Visible Photocatalytic Water Splitting and Photocatalytic Two-Electron

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Article

Visible Photocatalytic Water Splitting and Photocatalytic Two-Electron Oxygen Formation over Cu and Fe Doped CN 3

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Zhen Li, Chao Kong, and Gongxuan Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09469 • Publication Date (Web): 08 Dec 2015 Downloaded from http://pubs.acs.org on December 9, 2015

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The Journal of Physical Chemistry

Visible Photocatalytic Water Splitting and Photocatalytic Two-Electron Oxygen Formation over Cu and Fe Doped g-C3N4 Zhen Li a,b, Chao Kong a,b, Gongxuan Lua* a

State Key Laboratory for Oxo Synthesis and Selective Oxidation Lanzhou Institute

of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, China. b

University of Chinese Academy of Science, Beijing 100049, China.

Abstract: Water splitting via two two-electron processes (the H2O first photocatalytically converted to H2 and H2O2 under visible light irradiation, then the H2O2 disproportionation to H2O and O2 by a thermal catalytic process) has attracted extensive attentions recently. 1-2 Contrary to these reports, we found that not only the photocatalytic H2 generation could be driven by visible light, but also the two-electron H2O2 disproportionation to form H2O and O2 could also be photocatalyzed by visible light over g-C3N4 catalysts. Photocatalytic H2, O2 generation and simultaneous H2O2 formation in Cu/C3N4 and Fe/C3N4 dispersions were confirmed, about 2.1 and 1.4 µmol of H2 and 0.8 and 0.5 µmol of O2 evolved over Cu/C3N4 and Fe/C3N4 in 12 hours, respectively. To prove the photocatalytic process of H2O2 disproportionation, the H2O2 was added as a regent in g-C3N4, Cu/C3N4 and Fe/C3N4 dispersions. The results showed that the activity of H2 evolution decreased with the increase of H2O2 concentration, the corresponding AQEs of oxygen formation were 16.1%, 42.6% and 78.5% at 400 nm, respectively. The remarkable increase of anodic photo-currents over Fe/C3N4/ITO and Cu/C3N4/ITO electrodes indicated that the two-electron H2O2 disproportionation was catalyzed via surface photocatalytic mechanism. The ESR results implied reaction occurred by O2-• radical path over g-C3N4 under irradiation.

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Introduction Photocatalytic water splitting for hydrogen and oxygen generation driven by sunlight irradiation is one of the most promising routes to store the solar energy.1-5 The overall water splitting could be realized by applying an electrolyzer driven by a solar cell (photovoltaic (PV) electrolysis), or by using photocatalytic electrodes with additional photovoltaic force (photoelectrolysis (PE)) and light-irradiated suspended catalysts in water (photocatalysis (PC)). 6-9 It is well known that the water oxidation is a mult-electron process. For example, Pinaudet al. was a four-electron process. While Kohl et al.

1

10-12

reported the water oxidation

reported that the water oxidation to

form O2 first occurred in a two-electron process, leading to formation of H2O2, then it followed another two-electron process, H2O2 disproportionation to H2O and O2. In nature photosynthesis, the whole process was also occurred in a two-step manner (photosystem I and II (PS I and II)). In the PS II, the photosynthetic enzymes capture sunlight at around 700 nm to split water into O2 and [H]. In the PS I, the [H] is used to finish the carbon dioxide reduction. 13-16 It is known that some inorganic semiconductors are active catalysts for photocatalytic water splitting,

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such as metal and nonmetal mixed oxides,

18-19

sulfides and nitrides, 20-21 doped perovskites, 22 and nitridepyrochlores. 23 Among them, g-C3N4 attracted extensive attentions because it has suitable reduction potential, unique physical properties, promising electronic features and good light absorption in the visible region.24-25 In the view of these advantages, g-C3N4 was also widely used in other emerging fields, such as CO2 reduction,26 environmental remediation,27 the oxygen reduction reaction,28 bioimaging,29 disinfection25 and so on. For the photocatalytic water splitting, the catalytic properties of g-C3N4 could be improved by metallic and nonmetallic doping, 30-33 copolymerization, 34-35 and integrating g-C3N4 with other semiconductors, graphene oxide),

40-41

36-38

polymer,

39

and electron acceptors (grapheme and

or introducing mesoporous in its structure.

