Ti3C2-MXene/Bismuth Ferrite Nanohybrids for Efficient Degradation of Organic Dyes and Colorless Pollutants.

The current environmental and potable water crisis requires technological advancement to tackle the issues caused by different organic pollutants. Herein, we report the degradation of organic pollutants such as Congo Red and acetophenone from aqueous media using visible light irradiation. To harvest the solar energy for photocatalysis, we fabricated a nanohybrid system composed of bismuth ferrite nanoparticles with two-dimensional (2D) MXene sheets, namely, the BiFeO3 (BFO)/Ti3C2 (MXene) nanohybrid, for enhanced photocatalytic activity. The hybrid BFO/MXene is fabricated using a simple and low-cost double-solvent solvothermal method. The SEM and TEM images showed that the BFO nanoparticles are attached onto the surface of 2D MXene sheets. The photocatalytic degradation achieved by the hybrid is found to be 100% in 42 min for the organic dye (Congo Red) and 100% for the colorless aqueous pollutant (acetophenone) in 150 min. The BFO/MXene hybrid system exhibited a large surface area of 147 m2 g-1 measured via the Brunauer-Emmett-Teller sorption-desorption technique, which is found to be the largest among all BFO nanoparticles and derivatives. The photoluminescence spectra indicate a low electron-hole recombination rate. Fast and efficient degradation of organic molecules is caused by two factors: larger surface area and lower electron-hole recombination rate, which makes the BFO/MXene nanohybrid a highly efficient photocatalyst and a promising candidate for many future applications.

Introduction

Photocatalysis is a low-cost and environment-friendly technique to purify the wastewater from pollutants such as organic dyes, thus splitting out the compounds to form water molecules and carbon monoxide. In the past, semiconductors have been widely used as the photocatalysts to degrade organic dyes owing to the fact that they are not easily biodegradable.[1] For the last decade, TiO2 is used as a photocatalyst; however, its activity is limited to ultraviolet (UV) light as it has a wider band gap (∼3.2 eV) and lower activity under visible light irradiation. TiO2 due to its photocatalytic nature is also used in water splitting for the production of hydrogen (H2) but is still impeded due to its inferior visible light absorption and requirement of doping with other materials to increase its performance.[2] As UV and visible light form 4 and 43% of the solar spectrum, respectively, it is required to develop materials that could be used under the visible light spectrum.[3−5] For this purpose, bismuth ferrites are the potential candidates under visible light irradiation due to them having a narrow band gap.[6,7]

Bismuth ferrites (BiFeO3 or BFO) are a family of transition-metal oxides used in several applications.[8−10] At room temperature, it is a perovskite-type material, which shows multiferroic behavior,[11−13] with a 2.01 eV band gap.[14,15] According to recent studies, BFO compounds showed photocatalytic activity to degrade organic pollutants such as dyes.[7,16−18] In a recent report, thermally reduced graphene oxide is used in solution to absorb the methyl orange dye.[19] It has been shown in recent works that the nanocomposites of BiFeO3/graphene and metal co-doped BiFeO3 are highly efficient for the photocatalytic applications.[13,20,21] The reported result showed that the two-dimensional (2D) materials have better and extraordinary properties because of their enhanced effects; therefore, they are taken as promising materials for many applications.[22−28] Many 2D materials such as metal chalcogenides,[29,30] boron nitride,[31] and oxides and hydroxides[32,33] are prepared by exfoliating their three-dimensional (3D) structures.

