Direct Z-Scheme g-C3N5/Cu3TiO4 Heterojunction Enhanced Photocatalytic Performance of Chromene-3-Carbonitriles Synthesis under Visible Light Irradiation

Abstract

In order to make the synthesis of pharmaceutically active carbonitriles efficient, environmentally friendly, and sustainable, the method is regularly examined. Here, we introduce a brand-new, very effective Cu₃TiO₄/g-C₃N₅ photocatalyst for the production of compounds containing chromene-3-carbonitriles. The direct Z-Scheme photo-generated charge transfer mechanism used by the Cu₃TiO₄/g-C₃N₅ photocatalyst results in a suppressed rate of electron-hole pair recombination and an increase in photocatalytic activity. Experiments showed that the current method has some advantages, such as using an environmentally friendly and sustainable photocatalyst, having a simple procedure, quick reaction times, a good product yield (82–94%), and being able to reuse the photocatalyst multiple times in a row without noticeably decreasing its photocatalytic performance.

1. Introduction

Multi-component reactions (MCRs) have garnered a lot of interest recently as a method for creating a variety of heterocyclic compounds with applicability in medicine, pharmaceuticals, and agrochemistry [1,2]. One-pot MCRs, which induce organic reactants to react together in a single step, have emerged as an alternative platform for organic chemists due to their simple operation, effective purification techniques, side products, and quick turnaround times [3,4]. In simple terms, a multi-component reaction is one in which three or more reactants mix in a single vessel to create a single output that effectively contains every atom of the starting elements (with the exception of condensation products, such as water molecules, hydrochloric acid, or methanol) [5,6,7].

A number of naturally occurring O-bearing heterocyclic organic moieties, which are widely present in edible fruits and vegetables, contain the chromene and related structural skeletons as their major structural motifs [8,9]. Numerous industries, like agrochemistry, medicine, as well as material sciences, use carbonitrile-based heterocycles and their derivatives as ligands and catalysts [10]. Carbonitrile compounds have a high level of therapeutic activity and can be employed as antibacterial [11,12], anti-tumor [13,14], anti-inflammatory, anti-cancer [15,16,17], and anti-convulsant drugs [18]. Compounds with a carbonitrile base are also employed to make electrical and optical materials as well as anion sensors [19,20]. Given the many different uses for chromenes, synthetic chemists have been exploring new and effective catalysts. In the traditional way of making carbonitrile-based chemicals, the product needed to be made at a high temperature and through a long process called “synthesis”.

As a result of growing environmental worry, a lot of work has been put into creating innovative procedures that reduce emissions in chemical synthesis. From the perspective of green and sustainable chemistry [21,22,23], designing effective and affordable chemical processes utilising heterogeneous catalysts to create fine chemicals and pharmaceutical goods through MCRs has recently attracted a lot of attention from academics and businesses. The fundamental drawback of heterogeneous catalysts is, however, their decreased catalytic activity. The best way to deal with this issue is to minimize the size of the particles in the heterogeneous catalyst [24,25]. Metal oxide nanocatalysts combine the benefits of photo and heterogeneous catalysts. Although this type of material has many valuable advantages, including a high surface area, efficient particle distribution, a straightforward recovery procedure, effective light absorption property, good stability, and non-toxic behaviour, their propensity to agglomerate and oxidise is a drawback [26,27,28]. Immobilizing organic molecules on metal oxide nanoparticles is thought to be a workable way to address this drawback.

Given the significance of metal oxide, nanoparticles (metal oxide NPs) have received a lot of attention and are widely employed in water splitting, catalysis, photocatalysis, energy, and environmental remediation [29,30,31,32]. Titanium dioxide (TiO₂) and copper oxide (CuO) nanoparticles have attracted a great deal of interest. It was discovered that CuO and TiO₂ NPs worked effectively as catalysts for the oxidation of organic molecules as well as CO and NO [33,34]. Additionally, in last decade, a lot of attention has been focused on the use of CuO and TiO₂ NPs for chemical reactions [35]. CuO NPs’ high levels of electron-hole pair recombination and TiO₂’s less obvious active nature are some of its disadvantages. We created the Cu₃TiO₄ NPs for the organic reaction based on the aforementioned investigative information. In the context of heterogeneous nanocatalysts, this class of catalysts seems to be one of the most promising approaches toward effective reactions under mild and ecologically nonthreatening circumstances.

