Materials Science in Semiconductor Processing 40 (2015) 720–726

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Visible and infrared photocatalytic activity of TiOx thin films preparedby reactive sputtering

M. Zapata-Torres a,n,1, E. Hernández-Rodríguez b, R. Mis-Fernandez b, M. Meléndez-Lira c,O. Calzadilla Amaya d, D. Bahena e, V. Rejon b, J.L. Peña b

a Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada del Instituto Politécnico Nacional, Legaria # 694 Col. Irrigación, C.P. 11500 México D.F., Mexicob Departamento de Física Aplicada, CINVESTAV-IPN, Unidad Mérida, km. 6 Carretera Mérida-Progreso, C.P. 97310 Mérida, Yucatán, Mexicoc Departamento de Física, CINVESTAV-IPN, Apartado Postal 14-740, 07000 México D.F., Mexicod Facultad de Física, Universidad de La Habana, San Lázaro y L, C.P. 10400, La Habana, Cubae LANE, CINVESTAV-IPN, Apartado Postal 14-740, 07000 México D.F., Mexico

a r t i c l e i n f o

Article history:Received 17 May 2015Received in revised form14 July 2015Accepted 24 July 2015

Keywords:TiOx

Red-shift band gapNIR photocathalitic activity

x.doi.org/10.1016/j.mssp.2015.07.07201/& 2015 Elsevier Ltd. All rights reserved.

esponding author.n sabbatical leave at Departamento de FísicMérida, Mexico.

a b s t r a c t

TiOx thin films have been deposited on corning glass substrates, using RF magnetron reactive sputteringfrom a Ti target. The effect of oxygen mass flow (OMF) used during the growth of the films on theirphysical and photocatalytic properties was investigated. The properties of the samples were analyzed byX-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM),X-ray photoelectron spectroscopy (XPS) and UV–vis spectroscopy. The films presented a stratifiedstructure with the surface layer having the stoichiometric characteristics of TiO2 while the inner layercontained a mixture of TiO2, Ti2O3 and TiO oxides. The photocatalytic efficiency was evaluated by thephotodegradation of a methylene blue aqueous solution. The samples showed a photocatalytic activity inthe visible and visible-NIR light spectrum, depending on the phases of titanium oxides present in thefilm.

& 2015 Elsevier Ltd. All rights reserved.

1. Introduction

TiO2 is a widely used photocatalyst with diverse applicationssuch as air purification, deodorization, water purification and self-cleaning coatings [1-4]. TiO2 is a wide band gap (3.2 eV) semi-conductor material that shown photocatalytic activity under UVlight irradiation. However, the UV portion in the solar radiationreaching the surface of the earth is very faint, just around 5% of theincident solar energy. Then, producing a photocatalytic TiO2 basedmaterial with a lower band gap could increase its efficiency.Finding novel photocathalytic semiconductor materials that ab-sorb lower energy photons, in the visible and infrared regions ofthe solar spectrum, would allow to use a wider portion of the solarspectrum and as consequence increasing the photocatalytic effi-ciency. Several studies have reported the red-shift of the band gapof TiO2 employing several approaches such as doping with diversemetals (e.g., V [5], Cr [6], Fe [7], etc); doping with non metallic ions(e.g., N [8], C [9]); dye sensitization [10]; mixing with semi-conductor quantum dots (e.g., CdS [11], PbSe [12], etc). More

a Aplicada, CINVESTAV-IPN,

recently, oxygen deficient titanium dioxide, represented as TiOx,has been reported as an effective way to extend the light ab-sorption of TiO2 based materials into the visible region of theelectromagnetic spectrum, due to the fact the energies of localizedstates are (0.75–1.18 eV) below the conduction band minimum[13–15]. It has been reported the production of TiOx powder bydifferent techniques such as reactive pulsed DC magnetron sput-tering [16], thermal annealing of TiO2 nanoparticles with reducinggases [17], solvothermal method [18], etc. However, for practicalapplication, TiOx thin films are more adequate than powders.Several methods can be used to prepare TiOx thin films, such asblow arc discharge [19], pulsed laser deposition [20], plasma as-sisted chemical vapor deposition [21,22], plasma surface treatment[23], anodization [24] and reactive magnetron sputtering [25–27].Among these methods, the magnetron sputtering method allowsan easy control of the films structure and composition besidesprovides many advantages in reproducibility, high mechanicaldurability, strong adhesion and uniformity and it is applicable tolarge area deposition.

It has been reported that the TiOx thin films had photoresponsein the visible region of the electromagnetic spectrum. Nakanoet al. [23] reported visible light photocatalytic activity of of oxygendeficient TiO2 films obtained by Ar/H2 plasma surface treatment;Dong et al. [24] synthesized defective black TiO2 films via

M. Zapata-Torres et al. / Materials Science in Semiconductor Processing 40 (2015) 720–726 721

anodization for visible light photocatalysis; Lee et al. [26] grewN-doped TiOx films using reactive sputtering reporting a red-shiftband gap; and Leichweiss et al. [20] grewn highly nonstoichio-metric TiOx thin films, that shown high efficiency for water split-ting reaction, using pulsed laser deposition at room temperature.Related to the activation of TiO2 by infrared radiation, we onlyfound the work of Gannoruwa et al. [28], they reported that thenanocomposite Ag2O/TiO2 was infrared active in the range from800 nm to 1200 nm. To the best of our knowledge, there is not anyreport to date about infrared radiation photocatalytic activity ofTiOx thin films, which is the subject of the present paper.

In this work, we prepared TiOx films by using RF magnetronreactive sputtering from a Ti target. We varied the oxygen massflow concentration and study the effect of the different titaniumoxide phases on the photocatalytic properties of the films.

2. Experimental procedures

2.1. Sample preparation

TiOx thin films were grown by the reactive RF magnetronsputtering technique from a Ti metal target in an Ar–O2 gas mix-ture. Deposition was made onto glass substrates (size 1″�1″) byusing an off-axis configuration and a base pressure of5�10�6 Torr or better. The RF power applied to the target was setto 300 W and the working pressure was 30 mTorr. The distancebetween the substrate holder and the target was 80 mm and thesubstrate was rotated at 100 rev/min in order to promote filmuniformity. The thickness of the film was monitored by a quartzmicrobalance in order to obtain samples 100 nm thick. Depositionwas performed at room temperature. In order to obtain films withdifferent chemical composition, the oxygen and argon mass flowswere changed as shown in Table 1 (also, the identification code ofthe samples can be seen in this Table), but the total mass flow wasset to 50 sccm for all of them. As a control sample, a stoichiometricTiO2 film was deposited onto an ITO-coated glass substrate byusing the methodology reported by Hocevar et al. [29].

2.2. Characterization

X ray diffraction (XRD) experiments were carried out in a Sie-mens D-5000 diffractomer using the Cu Kα radiation(λ¼1.5406 Å). Resistivity measurements were performed in aKeithley 4200 Semiconductor Characterization System by usingthe four point probe technique. The morphology and cross-sec-tional characterizations were made with a high resolution scan-ning microscope (HRSEM) Carl Zeiss Auriga. High resolutiontransmission electron microscope (HR-TEM) and selected-areaelectron diffraction (SAED) were carried out with a transmission

Table 1Growth conditions and bandgap energies of the studied samples.

Sample MFArgon(sccm)

MFOxygen(sccm)

Substrate Bandgapenergy

P1 50 0 Glass –

P2 49.80 0.20 Glass 2.91P3 49.58 0.42 Glass 2.91P4 49.56 0.44 Glass 2.90P5 49.54 0.46 Glass 2.93P6 49.52 0.48 Glass 2.91; 3.2P7 48.92 1.08 Glass 3.2P8 49.56 0.44 Glass coated with

ITO2.91

P9 48.92 1.08 Glass coated withITO

3.2

electron microscope JEOL model JEM-ARM200F operated at200 kV. A Shimadzu UV–vis spectrophotometer was used for col-lecting transmittance spectra of the films in the spectral range of320–1000 nm. In order to understand the optical properties andits correlation with the photocatalytic activity, the titanium oxidespresent in the films were characterized by means of X-ray Pho-toelectron Spectroscopy (XPS) (model K alpha by Thermo Scien-tific) for selected samples. The general survey as well as the highresolution spectra in the regions of the C 1s, O 1s, Ti 2p wereobtained at the surface of the films and after 30 s of etching withAr ions. The binding energy of the C 1s line at 284.5 eV was takenas the reference peak to calibrate the obtained spectra. The Ti 2ppeaks were fitted by employing asymmetric Gaussian-Lorentzianfunctions. The background subtraction was performed by using themathematical model derived by Shirley.

