Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2019 Nov 25;4(24):20530–20539. doi: 10.1021/acsomega.9b02359

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

M Abdullah Iqbal †, Ayesha Tariq †, Ayesha Zaheer †, Sundus Gul †, S Irfan Ali ‡,§, Muhammad Z Iqbal ∥,*, Deji Akinwande ⊥, Syed Rizwan †,*
PMCID: PMC6906764  PMID: 31858037

Abstract

graphic file with name ao9b02359_0009.jpg

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.

1. 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 hydroxides32,33 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 Mn+1XnTx (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.

2. Results and Discussion

2.1. 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 1a 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 1b, 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 1b, indicating the etching of Al from the structure.46,47 In Figure 1c, 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 1c.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 1d. 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.

Figure 1

Open in a new tab

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.

2.2. 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 2a–f and Figure 2g,h, respectively. The Ti3C2Tx–MXene sheets after etching show partial splitting due to aluminum removal from the parent Ti3AlC2 compound as seen in Figure 2a,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 2c–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 2g,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.

Figure 2

Open in a new tab

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.

2.3. 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 3. Figure 3a 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 3b–f. The high-resolution spectrum of Bi 4f is shown in Figure 3b, 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 3c, 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 3d. 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.67Figure 3e 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 3f; 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.

Figure 3

Open in a new tab

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.

2.4. 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

2.4. 1

In eq 1, 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 4a. 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 4b.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 4b, which indicates that the produced nanohybrid is better suitable for photocatalytic application than its pure BFO counterpart.

Figure 4.

Figure 4

Open in a new tab

(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 4c 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 5a, 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 5b.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.

Figure 5

Open in a new tab

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

2.5. Photocatalytic Activity of Pure BFO and BFO/MXene

Figure 6a shows the absorbance spectra of the BFO/MXene nanohybrid. Figure 6b 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 6c, 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.

Figure 6

Open in a new tab

(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 6d,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.

2.6. 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 7 explains the photocatalytic degradation mechanism. The photo-excited BFO/MXene produces electron–hole paired charge carriers (eq 2). 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.

Figure 7

Open in a new tab

BFO/MXene nanohybrid degrading organic dye molecules.

The following equations express the whole photodegradation mechanism:

2.6. 2
2.6. 3
2.6. 4
2.6. 5
2.6. 6
2.6. 7

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.

2.7. Catalytic and Photocatalytic Degradation of Acetophenone

Figure 8 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 8b 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 8c. 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 8b. 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.

Figure 8

Open in a new tab

(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.

3. 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.

4. Methods

4.1. 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.

4.2. 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.

4.3. 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

4.4. 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.

4.5. 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.

4.5.1. 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 8(20,45,87,88)

4.5.1. 8

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.

Acknowledgments

Part of the research work carried out in Pakistan was funded by the Higher Education Commission (HEC) of Pakistan under project nos.6040/Federal/NRPU/R&D/HEC/2016 & HEC/R&D/PAKUS/2017/783. Part of this work was also funded by a grant from the United States Government under the program United States Agency for International Development (USAID) under the Pakistan–U.S. Science & Technology Cooperation Program. The contents do not necessarily reflect the views of the U.S. Government. This work was also partially supported by a research startup grant from United Arab Emirates University (grant no. 31N269).

Author Contributions

A.I., A.T., A.Z., and S.G. carried out the experimentation, S.I.A. helped in characterization, M.Z.I. helped in analyzing the data, and D.A. and S.R. conceived the idea and supervised the research project.

The authors declare no competing financial interest.

