Chemical Engineering Journal 243 (2014) 537–548

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Chemical Engineering Journal

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How to modify the photocatalytic activity of TiO2 thin films throughtheir roughness by using additives. A relation between kinetics,morphology and synthesis

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.11.031

⇑ Corresponding author. Tel.: +32 4366 3540; fax: +32 4366 3545.E-mail address: charline.malengreaux@ulg.ac.be (C.M. Malengreaux).

Charline M. Malengreaux a,⇑, Géraldine M.-L. Léonard a, Sophie L. Pirard a, Iolanda Cimieri b,Stéphanie D. Lambert a, John R. Bartlett c, Benoît Heinrichs a

a Laboratoire de Génie Chimique, B6a, Université de Liège, B-4000 Liège, Belgiumb LumiLab, Department of Solid State Sciences, Ghent University, B-9000 Ghent, Belgiumc Faculty of Science, Health, Education and Engineering, University of the Sunshine Coast, Marochydore DC, Qld 4558, Australia

h i g h l i g h t s

� TiO2 anatase films were produced by two controlled sol–gel method and dip-coating.� Aim: improve the photocatalytic activity of TiO2 thin films by using organic additive.� Films were transparent, adherent, homogeneous with thickness from 30 nm to 200 nm.� Organic additive induces a modification of the active surface of the films.� Reaction rate constant can be multiplied by two due to the presence of the additive.

a r t i c l e i n f o

Article history:Received 10 April 2013Received in revised form 11 November 2013Accepted 15 November 2013Available online 23 November 2013

Keywords:TiO2

Thin filmAdditivePhotocatalysisSol–gelDip-coating

a b s t r a c t

The possibility to improve the photocatalytic activity of TiO2 thin films by using organic additives isinvestigated. Two non-aqueous sol–gel routes (In Situ and Ex Situ) have been selected to obtainorganic/inorganic stable sols and TiO2 thin films have been produced using the dip-coating processoptimized in a previous study. The influence of the additive on the physico-chemical properties ofthe catalysts has been evaluated using XRD, GIXRD, UV–Vis spectroscopy, krypton and nitrogenadsorption–desorption, profilometry and SEM. The photocatalytic activity has been assessed bymonitoring the degradation of 4-nitrophenol (4-NP) under UV light and UV–Visible light over time.The performances of the catalysts have been evaluated using the values of the reaction rate coefficientestimated through the adjustment of the first order kinetic model taking in account the variation ofthe volume inside the reactor due to sampling. The results highlight that the performances of thecatalysts were better that the performances of the commercial photocatalytic coating (Saint Gobain GlassBioclean�) which constitutes a significant step forwards for the production of photocatalytic coating.Moreover, the results highlight that the organic additive induces an alteration of the surface roughnessand hence, a modification of the active surface of the films. In the case of the In Situ synthesis an increaseof the active surface is associated to a 100% improvement of the photocatalytic activity. On the opposite,for the Ex Situ synthesis, a decrease of the active surface is associated to a 50% decrease of the photocat-alytic activity due to the presence of the additive.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Environmental pollution has become a critical issue for oursociety. In the case of water pollution, various methods such asbiodegradation, coagulation, adsorption, advanced oxidationprocesses (AOP) and membrane processes have been investigatedto remove the pollutants from water [1]. Heterogeneous

semiconductor photocatalysis is an AOP, which was proven to bea promising technology for the total mineralization of most ofthe organic pollutants by using natural or artificial light [2]. A qua-si-exhaustive list of organic pollutants, which can be eliminated byusing heterogeneous photocatalysis, was reported by Blake [3].One of these organic pollutants, 4-nitrophenol (C6H5NO3, 4-NP),which is used in the chemical industry for the manufacture ofinsecticides, herbicides, synthetic dyes and pharmaceuticals, isone of the most refractory substances present in industrial waste-waters because of its high stability and solubility in water [4]. This

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pollutant is highly toxic for humans and is therefore considered asa priority toxic pollutant by the United States Environmental Pro-tection Agency (USEPA) [5].

In the case of heterogeneous photocatalysis, the most fre-quently used semiconductor is titanium dioxide, TiO2, due to itshigh oxidative potential, chemical stability, availability and its rel-atively low price [6]. When TiO2 semiconductor is exposed to UVlight, electron–hole pairs are generated at the surface of the cata-lyst as well as in the bulk. The electrons (e�cb) and holes (hþvb) canreact with H2O and O2, respectively to produce radical species suchas OH� and O��2 superoxides, which can then reduce and oxidize thepollutant adsorbed on the photocatalyst surface [1,7]. In order toinsure the efficiency of the semiconductor photocatalyst, the dif-ferent interfacial processes involving e�cb and hþvb must competeeffectively with the major deactivation process, which correspondsto the e�cb � hþvb recombination, occurring in the bulk or at the sur-face of the TiO2 particles [8–10]. Anatase and rutile are the mostcommonly used titania polymorphs, however, a major drawbackof pure anatase is that the bandgap energy (Eg) is about 3.2 eV,which requires the use of UV light (k 6 387 nm) to generate elec-tron–hole pairs in the TiO2 crystallites and hence, only 5–8% of so-lar photons have sufficient energy to activate the catalyst.Numerous studies have been undertaken to extend the activity ofthe TiO2 towards the visible range by either enlarging the rangeof light which can be absorbed by the catalysts or by enhancingthe electron–hole separation efficiency. Several approaches havebeen reported such as dye photosensitization on the TiO2 surface[11,12] or in the bulk [13], sensitization by electron transfer fromtransition metal salts [14,15], immobilization of metal particles[16], combination of different semiconductors [2] as well as theintroduction of metal ion into the TiO2 lattice using impregnation[17] or atomic substitution [18] methods. In the case of thin films,another approach reported in the literature to improve the activityof TiO2 catalyst is the modification of the thin film morphology byusing organic molecules, which can be degraded during the calci-nation step and hence, inducing an increase of the porosity androughness of the surface of the films [19–21].

The sol–gel route is an interesting approach for the synthesis ofpure and modified TiO2-based photocatalysts with controllednanostructures and surface properties. When precursors such astitanium alkoxides are used, the hydrolysis and condensation reac-tions lead rapidly to the formation of a white precipitate by reac-tion with water or even atmospheric humidity. The hydrolysisand condensation reactions being exothermic, favors the precipita-tion reaction due to the increase of the temperature. In order to en-sure a better control over the evolution of the reacting system andto prevent precipitation, it is desirable to slow down the hydrolysisand condensation reactions. In order to achieve this goal, sol–gelprocesses are performed using organic solvents to prevent precip-itation and complexing ligands (e.g. b-diketones or carboxylicacids) are used to lower the reactivity of the titanium alkoxideallowing the control of the evolution of the structure [8,22,23].

