Crystalline Copper Phosphide Nanosheets as an Efficient Janus

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Crystalline Copper Phosphide Nanosheet as an Efficient Janus Catalyst for Overall Water Splitting Ali Han, Hanyu Zhang, Ruihan Yuan, Hengxing Ji, and Pingwu Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10983 • Publication Date (Web): 23 Dec 2016 Downloaded from http://pubs.acs.org on December 27, 2016

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ACS Applied Materials & Interfaces

Crystalline Copper Phosphide Nanosheet as an Efficient Janus Catalyst for Overall Water Splitting

Ali Han, Hanyu Zhang, Ruihan Yuan, Hengxing Ji, Pingwu Du* CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, iChEM (the Collaborative Innovation Center of Chemistry for Energy Materials), University of Science and Technology of China (USTC), Hefei, Anhui Province, 230026, China *Corresponding author: [email protected]

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Abstract: Hydrogen is essential to many industrial processes and could play an important role as an ideal clean energy carrier for future energy supply. Herein, we report for the first time the growth of crystalline Cu3P phosphide nanosheets on conductive nickel foam (Cu3P@NF) for electrocatalytic and visible light-driven overall water splitting. Our results show that the Cu3P@NF electrode can be used as an efficient Janus catalyst for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). For OER catalysis, a current density of 10 mA/cm2 requires an overpotential of only ~320 mV and the slope of the Tafel plot is as low as 54 mV/dec in 1.0 M KOH. For HER catalysis, the overpotential is only ~105 mV to achieve a catalytic current density of 10 mA cm-2. Moreover, overall water splitting can be achieved in a water electrolyzer based on the Cu3P@NF electrode, which showed a catalytic current density of 10 mA/cm2 under an applied voltage of ~1.67 V. The same current density can also be obtained using a silicon solar cell under ~1.70 V for both HER and OER. This new Janus Cu3P@NF electrode is made of inexpensive and non-precious metal-based materials, which opens new possibilities based on copper to exploit overall water splitting for hydrogen production. To the best of our knowledge, such high performance of a copper-based water oxidation and overall water splitting catalyst has not been reported to date. Keywords: Copper; Water Oxidation; Hydrogen production; Janus catalyst; Water splitting


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Introduction Overall water splitting to produce hydrogen and oxygen assisted by a catalyst has received much attention due to its potential for the production of clean hydrogen energy.1-3 Efficient water splitting is mainly hindered by the OER half-reaction due to its four-electron transfer process, coupled with the removal of four protons to form a new O-O bond.2,4-5 To overcome this bottleneck, robust, low-cost, and highly effective water oxidation catalysts are highly desired. In previous studies, precious metal-based catalysts, including homogeneous molecular complexes (Ru,6-10 and Ir11-12) and heterogeneous metal oxides (RuO213-14 and IrO215-16), have been widely studied and exhibited low overpotentials and high turnover rates for OER in basic or acidic solutions. However, the high cost associated with these catalysts will probably limit their large scale application. Designing highly active catalysts made of earth-abundant elements for water splitting will provide a cheap hydrogen source without increasing atmospheric CO2 levels. Therefore, many low-cost electrocatalysts have been developed for water oxidation based on earth-abundant transition metals such as manganese,17-18 cobalt,19-21 nickel,22-25 molybdenum,26-27 and iron.28-29 Although copper is a very cheap element with high abundance, it has attracted much less attention for OER and its performance generally requires a quite high overpotential.30-34 In 2014, our group reported nanostructured copper oxide electrodeposited from copper(II) complexes can serve as an active catalyst for water oxidation.35 Later, CuO nanomaterials directly synthesized from a simple copper salt also exhibited good catalytic activity for OER.36 Moreover, copperbased catalyst composite film was found to be an interesting electrocatalyst for both HER and OER in the same electrolyte.37 Such a catalyst made of an earth-abundant element is quite


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attractive for designing water splitting devices because one material plays a dual role, on the cathode catalyzing HER and on the anode catalyzing OER.38-39 The catalyst activity switching is explained by the transformation of morphology and composition of the catalyst surface, as evidenced by recent studies using cobalt oxide films,38 nickel oxide,39 and nickel phosphide for both OER and HER.24,40 Metal phosphides have been recently studied in both electrocatalysis and photocatalysis for HER, such as CoP,41 Ni2P,42 Cu3P,43 and FeP.44 However, the application of copper phosphide nanosheets material as a Janus catalyst for both HER and OER has not been received prior investigation. Herein, we report novel self-supported crystalline Cu3P nanosheets on 3D nickel foam as an excellent Janus catalyst for both HER and OER and subsequent overall water splitting. To the best of our knowledge, this is the first proof that crystalline Cu3P is an efficient catalyst for OER, HER, and overall water splitting.

