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Colloids and Surfaces A: Physicochem. Eng. Aspects 443 (2014) 52– 59

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jo ur nal ho me p ag e: www.elsev ier .com/ locate /co lsur fa

icrowave assisted synthesis of ZnPc-COOH and SiO2/ZnPc-COOHanopaticles: Singlet oxygen production and photocatalytic property

ang Qiana, Wu Weia,∗, Meng Hongc, Chen Jianfenga,b, Chu Guangwenb, Zou Haikuib

Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, ChinaResearch Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029,hinaCollege of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China

i g h l i g h t s

Zinc phthalocyanine was prepared bymicrowave irradiation method.SiO2/ZnPc-COOH was synthesizedwith APTES as the coupling agent.Singlet oxygen production ability,improved water solubility, good pho-toactivity.Postulated mechanism of the visible-light-induced photodegradation ofRhB.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 1 June 2013eceived in revised form 22 October 2013ccepted 28 October 2013vailable online 4 November 2013

a b s t r a c t

In this paper, we synthesized �-tetra-(p-carboxyphenoxy) phthalocyanine zinc (ZnPc-COOH) bymicrowave irradiation method, and SiO2/ZnPc-COOH by using 3-aminopropyltriethoxysilane (APTES)as the coupling agent. Their structural and optical properties were characterized by 1HNMR, MS, IR,SEM, TEM, XRD, TG, etc. The synthesized SiO2/ZnPc-COOH has a porous structure with a 13 nm film ofZnPc-COOH attached on the surface of SiO2. Infrared spectroscopy and TG analysis demonstrated that

eywords:iO2/ZnPc-COOHinglet oxygenhoto degradationhodomine B

ZnPc-COOH has been covalently bonded to SiO2 particles. The singlet oxygen quantum yields of ZnPc-COOH and SiO2/ZnPc-COOH are 0.54 and 0.21, respectively. The photocatalytic studies suggested anenhanced activity of SiO2/ZnPc-COOH nanoparticles due to its good absorption, better water solubility,enhanced photo stability and accelerated photogeneration of superoxide radicals (O−

2 •) by ZnPc-COOH.The present work shows that SiO2/ZnPc-COOH has great potential for application in photodynamic cancertherapy as well as in photo-degradation of water pollutes.

. Introduction

Phthalocyanines have attracted considerable interest for manyecades because of their remarkable optical and lasing properties,

hermal and chemical stabilities and they have been used as excel-ent dyes and pigments for many years [1]. Moreover, they haveot various applications in modern science and technology, such as

∗ Corresponding author. Tel.: +861064443134.E-mail address: wuwei@mail.buct.edu.cn (W. Wei).

927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.colsurfa.2013.10.056

© 2013 Elsevier B.V. All rights reserved.

photovoltaic materials in solar cells [2,3], systems for fabricationof light emitting diodes (LED) [4], photoconductors in xerography[5], nonlinear optical materials [6,7], sensitizers for photodynamiccancer therapy (PDT) [8–10], dyes at recording layers [11], as wellas diverse catalytic systems [12]. Although the singlet oxygen pro-duction ability and water solubility are always used to evaluatethe synthesized phthalocyanine [13], the production of superoxideradical O−

2 • by irradiation of phthalocyanine also plays a major role

for the photo-degradation of some pollutants [14–16].

A drawback for non-substituted phthalocyanines is that they areinsoluble in most common solvents, such as water or ethanol, andthe aggregation of phthalocyanine species has deleterious effects

Physi

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imopnictnipmc3fIsaSidas

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W. Qian et al. / Colloids and Surfaces A:

n their desired optical, electrical, and catalytic properties [17].hus, for the purpose of heterogeneous catalysts, researches focusn designing and synthesizing phthalocyanine with appropriateubstituent groups (e.g.–SO3H and –COOH) at the periphery ofhe macrocycle, which can be soluble in the polar solvents men-ioned above. Silica nanoparticles have been widely used as aarrier vehicle for drug delivery due to their unique advantages,uch as small yet uniform pore size, large specific surface areand pore volume, good water solubility and biocompatibility [18].n recent years, microwave-assisted organic synthesis (MAOS) haseen widely used for efficient synthesis of phthalocyanine [19,20].sing a microwave oven in MAOS not only reduces chemical reac-

ion times from hours to minutes, but also reduces side reactions,ncreases the yield, and improves reproducibility [21]. These advan-ages made MAOS as a forefront technology for rapid reactionptimization, for efficient synthesis of new chemical entities, asell as for discovering and probing new chemical reactivity.

