Environmental applications of titania-graphene photocatalysts

Unveiling bisphenol A toxicity: human health impacts and sustainable treatment strategies

INTRODUCTION

The widespread presence of bisphenol-A (BPA) in consumer goods like water bottles and eyeglass frames raises serious concerns about the chemical's ability to accumulate in human tissues. Molecular filtration and activated carbon adsorption are two of the many BPA treatment technologies that have emerged in response to these issues; both are essential in the removal or degradation of BPA from water sources and industrial effluents.

CONTENT

To secure the long-term health and environmental advantages of BPA treatment approaches, sustainable development is essential. Both the efficient elimination or destruction of BPA and the reduction of the treatment operations' impact on the environment are important components of a sustainable approach. Different search engines like Pub-Med, MEDLINE, Google Scholar and Scopus are used for these systematic reviews and analyzed accordingly. This can be accomplished by making treatment facilities more energy efficient and using environmentally friendly materials. Greener ways to deal with BPA pollution are on the horizon, thanks to innovative techniques like bioremediation and improved oxidation processes. Reducing dependence on conventional, resource-intensive procedures can be achieved by investigating the use of bio-based materials and natural adsorbents in treatment processes.

SUMMARY AND OUTLOOK

This review article tackling the health and environmental concerns raised by BPA calls for an integrated strategy that incorporates sustainable development principles and technology progress. We can reduce the negative impacts of BPA contamination, improve environmental stewardship in the long run, and ensure human health by combining cutting-edge treatment technologies with sustainable behaviours.

TiO2/graphene composite nanofibers for efficient photocatalytic degradation of pharmaceutical compounds: Rifampin, Phenazopyridine, Azathioprine

Pharmaceutical wastewater treatment is an essential component of environmental protection and sustainable development. In this study, our aim was to investigate the morphology, characterization, and effectiveness of TiO2/graphene composite nanofiber photocatalysts in the treatment of pharmaceutical wastewater containing three different pharmaceutical groups, such as an antibiotic (rifampin), painkiller (phenazopyridine), and immunosuppressant (azathioprine). Various parameters such as pH, salt concentration, and initial pharmaceutical compound concentration were optimized to achieve maximum degradation kinetics and efficiency. The optimum conditions were determined to be 1.5% graphene content, 30 ppm initial concentration of pharmaceutical compound, pH=5, and a 0.5 g/L photocatalyst dose. The presence of salt slightly decreased the degradation kinetics, but it did not significantly affect the performance of the TiO2/graphene composite nanofibers photocatalyst. At optimum condition, TiO2/1.5% graphene composite nanofibers degraded 50% of phenazopyridine, 86.89% of rifampin, and completely azathioprine. Comparing with phenazopyridine, N heteroatom-rich molecule of azathioprine and hydroxyl-rich molecule of rifampin lead to being susceptible to photocatalytic degradation. The reuse of the photocatalyst in multiple cycles showed consistent performance, indicating its potential for practical and economic applications.

Oxygen-Deficient Bioceramics: Combination of Diagnosis, Therapy, and Regeneration

The journey of ceramics in medicine has been synchronized with an evolution from the first generation-alumina, zirconia, etc.-to the third -3D scaffolds. There is an up-and-coming member called oxygen-deficient or colored bioceramics, which have recently found their way through biomedical applications. The oxygen vacancy steers the light absorption toward visible and near infrared regions, making the colored bioceramics multifunctional-therapeutic, diagnostic, and regenerative. Oxygen-deficient bioceramics are capable of turning light into heat and reactive oxygen species for photothermal and photodynamic therapies, respectively, and concomitantly yield infrared and photoacoustic images. Different types of oxygen-deficient bioceramics have been recently developed through various synthesis routes. Some of them like TiO2- x , MoO3- x , and WOx have been more investigated for biomedical applications, whereas the rest have yet to be scrutinized. The most prominent advantage of these bioceramics over the other biomaterials is their multifunctionality endowed with a change in the microstructure. There are some challenges ahead of this category discussed at the end of the present review. By shedding light on this recently born bioceramics subcategory, it is believed that the field will undergo a big step further as these platforms are naturally multifunctional.

Carbon-Based Nanocatalysts (CnCs) for Biomass Valorization and Hazardous Organics Remediation

The continuous increase of the demand in merchandise and fuels augments the need of modern approaches for the mass-production of renewable chemicals derived from abundant feedstocks, like biomass, as well as for the water and soil remediation pollution resulting from the anthropogenic discharge of organic compounds. Towards these directions and within the concept of circular (bio)economy, the development of efficient and sustainable catalytic processes is of paramount importance. Within this context, the design of novel catalysts play a key role, with carbon-based nanocatalysts (CnCs) representing one of the most promising class of materials. In this review, a wide range of CnCs utilized for biomass valorization towards valuable chemicals production, and for environmental remediation applications are summarized and discussed. Emphasis is given in particular on the catalytic production of 5-hydroxymethylfurfural (5-HMF) from cellulose or starch-rich food waste, the hydrogenolysis of lignin towards high bio-oil yields enriched predominately in alkyl and oxygenated phenolic monomers, the photocatalytic, sonocatalytic or sonophotocatalytic selective partial oxidation of 5-HMF to 2,5-diformylfuran (DFF) and the decomposition of organic pollutants in aqueous matrixes. The carbonaceous materials were utilized as stand-alone catalysts or as supports of (nano)metals are various types of activated micro/mesoporous carbons, graphene/graphite and the chemically modified counterparts like graphite oxide and reduced graphite oxide, carbon nanotubes, carbon quantum dots, graphitic carbon nitride, and fullerenes.

Enhanced Photocatalysis of Black TiO2/Graphene Composites Synthesized by a Facile Sol-Gel Method Combined with Hydrogenation Process

In this study, in situ TiO2 was grown on the surface of graphene by a facile sol-gel method to form black TiO2/graphene composites with highly improved photocatalytic activity. The combination of graphene and TiO2 was beneficial to eliminate the recombination of photogenerated electron holes. The self-doping Ti3+ was introduced, accompanied by the crystallization of amorphous TiO2, during the hydrogenation process. Consequently, the narrowed bandgap caused by self-doping Ti3+ enhanced the visible light absorption and thus made the composites appear black. Both of them improved the photocatalytic performance of the synthesized black TiO2/graphene composites. The band structure of the composite was analyzed by valence band XPS, revealing the reason for the high visible light catalytic performance of the composite. The results proved that the black TiO2/graphene composites synthesized show attractive potential for applications in environmental and energy issues.

Effective Carbon/TiO2 Gel for Enhanced Adsorption and Demonstrable Visible Light Driven Photocatalytic Performance

A new strategy to synthesise carbon/TiO2 gel by a sol–gel method is proposed. Textural, morphological, and chemical properties were characterised in detail and the synthesised material was proven to be an active adsorbent, as well as a visible light photocatalyst. Homogenously distributed TiO2 is mesoporous with high surface area and, hence, exhibited a high adsorption capacity. The adsorption equilibrium experimental data were well explained by the Sips isotherm model. Kinetic experiments demonstrated that experimental data fitted a pseudo second order model. The modification in electronic structure of TiO2 resulted in a reduced bandgap compared to commercial P25. The absorption edge studied through UV-Vis shifted to the visible region, hence, daylight photocatalytic activity was efficient against degradation of MB dye, as an example pollutant molecule. The material was easily removed post treatment, demonstrating potential for employment in industrial water treatment processes.

Synthesis and characterization of ternary chitosan-TiO2-ZnO over graphene for photocatalytic degradation of tetracycline from pharmaceutical wastewater

Various nanocomposites of TiO₂-ZnO, TiO₂-ZnO/CS, and TiO₂-ZnO/CS-Gr with different molar ratios were synthesized by sol-gel and ultrasound-assisted methods and utilized under UV irradiation to enhance the photocatalytic degradation of tetracycline. Characterization of prepared materials were carried out by XRD, FT-IR, FE-SEM, EDX and BET techniques. The TiO₂-ZnO with the 1:1 molar ratio supported with 1:2 weight ratio CS-Gr (T1‒Z1/CS1‒Gr2 sample) appeared as the most effective material at the optimized operational conditions including the tetracycline concentration of 20 mg/L, pH = 4, catalyst dosage of 0.5 g/L, and 3 h of irradiation time. As expected, the graphene had a significant effect in improving degradation results. The detailed performances of the T1‒Z1/CS1‒Gr2 were compared with ternary nanocomposites from EDX and BET results as well as from the degradation viewpoint. This novel photocatalyst can be effective in actual pharmaceutical wastewater treatment considering the applied operational parameters.

