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Molten-salt synthesis of g-C3N4-Cu2O heterojunctions with highly enhanced photocatalytic performance
Accepted Manuscript Title: Molten-salt synthesis of g-C3 N4 -Cu2 O heterojunctions with highly enhanced photocatalytic performance Authors: Shiyu Zuo, Haiming Xu, Wei Liao, Xiangjuan Yuan, Lei Sun, Qiang Li, Jie Zan, Dongya Li, Dongsheng Xia PII: DOI: Reference:
S0927-7757(18)30183-3 https://doi.org/10.1016/j.colsurfa.2018.03.013 COLSUA 22338
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
6-2-2018 4-3-2018 4-3-2018
Please cite this article as: Zuo S, Xu H, Liao W, Yuan X, Sun L, Li Q, Zan J, Li D, Xia D, Molten-salt synthesis of g-C3 N4 -Cu2 O heterojunctions with highly enhanced photocatalytic performance, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2010), https://doi.org/10.1016/j.colsurfa.2018.03.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Molten-salt synthesis of g-C3N4-Cu2O heterojunctions with highly enhanced photocatalytic performance
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Shiyu Zuo 1, Haiming Xu2*, Wei Liao1, Xiangjuan Yuan1, Lei Sun1, Qiang Li1, Jie Zan1,
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Dongya Li1, 2*, Dongsheng Xia1*
School of Environmental Engineering, Wuhan Textile University, Wuhan,
Engineering Research Center Clean Production of Textile Dyeing and Printing,
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430073, P.R. China.
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Ministry of Education
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*Corresponding author.
E-mail address: [email protected]; [email protected]
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Graphical Abstracts
Highlights
A novel g-C3N4-Cu2O heterojunction was synthesized by a simple and efficient way. A molten-salt process was developed to form the heterojunctions.
The g-C3N4-Cu2O heterojunctions can significantly enhance photocatalytic
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activity.
The large specific area of the heterojunction providing more active sites to
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efficient separation of photogenerated e--h+ pairs
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Abstract
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A novel heterojunction composite photocatalyst, g-C3N4-Cu2O, was fabricated via
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a high-temperature solid-phase molten salt method, hydrothermal method, and high-
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temperature calcination method. The structures and properties of as-synthesized samples were characterized using a range of techniques, such as X-ray photoelectron
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spectroscopy, scanning electron microscopy, UV-Vis diffuse reflectance spectra and the Brunauer Emmet Teller (BET) theory. Their photocatalytic activity was evaluated based
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on the degradation of methyl orange (MO) under visible light irradiation. Based on the
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results obtained via TEM, XPS, EPR, and the other techniques, we verified that a heterojunction had formed. The g-C3N4-Cu2O heterojunction had the largest specific surface area, which provided plentiful activated sites for photocatalytic reaction. Moreover, g-C3N4-Cu2O showed the highest photocurrent effect and the minimum charge-transfer resistance. Furthermore, the g-C3N4-Cu2O heterojunction exhibited the
highest MO photodegradation rate. After a series of radical trapping experiments and EPR analysis, we demonstrated that the holes and •O2- radicals could be the main active species involved in MO photodegradation. The molten-salt process can improve the BET surface area to form abundant heterojunction interfaces, which serve as
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channels for photogenerated carrier separation and thereby enhance its light utilization
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and quantum efficiency.
Keywords: Photocatalytic activity; g-C3N4-Cu2O; heterojunction; molten-salt
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synthesis
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1. Introduction
Graphitic carbon nitride (g-C3N4) has attracted attention as an n-type
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semiconductor in the photocatalysis field, owing to its good visible light absorption properties (Eg = 2.7 eV) and its photocatalytic stability, which is necessary for good
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photocatalytic performance [1, 2]. However, the photocatalytic efficiency of pure g-
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C3N4 remains relatively low because of the fast recombination of photogenerated electron-hole pairs [3, 4]. Cu2O has great potential for harvesting solar energy because
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of its low toxicity, low cost, high abundance, easy preparation process, better absorption of visible light, and adjustable band gap [5-7]. However, as a result of the fast recombination of the photogenerated electron-hole pairs, its photocatalytic efficiency still needs further improvement [8, 9].
It has been proven that the formation of a heterojunction between an n-type semiconductor and a p-type semiconductor is a more effective strategy to significantly improve their photocatalytic performance owing to the existence of an internal electric field built at the p–n heterojunction [3, 10, 11]. Up to now, some g-C3N4-Cu2O
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heterojunction complex photocatalysts have been reported [12, 13]. The photocatalytic
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activity of photocatalysts is higher than that of monomeric materials because of the
effective separation and migration of the photogenerated carriers, and research to improve the composition of the two phases has been undertaken by developing the
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nanowires [14], octahedrons [15], core-shell structures, and the like [16]. However,
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owing to g-C3N4 block agglomeration, poor dispersion, and other defects, the
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photocatalytic properties and stability of g-C3N4-Cu2O still need to be improved. The traditional thermal polymerization of g-C3N4 results in small specific surface
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areas, low exciton binding energy, low crystallinity, photogenerated carrier complexes, low quantum efficiency, and limited utilization of visible light, which severely restricts
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its practical application in the field of photocatalysis. The modification of g-C3N4 via a
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molten salt method has resulted in g-C3N4 materials with improved photocatalytic activity [17, 18], as they have a larger specific surface area and better dispersion. Using
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g-C3N4 leads to the formation of abundant heterojunction interfaces, which effectively separate photogenerated carriers to enhance the heterojunction’s photocatalytic properties. A novel heterojunction-based composite photocatalyst, g-C3N4-Cu2O, was
fabricated via a high-temperature solid-phase molten salt method, a hydrothermal method, and a high-temperature calcination method. The photocatalytic activity of the photocatalysts was evaluated by degradation of a methyl orange (MO) aqueous solution under visible light irradiation. A photocatalytic mechanism was proposed based on the
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characterization and photocatalytic performance of the photocatalyst and on radical
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trapping experiments and electron paramagnetic resonance (EPR) analysis.
2. Experimental 2.1 Materials
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All reagents supplied by Sinopharm Chemical Reagent Co., Ltd. and Shanghai
2.2 Synthesis of materials
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received without further purification.
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Aladdin Biochemical Technology Co., Ltd. are of analytical grade and were used as
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2.2.1. Synthesis of g-C3N4
0.5 g of dicyandiamide were ground in 2.5 g of LiCl and KCl eutectic salt (molar
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ratio 6: 4) for 10 min and transferred to a covered corundum crucible. The crucible was
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placed in a muffle furnace, heated to 500 °C at a rate of 5 °C/min in an air atmosphere, and incubated for 4 h until the product had naturally cooled. The resulting mixture was
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milled, washed with hot water at 80 °C for 1 h, and dried for 12 h at 60 °C. Then, the resulting sample was labeled ms-g-C3N4 0.5 g of dicyandiamide were ground for 30 min and transferred to a covered corundum crucible. The crucible was placed in a muffle furnace, heated to 500 °C at a
rate of 5 °C/min in an air atmosphere, and incubated for 4 h. After the product had naturally cooled, the resulting mixture was ground and the resulting sample was labeled g-C3N4. 2.2.2. Synthesis of the g-C3N4-Cu2O composite photocatalyst
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1.25 g of CuSO4·5H2O and different proportions of ms-g-C3N4 were added to 40
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mL of deionized water. Then, 1.5 g of xylitol were added to the suspension and the pH
of the suspension was adjusted to 13. After the above-mentioned mixed solution was transferred to a polytetrafluoroethylene liner, the autoclave was heated to 180 °C for 30
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h. The samples were separated and washed with deionized water and dried in a vacuum
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oven at 60 °C. Finally, the obtained sample was heated to a temperature of to 200 °C in
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argon and incubated for 2 h at a constant temperature to prepare the ms-g-C3N4-Cu2O heterojunction.
