Materials Letters 117 (2014) 260–262
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Al-doping effects on structure and optical properties of ZnO nanostructures Yan Li, Jian Meng n College of Chemical and Environment, North University of China, Taiyuan 030051, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 October 2013 Accepted 29 November 2013 Available online 7 December 2013
Al-doped ZnO (AZO) nanoparticles are prepared by the homogeneous precipitation method, which were characterized by X-ray diffraction, scanning electron microscope, ultraviolet and visible spectrophotometer and an infrared spectrometer. It is found that the AZO particles are of hexagonal wurtzite phase. With increase of Al doping, the particle size become smaller. A minimum diameter of 20 nm is obtained doped with 5 mol% Al. The band gap is found to broaden with increasing dopant concentration, and the ultraviolet absorption peak appeared blue shift. When doped with 5 mol% Al, the ultraviolet absorption rate and the visible light transmittance are 98% and 90%, respectively. & 2013 Elsevier B.V. All rights reserved.
Keywords: Al-doped ZnO Structural properties Optical properties Homogeneous precipitation method
1. Introduction Zinc oxide is one of the most promising materials for optoelectronic applications because of its wide direct band gap (3.37 ev) and large excitation binding energy of 60 meV . Particle size and morphology have a strong effect on their properties and application . Thus, various ZnO structures including nanostructures, nanowires, nanobowls, and nanopellets have been produced . They are widely used in many important areas, such as solar cells, pigments, gas sensors, electronics and photocatalysts. Different methods have been used to prepare ZnO nanostructures, such as hydrothermal, sol–gel, mechanical milling, and chemical vapor deposition . The above methods have been widely studied, but prepare of nano-zinc oxide by the homogeneous precipitation method is studied less. Doping is an effective and facile method to modify the physical properties (e.g. optics, magnetics and electricity) of the base materials and this will extend the applications of the base materials. The promising oxide Al-doped ZnO (AZO) particles have attracted more and more attentions due to its low cost, nontoxic, superior electrical conductivity and especially the high transparent in visible region and the high absorptive in ultraviolet region. Many research groups have prepared AZO thin ﬁlms, AZO nanorods, AZO nanowires by different methods (e.g. magnetron sputtering, sol–gel method, hydrothermal method and pulsed laser deposition) and investigated the effect of Al doping concentrations on the microstructure, optical property and electrical conductivity of AZO particles. However, there are few reports on the preparation of AZO particle by the homogeneous precipitation method. n
Corresponding author. E-mail addresses: [email protected]
(Y. Li), [email protected]
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.11.126
In this paper, the AZO particles are prepared by the homogeneous precipitation method and the effect of Al doping concentrations on the microstructure and optical property are investigated.
2. Experiment All the chemicals used in this work were of analytical reagent grade and used as received without further puriﬁcation. In a typical synthesis, 9.22 g Zn(NO3)2 6H2O, 5.59 g CO(NH2)2 and a certain amount of OP-10 were dissolved in 100 ml deionized water under constant stirring in the thermostat water bath at 95 1C. Four hours later, the precipitate was obtained after ﬁltering and washing several times. Finally, the pure ZnO was obtained after drying at 90 1C and calcining at 450 1C both in air for 2 h. The products AZO were synthesized by the same steps except adding a certain amount of Al(NO3)3 9H2O to the mixture solution in the ﬁrst step. The structure of the samples was characterized by X-ray diffraction (XRD) using a D/MAX-RB diffractometer with nickel ﬁltered Cu Kα radiation (40 kV, 30 mA and sweep speed ¼6 1/min). The surface morphologies were taken on scanning electron microscope (SEM). The optical properties were investigated by an ultraviolet and visible spectrophotometer and an infrared spectrometer.
3. Results and discussion Fig. 1 shows the XRD patterns of ZnO nanostructures with and without Al doping. All of the diffraction peaks can be indexed as hexagonal wurtzite ZnO for products, which is in good agreement with the reported data for ZnO of JCPDS Card . The very sharp diffraction peaks were indicated the good crystallinity of the
Y. Li, J. Meng / Materials Letters 117 (2014) 260–262
prepared crystals and no characteristic peaks were detected for the other impurities such as Zn(NO3)2 6H2O and Zn(OH)2. No signiﬁcant differences were observed for the undoped and Al-doped ZnO particles with the exception of the weaker peak intensity of the later. The sizes of ZnO nanostructures with and without Al doping calculated according to the XRD date were 20 nm and 70 nm, respectively. We attribute this phenomenon to a substitution of zinc by aluminum in the hexagonal lattice .
