UV and visible light photocatalytic activity of Au/TiO2 nanoforests with Anatase/Rutile phase junctions and controlled Au locations

Emerging ZnO Semiconductors for Photocatalytic CO2 Reduction to Methanol

Carbon recycling is poised to emerge as a prominent trend for mitigating severe climate change and meeting the rising demand for energy. Converting carbon dioxide (CO2) into green energy and valuable feedstocks through photocatalytic CO2 reduction (PCCR) offers a promising solution to global warming and energy needs. Among all semiconductors, zinc oxide (ZnO) has garnered considerable interest due to its ecofriendly nature, biocompatibility, abundance, exceptional semiconducting and optical properties, cost-effectiveness, easy synthesis, and durability. This review thoroughly discusses recent advances in mechanistic insights, fundamental principles, experimental parameters, and modulation of ZnO catalysts for direct PCCR to C1 products (methanol). Various ZnO modification techniques are explored, including atomic size regulation, synthesis strategies, morphology manipulation, doping with cocatalysts, defect engineering, incorporation of plasmonic metals, and single atom modulation to boost its photocatalytic performance. Additionally, the review highlights the importance of photoreactor design, reactor types, geometries, operating modes, and phases. Future research endeavors should prioritize the development of cost-effective catalyst immobilization methods for solid-liquid separation and catalyst recycling, while emphasizing the use of abundant and non-toxic materials to ensure environmental sustainability and economic viability. Finally, the review outlines key challenges and proposes novel directions for further enhancing ZnO-based photocatalytic CO2 conversion processes.

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.

Controlling nanoparticle placement in Au/TiO2 inverse opal photocatalysts

Gold nanoparticle-loaded titania (Au/TiO2) inverse opals are highly ordered three-dimensional photonic structures with enhanced photocatalytic properties. However, fine control over the placement of the Au nanoparticles in the inverse opal structures remains challenging with traditional preparative methods. Here, we present a multi-component co-assembly strategy to prepare high-quality Au/TiO2 inverse opal films in which Au nanoparticles are either located on, or inside the TiO2 matrix, as verified using electron tomography. We report that Au nanoparticles embedded in the TiO2 support exhibit enhanced thermal and mechanical stability compared to non-embedded nanoparticles that are more prone to both leaching and sintering.

Plasmons Enhancing Sub-Bandgap Photoconductivity in TiO2 Nanoparticles Film

The coupling between sub-bandgap defect states and surface plasmon resonances in Au nanoparticles and its effects on the photoconductivity performance of TiO2 are investigated in both the ultraviolet (UV) and visible spectrum. Incorporating a 2 nm gold nanoparticle layer in the photodetector device architecture creates additional trapping pathways, resulting in a faster current decay under UV illumination and a significant enhancement in the visible photocurrent of TiO2, with an 8-fold enhancement of the defects-related photocurrent. We show that hot electron injection (HEI) and plasmonic resonance energy transfer (PRET) jointly contribute to the observed photoconductivity enhancement. In addition to shedding light on the below-band-edge photoconductivity of TiO2, our work provides insight into new methods to probe and examine the surface defects of metal oxide semiconductors using plasmonic resonances.

Photochromic Uranyl-Based Coordination Polymer for Quantitative and On-Site Detection of UV Radiation Dose

A highly sensitive detection of ultraviolet (UV) radiation is required in a broad range of scientific research, chemical industries, and health-related applications. Traditional UV photodetectors fabricated by direct wide-band-gap inorganic semiconductors often suffer from several disadvantages such as complicated manufacturing procedures, requiring multiple operations and high-cost instruments to obtain a readout. Searching for new materials or simple strategies to develop UV dosimeters for quantitative, accurate, and on-site detection of UV radiation dose is still highly desirable. Herein, a photochromic uranyl-based coordination polymer [(UO2)(PBPCA)·DMF]·DMF (PBPCA = pyridine-3,5-bis(phenyl-4-carboxylate), DMF = N,N'-dimethylformamide, denoted as SXU-1) with highly radiolytic and chemical stabilities was successfully synthesized via the solvothermal method at 100 °C. Surprisingly, the fresh samples of SXU-1 underwent an ultra-fast UV-induced (365 nm, 2 mW) color variation from yellow to orange in less than 1 s, and then the color changed further from orange to brick red after the subsequent irradiation, inspiring us to develop a colorimetric dosimeter based on red-green-blue (RGB) parameters. The mechanism of radical-induced photochromism was intensively investigated by UV-vis absorption spectra, EPR analysis, and SC-XRD data. Furthermore, SXU-1 was incorporated into an optoelectronic device to fabricate a novel dosimeter for convenient, quantitative, and on-site detection of UV radiation dose.

Boosting Fenton's Oxidation Reaction by a Food Waste-Derived Catalyst for Oxidizing Organic Dyes: Synergistic Effect of Complex Iron Oxides and the Layer Carbon Structure

The constant increase in the human population drives the demand for food supply and thereby increasing the food wastage dramatically all over the world. Especially, around 60% of banana biomass has been generated as inedible domestic waste. Herein, we successfully employed banana waste as a catalyst for Fenton's oxidation reaction. The biomass-derived catalysts were subjected to various characterization techniques such as XRD, ATR-FTIR, confocal Raman spectroscopy, and XPS, XRF, BET, SEM, and TEM analyses. The XRD results revealed that, after carbonization of the dried banana bract material, a perloffite-like metal oxide phase was formed due to the aerial oxidation reaction. Characterization results of Raman and ATR-FTIR confirm that the carbonized catalyst possesses a layer-like structure with different types of functional groups. The calcium, magnesium, potassium, sodium, and iron are the dominating metal species in the resultant material, which was evident from the XRF and EDAX analyses. The carbonized banana bract catalyst is successfully utilized for the Fenton's oxidation reaction at neutral pH. The experimental results showed that the degradation efficiency of the fresh catalyst was 95% in 4 h of reaction time, and the stability of the catalyst was retained up to nine consecutive cycles. The high activity of MB, methylene blue, is mainly attributed to the strong interaction between oxy functional groups of the catalyst and MB molecule as compared to RhB. Further, the calculated efficiency of the hydrogen peroxide was found to be 99% and the self-decomposition of hydrogen peroxide by the formed metal oxides was highly limited.

Fabrication of Z-Type TiN@(A,R)TiO2 Plasmonic Photocatalyst with Enhanced Photocatalytic Activity

Plasmonic effect-enhanced Z-type heterojunction photocatalysts comprise a promising solution to the two fundamental problems of current TiO₂-based photocatalysis concerning low-charge carrier separation efficiency and low utilization of solar illumination. A plasmonic effect-enhanced TiN@anatase-TiO₂/rutile-TiO₂ Z-type heterojunction photocatalyst with the strong interface of the N-O chemical bond was synthesized by hydrothermal oxidation of TiN. The prepared photocatalyst shows desirable visible light absorption and good visible-light-photocatalytic activity. The enhancement in photocatalytic activities contribute to the plasma resonance effect of TiN, the N-O bond-connected charge transfer channel at the TiO₂/TiN heterointerface, and the synergistically Z-type charge transfer pathway between the anatase TiO₂ (A-TiO₂) and rutile TiO₂ (R-TiO₂). The optimization study shows that the catalyst with a weight ratio of A-TiO₂/R-TiO₂/TiN of approximately 15:1:1 achieved the best visible light photodegradation activity. This work demonstrates the effectiveness of fabricating plasmonic effect-enhanced Z-type heterostructure semiconductor photocatalysts with enhanced visible-light-photocatalytic activities.

Optimizing the performance of Auy/Nix/TiO2NTs photoanodes for photoelectrochemical water splitting

Water splitting using photoelectrochemical (PEC) techniques is thought to be a potential method for creating green hydrogen as a sustainable energy source. How to create extremely effective electrode materials is a pressing concern in this area. In this work, a series of Nix/TiO2 anodized nanotubes (NTs) and Auy/Nix/TiO2NTs photoanodes were prepared by electrodeposition via cyclic voltammetry and UV-photoreduction, respectively. The photoanodes were characterized by several structural, morphological, and optical techniques and their performance in PEC water-splitting for oxygen evolution reaction (OER) under simulated solar light was investigated. The obtained results revealed the nanotubular structure of TiO2NTs was preserved after deposition of NiO and Au nanoparticles while the band gap energy was reduced allowing for effective utilization of solar light with lower charge recombination rate. The PEC performance was monitored and it was found that the photocurrent densities of Ni20/TiO2NTs and Au30/Ni20/TiO2NTs were 1.75-fold and 3.25-fold that of pristine TiO2NTs, respectively. It was confirmed that the performance of the photoanodes depends on the number of electrodeposition cycles and duration of photoreduction of gold salt solution. The observed enhanced OER activity of Au30/Ni20/TiO2NTs could be attributed to the synergism between the local surface plasmon resonance (LSPR) effect of nanometric gold which increased solar light harvesting and the p-n heterojunction formed at the NiO/TiO2 interface which led to better charge separation and transportation suggesting its potential application as an efficient and stable photoanode in PEC water splitting for H2 production.

Improved Simulated-Daylight Photodynamic Therapy and Possible Mechanism of Ag-Modified TiO2 on Melanoma

Simulated-daylight photodynamic therapy (SD-PDT) may be an efficacious strategy for treating melanoma because it can overcome the severe stinging pain, erythema, and edema experienced during conventional PDT. However, the poor daylight response of existing common photosensitizers leads to unsatisfactory anti-tumor therapeutic effects and limits the development of daylight PDT. Hence, in this study, we utilized Ag nanoparticles to adjust the daylight response of TiO₂, acquire efficient photochemical activity, and then enhance the anti-tumor therapeutic effect of SD-PDT on melanoma. The synthesized Ag-doped TiO₂ showed an optimal enhanced effect compared to Ag-core TiO₂. Doping Ag into TiO₂ produced a new shallow acceptor impurity level in the energy band structure, which expanded optical absorption in the range of 400-800 nm, and finally improved the photodamage effect of TiO₂ under SD irradiation. Plasmonic near-field distributions were enhanced due to the high refractive index of TiO₂ at the Ag-TiO₂ interface, and then the amount of light captured by TiO₂ was increased to induce the enhanced SD-PDT effect of Ag-core TiO₂. Hence, Ag could effectively improve the photochemical activity and SD-PDT effect of TiO₂ through the change in the energy band structure. Generally, Ag-doped TiO₂ is a promising photosensitizer agent for treating melanoma via SD-PDT.

Effect of Ag modification on TiO2 and melem/g-C3N4 composite on photocatalytic performances

Here, the comparison of two different semiconductor materials is demonstrated, TiO₂ and melem/g-C₃N₄ composites-modified with balls of approximately 5 nm Ag nanoparticles (NPs) as photocatalysts for the degradation of the model dye acid orange 7 (AO7). The melem molecule synthesized here is one of a series of organic compounds consisting of triazine ring compounds with a structure similar to that of melam and melamine. The photodegradation process of AO7 was carried out to examine all powder materials as a potential photocatalyst. Additionally, two different lamps of wavelengths 368 nm (UV light) and 420 nm (VIS light) were applied to compare the photodegradation tests. A new synthesis route for the acquisition of Ag NPs (Ag content 0.5, 1.0 and 2.5 wt%), based on a wet and low temperature method without the use of reducing reagents was proposed. The best photocatalytic performances under UV and VIS light were obtained for both, TiO₂ and melem/g-C₃N₄ materials (new synthesis route) modified with a very low Ag content-0.5 wt%. The photodegradation activities using UV lamp (3 h, 368 nm irradiation) for samples with 0.5 wt% of Ag: TiO₂ and melem/g-C₃N₄, in excess of 95 and 94%, respectively, were achieved. The highest photoactive materials melem/g-C₃N₄ with 0.5 and 1 wt% Ag revealed 98% of activity under the VIS lamp after 3 h long irradiation. Our work demonstrates a novel, environmentally acceptable, and cost-effective chemical strategy for preparation of photocatalysts suitable for degradation of organic contaminants in wastewater treatment.

Hyaluronic acid-covered piezoelectric nanocomposites as tumor microenvironment modulators for piezoelectric catalytic therapy of melanoma

Increasing the formation of reactive oxygen species (ROS) and reducing the elimination of ROS are the two main objectives in the development of novel inorganic sonosensitizers for use in sonodynamic therapy (SDT). Therefore, BTO-Pd-MnO₂-HA nanocomplexes with targeted tumor cells and degradable oxygen-producing shells were designed as piezoelectric sonosensitizers for enhancing SDT. The deposition of palladium particles (Pd NPs) leads to the formation of Schottky junctions, promoting the separation of electron-hole pairs and thereby increasing the efficiency of toxic ROS generation in SDT. The tumor microenvironment (TME) triggers the degradation of MnO₂, and the released Mn²⁺ ions catalyze the generation of hydroxyl radicals (•OH) from H₂O₂ through a Fenton-like reaction. BTO-Pd-MnO₂-HA can continuously consume glutathione (GSH) and generate O₂, thereby improving the efficiency of SDT and chemodynamic therapy (CDT). A multistep enhanced SDT process mediated by the piezoelectric sonosensitizers BTO-Pd-MnO₂-HA was designed, targeted by hyaluronic acid (HA), activated by decomposition in TME, and amplified by deposition of Pd. This procedure not only presents a new alternative for the improvement of sonosensitizers but also widens the application of piezoelectric nanomaterials in biomedicine.

A Review on Nano Ti-Based Oxides for Dark and Photocatalysis: From Photoinduced Processes to Bioimplant Applications

Catalysis on TiO₂ nanomaterials in the presence of H₂O and oxygen plays a crucial role in the advancement of many different fields, such as clean energy technologies, catalysis, disinfection, and bioimplants. Photocatalysis on TiO₂ nanomaterials is well-established and has advanced in the last decades in terms of the understanding of its underlying principles and improvement of its efficiency. Meanwhile, the increasing complexity of modern scientific challenges in disinfection and bioimplants requires a profound mechanistic understanding of both residual and dark catalysis. Here, an overview of the progress made in TiO₂ catalysis is given both in the presence and absence of light. It begins with the mechanisms involving reactive oxygen species (ROS) in TiO₂ photocatalysis. This is followed by improvements in their photocatalytic efficiency due to their nanomorphology and states by enhancing charge separation and increasing light harvesting. A subsection on black TiO₂ nanomaterials and their interesting properties and physics is also included. Progress in residual catalysis and dark catalysis on TiO₂ are then presented. Safety, microbicidal effect, and studies on Ti-oxides for bioimplants are also presented. Finally, conclusions and future perspectives in light of disinfection and bioimplant application are given.

Oxygen Vacancy Mediated Band-Gap Engineering via B-Doping for Enhancing Z-Scheme A-TiO2/R-TiO2 Heterojunction Photocatalytic Performance

Fabrication of Z-scheme heterojunction photocatalysts is an ideal strategy for solving environmental problems by providing inexhaustible solar energy. A direct Z-scheme anatase TiO₂/rutile TiO₂ heterojunction photocatalyst was prepared using a facile B-doping strategy. The band structure and oxygen-vacancy content can be successfully tailored by controlling the amount of B-dopant. The photocatalytic performance was enhanced via the Z-scheme transfer path formed between the B doped anatase-TiO₂ and rutile-TiO₂, optimized band structure with markedly positively shifted band potentials, and the synergistically-mediated oxygen vacancy contents. Moreover, the optimization study indicated that 10% B-doping with the R-TiO₂ to A-TiO₂ weight ratio of 0.04 could achieve the highest photocatalytic performance. This work may provide an effective approach to synthesize nonmetal-doped semiconductor photocatalysts with tunable-energy structures and promote the efficiency of charge separation.

Controlling Mineralization with Protein-Functionalized Peptoid Nanotubes

Sequence-defined foldamers that self-assemble into well-defined architectures are promising scaffolds to template inorganic mineralization. However, it has been challenging to achieve robust control of nucleation and growth without sequence redesign or extensive experimentation. Here, peptoid nanotubes functionalized with a panel of solid-binding proteins are used to mineralize homogeneously distributed and monodisperse anatase nanocrystals from the water-soluble TiBALDH precursor. Crystallite size is systematically tuned between 1.4 and 4.4 nm by changing protein coverage and the identity and valency of the genetically engineered solid-binding segments. The approach is extended to the synthesis of gold nanoparticles and, using a protein encoding both material-binding specificities, to the fabrication of titania/gold nanocomposites capable of photocatalysis under visible-light illumination. Beyond uncovering critical roles for hierarchical organization and denticity on solid-binding protein mineralization outcomes, the strategy described herein should prove valuable for the fabrication of hierarchical hybrid materials incorporating a broad range of inorganic components.

A Modified Sol-Gel Synthesis of Anatase {001}-TiO2/Au Hybrid Nanocomposites for Enhanced Photodegradation of Organic Contaminants

A sol-gel synthesis technique was employed for the preparation of anatase phase {001}-TiO2/Au hybrid nanocomposites (NCs). The scalable, schematic, and cost-efficient method was successfully modified using HF and NH4OH capping agents. The photocatalytic activity of the as-synthesized {001}-TiO2/Au NCs were tested over 2-cycle degradation of methylene blue (MB) dye and pharmaceutical active compounds (PhACs) of ibuprofen and naproxen under direct sunlight illumination at 35 °C and 44,000 lx. Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HR-TEM), fast Fourier transform (FFT), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDS), and ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) were employed for the characterization of the as-prepared sample. The characterization results from the TEM, XPS, and XRD studies established both the distribution of Au colloids on the surface of TiO2 material, and the presence of the highly crystalline structure of anatase {001}-TiO2/Au NCs. Photodegradation results from the visible light irradiation of MB indicate an enhanced photocatalytic performance of Au/TiO2 NCs over TiO2. The results from the photocatalytic activity test performed under direct sunlight exposure exhibited promising photodegradation efficiencies. In the first cycle, the sol-gel synthesized material exhibited relatively better efficiencies (91%) with the MB dye and ibuprofen, while the highest degradation efficiency for the second cycle was 79% for the MB dye. Pseudo first-order photodegradation rates from the first cycle were determined to be comparatively slower than those from the second degradation cycle.

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.

Zirconia-Platinum Nanohybrids for Ultrasound-Activated Sonodynamic-Thermodynamic Bimodal Therapy by Inducing Intense Intracellular Oxidative Stress

The therapeutic exploration of nano-zirconia semiconductor largely remains untouched in the field of fundamental science to date. Here, a robust nano-sonosensitizer of ZrO_{2- x} @Pt is strategically formulated by in situ growth of Pt nanocrystal onto the surface of oxygen-deficient ZrO_{2- x} . Compared to 3.09 eV of nano-ZrO_{2- x} , the bandgap of ZrO_{2- x} @Pt Schottky junction is narrowed down to 2.74 eV. The band bending and bandgap narrowing enables an enhanced e^- /h⁺ separation in the presence of aPt electron sink, which facilitates a high yield of singlet oxygen (¹ O₂ ) and hydroxyl radicals (·OH) under ultrasound (US) irradiation. Moreover, nanozyme Pt with catalase-mimic activity can promote ¹ O₂  generation by relieving the hypoxic tumor microenvironment. Upon further modification of 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (AIPH), US-stimulated local thermal shock can disintegrate AIPH to create cytotoxic alkyl radicals (^• R). US-triggered reactive oxygen species generation and hyperthermia-induced alkyl radical production lead to severe and irreversible tumor cell death. Such combinatorial sonodynamic-thermodynamic therapy benefits the tumor eradication and metastasis inhibition at the animal level, with the aid of immunogenetic cell death and immune checkpoint blockade. Taken together, this proof-of-concept paradigm expands the medical use of nano-zirconia and provides useful insights for its therapeutic perspectives.

Platinum-Titania Schottky Junction as Nanosonosensitizer, Glucose Scavenger, and Tumor Microenvironment-Modulator for Promoted Cancer Treatment

To date, the construction of heterogeneous interfaces between sonosensitizers and other semiconductors or noble metals has aroused increasing attention, owing to an enhanced interface charge transfer, augmented spin-flip, and attenuated activation energy of oxygen. Here, a smart therapeutic nanoplatform is constructed by surface immobilization of glucose oxidase (GOx) onto a TiO₂@Pt Schottky junction. The sonodynamic therapy (SDT) and starvation therapy (ST) mediated by TiO₂@Pt/GOx (TPG) promote systemic tumor suppression upon hypoxia alleviation in tumor microenvironment. The band gap of TiO₂@Pt is outstandingly decreased to 2.9 eV, in contrast to that of pristine TiO₂. The energy structure optimization enables a more rapid generation of singlet oxygen (¹O₂) and hydroxyl radicals (^•OH) by TiO₂@Pt under ultrasound irradiation, resulting from an enhanced separation of hole-electron pair for redox utilization. The tumorous reactive oxygen species (ROS) accumulation and GOx-mediated glucose depletion facilitate oxidative damage and energy exhaustion of cancer cells, both of which can be tremendously amplified by Pt-catalyzed oxygen self-supply. Importantly, the combinatorial therapy triggers intense immunogenetic cell death, which favors a follow-up suppression of distant tumor and metastasis by evoking antitumor immunity. Collectively, this proof-of-concept paradigm provides an insightful strategy for highly efficient SDT/ST, which possesses good clinical potential for tackling cancer.

Constructing epitaxially grown heterointerface of metal nanoparticles and manganese dioxide anode for high-capacity and high-rate lithium-ion batteries

Low ion migration rate and irreversible change in the valence state in transition-metal oxides limit their application as anode materials in Li-ion batteries (LIBs). Interfacial optimization by loading metal particles on semiconductor can change the band structure and thus tune the inherent electrical nature of transition-metal oxide anode materials for energy applications. In this work, Au nanoparticles are epitaxially grown on MnO₂ nanoroads (MnO₂-Au). Interestingly, the MnO₂-Au anode shows excellent electrochemical activity. It delivers high reversible capacity (about 2-3 fold compared to MnO₂) and high rate capability (740 mA h g^-1 at 1 A g^-1). The electron holography and density functional theory (DFT) results demonstrate that the Au particles on the surface of MnO₂ can form a negative charge accumulation area, which not only improves the Li ion migration rate but also catalyzes the transition of MnO_x to Mn⁰. This study provides a direction to heterointerface fabrication for transition-metal oxide anode materials with desired properties for high-performance LIBs and future energy applications.

Materials characterization of TiO2 nanotubes decorated by Au nanoparticles for photoelectrochemical applications

The structural and chemical modification of TiO2 nanotubes (NTs) by the deposition of a well-controlled Au deposit was investigated using a combination of X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Scanning Transmission Electron Microscopy (STEM), Raman measurements, UV-Vis spectroscopy and photoelectrochemical investigations. The fabrication of the materials focused on two important factors: the deposition of Au nanoparticles (NPs) in UHV (ultra high vacuum) conditions (1-2 × 10-8 mbar) on TiO2 nanotubes (NTs) having a diameter of ∼110 nm, and modifying the electronic interaction between the TiO2 NTs and Au nanoparticles (NPs) with an average diameter of about 5 nm through the synergistic effects of SMSI (Strong Metal Support Interaction) and LSPR (Local Surface Plasmon Resonance). Due to the formation of unique places in the form of "hot spots", the proposed nanostructures proved to be photoactive in the UV-Vis range, where a characteristic gold plasmonic peak was observed at a wavelength of 580 nm. The photocurrent density of Au deposited TiO2 NTs annealed at 650 °C was found to be much greater (14.7 μA cm-2) than the corresponding value (∼0.2 μA cm-2) for nanotubes in the as-received state. The IPCE (incident photon current efficiency) spectral evidence also indicates an enhancement of the photoconversion of TiO2 NTs due to Au NP deposition without any significant change in the band gap energy of the titanium dioxide (E g ∼3.0 eV). This suggests that a plasmon-induced resonant energy transfer (PRET) was the dominant effect responsible for the photoactivity of the obtained materials.

The Role of Common Alcoholic Sacrificial Agents in Photocatalysis: Is It Always Trivial?

Photocatalytic hydrogen production is proposed as a sustainable energy source. Simultaneous reduction and oxidation of water is a complex multistep reaction with high overpotential. Photocatalytic processes involving semiconductors transfer electrons from the valence band to the conduction band. Sacrificial substrates that react with the photochemically formed holes in the valence band are often used to study the mechanism of H₂ production, as they scavenge the holes and hinder charge carrier recombination (electron-hole pairs). Here, we show that the desired sacrificial agent is one forming a radical that is a fairly strong reducing agent, and whose oxidized form is not a good electron acceptor that might suppress the hydrogen evolution reaction (HER). In an acidic medium, methanol was found to fulfill both these requirements better than ethanol and propan-2-ol in the TiO₂ -(M⁰ -NPs) (M=Au or Pt) system, whereas in an alkaline medium, the alcohols exhibit a reverse order of activity. Moreover, we report that CH₂ (OH)₂ is by far the most efficient sacrificial agent in a nontrivial mechanism in acidic media. Our study provides general guidelines for choosing an appropriate sacrificial substrate and helps to explain the variance in the performance of alcohol scavenger-based photocatalytic systems.

Color Tuning of Electrochromic TiO2 Nanofibrous Layers Loaded with Metal and Metal Oxide Nanoparticles for Smart Colored Windows

Co-axial electrospinning was applied for the structuring of non-woven webs of TiO2 nanofibers loaded with Ag, Au, and CuO nanoparticles. The composite layers were tested in an electrochromic half-cell assembly. A clear correlation between the nanoparticle composition and electrochromic effect in the nanofibrous composite is observed: TiO2 loaded with Ag reveals a black-brown color, Au shows a dark-blue color, and CuO shows a dark-green color. For electrochromic applications, the Au/TiO2 layer is the most promising choice, with a color modulation time of 6 s, transmittance modulation of 40%, coloration efficiency of 20 cm2/C, areal capacitance of 300 F/cm2, and cyclic stability of over 1000 cycles in an 18 h period. In this study, an unexplored path for the rational design of TiO2-based electrochromic device is offered with unique color-switching and optical efficiency gained by the fibrous layer. It is also foreseen that co-axial electrospinning can be an alternative nanofabrication technique for smart colored windows.

Are photocatalytic processes effective for removal of airborne viruses from indoor air? A narrative review

A wide variety of methods have been applied in indoor air to reduce the microbial load and reduce the transmission rate of acute respiratory diseases to personnel in healthcare sittings. In recent months, with the occurrence of COVID-19 pandemic, the role of portable ventilation systems in reducing the load of virus in indoor air has received much attention. The present study delineates a comprehensive up-to-date overview of the available photocatalysis technologies that have been applied for inactivating and removing airborne viruses. The detection methods for identifying viral particles in air and the main mechanisms involving in virus inactivation during photocatalysis are described and discussed. The photocatalytic processes could effectively decrease the load of viruses in indoor air. However, a constant viral model may not be generalizable to other airborne viruses. In photocatalytic processes, temperature and humidity play a distinct role in the inactivation of viruses through changing photocatalytic rate. The main mechanisms for inactivation of airborne viruses in the photocatalytic processes included chemical oxidation by the reactive oxygen species (ROS), the toxicity of metal ions released from metal-containing photocatalysts, and morphological damage of viruses.

Magnetic photocatalysts from δ-FeOOH and TiO2 and application in reactions for degradation of methylene blue and paracetamol with UV-C and sunlight

Water contamination is a common problem, especially considering dyes and drugs disposal. A possible and effective treatment method to remove these organic pollutants from water is photocatalytic reaction. This study aimed to improve the photocatalytic properties of TiO₂ using iron oxides (Ti/Fe composite). Different magnetic photocatalysts based on commercial TiO₂ were obtained with 30, 50, and 80% (wt./wt.) of TiO₂ supported on maghemite. X-ray diffraction with Rietveld refinement confirms the presence of γ-Fe₂O₃, α-Fe₂O₃, anatase, and rutile, as well as the relative percentages of the phases present in each photocatalyst. The magnetic properties were certified by VSM and sedimentation kinetics in the presence of a magnetic field. Besides their magnetic properties, UV-vis DRS shows that the obtained photocatalysts presented lower bandgap values when compared with TiO₂. These factors allowed the materials to absorb radiation in the visible-light region and the separation from the reaction medium by the application of magnetic field. It was observed an enhancement of photodegradation reaction of methylene blue (MB) and paracetamol (PC). For example, when the content of TiO₂ increased from 30 to 80% (wt./wt.), the efficiency increased from 58 to 99% (for MB) and 39 for 80% (for PC) under UV (λ = 254 nm). The reactions carried out with solar radiation showed 56 to 95% efficiency to discolor MB. In addition, the results of sedimentation kinetics and characterization confirmed the goals of the synthesis.

Photodehydrogenation of Ethanol over Cu2O/TiO2 Heterostructures

The photodehydrogenation of ethanol is a sustainable and potentially cost-effective strategy to produce hydrogen and acetaldehyde from renewable resources. The optimization of this process requires the use of highly active, stable and selective photocatalytic materials based on abundant elements and the proper adjustment of the reaction conditions, including temperature. In this work, Cu₂O-TiO₂ type-II heterojunctions with different Cu₂O amounts are obtained by a one-pot hydrothermal method. The structural and chemical properties of the produced materials and their activity toward ethanol photodehydrogenation under UV and visible light illumination are evaluated. The Cu₂O-TiO₂ photocatalysts exhibit a high selectivity toward acetaldehyde production and up to tenfold higher hydrogen evolution rates compared to bare TiO₂. We further discern here the influence of temperature and visible light absorption on the photocatalytic performance. Our results point toward the combination of energy sources in thermo-photocatalytic reactors as an efficient strategy for solar energy conversion.

Branching TiO2nanowire arrays for enhanced ethanol sensing

Nanostructure modulation is effective to achieve high performance TiO₂-based gas sensors. We herein report a wet-chemistry route to precipitate directly branched TiO₂nanowire arrays on alumina tubes for gas sensing applications. The optimized branched TiO₂nanowire array exhibits a response of 9.2 towards 100 ppm ethanol; whilst those of the pristine TiO₂nanowire array and the branched TiO₂nanowire powders randomly distributed are 5.1 and 3.1, respectively. The enhanced response is mainly contributed to the unique porous architecture and quasi-aligned nanostructure, which provide more active sites and also favor gas migration. Phase junctions between the backbone and the branch of the branched TiO₂nanowire arrays help the resistance modulation as a result of potential barriers. The facile precipitation of quasi-aligned arrays of branched TiO₂nanowires, which arein situgrown on ceramic tubes, thus provides a new economical synthetic route to TiO₂-based sensors with excellent properties.

Metal Oxide-Based Photocatalytic Paper: A Green Alternative for Environmental Remediation

The interest in advanced photocatalytic technologies with metal oxide-based nanomaterials has been growing exponentially over the years due to their green and sustainable characteristics. Photocatalysis has been employed in several applications ranging from the degradation of pollutants to water splitting, CO2 and N2 reductions, and microorganism inactivation. However, to maintain its eco-friendly aspect, new solutions must be identified to ensure sustainability. One alternative is creating an enhanced photocatalytic paper by introducing cellulose-based materials to the process. Paper can participate as a substrate for the metal oxides, but it can also form composites or membranes, and it adds a valuable contribution as it is environmentally friendly, low-cost, flexible, recyclable, lightweight, and earth abundant. In term of photocatalysts, the use of metal oxides is widely spread, mostly since these materials display enhanced photocatalytic activities, allied to their chemical stability, non-toxicity, and earth abundance, despite being inexpensive and compatible with low-cost wet-chemical synthesis routes. This manuscript extensively reviews the recent developments of using photocatalytic papers with nanostructured metal oxides for environmental remediation. It focuses on titanium dioxide (TiO2) and zinc oxide (ZnO) in the form of nanostructures or thin films. It discusses the main characteristics of metal oxides and correlates them to their photocatalytic activity. The role of cellulose-based materials on the systems’ photocatalytic performance is extensively discussed, and the future perspective for photocatalytic papers is highlighted.

A Direct Z-Scheme for the Photocatalytic Hydrogen Production from a Water Ethanol Mixture on CoTiO3/TiO2 Heterostructures

Photocatalytic H₂ evolution from ethanol dehydrogenation is a convenient strategy to store solar energy in a highly valuable fuel with potential zero net CO₂ balance. Herein, we report on the synthesis of CoTiO₃/TiO₂ composite catalysts with controlled amounts of highly distributed CoTiO₃ nanodomains for photocatalytic ethanol dehydrogenation. We demonstrate these materials to provide outstanding hydrogen evolution rates under UV and visible illumination. The origin of this enhanced activity is extensively analyzed. In contrast to previous assumptions, UV-vis absorption spectra and ultraviolet photoelectron spectroscopy (UPS) prove CoTiO₃/TiO₂ heterostructures to have a type II band alignment, with the conduction band minimum of CoTiO₃ below the H₂/H⁺ energy level. Additional steady-state photoluminescence (PL) spectra, time-resolved PL spectra (TRPLS), and electrochemical characterization prove such heterostructures to result in enlarged lifetimes of the photogenerated charge carriers. These experimental evidence point toward a direct Z-scheme as the mechanism enabling the high photocatalytic activity of CoTiO₃/TiO₂ composites toward ethanol dehydrogenation. In addition, we probe small changes of temperature to strongly modify the photocatalytic activity of the materials tested, which could be used to further promote performance in a solar thermophotocatalytic reactor.

Influencing the Electron Density of Nanosized Au Colloids via Immobilization on MgO to Stimulate Surface Reaction Activities

Heterogenization of colloidal gold on MgO is demonstrated to facilitate its catalytic surface reactivity. We show that the electron density on Au influenced by its immobilization on MgO along with the ensued metal-support interaction is one of the key parameters to obtain high activity. As elucidated by X-ray absorption spectroscopic (X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, and extended X-ray absorption fine structure) studies, the presence of well-dispersed nanosized Au on MgO is observed to result in an enhancement in the electron density of Au. The consequence of this electron-rich gold on the catalytic activity is then investigated using the nitroarene reduction as a model reaction with a detailed kinetic study. The kinetic study is an attempt to use a true heterogeneous system rather than the usually studied quasi-homogeneous systems. The results obtained reveal that the Au/MgO catalyst has a surface rate constant of ∼1.39 × 10^-3 mol m^-2 s^-1, which is significantly higher than those of the reported catalysts. While it validates the higher catalytic activity with a TOF of 9456 h^-1 observed for Au/MgO, the increased adsorption constant for 4-nitrophenol on Au/MgO further reflects the efficacy of MgO as the support. This not only allows effective heterogenization of the Au nanoparticles keeping the catalyst stable under the reaction conditions and being reused several times but also renders a capability in reduction of other nitro group-containing substrates. Therefore, the results are believed to be of importance in designing heterogeneous catalysts utilizing the distinctive properties of the nanosized colloids and tuning their surface reactivity as well.

Influence of lattice strain on Fe3O4@carbon catalyst for the destruction of organic dye in polluted water using a combined adsorption and Fenton process

In this study, 8, 25 and 50 wt% Fe3O4@activated carbon (AC) catalysts were prepared by simple coprecipitation method. The efficiency of the catalysts for the advanced Fenton's oxidation process using methylene blue (MB) as a model substrate was tested. Both modified and unmodified activated carbon catalysts exhibited similar activity towards the Fenton's oxidation process. Therefore, it is difficult to identify the role of the catalyst in this dye removal process. Hence, we proposed a new methodology to remove the MB by adopting the adsorption process initially, followed by the Fenton's oxidation process. The proposed process significantly improved the methylene blue decomposition reaction over the 25 wt% Fe3O4@AC catalyst. However, this trend was not seen in pure activated carbon and Fe3O4@AC (8 and 50 wt%) catalysts due to the instability of the material in the oxidizing medium. The possible reason for the deactivation of the catalysts was evaluated from lattice strain calculations, as derived from the modified W-H models (Uniform Deformational Model (UDM), Uniform Stress Deformation Model (USDM) and Uniform Deformation Energy Density Model (UDEDM)). These results provided a quantitative relationship between the experimentally calculated lattice strain values and Fenton's catalytic activity. Furthermore, the optimized strain value and crystalite size of Fe3O4 on the activated carbon matrix are responsible for the high catalytic activity.

Plasma Nitriding of TiO2 Nanotubes: N-Doping in Situ Investigations Using XPS

The nitrogen doping of titanium dioxide nanotubes (TiO₂ NTs) was investigated as a result of well-controlled plasma nitriding of TiO₂ NTs at a low temperature. This way of nitrogen doping is proposed as an alternative to chemical/electrochemical methods. The plasma nitriding process was performed in a preparation chamber connected to an X-ray photoelectron spectroscopy (XPS) spectrometer, and the nitrogen-doped TiO₂ NTs were next investigated in situ by XPS in the same ultrahigh vacuum (UHV) system. The collected high-resolution (HR) XPS spectra of N 1s, Ti 2p, O 1s, C 1s, and valence band (VB) revealed the formation of chemical bonds between titanium, nitrogen, and oxygen atoms as substitutional or interstitial species. Moreover, the results provided a characterization of the electronic states of N-TiO₂ NTs generated by various plasma nitriding and annealing treatments. The VB XPS spectrum showed a reduction in the TiO₂ band gap of about 0.6 eV for optimal nitriding and heat-treated conditions. The TiO₂ NTs annealed at 450 or 650 °C in air (ex situ) and nitrided under UHV conditions were used as reference materials to check the formation of Ti-N bonds in the TiO₂ lattice with a well-defined structure (anatase or a mixture of anatase and rutile). Scanning electron microscopy microscopic observations of the received materials were used to evaluate the morphology of the TiO₂ NTs after each step of the nitriding and annealing treatments.

Bidirectional Photochromism via Anchoring of Carbon Dots to TiO2 Porous Films

Photochromic materials present photocontrollable properties, which is of great interest for potential applications including high-density storage and optical displays. Herein, we demonstrate a promising pathway toward smart photochromic nanocomposite exploration by anchoring of carbon dots (CDs) to titanium dioxide (TiO₂) porous films. This study reveals that the color of the CDs/TiO₂ film obtained by dropping anchoring becomes darker and that obtained by immersion anchoring becomes lighter, both under blue light irradiation. For the photobleaching material system, the spectral response is strongly dependent on wavelength and polarization of the exciting light, which provides new dimensions for optical information encryption and memory. This work lays the foundation for the materials platform in the integration of advanced information processing in the future.

Utilising anatase nano-seeds coupled with a visible-light antennae system (Cu–Pd–N) for effective photo-organic transformations

Bandgap tuning TiO₂ nano-seeds with a three-component strategy (Cu, Pd, and N) has facilitated the selective photo-oxidation of cyclic alcohols.

Self-Optimized Catalysts: Hot-Electron Driven Photosynthesis of Catalytic Photocathodes

Photogenerated hot electrons from plasmonic nanostructures are very promising for photocatalysis, mostly due to their potential for enhanced chemical selectivity. Here, we present a self-optimized fabrication method of plasmonic photocathodes using hot-electron chemistry, for enhanced photocatalytic efficiencies. Plasmonic Au/TiO₂ nanoislands are excited at their surface plasmon resonance to generate hot electrons in an aqueous bath containing a platinum (cocatalyst) precursor. Hot electrons drive the deposition of Pt cocatalyst nanoparticles, without any nanoparticle functionalization and negligible applied bias, close to the hotspots of the plasmonic nanoislands. The presence of TiO₂ is crucial for achieving higher chemical reaction rates. The Au/TiO₂/Pt photocathodes synthesized using hot-electron chemistry show a photocatalytic activity of up to 2 times higher than that of a control made with random electrodeposited Pt nanoparticles. This light-driven positioning of the cocatalyst close to the same positions where hot electrons are most efficiently generated and transferred represents a novel and simple method for synthesizing complex, self-optimized photocatalytic nanostructures with improved efficiency and selectivity.

Pathways to Tailor Photocatalytic Performance of TiO2 Thin Films Deposited by Reactive Magnetron Sputtering

TiO₂ thin films are used extensively for a broad range of applications including environmental remediation, self-cleaning technologies (windows, building exteriors, and textiles), water splitting, antibacterial, and biomedical surfaces. While a broad range of methods such as wet-chemical synthesis techniques, chemical vapor deposition (CVD), and physical vapor deposition (PVD) have been developed for preparation of TiO₂ thin films, PVD techniques allow a good control of the homogeneity and thickness as well as provide a good film adhesion. On the other hand, the choice of the PVD technique enormously influences the photocatalytic performance of the TiO₂ layer to be deposited. Three important parameters play an important role on the photocatalytic performance of TiO₂ thin films: first, the different pathways in crystallization (nucleation and growth); second, anatase/rutile formation; and third, surface area at the interface to the reactants. This study aims to provide a review regarding some strategies developed by our research group in recent years to improve the photocatalytic performance of TiO₂ thin films. An innovative approach, which uses thermally induced nanocrack networks as an effective tool to enhance the photocatalytic performance of sputter deposited TiO₂ thin films, is presented. Plasmonic and non-plasmonic enhancement of photocatalytic performance by decorating TiO₂ thin films with metallic nanostructures are also briefly discussed by case studies. In addition to remediation applications, a new approach, which utilizes highly active photocatalytic TiO₂ thin film for micro- and nanostructuring, is also presented.

Electrospinning Fabricating Au/TiO2 Network-like Nanofibers as Visible Light Activated Photocatalyst

Exploiting photocatalysts with characteristics of low cost, high reactivity and easy recovery offer great potentials for complete elimination of toxic chemicals and environmental remediation. In this work, Au/TiO2 network-like nanofibers were fabricated using a facile electrospinning technique followed by calcinations in air. Photocatalytic tests indicate that the Au/TiO2 network-like nanofibers possess an excellent photodegradation rate of rhodamine B (RB) under UV, visible and natural light radiation. The enhanced photocatalytic activity can be attributed to the plasmonic resonance absorption of Au nanoparticles, and photogenerated electrons and holes are effectively separated by the Au/TiO2 heterojunction structures. Furthermore, the three-dimensional network structure can provide a large number of active sites for RB degradation.

TiO2 nanotrees for the photocatalytic and photoelectrocatalytic phenol degradation

Constructing nanotrees is an effective method to enhance the photoelectrocatalytic efficiency of TiO₂ using single-crystalline trunks with an appropriate length.

Reduced graphene oxide (rGO) supported electron deficient B-doped TiO2 (Au/B-TiO2/rGO) nanocomposite: An efficient visible light sonophotocatalyst for the degradation of Tetracycline (TC)

Incorporation of electron deficient boron atoms along with Au doped TiO₂ in the presence of rGO support was synthesized by hydrothermal method and demonstrated for the sonophotocatalytic degradation of TC under visible light illumination. The successful incorporation of electron deficient boron atoms and Au on TiO₂ was considerably enhanced the optical absorption towards visible region due to the formation acceptor energy levels below to the conduction band of TiO₂ by boron doping and surface plasmonic effect of Au. Moreover, formation of acceptor energy levels and introduction of reduced graphene oxide (rGO) support significantly improved the electron-hole pair separation and transportation which were supported by UV-vis-DRS, photo-current and photoluminescence measurements. The individual effect of photocatalysis and ultrasound for the TC degradation was found to be 45% and 12%, respectively. Importantly, a complete degradation (100%) of TC was achieved with 1.3 folds synergistic effect when ultrasound coupled with photocatalysis in 1 h. The enhanced degradation activity was mainly attributed to combined effect of rapid electron-hole pair separation facilitated by electron deficient B-atoms and rGO support and physical forces of ultrasound as well. In addition, ∼74% of Total Organic Carbon (TOC) removal was achieved within 1 h which further confirmed the effective demineralization of TC by the Au/B-TiO₂/rGO composite.