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Simultaneous Photocatalytic Oxygen Production and Hexavalent Chromium Reduction in Ag3PO4/C3N4 S-scheme Heterojunction

Tao Yang ,  Pengke Deng ,  Lele Wang ,  Jie Hu ,  Qinqin Liu ,  Hua Tang

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Citation:  Tao Yang, Pengke Deng, Lele Wang, Jie Hu, Qinqin Liu, Hua Tang. Simultaneous Photocatalytic Oxygen Production and Hexavalent Chromium Reduction in Ag3PO4/C3N4 S-scheme Heterojunction[J]. Chinese Journal of Structural Chemistry, 2022, 41(6): 220602. doi: 10.14102/j.cnki.0254-5861.2022-0062 shu

Simultaneous Photocatalytic Oxygen Production and Hexavalent Chromium Reduction in Ag3PO4/C3N4 S-scheme Heterojunction

English

  • INTRODUCTION

    Environmental pollution by heavy metals has become a great concern because of their toxic effects on plants, animals and human being. Among the numerous heavy metal ions, hexavalent chromium (Cr(Ⅵ)) is a major critical contaminant in drinking water source because of its highly toxic, cancerogenic and mobile characteristics.[1-3] The reduction of Cr(Ⅵ) into Cr(Ⅲ) has been applied to treat Cr(Ⅵ)-bearing wastewaters due to that Cr(Ⅲ) is nontoxic with low solubility and mobility. Photocatalytic reduction as one of the promising strategies has drawn wide attention due to its high efficiency and environment friendly for removing Cr(Ⅵ).[4-6] During photocatalytic Cr(Ⅵ) reduction in water, the photogenerated electrons were used to drive the reduction reaction, leaving the holes being consumed by sacrifice agents. Such a reductive half-reaction resulted in lower solar light conversion efficiency. Once suitable oxidative half-reaction was designed in the Cr(Ⅵ) reduction reaction system, the utilization of photons should be improved accompanied by the generation of useful products. Water oxidation is a typical oxidation reaction which could take advantage of the photogenerated holes.[7-10] Generally, the multi-electron process of water oxidation leads to slow kinetics which limits the O2 generation efficiency. Therefore, the combination of photocatalytic O2 generation with Cr(Ⅵ) reduction will not only accelerate the charge separation but also promote the quantum efficiency of the photocatalysts.

    Ag3PO4 as an ideal semiconductor photocatalyst has demonstrated excellent performance in oxygen production and Cr(Ⅵ) removal. Liu et al prepared porous Ag/Ag3PO4/GO microspheres with a high reduction performance of hexavalent chromium after 30 h of continuous light irradiation.[11] Megala et al constructed a double direct Z-type photocatalyst NiAl LDH/g-C3N4/Ag3PO4 to achieve improved performance of overall water splitting compared to the NiAl LDH/g-C3N4 binary photocatalyst.[12] All such work indicated that Ag3PO4-based composites can serve as the potential photocatalysts in O2 production or Cr(Ⅵ) removal. However, there are few relevant reports on Ag3PO4-based photocatalysts for simultaneously realizing Cr(Ⅵ) reduction and water oxidation. Recently, S-scheme heterojunctions have achieved a success in photocatalysis because of their strong redox ability and unique charge carrier transfer efficiency.[13-16] Considering the advantages of S-scheme heterojunction, constructing Ag3PO4-based S-scheme heterojunction is a promising strategy to make full use of photogenerated electron/hole and realize simultaneously photocatalytic production of oxygen and Cr(Ⅵ) reduction. Graphitic carbon nitride (g-C3N4), a stable and inexpensive metal-free polymeric semiconductor, can couple with Ag3PO4 to construct an S-scheme heterojunction due to their matched energy band structure.[17-20] In addition, C3N4 hollow spheres affording large specific surface areas and abundant reactive sites have been studied extensively. Zhang et al. prepared a porous hollow spherical g-C3N4 catalyst decorated with single Cu atoms for selective oxidation of benzene to phenol.[21] Combining C3N4 hollow spheres with Ag3PO4 to construct an S-scheme heterojunction demonstrates great potential of simultaneous Cr(Ⅵ) reduction and oxygen production.

