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Review

Progress in the Synthesis and Applications of C3N5-Based Catalysts in the Piezoelectric Catalytic Degradation of Organics

by
Shupeng Yin
1,
Huiguo Yu
1,
Haifeng Fu
1,
Yinglong Wang
2,* and
Fanqing Meng
2,*
1
Department of Biological and Chemical Engineering, Shandong Vocational College of Science and Technology, Weifang 261053, China
2
College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(12), 854; https://doi.org/10.3390/catal14120854
Submission received: 28 September 2024 / Revised: 6 November 2024 / Accepted: 11 November 2024 / Published: 25 November 2024
(This article belongs to the Section Catalytic Materials)
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Abstract

:
Piezoelectric catalysis has shown great potential for application in green chemistry due to its “clean” properties. By applying external mechanical force, this method can induce rapid charge transfer, providing an important reaction pathway for carbon neutrality and carbon peaking. Carbon nitride (C3N5)-based catalysts, as a novel material, have received widespread attention for their synthesis and application in the piezoelectric catalytic degradation of organic compounds. This review summarizes the latest research progress of C3N5-based catalysts, covering their applications in environmental governance and resource utilization, including the removal of organic pollutants in water. We focused on the synthesis strategy, characterization methods, and application progress of C3N5-based catalysts in the degradation of organic pollutants. The quantitative results show that some C3N5-based catalysts had removal efficiencies of over 85% in the treatment of specific pollutants. In addition, this article also discusses the piezoelectric effect and its degradation mechanism, providing direction for future research. Finally, the application prospects and potential development directions of C3N5-based catalysts in environmental governance are discussed.

1. Introduction

Water is the source of human life and an indispensable necessity in daily life [1]. As the global population continues to grow and urbanization and industrialization accelerate, the demand for water resources continues to increase [2]. Additionally, a large amount of waste water is produced. Untreated sewage, if it enters the human body directly through the diet and other ways, poses a great threat to health [3,4]. Wastewater treatment involves the removal of pollutants from the water used by households, businesses, and industries, and improper treatment can lead to serious environmental and health problems, including water-borne diseases, the contamination of surface and groundwater waters, and environmental damage to aquatic ecosystems [5,6,7]. Organic pollutants produced by the chemical industry have been detected in natural water bodies around the world, endangering the ecological environment, aquatic life, and human health. Therefore, it is urgent to effectively remove emerging drugs and organic pollutants from water, especially to reduce their toxicity.
In academic research, constantly improving wastewater treatment technology and strengthening the construction and management of wastewater treatment facilities are key measures to ensure sustainable development and human well-being [8]. The sustainable use and protection of water resources can only be achieved through the comprehensive use of various technical means jointly committed to improving the efficiency of wastewater treatment and water purification. To date, various methods have been developed to remove organic pollutants from water, including physical techniques, biological techniques, and chemical techniques [9,10]. Although the physical technology has strong adsorption selectivity, it has different adsorption efficiencies for organic matter types, resulting in a poor treatment effect for some specific organic matter. At the same time, the adsorbent may cause structural damage or dissolution after adsorbing organic matter, and its long-term stability is doubtful [11,12]. The biological technology mainly includes biological injection, biological stimulation, biological ventilation, and other methods. Although green and pollution-free, the drawbacks of these methods include the following: Unstable environmental conditions may lead to reduced biological treatment effects. In the treatment of highly concentrated organic pollution, a long treatment time is required. Biological treatment systems require regular monitoring and regulation of biological communities to maintain system stability, which increases operating costs. Among other problems, some specific organic matter may not be biodegradable [13,14]. Chemical technology is mainly dominated by advanced oxidation processes. Methods such as Fenton and ultrasonic oxidation can oxidize free radicals and decompose specific pollutants into environmentally friendly compounds, but their toxicity is also inevitable [15]. Piezocatalysis is an innovative method for treating wastewater by transforming mechanical vibration energy into electrochemical energy to enhance chemical reactions [16]. During the piezocatalysis process, vibration energy induces polarization within the piezoelectric material, generating positive and negative charges. These charges react with substances dissolved in a solution, producing various active species with strong redox capabilities for wastewater treatment. Ruan et al. synthesized metal–organic framework ZIF-8 nanoparticles using a liquid-phase method, demonstrating excellent piezocatalytic performance in degrading rhodamine B (RhB) dye under vibration. The principle of the degradation of RhB by ZIF-8 nanoparticles may be achieved through adsorption and the generation of active substances and their reaction, along with other mechanisms. The piezocatalytic degradation rate of 5 mg/L RhB dye can reach approximately 94.5% after 90 min of vibration [17]. Liu et al. combined the semiconductor material BiWO with AgO to form the AgO/BiWO piezoelectric catalyst, characterized it, and investigated its removal effect on ciprofloxacin (CIP). According to the analysis, when the AgO/BiWO composite ratio was 5%, the rate constants of piezoelectric catalysis to remove CIP were about 1.40 and 4.87 times higher than when BiWO and piezoelectric catalysis technology were used alone [18]. Shen et al. combined several layers of MoS nanosheets with a high-porosity flexible carbon matrix derived from melamine sponges to synthesize a high-performance stand-alone composite piezoelectric catalyst that can harvest large amounts of mild mechanical energy and trigger the piezoelectric effect, thereby effectively degrading dyes [19].
Nitrogen-rich graphitic carbon nitride (g-C3N5), as a well-known piezoelectric material, has attracted much attention because it does not contain metal and reduces the use cost. Additionally, it has outstanding characteristics, such as thermal stability and structural stability, strong large-scale manufacturing capacity, low toxicity, and a suitable energy band structure. Fu et al. utilized a two-step thermal polymerization/etching technique to fabricate a highly efficient piezocatalytic semiconductor, namely C3N5−x-O [20]. Experimental analyses and structural simulations have shown that incorporating dual defects in C3N5, including O doping and N vacancy, enhances its asymmetry and exposes triangular pores. Additionally, an optimized band structure and charge distribution in the thin-layered C3N5−x-O resulted in improved piezoelectric properties and expanded active surfaces. Zhao et al. designed an eco-friendly piezoelectric tube functionalized with CN/polyvinylidene fluoride composites for pollutant decomposition. The combined effects of CN and polyvinylidene fluoride significantly enhanced the piezoelectric properties of the materials and accelerated the separation efficiency of charge carriers [21].
In this paper, we provide an overview of the state-of-the-art research on C3N5-based catalysts, and we focus on the application of C3N5-based catalysts in the piezoelectric catalytic degradation of pollutants. The main contents include the piezoelectric effect, degradation mechanism, and synthesis strategy of C3N5-based catalysts, along with the characterization means and application progress of degrading organic pollutants. Finally, we present conclusions and future perspectives on C3N5-based piezocatalysis to guide future research directions.