The immobilization of phthalocyanines on a large area of sil-ca (SiO2) has been widely studied, taking into account its high

echanical, chemical and thermal resistances. Many works focusn finding an effective way of trapping phthalocyanines insideorous silica. Pal [22] prepared phthalocyanine and Silver/Goldanoparticles incorporated MCM-41with an in situ physical trapp-

ng method for the electrocatalytic reductions of O2 and CO, whichan lead to an enhanced electro-catalytic reduction for O2 reduc-ion. Ribeiro [23] covalently trapped the phthalocyanine into silicaetwork using the sol-gel process, it shows that the compos-

te retained all the optical property and thermal stability as forhthalocyanine. In most studies, the SiO2 nanoparticles were firstlyodified and then they were covalently bonded with phthalo-

yanine by sol–gel or adsorbing method [24–26]. The reagent of-aminopropyltriethoxysilane (APTES) has been most widely usedor the immobilization of phthalocyanine in silica network [27,28].n this study, ZnPc-COOH and SiO2/ZnPc-COOH nanoparticles wereynthesized and their morphology, structure, thermal stabilitynd optical property etc were investigated. The as-synthesizediO2/ZnPc-COOH shows the capability to produce singlet oxygen,mproved water solubility and photo-catalytic ability for the degra-ation of Rhodamine B in aqueous solution. We also proposed

mechanism that may explain the enhanced properties of as-ynthesized SiO2/ZnPc-COOH.

. Experimental section

.1. Materials

N,N-dimethylformamide (DMF), methanol (MeOH), acetoneC3H6O), hydrochloric acid (HCl), potassium carbonate (K2CO3),inc chloride (ZnCl2), 4-hydroxybenzoic acid (C7H6O3), hexade-yl trimethyl ammonium bromide (CTAB), tetraethylorthosilicateTEOS), rhodamine B (RhB) and ammonia water (25%) were pur-hased from Beijing Chemical Reagent Company (China) and useds received. 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU, 99%) and-nitrophthalonitrile (C8H9N3O2, 99%) were purchased from Huaeng agent company. 1,3-Diphenylisobenzofuran (DPBF, 95%) and-aminopropyltriethoxysilane (APTES, 99%) were obtained fromCROS and used as received.

.2. Synthesis

.2.1. Synthesis of 3-(4-carboxyphenoxy) phthalonitrile (3)

3-Nitrophthalonitrile (2.2 g, 12.7 mmol) 1, 4-hydroxybenzoic

cid (1.75 g, 1 mmol) 2, finely ground K2CO3 (7.9 g, 57 mmol) andry DMF (10 mL) were mixed and irradiated at 400 W for 10 minnder N2 atmosphere in a microwave oven (WBFY205, 2.45 GHz,

cochem. Eng. Aspects 443 (2014) 52– 59 53

Beijing Rui Chen Wei Ye Equipment Company) equipped with areflux condenser. Pure water (150 mL) was poured into the solu-tion after cooling down and the pH of the mixture was adjustedto 1.00 by HCl. Flocculent precipitate was obtained and filtered toget the solid products. Methanol was used many times for productrecrystallization. The yield was 62.8%.

2.2.2. Synthesis of ˛-tetra-(p-carboxyphenoxy) phthalocyaninezinc (ZnPc-COOH) (4)

The synthetic route for ZnPc-COOH is shown in Scheme 1. 3-(4-Carboxyphenoxy) phthalonitrile (3) (450 mg, 1.67 mmol) and ZnCl2(57 mg, 0.42 mmol) were dissolved in dry DMF (4 mL) and 1, 8-diazabicyclo [5,4,0] undec-7-ene (DBU) (0.6 mL) was added. Themixture was then heated and stirred at 400 W in the microwavefor 5 min under N2 atmosphere. After reaction, the mixture wascooled to 30 ◦C and precipitated by adding HCl (1 M). Solid productwas filtered and washed with water until the pH was 7. The productwas then extracted by acetone in a Soxhlet extractor. Yield: 270 mg(58%). 1H NMR (DMSO-d6) 1 ppm: 8.027 (d, 2H), 7.952 (d, 1H), 7.885(m, 1H), 7.494 (d, 2H), 7.313 (d, 2H). Calc. for C60H32N8O12Zn: %C64.23, %H 2.88, %N 9.99; Found: %C 64.68, %H 3.01, %N10.13. MS(ESI-MS) m/z: Calc. 1121.9; Found: 1116.5.