    In this paper, Ag3PO4 was coupled with hollow spherical C3N4 to construct an S-scheme heterojunction by in situ precipitation method. The Ag3PO4/C3N4 S-scheme heterojunction can realize the reduction of Cr(Ⅵ) to be Cr(Ⅲ) and oxidation of water to produce oxygen in one reaction system. As a result, the optimized composite afforded high oxygen evolution of 803.31 µmol·g-1·h-1 and Cr(Ⅵ) conversion of 87.9% under visible light. The elemental valence and electron transfer route in Ag3PO4/C3N4 have been studied by XPS and ESR spectra. The possible reaction mechanism of the simultaneous reduction Cr(Ⅵ) and oxygen production over Ag3PO4/C3N4 S-scheme heterojunction was systematically studied.

    RESULTS AND DISCUSSION

    XRD as well as FTIR was performed to analyze the crystal structure and constitution of the catalysts. Figure 1a displays the XRD patterns of C3N4, Ag3PO4 and Ag3PO4/C3N4 composites. The characteristic peaks of C3N4 located at 27.3° and 13.1° were attributed to the (002) and (100) faces of C3N4, which is in agreement with the reported literature.[22, 23] The diffraction peaks of pure Ag3PO4 also agreed with the standard PDF card for silver phosphate (JCPDS No: 06-0505).[24] For composite samples, only the characteristic peaks of Ag3PO4 were observed since the mass content of C3N4 was much lower than that of Ag3PO4.[25] However, the FTIR spectrum of APCN5 showed both the characteristic peaks of Ag3PO4 and C3N4 samples in Figure 1c. The signals at 1015 and 559 cm-1 were assigned to the stretching vibrations of the P-O bond in PO43-. The peaks at 1657 and 1390 cm-1 were due to the stretching and bending of H-O bond in water. The characteristic peaks at 1240-1639 cm-1 were the stretching vibrations of the CN heterocycle.[26] The peak at 809 cm-1 originated in the breathing vibration of the triazine unit.

    Figure 1

    Figure 1.  (a, b) XRD patterns of Ag3PO4, C3N4 and Ag3PO4/C3N4 composites. (c) FTIR spectra of Ag3PO4, C3N4 and APCN5 nanocomposites.
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    The morphology of Ag3PO4, C3N4 and the Ag3PO4/C3N4 composites was also investigated using the scanning electron microscopy (SEM). Ag3PO4 is composed of irregular agglomerate grains (Figure 2a and b) with size of 300 nm. C3N4 sample features the regular hollow sphere structure with the average diameter of 3-4 µm. In Ag3PO4/C3N4, the C3N4 hollow spheres could be preserved while Ag3PO4 particles are dispersed on the surface of C3N4 hollow spheres. As shown in the EDS mapping, it is clear that Ag, P, O, C and N elements homogeneously distributed in the Ag3PO4/C3N4 composite.

    Figure 2

    Figure 2.  SEM pictures of Ag3PO4 (a), C3N4 (b), APCN5 (c) and corresponding elemental mapping images of Ag (d), P (e), O (f), C (g), N (h) elements in APCN5.
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    XPS spectra were tested for studying the chemical composition. In the survey spectrum (Figure 3a), Ag, P, O, C and N were detected, which is consisted with the FTIR results. In Figure 3b, the bands at 367.74 and 373.74 eV can be referred to Ag 3d5/2 and Ag 3d3/2 in Ag3PO4.[27-29] Figure 3c shows that the P 2p peak at 132.68 eV belongs to the signal of P5+ in Ag3PO4. The O 1s spectrum (Figure 3d) depicts two different peaks at 530.41 and 532.20 eV, corresponding to the lattice oxygen and surface oxygen of Ag3PO4, respectively. Figure 3e shows the C 1s spectrum and the two bands located at 284.55 and 287.96 eV attributed to the sp2 C-C and N-C=N bonds of C3N4 sample, respectively.[30] The peak at 286.38 eV appears to be the C=N bond of C3N4 sample. The N 1s spectrum displays four peaks at 398.43, 399.39, 400.78 and 404.35 eV which belong to the pyridinic N, pyrrolic N, graphitic N and N2, respectively (Figure 3f).

