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fly ash cenospheres with enhanced visible-light photocatalytic activity
Optical Materials 53 (2016) 73–79
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and characterization of cube-like Ag@AgCl-doped TiO2/fly ash cenospheres with enhanced visible-light photocatalytic activity Shaomin Liu ⇑, Jinglin Zhu, Qing Yang, Pengpeng Xu, Jianhua Ge, Xuetao Guo School of Earth Science and Environmental Engineering, Anhui University of Science and Technology, Huainan 232001, PR China
a r t i c l e
i n f o
Article history: Received 12 November 2015 Received in revised form 3 January 2016 Accepted 8 January 2016
Keywords: Ag@AgCl TiO2 Fly ash cenospheres Photocatalysis
a b s t r a c t A cube-like Ag@AgCl-doped TiO2/fly ash cenosphere composite (denoted Ag@AgCl–TiO2/fly ash cenospheres) was successfully synthesized via a two-step approach. The as-prepared catalysts were characterized by scanning electron microscopy, X-ray diffraction, diffuse reflectance ultraviolet–visible spectroscopy, Brunauer–Emmett–Teller, and X-ray photoelectron spectroscopy. The photocatalytic experiment showed that the rhodamine B degradation rate with Ag@AgCl–TiO2/fly ash cenospheres was 1.56 and 1.33 times higher than that with AgCl–TiO2/fly ash cenospheres and Ag@AgCl, respectively. The degradation ratio of rhodamine B with Ag@AgCl–TiO2/fly ash cenospheres was nearly 100% within + 120 min under visible light. Analysis of active species indicated that O 2 and h dominated the reaction, and OH participated in the photocatalytic reactions as an active species. A mechanism for the photocatalytic degradation by the Ag@AgCl–TiO2/fly-ash cenospheres was also proposed based on the experimental results. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction In the last two decades, titanium dioxide has attracted considerable interest for degradation of various organic pollutants because of its high photocatalytic activity, good stability, and low cost [1–3]. However, the limited UV-induced activity of TiO2 because of its wide bandgap (i.e., 3.2 eV for anatase and 3.0 eV for rutile) has considerably prevented its practical application under natural sunlight. To overcome this drawback, numerous methods have been used to broaden the absorption threshold of TiO2 into the visible region, including nonmetal doping [4–6], metal deposition [7–9], and coupling with other semiconductors [10–12]. Among these methods, the use of noble metal nanoparticles (N-Ps) have received extensive research attention because of the surface plasmon resonance of N-Ps, which accelerates separation of photo-generated electrons and holes on the surface of the semiconductor catalyst. Moreover, Ag N-Ps deposited on semiconductors are believed to show efficient plasmon resonance under visible light. Bi and Ye prepared core–shell Ag/AgCl hetero-nanowires with uniform structures, which were then used for the decomposition of methyl orange dye under visible light by an in situ oxidation reaction at room temperature [13]. Yu et al. deposited Ag/AgCl onto TiO2 nanotube arrays and then reduced partial Ag+ ions of AgCl particles to ⇑ Corresponding author. E-mail address: [email protected] (S. Liu). http://dx.doi.org/10.1016/j.optmat.2016.01.010 0925-3467/Ó 2016 Elsevier B.V. All rights reserved.
Ag0 species, which is efficient for the photocatalytic degradation of methyl orange under xenon lamp irradiation [14]. Hu et al. prepared Ag/AgBr/TiO2 by deposition–precipitation method; this material showed excellent photocatalytic activity for the destruction of azo dyes and bacteria under visible light [15]. However, silver halide-modified titanium dioxide catalysts are difficult to recycle, which limits their practical applications. In addition, such catalysts have low light usage rate because its relatively large size leads particles to sink or become suspended in the solution. To solve these problems, the TiO2/fly ash cenospheres (FAC) were synthesized via a simple sol–gel method and then Ag@AgCl N-Ps were deposited on TiO2/FAC via a two-step approach. The prepared composite showed high photocatalytic activity for the degradation of rhodamine B in aqueous solution under visible light. To elucidate the mechanism of the photocatalytic process, the main oxidative species in the catalytic process were detected through OH analysis and radical trapping experiments. A mechanism for the photocatalytic degradation by Ag@AgCl–TiO2/FAC was also proposed. 2. Experiment 2.1. Chemicals Fly ash was supplied by the Huainan Fossil Fuel Power Station and used with acid pretreatment. Tetrabutyl titanate, absolute ethanol, acetylacetone, polyethylene glycol (PEG), ethanol, AgNO3,
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polyvinylpyrrolidone (PVP), NaCl, and HCl were purchased from Shanghai Sinopharm. Deionized water was prepared by our laboratory.
to record the absorbance of the samples at 550 nm. The photocatalytic degradation rate (DR) was calculated with the following formula:
2.2. Preparation of TiO2/FAC
DR ¼ ½ð1 Ai =A0 Þ 100%
In a typical process, TiO2/FAC was prepared via a simple sol–gel method [16]. In brief, 90 mL of absolute ethanol, 34 mL of tetrabutyl titanate, and 10 mL of acetyl acetone were mixed to form a solution. Another mixing solution containing 50 mL of absolute ethanol and 1.8 mL of deionized water was added drop-wise into the first solution. The pH of the mixed solution was adjusted to 5 with 67% HNO3 with stirring for 1 h at room temperature. Subsequently, 2 g of PEG was added as surfactant with continuous stirring for 1 h in a water bath at 50 °C, and FAC was added with further stirring for 1 h. The abovementioned solution was evaporated until nearly dry at 80 °C in a water bath after aged for 24 h at room temperature, and the remaining solid matter was dried at 80 °C in an oven. Finally, calcination for 2 h resulted in TiO2/ FAC at 500 °C.