42

Liu et al.

43

reported that doping of g-C3N4 with nonmetallic elements could extend its optical absorption to the visible region, modify its electronic properties by creating 2


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localized/delocalized states in the band gap of g-C3N4 and enhance its photocatalytic activity. From point of thermodynamic view, the four-electron process for O2 evolution is more favorable, due to the barrier for four-electron process (1.23 eV, 2H2O → O2+ 4H+ + 4e-) is lower than that of two-electron process (1.78eV, 2H2O→H2O2+ 2H+ + 2e-). But the catalyst induced water splitting via forming H2O2 is kinetically favored. So the water splitting reaction could take place in two two-electron paths over CDot-C3N4 with aid of catalytic H2O2 disproportionation.2 In this process, H2O2 disproportionation was the rate-controlling step, which limited the efficiency of overall water splitting. In Liu’s work, CDot worked as a thermal catalyst to promote H2O2 disproportionation. It is known that manganese, 44 copper 45 and iron 46 can also catalyze H2O2 disproportionation. However, the H2O2 disproportionation is a slow reaction. Enhancing the rate of H2O2 disproportionation can help to solve the critical and difficult problem of O2 evolution in photocatalytic overall water splitting, as well as further reveal the mechanism of some oxidation reaction related to H2O2, because O2-• radicals generation step is rate-determined step. 47-48 Contrary to previous reports, we found the two-electron disproportionation of H2O2 could be catalyzed by doped g-C3N4 under visible light irradiation. The activities of O2 evolution over g-C3N4, Fe/C3N4 and Cu/C3N4 photocatalysts under visible light irradiation were significant higher than that under dark condition, suggesting that the critical and difficult issue of O2 evolution in photocatalytic overall water splitting can be overcome by photocatalyst design for two two-electron oxygen evolution reaction, thus the efficiency of overall water splitting could be further enhanced. According to the characterization of the linear sweep voltammetry, the anode currents of Fe/C3N4/ITO and Cu/C3N4/ITO electrode were higher than that of g-C3N4/ITO electrode, light irradiation resulted in further increase of anodic current on both Fe/C3N4/ITO and Cu/C3N4/ITO electrodes, indicating nano-metal Fe and Cu could promote efficiently the O2 evolution. Mechanism analysis indicated that the two-electron H2O2 disproportionation process occurred via surface catalytic mechanism. 3



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Results and discussions The time courses of H2 and O2 evolution catalyzed by Fe/C3N4 (0.37wt%) and Cu/C3N4 (0.42wt%) in 100 ml water under visible light irradiation (λ≥420 nm) were shown in Fig. 1 and Fig. S1. There were H2 and O2 evolved over Cu/C3N4 photocatalyst under visible light irradiation, corresponding to 1.4 and 0.5 µmol in 12 h, respectively. However, the Fe/C3N4 photocatalyst showed a higher activity for H2 and O2 evolution, about 2.1 and 0.8 µmol generated in 12 h under visible light irradiation, respectively. In addition, the H2O2 formation was also checked by the UV-Vis absorption in the Fe and Cu decorated g-C3N4 dispersions and the results were shown

in Fig. 2 and Fig. S2. After 12 h visible light irradiation, a peak centered at 438 nm was appeared, due to the toluidine oxidized by formed H2O2, the measured concentrations of

H2O2 were 24.5 and 13.7 µmol/L in Fe/C3N4 and Cu/C3N4 dispersions, respectively. However, no H2O2 were detected without visible light irradiation in the blank test. This result indicated that H2O2 was formed in the water splitting process in the Fe/C3N4 and Cu/C3N4 dispersions under visible light irradiation, which was similar to the phenomenon Kang observed in CDot-C3N4 dispersion.2

