Microwave-assisted synthesis of bismuth oxide

Single phase and ultrafi ne bismuth oxide was synthesized via microwave-assisted hydrothermal synthesis. The effect of reaction parameters (temperature/pressure and pH) on the product phase composition and morphology was discussed. The transformation of bismuth hydroxide into bismuth oxide was controlled by pH value and it was accelerated by time and temperature. The phase composition of reaction products was strongly dependent on pH value. The amorphous products were obtained at acidic pH conditions and the crystalline single phase product α-Bi2O3 phase was obtained at pH ≥12. The particle size was reduced from micrometric to nanometric size in the presence of a chelating agent. The bismuth hydroxides into bismuth oxides transformation mechanism, consisting in polycondensation of Bi – OH bounds to Bi – O – Bi bridges and crystallization of Bi2O3, was proposed.


I. Introduction
Bismuth oxide and binary or ternary systems based on bismuth oxide are often used as catalytic materials for partial oxidation of hydrocarbons, such as propene, isobutene or methane [1], and for photo catalytic applications [2,3].The low particle size or high specifi c surface area and also the good chemical and phase purity are the most important properties of bismuth oxide.Bismuth oxide exists in fi ve polymer phic modifi cations.The low temperature α and high temperature δ phases are stable, whereas β, γ and ω are metastable at high temperatures [4][5][6].Crystalline bismuth oxide was prepared by conventional high temperature solid state reaction of bismuth salts [7] or by means of mechanochemical synthesis [8,9].Non-conventional methods such as hydrothermal synthesis [10] and sonochemical synthesis [3] were also used for bismuth oxide preparation.However most of the methods are however based on precipitation reactions of bismuth salts and calcination of bismuth products [5,6,[12][13][14][15].Nucleation and growth of particles in the course of hydrolysis / condensation of bismuth salts can be controlled by means of additives such as polyethylen glycol [5], polyvinylpy-rollidon [3], urea [14] and / or citric acid [13].However, no information is available about bismuth oxide synthesis in the microwave fi eld.The study of direct synthesis of bismuth oxide by hydrolysis and condensation of bismuth nitrate in microwave fi eld is the main goal of this paper.

Preparation of samples
Two different routes of bismuth oxide preparation were studied.The fi rst one consists of two steps.In the fi rst step, the bismuth hydroxide was prepared by precipitation reaction of bismuth nitrate with ammonium hydroxide solution (1 M).The precipitated bismuth hydroxide was separated and washed with water to neutral pH.In the second step, the pH of bismuth hydroxides was adjusted to a value in the range of 6-12 by nitric acid and / or ammonium hydroxide.Suspensions were placed in microwave fi eld and transformed under hydrothermal conditions at temperatures from 100 to 220°C.The microwave reactor (Multiwave 3000, Anton Paar, Austria) wor king at a frequency of 2.45 GHz and a maximum power of 1400 W was used for experiments.
In the second synthesis way the infl uence of the chelating agent (polyethyleneglycol) on product precipitation was investigated.Aqueous solutions of reactants prepared by dissolving bismuth nitrate (2.0 M), sodium hydroxide (4.0 M) and polyethyleneglycol (PEG 4000 -0.05 M) were cooled to a temperature below 100°C and then mixed in molar ratio 0.12 / 0.44 / 0.002, respectively.The suspensions created immedi ately after the mixing of reactants were placed in the microwave reactor and treated at temperatures from 40 to 60°C, with or without stirring during treatment.All the products were separated by centrifugation, washed with water to neutral pH and dried at 110°C.All chemicals used in the experiments were of AR grade and bought from Sigma Aldrich.

Characterization of powders
The phase composition was determined by X-Ray diffraction analysis, K Co wavelength (X´pert, Philips, Netherlands).The morphology of powder products was studied by scanning microscopy (SEM XL 30, Philips, Netherlands).The specifi c surface area (SSA BET ) was determined by the BET method (Chem bet, Quantachrome, USA).For the estimation of the OH group con-tent in Bi X (OH) Y (O) Z infrared spectroscopy (Nicolet, Thermo Corporation, USA) and thermal analysis Pyris 1 (Perkin Elmer, USA) were used.

