Investigation of Dynamics of Radiolytic Formation of CdSe

ARTICLE pubs.acs.org/JPCA

Investigation of Dynamics of Radiolytic Formation of CdSe Nanoparticles in Aqueous Solutions Shalini Singh, M. C. Rath,* and S. K. Sarkar Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India ABSTRACT:

The formation of cadmium selenide, CdSe, nanoparticles in aqueous solutions containing equimolar ammoniated cadmium sulfate, [Cd(NH3)]4SO4 and sodium selenosulfate, Na2SeSO3 as the starting materials, has been investigated by electron pulse radiolysis coupled with kinetic spectrometry. The formation of CdSe nanoparticles was found to proceed through the generation of short-lived transient intermediate species having an absorption peak at 520 nm, which is formed only upon the reaction of hydrated electrons, eaqh with the precursor ions under deaerated conditions. The transient intermediate species decays with a weighted average rate constant, 1.2
107 s1. The transient intermediate species formed in the case of individual precursors did not match with the transients formed when both the precursors are taken together in the solutions under the present experimental conditions. The reaction rate constants between the precursor ions, [Cd(NH3)4]2+ and the transient intermediate species formed from [SeSO3]2 was 1.9
1010 M1 s1. Similarly, the reaction rate constants between the precursor ions, [SeSO3]2 and the transient intermediate species formed from [Cd(NH3)4]2+ was 5.5
1010 M1 s1. This clearly indicates that the formation of CdSe nanoparticles occurs through both reaction channels. However, the major reaction channel is through the reaction of eaqh with the [Cd(NH3)4]2+ ions (k = 3.1
10 10 M1 s1), as its rate constant is one order higher than that of the reaction of eaqh with the [SeSO3]2 ions (k = 2.3
109 M1 s1).

1. INTRODUCTION The synthesis of semiconductor nanoparticles and Quantum Dots (QDs), has received extensive research interest in recent years, due to their special optical and electronic properties.1 Owing to promising functions and properties displayed by colloid semiconductor nanocrystals, especially IIVI quantum dots and their related nanostructures, they have numerous potential applications in optoelectronics, quantum dot lasers, and biolabeling.2 Cadmium selenide (CdSe), which is a medium band gap material with an energy band gap of 1.75 eV at 300 K, is one of the most important IIVI semiconductors.35 This material has been widely used for optoelectronic devices. Among others, the synthesis of these nanomaterials through the radiation-chemical route has been increasingly in demand for its simplicity and high efficiency.69 Two important factors often control the morphology and growth of the nanomaterials, i.e., absorbed dose and dose rate of the irradiation.10,11 The solvent medium plays a very important r 2011 American Chemical Society

role in the synthesis, as in the radiation chemistry the high-energy radiation first interacts with the solvent to form the primary radicals or excited states of the solvent depending on their polarity and refractive index.12,13 These primary radicals are basically the reactive species for any further reaction to take place. In the case of aqueous solutions, the hydrated electrons, eaqh are the key primary radicals for the growth of metallic and semiconductor nanomaterials. In the case of polar nonaqueous solvents the solvated electrons esolvh induce the growth, whereas very little is known in the case of nonpolar nonaqueous solvents, due to the formation of excited states of the solvent molecules instead of any reactive radicals. Electrons being the best reducing agents, the synthesis of nanomaterials is often carried out by utilizing these reactive species. Several researchers have investigated Received: June 27, 2011 Revised: October 11, 2011 Published: October 13, 2011 13251

