Photocatalytic Activity of Amorphous−Anatase Mixture of Titanium(IV) Oxide Particles Suspended in Aqueous Solutions

Crystal-Phase Engineering in Heterogeneous Catalysis

The performance of a chemical reaction is critically dependent on the electronic and/or geometric structures of a material in heterogeneous catalysis. Over the past century, the Sabatier principle has already provided a conceptual framework for optimal catalyst design by adjusting the electronic structure of the catalytic material via a change in composition. Beyond composition, it is essential to recognize that the geometric atomic structures of a catalyst, encompassing terraces, edges, steps, kinks, and corners, have a substantial impact on the activity and selectivity of a chemical reaction. Crystal-phase engineering has the capacity to bring about substantial alterations in the electronic and geometric configurations of a catalyst, enabling control over coordination numbers, morphological features, and the arrangement of surface atoms. Modulating the crystallographic phase is therefore an important strategy for improving the stability, activity, and selectivity of catalytic materials. Nonetheless, a complete understanding of how the performance depends on the crystal phase of a catalyst remains elusive, primarily due to the absence of a molecular-level view of active sites across various crystal phases. In this review, we primarily focus on assessing the dependence of catalytic performance on crystal phases to elucidate the challenges and complexities inherent in heterogeneous catalysis, ultimately aiming for improved catalyst design.

Bismuth Tungstate Nanoplates-Vis Responsive Photocatalyst for Water Oxidation

The development of visible-light-responsive (VLR) semiconductor materials for effective water oxidation is significant for a sustainable and better future. Among various candidates, bismuth tungstate (Bi2WO6; BWO) has attracted extensive attention because of many advantages, including efficient light-absorption ability, appropriate redox properties (for O2 generation), adjustable morphology, low cost, and profitable chemical and optical characteristics. Accordingly, a facile solvothermal method has been proposed in this study to synthesize two-dimensional (2D) BWO nanoplates after considering the optimal preparation conditions (solvothermal reaction time: 10-40 h). To find the key factors of photocatalytic performance, various methods and techniques were used for samples' characterization, including XRD, FE-SEM, STEM, TEM, HRTEM, BET-specific surface area measurements, UV/vis DRS, and PL spectroscopy, and photocatalytic activity was examined for water oxidation under UV and/or visible-light (vis) irradiation. Famous commercial photocatalyst-P25 was used as a reference sample. It was found that BWO crystals grew anisotropically along the {001} basal plane to form nanoplates, and all properties were controlled simultaneously by tuning the synthesis time. Interestingly, the most active sample (under both UV and vis), prepared during the 30 h solvothermal reaction at 433 K (BWO-30), was characterized by the smallest specific surface area and the largest crystals. Accordingly, it is proposed that improved crystallinity (which hindered charge carriers' recombination, as confirmed by PL), efficient photoabsorption (using the smallest bandgap), and 2D mesoporous structure are responsible for the best photocatalytic performance of the BWO-30 sample. This report shows for the first time that 2D mesoporous BWO nanoplates might be successfully prepared through a facile template-free solvothermal approach. All the above-mentioned advantages suggest that nanostructured BWO is a prospective candidate for photocatalytic applications under natural solar irradiation.

Novel gC3N4/MgZnAl-MMO derived from LDH for solar-based photocatalytic ammonia production using atmospheric nitrogen

The development of catalysis technologies for sustainable environmental applications, especially an alternative to ammonia (NH3) production under the Haber-Bosch process, has gained a lot of scope in recent days. The current work demonstrated a green synthesis of graphitic carbon nitride (gC3N4) containing magnesium-zinc-aluminium mixed metal oxides (MgZnAl-MMO) derived from layered double hydroxide (LDH) for visible light aided catalytic production of ammonia. Pyrolysis-hydrothermal techniques were adopted for the synthesis and fabrication of the gC3N4/MgZnAl-MMO catalytic composite. Characterization results of field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), UV-visible spectroscopy, photoluminescence (PL), etc. showed the desired properties and functionalities like semi-crystalline structure with rough surface morphology that enhance the sorption reactions. Catalytic composite gC3N4/MgZnAl-MMO showed a bandgap energy of 2.16 eV that is considerably shifted toward the visible range when compared to gC3N4 (2.39 eV) and MgZnAl-MMO (2.93 eV). The results were also well complied with XPS results obtained that promote solar-based photocatalysis. The gC3N4/MgZnAl-MMO assisted photocatalytic production of NH3 in an aqueous media proved to be acceptable by the production of a maximum 47.56 μmol/L NH3 under visible spectrum employing a light emitting diode (LED) source. The results showed that the advancement of catalyst for desired functionalities and NH3 production using LED simulating solar light-aided catalysis would be an alternative to the Haber-Bosch process and solar-based sustainable processes for NH3 production.

Photocatalytic transfer of aqueous nitrogen into ammonia using nickel-titanium-layered double hydroxide

The development of solar-driven transfer of atmospheric nitrogen into ammonia is one of the green and sustainable strategies in industrial ammonia production. Nickel-titanium-layered double hydroxide (NiTi-LDH) was synthesised using the soft-chemical process for atmospheric nitrogen fixation application under photocatalysis in an aqueous system. NiTi-LDH was investigated using advanced characterisation techniques and confirmed the potential oxygen vacancies and/or surface defects owing to better photocatalytic activity under the solar spectrum. It also exhibited a bandgap of 2.8 eV that revealed its promising visible-light catalytic activities. A maximum of 33.52 µmol L-1 aqueous NH3 was obtained by continuous nitrogen (99.9% purity) supply into the photoreactor under an LED light source. Atmospheric nitrogen supply (≈78%) yielded 14.67 µmol L-1 aqueous NH3 within 60 min but gradually reduced to 3.6 µmol L-1 at 330 min. Interestingly, in weak acidic pH, 20.90 µmol L-1 NH3 was produced compared to 11.51 µmol L-1 NH3 in basic pH. The application of NiTi-LDH for visible-light harvesting capability and photoreduction of atmospheric N2 into NH3 thereby opens a new horizon of eco-friendly NH3 production using natural sunlight as alternative driving energy.

