3.1 Structure analysis
Room temperature X-ray diffraction patterns of Bi1 − xCaxFe0.99Nb0.01O3 (BCFNO, x = 0.00 ~ 0.25) are shown in Fig. 1, where Nb1% means BiFe0.99Nb0.01O3 sample. No major impurity phase was observed in the prepared samples. The XRD patterns of pure BiFeO3 and Nb1% and x = 0.05, x = 0.10 samples in the BCFNO series could be labeled in a rhombohedral perovskite structure with R3c phase (JCPDS no.86-1518). With the increasing of Ca doping concentration x, the (104) and (110) peaks tend to merge together when x ≤ 0.01 as shown in the inset of Fig. 1. It is found that the crystal structure of x = 0.25 sample(Bi0.75Ca0.25Fe0.99Nb0.01O3) is a standard cubic perovskite structure with Pm3m phase(JCPDS no.71–0878). This structural phase change is completely consistent with the change trend of the crystal structure of Ca-doped BFO alone[18], which shows that the main contribution of the change in the crystal structure of the Ca-Nb co-doped BCFNO sample comes from the replacement of Bi ions by Ca ions. With an increasing Ca concentration x, all diffraction peaks shift toward higher angle since the twelve coordination ionic radius of Ca2+ (134pm) is slightly smaller than that of Bi3+ (139pm) and the six coordination ionic radius of Nb5+ (64pm) is slightly smaller than that of Fe3+ (64.5pm).
To studied structural properties of Ca and Nb co-doped BFO in detail, Rietveld refinement was adopted using the GSAS program. Patterns of refinements on XRD data were shown in Fig. 2. R3c space group was engaged in refining. As shown the remnant weight profile(Rwp) and and the fitness factor χ2 ≤ 2.564 in Fig. 2, it suggested that the single phase with R3c space group for 0 ≤ x ≤ 0.10 and with Pm3m phase group for x = 0.25, while the double phase for 0.10 < x < 0.25. The best fits was well consistent with XRD data and the phase transition in Ca-doped BFO[18]. The variation of lattice constant as a function of Ca concentration (x) of the BCFNO samples are shown in Fig. 3. With the increase of Ca content x, the lattice constant of BCFNO decrease which the overall trend was consistent with the Ca-doped BCFO sample[18].
Figures 4 shows the SEM images of BiFeO3, BiFe0.99Nb0.01O3, Bi0.90Ca0.10FeO3, Bi0.90Ca0.10Fe0.99Nb0.01O3 samples. The particle size of Bi0.90Ca0.10FeO3 sample is larger than BiFe0.99Nb0.01O3. The particles of Bi0.90Ca0.10Fe0.99Nb0.01O3 are arranged more compactly, the particle size is about 70nm. It is found that a small amount(0.01) of Nb5+ co-doping with Ca2+ further refines the particles of the sample. The average grain sizes D of the samples can be estimated by X-rays data using the Debye-Scherrer formula and the grain size corresponding to the BCFNO(x = 0.00, x = 0.05, 0.10, 0.15, 0.20, 0.25) sample is 27.1nm,49.5nm, 50.7nm, 46.1nm, 54.1nm, 53.1nm. The obtained results confirm that 1% Nb5+ ions are the main cause of particle refinement, and the increased Ca2+ dopants have little effect on the particle size of the samples.
The structure and phase transition confirmed by XRD can also been characterized by the Raman peaks. Figure 5 represents the Raman spectra of BCFNO nanoparticles. For BFO, Raman Raman active modes can be summarized using the following irreducible representation: 4A1 + 9E[19][20]. The main 13 vibrating modes observed are consistent with the results in Ca-doped BFO[18]. It can be seen that the 13 Raman activity peaks of pure BFO decrease and disappear gradually as the Ca doping amount increases to 0.25 in BCFNO, which indicates that the structure change from R3c structure to Pm3m structure in the Ca-Nb co-doped sample without Raman activity peak. Compare Raman Scattering Spectra of BCFNO and BCFO Samples, the similar evolution trend of Raman peaks also proves the structural phase transition is mainly caused by Ca2+ doping[18]. Apparently, these Raman results are consistent with the XRD observations.