In this work, organic/inorganic hybrid TiO2 thin films have beenproduced using an optimized dip-coating process [22] and anorganophosphorylated additive (diethyl-(2-N-(aminoethyl))phos-phonate, EDAP) introduced by non-aqueous sol–gel routes (In Situand Ex Situ). The influence of the additive on the physico-chemicalproperties of the materials has been investigated using a suite ofcomplementary techniques and the photocatalytic activity hasbeen measured using a custom-designed multisamples photoreac-tor. The performance of the catalysts was compared using the reac-tion rate coefficient estimated through the adjustment of a firstorder kinetic model on the experimental data. It is really importantto note that, in the case of thin films, the determination of thekinetic parameters through the adjustment of a kinetic model onexperimental data collected during several hours has to be done

carefully. A previous study [24] has established that in order todetermine the kinetics parameters, the variation of the volume in-side the photoreactor due to sampling has to be taken in account inorder to determine the actual reaction rate coefficient value inde-pendently of the experimental conditions.

Few studies of the 4-NP photocatalytic degradation under UVlight in the presence of commercial TiO2 powder catalyst havebeen reported in the literature [9,25,26]. To our knowledge, a studyof the 4-NP photocatalytic degradation in the presence of modifiedTiO2 thin films catalyst under UV–Visible light and taking into ac-count the variation of the volume inside the reactor due to sam-pling has never been reported. The purpose of this article is toevaluate the influence of the organic additive on the physico-chemical properties of the films and to understand the relationshipbetween the synthesis route, the morphology of the films and thekinetics of the photodegradation of an organic pollutant (4-NP) inaqueous solution in the presence of the TiO2 thin films.

2. Experimental

2.1. Preparation of sols

All sampling of reagents and manipulations were performed atroom temperature under a constant nitrogen flow in order to avoidthe precipitation of the titanium alkoxide (titanium tetraisoprop-oxide, TTIP) precursor by reaction with the air humidity whenthe atmosphere is uncontrolled. Different molar ratios were usedin both syntheses. The dilution ratio, D, is defined as:

D ¼ ½solvent�ð½TTIP� þ ½additive�Þ ð1Þ

and the hydrolysis ratio, H, is defined as:

H ¼ ½H2O�ð½TTIP� þ n

4 ½additive�Þ ð2Þ

where n is the number of alkoxy functions bound to phosphorusatoms (2 for EDAP) [27].

2.1.1. In situ synthesisStable TiO2 sols were prepared based on the synthesis reported

in a previous study [22]. The chosen additive (EDAP) was dissolvedin dry isopropanol (IPA, Thermofisher) and vigorously stirred atroom temperature for 10 min. Titanium tetraisopropoxide(TTIP > 98%, Merck) was then dissolved into the mixture and vigor-ously stirred at room temperature for another 30 min. Pure acetyl-acetone (AcAc, Thermofisher) was added (AcAc:TTIP molar ratio,0.3) and the mixture was stirred for 1 h. Finally glacial acetic acid(HAc, Thermofisher) was slowly added into the solution (HAc:TTIPmolar ratio, 0.2) to initialize the hydrolysis via an esterificationreaction which leads to the In Situ production of water. In all syn-theses, D and H were equal to 40 and 0.2, respectively, and the mo-lar ratio [EDAP]/[TTIP] was equal to 0, 0.01 or 0.1 for samplesrespectively labeled: IN-Pure-TiO2; IN-1%-TiO2 and IN-10%-TiO2

sols. Transparent yellow solutions were obtained and entered in-side a glovebox (MB 200B, mBrAun, argon atmosphere, T = 17 �C,[H2O] < 0.1 ppm, [O2] < 0.1 ppm) where they remained liquid fortwo months.

2.1.2. Ex situ synthesisStable TiO2 sols were prepared through a synthesis adapted

from the cogelation method reported in [27]. EDAP was dissolvedin half of the total amount of 2-methoxyethanol (MetOH > 99%,Thermofisher) and vigorously stirred for 10 min. Titaniumtetraisopropoxide (TTIP > 98%, Merck) was then added to the mix-ture and vigorously stirred for another 30 min. The other half of 2-

C.M. Malengreaux et al. / Chemical Engineering Journal 243 (2014) 537–548 539

methoxyethanol was mixed with reagent-grade water in a secondvessel and also stirred vigorously for 30 min. The solution contain-ing the water was then added dropwise into the first vessel and themixture was stirred vigorously for 1 h. In all syntheses, D and Hwere equal to 40 and 2.2, respectively, and the molar ratio[EDAP]/[TTIP] was equal to 0, 0.01 or 0.1 for samples respectivelylabeled: EX-Pure-TiO2 ; EX-1%-TiO2 and EX-10%-TiO2 sols. Trans-parent brown solutions were obtained and entered inside theglovebox where they remained liquid for one month.

2.2. Preparation of thin films

Single layer and multilayer thin films were prepared inside theglove-box by using a dip-coating process (Dip-Coater KSV-DC, KSVInstrument) which has been optimized previously [22]. The opti-mal withdrawing speed was 60 mm min�1 and alkaline free glasssubstrates (Schott Glass AF37, 25 mm � 75 mm � 0.7 mm, alka-line–earth aluminosilicate glass, alkali- and arsenic free in synthe-sis) were used to prevent the migration of sodium ions from thesubstrate into the film during the heat treatment at 500 �C. Afterwithdrawing, the films were dried at room temperature for 5 hand then calcined in air at 500 �C for 1 h using a heating rate of10 �C min�1. Then the films were cooled down to room tempera-ture at a similar cooling rate. For the multilayer films the heattreatment was conducted between each dipping/withdrawing cy-cle into the precursor solution.

A coating cycle is then defined as the process (schematized inFig. 1) from the dipping/withdrawing steps to the calcination stepat 500 �C and the final substrates have a TiO2 covered surface of15 � 10�4 m2 on each side of the substrate. For each set of experi-mental variables, 10 replicates of the same film were produced inorder to evaluate the reproducibility of the process as well as thephotocatalytic experiments and therefore allowing statisticaltreatments of the measured data.

2.3. Film characterization

The adhesion of the coating to the substrates was assessed by ascotch tape test (3 M’s scotch tape). For each set of experimentalvariables, three replicates of each film were covered with a pieceof scotch tape and the adherence was considered adequate if theremoval of the tape did not cause visible damages to the coating[21].

Crystallographic properties of the films were investigated byusing grazing incidence X-ray diffraction (GIXRD, Bruker D8). The

Fig. 1. Experimental procedure for the production of single layer and multilayerfilms of TiO2.

diffractometer using the Cu Ka radiation was operated at 40 kV,40 mA with an incident beam angle of 0.3�. The size of TiO2 crystal-lites, dc, was estimated from GIXRD peak broadening by the Scher-rer method:

dc ¼ 0:9k

ðB cosðhÞÞ ð3Þ

with B the full-width at half maximum after correction of theinstrumental broadening, k the wavelength (nm) and h the Braggangle (rad) [28].

The thickness and roughness of the different film were mea-sured using a surface profilometer (Veeco, Dektak 150f, Stylus Pro-filer). In order to have representative values for the films thicknessand roughness, the measure was performed 10 times on one pointof each film and this procedure was repeated in three differentpoints of each measured films. The reported values of thicknessand roughness are the mean values calculated. The surface profi-lometer used was also equipped with a video camera allowing toobserve the surface of the films and to take pictures with an ampli-fication of 185�.

