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Ti6O13) synthesized by ultrasound: Photocatalytic activity, photocorrosion resistance and degradation mechanism
Separation and Purification Technology xxx (2016) xxx–xxx
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
High visible light intercalated nanophotocatalyst (PbS-CdS/Ti6O13) synthesized by ultrasound: Photocatalytic activity, photocorrosion resistance and degradation mechanism S. Sehati a, M.H. Entezari a,b,⇑ a b
Sonochemical Research Center, Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, 91779 Mashhad, Iran Environmental Chemistry Research Center, Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, 91779 Mashhad, Iran
a r t i c l e
i n f o
Article history: Received 3 July 2016 Received in revised form 19 October 2016 Accepted 24 October 2016 Available online xxxx Keywords: Ultrasound Degradation RB5 Photocatalyst Intercalation Nanocomposite
a b s t r a c t In this study, the photocatalytic activity of the PbS-CdS/Ti6O13 nanocomposite prepared by sonointercalation method was evaluated by the degradation of Reactive Black 5 (RB5) as a water pollutant under sunlight irradiation. Strong photocatalytic activity was observed due to the improved light harvesting property of photocatalyst, significant decrease in bandgap energies and heterogeneous electron transfer from CdS and PbS as a guest to K2Ti6O13 as a host. The RB5 concentrations in the solution and on the surface of PbS-CdS/Ti6O13 nanocomposite were measured during the experiment. The effects of some important parameters such as ultrasonic amplitude and initial composite ratio were studied in the synthesis of nanocomposite. The activity of synthesized nanocomposites on the dye degradation was evaluated and the stability and photocorrosion resistance of the catalyst were studied in this work too. In addition, the mechanism of separation of the photogenerated electrons and holes in the PbS-CdS/ Ti6O13 nanocomposite was investigated. The responsible superior active species for the photodegradation activity were explored and a possible mechanism was suggested. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Water pollutant treatment and the effect of water pollution on the environment are of special concerns to scientists working in remediation of wastewaters [1,2]. Dyes are considered as one of the most important classes of contaminants in industrial waste waters due to their toxic nature, large production volume and low rate of treatment by conventional processes involving biological and chemical methods [3]. Therefore, it is essential to remove these organic pollutants from wastewaters before discharge to major water bodies [4]. Degradation of dyes, regardless of the used technique(s) leads to formation of biodegradable intermediates and smaller compounds, which can be mineralized under ideal conditions [5,6]. Photocatalytic degradation of organic pollutant in water using solar energy and semiconductor catalysts has been widely researched [7,8]. Oxides with regularities such as layered and tunneling structures are considered as favorable materials for effective photo degradation of water pollutants. The high activ⇑ Corresponding author at: Sonochemical Research Center, Environmental Chemistry Research Center, Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, 91779 Mashhad, Iran. E-mail addresses: [email protected], [email protected] (M.H. Entezari).
ity of these layered compounds is related to easy migration and separation of the photo generated electron-holes through the layers [9–11]. K2Ti6O13 can be considered as one of the most important alkali-metal titanate which is a significant candidate for application in photocatalysis, fuel cells and plastics [12]. These kinds of titanate compounds do not show efficient photocatalytic activity under visible light owing to their wide bandgap (3.2– 3.5 eV) [13]. Fabricating layered composites, consisting of titanate and narrow bandgap semiconductors can promote the photocatalytic activities of these compounds in the presence of visible light [14,15]. Metal sulfides are well-known semiconductor photocatalysts because of their suitable band structures and good photoresponses under visible light irradiation for photodegradation of chemical contaminants in water and air [16,17]. PbS with a bulk bandgap of about 0.41 eV, considering as an indirect semiconductor, can absorb the light in the whole visible region [18,19]. The most drawback for using the PbS is fast recombination of photoinduced hole-electron (of the order microseconds and less) due to the very low distance between the valence band and conduction band. Catalytic properties of PbS can be modified by mixing by other semiconductors [20,21]. CdS is another semiconductor material investigated for photocatalytic reactions [22]. The ternary/coupled semiconductor systems such as Pt/PdS/CdS [23], ZnS/CdS/ZTP
http://dx.doi.org/10.1016/j.seppur.2016.10.045 1383-5866/Ó 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Sehati, M.H. Entezari, High visible light intercalated nanophotocatalyst (PbS-CdS/Ti6O13) synthesized by ultrasound: Photocatalytic activity, photocorrosion resistance and degradation mechanism, Separ. Purif. Technol. (2016), http://dx.doi.org/10.1016/j. seppur.2016.10.045
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2.4. Photocatalytic testing
[16], CdS/ZnS/TiO2 [24], exhibit higher photocatalytic activity in comparison with their sulfides or oxide components. Preparing and understanding such nanocomposites from the interaction between different components and examination of charge carrier under irradiation aspects, are still challenging task for scientists. In the present work, we have attempted to improve stability and light-harvesting properties of co-incorporated the PbS and CdS nanoparticles into the interlayer of K2Ti6O13. The composite synthesized by sonochemical approach and its catalytic activity was checked through investigation on the photocatalytic degradation of RB5 under sunlight irradiation. The effects of important factors, affecting the photocatalytic process were studied. Finally, a reaction mechanism was proposed and discussed.
The photocatalytic activity of the prepared PbS-CdS/Ti6O13 photocatalyst was tested for the decolorization of RB5 using a Pyrex glass vessel containing 30 mL RB5 (40 mg L1). An appropriate amount of the photocatalyst in powder form was added at natural pH of RB5 (pH = 6.5) and magnetically stirred under direct sunlight radiation in sunny days in July 2015 (without staying in dark condition). The temperature of the dye solution was kept in the range of 25–30 °C. At specific time intervals, 5 mL of the sample solution was withdrawn and centrifuged prior measurements for the remaining RB5 concentration by a UV–vis spectrophotometer using the characteristic absorption band at 600 nm.
2. Experimental
3. Result and discussion
2.1. Materials
3.1. Characterization of catalyst
Tetrabutyltitanate (Ti(O-Bu)4), potassium hydroxide (KOH) and stearic acid (CH3(CH2)16CO2H) from Merck, have been used without further purification for the synthesis of potassium hexatitanate. Cadmium nitrate (Cd(NO3)2), Lead nitrate (Pb(NO3)2), thioacetamide (SC(CH3)(NH2)), and RB5 were supplied from Sigma-Aldrich, Germany. Dye solutions were prepared by dissolving the dye in de-ionized water. The pH was adjusted with sodium hydroxide (NaOH) and hydrochloric acid (HCl).
