BiNaO₃ (Sigma Aldrich) was dissolved in ethylene glycol (Reasol) to produce a 0.03M solution and maintained continuous stirring at 700 rpm for 15 min, then, the resulting solution was transferred into a digestion vessel (Parr mod. 4781). After that, the vessel was introduced into a microwave oven (Amana Commercial Mod. HDC12A2), the solution was irradiated at 1200 Watts for 10 seconds. Finally, the precipitates were centrifuged and washed several times using distilled water and isopropyl alcohol intercalary. The resulting precipitates were dried for 1 h in an oven (Neytech Qex mod. V1.2) at 100˚C for 1 hour.

The surface morphology of the samples of commercial Bi₂O₃, Bi₂O₂CO₃ synthesized and commercial TiO₂ was characterized by scanning electron microscopy (SEM) in a JEOL microscope model JSM-7800F, with a resolution of 0.8 nm at 15 kV of acceleration in secondary electrons mode. A Rigaku diffractometer, model Miniflex 600 with X ray tube with Cu Ka (l = 1.54 A˚) radiation and lineal focus was used for XRD, with 40 kV y 15 mA. Symmetric measurements from 3 to 70 degrees with step of 0.01 and a speed or 3 degrees/minute. Nitrogen adsorption and desorption isotherms were measured at 77 K using Quantachrome Autosorb 1. Prior to the measurements, the Bi₂O₃ 0.03 M and TiO₂ samples were outgassed at 100˚C for 25 h. and the commercial Bi₂O₃ sample was outgassed as the same temperature like the previous samples for 43 h. The specific surface area and the pore size distribution (average pore diameter and mean pore volume) were measured from the adsorption isotherm using the Brunauer-Emmett-Teller method and Barrett-Joyner-Halenda method, respectively.

To verify the degradation of methylene blue (C₁₆H₁₈ClN₃S), it was prepared a 25-ppm solution (Sigma Aldrich). First a photolysis was made using 50 ml of the 25-ppm solution that was transferred to a quartz reactor. To make this experiment it was used a closed equipment that contained 8 UV lamps (50 W), and the solution was stirred at 600 rpm. During irradiation, 5 ml of the solution was removed every 10 min for 2 h and went back inside the solution after being analyzed using a Perkin Elmer spectrophotometer 365 in a range of 400 - 700 nm to measure the absorbance of each sample. The commercial Bi₂O₃, synthesized Bi₂O₂CO₃ and TiO₂ catalyst were also studied under the same conditions.

The morphology of the samples was observed by SEM. Figure 1(a) shows the surface of the synthesized Bi₂O₃ sample composed by agglomerated foils of 5 µm maximum that might have very good results in photocatalytic activity because of the porosity found on the structure. These foils started to grow during the first 10 seconds of irradiation. The next two images are from the commercial Bi₂O₃ (Figure 1(b)) and TiO₂ (Figure 1(c)) respectively, these structures show difference between morphology and how it affects the degradation of methylene blue because the images show that TiO₂ particles are sphere shaped with an average diameter between 100 to 200 nm, while Bi₂O₃ shows compact agglomerate structures with a diameter of 10 µm.

According to diffraction pattern, Figure 1(a) corresponds to a body centered tetragonal crystalline structure with unit cell parameters a = b = 3.865 and c = 13.675 and angles of α. β, Υ de 90˚, this information matches accurately to the crystallographic PDF chart 41-1488, of the bismuth oxide carbonate (Bi₂O₂CO₃). Figure 1(b) shows the diffraction pattern of the alpha bismuth oxide (Bi₂O₃), where the corresponding PDF chart is 71-0465 and demonstrated the presence of a monoclinic system with space group P-21/c(14) and lattice parameters of a = 5.849, b = 8.164 and c = 7.510 and the angle β = 112.97. Finally, Figure 1(c) show the XRD of commercial TiO₂ anatase phase agree with the PDF chart 89-4921 where tetragonal system is reported with lattice parameters of a = b = 3.77 and c = 9.501, angles of 90˚ and space group I41/ad(141).

The nitrogen adsorption-desorption isotherms of TiO₂ and Bi₂O₃ structures are shown in Figure 2. The TiO₂ and synthesized Bi₂O₃ are a type II isotherm curves. It is observed that the synthesized Bi₂O₃ adsorbed better the N₂ due to a bigger surface area and pore radius. The commercial Bi₂O₃ is a type III isotherm due to the small surface area and pore volume in comparison to the other isotherms.

The corresponding average pore diameter and total pore volume are calculated from the BJH pore size distribution curves and the values are given in Table 1, it is clear that the molecules synthesized of Bi₂O₂CO₃ exhibit a superior surface area and also a higher pore diameter and pore volume than TiO₂ particles. The commercial Bi₂O₃ surface area is 5.64 times smaller than the synthesized sample and it is because a precursor solution with low concentration was used for the synthesis and also the small period of time on the microwave affected positively the porosity of the structure. The specific surface area for the Bi₂O₂CO₃, Bi₂O₃ and TiO₂ materials is also listed in Table 1.

