Laser Irradiance and Wavelength-Dependent Compositional Evolution

Feb 5, 2008 - Laser irradiance and wavelength-dependence controlling in the composition of ZnO−ZnOOH composite nanoparticles in sodium dodacyl ...
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J. Phys. Chem. C 2008, 112, 2812-2819

Laser Irradiance and Wavelength-Dependent Compositional Evolution of Inorganic ZnO and ZnOOH/Organic SDS Nanocomposite Material Subhash C. Singh and Ram Gopal* Laser and Spectroscopy Lab, Physics Department, UniVersity of Allahabad, Allahabad-1002, India ReceiVed: July 10, 2007; In Final Form: October 13, 2007

Laser irradiance and wavelength-dependence controlling in the composition of ZnO-ZnOOH composite nanoparticles in sodium dodacyl sulfate (SDS) matrix is presented. Varying the energy of the laser and its wavelength used for ablation, the ratio of ZnO to ZnOOH, their particle sizes, and distributions can be widely controlled. Second and third harmonics of pulsed Nd:YAG laser is used to ablate a zinc target placed in aqueous media of SDS, with varying energies and simultaneous flow of pure oxygen in the closed vicinity of the laser-ablated plasma plume. A colloidal solution produced shows two UV-visible absorption peaks at 365 and 490 nm. The ratio of absorbance peaks at 365 and 490 nm decreases with an increase in energy, but the decreasing rate of their ratios is higher for the third harmonic compared to that of the second. XRD UV-visible absorption, transmission electron microscopy, and FTIR spectroscopic techniques are used for the characterization of the produced colloidal solution. A possible mechanism of the synthesis of ZnO and ZnOOH nanoparticles is proposed.

1. Introduction Nanodimensional inorganic/organic composite materials have attracted great attention because of their tunable composition and size-dependent optical and electronic properties.1 Metallic nanoparticles and their oxides have shown their potential applications in the fabrication of microelectronics, optoelectronics, photonics, and sensing devices.2 Zinc oxide,3 hydroxide,4 and oxyhydroxide materials and their compositions are found to be important for the same. Zinc oxide is mostly a n-type, II-VI, wide direct band gap, semiconducting material. Because of its high optoelectronic efficiencies relative to the indirect band gap group IV crystals, it is considered a reliable material for applications in the visible and near-ultraviolet region. As wurtzite zinc oxide has a wide band gap (3.37 eV), high exciton binding energy (≈60 meV) at room temperature, and high dielectric constant, and therefore possesses important applications in the fabrication of electronic and optical devices, mainly UV/blue lasers.5-6 ZnO has a noncentrosymmetric C46v wurtzite crystal symmetry, hence it is found to be an interesting material for nonlinear secondharmonic generation and wave mixing in nanoscale cavities.7 It is transparent to most of the solar spectrum, therefore, widely used as window material in solar cells, optical waveguides, light modulators, and optical sensors. Application of ZnO has increased in current decades in the fabrication of switching elements, transistors, lasers, and detectors, and therefore controlled synthesis of good quality zinc oxide nanostructures such as nanocrystals,8 nanowires,9 nanobelts,10 and other nanoarchitectures11,12 has been in great demand. Several routes are employed for the production of ZnO nanomaterials including solvothermal,13 thermal evaporation,14,15 solid-state pyrolysis,16 sol-gel synthesis,17 sputtering,18 chemical vapor deposition,19 and molecular beam epitaxy,20 etc. * To whom correspondence should be addressed. E-mail: [email protected]; [email protected]. Phone: +91-5322460764. Fax: +91-532-2460993.

