Enhanced Photocatalytic Activity Based on Composite Structure with

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Enhanced Photocatalytic Activity Based on Composite Structure with Downconversion Material and Graphene Ke Fu,† Jinzhao Huang,*,† Nannan Yao,† Xijin Xu,*,† and Mingzhi Wei‡ †

School of Physics and Technology, University of Jinan, Jinan 250022, Shandong Province, People’s Republic of China School of Material Science and Engineering, Qilu University of Technology, Jinan 250353, Shandong Province, People’s Republic of China

‡

ABSTRACT: TiO2 nanorods arrays (TiO2 NRAs):Eu3+, Tb3+ decorated with Cs2CO3/CdS grown on carbon textiles coated with a TiO2−graphene (TiO2-G) thin film (TiO2-G/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3/CdS) was fabricated. The as-prepared sample exhibits an excellent photodegradation performance under xenon (Xe) lamp light irradiation. The result indicates that the presence of Eu3+, Tb3+ can be excited by ultraviolet light to produce a high intensity visibe light absorbed by CdS, which can generate more photogenerated electron hole pairs. In addition, the presence of graphene is an important factor in accelerating the transferring of photogenerated electron to carbon textiles for photocatalytic reaction. This work can offer an alternative route for designing a hybrid structure system to utilize effectively broad-spectrum solar light and suppress the recombination of photogenerated electron−hole pairs.

1. INTRODUCTION Since A. Fujishima and K. Honda reported that the hydrogen production phenomenon on the TiO2 electrode in 1972,1 the semiconductor photocatalysis has been extensively studied on the application of environmental pollution abatement due to its physical and chemical stability, environmental friendliness, low cost and its strong oxidizing power under ultraviolet (UV) light and so on.2−5 Recent works have focused on one-dimensional TiO2 nanostructures, particularly TiO2 nanorods arrays (NRAs) because of their superior photocatalytic and photoelectronic performance.6−11 In addition, metal oxides such as TiO2, ZnO are the wide band gap semiconductors determining that they can only absorb UV light (UV light only occupies 5% of the solar energy) to stimulate electron−hole pairs, which results in a low energy conversion efficiency.12 To increase the utilization of solar energy, a series of innovative work including dye sensitization, doping by metals or nonmetals, coupling with low band gap semiconductors and modifying with carbon-based materials have been investigated.13−17 Among them, decoration with CdS has been considered to be an important technique because CdS with a band gap of 2.4 eV matched well with the visible part of solar energy, which can enhance the visible light harvesting ability of TiO2 and ZnO photocatalysts.18−25 Moreover, doping of relevant element in TiO2 nanostructures can alter its intrinsic physical properties, such as luminescence and electronic properties. Rare earth ions can become the better luminescence center based on its special 4f electron transition at difference energy level.26 Rare earth doped TiO2 has become one important way to extend the optical absorption edge of TiO2.27,28 Our earlier work proved that multiphoton upconversion processes can enhance the photocatalytic performance of TiO2 by converting near-infrared light to visible light.29 However, a problem of the lower utilization efficiency of luminescence intensity of upconversion compared to the downconversion process still exists in such © XXXX American Chemical Society

photocatalytic systems. Thus, it is necessary to improve the photocatalytic activity of composite structure by doping with downconversion materials. Among the downconversion materials, Eu and Tb are more attractive due to their intensive luminescence. Downconversion can improve the energy conversion efficiency and generate more photogenerated carriers by stimulating a stronger visible light absorbed by CdS, thus the photocatalytic activity can be enhanced effectively. Meanwhile, much attention has been paid to graphene due to its fascinating physical properties and potential various applications.30 In the photocatalytic reactions, it is postulated that graphene can accelerate the separation of photogenerated electron−hole, resulting in a higher photocatalytic performance.31 Considering the excellent electron mobility, it is expected that the photocatalytic performance of photocatalysts can be further improved by the effective modification of graphene. On the other hand, TiO2 NRAs provide better conductivity than TiO2 particles for enhancing charge transport due to the single crystal structure, however, which is not enough to obtain even a reasonable performance for a large amount of surface defects.32 The Cs2CO3 with relatively high electron mobility and environmental stability determine that it can be used to reduce defects on the surface of the ordered TiO2 NRAs.33 To the best of our knowledge, the composite structure of TiO2-G/ TiO2 NRAs:Eu3+, Tb3+/Cs2CO3/CdS grown on carbon textiles used in photocatalytic degradation is rare. Carbon textiles are woven by carbon fibers with high conductivity and have a higher specific surface area, increasing the contact area of catalyst with the pollutants. Received: October 28, 2015 Revised: January 10, 2016 Accepted: January 26, 2016

