High-Efficacy Hierarchical Dy2O3/TiO2 Nanoflower toward Wastewater Reclamation: A Combined Photoelectrochemical and Photocatalytic Strategy.

Developing a sustainable photocatalyst is crucial to mitigate the foreseeable energy shortage and environmental pollution caused by the rapid advancement of global industry. We developed Dy2O3/TiO2 nanoflower (TNF) with a hierarchical nanoflower structure and a near-ideal anatase crystallite morphology to degrade aqueous rhodamine B solution under simulated solar light irradiation. The prepared photocatalyst was well-characterized using powder X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, energy-dispersive spectroscopy, scanning electron microscopy, Brunauer-Emmett-Teller, diffuse reflectance UV-vis spectra, and X-ray photoelectron spectroscopy. Further analysis was performed to highlight the photoelectrochemical activity of the prepared photocatalysts such as electrochemical impedance spectroscopy, linear sweep voltammetry, photocurrent response, and a Mott-Schottky study. The crystalline Dy2O3/TNF exhibits superb photocatalytic activity attributed to the improved charge transfer, reduced recombination rate of the electron-hole pairs, and a remarkable red-shift in light absorption.

Introduction

Synthetic dyes present in wastewater pose a significant hazard to the health of both the ecosystem and humans. The total production of synthetic dyes used in the industry to color plastics, leather, fabric, cosmetics, foodstuffs, and other consumer products is ∼7 × 105 tons per annum. The rating indicates that 2–10% of the produced dyes are ultimately discharged as effluent wastes into rivers and streams.[1−6] Advanced Oxidation Processes (AOPs) are a developed wastewater treatment, which generates highly reactive radicals (·OH, ·O2–, and so on) using photocatalysts to oxidize O2 and H2O molecules by irradiation with UV–vis light. These radicals interact with organic pollutants and convert them to nontoxic small molecules (H2O and CO2). However, the main challenge facing AOPs since Glaze et al. formally defined the concept in 1987 is the high ultraviolet (UV) light required as energy input to produce the highly active radicals.[1,7,8] Therefore, finding a catalyst to lower the efficient activation energy is the basis of the next-generation AOPS. Previous studies have focused mainly on an activation energy strategy through the single-electron redox cycle of the exposed transition metals such as Cu,[9] Fe,[10] and Mn,[11] on various supports (activated carbon, zeolites, and magnetite). Fenton’s reagent as an example of a homogeneous catalyst suffers from drawbacks, including the recyclability of Fe2+, the requirement of a very low pH medium, and the accumulation of iron-containing sludge.[9−12]

Recently, semiconductor photocatalysis has emerged as an important solution to energy scarcity and environmental protection. The materials’ bandgap energy, specific surface area, morphology, and conductivity are the critical factors for the photocatalytic property.[13−18] Irradiation of photocatalysts produces holes in the valence band (VB) and electrons in the conduction band. Holes undergo an oxidation reaction to generate hydroxyl radicals (·OH), while electrons react with oxygen in the air through a reduction process to produce superoxide radical anions (·O2–). The organic pollutants can be degraded by these active radicals to create H2O and CO2.[19−21] Several strategies have been studied to enhance the photocatalytic efficiency of visible-light photocatalysts, in particular, doping, band gap regulation, structural control, surface sensitization, and phase transfer.[22−24] Characteristic features of efficient photocatalysts include photoactivity to biological and chemical matters, nontoxicity, ability to absorb near UV–visible regions, photostability, and cost-effectiveness.

