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Applied Surface Science 298 (2014) 182–191

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

iO2-SnO2 heterostructures applied to dye photodegradation:he relationship between variables of synthesis andhotocatalytic performance

agner R. de Mendonc aa,∗, Osmando F. Lopesa, Raul P. Fregonesib,ania R. Giraldi c, Caue Ribeirod

Departamento de Química-UFSCar, Rod. Washington Luiz, km 235, CEP: 13565-905 São Carlos SP, BrazilDepartamento de Física–UFSCar, Rod. Washington Luiz, km 235, CEP: 13565-905 São Carlos SP, BrazilInstituto de Ciência e Tecnologia - UNIFAL, Rod. José Aurélio Vilela, 11.999, CEP: 37715-400 Poc os de Caldas MG, BrazilEmbrapa Instrumentac ão Agropecuária, Rua XV de Novembro, 1452, CEP: 13560-970, São Carlos SP, Brazil

r t i c l e i n f o

rticle history:eceived 31 August 2013eceived in revised form 9 December 2013ccepted 25 January 2014vailable online 31 January 2014

eywords:eterostructures

a b s t r a c t

This paper describes the synthesis of TiO2-SnO2 heterostructures and their application to water decon-tamination based on the photodegradation of Rhodamine B (RhB). The heterostructures were fabricatedthrough two different routes, a hydrolytic sol gel and the polymeric precursor method, both of whichinduced the growth of SnO2 on commercial TiO2. The results show that the heterostructures presentedhigher photoactivity behaviors than commercial TiO2 nanopowders. The achievement of homogeneityduring phase formation (i.e., of the SnO2 dispersion over the TiO2 nanoparticles) was a key parameterfor obtaining higher photocatalytic activities per unit area. The main degradation mechanism was corre-

•

ater decontamination

ol–gel synthesishotocatalysis

lated with the process of OH radical generation, which was related to the concentration and nature of thesurface hydroxyl groups. Accordingly, the polymeric precursor method was shown to be more adequatefor dispersing higher amounts of SnO2 in comparison with the hydrolytic sol gel method. Additionally,the polymeric precursor method delivered higher proportions of bonded surface hydroxyl groups, whichwere responsible for radical formation; in contrast, the hydrolytic sol gel method demonstrated the

ed w

highest amount of adsorb

. Introduction

Anatase TiO2 has been widely studied as a photocatalyst forater decontamination due to its properties of low solubility and

hemical and physical stability in both aqueous and gaseous sys-ems [1,2]. However, its photocatalytic activity remains insufficientor practical application [3]. One of the major drawbacks to thepplication of TiO2 lies in the recombination of electron–hole pairs,hich are generated after photon absorption when the semicon-uctor is irradiated with energy equal to or higher than its bandap. The recombination process reduces the photocatalytic effi-iency of TiO2. Several strategies have been developed to minimize

r avoid the effect of recombination. In particular, the associationf titanium dioxide with metals or other semiconductors to formeterostructures is a promising approach [4–7]. There is supporting

∗ Corresponding author. Tel.: +55 16 2107 2883; fax: +55 16 2107 2902.E-mail addresses: vagneromito@yahoo.com.br, vagnerqui03@hotmail.com

V.R. de Mendonc a), osmando iq@hotmail.com (O.F. Lopes), raulfisica@gmail.comR.P. Fregonesi), tania.giraldi@unifal-mg.edu.br (T.R. Giraldi),aue.ribeiro@embrapa.br (C. Ribeiro).

169-4332/$ – see front matter © 2014 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2014.01.157

ater.© 2014 Elsevier B.V. All rights reserved.

evidence of the effectiveness of strategy in terms of the synergis-tic effects of rutile and anatase mixed phases in photocatalysis,because the coupling of different phases can be interpreted as aheterostructure [8].

In this sense, the TiO2-SnO2 system, where the components arein the anatase and rutile phases, respectively, is of interest dueto the electronic and crystallographic similarities between thosematerials [9], as evidenced by the similar values of the interplanardistances in some crystallographic directions [10a,b]. When cou-pling these two oxides, the similarity between lattice parametersmakes possible the formation of a heterojunction. This structureis characterized by the fact that under equilibrium conditions, theFermi level should be the same throughout the interface betweenthe different phases, because the Fermi level reflects the chemi-cal potential of the electrons in the solid [11]. Then, because thematerials have different work functions, the spatial separation ofcharges (electron/hole) is promoted, which leads to an increase inthe lifetime of the photogenerated charges.

However, the proper synthesis of this system remains a prob-lem, based on the adequacy of different routes for the precipitationof the desired crystalline phases in the optimal fashion. Sol gel-based methods are significantly influenced by various variables,

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V.R. de Mendonc a et al. / Applied

uch as the relative concentration, precipitation times and calci-ation conditions, where changes in these variables may lead tohase separation. Consequently, the final material following such

preparation method is a mixture of individual phases and is not aroper heterostructure. To study the influence of those characteris-ics on the final properties of the materials, two different routes forhe production of TiO2-SnO2 heterostructures, both based on therowth of SnO2 on crystalline TiO2 nanoparticles at different phaseatios, were studied here, including the hydrolytic sol gel methodH) and the polymeric precursor method (P), which is a modifiedol gel method.

To probe the photocatalytic properties of the materials pro-uced and to determine the relative influences of the preparationethod and the concentration of SnO2 on the photocatalytic

ctivities, the photodegradation of Rhodamine B (RhB) under UVadiation was used as a model dye system. This dye is one ofhe most common chemicals used in industrial production meth-ds and often causes environmental pollution [12]. Therefore, itas chosen as a representative organic substrate to evaluate thehotocatalytic activity of the synthesized heterostructures [3]. Fur-her, due to the features of this dye and its behavior in aqueousolutions, the highest degradation rate corresponds to the highestemiconductor photocatalytic activity [13]. Additionally, to clarifyhe degradation mechanism, the formation rate of active hydroxyl

adicals was evaluated and correlated with the rate of RhB pho-odegradation.

ig. 1. A schematic of the coating synthesis process based on the two methodologies. (A)

y fast precipitation over dispersed TiO2 nanoparticles; (B) in the polymeric precursor mormation of SnO2 over TiO2 is achieved by controlled calcination.

ce Science 298 (2014) 182–191 183

2. Experimental

TiO2-SnO2 coatings were prepared by two different chemicalroutes, based on the growth of SnO2 on commercial crystalline TiO2nanoparticles at different ratios. In both methods, colloidal disper-sions containing 2.0 g of commercial TiO2 nanoparticles (Aldrich,nanopowders of 99.7% purity containing 92% anatase and theremainder, rutile, as determined by XRD analysis using the relationbetween the peak intensities of the anatase (101) and rutile (110)[14,15] planes [16]) were initially prepared in 50 mL of deionizedwater.

To prepare the heterostructures using the hydrolytic sol gelmethod [17], precursor solutions were prepared by dissolvingtin (II) chloride dihydrate (SnCl2·2H2O, Mallinckrodt) in abso-lute ethanol at room temperature, yielding a tin concentration of0.025 mol L−1. The solution was added drop-wise to the previouslyprepared TiO2 colloidal dispersions at the desired proportions.Hydrolysis and polycondensation reactions occurred immediately,yielding SnO2-coated TiO2 nanoparticles. Chloride ions were elimi-nated by dialysis, until the precipitate was no longer detected aftermixing with AgNO3 solution.

Alternatively, in the polymeric precursor route [18], a tin poly-meric solution was obtained by chelating a metal ion from a salt(SnCl2·2H2O, Mallinckrodt) using citric acid (JTBaker), followed by

polymerization with ethylene glycol (Synth), and then stirring thesolution for 12 h at 80 ◦C.

In the hydrolytic sol gel method, the formation of SnO2 nanoparticles is carried outethod, a precursor resin of Sn is deposited by drying in a rotoevaporator, and the

184 V.R. de Mendonc a et al. / Applied Surfa

Table 1The samples studied and the mass proportions between the oxides.

Method Sample %TiO2 (weight) %SnO2 (weight)

Reference Sample TiO2 100 0

Polymeric Precursor SAM0P81.6 18.4

Hydrolytic Sol Gel SAM0H

Polymeric Precursor SAM1P52.6 47.4

Hydrolytic Sol Gel SAM1H

Polymeric Precursor SAM2P35.7 64.3

rMNm(tmrme

tipatssm(3r

mcuswpdtt

rnwca(t

b2o

Ewsb

Experiments to determine hydroxyl radical rate formationwere performed indirectly by detecting the formation of 2-hydroxyterephthalic acid, which develops as a result of the reactionbetween hydroxyl radicals and terephthalic acid (TPA–Aldrich, 98%

20 30 40 50 60 70

Reference

SAM0 P

SAM1P

SAM 0H

SAM1H

## **

*

Inte

nsity

/ (a

.u.)