In-situ responses of phytoplankton to graphene photocatalysis in the eutrophic lake Xingyun, southwestern China

Graphene photocatalysis is receiving increased attention for its potential to be used as a novel green technology for mitigating harmful algae in highly eutrophic waters. However, graphene is seldom applied to in situ aquatic ecosystems for environmental applications. Here, the impacts of graphene photocatalysis on phytoplankton and environmental conditions were evaluated through an in situ macrocosm experiment in the eutrophic Lake Xingyun, southwestern China. The graphene photocatalysis treated area had significantly reduced conductivity, total nitrogen (TN), total phosphorus (TP) and dissolved phosphorus concentrations, as well as increased dissolved oxygen (DO) concentrations. The abundances of all species of the genus Microcystis were significantly reduced in the graphene photocatalysis-treated area; in contrast, the abundances of all species of the diazotrophic genera, including Anabaena and Aphanizomenon, greatly increased after treatment with graphene photocatalysis. Eukaryotic algae, especially Chlorophyta, Euglenophyta and Pyrrophyta, as well as Cryptophyta, had significantly higher abundances in the graphene photocatalysis-treated area, whereas most of the eutrophic diatom species had lower abundances in the treated area. These observed differences in eukaryotic algae between the two groups might be related to their sensitivity to graphene photocatalysis and their tolerance of nutrients. Generally, graphene photocatalysis can make a great contribution to the improvement of eutrophic water, as evidenced by the reduction in cyanobacteria abundance and phosphorus concentration, as well as the increase in species richness and the dissolved oxygen concentration in the treated area. However, the mechanisms underlying these differences in phytoplankton community structure and environmental conditions require further study.

Prospects of Synthesized Magnetic TiO2-Based Membranes for Wastewater Treatment: A Review

Global accessibility to clean water has stressed the need to develop advanced technologies for the removal of toxic organic and inorganic pollutants and pathogens from wastewater to meet stringent discharge water quality limits. Conventionally, the high separation efficiencies, relative low costs, small footprint, and ease of operation associated with integrated photocatalytic-membrane (IPM) technologies are gaining an all-inclusive attention. Conversely, photocatalysis and membrane technologies face some degree of setbacks, which limit their worldwide application in wastewater settings for the treatment of emerging contaminants. Therefore, this review elucidated titanium dioxide (TiO2), based on its unique properties (low cost, non-toxicity, biocompatibility, and high chemical stability), to have great potential in engineering photocatalytic-based membranes for reclamation of wastewater for re-use. The environmental pathway of TiO2 nanoparticles, membranes and configuration types, modification process, characteristics, and applications of IPMs in water settings are discussed. Future research and prospects of magnetized TiO2-based membrane technology is highlighted as a viable water purification technology to mitigate fouling in the membrane process and photocatalyst recoverability. In addition, exploring life cycle assessment research would also aid in utilizing the concept and pressing for large-scale application of this technology.

Graphene-TiO2 hybrids for photocatalytic aided removal of VOCs and nitrogen oxides from outdoor environment

Outdoor and indoor air pollution has become a global concern in modern society. Although many policies and regulations on air quality have been promulgated worldwide over the past decades, airborne pollution still negatively affects health and therefore the life-style of human beings. One of the strategies to challenge this problem might be reducing the amount of airborne pollutant by mineralising them via photoinduced reactions. Photocatalytic oxidation of gaseous pollutants via titanium dioxide is one of the most promising solar photochemical reactions. In this research work, by means of a green sol-gel procedure, we have coupled titania to graphene (0.5 and 1.0 wt%) aiming to increase the solar photocatalytic activity of the produced hybrid materials. Transient paramagnetic species formed upon UV-A irradiation were detected by means of EPR spectroscopy. The photocatalytic reactions were assessed by monitoring the removal of nitrogen oxides and two different volatile organic compounds (benzene and isopropanol), which has never been assessed before. Our results highlight the exceptional characteristics of the TiO₂/graphene hybrid material synthesised with 1.0 wt% graphene, and its excellent suitability for multi-purpose applications in the field of environmental remediation. Compared to unmodified titania, it shows a clear enhancement in the photocatalytic removal of those hazardous pollutants, having a photocatalytic degradation rate twice higher. In addition, the same material is highly stable and shows fully recyclability over repeated tests. Hybrid titania-graphene materials could thus be exploited to grant safer outdoor and indoor environments, having thus a beneficial impact on public health and on the quality of our lives.

Application of graphene-based materials for removal of tetracyclines using adsorption and photocatalytic-degradation: A review

Tetracyclines are extensively used to treat human and animal infectious diseases due to its effective antimicrobial activities. About 70-90% of its parent materials are released into the environment through urine and feces, implying they are the most frequently detected antibiotics in the environment with high ecological risks. Adsorption and photocatalysis have been promising techniques for the removal of tetracyclines due to effectiveness and efficiency. Graphene-based materials provide promising platforms for adsorptive and photocatalytic removal of tetracyclines from aqueous environment owning to distinctive remarkable physicochemical, optical, and electrical characteristics. Herein, we intensively reviewed the available literatures in order to provide comprehensive insight about the applications and mechanisms of graphene-based materials for removal of tetracyclines via adsorption and phototocatalysis. The synthesis methods of graphene-based materials, the tetracycline adsorption and photocatalytic-degradation conditions, and removal mechanisms have been extensively discussed. Finally concluding remarks and future perspectives have been deduced and recommended to stimulate further researches in the subject. The review study can be used as theoretical guideline for further researchers to improve the current approaches of material synthesis and application towards tetracyclines removal.

