Application of Polydopamine-Based Photocatalysts in Energy and Environmental Systems

Wang, Hui; Jing, Liquan; Varamesh, Amir; Yan, Yubo; Zhang, Hongguang; Gao, Qi; Hu, Jinguang

doi:10.70322/prp.2024.10007

Photocatalysis: Research and Potential

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Application of Polydopamine-Based Photocatalysts in Energy and Environmental Systems

Hui Wang ¹ Liquan Jing ^1,*

Amir Varamesh ¹ Yubo Yan ^1,2 Hongguang Zhang ¹ Qi Gao ¹ Jinguang Hu ^1,*

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Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive, NW, Calgary, AB T2N1N4, Canada

School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huai’an 223300, China

Authors to whom correspondence should be addressed.

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Photocatalysis: Research and Potential 2024, 1 (3), 10007; https://doi.org/10.70322/prp.2024.10007

Received: 29 July 2024 Accepted: 04 September 2024 Published: 09 September 2024

ABSTRACT: Polydopamine (PDA) is also widely sought after in photocatalytic applications due to its fascinating properties such as simple preparation, templating agent, near-infrared absorption, high photothermal conversion efficiency, abundant functional groups, and strong chelating effect of metal ions. This review will present the structural features and synthetic methods of PDA, the advantages of PDA for photocatalytic applications (templating agent effect, light absorption properties, film-forming properties, hydrophilicity, conductivity, etc.), the modulation strategies of PDA for photocatalytic applications, and the use of PDA-based photocatalytic materials for solar-powered water purification (heavy metal adsorption and reduction, catalytic degradation of organic pollutants, and antimicrobial properties), hydrogen production, hydrogen peroxide production, CO₂ reduction, and organic conversion. Finally, this review will provide valuable information for the design and development of PDA-based photocatalytic materials.

Keywords: Polydopamine; Photocatalyst; Energy; Environment

Graphical Abstract

1. Introduction

Photocatalytic technology has important applications, especially in the fields of environmental remediation, energy conversion, and synthetic chemistry, by using light energy to excite semiconductor materials to generate electron and hole pairs and initiate chemical reactions [1,2,3,4]. The photocatalytic process involves light absorption, separation and migration of electron-hole pairs, and reaction with molecules at the material surface [5]. Despite the challenges of limited light absorption range, and material stability, high photogenerated carrier complexity, the development of new materials will promote its application in the environmental and energy fields [6,7]. Polydopamine (PDA), as an emerging material, has gained much attention in photocatalysis in recent years. Further, PDA exhibited great potential in the field of photocatalysis as a melanoidin-like material with structural diversity (rich functional groups such as phenolic hydroxyl, amino, and amide groups), biocompatibility, good adhesion (stable coating and film-forming properties), and broad-spectrum absorption (capable of absorbing ultraviolet, visible, and near-infrared light) [8,9]. Currently, PDAs have the potential to be used in the effective degradation of organic pollutants, decomposition of water to produce hydrogen, and reduction of carbon dioxide, etc. To improve its photocatalytic performance, researchers have mainly used the strategy of constructing heterojunctions to modulate the electronic structure of PDA and enhance the separation efficiency of photogenerated carriers [10,11,12,13]. However, PDA also has drawbacks such as photocorrosion and structural instability, especially in strong acid or alkali environments. In addition, the poor dispersion of PDA nanoparticles will affect the modification of the photocatalyst and reduce the catalytic efficiency [14,15]. Despite some challenges, the continuous optimization and design of PDA makes it a promising prospect for applications in the fields of environmental pollution control, clean energy conversion, and green chemical synthesis. Future research will focus on enhancing their efficiency and stability for a wider range of applications. In recent years, the preparation and extensive applications of PDA-based materials on photocatalytic have rapidly developed, as shown by the growing publications over past 10 years (Figure 1a). As impressive as the volume of publications on photocatalysis is its broad interdisciplinary contribution. An analysis of the same dataset of photocatalysis publications by discipline revealed that nearly 75% of the research focused in the fields of water purification and water splitting. In addition to that, significant contributions came from CO₂ reduction and H₂O₂ production (Figure 1b).

Figure 1. (a) Number of publications on PDA-based photocatalysts during the past 10 years (source: Web of Science; date: 30 July. 2024; keywords: polydopamine photocatalytic). (b) Pie chart of “PDA-based photocatalysts” by different research areas using the same data as in application.

2. Molecular Structure and Polymerization Mechanisms of PDA

PDA (Polydopamine) is a synthetic polymer inspired by the adhesive proteins found in mussels. Its structure and polymerization process are of significant interest due to its adhesive properties and biocompatibility. The specific process includes dissolving dopamine hydrochloride in a Tris-HCl buffer solution, usually at a pH of 8.5. The solution is stirred at room temperature, allowing the dopamine to undergo self-oxidation in the presence of oxygen in the air, gradually forming polydopamine [16,17]. In PDA, dopamine functions as a building block, consisting of an amine group (a primary amine attached to an ethyl chain) and a catechol group (an ethyl ring). The multiple chemical reaction sites in PDA, complex redox processes, and generates numerous intermediates, resulting in a diverse synthesis mechanism with no definitive conclusion yet. Lee et al. proposed two polymerization mechanisms for PDA: non-covalent self-assembly and covalent polymerization, which have been widely accepted [8,18]. In the covalent reaction pathway, dopamine is first oxidized to a quinone structure. The quinone structure may then be further oxidized to form 5,6-dihydroxyindole (DHI) (Figure 2a), or undergo intermolecular reactions through catechol-catechol coupling. Finally, cross-linking reactions between these intermediates occur, forming the final PDA structure. In addition to covalent polymerization, the formation of PDA also involves a non-covalent self-assembly pathway [19]. As illustrated in Figure 2b, PDA is considered to be an aggregate of monomers that are primarily cross-linked through strong intrachain and interchain non-covalent forces, encompassing hydrogen bonding, charge transfer and π-π stacking, which is similar to other synthetic or biological supramolecular polymers [8,17].

Figure 2. (a). Dopamine oxidation and subsequent cyclization and rearrangement; (b) The different pathways of PDA formation.

3. Advantages of PDA in the Field of Photocatalysis

3.1. Light Absorption and Excellent Photothermal Conversion Properties PDA, a black polymer generated through the self-assembly of dopamine, exhibits an expansive light absorption spectrum that spans from the ultraviolet to the near-infrared spectrum. This broad absorption capability stems from the intricate conjugated system within its molecular structure, which consists of aromatic rings intertwined with nitrogen-containing moieties. Additionally, the synergistic conjugation of phenolic hydroxyl and amino groups further facilitates electron transition at diminished energy levels [17]. The optical absorption ability could be boosted by changing the bandgap of PDA, for widely using in photothermal conversion and energy harvesting applications. As reported by Zou et al., doping TEMPO moiety into the PDA microstructure by covalently connecting with DHI/IQ oligomers could enhance the light absorption across a broad spectrum by decreasing the energy bandgap and increasing electron delocalization (Figure 3a) [20]. Later, Bai and co-workers proposed that introducing nitrogen-containing heterocycles on PDA significantly lowers the LUMO, promoting electron delocalization in the local regions, which also resulted in the visible light absorption [21]. Moreover, the broad-spectrum light absorption capability of PDA is highly beneficial in photocatalysis. Utilizing PDA in semiconductor modification or constructing heterojunctions can expand the absorption range of semiconductors and promote charge separation. Some researches demonstrated that PDA could be used as photonic sensitizers for enhanced photocatalysis of ZnO in the range of visible light. For instance, Dong et al. fabricated a water purification membrane (PDA/ZnO-NWs/PVDF) by DA polymerization on the ZnO-NWs/PVDF surface [22]. The UV-vis-DRS was shown in Figure 3b, the introduction of PDA has significantly improved the material’s absorbance to light in the range of 396−1750 nm. Similarly, by modifying PDA, the light utilization range can be extended from UV to visible light, which can also be applied to other semiconductors, such as TiO₂ and graphitized carbon nitride (g-C₃N₄) [23,24,25].

Figure 3. (a) The schematic illustration of the bandgap and light absorption ability of PDA and TEMPO-doped PDA [20]. (b) Light absorption ability of PDA/ZnO-NWs/PVDF and ZnO-NWs/PVDF [22]. (c) TEM image of CuSnO-DopaCube and distribution of corresponding elements. (d,e) UV-Vis absorbance spectrum and the evaluation of photothermal properties of CuSnO-DopaCube [26]. (f) Schematic illustration of ZIF-8@PDA/C3N4 coating on the wood hydrogel. (g) IR images of the ZP-C-W in different solar illuminations. (h) The corresponding evaporation rates and solar evaporation efficiencies of different photocatalysts [27].

