Nanoarchitectural Evolution from Laser-Produced Colloidal Solution

J. Phys. Chem. C 2010, 114, 9277–9289

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Nanoarchitectural Evolution from Laser-Produced Colloidal Solution: Growth of Various Complex Cadmium Hydroxide Architectures from Simple Particles Subhash C. Singh*,† Laser Spectroscopy & Nanomaterials Laboratory, Department of Physics, UniVersity of Allahabad, Allahabad-211002, India

Ram Gopal Laser Spectroscopy & Nanomaterials Laboratory, Department of Physics, UniVersity of Allahabad, Allahabad-211002, India ReceiVed: March 2, 2010; ReVised Manuscript ReceiVed: April 3, 2010

Complex nanostructures and nanoassemblies have exhibited their potential application in the fabrication of future molecular machines and molecular devices. Liquid phase pulsed laser ablation (LP-PLA) is an easy, versatile, environmentally friendly, and rapidly growing method for the synthesis of nanostructured materials. Several experimental laser and liquid media parameters have been developed, but others are under development. The interaction of an anionic surfactant with the nanomaterials having a positive surface charge density is a key parameter, but an unanswered question until now, in the field of LP-PLA. Nanosecond pulsed laser ablation of a cadmium rod placed on the bottom of a glass vessel containing aqueous media of sodium dodecyl sulfate at different concentrations is used to produce a variety of cadmium hydroxide nanostructures from nanoparticles to nanorods, nanotetrapods, nanoflower buds, and 2D and 3D nanoflowers in order to investigate the above liquid media parameter. It is suggested that initially produced spherical nanoparticles get selfassembled into 1D nanorods, which themselves also get assembled into their successor nanoarchitectures. An aqueous medium of 20 mM SDS is found most suitable for the growth of such nanostructures. An increase of the surfactant concentration induces the synthesis of higher aspect ratio 1D nanorods with a larger tendency of aggregation and agglomeration. The rate of increase of agglomeration and aggregation with the surfactant concentration is so high that the nanomaterials produced in 100 mM surfactant concentration lose their individual identity. A detailed investigation on the evolution, growth, and self-assembly of various nanostructures is presented. 1. Introduction Materials with complex geometries and having sizes down to the nanometer scale exhibit significantly enhanced functionality in their properties over their bulk counterparts. The potential examples may include semiconducting nanowire quantum dot systems for gas sensing1 and self-assembled flower-like architectures for catalytic applications.2 Tunable LED applications of semicoducting nanowires3 and improvement in the understanding of entangled quantum states and quantum information processing by branched quantum dots in an individual nanostructure4 are some other renowned applications of such materials. Self-assembly of nanoparticles into specific structures may provide controlled fabrication of nanometer-sized building blocks and devices with unique and useful electronic, optical, and magnetic properties.5,6 Self- or induced assembling of simple nanoparticles and rods into complex geometries, such as nanoflowers,7 binary superlattices,8 optical grating,9 and other nanopatterns for various scientific and technological applications have attracted the attention of nanotechnologists and material scientists in recent times. After several years of achievements, scientists have developed various chemical approaches for the * To whom correspondence should be addressed. E-mail: [email protected] (S.C.S.), [email protected] (S.C.S.), [email protected] (R.G.). † Present address: National Centre for Plasma Science and Technology, Dublin City University, Dublin-9, Ireland.

synthesis of nanomaterials and their complex self-assembled nanoarchitectures. Chemical synthesis of nanomaterials or their surface modification to induce self-assembly causes chemical adsorption on the reactive surface sites of nanomaterials, which limits their functionalities. Instead of it, chemical methods involve complex processes and have limited control parameters and different reaction mechanisms for the synthesis of different nanomaterials. Owing to these problems, pulsed laser ablation of a sold target in vacuum, gases, or liquid media in the absence of chemicals, are developed for the synthesis of various nanostructures. Pulsed laser ablation at the solid-liquid interface has shown its revival of interest over the past few years and is known as liquid phase pulse laser ablation (LP-PLA), which involves the firing of laser pulses on the surface of a solid target immersed into liquid media transparent to the ablating laser wavelength. Interaction of the front part of the laser pulse with the target creates vapors on the target surface, which are irradiated by the tail part of the same pulse. It causes photoionization and the generation of a dense, high-temperature, and high-pressure laser plasma plume, which expands perpendicular to the target surface into the liquid. This expanding plume interacts with the surrounding liquid particles, creating cavitation bubbles, which, upon their collapse, give rise to extremely high temperature and pressure. These conditions are, however, very localized and exist across the nanoscale. Indeed, the LP-PLA has proven to be an