Photocatalytic activity of P-doped TiO2 photocatalyst

In this study, P-doped TiO₂ photocatalysts with different molar percentages (in the range 0.071-1.25 mol %) of the non-metallic element were prepared and their photocatalytic activity under visible light irradiation was tested. All achieved samples were characterized by XRD, Raman, UV-Vis DRS and SEM-EDX techniques. XRD and Raman analysis showed that all doped photocatalysts were in anatase phase and evidenced that P ions were successfully incorporated into the TiO₂ crystal lattice, affecting also the crystallinity degree of the P-doped TiO₂ photocatalysts. Noticeably, the UV-Vis DRS spectra evidenced that the highest redshift in absorption edge was observed for the photocatalyst with the lowest P content (0.071PT), which showed also the lowest bandgap (2.9 eV). The photocatalytic performances of all P-doped TiO₂ samples were compared with that of commercial TiO₂ by evaluating the decolorization of methylene blue (MB) dye under visible light irradiation. Results showed that phosphorus doping strongly promoted photocatalytic activity in the presence of visible light. Furthermore, the most active photocatalyst in visible light tests (0.071PT) also showed better photocatalytic activity than commercial TiO₂ in the decolorization of MB under simulated sunlight irradiation. Finally, 0.071PT photocatalyst was preliminarily tested against Escherichia coli (E. coli) under simulated solar light, showing an inactivation efficiency of 90% after 2 h of treatment time.

Formation of a Nanorod-Assembled TiO2 Actinomorphic-Flower-like Microsphere Film via Ta Doping Using a Facile Solution Immersion Method for Humidity Sensing

This study fabricated tantalum (Ta)-doped titanium dioxide with a unique nanorod-assembled actinomorphic-flower-like microsphere structured film. The Ta-doped TiO₂ actinomorphic-flower-like microsphere (TAFM) was fabricated via the solution immersion method in a Schott bottle with a home-made improvised clamp. The samples were characterised using FESEM, HRTEM, XRD, Raman, XPS, and Hall effect measurements for their structural and electrical properties. Compared to the undoped sample, the rutile-phased TAFM sample had finer nanorods with an average 42 nm diameter assembled to form microsphere-like structures. It also had higher oxygen vacancy sites, electron concentration, and mobility. In addition, a reversed double-beam photoacoustic spectroscopy measurement was performed for TAFM, revealing that the sample had a high electron trap density of up to 2.5 μmolg^-1. The TAFM showed promising results when employed as the resistive-type sensing film for a humidity sensor, with the highest sensor response of 53,909% obtained at 3 at.% Ta doping. Adding rGO to 3 at.% TAFM further improved the sensor response to 232,152%.

Recent Advances in Photocatalytic Oxidation of Methane to Methanol

Methane is one of the promising alternatives to non-renewable petroleum resources since it can be transformed into added-value hydrocarbon feedstocks through suitable reactions. The conversion of methane to methanol with a higher chemical value has recently attracted much attention. The selective oxidation of methane to methanol is often considered a “holy grail” reaction in catalysis. However, methanol production through the thermal catalytic process is thermodynamically and economically unfavorable due to its high energy consumption, low catalyst stability, and complex reactor maintenance. Photocatalytic technology offers great potential to carry out unfavorable reactions under mild conditions. Many in-depth studies have been carried out on the photocatalytic conversion of methane to methanol. This review will comprehensively provide recent progress in the photocatalytic oxidation of methane to methanol based on materials and engineering perspectives. Several aspects are considered, such as the type of semiconductor-based photocatalyst (tungsten, titania, zinc, etc.), structure modification of photocatalyst (doping, heterojunction, surface modification, crystal facet re-arrangement, and electron scavenger), factors affecting the reaction process (physiochemical characteristic of photocatalyst, operational condition, and reactor configuration), and briefly proposed reaction mechanism. Analysis of existing challenges and recommendations for the future development of photocatalytic technology for methane to methanol conversion is also highlighted.

Flexible Nano-TiO2 Sheets Exhibiting Excellent Photocatalytic and Photovoltaic Properties by Controlled Silane Functionalization-Exploring the New Prospects of Wastewater Treatment and Flexible DSSCs

TiO₂ nanoparticles surface-modified with silane moieties, which can be directly coated on a flexible substrate without the requirement of any binder materials and postsintering processes, are synthesized and characterized using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, Brunauer-Emmett-Teller, X-ray photoelectron spectroscopy, Raman spectroscopy, photoluminescence spectroscopy, time-correlated single-photon counting, and transmission electron microscopy. The viability of the prepared surface-modified TiO₂ (M-TiO₂) sheets as a catalyst for the photo-induced degradation of a model dye, methylene blue, was checked using UV-visible absorption spectroscopy. The data suggest that, compared to unmodified TiO₂, M-TiO₂ sheets facilitate better dye-degradation, which leads to a remarkable photocatalytic activity that results in more than 95% degradation of the dye in the first 10 min and more than 99% of the degradation in the first 50 min of the photocatalytic experiments. We also demonstrate that M-TiO₂ can be recycled with negligible reduction in photocatalytic activity. Further, the photovoltaic properties of the developed M-TiO₂ sheets were assessed using UV-visible absorption spectroscopy, electrochemical impedance spectroscopy (EIS), and photochronoamperometry. Dye-sensitized solar cells (DSSC) fabricated using M-TiO₂ as the photoanode exhibited a photoconversion efficiency of 4.1% under direct sunlight. These experiments suggested that M-TiO₂ sheets show enhanced photovoltaic properties compared to unmodified TiO₂ sheets, and that, when N-719 dye is incorporated, the dye-TiO₂ interaction is more favorable for M-TiO₂ than bare TiO₂. The simple solution processing method demonstrated in this paper rendered a highly flexible photoanode made of M-TiO₂ with superior charge-separation efficiency to an electrode made of bare TiO₂. We propose that our findings on the photovoltaic properties of M-TiO₂ open up arenas of further improvement and a wide scope for the large-scale production of flexible DSSCs on plastic substrates at room temperature in a cost-effective way.

H2O2 Exfoliation of TiO2 for Enhanced Hydrogen Production from Photocatalytic Reforming of Methanol

Hydrogen is considered a future energy carrier for clean and sustainable technology. Photocatalytic reforming of methanol produced hydrogen using water and energy from sunlight. This study reported enhanced activity of TiO2 without metal co-catalyst for hydrogen production following H2O2 exfoliation. TiO2 was transformed into peroxo-titania species on the outer layer of the particles, resulting in surface exfoliation. The exfoliation reduced TiO2 crystallite sizes enhanced the surface hydroxyl group and reduced the band gap to 3.0 eV. Hydrogen production from methanol-water mixtures on the TiO2 after four consecutive exfoliations was measured at 300 µmol, significantly higher than the fresh TiO2 (50 µmol).  H2O2 exfoliated TiO2 reduced the pathway for charge migration to the surface.  A high concentration of surface hydroxyl group trapped the charge carriers for efficient hydrogen production. Copyright © 2022 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Degradation of Reactive Yellow 18 Using Ionizing Radiation Based Advanced Oxidation Processes: Cytotoxicity, Mutagenicity and By-Product Distribution

The degradation of Reactive Yellow 18 (RY-18), induced by gamma radiation in aqueous medium, was carried out as a function of gamma radiation dose (5–20 kGy) and concentration of hydrogen peroxide, the initial dye concentration and pH of the solution were optimized for the maximum degradation efficiency. Gamma radiations alone and in combination with H2O2 were used to degrade the RY-18. A degradation rate of 99% was achieved using an absorbed dose of 20 kGy, 0.6 mL H2O2 in acidic pH. Variations in the functional groups of untreated and treated RY-18 were determined by FTIR analysis. The LCMS technique was used to determine the intermediates formed during the degradation process. The cytotoxicity and mutagenicity of RY-18 were studied by hemolytic and Ames tests, respectively. There were significant reductions in cytotoxicity and mutagenicity in response to gamma radiation treatment. Cytotoxicity was reduced from 15.1% to 7.6% after treatment with a 20 kGy absorbed dose of gamma radiations with 0.6 mL H2O2. Mutagenicity was reduced by 81.3% and 82.3% against the bacterial strains TA98 and TA100 after treatment with a 20 kGy absorbed dose with 0.6 mL H2O2. The advanced oxidation process efficiency was evaluated using the byproduct formations, which were low-molecular-weight organic acid units, which through further oxidation were converted into carbon dioxide and water end products. Based on RY-18 degradation, cytotoxicity and mutagenicity reduction, the gamma radiation in combination with H2O2 has potential for the removal of dye from the effluents.

Fabrication of Fe-Si-B Based Amorphous Powder Cores by Spark Plasma Sintered and Their Magnetic Properties

Mechanical ball milling was used to coat SiO₂ nanopowder on a Fe-Si-B amorphous powder in this study. The Fe-Si-B/SiO₂ core–shell amorphous composite powder was obtained after 6h of ball milling. At 490 °C, the amorphous powder is thermally stable. Discharge plasma sintering was used to create a Fe-Si-B/SiO₂ magnetic powder core (SPS). At a sintering temperature of 420 to 540 °C, the phase composition and magnetic characteristics of the magnetic particle core were investigated. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to examine the structural features of the magnetic particle core. A precision resistance tester and a vibrating sample magnetometer were used to assess the resistivity and magnetic characteristics of the magnetic particle core. The findings showed that Fe₃Si and Fe₂B are the phases generated during spark plasma sintering. High-frequency power loss increases as density rises. However, at the measured frequency, the magnetic permeability of the magnetic particle core changes slightly and has excellent frequency characteristics, making it appropriate for use in high-frequency components.

Structural analysis and dye removal behavior of amorphous titania embedded poly(vinyl butyral) hybrid fiber

This study reports on the efficient methylene blue (MB) dye removal properties of a polyvinyl butyral (PVB)-amorphous titania (amTiO₂) hybrid fiber (PVB-amTiO₂F) made by air-gap spinning in acetone solvent. The successful fabrication of PVB-amTiO₂F was confirmed by employing Fourier transform infrared, scanning electron microscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, and energy dispersive X-ray measurement. Batch experiments were used to examine the cationic MB dye adsorption performance in the dark. The observed data showed that the developed PVB-amTiO₂F exhibited moderate adsorption efficiency (68-70%) which is comparable to other amorphous titania-rich adsorbents. The adsorption kinetics was well fitted with a pseudo-second-order model, suggesting that adsorption is mainly led by chemisorption. In addition, the MB degradation properties under visible light were also studied afterwards. A possible adsorption mechanism is discussed. Moreover, the as-fabricated fiber exhibited average to good reusability after 6 cycles. Only cationic MB dye solution was able to demonstrate such properties.

