Efficient removal of oxytetracycline antibiotic from aqueous media using UV/g-C3N4/Fe3O4 photocatalytic process

Residual pharmaceuticals in the environment are a class of emerging pollutants that endanger human health. Tetracycline's family, including oxytetracycline (OTC), are known as one of the most produced and consumed antibiotics worldwide. The g-C3N4/Fe3O4 nanocomposite with high level of catalytic efficiency features suitable performance in water/wastewater treatment. Therefore, in the present study, this nanocomposite was applied to remove the oxytetracycline from the aqueous environment. In this research study, g-C3N4/Fe3O4 nanocomposite (serving as catalyst) was initially synthesized by a simple hydrothermal method. The effect of key operating parameters such as initial solution pH, dose of catalyst, contact time and initial concentration of OTC in aqueous solutions was investigated under UV irradiation. In addition, COD and TOC tests, the kinetics and the effect of radical scavengers on the applied photocatalytic process were all evaluated. The maximum removal efficiency of OTC (99.8 %) was achieved under the following conditions: neutral solution pH 7; catalyst dose, 0.7 g/L; and an initial OTC concentration of 5 mg/L. The data showed that the kinetics of the reaction followed the first-order model with R2 of 0.9755. The respective COD and TOC efficiency values for the applied photocatalytic process were determined to be 87 and 59 %, respectively. In addition, the lowest removal efficiency of OTC was observed in the presence of tert-butanol radical scavengers, and OH radicals played a main role. The UV/g-C3N4/Fe3O4 photocatalytic process proved to be highly efficient for the removal of OTC antibiotic and could be potentially applied for the removal of other pollutants from aqueous solutions.

From Pristine to Heteroatom-Doped Graphene Quantum Dots: An Essential Review and Prospects for Future Research

Graphene quantum dots (GQDs) are carbon-based zero-dimensional materials that have received considerable scientific interest due to their exceptional optical, electrical, and optoelectrical properties. Their unique electronic band structures, influenced by quantum confinement and edge effects, differentiate the physical and optical characteristics of GQDs from other carbon nanostructures. Additionally, GQDs can be synthesized using various top-down and bottom-up approaches, distinguishing them from other carbon nanomaterials. This review discusses recent advancements in GQD research, focusing on their synthesis and functionalization for potential applications. Particularly, various methods for synthesizing functionalized GQDs using different doping routes are comprehensively reviewed. Based on previous reports, current challenges and future directions for GQDs research are discussed in detail herein.

Visible-light-driven Oxygen Vacancy and Carbon Doping of C@TiO2-x Photocatalyst for Enhanced Pollutants Degradation Performance

Oxygen Vacancy (OVs) and carbon doping of the photocatalyst body will significantly enhance the photocatalytic efficiency. However, synchronous regulation of these two aspects is challenging. In this paper, a novel C@TiO2-x photocatalyst was designed by coupling the surface defect and doping engineering of titania, which can effectively remove rhodamine B (RhB) and has a relatively high performance with wide pH range, high photocatalytic activity and good stability. Within 90 minutes, the photocatalytic degradation rate of RhB by C@TiO2-x (94.1 % at 20 mg/L) is 28 times higher than that of pure TiO2 . Free radical trapping experiments and electron spin resonance techniques reveal that superoxide radicals (⋅O2- ) and photogenerated holes (h+ ) play key roles in the photocatalytic degradation of RhB. This study demonstrates the possibility of regulating photocatalysts to degrade pollutants in wastewater based on an integrated strategy.

Progress in the preparation, application, and recycling of black phosphorus

Black phosphorus nanosheets (BPNSs) are a new member of the nanomaterial family, and they have good development potential in electrochemistry, electronics, optoelectronics, environmental protection, biomedical, and other fields because of their bandgap width, high anisotropy, broad optical absorption, high carrier mobility and many other features. Although many articles have been published about the preparation and application of BPNSs, these aspects have not been elucidated, and we aimed to fill this knowledge gap in this review. First, we used VOSviewer software to sort out articles published in the past 5 years and drew a literature map, which allowed us to sort out the relationship between various studies related to BPNSs, and reflect on the research focus in recent years. Because BPNSs must be made from black phosphorus (BP), and BPNSs are a nano form of BP, the collation of the BP preparation scheme was also helpful for the related research on BPNSs. This paper introduces the preparation of bulk BP and BPNSs, analyzes and compares the advantages and disadvantages of each method, and points out the most promising methods in the future. Then, we propose improvement directions for this method. We also introduce the characterization of BPNSs and combine it with the subsequent photocatalytic application of BPNSs. As a new material, the effect of BPNSs on the environment is still unknown; thus, an end treatment scheme for BPNSs is summarized according to existing methods. Based on the experience of nanomaterial treatment, this paper proposes a research focus for the end treatment of BPNSs in the future, providing a reference scheme for the end treatment of other nanomaterials. Finally, we summarize the full text and propose recommended methods and improvement plans.

