Enhanced visible-light photodegradation of fluoroquinolone-based antibiotics and E. coli growth inhibition using Ag–TiO2 nanoparticles

Antibiotics in wastewater represent a growing and worrying menace for environmental and human health fostering the spread of antimicrobial resistance. Titanium dioxide (TiO₂) is a well-studied and well-performing photocatalyst for wastewater treatment. However, it presents drawbacks linked with the high energy needed for its activation and the fast electron–hole pair recombination. In this work, TiO₂ nanoparticles were decorated with Ag nanoparticles by a facile photochemical reduction method to obtain an increased photocatalytic response under visible light. Although similar materials have been reported, we advanced this field by performing a study of the photocatalytic mechanism for Ag–TiO₂ nanoparticles (Ag–TiO₂ NPs) under visible light taking in consideration also the rutile phase of the TiO₂ nanoparticles. Moreover, we examined the Ag–TiO₂ NPs photocatalytic performance against two antibiotics from the same family. The obtained Ag–TiO₂ NPs were fully characterised. The results showed that Ag NPs (average size: 23.9 ± 18.3 nm) were homogeneously dispersed on the TiO₂ surface and the photo-response of the Ag–TiO₂ NPs was greatly enhanced in the visible light region when compared to TiO₂ P25. Hence, the obtained Ag–TiO₂ NPs showed excellent photocatalytic degradation efficiency towards the two fluoroquinolone-based antibiotics ciprofloxacin (92%) and norfloxacin (94%) after 240 min of visible light irradiation, demonstrating a possible application of these particles in wastewater treatment. In addition, it was also proved that, after five Ag–TiO₂ NPs re-utilisations in consecutive ciprofloxacin photodegradation reactions, only a photocatalytic efficiency drop of 8% was observed. Scavengers experiments demonstrated that the photocatalytic mechanism of ciprofloxacin degradation in the presence of Ag–TiO₂ NPs is mainly driven by holes and ˙OH radicals, and that the rutile phase in the system plays a crucial role. Finally, Ag–TiO₂ NPs showed also antibacterial activity towards Escherichia coli (E. coli) opening the avenue for a possible use of this material in hospital wastewater treatment.

Ag nanoparticles decorated-TiO₂ P25 are a viable alternative for the degradation, through a rutile-mediated mechanism, of fluoroquinolone-based antibiotics under visible light irradiation and, at the same time, for bacteria inactivation in water.     

Construction of H4xK2xSn2−xS4+x/TiO2 nanocomposites with enhanced visible light-driven photocatalytic performance

In this paper, a new photocatalyst with TiO₂ nanospheres decorated on ultrathin layered thiostannate H_4xK_2xSn_2−xS_4+x (X = 0.5–0.6, HKTS) nanosheets was successfully synthesized by a facile solvothermal method combined with the hydrolysis of tetrabutyl titanate and it was denoted as HKTS/TiO₂. By adjusting the content of tetrabutyl titanate, composites with different Sn/Ti molar ratios were prepared. The composites were applied for RhB degradation under visible light irradiation, and the optimum proportion of HKTS/TiO₂ was obtained. The results of X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy and scanning electron microscopy confirmed that TiO₂ was successfully decorated on HKTS nanosheets. The combination of TiO₂ and HKTS extended the absorption wavelength of TiO₂ from UV to visible light range, and the separation efficiency of photoexcited electron–hole pairs was also enhanced. The photocatalytic degradation rate of RhB over HKTS/TiO₂-1.0 was almost 97.9% after 60 min illumination, which was higher than those of HKTS and pure TiO₂. The photocatalyst exhibited excellent reusability and stability as the degradation rate of RhB was 95.7% even after three cycles. The photocatalytic mechanism experiment indicated that ·O₂⁻ and h⁺ played a dominant role in the photocatalytic process. All these results indicate that the newly fabricated HKTS/TiO₂ composites provide a high-performance photocatalyst for waste water treatment, and the application of thiostannate can be extended to the field of photocatalytic materials.

The enhanced visible light photocatalytic performance of a newly fabricated nanostructure combined TiO₂ with HKTS.     