References

  1. Raffainer I. I.; Rudolf von Rohr P. Promoted wet oxidation of the azo dye orange II under mild conditions. Ind. Eng. Chem. Res. 2001, 40, 1083–1089. 10.1021/ie000629a. [DOI] [Google Scholar]
  2. Cai J.; Shen J.; Zhang X.; Ng Y. H.; Huang J.; Guo W.; Lin C.; Lai Y. Light-Driven Sustainable Hydrogen Production Utilizing TiO2 Nanostructures: A Review. Small Methods 2019, 3, 1800184. 10.1002/smtd.201800184. [DOI] [Google Scholar]
  3. Vijayan B.; Dimitrijevic N. M.; Rajh T.; Gray K. Effect of Calcination Temperature on the Photocatalytic Reduction and Oxidation Processes of Hydrothermally Synthesized Titania Nanotubes. J. Phys. Chem. C 2010, 114, 12994–13002. 10.1021/jp104345h. [DOI] [Google Scholar]
  4. Shu X.; He J.; Chen D. Visible-light-induced photocatalyst based on nickel titanate nanoparticles. Ind. Eng. Chem. Res. 2008, 47, 4750–4753. 10.1021/ie071619d. [DOI] [Google Scholar]
  5. Asahi R.; Morikawa T.; Ohwaki T.; Aoki K.; Taga Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 2001, 293, 269–271. 10.1126/science.1061051. [DOI] [PubMed] [Google Scholar]
  6. Wang X.; Lin Y.; Ding X.; Jiang J. Enhanced visible-light-response photocatalytic activity of bismuth ferrite nanoparticles. J. Alloys Compd. 2011, 509, 6585–6588. 10.1016/j.jallcom.2011.03.074. [DOI] [Google Scholar]
  7. Gao F.; Chen X.; Yin K.; Dong S.; Ren Z.; Yuan F.; Yu T.; Zou Z.; Liu J. M. Visible-Light Photocatalytic Properties of Weak Magnetic BiFeO3 Nanoparticles. Adv. Mater. 2007, 19, 2889–2892. 10.1002/adma.200602377. [DOI] [Google Scholar]
  8. Remya K.; Suresh C. H. Non-covalent intermolecular carbon–carbon interactions in polyynes. Phys. Chem. Chem. Phys. 2015, 17, 27035–27044. 10.1039/C5CP04467G. [DOI] [PubMed] [Google Scholar]
  9. Dutta D. P.; Jayakumar O. D.; Tyagi A. K.; Girija K. G.; Pillai C. G. S.; Sharma G. Effect of doping on the morphology and multiferroic properties of BiFeO3 nanorods. Nanoscale 2010, 2, 1149–1154. 10.1039/c0nr00100g. [DOI] [PubMed] [Google Scholar]
  10. Wang J.; Neaton J. B.; Zheng H.; Nagarajan V.; Ogale S. B.; Liu B.; Viehland D.; Vaithyanathan V.; Schlom D. G.; Waghmare U. V.; Spaldin N. A.; Rabe K. M.; Wuttig M.; Ramesh R. Epitaxial BiFeO3 multiferroic thin film heterostructures. Science 2003, 299, 1719–1722. 10.1126/science.1080615. [DOI] [PubMed] [Google Scholar]
  11. Vijayasundaram S. V.; Suresh G.; Mondal R. A.; Kanagadurai R. Substitution-driven enhanced magnetic and ferroelectric properties of BiFeO3 nanoparticles. J. Alloys Compd. 2016, 658, 726–731. 10.1016/j.jallcom.2015.10.250. [DOI] [Google Scholar]
  12. Mao W.; Wang X.; Chu L.; Zhu Y.; Wang Q.; Zhang J.; Yang J.; Li X. a.; Huang W. Simultaneous enhancement of magnetic and ferroelectric properties in Dy and Cr co-doped BiFeO3 nanoparticles. Phys. Chem. Chem. Phys. 2016, 18, 6399–6405. 10.1039/C5CP07327H. [DOI] [PubMed] [Google Scholar]
