Bismuth oxide (Bi₂O₃) particles have attracted attention as candidate materials for helping to solve energy and environmental problems. Practical research is being actively conducted on photocatalysts [1] - [8], supercapacitors [9] [10] [11], and Li-ion and Na-ion battery materials [12] [13]. Such practical research involves implementation of morphological control of Bi₂O₃ particles as a way to improve their properties. Various shapes of Bi₂O₃ particles have been reported, such as nanowires [2], nanorods [5] [10] [11], nanotubes [14], and flowers [1] [6] [8] [15] [16] [17]; these shapes have been reported to lend excellent properties to the particles. Works such as those outlined next have specifically reported that the flower shape of Bi₂O₃ particles shows potential to lend excellent properties to the particles, on account of the large specific surface area in the case of this shape. Zhou et al. [1] succeeded in synthesizing flower-shaped particles of δ-Bi₂O₃ in sheet shape in aqueous solution in the presence of VO 3 − . Tseng et al. [15] succeeded in synthesizing flower-shaped γ-Bi₂O₃ particles from petals produced by the self-organization of nanosized triangular and pyramidal structures formed by a reaction in aqueous solution by using polyethylene glycol as a capping agent. Wang et al. [16] synthesized flower-shaped α-Bi₂O₃ by using a mixture of glycerine and oleic acid in an ethanol-water medium as a capping agent. Zhang et al. [17] synthesized flower-shaped Bi₂(CO₃)O₂ particles consisting of nanosheet petals in the presence of citric acid. By calcining the particles, they succeeded in converting them to flower-shaped α-Bi₂O₃ while retaining the morphology of the Bi₂(CO₃)O₂ particles. After synthesizing flower-shaped CH₃COO(BiO) from nanosheets by using glacial acetic acid, Zhang et al. [8] obtained flower-shaped δ-Bi₂O₃ particles via calcination. By performing synthesis in the presence of L-asparagine, Xiao et al. [6] further succeeded in synthesizing bismuth-asparagine complex microspheres and subsequently converting them to flower-shaped α-Bi₂O₃ particles via calcination. Therefore, it is clear that studies conducted thus far have obtained different flower-shaped Bi₂O₃ or Bi₂O₃ precursor particles according to the type of additives such as the inorganic ions or organic molecules added in the reaction solution.

The present paper reports that α-Bi₂O₃ microflowers composed of nanometre-sized rod-shaped particles can be synthesized by an extremely easy and simple approach of inducing a reaction at a suitable reaction temperature and for a suitable reaction time through the addition of NaOH aqueous solution to mixed aqueous solution of Bi(NO₃)₃·5H₂O and HNO₃ without using any particular additive. This paper also reports that the shape of α-Bi₂O₃ particles can be controlled via the addition of an additive such as NaF or Na₂CO₃ to the reaction solution.

Transparent reaction solution was obtained by mixing Bi(NO₃)₃∙5H₂O (Wako Pure Chemical Industries, Ltd.) (0.001 mol), 20 ml of distilled water, and 7 ml of nitric acid (69 wt%, SIGMA-Aldrich). Then, 20 ml of 10 mol/dm³ NaOH (Wako Pure Chemical Industries, Ltd.) aqueous solution was added to the reaction solution, and the reaction was induced at certain temperatures (0.2˚C, 20˚C, 40˚C, 80˚C, 120˚C, and 160˚C) and times (0 min [i.e. it was recovered immediately after mixing the NaOH aqueous solution], 10 min, 1 h, and 24 h). When the reaction temperature was 80˚C or lower, the reaction was induced in a flask by using a water bath, and when the reaction temperature was 120˚C or higher, the reaction was induced in a hydrothermal reaction vessel (sealed stainless steel container with interior Teflon coating) by using an oil bath. The solid and liquid phases of the obtained product were separated by centrifugation at 3000 rpm for 5 minutes, after which they were washed 5 times with distilled water and then freeze-dried. The use of NaF (SIGMA-Aldrich) or Na₂CO₃ (Wako Pure Chemical Industries, Ltd.) as an additive was also considered. 0.05 mol of NaF or Na₂CO₃ was added to 60 ml of distilled water in order to create an aqueous solution containing the additive. After mixing of the aqueous solution containing the additive with the previously mentioned transparent reaction solution containing Bi(NO₃)₃·5H₂O and HNO₃, the reaction was induced via the addition of 10 mol/dm³ NaOH solution for 1 h at 20˚C. The solid and liquid phases of the obtained product were separated by centrifugation, after which they were washed with distilled water and then freeze-dried. To evaluate the obtained product, crystal structure analysis by powder X-ray diffraction (XRD) measurement (Shimadzu XRD-6100) and shape observation by scanning electron microscopy (SEM; Hitachi S-3000N) and transmission electron microscopy (TEM; JEOL JEM-2100) were performed.

