Effective Interfacing of Surface Homojunctions on Chemically Identical g-C3N4 for Efficient Visible-Light Photocatalysis without Sacrificial Agents

Developing efficient homojunctions on g-C3N4 promises metal-free photocatalysis to realize truly sustainable artificial photosynthesis. However, current designs are limited by hindered charge separation due to inevitable grain boundaries and random formation of ineffective homojunctions embedded within the photocatalyst. Here, efficient photocatalysis is driven by introducing effective surface homojunctions on chemically and structurally identical g-C3N4 through leveraging its size-dependent electronic properties. Using a top-down approach, the surface layer of bulk g-C3N4 is partially exfoliated to create sheet-like g-C3N4 nanostructures on the bulk material. This hierarchical design establishes a subtle band energy offset between the macroscopic and nanoscopic g-C3N4, generating homojunctions while maintaining the chemical and structural integrities of the original g-C3N4. The optimized g-C3N4 homojunction demonstrates superior photocatalytic degradation of antibiotic pollutants at >96% efficiency in 2 h, even in different real water samples. It achieves reaction kinetics (≈0.041 min-1) up to fourfold better than standalone materials and their physical mixture. Mechanistic studies highlight the importance of the unique design in boosting photocatalysis by effectively promoting interfacial photocarrier manipulation and utilization directly at the point-of-catalysis, without needing co-catalysts or sacrificial agents. This work presents enormous opportunities for developing advanced and green photocatalytic platforms for sustainable light-driven environmental, energy, and chemical applications.

G-C3N4 sheet nanoarchitectonics with island-like crystalline/amorphous homojunctions towards efficient H2 and H2O2 evolution

Photocatalystic evolution of H2O2 from water and oxygen has attracted significant attention because of environmentally friendly. The absorption in visible and hydrophilic feature of graphitic carbon nitride (g-C3N4) make it a good candidate. In this paper, a rapid post-treatment at high temperature was developed to obtain g-C3N4 nanosheets with abundant crystalline/amorphous interfaces to form homojunctions, which optimized uniplanar carrier mobility dynamics. The conversion from bulk to two-dimensional g-C3N4 resulted from the breakage of interplanar hydrogen bonds and interlayer Van der Waals force. The unique morphology not only rendered photocatalyst with larger specific surface area but also inhibited the robust volume recombination of charge carriers. The accelerated charge carriers flow at the interface, interplane and interlayer together ameliorated the separation and transfer of electrons and holes. A new-emerged n→π* transition ameliorated the poor light utilization efficiency. Beyond the increased photocatalytic H2 evolution property (779.2 μmol g-1 h-1), optimized sample displayed a H2O2 evolution activity as high as 4877.1 μM g-1 h-1 under visible light illumination, which was ∼5.8 times of that of bulk g-C3N4. Detailed photocatalytic mechanism investigation manifested that the two-step single-electron oxygen reduction process occupied the dominant status in H2O2 evolution. This work proposed a novel strategy for obtaining g-C3N4 homojunctions as a promising bi-functional metal-free catalyst to be applied in clean energy production field.

Recent advances of semiconductor photocatalysis for water pollutant treatment: mechanisms, materials and applications

Semiconductor photocatalysis has become an increasing area of interest for use in water treatment methods. This review systematically presents the recent developments of emerging semiconductor photocatalysis system and their application in the removal of water pollutants. A brief overview of the semiconductor photocatalysis mechanism involved with the generation of reactive oxygen species (ROS) is provided first. Then a detailed explanation of the development of TiO2-based, g-C3N4-based, and bismuth-based semiconductor materials and their applications in the degradation of water pollutants are highlighted with recent illustrative examples. Furthermore, the future prospects of semiconductor photocatalysis for water treatment are critically analyzed.

Potassium Molten Salt-Mediated In Situ Structural Reconstruction of a Carbon Nitride Photocatalyst

Metal-free polymeric carbon nitride (PCN) materials are at the forefront of photocatalytic applications. Nevertheless, the overall functionality and performance of bulk PCN are limited by rapid charge recombination, high chemical inertness, and inadequate surface-active sites. To address these, here, we employed potassium molten salts (K⁺X^-, where X^- is Cl^-, Br^-, and I^-) as a template for the in situ generation of surface reactive sites in thermal pyrolyzed PCN. Theoretical calculations imply that addition of KX salts to PCN-forming monomers causes halogen ions to be doped into C or N sites of PCN with a relative trend of halogen ion doping being Cl < Br < I. The experimental results show that reconstructing C and N sites in PCN develops newer reactive sites that are beneficial for surface catalysis. Interestingly, the photocatalytic H₂O₂ generation rate of KBr-modified PCN was 199.0 μmol h^-1, about three times that of bulk PCN. Owing to the simple and straightforward approach, we expect molten salt-assisted synthesis to have wide exploration in modifying PCN photocatalytic activity.

Functionalized Regulation of Metal Defects in ln2S3 of p-n Homojunctions

The introduction of metal vacancies into n-type semiconductors could efficiently construct intimate contact interface p-n homojunctions to accelerate the separation of photogenerated carriers. In this work, a cationic surfactant occupancy method was developed to synthesize an indium-vacancy (V_In)-enriched p-n amorphous/crystal homojunction of indium sulfide (A/C-IS) for sodium lignosulfonate (SL) degradation. The amount of V_In in the A/C-IS could be regulated by varying the content of added cetyltrimethylammonium bromide (CTAB). Meanwhile, the steric hindrance of CTAB produced mesopores and macropores, providing transfer channels for SL. The degradation rates of A/C-IS to SL were 8.3 and 20.9 times higher than those of crystalline In₂S₃ and commercial photocatalyst (P25), respectively. The presence of unsaturated dangling bonds formed by V_In reduced the formation energy of superoxide radicals (^•O₂^-). In addition, the inner electric field between the intimate contact interface p-n A/C-IS promoted the migration of electron-hole pairs. A reasonable degradation pathway of SL by A/C-IS was proposed based on the above mechanism. Moreover, the proposed method could also be applicable for the preparation of p-n homojunctions with metal vacancies from other sulfides.

Synergistic effects of the hybridization between boron-doped carbon quantum dots and n/n-type g-C3N4 homojunction for boosted visible-light photocatalytic activity

Dye wastewater has raised a prevalent environmental concern due to its ability to prevent the penetration of sunlight through water, thereby causing a disruption to the aquatic ecosystem. Carbon quantum dots (CQDs) are particularly sought after for their highly tailorable photoelectrochemical and optical properties. Simultaneously, graphitic carbon nitride (g-C₃N₄) has gained widespread attention due to its suitable band gap energy as well as excellent chemical and thermal stabilities. Herein, a novel boron-doped CQD (BCQD)-hybridized g-C₃N₄ homojunction (CN) nanocomposite was fabricated via a facile hydrothermal route. The optimal photocatalyst sample, 1-BCQD/CN (with a 1:3 mass ratio of boron to CQD) accomplished a Rhodamine B (RhB, 10 mg/L) degradation efficiency of 96.8% within 4 h under an 18 W LED light irradiation. The kinetic rate constant of 1.39 × 10^-2 min^-1 achieved by the optimum sample was found to be 3.6- and 2.8-folds higher than that of pristine CN and un-doped CQD/CN, respectively. The surface morphology, crystalline structure, chemical composition and optical properties of photocatalyst samples were characterized via TEM, FESEM-EDX, XRD, FTIR, UV-Vis DRS and FL spectrometer. Based on the scavenging tests, it was revealed that the photogenerated holes (h⁺), superoxide anions (∙O₂^-) and hydroxyl radicals (∙OH) were the primary reactive species responsible for the photodegradation process. Overall, the highly efficient 1-BCQD/CN composite with excellent photocatalytic activity could provide a cost-effective and robust means to address the increasing concerns over global environmental pollution.

Silver nanoparticles embedded 2D g-C3N4nanosheets toward excellent photocatalytic hydrogen evolution under visible light

Photocatalytic water splitting is considered to be a feasible method to replace traditional energy. However, most of the catalysts have unsatisfactory performance. In this work, we used a hydrothermal process to grow Ag nanoparticlesin situon g-C₃N₄nanosheets, and then a high performance catalyst (Ag-g-C₃N₄) under visible light was obtained. The Ag nanoparticles obtained by this process are amorphous and exhibit excellent catalytic activity. At the same time, the local plasmon resonance effect of Ag can effectively enhance the absorption intensity of visible light by the catalyst. The hydrogen production rate promote to 1035μmol g^-1h^-1after loaded 0.6 wt% of Ag under the visible light, which was 313 times higher than that of pure g-C₃N₄(3.3μmol g^-1h^-1). This hydrogen production rate is higher than most previously reported catalysts which loaded with Ag or Pt. The excellent activity of Ag-g-C₃N₄is benefited from the Ag nanoparticles and special interaction in each other. Through various analysis and characterization methods, it is shown that the synergy between Ag and g-C₃N₄can effectively promote the separation of carriers and the transfer of electrons. Our work proves that Ag-g-C₃N₄is a promising catalyst to make full use of solar energy.

