Plasmonic silver quantum dots coupled with hierarchical TiO2 nanotube arrays photoelectrodes for efficient visible-light photoelectrocatalytic hydrogen evolution

Etching processes of Si(111) surfaces with bunched steps via atomic hydrogen irradiation intermittently observed by atomic force microscopy

Silicon (Si) (111)-(7 × 7) surfaces with wide terraces and bunched steps were passivated with atomic hydrogen (H) and subsequently etched by irradiation of atomic H. The atomic H can suppress the reactivity of Si surfaces by terminating the dangling bonds of Si surfaces. Meanwhile, atomic H can break the periodic atomic structures such as (7 × 7) on the Si surfaces. In the present study, we intermittently repeated the atomic H irradiation to the H-terminated Si(111) surface and frequency-modulation atomic force microscopy observation in a conventional vacuum chamber. When H2gas was introduced to the cracker with a gas flow rate of 1 sccm (≈1.7 × 10-8m3·s-1), corresponding to an atomic H flux of 2.5 × 1016cm-2·s-1, one hour of the atomic H irradiation increased the roughness of the terrace from 1.4 nm up to 1.9 nm. With increasing the gas flow rate to 10 sccm, pits were formed on the surface and enlarged to 20-40 nm diameters across the bunched step with shallow flat bottoms and non-uniform winding edges. The surface etching probably starts from the adsorption of H on the lower-coordinated Si atoms exposed at the bunched steps. The side walls of pits seemed to consist of {110} and {100} facets that are readily etched due to the smaller number of the back bonds of Si atoms.

Recent Progress of Ion-Modified TiO2 for Enhanced Photocatalytic Hydrogen Production

Harnessing solar energy to produce hydrogen through semiconductor-mediated photocatalytic water splitting is a promising avenue to address the challenges of energy scarcity and environmental degradation. Ever since Fujishima and Honda's groundbreaking work in photocatalytic water splitting, titanium dioxide (TiO2) has garnered significant interest as a semiconductor photocatalyst, prized for its non-toxicity, affordability, superior photocatalytic activity, and robust chemical stability. Nonetheless, the efficacy of solar energy conversion is hampered by TiO2's wide bandgap and the swift recombination of photogenerated carriers. In pursuit of enhancing TiO2's photocatalytic prowess, a panoply of modification techniques has been explored over recent years. This work provides an extensive review of the strategies employed to augment TiO2's performance in photocatalytic hydrogen production, with a special emphasis on foreign dopant incorporation. Firstly, we delve into metal doping as a key tactic to boost TiO2's capacity for efficient hydrogen generation via water splitting. We elaborate on the premise that metal doping introduces discrete energy states within TiO2's bandgap, thereby elevating its visible light photocatalytic activity. Following that, we evaluate the role of metal nanoparticles in modifying TiO2, hailed as one of the most effective strategies. Metal nanoparticles, serving as both photosensitizers and co-catalysts, display a pronounced affinity for visible light absorption and enhance the segregation and conveyance of photogenerated charge carriers, leading to remarkable photocatalytic outcomes. Furthermore, we consolidate perspectives on the nonmetal doping of TiO2, which tailors the material to harness visible light more efficiently and bolsters the separation and transfer of photogenerated carriers. The incorporation of various anions is summarized for their potential to propel TiO2's photocatalytic capabilities. This review aspires to compile contemporary insights on ion-doped TiO2, propelling the efficacy of photocatalytic hydrogen evolution and anticipating forthcoming advancements. Our work aims to furnish an informative scaffold for crafting advanced TiO2-based photocatalysts tailored for water-splitting applications.

Surface plasmon resonance Bismuth-modified NH2-UiO-66 with enhanced photocatalytic tetracycline degradation performance

For nearly a century, the misuse of antibiotics has gradually polluted water and threatened human health. Photocatalysis is considered an efficient way to remove antibiotics from water. Zirconium-based metal-organic frameworks have attracted much attention as promising photocatalysts for the degradation of antibiotics. However, single Zirconium-based metal-organic frameworks can still not achieve a more satisfactory photocatalytic efficiency, due to poor light absorption and charge separation efficiency. In this study, a novel metal-loaded metal-organic frameworks material was explored. As a potential photocatalytic material, the performance of NH2-UiO-66 in the photocatalytic degradation of tetracycline was greatly improved just by the loading of a single metal. Bismuth/NH2-UiO-66 photocatalysts of various compositions were physicochemically (TEM, SEM, XRD, XPS, BET, FTIR, UV-VIS, PL), and electrochemically (electrochemical impedance spectroscopy, photocurrent response) characterized. We evaluated the photocatalytic performance of Bismuth/NH2-UiO-66 composites by measuring their ability towards tetracycline decomposition in simulated sunlight irradiation conditions. The experimental results indicated that the introduction of metal Bismuth significantly boosts the photocatalytic activity of the composite catalysts. The final degradation rate of Bismuth/NH2-UiO-66 for tetracycline was found to be 95.8%, namely 2.7 times higher than pure NH2-UiO-66. This behavior is due to the surface plasmon resonance effect of Bismuth, which ameliorates the photocatalyst's electron-hole separation and strengthens the charge transfer. Apart from that, the presence of Bismuth magnifies the visible-light absorption range of Bismuth/NH2-UiO-66. In this study, an innovative approach for designing efficient and cost-effective metal-modified metal-organic frameworks photocatalysts is proposed.

