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Chemical Physics Letters 415 (2005) 85–88

Visible light detoxification by 2,9,16,23-tetracarboxylphthalocyanine copper modified amorphous titania

Feng Chen a,*, Zhigang Deng a, Xiaopei Li a, Jinlong Zhang a,*, Jincai Zhao b

a Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR Chinab Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China

Received 5 July 2005; in final form 15 August 2005Available online 19 September 2005

Abstract

Visible light detoxification of methyl orange (MO) was achieved with a photo-stable 2,9,16,23-tetracarboxyl phthalocyanine (TcPc)/amorphous TiO2 hybrid photocatalyst. TcPc/amorphous TiO2 exhibits an excellent photocatalytic activity under visible irradiation(k > 550 nm). Besides the active oxygen species, sensitizer radical cation, TcPc+�, was also found to react with MO directly and inducethe photodegradation of MO significantly for the first time in dye sensitized photocatalytic system.� 2005 Elsevier B.V. All rights reserved.

1. Introduction

Titanium dioxide (TiO2) is one of the most popular andpromising materials as photocatalyst for air treatment anddetoxification of waste water [1–3]. However, because of itslarge band gap, 3.2 eV for bulk TiO2, only about 3.0% ofsolar light reaching the earth surface can be utilized byTiO2. In order to obtain a more efficient utilization of solarlight, diverse efforts have been made to shift the response ofTiO2 (k 6 387 nm) into the visible region, which includeimpunity doping [4–7] and dye sensitization [8–12].

Impurity doping of TiO2 with transitional metal ionssuch as V, Mo, Fe, Co, or Ni can extend light absorptionof TiO2 fairly to the visible region, but the effect of dopanttransitional metal ions to the photocatalytic activity ofTiO2 is still in confusing [6,7]. Some authors reported therecombination rate of charge carriers of doped TiO2 in-creased with ion doping with respect to the bare TiO2,while others found it decreased. Dye sensitization has beenextensively studied for solar cell, but seldom used in consti-tuting a photocatalyst for detoxification of waste water.Although dyes have a considerable photoabsorption coeffi-

0009-2614/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2005.08.107

* Corresponding authors. Fax: +86 21 64252062.E-mail address: fengchen@ecust.edu.cn (F. Chen).

cient to the visible light, they are always not stable andphotodegraded readily under the illumination with TiO2

[8]. Recently, several reports indicated some special dyes,such as ruthenium bipyridyl complexes [9,10] and phthalo-cyanine derivates [11], can be used to sensitize TO2 for or-ganic detoxification. A photoinduced electron transferfrom excited dye to TiO2 and the surface reaction ofTiO2 conductive band electron were suggested as the keyprocesses in those works [9,10]. However, little attempthas been made to look into the chemical process of dyecation radical. What�s more, although ruthenium bipyridylcomplexes/TiO2 system was only present in anaerobicenvironment, ruthenium bipyridyl complexes were stillunstable under visible irradiation [9]. Some phthalocyaninederivates are believed stable in dye sensitization photoca-talysis, but crystalline anatase and rutile are necessary forconstituting phthalocyanine/TiO2 photocatalyst [11,12],while amorphous TiO2 was reported to be photo-inactive[13,14]. On the other hand, typical preparation for phthalo-cyanine/TiO2 photocatalyst always involved an uncharita-ble solvent such as sulfuric acid, which is destructive toTiO2 and possibly lead to some unpredictable changes tothe physicochemical properties of TiO2.

Azo dye effluents from textile industries are becoming aserious environment problem because of their unacceptable

Fig. 1. The diffuse reflectance spectra of am-TiO2 (� � �) and TcPc/am-TiO2

(—) and the UV–Vis absorption spectra of TcPc (pH = 8.0; - -) and MO(-Æ Æ-) aqueous solution.

86 F. Chen et al. / Chemical Physics Letters 415 (2005) 85–88

color, high chemical oxygen demand content, resistant tochemical, photochemical and biological degradation.Decolorization and decomposition of azo dyes are a high-light for environmental chemists. Therefore, MO is selectedas a reliable model azo dye pollutant to investigate the pho-tocatalytic activity of the photocatalysts in this work. Astable dye sensitized mesoporous amorphous TiO2 (am-TiO2) was prepared in a mild way. MO was smoothlyphotodegraded with this dye sensitized amorphous TiO2

under visible irradiation (k > 550 nm).