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DOI: 10.1021/acs.iecr.5b04076 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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electron microscopy (FE-SEM, Quanta FEG250). Using a D8 ADVANCE with Cu Kα at λ = 0.154 06 nm invested the structure of samples. The UV−vis diffuse reflectance spectra (DRS) and UV−vis absorption spectra of samples were examined with a UV−vis spectrophotometer (TU-1901). XPS spectra were recorded on a Thermo Fisher ESCALAB 250Xi system with Al Kα radiation, operated at 150 W. 2.4. Potocurrent Measurement. The photocurrent by irradiating the photoanode with a xenon (Xe) lamp (153 mW/ cm2) was recorded with an electrochemical workstation (CS2350) with 1 M KOH as an electrolyte. The photoelectrochemical cell was a three-electrode system: The prepared samples of the composite structure, Ag/AgCl electrode, and Pt electrode acted as the working, reference, and counter electrodes, respectively. 2.5. Measurement of Photocatalytic Activity. The photocatalytic activity of samples were evaluated by the degradation of RhB. A 500 W Xe lamp was used as the solarsimulated light source. The samples were placed into 10 mL RhB solution (5 mg/L). After irradiation for a designated time (20 min), 3 mL of the RhB aqueous solution was taken out to identify the concentration of RhB. All of these measurements were carried out at room temperature.

In this study, the composite structure of TiO2-G/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3 /CdS has been prepared (Figure 1). The significant enhancement of photocatalytic activities was

Figure 1. Schematic illustration for the preparation of TiO2-G/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3 /CdS.

observed. The photoelectrochemical properties of the samples were analyzed. The proposed mechanism of the photocatalyst was also discussed.

3. RESULTS AND DISCUSSION 3.1. Characterization and Morphology. Figure 2 shows the SEM images of the synthesized catalysts. Figure 2b clearly shows that the TiO2-G thin film has been successfully coated on the carbon microfiber cores and the graphene is evenly dispersed. In addition, the addition of Eu3+ and Tb3+ has no effect on the morphology of the TiO2 NRAs (Figure 2d). As shown in Figure 2c,e, the TiO2 NRAs has grown on TiO2-G and TiO2 thin film successfully, and their morphologies are almost the same. Above all, the TiO2 NRAs uniformly distributed on the carbon microfiber cores and the diameter of the TiO2 NRAs is uniform. At the same time, the CdS thin film is uniformly packed on the surface of TiO2 NRAs, as is shown in the inset of Figure 2h. These results indicate that morphology has no impact on the photocatalytic performance of the catalyst. The XRD patterns for samples are shown in Figure 3. All the samples were composed of anatase TiO2 and rutile TiO2. In curve a, the diffraction peaks at 25.4°, 27.4°, 36.1°, 41.2°, 53.8°, 62.6°, and 69.0° are observed. These peaks could be perfectly indexed to the (101), (110), (004), (111), (105), (204), (116), and (220) planes of anatase and rutile TiO2 (JCPDS 21-1272). Among them, the majority of crystal phase was anatase and the crystallite size of TiO2 is 19.6 nm. From Figure 3, we do not find the diffraction peaks indicative of Eu3+, Tb3+, which may be attributed to the amount of doping is extremely low or the rare earth ions are doped into the TiO2 crystal lattice.34 In the XRD pattern of the TiO2-G/TiO2 NRAs:Eu3+, Tb3+ sample, graphene peaks were not observed and structure of TiO2 was nearly unchanged. It may be due to the fact that the characteristic (002) peak at 25.9° of graphene is weak and might overlaps with the (101) peak of anatase TiO2 (25.4°) and the graphene does not affect the crystal structure of TiO2.35 As shown in Figure 4, the chemical consists of TiO2 NRAs:Eu3+, Tb3+ were confirmed by XPS results. The presence of Tb3+ was confirmed by the characteristic peaks at 1239.77 eV (Figure 4a). Figure 4b shows the Eu 3d XPS spectrum of TiO2 NRAs:Eu3+, Tb3+. There is a peak located at 1134.065 eV, which is attributed to Eu 3d5/2.36 The peak located at 529.13 eV

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Carbon textiles (W0S1002) were purchased from TaiWan CeTech Co., Ltd. Graphene were purchased from Jining Leader Nano Technology Co., Ltd. All reagents were of analytical grade and the solvent was ultrapure water. Before use, carbon textiles was ultrasonically cleaned for 20 min with acetone, ethanol, and distilled water respectively, and then dried in an oven at 80 °C. 2.2. Preparation of Photocatalysts. The TiO2 sol and TiO2 NRAs were prepared using a previously reported method.29 For the graphene doping experiment, a certain amount of graphene was added into the transparent faint yellow solution. The concentration of graphene is 1 mg mL−1 (20 mL) was used in this doping experiment. Finally, the solution is sonicated for 3 h to form a uniform dispersion of TiO2-G sol. To facilitate the nucleation of TiO2 NRAs, the TiO2 sol was deposited on the carbon microfiber cores by immersing method, and the optimum coating condition of a 2 cm × 3 cm carbon textiles substrate was immersed in the TiO2 sol in for 30 s. And then the seed layer was calcinated at 400 °C for 1 h. For the Eu3+ and Tb3+ doping experiment, the repuired amount of the Eu3+ and Tb3+ were added into the nitric acid− ethanol solution. The Ti:rare earth mole ratio 1:0.01 and the mole ratio of Eu3+ to Tb3+ is 1:1 used in this doping experiment. Cs2CO3 thin film was fabricated on the TiO2 NRAs by immersing method. The TiO2 NRAs were immersed in the ethylene glycol monomethyl ether solution containing Cs2CO3 (2 mg/mL) for 20 S. And then the thin film was calcinated at 170 °C for 30 min. To deposit CdS, a TiO2 NRAs substrate was immersed in an ethanol solution containing Cd(NO3)2 (0.05 M) for 10 s, and rinsed with ethanol. It was then immersed in a Na2S methanol solution (0.05 M) for another 10 s, and rinsed with methanol. The incorporated amount of CdS can be increased by repeating the deposition cycles. 2.3. Characterization. The surface morphology of the samples was investigated using a field emission scanning B