2. Synthesis and Characterization of C3N5-Based Catalysts

In recent years, with the continuous emergence of novel theories and experimental evidence, the CxNy family has attracted significant attention. The rise of this trend is due to the remarkable characteristics and performance exhibited by these materials, which have shown strong adaptability and influence in various fields. It has been confirmed that the performance of CxNy materials mainly depends on several key factors, such as nitrogen content, structural characteristics, and crystal structure. Notably, the increased nitrogen content can enhance the electronic properties of CxNy materials by introducing a higher density of active nitrogen sites, thereby improving the overall performance. Among these materials, C3N5 has garnered particular attention due to its outstanding physical and chemical properties, establishing its importance in energy and environmental applications [22,23]. Therefore, our research focuses on comprehensively exploring the intrinsic physical and chemical properties of C3N5-based materials [24].
Melem (2,5,8-triamino-s-heptazine) is a precursor used in the synthesis of C3N5 polymers. To synthesize Melem, melamine is heated overnight at 425 °C and subsequently purified by boiling in water. Maleim, obtained from melamine, is reacted with hydrazine hydrate (55% NH2NH2·H2O) in a high-pressure reactor at 140 °C for 24 h. This reaction converts the amino (-NH2) groups into hydrazine (-NH-NH2) groups, yielding Maleim hydrazine (MH, 2,5,8-trihydrazine heptazine), which is a white powder. MH is further processed by heating at 450 °C for 2 h to produce an orange C3N5 polymer (Figure 1). The surface chemistry of the synthesized material was analyzed using X-ray photoelectron spectroscopy (XPS), as depicted in Figure 1e–h. The presence of Na 1s and Cl 2p signals is attributed to Na+ ions trapped within the polymer structure and residual NaCl from MH purification. Excluding the contributions of Na 1s, Cl 2p, and O 1s peaks, the atomic percentages of C and N in C3N5 were determined to be 36.76% and 63.24%, respectively, which align closely with the theoretical stoichiometry of C3N5 (C-37.50 at.% and N-62.50 at.%). High-resolution XPS analysis of C3N5 in the C 1s region revealed two peak components at 284.8 and 287.9 eV, corresponding to sp3 and sp2 hybrid carbon, respectively. The sp3 peak originates from various carbon configurations within the polymer matrix, while the strong sp2 peak indicates the presence of N Ü C-N aromatic carbon, forming the backbone of C3N5. The high-resolution XPS spectrum of the N 1s core energy level reveals two distinct peaks at 398.7 and 400.2 eV after deconvolution. The 398.7 eV peak corresponds to tertiary N in the aromatic ring structure and secondary C-π N-C nitrogen, while the 400.2 eV peak is attributed to primary residual NH2 and bridging C-N π N-C type nitrogen. The atomic percentages of N in the aromatic ring (Nring) and bridging (Nbridging) were determined to be 60.47% and 39.53%, respectively, supporting the proposed heptazine units and azo (-N≡N-) motif, consistent with the theoretical C3N5 structure. The Na1s region in HR-XPS shows a peak at 1071.9 eV due to Na+ ions and residual NaCl. Peaks at 531.6 and 532.4 eV in the O 1s region correspond to surface-adsorbed oxygen and -OH groups. Electron energy loss spectroscopy (EELS) was employed to characterize C and N bonds in C3N5. The normalized EELS spectra exhibit two symmetrical peaks attributed to C and N K edge losses. Peaks at 284.6 and 293.2 eV in the C-K edge signal indicate π* and σ* electronic transitions of sp2 hybridized carbon coordinated with nitrogen in the s-heptazine nucleus. The relative intensity of the π*-π* peak area ratio in C3N5 is higher than in g-C3N4, indicating enhanced conjugation arising from extended π-orbital overlap between the bridging azo functional group and the heptazine moiety. The development of an extended π-conjugated network in C3N5 is evidenced by increased UV–vis absorption and reduced time-resolved photoluminescence (TRPL) lifetime decay. N K-edge energy loss peaks at 399.8 and 408.5 eV for C3N5 confirm sp2 hybridized nitrogen in the heptazine ring. No new peaks in C3N5’s N K-edge loss indicate a similar electronic environment for bridging nitrogen as in g-C3N4. While the π* signal intensity at the N K edge of C3N5 is slightly lower, suggesting enhanced azo motif contribution to σ* transitions, the increase in total peak area of C3N5’s N K edge peak suggests additional nitrogen in the framework. The N:C atomic ratio in C3N5 matches both theoretical and CHNS analysis values, though slight nitrogen loss may occur due to azo bond breakage under high-energy electron beams.
The C3N5 monolayer consists of triazole and triazine units, forming a two-dimensional monoclinic lattice (Figure 2). Optimized lattice parameters (a = 5.64 Å, b = 6.68 Å, and γ = 113°) match experimental data. Four types of nitrogen atoms are present: sp3 hybridized nitrogen (N1 and N5), pyridine nitrogen (N2, N4, N6, and N7), pyrrole nitrogen (N8), and patterned nitrogen (N3). The Sp3 hybridized nitrogen induces lattice buckling (buckling height: 1.91 Å). The N-N bond length between sp3 hybridized nitrogen atoms (N1, N5) is about 1.40 Å, slightly shorter than (N1, N8) in triazole (1.42 Å). C-N bond length ranges from 1.32 Å to 1.45 Å, close to C3N4 (1.33 and 1.39 Å) and C2N (1.34 Å) values. Formation energy of the C3N5 monolayer (−2.39 eV/atom) closely matches theoretical calculations. Phonon spectrum analysis and ab initio molecular dynamics (AIMD) simulations confirm the dynamic and thermodynamic stability of the material. First principles calculations are performed using the Vienna ab initio simulation package (VASP) based on density functional theory (DFT). The electron–ion interactions were described using the Projection Enhanced Wave (PAW) potential, with the Perdew Burke Ernzerhof (PBE) formulation of the Gradient Approximation (GGA) as the exchange correlation functional. The DFT-D3 method was employed to account for van der Waals interactions, effectively reproducing experimentally observed lattice constants. Hybrid functionals based on the HSE06 approximation were used to obtain more accurate electronic band structures and light absorption properties. The Kohn Sham electron wave function is developed using a 520 eV plane wave, with energy convergence set at 10−5 eV and residual force of 0.01 eV Å −1. The Brillouin zone integration uses a 7 × 7 × 1 k-point grid. To eliminate the interaction between adjacent images, a vacuum space of over 20 Å was applied in the z-direction. The electrocatalytic reaction calculation uses 2 × 2 super units. The formula for calculating the formation energy of C3N5 monolayer is Ef = E(C3N5) − (mμc + nμn), where E (C3N5) is the total energy of the monolayer, m and n are the number of carbon and nitrogen atoms per unit cell, and μC and μN are obtained from graphene and N2, respectively. The photon spectrum is calculated through the Phonopy code and VASP interface, and first principles molecular dynamics simulations are conducted under the gauge ensemble (NVT) with a time step of 1.0 fs [26].

