Optics & Laser Technology 51 (2013) 17–23

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Optics & Laser Technology

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n CorrE-m

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Study of the fragmentation phenomena of TiO2 nanoparticles producedby femtosecond laser ablation in aqueous media

S.I. Alnassar a, E. Akman b, B.G. Oztoprak b,c, E. Kacar b, O. Gundogdu d, A. Khaleel e, A. Demir d,n

a Institute of Laser for Post Graduate Studies, University of Baghdad, Iraqb Kocaeli University, Laser Technologies Research and Application Center, 41275 Kocaeli, Turkeyc BEAM Ar-Ge Optics and Laser Technologies Ltd., KOUTechnopark, Basiskele 41275, Kocaeli, Turkeyd Kocaeli University, Electro-Optics Systems Engineering, 41380 Umuttepe, Kocaeli, Turkeye Diyala University, College of Engineering, Diyala, Iraq

a r t i c l e i n f o

Article history:Received 19 November 2012Received in revised form8 February 2013Accepted 12 February 2013Available online 13 April 2013

Keywords:Laser ablationMetal oxide nanoparticlesUltrafast Ti:Sapphire laser

92/$ - see front matter & 2013 Elsevier Ltd. Ax.doi.org/10.1016/j.optlastec.2013.02.013

esponding author. Tel.: þ90 262 3031061; faxail addresses: arifd@kocaeli.edu.tr, arifdkou@g

a b s t r a c t

Since last decade, Pulsed Laser Ablation in Liquid (PLAL) has become an increasingly important techniquefor the production of the nanoparticles (NPs) since it usually provides high purity nanoparticle systems.This paper reports on the production and fragmentation of titanium oxide TiO2 nanoparticles by pulsedlaser ablation of a titanium target immersed in Sodium Dodecyl Sulfate (SDS) solution using an ultrafastTi:Sapphire laser. After the production of TiO2 nanoparticles for 30 min of laser irradiation, secondharmonics of the laser wavelength are re-applied for different energies (180,120, 60 mJ) to SDS solutioncontaining TiO2 colloids in order to fragment relatively large pieces to obtain smaller ones. It was foundthat size of nanoparticles after the treatment is independent of the initial characteristics of colloids, butdepends strongly on laser parameters especially pulse energy and on the presence of chemically activespecies in the solution. It was reported that particle size and size distribution range can be decreasedusing second harmonics of Ti:Sapphire laser wavelengths by using different values of energy.Re-irradiation process at average energy value of 180 μJ decreased average particle size from 185 nmto 110 nm. Characterization of the NPs was studied by applying various techniques such as UV–visible(UV–vis.), Transmission Electron Microscope (TEM), Dynamic Light Scattering (DLS) and Fourier Trans-form Infra-Red (FTIR).

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Nanomaterials display unique and superior properties whichare different from those of their bulk materials, because of theirhigh surface area to volume ratio [1]. The vastly increased ratio ofsurface area to volume leads to new quantum mechanical effectssuch as the “quantum size effect” where the electronic propertiesof solids are altered with great reductions in particle size as thesize of the particle moves to a regime where quantum confine-ment effects are predominant [2]. Plasmonic behavior in theUV-region especially with controlled morphology and particle sizemakes TiO2 nanoparticles an attractive prospect for use as a goodUV absorber not only for pharmaceutical applications but also insolar cell applications with extended spectral range. Synthesis ofhigh quality nanostructured materials is a very active area asnanoparticles represent an important class of material develop-ment field for novel devices that can be used in many applications

ll rights reserved.

: þ90 262 3031013.mail.com (A. Demir).

such as: photothermal [3], therapy [4], surface-enhanced Ramanspectroscopy [5], biochemical sensors [6], solar cells etc. [7].

There are two main approaches to produce nanomaterials: top-down and bottom-up. In the top-down approach the production ofnanoparticles is realized by etching smaller structures from largerones. Laser ablation and milling are two of the typical examples totop-down approach. On the other hand, bottom-up approachrefers to the build-up of a material: atom by atom, molecule bymolecule, or cluster-by-cluster [8].