Rational design of S-scheme AgI/ZrTiO4-x heterojunctions for remarkably boosted norfloxacin degradation

Emerging S-scheme heterojunction photocatalysts endowed with efficient charge separation and strong redox capacity have stimulated wide interests in dealing with environmental issues nowadays. In this work, we firstly fabricated the oxygen vacancy modified ZrTiO_4-x nanocrystals, which was further combined with AgI to build the defective S-scheme AgI/ZrTiO_4-x heterojunctions for visible-light photocatalytic norfloxacin degradation. The synthesized ZrTiO_4-x nanocrystals and AgI/ZrTiO_4-x heterojunctions displayed remarkably boosted norfloxacin degradation performance under visible-light irradiation. The reaction rate constant of the optimized AgI/ZrTiO_4-x-5% heterojunction is as high as 0.01419 min^-1, which is approximately 43.35 times that of AgI and 7.93 times that of ZrTiO_4-x nanocrystals, and far superior to those of commercial TiO₂ and commercial ZrO₂. The high-performance photocatalytic norfloxacin degradation could be mainly attributed to the formation of S-scheme charge transfer pathways and oxygen vacancy defects. More significantly, AgI/ZrTiO_4-x could also realize the effective photo-decomposition of other emerging pollutants. Finally, the visible-light photocatalytic performance and photocatalysis mechanism were investigated.

Nitrogen and Sulfur Co-Doped Graphene Quantum Dots Anchored TiO2 Nanocomposites for Enhanced Photocatalytic Activity

Herein, nitrogen (N) and sulfur (S) co-doped graphene quantum dots (GQDs) using different one-dimensional (1-D) carbon nanomaterials as precursors were synthesized, followed by heterojunction formation with TiO2. GQDs exhibit unlike physiochemical properties due to the disproportionate ratio of N and S heteroatoms and dissimilar reaction parameters. Tailored type-II band gap (Eg) alignment was formed with narrowed Eg value that improves photogenerated electron transfer due to π-conjugation. GQDs-TiO2 nanocomposites exhibit remarkably high methylene blue (MB) degradation up to 99.78% with 2.3–3 times elevated rate constants as compared with TiO2. CNF-GQDs-TiO2 demonstrates the fastest MB degradation (60 min) due to the synergistic effect of nitrogen and sulfur doping, and is considered the most stable photocatalyst among prepared nanocomposites as tested up to three cyclic runs. Whereas, C–O–Ti bonds were not only responsible for nanocomposites strengthening but also provide a charge transfer pathway. Moreover, charge transport behavior, generation of active species, and reaction mechanism were scrutinized via free-radical scavenger analysis.

Facile Fabrication of Durable Biochar/H2-TiO2 for Highly Efficient Solar-Driven Degradation of Enrofloxacin: Properties, Degradation Pathways, and Mechanism

Widespread application of TiO₂ for degradation of antibiotics is restricted by mainly the low photodegradation efficiency under solar irradiation. To expand the application of TiO₂, the key factors that should be improved are visible-light response, yield of electrons and holes, and durability. Herein, we report a visible-light responsive and durable sugarcane-bagasse-derived biochar supported hydrogenated TiO₂ (HSCB/H₂-TiO₂) photocatalyst with higher electron production fabricated by a facile one-pot hydrogenation. Mild hydrogenation temperature preserved the lotus-stem-like structure of sugarcane bagasse and gave the photocatalyst great separability. The superior durability of HSCB/H₂-TiO₂ was demonstrated by 12 rounds of repeated degradation of methylene blue (MB). In addition, the electron paramagnetic resonance (EPR) results demonstrated that the biochar skeleton contains abundant persistent free radicals (PFRs), which can provide excess electrons to form more ^•O₂ ^-. Meanwhile, radical quenching experiment and EPR radical trapping results also revealed that ^•O₂ ^- was the most dominant species for enrofloxacin (ENR) degradation. Thus, the as-fabricated photocatalyst shows excellent solar-driven degradation of ENR, and 95.6% of ENR was degraded in 180 min under simulated solar irradiation. Possible ENR degradation pathways and mechanism are also proposed based on the identified intermediates.

