Cu2O Cathode

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Energy, Environmental, and Catalysis Applications

Surface Assistant Charge Separation in PEC Cu2S-Ni/Cu2O Cathode Wan Zhang, Ruotian Chen, Zhiguang Yin, Xinyu Wang, Zenglin Wang, Fengtao Fan, and Yi Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11976 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 24, 2019

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ACS Applied Materials & Interfaces

Surface Assistant Charge Separation in PEC Cu2S-Ni/Cu2O Cathode Wan Zhang1, Ruotian Chen2, Zhiguang Yin1, Xinyu Wang1, Zenglin Wang1, Fengtao Fan2, Yi Ma1* 1

Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical

Engineering, Shaanxi Normal University, Xi’an 710119, Shaanxi, China

2 State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, The Collaborative Innovation Centre of

Chemistry for Energy Materials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road

457, Dalian 116023, China

*Corresponding author: [email protected]

Keywords: Photoelectrocatalysis; H2 Evolution; Cuprous oxide; Charge Dynamics; Surface Charge Separation

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ABSTRACT Fabrication of high efficiency photocathode is a challenge issue in photoelectrocatalysis (PEC). In this work, Cu2S-Ni/Cu2O photocathode was constructed via electrodeposition followed by a two-step overlayer deposition procedure including direct-current magnetron sputtering (DCMS) and ion exchange reaction. We found that the presence of Ni in the inner-layer could not only affect the morphology but also enhance the formation rate of the outer-layer Cu2S. The XPS results indicate that the Ni exist as NiOx instead of Ni0. The photocurrent of Cu2S-Ni/Cu2O achieved 2 times of it on the pristine Cu2O. The charge dynamic characterizations, including electrochemical impedance spectroscopy (EIS), Tafel slopes and photoluminescence (PL) spectra, demonstrated that the Ni can promote the hydrogen evolution reaction follow the Heyrovsky reaction, while Cu2S shows a crucial role on the surface charge separation. At last, surface photovoltage microscopy (SPVM) technology was used to reveal the function of each overlayers. It gives a direct evidence for the charge transportation pathway in the system and explains the function of each component.


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1. INTRODUCTIUON As the increasing serious energy and environment problems, energy conversion and storage become urgent issues in recent years.1,

2

Generation of clean fuels from

sunlight offers a promising approach for the sustainable energy storage. Hydrogen is considered as an ideal energy carriers with a high calorific value and can be combusted in a clean and efficient manner. Therefore, direct splitting of water into hydrogen and oxygen by solar light appears to be one of the most attractive strategies, which could be a potential technology to transform our current fossil fuel dependent society into a sustainable one.3 Hydrogen production via water splitting with the help of solar energy can be achieved by photocatalytic (PC) process4-8, photoelectrocatalytic (PEC) process,9-11 and solar-driven electrochemical process.12 Apparently, the PEC process exhibits more advantages than PC process. For example, the catalyst can be easily recycled; the higher charge separation efficiency can be obtained with an additional driving force; the back reaction can be greatly suppressed with also lower cost for follow-up product separation.13 A PEC electrode is usually based on a semiconductor catalyst, which should be efficient for light absorption, low-cost and environmental friendly. Cuprous oxide, Cu2O, is an attractive candidate for the PEC hydrogen production with a direct bandgap of 2.0 eV. The theoretical photocurrent of Cu2O can reach up to 14.7 mA/cm2 and a light-to-hydrogen conversion efficiency of 18% based on the AM1.5 spectrum.14 It has been considered the only candidate that meets all the requirements concerning to both economical and practical factors.15 However, there



are some challenges restricting its application. One is the activity. Although the theoretical photocurrent of Cu2O can reach up to 14.7 mA/cm2, the facile charge recombination in the bulk and on the surface of Cu2O limit its performance. The other is the stability. The redox potentials for the reduction and oxidation of Cu2O locate within the water splitting potentials. If the photo-generated charges could not transport and depart from Cu2O quickly, the Cu2O would suffer from serious photocorrosion. As can be seen, the above issues can be both ascribe to the charge dynamic performance in the photoelectrode, which always be a crucial factor for the property of photoelectrodes.16-18 Up to now, many efforts have been made to solve the problems from the charge dynamic aspect. Construction of heterojunction materials such as Cu2O/CuO,19-21 Cu2O/TiO2,22-24 Cu2O/g-C3N4,23 CdS/Cu2O25 and Cu2O-CuO-TiO226 etc. have been investigated, which was considered to be beneficial for the charge separation. Besides, deposition of cocatalyst such as NixPx,27 RuOx,15 NiOx,28 Ni,29 and Pt30-32 were also reported, which could not only promote the charge separation but also function as the active sites for hydrogen evolution. Furthermore, the overlayer deposition technology was considered to be another efficient way for improving the stability of Cu2O, which could isolate Cu2O from the electrolyte and block the corrosion reaction.33-37 Although the performance of Cu2O-based photocathode has been improved in the reported literatures, the charge dynamics in these systems were still unclear. Most of the researchers could only give a hypothetic mechanism with indirect evidence, which could provide limited information for the design of an efficient photoelectrocatalyst.


