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Applied Surface Science 258 (2012) 4339–4346

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Applied Surface Science

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The influence of surfactants on the crystalline structure, electrical andphotocatalytic properties of hybrid multi-structured (SnO2, TiO2 and WO3) thinfilms

Alexandru Enesca ∗, Luminita Andronic, Anca DutaThe Centre: Product Design for Sustainable Development, Transilvania University, Eroilor 29, 500036 Brasov, Romania

a r t i c l e i n f o

Article history:Received 20 April 2011Received in revised form19 December 2011Accepted 26 December 2011Available online 8 January 2012

Keywords:Spray pyrolysis depositionHybrid layersMorphologyElectrical propertiesPhotocatalysis

a b s t r a c t

The paper presents the influence of surfactants additives (anionic and cationic) on the crystalline struc-ture (XRD), morphology (AFM), surface energy (contact angle), optical (absorbance and reflectance) andelectrical properties of mono and multi-structured thin films containing SnO2, TiO2 and WO3 layers. Theanionic surfactant supports the structure densification with regular grains distribution. Contrary, thefilms obtained using cationic surfactants have defects in the layer morphology and the samples are non-homogeneous and non-uniform. The samples have photocatalytic activity with an efficiency of 80% forthe Ti sample and up to 65% for bi (Sn Ti, Sn W) and tri-component (Sn W Ti) samples.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The photo-electrochemical conversion of the solar energy usingsemiconductors has attracted considerable interest over the pastdecades. The photo-induced charge separation, using the band gapexcitation of the semiconductor films, plays a significant role inmaximizing the photocurrent generation efficiency if recombina-tion is limited.

Bi-component semiconductors’ thin films are considered asimportant technological materials because of their prime applica-tions in various optical and electronic devices [1–3]. Interest in thephysical and opto-electrical properties of the transition oxide fam-ily has grown rather sharply during the past few years, since thediscovery of the electrochromic behaviour in these materials [4–6].

The structures coupling semiconductors represent some of themost promising candidates for a photocatalyst and have beenfound to be suitable for a wide range of processes, including solarenergy conversion and wastewater degradation of organic com-pounds by heterogeneous photocatalysis [7–9]. By coupling twodifferent semiconductor materials such as TiO2/SnO2, SnO2/WO3and WO3/TiO2, enhanced charge separation is possible, by

∗ Corresponding author. Tel.: +40 726680794; fax: +40 268410525.E-mail addresses: aenesca@unitbv.ro (A. Enesca), a.duta@unitbv.ro (A. Duta).

accumulating electrons and holes in two different semiconductorlayers with the suppression of charge recombination [10–12].

This paper investigates the influence of surfactant addi-tives on tailoring the morphology of mono-component (SnO2,TiO2 and WO3), bi-component (SnO2/TiO2 and SnO2/WO3) andtri-component (SnO2/WO3/TiO2) thin films obtained by spraypyrolysis deposition (SPD). New correlation between the crys-talline structures, morphologic and photocatalytic properties arepresented. A detailed study on the electric properties of bi- and tri-component samples indicates how the investigated structures canprevent the recombination of the electron–hole pairs and expandthe range of useful excitation light towards the visible part of thelight spectrum. Also, the paper points out oxide combinations withunfavourable band energy position, where the electrical resistanceof overall system increases.

2. Experimental

2.1. Precursors preparation

Six types of layers were deposed by spray pyrolysis (SPD), usingidentical quantities (30 mL, 0.025 molar concentrations) of theprecursor’s solution, prepared using absolute ethanol as solvent(C2H5OH, 100%, Alfa Aesar):

0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.apsusc.2011.12.110

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Table 1Deposition parameters.

Sample Metal oxide Additive Deposition temperature (◦C)

Type Concentration (ppm)

1st layerSn SnO2 SDS 400 350Ti TiO2 HTAB 200 500W WO3 HTAB 25 300Sn Ti SnO2 SDS 400 350Sn W SnO2 SDS 400 350Sn W Ti SnO2 SDS 400 350

2nd layerSn Ti TiO2 HTAB 200 500Sn W WO3 HTAB 25 300Sn W Ti WO3 HTAB 25 300

3rd layerSn W Ti TiO2 HTAB 200 500

- SnO2 layers, using tin chloride (SnCl4, 99.8%, Alfa Aesar) as metalprecursor;

- TiO2 layers, using titanium chloride (TiCl4, 99.9%, Alfa Aesar);- WO3 layers, using tungsten chloride (WCl6, 99.8%, Alfa Aesar).