3. Synthesis of C3N5-Based Catalysts and Application of Piezoelectric Catalytic Degradation of Organic Matter

Nitrogen-rich carbonitrides, such as C3N5, are considered as emerging semiconductors due to their unique triazine and triazole units, which make them highly appealing for photocatalytic applications. Research efforts are increasingly focused on enhancing the piezoelectric catalytic efficiency of nitrogen-rich carbonitrides in the degradation of organic pollutants, highlighting their significant potential [23,27,28].
Liu et al. [18] synthesized porous C3N5 (p-C3N5) to activate periodate (PI) for the degradation of tetracycline (TC) under visible light conditions. Experimental results confirmed that the porous structure of C3N5 introduces abundant defects, which serve as active sites. These defects enhance both photoelectric efficiency and electron-hole separation, promote PI activation, and facilitate the generation of reactive oxygen species, significantly improving the photocatalytic performance. Li et al. [29] constructed a built-in electric field (BIEF) that facilitates the transport and separation of photo-generated carriers thereby enhancing its photocatalytic activity. This synergistic effect, coupled with increased oxygen adsorption capacity, resulted in a levofloxacin hydrochloride (LFH) removal efficiency of 0.0299 min−1 under visible light, which is 3.8 times higher than that of conventional carbon nitride (CN, 0.0086 min−1). Wang et al. [30] developed a novel oxygen-doped C3N5 (O-C3N5) with multiple defect sites through a simple one-step calcination method. The findings demonstrated that oxygen doping enhances molecular oxygen photoactivation, enabling the effective treatment of complex wastewater. Similarly, Fu et al. [20] synthesized dual-defect C3N5 materials with oxygen doping and nitrogen vacancies using thermal polymerization coupled with thermal etching. This study showcased the superior piezoelectric catalytic performance of C3N5 compared to C3N4, revealing that defect engineering amplifies piezoelectric catalytic degradation, Fenton reactions, and H2O2 generation. Additionally, defect engineering is effective in augmenting the asymmetry of catalyst molecular structure, thereby increasing polarization and piezoelectric performance [20]. The C3N5−x-O piezoelectric catalytic system effectively degrades tetracycline (TC) and Rhodamine B (RhB) with the aid of ultrasound, and the degradation efficiency of the piezoelectric catalytic Fenton system is further improved. The principle of the electrocatalytic system effectively interacting with Rhodamine B with the assistance of ultrasound is that ultrasound can promote mass transfer and reaction kinetics, and can also cause physical and chemical effects on the reaction system, such as generating cavitation bubbles that can enhance the contact and reaction between the electrocatalyst and RhB. It is important to highlight that even when there are high levels of background ions or elevated concentrations of pollutants, piezoelectric catalytic systems exhibit excellent performance, demonstrating their enormous potential in practical applications. In addition, without adding any sacrificial agents, a high H2O2 yield of 0.615 mM/g/h was achieved through piezoelectric catalytic reduction of O2.
In order to study the pressure catalytic performance of different catalysts, a series of degradation systems were first established using RhB as a model substrate. As illustrated in Figure 3a, C3N5−x-O demonstrates superior catalytic performance under pressure, achieving complete degradation of 20 ppm of RhB within 40 min with a rate constant of 0.0965 min−1. Double-defect C3N5 has more effective pressure catalytic performance than single-defect C3N5 or original C3N5. After double-defect modification of traditional C3N4, its pressure catalytic degradation rate of RhB increased from 62.8% to 73.5%, but it was still significantly lower than that of C3N5-based materials. In addition, without a catalyst, the degradation rate of RhB is only 7.7%, and the kinetic constant value is 0.0019 min−1, indicating the critical role of the catalyst. Catalysts with O sites exhibit superior O2 adsorption energy, longer O-O bonds, and increased electron transfer with O2 compared to catalysts with N vacancies, leading to enhanced capability in activating O2 [31]. Further investigation into degradation conditions was conducted utilizing TC as the representative substrate. TC is a wide-spectrum antibiotic extensively applied in medical and aquaculture fields. Due to its challenging degradability and negative impact