Journal of Nanoparticle Research

, 11:1831

Laser ablative approach for the synthesis of cadmium hydroxide–oxide nanocomposite

Authors

    • Laser Spectroscopy and Nanomaterials Lab (LSNL), Department of Physics, UGC-CAS (Material Characterization)University of Allahabad
  • R. K. Swarnkar
    • Laser Spectroscopy and Nanomaterials Lab (LSNL), Department of Physics, UGC-CAS (Material Characterization)University of Allahabad
  • R. Gopal
    • Laser Spectroscopy and Nanomaterials Lab (LSNL), Department of Physics, UGC-CAS (Material Characterization)University of Allahabad
Brief Communication

DOI: 10.1007/s11051-009-9696-9

Cite this article as:
Singh, S.C., Swarnkar, R.K. & Gopal, R. J Nanopart Res (2009) 11: 1831. doi:10.1007/s11051-009-9696-9

Abstract

Cadmium hydroxide/oxide nanocomposite material is synthesized by pulsed laser ablation of cadmium metal in double distilled water. As-synthesized cadmium hydroxide/oxide particles transforms into pure oxide after annealing at 350 °C for 9 h. As-obtained particles are spherical in shape with 15 nm average diameter, while spherical as well as rod shaped nanostructures are formed after annealing. PL spectrum of annealed powder has peaks corresponding to the defect levels rather than the band gap transitions.

Keywords

Cadmium hydroxide/oxide nanocompositePulsed laser ablationX-ray diffractionThermal analysisNanoparticle synthesis

Introduction

Metal hydroxides such as layered double hydroxides, hydroxide double salts, and single metal hydroxides have excellent anion exchange capacity (Meyn et al. 1990; Clearfield et al. 1991; Khan and Hare 2002) with advanced technological applications (Reichle et al. 1989; Tagaya et al. 1993; Fujita and Awaga 1997; Newman and Jones 1998). Hydroxides of divalent metals such as Ni, Co, Cu, and Zn are widely investigated using coprecipitation (Takahashi et al. 1997) or organo derivatization (Ogata et al. 1998) methods. Materials achieved by these methods have chemically contaminated surfaces, poor crystallinity as well as turbostratic disorder. Pulsed laser ablation (PLA) under liquid confinement has advantages to overcome these problems. Metal oxide nanostructures obtained by heat treatment of corresponding PLA produced hydroxides has above-mentioned advantages over that obtained through heat treatment chemically produced hydroxides. Synthesis of colloidal solution of noble metal nanocrystals using PLA of corresponding metal target in aqueous media and study of their size, shape, and other properties on ablation parameters (laser wavelength, irradiance, pulse width, nature of ablation media etc.) are intense field of research now-a-days (Mafune et al. 2000; Tsuji et al. 2001; Kabashin et al. 2003; Compagini et al. 2003). Lasers are also used for controlled resizing and reshaping of already synthesized nanomaterials by melting and fragmentation mechanisms (Fujiwara et al. 1999; Takami et al. 1999; Hodak et al. 2000). Laser ablation of active metals such as Zn (Zeng et al. 2005; Liang et al. 2004a, b; Ishikawa et al. 2006; Yang et al. 2007; Ajimsha et al. 2008), Sn (Liang et al. 2003), Mg (Liang et al. 2004a, b), Ti (Singh et al. 2009), etc. in different confining liquids is done in order to synthesize oxides, hydroxides, as well as oxyhydroxides of these metals.

PLA process has several advantages over other conventional routes including (a) large number of available ablation parameters for controlling the size, shape, and composition of nanomaterials, (b) produced nanomaterials have inherent stochiometry from their mother targets therefore, capability to produce nanomaterials of desired chemical composition, (c) ability of producing nanomaterials having surfaces free from chemical contamination.

Recently we have synthesized ZnO/ZnOOH composite nanomaterials using second and third harmonics of pulsed Nd:YAG laser at different laser irradiance with continuous flow of pure oxygen in the closed vicinity of laser ablated plasma plume (Singh and Gopal 2008). Synthesis of CdO nanocrystals by PLA and its structural, thermal, and optical characterization is main theme of the present investigation. Laser ablation of cadmium metal in pure water produces 18.6 nm average sized particles of Cd(OH)2/CdO nanocomposite, which converted into CdO nanocrystals with 15.9 nm average size after annealing at 350 °C for 9 h.

