The search for active semiconductor photocatalysts that directly split water under visible-light irradiation remains one of the most challenging tasks for solar-energy utilization. Over the past 30 years, the search for such materials has focused mainly on metal-ion substitution as in In(1-x)Ni(x)TaO(4) and (V-,Fe- or Mn-)TiO(2) (refs 7,8), non-metal-ion substitution as in TiO(2-x)N(x) and Sm(2)Ti(2)O(5)S(2) (refs 9,10) or solid-solution fabrication as in (Ga(1-x)Zn(x))(N(1-x)O(x)) and ZnS-CuInS(2)-AgInS(2) (refs 11,12). Here we report a new use of Ag(3)PO(4) semiconductor, which can harness visible light to oxidize water as well as decompose organic contaminants in aqueous solution. This suggests its potential as a photofunctional material for both water splitting and waste-water cleaning. More generally, it suggests the incorporation of p block elements and alkali or alkaline earth ions into a simple oxide of narrow bandgap as a strategy to design new photoelectrodes or photocatalysts.
The
electronic band structure of a semiconductor photocatalyst
intrinsically controls its level of conduction band (CB) and valence
band (VB) and, thus, influences its activity for different photocatalytic
reactions. Here, we report a simple bottom-up strategy to rationally
tune the band structure of graphitic carbon nitride (g-C3N4). By incorporating electron-deficient pyromellitic
dianhydride (PMDA) monomer into the network of g-C3N4, the VB position can be largely decreased and, thus, gives
a strong photooxidation capability. Consequently, the modified photocatalyst
shows preferential activity for water oxidation over water reduction
in comparison with g-C3N4. More strikingly,
the active species involved in the photodegradation of methyl orange
switches from photogenerated electrons to holes after band structure
engineering. This work may provide guidance on designing efficient
polymer photocatalysts with the desirable electronic structure for
specific photoreactions.
Despite the fact that Ta3N5 absorbs a major fraction of the visible spectrum, the rapid decrease of photocurrent encountered in water photoelectrolysis over time remains a serious hurdle for the practical application of Ta3N5 photoelectrodes. Here, by employing a Co3O4 nanoparticle water oxidation catalyst (WOC) as well as an alkaline electrolyte, the photostability of Ta3N5 electrode is significantly improved. Co3O4/Ta3N5 photoanode exhibits the best durability against photocorrosion to date, when compared with Co(OH)x/Ta3N5 and IrO2/Ta3N5 photoanodes. Specifically, about 75% of the initial stable photocurrent remains after 2 h irradiation at 1.2 V vs. RHE (reversible hydrogen electrode). Meanwhile, a photocurrent density of 3.1 mA cm−2 has been achieved on Co3O4/Ta3N5 photoanode at 1.2 V vs. RHE with backside illumination under 1 sun AM 1.5 G simulated sunlight. The reason for the relatively high stability is discussed on the basis of electron microscopic observations and photoelectrochemical measurements, and the surface nitrogen content is monitored by X‐ray photoelectron spectroscopic analysis.
Surface exfoliation: A Ta3 N5 photoanode prepared by a thermal oxidation and nitridation method shows a high solar photocurrent. This photocurrent is currently the highest achieved by a Ta3 N5 photoanode. The photocurrent is obtained mainly because of facile thermal and mechanical exfoliation of the surface passivation layer of the Ta3 N5 photoanode.
The surface pretreatment by electrochemical cyclic voltammetry
(CV) in the dark was found to remarkably enhance the photocurrent
of Mo-doped BiVO4 from the front side illumination. The
variation of the samples before and after the surface pretreatment
was investigated by scanning electron microscopy, X-ray photoelectron
spectroscopy, and Mott–Schottky methods. The results showed
that the photocurrent enhancement came from both the removal of the
surface recombination center, including Mo6+ ions, and
reoxidation of the reduced species. The part of the reduced ions can
be reoxidized in air. However, the photocurrent enhancement from the
Mo6+ dissolution can be kept at high potential under illumination.
A possible mechanism was also proposed to explain the reason for the
photocurrent enhancement.
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