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The Journal of Physical Chemistry A the reaction pathways of the formation of either metallic or semiconductor nanomaterials through the radiation chemical reactions in aqueous and/or polar organic solvents.1416 The reaction pathways depend on the nature of the precursor ions/ molecules and the solvent used.1723 In this context, the reaction mechanism of the radiolytic formation of several metallic nanomaterials of Ag, Au, Pt, etc. and semiconductor nanomaterials of CdS, ZnS in aqueous solutions have been reported in the literature.2429 Nada M. Dimitrijevic has shown electron-transfer reactions on CdSe colloids by pulse radiolysis.30 Souici and coworkers have synthesized lead sulfide nanoparticles with diameter in the range of 945 nm by the radiolytic method in aqueous solutions containing Pb2+ and thiol. They have shown that the irradiation dose plays a crucial role to control the size of the nanoparticles and consequently to modify their optical properties.31 ZnS nanoparticles were also synthesized by γ-irradiation of aqueous solution containing Zn2+ and thiol (RSH).32 Monodispersed ultrasmall ZnS particles with 1.5 nm diameter, characterized by a band edge at 238 nm are produced at low doses. At higher doses, larger particles absorbing at λ > 260 nm are obtained. The early steps of formation and coalescence of ZnS particles have been investigated by pulse radiolysis studies. In all of these studies, the formation of these nanomaterials initiated through the reaction of the eaqh with the metallic cations and anions. Several groups, including ours, have reported the radiolytic synthesis and study of CdSe nanomaterials in aqueous and organic solvents.3339 Different groups use different precursors for their synthesis and explain their results accordingly.4043 In the past decade, a great deal of research has been done on controlling the size, shape, and crystal structure of CdSe nanocrystals because these parameters strongly affect their electrical and optical properties.2,4 However, the reaction mechanism leading to the formation of these semiconductor nanomaterials using the above-mentioned precursors is still to be understood. In this study, we have reported the reaction mechanism of the formation of CdSe nanoparticles in aqueous solutions containing the precursors, such as ammoniated cadmium sulfate ([Cd(NH3)]4SO4) and sodium selenosulfate (Na2SeSO3) in aqueous solutions using electron pulse radiolysis. The transient intermediate species involved in the reaction processes are investigated under various experimental conditions such as in the presence of OH• radical quencher, tertbutanol (i.e., reducing condition) and in the presence of eaqh quencher, a N2O saturated solution (i.e., oxidizing condition) for getting a clear understanding of the reaction mechanism and the formation of these nanoparticles.44

2. EXPERIMENTAL SECTION 2.1. Chemicals. The starting reagents, ammoniated CdSO4 ([Cd(NH3)4]SO4) and Na2SeSO3 solutions were freshly prepared from high purity chemicals obtained from Aldrich. Ammoniated CdSO4 solutions were prepared by adding desired quantity of 25% ammonia to freshly prepared CdSO4 solutions until clear transparent solutions appeared. Na2SeSO3 solution was prepared by refluxing the solution containing 1 g Se powder and 10 g Na2SO3 in 50 mL nanopure water at 70 °C for 7 h.45 Nanopure water from Milipore water purifier system was used for preparing the solutions. In one set of experiments 1 M tertbutanol (CH3 (CH3)2COH) was added to these solutions to quench the hydroxyl radicals, OH•• and allow only the hydrated electron, eaqh, reactions to occur during the radiolytic processes.

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Scheme 1. Structural Formula of (a) Ammoniated Cadmium and (b) Selenosulfate Ions

Equimolar solutions of both of the starting reagents were mixed together just before the radiolysis experiments. These mixed solutions (called reaction mixtures) as well as the individual precursor solutions were deaerated by purging with high purity N2 gas for carrying out the radiolytic studies. In another set of experiments, equimolar (0.5 and 10 mM each) aqueous solutions of ammoniated CdSO4 and Na2SeSO3 without containing tertbutanol were mixed together just before the radiolysis experiments. These reaction mixtures as well as the individual precursor solutions were saturated with N2O gas to quench the hydrated electrons, eaqh and allow the hydroxyl radicals, OH• reactions to occur during the radiolysis processes. The pH values of these solutions were between 9 to 10. 2.2. Pulse Radiolysis. The reaction mixtures were used in the pulse radiolysis experiments. Pulse radiolysis experiments were carried out with a 7 MeV linear electron accelerator (LINAC) coupled with a kinetic spectrometer. The solutions were irradiated with electron pulses of fwhm about 200 ns inside a 10
10
10 mm flow quartz cell with all the sides transparent to the visible light. The divergence of the electron beam at the sample position is slightly more than 10 mm and it is of Gaussian shape. Therefore, a uniform irradiation is expected in this set up. The white light from a 450 W Xenon lamp at a 90° angle to the electron beam irradiation was used for the detection of the transient species produced upon radiolysis. The diameter of the Xenon light falling on the sample cell is about 10 mm. Therefore, the absorbencies of the transient species are measured uniformly throughout the cell. The details of the set up are given elsewhere.46 The absorbed dose was determined by using a chemical dosimeter, 10 mM potassium thiocyanate, KSCN solution kept in a quartz cell of similar dimensions. The absorbed dose in the pulse radiolysis experiments were 50 Gy. The time-resolved spectra and the kinetic profiles at different probe wavelengths for the formation and decay of the transient intermediate species produced during radiolysis were obtained during the pulse radiolysis experiments. 2.3. Characterization. The radiolysis products, CdSe nanoparticles, were obtained upon the repetitive irradiations in the LINAC with an absorbed dose of about 40 kGy. The products were characterized by XRD and TEM measurements. X-ray diffraction (XRD) measurements were recorded on a Phillips X-ray diffractometer, model PW 1710 system, using a monochromatic Cu Kα source (λ = 0.154 nm). The electron diffraction and high-resolution transmission electron microscopic (HRTEM) images were acquired on a TEM, model no. FEI, TECNAI-F30. The preparation of samples for HRTEM analysis involved sonication in methanol for 25 min and deposition on a carbon-coated copper grid. The accelerating voltage of the electron beam was 300 kV. DC magnetization measurements, as a function of field were carried out using an E.G. and G.P.A.R. vibrating sample magnetometer (model 4500). 13252

