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Performance of solvothermally grown Bi2MoO6 photocatalyst toward degradation of organic azo dyes and fluoroquinolone antibiotics
Materials Letters 258 (2020) 126764
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Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Performance of solvothermally grown Bi2MoO6 photocatalyst toward degradation of organic azo dyes and fluoroquinolone antibiotics Tammanoon Chankhanittha, Varanya Somaudon, Jidapa Watcharakitti, Voranan Piyavarakorn, Suwat Nanan ⇑ Materials Chemistry Research Center, Department of Chemistry and Center of Excellence for Innovation in Chemistry (PERCH-CIC), Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand
a r t i c l e
i n f o
Article history: Received 13 September 2019 Received in revised form 30 September 2019 Accepted 30 September 2019 Available online 11 October 2019 Keywords: Bi2MoO6 Photocatalytic degradation Organic azo dyes Antibiotics
a b s t r a c t Preparation of Bi2MoO6 catalyst at low temperature via a solvothermal method was reported. Neither surfactant nor capping agent was used. The orthorhombic Bi2MoO6 catalyst showed a very high efficiency of 90% toward photodegradation of two azo dyes (reactive red dye and Congo red dye) and two antibiotics (ofloxacin and norfloxacin) under both ultraviolet (UV) and visible light irradiation. The photodegradation reaction of these organic pollutants correlates well with the pseudo first-order kinetics model. Both electron and hole play a major role in degradation of the pollutants. The Bi2MoO6 photocatalyst still shows a very high photocatalytic performance even after five cycles of use. This indicates the advantages of structural stability and reusability of the catalyst. The prepared Bi2MoO6 could be a promising photocatalyst for degradation of organic dyes and antibiotics present in water. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor photocatalysis has attracted a great interest due to its promising potential for environmental remediation [1]. Traditional photocatalyst such as TiO2, with a large band gap of 3.2 eV, only shows ultraviolet (UV) responsive photodegradation of the organic pollutants. Interestingly, the semiconductor bismuth molybdate (Bi2MoO6), an important Aurivillius oxide with a direct band gap of 2.6–2.8 eV, has been received considerable attention due to its application in visible-light-driven photocatalysis [2,3]. Recently, the synthesis of bismuth molybdate with different morphology including spherical structure, York-shell structure, and rod-shaped structure via physical and chemical methods has been reported [3–5]. In some cases, heterojunction photocatalysts based on Bi2MoO6 were also fabricated [6,7]. Among various preparation methods, a hydrothermal/solvothermal method is the most promising synthetic route for fabrication of Bi2MoO6 due to its simplicity and low cost [8–10]. The present work reports the solvothermal synthesis of Bi2MoO6 catalyst. Neither surfactant nor capping agent is needed. The prepared catalyst showed a uniform plate-like morphology of 390 nm 940 nm. The blue emission over 460–500 nm due to ⇑ Corresponding author. E-mail address: [email protected] (S. Nanan). https://doi.org/10.1016/j.matlet.2019.126764 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
surface defect states was observed. The photodegradation efficiency of the catalyst was determined by using azo dyes and antibiotics as model pollutants. A very high efficiency of 90% was obtained. To the best of our knowledge, this is the first work demonstrating the visible-light-responsive photodegradation of both anionic azo dyes and fluoroquinolone antibiotics by Bi2MoO6 photocatalyst. 2. Experimental sections All chemicals including Bi(NO3)35H2O (Fluka), ethylene glycol (Sigma Aldrich), (NH4)6Mo7O24 (Fluka), and absolute ethanol were used without further purification. The de-ionized water was used. In a typical synthesis, 3.9855 g of Bi(NO3)35H2O was dissolved in 30 mL of ethylene glycol (solution A). Meanwhile, 0.7138 g of (NH4)6Mo7O24 was dissolved in 30 mL of distilled water and stirred under heating at 100 °C for 15 min (solution B). Solution A was added dropwise into solution B and then 10 M NaOH solution was added to adjust the pH value to about 9. Afterward, the mixture was transferred to a 100 mL Teflon-lined stainless-steel autoclave and maintained at 160 °C for 24 h. The product was naturally cooled down to room temperature and washed with ethanol. The sample was then dried in an oven at 60 °C for 24 h. A paleyellow powder of Bi2MoO6 was obtained. Characterization and photocatalytic study can be found in supporting information.
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3. Results and discussion The XRD pattern (Fig. 1a) of Bi2MoO6 belongs to orthorhombic phase (JCPDS No. 72–1524). The XRD peaks found at 2h = 23.50°, 28.24°, 32.59°, 33.13°, 36.00°, 39.65°, 46.74°, 47.11°, 55.33°, 56.23°, and 58.41° corresponded to the reflection from (1 1 1),
(1 3 1), (0 0 2), (0 6 0), (1 5 1), (0 4 2), (2 0 2), (0 6 2), (3 3 1), (1 9 1), and (2 6 2) planes, respectively [11]. No peak due to impurity was detected. The FT-IR spectrum of Bi2MoO6 (Fig. 1b) showed a broad band at about 3389 cm 1 corresponding to the OAH stretching vibration of water. The peak at 1384 cm 1 corresponded to the C@O stretching
Fig. 1. XRD pattern (a), FT-IR spectrum (b), UV–vis spectrum (c), PL spectrum (d), SEM micrograph (e), EDX spectrum (f), SEM micrograph of the mapping area and EDX elementary mapping of Bi, Mo, and O of the as-synthesized Bi2MoO6 (g).
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modes due to the absorption of CO2 on the surface of the Bi2MoO6 catalyst. The peak at 846 cm 1 indicated Mo-O-Mo bridging bond. The peak found at 730 cm 1 and 578 cm 1 were attributed to the presence of Mo-O bond and Bi-O bond, respectively [12]. This confirms the purity of the sample. The UV–vis spectrum of Bi2MoO6 was shown in Fig.1c. The energy band gap (Eg) of 2.62 eV was estimated by Tauc plot [13] as shown in the inset of Fig.1c. Photoluminescence (PL) spectrum of Bi2MoO6 (using excitation wavelength of 300 nm) was shown in Fig. 1d. In principle, both near band edge (NBE) emission and trapped emission are expected in PL spectrum of semiconductor
3
photocatalyst. The former is sharp and found near the absorption edge. The latter is broad and found at longer wavelength [14,15]. In the present work, the blue emission over 460–500 nm comprising a strong peak at 470 nm and three small peaks at 473, 483 and 493 nm was observed. This is due to surface defect states found in the Bi2MoO6 catalyst [16]. The prepared Bi2MoO6 catalyst showed a uniform morphology composed of plate-like structure with the grain size of about 390 nm 940 nm (Fig. 1e). The EDX spectrum (Fig. 1f) indicated the existence of bismuth (Bi), molybdenum (Mo) and oxygen (O). The mapping study was also included. The SEM micrograph of
Fig. 2. Lowering of concentration (C/C0) vs time due to photodegradation of dyes under visible light irradiation (a), plot of C/C0 due to photodegradation of dyes under UV light (b), plot of C/C0 due to photodegradation of antibiotics under visible light (c), plot of C/C0 due to photodegradation of antibiotics under UV light (d), determination of rate constant under visible light (e), and determination of rate constant under UV light (f).
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Fig. 3. Effect of some scavengers on photodegradation of Congo red (CR) dye (a) and ofloxacin (OFL) antibiotic (b), hydroxyl radical trapping PL spectra of Bi2MoO6 under visible light irradiation (c), possible degradation mechanism of organic pollutant with Bi2MoO6 catalyst (d), reusability of Bi2MoO6 catalyst for degradation of CR dye (e) and OFL antibiotic (f), XRD patterns of the Bi2MoO6 catalyst before and after photodegradation of the pollutants (g), and SEM micrographs of the catalyst after photodegradation of CR dye and OFL antibiotic (h). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the mapping area was shown in Fig. 1g. In addition, elemental colour mapping of the Bi2MoO6 catalyst showed the homogeneity of the particles supporting the uniform distribution of Bi, Mo and O (Fig. 1g).
Photodegradation efficiency of Bi2MoO6 was investigated by using azo dyes and fluoroquinolone antibiotics as model pollutants. The chemical structures of these organic pollutants namely reactive red (RR141) dye, Congo red (CR) dye, ofloxacin (OFL),
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and norfloxacin (NOR) antibiotics were shown in Table S1. On examining the dye degradation, lowering of concentration (C/C0) with increasing time (t) under both visible and UV light were observed as shown in Fig. 2a and b, respectively. According to the blank test (no catalyst), the RR141 dye concentration remained constant over 240 min indicating hardly self-photodegradation of this dye. RR141 dye solution with Bi2MoO6 in the dark provided C/C0 of 0.77 after 240 min indicating low contribution of dye removal by adsorption process. In the presence of Bi2MoO6, however, lowering of C/C0 with time was observed. The efficiency reached 45% and 37% under visible light and UV light irradiation, respectively. In the case of CR dye, a huge contribution (77%) of dye removal by adsorption process was observed after 240 min. Additionally, the efficiency reached 91% after photo irradiation. On examining removal of antibiotics (Fig. 2c and d), both OFL and NOR showed low contribution of adsorption process (25% and 35% for removal of OFL and NOR, respectively). OFL showed photodegradation efficiency of 85% under both UV and visible light irradiation. In contrast, NOR showed efficiency of 70% and 90% under visible light and UV light irradiation, respectively. The results indicated the role of both light and photocatalyst for removal of these organic pollutants. It is well known that the commercial TiO2 (P25) photocatalyst has been used as a main standard photocatalyst for water treatment with an excellent photocatalytic performance. In the present work, the photocatalytic efficiency of the prepared Bi2MoO6 catalyst was evaluated and compared with that of commercially available TiO2 (P25) photocatalyst as shown in Fig. 2a and c. After 240 min of visible light irradiation, TiO2 (P25) displayed efficiency of 46%, 55% 65% and 60% toward degradation of RR141, CR, OFL and NOR, respectively. The efficiency of the prepared Bi2MoO6 catalyst is about 1.65 and 1.3 times greater than that of TiO2 toward degradation of CR and OFL, respectively. This supports better visiblelight-responsive photocatalytic activity of the prepared Bi2MoO6 catalyst in comparison to that of commercial TiO2 (P25). The performance of Bi2MoO6 catalyst can also be investigated by determining the degradation rate of the process. Under visible light, a graph of ln (C0/C) versus time (Fig. 2e) showed a straight line indicating the pseudo first-order kinetics of photodegradation reaction. The corresponding rate constant (k1) can be calculated from the slope of each plot. OFL gave the highest rate constant of 0.0084 min 1 (R2 = 0.9747). The rate constants under UV light were also presented in Fig. 2f. The first order kinetics was still observed. OFL showed the highest rate constant of 0.0143 min 1. This correlated well with the results obtained from visible light irradiation. All in all, the enhanced photocatalytic performance of the plate-like Bi2MoO6 catalyst can be described to the presence of numerous nanoplates with high specific surface area. The individual component of nanoplates can generate large interspaces. This leads to the improvement of organic pollutant adsorption, transportation and light harvesting. Additionally, a very high photocatalytic efficiency found in the catalyst is also attributed to enhancement of charge separation at the interface resulted from the layer structure of [Bi2O2]2+ slabs in the prepared Bi2MoO6 semiconductor [3]. This resulted in not only lowering electronhole recombination efficiency but also increasing lifetime of the photogenerated charge carriers. To understand the photocatalytic degradation mechanism and active species involved in the degradation of CR dye and OFL antibiotic, trapping experiments using various scavengers have been investigated [17]. The prepared Bi2MoO6 catalyst was used to degrade CR dye and OFL antibiotic after incorporation of different scavengers. Fig. 3a and b showed that after addition of EDTA-2Na and K2Cr2O7, a dramatic decrease of photodegradation efficiency was observed indicating that both hole and electron play a crucial role in degradation of CR dye and OFL antibiotic. In