Fig. 1 shows the XRD patterns of K2Ti6O13and PbS-CdS/Ti6O13 synthesized via ultrasound. The synthesis of K2Ti6O13with high purity was confirmed by the X-ray diffractions and shown in Fig. 1a. It could be clearly observed that the synthesized layered K2Ti6O13was highly crystalline. Fig. 1b indicates the XRD pattern of intercalated nanocomposite (PbS-CdS/Ti6O13) in the presence of ultrasound (amplitude 60%). Interlayer spacing of titanate sheets (2 0 0) was expanded from 0.75 nm to 0.81 nm due to the incorporation of cubic PbS and CdS nanoparticles between layers and slightly distortion of the crystal lattice. Besides that, the intensity of 2 0 0 peak was decreased because of partially covering the host surface by guest nanoparticles. The morphologies and sizes of the as-synthesized K2Ti6O13 and PbS-CdS/Ti6O13 samples were examined using the field-emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). Fig. 2(a) and (c) illustrates the nanobelt structure of K2Ti6O13 preparing via ultrasound with a width of 40– 80 nm, and length of 600–800 nm. Fig. 2(b) and (d) indicate the FESEM and TEM images of PbS-CdS/Ti6O13 nanocomposite, respectively [25]. Obviously, the shape of host molecule (K2Ti6O13) did not change during the sono-intercalation process and PbS-CdS nanoparticles uniformly covered the host surface. Besides that, guest molecules incorporated into the titanate layers. Fig. 3 shows the UV–vis spectra of as-prepared samples. The sample K2Ti6O13 possessed a strong absorption in the UV region and a weakly absorption in the visible region with estimated band
2.2. Preparation methods The K2Ti6O13 and intercalated PbS-CdS/Ti6O13 nanocomposite were prepared according to the method presented in our recent publication [25]. A brief description of procedure is as follows: firstly the appropriate amount of stearic acid was melted at 90–120 °C. Then, saturated aqueous solution of KOH was suddenly added. The mixture vigorously stirred while was kept at 90–120 °C. After addition of Ti(O–Bu)4, the reaction mixture was sonicated for 30 min until a homogenous transparent solution was formed. Ignition of solution was done at 300 °C in furnace and then calcined at 750 °C for 2 h. In order to incorporate PbS and CdS nanoparticles into the interlayer of K2Ti6O13, 0.1 g of as-prepared titanate nanoparticles was added to the mixture containing the NaOH and different amount of Pb(NO3)2 and Cd(NO3)2. Then, the mixture was exposed under ultrasonic irradiation at various amplitudes (20–80%) for 1.5 h at 25 °C. The sample was separated and washed several times with de-ionized water. After that, SC(CH3)(NH2) solution was added to sample and sonicated for 30 min. Finally, the precipitates were centrifuged and washed with ethanol and distilled water. The particles were dried in an oven at 110 °C for 4 h. 2.3. Characterization and equipment The crystalline structures of the catalysts at different ultrasound intensities were studied by XRD recorded on a X-ray analyzer (PHILIPS PW1800) at a 2h angle ranging from 5° to 60°. The morphological properties of the PbS-CdS/Ti6O13 photocatalyst was examined by TEM (CM120-Philips). The topological properties of the catalyst was observed by FESEM(Mira 3–XMU). The optical absorption properties of the photocatalyst and absorption spectrum of RB5 were obtained using a UV–vis spectrophotometer (Unico 2800) in the range of 200–800 nm at room temperature. The atomic absorption spectrophotometer (AAS, Varian, spectra-110 880/220-Australia Pty Ltd.) was used to determine the concentration of Pb2+ and Cd2+ ions in the solution during the photocatalytic process.
Fig. 1. XRD patterns of (a) K2Ti6O13, (b) PbS-CdS/Ti6O13.
Please cite this article in press as: S. Sehati, M.H. Entezari, High visible light intercalated nanophotocatalyst (PbS-CdS/Ti6O13) synthesized by ultrasound: Photocatalytic activity, photocorrosion resistance and degradation mechanism, Separ. Purif. Technol. (2016), http://dx.doi.org/10.1016/j. seppur.2016.10.045
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Fig. 2. (a) FE-SEM image of K2Ti6O13 nanobelt, (b) FE-SEM image of PbS-CdS/Ti6O13, (c) TEM image of K2Ti6O13, (d) TEM image of PbS-CdS/Ti6O13.
showing that the guest molecules have a strong impact on the light absorption performance layered compound.
3.2. Photocatalytic degradation of RB5 under sunlight irradiation 3.2.1. Effect of ultrasonic amplitude Fig. 4 shows the degradation of RB5 with PbS-CdS/Ti6O13 photocatalyst, preparing at various ultrasonic amplitudes, under sunlight
Fig. 3. UV–vis absorption spectra of K2Ti6O13 and PbS-CdS/Ti6O13 samples.
gap about 3.42 eV. Comparing with the potassium hexatitanate, the UV–vis absorption edges of the PbS-CdS/Ti6O13 nanocomposite demonstrated obviously red shift corresponding to a band gaps of about 2.93, 2.58 and 1.98 eV. The incline in host band gap from 3.42 to 2.98 presumably attributed to slightly distortion of the crystalline lattice of titanate layers duo to the intercalation of guest molecules. The light absorbance intensity of the PbS-CdS/Ti6O13 in the visible region was greater than that of the precursor K2Ti6O13,
Fig. 4. Degradation of RB5 by photocatalysts prepared in different sonication amplitudes. (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
Please cite this article in press as: S. Sehati, M.H. Entezari, High visible light intercalated nanophotocatalyst (PbS-CdS/Ti6O13) synthesized by ultrasound: Photocatalytic activity, photocorrosion resistance and degradation mechanism, Separ. Purif. Technol. (2016), http://dx.doi.org/10.1016/j. seppur.2016.10.045
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irradiation. Based on the results of sono-synthesized nanocomposites at different amplitudes, reporting at our recent published work [25], the amount of guest molecules incorporated between titanate layers was increased from 20% to 60% amplitudes and this quantity was dropped at 80% amplitude. The photocatalytic activity of intercalated nanocomposites is directly related to the amount of incorporated PbS and CdS nanoparticles. This is due to reducing the hole-electron pair recombination through the interfacial charge transfer [26,27]. Therefore, the recombination of photo induced h+ and ein photocatalyst synthesized at 20% and 80% amplitudes were greater than that of prepared at 60% amplitude. Accordingly, the degradation efficiency of RB5 was maximized using photocatalyst synthesized at 60% amplitude. Thus, all the next experiments were done with the sample, preparing at 60% amplitude.
3.2.2. Effect of initial composite ratio The intercalated nanocomposites of PbS-CdS/Ti6O13 (60% amplitude) were prepared with different initial concentration ratios of guest molecule’s precursors (Pb(NO3)2 and Cd(NO3)2) (1:3, 1:2, 1:1, 2:1) and constant amount of host nanoparticles. Then, the photocatalytic activities was studied for different samples in RB5 removal. As shown in Fig. 5, the photocatalytic activities of the nanocomposite, co-intercalating by PbS and CdS, were higher than that of PbS/Ti6O13 or CdS/Ti6O13. It’s owing to the high charge separation efficiency of PbS-CdS/Ti6O13 (with various ratios) due to distortion of crystal structure [28]. In fact, the co-incorporating of PbS and CdS into the interlayer of titanate played an important role in photocatalytic degradation of RB5. The intercalating of the PbS and CdS guest nanoparticles into the layers of potassium hexa-titanate could suppress the particle growth and induced a quantum size effect. Besides that, the photo-generated electrons could quickly be transferred through the guest particles, and the recombination of the photoinduced charge carriers was effectively decreased in comparison with single guest intercalated nanocomposites. These results also confirm that an initial precursor’s concentrations with the (2:1) proportion of PbS-CdS in the composite has the highest removal efficiency. More increase in the ratio CdS/PbS in the composite suppressed the photocatalytic activities. Generally, further presence of higher bandgap semiconductor (CdS) in the ratio decreases the extent of light absorption in comparison with other samples with more content of PbS. Therefore, all experiments were continued with optimized initial composite ratio (PbS-CdS (2:1)/Ti6O13).