Three mono-component reference layers were deposited usingsingle precursor sources. The other three samples were deposited ina wafer structure, consisting of two or three mono-component lay-ers. Most of the layered structures used in photo-electrochemicalapplications use tin-oxide based substrates (FTO, ITO, etc.), there-fore the multi-component samples have the first layer based onSnO2. The layer deposition order can be depicted from the samplesname and is described in Table 1.

For tailoring the film morphology, one anionic surfactant (SDS –sodium n-dodecylsulfate, 99%, Alfa Aesar) and one cationic surfac-tant (HTAB – 1-hexadecyl-trimethylammonium bromide, 98%, AlfaAesar) were mixed, in various concentrations, into the precursors(see Table 1). The type and the concentration values where selectedbased on previous experimental work, considering the limiting sur-factant amount above which morphological and electrical changesare expected.

The microscopic glass (Heinz Herenz) and FTO (F doped SnO2coated glass – Libbey Owens Ford TEC 20/2.5 nm) were used as sub-strates for all six types of samples. Pieces of 2 cm × 2 cm glass andFTO were cleaned by successive immersion in ethanol and acetone,using an ultrasonic bath and were further dried with compressedair.

After deposition, each layer was cooled down at room temper-ature and then annealed in air at 500 ◦C for 5 h.

2.2. Film characterization

The crystalline structure of the samples was studied by XRDanalysis using a Bruker D8 Discover Advanced Diffractometerwith locked coupled continuous scan, using a scintillation counter(12,800 steps, 2 s/step) and a radiation with 1.5406 A wavelength(CuK�1, at 40 kV, 20 mA).

The morphology of the nanocomposite structures was inves-tigated using an Atomic Force Microscope (AFM, NT-MDT modelBL222RNTE). The images were taken in semicontact mode withSi-tips (NSG10, force constant 0.15 N/m, tip radius10 nm).

Absorbance and reflectance spectra were recorded using aUV–VIS spectro-photometer (Perkin Elmer Lambda 25 UV/VIS).The analysis covered the 200–800 nm range, with a scan rate of60 nm/min (lamp changes at 326 nm).

The current-voltage analyses were performed using a multi-channel potentiostat (PAR Instruments, model HM 8143) withfrequency analyser using two graphite contacts positioned on the

top of the layers and on the substrate surface. One contact was usedfor applying voltage (on the substrate) and the second contact wasused as a receiver, for recording the current intensity (on the layers).

2.3. Photocatalysis experiments

The photodegradation reactor consists of a static cylindricalflask, open to air. Three F18W/T8 black light tubes (Philips) (UVAlight, typically 340–400 nm, with �max (emission) = 365 nm), placedannular to the photoreactor were used for photodegradation exper-iments. The mean value of the radiation flux intensity, reachingat the sample position, was of 3 Lx (digital Luxmeter Mavolux5032 C/B USM).

The pollutant testing dye used in this work was 0.0125 mMmethylene blue (99.8%, Merck) prepared by dissolving the pow-der in ultra-pure water (Direct-Q 3 Water Purification System). Inall experiments, hydrogen peroxide (4 mL H2O2 30%/L of dye solu-tion) was used as electron acceptor to improve the photocatalyticefficiency on the semiconductors.

Absorbance spectra (Perkin Elmer Lambda 25 UV/VIS) wererecorded in the range of 200–600 nm, and the calibration curve wasdeveloped at the maximum absorption wavelength experimentallyregistered for methylene blue (� = 665 nm).

The efficiency of the methylene blue photo-degradation wascalculated using the following equation:

� = c0 − c

c0× 100 (1)

where c0 represents the initial concentration and c represents theconcentration after t minutes of photocatalysis, calculated basedon the calibration curve.

3. Results and discussions

3.1. Crystalline structure and morphology

For the Sn sample, the diffraction data (Fig. 1) of the mono-component layers show the formation of tetragonal tin oxide(SnO2). Several papers [13–15], report the formation of stannousoxide (SnO) simultaneously with the formation of stannic oxide(SnO2). To prevent this, the Sn sample was annealed at 500 ◦C andthe results confirm that only the higher oxidation state compoundis formed.

