driven photocatalytic water oxidation

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Fe2O3/C-C3N4 based tight heterojunction for boosting visible-light-driven photocatalytic water oxidation Lingqiao Kong, Junqing Yan, Ping Li, and Shengzhong (Frank) Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01799 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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ACS Sustainable Chemistry & Engineering

Fe2O3/C-C3N4 based tight heterojunction for boosting visible-lightdriven photocatalytic water oxidation Lingqiao Konga, Junqing Yana*, Ping Lia, and Shengzhong (Frank) Liua,b a

Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education; Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, No. 620, West Chang'an Avenue, Chang'an District, Xi'an, 710119, People’s Republic of China E-mail: [email protected]

b

iChEM, Dalian Institute of Chemical Physics, Dalian National Laboratory for Clean Energy, Chinese Academy of Sciences, 457 Zhongshan Road Dalian, Dalian, 116023, People’s Republic of China

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Recently, the photocatalytic water splitting for the clean hydrogen (H2) energy through the use of solar energy has been considered to be a promising means of renewable energy conversion. Owing to the high activation barriers of oxygen (O2) generation, the half reaction i.e. water oxidation is the rate-limiting steps of the overall water splitting efficiency. The photocatalysts with the high activity and nonmetal usage are needed. Herein, we report one new thin layered heterojunction sample of Fe2O3/C-C3N4, containing layered α-Fe2O3 and carbon-coated g-C3N4, obtained through one simple repeatable solid-state synthesis strategy. The layered FeOOH and g-C3N4 are first synthesized working as the precursors. Under the N2 atmosphere and 580 ˚C, the dehydration of FeOOH happens and transfers into layered α-Fe2O3, the water vapor destroys the Van der Waals force of g-C3N4 and induces the parts of edge carbonization. Under the synergistic effect of vapor and heating, the thin layered Fe2O3/C-C3N4 heterojunction is obtained. Without co-catalysts adding, the obtained sample shows the efficient visible-light driven photocatalytic water oxidation performance, i.e. a 22.3 µmol/h oxygen evolution rate under the LED lamp of λ=420 nm illumination, 3, 16 and 30 times higher than reference Fe2O3/C3N4, bare α-Fe2O3 and g-C3N4, respectively. The key parameters for the enhanced photocatalytic activity can be attributable to the carbon layer and the tight contact structure, which can work as the carriers (electrons from g-C3N4 and holes from α-Fe2O3) collection center and provide the small migration resistance. Moreover, the carbon shows the different migration rate for electrons and holes and then facilitates the separation of carriers to some extent. To our knowledge, no other papers on Fe2O3/C-C3N4 based photocatalytic water oxidation are reported. This work can provide a new insight for synthesis of g-C3N4 based photocatalysts, also help us understand the water oxidation reaction.

Keywords: α-Fe2O3; g-C3N4; heterojunction; water oxidation; photocatalysis


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Introduction The water splitting to generate green hydrogen (H2) fuel through photocatalytic process using one semiconductor is thought as the promising strategy to solve the future or near global energy and environmental issues [1-14]. However, the solar-toH2 efficiency is still limited, the half reaction of oxygen (O2) generation of water splitting is difficult owing to requirement of four-hole of photogenerated carriers and decides the final H2 evolution efficiency. The photocatalytic O2 generation has attracted enormous attention and achieved some significant progresses in the past decades. To date, it remains a great challenge to study and improve the oxygen evolution through fabricating new/novel, high-performance, low-cost and stable sunlight photocatalytic system. A good O2 generation photocatalyst should own the below prominent properties [15-19]: 1) a narrow bandgap, which can provide the usage of sunlight energy as much as possible; 2) the fast carrier migration, especially the hole; 3) a large surface area, which can give abundant active and incident photon absorption sites; 4) a suitable valence band maximum (VBM) energy level, which should more positive than the oxidation potential of O2/H2O (Normal Hydrogen Electrode, NHE). It is worth mentioning that the desirable photocatalyst should own the following merits for satisfying the above mentioned requirements, i.e. a appropriate structure, such as metal-semiconductor and semiconductor-semiconductor heterojunction or homojunction, and the applicable semiconductors. Recently, on oxygen evolution reaction (OER), the most active catalysts are mainly based on precious-metal oxides, such as IrO2 and RuO2, which suffer from high cost, scarce and then limiting the widespread application [20-24]. Therefore, considerable effort has been devoted to designing and developing new noble metal-free materials such as carbon and transition-metal compounds [25-27]. Among them, graphitic carbon nitride (g-C3N4) has been developed and applied as an efficient OER-catalyst owing to its high content of pyridinic-like nitrogen (theoretically up to 60 wt.%), which can work as the active sites for oxygen generation [28-34]. However, g-C3N4 is not one metallic material but one n-type semiconductor with a bandgap of 2.7 eV [3540]. And thus, its electrocatalytic performance is limited. To this point, many developed ideas concerning the improvement of OER-activity of g-C3N4 have been reported, such as, introducing one conductive supporter carbon-fiber paper [41], titanium carbide nanosheets [42], carbon nanotube [43], crystalline carbon [44] and