Experimental setup

Experimental arrangement for the synthesis of colloidal solution of nanomaterials using PLA in aqueous medium is described elsewhere (Singh and Gopal 2007). Briefly high purity cadmium target (99.99 %, Johnston Mathey, U.K.), placed on the bottom of glass vessel containing 30 mL double distilled water, was allowed to irradiate with focused output of 1,064 nm from pulsed Nd:YAG laser (Spectra Physics, Quanta Ray, USA) operating at 35 mJ/pulse energy, 10 ns pulse width, and 10 Hz repetition rate for 1 h. As-synthesized colloidal suspension was brown in color and found stable for 1 week. Solution was centrifuged at 4,000 rpm and obtained residue was dried at 60 °C in oven for 24 h. As-obtained powder was used for TGA, DTA, FTIR, XRD, and annealed at 350 °C for 9 h. UV–visible absorption spectra of as-synthesized colloidal solution and annealed powder dispersed in methanol was recorded with Perkin Elemer Lambda 35 spectrophotometer.

Transmission electron microscopic (TEM) images of as-synthesized and annealed samples were recorded with Technai G-20 Stwin transmission electron microscope. Scanning electron microscopic image of the powder was recorded with JEOL-SEI scanning electron microscope. Powder X-ray diffraction pattern of the as-obtained and annealed powder was recorded with PAN Alytical (Philips model) using Cu-Kα radiation (λ = 1.5406 Ǻ). The thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) of as-synthesized powder was carried out on Perkin Elmer model No. 7 under the atmosphere of nitrogen at the heating rate of 10 °C/min. As-synthesized powder was dispersed into KBr matrix and palletized at 10 ton pressure. Fourier transform infrared Raman (FTIR) spectrum of the powder was recorded by placing the pellet in the path of the IR beam of IR spectrometer (FTIR Spectrum RX-1, Perkin Elmer).

Thermally annealed sample is excited with 266 nm laser light operating at the energy of 15 mJ/pulse energy. The laser beam is focused on the surface of the powder sample, pressed on the glass slide, with 6 cm focal length convex lens. Emitted PL signal is collected on the entrance slit of Spex TRIAX 320M monochromator using 10 cm focal length cylindrical lens. The light signal is dispersed with 1,800 grooves/mm grating and recorded with TE cooled ICCD detector.

Results and discussion

Figure 1 depicts TGA and DTA spectra of as-synthesized powder sample obtained after centrifugation and drying the residue at 60 °C in oven. TGA of the sample illustrates two steps of the mass decay. In the first step from 337 to 400 °C, there is almost 18% weight loss, which is almost equal to the loss of one water molecule from each of the Cd(OH)2 molecules. There is a sharp endothermic peak at 373 °C in the DTA spectrum corresponding to the first step of the TGA, which confirms that sample absorbs heat at this temperature and transition of Cd(OH)2 into CdO is done. Almost 33% weight loss is observed in the temperature range of 475–640 °C.
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Fig. 1

a Thermo gravimetric analysis (TGA) and b differential thermal analysis spectra of as-synthesized dried powder

X-ray diffraction pattern of as-obtained and annealed powder samples are illustrated in Fig. 2. The XRD peaks for as-synthesized powder lying at 2θ = 29.50°, 35.18°, 49.00° are assigned as [100], [101], and [102] miller indices of hexagonal symmetry of Cd(OH)2, while that of peaks at 2θ = 33° and 38° are attributed for [111], [200] indices for the cubic symmetry of CdO nanocrystals (Fig. 2a). Peak at 2θ = 48° corresponds to the unreacted cadmium metal. Diameter of the nanocrystals is measured from XRD peaks using Scherrer’s equation (D = 0.9λ/βcosθ; where D is the crystallite diameter, λ the wavelength of X-ray used for diffraction, and β the FWHM at the selected 2θ). XRD data of as-synthesized powder corroborate that the as-obtained powder is Cd(OH)2/CdO nanocomposite having 18.6 nm average diameter for Cd(OH)2 nanocrystals while that of 21.8 nm for CdO. XRD data of annealed sample at 350 °C for 9 h (Fig. 2b) illustrates that almost all the peaks corresponding to Cd(OH)2 disappears and CdO peaks become dominant. Sharp and intense peaks at 2θ = 33.0°, 38.32°, and 55.30° prove the synthesis of good quality crystalline CdO nanoparticles (NPs) with 15.9 nm average diameter. Transition of Cd(OH)2/CdO nanocomposite material into pure CdO NPs at 350 °C, as evidenced by XRD data, strongly supports our TGA and DTA observations of elimination of a water molecule from each of the Cd(OH)2 molecules as depicted below:
$$ {\text{Cd}}\left( {\text{OH}} \right)_{ 2} \to {\text{CdO}} + {\text{H}}_{ 2} {\text{O}} . $$
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Fig. 2

X-ray diffraction (XRD) spectra of (a) as-synthesized dried powder and (b) annealed at 350 °C for 9 h

TEM images and SAED pattern of as-synthesized colloidal solutions of NPs produced by PLA of cadmium in double distilled water are illustrated in Fig. 3. Particles are spherical in shape with 10–20 nm diameters, which is also almost equal to that calculated from XRD data. SAED pattern shows bright spots arrange in three parallel lines indicates that synthesized particles are single crystalline in nature. Low magnification SEM image of as-synthesized Cd(OH)2 nanopowders is shown in Fig. 4. TEM images of cadmium oxide nanomaterials produced by thermal annealing of as-synthesized powder at 350 °C for 9 h are displayed in Fig. 5. It is evident from these images that heat treated CdO nanomaterial has spherical and rod shaped nanostructures. Spherical particles have two size distributions lie in the range of 5–10 nm with 7 nm average diameters and 20–30 nm with 24 nm average sizes. Spherical particles are assembled in the linear manner and make rod shaped nanostructure of 500 nm in length and 55 nm diameter with arrow like structure at the end.
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Fig. 3

TEM images and SAED pattern of as-synthesized Cd(OH)2 nanoparticles

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Fig. 4

Low magnification SEM images of as-synthesized Cd(OH)2 nanoparticles

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Fig. 5

TEM images of CdO nanoparticles produced after thermal annealing of as-synthesized Cd(OH)2 powder at 350 °C for 9 h

UV–visible absorption spectra of as-synthesized colloidal solution of NPs produced just after the ablation and solution obtained by dispersing 350 °C, 9 h annealed powder are displayed in Fig. 6. UV–visible absorption of as-synthesized solution of NPs illustrates a wide absorption peak centered at 300 nm with a long tail toward a higher wavelength side. The absorption peak lies in between the surface plasmon resonance absorption peaks of Cd metal nanocrystals (243 nm) and that of the CdO nanocrystals (344); therefore, it may be due to the Cd(OH)2 nanocrystals, which is evidenced by XRD and DTA analyses. Baseline modified UV–visible absorption curve of methanol dispersed CdO NPs is shown in Fig. 6a with dotted line in order to comparison with as-synthesized colloidal solution, while original spectrum is displayed in the inset. As-synthesized colloidal solution, i.e., Cd(OH)2 has significant absorption in the visible spectral range, therefore, limited applications as window materials and waveguides, while that of the annealed sample (CdO NPs) exhibited almost negligible absorption in the optical range, hence can be used as TCOs and waveguides.
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Fig. 6

UV–visible absorption spectra of quantum dots synthesized by laser ablation of cadmium in water: a as-synthesized colloidal solution of nanocrystals in water (solid line) and base line modified 350 °C for 9 h annealed methanol dispersed powder (dotted line). Original spectrum of 350 °C for 9 h annealed sample is illustrated in the inset b Tauc plot of as-synthesized powder (solid line) and 350 °C for 9 h annealed methanol dispersed powder (dotted line). (c) Line intercepts at α = 0 for 350 °C, 9 h annealed methanol dispersed powder. (d) Line intercepts at α = 0 for as-synthesized colloidal solution

The band gap of semiconductor materials increases with the decrease in particles size, which leads to the shift of the absorption edge toward high energy; this is the so-called quantum size effect. The optical band gap of as-synthesized colloidal solution, i.e., Cd(OH)2 NPs as well as annealed sample, i.e., CdO nanocrystals are studied by UV–visible optical absorbance spectra. The optical band gap, Eg, of the samples are determined from the absorbance spectra, where a steep increase in the absorption is observed due to the band–band transition, from the general relation (αhν)n = B(E − Eg), where B is the constant related to the effective masses of charge carriers associated with valance and conduction bands, Eg the band gap energy, E = hν the photon energy, and n = ½ or 2, depending on whether the transition is indirect or direct, respectively. Figure 6b depicts plot of (αhν)2 versus the photon energy hν having solid line for as-synthesized sample and dotted line for that of annealed one. Nature of the plot is in such a way that there are two intercepts corresponding to α = 0 at 2.70 eV corresponding to band–band transition, while that of intercepts at 3.89 is due to transition of deep level electrons to conduction band. Plot corresponding to UV–visible absorption spectrum of as-synthesized colloidal solution is illustrated in Fig. 6d, making intercept at 2.6 eV for α = 0, predicting that Cd(OH)2 nanocrystals have comparatively smaller band gap energy.

