Aerobic Oil-Phase Cyclic Magnetic Adsorption to Synthesize 1D Fe2O3@TiO2 Nanotube Composites for Enhanced Visible-Light Photocatalytic Degradation

Abstract

In this work, Fe₂O₃@TiO₂ nanostructures with staggered band alignment were newly designed by an aerobic oil-phase cyclic magnetic adsorption method. XRD and TEM analyses were performed to verify the uniform deposition of Fe₂O₃ nanoparticles on the nanotube inner walls of TiO₂. The steady-state degradation experiments exhibited that 1FeTi possessed the most superior performance, which might be ascribable to the satisfying dark adsorption capacity, efficient photocatalytic activity, ease of magnetic separation, and economic efficiency. These results indicated that the deposition of Fe₂O₃ into TiO₂ nanotubes significantly enhanced the activity of Fe₂O₃, which was mainly ascribed to the Fe₂O₃-induced formation of staggered iron oxides@TiO₂ band alignment and thus efficient separation of h⁺ and e⁻. Furthermore, the PL intensity and lifetime of the decay curve were considered as key criterions for the activity’s evaluation. Finally, the leaching tests and regeneration experiments were also performed, which illustrated the inhibited photodissolution compared with TiO₂/Fe₃O₄ and stable cycling ability, enabling 1FeTi to be a promising magnetic material for photocatalytic water remediation.

1. Introduction

In recent years, water pollution has become an international environmental problem that constricts the development of human health, economy and sustainability [1,2]. To solve this problem, various efficient technologies have been developed and employed to purify wastewater, such as photocatalysis, adsorption, chemical precipitation, and membrane filtration [3,4,5,6]. Among them, photocatalysis has been regarded as a promising approach due to its convenient operation, effective remediation and low environmental impacts [7,8]. Among the well-studied photocatalysts, nano iron oxides including α-Fe₂O₃, γ-Fe₂O₃ and Fe₃O₄ have received extensive investigations and increasing attentions in the field of photocatalytic decontamination, benefiting from low cost, nontoxicity, large surface area, and especially strong absorption in the visible light region [9,10]. Moreover, the favorable magnetism of iron oxides makes the recovery of catalysts more convenient and economical via applying an external magnetic field for cyclic utilization after wastewater treatment [11].

However, the major drawbacks of iron oxides including low charge carriers’ mobility and rapid photogenerated electron–hole recombination rate restrict its practical applications in photocatalysis [12]. Consequently, various strategies were developed to enhance its photocatalytic properties, such as controlling diverse morphologies (e.g., nanoparticles, nanocubes and nanodiscs) [13], doping metal or non-metal ions such as Ca and I [14,15], and coupling with other semiconductors to generate satisfactory photocatalytic heterostructures. Among them, the coupling strategy with other semiconductors has been receiving extensive attentions since these heterostructures can efficiently suppress recombination rates and promote transportation rates of photo-generated charge carriers. For instance, the heterogeneous photocatalysts including α-Fe₂O₃@AgCO₃ [16], γ-Fe₂O₃@Mn₃O₄ [17] and Fe₃O₄@TiO₂ [18] all displayed significantly increased photocatalytic performance compared with pure iron oxides.

Among the various semiconductors combined with iron oxides, TiO₂ has been extensively investigated for their stable, nontoxic and economical properties [19,20]. Particularly, abundant attentions have been given to 1D TiO₂ nanotubes (abbreviated as TNT in this article) due to their unique 1D nanotube architectures accompanied with superior charge transport property and large internal surface to disperse doped iron oxides nanoparticles [21,22,23]. Especially, the constructed heterostructures of TiO₂ and iron oxides could further enhance the separation of charge carriers in this heterostructure. Recently, the majority of the researches on Fe_xO_y@TNT nanocomposites focused on Fe₃O₄@TNT composites [24,25]. Unfortunately, the narrow bandgap of Fe₃O₄ brought serious photodissolution of Fe₃O₄, which would deactivate the composites and induce the decrease of photocatalytic performance [25]. As a result, iron oxide species with larger bandgaps such as Fe₂O₃ (bandgap ~2.3 eV) rather than Fe₃O₄ with narrow bandgap (0.1 eV), are more beneficial for the separation of photo-induced charge carriers in the heterojunctions [26,27]. On the other hand, Fe₂O₃ possesses higher chemical stability than Fe₃O₄ at room temperature, which can be oxidized in the presence of oxygen. From this point of view, it is reasonable to believe that introducing Fe₂O₃ nanoparticles into TiO₂ nanotubes would significantly enhance the separation of charge carriers in Fe₂O₃@TiO₂ heterojunctions. Furthermore, the excellent magnetic property of Fe₂O₃ would make them easier to be recycled and regenerated in the photocatalytic decontamination research, which is considered as a sustainable material for future applications [11]. Although Fe₂O₃@TNT composites have bright prospects and potential applications in photocatalytic water remediation, efficient methods to synthesize TiO₂ nanotubes with uniform Fe₂O₃ nanoparticles deposition still need to be developed since the agglomeration of Fe₂O₃ nanoparticles would act as the recombination center and accelerate the destruction of excitons. The extensively used manner to prepare this heterostructure is the dipping method, which is limited by the low-dispersion loading and severe agglomeration under high deposition [28,29]. In our previous work, an anaerobic oil-phase cyclic magnetic adsorption (OCMA) method was developed to uniformly deposit Fe₃O₄ nanoparticles into TNT, which would provide inspiration to inhibit the agglomeration of Fe₂O₃ nanoparticles in this work. Especially, although the carbon species was introduced into the designed Fe₃O₄@TiO₂ composites to inhibit the migration of e^- towards Fe₃O₄, 13.5% (in 3 h) of Fe₃O₄ in the prepared Fe₃O₄@C@TiO₂ composites were still photo-dissolved, which might induce poor recycling performance [27]. As a result, the wide-bandgap Fe₂O₃ nanoparticles, instead of narrow-bandgap Fe₃O₄, were designed for fabricating Fe₂O₃@TiO₂ composites with enhanced photocatalytic performance and inhibited photo dissolution. Especially, the designed Fe₂O₃@TiO₂ heterostructures avoid the incorporation of carbon materials, which might have unassessed toxicity to the environment. Nevertheless, there was little work on the synthesis method and photocatalytic investigation of TiO₂ nanotubes with uniform deposition of Fe₂O₃ nanoparticles.

