The 3D porous structure of graphene functional materials possesses characteristics such as high porosity and large specific surface area, enabling dye molecules to easily enter and diffuse within the 3D network, thereby emerging as a promising solution for treating dye wastewater pollution. In this study, graphene@MXene composite hydrogels with a 3D porous structure were prepared using the hydrothermal reduction method with MXene and graphene oxide as raw materials, and their adsorption performance towards MB was investigated. The morphology and structure of the graphene@MXene composite hydrogels were examined through SEM, EDX,FT-IR, and BET analysis techniques. The graphene@MXene composite hydrogels doped with 20 wt% MXene exhibited the best removal efficiency for MB, showcasing excellent adsorption performance following the introduction of MXene. It is noteworthy, however, that as the proportion of MXene increased, there was a noticeable decrease in the adsorption performance of the composite hydrogels towards MB. During the preparation of the composite hydrogel, the introduction of Ca2+ as a cross-linking agent not only reduced the preparation time but also adjusted the specific surface area and pore size distribution of the hydrogel. As a result, there was an approximately 30% increase in the MB adsorption rate. The adsorption kinetics results revealed that the adsorption of MB on the graphene@MXene composite hydrogels, both pre- and post-introduction of calcium ions, conformed to a pseudo-second-order kinetic model. This observation suggests that the adsorption process is influenced by various mechanisms such as external diffusion, surface adsorption, and internal diffusion. Our study further revealed that the microwave irradiation spectra of 3D graphene@MXene composite aerogels changed before and after MB adsorption, exhibiting characteristic features of graphene or MXene. This finding confirms the adsorption of MB, suggesting a potential new method for its detection. This research demonstrates promising applications in both dye wastewater treatment and the detection of MB.
Research Article
Facile preparation of 3D porous graphene-based composite hydrogels doped with different ratios of MXene for MB adsorption
https://doi.org/10.21203/rs.3.rs-4702283/v1
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graphene@MXene
composite hydrogels
3D porous structure
dyes adsorption
calcium ion
tunable microwave irradiation spectra
With increasing industrialization, water pollution caused by dyes has become a serious problem, threatening the health of humans, animals and plants [1–3]. Methylene blue (MB) is a typical cationic dyestuff in dyestuffs, which is widely used in textile, leather and medical industries. Due to the high coloration, stable structure and difficult degradation, dyestuff wastewater has become one of the most difficult industrial wastewaters to treat[4]. Various physical, chemical and biological methods have been applied to the treatment of synthetic dye-containing wastewater over the past decades [5–7]. Among these methods, adsorption has become the most desirable method for the removal of synthetic dyes from wastewater due to the advantages of low cost, simplicity of operation, and reusability [8]. Many materials, such as active carbon, kaolinite, natural materials polymer, etc, have also been used as adsorbents to study the adsorption of dyes [9–14].
Due to large specific surface area, surface hydrophobic π-π interactions, good hydrophilicity, and high negative charge density, graphene and its derivatives has been proven to be a good adsorbent material for dyes, and can efficiently adsorb dye in solution through electrostatic and π-π interactions [15–18]. In practical applications, graphene has the disadvantage of being difficult to recycle when used directly as an adsorbent, resulting in serious secondary pollution. Preparing it into a carbon gel with an interconnected three-dimensional (3D) skeleton can effectively solve this problem. Shimizu et al. prepared CNT-graphene aerogels by hydrothermal method, which exhibited good adsorption capacity for MB (96.1mg/g), yet low capacity for anionic dyes (MO 51.6mg/g) [19]. Han et al. synthesized RGO-based aerogels by bacterial reduction and chemical cross-linking, and the adsorption capacity of the obtained aerogels for MB and CR was 135.17 mg/g and 134.24 mg/g, respectively [20], while their preparation process was complicated.
Recently, the emergence of MXene (layered transition metal carbons and nitrides) has attracted great interest in the field of materials science [21, 22]. Due to their good electrical conductivity, hydrophilicity, large specific surface area and high number of surface active sites, MXenes (e.g., Ti3C2Tx, V2CTx, etc.) have also been shown to be good dye adsorbents for the effective removal of MB from solution via electrostatic and hydrogen bonding interactions [23–26]. Previous studies have shown that the introduction of MXene into graphene and its derivatives allows the preparation of 3D graphene@MXene composites [27–29]. However, the doping ratio of MXene on the MB adsorption properties of the prepared 3D graphene@MXene composites needs to be further explored.