Results and discussion The Cu3P@NF electrode was facilely synthesized by a simple two-step method (details are given in the Supporting Information, SI). The schematic illustration in Figure 1a shows the transformation from Cu(OH)xF@NF to Cu3P@NF, presenting an obvious color change from silver white to bronze to dark grey after phosphorization (Figure 1b). The formation of nickel phosphide on the surface of nickel foam could be avoided based on the following: (1) first, the direct formation of nickel phosphide on the nickel foam using red phosphorous as the P source was obtained at much higher temperature.45-46 (2) second, the phosphorization temperature using hypophosphite as the P source was higher than 400°C under the same condition and the


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phosphorization time was longer than one hour.47 (3) third, the copper oxide/hydroxide is much easier for phosphorization transformation than nickel metal at a low temperature.43 (4) fourth, the amount of the hypophosphite as the P sources in this present study was only enough to transform the Cu(OH)xF on the surface. Figure 2a shows the powder X-ray diffraction (XRD) data of the as-prepared Cu3P@NF electrode (orange plot) and pure Cu3P (green plot). The diffraction patterns of Cu3P@NF and Cu3P are consistent with crystalline Cu3P (PDF#71-2261). The black plot pattern represents the nickel foam substrate (PDF#03-1043). The scanning electron microscopy (SEM) image of the Cu3P nanosheet layer on nickel foam is shown in Figure 2b-2c. The results show that Cu3P nanosheets uniformly cover the nickel foam surface with an average length of 2-3 µm and a thickness of ~40 nm (Figure 2c). To further characterize the Cu3P nanosheets, high-resolution TEM (HRTEM) was performed. One single nanosheet is shown in Figure 2d. The lattice fringe spacings are correspond to the (-1-12) and (113) planes, respectively (Figure 2e). The result indicates that the exposed surface is the [1-10] facet. Moreover, the angle between the (-1-12) and (113) planes is 79.4°, which is in good agreement with the theoretical value of the angle (80.1°) between the (-112) and (113) planes. The lattice fringes in the SAED pattern confirms the interplanar lattice spacings of the (-1-12) and (113) atomic planes in the Cu3P nanosheet (Figure 2f). Single crystalline character can be observed over the Cu3P nanosheet. The EDX spectrum in Figure S1a shows that the Cu3P@NF electrode consists of Cu, P, and Ni (from the nickel foam) elements, and the mole ratio of Cu and P is close to 3:1, indicating the composition of the nanosheet is Cu3P. The ICP-AES result shows that the mole ratio of Cu:P in Cu3P after removing the nickel foam substrate is close 3:1 and only slight nickel elements is detected (Cu:P:Ni = 30:10:3).


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Therefore, significant interference by nickel phosphide was successfully avoided, which is also consistant with the XRD results (Figure 2a) and HRTEM image (Figure 2d). The XPS spectrum of the Cu3P@NF electrode was measured to identify the surface chemical composition and valence states. The survey scan in Figure S1b shows the presence of Cu, Ni, P, and O elements, either from the Cu3P material or the nickel substrate. The spectra are referenced to the C 1s peak (285.0 eV) and the Ni 2p peak is from the NF substrate. Figure S1c shows the high-resolution XPS spectrum of Cu 2p. The peaks at 932.1 and 952.1 eV correspond respectively to the Cu 2p3/2 and Cu 2p1/2 binding energies in copper phosphide.43 One peak in the high resolution spectrum of P 2p is located at 129.3 eV, corresponding to P 2p3/2, which is assigned to P character in Cu3P.43 Another peak at 132.9 eV can be assigned to metal phosphate, probably due to the oxidation of phosphide on the surface of the electrode.41,43-44 The application of the Cu3P@NF electrode for hydrogen production catalysis was measured in an alkaline solution (1.0 M KOH, pH ~13.6) using a standard three-electrode electrochemical cell. Linear sweep voltammetry (LSV) of the Cu3P@NF was recorded at a scan rate of 5 mV/s between 0 V and -0.25 V vs. RHE (Figure 3a). A significant cathodic current density was observed after -50 mV vs. RHE, accompanied by evolution of large amount of gas bubbles, which was confirmed to be hydrogen by gas chromatography. To achieve a catalytic current density of 10 mA cm−2, the overpotential (η) required is ~105 mV for Cu3P@NF, which is only 60 mV higher than for the high performance Pt/C@NF electrode. In contrast, a blank nickel foam electrode showed no obvious current density before -0.2 V vs. RHE. Figure 3b shows the Tafel plots for the Cu3P@NF and Pt/C@NF electrodes. Based on the Tafel plot, a slope of 42 mV/dec was obtained for Cu3P@NF, which is quite close to Pt/C@NF (31 mV/dec) and even better than many reported HER catalysts (Table S1), such as CoP/CC (129 mV/dec),41 Ni/NiO-


