Synthesis of TiO2 on SnO2 bicomponent system and investigation of its structure and photocatalytic activity

Materials Chemistry and Physics 220 (2018) 249–259

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Synthesis of TiO2 on SnO2 bicomponent system and investigation of its structure and photocatalytic activity

T

M. Shipochkaa,∗, A. Eliyasb, I. Stambolovaa, V. Blaskova, S. Vassilevc, S. Simeonovad, K. Balashevd Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, “Acad. G. Bonchev” Str., Bld. 11, Sofia, 1113, Bulgaria Institute of Catalysis, Bulgarian Academy of Sciences, “Acad. G. Bonchev” Str., Bld. 11, Sofia, 1113, Bulgaria c Institute of Electrochemistry and Energy Systems, Bulgarian Academy of Sciences, Acad. G. Bonchev, bl. 10, Sofia, 1113, Bulgaria d Sofia University, Faculty of Chemistry and Pharmacy, Department of Physical Chemistry, 1 James Bourchier Blvd, Sofia, 1164, Bulgaria a

b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

Thin SnO /TiO sprayed bilayers ex• hibit high visible light photocatalytic 2

2

activity.

higher SnO content influences • The positive photocatalytic activity. roughness and higher content • Higher of OH groups results in higher reac2

−

tion rate.

Double layered SnO /TiO system en• sures effective separation of charge 2

2

carriers.

SnO /TiO sprayed bilayers have • Thin lower band gap energy than pure TiO 2

2

2

films.

A R T I C LE I N FO

A B S T R A C T

Keywords: Photocatalyst Oxide films Photocatalysis Optical band gap

Photocatalytically visible light active thin SnO2/TiO2 bicomponent coatings with SnO2 underlayer, were deposited by spray pyrolysis. XRD, AFM, XPS and DRS analyses were used to characterize their morphology, phase and chemical composition, structure and band gap width. The model wastewater contaminant was diazo dye and the photocatalytic properties were tested by discoloration oxidation reaction. The photocatalytic activity of the SnO2/TiO2 system depends on the SnO2 precursor solution parameters: sprayed volume and the concentration of the SnO2 solution. It was revealed that the bicomponent coatings with SnO2 underlayer, which has been prepared at large spray solution volume and low tin precursor concentration exhibit increased photocatalytic activity. The combination of several factors ensures high rate of the oxidative reaction of degradation: the lower recombination degree of electron-hole pairs due to the separation of charge-carriers; decreased band gap energy, higher roughness and larger content of hydroxil groups on the surface of SnO2/TiO2 bilayers in comparison to the TiO2 films as well as the synergism between anatase and rutile phases.

1. Introduction During the last decades, heterogeneous photocatalysis using TiO2 powders and films has made great progress in purification of water and ∗

air by decomposition of various organic compounds including light absorbance of the semiconductor, excitation and migration of the charge carriers, and redox reaction on the semiconductor surface. The TiO2 has been attracting worldwide attention due to its chemical and

Corresponding author. E-mail address: [email protected] (M. Shipochka).

https://doi.org/10.1016/j.matchemphys.2018.08.054 Received 25 October 2017; Received in revised form 30 May 2018; Accepted 19 August 2018 Available online 28 August 2018 0254-0584/ © 2018 Elsevier B.V. All rights reserved.

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investigated by X-ray photoelectron spectroscopy (XPS). The measurements were carried out on AXIS Supra electron-spectrometer (Kratos Analitycal Ltd.) using monochromatic AlKα radiation with a photon energy of 1486.6 eV and charge neutralisation system. The binding energies (BE) were determined with an accuracy of ± 0.1 eV. The chemical compositions in the depth of the films were determined monitoring the areas and binding energies of C1s, O1s, Ti2p and Sn3d photoelectron peaks. Using the commercial data-processing software of Kratos Analytical Ltd. the concentrations of the different chemical elements (in atomic %) were calculated by normalizing the areas of the photoelectron peaks to their relative sensitivity factors. UV–vis diffuse-reflectance spectroscopy (DRS) has been applied to determine the band gap energy Eg of reference pure TiO2 and doublelayered system SnO2/TiO2, aiming at comparison of the so obtained their values. The diffuse-reflectance spectra (DRS) have been recorded on Thermo Evolution 300 UV-VIS spectrometer within the wavelength range 190 nm up to 1100 nm. It is known that diffuse reflection occurs upon focusing the beam of the spectrometer on the sample. The diffusescattered light (redirected by mirrors) is detected by a sensor. The changes in the form of the bands and in their relative intensity (caused by the mirrors) have to be taken into account by Kubelka-Munk transformation. The equation of Kubelka-Munk has the following analytical expression:

