaches, photocatalysis with its simplicity and eco-safety has the most scientific attention in aqueous media[8]. Particularly, metal oxide nanoparticles have been widely used in the photocatalytic degradation of these organic pollutant dyes[9]. TiO2 with its high photosensitivity, nontoxicity, stability, low cost, and abundance has great advantages[10]. However, TiO2 has not a sufficient photocatalytic efficiency under visible light due to its large valenceconductive band (VB-CB) gap[11]. In order to reduce the band gap and increase the solar light absorption, numerous attempts have been made such as noble metal deposition, doping with metal or non-metals, ion implantation, and dye sensitization[12-16]. Recently, the organometallic phthalocyanine (Pc) dyes have been successively used for the activation of TiO2 under the visible light. In this context, new Pc-TiO2 nanocomposites are still desirable for diversity and improvement of photosensitization[17-19]. Pcs have numerous opportunities owing to the structural modification ability, extensively delocalized 18 π-electronic structure, chemical and thermal stability, availability for introducing different peripheral/non-peripheral substituents and exchange simplicity of central ions[20-22]. Pcs are very stable at visible light region of 400-800 nm, have high molar absorption coefficient in this region and show catalytic properties[23-25]. These features make Pcs as perfect candidates for Pc-TiO2 nanocomposites which cause photocatalytic degradation of organic pollutants in wastewater.

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Methylene blue (MB) is one of the most studied organic pollutants due to its solubility in water and its common use as the coloring agent in paper, printing and textile industries. It is also present in wastewater from other industries. The over exposure to methylene blue dye may cause some harmful effects such as increased heart rate, jaundice, shock, vomiting, and tissue necrosis in humans[26, 27]. Herein, we synthesized and characterized five non-peripherally 2-hydroxymethyl-1,4benzodioxane tetra-substituted phthalocyanines and their TiO2 nanocomposites. The nanocomposites have been investigated for their photocatalytic activities against methylene blue under visible light irradiation. 2 2.1

Experimental Part Materials and Equipment

All chemicals and solvents were purchased with high purity from commercial suppliers. All solvents were dried and stored over molecular sieves. In the synthesis and characterization of Pc compounds, the progress of the reactions were monitored by TLC. Melting points of the substances were determined using an electrothermal Gallenkamp device. The IR spectra were recorded using a Perkin Elmer spectrophotometer with ATR sampling accessory. The UV-Vis spectra were recorded on an Agilent 8453 UV/Vis spectrophotometer. Elemental analysis was carried out by a LECO CHNS 932 instrument. A Varian Unity Inova 500 MHz spectrometer was used for 1H-NMR and

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C-NMR spectra. MALDI-TOF mass spectra were performed

using a Bruker Microflex LT MALDI-TOF-MS. The GC–MS spectrum was obtained on an Agilent 6890N GC-System-5973 IMSD. The Synthesis of pure TiO2 and PhthalocyanineTiO2 nanocomposites were carried out by a high pressure and high temperature reactor (Berghof Pressure Digestion DAB- 3, Germany). In the characterization of TiO2 nanocomposites, following techniques were used. The XRD patterns were recorded on a 4

Rigaku miniflex X-ray diffractometer, using Cu-Kα radiation (1.54 Å, 40 kV, 55 mA). The morphologies of the samples were analyzed by the field-emission gun scanning electron microscope (FEG-SEM) on Philips XL 30 SFEG. Energy dispersive X-ray (EDX) spectroscopy attached to SEM was used to analyze the composition of samples. The UV–vis diffuse reflectance spectroscopy (UV-DRS) was used by Shimadzu UV-3600 UV–vis NIR spectrophotometer. For Brunauer–Emmett–Teller (BET) surface areas, nitrogen adsorptiondesorption analysis were carried out at 77 K using a NOVA3000 series instrument (Quantachrome Instruments). The samples were out-gassed under vacuum at 473 K for 3 h before the adsorption of nitrogen. 2.2

Synthesis of compounds

3-((2,3-dihydrobenzo[b][1,4]dioxin-2-yl)methoxy)phthalonitrile (1) The 2-hydroxymethyl-1,4-benzodioxane (0.52 g, 2.89 mmol) was dissolved in N,N′dimethylformamide (DMF) (30 ml) under nitrogen atmosphere, and 3-nitrophthalonitrile (0.5 g, 2.89 mmol) was added to the solution. After stirring for 25 min at room temperature, finely grinded anhydrous potassium carbonate (K2CO3) (1.96 g, 14.45 mmol) was added to this mixture in small portions for 2 h with efficient stirring. The reaction mixture was then stirred in nitrogen atmosphere for a total of 24 h at 45 °C. Following that, it was cooled to room temperature and poured into 250 ml ice-water. The precipitate formed was filtered off and washed by successive with water, n-hexane and diethyether before drying. The creamy crude product was purified by recrystallization in methanol (MeOH). Finally, the pure powder obtained was dried in vacuum. This compound is soluble in ethanol (EtOH), MeOH, acetone, dichloromethane (DCM) and chloroform (CHCI3 ). Yield: 87% (734 mg). Mp: 136-137 °C. Anal. calc. for C17H12N2O3: C, 69.86; H, 4.14; N, 9.58. Found: C, 69.52; H, 4.01; N, 9.32%. FT-IR (ATR, cm−1): 3086.68 (Ar–CH); 2976.41, 2935.69, 2880.29 (Aliphatic-CH, CH2); 2230.94 (C≡N); 1578.41, 1495.09, 1471.57 (C═C), 1253.67, 1242.82 (C–O–C), 1054.34; 5