The most efficient physical method for nanofabrication is the laserablation process because of a number of advantages compared toconventional methods. The advantages of this method are simplicity ofthe procedure and the absence of chemical reagents in solution [1].This method also gives certain flexibility over other techniques as alltypes of materials can be processed and ablated due to the very highenergy density. Controlling the size of produced NPs by optimizing theprocess parameters such as irradiation time, pulse duration, energydensity and laser wavelength etc. [9].

Laser ablation of materials from a solid target occurs eitherin a vacuum or in a liquid environment to produce nanoclusters.In the former method nanoclusters can be deposited onto a solid

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substrate resulting in a formation of a nanostructured film [10].This method has some disadvantages such as the difficulty ofcontrolling the production of NPs. In the latter method, nanoclus-ters can be released into the liquid forming a colloidal nanoparticlesolution leading to a more effective collection of synthesizedparticles. PLAL does not need a vacuum system and has a highcollection yield making it more efficient compared to the laserablation in gas phase [11]. In other words the solvent can providepositive physical and chemical effects such as plasma confinement,cooling actions, oxidation or reduction leading to enhancement ofablation efficiency [12].

The concept of producing oxide using laser irradiation of metaltargets in water was demonstrated in 1987 where iron and tantalumoxides were formed on target surfaces in water using a Q-switchedruby pulsed laser by using a third harmonic of a pulsed Nd:YAG laserPLAL of Ti in water and SDS solution [13]. Sasaki et al. [14] havesynthesized TiO2 in both deionized water and sodium dodecyl sulfate(SDS) solutions and they have explained crystallinity of the nano-particles strongly depended on the SDS concentration in the solution.The metal oxide nanoparticles have many applications in nonlinearoptics, optoelectronics, biomedical engineering, electro-optical devicesand chemical catalysts [15].

The production of NPs by femtosecond laser has been gettingmore common due to its efficiency in ablation of materials andeffective control of particle size compared with nanosecond laserablation. Tan et al. [16] have explained that use of femtosecond lasercan effectively minimize laser–plume interaction and reduce the heataffected zones. Moreover the limited heating effect which resultsfrom the interaction of ultra-short pulses with the matter benefits afaster cooling of ablated particles and prevents them from aggregat-ing. For all the reasons stated above, the ultra-short laser pulses arefavorable for the synthesis of smaller particles as mentioned byKabashin and Meunier [17]. It is also possible to control thenanoparticle growth with different amounts of concentrations ofsurfactants. Tilaki et al. [18]; Mafune et al. [19] and Chen and Yeh [20]have reduced the nanoparticle size and prevented their agglomera-tion by changing the surfactant and its concentration.

Several authors tried to control of nanoparticle size distributionand density by optimizing the laser parameters such as pulse energy,pulse repetition rate, laser wavelength, pulse duration and focusing

Fig. 1. Experimental set up of femtose

conditions of femtosecond beam [21]. Akman et. al. [22] studied theeffect of Ti:sapphire laser parameter on size and morphologicalproperties of silver nanoparticles with repeated pulses in the secondharmonics. In another paper, Akman et. al [23] and Khaled [24] alsoreport the effect of various laser parameters on size and morphologyof gold nanoparticles where laser energy range was selected to coverthe plasmonic properties of gold. Barcikowski [25] examined theinfluence of pulse energy and micromachining speed revealing thatin some cases the effect of laser fluence on the nanoparticle sizedistribution is very weak, in other cases the use of higher laserfluences produce a distribution shift toward the smaller particles.

