IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 18 (2007) 375709 (7pp) doi:10.1088/0957-4484/18/37/375709

Photocatalytic and antibacterial activity ofTiO2 and Au/TiO2 nanosystemsLidia Armelao1, Davide Barreca1,4, Gregorio Bottaro1,Alberto Gasparotto2, Chiara Maccato2, Cinzia Maragno2,Eugenio Tondello2, Urska Lavrencic Stangar3, Martina Bergant3

and Dunja Mahne3

1 ISTM-CNR and INSTM, Department of Chemistry, Padova University, Via Marzolo,1-35131 Padova, Italy2 Department of Chemistry, Padova University and INSTM, Via Marzolo, 1-35131 Padova,Italy3 Laboratory for Environmental Research, University of Nova Gorica, Vipavska 13,5001 Nova Gorica, Slovenia

E-mail: davide.barreca@unipd.it

Received 11 July 2007, in final form 24 July 2007Published 22 August 2007Online at stacks.iop.org/Nano/18/375709

AbstractThis work focuses on the photocatalytic performances and antibacterialactivity of TiO2 and Au/TiO2 nanosystems. While the former are obtained bya sol–gel route, the latter are synthesized by an innovative hybridRF-sputtering/sol–gel approach, followed by ex situ annealing in air up to600 ◦C. Important information on nanoparticle size, shape and distribution isobtained by the combined use of glancing incidence x-ray diffraction(GIXRD) and field emission-scanning electron microscopy (FE-SEM).Subsequently, the photocatalytic performances of the obtained nanosystemsin the decomposition of the azo-dye Plasmocorinth B and their antibacterialactivity in the elimination of Bacillus subtilis are illustrated and discussed incomparison with films obtained from standard Degussa P25 powders. Theobtained results show a significant dependence of the functionalperformances on the system’s compositional, structural and morphologicalproperties. In particular, the dispersion of gold nanoparticles on the TiO2matrix has a beneficial influence on the azo-dye photodegradation, whereasthe antimicrobial activity of Au/TiO2 films is retarded with respect to pureTiO2.

1. Introduction

Semiconductor photocatalysts have recently attracted consid-erable attention for advanced oxidation processes (AOPs) usedin decontamination, purification and deodorization of air, wa-ter and industrial effluents [1–6]. In this context, TiO2 is oneof the most studied materials [7–10] thanks to its stability andphotosensitivity [11, 12] both in the form of powders and thinfilms [13, 14]. The use of the latter has become an attractivealternative to circumvent the technological difficulty and thehigh costs related to catalyst recovery [1, 4] as well as to de-velop self-cleaning and self-sterilizing systems operating un-

4 Author to whom any correspondence should be addressed.

der UV light in living areas [1, 2]. In particular, TiO2 filmshave already been proposed for use in hospitals, hotels andcommercial facilities [1, 8] thanks to their capability of killingfungi, viruses, algae and bacteria [1, 14, 15] and simultane-ously producing both the nullification of bacteria viability andthe destruction of their cells [16]. In fact, when TiO2 catalystsare subjected to irradiation with photons of energy equal toor higher than their bandgap (3.2 eV), the generated electron–hole pairs can induce the formation of reactive oxygen species(ROS), such as ·OH and O.−

2 , that are directly involved in theoxidation processes leading to the degradation of both contam-inants and microorganisms [4, 6, 8, 9, 13, 15, 17].

The efficiency of TiO2-based photocatalyst and anti-bacterial systems directly depends on the ability to obtain

0957-4484/07/375709+07$30.00 1 © 2007 IOP Publishing Ltd Printed in the UK

Nanotechnology 18 (2007) 375709 L Armelao et al

nanostructured materials with tailored features and to generateelectron–hole pairs with a reduced recombination rate [13].As a matter of fact, several methods have been employedto improve the system’s performance, including tailoringof titania particle size and its surface modification withsemiconducting or metal nanoparticles [5, 11, 13, 18]. Theintroduction of the latter has been motivated by the requirementof an improved absorption in the visible light region, sinceTiO2 absorbs mainly the UV component of solar radiation dueto its relatively large bandgap [18, 19]. Moreover, in the caseof metal–TiO2 composites, an increase in the photocatalyticefficiency has also been observed thanks to a reduction inthe electron–hole recombination rate, due to a better chargeseparation between electrons and holes [2, 6, 11, 13]. Amongthe different metals, gold nanoparticles have been extensivelyused to obtain Au/TiO2 nanocomposites, yielding attractivesystems for the photocatalytic degradation of several aromaticpollutants [5, 10, 12, 18, 20], organic dyes such as acidgreen 16 [11] and azo-compounds [2], as well as for use inantimicrobial systems [13].

