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Solar Energy 122 (2015) 11–23

Photocatalytic TiO2 sol–gel thin films: Optical andmorphological characterization

E. Blanco, J.M. Gonzalez-Leal ⇑, M. Ramırez-del Solar

Department of Condensed Matter Physics and Institute of Electron Microscopy and Materials, University of Cadiz,

Campus Universitario de Puerto Real, 11510 Puerto Real, Cadiz, Spain

Received 16 June 2015; received in revised form 29 July 2015; accepted 31 July 2015

Communicated by: Associate Editor Gion Calzaferri

Abstract

TiO2 thin films have been deposited onto glass substrates by sol–gel dip coating technique. The morphological and optical character-ization of the as-deposited and annealed films at different temperatures in the range 150–700 �C have been carried out by UV–vis spec-troscopy and ellipsometry. An original rigorous optical characterization method based on both the transmission spectrum of a tri-layeroptical system and the Tauc-Lorentz dispersion model, has been developed and successfully applied to calculate the thickness, opticalconstants, absorption edges and optical bandgaps of the dip-coated TiO2 films. Micro-Raman spectroscopy and thermoanalyticalmeasurements have been also performed to complete the characterization of the films and support the morphological and optical results.As-deposited films have been proved to be amorphous and the onset of the formation of anatase phase has been found at annealingtemperature of 400 �C. The occurrence of anatase-phase and the increase of crystal size with increasing annealing temperature have beenboth evidenced by Raman spectroscopy measurements performed in the samples. Such evolution has been accompanied by larger valuesof the refractive index and a slight decrease in the values of the optical bandgap. Results indicate that samples annealed at 500 �C areorganic residues free, and they shows small anatase crystals, highly porous network, as well the lower electron–hole recombination ratewithin the annealing temperature series.� 2015 Elsevier Ltd. All rights reserved.

Keywords: TiO2; Sol–gel growth; Photocatalysis; Optical properties

1. Introduction

Access to clean water and sanitation is one of the mostimportant problems affecting people over the world, evenin regions traditionally considered water-rich. Newresearch is done in methods of purifying water at lower costand minimizing the use of chemicals. Additionally to tradi-tional compounds such as heavy metals, new emergingmicropollutants (endocrine disrupters, pesticides, pharma-

http://dx.doi.org/10.1016/j.solener.2015.07.048

0038-092X/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +34 956 016569/2808/6317.E-mail address: juanmaria.gonzalez@uca.es (J.M. Gonzalez-Leal).

ceutical products, etc.) are present in the aquatic environ-ment and their toxic, persistent and bioaccumulativeproperties may have chronic direct or indirect effects onecosystems and on human health (Richardson, 2008).

Advanced Oxidation Processes (AOPs) may be used fordecontamination of water containing organic pollutantsand/or for disinfection removing current and emergingpathogens (Malato et al., 2009). They are characterizedby the same chemical feature: production of hydroxyl rad-icals (�OH) that are known as indiscriminate oxidizingagents and they are able to oxidize and mineralize almost

12 E. Blanco et al. / Solar Energy 122 (2015) 11–23

any organic molecule yielding CO2, H2O and diluted inor-ganic acids.

Heterogeneous solar photocatalytic detoxificationallows the production of hydroxyl radicals when they areimmersed in water, by the combination of a wide-bandgap semiconductor and irradiation with UV–vis light fromthe sun. Absorption of photon of energy greater than thebandgap energy leads to the formation of an electron/holepair. Redox reactions may occur provided recombination isprevented: valence band holes are powerful oxidants whilethe conduction bands electrons are good reductants.

Whenever different semiconductor materials have beentested under comparable conditions for the degradationof the same compounds, titanium dioxide, TiO2, has beendemonstrated to be the most efficient photocatalyst todestroy pollutants because of its following properties: sta-bility, low cost, non-toxic, high turnover, versatile in theintegration with different substrates, complete mineraliza-tion of organic pollutants, high catalytic activity, strongoxidizing power, stable against photo corrosion and chem-ical attack (Singh et al., 2013). These properties make it asuitable platform to be employed for water and air purifi-cation through utilization of free solar energy (Pelaezet al., 2012).

TiO2 exits as three different polymorphs: anatase, rutileand brookite. The most stable form is rutile and typicallythe metastable anatase and brookite will transform to thethermodynamically stable rutile by calcination upper600 �C (Pelaez et al., 2012). It is an n-type semiconductordue to oxygen deficiency with a band gap around of 3.2–3.0 eV depending on which of the forms is presented(Lan et al., 2013).

Intensive research has been developed about the photo-catalytic properties of TiO2 nanoparticles (Bhosale et al.,2014). Nevertheless, they present problems in the use incontinuous flow systems for water purification and, addi-tionally, the need of separation or filtration steps in theseapplications. This post-treatment recovery is both timeand money consuming. To overcome these drawbacks, ithas been proposed as an alternative the photocatalystimmobilization in the form of thin film on a suitable sub-strate as glass, quartz, stainless steel or ceramics (Chenand Dionysiou, 2006).

Dip coating is the deposition of a solid film on a sub-strate by immersion in a sol or solution, withdrawal anddrying (Brinker et al., 1991). Optical-quality films of con-trolled index and thickness are readily obtainable by con-trolling the microstructure via the porosity, with simple,inexpensive apparatus. More or less complex and bigshapes, like cylinder tubes, can be coated in one step; thissimplicity is not always possible with evaporative or sput-tering techniques. Moreover, the purity of solution chem-istry, such as sol–gel route, can be exploited.