photo-stability, non-toxicity, low cost and suitable optical band gap [1]. However, it can utilize only 6% of the total solar radiation due to the relatively wide intrinsic band gap [2]. To resolve the above problem, many efforts have been exerted to extend the light absorbance of TiO2 into the visible light region, such as: (i) transition metals or non metal ion doping [3,4], (ii) metal deposition [5], (iii) semiconductor composites [6], (iv) modifying of TiO2 particles with electron beam [7] etc. Recently, the gas sensing and photocatalytic properties of TiO2–SnO2 system is widely studied, due to its interesting physicochemical properties [8,9]. It has been proved that the TiO2–SnO2 composite nanoparticles via the combination of anatase and rutile-type of phases is effective for achieving a higher degree of charge separation, an increased lifetime of the free charge carriers and an enhanced inter-phase charge transfer to absorbed substrates [10]. On the other hand, this system offers larger number of active adsorption sites as the electrons transfer from TiO2 to SnO2 grains. Recently, the reserchers have focused their efforts on the bilayered systems: TiO2 deposited on SnO2 film. Only few data are available in the literature about SnO2/TiO2 films prepared by sol-gel method [11–13]. Shang et al. [11] have reported that TiO2/SnO2 heterostructure exhibits a higher photocatalytic performance than TiO2 in formaldehide degradation. Huang et al. [14] proved that bilayered structures TiO2/SnO2 showed decrease in the width of band gap in comparison with that of pure TiO2. In the available literature systematic data about the effect of the tin dioxide underlayer features on the photocatalytic efficiency of the sprayed SnO2/TiO2 bilayers are missing. The aim of this work was to study the effect of SnO2 films spray deposition parameters in order to optimize the preparation conditions of highly active visible light excited photocatalyst.

F (R) =

(1 − R)2 k = 2R s

where R denotes absolute reflectance of the studied layer, while s is the scattering coefficient and k is the coefficient of molar absorbability [15]. The optical band gap is evaluated using the equation of Tauc:

2. Experimental

1

(αhv ) n = A (hv − Eg ) The aluminium foil plates (75 × 25 mm, thickness 0.3 mm) were used for the deposition of thin SnO2/TiO2 bilayered system. The substrates were cleaned successively in hot ethanol and then in acetone. Afterwards tin dioxide film was deposited by spray pyrolysis on the cleaned foil. Tin tetrachloride (SnCl4, Sigma Aldrich, 99.99% purity) was diluted with isopropanol to obtain 0.25 or 0.125 M solutions. The experimental conditions for preparing SnO2 underlayer are shown in Table 1. Titanium tetrachloride TiCl4 was diluted with a mixture of isopropanol and butyl carbitol (C4H9OC2H4OC2H4OH) (1:4 v/v) to obtain 0.1 M solution. The aerosol of titanium precursor solution (25 ml) was sprayed onto the SnO2 film. The aerosol was generated by pneumatic glass nebulizer to the substrate heated at 280–300 °C. All the samples were thermally treated at 400 °C. For comparison TiO2 film was deposited on the same aluminium foil plates and thermally treated at 400 °C. The phase composition of the samples was studied by X-ray diffraction (XRD) with CuKα-radiation (Philips PW 1050 apparatus). The average crystallite sizes of the films were estimated according to Scherrer's equation: The film composition and electronic structure were

1 2 3 4 5 6

SnCl4 solution concentration

– 0.25 0.25 0.25 0.125 0.125

Sprayed ml of solution

– 25 15 5 25 30

SnO2 content mg/cm2

Crystallites size (nm) TiO2 anatase

SnO2 cassiterite peak (110)

