Methylene blue photocatalytic degradation under visible light irradiation on copper phthalocyanine-sensitized TiO2 nanopowders

Methylene blue photocatalytic degradation under visible light irradiation on copper phthalocyanine-sensitized TiO2 nanopowders

Materials Science and Engineering B 224 (2017) 9–17 Contents lists available at ScienceDirect Materials Science and Engineering B journal homepage: ...

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Materials Science and Engineering B 224 (2017) 9–17

Contents lists available at ScienceDirect

Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb

Methylene blue photocatalytic degradation under visible light irradiation on copper phthalocyanine-sensitized TiO2 nanopowders Beyza Cabir a, Mehmet Yurderi b, Nurdan Caner b, Mehmet Salih Agirtas a,⇑,1, Mehmet Zahmakiran b,⇑,1, Murat Kaya c a b c

Department of Chemistry, Yuzuncu Yil University, 65080 Van, Turkey Nanomaterials and Research Group, Department of Chemistry, Yuzuncu Yil University, 65080 Van, Turkey Department of Chemical Engineering and Applied Chemistry, Atilim University, 06836 Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 10 April 2017 Received in revised form 28 June 2017 Accepted 30 June 2017

Keywords: Copper Phthalocyanine Titania Nanopowder Photocatalysis

a b s t r a c t Described herein is a new photocatalytic material that shows remarkable catalytic performance in terms of activity and reusability in the photocatalytic degradation of methylene blue (MB) in water. The new catalyst system comprised of copper phthalocyanine modified titanium(IV) oxide (TiO2) nanopowders (CuPc-TiO2) was prepared by the wet chemical impregnation method to improve the photocatalytic activity of TiO2 and characterized by the combination of various spectroscopic tools including ICP-OES, P-XRD, DR/UV–Vis, FTIR, FE-SEM, SEM-EDX, BFTEM, HRTEM and N2-adsorption-desorption techniques. The photocatalytic performance of the resulting CuPc-TiO2 in terms of activity and stability was evaluated by the photocatalytic degradation of MB in aqueous solution under mild conditions. Our results revealed that CuPc-TiO2 photocatalyst displayed remarkable activity (TOF = 3.73 mol MB/(mol CuPc + mol TiO2)  h) in the complete (100%) photocatalytic degradation of MB under visible light irradiation (150 W). Moreover, CuPc-TiO2 photocatalyst showed excellent stability against to sintering and clumping throughout the reusability experiments and it retained >80% of its initial activity even at 5th reuse, which makes it reusable photocatalyst in the photocatalytic degradation of MB. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction The release of several hazardous organic dyes from many textiles industries in waste water is a main cause for serious environmental problems that concerned with human health and the aquatic medium due to the toxicity and the carcinogenic effect of these dyes [1]. Additionally, many of these dyes are visible in water at concentrations as low as 1.0 ppm, which is enough to present an aesthetic problem [2,3]. In this context, various approaches such as adsorption on high surface area supports [4], chemical precipitation [5], sedimentation [6], biological membranes [7], ionexchange [8] and electrochemical [9] processes have been tested to remove these toxic dyes from water. However, these methods usually proceed slowly, require expensive equipment and may lead to transfer of the main pollutant into minor pollutants that requires further elimination [10]. Furthermore, the majority of these organic textile dyes are photocatalytically stable and ⇑ Corresponding authors. E-mail addresses: [email protected] (M.S. Agirtas), [email protected] (M. Zahmakiran). 1 www.nanomatcat.com. http://dx.doi.org/10.1016/j.mseb.2017.06.017 0921-5107/Ó 2017 Elsevier B.V. All rights reserved.