  13. Wang X.; Mao W.; Wang Q.; Zhu Y.; Min Y.; Zhang J.; Yang T.; Yang J.; Li X. A.; Huang W. Low-temperature fabrication of Bi25FeO40/rGO nanocomposites with efficient photocatalytic performance under visible light irradiation. RSC Adv. 2017, 7, 10064–10069. 10.1039/C6RA27025E. [DOI] [Google Scholar]
  14. Li Z.; Shen Y.; Guan Y.; Hu Y.; Lin Y.; Nan C.-W. Bandgap engineering and enhanced interface coupling of graphene–BiFeO3 nanocomposites as efficient photocatalysts under visible light. J. Mater. Chem. A 2014, 2, 1967–1973. 10.1039/C3TA14269H. [DOI] [Google Scholar]
  15. Choi T.; Lee S.; Choi Y. J.; Kiryukhin V.; Cheong S. W. Switchable ferroelectric diode and photovoltaic effect in BiFeO3. Science 2009, 324, 63–66. 10.1126/science.1168636. [DOI] [PubMed] [Google Scholar]
  16. Sun B.; Wei L.; Li H.; Chen P. White-light-controlled ferromagnetic and ferroelectric properties of multiferroic single-crystalline BiFeO3 nanoflowers at room temperature. J. Mater. Chem. C 2014, 2, 7547–7551. 10.1039/C4TC01196A. [DOI] [Google Scholar]
  17. Papadas I.; Christodoulides J. A.; Kioseoglou G.; Armatas G. S. A high surface area ordered mesoporous BiFeO3 semiconductor with efficient water oxidation activity. J. Mater. Chem. A 2015, 3, 1587–1593. 10.1039/C4TA05272B. [DOI] [Google Scholar]
  18. An J.; Zhu L.; Wang N.; Song Z.; Yang Z.; Du D.; Tang H. Photo-Fenton like degradation of tetrabromobisphenol A with graphene BiFeO3 composite as a catalyst. Chem. Eng. J. 2013, 219, 225–237. 10.1016/j.cej.2013.01.013. [DOI] [Google Scholar]
  19. Iqbal M. Z.; Abdala A. A. Thermally reduced graphene: synthesis, characterization and dye removal applications. RSC Adv. 2013, 3, 24455–24464. 10.1039/c3ra43914c. [DOI] [Google Scholar]
  20. Irfan S.; Rizwan S.; Shen Y.; Li L.; Asfandiyar; Butt S.; Nan C. W. The Gadolinium (Gd3+) and Tin (Sn4+) Co-doped BiFeO3 Nanoparticles as New Solar Light Active Photocatalyst. Sci. Rep. 2017, 7, 42493. 10.1038/srep42493. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Soltani T.; Lee B.-K. Sono-synthesis of nanocrystallized BiFeO3/reduced graphene oxide composites for visible photocatalytic degradation improvement of bisphenol A. Chem. Eng. J. 2016, 306, 204–213. 10.1016/j.cej.2016.07.051. [DOI] [Google Scholar]
  22. Yoon Y.; Ganapathi K.; Salahuddin S. How good can monolayer MoS2 transistors be?. Nano Lett. 2011, 11, 3768–3773. 10.1021/nl2018178. [DOI] [PubMed] [Google Scholar]
  23. Yoo J. J.; Balakrishnan K.; Huang J.; Meunier V.; Sumpter B. G.; Srivastava A.; Conway M.; Reddy A. L.; Yu J.; Vajtai R.; Ajayan P. M. Ultrathin planar graphene supercapacitors. Nano Lett. 2011, 11, 1423–1427. 10.1021/nl200225j. [DOI] [PubMed] [Google Scholar]
  24. Schwierz F. Graphene transistors. Nat. Nanotechnol. 2010, 5, 487–496. 10.1038/nnano.2010.89. [DOI] [PubMed] [Google Scholar]
  25. Fowler J. D.; Allen M. J.; Tung V. C.; Yang Y.; Kaner R. B.; Weiller B. H. Practical chemical sensors from chemically derived graphene. ACS Nano 2009, 3, 301–306. 10.1021/nn800593m. [DOI] [PubMed] [Google Scholar]
