Ni3S2 Nanosheet Flowers Decorated with CdS Quantum Dots as a

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Ni3S2 Nanosheet Flowers Decorated with CdS Quantum Dots as a Highly Active Electrocatalysis Electrode for Synergistic Water Splitting Shanqing Qu, Jun Huang, Jinsong Yu, Guangliang Chen, Wei Hu, Mengmeng Yin, Rui Zhang, Sijun Chu, and Chaorong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06377 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

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ACS Applied Materials & Interfaces

Ni3S2 Nanosheet Flowers Decorated with CdS Quantum Dots as a Highly Active Electrocatalysis Electrode for Synergistic Water Splitting Shanqing Qu†§, Jun Huang‡§, Jinsong Yu#, Guangliang Chen†⋇, Wei Hu†, Mengmeng Yin†, Rui Zhang†, Sijun Chu†,Chaorong Li† †

Key Laboratory of Advanced Textile Materials and Manufacturing Technology and

Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China ‡

College of Physics and Electronic Information, Gannan Normal University,

Ganzhou, Jiangxi 341000, P. R. China #

Department of Chemistry and Chemical Engineering, University of New Haven,

300 Boston Post Road, West Haven, CT 06516, USA

Abstract A facile and effective strategy for fabricating a three-dimensionally (3-D) structured nanocomposite catalyst based on nonprecious metals for water splitting in alkaline electrolyzers is reported in this paper. This nanocomposite catalyst consists of the CdS quantum dots (QDs) decorated Ni3S2 nanosheet flowers deposited on the plasma-treated nickel foam (PNF). The NiO formed during the plasma treatment is shown to play an important role for pushing the hydrogen and oxygen evolution reactions (HER and OER) in alkaline media. The enhanced exposure of active sites on the nano-petalages results in superior catalytic performance for promoting HER and OER in alkaline electrolyzers. Specifically, a current density of 10 mA cm-2 can be achieved for the HER with a 121 mV overpotential when the working electrode based on the 1 mM CdS/Ni3S2/PNF catalyst is employed in 1 M KOH. The corresponding Tafel slope is 110 mV/decade. For the OER, the onset potential can be as low as 1.25 V vs reversible hydrogen electrode (RHE) reference electrode, which is substantially lower than the commercial IrO2 catalyst (∼1.47 V). This nano-structured catalyst has excellent long-term stability, and the linear scan voltammogram (LSV) curves of the 1

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HER and OER in 1 M KOH solution show negligible decay after undergoing 104 cycles of cyclic voltammogram. The nanocomposite material developed in this study is an ideal candidate as a catalyst for splitting water in alkaline media with relatively low overpotentials at reasonably high current densities (≥100 mA cm-2).

Keywords: eletrocatalysis, water splitting, Quantum dots, Nanosheet flower, plasma treatment, nanocomposite catalyst

1. Introduction Facing the challenges of the energy crisis and environment pollution resulting from fossil fuel utilization, many research communities1-4 were triggered to find an abundant, everlasting, and green energy. Among the various green energy technologies, H2 based fuel cells are particularly attractive because of the high efficiency and minimum environmental impact.5, 6 H2 based fuel cells typically require high pure of H2 fuel because of the poisoning effects on the Pt based anode catalysts by the CO and sulfur species (such as H2S) that are present in the H2 fuels generated via typical processes such as auto-reform. Fortunately, electrochemical or photoelectrochemical water electrolysis can produce ultrapure H2 for this purpose. The electrocatalytic water splitting typically involves two half-cell reactions, namely the hydrogen and oxygen evolution reactions. Depending on the medium that is used, two different mechanisms have been widely accepted. Briefly, when an acidic medium is present: HER: 2 + 2 → and OER: 2 → + 4 + 4 , respectively. When an alkaline medium is present: HER: 2 + 2 → + 2

and

OER: 4 → +

2 + 4 , respectively. One of the main challenges for electrocatalytic water splitting, however, is to improve the overall energy efficiency as substantial amount of energy would have to be consumed by the electrolysis process. While photovoltaic or photoelectrochemical approaches can be employed and the efficiency of sunlight-driven water splitting nearly reached 12.3% for generating H2,1 widespread implementation of these approaches is still hampered by the high expenses of the necessary apparatus. 2