2 / (deg ree )

*

A

SAM2 H

SAM2 P

200 30 0 40 0 50 0 60 0 70 0 80 00,0

0,2

0,4

639cm-1

Inte

nsity

/ (a

.u.)

Raman Shift / (cm-1 )

144cm-1 395c m-1 515c m-1

SAM0 P

SAM0H

SAM 1P

SAM 2P

SAM2H

SAM1H

B

Hydrolytic Sol Gel SAM2H

In both of the methodologies presented in Fig. 1, the final mate-ials were obtained after solvent evaporation under low pressure.aterial loss during the process was expected to be insignificant.ext, the materials were calcined at 400 ◦C to yield the P (poly-eric precursor method) samples and at 200 ◦C to obtain the H

hydrolytic sol gel method) samples. Different annealing tempera-ures were used due to the requirement of the polymeric precursor

ethod to eliminate the organic residues that remain from the cit-ic acid and ethylene glycol. In contrast, in the hydrolytic sol gelethod, alcohol solutions were produced; therefore, it was not nec-

ssary to use high temperatures to eliminate the organic solvent.Fig. 1 shows a schematic of the methods applied in the syn-

hesis of the materials. Table 1 lists the synthesis method, thenitial mass ratio and the acronyms used to identify the sam-les with the corresponding text. The proportions were chosenccording to the characteristics of the materials. The lower propor-ion of SnO2 (SAM0) was described in the literature as providingignificant improvements to the photocatalytic properties of theystem [19]. The other proportions studied represent approxi-ately the same mass of material (SAM1) and the same volume

SAM2), according to the oxide mass density values, which are.8 g cm−3 for anatase TiO2 and 6.9 g cm−3 for rutile SnO2 [20],espectively.

X-ray diffraction (XRD) analyses were carried out using a Shi-adzu XRD6000 diffractometer with radiation at 0.15456 nm,

orresponding to the Cu K� emission wavelength. The conditionssed in the analyses were a 2� scan between 20◦ and 70◦, an expo-ure time of 1 s and an angular pass of 0.02◦. The crystallite sizesere calculated according to Scherrer’s equation [21], where eacheak was deconvoluted using a Pseudo-Voigt approximation toetermine the full-width at half-maximum (FWHM). Raman spec-roscopy was performed with a FT-Raman Bruker RFS 100/s usinghe 1063 nm line of a YAG laser.

The particle morphology was characterized with high-esolution transmission electron microscopy (HRTEM) (FEI Tec-ai20) performed at 200 kV. TEM samples were prepared byetting carbon-coated copper grids with a drop of the aqueous

olloidal suspensions, followed by drying in air. Semi-quantitativetomic compositions were evaluated by energy-dispersive X-rayEDX) spectrometry using an EDX Link Analytical device (Isis Sys-em Series 200) coupled to a LEO 440 SEM microscope.

The specific surface area (SA) of each sample was determinedy nitrogen physical adsorption at 77 K, using a Micromeritics ASAP000 particle size analyzer. The surface areas were evaluated basedn the standard BET procedure.

Photoluminescence spectra (PL) were obtained in a Perkinlmer luminescence spectrometer (model LS-50b). The samples

ere dispersed in water at 0.5 g L−1 and excited at 254 nm. The

ample emissions were collected at 90◦ relative to the incidenteam.

ce Science 298 (2014) 182–191

The surface-adsorbed hydroxyl groups on the photocatalystsamples were studied by Fourier-transform infrared absorptionspectroscopy (FTIR–PerkinElmer Spectrum 1000). The sampleswere mixed with potassium bromide (KBr) at a concentration of0.3 weight%. The mixture was pressed under 5 tons cm−2 to obtaina pellet. The samples were analyzed after drying for 24 h at 110 ◦C,and 32 scans were conducted at a resolution of 2 cm−1 for eachpellet.

