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Article

BioTemplated Fe3+-Doped g-C3N4 Heterojunction Micromotors for the Degradation of Tetracycline through the Photo-Fenton Reaction

by
Qingbao Gan
1,
Jianwei Zhang
1,
Jinglin Wang
1,
Yuntian Wei
1,
Shikun Chen
1,
Shuguang Cai
1,
Xueqing Xiao
1 and
Chan Zheng
1,2,*
1
School of Materials Science and Engineering, Fujian University of Technology, Fuzhou 350118, China
2
Institute of Biology and Chemistry, Fujian University of Technology, Fuzhou 350118, China
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(9), 579; https://doi.org/10.3390/catal14090579
Submission received: 25 July 2024 / Revised: 15 August 2024 / Accepted: 20 August 2024 / Published: 30 August 2024
(This article belongs to the Section Catalytic Materials)
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Abstract

The excessive discharge of antibiotics into aquatic systems is a major issue in many countries worldwide and poses a threat to human health and the sustainable development of society. Hence, developing efficient treatment methods and purification technologies to degrade antibiotics is essential. Herein, we present the synthesis of low-cost, self-propelled tubular Fe3+-incorporated graphitic carbon nitride (g-C3N4-Fe@KF) micromotors using kapok fibers (KFs) as templates and their application as photo-catalysts for the photo-Fenton degradation of tetracycline (TC) under visible-light irradiation. The g-C3N4-Fe@KF micromotors moved rapidly when being propelled by oxygen bubbles generated in a hydrogen peroxide (H2O2) solution as a result of a photo-assisted Fenton reaction. The motion behavior of the g-C3N4-Fe@KF micromotors was dependent on the concentration of H2O2 and the length of the micromotors. The propulsion mechanism was discussed in detail. The micromotors efficiently degraded antibiotics via the photo-Fenton process. Photo-Fenton degradation efficiency was attributed to the synergistic effects of the doped Fe3+ and g-C3N4 under visible-light irradiation and self-propulsion of the micromotors. In addition, the micromotors possessed good reusability, thereby efficiently realizing multiple cycles of degradation. The current work offers an avenue for the design of micromotors, using inexpensive approaches, for various potential environmental applications.