TiO2 photocatalyst is widely used for photo-oxidation of organic pollutants. The valence and conduction band of TiO2 anatase is at +2.7 and −0.5 eV versus standard hydrogen electrode (SHE), respectively.[2−5] The solar energy spectrum consists of only 5–7% ultraviolet (UV), 47% infrared (IR), and 46% visible radiation.[25,26] Although TiO2 is active in the UV region, developing a novel TiO2-based photocatalyst with an improved band gap has gained importance for advancing visible-light photocatalysis. Recently, pure anatase nanorods were synthesized by a thermal treatment of H3Ti3O7 nanotubes at 700 °C and showed superior performance with regard to dye degradation by comparison with Evonik P25 TiO2 (15% rutile + 85% anatase), reflecting the role of morphology in the photocatalyst performance.[27−29] Lui et al. studied the photocatalytic efficiency of graphene-wrapped TiO2 nanoflowers (TNFs) with 5 wt % graphene loading for methylene blue (MB) solution degradation, and the results outperformed P25 TiO2 by threefold.[29] Harris et al. investigated the efficiency of a series of TiO2 nanoflowers as a catalyst for the photo-oxidation of MB with remarkable activity under UV irradiation.[2] Various morphologies of TiO2 have been studied extensively as a photocatalyst for the degradation of organic pollutants, including nano- and microparticles, nanowires, nanotubes, and hierarchical morphologies.[27,30,31] The later morphology is of great interest due to their ability to combine the high surface area of small particles (increase the material’s capacity for adsorption) with the light-scattering ability of the larger particles (increase the probability of photon absorption).[2,29] Liao et al. synthesized a flower-like morphology as an example of hierarchical materials with a relatively large surface area, large size, and enhanced charge transfer for use in dye-sensitized solar cells.[32]

Although lanthanide materials are familiar insulators, incorporating semiconductors with lanthanides enhanced the photocatalytic performance of the material due to the presence of f-orbitals that trap the photogenerated electrons on irradiation with other semiconductors.[25,33] The doping of ZnO with a lanthanide material has been recently studied by Josephine et al. as a photocatalyst under visible-light radiation.[33] The synthesized Dy2O3@SiO2@ZnO was investigated as a highly visible active photocatalyst for degradation of the endocrine disruptor 2,4-D.[25] Sheydaei et al. studied the photocatalytic ozonation of wastewater via the immobilization of TiO2, graphite, and Dy2O3 and achieved 90% decolorization efficiency.[34]

Herein we present Dy2O3/TNF as a wastewater treatment photocatalyst that successfully tackles the aforementioned challenges regarding current AOPs. This photocatalyst is enabled by incorporating Dy2O3 with TNF to solve the current photocatalysts’ challenges and exhibits superior activity to generate ·O2– and ·OH radicals at pH 8. The specific aims of the present study are (1) a hydrothermal synthesis of TiO2 nanoflower, (2) modification of TNF via doping with Dy2O3 and calcination at 700 °C, (3) detailed physicochemical and photoelectrochemical characterization of the synthesized Dy2O3/TNF, and (4) investigate the photocatalytic efficiency of Dy2O3/TNF for wastewater treatment. To fully understand the photocatalytic degradation process, the effects of solution pH, initial dye concentration, adsorbent dosage, and contact time on the photodegradation of RhB dye under simulated solar light irradiation were successfully studied.

Experimental Section

Chemicals

Titanium butoxide, dysprosium(III) nitrate pentahydrate, glacial acetic acid, absolute ethanol, HCl, NaOH, and rhodamine B (RhB) dye were all obtained from Sigma-Aldrich and used without further treatment.

Synthesis of TiO2 and Dy2O3/TiO2 Nanoflower

TNFs were hydrothermally synthesized based on a typical procedure reported by Harris et al.[2] Briefly, 2 mL of titanium butoxide was slowly added and stirred vigorously with 60 mL of glacial acetic acid at room temperature. After 15 min, the white suspension was placed in a 100 mL Teflon-lined autoclave and transferred into a preheated oven at 140 °C for 12 h. The resultant white product was separated by centrifugation (7000 rpm, 5 min) and washed several times by deionized water. Then the solvent was exchanged by ethanol (total amount 100 mL), and the obtained product was dried overnight at 60 °C. The dried powder was placed in a crucible and calcined in a muffle furnace for 3 h at 700 °C with a heating rate of 5 °C/min. In order to prepare Dy2O4/TiO2 nanoflower samples, the same previous steps were followed, except for adding specific amounts of a certain amount of Dy(NO)3·5H2O to finally obtain (1–10%) of Dy2O3/TiO2 nanoflowers after burning at 700 °C for 2 h.

Characterization

The as-synthesized photocatalysts were characterized by Fourier transform infrared (FT-IR) recorded on a Mattson FT-IR-5000S spectrophotometer using a KBr pellet with 4 cm–1 resolution. X-ray diffraction (XRD) patterns were implemented at a scanning rate of 2°·min–1 using monochromatic Cu Kα radiation (40 kV, 40 mA). Rutile (R) and anatase (A) crystallite sizes (l) were investigated by the Scherrer equation depending on the main reflection peak of rutile (110) and anatase (101) at 2θ = 27.3 and 24.9°, respectively. The rutile phase ratio (%R) in the prepared catalysts was determined using the following equation[2]where IA and IR are the peak intensities for the rutile and anatase reflections, respectively. The surface compositions and the oxidation state of the catalysts were analyzed by the X-ray photoelectron spectroscopy (XPS) technique. The analyses used a Kratos Axis Ultra DLD equipped with a monochromatic Al Kα X-ray source (1486.69 eV) and a hemispherical electron energy analyzer. Survey scans were acquired at a pass energy of 80 eV, and core-level scans were acquired at a pass energy of 20 eV. For binding energy scale calibration, adventitious hydrocarbon referencing (C 1s signal at 285.0 eV) was used. The morphology of prepared materials was studied by using transmission electron microscopy (TEM), a JEOL-JEM-2100 instrument with a slow-scan charge-coupled device (CCD), which was operated at 120 kV, and scanning electron microscopy (SEM, JEOL-JSM-6510 LV) at 5–10 kV. A nitrogen adsorption study over the prepared catalysts was performed at −196 °C using QuantaChrome NOVA touch LX4 equipment. Before the measurement, the catalysts were activated at 300 °C for 3 h under vacuum. Diffuse reflectance spectra (UV–vis–NIR (NIR = near-infrared)) referenced to BaSO4 were obtained using a Fischer Scientific spectrometer EVO 300. The band gaps of TNF and Dy2O3/TNF composites were directly calculated by a Tauc plot applying the Kubelka–Munk function.[2]

Electrochemical Measurements

Photoelectrochemical measurements were obtained at room temperature in a 0.5 M Na2SO4 solution using a three-electrode configuration CHI 6005E (CH Instruments) Electrochemical Analyzer. TNF and Dy2O3/TNF samples were completely covered with a fluorine-doped tin oxide (FTO) sheet and used as the working electrode. Ag/AgCl (KCl, saturated) and a Pt sheet were used as reference and counter electrodes, respectively. The photocatalyst sample was well-dispersed in a 1 mL-sized tube under sonication with 450 μL of absolute ethanol and 50 μL of 5 wt % Nafion solution. After sonication for 1 h, a 10 μL suspension was dropped over a well-cleaned FTO sheet based on the experiment type and thoroughly dried at room temperature. The photocurrent response intensity of TNF and Dy2O3/TNF composites was completely distinct for several intermittent switching turning to light. The photocurrent density (J-v curves) and linear sweep voltammetry (LSV) test that provide the background capacitive current of the synthesized materials were displayed under 1 Sun illumination (AM 1.5G) in a 0.5 M Na2SO4 aqueous solution. Electrochemical impedance spectroscopy (EIS) measurements were conducted in a frequency range between 0.1 Hz and 100 kHz at 298 K in a 250 mL cell containing 0.1 M KOH as electrolytic solution saturated with N2 for 15 min to remove other dissolved gas. Mott–Schottky (MS) measurements were utilized to investigate the band edge potential of the absolute semiconductor components and the type of semiconductivity. The Applied Bias Photon to current conversion Efficiency (% of ABPE) was estimated using eq .[36]Here, η and Jph are the efficiency and the obtained photocurrent density, respectively. Ptotal is the total incident power density (100 mW/cm2), and ERHE is the applied bias versus reversible hydrogen electrode and calculated using eq .