on biodegradation technology, the piezoelectric catalytic degradation of this pollutant holds greater significance in water treatment. Figure 3c shows the dependence of the degradation rate on the mass ratio of catalyst to TC. When using TC as a representative substrate, 5 ppm TC was fully degraded within 100 min at a catalyst-to-TC mass ratio of 100. Interestingly, higher temperatures inhibited degradation, likely due to reduced oxygen solubility and the generation of reactive oxygen species at elevated temperatures. Practical application studies revealed that common inorganic substances up to 10 ppm had negligible effects on TC degradation. However, at concentrations exceeding 500 ppm, inhibitory effects became pronounced [32]. As shown in Figure 3d, effective degradation of TC can be achieved over a wide pH range. The practical application of piezoelectric catalytic systems was studied by adding common inorganic substances and organic macromolecules to simulate water. Figure 3e shows that the effect of inorganic substances up to 10 ppm on TC degradation can be ignored. It is worth noting that only inorganic compounds up to 500 ppm can exhibit relatively high inhibitory effects on TC degradation.
Ma et al. [33] developed an eco-friendly and effective advanced oxidation method by combining sodium percarbonate (SPC) with an ultrathin C3N5 photocatalyst. With visible light exposure, a removal efficiency of up to 93.97% for SMZ was achieved in 120 min. Figure 3a illustrates that in the SPC/Dark, SPC/Light, and U-C3N5/SPC/Dark systems, the SMZ concentration remains unchanged after 120 min of reaction. These findings suggest that SPC, U-C3N5, or visible light lack the capability to oxidize SMZ. However, in the U-C3N5/Light photocatalytic system, SMZ was degraded, achieving a removal rate of 38.08%, indicating that U-C3N5 can generate electron-hole pairs when exposed to visible light. Notably, when SPC is combined with the photocatalytic system (U-C3N5/SPC/Light), the degradation efficiency of SMZ increases, reaching a removal rate of 76.23% within 120 min. The degradation rate constant is determined using Equation (1), where Co and Ct represent the concentration of SMZ at 0 min and t min, respectively. K (min−1) is the reaction rate constant, and T (min) is the reaction time. The degradation process of SMZ follows a pseudo-first-order reaction, with the rate constant for U-C3N5/SPC/Light (0.0118 min−1) being 3.37 times that of U-C3N5/Light (0.0035 min−1), as shown in Figure 3b.
In addition, we investigated the effects of the dosage of UC-C3N5 and SPC, anions, and initial pH on the degradation of SMZ. As shown in Figure 4a, when the SPC concentration increased from 50 mg/L to 200 mg/L, the degradation efficiency of SMZ increased from 56.84% to 81.08%, accompanied by a corresponding increase in the rate constant from 0.0069 to 0.0133 min−1, as depicted in Figure 4b. The enhancement in elimination efficiency can be attributed to the generation of increased active substances with higher levels of SPC. However, at a concentration of 300 mg/L, the removal rate dropped to 76.96%, likely due to excess SPC resulting in excess H2O2, which acts as an ·OH scavenger [34]. Similarly, with an increase in U-C3N5 dosage from 10 mg to 40 mg, the degradation efficiency escalated from 54.45% to 93.97% (Figure 4c). Concurrently, the rate constant surged from 0.0062 to 0.0230 min−1 (Figure 4d). The improvement in elimination efficiency can be attributed to the increased activation of SPC by more electron-hole pairs (e−/h+) at elevated U-C3N5 dosages. Nevertheless, further increasing the U-C3N5 dosage from 40 mg to 60 mg led to a decrease in removal efficiency from 93.97% to 84.08%. This reduction is likely due to shielding effects caused by excess U-C3N5, which limits light penetration and reduces the generation of e−/h+ pairs to activate SPC.
The efficiency of catalysis is also linked to the initial pH of the solution. As illustrated in Figure 4e, when the pH value increased from 3.00 to 7.08, the removal efficiency of SMZ improved from 73.69% to 82.97%. However, with a further increase in pH from 7.08 to 9.17, the efficiency declined from 82.97% to 78.14%. This is because at pH levels (>7.0), the oxidation potential of ∙OH decreases, leading to the self-decomposition of H2O2 into O2 and H2O [31]. At pH values below 7.0, U-C3N5 is positively charged, while SMZ exists as a neutral or zwitterionic compound. The weak interaction between SMZ and U-C3N5 limits SMZ degradation, resulting in lower efficiency at acidic pH levels. Anions and organic compounds in solution are