Vacancy
engineering, that is, self-doping of vacancy in semiconductors,
has become a commonly used strategy to tune the photocatalytic performances.
However, there still lacks fundamental understanding of the role of
the vacancies in semiconductor materials. Herein, the g-C3N4 nanosheets with tunable nitrogen vacancies are prepared
as the photocatalysts for H2 evolution and CO2 reduction to CO. On the basis of both experimental investigation
and DFT calculations, nitrogen vacancies in g-C3N4 induce the formation of midgap states under the conduction band
edge. The position of midgap states becomes deeper with the increasing
of nitrogen vacancies. The g-C3N4 nanosheets
with the optimized density of nitrogen vacancies display about 18
times and 4 times enhancement for H2 evolution and of CO2 reduction to CO, respectively, as compared to the bulk g-C3N4. This is attributed to the synergistic effects
of several factors including (1) nitrogen vacancies cause the excitation
of electrons to midgap states below the conduction band edge, which
results in extension of the visible light absorption to photons of
longer wavelengths (up to 598 nm); (2) the suitable midgap states
could trap photogenerated electrons to minimize the recombination
loss of photogenerated electron–hole pairs; and (3) nitrogen
vacancies lead to uniformly anchored small Pt nanoparticles (1–2
nm) on g-C3N4, and facilitate the electron transfer
to Pt. However, the overintroduction of nitrogen vacancies generates
deeper midgap states as the recombination centers, which results in
deterioration of photocatalytic activities. Our work is expected to
provide new insights for fabrication of nanomaterials with suitable
vacancies for solar fuel generation.
The development of efficient photocatalytic material for converting solar energy to hydrogen energy as viable alternatives to fossil-fuel technologies is expected to revolutionize energy shortage and environment issues. However, to date, the low quantum yield for solar hydrogen production over photocatalysts has hindered advances in the practical applications of photocatalysis. Here, we show that a carbon nitride intercalation compound (CNIC) synthesized by a simple molten salt route is an efficient polymer photocatalyst with a high quantum yield. We found that coordinating the alkali metals into the C-N plane of carbon nitride will induce the un-uniform spatial charge distribution. The electrons are confined in the intercalated region while the holes are in the far intercalated region, which promoted efficient separation of photogenerated carriers. The donor-type alkali metal ions coordinating into the nitrogen pots of carbon nitrides increase the free carrier concentration and lead to the formation of novel nonradiative paths. This should favor improved transport of the photogenerated electron and hole and decrease the electron-hole recombination rate. As a result, the CNIC exhibits a quantum yield as high as 21.2% under 420 nm light irradiation for solar hydrogen production. Such high quantum yield opens up new opportunities for using cheap semiconducting polymers as energy transducers.
Water adsorption and dissociation on the perfect, oxygen containing and nitrogen vacancy containing Ta3N5(100) surfaces are systematically studied by density functional theory calculations. The results show that the perfect Ta3N5(100) surface is very active for water dissociation because of the dangling bonds formed on the perfect Ta3N5(100) surface. The presence of oxygen on the surface is able to stabilize the Ta3N5(100) surface but not to facilitate water dissociation, which may be ascribed to the saturation of surface dangling bonds by oxygen. The presence of a nitrogen vacancy on the surface is able to facilitate water dissociation, but Ta3N5(100) surfaces with nitrogen vacancies are not stable. We found that keeping the impurity oxygen as less as possible is one effective approach to enhance the water splitting ability of Ta3N5. We propose that doping with foreign elements is one potential method to obtain a clean Ta3N5(100) surface, since the oxygen concentration may be adjusted by competition between oxygen and foreign elements.
Oxygen is a natural impurity in the Ta3N5 semiconductor photocatalyst and very difficult to be completely eliminated in different growth conditions. In this study, density functional theory calculations are performed to unravel the cause of natural existence of oxygen impurity in Ta3N5 from the perspectives of mechanical stability and atomic cohesion. The elastic properties calculations show that the oxygen impurity in Ta3N5 is able to remedy the weakened mechanical stability induced by the nitrogen vacancy in Ta3N5. The atomic cohesion calculations show that the oxygen impurity in Ta3N5 enlarges the valence band width of Ta3N5, suggesting that the oxygen impurity is able to strengthen the atomic cohesion of Ta3N5. Based on our calculation results, we propose that the charge-compensation codoping is a promising strategy to improve the water splitting ability of Ta3N5 and simultaneously maintain the mechanical stability and enhanced atomic cohesion of Ta3N5.
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