TiO2 photocatalysis: Design and applications

Graphene-based porous nanohybrid architectures for adsorptive and photocatalytic abatement of volatile organic compounds

Volatile organic compounds (VOCs) represent a considerable threat to humans and ecosystems. Strategic remediation techniques for the abatement of VOCs are immensely important and immediately needed. Given a unique set of optical, mechanical, electrical, and thermal characteristics, inimitable surface functionalities, porous structure, and substantial specific surface area, graphene and derived nanohybrid composites have emerged as exciting candidates for abating environmental pollutants through photocatalytic degradation and adsorptive removal. Graphene oxide (GO) and reduced graphene oxide (rGO) containing oxygenated function entities, i.e., carbonyl, hydroxyl, and carboxylic groups, provide anchor and dispersibility of their surface photocatalytic nanoscale particles and adsorptive sites for VOCs. Therefore, it is meaningful to recapitulate current state-of-the-art research advancements in graphene-derived nanostructures as prospective platforms for VOCs degradation. Considering this necessity, this work provides a comprehensive and valuable insight into research progress on applying graphene-based nanohybrid composites for adsorptive and photocatalytic abatement of VOCs in the aqueous media. First, we present a portrayal of graphene-based nanohybrid based on their structural attributes (i.e., pore size, specific surface area, and other surface features to adsorb VOCs) and structure-assisted performance for VOCs abatement by graphene-based nanocomposites. The adsorptive and photocatalytic potentialities of graphene-based nanohybrids for VOCs are discussed with suitable examples. In addition to regeneration, reusability, and environmental toxicity aspects, the challenges and possible future directions of graphene-based nanostructures are also outlined towards the end of the review to promote large-scale applications of this fascinating technology.

Pulmonary dust foci as rat pneumoconiosis lesion induced by titanium dioxide nanoparticles in 13-week inhalation study

Background

Most toxicological studies on titanium dioxide (TiO₂) particles to date have concentrated on carcinogenicity and acute toxicity, with few studies focusing of pneumoconiosis, which is a variety of airspace and interstitial lung diseases caused by particle-laden macrophages. The present study examined rat pulmonary lesions associated with pneumoconiosis after inhalation exposure to TiO₂ nanoparticles (NPs).

Methods

Male and female F344 rats were exposed to 6.3, 12.5, 25, or 50 mg/m³ anatase type TiO₂ NPs for 6 h/day, 5 days/week for 13 weeks using a whole-body inhalation exposure system. After the last exposure the rats were euthanized and blood, bronchoalveolar lavage fluid, and all tissues including lungs and mediastinal lymph nodes were collected and subjected to biological and histopathological analyses.

Results

Numerous milky white spots were present in the lungs after exposure to 25 and 50 mg/m³ TiO₂ NPs. Histopathological analysis revealed that the spots were alveolar lesions, characterized predominantly by the agglomeration of particle-laden macrophages and the presence of reactive alveolar epithelial type 2 cell (AEC2) hyperplasia. We defined this characteristic lesion as pulmonary dust foci (PDF). The PDF is an inflammatory niche, with decreased vascular endothelial cells in the interstitium, and proliferating AEC2 transformed into alveolar epithelial progenitor cells. In the present study, the AEC2 in the PDF had acquired DNA damage. Based on PDF induction, the lowest observed adverse effect concentration for pulmonary disorders in male and female rats was 12.5 mg/m³ and 6.3 mg/m³, respectively. The no observed adverse effect concentration for male rats was 6.3 mg/m³. There was a sex difference in lung lesion development, with females showing more pronounced lesion parameters than males.

Conclusions

Inhalation exposure to TiO₂ NPs caused PDF, an air-space lesion which is an alveolar inflammatory niche containing particle-laden macrophages and proliferating AEC2. These PDFs histopathologically resemble some pneumoconiosis lesions (pulmonary siderosis and hard metal pneumoconiosis) in workers and lung disease in smokers, suggesting that PDFs caused by exposure to TiO₂ NPs in rats are an early pneumoconiosis lesion and may be a common alveolar reaction in mammals.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12989-022-00498-3.

Enhancing the Photocatalytic Activity of TiO2/Na2Ti6O13 Composites by Gold for the Photodegradation of Phenol

This study aims to synthesize Au/TiO2/Na2Ti6O13 composites to reduce the occurrence of recombination and increase photocatalytic activity in phenol degradation. Gold was used due to its high stability and strong surface plasmon resonance (SPR) properties which make it operate effectively in the visible light spectrum. The prepared composites were characterized using XRD, SEM, TEM, FTIR, and DRS. The results showed that the composite consisted of rutile TiO2 with a crystal size of 38–40 nm and Na2Ti6O13 with a crystal size of 25 nm. The gold in the composite has a crystallite size of 16–19 nm along with the percentage of gold added. Morphological analysis shows that the composite has the form of inhomogeneous spherical particles with gold spread among composites with sizes less than 20 nm. FTIR analysis showed the presence of Na–O and Ti–O–Ti bonds in the composite. The best composite was 3% Au/TiO2/Na2Ti6O13 which had high crystallinity, small particle size, and bandgap energy of 2.59 eV. Furthermore, it had an efficiency 205% better than without gold. After that, cost estimation is proposed as a large-scale application. This study describes the total cost, break-even analysis, and payback analysis for the commercialization needs of the designed photocatalytic catalyst.