– 20 12 4.1 8.2 12

33 31 30 29 30 –

– 8 8 12 11 9

(2)

In equation (2) α is the coefficient of absorbance, А is a constant of proportionality, h is the constant of Planck, ν is the frequency of the photon. Then Eg is the optical band gap and in our case of direct transition n equals ½ (it acquires value 2 in the case of indirect transition) [16]. Now the so obtained DRS spectrum acquires the form of the Kubelka-Munk's function. We can substitute F(R∞) in the equation of Tauc for coefficient of absorbance α. Therefore we obtain a new relationship: 1

(hvF (R∞)) n = A (hv − Eg )

(3)

There follows extrapolation of the linear region of the plot (hνF (R∞))1/n with respect to the ordinate in regard to the energy of the photon hν on the abscissa gives in result the value of the optical band gap energy Eg. The Eg is obtained from the intersection point of the tangent to the curve with the abscissa. The Tauc's types of plots, used for calculating the Eg values of the various films (based on the equation of the extrapolation in the linear region y = a+b.x, one can determine „x“, which is the value of the band gap energy). The surface topography was studied by means of Atomic Force microscope (AFM) (NanoScopeV system, Bruker Inc.) operating in tapping mode in air. The silicon cantilevers (Tap 300 Al-G, Budget Sensors, Innovative Solutions Ltd., Bulgaria with resonance frequency of 150 ± 75 kHz; tip radius was less than 10 nm) were used with 30 nm thick aluminum reflecting coating. The scanning rate was set at 1 Hz. Subsequently, all the images were flattened by means of the Nanoscope software. The photocatalytic experiments were conducted with powerful visible-light irradiation source (Tungsram lamp; 500 W, light intensity 9700 L m). The sample was dipped into the reaction vessel containing 150 ml solution of Reactive Black 5 dye (20 ppm concentration) and stirred continuously under oxidative conditions at room temperature. Initially, the continuously stirred solution was left for 30 min in a dark to reach adsorption-desorption equilibrium in order to evaluate the

Table 1 SnO2 underlayer deposition parameters and crystallites size of anatase and cassiterite phase. Sample

(1)

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Fig. 1. XRD spectrum of SnO2/TiO2 bilayers (sample 3).

Fig. 3. XRD spectrum of SnO2/TiO2 bilayers (sample 6).

adsorption capacity. Then the visible light irradiation was swiched on and the photocatalytic reaction is started. The photocatalytic degradation degree was evaluated by taking aliquot of the solution and measuring the residual concentration using spectrophotometer UV1600PC in the wavelength range from 200 to 800 nm at regular time intervals.

phases a relatively small amount of rutile phase appears (Fig. 3). This result indicates that the increased content of SnO2 accelerates the transformation of anatase into rutile phase. In our previous study we have found out that the addition of SnO2 in TiO2 sprayed films has a promoting effect on the transformation anatase into rutile crystalline phase due to the rutile-like structure of the cassiterite phase [17]. Table 1 represents the sizes of the crystallites of the anatase and cassiterite phase of the samples. The crystallites sizes of the anatase phase and cassiterite phase are varying sligthly i.e. are not dependent on the SnO2 underlayer deposition parameters. The surface composition and chemical state of the SnO2/TiO2 bilayers were investigated by XPS. The XPS analysis detects peaks of O1s, C1s and Ti2p on the surface of the films. The SnO2/TiO2 bilayers prepared under different conditions show similar features. The Ti2p3/2 peaks have a maximum at 458.8 eV, typical of Ti4+ oxidation (Fig. 4a). The O1s spectra of the samples are shown on Fig. 4b. The Ols peak is deconvoluted by Lorentzian–Gaussian curve fitting into three components (for sample 1) and two components (for samples 3 and 6) with increasing binding energy (Fig. 5). The first one 530 eV is assigned to lattice oxygen in TiO2 and the second one at ∼531.7 eV is attributed to adsorbed hydroxyl species. It is known, that by means of XPS one measures the kinetic energy and number of electrons that escape from the top layer: 0–5 nm of the material being analyzed. For this reason no peaks, corresponding to underlayer SnO2, are observed in the spectra. The Ototal/Ohydroxyl groups atomic ratio is obtained from the deconvoluted O1s peaks (Table 2). The values of Ototal/Ohydroxyl groups atomic ratio are decreasing upon increasing quantity of tin in precursor solution, which means that by increasing the amount of hydroxyl groups the photocatalytic activity of the top layers is increased. The presence of a photoelectron peak around 198 eV, was also registered, which reveals the existence of chlorine on the films surface (Fig. 4a, inset). The Cl doping of titania was ascertained to induce appearance of electron level in the band gap. Some authors [18,19] report shifting of the absorption edge to visible region. The Cl¯ anions are supposed to introduce some more surface defects [20] and produce more hydroxyl radicals [21], which results in higher conversion degree of the model contaminant. So far no mechanism of visible light excited activity for chlorine doped TiO2 has been reported [22] to the best of our knowledge. UV–vis diffuse-reflectance spectroscopy (DRS) for the reference pure TiO2 and double-layered system SnO2/TiO2 is presented on Fig. 6 and Kubelka-Munk transformation on Fig. 7. The band gap energy of the TiO2 films is estimated on the basis of the plot in Fig. 8. Fig. 9 represents Tauc's types of plots and estimated band gap values are within the range 2.76–3.48 eV. The energy of the optical band gap is