10.1021/jp1018907  2010 American Chemical Society Published on Web 05/03/2010

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effective method for preparation of a variety of nanostructured materials, including nanocrystalline diamond;10 cubic boron nitride;11 Ag,12,13 Au,14,15 and Pt16 noble metals; ZnO,17-19 TiO2,20 SnO2,21 and Al2O322 as the active metal oxides; and metal carbide.23 Cadmium hydroxide, Cd(OH)2, is a wide band-gap (3.2 eV), n-type semiconductor material with potential applications in solar cells, photodetectors, transparent electrodes, sensors, and cathode materials for battery applications.24 Owing to its high positive surface charge density, it adsorbs negatively charged dyes or DNA molecules on its surface and is a potential candidate for the purification of water and separation of negatively charged molecules from matrices.25 These important applications of cadmium hydroxide depend on its specific optical and electrical properties, such as its transparency in the visible range of the solar spectrum as well as high electrical conductivity. Despite these, cadmium hydroxide is also an important precursor material that can be easily converted into cadmium oxide by dehydration or other functional optoelectronic materials, such as CdS and CdSe, by reaction with appropriate precursors. Several chemical and physical approaches, including hydrothermal,26 solvothermal,27 microwave-assisted,28 chemical bath deposition,29 and solution adduct methods,30 are employed for the synthesis of cadmium hydroxide nanostructures. Products obtained from these methods have chemically contaminated surfaces, which limits their technological applications. LP-PLA has several advantages over conventional chemical routes, including (a) the availability of a large number of laser and liquid media parameters to control the size, shape, and composition of nanostructures, (b) no chemical, except surfactant, required, which provides materials having surfaces free from contaminations, and (c) the availability of a large number of solid target materials in the periodic table could provide nanomaterials of any element or its oxide using LP-PLA. Recently, we have reported the synthesis of Cd(OH)2 spherical nanoparticles and their thermal conversion into CdO spherical and rod-shaped nanostructures.31 The growth of cadmium hydroxide spherical nanoparticles and their self-assembly into 1D nanorods and nanoflower buds, 2D nanoflowers and nanosheets, and 3D nanotetrapods and nanoflower-like structures from the aqueous solution produced by the LP-PLA of cadmium rods in the aqueous media of different concentrations with some novel findings is the subject of present investigation. To the best of our knowledge, the use of the LP-PLA technique has not yet been reported for the synthesis and self-assembly of such Cd(OH)2 nanoarchitechtures. 2. Experimental Details The experimental arrangement for the synthesis of cadmium hydroxide nanostructures by LP-PLA is similar as that described previously.11-23 Briefly, the surface of a high-purity (99.99%) cadmium rod (Spec Pure, Johnston Mathey), immersed into the aqueous media of 10 mL of an anionic SDS surfactant at different (20, 50, and 100 mM) concentrations, was allowed to irradiate with a focused output from a pulsed Nd:YAG laser (Spectra Physics, Quanta Ray, U.S.A.) operating at a 1064 nm wavelength, 55 mJ/pulse energy, 10 ns pulse width, and 10 Hz repetition rate for 1 h. The laser beam was condensed into a 0.6 mm spot diameter on the target surface using a 25 cm focal length quartz lens through a 8.5 mm length of the liquid media. The vessel with the target was translated along the length of the rod in order to get homogeneous ablation and to avoid crater formation. All the LP-PLA experiments were performed at room temperature and 1 atmospheric pressure. Obtained colloidal solutions of nanoparticles were stored in sealed plastic containers.

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Figure 1. Representative XRD pattern of the nanostructures synthesized by LP-PLA in the aqueous medium of 20 mM SDS.