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CNT (85 mV/dec),48 and CoOx@CN (115 mV/dec).19 The results demonstrate the as-synthesized Cu3P@NF material is an excellent cathode for HER. The Faraday efficiency for H2 evolution using the Cu3P@NF electrode was measured under an applied overpotential of 300 mV. Subsequently, two methods were applied to evaluate the durability of the Cu3P@NF cathode for HER performance. First, long-term bulk electrolysis was conducted under an overpotential of ~100 mV. The LSV plot shows no significant decrease for 24 hours, indicating its robustness for hydrogen evolution (Figure 3a, green plot). Second, the chronopotentiometry method was applied for HER catalysis. Under a fixed catalytic current density at 10 mA cm−2, the overpotential (η) was monitored over 24 hours (Figure 3c). The results show that the decrease in η was less than 10.0 ± 5.0 mV, which is consistent with the bulk electrolysis method and confirms good durability of Cu3P@NF for HER. The electrolysis experiment produced ~1200 µmol H2 in one hour. Compared with the theoretical amount of H2, the Faraday efficiency was > 98 % (Figure 3d). Figure S2a show the SEM image of the Cu3P@NF electrode after HER catalysis for 2 hours. No significant difference of the nanosheet morphology was observed on the electrode before and after catalysis, indicating good stability of the material. The XPS spectrum of the nanosheets after HER catalysis demonstrates consistent peaks of Cu3P. XPS Cu 2p spectrum shows two major peaks of Cu 2p3/2 (931.9 eV) and Cu 2p1/2 (951.8 eV), and P 2p is represented by two typical peaks at P 2p3/2 (128.8 eV) and P 2p (132.5 eV) (Figures S2b-2c). All of the above results confirm that the as-synthesized Cu3P nanosheet material is quite stable during HER catalysis for hydrogen evolution.


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Showing its capacity in a dual role, the Cu3P@NF electrode showed excellent catalytic activity for OER catalysis during anodic scans. The electrocatalytic experiments were performed in an alkaline solution (1.0 M KOH) by LSV at a scan rate of 5 mV/s between 1.2 V and 1.7 V vs. RHE (Figure 4a). In the initial scan using the Cu3P@NF as an anode, the result shows significant OER catalytic activity with an obvious onset potential at ~1.52 V vs. RHE, accompanied by a significant amount of gas bubbles released from the surface. The gas was confirmed to be oxygen by both gas chromatography and a fluorescence-based oxygen sensor. Moreover, 50 LSV scans made the electrode became even more active (Figure S3b), presenting an obvious difference between the first and the 50th scans (Figure 4a and Figure S3b). The onset catalytic potential decreased to ~1.50 V after activation for continuous 50 scans and this difference indicates the formation of more active catalyst sites during the CV scans.24 Note, the onset overpotential was signed after the obvious Ni-OOH peak.29,49-50 Through analysis of continuous scans of blank NF electrode (Figure S4a), the new peaks at ~1.38 V can be ascribed to the oxidation of the Ni substrate, as also appeared in other nickel foam based OER catalysts.4950

The activity of IrO2@NF and blank NF electrode were also measured for comparison. The

blank NF electrode shows very poor catalytic activity toward OER at 1.2-1.7 V vs. RHE (Figure S5), indicating the important role of the Cu3P for catalytic water oxidation. In addition, the IrO2@NF electrode showed an onset catalytic potential at ~1.58 V for OER. Therefore, the Cu3P@NF electrode exhibited better performance for OER after activation than IrO2@NF and blank Ni foam in our present study (Figure S6). The reaction kinetics of Cu3P@NF for OER was further studied by Tafel plot (Figure 4b). The current densities (j) under various potentials were obtained from the LSV data. Impressively, the slope of the Tafel plot for Cu3P@NF is only ∼54 mV/dec (fitted from the LSV data in Figure