3. Results and discussion The XRD spectrum of a TiO2 thin film, annealed at 400 °C (not shown here) registers well crystallized anatase phase with strongest diffraction peak, associated with the crystalline direction (101). The diffraction peaks, corressponding to orientations (004) and (112), have weaker intensities. The bilayers numbered 2, 3, 4 and 5 contain mixture of two crystallographic phases: anatase and cassiterite. Fig. 1 represents the typical XRD spectrum of SnO2/TiO2 bilayered structure: sample 3. With increasing the quantity of SnO2 the intensity of the peak belonging to cassiterite phase becomes stronger, while the intensity of anatase phase weakens (Figs. 2 and 3). Huang et al. [14] have also established for the TiO2-SnO2 composites that intensity of cassiterite peak is expected to be increased with the SnO2 content increase, while the diffraction peak of anatase phase gradually decreases. In sample 6 besides these two

Fig. 2. XRD spectrum of SnO2/TiO2 bilayers (sample 5). 251

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Fig. 4. Ti2p (a), O1s (b) and Cl2p (b, inset) core level spectra of SnO2/TiO2 bilayers.

decreasing upon increasing the tin content in the initial solution as well as upon increasing the quantity of the sprayed solution (Table 2). The charge carrier transfer between the conduction bands of TiO2 and SnO2 results in promotion of the photocatalytic efficiency of the layers, due to shifting of the absorption edge to higher wavelengths in the cases of samples 4 and 6. Other research groups proved also that TiO2/SnO2 films have lower band gap value than that of the pure TiO2 [14]. For the pure TiO2 sample the value of Eg is 3.48 eV and it is in correspondence with the values, published by other authors [23–27]. The surface morphologies of the TiO2 and SnO2/TiO2 bilayers were investigated by Atomic Force Microscopy (AFM). Typical topographical images of the surface of TiO2 coating (sample 1) are represented in 2D and 3D format (Fig. 10). It can be seen from the figure that the film

Table 2 XPS data for chemical composition, absorption edge and band gap values of the samples. Sample

Ti, at.%

O, at.%

Ototal/OOH

1 3 6

19.1 28.8 28.6

81.1 70.1 71.4

1.74 1.41 1.46

groups

absorption edge, nm

Еg, eV

375 448 481

3.48 2.98 2.76

follows the topography of the aluminium substrate. The film is relatively dense without any visible pores and cracks. The surface root mean square (RMS) value of the surface roughness of selected region

Fig. 5. Deconvoluted O1s peaks of SnO2/TiO2 bilayers for samples 1(a), 3 (b) and 6 (c). 252

