Appl Phys A (2011) 102: 91–97DOI 10.1007/s00339-010-6151-9

R A P I D C O M M U N I C AT I O N

Effect of the excimer laser irradiation on sol–gel derivedtungsten–titanium dioxide thin films

Yasir F. Joya · Zhu Liu

Received: 26 August 2010 / Accepted: 24 November 2010 / Published online: 15 December 2010© Springer-Verlag 2010

Abstract A novel technique based on the excimer laser in-duced crystallization and modification of TiO2 thin filmsis being reported. W+6 ions loaded TiO2 (WTO) precursorfilms were prepared by a modified sol–gel method and spin-coated onto microscopic glass slides. Pulsed KrF (248 nm,13 ns) excimer laser was used to irradiate the WTO amor-phous films at various laser parameters. Mesoporous andnanostructured films consisting of anatase and rutile wereobtained after laser irradiation at room temperature. The ef-fect of varying W+6 ions concentrations on structural andoptical properties the WTO films was analyzed by X-raydiffraction, field-emission scanning electron microscope,UV-Vis spectrophotometer and transmission electron micro-scope before and after laser treatment. Films irradiated for10 pulses at 65–75 mJ/cm2 laser fluence, exhibited anatasewhereas higher parameters promoted the formation of rutile.XPS results revealed WO3 along with minor proportion ofWO2 compounds after laser irradiation. Photo-absorbanceof the WTO films was increased with increase in W+6 ionsconcentration in the film. TEM results exhibited a crystal-lite size of 15 nm which was confirmed from SEM results aswell.

1 Introduction

Titanium dioxide (TiO2) is a widely known semiconductordue to excellent properties in photocatalysis, self-cleaning

Y.F. Joya (�) · Z. LiuCorrosion and Protection Centre, The Mill, School of Materials,The University of Manchester, Manchester, M13 9PL, UKe-mail: yasir.joyah@gmail.comFax: +44-161-3064865

Z. Liue-mail: zhu.liu@manchester.ac.uk

surfaces, antibacterial coatings, gas sensors, organics de-composition and water purification, non-toxicity and lowcost, etc. [1, 2]. TiO2 exists in three polymorphs of whichanatase (tetragonal) and rutile (tetragonal) are the most de-sired. However, anatase-based titania films have found mostapplications owing to their highest photo-activity and an-tibacterial properties [3, 4]. The energy band gap of anataseis 3.32 eV, so it absorbs light up to a wavelength of 385 nm.As the UV light constitutes only up to 10% of solar spec-trum, it is necessary to decrease the energy band gap of TiO2

to increase its photo-absorbance into visible region of thesolar spectrum.

It is believed that the absorbance of anatase may be in-creased by adding certain dopant ions into pure TiO2 as wellas by decreasing the particle size and increasing the sur-face area [5]. Among others, tungsten trioxide (WO3) hasexhibited promising results in enhancing the photocatalysisof TiO2 [6, 7]. On the other hand, it may also be useful togenerate nanocrystalline TiO2 films with diverse microstruc-tures to improve the photocatalysis and other properties ofinterest [1, 8]. As the photocatalytic behavior was improvedby fabricating anatase mesoporous films with high surfacearea [9, 10].

To date, various methods have been discovered to prepareTiO2 nanocrystalline thin films, including hydrothermal,CVD and laser ablation, to name a few [11–13]. However,wet chemical method, e.g. sol–gel, is mostly used to pre-pare mesoporous TiO2 films by using surfactants or copoly-mer templates [14]. Novel techniques have been exploredrecently due to the rapid innovation in laser processing ofadvanced materials [15]. This approach not only can micro-machine/pattern thin films at nano-dimension but can alsoinduce photothermal reactions and chemical transformationin sol–gel precursor films [16–20]. Among various lasers,continuous wave CO2 (10.6 μm) and excimer laser (193–

92 Y.F. Joya, Z. Liu

308 nm) are optimal choices as visible laser light transmitsthrough the sol–gel derived organic films [21] . Pulsed ex-cimer laser keeps high photon energy (∼5 eV) coupled withnanosecond pulse width (10–23 ns). The organic bonds insol–gel film are stimulated by high energy photons, trig-gering nucleation and growth of a new phase within a fewnanoseconds in contrary to the extensive conventional heattreatments [22].