  26. Bizeto M. A.; Shiguihara A. L.; Constantino V. R. L. Layered niobate nanosheets: building blocks for advanced materials assembly. J. Mater. Chem. 2009, 19, 2512. 10.1039/b821435b. [DOI] [Google Scholar]
  27. Stoller M. D.; Park S.; Zhu Y.; An J.; Ruoff R. S. Graphene-based ultracapacitors. Nano Lett. 2008, 8, 3498–3502. 10.1021/nl802558y. [DOI] [PubMed] [Google Scholar]
  28. Stankovich S.; Dikin D. A.; Dommett G. H.; Kohlhaas K. M.; Zimney E. J.; Stach E. A.; Piner R. D.; Nguyen S. T.; Ruoff R. S. Graphene-based composite materials. Nature 2006, 442, 282–286. 10.1038/nature04969. [DOI] [PubMed] [Google Scholar]
  29. Novoselov K. S.; Jiang D.; Schedin F.; Booth T. J.; Khotkevich V. V.; Morozov S. V.; Geim A. K. Two-dimensional atomic crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451–10453. 10.1073/pnas.0502848102. [DOI] [PMC free article] [PubMed] [Google Scholar]
  30. Joensen P.; Frindt R.; Morrison S. R. Single-layer MoS2. Mater. Res. Bull. 1986, 21, 457–461. 10.1016/0025-5408(86)90011-5. [DOI] [Google Scholar]
  31. Pacilé D.; Meyer J. C.; Girit Ç. Ö.; Zettl A. The two-dimensional phase of boron nitride: Few-atomic-layer sheets and suspended membranes. Appl. Phys. Lett. 2008, 92, 133107. 10.1063/1.2903702. [DOI] [Google Scholar]
  32. Takahashi N.; Hata H.; Kuroda K. Exfoliation of Layered Silicates through Immobilization of Imidazolium Groups. Chem. Mater. 2011, 23, 266–273. 10.1021/cm102942s. [DOI] [Google Scholar]
  33. Ma R.; Sasaki T. Nanosheets of oxides and hydroxides: Ultimate 2D charge-bearing functional crystallites. Adv. Mater. 2010, 22, 5082–5104. 10.1002/adma.201001722. [DOI] [PubMed] [Google Scholar]
  34. Gogotsi Y.Chemical vapour deposition: Transition metal carbides go 2D. Nat. Mater. 2015. [DOI] [PubMed] [Google Scholar]
  35. Anasori B.; Xie Y.; Beidaghi M.; Lu J.; Hosler B. C.; Hultman L.; Kent P. R.; Gogotsi Y.; Barsoum M. W. Two-dimensional, ordered, double transition metals carbides (MXenes). ACS Nano 2015, 9, 9507–9516. 10.1021/acsnano.5b03591. [DOI] [PubMed] [Google Scholar]
  36. Ghidiu M.; Naguib M.; Shi C.; Mashtalir O.; Pan L. M.; Zhang B.; Yang J.; Gogotsi Y.; Billinge S. J.; Barsoum M. W. Synthesis and characterization of two-dimensional Nb4C3 (MXene). Chem. Commun. 2014, 50, 9517–9520. 10.1039/C4CC03366C. [DOI] [PubMed] [Google Scholar]
  37. Naguib M.; Halim J.; Lu J.; Cook K. M.; Hultman L.; Gogotsi Y.; Barsoum M. W. New two-dimensional niobium and vanadium carbides as promising materials for Li-ion batteries. J. Am. Chem. Soc. 2013, 135, 15966–15969. 10.1021/ja405735d. [DOI] [PubMed] [Google Scholar]
  38. Khazaei M.; Arai M.; Sasaki T.; Chung C.-Y.; Venkataramanan N. S.; Estili M.; Sakka Y.; Kawazoe Y. Novel Electronic and Magnetic Properties of Two-Dimensional Transition Metal Carbides and Nitrides. Adv. Funct. Mater. 2013, 23, 2185–2192. 10.1002/adfm.201202502. [DOI] [Google Scholar]
  39. Naguib M.; Mashtalir O.; Carle J.; Presser V.; Lu J.; Hultman L.; Gogotsi Y.; Barsoum M. W. Two-dimensional transition metal carbides. ACS Nano 2012, 6, 1322–1331. 10.1021/nn204153h. [DOI] [PubMed] [Google Scholar]