Metal-Induced Crystallization in Metal Oxides

ConspectusThe properties of a material depend upon its physical characteristics, one of these being its crystalline state. Next generation solid-state technologies will integrate crystalline oxides into thermal sensitive processes and composite materials. Crystallization of amorphous phases of metal oxides in the solid state typically requires substantial energy input to induce the amorphous to crystalline phase transformation. In the case of silica, the transformation to α-quartz in a furnace occurs above 1300 °C and that of titania, above 400 °C. These calcination processes are costly in energy but also often degrade complex material architectures or compositions.Thus, low temperature crystallization techniques are required that preserve macro- and mesostructures and complex elemental composition (e.g., organic-, metal-, and semiconductor-metal oxide hybrids/composites). Some solution-based techniques exist to directly fabricate crystalline metal oxides. However, these are not always compatible with the specificities of the system or industrial constraints. A postsynthetic, solid-state approach that reduces crystallization temperature in metal oxides is metal-induced crystallization (MIC).MIC is the introduction of catalytic amounts of a cation, which can be an s-block, p-block, or d-block cation, that migrates through the solid metal oxide lattice. The cation is thought to temporarily break metal oxide bonds, allowing [MOx] polyhedra to rotate and reform bonds with neighboring [MOx] groups in a lower energy crystalline phase. Depending on the system, the cation can favor or defavor the formation of a particular crystalline phase, providing a means to tune the purity and crystalline phase ratios. An advantage of MIC is that, although the crystallization occurs in the solid state, the crystallization process can be accomplished for particle suspensions in liquid media. In this case, the energy required to induce the crystallization can come from, for example, a microwave or an ultrasound bath. The crystallization of particles in suspension avoids aggregation from particle-particle sintering. In the case of thin films, the energy for crystallization typically comes from a laser or calcination.MIC is only recently being used as a low temperature metal oxide crystallization technique, despite being widely used in the semiconductor industry. Here, the mechanism and previous studies in MIC are presented for titania, silica, and other oxides. The beauty of this technique is that it is extremely easy to employ: cations can be incorporated into the system postsynthetically and then are often expelled from the lattice upon phase conversion. We expect MIC to enrich materials for photochromic, optoelectronic, catalyst, biological, and other applications.

Influence of rGO and Preparation Method on the Physicochemical and Photocatalytic Properties of TiO2/Reduced Graphene Oxide Photocatalysts

In this study, a series of TiO2/rGO photocatalysts were obtained with a two-step procedure: a solvothermal method and calcination at 300–900 °C in an argon atmosphere. It was noted that the presence of rGO in photocatalysts had an important role in the changes in crystallite size and specific surface area. In TiO2/rGO samples, different surface functional groups, such as C−Cgraph, C−Caliph, C−OH, C=O, and CO(O), were found. It was observed that rGO modification suppressed the anatase-to-rutile phase transformation. The photocatalytic activity of the obtained nanomaterials was investigated through the decomposition of methylene blue under UV and artificial solar light irradiation. It was found that the adsorption degree played an important role in methylene blue decomposition. The experimental results revealed that TiO2/rGO samples exhibited superior removal efficiency after calcination for methylene blue compared toTiO2 without rGO, as well as a commercial photocatalyst KRONOClean 7000. It was noted that photocatalytic activity increased with the increase in the calcination temperature. The highest activity was observed for the sample calcined at 700 °C, which consisted of 76% anatase and 24% rutile. This study clearly demonstrated that TiO2/rGO samples calcined in argon can be used as efficient photocatalysts for the application of methylene blue decomposition.

Morphology-Governed Performance of Multi-Dimensional Photocatalysts for Hydrogen Generation

In the past few decades, extensive studies have been performed to utilize the solar energy for photocatalytic water splitting; however, up to the present, the overall efficiencies reported in the literature are still unsatisfactory for commercialization. The crucial element of this challenging concept is the proper selection and design of photocatalytic material to enable significant extension of practical application perspectives. One of the important features in describing photocatalysts, although underestimated, is particle morphology. Accordingly, this review presents the advances achieved in the design of photocatalysts that are dedicated to hydrogen generation, with an emphasis on the particle morphology and its potential correlation with the overall reaction performance. The novel concept of this work—with the content presented in a clear and logical way—is based on the division into five parts according to dimensional arrangement groups of 0D, 1D, 2D, 3D, and combined systems. In this regard, it has been shown that the consideration of the discussed aspects, focusing on different types of particle morphology and their correlation with the system’s efficiency, could be a promising route for accelerating the development of photocatalytic materials oriented for solar-driven hydrogen generation. Finally, concluding remarks (additionally including the problems connected with experiments) and potential future directions of particle morphology-based design of photocatalysts for hydrogen production systems have been presented.