PDA excels in photothermal conversion due to its remarkable light absorption capacity, effectively absorbing light from the visible to the near-infrared spectrum. It converts absorbed light into heat energy through non-radiative relaxation processes, where electron excitation energy is transformed into molecular vibrational energy, subsequently released as heat. The molecular structure of PDA not only facilitates efficient photothermal conversion but also enables rapid heat transfer via intermolecular interactions. Additionally, its robust thermal stability allows it to retain structural integrity at elevated temperatures, establishing PDA as a potent material for photothermal applications [28]. Gao and co-workers developed a Cu-Sn-PDA photocatalytic Fenton system [26]. As the TEM images shown in Figure 3c, the heterojunction structure fabricated by Cu₂O and SnO₂ significantly as a core, which was closely wrapped by PDA. The mapping images also confirmed the successful synthesis of the three-layered heterostructure, the element mapping showed a uniform mixture of Cu, Sn, O, and C elements, indicating that the Cu/Sn heterostructure was tightly wrapped by PDA. Firstly, the CuSnO-DopaCube’s optical density (OD) values were detected through UV full spectrum (Figure 3d). The CuSnO-DopaCube (500 μg·mL⁻¹) had an OD value of 1.04 at 808 nm and 0.82 at 980 nm, indicating better photo-thermal conversion efficiency in the NIR region. In addition, the PDA coating in CuSnO-DopaCube significantly improved the photothermal performance (Figure 3e). Based on the excellent light absorption properties across a wide spectrum of light and high photothermal conversion efficiency, PDA as a light absorption layer was well developed in solar evaporator. The solar thermal evaporation process offers a promising technology for producing thermal energy for promoting reaction dynamics and thermodynamics [29]. Lu et al. designed and fabricated a wood/ZIF-8/PDA composite and applied to the solar evaporation and water purification with the photothermal conversion efficiency of 92%. Among them, the black PDA coating on the wood surface used as light absorption layer [30]. Based on the wood/ZIF-8/PDA composite, the group members designed a wood-based hydrogel evaporator modified by C₃N₄ and PVA/chitosan hydrogel, which accelerate solar desalination and remove pollutants from wastewater (Figure 3f) [27]. As displayed in Figure 3g,h, the evaporator surface temperature increased with the increasing solar intensity. The highest surface temperature and energy conversion efficiency under 6 kW/cm⁻² after 60 min could reach up to 82.7 °C and 94.42%, respectively. The excellent solar evaporation performance was contributed to the solar spectra utilization of PDA. In brief, the great light absorption and photothermal conversion properties allow PDA to improve solar energy utilization. Apart from PDA, the light absorption capacities of other common photocatalytic polymers include polyaniline (PANI) [19,31], polypyrrole (PPy) [32] and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) [33,34] are more limited than that of PDA, and it has fewer active sites [35]. For example, PANI and PPy have good light absorption ability in the UV region, but their performance in the visible region is relatively weak. Some polymers can enhance their photocatalytic properties in the form of doping or composites, but in general additional modifications are required to achieve the effect of PDA. 3.2. Surface Adhesion The catechol and amine groups on the PDA surface make it a good substrate or template for the chelating metal ions [36]. Wang et al. synthesized PDA/CdS nanocomposite for Rhodamine B (RhB) removal from aqueous solutions [37]. First, the PDA nanoparticles were synthesized uniformly with a spherical structure (Figure 4a). Then followed by depositing CdS nanoparticles on the PDA substrate. A possible explanation may be that Cd²⁺ ions chelated with the PDA substrate, which promoted a heterogeneous nucleation of CdS nanoparticles (Figure 4b). In photocatalytic in removing RhB process, the PDA NPs in nanocomposite with the functional groups, such as −OH and −NH₂, exhibited the excellent absorption effect for RhB. In conclusion, it was inferred that there was a significant synergy between adsorption and degradation to efficiently remove RhB from the reactions. Additionally, PDA served as an effective shell material for constructing core-shell structures, encapsulating nanoparticles to enhance their stability. Throughout its formation, dopamine monomers polymerize at various sites to create a cross-linked network, boosting the mechanical stability and adhesive properties of the polymer film. PDA demonstrated superior interfacial activity, capable of self-assembling onto diverse material surfaces to form a uniform coating. Its molecules naturally orient at the liquid-solid interface, creating a dense film that underpins its exceptional adhesive qualities [38]. As shown in Figure 4c, the PDA coating of the core-shell structure was spontaneously formed by immersing the core structure material in an alkaline aqueous solution of dopamine [39]. The sandwich structure was a three-layer concentric heterostructure composed of two or more materials, which exhibited stronger synergistic effects comparing with traditional two-component core-shell structures [10,40,41,42]. Shang et al. synthesized the novel recyclable Fe₃O₄@PDA/CuS core-shell structure photocatalyst through by in situ reduction method [43]. Among them, the Fe₃O₄ was used as a magnetic core to provide recovery ability, then the PDA coated on Fe₃O₄ by self-polymerization. The PDA coating can prevent Fe₃O₄ from being oxidized, and the abundant -OH and -NH₂ can chelate copper ions. The Fe₃O₄@PDA/CuS exhibited excellent photocatalytic performance and cycle stability for MB degradation. Therefore, the rich active groups on the surface of PDA enable it to form strong adhesion with a variety of material surfaces, which can be used as a bonding layer to immobilize the photocatalyst on various substrates and increase the stability and service life of the photocatalyst. Compared with PDA, PANI, PPy and PEDOT: PSS exhibited weak adhesion, especially in wet environment, but it can still be enhanced through chemical modification or by introducing a hydrophilic polymer adhesive layer to improve its surface adhesion properties [44,45].

Figure 4. (a) The preparation process of PDA NPs/CdS nanocomposite and (b) the RhB degradation process [37]. (c) Preparation process of the PDA@WO3 nanocomposite [39].

3.3. Film Forming Properties As inspired by mussels’ foot proteins mfp, a strong attachment to almost all substrate types have made PDA one of the most popular coating methods. So far, PDA coatings have been used as mechanical strength coatings combined with porous materials, such as hydrogel [46], foam [47], sponges [48,49] and cotton fabrics [50,51]. On the one hand, this ability to uniformly coat different materials ensures that the light absorption layer is stable and durable, maintaining its efficiency over time. On the other hand, the modification of PDA on the substrates will further enable strong binding of photocatalysts on the substrate, benefiting the stability and recyclability of the photocatalyst. Li et al. reported a novel recyclable cotton fabric/PDA/BiOBr (CFPB) photocatalyst prepared by immobilizing the BiOBr component on PDA modified cotton fabric, which was used for RhB removal and Cr(VI) reduction in visible light irradiation (Figure 5a) [52]. Gong et al. synthesized a novel coated photocatalyst based on the polydopamine modified melamine sponge and then adhering Ag-AgCl/WO₃/g-C₃N₄ (AWC) nanoparticles (Figure 5b) [53]. The AWC/PDA/MS removed 99.9% of trimethoprim (TMP) under visible light in 90 min. The recyclable photocatalyst based on a PDA modified flexible substrate provides a green alternative to powdered photocatalysts with low recyclability. PDA have been used as a component in a number of different filtration membranes, such as PAN and PVDF [54,55]. For instance, Wang et al. designed a 3D layered photocatalytic fiber membrane PAN@PDA/Tb-g-C₃N₄/ZnIn₂S₄ (PPTZ), by electrospinning and coating process (Figure 5c). First, the PAN@TCN fiber films (layer 1) were obtained via electrostatic spinning. Then PDA was deposited on the PT membrane via in situ self-polymerization, which was acted as an intermediate linker. Lastly, the ZnIn₂S₄ was uniformly deposited on the membrane surface, then the three-dimensional layered membrane was labelled as PPTZ. Such a fibrous film had excellent photocatalytic degradation efficiency of antibiotics. In conclusion, PDA forms stable coatings on various substrates with its simple film formation process, excellent adhesion, and surface uniformity, while the film formation of other polymers is more complex, often requiring specific control of conditions, and has advantages in electrical conductivity and mechanical properties [45]. Therefore, PDA is relatively more advantageous in the field of photocatalysis.