In this study, we prepared graphene @MXene hydrogels by hydrothermal method and investigated the adsorption characteristics of graphene @MXene hydrogels with different doping ratios for MB. The results showed that the adsorption characteristics of MB by the prepared graphene@MXene composite hydrogels deteriorated with an increase in MXene content. Notably, the graphene@MXene composite hydrogels prepared with 20 wt% MXene doping exhibited superior MB removal efficiency. Moreover, the incorporation of Ca2+ as a cross-linking agent during the preparation of graphene@MXene composite hydrogels not only reduced the preparation time by 2/3 but also adjusted the specific surface area and pore size distribution of the composite hydrogels, thereby enhancing the MB removal rate. Kinetic tests revealed that the adsorption of MB by the graphene@MXene hydrogels followed a pseudo-second-order kinetic model, with the adsorption process being influenced by external diffusion, surface adsorption, and internal diffusion. Additionally, our study demonstrated that the microwave irradiation spectra of graphene@MXene aerogel changed before and after MB adsorption, showcasing the distinct properties of graphene and MXene, respectively. This observation not only confirmed the adsorption of MB but also introduced a novel method for MB detection. The findings of this study hold promise for applications in dye wastewater treatment and MB detection.
2.1 Materials
Graphene oxide (GO, 2mg/mL) and MXene (Ti3C2, 5 mg/mL) dispersions were respectively obtained from Angstar New Carbon Materials Changzhou Co., Ltd and Suzhou Beike Nano Technology Co., Ltd. Methylene blue (MB, C16 H18 ClN3S•3H2O,90%), and calcium chloride (CaCl2) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Ultrapure water, which was obtained by an ultrapure water machine (ULUPURE), was used to prepare MB solutions.
2.2 Preparation of graphene@MXene hydrogels
The preparation of graphene@MXene composite hydrogels was carried out in the following steps: firstly, according to the predetermined doping ratio, the GO solution and the MXene dispersion were removed separately and poured into a beaker. Then, a magnetic stirrer (1300 r/min for 10 min) was used to mix the two. Subsequently, a polyvinyl chloride film was used to seal the beaker, and the beaker was placed in a constant temperature drying oven at 100 ℃ for 15 h to obtain the graphene@MXene composite hydrogels. Finally, the resulting composite hydrogels were removed and washed three times in ultrapure water for subsequent studies.
2.3Preparation of calcium ion-modulated graphene@MXene composite hydrogels
Firstly, 0.2 g of calcium chloride was added to the mixed solution of GO and MXene. Then, it was stirred by a magnetic stirrer (1300 r/min, 10 min) to mix it thoroughly. Subsequently, the beaker was sealed with polyvinyl chloride film and placed in a constant temperature drying oven at 100 ℃ for 5 hours to obtain calcium ion-regulated graphene@MXene composite hydrogels. Finally, the prepared composite hydrogels was taken out and washed three times in ultrapure water for subsequent research.
2.4 Microwave irradiation of graphene@MXene composite aerogel
The graphene@MXene composite hydrogels before and after adsorption of MB were put into ultrapure water for cleaning, and then the graphene@MXene aerogels were obtained by freeze-drying. Subsequently, the prepared aerogels were placed in a household microwave oven, and the luminescence spectra of the graphene@MXene aerogel were collected according to the method reported in Ref [30].
2.5 Characterisation
Scanning electron microscopy (SEM, SU 8000) and energy dispersive X-ray spectrometry (EDX, LXRF Systems) were used to study the morphology of the samples and the distribution of elements. Nitrogen adsorption/desorption isotherms at 77 K were determined using a JW-BK 200 C. Specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method and pore size distributions were obtained using the BJH model. A spectrometer (QE 65 Pro, Ocean Optics) was used to measure and record the spectra of the composite aerogels. A UV-Vis spectrophotometer (TU-1810PC) was used to determine the residual concentrations of MB.