    Figure 3

    Figure 3.  (a) XPS survey spectrum of the APCN5 composite. (b-f) High-resolution XPS spectrum of Ag 3d, P 2p, O 1s, C 1s and N 1s for the APCN5 sample.
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    Photocatalytic activities of Ag3PO4, C3N4 and Ag3PO4/C3N4 composites were evaluated by photocatalytic O2 generation coupled with Cr(Ⅵ) reduction. As displayed in Figure 4a and b, when potassium dichromate is used as a Cr(Ⅳ) source, Ag3PO4/C3N4 sphere composites exhibit a higher photocatalytic oxygen production performance than the pure Ag3PO4 sample. The highest O2 production is up to 803.31 μmol·h-1·g-1 for APCN5. At the same time, the Cr(Ⅵ) reduction yield for APCN5 could get to 87.9% after 75 min light irradiation. Under the visible-light irradiation, the Cr(Ⅵ) reduction efficiency of the as-prepared photocatalysts follows the order of Ag3PO4 < APCN9 < APCN1 < APCN7 < APCN5, which is consistent with that in the photocatalytic OER results. As shown in Figure S1, only 66.9% Cr(Ⅵ) conversion was obtained in the single Cr(Ⅵ) reduction reaction for APCN5 sample. When the sacrificial agent is replaced by AgNO3, the average photocatalytic O2 evolution rates over Ag3PO4/C3N4 composites are found to be 817.99, 916.05, 1051.33, 850.5 and 765.03 µmol·g-1·h-1 for APCN1, APCN3, APCN5, APCN7 and APCN9, respectively. Obviously, with increasing the C3N4 sphere mass ratio, the O2 evolution activity increases greatly and then decreases gradually. The APCN5 affords the highest O2 generation of 1051.33 µmol·g-1·h-1, which is 1.8 times of Ag3PO4 (Figure 4c and d). As shown in Figure 4e and f, photocatalytic synchronous reduction of Cr(Ⅵ) and oxy-gen evolution with the optimal APCN5 sample at different wavelengths of 500, 550 and 700 nm were performed. The oxygen production at 500 nm reaches 484 µmol·g-1·h-1 at 75 min and the conversion rate of Cr(Ⅵ) is 65.9%, while the oxygen production at 550 nm is 114 µmol·g-1·h-1 and the conversion rate of Cr(Ⅵ) is 34.0%. The apparent quantum efficiency (AQY) of APCN5 is 5.5%, 14.8%, 1.8%, 0.8% and 0.05% at 365, 420, 500, 550, and 700 nm, respectively (Figure S2). All these results confirmed that the APCN5 could afford higher O2 generation and Cr(Ⅵ) reduction activity than the reported photocatalysts listed in Table S1 and S2.

    Figure 4

    Figure 4.  (a, b) Photocatalytic activities for O2 generation coupled with Cr(Ⅵ) reduction of different samples. (c, d) Photocatalytic O2 production activity with AgNO3 as the sacrificial agent. (e, f) Photocatalytic activities for O2 generation coupled with Cr(Ⅵ) reduction of APCN5 composites at different wavelengths.
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    Different sacrificial agents were tested in the photocatalytic OER reaction. When potassium dichromate and nitrobenzene were used as electron-trapping sacrificial agents (Figure 5a and b), the APCN5 demonstrates lower photocatalytic OER activity (809.1 and 199.2 µmol·g-1·h-1) than AgNO3 (1069.5 µmol·g-1·h-1). This indicates that AgNO3 has the significant electron-trapping ability. As shown in Figure 5c and d, even after 120 min UV-visible and visible light irradiation with AgNO3 as the sacrificial agent, APCN5 reaches 1901 and 1339 µmol g-1 O2 production, respectively. When Cr(Ⅵ) was used as the sacrificial agent, the oxygen production is 1588 and 1072 µmol g-1, and the conversion rate of Cr(Ⅵ) is 92.1% and 89.7%, respectively. These results revealed that the Ag3PO4/C3N4 composite showed good photo-stability under UV-visible and visible light irradiation. The morphology of Ag3PO4 and C3N4 spheres of the composite could maintain their initial morphology before and after photocatalytic reaction (Figure S3), confirming their good stability.