Figure 5. (a) The preparation process of photocatalyst fabrics CFPB [52]. (b) The schematic of photocatalytic degradation of TMP with AWC/PDA/MS photocatalyst coatings [53]. (c) The synthetic pathway of the PPTZ film [56].

3.4. Conductivity Polydopamine (PDA), as an organic polymer, is not a conductive material in the traditional sense, but it has certain conductivity. This is mainly due to the aromatic ring structure contained in the PDA molecule forming a certain conjugated system. These conjugated systems allow π electrons to move easily between molecules, thereby giving PDA a certain conductivity [57]. Second, PDA exhibits significant π−π interactions due to its unique molecular structure, which contains aromatic rings. These π−π interactions contribute to several important properties and applications of PDA [38]. For example, the π−π interactions enable PDA to strongly adsorb onto surfaces containing aromatic rings or π-conjugated systems [58]. Additionally, the strong interactions facilitate electron transfer processes, contributing to the overall catalytic performance. Graphitized carbon nitride (g-C₃N₄) is a 2D metal-free π-conjugated polymer semiconductor photocatalyst with a wide range of applications in photocatalysis [59]. The conjugated π structure shared between PDA and g-C₃N₄ allows π-π^∗ electrons delocalization, which synergistically enhanced carrier transport. Therefore, modifying g-C₃N₄ with PDA can enhance the activity [15,60]. Furthermore, the bridge-like channel structure formed by the PDA could induce charge transfer through the additional electron transfer channel (PDA), forming the direct Z-scheme charge transfer mechanism [61,62,63]. Wu and colleagues constructed a Z-Scheme Bi@Bi₂O₂CO₃/PDA/g-C₃N₄ (BPCN) heterojunction [64]. As illustrated in Figure 6a, during the polymerization process, PDA adhered to g-C₃N₄ through strong covalent and non-covalent interactions, as well as π-π^∗ stacking, and provided additional pathways for electron transfer. Thereby enhancing photocatalytic performance through synergistic effects on light absorption and charge carrier separation. The possible photocatalytic mechanism of BPCN heterojunction was proposed in Figure 6b. In the internal electric field formed between Bi₂O₂CO₃ and g-C₃N₄, electrons rapidly flow from the conduction band of Bi₂O₂CO₃ to the valence band of g-C₃N₄. They then recombine with the holes on g-C₃N₄ via PDA acting as an electronic bridge. The conjugated π-π^∗ structure between g-C₃N₄ and PDA facilitated electron delocalization and carrier transfer, balancing the Fermi levels between the semiconductors. Similarly, PDA bridge acted as cross-link of the active phase on carbon nitrates (g-C₃N₄) to ZnAl layered double hydroxide nanosheets to construct VL-driven Z-type heterojunction catalysts [61].

Figure 6. (a) The preparation process of PDA/g-C3N4 and (b) the possible mechanism of charge transfer of BPCN under visible light irradiation [64]. (c) The sandwich structure of CdS@PDA@SnO2−x and (d) the photoelectron transfer of CdS@PDA@SnO2−x [65].

In sandwich structure composites, PDA can not only prevent photocorrosion of the nuclear structure, but also served as photosensitizer and electron transfer mediator in the heterojunction [51,66]. Liu and co-workers constructed a CdS@PDA@SnO_2−x heterojunction with sandwich structure (Figure 6c) [65]. In this structure, PDA molecules wraped around the surface of CdS, protecting the core from photocorrosion. Simultaneously, the catechol structure of PDA can anchor Cd²⁺ and Sn⁴⁺ ions at specific positions, leading to quick electron transfer, as illustrated in Figure 6d. The as- prepared photocatalyst exhibited an excellent photocatalytic hydrogen (H₂) production activity. Qin et al. synthesized a CdS@PDA/NiS ternary composite for H₂ production [40]. The strong Cd–O chemical bond between CdS and PDA served as high-speed electron transmission channels, while the Schottky junction accelerated the spatial separation of carriers and improved the photocatalytic hydrogen production performance. After four experiment cycles, the obtained CdS@PDA/NiS maintained good photocatalytic activity and structural integrity. PDA with conjugated systems has excellent electron transport capability, which can reduce the recombination of electron-hole pairs during photocatalysis. Apart from PDA, other common photocatalytic polymers (such as PANI and PPy) also exhibited excellent electrical conductivity and stability [67,68]. This property allows them to conduct electrons efficiently and promote the separation of electrons from holes in the photocatalytic reaction, thereby improving the photocatalytic efficiency.

4. Regulation Strategies of PDA in the Field of Photocatalysis (Mainly Composite Materials with Various Semiconductors)

4.1. Directly Construct Composite Materials with PDA and Other Semiconductors PDA is a kind of polymer material with unique chemical structure and functional groups, which has received wide attention for its potential application in photocatalysis. At present, the photocatalytic performance of PDA can be improved by metal ion doping and constructing composites with other semiconductors (or plasma metal monomers) or surface modification [69,70,71,72,73,74,75]. In the construction of photocatalytic composites, PDAs are often mainly used to form heterojunctions with other semiconductor materials (the main types include: direct to electron conduction type, type II, Z-scheme and S-scheme) to improve the photocatalytic performance. Commonly, g-C₃N₄ [58,76,77,78,79], metal oxides (TiO₂, ZnO, Cu₂O, CeO₂, etc.) [80,81,82,83,84,85,86], Bi-based materials (BiOCl, Bi_2.15WO₆ etc.) [87,88,89,90], metal sulfides (CdS, ZnS, ZnCdS, ZnIn₂S₄, etc.) [91,92,93,94], MOFs [95,96], metal alloys [97,98,99] and carbon dots [100] are used to construct composites directly with PDA. By compositing PDA with these semiconductor materials, it is possible to enhance the photocatalytic efficiency and at the same time extend its application in the visible light range, thus realizing more efficient photocatalytic reactions. Jing and co-workers designed a 1D CdS/PDA heteronanotubes (HNTs) photocatalyst for antibiotic degradation (Figure 7a) [93]. Under the light excitation, the oxygen was reduced into •O₂⁻ by photoexcited electrons or continuously converted into •OH to attack the antibiotics. On the other hand, h⁺ in the valence band can also attack the antibiotics directly and trigger continuous degradation. A Cu₂O@PDA nanospheres composite was designed by Zhou and co-workers for methylene orange (MO) dye degradation (Figure 7b) [84]. Cu₂O and PDA exhibit energy level alignment suitable for the construction of a type II heterojunction. Based on this mechanism, the electrons accumulated on the PDA surface and reacted with water and oxygen to produce ROS (including •OH and •O₂⁻), while the h⁺ focused on the valence band (VB) of Cu₂O. Under the oxidation of ROS and h⁺, MO was degraded into CO₂, H₂O, etc. Shi et al. constructed a polydopamine-coated tungsten oxide (PDA@WO₃) nanomaterial with special yolk-shell structures (Figure 7c) [39]. The WO₃ and PDA can form a Z-Scheme transport mechanism in photocatalytic reactions and lead to a lifetime extension of the carriers. Specifically, under light illumination, the PDA in PDA@WO₃ induces a π–π transition, where excited-state electrons are transferred from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of PDA, leaving behind a large number of holes (h⁺). The electrons generated from WO₃ are transferred to the HOMO orbital of PDA through interfacial transport under the effect of built-in electric field, and these electrons are combined with holes in HOMO. The electrons in the LUMO orbital of PDA could reduce O₂ to •O₂⁻, which react directly with the MB. Zhang et al. synthesized a novel S-scheme heterojunction photocatalyst comprised of ultrathin g-C₃N₄ (U-CN) and PDA [12] (Figure 7d). Under light irradiation, electrons in U-CN were excited from their VB to conduction band (CB) and eventually recombined with the holes in the HOMO of PDA driven by the built-in electric field, manifesting the S-scheme mechanism.

Figure 7. (a) Schematic diagram of the reaction mechanism during the catalytic decomposition of antibiotics by CdS/PDA [93]. (b) Schematic diagram of the possible photodegradation mechanism of Cu2O@PDA via visible light irradiation [84]. (c) Schematic diagram of the photodegradation mechanism of MB on PDA@WO3 sample [39]. (d) Schematic diagram of the S-type photocatalytic mechanism of C3N4/PDA [12].

