Light-Driven Integration of Graphitic Carbon Nitride into Polymer Materials ^†

Abstract

As a metal-free polymeric semiconductor with an absorption in the visible range, carbon nitride has numerous advantages for photo-based applications spanning hydrogen evolution, CO₂ reduction, ion transport, organic synthesis and organic dye degradation. The combination of g-C₃N₄ and polymer networks grants mutual benefit for both platforms, as networks are upgraded with photoactivity or formed by photoinitiation, and g-C₃N₄ is integrated into novel applications. In the present contribution, some of the recently published projects regarding g-C₃N₄ and polymeric materials will be highlighted. In the first study, organodispersible g-C₃N₄ were incorporated into a highly commercialized porous resin called poly(styrene-co-divinylbenzene) through suspension photopolymerization, and performances of resulting beads were investigated as recyclable photocatalysts. In the other study, g-C₃N₄ nanosheets were embedded in porous hydrogel networks, and so-formed hydrogels with photoactivity were transformed either into a ‘hydrophobic hydrogel’ or pore-patched materials via secondary network introduction, where both processes were accomplished via visible light. Since g-C₃N₄ is an organic semiconductor exhibiting sufficient charge separation under visible light illumination, a novel method for the oxidative photopolymerization of EDOT was successfully accomplished. As a result of the absence of dissolved anions during polymerization, so-formed neutral PEDOT is a highly viscous liquid that can be processed and post-doped easily, and grants facile coating processes.

1. Introduction

In this proceeding, recently published projects based on the combination of metal-free semiconductor graphitic carbon nitride (g-C₃N₄) and polymers will be summarized. The proceeding will briefly focus on how graphitic carbon nitride is successfully integrated into different reaction environments, such as in dispersion, or as a heterogeneous photo initiator, to evolve the properties and synthesis of polymer materials.

A relatively old material, g-C₃N₄ is a polymeric semiconductor discovered in 2009. Its synthesis is based on thermal polymerization (around 550 °C) of abundant and nitrogen-rich molecules, such as urea and melamine, that result in a semiconductor with photoactivity in the visible and UV range [1]. Its thermal reaction mechanism can be considered as recondensation of decomposed gaseous molecules of precursors to form a thermodynamically stable matter at elevated temperatures. It is different than a typical solid state condensation reaction; therefore, reaction conditions such as temperature ramp and applied atmosphere play a curial role in forming the desired and reproducible g-C₃N₄. Since it is a type of polymerization, the condensation degree can be also tuned by the variability of reaction conditions (mainly temperature). Moreover, further modifications, such as edge functioning and tuning repeating motifs, are just some possibilities to form diverse g-C₃N_4′s. g-C₃N₄ exhibits high stability in acidic and basic environments, as well as thermal stability up to 620 °C.

Selected precursor and preparation methods directly affect all final properties, such as surface area, morphology, surface charge, crystallinity and optical properties. Photophysical features of the resulting g-C₃N₄ may vary depending on the performed techniques. Synthesized g-C₃N₄s in powder form demonstrates a slightly negative surface charge; however, the synthesis of g-C₃N₄ thin films and membranes are known in the literature as well [2,3]. Furthermore, as a typical semiconductor, g-C₃N₄ exhibits reductive and oxidative pathways upon photo-formed electrons and holes, which are a vital role of photoredox-based science. g-C₃N₄ has been a haven for photocatalysis, and has been integrated into many appealing applications by acting as a heterogeneous photocatalyst [4]. Furthermore, under light irradiation, it can form radical species which can be harnessed to conduct radical photopolymerization techniques. From a polymer perspective, g-C₃N₄ was employed as a heterogeneous photoinitator for free radical and controlled radical polymerization methodologies.

However, some drawbacks based on dominant π-π interactions between g-C₃N₄ sheets cause problems, such as processability and dispersion preparation, which hinder its further possibilities. Recently, the surface modification of g-C₃N₄ materials introduced dispersible g-C₃N₄ colloids which, addressed both in aqueous and organic environments, are attractive for new possibilities [5,6]. There are numerous well-established techniques that deal with the integration of g-C₃N₄ colloids in soft materials, membranes and polymer particles to form hybrid materials. Yet, there are still many materials to be explored that are based on g-C₃N₄ and polymer materials.

