Heterojunction tuning and catalytic efficiency of g-C3N4-Cu2O with

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Kinetics, Catalysis, and Reaction Engineering

Heterojunction tuning and catalytic efficiency of g-C3N4-Cu2O with glutamate Dongya Li, Jie Zan, Liping Wu, Shiyu Zuo, Haiming Xu, and Dongsheng Xia Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04581 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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Industrial & Engineering Chemistry Research

Heterojunction tuning and catalytic efficiency of g-C3N4-Cu2O with glutamate Dongya Li1, 2*, Jie Zan1, Liping Wu1, Shiyu Zuo1, Haiming Xu2, Dongsheng Xia1* 1 School

of Environmental Engineering, Wuhan Textile University, Wuhan, 430073, P.R. China.

2 Engineering

Research Center Clean Production of Textile Dyeing and Printing, Ministry of Education, Wuhan, 430073, P.R. China.

*Corresponding author. E-mail: [email protected] and [email protected]

ABSTRACT The g-C3N4-Cu2O (CNCu2O-G) with a p-n heterojunction structure in the presence of glutamate was successfully developed through hydrothermal synthesis and high-temperature calcination. When addition the glutamate, the surface morphology of the heterojunction was regulated, and its specific surface area increased by a factor of approximately 1.5, and reaction sites were increased. Additionally, the charge recombination rate of the heterojunction was reduced, and the absorption band was redshifted to 460 nm, leading to stronger heterostructure contact interfaces, thus the visible-light utilization efficiency was enhanced. The catalytic performance of the CNCu2O-G was very stable, with no significant change in the photocatalytic efficiency of CNCu2OG after six recycles, the Fourier transform infrared patterns and X-ray diffraction of CNCu2O-G exhibited no significant changes after the photocatalytic reaction. Furthermore, electron paramagnetic resonance and free-radical removal experiments indicated that the holes and superoxide radicals were dominant species during the reaction.

Keywords: photocatalytic; g-C3N4-Cu2O; heterojunction; glutamate. 1. INTRODUCTION The energy crisis and environmental issues have been regarded as major topics of concern in recent years1-2. Photocatalytic technology can directly use light energy to produce free radicals with high activity, thereby efficiently degrading organic pollutants; thus, it has received wide attention. However, conventional photocatalytic technology still faces problems such as low light utilization efficiency and poor catalyst stability3-4; thus, the search for new catalysts has become a popular research topic. For instance, Zhang et al. synthesized a TiO2-graphene material which can photocatalytically remove bisphenol A with a higher efficiency5, while Liang et al. synthesized an Ag3PO4 @ UMOFNs material with the core-shell structure and showed high photoinduced charge separation efficiency and photocatalytic efficiency6. Cui et al. synthesized a series of 3D hydrogel materials for photocatalysis, and showed that the highly efficient photocatalytic performance of these materials provides a green and convenient method for the degradation of pollutants7-13. Guo et al. explored the high-efficiency photocatalytic degradation of tylosin with the g-C3N4 material14-15. As a new n-type semiconductor ， Graphite carbon nitride (g-C3N4) has drawn widely attentions in photocatalysis area recently as a result of its low preparation cost, low toxicity and suitable band gap. Cui et al. synthesize a g-C3N4/rGH hybrid hydrogel, and showed that this material could synergistically achieve self-regeneration through adsorption and photocatalytic