where A0 is the initial absorbance of rhodamine B solution at the absorption–desorption equilibrium and Ai is the absorbance of the reaction solution. In addition to the photocatalytic experiment, a similar radical trapping experiment was also performed. The only difference is certain amounts of scavengers were added to the reaction solution to reach the absorption–desorption equilibrium before the operating experiment. The formation of hydroxyl radical (OH) was detected by the PL technique. Terephthalic acid readily serves as a probe molecule that reacts with OH to produce a highly fluorescent product (2-hydroxyterephthalic acid). The PL intensity of 2hydroxyterephtalic acid is proportional to the quantity of OH radicals produced in solution [17–19]. The experimental process is similar to the photocatalytic experiment; the mixed solution, which contains 5 104 M terephthalic acid and 2 103 M NaOH aqueous solution, replaced the rhodamine B solution during the photocatalytic experiment. After visible-light irradiation every 20 min, the reaction solution was applied to determine the increase of the PL intensity at 425 nm excited by 315 nm light [20]. The PL spectra of generated 2-hydroxyterephthalic acid were recorded on a Shimadzu RF-5301PC fluorescence spectrophotometer.
2.3. Synthesis of plasmonic Ag@AgCl–TiO2/FAC composite photocatalysts The Ag@AgCl–TiO2/FAC composite photocatalysts were prepared via a two-step approach. Typically, 0.170 g of AgNO3 was added into a mixture containing 1.0 g of TiO2/FAC, 0.111 g of PVP, 30 mL of deionized water, and 50 mL of ethanol under magnetic stirring, which were transferred into Teflon-lined stainless steel autoclave and heated at 130 °C for 3 h after stirring for 30 min. A 20 mL solution with 0.0293 g of NaCl and an HCl aqueous solution (0.086 mL, 36 wt.%) were gradually added into the abovementioned mixture with further stirring for 24 h in the dark, followed by irradiation for 30 min with a 500 W Xe lamp to deoxidize Ag+ to Ag0. Finally, the obtained particle was collected by centrifugation, washed, and dried at 80 °C for 8 h to obtain the plasmonic Ag@AgCl–TiO2/FAC composite photocatalysts. For comparison, pure Ag@AgCl was synthesized similarly by the abovementioned method without TiO2/FAC. 2.4. Characterization The surface microstructure was investigated with a JSM-6700F by field-emission scanning electron microscopy (SEM). The phase structure was obtained from X-ray diffraction (XRD; Bruker D8 advance) with Cu Ka radiation (40 kV, 200 mA). UV–vis spectra were obtained using a UV2550 spectrometer (Shimadzu, Japan). XPS measurements were recorded on a KRATOA XSAM800 XPS system with a Mg Ka source. 2.5. Photocatalytic experiment Photocatalytic reactions were conducted in a 250 mL glass photoreactor, which was initiated by a high-pressure xenon long-arc lamp (GXH500W; Beijing NBET Technology Co., Ltd.). The photocatalytic activity of the Ag@AgCl–TiO2/FAC composite was evaluated from the degradation of rhodamine B. In each photocatalytic experiment, a certain amount of catalyst was added to 250 mL of rhodamine B (2 mg/L). After stirring the suspensions for 30 min in the dark, the absorption–desorption equilibrium between the catalysts and rhodamine B was established. During the reaction, a 10 mL sample of the reaction solution was obtained every 20 min, followed by centrifugation to remove the catalysts. UV– vis spectrophotometry (UV2550, Shimadzu, Japan) was conducted
ð1Þ
3. Results and discussion 3.1. Morphology and phase structures The morphology of the as-prepared photocatalysts was observed by SEM. Fig. 1 shows the SEM images of FAC, TiO2/FAC, and Ag@AgCl–TiO2/FAC, as well as the local enlargement of Ag@AgCl–TiO2/FAC. All photocatalysts showed spherical structure, and the diameter of the catalysts was approximately 150 lm, thereby indicating that the catalysts retain in their original morphology during the preparation. Fig. 1b shows that a thin film was loaded on the FAC surface; the surface of the TiO2/FAC became rougher, which is beneficial for photodegradation. In addition, a typical SEM micrograph of the Ag@AgCl particles with cube-like structure was found on the TiO2/FAC surface (Fig. 1d). A certain amount of Ag@AgCl particles was loaded on the TiO2/FAC surface, thereby suggesting that these particles were coated well and still maintained their good shape. Fig. 2 shows the XRD patterns of FAC, TiO2/FAC, pure Ag@AgCl, and Ag@AgCl–TiO2/FAC. The diffraction peaks of TiO2/FAC and Ag@AgCl–TiO2/FAC are distinctly different from those of FAC, which is mainly due to the coating of Ag@AgCl and TiO2 film onto the FAC surface. The main peaks of TiO2/FAC and Ag@AgCl–TiO2/ FAC composites located at 25.28°, 37.8°, 48.05°, 53.89°, 55.06°, 62.69°, and 68.76° were indexed to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), and (1 1 6) crystal planes, respectively, of TiO2 anatase phase (JCPDS No. 21-1272). Furthermore, the diffraction peaks attributed to AgCl (JCPDS No. 31-1238) were observed. The main peak of AgCl located at 32.24° corresponds to a crystalline cubic phase with a distance of 2.95 nm, which is indexed as the (2 0 0) peak. In addition, a weak and broad diffraction peak attributed to metallic Ag was observed according to the cubic phase Ag (JCPDS No. 65-2871). That is, metallic Ag was successfully produced by visible irradiation.