TiO2–Au composite nanofibers for photocatalytic hydrogen evolution

TiO₂-based materials for photocatalytic hydrogen (H₂) evolution have attracted much interest as a renewable approach for clean energy applications. TiO₂–Au composite nanofibers (NFs) with an average fiber diameter of ∼160 nm have been fabricated by electrospinning combined with calcination treatment. In situ reduced gold nanoparticles (NPs) with uniform size (∼10 nm) are found to disperse homogenously in the TiO₂ NF matrix. The TiO₂–Au composite NFs catalyst can significantly enhance the photocatalytic H₂ generation with an extremely high rate of 12 440 μmol g⁻¹ h⁻¹, corresponding to an adequate apparent quantum yield of 5.11% at 400 nm, which is 25 times and 10 times those of P25 (584 μmol g⁻¹ h⁻¹) and pure TiO₂ NFs (1254 μmol g⁻¹ h⁻¹), respectively. Furthermore, detailed studies indicate that the H₂ evolution efficiency of the TiO₂–Au composite NF catalyst is highly dependent on the gold content. This work provides a strategy to develop highly efficient catalysts for H₂ evolution.

The H₂ production rate of TiO₂–Au nanofibers is dramatically improved to 12 440 μmol g⁻¹ h⁻¹, 10 times that of pure TiO₂.     

Fabrication of a magnetic ternary ZnFe2O4/TiO2/RGO Z-scheme system with efficient photocatalytic activity and easy recyclability

A magnetic composite based on TiO₂ nanosheets, ZnFe₂O₄ and reduced graphene oxide (RGO) was synthesized by a one-step hydrothermal synthesis method, which possessed the band structure of a Z-scheme photocatalytic system. The properties and structures of the samples were characterized by XRD, UV-Vis DRS, Raman spectroscopy, SEM, EDS, XPS and PL spectroscopy. Compared with TiO₂ nanosheets and the TiO₂/RGO composite, the obtained ternary composite with 3 wt% RGO exhibited a significant enhancement in photocatalytic activities, attributed to the efficient charge separation induced by the fabricated Z-scheme system. About 99.7% of p-nitrophenol (p-NP) degraded within 60 min under simulated solar irradiation. Trapping experiments showed that superoxide anions (˙O₂⁻) and hydroxyl radicals (˙OH) were the main active species in the p-NP photocatalytic degradation. Finally, a possible photocatalytic mechanism of Z-scheme ZnFe₂O₄/TiO₂/RGO was proposed based on the results of trapping experiments and the energy bands of TiO₂ and ZnFe₂O₄.

A magnetic separable Z-scheme composite based on ZnFe₂O₄, TiO₂ nanosheets and RGO exhibits efficient photocatalytic degradation of p-NP.     

Fabrication,characterization and high photocatalytic activity of Ag–ZnO heterojunctions under UV-visible light

Pure ZnO and Ag–ZnO nanocomposites were fabricated via a sol–gel route, and the obtained photocatalysts were characterized by XRD, SEM, TEM, BET, XPS, PL and DRS. The results showed that Ag⁰ nanoparticles deposit on the ZnO surface and Ag modification has negligible impact on the crystal structure, surface hydroxyl group content and surface area of ZnO. However, the recombination of photogenerated electrons and holes was suppressed effectively by Ag loading. The photocatalytic activity was investigated by evaluating the degradation of MB under xenon lamp irradiation as the UV-visible light source, and the results show that the photocatalytic activity of ZnO significantly improved after Ag modification. Ag–ZnO photocatalysts exhibit higher photocatalytic activity than commercial photocatalyst P25. The degradation degree of MB for 1%Ag–ZnO was 97.1% after 15 min. ˙O₂⁻ radicals are the main active species responsible for the photodegradation process, and Ag–ZnO heterojunctions generate more ˙O₂⁻ radicals, which is the primary reason for the improved photocatalytic performance.

Ag–ZnO heterojunction promotes the separation of photogenerated pairs and thus exhibits high catalytic activity under UV-visible light.     