Scanning electron microscopy (SEM FEI Quanta 200 F) observa-tion enabled the morphology of the films surface to be analyzed inmore details.

Krypton adsorption–desorption measurement have been per-formed in order to evaluate the active surface area involved inthe photocatalytic degradation for different films (IN-Pure-TiO2,IN-1%-TiO2 and IN-10%-TiO2) at 77 K after outgassing using aMicromeritics ASAP 2020 equipped with a specifically designedsample holder. From the measured adsorbed volume of krypton,an estimation of the active surface area, Sactive (m2

active) per surfaceunit of covered substrate Ssubstrate (m2

substrate) can be obtained.

2.4. Preparation of powders

Some characterization techniques cannot be applied directly tothin films therefore, in order to perform diffuse reflectance spec-troscopy measurements, powders corresponding to the differentsols were prepared. It is important to note that films and powderscould present significant differences in term of morphology, crys-tallite size and structure and therefore the characterization of thepowders cannot be straightforward extrapolated to the films. Theresults obtained for the powder can only be used to determine ageneral tendency in the case of thin film.

In each case, 100 mL of sol were dried in a Petri dish at roomtemperature in order to obtain a gel. The gels were then calcinedwith the same thermal treatment as the corresponding films.

2.5. Powder characterization

The optical properties of the TiO2 powders were evaluated byperforming diffuse reflectance spectroscopy measurements in theregion 200–800 nm with a Varian Cary 500 UV–Vis-NIR spectro-photometer, equipped with an integrating sphere (Varian ExternalDRA-2500) and using BaSO4 as reference. The UV–Vis spectra re-corded in diffuse reflectance (Rsample) mode were transformed byusing the Kubelka–Munk function and then the direct opticalband-gap values, Eg (eV), were obtained using Eqs. (A1) and (A2)detailed in Appendix (see Appendix A.1).

2.6. Photocatalytic activity of the films

The photocatalytic activity of the different photocatalysts wasevaluated by monitoring the degradation of 4-nitrophenol (4-NP)in aqueous solution under two different light sources. A stock solu-tion of 4-NP with a concentration of 1.0 � 10�4 kmol m�3 was

540 C.M. Malengreaux et al. / Chemical Engineering Journal 243 (2014) 537–548

prepared by dissolving 4-NP in reagent-grade water. In this case,the pH is equal to 4.1 and only the protonated form of 4-NP is pres-ent in the solution. The measurements of the photocatalytic degra-dation of the pollutant under UV and UV–Visible light irradiationswere carried out using the experimental device, in which the lightsources are interchangeable. In order to assess the photocatalyticactivity of the films under UV light, the light sources used are UVfluorescent lamps (Osram Sylvania, Blacklight-Bleu Lamp, F18W/BLB-T8). The spectrum of the lamp has been measured using aspectrophotometer (mini-spectrophotometer Hamamatsu TG)and it was found that the light source yields a light covering thewhole UVA range (315–400 nm) with a peak emission atk = 365 nm (see Fig. A1 in Appendix A.2). In the case where thephotocatalytic activity of the films is evaluated under UV–Visiblelight, the light sources used are halogen lamps (Sensys, spots50 W, 12 V) and the spectrum of the lamp yielded a continuousspectrum from 330 nm to 800 nm (see Fig. A1 in Appendix A.2).Fig. 2 schematized the experimental set-up in the case of photocat-alytic experiments performed under UV–Visible light.

In each set up, four batch reactors (V0 = 24 � 10�6 m3 each)were placed at equal distances from the light source inside a blackbox and constantly homogenized using orbital stirring plates. In atypical experiment, two replicates of one film were placed in twoof the four batch reactors. In a third batch reactor an uncoated sub-strate was placed in order to performed blank reference measure-ment. All batch reactors were hermetically closed using lid, whichare transparent to both types of light, to avoid evaporation and theinternal temperature was maintained at 25 �C using air circulationensured by a fan. It has been calculated that, in each batch reactor,the quantity of O2 is in large excess with regard to 4-NP allowing usto assume that the concentration of O2 dissolved in the 4-NP solu-tion remains essentially constant during the degradation process.

Initially, the batch reactors were kept in the dark and the 4-NPconcentration and pH were monitored over a 6 h period to deter-mine if the 4-NP is adsorbed by the films or the substrates andto confirm that only the protonated form of 4-NP was present inthe mixture using the UV–Vis spectrum measured between 200and 500 nm. Indeed, if only the protonated form of 4-NP is presentin the solution, a single peak with a maximum absorbance atk = 317 nm is observed.

After these dark tests, the reactors were irradiated over 6 h andan aliquot of 1.0 � 10�6 m3 of 4-NP solution was sampled everyhour in each batch reactors using a micropipette. Blank measure-ments highlighted that no spontaneous 4-NP breakdown occursunder illumination in the absence of catalyst. Hence, the actualdegradation of 4-NP was determined by measuring the absorbanceof the solution using a Genesys 10S UV–Vis spectrophotometer(Thermo Scientific) at k = 317 nm (maximum of absorbance ofthe protonated form of 4-NP).

Fig. 2. Experimental design for measuring photoca

In order to compare the performance of the different TiO2 thinfilms produced with a commercial photocatalytic glass, Saint Gob-ain Glass Bioclean� was purchased and its photocatalytic activitywas measured without any treatment of the surface.

2.7. Total Organic Carbon measurements (TOC)

In order to confirm the complete mineralization of the pollutantdue to the photocatalytic activity of the different catalysts consid-ered, total organic carbon (TOC) measurements have been per-formed using a Hach DRB200 Reactor and a Total Organic CarbonDirect Method Test N Tube™ Reagent Set.

The TOC is determined by first sparging the sample underslightly acidic conditions to remove the inorganic carbon. In anoutside vial, the organic carbon contained in the sample is digestedby persulphate and acid to form carbon dioxide. During digestion,the carbon dioxide diffuses into a pH indicator reagent in an innerampoule. The adsorption of carbon dioxide into the indicator formscarbonic acid. Carbonic acid influences the pH of the indicator solu-tion which, in turn, changes the color. The amount of color changeis related to the original amount of carbon present in the sampleand can be evaluated using a Hach DR 2800 Spectrophotometerat k = 430 nm.

3. Results

3.1. Macroscopic observations on films

The six different sols corresponding to undoped and doped TiO2

enabled the production of homogeneous and transparent filmswith a smooth aspect and uniformly colored reflections due to lightinterference effects. The performed scotch tape (3 M’s scoth tape)tests showed that the films were well adhered to the substrates.

3.2. Grazing incidence X-ray diffraction on films

The effects of the synthesis method, the additive content andthe number of coating cycles on the crystallographic propertiesof the films were investigated. No reflections and especially no dif-fraction peaks corresponding to TiO2-rutile were detected. GIXRDpatterns show diffraction peaks corresponding only to TiO2-ana-tase phase for all the films after calcination at 500 �C.