2.6 Dye adsorption studies
The removal of methyl bromide from aqueous solutions is measured by testing the adsorption characteristics of the sample. 30mg of sample was added to 30 ml of 100 mg/L MB solution and placed in a water bath shaker at a temperature of 25°C. After stabilization of adsorption, the real-time concentration of MB was measured by UV-Vis spectrophotometer using the absorbance at the MB characteristic absorption wavelength (665 nm). The adsorption performance of graphene@MXene hydrogels was further investigated by varying different experimental conditions including contact time (0-84h). The results are the mean and standard deviation of 3 ~ 5 parallel experiments. The removal and adsorption capacity of MB were calculated using equations (1) and (2), respectively.
where C0 and Ct are the initial concentrations and the concentrations of MB at time t, m is the mass (g) of adsorbent, V is the volume (L) of MB and Qt represents the adsorption capacity of MB at t h.
3.1 Preparation of graphene@MXene composite hydrogels
Figure 1(a) schematically shows the graphene@MXene composite hydrogels. First, GO solution (2 mg/ml) and MXene solution (5 mg/mL) were poured into a beaker according to a certain ratio and the solutions were mixed homogeneously. Then, the beaker was left to stand for 15 h in a constant-temperature drying oven at 100 ℃ after sealing the beaker with a polyvinyl chloride film. Finally, the obtained graphene@MXene composite hydrogels were cleaned with ultrapure water for three times, as shown in Fig. 1 (b). The graphene@MXene aerogel obtained after the freeze-drying method is shown in Fig. 1(c). It should be noted that when only GO or MXene was available, the molded hydrogels could not be obtained even if the heating time was further extended (Figure S1). According to a previous report [27], This may be because GO is reduced to RGO by MXene in this process, which increases the hydrophobicity as well as the π-conjugated structure of RGO, thus constituting a 3D graphene framework. Meanwhile, MXene is embedded into the framework by self-assembly in this process with the help of interfacial interactions with RGO, thus obtaining graphene@MXene hydrogels. The results of Figure S2 showed the volume of the obtained graphene@MXene composite hydrogels gradually became smaller as the MXene content increased, and this result also confirms to some extent that the framework of the composite hydrogels depends on the GO content. It is well known that GO is non-conductive, but we found that the GO-MXene composite film prepared by doping 20 wt% MXene showed conductivity, and the sheet resistance of the obtained GO-MXene film became smaller and smaller as the MXene content increased (table S1). These results could also confirm to some extent that MXene could reduce GO.
To investigate the changes in the functional groups on the surface of the composite aerogel, FT-IR was employed to characterize the samples. Figure 2 illustrated that the change in the absorption peaks of the composite aerogel after the introduction of MXene indicated the successful synthesis of the prepared material. The spectra of the graphene aerogel at 3348, 1711, 1580, 1389, 1231, and 1067 cm− 1 corresponded to -OH, -C = O, C = C, C-OH, -OH, and -C-O stretches, respectively[31]. This indicated the presence of hydroxyl, carboxyl, and epoxy groups in the graphene aerogel. The reduction in the intensity of the -C = O (1711 cm− 1) and -C-O (1067 cm− 1) peaks compared to that of graphene aerogel indicated that GO was reduced to RGO during the reaction process. The reduction in the intensity of the C-OH (1389 cm− 1) peak and the absence of the -OH (3348 cm− 1, 1231 cm− 1) peaks of the composite aerogel indicated that the surface of GO underwent a process during the reaction, resulting in the removal of some of the hydrophilic oxygen-containing functional groups. This process constituted the 3D RGO framework, which exhibits a hydrophilic-hydrophobic equilibrium. The decrease in the intensity of the -OH (3348 cm− 1, 1231 cm− 1) peaks of the composite aerogel indicated that the surface of GO underwent a reaction during which some of the hydrophilic oxygen-containing functional groups were removed. The resulting hydrophilic-hydrophobic equilibrium constituted the 3D RGO framework. This was consistent with the results reported in the literature[27]. Furthermore, the prominent peak at 449 cm− 1 corresponded to the -Ti-O of MXene, which demonstrated that the MXene nanosheets have been successfully embedded in the RGO framework.