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Some insight questions are still unclear, such as what the real function of each component in the photocathode is, whether the overlayer only acts as a neutral protector, what the preferred deposition order is when using more than two components and how it effects the charge transportation and distribution. Besides, although a PEC process shows a better charge separation efficiency via an additional driving force, the catalyst on the electrode still suffers from serious charge recombination on the surface. Because the additional electrical driving force becomes weaker in the direction from substrate to catalyst surface, the bulk recombination can be efficiently suppressed but the surface recombination is not. In this research, a pseudo-sandwich-structured photocathode containing Cu2O, Cu2S and Ni was fabricated, which showed an obviously improved activity and stability compared to a pristine Cu2O in PEC hydrogen production reaction. The results showed that the deposition order could affect the morphology and charge transportation of the photocathode. The functions of each component in the photocathode were revealed by series of charge dynamic characterizations. It was evidenced that Ni can decide the rate determine step for the reaction, while Cu2S not only acted as a protector, but also responsible for the surface charge separation.

2. EXPERIMENTAL 2.1 Chemicals and Reagents

Copper(Ⅱ) sulfate pentahydrate (Cu2SO4•5H2O, ≥99.0%), Lactic acid(C3H6O3, ≥85.0%), sodium hydroxide (NaOH, ≥96.0%), ethanol (C2H6O, ≥99.7%), Acetone (C3H6O, ≥99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd



(Shanghai, China). The fluorine-doped tin oxide (FTO) conductive glass was purchase from Aoge Co., Ltd (Wuhan, China). All chemicals reagents were in analytical grade and used as received without any further purification process. Deionized water was used in all experiments.

2.2 Catalyst Preparation

Cu2O flim deposition ： Cu2O photoelectrodes were prepared according to the literature via a modified electrodeposition method.38 The fluorine-doped tin oxide (FTO) substrate was ultrasonic cleaned with acetone, alcohol and deionized water in sequence , followed by a layer of Au deposition using ion sputtering method with a sputtering time of 80 s prior to use. Typically, the electrodeposition process was performed in a solution with 0.3 mol/L Cu2SO4 and 3 mol/L lactic acid aqueous solution. The pH value of the solution was adjusted to 12.0 by NaOH aqueous solution. Cu2O thin films were deposited at a constant potential of 0.6 V for 500 s in a three-electrode configuration at 50 oC using FTO as working electrode, Pt foil as counter electrode and Ag/AgCl/KCl (saturated) as reference electrode. Ni overlayer deposition: The cocatalyst Ni were deposited on the surface of the Cu2O electrode or Cu2S/Cu2O electrode using direct-current magnetron sputtering (DCMS) at room temperature. The pressure of Argon sputtering gas was maintained at 0.2 Pa during deposition , the power were held constantly at 20 W, the deposition time were controlled for 20 s, 1 min, 2 min and 2.5 min, respectively.


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Cu2S overlayer deposition: Fabrication of Cu2S overlayers on Cu2O electrode or Ni/Cu2O electrode were performed via an ion exchange reaction by dipping the electrode in a 5 mM Na2S solution for 90 s at room temperature, followed by annealing in N2 atmosphere at 200 ℃ for 2 hours. The reaction for the formation of Cu2S are given as follows33, 39: Cu2O + S2- + H2O → Cu2S + 2OHThe surface of the electrode turned brown, suggesting that the Cu2S was successfully fabricated. The as-prepared photoelectrodes were denoted as ‘Ni-Cu2S/Cu2O-T’ or ‘Cu2S-Ni/Cu2O-T’ for different fabrication order. For example, Ni-Cu2S/Cu2O indicates the Cu2S is deposited prior to the Ni deposition, while Cu2S-Ni/Cu2O means the Ni is fabricated before the deposition of Cu2S. T is the sputtering time of Ni deposition and T = 2 min if not specially mentioned.

