Introduction
Characteristics of copper sulfide
- What is copper sulfide
- Crystal structure of copper sulfide
- Optical properties of copper sulfide nanocrystals and plasmonics
- Cu 2-x S nanocrystals as templates for topotactic nanoscale cation-exchange
- Colloidal Synthesis of copper sulfide nanocrystals
Thus, research on the colloidal synthesis of copper sulfide inspires many researchers for creating well-controlled copper sulfide nanocrystals in size and shape. Cu2-xS exhibits localized surface plasmon resonances (LSPR) because Cu2-xS nanocrystals have a high concentration of hole carriers caused by Cu deficiencies. The LSPR band of Cu2-xS can be tuned by addition of Cu+ ions due to the reduced hole concentration.
LSPR can also be removed by adding other metal cations (In3+).[7] It means that Cu2-xS has tunable LSPR and unique exciton properties. Colloidal Cu2-xS nanocrystals are synthesized by direct synthesis, which can be used as cation exchange reaction templates for other metal sulfides. Recently, it has been found that the crystallographic phase and chemical composition of Cu2-xS nanocrystals can be controlled by tuning the reaction temperature, molar ratio of precursors and volume ratio between ligand and solvent [13].
The ligand is attached in the thermodynamically preferred aspect, which means that the shape of the Cu2-xS nanocrystals can be controlled by choosing the ligand appropriately. Size and shape control of colloidal Cu2-xS nanocrystals. a,b) Spherical nanocrystals, (c) hexagonal nanoplatelets, (d) bistrums, (e) bipyramids, (f) ultrathin nanosheets.
Charateristics of two-dimensional materials
- Characteristics of reduced graphene oxide (rGO)
- Characteristics of molybdenum disulfide (MoS 2 )
In this structure, metal atoms are sandwiched between two layers of sulfur atoms that form a covalent bond with metal atoms. It exhibits unique anisotropic properties in electrical, chemical, mechanical, and thermal properties.[18] In the layered structure, each layer is connected by uge van der Waals interactions. MoS2 is commonly stable in '2H phase' stacking of AbA BaB with trigonal prism structure and often 3R phase.
Specifically, 1T phase which is meta-stable state is rarely found in nature stacking AbC AbC sequence and formed with octahedral geometry.(Figure 5)[19] In 2H phase, D3h. Specifically, 1T polymorph of MoS2 shows semi-metallic property caused by electronic configuration difference in Oh geometry. In general, bulk MoS2 can be exfoliated to monolayer or few layers of MoS2 by ultrasonication in various organic surfactants such as NMP (n-methylpyrrolidone) and IPA.
Monolayer MoS2 shows a direct band gap of 1.23 eV compared to an indirect band gap of 1.83 eV in the bulk state. Furthermore, the added lithium ion in the 2H phase of MoS2 can transform the metastable 1T phase with a donated electron. Interestingly, the 1T phase of MoS2 has semimetallic properties that are very useful for energy storage, electrocatalysts, and electronic devices.
Hydrogen evolution reaction of CuS
- Introduction of hydrogen evolution reaction
- Electrocatalytic activity of copper sulfide for HER
- Necessity of CuS/rGO hybrid structure
- Necessity of copper sulfide and molybdenum disulfide hybrid structure
In this respect, graphene can be the best candidates for the conduction channel of CuS because graphene has 2-D layered structure which can be templates for CuS/rGO hybrid structure. Finally, Cu2-xS/MoS2 hybrid structure made by ligand exchange of CuS with (NH4)2MoS4 can increase catalytic activity for HER. The two solutions of CuS in hexane (green) and MoS42- in DMF (brown) were mixed and sonicated in bath sonication for 5 minutes.
The simple mixture of CuS dispersed in toluene and rGO in water was stirred for 1 h in order to produce the CuS/rGO hybrid structure. As shown in Figure 10.a, the XRD measurement of CuS/rGO retains the CuS peak of the covelite phase after the hybrid with rGO. To synthesize the Cu2-xS/MoS2 hybrid structure, ligand exchange of CuS was performed with the MoS42- hybrid structure for CuS/MoS2.
The chemical composition of CuS and CuS@MoSx was investigated by X-ray photoelectron spectroscopy (XPS) analysis and inductively coupled plasma with optical emission spectrometer (ICP-OES). After the ligand exchange reaction, the onset potential and Tafel slope of CuS@MoSx is lower (344 mV and 11.8 mV/dec) than original CuS. To fabricate the Cu2-xS/MoS2 heterostructure, an annealing experiment of CuS@MoSx was performed after ligand exchange.
