Metal-free Photocatalyst Research Articles

Unveiling the role of a ground state charge transfer complex in carbon nanoparticles for highly efficient metal-free solar hydrogen production

A ground state charge transfer complex of N, P-CNPs and TEOA promotes long-lived free carriers and modifies the electronic energy levels for complete metal-free solar H2 generation. Free TEOA molecules act as effective SEDs during photocatalysis.

Unveiling the solar-driven synergistic production of a cyclic fuel-additive and carbon-free solar fuel from biogenic furfural: mediated by a metal-free organic semiconductor

This study offers an intriguing avenue to harness metal-free photocatalysts for the production of a bio-fuel additive (2-furyl-1,3-dioxolane) integrated with H2O2 production by effectively tapping the potential of both the charge carriers.

Aerobic Benzylic C(sp3)-H Bond Oxygenations Catalyzed by NBS under Visible Light Irradiation.

An efficient photocatalytic oxidation of benzylic C(sp3)-H bonds to ketones, esters, and amides has been developed using NBS as a metal-free photocatalyst and O2 as an oxidant. A variety of synthetically and biologically valuable compounds are assembled in moderate to excellent yields. The synthetic utility of this approach has been demonstrated by gram-scale experiments. A possible free radical mechanism was proposed to rationalize the reaction procedure.

Visible-light-driven efficient photocatalytic oxidation of 5-hydroxymethylfurfural over a metal-free recyclable anthraquinone derivative

Visible-light-driven upgrading of biomass and its derivatives into value-added chemicals under mild reaction conditions is an economical and sustainable strategy for biomass valorization. In this contribution, industrially-accessible anthraquinone and its derivatives (AQs) have been developed as efficient metal-free photocatalysts for the photo-oxidation of 5-hydroxymethylfurfural (HMF). Especially, a quantitative furans selectivity containing 86.0 % of 2,5-diformylfuran (DFF) and 13.6 % of 5-formyl-2-furancarboxylic acid (FFCA) was implemented at HMF conversion of 88.8 % over 2,6-dibromoanthraquinone (2,6-DB-AQ) catalyst under visible light illumination. In addition, the homogeneous 2,6-DB-AQ catalyst can be readily separated and recycled from the reaction mixture by a “water-assisted” precipitate method. Photophysical characterizations indicated that the introduction of bromine substituents on AQ enhances its visible light absorption ability, decrease its band gap energy and enables the catalyst with a more stable photoexcitation state. It has also been demonstrated that the lower photoexcitation energy of O2 over 2,6-DB-AQ endowed the catalyst with better capacity for the activation of O2 than that of AQ. These contributions should be in charge of the excellent catalytic activity of 2,6-DB-AQ for the photo-oxidation of HMF. This work demonstrates an efficient and recyclable organo-photocatalyst for the visible-light-mediated valorization of biomass-derived alcohols and aldehydes to value-added chemicals.

Efficient pollutant removal using tetrahydrofuran functionalized carbon nitride nanosheets with enhanced photocatalytic performance

Photocatalysis offers a green and promising strategy for light-driven pollutant degradation to address water pollution. Yet, challenges in conversion efficiency and cost-efficiency propel the search for effective metal-free photocatalysts. Here, we employ a ball milling technique using organic solvent molecules to enhance the exfoliation of g-C3N4 nanosheets. Incorporating tetrahydrofuran, with its distinctive five-membered monooxygenase ring structure, endows exfoliated CN nanosheets (TCN) with remarkable electron-donating capabilities, expanding specific surface area and active sites, essential for intricate photocatalytic reactions. Comparative assessments unequivocally establish TCN's superiority, revealing a remarkable 3.6-fold enhancement in RhB dye degradation and a remarkable 99.7 % removal efficiency. TCN demonstrates rapid, complete reduction of heavy metal ions (Cr6+) under natural sunlight within 10 min and exhibits a significant 2.3-fold antibiotic degradation enhancement over pristine CN. This pioneering work unveils the mechanisms behind TCN's superior photocatalytic performance, offering insights into designing advanced photocatalysts for crucial water purification applications.

Rejoint of Carbon Nitride Fragments into Multi-Interfacial Order-Disorder Homojunction for Robust Photo-Driven Generation of H2 O2.