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underlayer obtained by spraying of 30 ml SnCl4 solution (0.125 M) is represented in Fig. 11. The image displays the vertical growth of agglomerates that makes the film rough. Fig. 12 illustrates the topography of TiO2 films with SnO2 underlayer (sample 3). The AFM images of SnO2/TiO2 bilayers display more rough surface than those of the TiO2 film. The calculated mean roughness Ra values for sample 3 is about 40 nm, while for the sample 6 this value increases to 62 nm. The sample 6 possesses more developed surface, which consists of many grains with circular shapes and different diameters (Fig. 13). Figs. 14 and 15 illustrate the reaction course of the azo dye degradation with the illumination time interval for the samples with different SnO2 content in the spray solution. The photocatalytic studies have proved that the kinetic curves for RB5 degradation follow the pseudo-first order kinetics of the Langmuir - Hinshelwood model as confirmed by plotting of ln(C/Co) vs reaction time t. The slope of the straight line is the rate constant k. The R2 is the correlation coefficient, which has value higher than 0.9 as it was obtained. The photocatalytic efficiency of the SnO2/TiO2 system depends on the SnO2 films deposition parameters. The positive effect of the increase in the SnO2 content on the photocatalytic activity is significant. It is obvious, that increasing SnO2 content accelerates considerably the photocatalytic reaction after 180 min visible irradiation. Fig. 14 illustrates the efficiency of the structures with SnO2, obtained from 0.25 M solution (2, 3 and 4). One has the impression that the higher content of SnO2 leads to higher photocatalytic activity. It can be supposed that the higher number of holes reaching TiO2 surface leads to more intensive inter-phase electron transfer. The samples 5 and 6 with SnO2 prepared from 0.125 M solution have shown the same correlation between the SnO2 content and activity, but they have manifested higher activity than those obtained with other bilayered structures and TiO2 single layer. The evaporation rate of solution droplets is higher for the lower SnCl4 solution concentration (0.125 M) and the degree of supersaturation is reached faster [28]. This leads to formation of a abundance of nanosized crystallites acting as nucleation sites for TiO2 film growth. For this reason SnO2/ TiO2 films display rougher surface than those of TiO2 film, ensuring more active centers for dye adsorption. Fig. 16 shows the adsorption capacity and degree of dye degradation of the samples. The data for adsorption capacity were calculated using the results of photocatalytic experiments with the films in dark conditions for 30 min, plotted in coordinates C/Co vs t, where Co is the initial concentration of the pollutant in the treated solution and C the concentration on the 30th minute. As it can be seen from the figure the photocatalytic activity does not follow the changes of the sorption properties. Similar results were obtained for TiO2 films, used in photocatalytic degradation of Methylene Blue [29]. The highest photodegradation rate constant was estimated for sample 6, which does not exhibit the highest adsorption ability. The increase of the photocatalytic activity of the SnO2/TiO2 double layers could be due to the several factors:

Fig. 6. Diffuse-reflectance spectra of SnO2/TiO2 bilayers.

Fig. 7. Absorbance spectrum of the SnO2/TiO2 bilayers with Kubelka-Munk transformation.

(i) inter-phase charge transfer between SnO2 and TiO2 films (Fig. 17). Electron hole pairs are being generated during the illumination time interval. The photogenerated electrons are transferred from the conduction band of TiO2 to the conduction band of SnO2 having lower energy position. Thus the electrons are being accumulated in the SnO2 underlayer. The holes are migrating in the opposite direction – to the valence band of TiO2. In this way a lower recombination rate is achieved. Hassan et al. [30] have proved that the addition of Sn ions to TiO2 has significantly reduced the recombination rate, thus improving the photocatalytic efficiency of SnO2/TiO2 nanocatalysts. Similar results were presented by other research groups for the bicomponent oxides on the base of TiO2 [31,32]. (ii) the increased degree of light absorbance of the smaller band gap of the bilayers, compared to that of pure TiO2 [14,33]. In addition to this effect, the transfer of the photo-excited electrons to the

Fig. 8. Comparative diagram of (F(R)hν)2 as a function of hν, of SnO2/TiO2 bilayers.

amounted to 17 nm. The surface of SnO2 underlayer obtained by spraying of 0.25 M SnCl4 is realtively smooth and it follows the topography of the substrate (not shown). The surface morphology of SnO2