Various strategies have been adopted for improving thephotocatalytic efficiency of TiO2. They can be summarizedas either morphological modification, such as increasing

surface area and porosity, or as chemical modifications,by incorporation of additional components in the TiO2

structure.The aim of the present paper is to report the optical and

morphological characteristics of TiO2 thin films preparedby dip-coating deposition, to be applied for water decon-tamination. Morphological and optical results have beencompleted with others coming from calorimetric andRaman spectroscopic measurements in order to supportthe discussion. Results of the photocatalytic activity ofthese TiO2 films, coated over cylinders are actually underprogress, and they will be published in due course.

For the optical and morphological characterization ofthe samples, an original optical method based on boththe rigorous equations modeling the optical transmissionspectra at normal incidence of a tri-layer (film-substrate-film) optical system and the Tauc-Lorentz dispersion equa-tions (Jellison and Modine, 1996a,b), has been developed.The method allows determining the optical constants,absorption edges, bandgap energies and thicknesses ofthe dip-coated TiO2 films.

To the best of our knowledge, works in the literaturereporting the optical characterization of TiO2 samples onthe basis of Tauc-Lorentz model are based on spectro-scopic ellipsometry measurements (Lewkowicz et al.,2014; Saha et al., 2014).

Alternatively, works based on the optical transmissionspectra of tri-layer samples are typically based on approx-imated equations for this multilayer optical system andCauchy dispersion is used to model the refractive index(Sreemany and Sen, 2004). In such cases the dispersion ofthe refractive index fits relatively well the transparent spec-tral region, where the extinction coefficient is considered tobe zero, but fails to reliably model the behavior at lowwavelengths where the absorption starts being significant.

We have also found that the analysis of the opticalabsorption for determining the optical bandgap is widelyperformed by using the equation a = �1/d Log(T), a beingthe absorption coefficient, d the film thickness and T thetransmittance. It is worth mentioning that this equation isvalid provided the light attenuation is totally produced overthe light beamwhen passing through the film thickness once,a no significant contributions to the intensity of the transmit-ted light come from the reflections at the film interfaces, as itis usually the case for thin films as the ones studied here. Itshould be pointed out that the determination of both non-direct and direct optical bandgap from Tauc’s plot on thebasis of the so-calculated a values will be affected by theseinterferences and it could lead to wrong values for Eg

(Hishikawa et al., 1991). The optical characterizationmethod developed and used here avoids such error sources,which highlight both the originality and the reliability of theresults reported in the present paper.

On the other hand, porosity of the films has been evalu-ated from a model based on effective media approximation.It is possible to find in the literature the application of dif-

Fig. 1. Sketch of the tri-layer optical system describing the samples understudy: n stands for the refractive index, k for the extinction coefficient andd for the thickness of the TiO2 films; ns stands for the refractive index, ksfor the extinction coefficient, and ds for the thickness of the glass substrate.

E. Blanco et al. / Solar Energy 122 (2015) 11–23 13

ferent mixing models to the microstructure of TiO2 thinfilms based on the effective media approximations.Recently it has been published a study about the compar-ison of experimental data with different effective mediamodels (linear and nonlinear mixture models) applied toTiO2 thin films (Matthias et al., 2014). They demonstratethat an empirical power law expression holds from the Lor-entz–Lorenz theory and allows calculating effective poros-ity of TiO2 thin films from the effective refractive indexwith high accuracy.

2. Experimental

TiO2 precursor sol was prepared by hydrolysis of tita-nium n-butoxide (TBOT). For this purpose a solution ofthe alkoxide containing acetylacetone (acac), used aschelating agent to reduce titanium alkoxide functionality,and ethanol, as a solvent, in a molar ratio 1:0.5:35 was pre-pared. Then, HNO3 acidified water (pH = 1) was addeddrop wise to this solution under stirring in a molar ratioof 2. Modified Ti(OR)4�x(acac)x undergoes a more con-trolled hydrolysis and condensation than reactive Ti(OR)4 precursor, and precipitation of undesired phasesare thus prevented. After 1 h stirring at room temperature(RT), sol was kept at RT for ageing 100 h prior to deposi-tion. Sol was observed to be stable for weeks without sig-nificant evolution (no evidence of flocculation).

TiO2 films were deposited on both sides of planar high-quality borosilicate glass substrates (BOROFLOAT@)which tolerate heating temperatures up to 700 �C. Theywere previously cleaned in a surfactant solution underultrasonic agitation followed by rinsing and drying (Birchet al., 1995). Dip coating deposition was performed fromthe precursor solution, by using a homemade apparatusat precisely controlled withdrawal speed. Fresh films weresubsequently dried at 150 �C for 30 min. This procedurewas repeated until reaching the total number of accumu-lated layers desired. Finally, samples were annealed inair, into an oven at the different temperatures in the range150–700 �C.

The experimental parameters analyzed were the numberof layers N = 3, 4, 5, withdrawal speed (U = 75, 100, 110and 125 mm/min) and the annealing temperature (150,300, 400, 500, 550, 600, 650, 675 and 700 �C). Samplesannealed at 500 �C are coded as U_N. When the annealingtemperature is different than 500 �C, the correspondingvalue is indicated.

Thermogravimetric analysis and differential scanningcalorimetry (TGA–DSC) were performed in a PerkinElmerSTA6000 coupled to a PerkinElmer SpectrumTM 100 infra-red (IR) spectrometer by the TL 8000 transfer line forevolved gas analysis (EGA) under oxygen atmosphere ona powdered gel sample obtained after gel and drying at50 �C a small fraction of the TiO2 sol.