Because the surfactant that surrounds each nanoparticle pre-vents a direct contact between them, many researchers haveconcentrated towards studying the effect of the best surfactantto reduce the particle size or stabilizing the colloids [19,26].Mafune et al. [26] have reported the effect of SDS in determiningstability and size of the nanoparticles, where controlling thenanoparticle growth was achieved by diffusion and attachmentrates of SDS to the nanoparticle surface. As a result, size distribu-tion and stability of the nanoparticles depend critically on theproperties of the used surfactants. Many researchers use “laser-assisted size control” method to produce smaller and monodis-perse nanoparticles from different materials using nanosecondNd:YAG laser and its harmonics. In this method, nanoparticles aregenerated by the laser beam whose photon energy corresponds toabsorption band energy of the nanoparticles [27–29].

Later experiments showed that the morphology of nanoparti-cles prepared by laser ablation can be further modified by frag-mentation caused by the impact of subsequent laser pulses [22].Adequate understanding of the fragmentation process couldenable a better control of the laser ablation fragmentation processnamely with respect to its maximum efficiency and the desiredcharacteristics of the nanoparticles, this process is also called“two-step laser-assisted method” [22,30]. Some researchers whoused picosecond photo absorption spectroscopy concluded thatthe main reason of the size reduction is the fragmentation via theCoulomb explosion of the photoionized metal nanoparticle. Plechet al. [31] used resolved X-ray scattering to study the changes innanoparticle structures and the water molecules in the vicinity ofthe nanoparticles. They found that during a time scale of 1 ns,

cond laser ablation method.

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these particles undergo a melting transition due to the thermalchanges. Besner et al. [30] have also described this method toreduce the gold nanoparticle size using a femtosecond laser indeionized water. Numerous studies have been carried out on thefragmentation of the metal oxide nanoparticles in nanosecond,picoseconds and femtosecond regimes [22,28].

This paper reports results on the effect of the laser pulse energyon size and stability of nanoparticles through fragmentation, and/or size reduction of the agglomerates. Results of chemical bondingof TiO2 nanoparticle by FTIR analysis are also presented.

0

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µJµJµJ

Fig. 2. Absorption spectrum of TiO2 nanoparticles produced in aqueous solution ofSDS using two steps process by (a) 1st-step—800 nm and (b) 2nd-step—400 nm.

2. Experimental work

Experiments for the synthesis of colloidal solution of nanopar-ticles by using pulsed laser ablation in aqueous media were carriedout with a pulsed Ti/Sapphire laser beam (Quadronix IntenC laser)at Kocaeli University Laser Technologies Research and ApplicationCenter (LATARUM). Laser operates at 1 kHz repetition rate with apulse width of ≤130 fs at 2.5 mJ/pulse maximum laser beamoutput. Fig. 1a shows the experimental setup of femtosecond laserablation schematically.

The experiments were performed in two steps. The first stepinvolved the production of nanoparticles in liquid media usingfundamental wavelength (800 nm) of Ti:Sapphire laser at 1 kHz.The laser beam was focused onto a titanium target sample (purity99.99%). It was then cleaned by ultrasonic cleaning device andwiped with acetone and ethanol. After the cleaning process, targetwas mounted into a fused silica container filled with distilledwater and (SDS) (10−3 M).

Titanium target was fixed on a plate attached to a motor to rotate itin order to prevent laser irradiation on the same spot. The systemwasmounted on a magnetic stirrer rotator (ARCE-model) with a range ofrotation speed of 0–1300 rpm as shown in Fig. 1. Purpose of therotation was to ensure a uniform irradiation on the target andmovement of water that can enhance ablated particle diffusion aswell as to disperse produced NPs. The rotation speed of magneticstirrer was set to 600 rpm. The laser beamwas focused by a lens witha focal length of 100mm in order to get sufficient laser fluence for theablation. Laser beam waist was set to 6 mm diameter using anadjustable pinhole; the depth of the liquid volume above the targetwas 10 mm. The experiments were carried out in stirred liquid for30 min, with 0.6 mJ/pulse energy and the fluence was 1 J/cm2. Thelaser power was measured with a power meter (Newport Model841-PE) before each experiment as schematically shown in Fig. 1.The measurement point was below the lens after laser guiding equip-ment to ensure the amount of actual laser power hitting the targetwhich differs from the amount of laser power emitting from thesource due to loss associated with mirrors and air-dust. During thefirst step of the PLAL of titanium plate in SDS, the solution appearedcolorless and it began to change to violet within a few minutes.Change of color of colloidal solution of TiO2 to violet color can beconsidered to be a first indication that nanosized colloidal particleshave been produced.

