Efficient α/β-Bi2O3 composite for the sequential ...

Ceramics International 42 (2016) 13065–13073

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Efficient α/β-Bi2O3 composite for the sequential photodegradation of two-dyes mixture Tanveer A. Gadhi a, Agileo Hernández-Gordillo b,1,n, Monserrat Bizarro b, Pravin Jagdale a, Alberto Tagliaferro a, Sandra E. Rodil b a

Department of Applied Science and Technology, Politecnico di Torino, Italy Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito Exterior SN, Ciudad Universitaria, CP 04510 México D.F., Coyoacán, México

b

art ic l e i nf o

a b s t r a c t

Article history: Received 19 April 2016 Received in revised form 13 May 2016 Accepted 14 May 2016 Available online 15 May 2016

A mixture of α/β-Bi2O3 and α-Bi2O3 powders were obtained by a simple solid state reaction–annealing route at 550 °C. The structure, optical properties and surface area of the commercial α and β-Bi2O3 and the synthesized α-phase and α/β-composite were well characterized by X-ray diffraction, diffuse reflectance spectra and N2 physisorption. The annealed sample at 550 °C showed 20% of β-phase, forming a heterojunction of α/β-Bi2O3 whereas annealing at elevated temperature (650 °C) lead to the α-phase. Optical properties showed that the presence of the β-phase is mainly responsible for narrowing the energy band gap. The photocatalytic activity of the commercial α and β-Bi2O3 and the synthesized αphase and α/β-composite were investigated in degradation of single dyes, Indigo Carmine (IC) and Rhodamine-B (RhB) under both UV and visible light-induced photocatalysis. For the best photocatalyst, the photodegradation in a two-dye mixture solution was systematically studied considering the type of dye, the adsorption capacity of the samples and the behavior of dye photodegradation. The photocatalytic performance of α/β-Bi2O3 was comparatively much higher than the commercial α and β-Bi2O3, indicating that better performance of efficient charge separation and transfer across α/β-Bi2O3 composite was obtained. Possible mechanism of the single dye and two-dye mixture degradation was given by using α/β-Bi2O3 composite. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: α/β-Bi2O3 composite Solid-state reaction Dyes mixture Photodegradation

1. Introduction Increasing concerns on water pollution encourage the research for developing optimized techniques to re-utilize water in the industrial processes [1–4] with an efficient use of the energy and resources. Heterogeneous photocatalysis is reported as a green, sustainable and clean process which uses metal oxide semiconductors for degrading dyes, medicines and insecticides from water [5]. Among non-titanium based oxide semiconductors, bismuth oxide (Bi2O3) has been proposed as a potential visible-light photocatalytic material [6–12]. There are six polymorphs reported for Bi2O3: α-Bi2O3 (monoclinic), β-Bi2O3 (tetragonal), γ-Bi2O3 (body centered cubic), δ-Bi2O3 (cubic), ω-Bi2O3 (triclinic) and ε-Bi2O3 (triclinic) [13–15], exhibiting significantly different optical and electrical properties [16], but most important for the photocatalytic applications is the higher optical absorption in the visible n

Corresponding author. E-mail addresses: [email protected], [email protected] (A. Hernández-Gordillo). 1 CONACYT Research Fellow. http://dx.doi.org/10.1016/j.ceramint.2016.05.087 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

range compared to the TiO2. The room temperature α-phase and the high-temperature ( 4725 °C) δ-phase are thermodynamically stable, while the others are metastable phases [16] obtained during cooling. Nevertheless, both α- and β-phases are commonly obtained during chemical synthesis of Bi2O3 micro and nanostructures [6,9,17–19]. The direct optical band gap of the monoclinic α-Bi2O3 is reported around 2.7–2.9 eV and for the β-Bi2O3 (tetragonal), the reported value is slightly lower around 2.5 eV; in both cases, the precise value depends on the impurity caused by the synthesis method used. Similarly, the photocatalytic activity measured through the visible-light driven degradation or bleaching of dyes depends strongly on the crystalline phase, surface area and morphology. Some reports indicated a good activity for the αphase [18,20–23], while others define the β-phase as the most active [6,8–10] due to the lower energy gap, which allows the use of a larger window of the solar spectrum. More recently, two groups have reported superior photocatalytic performance of α/β-Bi2O3 heterojunctions but no information about the relative amount of each phase was given [24,25]. Hou et al. [24] reported the degradation of cationic rhodamine B (RhB) and anionic methyl orange (MO) under visible-light irradiation by β-Bi2O3 sheets in

