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Study of the fragmentation phenomena of TiO2 nanoparticles produced by femtosecond laser ablation in aqueous media
Optics & Laser Technology 51 (2013) 17–23
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Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Study of the fragmentation phenomena of TiO2 nanoparticles produced by 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, Iraq Kocaeli University, Laser Technologies Research and Application Center, 41275 Kocaeli, Turkey c BEAM Ar-Ge Optics and Laser Technologies Ltd., KOUTechnopark, Basiskele 41275, Kocaeli, Turkey d Kocaeli University, Electro-Optics Systems Engineering, 41380 Umuttepe, Kocaeli, Turkey e Diyala University, College of Engineering, Diyala, Iraq b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 November 2012 Received in revised form 8 February 2013 Accepted 12 February 2013 Available online 13 April 2013
Since last decade, Pulsed Laser Ablation in Liquid (PLAL) has become an increasingly important technique for 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 pulsed laser ablation of a titanium target immersed in Sodium Dodecyl Sulfate (SDS) solution using an ultrafast Ti:Sapphire laser. After the production of TiO2 nanoparticles for 30 min of laser irradiation, second harmonics of the laser wavelength are re-applied for different energies (180,120, 60 mJ) to SDS solution containing TiO2 colloids in order to fragment relatively large pieces to obtain smaller ones. It was found that size of nanoparticles after the treatment is independent of the initial characteristics of colloids, but depends strongly on laser parameters especially pulse energy and on the presence of chemically active species in the solution. It was reported that particle size and size distribution range can be decreased using 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 nm to 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 Transform Infra-Red (FTIR). & 2013 Elsevier Ltd. All rights reserved.
Keywords: Laser ablation Metal oxide nanoparticles Ultrafast Ti:Sapphire laser
1. Introduction Nanomaterials display unique and superior properties which are different from those of their bulk materials, because of their high surface area to volume ratio [1]. The vastly increased ratio of surface area to volume leads to new quantum mechanical effects such as the “quantum size effect” where the electronic properties of solids are altered with great reductions in particle size as the size of the particle moves to a regime where quantum confinement effects are predominant [2]. Plasmonic behavior in the UV-region especially with controlled morphology and particle size makes TiO2 nanoparticles an attractive prospect for use as a good UV absorber not only for pharmaceutical applications but also in solar cell applications with extended spectral range. Synthesis of high quality nanostructured materials is a very active area as nanoparticles represent an important class of material development field for novel devices that can be used in many applications
n
Corresponding author. Tel.: þ90 262 3031061; fax: þ90 262 3031013. E-mail addresses: [email protected], [email protected] (A. Demir).
0030-3992/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2013.02.013
such as: photothermal [3], therapy [4], surface-enhanced Raman spectroscopy [5], biochemical sensors [6], solar cells etc. [7]. There are two main approaches to produce nanomaterials: topdown and bottom-up. In the top-down approach the production of nanoparticles is realized by etching smaller structures from larger ones. Laser ablation and milling are two of the typical examples to top-down approach. On the other hand, bottom-up approach refers to the build-up of a material: atom by atom, molecule by molecule, or cluster-by-cluster [8]. The most efficient physical method for nanofabrication is the laser ablation process because of a number of advantages compared to conventional methods. The advantages of this method are simplicity of the procedure and the absence of chemical reagents in solution [1]. This method also gives certain flexibility over other techniques as all types of materials can be processed and ablated due to the very high energy density. Controlling the size of produced NPs by optimizing the process parameters such as irradiation time, pulse duration, energy density and laser wavelength etc. [9]. Laser ablation of materials from a solid target occurs either in 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 of controlling the production of NPs. In the latter method, nanoclusters can be released into the liquid forming a colloidal nanoparticle solution leading to a more effective collection of synthesized particles. PLAL does not need a vacuum system and has a high collection yield making it more efficient compared to the laser ablation in gas phase [11]. In other words the solvent can provide positive physical and chemical effects such as plasma confinement, cooling actions, oxidation or reduction leading to enhancement of ablation efficiency [12]. The concept of producing oxide using laser irradiation of metal targets in water was demonstrated in 1987 where iron and tantalum oxides were formed on target surfaces in water using a Q-switched ruby pulsed laser by using a third harmonic of a pulsed Nd:YAG laser PLAL of Ti in water and SDS solution [13]. Sasaki et al. [14] have synthesized TiO2 in both deionized water and sodium dodecyl sulfate (SDS) solutions and they have explained crystallinity of the nanoparticles strongly depended on the SDS concentration in the solution. The metal oxide nanoparticles have many applications in nonlinear optics, optoelectronics, biomedical engineering, electro-optical devices and chemical catalysts [15]. The production of NPs by femtosecond laser has been getting more common due to its efficiency in ablation of materials and effective control of particle size compared with nanosecond laser ablation. Tan et al. [16] have explained that use of femtosecond laser can effectively minimize laser–plume interaction and reduce the heat affected zones. Moreover the limited heating effect which results from the interaction of ultra-short pulses with the matter benefits a faster cooling of ablated particles and prevents them from aggregating. For all the reasons stated above, the ultra-short laser pulses are favorable for the synthesis of smaller particles as mentioned by Kabashin and Meunier [17]. It is also possible to control the nanoparticle growth with different amounts of concentrations of surfactants. Tilaki et al. [18]; Mafune et al. [19] and Chen and Yeh [20] have reduced the nanoparticle size and prevented their agglomeration by changing the surfactant and its concentration. Several authors tried to control of nanoparticle size distribution and density by optimizing the laser parameters such as pulse energy, pulse repetition rate, laser wavelength, pulse duration and focusing