Typically, nucleation and growth occur within the aggregatesof amorphous inorganic particles that are partially stabilized bysurface-adsorbed SDS surfactant molecules. Under these condi-tions, the crystallization reactions of the precursors can result incomplex forms due to emergent processes that radically transformthe systems [8]. The diffraction data give no evidence of segregated

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Fig. 1. Diffraction patterns of the samples.

sodium oxide formation following the SDS pyrolysis process; still,sodium cation is part of the systems and can act as dopant in theSnO2 layer, according to the following lattice reaction (Kroger–Vinknotation was used):

2SnO2 −→Na2O

2Na′ ′′Sn + 3V ··

O + OxO (2)

The Ti samples contain both the rutile and anatase TiO2 poly-morphs, as consequence of the deposition temperature (500 ◦C),although anatase has superior optical and electrical properties inphotocatalytic applications.

The influence of HTAB cationic surfactant during the pyrolysisprocess can be described in two steps: the first one occurs veryfast and involves the coil structure formation, while the second isrepresented by the thermal decomposition. In this case the growthprocess across a relatively small number of nuclei is likely, consider-ing the steric hindrance imposed by the branched chain surfactantstructure, and consequently the formation of larger crystallites.

The W sample contains orthorhombic WO3 and a non-stoichiometric form of tungsten oxide (WO2.92). The formation ofthe non-stoichiometric compounds was already reported [16,17],and was correlated with the oxygen deficit during the depositionprocess (the carrier gas was compressed air at 1.5 bar). Consider-ing the surfactant decomposition as an oxygen consuming processduring the pyrolysis process, the HTAB concentration was furtherdecreased at 25 ppm.

The XRD pattern of the Sn Ti bi-component sample presents thepeaks corresponding to tetragonal SnO2, anatase TiO2 and new TiO2crystalline structure, while no rutile structure is identified, indi-cating that the crystallization process is different, comparing withthe mono-component samples. Similar observations are valid forthe Sn W bi-component sample which mainly contains SnO2 andWO3 and only one peak can be assigned to the non-stoichiometrictungsten oxide (WO2.92). The results obtained for bi-componentsamples are confirmed by the three-component sample, contain-ing SnO2, anatase TiO2 and WO3 (one single peak corresponding toWO2.92).

The diffraction analyses are not indicating the formation ofmixed oxides, but there still exists the possibility of inter-dopingat the layers’ interface, through diffusion process, considering thedeposition temperatures. The values of the crystallite sizes (Table 2)were calculated with Scherrer formula and are higher for layers

obtained directly on microscopic glass comparing with the layersobtained on oxide substrates.

D = K�

FWHM × cos �= 0.9 × �

FWHM × cos �(3)

These differences are the result of a larger amount of high energysites on the oxide surfaces, acting as nucleation centres and promot-ing a large number of small sized crystallites. Contrary, the layersobtained directly on the smooth microscopic glass (average rough-ness 10 nm) can have a slower nucleation rate, thus larger crystallitesizes. Annealing enhances the crystallization process in oxygen richatmosphere and eliminates (most of) the carbon species that canbe formed during the deposition, as result of a possible incompletesurfactant decomposition.

The AFM analyses (Fig. 2) show the formation of dense mor-phologies in the case of Sn sample with a relatively small roughness(around 15 nm, Table 2). This is a consequence of the SDS structure,with a linear hydrocarbon tail that favours a uniform distribu-tion of nuclei on the substrate and the development of regularmorphologies. The samples where the HTAB surfactant was addedare characterized by a non-homogeneous morphology with highroughness, as result of larger aggregates. The bi-component Sn Tisample has a fractal morphology which is a particular propertyof anatase TiO2 layer deposed by SPD on oxide substrates [18,19].Larger aggregates are also obtained involving the smaller crystal-lites, as in the Sn W sample, resulting in random grains’ distributionand non-uniform morphologies. The three-component sample hasa porous morphology which is the consequence of the differentlayers superposition. The TiO2 thin film has a “smoothing effect”(during the deposition, the larger WO3 pores are filled by the nextlayer) and large interfacial areas can be expected in the two- andthree components films.

