Fabrication of novel magnetically separable nanocomposites using graphitic carbon nitride, silver phosphate and silver chloride and their applications in photocatalytic removal of different pollutants using visible-light irradiation

Interstitially O-doped CdxZn1−xS solid solution derived from chalcogenide molecular clusters for photocatalytic hydrogen evolution

An Oi-doped CdxZn1−xS solid solution with rich VS can trap holes and electrons for carrier separation to enhance photocatalytic activity.

The enhancement mechanism of ultra-active Ag3PO4 modified by tungsten and the effective degradation towards phenolic pollutants

A novel strategy of W modification was applied to overcome the disadvantages of Ag₃PO₄. Ultra-active Ag₃PO₄ with different W doping ratios were successfully synthesized by facile chemical precipitation method, among which 0.5%W-AP showed the best results. Meanwhile, the stability and yield were enhanced. XRD, Raman and ESR etc. were employed to investigate the morphology, structure and optical properties of samples. It was proved W⁶⁺ entered into the Ag₃PO₄ lattice, occupied the position of P⁵⁺ and doped in the form of WO₄^2-. The significant improvement of photocatalytic performance of W doped Ag₃PO₄ was attributed to the change of morphology, the decrease of particle size, the increase of crystallinity, the shrink of band gap energy and the reduction of photo-induced carriers recombination rate with W doping. The photocatalytic mechanism analysis showed h⁺ was the main oxidative species in the photocatalytic process, •O₂^- and •OH played minor roles. Under visible light irradiation, the impacts of the important operating parameters on the typical phenolic pollutants, phenol and bisphenol A, were evaluated with 0.5%W-AP. It was confirmed that 68% and 82% of phenol and bisphenol A were respectively degraded within 15 min and 40 min under optimized photocatalytic parameters: 0.4 g/L catalyst dosage, 20 mg/L pollutant concentration, pH 5.7 and 125 mW/cm² irradiation intensity, and the corresponding K' were 2.14 and 5.50 times of undoped samples. This work provides a new approach for effective degradation towards phenolic pollutants by Ag₃PO₄ with ultra-high photocatalytic activity, high applicability and enhanced stability and yield.

Synthesis and characterization of CuxO/Bi2O3 oxides for removal of HCHO under visible light irradiation

Cu_xO/Bi₂O₃ oxides grown on nickel foam were synthesized via an electrodeposition method to degrade indoor HCHO under visible light irradiation and fully characterized by XRD, SEM, FT-IR, and UV-Vis technologies. The characterization results showed that the Cu_xO/Bi₂O₃ oxides were successfully loaded on nickel foam and the visible light response spectrum was expanded to 740 nm. Plackett-Burman design combined with central composite design has been used to optimize factors that affect HCHO removal performance. The results demonstrated that bismuth nitrate content, polyethylene glycol 600 content, sintering time, and lactic acid concentration were the four most important factors affecting the HCHO removal performance over Cu_xO/Bi₂O₃ sample. The optimum Cu_xO/Bi₂O₃ sample could degrade 88.796% of HCHO in 300 min at the conditions of 4.28 mol/L lactic acid, 4.86% polyethylene glycol 600, 194.03 min sintering time, and 45.83 g bismuth nitrate, and the HCHO removal rate remained 82.3% after five cycles. A plausible mechanism for the degradation of HCHO under visible light irradiation was proposed. This work provides a feasible solution for removing indoor formaldehyde in the field of photocatalysis.

Sunlight driven photocatalytic degradation of organic pollutants using a MnV2O6/BiVO4 heterojunction: mechanistic perception and degradation pathways

In the field of photocatalysis, fabrication of a heterojunction structure with effective charge separation at the interface and charge shift to enhance the photocatalytic activity has acquired extensive consideration. In the present investigation, MnV₂O₆/BiVO₄ heterojunction samples with excellent photocatalytic performance under sunlight irradiation were conveniently synthesized by a hydrothermal technique, and characterized by UV-Vis, FTIR, XRD, FESEM, HRTEM, PL, BET and XPS techniques. The prepared samples were investigated as photocatalysts for degrading MB and RhB dyes under sunlight. Among various samples of MnV₂O₆/BiVO₄, the S-V hetero-junction sample exhibited maximum photocatalytic activity with 98% and 96% degradation of MB and RhB dyes, respectively, in 6 and 35 min. The high photocatalytic activity of MnV₂O₆/BiVO₄ may be due to the successful generation and shift of charges in the presence of visible light. The average reduction of chemical oxygen demand (COD) was found to be 75% after irradiation with direct sunlight. In the degradation process of dyes, superoxide anion radicals were the main responsive species, as revealed by trapping experiments. The degradation efficiency of MnV₂O₆/BiVO₄ heterojunction did not diminish even after four cycles. In addition, the catalytic performance of the fabricated heterojunction was also explored for reducing 4-nitrophenols (4-NP) by using NaBH₄. Absolute conversion of 4-NP to 4-aminophenol (4-AP) occurred without the production of intermediate byproducts.

Gibberellic acids promote growth and exopolysaccharide production in Tetraselmis suecica under reciprocal nitrogen concentration: an assessment on antioxidant properties and nutrient removal efficacy of immobilized iron-magnetic nanoparticles

The present study was aimed to assess the effect of gibberellic acids to enhance the growth, biomass, pigment, and exopolysaccharides production in Tetraselmis suecica under reciprocal nitrogen concentrations. For this study, the seven types of experimental media (N-P, NL-P/2GA3, N0-P/2GA3, NL-P/4GA3, N0-P/4GA3, NL-P/6GA3, and N0-P/6GA3) were prepared with the addition of gibberellic acids under various nitrogen concentrations. The experiment lasted for 15 days and the cell density, biomass, chlorophyll 'a', and exopolysaccharides (EPS) concentration of T. suecica were estimated for every 3 days. Then the EPS was subjected to the analyses of chemical (carbohydrate, protein, sulfate, and uronic acid), and antioxidant activity. In addition, nutrient removal efficiency was evaluated using different concentration of EPS. The highest DPPH (2,2-diphenyl-1-picrylhydrazyl) (86.7 ± 0.95%) and hydroxyl radical activity (85.7 ± 2.48%) were observed at the EPS concentrations 2.5 and 1.2 mg/mL, respectively. The immobilized magnetic Fe₃O₄-EPS (ferric oxide-exopolysaccharides) nanoparticles (5.0 and 10.0 g/L) have efficiently removed the excessive phosphate (89.5 ± 1.65%) and nitrate (73.5 ± 1.72%) from the Litopenaeus vannamei cultured wastewater. Thus, the application of gibberellic acids combined with limited nitrogen concentration could produce higher EPS that could exhibit excellent antioxidant activity, and nutrient removal efficacy in the form of Fe₃O₄-EPS magnetic nanoparticles.

Competitive role of nitrogen functionalities of N doped GO and sensitizing effect of Bi2O3 QDs on TiO2 for water remediation

