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Constructing the Band Alignment of g-C3N4/Cu2O Composites by adjusting the Contact Facet for Superior Photocatalytic Activity sancan Han, Xiaoyi Hu, Wei Yang, Qingren Qian, Xiaosheng Fang, and Yufang Zhu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01930 • Publication Date (Web): 24 Dec 2018 Downloaded from http://pubs.acs.org on December 26, 2018

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ACS Applied Energy Materials

Constructing the Band Alignment of Graphitic Carbon nitride (g-C3N4)/Copper(I) oxide (Cu2O) Composites by adjusting the Contact Facet for Superior Photocatalytic Activity Sancan Han†, Xiaoyi Hu†, Wei Yang‡, Qingren Qian†, Xiaosheng Fang*‡, Yufang Zhu*†

†School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, P. R. China.

‡Department of Materials Science, Fudan University, Shanghai 200433, P. R. China.

Corresponding Author：

*E-mail: [email protected]；[email protected]

ACS Paragon Plus Environment

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Abstract: g-C3N4/Cu2O p-n junction composites with different crystal facets were prepared by a simple one-step route, which showed strong facet-dependent photoactivity, where the degradation rate value k for 20% C3N4-Cu2O truncated cubes composites with {100} facets heterojunction was ~ 2.5 times higher than that of pure Cu2O truncated cubes. Meanwhile, k value for 20%C3N4-Cu2O octahedra with {111} facets heterojunction exceeds that of pure Cu2O octahedra by a factor of ~ 1.9. P-n heterojunctions with typeII energy alignment is determined by XPS analysis. Larger band energy offset (0.96 eV for △ECBO,1.73 eV for △EVBO) was observed in 20%C3N4-Cu2O truncated cubes composites compared with that of 20%C3N4-Cu2O octahedra composites (0.85 eV for △ECBO, 1.64 eV for △EVBO). The bigger band offset means stronger driving force of the electron transfer between Cu2O truncated cubes with {100} facets and C3N4, indicating band alignment of the heterojunction was facet-dependent, the properly larger band offsets between Cu2O truncated cubes and C3N4 result in the stronger driving force to the transfer of electron and hole in C3N4-Cu2O truncated cubes composites, which is beneficial to the photocatalytic performance. The study provides a new prospect for the


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rational design of highly efficient photocatalyst in terms of the facet-dependent composites.

Keywords: g-C3N4/Cu2O Truncated Cubes Composites; C3N4-Cu2O octahedra Composites; P−n Heterojunctions; Facet-Dependent Photoactivity; Band Offsets;

1. Introduction In recent decades, the design of semiconductor-based photocatalysts has extensively attracted attention in the photo-conversion and degradation fields.1-8 Semiconductor-based composites with appropriate energy band alignments at heterojunctions possess stronger driving force for effective charge separation, which play a vital role in enhancing photocatalytic performance.9-11 Generally, p-n heterojunction could effectively increase the electron-hole separation, improving the photocatalytic


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performance.12-14 Thus, it is highly desired to design p-n heterojunction composites with defined band alignment.

Cuprous oxide (Cu2O) is considered as typical p-type semiconductor with direct bandgap of 1.9 eV - 2.2 eV,4,5 which can take full use of the sunlight. Meanwhile, Cu2O is endowed with low toxicity, good abundance, good environmental acceptability, high optical absorption coefficiency,6-8 which make it a promising candidate in photoenergy conversion and photocatalytic application. W.X. Huang et al.15 studied the morphological evolution of Cu2O nanocrystals, which showed that the stability of Cu2O nanocrystals with different facets follows the order: cubic Cu2O nanocrystals {100} facets > octahedral Cu2O nanocrystals {111} facets > rhombic dodecahedral {110} facets. M. H. Huang et al. 16demonstrated

that the photocatalytic performance of Cu2O crystals with {111} facets

was more active than Cu2O with {100} facets. Furthermore, they synthesized Cu2O with exposed {110} facets, showing an excellent photocatalytic performance due to the high content of surface Cu atoms.17 Despite these efforts on the facets-dependent photocatalysis, it still suffers from fast charge carriers recombination efficiency in single


