Thin Solid Films 534 (2013) 301–307

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Degradation of TiO2 and/or SiO2 hybrid films doped with different cationic dyes

Violeta Purcar a,b, Simona Caprarescu c,⁎, Dan Donescu b, Cristian Petcu b, Ioan Stamatin a,Raluca Ianchis b, Hermine Stroescu d

a University of Bucharest, Faculty of Physics, 3Nano-SAE Research Centre, P.O. Box MG-38, 077125 Magurele, Romaniab National R&D Institute for Chemistry and Petrochemistry ICECHIM – Polymers Department, Splaiul Independentei 202-6, PO Box 174/35 Bucharest, Romaniac University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-7 Polizu St., 011061, Bucharest, Romaniad Institute of Physical Chemistry “Ilie Murgulescu” of the Romanian Academy, Splaiul Independentei 202, 060021 Bucharest, Romania

⁎ Corresponding author. Tel.: +40 766777128; fax: +E-mail addresses: violetapurcar@yahoo.com (V. Purc

(S. Caprarescu), ddonescu@chimfiz.icf.ro (D. Donescu), cistarom@3nanosae.org (I. Stamatin), ralumoc@yahoo.comhermine25@yahoo.com (H. Stroescu).

0040-6090/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tsf.2013.01.098

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 February 2012Received in revised form 20 January 2013Accepted 25 January 2013Available online 4 February 2013

Keywords:PhotocatalystsSilica hybridsDyeSol–gel

Hybrid thin films, silica–titanium oxides and silica–aluminum oxides, designed based on the sol–gel process areevaluated as catalysts in the photo-degradation of the cationic dyes. Silica matrices from different precursorswith various organic functional groups and cross-linked with titanium or aluminum agents (tetraisopropylorthotitanate and aluminum sec-butoxide) allow the surface property tailoring related to the high capacity ofthe dye adsorption respective, high photo-degradation activity. The cationic dyes (methylene blue, rhodamine B,crystal violet, malachite green) embedded on the hybrid silica matrix, under ultraviolet light, have a first order ki-netics of photodegradation. The cross-linking agents play a key role in the photocatalytic degradation and silicama-trix as dye absorbent. The photo-degradation rate for the binary system derived from methyltriethoxysilane/vinyltriethoxysilane precursors with both cross linkers showed a significant improvement by comparison withother hybrid materials. The significant increasing in the photodecomposition rate is related to the capacity togenerate additional oxidizing species by each silica hybrid compounds.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Dyes are extensively use in industry ranging from textile, leather,paper, printing inks, plastics, paints to cosmetics, pharmaceutical, andfood industries [1–3]. Often, the waste water is distinctively coloredto identify the origin of the pollutant source or to make distinctionbetween waste water and tap water [4]. Unfortunately, even naturaldyes have a toxicity level and often are not environmentally friendly.Regarding to the synthetic dyes, they have an increased toxicity evenat very low concentration on the ecosystems. The research addressesdifferent methods based on photocatalysis to oxidize the syntheticdyes in less toxic and biocompatible products. The photocatalysis in-volves high reactive radical species (such as hydroxyl and dioxygenanion radicals) which oxidize dye molecules leading to CO2, H2O, andother byproducts. In addition, by mineralization on the inorganicphotocatalyst with subsequent degradation, the water toxicity and itscolor are largely removed [5–7]. The yield of the dye degradation byphotocatalysis is dependent of: 1) photocatalyst particle size and theconcentration; 2) the dye concentration loaded on catalyst; 3) thesolution pH, temperature; 4) the light intensity and irradiation time;

40 214023939.ar), scaprarescu@yahoo.competcu@chimfiz.icf.ro (C. Petcu),(R. Ianchis),

rights reserved.

and 5) the amount of oxidants, impurities—organics and inorganics[8,9]. Each photocatalyst has an appropriate specificity for a range ofdyes and two methods are used: aqueous colloidal suspensions andthin films. Oxides and sulfur semiconductors (SnO2, ZrO2, CdS andZnO) showed photocatalytic activity under light irradiation but titani-um dioxide still remains representative due to its ability to remove alarge class of organic pollutant [4,10,11]. TiO2-hollow spheres in asso-ciation with peroxydisulfate (S2O8

