Hydrogen Evolution Catalyzed by Cu2WS4 at LiquidâLiquid Interfaces

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Hydrogen Evolution Catalyzed by Cu2WS4 at Liquid–Liquid Interfaces Faruk Ozel, Emre Aslan, Adem Sarilmaz, and Imren Hatay Patir ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 9, 2016

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ACS Applied Materials & Interfaces

Hydrogen Evolution Catalyzed by Cu2WS4 at Liquid–Liquid Interfaces Faruk Ozel1*, Emre Aslan2, Adem Sarilmaz1, Imren Hatay Patir2* 1

Karamanoglu Mehmetbey University, Department of Metallurgical and Materials

Engineering, 70200, Karaman, Turkey 2

Selcuk University, Department of Chemistry, 42030, Konya, Turkey

KEYWORDS. Cu2WS4 nanocubes, hot injection, liquid/liquid interfaces, catalytic hydrogen evolution, two-phase reactions, 4-electrode voltammetry.

ABSTRACT. The present study reports, for the first time, both a facile synthesis for ternary Cu2WS4 nanocubes, which synthesized by a simple and low cost hot-injection method, and the hydrogen evolution reaction at a biomembrane-like polarized water/1,2-dichloroethane interface catalyzed by Cu2WS4 nanocubes. The rate of hydrogen evolution reaction is increased by about 1000 times by using Cu2WS4 nanocubes when comparing an uncatalyzed reaction.

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1. Introduction Considerable efforts are currently being devoted to search for efficient electrocatalysts for the hydrogen evolution reaction (HER) and studies have recently been focus intensively on low-cost Pt-like alternatives, preferably based on Earth’s abundant elements. For example, catalysts based on tungsten or molybdenum sulfides (WS2 and MoS2), which are resembled with enzyme structures, have been extensively investigated for the photocatalytic and electrocatalytic HER[1-8] due to their stability over a wide range of pH values and relatively low overvoltage requirements to catalyze HER. The catalytic HER activities of MoS2 and WS2 materials were also enhanced by adding Co and Ni ions as promoters within their structures[2,9]. For this reason, the catalytic activity of ternary sulfides such as MWSx and MMoSx, M= Co, Ni, Cu, have been recently attracted more interest for the HER

[10-15]

. For example, it has been shown by Tran et. al. in

2013[10] that CoWSx and NiWSx are efficient and robust electrocatalysts for the HER. Moreover, Cu2MoS4 catalyst was found to be highly active and stable for the electrocatalytic and photocatalytic HER[11-13]. The photocatalytic hydrogen evolution activity of Cu2WS4 has also been reported previously[14-15]. However, there has been no report on the application of Cu2WS4 material as H2 evolution electrocatalysts. The electrocatalytic activity of catalysts for HER has been generally investigated by using the modified electrode methodology. Electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) offers also another approach to study electrocatalytic reactions without a solid working electrode, which formed between two solvents of a low mutual miscibility, such as water and 1,2-dichloroethane (DCE), each containing an electrolyte. This methodology has been applied intensely to the study of HER since 2008[16-26].


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Herein, the interfacial Galvani potential difference (with chemical or potentiostatic control) can be used to pump protons across the interface. Thus, protons transferred from the water to the organic DCE phase are reduced to hydrogen by electron donors in the organic phase, such as decamethylferrocene (DMFc)

[16]