Phyto-Extract-Mediated Synthesis of Silver Nanoparticles Using Aqueous Extract of Sanvitalia procumbens, and Characterization, Optimization and Photocatalytic Degradation of Azo Dyes Orange G and Direct Blue-15

Green synthesis of silver nanoparticles (AgNPs) employing an aqueous plant extract has emerged as a viable eco-friendly method. The aim of the study was to synthesize AgNPs by using plant extract of Sanvitalia procumbens (creeping zinnia) in which the phytochemicals present in plant extract act as a stabilizing and reducing agent. For the stability of the synthesized AgNPs, different parameters like AgNO3 concentration, volume ratios of AgNO3, temperature, pH, and contact time were studied. Further, AgNPs were characterized by UV-visible spectroscopy, FT-IR (Fourier Transform Infrared Spectroscopy), XRD (X-ray Diffraction), SEM (Scanning Electron Microscopy), and EDX (Energy Dispersive X-ray Spectrometer) analysis. FT-IR analysis showed that the plant extract contained essential functional groups like O-H stretching of carboxylic acid, N-H stretching of secondary amides, and C-N stretching of aromatic amines, and C-O indicates the vibration of alcohol, ester, and carboxylic acid that facilitated in the green synthesis of AgNPs. The crystalline nature of synthesized AgNPs was confirmed by XRD, while the elemental composition of AgNPs was detected by energy dispersive X-ray analysis (EDX). SEM studies showed the mean particle diameter of silver nanoparticles. The synthesized AgNPs were used for photocatalytic degradation of Orange G and Direct blue-15 (OG and DB-15), which were analyzed by UV-visible spectroscopy. Maximum degradation percentage of OG and DB-15 azo dyes was observed, without any significant silver leaching, thereby signifying notable photocatalytic properties of AgNPs.

Quasi-homogenous photocatalysis of quantum-sized Fe-doped TiO2 in optically transparent aqueous dispersions

In this study, the preparation of anatase TiO₂ nanocrystals via a facile non-aqueous sol-gel route and their characterization are reported. The 3-4 nm particles are readily dispersable in aqueous media and show excellent photoreactivity in terms of rhodamine B degradation. The catalytic performance can be further increased considerably by doping with iron and UV-light irradiation as a pre-treatment. The effect of surface ligands (blocked adsorption sites, surface defects etc.) on the photoreactivity was thoroughly probed using thermogravimetric analysis combined with mass spectrometry. Photoelectrochemical characterization of thin-film electrodes made from the same TiO₂ nanocrystals showed the opposite trend to the catalytic experiments, that is, a strong decrease in photocurrent and quantum efficiency upon doping due to introduction of shallow defect states.

Advanced Functional Metal-Ceramic and Ceramic Coatings Deposited by Low-Pressure Cold Spraying: A Review

Based on the recent analysis of various databases, cold spray (CS), the newest method among thermal spraying technologies, has received the unabated attention of hundreds of researchers continuously since its invention in the 1980s. The significance of CS lies in the low process temperature, which usually ensures compressive residual stresses and allows for the formation of coatings on a thermally sensitive substrate. This paper concerns the low-pressure cold spray (LPCS) variant employed for forming metal matrix composites (MMCs) with high ceramic contents and all-ceramic coatings. At the very beginning, the influence of LPCS process parameters on deposition efficiency (DE) is analysed. In the next part, the most useful feedstock powder preparation techniques for LCPS are presented. Due to the combination of bottom-up powder production methods (e.g., sol-gel (SG)) with LCPS, the metal matrix that works as a binder for ceramic particles in MMC coatings can be removed, resulting in all-ceramic coatings. Furthermore, with optimization of spraying parameters, it is possible to predict and control phase transformation in the feedstock material. Further in the paper, differences in the bonding mechanism of metal–ceramic mixtures and ceramic particles are presented. The properties and applications of various MMC and ceramic coatings are also discussed. Finally, the exemplary direction of CS development is suggested.

Preparation of High-Performance Metal-Free UV/Near Infrared-Shielding Films for Human Skin Protection

A series of metal-free UV/near infrared (NIR)-shielding coatings are successfully fabricated by shielded cathodic arc plasma evaporation (CAPE) and substrate-biased RF magnetron sputtering processes. The UV/NIR-shielding coatings comprising quarter-wave stacks of TiO2/SiO2 multilayers and high-conductivity sputter-deposited ITO films with a thickness in the range of 200-600 nm could block IRA and IRB radiations, respectively. The total thicknesses of UV/near infrared-shielding films are in the range from 375 nm to 1513.8 nm. The anatase-phase TiO2 films with absorption edge located at ∼375 nm were deposited by shielded CAPE at ∼100 °C. Further, the well-crystallized ITO films were found to have high free-electron concentrations (1.12 × 1021 cm-3), resulting in strong absorption of IRB due to the plasmon resonance absorption. The optimal optical design and ITO film thickness were investigated, and the TiO2(SiO2/TiO2)3 multilayer combined with an ITO film thickness of 400 nm was found to provide a high NIR-shielding rate of 94.8%, UVB to UVA-shielding rate of 92.7%, and average visible light transmittance of 68.1%. Further, human skin cells protected by a UV/NIR-shielding coating showed significantly decreased reactive oxygen species generation and inflammatory cytokine expression as compared to those of unprotected cells. The results demonstrate that the development of multifunction coatings have potential for transparent heat insulation windows and human skin protection against UV/IR radiations.

TiO2 as a Photocatalyst for Water Splitting-An Experimental and Theoretical Review

Hydrogen produced from water using photocatalysts driven by sunlight is a sustainable way to overcome the intermittency issues of solar power and provide a green alternative to fossil fuels. TiO₂ has been used as a photocatalyst since the 1970s due to its low cost, earth abundance, and stability. There has been a wide range of research activities in order to enhance the use of TiO₂ as a photocatalyst using dopants, modifying the surface, or depositing noble metals. However, the issues such as wide bandgap, high electron-hole recombination time, and a large overpotential for the hydrogen evolution reaction (HER) persist as a challenge. Here, we review state-of-the-art experimental and theoretical research on TiO₂ based photocatalysts and identify challenges that have to be focused on to drive the field further. We conclude with a discussion of four challenges for TiO₂ photocatalysts-non-standardized presentation of results, bandgap in the ultraviolet (UV) region, lack of collaboration between experimental and theoretical work, and lack of large/small scale production facilities. We also highlight the importance of combining computational modeling with experimental work to make further advances in this exciting field.

