Titanium glycolate-derived TiO2 nanomaterials: Synthesis and applications

Titanium oxide (TiO₂) is one of the most widely studied materials due to its fascinating properties and versatile applications in environmental and energy fields ranging from photocatalysis to solar cells and lithium ion batteries. The significance and variety of these applications have attracted great attention and spurred substantial progress in the synthesis and fundamental understanding of TiO₂-based nanomaterials, nanocomposites, and nanoderivatives. This review summarizes the recent advances in the design and preparation of TiO₂-based nanomaterials, nanocomposites, and nanoderivatives obtained from titanium glycolate precursor. Utilizing different fabrication strategies, titanium glycolate precursor with controllable morphology and size has been successfully produced, and it can be directly transformed into crystalline TiO₂ nanomaterials through diverse post-treatments, including calcination thermal-decomposition, and refluxing, hydrothermal, and microwave treatment-assisted hydrolysis. Furthermore, doped TiO₂, TiO₂-composites, and other derivatives could be simply achieved by adding additional chemicals during transformation. The favorable properties of the resulting TiO₂-based materials are also discussed, which are relevant to energy and environmental applications in the areas of dye-sensitized solar cells, lithium ion batteries, photocatalytic hydrogen evolution, photocatalytic CO₂ reduction, photocatalytic degradation, and adsorption removal of pollutants.     

IR‐Driven Ultrafast Transfer of Plasmonic Hot Electrons in Nonmetallic Branched Heterostructures for Enhanced H2 Generation

The ultrafast transfer of plasmon‐induced hot electrons is considered an effective kinetics process to enhance the photoconversion efficiencies of semiconductors through strong localized surface plasmon resonance (LSPR) of plasmonic nanostructures. Although this classical sensitization approach is widely used in noble‐metal–semiconductor systems, it remains unclear in nonmetallic plasmonic heterostructures. Here, by combining ultrafast transient absorption spectroscopy with theoretical simulations, IR‐driven transfer of plasmon‐induced hot electron in a nonmetallic branched heterostructure is demonstrated, which is fabricated through solvothermal growth of plasmonic W₁₈O₄₉ nanowires (as branches) onto TiO₂ electrospun nanofibers (as backbones). The ultrafast transfer of hot electron from the W₁₈O₄₉ branches to the TiO₂ backbones occurs within a timeframe on the order of 200 fs with very large rate constants ranging from 3.8 × 10¹² to 5.5 × 10¹² s^?1. Upon LSPR excitation by low‐energy IR photons, the W₁₈O₄₉/TiO₂ branched heterostructure exhibits obviously enhanced catalytic H₂ generation from ammonia borane compared with that of W₁₈O₄₉ nanowires. Further investigations by finely controlling experimental conditions unambiguously confirm that this plasmon‐enhanced catalytic activity arises from the transfer of hot electron rather than from the photothermal effect.     

Nanostructured Dielectric Fractals on Resonant Plasmonic Metasurfaces for Selective and Sensitive Optical Sensing of Volatile Compounds

Advances in the understanding and fabrication of plasmonic nanostructures have led to a plethora of unprecedented optoelectronic and optochemical applications. Plasmon resonance has found widespread use in efficient optical transducers of refractive index changes in liquids. However, it has proven challenging to translate these achievements to the selective detection of gases, which typically adsorb non‐specifically and induce refractive index changes below the detection limit. Here, it's shown that integration of tailored fractals of dielectric TiO₂ nanoparticles on a plasmonic metasurface strongly enhances the interaction between the plasmonic field and volatile organic molecules and provides a means for their selective detection. Notably, this superior optical response is due to the enhancement of the interaction between the dielectric fractals and the plasmonic metasurface for thickness of up to 1.8 μm, much higher than the evanescent plasmonic near‐field (≈30 nm) . Optimal dielectric–plasmonic structures allow measurements of changes in the refractive index of the gas mixture down to <8 × 10^?6 at room temperature and selective identification of three exemplary volatile organic compounds. These findings provide a basis for the development of a novel family of dielectric–plasmonic materials with application extending from light harvesting and photocatalysts to contactless sensors for noninvasive medical diagnostics.     

Structure,Synthesis, and Applications of TiO2 Nanobelts

TiO₂ semiconductor nanobelts have unique structural and functional properties, which lead to great potential in many fields, including photovoltaics, photocatalysis, energy storage, gas sensors, biosensors, and even biomaterials. A review of synthetic methods, properties, surface modification, and applications of TiO₂ nanobelts is presented here. The structural features and basic properties of TiO₂ nanobelts are systematically discussed, with the many applications of TiO₂ nanobelts in the fields of photocatalysis, solar cells, gas sensors, biosensors, and lithium‐ion batteries then introduced. Research efforts that aim to overcome the intrinsic drawbacks of TiO₂ nanobelts are also highlighted. These efforts are focused on the rational design and modification of TiO₂ nanobelts by doping with heteroatoms and/or forming surface heterostructures, to improve their desirable properties. Subsequently, the various types of surface heterostructures obtained by coupling TiO₂ nanobelts with metal and metal oxide nanoparticles, chalcogenides, and conducting polymers are described. Further, the charge separation and electron transfer at the interfaces of these heterostructures are also discussed. These properties are related to improved sensitivity and selectivity for specific gases and biomolecules, as well as enhanced UV and visible light photocatalytic properties. The progress in developments of near‐infrared‐active photocatalysts based on TiO₂ nanobelts is also highlighted. Finally, an outline of important directions of future research into the synthesis, modification, and applications of this unique material is given.     

Cobalt Plasmonic Superstructures Enable Almost 100% Broadband Photon Efficient CO2 Photocatalysis

The efficiency of heterogeneous photocatalysis for converting solar to chemical energy is low on a per photon basis mainly because of the difficulty of capturing and utilizing light across the entire solar spectral wavelength range. This challenge is addressed herein with a plasmonic superstructure, fashioned as an array of nanoscale needles comprising cobalt nanocrystals assembled within a sheath of porous silica grown on a fluorine tin oxide substrate. This plasmonic superstructure can strongly absorb sunlight through different mechanisms including enhanced plasmonic excitation by the hybridization of Co nanoparticles in close proximity, as well as inter- and intra-band transitions. With nearly 100% sunlight harvesting ability, it drives the photothermal hydrogenation of carbon dioxide with a 20-fold rate increase from the silica-supported cobalt catalyst. The present work bridges the gap between strong light-absorbing plasmonic superstructures with photothermal CO₂ catalysis toward the complete utilization of the solar energy.     

Light-induced ultrafast proton-coupled electron transfer responsible for H2 evolution on silver plasmonics

Light-driven proton-coupled electron transfer (PCET) reactions on nanoplasmonics would bring temporal control of their reactive pathways, in particular, prolong their charge separation state. Using a silver nano-hybrid plasmonic structure, we observed that optical excitation of Ag-localized surface plasmon instigated electron injection into TiO₂ conduction band and oxidation of isopropanol alcoholic functionality. Femtosecond transient infrared absorption studies show that electron transfer from Ag to TiO₂ occurs in ca. 650?fs, while IPA molecules near the Ag surface undergo an ultrafast bidirectional PCET step within 400?fs. Our work demonstrates that ultrafast PCET reaction plays a determinant role in prolonging charge separation state, providing an innovative strategy for visible-light photocatalysis with plasmonic nanostructures.     

Fundamentals of TiO2 Photocatalysis: Concepts,Mechanisms, and Challenges

Photocatalysis has been widely applied in various areas, such as solar cells, water splitting, and pollutant degradation. Therefore, the photochemical mechanisms and basic principles of photocatalysis, especially TiO₂ photocatalysis, have been extensively investigated by various surface science methods in the last decade, aiming to provide important information for TiO₂ photocatalysis under real environmental conditions. Recent progress that provides fundamental insights into TiO₂ photocatalysis at a molecular level is highlighted. Insights into the structures of TiO₂ and the basic principles of TiO₂ photocatalysis are discussed first, which provides the basic concepts of TiO₂ photocatalysis. Following this, details of the photochemistry of three important molecules (oxygen, water, methanol) on the model TiO₂ surfaces are presented, in an attempt to unravel the relationship between charge/energy transfer and bond breaking/forming in TiO₂ photocatalysis. Lastly, challenges and opportunities of the mechanistic studies of TiO₂ photocatalysis at the molecular level are discussed briefly, as well as possible photocatalysis models.     

p-n异质结NiO/TiO2纳米复合材料的构建及应用研究进展

TiO_2和NiO分别作为n型和p型半导体材料,通过复合构建p-n异质结NiO/TiO_2纳米复合材料可以促进光生电子-空穴对的分离,从而提高光电性能。综述了不同形貌的p-n异质结NiO/TiO_2纳米复合材料的构建,如纳米球、纳米棒和纳米带等,以及其在光催化、锂离子电池、染料敏化太阳能电池和传感器等领域中的最新研究进展。     