Oxygen-containing groups and P doped porous carbon nitride nanosheets towards enhanced photocatalytic activity

Metal-free polymer graphite carbon nitride (CN) is a promising photocatalyst that has garnered significant research attention. However, unmodified CN possesses several shortcomings such as low specific surface area, poor dispersibility in water, and rapid photogenerated electron-hole recombination, which have severely impacted its mass adoption. Here, this study proposed a two-step heat treatment method to incorporate P dopant and the containing-oxygen groups successively into CN. The final product, denoted as PO-CN, possessed a porous ultrathin nanosheet-like morphology. The introduction of P dopant altered the intrinsic electronic structure of CN. Meanwhile, the presence of oxygen-containing groups improved the dispersibility of PO-CN in water. Also, it led to the formation of a porous ultrathin structure that could provide more active sites. Through the synergistic effects of these two methods, PO-CN demonstrated superior photocatalytic performance compared to the unmodified counterpart. Based on the collective results obtained experimentally and theoretically, PO-CN possessed a porous ultrathin structure, low resistance, and low carrier recombination. The results show an optimal hydrogen evolution rate of PO-CN (997.7 mol h^-1 g^-1), which was 11.2 times and 3.22 times that of the CN (88.89 mol h^-1 g^-1) and PCN (310.3 mol h^-1 g^-1). Moreover, PO-CN was then used in the degradation of Rhodamine B, and a degradation kinetic constant (k) of 0.15009 was calculated, which was 18.42 times and 8.22 times higher as compared to those of CN (0.00815) and PCN (0.01826). Hence, this work provides a new strategy for the alteration of the morphology and electronic structure of CN.

Structure modulation of g-C3N4 in TiO2{001}/g-C3N4 hetero-structures for boosting photocatalytic hydrogen evolution

Structure design of photocatalysts is highly desirable for taking full advantage of their abilities for H₂ evolution. Herein, the highly-efficient TiO₂{001}/g-C₃N₄ (TCN) heterostructures have been fabricated successfully via an in situ ethanol-thermal method. And the structure of g-C₃N₄ in the TCN heterostructures could be exfoliated from bulk g-C₃N₄ to nanosheets, nanocrystals and quantum dots with the increase of the synthetic temperature. Through detailed characterization, the structural evolution of g-C₃N₄ could be attributed to the enhanced temperature of the ethanol-thermal treatment with the shear effects of HF acid. As expected, the optimal TCN-2 heterostructure shows excellent photocatalytic H₂ evolution efficiency (1.78 mmol h⁻¹ g⁻¹) under visible-light irradiation. Except for the formed built-in electric field, the significantly enhanced photocatalytic activity of TCN-2 could be ascribed to the enhanced crystallinity of TiO₂{001} nanosheets and the formed g-C₃N₄ nanocrystals with large surface area, which could extend the visible light absorption, and expedite the transfer of photo-generated charge carriers further. Our work could provide guidance on designing TCN heterostructures with the desired structure for highly-efficient photocatalytic water splitting.

The structure of g-C₃N₄ in the prepared TiO₂{001}/g-C₃N₄ hetero-structures could be modulated from BCN to CN-NS, CN-NC or CN-QD by tuning the synthetic temperature.

Pt nanoparticles embedded spine-like g-C3N4 nanostructures with superior photocatalytic activity for H2 generation and CO2 reduction

Conventional two-dimensional (2D) graphitic carbon nitride, 2D g-C₃N₄ with its layered structures and flat and smooth 2D surface possesses certain disadvantages that is affecting their photocatalytic performances. In this paper, new nanostructured spine-like three-dimensional (3D) g-C₃N₄ nanostructures are created for the first time via a new three-step synthesis method. In this method, self-assembly of layered precursors and H⁺ intercalation introduced by acid treatment play an important role for the unique nanostructure formation of 3D g-C₃N₄ nanostructures. The spine-like 3D g-C₃N₄ nanostructures show a superior photocatalytic performance for H₂ generation, achieving 4500 μmol·g^-1·h^-1, 8.2 times higher than that on conventional 2D g-C₃N₄. Remarkably spine-like 3D g-C₃N₄ nanostructures demonstrate a clear photocatalytic activity toward CO₂ reduction to CH₄ (0.71 μmol·g^-1·h^-1) in contrast to the negligible photocatalytic performance of conventional 2D g-C₃N₄ for the reaction. Adding Pt clusters as co-catalysts substantially enhance the CH₄ generation rate of the 3D g-C₃N₄ nanostructures by 4 times (2.7 μmol·g^-1·h^-1). Spine-like 3D g-C₃N₄ caged nanostructure leads to the significantly increased active sites and negatively shifted conduction band position in comparison with conventional 2D g-C₃N₄, favorable for the photocatalytic reduction reaction. This study demonstrates a new platform for the development of efficient photocatalysts based on nanostructured 3D g-C₃N₄ for H₂ generation and conversion of CO₂ to useful fuels such as CH₄.

