N-Doped and CdSe-Sensitized 3D-Ordered TiO2 Inverse Opal Films

Subscriber access provided by READING UNIV

Article

N-doped and CdSe sensitized 3D ordered TiO2 inverse opal films for synergistically enhanced photocatalytic performance Yun Song, Na-Jun Li, Dongyun Chen, Qing-Feng Xu, Hua Li, Jing-Hui He, and Jian-Mei Lu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04395 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

N-doped and CdSe sensitized 3D ordered TiO2 inverse opal films for synergistically enhanced photocatalytic performance Yun Song a, Najun Li*, a, b, Dongyun Chen a, b, Qingfeng Xu a, b, Hua Li a, Jinghui He a, Jianmei Lu*, a, b a

College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation

Center of Suzhou Nano Science and Technology, Soochow University, No. 199 Ren’ai Road, Suzhou Industrial Park, Suzhou 215123, China b

State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials,

Suzhou, Jiangsu 215123, China *Correspondence to: N. J. Li (E-mail: [email protected]) & J. M. Lu (E-mail: [email protected]), Tel/Fax: 86 0512 65880367

KEY WORDS: TiO2 inverse opal, nitrogen-doping, CdSe sensitization, visible-light-driven photocatalyst ABSTRACT An efficient method for fabricating a novel visible-light-driven photocatalyst by uniformly decorating CdSe nanoparticles on the framework of an N-doped TiO2 inverse opal (CdSe/N-TiO2 IO) is reported. Effects of different pore sizes, calcination temperature for nitrogen doping and loading amount of CdSe nanoparticles are investigated and the optimal conditions are obtained. The highly ordered mesoporous inverse opal structure exhibits excellent light-harvesting efficiency and fast mass transport. The incorporation of nitrogen into the TiO2 lattice and sensitization with CdSe nanoparticles effectively decrease the recombination rate of electronhole pairs. UV-vis DRS reveals that the absorbance edge of CdSe/N-TiO2 IO is red-shifted to the

ACS Paragon Plus Environment

1

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 43

visible-light region, and the bandgap is determined to be 2.2 eV. The degradation experiment using RhB as a model substance revealed that the N-doping and CdSe sensitization synergistically enhanced the photocatalytic efficiency of CdSe/N-TiO2 IO heterostructure by 5.5 times for photocatalytic degradation of

RhB in water under visible light. The remarkable

photocatalytic ability of CdSe/N-TiO2 IO can be attributed to the combination of chemical and morphological modifications of TiO2, which synergistically improve the visible-light absorption ability. The cycling experiments indicate the favorable photostability and excellent cycling stability of CdSe/N-TiO2 IO. The effects of the active species participated in photocatalytic process have also been tested, and the possible reaction mechanism of degradation of RhB with CdSe/N-TiO2 IO was investigated. Moreover, the CdSe/N-TiO2 IO films have good adhesion to FTO glass, which makes it a promising material for photoelectrocatalysis.

INTRODUCTION Semiconductor photocatalysts have received tremendous attention as promising technology in many applications, including photovoltaic cells, hydrogen production by splitting water and removal of hazardous substances.1-7 These applications play a critical part in dealing with current environmental issues and the worldwide energy crisis. Among various types of semiconductor, titanium dioxide (TiO2), as a typical and attractive material, has been extensively studied for its high photocatalytic activity, low cost and stable chemical properties.7-9 However, only a low ratio of solar photons can be harvested by TiO2, which limits its utilization. Normally, actions to improve the photocatalytic performance of a semiconductor are including enhancing light absorption properties, suppressing the recombination of photo-generated electrons and holes, and so on. In order to improve the light-harvesting ability of TiO2, three-


2

Page 3 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dimensional (3D) inverse opal (IO) has been employed in photocatalytic performance for their prominent optical properties.9-11 Generally, the group velocity of incident photons near the photonic stop band is distinctly slowed, and thus, the slow photons in inverse opals effectively increase the optical path length, therefore enhancing the interaction of light with the photocatalyst (slow photon effect).12-14 On account of multiple scattering effects of inverse opals, fewer photons tend to escape, which effectively raises the propagation rate of photogenerated electrons and holes and results in the improvement of optical absorption. In addition to these optical properties, inverse opal possesses a face-centered-cubic (FCC) structure, which offers high porosity (74% void volume) and specific surface area, providing more active sites for mass transfer. In addition to enhancing light-harvesting properties, great efforts have been devoted to inhibit the electron-hole recombination rate of TiO2 IO and broaden the absorption of TiO2 IO to visiblelight region. Generally, three main methods have been used to improve photocatalytic activity, such as doping elements,15,16 loading with noble metals,17-19 and sensitizing with narrow bandgap semiconductor.20-22 In recent years, doping TiO2 IO with metallic ions or non-metal elements has found to be an effective method, and among these investigations, nitrogen doping has been proven to be a simple and effective approach2,23 by creating a narrow N 2p band above the valence band of TiO2 to extend its optical absorption.24 Among reported photocatalysts, CdSe is a promising material for investigation since it can effectively absorb visible light based on its narrow band gap of 1.7 eV and has been widely investigated in photoelectrochemical and photocatalytic applications.25-31 In addition, CdSe has a relatively high conduction band with respect to TiO2, which is favorable for electron injection from CdSe to TiO2.32 Nevertheless, there have been few reports exploring the incorporation of CdSe into nitrogen-doped TiO2


3


Page 4 of 43

inverse opals. Herein, the ordered structure of inverse opals is utilized to improve lightharvesting ability of TiO2, as well as doping non-metal element and coupling with narrow band gap semiconductor in order to synergistically enhance visible-light absorption and decrease the recombination rate of electron-hole pairs. In this work, we present a relatively simplified route to prepare CdSe sensitized nitrogendoped TiO2 inverse opals (CdSe/N-TiO2 IO), as shown in Scheme 1. The photodegradation of Rhodamine B (RhB) in aqueous solution is conducted to evaluate the photocatalytic activity of CdSe/N-TiO2 IO under visible-light irradiation. For comparison, TiO2 IO and N-TiO2 IO are prepared and used as references. Synergistic effects among N-doping, CdSe sensitization, and the inverse opal structure are beneficial to charge carrier transfer and inhibit the recombination of photogenerated electrons and holes, thus enhancing the photocatalytic activity. Additionally, CdSe/N-TiO2 IO exhibits excellent cycling stability and is easy to recycle. This technique, based on the combination of N-doping and CdSe sensitizing TiO2 inverse opal structures, may provide new insights for photocatalytic applications.

Scheme 1. Schematic illustration of the synthetic process of CdSe/N-TiO2 IO.


4

Page 5 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EXPERIMETAL SECTION Materials. Anhydrous ethanol (>99.7%), tetrabutyl titanate (TBT), Rhodamine B (RhB), urea, potassium persulfate, cadmium chloride, selenium powder and sodium sulfite were purchased from the Sinopharm Chemical Reagent Co., Ltd. (China), polyvinylpyrrolidone K30 (PVP), styrene and acetylacetone were purchased from Sigma Aldrich (America). Fluorine-doped tin oxide (FTO) glass as substrates were obtained from Kun Shan Jia Yi Sheng Co., Ltd. (China). Characterization. The morphology and microstructures of all the samples were examined by Hitachi S-4700 scanning electron microscope and Hitachi H600 transmission electron microscope. The elemental analysis of samples was detected by HRTEM-Mapping and X-ray photoelectron spectroscopy (ESCALAB MK II) with Al-Kα radiation. XRD analysis was carried out using X’ Pert-Pro MPD with Cu Kα radiation. The optical properties were performed with UV-vis spectrophotometer (Shimadzu UV-3600). The nitrogen adsorption-desorption curve and specific surface area were performed on Micromeritics ASAP 2010 system. The photoluminescence (PL) spectra were obtained from fluorescence spectrophotometer (FLS920) with an excitation wavelength of 250 nm. Electron spin-resonance spectroscopy (EPR, JESX320, JEOL, Japan) was carried out to detect radicals spin-trapped by 5,5'-dimethyl-1-pyrrolineN-oxide (DMPO) with the settings of center field (326 mT), microwave frequency (9.15 GHz), and power (20 mW). Preparation of PS spheres and PS opals. Monodispersed polystyrene (PS) spheres were prepared with a surfactant-free emulsion polymerization method as reported elsewhere.33 Styrene was purified by vacuum distillation. Typically, 12.18 g styrene and 0.5 g PVP were added to 100 mL deionized water in a three-necked flask at room temperature. The mixture was stirred at about 300 rpm for 15 min. After a 1.5 wt% potassium persulfate aqueous (20 mL) was added, the


5


Page 6 of 43

reaction mixture was deoxygenated by bubbling nitrogen gas at room temperature for 30 min. Then, the mixture was gradually heated to 70 oC and kept at this temperature for 10 h to obtain PS spheres. In order to study the effect of air cavity diameter of N-TiO2 IO, PS templates with different particle size of 130 nm, 280 nm and 500 nm (indexed as PS130, PS280 and PS500) were obtained by varying the amount of PVP. And then PS opals were fabricated using PS templates (PS130, PS280 and PS500, respectively) via the vertical deposition method. The monodispersed PS spheres were added to 30 mL deionized water and adjusted to a 0.125 wt% dispersion. To obtain the opal-structured templates, the clean FTO glass (15×10×1 mm) was held vertically in a 5 mL beaker containing the PS sphere dispersion and kept at 45 °C for 36 h. The amount of PS dispersion depended on the thickness of the templates and thus the N-TiO2 IO. Finally, the opal templates were sintered at 80 °C for 30 min to enhance their physical strength. Preparation of N-TiO2 IO. In a typical procedure, acetylacetone (0.125 g) was dissolved in 60 mL anhydrous ethanol, followed by continuous stirring for 15 min. Then 0.5 g TBT was added to the mixture. The solution was indexed as Ti precursor. Subsequently, 0.29 g urea, as nitrogen source, was added to 60 mL anhydrous ethanol, and the solution was denoted as N precursor. 2 mL Ti precursor and 0.1 mL N precursor were mixed homogeneously. The PS opals were immersed into the mixed precursor solution for 2 h, and then dried in air. This infiltration procedure was repeated three times to increase the filling of the interstices of templates. Finally, the PS templates were removed via calcination in air at 500 oC by 2 oC min-1 for 2 h. N-TiO2 IO films with different diameters of air cavities were obtained using the different particle sizes of PS templates. Preparation of CdSe/N-TiO2 IO. Finally, N-TiO2 IO was decorated with CdSe nanoparticles via a hydrothermal method for photocatalytic sensitization. To investigate the effect of CdSe


6

Page 7 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


loading amount on enhancement of photocatalytic activities, CdSe (х)/N-TiO2 IO (x≈5, 10 and 15 wt%, respectively on behalf of the weight percentage of CdSe analyzed by X-ray photoelectron spectroscopy) were obtained by varying the loading amount of CdSe on N-TiO2 IO. Briefly, 0.1830 g CdCl2, 0.0796 g Se and 0.2520 g Na2SO3 (x=10 wt%) were added to 35 mL deionized water. After vigorously stirring for 30 min, the suspension was transferred into 50 mL Teflon-lined stainless steel autoclave along with N-TiO2 IO. The autoclave was heated to 180 oC for 8 h. After naturally cooling to room temperature, the FTO glass coated with CdSe/N-TiO2 IO were collected and washed with deionized water. Photocatalytic activity test. The photocatalytic activity of CdSe/N-TiO2 IO was evaluated by the photodegradation of RhB under visible-light irradiation. In each experiment, 10 mg photocatalyst was placed horizontally at the bottom of a beaker filled with 5 mg/L RhB solution (20 mL). A 300 W Xenon lamp with a cut-off filter (λ>420 nm) was used as light source. Before illumination, the samples were treated in the dark for 30 min at room temperature to achieve adsorption-desorption equilibrium. At each 10 min interval, 1 mL of solution was extracted and analyzed by recording the variations in the absorption band maximum (554 nm) of RhB using UV-Vis spectrometer. What’s more, different scavengers are applied in photocatalytic experiments to investigate the role of active species. The cycling performance was also examined to assess the photocatalytic stability of CdSe/NTiO2 IO films: the films were rinsed thoroughly with deionized water and dried at 60 oC after each cycle for next utilization. Photocatalytic experiments were also performed on P25, TiO2 IO and N-TiO2 IO to evaluate the performance of different catalysts, the test was similar to the above.