Photocatalytic technology based on carbon nitride (C3 N4 ) offers a sustainable and clean approach for hydrogen peroxide (H2 O2 ) production, but the yield is severely limited by the sluggish hot carriers due to the weak internal electric field. In this study, a novel approach is devised by fragmenting bulk C3 N4 into smaller pieces (CN-NH4 ) and then subjecting it to a directed healing process to create multiple order-disorder interfaces (CN-NH4 -NaK).The resulting junctions in CN-NH4 -NaK significantly boost charge dynamics and facilitate more spatially and orderly separated redox centers. As a result, CN-NH4 -NaK demonstrates outstanding photosynthesis of H2 O2 via both two-step single-electron and one-step double-electron oxygen reduction pathways, achieving a remarkable yield of 16675µmol h-1 g-1 , excellent selectivity (>91%), and a prominent solar-to-chemical conversion efficiency exceeding 2.3%. These remarkable results surpass pristine C3 N4 by 158 times and outperform previously reported C3 N4 -based photocatalysts. This work represents a significant advancement in catalyst design and modification technology, inspiring the development of more efficient metal-free photocatalysts for the synthesis of highly valued fuels.

Ti-Modified Imogolite Nanotubes as Promising Photocatalyst 1D Nanostructures for H2 Production.

Imogolite nanotubes (INTs) are predicted as a unique 1D material with spatial separation of conduction and valence band edges but their large band gaps have inhibited their use as photocatalysts. The first step toward using these NTs in photocatalysis and exploiting the polarization-promoted charge separation across their walls is to reduce their band gap. Here, the modification of double-walled aluminogermanate INTs by incorporation of titanium into the NT walls is explored. The precursor ratio x = [Ti]/([Ge]+[Ti]) is modulated between 0 and 1. Structural and optical properties are determined at different scales and the photocatalytic performance is evaluated for H2 production. Although the incorporation of Ti atoms into the structure remains limited, the optimal condition is found around x = 0.4 for which the resulting NTs reveal a remarkable hydrogen production of ≈1500µmol g-1 after 5h for a noble metal-free photocatalyst, a 65-fold increase relative to a commercial TiO2-P25. This is correlated to a lowering of the recombination rate of photogenerated charge carriers for the most active structures. These results confirm the theoretical predictions regarding the potential of modified INTs as photoactive nanoreactors and pave the way for investigating and exploiting their polarization properties for energy applications.

Preparation of g-C3N4/Bismuth Iodide/Tourmaline Composites and Their Photodegradation Performance of Amaranthine

In this study, g-C3N4/BiOI/tourmaline composites were synthesized by loading BiOI and tourmaline on g-C3N4 nanofilms using g-C3N4, BiOI, and tourmaline as raw materials, aiming to optimize the photocatalytic degradation of amaranth red dye wastewater (AR). Single-factor experiments were conducted to optimize the degradation process. The findings indicate that a degradation rate of 95.88% can be achieved for AR within 150 min of light exposure, given a g-C3N4/BiOI/tourmaline dosage of 1 g/L, an initial pH level of 3 for the AR, and an initial AR concentration of 10 mg/L. Upon repeating the experiment for five cycles, the composite exhibited 83.10% activity, demonstrating its stability. Various analytical techniques were employed for material characterization: X-ray diffractometry (XRD) to ascertain the crystalline structure, field-emission scanning electron microscopy (FESEM) to evaluate morphology, X-ray photoelectron spectroscopy (XPS) for elemental analysis, ultraviolet–visible diffuse reflectance spectroscopy (UV-vis DRS) for optical properties, and photoluminescence (PL) spectroscopy to analyze the recombination of excited electrons and holes. Free radical masking experiments further elucidated the photocatalytic mechanism, revealing that the primary active species in the degradation process is the superoxide anion (O2−). This research offers a novel approach to address the limitations of n-type metal-free semiconductor photocatalysts.

Visible light-promoted metal-free aerobic photooxidation of xanthenes, thioxanthenes and dihydroacridines in deep eutectic solvents

Benzylic systems such as 9H-xanthenes, 9H-thioxanthenes and 9,10-dihydroacridines can be easily oxidized to the corresponding xanthones, thioxanthones or acridones, respectively, in deep eutectic solvents by a visible blue light-promoted photooxidation procedure carried out using ambient air as oxidant in the presence of riboflavin tetraacetate as a metal-free photocatalyst. The obtained yields are high or almost quantitative, and the reaction media can be recovered and reused. The environmental friendliness of the protocol is demonstrated based on several green metrics.