Organic synthesis research in the area of photocatalysis, notably with regard to visible light, is appealing [36]. Heterogeneous photocatalysis has been given a significant role by carbon-based materials. Due to a variety of characteristics, including stability, narrow bandgap, 2D nanomaterial, excellent visible-light active absorption, low density, and low toxicity, graphitic-carbon nitride (g-C₃N₄) is an effective catalyst under visible light [37,38]. Additionally, its layered structure makes the anchor metal an appropriate substrate for its direct use as a heterogeneous photocatalyst. A variety of modified carbon nitrides, including nanosheets, nanorods, metal-doped and non-metal modified carbon nitrides are effective nanomaterials [39,40,41]. An intriguing method has been recently created to modify the polymeric structure of C₃N₄ in order to create novel carbon nitrides with a wider visible-light sensitive range. For instance, integrating an N-rich 3-amino-1,2,4-triazole unit into the s-heptazine motif led to the highly ordered C₃N₅, which can increase the C:N atom ratio from 3:4–3:5. The lower bandgap energy is a result of the extra N-atoms connected to the terminal NH₂ groups in the form of 1,2,4-triazole, which are different from the usual C₃N₄ in that they are bridged by a N-atom through sp² hybridization [42,43,44]. The construction of heterojunction photocatalysts based on C₃N₄ has received extensive research, due to its viability and efficiency for the spatial separation of photogenerated electron-hole pairs. Appropriate catalyst materials can generate heterojunctions, which can lower the activation energy needed for the reduction reaction. The secondary target is to encourage the division of photogenerated charge carriers while simultaneously providing more active locations for the photocatalytic activity. Meticulous design of a heterojunction is a promising method to increase photocatalytic activity, because of its rapid charge carrier separation caused by the integrated electric field [45,46,47,48]. A switch from photocatalysts based on C₃N₅ to those based on the organic transformation reaction is driving recent advancements for cleaner, more sustainable chemistry. The heterojunction is followed by carbon nitride-based catalysts like Type-II and Z-Scheme. A Z-scheme system has also been built with a pertinent shuttle mediator to obtain better light-harvesting potential with high charge separation performance and sturdy redox ability, furthermore to photocatalysts with conventional heterojunctions that can successfully separate photogenerated electron-hole pairs via band alignment [49,50]. Numerous studies on g-C₃N₄ linked to the Z-Scheme electron transfer mechanism have been reported, including those on TiO₂/g-C₃N₄ [51], CuO/g-C₃N₄ [52], ZnO/g-C₃N₄ [53], WO₃/g-C₃N₄ [54], V₂O₅/C₃N₄ [55], MnO₂/C₃N₄, [56] and Bi₂O₃/g-C₃N₄ [57], among others. In recent years, g-C₃N₅-based heterojunction production has become increasingly significant in photocatalysis and other applications. There are currently very few studies on Z-Scheme mechanisms for g-C₃N₅-based heterojunctions, including Ag₃PO₄/C₃N₅ [58], FeOCl/g-C₃N₅ [59], Bi₄O₅I₂/g-C₃N₅ [60], LaCoO₃/g-C₃N₅ [61] and Bi₂WO₆/g-C₃N₅ [62,63]. Still, there is no report carried out on Cu₃TiO₄/g-C₃N₅ heterojunction fabrication and photocatalytic application.

Light-emitting diodes (LEDs) are the best type of light source for photocatalytic applications. BLUE LEDs have benefits over other light sources in terms of efficiency, the qualities of power, compatibility, longevity, and environmental friendliness. Due to their unique characteristics, BLUE LEDs offer an improvement over traditional lighting sources and allow for more innovative scope when designing different photochemical reactions [64].

In this research, we have developed a novel and very effective Cu₃TiO₄/g-C₃N₅ photocatalyst for the construction of chromene-3-carbonitrile molecules under BLUE LED light. The direct Z-scheme charge separation mechanism employed by the Cu₃TiO₄/g-C₃N₅ hetero-junction photocatalyst reduces the rate of electron-hole (e⁻/h⁺) pair recombination of Cu₃TiO₄ and increases its photocatalytic activity. The as-prepared g-C₃N₅/Cu₃TiO₄ nanocomposites showed better photocatalytic activity and stability when compared to pure Cu₃TiO₄ and pure C₃N₅. The spatial separation of photoinduced charge carriers could be attributed to the improved photocatalytic performance. The characteristics of the obtained catalysts have been studied with the help of XRD, UV-Vis DRS, FT-IR, FE-SEM, TEM, Zetapotential, and XPS. The applications for the catalyst have also been demonstrated and characterized. This work will be opening up a new path in the field of photocatalysts and MCRs and can further be developed for a wide range of applications other than the proposed one.