847.13, 755.61. 1H-NMR (Acetone-d6), (δ: ppm): 7.81–7.78 (t, 1H, Ar-H), 7.64–7.63 (d, 1H, Ar-H), 7.53–7.52 (d, 1H, Ar-H), 6.79–6.73 (m, 4H, Ar-H), 4.62-4.51 (s, 1H, Aliphatic-CH), 4.42-4.18 (m, 4H, Aliphatic-CH2).

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C-NMR (CDCl3), (δ: ppm): 160.61, 142.99, 142.35,

132.67, 125.12, 122.12, 121.99, 117.54, 117.43, 117.30, 117.00, 115.16 (C≡N), 112.69 (C≡N), 105.50, 70.77, 67.55, 64.52. MS (GC–MS) m/z, Calc.: 292.08, Found: 292 [M]+. Non-peripherally

tetra-2-hydroxymethyl-1,4-benzodioxane

substituted

phthalocyanine

derivatives (2–6) A general procedure was followed for the synthesis of non-peripherally phthalocyanine derivatives (2–6): The phthalonitrile compound (1) (200 mg. 0.69 mmol), n-pentanol (4 mL) and 1,8-diazabicyclo[4.5.0]undec-7-ene (DBU) (5 drops) were mixed for the synthesis of 2. For the synthesis of 3-6, equivalent (0.17 mmol) amounts of corresponding anhydrous metal acetate salts were also added into the mixtures (31.6 mg Zn(CH3COO)2 for 3; 29.4 mg Co(CH3COO)2 for 4; 30.4 mg Ni(CH3COO)2 for 5; and 31.3 mg Cu(CH3COO)2 for 6). The mixtures were then heated to 160 °C and stirred for 24 h at this temperature under N2 atmosphere. After cooling at ambient conditions, the reaction mixtures were precipitated by the addition of n-hexane and filtered off. by washing with hot MeOH and EtOH, the products were purified via column chromatography using silica gel and tetrahydrofuran (THF)/MeOH solvent system. All the phthalocyanine compounds (2-6) were soluble in THF, CHCI3, dichloromethane, DMF and dimethyl sulfoxide (DMSO) solvents. Metal-free phthalocyanine (2) The solvent system for column chromatography was THF: MeOH (100:4). Yield: 30% (60.6 mg), Mp: > 200 °C, Anal. calc. for C68H50N8O12: C, 69.74; H, 4.30; N, 9.57; Found: C, 69.41; H, 4.15; N, 9.25%. FT-IR (ATR, cm−1): 3289.84 (single bond N-H), 3042.38 (Ar-CH), 2919.83, 2870.98 (Aliphatic-CH, CH2), 1591.11, 1523.07, 1481.05 (C═C), 1262.59, 1246.08 6

(C-O-C), 1063.98, 841.95, 742.90. 1H-NMR (CDCI3), (δ: ppm): 7.73-7.23 (bm, 6H, Ar-H), 7.19-7.04 (bm, 6H, Ar-H), 6.91-6.49 (bm, 16H, Ar-H), 4.94 (bs, 4H, Aliphatic-CH), 4.66-4.21 (bm, 16H, Aliphatic-CH2). UV–vis (THF, 1×10 −5 M): λmax/nm (log ε): 722 (5.00), 689 (4.98), 656 (4.60), 625 (4.48), 316 (4.79). MS (MALDI-TOF), (m/z): Calc.: 1171.17, Found: 1171.18 [M]+. Zinc(II) phthalocyanine (3) The solvent system for column chromatography was THF: MeOH (100:3). Yield: 39% (83.0 mg), Mp: > 200 °C, Anal. calc. for C68H48ZnN8 O12: C, 66.16; H, 3.92; N, 9.08; Found: C, 65.85; H, 3.61; N, 8.83%. FT-IR (ATR, cm−1): 3041.35 (Ar-CH), 2923.20, 2870.36 (Aliphatic-CH, CH2), 1588.72, 1489.35, 1465.60 (C═C), 1262.26, 1244.34(C-O-C), 1043.18, 841.07, 742.12. 1H-NMR (CDCI3), (δ: ppm): 7.72-7.50 (bm, 6H, Ar-H), 7.38-7.29 (bm, 6H, Ar-H), 7.09-6.43 (bm, 16H, Ar-H), 4.93 (bs, 4H, Aliphatic-CH, CH2), 4.49-4.18 (bm, 16H, Aliphatic-CH2). UV–vis (THF, 1×10−5 M): λmax/nm (log ε): 695 (5.01), 626 (4.29), 319 (4.35). MS (MALDI-TOF), (m/z): Calc.: 1234.54, Found: 1234.14 [M]+. Cobalt(II) phthalocyanine (4) The solvent system for column chromatography was THF: MeOH (100:3). Yield: 35% (73.5 mg), Mp: > 200 °C, Anal. calc. for C68H48CoN8O12: C, 66.50; H, 3.94; N, 9.12; Found: C, 66.19; H, 3.51; N, 8.89%. FT-IR (ATR, cm−1): 3042.00 (Ar-CH), 2927.06, 2872.91 (Aliphatic-CH, CH2), 1591.81, 1516.84, 1489.90 (C═C), 1262.07, 1245.94 (C-O-C), 1043.22, 841.23, 740.83. UV–vis (THF, 1×10−5 M): λmax/nm (log ε): 680 (5.01), 615 (4.41), 312 (4.65). MS (MALDI-TOF), (m/z): Calc.: 1227.27, Found: 1227.79 [M]+, 1245.41 [M+H2O]+. Nickel(II) phthalocyanine (5) The solvent system for column chromatography was THF: MeOH (100:5). Yield: 30% (63.5 mg), Mp: > 200 °C, Anal. calc. for C68H48N8NiO12: C, 66.52; H, 3.94; N, 9.13; Found: C, 7

