The pivotal roles of spatially separated charge localization centers on the molecules activation and photocatalysis mechanism

Applied Catalysis B: Environmental 262 (2020) 118251

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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

The pivotal roles of spatially separated charge localization centers on the molecules activation and photocatalysis mechanism

T

Wen Cuia,b, Lvcun Chena,b, Jianping Shengb, Jieyuan Lib,c, Hong Wangb,d, Xing’an Dongb,d, ⁎ Ying Zhoua, Yanjuan Sunb,d, Fan Donga,b, a

The Center of New Energy Materials and Technology, School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China Research Center for Environmental Science & Technology, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 611731, China c College of Architecture and Environment, Sichuan University, Chengdu, Sichuan 610065, China d Chongqing Key Laboratory of Catalysis and New Environmental Materials, College of Environment and Resources, Chongqing Technology and Business University, Chongqing 400067, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Heterostructure Separated charge localization centers Heterogeneous photocatalysis Molecules activation Reaction mechanism

Even though g-C3N4-based heterostructure have been constructed widely for remarkable photocatalytic performance, the function of active sites on different sides of heterostructure remains to be elucidated. Here, we develop (BiO)2CO3 nanospheres decorated g-C3N4 hybrid heterostructure (CN-BOC), aiming to elaborate the role of different active sites between CN and BOC and unravel the promotion mechanism. It is revealed that the spatially separated charge localization centers resulting from diverse active sites have been firstly identified in the heterostructure. The promoted adsorption/activation of NO and O2 molecule in different sites and accelerated spatial charge carriers separation have been confirmed. Correspondingly, the NO molecules prefer to adsorb on the electron-deficient areas and donate electrons to achieve a higher valance state. Inversely, the O2 molecule tend to adsorb on the electron-efficient areas to receive more electrons for production of •O2− radicals, which contributes to the efficient photocatalytic conversion of NO into nitrites/nitrates. The present work could broaden insights into the function of spatially separated active sites for the adsorption/activation of reactants on hybrid heterostructure and provide a novel strategy to advance the development of photocatalytic technology.

1. Introduction The landmark event of photocatalysis under ultraviolet (UV) light was ignited by the pioneering study of Fujishima and Honda in 1972 [1]. Since then, there has been substantial development in heterogeneous photocatalysis for photocatalytic degradation of pollutants and conversion of solar energy in response to addressing the environmental and energy issues [2–5]. The design of visible-light-responsive photocatalysts is vastly pursued for effective utilization of the solar spectra (ca. 43 %) [6–8]. Recently, a metal-free layered conjugated semiconductor, namely graphitic carbon nitride (labeled as CN), has become an alternative and attractive photocatalyst in view of its facile synthesis, appealing electronic structure and high physicochemical stability [9–11]. The heterogeneous photocatalysis involves successive procedures of molecular reactants adsorption, photoactivated reaction, and products

desorption occurring at the catalyst surface [12–14]. Apparently, the adsorption of reactants on the surface of catalyst is the prerequisite condition for heterogeneous photocatalysis. Subsequent photocatalytic reaction in terms of visible-light utilization, charge carrier separation, and redox ability of radicals also serves for photocatalytic performance [15]. Considering the photocatalytic NO removal on CN, the oxidation of NO molecule for pollutant activation and the reduction of O2 molecule for reactive oxygen species (ROS) generation are crucial to deplete the acidic contaminant. Nevertheless, rarely exposed active sites and random transfer of charge carriers in CN, arising from intrinsic graphitic sp2-hybridized array of tri-s-triazine repeating units and inert stack of layers, hinder the adsorption/activation of reactants and the separation of charge carriers and thus gives rise to unsatisfactory performance [16–19]. Therefore, several modifications of bare CN have been systematically conducted to optimize the photoactivity. The introduction of

⁎ Corresponding author at: The Center of New Energy Materials and Technology, School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China. E-mail address: [email protected] (F. Dong).

https://doi.org/10.1016/j.apcatb.2019.118251 Received 28 May 2019; Received in revised form 25 September 2019; Accepted 1 October 2019 Available online 20 October 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.