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Fig. 9. Tauc's types of plots: (F(R)hν)2 as a function of hν for samples 1(a), 3 (b) and 6 (c).

adsorbed oxygen (O2−●) and oxidation of H2O molecules into OH● radicals by the holes [34] also facilitates the reaction (probably by radical-chain mechanism in the bulk water phase). (iii) rougher surface of the bilayered structure ensures a larger number

of adsorption sites (as it can be seen in the AFM images) and increase of UV irradiated region. Yoshida et al. have also proved that photocatalytic efficiency enhances due to the increased surface roughness, achieved by chemical etching of the substrate [35].

Fig. 9. (continued) 254

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Fig. 9. (continued)

Fig. 10. (continued)

surface of the catalyst, which leads to complete oxidation [38]. (v) probably, there is also synergistic effect between anatase and rutile phases. The separate phases of rutile and anatase display lower activity in comparison to their mixtures. Moreover the presence of inter-phase conjunctions between the two TiO2 polymorphic phases can effectively enhance the separation of the photoinduced electron-hole pairs and reducing the degree of their recombination [39]. (vi) Concerning the role of the metallic substrate we should underline two important aspects: - the metallic sheet offerring more nucleation sites for film growth during the spray pyrolysis which ensures the formation of rougher films surface. - the metal support is conductor and the photo excited electrons can penetrate in the aluminum, which traps the electrons and the recombination rate is minimized.

Fig. 10. AFM images of TiO2 film in 2D (left) and 3D (right) format.

(iv) the enhanced photoactivity of the SnO2/TiO2 films can be explained also based on the lower values of Ototal/Ohydroxyl ratio. Hydroxyl anions on the photocatalyst surface are oxidized by the separated holes (h+) into OH● hydroxyl radicals being transferred to the bulk water phase and these radicals enhance the photocatalytic reaction [36,37]. The free OH radicals are active enough to extract hydrogen atoms from the dye molecules, adsorbed on the 255

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Fig. 12. (continued)

Fig. 11. AFM images of SnO2 underlayer of sample 3 (left) and sample 6 (right) in 3D format.

Fig. 11. (continued)

Fig. 13. AFM images of SnO2/TiO2 bilayers of sample 6 in 2D (left) and 3D (right) format.

Fig. 12. AFM images of SnO2/TiO2 bilayers of sample 3 in 2D (left) and 3D (right) format.

Fig. 13. (continued)

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Fig. 14. Kinetic curves of RB5 degradation for reference TiO2 film and SnO2/TiO2 bilayers with SnO2 layer, obtained from 0.25 M solution.

Fig. 15. Kinetic curves of RB5 degradation for SnO2/TiO2 bilayers with SnO2 layer, obtained from 0.125 M solution.

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Fig. 16. Adsorption ability for 30 min under dark conditions and degree of dye degradation for 3 h of illumination.

