The evergrowing energy crisis and environmental degradation have instigated scientists to explore sophisticated, versatile energy conversion systems. Photoconversion, which conveys solar power to chemical energy, has emerged as an eminent energy conversion approach to accomplish the demands. Recent years have seen the rocketing rise of semiconductor heterostructures as an ideal material paradigm for the realization of miscellaneous photoconversion applications ranging from water splitting, CO 2 reduction and environmental purification to photosynthesis. With tailored functionalities from synergetic effects, semiconductor heterostructures come into prominence as the forefront of photocatalyst development. This topical review summarizes the photoconversion applications developed so far by employing semiconductor heterostructures, with a focus on the heterostructure design principle, interfacial charge dynamics and key factors dictating the overall performance. Future research outlooks and perspectives on the progress of photoconversion technology are also presented.
SrTiO3 cubes with tunable sizes of 160–290 nm have been synthesized by mixing
TiCl4, SrCl2, and LiOH in pure ethanol or a
water/ethanol mixed solution at just 70 °C for 3 h. Replacing
water/ethanol with water/hexanol and water/ethylene glycol, and fine
tuning the amounts of other reagents, resulted in the formation of
edge-truncated cubes and {100}-truncated rhombic dodecahedra, respectively.
X-ray diffraction and transmission electron microscopy characterization,
supported by Rietveld refinement analysis, have revealed shape-dependent
tuning in lattice parameters. The cubes display slight size-related
optical band shifts, and they show clearly more blue-shifted light
absorption than the other particles exposing significant {110} faces.
The {100}-truncated rhombic dodecahedra are far more efficient than
cubes at photodegradation of methylene blue and photocatalyzed hydrogen
evolution from water in the presence of methanol. The photocatalytic
activity variation should arise from different degrees of surface
band bending for the {100} and {110} faces of SrTiO3, suggesting
surface facet control as a strategy for enhancing photocatalyzed hydrogen
production.
An Au-mediated Cu2O-based Z-scheme heterostructure system
is demonstrated for use as efficient photocathodes in photoelectrochemical
(PEC) reduction. The samples are prepared by electrodepositing a Cu2O layer on the surface of Au particle-coated TiO2 nanorods. For TiO2-Au-Cu2O, the embedded Au
particles function as a charge transfer mediator to enhance the electron
transportation from the conduction band of TiO2 to the
valence band of Cu2O. Such a vectorial charge transfer
leads to the concentration of electrons at the conduction band of
Cu2O and the collection of holes at the valence band of
TiO2, providing TiO2-Au-Cu2O with
substantially high redox abilities for reduction applications. Time-resolved
photoluminescence spectra and electrochemical impedance spectroscopy
analysis suggest that interfacial charge transfer is significantly
improved because of the Au-mediated Z-scheme charge transfer mechanism.
By virtue of the high redox ability and improved interfacial charge
transfer, TiO2-Au-Cu2O performs much better
as a photocathode in H2 production and CO2 reduction
than pure Cu2O and binary TiO2-supported Cu2O do. Remarkably, the photocurrent density of TiO2-Au-Cu2O toward PEC CO2 reduction can reach
as high as −1.82 mA/cm2 at +0.11 V vs RHE. The incident
photon-to-current conversion efficiency data manifest that TiO2-Au-Cu2O surpasses both pure Cu2O and
binary TiO2-supported Cu2O in PEC reduction
across the whole photoactive region. The current study paves a valuable
approach of devising Z-scheme photocathode for the construction of
sophisticated artificial photosynthesis systems capable of solar-to-fuel
conversion.
We
synthesized nitrogen (N)-doped graphene quantum dots (N-GQDs)
using a top-down hydrothermal cutting approach. The concentration
of N dopants was readily controlled by adjusting the concentration
of the N source of urea. When N dopants were incorporated into GQDs,
visible absorption was induced by C–N bonds, which created
another pathway for generating photoluminescence (PL). Time-resolved
PL data revealed that the carrier lifetime of GQDs was increased upon
doping with the optimized N concentration. The photoelectrochemical
properties of N-GQDs toward water splitting were studied, and the
results showed that 2N-GQDs prepared with 2 g of urea produced the
highest photocurrent. The photocatalytic activity of 2N-GQDs powder
photocatalyst for hydrogen production was also examined under AM 1.5G
illumination, showing substantial enhancement over that of pristine
GQDs. Electrochemical impedance spectroscopy data further revealed
a significant improvement in charge dynamics and reaction kinetics
and an increased carrier concentration as a result of N doping.
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