Since the photocatalytic efficiency of Au/TiO2 nanosys-tems depends on both the metal loading and the prepara-tion route, the mechanism of nucleation and growth of goldnanoparticles may play a dominant role [6]. To controlthe system morphology, several preparation strategies havebeen attempted, including liquid phase routes [13, 21], sol–gel [22–24], chemical vapor deposition [25, 26], sputter-ing [27], thermal oxidation [28], evaporation [29] and pulsedlaser deposition [19].

In recent years, our research group has developed an in-novative RF-sputtering/sol–gel route to Au/TiO2 nanocompos-ites based on the RF sputtering of gold on sol–gel titania xe-rogels and subsequent annealing in air. The peculiarities ofthis synthetic strategy in producing nanocomposites with tai-lored features have already been described [23, 30]. Takingadvantage of such results, the present work is devoted to in-vestigate the functional applications of the obtained nanosys-tems as photocatalysts and antibacterial agents. To the best ofour knowledge, such measurements have never been performedon gold–titania nanocomposites obtained by a similar syntheticapproach. With respect to photocatalysis in water cleaning, thedegradation of the azo-dye Plasmocorinth B was chosen as atest reaction. The choice of Plasmocorinth B is due to its stabil-ity under environmental conditions and its moderate adsorptionon the photocatalyst surface, so that the discoloration of the dyesolution can be directly related to its decomposition [22]. Fur-thermore, the antibacterial activity of the obtained systems wasinvestigated using a Gram-positive bacterium, Bacillus subtilis,a widely used model organism for laboratory studies [15]. Themain aim of the present paper is to discuss the performance ofAu/TiO2 nanosystems as a function of their structural and mor-phological features, with particular attention to the role of goldparticle size, shape and distribution. Furthermore, the func-tional behavior was compared to that of pure TiO2 sol–gel filmsand layers obtained from Degussa P25 powders, used as refer-ence materials for comparison purposes.

2. Experimental details

Au/TiO2 systems were obtained according to a hybrid RF-sputtering/sol–gel procedure originally developed by our

research group [23]. Titania xerogels were deposited oncleaned Herasil silica slides (25 mm × 75 mm × 1 mmeach, Heraeus®, Quarzschmelze, Hanau, Germany) andsubsequently used as substrates for gold deposition fromAr plasmas. All depositions were performed adopting thesame experimental parameters (RF power = 5 W, totalpressure = 0.08 mbar, sputtering time = 10 min), yieldingthus the same gold amount for all the obtained samples. Theresulting systems were annealed ex situ at temperatures of 200,400 and 600 ◦C for 1 h. For comparison, pure TiO2 coatingswere prepared by sol–gel from ethanolic solutions of Ti(OPri)4

(OPri = i so-propoxy) and Hacac (2,4-pentanedione) [23] andsubjected to the same thermal treatments.

A commercially available Degussa P25 TiO2 was used asa reference photocatalyst. The immobilized particulate TiO2

layers were prepared on silica slides by sedimentation froman aqueous suspension of TiO2 P25 (10 g l−1), with additionaldrying and annealing at 500 ◦C for 15 min [31, 32].

GIXRD patterns were recorded by a Bruker D8 Advancediffractometer equipped with a Gobel mirror and a Cu Kα

source (40 kV, 40 mA), at a fixed incidence angle of 0.5◦. Theaverage crystallite dimensions were estimated by means of theScherrer equation as reported elsewhere [23].

FE-SEM measurements were performed by a ZeissSUPRA 40VP instrument operated at acceleration voltageslower than 20 kV, equipped with an Oxford INCA x-sight x-ray detector. ImageJ picture analyzer softwarewas used to estimate the titania surface coverage by goldnanoparticles [33].

