Tunable poly(aryleneethynylene) networks prepared by emulsion templating for visible-light-driven photocatalysis
Graphical abstract
Introduction
During the past few decades, various strategies have been developed for viable water and waste-water treatment technologies. Heterogeneous photocatalysis is considered as an emerging branch of Advanced Oxidation Processes (AOPs) due to its ability of non-selective degradation of almost all water pollutants with the aid of a solid-photocatalyst [1,2]. Powdered semiconducting oxides (e.g. TiO2 or ZnO) are certainly the most widespread solid photocatalysts in heterogeneous photocatalysis, since these materials are cheap and efficient in transformation of contaminants into H2O and CO2 [3,4]. Basic principle of action relies on the formation of highly reactive charge carriers (i.e. electron/hole (e−/h+) pairs) upon the absorption of a photon with energy equal to or greater than the semiconductors’ band-gap energy, which are then immediately involved in the reductive or oxidative reactions at the semiconductors’ surface forming the Reactive Oxygen Species (ROS) [5]. Therefore, for an efficient photocatalysis the charge-carriers should be shuttled towards the semiconductors’ surface, which has to effectively compete with a deactivation route, i.e. e−/h+ recombination [6,7]. The trade-off among inherently fast recombination of charged carriers, non-absorbing ability in the visible-light region and possible leaching of the heavy metals into the water, remain a challenging hurdle in the practical application of semiconducting oxides for the wastewater treatment.
Recently, metal-free and heterogeneous organic (polymeric) photocatalysts based on spatially extended π-π and π-n-π bonding systems emerged as a new kind of photocatalysts, where the presence of π-electrons allow formation of different types of charge carriers without cleavage of the macromolecular backbone [8,9]. The main advantages of semiconducting π-conjugated polymeric systems are their optical and electronic properties that can be controlled via structural design on a molecular level [10]. Such fully sp2-carbonlinked π-conjugated polymers in particular, exhibit excellent stability against strong bases, acids and photodegradation [11]. Thus semiconducting π-conjugated polymeric photocatalysts have drawn great attention in recent years as promising alternative to the traditional inorganic semiconductors in photocatalysis [12]. Graphitic carbon nitrides (g-C3N4), a state-of-the art non-metal-based visible-light active polymeric photocatalyst, have been intensively studied for visible light-driven reactions such as water splitting [13,14], CO2 photoreduction [15,16], or photodisinfection [17,18]. Moreover, since new properties of g-C3N4 being continuously discovered, applications have been extended from photocatalysis to bioimaging, solar cells, nanoarchitecture processes, and non-volatile memory devices [19,20]. However, especially intriguing among semiconducting polymeric photocatalysts are conjugated microporous polymers (CMPs) due to a number of exciting potential applications, stemming from the unusual combination of π-conjugation, large physical surface areas and good chemical and thermal stability [21]. Among CMPs, poly(aryleneethynylene) (PAE) networks with high specific surface area [[22], [23], [24]] have been attracted intense interest in many advanced applications like hydrogen evolution, light harvesting and energy conversion and storage [[25], [26], [27], [28], [29], [30]]. However, literature seldom reports on applying such π-conjugated-based porous polymeric semiconductors as the photocatalysts for the wastewater treatments [[31], [32], [33]]. Despite significant progress, the CMPs suffer from a low permeability and adverse loss of permeation flux over a prolonged usage due to the fouling of microporous morphology. The 3D-interconnected macroporous morphology of the polymerized HIPEs (high internal phase emulsion), the so-called polyHIPEs, is expected to enhance the permeability, since the macroporous morphology of polyHIPEs is known for a convective mass transfer, allowing an efficient kinetics. Therefore, combining a 3D-interconnected macroporous morphology of polyHIPEs and the photoactive π-electron backbone hold great promise for the wastewater treatment via heterogeneous photocatalysis under visible-light illumination. Recently, the synthesis of fully π-conjugated polyHIPEs has been reported [34] and they have been employed in applications such as singlet oxygen generation [35], selective oxidation of organic sulfides [36] or as photoinitiator for free radical polymerization [37]. However, there is still a need to maximize the performance of π-conjugated polyHIPEs towards the wastewater treatment under visible-light illumination by tailoring both the porous architecture and photocatalytic activity, which remains a great challenge.
It was previously shown that the conjugated macromolecular systems could mediate electron transfer under the visible-light irradiation and thus act as the photosensitizers. Thus, the Sonogashira cross-coupling polycondensation of the continuous phase of HIPE as reported herein, delivers the hierarchically porous functional poly(aryleneethynylene) (PAE)-based polyHIPEs with high synthetic control over the optical gaps. The ability of derived materials for visible-light harvesting was examined in the process of photocatalytic oxidation of water-dissolved bisphenol A.
Section snippets
Materials
1,3,5-triethynylbenzene was obtain from TCI, 2,6-diiodo-4-nitrophenol was purchased from ABCR and 1,4-diiodo-2,5-dimetoxybenzene from Fluorochem. Potassium carbonate, 1,4-diiodobenzene, Span®80 (Sorbitan monooleate), copper (I) iodide, and tetrakis(triphenylphosphine) palladium, were provided from Sigma-Aldrich and used as received. Toluene was purchased from Merck.
Synthesis of poly(aryleneethynylene)-based polyHIPE networks
HIPEs were synthesized at a fixed molar concentration of the monomer of 0.133 mol/L and a constant molar ratio of ethynyl to
Chemical, morphological, and photo-physical properties of PAE-based polyHIPEs
Each poly(aryleneethynylene) (PAE)-based polyHIPE network is based on the 1,3,5-triethynylbenzen nodes and substituted diiodo-monomers synthesized through the Pd-catalyzed and Cu-cocatalyzed Sonogashira cross-coupling polycondensation within a continuous phase of the high internal phase emulsion (HIPE). The resulting cross-linked polyHIPEs are insoluble dark yellow monoliths, obtained in a ∼70 % yield. The polymers were characterized on a molecular level by a solid-state 13C CP/MAS NMR
Conclusions
In summary, we present a simple structural design principle of highly porous 3D-interconnected poly(aryleneethynylene) (PAE)-based polyHIPEs as pure organic, heterogeneous photocatalytic system, allowing the fine tuning of the photoredox potential by using different disubstituted diiodobenzene monomers. Via altering the substitution pattern on the diiodobenzene units, the resulted valence- and conduction-band positions of the polyHIPEs can be optimally aligned to adjust the required reductive
CRediT authorship contribution statement
Sarah Jurjevec: Investigation, Writing - original draft. Gregor Žerjav: Investigation, Writing - original draft. Albin Pintar: Writing - review & editing, Conceptualization. Ema Žagar: Writing - review & editing, Conceptualization, Methodology, Funding acquisition. Sebastijan Kovačič: Conceptualization, Methodology, Writing - original draft, Project administration, Supervision.
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.
Acknowledgements
The authors gratefully acknowledge the financial support of the Ministry of Education, Science and Sport of the Republic of Slovenia, and the Slovenian Research Agency (Programs P2-0145 and P2-0150).
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