  40. Naguib M.; Kurtoglu M.; Presser V.; Lu J.; Niu J.; Heon M.; Hultman L.; Gogotsi Y.; Barsoum M. W. Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2. Adv. Mater. 2011, 23, 4248–4253. 10.1002/adma.201102306. [DOI] [PubMed] [Google Scholar]
  41. Naguib M.; Mochalin V. N.; Barsoum M. W.; Gogotsi Y. 25th Anniversary Article: MXenes: A New Family of Two-Dimensional Materials. Adv. Mater. 2014, 26, 992–1005. 10.1002/adma.201304138. [DOI] [PubMed] [Google Scholar]
  42. Khazaei M.; Arai M.; Sasaki T.; Estili M.; Sakka Y. Two-dimensional molybdenum carbides: potential thermoelectric materials of the MXene family. Phys. Chem. Chem. Phys. 2014, 16, 7841–7849. 10.1039/C4CP00467A. [DOI] [PubMed] [Google Scholar]
  43. Kurtoglu M.; Naguib M.; Gogotsi Y.; Barsoum M. W. First principles study of two-dimensional early transition metal carbides. MRS Communications 2012, 2, 133–137. 10.1557/mrc.2012.25. [DOI] [Google Scholar]
  44. Dai J. F.; Xian T.; Di L. J.; Yang H. Preparation of BiFeO3-graphene nanocomposites and their enhanced photocatalytic activities. J. Nanomater. 2013, 2013, 1. 10.1155/2013/642897. [DOI] [Google Scholar]
  45. Irfan S.; Li L.; Saleemi A. S.; Nan C.-W. Enhanced photocatalytic activity of La3+ and Se4+ co-doped bismuth ferrite nanostructures. J. Mater. Chem. A 2017, 5, 11143–11151. 10.1039/C7TA01847A. [DOI] [Google Scholar]
  46. Mashtalir O., Chemistry of two-dimensional transition metal carbides (MXenes); Drexel University: 2015. [Google Scholar]
  47. Luo J.; Tao X.; Zhang J.; Xia Y.; Huang H.; Zhang L.; Gan Y.; Liang C.; Zhang W. Sn4+ ion decorated highly conductive Ti3C2 MXene: promising lithium-ion anodes with enhanced volumetric capacity and cyclic performance. ACS Nano 2016, 10, 2491–2499. 10.1021/acsnano.5b07333. [DOI] [PubMed] [Google Scholar]
  48. Irfan S.; Rizwan S.; Shen Y.; Tomovska R.; Zulfiqar S.; Sarwar M. I.; Nan C.-W. Mesoporous template-free gyroid-like nanostructures based on La and Mn co-doped bismuth ferrites with improved photocatalytic activity. RSC Adv. 2016, 6, 114183–114189. 10.1039/C6RA23674J. [DOI] [Google Scholar]
  49. Cheng X.; Gu S.; Centeno A.; Dawson G. Plasmonic enhanced Cu2O-Au-BFO photocathodes for solar hydrogen production. Sci. Rep. 2019, 9, 5140. 10.1038/s41598-019-41613-3. [DOI] [PMC free article] [PubMed] [Google Scholar]
  50. Tian Y.; Xue F.; Fu Q.; Zhou L.; Wang C.; Gou H.; Zhang M. Structural and physical properties of Ti-doped BiFeO3 nanoceramics. Ceram. Int. 2018, 44, 4287–4291. 10.1016/j.ceramint.2017.12.013. [DOI] [Google Scholar]
  51. Iqbal M. A.; Ali S. I.; Amin F.; Tariq A.; Iqbal M. Z.; Rizwan S. La-and Mn-Codoped Bismuth Ferrite/Ti3C2 MXene Composites for Efficient Photocatalytic Degradation of Congo Red Dye. ACS Omega 2019, 4, 8661–8668. 10.1021/acsomega.9b00493. [DOI] [PMC free article] [PubMed] [Google Scholar]
  52. Coondoo I.; Panwar N.; Bdikin I.; Puli V.; Katiyar R.; Kholkin A. Structural, morphological and piezoresponse studies of Pr and Sc co-substituted BiFeO3 ceramics. J. Phys. D: Appl. Phys. 2012, 45, 055302 10.1088/0022-3727/45/5/055302. [DOI] [Google Scholar]