66.19; H, 3.61; N, 8.81%. FT-IR (ATR, cm−1): 3042.25 (Ar-CH), 2927.98, 2871.74 (Aliphatic-CH, CH2), 1591.98, 1525.26, 1489.41 (C═C), 1260.33, 1244.16(C-O-C), 1040.78, 840.50, 740.17. 1H-NMR (CDCI3), (δ: ppm): 7.75-7.55 (bm, 6H, Ar-H), 7.41-7.38 (bm, 6H, Ar-H), 6.69-6.70 (bm, 16H, Ar-H), 4.96 (bs, 4H, Aliphatic-CH, CH2), 4.59-4.28 (bm, 16H, Aliphatic-CH2). UV–vis (THF, 1×10−5 M): λmax/nm (log ε): 693 (5.00), 624 (4.36), 339 (4.38). MS (MALDI-TOF), (m/z): Calc.: 1227.85, Found: 1227.70 [M]+. Copper(II) phthalocyanine (6) The solvent system for column chromatography was THF: MeOH (100:5). Yield: 31% (65.9 mg), Mp: > 200 °C, Anal. calc. for C68H48CuN8O12: C, 66.26; H, 3.92; N, 9.09; Found: C, 65.97; H, 3.45; N, 8.81%. FT-IR (ATR, cm−1): 3041.51 (Ar-CH), 2929.14, 2873.92 (Aliphatic-CH, CH2), 1590.70, 1488.87, 1464.74 (C═C), 1262.05, 1244.01 (C-O-C), 1042.32, 841.38, 738.91. UV–vis (THF, 1×10−5 M): λmax/nm (log ε): 696 (5.00), 626 (4.25), 318 (4.26). MS (MALDI-TOF), (m/z): Calc.: 1232.70, Found: 1232.79 [M]+. 2.3

Synthesis of nanocomposites

The same procedure was followed for the synthesis of all the TiO2 nanocomposites. The titanium(IV) isopropoxide {Ti[OCH(CH3)2]4} (5 mL) and isopropyl alcohol {2-Propanol} (50 mL) were mixed and stirred for 15 min. Next, 20 mg of aforementioned Pc compound (2-6) dissolved in CHCl3 (0.5 mL) was added into the mixture to form each Pc-TiO2 nanocomposite (e.g. 20 mg of 2 in 0.5 mL CHCl3 to form 2-TiO2). For the pure TiO2, no Pc was added at this step. After, distilled water (2.5 mL) was added dropwise to this mixture. The mixture was then stirred at room temperature for 2 h. Next, it was kept in the hydrothermal reactor for 6 h at 180 °C in order to facilitate nanocomposite formation. After cooling at room temperature, the nanocomposite was washed with water and was centrifuged for precipitation (10.000 ppm;

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20 min). Then, a centrifuge was repeated more after washing with ethanol. The product was finally dried at 80 °C under vacuum for one day. 2.4