, osmocene[17] and decamethylosmocene[18]. To date, it has

been also shown that HER at water-DCE interfaces can be efficiently catalyzed by noble metals, such as Pt and Pd[19] and non-noble metal Cu-nanoparticles formed by electrodeposition at the water/DCE interface[20], non-precious, earth abundant nanocrystalline catalysts such as molybdenum disulfide (MoS2)[21], tungsten disulfide (WS2)[22], molybdenum carbide (Mo2C), and molybdenum boride (MoB)[23], Cu2ZnSnS4, Cu2CoSnS4 nanofibers[24] and nanocomposites of Cu- and CoS-nanoparticles on the carbon nanotubes Cu/CNT[25] and CoS/CNT[26]. However, the catalytic behavior of Cu2WS4 on the HER at water/DCE interfaces still remains unexplored. There are a few reports on synthesis of colloidal Cu2WS4 nanomaterials. However, most of the routes require high pressure and long reaction time[27-29]. For example, Crossland et al. reported that Cu2WS4 was synthesized by hydrothermal route at temperatures between 110 and 250 ˚C for 48 h[27]. Jing et al. observed that Cu2WS4 was synthesized by hydrothermal route at temperatures between 160 and 240 ˚C for 72 h or at 200 ˚C for between 24 and 96 h[15]. According to the study of Li et al., Cu2WS4 was synthesized with hydrothermal method at 200 ˚C for 72 h[14]. Crossland et al. observed that Cu2MX4 (M=Mo or W; X=Se, S or S/Se) was synthesized with solvothermal method at reaction temperature between 110 and 220 ˚C for time frame from 6 to 96 h[28]. Moreover, molybdenum, instead of tungsten, has been used in different studies[11,29]. For instance, Cu2MoS4 nanosheets were synthesized with solvothermal route at temperatures between 190 and 210 ˚C for 24 h by Chen et al.[29]. Liang and Guo reported that Cu2MoS4 was synthesized by hydrothermal method at the range between 140 and 200 ˚C for 24 h[11].


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In this study, we reported a novel approach to synthesize Cu2WS4 through hot injection method to minimize the reaction time and required pressure. As mentioned above, the synthesis routes of Cu2WS4 nanomaterials are scarce. We aimed to apply a simple hot-injection method to synthesize Cu2WS4. Herein, cubic and cubic-like Cu2WS4 nanocrystals were synthesized by using hot-injection method in 30 min instead of 24 h, which is the fastest method in the literature to date, and they are called as nanocubes in this study. In addition, high-quality nanocubes can be obtained, and also composition and size distribution of the compound can be controlled with this method. Herein, we report that these Cu2WS4 nanocubes efficiently catalyze the HER by the organic reducing agent DMFc at the water/DCE biphasic systems. This catalytic reaction is investigated by four-electrode voltammetry and two-phase reactions. The catalytic effect of Cu2WS4 nanocubes as a ternary sulfide structure for HER has been firstly reported in biphasic systems. When compared with the other catalysts such as MoS2, WS2 microparticles, the HER rate considerably enhanced in the presence of the Cu2WS4 catalysts with 4.9 µmol H2 detected after only 5 min of biphasic reactions (the maximum stoichiometric amount of H2 is 5 µmol). However, in the same conditions, the formation of H2 by using MoS2 or WS2 catalysts is detected about 0.9 and 0.8 µmol as reported by our group[24-25]. The presence of Cu2WS4 catalyst results in the enhanced catalytic activity by nearly 1000-fold, in comparison with non-catalysed reaction. In the case of MoS2 or WS2 catalyst used in the same conditions, the catalytic activity is enhanced about 140 and 171-fold, respectively.


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2. Materials and Methods 2.1. Materials Ethanol, copper (II) chloride dihydrate (CuCl2.2H2O), tungsten (IV) chloride (WCl4), sulfur powder and decamethylferrocene (DMFc, 99%) were obtained from Sigma-Aldrich. Oleylamine 80-90% (OLA) was purchased from Acros Organic. Oleic acid (C18H34O2) was attained from Fisher Scientific. Hydrochloric acid (HCl, 37%), sulphuric acid (98%), 1,2-dichloroethane and toluene were procured from VWR. Tetraethylammonium chloride (TEACl, ≥ 99%) and bis(triphenylphosphoranylidene)-ammonium chloride (BACl, 97%) were provided from Aldrich. Lithium tetrakis(pentafluorophenyl)borate (LiTB) diethyletherate was supplied from TCI. Organic

electrolyte,

which

is

Bis(triphenylphosphoranylidene)ammonium

tetrakis

(pentafluorophenyl) borate (BATB), was attained in methanol:water volume rate 2:1 by metathesis about 1h stirring conditions as reported previously [20-23]. 2.2. Hot Injection Synthesis of Colloidal Cu2WS4 Nanocubes Hot injection method was utilized for the synthesis of Cu2WS4 nanocubes as well as tungstencopper salts and OLA precursors were used as in turn starting materials and capping agent, respectively. The synthesis procedure similar to previously published procedure [30]. Typically; 1 mmol CuCl2.2H2O, 0.5 mmol WCl4 were mixed with 10 ml of OLA in a three-neck flask and heated to 180 ˚C under Ar flow. When a greenish-blue solution was observed, a mixture of 2 ml OLA including 64 mg sulfur powder quickly injected into the solution under Ar flow. Then, the solution was subsequently heated up to 300 ˚C and kept for 30 minutes by stirring. After the reaction, the mixture was cooled to 120 ˚C and 2 ml oleic acid was added to reaction flask. Then the nanocubes were isolated by precipitation with 40 mL toluene:ethanol (7:1) mixtures followed