To study the mechanism of H2O2 disproportionation over the g-C3N4, Cu/C3N4 and Fe/C3N4 photocatalysts, the H2O2 was added as a reference reagent in g-C3N4, Cu/C3N4 and Fe/C3N4 dispersions. The time courses of O2 evolution catalyzed by g-C3N4, Fe/C3N4 (0.37wt%) and Cu/C3N4 (0.42wt%) in 2.5% (v/v) H2O2 aqueous solution under dark and visible light irradiation (λ≥420 nm) conditions were shown in Fig 3. There was a trace amount of O2 evolved over g-C3N4 photocatalyst under dark condition, which suggested the g-C3N4 photocatalyst itself was inactive for O2 evolution in this condition. Under irradiated with visible light (λ≥420 nm), the activity of O2 evolution over g-C3N4 photocatalyst has been enhanced obviously, about 113.4 µmol in 2h. These results revealed the two-electron process of H2O2 disproportionation was a light driven reaction over g-C3N4 photocatalyst. With loading of Cu and Fe NPs on g-C3N4, the activities of O2 evolution were further enhanced under visible light irradiation. 242.4 and 363.5 µmol of O2 were generated 4


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over Cu/C3N4 and Fe/C3N4 photocatalysts in 2 h, respectively. Without light irradiation, only 16.7 and 46.8 µmol of O2 could be detected over Cu/C3N4 and Fe/C3N4 photocatalysts in 2 h. These results further indicated that the light irradiation was an important factor for efficient two-electron H2O2 disproportionation reaction and Cu and Fe species could catalyze this process. The effect of Fe and Cu concentrations on the activity of O2 evolution over Fe/C3N4 and Cu/C3N4 photocatalysts were studied under visible light irradiation (λ≥420 nm) and the results were shown in Fig. S3. Under visible light irradiation, the activity of O2 evolution over Fe/C3N4 photocatalyst was increased obviously with the Fe concentration increase from 0 to 0.37wt%. Further increasing the concentration of Fe, the activity increased slowly. Corresponding results under dark conditions showed typical thermal catalytic characteristic over Fe/C3N4 catalysts, the detected amount of O2 evolution continue to increase with the increase of Fe concentration from 0 to 0.92wt%, and reached a platform at around 1wt% Fe loading amount. In the Fig. S3b, the O2 evolution over Cu/C3N4 photocatalysts was also enhanced significantly under light irradiation. And Cu concentration increase led to increase of O2 evolution. In Fig. 4 and Fig. S4, the results in different H2O2 concentrations were given. The amounts of O2 generation over Fe/C3N4 and Cu/C3N4 photocatalysts increased with the H2O2 concentration increase, and were linearly dependent on the H2O2 concentration in the range of 0 to 3%. Interestingly, the hydrogen evolution activity was decreased with the H2O2 concentration increase (seeing Fig. S17). This result showed the H2O2 could inhibit the hydrogen generation, which further proved the water splitting over Fe/C3N4 and Cu/C3N4 catalysts were two two-electron process. Apparent quantum efficiencies (AQEs) of oxygen evolution over g-C3N4, Fe/C3N4 and Cu/C3N4 photocatalysts were measured under a wide range of visible light irradiation wavelength from 400 to 730 nm, as shown in Fig. 5. The highest AQEs values were achieved at 400 nm, 16.1%, 42.6% and 78.5% corresponded to g-C3N4, Fe/C3N4 and Cu/C3N4 photocatalysts, respectively. The AQEs value decreased gradually with the incident light wavelength increased, which were coincided with the result of UV-vis absorption (as shown Fig. S5), implying the H2O2 disproportionation 5



