Nitrogen-doped Carbon Nanospheres-Modified Graphitic Carbon Nitride with Outstanding Photocatalytic Activity

How can heteroatoms boost the performance of photoactive nanomaterials for wastewater purification?

Photocatalysis, as an alternative for treating persistent water pollutants, holds immense promise. However, limitations hinder sustained treatment and recycling under varying light conditions. This comprehensive review delves into the novel paradigm of metal and non-metal doping to overcome these challenges. It begins by discussing the fundamental principles of photocatalysis and its inherent limitations. Understanding these constraints is crucial for developing effective strategies. Band gap narrowing by metal and non-metal doping modifies the band gap, enabling visible-light absorption. Impurity energy levels and oxygen vacancies influenced the doping energy levels and surface defects. Interfacial electron transfer and charge carrier recombination are the most important factors that impact overall efficiency. The comparative analysis of nanomaterials are reviewed on various, including nanometal oxides, nanocarbon materials, and advanced two-dimensional structures. The synthesis process are narratively presented, emphasizing production yields, selectivity, and efficiency. The review has potential applications in the environment for efficient pollutant removal and water purification, economic cost-effective and scalable production and technological advancement catalyst design, in spite of its challenges in material stability, synthesis methods and optimizing band gaps. The novelty of the review paper is on the proposal of a new paradigm of heterojunctions of doped metal and non-metal photocatalysts to promise highly efficient water treatment. This review bridges the gap between fundamental research and practical applications, offering insights into tailored nano photocatalysts.

Nanoarchitectonics on Z-scheme and Mott-Schottky heterostructure for photocatalytic water oxidation via dual-cascade charge-transfer pathways

The bottleneck for water splitting to generate hydrogen fuel is the sluggish oxidation of water. Even though the monoclinic-BiVO₄ (m-BiVO₄)-based heterostructure has been widely applied for water oxidation, carrier recombination on dual surfaces of the m-BiVO₄ component have not been fully resolved by a single heterojunction. Inspired by natural photosynthesis, we established an m-BiVO₄/carbon nitride (C₃N₄) Z-scheme heterostructure based on the m-BiVO₄/reduced graphene oxide (rGO) Mott-Schottky heterostructure, constructing the face-contact C₃N₄/m-BiVO₄/rGO (CNBG) ternary composite to remove excessive surface recombination during water oxidation. The rGO can accumulate photogenerated electrons from m-BiVO₄ through a high conductivity region over the heterointerface, with the electrons then prone to diffuse along a highly conductive carbon network. In an internal electric field at the heterointerface of m-BiVO₄/C₃N₄, the low-energy electrons and holes are rapidly consumed under irradiation. Therefore, spatial separation of electron-hole pairs occurs, and strong redox potentials are maintained by the Z-scheme electron transfer. These advantages endow the CNBG ternary composite with over 193% growth in O₂ yield, and a remarkable rise in ·OH and ·O₂^- radicals, compared to the m-BiVO₄/rGO binary composite. This work shows a novel perspective for rationally integrating Z-scheme and Mott-Schottky heterostructures in the water oxidation reaction.

Highly efficient removal of organic contaminant with wide concentration range by a novel self-cleaning hydrogel: Mechanism, degradation pathway and DFT calculation

A novel carbon nitride based self-cleaning hydrogel photocatalyst (KI-PCN gel, potassium and iodine co-doped carbon nitride confined in alginate) has been successfully constructed by a facile method. Fabricated photocatalyst showed enhanced synergistic adsorption-photocatalytic degradation property on a high concentration of methylene blue (HMB) because of enhanced carrier separation efficiency and improved light adsorption capacity of KI-PCN. As expected, the KI-PCN gel showed the highest apparent rate constant value ₍K_app =0.0310 min^-1), which was about 38.8 and 5.8 times as that of blank hydrogel (K_app=0.0008 min^-1) and PCN gel (K_app=0.0053 min^-1), respectively. Meanwhile, KI-PCN gel can continuously adsorb low concentration of MB (LMB), and the MB-adsorbed KI-PCN gel can self-clean under light irradiation. The bench-scale experiments simulating real river showed that KI-PCN gel can effectively and continuously remove LMB (0.1-20 ppm), indicating the possibility for the removal of contaminants in natural rivers. Furthermore, the possible degradation pathways were proposed by combining the density functional calculations (DFT) and intermediates identified by liquid chromatography-mass spectrometry (LC-MS). This work proposed a new perspective to acquire a novel self-cleaning and easily recyclable photocatalyst for treatment of wide concentration range organic wastewater as well as remediation of natural waterbody.

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.