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Figure 1. Transient absorption spectra obtained from the pulse radiolysis studies for the aqueous solutions containing 10 mM CdSO4 and ammonia solution (a) in the presence of 0.1 M t-BuOH, under N2 purged and (b) N2O saturated conditions. Pulse width 200 ns and dose 50 Gy. Inset: Kinetic profile monitored at peak position of Band  I (i.e., 350 nm) along with its decay curve fit.

3. RESULTS AND DISCUSSION The radiation-induced synthesis of a material in the aqueous solution is mainly governed by the radiolysis of water, as water being the solvent and is present in abundance. Radiolysis of water produces three major primary radicals, eaqh, OH• and H•, out of which eaqh and H• are reducing and OH• is oxidizing in nature.44 By suitably modifying the solvent medium, it is possible to investigate the reaction of any one of these three primary radicals with the reagents of our interest. The reducing environment can be achieved by adding 1 M tert-butanol (CH3 (CH3)2COH) to the aqueous solution, whereas the oxidizing environment is made by saturating the solutions with N2O gas. The synthesis of CdSe nanoparticles through electron beam irradiation in aqueous solutions containing equimolar ammoniated cadmium sulfate and sodium selenosulfate have been recently reported by our group.35,36 Therefore, the further characterization of the radiolytic products, CdSe nanoparticles was not performed in this study. The kinetics and dynamics of the radiolytic formation of CdSe nanoparticles have been investigated using electron pulse radiolysis studies. Below are the few important reactions that occur during the radiolysis processes. Radiolysis of water:

In the presence of tert-butanol: OH• ðH• Þ þ CH3 ðCH3 Þ2 COH f CH2 ðCH3 Þ2 COH þ H2 OðH2 Þ

ð2Þ

In the presence of N2O: H2 O

eaqh þ N2 O sf N2 þ OHh þ OH• ðk ¼ 9:1
109 M1 s1 Þ47

ð3Þ 3.1. Pulse Radiolysis Studies of Cadmium Precursor. The pulse radiolysis studies were performed only with the cadmium precursors, [Cd(NH3)4]2+, in the aqueous solutions with pH 910 (Scheme 1). In the first case, N2 purged aqueous solutions containing 10 mM CdSO4, ammonia and 1 M tert-butanol were studied. The transient intermediate species formed upon the reaction of the hydrated electrons, eaq, with the [Cd(NH3)4]2+ ions exhibit a strong absorption peak at 350 nm, as shown in Figure 1a, which was assigned as Band-I. The absorbance (ΔOD)

value was 0.2 at λmax = 350 nm and at within 1 μs time scale. The transient species decay with a pseudofirst order decay rate constant, 2.3
106 M1 s1. The redox potential (E) of eaq is 2.77 V vs NHE44 which is very high and the reaction rate constant between the eaq and [Cd(NH3)4]2+ ion is as high as 3.1
1010 M1 s1.48 eaqh þ ½CdðNH3 Þ4 2þ f ½CdðNH3 Þ4 •þ ðk ¼ 3:1
1010 M1 s1 Þ