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Currently, Pt-based catalysts remain the most efficient choices for the HER,7 while Ir oxide remains the benchmark electrocatalyst for the OER.8 However, the scarcity in the available Pt or Ir elements has rendered them prohibitively expensive. Apparently, cheaper catalysts for the HER and OER are highly desirable and the development of such catalysts remains a crucial task when practical implementation of H2 based fuel cells is concerned. 9 Because of such considerations, substantial efforts have been devoted to developing nanostructured non-noble catalysts based on transition metals, such as transition metal sulfides,8,10-13 nitrides,14 and phosphides15 for the HER or the OER in acidic media. Unfortunately, their performances are far from satisfactory when compared with the Pt based catalysts, mainly due to their poor electrical conductive properties. Another challenge related to the development of nanostructured non-noble catalyst based electrodes is associated with the immobilization of the nanomaterials, whose conventional protocol involves the utilization of a polymer binder.16,17 The polymer binder not only renders many of the active sites of the catalysts inaccessible and thus useless, but also typically hinders the electron transport, and thus reduces the overall electrocatalytic activity of such electrodes towards water splitting applications.10 While carbon based nanomaterials such as carbon nanotube-reduced graphene oxide (CNT-rGO)18 and one-dimensional (1D) electrospun carbon nanofibers (CNFs)19 have been evaluated and have shown improved electrode conductivities, the complexity in the electrode fabrication process and the corresponding high cost have determined that their practical applications are still in the infancy stage. In addition, such electrodes typically underperform when alkaline electrolyzers are present, which is a more preferable environment when water splitting is concerned. Recent studies indicate that nickel sulfide/phosphide catalysts are more economically viable options for water splitting in strong alkaline electrolyte solutions than in acidic or neutral media. For example, Sun et al. reported a three-dimensional, bifunctional electrocatalyst based on hierarchically porous urchin-like Ni2P microsphere superstructures anchored on nickel foam (Ni2P/Ni/NF).20 Their results 3



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showed that in 1.0 M KOH solution a current density of 10 mA cm-2 can be achieved with a low overpotential of -98 mV for HER or 200 mV for OER. Feng et al. claimed that the performance for both the HER and OER in an alkaline medium can be improved if the {2 10} high-index facets of the Ni3S2/NF can be exposed and accessed.8 Despite these exciting progresses, however, the overpotentials of most of the reported bifunctional electrocatalysts are still relatively higher than those of the noble metal based catalysts. Should the catalytic performances of such bifunctional catalysts be improved, they would become more attractive when compared with the noble metal catalysts. One approach is to further increase the number of active sites by altering the nanocomposite structure, and another route is to tune the Fermi levels of the HER and OER catalysts by dotting with quantum dots (QDs).21 Based on such considerations, for the first time, the CdS QDs decorated, hydrangea-like flowers assembled with the hexagonal Ni3S2 nanosheets were constructed on low purity nickel foam (NF) instead of high purity NF (99.99%, high purity NF is quite expensive).20 The NF is an important substrate for the fabrication of HER and OER catalysts in alkaline electrolyte.22,

23

The low purity NF was pretreated by a dielectric barrier

discharge (DBD) plasma under ambient conditions. The density states of the Ni3S2 nanosheet can be tuned by the CdS QDs and thus possess better catalytic capability towards HER and OER under the visible light. Experimental results showed that the heterogeneous electrodes fabricated in this work exhibited high catalytic efficiencies towards both HER and OER and ultra-stable electrocatalytic activities under visible light for continuous operations. While it is not presented in this work, the presence of visible light helps to promote the generation of electrically conductive species at the surface of the nanocomposite catalyst electrodes and in turn improves the overall catalytic efficiency for the water splitting.

2.Experimental section 2.1. Chemicals and Reagents Ni foam (NF) (thickness: 1.5 mm, bulk density) was purchased from Kunshan Jiayisheng Electronics Co., Ltd. Acetone, ethyl alcohol, nitric acid (HNO3), and 4


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hydrochloric

acid

(HCl),

Cadmium

acetate

dehydrate

(C2H6CdO4•2H2O),

mercaptoacetic acid (C2H4O2S), thiourea (H2NCSNH2), potassium dihydrogen phosphate (KH2PO4), dipotassium phosphate (K2HPO4), sodium sulfide (Na2S•5H2O) and Nafion (5 wt%) were purchased from the Hangzhou Mike Chemical Instrument Co., Ltd. In order to ensure the quality of the prepared materials, all the used reagents were analytical grade and the water used in the sample fabricationg processes was ultra-pure water (deionized water). 2.2. Synthesis of Ni3S2/PNF The method for fabricating Ni3S2/PNF catalyst is similar with that reported in the reference.8 Firstly, a piece of cutted nickel foam (NF) (10 × 30 mm2) was cleaned by ultrasonically with acetone (15 mL) and followed with 3 M hydrochloric acid (15 mL) for 10 min, respectively. Subsequently, it was washed with the mixed solution of water and ethanol (V%, 1:1) for three times. Then, the cleaned nickel foam was placed into the oven at 50 ℃ until it was sufficiently dry, and the both sides were treated with a dielectric barrier discharge (DBD) plasma for 0, 5, and 10 min, repectively. The pretreated nickel foam (PNF) and 20 mL of 1.445 mM thiourea solution

were

transferred

into

a

30

mL

stainless

autoclave

with

a

poly(tetrafluoroethylene) (PTFE) container and maintained at 150 ℃ for different reacting time. The as-prepared material was rinsed with ethanol and deionized water (1:1) solution for three times. Finally, the so-obtained product (Ni3S2/PNF) was dried in vacuum for 10 h at room temperature. 2.3. Decorating CdS quantum dots (QDs) onto Ni3S2/PNF nanosheet CdS QDs were prepared via a hydrothermal approach. In this process, 1 mM Cd(CH3COO)2·2H2O and 1 mM Na2S were dissolved into 30 mL deionized water, and 50 µL of thioglycolic acid was then introduced into the above solution while being stirred. Next, the Ni3S2/PNF samples were added to a 30 mL stainless autoclave with a poly(tetrafluoroethylene) (PTFE) container containing the former mixed solution. The autoclave was sealed and maintained at 160 ℃ for 10 h. Subsequently, 5



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the autoclave was naturally cooled down to room temperature, and then the samples were washed with ethyl alcohol and deionized water three times before being oven-dried at 50 ℃ for 10 h, and thus the prepared sample was named 1.0 mM CdS/Ni3S2/PNF. The fabrication of 3.0 or 5.0 mM CdS/Ni3S2/PNF followed the same reaction procedure as mentioned above. It should be noted that the pretreatment time of the NF by the DBD plasma used for the CdS QDs deposition was 10 minutes, and the sample used for HER and OER is the 1 mM CdS/Ni3S2/PNF catalyst, unless otherwise specified. 2.4. Characterization The morphologies of the CdS/Ni3S2/PNF nanocomposite catalysts fabricated under various conditions were characterized by using a JSM-6700F field-emission scanning electron microscopy (FE-SEM, JEOL, Japan), and the scratched powders of CdS/Ni3S2/PNF were analyzed with a JSM-2100 transmission electron microscopy (TEM, JEOL, Japan). The profile of Ni3S2 nanosheets were examined by using an atomic force microscopy (AFM, XE-100E, Korea). X-ray diffraction (XRD, Thermo Fisher Scientific, USA) was utilized to examine the crystalline phases of the CdS/Ni3S2/PNF nanocomposite with a Cu Kα radiation source at 35 kV. The surface chemical composition of the CdS/Ni3S2 was investigated by X-ray photoelectron spectroscopy (XPS, K-Alpha, USA) with the X-ray source operating at 300 W. Determination of the mass of Ni3S2 nanosheets grown on the nickel foam [m(Ni3S2)] was carried out as follows. First, the weight increment (x mg) of nickel foam can be directly obtained by comparing the weight of nickel foam before and after the synthesis of Ni3S2/PNF. Based on this chemical composition of Ni3S2, the mass of Ni3S2 nanosheets [m(Ni3S2)] was then calculated according to the equation: m(Ni3S2) = x mg * (MNi3S2/2MS) = x mg * (240/64) = 3.75x mg, where M is the molecular weight or atomic weight. The loading amount of Ni3S2 on NF was about 1.75 mg/cm2. The mass of CdS quantum dots deposited on the Ni3S2/PNF [m(CdS)] was similarly calculated. 2.5. Electrochemical Characterization: 6


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The electrochemical experiments were carried out with a CHI750E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd.). All electrochemical measurements were performed with a three-electrode system, in which PNF, Ni3S2/PNF or CdS/Ni3S2/PNF was used directly as the working electrode, and the working surface area was about 1 cm2. For electrocatalytic HER and OER in alkaline or neutral media, a Hg/HgO electrode and a platinum electrode were used as the reference electrode and counter electrode, respectively. All tests were carried out at room temperature and atmospheric pressure. The measurements were performed in a pH 7 corrected with 0.1 M KH2PO4/K2HPO4 solution, pH 10 corrected with 0.1 M KH2PO4/K2HPO4 and 1 M KOH solution, and 1 M KOH solution (pH14), respectively. We carried out electrochemical tests for cyclic voltammetry curve (CV), and linear scan voltammogram (LSV) with a scan rate of 20 mV s-1 in Ar-saturated 1 M KOH medium. The obtained LSV and Tafel curve were used to analyze the HER and OER performances, and all the potential values measured were calibrated by using the following equation: E. vs RHE = E. vs Hg/HgO +0.095 + 0.059pH. 8 In order to prove the electrocatalytic stability of fabricated catalysts with a commercial battery (1.5V), the chronoamperometric curves of HER and OER for samples were measured with -1.5 and 1.5 V, respectively. The electrochemical impedance spectroscopy (EIS) at frequencies ranging from 0.01 to 105 Hz was used to analyze the hydrogen evolution activity. For comparative purposes, the 20 wt % Pt/C powders loaded PNF electrode (1 cm2) was prepared with the same loading amount (1.75 mg/cm2) as CdS/Ni3S2/PNF, and the typical procedure was described as follows: (1) 7 mg of Pt/C powders and 10 µL 5 wt% Nafion solution were first dispersed in 1 mL of isopropanol with 60 min sonication to form a homogeneous ink, (2) 250 µL of this solution was pipetted onto PNF surface and dried sample in air for 2 hours, and (3) 50 µL of 0.5 wt% Nafion solution dissolved in isopropanol was drop-cast on the top of PNF electrode to protect the film. According to the methods reported previously,12, 24 the double-layer capacitance (Cdl) of catalysts through collecting cyclic voltammograms (CVs) in a non-Faradaic region was used to indicate electrochemically active surface area (ECSA). Cdl of 7



