Size controlling and tailoring the properties of GdxZn1-xO nanoparticles
Graphical abstract
Introduction
Zinc oxide (ZnO), a wide band gap semiconductor with large exciton binding energy (∼60 meV), has been extensively used in transparent conducting electrodes for solar cells, ultraviolet protection films, low-voltage, and short-wavelength electro-optical devices, photocatalysis, biomedicine, etc. [1]. In addition, ZnO-based nanomaterials have become the focus of intense scientific and technological interest due to their potential application in spintronics [2]. Unlike traditional electronics, in spintronics, both electron charge and spin are expected to carry on the information. For this purpose, diluted magnetic semiconductors (DMSs), which can be achieved by doping a semiconductor host crystal with magnetic ions, are natural candidates as they can potentially combine the electronic conduction with controlled spin polarization [3,4]. Nonetheless, for the production of functional magnetic devices, these materials must show a ferromagnetic (FM) order with a Curie temperature (TC) above 300 K [5]. Although weak room-temperature ferromagnetism has been widely reported in DMSs, including doped and even undoped zinc oxide [[6], [7], [8]], this issue still remains controversial, with no consensus on the nature of the observed long-range magnetic order [9].
Regarding the choice of the magnetic ions, transition metals (TM) and rare-earths (REs) have been applied for semiconductor doping, aiming fabrication of DMSs. However, unlike TM, in RE elements the 4f-electrons are strongly localized, which leads to indirect interactions (via 5d or 6s conduction electrons) that produce high magnetic moment per atom [10,11]. The potential of REs in this field has stimulated new research and promising results have been reported on Eu-, Dy-, Er-, Yb- and Ho-doped ZnO [[12], [13], [14], [15], [16]]. Moreover, scientific investigations on Gd-doped zinc oxide, in different shapes (thin films, nanopowders, nanorods, etc.) and synthesized by different methods (co-precipitation, combustion reaction, thermal decomposition, etc.), have provided different results in terms of magnetic properties at room temperature. Among them, pure paramagnetic (PM) behavior or the coexistence of paramagnetism and ferromagnetism are the most common observations [[17], [18], [19], [20]]. Indeed, considering that this topic is still not completely clarified, new efforts are required to investigate the Gd-doped ZnO, particularly in the nanosize regime, which can afford better solution of dopant and Zn ions in the crystal lattice, limiting the formation of dopant aggregates and avoiding ambiguous interpretation of the magnetic properties. Aiming to contribute to this important topic, the present study reports on a systematic study about the effects of Gd-doping on the structural, optical and magnetic properties of ZnO as mixed GdxZn1-xO nanoparticles (), synthesized by the polymer precursor method.
Section snippets
Materials and methods
GdxZn1-xO nanoparticles (with x = 0.000, 0.010, 0.020, 0.030, 0.040, 0.050, 0.075, and 0.100) were synthesized by the polymer precursor method (Pechini's method) using zinc nitrate hexa-hydrate (Zn(NO3)2·6H2O), gadolinium nitrate penta-hydrate (Gd(NO3)3·5H2O), ethylene glycol (C2H4(OH)2) and citric acid (C6H8O7) as reactants. These materials were purchased from Sigma-Aldrich and were used without any further purification. In short, citric acid (47.7 wt %) and zinc nitrate hexa-hydrate (31.7 wt
Results and discussion
In order to evaluate the annealing temperature required for the production of the GdxZn1-xO NPs from the precursor resin, thermogravimetric analysis/differential scanning calorimetry (TG/DSC) measurements of the as-produced gel were performed. The results are shown in Fig. 1. As can be seen, the complete resin decomposition process occurs after a few stages (c.a. vertical dotted lines): from RT to ∼160 °C (i), from ∼160 °C to ∼370 °C (ii), from ∼370 °C to ∼480 °C (iii), and from ∼480 °C to
Conclusion
GdxZn1-xO nanoparticles (0.000≤x≤0.100), with wurtzite structure, were successfully synthesized by the polymer precursor method. TG/DSC measurements confirm the crystallization at temperatures above 480 °C. XRD data analysis provides evidences for the progressive substitutional solution of Zn and Gd ions in the wurtzite crystalline structure as the Gd-content is increased. However, a secondary Gd2O3 phase is formed for samples with x≥0.075. The particle size shows a decreasing tendency as the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors thanks the Instituto de Ciências Biológicas (IB-UnB) for the TEM images. TJC also acknowledges Pró-Reitoria de Pesquisa e Inovação (PRPI-IFB) for the financial support in terms of the “Edital N° 41/RIFB 2018”. JAHC thanks the brazilian agencies CNPq and FAPDF for financial support.
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