The contact angle measurements (Fig. 3) were performed toinvestigate the influence of the morphology and the crystallinityon the surface properties. For accurate measurements, two liq-uids with different polarity were used: glycerol (with the polarcomponent of the surface energy �p = 41.50 mN/m and the dis-persive component �d = 21.20 mN/m) and ethyleneglycol (with thepolar component �p = 19 mN/m and the dispersive component�d = 29 mN/m).

According to the AFM data, the use of SDS allows a structuredensification. This result is confirmed by the contact-angle mea-surements, proving that the samples containing the SnO2 layerhave a low absorption/adsorption rate (0.03◦/s), comparing to TiO2(0.17◦/s) and WO3 (0.12◦/s) mono-component samples. For all thesamples, the wetting equilibrium is reached within a minute as theresult of the dense morphologies and low roughness values.

Using the Fowkes equation (Eq. (4)), that includes the dispersiveand polar components of the liquid-solid interfaces; the surfaceenergy values for each sample were calculated and are presentedin Table 3.

According to Fowkes:

�LV (1 + cos �) = 2[(�pLV �p

SV )1/2 + (�d

LV �dSV )

1/2] (4)

where �pLV , �d

LV , �pSV , �d

SV represent the polar and the dispersive com-ponents of the liquid and of the solid surface energies, respectively.

The surface energy investigations show that all samples have apredominant polar component, as expected for the oxide surface.This behaviour is more evident for the W mono-component sam-ples and can be explained by the slightly acidic character and/orthe surface non-homogeneity induced by the HTAB surfactant, withhigh energy surface areas (located mostly at the grains’ boundaries).The highest surface energy corresponds to the samples where theHTAB surfactant was used, inducing large, non-uniform areas.

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Fig. 2. 2D and 3D AFM images of the samples obtained by SPD: (a) Sn, (b) Ti, (c) W, (d) Sn Ti, (e) Sn W and (f) Sn W Ti.

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Table 2Crystallite size and roughness analysis.

Sample Crystallite size (A) Roughnessa (nm)

SnO2 TiO2 TiO2 WO3 WO2.92

Anatase Rutile2� = 26.59◦ 2� = 25.32◦ 2� = 27.43◦ 2� = 33.57◦ 2� = 22.96◦ 2� = 32.66◦

Sn 85.5 – – – – – 15.40Ti – 177.7 182.1 – – – 24.86W – – – – 111.4 108.2 68.81Sn Ti 81.8 81.3 – 78.4 – – 25.27Sn W 90.8 – – – 80.2 89.6 36.49Sn W Ti 87.5 82.9 – – 81.5 78.4 20.58

a Evaluated from AFM analysis (acceptable error 0.05 nm).

Fig. 3. Contact-angle measurement with glycerol.

The hydrophilic character proved by all layers represents anadvantage for further applications using water (aqueous systems)as working environment, such as photocatalysis.

One of the proposed mechanisms for the photoinducedhydrophilicity is initiated by holes, which react with the surfacelattice oxygen species, promoting the reconstruction of the surfacehydroxyl groups [20,21]. This phenomenon has various industrialapplications such as self-cleaning exterior tiles and anti-foggingmirrors. Another advantage is represented by the outer TiO2 layerthat increases the chemical stability of the bi- and tri-componentsamples and improves the lifetime of the material in aqueous solu-tion (especially for wastewater treatment by photocatalysis).

Table 3The values of polar and dispersive components of the surface energy.

Sample Surface energy� (mN/m)

Polarcomponent �p

(mN/m)

Dispersivecomponent �d

(mN/m)

Sn 45.51 28.97 16.54Ti 57.78 49.98 7.81W 77.89 66.73 11.16Sn Ti 51.59 34.01 17.57Sn W 48.96 31.80 17.16Sn W Ti 62.14 39.11 23.03

3.2. Opto-electric properties of the layers

The experimental data (Fig. 4) show that the absorption rangeof the samples is mostly located in the UV region. The WO3 layersinduce a slight shift to the visible region, thus more charge carriercan be generated into the materials by photoconversion.

Firstly, data were recorded on single component samples to geta correct estimation of the energetic levels in each material. Theextrapolation of this calculus on bi and tri-component samples canbe subject of possible errors, mainly as result of the ionic diffusionprocess at the layers’ interfaces, but the overall results in terms ofelectrical conductivity remain unchanged if the material’s compo-sition remains unchanged.