A critical review of the state-of-the-art green ammonia production technologies- mechanism, advancement, challenges, and future potential

Ammonia is significant in various industrial and transportation applications: carbon-free fuel, synthetic fertilizers, mining, medicines, water treatment, polymers, and textiles. In recent times, the utilization of green ammonia as an environmentally sustainable fuel with zero carbon emissions has garnered significant attention. Electrocatalysis, photocatalysis, photoelectrocatalysis, and biocatalysis are the most common catalytic methods in ammonia production. However, a substantial financial investment, lengthy time frame, eco-friendliness, energy-intensiveness, and ammonia yield need to be considered, during long-term application within the ammonia economy. Therefore, this review delivers insights into the comprehensive examination of existing scholarly literature on ammonia characteristics and different methods of ammonia production. Specifically, principle, strategies, reaction mechanism, catalysts engineering, and state-of-the-art photocatalysis, photoelectrocatalysis, electrocatalysis, and biocatalysis methods in ammonia production has been extensively articulated in this review. The challenges and advantages of each green ammonia production method have been highlighted. In the photocatalytic ammonia synthesis, comprehensive analysis and discussion of bismuth-based materials, Noble metal-modified photocatalysts, Metal-free semiconductor photocatalysts, and Metal sulphide-based materials were undertaken to lay a podium for green ammonia as promising material for various applications.

Rhombohedral boron monosulfide as a metal-free photocatalyst

Most of previous photocatalysts contain metal species, thus exploring a metal-free photocatalyst is still challenging. A metal-free photocatalyst has an advantage for the development of economical and non-toxic artificial photosynthesis system and/or environmental purification applications. In this study, rhombohedral boron monosulfide (r-BS) was synthesized by a high-pressure solid-state reaction, and its photocatalytic properties were investigated. r-BS absorbed visible light, and its photocurrent action spectrum also exhibited visible light responsivity. The r-BS evolved hydrogen (H2) from water under ultraviolet (UV) as well as under visible light irradiation, and its internal quantum efficiency reached 1.8% under UV light irradiation. In addition to the H2 evolution reaction, the r-BS photocatalyst drove carbon dioxide (CO2) reduction and dye oxidation reactions under UV irradiation. Although bare r-BS was not so stable under strong light irradiation in water, cocatalyst modification improved its stability. These results indicate that r-BS is a new class of non-metal photocatalyst applicable for H2 production, CO2 reduction, and environmental purification reactions.

Cathode materials in microbial electrosynthesis systems for carbon dioxide reduction: recent progress and perspectives

Microbial electrosynthesis (MES) is an emerging technology that enables the synthesis of value-added chemicals from carbon dioxide (CO2) or inorganic carbon compounds by coupling renewable electricity to microbial metabolism. However, MES still faces challenges in achieving high production of value-added chemicals due to the limited extracellular electron transfer efficiency at the biotic-abiotic interfaces. To overcome this bottleneck, it is crucial to develop novel cathodes and modified materials. This review systematically summarizes recent advancements in cathode materials in the field of electrocatalyst-assisted and photocatalyst-assisted MES. The effects of various material types are further investigated by comparing metal-free and metal materials and photocatalyst materials of different semiconductor types. Additionally, the review introduces the maximum production rate of value-added chemicals and conversion efficiency achieved by these cathode materials while highlighting the advantages and disadvantages of different material types. To the best of our knowledge, in electrocatalyst-assisted systems, the maximum CH4 yield on graphene aerogel/polypyrrole cathode achieved 1,672 mmol m-2 d-1, and the maximum Faraday efficiency (FE) of CH4 reached up to 97.5% on graphite plate. Meanwhile, the maximum acetate yield achieved 1,330 g m-2 d-1 with CO2 conversion efficiency into acetate close to 100% on carbon nanotube cathodes. In photocatalyst-assisted systems, the maximum acetate yield could reach 0.51 g L-1 d-1 with the coulombic efficiency of 96% on the MnFe2O4/g-C3N4 photocathode. Finally, prospects for future development and practical applications of MES are discussed, offering theoretical guidance for the fabrication of cathode materials that can improve production efficiency and reduce energy input.

Metal-Free Photocatalytic CO2 Reduction to CH4 and H2 O2 under Non-sacrificial Ambient Conditions.