Raman spectra were measured at 6 cm�1 resolution, inthe range 30–800 cm�1, using a dispersive Raman spec-trometer (Jasco, model NRS-7200) with a 100� objective

coupled to a 532 nm Nd-YAG laser excitation source oper-ating at 5.1 mW power, and collected for 1 min (3 accumu-lations of 20 s exposure each).

Optical transmission spectra of the samples were mea-sured at normal incidence in the 300–850 nm spectral rangeby using an Avantes AVS-USB2000 fiber optic spectrome-ter connected to a deuterium–halogen tungsten DT-Mini-2-GS light source from Ocean Optics.

Evaluation of the films thickness profiles was carried outby reflection ellipsometry with a PLASMOS SD-2300automatic ellipsometer with 2 mm2 spot size and 632.8 nmwavelength. Ellipsometric measurements were carried outat room temperature at Brewster condition as incident angle.

3. Optical characterization method

A sketch of the tri-layer optical system describing thesamples under study is illustrated in Fig. 1. Explicit expres-sions for the reflectance and transmittance of multi-layeroptical systems are complicated and difficult to manage.However, recurrence relations can be derived fromtransfer-matrix methods for optics, which are easily imple-mented in programming languages (Heavens, 1955). Suchrecurrence relations are introduced in Appendix A. Thetransmittance of a tri-layer optical system (film-substrate-film), and equivalently 4 interfaces (air-film, m = 1, film-substrate, m = 2, substrate-film, m = 3, film-air, m = 4),can be derived from these recurrence relations, performingthe following calculation:

T �ðns; ks; c2; n; k; d; kÞ ¼Q4

m¼1 tmp21;4 þ q21;4

; ð1Þ

We will assume in this work that films on both sides of thesubstrate have identical values of the refractive index, n,extinction coefficient, k, and thickness, d (see Fig. 1).

14 E. Blanco et al. / Solar Energy 122 (2015) 11–23

Typically, the substrate is a thick layer of the order ofseveral millimeters. Because of this, the interferences pro-duced in its interfaces, due to the phase c2 ¼ 2p=knsds,show very high spatial frequencies that cannot be resolvedby the spectrophotometer (ns and ks stand for the refractiveindex and extinction coefficient of the substrate, respec-tively). Such a fact can be considered by averaging Eq.(1) over c2, i.e. solving the following integral:

T ns; ks; n; k; d; kð Þ ¼ 1

2p

Z 2p

0

T � ns; ks; c2; n; k; d; kð Þ dc2 ð2Þ

Values of ns and ks at wavelengths within working spectralrange can be derived by using both the transmission andreflection spectra of bare substrate, T s;expðkÞ and Rs;expðkÞ,respectively, by solving the equation system described inAppendix A (Gonzalez-Leal et al., 2002). Once the disper-sions of both nsðkÞ and ksðkÞ are determined, the transmit-tance of the tri-layer optical system will be a function of theoptical constants, n and k, and thickness, d, of the TiO2

films, i.e., T ¼ T ðn; k; d; kÞ.Assuming that the dispersion of nðkÞ and kðkÞ can be

modeled by some parametrical function, a best-fitting algo-rithm can be implemented to specifically determine both ofthem for a particular optical material. In particular, theTauc-Lorentz model, which has been widely used to char-acterize amorphous semiconductors (Jellison andModine, 1996a,b; Ferlauto et al., 2002), as well as severalkinds of polycrystalline and crystalline-amorphous mixed-phase materials (Orava et al., 2008l Langereis et al.,2009; He et al., 2005; Saha et al., 2014), has been used inthe present work. Mathematical expressions of the refrac-tive index and extinction coefficient according to theTauc-Lorentz model are introduced in Appendix A. Sucha model is defined on the basis of 5 parameters, namely,Eg is the optical bandgap, e1 is the value of the real partof the dielectric function at high frequencies, Eo is the oscil-lator energy, and A and C are related to the strength andthe width of the absorption peak, respectively.

Fig. 2. Thermogravimetric (ATG) and calorimetric analysis curves of powdereand weight loss changes.

The following multivariable function can be defined inorder to determine the set of values for the Tauc-Lorentzparameter, which better reproduce the experimental trans-mission spectrum under analysis in the working spectralrange k0 6 ki 6 k1.

f Eg; e1;Eo;A;C; d� � ¼ Xki

ki¼k0

T Eg; e1;Eo;A;C; d; ki� �� T exp kið Þ� �2 ð3Þ

The values of the fitting parameters Eg; e1;Eo;A;C and d

can be determined by using some numerical method forminimizing f. In particular, the so-called Nelder–Meadmethod (Nelder and Mead, 1965), as implemented in Wol-fram Mathematica 10.1, has been used to perform the cal-culations for this work.

4. Results and discussion

Smooth, continuous and homogeneous TiO2 films wereobtained under the preparation conditions describedabove.

In order to design the coating processing, a preliminarythermal study was carried out. For this purpose, an aliquotof the sol was gelled at 50 �C and dried for several hours.TGA/DSC was performed on this TiO2 gel in order toestablish the main thermal events that take place onheating. Experiments were performed at 5�/min until900 �C in oxygen atmosphere. Fig. 2 shows TG/DTGtypical profiles where three main processes can bedistinguished contributing to a total weight loss of 38%.FIR-EGA spectra recorded during TGA experiments allowus to elucidate about the nature of the thermal events.

Fig. 3 presents the spectra recorded specifically at thetemperatures of the main DTG/DSC peaks. Initially, anendothermic process was observed in the 50–150 �C range(4.4 wt%, DH = 63 J/g), corresponding to desorption ofwater and residual alcohol remaining physically adsorbedwhich depends on the environmental conditions. For that

d TiO2 gel. DTG curve is also included for a better comparison of enthalpy

Fig. 3. IR spectra of the gases evolved from thermo balance at thetemperatures corresponding to the main DTG peaks.

Fig. 4. Raman spectra evolution on annealing temperature of a repre-sentative sample of the 100_4 series. Values of the FWHM of the band at144 cm�1 as a function of annealing temperature are plotted in the inset.

E. Blanco et al. / Solar Energy 122 (2015) 11–23 15

reason, experiment included an initial thermal stabilizationat 100 �C for 5 min, in order to establish a regular startpoint and focus on the thermal processes at highertemperature.

The main weight mass loss (see Table 1) occurs in the150–320 �C range that is assigned to the exothermic oxida-tion of unreacted substituents and chemisorbed organics.The corresponding IR absorption spectrum shows intensebands of alkyl group (2969, 1368, 1230 cm�1), ketone car-bonyl (1751 cm�1), CO2 (2358, 2310 cm�1), C–O (1230,1050 cm�1) (Leaustic et al., 1989), revealing intensiveevolution of alkoxy groups and acetylacetone/acetic acidwith minor evolution CO2. A new exothermic process, cen-tered at 371 �C, can be related with additional decomposi-tion of acetylacetonate groups bonded to TiO2 species(Acik et al., 2009). At 470 �C a sharp and intense DSCexothermic peak with no weight loss associated is clearlydue to crystallization of the amorphous TiO2 in anatasephase (Attar et al., 2007).

Finally, the exothermic peak present at 525 �C in DSCdiagram and the broad weight loss in this range(460–675 �C) are usually associated with additionalcondensation reactions, that takes place at the surface ofthe inorganic skeleton, leading to further crosslinking andwater release (Brinker et al., 1985). FTIR spectra above400 �C confirm the elimination of acetylacetone groupsand only small evolution of CO2, CO and water.

Sintering process from the amorphous gel dried at 150 �C to the crystalline TiO2 has been monitored by Ramanspectroscopy. Fig. 4 shows Raman spectra of a representa-

Table 1Main features of the thermal processes identified in thermal analysis.

Temp. range (�C) Weight loss (%) DH (J/g) DSC peak (�C)

150–320 15.4 �137.5 265320–460 10.9 �292 350– – �284 472460–675 6.2 �610 525

tive sample of the set 100_4, which was annealed at 300,400, 500, 600 and 700 �C.

It is observed in the figure that samples annealed belowor equal to 300 �C show no significant Raman features.Raman bands occur in samples heated at 400 �C andabove. The spectra show in all cases the characteristicsbands of the TiO2 anatase phase at 148 cm�1 (Eg1 mode),389 cm�1 (B1g1 mode), 523 cm�1 (A1g mode) and646 cm�1 (Eg3 mode). The Raman band at 200 cm�1,corresponding to Eg2 mode, becomes also visible afterannealing at 600 and 700 �C.

It is well-known that both the Raman band position andwidth (FWHM) are sensible to crystal size and the degreeof oxidation. This fact has been analyzed in the mostintense Eg anatase band occurring at 144 cm�1. Althougha quantitative relation between crystal size and the FWHMvalue is not straightforward (Li Bassi et al., 2005), the sig-nificant linear decrease observed with annealing tempera-ture (inset of Fig. 3) indicates a structural improvement(Mathews et al., 2009) and nanocrystals growth whenincreasing the annealing temperature.

It is important to note that conclusions about the evolu-tion of the Raman band positions are more complex,because the position of the band is more affected by oxygenstoichiometry, which concurrently evolves on heating.

Optical transmission spectra of a sample set preparedwith different withdrawal speed and different number offilm layers, all annealed at 500 �C, were measured. Thistemperature was chosen according to a compromisebetween the elimination of unwanted residues (resultsreported in Figs. 2 and 3), TiO2 crystallization (resultsreported in Figs. 2 and 4) and keeping an extensive porenetwork, as we will see later.

In all cases the transmission spectra of the as-depositedand annealed films exhibited interference fringes in the vis-ible region (the number of them increasing with increasingthe number of coating layers) and a sharp fall in the UVregion (evidencing the onset of the absorption region), asillustrated in Fig. 5.

16 E. Blanco et al. / Solar Energy 122 (2015) 11–23

Table 2 summarizes the film thicknesses derived fromthe best-fitting procedure based on Eq. (42), as well asthe values of the refractive index at 632.8 nm wavelengthfor these samples. Regarding the values of the refractiveindex, they do not present a regular trend with U and N,but all the values are in the 2.06 ± 0.06 range. Comparisonof the refractive index results with data published in the lit-erature shows close similarity to other measurementsobtained from sol–gel deposited thin films (Hou et al.,2003; Sreemany and Sen, 2004).

Film thicknesses follow a linear trend with the numberof layers giving rise to an average thickness per layer foreach withdrawal speed. The average layer thickness hasbeen plotted in Fig. 6 as a function of withdrawal speed.These results are in accordance with other authors at thisrange of withdrawal speed (Mohallem and Aegerter,1988; San Vicente et al., 2001).

The optical transmission spectra at normal incidence ofsamples annealed at different temperatures in the range150–700 �C were also measured. Spectra were analyzedby using the optical characterization method based onEq. (42). Results of the Tauc-Lorentz fitting parametersfor a representative 100_4 sample are listed in Table 3.

