Role of vanadium ions, oxygen vacancies, and interstitial zinc in room temperature ferromagnetism on ZnO-V2O5 nanoparticles
© Olive-Méndez et al.; licensee Springer. 2014
Received: 17 January 2014
Accepted: 29 March 2014
Published: 7 April 2014
In this work, we present the role of vanadium ions (V+5 and V+3), oxygen vacancies (VO), and interstitial zinc (Zni) to the contribution of specific magnetization for a mixture of ZnO-V2O5 nanoparticles (NPs). Samples were obtained by mechanical milling of dry powders and ethanol-assisted milling for 1 h with a fixed atomic ratio V/Zn?=?5% at. For comparison, pure ZnO samples were also prepared. All samples exhibit a room temperature magnetization ranging from 1.18?×?10−3 to 3.5?×?10−3 emu/gr. Pure ZnO powders (1.34?×?10−3 emu/gr) milled with ethanol exhibit slight increase in magnetization attributed to formation of Zni, while dry milled ZnO powders exhibit a decrease of magnetization due to a reduction of VO concentration. For the ZnO-V2O5 system, dry milled and thermally treated samples under reducing atmosphere exhibit a large paramagnetic component associated to the formation of V2O3 and secondary phases containing V+3 ions; at the same time, an increase of VO is observed with an abrupt fall of magnetization to σ?~?0.7?×?10−3 emu/gr due to segregation of V oxides and formation of secondary phases. As mechanical milling is an aggressive synthesis method, high disorder is induced at the surface of the ZnO NPs, including VO and Zni depending on the chemical environment. Thermal treatment restores partially structural order at the surface of the NPs, thus reducing the amount of Zni at the same time that V2O5 NPs segregate reducing the direct contact with the surface of ZnO NPs. Additional samples were milled for longer time up to 24 h to study the effect of milling on the magnetization; 1-h milled samples have the highest magnetizations. Structural characterization was carried out using X-ray diffraction and transmission electron microscopy. Identification of VO and Zni was carried out with Raman spectra, and energy-dispersive X-ray spectroscopy was used to verify that V did not diffuse into ZnO NPs as well to quantify O/Zn ratios.
The complex mechanisms that allow ferromagnetic order at room temperature in diluted magnetic oxides (DMO) are a controversial subject as magnetic behavior is strongly dependent on the synthesis method and it is very difficult to obtain reproducible homogeneity on samples. It has been widely supported that ferromagnetism is originated by structural defects [1, 2], mainly oxygen vacancies , but there exist some other structural defects such as interstitial cations [1, 4], cation vacancies , impurities , and if we consider so, the common doping with 3d ions . It has been shown theoretically and experimentally ( and references there in, ) that almost all of these defects have magnetic moment. On the other hand, some other systems report the absence of room temperature ferromagnetism on the same material combination. Coey et al. reported the construction of a phase diagram  for DMO, including percolation thresholds for oxygen vacancies (VO) and doping cations. Depending on the combination of these important defects, ferromagnetic, paramagnetic, or antiferromagnetic order can be presented on semiconducting or insulating oxides. Structural disorder can also be present in epitaxial thin films where crystalline order does not mean absence of Schottky and Frenkel defects. Epitaxial films are normally grown under thermodynamic equilibrium, avoiding an excessive formation of punctual defects higher than that intrinsically found: interstitial cations or VO in ZnO, TiO2, or SnO2.
The most popular mechanism for ferromagnetic order in DMO is the bound magnetic polaron (BMP) where a trapped electron at the site of the VO, with a hydrogenic radius (0.4 to 0.6 nm), intercepts and polarizes the magnetic moment from 3d ions creating ferromagnetic order. Percolation of such BMPs creates a spin-polarized impurity band. The polarization of this band depends on the energetic overlapping with the spin split 3d bands of the cation. This is a reason which holds that no ferromagnetism would be expected for certain systems such as SnO2: Sc, Ti, and Zn  or ZnO: Cr . On the other hand, ferromagnetism evidence on SnO2:Zn nanorods  was recently reported. It was proposed that substitutional Zn induced the formation of Sni defects to which is attributed the magnetic moment. This model is reinforced by theoretical calculations carried out by several groups [13, 14]. The model used to refer the origin of magnetism based on interstitial cations is named BMP’ . Structural defects do not mean partially amorphous material, but a concentration of punctual defects in monocrystalline structures.