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2.3 Characterization and analysis
X-ray diffraction (XRD) analysis was carried out on an X-ray diffractometer
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(X'Pert Pro, Netherlands Panalytical Company) using Cu Ka as the radiation source.
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Scanning electron microscopy (SEM) images were obtained using a Hitachi S-4800 field-emission scanning electron microscope with an accompanying energy dispersive
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X-ray spectrometer (EDS, 15 kV). Transmission electron microscope (TEM) images were obtained on a JEOL JEM-2010 electron microscope. UV-vis/DR spectra were recorded using a UV-vis spectrometer (UV-3600, Japan Shimadzu Corporation) equipped with an integrating sphere accessory in the diffuse reflectance mode (R). The
Brunauer–Emmett–Teller (BET) surface area and pore structure of the samples were obtained via nitrogen adsorption-desorption isotherm measurements (ASAP 2020, USA). The photoluminescence (PL) spectra of the samples were measured on an F4600 fluorescence spectrometer (VARIAN, Agilent, USA) with an excitation
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wavelength of 385 nm. X-ray photoelectron spectroscopy (XPS) analysis was
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performed using a VG Multilab 2000 X-ray photoelectron spectroscope (XPS,
BRUKER, Germany). Fourier-transform infrared (FT-IR) spectra were recorded on a Nicolet Avatar 470 spectrophotometer using the standard KBr disk method. Electron
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paramagnetic resonance (EPR) spectra were obtained using a Bruker ER 070 EPR
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Spectrometer (Karlsruhe, Germany).
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2.4 Photocatalytic activity measurements
Photocatalytic degradation experiments were conducted using a LED lamp as a light In summary, 50 mg of the photocatalyst were added to 100 mL of a 20 mg/L
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source.
MO solution in a self-made reactor. Before the photocatalytic reaction, the suspension
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was stirred for 30 min in a dark environment to ensure a balanced adsorption. Then, the
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suspension was illuminated by the LED lamp while being magnetically stirred. At given time intervals, about 1.2 mL of the solution suspension was filtered with 0.45-μm water
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syringe filter and then analyzed on a UV-Vis spectrophotometer.
3. Results and discussion 3.1 Characterization The phases of the g-C3N4, ms-g-C3N4, Cu2O, and ms-g-C3N4-Cu2O samples were
determined via XRD analysis. Fig. 1 shows their typical XRD patterns. g-C3N4 showed two diffraction peaks at 2θ=13.04° and 27.38°. This means that there is a tri-triazine unit in our synthetic sample [19, 20]. The XRD pattern of ms-g-C3N4 showed a series of peaks at 12.0°, 21°, 24.3°, 27.43°, 29.1°, and 32.4°, which can be attributed to
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deflections from the (100), (110), (200), (002), (102), and (210) planes of the
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poly(triazine imide) (PTI) phase (i.e., triazine-based ms-g-C3N4). The (100) and (002) diffraction planes correspond to the in-plane arrangement of nitrogen-linked heterocyclic rings and the stacking of conjugated aromatic rings, respectively [18]. The
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diffraction peaks of pure Cu2O appearing at 29.582°, 36.441°, 42.328°, 61.406°,
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73.556°, and 77.414° correspond to the (110), (111), (200), (220), (311), and (222)
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crystal planes of pure Cu2O, respectively [21]. For ms-g-C3N4-Cu2O composites, the XRD patterns revealed the coexistence of ms-g-C3N4 and Cu2O, indicating a two-phase
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composition of ms-g-C3N4 and Cu2O in these heterojunctions. The chemical compositions of the ms-g-C3N4-Cu2O samples were further
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analyzed via their FT-IR spectra, shown in Fig. S1. The similarities in these FT-IR
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spectra are in good agreement with the XRD results (Fig. 1). From the FT-IR spectrum of ms-g-C3N4, distinct bands at ~2175 cm-1 in ms-g-C3N4 indicate the presence of
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terminal cyano groups [18]. Some characteristic peaks in the FT-IR spectra, e.g., the peak at 890 cm-1 in g-C3N4 and the peaks at 995 and 2175 cm-1 in ms-g-C3N4, can be used to corroborate the differences in the surface chemical compositions, although it is difficult to attribute them to any specific species. In the case of the Cu2O sample, the
band at about 623 cm-1 corresponds to the stretching vibration of Cu (I)–O bonds [22]. These characteristic absorptions of ms-g-C3N4 and Cu2O all appeared in the spectra of ms-g-C3N4-Cu2O heterojunctions,
indicating
the coexistence of these two
semiconductors.
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The morphologies of the Cu2O, ms-g-C3N4, and ms-g-C3N4-Cu2O samples were
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comparatively analyzed via SEM. Cu2O has a relatively polyhedral shape (Fig. 2a), which may show better photocatalytic degradation effects on negatively charged MO [23]. Additionally, its larger particle size may accelerate the settling of the solid catalyst,
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which is conducive to recycling. As for ms-g-C3N4 (Fig. 2b), the bulks seem to be loose
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aggregates of nanoplates. Comparatively speaking, it had the appearance of flakes, with
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a very compact stack between layers. Fig. 2c shows SEM images of the composites. Scattered ms-g-C3N4 particles might have been deposited on the surface of Cu2O in the
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ms-g-C3N4-Cu2O composite, wrapped tightly to form a more compact heterogeneous junction structure. Fig. 2d shows typical EDS images, which indicate that C, N, O, and
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Cu were present in the composite. Their mass ratio was compared with that of ms-g-
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C3N4-Cu2O. Additionally, the EDS mapping clearly revealed that Cu, O, C and N were
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uniformly distributed on the surface of the sample [24]. To further understand the microstructures, we observed the samples of ms-g-C3N4-
Cu2O using a TEM and HR-TEM. As shown in Fig. 3a, ms-g-C3N4 was evenly anchored to the Cu2O surface layer, forming ms-g-C3N4-Cu2O heterojunctions. An HRTEM image is displayed in Fig. 3b. A 0.247-nm interplanar spacing was clearly
observed, corresponding to the (111) crystal plane of Cu2O. The two phases of ms-gC3N4 and Cu2O were clearly observed, and the formation of the ms-g-C3N4-Cu2O heterojunction was confirmed. This unique architecture can not only prevent the oxidation of elemental copper, but also greatly promote an efficient separation of
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electron-hole pairs. Thus, photocatalytic reactions can be greatly enhanced [25].
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The specific bonding and surface chemical states of the ms-g-C3N4-Cu2O sample was further characterized via XPS. Fig. 4 shows the typical XPS spectra obtained. The survey scan shown in Fig. 4a indicates that C, N, O, and Cu were present in the
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composite. All the individual diffraction peaks of both ms-g-C3N4 and Cu2O were
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present in the XPS patterns of the as-prepared ms-g-C3N4-Cu2O heterojunctions,
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indicating the two-phase composition of ms-g-C3N4 and Cu2O in these heterojunctions. Fig. 4b shows the characteristic peaks of Cu 2p at 932.6 and 952.4 eV, which were
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attributed to the binding energies of Cu 2p3/2 and Cu 2p1/2, respectively. However, it is difficult to differentiate Cu2O and Cu by the XPS features of Cu 2p3/2 and Cu 2p1/2
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because their binding energies are very close. According to previous reports, the Cu
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LMM peak region provides a clear means to distinguish between the two oxidation states. Fig. 4c shows that the Cu LMM peaks of the sample occurred at 570 eV, which
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is consistent with Cu2O [26]. The Cu 2p and Cu LMM binding energies after introducing g-C3N4 indicate changes in the chemical environments of Cu+ ions in the Cu2O structure and suggest the presence of a chemically bound interface between Cu2O and g-C3N4 rather than just physical contact between the two separate Cu2O and g-C3N4
phases. Fig. 4d shows the high-resolution XPS spectrum of C 1s, in which there were peaks at 284.7 and 288.2 eV according to the peak fitting results. The peak at 288.7 eV corresponds to C–N=C in the heterocyclic rings, and the peak at 284.4 eV can be attributed to C=C in adventitious carbon [27]. The corresponding binding energies of
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N 1s spectra were determined to be 398.8 eV, 400.0 eV, and 401.3 eV (Fig. 4e) [20].