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samples were derived from a sequence of CV curves with different scan rates (20-200 mV s-1, step: 20 mV s-1). By plotting the difference in current density (J) between the anode and cathode (Janodic − Jcathodic) at a certain potiental versus RHE against the scan rate, a linear slope was calculated as Cdl. 22 In addition, we collected the evolved O2 and H2 gases with a water drainage method, and then calculated the moles of gases generated from the reactions with an ideal gas law. The relationship between the Faradic efficiency of CdS/Ni3S2/PNF and the gas amount was defined according to reference 8. The photoelectrocatalytic influence of the CdS QDs on the HER efficiency was performed by an visible light source (CEL-HXF300/CEL-HXUV300, Beijing Zhongjiaojinyuan Co. Ltd.).

3. Results and discussion Thanks to the earth abundance and porous three-dimensional structure, the nickel foam (NF) is an important substrate for fabricating catalyst for HRE and ORE in the alkaline electrolyte.20,

22-25

However, nanoscale engineering of NF for efficiently

spliting water is still on the way. In this work, the NF was pretreated by a simple DBD system (see supporting information, Figure S1) and many microsized trenches were introduced on the NF framework, as shown in Figure S2. Moreover, the NiO compound was formed on the NF surface by the air-DBD plasma, and the cubic crystalline NiO phase was proven by the occurance of crystal planes in XRD ( Figure.S3) and Ni2+ peak in XPS (Figure S4).26 Under direct sulfurization of the plasma pretreated nickel foam (PNF) using thiourea, as the source of sulfur, the Ni3S2 nanosheets can be grown and anchored onto the PNF surface. Such a facile processing approach eliminated the need of a polymer binder which conventially was used for immobilizing the nanosized catalyst particles. The sulfurized PNF could undergo substantial deforming without suffering from any apparent structural damage (Figure 1(a)). Undergoing a similar sulfurization process, the Ni3S2 nanosheets were formed rather homogeneously on the NF (vertically) with the nanosheets spatially interconnected into a uniform network structure (Figure 1(b)). When the NF was treated by DBD plasma for 5 minutes, the hexagonal Ni3S2 nanosheets would grow 8


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vertically over the 3D framework of PNF, and some Ni3S2 nanosheets assembled and formed void spaces (Figure 1(c)). To our surprise, the Ni3S2 nanosheets would form highly ordered structure similar to hydrangea-like flowers with a size around 8.0-15.0 µm on the PNF surface (Figure 1(d)). Such porous, scaffold structure is beneficial for exposing substantially more active sites, resulting in the significant improvement of the catalytic performance for HER or OER.27 Figures 1(e) and 1(f) show the corresponding elemental mapping images of the Ni and the S elements of the Ni3S2/PNF, respectively, demonstrating that both Ni and S were uniformly distributed, especially on the Ni3S2 flower surfaces. It should be noted that the hexagonal phase of the Ni3S2 nanosheets was verified by the XRD pattern of the as-obtained material (Figure S5(a)), in which all the peaks can be well-indexed to the heazlewoodite phase of Ni3S2 with the R32 space group and the diffraction peaks at 44.5°, 51.8°, and 76.5° correspond to the metallic Ni (JCPDS No.65−2865). No other peak, such as NiS or NiS2, was observed, implying that the surface Ni atoms of the PNF were sulfurized to the Ni3S2 entirely. In order to better understand the growth process of the Ni3S2 nanosheets, the effect of the sulfurization reaction time on the morphology of Ni3S2 nanosheets was also investigated (Figure S6). Experimental results show that as the sulfurization reaction time increased, the Ni3S2 nanosheets formed horizontal layers instead, hindering the accessibility of the active sites to the reactive species (see Figure S6(c)). Therefore, longer reaction time (7 h) was not considered beneficial. The overlapped nano-plates had an average thickness of about 22.0 nm (Figure S6(d)) and may be deprived during the HER or the OER process. Interestingly, the hydrangea-like flowers consisted of the Ni3S2 nanosheets were formed readily on the PNF samples, and they did not appear when the original NF substrates were used. In order to obtain an explanation for such a phenomenon and gain better understanding of the possible growth mechanism, the images of the Ni3S2 flowers with different quantities of the hexagonal petalages were captured in FE-SEM images (Figure S7), and these images suggested that the formation of Ni3S2 flowers may be initiated at the micro-trenches on the NF introduced by the plasma treatment (centers of growth), where the growth 9



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rate of Ni3S2 nanosheets far exceeded those of the neighboring places. In fact, many Ni3S2 crystal nuclei could be distributed homogeneously on the surfaces of the microtrenches. Previously formed layer of the Ni3S2 nanosheets appeared to be pushed out by the newly formed ones because of the limited space of the micro-trenches. Although the Ni3S2 flower does not fit the Fibonacci number pattern, the close-packed Ni3S2 spherules follow the natural principle of the spherical tendency to minimize the strain energy.28

Figure1. (a) The flex property of sulfurized PNF. (b-d): top-view SEM images of Ni3S2 on the NF substrates treated by DBD plasma with different time (b: 0 min, c: 5 min and d: 10 min). (e) and (f): Ni and S mapping at selected Ni3S2 flower in image (d). The characteristic morphology and structure of CdS/Ni3S2/PNF nanocomposite catalyst are shown in Figure 2(a), and the undeformed Ni3S2 flowers indicate the Ni3S2/NF nanosheets were very stable even undergoing a second hydrothermal process for fabricating CdS. Under a higher resolution FE-SEM, it can be seen that the CdS QDs were uniformly distributed on the hexagonal Ni3S2 nanosheets with a average length around 120 nm. In TEM image (Figure 2(c)), two kinds of lattice fringes were observed, and the interplanar distances were about 0.23 and o.24 nm, corresponding to the (021) and (003) crystallographic planes of hexagonal Ni3S2 phase, respectively. Moreover, the observed angle between the (021) and (003) facets was about 71°, and it indicates that the maninly exposed facet of Ni3S2 nanosheet was {210}.8 In HRTEM images (Figure 2(d) and 2(e)), the size of CdS QDs decorated on Ni3S2 nanosheets was approximately 8 nm, and the d-spacing of lattice fringes was 10


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0.33 nm, assigning to the CdS (111) plane.29-31 Meanwhile, the facets of (021) for Ni3S2 and (111) for CdS were also confirmed by fast Fourier transform image (Figure 2(f)), and the heterostructured CdS/Ni3S2 catalyst may be show a higher catalytic activity for water spliting under the bifunction of visible light and electric battery.32,8 The crystalline structure of the CdS/Ni3S2/PNF, as shown in Figure S5(b), exhibited little variation from that of Ni3S2/PNF except for the region between 25° and 29° (inset of Figure S5(b)). In this region, three diffraction peaks at 24.8°, 26.5°and 28.2° could be observed, which were attributed to the (100), (002) and (101) crystallographic planes of the hexagonal phase of the CdS (JCPDS card no.41-1049).33,34 Figures 2(g)– 2(i) exhibit the corresponding mapping of Ni, S, and Cd atoms on the surface of CdS/Ni3S2/PNF nanocomposite catalyst, and the distribution of these elements were fairly uniform, which was consistent with that analyzed with XPS (Table S1).

Figure 2. (a) Low- and (b) high-resolution SEM images of CdS/Ni3S2. TEM images of (c) Ni3S2/PNF and (d) CdS/Ni3S2/PNF. Insets of (e) and (f) in (d) are HRTEM and the corresponding FFT of CdS/Ni3S2/PNF, and the images of (g-i) are the element distribution of Ni, S, and Cd on the selected flower in photograph (a). 11



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The electrocatalytic activity of Ni3S2/PNF dotted with different content of CdS for HER was investigated in a typical three-electrode system using CdS/Ni3S2/PNF as the working electrode directly. For comparison, the electochemcial characterization of NF, NF treated for 10 min by DBD plasma (PNF), Ni3S2/PNF and Pt/C in different media were also measured. The overpotentials of the NF and the PNF at a current density of 10.0 mA cm-2 were 252 and 225 mV (Figure S8(a)), significantly higher than those for the Ni3S2/PNF with different plasma pretreatment time (Figure S8(b)). The Ni3S2/PNF exhibited a high catalytic activity toward the HER in alkaline media (black line in Figure 3(a)), and a current density of 10 mA cm-2 could be achieved at an overpotential of 121 mV, lower than that for the NiS nanopaticles,35 the NiOH nanosheet,22 the NiFe(OH)x,1 and many other electrocatalysts (see Table S2). The LSV curve of 1 mM CdS/Ni3S2/PNF was almost identical to that of Ni3S2/PNF at low current densities (40 mA cm-2), except the 3 and 5 mM CdS/Ni3S2/PNF catalysts. Such an observation could be attributed to the possibility that depositing higher content of the CdS could destroy the microstructure of the Ni3S2 nanosheets during the second hydrothermal processing and the loss of the Ni3S2 nanosheets would then reduce the stability of the nanocomposite catalyst and thus the overall electrocatalytic performances. In fact, fine black powders could be found to appear in the electrolyte solution during the water splitting process, and these powders were confirmed to be CdS/Ni3S2. Figure 3(b) shows the Tafel slopes of samples presented in Figure 3(a), and the 3 and 5mM CdS/Ni3S2/PNF samples exhibited higher Tafel slopes than that of Ni3S2/PNF. Compared to Ni3S2/PNF (118 mV/decade), the Tafel slope of 1 mM CdS/Ni3S2/PNF sample was only 110 mV/decade, indicating much improved catalytic activity for the HER at a low current density.19 The kinetics improvement of Ni3S2 nanosheets decorating by CdS quantum dots for HER may be attributed to the reduced conduction band minimum of CdS/Ni3S2/PNF heterostructure31 or the fact that the increased S sites (Table S1) on the (210) surface reside at the step edges and thus have a lower coordination number and a less crowded local environment.8 It should be noted that 12