2.3. Photocatalytic activity

The photocatalytic activity of the TiOx thin films was in-vestigated, by monitoring the decomposition of methylene blue inan aqueous solution (8 ppm in water) under light irradiation; a300 W Oriel Xe arc lamp was used as light source. To avoid pho-tocatalytic activity on films due to the band gap of the TiO2 che-mical compound (387 nm), an interference longpass filter wasutilized to select wavelengths longer than 400 nm. To investigatethe origin of photocatalytic activity of the TiOx films at longerwavelengths, photodegradation experiments with and without anIR Oriel 61945 filter were performed; this filter allowed to cut-offthe radiation for λ4950 nm. In these experiments, TiOx sampleswith dimensions of 5 mm�10 mm were placed into a 8 ml glassvial with methylene blue solution and were illuminated perpen-dicularly to their surface. In order to obtain the degradation of themethylene blue solution as function of time, 0.5 mL of the solutionwas taken and placed in a cuvette each 10 min, the absorbancespectra was measured and then it was returned into the glass vial;this process was repeated until reaching a cumulative time of120 min. Photodegradation experiments were conducted by threetimes in order to minimize experimental error. The photoelec-trochemical experiments were carried out in a cylindrical Teflonreactor (10 mm diameter and 20 mm length) equipped with aquartz window. A two-electrode system was used: a platinumcounter electrode and a TiOx film deposited over a glass coatedwith ITO (as working electrode) immersed in an aqueous 0.1 MNaOH electrolyte. The spectral response was measured by illumi-nating the samples with monochromatic light (monochromator,Jobin Yvon H10 IR), in the range from 300 nm to 1300 nm. Thephotocurrent at each wavelength was recorded at constant elec-trode potential of 0 V using a Keithley 6517A Electrometer.

3. Results and discussion

Fig. 1 shows the XRD patterns of the samples (a) P1 and (b) P3.The XRD pattern in Fig. 1(a) shows that for OMF¼0 sscm (sampleP1), a titanium film with hexagonal phase was obtained, the dif-fraction peaks were indexed by using the card No. 441294 of JointCommittee of Powder Diffraction File database (PDF). All thesamples grown with OMF values greater than 0 sccm, showed aXRD pattern similar to that shown in Fig. 1(b), which is char-acteristic for an amorphous material.

The surface and cross sectional SEM images of P4 sample isshown in Fig. 2(a) and (b), respectively. The surface has a grain-likemorphology, with regular size. All the samples showed similarmorphology. As can be observed in Fig. 2(b), the thickness for thefilm was approximately 100 nm; it can be seen that the film iscomposed by two layers; the inner layer could be associated with

Fig. 1. X-ray diffraction patterns of samples (a) P1; (b) P3.

Fig. 2. SEM images of sample P4. (a) Surface and (b) cross sectional.

Fig. 3. (a) TEM image and (b) SAED pattern of sample P4.

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the mixture of titanium oxide phases (TiOx) and the outer layercould be associated to fully oxidized TiO2 compound due to thecontact with the air of the environment.