Photo-reduction of heavy metal ions and photo-disinfection of pathogenic bacteria under simulated solar light using photosensitized TiO2 nanofibers

We report the photosensitization of electrospun titania nanofibers, with a mean diameter of 195 nm, by low bandgap silver sulfide nanoparticles of 11–23 nm mean size with the aim of treating heavy metal ions and pathogenic bacteria simultaneously under simulated solar light irradiation. The 17 nm Ag₂S/TiO₂ nanofibers showed 90% photocatalytic reduction of Cr(vi) at pH of 3 with a pseudo-first order rate constant of 0.016 min⁻¹ which is significantly better than the previously reported for Ag–Ag₂S/TiO₂ composite particles. The antibacterial capability of the Ag₂S/TiO₂ nanofibers was evaluated via photo-disinfection of the Gram-positive and Gram-negative bacterial strains. The smallest sized 11 nm Ag₂S/TiO₂ nanofiber showed the best bactericidal efficiency of 100% and 90.6% against Gram-negative E. coli and Gram-positive S. aureus after 1 h of irradiation, respectively, whereas, only 50% E. coli and 41% S. aureus were found to be inactivated in dark. Furthermore, a UV–O₃ treatment of the 11 nm Ag₂S/TiO₂ nanofibers remarkably enhanced the antibacterial activity where 89% E. coli and 81% S. aureus were inactivated in just 10 min of the irradiation. Enhanced photocatalytic activity is attributed to the efficient charge separation and transfer and reduced electron–hole recombination induced by the effective heterojunction formation between TiO₂ and the optimally sized Ag₂S nanoparticles. The disinfection nature of the Ag₂S nanoparticles, role of the generated hydroxyl species under irradiation, and the cell wall damage mechanism is also discussed. This study demonstrates the potential use of these multifunctional composite TiO₂ nanofibers for water remediation.

Photosensitization of titania nanofibers by low bandgap silver sulfide nanoparticles for treating heavy metal ions and pathogenic bacteria simultaneously under simulated solar light irradiation.     

Efficient detoxification of triclosan by a S–Ag/TiO2@g-C3N4 hybrid photocatalyst: process optimization and bio-toxicity assessment

Owing to their persistency and toxicity, development of an effective strategy to eliminate antibiotic residues from the aquatic system has become a major environmental concern. Doping TiO₂ with hetero atoms and forming a hybrid structure with g-C₃N₄ could serve as an efficient visible light active photocatalytic candidate. In this study, a novel S–Ag/TiO₂@g-C₃N₄ hybrid catalyst was prepared for visible light degradation and detoxification of triclosan (TS) antibiotic. The effect of various operational parameters towards the photocatalytic degradation was systematically evaluated through response surface methodology (RSM) based on central composite design (CCD). The highest TS degradation (92.3%) was observed under optimal conditions (TS concentration = 10 mg L⁻¹, pH = 7.8, and catalyst weight = 0.20 g L⁻¹) after 60 min. Efficient charge separation resulted from the doped nanoparticles (silver and sulphur), the existing integrated electric field of the heterojunction and the overlying light response of hybridized TiO₂ and g-C₃N₄, thus the S–Ag/TiO₂@g-C₃N₄ composite showed impressively higher activity. The main degradation products of TS were identified by LC/ESI-MS analysis. In addition, the toxicity of the degradation products was investigated through an Escherichia coli (E. coli) colony forming unit assay and the results revealed that under optimal conditions a significant reduction in biotoxicity was noticed.

Owing to their persistency and toxicity, development of an effective strategy to eliminate antibiotic residues from the aquatic system has become a major environmental concern.     