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Fig. 1. SEM of (a) FAC, (b) TiO2/FAC, (c) Ag@AgCl–TiO2/FAC and (d) the local enlargement of Ag@AgCl–TiO2/FAC.
the plasmon resonance of the formed Ag N-Ps on the composite surface under visible-light irradiation during material preparation [21]. The bandgap energies of different photocatalysts were calculated based on the optical absorption edge obtained from UV–vis DRS spectra by using the following equation:
aht ¼ kðht EgÞn
Fig. 2. XRD spectra of FAC, TiO2/FAC, pure Ag@AgCl and Ag@AgCl–TiO2/FAC composites.
3.2. DRS and BET surface analysis The optical absorption of the as-prepared Ag@AgCl–TiO2/FAC samples was measured by DRS UV–vis spectroscopy. The absorption peaks of the TiO2/FAC, Ag@AgCl–TiO2/FAC, AgCl–TiO2/FAC, and Ag@AgCl photocatalysts in the visible region are shown in Fig. 3a. The absorption edges of TiO2/FAC and AgCl–TiO2/FAC are approximately 450 nm, thereby showing strong absorption in the UV light region, whereas Ag@AgCl exhibited strong absorption in the visible light range. When Ag@AgCl combined with TiO2/FAC, the absorption edge of Ag@AgCl–TiO2/FAC composites was slightly red-shifted. The visible light adsorption was also greatly enhanced compared with the TiO2/FAC composites. This result may be due to
ð2Þ
where a is the absorption coefficient, k is the parameter that is related to the effective masses associated with the valence and conduction bands, n is 1/2 for a direct transition, ht is the absorption energy, and Eg is the band gap energy [22–25]. The calculated bandgap energy of Ag@AgCl–TiO2/FAC is approximately 2.90 eV, which is consistent with the values reported in literature [26,27]. The DRS UV–vis spectra show that the as-prepared Ag@AgCl–TiO2/FAC is expected to exhibit excellent photocatalytic activity in degrading organic contaminants under visible light. Fig. 3b shows the nitrogen adsorption isotherm for FAC and Ag@AgCl–TiO2/FAC composites. The nitrogen adsorption isotherm clearly belongs to the slow adsorption and desorption (type-III isotherm). As shown in the inset of Fig. 3b, the pore size distribution plot shows that the composites have a broad distribution of pore size; the mean pore size of the composites is slightly larger than that of FAC, which is ascribed to the effect of PVP in the preparation of catalysts. Furthermore, the N2 absorption–desorption parameters of catalysts are displayed in Table 1; the BET specific surface area of Ag@AgCl–TiO2/FAC was 4.48 m2 g1, which is larger than that of FAC (0.38 m2 g1). 3.3. XPS characterization XPS characterization was performed to determine the chemical composition of Ag@AgCl–TiO2/FAC composites and identify the chemical state of elemental Ti and Ag. Fig. 4a shows the XPS survey spectra of Ag@AgCl–TiO2/FAC composites. The composite clearly contains Ti, O, Ag, Cl, and C elements. Fig. 4b illustrates that the binding energies of Ti 2p3/2 and Ti 2p1/2 attributed to Ti4+ (TiO2) are 458.13 and 464.08 eV, respectively, thereby suggesting that
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Fig. 3. (a) UV–Vis DRS spectrum of TiO2/FAC, AgCl–TiO2/FAC, Ag@AgCl–TiO2/FAC and Ag@AgCl composites; (b) N2 adsorption–desorption isotherm and the pore size distribution plot for FAC and Ag@AgCl–TiO2/FAC. The pore size distribution was estimated from the desorption branch of the isotherm.
Table 1 The N2 absorption–desorption parameters of catalysts. Catalysts
BET surface area (m2/g)
Pore volume (cm3/g)
Pore size (nm)
FAC Ag@AgCl–TiO2/FAC
0.38 4.48
0.0109 0.0192
114.51 17.16
photocatalytic DR decreased from 90.12% in the first run to 79.59% in the fourth run, which is ascribed to catalyst washout in the reuse cycles. However, the results indicate that Ag@AgCl– TiO2/FAC composites displayed excellent stability in terms of photocatalytic performance.
3.5. Possible photocatalytic mechanism TiO2 was successfully deposited on FAC via the sol–gel method. Fig. 4c shows the Ag 3d level spectrum. The Ag 3d5/2 and Ag 3d3/2 spin–orbital photoelectrons appeared at binding energies of 366.68 and 372.58 eV, respectively. The Ag 3d5/2 peak is divided into two different peaks of 366.57 and 367.59 eV; the peak of Ag 3d3/2 is divided into 373.36 and 372.45 eV [26,28]. The peaks at 367.59 and 373.36 eV were attributed to metallic silver; the peaks located at 366.57 and 372.45 eV belonged to Ag+, thereby indicating that Ag+ ions were successfully reduced to metallic Ag by visible irradiation.
3.4. Photocatalytic experiment The catalytic activity of Ag@AgCl–TiO2/FAC was evaluated by the degradation of rhodamine B. Fig. 5 shows the photocatalytic degradation rate of rhodamine B over TiO2/FAC, Ag@AgCl, and Ag@AgCl–TiO2/FAC composites. The Ag@AgCl–TiO2/FAC composites clearly exhibited better activity, which was higher than that of other composites. After irradiation for 120 min under visible light, the degradation rates of rhodamine B over Ag@AgCl–TiO2/ FAC, AgCl–TiO2/FAC, and Ag@AgCl were approximately 96.68%, 62.15%, and 72.68%, respectively. For the TiO2/FAC composites and blank controlled experiment, rhodamine B was almost not degraded. The rhodamine B degradation rate with Ag@AgCl–TiO2/ FAC was apparently 1.56 and 1.33 times higher than that with AgCl–TiO2/FAC and Ag@AgCl. The AgCl–TiO2/FAC showed photodegradation of rhodamine B because the Ag+ ions in AgCl–TiO2/ FAC were reduced to metallic Ag by the photocatalytic reaction under visible light. By comparison, Ag@AgCl–TiO2/FAC composites showed better photocatalytic performance because of the effect of Ag N-Ps. Photocatalyst stability is an important factor in practical application. The photocatalytic performance of Ag@AgCl–TiO2/FAC composite in the first four reuse cycles is shown in Fig. 6. The
OH analysis and radical trapping experiments were performed to investigate the degradation mechanism of rhodamine B over the Ag@AgCl–TiO2/FAC photocatalyst. KI and 1,4-benzoquinone (BQ) served as the h+ and O 2 scavengers [29–31], respectively. Fig. 7 shows that the PL emission spectra of the TA solution were obtained at 315 nm excitation for a given duration of visible-light irradiation. A slow increase in the PL intensity at 425 nm was clearly observed with the increasing irradiation time, thereby indicating that fluorescence was produced during the photocatalytic reactions. This trend implies that OH is an active species that participates in photocatalytic reactions. In addition, the photocatalytic degradation of rhodamine B with Ag@AgCl–TiO2/FAC was investigated in the presence of KI or BQ in Fig. 8. The rhodamine B removal efficiency reached 96.68% without KI and BQ within 120 min. However, only 48.26% and 48.09% rhodamine B were decomposed in the presence of 2 mM KI and BQ, respectively. That is, h+ and O 2 played an important role in the photocatalytic degradation of rhodamine B. As discussed above, the Ag@AgCl–TiO2/FAC composites possess excellent photocatalytic activity for decomposing rhodamine B as compared with Ag@AgCl and TiO2/FAC. The enhanced activity should be ascribed to the effect of Ag N-Ps. According to the band structure of Ag@AgCl–TiO2/FAC composites and the results of radical trapping experiments, a photocatalytic activity enhancement mechanism of Ag@AgCl–TiO2/FAC was proposed (Fig. 9). For fly ash cenospheres, the composite provided more adsorption sites for the pollutant than AgCl and TiO2 because of its larger specific surface area; the adsorbed pollutant can also migrate to the decomposition centers located on the catalyst surface. On the other hand, the fly ash cenospheres make photocatalytic reactions react more easily by maintaining a certain number of absorbed intermediate products on their surface. Under visible light, only Ag N-Ps are excited to produce photogenerated electrons and holes. The photogenerated electrons transfer from the valence band to the conduction band and are injected into the CB of TiO2 because of
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Fig. 5. Degradation rates of rhodamine B under visible light irradiation using TiO2/ FAC, Ag@AgCl, AgCl–TiO2/FAC and Ag@AgCl–TiO2/FAC composites.
Fig. 6. Photocatalytic performance of Ag@AgCl–TiO2/FAC composite photocatalyst in the first four reuse cycles.
Fig. 4. XPS spectra of Ag@AgCl–TiO2/FAC photocatalyst (a) survey spectrum, (b) Ti 2p and (c) Ag 3d.
the formed Schottky barrier at the Ag/TiO2 interface [32]. TiO2 possessed a band gap of 3.2 eV; the conduction band gap of TiO2 is 0.7 eV (vs. NHE) [33], which is more negative than the standard redox potential of O2/O 2 (0.046 eV). Consequently, the photogenerated electrons could be transferred to the conduction band of TiO2 and trapped by the absorbed O2 to produce O 2 [34]. In addition, with a large band gap of 3.25 eV, AgCl cannot be excited by visible light, but the lower conduction band potential (0.06 vs. NHE) enabled the transfer of photogenerated electrons over Ag to the conduction band of AgCl. The holes were scavenged by Cl to form Cl0 on the AgCl surface [35] and could be combined with
Fig. 7. OH trapping PL spectra of the Ag@AgCl–TiO2/FAC composite on TA solution under visible light irradiation.
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Acknowledgments This research project was financially supported by: Young and Middle-aged Academic Key Members of Anhui University of Science and Technology, Doctor’s degree Innovation Training Program (2013bj1105), International cooperative project of Anhui Province (12030-603003). References
Fig. 8. Photocatalytic degradation of rhodamine B over the Ag@AgCl–TiO2/FAC particles under different conditions with exposure to visible light.
Fig. 9. Proposed mechanism of photocatalytic degradation for Ag@AgCl–TiO2/FAC under visible light.
H2O molecules to produce OH, which induced the degradation of rhodamine B. These active species decomposed rhodamine B into the final product (i.e., carbon dioxide) or other intermediate products.
4. Conclusion In summary, the Ag@AgCl–TiO2/FAC photocatalysts were prepared using a two-step approach. The catalysts exhibited high photocatalytic activity for degradation of rhodamine B under visible light. The photocatalytic activity of Ag@AgCl–TiO2/FAC photocatalyst can be ascribed to Ag N-Ps, which effectively accelerated electron–hole separation on the surface of the catalyst. Investigation of + the main reactive species indicated that O 2 and h perform key functions in the photocatalytic degradation of rhodamine B, and OH participated in the photocatalytic reactions as an active species. This study is helpful in investigating plasmon photocatalysts activated by visible light.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and characterization of cube-like Ag@AgCl-doped TiO2/fly ash cenospheres with enhanced visible-light photocatalytic activity Shaomin Liu ⇑, Jinglin Zhu, Qing Yang, Pengpeng Xu, Jianhua Ge, Xuetao Guo School of Earth Science and Environmental Engineering, Anhui University of Science and Technology, Huainan 232001, PR China
a r t i c l e
i n f o
Article history: Received 12 November 2015 Received in revised form 3 January 2016 Accepted 8 January 2016
Keywords: Ag@AgCl TiO2 Fly ash cenospheres Photocatalysis
a b s t r a c t A cube-like Ag@AgCl-doped TiO2/fly ash cenosphere composite (denoted Ag@AgCl–TiO2/fly ash cenospheres) was successfully synthesized via a two-step approach. The as-prepared catalysts were characterized by scanning electron microscopy, X-ray diffraction, diffuse reflectance ultraviolet–visible spectroscopy, Brunauer–Emmett–Teller, and X-ray photoelectron spectroscopy. The photocatalytic experiment showed that the rhodamine B degradation rate with Ag@AgCl–TiO2/fly ash cenospheres was 1.56 and 1.33 times higher than that with AgCl–TiO2/fly ash cenospheres and Ag@AgCl, respectively. The degradation ratio of rhodamine B with Ag@AgCl–TiO2/fly ash cenospheres was nearly 100% within + 120 min under visible light. Analysis of active species indicated that O 2 and h dominated the reaction, and OH participated in the photocatalytic reactions as an active species. A mechanism for the photocatalytic degradation by the Ag@AgCl–TiO2/fly-ash cenospheres was also proposed based on the experimental results. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction In the last two decades, titanium dioxide has attracted considerable interest for degradation of various organic pollutants because of its high photocatalytic activity, good stability, and low cost [1–3]. However, the limited UV-induced activity of TiO2 because of its wide bandgap (i.e., 3.2 eV for anatase and 3.0 eV for rutile) has considerably prevented its practical application under natural sunlight. To overcome this drawback, numerous methods have been used to broaden the absorption threshold of TiO2 into the visible region, including nonmetal doping [4–6], metal deposition [7–9], and coupling with other semiconductors [10–12]. Among these methods, the use of noble metal nanoparticles (N-Ps) have received extensive research attention because of the surface plasmon resonance of N-Ps, which accelerates separation of photo-generated electrons and holes on the surface of the semiconductor catalyst. Moreover, Ag N-Ps deposited on semiconductors are believed to show efficient plasmon resonance under visible light. Bi and Ye prepared core–shell Ag/AgCl hetero-nanowires with uniform structures, which were then used for the decomposition of methyl orange dye under visible light by an in situ oxidation reaction at room temperature [13]. Yu et al. deposited Ag/AgCl onto TiO2 nanotube arrays and then reduced partial Ag+ ions of AgCl particles to ⇑ Corresponding author. E-mail address: [email protected] (S. Liu). http://dx.doi.org/10.1016/j.optmat.2016.01.010 0925-3467/Ó 2016 Elsevier B.V. All rights reserved.
Ag0 species, which is efficient for the photocatalytic degradation of methyl orange under xenon lamp irradiation [14]. Hu et al. prepared Ag/AgBr/TiO2 by deposition–precipitation method; this material showed excellent photocatalytic activity for the destruction of azo dyes and bacteria under visible light [15]. However, silver halide-modified titanium dioxide catalysts are difficult to recycle, which limits their practical applications. In addition, such catalysts have low light usage rate because its relatively large size leads particles to sink or become suspended in the solution. To solve these problems, the TiO2/fly ash cenospheres (FAC) were synthesized via a simple sol–gel method and then Ag@AgCl N-Ps were deposited on TiO2/FAC via a two-step approach. The prepared composite showed high photocatalytic activity for the degradation of rhodamine B in aqueous solution under visible light. To elucidate the mechanism of the photocatalytic process, the main oxidative species in the catalytic process were detected through OH analysis and radical trapping experiments. A mechanism for the photocatalytic degradation by Ag@AgCl–TiO2/FAC was also proposed. 2. Experiment 2.1. Chemicals Fly ash was supplied by the Huainan Fossil Fuel Power Station and used with acid pretreatment. Tetrabutyl titanate, absolute ethanol, acetylacetone, polyethylene glycol (PEG), ethanol, AgNO3,
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polyvinylpyrrolidone (PVP), NaCl, and HCl were purchased from Shanghai Sinopharm. Deionized water was prepared by our laboratory.
to record the absorbance of the samples at 550 nm. The photocatalytic degradation rate (DR) was calculated with the following formula:
2.2. Preparation of TiO2/FAC
DR ¼ ½ð1 Ai =A0 Þ 100%
In a typical process, TiO2/FAC was prepared via a simple sol–gel method [16]. In brief, 90 mL of absolute ethanol, 34 mL of tetrabutyl titanate, and 10 mL of acetyl acetone were mixed to form a solution. Another mixing solution containing 50 mL of absolute ethanol and 1.8 mL of deionized water was added drop-wise into the first solution. The pH of the mixed solution was adjusted to 5 with 67% HNO3 with stirring for 1 h at room temperature. Subsequently, 2 g of PEG was added as surfactant with continuous stirring for 1 h in a water bath at 50 °C, and FAC was added with further stirring for 1 h. The abovementioned solution was evaporated until nearly dry at 80 °C in a water bath after aged for 24 h at room temperature, and the remaining solid matter was dried at 80 °C in an oven. Finally, calcination for 2 h resulted in TiO2/ FAC at 500 °C.
where A0 is the initial absorbance of rhodamine B solution at the absorption–desorption equilibrium and Ai is the absorbance of the reaction solution. In addition to the photocatalytic experiment, a similar radical trapping experiment was also performed. The only difference is certain amounts of scavengers were added to the reaction solution to reach the absorption–desorption equilibrium before the operating experiment. The formation of hydroxyl radical (OH) was detected by the PL technique. Terephthalic acid readily serves as a probe molecule that reacts with OH to produce a highly fluorescent product (2-hydroxyterephthalic acid). The PL intensity of 2hydroxyterephtalic acid is proportional to the quantity of OH radicals produced in solution [17–19]. The experimental process is similar to the photocatalytic experiment; the mixed solution, which contains 5 104 M terephthalic acid and 2 103 M NaOH aqueous solution, replaced the rhodamine B solution during the photocatalytic experiment. After visible-light irradiation every 20 min, the reaction solution was applied to determine the increase of the PL intensity at 425 nm excited by 315 nm light [20]. The PL spectra of generated 2-hydroxyterephthalic acid were recorded on a Shimadzu RF-5301PC fluorescence spectrophotometer.
2.3. Synthesis of plasmonic Ag@AgCl–TiO2/FAC composite photocatalysts The Ag@AgCl–TiO2/FAC composite photocatalysts were prepared via a two-step approach. Typically, 0.170 g of AgNO3 was added into a mixture containing 1.0 g of TiO2/FAC, 0.111 g of PVP, 30 mL of deionized water, and 50 mL of ethanol under magnetic stirring, which were transferred into Teflon-lined stainless steel autoclave and heated at 130 °C for 3 h after stirring for 30 min. A 20 mL solution with 0.0293 g of NaCl and an HCl aqueous solution (0.086 mL, 36 wt.%) were gradually added into the abovementioned mixture with further stirring for 24 h in the dark, followed by irradiation for 30 min with a 500 W Xe lamp to deoxidize Ag+ to Ag0. Finally, the obtained particle was collected by centrifugation, washed, and dried at 80 °C for 8 h to obtain the plasmonic Ag@AgCl–TiO2/FAC composite photocatalysts. For comparison, pure Ag@AgCl was synthesized similarly by the abovementioned method without TiO2/FAC. 2.4. Characterization The surface microstructure was investigated with a JSM-6700F by field-emission scanning electron microscopy (SEM). The phase structure was obtained from X-ray diffraction (XRD; Bruker D8 advance) with Cu Ka radiation (40 kV, 200 mA). UV–vis spectra were obtained using a UV2550 spectrometer (Shimadzu, Japan). XPS measurements were recorded on a KRATOA XSAM800 XPS system with a Mg Ka source. 2.5. Photocatalytic experiment Photocatalytic reactions were conducted in a 250 mL glass photoreactor, which was initiated by a high-pressure xenon long-arc lamp (GXH500W; Beijing NBET Technology Co., Ltd.). The photocatalytic activity of the Ag@AgCl–TiO2/FAC composite was evaluated from the degradation of rhodamine B. In each photocatalytic experiment, a certain amount of catalyst was added to 250 mL of rhodamine B (2 mg/L). After stirring the suspensions for 30 min in the dark, the absorption–desorption equilibrium between the catalysts and rhodamine B was established. During the reaction, a 10 mL sample of the reaction solution was obtained every 20 min, followed by centrifugation to remove the catalysts. UV– vis spectrophotometry (UV2550, Shimadzu, Japan) was conducted
ð1Þ
3. Results and discussion 3.1. Morphology and phase structures The morphology of the as-prepared photocatalysts was observed by SEM. Fig. 1 shows the SEM images of FAC, TiO2/FAC, and Ag@AgCl–TiO2/FAC, as well as the local enlargement of Ag@AgCl–TiO2/FAC. All photocatalysts showed spherical structure, and the diameter of the catalysts was approximately 150 lm, thereby indicating that the catalysts retain in their original morphology during the preparation. Fig. 1b shows that a thin film was loaded on the FAC surface; the surface of the TiO2/FAC became rougher, which is beneficial for photodegradation. In addition, a typical SEM micrograph of the Ag@AgCl particles with cube-like structure was found on the TiO2/FAC surface (Fig. 1d). A certain amount of Ag@AgCl particles was loaded on the TiO2/FAC surface, thereby suggesting that these particles were coated well and still maintained their good shape. Fig. 2 shows the XRD patterns of FAC, TiO2/FAC, pure Ag@AgCl, and Ag@AgCl–TiO2/FAC. The diffraction peaks of TiO2/FAC and Ag@AgCl–TiO2/FAC are distinctly different from those of FAC, which is mainly due to the coating of Ag@AgCl and TiO2 film onto the FAC surface. The main peaks of TiO2/FAC and Ag@AgCl–TiO2/ FAC composites located at 25.28°, 37.8°, 48.05°, 53.89°, 55.06°, 62.69°, and 68.76° were indexed to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), and (1 1 6) crystal planes, respectively, of TiO2 anatase phase (JCPDS No. 21-1272). Furthermore, the diffraction peaks attributed to AgCl (JCPDS No. 31-1238) were observed. The main peak of AgCl located at 32.24° corresponds to a crystalline cubic phase with a distance of 2.95 nm, which is indexed as the (2 0 0) peak. In addition, a weak and broad diffraction peak attributed to metallic Ag was observed according to the cubic phase Ag (JCPDS No. 65-2871). That is, metallic Ag was successfully produced by visible irradiation.
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Fig. 1. SEM of (a) FAC, (b) TiO2/FAC, (c) Ag@AgCl–TiO2/FAC and (d) the local enlargement of Ag@AgCl–TiO2/FAC.
the plasmon resonance of the formed Ag N-Ps on the composite surface under visible-light irradiation during material preparation [21]. The bandgap energies of different photocatalysts were calculated based on the optical absorption edge obtained from UV–vis DRS spectra by using the following equation:
aht ¼ kðht EgÞn
Fig. 2. XRD spectra of FAC, TiO2/FAC, pure Ag@AgCl and Ag@AgCl–TiO2/FAC composites.
3.2. DRS and BET surface analysis The optical absorption of the as-prepared Ag@AgCl–TiO2/FAC samples was measured by DRS UV–vis spectroscopy. The absorption peaks of the TiO2/FAC, Ag@AgCl–TiO2/FAC, AgCl–TiO2/FAC, and Ag@AgCl photocatalysts in the visible region are shown in Fig. 3a. The absorption edges of TiO2/FAC and AgCl–TiO2/FAC are approximately 450 nm, thereby showing strong absorption in the UV light region, whereas Ag@AgCl exhibited strong absorption in the visible light range. When Ag@AgCl combined with TiO2/FAC, the absorption edge of Ag@AgCl–TiO2/FAC composites was slightly red-shifted. The visible light adsorption was also greatly enhanced compared with the TiO2/FAC composites. This result may be due to
ð2Þ
where a is the absorption coefficient, k is the parameter that is related to the effective masses associated with the valence and conduction bands, n is 1/2 for a direct transition, ht is the absorption energy, and Eg is the band gap energy [22–25]. The calculated bandgap energy of Ag@AgCl–TiO2/FAC is approximately 2.90 eV, which is consistent with the values reported in literature [26,27]. The DRS UV–vis spectra show that the as-prepared Ag@AgCl–TiO2/FAC is expected to exhibit excellent photocatalytic activity in degrading organic contaminants under visible light. Fig. 3b shows the nitrogen adsorption isotherm for FAC and Ag@AgCl–TiO2/FAC composites. The nitrogen adsorption isotherm clearly belongs to the slow adsorption and desorption (type-III isotherm). As shown in the inset of Fig. 3b, the pore size distribution plot shows that the composites have a broad distribution of pore size; the mean pore size of the composites is slightly larger than that of FAC, which is ascribed to the effect of PVP in the preparation of catalysts. Furthermore, the N2 absorption–desorption parameters of catalysts are displayed in Table 1; the BET specific surface area of Ag@AgCl–TiO2/FAC was 4.48 m2 g1, which is larger than that of FAC (0.38 m2 g1). 3.3. XPS characterization XPS characterization was performed to determine the chemical composition of Ag@AgCl–TiO2/FAC composites and identify the chemical state of elemental Ti and Ag. Fig. 4a shows the XPS survey spectra of Ag@AgCl–TiO2/FAC composites. The composite clearly contains Ti, O, Ag, Cl, and C elements. Fig. 4b illustrates that the binding energies of Ti 2p3/2 and Ti 2p1/2 attributed to Ti4+ (TiO2) are 458.13 and 464.08 eV, respectively, thereby suggesting that
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Fig. 3. (a) UV–Vis DRS spectrum of TiO2/FAC, AgCl–TiO2/FAC, Ag@AgCl–TiO2/FAC and Ag@AgCl composites; (b) N2 adsorption–desorption isotherm and the pore size distribution plot for FAC and Ag@AgCl–TiO2/FAC. The pore size distribution was estimated from the desorption branch of the isotherm.
Table 1 The N2 absorption–desorption parameters of catalysts. Catalysts
BET surface area (m2/g)
Pore volume (cm3/g)
Pore size (nm)
FAC Ag@AgCl–TiO2/FAC
0.38 4.48
0.0109 0.0192
114.51 17.16
photocatalytic DR decreased from 90.12% in the first run to 79.59% in the fourth run, which is ascribed to catalyst washout in the reuse cycles. However, the results indicate that Ag@AgCl– TiO2/FAC composites displayed excellent stability in terms of photocatalytic performance.
3.5. Possible photocatalytic mechanism TiO2 was successfully deposited on FAC via the sol–gel method. Fig. 4c shows the Ag 3d level spectrum. The Ag 3d5/2 and Ag 3d3/2 spin–orbital photoelectrons appeared at binding energies of 366.68 and 372.58 eV, respectively. The Ag 3d5/2 peak is divided into two different peaks of 366.57 and 367.59 eV; the peak of Ag 3d3/2 is divided into 373.36 and 372.45 eV [26,28]. The peaks at 367.59 and 373.36 eV were attributed to metallic silver; the peaks located at 366.57 and 372.45 eV belonged to Ag+, thereby indicating that Ag+ ions were successfully reduced to metallic Ag by visible irradiation.