SEM was used to observe the surface morphology of the prepared samples and the results were shown in Fig. 3. According to the results in Fig. 3(a)-Figure 3(d), the surface of the composite hydrogels was relatively smooth when the MXene content was low (20%). With the increase of MXene content, the lamellar fragments on the surface of the composite hydrogels increased significantly and the morphology became more and more complex, which could be attributed to the self-stacking of MXene[32]. The EDX mapping results in Fig. 3(e)-Figure 3(g) showed that C, O, and Ti existed in the hydrogels. These results also further confirmed that the prepared hydrogels were graphene@MXene composite hydrogels.
|
Samples |
Surface area (m2/g) |
Average pore width (nm) |
|---|---|---|
|
graphene@MXene-20% |
195.801 |
3.669 |
|
graphene@MXene-60% |
138.607 |
4.2553 |
|
graphene@MXene-CaCl2-20% |
223.034 |
3.8672 |
|
graphene@MXene-CaCl2-60% |
126.241 |
5.9227 |
3.2 Adsorption of MB based on graphene@MXene composite hydrogels
Considering the variation in morphology of graphene@MXene hydrogels and the excellent dye adsorption properties of graphene and MXene reported previously in the literature[24, 33, 34], we investigated the removal of MB by composite hydrogels with different MXene contents, and the results are shown in Fig. 4(a). It could be seen from Fig. 4(a) that the removal effect of graphene@MXene hydrogels on MB deteriorates with the increase of MXene content. When the MXene content reached 80%, the removal effect of graphene@MXene hydrogels on MB decreased dramatically. It may be attributed to two factors. On the one hand, it may be because the framework of the prepared 3D hydrogels became smaller with the decrease in GO content. Self-stacking caused by the increased MXene content embedded in the 3D graphene framework further reduces the specific surface area and reaction sites of the 3D hydrogels, which in turn reduced the MB scavenging effect of the graphene@MXene composite hydrogels. On the other hand, considering that MB was a cationic dye, the MB removal effect of GO was superior to that of RGO and graphene. Therefore, we speculated that the deterioration of the MB removal effect of graphene@MXene composite hydrogels may also be related to the degree of GO reduction: as the content of MXene increased, GO was reduced more effectively. Table 1 showed the results of specific surface area measurements (nitrogen adsorption method) of composite hydrogels with 20% and 60% MXene content. According to the results of Table 1, the composite hydrogels with high MXene content exhibited a smaller specific surface area. Table S2 showed the measurement results of the sheet resistance of different graphene@MXene aerogels, and the results indicated that the sheet resistance of the aerogels became smaller with increasing MXene content, implying better conductivity and better GO reduction. These results suggested that the variation in the MB removal effect of graphene@MXene composite hydrogels may be a result of the combined effect of these two factors.
Considering that pure graphene hydrogels could not be obtained by this method, we prepared pure graphene hydrogels (Figure S3) with a hydrothermal reactor and investigated its removal effect on MB, and the results are shown in Figure S4. It can be seen from Figure S6 that the removal effect of MB by pure graphene hydrogels was weaker than that of graphene@MXene hydrogels with 20% MXene content under the same conditions. These results also further indicated that the introduction of MXene can not only achieve the simple preparation of graphene@MXene hydrogels but also enhance the MB removal effect of graphene@MXene hydrogels.
Further, we found that the introduction of Ca2+ as a cross-linking agent during the preparation of the composite hydrogels not only promoted the forming of graphene@MXene composite hydrogels and shortened the preparation time of hydrogels from 15h to 5h, but also adjusted the specific surface area and pore size distribution of the composite hydrogels, thereby improving the MB adsorption rate. Figure S5 shows graphene@MXene composite hydrogels prepared by calcium ion modulation, and the results showed that the calcium ion-modulated hydrogels had a smoother and flatter morphology than the graphene@MXene hydrogels, we deduced that it may be due to the fact that the cross-linking of calcium ions provides more space for the embedding of MXene. The SEM results of graphene@MXene aerogel modulated by calcium ions were shown in Figure S6, and the results indicated that the samples prepared after the introduction of calcium ions had significantly smaller surface fragments than graphene@MXene aerogels, indicating that the self-stacked MXene became less.