A continuous flow reactor, whose main characteristicshave already been described [22], was employed forphotocatalytic experiments. A sample film on one side ofa silica glass support was immersed in the dye solutionnext to the wall of the photocatalytic cell and irradiated(23 mm × 23 mm surface) along the normal direction. A10 mm thick solution of NaBr (110 g) and Pb(NO3)2 (0.69 g)in water (230 g) was used as a 335 nm cutoff filter infront of the photocatalytic cell, irradiated by a 125 W Xelamp (Cermax xenon parabolic lamp). The PlasmocorinthB (40 mg l−1) aqueous solution (total volume = 6 ml) wascontinuously purged with oxygen during the irradiation. Aperistaltic pump (Heidolph PD 5001) with a silicon hose wasused to drive the solution from the photocatalytic reactor tothe cell positioned in the UV–vis spectrophotometer for on-line absorbance measurements and back to the reactor at a flowrate of 8 ml min−1. The photocatalytic activity was evaluatedby monitoring the dye absorbance maximum at λ = 527 nmversus irradiation time.

The system antibacterial activity was tested using theantibacterial drop test in two independent experiments thatgave similar results. Suspension culture of the model organismBacillus subtilis was cultured in liquid PYE (peptone, yeastextract) medium at 37 ◦C for 20–24 h. Bacterial cells thatsurvived after the treatment and were still able to proliferateformed visible colonies and are therefore called colony-forming units (CFUs). The bacterial suspension was thenwashed and diluted to 107 CFU ml−1 in saline solution. Silicaslides covered with pure TiO2 or Au/TiO2 systems weresterilized and placed in sterile Petri dishes. Drops of bacterialsuspension (350 μl in total) were added onto the surface of

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Nanotechnology 18 (2007) 375709 L Armelao et al

Figure 1. GIXRD patterns of Au/TiO2 nanocomposites as a functionof the annealing temperature. Peak positions for anatase TiO2 ( )and Au ( ) are indicated.

each system. The samples were subsequently illuminated bya 40 W long wavelength UV lamp (310–390 nm, peak at355 nm) at room temperature for 15, 30 and 45 min. Thephoton irradiance in the sample compartment was estimatedby potassium ferrioxalate actinometry [34] and determined tobe 8.67 × 10−9 einstein cm−2 s−1. After each exposure time,100 μl of tested bacterial suspension was diluted tenfold in asaline solution. Subsequently, aliquots (100 μl each) of dilutedbacterial solution were spread over PYE agar plates. After 24 hof incubation at 37 ◦C, the number of bacterial colonies wascounted. In addition to TiO2-based systems, uncoated silicaslides were included in each experiment in order to evaluatethe bacterial susceptibility to UV illumination alone. In orderto test the antibacterial efficiency of TiO2 systems withoutirradiation, some of the bacteria samples were exposed to themunder darkness during the entire experiment.

3. Results and discussion

On the basis of previous results [23], gold deposition on TiO2

was performed under suitable conditions aimed at producingdispersed gold nanoparticles on titania (see section 2). All as-prepared samples were homogeneous, crack-free and bluish-colored. Herein, pure and Au-containing specimens aredenoted as TiXXX and AuXXX, where XXX indicates theannealing temperature. In particular, attention was devotedto the influence of thermal treatments on the properties ofspecimens characterized by the same Au total amount.

The structural evolution upon annealing was preliminarilyinvestigated by GIXRD, which displayed TiO2 crystallizationin the anatase phase at T = 400 ◦C and a subsequentincrease of the diffracted intensity at 600 ◦C, with no signalsrelated to the other TiO2 polymorphs. It is worth highlightingthat anatase is highly desirable in view of photocatalyticapplications, being characterized by a higher activity thanrutile TiO2 [13, 35].

The anatase crystallization was almost unperturbed alsoin the patterns of Au/TiO2 systems (figure 1), as indicated

200 nm

(a)

200 nm

(a)

200 nm

(b)

200 nm

(b)

200 nm

(c)

200 nm200 nm

(c)

Figure 2. Selected plane-view FE-SEM micrographs of Au/TiO2

specimens as a function of annealing temperatures: (a) 200 ◦C;(b) 400 ◦C; (c) 600 ◦C.

by the diffraction peaks at 2ϑ = 25.3◦ (101) and 48.0◦(200). Notably, such diffractograms were dominated by thereflections of fcc gold at 2ϑ = 38.4◦ (111) and 44.5◦ (200),undergoing an intensity increase and a progressive sharpeningwith the annealing temperature. These trends correspondedto nanocrystal sizes between 12 and 15 nm for Au andlower than 20 nm for anatase, and suggested a progressivestructural/morphological evolution of the obtained systemsupon thermal treatment.