2. Results and Discussion

2.1. Characterization of Prepared Nanomaterials

The synthesized nanocomposite nanomaterial crystalline phase structure was investigated using the X-ray diffraction technique. In Figure 1, the pristine g-C₃N₅ sheets peak at 13.1° was related to (100), and another strong peak at 27.2° was related to (002) for graphitic g-C₃N₅ sheets (JCPDS card-87-1526) [65]. Peaks for Cu₃TiO₄ nanoparticles were found at 2theta = 26.4, 34.2, 38.6, 51.5, 59.1, 63.2, and 70.3° in relation to planes (004) (101) (102) (104) (006) (110) (112) (107), which corresponds well to JCPDS card no. 83-1285 [66]. Both are well-reflected in all CN5CT nanocomposites. Figure 1 shows the p-XRD patterns obtained for g-C₃N₅/Cu₃TiO₄ with varying wt. % Cu₃TiO₄. The XRD patterns of the 5% CN5CT composite showed no discernible change, while less Cu₃TiO₄ has been deducted because of the low loading percentage. The intensity of Cu₃TiO₄ nanoparticles peaks increased as the Cu₃TiO₄ nanoparticles ratio was increased to 10%, 20%, and 30%. Finally, Cu₃TiO₄ nanoparticles were supported on g-C₃N₅ nanosheets. As confirmed, they have shown good crystalline nature.

In BET analysis the surface area of the designed nanocomposites was examined, and depicted in Figure 2a. The surface area of Cu₃TiO₄ nanoparticles, g-C₃N₅ and g-C₃N₄ values are 10, 15, and 14 m²/g respectively. Then 10% CN5CT nanocomposite has 19 m²/g after supporting Cu₃TiO₄-nanoparticles, which thereby increased the surface area. The Cu₃TiO₄/g-C₃N₄ has less surface area compared to 10% CN5CT, as shown in Figure 2a. From BET analysis, 10%CN5CT exhibited more surface and active sites compared to others. In FT-IR spectroscopy (Figure 2b), g-C₃N₅ sheets showed peaks at 808, 1234, 1405, 1575, and 2160 cm⁻¹, respectively. The signal at 808 cm⁻¹ corresponds to the existence of triazine units in g-C₃N₅ nanosheets. NH₂ and NH peaks were obtained at 3500 cm⁻¹ and triazine unit stretching bands were found at 1200 cm⁻¹. The g-C₃N₅ nanosheet FT-IR report is consistent with previous literature. [67,68]. The FT-IR spectrum of Cu₃TiO₄ exhibited a range between 400–700 cm⁻¹ bands, which is owing to Ti-O-Ti and Cu-O stretching vibration [69]. After that, Cu₃TiO₄ nanoparticles supported the slight shift that happened in 5%, 10%, 20%, and 30% CN5CT nanocomposite.

In XPS, the chemical state of the prepared 10% CN5CT was exposed in Figure S1. Figure S1a shows the survey spectrum of the synthesized 10% CN5CT nanocomposite and all the signals confirmed the presence of C, N, Cu, Ti, and O. For C 1s, in Figure S1b, the three strong peaks at ~287, 285.8 and 283 eV show the presence of C 1s in nanocomposite, which corresponds to N-C-N, C-N and C-C bonds [70]. Then N 1s displayed two peaks at 399 and 397 eV, providing evidence for the presence of N 1s species (Figure S1c) with different environments of N-C and C-N-C bonds [71]. Cu 2p exposed two major peaks at 953 and 933 eV (Figure S1d) combined with two satellite lines, confirming the presence of Cu 2p species in the CN5CT composite [72]. The two peaks (Figure S1e) at 462 and 456 eV confirm the presence of Ti 2p species in CN5CT nanocomposite [72]. Finally, for O1s species, two strong peaks (Figure S1f) at 529 and 531 eV confirmed the existence of oxygen-metal and oxygen-hydrogen bonds in the composite.