Fig. 3(a) shows the HRTEM image of the sample P4. The TEMimage reveals the characteristics of polycrycristalline grain struc-ture; the contrast of the atomic structure of some grains immersedin an amorphous region were visible, the small crystallites are notconnected and appear to be randomly orientated. Selected areaelectron diffraction (SAED) is presented in Fig. 3(b), it reveals thepresence of amorphous (bright halo) and crystalline phases. Usingthe PDF number 10-0063 and PDF number 21-1272, correspondingto Ti2O3 and TiO2 (anatase phase), the SAED pattern was indexed. Itcan be seen the rings associated with TiO2 (labeled by the letter Aof anatase) and Ti2O3 phases; however, owing to the in-homogeneous and only partly crystalline nature of the material,

additional titanium oxides phases may remain undetected.The films deposited with different oxygen mass flow (OMF) had

different colors; the sample P1 had a metallic color, while thesamples P2 to P6 were dark, with darkness decreasing as OMFincreases. Finally, the sample P7 was transparent. The transmit-tance spectra of the TiOx films prepared with different OMF areshown in Fig. 4; it can be seen a strong dependence of the trans-mittance with the OMF; it increases with the raise of OMF. Theband gap energy was determined by using the method reported in[30], the values obtained are presented in Table 1. Samples P2 toP5 had a band gap near to 2.91 eV, this value is reported in [31] forTiO thin films. The transmittance spectrum of sample P6 presenteda double feature near the absorption edge that could be associatedto band gap values of 2.91 eV and 3.2 eV; the 3.2 eV values is as-sociated to the anatase phase of TiO2. The poor transmittance ofsome samples could be due to the fact that absorption of thephotons was extended in the visible and near infrared regions ofthe spectrum.

The optical transmittance spectra of the films was fitted by

Fig. 4. Optical transmittance of the samples grown with different oxygen massflow.

Table 2Resistivity of the samples obtained by the fourpoint technique.

Sample Resistivity (Ω cm)

P1 1.29�10�2

P2 3.77�10�1

P3 3.90P4 5.54�10�1

P5 3.80�102

P6 1.02�102

P7 5.01�105

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employing a dielectric model [32] to calculate their optical con-stants. The optical properties of TiOx thin films in the spectralrange of interest, 320 nm to 1000 nm, were fitted taking in ac-count three types of electronic excitations: band gap transitions,interband transitions into the upper half of the conduction band,and intraband transitions of the electrons in the conduction band.These transitions were modeled with standard formulae availablein the SCOUT 98 program. For the band gap transitions, we usedthe O´Leary-Johnson-Lim model [33] that has been proposed tomodel the band gap; the interband transitions into the upper halfof the conduction band were modeled by a harmonic oscillator;and for the intraband transitions we used the Drude model tocalculate the contribution of free electrons to the susceptibility. Allprocedure used in the fitting are implemented in the SCOUT 98program. The experimental data were fitted with a standard de-viation better than 0.2. With this procedure we obtained the re-fractive index n and absorption index k, for the samples P1–P7.The values obtained for sample P1 are typical of a metal. In orderto obtain the absorption coefficient α of the films, we use the re-lation:

k4α πλ

=

where: k is the absorption coefficient and the wavelenght.λ InFig. 5 the absorption coefficient for samples P2–P7 is shown. It canbe seen that α decrease with the increase of OMF in the vacuumchamber. It is important to mention that α is related to the amount

Fig. 5. Absorption coefficient for the samples P2 to P7.

of light absorbed in the material which is able to generateelectron-hole pairs; the values obtained for α were bigger thanthe reported for TiO2, Ti2O3 and TiO [30,31]; then probably oursamples have another type of transitions (states below the con-duction band) or large amounts of lattice disorder, that could yieldmid-gap states.

Table 2 presents the measured resistivity values of the samples.It can be seen that samples with low optical transmittance (P1 toP4) presented the lowest resistivity values; when the opticaltransmittance of the films increases (P5 and P6) so does the re-sistivity. For the sample with the band gap corresponding to TiO2

(P7), the resistivity was in the order of 105 Ω cm. Similar resultshas been reported in the literature [20], where the samples withdark color exhibit low electrical resistivity; after a thermal an-nealing at 600 °C the samples become transparent (due to the fulloxidation of the TiOx film), increasing electrical resistivity in fourorders of magnitude.