The band gap energy was measured using a Perkin Elmer spectrophotometer 365 with integration sphere at wavelength of 500 to 300 nm in Absorbance mode. The Kubelka-Munk method was used to determine the band gap energy. According to the Kubelka-Munk method the Equation (1) was be used:

( α h υ ) n = A ( h υ − E g ) (1)

where α, υ, A, E_g are the absorption coefficient, light frequency, a constant and the band gap energy, the n is determined by the type of optical transition in the semiconductor (i.e. n = 2 for the direct transition and n = 1/2 for the indirect transition). Taking in consideration that the samples are an indirect semiconductor, the band gap energy is determined from a plot of ( α h υ ) 1 2 vs. Band gap energy, obtaining the results shown in Table 2.

As it can be observed, the value of Bi₂O₂CO₃ band gap is inside the range of the theoretical values which supports the theory that it is actual Bi₂O₂CO₃ and not another compound, and because of its higher value, it needs more energy than anatase, that is one of the reasons that degradation percentage is slower than anatase.

The degradation of methylene blue is until the final stage of the process shown in Figure 3, give as result the formation of CO₂ and water, removing the dye in the original solution. In this work, we follow the reaction only with the dye concentration respect the time.

The experimental section of the application was carry out following the next procedure: first a calibration curve was obtained, in order to know the concentration of dye in the solution, a photolysis to know the degradation caused by the lamps and finally, the three catalyst were proved at the same conditions.

The calibration curve of methylene blue was obtained to calculate the concentration of the solutions in relation to time. It is clearly observed that the adjusted R-squared value is equal to 0.99756, with an adjusted equation y = 0.1103x − 0.01103. This data allows to obtain the concentration of each sample of methylene blue solutions that were taken during the degradation experiments.

To obtain the data for Table 3 some equations were used. Langmuir-Hinshelwood was employed to obtain the first approximation, considering a first order kinetics, as is used in organic dyes in low concentrations. The different concentrations of each solution were calculated using the adjusted equation of the calibration curve (Equation (2)).

C M B = A b s + 0.1148 0.1148 (2)

The equation above needs the Absorbance in different time (Abs), and the result is the concentration of methylene blue in the solution (C_MBS). Then, the percentage of methylene blue in solution was calculated in order to obtain the degradation efficiency, the Equation (3) was used

% C M B = M B S ∗ 100 C 0 (3)

C_O is the initial concentration of each experiment and finally, the degradation efficiency of each catalyst was calculated by the Equation (4):

% D M B = 100 − % M B S (4)

where D_MB is the degradation efficiency of catalysts and MBs is the percentage of methylene blue in solution. The table below shows the degraded methylene blue percentage in relation to time.

Figure 4 shows the comparison of the different experiments to show the degradation percentage of the methylene blue along the 120 min. TiO₂ degrades 85.64% of the solution while Bi₂O₂CO₃ shows a degradation percentage of 68.25, showing that the morphology affects positively on the photocatalytic activity. Morphology plays an important role in photocatalytic activity because commercial Bi₂O₃ is formed by agglomerated compacted structure that reduces its surface area. A serial of factors influence the photocatalytic reaction with Bi₂O₃, among which are: due to its hybridation (Bi 6s and O 2p orbital), it is a strong oxidizer and can successfully degrade organic compounds and favors the mobility of photoholes in the valence band [17].

According to Levenspiel, the equation used to get the parameters come from the reaction rate equation in terms of the concentration of the reactant

− r A = − d C A d t = k C A n (5)

where n means the order of the reaction, k is the rate constant and the C_A is the concentration of the reactant, which in this case is methylene blue.

It is well known that the concentration of the reactants in time t can be explained by the Equation (6)

C A = C A 0 ( 1 − X A ) (6)

C_A0 is the initial concentration of the catalyst and X_A is the degree of conversion of the reactant. Then, the substitution of Equation (6) in Equation (5) is next

− r A = C A 0 d X A d t = k ( C A 0 ( 1 − X A ) ) n (7)

Knowing the time of the experiment, the change of concentration in time t due to absorbance measurement and the initial concentration of each experiment, the equation to know the rate constant is shown below as Equation (8).

k = 1 t ∫ 0 X A d X A C A 0 n − 1 ( 1 − X A ) n (8)

Table 4 shows the results of the calculus made to get the approximately kinetic constant of each catalyst and to understand their behavior during the photocatalysis. Even that they are first order kinetics, the speed constant of Bi₂O₂CO₃ shows a bigger constant than TiO₂ Anatase because during the first 20 minutes of UV radiation, Bi₂O₂CO₃ degrades better than TiO₂. The same behavior is shown with Commercial Bi₂O₃. For the experiment anatase TiO₂ was used because its properties are well known, the superficial area has a value of 10 - 17 m²/g [18] [19]. TiO₂ anatase band gap experimental value is 3.0 eV which shows that needs more energy to get excited and to degrade in a higher way.