The experimental ellipsometrics angles W and D can becorrelated with the thickness and optical constants of a filmby:

tanWeiD ¼ rp01 þ rp12e�2id

1þ rp01rp12e�2id

1þ rs01rs12e

�2id

rs01 þ rs12e�2idð4Þ

where d in degrees is

d ¼ 360

kdðn21 � sin2 /Þ ð5Þ

and the Fresnel reflection coefficients are given by:

rp12 ¼n1 cos/2 � n2 cos/1

n1 cos/2 þ n2 cos/1

ð6Þ

Fig. 5. Experimental UV–vis transmission spectra of a representative100_3 sample and the bare substrate. The theoretical transmissionspectrum, as synthesized by using the best-fitting algorithm based onEq. (3), is also plotted. Results of the fitting parameters are reported in thefigure.

rs12 ¼n1 cos/1 � n2 cos/2

n1 cos/1 þ n2 cos/2

ð7Þ

Then, W and D are given explicitly as functions of theangle of incidence, the light wavelength in air, the opticalconstants of the film and the substrate, as well as the thick-ness of the film. The separation of Eq. (4) into its real andimaginary parts yields one equation for D and one for W.As it is known, W and D are cyclic functions of thicknessand the curves repeat periodically with every 180� changein d, with a period (a thickness value) depending on theoptical system. Thus, the determination of unique valuesof thickness and refractive index of an unknown transpar-ent film from a single measurement of W and D is notpossible.

In our case, the refractive index and average thickness ofthe film as determined from the optical transmissionspectra, it gives us the chance of applying these startingvalues in the ellipsometric calculations. Then, assuming afix refractive index (nfix mode) it is possible to make a rasterscan of the samples giving rise to a high accuracy profile ofthe films. Average thicknesses calculated from ellipsometricmeasurements are also included in Table 2.

The raster scan of each sample can give informationabout films homogeneity. As an example, Fig. 7 showsthe scan of a representative 100_3 sample where the filmexhibits no-cracked flat surface. As it can be seen in thisfigure, the ellipsometric angles distributions along the filmalso show a great homogeneity. The raster scan of thick-ness profile obtained from W and D shows a variation of12 nm between the highest and the lowest point along thefilm surface. In the transversal section, film shows a para-bolic profile with a higher thickness in the central regionand the thinner part close to the borders. At the same time,there is a softly increase of the thickness along the wettingdirection induced by gravitational forces during the dip-ping process, as expected for dip-coated films (Brinkeret al., 1991).

Fig. 8 shows the superposition of three raster scans forthe 75_3, 75_4 and 75_5 samples. It is pointed out thethickness linear increase with the number of layers. Noticethe relatively low relevance of the roughness, whichdecreases with the film thickness as 6.2%, 4.9% and 3.5%,respectively.

On the other hand, the optical method allows determin-ing the refractive-index dispersion, nðkÞ, as well as theabsorption edges of the films, að�hxÞ, on the basis of theTauc-Lorentz model.

Fig. 9(a) shows the dispersion curves for all the samplesannealed at 500 �C. An outstanding analogous behavior isobserved for all the samples, which is indicative ofstructural and compositional similarity at this particularannealing temperature, as well as a good indicator of thereproducibility of the dip-coating deposition technique.

For the representative 100_4 sample annealed at differ-ent temperatures in the range 150–700 �C the dispersioncurves (Fig. 9(b)) show a clear overall increase with increas-

Table 2Values of the film thickness, d, single-coating-layer thickness, d/N, average single-coating-layer thickness, d=N , determined from the optical transmissionspectra and from ellipsometric measurements. Refractive-index value at 632.8 nm, n(632.8), determined by the optical characterization method is alsoreported.

UV–vis transmission Ellipsometry

N n(632.8) d (nm) d/N (nm) d=N (nm) d (nm) d/N (nm)

Serie 75

3 2.04 142 47 46 ± 2 159 53.04 2.07 177 44 184 46.05 2.01 240 48 235 47.0

Serie 100

3 2.06 160 53 53 ± 1 162 54.04 2.04 217 54 205 51.35 2.08 264 53 271 54.2

Serie 110

4 2.01 230 58 58 234 58.5

Serie 125

3 2.05 186 62 62 ± 1 177 59.04 2.12 245 61 240 60.05 2.04 312 62 323 64.6

Fig. 6. Film thickness as a function of withdrawal speed.

E. Blanco et al. / Solar Energy 122 (2015) 11–23 17

ing annealing temperature. Very significant changes areobserved at lower annealing temperatures, while changesobserved at temperatures above 500 �C are gradual.

Changes at lower temperatures cannot be totallyexplained on the basis of thermal densification arguments,as can be evidenced from Fig. 10 which shows the plot of

Table 3Values of the Tauc-Lorentz fitting parameters for the representative 100_4 samperror of the fitting curves are reported.

Temp. (�C) Eg (eV) e1 A (eV)

150 2.55 1.90 27300 3.32 2.18 102400 3.30 2.20 107500 3.29 2.14 139550 3.28 2.33 134600 3.25 2.63 121650 3.23 2.66 124675 3.23 2.78 122700 3.14 3.19 89

dðn2 � 1Þ=ðn2 þ 2Þ as a function of the annealing tempera-ture, on the basis of Lorentz–Lorenz relation (Kittel, 2005),

n2 � 1

n2 þ 2¼ 4p

3Nap ) d

n2 � 1

n2 þ 2¼ 4p

3

mAap ð8Þ

where n is the refractive index, N ¼ m=V ¼ m=dA is thenumber of molecules per unit volume, d is the film thick-ness, A is the spectrophotometer light spot area and ap isthe mean polarizability. Fig. 10 highlights the differentstructural conformation of the sample when annealed at150 �C and 300 �C in comparison with the effect of theannealing at higher temperatures. This result is indeed con-sistent with the evolution observed in the Raman spectra(Fig. 4), which indicates microstructural evolution of thesample taken place during annealing, as well as the pres-ence of solvent remnants at the first stages of sample calci-nation. Changes in the refractive index at temperaturesabove 300 �C seem to be driven by film densification, asinferred from the almost constant values found for thefactor dðn2 � 1Þ=ðn2 þ 2Þ, which suggest that the numberand types of molecular dipoles, related to TiO2 anatasephase, do not show a significant evolution in the range400–700 �C.