Photocatalytic CO2 reduction to CH4 requires photosensitizers and sacrificial agents to provide sufficient electrons and protons through metal-based photocatalysts, and the separation of CH4 from by-product O2 has poor applications. Herein, we successfully synthesize a metal-free photocatalyst of a novel electron-acceptor 4,5,9,10-pyrenetetrone (PT), to our best knowledge, this is the first time that metal-free catalyst achieves non-sacrificial photocatalytic CO2 to CH4 and easily separable H2 O2 . This photocatalyst offers CH4 product of 10.6 μmol ⋅ g-1 ⋅ h-1 under non-sacrificial ambient conditions (room temperature, and only water), which is two orders of magnitude higher than that of the reported metal-free photocatalysts. Comprehensive in situ characterizations and calculations reveal a multi-step reaction mechanism, in which the long-lived oxygen-centered radical in the excited PT provides as a site for CO2 activation, resulting in a stabilized cyclic carbonate intermediate with a lower formation energy. This key intermediate is thermodynamically crucial for the subsequent reduction to CH4 product with the electronic selectivity of up to 90 %. The work provides fresh insights on the economic viability of photocatalytic CO2 reduction to easily separable CH4 in non-sacrificial and metal-free conditions.

Photoreduction of Aqueous Protons Coupling with Alcohol Oxidation on a S-Scheme Heterojunction Photocatalyst MnO/Carbon Nitride.

Crystalline carbon nitride (CCN), derived from amorphous polymeric CN, is considered as a new generation of metal-free photocatalyst because of its high crystallinity. In order to further promote the photocatalytic performance of CCN, p-type MnO nanoparticles are in situ synthesized and merged with n-type CCN through a one-pot process to form p-n heterojunction. The formed interfacial electric field between the semiconductors with different work functions efficiently breaks the coulomb interaction between MnO and CCN. The prepared catalysts exhibit drastically increased photocatalytic hydrogen evolution (PHE) activity integrated with oxidation of alkyl and aryl alcohols under irradiation of visible light. In the aqueous solution of benzyl alcohol (BzOH), the hydrogen generation rate over MnO/CCN (39.58 µmol h-1) is nearly 7 times and 37 times that of pure CCN (5.76 µmol h-1) and CN (1.06 µmol h-1), respectively, combining with oxidation of BzOH to benzaldehyde. This work proposes an avenue for in situ construction of a novel 2D material-based S-scheme heterojunction and extends its application in solar energy conservation and utilization.

Polymer Photocatalysts Containing Segregated π-Conjugation Units with Electron-Trap Activity for Efficient Natural-light-driven Bacterial Inactivation.

Development of highly efficient and metal-free photocatalysts for bacterial inactivation under natural light is a major challenge in photocatalytic antibiosis. Herein, we developed an acidizing solvent-thermal approach for inserting a non-conjugated ethylenediamine segment into the conjugated planes of 3,4,9,10-perylene tetracarboxylic anhydride to generate a photocatalyst containing segregated π-conjugation units (EDA-PTCDA). Under natural light, EDA-PTCDA achieved 99.9 % inactivation of Escherichia coli and Staphylococcus aureus (60 and 45 min), which is the highest efficiency among all the natural light antibacterial reports. The difference in the surface potential and excited charge density corroborated the possibility of a built-in electron-trap effect of the non-conjugated segments of EDA-PTCDA, thus forming a highly active EDA-PTDA/bacteria interface. In addition, EDA-PTCDA exhibited negligible toxicity and damage to normal tissue cells. This catalyst provides a new opportunity for photocatalytic antibiosis under natural light conditions.

Construction of Mo/Mo2C@C modified ZnIn2S4 Schottky junctions for efficient photo-thermal assisted hydrogen evolution

Photocatalytic water splitting on noble metal-free photocatalysts for H2 generation is a promising but challenging approach to realize solar-to-chemical energy conversion. In this study, Mo/Mo2C nanoparticles anchored carbon layer (Mo/Mo2C@C) was obtained by a one-step in-situ phase transition approach and developed for the first time as a photothermal cocatalyst to enhance the activity of ZnIn2S4 photocatalyst. Mo/Mo2C@C nanosheet exhibits strong absorption in the full spectrum region and excellent photo-thermal conversion ability, which generates heat to improve the reaction temperature and accelerate the reaction kinetics. Moreover, metallic Mo/Mo2C@C couples with ZnIn2S4 to form ZnIn2S4–Mo/Mo2C@C Schottky junction (denoted as ZMM), which prevents the electrons back transfer and restrains the charge recombination. In addition, conductive carbon with strong interfacial interaction serves as a fast charge transport bridge. Consequently, the optimized ZMM-0.2 junction exhibits an H2 evolution rate of 1031.07 μmol g−1 h−1, which is 41 and 4.3 times higher than bare ZnIn2S4 and ZnIn2S4–Mo2C, respectively. By designing novel photothermal cocatalysts, our work will provide a new guidance for designing efficient photocatalysts.

Persistent radical anion of perylene dianhydride: an emerging metal-free photocatalyst for near-infrared photocontrolled RAFT polymerization

Persistent radical anion of perylene dianhydride: an emerging metal-free photocatalyst for near-infrared photocontrolled RAFT polymerization

Metal-free redox-active diphenylpyrenediimides: A comparison between the photocatalytic performance of molecular and polymeric catalysts

Herein we described a donor–acceptor porous conjugated polydiphenylpyreneimide (DPPy-PI) with high thermal stability (530 °C) and surface area of 710 m2.g−1 prepared by condensation of 3,8-diphenylpyrene-1,2,6,7-tetracarboxylic dianhydride (DPPyDA) and tris(4-aminophenyl)benzene and a series of pyrenediimides (DPPy-DIs) as molecular references for comparative purposes. To evaluate the effect of the photoactive pyrene unit the polydiphenylperyleneimide (DPP-PI) having perylene units was also prepared.DPPy-PI has shown to see an efficient metal-free heterogeneous photocatalyst in the selective oxidation of aromatic sulfides and for Diels-Alder cycloadditions reaching almost full conversions with low reaction times and high selectivity that can be reused for at least fifteen cycles without significant loss of catalytic activity. Characterization data reveals that imide radicals are the important active intermediates during the redox processes of these PIs.

Metal-free 2D/2D VdW heterojunction fabricated by amorphous covalent triazine frameworks with nitrogen-defect on polymeric carbon nitride towards enhanced photocatalytic performance

The structural design of Van der Waals (VdW) heterojunction photocatalysts has still been a challenging issue, because it usually plays a decisive role in improving photocatalytic performance. Herein, a unique metal-free 2D/2D VdW heterojunction is fabricated by decoration of amorphous covalent triazine frameworks with nitrogen defect (VN-CTFs) on the surface of polymeric carbon nitride (PCN) via a simple acid stripping, self-assembly and heat-treatment process, which demonstrates high-efficiency and stable photocatalytic performance for hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR). The results of experiments reveal that the synergy produced by strong electron-withdrawing effect from cyano groups in VN-CTFs induces charge redistribution at the interface with PCN through π-π interaction of 2D/2D VdW heterojunction, resulting in the formation of built-in electric field, which effectively drives the transfer and separation of photogenerated carriers to enhance the photocatalytic performance. The optimal PCN/VN-CTFs-10 sample exhibits that the average HER rate reaches up to 75.1 and 3.7 times higher than that of onefold VN-CTFs and PCN, respectively, as well as achieves apparent quantum yield (AQY) of 21.5 % at 420 nm and stable operation of 10 cycles totaling 40 h. Meanwhile, CO yields of 28.3 and 2.5 times as high as that of pristine VN-CTFs and PCN, respectively, are also obtained over the optimal PCN/VN-CTFs-10 sample for CO2RR. This work exploits a new structural unit design strategy for developing the metal-free PCN-based VdW heterojunction photocatalysts to boost the photocatalytic performance.

A metal-free photoactive nitrogen-doped carbon nanosolenoid with broad absorption in visible region for efficient photocatalysis

Riemann surfaces inspired chemists to design and synthesize such multidimensional curved carbon architectures. It has been predicted that carbon nanosolenoid materials with Riemann surfaces have unique structures and novel physical properties. Here we report the first synthesis of a nitrogen-doped carbon nanosolenoid (N-CNS) using bottom-up approach with a well-defined structure. N-CNS was obtained by a rational Suzuki polymerization, followed by oxidative cyclodehydrogenation. The successful synthesis of N-CNS was fully characterized by GPC, FTIR, solid-state 13C NMR and Raman techniques. The intrinsic single-strand molecular structures of N-CNS helices can be clearly resolved using low-dose integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) technique. Possessing unique structural and physical properties, this long π-extended polymer N-CNS can provide new insight towards bottom-up syntheses of curved nanoribbons and potential applications as a metal-free photocatalyst for visible-light-driven H2 evolution and highly efficient photocatalyst for photoredox organic transformations.