Recently, reported well-synthesized 2D layered materials containing transition-metal elements with carbide and nitride, also called MXene, have attracted considerable attention for their carbon-based 2D layered structures.[34−40] They are characterized by the formula MXTx (n = 1, 2, 3) where M corresponds to various transition metals like titanium, chromium, and so on, X corresponds to carbon, nitrogen, and so on, and the functional groups of MXenes are represented by Tx (OH, O, F).[41] The first MXene compound reported was Ti3C2Tx, and today, there are 19 more such compounds from the MXene family being synthesized and many of them have been predicted for various applications using first-principles calculations.[35,38,40,42,43] Recently, Soltani and Lee studied the photodegradation of BiFeO3/reduced graphene oxide (rGO) nanocomposites and found that it degrades completely aqueous bisphenol A in 70 min under visible light irradiation.[21] Wang et al. reported the 100% degradation efficiency for Bi25FeO40/rGO nanocomposites in 180 min under visible light irradiation for methyl orange.[13] Also, Dai et al. have found only 50% degradation of methyl orange in 6 h, which shows the very poor photocatalytic efficiency of the BiFeO3/GO nanohybrid.[44] Irfan et al. reported the 17% photodegradation of acetophenone using La3+- and Se4+-doped BFO nanostructures.[45] Many researchers are working on various metal-doped BiFeO3 materials and their hybrids with graphene to be used as photocatalysts since the pure BiFeO3/graphene photocatalysts have reached its efficiency limit. Therefore, there is a dire need for finding highly efficient pure BiFeO3-based 2D nanohybrid structures. In an effort to achieving this, we present here a very highly efficient BiFeO3/Ti3C2Tx–MXene nanohybrid structure that showed 100% photocatalytic activity within only 42 min for Congo Red dye removal at room temperature. However, no research has been found on doped BiFeO3/Ti3C2Tx-MXene nanohybrid or composites exhibiting the photocatalytic degradation of acetophenone because of its difficulty degrading from solution. Moreover, this work reports the band gap tuning of the proposed nanohybrid structure up to 1.96 eV, having the largest BET surface area of 147 m2 g–1 among any BiFeO3 structures or derivatives reported to date.

Results and Discussion

Structure of Doped BFO Nanoparticles and BFO/MXene Nanohybrid

The structural analysis was done by X-ray diffraction (XRD) (XRD, Rigaku 2500, Japan) with Cu Kα radiation. The scan angle range was 2θ ≈ 5–65° using Cu Kα radiation operating at a voltage of 40 kV and current of 20 mA. Figure a shows the XRD pattern of the MAX phase (Ti3AlC2) in red and the etched MAX phase (MXene, Ti3C2Tx) in black where Tx is the surface terminations (O–, OH–, and F–) on the MXene sheet.[40] In Figure b, a closer view of the MXene peaks is shown where three characteristic peaks are observed at 2θ ≈ 9.7, 19.1, and 39°corresponding to the (002), (004), and (104) planes, respectively, representing etched MAX; the decrease in peak intensity showed the crystallinity loss after the removal of aluminum from the MAX phase, as shown in SEM images.[39,40] The most intense peak in (Ti3C2Tx) is at 2θ ≈ 39°, which corresponds to the (104) plane as shown in Figure b, indicating the etching of Al from the structure.[46,47] In Figure c, the X-ray diffraction pattern of pure BFO nanoparticles is given in blue and that of the BFO/MXene nanohybrid structure is given in black. The diffraction peaks confirm the perovskite crystalline structure of pristine BFO, a distorted rhombohedral structure with the space group R3c. All the diffraction peaks of BFO are indexed to JCPDS 71-1518 as reported in the literature.[48−51] Along the characteristic peaks of BFO, a small amount of secondary phase also occurs at 2θ ≈ 27–28°. In non-stoichiometry, the Bi2O3 volatilization results are well known and also the secondary phase formation in BFO. The XRD peaks of the impurity phase in this BFO structure match with the Bi2Fe4O9 phase as shown in Figure c.[52−55] The effect of addition of BFO to the MXene sheet can be seen by the suppressed behavior of peaks where they broadened, became less intense, and slightly shifted toward higher diffraction angles as shown in Figure d. The slight shift of the XRD peaks of the BFO/MXene nanohybrid along 2θ toward the right as compared to the peaks of pure BFO might be due to compression of one of the lattice of the unit cell caused by the addition of BFO to the MXene sheet.[54] The doublet sharp peaks at 2θ ≈ 32° attributed to the (104) and (110) planes of BFO shifted to higher angles, which may be due to the fact that there is stress on the unit cell that causes the decrease of the lattice constant and interplanar distance; as a result, contraction of the unit cell volume occurs. However, the structure of the BFO nanoparticles remains the same, which has a rhombohedral distorted shape with the space group R3c, as the doublet peaks are clearly separated even after nanohybrid formation.[56] The particle sizes of BFO and BFO/MXene were calculated by Scherrer’s formula, and they are 45 and 43 nm, respectively.[57] The grain size decreases due to the decrease of the lattice constant or the lattice distortion that takes place upon doping or by hybrid formation, which always happens. This reduction in grain size causes enhancement of the surface-to-volume ratio of the prepared nanohybrid.[20] During lattice parameter calculation, a similar decreasing trend was observed for the BFO/MXene nanohybrid to that of pure BFO particles. The calculated values of lattice parameters a and c decreased from 4.58 to 4.55 and 7.31 to 7.29 Å, respectively. The measured values of BFO/MXene have a slight difference compared to the actual calculated values of BFO; this small difference in lattice parameters is due to the presence of a minute content of secondary phase.[58] The grain size was calculated using Scherrer’s formula; the reduction in grain size of BFO/MXene compared to BFO is due to the attachment of the BFO nanoparticles onto the Ti3C2Tx surfaces.