le annealed at different temperatures. Also, values of the root-mean-square

Eo (eV) C (eV) r.m.s. error

4.72 0.44 0.00124.41 1.25 0.00054.31 1.35 0.00084.24 1.13 0.00084.23 1.07 0.00084.17 0.74 0.00034.16 0.78 0.00034.15 0.72 0.00044.11 0.48 0.0006

Fig. 7. D (a) and W (b) maps determined from ellipsometric measure-ments. Film thickness profile calculated from D and W (c).

Fig. 8. Superposition of the raster scans for the 75_3, 75_4 and 75_5samples, showing the homogeneity of the sample surfaces and the increaseof thickness with increasing the number of dip-coated layers.

Fig. 9. Refractive-index dispersion of samples with different number ofdip-coated layers and prepared at different withdrawal speed, all annealedat 500 �C (a), and the ones corresponding to a representative 100_4 sampleannealed at temperatures in the range 150–700 �C (b).

18 E. Blanco et al. / Solar Energy 122 (2015) 11–23

On the other hand, a clear shift towards low photonenergies has been found in the absorption edges of thefilms, as well as a slight decrease in the band gap energywhen the annealing temperature is increased above 300 �C, as illustrated in Fig. 11(a) and (b), respectively. Changesare also more significant a lower annealing temperatures, inthe same way as observed in the refractive-index dispersioncurves of these samples. Absorption edges show a clearthermal-darkening occurring on the samples when increas-ing annealing temperature. Sample annealed at 150 �Cshows an absorption edge with a different behavior, whichreflects in a significantly low value for Eg, presumably dueto the presence of organic residua.

As it has been previously mentioned, the microstructureevolution with temperature can be interpreted on the basisof a sintering process, leading to a decrease of the porosityand an increase of the film density. The evolution of thefilms refractive index during sintering has been followed

Fig. 10. Plot of the Lorentz–Lorenz factor dðn2 � 1Þ=ðn2 þ 2Þ as afunction of the annealed temperature.

Fig. 11. Optical absorption edges (a) and values of the optical bandgap(b) for the 75_4 sample series annealed at temperatures in the range 150–700 �C.

E. Blanco et al. / Solar Energy 122 (2015) 11–23 19

by the changes induced in the UV–vis transmission spectra.A model based on the effective media concept can beapplied to these uncracked films, allowing the evaluationof the porosity of the films from their refractive indices.The application of effective medium approximation tomesoporous films involves their analysis from the contribu-tions of the volume fractions of their components. Themixture of a dispersed pore network (refractive index np)with a pure TiO2 skeleton film (refractive index ns) resultsin a effective refractive index neff of the porous film givenby:

f neffð Þ ¼ 1� Pð Þf nsð Þ þ Pf ðnpÞ ð9Þwhere P is the volume fraction of pores or film porosityand (1� P ) would be the volume fraction of pore freeTiO2. Several mixture models have been proposed to ana-lyze a heterogeneous thin film as homogeneous with arefractive index neff . Matthias et al. (2014), proposed theLorentz–Lorenz model as the most consistent with theirexperimental results for TiO2 anatase thin film. Thisapproximation considers a refractive index function,f ðniÞ, given by:

f ðiÞ ¼ n2i � 1

n2i þ 2ð10Þ

Then, the porosity equation, obtained by consideringthe Lorentz–Lorenz model in an effective medium, results:

P ¼ ðn2eff � n2s Þðn2eff þ 2Þ

ðn2p þ 2Þðn2p � n2s Þ

ð11Þ

Considering that the experimental refractive indices arethe effective refractive indices, neff , in Fig. 12, the evolutionduring heating of the refractive index of 100_4 sample iscompared with the corresponding porosity deduced fromEq. (11). In this case, it has been taken ns = 2.52 of dense(pore free) anatase phase and np = 1 for the porous phaseconsidering they are filled of air. It can be observed thatthe porosity is reduced during heating from 30% at 150 �C until 13% at 700 �C.

According to Brinker et al. (1992), in dip-coated filmsthe thickness results from the viscous drag and gravityforce balance. When the substrate speed and viscosity arelow (as the case for sol–gel) this is modulated by the ratioof viscous drag to liquid–vapor surface tension, cLV,according to the relationship derived by Landau andLevich (1942):

h ¼ 0:94ðgU 0Þ2=3c1=6LV ðqgÞ1=2

ð12Þ

This effect has been evaluated in our samples and it isrepresented in Fig. 13. It plots, in logarithm scales, the pro-duct of thickness and effective refractive index (neff is pro-portional to density) of annealed films at 500 �C versusthe withdrawal speed, U. A good linear fit (R = 0.996) witha slope of 0.65 is obtained, indicating that the mass of filmper unit area varies as U2/3 in agreement with that is foundin the literature for most of the investigated systems.According to Brinker et al. (1991) the slope value obtainedcorresponds with weakly branched non-particulate films.This theory, developed for gravitational draining of purefluids, has been shown to adequately model the thicknessof newly deposited by dip-coating films. In our case, wefind a reasonable agreement with the theory remainingeven when the samples were heat-treated at hightemperature.