The edge-epitaxial growth of yellow g-C3N4 on red g-C3N4 nanosheets with superior photocatalytic activities

g-C₃N₄ has been used as a photocatalyst to overcome the issues of environmental crises and energy shortages. Here, red g-C₃N₄ nanosheets (E_g: ∼ 1.89 eV) were used as seeds for the edge-epitaxial growth of yellow g-C₃N₄ (E_g: ∼ 2.59 eV) to form type II heterostructures. The heterostructures revealed superior photocatalytic activity for enhanced H₂ (3996 μmol g^-1 h^-1), CO (3.8 μmol g^-1 h^-1), and CH₄ (1.8 μmol g^-1 h^-1) evolution.

The pivotal role of defects in fabrication of polymeric carbon nitride homojunctions with enhanced photocatalytic hydrogen evolution

Fabrication of homojunctions is a cost-effective efficient way to enhance the photocatalytic performance of polymeric carbon nitride (CN), but the generation of defects upon synthesizing CN homojunctions and their roles in the homojunction fabrication were hardly reported. Herein, nitrogen-deficient CN homojunctions were simply synthesized by calcining dicyandiamide-loaded CN (prepared from urea and denoted as UCN) with dicyandiamide polymerizing into CN (denoted as DCN) and simultaneous formation of nitrogen vacancies in the surface of UCN. Fabrication of the nitrogen-deficient UCN (dUCN)/DCN homojunction depends on the nitrogen vacancy content in dUCN which can tune the energy band structure of dUCN from not matching to matching with that of DCN. The dUCN/DCN homojunction exhibits extended optical absorption and remarkably enhanced charge separation and photocatalytic H₂ evolution, compared with UCN and DCN. This work illustrates the pivotal role of defects in fabricating CN homojunctions and supplies a new facile way to synthesize nitrogen-deficient CN.

Design of earth-abundant Z-scheme g-C3N4/rGO/FeOOH ternary heterojunctions with excellent photocatalytic activity

A ternary Z-scheme g-C₃N₄/rGO/FeOOH heterostructure photocatalyst for H₂ production was designed and fabricated, which exhibited photocatalytic H₂ production of 124.9 and 869.8 μmol h⁻¹ g⁻¹ under visible and UV-Vis light irradiation, respectively.

Fusiform-Shaped g-C3 N4 Capsules with Superior Photocatalytic Activity

Carbon nitride (g-C₃ N₄ ) nanostructure rebuilding is an effective means to modify its photocatalytic properties, especially the hollow micron-nanostructure. The increased scattering in the body effectively improves the light utilization efficiency and improves catalytic properties. In this work, fusiform-shaped g-C₃ N₄ capsules are created by controlling the nucleation kinetics of supramolecular assemblies. The fusiform-shaped capsule micron-nanostructure is synthesized with ultrathin wall thickness and adjusted carbon/nitride ratios which decrease the recombination rate of photo-generated carriers. The hollow nanostructure and relatively higher specific surface area of the fusiform-shaped capsule effectively enhance light scattering inside body and lead to an enhanced carrier utilization efficiency. Moreover, the decrease of bandgap and relatively negative conduction band position affect the response of hollow fusiform-shaped g-C₃ N₄ capsules (Hf-g-C₃ N₄ ) in visible light region and improve the photo-reducing performance. In term of H₂ evolution property, Hf-g-C₃ N₄ has been improved to 7052 µmol g^-1 h^-1 , which is 10.9 times higher compared with bulk structure. More importantly, Hf-g-C₃ N₄ can produce CH₄ at the rate of 1.63 µmol g^-1 h^-1 without help of co-catalyst and hole sacrificial agent in the photocatalytic reduction reaction of CO₂ to CH₄ . At same time, the selective photocatalytic reduction of CO₂ is another advantage of Hf-g-C₃ N₄ .

Nanosheet-assembled hierarchical flower-like g-C3N4 for enhanced photocatalytic CO2 reduction activity

Nanosheet-assembled hierarchical flower-like g-C₃N₄ prepared by a molecular self-assembly and ethanol insertion strategy shows enhanced photocatalytic CO₂ reduction activity.

One-Step Growth of Amorphous/Crystalline Ga2O3 Phase Junctions for High-Performance Solar-Blind Photodetection

The pursuit of high-performance photodetectors functioning in the solar-blind spectrum is motivated by both scientific and practical applications ranging from secure communication, monitoring, sensing, etc. In particular, the fabrication of heterojunctions based on the wide band gap semiconductors has emerged as an attractive strategy to promote the high-efficient photogenerated electron/hole pair separation. However, the precisely controlled growth of heterojunctions remains a huge challenge. The lattice mismatch leads to the formation of defects and/or dislocations at the interface, deteriorating the performance of devices and limiting their envisioned applications. Here, we demonstrate a simple one-step growth of amorphous/crystalline Ga₂O₃ phase junctions by using sputtering technique, yielding a large responsivity of 0.81 A/W, a superior photo-to-dark current ratio over 10⁷, and an ultrahigh response speed of ∼12 ns. Compared to the previous reported solar-blind photodetectors, the obtained detectivity ≈ 5.67 × 10¹⁴ Jones is increased by 2 orders of magnitude. Such excellent photoresponse characteristics can be understood by the interfacial built-in field-promoted electron/hole pair separation for the amorphous/crystalline Ga₂O₃ phase junctions. Our results provide a novel path toward realizing high-performance optoelectronics functioning in the solar-blind spectrum.

Preparation of graphene oxide-graphene quantum dots hybrid and its application in cancer theranostics

Currently, an enormous amount of cancer research based on two-dimensional nano-graphene oxide (GO), as well as zero-dimensional graphene quantum dots (GQDs), is being carried out in the fields of therapeutics and diagnostics. However, the exploration of their hybrid "functional" nanomaterials in the theranostic system is still rare. In the current study, a stable complex of GO and GQDs was formed by an electrostatic layer-by-layer assembly via a polyethylene imine bridge (GO-PEI-GQDs). Furthermore, we compared separate mono-equivalents of the GO-PEI-GQDs complex - GO and GQDs, in terms of cell imaging (diagnostics), photothermal, and oxidative stress response in breast cancer cells (MDA-MB-231). GO-PEI-GQDs showed an excellent photothermal response (44-49 °C) upon 808 nm laser (0.5 W cm^-2) exposure for 5 min at a concentration up to 50 μg/mL. We report new synergistic properties of GO-PEI-GQDs such as stable fluorescence imaging and enhanced photothermal and cytotoxic activities on cancer cells. Composite materials made up of GO and GQDs combining diverse properties help to study 2D-0D heterosystems and improve specific therapeutic systems in theranostics.

Rh/polymeric carbon nitride porous tubular catalyst: visible light enhanced chlorophenol hydrodechlorination in base-free aqueous medium

Novel porous tubular Rh/PCN exhibited highly promoted activity in hydrodechlorination of harmful chlorophenols to high-value cyclohexanol and cyclohexanone with base-free aqueous medium and balloon H₂ pressure under the irradiation of visible light.

Synthesis of Bi-deficient monolayered Bi2WO6 nanosheets with enhanced photocatalytic activity under visible light irradiation

The strong protonated hydroxyl groups around Bi vacancies could efficiently promote the separation of photoexcited electron–hole pairs.

Construction of 2D g-C3N4 lateral-like homostructures and their photo- and electro-catalytic activities

New g-C₃N₄ crystalline/amorphous lateral-like homostructures show excellent photocatalytic activity due to the effective separation of photogenerated carriers.

Controllable synthesis of graphitic carbon nitride nanomaterials for solar energy conversion and environmental remediation: the road travelled and the way forward

This review discusses advances in the synthesis and design of g-C₃N₄-based nanomaterials and their various photocatalytic and photoredox applications.

Ultrastable g-C3N4 assemblies with high quantum yield and reversible photoluminescence

Ultrastable g-C₃N₄ assemblies consisting of amorphous/crystalline nanosheets with high quantum yields up to 78% were prepared for the first time. A reversible photoluminescence was observed from green to blue once the pH was adjusted. These assemblies exhibit high stability in PL devices.