Enhanced photocatalytic H2 evolution of ultrathin g-C3N4 nanosheets via surface shuttle redox

Journal of Alloys and Compounds 810 (2019) 151918

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Enhanced photocatalytic H2 evolution of ultrathin g-C3N4 nanosheets via surface shuttle redox Peiyao Lin a, Jun Shen b, Hua Tang a, *, Zulfiqar a, Zixia Lin c, Yan Jiang a, ** a

School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, PR China School of Pharmacy, Suzhou Vocational Health College, Suzhou, 215009, China c Testing Center, Yangzhou University, Yangzhou, 225009, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 May 2019 Received in revised form 17 August 2019 Accepted 17 August 2019 Available online 17 August 2019

Graphitic carbon nitride (g-C3N4) is sprouted as an efficient and cost-effective visible-light-responsive photocatalyst for yielding hydrogen from water splitting. However, a brisk recombination of lightinduced electron-hole (e
- hþ) pairs in the bulk g-C3N4 effectuate poor quantum efficiency in the hydrogen evolution reaction (HER). The shuttle redox mediator manifests ample potential in accelerating photo-induced carrier segregation and in boosting charge transport in the HER. Here, we report that exploiting Ag/Ag(I) and Fe(III)/Fe(II) shuttle redox mediators, the hydrogen-evolving rate of the aforementioned g-C3N4 nanosheets can reach 3213.3 mmol g
1 h
1 under visible-light irradiation, which is eight times higher than that of pure g-C3N4 (404.82 mmol g
1 h
1). The effective coupling between a hydrogen-evolving catalyst and appropriate shuttle redox mediators significantly improves the HERphotocatalytic performance of native g-C3N4 nanosheets. Density functional theory calculations show that the presence of Ag/Ag(I) and Fe(III)/Fe(II) shuttle redox mediators can effectively promote H atom adsorption and facilitate a H2O reduction reaction. This work envisages a new and deft approach for contriving high-performance geC3N4ebased photocatalysts for highly efficient solar-to-fuel conversion. © 2019 Elsevier B.V. All rights reserved.

Keywords: Shuttle redox G-C3N4 Electron mediator Hydrogen evolution reaction Photocatalytic water splitting

1. Introduction Photocatalytic hydrogen generation over particulate semiconductor photocatalysts is rated as a budding alternative to effectively address the aggravating global energy crisis and environmental disruptions [1e6]. Thus, developing cost-effective, highperformance visible-light-responsive semiconducting photocatalysts is highly desirable [7]. Thus far, the most commonly explored photocatalytic materials are the metal oxide (MO)-based semiconductors. However, these photocatalysts have relatively wide bandgaps which restrict their solar light utilisation efficiency [8e17]. Although strenuous endeavours have been completed to synthesise an assortment of solar-driven H2-evolving photocatalysts, photocatalytic activities of predominant particulate photocatalysts remain far from the theoretical solar-to-fuel conversion efficiency. Numerous advances in the (HER) activity of g-C3N4 have been

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Tang), [email protected] (Y. Jiang). https://doi.org/10.1016/j.jallcom.2019.151918 0925-8388/© 2019 Elsevier B.V. All rights reserved.

substantiated since 2009 [18e24]. However, the HER efficiency of g-C3N4 is strongly affected by the conduction band (CB) and valence band (VB) potentials, as well as its bandgap energy. Furthermore, fast recombination of e
- hþ pairs, inefficient charge transfer, and severe backward reactions restrict further advancement in g-C3N4 HER efficiency [25e28]. In addition, photocatalytic hydrogen production is essentially a two-electron reaction that requires injection of photo-induced charge carriers into an adsorbate. Therefore, an improvement in the quantum efficiency mainly relies on increasing the availability of active electrons via the photoexciting electron to form spatially separated electron/hole pairs and simultaneously inhibit recombination [29e34].
3þ 2þ Shuttle redox mediators (I
3 /I , Fe /Fe , etc.) have excellent mobility to diffuse into a solution after electron capture, leading to spatially separated charge carriers [35e38]. Among previously reported redox mediators, Fe(III)/Fe(II) is regarded as the most commonly used electron acceptor/donor (A/D) pair for redox potential optimisation of a photocatalytic system [36]. Recently, Kudo and co-workers reported that Fe(III)/Fe(II) could act as an efficient shuttle redox mediator in the establishment of Z-scheme systems for overall water splitting [37]. In addition, a Fe(III)/Fe(II) shuttle

