Scalable Route toward Superior Photoresponse of UV-Laser-Treated TiO2 Nanotubes

Volatile organic compound removal by post plasma-catalysis over porous TiO2 with enriched oxygen vacancies in a dielectric barrier discharge reactor

Non-thermal plasma (NTP) degradation of volatile organic compounds (VOCs) into CO₂ and H₂O is a promising strategy for addressing ever-growing environment pollution. However, its practical implementation is hindered by low conversion efficiency and emissions of noxious by-products. Herein, an advanced low-oxygen-pressure calcination process is developed to fine-tune the oxygen vacancy concentration of MOF-derived TiO₂ nanocrystals. Vo-poor and Vo-rich TiO₂ catalysts were placed in the back of an NTP reactor to convert harmful ozone molecules into ROS that decompose VOCs via heterogeneous catalytic ozonation processes. The results indicate that Vo-TiO₂-5/NTP with the highest Vo concentration exhibited superior catalytic activity in the degradation of toluene compared to NTP-only and TiO₂/NTP, achieving a maximum 96% elimination efficiency and 76% CO_x selectivity at an SIE of 540 J L^-1. Mechanistic analysis reveals that the ¹O₂, ˙O₂^- and ˙OH species derived from the activation of O₃ molecules on Vo sites contribute to the decomposition of toluene over the Vo-rich TiO₂ surface. With the aid of advanced characterization and density functional theory calculations, the roles of oxygen vacancies in manipulating the synergistic capability of post-NTP systems were explored, and were attributed to increased O₃ adsorption ability and enhanced charge transfer dynamics. This work presents novel insights into the design of high-efficiency NTP catalysts structured with active Vo sites.

Scanning with Laser Beam over the TiO2 Nanotubes Covered with Thin Chromium Layers towards the Activation of the Material under the Visible Light

This work presents pulsed UV laser treatment (355 nm, 2 Hz) of TiO₂ nanotubes decorated with chromium oxides. The modification was performed in a system equipped with a beam homogenizer, and during the irradiation, the samples were mounted onto the moving motorized table. In such a system, both precisely selected areas and any large area of the sample can be modified. Photoelectrochemical tests revealed photoresponse of laser-treated samples up to 1.37- and 18-fold under the illumination with ultraviolet-visible and visible light, respectively, in comparison to bare titania. Optimal beam energy fluence regarding sample photoresponse has been established. Scanning electron microscopy images, X-ray diffraction patterns, along with Raman and X-ray photoelectron spectra, suggest that the enhanced photoresponse results from changes solely induced in the layer of chromium oxides. It is believed that the results of the present work will contribute to a wider interest in laser modification of semiconductors exhibiting improved photoelectrochemical activity.

The Impact of Side-Selective Laser Tailoring of Titania Nanotubes on Changes in Photoelectrocatalytic Activity

Over the last few decades, titanium(IV) oxide-based materials have gained particular attention due to their stability, corrosion resistance, photocatalytic activity under UV light, and possibilities for modification. Among various structures, TiO₂ nanotubes (NTs) grown on Ti foil or glass substrates and obtained through a simple anodization process are widely used as photocatalysts or photoanodes. During the anodization process, the geometry of the nanotubes (length, distribution, diameter, wall thickness, etc.) is easily controlled, though the obtained samples are amorphous. Heat treatment is required to transform the amorphous material into crystalline material. However, instead of time- and cost-consuming furnace treatment, fast and precise laser annealing is applied as a promising alternative. Nonetheless, laser treatment can result in geometry changes of TiO₂ NTs, consequently altering, their electrochemical activity. Moreover, modification of the TiO₂ NTs surfaces with transition metals and further laser treatment can result in materials with unique photoelectrochemical properties. In this regard, we gathered the latest achievements in the field of laser-treated titania for this review paper. We mainly focused on single structural and morphological changes resulting from pulsed laser annealing and their influence on the electrochemical properties of titania. Finally, the theoretical basis for and combination of laser- and metal-modifications and their impact on the resulting possibilities for electrochemical water splitting are also discussed.