Composite materials show attractive characteristics of catalytic and sensing properties and tunable luminescence.21-23 The physical and chemical properties of these materials are tunable and composition dependent. Among the all reported physical and chemical routes to produce nanomaterials, laser ablation is widely used for the production of metallic and nonmetallic nanostructures in the gas phase.24 Henglein and Cotton25-27 used several liquids as ablating media to produce colloidal nanoparticles, which was found to be a simple and versatile tool for production of highly stable, monodispersed, chemical contamination-free particles. Laser ablation in liquid media is also used for the resizing and reshaping of nanoparticles produced by other conventional routes.28-30 The production of noble metal nanoparticles31-35 by laser ablation in liquid media is extensively studied, but there are few reports on active transition metals and their oxide nanoparticles using this approach. Liang et al. was the first to report charging-directed synthesis of interesting ZnDS and ZnO composite nanomaterials by the laser ablation of zinc in aqueous media of SDS with the third harmonic of a Nd:YAG laser.36,37 Usui et al.38-40 have ablated a zinc target with same wavelength of Nd:YAG laser in aqueous media of different surfactants and synthesized a layered structure of Zn(OH)2. Adopting a nearly similar method, Zeng et al. have synthesized Zn/ZnO core/ shell nanoparticles using the fundamental wavelength of the same laser.41,42 Very recently we have reported the synthesis of Zn-ZnO composite nanomaterials by laser ablation of a zinc plate in aqueous solution of SDS using the second harmonics of a Nd:YAG laser.43 The present work deals with laser ablation of a Zn plate in an aqueous solution of SDS with simultaneous flow of pure oxygen gas in the closed vicinity of the laser-ablated plasma plume. In the synthesis of ZnO nanoparticles in gas phase or deposition of thin films of ZnO, laser ablation of Zn or ZnO targets in the rich environment of oxygen and inert gas is done to improve crystal quality and lasing of ZnO nanomaterials. These types of works have motivated us to use this phenomenon

10.1021/jp0753676 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/05/2008

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Figure 1. X-ray diffraction spectrum of ZnO nanoparticles produced by laser ablation of zinc in aqueous media with 100 mJ/pulse energy and continuously flowing oxygen gas.

by increasing the concentration of dissolved oxygen in water or flowing oxygen gas in the closed vicinity of the laser-ablated plasma plume, and we have observed two new results. The first is that the absorption spectra of the obtained colloidal solutions exhibits an extra peak at 490 nm instead of 365 nm for ZnO as reported previously,42 and the second is that the dimension of produced particles in solution decreases with increase of laser irradiance, which is opposite to all previously reported literature. The produced colloidal solution shows an interesting evolution of two peaks in the absorption at 365 and 490 nm, indicating production of two different species in the solution. Ratios of the two species, their average sizes, and distribution are highly dependent on laser wavelength and irradiance used for ablation. The average sizes and distribution of particles produced by this method are smaller compared to that of other conventional methods. 2. Experimental Section The typical experimental setup was the combination of pulsed laser ablation of a pure zinc plate in aqueous solution of sodium dodecyl sulfate (Glaxo Smith Kline; denoted as SDS hereafter) with a simultaneous flow of pure oxygen gas. A zinc plate (99.99%, Spec-pure Johnson Mathey, U.K.) was placed in the bottom of a glass vessel containg 10 mL of 0.05 mM aqueous solution of SDS. Second and third harmonics of a pulsed Nd:YAG laser (Spectra Physics, Quanta-Ray) operated at 10 Hz with a pulse width of 8 and 5 ns, respectively, was used. The laser beam with varying energy in between 60-100 mJ/pulse for the second and 25-70 mJ/pulse for third harmonics was focused on the metal plate with a spot size of about 500 µm in diameter using a convex lens of 25 mm focal length. Oxygen gas was allowed to flow through a glass capillary having a jet at the second end so that gas reaches in the closed vicinity of laser-ablated plasma plume easily. The second end of the capillary was being kept near to the laser spot, and flow rate of oxygen was being controlled with the valve. Laser ablation was