As the binding energy of Ti 2p core level depends on the oxi-dation state of the Ti cation, curve fitting of the Ti 2p XPS signalcan be used to detect the different titanium cation oxidation statespresent in titanium oxides. The XPS data analysis was performedwith the XPSPeak 4.1 program [34]. The Ti 2p XPS signals werefitted with a mixed Lorentzian-Gaussian curves. The Ti 2p3/2 and Ti2p1/2 signal areas have a 2:1 ratio, a proportion which is used inthe curve fitting of all Ti 2p XPS spectra. For the fit, the peak po-sitions were maintained fixed according to values showed in Ta-ble 3 (the values were taken from the XPS database of the NationalInstitute of Standards and Technology [35]). Also, the full width athalf maximum (FWHM) was fixed for the all peaks used for fittingthe spectra, and the adjustable parameter was the intensity of thepeaks. The experimental XPS peaks corresponding to the Ti 2pcore level emission from the surface of P2 sample and after 30 s ofetching of P2, P4 and P7 samples are presented in Fig. 6; also it canbe seen the fitted curves after deconvolution of experimental data,corresponding to different chemical states of titanium. Fig. 6(a) shows that surface of P2 sample consists only of TiO2, withoutno evidence for the presence of Ti2O3 and TiO. Similar results wereobtained for the surface of P4 and P7 samples. This result meansthat air contact with the TiOx thin films forms a thin oxidized layerat the surface, having the stoichiometry of the TiO2. Fig. 6(b) and(c) show that the XPS spectra from the inner layer of P2 and P4

Table 3Binding energies for Ti 2p3/2 and Ti 2p1/2, of different titanium oxides.

Atom Electronic state Titanium compound Binding energy (eV)

Ti 2p1/2 TiO2 464.60Ti 2p1/2 Ti2O3 462.40Ti 2p1/2 Ti 460.10Ti 2p1/2 TiO 460.20Ti 2p3/2 TiO2 458.70Ti 2p3/2 Ti2O3 457.80Ti 2p3/2 Ti 454.00Ti 2p3/2 TiO 455.90

Fig. 6. XPS spectra of the (a) surface of the sample P2; (b) inner layer of the sample P2; (c) inner layer of the sample P4 and (d) inner layer of the sample P7.

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samples contains the contributions of Ti2þ , Ti3þ and Ti4þ , whichindicates the presence of TiO, Ti2O3 and TiO2 titanium oxides be-low the surface layer. In the case of sample P7, the XPS spectra(Fig. 6(d)) shows that the inner layer is composed by Ti2O3 andTiO2. In the XPS analysis it was necessary to remove the surfacelayer of the films in order to obtain the true bulk composition;however, the ion bombardment changes the chemical state andthe composition of the titanium oxides and additional structuresappear in the XPS spectrum. Hashimoto et al. [36] reported thatthe original state of TiO2 was changed to Ti2O3 and TiO by ionbombardment; therefore, in order to determine the relative con-centration of TiO, Ti2O3 and TiO2 compounds in our samples, it wasnecessary to apply a correction algorithm by taking into accountthe transformation of TiO2 compound into TiO and Ti2O3. In arough approximation, it can be considerate that 40% and 0% of theTiO2 was changed into Ti2O3 and TiO, respectively, after 30 s ofetching according to the proposed formulation by Hashimoto et al.[36]. Therefore, in order to apply this correction to the relativepercentages of TiO, Ti2O3 and TiO2 compounds determined fromthe experimental data, it can be used the following considerations.Let A1, A2 and A3 the integrated intensity of Ti4þ , Ti3þ and Ti2þ inthe experimental XPS spectra, respectively, and AT¼A1þA2þA3.

Also, considerate the addition of integrated intensity of Ti4þ andTi3þ as A4 (A4¼A1þA2). The corrected integrated intensity of Ti4þ

(A1c) could be approximated by A1/0.6; hence the corrected in-tegrated intensity of Ti3þ (A2c) is A4�A1c. Then, the amount of

each titanium oxide phase could be obtained by using:

AA

% Ti x100c4 1

T=+

AA

% Ti x100c3 2

T=+

AA

% Ti x1002 3

T=+

From the fractional analysis determined by the integrated in-tensity for each contribution, the percentages of Ti 2p3/2 peaks dueto Ti2þ , Ti3þ and Ti4þ for the samples P2 (OMF 0.2), P4 (OMF 0.44)and P7 (OMF 1.08) are plotted in Fig. 7. The percentage of Ti4þ

becomes larger with the increase of OMF in the TiOx films; thefractions of Ti2þ and Ti3þ decrease with OMF. These results showthat OMF influence strongly the formation of different titaniumoxides below the surface layer of the samples. As shown in Fig. 7,for all the samples grown by RF reactive magnetron sputtering itwas not possible to obtain only the TiO2 chemical compound inthe bulk composition, even when OMF was as large as 1.08 sccm(sample P7), where a small fraction of Ti2O3 was present (10%).Similar results were reported by Lee et al. [37].