3.4. Photocatalytic experiment The catalytic activity of Ag@AgCl–TiO2/FAC was evaluated by the degradation of rhodamine B. Fig. 5 shows the photocatalytic degradation rate of rhodamine B over TiO2/FAC, Ag@AgCl, and Ag@AgCl–TiO2/FAC composites. The Ag@AgCl–TiO2/FAC composites clearly exhibited better activity, which was higher than that of other composites. After irradiation for 120 min under visible light, the degradation rates of rhodamine B over Ag@AgCl–TiO2/ FAC, AgCl–TiO2/FAC, and Ag@AgCl were approximately 96.68%, 62.15%, and 72.68%, respectively. For the TiO2/FAC composites and blank controlled experiment, rhodamine B was almost not degraded. The rhodamine B degradation rate with Ag@AgCl–TiO2/ FAC was apparently 1.56 and 1.33 times higher than that with AgCl–TiO2/FAC and Ag@AgCl. The AgCl–TiO2/FAC showed photodegradation of rhodamine B because the Ag+ ions in AgCl–TiO2/ FAC were reduced to metallic Ag by the photocatalytic reaction under visible light. By comparison, Ag@AgCl–TiO2/FAC composites showed better photocatalytic performance because of the effect of Ag N-Ps. Photocatalyst stability is an important factor in practical application. The photocatalytic performance of Ag@AgCl–TiO2/FAC composite in the first four reuse cycles is shown in Fig. 6. The
OH analysis and radical trapping experiments were performed to investigate the degradation mechanism of rhodamine B over the Ag@AgCl–TiO2/FAC photocatalyst. KI and 1,4-benzoquinone (BQ) served as the h+ and O 2 scavengers [29–31], respectively. Fig. 7 shows that the PL emission spectra of the TA solution were obtained at 315 nm excitation for a given duration of visible-light irradiation. A slow increase in the PL intensity at 425 nm was clearly observed with the increasing irradiation time, thereby indicating that fluorescence was produced during the photocatalytic reactions. This trend implies that OH is an active species that participates in photocatalytic reactions. In addition, the photocatalytic degradation of rhodamine B with Ag@AgCl–TiO2/FAC was investigated in the presence of KI or BQ in Fig. 8. The rhodamine B removal efficiency reached 96.68% without KI and BQ within 120 min. However, only 48.26% and 48.09% rhodamine B were decomposed in the presence of 2 mM KI and BQ, respectively. That is, h+ and O 2 played an important role in the photocatalytic degradation of rhodamine B. As discussed above, the Ag@AgCl–TiO2/FAC composites possess excellent photocatalytic activity for decomposing rhodamine B as compared with Ag@AgCl and TiO2/FAC. The enhanced activity should be ascribed to the effect of Ag N-Ps. According to the band structure of Ag@AgCl–TiO2/FAC composites and the results of radical trapping experiments, a photocatalytic activity enhancement mechanism of Ag@AgCl–TiO2/FAC was proposed (Fig. 9). For fly ash cenospheres, the composite provided more adsorption sites for the pollutant than AgCl and TiO2 because of its larger specific surface area; the adsorbed pollutant can also migrate to the decomposition centers located on the catalyst surface. On the other hand, the fly ash cenospheres make photocatalytic reactions react more easily by maintaining a certain number of absorbed intermediate products on their surface. Under visible light, only Ag N-Ps are excited to produce photogenerated electrons and holes. The photogenerated electrons transfer from the valence band to the conduction band and are injected into the CB of TiO2 because of
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Fig. 5. Degradation rates of rhodamine B under visible light irradiation using TiO2/ FAC, Ag@AgCl, AgCl–TiO2/FAC and Ag@AgCl–TiO2/FAC composites.
Fig. 6. Photocatalytic performance of Ag@AgCl–TiO2/FAC composite photocatalyst in the first four reuse cycles.
Fig. 4. XPS spectra of Ag@AgCl–TiO2/FAC photocatalyst (a) survey spectrum, (b) Ti 2p and (c) Ag 3d.
the formed Schottky barrier at the Ag/TiO2 interface [32]. TiO2 possessed a band gap of 3.2 eV; the conduction band gap of TiO2 is 0.7 eV (vs. NHE) [33], which is more negative than the standard redox potential of O2/O 2 (0.046 eV). Consequently, the photogenerated electrons could be transferred to the conduction band of TiO2 and trapped by the absorbed O2 to produce O 2 [34]. In addition, with a large band gap of 3.25 eV, AgCl cannot be excited by visible light, but the lower conduction band potential (0.06 vs. NHE) enabled the transfer of photogenerated electrons over Ag to the conduction band of AgCl. The holes were scavenged by Cl to form Cl0 on the AgCl surface [35] and could be combined with
Fig. 7. OH trapping PL spectra of the Ag@AgCl–TiO2/FAC composite on TA solution under visible light irradiation.
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Acknowledgments This research project was financially supported by: Young and Middle-aged Academic Key Members of Anhui University of Science and Technology, Doctor’s degree Innovation Training Program (2013bj1105), International cooperative project of Anhui Province (12030-603003). References
Fig. 8. Photocatalytic degradation of rhodamine B over the Ag@AgCl–TiO2/FAC particles under different conditions with exposure to visible light.
Fig. 9. Proposed mechanism of photocatalytic degradation for Ag@AgCl–TiO2/FAC under visible light.
H2O molecules to produce OH, which induced the degradation of rhodamine B. These active species decomposed rhodamine B into the final product (i.e., carbon dioxide) or other intermediate products.
4. Conclusion In summary, the Ag@AgCl–TiO2/FAC photocatalysts were prepared using a two-step approach. The catalysts exhibited high photocatalytic activity for degradation of rhodamine B under visible light. The photocatalytic activity of Ag@AgCl–TiO2/FAC photocatalyst can be ascribed to Ag N-Ps, which effectively accelerated electron–hole separation on the surface of the catalyst. Investigation of + the main reactive species indicated that O 2 and h perform key functions in the photocatalytic degradation of rhodamine B, and OH participated in the photocatalytic reactions as an active species. This study is helpful in investigating plasmon photocatalysts activated by visible light.
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