1. Introduction

Rapid industrialization over the past decades has led to an accelerated depletion of energy resources and environmental issues owing to substantial production and the excessive release of toxic and nonbiodegradable organic pollutants into aquatic systems [1,2]. Although antibiotics have significantly promoted human health, they are regarded as dangerous contaminants stemming from constant release from agricultural activities and animal husbandry, along with their persistent presence in aquatic systems [3,4]. The excessive discharge of antibiotics inhibits the growth of aquatic organisms and leads to the development of antibiotic-resistant bacteria, endangering human health and the sustainable development of society. To remove antibiotics from the above-mentioned systems or to maintain them at safe levels, numerous strategies, such as physical or chemical adsorption, electrochemical oxidation, biodegradation, the Fenton reaction, and photocatalytic degradation, have been explored [5,6]. However, traditional biological and physical treatments can only partially degrade pollutants, resulting in residual trace organic pollutants remaining in aquatic systems [7,8]. Therefore, it is important to explore treatment technologies that can effectively degrade and eradicate antibiotics from aquatic systems.
Micromotors are self-propelled microdevices that can convert external energy, such as magnetic fields, electric fields, light sources, ultrasound energy, or chemical reactions, into propulsion forces. Compared with traditional static decontamination techniques, autonomously moving micromotors have been demonstrated to effectively and efficiently degrade contaminants. Owing to their continuous swimming mobility, micromotors can move through polluted systems, transport, and release remediation agents over a long range, and realize effective mixing of target pollutants without additional stirring. Therefore, self-propelled micromotors offer a platform for effective environment remediation. For example, the use of micromotors has been demonstrated in heavy organic removal [9,10], metal removal [11,12], and bacterial disinfection [13,14]. However, several challenges remain before micromotor-based water-solving technologies can be applied in practical applications. A major challenge is the high cost associated with the use of noble metals for micromotor propulsion. In addition, scaling up using existing fabrication methods is challenging and costly, typically involving multistep procedures and costly equipment, thereby hindering the practical application of micromotors in various fields. Accordingly, current research has focused on the development of cost-effective noble metal-free micromotors. For instance, MnO2-, Ag-, Mg-, and Fe-based micromotors propelled by bubbles have been exploited [15,16,17]. Among these metals, Fe is a cost-effective potential metal candidate. However, only a few studies have reported on Fe-based micromotors, and thus, these micromotors remain to be further examined.
Graphitic carbon nitride (g-C3N4), a stable carbon nitride allotrope, has been widely examined for diverse environmental applications [18,19,20]. Unlike most photocatalytic materials, g-C3N4 has a metal-free structure, which offers several advantages such as chemical and thermal stability, biocompatibility, visible-light absorption, and low cost. Hence, g-C3N4-based micromotors have attracted increasing research attention [21,22,23]. However, owing to its polymeric nature, g-C3N4 faces some issues as a photocatalyst, such as photogenerated electron–hole recombination, low specific surface area, wide bandgap, and low quantum efficiency, which impede the efficient utilization of sunlight [24,25]. These nonoptimal properties hinder the use of g-C3N4 in environmental photocatalysis. To enhance the photocatalytic response of g-C3N4, different strategies have been adopted, including surface functionalization, morphology control, metal and nonmetal doping, and composite formation [26,27]. Among these methods, doping is the most extensively used method to optimize light absorption and promote the separation of photo-generated electron–hole pairs.
Iron is the most abundant transition metal in the crust of the earth. It displays high activity and is relatively inexpensive and environmentally friendly. Moreover, owing to the excellent characteristics of iron, Fe doping has been extensively investigated. The hexagonal ring structure of g-C3N4 features abundant lone-pair electrons, which are helpful for bonding with metal cations [28,29]. Importantly, the valence ion pairs in the Fe element play a major role in photocatalytic reactions and, consequently, enhance the degradation efficiency. For instance, Jin et al. reported the synthesis of an Fe-C3N4 composite via high-temperature calcination and its successful application in the degradation of methylene blue (a thiazine dye) via a Fenton-like process [30]. Shi and co-workers reported the preparation of an Fe-doped g-C3N4 photocatalyst by a typical thermal polymerization process for the photo-Fenton degradation of tetracycline (TC) under visible-light irradiation [31]. Guan and colleagues reported the synthesis of Fe-doped g-C3N4 materials via a one-step thermal condensation reaction and their application in the degradation of rhodamine B under visible-light irradiation; the Fe-doped g-C3N4 materials displayed enhanced activity and excellent stability when compared with bare g-C3N4 [32]. However, further improvements are required in the degradation performance of current Fe-doped g-C3N4 photocatalysts. We propose the use of self-propelled micromotors as a strategy to address this issue. The use of micromotors can enhance mass transfer in polluted solutions and, therefore, accelerate the chemical detoxification of pollutants, subsequently providing an advanced technology for removing harmful substances. Furthermore, the integration of Fenton-like catalysts (Fe3+) and photocatalysts (g-C3N4) to construct heterojunction micromotors enables the micromotors to act as a microstirrer for enhancing intermixing, as well as a mobile microcleaner for catalytic oxidations, owing to photo-Fenton-like catalytic activity. The synergistic effect between the intrinsic catalytic activity of the catalyst and the propulsion of micromotors is expected to considerably improve the degradation efficiency.
In the present study, kapok fiber (KF), an inexpensive and readily available biological material, was chosen as a template to explore a simple and feasible process to synthesize micromotors. The tubular micromotors based on Fe3+-doped g-C3N4 heterostructures deposited on KFs (denoted as g-C3N4-Fe@KF) were prepared via a dipping and calcination procedure. The combination of high photo-Fenton-like catalytic activity and the self-propelled motion behavior of g-C3N4-Fe@KF produced an efficient photocatalyst for the degradation of antibiotics. TC was chosen as a target pollutant to study the degradation ability of the synthesized g-C3N4-Fe@KF micromotors. The g-C3N4-Fe@KF micromotors functioned as self-propelled heterogeneous photo-Fenton catalysts in the presence of hydrogen peroxide (H2O2), which served as both a fuel to drive the micromotors and a reagent for the Fenton oxidation of TC, as depicted in Scheme 1.

