CuBi2O4 Photocathodes for

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N, Cu-Codoped Carbon Nanosheets/Au/CuBi2O4 Photocathodes for Efficient Photoelectrochemical Water Splitting Na Xu, Feng Li, Lili Gao, Haiguo Hu, Yiping Hu, Xuefeng Long, Jiantai Ma, and Jun Jin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04133 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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ACS Sustainable Chemistry & Engineering

N, Cu-Codoped Carbon Nanosheets/Au/CuBi2O4 Photocathodes for Efficient Photoelectrochemical Water Splitting Na Xu, Feng Li, Lili Gao, Haiguo Hu, Yiping Hu, Xuefeng Long, Jiantai Ma and Jun Jin *

State Key Laboratory of Applied Organic Chemistry, The Key Laboratory of Catalytic Engineering of Gansu Province and Chemical Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, P. R. China. E-mail: [email protected]; Fax: +86-931-891-2582; Tel: +86-931-8912577

Keywords: N, Cu-codoped carbon nanosheets; Passivation layer; Au nanoparticles; Electron relay; CuBi2O4; Photocathode; PEC water splitting

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Abstract In this paper, we successfully fabricated CuBi2O4 and nitrogen, cuprum-codoped carbon nanosheets (N, Cu-C) heterostructure connected by Au nanoparticles, which was applied in photoelectrochemical (PEC) water splitting for the first time. Wherein the Au nanoparticles in situ decoration on the material surface was carried out by simple photo-reduction method. Owing to the plasmon resonance effect and low charge transfer resistance of Au nanoparticles, CuBi2O4/Au/N, Cu-C hybrids exhibited enhanced photocurrent compared with traditional CuBi2O4 and CuBi2O4/N, Cu-C. Typically, the as-prepared photoelectrode displayed the optimal photoelectric conversion (0.17%, at 0.4 V vs RHE) and PEC photocurrent (0.31 mA cm-2, at 100 mW cm-2 and 0.5 V vs RHE) that is about 5 times than pure CuBi2O4. The enhanced PEC water splitting ability can be mainly attributed to the N, Cu-codoped carbon nanosheets as the passivation layer and Au nanoparticles as an electron relay for electron transport and a plasmonic photosensitizer for increased light absorption. Introduction Environmental pollution and energy crisis have become inevitable threats to human development. It is an urgent need to find a green and sustainable alternative energy. Since Honda and Fujishima discovered the Titanium dioxide (TiO2) photoelectrodes to produce hydrogen in a photoelectrochemical cell (PEC) in 19721, the great potential for hydrogen production from water by solar radiation has attracted extensive attention, which is seen as a possible solution to environmental pollution and energy crisis. Nevertheless, the wide energy gap of TiO2 (3.2 eV) made it only absorb UV-light, accounting for ca. 4% of the solar energy, which led to a low light 2


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utilization efficiency2. Therefore, it is crucial to develop narrow bandgap semiconductor materials with effective visible light activity. Abundant metal oxide semiconductor CuBi2O4 has been considered as one of effective matrixes for water splitting due to its unique natures, such as sufficiently narrow direct bandgap (1.8 eV), appropriate band edge positions and low cost3. Assuming that all the energy above the bandwidth of the photons are absorbed and utilized with 100% efficiency, the maximum theoretical photocurrent density under the AM1.5 illumination can reach 19.7-29.0 mA cm-2. Regretfully, relatively poor photoelectric conversion efficiency caused by poor charge carrier transport and reaction kinetics of CuBi2O4 and its instability due to self-photoelectric-corrosion in contact of electrolyte solution, limit its application and competitiveness in PEC water splitting4. Various strategies have been explored to improve CuBi2O4 photoelectrodes with optimal solar-to-chemical energy conversion efficiency, which could be realized by topography designing5-6, selective elements doping7-8, heterojunction forming with some semiconductors9 or noble metals decorating10. These improvements can assist the rapid separation and transportation of photogenerated electrons to the surface for protons reduction, and thus, the photoconversion efficiency of CuBi2O4 can be improved in electrolyte solutions. However, these results cannot be compared with its theoretical value, how to obtain CuBi2O4 photocathode with higher photoelectric conversion efficiency and photoelectric stability as well as through a simple and inexpensive way is still a challenge. The efficiency and stability of photoelectrodes was previously improved by using