The presence of the different phases of titanium oxides in allthe samples is fundamental for the observed photocatalytic

Fig. 7. Fractions of Ti2þ , Ti 3þ , Ti 4þ and the sum of Ti2þ and Ti 3þ; from thesamples P2, P4 and P7.

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response reported in this work. These characteristics are asso-ciated to the OMF employed during the sputtering depositionprocess. In order to compare the performance of the samplesagainst a pure TiO2 sample, one film of TiO2 on aITO/glass sub-strate were prepared using the methodology reported by Hocevaret al. [29].

The photocatalytic activity, in the visible and near infrared re-gions, of the TiO2, P2, P4 and P7 samples was investigated by thedegradation of a methylene blue solution. Two two types of ex-periments were carried out: a) with infrared filter, and b) withoutinfrared filter; in both cases the longpass filter was used. Themeasurements employing TiO2 sample with or without the infra-red filter, did not exhibit any photocatalytic activity, which is ex-plained by the fact that stoichiometric TiO2 compound presents aphotoresponse only for wavelengths below its band gap (387 nm).The change in methylene blue concentration for samples P2, P4and P7, as a function of time, is plotted in Fig. 8. The blank symbolscorrespond to the experiment carried out employing the infraredfilter; and the filled symbols correspond to the experimentswithout it. The TiOx thin films showed photocatalytic activity un-der visible and near infrared illumination. This finding is in ac-cordance with the observed transmittance spectra, where thesesamples absorb light in the visible and infrared regions. For thesample P7, we had a mixture of TiO2 and Ti2O3 according to theXPS measurements; it has been reported that the binary TiO2–

Fig. 8. Methylene blue concentration vs. irradiation time of the samples P2, P4 andP7; with (F) and without IR filter.

Ti2O3 extended the visible response of photocatalyst [38], andtherefore the degradation of methylene blue could be due to thepresence of these titanium oxides phases. For this sample, we didnot find a difference in degradation of methylene blue by using orremoving the IR filter. The samples P2 and P4, showed a decreasein the degradation of the methylene blue solution when the in-frared filter was used. The sample P2 showed the highest photo-catalytic activity. The samples P2 and P4 had a mixture of TiO,Ti2O3 and TiO2 as showed by the XPS measurements and the darkcolor could be related with the presence of TiO. Therefore, becauseTiO2 compound do not show photocatalytic activity under irra-diation with light having a wavelenght higher than 400 nm, thephotocatalytic activity in the TiOx films must be related to thepresence of TiO and Ti2O3 compounds, as can be seen fromFigs. 6 and 7.

TiOx films, with the same experimental conditions of samplesP4 and P7 were deposited on glass coated with ITO, these sampleswere labeled as P8 and P9. The films were used as anodes inphotoelectrochemical cells. While current intensity is measuredduring monochromatic sample illumination with the full spectrumof the light source, the incident photon to current intensity isuseful to determine the wavelength dependence of the photo-response activity. When a working electrode is illuminated, theelectrons and holes, generated by the light, are separated by mi-gration within the depletion layer and by diffusion; then the holesreact at the interface of the semiconductor and the electrolyte(oxidizing the reduced species) and the electrons are detected asphotocurrent in the external circuit. The spectral photoresponse ofthe samples P8 and P9 is shown in Fig. 9(a) and (b), respectively.The intensities were normalized and the background subtracted inthe measurements, only for clarify purpose. Notable photocurrentresponse can be found for both samples below 400 nm, corre-sponding to the fundamental absorption edge in TiO2, which is oneof the compounds constituting the samples (as determined fromXPS and SAED analysis). Also, at about 680 nm a local maximum ofthe spectral response was presented; this photoresponse can beassociated with the photocatalytic activity of the samples observedfor illumination in the visible region. Moreover, the sample P8 hada peak at about 1028 nm, these wavelength is in the IR spectrum.Because photocatalytic degradation decreases when the IR filter isused in samples P2 and P4, the result described above (for sampleP8) confirms that samples with the presence of TiO compound arephotoactive under IR radiation. By comparison, the control sampleof stoichiometric TiO2 only presented a peak photoresponse below400 nm, which explains the non presence of photocatalytic