4.2. Composite Materials Constructed with PDA as an Electronic Bridge Structure PDA as an electronic bridge plays multiple advantages in photocatalytic composites, including improving the separation efficiency of photogenerated electrons and holes and forming multifunctional composite structures with a variety of semiconductors to realize diversified photocatalytic reactions, which shows its potential for application in building highly efficient and stable photocatalytic materials [101,102,103,104]. Zhang et al. developed a ZnO@PDA-Au photocatalytic system for organic pollutant removal (Figure 8a) [105]. When Au contacts ZnO, a Schottky barrier forms due to work function differences, and ZnO under UV-visible light excitation generates electron-hole pairs. Electrons transfer from ZnO to Au and PDA, reducing dissolved oxygen to create •O₂⁻, while holes oxidize water to form •OH. These radicals contribute to organic molecule degradation. The PDA and local surface plasmon resonance (LSPR) effect of Au enhanced visible light absorption. The Schottky barrier served as an electron trap utilizing charge separation efficiency and significantly and boosting the catalytic performance of ZnO@PDA-Au. Figure 8b illustrates that AgNPs/PDA/g-C₃N₄ enhances the charge step transfer route via the built-in electric field, with PDA capturing molecular oxygen to generate free radicals and improve pollutant adsorption. The LSPR effect of AgNPs was triggered at a wavelength of 390–420 nm, producing hot electrons and reducing the electron-hole recombination rate, which increased the generation rate of charge carriers. This synergy between PDA and AgNPs significantly boosted the photocatalytic MB degradation activity of g-C₃N₄ [106]. Figure 8c illustrated the photocatalytic hydrogen production mechanism developed by Meng et al. using a g-C₃N₄ nanotube@PDA/NiCo-LDH composite. Under visible light, g-C₃N₄ and NiCo-LDH generated electron-hole pairs, with PDA acting as a photosensitizer and interface medium to boost light absorption and facilitate charge transport. The electrons from g-C₃N₄ were drawn to the higher energy levels of NiCo-LDH, while the holes migrated to g-C₃N₄. Pt nanoparticles catalyzed the reaction of electrons on NiCo-LDH with H⁺ ions in water to produce hydrogen, whereas holes on g-C₃N₄ were quenched by TEOA. The combined action of the type II heterojunction and PDA significantly enhances carrier separation and transport, greatly improving the efficiency of hydrogen production [107]. Yang et al. developed a type II heterostructure by combining carbon nitride nanosheets (CNNS) with CdS, which under light exposure, facilitated the production of photogenerated electron-hole pairs (Figure 8d) [15]. Electrons were transferred from the CB of CNNS to the CB of CdS, while holes migrated from the VB of CdS to the VB of CNNS. The electrons in CdS reacted with dissolved oxygen to produce superoxide radicals (•O₂⁻), and the holes in CNNS directly participated in the degradation of RhB. The inclusion of polydopamine (PDA) in this setup enhanced charge migration, minimized electron-hole recombination, and thus significantly bolstered the photocatalytic efficiency. A novel g-C₃N₄@PDA/BiOBr Z-scheme photocatalytic system was constructed by Guo et al. (Figure 8e) [62]. Here, electrons move from the CB of BiOBr to the VB of g-C₃N₄ through PDA, leading to effective charge separation. This movement resulted in a higher concentration of electrons in the CB of g-C₃N₄ and holes in the VB of BiOBr. The holes in BiOBr’s VB are potent enough to directly oxidize sulfamethoxazole (SMX), while the accumulated electrons in g-C₃N₄’s CB reduce O₂ to form superoxide radicals (•O₂⁻). Concurrently, the VB holes in BiOBr can also oxidize H₂O/OH− to generate hydroxyl radicals (•OH). These radicals collaborate to decompose SMX into CO₂ and H₂O. PDA serves a crucial role as an electron mediator, enhancing carrier separation and thereby boosting the photocatalytic redox capabilities of the composite material.

Figure 8. (a) Mechanism diagram of ZnO@PDA-Au under UV-visible light irradiation [105]. (b) Schematic diagram of charge migration of AgNPs/PDA/g-C3N4 [106]. (c) Possible mechanism of photocatalytic hydrogen production by g-C3N4@pDA/NiCo-LDH [107]. (d) Possible mechanism of photocatalytic removal of RhB by CNNS/PDA/CdS [15]. (e) Possible mechanism of photodegradation of SMX by Z-scheme heterojunction g-C3N4@PDA/BiOBr under visible light irradiation [62].

4.3. Comprehensive Photocatalytic System Constructed by Combining PDA, Semiconductor and Membrane Materials (Other Carrier Materials) The photocatalytic system fabricated by combining PDA, semiconductor and membrane materials utilizes the excellent film-forming and surface modification capabilities of PDA to form a uniform protective layer on the surface of the semiconductor material, which enhances stability and durability [108,109,110,111,112,113,114]. The hydrophilicity and anti-pollution properties of the PDA-modified membranes enhance the efficiency of wastewater treatment and simplify the cleaning and regeneration process. The biocompatibility and environmentally friendly synthesis process of PDA support the construction of sustainable photosynthetic systems. Membrane materials provide physical protection for the photocatalysts and prolong the service life, while PDA enhances the surface activity and reaction sites to accelerate the photocatalytic reaction rate. The composite system of PDA, semiconductor, and membrane materials (PES, PVDF, cellulose membrane, PVA, cellulose acetate membrane, PET, poly (arylene ether nitrile) (PEN), polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), etc.) is simple to prepare, easy to produce on a large scale, and multifunctional. Overall, the photocatalytic system constructed by PDA, semiconductor and membrane materials significantly improves the photocatalytic efficiency by improving the photocatalytic activity, stability, enhanced the surface properties and multifunctionality, and shown a wide range of application potential [115,116,117,118,119,120,121]. Shi et al. developed a poly (vinylidene fluoride) (PVDF) composite membrane enhanced with polydopamine and titanium dioxide nanoparticles, designed for efficient and eco-friendly treatment of organic wastewater (Figure 9a) [55]. After UV irradiation for 60 min, this membrane exhibited a water flux three times higher than that of the standard PVDF membrane. It showed remarkable pollutant retention, removing approximately 99.0% of Congo red and Coomassie Brilliant Blue G250. Post-treatment, the water flux of the membrane recovered over 87.2%. Additionally, the composite membrane demonstrated excellent retention and antifouling capabilities in oil-water emulsion filtration, underscoring its versatility in various wastewater treatment and separation applications. Zheng et al. developed a novel photocatalytic nanofiltration (NF) membrane by applying interfacial polymerization on PVDF membranes to introduce a polyamide layer and incorporating polydopamine with BaTiO₃/Ti₃C₂T_x nanoparticles (Figure 9b) [110]. This combination achieved both physical separation and photocatalytic degradation functionalities. The membrane demonstrated high water permeability (34.0 L·m⁻²·h⁻¹·bar⁻¹) and excellent Na₂SO₄ retention (94.7%). Under irradiation from a 300 W Xe lamp for 180 min, it showed substantial photocatalytic degradation efficiencies for various azo dyes: 95.3% for methyl orange (MO), 92.7% for alizarin yellow GG, and 83.5% for methyl red. This research introduces an innovative approach for creating bifunctional photocatalytic nanofiltration membranes with effective degradation capabilities. In addition to the membrane materials mentioned above, more types of carrier materials (polyurethane sponge, cellulose fibers, nickel foam, hydrogel, woven cotton fabric, wood, melamine foam, filter paper, etc.) can replace membrane materials [122,123,124,125,126,127,128], further promoting the practical application value of PDA-based photocatalytic materials.

Figure 9. (a) PVDF/PDA-TiO2 composite membrane for filtration of pollutants photodegradation [55]. (b) Schematic diagram of the BaTiO3/Ti3C2Tx@PDA membrane manufacturing process and the photodegradation mechanism of the BaTiO3/Ti3C2Tx@PDA membrane [110].