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carbon coating11, Al2O312, and NiO13 as a protecting layer for hydrogen evaluation. In particular, carbon nitride materials, such as g-C3N4 and N-doped graphene, have elicited widespread interest owing to its better performances containing excellent optical properties, low cost, and the most stabilization under thermal and chemical conditions, etc14-15. However, the conductivity of nitrogen-doped carbon coating is relative lower than graphite-like carbon layer16, which also impedes the charge transfer between nitrogen-doped carbon layer and semiconductor matrixes. Therefore, it would require other strategy to further optimize the interface between nitrogen-doped carbon layer and inorganic semiconductor. Au would be an appropriate choice for its excellent electronic transmission capability and chemical stability17. Moreover, Au could also endow it strong absorption in visible spectrum and effectively restrain the recombination of photogenerated electron and hole pair of CuBi2O4 due to localized surface plasma resonance (LSPR)18 and electron relay19 of Au nanoparticles. Hence, it can support the charge separation between semiconductor and the passivation layer, and in general facilitate the photoelectrocatalytic activities. In this work, we fabricate a ternary hybrid structure CuBi2O4/Au/N, Cu-C to import the PEC water splitting. The method involves the thermal oxidation of CuBi2O4 film on FTO and Au modified via photoreduction method, then it would be overlaid by N, Cu-codoped carbon layer through dip coating containing an aqueous solution of glucose, cupric nitrate and urea as precursor. The CuBi2O4 serves as a supported platform providing a possibility to build a composite structure on FTO for PEC water splitting, while N, Cu-codoped carbon layer could act as passivation layers to

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suppress surface recombination and improve reaction kinetics. More importantly, Au nanoparticles induced between CuBi2O4 and N, Cu-codoped carbon layer can be a bridge to facilitate the charge transfer between N, Cu-codoped carbon layer and inorganic semiconductor materials. The PEC performance and hydrogen evolution efficiency have been effectively improved compared with bare CuBi2O4 photoelectrode. The electrochemical experimental data shows that a co-doping effect of Cu leads to a higher photocurrent density, possibly attributed to the presence of Cu2+ as the assisted active center. Experimental section Preparation of CuBi2O4 film Scheme of the fabrication processes of the photoelectrode has been shown in Figure S1. Firstly, the CuBi2O4 film was grown on FTO glass substrates directly by thermal oxidation. FTO was cleaned by ultrasound in Triton X-100, deionized water and ethanol for every 15 min in turn and then dried with nitrogen flow. The cheap inorganic salts were chosen as CuBi2O4 precursors such as Cu(NO3)2·3H2O and Bi(NO3)3·5H2O. In a typical synthesis procedure, 0.097 mg Bi(NO3)3·5H2O was dissolved in 2 mL acetic acid and 0.012 mg Cu(NO3)2·3H2O was dissolved in 4 mL ethanol. Next above both were mixed to make a precursor solution with the volume proportion of 1:4. In the next 50 µL cm-2 of the precursor solution was drop-cast onto each substrate (1 cm × 2 cm FTO) lying flat in a muffle furnace under ambient conditions and immediately heated to 60 °C and kept for 1 h, then heated to 450 °C and kept for 2 h. The same way was repeat using previously CuBi2O4 film as the 5