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by centrifugation at 2,500 rpm for 1 min. Finally; Cu2WS4 nanocubes were washed four times with ethanol to remove excess amount of surfactants and left to dry at 70 ºC for 2 h. 2.3. Structural and Optical Characterization The phase purity and structure were analyzed by Renishaw in via confocal-Raman spectrometer and Bruker D8 X-ray diffractometer (CuKα source, λ=1.5406 nm). The composition of the nanocubes and the elemental maps of the individual atoms were characterized by using Zeiss-Evo model scanning electron microscope (SEM). The morphology, crystalline structure and size distribution were investigated using JEOL JEM-2100F transmission electron microscope (TEM). Absorption spectra and optic band gap of nanocubes solutions were measured by Shimadzu Uv-3600 UV-Vis-NIR spectrometer. 2.4. Two Phase Reaction Experiments Two phase reaction experiments were actualized in an 11 ml total-volumed glass flask under stirring conditions as described previously by Aslan et al[22] in the absence and presence of Cu2WS4. The content of the flasks and chemical polarization of the interface by using a common anion (TB–)[22] are also illustrated in Scheme 1. After two phase reactions, the headspace gas was injected into the gas chromatography (GC) (Shimadzu GC 2010Plus; column temperature, at 50 ºC; column, RESTEK molecular sieve 5A, 30 m, 50 µm film thickness; detector, TCD (thermal conductivity detector) at 250 ºC and argon as a carrier gas). 2.5. Electrochemical Measurements at the Water/DCE Interface A four-electrode cell with the geometric area of 1.53 cm2 was used to carry out electrochemical measurements by the CHI760D potentiostat. The cell composition is


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demonstrated in Scheme 2. Herein, two platinum counter electrodes provide the current flow. Silver/silver chloride (Ag/AgCl) and silver/silver sulfate (Ag/Ag2SO4) reference electrodes located into the organic and aqueous phases, respectively, through a luggin capillary, polarize the interface by applying the external potential. Electrochemical experiments were achieved in the glove-box system with oxygen-free solutions at room temperature (25±2 °C).

3. Results and Discussion 3.1 Characterization of Cu2WS4 Nanocubes Figure 1(a) shows the powder X-ray diffraction pattern of the as-synthesized Cu2WS4 nanocubes. The diffraction peaks at 17.6˚, 18.5˚, 29.2˚, 31.3˚, 32.9˚, 35.6˚, 37.5˚, 37.9˚, 42.9˚, 46.0˚, 47.2˚, 48.1˚, 49.3˚ and 51.0˚ correspond to the respective (002), (101), (112), (103), (200), (004), (202), (211), (114), (220), (105), (204) and (301) planes of the nanostructured tetragonal type Cu2WS4 (PDF No: 01-074-3742). The most intense peak at 17.6˚ is pointed that crystalline structure of Cu2WS4 nanocubes are oriented in the (002) direction. All diffraction peaks associated with fabricated nanocubes are both intense and neat, which is the indication of tetragonal crystal structure (I-Cu2WS4, body centered space group: I-42m) with effective crystallization and freeness of impurity. The structure of I-Cu2WS4 nanocubes composed from WS4 and CuS4 tetrahedra, which are separated by a van der Waals gap[27]. At this stage, electrostatic interactions are reduced between neighboring layers due to tungsten atoms in one layer lie above metal vacancies. Herewith, the length of the primitive form (P-Cu2WS4) is doubled by a c axis[5].


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The phase purity of the nanocubes was further studied by Raman analysis. As shown in Figure 1(b), three main Raman peaks centered at 216, 277 and 451 cm-1 corresponding to the A1, B1 and E mode of Cu2WS4[31-32]. Displacement of the Mo and W atoms in the lattice structure does not cause any change in the arrangement. On account of this, the similar peaks are clearly observed on the spectrum[32]. The reason for these similar values is that all these modes involve vibrations of the sulfur atoms. Therefore, the dominant peak at 451 cm-1 is stemmed from W–S/Cu–S force constants. Moreover, no Raman peaks related to other crystalline forms were detected. This result indicates that our samples have high purity and crystallinity.