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addition, a remarkable reduction of efficiency was also noticed with addition of KI, verifying a huge contribution from surface hydroxyl radical and photogenerated hole toward degradation of these organic pollutants. The terephthalic acid (TA) trapping method was also applied to study the hydroxyl radicals (∙OH) produced during photocatalytic process, in which TA can react with ∙OH to generate 2hydroxyterephthalic acid (TA-OH) with PL emission peak at 425 nm using a spectrofluorometer [18,19]. It can be seen from Fig. 3c that increasing photo irradiation time resulted in an increase in PL peak intensity suggesting the continuous production of hydroxyl radicals. This result strongly supports the trapping experiment regarding the role of hydroxyl radicals as one of the reactive species involved in photodegradation of the pollutants. The possible photodegradation mechanism was also proposed as shown in Fig. 3d. The recycling ability of the prepared Bi2MoO6 photocatalyst was also examined by measuring the catalytic efficiency for five cycles [20]. The efficiency remained the same after five times of use, as shown in Fig. 3e and 3f for degradation of CR dye and OFL antibiotic, respectively. CR dye degradation showed a slight decrease of efficiency from 91% (the first cycle) to 74% (the fifth cycle). In the case of OFL antibiotic degradation, the change of efficiency from 85% (the first run) to 70% (the fifth run) did support the recycling ability of the catalyst. Structural stability of the Bi2MoO6 catalyst after photodegradation of the pollutants was also confirmed. The same XRD patterns of both fresh and used Bi2MoO6 catalysts were observed as shown in Fig. 3g. In addition, The SEM micrographs of the virgin catalyst (Fig. 1e) and the catalyst after photodegradation (Fig. 3h) were similar. These results indicated the structural stability and a very high degradation efficiency of the Bi2MoO6 photocatalyst after five times of use. This is crucial for practical application. 4. Conclusions Preparation of a plate-like Bi2MoO6 catalyst at low temperature by a solvothermal method has been reported. Neither surfactant nor capping agent was used. Bi2MoO6 belongs to orthorhombic structure with a very high efficiency of 90% after photo irradiation. The degradation reaction correlates well with the first-order kinetics model. Bi2MoO6 catalyst still shows a very high efficiency even after the fifth cycle of use. This indicates the advantages of structural stability and reusability of the catalyst. This work demonstrates the promising potential of the Bi2MoO6 catalyst for removal of dyes and antibiotics. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Financial support from Materials Chemistry Research Center, Department of Chemistry and Center of Excellence for Innovation in Chemistry (PERCH-CIC), Faculty of Science, Khon Kaen is acknowledged. T. Chankhanittha wishes to thank scholarship from SAST. J. Watcharakitti and V. Somaudon would like to thank DPST scholarship from the Royal Thai Government. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.126764.
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Performance of solvothermally grown Bi2MoO6 photocatalyst toward degradation of organic azo dyes and fluoroquinolone antibiotics Tammanoon Chankhanittha, Varanya Somaudon, Jidapa Watcharakitti, Voranan Piyavarakorn, Suwat Nanan ⇑ Materials Chemistry Research Center, Department of Chemistry and Center of Excellence for Innovation in Chemistry (PERCH-CIC), Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand
a r t i c l e
i n f o
Article history: Received 13 September 2019 Received in revised form 30 September 2019 Accepted 30 September 2019 Available online 11 October 2019 Keywords: Bi2MoO6 Photocatalytic degradation Organic azo dyes Antibiotics
a b s t r a c t Preparation of Bi2MoO6 catalyst at low temperature via a solvothermal method was reported. Neither surfactant nor capping agent was used. The orthorhombic Bi2MoO6 catalyst showed a very high efficiency of 90% toward photodegradation of two azo dyes (reactive red dye and Congo red dye) and two antibiotics (ofloxacin and norfloxacin) under both ultraviolet (UV) and visible light irradiation. The photodegradation reaction of these organic pollutants correlates well with the pseudo first-order kinetics model. Both electron and hole play a major role in degradation of the pollutants. The Bi2MoO6 photocatalyst still shows a very high photocatalytic performance even after five cycles of use. This indicates the advantages of structural stability and reusability of the catalyst. The prepared Bi2MoO6 could be a promising photocatalyst for degradation of organic dyes and antibiotics present in water. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor photocatalysis has attracted a great interest due to its promising potential for environmental remediation [1]. Traditional photocatalyst such as TiO2, with a large band gap of 3.2 eV, only shows ultraviolet (UV) responsive photodegradation of the organic pollutants. Interestingly, the semiconductor bismuth molybdate (Bi2MoO6), an important Aurivillius oxide with a direct band gap of 2.6–2.8 eV, has been received considerable attention due to its application in visible-light-driven photocatalysis [2,3]. Recently, the synthesis of bismuth molybdate with different morphology including spherical structure, York-shell structure, and rod-shaped structure via physical and chemical methods has been reported [3–5]. In some cases, heterojunction photocatalysts based on Bi2MoO6 were also fabricated [6,7]. Among various preparation methods, a hydrothermal/solvothermal method is the most promising synthetic route for fabrication of Bi2MoO6 due to its simplicity and low cost [8–10]. The present work reports the solvothermal synthesis of Bi2MoO6 catalyst. Neither surfactant nor capping agent is needed. The prepared catalyst showed a uniform plate-like morphology of 390 nm 940 nm. The blue emission over 460–500 nm due to ⇑ Corresponding author. E-mail address: [email protected] (S. Nanan). https://doi.org/10.1016/j.matlet.2019.126764 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
surface defect states was observed. The photodegradation efficiency of the catalyst was determined by using azo dyes and antibiotics as model pollutants. A very high efficiency of 90% was obtained. To the best of our knowledge, this is the first work demonstrating the visible-light-responsive photodegradation of both anionic azo dyes and fluoroquinolone antibiotics by Bi2MoO6 photocatalyst. 2. Experimental sections All chemicals including Bi(NO3)35H2O (Fluka), ethylene glycol (Sigma Aldrich), (NH4)6Mo7O24 (Fluka), and absolute ethanol were used without further purification. The de-ionized water was used. In a typical synthesis, 3.9855 g of Bi(NO3)35H2O was dissolved in 30 mL of ethylene glycol (solution A). Meanwhile, 0.7138 g of (NH4)6Mo7O24 was dissolved in 30 mL of distilled water and stirred under heating at 100 °C for 15 min (solution B). Solution A was added dropwise into solution B and then 10 M NaOH solution was added to adjust the pH value to about 9. Afterward, the mixture was transferred to a 100 mL Teflon-lined stainless-steel autoclave and maintained at 160 °C for 24 h. The product was naturally cooled down to room temperature and washed with ethanol. The sample was then dried in an oven at 60 °C for 24 h. A paleyellow powder of Bi2MoO6 was obtained. Characterization and photocatalytic study can be found in supporting information.
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3. Results and discussion The XRD pattern (Fig. 1a) of Bi2MoO6 belongs to orthorhombic phase (JCPDS No. 72–1524). The XRD peaks found at 2h = 23.50°, 28.24°, 32.59°, 33.13°, 36.00°, 39.65°, 46.74°, 47.11°, 55.33°, 56.23°, and 58.41° corresponded to the reflection from (1 1 1),
(1 3 1), (0 0 2), (0 6 0), (1 5 1), (0 4 2), (2 0 2), (0 6 2), (3 3 1), (1 9 1), and (2 6 2) planes, respectively [11]. No peak due to impurity was detected. The FT-IR spectrum of Bi2MoO6 (Fig. 1b) showed a broad band at about 3389 cm 1 corresponding to the OAH stretching vibration of water. The peak at 1384 cm 1 corresponded to the C@O stretching
Fig. 1. XRD pattern (a), FT-IR spectrum (b), UV–vis spectrum (c), PL spectrum (d), SEM micrograph (e), EDX spectrum (f), SEM micrograph of the mapping area and EDX elementary mapping of Bi, Mo, and O of the as-synthesized Bi2MoO6 (g).
T. Chankhanittha et al. / Materials Letters 258 (2020) 126764
modes due to the absorption of CO2 on the surface of the Bi2MoO6 catalyst. The peak at 846 cm 1 indicated Mo-O-Mo bridging bond. The peak found at 730 cm 1 and 578 cm 1 were attributed to the presence of Mo-O bond and Bi-O bond, respectively [12]. This confirms the purity of the sample. The UV–vis spectrum of Bi2MoO6 was shown in Fig.1c. The energy band gap (Eg) of 2.62 eV was estimated by Tauc plot [13] as shown in the inset of Fig.1c. Photoluminescence (PL) spectrum of Bi2MoO6 (using excitation wavelength of 300 nm) was shown in Fig. 1d. In principle, both near band edge (NBE) emission and trapped emission are expected in PL spectrum of semiconductor
3
photocatalyst. The former is sharp and found near the absorption edge. The latter is broad and found at longer wavelength [14,15]. In the present work, the blue emission over 460–500 nm comprising a strong peak at 470 nm and three small peaks at 473, 483 and 493 nm was observed. This is due to surface defect states found in the Bi2MoO6 catalyst [16]. The prepared Bi2MoO6 catalyst showed a uniform morphology composed of plate-like structure with the grain size of about 390 nm 940 nm (Fig. 1e). The EDX spectrum (Fig. 1f) indicated the existence of bismuth (Bi), molybdenum (Mo) and oxygen (O). The mapping study was also included. The SEM micrograph of
Fig. 2. Lowering of concentration (C/C0) vs time due to photodegradation of dyes under visible light irradiation (a), plot of C/C0 due to photodegradation of dyes under UV light (b), plot of C/C0 due to photodegradation of antibiotics under visible light (c), plot of C/C0 due to photodegradation of antibiotics under UV light (d), determination of rate constant under visible light (e), and determination of rate constant under UV light (f).