This property can be expected only if the energy levels (bandgap, conduction band and valence band) are properly displaced inorder to insure the charge carrier flow into the multi-materials sys-tem. The evaluation was made by a study where the film thicknesswas estimated from reflectance spectra, and the band gap (Fig. 5)was calculated from the transmittance spectra (or 1/A) using thefollowing formula:

˛h� = A(h� − Eg)n (5)

where ˛ is a constant, h� is the photon energy, Eg is the semicon-ductor band gap energy and n is a constant [n = 1/2 for a direct gap(SnO2, WO3) and 2 for an indirect band gap semiconductor (TiO2)].Extrapolating the straight line part of the plot – either (˛h�)2 or(˛h�)1/2 vs. h� – to the energy axis for a zero absorption coefficientgives the optical band gap energy of the material (Fig. 5).

Fig. 4. Absorbance and the related reflectance spectra of the samples.

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Fig. 5. Band gap estimation based of absorption/transmittance spectra: (a) Sn, (b) W and (c) Ti.

The diagrams of the carrier charge transfer processes, Fig. 6,describe the bi and tri-component systems, using the band gapenergy Eg, the valence band (VB) potential EVB and the conductionband (CB) potential ECB vs. normal hydrogen electrode.

According to Andronic et al. [22], one method for calculating thevalence band and conduction band potentials of the semiconduc-tors uses the following empirical equation [23]:

EVB = �semiconductor − Ee + 0.5Eg (6)

where EVB is the VB edge potential, �semiconductor is the semiconduc-tor electronegativity, Ee is the energy of free electrons vs. hydrogen,Eg is the band gap energy of the semiconductor (experimentallymeasured from the absorbance spectrum of each thin film, Fig. 5),and ECB can be calculated by:

ECB = EVB − Eg (7)

The absolute semiconductor electronegativity �semiconductor (eV)and the absolute cationic electronegativity (eV) �cation (eV) can be

Fig. 6. The diagram of the carrier charge transfer process.

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calculating using Eq. (8), respectively Eq. (9), where �cation(P.u.)represents the cationic electronegativity expressed as Paulingunits.

�semiconductor (eV) = 0.45 × �cation (eV) + 3.36 (8)

�cation (eV) = �cation (P.u.) + 0.2060.336

(9)

The valence and conduction band potentials of SnO2, WO3 andTiO2 were calculated using Eqs. (6)–(9) [24] and are presented inFig. 6.

Under irradiation, charge carriers are generated into the mate-rials and, according to the band diagram in the case of Sn Ti sample,the electrons from tin oxide valence band are promoted on thetitanium valence band which is the closest energy level. The con-duction band (CB) edges of TiO2 and SnO2 are conveniently locatedat −0.46 and −0.10 V vs. normal hydrogen electrode (NHE). Thevalence band (VB) edge of SnO2 (+3.52 V) is much lower than thatof TiO2 (+2.86 V). In terms of conduction, electrons flow into theSnO2 under-layer, while holes oppositely diffuse into the TiO2 over-layer. A better charge separation in the coupled film is the resultof the fast electron-transfer process from the conduction band ofTiO2 to that of SnO2. Thus, the charge recombination is bettersuppressed in the coupled film than in the single film [25]. Anopposite situation can be described for the Sn W sample wheretin oxide conduction band represents the highest excitation state.The electrons generated in the conduction band of WO3 cannotmigrate to the higher-positioned conduction band of SnO2 due tothe unfavourable position of the energy bands. The energy levelsin the tri-component sample are also unfavourable positioned forthe charge carriers’ flow, as result of the energy gap induced by theWO3 component inserted between SnO2 and TiO2 layers.

These results are confirmed by the current density–voltage mea-surements (Fig. 7) which show that the tri-component and theSn W samples have a resistive electrical behaviour, while Sn Tishows a diode response. The diode response can be explainedconsidering that, even if tin oxide is usually regarded as an oxygen-deficient n-type semiconductor, the sodium ions from the SDSsurfactant can act as dopants, increasing the holes concentrationduring the annealing treatment, according to the following equa-tion [10]:

V ··O + 1

2O2 → Ox

O + 2h· (10)

The association of two semiconductors with convenient bandgaps and with conduction and valence bands suitably positioned(Table 4), can lead to a simultaneous electron transfer betweenthe coupled semiconductors. The slightly higher conductivity of

Fig. 7. Current density–voltage measurements in dark.

