Co-Pi

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Strongly coupled Metal oxide/reassembled carbon nitride/Co-Pi heterostructures for efficient photoelectrochemical water splitting Xiaoqiang An, Chengzhi Hu, Huachun Lan, Huijuan Liu, and Jiuhui Qu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01070 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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ACS Applied Materials & Interfaces

Strongly Coupled Metal oxide/Reassembled Carbon nitride/Co-Pi

Heterostructures

for

Efficient

Photoelectrochemical Water Splitting Xiaoqiang An†, Chengzhi Hu†,‡,*, Huachun Lan※∮, Huijuan Liu#,‡, and Jiuhui Qu†,‡

†

Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-

Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China. ※

School of Environment, State Key Joint Laboratory of Environment Simulation and Pollution

Control, Tsinghua University, Beijing, 100084, China. ∮

Research Center for Water Purification and Water Ecological Restoration, Tsinghua University,

Beijing, 100084, China. #

State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-

Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

KEYWORDS: Carbon nitride, photoelectrochemical, water splitting, metal oxides, photoanodes.

ACS Paragon Plus Environment

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ABSTRACT: The photoelectrochemical application of carbon nitride is extremely exciting, due to the meta-free components, low cost, nontoxicity and appropriate band positions. To construct carbon nitride-based heterostructures, a conventional ultrasonic exfoliation method is usually used to fabricate dispersion of ultrathin nanosheets. However, the outstretched structure and the poor dispersity inevitably results in the poor interfacial contact between different materials. To solve this problem, hydrolyzed carbon nitride suspension was used as homogenous precursor for the fabrication of composite photoanodes. The in-situ reassembly of 1-D nanofibers resulted in the formation of uniform and ultrathin carbon nitride nanoarchitectures on the surface of Fe2O3 nanorod arrays. Due to the strongly coupled interfaces and the deposition of Co-Pi water oxidation cocatalysts, as-synthesized heterostructured photoanodes exhibited three-fold increased photocurrent density and good stability, compared to pristine Fe2O3. The significantly improved photoactivity of Fe2O3/reassembled carbon nitride/Co-Pi heterostructures was ascribed to the decreased interfacial conductivity and facilitated charge separation. This material designing strategy was further used to construct TiO2/carbon nitride, ZnO/carbon nitride and WO3/carbon nitride heterostructures. The incorporation of hydrolyzed carbon nitride could remarkably enhance the PEC performance of these metal oxide photoanodes. Thus, this work provides a new paradigm for designing carbon nitride-based composite nanostructures for efficient and stable solar fuel production.

1. Introduction

Photoelectrochemical (PEC) water splitting has attracted much attention due to its environmental sustainability and cost effectiveness. Despite the significant recent progress in this research, the most challenging task is still to develop efficient and stable photoanodes that can oxidize water


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into oxygen.1 Among various photoanodes, Fe2O3, with a band gap of 2.2 eV and theoretical solar-to-hydrogen efficiency of 16.8%, is one of the most promising materials. However, charge recombination caused by short carrier diffusion length and slow interfacial kinetics seriously limits the practical applications of hematite.2,3 Great efforts have been devoted to reduce recombination losses, while constructing heterojunctions has been reported to be an effective means of addressing the above limitations.

Configuration of hematite nanoarray integrates the advantages of enhanced light-harvesting, improved charge separation, improved transport of photoexcited charge carriers, and increased surface area. So far, several works have been focused on the coupling of Fe2O3 nanorod arrays with various inorganic oxides, such as TiO2, ZrO2 and BiV1−xMoxO4.4-7 Due to the metal-free components, low-cost and well-matched bandgap, it is of particular interest to construct Fe2O3/carbon nitride heterojunctions.8,9 However, the outstretched structure and poor dispersity of bulk carbon nitride in water inevitably results in the poor interfacial contact between 1-D nanorods and 2-D nanosheets.10,11 New material strategy is highly desirable for the construction of strongly coupled Fe2O3/carbon nitride heterostructures with intimate interfacial contact. In principle, strongly coupled heterojunctions not only increase the accessible area for interfacial charge transfer, but also shorten the charge transport distance for directional migration of photogenerated carriers.12 For the typical solution route, homogenous carbon nitride dispersion is a prerequisite for the construction of strongly coupled Fe2O3/carbon nitride heterojunctions. Although dispersed carbon nitride with decreased thickness can be synthesized by ultrasonic exfoliation, the size of nanosheets is usually not uniform.13,14 Moreover, the folded and curled edges of 2-D nanosheets might result in the poor interfacial contact between Fe2O3 and carbon nitride.15 In this regard,