2. Results

2.1. Fabrication and Characterization of g-C3N4-Fe@KF Micromotors

The combination of the tubular structure of KFs, a photocatalytic characteristic of g-C3N4, and active self-propulsion generated a photocatalysis platform for the efficient degradation of TC. The self-propelled tubular g-C3N4-Fe@KF micromotors were fabricated via a dipping and calcination procedure, as demonstrated in Figure 1a. The tubular structure of KFs, which were used as templates, was replicated in the g-C3N4-Fe@KF micromotors. Specifically, KFs were dipped in ferric nitrate nonahydrate and urea solutions under continuous stirring. Subsequent calcination of the treated KFs resulted in the simultaneous deposition of Fe3+ and g-C3N4 on the surface of KFs. Such a synthesis procedure is simple and versatile, suitable for high-yield production at a low cost. The final tubular structure consists of g-C3N4-Fe heterostructures on the outer and inner surfaces of KFs. The micromotors not only are mobile in an H2O2 solution but also can be applied as a heterogeneous catalyst for photo-Fenton processes and the degradation of typical organic pollutants.
The morphology of the micromotors was investigated by scanning electron microscopy (SEM). Figure 1b shows the outer surface of a bare KF, which was smooth, and no other substances were detected. Figure 1c shows that the tubular structure of the KFs was preserved in g-C3N4-Fe@KF, which was obtained after the dipping and calcination treatments. A layer of substance was deposited on both the inner and outer walls of the KFs (Figure 1d,f). Energy-dispersive X-ray (EDX) spectroscopy elemental mapping of the micromotors revealed the well-dispersed distribution of C, N, Fe, and O elements (Figure 1f–i). The Fe component accounted for 9.4% of the total weight of the micromotors (Table 1), corresponding to 2.23 mol% of Fe in the g-C3N4-Fe@KF micromotors. The small detected amount of O element can be ascribed to adsorbed water on the surface of the micromotors.
To investigate the crystal properties of bare KFs, g-C3N4@KF, and g-C3N4-Fe@KF micromotors, XRD was performed (Figure 2a). The XRD pattern of KFs only featured a broad band, which was attributed to the (002) plane in the carbon material. In contrast, two distinct diffraction peaks were detected in the XRD pattern of g-C3N4@KF, indexed to the (100) and (002) crystal planes of g-C3N4 (Joint Committee on Powder Diffraction Standards JCPDS Card No. 87–1526), owing to in-plane repeating s-triazine moieties and the layered accumulation of π-conjugate planes, respectively [33]. In the representative XRD pattern of g-C3N4-Fe@KF micromotors, no diffraction peak associated with Fe could be detected, indicating that Fe3+ ions were successfully integrated into the g-C3N4 framework, probably through the formation of chemical bonds with N atoms in the g-C3N4 framework. Moreover, the peak associated with the (100) crystal plane was no longer apparent, and the intensity of the peak associated with the (002) crystal plane was less intense and broader relative to that observed for gC3N4@KF. These observations indicate that Fe3+ is immobilized within the CN framework and forms Fe–N coordination bonds during calcination. These phenomena would disrupt the periodic arrangement of the units in g-C3N4, as consistent with a previous study [34].
Figure 2b shows the Fourier-transform infrared (FTIR) absorption spectra of g-C3N4@KF and g-C3N4-Fe@KF micromotors. The spectrum of g-C3N4@KF featured a peak at 810 cm−1, which was attributed to the s-triazine ring [35], and broad bands at 1200–1700 cm−1, ascribed to C=N, C–N in the triazine rings, and C–N outside the triazine rings [36]. The weak absorption bands at 3000–3700 cm−1 were attributed to –NH–, –NH2, and O–H, corresponding to adsorbed water on the surface [37]. The FTIR spectra of g-C3N4@KF and g-C3N4-Fe@KF were comparable, indicating that the incorporated Fe3+ did not alter the structure of g-C3N4. However, the intensity of the band related to the triazine ring decreased after Fe3+ doping. This decrease is likely due to the interaction between Fe and N, inducing the destruction of the s-triazine unit [38], which confirms that Fe is bonded to N atoms in the triazine ring.
The pore structures of g-C3N4@KF and g-C3N4-Fe@KF micromotors were evaluated by N2 adsorption–desorption. The adsorption–desorption curves of both micromotors were identified as Type IV isothermal curves (Figure 2c), which indicates the existence of a mesoporous structure. This mesoporous structure was also deduced from the pore size distributions of the micromotors in Figure 2d; a pore size distribution ranging between 2 and 160 nm and centered at 10 nm was observed. The specific surface area of g-C3N4-Fe@KF micromotors was calculated to be 26.2 m2 g−1, which was lower than that of g-C3N4@KF (40.4 m2 g−1). The lower surface area observed indicates that the incorporated Fe3+ ions disrupt the layered structure of g-C3N4 together with the filling effect.
The element composition and chemical state of the g-C3N4-Fe@KF micromotors were also investigated by X-ray photoelectron spectroscopy (XPS). The XPS survey spectrum in Figure 3a shows peaks corresponding to C, N, Fe, and O elements. The N 1s high-resolution spectrum of the micromotors could be deconvoluted into four main peaks at 398.49, 399.14, 400.14, and 401.09 eV, respectively (Figure 3b). The peak at 398.49 eV was attributed to sp2 N (C–N=C) in the triazine units, and the peaks at 400.14 and 401.09 eV were ascribed to tertiary nitrogen N–(C)3 and amino functional groups (C–N–H) [39], respectively. The peak at 399.14 eV could be attributed to Fe–N formed upon chelation of the Fe3+ center with the N atom [40]. The XPS spectrum in Figure 3c revealed two Fe 2p peaks at 724.06 and 710.84 eV, which were attributed to Fe 2p1/2 and Fe 2p3/2, further confirming the incorporation of the Fe element in the CN structure. In addition, the peak observed at the binding energy of 710.84 eV is within the binding energy range of the Fe3+ valence state and comparable to that of Fe3+ in porphyrin (710.5 eV) [41], wherein Fe3+ binds to N via coordination. The above results confirm that Fe exists in the form of Fe3+, which interposes between the layers of g-C3N4 and forms Fe–N coordination bonds with N atoms.
The linear optical absorption properties of g-C3N4@KF and g-C3N4-Fe@KF micromotors were characterized by UV–visible diffuse reflectance spectroscopy (UV–vis DRS). The UV–vis spectra of the micromotors in Figure 4a featured a number of noncharacteristic peaks below 400 nm, which could be due to switching the light source during the test, as well as absorption bands at approximately 480 and 550 nm. The integration of Fe within g-C3N4 caused a red shift in the absorption edge of the g-C3N4-Fe@KF micromotors. Band-gap determination from the UV–vis DRS spectra and Tauc plots (Figure 4b) was performed using the Tauc relation [42] in Equation (1):
αhυ = A(hυ − Eg)1/n,
The band gaps of g-C3N4@KF and g-C3N4-Fe@KF micromotors were determined as 2.10 and 1.91 eV, respectively, by the extrapolation of linear regions to the abscissa. The narrowed band gap of g-C3N4-Fe@KF suggests that Fe3+ doping alters the electronic structure of g-C3N4 and that more photogenerated carriers can be excited under visible light, potentially resulting in an increased rate of photogenerated charge carriers and an enhanced photocatalytic activity.

2.2. Motion Behavior of g-C3N4-Fe@KF Micromotors

The motion behavior of the g-C3N4-Fe@KF micromotors was investigated by optical microscopy. Time-lapse images (Video S1) of the self-propelled movement of a micromotor captured over 9 s at 3 s intervals in an aqueous solution of 15 wt% H2O2 and 0.1 wt% sodium lauryl sulfate (SDS) are presented in Figure 5a. A tail of bubbles generated and released from both sides of the micromotor was observed, as it moved through the solution. The trail of bubbles illustrates the trajectory of the g-C3N4-Fe@KF micromotor. The generation of bubbles within the micromotors by the Fenton reaction between Fe3+ ions and H2O2 is described in Equation (2). In addition, owing to the band gap of the micromotors (1.91 eV) being lower than visible-light energy, visible light in the ambient environment can induce the generation of photon-induced carriers, see Equation (3). Subsequently, on the surface of the micromotors, electron–hole pairs can also react with H2O2 to generate O2, as described in Equations (4) and (5). To confirm the propulsion mechanism of g-C3N4-Fe@KF micromotors, the motion behavior of Fe@KF micromotors was examined under similar conditions (Video S2). The amount of generated bubbles was significantly lower, demonstrating the promoting effect of photo-induced carriers in the generation of bubbles. The proposed propulsion mechanism of g-C3N4-Fe@KF micromotors is depicted in Figure 5c. Equations (2)–(5) are as follows:
H2O2 + 2Fe3+ → 2Fe2+ + O2 + 2H+,
g-C3N4 + hυ → g-C3N4 (e− + h+),
H2O2 + 2h+ → O2 + 2H+,
H2O2 +2e− → OH· + OH−.
The g-C3N4-Fe@KF micromotors displayed two types of trajectories/motion, namely curved and self-rotating motions, as shown in Figure 5a,b (Videos S1 and S3, respectively). These different trajectories mainly derive from the differing lengths of the micromotors. Owing to the lower nucleation energy of the inner surface bubble embryo compared with that on the outer surface, the micromotor bubbles are mainly generated from the inside of the tube and are released from both ends of the tube [43,44]. For longer micromotors, the bubbles need to travel a longer distance before being released from the ends of the tube. In addition, the nonhomogeneously deposited Fe and g-C3N4 on the inner surface of KFs result in a difference in the number of bubbles released at both ends. Generally, the tube end with more bubbles will generate a greater driving force, leading to the micromotors moving forward with fewer bubbles and presenting a curved motion. In contrast, for shorter micromotors, the distance covered by the bubbles before reaching the ends of the tube structure is shorter, and hence, the bubbles are less affected by the uneven load distribution. Therefore, the number of bubbles released from both ends of the tube is similar, resulting in a self-rotating motion. Generally, the speed of the bubble-propelled micromotor positively correlates to the concentration of H2O2. Thus, the velocity of the g-C3N4-Fe@KF micromotors can be controlled by altering the concentration of H2O2. As observed in Figure 5d, g-C3N4-Fe@KF micromotors displayed increased speeds with increasing fuel concentrations. The results reveal that the mobility of the g-C3N4-Fe@KF micromotors depends on the fuel concentration and the length of the g-C3N4-Fe@KF micromotors. The motion behaviors displayed by g-C3N4-Fe@KF micromotors are expected to facilitate the degradation of TC.

2.3. Photocatalytic Activity of g-C3N4-Fe@KF Micromotors

To evaluate the performance of the synthesized g-C3N4-Fe@KF micromotors toward the photocatalytic degradation of organic pollutants, TC was chosen as the target contaminant. As g-C3N4 has been typically employed as a visible-light photocatalyst, it is believed that the prepared micromotors have the potential to enhance the degradation efficiency of g-C3N4 via a photo-Fenton reaction [45,46]. A series of control experiments to study the role of each component in the degradation of TC was performed. Figure 6a shows the degradation of TC by the micromotors in the presence/absence of H2O2 and/or visible-light irradiation. It was previously reported that the addition of surfactants to a H2O2 solution could minimize the surface tension and stabilize bubbles during micromotor movement [47]. However, considering that surfactants can be a source of additional pollution, in the present work, the degradation of TC was conducted in the absence of surfactants.
As shown in Figure 6a, only 4.2% of the TC degraded after 70 min in the presence of 0.3% H2O2 + Xenon lamp irradiation, indicating the limited degradation activity of H2O2 under Xenon lamp irradiation. Likewise, the removal of TC by hydrogen peroxide alone or Xenon lamp irradiation alone was minimal. In contrast, in the presence of g-C3N4@KF + visible-light irradiation, TC degradation efficiency increased to 28.82% after 70 min, which is attributed to the photocatalytic activity of g-C3N4. TC degradation efficiency increased further to 83.58% after 70 min in the presence of g-C3N4-Fe@KF micromotors + H2O2, which indicates that the presence of free radicals generated through the Fenton reaction can efficiently degrade TC. To achieve a higher degradation efficiency, the effect of the photo-Fenton process by combining light irradiation and the Fenton process induced by H2O2 and Fe was examined. The degradation efficiency of TC reached 96.79% after 70 min of reaction in the presence of g-C3N4-Fe@KF micromotors + H2O2 + light irradiation. The results demonstrate that light irradiation promotes the generation of reactive oxygen species and microbubbles and self-propulsion, which improves mass transfer within the photocatalyst system and enhances the degradation efficiency.
Each component of the micromotors likely plays a distinct role in the photo-Fenton process. In the presence of H2O2, the Fe2+ doped in the g-C3N4-Fe@KF micromotors is converted into Fe3+ through the Fenton process [48]. Concurrently, strong oxidizing hydroxyl radicals (OH·) are generated and can achieve the efficient decomposition of TC. The degradation of TC by the Fenton reaction is described by Equation (6):
Fe2+ + H2O2 → Fe3+ + ·HO + HO−,
The deposited g-C3N4 can induce the production of electron–hole pairs and active free radicals under Xenon lamp irradiation, thus accelerating the degradation of TC. The related degradation reactions are displayed in Equations (7)–(9):
g-C3N4-Fe@KF + hυ → g-C3N4-Fe@KF (e− + h+),
e− + O2 → ·O2−,
H2O2 + e− → HO· + HO−,
The generated ·HO and ·O2− can oxidize TC molecules through an oxidative decomposition process, as shown in Equation (10):
·HO/·O2− + TC → degradation products,
The proposed mechanism for the photocatalytic and Fenton degradation of TC over g-C3N4-Fe@KF micromotors is depicted in Figure 6b. In addition, charge transfer between Fe and g-C3N4 can promote the degradation process. Figure 6c shows that the intensity of the UV absorption peak of TC significantly decreased when both H2O2 and visible-light irradiation were present in the photocatalytic system. This reduction is mainly attributed to the presence of more reactive oxygen species generated via the photo-Fenton reaction and efficient self-propulsion. The decrease in the intensity of the H2O2 characteristic peak at 230–240 nm observed in Figure 6d indicates that H2O2 is consumed during the degradation process. The degradation of TC in the presence of sodium chloride (NaCl) at different concentrations was also evaluated and shown in Figure 6e. The micromotors displayed higher degradation efficiency rates in a salt solution than in deionized water. This result may be due to the reaction of Cl− (in a certain concentration range) and ·OH to form Cl· and ·Cl2− [49], which have strong oxidation capacity. Furthermore, varying the concentration of the NaCl solution had a negligible effect on the TC degradation rate of the micromotors, which is conducive to the application of micro-nanomotors in complex environments. Furthermore, the TC degradation efficiency displayed by g-C3N4-Fe@KF micromotors was higher than that displayed by static g-C3N4-Fe and comparable to those of the reported micromotors, as presented in Table 2, indicating the potential of g-C3N4-Fe@KF micromotors in the treatment of organic pollutants.
To examine the reusability of the g-C3N4-Fe@KF micromotors, degradation cycle experiments were conducted, see Figure 6f. The same conditions were employed in all degradation cycles. Relative to the TC degradation efficiency (96.79%) obtained in the first cycle, that of the second cycle decreased slightly to 85.88%. A further decrease to ~65% was observed after the third cycle. These findings demonstrate that the heterostructured micromotors show good reusability in the photo-Fenton-like degradation of antibiotics.

3. Materials and Methods

3.1. Materials

All chemicals were analytic grade reagents and used as is without further purification. Ferric nitrate nonahydrate, NaCl, and SDS were obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). Ethanol was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Urea and glacial acetic acid were purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd. (Tianjin, China). H2O2 solution was purchased from Silon Science Co., Ltd. (Shenzhen, China). KFs were purchased from Shanghai Panda Machinery (Group) Co., Ltd. (Shanghai, China). Deionized water was obtained using a Milli-Q Direct 8 system (Beijing, China).

3.2. Preparation of g-C3N4-Fe@KF Micromotors

The g-C3N4-Fe@KF micromotors were prepared via a dipping and calcination process. First, KFs were pretreated as follows. KFs (300 mg) were cut, using scissors, into pieces of lengths of 1–3 mm and then mixed with 20 mL of acetic acid and absolute ethanol mixed solution (1:1 v/v) in a beaker. The beaker was then placed in an oil bath set at 80 °C, and the mixture was stirred for 0.5 h. The resulting solution was filtered with deionized water and anhydrous ethanol twice. Second, in a separate recipient, 7 g of urea were added to 10 mL of deionized water and stirred until a clear solution was obtained. The pretreated KFs were added to the above solution, and the vessel containing the obtained mixture was placed in an oil bath at 80 °C and stirred magnetically for 10 min. Subsequently, 0.7 g of ferric nitrate pentahydrate were added to the mixture, which was stirred for another 20 min to achieve a precursor solution. Third, the precursor solution was placed in an ultra-low temperature refrigerator to solidify before being transferred to a vacuum freeze-drying box. The block precursor was removed after lyophilization for 2 days and the precursor was ground into a powder with a mortar and pestle. Fourth, the powder was transferred to a crucible and calcined in a muffle furnace at 500 °C for 2 h (heating rate of 10 °C min−1). The g-C3N4-Fe@KF micromotors were obtained after cooling. For comparison purposes, KF-templated g-C3N4 and KF-templated Fe (i.e., g-C3N4@KF and Fe@KF) were also synthesized.