    Figure 5

    Figure 5.  (a, b) Photocatalytic oxygen production of APCN5 with different sacrificial agents. The reactivity of oxygen production of the APCN5 composite under UV-vis (c) and visible light (d) excitation with different sacrificial agents.
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    To investigate the effect of C3N4 morphologies on the photocatalytic performance, the Ag3PO4/C3N4 composites with C3N4 tube, C3N4 bulk and C3N4 sheets were synthesized. As shown in Figure 6a-c, the SEM images confirm the successful synthesis of Ag3PO4/C3N4 tube, Ag3PO4/C3N4 sheet and Ag3PO4/C3N4 bulk composites. The corresponding BET specific surface area is shown in Table S3. The C3N4 hollow sphere has the largest specific surface area (1617 m2/kg) than other morphologies (493.8, 479.3 and 507.3 m2/kg for C3N4 tube, C3N4 bulk and C3N4 sheets, respectively). The photocatalytic O2 evolution efficiency of these samples follows the order: APCN5 > Ag3PO4/C3N4 sheet composite > Ag3PO4/C3N4 tube composite > Ag3PO4/C3N4 bulk composite, indicating the superiority of hollow sphere structure of C3N4 as the reduction semiconductor in the S-scheme he-terojunction (Figure 6d).

    Figure 6

    Figure 6.  SEM images of Ag3PO4/C3N4 tube (a), Ag3PO4/C3N4 bulk (b) and Ag3PO4/C3N4 sheet (c) composites. Photocatalytic activity of different samples (d).
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    To reveal the reactive species and understand the reaction mechanism, different kinds of trapping agents such as 1, 4-benzoquinone (BQ), 1-butanol (NBA) and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) were applied as the scavenger to catch the superoxide radical (·O2-), hydroxyl radical (·OH) and hole (h+), [31-34] respectively. As listed in Figure 7a, the O2 evolution efficiency of APCN5 declined in the presence of EDTA-2Na, BQ and NBA, suggesting that h+, ·O2- and OH all played important roles in the photocatalytic O2 evolution. The Cr(Ⅵ) reduction was almost inhibited with the addition of BQ (Figure 7b), which verified that the ·O2- also has a positive effect on the reduction of Cr(Ⅵ). The addition of NBA and EDTA-2Na resulted in decreased reactivity in Cr(Ⅵ) reduction, indicating that ·OH and h+ are not the main active species during Cr(Ⅵ) reduction.

    Figure 7

    Figure 7.  Trapping experiments for photocatalytic O2 generation coupled with Cr(Ⅵ) reduction over APCN5.
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    The charge separation and migration efficiency were also investigated by the photocurrent response, PL spectra, time-resolved PL (TRPL) decay spectra and the EIS Nyquist plots of the samples.[35-39] As can be seen in the photocurrent spectra in Figure 8a, the photocurrent intensity of APCN5 is higher than that of Ag3PO4 and C3N4, illustrating the boosted charge separation in the composite. As shown in Figure 8b, pure C3N4 spheres show stronger PL intensity than APCN5, which indicates low recombi-nation rate of photogenerated carriers in APCN5 owing to the for-mation of close contact interface. The fluorescence lifetime of Ag3PO4 and APCN5 is 0.008 and 0.483 ns, respectively (Figure 8c). The prolonged fluorescence lifetime of APCN5 implies enhanced separation of photogenerated charge carriers. The EIS plots show that the arc radius of APCN5 is smaller than that of Ag3PO4 and C3N4, which improved charge separation and migration efficiency in the composite.

