Planar tetrapods ZnO (PTP-ZnO) or cross shaped tetrapod nanostructures were synthesized by a cethyltrimethylamonium hydroxide (CTAOH)-assisted hydrothermal method at low temperature (120°C). The XRD diffractogram showed that the PTP-ZnO nanostructures showed a hexagonal wurtzite phase. The studies with high resolution transmission electron microscopy (HRTEM) and select area specific diffraction (SAED) revealed that the ZnO pods were single crystals and preferentially grew up along [002] direction. The growth mechanism of the CTAOH assisted-hydrothermal synthesized PTP-ZnO nanostructures is explained using the final shape guiding of materials nanostructured and surfactant-action theories.
Actually, the semiconductor zinc oxide ZnO is playing a protagonist roll between an oxide metallic family such as ZrO2, CaO, MgO, SiO2, Ga2O3, In2O3 TiO2 and SnO2 due to its wide band gap (3.2 eV) and large exciton binding energy of 60 meV [
The source materials used in this study were analytical reagent grade (Sigma-Aldrich) and used without further purification. Zinc acetate [Zn(CH3COO)2] was used as source material of Zn and cetyltrimethylammonioum hydroxide (CTAOH) as surfactant and catalyst. Initially, 0.01 M of zinc acetate was dissolved in ethyl alcohol and magnetically stirred at 60˚C for 1 h. Then, cetyltrimethylammonioun hydroxide was mixed into the solution with Zn/CTAOH molar ratio of 1/1.6 and then refluxed at 120˚C for 2 h. It was observed, as a product of the reaction, a white ZnO powder precipitated at the flask bottom. Then the resulting white powder products were centrifuged, washed with deionized water to remove the organic residues possibly remaining in the final products and finally dried at 60˚C in air.
The reaction mechanism proposed for hydrothermal synthesis is already reported by J. Zhung and coworkers [
In reaction (1) Zn2+ ions are combined with OH− radicals in the alcoholic solution to form a Zn(OH)2 colloid through the reaction Zn2+ + 2OH− → Zn(OH)2. Later, in the hydrothermal process, the Zn(OH)2 is separated into Zn2+ ions and OH− radicals according to reaction (2). Then, ZnO nuclei are formed according reaction (3), when the concentration of Zn2+ ions and OH− radicals reaches a super-saturation grade. Finally, the growth unites of [Zn(OH)4]2− radicals are obtained through the reaction (4). The dissolution-nucleation cycle according to reac- tions (5) and (6) respectively produces:
The X-ray diffraction (XRD) pattern of the PTP-ZnO nanostructures was obtained with a X-ray diffractometer (SIEMENS D 5000) using the CuKα (1.5406 Ǻ) radiation, with a scanning speed of 1˚ per min. at 35 KV and 30 mA. The morphology of the sample was studied using a JEM5600-LV scanning electron microscope. The single-crystal structure of the PTP-ZnO nanostructures was analyzed using a JEOL FEG 2010 Fast TEM electron microscope with a 2.1 Ǻ resolution (point to point).
thermal method at low temperature (120˚C). The XRD diffractograms reveal that this corresponds to ZnO system with wurtzite structure. All diffraction peaks of the PTP-ZnO nanostructures were indexed to the hexagonal phase of ZnO having lattice parameter a = 3.249 and c = 5.206 Å (JCPD file No 36-1451).
Actually it is possible shape guiding of materials nanostructured by controlling critical parameters in assisted hydrothermal surfactant synthesis technique. Lee [
In the before context on shape guiding of nanostructured materials, in this work PTP-ZnO nanostructures
(cross shaped tetrapod) has been synthesized at 120˚C by the CTAOH-assisted hydrothermal method taking account as the key parameters in the hydrothermal process the temperature and the capping molecules. Thus in the hydrothermal process to obtain the PTP-ZnO nanostructures at low temperature (120˚C) it is assumed that the seed unit cell formed in the initial nucleation process is an truncated octahedron with faces which have an crystallographic hexagonal wurtzite structure formed in the central region of the cross-like and from them the growth of the ZnO pods is possible through the preferential growth on the four opposite faces of the {100} faces of the truncated octahedron seed. Finally the ZnO pods are grown along the {002} directions (c axes) with interplanar separation distance of 0.26 nm. In other words the phase of the ZnO pods nanocrystals was determined by the temperature-mediated phase control of the initial seed. However, the played roll of the CTMAOH is as surfactant and catalyst.
Once that an determined crystalline phase has been selected during the initial nucleation stage the delicate balance between the growth kinetic and the thermodynamic growth regimes will affect strongly the final architecture of the ZnO nanocrystals, in our hydrothermal experiment the temperature used was low (120˚C) and in non-equilibrium condition selective anisotropic growth between different crystallographic faces is established: in our case since the surface energy of the (001) face of the wurtzite phase is higher than that other faces, selective growth through the c axes of the hexagonal ZnO pod is induced.
To observe the influence of the CTAOH on the hydrothermal process other two experiments were made: one without CTAOH and second one at higher temperature (200˚C): in the first experiment only particles of about 100 nm were obtained see
nanostructures thus obtained, it clearly shows that ZnO nanorods but no PTP-ZnO nanostructures are obtained, this is because the capsules of CTAOH are normally destroyed at 160˚C in the hydrothermal process and as a consequence only an individual CTAOH molecule interact with a growth unit [Zn(OH)4]2− to form a complex agent which is adsorbed on the surface of the ZnO seed an active site will form and ZnO nanorods preferentially grows in this site.
In this work planar tetrapod (shaped cross-like nanostructures) of ZnO (PTP-ZnO) were synthesized by the CTAOH-assisted hydrothermal method at 120˚C. The ZnO phase obtained for the pods belonging to the PTP- ZnO was the wurtzite hexagonal phase; the ZnO pods crystal single grown along the (002) direction with a separation distance between the (002) faces of 0.26 nm. The total influence of the temperature in the hydrothermal reaction and the initial nuclei seed obtained join with the effect of the capping molecules on the shape of the end architecture for the ZnO nanostructures confirmed the validity of the empirical theory on guiding shape final of nanostructures in a surfactant-assisted hydrothermal process.
The authors wish to thank to Adriana Tejeda (IIM) for the XRD measurements. To Omar Novelo Peralta (IIM), Carlos Flores (IIM) and Luis Rendón (IF) for their support in the SEM and TEM characterization. The authors are also thankful to the Central Microscopy facilities of the Institute of Physics, UNAM, for providing the microscope tools used in this work.
SebastiánLópez-Romero,ManuelGarcía-Hipólito,Mayte Saraí ValverdeLabastida,María de Jesús QuirozJiménez, (2015) ZnO Planar-Tetrapod Synthesized by Cethyltrimethylamonium Hydroxide-Assisted Hydrothermal Method at Low Temperature. World Journal of Condensed Matter Physics,05,339-345. doi: 10.4236/wjcmp.2015.54034