2−) [12] or with oxyhalogens(KClO3, KBrO3, KIO4) and H2O2 as electron scavenger [13] showed ahigh degradation efficiency for methylene blue. Nitrogen dopedTiO2 also is reported as photocatalyst for the methylene blue degra-dation [14]. ZnS–CdS composites [15] and photo-Fenton systems(Fe+ and Fe+/H2O2) under visible irradiation (λ>470 nm) [16]were used formalachite green and brilliant green degradation. Aqueoussuspensions with TiO2 nanoparticles under aerobic conditions decom-pose the malachite in 1 h exposure to UV light. The surface topographyand high specific surface area needed to get a high efficiency [17]. Also,rhodamine B was degraded by photocatalysis with TiO2 and SiO2/TiO2

nanoparticles [18] as well as microporous TiO2 thin films under UVirradiation [19]. Thin films prepared by sol–gel process lead toimproved efficiency for degradation other dyes such as crystal violet(CV, hexamethyl-p-rosaniline chloride in aqueous solutions): Ag+

doped TiO2 under UV solar light [20], TiO2 ultrafine powder (Ti–SG)100% anatase [21]. The photocatalytic thin films, deposited on differentsupports by sol–gel process, have some advantages over aqueoussuspensions: 1) no recovery of photocatalyst powder in water, 2) one

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less contamination during water purification and 3) photocatalysts canbe easily reconditioned. In addition, using the sol–gel process, a series ofstructured thin films, appropriate for each dye can be designed. In thisrespect the couple silica–titanium and silica–aluminum oxides derivedfrom the sol–gel process are designed for the dye photo-degradationand their photocatalytic reaction kinetics is evaluated by UV–vis spec-troscopy. This study focused on a few representative cationic dyes:methylene blue BP, rhodamine B, crystal violet, and malachite green.Silica–titania and silica–alumina hybrid materials work in synergicway by combining two functions: high rate of the dye absorption fromsilica and high rate of photo-degradation from titania/alumina underUV–irradiation. The silica matrices from single and binary precursors,cross-linked with titania and alumina containing agents are designedin a common sol–gel scheme (see Scheme 1).

2. Experimental details

2.1. Materials

Precursors for silica: methyltriethoxysilane (MTES), vinyltriethoxy-silane (VTES), Merck; phenyltriethoxysilane (PTES), octyltriethoxy-silane (OTES) 3-glycidyloxypropyltrimethoxysilane (GMPS), Flukareagents.

Cross-linkers: tetraisopropyl orthotitanate (TIP) and aluminumsec-butoxide (ASB), Fluka reagents.

Complexing reagent: maleic anhydride (MA), Fluka.Benzoinedimethylacetal (FIN) (ICPAO), was used for the sol–gel

formulation with VTES as UV photo initiator.Dyes solutions (concentration ~10−3 mol/l): methylene blue BP

(MB), crystal violet (CV), Loba Feinchemie, rhodamine B (RB), GliwicePolska, malachite green (MG), Kordon Corp. are used for references inthe spectroscopic analysis and as doping agents.

Scheme 1. Hybrid films from sol–gel process, doped w

2.2. Synthesis of hybrid films doped with cationic dyes

The hybrid films were prepared by the sol–gel method in similarway to that reported elsewhere [22,23]. Typically, MTES (9 ml) ispre-hydrolyzed in acidic conditions with ethanol (5 ml) and 0.1 MHCl (0.56 ml) for 1 h under continuous stirring followed by thecomplexing agent addition, MA (0.152 g). In the next stage, undercontinuous stirring, the cross-linking agent [TIP (0.8 ml) or ASB(0.7 ml) dissolved in 3 ml CCl4] with 0.1 M HCl (1.2 ml) and mixedfor another hour is added in dropwise. (ASB is very viscous and astock solution of ASB–CCl4 (0.7 ml) was used as cross-linker agent).For the binary precursors (MTES/OTES, MTES/GMPS, MTES/VTES andMTES/PTES), they are mixed in 1:1 molar ratio and the procedure isrepeated as described above for MTES keeping the same quantities.The binary mixture MTES/VTES is photo-polymerized under UV, there-fore an amount of 0.24 g FIN was added in the second step in sol solu-tion to initiate VTES-radical polymerization. In the final stage each solwas doped with a diluted solution of dye (~10−3 mol/l) as follows:0.5 ml sol solution is mixed with 0.5 ml dye solution (5 μmol dye) for15 min (Scheme 2). Each solution spread on the glass slides is driedovernight at room temperature to obtain hybrid films doped. Eachfilm is exposed to the UV lamp in a black box from an hour to 8 h andanalyzed by UV–vis spectroscopy estimating the degradation kinetics.