ð4Þ In the second case, N2O saturated aqueous solutions containing 10 mM CdSO4 and ammonia were studied. The transient intermediate species exhibited an absorption peak at 350 nm too, but with a reduction of the absorbance (ΔOD) value by about 30% to 0.14 measured within 1 μs time scale, as shown in Figure 1b. However, the molar extinction coefficients in both cases were very much similar (Table 1). In this case, eaqh has two reaction channels, reactions 3 and 4. The concentration of dissolved N2O gas in aqueous solutions under a saturated condition is 25 mM. On the basis of competetion kinetics, a 30% reduction in reaction 4 is estimated, which matches with the observed results mentioned above. It is now confirmed that there is a reaction taking place between eaq and [Cd (NH3)4]2+ in both conditions to give a similar transient intermediate species. The transient intermediate species in the second case decays faster, k = 2.2
107 M1 s1 as compared to that in the first case, shown in Table 1. This could be possibly due to an additional reaction pathway between the OH• radicals and the intermediate species. 3.2. Pulse Radiolysis Studies of Selenium Precursor. The pulse radiolysis studies were also carried out only with the selenium precursors, [SeSO3]2+, in the aqueous solutions with pH 910 (Scheme 1). Similar to the cadmium precursor case as mentioned above, in this case also the pulse radiolysis experiments were carried out in two different conditions. In the first case, N2 purged aqueous solutions containing 10 mM Na2SeSO3 and 1 M tert-butanol was studied. The transient intermediate species formed upon the reaction of the hydrated electrons, eaq with the [SeSO3]2‑ ions exhibit two absorption peaks at 370 and 470 nm, as shown in Figure 2a, which were assigned as Band-I and Band-II, respectively for these spectra. The absorbance (ΔOD) value was much less than that in the case of cadmium. Furthermore, the absorbance value of Band-I is higher as compared to that of Band-II. The decay kinetics of the transient species was monitored at these two bands. It was observed that the decay kinetics monitored at these two bands was substantially different, and hence could be due to two different species 13253

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Table 1. Spectral and Kinetic Parameters for the Transient Intermediates Obtained in the Cd and Se Precursors Separately and Present Together in the Aqueous Solution from the Pulse Radiolysis Studiesa λmax (nm) (ε (M1 cm1))

systems studied

Band-I [Cd(NH3)4]2+ (N2) 2+

[Cd(NH3)4]

(N2O)

2

[SeSO3] (N2) [SeSO3]2 (N2O) [Cd(NH3)4]2+/[SeSO3]2 (N2) 2+

2

[Cd(NH3)4] /[SeSO3] a

(N2O)

Band-II

rate constants 1 1

Band-I k (M

s )

350 (15 770)

1.4
107

350 (15 500)

3.3
107

Band-II k (M1 s1)

370 (1716) 330 (1800)

470 (771) 500 (732)

7.5
104 8.6
105

1.0
107 6.9
106

390 (1,027)

520 (2224)

1.7
107

2.8
107

510 (2150)

2.1
10

3.4
107

340 (1100)

7

Pulse width 200 ns and dose 50 Gy.

Figure 2. Transient absorption spectra obtained from the pulse radiolysis studies for the aqueous solutions containing 10 mM Na2SeSO3 (a) in the presence of 0.1 M t-BuOH, under N2 purged and (b) N2O saturated conditions. Pulse width 200 ns and dose 50 Gy. Inset: Kinetic profile monitored at peak position of Band-I (i.e., 350 nm) along with its decay curve fit.

