Zn(O,S) has been successfully doped with different amounts of Ho and characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and transient photocurrent (TPC). The as-prepared Ho-doped Zn(O,S) catalysts with different Ho amounts are evaluated for hydrogen evolution reaction. The catalyst with the best performance in evolving hydrogen is further utilized to hydrogenate 4nitrophenol (4-NP) to 4-aminophenol (4-AP). It is found that doping with Ho obviously enhanced charge transfer and photoresponse properties of the catalyst. Therefore, the modified Zn(O,S) can optimally evolve hydrogen by 18 624 μmol/g, which is 20% higher than that of pristine Zn(O,S). Subsequently, the in situ generated hydrogen ions on catalyst surfaces also play an important role as a hydrogen source to hydrogenate 4-NP to 4-AP without any reducing agents such as NaBH 4 , which is commonly used as a hydrogen source. As Ho is doped in the lattices of Zn(O,S), it acts not only to separate photocarriers and to enhance the charge transfer but also to shorten the diffusion time of nitrophenolate ions to catalyst surfaces for further photocatalytic hydrogenation reaction (PHR) process. A plausible PHR mechanism has been provided to elucidate the great performance of Ho-modified Zn(O,S) for photocatalytic hydrogenation.
A low-temperature radio-frequency-sputtered
Ag-modified MoS
x
lamellar thin-film electrode
with a home-made
target was deposited on a carbon-cloth substrate and electrochemically
modified with amorphous Ni(OH)2. All the as-prepared thin-films’
properties were characterized by X-ray diffraction, scanning electron
microscopy, transmission electron microscopy, Raman spectroscopy,
X-ray photoelectron spectroscopy, and electrochemical measurements.
Electrochemical measurements of a-Ni(OH)2/Ag-modified MoS
x
lamellar thin films exhibited promising
hydrogen evolution reaction properties in alkaline solutions. The
linear sweep voltammetry and electrochemical impedance spectroscopy
measurements indicated a relatively low Tafel slope of 95.3 mV/dec
and a low electron-transfer resistivity of 12.26 Ω, respectively.
The observed overpotentials were 0.12 and 0.26 V at 10 and 100 mA,
respectively. Furthermore, cyclic voltammetry measurements revealed
that the thin-film electrode had the highest double-layer capacity
(C
dl) of 11.3 mF/cm2, which
also implied a highly active electrochemical surface area. Finally,
the stability of a-Ni(OH)2/Ag-modified MoS
x
thin films was examined with a constant current
density of 10 mA/cm2 for 20 h. It was found that the non-stoichiometric
MoS
x
played an essential role in the water-activation
process due to the presence of sulfur-vacancy sites (VS
2+). During the electrochemical process, VS
2+ acts as an active site to pin water and weaken the
OH–H bonding, in which the water-originated OH is further bonded
with periodically terminated surface defect of Ni(OH)
x
on the electrode. The defected Ni(OH)
x
on the catalyst surface helps the dissociation of
OH bonding in water adsorbed on Ag-MoS
x
to generate H* intermediates for further Heyrovsky steps. The low-cost
thin-film electrode with a relatively low overpotential is promising
for industrial applications based on the experimental data.
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