In the second step, the solution containing titanium oxidenanoparticles was re-irradiated with the second harmonic(400 nm) wavelengths of the Ti:Sapphire laser beam focused tothe middle of the solution by using a 50.2 mm lens at differentvalues of energy 180,120 and 60 mJ for 45 min. In the second step,only 5 ml of the solution containing TiO2 nanoparticles filled in anew container. The depth of the solution, containing nanoparticlesin the new container was now 8 mm. Magnetic stirrer was againused to ensure homogeneous particle distribution.

UV–visible extinction spectrum of the colloidal solutions wasrecorded using (Varian Cary -50 UV–Visible spectrophotometer) andbefore this test, we put the sample in ultrasonic cleaner (EMAG 50 HC)

to ensure the homogeneity of the NP solution. Size, morphology anddistribution of TiO2 nanoparticles were examined by TEM images;however, in order to have a quick measurement of size distribution ofnanoparticles, DLS techniquewas used as it is a fast, on-line and in situmethod. We found out that size distribution results from DLS areusually consistent with the ones obtained from TEM, although TEMprovides more definitive answers. We have used Malvern Nano ZS90for DLS and zeta potential measurements and JEOL JSM 6400F forelectron microscopy. Xu et al.[32] have used DLS to determine size andzeta potential of the polymer nanoparticles, Gao et al. [33], Calzolaiet al. [34] also used the DLS system to determine the diameter of thegold nanoparticles. Other analytical techniques such as (FTIR) spectro-scopy (The PerkinElmer Spectrum 100 Series FT-IR spectrometer) arealso used to study the adsorption of organic species on the TiO2

nanoparticles. FTIR spectra were measured at room temperature withthe spectrometer using the KBr Pellet technique [1]. Samples werelyophilized, gently mixed with 300 mg of KBr powder and compressedinto discs at a pressure of 40 MPa for 5 min, FTIR spectrum wasrecorded in the spectral range of 400–4000 cm−1 to know thechemical bonding of the produced nanoparticles.

3. Results and discussion

Underlying mechanisms can be summarized in three-steps: thefirst step is the generation of plasma due to high pressure and

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temperature as a result of the interaction of laser with matter.Second step is an ultrasonic adiabatic expansion of the plasma thatleads to a quick cooling of the plume region and hence to theformation of titanium clusters. Finally, the plasma is extinguishedand formed titanium clusters encounter and interact with thesolvent and surfactant molecules in the surrounding solutioninducing some chemical reactions [35]. The process and chemicalreaction can be described as below.

Ti(clusters)þ4H2O-Ti(OH)42H2 (1)

Ti(OH)4-TiO2þ2H2O (2)

Fig. 2a shows an absorption spectrum of TiO2 from the first stepwhich consists of a single broad intense cut-off absorptionwavelength around 510 nm due to the charge-transfer from thevalence band (mainly formed by 2p orbitals of the oxide anions) tothe conduction band (mainly formed by 3d t2 g orbitals of the Ti4þ

cations) [36].After the production of NPs, the fragmentation processes were

applied for three different laser pulses energies 180, 120, and 60 mJ.Cut-off absorption wavelengths were observed at 436, 557 and587 nm for energies 180, 120 and 60 mJ, respectively. This shows ablue shift from that of the TiO2 NPs produced at the first step

Fig. 3. TEM and size distribution o TiO2 nanoparticles produced using ultrashort Ti:sapph(c) 2nd-step—120 mJ and (d) 2nd-step—180 mJ.