Fig. 5. Effect of initial composite ratio. (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
3.2.3. Catalyst loading In order to investigate the effect of catalyst loading on the efficiency of photocatalytic degradation, various amounts of catalyst (0.01–0.07 g) were used while keeping all other parameters constant and the results are shown in Fig. 6. The results demonstrate that with enlarging the amounts of catalyst, the rate of pollutant’s degradation was continuously increased duo to availability of more active sites in the catalyst for the absorption of photon and more interaction with RB5 molecules [26]. Moreover, increase in photo-degradation efficiency was also related to an enhanced generation of the oxidizing radicals, which consequently increased the rate of photocatalytic degradation of dye [29]. The effects of catalyst loading on rate of degradation was insignificant at higher dosage (>0.05 g) and there was no remarkably change in removing of the dye. Hence, the whole experiments were continued with 0.05 g of the PbS-CdS/Ti6O13 photocatalyst. 3.2.4. Effect of initial pH The influence of initial pH value on the RB5 degradation efficiency was investigated at different pH values in the range of 2.5–11 and the results are shown in Fig. 7. The effect of pH on the degradation of RB5 by optimized PbS-CdS/Ti6O13 photocatalyst can be mainly explained by the surface charge of catalyst (pHpzc = 8.76) and its relation to the constants of acid dissociation of sulfonic and ethyl sulfonic groups in RB5 molecule [30]. At low pH (2.5) a strong adsorption of dye on the catalyst surface was observed due to the electrostatic attraction of the high positively charged PbS-CdS/Ti6O13 (pH < pHpzc) with the ionized sulfonic and ethyl sulfonic groups in RB5 molecules (pka value 1) which act as strong acid. In fact more adsorption of dye caused the higher degradation efficiency (100%). With increasing the pH from acidic to neutral pH of dye (6.5), the amount of positive charge on the surface of catalyst slightly declined so the adsorption of negatively charged RB5 on the catalyst surface and removal efficiency was decreased (94%). As can be seen, at high pH (above pHpzc = 8.76) the surface of catalyst tended to be negatively charged and a decrease in the degradation rate was observed. A minimum at pH = 11, reflecting the complexity of the sulfonic and ethyl sulfonic anions in approaching the negatively charged PbS-CdS/Ti6O13 surface (pH > pHpzc) when increasing the solution pH. Although the best results in RB5 degradation was observed at pH = 2.5, but the whole experiments were continued at neutral pH of dye because of the insignificant differences between their results. 3.2.5. RB5 removal in dark and light In order to compare the catalytic activity of PbS-CdS/Ti6O13 in dark and light, one sample contains of catalyst and RB5 was magnetically stirred in dark for 45 min to attain adsorption–desorption equilibrium. As shown in Fig. 8, the color of catalyst changed from
Fig. 6. Effect of different catalyst loading on degradation time. (Initial pH = 6.5, temp = 25 °C).
Please cite this article in press as: S. Sehati, M.H. Entezari, High visible light intercalated nanophotocatalyst (PbS-CdS/Ti6O13) synthesized by ultrasound: Photocatalytic activity, photocorrosion resistance and degradation mechanism, Separ. Purif. Technol. (2016), http://dx.doi.org/10.1016/j. seppur.2016.10.045
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Fig. 7. Effect of different pH on RB5 degradation. Outset shows the estimated pHpzc of PbS-CdS/Ti6O13. (Dose of catalyst = 0.05 g, temp = 25 °C, initial dye concentration = 40 ppm).
Fig. 8. UV–vis absorption spectra of RB5 solution in blank, in the presence of catalyst at dark, and color change of the catalyst during the adsorption process (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
Please cite this article in press as: S. Sehati, M.H. Entezari, High visible light intercalated nanophotocatalyst (PbS-CdS/Ti6O13) synthesized by ultrasound: Photocatalytic activity, photocorrosion resistance and degradation mechanism, Separ. Purif. Technol. (2016), http://dx.doi.org/10.1016/j. seppur.2016.10.045
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Fig. 9. UV–vis absorption spectra of (a) RB5 solution at different contact times under sunlight, (b) color change of the catalyst during the photocatalytic process (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
greenish yellow to blue due to the adsorption of RB5 on the catalyst surface and decolorization was not completely done even after 45 min. It was confirmed that the removal of RB5 was done by the adsorption process. On the other hand, another sample containing catalyst and RB5 solution directly was taken place under sunlight without keeping in the dark. To explain the changes of RB5 solution resulting from photocatalytic degradation, representative UV–vis spectra changes of the dye solution is presented in Fig. 9 as a function of reaction time. According to Fig. 9, absorption spectrum of the original RB5 solution (blank) can be described by three main peaks at wavelengths of 600, 312, and 254 nm. During photocatalytic degradation under sunlight, the absorbance values decreased continuously with time due to the fragmentation of bands and finally completely eliminated in 20 min. This indicates that the decolorization rate was higher under sunlight irradiation than that at dark. Besides that, when the catalyst was exposed to the solution, the color of catalyst changed to blue-green, demonstrating the presence of dye contaminants on the surface of catalyst. Although, complete decolorization was achieved in the solution after 20 min, but the catalyst surface color was still undertone blue-green (Fig. 9b). Over the time and in complete photocatalytic process, the appearance color of catalyst changed to greenish yellow because of decomposing of adsorbed RB5 on the surface of the catalyst. 3.2.6. Adsorption, desorption, and degradation on the catalyst surface In order to investigate the photocatalytic decomposition and adsorption of RB5 under solar light, the concentration of RB5
remained in the solution and the concentration of RB5 on the surface (through desorption) were measured at different interval times. Desorption was done by dispersing the catalyst in an aqueous solution at acidic medium (pH = 2.5, the pH with maximum desorption). Based on Fig. 10, the results of desorption show that the normalized concentration changes of adsorbed RB5, remaining on the catalyst surface after 20 min and 60 min were about 0.21 and 0.04, respectively. Hence, the photocatalytic decomposition was done continuously to 180 min under applied conditions to complete degradation and mineralization of the RB5 on the catalyst surface. The real degradation was estimated by the following equation:
Real degradation ¼ ½1 CðtÞ CðdÞ
ð1Þ
where C(t) and C(d) are the concentration of RB5 (ppm) at time (t) in the solution and on the surface for the desorption, respectively. As shown in Fig. 11, the amount of desorbed RB5 (ppm) reduced over the time and reached zero at 180 min. In addition, the color of desorbed solutions were quite different from primary original RB5, representing the color splitting during the complete degradation of dye pollutant (Fig. 11). Also it was reported the color of the desorbed solution are owing to the production of nitrate and sulfate ions from sulfone, sulfonate and amine groups of RB5 during the photocatalytic process [31,32]. It seems that the catalyst has a very active surface and the degraded dye molecules remained on the catalyst surface and the destruction continued until the mineralization completely done while the color of RB5 solution (bulk) was changed from dark blue to colorless solution. Fig. 12 shows the UV–vis spectra of desorbed solution during the degradation
Please cite this article in press as: S. Sehati, M.H. Entezari, High visible light intercalated nanophotocatalyst (PbS-CdS/Ti6O13) synthesized by ultrasound: Photocatalytic activity, photocorrosion resistance and degradation mechanism, Separ. Purif. Technol. (2016), http://dx.doi.org/10.1016/j. seppur.2016.10.045
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Fig. 10. Estimation of the amount of adsorption and degradation of RB5 during the photocatalytic process. (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
20 meanwhile the COD was completely eliminated at 180 min (Fig. 13). Obviously, the photocatalytic degradation efficiency is clearly higher than the COD removal efficiency. This is owing to the formation of some intermediates during the mineralization of RB5 on the surface and in the solution. The results indicate that the decay of the absorbance of chromophore in the RB5 molecule is a relatively fast process (disappearance of its peak at kmax = 600 in the UV–vis spectra). But, complete degradation requires more time. This may be due to different rates of competitive adsorption of reactants, containing initial dye, intermediate molecules and byproducts on the surface of photocatalyst that would oxidize at different rates from the parent molecule [32].
Fig. 11. Concentration of desorbed solution from the catalyst surface and the color of desorbed solution at different time. (Dose of catalyst = 0.05 g, temp = 25 °C, RB5 concentration = 40 ppm).
process. Obviously, with passing the time, the peek locating at 254 nm was created and finally eliminated after 180 min irradiation. 3.2.7. COD of RB5 solution The photocatalytic mineralization of RB5 was followed by measuring the COD during the process by dichromate method [33]. The RB5 in solution could be degraded in the photocatalytic reaction in
3.2.8. Photo-corrosion resistance and photostability of catalyst Obviously, the narrower band gap’s semiconductors in the bulk material like CdS and PbS are prone to photocorrosion which can seriously deactivate the photocatalysts [34,35]. Several efforts have been devoted to improve the photoactivity and photostability of sulfides semiconductors like preparation of composite particles [36,37]. In fact, the combination of these materials with other semiconductors is essential for an efficient charge transfer reaction and for the protection against photocorrosion. The photocorrosion behavior of PbS-CdS/Ti6O13 nanocomposite compared with the mixture of PbS and CdS nanoparticles, preparing in the same condition with composite. The photocorrosion was measured from the atomic absorption analysis for Pb2+and Cd2+ions in the filtrate after photocatalytic degradation reaction. The results are shown in Fig. 14 and confirmed that the amount of Cd2+ and Pb2+ released in solution during the photocatalytic reaction of nanocomposite were declined 90.3% and 89.4%, respectively, in comparison with the mixed sample. In order to further test the photocatalytic activity and stability of sample PbS-CdS/Ti6O13, the recycle experiments were conducted over the RB5 degradation via the photocatalyst under sunlight
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Fig. 12. UV–vis absorption spectra of desorbed solution from the catalyst surface at various time. (Dose of catalyst = 0.05 g, temp = 25 °C).