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redox mediator modification with a zero-approaching Gibbs free energy of an adsorbed hydrogen atom has been proven to facilitate the hydrogen-evolving process [38]. Compared to single shuttle redox, double shuttle redox has been proven to enable more efficient charge transfer and enhanced photocatalytic activity. Yu et al. [39] demonstrated that, compared to a single shuttle redox modification with Ti(IV) or Fe(III), regulation with both Ti(IV) and Fe(III) on the surface of AgBr resulted in faster interfacial charge transfer. Thus, it is speculated that with the aid of effective dual shuttle redox mediators, the HER efficiency of g-C3N4 could be further improved. Herein, we explored the preparation of modified ultrathin gC3N4 nanosheets by incorporating Ag/Ag(I) and Fe(III)/Fe(II) double shuttle redox mediators into the photocatalyst. The approach differs from conventional doping methods in that both redox shuttles are simultaneously integrated with g-C3N4 nanosheets. Subsequently, the obtained modified ultrathin g-C3N4 nanosheets were applied as photocatalysts with extended visible light absorption for the HER. Quite impressively, the shuttle redox-mediated g-C3N4 has an outstanding hydrogen-evolving rate of 3213.3 mmol g
1 h
1, which is 8-fold higher than that of pure g-C3N4. It enhanced fullspectrum light-harvesting, had more active redox sites, exhibited improved atomic hydrogen adsorption, and had higher electron mobility compared to g-C3N4 and Ag/g-C3N4 hybrid. In addition, a photocatalytic approach behind the melioration in the HER performance is proposed. This work envisions an artifice to engineer two-dimensional (2D) g-C3N4 nanomaterials for tunable charge separations and migrations in environmental crisis. 2. Experiment 2.1. Preparation of Ag/Ag(I)/Fe(III)/Fe(II)/g-C3N4 hybrid material First, the g-C3N4 nanosheets (shorted as CN) were prepared by

heating urea at 550  C [40]. Ag/g-C3N4 with 10 wt% Ag content was prepared using our previously reported chemical reduction method [41]. In a typical synthesis, 0.1 g g-C3N4 was introduced into the microemulsion and ultrasonicated for 12 h with subsequent addition of (0.1 M) aqueous AgNO3 solution and then stirred for 1 h in the dark. Then, 0.1 M NaBH4 solution was swiftly poured under incessant stringent stirring. Consequently, the solution was stirred for 24 h, washed and vacuum dried at 70  C to obtain an Ag/g-C3N4 (ACN) sample. The AFCN samples were prepared using an impregnation technique. As shown in Fig. 1, Ag/g-C3N4 (0.1 g) powders were dispersed into 40 mL of Fe(NO3)3 solution of different concentrations (2.5, 5, and 7.5 mmol L
1) under stirring for 15 min. Then, the resulting solution was maintained at 60  C for 2 h, filtrated, washed, and dried at 60  C. The AFCN samples with an Fe content of 6, 12, and 18 wt% were termed AFCN-1, AFCN, and AFCN-3, respectively. The Fe(III)/Fe(II)/g-C3N4 sample labelled FCN was also synthesised via a similar approach except exploiting g-C3N4 rather than of Ag/gC3N4. 2.2. Characterisations X-ray diffraction (XRD) results and the synthesised sample morphology were identified using an Ultima IV diffractometer (Rigaku) with Cu Ka radiation (l ¼ 1.5406 Å) and a field-emission transmission electron microscope (Tecnai G2 F30 S-TWIN, FETEM), respectively. X-ray photoelectron spectroscopy (XPS) spectra were recorded using a VG Scientific ESCALAB Mark II spectrometer, Ultraviolet (UVeVis) diffused reflectance spectra were obtained using a UVeVis spectrophotometer (Lambda 950, PE). The photoluminescence (PL) emission spectra were characterizated via an LS-55 (PerkinElmer) device. Photocurrent response curves and electrochemical impedance spectra (EIS) were recorded using an electrochemical station (CHI760, China) with a light-

Fig. 1. Schematic illustration of the fabrication of the AFCN hybrid.