    Figure 8

    Figure 8.  Transient photocurrent responses (a), PL spectra (b), Time-resolved PL decay spectra (c) and the EIS Nyquist plots (d) of C3N4 spheres, Ag3PO4 and APCN5 nanocomposites.
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    Figure 9 depicts the UV-vis DRS spectra of as-obtained samples. In comparison with pure Ag3PO4 and C3N4 spheres, the APCN composites show slightly red-shifted absorption in the visible light range with a light absorption threshold wavelength at 580 nm (Figure 9a). The band gaps Eg of Ag3PO4 and C3N4 spheres are 2.15 and 2.36 eV, respectively according to the reported calculation formula.[40-42] The positive slopes in the Mott-Schottky plots present n-type characteristic of Ag3PO4 and C3N4. The flat-band potentials (EFB) of Ag3PO4 and C3N4 are about -0.38 and -1.02 eV (vs. Ag/AgCl), respectively. According to the Nernst equation, the ECB values of Ag3PO4 and C3N4 are reckoned to be 0.21 and -0.42 eV, [43-46] respectively (Figure 9d).

    Figure 9

    Figure 9.  UV-Vis DRS (a), band gap energies (b), Mott-Schottky (M-S) plots (c) and energy band structure (d) of pure Ag3PO4 and C3N4 spheres.
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    The work functions of C3N4 and Ag3PO4 are further calculated, and the values of g-C3N4 and Ag3PO4 are 4.60 and 4.75 eV, respectively (Figure 10 a and b), indicating that the electrons will migrate from C3N4 to Ag3PO4 to form an internal electric field (IEF) from C3N4 to Ag3PO4.[47] IEF can work as a driving force for the migration of the photogenerated carriers. As a result, the energy band edges of C3N4 and Ag3PO4 bent upward and downward, respectively. EPR spectra were also investigated to study the photogenerated charge migration direction. As shown in the EPR spectra, the observed quartet peaks in Figure 10c and 10d are assigned to ·O2- and ·OH, respectively.[48-52] Since the CB edge of Ag3PO4 is more positive than that of O2/·O2- (-0.33 V vs. NHE) while the CB edge of C3N4 is more negative than -0.33 V, the ·O2- cannot be generated in the presence of Ag3PO4. In addition, the oxidation potential of H2O/·OH (1.99 V vs. NHE) is more negative but more positive than that of the VB edge of Ag3PO4 and C3N4, respectively. ·OH cannot be generated under the circumstance of bare C3N4. However, after the combination of C3N4 spheres with Ag3PO4, the composite could generate both ·O2- and ·OH, which confirms the S-scheme in the Ag3PO4/C3N4 sphere composite. The photogenerated e- on the CB of Ag3PO4 reacts with h+ on the VB of C3N4 while the e- on that of C3N4 and the h+ on the VB of Ag3PO4 are preserved for offering the stronger redox ability and fast e--h+ separation.

    Figure 10

    Figure 10.  (a, b) The calculated work functions of Ag3PO4 and C3N4. (c, d) EPR spectra of DMPO-·O2- and DMPO-·OH signals over Ag3PO4, C3N4, and APCN5 composites, respectively. (e) Schematic diagram of photogenerated charges in the Ag3PO4/C3N4 sphere composite.
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    According to the above results, the simultaneous oxygen production and Cr(Ⅵ) reduction reaction mechanism were discussed over the Ag3PO4/C3N4 sphere composite. As shown in Scheme 1, the Ag3PO4 and C3N4 spheres were excited by light. The photo-generated e- of Ag3PO4 recombined with the h+ of C3N4 spheres at the intimate interface driven by the formed IEF. At the same time, the e- and h+ with stronger redox ability at the CB of Ag3PO4 and the VB of C3N4 spheres tended to take part in the reduction of Cr(Ⅵ) and production of oxygen, respectively. In detail, the e- and h+ accumulated in the CB of C3N4 spheres and VB of Ag3PO4 reacted respectively with Cr(Ⅵ) and H2O to generate Cr(Ⅲ) and O2.