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Fig. 3. Effect of some scavengers on photodegradation of Congo red (CR) dye (a) and ofloxacin (OFL) antibiotic (b), hydroxyl radical trapping PL spectra of Bi2MoO6 under visible light irradiation (c), possible degradation mechanism of organic pollutant with Bi2MoO6 catalyst (d), reusability of Bi2MoO6 catalyst for degradation of CR dye (e) and OFL antibiotic (f), XRD patterns of the Bi2MoO6 catalyst before and after photodegradation of the pollutants (g), and SEM micrographs of the catalyst after photodegradation of CR dye and OFL antibiotic (h). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the mapping area was shown in Fig. 1g. In addition, elemental colour mapping of the Bi2MoO6 catalyst showed the homogeneity of the particles supporting the uniform distribution of Bi, Mo and O (Fig. 1g).
Photodegradation efficiency of Bi2MoO6 was investigated by using azo dyes and fluoroquinolone antibiotics as model pollutants. The chemical structures of these organic pollutants namely reactive red (RR141) dye, Congo red (CR) dye, ofloxacin (OFL),
T. Chankhanittha et al. / Materials Letters 258 (2020) 126764
and norfloxacin (NOR) antibiotics were shown in Table S1. On examining the dye degradation, lowering of concentration (C/C0) with increasing time (t) under both visible and UV light were observed as shown in Fig. 2a and b, respectively. According to the blank test (no catalyst), the RR141 dye concentration remained constant over 240 min indicating hardly self-photodegradation of this dye. RR141 dye solution with Bi2MoO6 in the dark provided C/C0 of 0.77 after 240 min indicating low contribution of dye removal by adsorption process. In the presence of Bi2MoO6, however, lowering of C/C0 with time was observed. The efficiency reached 45% and 37% under visible light and UV light irradiation, respectively. In the case of CR dye, a huge contribution (77%) of dye removal by adsorption process was observed after 240 min. Additionally, the efficiency reached 91% after photo irradiation. On examining removal of antibiotics (Fig. 2c and d), both OFL and NOR showed low contribution of adsorption process (25% and 35% for removal of OFL and NOR, respectively). OFL showed photodegradation efficiency of 85% under both UV and visible light irradiation. In contrast, NOR showed efficiency of 70% and 90% under visible light and UV light irradiation, respectively. The results indicated the role of both light and photocatalyst for removal of these organic pollutants. It is well known that the commercial TiO2 (P25) photocatalyst has been used as a main standard photocatalyst for water treatment with an excellent photocatalytic performance. In the present work, the photocatalytic efficiency of the prepared Bi2MoO6 catalyst was evaluated and compared with that of commercially available TiO2 (P25) photocatalyst as shown in Fig. 2a and c. After 240 min of visible light irradiation, TiO2 (P25) displayed efficiency of 46%, 55% 65% and 60% toward degradation of RR141, CR, OFL and NOR, respectively. The efficiency of the prepared Bi2MoO6 catalyst is about 1.65 and 1.3 times greater than that of TiO2 toward degradation of CR and OFL, respectively. This supports better visiblelight-responsive photocatalytic activity of the prepared Bi2MoO6 catalyst in comparison to that of commercial TiO2 (P25). The performance of Bi2MoO6 catalyst can also be investigated by determining the degradation rate of the process. Under visible light, a graph of ln (C0/C) versus time (Fig. 2e) showed a straight line indicating the pseudo first-order kinetics of photodegradation reaction. The corresponding rate constant (k1) can be calculated from the slope of each plot. OFL gave the highest rate constant of 0.0084 min 1 (R2 = 0.9747). The rate constants under UV light were also presented in Fig. 2f. The first order kinetics was still observed. OFL showed the highest rate constant of 0.0143 min 1. This correlated well with the results obtained from visible light irradiation. All in all, the enhanced photocatalytic performance of the plate-like Bi2MoO6 catalyst can be described to the presence of numerous nanoplates with high specific surface area. The individual component of nanoplates can generate large interspaces. This leads to the improvement of organic pollutant adsorption, transportation and light harvesting. Additionally, a very high photocatalytic efficiency found in the catalyst is also attributed to enhancement of charge separation at the interface resulted from the layer structure of [Bi2O2]2+ slabs in the prepared Bi2MoO6 semiconductor [3]. This resulted in not only lowering electronhole recombination efficiency but also increasing lifetime of the photogenerated charge carriers. To understand the photocatalytic degradation mechanism and active species involved in the degradation of CR dye and OFL antibiotic, trapping experiments using various scavengers have been investigated [17]. The prepared Bi2MoO6 catalyst was used to degrade CR dye and OFL antibiotic after incorporation of different scavengers. Fig. 3a and b showed that after addition of EDTA-2Na and K2Cr2O7, a dramatic decrease of photodegradation efficiency was observed indicating that both hole and electron play a crucial role in degradation of CR dye and OFL antibiotic. In
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addition, a remarkable reduction of efficiency was also noticed with addition of KI, verifying a huge contribution from surface hydroxyl radical and photogenerated hole toward degradation of these organic pollutants. The terephthalic acid (TA) trapping method was also applied to study the hydroxyl radicals (∙OH) produced during photocatalytic process, in which TA can react with ∙OH to generate 2hydroxyterephthalic acid (TA-OH) with PL emission peak at 425 nm using a spectrofluorometer [18,19]. It can be seen from Fig. 3c that increasing photo irradiation time resulted in an increase in PL peak intensity suggesting the continuous production of hydroxyl radicals. This result strongly supports the trapping experiment regarding the role of hydroxyl radicals as one of the reactive species involved in photodegradation of the pollutants. The possible photodegradation mechanism was also proposed as shown in Fig. 3d. The recycling ability of the prepared Bi2MoO6 photocatalyst was also examined by measuring the catalytic efficiency for five cycles [20]. The efficiency remained the same after five times of use, as shown in Fig. 3e and 3f for degradation of CR dye and OFL antibiotic, respectively. CR dye degradation showed a slight decrease of efficiency from 91% (the first cycle) to 74% (the fifth cycle). In the case of OFL antibiotic degradation, the change of efficiency from 85% (the first run) to 70% (the fifth run) did support the recycling ability of the catalyst. Structural stability of the Bi2MoO6 catalyst after photodegradation of the pollutants was also confirmed. The same XRD patterns of both fresh and used Bi2MoO6 catalysts were observed as shown in Fig. 3g. In addition, The SEM micrographs of the virgin catalyst (Fig. 1e) and the catalyst after photodegradation (Fig. 3h) were similar. These results indicated the structural stability and a very high degradation efficiency of the Bi2MoO6 photocatalyst after five times of use. This is crucial for practical application. 4. Conclusions Preparation of a plate-like Bi2MoO6 catalyst at low temperature by a solvothermal method has been reported. Neither surfactant nor capping agent was used. Bi2MoO6 belongs to orthorhombic structure with a very high efficiency of 90% after photo irradiation. The degradation reaction correlates well with the first-order kinetics model. Bi2MoO6 catalyst still shows a very high efficiency even after the fifth cycle of use. This indicates the advantages of structural stability and reusability of the catalyst. This work demonstrates the promising potential of the Bi2MoO6 catalyst for removal of dyes and antibiotics. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Financial support from Materials Chemistry Research Center, Department of Chemistry and Center of Excellence for Innovation in Chemistry (PERCH-CIC), Faculty of Science, Khon Kaen is acknowledged. T. Chankhanittha wishes to thank scholarship from SAST. J. Watcharakitti and V. Somaudon would like to thank DPST scholarship from the Royal Thai Government. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.126764.
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T. Chankhanittha et al. / Materials Letters 258 (2020) 126764
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