Research status and prospect of nano silver (Ag)-modified photocatalytic materials for degradation of organic pollutants

Nano silver (Ag) was metallic Ag monomers with particle size to the nanoscale. Photocatalyst was a kind of semiconductor material with photocatalytic function. Loading precious metal Ag onto semiconductor surfaces by microwave, laser-induced, solvent-thermal and hydrothermal methods could capture photogenerated electrons, reduced the compounding rate of holes and photogenerated electrons during the photocatalytic process, thereby improving the electron transfer efficiency of photocatalysis and enhancing the absorption of visible light by silver nanoparticles through the plasma resonance effect. The highly reactive free radicals produced by photocatalysts were used in the organic degradation process to degrade organic matter into inorganic matter and was a faster, more efficient and less polluting method of pollutant degradation, which has attracted a lot of attention from researchers. This review discussed the modification of various types of photocatalysts by nano Ag through different methods. The photocatalytic degradation of dyes, antibiotics and persistent organic pollutants by different modified composites was also analyzed. This review covered the several ways and means in which nano Ag has modified diverse photocatalytic materials as well as the photocatalytic degradation of dyes, antibiotics and persistent organic pollutants. This review identified the drawbacks of the existing nano Ag-modified photocatalytic materials, including their low yield and lack of recyclability, and it also offered suggestions for potential future directions for their improvement. The purpose of this review was to further research on the technology of nano Ag-modified photocatalytic materials and to encourage the creation of new modified photocatalytic nanomaterials for the treatment of organic pollutant degradation.

Surface plasmon-driven photoelectrochemical water splitting of a Ag/TiO2 nanoplate photoanode

A silver/titanium dioxide nanoplate (Ag/TiO2 NP) photoelectrode was designed and fabricated from vertically aligned TiO2 nanoplates (NP) decorated with silver nanoparticles (NPs) through a simple hydrothermal synthesis and electrodeposition route. The electrodeposition times of Ag NPs on the TiO2 NP were crucial for surface plasmon-driven photoelectrochemical (PEC) water splitting performance. The Ag/TiO2 NP at the optimal deposition time of 5 min with a Ag element content of 0.53 wt% demonstrated a remarkably high photocurrent density of 0.35 mA cm-2 at 1.23 V vs. RHE under AM 1.5G illumination, which was 5 fold higher than that of the pristine TiO2 NP. It was clear that the enhanced light absorption properties and PEC performance for Ag/TiO2 NP could be effectively adjusted by simply controlling the loading amounts of metallic Ag NPs (average size of 10-30 nm) at different electrodeposition times. The superior PEC performance of the Ag/TiO2 NP photoanode was attributed to the synergistic effects of the plasmonic Ag NPs and the TiO2 nanoplate. Interestingly, the plasmonic effect of Ag NPs not only increased the visible-light response (λ max = 570 nm) of TiO2 but also provided hot electrons to promote photocurrent generation and suppress charge recombination. Importantly, this study offers a potentially efficient strategy for the design and fabrication of a new type of TiO2 hybrid nanostructure with a plasmonic enhancement for PEC water splitting.

Recent Advances in Plasmonic Photocatalysis Based on TiO2 and Noble Metal Nanoparticles for Energy Conversion, Environmental Remediation, and Organic Synthesis

Plasmonic photocatalysis has emerged as a prominent and growing field. It enables the efficient use of sunlight as an abundant and renewable energy source to drive a myriad of chemical reactions. For instance, plasmonic photocatalysis in materials comprising TiO₂ and plasmonic nanoparticles (NPs) enables effective charge carrier separation and the tuning of optical response to longer wavelength regions (visible and near infrared). In fact, TiO₂ -based materials and plasmonic effects are at the forefront of heterogeneous photocatalysis, having applications in energy conversion, production of liquid fuels, wastewater treatment, nitrogen fixation, and organic synthesis. This review aims to comprehensively summarize the fundamentals and to provide the guidelines for future work in the field of TiO₂ -based plasmonic photocatalysis comprising the above-mentioned applications. The concepts and state-of-the-art description of important parameters including the formation of Schottky junctions, hot electron generation and transfer, near field electromagnetic enhancement, plasmon resonance energy transfer, scattering, and photothermal heating effects have been covered in this review. Synthetic approaches and the effect of various physicochemical parameters in plasmon-mediated TiO₂ -based materials on performances are discussed. It is envisioned that this review may inspire and provide insights into the rational development of the next generation of TiO₂ -based plasmonic photocatalysts with target performances and enhanced selectivities.