refractory towards chemical oxidation which make them resistant towards decolorization by conventional biochemical and physicochemical methods [11]. At this concern, a successful route for dye removal is concerned with utilizing nano-sized semiconductors such as TiO2 [12,13,14], ZnO [15], Fe2O3 [16] and CdS [17] that exhibit high photocatalytic reactivity in removal of several organic containments without transfer a primary pollutant into series of toxic materials. Of particular importance, titanium(IV) oxide (TiO2) has been extensively studied as a semiconductor photocatalyst for potential application in waste water treatment because of its high oxidative power, photostability, and nontoxicity [18,19]. However, one serious disadvantage of TiO2 is the large band gap of (3.2 eV) of its anatase form, which limits its photo-response to the ultraviolet (UV) region. Unfortunately, only a very small fraction (3.0–5.0%) of the solar spectrum falls in the UV region [20]. To efficiently utilize solar energy, many attempts have been undertaken to make TiO2 responsive to visible light, such as dye-sensitization [21,22], metal complex sensitization [23,24], coupling with a small band gap semiconductor [25,26], and doping transition metals or ions [27–31] and so on. In these methods, the organic dye photosensitization has been proved to be an inexpensive and efficient method

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to extend the absorption spectra of TiO2 into the visible region. In this approach, a dye molecule can absorb visible light to produce a singlet or triplet state (dye⁄). An electron is then injected from the singlet or triplet excited state of the dye into the conduction band (CB) of TiO2. The injected electron can reduce surface chemisorbed O2 to yield strong oxidizing radicals (such as [O2], [HO2], [OH]), which can degrade the organic pollutants [32]. Among these dyes, metallophthalocyanines (MPc), as a typical organic semiconductors, have been extensively investigated for this purpose, because of its nontoxicity, low cost, high thermal and chemical stability, and intensive absorption in the visible-light region [33,34]. The enhanced photocatalytic activities under visible light irradiation by using MPc modified TiO2 have already been reported in the literature [35–45] in which TiO2 has been used in the forms of micron sized particle [37,39,41], thin film/fiber [36,38], nanocrystalline [35,40,42] or porous material [43]. Expectedly, MPc-sensitized TiO2 nanomaterials exhibited a high photocatalytic activity among these TiO2 host materials because of their high surface area [46]. In the present paper, we report a successful attempt at the fabrication of 2,10,16,24-tetrakis(2-isopropyl-5-methylphenoxy phthalocyaninato) copper(II) (vide infra) supported TiO2 nanopowders, hereafter referred to as CuPc/TiO2, through wet chemical impregnation processing. CuPc/TiO2 photocatalyst was characterized by using inductively coupled plasma optical emission spectroscopy (ICP-OES), elemental analysis (EA), powder X-ray diffraction (P-XRD), UV–vis diffuse reflectance spectroscopy (DR/UV–Vis), fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), SEM-energy dispersive X-ray spectroscopy (SEM-EDX), bright field transmission electron microscopy (BFTEM), high resolution transmission electron microscopy (HRTEM) and N2 adsorption-desorption technique. The photocatalytic performance of CuPc/TiO2 in terms of activity and stability was investigated in the photocatalytic degradation of methylene blue (MB) under visible light irradiation. 2. Experimental 2.1. Materials Copper(II) chloride (CuCl2), tetrahydrofuran (THF; C4H8O), potassium carbonate (K2CO3), dimethylformamide (DMF; HCON (CH3)2), ethanol (C2H5OH), benzoquinone (C4H6O2) were purchased from Merck. The solvents were purified according to standard procedure [47] and stored over molecular sieves (4 Å). 2-Isopropyl-5-methylphenol (Thymol; 2-[(CH3)2CH]C6H3-5-(CH3) OH), 2-methyl-2-propanol ((CH3)3COH), 4-nitrophthalonitrile (O2NC6H3-1,2-(CN)2), titanium(IV) oxide nanopowders (TiO2 < 21 nm particle size) and tetramethylthionine chloride (Methylene blue; MB; C16H18ClN3SxH2O) were purchased from Sigma-AldrichÒ and used as received. Deionized water was distilled by water purification system (Milli-Q Water Purification System). All glassware and Teflon-coated magnetic stir bars were washed with acetone and copiously rinsed with distilled water before drying in an oven at 323 K. 2.2. Characterization Copper content of the samples was determined by ICP-OES (Leeman, Direct Reading Echelle) after each sample was completely dissolved in a mixture of HNO3/HCl (1/3 ratio). Powder X-ray diffraction (XRD) patterns were recorded with a MAC Science MXP 3TZ diffractometer using Cu-Ka radiation (wavelength 1.54 Å, 40 kV, 55 mA). TEM samples were prepared by dropping one drop of dilute suspension on copper coated carbon TEM grid and the solvent was then dried. BFTEM and HRTEM analyses were