Photocatalytic studies

Photocatalytic activity of synthesized nanocomposites was monitored by measuring of the change in the absorption spectra of MB due to the degradation by the photocatalyst under the visible light irradiation. A set of experiments were carried out to compare the effect of PcTiO2 nanocomposites and pure TiO2. To achieve this, we designed a photoreactor with a commercial UV filtering lamp (250 W, Philips brand), a magnetic stirrer, and a ventilator to keep the temperature constant. An initial aqueous solution of 100 mL MB (3 ppm) was prepared and mixed with the 0.1 g of Pc-TiO2 nanocomposite. The mixture was then homogenized in ultrasonic bath for 15 min. Before replacing to the photoreactor, the suspension was then stirred in dark for 1 h to reach the adsorption equilibrium. Photocatalytic study was performed at pH=6-7 and room temperature, under the visible light irradiation (250W) in the photoreactor. At the time intervals of 10 minutes, the small portions from the sample were taken and filtered by a 0.22 µm cellulose acetate membrane to remove the catalyst particles. The absorption spectra of the filtered solutions at 664 nm were measured to determine the degradation and concentration of MB. This process was repeated for all PcTiO2 nanocomposites (2-TiO2; 3-TiO2; 4-TiO2; 5-TiO2; and 6-TiO2) along with pure TiO2 nanoparticles. The reusability test was also performed using the same protocol described above by recycling the used photocatalyst for the next use. 3 3.1

Results and discussion Synthesis and characterization of compounds 1-6

In generally, the substituted phthalonitrile compounds is significant for the synthesis of phthalocyanine derivatives. The synthetic pathway of compounds 1-6 is demonstrated in 9

Scheme 1. The key compound, substituted phthalonitrile (1) was formed by a nucleophilic aromatic nitro displacement reaction of 3-nitrophthalonitrile with 2-hydroxymethyl-1,4benzodioxane in the presence of basic catalyst K2CO3 in dry DMF. The reaction was carried out at 45 °C under N2 atmosphere for 24 h. The structure of substituted phthalonitrile (1) was characterized by elemental analysis, FT-IR, 1 H-NMR,

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C-NMR and GC-MS techniques.

According to the FT-IR spectrum, the peak observed at 2230.83 cm−1 is due to the C≡N stretching vibration and indicates the formation of the compound 1. In the spectrum, the stretching vibrations of aromatic-CH peak at 3086.84 cm−1, aromatic–C═C peaks at 1578.41, 1495.09, 1471.57 cm−1, aliphatic –CH and –CH2 peaks at 2976.41, 2935.69, 2880.29 cm−1, and C–O–C peaks at 1253.67, 1242.82 cm−1 appeared at expected frequencies. In the 1 HNMR spectrum of the compound 1, the signals of aromatic -CH protons appeared in the range of 7.81-7.78, 7.64-7.63, 7.53-7.52 and 6.79-6.73 ppm, integrating into a total of 7 protons. In addition, the signals of the aliphatic CH and CH2 protons were observed at 4.56-4.52 and 4.42-4.09 ppm, integrating into a total of 5 protons. In the 13C-NMR spectrum of compound 1, the signals in the range of 160.61–64.52 ppm indicated the existence of a total of 17 carbon atoms. The nitrile carbon atoms were observed at 115.16 (–C≡N) and 112.69 (–C≡N) ppm. The GC–MS spectrum of displays the [M]+ (parent ion) peak at m/z =292.00, confirming the proposed structure. Non-peripherally

tetra

substituted

phthalocyanines

(2-6)

were

prepared

by

cyclotetramerization of compound 1. In the synthesis of novel metal-free phthalocyanine (2) and its metal complexes (3-6), corresponding anhydrous metal acetate salts (no metal salt for compound 2) were used and the reactions were carried out within 3 mL n-pentanol in the presence of DBU at 160 °C under N2 atmosphere for 24 h. Metal-free phthalocyanine and its metal (Zn, Co, Ni and Cu) complexes substituted with four (2,3-dihydrobenzo[b][1,4]dioxin2-yl)methoxy unit through oxy-bridges showed the enhanced solubility in a number of

10

organic solvents, e.g. THF, chloroform, dichloromethane, DMF and DMSO, etc. as expected. The structures of synthesized phthalocyanine compounds were verified by elemental analysis, UV–vis, FT-IR, 1H-NMR, and MALDI-TOF MS techniques. The FT-IR spectra of the phthalocyanine compounds are very similar to each other since they all possess the same functional groups. The only structural difference is the central metal ion. The proposed target structures of all new phthalocyanines were confirmed by disappearance of the -C≡N vibration at 2230.83 cm−1 in the FT-IR spectra of compound 1. The absence of C≡N vibration confirms cyclotetramerization reaction. For metal-free phthalocyanine (2), the inner core -NH vibration was observed at 3289.84 cm−1, and this situation is the main difference of the metal-free phthalocyanine from all metallophthalocyanines. In the FT-IR spectra of phthalocyanines (26), the stretching vibrations of aromatic –CH peaks at around 3042 cm−1, aliphatic CH and CH2 peaks at around 2929-2870 cm−1, and aromatic –C═C peaks at around 1590-1465 cm−1 appeared at expected frequencies.