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comparison to β/α composite samples sintered at different temperatures. The results showed an optimal dye degradation obtained from the β/α heterojunction sintered at 210 °C, where above 90% photodegradation was quickly obtained for anionic MO (15 min) and more slowly for cationic RhB dye (60 min). Shan et al. [25] produced α/β heterojunction using a two steps efficient method and evaluated it for the photodegradation of RhB dye under visible light where above 95% photodegradation was quickly obtained in 30 min The results were compared with the single phases (α and β) and the explanation about the better photocatalytic activity of the heterojunction was given in terms of efficient light harvesting and photogenerated carrier separation due to the proper alignment of the β/α electronic bands that were measured by X-ray photoelectron spectroscopy, which is in good agreement with the flat band potentials estimated by Hou et al. [24]. Despite the good explanation given in both papers about the better performance of the α/β-heterojunction, some issues are not clearly presented. In both papers, there is a large adsorption of the dye on the heterojunction photocatalyst surface, which occurs during the dark conditions and that leads to a decrease in the optical absorbance of the dye solution that cannot be considered as due to the photodegradation process. Actually, Hou et al. reported it (wrongly since decrements in the relative dye concentration, C/C0, of about 0.76 in the dark were reported as 24% and not as 76% adsorption) as the adsorption capacity of the heterojunction sample, but nevertheless the initial (dark) decrement of the C/C0 curve was included in the estimation of the reaction kinetic rate. A decreasing concentration of the reactant in the solution tends to increase the dye photodegradation rate because more active sites are available to the photogeneration of the reactive oxygen species [26,27]. Nevertheless, both works proposed the idea of using mixtures of phases taking advantage of the Bi2O3 polymorphism to improve the photocatalytic activity. In this paper, we want to explore further such idea producing well characterized α/β-Bi2O3 composite samples and compare the photocatalytic activity in the UV and visible light range for two different single dyes (cationic and anionic), as well as a mixture of two dyes, trying to approach to the real situation, where the wastewater contains more than a single dye.

2. Experimental details 2.1. Synthesis of Bi2O3 powders All chemicals were of analytical grade and used without further purification. Bi2O3 powders were synthesized through solid state reaction by direct heating of Bi(NO3)3
5H2O salt (Sigma-Aldrich) at 150 °C for 30 min in order to evaporate the water content; then the temperature was raised to 250 °C and kept for 2 h. The obtained powder was then annealed at different temperatures: 550 and 650 °C for 2 h followed with slow furnace cooling in order to obtain single or mixture of crystalline phases. For comparison, commercial α-phase (Farmaquimia) and β-phase (Sigma-Aldrich) Bi2O3 powder were used without any thermal treatment. 2.2. Characterization The phase composition of synthesized samples was characterized by X-ray diffraction (XRD) using a SIEMENS D500 X-ray diffractometer (Cu-Kα X-ray source). The diffraction patterns were analyzed using the PDXL2 software to identify each phase, determining the grain size (Halder-Wagner method) and the relative amount of each phase in the phase-mixtures (Relative Intensity Ratio, RIR method) [28]. The UV–vis diffuse reflectance spectra (DRS) were recorded on a UV–vis spectrophotometer equipped

with an integration sphere (Shimadzu 2600) and using BaSO4 as a reference. The spectra were converted from reflectance to absorbance by the Kubelka–Munk method [18]. The band-gap energy (Eg) was calculated considering an allowed direct transition for α-Bi2O3 phase, by extrapolating the linear portion of the (F(R)  hv)2 vs hv curves to F(R)¼ 0 [25]. The point of zero charge (pzc) was determined by potentiometric titration, using 0.01 M KCl as the background electrolyte to stabilize the ionic strength in the solution, and it was selected considering that the cations and anions of the background electrolyte do not compete with the absorption process and do not react with the catalyst surface generating complexes in the solution. The initial pH ¼2 was adjusted using 0.01 M of HCl and the titration was carried out from 2 to 12 pH by adding 0.1 mL of NaOH solution (0.01 M), which was measured using a potentiometer (Jeanway model 3540). Finally, plots of the change of the pH as a function the NaOH volume were done for the samples and the KCl background electrolyte. The pzc can be defined as the interception point between both plots [29] or using the differential technique proposed by Bourikas et al. [30]. 2.3. Photocatalysis tests The photocatalytic activity of synthesized Bi2O3 samples (550 °C, 650 °C) and commercially available α-phase and β-Bi2O3 phase was evaluated by degradation of indigo carmine (IC) and rhodamine B (RhB) dyes under either UV or white light (9 W with an irradiance of 26 W m  2, and 9 W with an irradiance of 33 W m  2, respectively), using 15 mL of the dye solution at pH ¼6–7 with a dye concentration of 10 mg/L. The powder load in the dye solution was 1 mg/mL and the suspension was stirred for 30 min in the dark to allow the adsorption-desorption equilibrium. The absorbance spectra of the dye solution were recorded as a function of time with a UV–vis spectrophotometer (Shimadzu 1800) by taking an aliquot of 3 mL and centrifuging at 5000 rpm for 5 min The powder sample and aliquot were returned to the vial to preserve the same amount of powder and solution. After selecting the sample with the best photocatalytic characteristics, it was evaluated for the degradation of a mixture of two dye solution containing IC and RhB dye. This mixed solution was prepared with 1.5  10  6 M concentration of each dye to ensure the same number of molecules in the solution. The powder loading was again 1 mg/mL and the pH solution was close to neutral (pH-6). The photocatalytic tests were carried out under either UV or visible light sources.

3. Results and discussion 3.1. Characterization of Bi2O3 3.1.1. Crystalline structure The XRD patterns of the commercial Bi2O3 and annealed samples are shown in Fig. 1. The commercial Bi2O3 COMα exhibited, as expected, the diffracted peaks of the monoclinic-α-Bi2O3 phase, with principal peaks at 2θ ¼27.06°, 27.52°, 33.17°, 33.9° (JCPDS card no. 01-071-0465). For the commercial β-Bi2O3 phase COMβ sample, the principal diffracted peaks at 2θ ¼ 27.96°, 31.78°, 32.72°, 46.22° and 55.48° can be indexed to tetragonal-β-Bi2O3 phase (JCPDS card no. 01-078-1793) [8,10]. The diffracted peaks of both α- and β-Bi2O3 commercial samples are narrow indicating that they are well crystallized and the average grain sizes, applying the Halder-Wagner method [28], are 91 and 53 nm, respectively. In the case of the synthesized B550 sample, most of the diffracted peaks (see Fig. 1) correspond to the α-Bi2O3 phase, but also peaks related to the β-Bi2O3 phase can be identified. This sample was composed of polymorphism α/β-Bi2O3 phase, forming a composite-like with

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Fig. 1. XRD patterns of all Bi2O3 commercial (COMα, COMβ) and synthesized (B550 and B650) samples, respectively.

Table 1. Composition phase, average crystal size and band gap energy of the commercial (COMα, COMβ) and synthesized (B550 and B650) samples. Sample

COMβ B550 B650 COMα

Composition phase (%) α

β

00 80 100 100

100 20 00 00

Energy band gap (eV)

2.3 2.8 2.9 2.8

low proportion of β-phase (20%) (Table 1). The width of the diffracted peaks of α/β-Bi2O3 composite suggests that it is formed by nanocrystallites with average size of 30 nm. The existence of both β- and α-phase in the Bi2O3 composite is reported to form a stable interface heterojunction [24,31]. The synthesized B650 sample, annealed at 650 °C, exhibited diffracted peaks corresponding to pure α-Bi2O3 phase, and its average grain size was determined to be 27 nm. The absence of the β phase suggests that its transformation into α-phase was completed. However, the average grain size was not increased. This can be explained as follows: during the annealing treatment, β-phase was formed inhibiting the sintering or growth of the formed α-phase. Thus a complete transformation of β- into α-phase was achieved without growing. 3.1.2. Optical property Fig. 2A shows the UV–vis DRS analysis of the various samples after using Kubelka-Munk function [18]. The inset shows the energy gap extraction. The commercial samples exhibit a wide absorption edge: from 410 to 450 nm for COMα and from 470 to 550 nm for COMβ. This absorption edge is characteristic of the α-Bi2O3 and β-Bi2O3 phase, respectively and is associated to the electronic transition from the valence band to the conduction band. The lower gap and the subsequent higher absorption coefficient in the visible region of β-Bi2O3 phase represents a great advantage for a better exploitation of solar light for photocatalytic reactions [10]. The annealed samples exhibit the absorption edge in the region from 410 to 450 nm similarly to the α-Bi2O3 phase. However, the B550 sample exhibits a red-shifted absorption probably due to the contribution of the β-Bi2O3 phase. Additional absorption in the 450–650 nm region has also been related to the presence of surface crystal defects, mainly oxygen vacancies [8,32], which in our case may be generated during the annealing treatment. The band-gap energy for the β-phase is 2.3 eV while for both α-phase commercial and annealed (B650) samples are in the