The photocatalytic activity in terms of the oxidation of RhBdye was tested under UV illumination. The different samples(150 mg L−1) were immersed in 20 mL of aqueous RhB solution(5.0 mg L−1). The systems were placed in a photo-reactor at a con-trolled temperature (20 ◦C) and then were illuminated by fourUV lamps (TUV Philips, 15 W, maximum intensity at 254 nm).Dye degradation was monitored by measuring the UV–vis spec-tra (Shimadzu-UV-1601 PC spectrophotometer, � = 554 nm [22]) ofthe solution at different times of light exposure. To test the directUV-photolysis of the dye, blank experiments were performed usinga RhB solution without any catalyst, which demonstrated the neg-ligible degradation of the dye by UV photoillumination alone. RhBwas chosen because the degradation of this dye depends only onthe parameters of the photocatalyst [13], hence the photocatalyticproperties of the synthesized materials can be described against asignificant range of various water contaminants. As a reference, thephotocatalytic behavior of pristine commercial TiO2 nanopowders,the same powders used to prepare the heterostructures, was alsoevaluated. In this way, we were able to see clearly the effect of SnO2on the material properties.

Fig. 2. (a) XRD patterns of the as-synthesized samples obtained by the differentmethods. (b) Raman spectra showing the characteristic shifts of anatase TiO2.

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V.R. de Mendonc a et al. / Applied

urity). For this experiment, particles were dispersed (150 mg L−1)n a solution of terephthalic acid (4 × 10−4 mol L−1), which was pre-ared in 2 × 10−3 mol L−1 NaOH. The suspension was irradiatednder the same conditions used in the photocatalytic experiments,nd •OH radical formation was evaluated by measuring the changen PL intensity emitted from the 2-hydroxyterephthalic acid at25 nm when excited with 315 nm radiation (Perkin Elmer lumi-escence spectrometer-model LS-50b) [23].

. Results and discussion

.1. Characterization

Fig. 2A shows the XRD patterns of the synthesized samples. The

natase TiO2 peaks are of similar intensity in all the samples. For theowest SnO2 proportion (SAM0H and SAM0P), the peaks related tohe crystalline SnO2 are not observable because of the low amountf SnO2 present; however, the increase in some peak intensities

ig. 3. HRTEM images of the as-synthesized samples. (A) and (B) SAM0P; (C) and (D) SAM

ce Science 298 (2014) 182–191 185

with an increase in SnO2 concentration is indicative of the crystal-lization of SnO2 on the TiO2 nanoparticle surfaces.

Even though SAM2P and SAM2H possessed the same amount oftin dioxide, the sample obtained by the hydrolytic sol gel method(SAM2H) presents sharper SnO2 peaks. In the SAM2H, the SnO2 wasformed as higher crystallites over the TiO2 surface, when comparedto SAM2P. This indicates that the polymeric precursor method,under the conditions applied here, was able to disperse higheramounts of SnO2, because XRD analysis presented poorly definedpeaks related to the SnO2 phase, different to that of SAM2H. Asnoted previously, the polymeric precursor method is less prone tothe formation of a significant fraction of agglomerates [24].

Table 2 shows the crystallite size measurements of the anatasephases in the uncoated TiO2, SAM0P and SAM0H. These three sam-ples are representative for this analysis, because they do not present

SnO2 peaks in addition to the TiO2 peaks. It should be noted thatthe samples treated at different temperatures also present no sig-nificant differences in these values. Although crystallite size is an

1P; (E) and (F) SAM1H. The highlighted regions correspond to SnO2 nanoparticles.

186 V.R. de Mendonc a et al. / Applied Surfa

Table 2The average crystallites sizes obtained using Scherrer’s equation.

Sample Uncovered TiO2 SAM0P SAM0H

ita

staswditStSS

Tiao

Average crystallite size (nm) 12.6 ± 0.1 12.9 ± 0.1 13.3 ± 0.3

mportant factor regarding the photoactivity of such materials, inhe samples studied in this work, crystallite size does not seem toffect the final properties of the materials.