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Scheme 1. Schematic representation of phthalonitrile and phthalocyanines

12

The 1H-NMR spectra of phthalocyanines 2, 3 and 5 were taken in CDCl3 at room temperature and these spectra are very similar to each other as a consequence of nearly same proton environments. In the 1H-NMR spectrum of metal-free phthalocyanine (2) in CDCl3, the aromatic protons were resonated between 7.73-7.23, 7.19-7.04 and 6.91-6.49 ppm, a total of 28 protons. The aliphatic –CH and –CH2 protons were observed at 4.94 and 4.66-4.21 ppm, a total of 20 protons respectively. The signals of the inner core protons of the –NH groups were not observed in the spectrum of metal-free phthalocyanine (2). This phenomenon is typical and caused by the strong aggregation between phthalocyanine molecules[28]. In the 1 H-NMR spectra of phthalocyanine 3 and 5 in CDCl3, the aromatic protons of the phthalocyanine core were resonated between 7.72 and 7.29 ppm for 3; and between 7.75 and 7.38 ppm for 5, integrating into 12 protons for each compound. The aromatic protons of substituted group were observed around 7.09-6.43 ppm for 3 and 6.69-6.70 ppm for 5, a total of 16 protons for each compound. For phthalocyanines 3 and 5, the aliphatic –CH protons were observed at around 4.93 and 4.96 ppm, respectively, a total of 4 protons for each compound. In these phthalocyanines; aliphatic -CH2 protons were observed around 4.49-4.18 ppm and 4.59-4.28 ppm, respectively, integrating into 16 protons for each compound. The signals of 1 H-NMR spectra of phthalocyanines (2, 3 and 5) are somewhat broader than corresponding signals in the starting phthalonitrile derivative due to the aggregation presence of phthalocyanine isomers. The 1H-NMR measurements of compounds (4 and 6) could not be performed because of the presence of paramagnetic copper(II) and cobalt(II) ions[29]. In the the MALDI-TOF mass spectra of the phthalocyanines (2-6), the characteristic peaks were observed at m/z= 1171.18 [M]+ for 2; 1234.14 [M]+ for 3; 1227.79 [M]+ and 1245.41 [M+H2O]+ for 4; 1227.70 [M]+ for 5 and 1232.79 [M]+ for 6, confirming the proposed structures (Fig. 1). The elemental analysis results for novel phthalonitrile and phthalocyanine compounds are satisfyingly close to calculated values.

13

Fig. 1. MALDI-TOF MS spectra of phthalocyanine compounds (2-6)

14

UV-vis spectroscopy is one of the most commonly used methods to verify the formation of phthalocyanines. The phthalocyanine compounds generally have two characteristic electronic transitions, a Q-band (600-750 nm in the visible region), assigned to the transitions of π-π* from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) in the phthalocyanine ring and a B band (300-350 nm in the UV region), resulting from the deeper π transitions [30, 31]. Fig. 2 shows the UV-vis absorption spectra of phthalocyanines (2-6) in THF. For the metal-free phthalocyanine (2), a split the Q band is observed at 722 and 689 nm with shoulders at 656 and 625 nm, while the B band appears at 316 nm. The Q band absorptions of metal-free phthalocyanines split to Qx and Qy due to the non-degenerate D2h symmetry by existing H atoms on two of the central N atoms. The Q band for metallophthalocyanines is also observed as a single peak due to the highly symmetrical D4h geometry around the metal center differently from metal-free phthalocyanines[32]. The synthesized metallophthalocyanines (3-6) have single, sharp, intense and well-defined Q band, peaking at 695 nm for 3; 680 nm for 4; 693 nm for 5; and 696 nm for 6. Substitute groups at the non-peripheral position cause red-shifting of the Q band[33]. The shoulders on the Q bands of 3-6 are observed at 626, 615, 624 and 626 nm, respectively. The other indicative absorption bands (the B bands) of 3-6 are observed between 312 and 339 nm.

15

Fig. 2. UV-vis spectra of synthesized phthalocyanine compounds (2-6)

3.2

Characterization of Pc-TiO2 nanocomposites

Fig. 3 shows the XRD patterns of pure TiO2 and Pc-TiO2 nanocomposites, synthesized by hydrothermal method. The pattern in pure TiO2 (Fig. 3a) has peaks observed at the angles of 2θ=25.35°, 2θ=37.86°, 2θ=48.03°, 2θ=53.91°, 2θ=55.06°, 2θ=62.67°, 2θ=68.75°, 2θ=70.25° and 2θ=75.07° corresponding to the planes of (101), (004), (200), (105), (211), (204), (116), (220) and (215), respectively. These values are typical for the anatase phase of TiO2 (JCPDS card no: 21-2172). This shows a successful synthesis of the anatase form of TiO2. In addition, the XRD pattern shows no indication for the formation of rutile phase. Upon the formation of Pc-TiO2 nanocomposites, the XRD patterns are not altered. The patterns in shown Fig. 3b-f clearly indicate that the crystalline phase of TiO2 is preserved without damage. On the other hand, the formation of Pc-TiO2 nanocomposites does not provide any additional characteristic 16

peaks on the XRD patterns. This has been attributed to the low concentration of guest Pc molecules on TiO2 surface in literature[15, 34]. The average crystal sizes of pure TiO2, 2TiO2, 3-TiO2, 4-TiO2, 5-TiO2 and 6-TiO2 were calculated using the Debye-Scherrer equation[35] as 19.77 nm, 18.28 nm, 18.24 nm, 19.66 nm, 19.43 nm and 17.04 nm, respectively. The small variations in sizes of different nanocomposites should be due to the interaction strength between the Pc and TiO2 particles[36].