graphene [45]. More importantly, Zhang and co-workers reported that Co-based samples could assist in generating O2 from water splitting of g-C3N4 [46-47]. It is acceptable to assume that carbon-based materials can improve the OER performance of g-C3N4 sample. And in this case, it is essential to construct a tight and suitable structure for the fast interface migration of carriers with low resistance [42]. The synthetic method is crucial for the material structure and the final catalytic performance and the in situ carbon loading strategy has been confirmed to be attractive and helpful [41, 42]. However, if the precursors of carbon and g-C3N4 are simultaneous treated, the morphology, structure and phase are not easily to control owing to the complicated interactions of precursors. Therefore, a simple method is still challenging and needed for g-C3N4 loading carbon-based samples. It should be mentioned that g-C3N4 belongs to the carbon-family materials and the carbonization can be occurred under a certain condition. This effect is still not reported to synthesize g-C3N4-carbon compound. On photocatalysis, hematite (α-Fe2O3) is an n-type semiconductor with a narrow bandgap (2.2 eV), and has been widely used in the visible-light-driven photocatalyst due to its low cost, thermodynamical stability and abundant feature [48-53]. Like other semiconductors, α-Fe2O3 also suffers the weaknesses of short lifetime of carriers and short hole migration distance [48, 51]. And the conjugation of coupling with other samples has been proved to be one efficient method for solving the above weaknesses [48]. To date, many α-Fe2O3-based heterojunctions have been studied and established, even with g-C3N4 [48, 50]. However, the synthesis strategies are always based on the impregnation process, and thus the tight combination between different components is limited owing to the relative weak interaction. In our present work, we developed one one-step effective, easy-reproduce and simple strategy for the conjugation of tight Fe2O3/C3N4 samples via the solid-state reaction under the nitrogen condition. Layered FeOOH and g-C3N4 were grinded thoroughly, and then treated via calcination at 580 °C. Under the dehydration affect, the FeOOH changeed into α-Fe2O3 and during these processes, the water vapour destroyed the Van der Waals' force between layered g-C3N4, then induced the exfoliation and a certain edge carbonization. The few layered and carbonized g-C3N4 with loading α-Fe2O3 showed the enhanced visiblelight-driven water oxidation performance without any usage of co-catalysts.


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Results and discussion Sample synthesis and characterization

Scheme 1. Schematic illustration of the preparation process of Fe2O3/C-C3N4.

At the initial stage, the layered FeOOH was synthesized according to literature with a little modify [54]. XRD pattern confirms the pure FeOOH phase and the TEM image shows the nanosheet structure (Figure S1). g-C3N4 was obtained from the thermolysis of urea at 550 °C. A certain proportion of FeOOH and g-C3N4 was mixed and grinded thoroughly (Scheme 1a), then the mixture was moved into one vacuum anneal furnace and heat treated under the N2 gas at the temperature of 580 °C (Scheme 1b). Please note that the heating rate was set to be a little high (50 °C /min) with a slow N2 flow for ensuring the enough interaction between FeOOH and g-C3N4. The water vapor from the thermal decomposition can enter into the layered g-C3N4 and then destroy the Van der Waals' force. The layered g-C3N4 was exfoliated and some edge sites were carbonized. And finally, the dissociated Fe2O3 was loaded on the surface thin carbonized g-C3N4 as shown in Scheme 1c, (referred to Fe2O3/C-C3N4). Also, during the rapid interaction, the relative tight coupling between Fe2O3 and C-C3N4 can be expected. A reference sample using g-C3N4 and Fe2O3 was also synthesized via above processes and named as Fe2O3/C3N4-r.



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Figure 1. XRD pattern of the Fe2O3/C-C3N4 and reference Fe2O3/C3N4-r.