Indeed, this prediction was confirmed by FE-SEM. Ascan be observed by the plane-view micrographs reported infigure 2, all samples were characterized by a homogeneousdistribution of gold nanoaggregates on the titania surface.Nevertheless, since the deposited Au amount was thesame for all samples (see above), their surface densityand morphological features were directly dependent on theannealing conditions. In particular, a progressive shapevariation of gold particles from island-like [36] to almost

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Nanotechnology 18 (2007) 375709 L Armelao et al

Figure 3. (a) Photobleaching of Plasmocorinth B (absorbance atλ = 527 nm versus irradiation time) in the presence of different TiO2

and Au/TiO2 nanosystems. The curves represent the stable repetitionfor each film, which was achieved after the first activation cycle(elimination of surface impurities, etc.). (b) Different photobleachingcycles of Plasmocorinth B for Au600 specimen.

spherical and an increase in the average interparticle distancetook place on increasing the treatment temperature. Suchmorphological and size changes of gold nanoparticles weremainly attributed to the influence of ex situ thermal treatments.In particular, upon increasing the annealing temperature largerparticles grew at the expense of the smaller ones (Ostwaldripening), and coalescence/agglomeration processes becameprogressively more marked [23, 30], with a concomitantevolution of the particle shape. For all the investigated systems,the observed particles were likely formed by the agglomerationof different crystallites. Image analysis indicated that the Aumorphological evolution induced by annealing resulted in aprogressively decreasing TiO2 coverage (44.0%, 32.0% and28.0% for samples reported in figures 2(a)–(c), respectively).This phenomenon was in good agreement with previouslyreported results [23, 30] which indicated a gold redistributiondue to particle agglomeration upon thermal treatment. Inparticular, a gold penetration even in the sub-surface titanialayers was shown and proved to be directly dependent on theselected annealing conditions.

These results suggested a direct influence of themorphological features on the photocatalytic performances

Table 1. First-order reaction rate constants and half-life times for thedegradation of Plasmocorith B for different TiO2 and Au/TiO2 filmsand Degussa P25.

Sample k (10−4 s−1) t1/2 (min)

Ti400 0.582 199Ti600 1.390 83Au400 0.694 166Au600 2.860 40Degussa P25 6.530 18

which are significantly affected by the size and shape of thegold particles [2]. To obtain a deeper insight into theserelations and show the peculiar role of Au nanoparticles inAu/TiO2 systems, the photocatalytic activity of TiO2 andAu/TiO2 specimens was investigated as a function of theannealing conditions. In figure 3(a), the photobleaching curvesfor Plasmocorinth B obtained in the presence of differentspecimens are compared with that pertaining to a standardDegussa P25 photocatalyst, prepared as described in section 2.The latter served as a reference of photoactive standardmaterial, but showed a poor adhesion to the substrate, whichprogressively worsened with prolonged cycling.

All the curves exhibited pseudo-first-order reactionkinetics, as already observed for azo-dye photodegradationpromoted by similar systems [2, 22]. The apparent first-orderrate constants and half-lives of the azo-dye (table 1) indicatedthat both thermal treatment and gold deposition have a markedinfluence on the system photocatalytic activity towards azo-compound degradation.

As for the pure TiO2 specimens, the photocatalytic activityincrease upon more severe thermal treatments was relatedto the parallel improvement in the system crystallinity (seeGIXRD results) that, in turn, reduced the content of the systemdefects, which act as recombination sites of photogeneratedholes and electrons [37]. The dispersion of gold nanoparticleson the TiO2 matrix enhanced the dye degradation with respectto pure titania systems and this effect was more pronouncedupon increasing the treatment temperature. The improvementresulting from gold introduction can be explained taking intoaccount that the obtained photocatalysts present a compositesurface acting both as a photon-capturing system and as a gold-promoted substrate. In particular, TiO2 irradiation results inthe production of electron–hole pairs that, in turn, react withelectron acceptors and OH− yielding O.−