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These correspond to the structure of ms-g-C3N4, further indicating the presence of ms-
g-C3N4 [28]. The fact that the binding energies of C 1s and N 1s in the g-C3N4-Cu2O composite were negatively shifted whereas the binding energies of Cu+ in g-C3N4-Cu2O
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were positively shifted suggests that Cu2O and g-C3N4 were chemically bound, while
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the shifts of the samples could probably be ascribed to electron transfers between Cu2O
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and g-C3N4 [25].
In our EPR measurements, the Cu2O sample yielded the highest EPR signal,
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shown in Fig. 5. In contrast, the EPR signal greatly decreased for ms-g-C3N4-Cu2O. The results above indicate that an interaction occurred between Cu2O and ms-g-C3N4,
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verifying the heterojunction formation in ms-g-C3N4-Cu2O [29].
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To further understand the electronic band structures and the photocatalytic properties of the samples, the optical properties of Cu2O, ms-g-C3N4, and ms-g-C3N4-
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Cu2O were characterized via their UV-vis absorption spectra, which were converted from the corresponding diffuse reflectance spectra via the Kubelka–Munk function. As shown in Fig. 6a, ms-g-C3N4 possessed an absorption edge of about 460 nm, which originated from its band gap of 2.81 eV [9]. Cu2O had broad absorption in the visible
region from 400 to 600 nm, which was attributed to the intrinsic band gap absorption of Cu2O (2.0 eV) [9, 30]. Compared with that of ms-g-C3N4, the absorption edges of the series of ms-g-C3N4-Cu2O composites were shifted to longer wavelengths, and the absorption intensity in the visible light region gradually increased as the Cu2O content
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increased. The dispersed ms-g-C3N4 brings ms-g-C3N4-Cu2O uptake closer to that of
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Cu2O, implying that the ms-g-C3N4-Cu2O heterojunction had an extended absorption in the visible wavelength region, which can increase its photocatalytic activity. It is well known that a higher PL intensity indicates a faster recombination of the
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charge carriers, resulting in lower photocatalytic activity [20]. Fig. 7 shows the PL
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spectra of the excited Cu2O, ms-g-C3N4, and ms-g-C3N4-Cu2O composites. The ms-g-
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C3N4 sample exhibited a strong intrinsic PL peak at ~460 nm [31]. In contrast, ms-gC3N4-Cu2O exhibited a weaker emission peak. This suggests that the recombination of
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electron-hole pairs was significantly inhibited after the addition of Cu2O to the ms-gC3N4 system. These PL results confirm the importance of the heterojunction effect of
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ms-g-C3N4-Cu2O on the effective separation of photo-induced charges and of how
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lower recombination rates enhance photocatalytic performance[32]. Fig. 8 shows the N2 adsorption-desorption isotherms and the corresponding pore
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size distribution curves (inset) of ms-g-C3N4 and a series of ms-g-C3N4-Cu2O composites. It can be seen that the samples can be classified as type IV, which indicates the presence of a mesoporous structure of ~12 nm [8]. The specific surface areas calculated using the BET method (SBET) were 14.5133, 219.2571, 4.9904, and 8.5108
m2·g-1 for g-C3N4, ms-g-C3N4, g-C3N4-Cu2O, and ms-g-C3N4-Cu2O, respectively, indicating that molten salt media are helpful for synthesizing ms-g-C3N4 with higher specific surface areas. The molten salt process can increase the BET surface area to form abundant heterojunction interfaces as channels for photogenerated carrier
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separation, thereby enhancing the photocatalytic performance of the composite [33, 34].
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3.2. Photocatalytic activity
To evaluate the photocatalytic activity of the as-synthesized ms-g-C3N4–Cu2O heterojunctions, we investigated the photodegradation of MO under different systems.
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As shown in Fig. 9, MO is very stable and almost no decomposition occurs in the
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absence of the catalyst; ms-g-C3N4 and Cu2O also showed poor activity. Notably, the
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ms-g-C3N4-Cu2O heterojunctions exhibited higher photocatalytic activity than either ms-g-C3N4 or Cu2O individually. It can be seen that the heterojunction structure formed
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by ms-g-C3N4-Cu2O effectively enhanced its photocatalytic activity. As shown in Fig. S2, the absorption of MO in the 270-nm and 463-nm wavelengths
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significantly decreased as irradiation time increased, and nearly disappeared after 30
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min. In the meantime, no additional absorption appeared, indicating the complete
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destruction of aromatic structures and coloring groups. As shown in Fig. S3, the photocatalytic activity first increased from ms-g-C3N4-
Cu2O(5:5) to ms-g-C3N4-Cu2O(1:5), and then remained relatively unchanged after further reduction of the ms-g-C3N4 content. The highest activity was obtained over the ms-g-C3N4-Cu2O (1:5) heterojunction, resulting in 84% degradation for MO within 30
min under visible light irradiation. This result implies that the ms-g-C3N4 content is a crucial factor for improving the photocatalytic activity of ms-g-C3N4–Cu2O heterojunctions. By using a suitable mass ratio, an efficient heterojunction interface between the two components could be formed, effectively suppressing the
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recombination of photogenerated charges [35].
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The stability of the photocatalysts is also crucial for practical applications. Cyclic
runs for the photodegradation of MO with ms-g-C3N4-Cu2O heterojunctions were performed, and the relationship between the degradation ratio of MO and the number
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of cycles is illustrated in Fig. 10. Note that the photodegradation of MO did not
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obviously decrease after reusing the photocatalysts for five cycles, implying that the
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ms-g-C3N4-Cu2O heterojunction was quite stable during the reaction. Furthermore, Fig. S4 and Fig. S5 show that the XRD and XPS patterns of the ms-g-C3N4-Cu2O
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heterojunction before and after the reactions underwent no evident changes, and that the crystal structure and the chemical valence of ms-g-C3N4-Cu2O did not change.
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Based on the above-mentioned analysis, the as-prepared ms-g-C3N4-Cu2O
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heterojunctions showed outstanding photocatalytic activity and stability.
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3.3 Mechanism To further enhance the photocatalytic performance of ms-g-C3N4-Cu2O and to
understand the photocatalytic mechanism of ms-g-C3N4-Cu2O (1:5) composites, trapping experiments were conducted to determine the main active species in the photocatalytic process (Fig. 11).
EDTA-2Na, BQ, and TBA were used as the scavengers for holes (h+), superoxide radicals
(•O2−), and hydroxyl radicals (•OH), respectively [31]. The photocatalytic
degradation efficiency of MO greatly decreased after EDTA-2Na and BQ were added, showing that both h+ and •O2− radicals were the main reactive species. However, after
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adding the •OH radical scavenger, the degradation efficiency of MO decreased only
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slightly, indicating that •OH radicals were not the main active species in the photocatalytic process.
The EPR technique was further used to investigate the photocatalytic mechanisms.