Fig. 3. XRD patterns of pure TiO2 and Pc-TiO2 nanocomposites

17

In order to too understand to the morphology of the nanocomposite surfaces, the FEG-SEM images were obtained. Fig. 4 shows the surface images of the synthesized pure TiO2 and PcTiO2 nanocomposites. In all of the nanocomposites, the spherical-like TiO2 structures are formed. The spherical-like particles on pure TiO2 surface are uniform whereas all the Pc-TiO2 nanocomposites contain aggregated Pc molecules on the surface. This shows moderate physical adsorption of Pc particles on TiO2 surface. In addition to the SEM images, the EDX analysis has been carried out to determine chemical composition of the synthesized pure TiO2 and Pc-TiO2 nanocomposites (insets in Fig. 4). The EDX image of the pure TiO2 has only peaks of the O and Ti atoms while all the Pc-TiO2 nanocomposites possess additional peaks of C and N atoms of the phthalocyanine rings and the corresponding metal center atom (Zn for 3-TiO2, Co for 4-TiO2, Ni for 5-TiO2 and Cu for 6-TiO2). The FT-IR spectra of pure TiO2 and Pc-TiO2 nanocomposites are presented in Fig. 5. Comparing with pure TiO2, there are some differences in the FT-IR spectra of Pc-TiO2 nanocomposites due to the phthalocyanine components. The spectra of all the nanocomposites include the Ti-O and Ti-O-Ti stretching vibrations observed at 650-500 cm-1. In addition, the peaks around 1631 cm-1 and 3400 cm-1 are due to the Ti-OH and H-OH vibrations of absorbed water molecules respectively. Differing from pure TiO2, the spectra of the Pc-TiO2 nanocomposites show additional peaks around 3040-2950 cm-1 due to the aromatic and aliphatic-CH vibrations, around 1550-1490 cm-1 due to the aromatic C=C vibrations and around 1450 cm-1 due to C-O-C vibrations, which all support the presence of Pc on the TiO2 surface. These peaks were also observed in the FT-IR spectra of all bare Pcs.

18

Fig. 4. FEG-SEM and EDX images of pure TiO2 and Pc-TiO2 nanocomposites

19

Fig. 5. FT-IR spectra of pure TiO2 and Pc-TiO2 nanocomposites

Fig. 6 shows the UV-DRS spectra of pure TiO2 and all the Pc-TiO2 nanocomposites. The spectrum of pure TiO2 exhibits a single absorption band in the UV region and no absorption in the visible region. The Pc-TiO2 nanocomposites, on the other hand, possess the B band in the region of 300-400 nm and the Q band in the region of 600-750 nm, which is typical for Pc skeleton. The energy band gap between the valence and conductive bands in pure TiO2 is too large for visible light absorption. Upon adsorbing the Pc molecules on the surface, the visible HOMO-LUMO transition in the Pc molecules lowers the gap. Thus, Pc-TiO2 nanocomposites gain importance in the visible light absorption. The band gaps of pure TiO2 and Pc-TiO2 nanocomposites were calculated using Kubelka-Munk approach[37, 38]. Fig. 7 obtained from the UV-DRS spectra shows the calculated band gaps. All of the Pc compounds in the Pc-TiO2 nanocomposites reduce the VB-CB band gap and thus the absorption spectrum of TiO2 extends into the visible region. This reduction is the least in 2-TiO2 nanocomposite and the 20

most in 6-TiO2. The metal center in the Pc molecules should play an important role in the absorption. The absence of metal center in 2-TiO2 could be speculated by the reason of having least effect on the absorption spectrum. The BET surface areas of pure TiO2 and Pc-TiO2 nanocomposites were measured under N2 atmosphere at the liquid nitrogen temperature (77 K) and incremental pressures. Prior to the measurements, the samples were degassed at 473 K under vacuum. The specific surface areas were calculated according to the BET method, pore volume and size were derived from desorption branch according to the BJH model. The surface areas, pore volumes and sizes are given in Table 1. The differences related to BET surface areas, pore sizes and pore volumes between pure TiO2 and Pc-TiO2 nanocomposites can be ascribed to deposition of Pc on the surface of TiO2. It should be indicated that the Pc deposition on the surface of TiO2 slightly decreased the surface area, pore size, pore volume and crystal size (Table 1). Table 1. Some structural parameters of pure TiO2 and Pc-TiO2 nanocomposites Catalyst name

Crystal size (nm)

SBET (m2/g)

Pore size (nm)

Pore volume (cm3/g)

Band gap energy (eV)