The Photocatalytical Properties of RGO/TiO2 Coated Fabrics

The aim of this work was to immobilize reduced graphene oxide (RGO) and titanium dioxide (TiO2) on the surface of selected fibrous structures. Textile fabrics made of cotton (CO) and polyamide (PA) were used as a carrier. The following modification methods were applied: coating for modification of PA and dip-coating for modification of CO. In the dip-coating method, no auxiliaries were used, which is a huge advantage. The RGO/TiO2 coated fabrics were characterized using several techniques: ultraviolet–visible (UV–VIS) spectroscopy, scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The obtained results showed the immobilization of RGO and TiO2 on the fabrics. Raw fabrics absorb much less radiation than coated ones, which is associated with strong absorption of radiation by applied modifiers (RGO and TiO2). Photocatalytic activity of functionalized textiles was determined using aqueous phenol solutions. Phenol removal efficiency obtained for RGO/TiO2 coated CO and RGO/TiO2 coated PA was 51% and 46%, respectively. The hydroxyl radicals play a major role in the phenol photocatalytic degradation. The phenol removal efficiency in the fifth cycle was higher (about 14% and 8% for RGO/TiO2 coated CO and RGO/TiO2 coated PA, respectively) compared to the first cycle.

Solar photocatalytic degradation of pesticides over TiO2-rGO nanocomposites at pilot plant scale

Two TiO₂-rGO nanocomposites were prepared by hydrothermal method from commercial TiO₂ (P25 and Hombikat UV100, HBK). In both cases TiO₂ nanoparticles appeared intimate and homogeneously distributed on rGO surface, but forming a dense network in P25-rGO nanocomposite, and a more open structure in HBK-rGO. Zeta potential and particle size distribution favored the ease of HBK-rGO nanocomposite to form stable suspensions. A comparative analysis of these two photocatalysts was performed on the pilot plant scale solar assisted photodegradation of a 200 μg·L^-1 or 5 mg·L^-1 mixture of persistent and biorecalcitrant pollutants in deionized water (methomyl, pyrimethanil, isoproturon and alachlor, all used as pesticides). Complete removal of pesticides was achieved, though faster with P25-rGO when O₂ was the oxidant. However, the use of hydrogen peroxide (H₂O₂) dosage as oxidant speeded up pesticides removal, but HBK-rGO performance resulted much improved. Finally, at realistic very low concentrations of 200 μg_{each pesticide}·L^-1, the complete removal of pesticides was achieved at very short times (<25 min), showing the efficiency of the synthetized TiO₂-rGO nanocomposites in this pilot-plat scale solar process to mitigate refractory and biorecalcitrant contaminants on effluents as a sustainable and efficient process.

Localized Building Titania-Graphene Charge Transfer Interfaces for Enhanced Photocatalytic Performance

Achieving high photocatalytic activity of titania-graphene composites calls for well-controlled titania size and efficient charge transfer interfaces. However, it is rather difficult because of easy restacking of graphene sheets and random nucleation and growth of titania nanoparticles in solution. Here, we reported a facile way to control the TiO₂ sizes and interfaces by localizing the nucleation and growth of titania on graphene sheets, which prohibits both restacking of graphene and random growth of TiO₂. As a result, a composite with controllably less than 10-nm-sized TiO₂ nanoparticles evenly distributed on thin graphene sheets was achieved. Thanks to the small size of titania and efficient charge transfer interfaces, the TiO₂/graphene composite exhibits a significant enhancement of photocatalytic H₂ evolution activity, reaching 1.35 mmol g^-1 h^-1. Furthermore, the composite also shows high photocatalytic activity on dye degradation under visible light illumination.

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.

Advances in the application, toxicity and degradation of carbon nanomaterials in environment: A review

Carbon nanomaterials (CNMs) are novel nanomaterials with excellent physicochemical properties, which are widely used in biomedicine, energy and sensing. Besides, CNMs also play an important role in environmental pollution control, which can absorb heavy metals, antibiotics and harmful gases. However, CNMs are inevitably entering the environment while they are rapidly developing. They are harmful to living organisms in the environment and are difficult to degrade under natural conditions. Here, we systematically describe the toxicity of carbon nanotubes (CNTs), graphene (GRA) and C₆₀ to cells, animals, humans, and microorganisms. According to the current research results, the toxicity mechanism is summarized, including oxidative stress response, mechanical damage and effects on biological enzymes. In addition, according to the latest research progress, we focus on the two major degradation methods of chemical degradation and biodegradation of CNTs, GRA and C₆₀. Meanwhile, the reaction conditions and degradation mechanisms of degradation are respectively stated. Moreover, we have prospects for the limitations of CNM degradation under non-experimental conditions and their potential application.