Synthesis of bismuth oxide by precipitation reaction in MW fi eld
The phase composition of reaction products prepared at pH = 7 at different reaction temperatures is given in Fig. 1.The crystalline products prepared at temperatures of 100°C, 180°C and 220°C containing two bismuth oxide phases (α-Bi 2 O 3 and δ-Bi 2 O 3 ) and also an unknown phase were found in the XRD spectra.From IR spectra given in Fig. 2 it follows that the unknown phase is probably bismuth hydroxo-oxide Bi X (OH) Y (O) Z , which can be transformed into single-phase Bi 2 O 3 by calcination at 600°C.Water bound as OH groups was released in the course of calcination as observed by two peaks (Fig. 3) at 505°C and 560°C probably because of different positions of OH groups in the bismuth hydroxide-oxide structure.The main infl uence on the composition of reaction products was that of the pH of reaction medium.Amorphous Bi X (OH) Y (O) Z products (Fig. 4) containing large amounts of OH groups (Fig. 5) with high specifi c surface area (SSA BET = 39.0 m 2 /g) were created under slightly acidic conditions -pH = 6.The morphology of the product prepared at 100ºC is given in Fig. 6a.The particles were of fl aky shape with a maximum particle size of 5 μm.Under basic conditions (at pH ≥ 12) singlephase α-Bi 2 O 3 products with small specifi c surface areas (SSA BET =0.2 m 2 /g) were created.IR spectra given in Fig. 5 showed no OH groups in the product.The morphology of product particles prepared at 100°C under the basic conditions (Fig. 6b) is similar to morphology of the particles prepared under acidic conditions (Fig. 6a).The effect of pH on the composition of reaction products is given in the Table 1.

Synthesis of bismuth oxide in the presence of chelating agent and MW fi eld
Nanometric particles of bismuth oxide prepared by precipitation of bismuth nitrate under the basic conditions and in the presence of polyethyleneglycol at 90 °C/2h were published by Li [5].The results given in this paper concern a similar synthesis but carried out in the stirred microwave reactor at lower temperatures (40°C and 60°C) and shorter time (30 min).XRD spectra of the products are given in Fig. 7.The crystalline products containing mainly α-Bi 2 O 3 and traces of δ-Bi 2 O 3 were prepared in both cases (40°C and 60°C).The needle-like particles of α-Bi 2 O 3 (Fig. 8) had the specifi c surface area in the range of 0.2-0.6 m 2 /g.In comparison with the results of Li [5] the nanometric spherical particles were not obtained.However, the synthesis carried out in the microwave fi eld without stirring the reaction mixture lead to α-Bi 2 O 3 nanoparticles with a small amount of OH groups (Fig. 9).The needle-shaped particles with high specifi c surface area (SSA BET =21.6 m 2 /g) are given in Fig. 10.

Reaction mechanism of Bi 2 O 3 formation
On the basis of the above results a possible reaction mechanisms is described bellow.Reaction of Bi 3+ salt started by hydration and was followed by hydrolysis.Bismuth hydroxide is created in this fi rst phase of reaction me chanism.In the second phase of reaction mechanism the condensation of OH groups proceeds and is acco mpanied by the formation of bismuth oxide species.In particular this second step is strongly dependent on the pH of reaction medium.The formation of bis-  Microwave fi eld accelerated the processes of hydrolysis and condensation under basic conditions and therefore α-Bi 2 O 3 was formed at temperatures and times substantially lower than α-Bi 2 O 3 synthesized without using microwave irradiation [5,13].The acceleration of reaction process is the cause of non-conventional heating of the whole volume of reaction mixture by microwave fi eld.

IV. Conclusions
The nanocrystalline bismuth oxide with SSA BET = 21.4 m 2 /g was successfully prepared via microwave-assisted synthesis by precipitation from bismuth nitrate solutions with admixture of chelating agent to control the reaction rate and the fi nal properties of powder product (such as particle size, morphology and phase composition).The time of preparation and reaction temperature was reduced using microwave-assisted synthesis.

Figure 5 .
Figure 5. IR spectrum showing the amount of OH groups contained in the product synthesized at 100°C and at different pH (6, 12)

Figure 6 .
Figure 6.SEM pictures showing the particle shape and size of product synthesized at 100°C, 2h: a) pH = 6 and b) pH = 12

Figure 10 .Figure 9 .Figure 8 .Figure 7 .
Figure 10.SEM picture showing the particle shape and size of product synthesized with chelating agent and without stirring during the procedure (40°C / 30 min / pH = 12)

Table 1 The effect of pH on the reaction product prepared at pH = 6 -8 -10 -12, 100°C, 2h
x (OH) y O z Bi x (OH) y O z Bi x (OH) y O z α-Bi 2 O 3