In this paper, we report results concerning the structural and magnetic behavior of pure ZnO NPs milled under different conditions, and on the second part, we present a complete analysis of ZnO-V2O5 NPs, getting a clear conclusion about the role of each structural defect.
Samples were obtained by mechanical milling using a high-energy SPEX mill (Spex Industries, Inc., Metuchen, NJ, USA) for 1, 8, and 24 h on a polymer jar with yttrium-stabilized zirconia balls. Powders 99.9% ZnO and 99.6% V2O5 (both from Sigma-Aldrich, St. Louis, MO, USA) were used on the stoichiometric proportion to have 5% at. of V atoms against the total amount of metallic atoms. Also, pure ZnO powders were milled for 1 h with and without ethanol to evaluate the contribution from interstitial zinc (Zni) to the magnetic moment of the samples. Thermal treatment under reducing atmosphere (TT), a mixture of Ar:H2 [10:1], at 680°C for 1 h was applied to some of the obtained samples, a temperature barely higher than 672°C, which is the V2O5 melting point. This temperature was selected to ensure reaction between H2 and O from ZnO to produce VO. Magnetic σ(H) measurements were performed for all samples with a physical properties measuring system (PPMS) from Quantum Design (San Diego, CA, USA) at room temperature and an applied field of 2 T. Structural characterization was obtained from X-ray diffraction patterns (XRD). Chemical composition was identified by energy-dispersive X-ray spectroscopy (EDS) from EDAX in a transmission electron microscope (TEM) and in form of green compressed pellets in a scanning electron microscope (SEM). Micro-Raman spectroscopy was used to identify the presence of VO and Zni.
To name the samples, we use the following nomenclature: for ZnO-V2O5 samples, a number followed by letter h will be used to identify milling time. Ethanol-milled samples will have the suffix .Et, while dry milled samples do not have any suffix. Thermally treated samples will have. Cal suffix. Sample ZnO.Com represents commercial ZnO powder without any treatment. For example, sample 1 h.Et.Cal is a mixture of ZnO and V2O5 milled for 1 h with ethanol followed by TT, while ZnO.Et is pure ZnO ethanol-milled for 1 h and ZnO is 1-h dry milled ZnO.
Results and discussion
Pure ZnO nanoparticles
For sample ZnO.Et, the O2 chemical potential is eliminated as the NPs are surrounded by ethanol molecules. Then, the amount of VO is kept constant while milling increases the concentration of Zni (source of magnetic moment); as a consequence, magnetization increases from 1.34?×?10−3 (ZnO.Com) to 1.42?×?10−3 emu/gr. There exist some reports that attribute ferromagnetic signal in DMO only to VO, but with these defects even if they have magnetic moment (as a consequence of antiferromagnetic coupling with the sources of magnetism: interstitial cations of 3d dopants [18, 19]), the role of VO is only to mediate ferromagnetic order between magnetic moment sources and not to produce magnetic signal. For pure oxide systems, the used model is the BMP’. Our samples were used to confirm the existence of Zni defects at which we attribute the ferromagnetic enhancement magnetization by ethanol-assisted mechanical milling.
Samples with TT have a reduction of the O/Zn ratio as a consequence of the creation of VO; these ratios are semiqualitative as EDS is not a completely quantitative technique. There is also a reduction of the V concentration as a consequence of V2O5 evaporation.