Effect of Biogenic Silica Behavior in the Incorporation of Mesoporous Anatase TiO2 for Excellent Photocatalytic Mineralization of Sodium Diclofenac

TiO2/SiO2 composites were synthesized via a simple sol gel method by surface reduction of Ti4+ ions to Ti3+ using titanium isopropoxide as a TiO2 precursor and rice husks (RHA) as a SiO2 source. The silica content and calcination temperature of the materials were evaluated. Thermal, crystallographic and physicochemical aspects suggest that biogenic silica (SiO2) can improve the thermal stability of the anatase phase of TiO2, when the SiO2 content reaches 20%. The N2 adsorption-desorption isotherms showed that the SiO2-modified samples have uniform pore diameters and a large specific surface area. The XPS analysis showed the surface reduction of Ti4+ ions to Ti3+ within the TiO2 network via oxygen vacancies after SiO2 introduction, which is beneficial for the photocatalytic reaction. Photocatalytic degradation of sodium diclofenac (SDFC) shows that TiO2/SiO2 composites have better activity compared to commercial P25. Mesoporous TiO2 composite modified with 20 wt% SiO2 showed better photocatalytic mineralization than P25 (83.7% after 2 h instead of 57.3% for P25). The excellent photocatalytic mineralization of the photocatalysts can be attributed to the high anatase crystallinity exhibited by XRD analysis, high specific surface area, surface hydroxyl groups, and the creation of oxygen vacancy, as well as the presence of Ti3+ ions.

No evidence for carcinogenicity of titanium dioxide nanoparticles in 26-week inhalation study in rasH2 mouse model

With the rapid development of alternative methods based on the spirit of animal welfare, the publications of animal studies evaluating endpoints such as cancer have been extremely reduced. We performed a 26-week inhalation exposure studies of titanium dioxide nanoparticles (TiO2 NPs) using CByB6F1-Tg(HRAS)2Jic (rasH2) mice model for detecting carcinogenicity. Male and female rasH2 mice were exposed to 2, 8 or 32 mg/m3 of TiO2 NPs for 6 h/day, 5 days/week for 26 weeks. All tissues and blood were collected and subjected to biological and histopathological analyses. TiO2 NPs exposure induced deposition of particles in lungs in a dose-dependent manner in each exposure group. Exposure to TiO2 NPs, as well as other organs, did not increase the incidence of lung tumors in any group, and pulmonary fibrosis and pre-neoplastic lesions were not observed in all groups. Finally, the cell proliferative activity of alveolar epithelial type 2 cells was examined, and it was not increased by exposure to TiO2 NPs. This is the first report showing the lack of pulmonary fibrogenicity and carcinogenicity (no evidence of carcinogenic activity) of TiO2 NPs in 26-week inhalation study in rasH2 mice exposed up to 32 mg/m3, which is considered to be a high concentration.

Cation-Selective Oxide Semiconductor Mesoporous Membranes for Biomimetic Ion Rectification and Light-Powered Ion Pumping

Artificial membranes precisely imitating the biological functions of ion channels and ion pumps have attracted significant attention to explore nanofluidic energy conversion. Herein, inspired by the cyclic ion transport for the photosynthesis in purple bacteria, a bilayer inorganic membrane (TiO₂ /AAO) composed of oxide semiconductor (TiO₂ ) mesopores on anodic alumina (AAO) macropores is we developed. This inorganic membrane achieves the functions of ion channels and ion pumps, including the ion rectification and light-powered ion pumping. The asymmetric charge distribution across the bilayer membrane contributes to the cationic selectivity and ion rectification characteristics. The electrons induced by ultraviolet irradiation introduce a built-in electric field across TiO₂ /AAO membrane, which pumps the active ion transport from a low to a high concentration. This work integrates the functions of biological ion channels and ion pumps within an artificial membrane for the first time, which paves the way to explore multifunctional membranes analogous to its biological counterpart.

Periodic Nanoporous Inorganic Patterns Directly Made by Self-Ordering of Cracks

Solution-processed inorganic nanoporous films are key components for the vast spectrum of applications ranging from dew harvesting to solar cells. Shaping them into complex architectures required for advanced functionality often needs time-consuming or expensive fabrication. In this work, crack formation is harnessed to pattern porous inorganic films in a single step and without using lithography. Aqueous inks, containing inorganic precursors and polymeric latexes enable evaporation-induced, defect-free periodic arrays of cracks with tunable dimensions over several centimeters. The ink formulation strategy is generalized to more than ten inorganic materials including simple and binary porous oxide and metallic films covering a whole spectrum of properties including insulating, photocatalytic, electrocatalytic, conductive, or electrochromic materials. Notably, this approach enables 3D self-assembly of cracks by stacking several layers of different compositions, yielding periodic assemblies of polygonal shapes and Janus-type patterns. The crack patterned periodic arrays of nanoporous TiO₂ diffract light, and are used as temperature-responsive diffraction grating sensors. More broadly, this method represents a unique example of a self-assembly process leading to long-range order (over several centimeters) in a robust and controlled way.

Synthesis and characterization of a novel composite of rice husk-derived graphene oxide with titania microspheres (GO-RH/TiO2) for effective treatment of cationic dye methylene blue in aqueous solutions

Graphene oxide (GO) was synthesized utilizing rice husk (RH) as the starting raw material via a modified Hummers' method. Ground pencil leads were used as a control powder of the starting raw material to monitor the consistency of the synthesis method. TiO₂ microspheres were synthesized via a precipitated method using the pluronic F127 solution as the pore template. GO derived from RH (GO-RH) was composited with TiO₂ microspheres as GO-RH/TiO₂ composites by an impregnation method with weight ratios of 3:1, 2:2, and 1:3. Characterized results revealed GO-RH formed a ternary phase material of graphene oxide, graphite oxide, and silica. A typical microstructure of the calcined TiO₂ microspheres was found as the agglomerated anatase nanoparticles. Furthermore, the composites belong to large surface areas and numerous oxygen-containing functionalities on their surfaces. Removal efficiencies of cationic dye methylene blue (MB) from aqueous solutions by the composites, GO-RH and TiO₂, were studied under UV illumination for 180 min. Due to the effective combination of adsorption and photodegradation for the MB removal, the composites provided the higher efficiencies (99-100%) faster than those of GO-RH and TiO₂ and could be reused at least 4 times. Finally, a mechanism of the MB removal by the composites was proposed.