As a typical example, Figure 1 shows SEM and TEM images of the product obtained after 1 h of reaction at 20˚C without any additive. The low- and high-magnification SEM photographs in Figure 1(a) and Figure 1(b) show that microflowers (flower-shaped particles) were formed when rod-shaped particles several micrometres long radiated outwards isotropically. In the TEM image of the rod-shaped particles (Figure 1(c)), the edge of the particles was smooth and a black image was observed, suggesting the single-crystal nature of the rods. The width of the rod-shaped particles ranged from several tens of nanometres to several hundreds of nanometres. The average size of the microflowers measured from the SEM image was 11 µm. The largest and smallest parts of the flower-shaped structure were, respectively, about 17 µm and about 5 µm in size. All the diffraction peaks seen in the XRD pattern of the sample (Figure 2) are attributable to α-Bi₂O₃ (JCPDS no. 71-2274); the microflowers obtained in this study were found to be α-Bi₂O₃. Although several studies have reported flower-shaped particles, as mentioned in the introduction section, ours is the first study to obtain flower-shaped particles as shown in Figure 1.

Next, the process of formation of the microflowers was studied. Block-shaped particles such as those shown in Figure 3(a) inset were observed in samples obtained immediately after mixing of Bi(NO₃)₃ and NaOH aqueous solutions.

These particles are fine particles (Figure 3(a)) that clumped upon drying, and they are thought to be produced by the formation of secondary particles. No sharp diffraction peaks were observed in the XRD pattern (Figure 2) of this sample, which reveals that an amorphous substance was generated. When the reaction time reached 10 min, rod-shaped particles were observed to assemble and grow regularly as shown in Figure 3(b); that is, rod-shaped particles were observed to radiate outwards symmetrically on the right and left sides as indicated by green arrows. Through progression of this kind of growth, when the reaction time reached 1 h, the rod-shaped particles radiating outwards isotropically were thought to have produced microflowers such as those shown in Figure 1. The factors that contribute to the formation of the flower shape are currently unknown, but the reaction mechanism is currently surmised to be as shown in Figure 4. It is possible that fine amorphous particles were generated immediately after mixing of Bi(NO₃)₃ solution with NaOH aqueous solution and that these particles immediately aggregated, crystallised, and generated rod-shaped particles. A highly reactive crystal surface exists at the tip ends of the rod-shaped particles. It is possible that as a result of the heterogeneous nucleation and its subsequent growth, the rod-shaped particles produced particles that radiated outwards isotropically. NO 3 − could also have been involved in the formation of the rod-shaped particles. As stated in the introduction, in studies thus far, when flower-shaped Bi₂O₃ was synthesized, inorganic ions such as VO 3 − and organic

molecules—which have comparatively stronger affinity towards Bi³⁺ ions—were added [1] [6] [8] [15] [16] [17]. In the present research, because HNO₃ was added to form transparent Bi(NO₃)₃ solution, high concentrations of NO 3 − existed in the solution. Zhou et al. [1] and Tseng et al. [15] also added HNO₃ to obtain transparent Bi(NO₃)₃ aqueous solution when synthesizing flower-shaped Bi₂O₃. Under ordinary circumstances, NO 3 − is known to be an ion that have weak affinity towards metal ions; in the present study, NO 3 − existed in high concentrations, even though it does not have any particular strong affinity towards Bi³⁺; therefore, it could have contributed to the formation of rod-shaped particles by weakly adsorbing to the surface of the particles when the Bi₂O₃ crystals were formed. It is also interesting to note that although a small number of microflowers such as those shown in Figure 3(c) were observed after 24 h of reaction, almost all of these particles were rod-shaped; the largest of the rod-shaped particles were several tens of micrometres in size and were significantly larger than those in the samples obtained after 1 h of reaction. Nothing but diffraction peaks attributable to α-Bi₂O₃ was observed in the XRD pattern of this sample. The disappearance of α-Bi₂O₃ microflowers and the increased particle sizes suggest that the dissolution-reprecipitation reaction of α-Bi₂O₃ particles, i.e. the Ostwald ripening reaction, tended to occur. In other words, with progress of the Ostwald ripening reaction, small rod-shaped particles that constituted the microflowers disappeared by dissolution and the other large rod-shaped particles grew in size by reprecipitation; as a result, the α-Bi₂O₃ microflowers transformed into a large number of α-Bi₂O₃ rod-shaped particles.