2 and ·OH radicals,that are both ROS species involved in the oxidation of organicmolecules [4, 6, 8, 9, 11, 13, 15, 17]. As regards Au/TiO2

composites, the photocatalytic activity enhancement has beenascribed to the interfacial charge transfer promoted by thegold particles. In fact, due to the difference in the workfunctions of TiO2 and Au, conduction band electrons can beattracted by the metal particles, thus preventing electron–holerecombination phenomena [2]. It is also worth noting thatthe Au/TiO2 contact has been reported to have a Schottkybarrier character [38]. As a consequence, electrons excited byillumination at the gold particles are likely to move to the metalsurface driven by the radiation electric field and light-inducedcharge separation becomes easier, producing consequently anincrease in the photocatalytic efficiency with respect to pureTiO2 [11, 37–39]. This mechanism is also compatible with the

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Nanotechnology 18 (2007) 375709 L Armelao et al

15 min 30 min 45 minU

V o

nly

Au6

00

Ti6

00

Figure 4. Decrease in the number of viable Bacillus subtilis bacteriaafter exposure to Au/TiO2 nanosystems and UV light as a function ofirradiation time.

increase of the photocatalytic activity up to an optimal Au/TiO2

loading [2, 11].In the present case, the different efficiencies of Au/TiO2

samples reported in figure 3(a) reveal a direct influence of theannealing temperature on the system’s performance. Besidesa progressive reduction in the system defect content, asalready described for pure TiO2 samples, such behavior canbe interpreted considering that thermal treatments affect thesurface coverage, size and shape of Au nanoparticles which,in turn, play a key role in influencing the interfacial chargetransfer during irradiation [2]. In fact, an inspection offigure 2 indicates that an increase in the annealing temperaturefrom 200 to 600 ◦C resulted in a decreased titania coverage,meaning that more titania surface is directly exposed to theirradiation (see above) and available for pollutant adsorptionand light absorption. Therefore, the beneficial effect ofgold nanoparticles on the photocatalytic activity was moreclearly evident after treatment at 600 ◦C. On this basis, anantagonism between the gold synergistic action following theabove-described mechanism and a screening resulting fromthe presence of a gold excess on the titania surface cannot bedefinitely ruled out.

Previous works [2] have focused on the deteriorationof the metal/semiconductor interface in nanocompositephotocatalysts, attributed to the metal oxidation by thephotogenerated holes and/or surface hydroxyl radicals. Tothis aim, the reproducibility of the photocatalytic activityon Au/TiO2 systems was examined in order to check theirpotential use in practical systems. Figure 3(b) reportssuccessive photocatalytic degradations performed with sampleAu600. As can be observed, the photocatalyst presented avery stable response and did not show appreciable activitylosses after repeated uses, making thus the present results veryencouraging for technological applications.

Subsequently, attention was devoted to bactericidalactivity tests using the model organism Bacillus subtilis,focusing in particular on the 600 ◦C annealed samples. ThePYE agar plates containing the bacteria that were recoveredafter exposure to pure and Au-containing TiO2 systems and

rela

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nu

mb

er o

fb

acte

ria

surv

ived

rela

tive

nu

mb

er o

f b

acte

ria

su

rviv

ed

exposure time (minutes)

(a)

(b)

Figure 5. (a) Effect of pure and Au-containing TiO2 nanosystems onthe viability of Bacillus subtilis. A bacterial suspension(107 CFU ml−1) was placed on microscope glasses coated with TiO2

films calcined at 600 ◦C with or without Au. Films obtained fromDegussa P25 powders and uncoated silica slides served as positiveand negative control, respectively. (b) Relative number of Bacillussubtilis bacteria after 15 min exposure to different Au/TiO2

specimens and/or UV light.

(This figure is in colour only in the electronic version)

UV illumination are shown in figure 4. Bacterial growthwas still confluent after 15 min exposure to UVA light onlyand subsequently underwent a slow decrease. Conversely, asignificant growth reduction was observed already after 15 minof irradiation, if TiO2 films annealed at 600 ◦C were used inaddition to UVA light. Under these conditions, almost nobacteria survived after 30 and 45 min of treatment. UsingAu/TiO2 samples, a similar trend was observed, but thereduction in CFU numbers appeared slower than for puretitania. The detrimental effect of gold nanoparticles on theantibacterial activity of TiO2 specimens can be explainedby the concurrence of different causes. First, compared tosimple organic molecules, the Bacillus subtilis dimensions areappreciably higher (1.1–1.5 μm width by 2.0–6.0 μm length),thus preventing its intimate contact with the titania surface forthe present Au/TiO2 nanosystems.