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4a), indicating the rate-limiting step is a one-electron transfer process. The result shows a very efficient reaction kinetics for water oxidation, a slope which is even lower than the noble-metal based IrO2@NF electrode (∼74 mV/dec). In addition, the Tafel plot for Cu3P@NF was also much lower than other copper based materials and other noble metal free based materials, such as Cu2S (63 mV/dec), Co3O4/N-rmGO (67 mV/dec), and CoCo LDH (59 mV/dec), as seen in Table S2. To test the durability of the Cu3P@NF as an anode for water oxidation, long-term bulk electrolysis by the chronopotentiometry method was performed for 100 hours. The overpotential (η) required was only ~320 mV (~1.55 V vs. RHE) to achieve 10 mA/cm2 per geometric area of the electrode with great stability (Figure 4c, top). Note that such a low overpotential is even better than many reported noble-metal-free electrocatalysts for OER, such as, Mn3O4/CoSe2 nanocomposite (450 mV, 0.1 M KOH),51 Co3O4/N-rmGO (310 mV),21, NiCo2O4 nanowires (450 mV),52 and CuCo2O4/NrGO (360 mV).53 The Cu3P@NF electrode shows the best OER performance among the Cu-based catalysts, as shown in Table 1. When fixing an overpotential at 370 mV, a quite stable current density of ~25 mA/cm2 was achieved in 1.0 M KOH, which only slightly decreased to 22. 5 mA/cm2 after 100 hours (Figure 4c, bottom). The Faraday efficiency for O2 evolution using Cu3P@NF anode was evaluated under an overpotential of 300 mV for 2 hours. The electrolysis experiment resulted in the evolution of ~180 µmol O2 for Cu3P@NF. Compared with the theoretical amount of O2, this value indicates a Faradic efficiency of > 98 % (Figure S7). The above results show that Cu3P@NF is an efficient catalyst for water oxidation with high catalytic activity and great durability. Electrochemical impedance spectroscopy (EIS) of the Cu3P@NF electrode was performed to gain more insight into the kinetics of these electrocatalytic reactions (Figure 4d). The Nyquist


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plots (Z′ vs -Z″) of the Cu3P@NF, NF, and IrO2@NF electrodes consist of a depressed semicircle in the high-frequency region, corresponding to charge transfer resistance, Rct. Obviously, the Cu3P@NF exhibits a much smaller Rct than that of blank NF and IrO2@NF electrodes. After 50 CV scans, an obvious decrease of Rct (from ∼4 Ω to ∼3 Ω) is observed for Cu3P@NF, indicating the activation process improves the electron transfer process. This observation can be ascribed to the formation of the active nanostructured CuO catalyst for subsequent water oxidation. The morphology of the Cu3P nanosheet was further studied by SEM and TEM after 50 anodic scans. As revealed in Figure S8a, the nanosheet structure was well remained. This observation is consistent with our previous study showing that the conversion between Ni2P and NiOx can be realized without material deformation and swelling.24 Extensive elemental analysis of Cu3P after the activation process of OER was studied by XPS. High resolution of Cu 2p and P 2p spectra showed that the elemental composition was maintained after a period of catalysis (Figures S8b-8c). However, the Cu 2p3/2 peak at 932.2 eV was decreased and the Cu 2p peak at 934.0 eV was increased, and two obvious satellite peaks at 940.7 eV and 943.3 eV appeared, demonstrating the formation of copper oxide on the surface of the Cu3P@NF after OER activation.35,37,53 Structural change of the Cu3P nanosheet for OER catalysis was further probed by HR-TEM (Figure 5). After OER activation, the elemental composition of the Cu3P nanosheet surface was significantly changed. The EDX line scan of Cu3P nanosheet in Figure 5a showed no appreciable phosphorous within 3 nm of the edge of the Cu3P nanosheet. The inset in Figure 5a is the STEM image of Cu3P nanosheet. In contrast, when probing distances more than 3 nm, the content of phosphorous element increased, as well as oxygen and copper (Figure 5a). Copper was detected