As-synthesized colloidal solutions of nanomaterials were subjected to UV-visible absorption characterizations using a PerkinElmer Lambda 35 double-beam spectrophotometer with the corresponding aqueous media as reference. Excitation and PL spectra of colloids were obtained with a PerkinElmer LS55 spectrofluorometer. A Technai G-20-stwin transmission electron microscope operating at a 100 kV accelerating potential with 1.44 Å point and 2.32 Å line resolution was utilized for the TEM investigation of colloids aged for 6 months. A drop of colloidal solution of nanoparticles was placed on the carboncoated copper grid and dried at 60 °C before imaging. The nanopowder, obtained after centrifugation and drying of the solution, was used for X-ray diffraction (Rigaku, D-max-2000, Mo KR source with λ ) 70930 Å) and diffuses reflectance measurement. Diffuse UV-vis reflectance spectra of the nanostructured materials were recorded using a PerkinElmer Lambda35 spectrophotometer equipped with a Labsphere RSA-PE-20 diffuse reflectance accessory with barium sulfate as a white standard. 3. Result and Discussion The leading edge of the laser pulse interacts with the cadmium target, immersed in the aqueous media, and causes its surface evaporation. Photons from the middle part of laser pulse get absorbed by cadmium vapor, which causes multiphoton ionization and, consequently, plasma processing. Thus produced plasma expands supersonically against liquid media under strong confinement by absorbing the tail part of laser pulse. Chemical reactions among cadmium ions and species inside the aqueous media occurred at the plasma-liquid interface, inside the plasma plume as well as inside the liquid media, which are responsible for the synthesis of nanostructured materials. With the passes of ablation time, the concentration of nanostructured material in the liquid media increases, which is visualized as the change in the color or concentration; that is, the transparent solution turns into a turbid colloid. 3.1. Structural Characterization of Nanomaterials Produced in 20 mM Aqueous Media of SDS. A typical XRD pattern of the nanostructured material synthesized by LP-PLA in a 20 mM aqueous medium of SDS is depicted in Figure 1. Diffraction peaks at 2θ ) 13.79, 16.49, 19.72, 21.56, 22.38, 25.83, 27.30, and 28.70° are assigned to the (001), (220), (311j), (240), (331j), (221), (132j), and (022) Miller indices of the

Growth of Cd(OH)2 Architectures from Simple Particles

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Figure 2. TEM images of Cd(OH)2 nanorods, nanoflowers, nanotetrapods, and nanoflower bud-like structures (A, B) at the scale of 50 nm, (C, D) at the scale of 2 µm with the SAED pattern in the inset (right) and corresponding FFT pattern (left), and (E, F) at the scale of 1 µm synthesized by LP-PLA in aqueous media of 20 mM SDS.

monoclinic, end-centered Cm (8) phase (JCPDS Card No. 400760) of the γ-Cd(OH)2 with lattice constants a ) 6.530, b ) 10.22, and c ) 3.404 Å. Some of the observed XRD peaks get exactly matched or are very close to the monoclinic/bodycentered phase (Lm (8), JCPDS Card No. 18-1594) of the cadmium oxide hydroxide hydrate (Cd2O(OH)2(H2O)) with lattice constants a ) 5.688 Å, b ) 10.28 Å, and c ) 3.420 Å. Therefore, synthesis of cadmium oxide hydroxide hydrate nanostructures may be also possible. 3.2. Microscopic Investigation of Cadmium Hydroxide Nanoarchitectures. Figure 2 reveals TEM images of γ-Cd(OH)2 nanostructures synthesized by LP-PLA in 20 mM aqueous media of SDS with various nanoarchitectures, including spherical nanoparticles, nanorods, nanotetrapods, nanoflower buds, and two- and three-dimensional nanoflowers having 4-10 petals oriented in 2D and 3D space, respectively. TEM images depicted in Figure 2A,B have a large number of isolated nanorods with an average length of 18.27 ( 2.96 nm and an average diameter of 6.15 ( 1.34 nm, which results in an average aspect ratio of 3.05 ( 0.49 of these nanorods. Instead of these isolated, there

are several nanorods assembled at one of the two ends to form 4-10 petal 2D and 3D nanoflowers (encircled by rectangles). Panels C-F of Figure 2 represent TEM images of nanoarchitectures having a longer dimension as with that displayed in Figure 2A,B. The average length of the isolated nanorods, petals of the nanoflowers, and edges of the nanotetrapods in these images is 2.2 ( 0.45 µm with an average diameter of 0.49 ( 0.085 µm, which results in an average aspect ratio of 4.49 ( 0.40 for these nanostructures. SAED (right inset of Figure 2D) and the corresponding FFT patterns (left inset of Figure 2D) confirm the growth of single-crystalline, cadmium hydroxide nanorods in the C direction with a high degree of crystallinity. Figure 2E illustrates isolated nanorods (1), nanoflower bud (2), and 2D (3) and 3D (4) nanoflower-like architectures. The histogram of the aspect ratios of the nanostructures shown in Figure 2 and the expanded view of TEM images (Figure 2A,B) are displayed in Figure 3, showing that there are two Gaussian distributions of abundances centered at 2.8 and 3.6 aspect ratios, which reflects that there are two different ages of nanostructures in the solution. It is evident from the expanded view of the

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Figure 3. TEM images of small spherical nanoparticles and aggregates along with 1D nanoarchitectures and the histogram of the aspect ratio of nanorods (shown in Figure 2A,B) synthesized by LP-PLA in the aqueous media of 20 mM SDS.

TEM images that there are a large number of spherical particles with 2-5 nm diameters along with other nanoarchitectures

(circled by the white line), which may be considered as basic building blocks or seeds for the continuous development of new

Figure 4. (A) TEM image of the four-petal 2D single-crystal Cd(OH)2 nanoflower. SAED patterns at the (A) center of the nanoflower (B), middle point (C), and (D) end point of the petal of the nanoflower shown in (A) with corresponding FFT patterns in the insets.