2.3 Material Characterization

X-ray diffraction (XRD) were obtained using a diffractometer (MiniFlex 600, Rigaku) with a Cu-Kα radiation at 400 KV and 15 mA with the speed of 10o/min, over the range of 10-80o. The scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDX) of the photoelectrodes were obtained using a Field Emission Scanning Electron Microscopy (FESEM, Hitachi High-Technologies, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) were conducted at an Axis Ultra DLD XPS instrument with Al Kα radiation (Kratos



Analytical Ltd.). Binding energies for the high resolution spectra were calibrated by setting C 1s to 284.6 eV and analyzed by XPSPeak software. Photoluminescence (PL) spectra were performed using a Hitachi F-4600 fluorescence spectrophotometer with the excitation and emission slits set to 2.5 nm with a 450 W xenon lamp as the excitation source. The surface potential (CPD) images of the samples were measured using Kelvin probe force microscopy (KPFM) (Bruker Dimension Icon) under ambient conditions in the AM-KPFM mode. A Bruker SCM-PIT probe was used for the KPFM scan and the lift height of probe was set at 50 nm during the KPFM scan. All scan parameters were optimized with respect to good signal-to-noise ratio. A 450-nm laser with light intensity of 5 mW/cm2 was used to excite the samples and to obtain the surface potential images under illumination. The samples were characterized by surface photovoltage microscopy (SPVM) by continuously mapping the surface potential images in the dark and under illumination using KPFM. The difference between the surface potential images obtained in illuminated and dark KPFM scans at the same location was extracted as a SPVM image.

2.4 Photoelectrochemical (PEC) Measurements

The PEC performance of the composite photocathodes were carried out in a CorrTest CS310 electrochemical workstation (Wuhan CorrTest Instrument Co. Ltd., China) in an aqueous Na2SO4 solution (0.5 M), the electrolyte was bubbled with N2 for 30 min before the test and using a three-electrode configuration, the pH was


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maintained at 6.8. The Cu2O-based thin film electrode, Pt foil and Ag/AgCl/KCl (saturated) electrode were used as working electrode, counter electrode and reference electrode, respectively. All the potential measured were converted to reversible hydrogen electrode (RHE) using the following equation: E(RHE) = E(Ag/AgCl) + 0.197 V + 0.059 pH. The active area of the electrode was fixed to 1.0 cm2 with the cover of silicone rubber. Linear sweep voltammetry was carried out at a scan rate of 5 mV s−1. Chronoamperometry measurements were performed at 0 V vs. RHE under chopped light with dark/light cycles of 30/30 s. Throughout the testing process, a 300 W Xe-lamp with a AM 1.5G filter was used as light source. The double-layer capacitance (Cdl) to evaluate the electrochemical active surface area (ECSA) was determined in the non-Faradaic potential range of −0.05 to 0.05 V at different scan rates of 5, 10, 25, 50, 100, and 200 mV s−1. Charging current density differences (Δj) plotted as a function of the scan rate. The linear slope was equivalent to the double-layer capacitance Cdl, representing the ECSA.40 The kinetics of interfacial charge transfer in the photocathodes were evaluated using electrochemical impedance spectroscopy (EIS). The Nyquist plots were obtained at an excitation signal of 10 mV amplitude, with a frequency range from 0.01 Hz to 100 kHz.

3. RESULTS AND DISCUSSION 3.1 Material Characterization

Fig. 1 illustrates the preparation process diagram of different Cu2O-based photocathodes. The FTO substrate was firstly coated with Au via ion sputtering method because of its well reproducibility.14 Secondly, Cu2O was electrodeposited on



Fig. 1 Schematic illustration for the fabrication procedure of the photocathodes.

the substrate in a three electrode system. After that, the Ni and Cu2S deposition were conducted sequentially or conversely, and the aimed electrodes were denoted as Cu2S-Ni/Cu2O/FTO and Ni-Cu2S/Cu2O/FTO, respectively. The photocathodes were firstly characterized by XRD patterns. As shown in Fig. S1a, all the electrodes show the similar XRD patterns. The typical XRD peaks of Cu2O are located at 29.5, 36.4, 42.3, 61.3 and 73.5o (JCPDS 05-0667). The standard XRD patterns of Cu2S (JCPDS 53-0522) and Ni (JCPDS 45-1027) are also listed as references. Besides the peaks for FTO substrate, all other peaks for these electrodes belong to Cu2O, in which the peak located at 36.4o showing the dominant growth in (111) orientation. For Cu2S/Cu2O, Ni/Cu2O, Ni-Cu2S/Cu2O and Cu2S-Ni/Cu2O electrodes, no peak corresponds to Ni or Cu2S were observed, indicating that the deposition amount of Ni and Cu2S was quite small or existed as amorphous structure. XRD patterns of Cu2S-Ni/Cu2O photocathodes with different content of Ni also show no other peak except for Cu2O (Fig. S1b).