Fig. 9. Spectral photoresponse of the samples (a) P8 and (b) P9.

M. Zapata-Torres et al. / Materials Science in Semiconductor Processing 40 (2015) 720–726726

activity under irradiation with wavelengths higher than 400 nm.The photoresponse in the TiOx thin films-electrolyte interfacecould be generated by electron transfer between the localizedstates below the conduction band, similar results have been re-ported by Kim et. al. [39].

For stoichiometric TiO2 its bandgap of 3.2 eV explains the lowabsorption of light for λ4400 nm, then it is expected that it doesnot have any photocatalytic activity at longer wavelengths. Theband gap energies of TiO and Ti2O3 are 2.91 eV and 3.18 eV [30,31],respectively; hence the change in the band gap obtained in thesamples with the strongest absorption could be related to thepresence of the TiO. The bulk conductivity of TiO has been re-ported around 1000 S cm�1, then the presence of this phase in thesamples, with a strong absorption, modulate the resistivity al-lowing a better transport of the photogenerated carriers. Thesamples have a double layer structure over the glass substrate, theinner layer is composed by the different titanium oxides (TiO,Ti2O3 and TiO2); while on the surface there is a TiO2 layer due to anatmospheric oxidation process, with a thickness in the order of10 nm [40]. When a semiconductor electrode is brought intocontact with the electrolyte, a space charge layer is formed nearthe interface. Since the effective separation of photogeneratedelectron-hole pair occurs very close the interface, the rest part ofthe electrode plays only a role for the conduction or recombinationof the generated charge carriers [41]. The visible light (or visibleand NIR light) is absorbed by the inner layer, and the generatedcharge carriers (the electron-hole pairs without recombination)are transferred to the TiO2 surface layer; where the photocatalyticactivity is performed. Hence because the samples with poortransmittance had resistivities below the resistivity of stoichio-metric TiO2 (or Ti2O3), it is expected that the photogeneratedcharges reach more easily the sample surface. We further believethat the photocatalytic activity has a strong dependence with theinfrared spectrum for samples with the TiO phase and could berelated to the spectral photoresponse observed at longer wave-lengths. Then the remarkable photocatalytic activity observed inthe samples may be explained by an effective charge transportfrom the inner TiOx layer to the surface TiO2 layer due to thepresence of a TiO phase.

4. Conclusion

We studied the influence of oxygen partial pressure on thephysical and photocatalytic properties of the TiOx thin films pre-pared by RF magnetron reactive sputtering. The photocatalyticproperties as well as other film properties such as transmittance,resistivity and produced phases depend on the OMF. Based on theXPS results, our films are composed by two layers, the inner layerwas composed by different titanium oxides (TiO2, Ti2O3 and TiO)and the surface layer was stoichiometric TiO2. The samples showna photocatalytic activity in the visible or visible-NIR light spectra,which depends of the titanium oxides presented in the inner layerof the film with TiO playing a fundamental role in the photo-catalytic response.

Acknowledgments

This work is supported by SEP-CONACyT (project 153245) andSIP-IPN 20150090. XPS measurements were performed at

LANNBIO Cinvestav Mérida, under support from projects FOMIX-Yucatán 2008-108160, CONACyT LAB-2009-01 Nos. 123913 andCB2012/178947. HRSEM, HRTEM and SAED measurements wereperformed at LANE-CINVESTAV. The authors gratefully acknowl-edge the technical assistance by W. Cahuich, J. Roque and A. Gar-cia. O. Calzadilla acknowledge the financial support from ICyT-CLAF.

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