The typical morphology of the g-C₃N₅ and g-C₃N₅/Cu₃TiO₄ nanocomposite is shown in Figure 3a,b, indicating that the prepared g-C₃N₅ has a sheet-like morphology. As seen in Figure 3a, exfoliation caused the evolution of 3-amino-1,2,4-triazole into a typical layered structure, showing the formation of a loosely packed 2D g-C₃N₅ structure. In Figure 3b, 10%CN5CT nanocomposite, and Cu₃TiO₄ nanoparticles are marked. The 10% CN5CT nanocomposites showed a strong connection between g-C₃N₅ and Cu₃TiO₄, indicating that there would be a physical adsorption. It also implied that g-C₃N₅ might prevent the aggregation of Cu₃TiO₄ NPs as compared to Figure 3b. The effectiveness of the electron-hole separation can be increased by closing interfacial contacts between g-C₃N₄ and Cu₃TiO₄ nanoparticles by excellent dispersion. According to the elemental mapping in Figure 3c–f, the elements C, N, Ti, Cu, and O were evenly distributed throughout the 2D layered and 3D nanoparticle structures.

The perfect morphology and structural nature of the g-C₃N₅ and 10% CN5CT were recorded by using TEM (Figure 4a,b). The agglomerated sheet-like structure of g-C₃N₅ was confirmed by a TEM image at 50 nm (Figure 4a). The surface of the g-C₃N₅ was smooth and had high porosity, which was confirmed by the sheet-like structure of the g-C₃N₅. The 3-AT was thermal disintegration, copolymerization, and elimination of CO₂ and NH₃, which may result in the development of g-C₃N₅ nanosheets, which have a smaller and thinner nanosheet like structure than g-C₃N₄. Both g-C₃N₄ and g-C₃N₅ are similar in appearance. The EDS of g-C₃N₅ provides information about the given sample, g-C₃N₅, which has a higher quantity of nitrogen atoms compared to carbon atoms (Figure 4c). On introducing Cu₃TiO₄ into g-C₃N₅ nanosheets, no changes were observed on the surface of g-C₃N₅. The Cu₃TiO₄ NPs are uniformly distributed on the g-C₃N₅ surface and form an effective heterojunction (Figure 4b). In Figure 4d, EDS analysis of 10%CNCT confirms the elements with weight percentage viz., N (52.4%), C (35.1%), O (8.7%), Ti (2%), and Cu (1.8%). Figure 4e depicts the SEAD pattern of the 10%CN5CT, demonstrating that the prepared composite has a good crystalline nature and corresponding planes of (111), (103), (102), and (106) in XRD. The average particle size of Cu₃TiO₄ (~26 nm) in 10%CN5CT was calculated and given in Figure 4f. From the TEM image, the surface area of the compound increased with the decrease in Cu₃TiO₄ particle size. The photocatalyst 10% CN5CT was found to coexist on the surface of g-C₃N₅ sheets. The Cu₃TiO₄ (dark-coloured) nanoparticles are uniformly scattered on the g-C₃N₅ matrix to produce an efficient heterojunction structure, with the average particle size of Cu₃TiO₄ being found to be around 50 nm. The large surface area enhances the photocatalytic activity. Due to their excellent surface area, both g-C₃N₅ and Cu₃TiO₄ have close contact with each other. Due to this nature, the 10%CN5CT nanocomposite has better photocatalytic activity.

The charge separation and transfer of the electron-hole pair were analysed by using photoluminescence (PL) emission spectroscopy. Figure 5a shows the PL results for the prepared CN, CT, and CN5CT at various loading percentages. The g-C₃N₅ nanosheets emission peak was observed at around ~470 nm with high intensity, which confirmed the high recombination rate. On the other hand, the introduction of Cu₃TiO₄ nanoparticles on the surface of g-C₃N₅ nanosheets reduced the intensity of the peak, which confirmed the suppression of recombination rate of the CN5CT nanocomposite. Notably, when compared to other CN5CT nanocomposite, the 10% CN5CT nanocomposite has a lower emission peak. The PL investigation is strong evidence that 10%CN5CT has less electron-hole pair recombination than 5%, 20%, and 30% of CN5CT.