The phase structure of the studied samples was first tested by X-ray diffraction (XRD) as shown in Figure 1. Clearly, the Fe2O3/C-C3N4 sample gives the relative low XRD signal intensity compared to Fe2O3/C3N4-r, suggesting the strong interaction between FeOOH and g-C3N4. Furthermore, the typical XRD peaks of g-C3N4 and α-Fe2O3 can be easily detected without any impurities. In the case of g-C3N4, two peaks loaded at 2 theta of 13° and 27.4°, which are assigned to (100) and (200) [54-55], respectively, appear for Fe2O3/C3N4-r sample, but the 13.3° peak disappears for Fe2O3/C-C3N4 sample, confirming the above hypothesis of the strong interaction during the synthesis process. Also, a wide peak centered at 2 theta of 21° generates for the sample of Fe2O3/C-C3N4, and this peak can belong to amorphous carbon [56-57], suggesting the carbonation. To get more information about the synthetic process, the conditions i.e. molar ratio and calcination temperature were changed to do the reference comparative samples as given in Figure S2. When the molar ratios were set to be 1:0.1 and 0.2, no and more carbonation happened (Figure S2a), respectively, further confirming the importance of FeOOH for obtaining carbonized g-C3N4. When changing the temperature to 500 or 600 °C, no α-Fe2O3 pure phase was obtained (Figure S2b) although the carbonation happened. The Raman spectrum was further carried out for confirming the carbon as shown in Figure S3, two distinct D and G bands located respectively at 1349 and 1579 cm−1 were found for Fe2O3/C-C3N4 sample, corresponding to amorphous and graphitic carbon layers [44]. No Raman signals can


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be found for the reference Fe2O3/C3N4 sample, further confirming that the importance of FeOOH precursor for the parts of carbonation of g-C3N4.

Figure 2. (a) SEM image, (b) TEM and (c) HRTEM images of Fe2O3/C-C3N4 sample.

Electron microscopy analysis was carried out to check the morphology and microstructure information. As shown in Figure 2a, the scanning electron microscopy (SEM) image gives the layered structure of the Fe2O3/C-C3N4 sample. Compared to the fresh thick g-C3N4 (Figure S4), also the FeOOH (Figure S1), the studied sample owns a relative thin two-dimensional (2-D) layered structure. Figure 2b shows the transmission electron microscope (TEM) result, clearly suggesting that the layered gC3N4 loads some amount of 2-D α-Fe2O3. Also we can detect the amorphous parts in the edge region and Figure S5 gives the corresponding typical electron diffraction result. Figure 2c gives the corresponding high-resolution TEM (HRTEM) with the measured interplanar crystal spacing of 0.27 nm, which can be assigned to (104) plane of α-Fe2O3. To get more information about the structure, detailed HRTEM analysis was further tested as shown in Figure S6. Based on the different lattice structures between α-Fe2O3 and g-C3N4, the two parts can be detected and pointed out. And the edge parts can be assigned to carbon. Figure S7 shows the TEM image of the reference sample of Fe2O3/C3N4, no obvious carbon layer can be detected. Element distribution of the sample was next carried out via the high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) and the corresponding energy dispersive spectxrmleter (EDS) elemental mapping. According to the results in Figure 3, element C distributes all the observed part, and the Fe and N elements only appears in some small parts, suggesting that the α-Fe2O3 was loaded



on the surface of g-C3N4. Meanwhile, compared to the C and N mapping, the C element gives more areas than N, further confirmed that the carbon has been generated during the synthesis process. The relative element mass ratio was further analyzed based on the HAADF-STEM as given in Figure S8. The atomic mass ratio of N to Fe was measured to be ca. 2:1, which is consistent with the theoretical value. In the case of C element, more proportion can be detected compared to nitrogen, suggesting the existence of amorphous carbon. We also checked the TEM images of reference samples (Figure S2). As shown in Figure S9, clearly, more FeOOH can induce the formation of amorphous carbon (in the case of molar ratios of 1:1.5 and 1:2.5 of g-C3N4 to FeOOH). And the calcination temperature can induce the pyrolysis of layered FeOOH into small α-Fe2O3.

Figure 3. HAADF-STEM image and the correspond EDS elemental mapping of Fe2O3/C-C3N4 sample.