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phase. To solve the problem, p-Cu2O was coupled with certain n-type semiconductor such as ZnO, WO3, TiO2 with more positive conduction band to form p-n heterojunction for achieving efficient electron transfer from Cu2O to the n-type semiconductor.18-21 However, there existed a big problem about the lattice matching between p-Cu2O and nsemiconductor. g-C3N4 is typical n-type semiconductors with excellent electric conductivity and large surface area. In comparison with other carbon materials, it not only forms the certain band alignment with p-type semiconductors but also provides more active centers when coupled with other semiconductors.22-26 There existed some reports in terms of the coupling between p-Cu2O with n-C3N4. Chattopadhyay et al.,27 demonstrate a room-temperature synthesis of Cu2O and C3N4 composites, and DFT calculation was carried out to investigate electronic property for the composites. Guo et al.,28 synthesized Cu2O nanoparticles on g-C3N4 nanosheets by in-situ reduction method, which showed the improved photocatalytic activity. Cui et al.,29 prepared core@shell Cu2O/g-C3N4 composites, showing efficient visible light hydrogen evolution. The abovementioned efforts showed that Cu2O/g-C3N4 composites were indeed effective in improving the photoactivity, revealing the promising potential in photocatalytic application.


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However, there were no reports on facets-dependent energy band alignment at the interface between p-Cu2O with different exposed crystal facets and n-C3N4.

In this work, C3N4-Cu2O octahedra composites with different weight ratios of C3N4 to Cu2O (10%:1, 20%:1, 25%:1), were synthesized by a simple one-step hydrothermal process, and 20%C3N4-Cu2O octahedra composites showed the best photocatalytic activity. Cu2O with different exposed facets can be adjusted by the concentration of PVP, which can be beneficial to the effective interaction between C3N4 and Cu2O, resulting in effective electron transfer from Cu2O to C3N4.The energy band alignments between Cu2O with different exposed facets and C3N4 heterointerfaces were systematically investigated by X-ray photoelectron spectroscopy (XPS) in detail. The bigger band offset means larger driving force of electron transfer between Cu2O truncated cubes with {100} facets and C3N4, indicating band alignment of the heterojunction was facet-dependent, which is consistent with photocatalytic results. The study showed that the formation of heterojunction on different exposed facets for photocatalysts could change optimized ratio


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of photocatalytic activity, which provids a simple and low-cost method to construct the photocatalytic composites.

Experimental Section

All chemicals were used as received without any further purification. Urea, CuCl2·2H2O, PVP, ascorbic acid, analytical alcohol (> 99.7 %), and hexane (> 97 %) were all purchased from Sinopharm Chemical Reagent Corp.

Material synthesis:

The synthesis of protonated C3N4: 10g urea were heated at 550 oC for 2h in muffle furnace with the heating rate of 10 oC/min. Then C3N4 was treated by 37% HCL for 3h at room temperature to produce protonated C3N4, which could provide higher ionic conductivity and better dispersion in the solution.30

The synthesis of C3N4-Cu2O composites: g-C3N4-Cu2O Octahedra was synthesized by reducing a copper-citrate complex solution with ascorbic acid in the presence of polyvinylpyrrolidone (PVP).21 Typically, a certain amount of protonated g-C3N4 (~ 70 mg,


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~ 140mg, ~ 175mg) was respectively ultrasonic dispersed into 100 mL of a mixture solution containing CuCl2· 2H2O (10 mM), sodium citrate (3.4 mM) and PVP (Mw = 40000, 0.034 g mL−1) for 0.5 h, then 10 mL solution of NaOH (2 M) was added dropwise into the mixture solution. After 0.5 h, 10 mL of an ascorbic acid solution (0.6 M) was added dropwise into the above solution. The resultant suspension was further aged at 80 °C for 3 h to form the C3N4-Cu2O octahedra composites, which are named as 10%C3N4-Cu2O octahedra, 20%C3N4-Cu2O octahedra and 25%C3N4-Cu2O octahedra, respectively. Pure Cu2O crystals were fabricated using the above procedure without addition of g-C3N4. Truncated cubes Cu2O were synthesized by only changing the concentration of PVP (0.015 g mL−1).