The thermal stability of the prepared nanocomposite was carried out by using thermo gravimetric analysis (TGA). Here, in Figure 5b, is depicted the high stability of Cu₃TiO₄ nanoparticles, because no characteristic decomposition occurred in the Cu₃TiO₄ nanoparticles and up to 98% of Cu₃TiO₄ nanoparticles were getting residue at 800 ^oC, showing the high stability nature of nanoparticles. At 530 ^oC, g-C₃N₅ nanosheets began decomposing, and all of the carbon and nitrogen in g-C₃N₅ was easily eliminated above 700 °C (1% residue at 800 °C). In 10% CN5CT nanocomposite, g-C₃N₅ nanosheets got decomposed at 530 ^oC but the residue obtained was 12%, which shows the presence of Cu₃TiO₄ nanoparticles in g-C₃N₅ nanosheets at a 10% ratio was confirmed. The 10%CN5CT nanocomposite has improved the thermal stability compared to g-C₃N₅.

2.2. Photocatalytic Activity of g-C₃N₅/Cu₃TiO₄ Nanocomposite

In this study, we concentrated on the synthesis of different carbonitriles using a g-C₃N₅/Cu₃TiO₄ (CN5CT) photocatalyst. The catalyst exhibited better efficiency for the synthesis of carbonitriles in the presence of light. Ethanol has been used as solvent, which generated a yield of 94% (Table 1, entry 6). The reaction was carried out in the presence of BLUE LED visible light using a photocatalyst employing diketone, malanonitrile, and aldehyde (Scheme 1). Based on the characterization results, we investigated the CN5CT nanocomposite, which has a strong light absorption capacity and thereby increases the life time of electrons in the conduction band. The high electron-hole recombination rate improves the activity of the catalyst in the production of 1.4a. In this case, 20 mg of the catalyst was combined with the reactants and exposed to Blue LED light. Simultaneously, radicals formed on the catalyst surface, resulting in the formation of activated complexes. These are highly active species that are rapidly degraded to create products on the catalyst surface. In Table 1, the reaction conditions were adjusted to determine the optimal conditions for carbonitrile synthesis. Here, the solvent, catalyst, condition, and time were varied to determine the optimization condition. (Table 1 entries 1 and 3), produced yields (62% and 33%) that were significantly lower than the CN5CT nanocomposite. CN5CT nanocomposite and ethanol solvent worked better in all situations in the presence of light for 3 min. In all situations, the CN5CT nanocomposite and ethanol solvent worked better in all situations in the presence of light for 3 min. The increase in catalyst percentage also did not affect the product yield and for 5%, 20% and 30%, CN5CT nanocomposites give good yields (Table 1, entry 5, 7, 8).

The optimal condition from Table 1 was utilized to synthesize different carbonitrile derivatives, which is shown in Table 2. The physicochemical properties and evidence (FT-IR, ¹H, ¹³C NMR, and GC-MS) of the synthesized carbonitrile derivatives 1.4a-g are shown in Supporting Information Figures S2–S29. Light excites the electrons on the catalyst surface, causing them to move into the excited conduction band. The radicals (^•O₂⁻ and ^•OH) then react with the reactants to generate intermediates, which are then broken down to form the desired products on the catalyst surface. The catalyst, CN5CT nanocomposite, showed exceptional activity in the preparation of compound 1.4a. The recovered catalyst also performed admirably in the subsequent runs, with better yields.

In Table 2, we studied how several heterocycles were synthesized. We successfully prepared several substituted aldehydes such as electron donating, electron withdrawing, and halogenated aldehydes here. When donating substitutions were compared to others, they produced moderate yields (Table 2, compound 1.4b). In the proposed procedure, napthaldehyde and benzaldehyde substitutions produced the highest yields of all. The multi-component technique made good yields and cut down on the amount of time needed to make 1.4a.

The reaction was carried out using CN5CT and irradiated for 3 min in an open environment at RT with a BLUE LED light. Remarkably, 94% of the targeted product (1.4a) was achieved. Nevertheless, when ambient O₂ was removed from the reaction mixture, the conversion was minimal and only a trace amount of product could be found if the reaction was completed in darkness or without photocatalysts. As a result, CN5CT photocatalyst, atmospheric oxygen, and BLUE LED visible light are all required for this transition.

2.3. Proposed Mechanism of 1.4a Synthesis

Based on the findings, a plausible mechanism was illustrated in Scheme 2. The more stable triplet state CN5CT* of the photoredox catalyst is then activated by the absorption of visible light and performs a single electron transfer (SET). This single electron transfer from 1.1 and 1.3 to the excited state of the CN5CT photocatalyst then lost a hydrogen atom and generated malononitrile radical (1.1a) and 1,3 cyclohexanedione radical (1.3a). The Knoevenagel condensation process between aldehyde (1.2) and activated malononitrile (1.1a) resulted in the synthesis of alkene intermediate (1.2a) [74]. Then, the intermediates 1.2b and 1.3a react in situ via Michael addition to give an adduct 1.3b, followed by intramolecular cyclisation of 1.3c, giving the desired product 1.4.