Figure 1

XRD spectra of (a) MAX (Ti3AlC2) and MXene (Ti3C2Tx), (b) closer view of the MXene curve, (c) XRD spectra of BFO and the BFO/MXene nanohybrid, and (d) zoomed image of peak shifting.

Morphological Analysis

The surface morphology including the shape and size of MXene and the BFO/MXene nanohybrid was studied using a field emission scanning electron microscope and a tunneling electron microscope as shown in Figure a–f and Figure g,h, respectively. The Ti3C2Tx–MXene sheets after etching show partial splitting due to aluminum removal from the parent Ti3AlC2 compound as seen in Figure a,b, forming a multilayer stack of MXene sheets.[39,40] The BFO nanoparticles seem to attach onto the MXene surface, indicating good nanohybrid formation as shown in Figure c–f taken at different zooming scales, that is, 2 μm, 300 nm, 200 nm, and 100 nm, respectively. The closer view at 100 nm shows that the BFO particles are in nonuniform round clusters attached to the MXene sheet, forming a porous network of BFO/MXene. TEM analysis of the BFO/MXene nanohybrid is shown in Figure h. It can be clearly seen that the BFO nanoparticles penetrate into the MXene layers and are adsorbed onto the surface of the MXene sheets, which is in good agreement with SEM results. The multilayer MXene sheets can be seen clearly, providing a smooth surface to the BFO nanoparticles. The particle size from TEM images is estimated using ImageJ software,[59] which is ∼25 to 50 nm.

Figure 2

Surface morphology of MXene sheets and BFO/MXene nanohybrids. (a, b) Morphologies of exfoliated MXene sheets and (c–f) BFO/MXene nanohybrids. (g, h) TEM images of BFO/MXene nanohybrids.

X-ray Photoelectron Spectroscopy (XPS)

The chemical composition and binding energies of the synthesized nanohybrid are determined using the X-ray photoelectron spectroscopy (XPS) technique. XPS analysis of the BFO/MXene nanohybrid is shown in Figure . Figure a shows the survey spectrum containing the peaks for the following elements: bismuth, iron, oxygen, titanium, and carbon, which are abbreviated hereafter as Bi, Fe, O, Ti, and C elements in the BFO/MXene nanohybrid with their respective binding energies. High-resolution scans were taken for Bi, Fe, O, Ti, and C and are shown in Figure b–f. The high-resolution spectrum of Bi 4f is shown in Figure b, spreading from 157 to 166 eV, having sharp peaks at 158.8 and 164.1 eV, and the spectrum shows the binding energies of two different states of Bi, which are Bi 4f7/2 and Bi 4f5/2. According to a report, this implies that Bi has an oxidation state of 3+.[60] In Figure c, the high-resolution spectrum of Fe is discussed, which spreads over a range from 706 to 729 eV. The exact peaks are measured at 710.8 and 724.3 eV, which are the binding energies of two states, that is, Fe 2p3/2 and Fe 2p1/2, respectively, which is related to the Fe2+ ions.[60−62] The presence of Fe ions causes more oxygen vacancies to form on BFO’s surface, which further increases the chances of adsorption of organic species on the surface.[63,64] The high-resolution spectrum of the oxygen O 1s is given in Figure d. The binding energy of oxygen ranges from 527 to 534 eV; deconvolution of the oxygen peak was done and fitted with a Lorentzian–Gaussian curve to further observe the bonding of the oxygen atom. Three different peaks were found from the curve deconvolution: the energy peaks were found at 529.7, 531.1, and 532.8 eV, which are attributed to lattice oxygen atoms, hydroxyl oxygen, and surface adsorbed oxygen species, respectively.[65,66] The results of XPS confirm the presence of oxygen and hydroxyl species on the surface of the BFO/MXene nanohybrid, which helps in the photocatalytic degradation mechanism; it is a series of redox reactions that occur on the surface of the materials.[67]Figure e shows the high-resolution spectrum of Ti 2p, and the peak spreads over 454 to 470 eV, which is attributed to the Ti bond; the binding energies for Ti 2p peak at 459.7 and 465.6 eV for Ti(IV) 2p3/2 and Ti(IV) 2p1/2, respectively. The evidence of Ti–O and Ti–C bonding can be seen from both of the peaks.[68,69] The high-resolution spectrum of the carbon C 1s is shown in Figure f; there is a single peak at 284.7 eV, which can be deconvoluted if required into two peaks for the C–C and C–O bonds.[22,68] Thus, elemental bonds present in the synthesized nanohybrid structure at specific binding energies confirm the successful formation of the BFO/MXene nanohybrid.

