Applied Surface Science 255 (2009) 6274–6278

Photocatalytic activity enhancement of TiO2 films by micro and nano-structuredsurface modification

M. Bizarro a,*, M.A. Tapia-Rodrıguez a, M.L. Ojeda b, J.C. Alonso a, A. Ortiz a

a Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma de Mexico, A.P. 70-360, Coyoacan 04510, D.F., Mexicob Departamento de Ciencias Naturales y Exactas, Universidad de Guadalajara, Centro Universitario de los Valles, C.P. 46600 Ameca, Jalisco, Mexico

A R T I C L E I N F O

Article history:

Received 6 November 2008

Received in revised form 29 January 2009

Accepted 30 January 2009

Available online 10 February 2009

PACS:

82.65.+r

Keywords:

Photocatalysis

Thin films

Titanium oxide

Water treatment

A B S T R A C T

Titanium oxide thin films were deposited by spin coating using a precursor solution of titanium oxide

(IV) acetylacetonate. To increase the contact surface area of the films, TiO2 microspheres were added to

the surface of the films. These spheres were 2 mm in diameter and formed agglomerates on the surface.

They did not spread uniformly across the substrate, creating different roughnesses and morphologies

along the surface of films. Photocatalytic properties of the samples were tested by the degradation of a

methyl orange solution. The degradation performance was compared between plain films, films with

microspheres and films covered with commercial TiO2 P25 powder. The results indicate that the samples

that were surface modified with TiO2 microspheres present a photodegradation reaction rate 62 times

higher than that obtained for plain TiO2 films. The rate of reaction of the samples covered with P25 was 2

times greater than that obtained for the samples with microspheres, but the adhesion to the film was

better in the case of microspheres. Moreover, samples with microspheres could be reused several times

maintaining the same structural and photocatalytic properties.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

One of the most important problems in the world is waterpollution. Great scientific efforts have been focused to solve thisproblem. During the last decade photocatalytic processes haveshown advantages for water treatment by decomposing organiccompounds using ultraviolet radiation. Among many candidatesfor photocatalysts, titanium dioxide (TiO2) presents the bestcharacteristics: it is the most efficient photocatalyst, it is verystable chemically and it is relatively cheap [1–3]; these propertiesmake TiO2 a potential material for water treatment at an industrialscale. At present, photocatalytic processes involve the use of TiO2

powders, which are efficient in dye bleaching but need furtherseparation processes like filtration, decantation and/or centrifuga-tion. This makes the final process slower and expensive. A practicalsolution is to have the catalyst deposited as a thin film [4–8]. In thisway, the photocatalyst is adhered to a substrate which can beeasily removed from the water container alter the bleachingprocess, with any loses of the catalytic material.

The principal limitation to change the use of TiO2 powders tothin films is the high efficiency obtained for the first ones. Powders

* Corresponding author. Tel.: +52 55 56224770x45671; fax: +52 55 56161251.

E-mail address: monserrat@iim.unam.mx (M. Bizarro).

0169-4332/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2009.01.094

have intrinsically much more surface area than a continuous solidfilm. It is worth mentioning that photocatalytic processes aresurface phenomena, which are directly related to the surface areaof the catalyst that is in contact with the molecules that will react[8,9]. Therefore, to obtain efficient films comparable to powders, itis desirable to produce films with structures such that enable moresurface area for catalytic purposes, instead of continuous andsmooth films. Good quality continuous TiO2 films have beendeposited by different techniques, such as: sputtering, pulsed laserdeposition, spray pyrolysis, sol–gel, etc. and present goodchemical, catalytical and electrical properties [6,7,10–13]. In thepresent work, we report the effect of surface structured TiO2 filmsby the adsorption of TiO2 microspheres, on the photocatalyticresponse. Besides, we consider a balance between stable andreusable films and a high photocatalytic efficiency to degradeorganic dyes, which are typical pollutants in the textile industrywaste water.

2. Experimental

Titanium oxide thin films were deposited by the spin coatingtechnique using a 0.1 M solution of titanium oxide (IV) acetyla-cetonate (TiOAAc) dissolved in anhydrous methanol. Diethanola-mine (DEA) was added to that solution as surfactant, in aproportion [DEA]/[TiOAAc] = 1.2. The films were formed by three

Table 1Characteristics of the studied samples.

Sample

name

Characteristics Annealing

temperature (8C)

TASC TiO2 film 400

SCTE TiO2 film + TiO2 microspheres 400

SCTP TiO2 film + TiO2 powder (Degussa P25) 400

Fig. 1. XRD patterns of (a) TiO2 films with microspheres, (b) microspheres, (c) TiO2

films with Degussa P25 powder, (d) Degussa P25 TiO2 powder and (e) TiO2 plain

film.