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Fig. 2 Top-view SEM images for the photocathodes: (a) Cu2O, (b) Cu2S/Cu2O, (c) Ni/Cu2O, (d) Ni-Cu2S/Cu2O, and (e) Cu2S-Ni/Cu2O.

The top-view SEM images of the composite photocathodes are shown in Fig. 2. As evidenced by XRD patterns, Cu2O showed a preferable growth in (111) orientation, which exposed triangular facets on the FTO substrate with dense and smooth surface structure (Fig. 2a). Fabrication of Cu2S apparently increased the roughness of the photoelectrode (Fig. 2b), implied a uniform and high coverage of Cu2S on Cu2O photocathode. DCMS provided a homogeneous deposition method for the Ni coating. Therefore, Ni/Cu2O photocathode exhibited a similar morphology to Cu2O (Fig. 2c). When the deposition of Cu2S was followed by a Ni coating process (Fig. 2d), the surface roughness of Ni-Cu2S/Cu2O electrode decreased compared to Cu2S/Cu2O, showing more uniform nanoparticle dispersion. Interestingly, if Cu2S and Ni were deposited in conversely order, Cu2S-Ni/Cu2O photoelectrode showed a quite diverse morphology. In the presence of Ni, the kinetics of the ion exchange reaction (Cu2O→ Cu2S) on the surface of Cu2O became obviously different. Cu2S preferred to grow into larger thin layers rather than nanoparticles as it on the bare Cu2O photocathode. Thus,



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it exhibited an interconnected Cu2S nanolayers on the top of the photocathode and the original triangular structure of Cu2O were totally covered. The nanolayer morphology of Cu2S was further confirmed by EDX mapping, in which the S signal was only observed on the nanolayers (Fig. S2). It appeared that, the growth kinetics of Cu2S could be strongly affected by the surface composition of the photoelectrodes. The resulted morphology may provide a unique electron transportation dynamics, which may directly influence the PEC performance of the photoelectrodes. Table 1 Elements distributions of Cu2O-based photocathodes analyzed by EDX spectra.

O

Cu

S

Ni

at%

at%

at%

at%

Cu2O

27.01

72.99

-

-

Cu2S/Cu2O

22.12

73.62

4.26

-

Ni/ Cu2O

25.58

73.96

-

0.46

Ni-Cu2S/Cu2O

33.63

63.53

2.43

0.42

Cu2S-Ni/Cu2O

11.52

71.65

16.71

0.13

Photocathodes

The elements distributions of the photocathodes were analyzed by EDX spectra (Fig. S3) and the data were summarized in Table 1. Most of the photocathodes contain Cu and O as major elements. The content of S for Cu2S/Cu2O is 4.26 at%, which decreases to 2.43 at% for Ni-Cu2S/Cu2O due to the partial coverage of Ni on top of Cu2S. Interestingly, for Cu2S-Ni/Cu2O, the amount of S increases to 16.71 at% which is ~ 4 times of it in Cu2S/Cu2O, showing that the present of Ni could not only affect the morphology but also enhance the formation rate of Cu2S. Apparently, the surface chemical environmental of Cu2O photoelectrode was totally changed after the deposition of Ni. It is proposed that the present of Ni may act as a catalyst for the ion exchange reaction, which


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results in an increased Cu2S formation rate on Cu2S-Ni/Cu2O photoelectrode. Besides, the atomic ratios of (O+S) to Cu in Cu2S/Cu2O (26:74) and Cu2S-Ni/Cu2O (28:72) are both close to the ratio of O to Cu in Cu2O (27:73), confirming that the Cu2S was totally obtained by the ion exchange reaction. The content of Ni on Ni/Cu2O is only 0.46 at%, which keep its value of 0.42 at% on Ni-Cu2S/Cu2O. When the Ni becomes an inner layer as it in Cu2S-Ni/Cu2O, it decreases to only 0.13 at%. The cross-section SEM image of Cu2S-Ni/Cu2O and corresponding EDX mapping were exhibited in Fig. 3. The deposition thickness of Cu2O layer was about 1 μm, which integrated firmly with the FTO substrate. The O signal was observed on the Cu2O and FTO region, while the Cu signal was only observed on Cu2O. The signal of Ni layer was hardly seen, only a faint signal was obtained on the top of Cu2O. S signal was observed in a broad region on the top of Cu2O and be not limited into a thin layer, which indicated some Cu2S nanolayers grew on the vertical direction to the substrate.