Figure 5. (A, B) TEM images and (C) SAED pattern of the nanostructured material shown in the inset of (B). (D) FFT pattern with encircled parallel planes. (E) TEM image at different mesh of the grid. (F) SAED pattern of the point shown by the circle in (E) with the coressponding FFT pattern in the inset of the nanomaterial produced in 50 mM aqueous media of SDS.

embryonic nanostructures (nanorods, nanoflowers, nanotetrapods, etc.) as well as growth in the dimensions of already developed ones. Analysis of the TEM images (Figure 2A-F) reveals that they have spherical nanoparticles (2-5 nm diameter), nanorods, and nanoflowers of smaller (av ) 18.27 ( 2.96 nm) as well as larger (av ) 2.2 ( 0.45 µm) dimensions, which suggest that the spherical nanoparticles are assembled in a linear manner to form smaller nanorods. These nanorods get selfassembled in different ways to evolve into nanoflowers, nanotetrapods, and other nanoarchitectures as their successors. Isolated spherical nanoparticles present in the system are used to increase the length and diameter of the nanorods and hence dimensions of the other derived nanoarchitectures. Therefore, it can be easily concluded that spherical nanoparticles are the basic building blocks of all the nanoarchitectures present in the solution. The image of a single four-petal 2D nanoflower-like structure, alongwith SAED patterns at three different positions are depicted in Figure 4A-D. Viewing petals of the nanoflower (Figure 4A), it can be concluded that several nanorods of the same dimension

get assembled side-by-side to form a single petal of the nanoflower. Four such petals further get self-assembled at a single point to make a 2D nanoflower-like structure. To make a deep structural investigation, SAED patterns from different points of a single nanoflower are recorded. The SAED pattern from the central point of the flower, as well as mid and end points (white line encircled) of a petal, are shown in Figure 4B-D, respectively, along with their corresponding FFT images in the insets. Electron diffraction from different positions of a nanoflower provides crystallographic orientation of atomic planes at those points. All the three recorded SAED patterns correspond to diffraction from different planes containing Cd2+ and OH- ions of the γ-Cd(OH)2 nanoarchitectures. The central part of the nanoflower is more crystalline and has dominated (100) and (002) planes, whereas the middle and end parts of the petal are comparatively less crystalline and with (22j1j), (102), (1j10) and (110), (102) planes, respectively. TEM images and SAED and corresponding FFT patterns of nanomaterials produced by LP-PLA in 50 mM aqueous media of SDS are depicted in Figure 5A-F. Nanomaterial produced

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Figure 6. (A, B) TEM images and (C) SAED pattern of the point marked in (B). (D) FFT pattern corresponding to the SAED of the nanomaterials produced by LP-PLA in 100 mM aqueous media of SDS.

in 50 mM aqueous media of the surfactant has rod-shaped nanostructures with an average length of 387 ( 121 nm and an average diameter of 80 ( 18 nm (aspect ratio ) 4.83 ( 1.13) along with aggregates of spherical nanoparticles. The SAED pattern of a nanorod (Figure 5C) depicts bright spots arranged in line, illustrating its single-crystalline nature with a columnar shape. The FFT pattern corresponding to the SAED (Figure 5C) is displayed in Figure 5D and has parallel planes (marked with circles) of almost equal interplanner spacing. Several sets of parallel planes are arranged symmetrically around a set of parallel planes at the center. There is a higher tendency of aggregation and agglomeration but lower to make an assembly as with the materials synthesized in 20 mM aqueous media. The TEM image of the same sample, but taken from a different position of the grid, is presented in Figure 5E, showing largersized (100-150 nm) cadmium hydroxide aggregates. The SAED pattern of an aggregate has bright spots (Figure 5F) arranged into lines as well as at the corners of the hexagon, depicting not only the highly crystalline nature but also the column shape of its constituents. The linear arrangement of bright spots illustrates that aggregates have dominated rod-shaped nanostructures, while the arrangement at the corners of the hexagon depicts that they have also some spherical particles. The FFT pattern of the corresponding SAED is illustrated in the inset of Figure 5F. Figure 6A,B illustrates TEM images of the product obtained by LP-PLA into 100 mM aqueous media of SDS. These images suggest that the rate of agglomeration and aggregation of nanomaterials in the aqueous media of 100 mM SDS is too high to detect interfaces between two individual nanomaterials. The electron diffraction pattern at a marked selected point of the aggregate (Figure 6C) shows an arrangement of bright spots in the hexagonal manner, which presents that aggregates are spherical in shape, hexagonal in symmetry, and polycrystalline in nature. There are also several diffuse circular rings in the

SAED pattern, which verify the presence of a smaller order of amorphous material at the same point. The FFT image corresponding to the SAED pattern (Figure 6C) is displayed in Figure 6D. TEM investigation of nanomaterials synthesized in the aqueous media of different concentrations of SDS reveals that the aspect ratio of nanorods and the nature of their aggregation and agglomeration increase with the increase of surfactant concentrations. The rate of increase of aggregation and agglomeration of nanostructured materials dominates over the rate of increase of their aspect ratios; therefore, they lose their individual identity at higher (100 mM) concentrations. In addition to these, the nature of making self-assembly and the degree of crystallization also decrease with the increase of SDS concentration. 3.3. UV-visible Absorption and Diffuse Reflectance Characterizations. UV-visible absorption spectra of as-synthesized colloidal solutions of nanomaterials produced by LP-PLA in 20, 50, and 100 mM aqueous media of SDS are depicted in Figure 7. Absorption spectra of all the samples have three absorption band maxima at 230, 300, and 350 nm spectral regions. Absorption at 230 nm corresponds to the interband transition of inner shell electrons to the conduction band, whereas that of peaks at 300 and 350 nm arise due to the coherent oscillations of free electrons (SPR absorption) along the transverse and longitudinal directions of the nanorods, respectively. Absorbance at all the peak positions increases with the increase of surfactant concentrations, showing the rate of increase of ablation with the addition of surfactant into the solution. The SPR absorption peak corresponding to the length and width of the nanorods shifted toward the longer and shorter wavelength side, rspectively, with the increase of surfactant concentration, which reveals that the length of the nanorods in the as-synthesized solution increases, while the width decreases



Figure 7. UV-visible absorption spectra of as-synthesized colloidal solutions of NPs produced by LP-PLA of a cadmium rod in 20, 50, and 100 mM aqueous media of SDS.

with the increase of surfactant concentration, resulting in the increase in the aspect ratio. UV-visible diffuse reflectance spectra (DRS) of nanomaterials, synthesized in 20 and 50 mM aqueous media of SDS, deposited on glass substrates are illustrated in Figure 8A. Bandgap energies of the these materials are calculated from DRS data using the Kubelka-Munk function. The diffuse reflectance, R∞, of the samples is related with the Kubelka-Munk function F(R) by the expression F(R∞) ) (1 - R∞)2/2R∞ ) K/S, where K and S are absorption and scattering coefficients, respectively. For a direct band-gap semiconductor material near its band edge, the F(R∞) relates to the band gap, Eg, as F(R∞) ) Bi(hυ - Eg)1/2/hυ. The plot of (F(R∞)hυ)2 against hυ and the extrapolation of the linear portion of the curve enable the value of band-gap energy (Eg) similar to the well-known Tauc plot. Kubelka-Munk plots, corresponding to the DRS (Figure 8A), are illustrated in Figure 8B. Extrapolation of the linear portions of these curves intercepts the F(R∞) ) 0 line at 3.47 and 3.55 eV for the nanostructures synthesized in 20 mM and 50 mM aqueous media of SDS, respectively, which are bandgap energies of the corresponding samples. The larger bandgap energies of these samples than bulk illustrate their high degree of quantum confinements. 3.4. Photoluminescence and Excitation Spectra of Cadmium Hydroxide Nanostructures. Excitation and photoluminescence (PL) spectra of the nanomaterials obtained through LP-PLA in 20 and 50 mM aqueous media of SDS are depicted in Figures 9 and 10, respectively. PL spectra of cadmium hydroxide nanostructures, obtained from the 20 mM surfactant solution at different excitation wavelengths, have four emission peaks centered at 3.22, 2.87, 2.57, and 2.35 eV. The intensity of all the PL peaks decreases with the decrease of excitation photon energy without any shift in the peak positions. The schematic representation of excitation and emission transitions is illustrated in Figure 9C. Emission peaks observed at 3.22 and 2.87 eV may arise due to the electronic transitions from the bottom of the conduction band to the top of the valence band (corresponding to the band gap) and to the Cd2+ vacancy level/OH- ion at interstitial positions (hole trap levels), respectively. The transition of electrons from energy levels corresponding to Cd at an interstitial position/OH- ion vacancy (electron trap levels) to the top of the valence band and to the

Figure 8. Diffuse reflectance spectra of nanostructured Cd(OH)2 materials synthesized by LP-PLA in the aqueous media of 20 and 50 mM SDS.

hole trap level makes emission peaks at 2.57 and 2.35 eVs, respectively. Excitation with higher energy (shorter wavelength of light) causes the transition of a larger number of electrons from the valence band to the conduction band than the lower energy (higher wavelength of light, hc/λ e ∆Eg) of light excitation. Electrons excited to the higher-energy levels of the conduction band get decayed to the bottom of the conduction band through nonradiative emission of light, followed by radiative transitions to electron, hole trap levels, and the top of conduction bands. Excitation with 233 nm of light may serve as its resonant absorption by the sample, which causes the transition of the maximum number of electrons from the valence band to the conduction band, followed by the appearance of more intense peaks as compared to other excitations. Emission peaks are intense and sharp at 233 nm excitation, showing random lasing behavior of the nanostructures synthesized by LP-PLA in 20 mM aqueous media of SDS. The excitation spectrum (Figure 10A) of the nanostructures synthesized by LP-PLA in 50 mM SDS has a wide peak at 340 nm, a comparatively low intensity and sharper peak at 250 nm, and a hump at 320 nm. The Gaussian fitted PL spectrum corresponding to 250 nm excitation is depicted in Figure 10B, whereas PL spectra corresponding to 250, 320, and 340 nm excitations are displayed in Figure 10C. Excitation with 250 nm of light induces PL emission corresponding to the band gap as well as different defect levels, while only defect level emissions are observed with 320 and 340 nm excitations. PL intensities related to different defect levels increase, while that corresponding to the

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Figure 9. (A) Excitation, (B) PL spectra at different excitations of Cd(OH)2nanostructures synthesized in 20 mM aqueous media of SDS and (C) schematic representation.

band gap decreases as the excitation wavelength approaches closer to the band-gap energy (∆Eg ≈ 3.55 eV). Despite the lowest excitation intensity of this sample at a wavelength of 250 nm, it produces the most intense emission at 3.48 nm when excited by this light. Excitation with 250 nm light causes the transition of electrons from the valence band to the higher-energy levels of the conduction band. Few of them may get decayed directly from higher-energy levels to the valence band and emit light (3.8, 3.58 eV) larger than that of the band-gap energy, while the rest of them may be decayed to the bottom of the conduction band by thermal relaxation, followed by their transitions to the valence band and hole trap level with an emission of 3.39 eV (band-band) and 2.87 eV (hole trap level) light energies, respectively. Some of them get trapped by the electron trap level, followed by transitions to the hole trap level and valence band with the emission of light energies of 2.84 and 3.13 eV, respectively. In the cases of 320 and 340 nm excitations, most of the electrons may get trapped by the electron trap level, consequently, a weak emission related to the band-band transition. Electronic transitions from the electron trap level to the valence band and hole trap level are the basis of PL peaks at 2.95 and 2.79 eV, respectively. Ratios of emission intensities for 320

and 340 nm excitations at a particular emission wavelength are almost equal to the ratios of their corresponding excitation intensities. 4. Synthesis Mechanism, Nucleation, Growth, and Self-Assembling 4.1. Basic Processes Involved for the Synthesis of Nanomaterials by LP-PLA. The synthesis of cadmium hydroxide nanorods and their self-assembly into various nanostructures includes several steps, starting from laser ablation of the cadmium target to produce cadmium plasma confined in the liquid media to various cadmium hydroxide nanoarchitectures as the final product. According to the works of Fabro et al.,32 the front part of the laser pulse interacts with the target, immersed in liquid media, vaporizes its surface, and creates plasma, referred to as laser induced plasma (LIP). Absorption of the tail part of the laser pulse by LIP and continual supply of vaporizing species from the solid target causes its adiabatic expansion with supersonic velocity under the liquid confinement and creation of a shock wave. The shock wave generated by the expansion of the LIP under the confinement of liquid pushes the LIP into a thermodynamic state of the higher



Figure 10. (A) Excitation, (B) Gaussian curve fitted PL spectrum corresponding to 250 nm excitation and (C) PL spectra at different excitations of Cd(OH)2nanostructures synthesized in 50 mM aqueous media of SDS.

temperature, higher pressure, and higher density than that of the initially generated plasma by creating an additional pressure and temperature increase in the LIP. Four types of chemical reactions, (a) between plasma species inside the plasma, (b) between the plasma and liquid species inside the plasma, (c) between the plasma and liquid species at the plasma-liquid interface, and (d) between plasma species and liquid species inside the liquid, may be taking place. Various reaction products get cooled down and condense to synthesize nanoparticles suspended on the surface of liquid media as well as inside the liquid. 4.2. Evolution, Growth, and Self-Assembly of Various Cadmium Hydroxide Nanostructures. Figure 11 displays the stepwise evolution and growth of various nanostructures and their self-assemblies in the solution, whereas Figure 12 presents a schematic view of the evolution and growth of those nanostructures and their assemblies. The laser produces cadmium plasma, which expands supersonically under the confinement of liquid media. Aqueous hydroxylation of the cadmium plasma takes place at the plasma-liquid interface to form cadmium hydroxide molecules, which act as seeds for the growth of cadmium hydroxide spherical nanoparticles with a diameter of 8 nm, as represented by step II of the scheme (Figure 12). These spherical Cd (OH)2 NPs get linearly self-assembled to form Cd (OH)2 nanorods, which are represented by steps III

and IV of the scheme (Figure 12) and displayed in Figure 11B. Nanorods thus formed get self-assembled in several ways to make various nanostructures, such as nanosheets, nanoterapods, nanoflowers, nanoflower buds, etc. Four nanorods get attached at one end with second ends directed toward edges of tetrahedral, forming a nanotetrapod-like structure (Figure 11C, Figure 12, step V), while a large number of small nanorods get attached at the end of a single, longer nanorod to form a nanoflower bud-like structure (Figure 11D, Figure 12, step V). In a similar way, nanorods get assembled side-by-side to make a nanosheetlike structure (Figure 11E, Figure 12, step V) and, at the one end with comparatively larger distances between their second end, to form a single petal of a flower (Figure 12, steps V and VI). Numbers of such petals are getting assembled in a plane to make 2D (Figure 11F, Figure 12, step VII) as well as out of plane for 3D (Figure 11G) nanoflower-like architectures. Small nanoparticles (2-3 nm) and aggregates, shown in Figure 3, are continuously used for the fabrication of smaller, new nanorods and nanoarchitectures by self-assembly as well as growth of already fabricated nanostructures by surface deposition. The evolution of a cadmium hydroxide embryo and the synthesis of its new spherical nanoparticles are continued until all the Cd(OH)2 molecules in the solution get consumed.

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Figure 11. TEM images of nanostructured materials in the order of their origin: (A) nanoparticles, (B) nanorods, (C) nanotetrapods, (D) nanoflower bud, (E) nanosheet, and (F) 2-dimmensional and (G) three-dimensional nanoflowers.

Figure 12. Schematic evolution of various nanostructures starting from laser ablation of the target.

Now, it is reasonable that several questions may arise in the mind of readers: “How do particles get self-assembled to form rods? How do these rods get self-assembled to make several other complex nanoarchitectures? What is the driving force in the evolution of rods from simple particles and other 2D and 3D architectures from 1D rods? Therefore, these issues are also the subject of discussion and are done in the following steps. One third of cadmium atoms located at the surface of cadmium hydroxide spherical nanoparticles are positively charged (Figure 13a) due to the protonation of hydroxyl group coordinated with the cadmium atom to give a composition of [Cd37(OH)68(H2O)]6- · 6X- [X- is a counter anion].25b The positively charged surface of the spherical cadmium hydroxide NPs electrostatically attracts the anionic head (SO4-2) of the

SDS molecule, making its tail away from the surface. A cadmium hydroxide nanoparticle attached with an SDS molecule acts as a unit for the growth of nanorods from the nanoparticles. Tails of SDS molecules, attached with different Cd(OH)2 NPs, get bound with each other and act as molecular fasteners and act as molecular fasteners between NPs to make nanorods (Figure 13b). The arrangement of SDS molecules around the cadmium hydroxide nanoparticles or nanorods determines the shape of the daughter nanoarchitectures and depends on the distribution of positively charged cadmium ions on the mother architecture. The most favorable condition to minimize Coulombic potential for the system of cadmium hydroxide particles and SDS molecules is their linear arrangement. Surfaces of the nanorods perpendicular to the growth



Figure 13. Cartoons illustrating the following questions: How do cadmium hydroxide nanoparticles get assembled into nanorods? What are the driving forces? How do nanorods get assembled into various complex architectures?

direction have a large number of positively charged cadmium atoms, which attract anionic heads of a number of pairs of SDS molecules. These pairs act as molecular linkers between surfaces of two rods for their side-by-side assembly and formation of nanosheets and petals of nanoflowers (Figure 13c). The ends of nanorods may also attract three pairs of SDS molecules from spatial directions corresponding to three edges of a tetrahedral. Anionic heads of counter SDS molecules of each pair also draw similar cadmium hydroxide nanorods to make nanotetrapodlike structures (Figure 13d). At last, anionic heads of a large number of SDS molecules are attached at the end of nanopetals and are oriented in 2D planes as well as 3D space to attract similar petals to form 2D and 3D nanoflower-like architectures, respectively (Figure 13e). In the formation of all of the nanoarchitectures, a pair of SDS molecules act as molecular fasteners between their mother architecture units and electrostatic forces drive these architectures. 4.3. Influence of Surfactant Concentration on the Growth, Assembly, and Agglomeration/Aggregation Processes. The growth of the above-described nanostructures and surfactantinduced assembly, agglomeration/aggregation are complex