conditions of femtosecond beam [21]. Akman et. al. [22] studied the effect of Ti:sapphire laser parameter on size and morphological properties of silver nanoparticles with repeated pulses in the second harmonics. In another paper, Akman et. al [23] and Khaled [24] also report the effect of various laser parameters on size and morphology of gold nanoparticles where laser energy range was selected to cover the plasmonic properties of gold. Barcikowski [25] examined the influence of pulse energy and micromachining speed revealing that in some cases the effect of laser fluence on the nanoparticle size distribution is very weak, in other cases the use of higher laser fluences produce a distribution shift toward the smaller particles. Because the surfactant that surrounds each nanoparticle prevents a direct contact between them, many researchers have concentrated towards studying the effect of the best surfactant to reduce the particle size or stabilizing the colloids [19,26]. Mafune et al. [26] have reported the effect of SDS in determining stability and size of the nanoparticles, where controlling the nanoparticle growth was achieved by diffusion and attachment rates of SDS to the nanoparticle surface. As a result, size distribution and stability of the nanoparticles depend critically on the properties of the used surfactants. Many researchers use “laserassisted size control” method to produce smaller and monodisperse nanoparticles from different materials using nanosecond Nd:YAG laser and its harmonics. In this method, nanoparticles are generated by the laser beam whose photon energy corresponds to absorption band energy of the nanoparticles [27–29]. Later experiments showed that the morphology of nanoparticles prepared by laser ablation can be further modified by fragmentation caused by the impact of subsequent laser pulses [22]. Adequate understanding of the fragmentation process could enable a better control of the laser ablation fragmentation process namely with respect to its maximum efficiency and the desired characteristics of the nanoparticles, this process is also called “two-step laser-assisted method” [22,30]. Some researchers who used picosecond photo absorption spectroscopy concluded that the main reason of the size reduction is the fragmentation via the Coulomb explosion of the photoionized metal nanoparticle. Plech et al. [31] used resolved X-ray scattering to study the changes in nanoparticle structures and the water molecules in the vicinity of the nanoparticles. They found that during a time scale of 1 ns,
Fig. 1. Experimental set up of femtosecond laser ablation method.
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Experiments for the synthesis of colloidal solution of nanoparticles by using pulsed laser ablation in aqueous media were carried out with a pulsed Ti/Sapphire laser beam (Quadronix IntenC laser) at Kocaeli University Laser Technologies Research and Application Center (LATARUM). Laser operates at 1 kHz repetition rate with a pulse width of ≤130 fs at 2.5 mJ/pulse maximum laser beam output. Fig. 1a shows the experimental setup of femtosecond laser ablation schematically. The experiments were performed in two steps. The first step involved the production of nanoparticles in liquid media using fundamental wavelength (800 nm) of Ti:Sapphire laser at 1 kHz. The laser beam was focused onto a titanium target sample (purity 99.99%). It was then cleaned by ultrasonic cleaning device and wiped with acetone and ethanol. After the cleaning process, target was mounted into a fused silica container filled with distilled water and (SDS) (10−3 M). Titanium target was fixed on a plate attached to a motor to rotate it in order to prevent laser irradiation on the same spot. The system was mounted on a magnetic stirrer rotator (ARCE-model) with a range of rotation speed of 0–1300 rpm as shown in Fig. 1. Purpose of the rotation was to ensure a uniform irradiation on the target and movement of water that can enhance ablated particle diffusion as well as to disperse produced NPs. The rotation speed of magnetic stirrer was set to 600 rpm. The laser beam was focused by a lens with a focal length of 100 mm in order to get sufficient laser fluence for the ablation. Laser beam waist was set to 6 mm diameter using an adjustable pinhole; the depth of the liquid volume above the target was 10 mm. The experiments were carried out in stirred liquid for 30 min, with 0.6 mJ/pulse energy and the fluence was 1 J/cm2. The laser power was measured with a power meter (Newport Model 841-PE) before each experiment as schematically shown in Fig. 1. The measurement point was below the lens after laser guiding equipment to ensure the amount of actual laser power hitting the target which differs from the amount of laser power emitting from the source due to loss associated with mirrors and air-dust. During the first step of the PLAL of titanium plate in SDS, the solution appeared colorless and it began to change to violet within a few minutes. Change of color of colloidal solution of TiO2 to violet color can be considered to be a first indication that nanosized colloidal particles have been produced. In the second step, the solution containing titanium oxide nanoparticles was re-irradiated with the second harmonic (400 nm) wavelengths of the Ti:Sapphire laser beam focused to the middle of the solution by using a 50.2 mm lens at different values 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 a new container. The depth of the solution, containing nanoparticles in the new container was now 8 mm. Magnetic stirrer was again used to ensure homogeneous particle distribution. UV–visible extinction spectrum of the colloidal solutions was recorded using (Varian Cary -50 UV–Visible spectrophotometer) and before this test, we put the sample in ultrasonic cleaner (EMAG 50 HC)
3. Results and discussion Underlying mechanisms can be summarized in three-steps: the first step is the generation of plasma due to high pressure and 4
1st-step-0,8 mJ 3.5 3
Absorption (a.u)
2. Experimental work
to ensure the homogeneity of the NP solution. Size, morphology and distribution of TiO2 nanoparticles were examined by TEM images; however, in order to have a quick measurement of size distribution of nanoparticles, DLS technique was used as it is a fast, on-line and in situ method. We found out that size distribution results from DLS are usually consistent with the ones obtained from TEM, although TEM provides more definitive answers. We have used Malvern Nano ZS90 for DLS and zeta potential measurements and JEOL JSM 6400F for electron microscopy. Xu et al.[32] have used DLS to determine size and zeta potential of the polymer nanoparticles, Gao et al. [33], Calzolai et al. [34] also used the DLS system to determine the diameter of the gold nanoparticles. Other analytical techniques such as (FTIR) spectroscopy (The PerkinElmer Spectrum 100 Series FT-IR spectrometer) are also used to study the adsorption of organic species on the TiO2 nanoparticles. FTIR spectra were measured at room temperature with the spectrometer using the KBr Pellet technique [1]. Samples were lyophilized, gently mixed with 300 mg of KBr powder and compressed into discs at a pressure of 40 MPa for 5 min, FTIR spectrum was recorded in the spectral range of 400–4000 cm−1 to know the chemical bonding of the produced nanoparticles.