Figure 3

XPS analysis (a) survey scan of the BFO/MXene nanohybrid and high-resolution scans of (b) Bi 4f, (c) Fe 2p, (d) O 1s, (e) Ti 2p (f) C 1s.

Band Gap Engineering

The absorption spectra of pure BFO nanoparticles and the BFO/MXene nanohybrid were obtained using a UV–vis spectrophotometer. The well-established Kubelka–Munk technique is used to estimate the absorption spectra of both samples.[70,71] It is well known from this theory that the relation between the absorption coefficient α and the photon energy hυ for the allowed transitions is

In eq , A is a constant function, Eg is the band gap energy, hν is the frequency times Planck’s constant, and n is a positive integer.

The optical absorption spectra for pure BFO nanoparticles and the BFO/MXene nanohybrid are measured at room temperature between the wavelength range of 350–700 nm as shown in Figure a. A sudden characteristic peak of pure BFO nanoparticles is observed at around 599 nm.[71] To calculate the band gap for pure BFO nanoparticles and the BFO/MXene nanohybrid, the Tauc plot method is used as shown in Figure b.[72,73] A clear red shift is observed in the absorption spectra for the BFO/MXene nanohybrid compared to the pure BFO nanoparticles. The calculated band gap from the Tauc plot for pure BFO nanoparticles at direct band transition is 2.01 eV, which agreed well with the previous reports.[7,74,75] The calculated band gap for BFO/MXene is 1.96 eV, and there is slight reduction in the band gap, as shown in Figure b, which indicates that the produced nanohybrid is better suitable for photocatalytic application than its pure BFO counterpart.

Figure 4

(a) Diffuse reflectance spectrum (DRS) for BFO and the BFO/MXene nanohybrid. (b) Tauc plot for band gap calculation. (c) Photoluminescence spectra (PL) of BFO and the BFO/MXene nanohybrid.

Figure c shows the photoluminescence spectra (PL) of pure BFO nanoparticles and the BFO/MXene nanohybrid. The PL intensity shows the recombination rate of the charge carries, and BFO shows a higher peak than the BFO/MXene nanohybrid, which means that the charge recombination rate for BFO is much higher than that of the BFO/MXene nanohybrid.

Once light is irradiated on the nanohybrid, electron–hole pairs are produced, which in the case of attached MXene surfaces are allowed to spread on MXene surfaces quickly, which may cause delay for recombination of charge carriers.

The phenomena of excitation occur for the BFO nanoparticles when light is irradiated, and the valence band (VB) electrons from a low energy level jump to the conduction band (CB). The CB in semiconductors has more than one energy band as explained in detail by Liqiang et al., which is because the photoluminescence mechanism in the semiconductors is complex, and their work also relates the PL intensity to the photocatalytic activity.[76] The nanohybrid when activated with light produces electron–hole pairs, and in aqueous media, the electron–hole pairs react to form radicals on the surface of the BFO/MXene nanohybrid.