The promising solar irradiated photocatalyst by pairing of bismuth oxide quantum dots (BQDs) doped TiO₂ with nitrogen doped graphene oxide (NGO) nanocomposite (NGO/BQDs-TiO₂) was fabricated. It was used for degradation of organic pollutants like 2,4-dichlorophenol (2,4-DCP) and stable dyes, i.e. Rhodamine B and Congo Red. X-ray diffraction (XRD) profile of NGO showed reduction in oxygenic functional groups and restoring of graphitic crystal structure. The characteristic diffraction peaks of TiO₂ and its composites showed crystalline anatase TiO₂. Morphological images represent spherical shaped TiO₂ evenly covered with BQDs spread on NGO sheet. The surface linkages of NO-O-Ti, C-O-Ti, Bi-O-Ti and vibrational modes are observed by Fourier transform infrared spectroscopy (FTIR) and Raman studies. BQDs and NGO modified TiO₂ results into red shifting in visible region as studied in diffused reflectance spectroscopy (DRS). NGO and BQDs in TiO₂ are linked with defect centers which reduced the recombination of free charge carriers by quenching of photoluminescence (PL) intensities. X-ray photoelectron spectroscopy (XPS) shows that no peak related to C-O in NGO/BQDs-TiO₂ is observed. This indicated that doping of nitrogen into GO has reduced some oxygen functional groups. Nitrogen functionalities in NGO and photosensitizing effect of BQDs in ternary composite have improved photocatalytic activity against organic pollutants. Intermediate byproducts during photo degradation process of 2,4-DCP were studied through high performance liquid chromatography (HPLC). Study of radical scavengers indicated that O₂^·- has significant role for degradation of 2,4-DCP. Our investigations propose that fabricated nanohybrid architecture has potential for degradation of environmental pollutions.

Photoelectrochemical Degradation of Organic Pollutants on a La3+ Doped BiFeO3 Perovskite

Towards nonconventional wastewater treatment methods for the degradation of organic pollutants in wastewater, a perovskite-based photoelectrochemical system was developed. Bismuth ferrite doped with lanthanum (La-BiFeO3, La-BFO) perovskite was synthesised through a hydrothermal method with low calcination temperature for the photoelectrochemical degradation of orange II dye and other cocktails of dyes. Photoanodes were prepared by the deposition of the perovskites on a fluorine-doped tin oxide (FTO) substrate. The photoanodes were characterised using XRD, FESEM, FTIR and UV-vis diffuse reflectance. The photoelectrochemical properties of the synthesised photoanodes were investigated with chronoamperometry and electrochemical impedance spectroscopy (including Mott–Schottky analysis). The results show that all La3+-doped BFO photoanodes exhibited a higher absorption edge in the visible light region than the undoped BFO. The photocurrent response of 10% La-BFO (the best performing electrode) exhibited a three times higher current response than the pure BFO. In addition, the electrode exhibited a good degradation efficiency of 84.2% within 120 min with applied bias potential of 2 V at a pH of 7. EIS studies showed a significant enhancement of the interfacial electron transfer of the charge carriers. The enhancements in electrode performances were attributed to the synergistic effect of the applied bias potential and the introduction of La3+ into the BFO matrix. This study therefore shows that the photoelectrocatalytic performance of BFO for water treatment can be improved by the introduction of perovskites-doping ions such as La3+.

Titanium Dioxide/Polyvinyl Alcohol/Cork Nanocomposite: A Floating Photocatalyst for the Degradation of Methylene Blue under Irradiation of a Visible Light Source

Photocatalytic degradation by the titanium dioxide (TiO2) photocatalyst attracts tremendous interest due to its promising strategy to eliminate pollutants from wastewater. The floating photocatalysts are explored as potential candidates for practical wastewater treatment applications that could overcome the drawbacks posed by the suspended TiO2 photocatalysis system. The problem occurs when the powdered TiO2 applied directly into the treated solution will form a slurry, making its reuse become a difficult step after treatment. In this study, the immobilization of titanium dioxide nanoparticles (TiO2 NPs) on the floating substrate (cork) employing polyvinyl alcohol (PVA) as a binder to anchor TiO2 NPs on the surface of the cork was carried out. Characterizations such as Fourier transformer infrared, X-ray diffraction (XRD), ultraviolet-visible spectroscopy (UV-vis), zeta potential, photoluminescence spectroscopy, femtosecond to millisecond time-resolved visible to mid-IR absorption spectroscopy, ion chromatography, and scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDX) analyses were employed. XRD analysis revealed the formation of anatase-phase TiO2 NPs. The results demonstrated that the crystallite size was 9.36 nm. The band gap energy of TiO2 NPs was determined as 3.0 eV. PL analysis verified that TiO2 NPs possessed a slower recombination rate of electron-hole pairs as compared to anatase TiO2. The result was attributed by the behavior of photogenerated charge carriers on TiO2 NPs, which existed as shallowly trapped electrons that could survive longer than a few milliseconds in this study. Furthermore, SEM-EDX analysis indicated that TiO2 NPs were well distributed on the surface of the cork. At the optimal mole ratio of TiO2/PVA (1:8), the TiO2/PVA/cork floating photocatalyst degraded at 98.43% of methylene blue (MB) under a visible light source which performed better than under sunlight irradiation (77.09% of MB removal) for 120 min. Besides, the mineralization result has measured the presence of sulfate anions after photocatalytic activities, which achieved 86.13% (under a visible light source) and 65.34% (under sunlight). The superior photodegradation performance for MB was mainly controlled by the reactive oxygen species of the superoxide radical (•O2 -). The degradation kinetics of MB followed the first-order kinetics. Meanwhile, the Langmuir isotherm model was fitted for the adsorption isotherm. The floating photocatalyst presented good reusability, resulting in 78.13% of MB removal efficiency even after five cycles. Our TiO2/PVA/cork floating photocatalyst fabrication and high photocatalytic performance are potentially used in wastewater treatment, especially under visible light irradiation.