2.5 2 1.5 1 0.5 0 200
300
400
500
600
700
800
Wavelength (nm) 4 2nd-step-180 µJ 3.5
2nd-step-120 µJ 2nd-step-60 µJ
3
Absorption (a.u)
these particles undergo a melting transition due to the thermal changes. Besner et al. [30] have also described this method to reduce the gold nanoparticle size using a femtosecond laser in deionized water. Numerous studies have been carried out on the fragmentation of the metal oxide nanoparticles in nanosecond, picoseconds and femtosecond regimes [22,28]. This paper reports results on the effect of the laser pulse energy on size and stability of nanoparticles through fragmentation, and/ or size reduction of the agglomerates. Results of chemical bonding of TiO2 nanoparticle by FTIR analysis are also presented.
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2.5
2
1.5
1
0.5
0 200
300
400
500
600
700
800
Wavelength (nm) Fig. 2. Absorption spectrum of TiO2 nanoparticles produced in aqueous solution of SDS using two steps process by (a) 1st-step—800 nm and (b) 2nd-step—400 nm.
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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 that leads to a quick cooling of the plume region and hence to the formation of titanium clusters. Finally, the plasma is extinguished and formed titanium clusters encounter and interact with the solvent and surfactant molecules in the surrounding solution inducing some chemical reactions [35]. The process and chemical reaction can be described as below. Ti(clusters) þ4H2O-Ti(OH)42H2
(1)
Ti(OH)4-TiO2 þ2H2O
(2)
Fig. 2a shows an absorption spectrum of TiO2 from the first step which consists of a single broad intense cut-off absorption wavelength around 510 nm due to the charge-transfer from the valence band (mainly formed by 2p orbitals of the oxide anions) to the conduction band (mainly formed by 3d t2 g orbitals of the Ti4 þ 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 and 587 nm for energies 180, 120 and 60 mJ, respectively. This shows a blue shift from that of the TiO2 NPs produced at the first step
which is 510 nm in especially for 180 mJ energy. However, for 120 and 60 mJ pulse energy there are also a shift towards red in spite of the decrease in the size of TiO2 nanoparticles. This is in contrast to the expected shift to the blue region. We have assumed that the reason of this shift towards red region is related with the agglomeration of the TiO2 nanoparticles [37]. Agglomeration might take place in time between the first and second steps, hence the time required between two processes may not be sufficient enough for a homogeneous fragmentation leading to smaller nanoparticles. The optical absorption spectra of the TiO2 nanoparticles, which were measured after each ablation and fragmentation process is shown in Fig. 2b. On the other hand the shift towards the higher energy (lower wavelength) that appears in the spectrum indicates a reduction in particle sizes. Similar results also have been observed by Akman et al. [22] with gold and silver NPs. This reduction in particle size with increasing energy in the second step is due to the interaction of generated particles from the metal plate with the laser beam. The blue shift is in agreement with the fragmentation of larger particles as also reported in [38]. Besner et al. [30] have suggested three mechanisms which may explain the absorption of radiation by nanoparticles; (i) a direct absorption of the laser radiation, (ii) absorption of energy of the
Fig. 3. TEM and size distribution o TiO2 nanoparticles produced using ultrashort Ti:sapphire laser beam in aqueous solution of SDS: (a) 1st-step—0.8 mJ, (b) 2nd-step—60 mJ, (c) 2nd-step—120 mJ and (d) 2nd-step—180 mJ.