Secondary phase formation containing V+3 ions for samples with TT is also supported by the high positive susceptibility measured on samples; the arrows in Figure 5a indicate the direction in which the susceptibility from samples 1 h and 1 h.Et has changed after TT, supporting the idea that γ-Zn3(VO4)2 and ZnV2O4 are formed during TT and/or cooling and not during milling. A combination of diamagnetic susceptibility from ZnO and paramagnetic susceptibility from γ-Zn3(VO4)2 and ZnV2O4 contributes to the approached value (arrows in Figure 5a). The paramagnetic change is stronger on sample 1 h.Et.Cal, which has lower V2O3 content after milling; then, the TT reduced some of the V+5 to V+3 ions and both of the secondary phases are formed. For sample 1 h, the behavior turns on the opposite way; susceptibility has a slight decrement suggesting separation of V2O3 NPs from ZnO surface to form secondary phases and V2O5.
Ferromagnetic components from typical DMO mechanisms for all samples are shown in Figure 5b. Samples 1 h and 1 h.Et have the highest specific magnetizations σ?~?3.5?×?10−3 emu/gr, but as sample 1 h has the largest paramagnetic component, attributed to V2O3, we can assume that not all V+5 or V+3 contribute to the ferromagnetic moment on the samples. Usually high doping concentration of magnetic ions forms antiferromagnetic complexes ; this is the reason that lower ion concentration produces the highest magnetic moment per doping ion. As V has a very low solubility limit on ZnO?~?0.2% , secondary phases are more easily formed instead of promotion of V diffusion into the ZnO matrix. After TT magnetization decays to σ?~?0.7?×?10−3 emu/gr, which has been already explained with the formation of secondary phases and is also due to a reduction of structural defects on ZnO, NPs from sample 1 h increase their average size by coalescence to reduce their surface free energy and a reorganization of the surface is promoted by atom diffusion, reducing the sources of magnetism; at the same time, reaction between ZnO and V oxides produces secondary phases, reducing the number of ZnO/V interfaces.
We prepared pure ZnO and a mixture of ZnO and V2O5 NPs by mechanical milling in different conditions: dry and ethanol-assisted milling. From Raman spectra of the pure ZnO dry milled sample, the increase of the signal of the A1(LO) mode, related to structural defects such as Zni, supports the fact that this defect is the source of magnetic moment as the sample has higher magnetization than that of commercial ZnO. On the other hand, dry milled samples exhibit a reduction of magnetization; even if milling increases the concentration of Zni, the exposure of the powders to oxygen from air during milling reduces the amount of VO, which mediates ferromagnetic order between Zni. The coupling between Zni through VO corresponds to the BMP’ model.
For the ZnO-V2O5 system, it was proven that V+5 ions added at the surface of the ZnO NPs form BMPs, increasing the magnetization from 1.42?×?10−3 to 3.5?×?10−3 emu/gr, demonstrating that V ions produces magnetic order in the system ZnO:V. TT induced the formation of ZnV2O4 secondary phase, containing V +3 ions, which is paramagnetic. V+3 ions are also present on ZnO-V2O5 dry milled sample as shown by a weak and broad peak on Raman spectra on the interval 750 to 1,000 cm−1, supporting the idea that dry milling, in some form, reduces the charge of some ions from V+5 to V+3. After TT, the amount of VO was increased but magnetization falls to 0.7?×?10−3, demonstrating that the intrinsic amount of VO on ZnO is enough to mediate ferromagnetic order.
All authors work at CIMAV Chihuahua, with the exception of RAGV who works at Honeywell Chihuahua as a design engineer. SFOM is working as a researcher in the field of nanostructured and magnetic materials. CRSR is working as a technician in charge of several magnetic measuring techniques. FEM is a professor working with theoretical simulation, and JAMA is a professor working with a wide variety of magnetic materials.
The authors thank the financial support received from PROIN-CONACYT grant no. 197000. Also want to thank Karla Campos Venegas for EDS acquisition, Enrique Torres Moye for XRD patterns, and Pedro Pizá Ruiz for Raman acquisition.
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