(Table 1). The decay monitored at Band-I is very slow as compared to that at Band-II. The reaction rate constant between the eaq and [SeSO3]2+ ion (reaction 5) was determined by monitoring the decay kinetics of the hydrated electrons at 650 nm to be 2.3
109 M1 s1. eaqh þ ½SeSO3 2 ½SO3 2 þ Se• ðk ¼ 2:3
109 M1 s1 Þ

ð5Þ

In the second case, N2O saturated aqueous solutions containing 10 mM Na2SeSO3 were studied. The transient intermediate species exhibit two absorption peaks at 330 and 500 nm, but with a reduction of the absorbance (ΔOD) value by about 30% similar to the case in the cadmium precursor, as shown in Figure 2b. However, the molar extinction coefficients in both the cases were very much similar (Table 1). In this case, eaqh has two reaction channels, reactions 3 and 5. On the basis of competetion kinetics, about 90% reduction in the reaction 6 is estimated which does not match with the observed results. Moreover, the decay kinetics monitored at these bands are very much in close comparison and faster as compared to that monitored at the Band-I in the first case. On the basis of these results, it is predicted that the OH• radicals formed under these conditions could be reacting with the [SeSO3]2‑ ions as well as the transient intermediate species formed by the reaction 5. Therefore, the transient intermediate species formed in this case certainly differ from those observed in the first case, i.e., solution containing tertbutanol and N2 purged. The decay constants estimated are shown in Table 1. 3.3. Pulse Radiolysis Studies of Both Cadmium and Selenium Precursors Together. The pulse radiolysis studies were carried out with the aqueous solutions containing both cadmium

precursors, [Cd(NH3)4]2+ and selenium precursors, [SeSO3]2‑ in 1:1 molar ratio. The pH of the solutions was between 9 to 10, where the hydrated electrons exist as eaq. The pulse radiolysis experiments were carried out in two different conditions. In the first case, N2 purged aqueous solutions containing 1 M tertbutanol and the precursors of two different concentrations. In one set, the precursor concentrations were kept 5 mM each and in another set these were 0.5 mM each. The transient intermediate species formed upon the reaction of the hydrated electrons, eaq, with the precursor ions exhibit two distinct absorption peaks at 390 and 520 nm, as shown in Figure 3a,b, which were assigned as Band-I and Band-II, respectively, for these spectra. The absorbance (ΔOD) value was much less than that in the case of the only cadmium precursor (Figure 1), but comparable to that observed in the case of the selenium precursor (Figure 2). It is observed that the absorbance value of the Band-II is higher as compared to that of the Band-I. The ratios of Band-II to Band-I as well as their absolute values remain constant in both the concentrations. The decay kinetics of the transient species was monitored at these two bands (Table 1). As the peak at 300 nm is not matching with the peaks obtained in the case of cadmium and selenium precursors individually, this peak cannot be assigned to their transient species. In the case of lower concentration of precursors (0.5 mM) in Figure 3b, the decay at 520 nm (Band-II) and the formation at 300 nm are quite comparable. So, therefore, the transient species (III) formed upon the reaction of both the precursors, get converted to another long-lived transient species with an absorption maximum at 300 nm, before the formation of stable nanoparticles. This observation is distorted in the case of high precursor 13254

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Figure 3. Transient absorption spectra obtained from the pulse radiolysis studies for the aqueous solutions containing CdSO4, Na2SeSO3 and ammonia solution (a) 10 mM each (b) 0.5 mM each, under N2 purged. Pulse width 200 ns and dose 50 Gy. (b) Inset: Temporal variation plot of absorbance measured at 300, 390, and 520 nm.

Figure 4. Transient absorption spectra obtained from the pulse radiolysis studies for the aqueous solutions containing CdSO4, Na2SeSO3 and ammonia solution (a) 10 mM each (b) 0.5 mM each, under N2O saturated conditions. Pulse width 200 ns and dose 50 Gy.

concentration in Figure 3a, where the possibility of other adduct species could interfere with this mechanism. In the case of the N2O saturated solution, the formation of the transient species III (Figure 4a) with the absorption peak at 520 nm is less due to the lower yield of eaq. Therefore, the similar observation as obtained in the case of N2 purged condition was not found here. However, in the case of lower precursor concentration (0.5 mM), there is a signature of absorption with a peak at 300 nm is seen in the Figure 4b. The reaction rate constants between the [Cd (NH3)4]•+ radicals and [SeSO3]2 ions (reaction 6) and the Se•‑ radicals and [Cd (NH3)4]2+ ions (reaction 7) were determined by monitoring the decay kinetics at the Band-II, by controlling the concentrations of the individual precursors in the pulse radiolysis experiments. ½CdðNH3 Þ4 •þ þ ½SeSO3 2 f ½CdðNH3 Þ4 •þ : ½SeSO3 2 ðk ¼ 1:9
1010 M1 s1 Þ Se• þ ½CdðNH3 Þ4 2þ f ½CdðNH3 Þ4 2þ : Se• ðk ¼ 5:5
1010 M1 s1 Þ