Fig. 3 Cross-section SEM image of Cu2S-Ni/Cu2O (a) and corresponding EDX mapping for all elements (b), O (c), Cu (d), Ni (e) and S (f).



Fig. 4 XPS of Cu2S-Ni/Cu2O photocathode: (a) Cu 2p , (b) Auger Cu LMM (c) Ni 2p, , (d) S 2p and (e) O 1s.

The XPS results for Cu2S-Ni/Cu2O photocathode are illustrated in Fig.4. The band of Cu 2p2/3 can be deconvoluted into two peaks (Fig.4a). The major peak located at 932.9 eV is assigned to Cu+ or Cu0 species41, 42 and the smaller peak at 934.7 eV is ascribed to Cu2+. As Cu 2p3/2 XPS cannot differentiate between Cu+ and Cu0, Auger Cu LMM spectra were performed, which confirmed the presence of Cu+ at bingding energy ~ 570 eV.43 The XPS and LMM results of Cu demonstrated that, although Cu+ was the doninent specises in the photocathode, a small amount of Cu2+ can still be observed. As shown in Fig. 4c, the Ni 2p spectrum can be fitted into two peaks of Ni 2p3/2 and Ni 2p1/2 with satellite peaks, respectively. The binding energy ~ 855.3 eV is higher than Ni0 and lower than Ni2+,44, 45 indicating that Ni is likely to be in a high-valent state and should be denoted as NiOx. The XP spectrum of S 2p are shown in Fig. 4d. Two bands located at 162.0 eV and 163.1 eV are ascribed to S2− and


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Sn2−, respectively.46,

47

S2− can be easily oxidized to Sn2− due to its strong

reducibility. Therefore, Sn2− can be observed in most of the disulfides. Besides, the XP spectrum of O exhibited a typicial O2− in Cu2O (Fig. 4e). The UV-Vis diffuse reflectance spectra of the photoelectrodes were performed to evaluate the light absorption properties (Fig. S4). All the samples show the similar profile, which reflects the typical spectra of Cu2O. Besides, Ni-Cu2S/Cu2O and Ni/Cu2O showing a decreased intensity demonstrates that the deposition of Ni at final step will slightly decrease the intensity of light absorption. 3.2. Photoelectrochemical Performance. The PEC performances of Cu2O-based photocathodes are shown in Fig 5 and the values were summarized in Table 2. In Fig. 5a, pristine Cu2O produced a photocurrent of only 0.85 mA/cm2 at 0 V vs. RHE. In the protection of Cu2S, the photocurrent increases to 1.13 mA/cm2 for Cu2S/Cu2O. The photocurrent for Ni/Cu2O can further increase to 1.28 mA/cm2. As can be seen, the deposition of Cu2S or Ni can both contribute to the enhancement of the activity. When the two components used together, the photocurrent can be further increase to 1.44 and 1.70 mA/cm2 for Ni-Cu2S/Cu2O and Cu2S-Ni/Cu2O, respectively, which is 1.7 and 2.0 times of it in the pristine Cu2O photocathode. The I-V curves of Cu2S-Ni/Cu2O photocathodes with different contents of Ni were also performed, showing that the optimized Ni content was at 2 min (Fig. S5). Further increases the content of Ni, the PEC performance deteriorates. There would be two consequences, when the content of Ni increased.



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Fig. 5 The PEC performance of Cu2O-based photocathodes under chopped light irradiation of a Xe-lamp with AM 1.5 G filter: (a) photocurrent density–voltage curves and (b) stability test at 0 V vs. RHE. Table 2 Summary of the PEC performance of Cu2O-based photocathodes based on Fig.5. Photocathodes

Photocurrent at 0 V vs. RHE (mA/cm2)*

Photocurrent maintained at 250 s

Photocurrent maintained at 500 s

Cu2O Cu2S/Cu2O Ni/Cu2O Ni-Cu2S/Cu2O Cu2S-Ni/Cu2O

0.85 1.13 1.28 1.44 1.70

30% 99% 12% 50% 81%

12% 42% 7% 13% 45%

*The

value was obtained from Fig.5a.