P. Lin et al. / Journal of Alloys and Compounds 810 (2019) 151918

emitting diode light source. Electron spin resonance (ESR) spectra were appraised using a Bruker model A300 spectrometer with 5, 5dimethyl-l-pyrroline N-oxide (DMPO) and 2, 2, 6, 6-tetramethyl-1piperidinyloxy (TEMPO) as spin-trapping reagents. Inductively coupled plasma (ICP) assessments were using an ICP-5000 optical emission spectrometer for elemental analysis. 2.3. Photocatalytic tests Photocatalytic H2 generation experiments were performed in a sealed reactor accompanied by a cooling unit. In a typical synthesis, 10 mg of the as-prepared sample, lactic acid (5 mL, 20 vol%), and Pt (3 wt%) were dispelled in distilled water (45 mL) and ultrasonicated for 15 min. The reactor was subsequently irradiated by a 300-W Xeilluminant and the H2 evolution was recorded. The discernible quantum efficiency was characterised by a certain amount of catalyst (10 mg) and a Xe lamp (300 W) equipped with different band-pass filters. The apparent quantum efficiency is defined as follows:

1

0 H2 molecules B 2  total evolved total time AQE ¼ B @total number of incident photons time of incidence

C C  100% A

3

3. Results and discussion To gain insight into the enhancement of photocatalytic H2 generation versus the AFCN sample, microstructural analysis was initially performed. The transmission electron microscopy (TEM) micrograph of pristine CN is shown in Fig. 2a, demonstrating that the CN nanosheets consisted of thin layers with wrinkles and irregular folding structures, offering a widespread attachment plane for metal ions. For the ACN sample (Fig. 2b), Ag nanoparticles (NPs) 10 nm in size are closely attached to the CN slices. When the CN is impregnated in the Fe(NO3)3 solution to form FCN, no obvious particles are observed (Fig. 2c), probably because Fe ions are immobilised on negatively charged CN nanosheets through electrostatic interaction instead of Fe particles. When the ACN sample is impregnated in the Fe(NO3)3 solution, the resultant AFCN sample shows a lamellar structure loading with small NPs (Fig. 2d). The TEM elemental mapping results (Fig. 2e) demonstrate that the Ag and Fe elements coexist on the slice of ultrathin CN nanosheets and show similar distribution patterns, which is also evidence of the simultaneous incorporation of Ag and Fe elements into the AFCN. ICP analyses show the Fe content in the as-synthesised AFCN is 10.2 wt%, which is nearly the same as the apparent 12 wt% loading amount. To characterize the sample thickness, atomic force microscopy (AFM) was used. Before ultrasonication, the bulk CN has a thickness

Fig. 2. TEM images of (a) bare CN; (b) ACN; (c) FCN and (d) AFCN; (e) Typical STEM image of AFCN and elemental mapping results of C, N, Ag, and Fe.

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P. Lin et al. / Journal of Alloys and Compounds 810 (2019) 151918

Fig. 3. AFM image of (a) CN sample before exfoliating. (b) exfoliated CN monolayer and (c) AFCN sample.

of 13e15 nm (Fig. 3a) while after ultrasonication, it is 3e5 nm (Fig. 3b), demonstrating successful preparation of CN nanosheets via ultrasonication. In addition, we characterised the AFCN sample (Fig. 3c) and found that its thickness relative to CN did not vary. The CN, ACN, and AFCN phase identification and crystal structure were analysed using XRD (Fig. 4a). The CN demonstrates two typical diffraction peaks centred at 13.1 and 27.8 , originating from the (100) plane of in-planar packing of the conjugated aromatic segments and (002) plane of interlayer structural packing units, respectively [42]. For ACN, the additional peaks belong to CN and four reflections occur at 38.1, 44.3 , 64.5 , and 77.5 , accordant with the (111), (200), (220), and (311) reflection planes of the facecentred cubic (fcc) structure of Ag (JCPDS 04-0783), and clearly show the coetaneousness of fcc-Ag and g-C3N4 in the ACN sample. However, for the AFCN sample, only CN peaks can be observed and those of Ag could not be clearly seen, which might be explained by the replacement reaction between Fe(III) when the ACN sample was impregnated in the Fe(NO3)3 solution, leading to a decrease in the amount of Ag NPs on CN. The chemical states and surface composition of the samples were further probed via XPS. As shown in the XPS survey spectra (Fig. 4b), all samples manifest the typical binding energy peaks of the C and N elements and exemplify the characteristic g-C3N4 phase [43]. In addition, Ag is found in the spectra of both ACN and AFCN, demonstrating the existence of Ag