2.2.3. Synthesis of SiO2/ZnPc-COOHCTAB (0.08 g) was added into a mixture of ethanol (13 mL) and

water (27.5 mL) at 30 ◦C. After 5 min stirring, TEOS (0.5 mL) wasdropped into the mixture and then ammonia water (0.5 mL) wasadded. The stirring speed was kept at 700 rpm for 3 h. The productwas centrifuged, washed with water and dried under vacuum at80 ◦C for 12 h. The product was then calcined at 500 ◦C for 3 h toremove the surfactant to get the SiO2 particles

To prepare SiO2/ZnPc-COOH, the SiO2 particles (1 g) was firstdispersed in 50 mL toluene, and APTES (0.35 mL) was added at80 ◦C. The reaction was kept for 12 h under continuous stirring,then the product was centrifuged, washed with water and driedin vacuum at 80 ◦C for 24 h. The modified SiO2 particles (1 g) weredispersed by ultrasonic irridation in DMF (15 mL) for 20 min, then0.187 g of ZnPc-COOH was added into the solution and the mix-ture was stirred in the dark for 6 h. The product was washed withwater and ethanol for three times. After drying under vacuum at80 ◦C for 6 h, the SiO2/ZnPc-COOH was obtained. The formation ofSiO2/ZnPc-COOH composite is shown in Scheme 2.

2.3. Characterizations

2.3.1. Products characterizationsThe morphologies of the synthesized particles were charac-

terized by transmission electron microscopy (TEM) (JEM-3010,Japan) and scanning electron microscopy (SEM) (S-4700, Japan).Thermal gravimetric (TG) analysis was conducted on a thermalanalysis instrument (STA-449C, Germany) in air at a scanning rateof 10 ◦C/min. X-ray powder diffraction (XRD) was carried out ona Rigaku 2304 diffractometer with CuKa radiation (Ni-filtered).Infrared spectra (IR) were recorded on a Nicolet-8700 FT-IR spec-trometer. 1H NMR spectra were recorded in DMSO-d6 solutions ona AV300/AV600 spectrometer (Bruker, Germany). Elemental analy-ses were obtained with a vario ELcube Instrument from Elementarcompany (Germany).

2.3.2. Singlet oxygen quantum yield measurementsThe determination of its Singlet oxygen quantum yield

(˚�) determinations was performed using the chemicaltrapping method. 2 mL ZnPc-COOH solution that containeddiphenylisobenzofuran (DPBF) with a concentration lower than∼3 × 10−5 mol/dm3 was irradiated at 670 nm in air-saturated

54 W. Qian et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 443 (2014) 52– 59

Scheme 1. Microwave assisted synthetic route to ZnPc-COOH.

SiO2/Z

Da

�

w(istrtw

2

aHeout3ttRTc

3

3

SCZ

Scheme 2. Scheme of

MSO. ϕ� values were obtained by the relative method using ZnPcs the reference (Eq. (1)):

� = �ref�

k

kref

Irefa

Ia(1)

here �ref�

is the singlet oxygen quantum yield for the standard0.67 for ZnPc in DMSO) [29], k and kref are the DPBF photo bleach-ng rate constants in the presence of the respective samples andtandard, respectively; Ia and Iref

a are the rates of light absorption athe irradiation wavelength of 670 nm by the samples and standard,espectively. The initial DPBF concentrations are kept the same forhe ZnPc reference and the samples. DPBF degradation at 415 nmas monitored.

