Figure 8 A) Scheme of synthesis method for Pt-CdS hybrids
(yellow) by synthesis in organic media, selective growth of Pt
nanoparticles (grey) at the tips, and phase transfer into aqueous medium
with cysteine and anchoring of the RuDTC catalyst to the side surfaces
of the nanorods. B) Hydrogen evolution by bare and tip-decorated CdS
nanorods. C) Comparison of average H2 and
O2 generation rates over the first hour of illumination
calculated from at least two measurements for each system. Error bars
represent s.d. of measured rates. Sacrificial agent (TEOA) was used only
in the first experiment (leftmost blue bar). A-C) Reproduced with
permission.[26a] Copyright 2018, Springer Nature.
3.6. Band gap structure
In the metal-semiconductor heterostructures, a photocatalyst with
excellent performance can be designed by the regulation strategy of the
energy band engineering. This is because the maximized synergistic
effect can be reached by blending the appropriate components to enhance
its catalytic effect.[27]
Zhuang et al. constructed a unique 1D binary
[ZnS-CdS]-ZnS-[ZnS-CdS]-ZnS structure, a hetero-structure
nanorod with multi-nodal sheath CdS, which can achieve better absorbance
and charge transport continuity and provide a smaller band gap
semiconductor - CdS node sheath.[28] Through
selective growth of the metal on the node sheath, the structure of
ternary hybrid [ZnS-(CdS/M)]-ZnS-[ZnS-(CdS/M)]-ZnS is obtained.
This unique structure enables the transfer of photoexcited electrons
from the node sheath of CdS to the metal surface and the ZnS nanorod,
and thus can improve the charge separation efficiency. Research on the
band gap alignment of materials shows that the binary multi-node sheath
nanostructures can be regarded as the connection between ZnS(111)
nanorods and
ZnS(\(\overline{\overset{\overline{}}{\overline{1}}10}\))/CdS(\(\overline{1}10\))
hybrid nodal sheath, and the energy gap of
ZnS(\(\overline{\overset{\overline{}}{\overline{1}}10}\))/CdS(\(\overline{\overset{\overline{}}{\overline{1}}10}\))
is located within ZnS(111). Therefore, the binary structure forms a
periodic straddling gap alignment (type I heterojunction)
(Figure 9B), but there might be an
accumulation of electrons and holes in CdS. Therefore, the ternary
multi-node sheath with metal loaded nanostructures are investigated
then. The calculated workfunction of
CdS(\(\overline{\overset{\overline{}}{\overline{1}}10}\)) is larger than
the (\(\overline{\overset{\overline{}}{\overline{1}}10}\)) surface of
Au, Pd and Pt, so the difference of Fermi energy level/workfunction
propels free electrons from Au, Pd or Pt to
CdS(\(\overline{\overset{\overline{}}{\overline{1}}10}\)), thereby
bending the CdS band so that it can be staggered with
ZnS(\(\overline{\overset{\overline{}}{\overline{1}}10}\)) and ZnS(111),
forming type II heterojunction (Figure 9C). Thus, photogenerated
electrons will be transferred to both the metal domain and ZnS- (111).
In addition, this structure can also highly suppress the recombination
between photoexcitation electron and hole. Tests on the catalytic effect
show that the ternary multi-node sheath nanostructures have the highest
hydrogen production rate than other structure (Figure 9A), and the best
effect is achieved when Pt replaces Au in the ternary structure, which
is consistent with the excellent performance of the Pt decorated hybrid
nanostructures described in section 3.1.[28]