The influence of duration of high-temperature exposure on the properties of carbon nitride obtained in molten salts

Graphtic carbon nitride (GCN) is one of the most stable carbon nitride polymorph. It has semiconductor properties, which allows to apply it as photocatalyst for organic matter degradation [1–3], synthetic reactions [4], carbon dioxide reduction [5], nitrogen fixation reactions [6], hydrogen evolution [7–9]. One of most frequently studied applications is wastewaters treatment where organic contaminants are being degraded by highly active hydroxyl radicals. Meanwhile, more sophisticated utilization may be performed in non-aqueous solutions, where formation of hydroxyl radicals is restricted. It is possible to selectively oxidize organic compounds or keep C-C bonds formation in mild, non-oxidative conditions [10].

Convenient ways of carbon nitride preparation by the means of powders calcinations are known to result weakly ordered materials [11]. Through this procedure linear heptazine polymer forms, which are connected to each other by hydrogen bonds [4]. This material has high hydrogen amount of about 2 % – this fact points to the incomplete condensation of functional groups. Low extent of carbon nitride polymerization leads to incomplete formation of conjugated electronic structure. Carbon nitride, prepared through this way represented by particles of irregular shape with significant share of amorphous phase– this lowers its photocatalytic efficiency due to hindered charge transport, easiness of charges recombination [12].

The efficiency of C₃N₄ photocatalytic application may be enhanced via the improvement or its structure [13]. Highly ordered material facilitatates charges migration and their accessibility to substrates [14]. High degree if charge migration is the result of π orbital conjugation, therefore highly ordered structure is necessary [15]. Ordered structure may be the result of templates application, but nevertheless that templates needed to be excluded from the structure of material after synthesis – this needs additional chemicals etc. [16], therefore template-free procedure is desired. Previously developed alternative route for improvement of carbon nitride crystallinity include ionothermal synthetic process, preformed through 2 steps of thermal treatment in molten salts with the subsequent heating in the sealed ampule. It allows to get uniform nanoparticles of C 3 N 4 semiconductor [17].

Additional way to improve carbon nitride performance is to dope it. Doping include: formation of nitrogen vacancies [18], carbon vacancies [19], doping with metal- or non-metal heteroatom [20], copolymerization with different monomers [21], molecular doping [22]. One of recently developed way is doping with cyano groups [23]. Cyano group boosts visible light absorption, hinders charge recombination and makes process of exfoliation easier due to the enhancement of distance between single layers of carbon nitride [24].

In this work highly crystalline carbon nitride, doped with cyano groups, was prepared via 1-step process in molten eutectic KCl/LiCl mixtures from 5 different precursors: melamine, urea, thiourea, me-lamine/urea and melamine/thiourea mixtures. This process does not utilize any templates to prepare uniform hexagonal particles of carbon nitride. Transmission electron microscopy reveal ordered structure of hexagonal particles without amorphous inclusions. Crystalline structure and uniform shape of C3N4 particles facilitate non-recombinational delivery of charges to substrates.

Efficiency of photocatalytic performance was evaluated with the model reaction of selective oxidation of benzyl alcohol to benzaldehyde – industrially relevant substrate utilized for dyes synthesis, cosmetic, pefume industries as denaturant, flavouring agent, fragrance, as well as in the food industry [25]. Convenient ways of benzaldehyde preparation are not environmentally friendly because they exploit tough reaction conditions and strong oxidants such as permanganates and chromates. Proposed process of oxidation of benzyl alcohol to benzaldehyde is activated by solar irradiation, does not utilize harmful metal based oxidants, and has no toxic byproducts. It matches to strict requirements of «green» chemistry. The enhanced efficiency of eutectic processed materials roots from their improved structure with uniform particles and less share of amorphous phase.

Experimental

Preparation of carbon nitride materials

5 samples of carbon nitride were prepared, differing by the exposure time in the muffle furnace. In each case, 9 g of LiCl + 11 g of KCl + 4 g of melamine were mixed. The mixture was placed in a crucible with a lid and heated in a muffle furnace to 600°C with various holding times at this temperature. Characteristics of the samples and mass yield are shown in Table 1.