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degradation16. However, g-C3N4 has shortcomings such as a high photogenerated electron-hole recombination rate, low utilization rate that limit its application and so on17. To date, many methods have been developed to boost the photogenerated carriers’ separation efficiency and the g-C3N4’s photocatalytic performance, e.g., increasing the specific surface area, changing the morphology, and forming different types of heterojunctions by loading metal oxides (e.g., Cu2O, etc.)18. Cu2O is a p-type semiconductor (Eg = 2.0 eV)19. It has attracted the attention of researchers because of its simple preparation, low cost, low toxicity, the wide range of visible-light absorption, and easy morphology control20. Thus, the g-C3N4-Cu2O heterojunctions has become a focus of research in recent years. Many studies have been reported that the g-C3N4 and Cu2O can be combined by different methods to form the typical p-ntype g-C3N4-Cu2O heterojunction to enhance the photocatalytic performance of the material. g-C3N4-Cu2O was prepared by Anandan et al., which showed efficient interparticle charge transfer efficiency and lower electron-hole recombination rate21. Yan et al. reported a simple self-assembly method for preparation of a g-C3N4-Cu2O-RGO composite 3D aerogel photocatalyst that influenced the g-C3N4-Cu2O surface structure, promoted the transfer of electrons, and improved the photocatalytic performance19. Zuo et al. reported some studies in which g-C3N4 was successfully modified by adding eutectic salt (LiCl and KCl)22 or adding acid to form a single layer of nanometer rice23. Then, the modified g-C3N4 was used to synthesize a g-C3N4-Cu2O heterojunction material with different morphological structures via the hydrothermal method. Finally, the surfactant polyethylene glycol was added to regulate the surface of the g-C3N4-Cu2O heterojunction material, forming a more abundant heterojunction interface as a channel for photogenerated carrier separation, effectively improving the light utilization efficiency24-25. However, the block agglomeration between Cu2O and g-C3N4, poor dispersion, and other defects hindered the realization of the full potential of the heterojunction. Therefore, we added glutamate to regulate the morphology of the heterojunction and improve its catalytic efficiency. In the study, a CNCu2O-G heterojunction with better photocatalytic properties compared with the CNCu2O was synthesized by adding glutamate to the g-C3N4-Cu2O structure. The effects of the glutamate addition on the specific surface area, band structure, absorption band edge, charge recombination rate, and performance of the CNCu2O-G were examined. Furthermore, the underlying photocatalytic mechanism was suggested according to the characterization and radical-trapping experiments, and it was proven that the main active species were the holes and superoxide radicals. This study provides a practical guide for increasing the charge-separation efficiency and photocatalytic performance.

2. EXPERIMENTAL SECTION 2.1 Materials. Glutamate, copper(II) sulfate pentahydrate (CuSO4·5H2O), melamine, xylitol, methanol, and ethanol were acquired from Sinopharm Chemical Reagent Co., Ltd. Moreover, the 1,4-benzoquinone (BQ), Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), tert-butyl alcohol (TBA), and 5,5-dimethyl-1-pyrroline-N-oxid (DMPO) were acquired from Shanghai Aladdin


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Biochemical Technology Co., Ltd. and were used as free-radical scavengers. In this work, the purity of all chemicals used was analytically pure.

2.2 Synthesis of the g-C3N4. The sample was obtained via a high-temperature calcination and a simple chemical exfoliation method26. First, some melamine was heated to 550 ℃ in the muffle furnace at the heating rate of 5 ºC∙min-1 and held for 4 h. After grinding, the paleyellow powder was successfully acquired. Then, the g-C3N4 was added to H2SO4 (98 wt%, 10 mL) and mixed well. Next, the mixture was added dropwise to deionized water (100 mL), followed by ultrasonication to remove the exfoliation. The resulting suspension was centrifuged, washed, and dried to obtain the further purified g-C3N4. Finally, the resulting product was subjected to condensate reflux at 65 °C in methanol for 6 h for the removal of the structural defects. The g-C3N4 nanosheets were obtained after drying.

2.3 Preparation of CNCu2O-G composite photocatalyst. First, the CuSO4·5H2O (1.25 g), xylitol (1.5 g), and the g-C3N4 powder (0.08 g) were added in water (30 mL). The prepared solution was mixed while glutamate (0.5 g) was added to adjust the pH to 13. After stirring for 0.5 h, the solution was transferred to polytetrafluoroethylenes liner to conduct the hydrothermal process with a temperature of 180 °C for 30 h. After cooling, using the deionized water and methanol to wash the products. Then, the sample was dried at 60 °C overnight to obtain the black powder. Finally, the powder was kept at 200 °C for 2 h in a tube furnace. The product obtained after washing and drying was denoted as CNCu2O-G. Pure Cu2O without the g-C3N4 particles and g-C3N4-Cu2O (CNCu2O) without glutamate were also prepared via the same procedure. The synthesis process was shown in Figure 1.