  53. Wang T.-H.; Tu C.-S.; Ding Y.; Lin T.-C.; Ku C.-S.; Yang W.-C.; Yu H.-H.; Wu K.-T.; Yao Y.-D.; Lee H.-Y. Phase transition and ferroelectric properties of xBiFeO3–(1– x)BaTiO3 ceramics. Curr. Appl. Phys. 2011, 11, S240–S243. 10.1016/j.cap.2011.01.037. [DOI] [Google Scholar]
  54. Mohanty B.; Parida B.; Parida R. Structural and conduction behaviour of (BaSr)0.5TiO3 modified in BFO perovskite. Mater. Chem. Phys. 2019, 225, 91–98. 10.1016/j.matchemphys.2018.12.076. [DOI] [Google Scholar]
  55. Awan M.; Bhatti A. Synthesis and multiferroic properties of BFO ceramics by melt-phase sintering. J. Mater. Eng. Perform. 2011, 20, 283–288. 10.1007/s11665-010-9663-8. [DOI] [Google Scholar]
  56. Singh S.; Ishiwara H.; Maruyama K. Room temperature ferroelectric properties of Mn-substituted BiFeO3 thin films deposited on Pt electrodes using chemical solution deposition. Appl. Phys. Lett. 2006, 88, 262908. 10.1063/1.2218819. [DOI] [Google Scholar]
  57. Ma Z.; Hu Z.; Zhao X.; Tang Q.; Wu D.; Zhou Z.; Zhang L. Tunable band structures of heterostructured bilayers with transition-metal dichalcogenide and MXene monolayer. J. Phys. Chem. C 2014, 118, 5593–5599. 10.1021/jp500861n. [DOI] [Google Scholar]
  58. Ashraf M. W.; Tayyaba S.; Ali S. M.; Ramay S. M.; Saleem M. BiFeO3 and La doped BiFeO3 nano-particles decorated anodic Al2O3 porous template fabricated with two step anodization. Mater. Lett. 2019, 244, 115–118. 10.1016/j.matlet.2019.02.061. [DOI] [Google Scholar]
  59. Rice S. B.; Chan C.; Brown S. C.; Eschbach P.; Han L.; Ensor D. S.; Stefaniak A. B.; Bonevich J.; Vladár A. E.; Walker A. R. H.; Zheng J. Particle size distributions by transmission electron microscopy: an interlaboratory comparison case study. Metrologia 2013, 50, 663. 10.1088/0026-1394/50/6/663. [DOI] [PMC free article] [PubMed] [Google Scholar]
  60. Meng W.; Hu R.; Yang J.; Du Y.; Li J.; Wang H. Influence of lanthanum-doping on photocatalytic properties of BiFeO3 for phenol degradation. Chin. J. Catal. 2016, 37, 1283–1292. 10.1016/S1872-2067(16)62449-X. [DOI] [Google Scholar]
  61. Mullet M.; Khare V.; Ruby C. XPS study of Fe(II)-Fe(III) (oxy)hydroxycarbonate green rust compounds. Surf. Interface Anal. 2008, 40, 323–328. 10.1002/sia.2758. [DOI] [Google Scholar]
  62. Ballirano P.; Pacella A.; Cremisini C.; Nardi E.; Fantauzzi M.; Atzei D.; Rossi A.; Cametti G. Fe (II) segregation at a specific crystallographic site of fibrous erionite: A first step toward the understanding of the mechanisms inducing its carcinogenicity. Microporous Mesoporous Mater. 2015, 211, 49–63. 10.1016/j.micromeso.2015.02.046. [DOI] [Google Scholar]
  63. Chuai H.; Zhou D.; Zhu X.; Li Z.; Huang W. Characterization of V2O5/MoO3 composite photocatalysts prepared via electrospinning and their photodegradation activity for dimethyl phthalate. Chin. J. Catal. 2015, 36, 2194–2202. 10.1016/S1872-2067(15)61002-6. [DOI] [Google Scholar]