Figure 1. (a) XRD patterns of single phase Cu2WS4 nanocubes and (b) room temperature Raman spectrum of Cu2WS4 nanocubes. Figure 2(a) and (b) shows low resolution and high resolution TEM (HR-TEM) images of assynthesized I-Cu2WS4 nanocubes at different magnifications. As can be clearly seen from Figure 2(a), the I-Cu2WS4 is formed as rectangular and square cubes with the average edge length ranging from 100 to 500 nm. Although the Cu2WS4 nanocubes exhibit polydispersed population, the synthesized nanocubes display good crystalline features. HR-TEM image of an individual


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interpalanar spacing (Figure 2(b)) showed lattice fringes with a spacing of d = 0.520 nm, which corresponds to the (002) lattice plane of the body centered I-Cu2WS4 structure[27,28]. The selected area diffraction (SAED) patterns of the nanocubes presented in Figure 2(c) match with the structure of tetragonal I-Cu2WS4 and demonstrate that nanocubes have a single crystalline in nature.


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Figure 2. TEM image (a), HR-TEM image (b), SAED pattern (c) and scanning electron microscopy (SEM) – elemental mapping (EDX) images (d) of Cu2WS4 nanocubes.


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The composition analysis was carried out to determine the elemental distribution and real composition in individual nanocubes. The energy-dispersive X-Ray (EDX) elemental map for the Cu, W and S atoms is shown in Figure 2(d). The mapping images revealed that the Cu, W, S atoms were homogeneously distributed throughout the nanocubes. Moreover, the average elemental composition (%) ratio of Cu2WS4 nanocubes were found as Cu1.9W1.1S4, which reveal that the sample shows copper poor, tungsten rich stoichiometric composition. It can be seen that the atomic percentages of the Cu:W:S was close to the desired 2:1:4 ratio and consistent with the theoretical formula (see SI Figure S1 for the EDX spectra). Uv-Vis absorption spectrum of the nanocubes is shown in Figure 3. It revealed that the nanocubes exhibited broad and strong absorption in the Ultraviolet-Visible region with tails extending to the red region. Remarkable differences in the optical properties of nanostructured Cu2WS4 materials were previously observed[14]. The shift in the bandgap to higher energies can be attributed to the size effect in the small semiconductor particles[33]. Furthermore, this broad absorption peak can be attributed to the cationic (copper) vacancies (Moss-Burstein effect caused by the copper deficiency in Cu2WS4), as the composition of the synthesized Cu2WS4 nanocubes, determined by EDX, is slightly copper-deficient (Cu1.9W1.1S4) compared with stoichiometric Cu2WS4[34]. The optical band gaps of these Cu2WS4 nanocrystalline cubes were measured at around 1.74 eV which proves itself to be a promising candidate in future fabrication of costeffective, high efficiency energy conversion and energy storage applications [35-37].


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Figure 3. Uv-Vis absorption spectra of Cu2WS4 nanocubes in chlorobenzene. The inset image shows the ahⱱ2 vs photon energy (eV). 3.2. Two-Phase Reactions Controlled by a Common Ion at the Liquid/Liquid Interface The catalytic activity of Cu2WS4 was initially studied by two-phase reactions

[16]

and the

Galvani potential difference was fixed approximately to 0.58 V because of the partition of TBanion in the two phase reaction[16]. In this case, the organic protons can be reduced by the electron donor DMFc (the standard redox potential of protons and DMFc in DCE is 0.55 V and 0.04 V vs SHE)

[16]

. Also, the protons in the water phase may be reduced by organic reducing

agent DMFc in the event of the Galvani potential difference bigger than 0.55 V [26]. The reaction is written as:

(1)

DMFc

o

+ H

+, w

Cu 2WS 4     → DMFc + , o + 1/2H

2

(1)


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o and w symbolize organic and water phases, respectively.

Figure 4. (a) Photographic representation of after 5 min of reaction in the absence (flask 1) and presence (flask 2) of Cu2WS4 nanocubes by two-phase reactions (b) Gas chromatograms of these flasks for H2 evolution at the water/DCE interface after 5 min.