In this work, uniform 1D magnetic Fe₂O₃@TNT composites with excellent dispersive Fe₂O₃ deposition were newly synthesized through a new aerobic OCMA method. This new synthesizing method is more convenient and economical compared with other reported methods. Moreover, the photocatalytic performance of these nanocomposites containing different amounts of Fe₂O₃ under visible light irradiation were evaluated and discussed. The results indicated that there was an optimum amount of Fe₂O₃ deposition, which could be well explained by the physicochemical properties including Fe₂O₃ dispersion, PL intensity and charge carriers’ lifetime. Finally, the photo dissolution of the obtained composites was further examined by the leaching experiments and the results indicated superior repression of photo dissolution and stable regeneration of Fe₂O₃@TiO₂ compared with Fe₃O₄@TiO₂ and Fe₃O₄@C@TiO₂. 2. Materials and Methods 2.1. Materials and Reagents Titanium foil (10 × 25 mm², 0.1 mm thickness, ≥99.5%) was purchased from Beijing Qianshuo Non-ferrous Metal Co., Ltd. (Beijing, China). FeCl₃·6H₂O (≥99 wt%), FeSO₄·7H₂O (≥99 wt%) and NH₄F (≥99.5 wt%) were supplied by Aladdin Co., Ltd. (Tianjin, China). n-hexane (C₆H₁₄), ethanol (C₂H₅OH), ethylene glycol ((CH₂OH)₂), oleic acid (C₁₈H₃₄O₂), Rhodamine B (RhB), and ammonia water (NH₃·H₂O, 25 wt%) were provided by Guangfu Co., Ltd. (Tianjin, China). All reagents were analytical grade and used as received without further purification. 2.2. Synthesis of TiO₂ Nanotubes

Electrochemical anodization was applied to synthesize self-organized porous TNT [30]. Prior to anodization, the Ti foils were ultrasonically treated in n-hexane, followed with dipping in ethanol and deionized water respectively, and then dried in a nitrogen stream. Then, the Ti foils were anodized under constant potential (60 V) for 4 h in a two-electrode system where Ti served as the working electrode and platinum (Pt) foil served as the counter electrode. The electrolyte was an ethylene glycol solution containing NH₄F (0.135 M) and H₂O (2% in volume). After anodization, the as-anodized samples were ultrasonically cleaned in ethanol for 5 min to remove surface debris and then stored in hexane for further deposition.

2.3. Synthesis of Fe₃O₄@oleic Acid

Fe₃O₄@oleic acid nanoparticles were prepared by chemical co-precipitation method [31]. The procedure was as follows: FeCl₃·6H₂O (140 mmol) and FeSO₄·7H₂O (70 mmol) were dissolved into 100 mL deionized water and then the solution was heated to 80 °C in 20 min with vigorous stirring. Then, 3 mL oleic acid and 15 mL ammonia water were added rapidly into this solution. After stirring for 1 h, the black magnetic gel was cooled to 25 °C and separated by magnetic decantation. The precipitated particles were washed several times by ethanol to remove the excess oleic acid, followed by drying in vacuum oven at 60 °C for 12 h.

2.4. Synthesis of 1D Fe₂O₃@TiO₂ Nanotube Composites

Fe₂O₃@TiO₂ nanotube composites were first synthesized via an aerobic OCMA method [27]. Firstly, certain amounts of Fe₃O₄@oleic acid were ultrasonically dispersed in hexane and this suspension was filtered to remove the undispersed particles thoroughly and prepare a homogeneous solution of 1 g/L Fe₃O₄@oleic acid (The preparation of 1 g/L Fe₃O₄@oleic acid solution: Firstly, certain amounts (marked as M₁) of Fe₃O₄@oleic acid were ultrasonically dispersed in hexane. Then, this suspension was filtered to remove the undispersed particles thoroughly. The mass of these undispersed particles can be determined and marked as M₂. Therefore, the real amounts of Fe₃O₄@oleic acid dissolved in hexane can be calculated by_M1−_M2 . After adding hexane to a certain volume, the concentration of “Fe₃O₄@oleic acid” in the homogeneous solution can be confirmed). Then, different volumes of the homogeneous solution (0.5 mL, 1 mL, 2 mL) were added onto the anodized TNT arrays under the effect of an external magnetic field beneath the TNT arrays, as shown in Scheme 1. It should be noticed that magnetic force played a vital role in loading Fe₃O₄@oleic acid nanoparticles into TiO₂ nanotubes and the volatilization of hexane also made a contribution. Finally, these 0.5, 1, and 2 mL suspension-loaded TNT arrays were annealed at 450 °C for 2 h in air atmosphere to obtain Fe₂O₃@TNT composites with high crystallinity, which were recorded as 0.5FeTi, 1FeTi and 2FeTi, respectively. In addition, Fe₃O₄@oleic acid was also annealed at the same conditions to prepare pure Fe₂O₃ nanoparticles for comparative experiments.

2.5. Photocatalytic Experiments For assessments of the photocatalytic activity of the samples, Rhodamine B (RhB) solutions in concentration of 10 mg/L were first prepared. Then, 70 mg as-prepared catalysts (Fe₂O₃, 0.5FeTi, 1FeTi and 2FeTi) were suspended into 70 mL RhB solutions, respectively. After dark adsorption for 2 h and reaching equilibrium, the mixed solutions were illuminated for 6 h under visible light (λ ≥ 400 nm) using a 300 W Xenon lamp (light intensity of 90 mW·cm⁻² at distance of 15 cm from the light source). During the photocatalytic experiments, 3 mL samples of the mixed solutions were taken out and centrifuged at a given time interval. Finally, the photocatalytic activity was analyzed by UV−vis absorption measurement of the characteristic peak of RhB at 554 nm. For each sample, all photocatalytic experiments were carried out for irradiation duration of 6 h at 20 °C and were repeated three times. 2.6. Characterizations The obtained samples were characterized by an XRD diffractometer (Rigaku, D/max 2500, Cu Kα radiation, λ = 1.5418 Å, Tokyo, Japan) within the scanning angle range of 15°–60° at rate of 8° min⁻¹. The morphology was studied by scanning electron microscopy (SEM, Nanosem 430, FEI, Eindhoven, The Netherlands) and high-resolution transmission electron microscopy (HRTEM, Tecnai G20&F20, FEI, Eindhoven, The Netherlands). UV-Vis measurements were performed on a Hitachi U4100 UV Spectrometer and Fluorescence spectra were measured with FLS980 Series of Fluorescence Spectrometers (excited at λ = 420 nm). X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, Thermo, MA, USA) with monochromatic Al Kα (1486.6 eV) X-ray source was employed to analyze elemental contents and the chemical states of these composites. All the binding energies were calibrated by contaminant carbon at 284.8 eV. BET surface areas, BJH pore volume and average pore size were determined by N₂ adsorption–desorption isotherms, which was measured by SSA-7000 (BJ Builder, Beijing, China). FTIR spectra were recorded on Bruker Alpha spectrometer from 4000 to 400 cm⁻¹ (Regensburg, Germany). The magnetic properties were measured by vibrating sample magnetometer (VSM, MPMS SQUID XL, Quantum Design, Santiago, MN, USA) in the field from −20,000 to 20,000 Oe. High-resolution mass spectra (HR-MS) were measured by a mass spectrometer (Bruker, solanX 70 FT-MS, Regensburg, Germany) in electrospray ionization (ESI) mode. 3. Results and Discussion 3.1. Structural and Elemental Characterizations

The crystal structures of these as-synthesized samples were determined by XRD analysis, as shown in Figure 1A. It can be found that the XRD patterns of as-prepared pure Fe₂O₃ contain diffraction peaks of both α-Fe₂O₃ at 33.2° (104) and γ-Fe₂O₃ at 30.2° (220), which indicates the coexistence of α-Fe₂O₃ and γ-Fe₂O₃ [32,33]. After the formation of a heterojunction, a series of new peaks, which can be well indexed to anatase TiO₂ (JCPDS 21-1272), appear in the 1D Fe₂O₃@TNT composites. Since the peaks related to iron species were hardly to be observed due to the small amount of deposited Fe₂O₃ in Figure 1A, the narrow-range XRD spectra with higher resolution were further performed to examine the iron oxide species. As shown in Figure 1B, the characteristic peak intensity at 35.6° increases with the rising contents of Fe₂O₃ in 0.5FeTi, 1FeTi and 2FeTi, which further confirms the successful deposition of iron oxides. Furthermore, 2FeTi and Fe₂O₃ samples were further selected and analyzed by selective area electron diffraction (SAED) to ensure the existence of Fe₂O₃ (Figure 1C,D). As shown in Figure 1C, the SAED pattern of Fe₂O₃ with characteristic rings at 3.023 Å, 2.736 Å and 2.565 Å (from inside to outside) is in good agreement with the d-spacing of γ-Fe₂O₃ (d₂₂₀ = 2.953 Å), α-Fe₂O₃ (d₁₀₄ = 2.700 Å) and γ-Fe₂O₃ (d₃₁₁ = 2.518 Å) [34]. For the SAED pattern of 2FeTi composite in Figure 1D, the diffractive rings of Fe₂O₃ are weak but still in agreement with corresponding crystal planes of Fe₂O₃, which might be due to the low and uniform dispersion of Fe₂O₃. Especially, it should be mentioned that the SAED patterns of 1FeTi and 0.5FeTi are also examined while the weak signals of Fe₂O₃ hinder their further characterization by this method. Nevertheless, the coexistence of α-Fe₂O₃ and γ-Fe₂O₃ in these composites is reasonable considering that all the samples are annealed at the same condition.

To further confirm the inner structures of the prepared samples, SEM, TEM and HRTEM analyses were employed and the results are shown in Figure 2. SEM image of anodized TiO₂ nanotubes in Figure 2A reveals that the nanotubes are highly ordered with a uniformed outer diameter distribution of ~108 nm. After doping with Fe₂O₃, the well dispersed black iron oxides nanoparticles on the inner walls of TNT can be clearly observed in the HRTEM image of 0.5FeTi, as presented in Figure 2B. The nanotubes with diameter of 105 nm also meet well with the SEM image in Figure 2A. With the increasing deposition, it is observed that the number of black Fe₂O₃ nanoparticles on the inner walls increases, as shown in Figure 2B,C,E. Furthermore, the Fe₂O₃ nanoparticles in 1FeTi (Figure 2C) and 2FeTi (Figure 2E) have similar inner structures with 0.5FeTi (Figure 2B) and are also well dispersed on TNT inner walls, indicating the successful deposition of Fe₂O₃ into TNT. Then, the crystalline forms of the synthesized composites were further examined by the HRTEM image of 1FeTi in Figure 2F. The lattice fringes with interplanar spacings of 0.187 nm, 0.205 nm and 0.482 nm are well ascribed to the (200) plane of TiO₂, (202) plane of α-Fe₂O₃ and (111) plane of γ-Fe₂O₃, respectively, which is consistent with the observations from XRD. Finally, pure Fe₂O₃ nanoparticles were also examined by TEM (Figure 2D) and the aggregation of Fe₂O₃ particles is quite obvious, indicating the uniform dispersion could be easily achieved after doping into TiO₂ nanotubes.

After the determination of crystalline forms and inner structures of the synthesized composites, XPS analysis was carried out to further characterize the doping contents and surface chemical states of the obtained products, and the results are displayed in Figure 3. Figure 3A is the full-range spectra of the synthesized Fe₂O₃@TNT nanocomposites and the presence of C, Ti, O, Fe elements could be ascertained according to the peaks of C 1s, Ti 2p, O 1s, and Fe 2p. Specifically, the characteristic peak of C 1s situated at ~284.8 eV is attributed to the contaminant carbon, which is used for the calibration of the XPS spectra [35]. The Ti 2p_1/2 and 2p_3/2 peaks are observed at 464.1 and 458.0 eV respectively, which is in agreement with the typical anatase TiO₂ [36]. The O 1s spectrum centered at 529.0 eV is ascribed to the metal oxides including Fe₂O₃ and TiO₂. The Fe 2p spectra at 710.2 and 724.1 eV are the characteristic peaks of Fe₂O₃ and they were selected to analyze the chemical states of the doped Fe₂O₃ [37], as shown in Figure 3B–D. The Fe 2p spectra exhibit two contributions of 2p_1/2 and 2p_3/2 located at 723.9 and 710.4 eV, which can be assigned to Fe(III) oxidation state [37]. The additional peaks at 729.6 and 716.4 eV are ascribable to the satellite peaks of Fe(III) oxidation state. Especially, the peak intensity of Fe 2p increases with the growth of the Fe₂O₃ content, which is well consistent with the XRD results in Figure 1.

In order to identify the elemental molar ratios of the samples, careful calculation based on Figure 3A was performed after subtracting the peak of contaminant carbon, and the results are listed in Table 1. The O/Ti ratios in 0.5FeTi, 1FeTi and 2FeTi are 2.02, 2.17 and 2.31 respectively, which are close to theoretical values (2). Especially, the O/Ti ratios increase with the rising depositions, which further confirms the successful loading of Fe₂O₃ on the inner walls of TNT. It should be mentioned that the calculated Fe content at low deposition (0.5FeTi) was smaller than the theoretical value, which might be due to the fairly slight and highly dispersed Fe₂O₃ nanoparticles on the surface of TiO₂ nanotubes. Except for the XPS detection, FTIR analysis was further carried out to characterize the chemical groups of the synthesized composites, which further confirms the successful deposition of Fe₂O₃ (shown in Figure S1).

To further determine the inner structures of the synthesized nanocomposites, nitrogen adsorption/desorption isotherms analyses were utilized to examine the surface area and the pore information, and the results are presented in Figure 4 and Figure S2. The specific surface areas and pore sizes as well as volumes were calculated according to BET and BJH methods and the results are displayed in Table 2. Figure 4 shows that all the samples exhibit typical type IV isotherms with H₃ hysteresis loop, suggesting the mesoporous structure of all the synthesized materials according to the IUPAC classification [38]. As shown in Table 2, the BET specific surface areas of these composites (0.5FeTi, 1FeTi, 2FeTi) increase significantly compared to bare Fe₂O₃. Considering the less BET-specific surface area of TNT (17.53 m²/g) than the composites, the enlargement of surface area is mainly ascribable to the uniformly doped Fe₂O₃ particles on TNT inner walls, which could not only enlarge the dispersibility of the iron oxides but also increase the contact area with the organic pollutants and thus enhanced photocatalytic performance. On the other hand, the BJH desorption branch pore sizes and volumes were selected to analyze the inner structures of the synthesized materials, as shown in Table 2. It can be observed that the pore sizes and volumes decrease with the increment of iron oxides, which also verifies the successful deposition of Fe₂O₃ in TiO₂ nanotubes. The large pore size and pore volume of 0.5FeTi reveal that the Fe₂O₃ nanoparticles are highly dispersed on the inner walls of TiO₂ nanotubes. With the increment of deposition, the pore sizes of 1FeTi and 2FeTi decrease sharply and fall into a range near pure Fe₂O₃, which is probably associated with the smaller inner structures between Fe₂O₃ nanoparticles, characterized at about 4 nm (as shown in the inset figure of Figure 4). Furthermore, it could be observed from the pore volume results of 1FeTi and 2FeTi that this value does not significantly decrease with the pore size, which further indicates more abundant pores in 1FeTi and 2FeTi and verifies the deduction mentioned above. Finally, it is observed that the peak area associated with Fe₂O₃ (marked by shadow ellipse in Figure 4) increases following this order: 1FeTi > 2FeTi > Fe₂O₃ > 0.5FeTi, which indicates the best deposition and dispersion of Fe₂O₃ nanoparticles in 1FeTi.

3.2. Photo-Chemical and Bandgap Characterizations

After the determination of the inner structures and chemical contents, the photo-chemical properties and bandgap information of the synthesized 1D nanocomposites were investigated through UV-Vis and PL spectra, as presented in Figure 5 and Figure 6.

Figure 5 shows the UV–vis absorption spectra of Fe₂O₃@TNT samples with variable Fe content and pure Fe₂O₃. The tangent lines at different wavelength range were used to determine the gap information of the synthesized materials. Fe₂O₃ possessed excellent absorption in both UV region and visible region. The tangent line of Fe₂O₃ in the wavelength from 240 nm to 255 nm correlated well with the bandgap of 2.33 eV, which is consistent with the reported value for Fe₂O₃ [26]. Besides the bandgap of Fe₂O₃ observed in Figure 5A, one more weak peak is observed in Figure 5C and the absorption bandgap energy can be extended to 0.79 eV, which is correlated with the midgap of Fe₂O₃. As for Fe₂O₃@TNT composites, the first peak that appeared in Figure 5A is mainly related to the doped iron oxides and it can be seen that the absorption bandgap energy improves slightly with increasing Fe content. Additionally, similar results are found in the third peak shown in Figure 5C. Furthermore, a new peak appeared in the range from 354 nm to 380 nm after the generation of Fe₂O₃@TNT heterojunction in Figure 5B, which is probably due to the migration of photogenerated charge carriers between TiO₂ and Fe₂O₃. After the correlation, it was found that the derived bandgap energy in Figure 5B decreases significantly with increasing loading of iron oxides.

Generally, single bandgap derived by UV-Vis spectrum cannot determine the band position, and further characterizations such as ultraviolet photoelectron spectroscopy (UPS) or Mott-Schottky measurement should be utilized to determine the position of valence band or conduction band [14,39]. Nevertheless, as the gaps derived in Figure 5B were regarded as the results of electron migration between Fe₂O₃ and TiO₂, the relative positions of their individual gaps could be obtained. Thus, based on the extrapolation of UV–vis spectra in Figure 5, the band structures of these four catalysts were derived and are schematically shown in Scheme 2. It is interesting to note that the band alignment of these heterostructures are staggered form, which is not in accordance with the native included alignment of Fe_xO_y@TiO₂ [40]. Such band alignments might be attributed to the coexistence α-Fe₂O₃ and γ-Fe₂O₃, resulting in the formation of staggered band alignment of α-Fe₂O₃/γ-Fe₂O₃ heterojunction. Finally, it should be mentioned that the staggered band form between Fe₂O₃ and TiO₂ would significantly hinder the recombination of charge carriers and thus enhance the photocatalytic performance [41].

Figure 6 shows the PL spectra and decay curves of the synthesized materials. PL spectra were traditionally applied to detect the photo-chemical properties of the synthesized materials and it was considered that higher PL intensity indicated more electron-hole pairs generated in the heterostructures containing iron oxides [42]. Figure 6A clearly shows that the PL spectra of Fe₂O₃ nanoparticles is lower than that of three Fe₂O₃@TNT composites, implying that combining Fe₂O₃ with TiO₂ nanotubes could effectively promote the generation of charge carriers. Except for the fluorescence intensity, the luminescence efficiency should also be considered by analyzing fluorescence lifetime. It can be observed in Figure 6B that the intensity of the decay curves fast diminishes after reaching the top of peak instead of a slow tailing trend, which might be due to the high charge carriers recombination rate of Fe₂O₃. Then, the fluorescence lifetimes of 0.5FeTi, 1FeTi, 2FeTi, and Fe₂O₃ were determined from the decay curves and fitted by a single exponential term. According to the corresponding equation (as shown in Figure S3), the lifetimes of 2FeTi, 1FeTi, 0.5FeTi, and Fe₂O₃ were calculated to be 0.6844 ns, 0.6759 ns, 0.6475 ns, and 0.6306 ns, respectively, indicating that the lifetimes also increase with the increment of Fe₂O₃ deposition. Additionally, these results also reveal the superb migration and effective separation of electrons and holes between Fe₂O₃ and TiO₂, which is also shown in Scheme 2. Under the consideration of both charge carriers’ generation and their lifetimes, 2FeTi has the highest fluorescence intensity and fluorescence lifetime, which indicates that 2FeTi might have the best photocatalytic activity, followed by 1FeTi or 0.5FeTi and lastly bare Fe₂O₃.

3.3. Magnetic Characterizations

Finally, the magnetic property of as-obtained products was evaluated by using a vibrating sample magnetometer, which is of great importance for practical applications. Figure 7 exhibits the magnetization curves measured at 300 K, and the insert graph shows the magnetic separation result of 1FeTi by using a magnet. The suspension containing well-dispersed particles of 1FeTi turned into clear solution without residues left in only several seconds via a magnet, which implies that the synthesized 1D Fe₂O₃@TNT composites possess good magnetic separation ability. In addition, the magnetization curves passing through the origin with the magnetic saturation (Ms) values of 0.23, 0.48 and 1.18 emu/g for 0.5FeTi, 1FeTi and 2FeTi indicated that they all possessed superparamagnetic behaviors. Accordingly, the molar contents of Fe₂O₃@TNT composites were estimated to be 1.06%, 2.23% and 5.54% for 0.5FeTi, 1FeTi and 2FeTi respectively, which is calculated by comparing the Ms values of the synthesized composites and pure Fe₂O₃ (the Ms value was 27.48 emu/g, shown in Figure S3). The results derived from Ms values are a little higher than the contents derived from the XPS spectra, which is ascribed to more excellent magnetism originating from the mini “magnet” formed along the nanotubes after the magnetic Fe₂O₃ nanoparticles are confined in the nanotubes. On the other hand, XPS technique is a surface-sensitive technique that excites electrons from the top surface of 1–12 nm thick, which might also be a reason for lower contents.

3.4. Photocatalytic Activity

The photocatalytic activities of as-synthesized composites under visible light were assessed by the degradation experiments of RhB (10 mg/L) using Xe light irradiation (300 W, λ ≥ 400 nm). The variation of the concentration with the time of irradiation is plotted in Figure 8. During dark equilibrium period, these Fe₂O₃@TNT composites possess more efficient adsorption capacity for RhB compared with pure Fe₂O₃ and TNT (Figure S5), which is mainly ascribed to the homogeneous dispersity of Fe₂O₃ on TNT, as illustrated in Figure 2 and Figure 4. Especially, it is observed that 1FeTi shows the optimal adsorption property, which might have resulted from the most dispersive Fe₂O₃ inner structures in 1FeTi (Figure 4). After the visible-light irradiation, Fe₂O₃@TNT composites show remarkably enhanced photocatalytic activity compared to pure Fe₂O₃, which is attributed to the efficient separation of charge carriers after doping onto TiO₂ nanotubes. Briefly, Fe₂O₃ would act as the electron trappers in the composites which would enhance separation of excitons and thus the photocatalytic performance. In addition, the performances of 1FeTi and 2FeTi are almost the same and much better than 0.5FeTi, which might be due to the more abundant electron trappers accompanied with the moderate increment of Fe₂O₃ deposition. Hence, 1FeTi and 2FeTi are considered as the candidates with the most potential for organics degradation. However, 1FeTi consumes less iron oxides and thus was more cost-effective. Above all, 1FeTi is considered to be a better Fe₂O₃@TNT composite under the consideration of dark adsorption capacity, photocatalytic activity and economic efficiency. Finally, the rate constant k_a is fitted according to the pseudo-first order reaction kinetics (Equation (1)), and the results are displayed in Table 3. The order of the rate constants is in good agreement with the curves in Figure 8B. Interestingly, it is observed that the trend of rate constant is coincident with the deduction of the PL results, which implies that fluorescence intensity and fluorescence lifetime could describe the photocatalytic property of Fe₂O₃ and its composites effectively. Finally, the comparison of this study with other similar Fe₂O₃-based materials reported in other literatures is given in Table 4. As seen in Table 4, the photodegradation activity of 1FeTi is significantly improved, which further demonstrates that 1FeTi could be a promising material for photocatalytic wastewater remediation.

ln_C0/C=kt

where C₀ and C are the original concentration of RhB and the corresponding concentration at the reaction time (t), respectively, and k is the pseudo-first degradation rate constant.

3.5. Intermediate Products and Degradation Pathway of RhB

Although it has been claimed that 84.96% of the RhB could be degraded by 1FeTi in 6 h in Section 3.4, the specific mineralization efficiency has not been determined. Consequently, the intermediate products during photocatalysis of RhB over 1FeTi for 0–6 h were identified by HR-MS. As shown in Figure S6, the HR-MS spectra of RhB solution (t = 0 h) shifted from m/z = 443 (characterized as RhB) to m/z = 415, 387, 359, 331, 304, 302, and 272 after photo illumination, which might be ascribable to the N-de-ethylating process under the effects of photogenerated radicals [49,50]. Then, the N-de-ethylated intermediates would react with functional radicals to produce opening-ring intermediates (e.g., phthalic acid), which could be further mineralized to CO₂ and H₂O [49,50]. Based on these deductions, the degradation pathway of RhB was proposed and is displayed in Scheme 3. Finally, it should be mentioned that the intensity of MS spectrum at t = 6 h is much weaker than that at t = 0 h, indicating the almost total mineralization of RhB in the wastewater.

3.6. Recyclable Performance

Before the regeneration experiments, the leaching test should be probed to examine the loss of supported Fe₂O₃ particles and to ensure the maintenance of superior photocatalytic performance [27]. As a result, 1FeTi after photocatalysis was sampled and tested by XPS, and the results are displayed in Figure S7 and Figure 9A. It could be observed from the Fe 2p spectra in Figure 9A that peak shape and intensity is almost unchanged after photocatalysis, indicating the effective inhibition of photo dissolution of the doped Fe₂O₃ [25]. Furthermore, the dissolution amount was determined to be 13.4% in 6 h according to Table 1 and Figure S7, which is much less than 17.5% in 0.5 h for the Fe₃O₄@TiO₂ and 13.5% in 3 h for Fe₃O₄@C@TiO₂ heterostructures [25,27], verifying the hypothesis that the photo dissolution could be significantly hindered by incorporating wide-gap Fe₂O₃ instead of narrow-gap Fe₃O₄.

Considering the good stability of 1FeTi during photocatalysis, the regeneration experiments were performed. As shown in Figure 9B, the recyclable performance is almost stable in three cycles, and the slight reduction might be due to the moderate dissolution of Fe₂O₃ from the composite. Generally, 1FeTi exhibits superior repression for photo dissolution and stable regeneration, which could be considered as a promising photocatalytic medium for water remediation.

4. Conclusions In this work, 1D Fe₂O₃@TiO₂ nanotube composites were first successfully synthesized by an aerobic OCMA method. The structural and photo-chemical properties of the prepared Fe₂O₃ and Fe₂O₃@TNT composites were characterized via XRD, XPS, FTIR, HRTEM, BET, UV-Vis, and PL spectra. Based on the XRD results, both α-Fe₂O₃ and γ-Fe₂O₃ were found to exist in pure Fe₂O₃. The successful deposition of Fe₂O₃ was further confirmed by XPS and FTIR spectra. In addition, HRTEM was also applied to evaluate the dispersity of the Fe₂O₃ nanoparticles, and the results indicated that the nanoparticles were well dispersed on the TNT inner walls, resulting in efficient separation of charge carriers and homogeneous dispersion of Fe₂O₃ nanoparticles, which was further verified by the PL and BET analysis. According to the UV–vis spectra, the band structures of these catalysts were derived and it was found that the band alignments of these heterostructures were staggered, which might have resulted from the coexistence of α-Fe₂O₃ and γ-Fe₂O₃. Benefiting from the superior structural and photo-chemical properties, the composites exhibited much higher photocatalytic degradation efficiency towards RhB than pure Fe₂O₃ under visible-light irradiation. The photocatalytic results were in good agreement with PL results, which revealed the photoactivity tendency of the heterojunctions could be well predicted through fluorescence intensity and fluorescence lifetime. Furthermore, the introduction of Fe₂O₃ in Fe₂O₃@TNT nanostructures made them easy to be separated and recovered in a magnetic field, which further confirmed that the synthesized Fe₂O₃@TNT composites are efficient and cost-effective materials. Finally, the photo dissolution phenomenon of 1FeTi was examined and the low leaching amount as well as stable recycling performance illustrated the promising prospects of 1FeTi for water remediation and rational design of Fe₂O₃ into TiO₂ instead of Fe₃O₄.

Supplementary Materials

The following are available online at https://www.mdpi.com/2079-4991/10/7/1345/s1, Figure S1: N2 adsorption-desorption isotherms, Figure S2: Decay curve fitting procedures for the synthesized samples; Figure S3: Magnetization curve of Fe2O3.

Author Contributions

Conceptualization, X.H. and J.B.; methodology, Q.T. and R.W.; formal analysis, Q.T., J.B. and C.X.; data curation, Q.T. and Y.Z.; resources, H.H. and J.W.; writing-original draft preparation, Q.T.; writing-review and editing, J.B., L.Y. and H.H.; supervision, H.H. and L.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by National Key Research and Development Program of China (No. 2016YFB0600504).

Acknowledgments

The authors would like to thank National Engineering Research Center of Industrial Crystallization Technology at Tianjin University for the technical support.

Conflicts of Interest

The authors declare no conflict of interest.

References

1. Rasool, K.; Pandey, R.P.; Rasheed, P.A.; Buczek, S.; Gogotsi, Y.; Mahmoud, K.A. Water treatment and environmental remediation applications of two- dimensional metal carbides (MXenes). Mater. Today 2019, 30, 80-102.

2. Shannon, M.A.; Bohn, P.W.; Elimelech, M.; Georgiadis, J.G.; Marĩas, B.J.; Mayes, A.M. Science and technology for water purification in the coming decades. Nature 2008, 452, 301-310.

3. Ouyang, J.; Wang, Y.; Li, T.; Zhou, L.; Liu, Z. Immobilization of carboxyl-modified multiwalled carbon nanotubes in chitosan-based composite membranes for U (VI) sorption. J. Radioanal. Nucl. Chem. 2018, 317, 1419-1428.

4. Li, G.; Wang, Y.; Bi, J.; Huang, X.; Mao, Y.; Luo, L. Partial Oxidation Strategy to Synthesize WS2/WO3 Heterostructure with Enhanced Adsorption Performance for Organic Dyes: Synthesis, Modelling, and Mechanism. Nanomaterials 2020, 10, 278.

5. Huang, X.; Yang, J.; Wang, J.; Bi, J.; Xie, C. Design and synthesis of core e shell Fe3O4@PTMT composite magnetic microspheres for adsorption of heavy metals from high salinity wastewater. Chemosphere 2018, 206, 513-521.

6. Bi, J.; Huang, X.; Wang, J.; Tao, Q.; Lu, H.; Luo, L.; Li, G.; Hao, H. Self-assembly of immobilized titanate films with different layers for heavy metal ions removal from wastewater: Synthesis, modeling and mechanism. Chem. Eng. J. 2020, 380, 122564.

7. Yang, B.; Bi, W.; Wan, Y.; Li, X.; Huang, M.; Yuan, R.; Ju, H.; Chu, W.; Wu, X.; He, L.; et al. Surface etching induced ultrathin sandwich structure realizing enhanced photocatalytic activity. Sci. China Chem. 2018, 61, 1572-1580.

8. Xu, Q.; Zhang, L.; Yu, J.; Wageh, S.; Al-Ghamdi, A.A.; Jaroniec, M. Direct Z-scheme photocatalysts: Principles, synthesis, and applications. Mater. Today 2018, 21, 1042-1063.

9. Hitam, C.N.C.; Jalil, A.A. A review on exploration of Fe2O3 photocatalyst towards degradation of dyes and organic contaminants. J. Environ. Manag. 2020, 258, 110050.

10. Wang, J.; Bai, Z. Fe-based catalysts for heterogeneous catalytic ozonation of emerging contaminants in water and wastewater. Chem. Eng. J. 2017, 312, 79-98.

11. Wu, W.; Jiang, C.; Roy, V.A.L. Recent progress in magnetic iron oxide-semiconductor composite nanomaterials as promising photocatalysts. Nanoscale 2015, 7, 38-58.

12. Chen, C.; Duan, F.; Zhao, S.; Wang, W.; Yang, F.; Nuansing, W.; Zhang, B.; Qin, Y.; Knez, M. Porous Fe2O3 nanotubes with α-γ phase junction for enhanced charge separation and photocatalytic property produced by molecular layer deposition. Appl. Catal. B Environ. 2019, 248, 218-225.

13. Kusior, A.; Michalec, K.; Jelen, P.; Radecka, M. Shaped Fe2O3 nanoparticles-Synthesis and enhanced photocatalytic degradation towards RhB. Appl. Surf. Sci. 2019, 476, 342-352.

14. Demirci, S.; Yurddaskal, M.; Dikici, T.; Sarıoğlu, C. Fabrication and characterization of novel iodine doped hollow and mesoporous hematite (Fe2O3) particles derived from sol-gel method and their photocatalytic performances. J. Hazard. Mater. 2018, 345, 27-37.

15. Guo, S.; Wang, H.; Yang, W.; Fida, H.; You, L.; Zhou, K. Scalable synthesis of Ca-doped α-Fe2O3 with abundant oxygen vacancies for enhanced degradation of organic pollutants through peroxymonosulfate activation. Appl. Catal. B Environ. 2020, 262, 118250.

16. Guo, R.; Qi, X.; Zhang, X.; Zhang, H.; Cheng, X. Synthesis of Ag2CO3/α-Fe2O3 heterojunction and it high visible light driven photocatalytic activity for elimination of organic pollutants. Sep. Purif. Technol. 2019, 211, 504-513.

17. Ma, Q.; Zhang, X.; Guo, R.; Zhang, H.; Cheng, Q.; Xie, M.; Cheng, X. Persulfate activation by magnetic γ-Fe2O3/Mn3O4 nanocomposites for degradation of organic pollutants. Sep. Purif. Technol. 2019, 210, 335-342.

18. Beketova, D.; Motola, M.; Sopha, H.; Michalicka, J.; Cicmancova, V.; Dvorak, F.; Hromadko, L.; Frumarova, B.; Stoica, M.; Macak, J.M. One-Step Decoration of TiO2 Nanotubes with Fe3O4 Nanoparticles: Synthesis and Photocatalytic and Magnetic Properties. ACS Appl. Nano Mater. 2020, 3, 1553-1563.

19. Peng, Y.; Li, M.; Zhang, S.; Nie, G.; Qi, M.; Pan, B. Improved performance and prolonged lifetime of titania-based materials: Sequential use as adsorbent and photocatalyst. Sci. China Chem. 2015, 58, 1211-1219.

20. Meng, A.; Zhang, L.; Cheng, B.; Yu, J. Dual Cocatalysts in TiO2 Photocatalysis. Adv. Mater. 2019, 31, 1-31.

21. Lee, K.; Mazare, A.; Schmuki, P. One-dimensional titanium dioxide nanomaterials: Nanotubes. Chem. Rev. 2014, 114, 9385-9454.

22. Wang, M.; Sun, L.; Lin, Z.; Cai, J.; Xie, K.; Lin, C. P-n Heterojunction photoelectrodes composed of Cu2O-loaded TiO2 nanotube arrays with enhanced photoelectrochemical and photoelectrocatalytic activities. Energy Environ. Sci. 2013, 6, 1211-1220.

23. Feng, J.X.; Xu, H.; Dong, Y.T.; Lu, X.F.; Tong, Y.X.; Li, G.R. Efficient Hydrogen Evolution Electrocatalysis Using Cobalt Nanotubes Decorated with Titanium Dioxide Nanodots. Angew. Chem. Int. Ed. 2017, 56, 2960-2964.

24. Challagulla, S.; Nagarjuna, R.; Ganesan, R.; Roy, S. Acrylate-based Polymerizable Sol-Gel Synthesis of Magnetically Recoverable TiO2 Supported Fe3O4 for Cr(VI) Photoreduction in Aerobic Atmosphere. ACS Sustain. Chem. Eng. 2016, 4, 974-982.

25. Li, Z.J.; Huang, Z.W.; Guo, W.L.; Wang, L.; Zheng, L.R.; Chai, Z.F.; Shi, W.Q. Enhanced Photocatalytic Removal of Uranium(VI) from Aqueous Solution by Magnetic TiO2/Fe3O4 and Its Graphene Composite. Environ. Sci. Technol. 2017, 51, 5666-5674.

26. Ong, W.J.; Tan, L.L.; Ng, Y.H.; Yong, S.T.; Chai, S.P. Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer to Achieving Sustainability? Chem. Rev. 2016, 116, 7159-7329.

27. Bi, J.; Huang, X.; Wang, J.; Wang, T.; Wu, H.; Yang, J.; Lu, H.; Hao, H. Oil-phase cyclic magnetic adsorption to synthesize Fe3O4@C@TiO2-nanotube composites for simultaneous removal of Pb(II) and Rhodamine B. Chem. Eng. J. 2019, 366, 50-61.

28. Chen, H.; Li, T.; Zhang, L.; Wang, R.; Jiang, F.; Chen, J. Pb(II) adsorption on magnetic γ-Fe2O3/titanate nanotubes composite. J. Environ. Chem. Eng. 2015, 3, 2022-2030.

29. Zhao, L.; Cao, Q.; Wang, A.; Duan, J.; Zhou, W.; Sang, Y.; Liu, H. Iron oxide embedded titania nanowires-An active and stable electrocatalyst for oxygen evolution in acidic media. Nano Energy 2018, 45, 118-126.

30. Roy, P.; Berger, S.; Schmuki, P. TiO2 nanotubes: Synthesis and applications. Angew. Chem. Int. Ed. 2011, 50, 2904-2939.

31. Yang, K.; Peng, H.; Wen, Y.; Li, N. Re-examination of characteristic FTIR spectrum of secondary layer in bilayer oleic acid-coated Fe3O4 nanoparticles. Appl. Surf. Sci. 2010, 256, 3093-3097.

32. Jang, J.T.; Lee, J.; Seon, J.; Ju, E.; Kim, M.; Kim, Y.I.L.; Kim, M.G.; Takemura, Y.; Arbab, A.S.; Kang, K.W.; et al. Giant Magnetic Heat Induction of Magnesium-Doped γ-Fe2O3 Superparamagnetic Nanoparticles for Completely Killing Tumors. Adv. Mater. 2018, 30, 1-8.

33. Liu, S.; Zheng, L.; Yu, P.; Han, S.; Fang, X. Novel Composites of α-Fe2O3 Tetrakaidecahedron and Graphene Oxide as an Effective Photoelectrode with Enhanced Photocurrent Performances. Adv. Funct. Mater. 2016, 26, 3331-3339.

34. Wu, H.; Wu, G.; Wang, L. Peculiar porous α-Fe2O3, γ-Fe2O3 and Fe3O4 nanospheres: Facile synthesis and electromagnetic properties. Powder Technol. 2015, 269, 443-451.

35. Ling, M.; Blackman, C.S. Gas-phase synthesis of hybrid nanostructured materials. Nanoscale 2018, 10, 22981-22989.

36. Shen, G.; Zhang, R.; Pan, L.; Hou, F.; Zhao, Y.; Shen, Z.; Mi, W.; Shi, C.; Wang, Q.; Zhang, X.; et al. Regulating the Spin State of Fe III by Atomically Anchoring on Ultrathin Titanium Dioxide for Efficient Oxygen Evolution Electrocatalysis Angewandte. Angew. Chem. Int. Ed. 2020, 59, 2313-2317.

37. Xie, J.; Jin, R.; Li, A.; Bi, Y.; Ruan, Q.; Deng, Y.; Zhang, Y.; Yao, S.; Sankar, G.; Ma, D.; et al. Highly selective oxidation of methane to methanol at ambient conditions by titanium dioxide-supported iron species. Nat. Catal. 2018, 1, 889-896.

38. Mou, H.; Song, C.; Zhou, Y.; Zhang, B.; Wang, D. Design and synthesis of porous Ag/ZnO nanosheets assemblies as super photocatalysts for enhanced visible-light degradation of 4-nitrophenol and hydrogen evolution. Appl. Catal. B Environ. 2018, 221, 565-573.

39. Guo, T.; Wang, K.; Zhang, G.; Wu, X. A novel α-Fe2O3@g-C3N4 catalyst: Synthesis derived from Fe-based MOF and its superior photo-Fenton performance. Appl. Surf. Sci. 2019, 469, 331-339.

40. Puthirath Balan, A.; Radhakrishnan, S.; Woellner, C.F.; Sinha, S.K.; Deng, L.; Reyes, C.D.L.; Rao, B.M.; Paulose, M.; Neupane, R.; Apte, A.; et al. Exfoliation of a non-van der Waals material from iron ore hematite. Nat. Nanotechnol. 2018, 13, 602-609.

41. Chen, Y.; Jiang, D.; Li, L.; Li, Z.; Li, Q.; Shi, R. Enhanced photoelectrochemical activity of α-Fe2O3/TiO2 nanoheterojunction by controlling hydrodynamic conditions. Nanotechnology 2020, 31, 174002.

42. Charanpahari, A.; Umare, S.S.; Gokhale, S.P.; Sudarsan, V.; Sreedhar, B.; Sasikala, R. Enhanced photocatalytic activity of multi-doped TiO2 for the degradation of methyl orange. Appl. Catal. A Gen. 2012, 443-444, 96-102.

43. Liang, H.; Liu, K.; Ni, Y. Synthesis of mesoporous α-Fe2O3 via sol-gel methods using cellulose nano-crystals (CNC) as template and its photo-catalytic properties. Mater. Lett. 2015, 159, 218-220.

44. Lei, R.; Ni, H.; Chen, R.; Zhang, B.; Zhan, W.; Li, Y. Growth of Fe2O3/SnO2 nanobelt arrays on iron foil for efficient photocatalytic degradation of methylene blue. Chem. Phys. Lett. 2017, 673, 1-6.

45. Wei, Z.; Wei, X.; Wang, S.; He, D. Preparation and visible-light photocatalytic activity of α-Fe2O3/γ-Fe2O3 magnetic heterophase photocatalyst. Mater. Lett. 2014, 118, 107-110.

46. Tyrpekl, V.; Vejpravová, J.P.; Roca, A.G.; Murafa, N.; Szatmary, L.; Nižňanský, D. Magnetically separable photocatalytic composite γ-Fe2O3@TiO2 synthesized by heterogeneous precipitation. Appl. Surf. Sci. 2011, 257, 4844-4848.

47. Costa, G.S.; Costa, M.J.S.; Oliveira, H.G.; Lima, L.C.B.; Luz, G.E.; Cavalcante, L.S.; Santos, R.S. Effect of the applied potential condition on the photocatalytic properties of Fe2O3/WO3 heterojunction films. J. Inorg. Organomet. Polym. Mater. 2020.

48. Park, Y.; Kim, H.; Lee, G.; Pawar, R.C.; Lee, J.; Lee, C.S. Photocatalytic evaluation of self-assembled porous network structure of ferric oxide film fabricated by dry deposition process. Mater. Chem. Phys. 2016, 181, 241-247.

49. He, Z.; Sun, C.; Yang, S.; Ding, Y.; He, H.; Wang, Z. Photocatalytic degradation of rhodamine B by Bi2WO6 with electron accepting agent under microwave irradiation: Mechanism and pathway. J. Hazard. Mater. 2009, 162, 1477-1486.

50. Natarajan, T.S. Enhanced photocatalytic activity of bismuth-doped TiO2 nanotubes under direct sunlight irradiation for degradation of Rhodamine B dye. J. Nanoparticle Res. 2013, 15, 1669-1671.