frequently present in real water systems, potentially influencing SMZ degradation efficiency. In this study, the impacts of different anions (Cl−, NO3−, and SO42−) and humic acid on the decomposition of SMZ were investigated. As shown in Figure 4f, low concentrations of anions (10 mg/L) had negligible effects on SMZ degradation. Interestingly, moderately elevated levels of anions (25 and 50 mg/L) slightly improved SMZ decomposition. This can be attributed to the oxidation of Cl− and SO42− by ·OH, generating reactive intermediates [35]. Additionally, NO3− exhibits photochemical activity and can generate ·NO2 and OH under light irradiation [31]. Conversely, humic acid inhibits degradation due to its competitive interaction with SMZ, as humic acid competes for reactive intermediates, thereby reducing SMZ degradation efficiency [36].
Li et al. [24] investigated the activation effect of Co-C3N5 on PMS for pollutant degradation. As shown in Figure 5a, in the presence of PMS, 96% of PCB 28 (0.5 mg/L) was degraded within 30 min, whereas significant degradation of PCB 28 was not observed when using PMS or Co-C3N5 alone. The activation efficiency of PMS was also examined at different synthesis temperatures (600–800 °C). The findings indicated that the degradation efficiency of PCB28 was 77%, 76%, and 80% when subjected to temperatures of 600 °C, 700 °C, and 800 °C, respectively. These results suggest that 500 °C is the optimal temperature for producing cobalt-doped C3N5 materials. This preference arises because higher synthesis temperatures cause the breakdown of the C3N5 structure, leading to the loss of active sites and the formation of cobalt oxide [37]. Furthermore, the decomposition of PMS (Co-C3N5-500 °C) was monitored during PCB 28 degradation. It was observed that 78% of the PMS decomposed within 10 min, closely correlating with the degradation trend of PCB 28. This suggests that the active components generated from PMS decomposition primarily contribute to the degradation of PCB 28.
Additionally, recycling and stability tests were carried out on Co-C3N5 to evaluate its reusability and durability. As illustrated in Figure 5b, the degradation efficiency of PCB 28 within 30 min remained high at 94.9%, 92.6%, 91.8%, and 92.8% over four cycles. This highlights the consistent catalytic performance of Co-C3N5 for activating PMS, attributed to the strong interaction between Co and N. The concentration of dissolved Co ions across all four cycles was below 0.01 mg/L, significantly lower than the Temporary Peer Review Cobalt Toxicity Value.
Furthermore, the impact of Co-C3N5 loading and PMS concentration on the degradation of PCB 28 in the Co-C3N5/PMS system was investigated. As depicted in Figure 6a, increasing the Co-C3N5 dosage from 0.1 g/L to 0.2 g/L significantly enhanced PCB 28 degradation efficiency, rising from 82% to 94%. This improvement can be attributed to the greater availability of Co (II)-N sites for PMS activation, leading to pollutant degradation. However, a slight decline in PCB 28 degradation efficiency was observed, from 94% to 78%, at higher loadings (0.2 g/L to 1.0 g/L). One possible explanation is that excess reductive active sites in Co-C3N5 compete with PCB 28 for free radicals, thereby reducing degradation efficiency.
As shown in Figure 6b, the degradation efficiency of PCB 28 increased from 65% to 94% as the PMS concentration increased from 0.5 mM to 2.0 mM. However, the efficiency declined to 77% at 3.0 mM PMS concentration. This behavior can be explained by the fact that, while higher PMS concentrations generate more free radicals, excess PMS may quench these radicals, leading to reduced PCB 28 degradation. The impacts of Co-C3N5 loading PMS concentration, initial reaction pH, and on the degradation of PCB 28 by Co-C3N5/PMS were also evaluated. As illustrated in Figure 6c, the catalytic performance of Co CNs was remarkable across various pH levels, achieving a PCB28 degradation rate exceeding 80% within 30 min in the pH range of 3.0 to 9.0. Nevertheless, as the pH value was raised to 11.0, the degradation rate of PCB28 decreased to 37%. This decline is attributed to chelation of OH and active Co sites at elevated pH levels, which impedes electron transfer from Co sites to PMS. Moreover, literature reports suggest that pH influences the degradation efficacy of pollutants [38]. However, this mechanism appears to be secondary in this system, as PCB 28 exhibits minimal dissociation across a wide range of pH values.
The relevant literature on the degradation of pollutants by C3N5-based piezoelectric catalysts are shown in Table 1.