3.3. Characterization of g-C3N4-Fe@KF Micromotors

The morphology, elemental distribution, crystal characteristics, composition, texture, and linear optical properties of the g-C3N4-Fe@KF micromotors were characterized using field-emission SEM (FEI Nova Navo 450, FEI Co., Hilsboro, OR, USA), EDX spectroscopy(Oxford X-MAN, Oxford, UK), XRD (Bruker D8-Advance, Billerica, MA, USA), FTIR spectroscopy (Nicolet 6700, Thermo Fisher Scientific Instruments, Waltham, MA, USA), XPS (Escalab 250, Thermo Fisher Scientific Co., USA), a surface area and porosimetry analyzer (3Flex, Micromeritics Instrument Corporation, Norcross, GA, USA), and UV–vis DRS (UV- 2600i, Shimadzu, Japan). The details can be found in previous work [21].

3.4. Motion Characterization of g-C3N4-Fe@KF Micromotors

A Nikon ECLIPSE E100 microscope (Nikon Instruments, Tokyo, Japan) equipped with a 10× eyepiece, and 4× and 10× objective lenses were used to investigate the immersion and motion of the g-C3N4-Fe@KF micromotors in an aqueous solution containing 0, 3, 9, and 15 wt% H2O2 and 1 wt% SDS. A digital camera (PSC601-10C, OPLENIC, Zhejiang, China) was used to observe the microstructure of the samples, and the movement of the g-C3N4-Fe@KF micromotors was assessed by a video capture program (OPLENIC×64).

3.5. Degradation of TC

The photocatalytic degradation of TC aqueous solutions was conducted in a glass vessel. In a typical photocatalytic experiment, a 300 W Xenon lamp (PLS-SXE300+, Beijing Perfectlight Technology Co., Ltd., Beijing, China) was used as the main light source. The optical power density was adjusted to 136 mW cm−1 and the spectrum of the Xenon lamp ranged from 300 to 1100 nm. A TC solution (50 mL, 50 mg L−1, pH 7) containing the micromotors (2 mg) was placed in a vessel. Before degradation, the mixture solution was allowed to stand in the dark for 30 min and under stirring to obtain adsorption–desorption equilibrium. Then, 0.3% H2O2 was added to stimulate the reaction. Within specified time intervals, 1 mL of solution was taken out with a syringe, and the drawing solution was immediately characterized to determine the concentration of TC (λ = 357 nm) by UV–vis spectroscopy. The concentration of TC was calculated for each test using the standard absorbance curve to obtain a degradation curve of TC concentration over time.
Degradation experiments were also conducted in the presence of salt using the same procedure described above, except that TC solutions were prepared with NaCl at different concentrations (0.05, 0.1, 0.2, and 0.3 mol L−1).

3.6. Reusability Study

After each cycle experiment, the micromotors were recovered by centrifugation at 8000 rpm, washed with deionized water twice and then with anhydrous ethanol, and dried in air. The micromotors were then dispersed in 0.3 mL of deionized water and added to the TC solution for the following degradation cycle using the same conditions. The residual TC concentration was measured after 30 min.

4. Conclusions

In this study, we demonstrated the application of tubular Fe-incorporated g-C3N4-based micromotors in the photo-Fenton degradation of TC in aqueous solutions. The micromotors were synthesized through a dipping and calcination procedure using KFs as biotemplates. This strategy could potentially be extended to the construction of diverse micromotors with various applications. The XRD, FTIR, XPS, and UV–vis spectroscopy results collectively confirmed the incorporation of Fe3+ into the g-C3N4 photocatalyst. The synthesized g-C3N4-Fe@KF micromotors underwent rapid movement when being propelled by oxygen bubbles generated in an aqueous H2O2 solution through a photo-assisted Fenton reaction. The motion behaviors of the g-C3N4-Fe@KF micromotors were influenced by the concentration of H2O2 and the length of the micromotors. The g-C3N4-Fe@KF micromotors were successfully applied to the degradation of antibiotics via the photo-Fenton reaction. The improved photo-Fenton degradation efficiency of the g-C3N4-Fe@KF micromotors was attributed to the synergistic effects of the doped Fe3+ and g-C3N4 under visible-light irradiation and the self-propulsion of the micromotors. In addition, the micromotors exhibited good reusability, realizing multiple cycles of degradation. The use of natural raw template materials and inexpensive chemical reagents, together with the facile fabrication method, self-propulsion behavior, high photocatalytic performance, and good recyclability, make the Fe3+-doped g-C3N4 micromotors promising for potential application in practical environmental remediation. The proposed strategy of combining self-propulsion behavior, photo-Fenton reaction, and biotemplate KF micromotors for the development of heterogeneous photocatalysts provides avenues for addressing environmental problems.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal14090579/s1: Video S1: Self-propelled movement of g-C3N4-Fe@KF micromotors (length 75 μm) in an aqueous solution of 15 wt% H2O2 and 0.1 wt% SDS; Video S2: Motion behavior of Fe@KF micromotors in an aqueous solution of 15 wt% H2O2 and 0.1 wt% SDS; Video S3: Motion behavior of g-C3N4-Fe@KF micromotors (length 38 μm) in an aqueous solution of 15 wt% H2O2 and 0.1 wt% SDS.

Author Contributions

Conceptualization, Q.G.; methodology, J.Z.; investigation, J.W.; data curation, Y.W. and S.C. (Shikun Chen); writing—original draft preparation, Q.G.; writing—review and editing, S.C. (Shuguang Cai); visualization, X.X.; supervision, C.Z.; project administration, C.Z.; funding acquisition, C.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Fujian Province University Industry University Research Joint Innovation Technology Program Project (Grant No. 2023H6035) and the Fujian Province Science and Technology Project (Grant No. 2021I00211).

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article and its Supplementary Materials.

Conflicts of Interest

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 work.

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Catalysts 14 00579 sch001
Scheme 1. Diagram showing the application of self-propelled g-C3N4-Fe@KF micromotors for photo-Fenton degradation of TC under visible-light irradiation.
Scheme 1. Diagram showing the application of self-propelled g-C3N4-Fe@KF micromotors for photo-Fenton degradation of TC under visible-light irradiation.
Catalysts 14 00579 sch001
Catalysts 14 00579 g001
Figure 1. (a) Bio-templated fabrication of g-C3N4-Fe@KF micromotors through a facile dipping and calcination protocol; (b) SEM image of a bare KF template; (c–e) SEM images of g-C3N4-Fe@KF micromotor, (f–i) EDX spectroscopy elemental mapping of g-C3N4-Fe@KF micromotor.
Figure 1. (a) Bio-templated fabrication of g-C3N4-Fe@KF micromotors through a facile dipping and calcination protocol; (b) SEM image of a bare KF template; (c–e) SEM images of g-C3N4-Fe@KF micromotor, (f–i) EDX spectroscopy elemental mapping of g-C3N4-Fe@KF micromotor.
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Catalysts 14 00579 g002
Figure 2. (a) XRD patterns of KFs, g-C3N4@KF, and g-C3N4-Fe@KF micromotors; (b) FTIR absorption spectra of g-C3N4@KF and g-C3N4-Fe@KF micromotors; (c) N2 adsorption–desorption isotherms of g-C3N4@KF and g-C3N4-Fe@KF micromotors; (d) pore size distributions of g-C3N4@KF and g-C3N4-Fe@KF micromotors.
Figure 2. (a) XRD patterns of KFs, g-C3N4@KF, and g-C3N4-Fe@KF micromotors; (b) FTIR absorption spectra of g-C3N4@KF and g-C3N4-Fe@KF micromotors; (c) N2 adsorption–desorption isotherms of g-C3N4@KF and g-C3N4-Fe@KF micromotors; (d) pore size distributions of g-C3N4@KF and g-C3N4-Fe@KF micromotors.
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Catalysts 14 00579 g003
Figure 3. (a) XPS survey spectrum of g-C3N4-Fe@KF micromotors; (b) N 1s high-resolution spectrum of g-C3N4-Fe@KF micromotors; (c) Fe 2p high-resolution spectrum of g-C3N4-Fe@KF micromotors; (d) O 1s high-resolution spectrum of g-C3N4-Fe@KF micromotors.
Figure 3. (a) XPS survey spectrum of g-C3N4-Fe@KF micromotors; (b) N 1s high-resolution spectrum of g-C3N4-Fe@KF micromotors; (c) Fe 2p high-resolution spectrum of g-C3N4-Fe@KF micromotors; (d) O 1s high-resolution spectrum of g-C3N4-Fe@KF micromotors.
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Figure 4. (a) UV–vis diffuse absorbance spectra of g-C3N4@KF and g-C3N4-Fe@KF micromotors; (b) Tauc plots of g-C3N4@KF and g-C3N4-Fe@KF micromotors.
Figure 4. (a) UV–vis diffuse absorbance spectra of g-C3N4@KF and g-C3N4-Fe@KF micromotors; (b) Tauc plots of g-C3N4@KF and g-C3N4-Fe@KF micromotors.
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Catalysts 14 00579 g005
Figure 5. (a,b) Propulsion behaviors of g-C3N4-Fe@KF micromotors with different lengths of 75 μm (a) and 38 μm (b) in the presence of 15 wt% H2O2. Scale bars, 100 μm. The time-lapse images are taken from Videos S1 and S3, respectively. (c) Proposed propulsion mechanism of g-C3N4-Fe@KF micromotors. (d) Influence of fuel (H2O2) concentration on the speed of the g-C3N4-Fe@KF micromotors.
Figure 5. (a,b) Propulsion behaviors of g-C3N4-Fe@KF micromotors with different lengths of 75 μm (a) and 38 μm (b) in the presence of 15 wt% H2O2. Scale bars, 100 μm. The time-lapse images are taken from Videos S1 and S3, respectively. (c) Proposed propulsion mechanism of g-C3N4-Fe@KF micromotors. (d) Influence of fuel (H2O2) concentration on the speed of the g-C3N4-Fe@KF micromotors.
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Figure 6. (a) Degradation of TC under various conditions; (b) schematic diagram of the proposed mechanism of the photo-Fenton degradation of TC by self-propelled g-C3N4-Fe@KF micromotors; (c) absorbance spectra of TC after degradation for 70 min in the presence of g-C3N4-Fe@KF micromotors; (d) absorbance spectra of H2O2 measured during TC degradation for 70 min in the presence of g-C3N4-Fe@KF micromotors; (e) degradation of TC in the presence of NaCl at different concentrations; (f) reusability of g-C3N4-Fe@KF micromotors during repeated degradation of TC. The pH of all the solutions in the experiments was 7, and the optical power density was adjusted to 136 mW cm−1.
Figure 6. (a) Degradation of TC under various conditions; (b) schematic diagram of the proposed mechanism of the photo-Fenton degradation of TC by self-propelled g-C3N4-Fe@KF micromotors; (c) absorbance spectra of TC after degradation for 70 min in the presence of g-C3N4-Fe@KF micromotors; (d) absorbance spectra of H2O2 measured during TC degradation for 70 min in the presence of g-C3N4-Fe@KF micromotors; (e) degradation of TC in the presence of NaCl at different concentrations; (f) reusability of g-C3N4-Fe@KF micromotors during repeated degradation of TC. The pH of all the solutions in the experiments was 7, and the optical power density was adjusted to 136 mW cm−1.
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Table 1. Weight percentage of elements in representative g-C3N4-Fe@KF micromotors.
Table 1. Weight percentage of elements in representative g-C3N4-Fe@KF micromotors.
ElementElement Proportion (wt%)
C49.8
N34.1
Fe9.4
O6.7
Table 2. Comparison of the organic pollutant degradation efficiency of different materials.
Table 2. Comparison of the organic pollutant degradation efficiency of different materials.
MaterialPollutantLight ConditionsDegradation Time and ConditionsDegradation RateReference
rGO/ZnO/BiOI/Co-Pi/Pt micromotors (40 mg/L)R6G (10 mg/L)Blue laser (435 nm)60 min, H2O294%[50]
Fe0.11Bi0.89OBr/Fe3O4/Mn3O4 micromotor (10 mg/L)MB (10 mg/L)Sunlight50 min, H2O298%[51]
α-Fe2O3/ZnFe2O/Mn2O3 micromotors (50 mg)MB (5 mg/L, 50 mL)Sunlight45 min, H2O295%[52]
PASP/Fe2O3-MnO2 micromotor (160 mg/L)TC (30 mg/L)-50 min, H2O290%[53]
BSA-NiCo2O4@MnO2/C micromotors (50 mg)TCH (10 mg/L, 50 mL)-180 min, H2O297.6%[54]
Fe3O4/CeO2/g-C3N4 composites (50 mg)TCH (50 mg/L)Visible light120 min, H2O296.63%[55]
Porous P, Fe-doped g-C3N4 nanostructure material (30 mg)TCH (20 mg/L)Visible light60 min, H2O298%[56]
α-Fe2O3@ g-C3N4 catalyst (50 mg)TC (40 mg/L, 100 mL)Visible light100 min, H2O292%[57].
Fe(II)-doped g-C3N4 catalyst (50 mg)MB (50 mg/L, 100 mL)Visible light90 min, H2O290% (5%-Fe/CN)
100% (10%-Fe/CN)		[58]
g-C3N4/Fe3O4 @MIL-100(Fe) (0.67 g/L)CIP (200 mg/L)Visible light150 min, H2O294.7%[59]
g-C3N4-Fe@KF micromotors (2 mg)TC (50 mg/L, 50 mL)Xenon lamp irradiation70 min, H2O296.79%Current work
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MDPI and ACS Style