Characterization: The wide-angle X-ray diffraction (WAXRD) patterns were obtained on a D8 ADVANCE powder diffractometer by Cu Kα radiation (λ=0.15405 Å). The morphology was

observed

by

scanning

electron

microscope

(SEM)

and

transmission electron microscope (TEM) using FEI Quanta 450 field emission scanning electron microscope and Tecnai G2 F30 transmission electron microscope with an


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acceleration voltage of 300 kV, respectively. Nitrogen adsorption-desorption isotherms were gained on a Micromeritics Tristar II 3020 surface area and pore size analyzer. The surface area, pore size and pore volume were determined by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods. X-ray photoelectron spectroscopy (XPS) were tested with an ESCALAB 250 X-ray photoelectron spectrometer using Al Kα (hν = 1486.6 eV) radiation.

Photocatalytic test: The degradation of Rhodamine B (RhB) was conducted under mercy lamp (300 W) radiation. Typically, 10 mg of photocatalysts were ultrasonic dispersed into the solution of RhB (20 mg/L) with additional 0.7 ml H2O2 ( ≥ 30 wt%). Before the degradation, adsorption-desorption equilibrium process was carried out about 40 min in the dark. After being exposed to the light, 3 ml of suspension was taken away for every 10 min, centrifuged and analyzed at a UV-Vis spectrophotometer at ~ 550 nm.31

Results and Discussion


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Figure 1. (a) SEM images for Cu2O truncated cubes; (b) HRTEM image; (c) the corresponding SAED pattern following [110] direction; (d) SEM images for Cu2O octahedra (inset: Low-magnification TEM); (e) HRTEM image; (f) the corresponding SAED pattern following [111] direction.

Cu2O nanocrystals with different morphology were synthesized by reducing Cu2+ with the addition of PVP. Figure 1a and 1d displayed the scanning electron microscopy (SEM) for pure Cu2O with well-defined facets. The average size of the Cu2O truncated cubes with six {100} facts was ~ 1.5 μm when 0.5 g PVP was added (Figure 1a). Meanwhile, the Cu2O octahedra with eight {111} facets when the addition of PVP increased to 3.5 g, and


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the length and the width can achieve ~ 3.0 μm and ~ 1.5 μm (Figure 1d). As the previous study reported17, 32, 33, more adsorbed PVP on {111} plane could hinder the growth of {100} plane, which is consistent with our study here, the Cu2O crystals with {100} facets finally changed to octahedra crystals with {111} facets with the increasing of PVP content during the reaction process. Meanwhile, PVP acts as a capping agent, which is beneficial to a strong interaction between C3N4 and Cu2O, resulting in effective electron transfer from Cu2O to C3N4. Figure 1b, 1c, 1e and 1f showed the high-resolution TEM (HR-TEM) images and selected-area electron diffraction (SAED) patterns for Cu2O truncated cubes and Cu2O octahedra along [110] and [111] direction, respectively. As can be seen in Figure 1b, the HR-TEM images displayed the distinct lattice fringes with d-spacing of ~ 0.29 nm and ~ 0.25 nm, corresponding to the (110) and (111) crystal plane, respectively. HRTEM image for Cu2O octahedra is displayed in Figure 1e, a clear lattice fringes of ~ 0.29 and ~ 0.25 nm respond to the (110) and (101) crystal plane. All the SAED pattern demonstrated the single crystalline nature of Cu2O.


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Figure 2．(a) The XRD patterns and (b) UV-vis absorption spectra of Cu2O octahedra and Cu2O truncated cubes samples (inset: Tauc plot from Figure 2b).

Figure 2a displayed the X-ray diffraction patterns (XRD) for both kinds of Cu2O samples, no obvious impurities were observed, all the diffraction peaks in the curves are belonged to the face-centered cubic (fcc) Cu2O phase (JCPDS card No. 05-0667). The sharp and strong peaks show the high-degree crystallinity, the peaks of 2θ values at 29.8 o,

36.4 o, 42.4 o, 61.5 o, 73.3

o

and 77.5

o

can be assigned to (110), (111), (200), (200),

(311) and (222) crystal planes for fcc Cu2O, respectively.34-36 The ratio of the diffraction peak intensity for (111)/(200) increases from 2.40 for truncated cubes-shaped to 2.66 for Cu2O octahedra, indicating the transition from {100} facts to {111} facts, which is consistent with the above SEM results. The optical absorption properties for Cu2O


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crystals were determined by the comparison of UV-visible spectra, in Figure 2b. The optical absorbance for Cu2O samples increases from the wavelength of ~ 640 nm, and the corresponding Tauc Plot for the samples was shown in the inset of Figure 2b, the bandgaps for Cu2O crystals were defined as ~ 1.93 eV and ~ 1.95 eV, where n was taken as 2 due to the indirect bandgap semiconductor of Cu2O, indicating that these two kinds of Cu2O samples own similar absorption efficiency.

Figure 3. (a) SEM images for 20% C3N4-Cu2O octahedra composites; (b) Lowmagnification TEM for 20% C3N4-Cu2O octahedra composites; (c) HRTEM image at the heterojunction; (d) Schematic illustration of 20% C3N4-Cu2O octahedra composites unit


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cell and atomic configuration.

Figure 4. (a) SEM images for 20% C3N4-Cu2O truncated cubes composites; (b) Lowmagnification TEM for 20% C3N4-Cu2O truncated cubes composites; (c) HRTEM image at the heterojunction; (d) Schematic illustration of 20% C3N4-Cu2O truncated cubes composites unit cell and atomic configuration.

The transmission electron microscopy (TEM) and SEM images were displayed to investigate the structure of C3N4-Cu2O composites in Figure 3 and Figure 4. As can be seen, Cu2O successfully coupled with C3N4 together. Meanwhile, Cu2O samples still maintain the original structure. In Figure 3a and 4a, the intimate interface at the


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heterojunction between two phases is formed, where all the surface of Cu2O is almost covered with irregular wrinkles of C3N4. The tight coupling between p-Cu2O and n-C3N4 is great vital to the charge transfer, leading to high charge separation, consequently improving the photocatalytic performance. TEM results taken near the particle edges were further evidenced the coupling between p-Cu2O and n-C3N4. The region of the C3N4Cu2O octahedra interface marked by the white line for HRTEM as shown In Figure 3b and 4b viewed along [110] and [001] direction, respectively. Figure 3c showed a lattice fringe with d=0.25 nm corresponds to the (111) plane for Cu2O, which also convinced its highly crystallization.37,38 Meanwhile, (100) plane with d=0.21 nm was shown in Figure 4c. These clearly observed interfaces showed the well formation of p-n heterojunction between pCu2O and n-C3N4. Figure 3d and Figure 4d displayed the atomic configuration of C3N4Cu2O composites, the number of copper atoms density per unit on the (111) face is 0.3825 Å-2, which is ~3.5 times higher than that of (100) face (0.1097 Å-2). Hence, Cu2O octahedra crystals were taken to investigate the role of C3N4 for C3N4-Cu2O octahedra composites (10%C3N4-Cu2O octahedra, 20%C3N4-Cu2O octahedra and 25%C3N4-Cu2O octahedra in detail.


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Figure 5 ． (a) the XRD pattern for pure Cu2O octahedra and C3N4-Cu2O octahedra composites; (b) N2 adsorption-desorption isotherm for pure C3N4, Cu2O octahedra and C3N4-Cu2O octahedra composites.


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Figure 6．(a) UV-vis absorption spectra for pure C3N4, Cu2O octahedra and C3N4-Cu2O octahedra composites; (b) Tauc plot from Figure 6a; (c) PL spectra for pure Cu2O octahedra and C3N4-Cu2O octahedra composites.

The XRD patterns of pure C3N4 and C3N4-Cu2O octahedra composites were


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displayed in Figure 5a and S1. In Figure 5a, the peaks centered at ~ 13.1 o, ~ 27.8 o and ~ 61.3 o can be ascribed to the (100), (002) and (220) diffraction planes, which respond to the in-plane characteristic interlayer-stacking peak for C3N439. However, no typical peaks from C3N4 were observed in C3N4-Cu2O octahedra composites, which can be explained by the small amount of C3N4 in the composites (Figure S1). In Figure 5b, A BET analysis of nitrogen adsorption/desorption isotherms were carried out to investigate the specific surface area and pore size distribution for pure Cu2O octahedra and C3N4-Cu2O octahedra composites. Table S1 displayed that the pure Cu2O octahedra crystals own the lowest specific surface area of ~ 0.3 m2/g. The specific surface area for C3N4-Cu2O octahedra composites increases with the increasing of C3N4 content, after the introduction of C3N4. As we all know, larger surface area is of great importance to offer abundant active sites to adsorb reactive species and trap charge carrier, which could be beneficial to the higher photocatalytic performance for C3N4-Cu2O octahedra composites. The optical properties of pure C3N4 and C3N4-Cu2O octahedra composites is shown in Figure 6. C3N4 can absorb the light wavelength from ~ 440 nm to ~ 300 nm, which is consistent with the previous results. The absorption background of C3N4-Cu2O octahedra


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composites decreased compared with pure Cu2O octahedra, which can be explained by the opacity of C3N4, meaning that C3N4 could weakens the optical absorption. As can be seen in Figure 6b, the bandgaps of the C3N4-Cu2O octahedra composites were calculated as ~ 2.0, which is all higher than that of pure Cu2O. The bandgap for pure C3N4 was defined as ~ 2.7 eV, where n was taken as 1/2 in the inset of the Figure 6b. The photoluminescence (PL) spectra were used to investigate the charge carriers transfer and recombination process for the samples with the excitation light of ~ 532 nm, which were revealed in Figure 6c. A similar PL peak centered at ~ 640 nm was displayed, no PL peak of C3N4 was observed because of the small amount of C3N4, which is consistent with XRD results. In comparison, the peaks of C3N4-Cu2O octahedra composites are all lower than that of pure Cu2O octahedra, indicating the recombination rate of the charge carries decreases after the introduction of C3N4. Indeed, C3N4 owns the high conductivity and electron withdrawing/storing ability, resulting in the better separation of the electron-hole pairs. Meanwhile, the intimate and large interface combination between Cu2O octahedra and C3N4 was also beneficial to the electron transfer, hence improving the photocatalytic performance. Among all the samples, 10%C3N4-Cu2O octahedra composites displayed


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the lowest PL emission, indicating the best charge transfer efficiency.

Figure 7．(a) The degradation rate and (b) kinetic curves of RhB degradation for pure C3N4, Cu2O octahedra and C3N4-Cu2O octahedra composites; (c) The degradation rate and (d) kinetic curves of RhB degradation for Cu2O octahedra, Cu2O truncated cubes, 20% C3N4-Cu2O octahedra and 20% C3N4-Cu2O truncated cubes composites.

The photocatalytic properties for pure C3N4, Cu2O octahedra and C3N4-Cu2O octahedra composites were displayed in Figure 7a-7b. The comparison of the photocatalytical performance for pure Cu2O octahedra, Cu2O truncated cubes, 20%C3N4Cu2O octahedra composites and 20%C3N4-Cu2O truncated cubes composites was


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presented in Figure 7c-7d. Figure S2a-2c displayed the change in intensity of UV-visible absorption at RhB characteristic peak of ~ 550 nm for pure Cu2O octahedra, Cu2O truncated cubes, 20%C3N4-Cu2O octahedra composites and 20%C3N4-Cu2O truncated cubes composites, respectively. The degree of degradation C/C0 for RhB against the irradiation time was plotted in Figure 7a. As can be seen, the photocatalytic activity for all the samples is superior to that of commercial P25, and C3N4-Cu2O octahedra composites displayed excellent degradation rate compared with pure Cu2O octahedra, which can be explained by effective electron transfer from Cu2O octahedra to C3N4. In particularly, 20%C3N4-Cu2O octahedra showed the best photocatalytic activity, the RhB degradation C/C0 can reach to 92% in 20 min, while 25%C3N4-Cu2O octahedra was 89%, which reveals that more addition of C3N4 in the composites does not mean better photoactivity because of the opacity of C3N4, which is consistent with the UV-vis absorption spectra results. To better compare the photocatalytic activity for the samples, the kinetic reaction can be expressed as the pseudo-first-order reaction:40

1

( )=𝑘 𝑡

Ln

c0 c

𝑎


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Where c and c0 are the concentration at a given time and t=0, respectively, ka is the photocatalytic rate constant, which is a basic parameter to characterize the photodegradation. In Figure 7b, the degradation rate constants follow the order: 20%C3N4-Cu2O octahedra (0.124 min-1) > 25%C3N4-Cu2O octahedra (0.107 min-1) > 10%C3N4-Cu2O octahedra (0.101 min-1) > Cu2O octahedra (0.067 min-1) > C3N4 (0.064 min-1), the k values of the C3N4-Cu2O composites are all higher that of pure Cu2O octahedra and C3N4, exhibiting the introduction of C3N4 could improve the photocatalytic performance effectively. Among these, the k value (0.124 min-1) of 20%C3N4-Cu2O octahedra composites exceeds that of pure Cu2O octahedra by a factor of ~ 1.9 (0.067 min-1), indicating the importance role to adjust the amount of C3N4 in order to obtain the synergistic role in achieving optimal photocatalytic performance. Based on the above results, 20%C3N4-Cu2O truncated cubes composites were taken to compare the photocatalytic performance. As shown in Figure 7c and 7d, the photoactivity for the samples followed the order: 20%C3N4-Cu2O octahedra (0.124 min-1) > 20%C3N4-Cu2O truncated cubes (0.112 min-1) > Cu2O octahedra (0.067 min-1) > Cu2O truncated cubes (0.044 min-1). In comparison, the degradation rate value k for 20%C3N4-Cu2O truncated


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cubes composites was ~ 2.5 times higher than that of pure Cu2O truncated cubes, the optimized the ratio (

𝑘𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒𝑠 𝑘𝑝𝑢𝑟𝑒 𝐶𝑢2𝑂

) is higher than that of 20%C3N4-Cu2O octahedra (~ 1.9),

showing the facet dependent photoactivity in the composites.

Figure 8. XPS spectra of CL and VB for (a) Cu2O octahedra; (b) Cu2O truncated cubes; (c) 20% C3N4-Cu2O octahedra composites; (d) 20% C3N4-Cu2O truncated cubes composites; (e) C3N4.


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XPS studies were employed to investigate the band alignment at the heterojunction, which was a powerful technique to determine band offset of the heterojunction interface.41 The core level (CL) and valence band of XPS spectra for Cu2O octahedra, Cu2O truncated cubes, C3N4, 20%C3N4-Cu2O octahedra and 20%C3N4-Cu2O truncated cubes composites were shown in Figure 8. The Cu 2p3/2 spin orbital photoelectron spectra were fitted with Gaussian function and shirley background subtraction. The CL of Cu 2p3/2 is located at ~ 932.50 eV and ~ 932.01 eV for Cu2O octahedra and Cu2O truncated cubes, in Figure 8a and 8b. The Cu 2p3/2 for the 20%C3N4-Cu2O octahedra and 20%C3N4-Cu2O truncated cubes composites were defined as ~ 932.94 eV and ~ 932.06 eV, respectively. Linear extrapolation method was applied to determine the valence band maximum (VBM), and the VBM value of ~ 0.05 eV and ~ 0.31 eV was determined for Cu2O Octahedra and Cu2O truncated cubes. The valence band offset (VBO) and conduction band offset (CBO) were calculated based on Figure 8 as bellowed:19, 42

𝐶𝑢2𝑂 𝐶3𝑁4 𝐶3𝑁4 2𝑂 ∆𝐸𝑉𝐵𝑂 = (𝐸𝐶𝑢 𝐶𝑢,2𝑝 ― 𝐸𝑉𝐵𝑀 𝐶𝑢) ― (𝐸𝑁 1𝑠 ― 𝐸𝑉𝐵𝑀 𝑁) +∆𝐸𝐶𝐿

𝐶3𝑁4 2𝑂 ∆𝐸𝐶𝐵𝑂 = (𝐸𝐶𝑢 𝑔𝑎𝑝 ― 𝐸𝑔𝑎𝑝 ) +∆𝐸𝑉𝐵𝑂

2

3


24

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𝑂 where (𝐸𝐶𝑢 𝐶𝑢,2𝑝 ― 2

2𝑂 𝐸𝐶𝑢 𝑉 𝐶𝑢 )

𝑁 and (𝐸𝐶𝑁 1𝑠 3

4

― 𝐸𝐶𝑉3𝑁𝑁4 ) are

and VBM for Cu2O and C3N4, respectively.

the energy difference between Cu 2p/N 1s

∆𝐸𝐶𝐿

is the energy difference between Cu 2p

― and N 2p for C3N4-Cu2O composites, which can be calculated as ∆𝐸𝐶𝐿 = (𝐸𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒𝑠 𝐶𝑢,2𝑝

). 𝐸𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒𝑠 𝑁 1𝑠

All the values for the core-level, VBM, band-gap, VBO and CBO of pure Cu2O octahedra, pure Cu2O truncated cubes, C3N4, 20%C3N4-Cu2O octahedra and 20%C3N4-Cu2O truncated cubes composites are summarized in Table S2.