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reaction proceed over catalyst surface. The UV-vis absorption spectra of g-C3N4, Fe/C3N4 and Cu/C3N4 showed almost same characteristics indicating that implanting Fe and Cu NPs over g-C3N4 did not impact the light absorption ability of g-C3N4. The estimated Eg values of C3N4, Fe/C3N4 and Cu/C3N4 catalysts were 3.00, 2.97 and 2.93 eV, respectively. This result also suggested the Fe and Cu NPs have an ignorable impact on the light absorption ability of g-C3N4. The different band gap of pristine g-C3N4 might be caused by the quantum confinement effect and/or due to the different thermal condensation of precursors. 26 Fluorescence lifetime data of g-C3N4, Fe/C3N4 and Cu/C3N4 photocatalysts were also measured in 2.5% H2O2 aqueous as shown in Table 1. The emission decay of g-C3N4 was single-exponential, suggesting that there was only one emitting species (g-C3N4).49 The average lifetime of g-C3N4 was 1.22 ns. However, after doping with Cu and Fe NPs on the surface of g-C3N4, the average lifetime data were prolonged obviously, corresponding to 2.17 and 4.72 ns. This result indicated the Cu and Fe NPs could promote the excited charge separation, and Fe NPs was more efficient for the excited charge separation than Cu NPs. In order to further investigate the roles of Fe and Cu NPs in H2O2 disproportionation reaction, the electrochemical O2 evolution activities of g-C3N4, Fe/C3N4 and Cu/C3N4 electrodes were also studied using the linear sweep voltammetry (LSV) technique. As shown in Fig. 6, the anode currents corresponding to the H2O2 disproportionation to O2 increased with the increase of the applied potential. The anode currents of Fe/C3N4/ITO and Cu/C3N4/ITO electrodes were higher than that of g-C3N4/ITO electrode. These results implied that loading Fe and Cu NPs on the surface of the g-C3N4 could catalyze the H2O2 disproportionation. Fe NPs was more active than Cu NPs because the anode current of Fe/C3N4/ITO was higher than that of Cu/C3N4/ITO. The onset potentials of g-C3N4, Fe/C3N4 and Cu/C3N4 electrodes in the presence of H2O2 were 1.134, 1.086 and 1.142 V, respectively, as shown in Fig. S6. Fe/C3N4 had the lowest onset potential. Considering Fe/C3N4 present the highest activity of O2 evolution from H2O2, this low onset potential reflected the Fe catalytic role on H2O2 disproportionation reaction. Under 6


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irradiation conditions, H2O2 disproportionation was also catalyzed by Fe or Cu/C3N4 electrodes, as shown in Fig. S7. The significant enhanced current was observed on light-irradiated g-C3N4, Fe/C3N4 and Cu/C3N4 electrodes. It is reasonable to assume the excited charges induce and promote H2O2 disproportionation reaction. Transmission electron microscopy (TEM) images of g-C3N4, Fe/C3N4 and Cu/C3N4 photocatalysts were shown in Fig. S8. In Fig. S8 A, C, E, the catalysts all showed the typical morphology and structure of g-C3N4. 2,50 The corresponding Selective Area Electron Diffraction (SAED) pattern (insert of Fig. S8 A) suggested the g-C3N4 was in amorphous structure. The high-resolution TEM (HRTEM) of these catalysts were shown in Fig. S8 B, D and F. The black regions represent doped Fe and Cu on g-C3N4 surface. The diameters of those doped Fe and Cu domains were in 3-5 nm. The elemental mapping revealed that the C, N, O, Fe and Cu were uniform distribution in Fig. S9. According to the insert of Fig. S8 D and F, the lattice spacing of 0.21nm could be assigned to the (110) facet of metal Fe (JCPDS#89-4186) and the lattice spacing of 0.21nm could be assigned to the (111) facet of metal Cu (JCPDS#89-2838). In addition, the SEM and TEM images were obtained to determine the microstructure of g-C3N4, Fe/C3N4 and Cu/C3N4 photocatalysts and the results were shown in Fig. S10 and S11. Fig. S10C showed a typical bulk g-C3N4. After doping Fe and Cu NPs, the g-C3N4 appeared sheet structures as shown in Fig. S10A and B. These results indicated the Fe and Cu NPs would impact the microstructure of g-C3N4, which might due to the interaction between Fe, Cu NPs and g-C3N4. The different microstructures of g-C3N4 were also observed in the result of TEM in Fig. S11. The XRD patterns for g-C3N4, Fe/C3N4 and Cu/C3N4 photocatalysts were showed in Fig. 7. The (100) facet diffraction peak at 13.6° related to the interplanar structural packing, such as the hole-to-hole distance of the nitride pores in the g-C3N4,

51

corresponding to an

interlayer spacing of 0.67 nm. The sharp diffraction peak at 27.9° represented the characteristic interlayer stacking structure of (002) facet, which revealed an interlayer spacing about 0.32 nm. However, no typical peaks of metal Fe and Cu were detected, indicating the Fe and Cu were highly dispersed on the surface of g-C3N4. The average 7