M. Bizarro et al. / Applied Surface Science 255 (2009) 6274–6278 6275

layers of the above mentioned solution at ambient temperatureand were annealed at 400 8C in air atmosphere for 1 h after eachlayer deposition in order to evaporate the organic species and formthe titanium oxide. Three different sets of samples were prepared.The first set (samples named TASC) was grown with the previousprocedure to form TiO2 films. The films of the second set (samplesnamed SCTE) were surface modified by adding approximately 5 mgof TiO2 microspheres to the last layer before annealing. These TiO2

spheres were 2 mm in diameter and were produced using titaniumisopropoxide (Ti(iOPr4)) and anhydrous ethanol as previouslyreported [14]. The last step of the microspheres synthesis is anannealing treatment at 500 8C for 1 h, so the thermal annealing ofthe final sample SCTE did not affect the properties of themicrospheres. The third group consisted of films covered with�5 mg of commercial Degussa P25 TiO2 powder, added to the lastlayer before annealing. This last set of samples (named SCTP) wasproduced for comparison purposes. A summary of the character-istics of the different samples is shown in Table 1. The films weredeposited onto 1 in. � 0.5 in. pyrex glass substrates ultrasonicallycleaned with trichloroethylene, acetone and methanol. Thedeposition parameters were the same in all cases: angular velocityof 2500 rpm for 10 s. The crystalline structure of the films wasdetermined by X-ray diffraction, with a Siemens D500 diffract-ometer using the Cu Ka1 wavelength (1.54056 A). The morphologyor the films was studied with a Leyca Cambridge 400 scanningelectron microscope. The thickness and the roughness of the filmswere measured with a Sloan DekTac IIA profilometer.

Photocatalytic properties of the films were tested by thedecomposition of a methyl orange solution (C14H14N3SO3Na). Twoof each set of samples were put back to back and immersed in10 ml of a 10�5 M methyl orange solution that was irradiated witha 7 W UV lamp with emission at wavelengths of 433, 408, 398 and364 nm. Ultraviolet illumination is needed because it is the energyrequired to produce the excitation of an electron from the valenceband to the conduction band of the TiO2, since its bandgap is of3.2 eV in the anatase crystalline phase. The optical absorption ofthe dye solution was measured in the range of 190–1100 nm with adouble beam UNICAM UV-300 spectrophotometer using deionizedwater in the reference beam. These spectra were obtained as afunction of time and analyzed to calculate the dye degradationpercentage as indicated in Eq. (1) and the speed of the reaction foreach case [15].

Degradation % ¼ 1� AðtÞA0

� �� 100 (1)

A(t) is the absorbance after a time t and A0 is the dye initialabsorbance. The absorbance given at each irradiation time wasrelated to a specific dye concentration using a calibration curve formethyl orange solutions with different concentrations. Thecalculated concentration was plotted as a function of theirradiation time to obtain the kinetic behavior of the photocatalyticreaction.

3. Results

Fig. 1 shows the X-ray diffraction spectra of the differentsamples. The samples TASC and SCTE presented the anatase phase

of TiO2, while the samples SCTP, that were covered with DegussaP25 powder, showed both anatase and rutile phases. To confirmthat the two phases came from the P25 Degussa powder, – as isestablished by the manufacturer – and not from a phase transitionduring the annealing process of the film, a sample of this powderwas also analyzed. The TiO2 microspheres presented the anatasephase only, as well as the plain TiO2 film.

SEM images of the surface morphology are shown in Fig. 2.Images 2a and 2b correspond to the sample with spheres adsorbedto the surface (SCTE); whereas Images 2c and 2d correspond to thesample covered with the commercial Degussa P25 powder (SCTP).It is observed the completely different surface that is generatedwith the addition of microspheres or powder to the surface. In thefirst case, the spheres formed agglomerates as they were adheredto the film by a small compression. Thus the spheres stackperpendicular to the substrate, giving a roughness of �4 mm,which is larger than the roughness obtained for the films with P25powder (�1 mm). In the last case, the powder spread over thesurface, maintaining a flatter shape. This is explained consideringthe different particle sizes that are of the order of 2 mm in the caseof the TiO2 microspheres and around 30 nm in the case of thepowder, as determined using the Scherrer’s formula [16]. In thecase of the TiO2 plain films the surface is continuous and smooth,and therefore no structure was perceptible with SEM. Anotherobservable characteristic is that the adsorption of the particles tothe substrate (both spheres and powder) is not homogeneous;there are places where no particles were adhered. This feature is aresult of the deposition procedure, as the films were literallycoated in the particles. The TiO2 films (TASC) present good opticaltransmission, higher than 80% in the visible region; but thesamples SCTE and SCTP, that were surface modified, present veryhigh dispersion and their transmittance is very low. For SCTE thetransmittance is about 20% in the visible region, but thetransmittance of samples SCTP decreases with shorter wave-lengths, as shown in Fig. 3.