TiO2

19.77

86.138

10.1

0.44

3.23

2- TiO2

18.28

82.299

8.69

0.35

3.04

3- TiO2

18.24

82.966

8.26

0.35

3.08

4- TiO2

19.66

81.125

8.37

0.36

2.97

5- TiO2

19.43

82.201

8.25

0.33

2.85

6- TiO2

17.04

85.302

9.05

0.37

2.81

21

Fig. 6. The UV-DRS spectra of pure TiO2 and Pc-TiO2 nanocomposites

Fig. 7. Plots of transformed Kubelka-Munk function

22

3.3

Photocatalytic degradation of MB

Methylene blue is widely used to study the effect of the phthalocyanine compounds on the catalytic activity of TiO2 nanocomposites[9]. We chose the degradation of MB as a model reaction to monitor the photocatalytic activity of pure TiO2 and Pc-TiO2 nanocomposites. Since MB has a characteristic absorption band at 664 nm, the intensity of the absorption can be easily related to the concentration of MB. The visible light absorption normally is not sufficient to initiate the VB-CB transition in TiO2. By addition of Pc, either the energy gap is reduced in Pc-TiO2 so that the transition becomes accessible or electron transfer occurs from Pc to CB of TiO2 with no need of VB-CB transition. When the transition or electron transfer occurs, the TiO2 composites produce the singlet oxygen anion (and hydroxyl radicals) in the aqueous media[19]. Then, this singlet oxygen anions and hydroxyl radicals react with MB for its degradation, eventually causing the absorption intensity to drop. For this purpose, we firstly prepared MB solution (3 ppm, 100 mL) and mixed with the photocatalyst (0.1 g). In order not to have any decomposition, the mixture was kept in dark for 1 h. At the first step, to confirm our hypothesis about the degradation by the singlet oxygen and hydroxyl radicals produced by TiO2, we tested MB-TiO2 mixture in the absence of Pc in the UV region. We observed a complete degradation by TiO2 photocatalyst in 1 h in the UV region. This is a complete agreement with the well-known catalytic activity of TiO2 in the UV region[39]. At the second step, to validate our hypothesis, we made sure that the same MBTiO2 mixture in the absence of Pc does not have any degradation in the visible region. In the final step, each Pc-TiO2 nanocomposite was individually tested at the same conditions for the degradation of MB. We accomplished this dynamically by monitoring the time for a complete degradation of MB for each Pc-TiO2 nanocomposite (Fig. 8 and Fig. 10a). The Pc-TiO2 nanocomposites were tested for full degradation (100%) of MB within 100-140 minutes. Among the 23

nanocomposites, 6-TiO2 (CuPc-TiO2) exhibits the best performance with a full degradation for 100 min (Fig. 8e) while 4-TiO2 (CoPc-TiO2) shows the least activity for 140 min (Fig. 8c). The performances of the nanocomposites are also presented in Fig. 8f by means of degradation efficiency, C/C0, where C and C0 are dynamic and initial concentrations of MB. The pure TiO2 photocatalyst shows almost no activity in either dark or visible light, as expected. All the Pc-TiO2 nanocomposites, on the other hand, have non-linear (exponential) degradation rates in the visible light whereas no degradation was observed in dark. The rates are very similar for each Pc-TiO2 nanocomposite. The least 95% of MB was degraded by all of the Pc-TiO2 nanocomposites within the first 100 minutes. Novel TiO2 photocatalysts including an oxy-bridged phthalocyanine derivatives have different metal ions in macrocyclic ring. As it is known, TiO2 has polar character. Therefore, substituted groups (oxy-bridged groups in non-peripherally position) were designed to increase the polarity of phthalocyanine for better adsorption of large phthalocyanine molecules on TiO2. The preparation of Pc-TiO2 nanocomposites is expected to produce more effective photocatalyst system by facilitating the electron transfer and shifting the wavelength of the light to visible region. The presence of metal ion in phthalocyanine ring can increases the electron transfer and intense absorption bands in the longer wavelength. The efficient photocatalytic properties of phthalocyanine containing metal ion are well known and some reasons are responsible for this extraordinarily outcome. The mentioned reasons are basically redox activity, high thermal stability and absence of toxicity. The d-electron configuration of metal ion in the phthalocyanine ring may important to achieve an efficient photocatalytic performance. If a paramagnetic (like Co(II) and Cu(II)) metal ion place in the ring of phthalocyanine, its photocatalytic performance becomes better than the employing a diamagnetic (like Zn(II) and Ni(II)) metal ion. The deactivation rate is higher in presence paramagnetic metallo phthalocyanines, but they are long-lived to survive under long periods 24

of irradiation[40, 41]. Other processes can occur when oxygen coordinates to the paramagnetic metal center[42, 43]. In this study, the photocatalytic activity of 6-TiO2 photocatalyst containing paramagnetic copper(II) ion slightly higher than the others photocatalyst, but the photocatalytic activity of 4-TiO2 photocatalyst containing paramagnetic cobalt(II) ion is slightly lower than the others photocatalyst. This situation can be explained as follows. The photocatalytic activity of Pc-TiO2 nanocomposites depends on many factor: the adsorption of phthalocyanine on TiO2 surface, surface area, crystal size, crystallinity, bandgap energy and electron-hole recombination rate. Therefore, an explanation of photocatalytic activity order is complicated. Because structural and textural properties of all Pc-TiO2 photocatalysts are similar, the photocatalytic performances of they are similar to each other.