on the ACN and AFCN samples. Meanwhile, a weak peak of Fe could be observed on AFCN, indicating that the AFCN hybrid was successfully fabricated. For the high-resolution spectra of Ag 3d (Fig. 4c), the distinctive peaks of Ag 3d5/2 and Ag 3d3/2 of ACN are at 368.12 and 374.11 eV, respectively, which can be mainly attributed to the silver (Ag) [44,45]. According to the Ag 3d spectrum of AFCN, the slightly rightward shift of Ag 3d5/2 and Ag 3d3/2 to higher binding energies indicates the collective effects between Ag and Fe. In addition, four Ag peaks can be fitted in the AFCN sample and the peak positions of Ag 3d5/2 and Ag 3d3/2 show a red shift. The peaks at 368.3 and 374.3 eV are attributed to Ag while the peaks at 368.9 and 375.0 eV belong to Ag (I), indicating that both metallic Ag and Ag (I) co-exist in the AFCN sample. The highly resolved spectra of Fe 2p of FCN and AFCN are shown in Fig. 4d. The satellite (Sat.) peak of Fe(III) can be seen in the FCN sample [46]. The existence of peaks peculiar to Fe(II) in the AFCN sample shows the extraction of Fe(III) from the aqueous suspension containing Fe(NO3)3 and its transformation to metallic Fe(II) during preparation, indicating that Fe(II) and Fe(III) simultaneously exist in the AFCN sample. The aforementioned results suggest that Ag/Ag(I) and Fe(III)/Fe(II) double shuttle redox have been effectively strewn on the CN surface in the AFCN composite. Fig. 5a and b show a comparison of the photocatalytic H2 generation activity over various samples when irradiated by visible

P. Lin et al. / Journal of Alloys and Compounds 810 (2019) 151918

5

Fig. 4. XRD patterns (a) and XPS spectra (b) of CN, ACN and AFCN; (c) high-resolution Ag 3d XPS spectra of CN, ACN and AFCN; (d) high-resolution Fe 2p XPS spectra of CN, FCN and AFCN.

Fig. 5. Photocatalytic H2 evolution over various samples (a, b) under visible light; Recycle runs (c) and quantum efficiency of H2 evolution (d) over the AFCN sample.

light (l > 420 nm). The ACN showed much higher photocatalytic H2 production activity (6432.41 mmol g
1) versus that of the CN (2009.47 mmol g
1). The photocatalytic H2 generation performance of AFCN was further enhanced to 16237.52 mmol g
1, indicating that

Fe addition could effectively improve the H2 evolution activity. In addition, there is a notable relationship between the H2 evolution performance and the Fe content; the highest H2 production rate of 3213.15 mmol g
1 h
1 is achieved compared to that of the AFCN

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P. Lin et al. / Journal of Alloys and Compounds 810 (2019) 151918

Table 1 A comparison between the as-obtained sample in this study and related catalysts in literature. Catalyst

Light

The First cycle (mmol$g
1h
1)

The Last cycle (mmol$g
1h
1)

Cycle times

Ref.

Pt2þ/Pt0 hybrid nanodots/g-C3N4 Pt/t-ZrO2/g-C3N4 g-C3N4 nanosheets/Amorphous NiS PdeAg decorated g-C3N4 Pt single atoms/g-C3N4 PtPd alloy/g-C3N4 FeP/g-C3N4 PdAg/g-C3N4 This work

visible light visible light visible light visible light visible light visible light Visible light visible light visible light

2500 800 515 1250 3000 1600 177.9 2925 3213

2475 600 466 1062 2750 1250 1.57% 2375 2733

3 4 4 4 4 4 3 4 5

[47] [48] [49] [50] [51] [52] [53] [54]

Fig. 6. EPR (a) and ESR spectra of radical adducts trapped by TEMPO and DMPO: TEMPO-hþ in CH3CN dispersion (b); DMPO-O
2 in MeOH dispersion (c); (d) DMPO-OH in aqueous dispersion over CN, CAN and AFCN under visible-light irradiation.