4. Conclusions and Prospects

In conclusion, N-rich polymer carbon nitride (C3N5)-based catalysts, as a kind of promising piezoelectric catalysts, offer advantages such as simple preparation, environmental friendliness, designability, unique electronic energy band structure, stability, and low toxicity, making them highly attractive to researchers in the field of organic pollutant degradation. In this paper, we systematically reviewed the synthesis methods and characterization techniques of C3N5-based catalysts, which exhibit excellent piezoelectric performance in the piezoelectric-catalyzed degradation of various organic pollutants and demonstrate significant developmental potential. Although C3N5-based catalysts have made great progress in the direction of degrading organic pollutant degradation, several challenges remain to be addressed
(1)
With the rapid development of society and growing environmental awareness, the need for enhanced prevention and control of organic pollutants is increasing, emphasizing the need for greater piezoelectric catalytic efficiency. Improvements in morphology modulation, elemental doping, and piezoelectric material innovation are necessary to advance the piezoelectric degradation capabilities of organic pollutants.
(2)
In environmental applications, research has predominantly focused on degrading antibiotics and dyes in wastewater. However, limited attention has been given to removing nitrogen oxides (NOx) and volatile organic compounds (VOCs) from the air. Future research should expand the applications of C3N5-based catalysts to include air purification and investigate their performance in addressing gaseous pollutants.
(3)
In the future, noise, micro-vibrations, and other ambient energy sources could be utilized to activate piezoelectric catalytic effects. This approach could not only address environmental pollution but also harness renewable energy, minimizing energy losses and improving overall system efficiency.
(4)
Photocatalytic technology can be used to purify volatile organic compounds in the air. This is because these small molecules can react with photo-generated electrons/holes with redox ability. The original C3N5 has been proven to be an excellent candidate for the adsorption of some small molecules due to its high nitrogen content as an active center. Therefore, further research on applying C3N5 photocatalytic technology to air pollution control is both necessary and meaningful.
(5)
Future efforts should prioritize sustainable synthesis methods, such as employing green chemistry techniques or novel catalysts, to improve synthesis efficiency and reduce environmental impact. Identifying and overcoming potential technical challenges in the synthesis process, such as optimizing reaction conditions, improving yield, and controlling material purity, are important steps in driving research. Efforts should be made to thoroughly explore the application of C3N5 in fields such as environmental protection, energy conversion, and materials science, to design specific experimental and research questions, and to verify its actual effectiveness. Interdisciplinary collaboration and promotion of in-depth research and widespread application of C3N5 by combining knowledge from fields such as chemistry, materials science, and engineering should be encouraged.

Author Contributions

S.Y.: Conceptualization, Methodology, Investigation, Writing—Original Draft. H.Y.: Conceptualization, Methodology, Investigation, Writing—Original Draft. H.F.: Conceptualization, Methodology, Investigation, Writing—Original Draft. Y.W.: Funding Acquisition. F.M.: Writing, Review and Editing. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the National Natural Science Foundation of China (No. 22078166).

Data Availability Statement

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.

Conflicts of Interest

No conflict of interest exits in the submission of this manuscript, and the manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and is not under consideration for publication elsewhere, in whole or in part.