As an example the influence of the number of coating cycles onGIXRD patterns is presented in Fig. 3 for films dipped one, two andthree times into the EX-Pure-TiO2 sol. As expected the intensity ofthe peak increases with the number of deposited layers due to theincrease of the film thickness. Indeed, by increasing the film thick-ness, the volume of TiO2 probed by the X-rays increases and a lar-

talytic activity under UV–Visible illumination.

Fig. 3. GIXRD patterns of TiO2 thin films corresponding to 1, 2 or 3 coating cyclesand calcined 1 h at 500 �C compared to the reference pattern of TiO2-anatase (ICDDfile No. 21-1272).

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ger number of crystallites are detectable leading to more intensediffraction peaks. Similar observations about the increasing inten-sity of the peak with increasing the number of coating cycles aremade for films obtained after one, two and three coating cycles,respectively, in each sol.

In order to determine the effect of the synthesis method on theGIXRD patterns, Fig. 4 presents the patterns corresponding to filmsdipped three times in the six different sols. A comparison betweenFig. 4a and b allows studying the influence of the synthesis meth-od. It can be observed that the intensity of the peak is higher for

Fig. 4. GIXRD patterns of TiO2 thin films obtained after 3 coating cycles andcalcined 1 h at 500 �C using the sols produced through (a) the Ex Situ synthesis or(b) the In Situ synthesis compared to the reference pattern of TiO2-anatase (ICDDfile No. 21-1272).

films obtained with the Ex Situ synthesis. Using GIXRD, the pene-tration depth of the X-rays into the films is really low and alwaysthe same if the angle is constant, which means that the intensity ofthe GIXRD peaks is related to the film thickness but also theamount of crystalline material contained in the volume of TiO2

probed by the X-rays. The films produced through the Ex Situ syn-thesis are then thicker and/or better crystallized than the ones ob-tained with the In Situ synthesis.

No effect of the additive content on the number and position ofthe diffraction peaks in the GIXRD patterns was observed in gen-eral. However, it can be observed on Fig. 4a that the intensity ofthe peak increases with the additive content for films obtainedafter three coating cycles. It is suggested that the addition of addi-tive in the TiO2 leads to an increase of the film thickness and/or to ahigher level of crystallization in the case of the Ex Situ synthesis,while no significant effect of the additive content was observedin the case of the In Situ synthesis.

The size of TiO2 crystallites, dc, estimated from GIXRD peakbroadening by the Scherrer method (Eq. (3)) for each film is pre-sented in Table 1. It can be observed that the estimated values ofdc are lower for all films produced through the In Situ synthesisthan for films produced through the Ex Situ synthesis. Table 1 alsohighlights that the dc values are not significantly influenced by theadditive content and neither by the number of coating cycles.Therefore, for each synthesis method, a mean value can be esti-mated and they were found to be equal to dc,InSitu = 14 ± 3 nmand dc,ExSitu = 23 ± 4 nm, respectively.

3.3. Profilometry and SEM microscopy

The thickness of the films was measured by profilometry forsingle and multilayers films corresponding to each sample. Table 1shows the influence of the synthesis method, the additive contentand the number of coating cycles on the film thickness. Table 1highlights that thickness increases linearly with the number ofcoating cycles suggesting that the thickness of the successive lay-ers deposited is similar. Table 1 also evidences that thickness isnot significantly influenced by the additive content. It can also beobserved in Table 1 than the thickness of the films obtainedthrough the Ex Situ synthesis is higher than the thickness of thefilms produced through the In Situ synthesis.

The surface morphology of the films such as presence cracks,adhesion to the substrate, and homogeneity of the layer was ob-served using the profilometer camera. The evolution of theroughness with the additive content is different according tothe synthesis method used to produce the sol. In the case ofthe In Situ synthesis, the roughness increases with the additivecontent and the number of coating cycles. For films producedthrough the Ex Situ synthesis, the opposite effect is observed. In-deed, the roughness decreases when increasing the additive con-tent and the number of coating cycles. As example, Fig. 5presents pictures of films obtained after three coating cycles intothe IN-Pure-TiO2 (Fig. 5a) and IN-10%-TiO2 (Fig. 5b) solsrespectively.

The surface morphology of the films was also observed in moredetails using SEM microscopy. Similar observations with the onesobtained by profilometry were made about the evolution of theroughness with the additive content for each synthesis method;the roughness increases with the additive content and the numberof coating cycles in the case of the In Situ synthesis, while theroughness decreases with the additive content and the numberof coating cycles in the case of the Ex Situ synthesis. As an exampleFig. 6a presents SEM pictures obtained for films obtained afterthree coating cycles in the different sols produced through the InSitu synthesis. The pictures show the appearance of small cracksat the film surface when the additive content increases in the case

Table 1Physico-chemical properties for the films obtained through the In Situ and the Ex Situ syntheses.

Sample Dopant content (% mol/mol) Number of coating cycles Thickness (nm) Roughness (nm) dc (nm)

EX-Pure-TiO2 0 1 69 ± 4 4.5 ± 1.9 19 ± 5EX-l%-TiO2 1 1 59 ± 3 1.1 ± 0.3 17 ± 5EX-10%-TiO2 10 1 78 ± 2 0.8 ± 0.1 24 ± 5

IN-Pure-TiO2 0 1 34 ± 7 0.7 ± 0.1 11 ± 5IN-l%-TiO2 1 1 42 ± 5 3.1 ± 0.6 11 ± 5IN-10%-TiO2 10 1 49 ± 6 24 ± 4 14 ± 5

EX-Pure-TiO2 0 2 111 ± 10 9.0 ± 0.7 24 ± 5EX-l%-TiO2 1 2 124 ± 7 1.9 ± 0.4 28 ± 5EX-10%-TiO2 10 2 112 ± 5 0.8 ± 0.2 28 ± 5

IN-Pure-TiO2 0 2 84 ± 3 0.9 ± 0.2 21 ± 5IN-l%-TiO2 1 2 122 ± 7 9.3 ± 2.4 14 ± 5IN-10%-TiO2 10 2 99 ± 7 30 ± 2 17 ± 5

EX-Pure-TiO2 0 3 181 ± 15 13 ± 1 18 ± 5EX-l%-TiO2 1 3 194 ± 17 4.5 ± 1.8 23 ± 5EX-10%-TiO2 10 3 204 ± 12 1.1 ± 0.7 26 ± 5

IN-Pure-TiO2 0 3 126 ± 12 2.0 ± 1.4 15 ± 5IN-l%-TiO2 1 3 177 ± 13 11 ± 5 11 ± 5IN-10%-TiO2 10 3 136 ± 7 37 ± 7 15 ± 5

Thickness and roughness measured by profilometry; dc, the crystallites size calculated by using the Scherrer equation.

Fig. 5. Pictures of films obtained after 3 coating cycles in (a) IN-Pure-TiO2 sol and (b) IN-10%-TiO2 sol.

Fig. 6. SEM micrographs for films (a) obtained after 3 coating cycles in the IN-Pure-TiO2, IN-1%-TiO2 and IN-10%-TiO2 sols (b) obtained after 1, 2 or 3 coating cycles in the IN-10%-TiO2 sol.

