Thin Solid Films 544 (2013) 571–575

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Photocatalytic activity of tin-doped TiO2 film deposited via aerosol assisted chemicalvapor deposition

Chin Sheng Chua a,b,⁎, Ooi Kiang Tan a, Man Siu Tse a, Xingzhao Ding b

a School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singaporeb Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, 638075, Singapore

⁎ Correspondingauthor at: Singapore Institute ofManufaDrive, Singapore 638075. Tel.: +65 96971154.

E-mail address: cschua@simtech.a-star.edu.sg (C.S. C

0040-6090/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tsf.2012.12.066

a b s t r a c t

a r t i c l e i n f o

Available online 2 January 2013

Keywords:Aerosol assisted CVDTiO2

PhotocatalystTin doping

Tin-doped TiO2 films are deposited via aerosol assisted chemical vapor deposition using a precursor mixturecomposing of titanium tetraisopropoxide and tetrabutyl tin. The amount of tin doping in the deposited filmsis controlled by the volume % concentration ratio of tetrabutyl tin over titanium tetraisopropoxide in themixed precursor solution. X-ray diffraction analysis results reveal that the as-deposited films are composedof pure anatase TiO2 phase. Red-shift in the absorbance spectra is observed attributed to the introductionof Sn4+ band states below the conduction band of TiO2. The effect of tin doping on the photocatalytic prop-erty of TiO2 films is studied through the degradation of stearic acid under UV light illumination. It is foundthat there is a 10% enhancement on the degradation rate of stearic acid for the film with 3.8% tin doping incomparison with pure TiO2 film. This improvement of photocatalytic performance with tin incorporationcould be ascribed to the reduction of electron-hole recombination rate through charge separation and an in-creased amount of OH radicals which are crucial for the degradation of stearic acid. Further increase in tindoping results in the formation of recombination site and large anatase grains, which leads to a decrease inthe degradation rate.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

TiO2 has attracted extensive attention because of its hassle free ap-proach to the removal of organic contamination from a surface [1],applicability as electrode for dye-sensitized solar cell [2], antibacterialapplications [3], or use as volume hologram recording material [4].However, one shortcoming of TiO2 is its large bandgap (3.2 eV), thatonly allows photo-reaction to proceed under UV illumination. To re-solve this shortcoming, the research has focused on various methodsto reduce the bandgap of TiO2. One such method is by doping withtransition metal ions.

Appropriate transition metal doping in TiO2 has been demonstratedto be effective in enhancing the photocatalytic property. Di Paola et al.[5] reported that cobalt doped TiO2 shows greater photoactivity in thedegradation of methanoic acid while tungsten doped TiO2 is more effec-tive in degrading benzoic acid and 4-nitrophenol. Doping TiO2 with iron[6], copper [7], silver [8] and vanadium [9] has also been reported tohave visible light photocatalytic effect. Among the transition metals,tin doping has shown great promise as it helps to suppress the recombi-nation of photogenerated electrons and holes. Sn4+ dopants introducean energy state in the bandgap of TiO2, at 0.4 eV below its conductionband [10]. The energy state traps photogenerated electrons resulting in

cturing Technology, 71Nanyang

hua).

rights reserved.

the separation of electrons and holes. This reduces recombination ofelectrons and holes, and thus, improves the photoactivity of TiO2 [11].It has also been shown that the incorporation of Sn4+ ions promotesthe crystallization of rutile phase as cassiterite SnO2 crystallizes in thesame tetragonal crystal structure as rutile TiO2 [12,13]. The crystalliza-tion of rutile phase forms anatase–rutile mixed phase TiO2 film [14],which can enhanced the photoactivity of the film as had been observedin P25 particles [15].

To dope Sn4+ ions into TiO2 film, aerosol assisted chemical vapordeposition (AACVD) is an attractive method as it uses single-sourceprecursor [16]. This allows for better control of the doping concentra-tion [17] and removes the need to match the reaction rate as requiredin a multi-component precursor mixture [18]. Another key advantageof AACVD is that non-volatile precursor may be used for deposition[16].

In this work, the aim was to deposit tin-doped TiO2 films on sodalime glass using AACVD. The effect of tin doping on the photoactivityof the as-deposited TiO2 thin film was investigated through thephotodegradation of stearic acid.

2. Experimental methods

2.1. Silicon dioxide (SiO2) barrier layer coating on soda-lime glass

Soda lime glass was pre-coated with a layer of SiO2 barrier layer toprevent sodium from diffusing into and poisoning the TiO2 film. The

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SiO2 barrier layer was deposited by dipping the glass into a solutioncontaining 60 ml tetraethyl orthosilicate (TEOS, 98%purity, Sigma-Aldrich), 94 ml ethanol and 20 ml distilled water. The pH of the TEOSsolution was adjusted to 3 through addition of acetic acid (Merck,100% purity). The dip-coating was carried out on MTS Synergie 200dip coater using a withdrawal speed of 12.7 cm/min. After coating, thefilmwas cured at 400 °C for 1 h in a box furnace. Ellipsometermeasure-ment (Model HS-190, J.A. Woollam Inc.) indicated that the depositedSiO2 film had a thickness of 240 nm.