Gan, Q.; Zhang, J.; Wang, J.; Wei, Y.; Chen, S.; Cai, S.; Xiao, X.; Zheng, C. BioTemplated Fe3+-Doped g-C3N4 Heterojunction Micromotors for the Degradation of Tetracycline through the Photo-Fenton Reaction. Catalysts 2024, 14, 579. https://doi.org/10.3390/catal14090579

AMA Style

Gan Q, Zhang J, Wang J, Wei Y, Chen S, Cai S, Xiao X, Zheng C. BioTemplated Fe3+-Doped g-C3N4 Heterojunction Micromotors for the Degradation of Tetracycline through the Photo-Fenton Reaction. Catalysts. 2024; 14(9):579. https://doi.org/10.3390/catal14090579

Chicago/Turabian Style

Gan, Qingbao, Jianwei Zhang, Jinglin Wang, Yuntian Wei, Shikun Chen, Shuguang Cai, Xueqing Xiao, and Chan Zheng. 2024. "BioTemplated Fe3+-Doped g-C3N4 Heterojunction Micromotors for the Degradation of Tetracycline through the Photo-Fenton Reaction" Catalysts 14, no. 9: 579. https://doi.org/10.3390/catal14090579

APA Style

Gan, Q., Zhang, J., Wang, J., Wei, Y., Chen, S., Cai, S., Xiao, X., & Zheng, C. (2024). BioTemplated Fe3+-Doped g-C3N4 Heterojunction Micromotors for the Degradation of Tetracycline through the Photo-Fenton Reaction. Catalysts, 14(9), 579. https://doi.org/10.3390/catal14090579

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