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white continuum and (iii) interband resonant multiphoton absorption. With the 2nd harmonic of the Ti:Sapphire laser wavelength used in the second step of our experiment, it is expected that direct absorption mechanism would be dominant for the size reduction of TiO2 nanoparticles since the laser beam photon energy is in the absorption band range of TiO2 nanoparticles. A number of studies have concentrated on decreasing the size of the particles by using various nanosecond and femtosecond lasers running at different wavelengths as smaller scale nanoparticles are important for lots of applications such as sensing technologies [38,39]. More detailed information on size properties of nanoparticles was obtained from TEM images. Fig. 3a shows a TEM micrograph and size distribution of TiO2 nanoparticles produced in the first step where the scale bar is the 200 nm. Fig. 3b, c and d shows particle size distributions for second step energies of 60 μJ, 120 μJ and 180 μJ with respected particle distributions. It can be seen that particle size distribution becomes narrower and average particle size becomes smaller. The size distribution shown in Fig. 3a with average particle size of 180 nm is in fact appears largely due to effects of nanoparticle agglomeration. The large size variation is expected especially in the first step as when the laser beam first ablated the surface, it is possible
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to have nanostructures on the larger side of the nanoscale. However, the incoming laser pulse can be absorbed or scattered by the dispersed nanoparticles in the liquid. This event causes the shielding effect which reduces the ablation efficiency with time [22]. However, it is anticipated that if higher laser energies as well as further steps are applied, it is possible to obtain even a smaller size range. On the other hand, in the case of 400 nm laser wavelength, another interaction occurs between TiO2 NPs and the laser beam and as a result of this interaction large particles will fragment and become smaller. By increasing the energy of laser, the efficiency of fragmentation will increase, therefore obtained particle sizes with 180 μJ will decrease from 180 nm to 110 nm as shown in Fig. 3d while the diameter of these NPs in the first step is 180 nm. This is because further fragmentation of the agglomerates takes place via a direct absorption of laser beam. By using and controlling the fragmentation process through interaction with the laser radiation would end in a smaller, virtually monodispersive nanoparticle size distribution. Experiments reported here showed that final size distribution was almost independent of the initial size and shape of nanoparticles, but depended largely on radiation parameters. Final size of the nanoparticles is determined by the chemical interaction of the
Fig. 4. FTIR spectra of TiO2 nanoparticles in 1st-step—800 nm Ti:Sapphire laser beam.
Fig. 5. Zeta potential distributions of TiO2 nanoparticles illuminated by 400 nm and 800 nm wavelength for different re-irradiation energy pulses (a) 60 mJ (b) 120 mJ (c) 180 μJ (d) 0, 8 mJ at the end of the first step.
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fragmented species in the solution [30]. Fig. 4 shows FTIR spectrum of TiO2 nanoparticles as FTIR was found to be very useful to understand bonding between Ti–O atoms or molecules. The FTIR peaks at (500–700 cm−1) is the characteristic vibrations of Ti–O corresponding to asymmetric, symmetric and anamorphic stretches, while the broad intense band below 1200 cm−1 is due to Ti–O–Ti vibrations [36,40]. This figure shows peaks corresponding to stretching vibrations of the O–H and bending vibrations of the adsorbed water molecules around 3350–3450 cm−1 and 1620– 1635 cm−1 respectively. Other strong absorption bands at 1522 and 1566 cm−1 are due to aromatic C–C stretching. Multiple bands between 1200 and 1000 cm−1 are the result of phenolic C–O stretching and aromatic C–H in-plane bending. Measurements of zeta potential was also carried out in order to study the stability of nanoparticles as this is extremely important for many applications [41]. The criteria of stability of NPs can be evaluated when the values of zeta potential ranges from higher than þ30 mV to lower than −30 mV [22]. Fig. 5a and d show zeta potential distributions of TiO2 nanoparticles illuminated by 800 nm in the first step and 400 nm wavelength in the second step for different values of energy measured zeta potential which varies 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 nanoparticles by focusing an ultrafast Ti:Sapphire laser onto a Ti target in liquid media. After the first stage of fragmentation of TiO2 nanoparticles induced by irradiation of the femtosecond laser pulse onto nanoparticles, the solution containing TiO2 nanoparticles has been resubjected to second harmonic, 400 nm wavelength of the laser at different pulse energies of 180, 120 and 60 μJ in the second step. When 180 μJ was used, size of the nanoparticles decreased from 180 nm to 100 nm. The TiO2 nanoparticles are size-reduced by the Coulomb explosion of the highly charged particles therefore the second process with the laser resolves the agglomeration problem and provides homogeneous nanoparticle distributions in liquids within a matter of 45 min. The optical absorption of TiO2 nanoparticle multiphase system was measured in order to investigate influence of absorbance of irradiation laser by ablated particles, a shift in the wavelength to blue takes place for energies of 180 μJ and 120 μJ with absorption wavelength of 410 and 430 nm respectively. However, at the pulse energy of 60 μJ, the value of absorption wavelength is 480 nm and there is a shift this time towards red when compared to its value of 450 nm of TiO2 NPs produced in the first step. We have assumed that the reason of shift toward red region is related to the agglomeration of the TiO2 nanoparticles; therefore the time required between two processes may not be sufficient enough for a homogeneous fragmentation towards smaller nanoparticles. The size distribution of TiO2 nanoparticles can be controlled by using 400 nm laser pulses during further fragmentation. This leads to a narrow size distribution as well as stable nanoparticle system. We are of the opinion that it is possible to obtain more stable and smaller nanoparticles by using higher energies or possibly changing the irradiation time [26,42]. The Zeta potential varies between (−49.5) mV and (−51.8) mV for 400 nm second step irradiation at different energies indicating a stable and low degree agglomeration nanoparticles in the solution. These results show that the agglomeration and nanoparticle sizes can be reduced by the second application of the laser beam.
This convenient synthesis strategy can be applied as a general approach to producing TiO2 NPs which have attracted a significant interest from materials scientists and physicists due to their special properties. TiO2 NPs have gained a great importance in several technological applications such as photocatalysis, sensors, dye-sensitized solar cells, biological and memory devices [7,43].