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hydrolyzed carbon nitride nanostructures should be a better choice for the heterojunction construction. If bulk carbon nitride nanosheets can be tailored into small and uniform assemblies and reassembled onto Fe2O3 nanostructures, strongly coupling-induced enhancement of PEC performance can be expected. To the best of our knowledge, few studies have focused on building composite photoanodes using reassembled carbon nitride nanoarchitectures. Recently, surface modification with electrocatalysts has been widely used to enhance PEC performance of semiconductor photoanode. This strategy can increase water oxidation efficiency by both increasing the photocurrent density and decreasing the photocurrent onset potential.16,17 Several types of electrocatalysts have been combined with Fe2O3, such as IrO2, Pt, NiOOH, FeOOH and cobalt phosphate (Co−Pi).18,19 Among them, Co−Pi has specifically attracted tremendous attention because its earth-abundant, efficient water oxidation and good stability due to the “self-healing” mechanism.20 For example, Hamann et al. found that Co−Pi catalysts can efficiently collect and store photogenerated holes from the hematite electrode, which contributed to the reduced surface state recombination and increased water oxidation efficiency.21 Therefore, it is anticipated that PEC performance of Fe2O3/reassembled carbon nitride could be further improved through Co-Pi deposition. Herein, hydrolyzed carbon nitride suspension was used as homogenous precursor for the fabrication of composite photoanodes. The in-situ reassembly of 1-D nanofibers resulted in the formation of ultrathin carbon nitride nanoarchitectures on the surface of Fe2O3 nanorod arrays, with strongly coupled interfaces. To improve water oxidation kineics, Co-Pi cocatalysts was further deposited onto the surface of heterostructures. Compared to pristine Fe2O3, assynthesized Fe2O3/reassembled carbon nitride/Co-Pi heterostructures exhibit three-fold increased photocurrent density. According to the experimental characterizations, the significantly


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improved photoactivity was ascribed to the decreased interfacial conductivity and facilitated charge separation. The improved PEC performance of other metal oxide photoanodes (TiO2, ZnO and WO3) indicated the good applicability of this material strategy. 2. Experimental section 2.1 Fabrication of hydrolyzed carbon nitride Pure bulk carbon nitride was prepared using melamine as precursor. Typically, 2 g of melamine was put into a crucible covered loosely with a lid, which was heated at 550 ℃for 4 h in static air in a muffle furnace. The ramping rate was 5 ℃/min. After cooled naturally to room temperature, the resultant yellow solid was collected and ground into powder for further use. For hydrolyzing bulk carbon nitride in alkaline conditions, 500 mg of as-synthesized powder was mixed with 20 mL of NaOH solution (with the concentration of 3 M). The mixture was stirred at 60 °C for 12 h. Then, hydrolyzed carbon nitride was dialyzed in a 3500 Da dialysis bag against deionized water for several days until neutral. The concentration of hydrolyzed carbon nitride solution was determined to be 3 mg/mL. 2. 2 Fabrication of Fe2O3 photoanodes Hematite nanorods on FTO glass were prepared by a simple hydrothermal method as reported by Vayssieres et al.22 In a typical procedure, 20 mL aqueous solution containing 0.1 M FeCl3 and 1 M NaNO3 was put into a Teflon-lined stainless steel autoclave. The pH value was adjusted to 1.3 by HCl. A piece of cleaned FTO glass was placed in the autoclave with the conducting side facing down. After a hydrothermal reaction at 95 ℃ for 10 h, FeOOH film formed on the FTO


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substrate. The obtained film was annealing in air at 550 ℃ for 2 h to convert FeOOH into Fe2O3. The sample was further annealed at 800 ℃ for 15 min to improve the crystallinity.

2.3 Fabrication of Fe2O3/reassembled carbon nitride/Co-Pi photoanodes Carbon nitride was reassembled onto Fe2O3 nanorod arrays by vertically dipping the Fe2O3coated substrates into the hydrolyzed carbon nitride solution. The dipping procedures were performed at a dip and withdraw rate of 2.5 mm/min. After each dipping, the freshly prepared film was dried at room temperature for 10 minutes. These procedures were repeated until a desired thickness was obtained. Finally, Fe2O3/reassembled carbon nitride electrodes were rinsed with deionized water and dried at 60 °C for 1 hour. Co-Pi was deposited onto Fe2O3/reassembled carbon nitride photoanodes by photo-assisted electrodeposition under AM 1.5 simulated solar irradiation. 0.1 M potassium phosphate buffer solution containing 0.5 mM cobalt nitrate was used as electrolyte. The deposition was carried out at the current of 6 mA/cm2 at pH 8. The composite photoanode fabricated from reassembled carbon nitride was denoted as Fe2O3/R-CN/Co-Pi. For comparison, the liquid exfoliation method was also used to fabricate carbon nitride dispersion. Accordingly, the composite sample was denoted as Fe2O3/E-CN/Co-Pi. 2.4 Characterizations X-ray diffraction (XRD) measurement was carried out on X'Pert PRO MPD, with a voltage of 40 kV. The morphology of the samples was characterized by field emission scanning electron microscope (FE-SEM, SIGMA) and high-resolution transmission electron microscope (HR-TEM, JEOL-2010). The elemental composition was analyzed by energy dispersive X-ray spectrometer


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(EDS). X-ray Photoelectron Spectroscopy (XPS) was carried out using the XPS spectrometers (ESCALab220I-XL). The diffuse reflectance spectra were characterized by UV-vis-NIR spectrophotometer (Cary 5000). Steady-state photoluminescence (PL) were measured at room temperature using a fluorescence spectrometer (Hitachi, F-4500). 2.5 Photoelectrochemical measurements PEC measurements were performed using a three-electrode configuration. A 150 W Xe lamp equipped with AM 1.5G filter was used as light source, the light intensity was calibrated to 100 mW/cm2. The FTO glass coated with samples was used as the working electrode, with a size of 1 cm * 1 cm. Platinum plate and Ag/AgCl electrode were used as counter and reference electrode, respectively. 1 M NaOH aqueous solution (pH=13.0) was used as the electrolyte. Photocurrent was measured with linear-sweep voltammetry (Gamry electrochemical workstation, Interface 1000). IPCE characteristics were measured with a monochromator (Oriel Cornerstone 130 1/8m). Electrical impedance spectroscopy (EIS) data were collected using a 10 mV amplitude perturbation at frequencies between 0.001 Hz and 1 MHz, either under dark condition or light irradiation. Mott-Schottky analysis was carried out in the dark at the frequency of 1000 Hz. The donor concentration was calculated with the following equation:23,24 Nd= (2/eɛɛ0)[d(1/C2)/dV]-1 where e=1.60×10-19 C is the electron charge, ɛ=80 is the dielectric constant of hematite, ɛ0 =8.85×10-14 F•cm-1 is the vacuum permittivity, C is the capacitance of the space charge region, V is the electrode applied potential, and Nd is the donor concentration.25 3. Results and discussion


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The morphology of carbon nitride before and after hydrolysis reaction in alkaline solution was observed by TEM. Figure 1a shows that bulk carbon nitride are composed of aggregated morphology, with an irregular and lamellar structure. The wrinkled structure at the edges agrees well with g-C3N4 synthesized by the polymerization method. Comparatively, hydrolyzed sample turned into networked agglomerates, which are consisted of a great deal of ultrathin nanofibers. These nanofibers have a uniform structure, with the diameter of about 10 nm and length of several micrometers (Figure 1b and Figure S1a). The dispersion characteristics of different carbon nitride samples were evaluated. The highly transparent solution of hydrolyzed carbon nitride (Figure 1c) indicates that as-synthesized sample is ultrathin and highly dispersed. Furthermore, the suspension can keep stable even after several months (Figure 1d). In contrast, ultrasonic exfoliation only results in the formation of 2-D graphene-like ultrathin nanosheets (Figure S1b). When isopropanol was used as solvent, dispersed suspension is achieved (Figure 1e). However, the nonuniform size distribution leads to the formation of precipitates on the bottle of container after several weeks (Figure 1f). Although these nanosheets can be dispersed in water, to some extent, the dispersity is poor (Figure 1g). The suspension can keep stable for only several days (Figure 1h). All these results indicate that hydrolyzed carbon nitride provides homogeneous suspension for the fabrication of heterostructures.