The standard potential of Ag/AgCl as a reference electrode, E0Ag/AgCl, equals 0.1976 V at 25 °C. The following Mott–Schottky equation was used to obtain the charge carrier density[37,38]where C is the space charge capacitance, e is an elemental charge, ND is the electron carrier density, ε is the relative permittivity of semiconductor, εo is the permittivity of vacuum, kB is the Boltzmann constant, T is the absolute temperature, Vs is the applied potential, and EFB is flat band potential. Herein, kB = 1.380 648 52 × 10–23 m2 kg s–2 K–1, e = −1.6 × 10–19 C, ε0 = 8.854 × 10–12 F m–1, and ε = 7.7 for TNF.[39,40]

Photocatalytic Activity

The whole photocatalytic process was performed by transferring 20 mg of the photocatalyst to 50 mL of RhB dye solution in a reaction quartz tube and irradiating it using (SciSun-300) Solar Simulator, Class AAA, 300 W, 50 × 50 mm, (100 mW/cm2) equipped with an AM 1.5G filter. The catalyst–dye mixture was first stirred by a magnetic stirrer in the dark for 1 h to attain adsorption–desorption equilibrium. After that, the xenon lamp was turned on, and the photocatalytic degradation of the RhB dye was studied for 2 h. At regular time intervals, 0.5 mL of the sample was taken out and diluted to 5 mL with distilled water, then centrifuged to remove the catalyst powder. The absorbance of the 2 mL centrifuged solution was acquired using a UV–vis spectrophotometer at 554 nm to investigate the remaining concentrations of RhB dye.

Results and Discussion

The powder XRD pattern was used to investigate the polymorph phases of TiO2 that exist in the TNF photocatalysts, and the results are displayed in Figure . According to Figure a, pure TNF shows well-defined and intense diffraction peaks at 2θ = 24.9, 37.1, 39.8, 49.8, and 55.5°, which are typical for the anatase phase.[40,41] Moreover, doping of Dy2O3 into TNF results in the appearance of new weak diffraction peaks at 2θ = 27.3 and 38.8° that are assigned to the rutile phase. Considering these results, calcination of TiO2 at 700 °C yielded TNF that was ∼100% anatase phase, while annealing with Dy2O3 results in TNF composites that consist of predominant anatase (>95%) and minority rutile (<5%) phases. The presence of Dy2O3 induces the rutile crystallite growth and preserves the anatase as the main phase with important implications for the photocatalytic performance, as discussed later. A Dy2O3 separate phase is not readily detected in the XRD patterns of Dy2O3/TNF composite due to its smaller particle size or low loading.[40,41] FT-IR spectra of TNF and Dy2O3/TNF photocatalysts in the range of 4000–400 cm–1 were shown in Figure b. The spectrum bands at 498.5, 814.8, 1142.7,1627.7, and 3431.9 cm–1 confirmed the successful preparation of pure anatase nanoflower compared with the previously reported literature.[40−43] The main characteristic observations from the FT-IR spectrum are (1) the absence of acetic acid and acetate bands used in the preparation of TNF after calcination at 700 °C, (2) clear bands below 900 cm–1 are related to anatase polymorph, and (3) the presence of surface O–H groups of TiO2 with stretching mode at ∼3500 cm–1. The FT-IR spectrum of Dy2O3/TNF is typically identical to that of bare TNF, and both of them reflect the presence of anatase as the main phase of TiO2.

Figure 1

(a) XRD and (b) FT-IR of some selected samples calcined at 700 °C.