In Figure a, Brunauer–Emmett–Teller surface area calculation is made using a multipoint BET method. It confirms that the material is mesoporous in nature.[77] The pore sizes are found to be ∼1.68 to 2.47 nm using the Barret–Joyner–Halenda method, as shown in Figure b.[78] The BFO/MXene nanohybrid in the produced system showed the highest reported BET surface area of ∼147 m2 g–1. Thus, understanding the effect of a larger surface area, two benefits can be attributed to it: the generated charge carriers reside over the surface of MXene sheets for a longer time, thus causing a lower recombination rate for the BFO/MXene nanohybrid than for the bare BFO nanoparticles, and a larger surface area provides more active sites for redox reactions to occur, which in turn improves the dye degradation process.[79]

Figure 5

(a) N2 gas isotherm for BFO/Ti3C2Tx, measured at 77 K. (b) Pore volume vs pore diameter.

Photocatalytic Activity of Pure BFO and BFO/MXene

Figure a shows the absorbance spectra of the BFO/MXene nanohybrid. Figure b shows the photocatalytic degradation capacity of bare BFO and the BFO/MXene nanohybrid. It can be observed that only 33% of the Congo Red (CR) was degraded by pure BFO under visible light irradiation in 42 min. This shows that pure BFO is very stable and not suitable for degradation. The previously reported photocatalytic degradation of CR using pure MXene (Ti3C2) was 12% in 120 min.[80] Crystal clear water was obtained in only 42 min by using BFO/MXene as a photocatalyst with visible light irradiation. From Figure c, it was observed that only 21% of the Congo Red dye was degraded from catalytic solution under dark experiments, which shows that BFO/MXene is a better photocatalyst rather than a catalyst in dark conditions. Degradation of the dye can be achieved in two conditions, dark and light conditions, and it occurs via a combination of surface adsorption and degradation mechanisms. The degradation mechanism is explained in a later section.

Figure 6

(a) Absorbance spectra of the BFO/MXene nanohybrid. (b) Photocatalytic degradation of CR by BFO and the BFO/MXene nanohybrid against time taken using light. (c) Degradation of CR by the BFO/MXene nanohybrid against time taken under dark condition experiments. (d) Four cycles of the degradation process. (e) BFO/MXene XRD curve before and after irradiation.

The photocatalytic activity also depends upon the electron and hole recombination process as stated earlier, and BFO/MXene’s PL intensity is quite weaker than BFO’s, which also supports the fact that a lower PL intensity means a higher photocatalytic activity of the system. Previous reports on BFO/graphene hybrids showed a higher surface area and higher photogeneration rate, in turn providing higher photocatalytic activity.[13,17,20,44] Here, the produced system has a higher BET surface area of 147 m2 g–1, in comparison with previously reported bare BFO, BFO/graphene hybrids, and their other derivatives, thus causing higher production of charge carries for the redox reactions and hence providing more effective photodegradation of the organic dye (CR). Thus, the improved photocatalytic activity should be attributed to the following factors as stated earlier: higher charge carrier generation of BFO; large separation time of electrons and holes; slight reduction in crystallite size; higher BFO/MXene surface area, which provides a larger number of active sites as compared to that of bare BFO nanoparticles; and higher surface area of the BFO/MXene nanohybrid, providing quick transfer of excited charges from BFO to MXene sheets.

The stability of photocatalysts is a key factor for their practical applications. From Figure d,e, it can be seen that, after four cycling runs, the crystal structure of BFO/MXene remains the same before and after photocatalytic degradation reactions, which makes BFO/MXene a stable visible light-induced catalyst and thus suitable for commercial applications.

Photocatalytic Degradation Mechanism

Visible light irradiation produces electron–hole pairs in the material; the electrons combine with O2, and the holes combine with OH– to produce super oxide and free hydroxyl radicals, respectively. These radicals are highly active species that degrade the organic pollutants and produce some harmless byproducts (CO2 and H2O). Figure explains the photocatalytic degradation mechanism. The photo-excited BFO/MXene produces electron–hole paired charge carriers (eq ). The MXene sheets quickly trap the electrons, reducing the chances of recombination, as shown by the lower PL intensity of BFO/MXene earlier. In aqueous media, electrons on combining with O2 produce superoxide anion radicals ·O2, and on the other hand, holes interact with OH– to produce free hydroxyl radicals ·OH–. The·O2 and ·OH– radicals are highly reactive toward the degradation of organic pollutants, thus degrading CR into harmless products (CO2 and H2O). Previous reports on dye degradation show the degradation mechanism as shown here.[45,81−86]

Figure 7

BFO/MXene nanohybrid degrading organic dye molecules.