Scheme 1. Schematic representation of the composition of shake flask reactions for catalytic H2 evolution at the water/DCE interface.


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As illustrated in Figure 4(a), an aqueous solution (including 10 mM LiTB and 100 mM HCl) in the absence (flask 1) and presence (flask 2) of Cu2WS4 catalyst was emulsified with the organic phase (including 5 mM DMFc, as the electron donor, in DCE) under oxygen-free stirring conditions for 5 min. The amount of hydrogen accumulated from the headspace of flasks 1 and 2 was monitored by gas chromatography. A very slow HER rate is observed by only DMFc, showing 0.011 µmol of H2 formation after 5 min of reactions as demonstrated in Figure 5(a). The amount of hydrogen at maximum theoretical stoichiometric rate (5 µmol) is delimitated by the DMFc concentration (10 µmol). The HER rate increased significantly by using Cu2WS4 catalyst with 4.9 µmol H2 detected after 5 min. The rate of HER in the presence and absence of catalyst is also displayed in course of time by GC as shown in Figure 5(a). H2 evolution rate is also associated with DMFc+ (λmax = 779 nm) concentration in the DCE phase. DMFc+ peak is more increased with the presence of Cu2WS4 catalyst compared to the absence of any catalyst (see SI Figure S2 for the UV-Vis absorption spectra). The rate of HER was presumed to be first order according to the DMFc concentration as defined previously[20,38] and can be written as: υ = k[DMFc]

(2)

where k shows the apparent rate constant of the HER. The integrated rate law is emphasized as[20,38]:

[DMFc] kt = ln

0

+

[DMFc] − [DMFc ] 0

(3)

[DMFc]0 indicates DMFc concentration at the beginning. The apparent rate constants (k/min-1) were computed by the data fitting according to the first-order rate equation and determined to be


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as 0.48009 and 0.00042 in the presence and absence of Cu2WS4 catalyst, respectively (Figure 5(b)). The presence of Cu2WS4 catalyst results in the enhanced catalytic activity by about 1005fold, when compared to the absence of Cu2WS4 catalyst (Figure 5(b)). Also, the stoichiometric amount of H2 is reached almost maximum value of 5 µmol after only 5 min of biphasic reactions in the presence of Cu2WS4 catalyst. However, the H2 production in the absence of Cu2WS4 catalyst occurs much slowly for 24 hours, eventually evolving about 0.8 µmol of hydrogen[21].

Figure 5. (a) Evolution of H2 by DMFc electron donor with time (min) in the presence and absence of Cu2WS4 (b) Definition of the rate constants: plots of the integrated rate law vs. time (min) for H2 production reactions with and without Cu2WS4, exhibited 1st order kinetics. 3.3. Voltammetry Studies at the Liquid/Liquid Interface The catalytic activity of Cu2WS4 catalyst for HER by DMFc was also investigated by polarizing the liquid/liquid interface (water/DCE) potentiostatically with a four-electrode voltammetry. The four-electrode electrochemical cell and its composition (Scheme 2) were used


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to perform cyclic voltammograms (CVs; Figure 6(a-b)) under anaerobic conditions. The potential window at the interface is limited by the transfer of supporting electrolytes. The highly hydrophobic salt, BATB, is used as organic supporting electrolyte. Thus, the potential window, which shows the blank CV, is restricted by the H+ transfer at the positive potentials and SO42– transfer at the negative potentials from the aqueous phase to the organic phase (Scheme 2, x, y, z=0, 0, 0.5, respectively). In the presence of the electron donor DMFc in DCE phase (Scheme 2, x, y, z=5, 0, 0.5, respectively), a little current wave increase is observed at positive potentials, indicating that assisted proton transfer by DMFc from water to organic phase is pursued by the proton reduction to evolve H2 and DMFc+[16]. A further indication of DMFc+ is also observed by reversible transfer peak at negative potentials -0.26 V.

Figure 6. CVs performed (a) in the presence of merely DMFc (green line, x=5, y=0, z=0.5), only Cu2WS4 (dotted black line, x=0, y=0.1, z=0.5), both DMFc and Cu2WS4 in DCE and water phases, respectively (black line, x=5, y=0.1, z=0.5), by using the cell composition in Scheme 2 (b) Effect of pH on the catalytic CV response (x=5, y=0.1, z= 0.5, 5, and 50).