Photocatalytic Degradation of Pharmaceuticals Carbamazepine, Diclofenac, and Sulfamethoxazole by Semiconductor and Carbon Materials: A Review

The presence of pharmaceutical compounds in the environment is a reality that calls for more efficient water treatment technologies. Photocatalysis is a powerful technology available but the high energy costs associated with the use of UV irradiation hinder its large scale implementation. More sustainable and cheaper photocatalytic processes can be achieved by improving the sunlight harvesting and the synthesis of semiconductor/carbon composites has proved to be a promising strategy. Carbamazepine, diclofenac, and sulfamethoxazole were selected as target pharmaceuticals due to their recalcitrant behavior during conventional wastewater treatment and persistence in the environment, as properly reviewed. The literature data on the photocatalytic removal of carbamazepine, diclofenac, and sulfamethoxazole by semiconductor/carbon materials was critically revised to highlight the role of the carbon in the enhanced semiconductor performance under solar irradiation. Generally it was demonstrated that carbon materials induce red-shift absorption and they contribute to more effective charge separation, thus improving the composite photoactivity. Carbon was added as a dopant (C-doping) or as support or doping materials (i.e., nanoporous carbons, carbon nanotubes (CNTs), graphene, and derived materials, carbon quantum dots (CQDs), and biochars) and in the large majority of the cases, TiO₂ was the semiconductor tested. The specific role of carbon materials is dependent on their properties but even the more amorphous forms, like nanoporous carbons or biochars, allow to prepare composites with improved properties compared to the bare semiconductor. The self-photocatalytic activity of the carbon materials was also reported and should be further explored. The removal and mineralization rates, as well as degradation pathways and toxicity of the treated solutions were also critically analyzed.

Indium Tin-Oxide Wrapped 3D rGO and TiO2 Composites: Development, Characterization, and Enhancing Photocatalytic Activity for Methylene Blue

A hybrid material of indium tin-oxide (ITO) wrapped titanium dioxide and reduced graphene oxide (ITO-rGO and TiO2) was prepared using a facile hydrothermal technique. TiO2 nanorods were in situ grown on the surface of rGO (rGO and TiO2), and which was then assembled onto ITO substrate layer by layer with formation of a 3D structure. ITO-rGO and TiO2 exhibit low charge transfer resistance at the electrode-electrolyte interface and have good photoresponsive ability. Methylene blue (MB) can be effectively adsorbed and enriched onto ITO-rGO and TiO2 surface. The adsorption kinetics and thermodynamics of ITO-rGO and TiO2 were evaluated, showing that the exothermic and entropy-driven reaction were the main thermodynamic processes, and the Langmuir isotherm was the ideal model for adsorption fitting. Meanwhile, ITO greatly improved degradation of rGO and TiO2 because electrons can be collected by ITO before recombination and MB can easily enter into the 3D structure of rGO and TiO2. The highest photodegradation rate of MB reached 93.40% for ITO-rGO and TiO2 at pH 9. Additionally, ITO-rGO and TiO2 successfully solved the problems of being difficult to recycle and causing secondary pollution of traditional TiO2 catalysts. Therefore, ITO-rGO and TiO2 may be a potential photocatalyst for degrading organic pollutants in water.

Utilization of Carbon Nanospheres in Photocatalyst Production: From Composites to Highly Active Hollow Structures

Titanium dioxide–carbon sphere (TiO₂–CS) composites were constructed via using prefabricated carbon spheres as templates. By the removal of template from the TiO₂–CS, TiO₂ hollow structures (HS) were synthesized. The CS templates were prepared by the hydrothermal treatment of ordinary table sugar (sucrose). TiO₂–HSs were obtained by removing CSs with calcination. Our own sensitized TiO₂ was used for coating the CSs. The structure of the CSs, TiO₂–CS composites, and TiO₂–HSs were characterized by scanning electron microscopy (SEM), infrared spectroscopy (IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance spectroscopy (DRS). The effect of various synthesis parameters (purification method of CSs, precursor quantity, and applied furnace) on the morphology was investigated. The photocatalytic activity was investigated by phenol model pollutant degradation under visible light irradiation (λ > 400 nm). It was established that the composite samples possess lower crystallinity and photocatalytic activity compared to TiO₂ hollow structures. Based on XPS measurements, the carbon content on the surface of the TiO₂–HS exerts an adverse effect on the photocatalytic performance. The synthesis parameters were optimized and the TiO₂–HS specimen having the best absolute and surface normalized photocatalytic efficiency was identified. The superior properties were explained in terms of its unique morphology and surface properties. The stability of this TiO₂–HS was investigated via XRD and SEM measurements after three consecutive phenol degradation tests, and it was found to be highly stable as it entirely retained its crystal phase composition, morphology and photocatalytic activity.

Advanced Photocatalysts Based on Reduced Nanographene Oxide-TiO2 Photonic Crystal Films

Surface functionalization of TiO₂ inverse opals by graphene oxide nanocolloids (nanoGO) presents a promising modification for the development of advanced photocatalysts that combine slow photon-assisted light harvesting, surface area, and mass transport of macroporous photonic structures with the enhanced adsorption capability, surface reactivity, and charge separation of GO nanosheets. In this work, post-thermal reduction of nanoGO–TiO₂ inverse opals was investigated in order to explore the role of interfacial electron transfer vs. pollutant adsorption and improve their photocatalytic activity. Photonic band gap-engineered TiO₂ inverse opals were fabricated by the coassembly technique and were functionalized by GO nanosheets and reduced under He at 200 and 500 °C. Comparative performance evaluation of the nanoGO–TiO₂ films on methylene blue photodegradation under UV-VIS and visible light showed that thermal reduction at 200 °C, in synergy with slow photon effects, improved the photocatalytic reaction rate despite the loss of nanoGO and oxygen functional groups, pointing to enhanced charge separation. This was further supported by photoluminescence spectroscopy and salicylic acid UV-VIS photodegradation, where, in the absence of photonic effects, the photocatalytic activity increased, confirming that fine-tuning of interfacial coupling between TiO₂ and reduced nanoGO is a key factor for the development of highly efficient photocatalytic films.

Reduced Graphene Oxide–P25 Nanocomposites as Efficient Photocatalysts for Degradation of Bisphenol A in Water

Reduced graphene oxide–titanium dioxide photocatalyst (rGO–TiO2) was successfully synthesized by the hydrothermal method. The rGO–TiO2 was used as photocatalyst for the degradation of bisphenol A (BPA), which is a typical endocrine disruptor of the environment. Characterization of photocatalysts and photocatalytic experiments under different conditions were performed for studying the structure and properties of photocatalysts. The characterization results showed that part of the anatase type TiO2 was converted into rutile type TiO2 after hydrothermal treatment and 1% rGO–P25 had the largest specific surface area (52.174 m2/g). Photocatalytic experiments indicated that 1% rGO–P25 had the best catalytic effect, and the most suitable concentration was 0.5 g/L. When the solution pH was 5.98, the catalyst was the most active. Under visible light, the three photocatalytic mechanisms were ranked as follows: O2•− > •OH > h+. 1% rGO–P25 also had strong photocatalytic activity in the photocatalytic degradation of BPA under sunlight irradiation. 1% rGO–P25 with 0.5 g/L may be a very promising photocatalyst with a variety of light sources, especially under sunlight for practical applications.

Phenol Abatement by Titanium Dioxide Photocatalysts: Effect of The Graphene Oxide Loading

Hetero-photocatalytic graphene-TiO2 materials have, in the literature, been found to possess better photocatalytic activity for environmental applications compared to pure TiO2. These types of materials can be prepared in different ways; however, their photocatalytic performance and quality are not easily controlled and reproduced. Therefore, we synthetized graphene oxide-TiO2 nanoparticles by sol-gel reaction from TiCl4, as precursor, with two different methods of synthesis and with a graphene oxide (GO) loading ranging from 0 to 1.0. This approach led to a good adhesion of GO to TiO2 through the Ti-O-C bonding, which could enhance the photocatalytic performances of the materials. Overall, 0.05 wt % GO loading gave the highest rate in the photodegradation of phenol under visible light, while higher GO loadings had a negative impact on the photocatalytic performances of the composites. The 0.05 wt % GO-TiO2 composite material was confirmed to be a promising photocatalyst for water pollutant abatement. The designed synthetic approach could easily be implemented in large-scale production of the GO-TiO2 coupling materials.

Graphene oxide/titania photocatalytic ozonation of primidone in a visible LED photoreactor

A graphene oxide-titania (GO/TiO₂) composite was synthesized via sol-gel method, and studied in aqueous Primidone mineralization with ozone and LED visible light. The photocatalyst was characterized by different techniques (XRD, TEM, SBET, TGA, UV-vis diffuse reflectance spectroscopy). The band gap value decrease from 3.14 eV for bare TiO₂ samples to 2.5 eV in GO/TiO₂ composites clearly shows the interaction of GO with TiO₂ structure. Approximately 20 mg L^-1 of Primidone was removed in less than 20 min if ozone was applied, regardless of the presence or absence of light and catalyst. However, reactivity tests show a synergism effect between photocatalysis and ozonation for mineralization purposes. The combination of ozone and GO improved the activation of TiO₂ under visible light. Process optimization led us to select a catalyst dosage of 0.25 g L^-1, a light radiance of 359 W m^-2 and a GO loading in the catalyst around 0.75%. At these conditions, with photocatalytic ozonation, the presence of GO in the catalyst improved mineralization up to 82% in 2 h compared to 70% reached with bare TiO₂. Catalyst reusability shows no decrease of photocatalytic activity. Scavenger tests point to hydroxyl radicals as the main species responsible for Primidone removal.

Towards visible-light photocatalysis for environmental applications: band-gap engineering versus photons absorption-a review

A range of different studies has been performed in order to design and develop photocatalysts that work efficiently under visible (and near-infrared) irradiation as well as to improve photons absorption with improved reactor design. While there is consensus on the importance of photocatalysis for environmental applications and the necessity to utilized solar irradiation (or visible-light) as driving force for these processes, it is not yet clear how to get there. Discussion on the future steps towards visible-light photocatalysis for environmental application is of great interest to scientific and industrial communities and the present paper reviews and discusses the two main approaches, band-gap engineering for efficient solar-activated catalysts and reactor designs for improved photons absorption. Common misconceptions and drawbacks of each technology are also examined together with insights for future progress.

Fabrication of 3D Hierarchical Byttneria Aspera-Like Ni@Graphitic Carbon Yolk-Shell Microspheres as Bifunctional Catalysts for Ultraefficient Oxidation/Reduction of Organic Contaminants

The search for earth-abundant, low-cost, recyclable, multifunctional as well as highly active catalysts remains the most pressing demand for heterogeneous catalytic elimination of pollutants in water environment remediation. Herein, a porous graphitic carbon-encapsulated Ni nanoparticles (NPs) hybrid (Ni@GC) is designed/constructed by direct pyrolysis of a Ni-based metal-organic framework (MOF) in N₂ . The resulting Ni@GC exhibits a unique 3D hierarchical byttneria aspera-like yolk-shell structure with a high surface area, abundant active sites as well as good microwave (MW)-absorbing performance. The outstanding MW-driven oxidation of norfloxacin to inorganic molecules and direct catalytic reduction of 4-nitrophenol to less toxic and more useful chemical raw material (4-aminophenol) can originate from the synergistic effects, including the presence of both zero-valent Ni NPs as the active center and graphitic carbon as the protective layer/electron acceptor as well as unique porous yolk-void-shell structure, which facilitates the MW energy-harvesting and/or rapid mass transfer of the reactant/product in the channels and cavities. Accordingly, the localized surface plasmon resonance-excitation and electronic relay mechanism are proposed to account for the catalytic oxidation/reduction, respectively. This work provides a new strategy for the design/assembly of multifunctional metal@GC hybrids with a unique architecture and elucidates new opportunities for remediation of environmental water.

Titanium Dioxide/Graphene and Titanium Dioxide/Graphene Oxide Nanocomposites: Synthesis, Characterization and Photocatalytic Applications for Water Decontamination

The use of titanium dioxide, TiO2 as a photocatalyst in water decontamination has witnessed continuous interest due to its efficiency, stability, low toxicity and cost-effectiveness. TiO2 use is limited by its large band gap energy leading to light absorbance in the UV region of the spectrum, and by the relatively fast rate of recombination of photogenerated electrons and positive holes. Both limitations can be mitigated by using carbon-TiO2 nanocomposites, such as those based on graphene (G) and graphene oxide (GO). Relative to bare TiO2, these nanocomposites have improved photocatalytic activity and stability under the UV–visible light, constituting a promising way forward for improved TiO2 photocatalytic performance. This review focuses on the recent developments in the chemistry of TiO2/G and TiO2/GO nanocomposites. It addresses the mechanistic fundamentals, briefly, of TiO2 and TiO2/G and TiO2/GO photocatalysts, the various synthesis strategies for preparing TiO2/G and TiO2/GO nanocomposites, and the different characterization techniques used to study TiO2/G and TiO2/GO nanocomposites. Some applications of the use of TiO2/G and TiO2/GO nanocomposites in water decontamination are included.

Photodegradation of pharmaceuticals and personal care products in water treatment using carbonaceous-TiO2 composites: A critical review of recent literature

The high concentrations of pharmaceuticals and personal care products (PPCP) that found in water in many locations are of concern. Among the available water treatment methods, heterogeneous photocatalysis using TiO₂ is an emerging and viable technology to overcome the occurrence of PPCP in natural and waste water. The combination of carbonaceous materials (e.g., activated carbon, carbon nanotubes and graphene nanosheets) with TiO₂, a recent development, gives significantly improved performance. In this article, we present a critical review of the development and fabrication of carbonaceous-TiO₂ and its application to PPCP removal including its influence on water chemistry, and the relevant operational parameters. Finally, we present an analysis of current priorities in the ongoing research and development of carbonaceous-TiO₂ for the photodegradation of PPCP.

Reusable Photocatalytic Optical Fibers for Underground, Deep-Sea, and Turbid Water Remediation

An approach for underground, deep, and turbid water remediation is presented based on optical fibers with a photocatalytic coating. Thus, photocatalytic TiO2 P25 nanoparticles immobilized in a poly(vinylidene difluoride) (PVDF) matrix are coated on polymeric optical fibers (POFs) and the photocatalytic performance of the system is assessed under artificial sunlight. To the best of our knowledge, poly(methyl methacrylate)-POF coated with TiO2/PVDF and the reusability of any type of POF for photocatalytic applications are not previously reported. The photocatalytic efficiency of the hybrid material in the degradation of ciprofloxacin (CIP) and its reusability are evaluated here. It is shown that 50 w/w% of TiO2 P25 achieves a degradation of 95% after 72 h under artificial sunlight and a reusability of three times leads to a loss of activity inferior to 11%. The efficient removal of ciprofloxacin and the stability of the POF coated with TiO2 P25 successfully demonstrate its suitability in the degradation of pollutants with potential application in regions with low light illumination, as in underground and deep water.

Evidence of anatase intergrowths formed during slow cooling of reduced ilmenite

Controlling the parameters during synthetic rutile production is essential to minimize production costs and ensure final product quality. Powder X-ray diffraction (PXRD) is typically used within the industry to guide process control. This work investigated the source of unusual features observed in the PXRD pattern of a slow-cooled reduced ilmenite (RI), which were not observed for a rapid-cooled RI. For the slow-cooled RI, the 002 peak ofM3O5(anosovite) had disappeared and the intensity of the \bar 203, 203, 204 and 402 peaks had decreased significantly compared to the pattern for the rapid-cooled RI. Using transmission electron microscopy, selected area electron diffraction (SAED) and pair distribution function (PDF) analysis, the authors attribute these features toM3O5–anatase intergrowth formation, which causes a loss in long-range order along theM3O5caxis. Strong diffuse streaking in the SAED patterns was also evident and supported the presence of disordered intergrowths from the oxidation ofM3O5. PDF analysis showed a significant improvement in the fit to the data for the slow-cooled RI, primarily in the <17 Å region, when anatase was added to the PDF model. The results presented here highlight the importance of the reduction and cooling stages during the formation of these industrially relevant RI minerals, which may be used to direct the production process and final TiO2product quality.

Preparation and characterization of self-assembly hydrogels with exfoliated montmorillonite nanosheets and chitosan

Novel montmorillonite-nanosheet/chitosan (MMTNS/CS) hydrogels fabricated via the self-assembly of exfoliated MMTNS and CS chains were investigated. The exfoliation of MMTNS, self-assembly mechanism and structure of MMTNS/CS hydrogels were characterized by an atomic force microscope, scanning electron microscope, transmission electron microscope, Fourier transform infrared spectroscope, energy-dispersive x-ray spectroscope and Brunauer-Emmett-Teller analyzer, respectively. The results indicated that MMT could be easily exfoliated to nanosheets with a thickness of 1 ∼ 5 nm in aqueous solution by an ultrasonic base upon interlayer hydration. The formation mechanism of the self-assembly hydrogels was due to the hydrogen bond (-OH ··· ⁺NH₃-) and electrostatic interaction between the MMTNS and CS. The MMTNSs were connected consecutively by CS in-plane to form a huge slice. The porous structure of the hydrogels was controllable by adjusting the MMTNS/CS mass ratio. The hydrogels could be used as adsorbents for sewage treatments, carriers for drugs, microorganisms and catalyzers due to their controllable porous structure and tremendous specific surface area which were derived from the completely exfoliated MMTNS.

Mutual-stabilization in chemically bonded graphene oxide–TiO2 heterostructures synthesized by a sol–gel approach

The Ti–O–C interactions between GO and TiO₂ are strong enough to induce mutual shaping during synthesis and thermal treatment.