Gas Sensors Based on Titanium Oxides (Review)

Nanostructured titanium compounds have recently been applied in the design of gas sensors. Among titanium compounds, titanium oxides (TiO2) are the most frequently used in gas sensing devices. Therefore, in this review, we are paying significant attention to the variety of allotropic modifications of titanium oxides, which include anatase, rutile, brukite. Very recently, the applicability of non-stoichiometric titanium oxide (TiO2−x)-based layers for the design of gas sensors was demonstrated. For this reason, in this review, we are addressing some research related to the formation of non-stoichiometric titanium oxide (TiO2−x) and Magnéli phase (TinO2n−1)-based layers suitable for sensor design. The most promising titanium compounds and hetero- and nano-structures based on these compounds are discussed. It is also outlined that during the past decade, many new strategies for the synthesis of TiO2 and conducting polymer-based composite materials were developed, which have found some specific application areas. Therefore, in this review, we are highlighting how specific formation methods, which can be used for the formation of TiO2 and conducting polymer composites, can be applied to tune composite characteristics that are leading towards advanced applications in these specific technological fields. The possibility to tune the sensitivity and selectivity of titanium compound-based sensing layers is addressed. In this review, some other recent reviews related to the development of sensors based on titanium oxides are overviewed. Some designs of titanium-based nanomaterials used for the development of sensors are outlined.

Nanostructure of the laser-modified transition metal nanocomposites for water splitting

Although hydrogen is considered by many to be the green fuel of the future, nowadays it is primarily produced through steam reforming, which is a process far from ecological. Therefore, emphasis is being put on the development of electrodes capable of the efficient production of hydrogen and oxygen from water. To make the green alternative possible, the solution should be cost-efficient and well processable, generating less waste which is a huge challenge. In this work, the laser-based modification technique of the titania nanotubes containing sputtered transition metal species (Fe, Co, Ni, and Cu) was employed. The characteristics of the electrodes are provided both for the hydrogen and oxygen evolution reactions, where the influence of the laser treatment has been found to have the opposite effect. The structural and chemical analysis of the substrate material provides insight into pathways towards more efficient, low-temperature water splitting. Laser-assisted integration of transition metal with the tubular nanostructure results in the match-like structure where the metal species are accumulated at the head. The electrochemical data indicates a significant decrease in material resistance that leads to an overpotential of only +0.69 V at 10 mA cm^-2for nickel-modified material.

Free-standing TiO2 nanotubes decorated with spherical nickel nanoparticles as a cost-efficient electrocatalyst for oxygen evolution reaction

Here, we report significant activity towards the oxygen evolution reaction (OER) of spherical nickel nanoparticles (NPs) electrodeposited onto free-standing TiO2 nanotubes (TNT) via cyclic voltammetry. It has been shown that simple manipulation of processing parameters, including scan rate and number of cycles, allows for formation of the NPs in various diameters and amounts. The polarization data with respect to transmission electron microscopy (TEM) allowed for determination of the diameter and propagation depth of the Ni NPs leading to the highest activity towards the OER with an overpotential of 540 mV at +10 mA cm-2 and Tafel slope of 52 mV per decade. X-ray photoelectron spectroscopy (XPS) indicates the presence of structure defects within Ni NPs whereas Mott-Schottky analysis provides information on the anodically shifted flat band potential and highly increased donor density. The obtained results along with literature studies allowed a proposal of the origin of the enhancement towards the OER. We believe that combination of transition metal-based NPs and TNT provides valuable insight on efficient and low-cost electrocatalysts.