542 C.M. Malengreaux et al. / Chemical Engineering Journal 243 (2014) 537–548

of films produced through the In Situ synthesis corresponding to anincrease of the surface roughness, while in the case of the Ex Situsynthesis, no cracks were observed regardless of the additive con-

tent. Fig. 6b shows the morphology of the film surface observed forfilms obtained after one, two and three coating cycles into theIN-10%-TiO2 sol. This picture evidences that the cracks were

Table 2Optical properties for the different powders obtained through the In Situ and the ExSitu syntheses.

Sample Theoretical dopant content (% mol/mol) Eg,direct (eV) ± 0.05

EX-Pure-TiO2 - 3.17EX-l%-TiO2 1 3.18EX-10%-TiO2 10 3.01

IN-Pure-TiO2 - 3.17IN-l%-TiO2 1 3.17IN-10%-TiO2 10 3.01

Eg,direct value obtained by using transformed Kubelka–Munk function calculatedfrom DR-UV–VIS measurement.

C.M. Malengreaux et al. / Chemical Engineering Journal 243 (2014) 537–548 543

already present after one coating cycle suggesting that they aredue to an intrinsic effect of the additive.

Finally, it has been observed that the coloration phenomenonobserved on the films is strongly dependent on the thickness andvary from brown to yellow, blue and pink when the thickness in-creases. Films produced under the same conditions always exhibitthe same interference color, due to their comparable thickness.

The results of the krypton adsorption–desorption measure-ments evidenced that in the case of IN-Pure-TiO2, IN-1%-TiO2 filmsand IN-10%-TiO2 films, the estimated active surface area Sactive,developed by the films is drastically influenced by the presenceof the additive. Indeed, the values of Sactive have been found to beequal to 1 ± 0.5 m2

active m�2substrate, 3 ± 0.5 m2

active m�2substrate and 5 ± 0.5

m2active m�2

substrate for the IN-Pure-TiO2, the IN-1%-TiO2 and theIN-10%-TiO2, respectively. These results reinforce the suggestionthat an increase of the roughness leads to a higher value of the ac-tive surface area. A similar value has been reported for pure TiO2

thin film [32].

3.4. UV–Vis diffuse reflectance on powders

Fig. 7 shows the evolution of the normalized Kubelka–Munkfunction F(R1) with the wavelength, k, for powders correspondingto the different additive content introduced in the Ex Situsynthesis.

In all cases, an absorption band maximum is observed at wave-lengths around 350 nm. By using the transformed Kubelka–Munkfunction (F(R1)hm)2 plotted as a function of the energy hm, esti-mated values of the direct band gap can be obtained at the inter-section between the linear fit and the energy axis. As anexample, the graph included in Fig. 7 presents the evolution ofthe transformed Kubelka–Munk function (F(R1)hm)2 with energyhm for the powders obtained through the Ex Situ synthesis. By usingthis procedure, direct band gap values can be estimated for eachseries of catalysts and the corresponding values are presented inTable 2.

In the case of the TiO2-Pure catalyst, direct band gap value(Eg,direct) was found to be 3.17 ± 0.05 eV, for both synthesis meth-ods which is in good agreement with values reported previously[22] for the TiO2-anatase phase. For both synthesis methods, thecatalysts modified with 10% of additive presents a lower value ofEg,direct (3.01 ± 0.05 eV) than the value obtained for the TiO2-Purecatalyst suggesting a possible activation under visible light, whilethe value of Eg,direct does not change when 1% of additive is addedin the TiO2 matrix.

Fig. 7. Normalized Kubelka–Munk function F(R1) calculated from DR-UV–Visspectra and transformed via the Kubelka–Munk function (F(R1)hm)2 for the TiO2

produced through the Ex Situ synthesis.

3.5. Photocatalytic activity of the films

For all photocatalysts, adsorption tests performed in the darkshowed that the 4-NP concentration and the pH of the solution re-mained constant for 6 h. The measured concentration of pollutantin the solution at the end of the dark test is not significantly differ-ent from the initial concentration, suggesting that there is no 4-NPadsorption on the catalysts and hence, it is not necessary to waitbefore turning on the illumination in order to reach adsorptionequilibrium.

The blank experiments established that there is no spontaneousbreak down of the 4-NP after 6 h under illumination in the absenceof TiO2. According to those results it has been concluded that themeasured degradation of 4-NP under UV or UV–Visible illumina-tion is only due to the photocatalytic activity of the films.

To enable statistical treatments of the measured data, two pho-tocatalytic experiments were performed with two replicatesof each film. As an example, Fig. 8 presents the evolution of the4-NP concentration over time for the IN-Pure-TiO2 andIN-10%-TiO2 catalysts obtained after three coating cycles comparedto the activity of the commercial Saint Gobain Glass Bioclean�. Thevalues of concentration presented, are the mean values calculatedfrom the two repetitions of the experiment.

The determination of intermediate species associated with thepartial degradation of 4-NP has been reported in the literature[9,33]. In those studies, it has been highlighted that the absenceof peaks corresponding to the intermediates (4-nitrocatechol,1,2,4-benzenetriol, hydroquinone) in the UV–Vis spectrum mea-sured between 200 and 500 nm after several hours under illumina-tion is consistent with the complete mineralization of thepollutant.

Fig. 8. Evolution of 4-NP concentration under UV light at 25 �C and adjustment ofthe first-order model (–) on the experimental data for the reference catalystBioclean (Saint Gobain Glass Bioclean�) compared to the IN-Pure-TiO2 and IN-10%-TiO2 catalysts obtained after three coating cycles.

544 C.M. Malengreaux et al. / Chemical Engineering Journal 243 (2014) 537–548

However, in order to confirm the complete mineralization of thepollutant due to the photocatalytic activity of the different cata-lysts considered, total organic carbon (TOC) measurements havebeen performed. The results of these measurements, presented inTable 3, evidenced that the remaining organic carbon (OC) presentin the solution correspond to the remaining 4-NP estimated usingthe UV–Vis absorbance measurements suggesting the completemineralization of the pollutant which has been photodegraded un-der illumination.

Based on these TOC measurements and the absence of supple-mentary peaks in the UV–Vis spectrum measured between 200and 500 nm after 6 h under illumination, we have concluded thatthe photocatalysts developed in the present study promote com-plete mineralization of the 4-NP.

3.6. Kinetic models

The aim of this section is to determine a kinetic rate expressionwhich can be used to model the photocatalytic degradation of 4-NPover time, and to determine the actual value of the reaction ratecoefficient corresponding to each film. As mentioned in the intro-duction, the kinetics of the 4-NP degradation in the presence ofphotocatalytic thin films is usually described using simplifiedexpressions of the Langmuir–Hinshelwood (L–H) model such aspseudo-first order reaction [1,9,25,34] without taking into accountthat the volume of the reactor decreases during the experimentdue to sampling of the solution at regular time interval.

In order to take into account the variation of the volume insidethe batch reactor, the actual mass balance for the pollutant (4-NP)in a semi-continuous flow reactor has to be considered. The math-ematical development of the model, as well as the experimentalvalidation, in the case of thin films is presented in [24]. In the pres-ent work, the kinetics of the reaction has been described using twoLangmuir–Hinshelwood (LH) models. In this case the general equa-tion can be express as follows for each LH models.