The powder of CuS@MoSx was placed on an alumina boat and heated under vacuum at 400 oC and 600 oC, respectively, for 3 h. The covellite phase of CuS nanoplatelets with size 90 nm and 200 nm was successfully prepared using colloidal synthetic method. The elevated and activated surface of CuS@MoSx exhibits greatly enhanced catalytic activity in HER.
In addition, the annealing treatment of CuS@MoSx could produce a heterostructure (Cu1.96S/MoS2) and an alloy structure (Cu1.96S/Cu2Mo6S8). Liu, L., et al., Phase transformations of copper sulfide nanocrystals: Towards highly efficient quantum dot-sensitized solar cells. Zhang, J., et al., Photocatalytic visible-light H(2)-producing activity of porous CuS/ZnS nanosheets based on photoinduced interfacial charge transfer.
Experimental section
Materials
Experimental procedure
- Synthesis of 90 nm covellite (CuS) nanoplatelt
- Synthesis of 200 nm covellite (CuS) nanoplatelet
- Synthesis of CuS/rGO hybrid structure
- Synthesis of CuS@MoS 2 hybrid structure
- Electrochemical measurement for HER
CuS dispersed in toluene (1 mg/mL) and rGO dispersed in water (0.5 mg/mL) were prepared. The MoS 4 2 solution was prepared by sonication of 10 mg (NH 4 ) 2 MoS 4 in 40 mL DMF, and clear dispersed solution was obtained by extraction of the upper solution. The CuS solution became clearly transparent and was carefully obtained and centrifuged at 8500 rpm for 20 min.
Platinum wires were used as counter electrode and 3 M Ag/AgCl (Pine Research Instrument) electrode as reference electrode. The electrocatalyst materials were prepared by mixing 1 mg of catalyst powder, 100 μL of deionized water, 25 μL of ethanol, and 10 μL of Nafion in a 5 mL vial, followed by sonication for 20 min. The prepared ink was poured onto a glassy carbon electrode, and the amount of material on the electrode was 400 μg/cm2.
Characterization
Then, small rGO deteriorates the catalytic activity of CuS because rGO template makes aggregation of CuS that can cover the active site. In the visible region, the excitonic absorption peak of CuS@MoSx becomes higher than original CuS, which means that the band gap became smaller due to band alignment of CuS@MoSx. Morales-Garcia, A., et al., First-principles calculations and topological analysis of the electron density of covellite (CuS).
Ha, D.H., et al., Solid-solid phase transformations induced through cation exchange and strain in 2D heterostructured copper sulfide nanocrystals. Yuan, K.D., et al., Fabrication and microstructure of p-type transparent conductive CuS thin film and its application in dye-sensitized solar cells. Midda, S., et al., Ab initio and density functional study of spectroscopic properties of CuO and CuS.
Yang, J., et al., Two-dimensional hybrid nanosheets of tungsten disulfide and reduced graphene oxide as catalysts for enhanced hydrogen generation. Sun, X., et al., Engineering interfaces in two-dimensional heterostructures: Towards an advanced catalyst for Ullmann junctions. Du, Y., et al., A general method for the large-scale synthesis of uniform ultrathin metal sulfide nanocrystals.
Ahn, C., et al., Low-temperature synthesis of large-scale molybdenum disulfide thin films directly on a plastic substrate using plasma-enhanced chemical vapor deposition.
Result and discussion
Synthsis of CuS nanoplatelet
Sulfur solution dissolved in OLA was rapidly transferred to copper solution to simultaneously nucleate Cu thiolate complex. The ordered copper thiolate complex acts as a precursor for growing CuS nanosheets.[33] As a result, ∼90 nm hexagonal nanosheets were self-assembled into the domino-like superstructures through face-to-face stacking along the c -axis. Not only SEM image but also relatively high intensity at (006) plane shows stacking of ordered nanosheets. Figure 8.b)[36] The localized surface plasmonic resonance caused by confinement effect of small crystal size shows absorption band around 1300 nm.
The distance of the lamellar structure of the copper thiolate complex increased with the addition of OTA.[35] As a result, the copper thiolate grows in the direction of the plane to form hexagonal nanoplatelets. As shown in Figure 9.a, the agglomerated CuS nanoplatelets were agglomerated, and the high intensity of the (006) plane caused by the ordered agglomeration was also confirmed as shown in Figure 9.c.
CuS/rGO hybrid structure for HER
Ligand exchange of copper sulfide (CuS) with tetrathiomolybdate (MoS 4 2- )
Synthesis of Cu 1.96 S/MoS 2 hybrid structure by annealing of CuS@MoS x
Jeunghee Park, et al., In situ growth of copper sulfide nanocrystals on multiwalled carbon nanotubes and their application as novel solar cell and amperometric glucose sensor materials.
Conclusion