  64. Ke S.; Lin P.; Zeng X.; Huang H.; Zhou L. M.; Mai Y. W. Tuning of dielectric and ferroelectric properties in single phase BiFeO3 ceramics with controlled Fe2+/Fe3+ ratio. Ceram. Int. 2014, 40, 5263–5268. 10.1016/j.ceramint.2013.10.098. [DOI] [Google Scholar]
  65. Da Silva L. F.; Lopes O. F.; Catto A. C.; Avansi W.; Bernardi M. I.; Li M. S.; Ribeiro C.; Longo E. Hierarchical growth of ZnO nanorods over SnO2 seed layer: insights into electronic properties from photocatalytic activity. RSC Adv. 2016, 6, 2112–2118. 10.1039/C5RA23824B. [DOI] [Google Scholar]
  66. Wang T.; Lang J.; Zhao Y.; Su Y.; Zhao Y.; Wang X. Simultaneous doping and heterojunction of silver on Na2Ta2O6 nanoparticles for visible light driven photocatalysis: the relationship between tunable optical absorption, defect chemistry and photocatalytic activity. CrystEngComm 2015, 17, 6651–6660. 10.1039/C5CE01125F. [DOI] [Google Scholar]
  67. Li H.; Zhou Y.; Tu W.; Ye J.; Zou Z. State-of-the-art progress in diverse heterostructured photocatalysts toward promoting photocatalytic performance. Adv. Funct. Mater. 2015, 25, 998–1013. 10.1002/adfm.201401636. [DOI] [Google Scholar]
  68. Myhra S.; Crossley J.; Barsoum M. Crystal-chemistry of the Ti3AlC2 and Ti4AlN3 layered carbide/nitride phases—characterization by XPS. J. Phys. Chem. Solids 2001, 62, 811–817. 10.1016/S0022-3697(00)00268-7. [DOI] [Google Scholar]
  69. Li Y.; Sun Y.; Ho W.; Zhang Y.; Huang H.; Cai Q.; Dong F. Highly enhanced visible-light photocatalytic NOx purification and conversion pathway on self-structurally modified g-C3N4 nanosheets. Sci. Bull. 2018, 63, 609–620. 10.1016/j.scib.2018.04.009. [DOI] [PubMed] [Google Scholar]
  70. Sing K. S. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984). Pure Appl. Chem. 1985, 57, 603–619. 10.1351/pac198557040603. [DOI] [Google Scholar]
  71. Kubelka P.; Munk F. Ein Beitrag Zur Optik der Farbanstriche. Zeitschrift fur Technische Physik 1931, 12, 593. [Google Scholar]
  72. Butler M. Photoelectrolysis and physical properties of the semiconducting electrode WO2. J. Appl. Phys. 1977, 48, 1914–1920. 10.1063/1.323948. [DOI] [Google Scholar]
  73. Xu X.; Lin Y. H.; Li P.; Shu L.; Nan C. W. Synthesis and photocatalytic behaviors of high surface area BiFeO3 thin films. J. Am. Ceram. Soc. 2011, 94, 2296–2299. 10.1111/j.1551-2916.2011.04653.x. [DOI] [Google Scholar]
  74. Li S.; Lin Y.-H.; Zhang B.-P.; Wang Y.; Nan C.-W. Controlled fabrication of BiFeO3 uniform microcrystals and their magnetic and photocatalytic behaviors. J. Phys. Chem. C 2010, 114, 2903–2908. 10.1021/jp910401u. [DOI] [Google Scholar]
  75. Joshi U. A.; Jang J. S.; Borse P. H.; Lee J. S. Microwave synthesis of single-crystalline perovskite BiFeO3 nanocubes for photoelectrode and photocatalytic applications. Appl. Phys. Lett. 2008, 92, 242106. 10.1063/1.2946486. [DOI] [Google Scholar]
  76. Afkhami A.; Moosavi R. Adsorptive removal of Congo red, a carcinogenic textile dye, from aqueous solutions by maghemite nanoparticles. J. Hazard. Mater. 2010, 174, 398–403. 10.1016/j.jhazmat.2009.09.066. [DOI] [PubMed] [Google Scholar]