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Figure 1. (a) TEM image of bulk carbon nitride; (b) TEM image of hydrolyzed carbon nitride; (c, d) Photograph of hydrolyzed carbon nitride suspension; (e, f) Photograph of exfoliated carbon nitride in isopropanol; (g, h) Photograph of exfoliated carbon nitride in water. The concentration of carbon nitride in (c)-(h) is about 1.5 mg/mL. The chemical composition and bonding nature of hydrolyzed carbon nitride was investigated by XPS. Two strong peaks at 284.7 eV and 288.4 are observed in the C 1s spectrum (Figure 2a), which can be ascribed to standard reference carbon (C-C) and N-C=N coordination in the framework of carbon nitride, respectively. The small peak at 286.2 eV is attributed to C-N bonds in the basic aromatic carbon nitride heterocycles.26 In Figure 2b, high resolution N 1s spectrum confirms the existence of sp2-hybridized nitrogen at 398.4 eV, the tertiary nitrogen at 399.4 eV and the amino functional groups at 400.9 eV, respectively.27,28 These results indicate that the morphology change of carbon nitride exhibits ignorable effect on the bond structures during the hydrolyzing procedures.


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Figure 2. High-resolution C 1s (a) and N 1s (b) spectra of hydrolyzed carbon nitride. Having achieved highly dispersed carbon nitride suspension, a dip-coating method was thereafter used to reassemble these nanofibers onto the surface of Fe2O3 photoanodes. The morphology of hematite film was firstly studied by SEM. Figure 3a proves that the FTO substrate is covered uniformly with dense and vertically aligned Fe2O3 nanorod arrays, with the average diameter of 120 nm. According to the cross-section view image in Figure 3b, the preferential growth direction of these nanorods is roughly perpendicular to the FTO substrate. The SEM image of Fe2O3/carbon nitride heterostructured film is presented in Figure 3c. Obviously, the surface of Fe2O3 nanorod arrays is uniformly covered by a flat and nearly transparent layer, indicating the reassembly of hydrolyzed carbon nitride nanofibers.29 According to the cross-section view in Figure S2a, the thickness of carbon nitride layer is only several nanometers, which is consistent with the ultrathin structure of nanofibers. Close observation proves the intimate interfacial contact between each nanorod and carbon nitride ultrathin layer. Due to the formation of small Co-Pi nanoparticles, the surface of Co-Pi cocatalyst loaded heterostructures becomes rougher (Figure 3d and Figure S2b). In contrast, the deposition of


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exfoliated carbon nitride only results in the formation of aggregated and disordered nanostructures on the surface of Fe2O3 film (Figure S3), with poor interfacial contact.

Figure 3. (a) Top-view SEM image of Fe2O3 photoanodes; (b) Cross-section view of Fe2O3 photoanodes; SEM images of Fe2O3/reassembled carbon nitride (c) and Fe2O3/reassembled carbon nitride/Co-Pi heterostructured (d) photoanodes. Figure S4 shows XRD patterns of different photoanodes. All the labeled peaks can be ascribed to the reflections from α-Fe2O3 hematite phase (JCPDS 33-0664) and SnO2 phase (JCPDS 01079-6887). The strong peak at 2θ=36 indicates the preferential orientation of hematite nanorods in the [110] direction.30 Due to the small amount and poor crystallinity, no obvious peak corresponding to carbon nitride and Co-Pi cocatalysts can be detected. The survey XPS spectrum of Fe2O3/reassembled carbon nitride/Co-Pi shows Fe, O, C, N, Co and P signals (Figure 4a), confirming the formation of heterostructures. In Figure 4b, the two


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strong peaks at 724.1 eV and 710.6 eV corresponds to Fe 2p1/2 and Fe 2p3/2 of Fe2O3, respectively. The appearance of satellite peak at 718.7 eV and 732.9 eV is the characteristic of Fe3+ in α-Fe2O3.31 In the C 1s (Figure S5a) and N 1s (Figure S5b) spectra, the deconvoluted peaks are consistent with those in the hydrolyzed carbon nitride, indicating the successful reassembly of carbon nitride onto Fe2O3. Due to the overlap with Fe L3VV Auger peak, a broad peak centered at 783.6 eV is observed in the high-resolution Co 2p spectrum (Figure 4c).32 The formation of Co-Pi cocatalysts on the surface of heterostructures can be further confirmed by the binding energy of P 2p at 133.3 eV, which matches well with that for P in the phosphate groups (Figure 4d).

Figure 4. (a) XPS survey scan from Fe2O3/reassembled carbon nitride/Co–Pi composite photoanodes; High-resolution Fe 2p (b), Co 2p (c) and P 2p (d) spectra of Fe2O3/reassembled carbon nitride/Co–Pi heterostructures.


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The impact of heterostructure formation on the PEC property of hematite photoanodes was investigated. Figure 5a shows the transient photocurrent-potential curves of α-Fe2O3, αFe2O3/reassembled carbon nitride and α-Fe2O3/reassembled carbon nitride/Co-Pi photoanodes under AM 1.5G sun irradiation in 1 M NaOH electrolyte. Upon sweeping the potential from 0.6 to 1.8 V vs. RHE, α-Fe2O3 exhibits a photocurrent density of 0.25 mA/cm2 at 1.23 V vs. RHE, with an onset potential of 0.6 V vs. RHE. The incorporation of hydrolyzed carbon nitride can indeed enhance the PEC performance, as all Fe2O3/reassembled carbon nitride heterostructures present superior photoactivity than pristine Fe2O3. When two-cycle spin-coating was carried out, increase of photocurrent density by a factor of about two is achieved. By further modifying the photoanodes with Co-Pi co-catalysts, cathodic shift of onset potential and significantly enhanced photocurrent density of 0.7 mA/cm2 are achieved. This value is 1.7 times higher than Co-Pi modified Fe2O3 (Figure S6, 0.42 mA/cm2), indicating the synergetic contribution of reassembled carbon nitride and Co-Pi cocatalysts to the improved PEC performance. It should be pointed out that the fabrication method of carbon nitride suspension shows great influence on the photoactivity of photoanodes. In Figure 5b, only 40% enhancement of photocurrent density is observed for the heterostructures fabricated from ultrasonic exfoliated carbon nitride. In comparison, coupling Fe2O3 with hydrolyzed carbon nitride results in three-fold increased photoactivity than pristine Fe2O3. Due to the strongly coupled heterostructured interfaces, Fe2O3/reassembled carbon nitride/Co-Pi even exhibits a superior photocurrent density than several reported Fe2O3-based photoanodes, which is presented in Table S1.


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Figure 5. (a) Photoelectrochemical performance of Fe2O3, Fe2O3/reassembled carbon nitride and Fe2O3/reassembled carbon nitride/Co-Pi photoelectrodes under chopped light illumination; (b) Photoelectrochemical performance of Fe2O3, Fe2O3/carbon nitride/Co-Pi heterostructures fabricated using reassembled carbon nitride and liquid exfoliated carbon nitride. To investigate the stability of heterostructured photoanodes, photocurrent was measured at a constant potential of 1.23 V vs. RHE. As can be seen in Figure 6a, photocurrent of αFe2O3/reassembled carbon nitride/Co-Pi photoanode keeps stable up to 24 h irradiation. The outstanding stability can be attributed to the strong adhesion of the reassembled carbon nitride on Fe2O3. It is important to clarify the underlying reasons for the significantly improved PEC performance of modified Fe2O3 photoelectrodes. Basically, light absorption and charge separation are the most important factors that determine the efficiency of a photoanode. Thus, UV–Vis diffuse reflectance spectroscopy was used to study the influence of reassembled carbon nitride on the optical absorption property of photoanodes. In Figure S7, the presence of carbon nitride exhibits negligible influence on the absorption edge of Fe2O3. This can be attributed to the relatively larger bandgap of hydrolyzed carbon nitride than hematite (Figure S8). Treatment


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methods show ignorable influence on the absorption property of carbon nitride, as both hydrolyzed and exfoliated carbon nitride exhibit slightly increased bandgap than bulk C3N4. The loading of carbon nitride resulted in the weak absorption of heterostructured photoanodes around 670 nm. Although the fundamental reason for this is still not clear, IPCE measurements indicate that this absorption does not contribute to the superior photoresponse of heterostructured photoanodes (Figure S9). One interpretation of this phenomenon is the complicated interactions between ultrathin carbon nitride layer and Fe2O3 arrays, which has also been observed in the other ordered nanostructures. 33 Thereafter, the impact of strongly coupled interfaces on the charge carrier behavior in the heterostructured photoanodes was investigated. In Figure S10, the superior photocurrent of hydrolyzed carbon nitride than bulk sample indicates the improved photoactivity caused by NaOH treatment. This can be further confirmed by the significantly reduced PL intensity, which proves the suppression of charge carrier recombination in hydrolyzed carbon nitride (Figure S11a). Furthermore, the flat band potential (Vfb) of hydrolyzed carbon nitride is much more negative than Fe2O3, as evidenced by the Mott-Schottky plots in Figure 6b and Figure S11b. The reassembly of carbon nitride onto Fe2O3 nanorods results in the formation of heterostructured interfaces with type II band alignment.34 From a thermodynamical point of view, this could facilitate the injection of separated electrons from the conduction band of carbon nitride to that of Fe2O3, while holes accumulate in the valence band of carbon nitride. In other words, photoinduced charge carriers could be efficiently separated through coupling Fe2O3 with reassembled carbon nitride. Compared to pristine Fe2O3, the 2-fold increased donor density of Fe2O3/reassembled carbon nitride/Co-Pi heterostructures (7.04×1019 cm−3) confirms the significant contribution of heterostructured interfaces to the enhanced electrical conductivity.35


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To investigate the charge transfer at the electrode/electrolyte interfaces, EIS measurements were carried out at open-circuit potential under dark condition. As presented in Figure 6c, the Nyquist plot of pristine Fe2O3 includes a minor semicircle part at high frequencies and a major straight line part at low frequencies. Since charge transfer at the semiconductor/electrolyte interface is normally slower than in the bulk, the high frequency region can be ascribed to the charge transfer resistance within the hematite bulk, and the low frequency response is assigned to the semiconductor–electrolyte charge transfer.36 The radii of low frequency response becomes smaller after the loading of carbon nitride, which means the improved interface charge transfer.37-39 The charge transfer across semiconductor/electrolyte interfaces is further decreased after the deposition of Co-Pi co-catalysts. It indicates that the charge transfer kinetics for water oxidation over Fe2O3/reassembled carbon nitride/Co-Pi photoanodes is much faster. The effect of surface modification on the charge carrier kinetics is more intuitively reflected in the Bode plots of α-Fe2O3 electrodes. Figure 6d−f shows the Bode plots of α-Fe2O3 and αFe2O3/reassembled carbon nitride/Co-Pi photoanodes at representative applied potentials under light irradiation. At 0.6 V in Figure 6d, both samples show two characteristic peaks centered at around 0.5 Hz and 40 Hz. The low frequency peak shows much higher phase value, suggesting that water oxidation is limited by charge transfer at the electrode/electrolyte interface.40 After the loading of reassembled carbon nitride and Co-Pi cocatalysts, the low frequency peak of Fe2O3 electrodes decreases and shifts towards the lower frequency. It confirms the longer and efficient electron lifetime in the strongly coupled heterostructures.41 At 1.0 V (Figure 6e), the further reduced low frequency peak of Fe2O3/reassembled carbon nitride/Co-Pi indicates the efficient trapping of photogenerated holes by the surface modification layer. Moreover, the high frequency peak of the heterostructures becomes much larger, suggesting that the surface reaction


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is predominantly limited by bulk transfer of carriers at this potential.42 At 1.4 V in Figure 6f, the low frequency peak of heterostructured electrode nearly disappears. This means that water oxidation is mainly limited by the intrinsic properties of Fe2O3 at this potential. Mott-Schottky and EIS analyses confirm that the increased carrier density and the improved interface charge transfer are the dominant reasons for the enhanced photoelectrochemical performance.