Insights in the Application of Stoichiometric and Non-Stoichiometric Titanium Oxides for the Design of Sensors for the Determination of Gases and VOCs (TiO2-x and TinO2n-1 vs. TiO2)

In this review article, attention is paid towards the formation of various nanostructured stoichiometric titanium dioxide (TiO2), non-stoichiometric titanium oxide (TiO2-x) and Magnéli phase (TinO2n-1)-based layers, which are suitable for the application in gas and volatile organic compound (VOC) sensors. Some aspects related to variation of sensitivity and selectivity of titanium oxide-based sensors are critically overviewed and discussed. The most promising titanium oxide-based hetero- and nano-structures are outlined. Recent research and many recently available reviews on TiO2-based sensors and some TiO2 synthesis methods are discussed. Some promising directions for the development of TiO2-based sensors, especially those that are capable to operate at relatively low temperatures, are outlined. The applicability of non-stoichiometric titanium oxides in the development of gas and VOC sensors is foreseen and transitions between various titanium oxide states are discussed. The presence of non-stoichiometric titanium oxide and Magnéli phase (TinO2n-1)-based layers in 'self-heating' sensors is predicted, and the advantages and limitations of 'self-heating' gas and VOC sensors, based on TiO2 and TiO2-x/TiO2 heterostructures, are discussed.

Anodic titania nanotubes decorated with gold nanoparticles produced by laser-induced dewetting of thin metallic films

Herein, we combine titania layers with gold species in a laser-supported process and report a substantial change of properties of the resulting heterostructures depending on the major processing parameters. Electrodes were fabricated via an anodisation process complemented with calcination to ensure a crystalline phase, and followed by magnetron sputtering of metallic films. The obtained TiO2 nanotubes with deposited thin (5, 10 nm) Au films were treated with a UV laser (355 nm) to form Au nanoparticles on top of the nanotubes. It was proven that selected laser working parameters ensure not only the formation of Au nanoparticles, but also simultaneously provide preservation of the initial tubular architecture, while above-threshold laser fluences result in partial destruction (melting) of the top layer of the nanotubes. For almost all of the samples, the crystalline phase of the nanotubes observed in Raman spectra was maintained independently of the laser processing parameters. Enhanced photoresponse up to ca 6 mA/cm2 was demonstrated by photoelectrochemical measurements on samples obtained by laser annealing of the 10 nm Au coating on a titania support. Moreover, a Mott-Schottky analysis indicated the dramatically increased (two orders of magnitude) concentration of donor density in the case of a laser-treated Au-TiO2 heterojunction compared to reference electrodes.

Formation of the hollow nanopillar arrays through the laser-induced transformation of TiO2 nanotubes

In the following article, we present a simple, two-step method of creating spaced, hollow nanopillars, from the titania nanotube arrays via pulsed laser-treatment. Due to the high ordering of the structure, the prepared material exhibits photonic properties, which has been shown to increase the overall photoefficiency. The optical and morphological changes in the titania nanotubes after pulsed laser-treatment with 532, 355, and 266 nm wavelengths in the 10-50 mJ/cm2 fluence range are studied. The investigation reveals, that by using appropriate wavelength and energy, the number of surface defects, geometrical features, or both can be tailored.

The Effect of Laser Re-Solidification on Microstructure and Photo-Electrochemical Properties of Fe-Decorated TiO2 Nanotubes

Fossil fuels became increasingly unpleasant energy source due to their negative impact on the environment; thus, attractiveness of renewable, and especially solar energy, is growing worldwide. Among others, the research is focused on smart combination of simple compounds towards formation of the photoactive materials. Following that, our work concerns the optimized manipulation of laser light coupled with the iron sputtering to transform titania that is mostly UV-active, as well as exhibiting poor oxygen evolution reaction to the material responding to solar light, and that can be further used in water splitting process. The preparation route of the material was based on anodization providing well organized system of nanotubes, while magnetron sputtering ensures formation of thin iron films. The last step covering pulsed laser treatment of 355 nm wavelength significantly changes the material morphology and structure, inducing partial melting and formation of oxygen vacancies in the elementary cell. Depending on the applied fluence, anatase, rutile, and hematite phases were recognized in the final product. The formation of a re-solidified layer on the surface of the nanotubes, in which thickness depends on the laser fluence, was shown by microstructure studies. Although a drastic decrement of light absorption was recorded especially in UV range, laser-annealed samples have shown activity under visible light even 20 times higher than bare titania. Electrochemical analysis has shown that the improvement of photoresponse originates mainly from over an order of magnitude higher charge carrier density as revealed by Mott-Schottky analysis. The results show that intense laser light can modulate the semiconductor properties significantly and can be considered as a promising tool towards activation of initially inactive material for the visible light harvesting.