.3.3. Photocatalytic decomposition monitoringIn a typical run, a suspension containing catalyst (50 mg)

nd 50 mL aqueous solution of RhB (5 mM/L) was prepared.2O2 (30 mM/L) was used throughout this experiment to accel-rate the degradation rate. Irradiation experiments were carriedut in a homemade reactor. A 500 W xenon arc lamp wassed as the irradiation source. Before the irradiation, the mix-ure of catalyst and dye solution was stirred in the dark for0 min to get an adsorption–desorption equilibrium. At certainime intervals, small aliquots (2 mL) were withdrawn and fil-ered to remove the catalyst particles. The concentrations of thehodomine B were analyzed by UV–Vis spectroscopy at 553 nm.he experiment was conducted at room temperature and ambientondition.

. Results and discussion

.1. SEM, TEM and HRTEM characterization

Fig. 1 shows the SEM, TEM and HRTEM images of the synthesizediO2 particles and SiO2/ZnPc-COOH composite particles. ZnPc-OOH loaded silica nano-particles were synthesized by graftingnPc-COOH onto the APTES modified silica spheres. From Fig. 1(a1,

nPc-COOH formation.

b1), we can see that no obvious change was observed for theSiO2/ZnPc-COOH particles except some of the composite particleswere broken. The reaction between –COOH and –NH2, which makesthe ZnPc-COOH firmly adhered to SiO2 surface, may cause thedeformation of the SiO2 particles. The TEM images in Fig. 1(a2, b2)clearly present the difference and the deformation on their surface.By analyzing 100 individual particles, we got the size distributionfor each image obtained by follows a Gaussian distribution (notshown), and the mean diameter for SiO2 is 243 nm and SiO2/ZnPc-COOH particles is 260 nm. Fig. 1(a3) shows a HRTEM image of SiO2,which exhibits a porous structure with round smooth surface. Anobvious film with a width of about 13 nm can be observed on thesurface of SiO2 from the HRTEM image of SiO2/ZnPc-COOH (b3),which should be due to the attachment of ZnPc-COOH on SiO2surface.

3.2. XRD and TG characterization

The respective narrow-angle X-ray diffraction patterns of thepure SiO2 nano-particles and SiO2/ZnPc-COOH are shown in Fig. 2(A), which should correspond to the (1 0 0) reflection. (1 1 0) and(2 0 0) reflections were not distinguishable. Using a d100 value of54.8 A for SiO2/ZnPc-COOH, the pore diameter of SiO2/ZnPc-COOHwas evaluated to be 4.8 nm, while the pore diameter of pure sil-ica was 2.1 nm (d100 = 37.6 A) [30,31]. The widening of the poresize might be due to the presence of ZnPc-COOH in the hexagonalmesoporous structure. Fig. 2 (B) shows the TG curves of (a) NH2-SiO2, (b) SiO2/ZnPc-COOH and (c) ZnPc-COOH. For the TG curve ofNH2-SiO2 (a), the weight loss is due to water evaporation and degra-dation of grafted APTES on the silica base. For ZnPc-COOH graftedSiO2 (curve b), one degradation stage starts around from 500 ◦Cand ∼7.2% weight loss can be observed, which can be attributed tothe degradation of the grafted ZnPc-COOH. As the curve c shows,ZnPc-COOH can be disintegrated over 300–400 ◦C. It is obvious that

the degradation temperature is higher for SiO2/ZnPc-COOH thanthat of ZnPc-COOH. The increased thermal stability indicates thatZnPc-COOH has been covalently bonded to the surface of the SiO2particles.

W. Qian et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 443 (2014) 52– 59 55

image

3

N(t1c3A1CtsCsOa(1mS

Fig. 1. SEM (a1, b1), TEM (a2, b2), and HRTEM (a3, b3)

.3. FT-IR spectra

The FTIR spectra of the pure SiO2 (curve a), APTES (curve b),H2–SiO2 (curve c) and SiO2/ZnPc-COOH (curve d) and ZnPc-COOH

curve e) are shown in Fig. 3. For the spectrum of ZnPc-COOH,he absorption peaks at about 706–863 cm−1, 921–985 cm−1,110 cm−1, 1135 cm−1 and 1427 cm−1 are assigned to phthalo-yanine skeletal vibration. Absorption peaks observed around423 cm−1, 3094 cm−1 and 1485 cm−1 are assigned to C–H, C–C,r–H stretch vibrations. The peaks at 1690 cm−1, 1606 cm−1,574 cm−1, 1284 cm−1 and 1324 cm−1 are assigned to the C=O,=N, C=C, C–O and C–N stretching vibrations respectively. All ofhese absorption peaks indicate the existence of the phthalocyaninekeletal as well as the designed structure of the synthesized ZnPc-OOH. The spectra for SiO2, NH2-SiO2 and SiO2/ZnPc-COOH presentome characteristic peaks, such as: (1) a broad band due to theH stretching of silanol groups bonded to the inorganic structure,nd also hydrogen bonds between adsorbed water at 3423 cm−1;

2) an angular vibration peak from adsorbed water molecules at632 cm−1; (3) an intense band related to the � as (Si–O–Si) asym-etric stretching at 1083 cm−1; (4) a shoulder band assigned to the

i–OH bending frequency at 956 cm−1; However, compared with

Fig. 2. (A) XRD spectra of SiO2 and SiO2/ZnPc-COOH; (B) TG cur

s of SiO2 (a1, a2, a3) and SiO2/ZnPc(COOH) (b1, b2, b3).

the spectrum of SiO2, NH2–SiO2 and SiO2/ZnPc-COOH show thescissor vibration of the NH2 terminal group of APTES (curve b) at1570 cm−1 [32] and the deformation mode of NH3

+ at 1472 cm−1.This clearly indicates that SiO2 has been modified by APTES andZnPc-COOH is anchored to the functionalized SiO2.

3.4. UV–Vis spectra study

The UV–Vis absorption spectra of ZnPc-COOH in acetone weregiven in Fig. 4. The phthalocyanine exhibits typical electronic spec-tra with two absorption regions, the Q band around 600–700 nmand the B band in the near UV region around 300–450 nm. Theaggregation behavior of ZnPc-COOH was investigated in acetone atdifferent concentrations. As the concentration increased, the inten-sity of the Q band also increased and there were no new bandsappeared due to the aggregated species observed [33]. As shownin the inset of Fig. 4, the Lamberte Beer law was obeyed, whichmeans that the composite was not significantly aggregated within

the concentration of 0.7 mmol/L.

Fig. 5 shows the UV–Vis absorption spectra of SiO2/ZnPc-COOHand ZnPc-COOH in four different solutions. In DMF, the dissolvedZnPc-COOH exhibits two clear peaks at 683 and 618 nm, which

ves of (a) SiO2, (b) SiO2/ZnPc-COOH, and (c) ZnPc-COOH.

56 W. Qian et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 443 (2014) 52– 59

F(

caoiamdsaSviTH

UC

Fig. 4. Aggregation behavior of ZnPc-COOH in acetone at different concentrations:

ig. 3. FT-IR spectra of (a) SiO2, (b) KH-550, (c) NH2–SiO2, (d) SiO2/ZnPc-COOH ande) ZnPc-COOH.

orrespond to the monomeric ZnPc-COOH, and one shoulder peakt 638 nm, which is attributed to dimers or aggregates. In the casef SiO2/ZnPc-COOH, the disappearance of the peak around 638 nmndicates that the mesoporous silica nano-particle provides a suit-ble environment for zinc(II) phthanolocyanine to be dispersedonomerically. The red shift of the monomers peaks is probably

ue to strong immobilization of ZnPc-COOH within mesoporousilica [34]. To test the difference of their solubility, ethanol, H2Ond PBS 7.4 buffer solutions were used. It needs to point out thatiO2/ZnPc-COOH showed good water solubility both in PBS 7.4 sol-ent and water. However, when ZnPc-COOH was dropped in water,t shows blurry peaks for its Q band due to its low solubility in water.he solubility behavior of SiO2/ZnPc-COOH improves a lot both in

2O and PBS buffer solution.

The photo stability study [35] was also investigated byV–Vis absorption spectra for both ZnPc-COOH and SiO2/ZnPc-OOH nanoparticles (T = 30 ◦C, under continuous illumination by a

Fig. 5. UV–Vis absorption spectra of (a) SiO2/ZnPc-COOH and (b) Z

0.5 × 10−6 (A), 1.5 × 10−6 (B), 3 × 10−6 (C), 4.5 × 10−6 (D), 7.0 × 10−6(E) mol/L (inset:plot of absorbance versus concentration).