3.2 Xps Analysis
XPS spectrum analysis of BiFe0.99Nb0.01O3(x = 0.00) and Bi0.90Ca0.10Fe0.99Nb0.01O3 (x = 0.10) samples were recorded to figure out valence state of Fe and oxygen vacancies(Vo) and the result is illustrated in Fig. 6(a)(b). As shown in Fig. 6(a), there is no shift of the fitting peaks(709.4/710.9eV) of Fe2p 3/2 which respectively could be ascribed to Fe2+ and Fe3+. The constant ratio of Fe3+ and Fe2+ indicates that the Ca2+ doping does not cause the variation of the valence state of Fe ion. The O1s XPS spectrum (see Fig. 6(b)) deconvoluted into two peaks, corresponding to the intrinsic O2− ions and the Vo [21]. As confirmed by the XPS patterns, the oxygen vacancies are increased with Ca concentration to maintain the electrical neutrality of the sample.
3.3 Optical Properties
BiFeO3 can be used as a narrow band gap metal oxide semiconductor in applications such as photocatalytic materials. The room-temperature UV-Vis absorption spectra of BCFNO samples are displayed in Fig. 7. The absorption band is comparatively broad in the wave-length range of 480–560 nm, which indicates the high availability of visible light. The absorption edge shifts to higher wavelengths for the doped samples. The band gap can be determined from Kubelka-Munk function[22]: (αhυ)n =A(hυ-Eg). (αhυ)2 as a function of hυ is plotted in Fig. 7(a), which provides the value of Eg by the tangent at (αhυ)2 = 0. The calculated optical band gap Eg decreases as x increases as shown in the inset (b) of Fig. 7. BFO has a maximum value of Eg (2.06 eV), which gradually decreases down to 1.78 eV for the Bi0.90Ca0.10Fe0.99Nb0.01O3 nanoparticles (see Fig. 7(b)). Two major reasons should be considered to make Eg shorten clear. As analyzed previously, on the one hand, the increased oxygen vacancy can act as a kind of defect induced energy levels below the conduction band(CB), which is closely associated with the shrinking of the band gap. Besides, it is reported the increased of the Fe-O-Fe angle, which could suppress the FeO6-octahedral tilting, decreases the bandwidth of CB and VB of BFO[23][24]. Ting Wang et al.[10] evaluated the optical properties of pure BiFeO3 and Bi0.8Nd0.2Fe0.95Nb0.05O3 nanoparticles prepared by a sol-gel technique and obtained that the values of Eg were approximately 2.16 and 2.05 eV for pure BiFeO3 and Bi0.8Nd0.2Fe0.95Nb0.05O3 nanoparticles. In addition, Kumar et al.[25] synthesized the Bi1 − xCaxFe1−xTixO3 nanoparticles (x = 0, 0.05, 0.10, 0.15 and 0.2) prepared by a tartaric acid modified sol-gel technique and the direct band gap values were found to be about 2.17, 2.16, 2.11, 2.09 and 2.05 eV, respectively. Accordingly, the present Eg values in our work compare favorably with the above-reported ones. Thanks to the energy band gap is located in the visible region, the as-prepared BiFeO3 nanopowders can be used for photocatalysis in the decomposition of organic compounds.
Figure 7 The room-temperature UV-vis spectra of Bi1 − xCaxFe0.99Nb0.01O3 (x = 0.00 ~ 0.25) samples. Inset (a) shows (αhυ)2 versus hυ plot. Inset (b) shows the band-gap Eg versus doping amount x plot.
