Po Wu

Structure defects in g-C3N4 limit visible light driven hydrogen evolution and photovoltage

Graphitic carbon nitride (g-C3N4) is a promising visible-light-responsive photocatalyst for hydrogen generation from water. As we show here, the photocatalytic activity of g-C3N4 is limited by structure defects generated during the calcination process. Specifically we find that the photocatalytic hydrogen production rate from aqueous methanol is inversely related to the calcination temperature (520–640 °C). The highest activity of 0.301 mmol h−1 g−1 is observed for the sample prepared at the lowest processing temperature. Surface photovoltage (SPV) spectroscopy shows that the maximum photovoltage is reduced (from 1.29 V to 0.62 V) as the processing temperature is increased, in accordance with higher defect concentrations and faster electron–hole recombination. The defects also produce additional optical absorption in the visible spectra and cause a red shifted, weakened photoluminescence (PL). Based on the sub-gap signal in the SPV and PL spectra, defect energy levels are +0.97 V and −0.38 V (vs. NHE) in the band gap of the material. According to Fourier transform infrared (FTIR) spectra, the defects are due to amino/imino groups in the g-C3N4 lattice.

Spatial engineering of photo-active sites on g-C3N4 for efficient solar hydrogen generation

ZnFe2O4 modified g-C3N4 was successfully synthesized by a simple one-pot method. The visible-light-driven photocatalytic hydrogen production activity of g-C3N4 was significantly enhanced due to spatial engineering of the photo-active sites via ZnFe2O4 modification and Pt loading. It is proposed that ZnFe2O4 does not function as visible light sensitizer but as oxidation active sites. In the present ZnFe2O4/g-C3N4 photocatalysts, the photo-induced holes in g-C3N4 tend to transfer to ZnFe2O4 due to the straddling band structures (Type I band alignment), while the photo-induced electrons in g-C3N4 prefer to transfer to the loaded Pt cocatalysts, which can function as reduction active sites for hydrogen production. As a result, the photoinduced electrons and holes in g-C3N4 are efficiently separated by spatial engineering of the photo-active sites, and hence enhanced photocatalytic hydrogen generation activity is obtained.

Nitrogen-doped CeOx Nanoparticles Modified Graphitic Carbon Nitride for Enhanced Photocatalytic Hydrogen Production

Nitrogen-doped CeOx nanoparticles (N-CeOx NPs) were directly formed on graphitic carbon nitride (g-C3N4) via a facile one-pot method by annealing a mixture of Ce(NO3)3 and melamine as CeOx and g-C3N4 precursors, respectively. Nitrogen was in situ doped into CeOx under NH3 atmosphere released by the decomposition of melamine during the annealing process. The physical and photophysical properties of N-CeOx NPs modified g-C3N4 photocatalysts were investigated to reveal the effects of N-CeOx NPs on the photocatalytic activities of g-C3N4. It was found that the one-pot annealing method was favorable for forming intimate interfacial contact between N-CeOx and g-C3N4. The visible light photocatalytic hydrogen production activity over g-C3N4 was enhanced by ca. 1.2 times with the N-CeOx NPs modification. The significant enhancement in photocatalytic performance for the N-CeOx/g-C3N4 heterojunction should be mainly because of the promoted charge transfer and charge separation between N-CeOx NPs and g-C3N4 resulting from the intimate interfacial contact and Type II band alignment. In addition, the improved visible light absorption of N-CeOx NPs induced by nitrogen doping could be another reason for the enhanced photocatalytic activity of the N-CeOx/g-C3N4 heterojunction.

A novel Sn2Sb2O7 nanophotocatalyst for visible-light-driven H2 evolution

AbstractA novel pure cubic-phase pyrochlore structure tin(II) antimonate nanophotocatalyst, stoichiometric Sn2Sb2O7, has been prepared by a modified ion-exchange process using an antimonic acid precursor, and employed in visible-light-driven photocatalytic H2 evolution for the first time. The physicochemical properties (crystal phase, chemical composition and state, textural properties, and optical properties) of the material were investigated by different instrumental techniques. Compared with the antimonic acid precursor, the as-prepared Sn2Sb2O7 had a narrower bandgap, smaller crystal size, and larger BET surface area. The as-prepared Sn2Sb2O7 was validated as a promising candidate for visible-light-driven photocatalytic H2 evolution with a constant rate of 40.10 μmol·h−1·gcat−1.

Tin(II) Antimonates with Adjustable Compositions: Effects of Band‐Gaps and Nanostructures on Visible‐Light‐Driven Photocatalytic H2 Evolution

A series of tin(II)–antimonate photocatalysts with varied Sn content were prepared by altering the ion‐exchange time and reaction temperature to control their physicochemical properties, especially their band‐gaps and nanostructures. Furthermore, the effect of these catalysts on visible‐light‐driven photocatalytic H2‐evolution was also investigated. With an increase in Sn content, the narrowed band‐gaps enhanced the absorption of photons to excite the photogenerated charge carriers. A decrease in nanocrystal size approaching stoichiometric compositions impeded the recombination of the photogenerated charge carriers; the increased surface areas and pore volumes, owing to the nanostructural transformation, accelerated the redox reactions. Consequently, the photocatalytic activities gradually improved and the highest rate was observed for stoichiometric Sn2Sb2O7. As a result, the as‐prepared tin(II) antimonates—especially Sn2Sb2O7—were confirmed to be stable and efficient photocatalysts for visible‐light‐driven H2 evolution. Moreover, the activities of these photocatalysts could be improved by tuning their physicochemical properties to jointly optimize all of the processes in the photocatalytic reaction.

A First‐Principles Investigation on Microscopic Atom Distribution and Configuration‐Averaged Properties in Cd1−xZnxS Solid Solutions

The structural, energetic, and electronic properties of zincblende and wurtzite phase Cd(1-x)Zn(x)S (0≤x≤1) solid solutions were investigated by first-principles calculations. It was revealed that the trend of atom distribution in configurations with the same x value can be quantitatively characterized by the average length of the Zn-S bonds. The origin of this trend was attributed to the strong interaction of the Zn-S bonds, which acted against the aggregation of Zn atoms in this solid solution. By using a configuration-averaged method, structural and energetic properties were estimated as a function of Zn content at the level of the generalized gradient approximation, whereas electronic properties were corrected by using a hybrid functional. Phase diagrams of both solid solutions were established. An optimal x value of approximately 0.5 for photocatalytic hydrogen production was determined by taking both the band edges and band gaps into consideration; this conclusion was supported by the results of a variety of experiments.

Revisiting the Zinc-Blende/Wurtzite Heterocrystalline Structure in CdS

The band offset at CdS zinc-blende (ZB)/wurtzite (WZ) heterocrystalline interface was revisited using the first principles calculations with the local density approximation (LDA), generalized gradient approximation (GGA), and Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional. It was revealed that, unlike most IV, III-V, and II-VI semiconductors, the band alignment at CdS ZB/WZ heterocrystalline interface was of type-I with straddling lineup of band edges, which was irrespective of the exchange-correlation energy functional, the thickness of ZB and WZ segments, and the ZB/WZ interface location. The partial charge densities of VBM and CBM states were separated around two adjacent interfaces in one unit cell of heterocrystalline superlattice. This type of carrier localization was mainly attributed to the spontaneous polarization occurring in the WZ segment rather than the band offset at the interface.