25

Fig. 8. Photocatalytic degradations monitored by the absorption spectra for MB using Pc-TiO2 nanocomposites (a-e) and degradation efficiencies (f)

26

The mechanism that TiO2 is activated by Pcs in the visible region for the degradation of organic pollutants has been proposed in several studies [18, 19, 44-47]. Based on the EPR studies [45], the hydroxyl radicals along with the superoxide radical anions have been produced by the absorption of visible light in Pc-TiO2 nanocomposites. A schematic representation of the mechanism is shown in Fig. 9. Because of the accessible band gap of HOMO-LUMO in Pc compounds, the photoexcitation on Pc-TiO2 nanocomposites occurs by the absorption of the visible light (λ>400 nm). Through inter system crossing, the energy is either transferred to the molecular O2 producing 1O2; or to CB of TiO2, which eventually produces ‧O2-. Once the singlet oxygen anion is generated, which is in equilibrium with the hydroxyl radicals, the dye is decomposed by these reactive reagents. The sequential reactions are presented in the equations ( 1-( 7)[19, 48]. Pc-TiO2 + hν  Pc*-TiO2

(1)

Pc*-TiO2 + O2  Pc-TiO2 + 1O2

(2)

Pc*-TiO2  Pc-TiO2* (CB)

(3)

Pc-TiO2*(CB) + O2  Pc-TiO2 + ·O2-

(4)

O2· - + H2O  HOO· + OH-

(5)

HOO· + H2O  3(·OH)

(6)

Dye (MB) + ·O2- (or OH·)  oxidized dye products

(7)

27

Fig. 9. Schematic view of the degradation mechanism for MB by Pc-TiO2 nanocomposites

In addition to the photocatalytic efficiency of the Pc-TiO2 nanocomposites, the reusability of the photocatalysts is another vital parameter for environmental applications. Therefore, we carried out further studies on reusability of the photocatalysts. To assess that, we firstly filtered the photocatalyst following the complete degradation of MB, washed with distilled water and ethanol and then dried for 1 h at 60 °C. The same procedure for the catalytic activity tests was sequentially repeated 5 times with these used (recycled) photocatalysts. The activity of all the Pc-TiO2 nanocomposites showed a gradual decrease down to 76%. (Fig. 10b). The activity loss by ~20-24% is common for these photocatalysts and attributed to mainly permanent adsorption of phthalocyanine intermediate species formed by partial degradation during the photocatalysis process on TiO2 surface, accumulation of end-products (or byproducts) of MB on the TiO2 surface and partly removal of phthalocyanine molecules (the concentration decrease) from the TiO2 surface[15, 18, 47]. The dominant reason is believed to be the decomposition of phthalocyanine compound during the photocatalysis process. Despite of the partial activity loss, we conclude that the remaining activity is still

28

considerably high even after the fifth recycle, and Pc-TiO2 nanocomposites could be sensitizer candidates for environmental applications.

Fig. 10. Full degradation times of MB for each nanocomposite (a) and reusability efficiency of photocatalysts up to five cycles (b)

29

4

Conclusion

In this study, we synthesized and characterized novel non-peripherally tetra substituted phthalocyanine compounds (2-6) which could be used to modify the TiO2 surface to produce Pc-TiO2 nanocomposites. We confirmed that both pure TiO2 and Pc-TiO2 nanocomposites were all in the anatase phase. We showed that all phthalocyanines activated TiO2 in the visible region for the 100% photocatalytic degradation of MB in just 100-140 min. The reusability tests showed that more than 76% of the activity retained even after the fifth recycle. Our results confirm the potential use of Pc-TiO2 nanocomposites in the removal industrial organic pollutants. Acknowledgements This study was supported by the Research Fund of Yildiz Technical University (Project no: 2016-01-02-DOP01). S. G. greatly acknowledges Abdulkadir Kocak from Gebze Technical University, Hakan Yilmaz from Ondokuz Mayis University and Ozge Koyun from Yildiz Technical University

30

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Highlights:



New non-peripherally tetra-substituted metal-free phthalocyanine and its metal complexes ( Zn(II), Co(II), Ni(II) and Cu(II)) were synthesized and characterized.



The TiO2 nanocomposites of the synthesized phthalocyanines (Pcs) were formed via the hydrothermal method and characterized.



All the nanocomposites showed significant photocatalytic activities on methylene blue organic pollutant under visible light irradiation.

33

Graphical Abstract Non-peripherally tetra substituted phthalocyanine derivatives along with their TiO2 nanocomposites were synthesized, and characterized. The photocatalytic activities of the synthesized TiO2 nanocomposites under the visible light irradiation were investigated by the degradation of methylene blue using UV-vis absorption spectroscopy.

34