The following equations express the whole photodegradation mechanism:

The improved photocatalytic activity of BFO/MXene is due to the higher electron–hole pair generation, lower recombination rate of charge carriers, wider absorption of photons, and enhanced redox reactions on the photocatalyst surface.

Catalytic and Photocatalytic Degradation of Acetophenone

Figure shows the degradation of acetophenone under dark and light conditions. The BFO/ MXene nanohybrid was also studied for degradation of acetophenone under dark and light conditions. To prepare the pollutant solution, 100 mg of the prepared BFO/MXene photocatalyst was added into 30 ppm acetophenone solution having a volume of 100 mL. After 2 h of stirring, we get a homogeneous saturated solution of pollutants and catalysts. In the first experiment, the solution was put under light and its degradation concentration was checked every 30 min for up to 150 min until the solution was clear and the pollutant was completely degraded. To check the remaining pollutant concentration, every 30 min, 5 mL of the prepared sample was taken out and centrifuged at 7000 rpm, and the supernatant was characterized using a UV–visible spectrophotometer to check the absorbance, which is related to the remaining pollutant concentration in the solution. Figure b shows the degradation of acetophenone under light; the pollutant was degraded completely in 150 min. In the second experiment, the homogeneous mixture of pollutants and catalysts was kept in dark conditions for 2 h and then under light. The same method stated above was repeated for checking the remaining pollutant concentration, and the BFO/MXene nanohybrid degraded the same pollutant in 150 min but with less efficiency, only 60% of the acetophenone was degraded in a similar time as shown in Figure c. In the second experiment with the dark phase, the adsorption of acetophenone on the catalyst’s surface reaches the equilibrium point. Although the large surface area of (BFO/MXene) allows more adsorption of organic molecules on the catalyst surface, in the process of achieving a homogeneous mixture of catalysts and organic pollutants, as a result, light may not interact completely with the catalyst, and it might be the reason for the less efficient degradation of the pollutant by the catalyst, which is supported by the first experiment in comparison with Figure b. It is difficult to degrade the pollutant acetophenone as compared to other organic pollutants such as organic dyes Congo Red (CR) and methyl violet due to the highly stable benzene ring in its structure. However, the degradation rate of acetophenone and Congo Red (CR) by the newly produced nanohybrid was almost the same due to the enhanced properties of the BFO/MXene nanohybrid. In a word, BFO/MXene is highly efficient in degrading colored compounds, such as CR, as well as colorless compounds, like acetophenone, because of its good physical and chemical properties.[45]

Figure 8

(a) Absorbance spectra of BFO/MXene showing the degradation efficiency of organic pollutants from solution at different times. (b) Catalytic and photocatalytic degradation of acetophenone in the presence of BFO/MXene. (c) Photocatalytic degradation of acetophenone in the presence of BFO/MXene in light-induced experiments.

Conclusions

BiFeO3 (BFO) nanoparticles were prepared using a sol–gel method. The two-dimensional (2D) MXene sheets were synthesized from their pure MAX phase (Ti3AlC2) by chemical etching of aluminum and hydrofluoric acid (HF) was used as the etchant. The reaction was done at room temperature. The BFO/MXene nanohybrid was fabricated using a double-solvent solvothermal technique. The nanohybrid was found to have the largest BET surface area of 147 m2 g–1, a low band gap of 1.96 eV, and a low recombination time. The nanohybrid was further tested for photocatalytic degradation of Congo Red and acetophenone. It comes out to be the best photocatalyst under visible light irradiation, which degraded the Congo Red dye in only 42 min. It can be seen that the degradation of the colorless pollutant acetophenone was also achieved completely in ∼150 min. Meanwhile, the degradation of the same pollutant under dark effects was only 60% in the same time. This shows that the present catalyst (BFO/MXene) is very efficient for the colorless organic pollutant acetophenone. Although colorless organic pollutants are difficult to degrade, the highly efficient BFO/MXene nanohybrid makes it possible. The high photocatalytic activity attained here is unique in itself, which makes it a potential candidate for commercial applications owing to its low-cost synthesis route.