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less than 3 nm from the edge due to the carbon-supported copper grid for HRTEM measurement. The HR-TEM image showed that small nanoparticles with average size of 2-3 nm are formed around the Cu3P nanosheet after 50 CV scans (Figure 5b). The spacing of the fringes is characteristic of specific facet of copper oxide (-111) (PDF# 89-5898). Besides, the core material remains as Cu3P, as evidenced by the fringes corresponding to the crystalline Cu3P. The fringes became slightly curvy (Figure 2e), indicating defects were created during the cycling process and the CuO formation. The elemental composition in the Cu3P nanosheet after OER catalysis was also revealed by EDX elemental mapping (Figures 5c-5f). The core-shell Cu3P/CuO composite can be observed. Copper is uniformly distributed on the whole nanosheet, while phosphorous is present only in the inside layer. Oxygen is also seen in the outside layer covering the whole surface of the Cu3P nanosheet. This core-shell structure is consistent with observation of the linear HRTEM scans in Figure 5a. Based on the above results using Cu3P@NF for both hydrogen evolution and water oxidation catalysis, we subsequently constructed an electrolyzer for overall water splitting using Cu3P@NF on both anode and cathode (Figure 6a-6b). The Cu3P@NF electrode was used as the anode after activation for 50 CV scans. The overall water splitting experiment was carried out in 1.0 M KOH at room temperature. A catalytic current density of 10 mA/cm2 required only a potential of 1.67 V, presenting an overpotential of only 440 mV to achieve overall water splitting. Impressively, the observed onset potential was as low as 1.50 V (η ~270 mV) for water electrolysis using the present noble-metal binder free Cu3P@NF catalyst. Our device to do overall water splitting driven by electricity can be also powered by a Si solar cell (1.7 V, Figure 6c and 6d, green plot). Notably, the Cu3P electrode maintained excellent stability as manifested by the bulk electrolysis with a controlled voltage of 1.70 V for 10 hours.


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Thus, Cu3P showed excellent Janus activity and stability for overall water splitting, and our electrolyzer using Cu3P and powered by a solar cell can be viewed as a device for photocatalytic water splitting.

Conclusions In summary, herein we report for the first time that Cu3P nanosheet material is capable of catalyzing both OER and HER under alkaline conditions. Specifically, the Cu3P@NF electrode showed an overpotential of only ~105 mV for HER to reach a current density of 10 mA/cm2 and an overpotential of ~330 mV for OER to reach a current density of 10 mA/cm2. Great durability has been achieved for both reactions in alkaline solution. To demonstrate the water splitting capability of the Cu3P@NF electrode, an efficient electrolyzer has been constructed, in which a current density of 10 mAcm-2 requires only 1.67 V with good stability. The voltage required by this electrolyzer can also be powered by a readily available commercial Si solar cell under a similar voltage. Hence, the present Cu3P nanosheet electrode made of low cost and earth abundant materials could be promising for applications in future water splitting devices.

Experimental Methods Materials. All chemicals and materials were purchased from Aldrich or Acros and used without further purification, includingnickel foam, copper nitrate hemi(pentahydrate) (Cu(NO3)23H2O, 99.0%), ammonium fluoride (NH4F, 96.0%), urea (CO(NH2)2, 99.0%), sodium hypophosphite (NaH2PO2 99.0%), and potassium hydroxide (KOH, 85.0%). Millipore water (resistivity: ~18 MΩ·cm) was used to prepare the electrolyte solutions and the pH of 1.0 M KOH solution was ~13.6.


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Synthesis of Cu3P nanosheets on nickel foam The precursor Cu(OH)xF@NF material was firstly prepared by a hydrothermal method on commercial nickl foam (2.5 × 5 cm). 2 mmol Cu(NO3)23H2O (483.1 mg), 7.5 mmol NH4F (279.0 mg), 15 mmol urea (900.0 mg), and 60 mL water were mixed into a 100 ml Teflon-lined autoclave and the mixture was sonication for 10 min at room temperature. Then, a clean nickel