    Scheme 1

    Scheme 1.  Schematic illustration of simultaneous production of oxygen and reduction of Cr(Ⅵ) over the Ag3PO4/C3N4 spheres S-scheme heterojunction under visible-light irradiation.
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    CONCLUSION

    Novel S-scheme heterojunction of Ag3PO4/C3N4 sphere composite was designed and fabricated via an in-situ precipitation method. The results revealed that APCN5 exhibits high-efficiency for the photocatalytic removal of Cr(Ⅵ) and oxygen production. Under optimal conditions, APCN5 could give an 87.9% conversion of Cr(Ⅵ) into Cr(Ⅲ) and an O2 production of 803.31 μmol·g-1·h-1, respectively. The improved activity is due to the following reasons: (i) the Ag3PO4/C3N4 sphere composite provided abundant reactive sites; (ii) the S-scheme transfer pathway contributed to the efficient separation of charges and strong redox capability. This research may provide a new strategy for developing an effective heterostructure toward photocatalytic O2 production as well as the wastewater purification.

    EXPERIMENTAL

    Photocatalyst Synthesis

    C3N4 Spheres. 0.5 g of melamine was added into 20 mL dimethyl sulfoxide (DMSO). After ultrasonic dispersion, the obtained solution is recorded as liquid A. Then, 0.51 g of melamine was dispersed in 10 mL of DMSO. After ultrasonication, the obtained solution is recorded as liquid B. The obtained solutions A and B were mixed under stirring for 10 min and the product was washed with anhydrous ethanol and deionized water for 2 times, respectively. After centrifuging and drying, the white powder was obtained. Then, the white powder was heated at 550 ℃ (2.5 ℃/min) under N2 atmosphere for 4 h to obtain the yellow g-C3N4 spheres.[53]

    g-C3N4 Sheets. 15 g of urea was treated at 550 ℃ in a muffle furnace for 4 h (2 ℃/min) to obtain the light-yellow g-C3N4 sheets powder.

    C3N4 Tubes. 1.5 g of urea and 0.4 g of melamine were mixed and fully ground for 30 min under N2 atmosphere. Then, the mixture was treated at 550 ℃ for 4 h (5 ℃/min) to obtain the light-yellow C3N4 tubes.

    C3N4 Bulk. 15 g of melamine was treated to 550 ℃ in a muffle furnace for 4 h (2 ℃/min) to get light-yellow C3N4 bulk.

    Ag3PO4/C3N4 Sphere Composites. Different contents of C3N4 spheres (4, 12, 20, 28 and 36 mg, respectively) with 0.51 g of AgNO3 were added into the mixed solution of 10 mL of N, N-dimethylformamide (DMF) and 10 mL deionized water under continuous ultrasonication. Afterwards, 0.1 M Na2HPO4 solution was poured and stirred for 30 min. The precipitate was washed with anhydrous ethanol and water respectively, centrifuged and dried at 70 ℃ for 4 h, obtaining a yellow powder. The obtained products were named as APCN1, APCN3, APCN5, APCN7 and APCN9 (The mass ratios of C3N4 spheres are 1%, 3%, 5%, 7% and 9%, respectively).

    Characterization. The XRD pattern was used to analyze the structure of the sample with an X-ray diffractometer instrument (D8 Advance, Bruker, Germany) in the 2θ range from 10° to 80°. The micromorphology and the corresponding elemental composition of the samples were examined by SEM (JSM-7001F). The Fourier transform infrared spectra of the sample were carried out on a Nicolet 460 Fourier transform infrared spectrometer (FTIR, Nicolet 460, USA). UV-vis DRS spectra were performed on a Shimadzu UV-2600 spectrometer. The XPS spectra were performed on a Scienta R3000 spectrometer. The photocurrent density curves (I-T), electrochemical impedance spectroscopy (EIS) as well as Mott-Schottky curves for the sample were checked on the electrochemical workstation (CHI 660E, ChenHua Instruments, Shanghai) with three electrodes (Pt electrode, Ag/AgCl electrode and working electrode).[54] EPR analysis was performed on a Bruker A 300 instrument.