7


Page 8 of 43

RESULTS AND DISCUSSION Effect of air cavity diameter. In order to study the effect of air cavity diameter of inverse opals, PS templates with different particle size of 130 nm, 280 nm and 500 nm (PS130, PS280 and PS500) were prepared and used to fabricate PS opals and N-TiO2 IO. Figure 1a1, a2 and a3 show that the monodispersed PS spheres are closely arranged, and the PS opals exhibit facecentered-cubic array. And all of the TiO2 IOs (Figure 1b1, b2 and b3) and N-TiO2 IOs (Figure 1c1, c2 and c3) have uniform air cavities intensively arranged with ordered periodicity. Since the PS templates are burned off, the frame structure of TiO2 IOs and N-TiO2 IOs is more compact than that of the PS opal templates. Well-structured 3D periodically ordered structures with few defects are clearly observed after calcination, which is consistent with the structure of opal templates. Figure 1d demonstrates the reflectance spectra of N-TiO2 IOs obtained from different templates (PS130, PS280 and PS500). According to previous studies,9,34,35 TiO2 inverse opals prepared by colloid crystal templates present dramatically improved photocatalytic activity due to multiple scattering processes and the slow photon effect. To confirm the slow photon effect of N-TiO2 IOs prepared from different diameter of PS opals, the reflectance spectra of N-TiO2 IO are investigated in water. The stop band reflectance peak indicates that when photocatalyst absorbs incident photons near the band edges, the group velocity of light is distinctly slowed, leading to a longer interaction time and stronger light absorption. As shown in Figure 1d, NTiO2 IO (PS280) has photonic band gap centered at 509 nm, while N-TiO2 IO (PS130) or NTiO2 IO (PS500) have no stop band peaks centered at visible region, so N-TiO2 IO prepared from PS280 is taken for further modification and photocatalytic investigation.


8

Page 9 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. SEM images of PS opals (a1, a2 and a3), TiO2 IO (b1, b2 and b3) and N-TiO2 IO (c1, c2 and c3) prepared from PS templates with different particle sizes and (d) reflectance spectra of NTiO2 IO obtained from PS130, PS280 and PS500.

Figure 2. SEM images of CdSe/N-TiO2 IO with different CdSe loading amount (a, b and c) and N2 adsorption-desorption isotherms and BJH pore-size distribution curve of CdSe/N-TiO2 IO (d). Effect of CdSe loading amount. CdSe nanoparticles with effective light absorption ability and narrow band gap are utilized to sensitize the photocatalytic activity of N-TiO2 IO with increasing CdSe loading amount. And CdSe/N-TiO2 IOs with different CdSe loading amount (indexed as CdSe(x)/N-TiO2 IO, x≈5, 10 and 15 wt%, respectively) are obtained, as shown in Figure 2a-c. When the concentration of Cadmium precursor is relatively low (x≈5 wt%), the skeleton of N-TiO2 IO cannot be covered completely (Figure 2a). While the Cadmium precursor is overdosing (x≈15 wt%), serious aggregation and pore clogging are occurred (Figure 2c).


9


Page 10 of 43

Therefore, an appropriate percentage of CdSe loading on N-TiO2 IO is important for this modification process. From the SEM image of CdSe/N-TiO2 IO in Figures 2b (x≈10 wt%), we can see that the well-ordered inverse opals are retained after the hydrothermal process and a homogeneous coverage of CdSe can be observed without pore clogging. So it is used for further investigation, and this optimal loading condition also can be supported by the photocatalytic experiments in Figure S1. By the way, the calcination temperature of N-TiO2 IO is also investigated. PS templates cannot be completely removed at relatively low temperature ( < 400 °C), while the excessive calcination temperature ( > 600 °C) leads to the reducing of N doping in the films. So 500 °C is selected as the certain calcination temperature for PS templates removing to obtain N-TiO2 IO. Morphology and structure of CdSe/N-TiO2 IO. The N2 adsorption-desorption isotherms and Barrett-Joyner-Halenda (BJH) pore-size distribution curve of CdSe/N-TiO2 IO is given in Figure 2d. The isotherms are identified as type ΙΙΙ isotherms with a H3 hysteresis loop according to the IUPAC classification, indicating the mesoporous nature of the photocatalyst. The inset shows the pore-size distribution curve determined from the adsorption branch, which reveals a relatively narrow pore-size distribution centered at 14.4 nm. The surface area and pore volume of CdSe/NTiO2 IO are calculated to be 66.56 m2 g-1 and 0.24 cm3 g-1. The nitrogen adsorption-desorption curves for the other samples are displayed in Figure S2. TEM images and the elemental mapping of CdSe/N-TiO2 IO are presented in Figure 3. Figure 3a further reveals that CdSe is densely and uniformly decorated on N-TiO2 IO. Its HRTEM image (Figure 3b) indicates the high crystallinity of CdSe/N-TiO2 IO. The measured lattice spacing of 0.352 nm corresponds to the (101) plane of anatase TiO2, while 0.371 nm corresponds to the (100) plane of hexagonal CdSe. The elemental mapping (Figures 3c-h)


10

Page 11 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


indicates the presence of Ti, O, N, Cd and Se atoms in the sample. The Cd and Se elemental mapping confirms that CdSe was uniformly grown on the N-TiO2 IO structure. And the TEM images of N-TiO2 IO films are shown in Figure S3.

Figure 3. (a) TEM and (b) HRTEM image of CdSe/N-TiO2 IO, and (c-h) HAADF-STEM-EDS elemental mapping of CdSe/N-TiO2 IO. Phase and composition. Figure 4 shows the XRD pattern of CdSe/N-TiO2 IO along with the TiO2 IO and N- TiO2 IO patterns as references. TiO2 IO has anatase (PDF#99-0008) and rutile (PDF#99-0090) crystal structures, and there is no phase change compared with N-TiO2 IO, which is in accordance with other reports on nitrogen-doped TiO2. Because the diffraction peak corresponding to the (002) plane of the CdSe phase is located at almost the same position as the (101) crystal plane of anatase TiO2, it is difficult to identify the (002) plane of the CdSe nanoparticles. To confirm the existence of CdSe nanoparticles, other peaks are analyzed. The peaks at 2θ values of 23.9, 27.0 and 41.9° can be indexed to the (100), (101) and (110) crystal


11


Page 12 of 43

planes, respectively, of hexagonal-phase CdSe (PDF#08-0459). Therefore, the XRD data confirm the presence of CdSe/N-TiO2 IO, which is in good agreement with the SEM and HRTEM results.