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the HER followed the Volmer−Heyrovsky mechanism, which comprised a fast discharge of one proton and a slow combination of the discharged proton with an additional proton [H2O(l)+e-1+⋇⇌H*+OH-(aq), H*+H2O(l)+e-⇌H2+OH-(aq)+⋇, H*+H*⇌H2(g)+2*].20,36 Therefore, the pH value of the electrolyzer should affect the LSV measurement outcomes. Such influences were illustrated by Figure S9(a), where the LSV curves were measured for the CdS/Ni3S2/PNF in three electrolyzers (pH: 7, 10, and 14), and the experimental data exhibited that at higher pH value indicating a higher transfer rate of ions generated in electrolyzer, less overpotential would be required to achieve the same current density. One key performance criterion for the HER electrochemical catalysts is the operational stability, which is illustrated by Figure 3(c). Figure 3(c) shows the chronoamperometric curves (I-t) of the HER catalyzed by the Ni3S2/PNF and the CdS/Ni3S2/PNF at a static potential of -1.50 V. Compared to the Ni3S2/PNF electrode, the 1.0 mM CdS/Ni3S2/PNF induced a higher current density (130 and 100 mA cm-2 for the CdS/Ni3S2/PNF and the Ni3S2/PNF, respectively) with much less drifting over 10 hours of electrochemical operation. In addition, the long-term durability of Ni3S2/PNF and CdS/Ni3S2/PNF electrodes was also evaluated by the LSV curves obtained after 104 CV cycles (Figure 3(d)), and the electrodes provided almost the same onset potential and current density, indicating the growth process of CdS QDs (1 mM) did not affect the long-term durability and activity of Ni3S2/PNF nanosheets toward HER. The excellent stability of the 1.0 mM CdS/Ni3S2/PNF was also observed morphologically (SEM images: insets of Figure 3(d)) and structurally (XRD: Figure S5). The catalyzed HER mechanism of 1.0 mM CdS/Ni3S2/PNF and Ni3S2/PNF was further explored through the electrochemical impedance spectroscopy (EIS), as shown in Figure 3(e). A much higher HER activity of the 1.0 mM CdS/Ni3S2/PNF was evidential from the significantly smaller diameter of the semicircle in the Nyquist plot, indicating a smaller contact resistance and a fast Faradaic process of CdS/Ni3S2/PNF. Figure 3(f) shows the amount of H2 evolved over CdS/Ni3S2/PNF in 1 M KOH for 60 min, and the hybrid catalyst afforded a high stable H2 evolution rate of 13.5 mmol/h. 13



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It should be noted that the experimental data matched well with the theoretical H2 amount, indicating a near 100% Faradaic yield during the HER in the alkline media.8 Meanwhile, the fast kinetics of HER on the CdS/Ni3S2/PNF catalyst was qualitatively illustrated by the inset of Figure 3(f), which shows the fast generation of large volume of hydrogen gas. Although the exact reason for explaining the high catalytic activity of CdS/Ni3S2/PNF for H2 generation is still unclear, the following facts should be not neglected. In brief, the surface electric resistance of alkline media (R1) is far smaller than that of origianl NF (R2), as shown in Scheme 1, and the transfer rate of interfacial electron increases rapidly when microtrenches induced by the DBD plasma on the NF surface are filled with alkline solution. Compared to the RGO-CdS-NixS system,36 the photocatalytic activity of CdS/Ni3S2/PNF (Figure S9(b) and Figure S10 ) was very marginal, and the effect of visible light on the HER mechanism of CdS/Ni3S2/PNF can be neglected.

Figure 3. (a) LSV curves of HER and (b) Tafel plots for the various catalysts derived from the data presented in (a). (c) Chronoamperometric curves of Ni3S2/PNF and CdS/Ni3S2/PNF at a static potential of -1.5 V. (d) LSV curves for Ni3S2/PNF and CdS/Ni3S2/PNF before and after 104 cycles between 0 and -0.8 V (the insets: the SEM images of CdS/Ni3S2/PNF under a HER for 104 cycles). (e) Nyquist plots (overpotential= 230 mV) of different samples. (f) Electrocatalytic efficiency of H2 14


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production over CdS/Ni3S2/PNP with a potential of -1.5 V in 1 M KOH for 60 min.