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for 12 h.

impurities into CN matrix via copolymerization and doping has been proposed, aiming to modify the electronic structure and energy band configuration [20–22]. Besides, the construction of CN-based hybrid nanocomposites including metal deposition, incorporation with nanomaterials and coupling with other semiconductors becomes an increasing requirement to introduce extra active sites [23–27]. Generally, the construction of semiconductor/CN hybrid heterostructure photocatalysts has been reported widely, such as metal oxide/CN hybrid nanocomposites [28,29], metal sulfide/CN hybrid nanocomposites [30,31], complex compound/CN hybrid nanocomposites [32,33], and metal organic framework/CN hybrid nanocomposites [34,35]. However, the function of active sites concerning the adsorption/activation of reactants and the separation of charge carriers are still unspecified on the hybrid heterostructure. This also makes the subsequent reaction pathway and promotion mechanism of photocatalysis unclear, which restricts the advance of heterogeneous photocatalysis. According to our previous work, CO32– from carbonates could attack the periodic arrangement of intralayer melon strands (hydrogen bonds) of CN in co-pyrolysis process, which induces more active sites to be exposed and shortens the charge carriers transportation distance [36,37]. The (BiO)2CO3 (BOC), which possesses an Aurivillius-layered structure consisting of [Bi2O2]2+ slabs interleaved between two slabs of CO32–, has been reported several times on the construction of BOC/CN hybrid heterostructure for photocatalytic pollutant removal [38–41]. Wang et al. discovered that partly self-sacrificing of CN could supply CO32– to help construct BOC in one-pot hydrothermal reaction with bismuth source and pure CN [38]. However, the interfacial structure and the active sites on the BOC/CN hybrid heterostructure are not revealed, which is of significant importance to understand the catalysis mechanism. In this work, we employ a one-step in situ co-pyrolysis of urea and BOC to destroy the intralayer melon strands and simultaneously induce the thermal reconstruction of BOC to develop (BiO)2CO3 nanospheres decorated g-C3N4 hybrid heterostructure photocatalysts (CN-BOC) in order to introduce abundant and diverse active sites. Furthermore, according to the experimental characterization and theoretical simulation, the spatially separated charge localization centers have been identified due to the construction of diverse activate sites between CN and BOC. This could promote the adsorption/activation of NO molecule and O2 molecule in different sites and accelerate the spatial separation of charge carriers to realize a highly efficient photocatalytic NO removal. Definitively, NO molecules prefer to adsorb on the electrondeficient areas and donate electrons to achieve a higher valance state, which favors further photocatalytic oxidation. On the contrary, O2 molecule tend to adsorb on the electron-efficient areas to receive more electrons for the production of •O2− radicals to participate in the photocatalytic reaction. The present work could broaden insights into the function of separated active sites for the adsorption/activation of reactants and spatial separation of carriers on hybrid heterostructure photocatalysts and provide a novel strategy to advance the development of photocatalytic technology.