carried out on a JEOL JEM-200CX transmission electron microscopes operating at 120 kV. The FTIR spectra of the samples were taken on Shimadzu IR-Affinity-1. DR/UV–Vis (with integrating sphere attachment) and UV–Vis spectra were taken by using Shimadzu UV-3600. FESEM analyses were performed on XL-30 ESEM FEG, Micro FEI Philips. The nitrogen adsorption-desorption analysis was carried out at 77 K using a NOVA3000 series instrument (Quantachrome Instruments). The sample was out-gassed under vacuum at 473 K for 3 h before the adsorption of nitrogen. 2.3. Synthesis of 2,10,16,24-tetrakis(2-isopropyl-5-methylphenoxy phthalocyanito) Copper(II) (CuPc) The synthesis of 2,10,16,24-tetrakis(2-isopropyl-5-methylphe noxy phthalocyaninato) copper(II) was conducted according to literature procedure [48] that was published recently by some of us. According to this synthesis protocol a mixture of 4-(2-isopropyl-5methylphenoxy) phthalonitrile (0.36 mmol; 100 mg) and copper (II) chloride (0.15 mmol; 20 mg) was powdered in a quartz crucible, then heated at 473 K in a sealed glass tube for 8 min under nitrogen atmosphere. After cooling to room temperature, the product was washed with ethanol and hexane. Next, the product was dissolved in THF to remove impurities and then THF was evaporated to obtain solid final product. The yield was found to be 37 mg (35%) and elemental analysis for C72H64CuN8O4 (calcd. C 73.98; H 5.52; N 9.59%) was found to be C 73.87; H 5.55; N 9.51%. UV–Vis (THF) kmax (loge) = 676 (5.32), 608 (4.41), 343 (5.1). FTIR m = 3080 (Ar-CH), 2958, 2870 (CH3), 1612, 1573, 1465, 1222, 1091, 1053, 956, 813, 744 cm1. 2.4. Preparation of CuPc/TiO2 CuPc/TiO2 catalyst was obtained by the conventional impregnation method [49], CuPc (26.3 mg and 55.6 mg for 4.7 and 9.1 wt% Cu loadings, respectively) for and TiO2 (500 mg) nanopowders were taken into 20 mL of dry THF and under reflux conditions the resulting slurry was allowed to stir for 12 h. The black solid was filtered by centrifugation (10,000 rpm  3 min) and put on Whatmann filter paper for suction filtration at room temperature and washed repeatedly with dry ethanol. CuPc/TiO2 powders was dried in a vacuum oven at 373 K under 101 Torr pressure and used for further application. 2.5. Photocatalytic test The photoreactor (Fig. S1, Supporting Information) was designed with an internal xenon lamp (XHA-150W) equipped with a cut-off glass filter transmitting k > 400 nm surrounded by a water-cooling quartz jacket to cool the lamp, where 30 mL of the MB solution with an initial concentration of 20 mM in the presence of solid CuPc/TiO2 catalyst (10 mg). The solution was stirred in the dark for 30 min to obtain a good dispersion and reach adsorption-desorption equilibrium between the organic molecules and the catalyst surface. Decreases in the concentrations of MB were analyzed by a Shimadzu UV-3600 spectrophotometer at k = 660 nm. At given intervals of illumination, the samples (1.5 mL) of the reaction solution were taken out and then centrifuged, filtered and the filtrates were analyzed. The photo-degradation rate of methylene blue was calculated by the following equation: photodegradation efficiency = C/C0  100%, where C0 is the initial concentration and C is the concentration of methylene blue as function of time irradiation. 2.6. Durability test After one complete reaction cycle, the photocatalyst was isolated by suction filtration using Whatmann filter paper and

