The challenge and opportunity of organic semiconductors in photocatalysis
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Abstract
Employing organic semiconductors to drive photocatalytic processes for chemical fuel production and pollutant degradation is a viable pathway for tackling the energy crisis and environmental pollution. In this review, we summarize the development of organic semiconductor photocatalysis so far and propose the future vision of organic semiconductors as state-of-the-art photocatalysts in practical applications. Compared to inorganic semiconductors, organic semiconductors display a large absorption coefficient and easily tunable topological and electronic structures, which set them apart from ordinary inorganic photocatalysts. However, the chemical instability, high exciton dissociation energy and low charge carrier mobility of organic semiconductors are the major obstacles to the improvement of their photocatalytic activity. Obviously, the opportunity and challenge coexist in the development of organic semiconductor photocatalysis. In light of this, we systematically compare the merits and shortcomings of organic semiconductors for heterogeneous photocatalysis and enumerate some feasible approaches to overcoming the bottlenecks hindering their photocatalytic performance. By carefully considering factors such as conjugated linkage types, building blocks, and electron donor-acceptor structures, highly reactive and stable organic semiconductor photocatalysts can be developed.
Keywords
INTRODUCTION
Since clean energy production via the sunlight-driven photocatalytic pathway opened up by Fujishima and Honda[1], great efforts have been made to exploit state-of-the-art photocatalysts for solar fuel production and environmental remediation[2-4]. Up to now, inorganic semiconductors including metal oxides[5], metal oxynitrides[6], metal oxyhalides[7], and chalcogenides[8] are most widely investigated. Photocatalysis involves several key processes, such as light absorption, exciton generation, diffusion, separation, charge carrier transport and recombination, and charge carrier injection to interfacial reactant[9]. Accordingly, structural optimization strategies for inorganic semiconductor photocatalysts have been developed to enhance each of these processes. To enhance the light harvesting and redshift the absorption edge of UV-responsive semiconductors, strategies such as dye-sensitizing[10], quantum dot-sensitizing[11], heteroatom doping[12], solid solution construction[13] are employed. Heterojunction formation[14], defect engineering[15], and crystal facet modification[16] are promising methods to optimize electron-hole separation efficiency and charge carrier dynamics. Nanoparticle size tuning[17] and cocatalyst loading[18] facilitate the charge carriers-involved interfacial chemical reactions. In addition to the traditional inorganic semiconductor materials, other materials with certain intrinsic merits also arouse interest in the community of heterogeneous photocatalysis. Plasmonic nanostructures (Au, Ag, Cu), as a category of photon antennae in solar energy conversion, have recently drawn great attention in photocatalysis owing to their large extinction coefficient and electromagnetic field confinement at nanoscale[19]. The hot carriers with high energy generated after the plasmon dephasing can be injected into the interfacial adsorbed reactant molecules and subsequently trigger chemical reactions[20]. Despite the large absorption cross section and high near-field enhancement in the vicinity of plasmonic nanostructures, in which the electron-electron scattering and electron-phonon relaxation take place at an extremely transient timescale, the lifetime of hot carriers is significantly minimized, resulting in low solar-to-chemical conversion efficiency on the plasmonic nanostructures[21]. Metal-organic frameworks (MOFs), as inorganic-organic hybrid materials with three-dimensional network structures, are also explored for heterogeneous photocatalysis because of their large specific surface area and high porosity[22-24]. The Metal-Oxo clusters isolated by the organic linkers in some MOFs are analogous to semiconductor quantum dots[25], thus endowing corresponding MOFs with semiconductor-like properties[26]. However, a considerable amount of these MOF materials are sensitive to moisture and aqueous solution[25], hindering their further application in photocatalysis. In this context, it is indispensable to develop new classes of photocatalysts to realize the practical application of photocatalysis in energy conversion and environmental remediation.
Why do organic semiconductors arouse great interest in the photocatalysis field?
As stated above, the structure-activity relationship in the inorganic semiconductor photocatalysis has been well established; attention needs to be paid to other promising photocatalysts. Recent years have witnessed the booming progress of organic semiconductor-based optoelectronic devices[27-30] owing to their merit of low cost, flexibility, and facile solution processability. Notably, organic photovoltaic (OPV) has seen substantial advancements, with power conversion efficiencies (PCEs) of single-junction or tandem OPV devices approaching 20%, nearing the PCEs of commercial inorganic solar cells[31]. Considering that the development of inorganic semiconductor photocatalysis draws on a set of methodologies from the inorganic semiconductor solar cells and electronic devices, organic semiconductor photocatalysis could similarly benefit from the recent progress in OPV. Moreover, the fundamental workings of OPV can be divided into light absorption, exciton generation and dissociation, charge carrier transport, and charge carrier collection in organic semiconductor devices[31]. Likewise, organic semiconductor photocatalysis follows the same steps as OPV, with the additional final procedure of charge carrier transfer to reactant molecules. As a result, organic semiconductors are emerging as promising photocatalysts for solar fuel production.
What are organic semiconductor photocatalysts?
Organic semiconductors are a group of organic molecules or polymers with a π-conjugated system constructed by the Pz orbitals of sp2-hybridized carbon or heteroatoms[32]. The transition between π bonding orbital and π* anti-bonding orbital (π-π*) has a much narrower energy gap compared to the σ-σ* transition in the molecules with σ-bonding backbones, endowing the organic semiconductors with visible light response[32]. Up to now, the widely investigated organic semiconductor photocatalysts mainly include supramolecular catalysts[33], conjugated polymers[34], Graphitic carbon nitride (g-C3N4)[35], and covalent organic frameworks[36] [Figure 1]. This review paper will focus on the advancement and outlook of organic semiconductor photocatalysis based on these four types of materials.