Highly crystalline anatase TiO2 nanocuboids as an efficient photocatalyst for hydrogen generation

Highly crystalline anatase titanium dioxide (TiO₂) nanocuboids were synthesized via a hydrothermal method using ethylenediamine tetraacetic acid as a capping agent. The structural study revealed the nanocrystalline nature of anatase TiO₂ nanocuboids. Morphological study indicates the formation of cuboid shaped particles with thickness of ∼5 nm and size in the range of 10–40 nm. The UV-visible absorbance spectra of TiO₂ nanocuboids showed a broad absorption with a tail in the visible-light region which is attributed to the incorporation of nitrogen atoms into the interstitial positions of the TiO₂ lattice as well as the formation of carbonaceous and carbonate species on the surface of TiO₂ nanocuboids. The specific surface areas of prepared TiO₂ nanocuboids were found to be in the range of 85.7–122.9 m² g⁻¹. The formation mechanism of the TiO₂ nanocuboids has also been investigated. Furthermore, the photocatalytic activities of the as-prepared TiO₂ nanocuboids were evaluated for H₂ generation via water splitting under UV-vis light irradiation and compared with the commercial anatase TiO₂. TiO₂ nanocuboids obtained at 200 °C after 48 h exhibited higher photocatalytic activity (3866.44 μmol h⁻¹ g⁻¹) than that of commercial anatase TiO₂ (831.30 μmol h⁻¹ g⁻¹). The enhanced photoactivity of TiO₂ nanocuboids may be due to the high specific surface area, good crystallinity, extended light absorption in the visible region and efficient charge separation.

Highly crystalline TiO₂ nanocuboids have been prepared and their photocatalytic hydrogen generation activity was evaluated via water splitting.     

Synergistic effect of Ni–Ag–rutile TiO2 ternary nanocomposite for efficient visible-light-driven photocatalytic activity

P25 comprising of mixed anatase and rutile phases is known to be highly photocatalytically active compared to the individual phases. Using a facile wet chemical method, we demonstrate a ternary nanocomposite consisting of Ni and Ag nanoparticles, decorated on the surface of XTiO₂ (X: P25, rutile (R)) as an efficient visible-light-driven photocatalyst. Contrary to the current perspective, RTiO₂-based Ni–Ag–RTiO₂ shows the highest activity with the H₂ evolution rate of ∼86 μmol g⁻¹ W⁻¹ h⁻¹@535 nm. Together with quantitative assessment of active Ni, Ag and XTiO₂ in these ternary systems using high energy synchrotron X-ray diffraction, transmission electron microscopy coupled energy dispersive spectroscopy mapping evidences the metal to semiconductor contact via Ag. The robust photocatalytic activity is attributed to the improved visible light absorption, as noted by the observed band edge of ∼2.67 eV corroborating well with the occurrence of Ti³⁺ in Ti 2p XPS. The effective charge separation due to intimate contact between Ni and RTiO₂via Ag is further evidenced by the plasmon loss peak in Ag 3d XPS. Moreover, density functional theory calculations revealed enhanced adsorption of H₂ on Ti₈O₁₆ clusters when both Ag and Ni are simultaneously present, owing to the hybridization of the metal atoms with d orbitals of Ti and p orbitals of O leading to enhanced bonding characteristics, as substantiated by the density of states. Additionally, the variation in the electronegativity in Bader charge analysis indicates the possibility of hydrogen evolution at the Ni sites, in agreement with the experimental observations.

Robust photocatalytic activity of Ni–Ag–RTiO₂ is attributed to the improved visible light absorption and effective charge separation due to intimate contact between Ni and RTiO₂via Ag, as evidenced by Ti³⁺ in Ti 2p XPS and energy dispersive mapping.     

Ag0/Au0 nanocluster loaded Bi2O4 photocatalyst for methyl orange dye photodegradation

Visible-light-sensitive Ag and Au nanocluster loaded Bi₂O₄ (Ag–Bi₂O₄ and Au–Bi₂O₄) semiconductor photocatalysts have been synthesized. The composite materials exhibited increased photocatalytic degradation of the azo-dye pollutant, Methyl Orange (MO). In addition, Au–Bi₂O₄ (Au-7% wt) showed the highest MO degradation rate (0.05904 min⁻¹) i.e. 7.69 times higher than the pristine Bi₂O₄ and 1.4 times higher than 1% Ag–Bi₂O₄. The optical properties of the composites showed that the band gaps of the composite samples 1% Ag–Bi₂O₄ and 7% Au–Bi₂O₄ were 1.96 eV and 2.09 eV, respectively. The increase in the degradation rate is attributed to the decrease of the recombination rate of photoinduced e⁻/h⁺, caused by the enhanced charge transfer between the metal nanoparticles and Bi₂O₄ as confirmed in the photocurrent measurements. The photocurrent measurements showed increase in the transients output by 8.25 times and 2.75 times for 1% Ag–Bi₂O₄ & 3% Au–Bi₂O₄, respectively as compared to that of the pristine Bi₂O₄. These features further aided the increase in the photocatalytic efficiency while retaining the original physical properties, thus showing the robustness of Bi₂O₄ as a photocatalyst.

Illustration of charge transfer between Bi₂O₄ and nanoclusters, and photocatalytic MO degradation by M/Bi₂O₄ under visible light irradiation (M = Ag or Au).     

Dye degradation performance,bactericidal behavior and molecular docking analysis of Cu-doped TiO2 nanoparticles

Copper-doped TiO₂ was prepared with a sol–gel chemical method. Various concentrations (3, 6, and 9 wt%) of Cu dopant were employed. Several techniques were implemented to assess the structural, optical, morphological and chemical properties of the synthesized samples. Evaluation of elemental composition using SEM-EDS and XRF techniques showed the presence of dopant element in the prepared samples. XRD analysis confirmed the presence of anatase (TiO₂) phase with interstitial doping. Incorporation of dopant was observed to enhance the crystallinity and increase the crystallite size of the synthesized products. SAED profiles revealed a high degree of crystallinity in the prepared specimens, which was also evident in the XRD spectra. Optical properties studied using UV-vis spectroscopy depicted a shift of the maximum absorption to the visible region (redshift) that signified a reduction in the band gap energy of Cu-doped TiO₂ samples. Examination of morphological features with scanning and high-resolution transmission electron microscopes revealed the formation of spherical nanoparticles with a tendency to agglomerate with increasing dopant concentration. Molecular vibrations and the formation of Ti–O–Ti bonds were revealed through FTIR spectra. PL spectroscopy recorded the trapping efficiency and migration of charge carriers, which exhibited electron–hole recombination behavior. Doped nanostructures showed enhanced bactericidal performance and synergism against S. aureus and E. coli. In summary, Cu-doped TiO₂ nanostructures were observed to impede bacteria effectively, which is deemed beneficial in overcoming ailments caused by pathogens such as microbial etiologies. Furthermore, molecular docking analysis was conducted to study the interaction of Cu-doped TiO₂ nanoparticles with multiple proteins namely β-lactamase (binding score: −4.91 kcal mol⁻¹), ddlB (binding score: −5.67 kcal mol⁻¹) and FabI (binding score: −6.13 kcal mol⁻¹) as possible targets with active site residues. Dye degradation/reduction of control and Cu-doped samples were studied through absorption spectroscopy. The obtained outcomes of the performed experiment indicated that the photocatalytic activity of Cu-TiO₂ enhanced with increasing dopant concentration, which is thought to be due to a decreased rate of electron–hole pair recombination. Consequently, it is suggested that Cu-TiO₂ can be exploited as an effective candidate for antibacterial and dye degradation applications.

Copper-doped TiO₂ was prepared with a sol–gel chemical method.     

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.     

Deposition of platinum on boron-doped TiO2/Ti nanotube arrays as an efficient and stable photocatalyst for hydrogen generation from water splitting

An efficient photocatalyst of boron-doped titanium dioxide/titanium nanotube array-supported platinum particles (Pt–B/TiO₂/Ti NTs) was fabricated for photocatalytic water splitting for hydrogen production through a two-step route. First, B/TiO₂/Ti NTs were prepared by anodic oxidation using hydrofluoric acid as electrolyte and boric acid as a B source. Then, Pt particles were deposited on the surface of B/TiO₂/Ti NTs by photo-assisted impregnation reduction. The structure and properties of Pt–B/TiO₂/Ti NTs were characterized by various physical measurements which showed the successful fabrication of Pt–B/TiO₂/Ti NTs. The Pt–B/TiO₂/Ti NTs, with a B-doping content of 15 mmol L⁻¹, showed the highest photocatalytic activity and exhibited a photocatalytic hydrogen-production rate of 384.9 μmol g⁻¹ h⁻¹, which was 9.2-fold higher than that of unmodified TiO₂/Ti NTs (41.7 μmol g⁻¹ h⁻¹). This excellent photocatalytic performance was ascribed mainly to the synergistic effect of Pt and B, which could enhance the photocatalytic activity of TiO₂/Ti NTs.