Herein, the following proceeding will highlight three recently published reports based on g-C₃N₄ and polymer combination. It will start with an organic dispersion of g-C₃N₄ to form crosslinked porous polymer beads named poly(styrene-co-divinylbenzene) through suspension polymerization, to sculpt photoactive bead materials. Aqueous dispersion of g-C₃N₄ will be employed for porous hydrogel synthesis, and the embedded g-C₃N₄ nanosheets will act as anchoring points for further light-induced modifications, i.e., pore substructuring and the synthesis of ‘hydrophobic hydrogel’. Finally, g-C₃N₄ will be utilized as a heterogeneous photoinitiator to polymerize 3,4-ethylenedioxythiophene (EDOT) to form a non-doped oligoEDOT material that is prone to processing prior to doping.

2. Results

2.1. Upgrading Poly(styrene-co-divinylbenzene) Beads through Suspension Photopolymerization via Carbon Nitride

Metal-free semiconductor graphitic carbon nitride (g-C₃N₄) can act as a heterogeneous photoredox polymer initiator. This study introduces g-C₃N₄ integration into the inner surface (interfaces) of porous poly(styrene-co-divinylbenzene) beads via one-pot suspension photopolymerization (Scheme 1).

Regarding suspension polymerization principles, reaction conditions such as crosslinking ratio (25, 35, 50 wt.% divinylbenzene), the presence of porogens (toluene as co-solvent, 1-octanol as dispersion agent of vTA-CMp), and mechanical agitation (slow and medium) are investigated. According to results, 25 wt.% divinylbenzene, the presence of toluene along with 1-octanol, and medium agitation speed, were elucidated as optimal conditions.

As shown in Figure 1, bead morphology was investigated via SEM, and the resulting images revealed a highly porous structure (Figure 1(a1,a2)). In addition, the EDX investigation exhibiting homogeneous nitrogen distribution indicated successful vTA-CMp corporation, since none of the reaction ingredients, except vTA-CMp, bear nitrogen (Figure 1b).

Photocatalytic properties of the selected beads were investigated by performing an aqueous Rhodamine B dye degradation experiment under visible light illumination and dark conditions (Figure 2a,b). Among the as-synthesized beads, the g-C₃N₄ photo-initiated bead (CBT5m) exhibited 89% degradation efficiency in 5 h, within the R² value of 0.971 (Figure 2c). Moreover, cyclic photocatalytic cycles ran up to 7 cycles and resulted in moderate loss of activity, 55.6% (Figure 2d). Lastly, the pH effect on photodegradation explicated the stability of the applied beads with no host degradation. In addition, dye adsorption/desorption properties were tested in both aqueous and organic dyes.

Afterwards, g-C₃N₄-incorporated beads were subjected to a photo-induced surface modification by employing vinylsulfonic acid (VSA) and 4-vinyl pyridine (VP) under visible light irradiation. According to the results, VSA photo-grafting was analyzed successfully by elemental mapping via EDX along with elemental analysis via detected sulfur content. On the other hand, VP photo-grafting was confirmed via combustive elemental analysis together with FT-IR successfully.

In other words, beads donated with photoactivity via metal-free semiconductor g-C₃N₄ integration can be exploited for dye photodegradation and as an acid-base catalyst after post-functionalization through a simple photo-induced surface modification.

2.2. Synthesis of g-C₃N₄ Integrated Hydrogels and Subsequent Post-Modifications

As a particular class of hydrophilic polymers, hydrogels possess a high water content through their crosslinked porous networks. A facile synthesis of a hydrogel can be simply described as a combination of a water-soluble monomer and a multi-functional crosslinker in the presence of an initiator (mainly radical-based systems). In this context, firstly, g-C₃N₄ embedded hydrogel synthesis, thereafter its further subsequent visible light-assisted post modifications will be described (Scheme 2).