Precisely Engineered Photoreactive Titanium Nanoarray Coating to Mitigate Biofouling in Ultrafiltration

To combat biofouling on membranes, diverse nanostructures of titanium dioxide (TiO₂) have emerged as effective antimicrobial coatings due to TiO₂'s abilities to transport charge and photoinduce oxidation. However, TiO₂ composite polymeric membranes synthesized using traditional methods of growing crystals have proven chemically unstable, with loss of coating and diminishing antimicrobial performance. Thus, new fabrication methods to enhance durability and efficacy should be considered. In this work, we propose a stepwise approach to construct a stable, uniform TiO₂ nanoarray of regularly spaced, aligned crystals on the surface of a polytetrafluoroethylene ultrafiltration membrane using precisely controlled atomic layer deposition (ALD) followed by solvothermal deposition. We demonstrate that ALD can uniformly seed TiO₂ nanoparticles on the membrane surface with atomic-scale precision. Subsequently, solvothermal deposition assembles and aligns a uniform TiO₂ nanoarray forest. In the presence of sunlight, this TiO₂ nanoarray effectively inactivates any deposited bacteria, increasing flux recovery after membrane cleaning. By systematically investigating this antimicrobial activity, we found that TiO₂ both physically damages cell membranes as well as produces reactive oxygen species in the presence of sunlight that inactivate bacteria. Our study provides an effective bottom-up synthesis scheme to optimize and tailor antifouling TiO₂ coatings for ultrafiltration and other surfaces for a wide range of applications.

Solid-State Synthesis of Direct Z-Scheme Cu2O/WO3 Nanocomposites with Enhanced Visible-Light Photocatalytic Performance

In this paper, we report the preparation of visible-light active direct Z-scheme Cu2O/WO3 nanocomposite photocatalyst by a solid-state reaction avoiding the otherwise inevitable formation of CuWO4 phase in wet syntheses. Structure, morphology, and thermal and optical properties of prepared WO3 nanoplatelets decorated by Cu2O were investigated by XRD, Raman spectroscopy, SEM/TEM, combined thermogravimetric (TG)/differential scanning calorimetry (DSC) analysis, and UV–VIS spectroscopy. The photocatalytic performance of the prepared samples under UV and visible light was studied through monitoring discoloration of methylene blue under illumination by selected wavelengths, allowing for the distinguishing between the contributions of the two semiconductive components. Experimental results showed that the decoration of WO3 nanoplates by Cu2O nanoparticles led to an improvement in photocatalytic performance, regardless of used LED (Light-Emitting Diode) wavelength, even at low concentrations. By using scavengers selectively blocking reactive species involved in the discoloration reaction, we determined that the Cu2O/WO3 nanocomposite exhibited the characteristics of direct Z-scheme-type photocatalyst.

Photocatalytic Decolorization of Methyl Red on Nanoporous Anodic ZrO2 of Different Crystal Structures

High surface area, self-organized nanoporous ZrO2 arrays with perfect adhesion to the Zr substrate were synthesized by anodization in an aqueous electrolyte containing (NH4)2SO4 and NH4F. The obtained semiconductor materials were tested as photocatalysts for decolorization of the methyl red (MR) as a model azo dye pollutant. It was demonstrated that as-synthesized anodic ZrO2 anodic layers are already crystalline and, therefore, do not require further thermal treatment to provide a high photocatalytic performance. However, photocatalytic efficiency could be improved by annealing at a relatively low-temperature of 350 °C. Higher annealing temperatures caused a gradual drop of photocatalytic activity. The photocatalytic behavior was correlated with the crystal phase transformation in anodic ZrO2. It was found that higher photocatalytic activity was observed for the tetragonal phase over the monoclinic phase (predominant at elevated temperatures). It results from the optimal and complex electronic structure of annealed ZrO2 with three different energy states having absorption edges at 2.0, 4.01 and 5.28 eV.

Contact Angle Relaxation on Amorphous, Mixed-Phase (Anatase + Rutile), and Anatase TiO2 Films and Its Mechanism

TiO₂ films generally undergo contact angle relaxation in the dark. It has been suggested that carbon contamination and the loss of surface OH generated by UV may be the major causes. However, the mechanisms for the long-lasting hydrophilicity have not been fully understood. Here, we studied contact angle relaxation of amorphous, mixed-phase, and anatase, and a new mechanism is proposed. After UV exposure and oxygen plasma treatment, the films' relaxation was observed over short-term (1 day) and long-term (>30 days) scales with XPS analysis using two quantitative parameters: relative amount and binding energy (B.E.) shifting. One day after plasma treatment, we observed that the donor-acceptor complex (DAC) and Ti-OH peaks of anatase shifted toward lower B.E., while the other films showed no shift or positive B.E. shifting. Interestingly, the relaxation of the amorphous and mixed-phase TiO₂ occurred over time despite the large number of total OH groups (I_OH/I_bulk > 75%) and DAC (I_DAC/I_bulk > 110%), and only the anatase film showed superhydrophilicity (∼10°) for 90 days. Also, the B.E. of all OH peaks increased over time, indicating that polarizable hydroxyls relaxed in the dark. Although the greater binding strength of Ti-OH and DAC on the anatase surface maintains long-lasting hydrophilicity, the loss of polarizable OH causes relaxation on the less-reactive TiO₂ films. Carbon contamination can also contribute to the relaxation over time. Taken together, we conclude that the surface energy, polarizable OH, and contaminants are the major factors affecting relaxation; this study gives a full picture of the mechanism integrated over some of the previously reported models.