Enzyme-Photocatalyst Tandem Microrobot Powered by Urea for Escherichia coli Biofilm Eradication

Urinary-based infections affect millions of people worldwide. Such bacterial infections are mainly caused by Escherichia coli (E. coli) biofilm formation in the bladder and/or urinary catheters. Herein, the authors present a hybrid enzyme/photocatalytic microrobot, based on urease-immobilized TiO₂ /CdS nanotube bundles, that can swim in urea as a biocompatible fuel and respond to visible light. Upon illumination for 2 h, these microrobots are able to remove almost 90% of bacterial biofilm, due to the generation of reactive radicals, while bare TiO₂ /CdS photocatalysts (non-motile) or urease-coated microrobots in the dark do not show any toxic effect. These results indicate a synergistic effect between the self-propulsion provided by the enzyme and the photocatalytic activity induced under light stimuli. This work provides a photo-biocatalytic approach for the design of efficient light-driven microrobots with promising applications in microbiology and biomedicine.

Recent advances in the breakdown of microplastics: strategies and future prospectives

Microplastics pollution is becoming a major environmental issue, and exposure to microplastics has been associated with numerous adverse results to both the ecological system and humans. This work summarized the state-of-the-art developments in the breakdown of microplastics, including natural weathering, catalysts-assisted breakdown and biodegradation. Characterization techniques for microplastic breakdown involve scanning electron microscopy, Fourier infrared spectroscopy, X-ray photoelectron spectroscopy, etc. Bioavailability and adsorption capacity of microplastics may change after they are broken down, therefore leading to variety in microplastics toxicity. Further prospectives for should be focused on the determination and toxicity evaluation of microplastics breakdown products, as well as unraveling uncultivable microplastics degraders via cultivation-independent approaches. This work benefits researchers interested in environmental studies, particularly the removal of microplastics from environmental matrix.

SARS-CoV-2 pharmaceutical drugs: a critical review on the environmental impacts, chemical characteristics, and behavior of advanced oxidation processes in water

This review summarizes research data on the pharmaceutical drugs used to treat the novel SARS-CoV-2 virus, their characteristics, environmental impacts, and the advanced oxidation processes (AOP) applied to remove them. A literature survey was conducted using the electronic databases Science Direct, Scopus, Taylor & Francis, Google Scholar, PubMed, and Springer. This complete research includes and discusses relevant studies that involve the introduction, pharmaceutical drugs used in the SARS-CoV-2 pandemic: chemical characteristics and environmental impact, advanced oxidation process (AOP), future trends and discussion, and conclusions. The results show a full approach in the versatility of AOPs as a promising solution to minimize the environmental impact associated with these compounds by the fact that they offer different ways for hydroxyl radical production. Moreover, this article focuses on introducing the fundamentals of each AOP, the main parameters involved, and the concomitance with other sources and modifications over the years. Photocatalysis, sonochemical technologies, electro-oxidation, photolysis, Fenton reaction, ozone, and sulfate radical AOP have been used to mineralize SARS-CoV-2 pharmaceutical compounds, and the efficiencies are greater than 65%. According to the results, photocatalysis is the main technology currently applied to remove these pharmaceuticals. This process has garnered attention because solar energy can be directly utilized; however, low photocatalytic efficiencies and high costs in large-scale practical applications limit its use. Furthermore, pharmaceuticals in the environment are diverse and complex. Finally, the review also provides ideas for further research needs and major concerns.

Fabrication of Graphene-Based TiO2@CeO2 and CeO2@TiO2 Core-Shell Heterostructures for Enhanced Photocatalytic Activity and Cytotoxicity

Development of light-harvesting properties and inhibition of photogenerated charge carrier recombination are of paramount significance in the photocatalytic process. In the present work, we described the synthesis of core-shell heterostructures, which are composed of titanium oxide (TiO₂) and cerium oxide (CeO₂) deposited on a reduced graphene oxide (rGO) surface as a conductive substrate. Following the synthesis of ternary rGO-CeO₂@TiO₂ and rGO-TiO₂@CeO₂ nanostructures, their photocatalytic activity was investigated toward the degradation of rhodamine B dye as an organic pollutant under UV light irradiation. The obtained structures were characterized with high-resolution transmission electron microscopy, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, Brunauer-Emmett-Teller, X-ray photoelectron spectroscopy surface analysis, and UV-Vis spectroscopy. Various parameters including pH, catalyst dosage, temperature, and contact time were studied for photocatalysis optimization. Heterostructures showed considerable advantages because of their high surface area and superior photocatalytic performance. In contrast, rGO-CeO₂@TiO₂ showed the highest photocatalytic activity, which is attributed to the more effective electron-hole separation and quick suppression of charge recombination at core-shell phases. A biological assay of the prepared heterostructure was performed to determine the cytotoxicity against breast cancer cells (MCF-7) and demonstrated a very low survival rate at 7.65% of cells at the 17.5 mg mL^-1 concentration of applied photocatalyst.