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Scheme 2. The electrochemical cell composition for catalytic H2 evolution at the liquid/liquid interface. The catalytic effect of Cu2WS4 catalysts was investigated by suspending these catalysts in the water phase by using DMFc electron donor in the organic phase (Scheme 2, x, y, z = 5, 0.1 and 0.5, respectively). In this case, the increased current wave (with the reduced onset potential) at positive potentials and the enlarged current wave of DMFc+ transfer at negative potentials provided evidence for the catalytic activity of Cu2WS4 catalysts on evolution of H2. The presence of merely Cu2WS4 catalyst does not lead to any change in the blank CV (Figure 6a). Moreover, as shown in Figure 6b, the observed current increase at positive potentials shifts with the aqueous pH (around 60 mV/pH) in accordance to the Nernst equation. It can be used for the facilitated ion transfer reactions taking place at liquid/liquid interfaces[23]. Voltammetry results show that evolution of H2 by DMFc occurs at a very low rate without using Cu2WS4 catalyst, which acts as an efficient H2 evolution catalyst. 4. Conclusion In this study, Cu2WS4 nanocubes have been synthesized for the first time by a simple hot injection method. Moreover, Cu2WS4 nanocrystalline cubes have been used to improve the HER activity by organic electron donor DMFc in water/DCE biphasic system. Excellent catalytic


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activity for the HER has been observed for Cu2WS4. This study opens new perspectives for the search of other ternary sulfide compounds for the evolution of H2 at the ITIES without using noble metals. ASSOCIATED CONTENT Supporting Information. EDX spectra of Cu2WS4 nanocubes and UV-Vis spectra before and after two phase reaction are in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *F.O. and I.H.P. e-mail: [email protected] and [email protected] Author Contributions I.H.P., F.O. and E.A. designed research; E.A. and A.S. performed research; I.H.P., F.O. and E.A. analyzed the data and wrote the paper. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally to this work. Funding Sources The authors would like to thank TUBITAK (The Scientific and Technological Research Council of Turkey) (215M309) for supporting this work and also Selcuk and Karamanoglu


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Mehmetbey University for Scientific Research Foundation (15401115 and 32-M-16, respectively).


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(9) Merki, D.; Vrubel, H.; Rovelli, L.; Fierro S.; Hu, X. Fe, Co, and Ni Ions Promote the Catalytic Activity of Amorphous Molybdenum Sulfide Films for Hydrogen Evolution Chem. Sci. 2012, 3, 2515–2525. (10) Tran, P. D.; Chiam, S. Y.; Boix, P. P.; Ren, Y.; Pramana, S. S.; Fize, J.; Artero, V.; Barber, J. Novel Cobalt/nickel–tungsten-sulfide Catalysts for Electrocatalytic Hydrogen Generation from Water Energy Environ. Sci. 2013, 6, 2452-2459. (11) Liang, H.; Guo, L. Synthesis, Characterization and Photocatalytic Performances of Cu2MoS4 Int. J. Hydrogen Energy 2010, 35(13), 7104–7109. (12) Yang, C.; Tran, P. D.; Boix, P. P.; Bassi, P. S.; Yantara, N.; Wong, L. H.; Barber, J. Engineering a Cu2O/NiO/Cu2MoS4 Hybrid Photocathode for H2 Generation in Water Nanoscale 2014, 6, 6506-6510. (13) Tran, P. D.; Nguyen, M.; Pramana, S. S.; Bhattacharjee, A.; Chiam, S. Y.; Fize, J.; Field, M. J.; Artero, V.; Wong, L. H.; Loob, J.; Barber, J. Copper Molybdenum Sulfide: A New Efficient Electrocatalyst for Hydrogen Production From Water Energy Environ. Sci. 2012, 5, 8912-8916. (14) Li, N.; Liu, M.; Zhou, Z.; Zhou, J.; Sun, Y.; Guo, L. Charge Separation in Facet-engineered Chalcogenide Photocatalyst: A Selective Photocorrosion Approach Nanoscale 2014, 6, 96959702. (15) Jing, D.; Liu, M.; Chen, Q.; Guo, L. Efficient Photocatalytic Hydrogen Production under Visible Light over a Novel W-based Ternary Chalcogenide Photocatalyst Prepared by a Hydrothermal Process Int. J. Hydrogen Energy 2010, 35(16), 8521-8527. (16) Hatay, I.; Su, B.; Li, F.; Partovi-Nia, R.; Vrubel, H.; Hu, X.; Ersoz, M.; Girault, H. H. Hydrogen Evolution at Liquid–Liquid Interfaces Angew. Chem., Int. Ed. 2009, 48, 5139–5142.