Graphene quantum dot and iron co-doped TiO2 photocatalysts: Synthesis, performance evaluation and phytotoxicity studies

The present study reports the synthesis, photocatalytic decolorization of reactive black 5 dye and phytotoxicity of graphene quantum dots (GQDs) and iron co-doped TiO₂ photocatalysts via modified sol gel method. GQDs were synthesized by direct pyrolysis of citric acid (CA). Scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS), Raman spectroscopy, atomic force microscopy (AFM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance spectroscopy (DRS), Brunauer-Emmett-Teller (BET) and photoluminescence spectroscopy (PL) were used to determine the physicochemical properties of the best performing photocatalysts. The results indicated improved physicochemical properties of GQD-0.1Fe-TiO₂-300 with root mean square roughness (Rz) (33.82 nm), higher surface area (170.79 m² g^-1), pore volume (0.08 cm³ g^-1), and bandgap (2.94 eV). Moreover, GQD-0.1Fe co-doping of TiO₂ greatly improved the photocatalytic decolorization efficiency for RB5 dye. The photocatalytic reaction followed the pseudo first order reaction with gradual decrease in K_app values for increment in RB5 concentration. The K_C value was obtained as 2.45 mg L^-1 min^-1 while the K_LH value was 0.45 L mg^-1 indicating the heterogeneous reaction system followed the Langmuir-Hinshelwood isotherm and simultaneously occurring adsorption and photocatalytic processes. Photocatalytic reaction mechanism studies exhibited the holes and OH radicals as the main active species in the GQD-0.1Fe-TiO₂-300 responsible for the decolorization of RB5. The proposed reaction pathway showed that both Fe-TiO₂ and GQDs play important role in generation of electrons and holes. Additionally, GQD-0.1Fe-TiO₂-300 were durable up to four cycles. Phytotoxicity assay displayed that treated water and best performing photocatalysts had no effect on Lycopersicon esculentum seed germination. Therefore, the proposed system can pave a viable solution for safe usage of dye loaded wastewater and effluent for irrigation after treatment.

Carbon quantum dot-sensitized hollow TiO2 spheres for high-performance visible light photocatalysis

TiO₂/CQD composites were synthesized through carbon quantum dots covalently attached to the surface of hollow TiO₂ spheres for visible light photocatalytic degradation of organics.

Preparation of carbon-coated brookite@anatase TiO2 heterophase junction nanocables with enhanced photocatalytic performance

One-dimensional TiO2@C nanocables with a heterophase junction have been successfully prepared by coating brookite@anatase TiO2 with a thin layer of hydrothermal carbon (HTC). Compared with anatase TiO2, the biphase brookite@anatase structure can reduce the recombination rate of the excited electron/hole pairs of TiO2. The HTC coating not only enhances the adsorption capability of the TiO2 catalyst for organic pollutants but also facilitates photogenerated electron transfer to further increase its photocatalytic activity. Therefore, compared with anatase TiO2, brookite@anatase TiO2, and TiO2@C, the brookite@anatase TiO2@C shows the highest photocatalytic activity for the photodegradation of tetracycline (TC) under the irradiation of UV-visible light. Moreover, ˙O2 has been proved to be the predominant active species for the photodegradation of TC, and the photocatalytic mechanism of brookite@anatase TiO2@C nanocables has also been proposed.

Spatial Heterojunction in Nanostructured TiO2 and Its Cascade Effect for Efficient Photocatalysis

A highly efficient photoenergy conversion is strongly dependent on the cumulative cascade efficiency of the photogenerated carriers. Spatial heterojunctions are critical to directed charge transfer and, thus, attractive but still a challenge. Here, a spatially ternary titanium-defected TiO₂@carbon quantum dots@reduced graphene oxide (denoted as V_Ti@CQDs@rGO) in one system is shown to demonstrate a cascade effect of charges and significant performances regarding the photocurrent, the apparent quantum yield, and photocatalysis such as H₂ production from water splitting and CO₂ reduction. A key aspect in the construction is the technologically irrational junction of Ti-vacancies and nanocarbons for the spatially inside-out heterojunction. The new "spatial heterojunctions" concept, characteristics, mechanism, and extension are proposed at an atomic-/nanoscale to clarify the generation of rational heterojunctions as well as the cascade electron transfer.

Interfacial co-existence of oxygen and titanium vacancies in nanostructured TiO2 for enhancement of carrier transport

The interfacial co-existence of oxygen and metal vacancies in metal oxide semiconductors and their highly efficient carrier transport have rarely been reported. This work reports on the co-existence of oxygen and titanium vacancies at the interface between TiO₂ and rGO via a simple two-step calcination treatment. Experimental measurements show that the oxygen and titanium vacancies are formed under 550 °C/Ar and 350 °C/air calcination conditions, respectively. These oxygen and titanium vacancies significantly enhance the transport of interfacial carriers, and thus greatly improve the photocurrent performances, the apparent quantum yield, and photocatalysis such as photocatalytic H₂ production from water-splitting, photocatalytic CO₂ reduction and photo-electrochemical anticorrosion of metals. A new "interfacial co-existence of oxygen and titanium vacancies" phenomenon, and its characteristics and mechanism are proposed at the atomic-/nanoscale to clarify the generation of oxygen and titanium vacancies as well as the interfacial carrier transport.

Quenching-Induced Structural Distortion of Graphitic Carbon Nitride Nanostructures: Enhanced Photocatalytic Activity and Electrochemical Hydrogen Production

Engineered nanomaterials are emerging in the field of environmental chemistry. This study involves the analysis of the structural, electronic, crystallinity, and morphological changes in graphitic carbon nitride (g-C₃N₄), an engineered nanomaterial, under rapid cooling conditions. X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, Brunauer-Emmett-Teller, Fourier transform infrared, Raman, band gap, and Mott-Schottky analyses strongly proved that the liquid N₂-quenched sample of g-C₃N₄ has structural distortion. The photocatalytic efficiency of engineered g-C₃N₄ nanostructures was analyzed through the degradation of reactive red 120 (RR120), methylene blue (MB), rhodamine B, and bromophenol as a representative dye. The photocatalytic dye degradation efficiency was analyzed by UV-vis spectroscopy and total organic carbon (TOC) analysis. The photocatalytic efficiency of g-C₃N₄ under different quenching conditions included quenching at room temperature in ice and liquid N₂. The degradation efficiencies are found to be 4.2, 14.7, and 82.33% for room-temperature, ice, and liquid N₂ conditions, respectively. The pseudo-first-order reaction rate of N₂-quenched g-C₃N₄ is 9 times greater than the ice-quenched g-C₃N₄. Further, the TOC analysis showed that 55% (MB) and 59% (RR120) of photocatalytic mineralization were achieved within a time duration of 120 min by the liquid N₂-quenched g-C₃N₄ nanostructure. In addition, the quenched g-C₃N₄ electrocatalytic behavior was examined via the hydrogen (H₂) evolution reaction in acidic medium. The liquid N₂-quenched g-C₃N₄ catalyst showed a lower overpotential with high H₂ evolution when compared with the other two g-C₃N₄-quenched samples. The results obtained provide an insight and extend the scope for the application of engineered g-C₃N₄ nanostructures in the degradation of organic pollutants as well as for H₂ evolution.

Defective Anatase TiO2-x Mesocrystal Growth In Situ on g-C3 N4 Nanosheets: Construction of 3D/2D Z-Scheme Heterostructures for Highly Efficient Visible-Light Photocatalysis

Environmental remediation by employing visible-light-active semiconductor heterostructures provides effective solutions for handling emerging contaminants by a much greener and lower cost approach compared with other methods. This report demonstrates that the in situ growth of nanosized single-crystal-like defective anatase TiO_2-x mesocrystals (DTMCs) on g-C₃ N₄ nanosheets (NSs) can produce a 3D/2D DTMC/g-C₃ N₄ NS heterostructure with the two components held together by chemical bonds to form tight interfaces. This nanostructured heterostructure displayed remarkably improved photocatalytic activity toward the removal of the model pollutants Methyl Orange (MO) and Cr^VI under visible-light irradiation in comparison with the pristine DTMC and g-C₃ N₄ NS components, which suggests that both the oxidation and reduction abilities of the DTMC/g-C₃ N₄ NSs were simultaneously enhanced after fabrication. On the basis of the results of a systematic characterization, a reasonable mechanism for the photocatalytic activity based on a direct Z-scheme heterojunction is proposed and further verified by the measurement of ^. OH. This novel Z-scheme heterojunction endows the heterostructure with improved photogenerated electron/hole pair separation and a strong redox ability for the efficient degradation of wastewater pollutants. This work will be useful for the design and fabrication of direct Z-scheme heterostructured photocatalysts with novel architectures for applications in energy conversion and environmental remediation.