We have demonstrated earlier on the excimer laser capa-bility to crystallize Ce and W doped TiO2 films and to gen-erate anatase nanostructure [20, 23]. The present researchis based on the effect of excimer laser processing on W+6-doped TiO2 films with varying WO3 concentrations. WO3

was selected due to its lower band gap (2.8 eV) as comparedto TiO2 (3.0 eV). The effect of WO3 concentration on TiO2

films phase transformation, structural and optical propertiesis discussed before and after excimer laser irradiation.

2 Experimental

WTO precursor solutions with varying W+6 ions concen-trations were prepared by sol–gel method as shown inTable 1. Titanium n-butoxide Ti(C4H9O)4 was used asTiO2 precursor whereas tungsten isopropoxide W(iPr)6 wasadded as a WO3 dopant precursor. Acetic acid (CH3COOH)and hydrochloric acid (HCl) were used as stabilizers. Thesol–gel solution was diluted with n-butanol (C4H9OH)and de-ionized (DI) water added to hydrolyze and forma homogeneous WTO precursor sol. The molar ratio ofTi:CH3COOH:H2O was fixed at 1:2:2. The chemicals wererefluxed and magnetically stirred together under argon

Table 1 Composition of WTO films prepared by sol–gel/laser tech-nique

Batch W+6, weight % Possible formula

1WTO 1.1 W0.005 Ti0.995O2

2WTO 2.2 W0.01 Ti0.99O2

3WTO 3.0 W0.014 Ti0.986O2

4WTO 4.0 W0.018 Ti0.982O2

purged atmosphere at room temperature to avoid any precip-itation. Refluxing was conducted continuously for 15 hoursleading to a 0.2 molar Ti sol for subsequent spin-coating.The obtained sols were then aged at room temperature for24 hours in a sealed beaker.

The aged sols were spin-coated onto the microscopicglass slides (2.5 × 2.5 cm2) at 500 rpm for 10 seconds andthen at 2000 rpm for 30 seconds respectively. Each samplewas prepared by coating up to four consecutive layers ofWTO where each layer was dried at 300◦C for 2 minuteson hot plate. GSI Lumonics PM-840 KrF excimer laser(λ = 248 nm, pulse width = 13–20 ns) was used to irradiatethe films through a mask of 0.5×0.5 cm2 without using anyfocusing lens (Fig. 1). The samples were moved across thestationary laser beam along x and y directions to cover theentire film area of 2.5 × 2.5 cm2. The laser processing wascarried out at ambient atmosphere. Laser fluence was fixedat 65 mJ/cm2 whereas the number of laser pulses receivedper unit area was varied from 10 to 100 at a fixed pulse rep-etition rate of 10 Hz.

After the laser irradiation, samples were characterized interms of structural changes by using low-angle XRD. Spec-tra were recorded from 20 to 70 degrees, 2-theta range with astep size of 0.05 and scan time of 6 seconds. The microstruc-tural and compositional analysis was done by FEG-SEMcoupled with energy dispersive X-ray spectrometer (EDX).The film thickness and nanocrystalline details were investi-gated by TEM. Samples for TEM were prepared by FIB withAr ions etching. The optical properties of films were deter-mined using a UV–visible spectrophotometer in the 300–1000 nm wavelength range.