Next, the effect of the reaction temperature on the shape of α-Bi₂O₃ was studied. Synthesis was performed at various reaction temperatures with a reaction time of 1 h. XRD measurement confirmed that only α-Bi₂O₃ was produced at all reaction temperatures. When the reaction temperature was 0.2˚C, as shown in Figure 5(a), microflowers with an average size of 9 µm (approximately 4 - 17 µm in size) were formed. The size distribution of the microflowers obtained at 0.2˚C seems to be broader than that obtained at 20˚C, as shown in Figure 1(a), Figure 5(a), and Figure 5(b). On the other hand, when the reaction temperature reached 40˚C, as shown in Figure 5(c), while some microflowers such as those observed at the reaction temperature of 20˚C were observed, most were rod-shaped particles several tens of micrometres in length. As shown in Figure 5(d), microflowers were not observed at the reaction temperature of 80˚C; only rod-shaped

particles several tens of micrometres in length were observed at this temperature. These particles were larger than those generated at 40˚C. Rod-shaped particles were also observed in samples obtained at 120˚C and 160˚C as shown in Figure 5(e) and Figure 5(f). The length of the particles formed at both these temperatures ranged from several tens of micrometres to several hundreds of micrometres, and these particles were longer and thicker than those formed at 80˚C.

Finally, the effects of additives were studied. The reaction was conducted for 1 h at a temperature of 20˚C. Three types of additives were examined: 60 ml of water solution, a solution of NaF added to 60 ml of water, and a solution of Na₂CO₃ added to 60 ml of water. The reason for using NaF and Na₂CO₃ as additives is that it is well known that F⁻ or CO 3 2 − ions strongly coordinate with metal ions to form unique oxides and hydroxides. XRD measurements confirmed that only α-Bi₂O₃ was generated after synthesis with the addition of any of these additives. Microflowers (Figure 1(a) and Figure 1(b)) such as those observed in particles produced by synthesis without the addition of an additive were not observed in particles produced by synthesis with the addition of the 60 ml of water (see Figure 6(a)). Rod-shaped particles clumped to form a shape similar to the feather of a bird (this can also be termed “fan-shaped”) were observed. These fan-shaped particles are probably those that were formed when the obtained product was washed and dried, and these fan-shaped particles were probably formed by the crumbling of the aggregate generated by synthesis. These results indicate that the concentration of ions such as Bi³⁺ and NO 3 − has a significant effect on the formation of assemblies of α-Bi₂O₃ rod-shaped particles. More microflowers were observed to be formed from the rod-shaped particles when the solution of NaF added to 60 ml of water was used as the additive (see Figure 6(b)) than when 60 ml of water solution was used as the additive; numerous rod-shaped particles that were believed to have been formed when the flower-shaped structure fell apart were observed. On the other hand, although a large number of rod-shaped particle assemblies were observed among the

particles formed using the solution of Na₂CO₃ added to 60 ml of water (see Figure 6(c)), a few microflowers were present that grew out isotropically from rod-shaped particles similar to those observed among the particles formed without any additive; further, numerous butterfly-shaped particles composed of rods radiating outwards in a two-dimensional plane. The length of these particles in the longitudinal direction was approximately 19 µm. Detailed observation of the butterfly-shaped particle (see Figure 6(d)) revealed that several rods as indicated by the yellow arrows were connected when the angle changed gradually. As mentioned previously, this image suggests that the tips of the rods are highly reactive, and subsequent nucleation and growth of rods with the tip of the rods as the origin are possible. The difference in the shapes of α-Bi₂O₃ particles resulting from employing these additives is thought to be a result of the difference in the adsorptions to the particle surfaces of F⁻ or CO 3 2 − , which are negative ions that coexist in the reaction solution during the growth of the α-Bi₂O₃ crystals. This indicates that size and shape of the flower structure can be controlled by selecting an appropriate additive.

This paper has reported that α-Bi₂O₃ microflowers composed of aggregates of nanorods can be synthesized by an extremely simple and easy approach of inducing a reaction through the addition of sodium hydroxide aqueous solution (with a reaction time of 1 h at a temperature of 20˚C) to a mixed aqueous solution of Bi(NO₃)₃∙5H₂O and HNO₃. The findings of the study have demonstrated that control of the reaction temperature and reaction time plays an important role in the formation of the microflowers; in particular, α-Bi₂O₃ microflowers can be formed easily with a short reaction time and low reaction temperature. The method employed in this study shows promise for synthesizing flower-shaped α-Bi₂O₃ from the standpoints of its low cost and low energy consumption. The findings also indicate that the shape of microflowers can be controlled by synthesizing them in the presence of an additive such as NaF or Na₂CO₃. Future study will be aimed at the evaluation of their characteristics of photocatalysts and Li-ion batteries.

The authors declare no conflicts of interest regarding the publication of this paper.

Yada, M., Yamanoi, T. and Watari, T. (2020) Simple Template-Free Synthesis of Bi₂O₃ Microflowers Composed of Nanorods. Advances in Materials Physics and Chemistry, 10, 319-327. https://doi.org/10.4236/ampc.2020.1012025