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size of g-C3N4, Fe/C3N4 and Cu/C3N4 photocatalysts were about 2.2 nm from the line width analysis of the (002) diffraction peak using Scherer equation. In order to further study the characteristics of g-C3N4, Fe/C3N4 and Cu/C3N4 photocatalysts, the FT-IR spectra of g-C3N4, Fe/C3N4 and Cu/C3N4 photocatalysts were carried out and the result was shown in Fig. S12. The peaks at 1200 and 1750 cm-1 corresponded to the characteristic stretching modes of C-N heterocycles. The peak at 809 cm-1 belonged to the breathing mode of triazine units. The characteristic features of the condensed C−N heterocycles, i.e., the typical breathing mode was centered at 806.5cm-1. In addition, the peaks at 1315.7 and 1241.2 cm-1 corresponded to the stretching vibrations of the connected units of N−(C)3 (full condensation) and C−N−C (partial condensation), respectively. While the peaks at 1645.3, 1564.6, and 1408.2 cm−1 represented the stretching vibration modes of heptazine-derived repeating units.52 All these peaks represented typical characteristics of g-C3N4. The XPS spectra of g-C3N4, Fe/C3N4 and Cu/C3N4 photocatalysts were shown in Fig. 8 and Fig. S13. The survey spectrum, C1s spectrum and N1s spectrum results suggested the C and N were in the same chemical state in the g-C3N4, Fe/C3N4 and Cu/C3N4 photocatalysts. Fig. 8a showed the C1s spectra. The peak at 288.3 eV was identified as sp2-bonded carbon (N–C＝N), while the weak one at 286.4 eV was identified as sp3-bonded carbon (N–C). 2 Fig. 8b showed the four N1s peaks. The peak centered at 398.8 eV corresponded to sp2 hybridized aromatic N bonded to carbon atoms (C＝N–C). The peak centered at 399.6 eV was assigned to the tertiary N bonded to carbon atoms in the form of N–(C)3 or H–N–(C)2. 2 The weaker peak with a high binding energy at 401.3 eV was attributed to quaternary N bonded to three carbon atoms in the aromatic cycles. 53 And the peak centered at 404.4eV belonged to the π excitation. 54 In Fig. 8c, the peaks centered at 706.8 and 720.0 eV were attributed to Fe 2p3/2 and 2p1/2, indicating Fe was in metallic state. The peaks centered at 929.1 and 949.0 eV in Fig.8d belonged to metal Cu 2p3/2 and 2p1/2. The Auger kinetic energy peak at 918.3 eV in the Fig. 8e further revealed the Cu was in metallic state. After reaction, the Fe and Cu XPS were tested again and the results were shown in Fig. S14. The peaks centered at 710.9 and 724.1 eV could be attributed to Fe 2p3/2 and 8


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Fe2p1/2, indicating that Fe existed in the oxidized state of Fe2O3. Consequently, the peaks centered at 933.3 and 953.3 eV belonged to Cu2p3/2 and Cu2p1/2, suggesting that Cu existed in the form of CuO. These results indicated the metallic Fe and Cu in fresh catalyst are oxidized by H2O2. The stabilities of O2 evolution over g-C3N4, Fe/C3N4 and Cu/C3N4 photocatalysts were also tested under visible light irradiation (λ≥420 nm). As shown in Fig. 9, the catalysts were stable in 360 min reaction. After each run, the catalysts were collected by centrifuging from the reaction mixture and redispersed in the fresh 2.5% H2O2 aqueous solution. In addition, the fresh and used catalysts were characterized by FT-IR and the results were given Fig. S15. No significant differences were observed in fresh and used Fe/C3N4. ESR technique, as an efficient method to determine short-lived radicals, was carried out for understanding the photocatalytic mechanism. In the Fig. 10, the ESR signals of DMPO-OH• and DMPO-O2-• were detected in our reaction systems. The typical 1:2:2:1 and 1:1:1:1:1:1 peaks corresponded to the DMPO-OH• (g=2.0048) and DMPO-O2-• (g=2.0001), respectively. Their intensity was obviously in agreement with the activity results as shown in Fig. 3. This result indicated the H2O2 disproportionation occurred in a radical mechanism. However, no signal was observed when DMPO existed only. Only OH• radical was observed without catalysts, which suggested the O2-• was a key radical for H2O2 disproportionation in our systems. The porposed mechanism for light-driven water spliting catalyzed by Fe/C3N4 and Cu/C3N4 was present in scheme 1. Under the visible light irradiation, the water was converted into hydrogen and H2O2 , and the H2O2 was further converted into oxygen and H2O via photocatalytic disproportionation route. The photocatalytic two-electron oxygen formation from H2O2 occurred along the following steps: H2O2 +e- =HO· +OH-