2.3. Characterization

The degradation of the dyes embedded in the hybrid films wasperformed under UV lamp irradiation, λ=365 nm (from VilberLourmat). The absorption spectra of the hybrid films as well as thekinetics of the process are evaluated using a UV–VIS spectrophotom-eter (Nicolet Evolution 500, Thermo Electron Corp.) in the range400–750 nm. The film thickness was determined by spectroscopic

ith dyes, photo-degradation under UV irradiation.

Scheme 2. Synthetic route for the hybrid films doped with different dyes.

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ellipsometry measurements in the UV–VIS–NIR spectral range, usinga J.A. Woollam Co. VASE (variable angle spectroscopic ellipsometer)with a wavelength resolution of 5 nm at two angles of incidence (65°and 70°) (Table 1). The surface areas of the samples deposited ontoslide glass were 25×75 mm. The decay of the photochromic effectwas monitored vs. time at the wavelength of the maximum absorp-tion of each sample at 25 °C. The kinetic constants were calculatedfrom the absorbance decay curves.

3. Results and discussion

3.1. UV–VIS

In Fig. 1 is shown the absorbance spectra under UV irradiationrecorded at 0, 1, 3, 5, and 8 h, exposure time, for the representativebinary system MTES–VTES. The absorbance gradually decreaseswith the irradiation; it shows degradation of dyes monomers at awavelength characteristic (the amount of dye dimerization adsorbedin organic–inorganic hybrid materials is insignificant at low concentra-tions [24]). Dyes with positive net charge interact electrostatically withnegative charged silicamatrix leading to higher degradation rate (meth-ylene blue>malachite green>crystal violet>rhodamine B, Fig. 1) inagreement with other reports [25]. The position of the band also de-pends on the nature of the organic constituents from the matrix withred shift respective blue shift. The red shift in the absorbance band in-creases from MTES/VTES to MTES/PTES crosslinked with TIP anddoped with different dyes. This effect can be due to the interactionsbetween dyes and the phenyl rings from PTES (i.e., π–π interactions)which can induce a red shift in wavelength [26] and lowest kineticconstant rate (Fig. 3). Similarly, a red-shift of the characteristic peak

Table 1The thickness of hybrid films doped with dyes, using different cross-linker agents.

Precursor Dyes Thickness (+/−0.2)microns

TIP ASB

MTES MB, RhB, CV, MG 3÷5 3.5÷4MTES+OTES MB, RhB, CV, MG 4÷8 6÷7MTES+GMPS MB, RhB, CV, MG 9÷10 5÷8MTES+VTES MB, RhB, CV, MG 4÷5 0.5÷5MTES+PTES MB, RhB, CV, MG 6÷7.5 5÷10

takes place for methylene blue-doped hybrid films from MTES/OTES,MTES/GMPS, and MTES/VTES with ASB crosslinking agent by compari-son with the samples TIP-crosslinked. That is consistent with other re-ports where C and/or S atom doping is correlated with the red-shiftphenomenon [27].

Previous studies explained why organic dyes have the tendency toform aggregates even at low concentration. Aggregates were shownto cause fast thermal relaxation of electronic excitation energy [28].Comparatively, a blue shift of the wavelength of the characteristicpeak was observed in particular case of dye-doped hybrid filmsbased on MTES and MTES/PTES in the presence of ASB, in comparisonwith the samples with TIP. This effect can be explained by a decreaseof polarity. In case of hybrid films doped with rhodamine B, crystal vi-olet and malachite green, no significant modifications were observedbetween the wavelengths of the samples regardless of the use of TIPor ASB. The incorporation of organic (R) groups into the silica matrixresults in a polarity decrease, which is a function of the R loading [29].The OH ratio in the pore surface has amuch lower effect on the polarity.Hydrolyzing the aluminum alkoxides in an organic solvent with con-trolled water quantities, two reactions can occur [30]. After a fewhours of aging, the precipitate is converted into pseudoboehmite andit is the end product for a water-to-aluminum ratio of 3.0±0.5 in theinitially amorphous phase. Comparatively, an aluminum trihydroxidebayerite phase is finally formed for water-to-aluminum molar ratios of20 or more when the temperature stays under 350 K [30]. However,these processes are influenced by the presence of anions that tend tocoordinate with aluminum atoms.