Periodical 2D Photonic–Plasmonic Au/TiOx Nanocavity Resonators for Photoelectrochemical Applications

Effective light trapping at the nanoscale is vital for efficient photoelectrochemical (PEC) applications. Photonic and plasmonic resonators are the two most promising approaches for this purpose, and the synergetic combination of these two resonators will tail the propagation lengths of incident light along with field enhancements, and thus presents further enhanced light‐trapping activity. Herein, a new hybrid photonic–plasmonic resonator is proposed through sputtering plasmonic Au nanoparticles (NPs) into the 2D photonic TiO_x nanocavity. Through facile control of the size of Au NPs, the matching of resonant wavelength of plasmonic Au NPs and photonic nanocavities maximize the light‐trapping intensity and thus further improve the PEC performance. Furthermore, for expanding the PEC applications, after functionalization of Au NPs with aptamer as a biomolecular recognition unit, a PEC aptasensor is also proposed and presents the highest sensitivity for antibiotic detection.     

Integration of Multiple Plasmonic and Co‐Catalyst Nanostructures on TiO2 Nanosheets for Visible‐Near‐Infrared Photocatalytic Hydrogen Evolution

Utilization of visible and near‐infrared light has always been the pursuit of photocatalysis research. In this article, an approach is developed to integrate dual plasmonic nanostructures with TiO₂ semiconductor nanosheets for photocatalytic hydrogen production in visible and near‐infrared spectral regions. Specifically, the Au nanocubes and nanocages used in this work can harvest visible and near‐infrared light, respectively, and generate and inject hot electrons into TiO₂. Meanwhile, Pd nanocubes that can trap the energetic electrons from TiO₂ and efficiently participate in the hydrogen evolution reaction are employed as co‐catalysts for improved catalytic activity. Enabled by this unique integration design, the hydrogen production rate achieved is dramatically higher than those of its counterpart structures. This work represents a step toward the rational design of semiconductor–metal hybrid structures for broad‐spectrum photocatalysis.     

Photocatalytic activity of nanostructured TiO2‐modified white cement

In this study, the photocatalysis of Alphazurine FG as a model water pollutant, by white Portland cement samples modified with different TiO₂ nanomaterials (TiO₂ in the form of Degussa P25, Millennium PC500, Millennium PC105 and anatase-Merck), was investigated. The pseudo first-order rate constants of Alphazurine FG photocatalysis were compared with the degradation rate by unmodified cement sample. A significant enhancement of the photocatalytic activity due to the addition of TiO₂ nanomaterials in the form of Degussa P25 to Portland cement was observed. The crystalline phase, average crystalline size and surface area of the photocatalyst were found to have a significant influence on the photocatalytic activity of the nanostructured TiO₂-modified cement samples. The photocatalysis of Alphazurine FG in the presence of TiO₂–P25/cement was experimentally studied through changing the UV light intensity and initial dye concentration. The electrical energy consumption of Alphazurine FG photocatalysis was estimated at different initial dye concentrations. The results open important perspective applications in the field of eco-friendly construction applications.     

Plasmon Modes Induced by Anisotropic Gap Opening in Au@Cu2O Nanorods

Integration of semiconductors with noble metals to form heteronanostructures can give rise to many interesting plasmonic and electronic properties. A number of such heteronanostructures have been demonstrated comprising noble metals and n‐type semiconductors, such as TiO₂, ZnO, SnO₂, Fe₃O₄, and CuO. In contrast, reports on heteronanostructures made of noble metals and p‐type semiconductors are scarce. Cu₂O is an unintentional p‐type semiconductor with unique properties. Here, the uniform coating of Cu₂O on two types of Au nanorods and systematic studies of the plasmonic properties of the resultant core–shell heteronanostructures are reported. One type of Au nanorods is prepared by seed‐mediated growth, and the other is obtained by oxidation of the as‐prepared Au nanorods. The (Au nanorod)@Cu₂O nanostructures produced from the as‐prepared nanorods exhibit two transverse plasmon peaks, whereas those derived from the oxidized nanorods display only one transverse plasmon peak. Through electrodynamic simulations the additional transverse plasmon peak is found to originate from a discontinuous gap formed at the side of the as‐prepared nanorods. The existence of the gap is verified and its formation mechanism is unraveled with additional experiments. The results will be useful for designing metal–semiconductor heteronanostructures with desired plasmonic properties and therefore also for exploring plasmon‐enhanced applications in photocatalysis, solar‐energy harvesting, and biotechnologies.     

Enhanced nanocatalysts

Rapid development of nanofabrication techniques has created many different types of advanced nanosized semiconductors. Photocatalytic materials used to degrade organic and inorganic pollutants now include, in addition to TiO₂, ZnO, Fe₂O₃, WO₃, MoS₂, and CdS. Nanoparticles’ unique properties, e.g. surface to volume ratio and quantum effects, continue to improve and expand photocatalysis’ role in areas like environmental remediation, odor control, sterilization, and renewable energy. Controlling semiconductor size, shape, composition, and microstructure promises to benefit future research and applications in these fields. This review examines recent advances at the interface of nanoscience and photocatalysis, especially pertaining to nanocatalyst enhancements, for current and future environmental applications.     

Multichannel Charge Transfer and Mechanistic Insight in Metal Decorated 2D–2D Bi2WO6–TiO2 Cascade with Enhanced Photocatalytic Performance

Promising semiconductor‐based photocatalysis toward achieving efficient solar‐to‐chemical energy conversion is an ideal strategy in response to the growing worldwide energy crisis, which however is often practically limited by the insufficient photoinduced charge‐carrier separation. Here, a rational cascade engineering of Au nanoparticles (NPs) decorated 2D/2D Bi₂WO₆–TiO₂ (B–T) binanosheets to foster the photocatalytic efficiency through the manipulated flow of multichannel‐enhanced charge‐carrier separation and transfer is reported. Mechanistic characterizations and control experiments, in combination with comparative studies over plasmonic Au/Ag NPs and nonplasmonic Pt NPs decorated 2D/2D B–T composites, together demonstrate the cooperative synergy effect of multiple charge‐carrier transfer channels in such binanosheets‐based ternary composites, including Z‐scheme charge transfer, “electron sink,” and surface plasmon resonance effect, which integratively leads to the boosted photocatalytic performance.     

Alternative Plasmonic Materials: Beyond Gold and Silver

Materials research plays a vital role in transforming breakthrough scientific ideas into next‐generation technology. Similar to the way silicon revolutionized the microelectronics industry, the proper materials can greatly impact the field of plasmonics and metamaterials. Currently, research in plasmonics and metamaterials lacks good material building blocks in order to realize useful devices. Such devices suffer from many drawbacks arising from the undesirable properties of their material building blocks, especially metals. There are many materials, other than conventional metallic components such as gold and silver, that exhibit metallic properties and provide advantages in device performance, design flexibility, fabrication, integration, and tunability. This review explores different material classes for plasmonic and metamaterial applications, such as conventional semiconductors, transparent conducting oxides, perovskite oxides, metal nitrides, silicides, germanides, and 2D materials such as graphene. This review provides a summary of the recent developments in the search for better plasmonic materials and an outlook of further research directions.     

TiO2-based photocatalysts prepared by oxidation of TiN nanoparticles and their photocatalytic activities under visible light illumination

To develop TiO₂-based photocatalysts with visible light activity for better solar energy utilization, a simple flash oxidation method was developed by calcining commercial TiN nanoparticle to prepare N-doped TiO₂ photocatalyst and TiN/TiO₂ composite photocatalysts through the modulation of the calcination time and temperature. It was found that more energy and processing time were needed to prepare N-doped TiO₂ photocatalyst than that of TiN/TiO₂ composite photocatalyst during this process, while TiN/TiO₂ composite photocatalyst had a better visible light absorption/photocatalytic performance than that of N-doped TiO₂ photocatalyst prepared from the oxidation of the same TiN precursor. Thus, the preparation of TiN/TiO₂ composite photocatalyst from TiN precursor should be a more preferred approach than the preparation of N-doped TiO₂ photocatalyst for visible-light-activated photocatalysis for its cost-effectiveness.     

TiO2 nanotubes: Self-organized electrochemical formation, properties and applications

The present paper gives an overview and review on self-organized TiO₂ nanotube layers and other transition metal oxide tubular structures grown by controlled anodic oxidation of a metal substrate. We describe mechanistic aspects of the tube growth and discuss the electrochemical conditions that need to be fulfilled in order to synthesize these layers. Key properties of these highly ordered, high aspect ratio tubular layers are discussed. In the past few years, a wide range of functional applications of the layers have been explored ranging from photocatalysis, solar energy conversion, electrochromic effects over using the material as a template or catalyst support to applications in the biomedical field. A comprehensive view on state of the art is provided.     

Fabricating a Homogeneously Alloyed AuAg Shell on Au Nanorods to Achieve Strong,Stable, and Tunable Surface Plasmon Resonances

Colloidal metal nanocrystals with strong, stable, and tunable localized surface plasmon resonances (SPRs) can be useful in a corrosive environment for many applications including field‐enhanced spectroscopies, plasmon‐mediated catalysis, etc. Here, a new synthetic strategy is reported that enables the epitaxial growth of a homogeneously alloyed AuAg shell on Au nanorod seeds, circumventing the phase segregation of Au and Ag encountered in conventional synthesis. The resulting core–shell structured bimetallic nanorods (AuNR@AuAg) have well‐mixed Au and Ag atoms in their shell without discernible domains. This degree of mixing allows AuNR@AuAg to combine the high stability of Au with the superior plasmonic activity of Ag, thus outperforming seemingly similar nanostructures with monometallic shells (e.g., Ag‐coated Au NRs (AuNR@Ag) and Au‐coated Au NRs (AuNR@Au)). AuNR@AuAg is comparable to AuNR@Ag in plasmonic activity, but that it is markedly more stable toward oxidative treatment. Specifically, AuNR@AuAg and AuNR@Ag exhibit similarly strong signals in surface‐enhanced Raman spectroscopy that are some 30‐fold higher than that of AuNR@Au. When incubated with a H₂O₂ solution (0.5 m ), the plasmonic activity of AuNR@Ag immediately and severely decayed, whereas AuNR@AuAg retained its activity intact. Moreover, the longitudinal SPR frequency of AuNR@AuAg can be tuned throughout the red wavelengths (≈620–690 nm) by controlling the thickness of the AuAg alloy shell. The synthetic strategy is versatile to fabricate AuAg alloyed shells on different shaped Au, with prospects for new possibilities in the synthesis and application of plasmonic nanocrystals.     

Hydrogen-Doping-Induced Metal-Like Ultrahigh Free-Carrier Concentration in Metal-Oxide Material for Giant and Tunable Plasmon Resonance

The practical utilization of plasmon-based technology relies on the ability to find high-performance plasmonic materials other than noble metals. A key scientific challenge is to significantly increase the intrinsically low concentration of free carriers in metal-oxide materials. Here, a novel electron–proton co-doping strategy is developed to achieve uniform hydrogen doping in metal-oxide MoO₃ at mild conditions, which creates a metal-like ultrahigh free-carrier concentration approaching that of noble metals (10²¹ cm⁻³ in H_1.68MoO₃ versus 10²² cm⁻³ in Au/Ag). This bestows giant and tunable plasmonic resonances in the visible region to this originally semiconductive material. Using ultrafast spectroscopy characterizations and first-principle simulations, the formation of a quasi-metallic energy band structure that leads to long-lived and strong plasmonic field is revealed. As verified by the surface-enhanced Raman spectra (SERS) of rhodamine 6G molecules on H_xMoO₃, the SERS enhancement factor reaches as high as 1.1 × 10⁷ with a detection limit at concentration as low as 1 × 10⁻⁹ mol L⁻¹, representing the best among the hitherto reported non-metal systems. The findings not only provide a set of metal-like semiconductor materials with merits of low cost, tunable electronic structure, and plasmonic resonance, but also a general strategy to induce tunable ultrahigh free-carrier concentration in non-metal systems.     

Green and reusable Ag/AgCl-TiO2 nanocomposites for visible light-triggered dye degradation

Ag/AgCl-TiO₂ plasmonic nanocomposites (NCs) are endowed with excellent visible-light photocatalytic activity. However, only a few studies investigated environmentally friendly approaches to their synthesis. In this work, Ag/AgCl-TiO₂ NCs at five different compositions were prepared in a single-step process by a green and cost-effective route, using Satureja khuzistanica Jamzad aqueous extract. The role of the aqueous plant extract as a reducing and stabilizing agent, and the formation of the NCs were evidenced by several techniques, including FT-IR, EDS, SEM, HRTEM, elemental mapping, and XRD. The morphological analysis demonstrated that the NCs formed nanoaggregates with an average size of 30 nm. The synthesized Ag/AgCl-TiO₂ NCs displayed a remarkable photoactivity in the visible light region, as confirmed by the significantly higher degradation rates of methyl orange (MO) compared to TiO₂. In particular, the 15% Ag/TiO₂ molar ratio sample revealed a MO degradation efficiency higher than 99% under visible light, and retained high photocatalytic activity even after several degradations runs. Overall, the green, cost-effective, and scalable synthesis of Ag/AgCl-TiO₂ NCs herein reported provides a novel, more sustainable strategy for the high-efficiency modification of TiO₂ photocatalyst in engineering and other environmental applications.