Figure 1. Main organic semiconductor photocatalysts: (A) Supramolecular materials. Adapted with permission[33], Copyright 2020 American Chemical Society; (B) Conjugated polymers. Adapted with permission[34], Copyright 2016 American Chemical Society; (C) Graphitic carbon nitride (g-C3N4). Adapted with permission[35], Copyright 2016 American Chemical Society; (D) Covalent organic frameworks. Adapted with permission[36], Copyright 2013 Royal Society of Chemistry. 2D: two-dimensional; 3D: three-dimensional.
Supramolecular materials
Supramolecular chemistry is a research field defined as “chemistry beyond the molecule”[37]. As the name suggests, supramolecular formation is an intermolecular assembly via the non-covalent interaction (hydrogen bonding, π-π stacking, dipoles-dipoles interaction, metal-ligand coordination, cation-π interaction)[38,39]. Supramolecular materials have been extensively studied as photocatalysts in recent years
Conjugated polymers
Conjugated polymers are a category of macromolecules with π-conjugated main chains composed of repeating building units, which is different from the traditional polymers with saturated main chain structures[53]. The backbones of conjugated polymers are made up of repeating monomers with unsaturated π bonds, such as aromatic, olefinic, or acetylenic building units[34,53]. The mobile π-electrons in the main chain endow conjugated polymers with semiconducting properties[54]. In 1985, Yanagida et al. first reported the application of linear polymer poly(p-phenylene) in the photocatalytic hydrogen evolution under 290 nm light illumination[55], opening up a novel era for the conjugated polymer in photocatalysis. Since then, multiple attempts have been made to probe the structure-property-activity relationship in conjugated polymer photocatalysis. Extending the oligomer length could redshift the optical absorption edge of 1,4-phenylene/2,5-thiophene copolymers[56]. Furthermore, decreasing the 1,4-phenylene ratio and increasing the 2,5-thiophene ratio in the copolymers can lower the band gap as well. However, a larger 2,5-thiophene ratio lowers the conduction band minimum potential of copolymers, leading to a smaller photocatalytic H2 evolution rate[56]. Consequently, a trade-off between thermodynamic driving force and optical band gap has to be taken into account when it comes to the molecular design of conjugated polymer photocatalysts. Changing the hydrophobicity or hydrophilicity of the main chain is also a feasible way to tune the conjugated polymers’ photocatalytic performance[57]. In addition to the main chain engineering, side chain hydrophilic modification can also boost the photocatalytic activity of conjugated polymers[58,59]. When poly(benzene-dibenzo[b,d]thiophene sulfone) acts as the backbones, the hydrophilic side chain tri(ethylene glycol) shows great advantage in photocatalytic hydrogen production over n-decyloxy and n-dodecyl side chains[60].
Graphitic carbon nitride
Graphitic carbon nitride (g-C3N4) is a graphite-like layered carbon nitride material prepared via a simple high-temperature condensation process in an air or inert gas atmosphere using several low-cost, nitrogen-rich precursors, such as cyanamide, dicyandiamide, melamine, thiourea, and urea[35,61]. The condensation of these N-containing monomers can form an in-plane large π-bonded conjugated system arising from sp2 hybridization between C and N atoms. This π-conjugation system in g-C3N4 is composed of basic structural units of triazine (C3N3) structure and heptazine ring (C6N7)[35]. The layers stack via a weak intermolecular interaction with an interlayer spacing of 0.326 nm. As a visible light-responsive semiconductor, g-C3N4 shows an optical absorption edge around 450 nm, equaling a band gap of 2.7 eV[61]. The valence band maximum and conduction band minimum energy level of g-C3N4 straddles the potentials of corresponding water oxidation reaction and hydrogen evolution, featuring as a promising photocatalyst for water splitting[62]. Since Wang et al. discovered the semiconductor photocatalytic performance of g-C3N4 in 2009[63], much attention has been paid to exploiting the state-of-the-art g-C3N4-based photocatalysts for solar energy fuel production and environmental remediation. Specifically, doping transition metals (Mn, Fe, Co, Ni, Cu)[64] or main group elements (B, S, P) into the skeletons of g-C3N4 boosts the photocatalytic activity by narrowing the band gap and enriching the accessible catalytic sites[65-67]. Coupling g-C3N4 with other semiconductors also improves the photocatalytic performance by facilitating the photoexcited electron-hole separation[68].
Covalent organic frameworks
Covalent organic frameworks (COFs) are covalent-bonding crystalline organic porous materials with highly ordered network structures, which are constructed by organic molecular building blocks via condensation reactions[69,70]. Similar to the metal-organic frameworks (MOFs), the two-dimensional and three-dimensional networks of COFs are linked by the nodes and linkers. The corresponding linkage ways vary based on different organic building units, such as boronic acid esters, olefin, triazine, imine, β-ketoenamine, hydrazone, azine, and acrylonitrile, among others[71]. Because of the rigid π backbones, COFs also show semiconductor properties[72] and even have advantages over the other three types of organic semiconductors in some aspects. Compared with organic supramolecules, COFs have a significantly larger specific surface area[73], creating more accessible catalytic sites for photocatalytic reactions. In contrast to the low crystallinity of conjugated polymers, COFs possess a much higher crystallinity, which is beneficial for charge carrier transport[74]. Compared to the triazine- or heptazine-based building units in the g-C3N4 structural backbone, the greater structural diversity endows COFs with easier tunable optical and electronic properties[72]. By virtue of so many advantages, COFs have become the most promising organic semiconductors for heterogeneous photocatalysis, as evidenced by the considerable amount of published works since the first reported photocatalytic hydrogen production on TFPT-COF in 2014[75-79].