After UV irradiation, a decrease of absorbance was observed for theall obtained hybrid systems. Amore evident decrease of the absorbancewas observed in the case of the hybrid films based onMTES/VTES dopedwith different dyes in the presence of cross-linking agents (TIP andASB)as compared with the other hybrid films. UV–VIS spectra of these hy-brid films recorded before and after irradiation at different times(1 h, 3 h, 5 h and 8 h) are represented in Fig. 1. When ASB is usedas cross-linking agent, the reaction took place very fast under UVirradiation in comparison to that of TIP. This effect shows that TiO2

acts as a photocatalyst in the process.

3.2. The photocatalytic kinetics

The photodegradation of hybrid films doped with dyes was exam-ined by employing a pseudo-first-order kinetic model.

ln At=A0ð Þ ¼ −kt

where A0 and At are the dye absorbances at 0 time and t time, respec-tively; k is the reaction rate constant.

Fig. 2 shows the photodegradation kinetics, ln(At/A0) vs irradia-tion time (t). All binary systems have a pseudo-first-order kineticsbehavior. The rate constants are summarized in Fig. 3.

The highest photoactivity is reached for MB by MTES and MTES/VTES crosslinked with TIP and MTES/VTES crosslinked with ASB(Fig. 3). It can be concluded that there is a strong correlation betweenthe photocatalytic activity and the content of titanium oxide (anatasecrystallinity in the titania hollow spheres has been proved to reachhigh photocatalytic activity [12]). Also MTES/VTES–TIP has a highphotoactivity for the decomposition of RhB, CV and MG.

The high photocatalytic activity can be assigned to the high specificarea of the films induced by silica clusters (Fig. 3, Scheme 1) respectivedue to very high reactivity of the vinyl-terminated alkoxide groups(VTES). The vinyl-terminated alkoxide (VTES) can be easily accommo-dated in a growing inorganic network [22]. In opposite the crosslinkerASB induces for MB, CV, and RhB a low photodegradation rate withthe exception of MTES/VTES. In this case, it seems that the vinyl groupfrom alkoxide has dominant role in photodegradation.

Fig. 1. Absorbance spectra measured on MTES+VTES hybrid film doped with dyes, prepared with different cross-linking agents (TIP and ASB), before irradiation (0 h) and afterirradiation at different times: 1 h, 3 h, 5 h and 8 h, respectively.

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In the case of RhB doped samples, the lowest RhB decompositionphotoactivity was obtained for the sample containing MTES/GMPSfor both crosslinkers TIP and ASB (Fig. 3). These results reveal thatthe alkoxy and epoxy groups of GMPS can lead to a strong interaction

between RhB and Si\O\Si bonds. The glycidyl derivative (GMPS)can react with RhB by opening the oxyranic cycle [31–33] or byde-ethylation of the chromophore structure [34]. According to thestudy by Zhang et al. [35] RhB (N,N,N,N′-tetraethylated rhodamine

Fig. 2. Photodegradation of the hybrid films doped with dyes against UV irradiation time.

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molecule) shows a λmax at 552 nm. In the case of rhodamine B, thehighly delocalized \CO band of COO groups is mainly affected byUV radiation, which can correlate with the faster photo degradationof samples containing MTES/GMPS. Analyzing Fig. 3, the lowestphotoactivities were obtained for the sample prepared with MTES/PTES in the presence of TIP as compared to other samples with TIP.Ph groups hinder the movement of the photochromic molecules in-side of the silica pores, resulting in slower isomerization kinetics incomparison with other groups.

During the hydrolysis and polycondensation, TIP converts in smallinterconnected network of titanium oxide with silica as well TiO2

nanoparticles (in anatase or rutile form) decorated with a significantnumber of functional groups such as \OH and\O\C3H7 (\OR). Theyrelease a large amount of Ti3+ ions under UV irradiation [36] which in-duce a high level of condensation and hydrogen photogeneration(Nishimoto et al. reported that anatase shows high activity for hydrogen

photogeneration from aqueous alcohols with respect to the amount ofthe active amorphous TiO2 and rutile phase) [37,38].