Pt–B/TiO₂/Ti NTs, prepared by anodic oxidation and photo-deposition methods, showed excellent photocatalytic activity.     

Three-dimensional graphene encapsulated Ag–ZnFe2O4 flower-like nanocomposites with enhanced photocatalytic degradation of enrofloxacin

Three-dimensional (3D) Ag–ZnFe₂O₄-reduced graphene oxide (rGO) was successfully synthesized using a hydrothermal and photo-reduction method, and the morphological differences of the materials were observed. Their photocatalytic activity was evaluated by photocatalytic degradation of enrofloxacin (ENR) under visible-light irradiation. The results indicated that Ag–ZnFe₂O₄–rGO exhibited superior photocatalytic properties and good stability. In this research, the enhancement of photocatalytic performance is mainly attributed to the electron channelization ability of rGO, which traps the photoexcited electrons of ZnFe₂O₄ on its π framework, and reduces the electron–hole recombination rate. Moreover, the high surface area of 3D pompon mum flower-like ZnFe₂O₄ provides more reactive sites. In addition, free radical capture and ESR experiments as well as pathway analysis of degradation also confirmed that superoxide radicals (˙O₂⁻) and photo-generated holes from Ag–ZnFe₂O₄–rGO were the main active species in the degradation progress of ENR.

Three-dimensional (3D) Ag–ZnFe₂O₄-reduced graphene oxide (rGO) was successfully synthesized using a hydrothermal and photo-reduction method, and the morphological differences of the materials were observed.     

Homojunction and defect synergy-mediated electron–hole separation for solar-driven mesoporous rutile/anatase TiO2 microsphere photocatalysts

The photocatalytic hydrogen evolution of TiO₂ is deemed to be one of the most promising ways of converting solar energy to chemical energy; however, it is a challenge to improve the photo-generated charge separation efficiency and enhance solar utilization. Herein, black mesoporous rutile/anatase TiO₂ microspheres with a homojunction and surface defects have been successfully synthesized by an evaporation-induced self-assembly, solvothermal and high-temperature surface hydrogenation method. The H500-BMR/ATM (HX-BMR/ATM, where X means the different hydrogen calcination temperatures) materials not only possess a mesoporous structure and relatively high specific surface area of 39.2 m² g⁻¹, but also have a narrow bandgap (∼2.87 eV), which could extend the photoresponse to the visible light region. They exhibit high photocatalytic hydrogen production (6.4 mmol h⁻¹ g⁻¹), which is much higher (approximately 1.8 times) than that of pristine mesoporous rutile/anatase TiO₂ microspheres (3.58 mmol h⁻¹ g⁻¹). This enhanced photocatalytic hydrogen production property is attributed to the synergistic effect of the homojunction and surface defects in improving efficient electron–hole separation and high utilization of solar light. This work proposes a new approach to improve the performance of photocatalytic hydrogen production and probably offers a new insight into fabricating other high-performance photocatalysts.

Mesoporous rutile/anatase TiO₂ microspheres with surface defects are fabricated and exhibit excellent solar-driven photocatalytic performance due to synergistic effect of the homojunction and surface defects favoring efficient e–h separation.     

Reduced graphene oxide-supported Ag-loaded Fe-doped TiO2 for the degradation mechanism of methylene blue and its electrochemical properties

Graphene oxide-based composites have been developed as cheap and effective photocatalysts for dye degradation and water splitting applications. Herein, we report reduced graphene oxide (rGO)/Ag/Fe-doped TiO₂ that has been successfully prepared using a simple process. The resulting composites were characterized by a wide range of physicochemical techniques. The photocatalytic activities of the composite materials were studied under visible light supplied by a 35 W Xe arc lamp. The rGO/Ag/Fe-doped TiO₂ composite demonstrated excellent degradation of methylene blue (MB) in 150 min and 4-nitrophenol (4-NP) in 210 min under visible light irradiation, and trapping experiments were carried out to explain the mechanism of photocatalytic activity. Moreover, electrochemical studies were carried out to demonstrate the oxygen evolution reaction (OER) activity on rGO/Ag/Fe-doped TiO₂ in 1 M of H₂SO₄ electrolyte, with a scan rate of 50 mV s⁻¹. The reductions in overpotential are due to the d-orbital splitting in Fe-doped TiO₂ and rGO as an electron collector and transporter.

Graphene oxide-based composites have been developed as cheap and effective photocatalysts for dye degradation and water splitting applications.     

Effects of chlorinated Pd precursors and preparation methods on properties and activity of Pd/TiO2 catalysts

We investigated the effects of Pd precursors and preparation methods on the physicochemical properties and performance of Pd/TiO₂ catalysts in the photocatalytic degradation of methyl violet. To confirm the influence of the precursors, Pd/TiO₂ catalysts were prepared via chemical reduction (CR) using four different Pd precursors. Additionally, to determine the effects of preparation methods, Pd/TiO₂ catalysts were fabricated using K₂PdCl₄ precursor via three different methods: CR, deposition–precipitation (DP), and impregnation. The CO chemisorption results showed that the catalyst prepared via DP using the K₂PdCl₄ precursor, i.e., Pd/TiO₂_K_DP, displayed the highest Pd dispersion of 12.42% owing to the stable formation of Pd(OH)₂, which strongly interacted with the –OH groups on the TiO₂ support. Although the catalyst prepared via CR using the Pd(NH₃)₄Cl₂·H₂O (PA) precursor, i.e., Pd/TiO₂_PA_CR, had the lowest Pd dispersion of 0.7%, it exhibited the highest absorption of 26% after 30 min in the dark. It was found that high Pd²⁺/Pd⁰ ratio in dark conditions adversely affected the absorption of MV owing to electrostatic repulsion between the cationic dyes and metal nanoparticles. However, the Pd dispersion and the specific surface area played a key role in the photocatalytic activity under UV irradiation. Pd/TiO₂_K_CR with higher Pd dispersion showed the highest photocatalytic activity and reaction rate of 0.0212 min⁻¹.

We investigated the effects of Pd precursors and preparation methods on the physicochemical properties and performance of Pd/TiO₂ catalysts in the photocatalytic degradation of methyl violet.     

In-plasma-catalysis for NOx degradation by Ti3+ self-doped TiO2−x/γ-Al2O3 catalyst and nonthermal plasma

In an attempt to realize the efficient treatment of NO_x, a mixed catalyst of Ti³⁺ self-doped TiO_2−x and γ-Al₂O₃ was constructed by reducing commercial TiO₂. The degradation effect on NO_x was evaluated by introducing the mixed catalyst into a coaxial dual-dielectric barrier reactor. It was found that the synthesized TiO_2−x could achieve considerable degradation effects (84.84%, SIE = 401.27 J L⁻¹) in a plasma catalytic system under oxygen-rich conditions, which were better than those of TiO₂ (73.99%) or a single plasma degradation process (26.00%). The presence of Ti³⁺ and oxygen vacancies in TiO_2−x resulted in a relatively narrow band gap, which contributed to catalyzing deeply the oxidation of NO_x to NO₂⁻ and NO₃⁻ during the plasma-induced “pseudo-photocatalysis” process. Meanwhile, the TiO_2−x showed an improved discharge current and promoted discharge efficiency, explaining its significant activation effect in the reaction. Reduced TiO_2−x could achieve an impressive degradation effect in a long-time plasma-catalysis process, and still maintained its intrinsic crystal structure and morphology. This work provides a facile synthesis procedure for preparing Ti³⁺ self-doped TiO_2−x with practical and scalable production potential; moreover, the novel combination with plasma also provides new insights into the low-temperature degradation of NO_x.