SEM and TEM images of Dy2O3/TNF confirm the nanoflower crystallinity of TiO2 with ultrafine Dy2O3 nanoparticles homogeneously distributed above its surface, as shown in Figures and S1. The spherical hierarchy of the as-prepared photocatalyst consists of a three-dimensional (3D) nanoflower (∼0.5 μm) with thin two-dimensional (2D) nanosheets (∼13 nm) that radiate from its center. Moreover, the elemental mapping for 5% Dy2O3/TNF photocatalyst (Figure c–f) confirms the homogeneity of the catalyst, where Dy2O3 nanoparticles are successfully and homogeneously distributed over the TNF surface without any kind of aggregations. The elemental mapping may explain the absence of any diffraction peaks of the Dy2O3 phase in the XRD study. The nitrogen adsorption–desorption isotherms as well as the specific surface area (SBET), average pore volume (cc/g), and pore radius DV (r) of TNF and Dy2O3/TNF are shown in Figure S2 and summarized in Table S1. The samples exhibit a type II adsorption isotherm with an H3 hysteresis loop, which may be due to some mesoporous nature of the catalysts.[42,43] As shown in Table S1, it was also observed that significant changes in the surface properties after the incorporation of Dy2O3, such as the specific surface area, is doubled in the case of the 5% Dy2O3/TNF sample compared to that of the TNF sample and then decreased after that, while the average pore volume (cc/g) and pore radius (nm) values remained almost constant at ∼0.32 cc/g and 1.62 nm, respectively. These results may also support the fine dispersion of Dy2O3 on the TNF surface and inside its pores.

Figure 2

(a, b) SEM, (c–f) EDS-mapping, and (g–i) TEM images of Dy2O3/TNF.

XPS was applied to probe the bonding and specification environments of titanium, dysprosium, and oxygen in TNF and Dy2O3/TNF photocatalysts (Figure a–d). The survey spectrum XPS of Dy2O3/TNF exhibits strong emission lines due to Ti, O, Dy, and a weak C line. The XPS data for high-resolution Ti 2p spectra containing signals at 458.2 and 463.9 eV are in an area ratio of 2:1, respectively. These peaks can be typically attributed to Ti 2p1/2 and 2p3/2 levels of Ti(IV) cations.[44] The XPS spectrum of O 1s for Dy2O3/TNF showed a peak at 529.6 eV attributed to the lattice oxygen peak in TiO2 and matching the presence of a surface hydroxyl peak on the FT-IR analysis. The high-resolution XPS for Dy2O3 exhibited two peaks at 152.8 and 156.7 eV attributed to Dy 4d3/2 and Dy 4d5/2, respectively.[35,45] The noticed C 1s peak in the survey should be likely due to the acetate organic compound present as traces of carbon on the surface of TNF after thermal treatment at 700 °C. The occupied electrons in Ti 2p1/2 and 2p3/2 levels can be readily excited into unoccupied Ti 3d and Ti 3d-O 2p hybrid orbitals. In addition, the excitation of electrons from O 1s to Ti 3d-O 2p hybrid orbitals of TNF.2 energy-dispersive spectroscopy (EDS) shown in Figure e reflects the high purity of the prepared sample, and the elemental composition weight ratio was found to be 62:4:34% for titanium, dysprosium, and oxygen, respectively.

Figure 3

(a–d) XPS and (e) EDS analysis of Dy2O3/TNF.

The diffuse reflectance UV–vis spectra (DRS) of TNF and different wt % Dy2O3 photocatalysts were used to explore the band gap energy and optical properties (Figure S3). The TNF sample absorbed firmly below 400 nm, concerning the surface defects in TiO2 nanoflower. We note that this absorption is red-shifted, and a new strong edge appeared in the visible wavelengths region after doping with Dy2O3, indicating the visible-light absorption enhancement and potent combination between Dy2O3 and TNF. This observation suggests that Dy2O3/TNF will have better photocatalytic performance than TNF under simulated solar light irradiation. Table S1 summarizes the estimated band gaps for each sample according to Tauc plots. For TNF, the band gap energy (Eg) was ∼3.24 eV and was typical for the anatase phase.[2] When the loading ratio of Dy2O3 is increased, the band gap drops to 2.95 eV with 5% Dy2O3, as shown in Figure S3.