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Catalysts 14 00854 g001
Figure 1. (a) He ion image of C3N5 and HR-TEM image of C3N5 (b) at 50 nm, (c) at 10 nm, and (d) at 5 nm scale; left and right insets displaying SAED diffraction patterns and interplanar spacing, respectively; the HR-XPS spectra of the core energy levels of C3N5 in the (e) C1s and (f) N1s regions, as well as the normalized EELS spectra of g-C3N4 and C3N5, exhibit π*, (g) XPS spectra of C3N5 and g-C3N4 and (h) N K edge losses σ*; the relative intensity of the peak; (i) the synthesis roadmap of C3N5 [25].
Figure 1. (a) He ion image of C3N5 and HR-TEM image of C3N5 (b) at 50 nm, (c) at 10 nm, and (d) at 5 nm scale; left and right insets displaying SAED diffraction patterns and interplanar spacing, respectively; the HR-XPS spectra of the core energy levels of C3N5 in the (e) C1s and (f) N1s regions, as well as the normalized EELS spectra of g-C3N4 and C3N5, exhibit π*, (g) XPS spectra of C3N5 and g-C3N4 and (h) N K edge losses σ*; the relative intensity of the peak; (i) the synthesis roadmap of C3N5 [25].
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Catalysts 14 00854 g002
Figure 2. Relaxation atomic structure of C3N5 monolayer (top view (a) and side view (b)): nitrogen and carbon atoms are represented by blue and brown spheres, respectively, with the circles indicating the atoms and corresponding bond lengths (Å) in the crystal cell; (c) the projected density of states of single-layer atomic orbitals [26].
Figure 2. Relaxation atomic structure of C3N5 monolayer (top view (a) and side view (b)): nitrogen and carbon atoms are represented by blue and brown spheres, respectively, with the circles indicating the atoms and corresponding bond lengths (Å) in the crystal cell; (c) the projected density of states of single-layer atomic orbitals [26].
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Catalysts 14 00854 g003
Figure 3. (a) Pressure degradation performance of the prepared catalyst. (b) Degradation curves for different refractory pollutants. (c) TC degradation curves under various conditions such as dosage, power, and temperature are presented. (d) Degradation curve for 20 parts per million at different pH values of C3N5−x-O is illustrated. (e) Degradation curves of 20 parts. (f) Comparative analysis of kinetic constants between this study and other literature is conducted [20].
Figure 3. (a) Pressure degradation performance of the prepared catalyst. (b) Degradation curves for different refractory pollutants. (c) TC degradation curves under various conditions such as dosage, power, and temperature are presented. (d) Degradation curve for 20 parts per million at different pH values of C3N5−x-O is illustrated. (e) Degradation curves of 20 parts. (f) Comparative analysis of kinetic constants between this study and other literature is conducted [20].
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Figure 4. The influences on degradation, including (a) SPC concentration, (b) U-C3N5 dosage, (c) initial pH of SMZ solution, (e) anions, and fulvic acid, (d,f) SPC concentration and U-C3N5 dosage. Test bar Piece: CMZ, initial 10 mg/L, T = 250.5 °C, visible light [33].