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Figure 9. (a) and (b) the band offset based on the calculated results for 20% C3N4-Cu2O octahedra composites and 20% C3N4-Cu2O truncated cubes composites; (c) the scheme of the transfer for the electron and hole at the heterojunction between Cu2O and C3N4.

The proper band alignment of two components could produce the driving force to the transfer of electron-hole pairs at the interface. The energy band alignment of the 20%C3N4-Cu2O octahedra and 20%C3N4-Cu2O truncated cubes composites is based on the above calculation results, which were displayed in Figure 9a and 9b. As can be seen, Cu2O octahedra showed an energy level of ~ 0.85 eV and ~ 1.64 eV, higher than that of g-C3N4 in 20%C3N4-Cu2O octahedra. As for C3N4-Cu2O truncated cubes composites, the VBO and CBO value were ~ 0.96 eV and ~ 1.73 eV, respectively, indicating the type-II alignment for C3N4-Cu2O composites was formed at the heterojunction, which is beneficial to the charge carrier transfer. The 20%C3N4-Cu2O truncated cubes composites own higher band offset value in comparison with C3N4-Cu2O octahedra composites. Hence larger driving force of photogenerated charge transfer from Cu2O to C3N4 occurs at the heterojunction, which is in agreement with the results that the optimal ratio (~2.5) of


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photocatalytic activity in C3N4-Cu2O truncated cubes composites is higher than that of C3N4-Cu2O octahedra composites (~1.9), which means large charge-separation driving force is beneficial for the photocatalytical performance in the C3N4-Cu2O system. Heterojunctions between two composites formed on different exposed facets can produce different band alignments, the composites with relatively favorite exposed facets could result in better photocatalytic activity, demonstrating that it is facet-dependent. Based on above, Figure 9c showed the mechanism of photogenerated electron-hole transfer. Type II heterojunction occurs at the interface once Cu2O and C3N4 are coupled together. The photogenerated electrons are transferred from n-C3N4 to p-Cu2O by the CBO. Meanwhile the VBO between Cu2O and C3N4 drive the photogenerated hole transfer from n-C3N4 to p-Cu2O until the fermi level is even. The redistribution of electrons and holes on the heterojunction could effectively decrease the recombination of electron-hole pairs,43 resulting in the higher photocatalytic performance. For the C3N4-Cu2O system, the properly higher band offset would produce stronger driving force to the transfer of electrons and holes, thus further improving the photocatalytic activity.


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Conclusions In this study, p-Cu2O with different exposed facets and n-C3N4 semiconductor were synthesized by one-step hydrothermal method, the intimate heterojunction on different facets is confirmed, which is beneficial to effective transfer of charge carrier. The type-II energy alignment for C3N4-Cu2O composites was determined by XPS analysis, and the formation of the band offset on different exposed facets demonstrates that it is facetdependent. In combination with the calculated XPS results, the photogenerated electrons are transferred from n-C3N4 to p-Cu2O by the CBO. Meanwhile the VBO between Cu2O and C3N4 drive the photogenerated hole transfer from n-C3N4 to p-Cu2O, meaning that properly higher band offset of C3N4-Cu2O truncated cubes composites would produce stronger driving force to the transfer of electrons and holes, thus further improving the photocatalytic performance, which is consistent with the photodegradation results that the optimal ratio (~2.5) of photocatalytic activity in C3N4-Cu2O truncated cubes composites is higher than that of C3N4-Cu2O octahedra composites (~1.9). Hence, the investigation about the formation of heterojunction on different facets is of great importance to further construct highly efficient photocatalyst, which provides a new prospect to further improve


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the photoactivity.

Notes

The authors declare no competing financial interest. Acknowledgements The authors are grateful for financial support from the National Natural Science Foundation of China (Grant No. 51802195), the Shanghai Sailing Program (17YF1412700), National Natural Science Foundation of China (Grant No. 51572172), Young Teachers in Shanghai Colleges and Universities (ZZslg16062) and Cultivation fund of University of Shanghai for Science and Technology (ZR18PY06).

ASSOCIATED CONTENT

Supporting Information This Supporting Information is available free of charge via the Internet at http://pubs.acs.org.: XRD images, the BET data, XPS data, UV-vis absorption spectra.


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For Table of Contents Only


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Cu2O Composites by

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