Figure 4. The XRD patterns of TiO2 IO, N-TiO2 IO and CdSe/N-TiO2 IO. The compositional information and chemical states of CdSe/N-TiO2 IO were further characterized by XPS (Figure 5). In Figure 5a, the survey spectrum demonstrates that the surface of CdSe/N-TiO2 IO is composed of N, Ti, O, Cd and Se elements. The primary peak in the N 1s spectrum located at 399.8 eV (Figure 5b) is attributed to N in the O-Ti-N linkage formed by the substitution of oxygen atoms in the TiO2 lattice. The weak peak at 396.9 eV can be attributed to the characteristic peak of the N-Ti-N bond, indicating that nitrogen atoms were successfully doped into the TiO2 crystal lattice. The peak appearing at 401.9 eV can be ascribed to chemisorbed N2 on the pores of CdSe/N-TiO2 IO. Moreover, the atomic concentration of N in CdSe/N-TiO2 IO is found to be 0.84%. The two peaks appearing at 464.3 and 458.6 eV (Figure 5c) are assigned to Ti 2p1/2 and Ti 2p3/2 and are attributed the Ti element in TiO2. Figure 5d shows the O 1s peaks centered at 532.1 and 529.9 eV. The dominant peak at 529.9 eV is the


12

Page 13 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


characteristic O 1s peak corresponding to the Ti-O linkage in TiO2. The weak peak appearing at approximately 532.1 eV can be attributed to OH groups. From Figure 5e, the binding energies of Cd 3d are determined to be 405.4 and 412.2 eV for Cd 3d5/2 and Cd 3d3/2, while the peak observed at 54.7 eV corresponds to Se 3d (Figure 5f). Clearly, these results prove the successful deposition of CdSe on N-TiO2 IO.

Figure 5. XPS spectra of CdSe/N-TiO2 IO (a) Survey, (b) N 1s, (c) Ti 2p, (d) O 1s, (e) Cd 3d, and (f) Se 3d. Optical properties. Figure 6a illustrates the optical properties of P25, TiO2 IO, N-TiO2 IO and CdSe/N-TiO2 IO. It should be noted that P25 shows only UV absorption. In comparison to P25, the absorption edge of TiO2 IO is shifted to the visible region because of slow photon effect and multiple scattering processes. For N-TiO2 IO, a wide and flat absorption tail that stretches to


13


Page 14 of 43

700 nm can be observed. This tail can be attributed to the fact that nitrogen is doped into the TiO2 lattice, producing intermediate energy levels, which effectively narrows the bandgap and broadens the optical absorption. The absorption of CdSe/N-TiO2 IO is found to shift to the visible region (400-600 nm) due to the contribution of CdSe nanoparticles absorbing visible-light and transferring electrons to N-TiO2 IO to ensure photo-generated carriers separated. Figure 6b shows the Tauc plots used to determine the bandgaps of all the samples. Consequently, the bandgap energies are found to be 3.2, 3.0, 2.8 and 2.2 eV for P25, TiO2 IO, N-TiO2 IO and CdSe/N-TiO2 IO. The band gap narrowing shows more efficient utilization of visible-light and consequently more photo-excited carriers generation to perform superior photocatalytic activity. It can be concluded that the nitrogen doping and CdSe sensitization can effectively improve the optical absorption of CdSe/N-TiO2 IO, which is revealed by photodegradation experiment below.

Figure 6. (a) UV-vis absorption spectra and (b) band gap energies of P25, TiO2 IO, N-TiO2 IO and CdSe/N-TiO2 IO. Figure 7a compares the photocurrent density of P25, TiO2 IO, N-TiO2 IO and CdSe/N-TiO2 IO. A larger photocurrent density implies that more photogenerated electrons are efficiently transferred from the photoanode to the counter electrode through an external circuit under visible-light irradiation. The maximum photocurrent density of TiO2 IO is 2.03 µA cm-1, which


14

Page 15 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


is 10 times higher than that of commercial P25. The introduction of a 3D ordered structure may enhance the light absorption due to multiple scattering processes and the slow photon effect, resulting in an improved photocurrent. In comparison to TiO2 IO, N-TiO2 IO has a larger photocurrent density (3.03 µA cm-1), which is attributed to N doping. In addition, CdSe/N-TiO2 IO exhibits the highest photocurrent density (6.20 µA cm-1). Photogenerated electrons and holes in CdSe/N-TiO2 IO tend to rapidly separate and transfer to the surface of the photocatalyst, which can be ascribed to synergistic effects of nitrogen doping and CdSe sensitization. In summary, the observed trend in the photocurrent density of CdSe/N-TiO2 IO > N-TiO2 IO > TiO2 IO > P25 is consistent with the UV-vis absorption spectroscopy results.

Figure 7. (a) The photocurrent density versus time curves for the different samples, and (b) Photoluminescence spectra of P25, TiO2 IO, N-TiO2 IO and CdSe/N-TiO2 IO. This conclusion can be further supported by the photoluminescence (PL) spectra, as presented in Figure 7b. All the samples were measured with a 250 nm excitation, and the emission peak is centered at 410 nm. Normally, the intensity of the emission peak depends on the recombination of photoinduced electron-hole pairs, and a low intensity is consistent with the enhanced separation and transfer of electrons and holes.36-38 Among the photocatalysts, CdSe/N-TiO2 IO


15


Page 16 of 43

exhibits the lowest photoluminescence intensity, indicating the migration of photo-excited electron-hole pairs and superior photocatalytic activity.

Figure 8 (a) RhB photodegradation curves for P25, TiO2 IO, N-TiO2 IO and CdSe/N-TiO2 IO under visible light and (b) Photocatalytic degradation of 20 mL of 5 mg/L RhB solution under visible light irradiation, in the case of 5, 10, 15, 20 mg CdSe/N-TiO2 IO. Photocatalytic performance for RhB degradation. The photocatalytic performance of CdSe/N-TiO2 IO with optimal synthetic condition has been further tested, and P25, TiO2 IO and N-TiO2 IO are evaluated for comparison under the same conditions. Figure 8a displays the RhB removal efficiency treated by different samples. The adsorption-desorption equilibrium curves of all the samples are displayed in Figure S4. After a 30-minutes absorption-desorption equilibrium for all the samples in dark, the photodegradation of RhB is carried out under visible-light irradiation. For TiO2 IO and N-TiO2 IO, the degradation efficiency under visible-light irradiation for 60 minutes is calculated as 18% and 28%, respectively to the total residual RhB after absorption-desorption equilibrium. It is worth mentioning that CdSe/N-TiO2 IO exhibits a relatively high degradation efficiency of up to 99.7% in 60 min, which is 5.5 times the efficiency of TiO2 IO and 3.5 times the efficiency of N-TiO2 IO, and it is superior to most other photocatalysts.39-43 Therefore, CdSe/N-TiO2 IO has excellent photodegradation efficiency for


16

Page 17 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RhB. This result further confirms the synergistic effect of nitrogen doping and CdSe sensitization. The UV-vis absorption spectra of RhB photodegrdaed by P25, TiO2 IO, N-TiO2 IO and CdSe/N-TiO2 are shown in Figure S5. The effect of catalyst dosage on RhB removal efficiency is studied by varying the weight of catalyst from 5 to 20 mg (Figure 8b). Increasing the amount of catalyst enhances the light absorption ability of CdSe/N-TiO2 IO and produces more active free radicals. Nevertheless, a further increase in the catalyst dosage leads to the decrease in the RhB removal efficiency, which could be reduced by light penetration of CdSe/NTiO2 IO.

Figure 9 (a) Cycling photodegradation of RhB by CdSe/N-TiO2 IO under visible light and (b) SEM images of CdSe/N-TiO2 IO after photodegradation cycling. To investigate the stability of CdSe/N-TiO2 IO, recycling experiments were carried out. As shown in Figure 9a, the degradation efficiency reaches 99.2%, 98.8%, 98.8%, and 98.7% after the first, second, third and fourth cycle. This indicates that CdSe/N-TiO2 IO shows excellent removal efficiency and good stability after each cycle. Figure 9b shows the morphology of CdSe/N-TiO2 IO after the RhB photodegradation cycles. CdSe nanoparticles remain attached to the N-TiO2 IO skeleton, suggesting the structural stability of CdSe/N-TiO2 IO. UV-vis absorption spectra of RhB cycling photodegrdaed by CdSe/N-TiO2 IO are presented in Figure


17


Page 18 of 43

S6. The kinetic behaviors of P25, TiO2 IO, N-TiO2 IO and CdSe/N-TiO2 IO in the photocatalytic degradation of RhB follow a Langmuir-Hinshelwood first-order kinetics model. Table 1 lists the kinetic constants for RhB photodegradation with P25, TiO2 IO, N-TiO2 IO and CdSe/N-TiO2 IO after the adsorption equilibrium is reached. CdSe/N-TiO2 IO exhibits larger kinetic constants than the other samples, which is consist with the above studies. Phenol photodegradation curves for all the samples are shown in Figure S7. It also revealed that the photocatalytic performance of CdSe/N-TiO2 IO is well improved due to the synergistic effect among N-doping, CdSe sensitization, and inverse opal structure. Table 1. Kinetic constants and correlation coefficients of P25, TiO2 IO, N-TiO2 IO and CdSe/NTiO2 IO for 5 ppm RhB degradation. Samples

k(min-1)

R2

P25

0.00180

0.992

TiO2 IO

0.00429

0.989

N-TiO2 IO

0.00827

0.988

CdSe/N-TiO2 IO

0.04231

0.996

Photocatalytic mechanism for RhB degradation. Based on the above analysis, a possible photocatalytic mechanism for CdSe/N-TiO2 IO is proposed in Scheme 2. Valence band XPS spectra of TiO2 IO, N-TiO2 IO and CdSe nanoparticles are shown in Figure S8. The incident light of inverse opals is trapped by multiple scattering processes, which enhances interactions between light and the photocatalyst, thereby generating many more photoexcited carriers in the inverse opal structures. Moreover, the 3D ordered structure can increase the surface area and


18

Page 19 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


improve the mass transfer of the organic dye. It is generally accepted that the incorporation of nitrogen into the TiO2 lattice modifies its electronic band structure, thus producing an N 2p band above valance band. This intermediate energy level induces a relatively narrow bandgap for TiO2 and leads to improvements in visible-light harvesting. When incident photons are absorbed by CdSe/N-TiO2 IO, both CdSe and N-TiO2 IO are activated by visible-light. Then, the photoexcited electrons are rapidly transferred from the conduction band of CdSe to that of NTiO2 IO, because conduction band level of CdSe is more negative than that of TiO2. Meanwhile, holes are injected away from the N 2p band to the valance band of CdSe. This process effectively hinders the recombination of electron-hole pairs and therefore enhances the photocatalytic activity. Subsequently, the carriers migrate to the respective surface of the photocatalyst and take part in reactions to generate active species. It is deemed that the valence band of CdSe is more negative than the standard redox potential of ·OH/OH-, as a result, hydroxyl radicals (·OH) can be hardly produced on CdSe. However, ·OH can be generated from reaction between electron and H2O2. The photogenerated electrons react with oxygen molecules in water to form superoxide radical anions (·O2-), which on protonation generate highly active H2O2 molecules. Subsequently, hydroxyl radicals are generated by the degradation of H2O2, and serve as strong oxidizing agent to decompose the organic dye to small molecules. Meanwhile, the holes injected to the valance band of CdSe can oxide the adsorbed dyes directly and inhibit the recombination of charge carriers. The photocatalytic process includes the following:


19


Page 20 of 43

Scheme 2. Schematic diagram of the reaction mechanism for RhB removal.

Figure 10. RhB photodegradation curves for CdSe/N-TiO2 IO under different conditions with exposure to visible light: no scavenger, adding BQ, adding t-BuOH and adding AO. To investigate the role of active species, different scavengers are applied in photocatalytic


20

Page 21 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


experiments. In this study, ammonium oxalate (AO), tert-butyl alcohol (t-BuOH) and 1,4benzoquinone (BQ) are used as hole scavenger, hydroxyl radical scavenger and superoxide radical scavenger, respectively. Figure 10 shows the photocatalytic activity of CdSe/N-TiO2 IO toward the degradation of RhB under different conditions. With no addition of scavengers, the photocatalytic degradation efficiency of RhB is 99.7% after 60 minutes of visible-light irradiation. When AO (1 mmol/L) is added into the reaction system, the degradation efficiency of RhB decreases obviously, indicating that photo-excited holes are important in photodegradation process. In the presence of t-BuOH (1 mmol/L), the degradation of RhB is also inhibited. As the addition of BQ (1mmlo/L), the degradation efficiency is greatly decreased, suggesting that ·O2is the significant active specie generated in degradation of RhB. From the above results, it can be concluded that the photocatalytic degradation of RhB is driven mainly by ·O2- . At the same time, ·OH and holes also promote the reaction. As is shown in Figure 11, EPR spin-trap with DMPO technique are employed to detect the signals of ·O2- and ·OH, which further supports the photocatalytic mechanism.

Figure 11. (a) DMPO spin-trapping EPR spectra of CdSe/N-TiO2 IO in methanol dispersion for DMPO-·O2- and (b) DMPO spin-trapping EPR spectra of CdSe/N-TiO2 IO in water dispersion for DMPO-·OH.


