Optical properties of ZnO/BaCO3 nanocomposites in UV and visible regions
© Zak et al.; licensee Springer. 2014
Received: 2 June 2014
Accepted: 7 August 2014
Published: 18 August 2014
Pure zinc oxide and zinc oxide/barium carbonate nanoparticles (ZnO-NPs and ZB-NPs) were synthesized by the sol–gel method. The prepared powders were characterized by X-ray diffraction (XRD), ultraviolet–visible (UV–Vis), Auger spectroscopy, and transmission electron microscopy (TEM). The XRD result showed that the ZnO and BaCO3 nanocrystals grow independently. The Auger spectroscopy proved the existence of carbon in the composites besides the Zn, Ba, and O elements. The UV–Vis spectroscopy results showed that the absorption edge of ZnO nanoparticles is redshifted by adding barium carbonate. In addition, the optical parameters including the refractive index and permittivity of the prepared samples were calculated using the UV–Vis spectra.
81.05.Dz; 78.40.Tv; 42.70.-a.
KeywordsOptical Composite materials Ceramic materials
Nanotechnology has the potential to create many new devices with a wide range of applications in the fields of medicine, electronics, and energy production. The increased surface area-to-volume ratios and quantum size effects are the properties that make these materials potential candidates for device applications. These properties can control optical properties such as absorption, fluorescence, and light scattering. Zinc oxide (ZnO) is one of the famous metal oxide semiconductors with a wide bandgap (3.36 eV) and large excitation binding energy. These special characteristics make it suitable to use in many applications, such as cancer treatments, optical coating, solar cells, and gas sensors. In fact, doping, morphology, and crystallite size play an important role on the optical and electrical properties of ZnO nanostructures, which can be controlled by methods of the nanostructure growth. Therefore, many methods have been created to prepare ZnO nanostructures including sol–gel, precipitation, combustion, microwave, solvothermal, spray pyrolysis, hydrothermal[13, 14], ultrasonic, and chemical vapor deposition (CVD)[16, 17]. As mentioned above, the doping of ZnO with selective elements offers an effective method to enhance and control its electrical and optical properties. The effects of several elements on the optical and electrical properties of ZnO material have been investigated. For example, Au2+, Ce3+, Eu3+, In3+, and Mg2+[22, 23] have been used in order to control the optical properties; Mn2+, Cr2+, Co2+, Ni2+, Fe3+, Cu2+, and V5+ have been used to enhance the magnetic properties; and Li1+ and Na1+ have been used to obtain a p-type form of ZnO.
In the present research, a modified sol–gel route was used to prepare ZnO/BaCO3 nanoparticles (x = 0, ZnO-NPs; x = 0.1, ZB10-NPs; x = 0.2, ZB20-NPs) using gelatin as a polymerization agent. The gelatin was used as a terminator for growing the ZnO/BaCO3-NPs because it expands during the calcination process and the particles cannot come together easily. The crystallite size and crystallinity of the resulting ZnO/BaCO3-NPs were investigated.
In order to synthesize zinc oxide/barium carbonate nanoparticles (ZB-NPs), analytical-grade zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O, Sigma-Aldrich, St. Louis, MO, USA), barium nitrate (Ba(NO3)2, Sigma-Aldrich), and gelatin [(NHCOCH-R1) n , R1 = amino acid, type b, Sigma-Aldrich] were used as starting materials and distilled water as solvent. To prepare 10 g of the final product (ZB-NPs), the appropriate amounts of zinc and barium nitrate were dissolved in 50 ml of distilled water. The amounts of the precursor materials were calculated according to the (1 - x)ZnO/(x)BaCO3 formula, where x = 0, 0.1, and 0.2. On the other hand, 8 g of gelatin was dissolved in 300 ml of distilled water, and the solution was stirred at 60°C to obtain a clear gelatin solution. Finally, the Zn2+/Ba2+ solution was added to the gelatin solution. The container was then moved into an oilbath; meanwhile, the temperature of the oilbath was kept at 80°C while being continuously stirred to achieve a viscose, clear, and honey-like gel. For the calcination process, the gel was slightly rubbed on the inner walls of a crucible and then placed into the furnace. The temperature of the furnace was fixed at 650°C for 2 h, with a heating rate of 2°C/min.
The phase evolutions and structure of the prepared pure zinc oxide nanoparticles (ZnO-NPs) and ZB-NPs were investigated by X-ray diffraction (XRD; Philips X'pert, Cu Kα, Philips, Amsterdam, the Netherlands). The transmission electron microscopy (TEM) observations were carried out on a Hitachi H-7100 electron microscope (Hitachi Ltd., Chiyoda-ku, Japan) to examine the shape and particle size of the nanoparticles and field emission Auger electron spectroscopy (AES; JAMP-9500 F, JEOL Ltd., Akishima-shi, Japan) for elemental analysis. The ultraviolet–visible (UV–Vis) spectra were recorded by a PerkinElmer Lambda 25 UV–Vis spectrophotometer (PerkinElmer, Waltham, MA, USA).
Results and discussion
UV–Vis diffuse reflectance spectra and bandgap
The derivative method has been found as an easy and accurate method to calculate the optical bandgap compared to the Kubelka-Munk method. In this method, the direct bandgap can be estimated from the maximum of the first derivative of the absorbance data plotted versus energy or from the intersection of the second derivative with energy axis.
Method of optical constant calculations
Auger spectroscopy of ZnO/BaCO3 nanocomposites
ZnO and ZnO/BaCO3 nanoparticles were synthesized by the sol–gel method. XRD was used to study the crystallite sizes and structures. The crystallite sizes of the prepared BaCO3 and ZnO nanoparticles were obtained to be 12 ± 2 and 21 ± 2 nm, respectively, for ZB20-NPs. The average particle size of the prepared ZB20-NPs was obtained to be 30 nm, which supports the XRD results. The optical properties of the prepared samples were studied using UV–Vis spectroscopy. The analyzed results showed that the resonance frequency of the refractive index and permittivity is redshifted by BaCO3 concentration increases. The bandgaps of the pure ZnO, ZB10, and ZB20 nanoparticles were estimated to be 3.3, 3.28, and 3.24, respectively.
A. Khorsand Zak thanks Universiti Teknologi Malaysia for the postdoctoral fellowship. This work was funded by Universiti Teknologi Malaysia.
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