Fig. 1. XRD pattern of AZO particles: (a) pure ZnO; (b) AZO doped with 1 mol% Al; and (c) AZO doped with 5 mol% Al.
Fig. 2(a) and (b) shows the SEM images of the ZnO nanostructures with and without Al doping, respectively. It is clear that the products possess high crystallinity . A granular structure with ﬁne particles on the surface is observed from the SEM micrographs. The Al-doped ZnO exhibit uniform shape and a narrow size distribution with an average size of 20 nm, which agrees with the XRD analysis, while the undoped ZnO samples present an uneven size range from 30 nm to 80 nm . It is clear that black points are presented on the SEM images of Al-doped ZnO. We also attribute this phenomenon to a substitution of zinc by aluminum in the hexagonal lattice. In addition, Al-doped ZnO, with gray color, have deeper color than undoped ZnO, we attribute the deeper color to more small particle size. The particle size variation of Al-doped ZnO with different doping concentrations is displayed in Fig. 3 . The particle size ﬁrst decreased with increased Al concentrations. A minimum particle size of 20 nm was obtained at a doping concentration of 5 mol%. However, with increase in the Al doping concentration above 5 mol%, the particle size started to increase signiﬁcantly. When a small amount of Al is introduced into the particle, the Al is ionized into Al3 þ and replaces Zn2 þ , the growth velocity of ZnO is restrained. The precursors with small particle size are produced. At higher Al concentrations, the particle size increases because increasing dopant atoms may form some kind of new phase instead of dissolving in solid solution with ZnO, which leading to cell parameters and spacing increases rapidly. The precursors with large particle size are produced [5-8].
Fig. 2. SEM images (a) AZO doped with 5 mol% Al; and (b) pure ZnO.
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Fig. 3. Particle size of Al-doped ZnO nanoparticles as a function of the dopant concentration.
The smaller particle size of Al-doped ZnO has a larger surface atom ratio. In the infrared irradiation, the lattice vibration is different. There are no other miscellaneous peaks with a doping concentration of 5 mol%, showing that the addition of Al did not form a new phase. The UV–vis absorption spectrum of the Al-doped ZnO with different doping concentrations is displayed in Fig. 5. In the range of visible light, Al-doped ZnO absorb less visible light than undoped ZnO. The former has more transmittance and transparency . In ultraviolet region, the absorption ability of Al-doped ZnO nanoparticles on the UV is much stronger than the undoped ZnO. This is because, in the premise of keeping the ZnO structure, with an increase of the concentration of Al, the forbidden gap of Al/ZnO solid solution also gradually becomes widen . But the doping amount is not the bigger the better, when the doping concentration is greater than 5 mol%, Al tend to form a new phase of Al2O3, rather than replace the position of Zn2 þ , which leading to lower absorption performance of UV. When the Al doping concentration is 5%, the UV absorption rate is the highest, up to 98%. The transmittance of visible light is 90%.
Fig. 4. Infrared spectrum curve of AZO particles with different Al doping concentrations.
In summary, ZnO nanoparticles had been successfully prepared by the homogeneous precipitation method using Zn(NO3)2 6H2O and CO(NH2)2 with 1:3 M concentration in deionized water. The structural analysis conﬁrms that the Al-doped ZnO structures are of hexagonal wurtzite phase with smaller particle size. ZnO nanoparticles doped with 5 mol% Al concentration had a smaller particle size of 20 nm. In the above conditions, the ultraviolet absorption rate and the visible light transmittance are both enhanced, up to 98% and 90%, respectively. At the same time, ultraviolet absorption peak appeared blue shift, also shows that the success of the Al doping. References
Fig. 5. Absorption spectra of AZO particles with different Al doping concentrations.
Fig. 4 shows the infrared spectrum curve of ZnO nanoparticles with different doping concentrations. All the absorption peaks of Al-doped ZnO are well assigned to those of undoped ZnO. The absorption peak in 3500 cm 1 is generally attributed to a certain amount of hydroxyl in the surface of nanometer zinc oxide [9–10]. The absorption peaks in the range of 1500 2000 cm 1 are generally attributed to the bending vibration of free water H–O– H, showing that the prepared ZnO is extremely easy to absorb water. The absorption peak near 450 cm 1 is characteristics of zinc oxide absorption peak. The absorption peak of the characteristic peak of Al-doped ZnO appears a red shift, indicating the enhancement of optical properties. The absorption peak shape is different.
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