Controllable Synthesis Heterojunction of g‑C₃N₄ and BiVO₄ to Enhance the Photocatalytic Oxygen Evolution Activity

Heterojunctions formed between semiconductors have been confirmed to efficiently enhance the separation of photogenerated carriers, thereby boosting the photocatalytic activity. However, achieving controllable synthesis of heterojunctions remains a challenge. In this study, g-C3N4 (CN) was positively charged by carefully adjusting the pH of the solution. Subsequently, it was precisely located on the (010) crystal facet of decahedral BiVO4 (BVO) under light irradiation, where photogenerated negative electrons accumulate on the (010) facet of BVO. This process results in the construction of a composite with a heterojunction between CN and the (010) facet of BVO. The optimal photocatalytic oxygen production activity of this composite reaches 2966.9 μmol/g/h, a remarkable 3.3 times better than that of BVO alone. This result shows that the heterojunction can significantly improve the oxygen production activity of the composite photocatalyst. By a combination of the Kubelka–Munk function, Mott–Schottky, and theoretical calculations, we found that the migration of photogenerated electrons from BVO to CN matches well with the S-scheme mechanism. This work provides valuable suggestions and guidance for the precise synthesis of heterojunction photocatalyst and is looking forward to being applied to other materials related to environmental and energy research

Band Structure Engineering: Insights from Defects, Band Gap, and Electron Mobility, from Study of Magnesium Tantalate

Anion doping of semiconductors with nitrogen is a strategy often adopted to narrow the band gap of semiconductors and increase the range of light absorption. However, the influence of nitrogen doping on the electron mobility in the semiconductor is not fully understood and characterized. In this work, we used magnesium tantalate MgTa2O6 as a model system and hybrid density-functional theory calculations to characterize the mobility of electrons using the small polaron model in the presence of nitrogen-doping defects as well as oxygen-vacancy defects. We found that electron mobility is not significantly affected when MgTa2O6 is doped with a molar ratio N/O of ∼2%. However, in the presence of oxygen vacancies combined with nitrogen doping with the same molar ratio N/O of ∼2%, the barrier to electron hopping in the vicinity of the defects is much lower than that in pristine MgTa2O6 and in MgTa2O6 with oxygen-vacancy defects only. These results suggest that nitrogen doping combined with anion vacancy not only narrows band gap but also enhances electron mobility, a finding that may lead to new strategies toward synthesizing more efficient photocatalysts

Interfacial Construction of Zero-Dimensional/One-Dimensional g‑C₃N₄ Nanoparticles/TiO₂ Nanotube Arrays with Z‑Scheme Heterostructure for Improved Photoelectrochemical Water Splitting

The 0D/1D graphitic carbon nitride (g-C3N4)/TiO2 heterostructures containing an interfacial oxygen vacancy layer were sequentially constructed by anodic oxidation, NaBH4 reduction, and vapor deposition methods. Visible light absorption was significantly improved via construction of the interfacial oxygen vacancy layer and coupling with g-C3N4. Thus, 0D/1D g-C3N4/OV-TiO2 showed an optimal photocurrent density as high as 0.72 mA/cm2 at 1.23 V versus reversible hydrogen electrode under visible light irradiation, 8-fold higher than the data of g-C3N4/TiO2 without interfacial oxygen vacancy layer. Electrochemical impedance spectroscopy (EIS) revealed the 0D/1D g-C3N4/OV-TiO2 heterostructured photoanode showed the lowest charge transfer resistance among all the prepared photoanodes. This improved photoelectrochemical (PEC) performance could be attributed to the generation of Z-scheme heterostructure via construction of an interfacial oxygen vacancy layer between TiO2 and g-C3N4. This interfacial layer can promote charge carrier separation and transportation processes. The formation of this Z-scheme heterostructure was confirmed by hydroxyl fluorescence capture characterization and spin-polarized density functional theory calculations. We believe that our work can help rationally design and construct highly efficient heterostructured photoanodes for PEC water splitting applications

Oxygen Evolution Reaction (OER) on Clean and Oxygen Deficient Low-Index SrTiO₃ Surfaces: A Theoretical Systematic Study

SrTiO3 (STO) is a widely used photocatalyst for water splitting, which has no photoactivity without a cocatalyst. The reason for this unclear. Here, we performed an oxygen evolution reaction (OER) on clean and oxygen deficient (100), (110), and (111) surfaces on STO by density functional theory. Combining our results with experimental results in the literature, we demonstrated that the overpotential is small enough for OER to occur on (100) surfaces. There is no photoactivity due to the photogenerated holes that cannot migrate to the (100) surfaces. On the (110) and (111) surfaces, the overpotential is very high, which prevents the OER from taking place on these two surfaces. Our work gives a guidance principle to understand the water splitting from the overpotential of OER and migrating photogenerated carriers. It may be helpful to design high efficiency photocatalysts based on STO

Water Oxidation on TiO₂: A Comparative DFT Study of 1e^–, 2e^–, and 4e^– Processes on Rutile, Anatase, and Brookite

Experimental studies of the surface reactions of photocatalyzed or photoelectrocatalyzed water oxidation on rutile, anatase, and brookite TiO2 show significant differences between the three polymorphs. Yet a fundamental understanding of the differences is still lacking. In this work, we carried out a systematic comparative density functional theory (DFT) investigation of the mechanisms and energetics of water oxidation on rutile TiO2 (110), anatase TiO2 (101), and brookite TiO2 (210) model surfaces. Our results indicate that for all three phases, the most facile mechanism of water oxidation proceeds as a two-electron/proton process toward H2O2 formation via surface peroxo O* intermediates. The calculated overall overpotentials toward H2O2 formation are ∼0.27, 0.51, and 0.62 V on rutile, anatase, and brookite, respectively. The rate-limiting steps toward H2O2 formation are the OH* formation step for all three phases. We studied also the effect of pH. pH alters the binding energies of the reaction intermediates and affects the threshold values for the 1-electron, 2-electron, and 4-electron processes but does not affect the selectivity. Overpotentials for the 4-electron O2 evolution range from 0.8, 1.04, and ∼1.15 V on rutile, anatase, and brookite, respectively, with the same rate-determining steps as for the 2-electron process. Under photocatalytic conditions of light irradiation corresponding to the redox potential versus NHE of photogenerated holes in the valence band of the materials (∼3.0 V for rutile, ∼3.2 V for anatase, and ∼3.3 V for brookite), there is enough energy to drive the 4-electron O2 evolution spontaneously as well. Under these conditions, product selectivity (H2O2 vs O2) may require characterizing the reaction kinetics rather than coming out from the thermodynamic overpotentials

Oxygen Vacancies Enriched Hollow Bi₂MoO₆ Microspheres for Efficient Photocatalytic Oxidation of Hydrocarbons

Photocatalytic aerobic oxidation of hydrocarbons to ketones is an attractive route for synthesizing high-value-added chemicals. However, the main challenge of photocatalytic oxidation reactions is their low activity. Herein, hollow Bi2MoO6 microspheres were synthesized by a facile two-step synthesis route combining ethylene glycol solvothermal with postannealing treatment. In the photocatalytic aerobic oxidation of ethylbenzene to the corresponding ketones under visible light irradiation using O2 as an oxidant, the hollow Bi2MoO6 microspheres exhibit a record acetophenone production rate of 1.1 mmol g–1 h–1 with 90% selectivity. The photoactivity of oxygen vacancy-enriched Bi2MoO6 is 61 times higher than that of uncalcined Bi2MoO6, which can be attributed to the effective separation of photogenerated carriers and the abundant catalytic active sites (i.e., oxygen vacancies) on hollow Bi2MoO6 microspheres. This work provides more insights into understanding how to construct highly efficient and active visible-light-responsive photocatalysts for the aerobic oxidation of organic compounds

Photocatalytic Facet Selectivity in BiVO₄ Nanoparticles: Polaron Electronic Structure and Thermodynamic Stability Considerations for Photocatalysis

Selective charge separation among different crystal facets of a semiconductor is an intriguing phenomenon for which there is no firm and full theoretical foundation currently. In this work, we report on a density functional theory + U characterization of band alignment and electron and hole polaron stabilities among the (010), (110), and (011) facets of bismuth vanadate BiVO4 (BVO). Computation-derived band alignment indicates that the conduction band minima are at nearly the same level among the three facets but that the valence band maxima exhibit a shift. We also modeled electron and hole polarons as localized electrons and holes on vanadium and oxygen, respectively, and determined their relative stabilities from a “bulk” region to a surface region. Calculated stabilities reveal similar stability profiles across the various facets, with electron polarons most stable when localized on subsurface V atoms and hole polarons most stable on surface O atoms. Calculations indicate a small stability preference for electron polarons toward the (011) facet and for hole polarons toward the (110) facet, whereas, experimentally, interfacial reduction is observed to take place selectively on the (010) facet and oxidation on the (110) and (011) facets. Facet selectivity could be occurring on the basis of thermodynamics (electron or holes showing a stronger affinity for some facets over others) or kinetics (electron or hole transport and/or redox processes being more efficient toward/on some facets over others) or a combination of both. This work establishes that thermodynamic stability alone is not responsible for the observed facet selectivity in BVO. Therefore, we surmise that polaron transport kinetics and interfacial redox kinetics are likely to have a role in facet selectivity in BVO. These issues will be the subject of future publications

Anthraceno-Perylene Bisimides: The Precursor of a New Acene

A controlled synthesis strategy for a anthracene-fused perylene bisimide was developed from the cyclization of an anthracene unit pendant to a perylene diimide scaffold. The direct cyclization led to a zigzag molecule, while a Diels–Alder strategy influenced the regiochemistry of cyclization to afford the linear precursor of a new acene