Fig. 13. Removal percentage of RB5 and COD during the photocatalytic process at different time. (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
irradiation. Fig. 15a, shows the photocatalytic stability measurement of PbS-CdS/Ti6O13 in deionized water solution of RB5. Every measurement cycle was for 20 min with new prepared system by adding fresh solution of RB5 and separated and washed pre-used photocatalyst for each cycle. As illustrated in Fig. 15a, after four cycles of photocatalytic reaction, the as-prepared catalyst did not exhibit any significant loss of activity, demonstrating it possessed good stability for repeated photocatalytic process. As far as 4th and 5th cycles are concerned, the catalytic activity of the PbSCdS/Ti6O13 had a slight decrease in initial cycles. Fig. 15b demonstrates the catalyst stability and reusability in the RB5 solution prepared in tap water. The catalyst is stable and reusable but, the photocatalytic degradation is longer in the sample of dissolved RB5 in tap water than deionized water. In fact, the salts which generally present in the tap water, may be adsorbed on the surface of
catalyst, and thus, some of them compete with the adsorption of RB5 on the surface of the PbS-CdS/Ti6O13 photocatalyst.
3.3. Kinetics of degradation To study the kinetics of RB5 photodegradation with PbS-CdS/ Ti6O13 nanocomposite, a series of experiments were performed with different initial concentrations of RB5 ranging from 30 to 60 ppm (Fig. 16). Lower efficiency of photocatalytic degradation was observed at higher concentration of RB5. This is due to the formation of several layers of adsorbed dye on the catalyst surface at high concentrations, inhibiting the direct contact of catalyst with light in order to generate the holes and electrons [38]. In general, the effect of the initial concentration of large organic compounds
Please cite this article in press as: S. Sehati, M.H. Entezari, High visible light intercalated nanophotocatalyst (PbS-CdS/Ti6O13) synthesized by ultrasound: Photocatalytic activity, photocorrosion resistance and degradation mechanism, Separ. Purif. Technol. (2016), http://dx.doi.org/10.1016/j. seppur.2016.10.045
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Fig. 14. Photocorrosion of PbS-CdS/Ti6O13 nanocomposite and mixed sample (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C, time of irradiation = 20 min).
ln
C ¼ kt C0
ð3Þ
Apparent rate constant (k), resulting from the slopes of the plot for ln (C/C0) as a function of time, confirmed that RB5 photodegradation catalyzed by PbS-CdS/Ti6O13 follows a pseudo first-order kinetics model. The k values, depending on the initial concentration of RB5, decreased gradually along with the increase in the initial concentration of dye and demonstrate the suitability of the system for low dye concentrations. Certainly, the concentration of dye in wastewater from textile industry effluents is always in the range of 10–50 ppm [40].
3.4. Mechanism of degradation
Fig. 15. Repetitive photocatalytic degradation of (a) deionized water solution of RB5, (b) tap water RB5 solution, during four sequential cycles (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
on the photocatalytic degradation rate is described by pseudo firstorder kinetics with simple following equation [39]:
lnC ¼ kt þ ln C0
ð2Þ
where k is the pseudo first-order rate, and C0 and C are the concentrations of RB5 at the start and at time t, respectively. The integration of Eq. (2) yields Eq. (3):
As far as intercalated material is concerned, the photo-induced electrons could rapidly be transferred through the host, and the recombination between the photo generated charge carriers was efficiently decreased. This was one of the reasons that PbS-CdS/ Ti6O13 intercalated nanocomposite has higher photocatalytic activity than that of unsupported K2Ti6O13. Fig. 17 shows the photocatalyst PbS, CdS and K2Ti6O13 generated holes and electrons transmission under sunlight. The migration of photogenerated electrons, locating on the conduction band of PbS, CdS could be done to the conduction band of K2Ti6O13 with the higher potential, and the photo induced holes on the valence band of titanate could migrate to the valence band of CdS. Therefore, photogenerated electrons amassed in the conduction band of K2Ti6O13 and holes accumulated in the valence band of PbS and CdS, facilitating the separation of the photo induced electron-holes pairs and photocatalytic activity. In order to determine the influence of photo induced radical species in photocatalytic degradation of RB5 under solar light and proposed mechanism, a series of control experiments were performed with adding various scavengers for extinguishing hydroxyl radicals (OH), superoxide radicals(O 2 ), holes (h+), and electrons (e) and in the inert Ar atmosphere. As shown in Fig. 18, the presence of Ar had a strong impact on the removal of RB5, confirming that oxygen in the photocatalytic system plays a critical role for oxidation of RB5. Addition of ammonium oxalate (AO) as a hole scavenger caused the degradation of RB5 was almost terminated under solar light irradiation. When benzoquinone (BQ) as an inhibitor was added into the reaction system in order to the quench of the O 2 , the photodegradation of RB5 was significantly decreased to 40% conversion. Besides
Please cite this article in press as: S. Sehati, M.H. Entezari, High visible light intercalated nanophotocatalyst (PbS-CdS/Ti6O13) synthesized by ultrasound: Photocatalytic activity, photocorrosion resistance and degradation mechanism, Separ. Purif. Technol. (2016), http://dx.doi.org/10.1016/j. seppur.2016.10.045
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Fig. 16. Kinetics of photocatalytic degradation of RB5 (dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
that, a remarkable inhibition in RB5 degradation (20% conversion) was observed with addition of K2S2O8 scavenger for electrons. In fact, the activation of molecular oxygen was prevented through the presence of K2S2O8 as an electron scavenger. It seems that
the electron could not be able to participate directly on oxidation of RB5. Whereas the superoxide radicals, playing an important role in RB5 degradation, generated from adopting the electrons by molecular oxygen. Obviously, in photo-degradation of RB5 over the PbS-CdS/Ti6O13, the positively charged holes play the direct role in photocatalytic oxidation of RB5 and the indirect role of negatively charged electrons is to take part in activating the molecular oxygen by creation of O 2 radicals. It worth mentioning that the addition of isopropyl alcohol (IPA) as a OH scavenger had no significant affect. This implies that hydroxyl radicals do not play a major role in the RB5 degradation. In summary, the adsorbed RB5 molecules on the surface of PbS-CdS/Ti6O13 can be directly oxidized to the corresponding cationic radicals and ring cleavage was done by the positive holes with forceful oxidation power [32,41]. Combined with the results of UV–visible absorption spectra of desorbed solution during the photocatalytic degradation of RB5 (Fig. 12), the naphthalene derivatives, forming during the oxidation, could be further cleaved to form benzene derivatives (the strong peak located at the wavelength of 254) by the h+ and these benzene by-products could be degraded to smaller organic acids with further reactions [32]. On the other hand, the oxygen or activated oxygen species (O 2 ) which formed by reaction of photogenerated electrons with the adsorbed O2 could oxidize the above cationic radicals.
Fig. 17. Schematic illustration for the probable structure of band edges for the efficient transport of excited electrons and holes in a PbS-CdS/Ti6O13 under sunlight irradiation.
Fig. 18. Control experiments of photocatalytic degradation of RB5 in the presence of PbS-CdS/Ti6O13 with the addition of different radical scavengers and in the inert Ar atmosphere.