5. Application of PDA in the Field of Photocatalysis

As a typical organic polymer material, PDA itself has very poor photocatalytic activity. Nevertheless, PDA-based photocatalysts have attracted much attentions owing to their excellent properties, such as light harvesting abilities, interfacial properties and electron transfer capabilities. Hence, PDA can synergize with semiconductor photocatalysts to enhance photocatalytic performance in various application fields. In this part, we give a brief overview of their application areas in water purification, water splitting, carbon dioxide reduction and other photocatalytic reactions. 5.1. Water Purification In recent years, the shortage of water resources has become increasingly serious, and water pollution has exacerbated the shortage problem. The main pollutants include organic pollutants, heavy metal ions, microbes and bacteria. Photocatalytic technology has become an important, economical and effective method to solve environmental pollution problems. 5.1.1. Organic Pollutants Degradation PDA, with rich functional groups and unique structure, exhibits significant absorptivity for organic pollutants, making it a valuable material in environmental remediation. For example, the aromatic rings of PDA molecular structure can engage in π-π interactions with the aromatic rings of organic pollutants molecules, improving adsorption capacity. This interaction is particularly effective for adsorbing organic pollutants with aromatic ring structures, such as azo dyes and aromatic compounds [129,130,131]. Besides, electrostatic attraction or repulsion between the surface charge of the catalyst and the pollutant molecules affects adsorption performance. For example, a positively charged catalyst surface is more likely to adsorb negatively charged pollutant molecules. The surface charges of PDA are controllable by adjusting the pH medium [63]. Recently, an Ag/PDA@Ag₂S smart pH-heterojunction photocatalyst with multi interface band structure modulation was designed and proposed through a simple and rapid in situ polymerization method (Figure 10a) [132]. The photocatalytic process was carried out with Ag/PDA@Ag₂S for the four cationic and anionic dyes and, at the same time, the PDA-free photocatalyst was used as a reference. As shown in Figure 10b,c, it was observed that the Ag/PDA@Ag₂S s could successfully degrade cationic dyes with remarkably higher efficiency than anionic dyes. Specifically, methylene blue (MB) was degraded by 80% after 10 min, while MO was degraded by about 4.0%, which was proven visually by the color change as well. In addition to improving charge transport and separation, the ultra-thin layer shell introduced by PDA increases the photostability of Ag₂S/Ag photocatalysts. The PDA has the unique capability to switch on and off Ag/Ag₂S hybrid photocatalysts depending on the pH or ionic strength of the reaction medium, resulting in selective photodegradation of organic dyes. Recently, Daniel et al. explored the influence of PDA coating on the electronic effects of Au particles and tested the photocatalytic performance with Rhodamine 6G (Rh6G) [133]. As shown in Figure 10d, the Au/PDA hybrid nanoplatform was synthesized by an AuNR core followed by the further self-polymerization of PDA. The thickness of the PDA was influenced by the concentration of dopamine hydrochloride. Figure 10e illustrated the RhG6 degradation efficiency under different photocatalysts, indicating the higher photocatalytic efficiency of Au/PDA composites than that of Au and PDA particles alone and increased with the with the PDA thickness, where the degradation of AuNRs/PDA3 could reach up to 50% in 60 min. It was analyzed that the Au/PDA nanoparticles could generate reactive oxygen species (ROS) by the absorbed heat both in the visible and NIR regime (Figure 10f). When combined with the Au, the heat pump into the PDA, which could strengthen and drive the production of ROS of •OH and •O₂⁻. In addition, due to the photothermal effect and the release of ROS, the PDA modified plasmonic metal photocatalyst has excellent antibacterial properties in waste water treatment.

Figure 10. (a) The Ag/PDA@Ag2S smart heterojunction photocatalyst for dyes photocatalytic degradation. (b) Photocatalytic degradation efficiency of MB and MO at pH = 10. (c) Plots of photodegradation of MB and MO mixture against Ag/PDA@Ag2S smart photocatalyst at pH = 2 [132]. (d) PDA shell thickness (with SD) versus log2[DA]. (e) Percentage of Rh6G concentration depleted versus time of different photocatalysis. (f) Band model of the general production of ROS during VIS irradiation [133].

5.1.2. Heavy Metal Ion Reduction (Cr(VI)) PDA has a large number of hydroxyl and amino functional groups, which can chelate with metal ions, effectively adsorbing and removing heavy metal ions from water, thereby improving water treatment efficiency. The reduction of Cr(VI) ions typically involves converting to Cr(III), which is less toxic and more stable in the environment [134]. For example, Lu et al. synthesized PDA/AgNPs through the self-polymerization of PDA and the in-situ reduction of Ag⁺ to AgNPs via PDA [135]. The reaction process of Cr(VI) removal was shown in Figure 11a, the large amino groups of PDA adsorbed Cr(VI) into the surface of photocatalyst, then improving the degradation efficiency under illumination. As demonstrated in Figure 11b, the Cr(VI) removal under dark conditions and Cr(VI) concentration after illumination indicated that PDA absorption was also beneficial to Cr(VI) removal. For the existence of plentiful amino and catechol groups, the homogeneous composites combined the PDA and semiconductors through coordination and DA self-polymerization. Additionally, PDA can absorb a broad range of wavelengths, increasing light capture capabilities, thereby enhancing the activity of photocatalysts under visible light. Li and colleagues reported a part of the TiO₂-like structure Ti-PDA NPs composite for Cr(VI) reduction (Figure 11c) [69]. As shown in Figure 11d, the photoreduction rate of Cr(VI) by Ti-PDA 3 was near 99% in 30 min, which was contributed to the sensitization effect of PDA, which kept at 95.02% after three cycles. Li and co-workers reported a 0D/1D heterostructure of PDA modified hexagonal carbon nitride tubes (HCN) for the photocatalytic Cr(VI) reduction [77]. 0D PDA nanoparticles were widely distributed inside and outside the open channels of 1D HCN to promote the sufficiently photoreduction of Cr(VI) on dual surfaces, and can achieve nearly 100% reduction of Cr(VI) after 75 min. This highly efficient 0D/1D heterostructure may serve as a guide for photocatalysts of other conjugated polymers, thereby enhancing their potential for environmental remediation and energy applications.

Figure 11. (a) The photocatalytic process of Cr(VI) through PDA/AgNPs composites. (b) Removal effiency of Cr(VI) under illumination or dark [135]. (c) Synthesis of Ti-PDA NPs and the process of Cr(VI) absorption and photoreduction. (d) The photoreduction curves of Cr(VI) by different photocatalyst [69]. (e) The possible mechanistic for tetracycline (TC) degradation and antibacterial by ZnO@PDA/Cu-POMs [136].

5.1.3. Sterilization As be mentioned before, PDA exhibited high antibacterial efficiency under near infrared irradiation owing to the photothermal effect and photodynamic-induced ROS [42,137]. Recently, the excellent physiochemical properties and effectiveness against pathogenic microorganisms of metallic oxides have enabled them to be used for preparing photocatalyst composites for environmental remediation [138,139,140]. Cui and co-workers synthesized a ZnO@PDA/Cu-POMs, with high-efficiency photocatalytic degradation of TC and extraordinary antibacterial activities [136]. As shown in Figure 11e, the ZnO@PDA/Cu-POMs heterostructure conformed to the Z-type charge transfer mechanism. Then under the attack of the reactive species, 90.75% of TC was effectively degraded. Additionally, the ZnO@PDA/Cu-POMs nanocomposite exhibited excellent antibacterial activity against 4 kinds of Gram-positive/negative bacteria, and the antibacterial rates were higher than 99% for all bacterial strains. The possible mechanistic of antibacterial action was that the ROS of ZnO@PDA/Cu-POMs could cause damage to the membrane and DNA of bacteria, then collectively induced bacterial cell death. PDA is rich in functional groups such as hydroxyl, amine, and catechol groups, which can interact with various photocatalysts like TiO₂, ZnO, and g-C₃N₄ to form composite materials. In water purification, PDA synergistically works with photocatalysts to significantly enhance the degradation efficiency of organic pollutants such as MO and MB. The presence of PDA facilitates the generation of ROS (e.g., •OH, •O₂⁻), which rapidly oxidize and decompose contaminants in water. PDA not only degrades organic pollutants but also chelate with metal ions, effectively adsorbing and removing heavy metal ions from water, thereby im-proving water treatment efficiency. Furthermore, PDA exhibits outstanding antibacterial properties, effectively inhibiting the growth of pathogenic microorganisms in water, thus achieving multifunctional water purification. In short, under synergy with the semiconductor photocatalyst (such as metal sulfide or metal oxide), PDA shows excellent photocatalytic effects in the fields of water purification, including organic pollutants degradation, Cr(VI) reduction and sterilization. 5.2. Water Splitting Hydrogen, as an efficient and pollution-free energy source, can be widely used in industry, transportation, and energy storage. By producing hydrogen through photocatalytic water splitting, we can harness the inexhaustible resource of solar energy, reduce dependence on fossil fuels, decrease greenhouse gas emissions, and help address global climate change and energy crises [141,142,143]. As one of the most widely used water splitting materials, g-C₃N₄ has attracted considerable attention for its adjustable bandgaps and excellent physiochemical properties. In addition, the hollow g-C₃N₄ nanospheres (HCNS) stand out in the research on various novel structures of g-C₃N₄, owing to their unique hollow structure and abundant active sites. Wang et al. reported a strategy comprising the g-C₃N₄ and PDA to a hollow flower-like nanosphere (g-C₃N₄@PDA), where monodisperse polystyrene microsphere using as template [144]. The photocatalyst construction process was shown in Figure 12a. the PDA@PS nanospheres was formed via DA self-polymerizes. Specifically, the g-C₃N₄ nanosheets were then introduced onto the outer surface of the PDA@PS nanospheres via interfacial self-assembly, then removing the PS template by in CHCl₃. A series of photocatalysts were used to evaluate the H₂ production rates (HER) as shown in Figure 12b. A significant improvement in the HER was observed after exposure to visible light irradiated with g-C₃N₄@PDA nanospheres, which was contributed by the conjugated system and the hollow nanostructure. In addition, the HER matched well with its light absorption curve (Figure 12c). When the incident light wavelength was extended to 700 nm, the g-C₃N₄@PDA still showed good photocatalytic activity (HER = 5 µmol h⁻¹). The mechanism of optical activity was illustrated in the Figure 12d,e, based on the synergistic effect, PDA continuously supplied electrons to the CB of g-C₃N₄ while accepting holes from g-C₃N₄, thus reducing electron-hole recombination. Additionally, the hollow nanospheres with multifaceted open surface areas can facilitate surface-dependent reactions by effectively shuttling electrons and chemicals. These factors collaboratively enhance the photocatalytic performance of the nanomaterials. Similar, Yang et al. fabricated a PDA/defective ultrathin mesoporous g-C₃N₄ (PDA/DCN) Z-scheme organic for the H₂ evolution and organic degradation (Figure 12f) [145]. It was demonstrated that N-vacancy and π-π interactions provide strong connection and Z-scheme fast charge transfer.

Figure 12. (a) The preparation process of g-C3N4@PDA nanosphere. (b) The H2 evolution efficiency of different samples in light irradiation. (c) Wavelength dependence of H2 evolution rate on g-C3N4@PDA photocatalyst. (d) Band positions of PDA and g-C3N4 and (e) electron transfer in the g-C3N4@PDA system (∗ means that the electron transuition to excited state) [144]. (f) Scheme diagram of PDA/DCN photocatalytic H2 production and organic contaminant degradation [145]. (g) The photocatalytic H2 production effiency under different PDA thickness. (h) Schematic representation of the ZnS/PDA heterojunction [94].

In addition to g-C₃N₄, transition metal sulfides, such as such as ZnS, are widely applied as an efficient photocatalyst in photocatalytic H₂ evolution, due to their high reduction potential, as well as the rapid generation of photocarriers abilities. However, the application of ZnS is hindered by photocorrosion and instability due to oxidation of sulfide ions by photogenerated holes. PDA has good adhesion and interface modification capabilities, allowing it to form uniform coatings on various material surfaces, thereby improving the surface activity and stability of the photocatalyst. Recently, nanometer-controlled PDA coatings have been shown to enhance photocatalytic activation wavelength, rather than hinder them. Kim and co-workers reported on a strongly coupled heterojunction photocatalyst comprising by a ZnS nanorod and ultrathin PDA (ZnS/PDA). Figure 12g,h shown that the highest H₂ production rate was 2162.5 μmol h⁻¹ g⁻¹ with PDA layer of 1.2 nm, and remained 78.7% of initial photoactivity after three cycling. In addition, since the effective carrier transmission length was limited to less than 2 nm, the excited carriers cannot be transmitted to the surface if the PDA layer was thicker than the transmission length. This research results may be applicable to other sulfonium catalyst and sensitive nanomaterials and light catalyst, such as selenide, boride, telluride, etc. PDA has shown significant potential in the application of photocatalytic water splitting, primarily due to its unique physicochemical properties and its synergistic effects with photocatalysts. The reductive properties and excellent conductivity of PDA facilitate the transfer of photogenerated electrons and reduce the recombination of electron-hole pairs, thereby increasing the rate of hydrogen production during water splitting. Moreover, the PDA coating can be used as a protective layer to prevent photocorrosion or chemical corrosion of the catalyst, thereby improving the photocatalytic stability. 5.3. CO₂ Reduction The burning of fossil fuels and industrial activities have increased carbon dioxide (CO₂) concentration in the atmosphere, which in turn has caused climate change and a series of environmental problems. Photocatalytic CO₂ reduction is considered a promising method for reducing carbon dioxide emissions and producing renewable fuels. It utilizes light energy to convert CO₂ into valuable chemical products, such as methanol, methane or carbon monoxide [2,146]. This process typically involves the use of a semiconductor photocatalyst, which absorbs light and generates electron-hole pairs. These carriers then participate in the reduction of CO₂ and the oxidation of a sacrificial agent or water. It is essential to use photocatalysts that can separate charge efficiently and adsorb CO₂ [147,148]. Research has shown that the amine-functionalized metal oxides can effectively capture CO₂ by forming strong chemical adsorptive species, which not only promoted the activation of CO₂ but also enhanced the CO₂ reduction in photocatalytic. A great deal of amino groups in PDA, can easily be anchored to most surfaces, and enhance the CO₂ adsorption and activation capacity. For example, it was demonstrated that the C₃N₄ nanospheres modified with PDA had significantly enhanced CO₂ adsorption capacity and high photocatalytic efficiency [149]. It has been reported that dopamine modification on TiO₂ nanoparticles can not only improve the charge separation efficiency, but also increase the reduction power of electrons by 100 mV [150]. In addition, compared with TiO₂ bulks, hierarchical structure of TiO₂ has a shorter distance charge transfer, thereby increasing the charge transfer, the duty carrier recombination. Meng et al. reported a series of PDA-modified TiO₂ hollow spheres (TiO₂@PDA) for photocatalytic CO₂ reduction (Figure 13a) [151]. As shown in Figure 13b, when photocatalytic CO₂ reduction reactions were performed using photocatalysts, CH₄ and CH₃OH were detected as products. For pristine TiO₂, the generating rates of CH₄ and CH₃OH were 0.30 and 0.63 μmol h⁻¹ g⁻¹. With the deposition of 0.05% PDA, the CH₄ yield increased to 1.14 μmol h⁻¹ g⁻¹, which was nearly four times as many as the generation rate of sample T. The higher yields and outstanding methane selectivity were contributed to the enhanced CO₂ adsorption capacity as well as efficient separation and transfer of charge carriers induced between TiO₂ and PDA (Figure 13c).

Figure 13. (a) Schematic illustration of TiO2@PDA composites. (b) Photocatalytic yields of pure TiO2 and TiO2@PDA photocatalysts. (c) The possible mechanism of S-scheme charge transfer of TiO2@PDA [151]. (d) The synthesis of PDA/ZnO/Co3O4 photocatalysts. (e,f) The HRTEM images of ZnO/Co3O4and PDA15/ZnO/Co3O4. (g) The CO and H2 evolution rate of different photocatalysts under UV-vis illumination (the arrow points to the CO selectivity). (h) The recycling performance of on PDA15/ZnO/Co3O4. (i) The mechanism diagram for photocatalytic CO2 reduction mechanism diagram via PDA15/ZnO/Co3O4 [152].

Band position and intrinsic properties make the metal oxide can combine well matching the heterojunction formation. Additionally, the MOF-derived metal oxide also inherits the porous structure and large specific surface area, making it a promising candidate material for efficient photocatalytic CO₂ reduction [66]. For instance, a hollow ZnO/Co₃O₄ photocatalyst was constructed through pyrolyzing bimetallic ZnCo-ZIFs and modifying them with PDA (Figure 13d–f) [152]. Figure 13g shown shows the production of CO on PDA_m/ZnO/Co₃O₄, with the amount of PDA increasing from 3 mg to 120 mg. Among them, the PDA₁₅/ZnO/Co₃O₄ (the content_PDA = 15 mg) showed the highest CO production in the photocatalytic CO₂ reduction, with a rate of 537.5 μmol/g/h and a selectivity of 97.7%. The enhanced CO₂ adsorption/activation capability was attributed to the PDA modification. Further increasing PDA to 30 mg and 120 mg resulted in a decrease in CO production, which might be attributed to the reduced light absorption by excess covering of semiconductor ZnO/Co₃O₄ surface. As a result of recycling experiments, the high photocatalytic activity and CO selectivity were maintained after five cycles (Figure 13h). PDA shows great potential in photocatalytic CO₂ reduction through its synergistic effects with photocatalysts, particularly in enhancing photocatalytic efficiency, improving stability, and controlling product selectivity. For example, PDA combined with the amine-functionalized metal oxides can effectively capture CO₂. By optimizing the thickness and surface chemistry of PDA, it is possible to improve the selectivity of specific products. 5.4. Photocatalytic H₂O₂ Production As an environmentally friendly oxidant and ideal carrier of energy, hydrogen peroxide (H₂O₂) has been widely used chemistries, such as chemical production, healthcare, and environmental treatment for many years [153]. As the industrial production of one of the primary methods of hydrogen peroxide, anthraquinone method is through the cycle of anthraquinone compounds REDOX reaction, high concentrations of hydrogen peroxide production. Nevertheless, the process of anthraquinone depends on precious metals and organic solvents, and consumes a lot of energy [154]. Photocatalytic H₂O₂ strategy is based on the most abundant water (H₂O) and oxygen (O₂) in nature, which uses a photocatalyst to break down water or other compounds to form hydrogen peroxide under light, is considered an emerging research direction [155,156]. Photocatalytic H₂O₂ production in the main pathways including direct photocatalytic decomposition of water oxidation reaction (WOR) and oxygen reduction reaction (ORR). Among them, ORR is considered light catalytic production of H₂O₂ main ways, where oxygen is acting as an electron acceptor. Therefore, improving the photocatalyst’s ability to effectively adsorb and desorb oxygen can prevent electron-hole recombination and enhance the efficiency of photocatalytic H₂O₂ generation. As reported that the reversible conversion between catechol and o-ben zoquinone in PDA during the photocatalytic H₂O₂ production, which was similarly to the transformation of anthraquinone. Wei et al. designed the CdS-PDA hybrid catalyst via coating PDA on CdS for H₂O₂ production (Figure 14a) [157]. The adsorption energy of O₂ on CdS (002) and CdS (002)-PDA models was estimated by performing density functional theory (DFT) calculations (Figure 14b), which was consistent with the O₂TPD result in that O₂ was more strongly adsorbed on CdS (002)-PDA (E(O₂)_ads = −2.41 eV) than that on CdS (002) (E(O₂)_ads = −1.25 eV). The H₂O₂ selectivity of CdS -PDA was significantly enhanced from 30% to 82%, achieving a 13.7-fold catalytic yield over the CdS (Figure 14c). In addition, the K_f and K_d values (corresponding to the formation and decomposition rate constant of H₂O₂) in Figure 14d indicated that the PDA coating could not only improve the ability of O₂ adsorption, but also inhibit the decomposition of H₂O₂, and enhance the stability of CdS light corrosion. Similarly, Huang and co-workers constructed a PDA-loaded ZnIn₂S₄ (ZIS/PDA) for H₂O₂ production [91]. The S-scheme photocatalyst ZIS/PDA exhibited outstanding H₂O₂ production performance under visible light in pure water, where the highest H₂O₂ yield (1747 μmol·g^–1·h^–1), which was 39 times the yield of pure ZnIn₂S₄ (Figure 14e). The transition intermediates and the corresponding free energies were shown in Figure 14f, the ΔG of ^∗OOH generated by ZIS/PDA_0.1 was much lower than that of ZIS, suggesting that the ZIS/PDA_0.1 could reduce the energy barrier and improve the overall reaction rate. The possible mechanism of photocatalytic H₂O₂ by the ZIS/PDA_0.1 was illustrated in Figure 14g. Under the influence of the internal electric field (IEF), the constructed ZIS/PDA_0.1 S scheme heterojunction rapidly separated and transfered photogenerated electrons and holes. The photogenerated electrons, which accumulated on the CB of PDA, are then consumed by oxygen and protons. The PDA coating enhanced the selectivity of H₂O₂, increased the overall reaction rate, and significantly inhibited the decomposition of H₂O₂, thereby promoting the accumulation of H₂O₂.

Figure 14. (a) The synthesis process of CdS–PDA. (b) Computational models for O2 adsorption on photocatalysts. (c) Long-term photocatalytic H2O2 production and (d) Kf and Kd values of photocatalysts [157]. (e) H2O2 production rate of ZnIn2S4 and ZIS/PDAx. (f) Free energy of the intermediate state in the H2O2 production of ZIS and ZIS/PDA0.1. (g) Mechanism of photocatalytic H2O2 by the sustainable transformation of ZIS/PDA0.1 [91]. (h) The possible reaction mechanism of H2O2 production under CN-PDA [158].

In photocatalytic H₂O₂ production process, Superoxide free radical (•O₂⁻) is generated photocatalytic oxygen reduction of important intermediates, has the strong oxidizing properties, could undermine the stability of the catalyst. Therefore, avoiding the accumulation of •O₂⁻ resulting in its rapid conversion to H₂O₂ is crucial to improve catalyst stability and H₂O₂ yield. Deng et al. developed a heterojunction of PDA and g-C₃N₄ (CN-PDA) for H₂O₂ synthesis [158]. The possible mechanism of CN-PDA photocatalytic synthesis of H₂O₂ was shown in Figure 14h, which included two synthesis pathways. In WOR part, two adjacent C-NH groups on DHI absorb two water molecules. Then, 2e⁻ WOR generated H₂O₂, leaving behind two protonated amine groups (NH₂⁺). On the other hand, NH₂⁺ on PDA acted as a key position for the adsorption and reduction of •O₂⁻, which can rapidly convert •O₂⁻ into H₂O₂. This prevented the accumulation of •O₂⁻ that could lead to the destruction of the catalyst structure and promotes the regeneration of the C-NH group sites, thereby achieving efficient continuous production of H₂O₂. During the photocatalytic production of hydrogen peroxide, the electrons in the conduction band react with oxygen in the solution, reducing it to O₂⁻. The superoxide anion further reacts with H⁺ to eventually form H₂O₂. PDA has strong adsorption capabilities, effectively capturing dissolved oxygen and inhibiting H₂O₂ decomposition. In addition, the catechol in PDA reacted with O₂ to produce H₂O₂ and obenzoquinone. Studies on the photocatalytic synthesis of hydrogen peroxide by PDA have the potential to develop green and sustainable hydrogen peroxide synthesis methods. This method can be used to produce hydrogen peroxide under environmentally friendly conditions using renewable energy sources such as sunlight and has broad application prospects. 5.5. Applications in Other Photocatalytic Reactions 5.5.1. N₂ Fixation As an essential energy source, ammonia (NH₃) has a significant impact in industrial and agricultural production. In recent years, the photochemical synthesis of NH₃ from atmospheric nitrogen (N₂) has become a hot research frontier [159]. However, photocatalytic nitrogen fixation materials face limitations such as the difficulty of N₂ adsorption and activation, and the challenge of preventing the recombination of photogenerated carriers [160]. PDA has shown promising application in photocatalytic nitrogen fixation due to its unique properties. Wang et al. constructed a ternary SiW₉Co₃/PDA/BWO photocatalyst, where PDA acting as an adhesive bridge between SiW₉Co₃ with BWO [161]. In this work, PDA had a broad light absorption range and acted as an electron transport chain to facilitate electron–hole migration through π–π^∗ electronic delocalization (Figure 15a). In addition, the SiW₉Co₃/PDA/BWO provided suitable binding sites for molecular nitrogen activation due to the synergy between three photocatalysts. The catalytic efficiency of Z-scheme ternary SiW₉Co₃/PDA/BWO was 6.0 times higher than that of BWO.

Figure 15. (a) Schematic illustration of separation and migration of photo-generated electron-hole pairs over the Z-scheme SiW9Co3/PDA/BWO heterojunction [161]. (b) The plausible mechanism for selective aerobic oxidation of thioanisole under visible light by combining TiO2@PDA photocatalysis and TEMPO catalysis [162].

5.5.2. Organic Synthesis PDA is extensively utilized in organic synthesis owing to its chemical properties and functionality. It exhibits catalytic activity and can generate active free radicals to facilitate oxidation, reduction, and coupling reactions. PDA possesses abundant surface functional groups, which allow for easy functionalization, making it an excellent substrate for developing catalysts and adsorbents. Additionally, PDA can serve as a support to immobilize metal or organic catalysts. Its functional groups can engage in reactions or alter substrate adsorption, thereby influencing reactivity. The biocompatibility of PDA also renders it particularly valuable in bioorganic synthesis, making it suitable for the synthesis of bioactive molecules or drug precursors [163,164,165]. Many studies have shown that PDA, as a multifunctional coating on the surface of metal oxide semiconductors, can significantly improve the photoelectric performance of these materials due to the presence of π-π^∗ electron transitions. Shi et al. constructed the TiO₂@PDA with a large π-conjugated PDA system and used for aerobic oxidation reaction of thioanisole [162]. The possible mechanism was shown in Figure 15b, it was worth noting that PDA was activated to an unstable intermediate PDA radical^•+, which can regenerate into PDA by transforming TEMPO into TEMPO⁺. Thioanisole, under the action of TEMPO⁺, was converted into S-centered radical cation, which prefer to react with O₂^•− to form persulfoxide. Then the persulfoxide combined with CH₃OH to yield the sulfoxide product. Hence, the strategy of surface modification of semiconductor by PDA had a great potential for photocatalytic organic transformations selectively. In addition, the synthesis of amide compounds holds great significant importance in organic synthesis. Xu and co-workers reported a magnetic Fe₃O₄/PDA/CdS active photocatalyst for the amidation of aromatic aldehydes under air, where PDA improved the photocatalytic activity by its light energy conversion [166]. In a word, PDA photocatalytic nitrogen fixation is an advanced research topic focusing on using polydopamine as a photocatalyst to convert N₂ into NH₃ under light irradiation. PDA can be used as a conductive adhesive to bridge to synthesize ternary heterojunctions with other catalysts. The PDA facilitates the separation of photogenerated electron-hole pairs from the other two semiconductors, accelerates electron transfer, and reduces recombination, which is essential for improving photocatalytic reaction efficiency. Alternatively, PDA photocatalysis system can be used to synthesize various organic compounds, possibly providing new routes to synthesize complex molecules.

6. Conclusions and Prospects

This review provides an in-depth discussion on the structural characteristics and synthesis methods of polydopamine (PDA). It highlights PDA’s advantages in photocatalytic applications, such as its templating agent effect, light absorption performance, film-forming ability, hydrophilicity, and conductivity. The article also addresses strategies for enhancing PDA’s photocatalytic performance and explores the application of PDA-based materials in various fields, including solar water purification (targeting heavy metal reduction, organic pollutant degradation, and antibacterial activity), H₂ production, H₂O₂ production, CO₂ reduction, and organic transformation. The ultimate goal of the review is to offer valuable insights for the design and development of PDA-based photocatalytic materials. While PDA holds significant promise in the field of photocatalysis, it also faces certain challenges that need to be addressed. (1) To address the complexity of the PDA synthesis process and the difficulty of functionalization, the following strategies can be used to improve the process: simplified synthesis methods, such as one-step synthesis and direct oxidation, as well as the use of microwave-assisted synthesis to accelerate the reaction; simplified methods of functionalization, including the use of pre-functionalized dopamine precursors and in-situ functionalization techniques; highly efficient methods of functionalization, such as surface self-assembly and cross-linking functionalization; green synthesis methods, such as bio templating and enzymatic polymerization; and template-assisted synthesis, including hard and soft template methods, to control the morphology and structure of PDA. These strategies help to improve the preparation efficiency and performance of PDA and enhance their maneuverability and economy in practical applications. (2) To improve the limited light absorption range and weak absorption capacity of PDA, it can be improved by various strategies to enhance its potential for photocatalytic applications. These strategies include: doping of metallic or nonmetallic elements to modulate the energy band structure and electronic structure; dye sensitization (e.g., phthalocyanine and porphyrin dyes) to broaden the light absorption range; construction of composites of PDA with other inorganic semiconductors to fabricate heterojunctions to improve light absorption capacity; surface modification and functionalization to introduce conjugated groups (e.g., benzene rings, alkynyl groups, etc.) or grafting of photosensitizing compounds to change the light absorption properties; design of different nanostructures such as nanoparticles and porous structures to improve light absorption efficiency; and other approaches such as composite with surface plasmon resonance effect materials and copolymer design. Together, these approaches can help to enhance the effectiveness of PDA in applications, such as environmental remediation and energy conversion. (3) To solve the problem of high PDA photogenerated carrier complexation rate, various strategies can be adopted, including metallic or nonmetallic doping to modulate the energy band structure and promote carrier separation, dye sensitization to enhance light absorption and assist in carrier separation, construction of heterojunctions to promote the transfer of electrons and holes, surface modification and functionalization to change the electronic structure, design of novel nanostructures to increase the carrier transport path, composite with surface plasmon resonance effect materials to enhance the electromagnetic field and promote carrier separation, optimization of synthesis methods to improve the electronic structure of the materials, application of external electric or magnetic fields to guide the directional movement of carriers, and optimization of photocatalytic reaction conditions to promote carrier separation. The combined application of these methods can significantly reduce the photogenerated carrier complexation rate in PDAs and enhance the photocatalytic efficiency. (4) To enhance the stability of PDA, the reaction conditions can be adjusted to optimize the pH to a weakly basic range and synthesize at low temperature to reduce the occurrence of side reactions; doping with metallic or non-metallic elements to enhance the stability; surface modification to reduce hydrolysis and degradation through the introduction of hydrophobic groups; the use of chemical cross-linking agents to form a stable three-dimensional network structure; designing the nanostructures of PDA, such as nanoparticles and porous structures, to enhance stability; optimization of the synthesis process such as shortening the reaction time and adjusting the stirring speed to reduce side reactions and improve the homogeneity of the reaction. The implementation of these strategies helps to improve the stability of PDA, which in turn provides better performance and wider application prospects in areas such as photocatalysis and environmental remediation. (5) PDA holds significant potential in the field of environmental remediation and energy conversion due to its unique chemical properties and biocompatibility. For instance, (a): PDA combined with sporous materials can serve as support materials for photocatalysts, enhancing their stability and recyclability. Its excellent adsorption properties allow it to effectively capture pollutants in water, such as organic dyes and heavy metal ions, facilitating their degradation under light irradiation; (b): In the process of water splitting for hydrogen production, PDA can act as a co-catalyst, promoting the separation of photogenerated electrons and holes, thereby enhancing hydrogen production efficiency. (c): PDA also has potential applications in the photocatalytic reduction of CO₂. It can interact with CO₂ molecules, increasing their adsorption on the photocatalyst surface, which facilitates the reduction reaction and improves the selectivity and efficiency of the process; (d): Leveraging PDA’s photocatalytic properties, self-cleaning and antimicrobial coatings can be developed. When exposed to light, PDA coatings can catalyze the degradation of organic pollutants and bacteria, maintaining surface cleanliness. This application is particularly promising in building materials, medical devices, and textiles; (e) The excellent biocompatibility, tunable photothermal conversion performance, and extensive surface modification ability make PDA become a potential material in the fields of biomedicine (photothermal therapy, antibacterial applications), energy (energy storage), and environmental science (solar-driven evaporation and desalination). In summary, improving the comprehensive performance of PDA includes the use of simplified synthesis, functionalization strategies, doping, sensitization, surface and nanostructure design to enhance their preparation efficiency, light absorption ability and photocatalytic potential. Meanwhile, the stability of PDA can be effectively enhanced by adjusting the reaction conditions, doping, surface modification, cross-linking, nano structuring, and synthesis process, thus expanding its application in photocatalysis.

Acknowledgments

The authors would like to thank Canada First Research Excellence Fund (CFREF) and Natural Sciences and Engineering Research Council of Canada-Discovery Grant for their funding. H.W. acknowledges the financial support from the China Scholarship Council.

Author Contributions

H.W.: Writing–original draft, Supervision, Formal analysis. L.J.: Writing–review & editing, Supervision. A.V.: Supervision. Y.Y.: Supervision. H.Z.: Supervision. Q.G.: Supervision. J.H.: Writing–review& editing, Supervision, Funding acquisition.

Ethics Statement

Not applicable.

Informed Consent Statement

Funding

This research was funded by Natural Sciences and Engineering Research Council of Canada, Discovery Grant [grant number: 10040079].

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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