Optimization of N doping in TiO2 nanotubes for the enhanced solar light mediated photocatalytic H2 production and dye degradation

Herein, we report the optimization of nitrogen (N) doping in TiO₂ nanotubes to achieve the enhanced photocatalytic efficiencies in degradation of dye and H₂ gas evolution under solar light exposure. TiO₂ nanotubes have been produced via hydrothermal process and N doping has been tuned by varying the concentration of urea, being the source for N, by solid-state dispersion process. The structural analysis using XRD showed the characteristic occupancy of N into the structure of TiO₂ and the XPS studies showed the existence of Ti-N-Ti network in the N-doped TiO₂ nanotubes. The obtained TEM images showed the formation of 1D tube-like structure of TiO₂. Diffuse reflectance UV-Vis absorption spectra demonstrated that the N-doped TiO₂ nanotubes can efficiently absorb the photons of UV-Vis light of the solar light. The optimized N-doped TiO₂ nanotubes (TiO₂ nanotubes vs urea @ 1:1 ratio) showed the highest degradation efficiency over methyl orange dye (∼91% in 90 min) and showed the highest rate of H₂ evolution (∼19,848 μmol h^-1.g^-1) under solar light irradiation. Further, the recyclability studies indicated the excellent stability of the photocatalyst for the durable use in both the photocatalytic processes. The observed efficiency was ascribed to the optimized doping of N-atoms into the lattices of TiO₂, which enhanced the optical properties by forming new energy levels of N atoms near the valence band maximum of TiO₂, thereby increased the overall charge separation and recombination resistance in the system. The improved reusability of photocatalyst is attributed to the doping-induced structural stability in N-doped TiO₂. From the observed results, it has been recognized that the established strategy could be promising for synthesizing N-doped TiO₂ nanotubes with favorable structural, optical and photocatalytic properties towards dye degradation and hydrogen production applications.

Physical Origin of Diminishing Photocatalytic Efficiency for Recycled TiO2 Nanotubes and Ag-Loaded TiO2 Nanotubes in Organic Aqueous Solution

Arrays of titania nanotubes (TiO2NTs) were developed by electrochemical anodization and doped with silver on their surface by photodeposition to achieve TiO2NTs/Ag. It is found that only anatase TiO2NTs were formed, with the preferential growth direction perpendicular to the titanium substrate, and with the length and diameter of ~2 µm and 90–120 nm, respectively. The presence of Ag on the surface of TiO2NTs was also confirmed. The TiO2NTs and TiO2NTs/Ag were used as photocatalysts to decolorize the methylene blue (MB) aqueous solution. The photodegradation efficiency (PDE) is as high as 83% for TiO2NTs and 98% for TiO2NTs/Ag photocatalysts. This work focused on the investigation of the stability and recyclability of these photocatalysts in terms of efficiency and its physical origin by surface analysis using X-ray photoelectron spectroscopy (XPS). It is found that PDE diminishes from 83% to 76% in TiO2NTs upon eight recycling runs and from 98% to 80% in TiO2NTs/Ag upon six recycling runs. The XPS analysis revealed that the physical origin of diminishing efficiency is the carbon contamination on the surface of recycled TiO2NTs and a combination of carbon contamination and Ag leaching in recycled TiO2NTs/Ag.

Synthesis and characterization of Ag-loaded p-type TiO2 for adsorption and photocatalytic degradation of tetrabromobisphenol A

A p-type TiO₂ with Ti vacancies (D-TiO₂ ) was synthesized by a facile solvothermal treatment, and Ag/TiO₂ with different Ag loading amount was prepared through a photo-reduction deposition method. The samples were characterized through scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The adsorption and photocatalytic characteristics of tetrabromobisphenol A (TBBPA) on D-TiO₂ and Ag/TiO₂ were investigated. The adsorption of TBBPA on Ag/TiO₂ was significantly enhanced and was five times greater than that of pure TiO₂ . The increase in pH significantly inhibited the adsorption of TBBPA. The 2%-Ag/TiO₂ nearly completely degraded TBBPA in 10 min under UV-Vis light (λ > 360 nm), and the apparent reaction rate constant (k_app ) reached 0.63 min^-1 . The significantly enhanced UV-Vis light catalytic properties of the Ag/TiO₂ in comparison with that of TiO₂ were attributed to the increased adsorption capacity and electron transfer ability of the Ag/TiO₂ . Free radical trap experiments results showed that holes and superoxide radicals play a major role in the catalytic degradation of TBBPA by Ag/TiO₂ . Moreover, the Ag/TiO₂ catalyst exhibits high stability during TBBPA degradation even after three cycles. PRACTITIONER POINTS: Ti-defected TiO₂ and Ag/TiO₂ were synthesized using a solvothermal and photo-reduction deposition, respectively. Ag/TiO₂ exhibited outstanding adsorption and photocatalytic activity for TBBPA removal under UV-Vis light. Holes and superoxide radicals play a major role in the photocatalytic degradation of TBBPA.