Fig. 2B shows the Raman spectra for all the samples. Ramanhifts were observed at 144, 395, 515 and 639 cm−1, correspondingo the vibrational modes with symmetries of Eg, B1g, A1g and Eg ofnatase TiO2 [13]. However, it is not possible separate the Ramanhifts for SnO2 because the main shift is expected at 632 cm−1 [25],hich overlaps the anatase phase shift at 639 cm−1. That said, theecay in the intensity of the bands related to the TiO2 anatase with

ncreasing fractions of SnO2 may be related to the SnO2 coating onhe TiO2. It is interesting to note that this effect is more evident inAM2P than SAM2H, according to the observations gleaned fromhe XRD patterns, which suggests that for SAM2H, the formation ofnO2 agglomerates occurred on the TiO2 surfaces. In contrast, forAM2P, the SnO2 presented higher dispersion.

TEM and HRTEM images of selected samples are shown in Fig. 3.

he observed structures show that both methods effectively mod-fied the TiO2 surface, despite the irregular distribution of thes-formed oxide. In this sense, all the samples showed evidencef coating formation.

Fig. 4. Representative EDS analysis of selected samples. The peaks related to Ti and

ce Science 298 (2014) 182–191

To confirm the relative amounts of both oxides in selectedregions of the samples, in addition to the dispersion of SnO2 over thesamples, EDX analyses were performed, as shown in Fig. 4. SAMPsamples showed more intense Sn peaks than those presented bySAMH samples, when comparing the same oxide proportions. Theatomic maps (not shown here) agreed with this observation andshow that the Sn may be located on the Ti, because both elementsare co-located on the maps. Because the analysis was performedon specific parts of the samples, it can be said that the samplesobtained using the polymeric precursor method showed higher dis-persions of SnO2 over the TiO2 surfaces than the samples obtainedbased on the hydrolytic sol gel method.

It is noteworthy that in the polymeric precursor method, SnO2particles are formed slowly during the thermal annealing phase, asrepresented in Fig. 1. This factor may contribute to the formationof SnO2 particles on the TiO2. In contrast, in the hydrolytic sol gelmethod, the SnO2 phase forms immediately when the Sn2+ ionscome into contact with water, in a process that is significantly fasteroverall. Thus, SnO2 particles can precipitate separately from TiO2,which formed particles of higher crystallites observed in both theXRD patterns and Raman spectra; more specifically, it was observedthat SAM2H presented better defined

3.2. Photocatalytic properties

Fig. 5 shows the color removal profiles of RhB in the presence ofthe as-synthesized samples. Table 3 shows the k value (in min−1),

Sn are identified in the plots. (A) SAM0H; (B) SAM0P; (C) SAM2H; (D) SAM2P.

V.R. de Mendonc a et al. / Applied Surfa

1801501209060300

0.6

0.8

1.0

C/C

0

Time / (min)

Blank SAM0H SAM1H SAM2H Reference

A

1801501209060300

0.6

0.8

1.0

C/C

0

Time / (min)

Blank SA M0P SA M1P SA M2P Reference

B

Fh

wa

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TTa

have compatible interplanar distances in specific crystallographicdirections, allowing significant lattice mismatches to be avoided.

(k/S

SA) /

(min

-1.m

-2.g

) Hydrolytic sol gel method Polymeric precursor method

SAM0 SAM1 SAM2

A

120100806040200

I λ=4

26nm

/ (a.

u.)

Time / (min)

r2=0.99968

d[.OH] dt

= k* (a.u. .min-1 )

B

1 .m-2

.g)

C

ig. 5. Photocatalityc tests comparing the different synthesis methods. (A) theydrolytic sol gel method; (B) the polymeric precursor method.

hen considering a first-order kinetic RhB degradation process [2]nd the specific surface area (SSA) of the samples.

All the as-synthesized samples were more active than pristineiO2. Additionally, it is known that SnO2 does not exhibit sig-ificant photocatalytic activity, most likely due to its high bandap (approximately 3.8 eV) and the relative position (reductionotential) of its conduction band, which is insufficient for reduc-

ng molecular oxygen (O2 + e− → •−O2 E = −0.33 eV)] [26]. However,hen associating this semiconductor with TiO2, a type II hetero-

unction is formed by the partial superposition of the forbiddenands in both oxides, which forces charge separation [7]. The effi-ient charge separation properties of this heterojunction accountor the increased activity of the heterostructure.

able 3he reaction rate constants for Rhodamine B photodegradation and specific surfacerea (BET).