foam plate was placed into the mixed solution and sealed. The Teflon-lined autoclave was heated under 120 °C for 6 hours. The nickel foam plate was then cleaned by ethanol and water for 10 times to remove the unreacted substance and the absorbed Cu2+ ion. To prepare Cu3P@NF electrode, a porcelain crucible boat loaded with NaH2PO2 powder was placed into the heating zone of a quartz tube (Figure 1a) and another boat loaded with Cu(OH)xF@NF electrode was placed at the upstream side of the furnace. The molar ratio for Cu to P was 1:10. Subsequently, the sample was heated to 350 °C for 1 hour under N2 atmosphere.41 After cooling to ambient temperature, the as-prepared Cu3P@NF material was obtained and the color of the foam plate was changed into dark grey (Figure 1b). Finally, the material was cleaned by deionized water until the water is neutral. The Cu3P@NF electrode material was then used to measure the electrocatalytic properties in 1.0 M KOH solution. Electrochemical methods. All electrochemical experiments were performed using the three-electrode method reported in our previous study.58 The Cu3P@NF electrode was used as the electrode for HER measurement. The Cu3P@NF electrode after OER activation was used as anode for OER catalysis. The activation process was conducted under continuous CV scans between 1.2 V - 1.7 V, according to a similar method in our previous study.24 The mass of Cu3P nanosheet generated on the nickel


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foam was weighted after dissolving nickel substrate in 6.0 M HCl. For Cu3P@NF electrode, the loading mass of Cu3P was about 1.2 mg/cm2. The nickel foam deposited with 20 wt% Pt/C or IrO2 (1.2 mg/cm2) were used as electrodes for comparison under the same conditions (the geometric area of the nickel foam was 0.5 cm2). There was several iR drop for compensation and no stirring was used for the polarization curve. All potentials in this manuscript were converted to the RHE. Electrochemical impedance spectroscopy (EIS) data were measured in the range of 105-0.05 Hz with 10 mV sinusoidal perturbations under different applied overpotentials. Activation process of Cu3P@NF CV scans recorded on Cu3P@NF electrode in 1.0 M KOH are shown in Figure S3a. The first cycle demonstrates an anodic peak at ~1.41 V (vs. RHE). After the first cycle, this feature disappears and strong anodic (Ea1) and cathodic (Ec1) peaks appear at ~1.38 V and ~1.29 V, respectively. Moreover, after 50 scans, the electrode is even more active (Figure S3a), presenting obvious difference between the first and the 50th scan (50th) (Figure S3b). Similar observations are found in blank NF electrode, as shown in Figures S4a-4b. The first CV scan shows no obvious anodic (Ea2) or cathodic (Ec2) peaks but such peaks were observed at ~1.37 V and ~1.30 V respectively after continuous CV scans, which can be attributed to the reduction/oxidation of nickel species. To avoiding the effect of the nickel substrate, glassy carbon was used as the substrate to evaluate the redox properties of Cu3P nanosheet in 1.0 M KOH (Figure S5). During continuous CV scans, obvious anodic (Ea3) peak at ~1.55 V and cathodic (Ec3) peaks at ~1.54 V were observed in the first scan, followed by intense OER catalysis. As the number of scans increases (after 10 CV cycles), neither anodic wave nor cathodic wave indicating the redox chemistry of the copper ions could be observed, which may be too weak and overlapped with the catalytic waves in CV scans.


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Characterization of Cu3P nanosheet The Cu3P nanosheet and Cu3P@NF materials were further characterized by powder X-ray diffraction (XRD) (D/max-TTR III, Cu Kα radiation at 1.54178 Å), X-ray photoelectron Spectroscopy (XPS, ESCALAB 250), scanning electron microscopy (SEM, JSM-6700F field emission scanning electron microscope), and energy dispersive X-ray spectroscopy (EDX). The operation conditions for these instruments and the procedures for sample preparation can be found in our previous study.58 The Cu3P nanosheet structure was also measured by scanning transmission electron microscopy (Jeol ARM 200F). The analysis of the chemical compositions of Cu3P was obtained by measuring the EDX data and elemental mapping. Selected area electron diffraction (SAED) was measured to evaluate the crystalline phase, as well as the growth direction of the nanostructures. Line scan was used to measure the changes of elemental compositions along the direction of the nanosheet. Samples for SAED, HRTEM, and line scan analyses were prepared by ultrasonicating the Cu3P@NF electrode material in ethanol. The resulting suspension was dropped onto a carbon-coated grid.

Supporting Information Available. Experimental details are available about SEM images, Faradaic efficiency, LSV scans, EDX, and XPS data. This material is available free of charge via the Internet at http://www.acs.org.

