Gold-decorated TiO2 nanofibrous hybrid for improved solar-driven photocatalytic pollutant degradation

TiO2-based photocatalysts from type-II to S-scheme heterojunction and their applications

Photocatalysis is a promising sustainable technology to remove organic pollution and convert solar energy into chemical energy. Titanium dioxide has drawn extensive attention in this field owing to its high activity under UV light, good chemical stability, large availability, low price and low toxicity. However, the poor quantum efficiency derived from fast electron/hole recombination, the limited utilization of sunlight, and a weak reducing ability still hinder its practical application. Among the modification strategies of TiO2 to enhance its performance, the construction of heterojunctions with other semiconductors is a powerful and versatile way to maximise the separation of photogenerated charge carriers and steer their transport toward enhanced efficiency and selectivity. Here, the research progress and current status of TiO2 modification are reviewed, focusing on heterojunctions. A rapid evolution of the understanding of the different charge transfer mechanisms is witnessed from traditional type II to the recently conceptualised S-scheme. Particular attention is paid to different synthetic approaches and interface engineering methods designed to improve and control the interfacial charge transfer, and several cases of TiO2 heterostructures with metal oxides, metal sulfides and carbon nitride are discussed. The application hotspots of TiO2-based photocatalysts are summarized, including hydrogen generation by water splitting, solar fuel production by CO2 conversion, and the degradation of organic water pollutants. Hints about less studied and emerging processes are also provided. Finally, the main issues and challenges related to the sustainability and scalability of photocatalytic technologies in view of their commercialization are highlighted, outlining future directions of development.

Structurally and surficially activated TiO2 nanomaterials for photochemical reactions

Renewable fuels and environmental remediation are of paramount importance in today's world due to escalating concerns about climate change, pollution, and the finite nature of fossil fuels. Transitioning to sustainable energy sources and addressing environmental pollution has become an urgent necessity. Photocatalysis, particularly harnessing solar energy to drive chemical reactions for environmental remediation and clean fuel production, holds significant promise among emerging technologies. As a benchmark semiconductor in photocatalysis, TiO2 photocatalyst offers an excellent solution for environmental remediation and serves as a key tool in energy conversion and chemical synthesis. Despite its status as the default photocatalyst, TiO2 suffers from drawbacks such as a high recombination rate of charge carriers, low electrical conductivity, and limited absorption in the visible light spectrum. This review provides an in-depth exploration of the fundamental principles of photocatalytic reactions and presents recent advancements in the development of TiO2 photocatalysts. It specifically focuses on strategic approaches aimed at enhancing the performance of TiO2 photocatalysts, including improving visible light absorption for efficient solar energy harvesting, enhancing charge separation and transportation efficiency, and ensuring stability for robust photocatalysis. Additionally, the review delves into the application of photodegradation and photocatalysis, particularly in critical processes such as water splitting, carbon dioxide reduction, nitrogen fixation, hydrogen peroxide generation, and alcohol oxidation. It also highlights the novel use of TiO2 in plastic polymerization and degradation, showcasing its potential for converting plastic waste into valuable chemicals and fuels, thereby offering sustainable waste management solutions. By addressing these essential areas, the review offers valuable insights into the potential of TiO2 photocatalysis for addressing pressing environmental and energy challenges. Furthermore, the review encompasses the application of TiO2 photochromic systems, expanding its scope to include other innovative research and applications. Finally, it addresses the underlying challenges and provides perspectives on the future development of TiO2 photocatalysts. Through addressing these issues and implementing innovative strategies, TiO2 photocatalysis can continue to evolve and play a pivotal role in sustainable energy and environmental applications.

Evaluation of the photocatalytic degradation mechanism of methylene blue using nascent and Ag+ ions-modified TiO2

In photocatalytic removal of organic pollutants, adsorption and degradation are two important processes that take place. Various instrumental techniques and trapping experiments have been used to identify the reactive species and the mechanism of photodegradation. The present work focuses on investigating the mechanism of photo-induced degradation from the comparative characterization of fresh and used samples, isotherm models, competitive adsorption, and desorption studies of pure and Ag+-modified TiO2 NPs. The comparative characterizations of fresh and used NPs were carried out with FT-IR, EDX, and XRF analyses after methylene blue (MB) degradation. The Ag+ doped TiO2 used in this study was fabricated using simple impregnation technique. The prepared NPs were characterized using techniques including XPS, XRD, SEM/EDX, XRF, UV-DRS, and pH point-zero charge analyses (pHPZC). The Ag+-modified TiO2 NPs showed improved efficiency compared to pure TiO2 NPs using normal compact fluorescent light (CFL). The Langmuir and Freundlich isotherm models were applied to test the adsorption behavior on the surface photocatalysts. The investigational data finest fitted to the Langmuir isotherms model compared to Freundlich model, suggesting the homogeneous monolayer adsorption followed by degradations. The competitive removal of MB in the presence of a photo-generated electrons trapper (Cd2+) was enhanced almost 3-folds (115 mg/L) compared to the removal from a single MB solution (40 mg/L). The characterization of the used samples as well as adsorption in the dark and negligible desorption of used samples support the involvement of the proposed photo-induced degradation mechanism.

Nanocomposites of Cu2O with plasmonic metals (Au, Ag): design, synthesis, and photocatalytic applications

Metal-semiconductor nanocomposites have been utilized in a multitude of applications in a wide array of fields, prompting substantial interest from different scientific sectors. Of particular interest are semiconductors paired with plasmonic metals due to the unique optical properties that arise from the individual interactions of these materials with light and the intercomponent movement of charge carriers in their heterostructure. This review focuses on the pairing of Cu2O semiconductor with strongly plasmonic metals, particularly Au and Ag. The design and synthesis of Au-Cu2O and Ag-Cu2O nanostructures, along with ternary nanostructures composed of the three components, are described, with in-depth discussion on the synthesis techniques and tunable parameters. The effects of compositing on the optical and electronic properties of the nanocomposites in the context of photocatalysis are discussed as well. Concluding remarks and potential areas for exploration are presented in the last section.

Enhancing mechanical and photocatalytic properties by surface microstructure regulation of TiO2 nanofiber membranes

In this work, TiO₂ nanofiber membrane (NFM) with a complete surface microstructure was prepared through regulating the surface microstructure of TiO₂ NFM by doping Zr. The crystal structures and morphological analyses indicated that the nanofiber membranes were consisted by disordered accumulation of Zr-doped TiO₂ nanofibers with a crack-free surface, small grain size and high aspect ratio. When the doping amount of Zr was 0.8 mL, the tensile strength of the doped membranes reached 1.27 MPa, which was 60.7% higher than that of pure TiO₂ NFM. The photocatalytic performance of Zr-doped TiO₂ NFM was evaluated by the degradation performance of Methylene orange (MO) under simulated sunlight irradiation. Compared with the undoped TiO₂ NFM, the 0.8-Zr/TiO₂ NFM presented a higher catalytic degradation efficiency (improved by 69.6%), and the photocatalytic performance maintained stable after five circulating. It was found that the doping of Zr ions effectively limited the surface crack size and grain size of TiO₂ nanofibers, thereby improving the tensile strength, and enhanced the surface area effect and carrier transfer efficiency of TiO₂ NFM. On the other hand, a narrow band-gap was obtained by doping a small amount of Zr ions, which expanded the visible light response range to improve the photocatalytic performance of TiO₂ nanofibers.

Au/Ti Synergistically Modified Supports Based on SiO2 with Different Pore Geometries and Architectures

New photocatalysts were obtained by immobilization of titanium and gold species on zeolite Y, hierarchical zeolite Y, MCM-48 and KIT-6 supports with microporous, hierarchical and mesoporous cubic structure. The obtained samples were characterized by X-ray diffraction (XRD), N2-physisorption, scanning and transmission electron microscopy (SEM/TEM), diffuse reflectance UV–Vis spectroscopy (DRUV-Vis), X-ray photoelectron spectroscopy (XPS), Raman and photoluminescence spectroscopy. The photocatalytic properties were evaluated in degradation of amoxicillin (AMX) from water, under UV (254 nm) and visible light (532 nm) irradiation. The higher degradation efficiency and best apparent rate constant were obtained under UV irradiation for Au-TiO2-KIT-6, while in the visible condition for the Au-TiO2-MCM-48 sample containing anatase, rutile and the greatest percent of Au metallic clusters were found (evidenced by XPS). Although significant values of amoxicillin degradation were obtained, total mineralization was not achieved. These results were explained by different reaction mechanisms, in which Au species act as e− trap in UV and e− generator in visible light.