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Fig. 2. (A) UV–vis DRS spectra and the optical band gap estimation (inset) of the Bi2O3 commercial (COMα, COMβ) and synthesized (B550 and B650) samples, and (B) emission spectrum of the UV and Visible Lamp.

range 2.8–2.9 eV (Table 1). For the β/α-Bi2O3 composite sample (B550), the small quantity of β-phase (20%) increases the visiblelight absorption of the β/α-heterojunction [31], decreasing the band-gap energy at 2.7 eV. Comparing the absorption properties of the samples and the emission spectra of the UV and visible light lamp (Fig. 2B), all Bi2O3 samples can fully absorb UV light; however, the COMα sample cannot fully absorb visible light and as consequence will show lower photoactivity.

3.1.3. PZC property and surface area The pH of point of zero charge (pzc) is an important property of Bi2O3 surface because it affects its ability to adsorb anionic or cationic molecules when they are dispersed into aqueous solution. When the pH is below the pzc value, the solid surface become protonated [33], and anionic molecule could be adsorbed. By using the first derivative method, the pH of pzc for the β-, α-phase and for α/β-composite samples (Fig. 3) was found in the range of 6.8– 7.8, which were lower than that reported (9.4) [29]. The lowest pH value of pzc for the α/β-composite, suggest that this sample can achieve higher protonation than the others samples at the same pH solution. The specific surface areas of all samples are in the range of 1.3–3.5 m2/g.

Fig. 3. Potentiometric titrations and its first derivative of each one indicating the pzc value of all Bi2O3 commercial (COMα, COMβ) and synthesized (B550 and B650) samples.

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Fig. 4. (A) Indigo carmine dye structure, (B) Absorbance spectra of the IC dye solution during its photodegradation using B550 sample under UV light irradiation.

3.2. Single dye photodegradation In order to determine the photocatalytic property of β-, αphase and of α/β-composite samples, indigo carmine (IC) and rhodamine-B (RhB) were used as anionic and cationic dye, respectively, for the photodegradation test. The absorbance spectrum of IC (Fig. 4A) dye photodegradation by using B550 sample under UV-light is presented in Fig. 4B. The characteristic

absorbance peak of IC dye at 610 nm [34], slightly decreased (10%) during the dark condition (30 min), suggesting that a small amount of IC dye was adsorbed on α/β-composite surface. Afterwards, this absorbance peak of the IC dye fully decreased during the same time of UV-irradiation (30 min), due to a fast photodegradation process. This process is associated to the destruction of the indigoid group (NHC ¼CNH) of the IC dye structure via oxidation, originating secondary products, which exhibited absorbance from the benzene and carboxylic groups in the region 210–260 nm. This secondary products, identified as Isatin sulfonic acid and 2-amine-5-sulfo-benzoic acid [35–37], exhibited additional isosbestic absorption at 251 nm and the persistence of this absorption peak indicates that the photodegradation process of the secondary organic acid products was not complete [35]. Fig. 5 shows the profile of relative concentration (C/C0) (Fig. 5A and C) and the kinetic curve plots of photodegradation of the IC dye (Fig. 5B and D), respectively, during the UV and visible light irradiation (and the preceding dark equilibration period) by using β-, α-phase and of α/β-composite samples. Considering that the pH of IC dye solution is near to 6.8, the surface charge of Bi2O3 samples must be poorly protonated and as result, small amount of IC dye could be adsorbed in the region of the amphoteric sites. Thus, the interaction of the anionic dye with the poorly protonated surface is due to hydrogen bonds and, as consequence, the dye adsorption is weak [38]. Afterwards, the C/C0 of IC dye decreased during the UV irradiation (Fig. 5A), reaching similar degradation efficiency (60–65%) at 45 min, when either commercial β- or αphase was used. In the case of synthesized α-phase or α/β-composite higher values of conversion, 85% and 96% respectively, were reached. In all cases, the kinetic curve obtained by plotting ln(C0/C) versus the reaction time (t) showed a straight line (Fig. 5B), suggesting that the IC dye degradation follows a pseudo first order reaction kinetic [39]. The highest apparent kinetic rate value was obtained for the α/β-composite (kapp ¼10.5  10  2 min  1). When the IC dye degradation was carried out under visible light for 60 min (Fig. 5C), the decrease of IC dye concentration was