∆E1= 1.07 eV

ref. 55

H2O2 +h+ +2OH- =O2-· +2H2O

∆E2= -3.86 eV

ref. 56

O2-· +HO· = O2 +OH-

∆E3= -0.27 eV

ref. 57

The first step was an electron catalytic process (ECP), C3N4 absorbed the visible light to form excited electrons and holes. The electrons transferred to metal Fe or Cu 9



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sites, due to the energy potential difference between g-C3N4 and Fe2O3 or CuO NPs. The potential of those electrons was about -0.25 eV, 2 which had sufficient force to drive H2O2 disproportionation to form HO· and OH-.The second step was a hole catalytic process (HCP), the OH- and H2O2 could form the O2-· and H2O species reacted with the holes. At last, the O2-· and HO· could recombine to form O2. As the potential curve schematic shown in the insert figure, the ECP was an energy-consumption process, while the HCP and the third step were an energy-releasing process. The ECP was the rate-controlling step.

Conclusions In this work, we found the two-electron pathway of H2O2 splitting was a light-driven process. The activities of O2 evolution over g-C3N4, Fe2O3/ C3N4 and CuO/ C3N4 photocatalysts under visible light irradiation were significant higher than that under dark condition. The results of XPS showed the Fe and Cu were in metallic states, but after reaction, they were oxidized by H2O2. They were implanted on the surface of g-C3N4, which proved by the results TEM. According to the characterizations of XRD, UV-vis absorption and XPS, the Fe and Cu NPs have no effect on the performance of g-C3N4 for visible light absorption. But loading Fe and Cu on the surface of g-C3N4 could promote the O2 evolution. The characterization of the linear sweep voltammetry showed the anode currents of Fe/C3N4/ITO and Cu/C3N4/ITO electrode were higher than that of g-C3N4/ITO electrode, indicating Fe and Cu NPs could promote efficiently the two-electron pathway, which led to high activity for H2O2 disproportionation. The highest AQEs of g-C3N4, Cu/C3N4 and Fe/C3N4 photocatalysts were achieved at 400 nm, corresponding to 16.1%, 42.6% and 78.5%, respectively. In addition, the potential of Fe/C3N4 photocatalyst was lower obviously than that of g-C3N4 and Cu/C3N4 photocatalysts, which might lead to the activity of O2 evolution over Fe/C3N4 photocatalyst was higher obviously than that over g-C3N4 and Cu/C3N4 photocatalysts. This study will give a new insight in designing efficient catalyst for overall water splitting. 10


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■ ASSOCIATED CONTENT Supporting Information The detail methods of preparation of the g-C3N4, Fe/C3N4 and Cu/C3N4 photocatalysts, photocatalytic O2 evolution activity and AQE measurements, working electrode preparation

and

photoelectrochemical

measurements,

the

meteorological

chromatography of products, hydrogen peroxide (H2O2) measurements, the effect of Fe and Cu concentration on the activity of O2 evolution, the effect of H2O2 concentration on the activity of O2 evolution, the UV-vis absorption spectra of photocatalysts, the onset potentials, the current-time curve, transmission electron microscopy images, HRTEM images, the elemental mapping images, the SEM images, the TEM images, the FT-IR spectra, the XPS survey spectra, the FT-IR spectra of photocatalysts, the results of energy calculation, the effect of H2O2 concentration on the activity of H2 evolution, The time-resolve florescence decay plots and static florescence spectra photocatalysts were present.(PDF)

The

Supporting Information is available free of charge on the ACS Publications website http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel.: +86-931-4968 178. ACKNOWLEDGMENT This work is supported by the NSF of China (grant no. 21173242 and 21433007) respectively.

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Scheme and Figure Captions Table 1 Fluorescence lifetime measurements of g-C3N4, Fe/C3N4 (0.37wt%) and Cu/C3N4 (0.42wt%) photocatalysts. Fig. 1 The time courses of H2 evolution catalyzed by Fe/C3N4 (0.37wt%) and Cu/C3N4

(0.42wt%) in water under visible light irradiation (λ≥420 nm) conditions. Fig. 2 The two-electron process for water oxidation to form peroxides. Fig. 3 The time courses of O2 evolution catalyzed by g-C3N4, Fe/C3N4 (0.37wt%) and Cu/C3N4 (0.42wt%)in 2.5% (v/v) H2O2 aqueous solution under dark and visible light irradiation (λ≥420 nm) conditions. The O2 evolution checked in argon atmosphere conditions. Fig. 4 The effect of H2O2 concentration on the activity of O2 evolution over Fe/C3N4 (0.37wt%) photocatalyst under visible light irradiation (λ≥420 nm). Fig. 5 The apparent quantum efficiencies (AQEs) of g-C3N4, Fe/C3N4 (0.37wt%) and Cu/C3N4 (0.42wt%) photocatalysts under a wide range of visible light irradiation from 400 to 730 nm. Fig. 6 LSV curves of g-C3N4, Fe/C3N4 (0.37wt%) and Cu/C3N4 (0.42wt%) photocatalysts coated on ITO glass in mixed solution of 2.5% (v/v) H2O2 and 0.1 M Na2SO4. Fig. 7 Experimental XRD patterns for g-C3N4, Fe/C3N4 (0.37wt%) and Cu/C3N4 (0.42wt%) photocatalysts. Fig. 8 The XPS spectra of g-C3N4, Fe/C3N4 (0.37wt%) and Cu/C3N4 (0.42wt%) photocatalysts. a) C1s spectrum. b) N1s spectrum. c) Fe 2p spectrum. d) Cu 2p spectrum. e) Cu LMM spectrum. Fig. 9 The stabilities of O2 evolution over g-C3N4, Fe/C3N4 (0.37wt%) and Cu/C3N4 (0.42wt%) photocatalysts under visible light irradiation (λ≥420 nm). The reaction was continued for 360 min. After every run, the catalysts were collected by centrifuging from the reaction mixture and redistributed in the fresh 2.5% H2O2 aqueous solution and then evacuated.

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Fig. 10 ESR signals of the DMPO-OH• (aN=aH=14.77G) and DMPO-O2-• (aN=15.39G, aH=13.12G) adducts.(H2O2 88 mM, DMPO 8.8 mM, catalysts

50mg/L, light

irradiation time was 2 min and λ≥420 nm) Scheme 1 The proposed reaction mechanism for visible-light-driven water splitting by Fe/C3N4 and Cu/C3N4 photocatalysts.

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Table 1

Lifetime,

Pre-exponential

Average

(ns)

factors B

lifetime, (ns)

C3N4

τ1=1.22

B1=1

1.22

0.9995

Cu/ C3N4

τ1=2.17

B1=1

2.17

1.0001

Fe/ C3N4

τ1=4.72

B1=1

4.72

1.0048

C3N4 systems

20


χ2

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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Fig. 7

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Fig. 8

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Fig. 9

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Fig. 10

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Scheme 1

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TOC graphic Water splitting via two two-electron processes (the H2O first photocatalytically converted to H2 and H2O2 under light irradiation, then the H2O2 disproportionation to H2O and O2 by a thermal catalytic process) has attracted extensive attentions recently. Contrary to these reports, we found that not only the photocatalytic H2 generation could be driven by visible light, but also the two-electron H2O2 disproportionation to H2O and O2 could also be photocatalyzed by visible light over C3N4 catalysts. Photocatalytic hydrogen, oxygen generation and simultaneous H2O2 formation in Cu/C3N4 and Fe/C3N4 dispersions were confirmed. The corresponding AQEs of oxygen formation were 16.1%, 42.6% and 78.5% at 400 nm, respectively.

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Visible Photocatalytic Water Splitting and Photocatalytic Two-Electron

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