Table 1 Temperature program and mass yield for carbon nitride samples

Name	Program	Mass yield, %
C 3 N 4 _2	10hours ↑600 °С, 2 → 600 °С	51.3
C 3 N 4 _4	10hours ↑600 °С, 4 → 600 °С	48.2
C 3 N 4 _6	10hours ↑600 °С, 6 → 600 °С	60.3
C 3 N 4 _8	10hours ↑600 °С, 8 → 600 °С	41.5
C 3 N 4 _10	10hours ↑600 °С, 10 → 600 °С	39.4

Photocatalytic experiment

The activity of carbon nitride as a photocatalyst for the selective oxidation of benzyl alcohol to benzaldehyde was investigated. The photocatalytic process was carried out in a quartz reactor transparent to the UV spectra, the reactor has a water cooling jacket into which water was supplied – the temperature of the reaction mixture was 20 °C. The initial concentration of benzyl alcohol equals to 2 mmol/L, the volume of the reaction mixture was 30 ml, the matrix of 18 UV LEDs with a power of 1 W each served as an irradiation source, their luminosity spectrum represented one narrow peak with a maximum at 365 nm. Length of exposure time was equal to 5 hours. The concentrations of benzyl alcohol, benzaldehyde, and benzoic acid were determined by the means of high performance liquid chromatograph with a UV detector.

Characterization

Morphological investigation of the prepared composite silica-titania spheres was performed by a field emission scanning electron microscope (SEM) JEOL JSM 7001F. XRD patterns were recorded on a Rigaku Ultima IV diffractometer operating at Cu Kα radiation (λ = 0.154 nm). Products of photocatalytic experiment were studied with high performance liquid chromatograph Shimadzu Prominence LC-20 with UV detector.

Results and discussion

The maximum mass yield of carbon nitride (60.3 %) was detected for sample C 3 N 4 _6, which had an exposure time of 6 hours in the furnace. When increasing or decreasing the high-temperature exposure time, the mass yield decreases, the minimum value is reached at 10 hours of high-temperature exposure.

Morphology characteristics was studied by the means of scanning electron, samples C 3 N 4 _2, C₃N₄_6, C₃N₄_10 are presented on Figs. 1–3 respectively. It is clearly seen that carbon nitride materials, subjected to short time of high thermal treatment demonstrate higher rate of order in comparison to 10-hours treated. C 3 N 4 _2 sample exhibit cubic morphology and C 3 N 4 _6 has hexagonal morphology of particles, while carbon nitride, treated during 10 hours reveal irregular shape of its particles.

Fig. 1. C 3 N 4 _2 sample

Fig. 2. C 3 N 4 _6 sample

Fig. 3. C 3 N 4 _10 sample

Duration of heat treatment

Fig. 4. Conversion and selectivity of photocatalytic experiment of benzyl alcohol oxidation

The values of benzyl alcohol conversion and selectivity for benzaldehyde are shown in Fig. 4 and Table 2. A significant decrease in the reaction rate catalyzed by carbon nitride, which is subjected to maximum exposure in the furnace for 10 hours, can be noted. The selectivity with respect to the formation of benzaldehyde for all samples is at a high level exceeding 86 %.

Table 2

Conversion and selectivity of photocatalytic experiment of benzyl alcohol oxidation

Name	Conversion, %	Selectivity, %
C 3 N 4 _2	79.2	92.4
C 3 N 4 _4	76.8	87.3
C 3 N 4 _6	77.1	92.3
C 3 N 4 _8	73.8	86.7
C 3 N 4 _10	48.6	97.3

In all cases, the amount of benzoic acid formed is negligible. When summing the amounts of substances contained in the reaction mixture after 5 hours of irradiation, a value of less than 2 mmol/L is obtained – this indicates that part of the benzyl alcohol undergoes complete oxidation to undetectable products (carbon dioxide, water).

2 theta

!0h

Fig. 5. XRD graph of carbon nitride materials

Powder X-ray diffractometry demonstrates the presence of the same crystalline phase of carbon nitride (Fig. 5). The degree of crystallinity of the materials is different; it reaches its maximum values for materials exposed to exposure at 600 °C for 2–6 hours. With a longer time of high-temperature exposure, the sizes of the ordering regions fall, as indicated by a decrease in the peak intensity and an increase in the width of the peaks at half maximum. A decrease in the degree of crystallinity established by powder x-ray diffractometry correlates with a decrease in the photocatalytic activity of the materials.

Conclusions

It was found that the optimal duration of high-temperature exposure can be called 2 to 6 hours. At a given exposure time, the formation of the most ordered material occurs, with a further increase in the exposure time, crystallinity decreases according to powder x-ray diffractometry and transmission electron microscopy. Materials held at high temperature for a given time demonstrate the best conversion characteristics of benzyl alcohol and selectivity for benzaldehyde in a photocatalytic experiment.

An increase in the high-temperature exposure time to 8 and, in particular, up to 10 hours leads to a decrease in the ordering sites determined by the method of powder x-ray diffractometry and transmission electron microscopy. Photocatalytic activity also decreases.