Figure 6. (a) Photocurrent versus time for Fe2O3/reassembled carbon nitride/Co-Pi photoanodes at 1.23 V vs. RHE; Mott−Schottky (b) and Nyquist (c) plots of Fe2O3, Fe2O3/reassembled carbon nitride and Fe2O3/reassembled carbon nitride/Co-Pi photoanodes; Bode plots of Fe2O3 and Fe2O3/reassembled carbon nitride/Co-Pi photoelectrodes at 0.6 V (d), 1 V (e) and 1.4 V (f).


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Based on the above results, the corresponding mechanism for PEC water splitting is illustrated in Figure 7a. Upon irradiation, photogenerated electrons and holes migrate to the conduction bands and valence bands of Fe2O3 and carbon nitride, respectively. As an oxygen evolution catalyst, Co-Pi can catalyze the water oxidation reaction at a lower overpotential, i.e. improves the water oxidation kinetics.43 Therefore, Co–Pi can use the valence band holes of carbon nitride to drive water oxidation reaction, accompanied by the fast output of photoholes from the junction to water molecules. This process would reduce the recombination of photogenerated electron/hole pairs, and the electrons transfer to the Pt counter electrode to reduce water under external electric field. It is believed that the formation of strongly coupled interfaces can significantly contribute to the charge separation, which is crucial for the enhancement of PEC performance.44 The applicability of strongly coupled heterostructures in other metal oxide photoanodes was finally investigated. As shown in Figure S12, thin layers of reassembled carbon nitride can be clearly observed on the surface of TiO2 nanorod, ZnO nanorod and WO3 nanosheet arrays, respectively. Compared to heterostructures fabricated from exfoliated carbon nitride (Figure S13), the in-situ reassembly leads to the formation of intimately contacted interfaces. MottSchottky measurements indicate the formation of Type II band alignment between these metal oxides with hydrolyzed carbon nitride (Figure S14). Due to the facilitated interfacial charge transfer, significantly improved PEC activities are achieved for these photoanodes (Figure 7b-7d). All these results indicate that strongly coupled carbon nitride provides an effective strategy to design high-performance heterostructured photoanodes.


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Figure 7. (a) Schematic illustration of the mechanism for photoelectrochemical water splitting over Fe2O3/reassembled carbon nitride/Co-Pi photoanodes. Variation of photocurrent density versus applied voltage over TiO2-based heterostructures (b), ZnO-based heterostructures (c) and WO3-based heterostructures (d). 4. Conclusion In summary, a novel material strategy was used to reassemble carbon nitride nannofibers onto the surface of hematite nanorod arrays. Compared to conventional carbon nitride nanosheets fabricated from ultrasonic exfoliation, the high dispersity of hydrolyzed carbon nitride is beneficial for the formation of strongly coupled interfaces between different components. To improve the kinetics of water oxidation, Co-Pi cocatalysts were further deposited on the surface of heterostructures. When used as photoanodes for water splitting, the highly ordered Fe2O3/carbon nitride/Co–Pi triple junction exhibits significantly enhanced PEC performance. A


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three-fold increased photocurrent density was achieved, with good stability for more than 24 hours. Electrochemical impedance analysis indicated that the improved interfacial charge separation and decreased conductivity for electron transportation contributed to the superior photoactivity of heterostructured photoanodes. This material designing strategy was further used to construct TiO2/C3N4, ZnO/C3N4 and WO3/C3N4 heterostructures. The incorporation of hydrolyzed carbon nitride could remarkably enhance the PEC performance of these metal oxide photoanodes. Thus, this work provides a new paradigm for designing carbon nitride-based composite nanostructures for efficient and stable solar fuel production. ASSOCIATED CONTENT Supporting Information Additional characterizations of materials. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Prof. Chengzhi Hu. E-mail: [email protected]. Tel.: +86 10 62918589 ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51438011, 51722811, 51578531). REFERENCES


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