PES-Ag3PO4/g-C3N4 Mixed Matrix Film Photocatalyst for Degradation of Methyl Orange Dye

In the present study, we explored the effectiveness of PES-Ag₃PO₄/g-C₃N₄ film photocatalyst in degrading methyl orange dye under visible light irradiation. The PES-Ag₃PO₄/g-C₃N₄ film photocatalyst was prepared via a non-solvent-induced phase inversion process and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), laser scanning microscopy (LSM), X-ray photoelectron spectra (XPS), UV-diffuse reflectance (DRS), and water contact angle. The incorporation of the Ag₃PO₄/g-C₃N₄ composite into the PES matrix improved the pristine PES film’s hydrophilicity, as evidenced by the reduction of water contact angle from 79.03° to 54.33° for a film containing 15 wt % of Ag₃PO₄/g-C₃N₄ composite. The film’s photoactivity showed that 13 wt % was the best loading of Ag₃PO₄/g-C₃N₄ composite, and the degradation performance was maintained up to three cycles. The •O₂⁻ and h⁺ were the predominant species responsible for the methyl orange degradation.

SrTiO3/Bi4Ti3O12 Nanoheterostructural Platelets Synthesized by Topotactic Epitaxy as Effective Noble-Metal-Free Photocatalysts for pH-Neutral Hydrogen Evolution

Low-temperature hydrothermal epitaxial growth and topochemical conversion (TC) reactions offer unexploited possibilities for the morphological engineering of heterostructural and non-equilibrium shape (photo)catalyst particles. The hydrothermal epitaxial growth of SrTiO3 on Bi4Ti3O12 platelets is studied as a new route for the formation of novel nanoheterostructural SrTiO3/Bi4Ti3O12 platelets at an intermediate stage or (100)-oriented mesocrystalline SrTiO3 nanoplatelets at the completed stage of the TC reaction. The Bi4Ti3O12 platelets act as a source of Ti(OH)62- species and, at the same time, as a substrate for the epitaxial growth of SrTiO3. The dissolution of the Bi4Ti3O12 platelets proceeds faster from the lateral direction, whereas the epitaxial growth of SrTiO3 occurs on both bismuth-oxide-terminated basal surface planes of the Bi4Ti3O12 platelets. In the progress of the TC reaction, the Bi4Ti3O12 platelet is replaced from the lateral ends toward the interior by SrTiO3, while Bi4Ti3O12 is preserved in the core of the heterostructural platelet. Without any support from noble-metal doping or cocatalysts, the SrTiO3/Bi4Ti3O12 platelets show stable and 15 times higher photocatalytic H2 production (1265 μmol·g-1·h-1; solar-to-hydrogen (STH) efficiency = 0.19%) than commercial SrTiO3 nanopowders (81 μmol·g-1·h-1; STH = 0.012%) in pH-neutral water/methanol solutions. A plausible Z scheme is proposed to describe the charge-transfer mechanism during the photocatalysis.

Facile synthesis of Fe3O4/CuO a core-shell heterostructure for the enhancement of photocatalytic activity under visible light irradiation

A magnetically separable Fe₃O₄/CuO core-shell heterostructure photocatalyst was synthesized by hydrothermal method. The obtained photocatalyst was characterized by Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and UV-visible diffuse reflectance (UV-DRS). The obtained photocatalyst was used for the degradation of azo dye Direct Red 89 (DR89), under visible light irradiation provided by fluorescent lamp of 100 W in the presence of 7 mL of H₂O₂ (30%); the results of the photocatalytic activity for Fe₃O₄/CuO photocatalyst showed that in the presence of 0.75 g dispersed in 250 mL of 40 mg/L of DR89 dye at pH 6 the dye was completely removed after 240 min. Moreover, the photocatalytic activity of the prepared Fe₃O₄/CuO was enhanced 11 and 9 times compared with the pure Fe₃O₄ or CuO. The effect of initial dye concentrations on the photocatalytic activity was studied in the range of 20-60 mg/L, and the results showed that the catalyst has a good photocatalytic activity of 89% even at high concentration (60 mg/L). Furthermore, the catalyst maintained its activity after 5 cycles, and its paramagnetic property facilitates its recovery. The excellent photodegradation activity of Fe₃O₄/CuO was attributed to the low band gap of the catalyst equal to 1.54 eV and the enhancement of light absorption in visible range of 330-780 nm, but also to a better charge carriers separation, due to the presence of Fe₃O₄ that reduces electron/hole recombination.

Construction of TiO2-MnO2 0D-2D nanostructured heterojunction for enhanced photocatalytic hydrogen production

The low transfer efficiency and high recombination loss of photo-induced carriers in TiO2 are significant issues that hinder its photocatalytic activity. Herein, TiO2 nanoparticles (∼5 nm) were loaded on MnO2 nanosheets (40-60 nm) to form TiO2-MnO2 nanostructured heterojunction (0D-2D nanostructure unit), possessing a high specific surface area. The separation/transfer efficiency of photocarriers and the solar absorptivity of TiO2-MnO2 were improved, thus enhancing solar energy conversion efficiency. The enhanced transfer efficiency of carriers is associated with the 2D network of MnO2 and abundant oxygen vacancies serving as media for electron transport. The enhanced visible absorption and reduced recombination should be attributed to the narrowed bandgap and modified energy band structure. The photocurrent of TiO2-MnO2 increased obviously and the H2 production rate increased to 0.38 mmol g-1 h-1, compared with that of pure TiO2 (0.25 mmol g-1 h-1). The enhanced photocatalytic properties are also associated with the excellent water oxidation kinetics caused by MnO2 nanosheets.

Pulsed laser synthesis of reduced graphene oxide supported ZnO/Au nanostructures in liquid with enhanced solar light photocatalytic activity

ZnO/Au/rGO ternary nanocomposites possessing a high photocatalytic response under solar irradiation were synthesized by a two-step process via a pulsed laser synthesis and a wet chemical process. The crystalline structure, surface morphology, size distribution, elemental composition, and optical properties of the prepared ZnO/Au/rGO ternary nanocomposites were characterized using X-ray diffraction, field-emission scanning electron microscope, high-resolution transmission electron microscope, energy-dispersive X-ray spectroscopy, UV-vis diffuse reflectance spectra, and photoluminescence analysis. The photocatalytic activity of the as synthesized nanocomposites was evaluated for the degradation of methylene blue (MB) under solar light irradiation (SLI). The density of the elemental and carbonaceous components, such as the Au nanoparticles (NPs) and the rGO nano-matrix on ZnO, could be altered by changing the concentration of HAuCl₄.3H₂O (5, 10, 15, and 20 wt%) or rGO (2.5, 5, and 7.5 wt%) using the same synthetic processes. The ZnO/Au15/rGO5 nanocomposite showed the highest photocatalytic degradation efficiency of 95% MB after 120 min under SLI, potentially due to the increased absorption of solar light or the efficient separation and migration of charge carriers by the anchored Au NPs and rGO onto the ZnO NPs. Further, the observed results and reusability of ZnO/Au15/rGO5 makes it an exceptionally promising material for diverse applications in the field of wastewater treatment and other types of environmental remediation.

Transforming Photocatalytic g-C3 N4 /MoSe2 into a Direct Z-Scheme System via Boron-Doping: A Hybrid DFT Study

Z-scheme photocatalytic systems are an ideal band alignment structure for photocatalysis because of the high separation efficiency of photo-induced carriers while simultaneously preserving the strong reduction activity of electrons and oxidation activity of holes. However, the design and construction of Z-scheme photocatalysts is challenging because of the need for appropriate energy band alignment and built-in electric field. Here, we propose a novel approach to a Z-scheme photocatalytic system using density functional theory calculations with the HSE06 hybrid functional. The undesirable type-I g-C₃ N₄ /MoSe₂ heterojunction is transformed into a direct Z-scheme system through boron doping of g-C₃ N₄ (B-doped C₃ N₄ /MoSe₂ ). Detailed analysis of the total and partial density of states, work functions and differential charge density distribution of the B-doped C₃ N₄ /MoSe₂ heterojunction shows the proper band alignment and existence of a built-in electric field at the interface, with the direction from g-C₃ N₄ to MoSe₂ , demonstrating a direct Z-scheme heterojunction. Further investigation on the absorption spectra reveals a large enhancement of the light absorption efficiency after boron doping. The results consistently confirm that electronic structures and photocatalytic performance can be effectively manipulated by a facile boron doping. Modulating the band alignment of heterojunctions in this way provides valuable insights for the rational design of highly efficient heterojunction-based photocatalytic systems.

Valorization of phosphate waste rocks to Ag3PO4/hydroxyapatite for photocatalytic degradation of Rhodamine B under visible light irradiation

A silver phosphate/hydroxyapatite (Ag₃PO₄/HA) composite was produced from phosphate waste rocks, firstly by the valorization of these wastes to HA and then by the treatment of this prepared HA with a silver nitrate solution. A type of response surface methodology, Box-Behnken experimental design, was used to find optimum synthesis parameters (silver to HA weight ratios, calcination temperature and calcination time). The visible light photodegradation of Rhodamine B in aqueous solution was used as the experimental response. The analysis of variance for the results showed that silver weight ratio is the most influential parameter on photoactivity of the synthesized photocatalyst. The optimum conditions were predicted to give an RhB degradation yield of 98.609%/4 hours under visible light conditions. In this context, a Ag/HA weight ratio of 14%, a calcination temperature of 300 °C, and a calcination time of 30 min were found to be the optimum conditions. Samples synthesized under the optimum condition were characterized by the use of X-ray diffraction, X-ray fluorescence spectrometer, Fourier transform infrared spectrum analysis, scanning electron microscopy, transmission electron microscopy and ultraviolet-visible diffuse reflection spectroscopy. By comparison with pure HA, the characterization results clearly showed the successful synthesis of the Ag₃PO₄/HA composite.

Mechanistic pathways for the degradation of SMX drug and floatation of degraded products using F–Pt co-doped TiO2 photocatalysts

This work presents smart pathways to enhance the photocatalytic activity of TiO₂via co-doping with fluorine (F) and platinum (Pt) to form F–Pt co-doped TiO₂ photocatalysts and investigates the unique and unusual fluorination of the floated products. Our investigations indicate that the crystalline structure of the photocatalysts was a mixture of anatase and brookite phases and that the nanoparticles of the synthesized nanocomposites had nanometric sizes (4–25 nm). The F–Pt co-doped TiO₂ nano-photocatalysts demonstrated degradation of sulfamethoxazole (SMX) drug of >93% within 90 min under direct solar light and 58% degradation within 360 min under a solar simulator. Thus, co-doping TiO₂ with F and Pt atoms to form F–Pt co-doped TiO₂ nanocomposite is an efficient pathway to achieve high photocatalytic performance escorted with the formation of floating metal-fluoropolymer, unlike pristine TiO₂ which has less photocatalytic degradation and no generation of a floating polymer. Our photocatalytic protocol demonstrates that the degradation of SMX started with redox reactions of oxygen and water absorbed on the surface of the prepared nanocomposites to form superoxide anions (O₂˙⁻) and hydroxy radicals (˙OH) which have oxidation superpower. The resultant products were subsequently fluorinated by fluoride radical ions and floated as metal-fluoropolymer.

This work presents smart pathways to enhance the photocatalytic activity of TiO₂via co-doping with fluorine (F) and platinum (Pt) to form F–Pt co-doped TiO₂ photocatalysts and investigates the unique and unusual fluorination of the floated products.

First-principles investigation of nonmetal doped single-layer BiOBr as a potential photocatalyst with a low recombination rate

Nonmetal doping is an effective approach to modify the electronic band structure and enhance the photocatalytic performance of bismuth oxyhalides.

Enhanced Photocatalytic Activity of Electrospun β-Ga2O3 Nanofibers via In-Situ Si Doping Using Tetraethyl Orthosilicate

β-Ga2O3 has attracted considerable attention as an alternative photocatalyst to replace conventional TiO2 under ultraviolet-C irradiation due to its high reduction and oxidation potential. In this study, to enhance the photocatalytic activity of β-Ga2O3, nanofibers are formed via the electrospinning method, and Si atoms are subsequently doped. As the Si concentration in the β-Ga2O3 nanofiber increases, the optical bandgap of the β-Ga2O3 nanofibers continuously decreases from 4.5 eV (intrinsic) to 4.0 eV for the Si-doped (2.4 at. %) β-Ga2O3 nanofibers, and accordingly, the photocatalytic activity of the β-Ga2O3 nanofibers is enhanced. This higher photocatalytic performance with Si doping is attributed to the increased doping-induced carriers in the conduction band edges. This differs from the traditional mechanism in which the doping-induced defect sites in the bandgap enhance separation and inhibit the recombination of photon-generated carriers.

Effects of K+, Mg2+, Ca2+, Zn2+, La3+, Cr3+, Ce3+, Ce4+, and Mo5+ Doping on the Adsorption Performance and Optical Properties of Sodium Titanate Nanotubes

Metal-doped titanate nanotubes (TNTs) were synthesized using a simple ion-exchange method. Cations with different valencies (Mg²⁺, Ca²⁺, Zn²⁺, K⁺, Cr³⁺, Ce³⁺, Ce⁴⁺, Mo⁵⁺, and La³⁺) were used to assess their effects on the adsorption performance and optical properties of the prepared nanotubes. All samples were characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDXS), and elemental mapping, and the optical properties were analyzed using photoluminescence (PL) spectroscopy. The adsorption performance of the prepared doped titanate nanotubes was tested against methylene blue (MB) as a model cation dye. The results revealed that all samples except Ce(IV)-TNT showed high removal efficiencies but at different equilibrium times. Mg-TNT, Zn-TNT, and K-TNT showed higher MB removal percentages (97%) after 45 min, while Ca-TNT, Cr-TNT, Ce(III)-TNT, Mo-TNT, La-TNT, and Na-TNT showed removal efficiencies of 95, 84, 95, 96, 94, and 96% after 65, 1200, 120, 300, 180, and 105 min, respectively. The titanate nanotubes were recycled by photocatalytic degradation of the adsorbed dye molecules. The band gaps, valence band, and conduction band were calculated and compared for the potential photocatalytic water splitting application.