21


Page 22 of 43

CONCLUSIONS In summary, N-doped TiO2 inverse opal films decorated with CdSe nanoparticles were prepared and exhibited efficient photocatalytic performance. An improved photocatalytic activity is observed for CdSe/N-TiO2 IO in comparison to that of P25, TiO2 IO and N-TiO2 IO, which is ascribed to the significant enhancement in light absorption due to synergistic effects of nitrogen doping and CdSe sensitization, as well as the slow photon effect and multiple scattering processes in the 3D ordered inverse opal structures. This study indicates that the combination of nitrogen doping and CdSe sensitization of TiO2 inverse opals films is an effective approach to improve the photoresponse and inhibit charge carrier recombination, which may be beneficial in further studies of environmental and energy applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.XXXXXXX. TEM, BET, XPS and UV-vis spectra data.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected].


22

Page 23 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We thank Dr. Guoqing Bian for kind help in electron spin-resonance spectroscopy measurement and discussion. We gratefully acknowledge the financial support provided by National Natural Science Foundation of China (51573122, 21722607, 21776190), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (17KJA430014, 17KJA150009), the National Key Technology R&D Program (2015BAG20B03-06), the Science and Technology Program for Social Development of Jiangsu (BE2015637) and the project supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

REFERENCES (1) Wu, N.; Wang, J.; Tafen, D.-N.; Wang, H.; Zheng, J.-G.; Lewis, J.-P.; Liu, X.-G.; Leonard, S.-S.; Manivannan, A. Shape-enhanced photocatalytic activity of single-crystalline anatase TiO2 (101) Nanobelts. J. Am. Chem. Soc. 2010, 132, 6679-6685. DOI: 10.1021/ja909456f. (2) Rahul, T. K.; Sandhyarani, N. Nitrogen-fluorine co-doped titania inverse opals for enhanced

solar

light

driven

photocatalysis. Nanoscale,

2015, 7,

18259-18270.

DOI:

10.1039/C5NR04663G. (3) Zhao, S.; Zhang, Y.; Zhou, Y.; Zhang, C.; Sheng, X.; Fang, J.; Zhang, M. Reactable polyelectrolyte-assisted synthesis of BiOCl with enhanced photocatalytic activity. ACS Sustainable Chem. Eng. 2017, 5, 1416-1424. DOI: 10.1021/acssuschemeng.6b01987.


23


Page 24 of 43

(4) Tong, H.; Ouyang, S.-X.; Bi, Y.-P.; Umezawa, N.; Oshikiri, M.; Ye, J.-H. Nanophotocatalytic materials: possibilities and challenges. Adv. Mater. 2012, 24, 229-251. DOI: 10.1002/adma.201102752. (5) Kim, H. N.; Yoo, H.; Moon, J. H. Graphene-embedded 3D TiO2 inverse opal electrodes for highly efficient dye-sensitized solar cells: morphological characteristics and photocurrent enhancement. Nanoscale 2013, 5, 4200-4204. DOI: 10.1039/C3NR33672G. (6) Yu, Y.; Ren, J.; Meng, M. Photocatalytic hydrogen evolution on graphene quantum dots anchored TiO2 nanotubes-array. Int. J. Hydrogen Energ. 2013, 38, 12266-12272. DOI: 10.1016/j.ijhydene.2013.07.039. (7) Weng, X.; Zeng, Q.; Zhang, Y.; Dong, F.; Wu, Z. Facile approach for the syntheses of ultrafine TiO2 nanocrystallites with defects and C heterojunction for photocatalytic water splitting. ACS

Sustainable

Chem.

Eng. 2016,

4,

4314-4320.

DOI:

10.1021/acssuschemeng.6b00828. (8) Wang, S.; Yi, L.-X.; Halpert, J.; Lai, X.-Y.; Liu, Y.-Y.; Cao, H.-G; Yu, R.-B.; Wang, D.; Li, Y.-L. A novel and highly efficient photocatalyst based on P25-graphdiyne nanocomposite. Small 2012, 8, 265-271. DOI: 10.1002/smll.201101686. (9) Cheng, C.-W.; Karuturi, S.-K.; Liu, L.-J.; Liu, J.-P.; Li, H.-X.; Su, L.-T.; Fan, H.-J. Quantum-dot-sensitized TiO2 inverse opals for photoelectrochemical hydrogen generation. Small 2012, 8, 37-42. DOI: 10.1002/smll.20110166. (10) Chen, Z.; Fang, L.; Dong, W.; Zheng, F.; Shen, M.; Wang, J. Inverse opal structured Ag/TiO2 plasmonic photocatalyst prepared by pulsed current deposition and its enhanced visible light photocatalytic activity. J. Mater. Chem. A 2014, 2, 824-832. DOI: 10.1039/C3TA13985A.


24

Page 25 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11) Sordello, F.; Duca, C.; Maurino, V.; Minero, C. Photocatalytic mmetamaterials: TiO2 inverse opals. Chem. Commun. 2011, 47, 6147-6149. DOI: 10.1039/C1CC11243K. (12) Chen, J.; Freymann, G.; Choi, S.; Kitaev, V.; Ozin, G. Amplified photochemistry with slow photons. Adv. Mater. 2006, 18, 1915-1919. DOI: 10.1002/adma.200600588. (13) Chen, J.; Freymann, G.; Kitaev, V.; Ozin, G. Effect of disorder on the optically amplified photocatalytic efficiency of titania inverse opals. J. Am. Chem. Soc. 2007, 129, 1196-1202. DOI: 10.1021/ja066102s. (14) Yu, K.; Zhang, C.; Chang, Y.; Feng, Y.; Yang, Z.; Yang, T.; Lou, Y.; Liu, S.-X. Novel three-dimensionally ordered macroporous SrTiO3 photocatalysts with remarkably enhanced hydrogen production performance. Appl. Catal. B: Environ. 2017, 200, 514-520. DOI: 10.1016/j.apcatb.2016.07.049. (15) Klosek, S.; Raftery, D. Visible light driven V-doped TiO2 photocatalyst and its photooxidation of ethanol. J. Phys. Chem. B 2001, 105, 2815-2819. DOI: 10.1021/jp004295e. (16) In, S.; Orlov, A.; Berg, R.; García, F.; Pedrosa-Jimenez, S.; Tikhov, M.; Wright, D.; Lambert, R. Effective visible light-activated B-doped and B, N-codoped TiO2 photocatalysts. J. Am. Chem. Soc 2007, 129, 13790-13791. DOI: 10.1021/ja0749237. (17) Daskalaki, V.; Antoniadou, M.; Li, G.; Kondarides, D.; Lianos, P. Solar light-responsive Pt/CdS/TiO2 photocatalysts for hydrogen production and simultaneous degradation of inorganic or organic sacrificial agents in wastewater. Environ. Sci. Technol. 2010, 44, 7200-7205. DOI: 10.1021/es9038962. (18) Tanaka, A.; Nakanishi, K.; Hamada, R.; Hashimoto, K.; Kominami, H. Simultaneous and stoichiometric water oxidation and Cr(VI) reduction in aqueous suspensions of functionalized