DOI: 10.1021/acs.iecr.5b04076 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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catalytic performances of different samples toward to the degradation of RhB dyes under a Xe lamp (500 W) irradiation are present in Figure 5. There is only little change in the concentration of the solution when the RhB solution is irradiated without any catalyst. After 100 min of a Xe lamp irradiation, the degradation ratio of RhB was almost 95% in the presence of TiO2-G/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3/CdS, whereas 60% and 20% of RhB was decomposed by TiO2-G/ TiO2 NRAs/Cs2CO3/CdS, and TiO2/TiO2 NRAs/CdS, respectively. The TiO2-G/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3/ CdS exhibits the best photocatalytic activities, compared to other samples. A very important reason for the obvious advantage of TiO2-G/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3/CdS on photodegradation of RhB solution is that Eu3+ and Tb3+ play a significant role in converting UV light to visible light. The converted visible light was absorbed by CdS, and more photon energy has been used to exicite CdS to generate more carriers. In addition, TiO2-G/TiO2 NRAs:Eu3+, Tb3+/CdS exhibits a slightly better photocatalytic activity than TiO2/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3/CdS, which could degrade 80% and 70% of RhB in 100 min, respectively. As for the decrease in the photocataiytic activity of TiO2-G/TiO2 NRAs:Eu3+, Tb3+/CdS and TiO2/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3/CdS, compared with TiO2-G/TiO2 NRAs:Eu3+, Tb3+ /Cs2CO3/CdS. It is likely that the lack of Cs2CO3 or graphene will weaken the performance of photocatalysis. The introduction of Cs2CO3 layer improving the efficiency of photoelectron transport in the TiO2 nanorods by reducing the defects acting as the recombination center of the photoelectrons on the surface of TiO2 nanorod, and graphene as an electron sink to accept the photoelectrons from the excited TiO2NRAs:Eu3+, Tb3+/ Cs2CO3/CdS, which reduces the recombination of photoelectron−hole pairs leading to higher photocatalytic activity. In order to investigate the stability of the TiO2-G/TiO2 NRAs:Eu3+,Tb3+/Cs2CO3/CdS in the process of photocatalysis, we also carried out repeated photodegradation experiments of the TiO2-G/TiO2 NRAs:Eu3+,Tb3+/Cs2CO3/CdS and the results (Figure 6) show that the photocatalytic efficiency of the TiO2-G/TiO2 NRAs:Eu3+, Tb3+/CdS during RhB degradation does not show significant decline after six cyclic RhB degradation tests, indicating that TiO 2 -G/TiO 2 NRAs:Eu3+,Tb3+/Cs2CO3/CdS have high stability under Xe lamp irradiation. 3.3. Optical and Photoelectrochemical Properties. To confirm the above discussion, the downconversion photoluminescence spectra and photoluminescence excitation spectra of TiO2 NRAs:Eu3+, Tb3+ were measured. The solar radiation spectrum is shown in the Figure 7a. The UV−vis spectroscopy of CdS reveals that CdS has a relatively strong absorption in the visible region (Figure 7d). The emission peaks of TiO2 NRAs:Eu3+, Tb3+ locate in the visible region and match well with the absorption region of the CdS which indicates that the presence of Eu3+, Tb3+ can be excited by UV light to produce a high intensity visibe light absorbed by CdS. Therefore, the addition of Eu3+, Tb3+ can increase the utilization of solar energy for TiO2-G/TiO2 NRAs:Eu3+, Tb3+ /Cs2CO3/CdS. The catalyst can absorb more solar energy to generate more carriers. Consequently, the photocatalytic performance of the composite structure is improved. To understand the role that the Eu3+ and Tb3+ played during the light absorption, the UV−vis absorption was investigated. Figure 8 shows the UV−vis absorption spectra in a wavelength range of 400−600 nm. The spectra of TiO2 /TiO2 NRAs/

Figure 2. SEM images of (a) carbon textiles; (b) TiO2-G; (c)TiO2/ TiO2 NRAs; (d) TiO2/TiO2 NRAs:Eu3+, Tb3+; (e) TiO2-G/TiO2 NRAs; (f) TiO2-G/TiO2 NRAs:Eu3+, Tb3+; (g) TiO2-G/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3; (h) TiO2/TiO2 NRAs/CdS. The inset of panel h shows high-magnification SEM image of TiO2/TiO2 NRAs/ CdS.

Figure 3. XRD patterns of (a) TiO2-G/TiO2 NRAs:Eu3+, Tb3+; (b) TiO2/TiO2 NRAs:Eu3+, Tb3+; (c) TiO2/TiO2 NRAs.

is related to the oxygen bonded with metal as Ti−O (Figure 4c).37 It can be seen that the spectra of catalysis showed two peaks at 458.3 and 464.1 eV. These peaks can be assigned to the 3d5/2 and 3d3/2 spin orbit components of Ti4+ species (Figure 4d).38 3.2. Photocatalytic Activities. For testing the photocatalytic performance of the as-obtained samples, the photoC

DOI: 10.1021/acs.iecr.5b04076 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. XPS spectra of (a) Tb 3d, (b) Eu 3d, (c) O 1s, and (d) Ti 2p of TiO2 NRAs:Eu3+, Tb3+.