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DMPO (5, 5-dimethyl-1-pyrroline-N-oxide) is generally used as a radical scavenger to
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trap •OH− and •O2− radicals [32]. As shown in Fig. 12, there was no EPR signal in the
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dark. However, the characteristic peaks of •O2− radicals were observed under visible light irradiation. Considering the band structure of Cu2O and g-C3N4, •O2− radicals (-
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0.056 eV for O2/•O2−) can be generated on the surfaces of the two semiconductors under visible light irradiation. The •O2− radicals are derived from the partial transformation
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of •O2− via the following reaction: O2+e-→•O2−. Our results are in good agreement with
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those of the trapping experiments. Hence, we further verified the dominant role of •O2−. On the basis of our experimental and theoretical results, a possible visible light
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photocatalytic mechanism in the ms-g-C3N4-Cu2O heterojunction is proposed, and is illustrated in Fig.13. When the dispersed ms-g-C3N4 was in even contact with the Cu2O surface, electrons diffused from ms-g-C3N4 to Cu2O, thus resulting in an internal electric field that formed until the Fermi levels of ms-g-C3N4 and Cu2O reached
equilibrium, forming a heterojunction [25, 28]. Once the heterojunction was irradiated with visible light, both ms-g-C3N4 and Cu2O were excited and produced photogenerated electron-hole pairs. Under the action of the internal electric field, the photogenerated electrons moved toward the positive field (n-g-C3N4), and the holes moved toward the
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negative field (p-Cu2O)[16], thus promoting photo-generated electron-hole transfer
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[32]. The enhanced photocatalytic efficiency obtained was attributed to the improvement of the more dispersed ms-g-C3N4 and the larger BET surface area
achieved, resulting in stronger heterostructure contact interfaces and more reaction sites.
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All of these factors effectively suppress the recombination of photogenerated electron-
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hole pairs, leading to enhanced photocatalytic activity.
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4 Conclusions
A novel heterojunction composite photocatalyst, g-C3N4-Cu2O, was fabricated via
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a high-temperature solid-phase molten salt method, hydrothermal method, and hightemperature calcination method, in which the heterojunction was formed by the
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junction of strong interactions. Among all the obtained samples, the ms-g-C3N4-Cu2O
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heterojunction exhibited the most enhanced photocatalytic performance for the photodegradation of MO molecules because its large specific surface area provided
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more active sites for a higher charge separation efficiency. The free radical capture experiments and EPR results show that the photogenerated electron-hole pair and·O2 radicals were the main active species in the g-C3N4-Cu2O photocatalytic degradation of MO. The molten salt process can be used to improve the BET surface area to form
abundant heterojunction interfaces, which serve as channels for photogenerated carrier separation and thereby enhance light utilization and quantum efficiency. Consequently, our work may provide new insights for the design of high-efficiency visible light heterojunction photocatalysts by performing coupling modifications on semiconductor
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technology into environmental applications such as pollutant purification.
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photocatalysts and for extending the industrial applications of photocatalysis
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Acknowledgements
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This work was financially supported from the International S&T Cooperation
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Program of Wuhan (2017030209020255), the Creative Research Groups
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Program of the Natural Science Foundation of Hubei (2017CFA026), and the
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Natural Science Foundation of Hubei (2015CFB706).
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heterojunction with high visible light activity and stability in degradation of pollutants, Applied Catalysis B: Environmental, 218 (2017) 443-451. [30] Y. Wang, J. Tao, X. Wang, Z. Wang, M. Zhang, G. He, Z. Sun, A unique Cu2O/TiO2 nanocomposite with enhanced photocatalytic performance under visible light irradiation, Ceramics International, 43 (2017) 4866-4872. [31] S. Liang, Y. Zhou, Z. Cai, C. She, Preparation of porous g‐C3N4/Ag/Cu2O: a new composite with enhanced visible ‐ light photocatalytic activity, Applied Organometallic Chemistry, 11 (2016) 932-938. [32] Y. Bao, K. Chen, A novel Z-scheme visible light driven Cu2O/Cu/g-C3N4 photocatalyst using metallic copper as a charge transfer mediator, Molecular Catalysis, 432 (2017) 187-195. [33] J. Fei, J. Li, Controlled preparation of porous TiO2 – Ag nanostructures through supramolecular assembly for plasmon‐enhanced photocatalysis, Advanced materials, 27 (2015) 314-319. [34] Z. Dong, Y. Wu, N. Thirugnanam, G. Li, Double Z-scheme ZnO/ZnS/g-C3N4 ternary structure for efficient photocatalytic H2 production, Applied Surface Science, 430 (2018) 293-300. [35] Y. Liu, X. Yuan, H. Wang, X. Chen, S. Gu, Q. Jiang, Z. Wu, L. Jiang, Y. Wu, G. Zeng, Novel visible light-induced g-C3N4–Sb2S3/Sb4O5Cl2 composite photocatalysts for efficient degradation of methyl orange, Catalysis Communications, 70 (2015) 1720.
List of Figures Fig. 1, The XRD patterns of the Cu2O, g-C3N4, ms-g-C3N4 and ms-g-C3N4Cu2Osamples.
and EDS, EDS mapping of ms-g-C3N4–Cu2O heterojunction.
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Fig. 2, SEM image (a) Cu2O, (b) ms-g-C3N4, (c) ms-g-C3N4–Cu2O heterojunction,
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Fig. 3, TEM image (a) and HR-TEM image (b) of ms-g-C3N4–Cu2O heterojunction. Fig. 4, XPS spectra of the ms-g-C3N4-Cu2O sample. Fig. 5, EPR spectra of the ms-g-C3N4-Cu2O sample.
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Fig. 6, UV- Vis diffuse reflectance spectra (a and b) of Cu2O, ms-g-C3N4 and ms-g-
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C3N4-Cu2O composite photocatalysts.
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Fig. 7, PL spectra of Cu2O, ms-g-C3N4 and ms-g-C3N4-Cu2O composite
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photocatalysts.
Fig. 8, N2 adsorption/desorption isotherm curve of the synthesized ms-g-C3N4Cu2O(1:5) catalyst and Inset is the pore size distribution of ms-g-C3N4-Cu2O(1:5).
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Fig. 9, Degradation of MO in different reaction sysrems.
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Fig. 10, Cycling runs for the photocatalytic degradation of MO over g-C3N4-Cu2O （1:5）sample.
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Fig. 11, Free radical capture experiment of g-C3N4- Cu2O (1: 5). Fig. 12, DMPO spin-trapping EPR spectra of ms-g-C3N4-Cu2O(1:5) composite with irradiation for 5 min in (a) methanol dispersion (for DMPO-·O2−) and (b) aquesous dispersion (for DMPO-·OH).
Fig. 13, Mechanism scheme of photocatalytic degration of MO over g-C3N4-Cu2O.
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Figures and figure captions
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Fig. 1. The XRD patterns of the Cu2O, g-C3N4, ms-g-C3N4 and ms-g-C3N4-Cu2Osamples.
Fig. 2. SEM image (a) Cu2O, (b) ms-g-C3N4, (c) ms-g-C3N4–Cu2O heterojunction, and EDS, EDS mapping of ms-g-C3N4–Cu2O heterojunction.
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Fig. 3. TEM image (a) and HR-TEM image (b) of ms-g-C3N4–Cu2O heterojunction.
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Fig. 4. XPS spectra of the ms-g-C3N4-Cu2O sample.
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Fig. 5. EPR spectra of the ms-g-C3N4-Cu2O sample.
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Fig. 6. UV- Vis diffuse reflectance spectra (a and b) of Cu2O, ms-g-C3N4 and ms-g-C3N4Cu2O composite photocatalysts.