25


Page 26 of 43

plasmonic photocatalyst Au/TiO2-Pt under irradiation of green light. ACS Catal. 2013, 3, 18861891. DOI: 10.1021/cs400433r. (19) Zhou, W.; Guan, Y.; Wang, D.-Z.; Zhang, X.-H.; Liu, D.; Jiang, H.-D.; Liu, X.-G.; Wang, J.-Y.; Liu, H.; Chen, S.-W. PdO/TiO2 and Pd/TiO2 heterostructured nanobelts with enhanced photocatalytic activity. Chem. Asian J. 2014, 9, 1648-1654. DOI: 10.1002/asia.201301638. (20) Challagulla, S.; Nagarjuna, R.; Ganesan, R.; Roy, S. Acrylate-based polymerizable sol-gel synthesis of magnetically recoverable TiO2 supported Fe3O4 for Cr(VI) photoreduction in aerobic

atmosphere.

ACS

Sustain.

Chem.

Eng.

2016,

4,

974-982.

DOI:

10.1021/acssuschemeng.5b01055. (21) Xiang, Q.; Yu, J.; Jaroniec, M. Synergetic effect of MoS2 and graphene as cocatalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles. J. Am. Chem. Soc. 2012, 134, 6575-6578. DOI: 10.1021/ja302846n. (22) Zhou, W.-J.; Yin, Z.-Y.; Du, Y.-P.; Huang, X.; Zeng, Z.-Y.; Fan, Z.-X.; Liu, H.; Wang, J.Y.; Zhang, H. Synthesis of few-layer MoS2 nanosheet-coated TiO2 nanobelt heterostructures for enhanced photocatalytic activities. Small 2013, 9, 140-147. DOI: 10.1002/smll.201201161. (23) Hu, Z.; Xu, L.; Wang, L.; Huang, Y.; Xu, L.; Chen, J. One-step fabrication of N-doped TiO2 inverse opal films with visible light photocatalytic activity. Catal. Commun. 2013, 40, 106110. DOI: 10.1016/j.catcom.2013.06.010. (24) Zhang, D.; Yang, M. Band structure engineering of TiO2 nanowires by n-p codoping for enhanced visible-light photoelectrochemical water-splitting. Phys. Chem. Chem. Phys. 2013, 15, 18523-18529. DOI: 10.1039/C3CP51044A. (25) Xiao, F.-X.; Miao, J.-W.; Wang, H.-Y.; Yang, H.-B.; Chen, J.-Z.; Liu, B. Electrochemical construction of hierarchically ordered CdSe-sensitized TiO2 nanotube arrays:


26

Page 27 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


towards versatile photoelectrochemical water splitting and photoredox applications. Nanoscale 2014, 6, 6727-6737. DOI: 10.1039/C4NR01380H. (26) Hensel, J.; Wang, G.; Li, Y.; Zhang, J.-Z. Synergistic effect of CdSe quantum dot sensitization and nitrogen doping of TiO2 nanostructures for photoelectrochemical solar hydrogen generation. Nano Lett. 2010, 10, 478-483. DOI: 10.1021/nl903217w. (27) Lee, Y.-L.; Chi, C.-F.; Liau, S.-Y. CdS/CdSe co-sensitized TiO2 photoelectrode for efficient hydrogen generation in a photoelectrochemical cell. Chem. Mater. 2010, 22, 922-927. DOI: 10.1021/cm901762h. (28) Mukherjee, B.; Smith, Y.; Subramanian, V. CdSe nanocrystal assemblies on anodized TiO2 nanotubes: optical, surface, and photoelectrochemical properties. J. Phys. Chem. C 2012, 116, 15175-15184. DOI: 10.1021/jp208879f. (29) Qiu, F.; Han, Z.-J.; Peterson, J.; Odoi, M.; Sowers, K.; Krauss, T. Photocatalytic hydrogen generation

by

CdSe/CdS

nanoparticles.

Nano

Lett

2016,

16,

5347-5352.

DOI:

10.1021/acs.nanolett.6b01087. (30) Zhou, M.; Li, J.; Ye, Z.; Ma, C.; Wang, H.; Huo, P.; Yan, Y.-S. Transfer charge and energy of Ag@CdSe QDs-rGO core-shell plasmonic photocatalyst for enhanced visible light photocatalytic activity. ACS Appl. Mater. Interfaces 2015, 7, 28231-28243. DOI: 10.1021/acsami.5b06997. (31) Haldar, K.; Sinha, G.; Lahtinen, J.; Patra, A. hybrid colloidal Au-CdSe pentapod heterostructures synthesis and their photocatalytic properties. ACS Appl. Mater. Interfaces 2012, 4, 6266-6272. DOI: 10.1021/am301859b. (32) Bang, J.; Kamat, P. Solar Cells by Design: Photoelectrochemistry of TiO2 nanorod arrays decorated with CdSe. Adv. Funct. Mater. 2010, 20, 1970-1976. DOI: 10.1002/adfm.200902234.


27


Page 28 of 43

(33) Du, X.; He, J.-H. Facile size-controllable syntheses of highly monodisperse polystyrene nano-and

microspheres

by

polyvinylpyrrolidone-mediated

emulsifier-free

emulsion

polymerization. J. Appl. Polym. Sci. 2008, 108, 1755-60. DOI: 10.1002/app.27774. (34) Seo, Y.; Woo, K.; Kim, J.; Lee, H.; Lee, W. Rapid fabrication of an inverse opal TiO2 photoelectrode for DSSC Using a binary mixture of TiO2 nanoparticles and polymer microspheres. Adv. Funct. Mater 2011, 21, 3094-3103. DOI: 10.1002/adfm.201002489. (35) Cai, J.; Wu, M.; Wang, Y.; Zhang, H.; Meng, M.; Tian, Y.; Li, X.; Zhang, J.; Zheng, L.; Gong, J. Synergetic enhancement of light harvesting and charge separation over surfacedisorder-engineered

TiO2

photonic

crystals. Chem 2017,

2,

877-892.

