Figure 10 A) Kinetic hydrogen evolution measurements by CdS-Au
hybrid nanorods for different surface coatings. Straight lines represent
the linear fits from which the %QY was extracted. B) Apparent
photocatalysis %QY values for a wide range of surface coatings
including thiolated alkyl ligands, GSH, and polymer coating. PEI
exhibits the highest QY. A-B) Reproduced with
permission.[30] Copyright 2014, Wiley-VCH.
Surface ligand plays an important role in the construction of catalysts.
It is used to provide colloidal stability to the photocatalyst in water
while maintaining good ability to be close to the active surface to
transfer photo-generated excitons. In addition, the passivation of
surface defect achieved by coating the ligands can also eliminate trap
state thus increasing the photocatalytic properties.
Ben-Shahar and co-workers prepared CdS-Au and CdSe/CdS hybrid nanorods,
respectively, with thiolated-alkyl
ligands and polymer coating to study the influence of surface ligands on
the catalytic performances. Firstly, hydrophobic phosphonic acid and
alkylamine ligands were exchanged to different types of thiolated alkyl
ligands, including mercaptocarboxylic acids of different chain lengths
(mercaptoundecanoic acid (MUA), mercaptohexanoic acid (MHA) and
mercaptopropionic acid (MPA), L-glutathione (GSH)) and thiol bound
ligands (mercaptosulfonic acid (MSA) and
O-(2-carboxyethyl)-O’-(2-mercaptoethyl) heptaethylene glycol (S-PEG)).
Another type is polymer encapsulation, which generally uses
polyethylenimine (PEI). PEI binds to the surface of nanorods with amine
groups through ligand exchange, and poly(styrene-co-maleic anhydride)
(PSMA), which can encapsulate the hybrid nanorods to maintain the
surface coating of the original nanorods with a hydrophilic end toward
the solution. CdS-Au catalysts with different surface coatings were used
for photocatalysis. The results of hydrogen production rate and QE are
shown in Figure 10A, 10B. Thiolated-alkyl ligands show very low hydrogen
production rates, while polymer-coated hetero-structures offer quite
high rates, with the hydrogen production rates of PEI coated hybrid
nanorods being even an order of magnitude higher than that of MUA coated
hetero-structures. In addition, the fastest charge transfer kinetics and
particle passivation, and the longer effective fluorescence lifetime of
surface ligands can be obtained with PEI coating. Stone and colleagues
used a similar surface encapsulation strategy in which the PEI coated
nanocrystals achieved excellent catalytic performance than thiolated
alkyl ligands.[11]Through the passivation of
surface defects and decreasing of available hole trapping sites, the
effective charge transfer across the semiconductor-metal interface can
be realized.[30]
3.8. Hole scavenger
It is well known that electrons and holes are generated when
semiconductor nanorods are irradiated with sunlight. The photoexcited
electrons are transferred to the metal domain to reduce water to
hydrogen, while the photogenerated holes require the addition of
scavenger to remove. The introduction of hole scavenger not only
suppresses electron-hole recombination but also prevents photo-corrosion
of the metal sulfide semiconductor. Therefore, the addition of
sacrificial agent is very important in photocatalytic reaction. In
addition, different hole scavenger will bring different degrees of
influence and improper scavenger may even reduce the performance of the
catalyst.[31]
Wu et al. used MUA-capped CdSe/CdS-Pt and CdS-Pt hybrid nanorods
to study the performance of methanol (MeOH) and sulfite as hole
scavengers for photocatalytic hydrogen generation (Figure 11A and B).
Both CdSe/CdS-Pt and CdS-Pt nanorods have achieved efficient charge
separation and long-life charge separation states, the hole transfer
rate is positively correlated with hydrogen generation QE. Combining the
characterization results and calculation data, the use of MeOH scavenger
on CdS-Pt nanocrystals exhibit the lowest rate in the hole transferring.
The hole transfer rate from CdSe/CdS-Pt to the MUA and MeOH is about 4.9
times faster than that of CdS-Pt. The combination of MUA and highly
reductive sulfites provides a greater driving force for hole removal,
making the effect of this group
better than the group using MeOH as an electron donor. The interaction
of the hole acceptor with the surface-capping ligand, the accessibility
of the surface sites, and the strong reducibility of the sulfite cause
more holes in the surface trap of the CdS-Pt nanorod to be removed,
which is reflected in the fact that the hole transfer time from CdS-Pt
to sulfite is about 2.5 times faster than that of
CdSe/CdS-Pt.[32] In addition, Berr et al.also studied the catalytic performances of four hole scavengers for
Pt-CdS nanorods such as sodium sulfite
(SO32-), disodium
ethylenediaminetetraacetic acid (EDTA4-),
triethanolamine (TEA) and MeOH, respectively.[33]As can be seen from Figure 11C, SO32-achieves the best results with a QE that can exceed an order of
magnitude of the worst one such as MeOH. They believe that this is the
result of different redox potentials of hole scavengers. As shown in
Figure 11D, SO32- has the strongest
reducibility and is more easily oxidized by the combination of holes.
Besides, the morphology of the catalyst after photocatalysis using
different hole scavengers indicates that the nanorods using MeOH as hole
scavenger are shortened and aggregated while the nanorods using
SO32- can remain their morphology
after catalysis due to the electron donor timely cleared the holes
captured by the surface traps of the nanorods, which preventing the
photooxidation of catalyst.[33]