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substrate, ultimately obtaining CuBi2O4 film with two layers. Preparation of Au modified CuBi2O4 film via photoreduction method The CuBi2O4 film decorated with Au nanoparticles was prepared by photoreduction procedure. Firstly, 0.2 g polyvinylpyrrolidone (PVP, K90, MW = 58000) was dissolved in 20 mL pure ethanol. Secondly, 2 mL of HAuCl4 aqueous solution was added to above solution drop by drop. Then, the as-prepared CuBi2O4/Au film was immersed into the solution and irradiated by a 200 W Xe lamp for 15 min. Lastly, the substrate was taken out, washed with ethanol and naturally dried in air. Preparation of framework N, Cu-codoped mesoporous carbon nanosheets The N, Cu-codoped carbon nanosheets was prepared via a simple calcination of glucose in the presence of urea and metal ion precursor. Exactly 5 g of urea, 250 mg of glucose and x% of metal salt (x = 5, 10 and 15 vs mass of glucose) were dissolved in 10 mL of deionized water and mixed homogeneously. Subsequently, the as-prepared Au modified CuBi2O4 film was immersed into the solution for 15 min and then taken out, dried under natural conditions. Finally, the substrate was heated to 550 °C and maintained for 2 h under a self-sustaining atmosphere. The samples prepared with x = 5, 10 and 15 were denoted as Cu-5, Cu-10 and Cu-15, respectively. Variations in the synthesis procedure such as different metal precursor (Cu, Fe, Co, Ni, Zn) and annealing temperatures (450, 550, 650 °C) of N, Cu-codoped mesoporous carbon nanosheets were indicated as well. Characterizations The as synthesized samples were characterized by X-ray diffraction (XRD, Rigaku

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D/max-2400), field emission scanning electron microscopy (SEM, Hitachi S-4800, Chiyoda-ku, Tokyo, Japan), X-ray photoelectron spectroscopy (XPS, PHI-5702, America Electrophysics Corporation), transmission electron microscope (TEM, FEI TecnaiTM F30, USA) and high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2). The optical properties of the samples were tested on a UV-vis spectrometer (Varian Cary 5000). The incident photon-to-current efficiency (IPCE) was measured with a 300W Xe light source with an aligned monochromator. PEC measurements for water splitting The PEC measurements were operated on a CHI 760D electrochemical workstation (CH instrument) with a standard three-electrode system in 0.3 M K2SO4/0.2 M phosphate buffer solution (pH 6.68). Saturated Ag/AgCl electrode and Pt electrode were used as reference and counter electrode respectively. The potential versus reversible hydrogen electrode (RHE) scale was converted by the equation: VRHE = VAg/AgCl + 0.059 × pH + 0.1976 V (1) (standard potential of Ag/AgCl/saturated KCl vs RHE). A 300 W Xe lamp with light intensity of 100 mW cm-2 was used as illumination source. The working electrode with an electrolyte exposed area of 1 cm2 was illuminated from the front side. The PEC responses were performed using linear sweep voltammetry (LSV) at a scan rate of 5 mV s-1 under chopped light illumination with 5 s light on/off cycles. The photoelectrochemical stability of the electrode was evaluated by the photocurrent densities at a fixed potential of 0.4 V vs RHE. Electrochemical impedance spectroscopy (EIS) carried out at - 0.4 V vs Ag/AgCl was applied to measure the conductivity of the photoelectrodes. Measurements with H2O2

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contained hydrogen peroxide 30% with a 0.3 M K2SO4 and 0.2 M phosphate buffer: 30% H2O2 volume ratio of 4:1. Results and discussions The XRD patterns of CuBi2O4 and CuBi2O4/Au/N, Cu-C are displayed in Figure 1. The characteristic diffraction peaks of CuBi2O4 can be well indexed to the (200), (211), (220), (002), (420), (213) and (332) crystal planes of pure kusachiite phase (PDF# 48-1886), and other small diffraction peaks marked with “*” are coincident with the FTO (PDF# 46-1088). Except the diffraction peaks of CuBi2O4, the weak and wide diffraction peak at about 35.6° on the patterns of CuBi2O4/Au/N, Cu-C can be resulted from diffraction peak of carbon. The same peak also can be found from other samples with different metal precursor as shown in Figure S2. The diffraction peak of Au (111) at 38.1° is also observed, corroborating the successful deposition of Au nanoparticles20. Additionally, the size of Au nanoparticles is estimated based on the Scherrer equation: D = 0.89 × λ/(β cos θ) (2)21, where D is the average grain diameter, λ is the wavelength of X-rays (λ = 0.154 nm for Cu Kα radiation), and β is the half-peak width of the diffraction peak at 2θ. The calculated value for Au nanoparticles is 15.6 nm, which is consistent with the estimated 17 nm from the TEM images in Figure S3. Insert Fig. 1 here The morphology of the samples has been observed on SEM images. Figure 2a reveals that the CuBi2O4 film is made up of many homogeneous nanoparticles with a certain surface roughness. It can easily form surface traps, which are prone to causing