Fig. 13. Plot of the product of film thickness and refractive index versuswithdrawal speed according to Eq. (12).

Fig. 14. Photoluminescence emission spectra of samples 100_4 annealedat the temperatures indicated in the legend, when excited at 330 nm.

20 E. Blanco et al. / Solar Energy 122 (2015) 11–23

On the other hand, although it is difficult to observe anyphotoluminescence (PL) phenomenon at room temperaturefor bulk TiO2, due to its indirect transition nature, this isnot the case for nanostructures like TiO2 thin films. Thisphotophysical process gives us information about dynamicbehavior of photoinduced charge carriers of TiO2 closelyrelated to photochemical processes.

Fig. 14 shows the PL emission spectra (kex = 330 nm)for 100_4 TiO2 thin films annealed at 400 �C, 500 �C,600 �C and 700 �C. Two main PL features appear at380 nm (3.27 eV) and 471 nm (2,63 eV), the former isattributed to band-band PL and the latter to bound exci-tons PL (Liqiang et al., 2006). The analysis of the excitonicPL can help us to extract some conclusions about the pho-tochemical behavior of TiO2 thin films annealed at differenttemperatures. The excitonic PL intensity decreases as theTiO2 crystallite size increases, from 500 �C to 700 �C,which can be ascribed to the decrease in the content of sur-face oxygen vacancy and defect with increasing the crystalsize. During the excitonic PL process, oxygen vacanciesand defects can easily bind photo-induced electrons tostabilized excitons, so the recombination of electrons andholes is inhibited, giving rise to a higher photocatalyticactivity of the sample annealed at 500 �C, associated tothe holes strong oxidizing power.

In summary, regarding to the photocatalytic activity ofthe samples it is crucial to achieve a phase transformationfrom amorphous to crystalline titania thin film, since theamorphous films have no catalytic activity (Han et al.,2009). We have shown from Raman measurements thatanatase nanocrystals begin to form above 400 �C. By theother side, we have to consider that we are limited by themaximum temperature (700 �C) tolerable for borosilicateglass substrate used in photo-purification reactors. Fur-thermore, when films are annealed upper 400 �C its poros-ity is greatly reduced from a 23% to a 14% at 700 �C,according to results obtained from the Lorentz–Lorenzeffective medium approximation. Since photocatalytic pro-cess is a surface and not a volume or mass phenomenon,the highest porosity of samples annealed at 400–500 �Cwould lead to an enhancement of the photocatalytic activ-ity (Chen and Dionysiou, 2006). Additionally, according to

Fig. 12. Refractive index at 632.8 nm and porosity derived from Eq. (11)as a function of annealing temperature for the representative 100_4sample.

the above discussion, the maximum excitonic PL intensity,a key factor to define films photochemical activity, takesplace on samples annealed at 500 �C.

Also, for water treatment applications, complete organicresidues removing from thin films should be assured thathas been shown to take place upper 500 �C. So, it can beasserted that TiO2 thin films annealed at 500 �C is the mostpromising photocatalyst to destroy pollutants in watertreatments applications.

5. Conclusions

A sol–gel approach has been used to prepare TiO2 dip-coated thin films to be applied in water depuration photo-catalysis. Films, annealed at 500 �C, are organic residuesfree crystalline anatase phase. An optical method basedon the transmission spectrum taken at normal incidenceof a multilayer air-film-substrate-film-air optical systemhas been developed. The dispersion of the optical constantshas been modeled on the basis of Tauc-Lorentz model.This optical method allowed determining the thickness,

E. Blanco et al. / Solar Energy 122 (2015) 11–23 21

refractive-index dispersion, absorption edges and the opti-cal bandgap of the dip-coated samples.

Refractive indices obtained are in good agreement withother published for similar systems and thicknesses are in agood linear correlation with both the number of layers andthe withdrawal speed. Results from the optical characteri-zation method applied to ellipsometric angles measure-ments allowed determining the high-resolution filmthickness profiles, which demonstrate their relatively flatprofiles with a characteristic dip-coated film shape.

The occurrence and the increase of anatase-phase crystalsize with increasing annealing temperature have been bothevidenced by Raman spectroscopy measurements per-formed in the samples. Such evolution has been accompa-nied by larger values of the refractive index.

The application of an effective medium approximation,based on Lorentz–Lorenz expression, permit the determi-nation of thin film porosities during microstructure evolu-tion on heating. Results indicate that annealing treatmentsat 500 �C are optimum for photocatalytic applications giv-ing rise to small anatase crystals and highly porousnetwork.

Appendix A

A.1. Recurrence relations

Recurrence relations for the determination of the trans-mittance of a multilayer optical system according to Eq. (1)are introduced as follows:

p1;1 ¼ v1;1 ¼ 1; ð13Þq1;1 ¼ w1;1 ¼ 0; ð14Þr1;1 ¼ t1;1 ¼ g1; ð15Þs1;1 ¼ u1;1 ¼ g1; ð16Þp1;m ¼ p1;m�1 pm � q1;m�1 qm þ r1;m�1 tn � s1;m�1 um; ð17Þq1;m ¼ q1;m�1 pm þ p1;m�1 qm þ s1;m�1 tm þ r1;m�1 um; ð18Þr1;m ¼ p1;m�1 rm � q1;m�1 sm þ r1;m�1 vm � s1;m�1wm; ð19Þs1;m ¼ q1;m�; rm þ p1;m�1 sm þ s1;m�1 vm þ r1;m�1wm; ð20Þt1;m ¼ t1;m�1 pm � u1;m�1 qm þ v1;m�1 tm � w1;m�1 um; ð21Þu1;m ¼ u1;n�1 pm þ t1;m�1 qm þ w1;m�1 tm þ v1;m�1 um; ð22Þv1;m ¼ t1;m�1 rm � u1;m�1 sm þ v1;m�1 vm � w1;m�1wm; ð23Þwhere subscripts m are referred to the interface, and