Figure 7. (a) Spectrum of solar radiation; (b) excitation spectrum of TiO2 NRAs:Eu3+, Tb3+; (c) photoluminescence spectrum of TiO2 NRAs:Eu3+, Tb3+ with excitation wavelength of 320 nm; (d) UV−vis absorption spectrum of CdS.

Figure 5. Photocatalytic degradation of RhB in the presence of various catalysts.

Cs2CO3/CdS and TiO2/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3/CdS show exhibit onset absorption at around 516 nm, which corresponds to the band gap of CdS (2.4 eV). However, the absorption intensity of each sample is remarkably different. Relatively, TiO2/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3/CdS exhibits an increasing absorption. The enhancement of visible light absorption is probably due to that Eu3+, Tb3+ acts as a medium for converting UV to visble light, thus producing a high intensity visibe light absorbed by CdS. Evidently, the higher intensity of visible-light absorption math well with result of photocatalytic test. TiO2-G/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3/CdS exhibit excellent photocatalytic performance. I-t and I−V curves of samples under a Xe lamp illumination were used to illustrate the changes of photogenerated carriers in photocatalytic activity process. I−t response curves for different samples are shown in Figure 9a. In the dark, the current responses were weak. Under Xe lamp illumination, all of the other samples exhibited stronger photocurrents compared with TiO2/TiO2 NRAs/CdS.

Figure 6. Recycle of TiO2-G/TiO2 NRAs:Eu3+,Tb3+/Cs2CO3/CdS under Xe lamp irradiation.

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DOI: 10.1021/acs.iecr.5b04076 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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more photogenerated electron hole pairs. It is also noted that the addition of graphene and Cs2CO3 contributes to the enhancement of photocurrent. The TiO2 nanorods decorated with Cs2CO3 accelerate the electronic collection efficiency and improve electron lifetimes, and the graphene as an excellent electron-transport material greatly enhanced the photogenerated electrons transfer from conduction band (CB) of TiO2 to carbon textiles, which reflects much higher separation efficiency separation of photogeneratad electrons and holes. Figure 9b I−V curves of the samples further confirm the photocurrent density of TiO2-G/TiO2 NRAs:Eu3+, Tb3+/ Cs2CO3/CdS is much higher than other samples. When the bias potential is larger than 0.3 V, the I−V curve of TiO2-G/ TiO2 NRAs:Eu3+, Tb3+/Cs2CO3/CdS exhibits the sharpest increase in current. The slopes of photocurrents are in the order TiO2-G/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3/CdS > TiO2G/TiO2 NRAs:Eu3+, Tb3+/CdS > TiO2/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3/CdS > TiO2-G/TiO2 NRAs/Cs2CO3/CdS > TiO2/TiO2 NRAs/CdS, suggesting better charge separation, electron collection, and faster interfacial charge transfer. These results related to the photocurrent were similar to the results of photocatalytic tests. Impedance spectroscopy is a powerful tool to characterize the electric properties of semiconductor nanocomposites and has been widely employed in photoelectrochemical systems, and the diameter of the Nyquist plot reflects the charge transfer resistance between the electrode and electrolyte. Figure 10 presents a decreased semicircle diameter

Figure 8. UV−vis absorption spectra of TiO2/TiO2 NRAs/Cs2CO3/ CdS and TiO2/TiO2 NRAs:Eu3+, Tb3+/Cs2CO3/CdS.

Figure 10. EIS spectra of TiO2/TiO2 NRAs and TiO2/TiO2 NRAs/ Cs2CO3 measured under light.

after the TiO2/TiO2 NRAs/Cs2CO3 measured under illumination, confirming that the Cs2CO3 thin film can enhance the carrier mobility at the interface of TiO2 NRAs and photoelectrolyte. Figure 11 shows the UV−vis DRS of TiO2 and TiO2-G. A slightly red shift of the absorption edge is seen for TiO2-G compared to TiO2. The inset shows the modified Kubelka− Munk function [(ahν)1/2] plotted against hν. Bandgap of TiO2 and the composites obtained from these plots are 2.87 and 3.12 eV for TiO2-G and TiO2, respectively. Thus, a clear decrease in bandgap of TiO2 is observed as a result of graphene doping in TiO2. It is interesting to explore the reasonable reaction mechanism for the enhanced photocatalytic performance of TiO2-G/TiO2 NRAs:Eu3+, Tb3+ /Cs2CO3/CdS. In general, the down-

Figure 9. Photoinduced I−t curves (a) and I−V curves (b) of the composite structures.

Among the other samples, the TiO2-G/TiO2 NRAs:Eu3+, Tb3+/ Cs2CO3/CdS possessed the hightest photocurrent density. Moreover, the lack of Eu3+ and Tb3+ resulted in the most obvious weakening of photocurrent density. It is postulated that TiO2 NRAs:Eu3+, Tb3+ absorb UV light, while also excited by UV light, acting as a medium for converting UV to visible light, which means CdS can absorb more visible light to produce E

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YCXS15006), National Natural Science Foundation of China (Grant No. 61106059, 11304120, 61504048, 21505050), the Encouragement Foundation for Excellent Middle-aged and Young Scientist of Shandong Province (Grant No. BS2014CL012), the Science-Technology Program of Higher Education Institutions of Shandong Province (Grant No. J14LA01), the Natural Science Foundation of Shandong Provience (Grant No. ZR2013AM008).

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(1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37. (2) Patel, N.; Jaiswal, R.; Warang, T.; Scarduelli, G.; Dashora, A.; Ahuja, B. L.; Kothari, D. C.; Miotello, A. Efficient Photocatalytic Degradation of Organic Water Pollutants Using V−N-codoped TiO2 Thin Films. Appl. Catal., B 2014, 150, 74. (3) Spasiano, D.; Rodriguez, L. D. P. P.; Olleros, J. C.; Malato, S.; Marotta, R.; Andreozzi, R. TiO2/Cu(II) Photocatalytic Production of Benzaldehyde from Benzyl Alcohol in Solar Pilot Plant Reactor Langmuir. Appl. Catal., B 2013, 136, 56. (4) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69. (5) Safa, S.; Azimirad, R. Enhanced UV-Detection and Photocatalytic Performance of TiO2-SWNTs Nanocomposite Fabricated by Facile Wetness-Impregnation Method. Chinese J. Phys. 2014, 52, 1156. (6) Murakami, N.; Katayama, S.; Nakamura, M.; Tsubota, T.; Ohno, T. Dependence of Photocatalytic Activity on Aspect Ratio of ShapeControlled Rutile Titanium(IV)Oxide Nanorods. J. Phys. Chem. C 2011, 115, 419. (7) Yun, H. J.; Lee, H.; Joo, J. B.; Kim, W.; Yi, J. Influence of Aspect Ratio of TiO2 Nanorods on the Photocatalytic Decomposition of Formic Acid. J. Phys. Chem. C 2009, 113, 3050. (8) Yan, K.; Wu, G.; Jarvis, C.; Chen, A. Facile Synthesis of Porous Microspheres Composed of TiO2 Nanorods with High Photocatalytic Activity for Hydrogen Production. Appl. Catal., B 2014, 148, 281. (9) Yu, Y.; Xu, D. Single-crystalline TiO2 Nanorods: Highly Active and Easily Recycled Photocatalysts. Appl. Catal., B 2007, 73, 166. (10) Cho, I. S.; Chen, Z.; Forman, A. J.; Kim, D. R.; Rao, P. M.; Jaramillo, T. F.; Zheng, X. Branched TiO2 nanorods for photoelectrochemical hydrogen production. Nano Lett. 2011, 11, 4978. (11) Kinadjian, N.; Bechec, M. L.; Henrist, C.; Prouzet, E.; Lacombe, S.; Backov, R. Varying TiO2 Macroscopic Fiber Morphologies toward Tuning Their Photocatalytic Properties. ACS Appl. Mater. Interfaces 2014, 6, 11211. (12) Wang, S.; Zhao, L.; Bai, L.; Yan, J.; Jiang, Q.; Lian, J. Enhancing Photocatalytic Activity of Disorder-engineered C/TiO2 and TiO2 Nanoparticles. J. Mater. Chem. A 2014, 2, 7439. (13) Jiang, Z.; Liu, D.; Jiang, D.; Wei, W.; Qian, K.; Chen, M.; Xie, J. Bamboo Leaf-assisted Formation of Carbon/nitrogen Co-doped Anatase TiO2 Modified with Silver and Graphitic Carbon Nitride: Novel and Green Synthesis and Cooperative Photocatalytic Activity. Dalton Trans. 2014, 43, 13792. (14) Vaiano, V.; Sacco, O.; Sannino, D.; Ciambelli, P. Nanostructured N-doped TiO2 Coated on Glass Spheres for the Photocatalytic Removal of Organic Dyes under UV or Visible Light Rrradiation. Appl. Catal., B 2015, 170, 153. (15) Pugazhenthiran, N.; Murugesan, S.; Anandan, S. High Surface Area Ag-TiO2 Nanotubes for Solar/Visible-light Photocatalytic Degradation of Ceftiofur Sodium. J. Hazard. Mater. 2013, 263, 541. (16) Lee, J. H.; Kim, I. K.; Cho, D.; Youn, J. I.; Kim, Y. J.; Oh, H. J. Photocatalytic Performance of Graphene/Ag/TiO2 Hybrid Nanocomposites. Carbon lett 2015, 16, 247. (17) Yu, T.; Tan, X.; Zhao, L.; Yin, Y.; Chen, P.; Wei, J. Characterization, Activity and Kinetics of a Visible Light Driven Photocatalyst: Cerium and Nitrogen Co-doped TiO2 Nanoparticles. Chem. Eng. J. 2010, 157, 86−92.