carried out for 30 min, and solution turned yellowish from brownish with increase of ablation time. Most of the solutions were turbid and found to be stable without any sedimentation for more than 6 months. The optical absorption spectra of obtained colloidal solutions were recorded as synthesized by a UNICAM 5625, UV-vis spectrophotometer. A pH meter (Century CP-901) was used for the pH measurement of the produced colloidal solutions. Transmission electron microscopic investigation was performed on an E.M-C.M-12 (Philips) transmission electron microscope, operating at 100 keV, by putting a drop of colloidal solution on a carbon-coated copper grid. For the Fourier transform infrared study colloidal solution obtained by laser ablation with a 60 mJ/pulse of the second harmonic and a 70 mJ/ pulse of the third harmonic was centrifuged at 10000 rpm, and settled powders were dried in the oven at 60 °C temperature. The obtained dried powders were dispersed with KBr discs and palletized at 10-ton pressure. FTIR spectra of SDS coated ZnO nanoparticles, in the form of a pellet, which was suspended in the path of an IR beam of a spectrometer (FTIR Spectrum RX1, Perkin-Elmer), were recorded in the far-mid infrared region. The X-ray diffraction pattern of obtained powder corresponding with a 100 mJ/pulse of the second harmonic was recorded with a Rigaku D/Max 2200 diffractometer with Cu KR radiation at λ ) 1.5406 Å. 3. Results 3.1. Characterization of the Colloidal Solution of Nanoparticles Produced by the Second Harmonic of the Nd:YAG Laser. The XRD pattern of the produced nanoparticles is shown in Figure 1. Indexing observed peaks with (100), (002), (101), (102), (110), (103), (200), (112), and (201) shows that produced particles are ZnO with a wurtzite crystalline structure. There are two extra and comparatively wide peaks observed at 2θ ) 33.5 and 59.5°, which may be due to the ZnOOH nanoparticles. Wider peaks of ZnOOH compared with ZnO indicate that oxy-

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Figure 3. Ratios of absorbances at 370 and 490 nm of colloidal solutions produced by laser ablation.

Figure 2. (A) UV-visible absorption spectra of a colloidal solution of nanoparticles produced by laser ablation with 532 nm laser light at (a) 60, (b) 85, and (c) 100 mJ/pulse energies; (B) normalized absorbance at peak intensity at 365 nm to observe changes in the shape of optical absorbance as a function of laser irradiance.

hydroxide nanoparticles of zinc are smaller than its oxide nanoparticles. Figure 2A shows UV-visible absorption spectrum of colloidal solutions produced by laser ablation of a zinc plate in a 0.05 mM aqueous solution of SDS at (a) 60, (b) 85, and (c) 100 mJ/pulse energies of 532 nm of a Nd:YAG laser. Absorption spectra of all the three samples show two peaks nearly at 365 and 490 nm. The first peak at 365 nm is a characteristic absorption peak of ZnO, and confinement in nanoscale is proved because of the blue shift in the absorption peak corresponding to the bulk. The second peak may be due to the synthesis of zinc oxyhydroxide nanoparticles. It is previously reported that laser ablation of Al44 and Ga45 in water produces AlOOH and GaOOH nanoparticles, respectively. Normalized absorption spectra of colloidal solutions are shown in Figure 2B, in which all three samples are normalized at the first peak position to study the shift in peak position and change in peak width with energy. It is observed that the position of both absorption peaks in all three spectra shifted toward blue with a decrease in peak width and hence area as laser irradiance increases. The shift in absorption peaks toward blue indicates that the size of both the species in the solution decreases, while the reduction in peak width shows that the distribution of particles becomes narrower with an increase in laser irradiance. It was previously reported, in the synthesis of noble metallic46,47 and metal oxide42 nanoparticles by laser ablation, that the surface Plasmon absorption peak shifted toward red and the peak width increased with increase in laser irradiance, which are opposite to the observed results. These findings indicate that the flowing of pure oxygen gas during ablation causes a new mechanism in particle synthesis. Intensities of both the absorbance peaks at 365 and 490 nm are highly dependent on laser irradiance. It is found that the absorbance of the first peak decreases while that

of the second peak increases with an increase in laser irradiance. Ratios of absorbance peaks at 365 and 490 nm are plotted with laser irradiances in Figure 3, which indicate that the ratio of two evolutes in solution decreases with increase in energy. The pH values of the produced colloidal solutions decrease with an increase of laser irradiance: (a) 6.8, (b) 6.4, and (c) 6.2. This shows that the solution turns acidic from neutral (pH ) 7) after ablation and that the concentration of H+ ions in solution increases with laser irradiance. TEM images of the produced nanoparticles with corresponding size distributions are given in Figure 4. Calculating the size of more than 500 particles it is found that the average size and distribution of nanoparticles decrease with an increase in energy. These changes are also justified by the UV-visible absorption spectra. The average size and distribution of produced particles are (a) 17.3 and 13.8, (b) 14.1 and 13, and (c) 13.7 and 5.8 nm at (a) 60, (b) 85, and (c) 100 mJ/pulse energies, respectively. It is found that nanoparticles produced by 100 mJ/pulse energy have maximum monodispersity, which is also supported by UV-visible absorption spectrum. It is found that there is sharp change in dispersity when the energy changes from 85 mJ/pulse to 100 mJ/pulse. A comparative size distribution of produced nanoparticles is plotted in Figure 5, which shows that all three curves have more than one maximum. It is interesting to note that for low laser fluence (F ) 60 mJ/pulse) and high laser fluence (F ) 100 mJ/pulse) the size distribution can be easily extrapolated by Gaussian functions. For lowest laser fluence (F ) 60 mJ/pulse) the distribution curve, represented by curve a in Figure 5, has three maximas centered at 12.5, 22.5, and 37 nm, showing three different mechanisms for particle synthesis. The average size and distribution of produced particles are maximum for this experimental condition. The size distribution of particles produced by the intermediate laser fluence (F ) 85 mJ/pulse) is presented with curve b, having two maximums at 7.5 and 35 nm, showing that nearly 82% of the particles have diameters of 2-20 nm, but there is a significant amount of particles that lies in the 20-45 nm dimensions. The highest laser fluence distribution curve is presented by (c), showing two Gaussian distributions, but the intensity of second peak is very small as compared to that of the first. Nearly 80% of the particles are smaller than 15 nm with the average diameter being 10 nm and least size distribution (2.5 nm) being in this range. The selected area electron diffraction pattern given in Figure 6 shows broad diffuse rings due to the small sizes of the ZnO nanoparticles. Indexing of the SAED pattern is attributed to the (100), (002), (102), (110), and (112) peaks of the hexagonal crystalline phase of the ZnO nanoparticles. Some diffused weaker rings remain unidentified; it may be possible that they are due to the zinc oxyhydroxide nanoparticles. 3.2. Characterization of the Colloidal Solution of Nanoparticles Produced by the Third Harmonic of a Nd:YAG Laser. UV-visible absorption spectra of colloidal solutions

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Figure 4. TEM images and corresponding size distribution of colloidal solution of nanoparticles produced by laser ablation with 532 nm laser light at (a) 60, (b) 85, and (c) 100 mJ/pulse energies.

Figure 5. Comparative particle size distribution of nanoparticles produced by laser ablation with 532 nm laser light at (a) 60, (b) 85, and (c) 100 mJ/pulse energies.

Figure 7. (A) UV-visible absorption spectra of colloidal solution of nanoparticles produced by laser ablation with 355 nm laser light at (a) 25, (b) 55, (c) 70 mJ/pulse energies; (B) normalized absorbance at peak intensity at 365 nm to observe changes in the shape of optical absorbance as a function of laser irradiance.

Figure 6. SAED pattern of colloidal solution produced by laser ablation with 532 nm laser light operating at 60 mJ/pulse energy.

produced by third harmonics of a Nd:YAG laser at (a) 25, (b) 55, and (c) 70 mJ/pulse energies are presented in Figure 7A. All three samples also exhibit two absorption peaks close to 365.0 and 490.0 nm. These spectra also show trends similar to those obtained by laser ablation using second harmonics. Peak positions shifted toward blue and decreases in bandwidth and area are observed, with increases in laser irradiance. Intensities of both the peaks decrease with an increase in laser irradiance, but the ratio of absorbances A365/A490 decreases, which shows

that the amount of ZnO production decreases, while that of the evolution of the second species in solution increases with laser irradiance. Normalized absorption spectra at 365 nm are shown in Figure 7B in order to observe the change in the position and width of absorption peaks with a change in energy. The reduction in peak width of the first peak at 365 nm is shown by an arrow, indicating the reduction in peak width with energy. Similar trends are observed for the second peak at 490 nm as for first peak. The rate of decrease of the ratio of the absorption peaks A365/ A490 nm with laser irradiance is higher for the third harmonics compared to that of the second harmonics (Figure 8). This means that we can control the composition of components in the nanocomposite more easily using third harmonics compared to using the second harmonics of a Nd: YAG laser. The pH values of the produced colloidal solutions decreases as laser irradiance rises, that is, the solution turns

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Figure 8. Ratios of absorbances, at 370 and 490 nm, of colloidal solutions produced by laser ablation with 355 nm of laser light.

Figure 9. TEM images of colloidal solution produced by laser ablation with 355 nm laser light at 25 mJ/pulse taken at (a) 29.5 and (b and c) 120 kX resolution showing nanoseed and nanodagger-type structure, respectively.