Fig. 5. Profile of relative concentration and kinetic curve plot of IC dye photodegradation under (A, B) UV light and (C, D) visible light, respectively, by using Bi2O3 photocatalysts.

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Fig. 6. (A) Rhodamine-B dye structure and (B) Absorbance spectra of the RhB dye solution during its photodegradation by using B550 sample under UV light irradiation.

slower than under UV light. Both commercial and synthesized αphase samples presented low value of conversion (25% and 10%, respectively). The kinetic curve obtained by plotting ln(C0/C) versus the reaction time (t) showed a straight line (Fig. 5D), suggesting that the IC dye degradation follows a reaction kinetic of pseudo first order: a low kapp value (0.3  10  2 min  1 for B650 sample) was observed. In contrast, for the samples COMβ and B550 that contain β-phase the conversion was of 50% and 90%, respectively (Fig. 5C). The α/β-composite sample (B550) exhibited the highest conversion under both UV and visible light, where the kapp value of the IC dye degradation under UV light (kapp ¼ 10.5  10  2 min  1) was 3.7 times larger than that under visible light (kapp ¼2.6  10  2 min  1, Fig. 5D). This difference in photoactivity could be due to the low intensity of the visible lamp used in comparison to the UV-lamp. Fig. 6 shows the absorbance spectra of the RhB dye photodegradation under UV light using the B550 sample (produced the best results). The RhB (Fig. 6A) dye solution exhibits a characteristic absorbance peak at 554 nm [40], which during the dark condition was reduced by 20%, suggesting that RhB dye concentration underwent an adsorption process on α/β-composite surface [41]. Afterwards, during the UV irradiation (Fig. 6B), the RhB dye concentration was slowly decreased in a long time period (210 min), indicating that RhB was hardly degraded. It is known that the RhB photodegradation generates several intermediary products [42]; however, in our case during its degradation not additional or hypsochromic shift of absorbance peak was detected (as can be observed in Fig. 6B), indicating that RhB was degraded without intermediary products formation. Considering that the pH of RhB dye solution is near to 6.8, the surface charge of Bi2O3 samples must be poorly protonated. It means that the surface is partially hydroxylated while RhB dye is preferentially adsorbed by amphoteric sites. Thus higher amounts of RhB dye were adsorbed on the α/β-composite surface (20%)

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than on other samples surfaces (not shown). In this case the interaction of the cationic dye with the hydroxylated surface is through oxygen bonds, which are stronger than for IC, however, RhB dye can be also partially adsorbed on the protonated surface of Bi2O3, via diethylamino group [40,43]. Fig. 7 shows the results for the RhB dye photodegradation under both UV and visible light only for the B550 sample, since this material produced again the best results. Fig. 7A shows the relative concentration of the RhB dye during the irradiation time, while Fig. 7B presents the corresponding kinetic analysis (and the preceding dark equilibration period). The photodegradation of RhB was similar to that achieved for the IC dye; at 210 min, 80% and 40% of conversion were achieved under UV and visible light, respectively. The kinetic curve showed a straight line (Fig. 7B), suggesting that the RhB dye degradation also follows a pseudo first order reaction [44]. In this case, the kapp values were 6.2  10  3 min  1 and 3  10  3 min  1 under UV and visible light, respectively; 2 times larger under UV irradiation. It is known that the sensitization of solid sample caused by the RhB dye adsorption is possible [45,46]; however, using UV light at 367 nm (Fig. 2B), this sensitization effect of RhB dye is completely negligible because the dye absorbs visible light at 550 nm, indicating that the cationic dye was degraded by the use of α/β-composite photocatalyst. The kapp for RhB dye photodegradation using α/β-composite under visible light was apparently lower than that reported by Hou et al. [24] and Shan et al. [25]; however, the high adsorption capacity of their α/β-composite to adsorb the RhB dye (76% and 59%, respectively) gives a false value of the kapp. In our case, the dye adsorption capacity is very low (20%) and therefore this phenomenon does not interfere in the real photodegradation rate. Comparison to other materials and reports is always difficult, since the photocatalytic experimental conditions are usually very different. For example, our kapp for RhB dye photodegradation was apparently much lower than that for K4Nb6O17 photocatalyst modified with 25 wt% of PbS, reported by Cui et al. [47]. In that work, the observed high photoactivity (kapp ¼ 11  10  3 min  1) was possible due to the large surface area and wide absorption in the visible region; however, it is also important to mention that they used a high power halide lamp (250 W) in comparison to the low power white lamp (9 W) used in this research. Therefore, in terms of the energy input, the α/β-composite is better photocatalyst for the RhB degradation, even though the reaction rate is lower. The kapp for IC and RhB dyes photodegradation (Fig. 8A and B, respectively), under both UV and visible light, showed that α/βcomposite (B550 sample) exhibited better performances than the other samples, including commercial β-Bi2O3 (COMβ), which has been reported as the most photoactive phase [6,48]. The high photoactivity of α/β-composite can be due to the presence of a heterojunction between α  and β-Bi2O3 phases, which lead to better dye degradation. As shown above, the photodegradation rate of IC and RhB dye strongly depends on the type of dye and its adsorption capacity by each sample. The weak interaction of anionic IC dye with the Bi2O3 surface can be responsible of the fast photodegradation rate favoring a refreshment of the dye near the surface. 3.3. Two-dyes mixture photodegradation The molar concentration of each dye (IC and RhB) in the twodye solution was 1.5  10  6 M to ensure the same number of dye molecules in the solution. Considering that the total absorption spectrum of the two-dyes mixed solution is a contribution of the absorbance of both IC and RhB dyes, the mixed solution exhibited two absorbance peaks at 610 and 554 nm, one associated to each dye. The two-dye mixture photodegradation was investigated by

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Fig. 7. (A) Profile of relative concentration and (B) kinetic curve plot of RhB dye photodegradation under UV and visible light, respectively, by using B550 composite sample.

Fig. 8. Apparent kinetic rate constant of pseudo first order reaction for all Bi2O3 samples in the; (A) IC and (B) RhB dye degradation under UV and visible light irradiation.

Fig. 9. Absorbance spectrum of the two-dyes mixture photodegradation by using B550 composite sample under visible light irradiation at: (A) short and (B) long time.

using the α/β-composite sample (B550) under visible light. The absorbance spectra are showed in two time scales: short time; from the dark-adsorption period (30 min) up to 60 min of irradiation (Fig. 9A) and long time; from 60 until 240 min of irradiation (Fig. 9B). During the dark adsorption period, both IC and RhB dyes were adsorbed on α/β-composite surface (10% and 15%, respectively), because of the protonated composite surface generated at the pH ¼6 of the mixed solution [49]. When the two-dyes solution was irradiated using visible-light, the absorbance peak of the IC dye decreased quickly in 60 min (Fig. 9A), and at the same

time, the absorbance peaks of the secondary products were raised and the RhB dye was adsorbed on composite surface. The IC dye degradation was almost completed in 60 min, while the absorbance peak of RhB dye was hardly decreased even for a large period of irradiation time (240 min, Fig. 9B); however, it suffered of hypsochromic blue-shift observed at 540 nm and an isosbestic absorption point at 502 nm appeared, indicating the formation of an intermediary product. This product has been identified as N,Ndiethyl-N-ethyl-rhodamine due to the N-de-ethylated intermediates of RhB dye [42], which appears when the carboxyl group

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Fig. 10. (A) Kinetic curve plot and (B) apparent kinetic rate constant of each dye degradation in the two-dyes solution using B550 composite under UV and visible light.