Methods

Materials

Bismuth nitrate pentahydrate (Bi (NO3)3·5H2O) (99% pure), iron nitrate (Fe (NO3)3·9H2O) (98.5% pure), ethylene glycol (C2H6O2) (99%), acetic acid (C2H4O2) (99.5%), Ti3AlC2 (MAX phase), and hydrofluoric acid (39 wt %) were used as received.

Synthesis of MXene Sheets

Exfoliated sheets of Ti3C2Tx were fabricated from the pure MAX phase (Ti3AlC2) using a chemical etching technique with hydrofluoric acid (HF, 39 wt %) at room temperature for 60 h with magnetic stirring; they were then washed with deionized (DI) water and kept in an oven at 60 °C for 6 h to evaporate the water molecules in the sample.

Synthesis of BiFeO3 Nanoparticles

The double-solvent sol–gel method was used to synthesize BiFeO3 (BFO) nanoparticles. Bismuth nitrate pentahydrate (99% pure) and iron nitrate non-hydrate (98.5% pure) were mixed at an equal ratio in ethylene glycol and acetic acid solution followed by stirring for 180 min. The detailed synthesis method is given elsewhere.[48]

Synthesis of BiFeO3/MXene Nanohybrid

The BiFeO3/MXene (BFO/MXene) nanohybrid was fabricated using a double-solvent solvothermal technique. MXene solution was made in DI water with a molarity of 0.5 mg/mL followed by ultrasonication for 10 min. The BFO nanoparticles were dissolved in a mixture of acetic acid and ethylene glycol with a 1:1 ratio and (0.01 M) molarity. The BFO solution was ultrasonicated for 1 h at 60 °C; after that, both the prepared solutions were mixed and transferred to a Teflon-lined steel autoclave for solvothermal synthesis at 160 ° C for 2 h. The final product was washed with deionized (DI) water several times and then dried it at 80 ° C for 3 h.

Characterization

The structural analysis was done by X-ray diffraction (XRD) (XRD, Rigaku 2500, Japan) with Cu Kα radiation. The scan angle range was 2θ ≈ 5–65° using Cu Kα radiation operating at a voltage of 40 kV and current of 20 mA. The surface morphologies of MXene and the BFO/MXene nanohybrid were investigated using a field emission scanning electron microscope (FESEM, JEOL7001F) and a transmission electron microscope (TEM, Hitachi HT7700, 100 kV). Platinum was sputter-coated before characterization on the nanohybrid to avoid any charging effect. The ultraviolet–visible diffuse reflectance spectra (UV–vis DRS) were obtained using a UV–vis spectrophotometer (Hitachi UV-3310, Japan) to calculate the band gap and photocatalytic activity of the BFO/MXene nanohybrid. The BET surface area and porosity of the sample were investigated by the Brunauer–Emmett–Teller (BET) method using a Quadrasorb-SI v. 5.06 by N2 sorption/desorption isotherms at a temperature of 77.35 K. For adsorption measurements, the BFO/MXene sample was degassed at 300 °C. The surface area was calculated using a multipoint BET method, and the Barret-Joyner-Halenda (BJH) method was used to calculate the pore size. X-ray photoelectron spectroscopy (XPS) was used to study the binding energies of all the elements. Photoluminescence spectroscopy was used to observe the charge carrier’s generation/recombination rate.

Photocatalytic Characterization

The photocatalytic measurements of pure and hybrid BFO structures were performed by the degradation of organic compound Congo Red (CR) under visible light illumination. A 100 mg/L aqueous solution of BFO and BFO/MXene samples was prepared. The photocatalyst (100 mg) was dispersed into the dye solution and stirred for nearly 2 h under dark conditions to get equilibrium adsorption–desorption between the photocatalyst and the organic dye molecules. To keep the system’s temperature moderate and also to avoid any thermal contact during magnetic stirring, an ice bath was provided. For the visible light source, a xenon lamp of 300 W was used. The solution (3 mL) was taken out after every 8 min during the whole photocatalytic reaction. After centrifugation, the supernatant was separated from the catalyst dye solution and further processed through UV–vis spectroscopy. The photocatalytic degradation efficiency was estimated by eq (20,45,87,88)where Co is the initial concentration of CR and C is the concentration of CR after time interval t. Visible light (420 nm < λ < 780 nm) was used to observe the photocatalytic efficiencies of pure BFO and the BFO/MXene nanohybrid.