2.1.2. Synthesis of CN-BOC-X samples The samples were synthesized via co-pyrolysis of urea and BOC. First, 10 g of urea and a known amount of BOC (0.01, 0.02, 0.03, 0.04, and 0.05 g) were added to an alumina crucible (50 mL) with 20 mL distilled water. The obtained solution was dried in oven at 60 ℃. The solid precursors were placed in a semi-closed alumina crucible with a cover and calcined at 550 ℃ for 2 h at a heating rate of 15 ℃/min in static air. After the thermal treatment, the obtained samples with different weight ratios of urea and BOC were collected and labeled as CNBOC-X (where X represents the amount of BOC). Prestine CN was synthesized by pyrolysis 10 g of urea. Detailed information on characterization of the catalysts is available in Supplementary Material. 2.2. Evaluation of photocatalytic activity The photocatalytic activity was continuously detected by a NOX analyzer (Thermo Environmental Instruments Inc., model 42c-TL) in a continuous-flow reactor. After adsorption-desorption equilibrium achieved, a 150 W commercial tungsten halogen lamp (the average light intensity was 0.16 W/cm2) was turned on and the removal ratio (η) of NO was calculated as η = (1 − C/C0) × 100%, where C and C0 are the concentrations of NO in the outlet steam and the feeding stream, respectively. Detailed descriptions about the apparatus and experimental method are available in the Supplementary Material. 2.3. In situ DRIFTS investigation In situ DRIFTS measurements were conducted using a TENSOR II FTIR spectrometer (Bruker) equipped with an in situ diffuse-reflectance cell (Harrick). A Xe lamp (MUA-210, Japan) was used as the irradiation light source. Detailed descriptions of the in situ DRIFTS apparatus and experimental method are available in the Supplementary Material. 2.4. DFT calculation All spin-polarized DFT-D2 calculations were performed with the “Vienna ab initio simulation package” (VASP 5.4), utilizing a generalized gradient correlation functional [42,43]. A plane-wave basis set with a cut-off energy at 450 eV within the framework of the projectoraugmented wave method was used [44,45]. The Gaussian smearing width was set to 0.2 eV. The Brillouin zone was sampled with a 3 × 3 × 1 K points. All atoms were converged to 0.01 eV/Å. The adsorption energy (Eads) is defined as: Eads = Etot – (Es + Emol) where Etot, Es, and Emol represent the total energy of the adsorption complex, the pure substrate, and the isolated molecule, respectively. 3. Results and discussion

2. Experimental 3.1. Microstructure and chemical composition 2.1. Sample preparation The crystal structure of CN and CN-BOC-X samples are examined by XRD patterns. As shown in Fig. 1a, two characteristic diffraction peaks at 13.1° and 27.2°, arising from in-plane structural repeating motifs of aromatic systems and interlayer reflection of graphite-like structure, reflect the layered conjugated of CN [46,47]. Notably, these two peaks for CN-BOC-X samples are gradually diminished with the addition of BOC, which indicates the destruction of in-plane periodicity of aromatic systems and the fluctuation of stacking interlayer structure. In addition, an obvious characteristic diffraction peak around 30° is observed in CNBOC sample, which can be attributed to the (013) crystal plane of BOC (PDF#41-1488) and thus indicates the introduction of BOC on CN.

All chemicals employed in this study were analytical grade and were used as obtained without further treatment. 2.1.1. Synthesis of (BiO)2CO3 BOC was obtained by a simple hydrothermal method. In a typical synthesis, 0.46 g Na2CO3 was added to 70 mL distilled water and stirred vigorously for 10 min. Then, 1.6 g C6H5BiO7 was added to the above aqueous solution and stirred continuously for 30 min. And the suspension was hydrothermally treated at 160 °C for 24 h. Subsequently, the obtained samples were filtered, washed with ethanol and dried at 60 °C 2

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Fig. 1. Microstructure and chemical composition of as-prepared samples. XRD patterns (a) and high-resolution XPS spectra (b–d) of samples; HRTEM images of CN (e) and CN-BOC-4 (f–h); HRTEM image (i) and corresponding line scanning (j) and high magnification EDS chemical composition maps for CN-BOC-4 sample (k–n), individual Bi (red), O (yellow), C (blue), and N (green) maps and their composites. The scale bar of HRTEM images are 20 nm (e, f), 5 nm (g), 50 nm (h, i, k–n) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