Atomic Layer Deposition of SnO2-Coated Anodic One-Dimensional TiO2 Nanotube Layers for Low Concentration NO2 Sensing

The continuous emission of nitrous oxides contributes to the overall air pollution and deterioration of air quality. In particular, an effective NO₂ sensor capable of low concentration detection for continuous monitoring is demanded for safety, health, and wellbeing. The sensing performance of a metal oxide-based sensor is predominantly influenced by the availability of surface area for O₂ adsorption and desorption, efficient charge transport, and size or thickness of the sensing layer. In this study, we utilized anodic one-dimensional (1D) TiO₂ nanotube layers of 5 μm thick which offer large surface area and unidirectional electron transport pathway as a platform to accommodate thin SnO₂ coatings as a sensing layer. Conformal and homogeneous SnO₂ coatings across the entire inner and outer TiO₂ nanotubes were achieved by atomic layer deposition with a controlled thickness of 4, 8, and 16 nm. The SnO₂-coated TiO₂ nanotube layers attained a higher sensing response than a reference Figaro SnO₂ sensor. Specifically, the 8 nm SnO₂-coated TiO₂ nanotube layer has recorded up to ten-fold enhancement in response as compared to the blank nanotubes for the detection of 1 ppm NO₂ at an operating temperature of 300 °C with 0.5 V applied bias. This is attributed to the SnO₂/TiO₂ heterojunction effect and controlled SnO₂ thickness within the range of the Debye length. We demonstrated in this work, a tailored large surface area platform based on 1D nanotubes with thin active coatings as an efficient approach for sensing applications and beyond.

Laser-induced crystallization of anodic TiO2 nanotube layers

In this study, crystallization of amorphous TiO₂ nanotube (TNT) layers upon optimized laser annealing is shown. The resulting anatase TNT layers do not show any signs of deformation or melting. The crystallinity of the laser annealed TNT layers was investigated using X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM). The study of the (photo-)electrochemical properties showed that the laser annealed TNT layers were more defective than conventional TNT layers annealed in a muffle oven at 400 °C, resulting in a higher charge recombination rate and lower photocurrent response. However, a lower overpotential for hydrogen evolution reaction was observed for the laser annealed TNT layer compared to the oven annealed TNT layer.

Laser treatment of amorphous TiO₂ nanotube layers leads to their crystallization without deformation or formation of the thermal oxide layer.

The In-Depth Studies of Pulsed UV Laser-Modified TiO2 Nanotubes: The Influence of Geometry, Crystallinity, and Processing Parameters

The laser processing of the titania nanotubes has been investigated in terms of morphology, structure, and optical properties of the obtained material. The length of the nanotubes and crystallinity, as well as the atmosphere of the laser treatment, were taken into account. The degree of changes of the initial geometry of nanotubes were checked by means of scanning electron microscopy, which visualizes both the surface and the cross-section. The phase conversion from the amorphous to anatase has been achieved for laser-treated amorphous material, whereas modification of calcined one led to distortion within the crystal structure. This result is confirmed both by Raman and grazing incident XRD measurements. The latter studies provided an in-depth analysis of the crystalline arrangement and allowed also for determining the propagation of laser modification. The narrowing of the optical bandgap for laser-treated samples has been observed. Laser treatment of TiO₂ nanotubes can lead to the preparation of the material of desired structural and optical parameters. The usage of the motorized table during processing enables induction of changes in the precisely selected area of the sample within a very short time.