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photogenerated carriers recombination, leading to subdued PEC performance22. The top view of CuBi2O4/Au/N, Cu-C indicates the top surface of framwork mesoporous carbon nanosheets appeared to contain many step edges, bringing in a smoother surface. The form of accumulation of CuBi2O4 structure growing on FTO by thermal treatment shown in Figure 2c displays a 323 nm thickness of CuBi2O4 film. When CuBi2O4 has been further modified by N, Cu-codoped carbon nanosheets as shown in Figure 2d, a thin film covers on the top of CuBi2O4, resulting in an increase in the thickness of the film to 381 nm. The side-view SEM mapping (Figure S4) and EDS from cross-sectional SEM image (Figure S5) for CuBi2O4/Au/N, Cu-C further certify the existence of N, Cu-codoped carbon nanosheets and Au nanoparticles, and also show that the Au nanoparticles is loaded between CuBi2O4 and N, Cu-codoped carbon nanosheets. At the same time, the approximate thickness of N, Cu-codoped carbon nanosheets can be estimated to tens of nanometers, which is consistent with the previous conclusion. Moreover, the N, Cu-codoped carbon nanosheets encapsulating on the surface of CuBi2O4 finally makes it unapparent, and the contents of Cu and Bi measured by the SAED are not consistent with the theory. Insert Fig. 2 here TEM is conducted to obtain detailed microstructure of photoelectrodes as shown in Figure 3. A few fragmentary coarse black materials in the image can be ascribed to the CuBi2O4. Inversely, microstructure of CuBi2O4/Au/N, Cu-C has been observed as a smooth structure, since the N, Cu-codoped carbon nanosheets has been evenly coated on the surface of CuBi2O4. The measure lattice spacing about 0.33 nm as shown in

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Figure 3d is corresponded to the (211) plane of CuBi2O43, and the lattice spacing about 0.23 nm represents for Au(111)23. Figure S3b shows structure of CuBi2O4/Au/N, Cu-C of other areas, from which some Au nanoparticles can be seen about 17 nm, indicating that the Au nanoparticles have been successfully assembled by photoreduction method. The corresponding SAED diagram also proves the existence of Cu, Bi, O, C, N and Au elements. In addition, the EDS elemental mapping of the CuBi2O4/Au/N, Cu-C composite as depicted in Figure 4 indicates that the elements are homogeneously distributed. Consequently, TEM give a further proof for structure of the ternary hybrid system based on CuBi2O4, Au and N, Cu-codoped carbon nanosheets, and the Au nanoparticles are sandwiched between CuBi2O4 and N, Cu-codoped carbon nanosheets. Insert Fig. 3 here Insert Fig. 4 here Figure 5 is a typical XPS spectrum for the chemical composite and valence state of samples. The peaks with different position are definitely distinguished in Figure 5. The binding energies of Bi 4f7/2 and 4f5/2 with the peaks at 158.5 and 163.4 eV respectively are in accordance with that in CuBi2O424. As displayed in Figure 5b, the higher binding energy of Cu 2p3/2 at 954.0 eV and the shake-up peak at about 941 – 944 eV are two major peaks of CuBi2O4, proving the existence of Cu2＋25. Figure 5c indicates the Au 4f7/2 and Au 4f5/2 with binding energies of 83.4 and 87.1 eV, respectively, which is exactly consistent with Au0 providing a valid evidence for the existence of metallic state26. The peak for O 1s as shown in Figure5d with the lower

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binding energy of 529.6 eV can be attributed to O2- in CuBi2O422. The XPS results indicate that the doping amounts of nitrogen and loading amount of Au are 0.42% and 0.5%, respectively. In order to obtain cuprum valence state in N, Cu-codoped carbon film, the study of XPS spectrum of N, Cu-codoped carbon film in the absence of CuBi2O4 (Figure S6) found that Cu is in the form of divalent in the carbon film, which would be a boost of hydrogen generation27. In addition, XPS spectra of CuBi2O4/N, Cu-C and CuBi2O4/Au/N, Cu-C have also been tested using Au reference as shown in Figure S7, indicating that Au is indeed present in the form of zero-valence. The peak of C 1s shows that C is in the form of graphite carbon, and the presence of C - N also shows the successful doping of nitrogen28-29. Insert Fig. 5 here Figure 6a indicates the UV–vis spectra of CuBi2O4 and CuBi2O4/N, Cu-C in the presence and absence of Au nanoparticles. The as-prepared CuBi2O4 film displays a wide absorption edge at around 300 - 800 nm, which well matches the bandgap of CuBi2O4. Passivation of CuBi2O4 with the N, Cu-codoped carbon nanosheets extended the absorption intensity. The Au nanoparticles sandwiched between CuBi2O4 and N, Cu-codoped carbon nanosheets further increased the absorption of visible light. The strong absorption band in the range of 480 - 620 nm corresponded to the LSPR of Au nanoparticles19, 30. The PEC performances of the samples are evaluated by the linear sweep scanning voltammetry (LSV) curves in 0.3 M K2SO4/0.2 M phosphate buffer solution (pH 6.68) under simulated solar irradiation (AM 1.5 G, 100 mW cm-2). As shown in Figure 6b, the current density of the CuBi2O4 is relatively low, while

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CuBi2O4/N, Cu-C shows higher photocurrent density than pure CuBi2O4 at whole potential range. In addition, CuBi2O4/N, Cu-C exerts higher photoelectric response compared to CuBi2O4/nitrogen-doped carbon nanosheets (CuBi2O4/N-C) as shown in Figure S8a, which proves that the introduction of Cu can indeed increase the PEC effect from the side. In particular, these samples have been further improved when Au nanoparticles are sandwiched between CuBi2O4 and N, Cu-codoped carbon nanosheets. The photocurrent density of CuBi2O4/Au/N, Cu-C composite electrode has achieved up to 0.31 mA cm-2 at 0.5 V (vs RHE), which is about 5 times than pure CuBi2O4 due to increased conductivity of CuBi2O4/Au/N, Cu-C and LSPR effect of Au nanoparticles (Figure S9). The deposition rate of Au, doping content of Cu, contrastive metal precursor (Cu, Fe, Co, Ni, Zn) and annealing temperatures have been screened and optimized as shown in Figure S8,10. The co-doping of Cu with nitrogen exhibits a more active photoelectric effect than other transition metal elements, which could be attributed to the role of Cu2+ with catalytic hydrogen evolution31-32. The charge transfer resistance and separation efficiency of photogenerated carriers in photoelectrode can be investigated by EIS spectra as shown in Figure 6c. Obviously, the resistance of CuBi2O4/N, Cu-C is smaller than that of CuBi2O4, while CuBi2O4/Au/N, Cu-C has the shortest radius of a circle, indicating that charges transfer fastest in the ternary composite structure33-34. The Bode phases shown in Figure 6d exhibit the electron lifetime of the samples. It can be seen that CuBi2O4/N, Cu-C photoelectrode has longer electron lifetime than CuBi2O4, and CuBi2O4/Au/N, Cu-C has longest electron lifetime among the samples35. In order to