pm ¼ expðam�1Þ cosðcm�1Þ; ð24Þqm ¼ expðam�1Þ senðcm�1Þ; ð25Þrm ¼ expðam�1Þ ½gm cosðcm�1Þ � hm senðcm�1Þ�; ð26Þsm ¼ expðam�1Þ ½hm cosðcm�1Þ þ gm senðcm�1Þ�; ð27Þtm ¼ expð�am�1Þ ½gm cosðcm�1Þ þ hm senðcm�1Þ�; ð28Þum ¼ expð�am�1Þ ½hm cosðcm�1Þ � gm senðcm�1Þ�; ð29Þvm ¼ expð�am�1Þ cosðcm�1Þ; ð30Þ

wm ¼ � expð�am�1Þ senðcm�1Þ; ð31Þg0 ¼ 1; ð32Þh0 ¼ a0 ¼ c0 ¼ 0; ð33Þ

gm ¼ n2m�1 þ k2m�1 � n2m � k2mðnm�1 þ nmÞ2 þ ðkm�1 þ kmÞ2

ð34Þ

hm ¼ 2ðnm�1km � nmkm�1Þðnm�1 þ nmÞ2 þ ðkm�1 þ kmÞ2

ð35Þ

am ¼ 2pk

kmdm; ð36Þ

and

cm ¼ 2pk

nmdm; ð37Þ

where subscripts for variables n; k; d, which describe theoptical properties of the layers, refer to the previous layerto the interface m : n stands for the refractive index, k theextinction coefficient and d the thickness. Normal incidenceis considered in Eqs. (34)–(37).

A.2. Calculation of the refractive index and extinction

coefficient of the substrate

The optical constants of the substrate can be derivedfrom its transmission and reflection spectra, T s;expðkÞ andRs;expðkÞ, respectively, by solving the following equationsystem (Gonzalez-Leal et al., 2002):

Rsðns; xs; kÞ � Rs;expðkÞ ¼ 0;

T sðs; xs; kÞ � T s;expðkÞ ¼ 0;

�ð38Þ

where

Rsðns; xs; kÞ ¼ Rð1þ ð1� 2RÞx2s Þ1� R2 x2s

; ð39Þ

T sðns; xs; kÞ ¼ ð1� RÞ2 xs1� R2 x2s

; ð40Þ

R ¼ 1� ns1þ ns

� �2

; ð41Þ

xs ¼ Expð�4pks=kÞ: ð42Þ

A.3. Tauc-Lorentz model for the dispersion of the refractive

index and extinction coefficient

The Tauc-Lorentz model considers that the real andimaginary parts of the dielectric function, ~e ¼ er þ iei, canbe mathematically modeled as follows

eiðEg;Eo;A;C; �hxÞ ¼1�hx

AEoCð�hx�EgÞ2

ðð�hxÞ2�E2oÞ

2þC2ð�hxÞ2; for �hx > Eg;

0; for �hx 6 Eg;

8<:

ð43Þ

22 E. Blanco et al. / Solar Energy 122 (2015) 11–23

and

ei Eg;e1;Eo;A;C;�hx� �¼ e1þ ACaln

2pf4aEo

LnE2oþE2

gþaEg

E2oþE2

g�aEg

" #

�Ap

aatanf4aEo

p�arctan2Egþa

C

� �þarctan

a�2Eg

C

� �

þ4AEoEgð �hxð Þ2� c2Þpf4a

arctan2Egþa

C

� �

þarctana�2Eg

C

� ��AEoC �hxð Þ2þE2

g

� �pf4�hx

Ln�hx�Eg

�hxþEg

� �

þ2AEoC

pf4EgLn

�hx�Eg

�hxþEg

� �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiE2oþE2

g

� �2

þE2gC

2

r2664

3775; ð44Þ

where

aln ¼ E2g � E2

o

� �ð�hxÞ2 þ E2

gC2 � E2

o E2o þ 3E2

g

� �; ð45Þ

aatan ¼ ð�hxÞ2 � E2o

� �E2o þ E2

g

� �þ E2

gC2; ð46Þ

a ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi4E2

o � C2q

; ð47Þ

c ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiE2o � C2=2

q; ð48Þ

f4 ¼ ð�hxÞ2 � c2� �2

þ a2C2=4: ð49ÞEg is the optical bandgap, e1 is the value of the real part ofthe dielectric function at high frequencies, Eo is the oscilla-tor energy, A and C are related to the strength and thewidth of the absorption peak, respectively, �hx is thephoton energy and �h is Planck’s constant, h, divided by 2p.

From the above equations for the real and imaginaryparts of the dielectric function, the dispersion of the refrac-tive index and the extinction coefficient can be respectivelymodeled as follows:

n Eg; e1;Eo;A;C; k� � ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffie2r þ e2i

qþ er

� �s; ð50Þ

k Eg; e1;Eo;A;C; k� � ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffie2r þ e2i

q� er

� �s; ð51Þ

where the relation �hx ¼ hc=k, with c being the speed oflight in vacuum, can be used to conveniently work withwavelengths instead of photon energies.

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