Plasmonic Hybrid Nanostructures in Photocatalysis: Structures, Mechanisms, and Applications

(Sun)Light is an abundantly available sustainable source of energy that has been used in catalyzing chemical reactions for several decades now. In particular, studies related to the interaction of light with plasmonic nanostructures have been receiving increased attention. These structures display the unique property of localized surface plasmon resonance, which converts light of a specific wavelength range into hot charge carriers, along with strong local electromagnetic fields, and/or heat, which may all enhance the reaction efficiency in their own way. These unique properties of plasmonic nanoparticles can be conveniently tuned by varying the metal type, size, shape, and dielectric environment, thus prompting a research focus on rationally designed plasmonic hybrid nanostructures. In this review, the term "hybrid" implies nanomaterials that consist of multiple plasmonic or non-plasmonic materials, forming complex configurations in the geometry and/or at the atomic level. We discuss the synthetic techniques and evolution of such hybrid plasmonic nanostructures giving rise to a wide variety of material and geometric configurations. Bimetallic alloys, which result in a new set of opto-physical parameters, are compared with core-shell configurations. For the latter, the use of metal, semiconductor, and polymer shells is reviewed. Also, more complex structures such as Janus and antenna reactor composites are discussed. This review further summarizes the studies exploiting plasmonic hybrids to elucidate the plasmonic-photocatalytic mechanism. Finally, we review the implementation of these plasmonic hybrids in different photocatalytic application domains such as H₂ generation, CO₂ reduction, water purification, air purification, and disinfection.

Preparation of uniform gold nanoparticles of different quantity deposited on zinc oxide nanorods for photoelectrochemical water splitting

For the photocatalytic test, gold nanoparticles (AuNPs) were prepared using trisodium citrate dehydrate (TCD), following which they were combined on the surface of zinc oxide (ZnO) to prepare ZnO decorated with uniform AuNPs (ZnO/AuNP) photocatalysts. The photocatalytic performance with the ZnO/AuNP was estimated through the rhodamine B (RB) dye degradation under solar irradiation. ZnO/AuNP-30 showed the greatest photocatalytic performance, achieving dye degradation efficiency up to 78.65%. Photoelectrochemical (PEC) measurements were performed using the ZnO/AuNP photoanodes. With AuNP doping amounts of 10, 20, and 30 mL on the ZnO surface, photocurrent densities of 47.46, 63.74, and 68.64 mA cm^-2, respectively, were achieved at an applied voltage of 1.5 V. These values indicated that the doping of AuNPs on the ZnO surface is advantageous for enhancing its PEC water-splitting activity. The highest solar-to-hydrogen (STH) efficiency is 22% with the ZnO/AuNP-30 photoanode at an applied voltage of 0.88 V. The interfacial charge-transfer resistances at the interface were 40 and 2.2 kΩ cm² for the ZnO and ZnO/AuNP-30 photoanodes, respectively.

Fabrication and Photocatalytic Properties of Electrospun Fe-Doped TiO2 Nanofibers Using Polyvinyl Pyrrolidone Precursors

For the removal of pollutants, a modified TiO₂ photocatalyst is attracting attention. Fe-doped TiO₂ nanofibers were prepared through a combination of electrospinning and calcination. Morphological characterization of the sample was conducted using field-emission scanning electron and transmission electron microscopy. The crystal structure of each sample was analyzed using high-resolution transmission electron microscopy, selected area electron diffraction, and Fast Fourier Transform imaging. The average diameter of the Fe-doped TiO₂ nanofibers was measured to be 161.5 nm and that of the pure TiO₂ nanofibers was 181.5 nm. The crystal phase when heat treated at 350 °C was anatase for TiO₂ nanofibers and rutile for Fe-doped TiO₂ nanofibers. The crystal phase of the TiO₂ matrix was easily transitioned to rutile by Fe-doping. The photocatalytic performance of each sample was compared via the photodegradation of methylene blue and acid orange 7 under ultraviolet and visible light irradiation. In the Fe-doped TiO₂ nanofibers, photodegradation rates of 38.3% and 27.9% were measured under UV irradiation and visible light, respectively. Although other catalysts were not activated, the photodegradation rate in the Fe-doped TiO₂ nanofibers was 9.6% using acid orange 7 and visible light. For improved photocatalytic activity, it is necessary to study the concentration control of the Fe dopant.

Kombucha SCOBY Waste as a Catalyst Support

It is established that food waste can be repurposed to extend its lifecycle and decrease its carbon footprint. In this work, SCOBY (symbiotic culture of bacteria and yeast) waste from kombucha tea production has been repurposed as a catalyst support. Copper nanoparticles (Cu NPs) have been embedded in a piece of treated SCOBY via an in-situ method which enabled the catalyst, inCu/t-SCOBY, to be easily recycled. In addition, inCu/t-SCOBY catalyzed the full reduction of 4-nitrophenol in an excess of sodium borohydride (NaBH₄ ) within 20 minutes. After 6 additional catalytic cycles, the catalyst maintained up to 50% of its performance in the first cycle. Characterization of the catalyst has also been done to understand the mechanism of action and interactions occurring between t-SCOBY and Cu NPs. The results of this work clearly present a proof-of-concept in utilizing porous wastes materials such as SCOBY as catalyst supports, allowing metallic NPs to be efficacious and practical heterogenous catalysts.

Dual Plasmonic Au-Cu2-x S Nanocomposites: Design Strategies and Photothermal Properties

Coupling two different materials to create a hybrid nanostructured system is a powerful strategy for achieving synergistically enhanced properties and advanced functionalities. In the case of Au and Cu_2-x S, their combination on the nanoscale results in dual plasmonic Au-Cu_2-x S nanocomposites that exhibit intense photon absorption in both the visible and the near-infrared spectral ranges. Their strong light-absorbing properties translate to superior photothermal transduction efficiency, making them attractive in photothermal-based applications. There are several nanostructure configurations that are possible for the Au-Cu_2-x S system, and the successful fabrication of a particular architecture often requires a carefully planned synthetic strategy. In this Minireview, the different synthetic approaches that can be employed to produce rationally designed Au-Cu_2-x S nanocomposites are presented, with a focus on the experimental protocols that can lead to heterodimer, core-shell, reverse core-shell, and yolk-shell configurations. The photothermal behavior of these materials is also discussed, providing a glimpse of their potential use as photothermally active agents in therapeutic and theranostic applications.

Recent advances in photocatalytic decomposition of water and pollutants for sustainable application

Photoinduced reduction and oxidation, the important processes in photocatalytic water splitting and organic degradation, have generated increasing interest to address the energy and environmental issues. In this review, the recent developments in bandgap and interfacial engineering for enhanced light absorption, efficient charge separation and interfacial reaction are focused toward the applications in photocatalytic water splitting and organic degradation. In photoinduced reduction for hydrogen evolution, three major strategies are discussed: cocatalysts, sacrificial agents and heterojunctions. In photoinduced oxidation for organic degradation, three types of emerging pollutants of current concerns are highlighted: organic dyes, pharmaceuticals and volatile organic compounds. The key challenges of promising photocatalysts are discussed for future development and practical application.

Tunable thickness of mesoporous ZnO-coated metal nanoparticles for enhanced visible-light driven photoelectrochemical water splitting

The insufficient utilization of sunlight of ZnO, due to its broad band gap, results in low efficiency for photocatalytic hydrogen production. In this work, plasmonic noble metal nanoparticles (NPs) with different shapes (spheres and rods) were combined with mesoporous ZnO forming core-shell nanostructure to enhance the photocatalytic efficiency of ZnO in visible-light region. The photoelectrochemical water splitting activities of the metal@ZnO core-shell nanocomposites (NCs) were investigated. The photocurrent response of metal@ZnO NCs was found higher than pure ZnO or the mixture of metal NPs and ZnO ascribed to the effective charge transfer mechanism. It was also found that the photocurrent of metal@ZnO NCs was related to the thickness of ZnO and there was optimized shell for each kind of metal cores. Moreover, the introduction of Ag shell can get a higher photoelectrocatalytic efficiency compared to pure Au NPs core due to lower Schottky barrier between Ag and ZnO and wider extinction range in the visible light of Au@Ag NPs.

Solar-Powered Photodegradation of Pollutant Dyes Using Silver-Embedded Porous TiO2 Nanofibers

Titanium dioxide (TiO₂) nanomaterials have been ubiquitously investigated as a photocatalyst for organic contaminant treatment in wastewater due to their exemplary semiconductor properties. However, their huge band gap remains a barrier for visible light absorption, limiting their utility in practical applications. The incorporation of noble metals in the TiO₂ scaffold would help mitigate the problem via plasmonic resonance enhancements. Silver (Ag) is the chosen noble metal as it is relatively cheap and has great plasmonic effects. In this study, the use of electrospun Ag-embedded TiO₂ nanofibers as a photocatalyst is shown to be effective in decomposing rhodamine B and methyl orange dyes under a solar simulator in 3 h, which is more efficacious as opposed to pristine TiO₂ nanofibers. This showcases the potential of a simple and economic wastewater treatment system for the removal of organic pollutants.