intralayer of CN to BOC NSs, which induces CN and BOC to function as electron-deficient center and electron-efficient center, respectively. Therefore, spatially separated localization centers of charge carriers are constructed and thus the distribution of charge carriers is regulated, which is in accord with the shifted XPS adsorption peaks. Correspondingly, greatly diminished PL peaks (Fig. 2b) also demonstrates that the migration and transformation of charge carriers have been promoted. Meanwhile, the light absorption properties of the as-obtained samples were investigated by UV–vis DRS spectroscopy. As shown in Fig. 2c, the pure BOC exhibits optical absorption mainly in UV region. The low visible-light absorption comes from its special hierarchical microsphere structure with multiple light surface scattering and reflecting effects [54]. Along with the decoration of BOC NSs, a redshift in the optical absorption band edge was observed for the CN-BOCX samples. The broadened absorption range arises from the co-polymerization of CN and BOC, which is also consistent with the gradually deepened color of as-prepared samples (Fig. 2d). Therefore, the introduction of BOC NSs can broaden the light absorption range and construct separated localization centers of charge carriers, which is beneficial to promote the molecules activation and carriers separation. Also, the band energies (Eg) estimated from the intercept of the tangents to the plots of (αhv)1/2 vs. photo-energy (Fig. S2a) are 2.61 and 3.21 eV for pure CN and BOC samples, respectively. The electrochemical Mott-Schottky measurements were used to obtain the energyband potential of the samples (Fig. S2b). The flat potential of CN and BOC obtained by extrapolation of the Mott-Schottky plot are roughly −1.48 and −0.89 V, respectively, versus the saturated calomel electrode (SCE), which is equivalent to −1.24 and −0.65 V, respectively, versus the normal hydrogen electrode (NHE). Since the flat band potential (quasi Fermi level) is 0.1 V lower than the conduction band minimum for n-type semiconductors [55], the conduction band (CB) minimum of CN and BOC are −1.34 and −0.75 V, respectively. Therefore, the valence bands (VB) of CN and BOC were determined to be 1.27 and 2.46 V, respectively.

Besides, as shown in HRTEM, clear lattice fringes with a lattice spacing of 2.952 Å are found in Fig. 1g, which match the spacing of the (013) crystal planes of the BOC nanospheres (BOC NSs, 3–10 nm). To further confirm the spatial elemental distribution of CN-BOC-4 sample, line scanning (Fig. 1j) and EDS chemical composition map (Fig. 1k–n) are provided [48,49], and it is can be clearly observed that BOC NSs are distributed on the surface of CN. Also, the chemical structure and composition of CN and CN-BOC-X samples are examined by XPS measurements (Fig. 1e–g). Two strong peaks at 159.6 and 164.8 eV (Fig. 1e) can be ascribed to Bi 4f7/2 and Bi 4f5/2 of Bi3+, which is the symbol of Bi3+ in BOC. The O 1s spectra are recorded and can be fitted by three peaks. The peaks at 530.3, 531.7 and 533.2 eV are characteristic of Bi-O, carbonate species and surface hydroxyl groups, respectively [50,51]. The binding energies of C 1s at 284.8 eV correspond to the sp2 CeC bonds of CN [52,53]. The enhanced peak of C 1s at 288.1 eV can be not only assigned to sp2-bonded carbon in the N-containing aromatic rings (NeC]N) but also ascribed to carbonate ion in BOC. The N 1s spectra are deconvoluted into three peaks at 398.6 eV, 400.3 eV and 401.5 eV (Fig. 1g), corresponding to the sp2bonded N involved in the triazine rings (CeN]C), tertiary nitrogen N–(C)3 groups and amino functions (CeNeH) in CN, respectively [52,53]. It's worth noting that the adsorption peaks of NeC]N and CeN]C shift toward higher binding energy. The left shift of these adsorption peaks indicates that the introduction of BOC NSs decreases the electron density in CN and then gives rise to the charge redistribution. 3.2. Photoelectric property Subsequently, we employ the theoretical simulation (Density Functional Theory, DFT) to explore the transfer and redistribution of charge carriers between CN and BOC NSs. Combing the experimental characterization analysis, the reasonable and optimized geometric structure of CN-BOC are constructed. As shown in differences in charge density distribution (Fig. 2a), electrons tend to transfer directedly from 3