2.4. Transport Process of Photo-Excited Charge Carriers

The prepared samples’ light absorption in the 200–800 nm region was examined in order to obtain an appearance in the optical harvest property. The absorption edge of g-C₃N₅ is located at approximately 680 nm, as shown in Figure 6a. In contrast, the 10% CN5CT nanocomposite clearly showed a shift, which may have been caused by the intense interface interaction between Cu₃TiO₄ and g-C₃N₅. As a result, the representative 10 weight percentage (10%) CN5CT nanocomposite photocatalyst exhibits a greater capacity to capture visible light than 2D g-C₃N₅.

As seen in Figure 6b, the band-gap values were also calculated from the intercept of tangents to plots of (Ahv)^1/2 versus photon energy. The optical band gap of g-C₃N₅, Cu₃TiO₄ and 10% CN5CT was calculated and found to be 2.02, 2.15, and 2.04 eV respectively. In comparison to pure g-C₃N₅ and Cu₃TiO₄, the 10% CN5CT nanocomposite exhibits a narrower band gap. According to the findings, the photocatalyst for the 10% CN5CT heterojunction complex has good reactivity to visible light, which may increase its photocatalytic activity. The results show that photo-generated electron-hole charge carriers are enhanced, which will boost photocatalytic activity compared to CN5 and CT.

To obtain the band levels of the g-C₃N₅/Cu₃TiO₄, the valance band (CB) and conduction band (CB) potentials for g-C₃N₅ & Cu₃TiO₄ were calculated with the aid of Equations (1) and (2) [75].

E_VB = χ − Ee + ((1/2) * Eg)(1)

E_CB = E_VB − Eg(2)

where, Eg-band gap calculated from Figure 6b (2.02 eV for g-C₃N₅, and 2.15 eV for Cu₃TiO₄); E_CB-CB potential; E_VB-VB potential; Ee-energy of free electrons evaluated on the H scale (4.5 eV); χ–electro negativity (6.89 eV for g-C₃N₅ and 5.63 for Cu₃TiO₄) calculated from literature [64]. The calculated E_VB and E_CB were +1.38 and −0.64 eV for g-C₃N₅, +0.055 and −2.095 eV for Cu₃TiO₄, respectively.

It is well known that the heterojunction photocatalyst g-C₃N₅/Cu₃TiO₄ will be produced when g-C₃N₅ and Cu₃TiO₄ are coupled. According to the band gap structures of g-C₃N₅ & Cu₃TiO₄, the separation processes of photo-induced electron–hole (e⁻/h⁺) is conveyed in Figure 7a,b, respectively.

According to the type-II heterojunction (Figure 7a), the holes of g-C₃N₅ formed by visible-light will transfer to the VB of Cu₃TiO₄ (0.055 eV). Altogether, e⁻s instantaneously migrate from the CB of Cu₃TiO₄ (−2.09 eV) to the CB of g-C₃N₅, (−0.64 eV). However, the potential energy (V vs. NHE) of e⁻s from g-C₃N₅ is much lower than O₂/O₂⁻ (−0.33 V) [76], which designates that these electrons are unable to reducing O₂ to ·O₂⁻. Correspondingly, holes of Cu₃TiO₄ have inadequate potential energy to oxidize H₂O to ·OH (·OH/H₂O = 2.34 V vs. NHE) [77]. Consequently, the type-II mechanism does not well define the photocatalytic activity by the g-C₃N₅/Cu₃TiO₄ heterojunction.

Our photocatalyst works in total tandem with the visible light wavelength from BLUE LED light sources. Both g-C₃N₅ and Cu₃TiO₄ can be stimulated to produce electrons and holes when exposed to light. The electrons in CB of g-C₃N₅ tend to migrate to VB of Cu₃TiO₄ and recombine with the holes there under the influence of the electron transport system, following the Z scheme charge transfer path. As a result, the highly redox active photogenerated electrons in CB of Cu₃TiO₄ and holes in VB of g-C₃N₅ are retained to participate in the photocatalytic activity. If the photo-induced charge transfer process follows the direct Z-Scheme mechanism as illustrated in Figure 7b, the photon-induced excited electrons would thermodynamically transfer from the CB of g-C₃N₅ to VB of Cu₃TiO₄. Meanwhile, the O₂ could be reduced into ⋅O₂⁻ (reduction reaction) by the electrons generated by CB of Cu₃TiO₄ because of the more negative potential (−2.09 eV). Further, the holes of g-C₃N₅ are arrested by OH to produce the ˙OH, and the ˙OH are moved to the superficial of the nanocomposite to oxidize molecules, because of the more positive potential (+1.38 eV). This charge transfer process in the direct Z-scheme greatly improves the efficiency of photo-generated electron-hole separation, resulting in the enhancement of the photocatalytic activity [78,79,80].