Fourier transform infrared spectra (FTIR) measurement was performed to show more information about the structure of samples. As shown in Figure S10, Fe2O3/C-C3N4 and the reference Fe2O3/C3N4 give the similar signals. Typically, they all give the peaks located at 813 cm-1 , 1000–1760 cm-1 and 2900–3400 cm-1, which can be attributed to condensed CN heterocycles, stretching and bending modes of the nitrogen containing chemical bond, and stretching modes of uncondensed terminal


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amino groups (–NH2 or –NH groups) respectively, and these results are consistent with the previous reports [54, 55]. No obvious IR vibration ascribed to Fe-O bond at 516 cm-1 [50] can be detected, suggesting the low loaded content of α-Fe2O3. Furthermore, no obvious change of FTIR spectrum of g-C3N4 and carbon layer can be detected, suggesting that the carbonization happened at the surface sites of g-C3N4.To investigate the optical absorption, interfacial interaction and carrier seperation of the studied samples, UV–vis diffuse reflectance spectra (DRS) and room-temperature photoluminescence (PL) emission spectrum were carried out and Figure S11 showed the results. It is clear that two absorbance edges can be found for the Fe2O3/C-C3N4 and Fe2O3/C3N4 samples. One is at ca. 450 nm, corresponding to a bandgap of 2.76 eV, which is slightly smaller than the bulk g-C3N4 [54-55], confirming the exfoliated structure [54]. The other locates at 560 nm, resulting from its intrinsic narrow bandgap of ca. 2.2 eV for α-Fe2O3 [48]. Furthermore, the Fe2O3/C-C3N4 sample shows the relative strong absorbance compared to Fe2O3/C3N4 sample (Figure S11a), suggesting that the fast carrier migration across the interface between α-Fe2O3 and gC3N4 occurs in the Fe2O3/C-C3N4 sample [58-59]. Figure S11b shows the roomtemperature PL results. The conditions of pure α-Fe2O3 and g-C3N4 were also tested to give the clear PL information as shown in Figure S12. All PL measurements were carried out under the 365-nm excitation. Clearly, the PL quenching occurs in the range of from 420 to 600 nm (Figure S11b) and in the case of Fe2O3/C3N4, the main PL peak locates at ca. 440 nm can be found. Meanwhile, there is one center peak located at ca. 460 nm for the sample of Fe2O3/C-C3N4. However, the main PL peaks of bare α-Fe2O3 and g-C3N4 locate at ca. 480 and 440 nm (Figure S12), respectively. Therefore, the PL emission of reference Fe2O3/C3N4 mainly originates from g-C3N4, that is, the emission of loaded α-Fe2O3 is weak and overlaid by the strong signals of gC3N4. Markedly different from reference Fe2O3/C3N4 result, there is a clear emission shift for Fe2O3/C-C3N4. The tight heterojunction structure can push the photogenerated carriers fast migration and then change the recombination pathways. Moreover, the relative PL intensity of Fe2O3/C-C3N4 is lower than that of the reference Fe2O3/C3N4, confirming the weak carrier recombination and further suggesting the strong interaction of Fe2O3/C-C3N4 heterojunction.



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Figure 4. The high-resolution XPS spectrum of Fe2O3/C-C3N4 and reference Fe2O3/C3N4 samples under study: (a) C 1s, (b) N 1s and (c) Fe 2p.

The X-ray photoelectron spectroscopy (XPS) measurement was conducted to further examine the surface and sub-surface states of the studied samples. As shown in Figure 4a, high-resolution XPS spectrum of C 1s displays two main peaks at ca. 284.8 and 287.8 eV, which can be assigned to the surface adsorbed carbon and sp2hybridized carbon in the aromatic ring attached to the –NH2 group [54-55]. However, for the main XPS peak located at 287.8 eV, some slight shifts to low binding energy for the sample of Fe2O3/C-C3N4 compared to Fe2O3/C3N4 can be detected, suggesting that some adventitious elements disturbs the original binding affect of skeleton carbon. No obvious amorphous carbon signals can be found. For the XPS results of N 1s in Figure 4b, the signals can be deconvoluted into three peaks, i.e. one main peak is centered at 398.0 eV, which can be attributable to sp2 N involved in triazine rings [54], whereas one peak at ca. 399.4 eV attributable to bridging nitrogen atoms N–(C)3 [60]. The last peak at 404 eV originates from charging effects in heterocycles [50, 60]. Similarly and interestingly, some small shift to higher energy of Fe2O3/C-C3N4 in comparison to reference sample for the 399.5 eV peak can be detected, further suggesting the slight change of the structural unit. The Fe 2p spectra in Figure 4c can be deconvoluted into two peaks at 710.6 and 724.3 eV correspond to Fe3+ 2p3/2 and 2p 1/2,

respectively. The binding energies were also observed to shift to higher binding

energies of Fe2O3/C-C3N4 sample as compared to those in reference Fe2O3/C3N4, this is indicative of the existence of strong interaction between Fe2O3 and C3N4 in