which is 510 nm in especially for 180 mJ energy. However, for 120and 60 mJ pulse energy there are also a shift towards red in spite ofthe decrease in the size of TiO2 nanoparticles. This is in contrast tothe expected shift to the blue region. We have assumed that thereason of this shift towards red region is related with theagglomeration of the TiO2 nanoparticles [37]. Agglomerationmight take place in time between the first and second steps,hence the time required between two processes may not besufficient enough for a homogeneous fragmentation leading tosmaller nanoparticles. The optical absorption spectra of the TiO2

nanoparticles, which were measured after each ablation andfragmentation process is shown in Fig. 2b. On the other hand theshift towards the higher energy (lower wavelength) that appearsin the spectrum indicates a reduction in particle sizes. Similarresults also have been observed by Akman et al. [22] with gold andsilver NPs. This reduction in particle size with increasing energy inthe second step is due to the interaction of generated particlesfrom the metal plate with the laser beam. The blue shift is inagreement with the fragmentation of larger particles as alsoreported in [38].

Besner et al. [30] have suggested three mechanisms which mayexplain the absorption of radiation by nanoparticles; (i) a directabsorption of the laser radiation, (ii) absorption of energy of the

ire laser beam in aqueous solution of SDS: (a) 1st-step—0.8 mJ, (b) 2nd-step—60 mJ,

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white continuum and (iii) interband resonant multiphotonabsorption. With the 2nd harmonic of the Ti:Sapphire laserwavelength used in the second step of our experiment, it isexpected that direct absorption mechanism would be dominantfor the size reduction of TiO2 nanoparticles since the laser beamphoton energy is in the absorption band range of TiO2 nano-particles.

A number of studies have concentrated on decreasing the sizeof the particles by using various nanosecond and femtosecondlasers running at different wavelengths as smaller scale nanopar-ticles are important for lots of applications such as sensingtechnologies [38,39]. More detailed information on size propertiesof nanoparticles was obtained from TEM images. Fig. 3a shows aTEM micrograph and size distribution of TiO2 nanoparticlesproduced in the first step where the scale bar is the 200 nm.Fig. 3b, c and d shows particle size distributions for second stepenergies of 60 μJ, 120 μJ and 180 μJ with respected particledistributions. It can be seen that particle size distribution becomesnarrower and average particle size becomes smaller. The sizedistribution shown in Fig. 3a with average particle size of 180 nmis in fact appears largely due to effects of nanoparticle agglomera-tion. The large size variation is expected especially in the firststep as when the laser beam first ablated the surface, it is possible

Fig. 4. FTIR spectra of TiO2 nanoparticles in 1

Fig. 5. Zeta potential distributions of TiO2 nanoparticles illuminated by 400 nm and(c) 180 μJ (d) 0, 8 mJ at the end of the first step.

to have nanostructures on the larger side of the nanoscale.However, the incoming laser pulse can be absorbed or scatteredby the dispersed nanoparticles in the liquid. This event causes theshielding effect which reduces the ablation efficiency with time[22]. However, it is anticipated that if higher laser energies as wellas further steps are applied, it is possible to obtain even a smallersize range.

On the other hand, in the case of 400 nm laser wavelength,another interaction occurs between TiO2 NPs and the laser beamand as a result of this interaction large particles will fragment andbecome smaller. By increasing the energy of laser, the efficiency offragmentation will increase, therefore obtained particle sizes with180 μJ will decrease from 180 nm to 110 nm as shown in Fig. 3dwhile the diameter of these NPs in the first step is 180 nm. This isbecause further fragmentation of the agglomerates takes place viaa direct absorption of laser beam. By using and controlling thefragmentation process through interaction with the laser radiationwould end in a smaller, virtually monodispersive nanoparticle sizedistribution.

Experiments reported here showed that final size distributionwas almost independent of the initial size and shape of nanopar-ticles, but depended largely on radiation parameters. Final size ofthe nanoparticles is determined by the chemical interaction of the

st-step—800 nm Ti:Sapphire laser beam.