  77. Rouquerol J.; Rouquerol F.; Llewellyn P.; Maurin G.; Sing K. S.. Adsorption by powders and porous solids: principles, methodology and applications; Academic Press: 2013. [Google Scholar]
  78. Ramsay R.; Dunford C.; Gillman P. Methylene blue and serotonin toxicity: inhibition of monoamine oxidase A (MAO A) confirms a theoretical prediction. Br. J. Pharmacol. 2007, 152, 946–951. 10.1038/sj.bjp.0707430. [DOI] [PMC free article] [PubMed] [Google Scholar]
  79. Cai J.; Huang J.; Wang S.; Iocozzia J.; Sun Z.; Sun J.; Yang Y.; Lai Y.; Lin Z. Crafting Mussel-Inspired Metal Nanoparticle-Decorated Ultrathin Graphitic Carbon Nitride for the Degradation of Chemical Pollutants and Production of Chemical Resources. Adv. Mater. 2019, 31, 1806314. 10.1002/adma.201806314. [DOI] [PubMed] [Google Scholar]
  80. Tariq A.; Ali S. I.; Akinwande D.; Rizwan S. Efficient Visible-Light Photocatalysis of 2D-MXene Nanohybrids with Gd3+- and Sn4+-Codoped Bismuth Ferrite. ACS Omega 2018, 3, 13828–13836. 10.1021/acsomega.8b01951. [DOI] [PMC free article] [PubMed] [Google Scholar]
  81. Irfan S.; Shen Y.; Rizwan S.; Wang H. C.; Khan S. B.; Nan C. W. Band-Gap Engineering and Enhanced Photocatalytic Activity of Sm and Mn Doped BiFeO3 Nanoparticles. J. Am. Ceram. Soc. 2016, 31–40. 10.1111/jace.14487. [DOI] [Google Scholar]
  82. Li X.; Yu J.; Jaroniec M.; Chen X. Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels. Chem. Rev. 2019, 119, 3962–4179. 10.1021/acs.chemrev.8b00400. [DOI] [PubMed] [Google Scholar]
  83. Low J.; Yu J.; Jaroniec M.; Wageh S.; Al-Ghamdi A. A. Heterojunction Photocatalysts. Adv. Mater. 2017, 29, 1601694. 10.1002/adma.201601694. [DOI] [PubMed] [Google Scholar]
  84. Li X.; Xie J.; Jiang C.; Yu J.; Zhang P. Review on design and evaluation of environmental photocatalysts. Front. Environ. Sci. Eng. 2018, 12, 14. 10.1007/s11783-018-1076-1. [DOI] [Google Scholar]
  85. Shen R.; Jiang C.; Xiang Q.; Xie J.; Li X. Surface and interface engineering of hierarchical photocatalysts. Appl. Surf. Sci. 2019, 471, 43–87. 10.1016/j.apsusc.2018.11.205. [DOI] [Google Scholar]
  86. Cai J. S.; Huang J. Y.; Lai Y. K. 3D Au-decorated BiMoO6 nanosheet/TiO2 nanotube array heterostructure with enhanced UV and visible-light photocatalytic activity. J. Mater. Chem. A 2017, 5, 16412–16421. 10.1039/C7TA02077E. [DOI] [Google Scholar]
  87. Irfan S.; Shen Y.; Rizwan S.; Wang H.-C.; Khan S. B.; Nan C.-W.; Xie R. J. Band-Gap Engineering and Enhanced Photocatalytic Activity of Sm and Mn Doped BiFeO3 Nanoparticles. J. Am. Ceram. Soc. 2017, 100, 31–40. 10.1111/jace.14487. [DOI] [Google Scholar]
  88. Fatima S.; Ali S. I.; Iqbal M. Z.; Rizwan S. The high photocatalytic activity and reduced band gap energy of La and Mn co-doped BiFeO3/graphene nanoplatelet (GNP) nanohybrids. RSC Adv. 2017, 7, 35928–35937. 10.1039/C7RA04281G. [DOI] [Google Scholar]

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES