Growth of BiFeO 3 Microcylinders under a Hydrothermal Condition

BiFeO 3 microcylinders were synthesized via a hydrothermal condition. SEM observation reveals that with increasing the hydrothermal reaction time from 6 to 15 h, the microcylinders grow from ∼0.7 to ∼4.1 μm in height, whereas their diameter remains to be 3.7-3.8 μmwith a minor change. The microcylinders are mainly made up of sphere-like grains of 100–150 nm in size. A possible growth mechanism of the BiFeO 3 microcylinders is proposed. The photocatalytic activity of the as-prepared BiFeO 3 samples was evaluated by the degradation of acid orange 7 under simulated sunlight irradiation, revealing that they possess an appreciable photocatalytic activity. Magnetic hysteresis loop measurement shows that the BiFeO 3 microcylinders exhibit a typical antiferromagnetic behavior at room temperature.

Various wet chemical methods have been widely used to achieve those nano/microstructures of BiFeO 3 .Among them, the hydrothermal route has special advantages in tailoring the product morphology.The variation of any parameter in the hydrothermal reaction process, such as reaction temperature and time, mineralizer type and its concentration, pH value, metal ion concentration, and organic additive, could result in morphologically different products.In this work, we report the synthesis of BiFeO 3 microcylinders under a hydrothermal route and a possible growth mechanism of the microcylinders is proposed.The photocatalytic activity of as-prepared samples was evaluated by the degradation of acid orange 7 (AO7) under simulated sunlight irradiation.

Experimental
0.005 mol of Bi(NO 3 ) 3 ⋅5H 2 O and 0.005mol of Fe(NO 3 ) 3 ⋅9H 2 O were dissolved in 20 mL of dilute nitric acid solution.Then to the mixture solution was added 60 mL KOH solution with a concentration of 9 mol⋅L −1 drop by drop under magnetic stirring, and immediately a dark brown suspension solution was formed.The suspension was ultrasonically treated for 8 min and subsequently stirred vigorously with a magnetic stirrer for 30 min to create a highly uniform mixture.The resulted mixture was sealed in a Teflon-lined stainless steel autoclave of 100 mL capacity and submitted to hydrothermal treatment at 200 ∘ C.After different reaction time, the autoclave was cooled naturally to room temperature.The resulting brown precipitate was collected and washed several times with distilled water and absolute ethanol and then dried in a thermostat drying oven at 80 ∘ C for 12 h to obtain final BiFeO 3 product.By varying the reaction time from 6 to 15 h, several BiFeO 3 samples were prepared.
The phase purity of the samples was examined by means of X-ray powder diffraction (XRD) with Cu K radiation.The morphology of the samples was observed by field-emission scanning electron microscope (SEM).The ultraviolet-(UV-) visible diffuse reflectance spectra were measured using a UV-visible spectrophotometer with an integrating sphere attachment.A vibrating sample magnetometer (VSM) was used to measure the magnetic hysteresis loops of the samples at room temperature.
The photocatalytic activity of the products was evaluated by degrading AO7 in aqueous solution under simulated sunlight irradiation from a 200 W xenon lamp.The initial AO7 concentration was 5 mg⋅L −1 and the BiFeO 3 loading was 0.1 g in 200 mL of AO7 solution.Before illumination, the mixed solution was ultrasonically treated for 15 min in the dark to make BiFeO 3 particles uniformly dispersed.During the photocatalysis process, the water-jacketed reactor was cooled with water-cooling system to keep the solution at room temperature.After irradiation for 8 h, the reaction solution was centrifuged for 10 min at 3000 r⋅min −1 to remove BiFeO 3 particles.The AO7 concentration was determined by measuring the absorbance of the solution at a fixed wavelength of 484 nm using an UV-visible spectrophotometer.The percentage degradation is defined as ( 0 −   )/ 0 × 100%, where  0 and   are the AO7 concentrations before and after irradiation, respectively.

Results and Discussion
Figure 1 shows the XRD patterns of BiFeO 3 samples prepared at the reaction time of 6, 9, and 15 h (the corresponding samples are termed S-6, S-9, and S-15, resp.).For these samples, all the diffraction peaks can be indexed in terms of the rhombohedral structure of BiFeO 3 with space group 3 (PDF card number 74-2016).No traces of other phases are detected in the XRD patterns, revealing high purity of the as-prepared products.The widths of the diffraction peaks undergo no obvious change between the samples, indicating that they have nearly the same grain or subgrain size.
Figure 2 shows the SEM images of the as-prepared BiFeO 3 samples, revealing the synthesis of cylinder-like microparticles.When the hydrothermal reaction time is 6 h, the resulting sample consists of microcylinders with an average diameter of ∼3.7 m and height of ∼0.7 m (Figure 2(a)).When the reaction time is increased to 9 h, the obtained microcylinders have a diameter of ∼3.8 m and height of ∼ 1.7 m (Figure 2(b)).Further prolonging of the reaction time up to 15 h leads to the formation of microcylinders with a diameter of ∼3.8 m and height of ∼4.1 m (Figure 2(c)).This implies the continuous growth of BiFeO 3 microcylinders along the direction of height with increasing the reaction time, whereas their diameter undergoes minor change.Figure 2(d) shows an enlarged view of the SEM image of Figure 2(b), revealing that the microcylinders are mainly made up of sphere-like grains of 100-150 nm in size.
Figure 3 shows a possible growth mechanism of the cylinder-like BiFeO 3 particles.Initially, Bi 3+ and Fe 3+ ions react with OH − ions to produce amorphous Bi(OH) 3 and Fe(OH) 3 precipitates, and then the hydroxides undergo an attack of the mineralizer, KOH, to dissolve and form ion groups.These species undergo nucleation and growth when critical supersaturation is reached, leading to the formation of BiFeO 3 nanocrystals.The nascent nanocrystals come together or self-assemble to form cylinder-like aggregates.This self-organization process can be understood according to the oriented-attachment growth mechanism, where the  driving force is the reduction of overall surface energy [20,21].At the initial stage of the self-organization, thin or disk-like microcylinders with a diameter of about 3.7-3.8m are constructed from BiFeO 3 nanocrystals.With increasing the reaction time, BiFeO 3 nanocrystals are continuously attracted to attach on the surface of the microcylinders, leading to their continuous growth along the height direction, whereas their diameter undergoes minor change.Figure 4(a) shows the UV-visible diffuse reflectance spectra of BiFeO 3 samples and Figure 4(b) gives the corresponding first derivative of the reflectance () with respect to wavelength  (i.e., /).It is seen that the samples have a similar absorption edge at ∼568 nm, which is assigned to the electron transition from valence band to conduction band according to the theoretical results [22].From the absorption edge, the bandgap energy of the samples is obtained to be 2.18 eV.
Figure 5 shows the photocatalytic degradation of AO7 over BiFeO 3 samples as a function of irradiation time (), along with the blank experimental result.AO7 appears to be stable under simulated sunlight irradiation in the absence of photocatalyst, and its degradation percentage is less than 7% after 8 h of exposure.When BiFeO 3 samples are used as the photocatalyst, the degradation of AO7 is obviously enhanced, indicating an appreciable photocatalytic activity of the samples.The degradation percentage of AO7 after 8 h irradiation with samples S-6, S-9, and S-15 is about 56%, 28%, and 19%, respectively.The sample S-6 exhibits the highest photocatalytic activity, which is mainly ascribed to its relatively small particle size.Generally small particle size and large surface area to volume ratio is required to achieve good photocatalytic activity since the photocatalytic reaction occurs dominantly on the catalyst surface [23].
Figure 6 shows the magnetic hysteresis loops of BiFeO 3 samples measured at room temperature, revealing a nearly linear dependence of the magnetization on applied magnetic field.This indicates that all the samples exhibit an antiferromagnetic behavior at room temperature.It is well known that BiFeO 3 has a helical antiferromagnetic spin structure with a period of ∼62 nm [24].This helimagnetic structure will be destroyed in nanosized BiFeO 3 , making the spin compensation incomplete.As a result, weak ferromagnetism is generally observed for nanosized BiFeO 3 and exhibits an increasing trend with reducing the grain size [10,25].However, BiFeO 3 samples synthesized in this work crystallize in relatively large-sized microcrystals, which inhibits the observation of macroscopic magnetization.

Conclusions
Cylinder-like BiFeO 3 particles were successfully prepared by a hydrothermal method.It is found that the height of the resulted BiFeO 3 microcylinders has a dependence on the hydrothermal reaction time.With prolonging the

Figure 3 :
Figure 3: Schematic illustration of the growth mechanism for cylinder-like BiFeO 3 particles.

Figure 4 :Figure 5 :
Figure 4: (a) UV-visible diffuse reflectance spectra of BiFeO 3 samples and (b) the corresponding first derivative of the diffuse reflectance spectra.

Figure 6 :
Figure 6: Magnetic hysteresis loops of BiFeO 3 samples measured at room temperature.