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Study of the fragmentation phenomena of TiO2 nanoparticles produced by 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, Iraq Kocaeli University, Laser Technologies Research and Application Center, 41275 Kocaeli, Turkey c BEAM Ar-Ge Optics and Laser Technologies Ltd., KOUTechnopark, Basiskele 41275, Kocaeli, Turkey d Kocaeli University, Electro-Optics Systems Engineering, 41380 Umuttepe, Kocaeli, Turkey e Diyala University, College of Engineering, Diyala, Iraq b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 November 2012 Received in revised form 8 February 2013 Accepted 12 February 2013 Available online 13 April 2013
Since last decade, Pulsed Laser Ablation in Liquid (PLAL) has become an increasingly important technique for 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 pulsed laser ablation of a titanium target immersed in Sodium Dodecyl Sulfate (SDS) solution using an ultrafast Ti:Sapphire laser. After the production of TiO2 nanoparticles for 30 min of laser irradiation, second harmonics of the laser wavelength are re-applied for different energies (180,120, 60 mJ) to SDS solution containing TiO2 colloids in order to fragment relatively large pieces to obtain smaller ones. It was found that size of nanoparticles after the treatment is independent of the initial characteristics of colloids, but depends strongly on laser parameters especially pulse energy and on the presence of chemically active species in the solution. It was reported that particle size and size distribution range can be decreased using 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 nm to 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 Transform Infra-Red (FTIR). & 2013 Elsevier Ltd. All rights reserved.
Keywords: Laser ablation Metal oxide nanoparticles Ultrafast Ti:Sapphire laser
1. Introduction Nanomaterials display unique and superior properties which are different from those of their bulk materials, because of their high surface area to volume ratio [1]. The vastly increased ratio of surface area to volume leads to new quantum mechanical effects such as the “quantum size effect” where the electronic properties of solids are altered with great reductions in particle size as the size of the particle moves to a regime where quantum confinement effects are predominant [2]. Plasmonic behavior in the UV-region especially with controlled morphology and particle size makes TiO2 nanoparticles an attractive prospect for use as a good UV absorber not only for pharmaceutical applications but also in solar cell applications with extended spectral range. Synthesis of high quality nanostructured materials is a very active area as nanoparticles represent an important class of material development field for novel devices that can be used in many applications
n
Corresponding author. Tel.: þ90 262 3031061; fax: þ90 262 3031013. E-mail addresses: [email protected], [email protected] (A. Demir).
0030-3992/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2013.02.013
such as: photothermal [3], therapy [4], surface-enhanced Raman spectroscopy [5], biochemical sensors [6], solar cells etc. [7]. There are two main approaches to produce nanomaterials: topdown and bottom-up. In the top-down approach the production of nanoparticles is realized by etching smaller structures from larger ones. Laser ablation and milling are two of the typical examples to top-down approach. On the other hand, bottom-up approach refers to the build-up of a material: atom by atom, molecule by molecule, or cluster-by-cluster [8]. The most efficient physical method for nanofabrication is the laser ablation process because of a number of advantages compared to conventional methods. The advantages of this method are simplicity of the procedure and the absence of chemical reagents in solution [1]. This method also gives certain flexibility over other techniques as all types of materials can be processed and ablated due to the very high energy density. Controlling the size of produced NPs by optimizing the process parameters such as irradiation time, pulse duration, energy density and laser wavelength etc. [9]. Laser ablation of materials from a solid target occurs either in 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 of controlling the production of NPs. In the latter method, nanoclusters can be released into the liquid forming a colloidal nanoparticle solution leading to a more effective collection of synthesized particles. PLAL does not need a vacuum system and has a high collection yield making it more efficient compared to the laser ablation in gas phase [11]. In other words the solvent can provide positive physical and chemical effects such as plasma confinement, cooling actions, oxidation or reduction leading to enhancement of ablation efficiency [12]. The concept of producing oxide using laser irradiation of metal targets in water was demonstrated in 1987 where iron and tantalum oxides were formed on target surfaces in water using a Q-switched ruby pulsed laser by using a third harmonic of a pulsed Nd:YAG laser PLAL of Ti in water and SDS solution [13]. Sasaki et al. [14] have synthesized TiO2 in both deionized water and sodium dodecyl sulfate (SDS) solutions and they have explained crystallinity of the nanoparticles strongly depended on the SDS concentration in the solution. The metal oxide nanoparticles have many applications in nonlinear optics, optoelectronics, biomedical engineering, electro-optical devices and chemical catalysts [15]. The production of NPs by femtosecond laser has been getting more common due to its efficiency in ablation of materials and effective control of particle size compared with nanosecond laser ablation. Tan et al. [16] have explained that use of femtosecond laser can effectively minimize laser–plume interaction and reduce the heat affected zones. Moreover the limited heating effect which results from the interaction of ultra-short pulses with the matter benefits a faster cooling of ablated particles and prevents them from aggregating. For all the reasons stated above, the ultra-short laser pulses are favorable for the synthesis of smaller particles as mentioned by Kabashin and Meunier [17]. It is also possible to control the nanoparticle growth with different amounts of concentrations of surfactants. Tilaki et al. [18]; Mafune et al. [19] and Chen and Yeh [20] have reduced the nanoparticle size and prevented their agglomeration by changing the surfactant and its concentration. Several authors tried to control of nanoparticle size distribution and density by optimizing the laser parameters such as pulse energy, pulse repetition rate, laser wavelength, pulse duration and focusing
conditions of femtosecond beam [21]. Akman et. al. [22] studied the effect of Ti:sapphire laser parameter on size and morphological properties of silver nanoparticles with repeated pulses in the second harmonics. In another paper, Akman et. al [23] and Khaled [24] also report the effect of various laser parameters on size and morphology of gold nanoparticles where laser energy range was selected to cover the plasmonic properties of gold. Barcikowski [25] examined the influence of pulse energy and micromachining speed revealing that in some cases the effect of laser fluence on the nanoparticle size distribution is very weak, in other cases the use of higher laser fluences produce a distribution shift toward the smaller particles. Because the surfactant that surrounds each nanoparticle prevents a direct contact between them, many researchers have concentrated towards studying the effect of the best surfactant to reduce the particle size or stabilizing the colloids [19,26]. Mafune et al. [26] have reported the effect of SDS in determining stability and size of the nanoparticles, where controlling the nanoparticle growth was achieved by diffusion and attachment rates of SDS to the nanoparticle surface. As a result, size distribution and stability of the nanoparticles depend critically on the properties of the used surfactants. Many researchers use “laserassisted size control” method to produce smaller and monodisperse nanoparticles from different materials using nanosecond Nd:YAG laser and its harmonics. In this method, nanoparticles are generated by the laser beam whose photon energy corresponds to absorption band energy of the nanoparticles [27–29]. Later experiments showed that the morphology of nanoparticles prepared by laser ablation can be further modified by fragmentation caused by the impact of subsequent laser pulses [22]. Adequate understanding of the fragmentation process could enable a better control of the laser ablation fragmentation process namely with respect to its maximum efficiency and the desired characteristics of the nanoparticles, this process is also called “two-step laser-assisted method” [22,30]. Some researchers who used picosecond photo absorption spectroscopy concluded that the main reason of the size reduction is the fragmentation via the Coulomb explosion of the photoionized metal nanoparticle. Plech et al. [31] used resolved X-ray scattering to study the changes in nanoparticle structures and the water molecules in the vicinity of the nanoparticles. They found that during a time scale of 1 ns,
Fig. 1. Experimental set up of femtosecond laser ablation method.
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Experiments for the synthesis of colloidal solution of nanoparticles by using pulsed laser ablation in aqueous media were carried out with a pulsed Ti/Sapphire laser beam (Quadronix IntenC laser) at Kocaeli University Laser Technologies Research and Application Center (LATARUM). Laser operates at 1 kHz repetition rate with a pulse width of ≤130 fs at 2.5 mJ/pulse maximum laser beam output. Fig. 1a shows the experimental setup of femtosecond laser ablation schematically. The experiments were performed in two steps. The first step involved the production of nanoparticles in liquid media using fundamental wavelength (800 nm) of Ti:Sapphire laser at 1 kHz. The laser beam was focused onto a titanium target sample (purity 99.99%). It was then cleaned by ultrasonic cleaning device and wiped with acetone and ethanol. After the cleaning process, target was mounted into a fused silica container filled with distilled water and (SDS) (10−3 M). Titanium target was fixed on a plate attached to a motor to rotate it in order to prevent laser irradiation on the same spot. The system was mounted on a magnetic stirrer rotator (ARCE-model) with a range of rotation speed of 0–1300 rpm as shown in Fig. 1. Purpose of the rotation was to ensure a uniform irradiation on the target and movement of water that can enhance ablated particle diffusion as well as to disperse produced NPs. The rotation speed of magnetic stirrer was set to 600 rpm. The laser beam was focused by a lens with a focal length of 100 mm in order to get sufficient laser fluence for the ablation. Laser beam waist was set to 6 mm diameter using an adjustable pinhole; the depth of the liquid volume above the target was 10 mm. The experiments were carried out in stirred liquid for 30 min, with 0.6 mJ/pulse energy and the fluence was 1 J/cm2. The laser power was measured with a power meter (Newport Model 841-PE) before each experiment as schematically shown in Fig. 1. The measurement point was below the lens after laser guiding equipment to ensure the amount of actual laser power hitting the target which differs from the amount of laser power emitting from the source due to loss associated with mirrors and air-dust. During the first step of the PLAL of titanium plate in SDS, the solution appeared colorless and it began to change to violet within a few minutes. Change of color of colloidal solution of TiO2 to violet color can be considered to be a first indication that nanosized colloidal particles have been produced. In the second step, the solution containing titanium oxide nanoparticles was re-irradiated with the second harmonic (400 nm) wavelengths of the Ti:Sapphire laser beam focused to the middle of the solution by using a 50.2 mm lens at different values 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 a new container. The depth of the solution, containing nanoparticles in the new container was now 8 mm. Magnetic stirrer was again used to ensure homogeneous particle distribution. UV–visible extinction spectrum of the colloidal solutions was recorded using (Varian Cary -50 UV–Visible spectrophotometer) and before this test, we put the sample in ultrasonic cleaner (EMAG 50 HC)
3. Results and discussion Underlying mechanisms can be summarized in three-steps: the first step is the generation of plasma due to high pressure and 4
1st-step-0,8 mJ 3.5 3
Absorption (a.u)
2. Experimental work
to ensure the homogeneity of the NP solution. Size, morphology and distribution of TiO2 nanoparticles were examined by TEM images; however, in order to have a quick measurement of size distribution of nanoparticles, DLS technique was used as it is a fast, on-line and in situ method. We found out that size distribution results from DLS are usually consistent with the ones obtained from TEM, although TEM provides more definitive answers. We have used Malvern Nano ZS90 for DLS and zeta potential measurements and JEOL JSM 6400F for electron microscopy. Xu et al.[32] have used DLS to determine size and zeta potential of the polymer nanoparticles, Gao et al. [33], Calzolai et al. [34] also used the DLS system to determine the diameter of the gold nanoparticles. Other analytical techniques such as (FTIR) spectroscopy (The PerkinElmer Spectrum 100 Series FT-IR spectrometer) are also used to study the adsorption of organic species on the TiO2 nanoparticles. FTIR spectra were measured at room temperature with the spectrometer using the KBr Pellet technique [1]. Samples were lyophilized, gently mixed with 300 mg of KBr powder and compressed into discs at a pressure of 40 MPa for 5 min, FTIR spectrum was recorded in the spectral range of 400–4000 cm−1 to know the chemical bonding of the produced nanoparticles.