Template g-C₃N₄-embedded hydrogel was formed by free-radical polymerization in the presence of a redox couple (ascorbic acid-hydrogen peroxide), water-soluble monomer (N,N-Dimethylacrylamide, DMA) with a crosslinker (N,N′-methylenebisacrylamide, MBA), in an aqueous dispersion of g-C₃N₄. Gelation proceeded in 3 h and the resulting hydrogel further employed surface hydrophobization by being immersed into 4-methyl-5-vinylthiazole (vTA) under visible light irradiation. Drastic morphology change over the pore closure, observed via SEM investigation (Figure 3(a1,a2)), and sulfur detection via EDX measurement were supportive evidences of performed surface photo-modification (Figure 3b). Moreover, resulting hydrophobized hydrogel showed an enhanced water contact angle result (58.5° over 40 s), unlike the initial hydrophilic hydrogel (Figure 3c). In addition, the sustained cation release performance by investigating several cations in both samples was confirmed, based on controlled release after surface hydrophobization (Figure 3d).

Considering the SEM results exhibited the highly porous structure of host hydrogel, pore substructuring was performed by taking advantage of the embedded g-C₃N₄ photoactivity. Regarding that, host hydrogel was swollen by various vinyl monomers and exposed to visible light. According to the SEM investigation, the closure of pores via polymer layer formation was observed, and the detection of functional groups via FT-IR underlined the efficient modification (Figure 4a–c).

The enrichment of hydrogels via embedding photoactive g-C₃N₄ nanosheets can be a promising subject with a variety of target applications for the future, especially when the simplicity of post-functionalization via light illumination is considered.

2.3. Oxidative Photopolymerization of 3,4-Ethylenedioxythiophene (EDOT) via g-C₃N₄

Conductive polymers have been playing a vital role in a broad range of materials, from optoelectronic to conductive composites. PEDOT is a functional polymer possessing high conductivity and stability even after doping process. However, the lack of processability of PEDOT requires external factors, which in return negatively influence its conductivity. As an alternative way to overcome this, g-C₃N₄ was employed as photoredox-type photo-initiator to achieve the oxidative photopolymerization of EDOT under visible light irradiation. A one-pot reaction in the presence of oxygen allowed neutral oligo-EDOT formation (path 1) and a reusable deposit of g-C₃N₄. The alternative addition of excess g-C₃N₄ in the reaction led to a hybrid heterojunction material which possessed the deposited oligo-EDOT on g-C₃N₄ power, based on surface polaron formation (path 2) (Scheme 3).

The eventual molecular weight was analyzed via UV-vis spectroscopy. According to the literature, the so-formed oxidative photopolymerization product explicit oligomer formation, in comparison to established PEDOT products, was thus named as ‘oligo-EDOT’ (n ≥ 3) based upon its intensified and slightly red-shifted UV-vis spectrum (Figure 5a). Furthermore, the enhanced viscosity within its darkened color after polymerization (as EDOT is a colorless monomer) supported success in photo-oxidative polymerization (Figure 5b). Resulting viscous oligo-EDOT was further post-doped with HBF₄, and finally the dried product was confirmed via FT-IR (Figure 5c). From a manufacturing perspective, a facile formation of film and coatings without the further addition of chemical substances yielded homogeneous conductive matter (Figure 5d).

Oxidative photopolymerization to form non-doped conducting polymers and conjugated systems, that are promising for optoelectronic and energy devices, was achieved via g-C₃N₄ acting as a photocatalyst. When used in excess, surface polaron-based novel hybrid materials can be accessible.

3. Conclusions

The combination of a metal-free semiconductor g-C₃N₄ with polymer materials is highly exciting, and offers many advanced materials, as shown in the present contribution. Photoactivity of g-C₃N₄ can be harnessed to synthesize polymers through photoredox chemistry (either forming active radicals through charge transfer processes, excited electrons, or a positive hole can be directly employed), or alternatively g-C₃N₄ can be dispersed and embedded in polymer networks to grant photoactivity. Merging polymers with colloidal g-C₃N₄ materials is still at the infant stage; however, one can distill versatile synthetic-polymer chemistry knowledge with semiconductor science to design functional materials that can address soft materials, conducting polymers, hybrid polymer materials, agricultural delivery systems and catalysis.