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Fig. 7. PL spectra of Cu2O, ms-g-C3N4 and ms-g-C3N4-Cu2O composite photocatalysts.
Fig. 8. N2 adsorption/desorption isotherm curve of the synthesized ms-g-C3N4-Cu2O(1:5) catalyst
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and Inset is the pore size distribution of ms-g-C3N4-Cu2O(1:5).
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Fig. 9. Degradation of MO in different reaction sysrems.
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Fig. 10. Cycling runs for the photocatalytic degradation of MO over g-C3N4-Cu2O（1:5）sample under visible light.
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Fig. 11. Free radical capture experiment of g-C3N4- Cu2O (1: 5).
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Fig. 12. DMPO spin-trapping EPR spectra of ms-g-C3N4-Cu2O(1:5) composite with irradiation for
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5 min in (a) methanol dispersion (for DMPO-·O2−) and (b) aquesous dispersion (for DMPO-·OH).
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Fig. 13. Mechanism scheme of photocatalytic degration of MO over g-C3N4-Cu2O.
S0927-7757(18)30183-3 https://doi.org/10.1016/j.colsurfa.2018.03.013 COLSUA 22338
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
6-2-2018 4-3-2018 4-3-2018
Please cite this article as: Zuo S, Xu H, Liao W, Yuan X, Sun L, Li Q, Zan J, Li D, Xia D, Molten-salt synthesis of g-C3 N4 -Cu2 O heterojunctions with highly enhanced photocatalytic performance, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2010), https://doi.org/10.1016/j.colsurfa.2018.03.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Molten-salt synthesis of g-C3N4-Cu2O heterojunctions with highly enhanced photocatalytic performance
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Shiyu Zuo 1, Haiming Xu2*, Wei Liao1, Xiangjuan Yuan1, Lei Sun1, Qiang Li1, Jie Zan1,
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Dongya Li1, 2*, Dongsheng Xia1*
School of Environmental Engineering, Wuhan Textile University, Wuhan,
Engineering Research Center Clean Production of Textile Dyeing and Printing,
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430073, P.R. China.
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Ministry of Education
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*Corresponding author.
E-mail address: [email protected]; [email protected]
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Graphical Abstracts
Highlights
A novel g-C3N4-Cu2O heterojunction was synthesized by a simple and efficient way. A molten-salt process was developed to form the heterojunctions.
The g-C3N4-Cu2O heterojunctions can significantly enhance photocatalytic
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activity.
The large specific area of the heterojunction providing more active sites to
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efficient separation of photogenerated e--h+ pairs
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Abstract
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A novel heterojunction composite photocatalyst, g-C3N4-Cu2O, was fabricated via
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a high-temperature solid-phase molten salt method, hydrothermal method, and high-
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temperature calcination method. The structures and properties of as-synthesized samples were characterized using a range of techniques, such as X-ray photoelectron
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spectroscopy, scanning electron microscopy, UV-Vis diffuse reflectance spectra and the Brunauer Emmet Teller (BET) theory. Their photocatalytic activity was evaluated based
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on the degradation of methyl orange (MO) under visible light irradiation. Based on the
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results obtained via TEM, XPS, EPR, and the other techniques, we verified that a heterojunction had formed. The g-C3N4-Cu2O heterojunction had the largest specific surface area, which provided plentiful activated sites for photocatalytic reaction. Moreover, g-C3N4-Cu2O showed the highest photocurrent effect and the minimum charge-transfer resistance. Furthermore, the g-C3N4-Cu2O heterojunction exhibited the
highest MO photodegradation rate. After a series of radical trapping experiments and EPR analysis, we demonstrated that the holes and •O2- radicals could be the main active species involved in MO photodegradation. The molten-salt process can improve the BET surface area to form abundant heterojunction interfaces, which serve as
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channels for photogenerated carrier separation and thereby enhance its light utilization
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and quantum efficiency.
Keywords: Photocatalytic activity; g-C3N4-Cu2O; heterojunction; molten-salt
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synthesis
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1. Introduction
Graphitic carbon nitride (g-C3N4) has attracted attention as an n-type
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semiconductor in the photocatalysis field, owing to its good visible light absorption properties (Eg = 2.7 eV) and its photocatalytic stability, which is necessary for good
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photocatalytic performance [1, 2]. However, the photocatalytic efficiency of pure g-
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C3N4 remains relatively low because of the fast recombination of photogenerated electron-hole pairs [3, 4]. Cu2O has great potential for harvesting solar energy because
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of its low toxicity, low cost, high abundance, easy preparation process, better absorption of visible light, and adjustable band gap [5-7]. However, as a result of the fast recombination of the photogenerated electron-hole pairs, its photocatalytic efficiency still needs further improvement [8, 9].
It has been proven that the formation of a heterojunction between an n-type semiconductor and a p-type semiconductor is a more effective strategy to significantly improve their photocatalytic performance owing to the existence of an internal electric field built at the p–n heterojunction [3, 10, 11]. Up to now, some g-C3N4-Cu2O
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heterojunction complex photocatalysts have been reported [12, 13]. The photocatalytic
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activity of photocatalysts is higher than that of monomeric materials because of the
effective separation and migration of the photogenerated carriers, and research to improve the composition of the two phases has been undertaken by developing the
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nanowires [14], octahedrons [15], core-shell structures, and the like [16]. However,
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owing to g-C3N4 block agglomeration, poor dispersion, and other defects, the
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photocatalytic properties and stability of g-C3N4-Cu2O still need to be improved. The traditional thermal polymerization of g-C3N4 results in small specific surface
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areas, low exciton binding energy, low crystallinity, photogenerated carrier complexes, low quantum efficiency, and limited utilization of visible light, which severely restricts
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its practical application in the field of photocatalysis. The modification of g-C3N4 via a
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molten salt method has resulted in g-C3N4 materials with improved photocatalytic activity [17, 18], as they have a larger specific surface area and better dispersion. Using
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g-C3N4 leads to the formation of abundant heterojunction interfaces, which effectively separate photogenerated carriers to enhance the heterojunction’s photocatalytic properties. A novel heterojunction-based composite photocatalyst, g-C3N4-Cu2O, was
fabricated via a high-temperature solid-phase molten salt method, a hydrothermal method, and a high-temperature calcination method. The photocatalytic activity of the photocatalysts was evaluated by degradation of a methyl orange (MO) aqueous solution under visible light irradiation. A photocatalytic mechanism was proposed based on the
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characterization and photocatalytic performance of the photocatalyst and on radical
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trapping experiments and electron paramagnetic resonance (EPR) analysis.
2. Experimental 2.1 Materials
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All reagents supplied by Sinopharm Chemical Reagent Co., Ltd. and Shanghai
2.2 Synthesis of materials
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received without further purification.