The one is the exposed Cu2O on the surface decreased after the Ni deposition, which directly leads to a decreased amount of Cu2S formation at the follow-up deposition step via ion-exchange reaction; the other is the thickness of Ni layer increased with the Ni content, which may affect the light absorption efficiency of Cu2O. These two results may lead to a decreased PEC performance as the Ni content further increases.


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In order to investigate the surface area on the activity, the ECSAs of the photoelectrodes evaluated by Cdl were measured in the non-Faradaic potential range. As shown in Fig. S6, the values of Cdl are 0.23, 0.23, 0.31, 0.25 and 0.21 mF cm-2 for Cu2O, Cu2S/Cu2O, Ni/Cu2O, Ni-Cu2S/Cu2O and Cu2S-Ni/Cu2O, respectively. All the photoelectrodes show the similar Cdl. The highest ECSA for Ni/Cu2O did not contribute to its photocurrent obviously. Thus, it is proposed that the ECSA of the photoelectrodes has little effect on PEC performances and the specific structure of each photoelectrodes should be the crucial factor.

The stability tests were performed at 0 V vs. RHE. As shown in Fig. 5b, the photocurrent of Cu2O gradually decreases to only 30% at 250 s and further decreases to 12% at 500 s. For Cu2S/Cu2O, the stability obviously increases with 99% photocurrent remained in 250 s and still 42% of the value in 500 s. Although Ni/Cu2O performs a higher photocurrent at the very beginning, it sharply decays to 12% at around 250 s and remain only 7% at 500 s. Ni-Cu2S/Cu2O exhibits a good activity at the beginning but also shows a fairly stability, the photocurrent decreases to 50% at 250 s and only 13% remains at 500 s. Cu2S-Ni/Cu2O exhibits the highest photocurrent at the beginning and also a good stability with 81% photocurrent maintaining at 250 s and 45% at 500 s. According to the PEC performance, it seems that Ni is mainly responsible for the initial activity but it suffers from serious deactivation problem. Cu2S could not remarkably enhance the activity but act as an excellent protector for stabilizing the Cu2O-based photocathode from deactivation. Besides, the deposition order is important for the PEC performence. When Ni was deposited as an outer layer,



the photocathode could have a poor stability. In a word, the co-existence of Ni and Cu2S with a proper deposition order could simultaneously enhance the activity and stability of the Cu2O photocathode. 3.3. Charge Dynamics and Mechanism of the PEC Process. An excellent PEC performance is usually derived from a good charge dynamic behavior. Many properties such as minute electronic structures brought by defects will influence the charge transport across the interface.48-51 EIS Nyquist plots can effectively reflects the charge transfer kinetics, as presented by charge‐transfer resistance (Rct), which is in parallel with a double layer capacitor (Cdl) and in series with a solution resistor (Rs) in the equivalent circuit representing the current‐flow

Fig. 6 Charge dynamic characterizations of the Cu2O-based photocathodes: (a) The EIS Nyquist plots in the dark and (b) under AM 1.5 light illumination at 0 V vs. RHE; (c) The Tafel slopes derived from linear sweep voltammetry curves under irradiation; (d) The PL spectra.


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pathway. Fig. 6a shows the EIS Nyquist plots in the dark. The fitted values of Rct are 41470, 6086, 3771, 4946, 3681 Ω for Cu2O, Cu2S/Cu2O, Ni/Cu2O, Ni-Cu2S/Cu2O, and Cu2S-Ni/Cu2O, respectively. The Cu2O alone shows the much larger radius of semi-circle, which is one order magnitude larger of the smallest one (Cu2S-Ni/Cu2O). Although the radius of Cu2O dramatically decreases under light irradiation (Fig. 6b), it still shows the largest radius among all the photocathodes, which indicates a relatively slower faradaic charge transfer at the interface mainly between the catalysts and the electrolyte. The fitted values of Rct under illumination are 1037, 753, 540, 571, 418 Ω for Cu2O, Cu2S/Cu2O, Ni/Cu2O, Ni-Cu2S/Cu2O, and Cu2S-Ni/Cu2O, respectively. The smallest arc radius is observed for Cu2S-Ni/Cu2O, which reveals a faster charge transfer under light irradiation. The EIS Nyquist results matches well with the PEC performance, indicating the Cu2S-Ni/Cu2O shows the best interfacial charge transfer behavior with the smallest Rct. Generally, the hydrogen evolution reaction (HER) steps initiate with the Volmer reaction (equation 1) followed by a Heyrovsky reaction (equation 2) or a Tafel reaction (equation 3) with Tafel slopes of 120 mV/dec, 40 mV/dec and 30 mV/dec, respectively.52,

53

Therefore, the value of the Tafel slopes can reflect the

rate-determine step (RDS) for a HER. As shown in Fig.6c, Cu2O and Cu2S/Cu2O exhibit the relatively higher Tafel slope values of 76 and 57 mV/dec, respectively. It demonstrats the RDS of the two photocathodes include both Volmer and Heyrovsky reactions. For the other photocathodes with the presence of Ni, the Tafel slope values decrease to 43, 45 and 39 mV/dec for Ni/Cu2O, Ni-Cu2S/Cu2O and Cu2S-Ni/Cu2O,



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respectively. The values are all close to 40 mV/dec, showing that the RDS of these photocathodes mainly follow the typical Heyrovsky reaction. Especially for the Cu2S-Ni/Cu2O with the smallest Tafel slope value, the RDS totally follows Heyrovsky reaction, which is the faster HER process compared with Volmer reaction. The Tafel values reveals that the presence of Ni and Cu2S can promote the RDS of the photocathodes to following Heyrovsky reaction, in which Ni is obviouly the better promoter. 2H+ + 2e− → 2H*(adsorbed H)

Volmer reaction

(1)

H* + H+ + e− → H2

Heyrovsky reaction

(2)

H* + H* → H2

Tafel reaction

(3)

PL spectroscopy is a useful and sensitive technology for revealing the charge-recombination information of photogenerated electrons and holes. Fig.6d shows PL spectra of five Cu2O-based photocathodes. All the spectra exhibit the similar profile with typical bands centered around 465 nm. The intensity of the bands decreases in the order of Cu2O > Ni/Cu2O > Ni-Cu2S/Cu2O > Cu2S/Cu2O > Cu2S-Ni/Cu2O. Normally, the PL spectrum is deriving from the charge recombination in the semiconductor. The band intensity can reflect the rate of electron-hole recombination. The results demonstrate that, Cu2O suffers from the most serious charge recombination. The present of out-layer Ni can slightly decrease the charge recombination, while Cu2S can effectively increase the charge separation efficiency. Therefore, Cu2S-Ni/Cu2O with the lowest PL intensity shows the best performance for the charge separation, in which the overlayer Cu2S plays the crucial role.


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Fig. 7 SPVM images (a)-(e) and corresponding signal distribution (f)-(j) of the photocathodes under irradiation of a 450-nm laser with light intensity of 5 mW/cm2: (a), (f) Cu2O; (b), (g) Cu2S/Cu2O; (c), (h) Ni/Cu2O; (d), (i) Ni-Cu2S/Cu2O; and (e), (j) Cu2S-Ni/Cu2O.

The overlayer Ni can promote the PEC reaction to a better pathway of Heyrovsky reaction, while Cu2S can mainly contribute to the increase of the charge separation efficiency. However, why these components can show respective function in a whole PEC process is still an enigmatic question. Surface photovoltage microscopy (SPVM) technology can directly provide the evidence for the charge separation, transportation, and recombination.54-59 Fig. 7 shows the SPVM images and corresponding signal distribution of the photocathodes. Cu2O, Cu2S/Cu2O and Ni/Cu2O show relatively narrow surface voltage distribution, the majority voltage value locate around −26, −15 and −32 mV, respectively. The surface photovoltage value on Cu2S/Cu2O shifts to a positive direction compared to that on Cu2O indicates that the photogenerated holes migrate from Cu2O to Cu2S. On the contrary, the surface photovoltage on Ni/Cu2O shows an obviously negative shift reveals that the electrons transport from Cu2O to Ni. For Ni-Cu2S/Cu2O, a relatively broader distribution of the photovoltage was obtained. It can be seen in Fig.7d that, the red points with lower photovoltage value derives