2.4 Characterization. The structures of the catalysts were received via the X-ray diffraction (XRD, PANalytical, Netherlands) under the 40 kV operating voltage with the Cu-Ka source diffractometer. Fourier transform infrared (FT-IR, Nicolet Avatar, USA) spectrum were analyzed using the spectrometer with the KBr pellets. Transmission electron microscopy (TEM, JEOL JEM-2010, Japan) and scanning electron microscopy (SEM, Hitachi S-4800, Japan) were used to observe the catalysts' morphological structure. Xray photoelectron spectroscopy (XPS, BRUKER, Germany) were obtained for analyzing the chemical compositions of the catalysts. The specific surface area was analyzed via Brunauer-Emmett-Teller (BET, ASAP 2020, USA). The photoluminescence (PL, VARIAN Agilent, USA) was investigated on F-4600 fluorescence spectrometer and the samples were excited at 385 nm. UV-vis diffuse reflectance spectra (DRS, Shimadzu, Japan) were recorded on the UV-3600 instrument. Electron spin resonance (EPR, JEOL, Japan) spectra were obtained using the JES FA200 EPR Spectrometer. Surface photovoltage (SPV, PLSPS/IPCE1000,) values were obtained using a steady-state surface photovoltage spectrometer. The photoelectrochemical was



measured on Perfect Light electrochemical analyzer (model PEC1000) with standard three‐electrode mode, and the electrolyte was 0.2 M Na2SO4 solution.

2.5 Photocatalytic activity. In this experiment, the LED lamp with a wavelength range of 400 to 770 nm was applied as a light source, and the experimental reaction temperature was 25.5 °C. The photocatalytic performance of the products were assessed using methyl orange (MO) as the simulated contaminant. The concentration of MO was 20 mg∙L-1, and the concentration of the catalyst was 500 mg∙L-1. Before the photocatalytic reaction, the catalysts were reacted in dark for 0.5 h to remove its own adsorption effect. Then, the reaction was continued for 0.5 hour under light, and 1 mL of the solution was obtained at different time. The collected solution was examined on the UV-vis spectrophotometer (UV-8000S) with the wavelength of 463 nm.

2.6 Determination of Cu2O and Cu+ content. The CNCu2O-G composite (5 mg) was dissolved in H2SO4 (10 mL, 98 wt %), and stirred at room temperature for 2 h to dissolve totally; then, the supernatant was filtered, and the Cu+ content of the solution was determined by atomic absorption spectrometry in order to calculate the Cu2O content. The Cu+ dissolution value during the reaction was determined in the following conditions: the catalyst content in the reaction was 500 mg∙L-1, the MO content was 20 mg∙L-1, the reaction time was adsorption for half an hour, and the visible light was irradiated for half an hour. The time point sampling was used to determine the Cu+ content by the atomic absorption method. Three sets of data were measured and average values were obtained.

3. RESULTS AND DISCUSSION 3.1 Characterization. The crystal structures of Cu2O, g-C3N4, CNCu2O, and CNCu2O-G were obtained via XRD analysis, as displayed in Figure 2(a). The peaks for g-C3N4 were observed at 13.04° and 27.25°, corresponding to the (100) and (002) diffraction planes, respectively. This could indicate that our synthetic sample contained triazine units27. For the pure Cu2O, peaks were observed at 29.58°, 36.44°, 42.32°, 61.40°, 73.55°, and 77.41°, corresponding to the (110), (111), (200), (220), (311), and (222) crystal planes, respectively. For the CNCu2O, peaks of g-C3N4 and Cu2O appeared simultaneously, indicating that two-phase substances of gC3N4 and Cu2O existed in CNCu2O. Adding glutamate did not change the phase composition of CNCu2O, whereas it was beneficial to the enhancement of CNCu2O-G crystallinity. The chemical ingredient of the Cu2O, g-C3N4, CNCu2O, and CNCu2O-G was determined using the FT-IR spectra (shown in Figure 2(b)). The convolution peak at 1,640 cm-1 corresponding to the modes of the C=N stretching vibration19. For the Cu2O sample, the peak at 630 cm-1 could be classified as the stretching vibration of the Cu(I)-O bond28. All of mentioned absorption peaks of g-C3N4 and Cu2O occurred in the spectra of the CNCu2O-G, indicating that both semiconductors were present in the