2.2. Aerosol assisted chemical vapor deposition of tin-doped TiO2 film

In-house built aerosol assisted chemical vapor deposition systemwas used for the deposition of tin-doped TiO2 film on soda limeglass. Fig. 1 shows the schematic diagram of the system. Prior tofilm deposition, aerosol was generated from a precursor mixture for15 min using a Novita NH 659 humidifier. The precursor mixturecontained 0–10% by volume of tetrabutyl tin (Merck) in titaniumtetraisopropoxide precursor (97% purity, Sigma Aldrich). During de-position, one channel of N2 carrier gas with a flow rate of 0.49 slmwas introduced into the bubbler to transport the aerosol to the depo-sition head. Another channel of N2 dilution gas (flow rate 5 slm) wasadded to the carrier gas and aerosol precursor mixture for better filmuniformity. The gas pipelines used for gas transportation were heatedto 65–80 °C to prevent condensation of the precursor on the pipewalls. At the deposition head, the gas and precursor mixture wasspread horizontally through a series of metal plates placed acrossthe gas flow path. A horizontal gas stream was emitted from the de-position head to deposit a coating on the heated substrate (500 °C)that moved underneath the deposition head. TiO2 film with a thick-ness of 60 nm was deposited on the SiO2-precoated soda-lime glass.

For convenience, the deposited tin-doped TiO2 films are labeledaccording to the concentration of tetrabutyl tin in the precursor mix-ture used for the deposition. For instance, 1% tin precursor concentra-tion is labeled as Sn01TiO2 and 2% as Sn02TiO2.

2.3. Sample characterization

Crystalline structure of the deposited tin-doped TiO2 films was ana-lyzed by X-ray diffraction (XRD) using Cu Kα as the radiation source(λ=1.54). The XRD measurements were carried out on a D8 BrukerX-ray diffractometer at a glancing angle of 1°. The sampleswere scannedover a 2θ angular range of 20–80° at a scan rate of 1.74° per min.

Atomic concentration of tin dopants in the deposited films wasmeasured with X-ray photoelectron spectroscopy (XPS, Kratos-Axisspectrometer) using Al Kα as the X-ray source. Shirley backgroundsubtraction was used for the removal of background from the XPSpeaks while Gaussian–Lorentzian function was used for its peakfitting. UV–vis absorption measurement for the tin-doped TiO2 filmwas analyzed from UV–vis Spectroscopy (Shimadzu UV-3101PC).Surface morphology was determined from atomic force microscopy(AFM, Nanoscope IIIa, Digital Instrument). Tapping mode was used

Fig. 1. Schematic drawing of aerosol assisted chemical vapor deposition.

for the atomic force microscopy imaging over a scanned area of 1 μmby 1 μm. Photoluminescence spectra of the tin-doped TiO2 films wereobtained using Nanometric RPM2000 Photoluminescence Mapper usingan excitation laser at 325 nm line at room temperature.

2.4. Photocatalytic test

Photocatalytic activity of the deposited tin-doped TiO2 films was in-vestigated according to the degradation behavior of stearic acid coatedon the film surface. In the test, the TiO2 films were first radiated over-night under UV illumination (Hitachi FL15BL-B, intensity 0.5 mW/cm2

at 365 nm). Next, a baseline infrared spectrumwas recorded using Fou-rier Transform Infrared Spectrometer (FTIR, IRPrestige-21 Shimadzu).After that, stearic acid with a concentration of 10 mmol in methanolwas spin-coated onto the sample surface. The spin speed for stearicacid coating was 1000 rpm, at an acceleration of 1000 rpm2 for 1 min.The sample was dried on hot plate at 50 °C for 5 min. The amount ofstearic acid remaining on the film was determined by the FTIR spec-trometer. The FTIR spectra of the samples coated with stearic acidwere recorded and the integrated area under the absorption band inthe range of 2830–2975 cm−1 was regarded as a measurement of theamount of stearic acid. Then the sample was placed under UV light illu-mination to initiate the photocatalytic reaction. Periodically, the samplewas analyzed with FTIR to determine the amount of the stearic acidremaining on the substrate. Fig. 2 illustrates the typical variation ofFTIR spectrum of stearic acid as a function of UV illumination time onthe as-deposited TiO2 film. The integrated area was plotted against UVillumination time. The rate of decrease of the integrated area gives thestearic acid degradation rate on the sample surface.

3. Results and discussion

3.1. X-ray photoelectron spectroscopy (XPS) analysis of tin-doped TiO2

film

Due to low concentration of tin dopants in TiO2 film and deep pene-tration depth of electron beam into the substrate, Energy DispersiveX-ray spectroscopy could not accurately determine the amount of tindopants. As such, XPS was employed to determine the tin content inthe deposited films. Fig. 3 shows the detailed XPS spectra of Ti 2ppeaks. The Ti 2p3/2 and 2p1/2 peaks located at around 458.7 and464.4 eV are in agreement with those of the reported work [19].The intensity of these peaks decreases with the increase of tin pre-cursor concentration used for the deposition indicating that a great-er amount of Ti4+ ions is substituted by the Sn4+ ions. In addition, aslight shift to higher binding energy of the Ti 2p XPS spectrum is ob-served with the increasing Sn precursor concentration used. As Sn4+

Fig. 2. Typical variation of FTIR spectrum of stearic acid on as-deposited TiO2 film withillumination time under UV light. Interval between each measurement is 2 min.

Fig. 3. High resolution Ti 2p XPS spectra of the tin-doped TiO2 film. Fig. 5. Percentage tin dopants in TiO2 film.

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ions are more electronegative than Ti4+ ions, the incorporation ofSn4+ ions is believed to be responsible for this shift in the bindingenergy [20].

Fig. 4 shows the Sn 3d XPS spectra of the deposited films, where Sn3d5/2 and 3d3/2 peaks are located at around 486.5 and 495.3 eV respec-tively.With the increased Sn precursor concentration used for the depo-sition, a gradual increase of the Sn 3d peak intensity is observed. Thisindicates an increase in the Sn content in the deposited films. Fig. 5shows the increase in atomic concentration of Sn4+ ions as determinedfromXPS spectroscopy. This increase in Sn4+ ions is expected given thatmore tetrabutyl tin precursors were available during the deposition pro-cess. Ti to O and Sn to Ti ratios are presented in Table 1. The as-depositedfilm has a Ti to O ratio of 0.5, which shows good stoichiometry. The Ti toO ratio remains at 0.5 at low tin concentration but decreases as more Tiions are substituted by the Sn ions, Sn to Ti ratio, on the other hand,shows a steady increase with the increasing Sn dopants.

3.2. X-ray diffraction (XRD) analysis of tin-doped TiO2 film

Fig. 6 shows the XRD pattern of the deposited tin-doped TiO2 films.It was found that all the films are composed of only anatase phase. Thediffraction peaks observed at 2θ values of 25.9°, 48.9° and 56.0° can beassigned to (101), (200) and (211) planes of anatase TiO2. No rutile

Fig. 4. High resolution Sn 3d XPS spectra of the tin-doped TiO2 film.

TiO2 and SnO2 peaks are detected from the XRD patterns. Generally,with an increase of tin precursor concentration used for film deposition,all the XRD peaks shift to a lower angle side. This shift in peak positionwas the result of the substitution of Ti4+ ions (ionic radius of 0.68 Å) bythe larger Sn4+ ions (ionic radius of 0.71 Å),which leads to the increasein crystal cell volume and the shift in XRD pattern. The Sn4+ ions aremore likely to bedoped into TiO2 via substitutionmode due to similarityin electronegativity and ionic radius of Sn4+ and Ti4+ ions [21].

3.3. UV–vis absorption measurement of tin-doped TiO2 film

UV–vis absorption spectra of plain soda lime glass, pure TiO2 coatingand tin-doped TiO2 films are presented in Fig. 7. Plain soda lime glassshows absorption in the UV region at a wavelength below 330 nm.After deposition with TiO2 films, the absorption edge was shifted tohigher wavelength at 350 nm. Bulk anatase TiO2 has a bandgap of3.2 eV which allows the absorption of photons with wavelength below387 nm. Here, the absorption occurs at a lowerwavelength due to quan-tum effect on nanocrystalline TiO2 film [22]. With increasing tin contentin the coatings, the absorption edge is observed to gradually shift tohigher wavelength. This shift in absorption edge was caused by Sn4+

dopants, which have an energy level at 0.4 eV below the conductionband of TiO2 [10].

Bandgap of the tin-doped TiO2 film was determined from a Taucplot using the relation:

αhvð Þn ¼ A hv−Eg� �

: ð1Þ

From the Tauc plot, the bandgap of the material was determined byextrapolating the slope of the curve to the photon energy axis. Table 1presents the bandgap of the tin-doped TiO2 film. The as-deposited filmhas a bandgap of 3.45 eV which is higher than the bulk bandgap. Thisis caused by quantum effect from nanocrystalline TiO2 thin film [22].With an increased tin doping, the bandgap was observed to decrease.

Table 1Bandgap, O:Ti and Sn:Ti ratios of tin-doped TiO2 films.

Ti to O Sn to Ti Bandgap

TiO2 0.50 – 3.45Sn01TiO2 0.51 0.021 3.45Sn02TiO2 0.50 0.040 3.44Sn03TiO2 0.50 0.053 3.42Sn05TiO2 0.47 0.058 3.38Sn10TiO2 0.47 0.089 3.35

Fig. 6. XRD analysis of tin-doped TiO2 film.Fig. 8. Photoluminescence spectra of tin-doped TiO2 film.

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Such reduction in bandgap suggests that photons with lower energycould be absorbed by tin-doped TiO2 film for photocatalytic reaction.

3.4. Photoluminescence analysis of tin-doped TiO2 film

Photoluminescence emission analysis is used to understand theseparation of electrons and holes and their recombination. Fig. 8shows the photoluminescence analysis of tin-doped TiO2 film. TheTiO2 spectrum shows two emissions at 418 and 485 nm that corre-spond to the free exciton of TiO2 and Ti4+\OH respectively [23].At low levels of tin doping, a slight decrease in the peak intensityis observed indicating a reduction in electron hole recombinationrate. This reduction is due to Sn4+ trapping sites, which capturephotogenerated electrons, thus, preventing recombinationwith the gen-erated holes. At higher tin doping concentration, the photoluminescenceintensity increases which shows greater recombination of electrons andholes with an increased tin doping.

3.5. Photocatalytic properties of tin-doped TiO2 film

Degradation rate of stearic acid by tin-doped TiO2 films is presentedin Fig. 9. For each different tin-doping content samples, three sampleswere prepared and tested. At low tin doping concentration, a gradual in-crease in photoactivity is observedwith an increasing tin doping concen-tration. The increase in photoactivity reaches amaximumwith Sn02TiO2,and this corresponded to 3.8% tin doping in the TiO2 film. Further in-crease in tin doping results in an adverse effect in the photoactivity

Fig. 7. UV–vis absorption spectra of tin-doped TiO2 film.

with Sn10TiO2 sample having a photoactivity of only 30% of the originaldegradation rate with TiO2 film.

The initial increase in photoactivity was the result of Sn4+ dopants,which introduced additional band states into the bandgap of TiO2. Theband states are located at 0.4 eV below the conduction band of TiO2.This allows photogenerated electrons to be trapped in the Sn bandstates, thus, resulting in a separation of electrons and holes. With theseparation of electrons and holes, their recombination rate decreased.As a result, a greater amount of electrons and holes was available forthe photocatalytic reaction process. Thus, a faster stearic acid degrada-tion rate was observed.

At tin-doping concentration above 5%, the photoactivity of theTiO2 film decreases. This decrease is the result of both an increase ingrain size of the TiO2 film and the introduction of recombinationsites by the Sn4+ dopants. Fig. 10 shows the surface morphology ofthe deposited tin-doped TiO2 film. For pure TiO2 film, the averagegrain size is around 40 nm. With tin doping concentration of up to3.8 at.%, the grain size remains fairly unchanged. At higher dopingconcentration of more than 5 at.%, an increase in grain size is ob-served with Sn03TiO2 having a size of 50 nm. Further increase ingrain size is observed with grains larger than 80 nm observed forSn10TiO2 film. Such increase in grain size has a detrimental effect tothe photocatalytic property of the film as it reduces the specific

Fig. 9. Stearic acid degradation rate by various tin-doped TiO2 films.

Fig. 10. AFM images of the tin-doped TiO2 film. (A) TiO2, (B) Sn02TiO2, (C) Sn03TiO2 and (D) Sn10TiO2.

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surface area of the film. The decrease in specific surface leads to a re-duced number of active sites for the photocatalytic reaction process.In addition to the increase in grain size, the increase in tin dopantsleads to a decrease in the average distance between the electron trap-ping sites. As the distance between the trapping sites decreased, thetin dopants become recombination sites, which leads to the decreasein photoactivity [24]. Due to the increase in grain size and recombina-tion sites, the photoactivity of TiO2 film decreases at tin doping levelsof more than 5%.

4. Conclusion

Tin-doped TiO2 films had been prepared through aerosol-assistedchemical vapor deposition. The tin dopants introduce band states inthe TiO2 bandgap which leads to a bandgap narrowing effect. At lowtin doping concentration, the Sn4+ band states trap photogeneratedelectrons and reduce the recombination of electron hole pairs. This re-sults in faster degradation of stearic acid with 3.8% tin doping showingthe greatest improvement in stearic acid degradation. At higher tin dop-ing concentration, the decrease in photoactivity was caused by the for-mation of recombination sites and an increased in grain size of the TiO2

film.

Acknowledgments

This work was supported by A*STAR under the grant number“SERC 082 101 0017”.

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