The photocatalytic activity of the different samples wasobserved by the degradation of a 10�5 M methyl orange solutionunder UV light, measuring its absorption spectra each 2 h. This dyewas selected as an optical indicator for the reaction process andbecause of its high stability under UV irradiation. A control sampleof the dye was exposed to UV light during the total time ofexposure for the studied samples and no change in the absorptionspectrum was observed. The response of the smooth films TASC

Fig. 2. SEM images of samples SCTE (a and b) and SCTP (c and d), at different amplifications.

M. Bizarro et al. / Applied Surface Science 255 (2009) 6274–62786276

was very low in the time interval studied, presenting only a littledecrease of the absorption spectra. On the other hand, the sampleswith microspheres adsorbed to the TiO2 surface (samples SCTE)showed a notable diminution of the absorbance from the firsthours of irradiation with UV light. A typical behavior is shown inFig. 4a for the sample SCTE, where the total dye degradation wasreached in 18 h. For the analysis of the spectra, Eq. (1) was appliedto the maxima of absorption of the methyl orange (located atl = 464 nm) to obtain the degradation percentage and it wasplotted as a function of time as shown in Fig. 4b for the samesample. The response of samples that were covered with DegussaP25 powder presented the same feature as in Fig. 4, but thedegradation of the dye was faster, reaching the 100% bleaching inonly 6 h. Using a calibration curve for solutions of differentconcentrations of methyl orange, the kinetics of the reaction wasobtained for all the samples by plotting ln(C0/C) as a function oftime. This plot gives a straight line where the slope is the rate

Fig. 3. Optical transmission of the films TASC, SCTE and SCTP.

Fig. 4. (a) Absorption spectra for sample SCTE measured each 2 h until the total

degradation of the MO; (b) Degradation of methyl orange as a function of time.

Fig. 6. Repetitions of the photodegradation experiment for the sample SCTE, where

a good performance of the material is maintained.

Fig. 5. Rate of reaction of the different samples. Surface modified samples present

rates of reaction three orders of magnitude larger than continuous films.

M. Bizarro et al. / Applied Surface Science 255 (2009) 6274–6278 6277

constant of the reaction as it is shown in Fig. 5. This ismathematically represented by Eq. (2):

lnC0

C

� �¼ kt (2)

where C0 is the initial dye concentration (M), C is theconcentration after an irradiation time t (h), k is a first order rateof reaction. The rate of reaction for the different samples is given inTable 2. Eq. (2) is valid for first order kinetics as it is explained bythe Langmuir–Hinshelwood model [17] for low dye concentrations[18].

Comparing these rates of reaction, one can notice a bigdifference between the photocatalytic response of the plain TiO2

film and those samples that have a modified surface, either withmicrospheres or with powder. The continuous film provides lesscontact surface area than the modified samples, making thephotocatalytic reactions slower, as indicates the rate of reaction of0.0032 h�1. When the TiO2 microspheres are adsorbed to the film,they form an irregular surface with more area that is in contactwith the dye molecules. The larger area allows more dye moleculesto react simultaneously increasing the discoloration rate 62 times.In the case of the samples covered with P25 powder, the rate ofreaction was 130 times greater than the plain films, just 2 times theobtained for adsorbed microspheres. This value can be related tothe smaller grain size of the adsorbed particles. The rates ofreaction of the powders and spheres are also shown. It is clearlyseen that the P25 powders in a suspension actuate faster thanwhen they are attached to the film. On the contrary, the spherespresent almost the same activity in both cases. This means that thespheres make contact with the film (and with each other) only in asmall part of their surface, and then their activity is not affected.

As it was mentioned before, the samples SCTE and SCTP thathave a rough surface presented good photocatalytic activity for thedecomposition of methyl orange compound. The quality of thesetwo samples was tested to determine which one is the best forfurther applications. However, the adherence and immobilizationprocess of photocatalyst powders may deteriorate the photo-

Table 2Rate of reaction for the different samples.

Sample name Rate of reaction, k (h�1) R2

TASC (TiO2 film) 0.0032 0.858

SCTE (TiO2 film + spheres) 0.2003 0.989

SCTP (TiO2 film + P25) 0.4162 0.977

Spheres 0.1622 0.978

P25 1.2443 0.999

activity [19]. For this reason, we are interested on the durability/stability of the samples and their performance after several cycles,thus, some experiments and results were obtained repeating thedegradation experiment 3 times in samples SCTE and SCTP.Qualitatively, a simple observation of the samples after the first‘‘cycle’’ (first degradation experiment) revealed that in the sampleSCTP considerable part of the TiO2 P25 powder was lost, leaving aclearer surface (more transparent samples). These losses of thesurface catalyst are due to the erosion of the surface of sampleswith the aqueous solution, and the particles that were detachedmaintained suspended in the liquid. This is undesirable becauserequires further separation (filtration) of the catalyst. On the otherhand, the sample SCTE looked the same as before the immersion tothe methyl orange solution. The way in which the particles aredeposited onto the films is the same for the spheres and for P25powder; however, as the particle size of the powder is smaller thanthe spheres only those that are in direct contact to the surface arefirmly attached, and the other (a larger fraction) are stuck bypressure only, which are more susceptible to be removed. Afterthree cycles, in which sample SCTE was immersed in a new methylorange solution, the degradation rate was almost constant, as it canbe observed in Fig. 6. This fact confirms that the adhered TiO2

microspheres are firmly tight to the film and the morphology of thesurface remains the same.

4. Discussion

The photocatalytic process starts with the generation of anelectron-hole pair as a result of the excitation of TiO2 with UV light(Eq. (3)). The positive hole in the valence band oxidizes either thepollutant dye directly (Eq. (5)) or water to produce an OH radical(Eq. (6)), whereas the electron in the conduction band reducesoxygen adsorbed to TiO2 (Eq. (4)). In the degradation of the dye, thereduction of oxygen (Eq. (4)) and the oxidation of the dye molecule(Eqs. (5) and (6)) should proceed at the same time. Therefore, theefficient consumption of electrons is essential to promotephotocatalytic oxidation (Eq. (7)) [9]. The results obtained showthat this process is surface dependent.

TiO2�!hn

e� þ pþ (3)

e� þO2 ! O2� (4)

pþ þdye ! ! CO2 (5)

pþ þH2O ! �OH þ Hþ (6)

ce Science 255 (2009) 6274–6278

�OH þ dye ! ! CO2 (7)

The rate of reaction for the methyl orange decompositionobtained for the TiO2 films was of the order of 10�4 h�1. This lowvalue is due to the flatness of the film and the small surface areathat is in contact with the dye solution. However, when the surfaceof the film was modified with TiO2 microspheres or TiO2 P25powder, the decomposition rate increased considerably. Thephotocatalytic activity achieved by the samples with P25 washigher and at a first sight it is better than the TiO2 microspheres.The higher photocatalytic activity presented by the samples SCTPis attributed to the smaller grain size of the powder compared tothe size of microspheres (that are around 10–100 times larger).Many reports point out the importance of the surface area incatalysis, thus, the trend is to produce smaller particles to increasethat surface area by diminishing the particle size [20–25].However, in the present case it was seen that the powder wasnot completely stuck to the surface of film, and it is easily removedfrom it just by being in contact with an aqueous solution. On theother hand, TiO2 microspheres were firmly adhered to the surface,providing a reusable sample with the same characteristics aftersome cycles. This property makes the system TiO2 film + TiO2

microspheres more efficient, with no losses of the surface modifiercatalyst and gives a clean final solution.

5. Conclusion

Spin coating technique was used to produce continuous andsurface modified TiO2 films with photocatalytic properties. Theadsorption of TiO2 microspheres or powder to the surface of theTiO2 films increases notably the photocatalytic activity, givingreaction rates for the methyl orange decomposition of more than62 and 130 times, respectively, the rate obtained for thecontinuous film. The increase of the photocatalytic activity isattributed to the larger surface contact area created by themodification of the surface with the microspheres or powder.Moreover, microspheres present better adhesion to the TiO2 filmsthan the P25 powder, making them more resistant to the watererosion. In films with microspheres reproducible results wereobtained after several test cycles, they present the same surfacemorphology and give almost the same reaction rate of decom-

M. Bizarro et al. / Applied Surfa6278

position of methyl orange. These durability properties make thismaterial a good prospect for further water treatment applications.

Acknowledgements

The authors wish to thank L. Banos and O. Novelo for theirtechnical support. This work was partially supported by DGAPA-UNAM under Projects IN109507 and IN116109.

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