sample and is an eightfold increase in comparison to that of the bare CN (404.82 mmol g
1 h
1) and a threefold increase compared to that of the ACN (1264.73 mmol g
1 h
1). The H2 evolution rate decreased in the case of AFCN-3 with a higher Fe(III)/Fe(II) shuttle redox content, probably because the excessive Fe(III)/Fe(II) shuttle redox may take over active sites occupying the faces of CN nanosheets and form recombination centres for e
/hþ [39]. Furthermore, the H2 production of AFCN hybrid demonstrates no evident diminution after five runs within 20 h (Fig. 5c). An intricate comparison between the as-obtained sample current study and related geC3N4ebased sample in the literature is shown in Table 1 [47e54]. Fig. 5d shows that the highest quantum efficiency (QE) of AFCN is 6.77% at 420 nm. Aiming towards the effect of dual shuttle redox on the CN electronic structure, electron paramagnetic resonance (EPR) experiments were conducted for the CN, ACN, and AFCN samples. Fig. 6a shows CN has a sole paramagnetic presage demonstrating g value of 2.0065 contributed by the single electrons of the sp2-carbon atoms inside the p-conjugated aromatic rings [55,56].

Moreover, the signal intensity of the AFCN sample is significantly enhanced in comparison to the CN or ACN samples, showing an increased number of lone electron pairs and enhanced charge delocalisation in AFCN. For the TEMPO-hþ signals (Fig. 6b), a control experiment of TEMPO shows a typical signal of three peaks with an intensity of 1:1:1. The signal intensity slightly decreases in the presence of CN, ACN, and AFCN under irradiation; such a disappearance of TEMPO signals indicates that the photogenerated holes were all produced from photoexcited samples [57]. The greatest reduction of the TEMPO signal for AFCN shows the highest concentration of photogenerated holes, most probably stemming from the expeditious detachment of the photo-induced carriers. The DMPO-O
2 and DMPO-OH signals are shown in Fig. 6c and d, respectively. No DMPO-O
2 or DMPO-OH signal could be observed in the dark. Following illumination, six emblematic peaks from the DMPO-O
2 adducts were identified for CN, ACN, and AFCN, while four distinctive peaks having an intensity proportion of 1:2:2:1 from the DMPO-OH adducts were only pinpointed for ACN and AFCN [58,59]. For the DMPO-O
2 or DMPO-OH signals, the AFCN

P. Lin et al. / Journal of Alloys and Compounds 810 (2019) 151918

7

Fig. 7. Transient photocurrent response curves (a) and electrochemical impedance spectroscopy results (b) of CN, ACN and AFCN; UVevis spectra (c), Plots of the (ahy)1/2 vs. photon energy (hy) (d), valence-band XPS (e) and band structures (f) of CN and AFCN.

Fig. 8. Optimised geometry structures and absorption energies for H2.

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P. Lin et al. / Journal of Alloys and Compounds 810 (2019) 151918

Fig. 9. Schematic diagrams illustrating possible photocatalytic mechanism of various photocatalysts for hydrogen generation: (a) CN; (b) ACN; (c) AFCN.

hybrid shows the highest intensity compared to those of other samples, implying that more active superoxide radicals and hydroxyl radicals are generated from the irradiated AFCN sample. The increase in available free radicals is generally because of the improvement in the detachment efficacy of photo-induced carriers. Accordingly, transient photocurrent responses and EIS measurements were undertaken to confirm the improved carrier separation ability of the AFCN sample. The photocurrent response curves of pure CN, ACN, and AFCN were recorded when visible light was irradiated; the measured impacts are shown in Fig. 7a. The AFCN sample shows much higher photocurrent density in contrast to that of the other two samples, indicating that the incorporation of Ag/ Ag(I) and Fe(III)/Fe(II) double shuttle redox could synergistically improve the photogenerated carrier separation [60e63]. EIS curves of the pure CN, ACN, and AFCN samples (Fig. 7b) are consistent with the photocurrent analysis. Compared to CN and ACN, the shortest arc radius is observed for AFCN, indicating the occurrence of the fastest interfacial charge transfer on the AFCN sample surface [64e67]. The fast electron-transfer rate from an electrolyte to an electrode surface indicates strong interaction between ion couples and CN nanosheets. Fig. 7c shows the UV/Vis light absorption spectra, showing that bare CN has absorption in the region of 450 nm [68]. Increased light absorption in the whole spectra is observed for the AFCN sample, which can mainly be attributed to the half-filled electronic configuration of the Fe ions [69]. In addition, the band edge positions of the VB CB potentials of CN and AFCN were confirmed. The bandgap energies (Eg) can be calculated using the a(hv) ¼ A(hv
Eg)2 equation [70], the Eg of the CN and AFCN are 2.63 and 2.52 eV (Fig. 7d), respectively. According to the valance band XPS spectra (Fig. 7e), the VB positions of the CN and AFCN are estimated as 1.74 and 1.83 eV, respectively. Finally, the bandgap structures of the CN and AFCN are shown in Fig. 7f. The

