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Combustion synthesis of porous bismuth oxide

Bismuth oxides with a micro porous structure were prepared by a simple combustion reaction between oxidizer (bismuth nitrate) and fuel (citric acid). These porous bismuth oxides were prepared at different values of the stoichiometric coefficient. The as-prepared products were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM). The results show that many pores distributed in tetragonal bismuth oxides (β-Bi2O3) granules which have varied crystallinity and particle sizes at different oxidizer/fuel ratio. Increasing the fuel concentration improved the crystallinity and purity of β-Bi2O3. A further calcination could also increase the particle size, crystallinity and transform the phase structure from β-Bi2O3 to pure α-Bi2O3.

Recently, different methods have been developed for preparing bismuth oxide, such as (co)precipitation [11], sol-gel method [12], solvothermal method [13], microwave-assisted method [14], sonochemical [15], laser irradiation [16], and others. Different structures of bismuth oxide have been prepared, for examples, nanoparticles [17], thin film [12], nanowires [18], nanoflakes [19], and so on. However, operating complexity, high costs, and disposal expense of the by-products often limit the large-scale application of these techniques. Combustion synthesis can produce large-scale metal oxide particles in a short time at low costs using simple equipment [20]. By far, there are not so many reports on the preparation of micro porous bismuth oxide through combustion synthesis.
In this study, micro porous bismuth trioxide (β-Bi 2 O 3 ) particles was prepared via a simple combustion reaction between bismuth nitrate pentahydrate (Bi(NO 3 ) 3 ⋅5H 2 O) and citric acid monohydrate (C 6 H 8 O 7 ⋅H 2 O) and high crystalline bismuth trioxide (α-Bi 2 O 3 ) obtained by a further thermal treatment. Product properties such as the microstructure, morphology and phase transformations, as well as the behavior of combustion were analyzed.

Materials
All reagents used were of analytical grade and were purchased without further purification. Bismuth nitrate pentahydrate (Bi(NO 3 ) 3 ⋅5H 2 O) was received from Beijing Sinopharm Chemical Reagent Co., Ltd, China. Citric acid monohydrate (C 6 H 8 O 7 ⋅H 2 O) was purchased from Guangdong Xilong Chemical Co., Ltd, China. Nitric acid (HNO 3 ) (67% in purity) was purchased from Beijing Chemical Factory, China. Deionized water was self-made in the laboratory.

Synthesis of porous bismuth oxides
Synthesis process was adapted from the method reported by Martirosyan et al [20][21][22]. Combustion synthesis of bismuth oxides produced micro porous powders, and highly crystallized and highly purified bismuth oxides were obtained by further calcination. Bismuth nitrate pentahydrate (Bi(NO 3 ) 3 ⋅5H 2 O) was used as the oxidizer and organic compound citric acid monohydrate (C 6 H 8 O 7 ⋅H 2 O) the fuel. Several types of bismuth oxides were produced by combustion synthesis by the reaction [20]: where, f is the stoichiometric coefficient of the fuel that shows if the combustion is oxygen enriched (f<1−2xy/15) or poor (f>1−2xy/15), ω is the bismuth nitrate pentahydrate weight percent in the original mixture, M b and M c are the molecular weight of bismuth nitrate and citric acid monohydrate, respectively. The terms (g) and (s) stand for gas and solid phase, respectively.
To avoid the hydrolysis of bismuth nitrate in water, we dissolved bismuth nitrate pentahydrate into a 24 mL nitric acid solution (V( The mixture boiled, followed by frothing. It then ignited with releasing a lot of gases, and producing fluffy powders. The products were collected and placed into muffle furnace to anneal at 500 °C for 2 h with a heating rate of 10 °C/min.

Characterization
The crystal phase of the as-synthesized products was obtained by X-ray diffraction (XRD) analysis using a Bruker D8 advance diffractometer, with the X-ray diffractometer using Cu Kα (λ=1.54056 Å) over a wide range of 2θ=(20º−70º) at 0.05° 2θ s -1 . The morphologies of the as-made products were observed on a scanning electron microscope (SEM, Hitachi S4800).