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washed with excess ethanol–water mixture and dried in vacuum– oven at 353 K (101 Torr). Then, the dried catalyst weighed and used for the next cycle of catalytic reaction with fresh MB substrate. 3. Results and discussion The synthesis of copper phthalocyanine (2,10,16,24-tetrakis(2isopropyl-5-methylphenoxy phthalocyaninato) copper(II); CuPc) was conducted through the procedure that recently published by some of us [48], which comprised of the reaction of 4-(2isopropyl-5-methylphenoxy) phthalonitrile (1) and copper(II) chloride to yield 2,10,16,24-tetrakis(2-isopropyl-5-methylphenoxy phthalocyaninato) copper(II) (2) (Scheme 1). The characterization of as-synthesized CuPc was done by EA, FTIR and DR/UV–Vis analyses and their results are in accordance with literature [48]. The preparation of CuPc/TiO2 was simply and reproducibly done by the conventional wet impregnation of CuPc onto TiO2 support in THF under reflux conditions. After centrifugation, copious washing dry ethanol, and drying under vacuum CuPc/TiO2 photocatalyst was obtained as powders and characterized by ICP-OES, P-XRD, FTIR, DR/UV–Vis, FE-SEM, SEM-EDX, BFTEM, HRTEM and N2 adsorption-desorption techniques. The copper content of the sample was found to be 4.7 wt% Cu and it corresponds to impregnation of 4.3 mmol CuPc onto TiO2 nanopowder support. P-XRD patterns of CuPc/TiO2 and TiO2 are given in Fig. 1. The Bragg peaks of CuPc/

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TiO2 at 2h of 25.2, 37.9, 48.1, 53.9, 55.0, 62.6, 68.7, 70.4 and 75.1 are consistent with those of anatase phase TiO2 (JCPDS Card No 21-1272) and no impurity peaks appeared. This result shows that the surface modification with CuPc did not alter the crystalline phase of TiO2, which can be attributed to the low concentration and high dispersion of guest CuPc on the surface of TiO2. The average crystallite size was measured on the basis of Debye–Scherrer equation [11]. The average crystallite sizes of TiO2 and CuPc/TiO2 are found to be 15.6 nm and 12.5 nm, respectively. There is a slight decrease in the crystallite size of TiO2 after surface modification with CuPc. This is due to the significant interaction between CuPc and TiO2 through the hydrogen bonding, which formed by imine groups of CuPc and surface hydroxyl groups of TiO2 [50,51], and this interaction stabilizes TiO2 nanopowders by preventing further growth of crystallization. The surface areas TiO2 nanopowders and the as-prepared CuPc/TiO2 were examined by N2 adsorptiondesorption technique and BET surface areas of TiO2 nanopowders and CuPc/TiO2 were found to be 63 and 59 m2/g from their N2 adsorption-desorption isotherms (Fig. S2 in the Supporting Information). The observed decrease in the BET surface area for CuPc/ TiO2 can be ascribed to deposition of CuPc on the surface of TiO2 nanopowders. The FTIR spectra of the pure TiO2 nanopowders and CuPc/TiO2 are shown in Fig. 2, as observed the FTIR spectrum of the TiO2 nanopowders only exhibited the absorption peaks around 500 and 660 cm1, which are assigned to the Ti-O vibrations. The FTIR

Scheme 1. The synthesis route of 2,10,16,24-tetrakis (2-isopropyl-5-methylphenoxy phthalocyaninato) copper(II) (CuPc).

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Fig. 1. The powder X-ray diffraction patterns of CuPc/TiO2 and TiO2 in the range of 2h = 20–80°.

Fig. 3. (a) UV–Vis diffuse reflectance spectra of CuPc/TiO2 and TiO2, (b) Tauc plots of CuPc/TiO2 and TiO2.

Fig. 2. The FTIR spectra of CuPc/TiO2 and TiO2 in the range of 400–4000 cm1.

spectrum of CuPc/TiO2 appeared several absorption peaks at around 1409, 1531, 1627, 2855 and 2921 cm1, which might be assigned to Pc skeletal and metal-ligand vibrations as these peaks were also observed in the FTIR spectrum of pure CuPc [48]. The results obtained from ICP-OES, P-XRD, FTIR and N2-adsorptiondesorption revealed that CuPc is supported successfully on the surface of TiO2 nanopowders. DR/UV–Vis spectra of the neat TiO2 nanopowders and CuPc/TiO2 were depicted in Fig. 3. As observed in Fig. 3(a), DR/UV–Vis spectrum of TiO2 nanopowders only exhibited the fundamental absorption band in the UV region, there was no more absorption in visible wavelengths, whereas CuPc/TiO2 exhibited absorption bands between 600 and 750 nm (Fig. 3(b)) that might be attributable to the Q-band of CuPc [52], which corresponds to excitation between the ground state a1u (p) HOMO to eg (p⁄) LUMO. These absorption bands between of 600–750 nm was also observed in the UV–Vis diffuse reflectance spectrum of pure CuPc [48]. Commonly, the band gap of semiconductors is linked to their range of absorption wavelength and the band gap decreases with increasing of absorption edge. The band gaps of TiO2 nanopowders and CuPc/TiO2 were evaluated using Tauc approach [53]. The band gap values of TiO2

nanopowders and CuPc/TiO2 (Fig. 3(b)) were found to be 3.46 eV and 3.31 eV, respectively. Hence, CuPc/TiO2 absorbs more visible light than that of TiO2. These results revealed that the sensitization of TiO2 with CuPc could extend the absorbance spectrum of TiO2 into visible region. The morphology and surface composition of the as-prepared CuPc/TiO2 material were investigated by SEM, SEM-EDX, BFTEM, and HRTEM analyses. SEM images of CuPc/TiO2 material given in Fig. 4(a)–(b) show only the spherical TiO2 support material and there is no clumped or agglomerated CuPc formed in observable size on the surface of the TiO2 nanopowders. Although no bulky CuPc was observed outside of TiO2 nanopowders, SEM-EDX analyses indicate the presence of CuPc elements Cu and N on the surface of TiO2 nanopowders (Fig. 4(c)). BFTEM images of CuPc/TiO2 in different magnifications were shown in Fig. 4(d)–(f) and they indicated the presence of non-agglomerated mostly spherical shaped TiO2 support material and the preservation of their highly crystalline nature in the as-prepared CuPc/TiO2 material was also confirmed by HRTEM analysis (Fig. S3 in the Supporting Information). The average grain diameter of TiO2 support seems to be in the range of 17–22 nm and this range is in good agreement with the P-XRD results (vide supra). The photocatalytic degradation of methylene blue (MB) (Scheme 2) was chosen as a model reaction to evaluate the photocatalytic activity of the CuPc/TiO2. Methylene blue aqueous solution absorbs light around 664 nm, and shows bright blue. As it is

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Fig. 4. (a–b) SEM images of CuPc/TiO2, (c) SEM-EDX spectrum of CuPc/TiO2 and (d–f) BFTEM images of CuPc/TiO2 in different magnifications.

Scheme 2. Structural formula of methylene blue (MB; C16H18ClN3S).

well-known, when the aqueous solution is subjected to the photocatalytic degradation sensitized by TiO2, the characteristic absorption peak of methylene blue around 664 nm decreases smoothly

with a slight shift toward a shorter wavelength and the solution eventually becomes colorless [54]. As a representative example Fig. 5 shows the changes observed in both UV–Vis spectrum and color of the MB solution during its CuPc/TiO2 catalyzed photocatalytic degradation. In the experiments, pure TiO2 nanopowder was used as a photocatalytic reference to study the photocatalytic activity of the CuPc/TiO2. The degradation efficiency of the as-prepared samples was defined as C/C0, where C and C0 stands for the remnants and initial concentration of MB, respectively. Firstly, we performed control experiments under different reaction conditions (Fig. 6):

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Fig. 5. The change in the UV–Vis spectrum and the color of the methylene blue solution during CuPc/TiO2 (10 mg and contains 4.7 wt% Cu) catalyzed photocatalytic degradation of methylene blue (80 mM in 30 mL H2O) at room temperature.

Fig. 6. The degradation efficiency (C/C0) versus time graphs for photocatalytic degradation of MB (a) in the presence of CuPc/TiO2 (10 mg and contains 4.7 wt% Cu) and TiO2 photocatalyst in the dark, (b) in the absence and presence of photocatalysts (CuPc/TiO2 and TiO2) under visible light irradiation at room temperature.

(i) in the presence of photocatalyst but in the dark and (ii) with visible-light irradiation but in the absence of the photocatalyst. These results illuminated that the adsorption-desorption equilibrium of MB in the dark was established within 30 min (Fig. 6 (a)) and, there was no appreciable degradation of MB after 2.5 h in the absence of photocatalyst (Fig. 6(b)). Fig. 6(b) shows the degradation curves of MB on the TiO2 and CuPc/TiO2 as observed from this graph, neat TiO2 nanopowders provided low photocatalytic activity under visible light, and the degradation was only 11% in 2.5 h, whereas CuPc/TiO2 exhibited 54% degradation within the same time interval. Additionally, when the amount of CuPc on TiO2 nanopowders increased from 4.7 wt% to 9.1 wt% in terms of copper loading, CuPc/TiO2 photocatalyst, whose surface functionality and modification were found to be identical with those of CuPc/ TiO2 photocatalyst (at 4.7 wt% Cu loading) by FTIR and DR/UV–Vis analyses (Figs. S4 and S5 in the Supporting Information), showed higher activity at almost complete degradation of MB within 2.5 h (Fig. 7). This result illuminated that the increase in CuPc obviously enhanced the photocatalytic activity of CuPc/TiO2. We also performed another control experiment in which the photocatalytic activity of CuPc/TiO2 photocatalyst with high CuPc loading (12.4 wt% Cu) examined in the degradation of MB, which provided almost the same photocatalytic activity with neat CuPc due to the corruption of the crystallinity of the host material TiO2 as evidenced by P-XRD (Fig. S6 in the Supporting Information). The importance of the host material crystallinity on the catalytic performance of guest active metal species has already been reported by some of us with solid oxide supported metal nanoparticle catalysts [55–60]. The observed initial activity of CuPc/TiO2 photocatalyst (at 9.1 wt% Cu loading) in terms of turnover frequency (TOF) was calculated to be 3.73 mol MB/(mol CuPc + mol TiO2)  h and this activity value is higher than those of previously found by using CuPcTs/HTlcs (0.72 mol MB/(mol CuPcTs + mol HTlcs)  h) [37], PcTcCu/TiO2 nanofiber (1.92 mol MB/(mol PcTcCu + mol TiO2)  h) [38], commercial TiO2 (0.006 mmol MB/(mol TiO2)  h) [44] and ZnO powders (0.424 mmol MB/(mol ZnO)  h) [45]. The photocatalytic activity of pure CuPc was also investigated, which was a little better than that of the TiO2 nanopowders and the corresponding degradation of MB reached to 28%. All above experimental results suggested that CuPc as visible-light sensitization improve the visible-light photocatalytic activity of CuPc/TiO2.

Fig. 7. The degradation efficiency (C/C0) versus time graphs for photocatalytic degradation of MB in the presence of CuPc/TiO2 (contains 4.7 wt% Cu), CuPc/TiO2 (contains 9.1 wt% Cu), CuPc, CuPc/TiO2 (10 mg and contains 4.7 wt% Cu) + tert-butyl alcohol and CuPc/TiO2 (10 mg and contains 4.7 wt% Cu) + benzoquinone at 298 K.