Sn Ti sample compared with the Ti layer is thus consequence ofthe electrons contribution from the SnO2 conduction band.

This process strongly depends on the intimate contact betweenthe semiconductors. According to the AFM analysis, the Sn samplehas a dense and uniform morphology which allows a better contactbetween the SnO2 and TiO2 layers.

The electrical conductivity in all six samples is strongly influ-enced by the polycrystalline structure and the formation of otherby-products (mostly undesired, especially for the samples contain-ing tungsten oxide compounds). Due to the poor oxygen conditionsduring the deposition, the final layer contains intrinsic induceddefects, especially oxygen vacancies. The annealing process willdecrease the concentration of the oxygen vacancies, simulta-neously with the holes injection.

3.3. Photocatalysis efficiency

The photocatalytic activity of the samples was investigated inthe degradation of a 0.0125 mM methylene blue (MB) solutionunder UV light, measuring the absorption spectra for each sample,after 6 h of irradiation. Fig. 8 presents the photocatalytic activityof various single (Ti, W, Sn) and coupled (Sn W, Sn Ti, Sn W Ti)photocatalysts.

Tin oxide can be excited by photons with the wavelengths below326 nm, but it shows only low photocatalytic activity (∼12%) underUV light in the experimental conditions. When SnO2 is coupled with

Table 4Electronegativity, energetic levels and thickness values.

Sample Constituent elements Element elec-tronegativity(Pauling units)a

Absolutecationic elec-tronegativity(eV)b

Semiconductorelectronegativ-ity(eV)c

Eg (eV)d EVB (eV)e ECB (eV)f Thickness (�m)g

Ti Ti 1.54 5.196 5.698 3.32 2.86 −0.46 5O 3.44

Sn Sn 1.96 6.446 6.261 3.52 3.42 −0.10 6O 3.44

W W 2.36 7.637 6.797 2.9 3.75 0.85 16O 3.44

a Tabulated values.b Calculated from Eq. (9).c Calculated from Eq. (8).d Experimentally measured from absorbance spectrum (Fig. 5).e Calculated from Eq. (6).f Calculated from Eq. (7).g Experimentally measured from reflection spectra (Fig. 4-inset).

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Fig. 8. The methylene blue photodegradation on the investigated thin layers.

TiO2, the CB of SnO2 (−0.14 eV) is higher than that of TiO2 (−0.6 eV),the photogenerated electrons of the TiO2 conduction band will betransferred to the conduction band of SnO2, while the holes willmove in the opposite direction; at the same time, photogener-ated holes might be trapped within the TiO2 particle. As result,the coupled SnO2 TiO2 photocatalyst exhibits higher photocatalyticactivity (∼69%) than SnO2.

Coupled SnO2 WO3 and SnO2 WO3 TiO2 have electrical resis-tive behaviour but the photocatalytic experiments shows higherefficiency values comparing to the SnO2 and WO3 films. Manyfactors can affect the photocatalytic activity, including coupledstructures and surface areas (grain size and distribution) of thesemiconductors. The higher photocatalytic efficiencies correspondto the samples where the HTAB surfactant supported the formationof large aggregates, thus higher films/liquid interfaces, opened tothe oxidation processes.

4. Conclusions

Using different types of additives allowed tailoring the layersmorphology: (1) the anionic surfactant (SDS) promotes a struc-ture densification and homogeneous morphology (large numberof nuclei, uniformly distributed on the substrate surface); (2) thecationic surfactant (HTAB) promotes a steric hindrance havingas consequence a non-uniform and high roughness morphology(small number of nuclei, favouring the growth process). The sam-ples obtained using the HTAB surfactant is characterized by highsurface energy (62 mN/m) with a predominant polar component.

The numerical models show that only one type of bi-componentsample has favourably positioned the energy bands, allowingelectrical conduction. These calculations are confirmed by the cur-rent density–voltage measurements. However, the photocatalyticexperiments show that, even if the energy levels are not prop-erly positioned the hybrid and mono-component thin films wherethe HTAB surfactant is used have high photocatalytic activity, withefficiency values that varies from 40% to 80%.

Acknowledgements

This paper is supported by the Sectoral Operational ProgrammeHuman Resources Development (SOP HRD) Post-Doctoral Studies,

financed by the European Social Fund and by the Romanian Gov-ernment under the contract number POSDRU 59323.

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