The nanocomposite exhibited excellent stability, visible absorption nature, high yields, a shortened reaction time, and reusability (Figure S30a). The photocatalyst 10%CN5CT does not affected by the reaction mixture and giving higher yields for the next few runs. Then after recycled catalyst was showing good crystalline nature in XRD analysis shown in Figure S30b. In the presence of visible light, the narrow bandgap feature demonstrates a visible active photocatalyst. In visible light, green synthesis of different heterocycles using CN5CT nanocomposite produced good yields in a short period. The photocatalyst can be used repeatedly without affecting the activity of the catalyst.

3. Materials and Methods

3.1. Materials Information

All of the chemicals and reagents were obtained from Avra Chemicals Private Limited, which is located in Andhra Pradesh, India. The column solvents and other chemicals were brought from Avra chemicals and used without further purification or processing. We purchased the lemons from the Vellore local market in Katpadi, Tamil Nadu, India. The NMR solvents came from Merck and was said to be 99.9% pure by the manufacturer.

3.2. Preparation of Citrus Limon Extract

Fresh lemons were split into half and squeezed to gather the pulp. There is some excess waste in the gathered juice (seeds and peel). To eliminate the additional trash, the pulp was centrifuged at 10,000 rpm. The clear liquid from the top layer was collected and it was stored in refrigerator.

3.3. Preparation of Cu₃TiO₄ NPs

Copper acetate (1 M) and titanium tert-butoxide (1 M) were taken in two separate beakers with 50mL of deionized water. A titanium tert-butoxide solution was added into the copper acetate solution with constant stirring. Then 5 mL of citrus extract obtained was added and the mixture was kept stirring overnight. Bluish-white coloured residue was separated and washed with a water-ethanol solution to remove the other impurities. The precipitate was heated at 500 °C for 4 h in a silica crucible. Finally, a black precipitate was obtained and subjected to characterization.

3.4. Preparation of g-C₃N₅ Nanosheets and Composite

In a beaker, 1 mmol of 3-amino-1,2,4-triazole was stirred well with 20 pellets of NaOH for 20 min. The colour of the mixture was altered to light brown after it was ground. For 4 h, the mixture was heated at 550 °C at a rate of 20 °C/min. Finally, grey g-C₃N₅ was collected and rinsed 10 times with distilled water to eliminate unreacted NaOH before being kept for characterization. The 5%, 10%, 20%, and 30% CN5CT were produced after processing g-C₃N₅ with Cu₃TiO₄ at various concentrations.

3.5. General Synthesis of Carbonitriles (1.4a-g) under Visible Light

1,3-cyclohexadione (1 mmol), corresponding aldehydes (1 mmol), malononitrile (1 mmol), CN5CT photocatalyst (20 mg), and 5 mL of ethanol were placed into a 25 mL round-bottom flask in the BLUE LED (12 W, 450 to 495 wavelength) light chamber. The reaction mixture was illuminated, and TLC was tested every minute to identify the result. After reaction completion, the reaction mixture was poured into crushed ice and precipitated out. The solids were removed from the ethanol and recrystallized.

4. Conclusions

In conclusion, we described the fabrication of direct Z-scheme Cu₃TiO₄/g-C₃N₅ heterojunction and their characterization. The Cu₃TiO₄/g-C₃N₅ photocatalyst has expressed excellent activity for the synthesis of carbonitriles derivatives. In an as-prepared heterojunction, 10% Cu₃TiO₄ is dispersed on surface the surface of g-C₃N₅ exhibited better photocatalytic activity. When compared to pure Cu₃TiO₄ and g-C₃N₅, Cu₃TiO₄/g-C₃N₅ nanocomposites perform better at photocatalysis due to their distinctive nanostructure and direct Z-scheme heterojunction. Low-energy consumption visible light BLUE LED lights were used for the irradiation. The reaction has been carried out immediately and the photocatalyst was showing excellent activity, reusability, easily recoverable, less recombination rate in visible light and high surface area.