2. Experimental

2.1. Preparation of hybrid photocatalyst

Dye of 2,9,16,23-tetracarboxyl phthalocyanine copper(TcPc) was synthesized and purified as the literatures men-tioned [15,16]. Am-TiO2 was prepared by a sol–gel processwith the hydrolysis of titanium alkoxides at room temper-ature, following with a mild drying process: one week atambient temperature and 2 h at 80 �C to remove water.The gained TiO2 has a BET area of 165.6 m2/g and apore-size distribution with a maximum at 2.8 nm (Microm-eritics ASAP 2010). XRD measurements shows TiO2 ismainly composed of amorphous TiO2 with very littleamount of anatase (Rigaku D/max 2550 VB/PC). The sur-face modification of am-TiO2 was carried out by dispersingam-TiO2 in the TcPc aqueous solution: Traditionally, a cal-culated amount of TcPc was dissolved in 1 L doubly dis-tilled water at pH 8.0. Then, 1 g TiO2 powder was addedinto the TcPc solution. The suspension was stirred for 3 hto reach an adsorption–desorption equilibrium at the sur-face of TiO2. Then the hybrid photocatalyst was separatedfrom the aqueous suspension by centrifugation, washedrepeatedly with doubly distilled water and dried at 80 �Cfor use. UV–Vis diffuse reflectance spectra of photocatalystpowder was obtained for the dry-pressed disk samplesusing a Scan UV–Vis-NIR spectrophotometer (VarianCary 500) equipped with an integrating sphere assembly,using BaSO4 as the reflectance sample.

2.2. Photocatalytic experiment

Photodegradation was carried out by adding 0.1 g pho-tocatalyst into a 160 mL glass tube reactor containing100 mL of 10 mg/L MO solution. Prior to irradiation, thesuspension was magnetically stirred in the dark for 1 h toensure the establishment of an adsorption–desorption equi-librium of MO on the surface of photocatalyst. Then thestirred suspension was illuminated with a 1000-W halogenlamp, which was placed in a glass cylindrical jacket withflowing water to cool the lamp. A cut-off filter was placedin front of the glass cylindrical jacket to remove the lightwith wavelength below 550 nm. At given intervals of illumi-nation, reaction solution (about 4.0 mL) was sampled andfiltered through a Millipore membrane filter (0.22 lmpores) and analyzed with a UV–Vis spectrophotometer

(Varian Cary 100). The concentration of MO was deter-mined from the absorbance at 464 nm.

3. Results and discussion

UV–Vis diffuse reflectance spectra of TcPc/am-TiO2 dis-plays TcPc molecules on the surface of am-TiO2 have asimilar absorption as what they have in the aqueous solu-tion (Fig. 1), which means TcPc are in monomer or dimerbut not in congeries or microcrystallite at the surface ofam-TiO2. Although the absorption of am-TiO2 is inhibitedat the wavelength longer than 390 nm for its large bandgap, TcPc/am-TiO2 hybrid photocatalyst shows a goodabsorbance from 500 nm to 750 nm.

To test the photocatalytic activity of TcPc/am-TiO2,photodegradation of a typical azo-dye, MO was investi-gated in this work. Although visible light assisted photocat-alytic degradation of MO has been reported in previousliteratures [17], the photobleaching of MO was notobserved with bare am-TiO2 under the light irradiation ofk > 550 nm (Fig. 2, curve c), since no MO excitation couldbe achieved with the illumination of wavelength longer than550 nm. The surface modification of am-TiO2 with TcPc en-abled the photodegradation of MO (Fig. 2, curve e). About80% of MO was photodegraded within 3 h in the presenceof TcPc/am-TiO2 under visible irradiation. In order toinvestigate the photocatalytic activity of as preparedTcPc/am-TiO2, surface modified P-25 was also synthesizedto photodegradeMO. As shown in Fig. 2 (curve d), the pho-todegradation of MO with TcPc/P-25 was a little slowerthan that with TcPc/am-TiO2, although the TiO2 preparedin this work is almost composed of amorphous TiO2 but notcrystalline anatase or rutile.

In order to confirm the effect of TiO2 in the hybrid TcPc/am-TiO2, SiO2 was used instead of TiO2 for the photodegra-dation of MO. However, no obvious change was observedfor MO in the presence of TcPc/SiO2 under illumination.Furthermore,MOwith only TcPc (pH = 8) in homogeneous

Fig. 3. The photodegradation of MO (10 mg L�1) aqueous solution(pH = 4.0) with TcPc/am-TiO2 (1 g L

�1) upon visible illumination (cut-offfilter k > 550 nm) under various of conditions: (a) TcPc/am-TiO2 only;(b) in the presence of BQ (1.0 · 10�3 M); (c) TcPc/am-TiO2 with N2

bubbling; (d) TcPc/am-TiO2 with N2 bubbling in the presence of AgNO3

(5.0 · 10�3 M). *The concentration variation before irradiation is becauseof MO adsorption at the surface of TcPc/am-TiO2. **The concentrationincrease or a slowered degradation rate of MO at first hour is a result ofphoto-desorption of MO on the surface of TcPc/am-TiO2 under visibleirradiation, which can also be observed in Fig. 2, curve e.