500-W xenon arc lamp, intensity 25 mW/cm2). The absorptionspectra were measured in the range � = 300–800 nm. DMF, andethanol were used as the solvents. The concentration of both ZnPc-COOH and SiO2/ZnPc-COOH was fixed between 1 and 1.5 to achieveinitial absorbance values. The effective reaction rate constant kparameter (At = A0exp(−kt) was used to evaluate and compare thephoto stability of the phthalocyanines explored in our experiments,where At and A0 are the absorbances measured for the Q-band ofthe complex at time t and t = 0, respectively. As shown in Fig. 6, bothZnPc-COOH and SiO2/ZnPc-COOH performed better photo stabilityin EtOH than in DMF. This is because that EtOH is a more effec-tive �-donating ligand, and it can reduce the polarization of the

�-electronic cloud inside of the macrocycle and raise the electronicdensity of the conjugated C–N bonding system [36]. As for DMF orEtOH, the SiO2/ZnPc-COOH composite always perform better photo

nPc-COOH in DMF, ethanol, H2O, and PBS 7.4 buffer solution.

W. Qian et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 443 (2014) 52– 59 57

FZC

sS

3

et4DnTCpod

3

gti(Z

ig. 6. Kinetic curves of photolyzed (a) SiO2/ZnPc-COOH in EtOH (ke = 0.0008), (b)nPc-COOH in EtOH (ke = 0.001), (c) SiO2/ZnPc-COOH in DMF (ke = 0.003), (d) ZnPc-OOH in DMF (ke = 0.022).

tability than ZnPc-COOH, this might because the bond betweeniO2 and ZnPc-COOH can protect the dye from being photolysed.

.5. Singlet oxygen quantum yields

As an efficient singlet oxygen capture agent, DPBF can form anndoperoxide upon cycloaddition with singlet oxygen, resulting inhe complete disappearance of its characteristic absorption band at15 nm. A family of spectra that we obtained for SiO2/ZnPc-COOH inMSO was shown in Fig. 7. The absorption of SiO2/ZnPc-COOH doesot change significantly until the majority of DPBF is decomposed.he singlet oxygen quantum yield of ZnPc-COOH and SiO2/ZnPc-OOH was found to be ˚� = 0.54 and ˚� = 0.21 respectively. Theresence of SiO2 decreased the singlet oxygen quantum yieldf ZnPc-COOH. This might because SiO2 stemmed the light andecrease the singlet oxygen product efficiency of ZnPc-COOH.

.6. Photo-catalytic activity

In our experiment, RhB was selected as the model to investi-ate the catalytic performance of SiO2/ZnPc-COOH. The changes of

he concentration of RhB under various conditions are displayedn Fig. 8. RhB was hardly degraded by H2O2 alone in the darkcurve a) or with light (curve b), as well as in the condition ofnPc-COOH with H2O2 (curve c) because of the strong aggregation

Fig. 7. Time evolution of the UV–Vis spectra of DPBF in DMSO solution.

Fig. 8. Degradation profiles of RhB at different conditions: a, H2O2 + dark; b,H2O2 + light; c, ZnPc-COOH + H2O2 + light; d, SiO2 + H2O2 + dark; e, SiO2/ZnPc-COOH + H2O2 + dark; f, SiO2/ZnPc-COOH + light; g, SiO2/ZnPc-COOH + H2O2 + light.

of the ZnPc-COOH in water, which had almost no catalytic activ-ity for the oxidation. In the present of SiO2 in the dark (curve e),the concentration of RhB was reduced rapidly within 30 min, andthen kept almost constant during the remaining time, suggestingthat the adsorption process had reached a dynamic equilibrium.When SiO2/ZnPc-COOH was also present in the dark, it reached theequilibrium within 30 min (curve d), which was similar as curvee; however, the adsorbed amount (41%) was less than the originalSiO2 (46%). In addition, as shown in Fig. 8g and f, in the experimentusing SiO2/ZnPc-COOH as the photo-catalyst but without hydro-gen peroxide, the degradation of RhB was much lower than thatobserved in presence of H2O2. We suppose that the effect of H2O2addition is not only in the formation of hydroxyl radicals, but also itcan accelerate the production of O−

