Synthesis and characterization of undoped and cobalt-doped TiO2 nanoparticles via sol–gel technique

Appl Nanosci (2015) 5:449–456 DOI 10.1007/s13204-014-0337-y ORIGINAL ARTICLE Synthesis and characterization of undoped and cobalt-doped TiO2 nanoparticles via sol–gel technique S. Mugundan • B. Rajamannan • G. Viruthagiri • N. Shanmugam • R. Gobi • P. Praveen Received: 13 October 2013 / Accepted: 3 July 2014 / Published online: 24 July 2014 Ó The Author(s) 2014. This article is published with open access at Springerlink.com Abstract TiO2 nanoparticles doped with different concentrations of cobalt (4, 8, 12 and 16 %) were synthesized by sol–gel method at room temperature with appropriate reactants. In general, TiO2 can exist in anatase, rutile, and brookite phases. In this present study, we used titanium tetra iso propoxide and 2-propanol as a common starting materials and the obtained products were calcined at 500 °C and 800 °C to get anatase and rutile phases, respectively. The crystalline sizes of the doped and undoped TiO2 nanoparticles were observed with X-ray diffraction (XRD) analysis. The functional groups of the samples were identified by Fourier transform infrared spectroscopy (FTIR). From UV–VIS diffuse reflectance spectra (DRS), the band gap energy and excitation wavelength of doped and undoped TiO2 nanoparticles were identified. The defect oriented emissions were seen from photoluminescence (PL) study. The spherical uniform size distribution of particles and elements present in the samples was determined using two different techniques viz., scanning electron microscopy (SEM) with energy-dispersive spectrometer (EDX) and transmission electron microscope (TEM) with selected area electron diffraction (SAED) pattern. The second harmonic generation (SHG) efficiency was also found and the obtained result was compared with potassium di hydrogen phosphate (KDP). B. Rajamannan (&) Department of Engineering Physics, (FEAT), Annamalai University, Annamalainagar, Chidambaram, Tamilnadu 608002, India e-mail: S. Mugundan  G. Viruthagiri  N. Shanmugam  R. Gobi  P. Praveen Department of Physics, Annamalai University, Annamalainagar, Chidambaram, Tamilnadu 608002, India Keywords Cobalt  Doped TiO2  Nanoparticles  Crystalline size  FTIR  Optical properties Introduction Titanium dioxide or titania (TiO2) was first produced commercially in 1923. It is obtained from a variety of ores. The bulk material of TiO2 is widely nominated for three main phases of rutile, anatase and brookite. Among them, the TiO2 exists mostly as rutile and anatase phases which both of them have the tetragonal structures. However, rutile is a high-temperature stable phase and has an optical energy band gap of 3.0 eV (415 nm), anatase is formed at a lower temperature with an optical energy band gap of 3.2 eV (380 nm) and refractive index of 2 (Thamaphat et al. 2008). Among these polymorphs, rutile and anatase have been widely studied. Brookite is rarely studied due to its complicated structure and difficulties in sample preparation (Hu et al. 2009). These three phases can be commonly described as constituted by arrangements of the same building block-Ti–O6 octahedron in which Ti atom is surrounded by six oxygen atoms situated at the corners of a distorted octahedron. In spite of the similarities in building blocks of Ti–O6 octahedra for these polymorphs, the electronic structures are significantly different (Guangshe et al. 2011). Photocatalysis using TiO2 as a catalyst has been widely reported as a promising technology for the removal of various organic and inorganic pollutants from contaminated water and air because of its stability, low cost, and non-toxicity (Liu et al. 2008). TiO2 is the promising material as semiconductor having high photochemical stability and low cost. Well-dispersed titania nanoparticles with very fine sizes are promising in 123 450 Appl Nanosci (2015) 5:449–456 many applications such as pigments, adsorbents, and catalytic supports (Ramakrishna and Ghosh 2003). Since Fujishima and Honda discovered the photocatalytic splitting of water on a TiO2 electrode under ultraviolet (UV) light, many synthesis methods for preparing TiO2 nanoparticles and their applications in the environmental (photo catalysis and sensors) and energy (photovoltaics, water splitting, photo/electrochromics, and hydrogen storage) fields have been investigated (Shan and Demopoulos 2010). Recently, fine particles of titania have attracted a great deal of attention, because of their specific properties as an advanced semiconductor material, such as a solar cell, luminescent material, and photocatalyst for photolysis of water or organic compounds and for bacteriocidal action (Sugimoto et al. 2003). The Co-doped TiO2 nanocrystals have consumed great attention due to its enhanced photocatalytic activity (Yang et al. 2007). In this paper, we report the preparation of different weight percentages of Co-doped TiO2 nanoparticles by a sol–gel route. Materials and methods Sample preparation Preparation of bare and cobalt-doped TiO2 nanopowder Sol–gel technique was used to prepare bare and cobaltdoped TiO2 samples. 90 ml of 2-propanol was taken as a primary precursor and 10 ml titanium tetra isopropoxide was added to it drop wise with vigorous stirring during the process of TiO2 formation. The solution was vigorously stirred for 45 min to form sols. Liquid solution cobalt nitrate of desired concentration (4, 8, 12, and 16 %) was poured slowly drop by drop to that mixture with continued stirring. To obtain nanoparticles, the obtained gels were dried at 80 °C for 5 h to evaporate water and organic material to the maximum extent. Finally, the powders were kept in muffle furnace and calcinated at 500 °C for 5 h for the harvest of anatase phase and 800 °C for rutile phase. The particle was pulverized to powder using an agate mortar at room temperature for further characterizations. 0.05°, counting time of 10.16 s per data point) equipped with a Cu tube for generating Cu Ka radiation (k = 1.5406 Å). The incident beam in the 2-theta mode over the range of 20°– 80°, operated at 40 kV and 30 mA. The chemical structure was investigated by AVATAR 330 Fourier transform infrared spectrometer (FTIR) in which the IR spectrum was recorded by diluting the mixed powder in KBr and in the wavelength between 4,000 and 400 cm-1. The band gap energy and the particle size were measured at wavelength in the range of 200-2,500 nm by UV–VIS–NIR spectrophotometer (varian/carry 5000) equipped with an integrating sphere and the baseline correction was performed using a calibrated reference sample of powdered barium sulfate (BaSO4). The photoluminescence spectra (PL) are recorded with Perkin Elmer LS fluorescence spectrophotometer. Scanning electron microscope (SEM) images were observed with a Hitachi S-4800 microscope, combined with energydispersive X-ray spectroscopy (EDX, Oxford 7021) for the determination of elemental composition. Transmission electron microscope (TEM) with selected area electron diffraction (SAED) images w (...truncated)