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further illustrate the enhancement of PEC performance for CuBi2O4/Au/N, Cu-C, photoluminescence (PL) spectrum and Mott-Schottky (M-S) curves have been measured. As shown in Figure S11, the lower PL peak of CuBi2O4/N, Cu-C displays lower recombination rate of photoexcited electron-hole and longer electron lifetime compared with CuBi2O436, while the introduction of Au nanoparticles makes the more lower peak of CuBi2O4/Au/N, Cu-C than that of CuBi2O4/N, Cu-C. The M-S curves are obtained at 1000 Hz frequency in the dark. As shown in Figure S12, all the samples show negative slopes in the M-S curves, indicating that they are p-type semiconductors. The slopes derived from the M-S curves are used to evaluate the carrier density using the following equation: ND = 2/e0εε0[d(1/C2)/dV]-1 (3)37, where e0 is electron charge, ND is donor density (cm-3), ε is dielectric constant of CuBi2O4, ε0 is vacuum permittivity, V is potential applied to the electrode. The calculated electron densities of all the samples are shown in Table S1, indicating that the carrier density of CuBi2O4/Au/N, Cu-C is higher than CuBi2O4. So, it can be concluded that the CuBi2O4/Au/N, Cu-C has a greater potential for PEC water splitting than CuBi2O4 (Figure S13). All of the results are exactly matched with that of photocurrent curves. Insert Fig. 6 here The photoconversion efficiency (η) of samples as shown in Figure 7b can be evaluated based on the equation: η = jph × (0 - Vapp)/Ptotal (4)38, where jph is the photocurrent density obtained under an applied potential Vapp vs RHE, and Ptotal is the light density of 100 mW cm-2. The maximum of photoconversion efficiency for CuBi2O4 is ~0.07% at 0.4 V vs RHE, and the CuBi2O4/N, Cu-C exhibits a higher

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maximum of 0.09% at 0.4 V vs RHE. The CuBi2O4/Au/N, Cu-C exhibits a highest value of 0.17% among the range from 0.3 to 0.4 V vs RHE. It can be assumed that the inserted Au nanoparticles can contribute to the electronic transmission. The IPCE values of the photocathodes are measured with a applied voltage of - 0.3 V vs Ag/AgCl calculated as follows: IPCE(λ) = (1240 × Jp(λ))/(λ× Iλ(λ)) (5)39, where Jp(λ) is the measured photocurrent density (mA cm-2) and Iλ(λ) is the incident light power density (mW cm-2) under the wavelength of λ(nm). As shown in Figure 7a, for wavelength below 450 nm there is a sharp increase in IPCE values for the CuBi2O4/Au/N, Cu-C photocathode. This indicates that CuBi2O4/Au/N, Cu-C is more efficient at producing photocurrent from higher energy photons. Insert Fig. 7 here In order to prove that it is the dynamic factors affecting the photoelectric effect of the materials, we introduce the sacrificial agent to the system. Figure 8a shows chopped (light/dark) LSV scans for CuBi2O4 and CuBi2O4/Au/N, Cu-C photocathodes without or with H2O2 as an electron scavenger. For measurements without H2O2, CuBi2O4/Au/N, Cu-C photocathode shows higher photocurrent than the bare CuBi2O4. Moreover, the chopped LSV scan for the CuBi2O4/Au/N, Cu-C photocathode shows large cathodic transient spikes presumably caused by surface recombination of the photogenerated species40. This suggests that CuBi2O4/Au/N, Cu-C has poor reaction kinetics in the surface for the proton reduction reaction. Under the same conditions, with the introduction of H2O2 the CuBi2O4/Au/N, Cu-C shows much higher photocurrent density without transient spikes indicating that the composite material

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improves the reaction rate at the surface. In other words, these current densities do indeed be obtained in the absence of surface reaction limitations. It would be worth looking forward to attaining much higher photocurrent densities closer to the theoretical value if the limitations can be overcome through strategies such as load of the cocatalyst. Compared with the LSV of CuBi2O4/N, Cu-C in the presence of H2O2 as shown in Figure S14, the CuBi2O4/Au/N, Cu-C also shows higher photocurrent which indicates that the introduction of Au nanoparticles does increase the charge transfer efficiency and the carrier separation efficiency (Figure S15). Insert Fig. 8 here Finally, the stability of the CuBi2O4/Au/N, Cu-C film is appraised by chronoamperometric measurement, the photocurrent is measured at fixed potential of 0.4 V vs RHE under continuous illumination as shown in Figure 8b. At the initial stage after illumination, the photocurrent of the CuBi2O4/Au/N, Cu-C shows a fast decaying feature, which is normal for the PEC system41. The CuBi2O4/Au/N, Cu-C is preserved after 3000 s of continuous testing compared with bare CuBi2O4, revealing that the electrode has good response ability for photochemical stability in PEC measurements. Conclusions In summary, a novel CuBi2O4/Au/N, Cu-codoped carbon nanosheets hybrid electrode has been successfully prepared for PEC water splitting. The N, Cu-codoped carbon nanosheets acting as a protective layer can form heterojunction with CuBi2O4, improving separation efficiency of e-/h+ and chemical stability of the photoelectrode.

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Importantly, the Au nanoparticles can serve as an electron relay between the CuBi2O4 and N, Cu-codoped carbon nanosheets for charge transfer, and the LSPR excited in the Au nanoparticles can act as a plasmonic photosensitizers, which can inject hot electrons into the N, Cu-codoped carbon nanosheets and effectively increase the absorption of light. The combination of charge transfer and light absorption enhancement is the reason for the improvement of photoelectric conversion efficiency. Acknowledgements This research was supported by the Natural Science Foundation of Gansu (No. 17JR5RA213), the Fundamental Research Funds for the Central Universities (Grant no. lzujbky-2016-k08), the Key Laboratory of Catalytic Engineering of Gansu Province and Resources Utilization, Gansu Province for financial support.

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ASSOCIATED CONTENT Supporting Information. Scheme of the fabrication processes; characterization of samples: XPS spectra, XRD, SEM, TEM, PL spectrum; PEC measurements of

photoelectrodes:

LSV

characteristics,

Cyclic

voltammograms,

Mott–Schottky plot, time-dependent photocurrent; schematic diagram of dual role of Au nanoparticles in photoelectrode structure.

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TOC:

For Table of Contents Use Only SYNOPSIS: a novel CuBi2O4/Au/N, Cu-codoped carbon nanosheets hybrid electrode has been successfully prepared for PEC hydrogen evolution.

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Fig. 1 XRD pattern of FTO, CuBi2O4 and CuBi2O4/Au/N, Cu-C photoelectrode.

Fig. 2 SEM images of CuBi2O4 (a from top view, c from cross-sectional view) and CuBi2O4/Au/N, Cu-C (b from top view, d from cross-sectional view).

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Fig. 3 TEM image of CuBi2O4 (a) and CuBi2O4/Au/N, Cu-C (b). HRTEM image of the CuBi2O4 (c) and CuBi2O4/Au/N, Cu-C (d).

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Fig. 4 STEM image mapping analysis for the CuBi2O4/Au/N, Cu-C.

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Fig. 5 XPS spectra for the CuBi2O4/Au/N, Cu-C photocathode showing regions for (a) Bi 4f, (b) Cu 2p and (c) Au 4f. (d) The peak of O 1s is showed in detail.

Fig. 6 Absorptance spectra (a), LSV characteristics (b), EIS (c) and Bode plot (d) for CuBi2O4, CuBi2O4/N, Cu-C and CuBi2O4/Au/N, Cu-C.

Fig. 7 IPCE spectra (a) and photoconversion efficiency (b) for CuBi2O4, CuBi2O4/N, Cu-C and CuBi2O4/Au/N, Cu-C.

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Fig. 8 LSV characteristics (a) with or without H2O2 for CuBi2O4 and CuBi2O4/Au/N, Cu-C photoelectrodes. Curves of the time-dependent photocurrent (b) measured for CuBi2O4 and CuBi2O4/Au/N, Cu-C photoelectrodes at 0.4 V vs RHE under AM 1.5G light illumination.

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CuBi2O4 Photocathodes for

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