The advantages of organic semiconductors over inorganic semiconductors in photocatalysis
As mentioned above, organic semiconductors constructed by organic building blocks with large π-conjugation show their promising capability in photocatalytic reactions. In comparison to typical inorganic semiconductor photocatalysts, organic semiconductors exhibit some advantages in the aspects below:
Large absorption coefficient
Light absorption is the prerequisite for photocatalysis and lays the foundation for the subsequent photo-excited charge carrier generation and interfacial catalysis. As a term describing the light attenuation per unit length in a given medium, absorption coefficient can reflect how light was harvested. A high absorption coefficient in a specific material represents that the light beam is mostly absorbed when it passes through the inner space of this material, while a low value indicates that this material has a limited impact on the light beam attenuation. In the layer-by-layer structure of photoelectrochemical working electrode, the thickness of photo-absorbing layer has to be balanced with the thickness of charge carrier mediator layer or catalyst layer[80,81]. A thicker photo-absorbing layer can guarantee high absorption efficiency, but impedes the charge carrier transport in the film or at the catalytic interface[80]. This trade-off strategy works for most of the semiconductors with low absorption coefficients and short charge carrier diffusion lengths. Compared to the typical inorganic semiconductors such as crystalline Si, multiple organic semiconductors possess a much higher absorption coefficient over 105 cm-1[82], meaning that thin layer thicknesses can simultaneously facilitate the charge carrier transport without sacrificing the light absorption efficiency, which is conducive for achieving the higher light harvesting efficiency on organic semiconductors. The high light absorption coefficient can be easily achieved on COFs by tuning building blocks with large π-conjugation systems [Figures 2 and 3C].
Figure 2. Topological structure and building blocks of stacked 2D COFs of the Lieb lattice with the nodes highlighted in blue and edges in orange. 2D: Two dimensional; COFs: covalent organic frameworks.
Figure 3. The electronic structure and optical properties of 2D COFs and their related molecular building blocks mentioned in Figure 2. (A) Energy band diagram of COFs; (B) HOMO and LUMO energy level diagram of the molecular building blocks; (C) Absorption coefficients of COFs. 2D: Two dimensional; COFs: covalent organic frameworks; HOMO: highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital.
Facile energy band structure engineering
The energy band structure of semiconductors is of crucial importance for photocatalysis. First and foremost, the band gap of a semiconductor determines which part of sunlight can be absorbed and utilized. As visible light accounts for nearly 43% of the solar spectrum[35], exploiting the visible light-responsive photocatalysts is widely accepted as a promising approach for highly efficient capturing of solar energy. Additionally, the conduction band minimum (CBM) and valence band maximum (VBM) energy levels dictate whether the chemical reactions can be initiated thermodynamically. For hydrogen evolution, the potential of the conduction band minimum should be negative compared to the reversible hydrogen evolution (RHE) potential[62]. Designing the solid solution semiconductor is a common approach to redshift the absorption edge of inorganic semiconductors from ultraviolet (UV) to visible light region. For example, the as-synthesized (Ga1-xZnx) (N1-xOx) solid solution decreases the band gaps of ZnO and GaN from 3.3 to 2.7 eV. However, this nitridation of Ga2O3 and ZnO mixture under NH3 flow takes place at 1,123 K[83]. Nitrogen doping is able to narrow the band gap of TiO2 as well, but the calcination process still occurs at 823 K under N2 atmosphere[12]. These harsh synthetic conditions limit the optional inorganic semiconductor materials for photocatalysts and purge us to seek other band gaps tunable materials that can be synthesized under ambient conditions. The band structure of the organic semiconductors is determined by the electron density in the conjugated π-systems and can be manipulated via localizing or delocalizing the π electrons in their conjugation π-backbones[84]. Normally, adjusting the π-conjugation length and changing the substituent groups of organic semiconductors are the two feasible ways to tune the optical band gap, lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy positions[85-87]. Taking oligothiophenes-based compounds as an example, their band gaps increase from 2.19 to 1.91 and 1.67 eV, with the Thiophene heterocyclic numbers increasing from 3 to 5 and 6. The extension of conjugated π-systems not only narrows the band gap but also upshifts the LUMO and HOMO energy levels concurrently[86]. Employing the building blocks (nodes and edges) with different electronic structures also easily enables the band structure engineering of COFs [Figures 2, 3A and B]. When using tetrakis (4-formylphenyl) phenyl as the node, changing the edge structures from phenyl to benzothiadiazole and thiadiazoloquinoxaline results in the shrinking of the band gap because of the extended π-conjugation system [Figures 2 and 3]. The band gap of the related COF remains almost unchanged with the edge building block benzothiadiazole replaced by fluorine-substituted benzothiadiazole, but the HOMO and LUMO positions are lowered because the electron-withdrawing group (-F) decreases the π-electron density of the conjugation system and thus enhances the electron affinity energies[88].
Easy tailoring of dielectric constant
Photoexcited electron-hole recombination via the radiative or non-radiative way is detrimental to enhancing the photocatalytic activity. According to the Langevin formula
Overall, organic semiconductors prevail over inorganic semiconductors in light absorption coefficient, facile modification of energy band structure, and dielectric constant [Figure 4], featuring as an indispensable category of photocatalysts for realizing practical solar fuel production.
Figure 4. The advantages of organic semiconductors over inorganic semiconductors in photocatalysis.
How can organic semiconductor photocatalysts be synthesized?
Multiple methods can be used for the synthesis of organic semiconductor photocatalysts. Taking porphyrin-based supramolecular materials as an example, porphyrin monomers can self-assemble into nanostructures via reprecipitation, ion self-assembly, coordination polymerization, and metal-ligand coordination pathways[93]. Using the reprecipitation method, SnIV5-(4-pyridyl)-10,15,20-triphenylporphyrin (SnPyTriPP) and zinc-containing DP(CH3COSC5H10O)2P porphyrin derivatives can form the supramolecular nanostructures in different solvents[94]. In addition, surfactants, stabilizing reagents, and triblock copolymers can also be used to assist the self-assembly formation of supramolecular well-defined nanostructures[93]. Owing to the electrostatic interactions, supramolecular aggregates can be formed by mixing a cationic porphyrin monomer with an anionic porphyrin monomer. Based on this fundamental, porphyrin nanotubes were successfully prepared by the self-assembly of tin meso-tetra(4-pyridyl) porphyrin cations and tetrakis(4-sulfonatophenyl) porphyrin anions[95]. Concerning the conjugated polymers, their structures are derived from the polymerization of π-conjugated monomers. The aromatic monomers with halogen, alkynyl, amino, carboxyl, and boric acid groups will be interconnected via Sonogashira-Hagihara Coupling, Suzuki-Miyaura Coupling, Yamamoto Coupling, Heck Coupling, Cyclotrimerization Reaction, Phenazine Ring Fusion, Schiff-Base Condensations, Heterocycle Linkages, Alkyne Metathesis, Oxidative Coupling, Buchwald-Hartwig Amination pathways, etc[96]. The typical synthetic processes of conjugated polymers are mixing monomers, catalysts, and solvents under inert gas protection, and then refluxed at a specific temperature for a period of time[97]. The design and synthesis of covalent organic frameworks are similar to that of conjugated polymers. The monomers with different terminal groups undergo the polycondensation processes to form the two-dimensional (2D) or three-dimensional (3D) framework structures. The symmetry of monomers is of great importance, as it determines the topologies and pore sizes of COFs[98]. 2D COFs usually contain planar building blocks and the construction of 3D COFs requires at least one building unit with a tetrahedral or orthogonal geometry[98]. COF materials can be fabricated by various synthetic approaches, such as solvothermal synthesis, microwave synthesis, ionothermal synthesis, and room-temperature solution synthesis[99]. The pristine g-C3N4 can be synthesized via thermal condensation of nitrogen-containing precursors including cyanamide, dicyandiamide, melamine, thiourea, and urea at 450-650 °C[100]. Thin-layer g-C3N4 can be synthesized by exfoliating bulk g-C3N4. Sonication-assisted liquid exfoliation is a promising method to prepare two-dimensional g-C3N4 structures, as g-C3N4 nanosheets with a thickness of less than 2 nm can be obtained in isopropyl alcohol, 1,3-butanediol, and concentrated sulfuric acid under continuous sonication[101]. Additionally, thermal oxidation exfoliation, ball milling, and flash-freezing methods can also be used to fabricate 2D g-C3N4 ultrathin nanosheets[100].
Characterization methods for organic semiconductor photocatalysts
Optical band gap
The optical band gap of organic semiconductors can be measured using a UV-Vis spectrometer. The absorption edge wavelength (λ) of the organic semiconductors is regarded as the absorption onset position in the UV-vis spectra. Accordingly, the band gap of organic semiconductors (Eg) will be calculated based on the equation: Eg = hc⁄λ (h and c denote Plank constant and light speed in vacuum, respectively).
Energy band potential
HOMO or VBM of organic semiconductors is regarded as ionization energy (IE), which can be measured directly by ultraviolet photoelectron spectroscopy (UPS)[102]. Additionally, LUMO or CBM of organic semiconductors is equivalent to the electron affinity (EA), and the corresponding measurement can be performed via the low-energy inverse photoelectron spectroscopy (LE-IPES) technique on thin-film samples. Moreover, cyclic voltammetry (CV) is also a widely used technique for estimating the IE (or HOMO) and EA (or LUMO) by measuring the redox potentials[103]. CV measurements can be performed both in solution and on thin films.
Dielectric constant
The dielectric constant (εr) of organic semiconductors can be measured by impedance spectroscopy at different frequencies[104]. By fabricating a parallel-plate-type capacitor, εr of organic semiconductors can be calculated by the equation shown as follows:
Exciton binding energy
The exciton binding energy Eb of organic semiconductors can be determined from the temperature dependence of photoluminescence (PL) measurements[105]. Thermal dissociation of excitons was assumed to fully contribute to the radiative recombination of electron-hole pairs, which can be reflected by PL intensity change. Since the PL intensity decreases with the enhancement of temperatures, the calculation of activation energy in the Arrhenius equation can be extrapolated to the determination of exciton binding energy. Based on this equation:
Charge carrier dynamics
The exciton dissociation of organic semiconductors can be probed by photoluminescence spectroscopy (PL), and the PL emission peak intensity declining at a certain wavelength indicates the exciton quenching[107]. The charge transfer dynamics of organic semiconductors can be investigated via time-resolved photoluminescence (TRPL) technique. PL emission peak intensity decay as a function of time can reflect the lifetime of the photo-excited electron-hole pair. Considering that TRPL mainly monitors the charge carrier dynamics at nanosecond (ns) timescale, more precise information about charge carrier dynamics in a picosecond timescale can be provided by the femtosecond pump-probe transient absorption spectroscopy (TAS)[107,108]. In this technique, the pump laser beam excites the organic semiconductors, generating excitons, and the probe beam then reaches the excited sample after a delay to trace exciton diffusion and dissociation[108,109].
Photocatalytic reactions for organic semiconductors
Hydrogen evolution reaction
Photocatalytic water splitting is considered a sustainable approach to producing clean hydrogen, which takes place when the conduction band minimum potential of a photocatalyst is more negative than the reversible hydrogen evolution (RHE) potential. Organic semiconductors with tunable energy band structures show their competencies for hydrogen production[110]. Using triethanolamine (TEOA) as a sacrificial agent, g-C3N4 was found to be active in hydrogen evolution with a reaction rate ranging from 0.1 to 4 μmol·h-1 under visible light illumination[63]. As bulk g-C3N4 still suffers from low visible light harvesting, high photo-excited charge carrier recombination, and limited surface catalytic sites, much effort has been made to its structural modification[110]. Doping sodium ions into the skeleton of g-C3N4 decreases the band gap of pristine g-C3N4 from 2.74 to 2.42 eV[111]. Because of the extended visible light response, as-synthesized Na-doped g-C3N4 displays a H2 evolution rate nearly 4 times higher than that of pristine g-C3N4. Moreover, g-C3N4 and sulfur-doped g-C3N4 can form the staggered gap heterojunction, which is able to separate the electrons and holes efficiently[112]. Compared to the shorter charge carrier lifetime in g-C3N4, time-resolved photoluminescence spectroscopy reveals that photo-excited charge carrier lifetime is prolonged in this isotype heterojunction, leading to a H2 evolution rate 11 times higher than that of pure g-C3N4. Furthermore, forming layered nanojunctions with MoS2 can enrich the surface catalytic sites on g-C3N4 for hydrogen production[113]. MoS2 not only serves as a charge carrier mediator to deliver the photogenerated electrons on g-C3N4 to proton, but also reduces the activation energy for hydrogen evolution. Designing mesoporous g-C3N4 is also a feasible way to improve the hydrogen evolution activity by enhancing the accessible catalytic sites[114]; this methodology also works for the optimization of covalent organic framework photocatalysts. The hydrophilic 3.2 nm mesopores in fused-sulfone-COF can adsorb dye photosensitizers[115]. Owing to its high specific surface area and wide visible light response, this dye-sensitized COF exhibits a hydrogen production rate of 16.3 mmol·g-1·h-1 and an apparent quantum efficiency of 2.2% at
CO2 reduction reaction
Photocatalytic CO2 reduction to useful chemicals or industrial feedstocks has aroused great interest in the community of organic semiconductor photocatalysis[120]. Much attempt has been made to design robust organic semiconductor photocatalysts for CO2 reduction, but the multiple proton-coupled electron transfer pathways in the reaction result in sluggish kinetics and low product selectivity[120,121]. ACOF-1 constructed by the condensation of hydrazine hydrate and 1,3,5-triformylbenzene generates methanol in photocatalytic CO2 reduction without any sacrificial agent[122]. HCOF-1 synthesized by the condensation of hydrazine hydrate and 1,3,5-triformylphenol shows the same azine-linkage type as ACOF-1, while producing CO and CH4 in photocatalytic CO2 reduction reaction using TEOA as sacrificial agent[123]. Moreover, LZU1-COF formed via condensation of p-phenylenediamine and 1,3,5-triformylbenzene generates only CO in pure water under light illumination[124]. All these results indicate that photocatalytic CO2 reduction reaction selectivity strongly depends on the building units of the organic semiconductor and the solvent microenvironment. In general, CO, HCOOH, CH4, CH3OH, and C2H5OH are common products in organic semiconductor photocatalytic CO2 reduction reactions. CO2 reduction to CO is the most kinetically preferable pathway because of its lowest quantity of involved protons and electrons[120]. The ultrathin imine-based 2D COF nanosheets (NSs) show excellent photocatalytic CO2 reduction to CO. Among them, COF-367-Co NSs display a CO production rate of 10,162 μmol·g-1·h-1[125]. CoO4 catalytic sites-modified 2,3-dihydroxybenzene-1,4-dicarboxaldehyde (2,3-DHTA)-based COF also shows an extremely high CO generation rate of 18,000 µmol·g-1·h-1[126], outperforming any other organic semiconductor photocatalysts. Similar to CO production, HCOOH generation also involves the transfer of two electrons and two protons[120]. Metal-nitrogen coordination catalytic sites are proved to be crucial for boosting the formation of HCOOH. The Ru-N2 incorporated covalent triazine framework (CTF-2) displays a HCOOH generation rate of 2,090 µmol·g-1·h-1[127], which, to the best of our knowledge, is the highest photocatalytic HCOOH production rate reported among all organic semiconductor photocatalysts. The ketoenamine-based COFs with the same TpBD framework and different functional groups show distinct photocatalytic HCOOH production activity. Functional group -OCH3 exhibits an advantage over -CH3 and -NO2, resulting in a higher HCOOH generation rate for TpBD-(OCH3)2 COF compared to other COFs[128]. Different from CO and HCOOH production with relatively facile reaction kinetics, CH4 generation is more difficult on reaction kinetics because of the involvement of eight electrons and protons[129]. By designing a D-A structure between tris(4-ethynylphenyl)amine (TPA) and phenanthraquinone (PQ) to enhance the intramolecular electron transfer, the synthesized TPA-PQ photocatalyst overcomes the sluggish kinetics of CH4 generation and displays a visible-light-driven photocatalytic CH4 production rate of 2.15 mmol·g-1·h-1[130], which is much higher than that of TEB-PQ (0.268 mmol·g-1·h-1) constructed from the building blocks of triethynylbenzene (TEB) and PQ. The production of alcohols via photocatalytic CO2 reduction is challenging, but can be realized by designing appropriate catalytic sites. Embedding PdIn bimetallic clusters into the pores of N3-COF facilitates the generation of CH3OH and C2H5OH simultaneously[131], resulting in CH3OH and production rates of 24.5 and 8.75 µmol·g-1·h-1, respectively. Tan et al. introduced N vacancies and O doping to create dual catalytic sites in the skeleton of a melon-based organic photocatalyst[132]. The localized charge polarization in the as-synthesized oxygen-doped and nitrogen-defective melon-based organic photocatalyst (ON-MOP) simultaneously promotes C-C coupling and lowers the exciton binding energy, thus yielding a C2H5OH production rate of 800
State-of-the-art organic semiconductor photocatalysts in CO2 reduction
| Photocatalysts | Product | Reaction rate (µmol·g-1·h-1) | Illumination source | Reaction solvent | Reference | ||
| COF-367-Co NSs | CO | 10,162 | 300 W Xe lamp, λ ≥ 420 nm | KHCO3 (aq) | [125] | ||
| Co-2,3-DHTA-COF | CO | 18,000 | 300 W Xe lamp, λ > 420 nm | CH3CN/H2O | [126] | ||
| Ru-CTF-2 | HCOOH | 2,090 | 8W LED lamp | N,N-dimethylacetamide/TEOA | [127] | ||
| TpBD-(OCH3)2 | HCOOH | 108.3 | 300 W Xe lamp, 800 nm ≥ λ ≥ 420 nm | CH3CN/TEOA | [128] | ||
| TEB-PQ | CH4 | 269 | 300 W Xe lamp, λ > 420 nm | CH3CN/H2O | [130] | ||
| TPA-PQ | CH4 | 2,150 | 300 W Xe lamp, λ > 420 nm | CH3CN/H2O | [130] | ||
| PdIn@N3-COF | CH3OH | 24.5 | 300 W Xe lamp, λ ≥ 400 nm | H2O | [131] | ||
| PdIn@N3-COF | C2H5OH | 8.75 | 300W Xe lamp, λ ≥ 400 nm | H2O | [131] | ||
| ON-MOP | C2H5OH | 800 | 300 W Xe lamp, λ > 420 nm | NaHCO3(aq)/K2SO3(aq) | [132] | ||
H2O2 production reaction
Hydrogen peroxide (H2O2) is an eco-friendly oxidant widely used in various fields[133]. Compared to the commercial anthraquinone process for H2O2 production, the synthesis of H2O2 via photocatalytic oxygen reduction is a more environmentally friendly pathway[134]. To date, supramolecular materials, conjugated polymers, covalent organic frameworks, and g-C3N4-based materials have all shown their potential for photocatalytic H2O2 production[134-137]. The more efficient approach toward H2O2 production is the two-electron oxygen reduction pathway (O2 + 2e- + 2H+ = H2O2, 0.695 V vs. NHE), as its oxidant potential of 0.695 VNHE is much lower than that of two-electron water oxidation reaction (2H2O + 2h+ = H2O2 + 2H+,
In addition to hydrogen evolution, CO2 reduction, and H2O2 production reactions, recent years have also seen the application of organic semiconductors in the photocatalytic degradation of pollutants and organic synthesis[142].
Shortcomings of organic semiconductor photocatalysts
Although organic semiconductors display their capability in solar fuel production and environmental remediation, some shortcomings of organic semiconductors still hinder the enhancement of their photocatalytic activity.
Large exciton binding energy
The exciton is a quasi-particle in semiconductors composed of an electron-hole pair bound together by the Coulomb interaction[143]. The energy separating the bounded electron-hole pair is called the exciton binding energy, which is affected by the strength of the Coulomb interaction[144-146]. When the size of an inorganic semiconductor goes down from bulk to subnanoscale, the Coulomb force in the exciton becomes significantly stronger. Specifically, both the monolayer of two-dimensional material and a quantum dot have a larger exciton binding energy than their bulk counterparts due to their stronger excitonic Coulomb interactions[144-146]. It is well known that the binding energy of exciton (Eb) is inversely correlated to the exciton Bohr radius
Figure 5. (A) Exciton binding energy in common inorganic semiconductors and its relationship with the exciton Bohr radius. Adapted with permission[147], Copyright 2013 American Physical Society; (B) Exciton binding energy in organic semiconductors; (C) Relationship between exciton binding energy and dielectric constant in organic semiconductors; (D) Exciton binding energy as a function of conjugation length in organic semiconductors[149], Copyright 2022 Wiley-VCH.
Short exciton diffusion length and low charge carrier mobility
The exciton diffusion length and charge carrier mobility in organic semiconductors are crucial factors influencing photon-to-electron conversion efficiency[150]. Due to their electrical neutrality, excitons in semiconductors primarily move by diffusion rather than drift. The exciton diffusion length, defined as the average distance an exciton can travel before recombination, determines how many excitons reach the interface and thus affects electron-hole recombination. Generally, a longer exciton diffusion length allows more excitons in the bulk of organic semiconductors to reach the interface, thereby inhibiting the electron-hole radiative or non-radiative recombination[151]. However, the exciton diffusion length in organic semiconductors is typically less than 10 nm[152].
After exciton diffusion and dissociation, the generated electrons and holes migrate either in the bulk or toward the surface of photocatalysts. Unlike the band-like charge carrier transport in crystalline inorganic semiconductors, charge transport in organic semiconductors primarily occurs through inter-molecular hopping, where carriers move from one molecule to another[153-156]. This mode of transport results in significant charge carrier scattering, leading to a shorter mean free time and mean free path in organic semiconductors compared to inorganic semiconductors. According to the classical Drude model µ = τe/m*, the charge carrier mobility (µ) scales positively with the mean free time (τ) and is inversely proportional to the effective mass of the carrier (m*). The shorter mean free time in organic semiconductors already reduces their mobility relative to inorganic semiconductors. Moreover, the effective masses of charge carriers in organic semiconductors are larger than those in inorganic semiconductors[157], further diminishing mobility. Consequently, the combination of shorter mean free times and larger effective masses results in charge carrier mobilities that are several orders of magnitude lower in organic semiconductors than in inorganic semiconductors under identical conditions. This intrinsic drawback tremendously limits the number of charge carriers that can reach the surface of organic semiconductors to trigger interfacial chemical reactions.
Chemical instability
The instability of organic semiconductors is a major disadvantage that hinders their practical application[158]. Although organic semiconductor solar cells are typically fabricated and assembled in an inert gas atmosphere with extremely low humidity[30], their practical application is still bottlenecked by issues related to device durability and lifespan[31]. Even when these devices are kept away from direct contact with inorganic or organic solvents, they still cannot maintain optimal performance for extended periods. This issue is even more pronounced for organic semiconductor photocatalysts, which are often tested in solvents or under harsher conditions, such as strong acids or alkaline environments. For example, boronate ester-linked COFs decompose when exposed to water or moisture[159]. The photostability of organic semiconductor photocatalysts is another significant drawback. Different from the rigid and stable Ti-O octahedra building units in TiO2[5], the C-N or C-O bonds in organic semiconductors are more prone to structural damage from surface-adsorbed hydroxyl radicals and photogenerated holes[160]. Graphitic carbon nitride (g-C3N4) has been extensively studied for solar water splitting and CO2 reduction due to its suitable conduction band minimum potential and visible light response[62]. However, the origin of the CO2 reduction products generated on g-C3N4 has not been traced in detail before. Chen et al. found that CO production occurs on g-C3N4 under light illumination, both in the presence and absence of a CO2 gas atmosphere[160]. In other words, g-C3N4 undergoes rapid self-decomposition during the gas-solid photocatalytic CO2 reduction process, accompanied by CO production. Specifically, hydroxyl groups attack the C−N=C structural units in g-C3N4, leading to the release of CO after a series of complex reactions. These findings highlight the possible structural damage to the backbones of ordinary organic semiconductors in photocatalysis, emphasizing the need to develop more robust organic semiconductor photocatalysts with improved chemical stability.
Solutions to the shortcomings of organic semiconductor photocatalysts
To overcome the shortcomings of organic semiconductors in photocatalysis, several strategies have been developed. π-conjugation extension is a feasible method to lower exciton binding energies in organic semiconductors. Donor-acceptor structure design and heterojunction can improve charge carrier dynamics. Modifying building blocks is also a viable strategy to extend exciton diffusion lengths and improve the chemical stability of organic semiconductors.
π-conjugation extension
After theoretically analyzing the electronic structures of 32 organic semiconductors, Sugie et al. identified a quasi-linear relationship between exciton binding energy and transport band gap, i.e., the exciton binding energy is one-quarter of the transport band gap[161]. It is well known that the band gaps of organic semiconductors decrease with the enlargement of π-conjugation. Accordingly, the exciton binding energy is also expected to decline with π-conjugation extension. Ma et al. directly proved this concept via theoretical calculations. They demonstrated that COFs formed by incorporating polyyne (−C≡C−)n groups between the tertiary amino group and the heptazine ring of the g-C3N4 skeleton show reduced exciton binding energies as the number of polyyne units increases[162]. Lan et al. further experimentally verified this strategy for lowering exciton binding energy in conjugated polymers. Using dibenzothiophene-S, S-dioxide (FSO) as an electron-deficient structural unit, they showed that replacing the electron-rich building block biphenyl (BP) with dibenzothiophene (FS) modulated the exciton binding energy in the resulting conjugated polymers[163]. The larger π-conjugation system in FS compared to BP endows the FSO-FS conjugated polymer with a lower exciton binding energy
Donor-acceptor structure design
The design of electron donor-acceptor (D-A) structures is a widely adopted strategy to facilitate exciton dissociation and charge carrier transport in organic semiconductor photocatalysts[164,165]. The difference in electron affinity between electron-rich donors and electron-deficient acceptors drives electron transfer from the donor to the acceptor in the excited state[166]. When electron donor and acceptor units coexist within the same molecule, an intramolecular D-A structure is formed, which can be realized in conjugated polymers and covalent organic frameworks [Figure 6A and B][166,167]. COFs featuring D-A designs have demonstrated exceptional capabilities in promoting exciton diffusion and charge delocalization, particularly in systems with fully π-conjugated skeletons and large crystalline domain sizes[168-172]. For example, coupling a π-conjugation electron donor system with a typical electron acceptor, such as benzothiadiazole, results in a D-A structure where photoexcited electrons accumulate at the benzothiadiazole unit. These electrons can then be transferred to protons through the catalytic sites provided by N heteroatoms in benzothiadiazole to generate H2[173]. Utilizing benzothiadiazole as the electron acceptor unit, an imine-linked D-A-D structured COF has achieved a photocatalytic hydrogen evolution rate of 5,458 µmol·g-1·h-1 under visible light illumination[174]. Moreover, built-in
Figure 6. Design of Donor-Acceptor (D-A) structure in organic semiconductor photocatalysts: (A) Conjugated polymers with D-A structure. Adapted with permission[166], Copyright 2023 Wiley-VCH; (B) Covalent organic frameworks with D-A structure. Adapted with permission[167], Copyright 2022 Elsevier Ltd; (C) Supramolecular composites with D-A structure. Adapted with permission[175], Copyright 2021 Wiley-VCH.
