G-c3n4 Preparation
Brian Scarano
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Until now, lives dependent on the water environment and human being have faced various crises due to the enormous consumption of dyes in various industries such as textiles, biology, pharmaceutical, cosmetics, food packaging, analytical chemistry, plastic-derived chemicals, and other aspects of daily life. In these manufactories, approximately 15% of dyes are released into water resources without any purification process1, 2. Methylene blue (MB) has various applications and is widely used in diverse fields like biological, medical, pharmaceutical and chemical industries. The release of methylene blue in wastewater effluent has harmful environmental effects on water, living organisms and fishes. Therefore, it seems necessary to remove such hazardous compounds from wastewater effluent.
Among the available light sources, light emitting diodes (LED) have recently attracted much attention. LEDs are semiconductors that can be used as an alternative light source for light-dependent applications such as photocatalytic detoxification/decontamination of air and water environments. Therefore, low-power LEDs have unique characteristics, which include strong and almost concentrated radiation, low electrical energy consumption, small size, simple intensity adjustment, free of mercury and harmful metals, flexibility for design. Different types of photocatalytic reactors with good quality. Controlled lighting conditions, no need for a cooling system in the reactor, and a longer half-life compared to conventional light sources12, 13. So far, many researchers have tried to use LED photocatalysts in drinking water purification systems as well as pollutant degradation. LEDs have poor homogeneity in the light spectrum emitted at full power. Meanwhile, high-power LEDs (more than 200 W) face challenges: (i) Traditional lamps generate too much heat, but heat generation by high power LEDs cannot be ignored. (ii) The emission time decreases with time, so that 50,000 h is the lifetime of the light, which is about 5 times longer than that of the mercury vapor lamp14. Therefore, it is very important that a photocatalyst has sufficient ability to absorb the emitted light in concert with the LEDs used until pollutant removal with high efficiency15.
Graphitic carbon nitride was prepared according to the method presented in our previous report36. In detail proper amount of melamine was heated at 520 C in a crucible with a cover for 2 h. The as prepared yellow powder then washed by ethanol for three times and denoted as CN.
In order to prepare O doped g-C3N4, 10 g melamine and 6 g Oxalic acid were dissolved in 50 ml solution mixing of distilled water and ethanol; under continuous stirring magnetically at 80 C. The heating process continued until the complete removal of the solvent. The obtained sample was calcined in a ceramic crucible with a lid from room temperature to 520 C at a heating rate of 5 C/min for 2 h. after that the product cooled to room temperature and were ground to a fine powder. The resulting sample was labeled as (OCN)37.
To produce g-C3N4, 10 g of thiourea and 10 g of melamine were calcined in a crucible with a lid at 520C with a heating rate of 5C for 2 h and then cooled to room temperature. The prepared sample then washed with water and ethanol and denoted as (SCN)38.
To detect the reactive species that generate in the catalytic reactions, 4 m.mol of three typical scavengers (i.e., BQ, BA, and AO preferred as the scavengers of \(\mathrmO_2^\cdot-\), \(\mathrmHO^\cdot\), and hole (h+), respectively) separately used in the sonocatalytic degradation of MB under the optimum condition. According to Eq. (1), the effectiveness of each active species on the MB degradation process was studied.
As depicted in Fig. 2, the diffraction peaks located at around 13.1 (100) and 26.8 (002) attributed to the in-plane structural packing of tri-s-triazine units and the typical graphene-like stacking of the conjugated aromatic motif, respectively. Following JCPDS No. 87-1526, these plates are attributed to hexagonal graphitic materials, confirming the lamellar nature of g-C3N439. The (100) peak positions of the modified samples had a negligible shift than CN, indicating that the Oxygen or Sulfur atoms were successfully replaced with Nitrogen and Carbon atoms in the tri-s-triazine units of g-C3N4. Besides this, the intensity of (100) peak weakened than CN, indicating that the in-plane repeating motifs enlarged. Meanwhile, the position of the (002) peaks was constant, illustrating the retention of the crystalline motif of g-C3N4. More importantly, the (002) diffraction peaks of the modified samples become sharper and narrower in comparison with CN, implying that the increased degree of polymerization of the precursors led to the higher crystalline structure of g-C3N4 in SCN and OCN than CN23,24,25. Scherer equation (\(D=\fracK\lambda \beta Cos\theta \), where D, K, λ, β, and θ represent the crystal size, the shape constant value of 0.9, the X-ray radiation wavelength of 1.5418 , the full line width at half-maximum height, and the half of the scanning angle range, respectively) uses to calculate the crystalline size of particles. Hence, the index of (002) in CN, SCN, and OCN samples were selected for measuring D, which determined 17.3, 27.7, and 35.9 nm, respectively40. Furthermore, no obvious signal of impurities was detected, which is related to the high purity of the prepared catalysts. In summary, the findings suggest that non-metallic doped g-C3N4 samples were successfully produced and their higher crystalline nature was effectively enhanced by introducing different dopant atoms into the g-C3N4 lattice structure41.