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Fe2O3/C-C3N4 sample. Figure S13a shows the O 1s XPS spectrum with one main peak at 531.2 eV, which is corresponded to lattice oxygen. No obvious shift can be detected. To get more information about the calcination process, the Fe 2p XPS spectrum of samples under five temperatures were tested as shown in Figure S13b. Under the low temperature of 200 °C, two typical Fe 2p signals of FeOOH can be found [53], and with the temperature increasing, the Fe 2p peaks shift to low binding energy. Under the dual function of iron oxide formation and exfoliation of g-C3N4, the strong interaction with a certain of C 1s, N1s and Fe 2p XPS signals shift can be expected. The XPS shift results suggest that the tight structure of Fe2O3/C-C3N4 sample and then the fast carriers migration across the interface of Fe2O3 and g-C3N4 can be expected. The atom ratios of the studied Fe2O3/C-C3N4 are calculated based on the integrated peak areas of wide XPS result as given in Table S1, in which the values consistent with the above EDS result.

Photocatalytic water splitting performance As discussed above, such a new Fe2O3/C-C3N4 sample with the strong interaction and parts of edge carbon, excellent optical absorption, outstanding carriers separation and migration across the interface should own the attractive visible-light driven photocatalytic activity. Recently, g-C3N4 with some conductive crystalline carbon has been confirmed to show a remarkable electrocatalytic performance toward water oxidation to oxygen evolution [44]. The synthesized studied Fe2O3/C-C3N4 sample own the suitable conduction and valence band positions for establishing a Z-Scheme heterojunction structure [48, 50], which can be used for obtaining considerable photocatalytic water splitting activity. On photocatalytic water oxidation, it is a multistep reaction that needs four electrons or holes to finish the whole reaction.


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Figure 5. Oxygen evolution performance of the studied samples, (a) λ=420 nm and (b) λ=450, 470 and 500 nm. Condition: 100 mL 0.01 M AgNO3 solution, 0.1g catalyst.

To test the photocatalytic performance of the Fe2O3/C-C3N4 sample, AgNO3 solution (0.01 M) and a LED-lamp (λ=420 nm) were first used as the electron scavenger and visible light for the water oxidation to oxygen evolution. The bare gC3N4 and α-Fe2O3 were also tested working as the reference samples. As shown in Figure 5a, all the samples show the photocatalytic water oxidation performance with visible difference of the oxygen evolution. Typically, bare g-C3N4 and α-Fe2O3 give the relative low activity with the reaction rate of ca. 0.7 and 1.4 µmol/h, respectively. For the heterojunction samples of Fe2O3/C3N4 and Fe2O3/C-C3N4, the enhanced oxygen generation values are obtained, i.e. 7.3 and 22.3 µmol/h, respectively, ca. 10 and 30 times higher than g-C3N4, suggesting that the heterojunction structure can promote the photogenerated carriers separation, migration and then reaction. Moreover, the Fe2O3/C-C3N4 sample shows ca. 3 times higher than the reference Fe2O3/C3N4, which owns the similar heterojunction, confirming the crucial affect of the strong interaction and amorphous carbon for further inducing electron-hole pairs and participating in the reactions. Also, the activity of the sample of Fe2O3/C-C3N4 under the λ>420 nm condition is higher than Co-based co-catalysts of g-C3N4 [39-40]. To get the wide application of the studied samples, three other LED-lamps i.e. λ=450, 470 and 500 nm were used and Figure 5b shows the corresponding results. Clearly, the samples of Fe2O3/C-C3N4 and Fe2O3/C3N4 show the decreasing activity from 450 to 500 nm, confirming to the light absorption in Figure S11a. Typically, they give the


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activity of 7 and 1.7 µmol/h, 4.3 and 1.1 µmol/h, 2.6 and 0.9 µmol/h for 450, 470 and 500 nm conditions, respectively. It should be mentioned that all the activities of Fe2O3/C-C3N4 are 3~4 times higher than the reference Fe2O3/C3N4, further confirming the beneficial effects of the amorphous carbon. The stability of one catalyst is fundamental need for the practical application. The Fe2O3/C-C3N4 was tested under the 12-h measurement. According to Figure S14a, no activity change can be detected after the four cycles with the suggestion of the stability of Fe2O3/C-C3N4 sample. Figure S15 shows the corresponding XPS results of the studied elements, no obvious change can be found, further suggesting the stability of the Fe2O3/C-C3N4. Like AgNO3 solution, Na2S2O3 and FeCl3 are also always employed in the photocatalytic water oxidation reaction. Figure S14b gives the corresponding test results. AgNO3 solution shows the highest oxygen generation followed by Na2S2O3 and FeCl3. According to above discussion, the overall water splitting can be expected. We next tried that overall water splitting performance of the sample of Fe2O3/C-C3N4. As shown in Figure S16, the hydrogen and oxygen with the ratio of ca. 2:1 are released from the water splitting steadily. Although the activity is not so high compared to literature [61], the sample was tried without using co-catalysts, especially noblemetals. To our best knowledge, it is a remarkable first observation that photocatalytic overall water splitting based on Fe2O3/C3N4 sample using any co-catalysts.

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Figure 6. Electrochemical performance measurements. (a) Polarization curves of the Fe2O3/C-C3N4 and Fe2O3/C3N4 sample. (b) Nyquist plots of the Fe2O3/C-C3N4 and



Fe2O3/C3N4 sample. Z′ is real impedance and Z″ is imaginary impedance. (c) Photocurrent response of the studied samples under the illumination of LED-420 nm. Note: All the measurements were performed in 0.5M Na2SO4 solution. On the photocatalytic water splitting based on one semiconductor, three steps are needed i.e. photogenerated carriers generation, migration from bulk to surface and pass across the solid/solution interface. And the last step is crucial for the finial efficiency, owing to the relation with the overpotential of water splitting and carrier migration resistance. Based on these considerations and to get more information about the mechanism of photocatalytic water oxidation, electrochemical measurements were carried out. The sample was first deposited onto a glass carbon electrode (GCE). Figure 6a shows the polarization curves of the samples of Fe2O3/C-C3N4 and Fe2O3/C3N4 under the potential of ranging from 0.4 to 1.6 V vs Ag/AgCl. For the sample of reference Fe2O3/C3N4, the electrocatalytic water oxidation occurs near 0.9V (overpotential) and then increase moderately. Meanwhile, the Fe2O3/C-C3N4 sample almost starts at the potential of 0.4V with a enhanced current density value compared to reference sample. The overpotential of water oxidation decrease suggests that the carriers can pass the interface of catalyst/solution easily. For Fe2O3/C-C3N4 sample, the existence of some amount of edge carbon can facilitate the interface migration [44]. And then the migration resistance of the carriers should be studied. As shown in Figure 6b, the two samples show the typical circular Nyquist plots of the Fe2O3/CC3N4 and Fe2O3/C3N4 sample. In particular, the Fe2O3/C-C3N4 gives a smaller circular radius than Fe2O3/C3N4, suggesting a smaller value of the total average resistance, also confirming the polarization results. The photocurrent response, which suggests the carrier separation was also carried out after polarization and nyquist measurements using the same prepared electrodes under the illumination of LED-420 nm as shown in Figure 6c. Both samples give the timely response without delay, suggesting the fast photoelectric response ability of the Fe2O3/C3N4 heterojunction [44, 48]. However, the sample of Fe2O3/C-C3N4 shows a higher photocurrent compared to Fe2O3/C3N4, confirming the low overpotential of water oxidation and small carrier migration resistance. Also, the low carrier recombination can give the high photocurrent response.


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1.0

Normalized intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


α-Fe2O3 Fe2O3/C3N4-r

0.8

1% IrOx/Fe2O3 1.5% IrOx/Fe2O3

g-C3N4

0.6

Fe2O3/C-C3N4

0.4

α-Fe2O3 0.2

0.0 0

5

10

15

20

Time (ns) Figure 7. The time-resolved PL spectra of Fe2O3/C-C3N4 and Fe2O3/C3N4 samples under study, the excitation and emission wavelengths were 365 nm and 520 nm, respectively. IrOx/Fe2O3 samples were also used as the reference samples. The pure compact disc-formed samples in infrared method were chosen for this PL measurement.

To investigate the carrier separation of the samples, the time-resolved PL spectra were carried out as shown in Figure 7. Bare α-Fe2O3, 1% and 1.5% IrOx/Fe2O3 were also used as the reference samples. Clearly, bare α-Fe2O3 shows the longest fluorescence quenching, followed by Fe2O3/C3N4-r, 1% IrOx/Fe2O3, Fe2O3/C-C3N4 and 1.5% IrOx/Fe2O3. The corresponding fitted fluorescence lifetime based on the

following equation: y = A e τ + A e τ + y ,

(1)

can be found in Table S2. The average lifetimes are 24.5, 19.9, 14.1, 8.55 and 10.1 nm for bare α-Fe2O3, Fe2O3/C3N4-r, 1% IrOx/Fe2O3, 1.5% IrOx/Fe2O3 and Fe2O3/CC3N4, respectively. For the samples of bare α-Fe2O3 and IrOx loaded α-Fe2O3, the reduced lifetimes result from the excellent transfer ability of IrOx for photogenerated



holes. And in the case of Fe2O3/C3N4-r, a smaller fluorescence lifetime compared to bare α-Fe2O3 also appears though slight higher than that of 1% IrOx/Fe2O3, suggesting that the heterojunction structure can promote the carriers separation as shown from the model in Figure 7 (inset). For Fe2O3/C-C3N4 sample, a interesting and important phenomenon can be found. Its fitted fluorescence lifetime shows smaller than 1% IrOx/Fe2O3, just slight higher than 1.5% IrOx/Fe2O3, direct and further suggesting that both of amorphous carbon and heterojunction play positive effect on carrier separation and migration.

Figure 8. (a) Proposed carrier separation and migration pathways of Fe2O3/C-C3N4 sample. (b) The corresponding Z-scheme of the Fe2O3/C-C3N4 with the relative valence and conduction bands positions.

Based on above analysis and discussion, the photogenerated carriers separation and migration of the Fe2O3/C-C3N4 and the corresponding Z-Scheme mode [62-64] can be proposed as shown in Figure 8. Under the visible-light (λ>420 nm) illumination (Figure 8a), both the semiconductors of α-Fe2O3 and g-C3N4 are excited to generate electron-hole pairs, which are then separated and moved to different directions under the action force of the work function difference. In detail, the electrons of g-C3N4 will move to the conduction band of α-Fe2O3, meanwhile, the holes of α-Fe2O3 will transfer to the valance band of g-C3N4. However, under the acting force of formed ZScheme as shown in Figure 8 (b), the electrons of α-Fe2O3 and the holes of g-C3N4 will move to each other and then finish the result of recombination. In the case of existence of the amorphous carbon, the transformation of the electrons or holes will first go through the carbon layer easily owing the fast migration rate of carbon. And during this step, the recombination can be occur in parts of electrons and holes and


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importantly, the electron and hole own the different migration rate on the surface of carbon, which can also promote the separation and movement to different sides. Then, the samples of α-Fe2O3 and g-C3N4 will work as the oxidation and reduction sites for the photocatalytic reaction, respectively. Such a sample of Fe2O3/C-C3N4 with the tight heterojunction structure, amorphous carbon layer and the two-dimensional structure, should have the excellent or outstanding carrier separation capability, and then the corresponding photocatalytic performance can be expected, also the relative challenging water oxidation reaction can be achieved. Conclusion In summary, one novel layered Fe2O3/C-C3N4 heterojunction sample was synthesized via one simple, reliable and reproducible method of solid-state reaction taking use of layered glucose coated FeOOH and g-C3N4 as the precursors for the visible-light driven photocatalytic water oxidation reaction. Under the effects of dehydration of layered FeOOH, the water vapor induced the thermal exfoliation of the layered g-C3N4 and parts of edge carbonization. To our knowledge, no other report on Fe2O3/C3N4 based heterojunction samples with the amorphous carbon connecting them currently exists. XRD and Raman measurements confirmed the existence of the carbon. The TEM and the corresponding mapping results proved the edge carbonization phenomenon. XPS suggested the tight heterojunction structure. The Fe2O3/C-C3N4 sample showed a 22.3 µmol/h oxygen evolution rate without using any co-catalyst under the visible (λ=420 nm) illumination, 3, 16 and 30 times higher than reference Fe2O3/C3N4, bare α-Fe2O3 and g-C3N4, respectively. Electrochemical measurements such as polarization curves, EIS and photocurrent response suggested that the synthesized Fe2O3/C-C3N4 sample owns the relative low overpotential, low carrier migration resistance of water oxidation, and also fast photo-to-current response. And the time-resolved PL spectrum confirmed the thin layered heterojunction structure owns the similar photogenerated carriered separation ability to IrOx-Fe2O3 samples. This work can provide new knowledge and development of g-C3N4-based catalysts, also help us design new heterojunction with the considerable and efficient carriers separation ability.