    Photocatalytic Tests. The photocatalytic O2 production was performed on an online gas chromatography system (Labsolar-6A, Perfect light) using a sealed air tight quartz reactor. The photocatalytic reactor contained 25 mg of samples and 50 mL of aqueous solution with 0.06 M AgNO3 solution and 20 mg/L of K2Cr2O7 solution (as the Cr source). The reaction system was irradiated for 75 min under a 300 W Xe lamp using a filter with λ ≥ 420 nm. The evolution rate of oxygen was measured by a gas chromatograph (GC-2014 Shimadzu). The concentration of Cr(Ⅵ) was analyzed by a UV-2600 spectrophotometer with its absorption wavelength to be at 356 nm.


    ACKNOWLEDGEMENTS: This work is supported by the National Natural Science Foundation of China (21975110, 21972058 and 22102064), and Prof. H. Tang gratefully acknowledges the financial support from Taishan Youth Scholar Program of Shandong Province. The authors declare no competing interests.
    COMPETING INTERESTS
    Supplementary information is available for this paper at http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0062
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  • Figure 1  (a, b) XRD patterns of Ag3PO4, C3N4 and Ag3PO4/C3N4 composites. (c) FTIR spectra of Ag3PO4, C3N4 and APCN5 nanocomposites.

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    Figure 2  SEM pictures of Ag3PO4 (a), C3N4 (b), APCN5 (c) and corresponding elemental mapping images of Ag (d), P (e), O (f), C (g), N (h) elements in APCN5.

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    Figure 3  (a) XPS survey spectrum of the APCN5 composite. (b-f) High-resolution XPS spectrum of Ag 3d, P 2p, O 1s, C 1s and N 1s for the APCN5 sample.

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    Figure 4  (a, b) Photocatalytic activities for O2 generation coupled with Cr(Ⅵ) reduction of different samples. (c, d) Photocatalytic O2 production activity with AgNO3 as the sacrificial agent. (e, f) Photocatalytic activities for O2 generation coupled with Cr(Ⅵ) reduction of APCN5 composites at different wavelengths.

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    Figure 5  (a, b) Photocatalytic oxygen production of APCN5 with different sacrificial agents. The reactivity of oxygen production of the APCN5 composite under UV-vis (c) and visible light (d) excitation with different sacrificial agents.

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    Figure 6  SEM images of Ag3PO4/C3N4 tube (a), Ag3PO4/C3N4 bulk (b) and Ag3PO4/C3N4 sheet (c) composites. Photocatalytic activity of different samples (d).

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    Figure 7  Trapping experiments for photocatalytic O2 generation coupled with Cr(Ⅵ) reduction over APCN5.

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    Figure 8  Transient photocurrent responses (a), PL spectra (b), Time-resolved PL decay spectra (c) and the EIS Nyquist plots (d) of C3N4 spheres, Ag3PO4 and APCN5 nanocomposites.

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    Figure 9  UV-Vis DRS (a), band gap energies (b), Mott-Schottky (M-S) plots (c) and energy band structure (d) of pure Ag3PO4 and C3N4 spheres.

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    Figure 10  (a, b) The calculated work functions of Ag3PO4 and C3N4. (c, d) EPR spectra of DMPO-·O2- and DMPO-·OH signals over Ag3PO4, C3N4, and APCN5 composites, respectively. (e) Schematic diagram of photogenerated charges in the Ag3PO4/C3N4 sphere composite.

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    Scheme 1  Schematic illustration of simultaneous production of oxygen and reduction of Cr(Ⅵ) over the Ag3PO4/C3N4 spheres S-scheme heterojunction under visible-light irradiation.

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  • 发布日期:  2022-06-20
  • 收稿日期:  2022-03-18
  • 接受日期:  2022-04-30
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