Besides, XPS valence band spectra for Bi1 − xCaxFe0.99Nb0.01O3 (x = 0.00, 0.05, 0.10) samples are also given in Fig. 8. For Bi0.90Ca0.10Fe0.99Nb0.01O3 sample, with the help of the optical band gap Eg = 1.94eV calculated by the UV-vis spectrum, we can determine the border between the Fermi energy (EF) and valence band energy (VB) is about ~ 0.49 eV, suggesting that the EF is quite near to the VB. According to related reports, BFO is a p-type semiconductivity material [26][27][28], where the EF is near the VB, which means that the proportion of holes is higher than that of electrons. A schematic diagram(Fig. 8(b)) is introduced to better understand it. As the doping concentration x increases, the Ca and Nb co-doped samples gradually becomes a p-type semiconductor material, which might lead to a potential application in photocatalysis[29][30].
3.4 Magnetic Properties
The M-H hysteresis loops for pure BFO and Bi1 − xCaxFe0.99Nb0.01O3 (x = 0.00,0.10,0.25) samples are shown in Fig. 9. All our samples present weak ferromagnetism at room temperature. In order to study the magnetic enhancement mechanism of the BCFNO sample more clearly, the contrast diagram according to the change of the Mr of BFO, BCFO, and BCFNO with the doping amount x is shown in Fig. 10. It can be seen that the change of the remanence Mr of Bi1 − xCaxFe0.99Nb0.01O3 (BCFNO) is similar to that of Bi1 − xCaxFeO3 (BCFO). The Mr is well improved to the maximum value (for x = 0.10) in R3c phase at the same concentration x. When the cubic Pm3m phase appears, with further Ca doping (0.10 < x ≤ 0.25) in pure Ca doped and CaNb co-doped BFO, Mr decreases continuously which is due to the increasing of the Pm3m phase in which the DM interaction vanishes since the Fe-O-Fe angle equals to 180° and the Fe3+ moments are antiparallel to their nearest neighbors[31][32][33].
For the previous BCFO and BFNO series samples, in-depth discussion and analysis have been taken that the magnetic enhancement of pure Ca2 + doped and Nb5 + doped BFO samples are two different mechanisms[17][18]. The incorporation of Ca impurity ions increases oxygen vacancies, forming bound magnetic polarons at the defects, which enhance the magnetization at low Ca2 + doping content x[18]. The enhancement of the magnetic property of the Nb doped BiFeO3 samples is mainly attributed to the large ratio of surface to volume induced by the reduction of the particles size (size effect)[17]. Interestingly, the particle size of the Ca-Nb co-doped BCFNO sample is slightly smaller than that of BCFO due to the small amount of Nb introduced as discussed above. Under the same doping amount x, the magnetic properties of the CaNb co-doped BFO samples are significantly better than that of BCFO. The remnant magnetization (Mr) of BCFNO at x = 0.10 reaches a maximum value (0.146 emu/g) which is about 15 times compared with pure BFO or larger than pure Nb-doped BFO. Therefore,the superior magneitic properties in BCFNO can be ascribed to the synergistic effect of A and B site ions doping:
1. One of the important contributions is the decreasing of the grain size by appropriate Nb doping at Fe stie. The particle sizes of proper fixd content(1%) Nb5+ doped samples decrease below the range of cycloid spin periodicity of BiFeO3 (62 nm) and show a large ratio of surface to volume induced by the reduction of the particles size (size effect).
2. Ca2+ dopants and the increased oxygen vacancies induce impurity levels within the forbidden band, and bound charge carriers to these impurity levels. The bound carriers polarize the localized magnetic cations in their neighbourhoods to form magnetic polarons (MPs), which interrupts the long range AF order[34][35].
As discussed above, the enhancement of magnetization in BCFNO is possible due to the synergistic effect of Ca2+ and Nb5+ ions doping, on the one hand, the size effect induced by Nb5+ ions, on the other hand, the magnetic polarons bounded to the impurities by Ca2+ ions.