Experimental section



Synthesis of Fe2O3/C-C3N4 and Fe2O3/C3N4: Pristine g-C3N4 was obtained by urea pyrolysis under N2 atmosphere. 0.5 g of urea was calcined at 550 °C in a vacuum tube furnace for 2 h using a heating rate of 20 °C min−1. Layered FeOOH was synthesized according to the previous report [53] with some modification. An aqueous solution of iron trichloride (FeCl3) was mixed with sodium nitrate (NaNO3) under the molar ratio of 2:1. The mixture was stirred for about 30 min, and 0.09 M hydrochloric acid (HCl) was added. After stirring for 30 min, 1.5g oxalic acid was added into the above mixture. And then the mixture was placed in an oven at 120 ˚C for about 2 h. After its natural cooling, the product was treated through centrifugation and washing by water and ethanol three times. The obtained layered FeOOH was mixed into the g-C3N4 and after the mixture was grinded thoroughly, they was transferred into the vacuum for the calcination at 580 °C 4h under the N2 atmosphere. For the synthesis of reference Fe2O3/C3N4, the equal mole to FeOOH of α-Fe2O3 was used and treated via the same process of Fe2O3/C-C3N4 sample.

Characterization of samples: Transmission electron microscopy (TEM) test was performed on a FEI Tecnai G2 F20 electron microscope at an acceleration voltage of 200 kV. Diffuse reflectance ultraviolet-visible (UV-Vis) spectra of studied samples were recorded in the air against BaSO4 in the region of 200-800 nm on a Perkin– Elmer Lambda 950 spectrophotometer. X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku Smartlab-9kW instrument using Cu Kα X-ray (λ = 1.54186 Å) radiation at a scanning rate of 4 o/min in the region of 2θ = 10-80o. Xray photoelectron spectra (XPS) were acquired on a Kratos Axis Ultra DLD spectrometer with Al Kα (hυ = 1486.6 eV) as the excitation source. Raman analysis was carried out on a Renishaw InVia Raman spectrometer with the green line of an Ar-ion laser (514.53 nm) in the micro-Raman confguration. Room-temperature photoluminescence (PL) and time-resolved fluorescence spectroscopy were recorded on a PicoQuant Fluo Time 300 fluorescence spectrophotometer. Fourier transform infrared reflectance (FTIR) was carried out on a Bruker V70 spectrometer. The photoelectrochemical and electrocatalytic water splitting performances were carried out in a conventional three-electrode cell on a Zennium Zahner electrochemical workstation. Ag/AgCl (saturated KCl) and Pt were used as the reference and counter electrode, respectively. For the preparation of electrode, 4mg catalyst was added into the 1 mL water/ethanol (v/v: 3:1) solution, and then 80 µL Nafion was added into


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above solution. The mixture was treated through ultrasound 1h to obtain one wellmixed ink, which was dripped onto the glassy carbon electrode.

Photocatalytic oxygen evolution: Photocatalytic water splitting to generate oxygen was performed in a home-made side-irradiation-type Pyrex reaction cell connected to a closed gas circulation and evacuation system. In a typical experiment, catalyst sample of 100 mg was suspended in 100 mL Na2S2O3, AgNO3 or FeCl3 aqueous solution in the reaction cell. After evacuated for 30 min, the reactor cell was irradiated by the LED lamps under stirring. The gaseous products were analyzed by an on-line gas chromatograph (Varian CP-3800) with thermal conductivity detector.

Supporting Information XRD pattern andTEM image of the FeOOH nanosheet; XRD patterns of the reference samples synthesized under different conditions; Raman spectrum of studied samples; SEM image of fresh g-C3N4; SEM image of fresh g-C3N4; HRTEM image of Fe2O3/C-C3N4 sample; TEM image of the reference Fe2O3/ C3N4 sample; EDS result; TEM images of reference samples; FTIR spectrum; UV-Vis diffuse reflectance spectra; room-temperature PL spectrum; Table S1 and S2; Cycle photocatalytic activity; XPS results of the sample after the cycle reaction; and overall water splitting performance.

Acknowledgements We thank the support from National Key Research Program of China (2017YFA0204800, 2016YFA0202403), Natural Science Foundation of China (No. 21603136, 61674098), the National Science Basic Research Plan in Shaanxi Province of China (2017JM2007), the Changjiang Scholar and Innovative Research Team (IRT_14R33). The 111 Project (B14041), the Fundamental Research Funds for the Central Universities (GK201602007, 2018CSLZ011), and the Chinese National 1000Talent-Plan program are also acknowledged.



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Graphical abstract

Visible-light water oxidation photocatalyst of Fe2O3/C-C3N4 heterojunction is reported for assisting the development of a sustainable H2 generation reaction.


driven photocatalytic water oxidation

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