Furthermore, the modification of surface charges of thefilms due to Au aggregates should also be taken into account.In fact, the TiO2 antimicrobial activity was recently explainedby the attraction between opposite surface charges (titaniapositive, microorganisms negative) [40]. Such an interactionwas probably diminished in the case of a titania surface coveredby gold nanoaggregates, as for the present Au/TiO2 samples.

Figure 5(a) shows the antibacterial activity of TiO2 andAu/TiO2 systems annealed at 600 ◦C. It is worth highlightingthat pure and transparent TiO2 samples were almost as effective

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Nanotechnology 18 (2007) 375709 L Armelao et al

in bacterial elimination as coatings obtained from commercialDegussa P25 powders. Despite Au introduction decreasing theantibacterial efficiency of TiO2 specimens, a significant drop inbacterial viability was observed even for Au/TiO2 systems after30 and 45 min of UV illumination. Even if Bacillus subtilis wassusceptible to UV light alone to some extent, especially afterprolonged exposure time, the combination of TiO2 samples andUV light proved to be appreciably more efficient, resulting in alower activation time for bacteria elimination. As expected,TiO2 films were ineffective as antimicrobial coatings underdark conditions, since no reduction in bacterial growth couldbe observed even after 45 min exposure to TiO2 films alone.

The results in figure 5(b) are consistent with the decreaseof antimicrobial activity for Au/TiO2 nanosystems with respectto pure TiO2. Furthermore, an increase in the annealingtemperature from 200 to 600 ◦C improved the antibacterialproperties of both TiO2 and Au/TiO2 systems, in agreementwith the progressive crystallinity increase (see above) and withthe results obtained from the degradation of the azo-dye.

4. Conclusions

This work was devoted to the investigation of the photocat-alytic and antimicrobial activity of TiO2 and Au/TiO2 nanosys-tems obtained by sol–gel and RF-sputtering/sol–gel routes, re-spectively. In particular, the obtained systems were annealedin air at temperatures ranging from 200 to 600 ◦C and charac-terized in their structure and morphology before investigationof their functional properties. A suitable choice of the process-ing conditions enabled us to obtain nanosystems endowed withphotocatalytic performances comparable to those of films ob-tained from standard Degussa P25, a highly active crystallinetitania powder. It is also worth highlighting that the presentsamples display a better adhesion to the adopted substrates,thus enabling us to prevent the problems connected to catalystrecovery from the used solutions.

As concerns the photocatalytic degradation of the azo-dye Plasmocorinth B, an appreciable improvement in thephotocatalytic efficiency of titania nanosystems was achievedupon gold deposition. This effect, which was stronglydependent on the shape, size and distribution of goldnanoparticles on the titania surface, was attributed to theirpeculiar action in preventing electron–hole recombinationphenomena.

Finally, concerning the antibacterial effect of the obtainedsystems, pure TiO2 specimens exhibited an efficiency almostas high as that displayed by reference coatings obtainedfrom standard Degussa P25. In this case, the introductionof gold had a detrimental effect on the TiO2 functionalproperties, which could be attributed: (i) to the appreciablyhigher bacteria dimensions with respect to the simple organicpollutant molecules, preventing its intimate contact with TiO2

surface; (ii) to an alteration of the titania surface charge,resulting in a less efficient interaction.

Acknowledgments

This work was financially supported by Research ProgramsCNR-INSTM PROMO, COFIN-PRIN 2005, FIRB-MIUR

RBNE033KMA ‘Molecular compounds and hybrid nanostruc-tured materials with resonant and nonresonant optical proper-ties for photonic devices’ and INSTM-PRISMA ‘Oxide filmswith high dielectric constant from liquid and vapor phaseroutes’. The photocatalytic part of the study was done inthe framework of a Slovene–Italian bilateral cooperation (BI-IT/05-08-014 ‘Mesoporous titania-based films as photocata-lysts for pesticides degradation’).

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