from Ni, while the green color represents Cu2S overlayer. Thus, on the surface of Ni-Cu2S/Cu2O, the photogenerated electrons and holes migrate to Ni and Cu2S, respectively. The SPVM image of Cu2S-Ni/Cu2O exhibits the more remarkable information for the surface charge distribution. In Fig. 7e, besides the basic voltage of −15 mV represents as green color, large amount of red (−60 mV) and pink (30 mV) signals appear, which reflects a bigger voltage difference within the surface of Cu2S-Ni/Cu2O photocathode. As revealed in Fig. 7b and c, the red area belongs to the electrons accumulated on Ni and the pink area derives from the holes collected on Cu2S. Compared with the even nanoparticle dispersion of Ni-Cu2S/Cu2O due to the Ni deposition, Cu2S-Ni/Cu2O exhibits a three-dimensional structure with interconnected Cu2S nanolayers. This structure may beneficial for the carrier separation by spatial separate the electron and holes and therefore lead to a bigger voltage difference. The right overlayer deposition order on Cu2S-Ni/Cu2O brings a larger voltage difference between Ni and Cu2S, that can definitely promote the surface charge separation and beneficial for the PEC performance. The SPVM images give the direct evidence for the surface charge transportation directions on Cu2S-Ni/Cu2O photocathode. Combined with the band structures, a proposed mechanism of the PEC process on Cu2S-Ni/Cu2O photocathode is illustrated in Fig. 8. First, electron-hole pairs are generated on Cu2O under light irradiation. Second, the electrons in the conduction band (CB) of Cu2O migrate to Ni because of its large work function compared with semiconductor; on the other hand, the holes in the valence band (VB) transport to Cu2S with higher VB position. Third, the hydrogen


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Fig. 8 Proposed mechanism of the PEC process on Cu2S-Ni/Cu2O photocathode. evolution reaction finally accomplishes on Ni and the holes further migrate to the substrate. As we known, Ni could efficiently collect the photogenerated electrons and perform as the hydrogen evolution active sites, therefore it can decide the RDS of the reaction in some way. On the other hand, although the CB and VB of Cu2S both locate within the Cu2O, the SPVM evidenced that the photogenerated holes on the surface would transport to Cu2S. Thus, Cu2S could actually behave as a hole acceptor, which therefore improves the surface charge separation efficiency and facilitate the reduction reaction via prolong the lifetime of electrons. As an extra electric field exists in the PEC system, the holes transport from Cu2O to Cu2S will finally back to Cu2O and reach to the substrate instead of trigger an oxidation reaction on the surface.

4. CONCLUSION In summary, we have successfully fabricated a Cu2S-Ni bi-overlayer modified Cu2O photocathode via magnetron sputtering method and ion exchange reaction. The results showed that the deposition order of the overlayer greatly affects the PEC performance. The Cu2S-Ni/Cu2O photocathode with an outer overlayer of Cu2S performed the highest photocurrent density, −1.70 mA/cm2, which is 2 times of that



for pristine Cu2O and 1.2 times of that for Ni-Cu2S/Cu2O. SPVM images of the photocathodes gave the direct evidence for the dynamics of photogenerated charges and revealed the function of each overlayers on the PEC process. It illustrates that, the photogenerated electrons migrate from Cu2O to Ni and complete the HER reaction. Therefore, Ni can decide the RDS of the reaction in some way and promotes the reaction follow Heyrovsky reaction as evidenced in the Tafel slopes. On the other hand, the holes transport from the VB band of Cu2O to Cu2S and further reach to the substrate. Thus, the overlayer Cu2S not only behaves as a protector but also plays a crucial role on the surface charge separation by efficiently collecting the holes. This work provides an efficient modification method for the construction of high performance Cu2O-based photocathode. Furthermore, it gives a direct evidence for the charge transportation pathway and reveals the function of each overlayers in the system.

ASSOCIATED CONTENT Supporting Information XRD patterns of series photoelectrodes, top-view SEM image and corresponding EDX mapping of Cu2S-Ni/Cu2O, elements distributions of series photoelectrodes analyzed by EDX spectra, UV-Vis diffuse reflectance spectra of series photoelectrodes, the I-V curves of Cu2S-Ni/Cu2O photocathodes with different content of Ni, charging current density differences plotted against scan rate of the series photoelectrodes are supplied as Supporting Information.

ACKNOWLEDGEMENTS


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This work was supported by the National Natural Science Foundation of China (21603134), Natural Science Basic Research Plan in Shaanxi Province of China (2016JQ2023), Open Fund of State Key Laboratory of Catalysis (N-17-06). The authors sincerely thank Prof. Can Li (DICP) for revising the manuscript and provide useful advices and also thank Prof. Shengzhong Liu and Dr. Zhou Yang (SNNU) for their help on the photocathode preparation using DCMS method.

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Cu2O Cathode

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