Graphitic carbon nitride (g-C
3
N
4
), as an intriguing earth-abundant visible light photocatalyst, possesses a unique two-dimensional structure, excellent chemical stability and tunable electronic ...structure. Pure g-C
3
N
4
suffers from rapid recombination of photo-generated electron-hole pairs resulting in low photocatalytic activity. Because of the unique electronic structure, the g-C
3
N
4
could act as an eminent candidate for coupling with various functional materials to enhance the performance. According to the discrepancies in the photocatalytic mechanism and process, six primary systems of g-C
3
N
4
-based nanocomposites can be classified and summarized: namely, the g-C
3
N
4
based metal-free heterojunction, the g-C
3
N
4
/single metal oxide (metal sulfide) heterojunction, g-C
3
N
4
/composite oxide, the g-C
3
N
4
/halide heterojunction, g-C
3
N
4
/noble metal heterostructures, and the g-C
3
N
4
based complex system. Apart from the depiction of the fabrication methods, heterojunction structure and multifunctional application of the g-C
3
N
4
-based nanocomposites, we emphasize and elaborate on the underlying mechanisms in the photocatalytic activity enhancement of g-C
3
N
4
-based nanocomposites. The unique functions of the p-n junction (semiconductor/semiconductor heterostructures), the Schottky junction (metal/semiconductor heterostructures), the surface plasmon resonance (SPR) effect, photosensitization, superconductivity,
etc
. are utilized in the photocatalytic processes. Furthermore, the enhanced performance of g-C
3
N
4
-based nanocomposites has been widely employed in environmental and energetic applications such as photocatalytic degradation of pollutants, photocatalytic hydrogen generation, carbon dioxide reduction, disinfection, and supercapacitors. This critical review ends with a summary and some perspectives on the challenges and new directions in exploring g-C
3
N
4
-based advanced nanomaterials.
This review summarizes recent advances in the design, synthesis, mechanistic understanding and multifunctional applications of g-C
3
N
4
based heterojunctions/heterostructures.
All-inorganic Pb-free bismuth (Bi) halogen perovskite quantum dots (PQDs) with distinct structural and photoelectric properties provide plenty of room for selective photoreduction of CO2. However, ...the efficient conversion of CO2-to-CO with high selectivity on Bi-based PQDs driven by solar light remains unachieved, and the precise reaction path/mechanism promoted by the surface halogen-associated active sites is still poorly understood. Herein, we screen a series of nontoxic and stable Cs3Bi2X9 (X = Cl, Br, I) PQDs for selective photocatalytic reduction of CO2-to-CO at the gas–solid interface. Among all the reported pure-phase PQDs, the as-synthesized Cs3Bi2Br9 PQDs exhibited the highest CO2-to-CO conversion efficiency generating 134.76 μmol g–1 of CO yield with 98.7% selectivity under AM 1.5G simulated solar illumination. The surface halogen-associated active sites and reaction intermediates were dynamically monitored and precisely unraveled based on in situ DRIFTS investigation. In combination with the DFT calculation, it was revealed that the surface Br sites allow for optimizing the coordination modes of surface-bound intermediate species and reducing the reaction energy of the rate-limiting step of COOH– intermediate formation from •CO2 –. This work presents a mechanistic insight into the halogen-involved catalytic reaction mechanism in solar fuel production.
Novel plasmonic Bi nanoparticles deposited in situ in (BiO)2CO3 microspheres (Bi/BOC) were fabricated via a one-pot hydrothermal treatment of bismuth citrate, sodium carbonate, and thiourea. ...Different characterization techniques, including XRD, SEM, TEM, XPS, UV–vis DRS, PL, time-resolved fluorescence spectra, and photocurrent generation, were performed to investigate the structural and optical properties of the as-prepared samples. The results indicated that the Bi nanoparticles were generated on the surface of (BiO)2CO3 microspheres via the in situ reduction of Bi3+ by thiourea. The Bi nanoparticle deposited (BiO)2CO3 microspheres were employed for the photocatalytic removal of NO in air under visible light irradiation, and the sample exhibited a drastically enhanced photocatalytic activity and oxidation ability. The highly enhanced activity was attributed to the cooperative contribution of the surface plasmon resonance (SPR) effect, the efficient separation of electron–hole pairs, and the prolonged lifetime of charge carriers by the Bi nanoparticles. The behavior of Bi nanoparticles as a cocatalyst for enhancing photocatalytic activity is similar to that of noble metals in photocatalysis. When the amount of thiourea was controlled at 5%, the corresponding Bi/BOC sample exhibited the highest photocatalytic activity and exceeded those of other types of visible light photocatalysts, such as nonmetal-doped TiO2, C3N4, BiOBr, N-doped (BiO)2CO3, and even Ag-deposited (BiO)2CO3. The visible light photocatalytic activity of Bi/BOC was also tested at different wavelengths and with different light sources. It was found that the high activity could be well maintained even under a 5 W energy-saving light, demonstrating its great potential in practical applications. On the basis of DMPO-ESR spin trapping, the active species produced from Bi/BOC under visible light were hydroxyl radicals. Bi/BOC could produce more hydroxyl radicals in comparison to BOC due to the SPR effect of Bi, contributing to the enhanced oxidation ability. Furthermore, the Bi/BOC sample displayed a high photochemical stability under repeated irradiation. This work demonstrated the great feasibility of utilizing low-cost Bi nanoparticles as a substitute for noble metals to enhance visible light photocatalysis.
Herein, we report a facile strategy for the creation of 2D layered heterostructures with intimate interfacial contact and exposed reactive facets. The 2D layered heterostructures with intimate ...contact by sharing the interfacial oxygen atoms and exposed reactive facets endowed the as-prepared BiOIO3/BiOI nanostructures with highly enhanced visible photocatalytic performance for NO removal.
A visible-light-induced charge transfer pathway and a molecular-level photocatalysis mechanism on Bi semimetal@defective BiOBr hierarchical microspheres were revealed by combined in situ DRIFTS and ...DFT calculation.
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•Bi semimetal@defective BiOBr hierarchical microspheres were constructed.•Bi semimetal@defective BiOBr exhibited highly enhanced photocatalysis.•The charge transfer pathway on Bi metal@defective BiOBr was unraveled.•The photocatalysis mechanism on Bi metal@defective BiOBr was revealed.•A combined theoretical and experimental method was developed.
Charge transfer pathway and catalysis mechanism are two major issues in a specific catalytic reaction process. To further probe these two aspects of photocatalytic NO oxidation to address the environmental problem, Bi metal@defective BiOBr hierarchical microspheres were fabricated and used as a visible light photocatalyst. The interfacial and surface properties of Bi metal@defective BiOBr were optimized to understand the SPR effect of Bi metal and the oxygen vacancies (OVs) formed in situ. It was found that the charge transfer pathway on Bi metal@defective BiOBr has been significantly changed from that on pristine BiOBr. The Bi semimetal could act both as a charge transfer bridge and as a hot electron donor. The OVs induced the formation of an intermediate level in the band structure of BiOBr and promote O2 activation and thus the generation of O2− species. Due to the synergistic effects of Bi metal and OVs, Bi metal@defective BiOBr demonstrated highly enhanced visible light photocatalytic performance for NO removal. The photocatalytic NO oxidation process has been monitored by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), which could reveal the reaction intermediate products accurately. On the basis of an investigation with in situ DRIFTS and the simulation of the electronic structure, a new photocatalysis mechanism involving Bi metal, OVs, and NO transformation was proposed. The perspectives on the charge transfer pathway and photocatalysis mechanism in the present work can be extended to other catalysts for tuning the interfacial properties and enhancing the photocatalytic performance to address environmental problems.
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•Bi-metal-decorated amorphous bismuth oxide photocatalysts were fabricated.•The Bi metal could activate the amorphous bismuth oxide via the SPR effect.•The photocatalytic performance ...can be well tuned via content of Bi metal.•Bi metal/amorphous bismuth oxide displayed enhanced photocatalytic activity.•The photocatalysis mechanism was revealed with ESR and in situ DRIFT.
Amorphous semiconductors are seldom exploited as effective photocatalysts, as they are restricted by abundant bulk defects as carrier recombination centers. To activate amorphous bismuth oxide for efficient visible-light photocatalytic performance, a novel and facile strategy was developed. Plasmonic Bimetal-decorated amorphous bismuth oxide (Bi–BiO) was prepared by partial reduction with NaBH4. The content of Bi metal and the photocatalytic activity of the catalysts can be modulated by controlling the concentration of NaBH4 solution. Various techniques were employed to explore the structural features, optical properties, and active species during photocatalysis. The as-synthesized Bi–BiO catalysts were applied in photocatalytic removal of NO in air under and exhibited highly enhanced visible light photocatalytic activity. The significantly increased photocatalytic capability can be attributed to the combined effects of the enhanced visible light absorption and the improved separation efficiency of the charge carriers attributed to the surface plasmon resonance conferred by Bi metal. The advanced Bi–BiO catalysts also exhibited high photochemical and structural stability under repeated irradiation. Moreover, in situ DRIFT was carried out to reveal the time-dependent evolution of reaction intermediates during photocatalytic NO oxidation. A molecular-level photocatalysis mechanism was first proposed for Bi–BiO based on ESR and in situ DRIFT. This work could provide a new perspective in utilizing non-noble-metal Bi as a key activation factor to trigger the photocatalytic ability of amorphous semiconductors.
The bismuth element synthesized by a facile chemical solution method exhibited an admirable and stable photocatalytic activity towards the removal of NO under 280 nm light irradiation due to the ...surface plasmon resonance mediated direct photocatalysis, and most strikingly, showed a catalytic "memory" capability following illumination.
The photocatalytic performance of the star photocatalyst g-C3N4 was restricted by the low efficiency because of the fast charge recombination. The present work developed a facile in situ method to ...construct g-C3N4/g-C3N4 metal-free isotype heterojunction with molecular composite precursors with the aim to greatly promote the charge separation. Considering the fact that g-C3N4 samples prepared from urea and thiourea separately have different band structure, the molecular composite precursors of urea and thiourea were treated simultaneously under the same thermal conditions, in situ creating a novel layered g-C3N4/g-C3N4 metal-free heterojunction (g-g CN heterojunction). This synthesis method is facile, economic, and environmentally benign using easily available earth-abundant green precursors. The confirmation of isotype g-g CN heterojunction was based on XRD, HRTEM, valence band XPS, ns-level PL, photocurrent, and EIS measurement. Upon visible-light irradiation, the photogenerated electrons transfer from g-C3N4 (thiourea) to g-C3N4 (urea) driven by the conduction band offset of 0.10 eV, whereas the photogenerated holes transfer from g-C3N4 (urea) to g-C3N4 (thiourea) driven by the valence band offset of 0.40 eV. The potential difference between the two g-C3N4 components in the heterojunction is the main driving force for efficient charge separation and transfer. For the removal of NO in air, the g-g CN heterojunction exhibited significantly enhanced visible light photocatalytic activity over g-C3N4 alone and physical mixture of g-C3N4 samples. The enhanced photocatalytic performance of g-g CN isotype heterojunction can be directly ascribed to efficient charge separation and transfer across the heterojunction interface as well as prolonged lifetime of charge carriers. This work demonstrated that rational design and construction of isotype heterojunction could open up a new avenue for the development of new efficient visible-light photocatalysts.
•The g-C3N4 was synthesized by direct pyrolysis of cheap urea at 550°C in air.•The g-C3N4 treated for longer time possessed high surface area of 288m2/g.•A layer exfoliation and splitting mechanism ...was proposed.•The nanoarchitectures of g-C3N4 can be simply engineered by pyrolysis time.•The activity enhancement was ascribed to the synergistic effects of multiple factors.
In order to develop g-C3N4 for better visible light photocatalysis, g-C3N4 nanoarchitectures was synthesized by direct pyrolysis of cheap urea at 550°C and engineered through the variation of pyrolysis time. By prolonging the pyrolysis time, the crystallinity of the resulted sample was enhanced, the thickness and size of the layers were reduced, the surface area and pore volume were significantly enlarged, and the band structure was modified. Especially for urea treated for 4h, the obtained g-C3N4 nanosheets possessed high surface area (288m2/g) due to the reduced layer thickness and the improved porous structure. A layer exfoliation and splitting mechanism was proposed to explain the gradual reduction of layer thickness and size of g-C3N4 nanoarchitectures with increased pyrolysis time. The as-synthesized g-C3N4 samples were applied for photocatalytic removal of gaseous NO and aqueous RhB under visible light irradiation. It was found that the activity of g-C3N4 was gradually improved as the pyrolysis time was prolonged from 0min to 240min. The enhanced crystallinity, reduced layer thickness, high surface area, large pore volume, enlarged band gap, and reduced number of defects were responsible for the activity enhancement of g-C3N4 sample treated for a longer time. As the precursor urea is very cheap and the synthesis method is facile template-free, the as-synthesized g-C3N4 nanoscale sheets could provide an efficient visible light driven photocatalyst for large-scale applications.
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