Laser ablative approach for the synthesis of cadmium hydroxide–oxide nanocomposite
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- Singh, S.C., Swarnkar, R.K. & Gopal, R. J Nanopart Res (2009) 11: 1831. doi:10.1007/s11051-009-9696-9
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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.
KeywordsCadmium hydroxide/oxide nanocompositePulsed laser ablationX-ray diffractionThermal analysisNanoparticle synthesis
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 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
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.
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.
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.
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.