and pharmaceutical contaminants in the nature by multicomponent systems” EBR SANI for the financial support. References [1] M. Kapilashrami, Y. Zhang, Yi-Sh Liu, A. Hagfeldt, J. Guo, Probing the optical property and electronic structure of TiO2 nanomaterials for renewable energy applications, Chem. Rev. 114 (2014) 9662–9707. [2] C. Kormann, D.W. Bahnemann, M.R. Hoffmann, Preparation and characterization of quantum-size titanium dioxide, J. Phys. Chem. 92 (1988) 5196–5201. [3] G. Liu, X. Zhang, Y. Xu, X. Niu, L. Zheng, X. Ding, The preparation of Zn2+-doped TiO2 nanoparticles by sol–gel and solid phase reaction methods, respectively and their photocatalytic activities, Chemosphere 59 (2005) 1367–1371. [4] L.G. Devi, R. Kavitha, A review on non metal ion doped titania for the photocatalytic degradation of organic pollutants under UV/solar light: role of photogenerated charge Carrier dynamics in enhancing the activity, Appl. Catal., B: Environmental 140–141 (2013) 559–587. [5] C.C. Chan, C.C. Chang, W.C. Hsu, S.K. Wang, J. Lin, Photocatalytic activities of Pdloaded mesoporous TiO2 thin films, Chem. Eng. J. 152 (2009) 492–497. [6] J.Y. Kim, C.S. Kim, H.K. Chang, T.O. Kim, Effects of ZrO2 addition on phase stability and photocatalytic activity of ZrO2/TiO2 nanoparticles, Adv. Powder Technol. 21 (2010) 141–144. [7] M.M. Khan, S.A. Ansari, D. Pradhan, M.O. Ansari, D.H. Han, J. Lee, M.H. Cho, Band gap engineered TiO2 nanoparticles for visible light induced photoelectrochemical and photocatalytic studies, J. Mater. Chem. 2 (2014) 637–644. [8] B. Lyson-Sypien, A. Kusior, M. Rekas, J. Zukrowski, M. Gajewska, K. MichalowMauke, T. Graule, M. Radecka, K. Zakrzewska, Nanocrystalline TiO2/SnO2 heterostructures for gas sensing, Beilstein J. Nanotechnol. 8 (2017) 108–122. [9] V.R. de Mendonc, O.F. Lopesa, R.P. Fregonesi, T.R. Giraldi, C. Ribeiro, TiO2-SnO2 heterostructures applied to dye photodegradation: the relationship between variables of synthesis and photocatalytic performance, Appl. Surf. Sci. 298 (2014) 182–191. [10] O. Carp, C.L. Huisman, A. Reller, Photoinduced reactivity of titanium dioxide, Prog. Solid State Chem. 32 (2004) 33–177. [11] J. Shang, W. Yao, Y. Zhu, N. Wu, Structure and photocatalytic performances of glass/SnO2/TiO2 interphase composite film, Appl. Catal., A 257 (2004) 25–32. [12] E.I. Velazquez-Cruz, K.M. Anaya-Castillejos, R. Martinez-Martinez, A.B. SotoGuzman, C. Falcony, Characterization of a heterostructure TiO2/SnO2:F/substrate with two different geometries, prepared by spray pyrolysis to be used as photocatalyst, Surf. Rev. Lett. 20 (2013) 1350042-1. [13] A. Hatori, Y. Tokihisa, H. Tada, N. Tohge, S. Ito, K. Hongo, R. Shiratsuchi, G. Nogami, Patterning effect of a sol-gel TiO2 overlayer on the photocatalytic activity of a TiO2/SnO2 bilayer-type photocatalyst, J. Sol. Gel Sci. Technol. 22 (2001) 53–61. [14] M. Huang, J. Yu, B. Li, Ch Deng, L. Wang, W. Wu, L. Dong, F. Zhang, M. Fan, Intergrowth and coexistence of TiO2-SnO2 nanocomposite with excellent photocatalytic activity, J. Alloy. Comp. 629 (2015) 55–61. [15] J.T. Kajiya, Ed P. Hanrahan, W. Krueger, Reflection from layered surfaces due to subsurface scattering, SIGGRAPH ’93 Proceedings, 27 1993, pp. 165–174.

Fig. 17. Scheme for charge carrier transfer of SnO2/TiO2 bilayers.

4. Conclusions Nanosized SnO2/TiO2 double layered structures were obtained by spray pyrolysis. The films manifested high photocatalytic activity in the degradation of azo dye under visible irradiation, which is attributed to the smaller band gap. Photocatalytic efficiency of the SnO2/TiO2 system depends on the SnO2 films spray deposition parameters: concentration of solution and SnO2 content. The lower solution concentration and higher SnO2 content result in higher photocatalytic activity. The lower rate of recombination of electron-hole pairs due to the separation of charge carriers ensures high photocatalytic efficiency of the double layered system. The coexistence of anatase and rutile phases, larger number of active sites available for adsorption of reactive species (due to the rougher surface of the structures) and larger content of OH− groups on the surface of SnO2/TiO2 system in comparison to the TiO2 films ensures higher rate of the oxidative reaction. Acknowledgements The authors are grateful for sponsorship under contract “Heterogeneous catalytical and photocatalytical destruction of organic 258

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