Effect of synergy between oxygen vacancies and graphene oxide on performance of TiO2 for photocatalytic NO removal under visible light

Spatial interfacial heterojunctions of TiO2 for photocatalytic degradation of toluene: Effects of interface amorphous region and oxygen vacancy

The catalytic activity of TiO2 is contingent upon its crystal structure and the optoelectronic properties associated with defects. In this study, a one-step method was used to synthesize TiO2 with a spatial interface of rutile/anatase phases, and a simple thermal annealing process was applied to optimize the amorphous regions and oxygen vacancies at the interface between the rutile and anatase phases of TiO2. High-resolution transmission electron microscopy (HRTEM) elucidates the evolution process of the amorphous domain at the interface, skillfully introducing oxygen vacancies at the heterojunction interface by modulating the amorphous domain. The obtained photocatalyst (TiO2-350 °C) after annealing exhibits an optimal interface structure, with its photocatalytic activity and stability in degrading toluene far superior to P25. Photocurrent and photoluminescence (PL) measurements affirm that the existence of interfacial oxygen vacancies heightens the efficiency of electron transfer at the interface, while surface oxygen vacancies significantly enhance the stability and mineralization rate of toluene degradation. The improved photocatalytic properties were attributed to the combined effects of surface/interface oxygen vacancies and spatial interface heterojunctions. The one-step synthesis method developed in this work provides a novel perspective on combining spatially interfaced anatase/rutile phases with surface/interfacial oxygen vacancies.

Construction of a TiO2/BiOCl heterojunction for enhanced solar photocatalytic oxidation of nitric oxide

TiO₂/BiOCl heterojunction photocatalysts with different molar ratios (Ti : Bi) were synthesized by a simple solvothermal method. Various spectroscopic techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), nitrogen adsorption-desorption, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and UV-Vis diffuse reflectance spectroscopy (UV-vis DRS) were used to characterize the prepared photocatalysts. The photocatalytic activity of the catalysts was investigated by removing low concentrations of nitrogen oxides. The characterization results show that the TiO₂/BiOCl composite photocatalyst exhibits superior visible light response performance than pure BiOCl and TiO₂. The optimized TiO₂/BiOCl heterojunction with a Ti : Bi molar ratio of 4 : 1 has the best photocatalytic performance. The removal rate of nitrogen oxides of the composite photocatalyst can reach 75%, which is 2.34 times higher than that of pure BiOCl. The observed photocatalytic degradation activity of nitrogen oxides outperforms current state-of-the-art functional photocatalysts. The TiO₂/BiOCl composite photocatalyst has a larger specific surface area, stronger visible light absorption and higher charge separation efficiency compared to other control samples, which contribute to the enhanced photocatalytic activity. The experimental results indicate that the combination of TiO₂ with BiOCl is a promising technique to design visible light-responsive photocatalysts.

Photocatalytic activity of graphene oxide-TiO2 nanocomposite on dichlorvos and malathion and assessment of toxicity changes due to photodegradation

Heterogeneous photocatalysis was used for the removal of two widely used organophosphorus pesticides, dichlorvos, and malathion from water. Graphene oxide-TiO₂ nanocomposite (GOT) was synthesized and used as a photocatalyst for the removal of these pesticides. Batch studies for optimizing photocatalytic degradation and mineralization of pesticides over 80 min were conducted by varying the pH (2-10), catalyst dose (20 mg/L-200 mg/L), and initial pesticide concentration (0.5 mg/L-20 mg/L), and the irradiation source (125 W UV and visible lamp). Degradation kinetics for the pesticides were evaluated. Ellman assay was used to estimate the toxic effect of pesticides and evaluate toxicity reduction due to treatment. The highest degradation and mineralization of dichlorvos and malathion was observed at pH 6 and the optimum catalyst dose was 60 mg/L. Under UV irradiation, 80% and 90% degradation were observed for dichlorvos and malathion, respectively for 0.5 mg/L initial pesticide concentration. The photocatalytic degradation reaction followed Langmuir-Hinshelwood kinetics. A high degree of mineralization was achieved for both the pesticides. Analysis of the results revealed that the residual toxic effect after photocatalysis was primarily due to the residual parent compound. A comparative study revealed that GOT yielded better pesticide degradation compared to commercially available TiO₂ under both UV and visible irradiation.

TiO2/activated carbon synthesized by microwave-assisted heating for tetracycline photodegradation

A furfural residue-derived activated carbon (AC) supported black-TiO₂ photocatalyst was successfully prepared by ultrasonic-assisted sol-gel treatment (USG) and solvothermal treatment (ST) combined with microwave-assisted heating (MH). The prepared composites were characterized and evaluated based on the degradation of tetracycline hydrochloride (TC) under ultraviolet (UV) illumination. The average TiO₂ nanoparticle size of the as-synthesized catalysts was between 9 and 11 nm. The bandgap of TiO₂-USGM was 1.6 eV, much lower than that of other reference catalysts. Organic carbon and AC in the catalyst play positive roles in reducing the band gap (e.g. 1.6∼2.6 eV) and improving visible-light absorption. The oxygen vacancies are responsible for UV-visible absorption. Adding AC into black TiO₂ resulted in a lower degree of recombination of photogenerated electrons. Mott-Schottky plots showed that AC-containing TiO₂@AC-STM reduced the value of conduction band value from -0.59 eV to -0.24 eV, which is beneficial to photogenerated electrons. Compared with TiO₂, the Ti-O-C and Ti-C- in TiO₂@AC remarkably improved the adsorption and catalytic efficiency of TC. In a near-neutral pH environment, TiO₂@AC-STM and TiO₂@AC-USGM exhibited high removal efficiencies (88.0% and 75.7%, respectively) and degradation rates (0.0418 and 0.0302 μmol/g/s, respectively) at a catalyst load of 0.25 g/L. Notably, the catalyst can be effectively used over a wide range of pH (6-9). The solution pH after treatment was close to neutral, which is advantageous for wastewater treatment. The activation energies were found to be approximately 3.47 kJ/mol. The thermodynamic parameters showed that the photodegradation process was non-spontaneous and endothermic. Based on the trapping experiments, O₂⋅^- was mainly responsible for TC photodegradation over TiO₂@AC-STM, followed by h⁺. The TC degradation pathways and catalyst stability were also investigated. Biomass-derived carbon-supported catalysts have great potential for waste biomass utilization as green, and low-cost catalysts.

N-doping TiO2 hollow microspheres with abundant oxygen vacancies for highly photocatalytic nitrogen fixation

Photocatalytic fixation of nitrogen to ammonia (NH₃) is a green but low-efficiency technology due to the high recombination of photo-generated carriers and poor light absorption of photocatalysts. Generally, the adsorption capacity for N₂ and the band position of TiO₂ are responsible for bandgap, light-adsorption, and the separation of photocarriers. Therefore, they play crucial roles to improve catalytic activity. Herein, N-doping TiO₂ hollow microspheres (NTO-0.5) with oxygen vacancies were synthesized via a hydrothermal method using phenolic resin microsphere as a template. The obtained NTO-0.5 achieves an impressive ammonia yield of 80.09 μmol g_cat^-1h^-1. Oxygen vacancies of NTO-0.5 were confirmed by ESR, Raman, XPS, Zeta potential, and H₂O₂ treatment for reducing oxygen vacancies. The ammonia yield of NTO-0.5 decreases to 34.78 μmol g_cat^-1h^-1 after reducing oxygen vacancies by H₂O₂ treatment, which demonstrates the importance of oxygen vacancies. The oxygen vacancies narrow the bandgap from 3.18 eV to 2.83 eV and impede the recombination of photo-generated carriers. The hollow microspheres structure is conducive to light absorption and utilization. Therefore, the synergistic effect between the oxygen vacancies and the hollow microspheres structure boosts the efficiency of photocatalytic nitrogen fixation. After four cycles, the ammonia production yield still maintains at 76.52 μmol g_cat^-1h^-1, meaning high stability. This work provides a new insight into the construction of catalysts with oxygen vacancies to enhance photocatalytic nitrogen fixation performance.