Combustion behavior
The ratio between the oxidizer (bismuth nitrate pentahydrate) and the fuel (citric acid monohydrate) had a great influence on the mixture combustion behavior, such as the combustion temperature, velocity. The combustion could not be maintained when the fuel stoichiometric coefficient f was below 0.2. At f=0.2 the combustion was self-sustained and propagated at a low velocity without flame. A lot of gas mainly composed of CO 2 , N 2 , H 2 O vapor and a small amount of nitrogen oxides (NO x ) that generated from the combustion reaction between bismuth nitrate and citric acid increased the product volume, and then faint yellow fluffy powders formed. Increasing the value of f within limits increased the combustion temperature and velocity. The effect of the stoichiometric coefficient f on the combustion mode of the reaction and products color is shown in Table 1. As we observed in the experiment, when 0.2<f<0.3 the combustion propagated at a low velocity without flame, and producing bismuth oxide precursor powders of different colors from white-yellow to orange-red formed with the f increase. At 0.3<f<0.4 the combustion proceeded very fast in a deflagration way with a few sparkles and generated a large amount gas, and it was difficult to collect the synthesized product as it spattered violently. At 0.4<f<0.7 it was very dangerous that the mixture exploded instantaneously with increase in temperature. While, the velocity of reaction descended and it burned with a lot of flame when the value of f increased to 0.7. Therefore, the value of f at 0.2<f<0.3 is suitable for the combustion synthesis and operating safely.

Phase structure
The powder X-ray diffraction (XRD) patterns of products and calcinated bismuth oxides prepared by combustion synthesis at different values of f are shown in Figure 1. As shown in Figure 1, the pattern features of the original precursors obtained at the combustion step correspond to the pure tetragonal phase of bismuth oxide (β-Bi 2 O 3 ) (JCPDS No. 78-1793) with the feature peaks 2θ = 27.9°, 32.7°, 46.2°. The crystallinity of the products powders was calculated by the equation C=(P/T)×100%. C is the degree of crystallinity; P the diffraction peak intensity and T the total intensity. The calculated crystallinity of the precursors powders prepared at f=0.2, 0.225, 0.25, 0.275 and 0.3 were 88%, 87%, 89%, 93% and 98%, respectively. It could be concluded that the content of β-Bi 2 O 3 was higher at elevated f. Hence, the precursors required further calcination to decompose the impurity and complete the phase conversion. Their morphologies and surface structures were then studied by SEM.

Morphology
The morphology and microstructure of the prepared β-Bi 2 O 3 powders were observed on scanning electron microscope (SEM) as shown in Figure 2 (a)-(b). It can be noticed that the morphology of the bismuth oxides particles are spongy porous that many irregular pores are distributed in a particle and not very homogeneous, especially in Figure 1 (a). It can be attributed to the generation of gas from the combustion reaction which increased the product volume. The diameter of bismuth oxide particle is about micrometer. It also can be found that the agglomerations of particles are increased with the increase of f. Some spherical particles were formed and the structures are more closely packed at high value of f. It should be noted that the porous structure of β-Bi 2 O 3 is well formed at low value of f.

Effect of calcination
It can be observed from Figure 3 that, after calcination at 500 °C, all diffraction peaks of the samples can be unambiguously assigned to the most stable monoclinic phase of bismuth oxide (α-Bi 2 O 3 ) with narrow and strong intensity peak at 2θ = 27.5° with lattice parameters a = 5.831 Å, b = 8.148 Å, c = 7.487 Å. No impurity peaks are observed, indicating a high purity of the products. The XRD pattern was unchanged a lot with the increase of f. This suggested that the value of f has little effect on the phase of the sample after calcination. The calculated crystallinity of the products powders prepared at different values of f were >99%, which indicate that the prepared particles have been well crystallized. Compared to the β-Bi 2 O 3 , annealing the precursors leads to the increase of crystallization and peak intensity, which indicate that the phase structure of the samples transformed from the tetragonal and amorphous phase into the pure monoclinic phase after calcination.   The morphology and microstructure of the as annealed pure α-Bi 2 O 3 powders were observed on scanning electron microscope (SEM) as shown in Figure 4(a)-(b). It can be seen that the diameter of bismuth oxide particle is about micrometer. Compared to β-Bi 2 O 3 , the morphology and microstructure of α-Bi 2 O 3 powders changed a lot. The microstructure of α-Bi 2 O 3 is dendritic particles which exhibit many irregular pores, as shown in Figure 3(a), whereas the obtained α-Bi 2 O 3 at high value of f consists of randomly structures, as shown in Figure 3

Conclusion
In this paper, we prepared micro porous β-Bi 2 O 3 by a simple combustion reaction between bismuth nitrate and citric acid, and high crystalline α-Bi 2 O 3 was obtained after a further thermal treatment. The oxidizer (bismuth nitrate) / fuel (citric acid) ratio and calcination have strong impacts on the bismuth trioxide phase structure and crystallinity. By changing the ratio between the oxidizer and fuel the purity could be improved. Compared to the β-Bi 2 O 3 , annealing the products not only leads to the increase of crystallite sizes and crystallinity, but also transforms the phase structure from the tetragonal and amorphous phase into the pure monoclinic phase.

Acknowledgments
This paper is supported by the project of State Key Laboratory of Explosion Science and Technology (Beijing Institute of Technology, China) (No.YBKT19-04).