The microscopic structural and surface morphology of CN, OCN, and SCN were investigated by FE-SEM. The FE-SEM images in Fig. 3a, c, e show that the structure of the prepared samples includes large grains and two-dimensional bulky sheets with irregular blocks and non-uniform morphology. Unlike CN, which has nearly smooth sheets with sizes of about a few micrometers, SCN and OCN show semi-bulky fractions with smaller sheet size, which is probably the result of the polymerization process. In fact, the thermal conditions of the precursors for the preparation of SCN and OCN have led to changes in the size and shape of the sheets. Smaller particle size and higher porosity network of modified samples compared to CN can lead to an increase in the light-based performance of these samples. In addition, according to the information in Fig. 3a, c, e and Fig. S2, the particle sizes of SCN and OCN are in the range of one micrometer, which shows that a significant reduction in the length and thickness of the modified samples has occurred compared to CN. In fact, when melamine and oxalic acid or thiourea were used as precursors, volatile H2O or H2S gases created inorganic nanocrystals with a microporous structure in the g-C3N4 framework. In addition, the release of these gases can exfoliate carbon nitride sheets with more wrinkles and irregular shapes. Therefore, these smaller sizes of carbon nitride can lead to increased mass transfer phenomenon in SCN or OCN compared to CN. And the catalytic responses of photocatalysts can be improved19, 30, 44,45,46,47,48,49.
Figure 3b, d, f revealed that Oxygen or Sulfur elements successfully doped in g-C3N4 structure as OCN or SCN, respectively. Besides this, the prepared samples had high purity without disturbing elements, helping the prepared samples can display the truth of catalytic activities without disruptive impurities44. Moreover, a comparison between SCN and OCN evinced that the Oxygen content was more than Sulfur, originating from the truth that AO exerted a potent force for inserting Oxygen atoms into the g-C3N4 structure. Then, it changed the elemental composition of the g-C3N4 motif during the polymerization step, interpreting the stronger electronegativity of Oxygen than Sulfur for doping into the g-C3N4 structure19, 41.
As shown in the HR-TEM images in Fig. 4a, c, e, the as-prepared samples showed a graphite-like shape with a layered configuration. In addition, the planar structure of CN was transformed into a porous structure in SCN or OCN due to the extraction of off-gases during the intense polymerization of melamine or AO20. It is worth noting that the porous structure improves the contact between the surfaces of the catalyst and reactant molecules and leads to the improvement of the catalytic activities of SCN or OCN compared to CN43. Significantly, the dark area in the images of SCN and OCN shows the higher crystallinity framework of g-C3N4, which indicates that more tris-s-triazine units are uniformly arranged in these samples than in CN38, 50. According to Fig. 4b, d, f, the absence of bright spots in the SAED patterns indicates that these samples have a crystalline nature. In addition, the identified rings match well with the double peaks obtained in the XRD patterns of Fig. 2.
In summary, some simple modification methods for the fabrication of SCN and OCN were carried out through a one-pot non-metallic doping process under special polymerization conditions, which ultimately led to the enhancement of the optical and photocatalytic properties of SCN and OCN, such as reducing Eg and improving light utilization ability, and reduced recombination of charge carriers. So far, the applied synthesis modifications resulted in a safe, inexpensive and simple approach to fabricate highly crystalline nonmetallic g-C3N4 with good structural and surface properties. Then, DRS analysis showed that white-LED light has good harmonics for carrier excitation. Cold-white-low-power LED light showed suitable photocatalytic responses for MB reduction. According to Figure S8, with the synergy between US and photocatalyst, the use of US successfully promoted the photodegradation of MB under optimized conditions. Apart from the role of US in accelerating the excitation of charge carriers by the emission of long-wavelength radiation, the spontaneous increase in temperature (as the first enhancer) showed that the crossing of the activation energy barrier could be improved to facilitate the catalytic reactions. The second booster (i.e., O2 gas injection) synergistically with sonophotocatalysis, ultimately leads to an increase in MB mineralization efficiency by trapping the most excited electrons to produce \(\mathrmOH^\cdot\) Therefore, the introduction of these enhancers into the sonophotocatalytic systems finally revealed the maximum capabilities of the designed heterogeneous AOPs. In conclusion, these practical implementations optimistically extended our insight into the preparation of long-lived g-C3N4-based catalysts and enhanced sonophotocatalytic performance for rapid decontamination of organic dyes under low-power radiation sources.
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