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In order to check whether hydroxyl radicals (OH) are involved in the degradation of MB, the photocatalytic degradation of MB over CuPc/TiO2 photocatalyst was carried out in the presence of tert-butyl alcohol (tert-BuOH), which is a commonly used as OH scavenger [61]. As depicted in Fig. 7, the presence of tert-BuOH significantly decreased the photocatalytic degradation efficiency of MB on CuPc/TiO2 and this result confirmed that the OH radicals produced in the presence of CuPc/TiO2 photocatalyst. In other words, the hydroxyl (OH) radicals answerable for the observed enhancement in the photocatalytic degradation efficiency of MB. Additionally, in a separate control experiment benzoquinone, which is an excellent superoxide anion scavenger [62], was added to the solution to investigate the effect of superoxide anion on the photocatalytic degradation of MB over CuPc/TiO2 photocatalyst. As given in Fig. 7, the photocatalytic degradation efficiency of MB decreased in the presence of benzoquinone confirming that the superoxide anion radicals generated during the photocatalytic degradation of MB over CuPc/TiO2 photocatalyst. In light of these aforementioned results and the previously published reports on the dye-sensitized photocatalytic oxidation of pollutants, a proposed mechanism of MB photocatalytic degradation under visible light irradiation on CuPc/TiO2 photocatalyst was elucidated in Scheme 3. The valence band (VB) and conduction band (CB) of TiO2 are located at 7.4 and 4.2 eV (vs vacuum level) [63,64], respectively in the meanwhile the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels of CuPc positioned at 3.5 and 5.2 eV (vs vacuum level) [65–67], respectively. Upon irradiation with visible light CuPc supported on TiO2 nanopowders is excited and generates 1 O2 via energy transfer (Eqs. (1) and (2)). The LUMO position of CuPc fits well with the energy requirements for efficient electron injection into CB of TiO2 so the excited charge is then injected from the excited state (LUMO) of CuPc into CB of TiO2, which is followed by generation of CuPc cation radical (CuPc+) and conduction band (e cb) electrons of TiO2 (Eq. (3)). The dissolved oxygen molecules and singlet oxygen react with CB electrons to yield superoxide radical anions (O 2 ) and their initial protonation produce hydroperoxy (HO2) radicals then hydroxyl radicals (OH) (Eqs. (4)–(8)), which acts as a strong oxidizing agent in the decomposition of organic dyes (Eq. (9)). Besides the active oxygen species, the by-produced

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CuPc+ radical cation also reacted with MB and induces the photo degradation of MB (Eqs. (10) and (11)).

CuPc þ hv ! CuPc

ð1Þ

CuPc þ O2 ! CuPc þ 1 O2

ð2Þ

CuPc þ TiO2 ! CuPcþ þ TiO2 ðecb Þ

ð3Þ

TiO2 ðecb Þ þ O2 ! TiO2 þ O 2

ð4Þ

TiO2 ðecb Þ þ 1 O2 ! TiO2 þ O 2

ð5Þ

  O 2 þ H2 O ! OH þ HO2

ð6Þ

HO2 þ H2 O ! H2 O2 þ OH

ð7Þ

H2 O2 ! 2OH

ð8Þ

MB þ OH ! CO2 þ H2 O

ð9Þ

CuPcþ þ MB ! CuPc þ MBþ

ð10Þ

MBþ þ OH ! CO2 þ H2 O

ð11Þ

The photocatalytic durability of CuPc/TiO2 photocatalyst was evaluated by its isolability and reusability performance tested in the photocatalytic degradation of MB. For this purpose, after the complete degradation of MB, CuPc/TiO2 photocatalyst was isolated as black powders, washed with water-ethanol mixture, dried and bottled under nitrogen atmosphere. The isolated CuPc/TiO2 photocatalyst was weighed and re-dispersed in aqueous MB solution and yet an active in the photocatalytic degradation of MB; as given in Fig. 8 CuPc/TiO2 catalyst retains >80% of its activity even at the 5th catalytic reuse. The elemental composition, surface functionality, crystallinity and morphological analyses of reused CuPc/TiO2 photocatalyst were done by DR/UV–Vis, FTIR, BFTEM, P-XRD and ICP-OES analyses. DR/UV–Vis and FTIR spectra of CuPc/TiO2 photocatalyst harvested after 5th catalytic reuse were found to be identical with those of fresh CuPc/TiO2 photocatalyst (Figs. S7 and S8 in the Supporting Information), which reveals that the protection