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4. Conclusion K2Ti6O13 photocatalyst incorporated with PbS and CdS semiconductors was used for photodecomposition of RB5 under sunlight irradiation. The photocatalytic test results proved that the PbSCdS/Ti6O13 nanocomposites, preparing at different ultrasonic amplitude, had different effects on RB5 degradation due to the impact on intercalated contents. The nanocomposite with initial composite ratio (PbS-CdS (2:1)/Ti6O13) showed the highest photocatalytic activity and was able to decolorize about 100% of RB5 in 20 min. The remarkable photo-corrosion inhibition was observed during the photocatalytic activity duo to the incorporation of sulfide nanoparticle between the titanate layers. Besides that, the relative COD in solution became zero after 180 min of sunlight irradiation. According to the suggested mechanism, the cleavage of the chromophore, naphthalene and benzene rings in RB5 structure were principally accomplished through the hole and O 2 , produced during the process.
Acknowledgments The support of Ferdowsi University of Mashhad, Iran (Research and Technology) for this work (code3/38606, date 13/10/2015) is appreciated.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
High visible light intercalated nanophotocatalyst (PbS-CdS/Ti6O13) synthesized by ultrasound: Photocatalytic activity, photocorrosion resistance and degradation mechanism S. Sehati a, M.H. Entezari a,b,⇑ a b
Sonochemical Research Center, Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, 91779 Mashhad, Iran Environmental Chemistry Research Center, Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, 91779 Mashhad, Iran
a r t i c l e
i n f o
Article history: Received 3 July 2016 Received in revised form 19 October 2016 Accepted 24 October 2016 Available online xxxx Keywords: Ultrasound Degradation RB5 Photocatalyst Intercalation Nanocomposite
a b s t r a c t In this study, the photocatalytic activity of the PbS-CdS/Ti6O13 nanocomposite prepared by sonointercalation method was evaluated by the degradation of Reactive Black 5 (RB5) as a water pollutant under sunlight irradiation. Strong photocatalytic activity was observed due to the improved light harvesting property of photocatalyst, significant decrease in bandgap energies and heterogeneous electron transfer from CdS and PbS as a guest to K2Ti6O13 as a host. The RB5 concentrations in the solution and on the surface of PbS-CdS/Ti6O13 nanocomposite were measured during the experiment. The effects of some important parameters such as ultrasonic amplitude and initial composite ratio were studied in the synthesis of nanocomposite. The activity of synthesized nanocomposites on the dye degradation was evaluated and the stability and photocorrosion resistance of the catalyst were studied in this work too. In addition, the mechanism of separation of the photogenerated electrons and holes in the PbS-CdS/ Ti6O13 nanocomposite was investigated. The responsible superior active species for the photodegradation activity were explored and a possible mechanism was suggested. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Water pollutant treatment and the effect of water pollution on the environment are of special concerns to scientists working in remediation of wastewaters [1,2]. Dyes are considered as one of the most important classes of contaminants in industrial waste waters due to their toxic nature, large production volume and low rate of treatment by conventional processes involving biological and chemical methods [3]. Therefore, it is essential to remove these organic pollutants from wastewaters before discharge to major water bodies [4]. Degradation of dyes, regardless of the used technique(s) leads to formation of biodegradable intermediates and smaller compounds, which can be mineralized under ideal conditions [5,6]. Photocatalytic degradation of organic pollutant in water using solar energy and semiconductor catalysts has been widely researched [7,8]. Oxides with regularities such as layered and tunneling structures are considered as favorable materials for effective photo degradation of water pollutants. The high activ⇑ Corresponding author at: Sonochemical Research Center, Environmental Chemistry Research Center, Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, 91779 Mashhad, Iran. E-mail addresses: [email protected], [email protected] (M.H. Entezari).
ity of these layered compounds is related to easy migration and separation of the photo generated electron-holes through the layers [9–11]. K2Ti6O13 can be considered as one of the most important alkali-metal titanate which is a significant candidate for application in photocatalysis, fuel cells and plastics [12]. These kinds of titanate compounds do not show efficient photocatalytic activity under visible light owing to their wide bandgap (3.2– 3.5 eV) [13]. Fabricating layered composites, consisting of titanate and narrow bandgap semiconductors can promote the photocatalytic activities of these compounds in the presence of visible light [14,15]. Metal sulfides are well-known semiconductor photocatalysts because of their suitable band structures and good photoresponses under visible light irradiation for photodegradation of chemical contaminants in water and air [16,17]. PbS with a bulk bandgap of about 0.41 eV, considering as an indirect semiconductor, can absorb the light in the whole visible region [18,19]. The most drawback for using the PbS is fast recombination of photoinduced hole-electron (of the order microseconds and less) due to the very low distance between the valence band and conduction band. Catalytic properties of PbS can be modified by mixing by other semiconductors [20,21]. CdS is another semiconductor material investigated for photocatalytic reactions [22]. The ternary/coupled semiconductor systems such as Pt/PdS/CdS [23], ZnS/CdS/ZTP
http://dx.doi.org/10.1016/j.seppur.2016.10.045 1383-5866/Ó 2016 Elsevier B.V. All rights reserved.
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2.4. Photocatalytic testing
[16], CdS/ZnS/TiO2 [24], exhibit higher photocatalytic activity in comparison with their sulfides or oxide components. Preparing and understanding such nanocomposites from the interaction between different components and examination of charge carrier under irradiation aspects, are still challenging task for scientists. In the present work, we have attempted to improve stability and light-harvesting properties of co-incorporated the PbS and CdS nanoparticles into the interlayer of K2Ti6O13. The composite synthesized by sonochemical approach and its catalytic activity was checked through investigation on the photocatalytic degradation of RB5 under sunlight irradiation. The effects of important factors, affecting the photocatalytic process were studied. Finally, a reaction mechanism was proposed and discussed.
The photocatalytic activity of the prepared PbS-CdS/Ti6O13 photocatalyst was tested for the decolorization of RB5 using a Pyrex glass vessel containing 30 mL RB5 (40 mg L1). An appropriate amount of the photocatalyst in powder form was added at natural pH of RB5 (pH = 6.5) and magnetically stirred under direct sunlight radiation in sunny days in July 2015 (without staying in dark condition). The temperature of the dye solution was kept in the range of 25–30 °C. At specific time intervals, 5 mL of the sample solution was withdrawn and centrifuged prior measurements for the remaining RB5 concentration by a UV–vis spectrophotometer using the characteristic absorption band at 600 nm.
2. Experimental
3. Result and discussion
2.1. Materials
3.1. Characterization of catalyst
Tetrabutyltitanate (Ti(O-Bu)4), potassium hydroxide (KOH) and stearic acid (CH3(CH2)16CO2H) from Merck, have been used without further purification for the synthesis of potassium hexatitanate. Cadmium nitrate (Cd(NO3)2), Lead nitrate (Pb(NO3)2), thioacetamide (SC(CH3)(NH2)), and RB5 were supplied from Sigma-Aldrich, Germany. Dye solutions were prepared by dissolving the dye in de-ionized water. The pH was adjusted with sodium hydroxide (NaOH) and hydrochloric acid (HCl).
Fig. 1 shows the XRD patterns of K2Ti6O13and PbS-CdS/Ti6O13 synthesized via ultrasound. The synthesis of K2Ti6O13with high purity was confirmed by the X-ray diffractions and shown in Fig. 1a. It could be clearly observed that the synthesized layered K2Ti6O13was highly crystalline. Fig. 1b indicates the XRD pattern of intercalated nanocomposite (PbS-CdS/Ti6O13) in the presence of ultrasound (amplitude 60%). Interlayer spacing of titanate sheets (2 0 0) was expanded from 0.75 nm to 0.81 nm due to the incorporation of cubic PbS and CdS nanoparticles between layers and slightly distortion of the crystal lattice. Besides that, the intensity of 2 0 0 peak was decreased because of partially covering the host surface by guest nanoparticles. The morphologies and sizes of the as-synthesized K2Ti6O13 and PbS-CdS/Ti6O13 samples were examined using the field-emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). Fig. 2(a) and (c) illustrates the nanobelt structure of K2Ti6O13 preparing via ultrasound with a width of 40– 80 nm, and length of 600–800 nm. Fig. 2(b) and (d) indicate the FESEM and TEM images of PbS-CdS/Ti6O13 nanocomposite, respectively [25]. Obviously, the shape of host molecule (K2Ti6O13) did not change during the sono-intercalation process and PbS-CdS nanoparticles uniformly covered the host surface. Besides that, guest molecules incorporated into the titanate layers. Fig. 3 shows the UV–vis spectra of as-prepared samples. The sample K2Ti6O13 possessed a strong absorption in the UV region and a weakly absorption in the visible region with estimated band
2.2. Preparation methods The K2Ti6O13 and intercalated PbS-CdS/Ti6O13 nanocomposite were prepared according to the method presented in our recent publication [25]. A brief description of procedure is as follows: firstly the appropriate amount of stearic acid was melted at 90–120 °C. Then, saturated aqueous solution of KOH was suddenly added. The mixture vigorously stirred while was kept at 90–120 °C. After addition of Ti(O–Bu)4, the reaction mixture was sonicated for 30 min until a homogenous transparent solution was formed. Ignition of solution was done at 300 °C in furnace and then calcined at 750 °C for 2 h. In order to incorporate PbS and CdS nanoparticles into the interlayer of K2Ti6O13, 0.1 g of as-prepared titanate nanoparticles was added to the mixture containing the NaOH and different amount of Pb(NO3)2 and Cd(NO3)2. Then, the mixture was exposed under ultrasonic irradiation at various amplitudes (20–80%) for 1.5 h at 25 °C. The sample was separated and washed several times with de-ionized water. After that, SC(CH3)(NH2) solution was added to sample and sonicated for 30 min. Finally, the precipitates were centrifuged and washed with ethanol and distilled water. The particles were dried in an oven at 110 °C for 4 h. 2.3. Characterization and equipment The crystalline structures of the catalysts at different ultrasound intensities were studied by XRD recorded on a X-ray analyzer (PHILIPS PW1800) at a 2h angle ranging from 5° to 60°. The morphological properties of the PbS-CdS/Ti6O13 photocatalyst was examined by TEM (CM120-Philips). The topological properties of the catalyst was observed by FESEM(Mira 3–XMU). The optical absorption properties of the photocatalyst and absorption spectrum of RB5 were obtained using a UV–vis spectrophotometer (Unico 2800) in the range of 200–800 nm at room temperature. The atomic absorption spectrophotometer (AAS, Varian, spectra-110 880/220-Australia Pty Ltd.) was used to determine the concentration of Pb2+ and Cd2+ ions in the solution during the photocatalytic process.
Fig. 1. XRD patterns of (a) K2Ti6O13, (b) PbS-CdS/Ti6O13.
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Fig. 2. (a) FE-SEM image of K2Ti6O13 nanobelt, (b) FE-SEM image of PbS-CdS/Ti6O13, (c) TEM image of K2Ti6O13, (d) TEM image of PbS-CdS/Ti6O13.
showing that the guest molecules have a strong impact on the light absorption performance layered compound.
3.2. Photocatalytic degradation of RB5 under sunlight irradiation 3.2.1. Effect of ultrasonic amplitude Fig. 4 shows the degradation of RB5 with PbS-CdS/Ti6O13 photocatalyst, preparing at various ultrasonic amplitudes, under sunlight
Fig. 3. UV–vis absorption spectra of K2Ti6O13 and PbS-CdS/Ti6O13 samples.
gap about 3.42 eV. Comparing with the potassium hexatitanate, the UV–vis absorption edges of the PbS-CdS/Ti6O13 nanocomposite demonstrated obviously red shift corresponding to a band gaps of about 2.93, 2.58 and 1.98 eV. The incline in host band gap from 3.42 to 2.98 presumably attributed to slightly distortion of the crystalline lattice of titanate layers duo to the intercalation of guest molecules. The light absorbance intensity of the PbS-CdS/Ti6O13 in the visible region was greater than that of the precursor K2Ti6O13,
Fig. 4. Degradation of RB5 by photocatalysts prepared in different sonication amplitudes. (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
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irradiation. Based on the results of sono-synthesized nanocomposites at different amplitudes, reporting at our recent published work [25], the amount of guest molecules incorporated between titanate layers was increased from 20% to 60% amplitudes and this quantity was dropped at 80% amplitude. The photocatalytic activity of intercalated nanocomposites is directly related to the amount of incorporated PbS and CdS nanoparticles. This is due to reducing the hole-electron pair recombination through the interfacial charge transfer [26,27]. Therefore, the recombination of photo induced h+ and ein photocatalyst synthesized at 20% and 80% amplitudes were greater than that of prepared at 60% amplitude. Accordingly, the degradation efficiency of RB5 was maximized using photocatalyst synthesized at 60% amplitude. Thus, all the next experiments were done with the sample, preparing at 60% amplitude.
3.2.2. Effect of initial composite ratio The intercalated nanocomposites of PbS-CdS/Ti6O13 (60% amplitude) were prepared with different initial concentration ratios of guest molecule’s precursors (Pb(NO3)2 and Cd(NO3)2) (1:3, 1:2, 1:1, 2:1) and constant amount of host nanoparticles. Then, the photocatalytic activities was studied for different samples in RB5 removal. As shown in Fig. 5, the photocatalytic activities of the nanocomposite, co-intercalating by PbS and CdS, were higher than that of PbS/Ti6O13 or CdS/Ti6O13. It’s owing to the high charge separation efficiency of PbS-CdS/Ti6O13 (with various ratios) due to distortion of crystal structure [28]. In fact, the co-incorporating of PbS and CdS into the interlayer of titanate played an important role in photocatalytic degradation of RB5. The intercalating of the PbS and CdS guest nanoparticles into the layers of potassium hexa-titanate could suppress the particle growth and induced a quantum size effect. Besides that, the photo-generated electrons could quickly be transferred through the guest particles, and the recombination of the photoinduced charge carriers was effectively decreased in comparison with single guest intercalated nanocomposites. These results also confirm that an initial precursor’s concentrations with the (2:1) proportion of PbS-CdS in the composite has the highest removal efficiency. More increase in the ratio CdS/PbS in the composite suppressed the photocatalytic activities. Generally, further presence of higher bandgap semiconductor (CdS) in the ratio decreases the extent of light absorption in comparison with other samples with more content of PbS. Therefore, all experiments were continued with optimized initial composite ratio (PbS-CdS (2:1)/Ti6O13).
Fig. 5. Effect of initial composite ratio. (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
3.2.3. Catalyst loading In order to investigate the effect of catalyst loading on the efficiency of photocatalytic degradation, various amounts of catalyst (0.01–0.07 g) were used while keeping all other parameters constant and the results are shown in Fig. 6. The results demonstrate that with enlarging the amounts of catalyst, the rate of pollutant’s degradation was continuously increased duo to availability of more active sites in the catalyst for the absorption of photon and more interaction with RB5 molecules [26]. Moreover, increase in photo-degradation efficiency was also related to an enhanced generation of the oxidizing radicals, which consequently increased the rate of photocatalytic degradation of dye [29]. The effects of catalyst loading on rate of degradation was insignificant at higher dosage (>0.05 g) and there was no remarkably change in removing of the dye. Hence, the whole experiments were continued with 0.05 g of the PbS-CdS/Ti6O13 photocatalyst. 3.2.4. Effect of initial pH The influence of initial pH value on the RB5 degradation efficiency was investigated at different pH values in the range of 2.5–11 and the results are shown in Fig. 7. The effect of pH on the degradation of RB5 by optimized PbS-CdS/Ti6O13 photocatalyst can be mainly explained by the surface charge of catalyst (pHpzc = 8.76) and its relation to the constants of acid dissociation of sulfonic and ethyl sulfonic groups in RB5 molecule [30]. At low pH (2.5) a strong adsorption of dye on the catalyst surface was observed due to the electrostatic attraction of the high positively charged PbS-CdS/Ti6O13 (pH < pHpzc) with the ionized sulfonic and ethyl sulfonic groups in RB5 molecules (pka value 1) which act as strong acid. In fact more adsorption of dye caused the higher degradation efficiency (100%). With increasing the pH from acidic to neutral pH of dye (6.5), the amount of positive charge on the surface of catalyst slightly declined so the adsorption of negatively charged RB5 on the catalyst surface and removal efficiency was decreased (94%). As can be seen, at high pH (above pHpzc = 8.76) the surface of catalyst tended to be negatively charged and a decrease in the degradation rate was observed. A minimum at pH = 11, reflecting the complexity of the sulfonic and ethyl sulfonic anions in approaching the negatively charged PbS-CdS/Ti6O13 surface (pH > pHpzc) when increasing the solution pH. Although the best results in RB5 degradation was observed at pH = 2.5, but the whole experiments were continued at neutral pH of dye because of the insignificant differences between their results. 3.2.5. RB5 removal in dark and light In order to compare the catalytic activity of PbS-CdS/Ti6O13 in dark and light, one sample contains of catalyst and RB5 was magnetically stirred in dark for 45 min to attain adsorption–desorption equilibrium. As shown in Fig. 8, the color of catalyst changed from
Fig. 6. Effect of different catalyst loading on degradation time. (Initial pH = 6.5, temp = 25 °C).
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Fig. 7. Effect of different pH on RB5 degradation. Outset shows the estimated pHpzc of PbS-CdS/Ti6O13. (Dose of catalyst = 0.05 g, temp = 25 °C, initial dye concentration = 40 ppm).
Fig. 8. UV–vis absorption spectra of RB5 solution in blank, in the presence of catalyst at dark, and color change of the catalyst during the adsorption process (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
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Fig. 9. UV–vis absorption spectra of (a) RB5 solution at different contact times under sunlight, (b) color change of the catalyst during the photocatalytic process (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
greenish yellow to blue due to the adsorption of RB5 on the catalyst surface and decolorization was not completely done even after 45 min. It was confirmed that the removal of RB5 was done by the adsorption process. On the other hand, another sample containing catalyst and RB5 solution directly was taken place under sunlight without keeping in the dark. To explain the changes of RB5 solution resulting from photocatalytic degradation, representative UV–vis spectra changes of the dye solution is presented in Fig. 9 as a function of reaction time. According to Fig. 9, absorption spectrum of the original RB5 solution (blank) can be described by three main peaks at wavelengths of 600, 312, and 254 nm. During photocatalytic degradation under sunlight, the absorbance values decreased continuously with time due to the fragmentation of bands and finally completely eliminated in 20 min. This indicates that the decolorization rate was higher under sunlight irradiation than that at dark. Besides that, when the catalyst was exposed to the solution, the color of catalyst changed to blue-green, demonstrating the presence of dye contaminants on the surface of catalyst. Although, complete decolorization was achieved in the solution after 20 min, but the catalyst surface color was still undertone blue-green (Fig. 9b). Over the time and in complete photocatalytic process, the appearance color of catalyst changed to greenish yellow because of decomposing of adsorbed RB5 on the surface of the catalyst. 3.2.6. Adsorption, desorption, and degradation on the catalyst surface In order to investigate the photocatalytic decomposition and adsorption of RB5 under solar light, the concentration of RB5
remained in the solution and the concentration of RB5 on the surface (through desorption) were measured at different interval times. Desorption was done by dispersing the catalyst in an aqueous solution at acidic medium (pH = 2.5, the pH with maximum desorption). Based on Fig. 10, the results of desorption show that the normalized concentration changes of adsorbed RB5, remaining on the catalyst surface after 20 min and 60 min were about 0.21 and 0.04, respectively. Hence, the photocatalytic decomposition was done continuously to 180 min under applied conditions to complete degradation and mineralization of the RB5 on the catalyst surface. The real degradation was estimated by the following equation:
Real degradation ¼ ½1 CðtÞ CðdÞ
ð1Þ
where C(t) and C(d) are the concentration of RB5 (ppm) at time (t) in the solution and on the surface for the desorption, respectively. As shown in Fig. 11, the amount of desorbed RB5 (ppm) reduced over the time and reached zero at 180 min. In addition, the color of desorbed solutions were quite different from primary original RB5, representing the color splitting during the complete degradation of dye pollutant (Fig. 11). Also it was reported the color of the desorbed solution are owing to the production of nitrate and sulfate ions from sulfone, sulfonate and amine groups of RB5 during the photocatalytic process [31,32]. It seems that the catalyst has a very active surface and the degraded dye molecules remained on the catalyst surface and the destruction continued until the mineralization completely done while the color of RB5 solution (bulk) was changed from dark blue to colorless solution. Fig. 12 shows the UV–vis spectra of desorbed solution during the degradation
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Fig. 10. Estimation of the amount of adsorption and degradation of RB5 during the photocatalytic process. (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
20 meanwhile the COD was completely eliminated at 180 min (Fig. 13). Obviously, the photocatalytic degradation efficiency is clearly higher than the COD removal efficiency. This is owing to the formation of some intermediates during the mineralization of RB5 on the surface and in the solution. The results indicate that the decay of the absorbance of chromophore in the RB5 molecule is a relatively fast process (disappearance of its peak at kmax = 600 in the UV–vis spectra). But, complete degradation requires more time. This may be due to different rates of competitive adsorption of reactants, containing initial dye, intermediate molecules and byproducts on the surface of photocatalyst that would oxidize at different rates from the parent molecule [32].
Fig. 11. Concentration of desorbed solution from the catalyst surface and the color of desorbed solution at different time. (Dose of catalyst = 0.05 g, temp = 25 °C, RB5 concentration = 40 ppm).
process. Obviously, with passing the time, the peek locating at 254 nm was created and finally eliminated after 180 min irradiation. 3.2.7. COD of RB5 solution The photocatalytic mineralization of RB5 was followed by measuring the COD during the process by dichromate method [33]. The RB5 in solution could be degraded in the photocatalytic reaction in
3.2.8. Photo-corrosion resistance and photostability of catalyst Obviously, the narrower band gap’s semiconductors in the bulk material like CdS and PbS are prone to photocorrosion which can seriously deactivate the photocatalysts [34,35]. Several efforts have been devoted to improve the photoactivity and photostability of sulfides semiconductors like preparation of composite particles [36,37]. In fact, the combination of these materials with other semiconductors is essential for an efficient charge transfer reaction and for the protection against photocorrosion. The photocorrosion behavior of PbS-CdS/Ti6O13 nanocomposite compared with the mixture of PbS and CdS nanoparticles, preparing in the same condition with composite. The photocorrosion was measured from the atomic absorption analysis for Pb2+and Cd2+ions in the filtrate after photocatalytic degradation reaction. The results are shown in Fig. 14 and confirmed that the amount of Cd2+ and Pb2+ released in solution during the photocatalytic reaction of nanocomposite were declined 90.3% and 89.4%, respectively, in comparison with the mixed sample. In order to further test the photocatalytic activity and stability of sample PbS-CdS/Ti6O13, the recycle experiments were conducted over the RB5 degradation via the photocatalyst under sunlight
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Fig. 12. UV–vis absorption spectra of desorbed solution from the catalyst surface at various time. (Dose of catalyst = 0.05 g, temp = 25 °C).
Fig. 13. Removal percentage of RB5 and COD during the photocatalytic process at different time. (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
irradiation. Fig. 15a, shows the photocatalytic stability measurement of PbS-CdS/Ti6O13 in deionized water solution of RB5. Every measurement cycle was for 20 min with new prepared system by adding fresh solution of RB5 and separated and washed pre-used photocatalyst for each cycle. As illustrated in Fig. 15a, after four cycles of photocatalytic reaction, the as-prepared catalyst did not exhibit any significant loss of activity, demonstrating it possessed good stability for repeated photocatalytic process. As far as 4th and 5th cycles are concerned, the catalytic activity of the PbSCdS/Ti6O13 had a slight decrease in initial cycles. Fig. 15b demonstrates the catalyst stability and reusability in the RB5 solution prepared in tap water. The catalyst is stable and reusable but, the photocatalytic degradation is longer in the sample of dissolved RB5 in tap water than deionized water. In fact, the salts which generally present in the tap water, may be adsorbed on the surface of
catalyst, and thus, some of them compete with the adsorption of RB5 on the surface of the PbS-CdS/Ti6O13 photocatalyst.
3.3. Kinetics of degradation To study the kinetics of RB5 photodegradation with PbS-CdS/ Ti6O13 nanocomposite, a series of experiments were performed with different initial concentrations of RB5 ranging from 30 to 60 ppm (Fig. 16). Lower efficiency of photocatalytic degradation was observed at higher concentration of RB5. This is due to the formation of several layers of adsorbed dye on the catalyst surface at high concentrations, inhibiting the direct contact of catalyst with light in order to generate the holes and electrons [38]. In general, the effect of the initial concentration of large organic compounds
Please cite this article in press as: S. Sehati, M.H. Entezari, High visible light intercalated nanophotocatalyst (PbS-CdS/Ti6O13) synthesized by ultrasound: Photocatalytic activity, photocorrosion resistance and degradation mechanism, Separ. Purif. Technol. (2016), http://dx.doi.org/10.1016/j. seppur.2016.10.045
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Fig. 14. Photocorrosion of PbS-CdS/Ti6O13 nanocomposite and mixed sample (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C, time of irradiation = 20 min).
ln
C ¼ kt C0
ð3Þ
Apparent rate constant (k), resulting from the slopes of the plot for ln (C/C0) as a function of time, confirmed that RB5 photodegradation catalyzed by PbS-CdS/Ti6O13 follows a pseudo first-order kinetics model. The k values, depending on the initial concentration of RB5, decreased gradually along with the increase in the initial concentration of dye and demonstrate the suitability of the system for low dye concentrations. Certainly, the concentration of dye in wastewater from textile industry effluents is always in the range of 10–50 ppm [40].
3.4. Mechanism of degradation
Fig. 15. Repetitive photocatalytic degradation of (a) deionized water solution of RB5, (b) tap water RB5 solution, during four sequential cycles (Dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
on the photocatalytic degradation rate is described by pseudo firstorder kinetics with simple following equation [39]:
lnC ¼ kt þ ln C0
ð2Þ
where k is the pseudo first-order rate, and C0 and C are the concentrations of RB5 at the start and at time t, respectively. The integration of Eq. (2) yields Eq. (3):
As far as intercalated material is concerned, the photo-induced electrons could rapidly be transferred through the host, and the recombination between the photo generated charge carriers was efficiently decreased. This was one of the reasons that PbS-CdS/ Ti6O13 intercalated nanocomposite has higher photocatalytic activity than that of unsupported K2Ti6O13. Fig. 17 shows the photocatalyst PbS, CdS and K2Ti6O13 generated holes and electrons transmission under sunlight. The migration of photogenerated electrons, locating on the conduction band of PbS, CdS could be done to the conduction band of K2Ti6O13 with the higher potential, and the photo induced holes on the valence band of titanate could migrate to the valence band of CdS. Therefore, photogenerated electrons amassed in the conduction band of K2Ti6O13 and holes accumulated in the valence band of PbS and CdS, facilitating the separation of the photo induced electron-holes pairs and photocatalytic activity. In order to determine the influence of photo induced radical species in photocatalytic degradation of RB5 under solar light and proposed mechanism, a series of control experiments were performed with adding various scavengers for extinguishing hydroxyl radicals (OH), superoxide radicals(O 2 ), holes (h+), and electrons (e) and in the inert Ar atmosphere. As shown in Fig. 18, the presence of Ar had a strong impact on the removal of RB5, confirming that oxygen in the photocatalytic system plays a critical role for oxidation of RB5. Addition of ammonium oxalate (AO) as a hole scavenger caused the degradation of RB5 was almost terminated under solar light irradiation. When benzoquinone (BQ) as an inhibitor was added into the reaction system in order to the quench of the O 2 , the photodegradation of RB5 was significantly decreased to 40% conversion. Besides
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Fig. 16. Kinetics of photocatalytic degradation of RB5 (dose of catalyst = 0.05 g, initial pH = 6.5, temp = 25 °C).
that, a remarkable inhibition in RB5 degradation (20% conversion) was observed with addition of K2S2O8 scavenger for electrons. In fact, the activation of molecular oxygen was prevented through the presence of K2S2O8 as an electron scavenger. It seems that
the electron could not be able to participate directly on oxidation of RB5. Whereas the superoxide radicals, playing an important role in RB5 degradation, generated from adopting the electrons by molecular oxygen. Obviously, in photo-degradation of RB5 over the PbS-CdS/Ti6O13, the positively charged holes play the direct role in photocatalytic oxidation of RB5 and the indirect role of negatively charged electrons is to take part in activating the molecular oxygen by creation of O 2 radicals. It worth mentioning that the addition of isopropyl alcohol (IPA) as a OH scavenger had no significant affect. This implies that hydroxyl radicals do not play a major role in the RB5 degradation. In summary, the adsorbed RB5 molecules on the surface of PbS-CdS/Ti6O13 can be directly oxidized to the corresponding cationic radicals and ring cleavage was done by the positive holes with forceful oxidation power [32,41]. Combined with the results of UV–visible absorption spectra of desorbed solution during the photocatalytic degradation of RB5 (Fig. 12), the naphthalene derivatives, forming during the oxidation, could be further cleaved to form benzene derivatives (the strong peak located at the wavelength of 254) by the h+ and these benzene by-products could be degraded to smaller organic acids with further reactions [32]. On the other hand, the oxygen or activated oxygen species (O 2 ) which formed by reaction of photogenerated electrons with the adsorbed O2 could oxidize the above cationic radicals.
Fig. 17. Schematic illustration for the probable structure of band edges for the efficient transport of excited electrons and holes in a PbS-CdS/Ti6O13 under sunlight irradiation.
Fig. 18. Control experiments of photocatalytic degradation of RB5 in the presence of PbS-CdS/Ti6O13 with the addition of different radical scavengers and in the inert Ar atmosphere.
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4. Conclusion K2Ti6O13 photocatalyst incorporated with PbS and CdS semiconductors was used for photodecomposition of RB5 under sunlight irradiation. The photocatalytic test results proved that the PbSCdS/Ti6O13 nanocomposites, preparing at different ultrasonic amplitude, had different effects on RB5 degradation due to the impact on intercalated contents. The nanocomposite with initial composite ratio (PbS-CdS (2:1)/Ti6O13) showed the highest photocatalytic activity and was able to decolorize about 100% of RB5 in 20 min. The remarkable photo-corrosion inhibition was observed during the photocatalytic activity duo to the incorporation of sulfide nanoparticle between the titanate layers. Besides that, the relative COD in solution became zero after 180 min of sunlight irradiation. According to the suggested mechanism, the cleavage of the chromophore, naphthalene and benzene rings in RB5 structure were principally accomplished through the hole and O 2 , produced during the process.
Acknowledgments The support of Ferdowsi University of Mashhad, Iran (Research and Technology) for this work (code3/38606, date 13/10/2015) is appreciated.
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Please cite this article in press as: S. Sehati, M.H. Entezari, High visible light intercalated nanophotocatalyst (PbS-CdS/Ti6O13) synthesized by ultrasound: Photocatalytic activity, photocorrosion resistance and degradation mechanism, Separ. Purif. Technol. (2016), http://dx.doi.org/10.1016/j. seppur.2016.10.045