Using the atomic absorption values, size of the nanocrystals can be determined from effective mass formula:
$$ E \cong E_{\text{g}}^{\text{bulk}} + {\frac{{\hbar^{2} \pi^{2} }}{{2er^{2} }}}\left( {{\frac{1}{{m_{\text{e}} m_{0} }}} + {\frac{1}{{m_{\text{h}} m_{0} }}}} \right) $$
where \( E_{\text{g}}^{\text{bulk}} \) is the band gap energy of the bulk material, r the radius of nanocrystals, me and mh the effective masses of electrons and holes, while m0 the mass of the free electron.
The infrared spectroscopy may be found useful for understanding bonding between Cd, O and Cd, OH atoms/molecules. FTIR spectrum of KBr dispersed as-synthesized CdO/Cd(OH)2 powder is illustrated in Fig. 7. The FTIR peaks at 565 and 715 cm−1 is the characteristic vibrations of Cd–O, while that of at 857, 1425.36, 1,603, and 3,400 cm−1 is for Cd–OH stretching, bending, and their overtones.
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Fig. 7

Fourier transform infrared (FTIR) spectra of as-synthesized Cd(OH)2/CdO nanocomposite material

The synthesis processes of noble metal nanocrystals using laser ablation technique in solution are extensively studied, while that of metallic oxides and hydroxides nanocrystals involves complex processes. On the basis of previous studies of laser ablation of solid target in solution at solid–liquid interface (reactive quenching zone), the interaction of laser light with solid target would generate hot plasma in the vicinity of laser spot on the target. Similar processes are taken place in the laser ablation in vacuum and gas phase during pulsed laser deposition. In the case of laser ablation in vacuum, ablated species fly out from one another and causes inconvenience in cluster formation and production of nanocrystals. However, during the laser ablation in background gases, plasma is being confined in gas during the pulse, plume expands adiabatically with supersonic velocity and creates a shock wave in front, which produces elevated pressure and further increase of plasma temperature. As liquid is denser compared to gas, therefore larger pressure is being experienced by expanding plume and plasma temperature becomes higher comparative to ablation in gases. High pressure in front of expanding plume provides better way for reaction of ablated species with that dissolved in the liquid media such as oxygen and OH ions. In the case of present study active cadmium atoms, ions, and clusters could react with dissolved oxygen gas at interfacial region of plasma and liquid and OH ions produced by laser induced breakdown (LIB) of water molecules during each of the laser pulse, which produces CdO and Cd(OH)2 molecules, act as nucleation centers for the growth of cadmium oxide and Cd(OH)2 nanocrystals, respectively:
$$ 2 {\text{Cd}}^{ + + } + {\text{O}}_{ 2} \left( {\text{dissolved}} \right) \to 2 {\text{CdO,}} $$
$$ {\text{Cd}}^{ + + } + 2 {\text{OH}}^{ - } \to {\text{Cd}}\left( {\text{OH}} \right)_{ 2} . $$

As there is limited amount of dissolved oxygen in the water, while significant amount of OH ions produced by laser induced breakdown of water molecule, therefore ratio of Cd(OH)2/CdO is very high.

Photoluminescence spectrum of cadmium oxide NPs produced by heat treatment of as-synthesized Cd(OH)2/CdO nanocomposite at 350 °C for 9 h is illustrated in Fig. 8. The recorded spectrum is Gaussian fitted into four peaks at 2.30, 2.29, 2.26, and 2.24 eV. As band gap of CdO is more than 2.5 eV and UV data provide band gap energy 2.70 for CdO powder. Therefore, emission peaks may be associated with O vacancy, Cd at interstitial position, Cd vacancy and O at interstitial position rather than band–band transition.
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Fig. 8

PL spectrum of CdO nanomaterial produced by heat treatment of Cd(OH)2/CdO nanocomposite material

Conclusions

We have tried to demonstrate a new method for the synthesis of CdO/Cd(OH)2 nanocrystals by PLA of cadmium metal plate in aqueous medium, which produces 18.6 nm average size for Cd(OH)2 nanocrystals with 21.6 nm sized CdO nanocrystals in trace. Annealing of the sample at 350 °C for 9 h converts most of the Cd(OH)2 nanocrystals into CdO nanocrystals, which is also verified by DTA data. Produced CdO nanocrystals after annealing have 2.7 eV band gap, very close to the CdO bulk band gap (≈2.5 eV), indicating that there is no quantum confinement effect, which is effective below the size of 10 nm. Possible formation mechanism of cadmium oxide and cadmium hydroxide nanocrystals is proposed. This method can provide an alternative, pollution free, way for the synthesis of oxides and hydroxide nanocrystals of other metals. Nanocrystals synthesized by this method have chemical contamination free surfaces, which can be used for biological applications.

Acknowledgments

Authors are thankful to MRC, IISc. Bangalore for TGA and DTA characterization, NCEMP, Allahabad University for XRD & SEM and Prof. O.N. Srivastava and Prof. S.B. Rai, Banaras Hindu University, Varanasi for TEM and IR facilities, respectively. Mr. Singh is grateful to CSIR, New Delhi for financial support to carryout this work.

Copyright information

© Springer Science+Business Media B.V. 2009