Conjugated polymers have recently been under active investigation as promising alternatives to traditional inorganic semiconductors for photocatalysis. This is due to their unique advantages of low cost, high chemical stability, and molecularly tunable optoelectronic properties. This critical review summarizes the recent advancements in π-conjugated polymers for visible-light-driven photocatalytic applications including water splitting, CO2 reduction, organic transformation and degradation of organic dyes. Special emphasis is placed on how the changes in the polymer structure could influence their physicochemical properties and photocatalytic activities. This structure–activity relationship analysis should guide rational molecular design of conjugated polymers for improved photocatalytic activity.

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Conjugated polymers for visible-light-driven photocatalysis

As an alluring metal-free polymeric semiconductor material, graphite-like carbon nitride (g-C3N4; abbreviated as GCN) has triggered a new impetus in the field of photocatalysis, mainly favoured from its fascinating physicochemical and photoelectronic structural features. However, certain inherent drawbacks, involving rapid reassembly of photocarriers, low specific surface area and insufficient optical absorption, limit the wide-range applicability of GCN. Generation of 0D point defects mainly by introducing vacancies (C and/or N) into the framework of GCN has spurred extensive consideration owing to their distinctive qualities to manoeuvre substantially, the optical absorption, radiative carrier isolation, and surface photoreactions. The present review endeavours to summarise a comprehensive study on vacancy defect engineered GCN. Starting from the basic introduction of defects and C/N vacancy modulated GCN, numerous advanced strategies for the controlled designing of vacancy rich GCN have been explored and discussed. Afterwards, light was thrown on the various substantial technologies which are useful for characterising and identifying the introduction of defects in GCN. The salient significance of defect engineering in GCN has been reviewed concerning its impact on optical absorption, charge isolation and surface photoreaction ability. Typically, the achievement of defect engineered GCN has been scrutinised toward various applications like photocatalytic water splitting, CO2 conversion, N2 fixation, pollutant degradation, and H2O2 production. Finally, the review ends with conclusions and vouchsafing future challenges and opportunities on the intriguing and emerging area of vacancy defect engineered GCN photocatalysts.

C-, N-Vacancy defect engineered polymeric carbon nitride towards photocatalysis: viewpoints and challenges

Photoreduction of N₂ is among the most interesting and challenging methods for N₂ fixation under mild conditions. In this study, we determined that nitrogen vacancies (NVs) could endow graphitic carbon nitride (g-C₃N₄) with the photocatalytic N₂ fixation ability. Photocatalytic N₂ fixation induced by NVs is free from the interference of other gases. The reason is that NVs could selectively adsorb and activate N₂ because NVs have the same shape and size as the nitrogen atom in N₂. In addition to this, NVs could also improve many other properties of g-C₃N₄ to support photocatalytic N₂ fixation. These new findings could elucidate the design of efficient photocatalysts for photocatalytic N₂ fixation.

Selective photocatalytic N₂ fixation dependent on g-C₃N₄ induced by nitrogen vacancies

Construction of LSPR-enhanced 0D/2D CdS/MoO3-x S-scheme heterojunctions for visible-light-driven photocatalytic H2 evolution

Carbon nitride (CN), a 2D material composed of only carbon (C) and nitrogen (N), which are linked by strong covalent bonds, has been used as a metal-devoid and visible-light-active photocatalyst owing to its magnificent optoelectronic and physicochemical properties including suitable bandgap, adjustable energy-band positions, tailor-made surface functionalities, low cost, metal-free nature, and high thermal, chemical, and mechanical stabilities. CN-based materials possess a lot of advantages over conventional metal-based inorganic photocatalysts including ease of synthesis and processing, versatile functionalization or doping, flexibility for surface engineering, low cost, sustainability, and recyclability without any leaching of toxic metals from photocorrosion. Carbon nitrides and their hybrid materials have emerged as attractive candidates for CO2 capture and its reduction into clean and green low-carbon fuels and valuable chemical feedstock by using sustainable and intermittent renewable energy sources of sunlight and electricity through the heterogeneous photo(electro)catalysis. Here, the latest research results in this field are summarized, including implementation of novel functionalized nanostructured CNs and their hybrid heterostructures in meeting the stringent requirements to raise the efficiency of the CO2 reduction process by using state-of-the-art photocatalysis, electrocatalysis, photoelectrocatalysis, and feedstock reactions. The research in this field is primarily focused on advancement in the synthesis of nanostructured and functionalized CN-based hybrid heterostructured materials. More importantly, the recent past has seen a surge in studies focusing significantly on exploring the mechanism of their application perspectives, which include the behavior of the materials for the absorption of light, charge separation, and pathways for the transport of CO2 during the reduction process.

Nanostructured Carbon Nitrides for CO2 Capture and Conversion

Two-dimensional graphitic carbon nitride (g-C3N4) has been widely explored as a promising photocatalyst for solar CO2 conversion. However, rapid charge recombination and low visible-light utilization are severely detrimental to photocatalytic CO2 conversion. Zero-dimensional/two-dimensional (0D/2D) heterostructures are considered the promising materials with size tunability and enhanced charge separation efficiency for photocatalysis. Herein, a 0D/2D heterostructure of oxygen vacancy-rich TiO2 quantum dots confined in g-C3N4 nanosheets (TiO2-x/g-C3N4) was prepared by in-situ pyrolysis of NH2-MIL-125 (Ti) and melamine. Charge dynamics analysis by time-resolved photoluminescence (tr-PL) and femtosecond and nanosecond pump-probed transient absorption (TA) spectra revealed that charges transfer occured from 2D-g-C3N4 to 0D-TiO2 at an ultrafast subpicosecond time scale (

Interfacial charge transfer in 0D/2D defect-rich heterostructures for efficient solar-driven CO2 reduction

Plasmonic nanostructures have tremendous potential to be applied in photocatalytic CO2 reduction, since their localized surface plasmon resonance can collect low‐energy‐photons to derive energetic “hot electrons” for reducing the CO2 activation‐barrier. However, the hot electron‐driven CO2 reduction is usually limited by poor efficiency and low selectivity for producing kinetically unfavorable hydrocarbons. Here, a new idea of plasmonic active “hot spot”‐confined photocatalysis is proposed to overcome this drawback. W18O49 nanowires on the outer surface of Au nanoparticles‐embedded TiO2 electrospun nanofibers are assembled to obtain lots of Au/TiO2/W18O49 sandwich‐like substructures in the formed plasmonic heterostructure. The short distance (< 10 nm) between Au and adjacent W18O49 can induce an intense plasmon‐coupling to form the active “hot spots” in the substructures. These active “hot spots” are capable of not only gathering the incident light to enhance “hot electrons” generation and migration, but also capturing protons and CO through the dual‐hetero‐active‐sites (Au‐O‐Ti and W‐O‐Ti) at the Au/TiO2/W18O49 interface, as evidenced by systematic experiments and simulation analyses. Thus, during photocatalytic CO2 reduction at 43± 2 °C, these active “hot spots” enriched in the well‐designed Au/TiO2/W18O49 plasmonic heterostructure can synergistically confine the hot‐electron, proton, and CO intermediates for resulting in the CH4 and CO production‐rates at ≈35.55 and ≈2.57 µmol g−1 h−1, respectively, and the CH4‐product selectivity at ≈93.3%.

Plasmonic Active “Hot Spots”‐Confined Photocatalytic CO2 Reduction with High Selectivity for CH4 Production

Fundamental photocatalytic limitations of solar CO2 reduction remain due to low efficiency, serious charge recombination, and short lifetime of catalysts. Herein, two-dimensional graphitic carbon nitride nanosheets with nitrogen vacancies (g-C3 Nx ) located at both three-coordinate N atoms and uncondensed terminal NHx species were prepared by one-step tartaric acid-assistant thermal polymerization of dicyandiamide. Transient absorption spectra revealed that the defects in g-C3 N4 act as trapped states of charges to result in prolonged lifetimes of photoexcited charge carriers. Time-resolved photoluminescence spectroscopy revealed that the faster decay of charges is due to the decreased interlayer stacking distance in g-C3 Nx in favor of hopping transition and mobility of charge carriers to the surface of the material. Owing to the synergic virtues of strong visible-light absorption, large surface area, and efficient charge separation, the g-C3 Nx nanosheets with negligible loss after 15 h of photocatalysis exhibited a CO evolution rate of 56.9 μmol g-1  h-1 under visible-light irradiation, which is roughly eight times higher than that of pristine g-C3 N4 . This work presents the role of defects in modulating light absorption and charge separation, which opens an avenue to robust solar-energy conversion performance.

Defects Promote Ultrafast Charge Separation in Graphitic Carbon Nitride for Enhanced Visible-Light-Driven CO2 Reduction Activity.

Low charge separation efficiency is the critical limitation for solar-driven CO2 conversion into chemicals fuels. Accelerating charge transfer in the interface of photocatalysts is an intriguing approach to suppress charge recombination. Herein, Co2+ doped 0D/2D TiO2 quantum dots confined in graphitic carbon nitride (CoTiCN) heterostructure was prepared by in-situ pyrolysis of MOFs and urea. Co2+ serves as the bridge of linking 0D TiO2 and 2D g-C3N4 in the interface, and consequently accelerates charge transfer in the interface from 2D g-C3N4 to 0D TiO2. As a result, CO evolution rate for photocatalytic CO2 reduction reached 290 μmol g−1 h−1, much higher than those of pure g-C3N4 and TiO2/g-C3N4. In addition, photocatalytic mechanism study indicates that [Co(bpy)3]Cl2 in the system functions as cocatalyst without any photocatalytic activity under visible light irradiation. Electron transfer occurs from heterogeneous photocatalyst to [Co(bpy)3]Cl2, which acts as the electron transporter as active sites to catalyze CO2 reduction into CO. This work provides an insight into the design of metal doped 0D/2D material towards visible light driven CO2 reduction from the viewpoint of promoting charge transfer in the interface and the understanding of the photocatalytic mechanism of cocatalyst in system.

Solar-driven CO2 conversion over Co2+ doped 0D/2D TiO2/g-C3N4 heterostructure: Insights into the role of Co2+ and cocatalyst

Singlet fission directly from the upper excited vibrational and electronic states of cofacial perylene dimers, bypassing the relaxed state S1, was detected within 50 fs. This process competes well with vibrational cooling in S1 (4.7–7.0 ps) and S2 → S1 internal conversion (380 fs). The singlet fission has the energy threshold E = 3.06 eV. Other competitive relaxation processes are excimer and dimer cation formation on an ultrafast time scale. Excitation to higher energy levels (4.96 eV) leads to a higher efficiency of singlet fission.

Wenjun Ni

Papers

Interfacial charge transfer in 0D/2D defect-rich heterostructures for efficient solar-driven CO2 reduction

Plasmonic Active “Hot Spots”‐Confined Photocatalytic CO2 Reduction with High Selectivity for CH4 Production

Defects Promote Ultrafast Charge Separation in Graphitic Carbon Nitride for Enhanced Visible-Light-Driven CO2 Reduction Activity.

Solar-driven CO2 conversion over Co2+ doped 0D/2D TiO2/g-C3N4 heterostructure: Insights into the role of Co2+ and cocatalyst

Singlet Fission from Upper Excited Electronic States of Cofacial Perylene Dimer.