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Fig. 2. The photoelectric property of samples. Charge density difference distribution of CN-BOC sample: charge accumulation is shown in blue and depletion in yellow (a); photoluminescence spectra of photocatalysts (b); UV–vis spectra of samples (c); pictures of as-prepared samples (d); gray, brown, purple and red and pink spheres stand for C, N, Bi and O atoms, respectively; the isosurfaces are set to 0.0015 eV Å−3 (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Note that another obvious absorption band of NO+ (at 2151 cm−1) arises gradually over CN-BOC-4 samples [56,61]. As evidenced by the DFT calculation, NO molecule prefer to donate electrons to electrondeficient areas (around the CN) to form NO+, which is beneficial to the transformation of NO into nitro compounds during photocatalytic NO oxidation process [59,62]. Also, final products (NO2−/NO3−) in dark reaction are detected, which mainly result from two-coordinated N atoms of CN that induce the formation of the activated oxygen species to participate in the oxidation of pollutants [63,64]. Therefore, the redistribution of charge carriers significantly promotes the adsorption and activation of NO molecules to facilitate subsequent photocatalytic NO oxidation reaction.

3.3. Adsorption and activation of NO molecule on the active site The adsorption and activation of NO molecule are subsequently probed by DFT calculations. The significantly increased adsorption energy (from −0.11 eV for CN to −-2.13/−2.48 eV for CN-BOC) implies that the adsorption of NO molecule is greatly promoted after the decoration of BOC NSs. Also, compared with the adsorption energy of CN-BOC sample in different adsorption sites (Figs. 3a, S3), the NO molecule tends to absorb on the side of CN. And much higher total charge of NO molecules (Δq = 2.06 e, calculated with Bader method [31]) has been achieved on this side site. Due to the formation of spatially separated localization centers of charge carriers, the activation of NO molecules is accelerated by donating electrons to electron-deficient areas, and importantly, it can simultaneously enable the NO conversion into a higher valance state for further favorable photocatalytic oxidation. Consequently, to experimentally characterize the adsorption and activation of NO molecules, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was employed to dynamically monitor intermediates and products on the photocatalyst surface in time sequence during NO adsorption processes. As shown in Fig. 3b and c, the absorption bands related to NO species (NO, NO2, N2O4) appeared once NO was introduced at 25 °C in dark [56–59]. Also, the absorption band approximately at 2182 cm−1 associated with nitrosyl species (NO+) is detected both on CN and CN-BOC-4 samples [60].

3.4. Activation of O2 molecule and evaluation of the photocatalytic performance The time-dependent IR spectra of CN and CN-BOC-4 under visiblelight irradiation were recorded once adsorption equilibrium was achieved. As shown in Fig. S4, abundant IR adsorption bands characteristic of final products are observed, demonstrating that accumulated intermediates are transformed into nitrites or nitrates (NO2−, NO3−) during photochemical process [56–58,61,65–68]. Correspondingly, reactive oxygen species (ROS), which is responsible for the conversion of the intermediates into the final products under visiblelight irradiation, is confirmed using the DMPO spin-trapping ESR 4

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Fig. 3. Adsorption and activation of NO molecules. Optimized geometric structure of NO molecules adsorption on CN and CN-BOC (a), gray, brown, purple and red and pink spheres stand for C, N, Bi and O atoms, respectively; all lengths are given in Å; Eads stand for the adsorption energy of adsorption molecule; in situ DRIFTS spectra on CN (b) and CN-BOC (c) samples during NO adsorption processes (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 4. Adsorption and activation of O2 molecules. DMPO spin-trapping ESR spectra (•O2−) of CN and CN-BOC-4 samples(a); optimized geometric structure of the adsorption of O2 molecules on CN and CN-BOC (b), gray, brown, purple and red and pink spheres stand for C, N, Bi and O atoms, respectively; all lengths are given in Å; Eads stand for the adsorption energy of adsorption molecule (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

experimental method. Much stronger DMPO-%O2− signals have been detected for CN-BOC-4 than that for pristine CN (Fig. 4a). Furthermore, the adsorption and activation of O2 molecule was simulated by DFT calculation. After the optimization, the O2 molecules can be favorably absorbed on the top site of CN-BOC. As evidenced by the increased adsorption energy of CN-BOC, O2 molecules prefer to adsorb in electron-efficient areas (around the BOC) and thus much higher total charge (Δq = −0.75 e) is achieved (Figs. 4b, S5). The redistribution of charge carriers in CN-BOC enables O2 molecules to receive more electrons and to be activated for the enhanced production of %O2− radicals, which is

accord with the increased signals associated with %O2− radicals. Increased consumption of photo electrons scavenger agent also has been confirmed under visible-light irradiation (Fig. S6a), corresponding to the promoted generation of %O2−. Therefore, the introduction of BOC induces the charge redistribution and the formation of separated localization centers of charge carriers to facilitate the adsorption and activation of reactants (NO and O2). In addition, increased signals characteristic for DMPO-%OH was detected (Fig. S6b), indicating the promoted generation of hydroxy radicals (%OH) on CN-BOC-4 sample. Since the valance band (VB) of CN 5

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Fig. 5. Evaluation and analysis of the photocatalytic performance. Photocatalytic activity comparison (a) and cycling runs of CN-BOC-4 sample under visible-light irradiation (b).

could promote adsorption/activation of NO and O2 molecules in different sites and accelerate the spatial separation of charge carriers. The roles of spatially separated charge localization centers and the reaction mechanism of photocatalytic NO oxidation CN-BOC on heterostructure are illustrated in Fig. 6. 4. Conclusion The present work develops a facile one-step co-pyrolysis method to prepare (BiO)2CO3 nanospheres decorated g-C3N4 for highly efficient photocatalytic NO removal. The introduction of BOC contributes to the formation of abundant and diverse active sites resulting in spatially separated charge carrier localization centers, which facilitate the adsorption and activation of reactants and accelerate the spatial separation of charge carriers. Specifically, the oxidation of NO molecule for the facilitated pollutant activation and the reduction of O2 molecule for the production of •O2− radicals have been realized efficiently on different active sites in terms of photocatalytic NO removal. This research reveals the pivotal roles of active sites on the hybrid heterostructure and the photocatalysis reaction mechanism and then provides a novel modification strategy to advance the development of photocatalytic technology for efficient air purification. Declaration of Competing Interest Fig. 6. The illustration about roles of the spatially separated charge localization centers in (BiO)2CO3 nanospheres decorated g-C3N4 hybrid heterostructure and the reaction mechanism of photocatalytic NO oxidation.

None. Acknowledgements

is not positive enough to form %OH, the generation of %OH should originate from the transformation %O2–, along the reduction path of %O2– → H2O2 → %OH. The singlet oxygen (1O2) was also identified via reacting with 4-oxo-TEMP to produce 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPONE, a three line spectrum with relative intensity ratio of 1:1:1). The significantly enhanced characteristic peaks of TEMPONE after irradiation confirmed the presence of singlet oxygen which could be the results that the hole traps one electron from •O2– (Fig. S6c). Thus, we can infer that the electron excitation and charge transportation on CN-BOC-4 are effectively promoted, contributing to the formation of ROS for highly efficient pollutant removal. The photocatalytic performance of prepared samples towards NO removal was evaluated under visible-light irradiation (λ ≥ 420 nm). As shown in Fig. S7, the maximum NO removal ratio was observed after ca. 5 min and all CN-BOC-X samples exhibited superior activity in comparison with the pristine CN. The CN-BOC-4 sample exhibits the highest NO removal ratio (53.28%) and noticeably stability after five circulating runs (Fig. 5). The highly efficient performance of CN-BOC can be attributed to the spatially separated charge localization centers that

This work was supported by the National Natural Science Foundation of China (21822601, 21777011 and 21501016), the Plan for "National Youth Talents" of the Organization Department of the Central Committee and “the Fundamental Research Funds for the Central Universities” (ZYGX2019Z021). The authors also acknowledge the AM-HPC in Suzhou, China for computational support. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2019.118251. References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu, X. Wang, Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances, Chem. Soc. Rev. 45 (2014) 5234–5244. [3] Y. Ren, D. Zeng, W. Ong, Interfacial engineering of graphitic carbon nitride (g-

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