N, P, O co-doped carbon filling into carbon nitride microtubes to promote photocatalytic hydrogen production

Carbon nitride (CN) as the photocatalytic hydrogen production catalyst has attracted great attentions but suffering from a poor performance due to the unsatisfied energy band gap and the low separation efficiency of photogenerated carriers. Herein, we create a simple method to construct a novel CN-based photocatalyst, i.e., the N, P, O co-doped carbon filled CN microtube, which presents a narrow band gap, a high separation efficiency of photogenerated carriers, and a good stability. In this novel structure, the tubular morphology of CN ensures a narrow band gap, and the N, P, O co-doped carbon facilitates the transfer of photogenerated electrons. Coupling these two further reduces the energy band gap and improves the separation efficiency. For the photocatalytic hydrogen evolution under the visible light, the optimal sample presents an ultrahigh hydrogen evolution rate of 1149.71 μmol g^-1 h^-1 ranking at the top level, which is 112.60 times that of traditional bulk CN. In addition, it also has a high reusability and good stability after four cycle experiments. This study has provided a new viewpoint to design or develop the high-efficient photocatalysts for hydrogen production.

Insight into the activity of TiO2@nitrogen-doped hollow carbon spheres supported on g-C3N4 for robust photocatalytic performance

Designing an advance nanostructure semiconductor is an efficient strategy to promote the charge separation and thus improve the photocatalytic activity. Herein, a relatively high recombination rate of electron-hole pairs and low specific surface area of g-C₃N₄ (GCN) were subjected to the surface deposition of the core shell nanoparticles composed of nitrogen doped hollow carbon spheres (N-HCSs) as the supporting scaffold and TiO₂ nanoparticles as the photoactive layer. The ternary composites with different TiO₂@N-HCS content were prepared through a simplified nanocasting method followed by the two consecutive hydrothermal process. The effects of nitrogen doping in carbon framework, and nanoparticles amount were evaluated on the photocatalytic ability through the photodegradation of tetracycline (TC) molecules under the visible light irradiation. At the optimum content of core shell nanoparticles (7 wt%), the solar-driven TC photocatalytic degradation for ternary composite was approximately 85%, which was much better (about three times) than that of the pure GCN. More interestingly, the experimental results revealed that doping of nitrogen atoms has a positive role on the charge separation and the resulting photocatalytic efficiency. The employed hollow carbon spheres here play three important roles: (1) providing a substrate to uniformly dispersion of TiO₂ nanoparticles without any aggregation; (2) reducing the combination of charge carriers and improving the separation of photoinduced carriers; (3) formation of larger surface area and more active sites on the photocatalyst surface. Furthermore, the underlying photocatalytic degradation mechanism was introduced by the controlled experiments using photoluminescent and radical scavenger tests.

Recent Advances and Challenges in Photoreforming of Biomass-Derived Feedstocks into Hydrogen, Biofuels, or Chemicals by Using Functional Carbon Nitride Photocatalysts

Photoreforming of biomass into hydrogen, biofuels, and chemicals is highly desired, yet this field of research is still in its infancy. Developing an efficient, novel, and environmentally friendly photocatalyst is key to achieving these goals. To date, the nonmetallic and eco-friendly material carbon nitride has found many uses in reactions such as water splitting, CO₂ reduction, N₂ fixation, and biorefinery, owing to its outstanding photocatalytic activity. However, a narrow light absorption range and fast charge recombination are often encountered in the pristine carbon nitride photocatalytic system, which resulted in unsatisfying photocatalytic activity. To improve the photocatalytic performance of pure carbon nitride in biomass reforming, modification is needed. In this Review, the design and preparation of functional carbon nitride, as well as its photocatalytic properties for the synthesis of hydrogen, biofuels, and chemicals through biomass reforming, are discussed alongside potential avenues for its future development.

Synthesis of mesoporous carbon spheres via a soft-template route for catalyst supports in PEMFC cathodes

Synthesis of carbon spheres via a soft-template route should be further improved for industrial applications especially in terms of time, cost and scalability. The present work reports on the relatively fast production of mesoporous carbon via an ammonia-catalyzed hydrothermal soft-template one-pot route denoted as C_FAH with m-aminophenol as the carbon source and triblock copolymer Pluronic® F127 as the template. For comparison, an acidic route with resol as the carbon precursor (C_FRH) was evaluated as well. The best results regarding particle size and pore distribution of the as-prepared C_FRH and C_FAH samples were obtained in 2 M HCl and 6 M NH₄OH at 120 °C for 12 h and 700 °C pyrolysis temperature, respectively. GDE with C_FRH and C_FAH supported platinum showed excellent ECSA retention of about 60-70% during accelerated degradation testing under half-cell conditions compared to only 13% for GDE with Pt/C_Vulcan reference material.

Converting Organic Wastewater into CO Using MOFs-Derived Co/In2O3 Double-Shell Photocatalyst

The photocatalytic conversion of organic wastewater into value-added chemicals is a promising strategy to solve the environmental issue and energy crisis. Herein, Co/In₂O₃ nanotubes with a double-shell structure, as a highly efficient photocatalyst, are synthesized by a one-step calcination method. The Co/In₂O₃ heterostructure shows an outstanding photocatalytic CO₂ reduction performance of 4902 μmol h^-1 g^-1. Notably, these Co/In₂O₃ photocatalysts also achieve CO₂ self-generation and in situ reduction conversion in acid organic wastewater (phenol solution), in which the high CO₂ (47.5 μmol h^-1 g^-1) and CO (0.9 μmol h^-1 g^-1) evolution rates are demonstrated under solar irradiation. Transient photovoltage (TPV) tests demonstrate that Co nanoparticles on Co/In₂O₃ double-shell heterostructure serve as the CO₂ reduction sites for the effective capture and stabilization of the photogenerated electrons.

Synergism of carbon quantum dots and Au nanoparticles with Bi2MoO6 for activity enhanced photocatalytic oxidative degradation of phenol

Localized surface plasmon resonance (LSPR) offers an opportunity to enhance the efficiency of photocatalysis. However, the photocatalysts's plasmonic enhancement is still limited, as most metals/semiconductors depend on LSPR contribution of isolated metal nanoparticles. In the present work, carbon quantum dots (CQDs) and Au nanoparticles (NPs) were simultaneously assembled on the surface of a three-dimensional (3D) spherical Bi₂MoO₆ (BMO) nanostructure with surface oxygen vacancies (SOVs). The collective excitation of CQDs and Au NPs demonstrated an effective strategy to improve the utilization of up-conversion emission and plasmonic energy. The contribution of CQDs and Au NPs assembled on the surface of BMO (7 wt% CQDs/Au/BMO) realized a photocatalytic phenol degradation enhancement (apparent rate constants, k _app/min^-1) of 56.5, 9.5 and 3.9, and 2.2-fold increase compared to BMO, BMO-SOVs, Au/BMO and CQDs/BMO, respectively. The as-fabricated 7 wt% CQDs/Au/BMO exhibited the highest mineralization rate for phenol degradation with 72.4% TOC removal rate in 120 min. The excellent photocatalytic performance of CQDs/Au/BMO was attributed to the synergistic effect of CQDs, Au NPs and SOVs. The CQD up-conversion emission synergetically boosts Au NPs' LSPR significantly promoting the separation and migration of photogenerated electron (e^-)/hole (h⁺) pairs, which could improve the oxygen molecule activation process and thereby their ability to generate reactive oxygen species (ROS). The present work is a step forward to understand and construct similar photocatalysts using an entirely reasonable hypothesis of activity enhancement mechanism according to the active species capture experiments and band structure analysis.

Porous cauliflower-like molybdenum disulfide/cadmium sulfide hybrid micro/nano structure: Enhanced visible light absorption ability and photocatalytic activity

Micro-/nanostructured materials can control the diffraction and propagation of light, thereby providing new optical properties that can be exploited to enhance photocatalytic processes. In this work, a series of the cauliflower-like MoS₂/CdS hybrid micro-/nanostructures is synthesized. These structures contain numerous cracks and pores that can enhance the absorption and utilization of light as well as shorten the distance for transferring photogenerated electrons to the catalyst surface. The results of ultraviolet-visible diffuse reflectance absorption spectra show that the composite material has enhanced absorption in the visible light region. Further investigation of the optical characteristics of the synthesized materials using a finite-difference time-domain (FDTD) simulation reveals that the cauliflower-like micro-/nanostructure increases the optical absorption intensity at the MoS₂/CdS interface. Notably, the MoS₂/CdS hybrid micro-/nanostructures exhibits high photocatalytic hydrogen production activity (9.5 mmol g^-1 h^-1) and long-lasting cycle stability. This work helps us to further understand the enhancement mechanism of light absorption and utilization by porous structural materials.

Non-metal sensory electrode design and protocol of DNA-nucleobases in living cells exposed to oxidative stresses

Sensory protocols for evaluation of DNA distortion due to exposure to various harmful chemicals and environments in living cells are needed for research and clinical investigations. Here, a design of non-metal sensory (NMS) electrode was built by using boron-doped carbon spherules for detection of DNA nucleobases, namely, guanine (Gu), adenine (Ad), and thymine (Th) in living cells. The key-electrode based nanoscale NMS structures lead to voids with a facile diffusion, and strong binding events of the DNA nucleobases. Furthermore, the NMS geometric structures would significantly create electrode surfaces with numerous centrally active sites, curvature topographies, and anisotropic spherules. The NMS shows potential as sensitive protocol for DNA-nucleobases in living cells exposed to oxidative stresses. In one-step signaling assay, NMS shows high signaling transduction of Gu-, Ad-, and Th-DNA nucleobases targets with ultra-sensitive and low detection limits of 3.0, 0.36, and 0.34 nM, respectively, and a wide linear range of up to 1 μM. The NMS design and protocol show evidence of the role of surface construction features and B-atoms incorporated into the graphitic carbon network for creating abundant active sites with facile electron diffusion and heavily target loads along with within-/out-plane circular spheres. Indeed NMS, with spherule-rich interstitial surfaces can be used for sensitive and selective evaluation of damaged-DNA to various dysfunctional metabolism in the human body.