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S. Li and R. Jin
2.3.2 Boosting HER Activity with Au NCs/MoS 2 Composite
Besides the doping effects, metal NCs are also reported to show strong synergetic
effects when loaded on other materials. In 2017, the synergetic effect was reported
by Zhao et al. [30]. In this work, Au 25 (SR) 18 and Au 25 (SePh) 18 NCs are loaded
on the MoS 2 ultra-thin nanosheets. These two NCs have a similar kernel structure but different protecting ligands. The TEM images and XPS spectra indicate
the successful loading of Au NCs on the surface of MoS 2 (Fig. 2.5). The Mo 3d
XPS of composites exhibits negative shifting compared with MoS 2 , while the Au
4f spectra of composites show obvious positive shifting compared with Au 25 NCs.
The XPS results clearly indicate that the electron density transfers from Au 25 to
MoS 2 . In the HER activity test, the thiolate-protected Au 25 NCs exhibit a more positive onset potential and higher current density compared with MoS 2 nanosheets. On
the other hand, the benzeneselenolate-protected Au 25 NCs loaded MoS 2 nanosheets
show similar synergetic effects. However, the enhancement is less obvious compared
with the thiolate-protected Au 25 NCs. To explain the different enhancement in HER
catalytic activity, Au 4f XPS spectra of Au 25 (SR) 18 /MoS 2 , Au 25 (SePh) 18 /MoS 2 and
pure MoS 2 are obtained. The Au 25 (SR) 18 /MoS 2 composites exhibit more positive
shifting compared with the Au 25 (SePh) 18 /MoS 2 composites, indicating stronger electron density transfer effects of thiol protected Au 25 . Therefore, the electron interaction
between MoS 2 nanosheets and Au NCs is a key factor for the HER activity. Based
on these results, the authors proposed a dual interfacial effect, where the core/ligand
interface of Au NCs and the MoS 2 /Au NCs interface are both important in the HER
catalytic activity.
Du et al. also studied the synergetic effects between Au 2 Pd 6 NCs and MoS 2 [31].
In this work, DFT calculation is used to investigate the origin of the enhanced HER
activity. The DFT calculation shows the G for a proton adsorbed at Au 2 Pd 6 /MoS 2
composite is more negative than that of MoS 2 , indicating better HER activity of the
composites. Besides, it is found that in the Au 2 Pd 6 composites, both Au atoms and S
atoms have appropriate G for proton adsorption. In contrast, only the Au–Pd bridge
site has proper G in Au 2 Pd 6 NCs, and no proper active site for proton adsorption can
be found in defect-free MoS 2 . Therefore, the significant increase in active sites in the
composites is a key factor for boosted HER activity. Meanwhile, the DOS analysis
also explained the enhanced activity of composites. The Au 2 Pd 6 composites have
a defect state near the Fermi level. This unique defect state narrows the band gap,
leading to a better electronic conductivity.
2.4 Oxygen Evolution Reaction with Metal NCs
Oxygen evolution reaction (OER) is the other half reaction in water splitting and is
indeed critical. Oxygen can be formed through several proton/electron-coupled steps
in OER. The reaction can be described as follows:
S. Li and R. Jin
2.3.2 Boosting HER Activity with Au NCs/MoS 2 Composite
Besides the doping effects, metal NCs are also reported to show strong synergetic
effects when loaded on other materials. In 2017, the synergetic effect was reported
by Zhao et al. [30]. In this work, Au 25 (SR) 18 and Au 25 (SePh) 18 NCs are loaded
on the MoS 2 ultra-thin nanosheets. These two NCs have a similar kernel structure but different protecting ligands. The TEM images and XPS spectra indicate
the successful loading of Au NCs on the surface of MoS 2 (Fig. 2.5). The Mo 3d
XPS of composites exhibits negative shifting compared with MoS 2 , while the Au
4f spectra of composites show obvious positive shifting compared with Au 25 NCs.
The XPS results clearly indicate that the electron density transfers from Au 25 to
MoS 2 . In the HER activity test, the thiolate-protected Au 25 NCs exhibit a more positive onset potential and higher current density compared with MoS 2 nanosheets. On
the other hand, the benzeneselenolate-protected Au 25 NCs loaded MoS 2 nanosheets
show similar synergetic effects. However, the enhancement is less obvious compared
with the thiolate-protected Au 25 NCs. To explain the different enhancement in HER
catalytic activity, Au 4f XPS spectra of Au 25 (SR) 18 /MoS 2 , Au 25 (SePh) 18 /MoS 2 and
pure MoS 2 are obtained. The Au 25 (SR) 18 /MoS 2 composites exhibit more positive
shifting compared with the Au 25 (SePh) 18 /MoS 2 composites, indicating stronger electron density transfer effects of thiol protected Au 25 . Therefore, the electron interaction
between MoS 2 nanosheets and Au NCs is a key factor for the HER activity. Based
on these results, the authors proposed a dual interfacial effect, where the core/ligand
interface of Au NCs and the MoS 2 /Au NCs interface are both important in the HER
catalytic activity.
Du et al. also studied the synergetic effects between Au 2 Pd 6 NCs and MoS 2 [31].
In this work, DFT calculation is used to investigate the origin of the enhanced HER
activity. The DFT calculation shows the G for a proton adsorbed at Au 2 Pd 6 /MoS 2
composite is more negative than that of MoS 2 , indicating better HER activity of the
composites. Besides, it is found that in the Au 2 Pd 6 composites, both Au atoms and S
atoms have appropriate G for proton adsorption. In contrast, only the Au–Pd bridge
site has proper G in Au 2 Pd 6 NCs, and no proper active site for proton adsorption can
be found in defect-free MoS 2 . Therefore, the significant increase in active sites in the
composites is a key factor for boosted HER activity. Meanwhile, the DOS analysis
also explained the enhanced activity of composites. The Au 2 Pd 6 composites have
a defect state near the Fermi level. This unique defect state narrows the band gap,
leading to a better electronic conductivity.
2.4 Oxygen Evolution Reaction with Metal NCs
Oxygen evolution reaction (OER) is the other half reaction in water splitting and is
indeed critical. Oxygen can be formed through several proton/electron-coupled steps
in OER. The reaction can be described as follows: