Dual-absorber photoelectrodes are attractive candidates for solar water splitting due to their broadened absorption spectrum and improved photovoltage compared to single-absorber systems.
Dual-absorber
photoelectrodes have been proved to possess greater
potential than the single-absorber systems in the applications of
photoelectrochemical (PEC) cells (e.g., solar-driven water splitting);
however, the mismatching of the energy bands and substantial carrier
recombinations at the two absorber interfaces are normally subsistent.
Here, we introduce an intermediate layer of conformal Al2O3 into the silicon/hematite (Si/α-Fe2O3) microwire photoanode for enriching the understanding
of the interaction among the interlayer, inner absorber, and outer
absorber. Our results show that the Si/Al2O3/α-Fe2O3 microwire photoanode with the
thickness-optimized Al2O3 can lead to a substantial
increase in the photocurrent from 0.83 to 2.08 mA/cm2 at
1.23 VRHE (under 1 sun irradiation) and an obvious decrease
in the onset potential relative to the counterpart without Al2O3. By analyzing the PEC responses under various
monochromatic lights, PEC impedance spectroscopy, and intensity-modulated
photocurrent spectroscopy, we ascribe the improvements to the fact
that the suitable-thickness Al2O3 can passivate
the Si microwire surfaces and the bottom surfaces of the α-Fe2O3 film and give rise to Al doping into the post-synthesized
α-Fe2O3. These essential causes promote
the carrier separation in α-Fe2O3, diminish
the photoanode surface recombination rate, and then increase the surface
charge-transfer efficiency.
Hematite (α-Fe 2 O 3 ) material is regarded as a promising candidate for solar-driven water splitting because of the low cost, chemical stability, and appropriate bandgap; however, the corresponding system performances are limited by the poor electrical conductivity, short diffusion length of minority carrier, and sluggish oxygen evolution reaction. Here, we introduce the in situ Sn doping into the nanoworm-like α-Fe 2 O 3 film with ultrasonic spray pyrolysis method. We show that the current density at 1.23 V vs. RHE (J ph@1.23V ) under one-sun illumination can be improved from 10 to 130 μA/cm 2 after optimizing the Sn dopant density. Moreover, J ph@1.23V can be further enhanced 25folds compared to the untreated counterpart via the post-rapid thermal process (RTP), which is used to introduce the defect doping of oxygen vacancy. Photoelectrochemical impedance spectrum and Mott-Schottky analysis indicate that the performance improvement can be ascribed to the increased carrier density and the decreased resistances for the charge trapping on the surface states and the surface charge transferring into the electrolyte. Xray photoelectron spectrum and X-ray diffraction confirm the existence of Sn and oxygen vacancy, and the potential influences of varying levels of Sn doping and oxygen vacancy are discussed. Our work points out one universal approach to efficiently improve the photoelectrochemical performances of the metal oxide semiconductors.
Surface engineering, as an efficient strategy, can improve
the
photoelectrochemical water splitting (PEC-WS) performance for converting
inexhaustible sunlight into clean hydrogen fuel. Oxyhydroxides and
p–n heterojunctions have been demonstrated as efficient catalysts
for the water oxidation reaction. In this work, to address the drawbacks
of poor conductivity and sluggish oxidation kinetics of hematite,
we introduce a p-type NiOOH overlayer as a surface catalyst onto n-type
Sn-doping hematite (Sn@α-Fe2O3) photoanode.
The oxygen vacancies (Ov) are reconstructed both in the
bulk of Sn@α-Fe2O3 and the surface decoration
layer of NiOOH via Ar plasma treatment, effectively reducing unavoidable
defects introduced by the NiOOH overlayer. Compared with the original
Sn@α-Fe2O3 photoanode, the Sn@α-Fe2O3/NiOOH–Ar photoanode exhibits a significant
increase in photocurrent density (at 1.23 VRHE) of ∼3
times and a decrease in the onset potential of ∼200 mV. The
performance improvement can be ascribed to the synergistic effect
of the p–n junctions formed by NiOOH decoration and improved
conductivity through oxygen vacancy reconstruction, which remarkably
improves carrier separation in the bulk of α-Fe2O3 and suppresses carrier recombination on the photoanode surface.
Moreover, the density functional theory (DFT) calculation proves that
the real active sites are farther from (rather than near) the oxygen
vacancies.
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