Figure 10. TEM images and corresponding size distribution of a colloidal solution of nanoparticles produced by laser ablation with 355 nm laser light at (a) 55 and (b) 70 mJ/pulse energies.

acidic from neutral. The pH values of colloidal nanoparticles are (a) 6.6, (b) 6.3, and 6.0 at (a) 25, (b) 55, and 70 mJ/pulse, respectively. TEM images of nanoparticles produced by laser ablation using 355 nm of laser light with corresponding histogram is shown in Figures 9 and 10. Figure 9 panels a, b, and c show TEM images of nanoparticles produced with the least laser irradiance. It produces some seed- and dagger-shaped nanoparticles. The TEM image shown in Figure 9a is recorded with 29.5 kX, while those of Figure 9b,c are taken at 120 kX resolution. Seed-type particles have a base width of 25 nm and length of 40 nm, while dagger-shaped particles have widths of 11.7 nm and lenghts of 60 nm. Some spherical-shaped particles with average sizes of 10 nm are also produced. Nanoparticles produced with 55 and 70 mJ/pulse energies are shown in Figure 10 panels a and b, respectively. It is observed that particles produced with a 55 mJ/pulse laser fluence have larger sizes and distributions compared to those produced with 70 mJ/pulse energy. But the

Singh and Gopal

Figure 11. Comparative particle size distribution of nanoparticles produced by laser ablation with 355 nm laser light at (a) 55 and (b) 70 mJ/pulse energies.

rate of particle yield decreases with energy, which is also confirmed with absorption spectra. The comparative size distribution of nanoparticles produced with 55 mJ/pulse and 70 mJ/pulse is shown in Figure 11; both shows nearly Gaussiantype distribution. 3.3. FTIR Spectroscopy of SDS-Coated Nanoparticles Produced by Second and Third Harmonics. Fourier transform infrared (FTIR) transmittance spectrum of SDS coated zinc oxide nanoparticles produced with (A) 100 mJ/pulse of 532 nm and (B) 25 mJ/pulse energy of 355 nm laser ablation is displayed in Figure 12. An intense and wide peak at 3440 cm-1 is assigned as H-OH stretching. The peaks at 2943.0, 2918.0, 2849.9, 1468, and 1061 cm-1 are due to the -C-H stretching and bending.48 An intense and quite wide peak at 1599.7 cm-1 may be due to the combined effect of H-OH bending at 1610 cm-1 and Cd O stretching at 1581 cm-1, which shows effective bonding between the oxygen of the surface ZnO molecule and the carbon of SDS. The band at 1232 cm-1 is related to -SO4 of the SDS molecule.48 Transmittance peaks at 1340, 1234, and 1078.4 cm-1 are assigned as CH3 bending, CH2 bending, and C-C stretching modes of SDS, respectively. Peaks at 1468.8, 829.0, and 630 cm-1 may be due to the harmonics of H-OH stretching, symmetric CH2 stretching, and CH2 bending, respectively. A transmittance peak at 470.0 cm-1 is characteristic of Zn-O stretching,49 which is observed in both the samples. A comparative FTIR spectrum of bulk zinc oxide, zinc oxide nanoparticles, and SDS powder is presented in Figure 13. H-OH stretching and bending peaks are present in all the three samples. GaOH stretching of GaOOH nanoparticles is reported at 3200 cm-1.45 Zinc is lighter compared to gallium, therefore the stretching frequency of Zn-OH should be higher as observed with a peak at ∼3500 cm-1 in bulk hydrated ZnO as well as in ZnO nanoparticles. 4. Discussion 4.1. Synthesis and Morphology of Colloidal Nanoparticles by Laser Ablation. Laser ablation in active liquid media, causes high nonequilibrium processing, which allows synthesis of novel phases of materials and is of particular interest. Synthesis of nanomaterials by laser ablation in liquid media is under in the development stage. Laser ablation at the solid liquid interface, interaction between laser light and target, creates local hightemperature and pressure plasma plumes above the target surface. It is reported that laser ablation of a graphite48 target in water using 70 mJ/pulse energy of laser, detected using emission spectra and simulation, produced a pressure and temperature inside the plasma plume of 1Gpa and 6000 K, respectively. At this high temperature and pressure several chemical reactions and physical processes, that are not possible at normal conditions, will take place among ablated species, solvent and surfactant molecules, which induce formation of

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Figure 12. FTIR spectra of nanoparticles produced by laser ablation at (A) 532 nm and (B) 355 nm light of laser.

Figure 13. Comparative FTIR Sspectra of (A) bulk ZnO, (B) ZnO nanopowder produced by laser ablation with 532 nm laser light, and (C) SDS powder to differentiate peaks of ZnO and SDS.

particles in the solution. The structure, morphology, size, distribution, and hence properties of nanoparticles are highly dependent on laser wavelength, irradiance, surfactant concentration, nature of solvent, and other ablation parameters. Therefore properties of materials can be tuned by tuning ablation parameters. Production of active metallic oxide and oxyhydroxide nanomaterials involves following processes in their formation: (a) generation of high temperature and high-pressure plasma at solid-liquid interface, after an interaction between pulsed laser light and solid target; (b) cluster formation by adiabatic and supersonic expansion leads to cooling of plasma plume; (c) interaction of theses reactive clusters with species in aqueous media and reactive oxygen. In the case of the present study active zinc atoms, ions, and clusters could react with oxygen gas at the interfacial region of plasma and liquid, which produces ZnO molecules and acts as the nucleation center for the growth of zinc oxide nanoparticles. Some of the active zinc oxide molecules may react with OH- ions, produced by laser-induced breakdown of water molecules, cause production of ZnOOH

molecules, and may be responsible for the growth of zinc oxyhydroxide nanoparticles. The pH measurement before and after ablation shows that solution turns acidic from neutral, which indicates consumption of hydroxyl ions from solution and provides evidence for the synthesis of zinc oxyhydroxide nanoparticles. As laser irradiance (355 and 532 nm both) increases this causes more and more ablation of water molecules and production of H+ and OH- ions. An increase of OH- ions with energy results in more hydroxylation of ZnO molecules, which causes a decrease in the amount of ZnO molecules and an increase of ZnOOH molecules in solution. Therefore, the number of zinc oxide nanoparticles in solution reduces, while that of zincoxyhydroxide increases with increases in energy. The rate of reduction of A365/A490 is greater for the 355 nm laser light as compared to that of the 532 nm laser light, which results in more laser-induced breakdown of water molecules with 355 nm as compared with 532 nm. Synthesis of zinc oxyhydroxide nanoparticles depends on the supply of OH- ions, which itself is being controlled by laser irradiance used for ablation. Therefore, varying the energy of the laser beam and focusing condition we can control the ratio of zinc oxide and zinc oxyhydroxide nanoparticles. Two different mechanisms can be taking place for the synthesis of ZnO and ZnOOH nanoparticles. One of the two mechanisms is that active zinc clusters can react with oxygen molecules in the reactive quenching zone and form unstable ZnO2 molecules. The unstable intermediate product reacts with hydroxyl ions by forming ZnOOH molecules and leaving oxygen cations, which can react with any positive zinc atom and produce ZnO molecules. Another mechanism is the aqueous oxidation of the zinc cluster and formation of zinc hydroxide as an intermediate product, which produces zinc oxide molecules after dissociation. Zinc oxide molecules may get hydrated and form zinc oxyhydroxide molecules. In the absence of oxygen flow during laser ablation, produced zinc clusters get oxidized aqueously and produce Zn(OH)2 as an intermediate product, which after dissociation produces ZnO as a final product as reported in previous literature.42 Simultaneous injection of oxygen inside the high-temperature zinc plasma plume produces unstable ZnO2 molecules, which react with

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TABLE 1: Dependence of Peak Position, Absorbance Ratio, Peak Widths, Average Size, and Distribution of Particles with Laser Wavelengths and Irradiance solution no.

laser wavelength (nm) and energy (mJ/ pulse)

peak positions P365, P490 (nm)

Aa 365/A 490 (arb.)

1 2 3 4 5 6

532 and 60 532 and 85 532 and 100 355 and 25 355 and 55 355 and 70

375.8, 497.8 365.7, 494.4 362.3, 491.0 369.0, 494.5 365.0, 491.1 362.3, 484.3

1.50 1.30 0.90 1.22 1.20 1.16

a

peak widths Wb 365, W 490 (nm) 64.4, 33.9 54.5, 20.35 33.9, 20.34 150, 120 75, 50 50, 25

average size and distribution (nm) 17.3 and 13.8 14.1 and 13.0 13.7 and 5.8 elongated 27.8 and 11 15.9 and 4.25

A, absorbance of colloidal nanoparticles. b W, width of plasmon absorption peak of corresponding solution.

hydroxyl ions and weaken the binding between two oxygen atoms of ZnO2 molecules. This mechanism synthesizes ZnOOH molecules with release of oxygen anion. Oxygen anions inside the plasma are used for the production of ZnO molecules. It may be possible that ZnO nanoparticles produced with oxygen injection produces nanoparticles rich in oxygen, while those produced with absence of oxygen flow are oxygen deficient. The ZnO nanoparticles produced with a continuous flow of oxygen may have a tendency to absorb laser light of subsequent pulses, which produces a melting and fragmentation mechanism of previously synthesized particles, but these are matters of further research in the field. Following are the two proposed competitive mechanisms used in the synthesis of ZnO nanoparticles in the present work.

Zn (cluster) + O2 f Znδ+ -O‚‚‚OδZnO2 (unstable) + OH- f OH-‚‚‚Znδ+ r O‚‚‚O- f ZnOOH + OZn+(cluster) + O- f ZnO

(i)

and

Zn (cluster) + 2H2O f Zn (OH)2 Zn (OH) 2 f ZnO + H2O Zn+O- + OH- f OH+‚‚‚Zn+‚‚‚O-

(ii)

The ratio of ZnO to ZnOOH nanoparticles depends on concentration of oxygen and hydroxyl ion in the solution. The decrease in nanomaterials size and distribution with increase of laser irradiance is due to the melting and fragmentation of larger particles produced by previous pulses. In the case of second harmonic, larger zinc oxide particles can absorb laser light by two-photon absorption, and the temperature of the nanoparticles rise, which causes melting, fragmentation, and production of smaller particles. Therefore, increasing laser irradiance causes an increase in the temperature of the nanoparticles in solution and production of smaller fragments with narrow size distribution. Ablation using the third harmonic of Nd:YAG laser causes a high temperature of the previous produced particles owing to the large absorption coefficient of zinc oxide, hence melting and fragmentation is easier and we can easily control composition, particle size, and distribution with different laser irradiances of this wavelength. The increase in laser irradiance causes more laser-induced breakdown of water molecules and production of H+ and OHions in the solution. The increase in concentration of hydroxyl ions in the solution with energy provides favorable conditions for production of zinc oxyhydroxide nanoparticles. Therefore,

at higher laser irradiance, production of ZnOOH dominates over that of ZnO and the ratio of ZnO/ZnOOH decreases with an increase of energy. 4.2. Optical Absorption Spectra of Produced Colloidal Nanoparticles in the Violet and Blue Regions. Figures 1 and 6 show absorption spectra of ZnO-ZnOOH nanoparticles produced by laser ablation using the second and third harmonics of Nd:YAG laser, respectively. All six samples of the colloidal solution of ZnO-ZnOOH composite nanoparticles show two absorption peaks at 365 and 490 nm. As the size of produced particles is very large compared to their Bohr radius (1.8 nm for ZnO) a small blue shift in the absorption peak is observed. At a small laser fluence the main product is zinc oxide with a small amount of ZnOOH because at that energy there is small production of OH- ions, therefore the peak that appeared at 365 nm is high compared to that at 490 nm. As laser irradiance increases, it causes production of OH- ions in the solution; therefore, production of ZnOOH dominates over that of ZnO. So the peak at 490 nm increases with increase in energy and the peak at 365 nm decreases. At high laser irradiance the main component may be ZnOOH nanoparticles. The absorption peak in the violet region decreases, while that in the blue region increases with increase in laser irradiance. Nearly similar results are observed in the absorption spectroscopy of colloidal solutions produced by 355 nm of laser ablation. 4.3. pH Measurements of Produced Colloidal Solutions. After laser ablation, produced colloidal solutions become acidic from neutral. As laser irradiance increases this causes an increase in the breakdown of water molecules and production of H+ and OH- ions in the solution. Hydroxyl ions get consumed in the synthesis of ZnOOH nanoparticles and leavie their counterions in solution, which is responsible for the rise in the pH of solution. These processes including the breakdown of water molecules, production of oxyhydroxide of zinc, and concentration of H+ ions in solution increase with an increase in laser energy. Therefore pH values of produced colloidal solutions increase with laser irradiance. Reduction in pH value from neutral is low for the colloidal solution produced by the third harmonic of the Nd:YAG laser than that of the second owing to more ablation of the water molecules with 355 nm and hence more H+ ions in the solution. 5. Conclusions We have experimentally demonstrated controlled synthesis of ZnO-ZnOOH nanocomposite material by pulsed laser ablation of zinc metal plate in aqueous solution of SDS. The ratio of ZnO-ZnOOH nanoparticles, particle size, and distribution are controlled by laser wavelength and irradiance used for ablation. Laser ablation of the target induces zinc plasma, which produces zinc clusters after adiabatic expansion. Produced zinc clusters could react with oxygen gas that flows simultaneously in the solution during ablation and form zinc oxide molecules,

Inorganic ZnO and ZnOOH/Organic Material which act as seed for growth of ZnO particles. Some of the ZnO molecules may react with hydroxyl ions in the solution and generate ZnOOH molecules, seeds for growth of ZnOOH nanoparticles. Zinc clusters can also get aqueously oxidized and lead to the formation of ZnO particles. The ratio of ZnO to ZnOOH nanoparticles decreases with an increase in laser irradiance for both laser wavelengths, but rate of decrease of the ratio with laser irradiance is higher for 355 nm compared to that of 532 nm. Therefore, the ratio of components in nanocomposite materials can be easily controlled by the third harmonic as compared to the second. Aggregation, agglomeration, and termination in particle growth are achieved by DSions of SDS. It is observed that particle size and distribution decreases with an increase in laser irradiance for both wavelengths. The size and distribution of produced nanoparticles decrease with an increase of laser irradiance, which is opposite to the previously reported results of noble metal46,47 and metal oxide42 nanoparticles. It means that oxygen injection during laser ablation induces a new mechanism in the research field of laser material processing. The pH measurements of produced colloidal solutions and their irradiance dependence provide direct evidence for the synthesis of ZnOOH nanoparticles and the dependence of its rate of production on laser irradiance and wavelengths. Average particle sizes, distributions, and positions of absorbance peaks and peak widths with laser wavelengths and irradiances are given in Table 1. We have expected that laser ablation of active metals in the aqueous media of suitable surfactants will be highly useful in the production of metallic oxide and oxyhydroxide composite nanomaterials. Varying laser irradiance and wavelength used for ablation size, shape, distribution, and composition of nanocomposite materials can be easily controlled. Acknowledgment. The authors are thankful to Prof. O. N. Srivastava and Prof. S. B. Rai, Physics Department, Banaras Hindu University, Varanasi, for TEM measurements and IR facilities, respectively, and helpful discussions. We are also thankful to Prof. Ram Kripal and the Nanophosphor Application Center, Physics Department, Allahabad University, for providing the facility to record UV-visible absorption and XRD spectra, respectively. References and Notes (1) Subramanian, V.; Wolf, E.; Kamat, P. V. J. Phys. Chem B 2001, 105, 1439. (2) Comini, E.; Faglia, G.; Ferroni, M.; Sberveglieri, G. Appl. Phys. A 2007, 88, 45. (3) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. AdV. Funct. Mater. 2003, 13, 9. (4) Usui, H.; Sasaki, T.; Koshizaki, N. Appl. Phys. Lett. 2005, 87, 063105. (5) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Yiying, W.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (6) Johnson, J. C.; Yan, H.; Schaller, R. D.; Haber, L.H.; Saykally, R. J; Yang, P. J. Phys. Chem. B 2001, 105, 113875. (7) Johnson, J. C.; Yan, H.; Schaller, R. D.; Petersen, P. B.; Yang, P.; Saykally, R. J. Nano Lett. 2002, 2, 279. (8) Lin, Y.; Tseng, Y.; Yang, S.; Wu, S.; Hsu, C.; Chang, S. Cryst. Growth Des. 2005, 5, 579. (9) Chang, S. S.; Yoon, S. O.; Park, H. J.; Sakai, A. Mater. Lett. 2002, 53, 432.

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