Heterojunction
Constructing a heterojunction between two organic semiconductors is also a feasible way to promote photoexcited electron-hole separation and transport[178]. Similar to heterojunctions formed between two inorganic materials, proper band alignment is a prerequisite for the formation of organic semiconductor heterojunctions. The staggered CBM and VBM energy levels drive charge transfer between the two organic semiconductors - electrons are collected by the semiconductor with the lower CBM position, while holes accumulate in the semiconductor with the higher VBM position. Specifically, the donor polymer PTB7-Th can form a type II heterojunction with EH-IDTBR, where the staggered bandgap enables highly efficient electron-hole separation. As a result, the PTB7-Th/EH-IDTBR organic heterojunction exhibits significantly higher photocatalytic hydrogen evolution activity than benchmark photocatalysts such as TiO2 and C3N4[179]. In addition to favorable band alignment, efficient sunlight harvesting is also essential for constructing effective heterojunctions. According to the U.S. Department of Energy’s solar-driven photocatalytic hydrogen production guidelines, commercial application of solar-driven photocatalytic hydrogen production is not feasible if a photocatalyst lacks activity at wavelengths above 500 nm[180]. In this scenario, an inorganic/organic S-scheme heterojunction was formed by growing CdS nanocrystals on the surface of a pyrene-based conjugated polymer[181]. Both the CdS nanocrystals and the pyrene-based polymer show absorption edges beyond 500 nm, thereby meeting the threshold for achieving high solar-to-chemical conversion efficiency. Furthermore, the staggered energy bands generate a built-in electric field that facilitates highly efficient hole-electron separation, resulting in a prominent apparent quantum efficiency of 24.3% under visible light irradiation[181]. Additionally, a ternary heterojunction composed of PM6, ICTT-M, and IDMIC-4F - organic semiconductors with strong absorption in the 600-800 nm wavelength range - achieved an apparent quantum yield of 5.9% at 600 nm. This further highlights the vital importance of heterojunctions with broad solar spectrum response in boosting exciton dissociation and charge carrier transport[182].
Building block modification
Modifying the building blocks (i.e., the backbones and substituent groups) of organic semiconductors can effectively extend the exciton diffusion length and improve charge carrier transport. For example, removing the electron-withdrawing thiadiazole units from the backbone of the photovoltaic material Y6
Figure 7. Modifying the building blocks of organic semiconductors to enhance charge carrier dynamics and chemical stability: (A) Tailoring the molecular backbone. Adapted with permission[151], Copyright 2022 American Chemical Society; (B) Modifying substituent groups. Adapted with permission[183], Copyright 2019 Wiley-VCH; (C) Introducing irreversible linkages to enhance the chemical stability of COFs[186], Copyright 2024 American Chemical Society.
CONCLUSION
As discussed above, organic semiconductors with large absorption coefficients, tuneable electronic band structures, and controllable dielectric constants have shown merits in boosting photocatalytic reactivity. However, drawbacks in exciton dissociation, exciton diffusion, charge carrier transport, and chemical stability still hinder their practical application in photocatalysis. In addition to well-established strategies for optimizing photocatalytic performance - such as π-conjugation extension, donor-acceptor structural design, interface engineering, and building block modification - the following aspects should also be considered to further improve exciton dissociation, exciton diffusion, surface catalytic activity, and chemical stability:
Enhancing the Dielectric Constant: A high dielectric constant can not only reduce exciton binding energy but also suppress electron-hole recombination in organic semiconductors. Therefore, greater efforts are needed to design and synthesize organic semiconductor photocatalysts with elevated dielectric constants. One approach involves incorporating functional groups with flexible permanent dipoles into the molecular structures of organic semiconductors[102]. For example, ethylene glycol (EG) chains, which possess permanent dipoles and maintain conformational flexibility in the solid state, can reorient rapidly and thereby help enhance the dielectric constant[102]. Additionally, the polarizability of the molecules significantly affects the dielectric constant. Incorporating non-fullerene building units with large fused-ring structures offers a promising route to increasing molecular polarizability and, consequently, the dielectric constant[188].
Optimizing Surface Catalysis in Porous Structures: Beyond light absorption, exciton dissociation, and charge carrier transport, surface catalysis plays a crucial role in improving photocatalytic performance. The catalytic sites in organic photocatalysts influence reactant adsorption, the efficiency of charge carrier transfer to adsorbed reactants, and product desorption. In porous organic photocatalysts, fine-tuning the chemical environment within pores and modifying the structure of pore walls are key strategies for enhancing photocatalytic efficiency[189-192]. Such designs can improve hydrophilicity and reactant accessibility. For instance, chiral pore environments can promote enantioselective reactions and facilitate product separation[193-195]. Moreover, rationally designed pore walls allow precise regulation of cocatalyst loading and distribution, which can lower the overpotential of specific reactions[18,99]. Pore size also significantly affects the photocatalyst’s surface area and the mass transport of reactants and products, ultimately influencing turnover rates and overall catalytic efficiency[196-199]. Larger and more flexible pores generally improve molecular dynamics, enhancing reactant utilization and product release. Therefore, future research should focus on constructing large and adaptable pore structures in organic semiconductors.
Balancing Stability and Activity: In some cases, enhancing photocatalytic activity may compromise the stability of organic semiconductors. For example, increasing hydrophilicity can significantly enhance photocatalytic H2 production[115,200], but the reversible linkage of hydrophilic building blocks may lead to the destruction of the semiconductor framework[159]. Therefore, achieving chemical stability without sacrificing catalytic activity is critical for designing robust organic semiconductor photocatalysts. In this context, employing irreversible linkages lays a solid foundation for enhancing long-term chemical stability. Furthermore, integrating hydrophilic building blocks with irreversible linkages, such as carboxyl-substituted quinoline or imidazole, could enable the construction of highly durable organic photocatalysts.
DECLARATIONS
Acknowledgments
This paper was supported by the Joint Usage/Research Center for Catalysis.
Authors’ contributions
Conceptualization, Formal analysis, Project administration, Writing-original draft, Writing-review & editing: Lin, R.
Formal Analysis, Writing-review & editing: Hsueh, C.H.; Yu, H.
Resources support: Heine, T.; Chen, C.
Administrative support, Writing-review & editing: Murayama, T.
Availability of data and materials
Not applicable.
Financial support and sponsorship
None.
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
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