Acknowledgment This work was financially supported by the National Natural Science Foundation of China (21271166, 21473170), the Fundamental Research Funds for the Central Universities


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(WK3430000001, WK2060140015, WK2060190026), the Program for New Century Excellent Talents in University (NCET), and the Thousand Young Talents Program.

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Figures:

Figure 1. (a) A schematic illustration of the growth setup of Cu3P@NF. (b) Optical photograph of NF, Cu(OH)xF@NF, and Cu3P@NF.


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Figure 2. (a) X-ray diffraction patterns of the as-prepared 3D Cu3P@NF (orange), Cu3P (green), Cu3P PDF#71-2261 (black). (b) low-magnification SEM images of Cu3P@NF, inset: the structure of Cu3P@NF. (c) High-magnification SEM image of Cu3P@NF. (d) TEM image of Cu3P nanosheet. (e) HRTEM image of Cu3P nanosheet. (f) SAED pattern obtained from the Cu3P material.


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Figure 3. (a) Polarization curves of Cu3P@NF electrode in 1.0 M KOH (pH = 13.6) with a scan rate of 5 mV s−1, along with NF (black) and Pt/C@NF electrode (red) for comparison. The green plot represents the Cu3P@NF after bulk electrolysis for 24 h. The iR drop was corrected with several ohms; (b) Tafel plot for Cu3P@NF electrode and Pt/C@NF electrode obtained from the polarization curves; (c) Chronoamperometry method with a controlled current density of 10 mA/cm2 in 1.0 M KOH using Cu3P@NF electrode; (d) Hydrogen evolution over time versus theoretical quantities using the Cu3P@NF electrode for the hydrogen evolution reaction. The applied potential was -0.3 V vs. RHE.


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Figure 4. (a) Polarization curves of Cu3P@NF electrode in 1.0 M KOH (pH = 13.6) with a scan rate of 5 mV s−1, along with NF (black and red) and IrO2@NF electrode (blue) for comparison. The dark green plot represents the Cu3P@NF first LSV between 1.2 and 1.7 V vs. RHE. The magenta plot represents the LSV of Cu3P@NF after OER activity. The iR drop was corrected with several ohms. (b) Tafel plot for Cu3P@NF electrode and IrO2@NF electrode obtained from the polarization curves. (c) Up: Chronoamperometry method with a controlled current density of 10 mA/cm2 in 1.0 M KOH using Cu3P@NF electrode; Bottom: Bulk electrolysis of Cu3P@NF electrode with applied potential of 1.6 V vs. RHE. (d) EIS Nyquist plots of the blank NF, IrO2@NF, and Cu3P@NF electrodes. The applied potential was 300 mV overpotential vs. RHE.


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Figure 5. (a) EDS elemental line scan in the scan direction showing the presence of P (orange), O (green), and Cu (blue) elements. Inset: STEM image of Cu3P nanosheet after OER activation. (b) HRTEM image of the Cu3P nanosheet after OER activity; (d)-(f) Corresponding EDX maps of the elements on the sample region shown in (c). (d) copper elemental mapping. (e) phosphorous elemental mapping. (f) oxygen elemental mapping.


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Figure 6. (a) Current-potential response of an alkaline electrolyzer using Cu3P@NF/Cu3P@NF as anode and cathode. Note: the Cu3P@NF anode was the freshly prepared Cu3P@NF after 50 CV scans between 1.2 V and 1.8 V vs. RHE. (b) Photograph of the system showing the hydrogen (left) and oxygen (right) production during water electrolysis. (c) Scheme of water-splitting device powered by a Si solar cell. (d) Bulk electrolysis of water using Cu3P@NF/Cu3P@NF (black, driven by electrochemical power), and Cu3P@NF/Cu3P@NF (red, driven by solar cell) at a constant voltage of 1.70 V in 1.0 M KOH.


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Table 1. Copper-based heterogeneous catalysts for oxygen evolution reaction (OER). Catalysts

η (mV)

η (mV) 2

References 2

at 1.0 mA/cm

at 10 mA/cm

Cu3P/CuO@NF

250

320

This work

CuO from Cu-TEOA

780

-

54

Cu-Janus

749

-

37

CuO from Cu-TPA

600

-

35

CuO nanowires

550

-

36

Cu-Bi

530

-

55

CuO on Cu foil

485

580

56

Cu2S NPs

-

428

57


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Table of contents


27

Crystalline Copper Phosphide Nanosheets as an Efficient Janus

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