of the RhB is preferentially adsorbed on the semiconductor surface rather than the diethylamino group and as consequence, the reactive oxygen species mainly attack the conjugated chromophore ring structure [43,50]. The RhB dye adsorption through the carboxylic groups could be favored by the pH modification toward the acidic condition (pH o6) due to the formation of the Isatin sulfonic acid and 2-amine-5-sulfo-benzoic acid during the IC dye degradation (this pH variation was experimentally confirmed). The results suggest that the IC dye was selectively degraded into its intermediary products by reactive oxygen species and after the IC full degradation, the RhB dye started to be degraded into N,N-diethylN-ethyl-rhodamine whose formation was favored by the presence of the secondary products of the IC dye which are still persistent after the two-dyes mixture degradation. The estimated kapp for the IC dye photodegradation occurring during the first 60 min was 3.5  10  2 min  1 whereas RhB dye was predominantly adsorbed (Fig. 10A); and from 60 to 240 min, the kapp obtained for the RhB dye photodegradation was 0.85  10  2 min  1; lower than for the IC dye. Similar behavior was obtained when the two-dye mixture photodegradation was performed under UV light irradiation (figures not shown). Fig. 10B compares the UV and visible kapp for the individual dyes in the mixed-solution, where it can be observed that the IC dye photodegradation was 10 times larger under UV irradiation.

3.4. Mechanism of two-dyes mixture photodegradation The mechanism for the single dye photodegradation by using semiconductor oxide has been well established; where the IC and RhB dye are photodegraded by the reactive oxygen species generated on semiconductor surface [36,37]. The, reactive oxygen species were generated on β-, α-Bi2O3 phase surface by the reaction between the photogenerated electrons and the dissolved O2 in the solution; however, more reactive oxygen species were generated by using α/β-composite. The fast generation of reactive oxygen species using α/β-Bi2O3 composite can be due to the electron transfer cascade formed in the heterojunction (Fig. 11A), where the photogenerated electrons of the conduction band of the α-Bi2O3 are transferred to β-Bi2O3, whereas the holes are transferred from β-phase to α-phase [24], improving the electron-hole separation due to the presence of 20% of β-phase. On the other hand, little information about the mechanism of two-dye degradation using β-, α-Bi2O3 phase and α/β-composite can be found in the literature. However, the results obtained from the C/C0 profile and the reaction rates allow us to suggest a possible mechanism to explain the two-dye photodegradation process. When the two dyes (anionic and cationic) are in a mixed solution at the same concentration, the reactive oxygen species generated on the α/β-composite, preferentially react with the adsorbed anionic IC dye, facilitating its fast photodegradation

Fig. 11. (A) Diagram of electrons transfer cascade on β/α-Bi2O3 composite, (B) mechanism of two-dyes mixture photodegradation by using β/α-Bi2O3 composite.

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(Fig. 11B) due to its weak interaction through of the hydrogen bond. After the full IC dye degradation, the reactive oxygen species generated nearby the adsorption sites of the RhB-groups started to react with the cationic RhB dye (Fig. 11B), which is adsorbed through the carboxylic groups, and then is preferably deethylated [43]. Therefore, using α/β-Bi2O3 composite with improved photocatalytic activity result better for the single dye or two-dye photodegradation under visible light irradiation.

4. Conclusion The α/β-Bi2O3 heterojunction was successfully formed by the solid state reaction from direct heating of Bi(NO3)3
5H2O annealing at 550 °C. The composite sample contains α-phase with 20% of β-Bi2O3 phase which contribute to the adsorption in the visible light region. In the single dye degradation process, IC dye was faster degraded than the RhB dye. In the two-dye mixed solution the α/β-Bi2O3 heterojunction selectively degraded anionic IC dye at first and only when its degradation was almost complete, the degradation of the cationic RhB dye was initiated. The organic acid products generated from IC dye induced the adsorption of RhB dye via carboxylic groups and as consequence RhB dye was only deethylated. The highest photoactivity of α/β-Bi2O3 composite is probably due to the enhanced electron-hole separation caused by the electron transfer cascade formation. The formed heterojunction performed better photocatalytic activity under visible light than that the single commercial β-phases.

Acknowledgments The authors thank to CONACYT for the Cátedras-Conacyt/1169 project. The authors also acknowledge the financial support from the PHOCSCLEEN-318977 and the DGAPA-PAPIIT IN106015 projects. Finally the authors recognized the support of A. Tejeda, for making possible the different measurements.

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