Model LH1, taking into account the variation of the volume:

dCdt¼ � kS;LH1C

1þ KLH1C� Ssubstrate

V0 � VStð4Þ

Model LH2, taking into account the variation of the volume:

dCdt¼ � kS;LH2C

ð1þ KLH2CÞ2� Ssubstrate

V0 � VStð5Þ

where C is the pollutant concentration (kmol m�3), kS,LHi is the reac-tion rate coefficient of the film (m3 h�1 m�2

substrate) corresponding toModel LHi, KLHi is the equilibrium adsorption constant of the 4-NP(m3 kmol�1) corresponding to Model LHi, t is the time (h), Ssubstrate

is the surface of the substrate covered by the catalytic film(Ssubstrate = 15 � 10�4 m2

substrate), V0 is the initial volume of the solu-

tion (V0 = 24 � 10�6 m3) and VS is the solution sampling flow rate

Table 3Photocatalytic activity of the catalyst obtained through the In Situ and the Ex Situsyntheses estimated using UV–Vis absorbance and TOC measurements.

Sample Remaining OC insolution (%)

Remaining 4-NPin solution (%)

EX-Pure-TiO2 87 88EX-l%-TiO2 87 91EX-10%-TiO2 91 91

IN-Pure-TiO2 87 86IN-l%-TiO2 83 85IN-10%-TiO2 82 85

Remaining Organic Carbon (OC) in solution obtained by TOC measurements;remaining 4-NP in solution obtained by UV–Vis absorbance measurements.

(VS = 1.0 � 10�6 m3 h�1). As mentioned previously, in the case ofthin films, the solution is sampled at regular time interval in orderto measure the remaining concentration of the 4-NP inducing a sig-nificative variation of the volume of the reactor during theexperiment.

3.7. Parameter estimation

On the basis of experimental data, kinetic parameters were esti-mated for all the photocatalysts obtained after 3 coating cyclesusing both models. A maximum likelihood formulation is adopted,thus minimizing the sum of squares of the differences between cal-culated and measured concentrations. The values obtained forparameters ks,LHi and KLHi associated with standard deviations arepresented in Table 4 for each catalysts in the case of experimentsperformed under UV and UV–Visible light sources. In all cases, ithas been evidenced that parameter KLHi is not significantly differ-ent from zero for both Model LHi suggesting that those modelscannot be discriminated.

According to these results, the kinetic of the reaction can beconsidered to be of the first order, meaning that the mass balanceexpression can be simplified. The mathematical development ofthe model taking the variation of the volume inside the reactor intoaccount, in the case of a first order kinetic reaction, is expressed bythe following equation [24]:

dCdt¼ �kSC � Ssubstrate

V0 � VStð6Þ

After integration, the equation of the model is obtained:

lnCC0

� �¼ SsubstratekS

VSln

V0 � VStV0

!ð7Þ

where kS is the reaction rate coefficient of the film (m3 h�1 m�2substrate)

and C0 the initial pollutant concentration (C0 = 1.0 �10�4 kmol m�3).

By plotting ln CC0

� �as a function of ln V0�VSt

V0

� �the value of ks can

be determined using the slope a of the straight line obtained:

kS ¼aVS

Ssubstrateð8Þ

The values of ks with standard deviation obtained, using Eqs. (7)and (8), are presented in Table 5 for the measurements performedunder UV and UV–Visible light sources for each photocatalytic film,respectively. As expected, it can be observed that the estimatedvalues of ks are the same than the values of ks,LHi estimated byadjustment of both Model LHi using Eqs. (4) and (5).

Furthermore, the different values of ks estimated by adjustmentof the model on the experimental data can be used in order tomodel the evolution of the 4-NP concentration (C) for each catalystas a function of the illumination time taking the variation of thevolume inside the reactor into account using the followingexpression:

C ¼ C0V0 � VSt

V0

!SsubstratekSVS

ð9Þ

As an example, the mean value calculated from replicatedexperiments as well as the curves obtained using (Eq. (9)) areshowed in Fig. 8 for both IN-Pure-TiO2 and IN-10%-TiO2 catalysts.It can be observed that for each catalyst, the model fits the exper-imental data.

Table 4Values of the reaction rate constant ks,LH,i and equilibrium adsorption constant KLH,i with standard deviation determined by adjustment of both Model LHi respectively on themean value of 4-NP concentration calculated from replicated experiments.

Sample Dopant content (% mol/mol) 3 Coating cycles

ks,LH1 KLH1 ks,LH2 KLH2

(�10�4 m3 h�1 m2substrate) (m3 kmol�1) (�10�4 m3 h�1 m2

substrate) (m3 kmol�1)

UV lightEX-Pure-TiO2 0 4.2 ± 0.4 0.0 ± 1.1 4.2 ± 0.8 0.0 ± 1.0EX-l%-TiO2 1 2.8 ± 0.3 0.1 ± 1.0 2.8 ± 0.6 0.1 ± 1.0EX-10%-TiO2 10 2.1 ± 0.2 0.1 ± 1.0 2.1 ± 0.4 0.1 ± 1.0

IN-Pure-TiO2 0 1.6 ± 0.2 0.0 ± 1.0 1.6 ± 0.3 0.1 ± 1.0IN-l%-TiO2 1 2.7 ± 0.3 0.1 ± 1.0 2.8 ± 0.5 0.2 ± 1.0IN-10%-TiO2 10 3.2 ± 0.3 0.1 ± 1.0 3.3 ± 0.6 0.2 ± 1.0

UV–Visible lightEX-Pure-TiO2 0 1.1 ± 0.1 0.0 ± 1.0 1.1 ± 0.2 0.0 ± 1.0EX-l%-TiO2 1 0.8 ± 0.2 0.0 ± 1.0 0.8 ± 0.2 0.0 ± 1.0EX-10%-TiO2 10 0.6 ± 0.1 0.0 ± 1.0 0.6 ± 0.1 0.0 ± 1.0

IN-Pure-TiO2 0 0.5 ± 0.1 0.0 ± 1.0 0.5 ± 0.1 0.0 ± 1.0IN-l%-TiO2 1 0.8 ± 0.1 0.0 ± 1.0 0.8 ± 0.2 0.0 ± 1.0IN-10%-TiO2 10 1.0 ± 0.1 0.0 ± 1.0 1.0 ± 0.2 0.0 ± 1.0

Table 5Values of the reaction rate constant ks with standard deviation determined by adjustment of the first-order model on the mean value of 4-NP concentration calculated fromreplicated experiments.

Sample Dopant content (% mol/mol) 1 Coating cycle 2 Coating cycles 3 Coating cyclesks ks ks

(�10�4 m3 h�1 m2substrate) (�10�4 m3 h�1 m2

substrate) (�10�4 m3 h�1 m2substrate)

UV lightEX-Pure-TiO2 0 –a – 4.20 ± 0.08EX-l%-TiO2 1 – – 2.74 ± 0.11EX-10%-TiO2 10 – – 2.05 ± 0.06IN-Pure-TiO2 0 – – 1.57 ± 0.04IN-l%-TiO2 1 – – 2.66 ± 0.05IN-10%-TiO2 10 1.27 ± 0.06 1.47 ± 0.01 3.19 ± 0.07

UV–Visible lightEX-Pure-TiO2 0 – – 1.05 ± 0.04EX-l%-TiO2 1 – – 0.76 ± 0.12EX-10%-TiO2 10 – – 0.60 ± 0.07IN-Pure-TiO2 0 – – 0.48 ± 0.03IN-l%-TiO2 1 – – 0.80 ± 0.05IN-10%-TiO2 10 – – 0.96 ± 0.04

a Adjustment not performed.

C.M. Malengreaux et al. / Chemical Engineering Journal 243 (2014) 537–548 545

4. Discussion

4.1. Model validation

A statistical F-test has been performed for the catalysts pre-sented in Table 6 in order to validate the kinetic model selectedafter parameter adjustment on the experimental data (Eq. (9)).

According to the F-test, a model is validated when the ratio be-tween the residual variance corresponding to the model (s2

r ), whichevaluates the square of the deviation between the model and theexperimental data, and the experimental variance (s2

e ) calculated

Table 6Model validation by statistical Fisher F-test for the different catalysts presented inTable 5.

Sample vr vY s2r s2

Ys2

r =s2Y

F0:95;vr ;vY

UV lightEX-Pure-TiO2 4 5 2.36 � 10�7 1.49 � 10�7 1.58 5.19EX-l%-TiO2 4 5 3.57 � 10�7 1.13 � 10�8 3.13 5.19EX-10%-TiO2 5 6 1.34 � 10�7 4.85 � 10�8 2.77 4.39

IN-Pure-TiO2 5 6 2.76 � 10�8 1.00 � 10�9 2.75 4.39IN-l%-TiO2 4 5 2.60 � 10�7 2.04 � 10�7 1.27 5.19IN-10%-TiO2 5 6 1.54 � 10�7 1.40 � 10�7 1.11 4.39

from replicated experiments, which evaluate the square of theexperimental error, is smaller than the tabulated Fisher variableF0:95;mr ;me with a confidence interval equal to 95%. In this case, theresidual variance and the experimental variance are not signifi-cantly different, which means that the deviation between the mod-el and the experimental data corresponds to the experimentalerror and therefore, the model does not present a lack of fit [35].

The number of degree of freedom (mr) corresponding to theresidual variance is the number of mean value calculated fromthe replicates performed at each given time, minus the numberof parameter of the first-order kinetic model, i.e. 1 (parameter k).The number of degree of freedom (me) corresponding to the exper-imental variance is the number of replicates performed along thetime, minus the number of mean value calculated from the repli-cates performed at each given time.

For each considered catalyst, the value of the residual variances2

r corresponding to the first order model as well as the experimen-tal variance s2

e can be estimated using Eqs. (A3) and (A4) detailed inAppendix A.3.

The variance of the mean values s2Y, which is equal to the exper-

imental variance s2e divided by the number of replicates performed

for each experiment i at a given time, i.e. 2 can be estimated di-rectly from the values s2

e for each considered catalyst using:

546 C.M. Malengreaux et al. / Chemical Engineering Journal 243 (2014) 537–548

s2Y¼ s2

e

2ð10Þ

Finally, the statistical F-test is performed by comparing the va-lue of the tabulated F0:95;mr ;me to the ratio between the residual var-iance s2

r and the variance of the mean values s2Y.

F0:95;6;7 >s2

r

s2Y

ð11Þ

Residual variance corresponding to the model s2r and variance of

the mean value s2Y

has been estimated for each catalysts presentedin Table 5 using Eqs. (A3), (A4), and (10). The values as well as thecorresponding tabulated Fisher variable F0:95;mr ;me are reported inTable 6.

In all cases, the statistical F-test performed on the experimentaldata validates the first-order kinetic model, meaning that thismodel is suitable to represent the photocatalytic degradation ofthe 4-NP under illumination.

4.2. Effect of the additive

Table 5 highlights the effect of the number of coating cycleson the value of the reaction rate coefficient obtained for theIN-10%-TiO2 catalyst. As expected, the value of ks increases withthe number of coating cycles due to the larger amount of materialdeposited on the same covered surface of substrate. In all cases, anincrease of the film thickness leads to an enhanced photocatalyticactivity.

However, as it has already been evidenced, it is important tonote that the active surface developed by the film can be differentthan the surface of substrate covered by the films due to the rough-ness of the surface. A higher roughness is believed to lead to agreater active surface and hence, to an improvement of the photo-catalytic activity. In Table 5, the effect of the modification of thesurface of the films due to the introduction of the organic additiveis highlighted.

For films produced through the In Situ synthesis, the introduc-tion of the additive induces a modification of the surface morphol-ogy, which presents an increasing number of small cracks(observed by SEM) as well as an increasing roughness with theadditive content, as it has been measured by profilometry. Theevaluation of the active surface area by krypton adsorption mea-surements has evidenced that the introduction of 10% of additiveusing the In Situ synthesis leads to an active surface area of thefilms of a factor of five higher. In this case, the values of ks foundvary from 1.57 � 10�4 m3 h�1 m�2

substrate to 3.19 � 10�4 m3 h�1

m�2substrate under UV light and from 0.48 � 10�4 m3 h�1 m�2

substrate to0.96 � 10�4 m3 h�1 m�2

substrate under UV–Visible light. This representan improvement of 100% of the photocatalytic activity due to thepresence of the additive, suggesting that the modification of thesurface morphology leading to a higher roughness and hence, toa higher active surface of catalyst favors to the photocatalyticactivity of the films.

In contrast, in the case of films obtained using the Ex Situ syn-thesis, the introduction of increasing contents of additive leads toa crack-free homogeneous surface morphology with a decreasingroughness, as evidenced by SEM and profilometry. In this case,the presence of the additive induces a diminution of the active sur-face area of the film and hence, a reduced photocatalytic activity. In-deed, the corresponding reaction rate coefficients vary from4.20� 10�4 m3 h�1 m�2

substrate to 2.05� 10�4 m3 h�1 m�2substrate under

UV light and from 1.05� 10�4 m3 h�1 m�2substrate to 0.60� 10�4 m3 h�1

m�2substrate under UV–Visible light, due to the presence of the addi-

tive. This suggests that the modification of the surface morphologyleading to a lower roughness and thus, a lower active surface of

catalyst is detrimental to the photocatalytic activity. These trendsare observed regardless of the light source used.

Table 5 highlights that the activity of the films is lower whenthe UV–Visible lamp is used. Indeed, the values of the reaction ratecoefficient estimated for films illuminated using UV–Visible lampare around 70% lower than the value estimated for the same cata-lyst illuminated with UV lamp. This effect can be explained by alower intensity of the light corresponding to the wavelengthk = 365 nm in the UV–Visible lamp, while in the case of the UVlamp, the entire intensity of the light corresponds to the wave-length k = 365 nm.

5. Conclusion

Organic/inorganic TiO2 thin films have been produced using theoptimized dip-coating process and six different stable sols. Thesestable sols have been obtained using two non-aqueous sol–gelmethods and different contents of an organic additive (EDAP) havebeen introduced. The first sol–gel method (In Situ) is a non-aque-ous sol–gel method involving (i) the complexation of the titaniumalkoxide precursor and (ii) an in situ production of water. The sec-ond sol–gel method (Ex Situ) involves the addition of a smallamount of water in order to induce the hydrolysis of the titaniumprecursor.

The influence of the additive on the physico-chemical proper-ties of the materials has been investigated using a suite of comple-mentary techniques, including XRD, GIXRD, UV–Vis spectroscopy,krypton and nitrogen adsorption–desorption, profilometry andSEM. The photocatalytic activity of the different films has beenevaluated by monitoring the photocatalytic degradation of 4-nitro-phenol (4-NP) under UV light and UV–Visible light. By adjusting afirst-order model taking into account the variation of the volumeinside the photoreactor due to sampling on the experimental data,the value of the reaction rate coefficient has been estimated foreach catalyst.

The results establish that the thin film catalysts are composedof anatase with crystallite size found to be equal to dc,InSitu = 14 ±3 nm and dc,ExSitu = 23 ± 4 nm, respectively and thicknesses varyingfrom 34 nm to 204 nm. The results of the photocatalyticexperiments highlight that the performances of the differentcatalysts produced were always better that the performances ofthe commercial photocatalytic coating (Saint Gobain Glass Biocle-an�) which constitutes a significant step forwards for the produc-tion of photocatalytic coating.

Regarding the effect of the additive, the results highlight thatthe photocatalytic performances can be related to the active sur-face of the films through their roughness.

For the In Situ synthesis, the additive induces a modification ofthe surface morphology by increasing the number of small cracksas well as the surface roughness, both of these effects leading toa higher active surface possibly involved in the photocatalytic reac-tion. In this case, the values of the reaction rate coefficient ks werefound to increase of 100%.

On the opposite, in the case of films obtained through the Ex Situsynthesis, the introduction of the additive leads to a crack-free andhomogeneous surface morphology presenting a reduced roughnessassociated to an extremely low active surface. In this case, a de-crease of 50% of the value of ks is observed suggesting that a loweractive surface is detrimental to the photocatalytic activity. Thesetrends can be observed regardless of the light source used.

Acknowledgments

C.M. Malengreaux, S.L. Pirard and S.D. Lambert are grateful tothe F.R.S.– FNRS for PhD grant, postdoctoral researcher and

C.M. Malengreaux et al. / Chemical Engineering Journal 243 (2014) 537–548 547

research associate positions respectively. G. M.-L. Léonard thanksthe Université de Liège for PhD grant. The authors acknowledgeProf. J.-P. Pirard and Prof. D. Poelman for their helpful advicesduring results interpretation as well as Michael Nielsen, Universityof the Sunshine Coast, who performed the TOC measurements. Theauthors are grateful to the Vitrerie Duchaine sprl who kindly sup-plied the Saint Gobain Glass Bioclean� samples. The authors alsothank the Interuniversity Attraction Pole (IAP-P6/17), the Ministèrede la Région Wallonne, the Fonds de la Recherche FondamentaleCollective and the Fonds Wetenschappelijk Onderzoek Vlaanderenfor their financial support.

Appendix A

A.1. Determination of the optical band-gap values

The optical properties were evaluated by performing diffusereflectance spectroscopy measurements in the region 200–800 nm with a Varian Cary 500 UV–Vis-NIR spectrophotometer,equipped with an integrating sphere (Varian External DRA-2500)and using BaSO4 as reference. The UV–Vis spectra recorded indiffuse reflectance (Rsample) mode were transformed by using theKubelka–Munk function:

FðR1Þ ¼ð1� R1Þ2

2R1ðA1Þ

R1 is defined as R1 = Rsample/Rreference, where Rreference is the reflec-tance measured for an uncoated alkaline free substrate [29,30].For comparison, all spectra were normalized in intensity to 1.0 bydividing each spectrum by its maximum [16].

Using the well-known equation:

ðFðR1ÞhmÞ2 ¼ Cðhm� EgÞ ðA2Þ

where C is a constant. The direct optical band-gap values, Eg (eV),were obtained by plotting (F(R1)hm)2 as a function of the photon en-ergy hm and by determining the intersection of the linear part of thecurve and the x-axis [31].

A.2. Spectrum of the lamps

Fig. A1 displays the spectra measured between 330 nm and800 nm using a mini-spectrophotometer Hamamatsu TG respec-tively for the UV fluorescent lamp (Osram Sylvania, Blacklight-BleuLamp, F18W/BLB-T8) and the halogen lamp (Sensys, spots 50 W,12 V).

Fig. 1A. Spectrum measured between 330 nm and 800 nm using a mini-spectro-photometer Hamamatsu TG respectively for the UV fluorescent lamp (OsramSylvania, Blacklight-Bleu Lamp, F18W/BLB-T8) and the halogen lamp (Sensys, spots50 W, 12 V).

A.3. Statistical F-test

According to the F-test, a model is validated when the ratio be-tween the residual variance corresponding to the model (s2

r ), whichevaluates the square of the deviation between the model and theexperimental data, and the experimental variance (s2

e ) calculatedfrom replicated experiments, which evaluate the square of theexperimental error, is smaller than the tabulated Fisher variableF0:95;mr ;me with a confidence interval equal to 95%. In this case, theresidual variance and the experimental variance are not signifi-cantly different, which means that the deviation between the mod-el and the experimental data corresponds to the experimentalerror and therefore, the model does not present a lack of fit [35].

For each considered catalyst, the value of the residual variances2

r corresponding to the first order model can be estimated using:

s2r ¼

Plji¼1ðYi � bY iÞ

2

lj � 1ðA3Þ

where subscript i refers to the experiment performed at a giventime and lj is the number of experiment performed along the time.Yi is the mean value of the 4-NP concentration at a given time cal-culated using the 2 replicates of experiment i performed at a giventime, bY i is the value of the 4-NP concentration at a given time esti-mated using the first order model (Eq. (9)) at a given time. The cor-responding number of degree of freedom (mr) is the number of meanvalues calculated using the two replicates of each experiment i per-formed along the time, i.e. lj, minus the number of parameters, i.e. 1.

For each considered catalyst, the experimental variance s2e can

be estimated by:

s2e ¼

Plji¼1

P2j¼1ðYij � YiÞ

2

2lj � ljðA4Þ

where subscript i refers to the experiment performed at a giventime and lj is the number of experiment performed along the time.Subscript j refers to the two replicates of experiment i performed ata given time. Yij is the value of the 4-NP concentration correspond-ing to the replicate j of experiment i performed at a given time. In allcases, the experiments have been replicated two times, meaningthat j = 1 or 2. The corresponding number of degree of freedom(me) is the total number of replicates, i.e. 2lj (lj experiments repli-cated 2 times), minus the number of mean values calculated fromthe 2 replicates of experiment i performed along the time, i.e. lj.

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