When ASBwas used as cross-linking agent, the highest photoactivityfor decomposition of MB, RhB, CV and MG was obtained for sampleswith MTES/VTES. MTES/OTES–ASB sample doped with MB (Fig. 3) hasthe lowest decomposition photoactivity. In this case, it is assumed thatOTES with eight carbon atoms promotes a slower hydrophobic associa-tion, in the presence of ASB. In the case of malachite green and crystalviolet (which have nonlinear structures), the lowest photoactivity fordecomposition of dye was observed for MTES/PTES as compared toother samples with ASB. These results were also obtained when TIPwas used as cross-linking agent and pointed out that the larger size ofthe Ph groups are probably unable to accommodate the organic dyedue to steric effects.

Taking in account Scheme 1, the interaction between dyes andaluminum is stronger than dyes—TiO2. Presumably, aluminum led to

Fig. 3. Reaction rate constant of hybrid films for the photodegradation of dyes under visible light irradiation as a function of composition.

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formation of Al2O3 outerlayer where the dye is adsorbed, phenomenonalready studied indye-sensitized solar cells [39–41]. TheAl2O3-outerlayeris a barrier electron donors (dye) and acceptor (oxide matrix).

Based on these experimental results it can be emphasized that sever-al common characteristics andmechanisms of the dye photodegradationadsorbed on silica matrices crosslinked with titanium and aluminumoxide. Under UV irradiation the rate of the electron transfer to hybridfilm increases with the level of the dye adsorbed on silica matrix. Theamount of oxygen content is also one dominant factor [42].

Free •OH and O2 reactive radicals via reaction of electron transferto the oxides and functional groups are generated in the hybridfilms. The photocatalytic degradation occurs on the surface of the cat-alyst mediated by free radicals. The formation of •OH and O2 increaseswith the irradiation time speeding up the dye degradation. Hydroxylsare not selective radicals; therefore, they contribute to enhancementof organic decomposition by photodegradation [13].

The cross-linking agent is an important factor in the photocatalyticdegradation of the dyes.

Highly delocalized bands (C_C of VTES) are mainly affected by UVradiation and can be the origin of the photodegradation (see Figs. 2,3) in the case of all dye-doped hybrid films that contain MTES/VTESin the presence of TIP and of ASB. In MTES/VTES materials, the reac-tion took place very fast under UV irradiation by comparison withthe other samples where the reaction is slow to very slow.

The fact that the degradation efficiency is higher with TIP bycomparison with ASB shows that TiO2 acts as a photocatalyst. Dyemolecules are oxidized by •OH radicals on the surface of the catalyst[43,44].

Discrepancy between the pseudo-first-order reaction rate con-stants may be either due to different photocatalytic degradationmechanisms or to a competition for degradation between the reac-tant and the intermediate products.

The nature and amount of organic groups (R) in the organic–inorganic hybrid materials determine the polarity of the pore sur-faces and induce changes in the spectral and kinetic behavior ofthe photochromic molecules [45].

4. Conclusions

Different hybrid materials doped with organic photochromic dyes(methylene blue (MB), rhodamine B (RhB), crystal violet (CV) andmal-achite green (MG)) were synthesized by means of a sol–gel method.UV–vis spectroscopy demonstrated that the cationic dye molecules

have the tendency to form aggregates on the hybrid surface. Only thecontribution of the monomers from the dye molecules was studied,being insignificant that of dimmers. Photocatalytic decomposition ofdyes was carried out to evaluate the photocatalytic activity of silicafilms, by focusing on the effect of the cross-linking agent. Results dem-onstrated that the degradation rate depends on the number of highlyreactive radical and nonradical intermediates which are involved inthe photodegradation of dyes. The photodegradation reaction followeda pseudo-first-order kinetic reaction in case of all hybrid films. Thehighest decomposition photoactivity for all dyes, both in the presenceof TIP and of ASB, was obtained for MTES/VTES, with respect to samplescontaining other precursors. The ability to control the spectrokineticproperties of the photochromic hybrid films by varying the chemicalcomposition of the embedding silica matrix will be of great interestfor the design of materials with defined properties.

Acknowledgments

Violeta Purcar acknowledges the financial support of strategicgrant POSDRU/89/1.5/S/58852, Project “Postdoctoral programme fortraining scientific researchers” cofinanced by the European SocialFund within the Sectoral Operational Program Human ResourcesDevelopment 2007–2013.

Raluca Ianchis acknowledges the financial support of the Execu-tive Agency for Higher Education, Research, Development and Inno-vation Funding postdoctoral grants PNII-RU no. 44 (PD_206).

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