Scheme 1. Postulated mechanism of the visible light-induced photodeg-radation of MO with TcPc/am-TiO2.

Fig. 2. The photodegradation of MO (10 mg L�1) aqueous solution(pH = 4.0) with various of photocatalysts (1 g L�1) under visible illumi-nation (1000-W halogen lamp, cut-off filter k > 550 nm). (a) No catalyst,(b) TcPc/SiO2, (c) am-TiO2, (d) TcPc/P25, (e) TcPc/am-TiO2. *Theconcentration variation before irradiation is because of MO adsorption atthe surface of photocatalysts.

F. Chen et al. / Chemical Physics Letters 415 (2005) 85–88 87

aqueous also exhibited good photostability for 8 h illumina-tion. These two above controlled experiments exclude defi-nitely the possibility of dye sensitization oxidation (type Iand type II) routes induced by excitedTcPc forMOdegrada-tion. All the photodegradation ofMOmust experience a dye(TcPc) sensitized TiO2 photocatalytic mechanism, in whicham-TiO2 acts as an electron acceptor to accept the electronfrom excited TcPc molecule and a microreactor for surfaceelectron reacting with oxidants.

For the dye self-sensitized photocatalytic oxidation withcrystalline TiO2, active oxygen species such as surperoxideradicals, H2O2 and hydroxyl radicals generated by the reac-tion of excited electron with oxygen were reported as theessential and only attackers that induce the degradation oforganics [8,18]. For checking if the reaction of excited elec-tron with surface adsorbed oxygen can still carried out effec-tively even for am-TiO2, photodegradation of MO withTcPc/am-TiO2 in existence of 1,4-benzoquinone (BQ) aswellas in an anaerobic environment were carried out as indicatedin Fig. 3. The photodegradation of MO was greatly de-pressed with the addition of BQ, which should be attributedto the reaction of BQ with the active surperoxide radical. Itsuggested that the surperoxide radical still did as one of themain active species for MO photodegradation, which wasalso confirmed by the seriously depressed photodegradationof MO in the absence of oxygen. In fact, even in the absenceof oxygen, the photodegradation ofMOwas still carried outto some extent under the visible irradiation. It indicated thatsomething apart from the active oxygen species also inducedthe degradation ofMO in this scene. In the hybrid TcPc/am-TiO2 photocatalytic system, besides active oxygen species,the rest excited species are excited TcPc, TiO2 conductiveband electron and TcPc radical cation. Excited TcPc itselfhas been proved inactive for MO degradation (Fig. 2, curveb); TiO2 conductive band electron can not drive the reactionwith MO. Thus the only rested TcPc radical cation has the

most possibility for the degradation of MO in the absenceof oxygen. The redox potentials of TcPc/TcPc+� and MO/MO+�weremeasuredwith cyclic voltammetricmethod (plat-inum and Ag/AgCl as work and reference electrode, respec-tively). The obtained redox potentials of TcPc/TcPc+� andMO/MO+� are 1.54 V and 0.74 V, respectively; which allowsthe reaction of TcPc+�withMO. It was also confirmed by theacceleration ofMOphotodegradation in the absence of oxy-genwith the addition ofAg+ (Fig. 3, curve d). The surface ad-sorbed Ag+ would tend to react with TiO2 conductive bandelectron, which depresses the recombination of excited elec-tron with TcPc+�, and thus significantly accelerate the reac-tion of TcPc+� with MO.

In summary, am-TiO2 functions as an electron trap forthe excited surface adsorbed TcPc dye. The trapped elec-tron subsequently induces the generation of active oxygenspecies. Besides the active oxygen species, the by-producedTcPc+� radical cation also reacts with MO and induces thephotodegradation of MO (Scheme 1). Since no valence

88 F. Chen et al. / Chemical Physics Letters 415 (2005) 85–88

band hole is produced in am-TiO2, the interior chargerecombination is avoided in dye sensitization photocataly-sis. Thus, am-TiO2 also exhibits an excellent photocatalyticactivity for TcPc/TiO2, which is even a little better thanthat of TcPc/P-25, partly due to its better adsorbabilityto MO (Fig. 2).

Acknowledgements

This work was supported by the National NatureScience Foundation of China (20407007); Program forNew Century Excellent Talents in University (NCET-04-0414); National Basic Research Program of China(2004CB719500) and SRF for ROCS, SEM.

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