Scheme 3. The proposed mechanism for the MB photocatalytic degradation under visible light irradiation on CuPc/TiO2 photocatalyst.

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CuPc/TiO2 photocatalyst show exceptional stability throughout the catalytic runs against leaching and sintering so that they retain >80% of their photocatalytic activity even at the 5th catalytic reuse. Overall, CuPc/TiO2 photocatalyst is available by a simple procedure and is found to be superior photocatalyst in terms of activity and stability in the photocatalytic degradation of MB under visible light irradiation. Acknowledgements MZ thanks to Yüzüncü Yıl University Office of Scientific Research Projects for the financial support to his research laboratory. Additionally, the partial supports by Fevzi Akkaya Scientific Activities Support Fund (FABED), Science Academy and Turkish Academy of Sciences (TUBA) are gratefully acknowledged.

Appendix A. Supplementary data Fig. 8. The degradation efficiency (C/C0) versus time graphs for the reusability experiments performed on the photocatalytic degradation of MB in the presence of CuPc/TiO2 photocatalyst under visible light irradiation at room temperature.

surface functionality of CuPc/TiO2 photocatalyst throughout the reusability experiments. BFTEM image of CuPc/TiO2 photocatalyst harvested after 5th catalytic reuse (Fig. S9 in the Supporting Information) indicated that a slight clumping of TiO2 nanopowders, which explains the slight decrease in the observed activity of CuPc/TiO2 photocatalyst. P-XRD pattern of same sample (Fig. S10 in the Supporting Information) gave the matching Bragg peaks of the host material TiO2 nanopowders so this result is indicative of the preservation of intact crystallinity throughout the reusability experiments. ICP-OES analyses of the same recovered CuPc/TiO2 photocatalyst gave us almost the identical Cu loading (9.06% wt Cu) amount with that of the fresh catalyst. More importantly, Cu was not detected in the filtrate collected from each cycle by the ICP-OES technique (with a detection limit of 29 ppb for Cu) confirming that there is no indication of leaching of CuPc into the reaction solution within the detection limit of ICP-OES. Additionally, the photocatalytic degradation of MB was completely stopped by the removal of CuPc/TiO2 photocatalyst from the reaction solution; these results confirm the retention of active CuPc on the support and no leaching of active species to the solution. 4. Conclusions In summary, TiO2 semiconductor in nanopowder form has been used, for the first time, as support material for guest copper phthalocyanine (2,10,16,24-tetrakis(2-isopropyl-5-methylphe noxy phthalocyaninato) copper(II) complex to fabricate CuPc/TiO2 in this study. CuPc/TiO2 can simply and reproducibly be prepared by conventional impregnation of CuPc onto TiO2 nanopowders support in dry THF under reflux conditions. The characterization of CuPc/TiO2 photocatalyst was done by using ICP-OES, P-XRD, FTIR, DR/UV–Vis, SEM, SEM-EDX, BFTEM, HRTEM and N2adsorption-desorption analyses. The results of these multipronged analyses revealed that the formation of well-dispersed CuPc on the highly crystalline TiO2 nanopowders support. The photocatalytic performance of CuPc/TiO2 in terms of activity and stability was tested in the photocatalytic degradation of MB under visible light irradiation. CuPc/TiO2 was found to be highly active photocatalyst in this transformation. They provide exceptional turnover frequency (TOF = 3.73 mol MB/(mol CuPc + mol TiO2)  h), which is higher than those of previously reported CuPc-TiO2 based photocatalysts employed in the same reaction. Moreover,

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