Photoelectrochemical Study of TNF and Dy2O3/TNF Photocatalysts

Photocurrent response measurements were used to study the charge-carrier transfer by TNF and Dy2O3/TNF photocatalysts. Figure a depicts the transient photocurrent curves (I-t plots) at a bias potential of 0 V with several on/off cycles of discontinuous visible-light irradiation. The results obtained under various conditions of darkness and light showed that the photocatalyst could produce the phenomenon of electrons and holes only by exposure to light.[46] The photocurrent response intensity of 5% Dy2O3/TNF was higher than twice that of bare TNF, indicating that the Dy2O3/TNF hybrid can enhance the lifetime of the photogenerated charges compared to the bare TNF. As can be seen in Figure b, the photocurrent density observed at the lower bias region for 5% Dy2O3/TNF (0.98 mA/cm2) was higher regarding that of bare TNF (0.44 mA/cm2). Such superb enhancement can be assigned to the cocatalytic activity of Dy2O3 played in the nucleophilic attack of oxygen molecules by the electrons transferred from its valence band. The Applied Bias Photon to current conversion Efficiency was estimated for TNF and Dy2O3/TNF at 0 V (vs Ag/AgCl) to be 0.27 and 0.60%, respectively (the applied equation is available in the Experimental Section).[36] The remarkable enhancement in % ABPE of TNF after Dy2O3 is added is attributed to the improvement in the efficient light scattering and charge-transfer properties within the nanoflower morphology.

Figure 4

(a) I-t and (b) J-v curves of as-prepared photocatalysts measured at a bias potential of 0 V vs Ag/AgCl under 1 Sun illumination (100 mW/cm2) in 0.5 M Na2SO4 solution, (c) MS, and (d) EIS plots.

MS measurements were performed using TNF and Dy2O3/TNF photoanodes at a frequency of 5 kHz and a potential range from −1 to 1.5 V (vs Ag/AgCl) with a scan rate of 50 mV/s. As shown in Figure c, the MS plots have a positive slope, reflecting the n-type semiconductivity of both the pristine TNF and Dy2O3/TNF.[38,39] The flat band potential (EFB) was determined by the intersection of the tangent drawn to the MS plots with the X-axis. The flat band potential of 5% Dy2O3/TNF shifted toward the more positive side (EFB = −0.32 ± 0.01 V vs Ag/AgCl) compared to that of bare TNF (EFB = −0.61 ± 0.01 V vs Ag/AgCl). EFB is considered the conduction band in the case of n-type semiconductors, so the valence band of Dy2O3/TNF can be calculated by subtracting the band gap energy to be 2.63 V versus Ag/AgCl. The Fermi level is a hypothetical level below which all the states are occupied, which means that it is always above the valence band. In intrinsic semiconductors, the Fermi level lies exactly in the middle of the VB and CB at absolute zero. However, in extrinsic semiconductors, Fermi levels shift toward the CB in n-type and the VB in p-type. Regarding our extrinsic n-type semiconductors, the positive shift in the Fermi level toward the conduction band after incorporating Dy2O3 within TNF improves the charge transfer density and is compatible with the other results. The Mott–Schottky equation applied to obtain the charge carrier density is available in the Experimental Section.[39,46] Dy2O3/TNF exhibited the carrier density (ND) of 1.28 × 1029/cm3, higher than that of bare TNF (0.9383 × 1029/cm3), supporting the above observation concerning efficient charge transfer/separation and, consequently, enhancement of the photocatalytic performance of Dy2O3/TNF. Moreover, EIS was used to obtain more understanding about the conductivity of the as-prepared materials, as shown in Figure d. Dy2O3/TNF composite has a smaller arc radius regarding TNF photocatalyst and reveals lower charge-transfer resistance on the surface of Dy2O3/TNF compared to TNF.

Effect of Dy2O3 on the Performance of TNF for the Photo-oxidation of RhB Dye Solution

The photodegradation studies were performed by degrading an aqueous RhB solution using TNF and Dy2O3/TNF at pH 8 under simulated solar light irradiation. As shown in Figure a, there was no change in the dye concentration in a control experiment performed in the absence of the photocatalyst, reflecting that RhB dye is resistant to self-photolysis. Before the photocatalytic reaction, TNF and Dy2O3/TNF composites’ adsorption performance was investigated over 60 min in RhB dye solution (10 mg L–1) at pH 8 with a photocatalyst weight of 0.4 g L–1. After irradiation for 1 h, the photocatalytic degradation of Dy2O3/TNF composite reached ∼88%, while that of TNF was only 30%. Figure S4 exhibited the absorption spectrum of 10 mg/L RhB dye solution after irradiation for 1 h using a Solar Simulator (100 mW/cm2) and different photocatalysts. The higher photocatalytic performance of the nanoflower structure, Dy2O3/TNF, compared to the bare TNF can be ascribed to the increase in the visible light absorption and the improved light harvesting due to the reduction in the band gap energy of TNF after the junction with Dy2O3. Furthermore, the influence of Dy2O3 was investigated by varying the loading weight percentages in the Dy2O3/TNF composite. As shown in Figure a, the photodegradation activity of Dy2O3/TNF composite increased with increases in the wt % of Dy2O3 until it attained the highest efficiency with 5 wt % loading with a sustainable band gap of 2.95 eV.

Figure 5

(a) Photocatalytic efficiency, (b) kinetic rates of RhB degradation using TNF and Dy2O3/TNF photocatalysts, (c) reusability of Dy2O3/TNF for photodegradation of RhB dye, and (d) photocatalytic performance of Dy2O3/TNF in the presence of different scavengers.

Generally, the photocatalytic degradation of RhB by irradiated TNF and Dy2O3/TNF follows pseudo-first-order kinetics, which is expressed as kt = ln(Co/Ct), where Co is the initial concentration of the dye, Ct is the concentration at time t, and k is the degradation rate constant (min–1).[2,29]Figure b shows data for each photocatalyst by plotting ln(Co/Ct) versus time with excellent linearity, confirming the perfect applicability of pseudo-first-order kinetics. Among these photocatalysts, 5 wt % Dy2O3/TNF exhibits the highest rate constant (k), which is obtained from the slope of the data in Figure b and is summarized in Table S1. TNF showed the lowest activity due to the lower excitation energy source than band gap for the sample. The Dy2O3/TNF photocatalyst with a rate constant of 0.026 min–1 represents one of the highest activities for RhB degradation compared to the previously reported TiO2-based photocatalyst.[46−54] These results reflect the positive influence of Dy2O3 on the photoexcited electron–hole separation in the crystal lattice. The optimum pH and photocatalyst dose for degradation of RhB dye using Dy2O3/TNF is shown in Figure S5. The effect of pH on the photodegradation of RhB dye was examined in the pH range of 2–10. After 120 min of irradiation, the percentages of MB degradation at pH 2, 4, 6, 8, and 10 were 37.2, 48.8, 78.5, 95.8, and 98.53%, respectively. The results showed a sharp increase in the removal efficiency of RhB dye with increases in the pH from 2 to 8 followed by a slight increase from pH 8 to 10. Such an adsorption enhancement at the basic medium can be ascribed to the strong electrostatic interaction between the RhB (cationic dye) and the negatively charged surface of the photocatalyst.[56] The chemical stability and reusability of the photocatalyst is a critical issue from an economic point of view. To highlight the stability of Dy2O3/TNF, the degradation of fresh RhB dye solutions was repeated for 10 cycles. As shown in Figure c, Dy2O3/TNF has remarkable photostability and almost maintains the original photocatalytic efficiency, reflecting the leaching resistance of Dy2O3/TNF. The reused catalyst was further characterized using an XRD pattern and SEM techniques, as shown in Figure S6. The XRD pattern of the fresh and reused Dy2O3/TNF have a similar inline shape and intensity, indicating the successful regeneration of the photocatalyst. SEM also confirmed that the nanoflower topology is retained without destruction. The Dy2O3/TNF photocatalyst exhibits good chemical and photostability considering these results.

Elucidation of Active Species

Irradiation of photocatalysts produces holes in the VB and electrons in the conduction band. Holes undergo an oxidation reaction to generate hydroxyl radicals (·OH), while electrons react with oxygen in the air through a reduction process to produce superoxide radical anions (·O2–). Different trapping experiments were performed to investigate the main active species responsible for photodegradation of RhB using Dy2O3/TNF as a photocatalyst. Isopropyl alcohol (IPA), p-benzoquinone (BQ), silver nitrate (SN), and ammonium oxalate (AO) were added to the RhB dye solution as a scavenger for ·OH, ·O2–, e–, and h+, respectively.[4,55] The photo-oxidation rate of RhB dye (10 mg/L) after 120 min of irradiation without the addition of any capture reagent was 98.5%, while by additions of IPA, BQ, SN, and AO this rate was 18.1, 48.3, 68.7, and 79.4%, respectively (Figure d). According to these results, ·OH and ·O2– play the most significant influence on the photodegradation of RhB, while h+ and e– have a secondary role in the degradation of RhB using Dy2O3/TNF as photocatalyst.

Mechanism of Degradation

The schematic proposed photodegradation mechanism of our wastewater treatment photocatalyst is shown below and includes four steps: (1) absorption of visible light by photocatalyst generates holes in the valence band and electrons in the conduction band. (2) Holes undergo an oxidation reaction to generate hydroxyl radicals (·OH). (3) Electrons react with oxygen in the air through a reduction process to produce superoxide radical anions (·O2–), then H2O2, and finally ·OH. (4) The generated active radicals degrade the organic pollutants to produce harmless molecules (H2O and CO2). Scheme and eqs –13 describe the formation of active radicals using Dy2O3/TNF under simulated solar light irradiation.[2,34,54]

Scheme 1

Proposed Photodegradation Mechanism of RhB Wastewater Treatment Using Dy2O3/TNF Photocatalyst

Comparison of Dy2O3/TNF with the Previously Reported Photocatalysts

To elucidate the catalyst efficiency, the photodegradation of RhB dye using different photocatalysts previously reported in the literature is recorded in Table . A comparison study includes the photocatalyst dosage, reaction contact time, and light source. Considering the literature values, Dy2O3/TNF reached satisfactory results in photocatalytic degradation of RhB dye.

Table 1

Comparison Study for Degradation of RhB Dye in the Presence of Dy2O3/TNF with Other Photocatalysts Reported in the Literature

photocatalyst	light source	dosage (g/L)	reaction time (min)	removal efficiency	ref
Dy2O3/TNF	Solar Simulator, 100 mW	0.4	120	98.5%	this work
Bi2O3/g-C3N4	Hg–Xe lamp, 15 mW	0.3	180	83.0%	(57)
P25	two lead NULITE, 50 W	1.0	180	78.6%	(58)
g-C3N4/Ag/Ag3VO4	xenon lamp, 250 W	1.0	105	62.9%	(59)
commercial TiO2	xenon lamp, 350 W	1.5	75	30.0%	(60)
ZnO-rGO	UV	0.4	120	92.0%	(61)

Conclusion

In conclusion, the developed photocatalyst has a highly crystalline morphology with a hierarchical nanoflower structure of TiO2 doped with a nanosized Dy2O3. The photocatalytic activity of Dy2O3/TNF was investigated by the photo-oxidation of aqueous rhodamine B solution under simulated solar light irradiation. The enhanced visible-light absorption of Dy2O3/TNF is attributed to the f-f trasitions present in the f-orbitals of Dy2O3. Upon irradiation of Dy2O3/TNF with visible light, the electrons in the TNF valence band outer shell are excited to the conduction band and are finally trapped in the f-shell of Dy2O3. Hence, the superior performance of Dy2O3/TNF is aroused from its acceptable band gap, enhanced charge transfer, improvement of charge separation, and high photogenerated charge stability. Additionally, the excellent chemical stability of the catalyst facilitates its separation from the dye solution and is recycled without any significant loss of photocatalytic efficiency. Notably, the photoelectrochemical properties and photo-oxidation mechanism of the Dy2O3/TNF photocatalyst are thoroughly revealed by several techniques, providing outstanding opportunities for the design of high-performance and cost-effective photocatalysts.