CdSe nanoparticles is possible through both reaction channels. The presence of excess sulfite ions in the selenosulfate solution is expected due to its synthesis procedure. Therefore, we have carried another set of pulse radiolysis experiments for the determination of the reaction rate constant between the [Cd (NH3)4] •+ radicals and the [SO3]2 ions (reaction 8). The reaction rate constant was found to be very less, 7.4
106 M1 s1 and hence it is expected that this reaction might not be interfering in the present system. ½CdðNH3 Þ4 •þ þ ½SO3 2 f transientintermediates

ðk ¼ 7:4
106 M1 s1 Þ

ð8Þ eaqh and

2

the [SO3] ions The reaction rate constant between (reaction 9) has been reported in the literature48 and which is very less, 1.3
106 M1 s1 and such reaction might not be taking place in the present system.

ð6Þ

eaqh þ ½SO3 2 f transientintermediates ðk ¼ 1:3
106 M1 s1 Þ48

ð9Þ ð7Þ

It was observed that the rate constants for the reactions 6 and 7 are compararable. Hence, it is expected that the formation of

In the second case, the pulse radiolysis experiments were performed with the N2O saturated aqueous solutions containing both precursors of the above-mentioned concentrations without 13255

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Scheme 2. Proposed Reaction Mechanism for the Radiolytic Formation of CdSe Nanoparticles in Aqueous Solutions

Figure 5. XRD pattern recorded at room temperature of the radiolysis product obtained from electron beam irradiation of deaerated aqueous solution containing 10 mM ammoniated CdSO4, 10 mM Na2SeSO3, and 1 M tert-butanol.

the one [Cd (NH3)4]•+:[SeSO3]2, is more probable. As the transient species, III, are formed from the reaction of eaqh, with the precursors, therefore, the radiolytic yield of these transient species would be equal to that of the eaqh, 2.9. Thus the formation of the CdSe nanoparticles could be taking place from the transient species III, through the nucleation and growth (reaction 10) as given below: transientspeciesIII f ðCdSeÞsolvated f ðCdSeÞnp

tert-butanol. Two peaks 340 and 510 nm were found in the transient absorption spectra and were assigned as Band-I and Band-II (Figure 4a,b) respectively, which was obtained in the case of N2-purged condition. Unlike in the previous case (Figure 3a and b), the ratio of the peaks Band-I to Band-II is lower in this case along with an overall reduction in the absorbance value. The reduction in the absorbance value (0.016) at the Band-II in the case of N2O saturated one is close to that expected (0.019) based on the competition kinetics of the most predominant reaction, reaction 4. Further, there was a substantial reduction in the intensity of the Band-II, when the precursor concentrations were reduced from 5 mM (0.016) to 0.5 mM (0.005), which was not observed in the case where the precursors were taken along with tert-butanol. This figure matches with that estimated from the competition kinetics of the most predominant reaction, reaction 4. Therefore, it is now clearly confirmed that the absorption Band-II is due to the transient intermediate species formed by the reaction of the precursors with the hydrated electrons and which act as the seed for the growth of CdSe nanoparticles. Thus, the overall reaction pathways leading to the formation of CdSe nanoparticles is summerised in the Scheme 2. The transient species I and II menetioned there could be [Cd(NH3)4]•+ and Se•‑ respectively. On the basis of various probable reaction pathways, which may lead to the formation of CdSe nanoparticles can be seen from the Scheme 2. In this scheme, the transient species III could be [Cd(NH3)4]•+:[SeSO3]2, [Cd(NH3)4]2+:Se•, and [Cd(NH3)4]•+:Se•. On the basis of the reaction rate constants shown in this scheme, it is predicted that out of the above-mentioned three possible transient species,

ð10Þ

In our previous study,35 bare CdSe nanoparticles were synthesized in aqueous solutions using equimolar ammoniated CdSO4 and Na2SeSO3 as the starting materials upon gamma irradiation. The yield of CdSe nanoparticles under N2O-saturated condition was found to be about 60% of that under a totally reducing environment. The ratio of yields in the presence and absence of N2O saturation was found to be about 60:100. At 0.5 mM precursor concentrations, there was a formation of CdSe nanoparticles only under a deaerated condition, due to the lower precursor concentration; however, there was no CdSe formation under a N2O saturated condition. Thus the formation of nanoparticles was found to occur through the reaction of the precursor ions with the hydrated electrons which are in well agreement with our present study. There, we have signified the unique properties of CdSe NP synthesized via radiolysis method. These nanoparticles exhibited interesting behavior of room temperature ferromagnetism (RTFM), which is otherwise known to be present when CdSe is either doped or surface capped with capping agents. The RTFM observed in the CdSe nanoparticles (23 nm) synthesized via electron beam irradiation was found to be higher as compared to those obtained via γ irradiation. 3.4. Characterization of CdSe Nanoparticles. The radiolysis product formed upon repetitive irradiation in the LINAC, were immediately recovered from the aqueous solution by centrifuge and air-dried for further characterizations by XRD (Figure 5) and TEM (Figure 6). The Miller indices for the diffraction patterns attributable to the cubic crystallites of CdSe (JCPDS Card No. 190191) are marked in this figure. The broadening of these peaks clearly indicates that the nanosized CdSe crystals are formed during the radiolysis. Figure 6 shows the TEM images and the inset shows the Selected Area Electron Diffraction (SAED) patterns of CdSe nanoparticles. The results confirmed 13256

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’ AUTHOR INFORMATION Corresponding Author

*Tel: +91-22-25590297; Fax: +91-22-25505331; E-mail: madhab@ barc.gov.in.

’ ACKNOWLEDGMENT The authors would like to acknowledge R. S. Gholap, National Chemical Laboratory, Pune, for his help with the TEM measurements and Prof. B. S. M. Rao, National Centre for Free Radical Research (NCFRR), University of Pune, Pune, for the pulse radiolysis experiments carried out at this center. The authors also acknowledge Dr. S. N. Achari, Chemistry Division, Bhabha Atomic Research Centre, for his help with the XRD measurements. Ms. Shalini Singh acknowledges Bhabha Atomic Research Center, for providing her a research fellowship.

Figure 6. TEM micrograph of the radiolysis product, CdSe nanoparticles, obtained from electron beam irradiation of deaerated aqueous solution containing 10 mM ammoniated CdSO4, 10 mM Na2SeSO3, and 1 M tert-butanol. Insert shows SAED pattern of the nanoparticles.

the formation of CdSe nanoparticle agglomerates with the primary nanoparticle size of around 23 nm, where the agglomerates are linked with each other giving rise to network structures. The agglomeration was due to absence of any capping agents in the present system. The particle size of the individual grains obtained from TEM measurements matches very well with the values obtained from the XRD and absorption measurements.

4. CONCLUSIONS In summary, we have reported the dynamics of the radiationinduced synthesis of CdSe nanoparticles in aqueous solutions by electron pulse radiolysis studies coupled with kinetic spectrometry. The characterization of these nanoparticles was done by XRD and TEM measurements. The transient intermediate species formed during the radiolytic processes were investigated under various experimental conditions, such as reducing conditions in the presence 1 M tert-butanol and an oxidizing condition in the presence of saturated N2O, for getting a clear understanding of the reaction mechanism and the formation of these nanoparticles. The reaction rate constants of the formation and decay of these transient intermediate species have been determined from the kinetic profiles obtained in the pulse radiolysis studies. The formation of CdSe nanoparticles was found to proceed through (i) the generation of short-lived transient intermediate species having an absorption peak at 520 nm and (ii) the transformation of this short-lived species to a long-lived transient intermediate species having an absorption peak at 300 nm. This only occurs through the reaction of the hydrated electrons, eaqh, with both precursor ions under deaerated conditions. The yield of formation of CdSe nanoparticles was found to be reduced upon N2O saturation, which agrees with our previous report. The XRD and TEM measurements confirm the formation of cubic phase CdSe nanoparticles of sizes between about 2 to 3 nm in the aqueous solutions. This study is expected to deliver a better understanding for the synthesis of a desired semiconductor nanoparticle through the radiation chemical route in aqueous solutions.

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