narrowed bandgap energy of the AFCN is advantageous to light adsorption and improves the H2 production activity. In addition to the enhanced separation of photogenerated carriers and light adsorption, adsorption or release of hydrogen on the photocatalyst is also an important step. The adsorption energy of atomic hydrogen (DE) can describe the hydrogen evolution activity to some extent and the value of DE should be near zero for an efficient photocatalyst [71e73]. Density functional theory (DFT) estimates were completed to simulate the hydrogen adsorption over CN, ACN, and AFCN. Based on the optimisation results shown in Fig. 7, the H atom would be adsorbed above on the interstitial site in CN, and the calculated adsorption energy in CN is 0.7998 eV. For the ACN sample, the Ag atom is on the site above the interstitial site in CN and the H atom would be adsorbed on another interstitial site in the same triazine ring of the CN. The calculated adsorption energy in ACN (0.4924 eV) decreases as compared to that of CN, indicating that the addition of Ag can facilitate the HER. For the AFCN sample, the Ag and Fe atoms would occur above on different interstitial sites in two different triazine rings of CN and the H atom would be adsorbed on the interstitial site in the same triazine ring with the Ag atom on CN. In contrast, the calculated adsorption energy in AFCN (0.2657 eV) is the lowest among the three samples, leading to a maximised hydrogen evolution, which is in substantial concordance with the photocatalytic hydrogen evolution performances shown in Fig. 8. Based on the aforementioned analysis, the proposed possible mechanism of photocatalytic H2 evolution for different samples is shown in Fig. 9. Upon exposure to visible-light radiation, CN could be easily excited to render light-induced (e
- hþ) pairs; however, it possesses a relatively weak hydrogen reduction ability as a consequence of the high recombination of e
- hþ pairs [74,75] (Fig. 9a). When Ag NPs are loaded on the CN nanosheet faces, the photo-

P. Lin et al. / Journal of Alloys and Compounds 810 (2019) 151918

induced electrons generated in CN may migrate to Ag NPs, expediting the detachment of the photo-induced (e
- hþ) pairs (Fig. 9b). For the AFCN hybrid (Fig. 9c), the light-induced electrons move to the Ag/Ag(I) and Fe(III)/Fe(II) double shuttle redox pairs and subsequently participate in the HER. The Ag/Ag(I) and Fe(III)/Fe(II) shuttle redox mediator offers two new electron transfer paths to inhibit the recombination of charges and thus enhance the photocatalytic activity [76]. The synergetic effect among the efficient electron-hole separation, improved light adsorption, and the optimised hydrogen adsorption energy results in the enhanced photocatalytic hydrogen evolution performance of the AFCN hybrid. 4. Conclusions In summary, the Ag/Ag(I) and Fe(III)/Fe(II) double-redox mediators were simultaneously loaded on g-C3N4 nanosheet faces via a combination of chemical reduction and impregnation. The photocatalytic performance was significantly enhanced and the AFCN photocatalyst demonstrated a H2 evolution rate of 3213.15 mmol g
1 h
1, which is 8-fold higher than that of the pure CN (404.82 mmol g
1 h
1). The significantly improved photocatalytic behaviour was accredited to the collective effect of the Ag/Ag(I) and Fe(III)/Fe(II) shuttle redox mediators, which can enhance the detachment of (e
- hþ) pairs, improving the light-harvesting property and optimising the hydrogen adsorption energy for photocatalytic hydrogen evolution from water splitting. These findings may provide a new concept for the delineation of high-efficiency geC3N4ebased photocatalytic materials with redox mediators for solar-to-fuel conversion.

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