processes. SDS plays an important role in controlling the size of the nanoparticles and their self-assembly, with different mechanisms below and above the critical miceller concentration (CMC), but has an almost opposite nature for nanomaterials having positive and negative surface charge densities. The size and polydispersity of nanoparticles, having a negative surface charge density, such as zinc oxide and silver, decreases, while the stability of their colloidal solution and nature of making self-assembly increases with the increase of SDS concentration. The positive surface charge density of the cadmium hydroxide nanoparticles in the colloidal solution may be responsible for the decrease of colloidal stability and self-assembling behavior of cadmium hydroxide nanoparticles in the solution with the increase of SDS concentration. If the surfactant concentration is higher than that of the CMC, there is competition between self-assembling of the surfactant, that is, making micelles and surface capping of nanomaterials to prevent agglomeration. Most of the surfactant molecules, in the system of positively surface charged nanomaterials suspended in the aqueous media of concentrated (concn > CMC) anionic surfactant, are used to make micelles rather than capping of nanomaterial surfaces. This

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tendency increases rapidly with the increase of surfactant concentration; consequently, surfactant molecules as well as nanomaterials make their own aggregates separately. As a result of these processes, the nature of making self-assembly and the colloidal stability of nanomaterials produced by LP-PLA in 50 and 100 mM SDS solution are very low; therefore, a larger number of aggregates are observed in these solutions as with that produced in 20 mM SDS after aging of all the colloidal solutions for 6 months. 5. Conclusion Laser ablation of a cadmium rod in the aqueous media of SDS enables a simple and versatile route for the fabrication of the γ-Cd(OH)2 nanoparticles, nanorods, nanotetrapods, nanoflower buds, and 2D and 3D nanoflower-like structures. The concentration of surfactant affects the size, morphology, selfassembling, and aggregation and agglomeration of synthesized nanomaterials. An aqueous medium of 20 mM SDS is found most suitable for the evolution, growth, and self-assembly of γ-Cd(OH)2 nanoarchitectures. The aspect ratio, yield of the product, and rate of aggregation and agglomeration of synthesized nanomaterials get increased with the increase of surfactant concentrations. At the concentration of 100 mM, the rate of agglomeration and aggregation is so strong that the synthesized nanostructures lose their identity. The band gap of the nanostructures synthesized in 50 mM SDS concentration is slightly higher than that synthesized in 20 mM solution. PL spectra of nanostructures obtained from 20 mM SDS concentration have band-gap as well as defect level emission peaks. Intensities corresponding to defect as well as band-gap levels decrease with the decrease of excitation photon energy (increase of excitation wavelength). In the other case, nanostructures synthesized in 50 mM SDS concentration have a band gap with a very low defect level emission for higher-energy photon (shorter wavelength) excitation, while only defect level emissions are observed for lower-energy (higher wavelength) excitations. These observations conclude that nanostructures synthesized in 20 mM SDS concentration have a higher degree of defect levels than that synthesized in 50 mM SDS concentration. Spherical nanoparticles are the basic building blocks of all types of nanoarchitectures, therefore, synthesized nanorods, and hence, other nanoarchitectures derived from nanorods are in the monoclinic/ end-centered phase instead of the hexagonal phase of most of the directly grown nanorods and 1D nanostructures.26,29 Acknowledgment. The authors are thankful to Prof. O. N. Srivastava, Banaras Hindu University, Varanasi, for providing the TEM facility. S.C.S. is grateful to CSIR, New Delhi (Grant No. 09/001/(0303)/2008/EMR-I), for providing a Senior Research Fellowship award to carry out this research. Supporting Information Available: XRD data, phase, system, point group, and cell parameters of Cd(OH)2 and Cd2O(OH)2(H2O) of different JCPDS card nos. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Joshi, R. K.; Hu, Q.; Alvi, F.; Joshi, N.; Kumar, A. Au decorated zinc oxide nanowires for CO Sensing. J. Phys. Chem. C 2009, 113, 16199– 16202. (2) Zhang, W.; Chen, J.; Wang, X.; Qi, H.; Peng, K. Self-assembled three-dimensional flower-like R-Fe2O3 nanostructures and their application in catalysis. Appl. Organomet. Chem. 2009, 23, 200–203. (3) Hayden, O.; Greytak, A. B.; Bell, D. C. Core-shell nanowire lightemitting diodes. AdV. Mater. 2005, 17, 701–704.

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Nanoarchitectural Evolution from Laser-Produced Colloidal Solution

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