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Aladdin Biochemical Technology Co., Ltd. are of analytical grade and were used as
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2.2.1. Synthesis of g-C3N4
0.5 g of dicyandiamide were ground in 2.5 g of LiCl and KCl eutectic salt (molar
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ratio 6: 4) for 10 min and transferred to a covered corundum crucible. The crucible was
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placed in a muffle furnace, heated to 500 °C at a rate of 5 °C/min in an air atmosphere, and incubated for 4 h until the product had naturally cooled. The resulting mixture was
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milled, washed with hot water at 80 °C for 1 h, and dried for 12 h at 60 °C. Then, the resulting sample was labeled ms-g-C3N4 0.5 g of dicyandiamide were ground for 30 min and transferred to a covered corundum crucible. The crucible was placed in a muffle furnace, heated to 500 °C at a
rate of 5 °C/min in an air atmosphere, and incubated for 4 h. After the product had naturally cooled, the resulting mixture was ground and the resulting sample was labeled g-C3N4. 2.2.2. Synthesis of the g-C3N4-Cu2O composite photocatalyst
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1.25 g of CuSO4·5H2O and different proportions of ms-g-C3N4 were added to 40
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mL of deionized water. Then, 1.5 g of xylitol were added to the suspension and the pH
of the suspension was adjusted to 13. After the above-mentioned mixed solution was transferred to a polytetrafluoroethylene liner, the autoclave was heated to 180 °C for 30
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h. The samples were separated and washed with deionized water and dried in a vacuum
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oven at 60 °C. Finally, the obtained sample was heated to a temperature of to 200 °C in
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argon and incubated for 2 h at a constant temperature to prepare the ms-g-C3N4-Cu2O heterojunction.
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2.3 Characterization and analysis
X-ray diffraction (XRD) analysis was carried out on an X-ray diffractometer
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(X'Pert Pro, Netherlands Panalytical Company) using Cu Ka as the radiation source.
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Scanning electron microscopy (SEM) images were obtained using a Hitachi S-4800 field-emission scanning electron microscope with an accompanying energy dispersive
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X-ray spectrometer (EDS, 15 kV). Transmission electron microscope (TEM) images were obtained on a JEOL JEM-2010 electron microscope. UV-vis/DR spectra were recorded using a UV-vis spectrometer (UV-3600, Japan Shimadzu Corporation) equipped with an integrating sphere accessory in the diffuse reflectance mode (R). The
Brunauer–Emmett–Teller (BET) surface area and pore structure of the samples were obtained via nitrogen adsorption-desorption isotherm measurements (ASAP 2020, USA). The photoluminescence (PL) spectra of the samples were measured on an F4600 fluorescence spectrometer (VARIAN, Agilent, USA) with an excitation
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wavelength of 385 nm. X-ray photoelectron spectroscopy (XPS) analysis was
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performed using a VG Multilab 2000 X-ray photoelectron spectroscope (XPS,
BRUKER, Germany). Fourier-transform infrared (FT-IR) spectra were recorded on a Nicolet Avatar 470 spectrophotometer using the standard KBr disk method. Electron
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paramagnetic resonance (EPR) spectra were obtained using a Bruker ER 070 EPR
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Spectrometer (Karlsruhe, Germany).
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2.4 Photocatalytic activity measurements
Photocatalytic degradation experiments were conducted using a LED lamp as a light In summary, 50 mg of the photocatalyst were added to 100 mL of a 20 mg/L
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source.
MO solution in a self-made reactor. Before the photocatalytic reaction, the suspension
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was stirred for 30 min in a dark environment to ensure a balanced adsorption. Then, the
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suspension was illuminated by the LED lamp while being magnetically stirred. At given time intervals, about 1.2 mL of the solution suspension was filtered with 0.45-μm water
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syringe filter and then analyzed on a UV-Vis spectrophotometer.
3. Results and discussion 3.1 Characterization The phases of the g-C3N4, ms-g-C3N4, Cu2O, and ms-g-C3N4-Cu2O samples were
determined via XRD analysis. Fig. 1 shows their typical XRD patterns. g-C3N4 showed two diffraction peaks at 2θ=13.04° and 27.38°. This means that there is a tri-triazine unit in our synthetic sample [19, 20]. The XRD pattern of ms-g-C3N4 showed a series of peaks at 12.0°, 21°, 24.3°, 27.43°, 29.1°, and 32.4°, which can be attributed to
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deflections from the (100), (110), (200), (002), (102), and (210) planes of the
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poly(triazine imide) (PTI) phase (i.e., triazine-based ms-g-C3N4). The (100) and (002) diffraction planes correspond to the in-plane arrangement of nitrogen-linked heterocyclic rings and the stacking of conjugated aromatic rings, respectively [18]. The
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diffraction peaks of pure Cu2O appearing at 29.582°, 36.441°, 42.328°, 61.406°,
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73.556°, and 77.414° correspond to the (110), (111), (200), (220), (311), and (222)
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crystal planes of pure Cu2O, respectively [21]. For ms-g-C3N4-Cu2O composites, the XRD patterns revealed the coexistence of ms-g-C3N4 and Cu2O, indicating a two-phase
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composition of ms-g-C3N4 and Cu2O in these heterojunctions. The chemical compositions of the ms-g-C3N4-Cu2O samples were further
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analyzed via their FT-IR spectra, shown in Fig. S1. The similarities in these FT-IR
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spectra are in good agreement with the XRD results (Fig. 1). From the FT-IR spectrum of ms-g-C3N4, distinct bands at ~2175 cm-1 in ms-g-C3N4 indicate the presence of
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terminal cyano groups [18]. Some characteristic peaks in the FT-IR spectra, e.g., the peak at 890 cm-1 in g-C3N4 and the peaks at 995 and 2175 cm-1 in ms-g-C3N4, can be used to corroborate the differences in the surface chemical compositions, although it is difficult to attribute them to any specific species. In the case of the Cu2O sample, the
band at about 623 cm-1 corresponds to the stretching vibration of Cu (I)–O bonds [22]. These characteristic absorptions of ms-g-C3N4 and Cu2O all appeared in the spectra of ms-g-C3N4-Cu2O heterojunctions,
indicating
the coexistence of these two
semiconductors.
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The morphologies of the Cu2O, ms-g-C3N4, and ms-g-C3N4-Cu2O samples were
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comparatively analyzed via SEM. Cu2O has a relatively polyhedral shape (Fig. 2a), which may show better photocatalytic degradation effects on negatively charged MO [23]. Additionally, its larger particle size may accelerate the settling of the solid catalyst,
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which is conducive to recycling. As for ms-g-C3N4 (Fig. 2b), the bulks seem to be loose
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aggregates of nanoplates. Comparatively speaking, it had the appearance of flakes, with
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a very compact stack between layers. Fig. 2c shows SEM images of the composites. Scattered ms-g-C3N4 particles might have been deposited on the surface of Cu2O in the
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ms-g-C3N4-Cu2O composite, wrapped tightly to form a more compact heterogeneous junction structure. Fig. 2d shows typical EDS images, which indicate that C, N, O, and
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Cu were present in the composite. Their mass ratio was compared with that of ms-g-
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C3N4-Cu2O. Additionally, the EDS mapping clearly revealed that Cu, O, C and N were
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uniformly distributed on the surface of the sample [24]. To further understand the microstructures, we observed the samples of ms-g-C3N4-
Cu2O using a TEM and HR-TEM. As shown in Fig. 3a, ms-g-C3N4 was evenly anchored to the Cu2O surface layer, forming ms-g-C3N4-Cu2O heterojunctions. An HRTEM image is displayed in Fig. 3b. A 0.247-nm interplanar spacing was clearly
observed, corresponding to the (111) crystal plane of Cu2O. The two phases of ms-gC3N4 and Cu2O were clearly observed, and the formation of the ms-g-C3N4-Cu2O heterojunction was confirmed. This unique architecture can not only prevent the oxidation of elemental copper, but also greatly promote an efficient separation of
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electron-hole pairs. Thus, photocatalytic reactions can be greatly enhanced [25].
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The specific bonding and surface chemical states of the ms-g-C3N4-Cu2O sample was further characterized via XPS. Fig. 4 shows the typical XPS spectra obtained. The survey scan shown in Fig. 4a indicates that C, N, O, and Cu were present in the
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composite. All the individual diffraction peaks of both ms-g-C3N4 and Cu2O were
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present in the XPS patterns of the as-prepared ms-g-C3N4-Cu2O heterojunctions,
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indicating the two-phase composition of ms-g-C3N4 and Cu2O in these heterojunctions. Fig. 4b shows the characteristic peaks of Cu 2p at 932.6 and 952.4 eV, which were
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attributed to the binding energies of Cu 2p3/2 and Cu 2p1/2, respectively. However, it is difficult to differentiate Cu2O and Cu by the XPS features of Cu 2p3/2 and Cu 2p1/2
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because their binding energies are very close. According to previous reports, the Cu
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LMM peak region provides a clear means to distinguish between the two oxidation states. Fig. 4c shows that the Cu LMM peaks of the sample occurred at 570 eV, which
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is consistent with Cu2O [26]. The Cu 2p and Cu LMM binding energies after introducing g-C3N4 indicate changes in the chemical environments of Cu+ ions in the Cu2O structure and suggest the presence of a chemically bound interface between Cu2O and g-C3N4 rather than just physical contact between the two separate Cu2O and g-C3N4
phases. Fig. 4d shows the high-resolution XPS spectrum of C 1s, in which there were peaks at 284.7 and 288.2 eV according to the peak fitting results. The peak at 288.7 eV corresponds to C–N=C in the heterocyclic rings, and the peak at 284.4 eV can be attributed to C=C in adventitious carbon [27]. The corresponding binding energies of
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N 1s spectra were determined to be 398.8 eV, 400.0 eV, and 401.3 eV (Fig. 4e) [20].
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These correspond to the structure of ms-g-C3N4, further indicating the presence of ms-
g-C3N4 [28]. The fact that the binding energies of C 1s and N 1s in the g-C3N4-Cu2O composite were negatively shifted whereas the binding energies of Cu+ in g-C3N4-Cu2O
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were positively shifted suggests that Cu2O and g-C3N4 were chemically bound, while
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the shifts of the samples could probably be ascribed to electron transfers between Cu2O
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and g-C3N4 [25].
In our EPR measurements, the Cu2O sample yielded the highest EPR signal,
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shown in Fig. 5. In contrast, the EPR signal greatly decreased for ms-g-C3N4-Cu2O. The results above indicate that an interaction occurred between Cu2O and ms-g-C3N4,
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verifying the heterojunction formation in ms-g-C3N4-Cu2O [29].
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To further understand the electronic band structures and the photocatalytic properties of the samples, the optical properties of Cu2O, ms-g-C3N4, and ms-g-C3N4-
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Cu2O were characterized via their UV-vis absorption spectra, which were converted from the corresponding diffuse reflectance spectra via the Kubelka–Munk function. As shown in Fig. 6a, ms-g-C3N4 possessed an absorption edge of about 460 nm, which originated from its band gap of 2.81 eV [9]. Cu2O had broad absorption in the visible
region from 400 to 600 nm, which was attributed to the intrinsic band gap absorption of Cu2O (2.0 eV) [9, 30]. Compared with that of ms-g-C3N4, the absorption edges of the series of ms-g-C3N4-Cu2O composites were shifted to longer wavelengths, and the absorption intensity in the visible light region gradually increased as the Cu2O content
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increased. The dispersed ms-g-C3N4 brings ms-g-C3N4-Cu2O uptake closer to that of
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Cu2O, implying that the ms-g-C3N4-Cu2O heterojunction had an extended absorption in the visible wavelength region, which can increase its photocatalytic activity. It is well known that a higher PL intensity indicates a faster recombination of the
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charge carriers, resulting in lower photocatalytic activity [20]. Fig. 7 shows the PL
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spectra of the excited Cu2O, ms-g-C3N4, and ms-g-C3N4-Cu2O composites. The ms-g-
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C3N4 sample exhibited a strong intrinsic PL peak at ~460 nm [31]. In contrast, ms-gC3N4-Cu2O exhibited a weaker emission peak. This suggests that the recombination of
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electron-hole pairs was significantly inhibited after the addition of Cu2O to the ms-gC3N4 system. These PL results confirm the importance of the heterojunction effect of
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ms-g-C3N4-Cu2O on the effective separation of photo-induced charges and of how
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lower recombination rates enhance photocatalytic performance[32]. Fig. 8 shows the N2 adsorption-desorption isotherms and the corresponding pore
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size distribution curves (inset) of ms-g-C3N4 and a series of ms-g-C3N4-Cu2O composites. It can be seen that the samples can be classified as type IV, which indicates the presence of a mesoporous structure of ~12 nm [8]. The specific surface areas calculated using the BET method (SBET) were 14.5133, 219.2571, 4.9904, and 8.5108
m2·g-1 for g-C3N4, ms-g-C3N4, g-C3N4-Cu2O, and ms-g-C3N4-Cu2O, respectively, indicating that molten salt media are helpful for synthesizing ms-g-C3N4 with higher specific surface areas. The molten salt process can increase the BET surface area to form abundant heterojunction interfaces as channels for photogenerated carrier
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separation, thereby enhancing the photocatalytic performance of the composite [33, 34].
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3.2. Photocatalytic activity
To evaluate the photocatalytic activity of the as-synthesized ms-g-C3N4–Cu2O heterojunctions, we investigated the photodegradation of MO under different systems.
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As shown in Fig. 9, MO is very stable and almost no decomposition occurs in the
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absence of the catalyst; ms-g-C3N4 and Cu2O also showed poor activity. Notably, the
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ms-g-C3N4-Cu2O heterojunctions exhibited higher photocatalytic activity than either ms-g-C3N4 or Cu2O individually. It can be seen that the heterojunction structure formed
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by ms-g-C3N4-Cu2O effectively enhanced its photocatalytic activity. As shown in Fig. S2, the absorption of MO in the 270-nm and 463-nm wavelengths
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significantly decreased as irradiation time increased, and nearly disappeared after 30
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min. In the meantime, no additional absorption appeared, indicating the complete
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destruction of aromatic structures and coloring groups. As shown in Fig. S3, the photocatalytic activity first increased from ms-g-C3N4-
Cu2O(5:5) to ms-g-C3N4-Cu2O(1:5), and then remained relatively unchanged after further reduction of the ms-g-C3N4 content. The highest activity was obtained over the ms-g-C3N4-Cu2O (1:5) heterojunction, resulting in 84% degradation for MO within 30
min under visible light irradiation. This result implies that the ms-g-C3N4 content is a crucial factor for improving the photocatalytic activity of ms-g-C3N4–Cu2O heterojunctions. By using a suitable mass ratio, an efficient heterojunction interface between the two components could be formed, effectively suppressing the
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recombination of photogenerated charges [35].
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The stability of the photocatalysts is also crucial for practical applications. Cyclic
runs for the photodegradation of MO with ms-g-C3N4-Cu2O heterojunctions were performed, and the relationship between the degradation ratio of MO and the number
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of cycles is illustrated in Fig. 10. Note that the photodegradation of MO did not
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obviously decrease after reusing the photocatalysts for five cycles, implying that the
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ms-g-C3N4-Cu2O heterojunction was quite stable during the reaction. Furthermore, Fig. S4 and Fig. S5 show that the XRD and XPS patterns of the ms-g-C3N4-Cu2O
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heterojunction before and after the reactions underwent no evident changes, and that the crystal structure and the chemical valence of ms-g-C3N4-Cu2O did not change.
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Based on the above-mentioned analysis, the as-prepared ms-g-C3N4-Cu2O
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heterojunctions showed outstanding photocatalytic activity and stability.
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3.3 Mechanism To further enhance the photocatalytic performance of ms-g-C3N4-Cu2O and to
understand the photocatalytic mechanism of ms-g-C3N4-Cu2O (1:5) composites, trapping experiments were conducted to determine the main active species in the photocatalytic process (Fig. 11).