Figure 4(b) showed the MB removal results of graphene@MXene hydrogels regulated by calcium ions, and its MB removal effect gradually deteriorates with the increase of MXene content, which is consistent with graphene@MXene. However, it can be seen from Fig. 4 that the introduction of Ca2+could also effectively improve the adsorption rate and the removal effect of MB, which enabled the MB removal rate to be enhanced by ~ 30%. In addition, the enhancement of the removal rate of MB by Ca2+modulated graphene@MXene hydrogels gradually decreased with the increase of MXene content. We speculated that it may be due to the fact that the molding process of the hydrogels after the introduction of Ca2+ was the result of a combination of MXene reduction and Ca2+cross-linking, which led to a change in the structure of the hydrogels, increasing the number of reactive sites, which in turn enhanced the removal of MB. It could be seen from Table 1 that compared to the graphene@MXene hydrogels with 20% MXene content, the introduction of calcium ions not only increased the pore size but also improved the specific surface area, which can provide more reaction sites and thus enhanced the MB removal rate. Figure S7 showed the results of the elasticity test of the aerogel prepared with or without Ca2+introduction at 20% MXene content, the results showed that the Ca2+ modulated graphene@MXene aerogel had a more fluffy structure, which was consistent with the change in pore size in Table 1, and to a certain extent, it also showed that the introduction of calcium ions did change the structure of the graphene@MXene hydrogels. However, when the MXene content was 60%, the specific surface area of the hydrogels obtained after the introduction of calcium ions became slightly smaller compared to that without the introduction of Ca2+. This result suggested that for composite hydrogels with high MXene content, the enhancement of the MB adsorption rate after calcium ion introduction was not caused by the change in the specific surface area.
Figure S8 showed the change in pore size of the prepared hydrogels before and after calcium ion introduction. From the results in Figure S8, it could be seen that the graphene@MXene hydrogels obtained with or without Ca2+ introduction contain both microporous (< 2 nm) and mesoporous (2–50 nm) structures. When the MXene content was low (20%), the cross-linking of Ca2+ with MXene and graphene was able to further alleviate the stacking of MXene, increasing both the specific surface area and the average pore size, which led to an increase in both the reaction sites and enhanced the removal effect of MB. When the content of MXene was higher, the introduction of Ca2+ changed the ratio of micropores and mesopores, which led to an increase in the number of mesopores, thus making the specific surface area smaller. However, the increase of mesopore structure could provide more transient reaction sites during the MB removal process, thus enhancing the MB removal rate. The adsorption of MB by the microporous structure occurred more at the later stage of the reaction. From the results in Fig. 4, it could be seen that the graphene@MXene hydrogels prepared with or without Ca2+ introduction were able to achieve complete removal of MB. However, with Ca2+ introduction, the reaction was faster and the removal rate varied significantly in the pre and post-periods, whereas without Ca2+ introduction, the removal rate varied more slowly throughout the process. These results also suggested that the pore structure in the hydrogels can be modulated by regulating the amount of MXene under the condition of calcium ion introduction.
3.3 Adsorption kinetics
The adsorption kinetic measurements were used to further investigate graphene@MXene and graphene@MXene regulated by CaCl2. In this study, the pseudo-first-order kinetic model (Eq. 3) and pseudo-second-order kinetic model (Eq. 4) were used to analyze and simulate the adsorption process, and the results were shown in Fig. 5(a), Fig. 5(c) and Table 2.
where Qt (mg·g− 1) and Qe (mg·g− 1) are the adsorption capacity of MB at time t(h) and at adsorption equilibrium, respectively, and k1 (h− 1) and k2 (mg·g− 1·h− 1) are the rate constants for the pseudo-first-order and pseudo-second-order models,respectively.
|
Pseudo-first-order model |
Pseudo-second-order model |
||||||
|---|---|---|---|---|---|---|---|
|
Qe (mg/g) |
k1(h− 1) |
R12 |
Qe (mg/g) |
k2(h− 1) |
R22 |
||
|
RGO-MXene |
97.447 |
0.07228 |
0.98828 |
115.33 |
7.221E-4 |
0.99661 |
|
|
RGO-MXene-CaCl2 |
95.81 |
0.21983 |
0.98578 |
103.41 |
0.00338 |
0.99993 |
|
According to the results in Fig. 5(a), Fig. 5(c), and Table 2, it can be seen that the pseudo-second-order kinetic model has a better fitting result for both graphene@MXene hydrogels and Ca2+ cross-linked graphene@MXene hydrogels. According to previous reports, the pseudo-second-order model was more suitable for adsorption processes with low initial concentrations or adsorbents with abundant active sites. In this study, the initial MB concentration was 100 mg/L, which is a high concentration compared to previous studies [33]. Therefore, it can be inferred that both graphene@MXene as well as Ca2+ modulated graphene@MXene composite hydrogels have abundant adsorption sites. In addition, since the pseudo-second-order model was also more suitable for fitting adsorption processes with chemisorption as the rate-limiting phase, there was electron sharing or transfer between MB and graphene@MXene as well as Ca2+ modulated graphene@MXene composite hydrogels adsorbents [26, 33].
The intraparticle diffusion model (Eq. 5) was used to investigate to further explain the adsorption mechanism and the results are shown in Fig. 5(b), (d) and Table 3.
where Qt (mg·g− 1) is the adsorption capacity at time t (h); ki (mg·g− 1·h− 0.5) is the rate constant of intraparticle diffusion; and Ci (mg·g− 1) is the intercept, which is related to the thickness of the adsorption boundary layer.
According to the fitting results, the adsorption process of MB by graphene@MXene hydrogels prepared before and after the introduction of Ca2+ could be divided into three stages. Since the linear relationships of all three stages do not exceed the origin and the linear coefficients are greater than 0.9, that indicates that intra-particle diffusion was not the only rate-limiting step. In addition, compared to graphene@MXene hydrogels, calcium ion-modulated graphene@MXene hydrogels had a larger slope at Stage I, indicating a faster adsorption rate, implying more adsorption sites. This was because the adsorption of MB at this stage is spreading transient adsorption, which mainly occurred on the outer surface or larger pores with a faster adsorption rate. At Stage II, the adsorption rate of Ca2+ modulated graphene@MXene hydrogels decreased sharply, mainly because the decrease in concentration gradient made it taked a longer time for the dye molecules to diffuse in the adsorbent micropore. This also explained why graphene@MXene modulated by Ca2+ with high MXene content could have better MB removal even though the specific surface area becomes smaller. These changes also indirectly indicated that the introduction of Ca2+ does lead to changes in the structure and properties of graphene@MXene hydrogels.
|
Adsorption Materials |
Stage I |
Stage II |
Stage III |
||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
|
K1 |
C1 |
R12 |
K2 |
C2 |
R22 |
K3 |
C3 |
R32 |
|||
|
RGO-MXene |
15.08 |
0.31 |
0.995 |
10.88 |
20.24 |
0.986 |
1.39 |
86.7 |
0.96 |
||
|
RGO-MXene-CaCl2 |
28.81 |
0.70 |
0.993 |
6.79 |
59.21 |
0.983 |
1.42 |
87.4 |
0.948 |
||
3.4 Microwave irradiation spectroscopy of RGO-MXene modulated by MB adsorption
In addition, we found that the adsorption of MB leads to changes in the microwave irradiation spectra of graphene@MXene aerogels. Figure 6 showed the luminescence spectra of graphene@MXene aerogels and graphene@MXene aerogels-CaCl2 before and after MB adsorption when subjected to microwave irradiation.
Figure 6(a) showed the luminescence spectra of graphene@MXene aerogels before and after MB adsorption. Comparing the results in Fig. 6(a) with the luminescence spectra of pure MXene in Figure S9, it could be found that the luminescence spectra of graphene@MXene aerogel before adsorption of MB exhibit the spectral characteristics of graphene. However, the luminescence spectrum of graphene@MXene aerogel after adsorption of MB exhibits the spectral characteristics of MXene when exposed to microwave radiation, and the features became more and more obvious with the increase of MXene content. We also collected the luminescence spectra of graphene aerogel with MB adsorbed, which did not change significantly compared to the luminescence spectra of graphene aerogel before MB adsorption. We speculated that it may be related to the absorption of microwaves by RGO and MXene. Although MXene had better electrical conductivity, our previous study on the microwave reduction effect of RGO prepared by thermal annealing at different temperatures showed that it was not the case that the higher the heat treatment temperature, the better the electrical conductivity, and the better the microwave absorption effect of RGO [35]. Therefore, we speculate that when MB is not adsorbed, RGO had better microwave absorption properties than MXene, and the aerogel as a whole showed the luminescence spectral characteristics of graphene. After adsorption of MB, the electrical conductivity of both RGO and MXene would be reduced to a certain extent, which inhibited the microwave absorption of RGO and enhanced the microwave absorption of MXene, thus making the aerogel as a whole present the spectral characteristics of MXene externally when subjected to microwave radiation. The changes in the luminescence spectra of graphene@MXene aerogels also provided a new method for the identification of MB.
Similar results were seen in the microwave radiation spectra of Ca2+ modulated graphene@MXene aerogels, as shown in Fig. 6(b). According to previous studies, the introduction of different ions led to changes in the luminescence spectra of graphene when subjected to microwave irradiation, and the luminescence spectra in Fig. 6(b) had the spectral characteristics of calcium ions introduced [36]. Comparing the results in Fig. 6(b), it could be found that the luminescence spectra of graphene@MXene aerogels modulated by calcium ions also undergo two very obvious changes after MB adsorption: one was the appearance of the spectral line at 670.16 nm, which may be related to Ti, according to the results of the National Institute of Standards and Technology (NIST)[35]. The other was the disappearance of the spectral line at 553.19 nm. The change in the luminescence spectra of MB adsorption also indirectly confirmed MB adsorption, which also provided a new method for the detection of MB.
In this study, we prepared graphene@MXene composite hydrogels with different ratios by regulating the content of MXene and investigated the adsorption performance of MB using the hydrothermal reduction method. With the increase of MXene content, the composite hydrogels showed poorer removal of MB, which was related to the change in the specific surface area of the hydrogels and the reduction of GO during the preparation process. It was found that the introduction of Ca2+ shortened the preparation time of the hydrogels by 2/3, and at the same time, the specific surface area and pore structure of the hydrogels were increased, and the adsorption rate of MB was improved by about 30%. The adsorption kinetics results showed that although the adsorption of MB by the hydrogels prepared by both methods was more consistent with the pseudo-second-order kinetic model, their intra-particle diffusion processes were significantly different. Compared with graphene@MXene hydrogels, Ca2+ modulated graphene@MXene hydrogels exhibited a larger slope in the first stage, which was attributed to their large specific surface area, which instantly allowed for more active adsorption sites and thus higher MB removal rates. In addition, we observed that MB adsorption altered the microwave irradiation spectra of graphene@MXene aerogels, displaying the spectral characteristics of graphene (pre-adsorption) and MXene (post-adsorption). This change offered a new method for MB detection. This study not only held significant application value in hydrogel preparation and MB adsorption but also introduced a novel method for MB detection.
MB Methylene blue
SEM Scanning electron microscope
FTIR Fourier transform infrared spectrometer
BET Brunner−Emmet−Teller measurements
GO Graphene oxide
RGO Reduced graphene oxide
EDX Energy-dispersive X-ray
Author Contributions
Wenshuai Jiang carried out conceptualization; Data curation collection and analysis were performed by Yaning Wang, Xiaoxia Yan, Yuxi Wang and Kai Yang; Methodology was carried out by Wenshuai Jiang, Fei Lin, Jiefen Shen, Xuming Sun and Zhigang Chen; Writing – original draft was carried out by Wenshuai Jiang and Yaning Wang; Writing – review & editing was carried out by Wenshuai Jiang and Yaning Wang. All authors reviewed the manuscript.
Acknowledgments
Not applicable.
Funding
This work was supported by the Natural Science Foundation of Henan Province (222300420267), the Open Project Program of the First Affiliated Hospital of Xinxiang Medical University (XZZX2022015).
Data availability
All data generated or analyzed during this study are included in this published article.
Declarations
Ethics approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Competing interests
The authors declare that they have no competing interests regarding the publication of this article.
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No competing interests reported.