800 nm wavelength for different re-irradiation energy pulses (a) 60 mJ (b) 120 mJ

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fragmented species in the solution [30]. Fig. 4 shows FTIRspectrum of TiO2 nanoparticles as FTIR was found to be veryuseful to understand bonding between Ti–O atoms or molecules.The FTIR peaks at (500–700 cm−1) is the characteristic vibrationsof Ti–O corresponding to asymmetric, symmetric and anamorphicstretches, while the broad intense band below 1200 cm−1 is due toTi–O–Ti vibrations [36,40]. This figure shows peaks correspondingto stretching vibrations of the O–H and bending vibrations of theadsorbed water molecules around 3350–3450 cm−1 and 1620–1635 cm−1 respectively. Other strong absorption bands at 1522and 1566 cm−1 are due to aromatic C–C stretching. Multiple bandsbetween 1200 and 1000 cm−1 are the result of phenolic C–Ostretching and aromatic C–H in-plane bending.

Measurements of zeta potential was also carried out in order tostudy the stability of nanoparticles as this is extremely importantfor many applications [41]. The criteria of stability of NPs can beevaluated when the values of zeta potential ranges from higherthan þ30 mV to lower than −30 mV [22]. Fig. 5a and d show zetapotential distributions of TiO2 nanoparticles illuminated by800 nm in the first step and 400 nm wavelength in the secondstep for different values of energy measured zeta potential whichvaries between (−49.5) mV and (−51.8) mV indicating a stable,with extremely low agglomeration nanoparticles solution.

4. Conclusions

The present work has successfully produced TiO2 nanoparticlesby focusing an ultrafast Ti:Sapphire laser onto a Ti target in liquidmedia. After the first stage of fragmentation of TiO2 nanoparticlesinduced by irradiation of the femtosecond laser pulse ontonanoparticles, the solution containing TiO2 nanoparticles has beenresubjected to second harmonic, 400 nm wavelength of the laserat different pulse energies of 180, 120 and 60 μJ in the second step.When 180 μJ was used, size of the nanoparticles decreased from180 nm to 100 nm.

The TiO2 nanoparticles are size-reduced by the Coulombexplosion of the highly charged particles therefore the secondprocess with the laser resolves the agglomeration problem andprovides homogeneous nanoparticle distributions in liquidswithin a matter of 45 min.

The optical absorption of TiO2 nanoparticle multiphase systemwas measured in order to investigate influence of absorbance ofirradiation laser by ablated particles, a shift in the wavelength toblue takes place for energies of 180 μJ and 120 μJ with absorptionwavelength of 410 and 430 nm respectively. However, at the pulseenergy of 60 μJ, the value of absorption wavelength is 480 nm andthere is a shift this time towards red when compared to its value of450 nm of TiO2 NPs produced in the first step. We have assumedthat the reason of shift toward red region is related to theagglomeration of the TiO2 nanoparticles; therefore the timerequired between two processes may not be sufficient enoughfor a homogeneous fragmentation towards smaller nanoparticles.

The size distribution of TiO2 nanoparticles can be controlled byusing 400 nm laser pulses during further fragmentation. This leadsto a narrow size distribution as well as stable nanoparticle system.We are of the opinion that it is possible to obtain more stable andsmaller nanoparticles by using higher energies or possibly chan-ging the irradiation time [26,42].

The Zeta potential varies between (−49.5) mV and (−51.8) mVfor 400 nm second step irradiation at different energies indicatinga stable and low degree agglomeration nanoparticles in thesolution. These results show that the agglomeration and nanopar-ticle sizes can be reduced by the second application of thelaser beam.

This convenient synthesis strategy can be applied as a generalapproach to producing TiO2 NPs which have attracted a significantinterest from materials scientists and physicists due to theirspecial properties. TiO2 NPs have gained a great importance inseveral technological applications such as photocatalysis, sensors,dye-sensitized solar cells, biological and memory devices [7,43].

Acknowledgments

This work was supported by (DPT Medical Electro-OpticsResearch Laboratory, Project no. 2011K120330) and the Ministryof Higher Education of Iraq.

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