2.5 2 1.5 1 0.5 0 200
300
400
500
600
700
800
Wavelength (nm) 4 2nd-step-180 µJ 3.5
2nd-step-120 µJ 2nd-step-60 µJ
3
Absorption (a.u)
these particles undergo a melting transition due to the thermal changes. Besner et al. [30] have also described this method to reduce the gold nanoparticle size using a femtosecond laser in deionized water. Numerous studies have been carried out on the fragmentation of the metal oxide nanoparticles in nanosecond, picoseconds and femtosecond regimes [22,28]. This paper reports results on the effect of the laser pulse energy on size and stability of nanoparticles through fragmentation, and/ or size reduction of the agglomerates. Results of chemical bonding of TiO2 nanoparticle by FTIR analysis are also presented.
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2.5
2
1.5
1
0.5
0 200
300
400
500
600
700
800
Wavelength (nm) Fig. 2. Absorption spectrum of TiO2 nanoparticles produced in aqueous solution of SDS using two steps process by (a) 1st-step—800 nm and (b) 2nd-step—400 nm.
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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 that leads to a quick cooling of the plume region and hence to the formation of titanium clusters. Finally, the plasma is extinguished and formed titanium clusters encounter and interact with the solvent and surfactant molecules in the surrounding solution inducing some chemical reactions [35]. The process and chemical reaction can be described as below. Ti(clusters) þ4H2O-Ti(OH)42H2
(1)
Ti(OH)4-TiO2 þ2H2O
(2)
Fig. 2a shows an absorption spectrum of TiO2 from the first step which consists of a single broad intense cut-off absorption wavelength around 510 nm due to the charge-transfer from the valence band (mainly formed by 2p orbitals of the oxide anions) to the conduction band (mainly formed by 3d t2 g orbitals of the Ti4 þ 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 and 587 nm for energies 180, 120 and 60 mJ, respectively. This shows a blue shift from that of the TiO2 NPs produced at the first step
which is 510 nm in especially for 180 mJ energy. However, for 120 and 60 mJ pulse energy there are also a shift towards red in spite of the decrease in the size of TiO2 nanoparticles. This is in contrast to the expected shift to the blue region. We have assumed that the reason of this shift towards red region is related with the agglomeration of the TiO2 nanoparticles [37]. Agglomeration might take place in time between the first and second steps, hence the time required between two processes may not be sufficient enough for a homogeneous fragmentation leading to smaller nanoparticles. The optical absorption spectra of the TiO2 nanoparticles, which were measured after each ablation and fragmentation process is shown in Fig. 2b. On the other hand the shift towards the higher energy (lower wavelength) that appears in the spectrum indicates a reduction in particle sizes. Similar results also have been observed by Akman et al. [22] with gold and silver NPs. This reduction in particle size with increasing energy in the second step is due to the interaction of generated particles from the metal plate with the laser beam. The blue shift is in agreement with the fragmentation of larger particles as also reported in [38]. Besner et al. [30] have suggested three mechanisms which may explain the absorption of radiation by nanoparticles; (i) a direct absorption of the laser radiation, (ii) absorption of energy of the
Fig. 3. TEM and size distribution o TiO2 nanoparticles produced using ultrashort Ti:sapphire laser beam in aqueous solution of SDS: (a) 1st-step—0.8 mJ, (b) 2nd-step—60 mJ, (c) 2nd-step—120 mJ and (d) 2nd-step—180 mJ.