EDTA-2Na, BQ, and TBA were used as the scavengers for holes (h+), superoxide radicals
(•O2−), and hydroxyl radicals (•OH), respectively [31]. The photocatalytic
degradation efficiency of MO greatly decreased after EDTA-2Na and BQ were added, showing that both h+ and •O2− radicals were the main reactive species. However, after
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adding the •OH radical scavenger, the degradation efficiency of MO decreased only
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slightly, indicating that •OH radicals were not the main active species in the photocatalytic process.
The EPR technique was further used to investigate the photocatalytic mechanisms.
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DMPO (5, 5-dimethyl-1-pyrroline-N-oxide) is generally used as a radical scavenger to
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trap •OH− and •O2− radicals [32]. As shown in Fig. 12, there was no EPR signal in the
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dark. However, the characteristic peaks of •O2− radicals were observed under visible light irradiation. Considering the band structure of Cu2O and g-C3N4, •O2− radicals (-
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0.056 eV for O2/•O2−) can be generated on the surfaces of the two semiconductors under visible light irradiation. The •O2− radicals are derived from the partial transformation
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of •O2− via the following reaction: O2+e-→•O2−. Our results are in good agreement with
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those of the trapping experiments. Hence, we further verified the dominant role of •O2−. On the basis of our experimental and theoretical results, a possible visible light
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photocatalytic mechanism in the ms-g-C3N4-Cu2O heterojunction is proposed, and is illustrated in Fig.13. When the dispersed ms-g-C3N4 was in even contact with the Cu2O surface, electrons diffused from ms-g-C3N4 to Cu2O, thus resulting in an internal electric field that formed until the Fermi levels of ms-g-C3N4 and Cu2O reached
equilibrium, forming a heterojunction [25, 28]. Once the heterojunction was irradiated with visible light, both ms-g-C3N4 and Cu2O were excited and produced photogenerated electron-hole pairs. Under the action of the internal electric field, the photogenerated electrons moved toward the positive field (n-g-C3N4), and the holes moved toward the
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negative field (p-Cu2O)[16], thus promoting photo-generated electron-hole transfer
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[32]. The enhanced photocatalytic efficiency obtained was attributed to the improvement of the more dispersed ms-g-C3N4 and the larger BET surface area
achieved, resulting in stronger heterostructure contact interfaces and more reaction sites.
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All of these factors effectively suppress the recombination of photogenerated electron-
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hole pairs, leading to enhanced photocatalytic activity.
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4 Conclusions
A novel heterojunction composite photocatalyst, g-C3N4-Cu2O, was fabricated via
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a high-temperature solid-phase molten salt method, hydrothermal method, and hightemperature calcination method, in which the heterojunction was formed by the
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junction of strong interactions. Among all the obtained samples, the ms-g-C3N4-Cu2O
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heterojunction exhibited the most enhanced photocatalytic performance for the photodegradation of MO molecules because its large specific surface area provided
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more active sites for a higher charge separation efficiency. The free radical capture experiments and EPR results show that the photogenerated electron-hole pair and·O2 radicals were the main active species in the g-C3N4-Cu2O photocatalytic degradation of MO. The molten salt process can be used to improve the BET surface area to form
abundant heterojunction interfaces, which serve as channels for photogenerated carrier separation and thereby enhance light utilization and quantum efficiency. Consequently, our work may provide new insights for the design of high-efficiency visible light heterojunction photocatalysts by performing coupling modifications on semiconductor
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technology into environmental applications such as pollutant purification.
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photocatalysts and for extending the industrial applications of photocatalysis
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Acknowledgements
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This work was financially supported from the International S&T Cooperation
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Program of Wuhan (2017030209020255), the Creative Research Groups
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Program of the Natural Science Foundation of Hubei (2017CFA026), and the
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Natural Science Foundation of Hubei (2015CFB706).
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List of Figures Fig. 1, The XRD patterns of the Cu2O, g-C3N4, ms-g-C3N4 and ms-g-C3N4Cu2Osamples.
and EDS, EDS mapping of ms-g-C3N4–Cu2O heterojunction.
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Fig. 2, SEM image (a) Cu2O, (b) ms-g-C3N4, (c) ms-g-C3N4–Cu2O heterojunction,
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Fig. 3, TEM image (a) and HR-TEM image (b) of ms-g-C3N4–Cu2O heterojunction. Fig. 4, XPS spectra of the ms-g-C3N4-Cu2O sample. Fig. 5, EPR spectra of the ms-g-C3N4-Cu2O sample.
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Fig. 6, UV- Vis diffuse reflectance spectra (a and b) of Cu2O, ms-g-C3N4 and ms-g-
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C3N4-Cu2O composite photocatalysts.
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Fig. 7, PL spectra of Cu2O, ms-g-C3N4 and ms-g-C3N4-Cu2O composite
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photocatalysts.
Fig. 8, N2 adsorption/desorption isotherm curve of the synthesized ms-g-C3N4Cu2O(1:5) catalyst and Inset is the pore size distribution of ms-g-C3N4-Cu2O(1:5).
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Fig. 9, Degradation of MO in different reaction sysrems.
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Fig. 10, Cycling runs for the photocatalytic degradation of MO over g-C3N4-Cu2O （1:5）sample.
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Fig. 11, Free radical capture experiment of g-C3N4- Cu2O (1: 5). Fig. 12, DMPO spin-trapping EPR spectra of ms-g-C3N4-Cu2O(1:5) composite with irradiation for 5 min in (a) methanol dispersion (for DMPO-·O2−) and (b) aquesous dispersion (for DMPO-·OH).
Fig. 13, Mechanism scheme of photocatalytic degration of MO over g-C3N4-Cu2O.
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Figures and figure captions
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Fig. 1. The XRD patterns of the Cu2O, g-C3N4, ms-g-C3N4 and ms-g-C3N4-Cu2Osamples.
Fig. 2. SEM image (a) Cu2O, (b) ms-g-C3N4, (c) ms-g-C3N4–Cu2O heterojunction, and EDS, EDS mapping of ms-g-C3N4–Cu2O heterojunction.
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Fig. 3. TEM image (a) and HR-TEM image (b) of ms-g-C3N4–Cu2O heterojunction.
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Fig. 4. XPS spectra of the ms-g-C3N4-Cu2O sample.
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Fig. 5. EPR spectra of the ms-g-C3N4-Cu2O sample.
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Fig. 6. UV- Vis diffuse reflectance spectra (a and b) of Cu2O, ms-g-C3N4 and ms-g-C3N4Cu2O composite photocatalysts.
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Fig. 7. PL spectra of Cu2O, ms-g-C3N4 and ms-g-C3N4-Cu2O composite photocatalysts.
Fig. 8. N2 adsorption/desorption isotherm curve of the synthesized ms-g-C3N4-Cu2O(1:5) catalyst
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and Inset is the pore size distribution of ms-g-C3N4-Cu2O(1:5).
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Fig. 9. Degradation of MO in different reaction sysrems.
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Fig. 10. Cycling runs for the photocatalytic degradation of MO over g-C3N4-Cu2O（1:5）sample under visible light.
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Fig. 11. Free radical capture experiment of g-C3N4- Cu2O (1: 5).
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Fig. 12. DMPO spin-trapping EPR spectra of ms-g-C3N4-Cu2O(1:5) composite with irradiation for
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5 min in (a) methanol dispersion (for DMPO-·O2−) and (b) aquesous dispersion (for DMPO-·OH).
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Fig. 13. Mechanism scheme of photocatalytic degration of MO over g-C3N4-Cu2O.