Co3O4-Ag photocatalysts for the efficient degradation of methyl orange

In this paper, a series of Co3O4-Ag photocatalysts with different Ag loadings were synthesized by facile hydrothermal and in situ photoreduction methods and fully characterized by XRD, SEM, TEM, FTIR spectroscopy, XPS, UV-vis and PL techniques. The catalysts were used for the degradation of methyl orange (MO). Compared with the pure Co3O4 catalyst, the Co3O4-Ag catalysts showed better activity; among these, the Co3O4-Ag-0.3 catalyst demonstrated the most efficient activity with 96.4% degradation efficiency after 30 h UV light irradiation and high degradation efficiency of 99.1% after 6 h visible light irradiation. According to the corresponding dynamics study under UV light irradiation, the photocatalytic efficiency of Co3O4-Ag-0.3 was 2.72 times higher than that of Co3O4 under identical reaction conditions. The excellent photocatalytic activity of Co3O4-Ag can be attributed to the synergistic effect of strong absorption under UV and visible light, reduced photoelectron and hole recombination rate, and decreased band gap due to Ag doping. Additionally, a possible reaction mechanism over the Co3O4-Ag photocatalysts was proposed and explained.

Electrochemical Engineering of Nanoporous Materials for Photocatalysis: Fundamentals, Advances, and Perspectives

Photocatalysis comprises a variety of light-driven processes in which solar energy is converted into green chemical energy to drive reactions such as water splitting for hydrogen energy generation, degradation of environmental pollutants, CO2 reduction and NH3 production. Electrochemically engineered nanoporous materials are attractive photocatalyst platforms for a plethora of applications due to their large effective surface area, highly controllable and tuneable light-harvesting capabilities, efficient charge carrier separation and enhanced diffusion of reactive species. Such tailor-made nanoporous substrates with rational chemical and structural designs provide new exciting opportunities to develop advanced optical semiconductor structures capable of performing precise and versatile control over light–matter interactions to harness electromagnetic waves with unprecedented high efficiency and selectivity for photocatalysis. This review introduces fundamental developments and recent advances of electrochemically engineered nanoporous materials and their application as platforms for photocatalysis, with a final prospective outlook about this dynamic field.

Electrochemical Fabrication of rGO-embedded Ag-TiO2 Nanoring/Nanotube Arrays for Plasmonic Solar Water Splitting

Effective utilization of hot electrons generated from the decay of surface plasmon resonance in metal nanoparticles is conductive to improve solar water splitting efficiency. Herein, Ag nanoparticles and reduced graphene oxide (rGO) co-decorated hierarchical TiO2 nanoring/nanotube arrays (TiO2 R/T) were facilely fabricated by using two-step electrochemical anodization, electrodeposition, and photoreduction methods. Comparative studies were conducted to elucidate the effects of rGO and Ag on the morphology, photoresponse, charge transfer, and photoelectric properties of TiO2. Firstly, scanning electron microscope images confirm that the Ag nanoparticles adhered on TiO2 R/T and TiO2 R/T-rGO have similar diameter of 20 nm except for TiO2 R-rGO/T. Then, the UV-Vis DRS and scatter spectra reveal that the optical property of the Ag-TiO2 R/T-rGO ternary composite is enhanced, ascribing to the visible light absorption of plasmonic Ag nanoparticles and the weakening effect of rGO on light scattering. Meanwhile, intensity-modulated photocurrent spectroscopy and photoluminescence spectra demonstrate that rGO can promote the hot electrons transfer from Ag nanoparticles to Ti substrate, reducing the photogenerated electron-hole recombination. Finally, Ag-TiO2 R/T-rGO photoanode exhibits high photocurrent density (0.98 mA cm-2) and photovoltage (0.90 V), and the stable H2 evolution rate of 413 μL h-1 cm-2 within 1.5 h under AM 1.5 which exceeds by 1.30 times than that of pristine TiO2 R/T. In line with the above results, this work provides a reliable route synergizing rGO with plasmonic metal nanoparticles for photocatalysis, in which, rGO presents a broad absorption spectrum and effective photogenerated electrons transfer.

Efficient photoelectrochemical water-splitting over carbon membrane linked Au and TiO2 nanotube arrays film based on multiple carriers transport paths

Herein, a carbon membrane and Au nanoparticles were combined to improve the efficiency of photoelectrocatalytic water splitting over a TiO₂ nanotube arrays film (TiO₂ NTAF). Two different ternary nanostructures were constructed by hydrothermal and photochemical deposition processes. One was carbon membrane bridged Au nanoparticles and TiO₂ nanotube arrays (Au/C/TiO₂ NTAF), while the other was Au nanoparticles sandwiched between carbon membrane and TiO₂ nanotube arrays (C/Au/TiO₂ NTAF). The two structures exhibited enhanced visible light harvesting ability, but they showed distinctly different photoelectric properties. The unique microstructure of C/Au/TiO₂ NTAF resulted in a much higher reduction of the electron cloud density of Au nanoparticles as carrier recombination centers, which were responsible for its poor photoelectrochemical performance. However, a champion photocurrent of Au/C/TiO₂ NTAF was observed (0.984 mA cm^-2), indicating superior ability of the photoelectrocatalytic water splitting. The great enhancement was attributed to multiple carriers transport paths, which can efficiently utilize the sensitization of the carbon membrane and the surface plasmon resonance effect of the Au nanoparticles.

Seebeck-voltage-triggered self-biased photoelectrochemical water splitting using HfOx/SiOx bi-layer protected Si photocathodes

The use of a photoelectrochemical device is an efficient method of converting solar energy into hydrogen fuel via water splitting reactions. One of the best photoelectrode materials is Si, which absorbs a broad wavelength range of incident light and produces a high photocurrent level (~44 mA·cm-2). However, the maximum photovoltage that can be generated in single-junction Si devices (~0.75 V) is much lower than the voltage required for a water splitting reaction (>1.6 V). In addition, the Si surface is electrochemically oxidized or reduced when it comes into direct contact with the aqueous electrolyte. Here, we propose the hybridization of the photoelectrochemical device with a thermoelectric device, where the Seebeck voltage generated by the thermal energy triggers the self-biased water splitting reaction without compromising the photocurrent level at 42 mA cm-2. In this hybrid device p-Si, where the surface is protected by HfOx/SiOx bilayers, is used as a photocathode. The HfOx exhibits high corrosion resistance and protection ability, thereby ensuring stability. On applying the Seebeck voltage, the tunneling barrier of HfOx is placed at a negligible energy level in the electron transfer from Si to the electrolyte, showing charge transfer kinetics independent of the HfOx thickness. These findings serve as a proof-of-concept of the stable and high-efficiency production of hydrogen fuel by the photoelectrochemical-thermoelectric hybrid devices.

Efficient Photocatalytic Fuel Cell via Simultaneous Visible-Photoelectrocatalytic Degradation and Electricity Generation on a Porous Coral-like WO3/W Photoelectrode

Photocatalytic fuel cells (PFCs) have proven to be effective for generating electricity and degrading pollutants with a goal to resolve environmental and energy problems. However, the degradation of persistent organic pollutants (POPs), such as perfluorooctanoic acid (PFOA), remains challenging. In the present work, a porous coral-like WO₃/W (PCW) photoelectrode with a well-designed energy band structure was used for the photoelectrocatalytic degradation of POPs and the simultaneous generation of electricity. The as-constructed bionic porous coral-like nanostructure greatly improved the light-harvesting capacity of the PCW photoelectrode. A maximum photocurrent density (0.31 mA/cm²) under visible light (λ > 420 nm) irradiation and a high incident photon conversion efficiency (IPCE) value (5.72% at 420 nm) were achieved. Because of the unique porous coral-like structure, the suitable energy band position, and the strong oxidation ability, this PCW photoelectrode-based PFC system exhibited a strong ability for simultaneous photoelectrocatalytic degradation of PFOA and electricity generation under visible-light irradiation, with a power output of 0.0013 mV/cm² using PFOA as the fuel. This work provides a promising way to construct a reliable PFC using highly toxic POPs to generate electricity.

Polymer nanocomposites having a high filler content: synthesis, structures, properties, and applications

The recent development of nanoscale fillers, such as carbon nanotubes, graphene, and nanocellulose, allows the functionality of polymer nanocomposites to be controlled and enhanced. However, conventional synthesis methods of polymer nanocomposites cannot maximise the reinforcement of these nanofillers at high filler content. Approaches for the synthesis of high content filler polymer nanocomposites are suggested to facilitate future applications. The fabrication methods address the design of the polymer nanocomposite architecture, which encompasses one, two, and three dimensional morphologies. Factors that hamper the reinforcement of nanostructures, such as alignment, dispersion of the filler and interfacial bonding between the filler and polymer, are outlined. Using suitable approaches, maximum potential reinforcement of nanoscale fillers can be anticipated without limitations in orientation, dispersion, and the integrity of the filler particle-matrix interface. High filler content polymer composites containing emerging materials such as 2D transition metal carbides, nitrides, and carbonitrides (MXenes) are expected in the future.

A flexible CdS nanorods-carbon nanotubes/stainless steel mesh photoanode for boosted photoelectrocatalytic hydrogen evolution

An innovative flexible reticular photoanode (CdS-nanorods/CNTs coated on stainless iron mesh) was designed for efficiently driving photoelectrocatalytic (PEC) hydrogen (H₂) evolution under visible-light irradiation.

Biocompatible FeOOH-Carbon quantum dots nanocomposites for gaseous NOx removal under visible light: Improved charge separation and High selectivity

Development of biocompatible photocatalysts with improved charge separation and high selectivity is essential for effective removal of air pollutants. Iron-containing catalysts have attracted extensive attention due to their low-toxicity and high natural abundance. Here, carbon quantum dots (CQDs) modified FeOOH nanocomposites fabricated using a facile hydrothermal route showed enhanced NO removal efficiency (22%) compared to pure FeOOH. Moreover, generation of toxic NO₂ intermediates was significantly inhibited using the nanocomposites, demonstrating high selectivity for final nitrate formation. Photo-electrochemical results showed that both charge separation and transfer efficiency were significantly improved by CQDs addition, and the lifetime of photo-generated carriers was increased eventually. Density functional theory calculations further elucidated that the suppressed recombination of photo-induced electron-hole pairs was due to enhanced electron migration from the FeOOH to CQDs. A NO degradation mechanism was proposed based on detection of the reactive oxygen species using electron paramagnetic spectroscopy. In addition, the nanocomposite showed good biocompatibility and low cytotoxity, ensuring minimal environmental impact for potential application in large-scale.

Fully Depleted Ti-Nb-Ta-Zr-O Nanotubes: Interfacial Charge Dynamics and Solar Hydrogen Production

Poor kinetics of hole transportation at the electrode/electrolyte interface is regarded as a primary cause for the mediocre performance of n-type TiO₂ photoelectrodes. By adopting nanotubes as the electrode backbone, light absorption and carrier collection can be spatially decoupled, allowing n-type TiO₂, with its short hole diffusion length, to maximize the use of the available photoexcited charge carriers during operation in photoelectrochemical (PEC) water splitting. Here, we presented a delicate electrochemical anodization process for the preparation of quaternary Ti-Nb-Ta-Zr-O mixed-oxide (denoted as TNTZO) nanotube arrays and demonstrated their utility in PEC water splitting. The charge-transfer dynamics for the electrodes was investigated using time-resolved photoluminescence, electrochemical impedance spectroscopy, and the decay of open-circuit voltage analysis. Data reveal that the superior photoactivity of TNTZO over pristine TiO₂ originated from the introduction of Nd, Ta, and Zr elements, which enhanced the amount of accessible charge carriers, modified the electronic structure, and improved the hole injection kinetics for expediting water splitting. By modulating the water content of the electrolyte employed in the anodization process, the wall thickness of the grown TNTZO nanotubes can be reduced to a size smaller than that of the depletion layer thickness, realizing a fully depleted state for charge carriers to further advance the PEC performance. Hydrogen evolution tests demonstrate the practical efficacy of TNTZO for realizing solar hydrogen production. Furthermore, with the composition complexity and fully depleted band structure, the present TNTZO nanotube arrays may offer a feasible and universal platform for the loading of other semiconductors to construct a sophisticated heterostructure photoelectrode paradigm, in which the photoexcited charge carriers can be entirely utilized for efficient solar-to-fuel conversion.

Black TiO2 nanotube arrays fabricated by electrochemical self-doping and their photoelectrochemical performance

Herein, black TiO2 nanotube arrays (NTAs) were fabricated using electrochemical self-doping approaches, and characterized systemically by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), UV-visible absorption spectroscopy and photoluminescence spectroscopy (PL). The as-obtained black TiO2 nanotube arrays (NTAs) exhibited stronger absorption in the visible-light region, a better separation rate of light-induced carriers, and higher electrical conductivity than TiO2 nanotube arrays (NTAs). These characteristics cause black TiO2 nanotube array (NTA) electrodes to have higher photoelectrocatalytic activity for degrading anthraquinone dye (reactive brilliant blue KN-R) than the TiO2 nanotube array (NTA) electrode. Furthermore, a synergetic action between photocatalysis and electrocatalysis was also observed. The black TiO2 nanotube array (NTA) electrode is considered to be a promising photoanode for the treatment of organic pollutants.

Hydrogenated TiO2 nanotube photonic crystals for enhanced photoelectrochemical water splitting

We report the design, fabrication and characterization of novel TiO₂ nanotube photonic crystals with a crystalline core/disordered shell structure as well as substantial oxygen vacancies for photoelectrochemical (PEC) water splitting. The novel TiO₂ nanotube photonic crystals are fabricated by annealing of anodized TiO₂ nanotube photonic crystals in hydrogen atmosphere at various temperatures. The optimized novel TiO₂ nanotube photonic crystals produce a maximal photocurrent density of 2.2 mA cm^-2 at 0.22 V versus Ag/AgCl, which is two times higher that of the TiO₂ nanotube photonic crystals annealed in air. Such significant PEC performance improvement can be ascribed to synergistic effects of the disordered surface layer and oxygen vacancies. The reduced band gap owing to the disordered surface layer and localized states induced by oxygen vacancies can enhance the efficient utilization of visible light. In addition, the disordered surface layer and substantial oxygen vacancies can promote the efficiency for separation and transport of the photogenerated carriers. This work may open up new opportunities for the design and construction of the high efficient and low-cost PEC water splitting system.

Oxygen vacancies confined in SnO2 nanoparticles for glorious photocatalytic activities from the UV, visible to near-infrared region

For the purpose of effectively utilizing solar energy, tailoring of the energy band configuration represents an effective approach to the exploration and development of full-spectrum-responsive photocatalysts with advanced performance.

Water splitting catalyzed by titanium dioxide decorated with plasmonic nanoparticles

The development of active, cheap, efficient and visible-light-driven water splitting catalysts is currently the center of intense research efforts. Amongst the most promising avenues, the design of titania and plasmonic nanoparticle hybrids is particularly appealing. Titania has been known for long to be an active photocatalyst, able to perform water splitting under light irradiation. However, this activity is limited to the ultraviolet spectrum and suffers from too rapid charge carrier recombination. The addition of plasmonic nanostructures enables to push absorption properties to the visible region and prevent unwanted charge recombination. In this review, we explain the principles behind the activity of such nanohybrids towards visible light water splitting and detail the recent research developments relying on plasmonic metals, namely Au, Ag and Cu.

Fabrication of Nanostructures with Bottom-up Approach and Their Utility in Diagnostics, Therapeutics, and Others

Nanofabrication has been a critical area of research in the last two decades and has found wide-ranging application in improvising material properties, sensitive clinical diagnostics, and detection, improving the efficiency of electron transport processes within materials, generating high energy densities leading to pulse power, novel therapeutic mechanisms, environmental remediation and control. The continued improvements in the various fabrication technologies have led to realization of highly sensitive nanostructure-based devices. The fabrication of nanostructures is in principle carried out primarily using top-down or bottom-up approaches. This chapter summarizes the important bottom-up nanofabrication processes for realizing nanostructures and also highlights the recent research conducted in the domain of therapeutics and diagnostics.

Heterostructured ZnFe2O4/Fe2TiO5/TiO2 Composite Nanotube Arrays with an Improved Photocatalysis Degradation Efficiency Under Simulated Sunlight Irradiation

To improve the visible light absorption and photocatalytic activity of titanium dioxide nanotube arrays (TONTAs), ZnFe₂O₄ (ZFO) nanocrystals were perfused into pristine TONTA pipelines using a novel bias voltage-assisted perfusion method. ZFO nanocrystals were well anchored on the inner walls of the pristine TONTAs when the ZFO suspensions (0.025 mg mL^-1) were kept under a 60 V bias voltage for 1 h. After annealing at 750 °C for 2 h, the heterostructured ZFO/Fe₂TiO₅ (FTO)/TiO₂ composite nanotube arrays were successfully obtained. Furthermore, Fe³⁺ was reduced to Fe²⁺ when solid solution reactions occurred at the interface of ZFO and the pristine TONTAs. Introducing ZFO significantly enhanced the visible light absorption of the ZFO/FTO/TONTAs relative to that of the annealed TONTAs. The coexistence of type I and staggered type II band alignment in the ZFO/FTO/TONTAs facilitated the separation of photogenerated electrons and holes, thereby improving the efficiency of the ZFO/FTO/TONTAs for photocatalytic degradation of methylene blue when irradiated with simulated sunlight.

A P25/(NH4)xWO3 hybrid photocatalyst with broad spectrum photocatalytic properties under UV, visible, and near-infrared irradiation

In this study, a series of hybrid nanostructured photocatalysts P25/(NH₄)_xWO₃ nanocomposites with the average crystallite size of P25 and (NH₄)_xWO₃ of the sample was calculated to be about 30 nm and 130 nm, were successfully synthesized via a simple one-step hydrothermal method. The as-obtained samples was characterized by transmission electron microscopy (TEM), which implies that the P25/(NH₄)_xWO₃ nanocomposites are fabricated with favourable nanosizd interfacial. The XPS results confirmed that the obtained sample consists of mixed chemical valences of W⁵⁺ and W⁶⁺, the low-valance W⁵⁺ sites could be the origin of NIR absorption. As revealed by optical absorption results, P25/(NH₄)_xWO₃ nanocomposites possess high optical absorption in the whole solar spectrum of 200–2500 nm. Benefiting from this unique photo-absorption property and the synergistic effect of P25 and (NH₄)_xWO₃, broad spectrum response photocatalytic activities covering UV, visible and near infrared regions on degradation of Rhodamine B have been realized by P25/(NH₄)_xWO₃ nanocomposites. Meanwhile, the stability of photocatalysts was examined by the XRD and XPS of the photocatalysts after the reaction. The results show that P25/(NH₄)_xWO₃ photocatalysts has a brilliant application prospect in the energy utilization to solve deteriorating environmental issues.

Enhanced visible-light-driven photocatalytic activity of Au@Ag core–shell bimetallic nanoparticles immobilized on electrospun TiO2 nanofibers for degradation of organic compounds

In this work, Au@Ag core–shell nanoparticles (NPs) with variable Ag shell thickness were synthesized and immobilized on TiO₂ nanofibers (TNF).

One-dimensional TiO2 Nanotube Photocatalysts for Solar Water Splitting

Hydrogen production from water splitting by photo/photoelectron-catalytic process is a promising route to solve both fossil fuel depletion and environmental pollution at the same time. Titanium dioxide (TiO2) nanotubes have attracted much interest due to their large specific surface area and highly ordered structure, which has led to promising potential applications in photocatalytic degradation, photoreduction of CO2, water splitting, supercapacitors, dye-sensitized solar cells, lithium-ion batteries and biomedical devices. Nanotubes can be fabricated via facile hydrothermal method, solvothermal method, template technique and electrochemical anodic oxidation. In this report, we provide a comprehensive review on recent progress of the synthesis and modification of TiO2 nanotubes to be used for photo/photoelectro-catalytic water splitting. The future development of TiO2 nanotubes is also discussed.

WO3-x sensitized TiO2 spheres with full-spectrum-driven photocatalytic activities from UV to near infrared

To make full use of solar energy for photocatalytic reactions, in this work, we developed full-spectrum-responsive photocatalysts with noteworthy photocatalytic activities under either UV, visible or even near infrared irradiation for the photodegradation of methylene blue. The core-shell structure of TiO₂@WO_3-x is designed from the consideration of combining the full-spectrum photo-absorption properties of WO_3-x with the excellent semiconductor properties of TiO₂. As expected, the WO_3-x sensitized TiO₂ sphere gives rise to a prominently strong optical absorption in the whole region of 300-2500 nm and thus displays desired photocatalytic properties for the full utilization of all solar energy, especially in the unexploited NIR part, which accounts for most of the sunlight. Encouraged by the above exciting photocatalytic outcome, we then go further to propose a plausible mechanism for interpreting the NIR-driven photocatalytic properties, which is based on the hypothesis of low-valent W⁵⁺ site induced free electrons and evidence-based ESR results.

Hydrogen Generation using non-polar coaxial InGaN/GaN Multiple Quantum Well Structure Formed on Hollow n-GaN Nanowires

This article demonstrates for the first time to the best of our knowledge, the merits of InGaN/GaN multiple quantum wells (MQWs) grown on hollow n-GaN nanowires (NWs) as a plausible alternative for stable photoelectrochemical water splitting and efficient hydrogen generation. These hollow nanowires are achieved by a growth method rather not by conventional etching process. Therefore this approach becomes simplistic yet most effective. We believe relatively low Ga flux during the selective area growth (SAG) aids the hollow nanowire to grow. To compare the optoelectronic properties, simultaneously solid nanowires are also studied. In this present communication, we exhibit that lower thermal conductivity of hollow n-GaN NWs affects the material quality of InGaN/GaN MQWs by limiting In diffusion. As a result of this improvement in material quality and structural properties, photocurrent and photosensitivity are enhanced compared to the structures grown on solid n-GaN NWs. An incident photon-to-current efficiency (IPCE) of around ~33.3% is recorded at 365 nm wavelength for hollow NWs. We believe that multiple reflections of incident light inside the hollow n-GaN NWs assists in producing a larger amount of electron hole pairs in the active region. As a result the rate of hydrogen generation is also increased.

Gold nanocluster sensitized TiO2 nanotube arrays for visible-light driven photoelectrocatalytic removal of antibiotic tetracycline

It is of technical interest to develop low-cost, high-quality and scalable photosensitizers that could efficiently harvest visible light. Here we design an efficient photoelectrocatalyst by integrating a recently developed gold nanocluster (AuNC, as a photosensitizer) into two types of highly ordered TiO2 nanotube arrays (TNAs, as substrates to host the photosensitizers). The TNA electrodes used in this study are a short TNA (∼0.5 µm in length, synthesized by the anodic oxidation in an aqueous hydrofluoric (HF) acid solution) and a long TNA (∼4.5 µm in length, synthesized by the anodic oxidation in a fluorinated ethylene glycol (EG) solution). A number of characterization techniques (e.g., FESEM, XRD and XPS) were applied to study the as-synthesized nanocomposites. In particular, diffuse reflectance spectroscopy and photochemical measurements suggest that the AuNC-coated TNA electrodes have successfully extended visible light absorption and improved their photochemical performance. Compared with the blank TNAs, the as-designed nanocomposites exhibit an evidently enhanced photoelectrocatalytic performance towards tetracycline (an emerging antibiotic contaminant in aquatic environment) decomposition, where the removal efficiency increases from 65% to 81% for AuNC/long-TNA and from 46% to 73% for AuNC/short-TNA electrodes, respectively. The improved performance is largely attributed to the photo-electro-chemical synergetic effect. The photochemical performance of the as-designed nanocomposites could be further improved by fine tuning the size, composition, and surface of the AuNC-based photosensitizers.

Preparation of core-shell Ag@CeO2 nanocomposite by LSPR photothermal induced interface reaction

The core-shell structure of Ag@CeO2 was prepared by a novel and facile method, which was based on the photothermal effect of localized surface plasmon resonance (LSPR). Nanoparticles (NPs) of Ag were dispersed in a solution containing citric acid, ethylene glycol and cerium nitrate, then under irradiation, Ag NPs generated heat from LSPR and the heat-induced polymerization reaction in the interface between Ag and the sol resulted in cerium gel formation only on the surface of the Ag NPs. After calcination, Ag@CeO2 was successfully obtained, then Ag@CeO2/SiO2 was prepared by loading Ag@CeO2 on SiO2. The resultant catalyst exhibited favorable activity and stability for CO oxidation. The preparation method proposed here should be extendable to other composites with metallic cores and oxide shells in which the metallic nanoparticle possesses LSPR properties.