Catalytic Metasurfaces Empowered by Bound States in the Continuum

Photocatalytic platforms based on ultrathin reactive materials facilitate carrier transport and extraction but are typically restricted to a narrow set of materials and spectral operating ranges due to limited absorption and poor energy-tuning possibilities. Metasurfaces, a class of 2D artificial materials based on the electromagnetic design of nanophotonic resonators, allow optical absorption engineering for a wide range of materials. Moreover, tailored resonances in nanostructured materials enable strong absorption enhancement and thus carrier multiplication. Here, we develop an ultrathin catalytic metasurface platform that leverages the combination of loss-engineered substoichiometric titanium oxide (TiO_2-x) and the emerging physical concept of optical bound states in the continuum (BICs) to boost photocatalytic activity and provide broad spectral tunability. We demonstrate that our platform reaches the condition of critical light coupling in a TiO_2-x BIC metasurface, thus providing a general framework for maximizing light-matter interactions in diverse photocatalytic materials. This approach can avoid the long-standing drawbacks of many naturally occurring semiconductor-based ultrathin films applied in photocatalysis, such as poor spectral tunability and limited absorption manipulation. Our results are broadly applicable to fields beyond photocatalysis, including photovoltaics and photodetectors.

Simple Quaternary Templating Systems for Direct Synthesis of Unique SBA-15 Mesopore Frameworks Embedded with High-Content TiO2 Nanoparticles as High-Performance Photocatalysts

This work presents a simple P123-based quaternary templating system using titanyl sulfate (TS) as the TiO₂ precursor and self-contained sulfuric acid as the catalyst (TS/TEOS/P123/H₂O). A unique structural configuration of SBA-15-type mesopore frameworks embedded with high-content TiO₂ nanoparticles can be directly obtained. Even with a high TiO₂ content (29.1 wt %), well-defined mesostructures free of pore blocking can be secured. A new structural formation mechanism is unveiled: a self-assembly process between inorganic species and P123 micelles yields ordered mesostructures catalyzed by self-contained TS in the low-temperature step, while sol-gel reaction and crystallization of TS coincide with processes of mesostructural re-organization and partial evacuation of P123 from mesopores. The incorporation of high-content TiO₂ nanoparticles into mesopore frameworks mainly happens during the hydrothermal treatment step. Not surprisingly, thanks to well-defined mesostructures containing high-content accessible TiO₂ nanoparticles, such TiO₂/SBA-15 composites show high activity and good reusability in photodegrading Rhodamine B and humic acids and photoreducing highly toxic Cr⁶⁺ in water under UV irradiation.

Near-UV light emitting diode with on-chip photocatalysts for purification applications

A new design for light-emitting diodes (LEDs) with on-chip photocatalysts is presented for purification applications. An array of disk-shaped TiO₂, with a diameter of several hundred nanometers, combined with SiO₂ pedestals was fabricated directly on the surface of an InGaN-based near-ultraviolet (UV) LED using a dry etching process. The high refractive-index contrast at the boundary and the circular shape can effectively confine the near-UV light generated from the LED through multiple internal reflections inside the TiO₂ nanodisks. Such a feature results in the enhancement of light absorption by the photocatalytic TiO₂. The degradation of the organic dye malachite green was monitored as a model photocatalytic reaction. The proposed structure of LEDs with TiO₂/SiO₂ nanodisk/pedestal array exhibited a photocatalytic activity that was three times higher than the activity of LEDs with a TiO₂ planar layer. The integration of photocatalytic materials with near-UV LEDs in a single system is promising for various purification applications, such as sterilization and disinfection.

Modifications of microplastics in urban environmental management systems: A review

Microplastics (MPs) are a worldwide environmental pollution issue. Besides the natural environmental stresses, various treatments in urban environmental management systems induce modifications on MPs, further affecting their environmental behavior. Investigating these modifications and inherent mechanisms is crucial for assessing the environmental impact and risk of MPs. In this review, up-to-date knowledge regarding the modifications of MPs in urban environmental management systems was summarized. Variations of morphology, chemical composition, hydrophilicity and specific surface area of MPs were generalized. The aging and degradation of MPs during drinking water treatment, wastewater treatment, sewage sludge treatment and solid waste treatment were investigated. A high abundance of MPs occurred in sewage sludge and aging solid waste, while digestion and composting contributed to significant decomposition and reduction of MPs. These treatments have become converters for MPs before entering the environment. Several novel technologies for MPs removal were listed; However, no appropriate methods can be put into actual application by now, except the membrane separation. The corresponding effects of degradation on the behaviors of MPs, including adsorption, sinking and contaminant leakage, were discussed. Finally, three priorities for research were proposed. This critical review provides viewpoints and references for risk evaluation of MPs after treatments in urban environmental management systems.

Recent Advances in TiO2-based Photoanodes for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting has attracted a great attention in the past several decades which holds great promise to address global energy and environmental issues by converting solar energy into hydrogen. However, its low solar-to-hydrogen (STH) conversion efficiency remains a bottleneck for practical application. Developing efficient photoelectrocatalysts with high stability and high STH conversion efficiency is one of the key challenges. As a typical n-type semiconductor, titanium dioxide (TiO 2 ) exhibits high PEC water splitting performance, especially high chemical and photo stability. But, TiO 2 has also disadvantages such as wide band gap and fast electron-hole recombination rate, which seriously hinder its PEC performance. This review focuses on recent development in TiO 2 -based photoanodes as well as some key fundamentals. The corresponding mechanisms and key factors for high STH, and controllable synthesis and modification strategies are highlighted in this review. We conclude finally with an outlook providing a critical perspective on future trends on TiO 2 -based photoanodes for PEC water splitting.

