Preparation and Ferroelectric Properties of Er Doped BaTiO3 Nanotubes

In this paper, erbium-doped barium titanate nanotubes with different concentrations were prepared by combining the anodic oxidation method with the low-temperature hydrothermal method. The TiO2 nanotubes produced by electrolysis using titanium flakes as substrate were put into barium hydroxide octahydrate solution for hydrothermal reaction, and different concentrations of Er-doped barium titanate nanotubes were prepared by adding different concentrations of Er(NO3)3·5H2O to the reaction solution. The samples with different doping concentrations were analyzed for their morphological structure, physical phase composition and ferroelectric properties. All samples were found to have complete barium titanate nanotube structure, and the diffraction peaks of XRD were compared with the standard Cope, and the doped samples were all tetragonal phase chalcogenide structure with no other impurities generated. From the hysteresis loop diagram of the samples measured by the TF-2000 ferroelectric analyzer, it can be found that the residual polarization intensity of the samples showed a characteristic of increasing and then decreasing with the increase of the doping concentration, and the residual polarization intensity was the largest at 0.151 uC/cm2 under the hydrothermal reaction condition of 180°C and 3 h with the Er3+ concentration of 0.0025 mol/L.


Introduction
BaTiO3 is a typical ABO3 chalcogenide structure material [1], which has excellent ferroelectricity, piezoelectricity and plasticity properties.Because of its excellent performance, barium titanate is widely used in multilayer ceramic capacitors (MLCC), positive temperature coefficient (PTC) thermistors, sensors and other electronic components, and is known as the "pillar of the electronic ceramic industry" [2][3][4][5].With the rapid development of electronic technology, the requirements for electronic materials are getting higher and higher, especially for electronic components such as multilayer ceramic capacitors, integrated circuits, drivers and sensors, which are increasingly demanding.The miniaturization of electronic components, large energy storage capacity and the improvement of breakdown strength have become an imminent need and the future development trend of the electronics industry.This also places higher demands on the performance of BaTiO3 materials [6][7][8].Because the electrical properties of barium titanate itself can hardly meet the needs of today's technological development, the study of barium titanate modification has become a hot topic of research today.Many cationic dopants are highly soluble in BaTiO3 and are used to design the electrical properties of the material [9] .Different cationic dopants replace BaTiO3 at different positions, and the dopants with different replacement positions can be classified into three types: A-site dopants (replacing the A-site), B-site dopants (replacing the B-site) and amphoteric dopants (which can replace both A-and B-site) [9][10][11].Different substitution positions can change the different properties of barium titanate.Nowadays, the research on using different elements to replace the A/B position of barium titanate to enhance the dielectric, piezoelectric, ferroelectric and optical properties of barium titanate materials has achieved some achievements.
There are three methods for preparing barium titanate: solid-phase, liquid-phase and gas-phase methods.The method used in this paper is the low-temperature hydrothermal method in the liquid-phase method, which has the advantages of low reaction temperature, low cost, small particle size and uniform composition of the product.Barium titanate nanotubes with different erbium doping concentrations were prepared by varying the concentration of the dopant Er(NO3)3• 5H2O.The samples were analyzed by SEM for microscopic morphology, XRD for the physical structure of the crystals, and TF-2000 analyzer for the ferroelectric properties of the samples.

Experiment
Firstly, the titanium plate of 99.9% purity was cut into small square pieces of 2*2 (cm), and then the titanium pieces were ultrasonically cleaned with acetone, isopropyl alcohol, anhydrous ethanol and deionized water in turn for 10 min.Next, 0.8g of ammonium fluoride was dissolved in 15ml of deionized water, and 150ml of ethylene glycol was added and stirred thoroughly for 15min to prepare an organic electrolyte.Using the anodic oxidation method, the platinum was clamped at the cathode and the titanium sheet was clamped at the anode in the organic electrolyte of ammonium fluoride and electrolyzed for 4h under the condition of about 50V.The titanium dioxide nanotube arrays formed after oxidation (on the titanium substrate) were first cleaned with ethylene glycol for 10 min, then rinsed with deionized water for 10 min, and dried in an oven at 80℃after the cleaning and set aside.
Weigh the corresponding mass of Ba(OH)2• 8H2O and Er(NO3)3• 5H2O into the polytetrafluoroethy-lene (PTFE) hydrothermal reactor with an electronic balance, and then pour 70 ml of deionized water into the PTFE hydrothermal reactor with a measuring cylinder and stir thoroughly.After sufficient stirring, the titanium dioxide nanotubes were put into it to reach complete submersion to ensure that the titanium dioxide nanotubes were in full contact with the reaction solution.Then put the reactor into the stainless steel reactor, and finally put it into the high-temperature sintering furnace with a heating rate of 2℃/min to 180℃, and keep it for 3h.After the temperature in the furnace dropped to room temperature, the sample was taken out and washed with deionized water, put it into the oven and dried.Then the sample was put into the crucible and annealed using a muffle furnace, which was controlled to heat up to 450℃ within 150 min, followed by constant temperature kept for 3 h.Finally, the sample was removed after natural cooling to room temperature to obtain Er-doped BaTiO3 nanotubes, and the experimental flow chart is shown in Figure 1.   Figure 2 shows the images observed using swept surface electron microscopy (SEM), and it is found in the lower panel that the Er-doped BaTiO3, generated using hydrothermal reaction, has a distinct nanotube-like structure.Compared with undoped barium titanate, doped barium titanate nanotubes become thicker, with smaller tube orifices, rougher surfaces, and larger specific surface area.This is because erbium ions are doped into the barium titanate lattice, which replaces the titanium ions at the B position, causing the octahedral lattice to deform.Moreover, the radius of erbium ions is larger than that of titanium ions.Therefore, when erbium ions are doped and replaced, larger lattices and agglomerations are generated, indicating the phenomenon of grain size increase in the figure.It shows that the erbium ions successfully replaced the cations in barium titanate, which caused the uneven phenomenon on the surface of nanotubes.It can be clearly seen that with the increase of erbium ion doping concentration, columnar particles are gradually formed at the mouth as well as the surface of the nanotubes, and with the gradual increase of columnar particles, leading to the blockage of some of the nanotubes mouth.This may be caused by the concentration of the doped erbium ions being too high and some of them failing to react completely.