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material. These results are similar to the XRD results revealed in Figure 2(a). The morphologies of the g-C3N4, Cu2O, CNCu2O and CNCu2O-G samples were comparatively analyzed by SEM, as shown in Figures 3(a)-3(d). Pure g-C3N4 (Figure 3(a)) had a significant sheet structure, and the layers were very closely stacked. The pure Cu2O exhibits a spherical structure in Figure 3(b). For CNCu2O (Figure 3(c)), g-C3N4 is scattered on the surface of Cu2O, and the two were combined, forming a two-phase structure. As exhibited in Figure 3(d), the particle size of CNCu2O-G was increased after the addition of glutamate. The larger particle size could promote the precipitation rate of the catalysts, which was advantageous for recycling22. To further explore the microstructure of the CNCu2O-G, we observed the CNCu2O and CNCu2O-G using a high-resolution TEM. The two-phase structure is observed in both Figures 3(e) and 3(f), indicating that a heterojunction was formed in both cases, preventing the oxidation of elemental copper and thereby improving the efficiency of electron-hole pair separation29. This will strongly promote the photocatalytic reaction. Moreover, the (111) crystal plane of Cu2O was clearly observed in CNCu2O-G with the crystal face spacing of 0.246 nm. The (111) crystal plane peak remained after the addition of glutamate, indicating that the lattice structure was not changed. The XPS spectra of Cu 2p (a), N 1s (b), C 1s (c), and Cu LMM (d) for the Cu2O, g-C3N4, CNCu2O, and CNCu2O-G samples were depicted in Figure 4. The XPS analysis confirms the chemical state and the specific binding of the CNCu2O-G sample. The survey scan indicates that the carbon, nitrogen, oxygen, and copper elements were all present in the CNCu2O-G composite, which indicated that Cu2O and g-C3N4 were simultaneously present in CNCu2O-G. XPS results combined with XRD and FT-IR results proved the successful synthesis of CNCu2O-G. As shown in Figure 4(a), there are two characteristic peaks in the Cu 2p orbit around at 932.4 and 952.3 eV, assigning to the binding energy of Cu 2p3/2 and Cu 2p1/2 spin-orbit photoelectrons, respectively28. However, the two binding energies are very close, so it is difficult to determine the valence state of Cu, which belongs to Cu+ or Cu0. But it can be distinguished by the Cu LMM peak30-31. Figure 4(d) exhibits that all of the Cu LMM peaks of the catalysts were found at approximately 570 eV, corresponding to Cu+ 24. For CNCu2O, the peak at 569.7 eV had a slight shift in comparison to that of the pure Cu2O, and the peak was further shifted (at 569.5 eV) upon the introduction of glutamate. This finding indicates that the chemical environment of the Cu+ ions changed, suggesting the existence of a chemical bonding interface between Cu2O and g-C3N4 rather than the presence of two independent Cu2O and g-C3N4 phases. The N 1s core-level spectrum (Figure 4(b)) contains three deconvoluted peaks at 398.8, 400.0, and 401.3 eV. The strong peak at 398.8 eV is mainly derived from the sp2-bonded N participated in the triazine rings (C-N=C), and the weak peaks at 400.0 eV and 401.3 are ascribed to the N-(C)3 and C-N-H groups of g-C3N4, respectively32. The peaks of CNCu2O-G in N 1s are located at 398.9, 400.01, and 401.4 eV, indicating the presence of g-C3N4. Figure 4(c) shown the C 1s XPS spectrum, where the peaks of CNCu2O-G are observed at 284.8 and 288.1 eV. The peak at 284.8 eV is allocated to sp2 C-C bonds33, and the peak at 288.1 eV is ascribed to the sp-bonded carbon in nitrogen-containing aromatic rings (N-C=N)28. The Cu 2p, C 1s and N 1s binding energies of CNCu2O-G all show a slight shift, possibly because of the electron transfer and the chemical bonding between Cu2O and g-C3N429。 The electron paramagnetic resonance (EPR) technique was used to acquire the electronic structure information for g-C3N4,