Author Contributions

Original draft preparation, data collection and writing, M.A. and T.C.; Writing and editing, R.M.; Data analysis, K.A.; Writing-review and editing, G.M. and S.M.R.; Supervision, S.M.R. All authors discussed the results and contributed to the final manuscript. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Acknowledgments

Our profound thanks go to the management of the Vellore Institute of Technology for providing the Sophisticated Instrument Facility (SIF) to aid in the research endeavour.

Conflicts of Interest

The authors declared no potential conflicts of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures, Schemes and Tables

Figure 1. XRD images of prepared CN, CT and various percentages CNCT nanocomposites.

Figure 2. (a) BET isotherm and (b) FT-IR spectra.

Figure 3. FE-SEM images of (a) g-C3N5 (b) 10% CN5CT nanocomposite and (c–g) elemental mapping of 10% CN5CT.

Figure 4. (a,b) TEM images, (c,d) EDS spectra of g-C3N5, 10% CN5CT nanocomposite (e) SEAD pattern of 10% CN5CT. (f) Average particle size of Cu3TiO4 nanoparticles in 10% CN5CT.

Figure 5. (a) PL emission spectra of g-C3N5, 5% CN5CT, 30% CN5CT, 20% CN5CT, 10% CN5CT (b) TGA analysis of Cu3TiO4, g-C3N5, and 10% CN5CT nanocomposite.

Scheme 1. Synthesis of hexahydroquinoline-2-carbonitrile using CN5CT nanocomposite in Blue LED light.

Scheme 2. Mechanism of hexahydroquinoline-2-carbonitrile using g-C3N5/Cu3TiO4.

Figure 6. (a) UV-Vis DRS spectra and (b) bandgap plot.

Figure 7. Proposed electron-hole pair transfer mechanism of g-C3N5/Cu3TiO4 nanocomposite (a) type-II, (b) Direct Z-Scheme.

Table 1

Optimization for the synthesis of Carbonitriles in BLUE LED visible light.

S.No	Catalyst	Solvent	Conditions	Time (minutes)	Yield a (%)
1.	CuO	EtOH	Light	60	62
2.	CuOAc	EtOH	Light	60	66
3.	TiO2	EtOH	Light	60	33
4.	Cu3TiO4	EtOH	Light	60	75
5.	5% CN5CT	EtOH	Light	3	67
6.	10% CN5CT	EtOH	Light	3	94
7.	20% CN5CT	EtOH	Light	3	81
8.	30% CN5CT	EtOH	Light	3	89
9.	10% CN5CT	H2O	Light	10	61
10.	10% CN5CT	THF	RT	60	23
11.	10% CN5CT	Acetonitrile	RT	60	21
12	10%CN5CT	EtOH	RT	10	95
13.	Without Catalyst	EtOH	RT	10	Trace
14.	10% CN5CT	Solvent free	Heat	10	78

Diketone (1 mmol), aldehyde (1 mmol), malononitrile (1 mmol), ethanol, photocatalyst (20 mg). ^a Yields—Isolated yields.

Table 2

Scope of different substrate.

[Image omitted. Please see PDF.]	Name	R	Time	TON a	TOF a	Yield b	Ref. c
	1.4a	-Ph	5	865	3.52	85	[73]
	1.4b	4-(OCH3)-Ph	5	832	3.26	82	[73]
	1.4c	2-NO2-Ph	3	876	3.78	88	[73]
	1.4d	4-OH-Ph	5	831	3.22	81	[73]
	1.4e	4-F-Ph	5	878	3.86	90	[73]
	1.4f	2-Cl-Ph	3	923	4.63	94	[73]
	1.4g	2-Nap	3	902	4.12	92	[73]

Diketone (1 mmol), aldehyde (1 mmol), malononitrile (1 mmol), ethanol, photocatalyst (20 mg). ^a TON—Turn Over Number; TOF—Turn Over Frequency. ^b Yields—Isolated yields. ^c—reference.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12121593/s1, Figure S1: XPS of Cu₃TiO₄/g-C₃N₅ nanocomposites; Figures S2–S29: FT-IR, ¹H, ¹³C NMR and GC-MS of carbonitrile derivatives 1.4a–g; Figure S30: Reusability and reused XRD.

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