X-ray diffraction (XRD) analysis of Er-doped BaTiO3 nanotubes
Figure 3 shows the X-ray diffraction patterns from 20 to 80 degrees corresponding to the erbium doping concentrations of 0, 0.0005 mol/L, 0.0025 mol/L, 0.005 mol/L and 0.01 mol/L.As can be seen from the figure, compared to undoped barium titanate, all Er doped BaTiO3 nanotubes obviously have a perovskite structure.Comparison with the JSPDF standard card spectrum shows that the samples produced are tetragonal phase barium titanate with no impurity phase generated.The peaks at around 35°, 40°, 53° and 62° are titanium elements, which are caused by the lack of uniform film thickness on the titanium base.It was confirmed that erbium ions have been successfully doped into the cationic vacancies of barium titanate to generate Er-doped BaTiO3 nanotubes.
Figure 4 shows the local magnification of the diffraction peaks from 43° to 48° (200), from which it can be found that compared with undoped barium titanate, with the increase of Er 3+ concentration, the diffraction peaks are shifted first to the low-angle direction and then to the high-angle direction, which is because erbium ion is an amphoteric dopant, and the specific doping position is barium site (A site) or titanium site (B site) is related to the concentration of doped erbium, Ba/Ti, the valence state of erbium ion and the solubility of erbium ion in barium site and titanium site [10][11][12][13][14].This may be due to the erbium ion doping bias B site, first into the titanium site on the radius of the erbium ion is greater than the radius of the titanium ion, so erbium ion doping into the titanium site, the diffraction peak to the low-angle direction, with the further increase in doping concentration, erbium ion doping concentration in the titanium site reached the upper limit, so the erbium ion into the barium site, and because the radius of the erbium ion is smaller than the barium ion, so the erbium ion doping into the barium site, the diffraction peak to the high-angle direction shift, further explained the erbium ion doping into the cationic vacancies of barium titanate.

Ferroelectric properties of Er-doped BaTiO3 nanotubes
Figure 5 shows the hysteresis line plots of Er-doped BaTiO3 nanotubes with different concentrations (measured by using TF-2000 ferroelectric analyzer).It is obvious from the figure that the residual polarization intensity of the nanotubes does not vary monotonically with the increase of doping concentration, reaching the maximum at the doping concentration of 0.0025 mol/L with the maximum residual polarization intensity of 0.151 (uC /cm 2 ).This may be due to the fact that with the increase of doping concentration, Er 3+ in doping into Ti 4+ sites, which leads to lattice distortion of octahedra and enhances its ferroelectric properties, and further increase of doping concentration, the residual  polarization intensity decreases due to the stabilizing effect of Er 3+ replacing Ti 4+ sites on Ti-O bonds [15].In agreement with the relevant literature report [16], because Er 3+ is more electropositive, the Ti-O bond between O 2-and the less electropositive Ti 4+ ions will be enhanced when Er 3+ is doped into the Ti 4+ site.Therefore, after the substitution with Er 3+ dopant ions, larger lattice as well as agglomeration is expected, and larger grain size appears at this time.This also explains well why the residual polarization intensity of the sample increases and then decreases with increasing Er ion concentration.And it corresponds to the blockage of the mouth of Er-doped BaTiO3 nanotubes and the appearance of columnar particles on the surface.

Conclusion
The surface morphology, phase structure, and residual polarization intensity of the sample were comprehensively analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and TF-2000 ferroelectric analyzer.It was found that under hydrothermal reaction conditions of 180 ℃ and 3 hours, the residual polarization intensity of the sample showed a characteristic of first increasing and then decreasing with the increase of doping concentration.Moreover, when the Er 3+ concentration was 0.0025mol/L, the residual polarization intensity reached 0.151uC/cm 2 .

Table 1 .
Preparation process of Er-doped BaTiO3 nanotubes