Cu2O, CNCu2O, and CNCu2O-G, as exhibited in Figure 5(a). The CNCu2O-G sample exhibited the highest EPR signals with the g value of approximately 2.01 indicating the existence of unpaired electrons on the surface of CNCu2O-G. Simultaneously, there was no EPR signal for the g-C3N4 and Cu2O sample, but the CNCu2O and CNCu2O-G sample had the distinct EPR signal. And the CNCu2O-G shown the strongest EPR signal, the increase of the EPR signal results from the strengthening of the electron density around Cu, thus indicating that glutamate plays a regulatory role in the electron distribution of CNCu2O, possibly resulting in a better heterojunction effect34-35. The specific surface area and pore-size distribution of the g-C3N4, Cu2O, CNCu2O and CNCu2O-G composites were analyzed by N2 adsorption-desorption technique. As exhibited in Figure 5(b), the four isotherms were recognized as type IV with an H3 hysteresis loop, indicating the mesoporous structure of the composite materials36. The corresponding pore-size distributions of the samples are displayed in the inset of Figure 5(b). The aperture of Cu2O, CNCu2O and CNCu2O-G did not change significantly, but the pore volume of CNCu2O-G increased compared to that of CNCu2O, providing more opportunities for adsorption of organic pollutants. The BET surface areas of the g-C3N4, Cu2O, CNCu2O and CNCu2O-G were 67.86, 1.52, 3.94 and 5.73 m2∙g-1, respectively. The specific surface area of CNCu2O is between g-C3N4 and Cu2O, indicating an effective combination of the two, and the specific surface area of the composite was further improved after the addition of glutamate, thereby increasing the amount of active sites36 of CNCu2O-G compared with CNCu2O. Due to the rise in the amount of the active sites of CNCu2O-G, a rich heterojunction interface was formed as a channel for photogenerated carrier separation; thus, photocatalytic performance was improved37. The UV-vis absorption spectra and PL emission spectra were used to analyze the optical properties of the catalysts. As revealed in Figure 5(c), the g-C3N4 bandgap energy was 2.45 eV. The pure Cu2O had broader absorption in the region from 400 to 600 nm that is ascribed to the bandgap absorption of Cu2O at 2.06 eV. The photoabsorption edge of CNCu2O-G exhibited a small shift from 1.99 eV to 1.78eV compared to that of CNCu2O; we suppose that the addition of glutamate enhanced the photoresponse range of CNCu2O so that more electron-hole pairs were produced under visible-light irradiation38, leading to enhanced photocatalytic activity. Photoluminescence (PL) spectroscopy was used to analyze the charge recombination rate, the charge transfer and separation to estimate the photocatalytic activity39. Figure 5(d) shows the PL spectra of the Cu2O, g-C3N4, CNCu2O, and CNCu2O-G composites under excitation. The pure g-C3N4 had a strong emission peak centered at approximately 460 nm40. Comparatively, Cu2O exhibits a weaker emission peak. The decoration of the Cu2O onto the g-C3N4 may prevent charge recombination of the opposite charge carriers. After adding glutamate, the PL intensity was further decreased, because the addition of glutamate caused the PL intensity to decrease. This is possible because the addition of glutamate significantly slowed down the recombination of electron-hole pairs, and promoted the heterojunction effect21. Consequently, the photogenerated electron-hole pairs of the CNCu2O-G could achieve efficient transfer, thereby boosted photocatalytic performance. To further explore the photoelectric performance of the catalysts, the transient photocurrent-time (I-t) curves and the


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electrochemical impedance spectra (EIS) of the g-C3N4, Cu2O, CNCu2O and CNCu2O-G heterostructure composite were obtained. Figure 5(e) display the I-t curve of the samples with four visible light illumination cycles. Notably, there is no photocurrent response in the dark state, and when there is visible light, the photocurrent value increases rapidly and exhibits good repeatability. At the same time, the photoelectrode of CNCu2O-G showed the highest value compared with the other photoelectrodes, indicating that the glutamate addition could effectively enhance the heterojunction effect between Cu2O and gC3N4, and achieved effective separation and transfer of the photogenerated electrons and holes41. The high conductivity was demonstrated by EIS as shown in Figure 5(f). CNCu2O-G shows the lowest EIS radius that was attributed to the regulation of the CNCu2O-G morphological structure by glutamate during the synthesis; this leads to the enhancement of the heterojunction effect and the decrease in the electronic impedance, so that the high conductivity and charge separation are enhanced30, 42, thus enhancing the photocatalysis performance. The behavior of the photogenerated charges in the CNCu2O-G heterostructures was determined via surface photovoltage (SPV) spectroscopy. The SPV amplitude can reflect the photoresponse wavelength range of the semiconductor material and the degree of separation of its photogenerated charge43. The SPV spectra of Cu2O, g-C3N4 and CNCu2O-G heterostructure are illustrated in Figure 6. Compared with pure Cu2O, the SPV response of CNCu2O-G was significantly reduced at approximately 350 nm, and the SPV signal of CNCu2O-G from 350 to 800 nm was between those of g-C3N4 and Cu2O. This may be due to the role of g-C3N4 in capturing the photogenerated electrons on the surface of Cu2O that may render the weak function of the space charge region on the surface of Cu2O. This leads to more light-induced electrons accumulating on the CNCu2O-G surface. Thus, the photocatalytic performance was enhanced.

3.2 Photocatalytic performance The comparison of different reaction systems confirmed that the CNCu2O-G material has good photocatalytic performance. As depicted in Figure 7(a), in the system using g-C3N4 or Cu2O alone, the MO was hardly degraded. However, after the CNCu2O heterojunction was formed, its catalytic activity was significantly improved. The degradation rate of MO after modification with glutamate was further increased from 80% to 98%, indicating that the glutamate enhanced the heterojunction effect, thus enhancing the photocatalytic performance. The UV-vis spectral changes in the degradation of MO for CNCu2O-G are revealed in Figure 7(b). The corresponding characteristic peaks at 270 and 463 nm gradually decreased. Meanwhile, there were no new feature peaks generation, indicating the aromatic structure and the coloring group had been completely destroyed The impacts of catalyst initial concentrations on the MO degradation are revealed in Figure 7(c). As the catalyst concentration increased, the active sites in the reaction increased, increasing the generation of free radicals. To assess the influence of the MO concentration on the performance of CNCu2O-G, we changed the MO concentration from 20 to 40 mg∙L-1, while keeping the catalyst concentration at 500 mg∙L-1. As shown in Figure 7(d), the initial concentration increase of MO is



inversely proportional to its degradation rate. It is possible that as the dye concentration increased, the surface active sites of the catalyst were gradually consumed, reducing the probability of generation of strong oxidizing radicals. Figure 7(e) shows that the CNCu2O-G composite exhibits good photocatalytic properties during photodegradation of different dyes. The Cu+ dissolution of the CNCu2O-G composite during the degradation of MO is shown in Figure 7(f). The CNCu2O-G content in the reaction system was 500 mg∙L-1. The Cu2O content in the system was determined by atomic absorption spectrometry to be 407 mg∙L-1 (the mass ratio of g-C3N4 to Cu2O in CNCu2O-G composite was approximately 1:9). At the same time, the method was used to determine the Cu+ content during the photocatalytic degradation. The dissolution value of Cu+ was 1.17 mg∙L-1 when 500 mg∙L-1 CNCu2O-G was dosed, which may be due to the Cu+ remaining on the surface of the material during the preparation process. A small amount of dissolution (0.1 mg∙L-1) was observed during the adsorption in 0.5 h. When irradiated with visible light, the amount of Cu+ dissolution increased due to some light corrosion, but tended to be stable at 0.5 h, and the total dissolved amount was 0.54 mg∙L-1, which was less than 1% of the whole system. In addition, stability is also an important factor in evaluating catalyst performance. Recycling experiments were performed on CNCu2O-G under the same conditions for six cycles, as shown in Figure 8. There is limited reduction of the catalytic performance in the first four cycles. Although the reaction activity decreased slightly in the last two cycles, 75% degradation rate was obtained in the last cycle, implying that the CNCu2O-G was stable during the reaction. Figure 9(a) and Figure 9(b) exhibit that the XRD and FT-IR patterns of the CNCu2O-G heterojunction exhibited no significant changes after the reaction, indicating that the samples structure and elemental components of the CNCu2O-G were not changed. According to this analysis, the as-prepared CNCu2O-G heterojunctions have outstanding stability.

3.3 Mechanism discussion For elucidating the photocatalytic mechanism of the CNCu2O-G composites, TBA (•OH scavengers), BQ (•O2- scavengers), and EDTA-2Na (h+ scavengers) were added for removal experiments to identify the main reactive species during the photocatalytic process44-45. Figure 10(a) shown that after the addition of TBA, BQ, and EDTA-2Na, the degradation rates of MO were reduced to 90%, 57%, and 39%, respectively. The degradation rates were remarkably reduced by adding EDTA-2Na and BQ compared with the TBA addition, thus suggested that the h+ and •O2- played major roles in the process. The EPR experiment was conducted to further explore the photocatalytic mechanism. The EPR signal was detected by DMPO-trapped to ensure the active species in the reaction46. In Figure 10(b), EPR signal was not observed under dark condition, whereas the characteristic signal of the DMPO-•O2- could be found with visible light, while the DMPO-•OH- was not detected (shown in Figure 10(a)). These results confirmed •O2- as one of the main active species. According to the previous discussion, the probable photocatalytic mechanism was displayed in Figure 11. The CNCU2O-G heterojunction material is excited by visible light to generate photogenerated electron-hole pairs. On the one hand, electrons react with oxygen in the air to form •O2-, degrading MO. On the other hand, holes react directly with MO to convert them into


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small molecules such as H2O and C2O. The conduction-band (CB) of g-C3N4 and Cu2O were -1.18 and -1.4 eV, respectively, while the valence-band (VB) were +1.27 and +0.6 eV, respectively.

The electrons transferred to g-C3N4 and the holes

transferred to Cu2O, establishing an internal electric field before the Fermi level of the two sides of the heterojunction reached equilibrium47. When glutamate was added, the binding energy of C 1s in the CNCu2O-G composite underwent a negative shift, while the binding energy of Cu 2p in CNCu2O-G underwent a positive shift, indicating a chemical bond present between Cu2O and g-C3N4, because the shift can reasonably result from the electron transfer from Cu2O to g-C3N4. By adding glutamate, the absorption band edge of the CNCu2O-G heterojunction was redshifted, and the band gap. Simultaneously, the charge recombination rate and the electrochemical impedance of the CNCu2O-G heterojunction decrease, which enhances the utilization rate of the photogenerated electron-hole pairs. Additionally, the addition of glutamate increased the specific surface area of CNCu2O-G from 3.93 to 5.73 m2∙g-1, resulting in a stronger heterostructure effect, thus the photocatalytic activity was increased.

4. CONCLUSIONS The CNCu2O-G with higher heterojunction effect was successfully synthesized. The g-C3N4 and Cu2O reacted strongly at their interface under the participation of glutamate, forming the heterojunction. Compared with Cu2O, g-C3N4 and CNCu2O, CNCu2O-G has the strongest stability and optimal photocatalytic performance under condition with visible light. The binding energies of C 1s and Cu 2p for the CNCu2O-G were shifted, which is reasonably ascribed to the electron transfer from Cu2O to g-C3N4. The addition of glutamate promoted the heterojunction effect of CNCu2O-G, effectually increasing the crystallinity, enhancing the specific surface area of the photocatalyst and the efficacy of visible light utilization. Simultaneously, the recombination rate of photogenerated electron-hole pairs and the electrochemical impedance reduced, further causing the enhanced photocatalytic performance. This study provides a way to increase the light utilization of heterostructure catalytic materials.

ACKNOWLEDGMENTS The research was financially supported from the Refractory Industrial Wastewater Treatment Innovation Technology Research Center (2018ZYYD024), the Construction of platform of academician workstation in Quanzhou, and the Natural Science Foundation of Hubei (2018CFB682).

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Graphical abstract


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For Table of Contents Only List of Figures Figure 1, Schematics showing synthesis process of CNCu2O-G. Figure 2, (a) XRD patterns and (b) FT-IR spectra of the Cu2O, g-C3N4, CNCu2O and CNCu2O-G samples. Figure 3, SEM images of (a) g-C3N4, (b) Cu2O, (c) CNCu2O, (d) CNCu2O-G heterojunction and TEM images of (e) CNCu2O, (f) CNCu2O-G heterojunction. Figure 4, XPS spectra of the samples: (a) Cu 2p, (b) N1s, (c) C1 s and (d) Cu LMM. Figure 5, (a) EPR spectra, (b) N2 adsorption-desorption isotherm with the pore size distribution curves (inset), (c) UV-Vis diffuse reflectance band gap spectra, (d) PL spectra, (e) Photocurrent‐time dependence curves under visible light irradiation and (f) EIS Nyquist plots of Cu2O, g-C3N4, CNCu2O and CNCu2O-G composite photocatalysts. Figure 6, Surface photovoltage (SPV) spectra of Cu2O, g-C3N4 and CNCu2O-G heterostructure. Figure 7, (a) Degradation of MO in different reaction systems under visible light, (b) Representative UV-vis spectra changes, (c) Effect of the CNCu2O-G concentration, (d) Effect of the MO concentration on the degradation of MO, (e) Degradation of different pollutants, (f) Dissolution of copper ions during the reaction (initial catalyst concentration: 500 mg∙L-1, initial pollutant concentration: 20 mg∙L-1). Figure 8, Cycling runs for the photocatalytic degradation of MO over the CNCu2O-G sample. Figure 9, (a) XRD patterns and (b) FT-IR spectra of the CNCu2O-G sample before and after the cycling photocatalytic experiments. Figure 10, (a) Free radical capture experiment of CNCu2O-G, (b) DMPO spin-trapping EPR spectra of CNCu2O-G composite with irradiation for 5 min in methanol dispersion (for DMPO-·O2−) and (c) aqueous dispersion (for DMPO-·OH). Figure 11, Mechanism of the photocatalytic degradation of MO over CNCu2O-G.



Figures and figure captions

Figure 1 Schematics showing synthesis process of CNCu2O-G.

Figure 2 (a) The XRD patterns and (b) FT-IR spectra of the Cu2O, g-C3N4, CNCu2O and CNCu2O-G samples.

Figure 3 SEM images of (a) g-C3N4, (b) Cu2O, (c) CNCu2O, (d) CNCu2O-G heterojunction and TEM images of (e) CNCu2O, (f) CNCu2O-G heterojunction.


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(a)

(b)

(c)

(d)

Figure 4 XPS spectra of the samples: (a) Cu 2p, (b) N1s, (c) C1 s and (d) Cu LMM.



Figure 5 (a) EPR spectra, (b) N2 adsorption-desorption isotherm with the pore size distribution curves (inset), (c) UV-Vis diffuse reflectance band gap spectra, (d) PL spectra, (e) Photocurrent-time dependence curves under visible light irradiation and (f) EIS Nyquist plots of Cu2O, g-C3N4, CNCu2O and CNCu2O-G composite photocatalysts

Figure 6 Surface photovoltage (SPV) spectra of Cu2O, g-C3N4 and CNCu2O-G heterostructure.


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Figure 7 (a) Degradation of MO in different reaction systems under visible light, (b) Representative UV-vis spectra changes, (c) Effect of the CNCu2O-G concentration, (d) Effect of the MO concentration on the degradation of MO, (e) Degradation of different pollutants, (f) Dissolution of copper ions during the reaction (initial catalyst concentration: 500 mg∙L-1, initial pollutant concentration: 20 mg∙L-1).

Figure 8 Cycling runs for the photocatalytic degradation of MO over the CNCu2O-G sample.



Figure 9 (a) XRD patterns and (b) FT-IR spectra of the CNCu2O-G sample before and after the cycling photocatalytic experiments.

Figure 10 (a) Free radical capture experiment of CNCu2O-G, (b) DMPO spin-trapping EPR spectra of CNCu2O-G composite with irradiation for 5 min in methanol dispersion (for DMPO-·O2-) and (c) aqueous dispersion (for DMPO-·OH).

Figure 11 Mechanism of the photocatalytic degradation of MO over CNCu2O-G.


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Heterojunction tuning and catalytic efficiency of g-C3N4-Cu2O with

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