III. Result and discussion
3.4. Synthesis of Cu 1.96 S/MoS 2 hybrid structure by annealing of CuS@MoS x
To fabricate Cu2-xS/MoS2 heterostructure, annealing experiment of CuS@MoSx was conducted after ligand exchange. The poweder of CuS@MoSx was placed on alumina boat and heated up to 400 oC and 600 oC respectively for 3 hours under vacuum. As a result, CuS transformed into Cu1.96S (djurleite) and MoSx crystallize into MoS2 as annealing at 400 oC, which was confirmed by XRD.
(Figure 14.a)[40, 41] Otherwise, higher temperature as annealing at 600 oC form alloy structure of Chevrel type (Cu2Mo6S8) as shown in Figure 14.a. However, high temperature make the nanoplatelet re-aggregated at 400 oC and even the nanoplatelet morphology is even collapsed at 600 oC. (Figure 14.b,c) Although surface-volume ratio is decreased, performance of HER is improved compared to as- prepared CuS@MoSx. The formation of stable junction and heterostructure can construct more interconnection between copper sulfide and MoS2.
24
Figure 11. (a) Schematic illustration of exfoliation by ligand exchange, (b) Optical image before and after ligand exchange, (c) SEM image, (d) XRD spectrum of CuS (blue), CuS@MoSx (red, after ligand exchange).
25
Figure 12. XPS measurement. (a) Mo 3d, (b) S 2p, (c) Cu 2p, (d) N 1s, CuS (blue), CuS@MoSx (red, after ligand exchange).
26
300 350 400 450 500
Intensity (a.u.)
Wavenumber (cm-1) E12g
379 CuS@MoSx
CuS
A1g 406 MoS2
300 600 900 1200 1500 1800 2100 2400 CuS@MoSx 449
Intensity (a.u.)
Wavelength (nm)
CuS 408
decreased LSPR
-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 -20
-15 -10 -5 0
CuS CuS@MoSx
Current Density (mA/cm2)
Potential (V vs. RHE)
0.002 0.004 0.1
0.3 0.3 0.4 0.5
CuS@MoSx 88.2 mV/dec
Potential (V vs. RHE)
log | j(mA/cm2) | CuS 100 mV/dec
a b
c d
Figure 13. (a) Raman spectrum, (b) UV-vis-NIR absorption spectrum, (c) linear sweep for HER (d) Tafel slope of CuS (blue) and CuS@MoSx (red, after ligand exchange).
27
1 μm 600 oC
1 μm 400 oC
10 20 30 40 50 60 70 80
Cu2Mo6S8 (121) (003) (021)
CuS (JCPDS 00-006-0464) Cu2Mo
6S
8(JCPDS 01-081-5171)
As prepared 400 oC 600 oC
Intensity (a.u.)
2 Theta ()
Cu1.96S (JCPDS 00-012-0224) MoS2 (002)
-
Cu1.96S/MoS2
Cu1.96S/Cu2Mo6S8
CuS@MoSx
a b
c
0.002 0.004 0.20
0.25 0.30 0.35 0.40
102.5 mV/dec
93.3 mV/dec As prepared
400 oC for 3 h 600 oC for 3 h
Potential (V vs. RHE)
log | j(mA/cm2) | 93.4 mV/dec
-0.6 -0.4 -0.2 0.0
-20 -15 -10 -5 0
Current Density (mA/cm2 )
Potential (V vs. RHE) As prepared 400 oC for 3 h 600 oC for 3 h
f e
Figure 14. Annealing of CuS@MoSx (a) XRD data, (b, c) SEM image of annealing at 600 oC and 400 oC (e) Linear sweep voltammetry (f) Tafel slope of as-prepared CuS@MoSx (black), annealing at 400 oC (wine) and 600 oC (orange).
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4. Conclusion
The covellite phase of CuS nanoplatelet that has 90 nm and 200 nm dimension were successfully prepared by colloidal synthetic method. This CuS was hybridized with rGO and improved HER property because conducting channel of rGO increased conductivity. Otherwise, ligand exchange of CuS with (NH4)2MoS4 under sonication synthesized CuS@MoSx. This method could disassemble stacked nanoplatelets. The increased and activated surface of CuS@MoSx shows highly improved catalytic activity in HER. Furthermore, annealing treatment of CuS@MoSx could make heterostructure (Cu1.96S/MoS2) and alloyed structure (Cu1.96S/Cu2Mo6S8). Although nanoplatelet was aggregated and even collapsed by heating, HER property was better than as-prepared CuS@MoSx because formation of stable heterostructure can make better junction and synergistic effect. To conclude, I synthesized Cu2- xS/MoSx hybrid structure by inorganic ligand exchange reaction and improved HER property. This simple inorganic ligand exchange with nanocrystal template can be a method of fabrication for target morphology oriented heterostructure.
29
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