Samples Reaction constantkx103 (min-1)

Specific surface area(m2 g−1)

SAM0H 1.4 114SAM0P 2.2 105SAM1H 3.5 74.4SAM1P 3.1 82.8SAM2H 1.9 45.0SAM2P 2.4 32.0Reference 1.4 128

ce Science 298 (2014) 182–191 187

It is important to note that in terms of the interface betweenthe constituents of the heterostructure, only establishing physi-cal contact between the oxides is insufficient for the occurrence ofelectronic exchange, i.e., it is essential to obtain chemical inter-action, as well, which may be achieved by thermal treatment.Additionally, to establish chemical interaction, the oxides must

(k*/S

SA) /

(a.u

. .m

in- Hydrolytic sol gel method

Polymeric precursor method

SAM0 SAM1 SAM2

Fig. 6. (A) the relationship between the k/SSA ratio and SnO2 concentration; (B) arepresentative plot showing the method proposed by Ishibashi et al. [23], wherethe induced fluorescence intensity measured at 425 nm indicates the formation of2-hydroxyterephthalic acid with light illumination time. This value is proportionalto OH• formation. The inset shows the experimental fluorescence changes duringillumination when using pristine TiO2 as the reference; (C) the relationship betweenk*/SSA ratio and SnO2 concentration.

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For the samples obtained based on the hydrolytic sol gel method,it was observed an optimum value of the degree of coverage atwhich the sample showed the highest photoactivity. In SAM1H,the reduction in specific surface area was compensated by the

88 V.R. de Mendonc a et al. / Applied

In the TiO2 (work function = 4.2 eV) and SnO2 (work func-ion = 4.9 eV) heterostructures produced here, the holes will tendo migrate to the TiO2 valence band, and the electrons, to the SnO2onduction band [7,19]. Hence, it is very important for both oxideso have accessibility to the solution, i.e., a complete SnO2 coatingver the TiO2 would trap the holes within the material, preventingheir availability for the oxidative process.

Upon further analysis of the photocatalytic results, with respecto the lower SnO2 concentration, only SAM0P exhibited a significantmprovement of photoactivity; in contrast, SAM0H shows almosthe same activity value as that of the reference material. This mightave occurred as a result of the lack of interface between TiO2 andnO2; when using the hydrolytic sol gel method, as proposed basedn the discussion of the analysis of the XRD and Raman results, ashown in Fig. 2, it is more likely that the SnO2 particles precipitatedeparately from TiO2, leading to the formation of high crystallites.his result may affect the interaction between the oxides, preven-ing the formation of an interface, even when utilizing a furtheralcination step. Despite both SAM2P and SAM2H exhibiting bettererformances than the reference material, the activities of theseamples were also lower than those of SAM1P and SAM1H, respec-ively.

Increasing the SnO2 coating affected the total surface area of theaterials produced, as one can see in Table 3. Because this phaseas produced by crystallization over a pre-formed TiO2 phase,

arger amounts would act as a cementing phase, leading to pow-er agglomeration. Another contributor to this result is the higherensity of SnO2 relative to TiO2 [20].

In the photocatalytic experiments, a heterogeneous process thatakes place on the surface of the photocatalyst was carried out usinghe same mass of materials. Considering this fact, a true analysis ofhe photocatalytic activity of these materials was obtained usinghe ratio of the rate constant and material specific surface area,nstead of using only the degradation percentage or the k valuemin−1). The relationship between the values of k/SSA and SnO2oncentration is shown in Fig. 6A. The reference sample exhibitedhe lowest value; because this sample presented the highest sur-ace area, the ratio for this sample is not shown in Fig. 6. The lowestalue presented by the reference sample indicates that the pos-ible formation of a heterostructure promotes an increase in thehotocatalytic performance of the materials.

All the systems were held in the dark for 12 h prior to furthernalysis to evaluate the possible effects of adsorption onto the cata-yst surfaces. However, no significant changes in the RhB absorptionpectra were observed for any of the samples. Therefore, the mostmportant dye degradation mechanism may be the generation ofydroxyl radicals on the catalyst surface; these radicals can thenxidize the dye in solution [2]. The hydroxyl radicals generatedame from the surface-bound OH− groups and not from the freeH− in solution. Such radicals are formed due to the significantxidation power of the holes in the semiconductor valence band,articularly in the anatase TiO2.

To confirm that the primary RhB degradation mechanism isttack by •OH radicals, the rate of ?OH formation during UV radi-tion was measured according the method proposed by Ishibashit al. [23], as shown in Fig. 6B. It can be seen that the fluorescencentensity related to the presence of 2-hydroxyterephthalic acid inhe solution increased linearly with time. Because the OH radicaloncentration is directly proportional to the concentration of 2-ydroxyterephthalic acid, it can be concluded that the rate of OHadical formation follows a zero-order kinetic law in the time rangetudied. This result demonstrates that the profile of the decrease in

hB concentration may be related only to dye concentration andot to changes in the features of the photocatalyst during UV irra-iation, which could lead to a different degradation mechanismuring the experiment.

ce Science 298 (2014) 182–191

The value of k*, represented in the equation in Fig. 6B, can bedetermined directly from the inclination of the curve and repre-sents the efficiency of the formation of OH radicals. In additionto the RhB degradation behavior, this value also should be pro-portional to the specific surface area of the material. Therefore,Fig. 6C presents a plot of the ratio between k* and SSA for eachsample. The trend is the same as that observed for k/SSA, as shownin Fig. 6A. By comparing the sets of data, it can be affirmed thatattack by OH radicals is the primary RhB degradation mechanism,thus the absorption of dye onto the catalyst surface, followed bythe direct transfer of electrons from RhB to the photocatalyst [12]likely did not play a significant role in this process, as previouslydiscussed.

Fig. 7. Photoluminescence spectra of representative samples. (A) TiO2 and SnO2;(B) SAM0P and SAM2P; (C) SAM0H and SAM2H.

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V.R. de Mendonc a et al. / Applied

ormation of an interface between the oxides. However, increasinghe degree of coverage had a deleterious effect on the photocatalyticerformance of the materials. This likely resulted due to the lack ofew points of charge separation between TiO2 and SnO2. Becausehe XRD and Raman analyses showed that SnO2 was formed as highrystallites on the TiO2 surface with poor dispersion, it can be saidhat the additional growth of SnO2 over previously formed SnO2ould not improve the charge separation behavior of the materialnder the conditions applied in this work.

However, for the samples obtained using the polymeric precur-or method, the photoactivity per unit area is directly proportionalo the degree of SnO2 coverage within the range studied. For thehree samples obtained by this method, the reduction in surfacerea was likely compensated by a higher degree of charge separa-ion, as well as an increase in the lifetime of the photogeneratedharges.

To verify this possibility, the electronic behavior of the samplesas investigated by obtaining the PL emission spectra of the sam-les. The spectra for representative samples with different amountsf SnO2 (SAM0 and SAM2) and pristine oxides are presented inig. 7. The role of SnO2 in promoting better charge separation was

onfirmed by the decrease in band intensity with increasing frac-ions of SnO2. It was found that SAM2 exhibited lower intensityalues than SAM0, independent of the synthesis method by whichhe samples were obtained, indicating that the recombination of

SAM1

Hydrolytic sol gel me Polymeric precursor m

(I 317

2cm

-1/S

SA) /

(a.u

. m2 .g

-1)

SAM0

2600280030003200340036003800

1,0

0,8

0,6

0,4

0,2

0,0

Nor

mal

ized

Inte

nsity

/ (a

.u.)

Wavenumb

A

SAM2P

SAM1P

SAM0P

ig. 8. FTIR analysis of the as-synthesized samples. The samples were obtained by (A) thentensity of the bound hydroxyl peak (3172 cm−1) for the SAMP and SAMH samples.

ce Science 298 (2014) 182–191 189

photogenerated charges is prevented by increasing the SnO2 pro-portion of these materials [7]. Additionally, the longer lifetime,which might have been promoted by the increased contact areabetween the oxides, may be one contributor to the better photo-catalytic performance of the samples containing higher amountsof SnO2. Another interesting result arises from a comparison of thespectra of the heterostructures with pristine TiO2 and SnO2. SAM0Hpresented similar spectra to TiO2. This observation may be indica-tive of the low dispersion of SnO2 over TiO2, in accordance withthe analysis presented previously in this work. Other samples ana-lyzed here showed bands comparable to those present in the TiO2and SnO2 spectra, as highlighted by the line at 375 nm.

Because we are considering that the primary photodegradationmechanism is driven by hydroxyl radicals, it is important to ver-ify the influence of TiO2 coverage on the amount of OH− groupspresent on the surface of the heterostructures. FTIR spectra of thesamples are shown in Fig. 8A and B. In the spectral range presented,two main shifts can be observed: the shift at 3400 cm−1 is relatedto the stretching of adsorbed water, while the other is located at3172 cm−1 and corresponds to the vibration of Ti-OH [2]. The shift ofthe latter band is likely due to hydrogen bonding and/or the Sn-OH

vibration. In such diluted samples, the intensity of the latter bandincreases linearly with the fraction of OH groups present on thesurface [27,28]. For the samples obtained based on the hydrolyticsol gel method, the band located at 3400 cm−1 is more intense than

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B

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polymeric precuror method and (B) the hydrolytic sol gel method. (C) the relative

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90 V.R. de Mendonc a et al. / Applied

he other band at 3172 cm−1. This fact indicates that the SAMHamples, which were treated at lower temperature, present highermounts of adsorbed water than SAMP. It is important to note thathe bands presented by SAM1P at approximately 3000 cm−1 areelated to C–H stretching, which comes from the precursor used inhe synthesis process.

Separate relative intensities were calculated for each methodased on the intensity of SAM0, oxides proportion which exhib-

ted the most intense spectrum, at 3172 cm−1. For both setsf samples, the intensity decreased with SnO2 concentration. Aecrease in the amount of OH groups present on the oxide sur-aces should have a deleterious effect on the photocatalysis results.owever, the recorded observations can be explained by the fact

hat the OH surface groups decrease in addition to the surfacerea.

Because there is a direct relationship between FTIR intensityt 3172 cm−1 and the fraction of OH surface groups, the relativentensity can be directly compared with the relative reduction inpecific surface area. However, for this analysis, the comparisonannot be made between the samples obtained by the differentethods, because the relative intensities were calculated using theost intense spectrum for each method. The intensities of SAM0P

nd SAM0H are not comparable, because the higher content ofater in SAMH may have influenced the others bands intensities.

he results are shown in Fig. 8B. When comparing the ratio of apecific fraction of the contents with the corresponding specificurface area, the spectra follow the same trend for the others pre-iously presented in Fig. 6. In this sense, the decrease in SSA isore significant than the decrease in FTIR intensity for all samples,

xcept for SAM2H, where both decreased by the same magnitude.his result most likely occurred due to the low dispersion of SnO2ver the TiO2 for the samples obtained based on the hydrolytic solel method.

The samples obtained by the polymeric precursor methodielded an increase in the SnO2 concentration, which further pro-oted an increase in the amount of OH surface groups per area unit.

his finding is in accordance with previous papers [7]. Both effects,.e., the longer lifetime of photogenerated charges, as evidenced byL, and the high concentration of OH surface groups, as demon-trated by FTIR, may contribute to the increased photoactivity pernit area of SAM2P.

. Conclusions

TiO2-SnO2 heterostructures were produced using different pro-ortions of Ti and Sn and based on different fabrication routes,he hydrolytic sol gel and the polymeric precursor method. Inoth cases, the formation of the type II heterojunctions promoted

ncreased water decontamination abilities of the materials, asetermined by RhB degradation experiments, compared to pristineiO2 nanoparticles. The key factor in the photoactivity behav-or of these materials was the homogeneity of phase formation.he polymeric precursor method was able to disperse highermounts of SnO2 during heterostructure formation, because thearticles were formed slowly during the thermal annealing step;his result is in contrast to material obtained from the hydrolyticol gel method, where the nucleation of SnO2 occurs over pre-ormed TiO2 nanoparticles or by homogeneous nucleation, leadingo phase segregation. The role of SnO2 in promoting enhancedharge separation was confirmed and was found to be indepen-ent of the synthesis method by which the sample was obtained,

ndicating that the recombination of photogenerated charges wasrevented by increasing the SnO2 proportion on those materi-ls. In both cases, the photocatalytic activity was governed byree radical formation; this mechanism was also correlated to the

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surface hydroxylation of the samples. Hence, the nature of the sur-face groups was important: in the polymeric precursor method,the formation of bonded surface hydroxyl groups was favored,whereas in the hydrolytic sol gel method, adsorbed water waspredominant.

Acknowledgements

The authors gratefully acknowledge the financial support ofthe Brazilian research funding agencies FINEP and CNPq-RECAMProject 64913/2010-3. We are also grateful to LCE DEMA-UFSCar-Brazil for providing the HRTEM facilities.

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