Figure 11. UV−vis DRS of TiO2 and TiO2-G. The inset shows the modified Kubelka−Munk function [(ahν)1/2] plotted against hν.

conversion materials TiO2 NRAs:Eu3+, Tb3+ can absorb the UV light to generate electron−hole pairs, under Xe lamp irradiation. The Eu3+ and Tb3+ also can be excited by UV light to produce emission peaks in the visible range. After illuminated by greater intensity of visible light, more electrons are excited from the valence band (VB) of CdS to its CB.21 Then the photoelectrons are injected into the CB of TiO2. The TiO2 NRAs decorated with Cs2CO3 thin film can lead to an effective hole−electron separation due to the reducing defects on the surface of TiO2 nanorod. Meanwhile, graphene was served as an electron sink for fast trapping electrons from TiO2 NRAs/Cs2CO3/CdS. The photogenerated electrons react with the absorbed O2 on the edge of carbon textiles to produce OH·.16,23 These hydroxyl radicals are known to be very reactive oxidative species that react with the organic or water pollutants that can be degraded to CO2 and H2O. Additionally, the remaining holes in the VB of CdS can take part in the redox reaction to generate OH·, which is further used to remove RhB.

4. CONCLUSIONS In summary, the hybrid structure TiO2-G/TiO2 NRAs:Eu3+, Tb3+ /Cs2CO3/CdS was successfully synthesized. The results revealed that TiO2-G/TiO2 NRAs:Eu3+, Tb3+ /Cs2CO3/CdS shows significantly enhanced photocatalytic activity compared to other samples. The success is attributed to the much improved electron−hole separation and light utilization of the TiO2-G/TiO2 NRAs:Eu3+, Tb3+ /Cs2CO3/CdS. The improvement of performance for photocatalysis gives us a new insight on studying photocatalytic materials. Furthermore, the photocatalysts grown on carbon textiles make the collection and recycle of photocatalysts much easier.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail address: [email protected] (J. Huang). *E-mail address: [email protected] (X. Xu). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Graduate Innovation Foundation of University of Jinan, GIFUJN, (Grant No. F

DOI: 10.1021/acs.iecr.5b04076 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.iecr.5b04076 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX