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Laser induced changes in Pt and Tl metal nanoparticles
23 November 2001
Chemical Physics Letters 349 (2001) 19±24 www.elsevier.com/locate/cplett
Laser induced changes in Pt and Tl metal nanoparticles Sudhir Kapoor *, Dipak K. Palit, Tulsi Mukherjee Radiation Chemistry and Chemical Dynamics Division, Chemistry Group, Bhabha Atomic Research Centre, Mumbai 400 085, India Received 17 September 2001; in ®nal form 17 September 2001
Abstract Eect of laser pulse irradiation on platinum and thallium metal nanoparticles was studied. Laser pulse excitation of d-electrons in Pt nanoparticles leads to the trapping of electrons at the interface without much change in the steadystate absorption spectrum. TEM results show that the particles undergo a shape transformation and fragmentation on laser pulse irradiation. Studies carried out on Tl particles after laser pulse excitation shows an initial blue shift in the absorption band indicating decrease in the particle size. However, longer irradiation leads to dissolution of the particles. The results are discussed on the basis of energy transfer from the electrons to the surface atoms. Ó 2001 Published by Elsevier Science B.V.
1. Introduction Synthesis of nanoparticles is an interesting ®eld in solid state chemistry [1]. Recently, much interest has been shown in ultra®ne metal particles because they have unique properties dierent from those in bulk metals [1±10]. New experimental approaches have been initiated for preparation of metal nanopartices. In recent years, a new approach, namely, use of ultrashort laser pulses has been exploited to understand the size dependent changes in the electronic and optical properties of metal nanoclusters by probing the electronic relaxation time after femtosecond laser pulse excitation. It has been established that on laser pulse excitation the size
*
Corresponding author. Fax: +91-22-5505151. E-mail address: [email protected] (S. Kapoor).
and shape of the nanoparticles change [11±14]. In this work we have extended our earlier study [14] to Pt and Tl metal nanoparticles. The results are compared with those obtained earlier on Cu and Cd nanoparticles. 2. Experimental Materials: Tl2 SO4 (Sigma) and gelatin (BDH) were used as received. Solutions were prepared with nano-pure water (conductivity 60:06 ls cm 1 ). Spectrophotometric measurements were carried out on a Hitachi-330 UV±VIS spectrophotometer. 2.1. Preparation of metal nanoparticles using c irradiation method The Tl and Pt metal particles were prepared using the radiolytic method [15,16] in the presence
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 1 9 5 - 2
20
S. Kapoor et al. / Chemical Physics Letters 349 (2001) 19±24
of a stabilizer. In brief, gelatin was allowed to swell in water for 15 min at an ambient temperature. The solution became clear by warming to 40±50 °C for 2±3 min in a water bath with continuous stirring. The c radiolysis experiments were done using 60 Co source with a dose rate of 20 Gy min 1 [17]. Samples for transmission electron microscopy (TEM) were prepared by putting a drop of the colloidal solution prepared in acetone on a copper grid coated with a thin amorphous carbon ®lm. Samples were dried and kept under vacuum in a desiccator before putting them in a specimen holder. The TEM characterization was carried out using a JEOL JEM-2000FX electron microscope. Particle sizes were measured from the TEM micrographs and found to be in the range 10±30 nm. The third (355 nm, 8 mJ) or second (532 nm, 12 mJ) harmonic output pulses of 35 ps duration from an active passive mode-locked Nd:YAG laser (Continuum model 501C-10) were used for exciting the sample solutions. Continuum probe pulses in 400±920 nm wavelength region were generated by focusing the residual fundamental into a H2 O±D2 O liquid mixture (50:50) of 10 cm path length. The probe pulses were delayed with respect to the pump pulses using a 1 m long linear motion translation stage and the transient absorption at dierent delay times up to 6 ns were recorded by a dual diode-array-based multichannel analyzer (Spectroscopy Instruments, Germany) interfaced with an IBMPC. For the laser irradiation experiment 4 cm3 of the solution was taken in a rectangular quartz cell of dimension 1 cm 1 cm 4 cm. The zero delay position of the linear motion translation stage was adjusted in such a way that the probe pulses reach the sample just after the end of the pump pulses. The laser pulse energy (10 mJ pulse 1 ) was measured by a Gentec ED-200 energy meter placed in front of the cell. The optical density of the samples used in the transient absorption measurements was between 0.5 and 1.0 per 1 cm optical path length at the excitation wavelength. 3. Results and discussion It is known that in the case of particles which are smaller than the wavelength of light, the ab-
sorption coecient can be obtained from Mie theory [18]. Surface plasmon absorption maximum for Tl comes in the same region as predicted [18± 20] at about 260 nm. However, for Pt it appears at a shorter wavelength [21] than predicted. 3.1. Transient absorption in Pt nanoparticles Sol of Pt nanoparticles was prepared [16,21] by c irradiation of N2 -bubbled aqueous solution containing 1:0 10 4 mol dm 3 K2 PtCl4 , 0.1 mol dm 3 methanol and 0.1% (wt/v) gelatin at pH 7. The particles showed a steady increase in the optical density below 500 nm as the wavelength was decreased up to 220 nm. The spectrum was very similar to that reported in the literature [16,21]. The laser pulse excitation was carried out at wavelength k 355 nm which is far away from the plasmon absorption maximum of Pt nanoparticles [21]. Fig. 1 shows the transient absorption spectrum obtained in aqueous
Fig. 1. Time-resolved absorption spectrum of Pt sol in water measured at 0 ps and 1.2 ns after excitation with 355 nm laser pulse. Inset: steady-state absorption spectrum of Pt sol before and after laser pulse irradiation.
S. Kapoor et al. / Chemical Physics Letters 349 (2001) 19±24
colloidal Pt solution on excitation at 355 nm by a ps laser pulse. Immediately after the photo-excitation, a weak transient absorption in the wavelength region 450±900 nm could be observed. The visible absorption appeared instantaneously, that is, just after the ps pulse and the positive absorption was seen to decrease with delay in the probe pulse. The visible absorption of the transient obtained is similar to that observed in the case of silver nanoparticles [22]. It has been suggested [22] that laser pulse excitation leads to the ejection of electrons which shows absorption spectrum with maximum at around 700 nm. Thus, in the present case also the observed transient can be assigned to electrons trapped at the surface of metals. The absorption of the platinum particles is mainly due to inter-band transitions as its d band lies close to the sp band. Therefore, the observed transient absorption in Pt nanoparticles can be attributed to the transition of electrons from the d band which gets trapped at the solid± liquid interface. The interesting feature is that no change in the ground state absorption spectrum was observed after the laser pulse excitation (Fig. 1, inset). To con®rm this further we have carried out TEM studies. Fig. 2 shows the TEM for Pt particles before and after laser pulse irradiation. It can be seen that the particles have undergone some fragmentation after laser pulse irradiation. The statistics measurement of the cluster size before and after laser pulse irradiation was calculated to be 25 and 15 nm, respectively. Thus, the observed results can be explained as the excitation with 355 nm ps pulses leads to the intense heating of the Pt lattice [14,23,24]. Subsequently, the heat transfer to the surrounding medium leads to the formation of smaller particles of Pt. For phenanthroline stabilized Pt nanoparticles [25] it has been shown that the absorption of 3 nm particles is more than the 35 nm particles in the wavelength region 300±800 nm. Thus, it appears that in the present case on laser pulse irradiation the average size of the particles has not reduced signi®cantly at which scattering of the electrons from the surface takes place [26]. This is evident from TEM ®gures also. This could be the reason for not observing any change in the steady-state absorption spectrum of Pt nanoparticles
21
Fig. 2. TEM images of Pt nanoparticles before (a) and after (b) laser pulse irradiation.
before and after laser pulse irradiation. Experiments were also done at dierent laser ¯uences 10 and 8 mJ cm 2 , respectively. In all the experiments roughly similar phenomenon was observed. Thus, it is not possible to comment on the threshold of the laser power for the fragmentation of the particles.
22
S. Kapoor et al. / Chemical Physics Letters 349 (2001) 19±24
3.2. Laser induced changes in Tl nanoparticles Thallium nanoparticles were prepared by irradiating N2 -bubbled aqueous solution of Tl2 SO4 containing 2-propanol and gelatin. The irradiation was done using 60 Co c radiolysis. The solution was irradiated to a dose of 1.68 kGy so that larger Tl nanoparticles were produced. The spectrum of the sol so obtained agreed well with the spectra of the Tl sol reported in the literature [15]. It was observed that excitation using 355 nm laser pulse caused bleaching in the optical density of the solution. The photolytic process was irreversible. This showed that some permanent bleaching of the particles took place on the laser pulse excitation. To monitor these permanent changes, steady-state absorption experiments were carried out. Sol of Tl nanoparticles was irradiated with laser pulses having ¯uence of 15 mJ cm 2 (unfocussed beam) at a repetition rate of 10 Hz for various duration of times. The solution was refreshed after each pulse. The absorption spectra were recorded before and after the irradiation. Typical results obtained on the pulsed laser irradiation are shown in Fig. 3. It can be seen from Fig. 3 that as the irradiation time increases the absorption due to large Tl particles decreases and the spectrum shifts towards blue showing the formation of smaller Tl particles. However, on irradiating the sol for longer times it was observed that the path of the laser pulse becomes colourless indicating the probability of dissolution of the particles. Experiments were also done at dierent laser ¯uences, that is, at 10, 8 and 6 mJ=cm2 , respectively. In all the cases ®rst shift of the absorption band towards lower wavelength followed by dissolution was observed. To determine whether the laser induced size reduction of the Tl particles is caused by the heating of the particles an attempt has been made to estimate the temperature of the Tl particles. The temperature of the Tl particles is estimated from the measured absorbed laser energy by the Tl particles. The absorbed laser energy by the Tl particles per unit mass of Tl atom and per pulse, Q J g 1 pulse 1 is calculated by Q E=RqV ;
1
Fig. 3. Absorption spectra of colloidal Tl particles after laser pulse irradiation for various duration of times. The Tl sol was prepared by c irradiating (dose 1.68 kGy) a N2 -bubbled aqueous solution containing 1:0 10 2 mol dm 3 Tl2 SO4 , 0.1% (wt/v) gelatin and 1.0 mol dm 3 2-propanol.
where E is the laser energy absorbed by the solution of the Tl particles per unit time (J s 1 ), R is the repetition rate of the pulsed laser (10 Hz), q is the mass concentration of Tl (g cm 3 ) and V is the irradiated volume of the solution (cm3 ). The temperature T (K), of the Tl particles can be estimated on the basis of the absorbed laser energy by Eqs. (2) and (3) for higher temperature than the boiling point and the melting point, respectively, using bulk physical constants. The initial temperature was considered to be at room temperature (293 K): T Q
DHmelt
T Q
DHmelt =Cp 293;
DHvap =Cp 293;
2 3
where DHmelt is heat of melting 4:142 kJ mol 1 , DHvap is heat of vaporization (164.08 kJ mol 1 ) and Cp is the speci®c heat. It is assumed that the heat loss will be negligible. It is suggested that the melting point of the particles decreases
S. Kapoor et al. / Chemical Physics Letters 349 (2001) 19±24
drastically only when the particle size is less than 5 nm [23]. Therefore, calculation of temperature with bulk physical constants may be justi®ed. Using Eqs. (2) and (3) the temperature of the Tl particles calculated after laser pulse irradiation for all laser ¯uences used was found to be higher than boiling point. Thus, the change in size of the Tl particles can be related qualitatively to the temperature. It is generally believed that metals do not possess any direct signi®cant photochemistry as the charge carriers generated by absorption of light rapidly thermallize. It has been reported that photochemical reactions with substantial quantum yield can be initiated by light absorption in silver nanoparticles [27]. As mentioned above it has been shown that on exposure to laser pulses, fragmentation of larger metal nanoparticles takes place, leading to the formation of smaller nanoparticles [12,14,24]. Based on these arguments the above observations can be rationalized as follows. On laser pulse excitation, bigger Tl clusters get excited and hence become charged. The accumulation of electrons at the surface of the particles leads to the charging of the nanoparticles. Subsequently, the repulsion of the accumulated charges within the nanoparticles leads to fragmentation into smaller clusters. The increase in temperature of the particles on laser pulse excitation also contributes to the fragmentation of the particles [14]. It is known that energetically unstable Au nanoparticles undergo thermal decomposition as well as photo-thermal melting and fragmentation [24,28] while Ag nanoparticles are known to undergo photo-fragmentation [22]. We have observed earlier that Cd nanoparticles undergo fragmentation [14] on laser pulse excitation. In the present study it has been observed that Tl nanoparticles initially undergo fragmentation which accompanied by dissolution. This is in spite of the fact that both Cd and Tl in nanometer regime have high electronegative potential and hence energetically unstable [19]. Thus, it appears that in addition to the concept that energetically unstable particles undergo more thermal decomposition some other phenomenon may also play a signi®cant role.
23
4. Conclusion In the case of Pt nanoparticles the excitation at wavelength far away from the surface plasmon absorption band leads to the trapping of electrons at the interface without any signi®cant dissolution, however, the size of the particles decreased marginally. Thallium nanoparticles exhibit blue shift with laser pulse irradiation indicating fragmentation of the nanoparticles. However, on irradiation for long period dissolution of the particles takes place. Acknowledgements The authors are thankful to Dr. J.P. Mittal, Director, Chemistry and Isotope Group, for encouragement. References [1] G.A. Ozin, Adv. Mater. 4 (1992) 612. [2] D.S. Wang, M. Kerker, H. Chew, Appl. Opt. 19 (1990) 2135. [3] L. Genzel, T.P. Martin, Surf. Sci. 34 (1973) 33. [4] B.G. Ershov, A. Henglein, J. Phys. Chem. 97 (1993) 3434. [5] A. Henglein, P. Mulvaney, A. Holtzworth, T. Sosebee, A. Fojtik, Ber. Bunsenges. Phys. Chem. 96 (1992) 754. [6] M. Gutierrez, A. Henglein, J. Phys. Chem. 97 (1993) 11 368. [7] P. Mulvaney, T. Linnert, A. Henglein, J. Phys. Chem. 95 (1991) 7843. [8] Y. Yonezawa, T. Sato, M. Ohno, H. Hada, J. Chem. Soc. Faraday Trans. 1 (83) (1987) 1559. [9] Y. Yonezawa, T. Sato, S. Kuroda, J. Chem Soc. Faraday Trans. 1 (87) (1991) 1905. [10] T. Sato, N. Maeda, H. Ohkoshi, Y. Yonezawa, Bull. Chem. Soc. Jpn. 67 (1994) 3165. [11] T.S. Ahmadi, S.L. Logunov, M.A. El-Sayed, J. Phys. Chem. 100 (1996) 8053. [12] S.L. Logunov, T.S. Ahmadi, M.A. El-Sayed, J.T. Khoury, R.L. Whetten, J. Phys. Chem. B 101 (1997) 3713. [13] T.W. Roberti, B.A. Smith, J.Z. Zhang, J. Chem. Phys. 102 (1995) 3860. [14] S. Kapoor, D.K. Palit, Mat. Res. Bull. 35 (2000) 2071. [15] (a) S. Kapoor, C. Gopinathan, Radiat. Phys. Chem. 49 (1997) 51; (b) J. Butler, A. Henglein, Radiat. Phys. Chem. 15 (1980) 603; (c) B.G. Ershov, E. Janata, A. Henglein, J. Phys. Chem. 98 (1994) 10891.
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[16] A.D. Belapurkar, S. Kapoor, S.K. Kulshreshtha, J.P. Mittal, Material Res. Bull. 36 (2001) 145. [17] T. Mukherjee, in: S. Ahmad (Ed.), Atomic, Molecular and Cluster Physics, Narosa, New Delhi, 1997 (Chapter 27). [18] G. Mie, Ann. Phys. 25 (1908) 377. [19] A. Henglein, J. Phys. Chem. 97 (1993) 5457. [20] J.A. Creighton, D.G. Eadon, J. Chem. Soc. Faraday Trans. 87 (1991) 3881. [21] A. Henglein, B.G. Ershov, M. Malow, J. Phys. Chem. 99 (1995) 14129. [22] P.V. Kamat, M. Flumiani, G.V. Hartland, J. Phys. Chem. B 102 (1998) 3123.
[23] A. Takami, H. Kurita, S. Koda, J. Phys. Chem. B 103 (1999) 1226. [24] S. Link, C. Burda, M.B. Mohamed, B. Nikoobakht, M.A. El-Sayed, J. Phys. Chem. A 103 (1999) 1165. [25] B.A. Smith, J.Z. Zhang, U. Giebel, G. Schmid, Chem. Phys. Lett. 270 (1997) 139. [26] S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 8410. [27] T. Linnert, P. Mulvaney, A. Henglein, Ber. Bunsenges. Phys. Chem. 95 (1991) 838. [28] M.B. Mohamed, K.Z. Ismail, S. Link, M.A. El-Sayed, J. Phys. Chem. B 102 (1998) 9370.
Chemical Physics Letters 349 (2001) 19±24 www.elsevier.com/locate/cplett
Laser induced changes in Pt and Tl metal nanoparticles Sudhir Kapoor *, Dipak K. Palit, Tulsi Mukherjee Radiation Chemistry and Chemical Dynamics Division, Chemistry Group, Bhabha Atomic Research Centre, Mumbai 400 085, India Received 17 September 2001; in ®nal form 17 September 2001
Abstract Eect of laser pulse irradiation on platinum and thallium metal nanoparticles was studied. Laser pulse excitation of d-electrons in Pt nanoparticles leads to the trapping of electrons at the interface without much change in the steadystate absorption spectrum. TEM results show that the particles undergo a shape transformation and fragmentation on laser pulse irradiation. Studies carried out on Tl particles after laser pulse excitation shows an initial blue shift in the absorption band indicating decrease in the particle size. However, longer irradiation leads to dissolution of the particles. The results are discussed on the basis of energy transfer from the electrons to the surface atoms. Ó 2001 Published by Elsevier Science B.V.
1. Introduction Synthesis of nanoparticles is an interesting ®eld in solid state chemistry [1]. Recently, much interest has been shown in ultra®ne metal particles because they have unique properties dierent from those in bulk metals [1±10]. New experimental approaches have been initiated for preparation of metal nanopartices. In recent years, a new approach, namely, use of ultrashort laser pulses has been exploited to understand the size dependent changes in the electronic and optical properties of metal nanoclusters by probing the electronic relaxation time after femtosecond laser pulse excitation. It has been established that on laser pulse excitation the size
*
Corresponding author. Fax: +91-22-5505151. E-mail address: [email protected] (S. Kapoor).
and shape of the nanoparticles change [11±14]. In this work we have extended our earlier study [14] to Pt and Tl metal nanoparticles. The results are compared with those obtained earlier on Cu and Cd nanoparticles. 2. Experimental Materials: Tl2 SO4 (Sigma) and gelatin (BDH) were used as received. Solutions were prepared with nano-pure water (conductivity 60:06 ls cm 1 ). Spectrophotometric measurements were carried out on a Hitachi-330 UV±VIS spectrophotometer. 2.1. Preparation of metal nanoparticles using c irradiation method The Tl and Pt metal particles were prepared using the radiolytic method [15,16] in the presence
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 1 9 5 - 2
20
S. Kapoor et al. / Chemical Physics Letters 349 (2001) 19±24
of a stabilizer. In brief, gelatin was allowed to swell in water for 15 min at an ambient temperature. The solution became clear by warming to 40±50 °C for 2±3 min in a water bath with continuous stirring. The c radiolysis experiments were done using 60 Co source with a dose rate of 20 Gy min 1 [17]. Samples for transmission electron microscopy (TEM) were prepared by putting a drop of the colloidal solution prepared in acetone on a copper grid coated with a thin amorphous carbon ®lm. Samples were dried and kept under vacuum in a desiccator before putting them in a specimen holder. The TEM characterization was carried out using a JEOL JEM-2000FX electron microscope. Particle sizes were measured from the TEM micrographs and found to be in the range 10±30 nm. The third (355 nm, 8 mJ) or second (532 nm, 12 mJ) harmonic output pulses of 35 ps duration from an active passive mode-locked Nd:YAG laser (Continuum model 501C-10) were used for exciting the sample solutions. Continuum probe pulses in 400±920 nm wavelength region were generated by focusing the residual fundamental into a H2 O±D2 O liquid mixture (50:50) of 10 cm path length. The probe pulses were delayed with respect to the pump pulses using a 1 m long linear motion translation stage and the transient absorption at dierent delay times up to 6 ns were recorded by a dual diode-array-based multichannel analyzer (Spectroscopy Instruments, Germany) interfaced with an IBMPC. For the laser irradiation experiment 4 cm3 of the solution was taken in a rectangular quartz cell of dimension 1 cm 1 cm 4 cm. The zero delay position of the linear motion translation stage was adjusted in such a way that the probe pulses reach the sample just after the end of the pump pulses. The laser pulse energy (10 mJ pulse 1 ) was measured by a Gentec ED-200 energy meter placed in front of the cell. The optical density of the samples used in the transient absorption measurements was between 0.5 and 1.0 per 1 cm optical path length at the excitation wavelength. 3. Results and discussion It is known that in the case of particles which are smaller than the wavelength of light, the ab-
sorption coecient can be obtained from Mie theory [18]. Surface plasmon absorption maximum for Tl comes in the same region as predicted [18± 20] at about 260 nm. However, for Pt it appears at a shorter wavelength [21] than predicted. 3.1. Transient absorption in Pt nanoparticles Sol of Pt nanoparticles was prepared [16,21] by c irradiation of N2 -bubbled aqueous solution containing 1:0 10 4 mol dm 3 K2 PtCl4 , 0.1 mol dm 3 methanol and 0.1% (wt/v) gelatin at pH 7. The particles showed a steady increase in the optical density below 500 nm as the wavelength was decreased up to 220 nm. The spectrum was very similar to that reported in the literature [16,21]. The laser pulse excitation was carried out at wavelength k 355 nm which is far away from the plasmon absorption maximum of Pt nanoparticles [21]. Fig. 1 shows the transient absorption spectrum obtained in aqueous
Fig. 1. Time-resolved absorption spectrum of Pt sol in water measured at 0 ps and 1.2 ns after excitation with 355 nm laser pulse. Inset: steady-state absorption spectrum of Pt sol before and after laser pulse irradiation.
S. Kapoor et al. / Chemical Physics Letters 349 (2001) 19±24
colloidal Pt solution on excitation at 355 nm by a ps laser pulse. Immediately after the photo-excitation, a weak transient absorption in the wavelength region 450±900 nm could be observed. The visible absorption appeared instantaneously, that is, just after the ps pulse and the positive absorption was seen to decrease with delay in the probe pulse. The visible absorption of the transient obtained is similar to that observed in the case of silver nanoparticles [22]. It has been suggested [22] that laser pulse excitation leads to the ejection of electrons which shows absorption spectrum with maximum at around 700 nm. Thus, in the present case also the observed transient can be assigned to electrons trapped at the surface of metals. The absorption of the platinum particles is mainly due to inter-band transitions as its d band lies close to the sp band. Therefore, the observed transient absorption in Pt nanoparticles can be attributed to the transition of electrons from the d band which gets trapped at the solid± liquid interface. The interesting feature is that no change in the ground state absorption spectrum was observed after the laser pulse excitation (Fig. 1, inset). To con®rm this further we have carried out TEM studies. Fig. 2 shows the TEM for Pt particles before and after laser pulse irradiation. It can be seen that the particles have undergone some fragmentation after laser pulse irradiation. The statistics measurement of the cluster size before and after laser pulse irradiation was calculated to be 25 and 15 nm, respectively. Thus, the observed results can be explained as the excitation with 355 nm ps pulses leads to the intense heating of the Pt lattice [14,23,24]. Subsequently, the heat transfer to the surrounding medium leads to the formation of smaller particles of Pt. For phenanthroline stabilized Pt nanoparticles [25] it has been shown that the absorption of 3 nm particles is more than the 35 nm particles in the wavelength region 300±800 nm. Thus, it appears that in the present case on laser pulse irradiation the average size of the particles has not reduced signi®cantly at which scattering of the electrons from the surface takes place [26]. This is evident from TEM ®gures also. This could be the reason for not observing any change in the steady-state absorption spectrum of Pt nanoparticles
21
Fig. 2. TEM images of Pt nanoparticles before (a) and after (b) laser pulse irradiation.
before and after laser pulse irradiation. Experiments were also done at dierent laser ¯uences 10 and 8 mJ cm 2 , respectively. In all the experiments roughly similar phenomenon was observed. Thus, it is not possible to comment on the threshold of the laser power for the fragmentation of the particles.
22
S. Kapoor et al. / Chemical Physics Letters 349 (2001) 19±24
3.2. Laser induced changes in Tl nanoparticles Thallium nanoparticles were prepared by irradiating N2 -bubbled aqueous solution of Tl2 SO4 containing 2-propanol and gelatin. The irradiation was done using 60 Co c radiolysis. The solution was irradiated to a dose of 1.68 kGy so that larger Tl nanoparticles were produced. The spectrum of the sol so obtained agreed well with the spectra of the Tl sol reported in the literature [15]. It was observed that excitation using 355 nm laser pulse caused bleaching in the optical density of the solution. The photolytic process was irreversible. This showed that some permanent bleaching of the particles took place on the laser pulse excitation. To monitor these permanent changes, steady-state absorption experiments were carried out. Sol of Tl nanoparticles was irradiated with laser pulses having ¯uence of 15 mJ cm 2 (unfocussed beam) at a repetition rate of 10 Hz for various duration of times. The solution was refreshed after each pulse. The absorption spectra were recorded before and after the irradiation. Typical results obtained on the pulsed laser irradiation are shown in Fig. 3. It can be seen from Fig. 3 that as the irradiation time increases the absorption due to large Tl particles decreases and the spectrum shifts towards blue showing the formation of smaller Tl particles. However, on irradiating the sol for longer times it was observed that the path of the laser pulse becomes colourless indicating the probability of dissolution of the particles. Experiments were also done at dierent laser ¯uences, that is, at 10, 8 and 6 mJ=cm2 , respectively. In all the cases ®rst shift of the absorption band towards lower wavelength followed by dissolution was observed. To determine whether the laser induced size reduction of the Tl particles is caused by the heating of the particles an attempt has been made to estimate the temperature of the Tl particles. The temperature of the Tl particles is estimated from the measured absorbed laser energy by the Tl particles. The absorbed laser energy by the Tl particles per unit mass of Tl atom and per pulse, Q J g 1 pulse 1 is calculated by Q E=RqV ;
1
Fig. 3. Absorption spectra of colloidal Tl particles after laser pulse irradiation for various duration of times. The Tl sol was prepared by c irradiating (dose 1.68 kGy) a N2 -bubbled aqueous solution containing 1:0 10 2 mol dm 3 Tl2 SO4 , 0.1% (wt/v) gelatin and 1.0 mol dm 3 2-propanol.
where E is the laser energy absorbed by the solution of the Tl particles per unit time (J s 1 ), R is the repetition rate of the pulsed laser (10 Hz), q is the mass concentration of Tl (g cm 3 ) and V is the irradiated volume of the solution (cm3 ). The temperature T (K), of the Tl particles can be estimated on the basis of the absorbed laser energy by Eqs. (2) and (3) for higher temperature than the boiling point and the melting point, respectively, using bulk physical constants. The initial temperature was considered to be at room temperature (293 K): T Q
DHmelt
T Q
DHmelt =Cp 293;
DHvap =Cp 293;
2 3
where DHmelt is heat of melting 4:142 kJ mol 1 , DHvap is heat of vaporization (164.08 kJ mol 1 ) and Cp is the speci®c heat. It is assumed that the heat loss will be negligible. It is suggested that the melting point of the particles decreases
S. Kapoor et al. / Chemical Physics Letters 349 (2001) 19±24
drastically only when the particle size is less than 5 nm [23]. Therefore, calculation of temperature with bulk physical constants may be justi®ed. Using Eqs. (2) and (3) the temperature of the Tl particles calculated after laser pulse irradiation for all laser ¯uences used was found to be higher than boiling point. Thus, the change in size of the Tl particles can be related qualitatively to the temperature. It is generally believed that metals do not possess any direct signi®cant photochemistry as the charge carriers generated by absorption of light rapidly thermallize. It has been reported that photochemical reactions with substantial quantum yield can be initiated by light absorption in silver nanoparticles [27]. As mentioned above it has been shown that on exposure to laser pulses, fragmentation of larger metal nanoparticles takes place, leading to the formation of smaller nanoparticles [12,14,24]. Based on these arguments the above observations can be rationalized as follows. On laser pulse excitation, bigger Tl clusters get excited and hence become charged. The accumulation of electrons at the surface of the particles leads to the charging of the nanoparticles. Subsequently, the repulsion of the accumulated charges within the nanoparticles leads to fragmentation into smaller clusters. The increase in temperature of the particles on laser pulse excitation also contributes to the fragmentation of the particles [14]. It is known that energetically unstable Au nanoparticles undergo thermal decomposition as well as photo-thermal melting and fragmentation [24,28] while Ag nanoparticles are known to undergo photo-fragmentation [22]. We have observed earlier that Cd nanoparticles undergo fragmentation [14] on laser pulse excitation. In the present study it has been observed that Tl nanoparticles initially undergo fragmentation which accompanied by dissolution. This is in spite of the fact that both Cd and Tl in nanometer regime have high electronegative potential and hence energetically unstable [19]. Thus, it appears that in addition to the concept that energetically unstable particles undergo more thermal decomposition some other phenomenon may also play a signi®cant role.
23
4. Conclusion In the case of Pt nanoparticles the excitation at wavelength far away from the surface plasmon absorption band leads to the trapping of electrons at the interface without any signi®cant dissolution, however, the size of the particles decreased marginally. Thallium nanoparticles exhibit blue shift with laser pulse irradiation indicating fragmentation of the nanoparticles. However, on irradiation for long period dissolution of the particles takes place. Acknowledgements The authors are thankful to Dr. J.P. Mittal, Director, Chemistry and Isotope Group, for encouragement. References [1] G.A. Ozin, Adv. Mater. 4 (1992) 612. [2] D.S. Wang, M. Kerker, H. Chew, Appl. Opt. 19 (1990) 2135. [3] L. Genzel, T.P. Martin, Surf. Sci. 34 (1973) 33. [4] B.G. Ershov, A. Henglein, J. Phys. Chem. 97 (1993) 3434. [5] A. Henglein, P. Mulvaney, A. Holtzworth, T. Sosebee, A. Fojtik, Ber. Bunsenges. Phys. Chem. 96 (1992) 754. [6] M. Gutierrez, A. Henglein, J. Phys. Chem. 97 (1993) 11 368. [7] P. Mulvaney, T. Linnert, A. Henglein, J. Phys. Chem. 95 (1991) 7843. [8] Y. Yonezawa, T. Sato, M. Ohno, H. Hada, J. Chem. Soc. Faraday Trans. 1 (83) (1987) 1559. [9] Y. Yonezawa, T. Sato, S. Kuroda, J. Chem Soc. Faraday Trans. 1 (87) (1991) 1905. [10] T. Sato, N. Maeda, H. Ohkoshi, Y. Yonezawa, Bull. Chem. Soc. Jpn. 67 (1994) 3165. [11] T.S. Ahmadi, S.L. Logunov, M.A. El-Sayed, J. Phys. Chem. 100 (1996) 8053. [12] S.L. Logunov, T.S. Ahmadi, M.A. El-Sayed, J.T. Khoury, R.L. Whetten, J. Phys. Chem. B 101 (1997) 3713. [13] T.W. Roberti, B.A. Smith, J.Z. Zhang, J. Chem. Phys. 102 (1995) 3860. [14] S. Kapoor, D.K. Palit, Mat. Res. Bull. 35 (2000) 2071. [15] (a) S. Kapoor, C. Gopinathan, Radiat. Phys. Chem. 49 (1997) 51; (b) J. Butler, A. Henglein, Radiat. Phys. Chem. 15 (1980) 603; (c) B.G. Ershov, E. Janata, A. Henglein, J. Phys. Chem. 98 (1994) 10891.
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