2 • by ZnPc-COOH, which will beshown in the following section. From the results described above,it could be inferred that the decoloration of RhB in the SiO2/ZnPc-COOH/H2O2 system involves two processes: adsorption of RhB ontothe SiO2/ZnPc-COOH/H2O2 and catalytic oxidation of the adsorbate.Absorption of SiO2 may drive the molecular RhB to a spot very nearthe ZnPc-COOH photo-sensitizer, which will increase the degra-dation opportunity of RhB. Meanwhile, SiO2/ZnPc-COOH has abetter water solubility and the catalytic activity can be significantlyenhanced compared with ZnPc-COOH for the photo-degradationof pollutes in water. The suspended SiO2/ZnPc-COOH particles canbe easily separated from the bulk solution. The reusability of theSiO2/ZnPc-COOH was verified in six recycling experiments. Thecatalyst was separated by centrifugation and reused in subse-quent reaction without further treatment. The catalytic activity wasnearly unaffected and the degradation rate in the next 5 reactionsrun was not suppressed.

3.7. Proposed mechanism for photodegradation of RhB

Although the synthesized ZnPc-COOH and SiO2/ZnPc-COOHhave high singlet oxygen quantum yield, which may make it appli-cable in PDT treatment, it is suggested that singlet oxygen plays nomajor role in the photodynamic activity of phthaloeyanines [37].On the basis of our results and earlier reports on the photocat-alytic oxidation of RhB by phthalocyanines, a proposed mechanismwas elucidated in Scheme 3 for the photodegradation of RhB in thepresence of SiO2/ZnPc-COOH/H2O2 system. When irradiated with

light, MPc complexes can be excited to the singlet state (1MPc*)(1), which then undergoes intersystem crossing to the triplet state(3MPc*) (2). The 3MPc* might produce radicals on interaction withthe ground state molecular oxygen (3). The 3MPc* species could

58 W. Qian et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 443 (2014) 52– 59

adatio

a(rsHwosiaoRt(

4

iopsttoddisrpsa

A

2

R

[

[

[

[

[

[

[

[

[

Scheme 3. Postulated mechanism of the visible-light-induced photodegr

lso interact with the substrate molecule generating radical ions4), which then further formed superoxide (O−

2 •)(5), hydroperoxyladicals (HOO•) (6) and hydroxyl radicals (OH•) (7). The latter sub-equently afforded oxidation of the substrate [38]. The present of2O2 can produce more OH• (8); however, H2O2 can also interactith OH• to form HOO•(9). Oxygen can be formed by the interaction

f OH• and HOO•(10). In conclusion, the photodegradation processtarts with the photogeneration of superoxide radicals (O−

2 •) by themmobilized phthalocyanine species anchored on SiO2 upon irradi-tion. The presence of H2O2 can accelerate the reaction in two ways:ne is that it can produce more OH• for direct photodegradation ofhB and the other way may be that it can produce more oxygen forhe system to accelerate the photogeneration of superoxide radicalsO2

− •) by ZnPc-COOH.

. Conclusions

We have successfully synthesized ZnPc-COOH using microwaverradiation assisted method and covalently bonded ZnPc-COOHnto SiO2 nanoparticles. The synthesized SiO2/ZnPc-COOH has aorous structure with a 13 nm film of ZnPc-COOH attached on theurface of SiO2. The pore structure of SiO2 was enlarged and thehermal stability and photo stability of ZnPc-COOH were enhancedhrough the formation of SiO2/ZnPc-COOH composite. The singletxygen quantum yield of ZnPc-COOH and SiO2/ZnPc-COOH wasetermined to be 0.54 and 0.21, respectively. Meanwhile, degra-ation performance of RhB shows that the composites exhibit

mproved photoactivity in aqueous solution because of its goodolubility in water and accelerated photogeneration of superoxideadicals (O−

2 •) by ZnPc-COOH. It is expected that our synthesizedroducts may have a great potential to perform as the photo-ensitizer in PDT treatment, as well as promote their industrialpplication to eliminate the organic pollutants from waste water.

cknowledgment

The authors acknowledge the financial supports from NSFC (No.1076021 and 21376025).

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