TiO_2−x has a smaller forbidden band width, more abundant Ti³⁺ and oxygen vacancies, so as to obtain a better and more stable degradation effect of NO_x in plasma-catalysis process.     

Enhancing the quantum yield of singlet oxygen: photocatalytic degradation of mustard gas simulant 2-chloroethyl ethyl sulfide catalyzed by a hybrid of polyhydroxyl aluminum cations and porphyrin anions

By combining the anionic salt meso-tetra(4-carboxyphenyl)porphyrin (TCPP⁴⁻) and the Keggin polyoxometalate cation cluster [Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺via a simple ion-exchange method, a hybrid (C₄₈H₂₆N₄O₈)[Al₁₃O₄(OH)₂₄(H₂O)₁₂]₂(OH)₁₀·18H₂O (Al₁₃–TCPP) was prepared and thoroughly characterized as a prototype of polyoxometalate–porphyrin hybrids for the photocatalytic degradation of the mustard gas simulant 2-chloroethyl ethyl sulfide (CEES). The experimental results showed that the catalytic degradation rate of CEES in the presence of Al₁₃–TCPP reached 96.16 and 99.01% in 180 and 90 min in methanol and methanol–water solvent mixture (v/v = 1 : 1), respectively. The reaction followed first-order reaction kinetics, and the half-life and kinetic constant in methanol and solvent mixture were 39.8 min, −0.017 min⁻¹ and 14.7 min, −0.047 min⁻¹. Mechanism analysis indicated that under visible light irradiation in air, CEES was degraded through a combination of oxidation and alcoholysis/hydrolysis in methanol and the methanol–water solvent mixture. The superoxide radical (O₂˙⁻) and singlet molecular oxygen (¹O₂) generated by Al₁₃–TCPP selectively oxidized CEES into a non-toxic sulfoxide. The singlet oxygen capture experiments showed that Al₁₃–TCPP (Φ = 0.236) had a higher quantum yield of singlet oxygen generation than H₄TCPP (Φ = 0.135) under visible light irradiation in air. The material Al₁₃–TCPP has good reusability, and the degradation rate of CEES can still reach 98.37% after being recycled five times.

A hybrid Al₁₃–TCPP was thoroughly characterized as a prototype of polyoxometalate–porphyrin hybrids for the photocatalytic degradation of the mustard gas simulant 2-chloroethyl ethyl sulfide (CEES).     

Composite photocatalysts based on Cd1−xZnxS and TiO2 for hydrogen production under visible light: effect of platinum co-catalyst location

Ternary composite photocatalysts based on titania and solid solutions of CdS and ZnS were prepared and studied by a set of physicochemical methods including XRD, XPS, HRTEM, UV-vis spectroscopy, and electrochemical tests. Two synthetic techniques of platinization of Cd_1−xZn_xS/TiO₂ were compared. In the first case, platinum was deposited on the surface of synthesized Cd_1−xZn_xS (x = 0.2–0.3)/TiO₂ P25; in the second one, Cd_1−xZn_xS (x = 0.2–0.3) was deposited on the surface of Pt/TiO₂ P25. The photocatalytic properties of the obtained samples were compared in the hydrogen evolution from TEOA aqueous solution under visible light (λ = 425 nm). The Cd_1−xZn_xS (10–50 wt%; x = 0.2–0.3)/Pt (1 wt%)/TiO₂ photocatalysts demonstrated much higher photocatalytic activity than the Pt (1 wt%)/Cd_1−xZn_xS (10–50 wt%; x = 0.2–0.3)/TiO₂ ones. It turned out that the arrangement of platinum nanoparticles precisely on the titanium dioxide surface in a composite photocatalyst makes it possible to achieve efficient charge separation according to the type II heterojunctions and, accordingly, a high rate of hydrogen formation. The highest photocatalytic activity was demonstrated by 20% Cd_0.8Zn_0.2S/1% Pt/TiO₂ in the amount of 26 mmol g⁻¹ h⁻¹ (apparent quantum efficiency was 7.7%) that exceeds recently published values for this class of photocatalysts.

The determination of the preferred location of platinum particles in TiO₂–Cd_1−xZn_xS systems was carried out for the first time.