3 Results and discussion

3.1 XRD results

XRD spectra obtained from laser irradiated WTO samplesare shown in Fig. 2. The laser pulses were varied from 10to 100 at a fixed repetition rate of 10 Hz. As-dried sam-ples (300◦C) revealed amorphous character. The films were

Fig. 1 Schematic diagram oflaser processing of WTO films

Effect of the excimer laser irradiation on sol–gel derived tungsten–titanium dioxide thin films 93

Fig. 2 XRD spectra of laser treated WTO films at 65 mJ/cm2,10 pulses, 10 Hz frequency

crystallized after receiving laser pulses. The phase transfor-mation from amorphous to anatase and rutile was effectedby W+6 ions concentration in TiO2 film. Pure titania filmstreated at the same conditions revealed anatase phase only.However, WTO films were transformed into anatase andrutile simultaneously after laser irradiation. This manifeststhat the W addition made the films to absorb laser energymore effectively as compared to pure TiO2 films.

In order to optimize the laser parameters, different num-ber of laser pulses and fluence was used as shown in Figs. 3aand b, for 1WTO films. Lower number of laser pulses andlaser fluence favored anatase (Figs. 3a and b) while at highermagnitudes, rutile was obtained along with anatase. Thetransformation behavior for other WTO samples exhibited asimilar trend. It may be inferred that W ions favored anataseto rutile transformation. The appearance of the samples be-came dark with higher W concentration at 4%. This may beattributed to the oxygen ion vacancies induced by the highenergy (5 eV) excimer laser [24].

The amorphous to anatase and rutile transformation un-der UV laser irradiation may be attributed to the photother-mal as well as photochemical phenomenon. The laser beamenergy induced atomic vibrations of active species in amor-phous films resulted in heat generation which was diffusedinto TiO2 layers. However, this process is non-linear and dif-ficult to interpret without considering the thermal propertiesof the film before and after laser beam irradiation. In addi-tion, optical properties are also an important factor.

3.2 XPS analysis

In order to determine the chemical state of the laser treatedsurface, samples were subjected to XPS analysis. Figure 4shows the spectra obtained after laser irradiation of 2WTO

Fig. 3 Effect of the number of pulses (a) and fluence (b) on 1WTO

film with 10 pulses at 65 mJ/cm2. It demonstrates the bind-ing energies from W4f7/2 peak at 33.95–34.0 eV (red)which correspond to WO2 chemical state at the surface ofthe film [25]. The peak between 35.4 and 35.7 eV (blue) isrelated to the binding energy associated with W4f7/2 stateof WO3 [26]. A high broad peak obtained at 37.16 eV (pink)may be attributed to W5p3/2 state of W metal and 3p stateof TiO2 respectively [27, 28]. These results lead to the pos-sibility that tungsten ions might replace titanium ions fromits lattice points because of a closer ionic radius of Ti+4

(0.605 Å) and W+6 (0.60 Å) [29].

3.3 FEG-SEM imaging and EDX analysis

FEG-SEM images obtained from WTO films before/afterlaser treatment, as well as furnace sintering, are shown inFigs. 5a–c. As-dried film surface appeared flat without anydefinite sample surface (Fig. 5a). The microstructure wasmodified after receiving 10 laser pulses at 65–70 mJ/cm2

fluence (Fig. 5c). A porous morphology was also observed

94 Y.F. Joya, Z. Liu

as shown in the inset of Fig. 4c. The pores were in differ-ent size range with an average pore diameter of 50 nm. Themesoporosity may be generated due to the decomposition oforganics/residual carbon during the laser exposure. An aver-age grains size of 15 nm was determined from SEM.

Fig. 4 XPS spectra of 2WTO film after being laser irradiated with10 pulses at 65 mJ/cm2

EDX obtained from the same sample revealed Ti and Walong with elements from the glass substrate. Some whiteparticles embedded in TiO2 matrix were also observed andverified by EDX line scan to confirm their chemical compo-sition. These particles actually came from the gold coater asverified by a clear Au peak in EDX line scan (Fig. 5c).

A sample of similar composition was also made by fur-nace heating at 700◦C/1 hr for comparison (Fig. 5b). It re-vealed a finer and denser microstructure in contrast to thelaser treated film. It may be inferred that the laser irradiatedfilms have a higher surface area as compared to the furnacesintered films.

3.4 Optical properties

The UV–visible optical transmittance exhibited by WTOfilms before and after laser irradiation is shown inFigs. 6a–d. As-dried films revealed 40% reflectivity at theKrF laser wavelength of 248 nm. Therefore, up to 60% ofthe incident energy from the laser was absorbed plus trans-mitted through the film. As-dried film revealed maximumtransmittance, which was significantly reduced after apply-ing 10 pulses of the laser.

The absorbance of pure TiO2 films was increased af-ter W addition (Figs. 6a and b). This became more promi-

Fig. 5 FEG-SEM image obtained from as-dried (a) and laser irradiated 2WTO (b) furnace sintered at 700◦C (c) films. Inset in (b) and (c) is thecorresponding high resolution image

Effect of the excimer laser irradiation on sol–gel derived tungsten–titanium dioxide thin films 95

Fig. 6 UV–visible spectra from laser irradiated WTO films at various parameters

nent at a 100 number of pulses which produced a dark col-

oration in 4% WTO. This may be attributed to a new sub-

stoichiometric phase from WOx compound resulting due to

the oxygen vacancies created after the excimer laser irradia-

tion [30].

In order to demonstrate the effect of pulses and fluence,

data obtained from 2% WTO was selected (Figs. 6c and d).

The pulses and fluence together controlled the transmittance

of the films. The number of laser pulses indicated an increas-

ing trend in the absorbance of film in the visible spectrum of

light. This may be related to two important factors. Firstly,

the number of laser pulses increased the rutile fraction which

keeps a lower band gap compared to anatase. Secondly, this

effect may be attributed to the change in surface morphology

as verified from SEM results.

3.5 TEM imaging and analysis

2WTO films irradiated by 10 laser pulses at 65 mJ/cm2 flu-ence were subjected to TEM imaging and EDX analysis.EDX line scan was obtained from the films cross sectionshowing the distribution of elements (Fig. 7a). As measured,film thickness from the line scan graph was 412 nm. Peaksignals from Ti and O were observed, however no signifi-cant W peak was detected. This may be due to the fact thattungsten was dispersed on surface of the films as shown ear-lier by SEM results. Au and Pt covered most surface of theTiO2 films and resulted in hiding the EDX signals from W.However, EDX results obtained from FEG-SEM, indicatedthe signals from W as already discussed.

The cross section of films with corresponding SAED isshown in Fig. 7c. Anatase was confirmed from the pattern,along with rutile peaks from various planes. Anatase particle

96 Y.F. Joya, Z. Liu

Fig. 7 TEM image (a), EDX line scan (b) and SAED (c) obtained from 2WTO films made at 10 laser pulses @ 65 mJ/cm2 fluence

size of 15 nm was determined, which is in close agreementwith the SEM results.

4 Conclusions

W-loaded TiO2 thin films were successfully transformedinto anatase and rutile by a modified sol–gel/laser irradia-tion technique. W ions addition to TiO2 produced uniformlydistributed mesoporous anatase in the absence of any tem-plating agent. Anatase phase with higher absorbance wasobtained under optimized number of laser pulses and flu-ence. Whereas, a higher number of laser pulses and flu-ence favored a rougher surface and larger features with rutileas a dominant phase. TEM results revealed 100 nm depthof crystallized region (anatase) only in the total thicknessof 412 nm. As discovered from TEM images, nanocrys-talline and mesoporous anatase phase with an average grainsize of 15 nm may be obtained by this technique. XPS re-

sults confirmed the presence of WO3 and WO2 and sub-stoichiometric compounds in the laser irradiated films.

Acknowledgements The authors would like to gratefully acknowl-edge the Higher Education Commission (HEC) of Pakistan for theirgenerous financial support in this project.

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