Improved degradation of etodolac in the presence of core-shell ZnFe2O4/SiO2/TiO2 magnetic photocatalyst

In the present study, susceptibility to photocatalytic degradation of etodolac, 1,8-diethyl-1,3,4,9 - tetrahydro pyran - [3,4-b] indole-1-acetic acid, which is a non-steroidal anti-inflammatory drug frequently detected in an aqueous environment, was for the first time investigated. The obtained p-type TiO₂-based photocatalyst coupled with zinc ferrite nanoparticles in a core-shell structure improves the separation and recovery of nanosized TiO₂ photocatalyst. The characterization of ZnFe₂O₄/SiO₂/TiO₂, including XRD, XPS, TEM, BET, DR/UV-Vis, impedance spectroscopy and photocatalytic analysis, showed that magnetic photocatalyst containing anatase phase revealed markedly improved etodolac decomposition and mineralization measured as TOC removal compared to photolysis reaction. The effect of irradiation and pH range on photocatalytic decomposition of etodolac was studied. The most efficient degradation of etodolac was observed under simulated solar light for a core-shell ZnFe₂O₄/SiO₂/TiO₂ magnetic photocatalyst at pH above 4 (pKa = 4.7) and below 7. The irradiation of etodolac solution in a broader light range revealed a synergetic effect on its photodegradation performance. After only 20 min of degradation, about 100% of etodolac was degraded. Based on the photocatalytic analysis in the presence of scavengers and HPLC analysis, the transformation intermediates and possible photodegradation pathways of etodolac were studied. It was found that ∙O₂^- attack on C2-C3 bond inside pyrrole ring results mostly in the hydroxylation of the molecule, which next undergoes -CH₂COOH detachment to give 1,9-diethyl-3,4-dihydro-pyrano[3,4-b]indol-4a-ol. The obtained compound should further undergo subsequent hydropyran and pyrrole ring breaking to give a family of benzene derivatives.

Photocatalytic degradation of pesticides by nanofibrous membranes fabricated by colloid-electrospinning

Photocatalytic degradation of organic pollutants is a promising way to clean wastewater. Herein, we develop and compare two processes for fabricating nanofibrous membranes with photocatalytic properties. Hybrid nanofibers are produced by colloid-electrospinning and composed of metal oxide nanoparticles on sintered SiO₂ nanoparticles. The latter serves as support for the photocatalyst and preserves the structural integrity of nanofibers. Adsorption of metal salts on crosslinked polymer/SiO₂ fibers followed by calcination allows for the obtention of fibers with large amounts of metal oxide. Nanofibrous membranes with supported ZnO, In₂O₃, or mixture of both, display photocatalytic activity upon UV irradiation. The membranes can degrade a dye and an organophosphate pesticide more effectively than membranes directly fabricated from the calcination of metal oxides.

Influence of TiO2 Morphology and Crystallinity on Visible-Light Photocatalytic Activity of TiO2-Bi2O3 Composite in AOPs

Solution combustion synthesis was used to produce a junction between different TiO2 supports (anatase TiO2 nanorods (TNR) and nanoparticles (TNP) and TiO2 with anatase core and amorphous shell (a-TNR)) and narrow bandgap (BG) semiconductor β-Bi2O3. β-Bi2O3 acted as a visible-light photosensitizer and enabled us to carry out photocatalytic oxidation of water dissolved bisphenol A (BPA) with TiO2 based catalysts under visible-light illumination. Heterojunction between TiO2 and β-Bi2O3 in TNR + Bi and TNP + Bi composites enables the transfer of visible-light generated holes from the β-Bi2O3 valence band (VB) to the upper lying TiO2 VB. A p–n junction, established upon close chemical contact between TiO2 and β-Bi2O3, enables the transfer of visible-light generated electrons in the β-Bi2O3 conduction band (CB) to the TiO2 CB. In TNR + Bi and a-TNR + Bi composites, the supplied heat energy during the synthesis of samples was not sufficient to completely transform (BiO)2CO3 into β-Bi2O3. A p–n junction between (BiO)2CO3 and β-Bi2O3 enables the transfer of electrons generated by β-Bi2O3 to (BiO)2CO3. Hindered charge carrier recombination originating from the crystallinity of TiO2 is a more important factor in the overall kinetics of BPA degradation than high specific surface area of the amorphous TiO2 and reduction/oxidation of surface adsorbed substrates.

Enhanced interfacial adhesion and osseointegration of anodic TiO2 nanotube arrays on ultra-fine-grained titanium and underlying mechanisms

The poor adhesion of anodic TiO₂ nanotubes (TNTs) arrays on titanium (Ti) substrates adversely affects applications in many fields especially biomedical engineering. Herein, an efficient strategy is described to improve the adhesion strength of TNTs by performing grain refinement in the underlying Ti substrate via high-pressure torsion processing, as a larger number of grain boundaries can provide more interfacial mechanical anchorage. This process also improves the biocompatibility and osseointegration of TNTs by increasing the surface elastic modulus. The TNTs in length of 0.4 µm have significantly larger adhesion strength than the 2.0 µm long ones because the shorter TNTs experience less interfacial internal stress. However, post-anodization annealing reduces the fluorine concentration in TNTs and adhesion strength due to the formation of interfacial cavities during crystallization. The interfacial structure of TNTs/Ti system and the mechanism of adhesion failures are further investigated and discussed. STATEMENT OF SIGNIFICANCE: Self-assembled TiO₂ nanotubes (TNTs) prepared by electrochemical anodization have a distinct morphology and superior properties, which are commonly used in photocatalytic systems, electronic devices, solar cells, sensors, as well as biomedical implants. However, the poor adhesion between the TNTs and Ti substrate has hampered wider applications. Here in this study, we describe an efficient strategy to improve the adhesion strength of TNTs by performing grain refinement in the underlying Ti substrate via high-pressure torsion (HPT) processing. The interfacial structure of TNTs/Ti system and the mechanism of adhesion failure are systematically studied and discussed. Our findings not only develop the knowledge of TNTs/Ti system, but also provide new insights into the design of Ti-based implants for orthopedic applications.