Polymorphic Biological and Inorganic Functional Nanomaterials

This perspective involves two types of functional nanomaterials, amyloid fibrils and metal oxide nanowires and nanogrids. Both the protein and the inorganic nanomaterials rely on their polymorphism to exhibit diverse properties that are important to sensing and catalysis. Several examples of novel functionalities are provided from biomarker sensing and filtration applications to smart scaffolds for energy and sustainability applications.

Synthesis of TiC@C-anatase/rutile@polyvinyl alcohol/xylan: a powerful photocatalyst for degradation of organic pollutant under visible light

In this study, a composite bearing titanium carbide (TiC), titanium dioxide (TiO₂), polyvinyl alcohol and xylan (TiC@C-anatase/rutile@polyvinyl alcohol/xylan) was synthesized and applied as a photocatalyst for the degradation of bromophenol blue (BPB) solution through several steps. Nanostructure of TiC and TiO₂ in the anatase and rutile phases was obtained through heat treatment of TiC at different times and temperatures (TiC@AR) which led to a reduction in energy bandgap from UV to visible light, in addition to the enhancement of the surface activity. After TiC@AR polymerization by xylan and polyvinyl alcohol and obtaining TiC@AR/PX, the energy bandgap reduced to IR range (52% of the sunlight) while showing an enhancement in the surface activity. The photocatalytic activity of the compounds was tested by studying the decomposition of BPB solution under visible light. The result illustrated the ability of TiC and TiC@AR to decrease the concentration of BPB after 150 min by 35% and 37%, respectively, while this reduction was 72% for TiC@AR/PX. Considering the effective parameters, the energy bandgap and the surface layer played key roles in photocatalytic degradation.

Photoreduction of Copper Ions Using Silica–Surfactant Hybrid and Titanium (IV) Oxide under Sulfuric Acid Conditions

Photoreduction of Cu2+ ions to Cu metal by titanium(IV) oxide (TiO2) was conducted in the presence of a silica–surfactant hybrid under sulfuric acid conditions. After irradiation, a dark-red color, reflections due to Cu metal in the X-ray diffraction pattern, and peaks due to Cu 2p1/2 and 2p3/2 in the X-ray photoelectron spectrum indicated the precipitation of Cu metal in the product. In addition, an increase in the Brunauer–Emmett–Teller specific surface area from 36 and 45 m2/g for the silica–surfactant and TiO2, respectively, to 591 m2/g for the product, and a decrease in the intensity of the C-H stretching band in the Fourier–transform infra-red spectra implied the removal of surfactant during the reaction. These characteristics were never observed when TiO2 was used solely. Therefore, this study indicated that the photoreduction of Cu2+ ions to Cu metal by TiO2 was facilitated under the sulfuric acid medium, where the surfactants extracted from silica–surfactant hybrids by protons in the acidic condition were successfully photo-oxidized by TiO2. Thus, this study presents a new application of the conversion of a silica–surfactant hybrid into mesoporous silicas.

Charge carrier dynamics and reaction intermediates in heterogeneous photocatalysis by time-resolved spectroscopies

Sunlight as the most abundant renewable energy holds the promise to make our society sustainable. However, due to its low power density and intermittence, efficient conversion and storage of solar energy as a clean fuel are crucial. Apart from solar fuel synthesis, sunlight can also be used to drive other reactions including organic conversion and air/water purification. Given such potential of photocatalysis, the past few decades have seen a surge in the discovery of photocatalysts. However, the current photocatalytic efficiency is still very moderate. To address this challenge, it is important to understand fundamental factors that dominate the efficiency of a photocatalytic process to enable the rational design and development of photocatalytic systems. Many recent studies highlighted transient absorption spectroscopy (TAS) and time-resolved infrared (TRIR) spectroscopy as powerful approaches to characterise charge carrier dynamics and reaction pathways to elucidate the reasons behind low photocatalytic efficiencies, and to rationalise photocatalytic activities exhibited by closely related materials. Accordingly, as a fast-moving area, the past decade has witnessed an explosion in reports on charge carrier dynamics and reaction mechanisms on a wide range of photocatalytic materials. This critical review will discuss the application of TAS and TRIR in a wide range of heterogeneous photocatalytic systems, demonstrating the variety of ways in which these techniques can be used to understand the correlation between materials design, charge carrier behaviour, and photocatalytic activity. Finally, it provides a comprehensive outlook for potential developments in the area of time-resolved spectroscopies with an aim to provide design strategies for photocatalysts.

The property-governed activity of silver-modified titania photocatalysts: The influence of titania matrix

Commercial titania photocatalysts were modified with silver nanoparticles (NPs) by the photodeposition method in the presence/absence of methanol. The obtained photocatalysts were characterized by XRD, XPS, diffuse reflectance spectroscopy, STEM, and time-resolved microwave conductivity (TRMC) methods. The photocatalytic activity was tested under UV/vis irradiation for (i) methanol dehydrogenation (during silver deposition), (ii) oxygen evolution with in situ silver deposition, and (iii) oxidative decomposition of acetic acid, as well as under vis irradiation for 2-propanol oxidation. The action spectra of 2-propanol oxidation were also performed. It has been confirmed that modification of titania with silver causes significant improvement of photocatalytic activity under both UV and vis irradiation as silver works as an electron scavenger (TRMC data) and vis activator (possibly by an energy transfer mechanism). The obtained activities differ between titania samples significantly, suggesting that the type of crystalline phase, particle/crystallite sizes, and electron traps’ density are crucial for both the properties of formed silver deposits and resultant photocatalytic activity. It might be concluded that, under UV irradiation, (i) high crystallinity and large specific surface area are recommended for rutile- and anatase-rich samples, respectively, during hydrogen evolution, (ii) mixed crystalline phases cause a high rate of oxygen evolution from water, and (iii) anatase phase with fine silver NPs results in efficient decomposition of acetic acid, whereas under vis irradiation the aggregated silver NPs (broad localized surface plasmon resonance peak) on the rutile phase are promising for oxidation reactions.

Designing Nanoengineered Photocatalysts for Hydrogen Generation by Water Splitting and Conversion of Carbon Dioxide to Clean Fuels

Semiconductor photocatalysis has received tremendous attention in the past decade as it has shown great promise in the context of clean energy harvesting for environmental remediation. Sunlight is an inexhaustible source of energy available to us throughout the year, although it is rather dilutely dispersed. Semiconductor based photocatalysis presents one of the best ways to harness this source of energy to carry out chemical reactions of interest that require external energy input. Photocatalytic hydrogen generation by splitting of water, CO₂ mitigation, and CO₂ conversion to green fuel have therefore become the highly desirable clean and sustainable processes for a better tomorrow. Although numerous efforts have been made and continue to be expended to search and develop new classes of photocatalyst materials in recent years, several significant challenges still remain to be resolved before photocatalysis can reach its commercial potential. Therefore, major attention is required towards improving the efficiencies of the existing photocatalysts by further manipulating them and parallelly employing newer strategies for novel photocatalyst designs. This personal account aims to provide a broad overview of the field primarily invoking examples of our own research contributions in the field, which include photocatalytic hydrogen generation and CO₂ reduction to value added chemicals. This account reviews the state-of-the-art research activities and scientific possibilities which a functional material can offer if its properties are put to best use through goal-oriented design by combining with other compatible materials. We have addressed fundamental principles of photocatalysis, different kind of functional photocatalysts, critical issues associated with them and various strategies to overcome the related hurdles. It is our hope that this current personal account will provide a platform for young researchers to address the bottleneck issues in the field of photocatalysis and photocatalysts with a sense of clarity, and to find innovative solutions to resolve them by a prudent choice of materials, synthesis protocols, and approaches to boost the photocatalysis output. We emphasize that a targeted or goal-directed photocatalyst nanoengineering as perhaps the only way to realize an early success in this multiparametric domain.

Photocatalytic Materials Obtained from E-Waste Recycling: Review, Techniques, Critique, and Update

Waste-derived materials obtained from the recovery and recycling of electronic waste (e-waste) such as batteries and printed circuit boards have attracted enormous attention from academia and industry in recent years, especially due to their eco-friendly nature and the massive increment in e-waste due to technological development. Several investigations in the literature have covered the advances achieved so far. Meanwhile, photocatalytic applications are especially of interest since they maintain mutual benefits and can be used for H2 production from solar water splitting based on semiconductor processing as a proper environmentally friendly technique for solar energy conversion. In addition, they can be utilized to degrade a variety of organic and non-organic contaminations. Nonetheless, to the best of the authors’ knowledge, there has not been any comprehensive review that has specifically been focused on e-waste-derived photocatalytic materials. In this regard, the present work is dedicated to thoroughly discussing the related mechanisms, strategies, and methods, as well as the various possible photocatalysts synthesized from e-wastes with some critiques in this field. This brief overview can introduce modern technologies and promising possibilities for e-waste valorization, photocatalytic processes, and new photocatalytic degradation methods of eco-friendly nature. This paper discusses various e-waste-obtained photocatalytic materials, synthesis procedures, and applications, as well as several types of e-waste, derived materials such as TiO2, ZnO, indium tin oxide, and a variety of sulfide- and ferrite-based photocatalytic materials.

Photocatalytic Reforming of Biomass: What Role Will the Technology Play in Future Energy Systems

Photocatalytic reforming of biomass has emerged as an area of significant interest within the last decade. The number of papers published in the literature has been steadily increasing with keywords such as 'hydrogen' and 'visible' becoming prominent research topics. There are likely two primary drivers behind this, the first of which is that biomass represents a more sustainable photocatalytic feedstock for reforming to value-added products and energy. The second is the transition towards achieving net zero emission targets, which has increased focus on the development of technologies that could play a role in future energy systems. Therefore, this review provides a perspective on not only the current state of the research but also a future outlook on the potential roadmap for photocatalytic reforming of biomass. Producing energy via photocatalytic biomass reforming is very desirable due to the ambient operating conditions and potential to utilise renewable energy (e.g., solar) with a wide variety of biomass resources. As both interest and development within this field continues to grow, however, there are challenges being identified that are paramount to further advancement. In reviewing both the literature and trajectory of the field, research priorities can be identified and utilised to facilitate fundamental research alongside whole systems evaluation. Moreover, this would underpin the enhancement of photocatalytic technology with a view towards improving the technology readiness level and promoting engagement between academia and industry.

Design of Photocatalytic Functional Coatings Based on the Immobilization of Metal Oxide Particles by the Combination of Electrospinning and Layer-by-Layer Deposition Techniques

This work reports the design and characterization of functional photocatalytic coatings based on the combination of two different deposition techniques. In a first step, a poly(acrylic acid) + β-Cyclodextrin (denoted as PAA+ β-CD) electrospun fiber mat was deposited by using the electrospinning technique followed by a thermal treatment in order to provide an enhancement in the resultant adhesion and mechanical resistance. In a second step, a layer-by-layer (LbL) assembly process was performed in order to immobilize the metal oxide particles onto the previously electrospun fiber mat. In this context, titanium dioxide (TiO2) was used as the main photocatalytic element, acting as the cationic element in the multilayer LbL structure. In addition, two different metal oxides, such as tungsten oxide (WO3) and iron oxide (Fe2O3), were added into PAA anionic polyelectrolyte solution with the objective of optimizing the photocatalytic efficiency of the coating. All of the coatings were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM) images, showing an increase in the original fiber diameter and a decrease in roughness of the mats because of the LbL second step. The variation in the wettability properties from a superhydrophilic surface to a less wettable surface as a function of the incorporation of the metal oxides was also observed by means of water contact angle (WCA) measurements. With the aim of analyzing the photocatalytic efficiency of the samples, degradation of methyl blue (MB) azo-dye was studied, showing an almost complete discoloration of the dye in the irradiated area. This study reports a novel combination method of two deposition techniques in order to obtain a functional, homogeneous and efficient photocatalytic coating.

Integration of microbial electrochemical systems and photocatalysis for sustainable treatment of organic recalcitrant wastewaters: Main mechanisms, recent advances, and present prospects

In recent years, microbial electrochemical systems (MESs) have demonstrated to be an environmentally friendly technology for wastewater treatment and simultaneous production of value-added products or energy. However, practical applications of MESs for the treatment of recalcitrant wastewater are limited by their low power output and slow rates of pollutant biodegradation. As a novel technology, hybrid MESs integrating biodegradation and photocatalysis have shown great potential to accelerate the degradation of bio-recalcitrant pollutants and increase the system output. In this review, we summarize recent advances of photo-assisted MESs for enhanced removal of recalcitrant pollutants, and present further discussion about the synergistic effect of biodegradation and photocatalysis. In addition, we analyse in detail different set-up configurations, discuss mechanisms of photo-enhanced extracellular electron transfer, and briefly present ongoing research cases. Finally, we highlight the current limitations and corresponding research gaps, and propose insights for future research.

A review on the generation, discharge, distribution, environmental behavior, and toxicity (especially to microbial aggregates) of nano-TiO2 in sewage and surface-water and related research prospects

This article reviews the nano-effects and applications of different crystalline nano‑titanium dioxide (nano-TiO₂), identifies their discharge, distribution, behavior, and toxicity to aquatic organisms (focusing on microbial aggregates) in sewage and surface-water, summarizes related toxicity mechanisms, and critically proposes future perspectives. The results show that: 1) based on crystal type, application boundaries of nano-TiO₂ have become clear, extending from traditional manufacturing to high-tech fields; 2) concentration of nano-TiO₂ in water is as high as hundreds of thousands of μg/L (sewage) or several to dozens of μg/L (surface-water) due to direct application or indirect release; 3) water environmental behaviors of nano-TiO₂ are mainly controlled by hydration conditions and particle characteristics; 4) aquatic toxicities of nano-TiO₂ are closely related to their water environmental behavior, in which crystal type and tested species (such as single species and microbial aggregates) also play the key role. Going forward, the exploration of the toxicity mechanism will surely become a hot topic in the aquatic-toxicology of nano-TiO₂, because most of the research so far has focused on the responses of biological indicators (such as metabolism and damage), while little is known about the stress imprint caused by the crystal structures of nano-TiO₂ in water environments. Additionally, the aging of nano-TiO₂ in a water environment should be heeded to because the continuously changing surface structure is bound to have a significant impact on its behavior and toxicity. Moreover, for microbial aggregates, comprehensive response analysis should be conducted in terms of the functional activity, surface features, composition structure, internal microenvironment, cellular and molecular level changes, etc., to find the key point of the interaction between nano-TiO₂ and microbial aggregates, and to take mitigation or beneficial measures to deal with the aquatic-toxicity of nano-TiO₂. In short, this article contributes by 1) reviewing the research status of nano-TiO₂ in all aspects: application and discharge, distribution and behavior, and its aquatic toxicity; 2) suggesting the response mechanism of microbial aggregates and putting forward the toxigenic mechanism of nanomaterial structure; 3) pointing out the future research direction of nano-TiO₂ in water environment.

Surface State-Assisted Delayed Photocurrent Response of Au Nanocluster/TiO2 Photoelectrodes

Gold nanoclusters (NCs) can be used as sensitizers to extend the absorption capabilities of TiO₂ as photoelectrodes. However, the adsorption of NCs also creates additional surface states on the TiO₂ surface, which gives rise to intricacies in the understanding of various interfacial phenomena occurring in NC-sensitized TiO₂. One of the complexities that have recently been discovered is the size-dependent hole-transfer mechanism. In this work, we reveal another anomalous behavior in the hole-transfer process that the hole scavenging ability of the electrolyte also plays a role in determining the hole-transfer mechanism in the NC-TiO₂ system, which is unprecedented in other photoelectrode systems. In the presence of an efficient hole scavenger (Na₂SO₃), the hole transfer in Au₁₈-TiO₂ occurs directly through the highest occupied molecular orbital (HOMO) of Au₁₈ NCs. However, in the presence of a less efficient hole scavenger (ethylenediaminetetraacetic acid), hole transfer in Au₁₈-TiO₂ does not occur through the HOMO and shifts to surface state-assisted hole transfer. Due to surface state charging, this surface state-assisted hole-transfer mechanism results in delayed photocurrent response in Au₁₈-TiO₂. Evidence for this exotic hole-transfer mechanism shift is provided by photoelectrochemical electrochemical impedance spectroscopy, and its implications are discussed.

The Sensitization of TiO2 Thin Film by Ag Nanoparticles for the Improvement of Photocatalytic Efficiency

The formation of Ag nanoparticles on the surface of TiO2 (AgNP/TiO2) to enhance photocatalytic efficiency was studied. The Ag nanoparticles (AgNP) size, form, and distribution dependence on the initial thickness of Ag thin films, annealing temperature, and time were analyzed. The optimal annealing temperature of 400 °C and annealing time of 60 min were chosen to form AgNP from the initial Ag thin films with a thickness of 5, 7.5, and 10 nm. The formation of AgNP was done on amorphous TiO2 (a-TiO2), which crystallized into the anatase phase after the annealing. The photocatalytic efficiency (k–degradation rate constant, Defi–degradation efficiency) was evaluated by the photodegradation of Rhodamine B aqueous solution. The results suggested that the highest photocatalytic efficiency of Rhodamine B aqueous solution was reached where the average diameter (DA) of AgNP was ~38 nm (k38 = 0.017 min−1, Defi_38 = 63.5%), compared to 27 and 82 nm (k27 = 0.012 min−1, Defi_27 = 51.2% and k82 = 0.011 min−1, Defi_82 = 52.1%, respectively). The acquired results did not show clear correlation between the size and distribution of the AgNP on the TiO2 surface and photocatalytic efficiency. Nevertheless, the results suggest that AgNP can enhance the photocatalytic efficiency of TiO2 thin films (kTiO2 = 0.008 min−1, Defi_TiO2 = 36.3%).

Photocatalytic disinfection of Coliforms and degradation of natural organic matters in river water using titanate nanotubes

In this study, we synthesized and modified titanate nanotubes (TNTs) under different conditions of acid rinsing and calcination. The produced materials were then characterized by transmission electron microscopy, X-ray diffraction, ultraviolet-visible spectroscopy, temperature programmed desorption, inductively coupled plasma atomic emission spectroscopy, and Brunauer-Emmett-Teller analysis. The activity of material was evaluated via its application for disinfection of Coliforms and removal of natural organic matters (NOMs) in river water. Results showed that TNTs rinsed at pH 1.6 and calcined at 500°C had the highest removal efficiency for the treatment of Coliforms and NOMs in Sai Gon river water, possibly due to its high surface area, crystallinity, and surface acidity. The application of this TNTs material for the treatment of waters from Dong Nai River and Mekong River also show high removal efficiency, which could meet the quality standard for supply water, suggesting the potential of TNTs for practical drinking water treatment.

Template assisted preparation of silicone (polydimethylsiloxane) elastomers and their self-cleaning application

The formation of self-cleaning functions on silicone elastomers is crucial for practical applications but still challenging. In this study, superhydrophobic silicone elastomers (SHSEs) with a 3D-hierarchical microstructure were achieved during the curing process with the assistance of a homemade template. The micro-nano structure formed by the assistance of the template makes the silicone elastomer surface achieve robust superhydrophobicity with a WCA at ∼163°, which can easily self-clean, removing surface contamination. Also, TiO2 particles transferred from the template endow the surface with photocatalytic functions, which can degrade organic pollutants under UV irradiation. After sandpaper abrasion, the formed SHSE can maintain its excellent hydrophobicity and show liquid repellency to wine and coffee droplets. The SHSEs with self-cleaning functions have promising applications in water treatment, medical facilities, and wearable devices.

Preparation of mesoporous nitrogen-doped titania comprising large crystallites with low thermal conductivity

Reducing the thermal conductivity (κ) of mesoporous N-doped titania (TiO2) is crucial for the development of TiO2-based materials that exhibit excellent electronic, photochemical, and thermoelectric properties. Mesopores can contribute to the reduction of κ via phonon scattering, and the scattering effect due to the randomness of crystal interfaces should be significantly reduced to clarify the role of mesopores in reducing thermal conductivity. Highly ordered mesoporous N-doped TiO2 comprising large crystallites was prepared with silica colloidal crystals as a template into which a Ti source was introduced, followed by calcination with urea. N-doped samples comprising large crystallites exhibiting random mesopores were also prepared and used for the investigation of the effects of the shape and arrangement of the mesopore on phonon scattering. The mesostructures of the two separately prepared N-doped TiO2 samples were retained after sintering at 873 K and 80 MPa to fabricate pellets. Furthermore, the effective suppression of the long mean-free-path phonon conduction by the thin pore walls at a nanometer scale thickness significantly reduced the thermal conductivities of both samples. The presence of ordered mesopores further contributed to the reduction of κ, which was probably due to the enhanced contribution of the backscattering of phonons caused by ordered pore wall surfaces.

A General Design Strategy Enabling the Synthesis of Hydrolysis-Resistant, Water-Stable Titanium(IV) Complexes

Despite its prevalence in the environment, the chemistry of the Ti⁴⁺ ion has long been relegated to organic solutions or hydrolyzed TiO₂ polymorphs. A knowledge gap in stabilizing molecular Ti⁴⁺ species in aqueous environments has prevented the use of this ion for various applications such as radioimaging, design of water-compatible metal-organic frameworks (MOFs), and aqueous-phase catalysis applications. Herein, we show a thorough thermodynamic screening of bidentate chelators with Ti⁴⁺ in aqueous solution, as well as computational and structural analyses of key compounds. In addition, the hexadentate analogues of catechol (benzene-1,2-diol) and deferiprone (3-hydroxy-1,2-dimethyl-4(1H)-pyridone), TREN-CAM and THP^Me respectively, were assessed for chelation of the ⁴⁵ Ti isotope (t_1/2 =3.08 h, β⁺ =85 %, E_β+ =439 keV) towards positron emission tomography (PET) imaging applications. Both were found to have excellent capacity for kit-formulation, and [⁴⁵ Ti]Ti-TREN-CAM was found to have remarkable stability in vivo.