DOI:

10.1016/j.chempr.2017.05.006. (36) Zha, R.-H.; Nadimicherla, R.; Guo, X. Ultraviolet photocatalytic degradation of methyl orange by nanostructured TiO2/ZnO heterojunctions. J. Mater. Chem. A 2015, 3, 6565-6574. DOI: 10.1039/C5TA00764J. (37) Zalfani, M.; Schueren, B.; Hu, Z.-Y.; Rooke, J.; Bourguiga, R.; Wu, M.; Li, Y.; Tendeloo, G.-V.; Su, B.-L. Novel 3DOM BiVO4/TiO2 nanocomposites for highly enhanced photocatalytic activity. J. Mater. Chem. A 2015, 3, 21244-21256. DOI: 10.1039/C5TA00783F. (38) Yu, J.-G.; Yu, H.-G.; Cheng, B.; Zhao, X.-J.; Yu, J.-C.; Ho, W.-K. The effect of calcination temperature on the surface microstructure and photocatalytic activity of TiO2 thin films prepared by liquid phase deposition. J. Phys. Chem. B 2003, 107, 13871-13879. DOI: 10.1021/jp036158y. (39) Li, Y.-J.; Wei, C. Photocatalytic degradation of Rhodamine B using nanocrystalline TiO2zeolite surface composite catalysts: effects of photocatalytic condition on degradation efficiency. Catal. Sci. Technol. 2011, 1, 802-809. DOI: 10.1039/C1CY00012H.


28

Page 29 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(40) Tao, B.; Zhang, Y.; Han, D.; Li, Y.; Yan, Z. Synthesis of corundum-type In2O3 porous spheres and their photocatalytic properties. J. Mater. Chem. A 2014, 2, 5455-5461. DOI: 10.1039/C4TA00139G. (41) Chen, Y.; Li, F.; Cao, W.; Li T. Preparation of recyclable CdS photocatalytic and superhydrophobic films with photostability by using a screen-printing technique. J. Mater. Chem. A 2015, 3, 16934-16940. DOI: 10.1039/C5TA04065E. (42) Wang, Y.; Zheng, Y.-Z.; Lu, S.; Tao, X.; Che, Y.; Chen, J.-F. Visible-light-responsive TiO2-Coated ZnO:I nanorod array films with enhanced photoelectrochemical and photocatalytic performance. ACS Appl. Mater. Interfaces 2015, 7, 6093-6101. DOI: 10.1021/acsami.5b00980. (43) Li, G.-L.; Zhang, H.-Y.; Lan, J.; Li, J.; Chen, Q.-W.; Liu, J.-Y. Hierarchical hollow TiO2 Spheres: facile synthesis and improved visible-light photocatalytic activity. Dalton T. 2013, 42, 8541-8544. DOI: 10.1039/C3DT50503K.


29


Page 30 of 43

Graphical Abstract (For Table of Contents Use Only)

An efficient and scalable approach was reported for fabricating N-doping TiO2 inverse opal films decorated with CdSe nanoparticles (CdSe/N-TiO2 IO). CdSe/N-TiO2 IO has a 3D ordered structure which can enhance the interaction between incident light and photocatalysts. The combination of nitrogen-doping and CdSe sensitization effectively inhibits the recombination of photogenerated electrons and holes and improves visible light absorption ability. What’s more, CdSe/N-TiO2 IO exhibits superior photocatalytic activity and excellent cycling stability for photodegradation of organic pollutants in water.


30

Page 31 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Schematic illustration of the synthetic process of CdSe/N-TiO2 IO. 32x16mm (300 x 300 DPI)



Figure 1. SEM images of PS opals (a1, a2 and a3), TiO2 IO (b1, b2 and b3) and N-TiO2 IO (c1, c2 and c3) prepared from PS templates with different particle sizes and (d) reflectance spectra of N-TiO2 IO obtained from PS130, PS280 and PS500. 24x9mm (300 x 300 DPI)


Page 32 of 43

Page 33 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. SEM images of CdSe/N-TiO2 IO with different CdSe loading amount (a, b and c) and N2 adsorption-desorption isotherms and BJH pore-size distribution curve of CdSe/N-TiO2 IO (d). 29x13mm (300 x 300 DPI)



Figure 3. (a) TEM and (b) HRTEM image of CdSe/N-TiO2 IO, and (c-h) HAADF-STEM-EDS elemental mapping of CdSe/N-TiO2 IO. 78x96mm (300 x 300 DPI)


Page 34 of 43

Page 35 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. The XRD patterns of TiO2 IO, N-TiO2 IO and CdSe/N-TiO2 IO. 54x46mm (300 x 300 DPI)



Figure 5. XPS spectra of CdSe/N-TiO2 IO (a) Survey, (b) N 1s, (c) Ti 2p, (d) O 1s, (e) Cd 3d, and (f) Se 3d. 80x102mm (300 x 300 DPI)


Page 36 of 43

Page 37 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. (a) UV-vis absorption spectra and (b) band gap energies of P25, TiO2 IO, N-TiO2 IO and CdSe/NTiO2 IO. 25x10mm (300 x 300 DPI)



Figure 7. (a) The photocurrent density versus time curves for the different samples, and (b) Photoluminescence spectra of P25, TiO2 IO, N-TiO2 IO and CdSe/N-TiO2 IO. 24x9mm (300 x 300 DPI)


Page 38 of 43

Page 39 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. (a) RhB photodegradation curves for P25, TiO2 IO, N-TiO2 IO and CdSe/N-TiO2 IO under visible light and (b) Photocatalytic degradation of 20 mL of 5 mg/L RhB solution under visible light irradiation, in the case of 5, 10, 15, 20 mg CdSe/N-TiO2 IO. 25x10mm (300 x 300 DPI)



Figure 9. (a) Cycling photodegradation of RhB by CdSe/N-TiO2 IO under visible light and (b) SEM images of CdSe/N-TiO2 IO after photodegradation cycling. 24x9mm (300 x 300 DPI)


Page 40 of 43

Page 41 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 2. Schematic diagram of the reaction mechanism for RhB removal. 37x21mm (300 x 300 DPI)



Figure 10. RhB photodegradation curves for CdSe/N-TiO2 IO under different conditions with exposure to visible light: no scavenger, adding BQ, adding t-BuOH and adding AO. 53x44mm (300 x 300 DPI)


Page 42 of 43

Page 43 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 11. (a) DMPO spin-trapping EPR spectra of CdSe/N-TiO2 IO in methanol dispersion for DMPO-·O2and (b) DMPO spin-trapping EPR spectra of CdSe/N-TiO2 IO in water dispersion for DMPO-·OH. 25x10mm (300 x 300 DPI)


N-Doped and CdSe-Sensitized 3D-Ordered TiO2 Inverse Opal Films

Recommend Documents