Influence of Mg, Cu, and Ni Dopants on Amorphous TiO2 Thin Films Photocatalytic Activity

The present study investigates Mg (0 ÷ 17.5 wt %), Cu (0 ÷ 21 wt %) and Ni (0 ÷ 20.2 wt %) dopants (M-doped) influence on photocatalytic activity of amorphous TiO₂ thin films. Magnetron sputtering was used for the deposition of M-doped TiO₂ thin films. According to SEM/EDS surface analysis, the magnetron sputtering technique allows making M-doped TiO₂ thin films with high uniformity and high dopant dispersion. Photocatalysis efficiency analysis was set in oxalic acid under UV irradiation. In accordance with the TOC (total organic carbon) measurements followed by the apparent rate constant (k_app) results, the dopants’ concentration peak value was dopant-dependent; for Mg/TiO₂, it is 0.9% (k_app—0.01866 cm⁻¹), for Cu/TiO₂, it is 0.6% (k_app—0.02221 cm⁻¹), and for Ni/TiO₂, it is 0.5% (k_app—0.01317 cm⁻¹). The obtained results clearly state that a concentration of dopants in TiO₂ between 0.1% and 0.9% results in optimal photocatalytic activity.

Photo-annealed amorphous titanium oxide for perovskite solar cells

Electron selective layers are important to the efficiency, stability and hysteresis of perovskite solar cells. Photo-annealing is a low-cost, roll-to-roll-compatible process that can be applied to the post-treatment fabrication of sol-gel based metal oxide layers. Here, we fabricate an amorphous titanium oxide electron selective layer at a low temperature in a dry atmosphere using a UV light annealing system and compare it with a thermal annealing process. Active oxygen species are created by using UV light to promote hydrolysis and condense the TiO₂ precursor, which removes organic ligands effectively. The photo-annealed TiO₂-based perovskite solar cell has a power conversion efficiency of 19.37% without hysteresis.

TiO2-based Photocatalytic Cementitious Composites: Materials, Properties, Influential Parameters, and Assessment Techniques

Applications of heterogeneous photocatalytic processes based on semiconductor particles in cement-based materials have received great attention in recent years in enhancing the aesthetic durability of buildings and reducing global environmental pollution. Amongst all, titanium dioxide (TiO₂) is the most widely used semiconductor particle in structural materials with photocatalytic activity because of its low cost, chemically stable nature, and absence of toxicity. Utilization of TiO₂ in combination with cement-based materials would plunge the concentration of urban pollutants such as NO_x. In fact, cementitious composites containing TiO₂ have already found applications in self-cleaning buildings, antimicrobial surfaces, and air-purifying structures. This paper aims to present a comprehensive review on TiO₂-based photocatalysis cement technology, its practical applications, and research gaps for further progression of cementitious materials with photocatalytic activity.

In-situ creating elastic lattice OO bonds over semicrystalline yellow TiO2 nanoparticles for significantly enhanced photocatalytic H2 production

Atomic defects (e.g., Ti³⁺, oxygen vacancies) have been intenstively investigated for modifying TiO₂ in order to reach visible light active photocatalytic H₂ production. However, the atomic defects within TiO₂ could easily act as photo-generated charge-carrier recombination centers, resulting in relatively low H₂ conversion efficiency. In this paper, semicrystalline yellow TiO₂ nanoparticles rich of superoxide ions are new synthesized by a simple aqueous solution method. Instead of introducing atomic defects, we show for the first time that catalytic performance can also be significantly improved via in-situ creating lattice OO bonds within metastable semicrystalline TiO₂. The synthesized semicrystalline yellow TiO₂ exhibits significantly enhanced photocatalytic activity for H₂ production after cycle tests. The formaldehyde in aqueous solution is used as target pollutant to simulate industrial wastewater. In-situ created elastic lattice OO bonds are proposed to improve catalytic performance through facilitating the breakage of CH bonds of HCHO. A series of internally consistent reaction equations is proposed that describes the role of in-situ created lattice OO bonds for improving the catalytic performance. This is strongly supported by that the H₂ production rate at the end of the fourth cycle test is significantly more than that of the beginning of the first cycle test.

Photocatalytic Abatement of Volatile Organic Compounds by TiO2 Nanoparticles Doped with Either Phosphorous or Zirconium

The aim of this work is to study the activity of novel TiO2-based photocatalysts doped with either phosphorus or zirconium under a UV-Vis source. A set of mesoporous catalysts was prepared by the direct synthesis: TiO2_A and TiO2_B (titanium oxide synthesized by two different procedures), P-TiO2 and Zr-TiO2 (binary oxides with either nonmetal or metal into the TiO2 framework). Complementary characterizations (N2 physisorption at 77 K, X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) analysis, X-ray Photoelectron Spectroscopy (XPS), and (DR)UV-Vis spectroscopy) were used to investigate the physicochemical properties of the prepared catalysts. Then, the photocatalysts were tested for the oxidation of propylene and ethylene under UV-Vis light. As a result, the most promising catalyst for both the propylene and ethylene oxidation reactions was the P-TiO2 (propylene conversion = 27.8% and ethylene conversion = 13%, TOS = 3 h), thus confirming the beneficial effect of P-doping into the TiO2 framework on the photocatalytic activity.