Figure 4. The influences on degradation, including (a) SPC concentration, (b) U-C3N5 dosage, (c) initial pH of SMZ solution, (e) anions, and fulvic acid, (d,f) SPC concentration and U-C3N5 dosage. Test bar Piece: CMZ, initial 10 mg/L, T = 250.5 °C, visible light [33].
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Figure 5. Co-C3N5 activates PMS for PCB28 degradation: (a) degrade kinetics, (b) the catalytic ability of Co-C3N5 to PCB28 after four cycles [24].
Figure 5. Co-C3N5 activates PMS for PCB28 degradation: (a) degrade kinetics, (b) the catalytic ability of Co-C3N5 to PCB28 after four cycles [24].
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Figure 6. The effects of Co CsNs loading (a) PMS concentration, (b) and initial reaction pH, (c) on the degradation of PCB28 by Co CsNs/PMS. Reaction Conditions: [Co CsNs = 1.0 g/L, [PCB28] = 0.5 mg/L, [PMS] = 0.5–3.0 mM, pH = 3.0–1.0 (buffer solution) [24].
Figure 6. The effects of Co CsNs loading (a) PMS concentration, (b) and initial reaction pH, (c) on the degradation of PCB28 by Co CsNs/PMS. Reaction Conditions: [Co CsNs = 1.0 g/L, [PCB28] = 0.5 mg/L, [PMS] = 0.5–3.0 mM, pH = 3.0–1.0 (buffer solution) [24].
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Table 1. References list.
Table 1. References list.
Catalytic SystemReaction ConditionsTarget PollutantsRemoval EfficiencyRef
RN-g-C3N50.02 g RN-g-C3N5
20 ML 20 mg/L MB		Methylene Blue
(MB)		98% (120 min)[32]
AgCl/g-C3N550 mg AgCl/g-C3N5
50 mL 10 mg/L RhB		Rhodamine B (RhB)96% (30 min)[39]
CeTiO6/g-C3N51.6 g/L CeTi2O6/g-C3N5 75 mL 10 ppm (λ > 420 nm)2,4-dichlorophenol (2,4-DCP)96% (120 min)[40]
CdS/g-C3N50.1 g/L CdS-MHP 50 mL 0.01 mM RhB1 sun AM1.5 GRhodamine B (RhB)90% (80 min) [41]
Xp-/g-C3N51 sun AM1.5 G 6P-g-C3N5 5 ppm RhB, 20 ppm Rhodamine B (RhB) Tetracycline (TC)100% (180 min)[42]
Ag3PO4/g-C3N51.0 g/L Ag3PO4/C3N5 50 mL 20 mg/L TCH 300 W Xe lamp (λ > 400 nm)Tetracycline hydrochloride (TCH)90.5% (60 min)[43]
FeOCl/g-C3N51.0 mg/mL Catalyst 75 mL 10 mg/L TC30% 200 μL H2O2Tetracycline (TC)95% (40 min)[44]
CDs/MoS2/g-C3N50.02 g/L Catalyst
50 mL 30 mg/L (λ > 420 nm)		Methylene blue (MB)94% (120 min)[45]
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Yin, S.; Yu, H.; Fu, H.; Wang, Y.; Meng, F. Progress in the Synthesis and Applications of C3N5-Based Catalysts in the Piezoelectric Catalytic Degradation of Organics. Catalysts 2024, 14, 854. https://doi.org/10.3390/catal14120854

AMA Style

Yin S, Yu H, Fu H, Wang Y, Meng F. Progress in the Synthesis and Applications of C3N5-Based Catalysts in the Piezoelectric Catalytic Degradation of Organics. Catalysts. 2024; 14(12):854. https://doi.org/10.3390/catal14120854

Chicago/Turabian Style

Yin, Shupeng, Huiguo Yu, Haifeng Fu, Yinglong Wang, and Fanqing Meng. 2024. "Progress in the Synthesis and Applications of C3N5-Based Catalysts in the Piezoelectric Catalytic Degradation of Organics" Catalysts 14, no. 12: 854. https://doi.org/10.3390/catal14120854

APA Style

Yin, S., Yu, H., Fu, H., Wang, Y., & Meng, F. (2024). Progress in the Synthesis and Applications of C3N5-Based Catalysts in the Piezoelectric Catalytic Degradation of Organics. Catalysts, 14(12), 854. https://doi.org/10.3390/catal14120854

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