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Whiskers:

High-performance

Flexible All-solid-state Asymmetric

Micro-

Supercapacitors via Inkjet Printing Nano Energy 2015, 15, 303-312. (37) Pang, H.; Zhang, Y.-Z.; Run, Z.; Lai, W.-Y.; Huang W. Amorphous Nickel Pyrophosphate Microstructures for High-performance Flexible Solid-state Electrochemical Energy Storage Devices Nano Energy 2015, 17, 339-347. (38) Bian, X.; Scanlon, M. D.; Wang, S.; Liao, L.; Tang, Y.; Liu, B.; Girault, H. H. Floating Conductive Catalytic Nano-rafts at Soft Interfaces for Hydrogen Evolution Chem. Sci. 2013, 4, 3432–3441.


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Cu2WS4 nanocubes have been synthesized by hot-injection method, which is fast, simple, lowcost method, and exhibited excellent catalytic activity for the HER activity by organic electron donor DMFc at the water/DCE biphasic system. The catalytic activity of Cu2WS4 on the HER has been investigated for the first time at the water/DCE interface.


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Figure 1. (a) XRD patterns of single phase Cu2WS4 nanocubes and (b) room temperature Raman spectrum of Cu2WS4 nanocubes. 505x214mm (96 x 96 DPI)


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Figure 2. TEM image (a), HR-TEM image (b), SAED pattern (c) and scanning electron microscopy (SEM) – elemental mapping (EDX) images (d) of Cu2WS4 nanocubes. 203x293mm (96 x 96 DPI)



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Figure 3. Uv-Vis absorbtion spectra of Cu2WS4 nanocubes in chlorobenzene. The inset image shows the ahⱱ2 vs photon energy (eV). 173x148mm (192 x 192 DPI)


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Figure 4. (a) Photographic representation of after 5 min of reaction in the absence (flask 1) and presence (flask 2) of Cu2WS4 nanocubes by two-phase reactions (b) Gas chromatograms of these flasks for H2 evolution at the water/DCE interface after 5 min. 941x448mm (72 x 72 DPI)



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Figure 5. (a) Evolution of H2 by DMFc electron donor with time (min) in the presence and absence of Cu2WS4 (b) Definition of the rate constants: plots of the integrated rate law vs. time (min) for H2 production reactions with and without Cu2WS4, exhibited 1st order kinetics. 1058x500mm (72 x 72 DPI)


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Figure 6. CVs performed (a) in the presence of merely DMFc (green line, x=5, y=0, z=0.5), only Cu2WS4 (dotted black line, x=0, y=0.1, z=0.5), both DMFc and Cu2WS4 in DCE and water phases, respectively (black line, x=5, y=0.1, z=0.5), by using the cell composition in Scheme 2 (b) Effect of pH on the catalytic CV response (x=5, y=0.1, z= 0.5, 5, and 50). 1378x500mm (72 x 72 DPI)



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Scheme 1. Schematic representation of the composition of shake flask reactions for catalytic H2 evolution at the water/DCE interface. 218x115mm (72 x 72 DPI)


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Scheme 2. The electrochemical cell composition for catalytic H2 evolution at the liquid/liquid interface. 423x107mm (72 x 72 DPI)



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Graphical Abstract: Cu2WS4 nanocubes have been synthesized by hot-injection method, which is fast, simple, low-cost method, and exhibited excellent catalytic activity for the HER activity by organic electron donor DMFc at the water/DCE biphasic system. The catalytic activity of Cu2WS4 on the HER have been investigated for the first time at the water/DCE interface. 234x135mm (96 x 96 DPI)


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Hydrogen Evolution Catalyzed by Cu2WS4 at LiquidâLiquid Interfaces

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Hydrogen Evolution Catalyzed by Cu2WS4 at LiquidâLiquid Interfaces