Scheme 1 The conceptual model for the different electrochemical impedance spectra (EIS) of NF treated by DBD plasma, (a) 0 min and (b)10 min. In order to investigate the chemical states of sample before and after HER process, the CdS/Ni3S2/PNF was characterized by XPS accordingly. As shown in Fig. S11 (a), the Ni 2p spectrum of Ni3S2 can be deconvoluted into four peaks. Two peaks were located at 855.8 eV( Ni 2p3/2) and 873.2eV(Ni 2p1/2 ), indicating the binding energy of Ni2+.24,34 the other two peaks at 861.8 and 879.5 eV are the satellite of NiO,37,38 which indicates the Ni3S2 nanosheets contained the NiO phase origianted from the DBD plasma pretreatment of the NF (Table S1and Figure S4), consistent with the XRD result (Figure S3). Meanwhile, the CdS-Ni3S2 exhibited coincident binding energies of 162.5 eV (2p3/2) and 163.9 eV (2p1/2) for bridging S22- (Fig. S11 (b)), implying unsaturated S atoms on Ni-S and Cd-S sites.24 In addition, the peaks of Cd 3d5/2 (405.5 eV) and Cd 3d3/2 (412.5 eV) also occurred in the CdS/Ni3S2/PNF sample (Fig. S11 (c)), implying the CdS QDs were factually deposited on the surface of the Ni3S2 nanosheets. After 104 circles of HER in 1 M KOH, the binding energies of Ni 2p(Fig. S11 (d)), S 2p (Fig. S11 (e)) and Cd 3d (Fig. S11 (f)) were almost identical to those without undergoing a HER process, indicating the CdS/Ni3S2/PNF catalyst possesses an excellent stabiltiy, consistent with what had been previously reported.8,24,39 Although the crystal structure (Fig.S5) and chemical state (Fig.S11) of CdS/Ni3S2/PNF showed no phase transformation after a longer HER process, the Ni and S atoms ratio on the CdS/Ni3S2/PNF surface changed noticeably, as shown in Table S1. Moreover, there was negligible change for the LSV curves after continuous CV scanning for 104 cycles (Fig.3d). The exact reason for such interesting observations are yet to be determined. Nonetheless, the following facts should be 15



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considered: (i) The NiO nanoislands (phase) could be formed on the NF under a function of air-DBD plasma, and these crystal phases were covered by CdS/Ni3S2 nanosheets after the sulfuration reaction. (ii) During the continuous CV scanning test, most of CdS/Ni3S2 nanosheets were exfliated (Insets of Fig.3(d)) and the active sites of NiO were exposed. Moreover, the NiC phase was incuced on the catalyst surface during the HER process (see Figure S12), and both (NiO and NiC) were believed to be highly active towards motivating for the HER and OER.40,41 As a result, there is a marginal change in the LSV curves of CdS/Ni3S2/PNF electrode before and after a long HER process, thanks to the yet to be explained synergetic effect of the CdS/Ni3S2, NiC and NiO. Similar to the HER activity evaluations, the OER activity of the 1.0 mM CdS/Ni3S2/PNF was also assessed in 1 M KOH solution using the same three-electrode system. As the LSV curves given in Figure 4(a), the PNF, CdS/Ni3S2/PNF and Ni3S2/PNF all exhibited an excellent catalytic activity toward the OER, and the earlier onset potentials for O2 generation was only 1.25 V (see Video SI), which was far lower than the RuO2 loaded on NF (1.49 V)42 and the commercial IrO2 (1.47 V).36 It should be noted that a plateau between 1.25-1.4 V occurred on the LSV curve, which may be attributed to the redox oxidation of Ni species.24 Under a similar testing conditions, however, the current density of CdS/Ni3S2/NF and Ni3S2/PNF was much greater than that of PNF under a certain of overpotential. Meanwhile, the OER activity of CdS/Ni3S2/NF was more powerful than the Ni3S2/NF after a critical potential (1.6 V). In addition, the CdS/Ni3S2/PNF afforded a current density of 10 mA cm-2 at an overpotential of 151 mV, which outperformed state-of-the-art electrolyzers utilizing Co or Ni based bifunctional electrorcatalysts for overall water splitting (Table S3). The OER Tafel slope for CdS/Ni3S2/PNF was calculated to be 174.0 mV/decade, lower than the Ni3S2/PNF (192 mV/decade) and the PNF electrodes (238 mV/decade), indicating more favorable OER kinetics for CdS/Ni3S2/PNF catalyst. Figure 4(c) presents the operational stability of the OER catalyzed by the PNF, Ni3S2/PNF, and CdS/Ni3S2/PNF in 1 M KOH solution at a static potential of 1.5 V, and a very stable current density of the OER process for over 16


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12 hours was achieved. Moreover, CdS/Ni3S2/PNF LSV curves for the first CV scanning cycle and the 104 cycles were almost identical, indicating excellent OER long-term durability. Figure 4(e) exhibts the O2 amount of CdS/Ni3S2/PNF with a potential of 1.5 V in 1M KOH, and the evolution rate of O2 was about 14.6 mmol/h. Meanwhile, the CdS/Ni3S2/PNF electrodes can be driven by a 1.5V battery for HER and OER processes in a 1M KOH solution (see Figure 4(f) and Video SII).

Figure 4. (a) LSV curves of OER for different samples. (b) Tafel slopes of different catalysts derived from the data of (a). (c) Chronoamperometric curves of PNF, Ni3S2/PNF and CdS/Ni3S2/PNF at a potential of 1.5 V. (d) LSV of CdS/Ni3S2/PNF for 104 cycles. (e)The electrocatalytic efficiency of O2 over CdS/Ni3S2/PNF with a potential of 1.5V. (f) Digital photograph of the CdS/Ni3S2/PNF electrodes driven by a 1.5V battery for bifunctional water splitting. The experimental results discussed above showed that the 1 mM CdS/Ni3S2/PNF nanocomposite catalyst showed excellent catalytic properties for both HER and OER, and such superb catalytic performance can be ascribed to the following facts: (i) Because of the intrinsic metallic character of the CdS/Ni3S2, the catalytically active 17



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sites on each nanosheet are easily accessible to electrons coming from the electrode.42 (ii) The hydrangea-like flowers constructured by CdS/Ni3S2 nanosheets largely increase the catalytic area, greatly facilitating the ion and electron diffusion, and it is proved by the calculated Cdl of CdS/Ni3S2/PNF (32.58 mF cm-2), higher than PNF ( Cdl: 22.88 mF cm-2) and Ni3S2/PNF (Cdl: 31.14 mF cm-2), as shown in Fig. S13. (iii) The cross-linked contact between CdS/Ni3S2 nanosheets and PNF facilitates interfacial electron transport, and the rapid electron transfer in the presence of CdS/Ni3S2/PNF is supported by electrochemical impedance spectroscopy (EIS, Figure 3(e)), which shows the material has much lower Faradaic impedance. (iv) The crystal plane manily exposed on the CdS/Ni3S2 nanosheets is (210), and it exhibits excellent electrocatalytic activity for HER and OER.8 (v) the γ−NiO(OH) and NiO present in the CdS/Ni3S2/PNF nanocomposite catalyst (evidential from Table S1, Figure S4 and Figure S11 are also the catalytic active species for the HER and OER,41,43,44 confirmed by the higher catalytic activity of the PNF for OER in 1 M KOH ( Red line in Figure 4(a)) as compared with other untreated NF materials.22,45

4. Conclusion In summary, we have successfully synthesized new 3D hydrangea-like flowers aggregated with CdS/Ni3S2 nanosheets as bifunctional electrocatalysts for overall water splitting by using the strain energy theory to grow the spherical CdS/Ni3S2 on PNF with a facile sulfidization process. This 3-D structured CdS/Ni3S2/PNF nanocomposite catalyst exhibits excellent catalytic activities towards both HER and OER in alkaline media. Besides the excellent catalytic activities towards HER and OER, the 1.0 mM CdS/Ni3S2/PNF catalyst also exhibits remarkable stability at a high working potentials (±1.5 V). For HER, 10 mA cm-2 current density can be achieved with a low overpotential of 121 mV using the 1 mM CdS/Ni3S2/PNF catalyst and a current density of as high as 100 mA cm-2 can be achieved at the overpotential of only 400 mV. When it serves as OER catalyst, the onset potential is only 1.25 V. The superior activity and strong stability of CdS/Ni3S2/PNF shown by the experimental results were attributed to the high electrochemical surface area, low Faradaic 18


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impedance, and the presence of the oxidized Ni(NiO). This study introduces a brand new route for producing high efficiency HER and OER catalysts based on nonprecious metals.

Supporting Information The DBD system for NF treatment, SEM images, XRD patterns, XPS spectra and cyclic voltammograms of NF, PNF and Ni3S2/PNF before and after a electrical process, the effects of pH and visible light on the PLV curves of Ni3S2/PNF and CdS/Ni3S2/PNF, the comparison of electrocatalytic activity of catalysts for HER and OER reported recently, and the videos (AVI) of HER and OER at 1.5V, and OER at 1.25V.

Author Information Corresponding Author ⋇Email:

[email protected]

Author Contributions §

S.Q. Qu and J. Huang contributed equally

Acknowledgement We thank the financial supports from National Natural Science Foundation of China (Grant Nos. 51672249, 11505032, 11665005,11547139), the Zhejiang Natural Science Foundations of China (LY16A050002), the Natural Science Foundation of Jiangxi Province (Grant Nos. 20171ACB21049, 20171BAB211012), the Science and Technology Project of Jiangxi Provincial Department of Education(GJJ150981), the Program for Innovative Research Team of Zhejiang Sci-Tech University, and the Bidding Project of Gannan Normal University (15zb05).

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Ni3S2 Nanosheet Flowers Decorated with CdS Quantum Dots as a

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