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white continuum and (iii) interband resonant multiphoton absorption. With the 2nd harmonic of the Ti:Sapphire laser wavelength used in the second step of our experiment, it is expected that direct absorption mechanism would be dominant for the size reduction of TiO2 nanoparticles since the laser beam photon energy is in the absorption band range of TiO2 nanoparticles. A number of studies have concentrated on decreasing the size of the particles by using various nanosecond and femtosecond lasers running at different wavelengths as smaller scale nanoparticles are important for lots of applications such as sensing technologies [38,39]. More detailed information on size properties of nanoparticles was obtained from TEM images. Fig. 3a shows a TEM micrograph and size distribution of TiO2 nanoparticles produced in the first step where the scale bar is the 200 nm. Fig. 3b, c and d shows particle size distributions for second step energies of 60 μJ, 120 μJ and 180 μJ with respected particle distributions. It can be seen that particle size distribution becomes narrower and average particle size becomes smaller. The size distribution shown in Fig. 3a with average particle size of 180 nm is in fact appears largely due to effects of nanoparticle agglomeration. The large size variation is expected especially in the first step as when the laser beam first ablated the surface, it is possible
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to have nanostructures on the larger side of the nanoscale. However, the incoming laser pulse can be absorbed or scattered by the dispersed nanoparticles in the liquid. This event causes the shielding effect which reduces the ablation efficiency with time [22]. However, it is anticipated that if higher laser energies as well as further steps are applied, it is possible to obtain even a smaller size range. On the other hand, in the case of 400 nm laser wavelength, another interaction occurs between TiO2 NPs and the laser beam and as a result of this interaction large particles will fragment and become smaller. By increasing the energy of laser, the efficiency of fragmentation will increase, therefore obtained particle sizes with 180 μJ will decrease from 180 nm to 110 nm as shown in Fig. 3d while the diameter of these NPs in the first step is 180 nm. This is because further fragmentation of the agglomerates takes place via a direct absorption of laser beam. By using and controlling the fragmentation process through interaction with the laser radiation would end in a smaller, virtually monodispersive nanoparticle size distribution. Experiments reported here showed that final size distribution was almost independent of the initial size and shape of nanoparticles, but depended largely on radiation parameters. Final size of the nanoparticles is determined by the chemical interaction of the
Fig. 4. FTIR spectra of TiO2 nanoparticles in 1st-step—800 nm Ti:Sapphire laser beam.
Fig. 5. Zeta potential distributions of TiO2 nanoparticles illuminated by 400 nm and 800 nm wavelength for different re-irradiation energy pulses (a) 60 mJ (b) 120 mJ (c) 180 μJ (d) 0, 8 mJ at the end of the first step.
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fragmented species in the solution [30]. Fig. 4 shows FTIR spectrum of TiO2 nanoparticles as FTIR was found to be very useful to understand bonding between Ti–O atoms or molecules. The FTIR peaks at (500–700 cm−1) is the characteristic vibrations of Ti–O corresponding to asymmetric, symmetric and anamorphic stretches, while the broad intense band below 1200 cm−1 is due to Ti–O–Ti vibrations [36,40]. This figure shows peaks corresponding to stretching vibrations of the O–H and bending vibrations of the adsorbed water molecules around 3350–3450 cm−1 and 1620– 1635 cm−1 respectively. Other strong absorption bands at 1522 and 1566 cm−1 are due to aromatic C–C stretching. Multiple bands between 1200 and 1000 cm−1 are the result of phenolic C–O stretching and aromatic C–H in-plane bending. Measurements of zeta potential was also carried out in order to study the stability of nanoparticles as this is extremely important for many applications [41]. The criteria of stability of NPs can be evaluated when the values of zeta potential ranges from higher than þ30 mV to lower than −30 mV [22]. Fig. 5a and d show zeta potential distributions of TiO2 nanoparticles illuminated by 800 nm in the first step and 400 nm wavelength in the second step for different values of energy measured zeta potential which varies 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 nanoparticles by focusing an ultrafast Ti:Sapphire laser onto a Ti target in liquid media. After the first stage of fragmentation of TiO2 nanoparticles induced by irradiation of the femtosecond laser pulse onto nanoparticles, the solution containing TiO2 nanoparticles has been resubjected to second harmonic, 400 nm wavelength of the laser at different pulse energies of 180, 120 and 60 μJ in the second step. When 180 μJ was used, size of the nanoparticles decreased from 180 nm to 100 nm. The TiO2 nanoparticles are size-reduced by the Coulomb explosion of the highly charged particles therefore the second process with the laser resolves the agglomeration problem and provides homogeneous nanoparticle distributions in liquids within a matter of 45 min. The optical absorption of TiO2 nanoparticle multiphase system was measured in order to investigate influence of absorbance of irradiation laser by ablated particles, a shift in the wavelength to blue takes place for energies of 180 μJ and 120 μJ with absorption wavelength of 410 and 430 nm respectively. However, at the pulse energy of 60 μJ, the value of absorption wavelength is 480 nm and there is a shift this time towards red when compared to its value of 450 nm of TiO2 NPs produced in the first step. We have assumed that the reason of shift toward red region is related to the agglomeration of the TiO2 nanoparticles; therefore the time required between two processes may not be sufficient enough for a homogeneous fragmentation towards smaller nanoparticles. The size distribution of TiO2 nanoparticles can be controlled by using 400 nm laser pulses during further fragmentation. This leads to a narrow size distribution as well as stable nanoparticle system. We are of the opinion that it is possible to obtain more stable and smaller nanoparticles by using higher energies or possibly changing the irradiation time [26,42]. The Zeta potential varies between (−49.5) mV and (−51.8) mV for 400 nm second step irradiation at different energies indicating a stable and low degree agglomeration nanoparticles in the solution. These results show that the agglomeration and nanoparticle sizes can be reduced by the second application of the laser beam.
This convenient synthesis strategy can be applied as a general approach to producing TiO2 NPs which have attracted a significant interest from materials scientists and physicists due to their special properties. TiO2 NPs have gained a great importance in several technological applications such as photocatalysis, sensors, dye-sensitized solar cells, biological and memory devices [7,43].
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