Application of Polydopamine-Based Photocatalysts in Energy and Environmental Systems

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. 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. 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. 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). 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. 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. 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.