Spectroscopic assessment of the role of hydrogen in surface defects, in the electronic structure and transport properties of TiO2, ZnO and SnO2 nanoparticles

Precursor- and waste-free synthesis of spark-ablated nanoparticles with enhanced photocatalytic activity and stability towards airborne organic pollutant degradation

Photocatalyst synthesis typically involves multiple steps, expensive precursors, and solvents. In contrast, spark ablation offers a simple process of electrical discharges in a gap between two electrodes made from a desirable material. This enables a precursor- and waste-free generation of pure metal oxide nanoparticles or mixtures of various compositions. This study presents a two-step method for the production of photocatalytic filters with deposited airborne MnOx, TiO2, and ZnO nanoparticles using spark ablation and calcination processes. The resulting MnOx and TiO2 filters demonstrated almost twice the activity with outstanding performance stability, as compared to sol-gel MnO2 and commercial TiO2. The introduced method is not only simple, precursor- and waste-free, and leads to superior performance for the case studied, but it also has future potential due to its versatility. It can easily produce mixed and doped materials with further improved properties, making it an interesting avenue for future research.

Effect of Urea as a Shape-Controlling Agent on the Properties of Bismuth Oxybromides

Bismuth oxybromides were prepared via a solvothermal method by applying different urea amounts during synthesis. The effects of the urea ratio on the morpho–structural properties and photocatalytic activity of the samples were investigated. X-ray diffraction, diffuse reflectance spectroscopy, infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, and surface tension measurements were carried out to characterize the samples. Their photoactivity was evaluated by the photocatalytic degradation of rhodamine B and ibuprofen under UV and visible light irradiations. The urea ratio notably influenced morphology, particle size distribution, and photoactivity. However, it only had a limited effect on the crystalline composition, primary crystallite size, and band gap of bismuth oxybromides. The formation of Bi-based complexes and degraded urea-based products were observed, which were deduced to influence band gap energies and hence, photoactivity. Predominantly, samples prepared at low urea ratios proved to be the best for both rhodamine B and ibuprofen degradations under both irradiations.

Application of BiOX Photocatalyst to Activate Peroxydisulfate Ion-Investigation of a Combined Process for the Removal of Organic Pollutants from Water

The persulfate-based advanced oxidation processes employing heterogeneous photocatalysts to generate sulfate radicals (SO4•−) from peroxydisulfate ion (PDS, S2O82−) have been extensively investigated to remove organic pollutants. In this work, BiOX (X = Cl, Br, and I) photocatalysts were investigated to activate PDS and enhance the transformation rate of various organic substances under UV (398 nm) and Vis (400–700 nm) radiation. For BiOCl and BiOBr, in addition to excitability, the light-induced oxygen vacancies are decisive in the activity. Although without organic substances, the BiOI efficiency highly exceeds that of BiOBr and BiOCl for PDS activation (for BiOI, 15–20%, while for BiOBr and BiOCl, only 3–4% of the PDS transformed); each BiOX catalyst showed enhanced activity for 1,4-hydroquinone (HQ) transformation due to the semiquinone radical-initiated PDS activation. For sulfamethoxypyridazine (SMP), the transformation is driven by direct charge transfer, and the effect of PDS was less manifested. BiOI proved efficient for transforming various organic substances even under Vis radiation. The efficiency was enhanced by PDS addition (HQ is wholly transformed within 20 min, and SMP conversion increased from 40% to 90%) without damaging the catalyst; its activity did change over three consecutive cycles. Results related to the well-adsorbed trimethoprim (TRIM) and application of biologically treated domestic wastewater as a matrix highlighted the limiting factors of the method and visible light active photocatalyst, BiOI.

Aerosol-Assisted Deposition for TiO2 Immobilization on Photocatalytic Fibrous Filters for VOC Degradation

Atomization and spraying are well-established methods for the production of submicrometer- and micrometer- sized powders. In addition, they could be of interest to the immobilization of photocatalytic nanoparticles onto supports because they enable the formation of microporous films with photocatalytic activity. Here, we provide a comparison of aerosol-assisted immobilization methods, such as spray-drying (SD), spray atomization (SA), and spray gun (SG), which were used for the deposition of TiO₂ dispersions onto fibrous filter media. The morphology, microstructure, and electronic properties of the structures with deposited TiO₂ were characterized by SEM and TEM, BET and USAXS, and UV-Vis spectrometry, respectively. The photocatalytic performances of the functionalized filters were evaluated and compared to the benchmark dip-coating method. Our results showed that the SG and SA immobilization methods led to the best photocatalytic and operational performance for the degradation of toluene, whereas the SD method showed the lowest degradation efficiency and poor stability of coating. We demonstrated that TiO₂ sprays using the SG and SA methods with direct deposition onto filter media involving dispersed colloidal droplets revealed to be promising alternatives to the dip-coating method owing to the ability to uniformly cover the filter fibers. In addition, the SA method allowed for fast and simple control of the coating thickness as the dispersed particles were continuously directed onto the filter media without the need for repetitive coatings, which is common for the SG and dip-coating methods. Our study highlighted the importance of the proper immobilization method for the efficient photocatalytic degradation of VOCs.

Investigation of the efficiency of BiOI/BiOCl composite photocatalysts using UV, cool and warm white LED light sources - Photon efficiency, toxicity, reusability, matrix effect, and energy consumption

BiOI, BiOCl, and their composites (BiOI:BiOCl) with molar ratios from 95:5 to 5:95 were synthesized and tested in the transformation of methyl orange (MO) and sulfamethoxypyridazine (SMP) antibiotic, using three various LED light sources: UV LEDs (398 nm), cool and warm white LEDs (400-700 nm). The 80:20 BiOI:BiOCl photocatalyst showed the best adsorption capacity for MO and enhanced activity compared to BiOI and BiOCl. The apparent quantum yield (Φ_app) of the MO and SMP transformation for cool and warm white light was slightly lower than for 398 nm UV radiation. The effect of methanol and 1,4-benzoquinone proved that the transformation is initiated mainly via direct charge transfer, resulting in the demethylation of MO and SO₂ extrusion from SMP. The change of photocatalytic efficiency was followed during three cycles. After the first one, the transformation rates decreased, but there was no significant difference between the second and third cycles. The decreased efficiency is most probably caused by the intermediates, whose continuous accumulation was observed during the cycles. Ecotoxicity measurements confirmed that no toxic substances were leached from the catalyst, but the transformation of both MO and SMP results in toxic intermediates. Using 80:20 BiOI:BiOCl and LED light source, the energy requirement of the removal is about half of the value determined using TiO₂ and a mercury vapor lamp. The effect of some components of wastewater (Cl^-, HCO₃^- and humic acids), pH, and two matrices on the composite photocatalysts' efficiency and stability were also investigated.

The Effect of the Reducing Sugars in the Synthesis of Visible-Light-Active Copper(I) Oxide Photocatalyst

In the present work, shape tailored Cu₂O microparticles were synthesized by changing the nature of the reducing agent and studied subsequently. d-(+)-glucose, d-(+)-fructose, d-(+)xylose, d-(+)-galactose, and d-(+)-arabinose were chosen as reducing agents due to their different reducing abilities. The morpho-structural characteristics were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and diffuse reflectance spectroscopy (DRS), while their photocatalytic activity was evaluated by methyl orange degradation under visible light (120 min). The results show that the number of carbon atoms in the sugars affect the morphology and particle size (from 250 nm to 1.2 µm), and differences in their degree of crystallinity and photocatalytic activity were also found. The highest activity was observed when glucose was used as the reducing agent.

NO2gas sensor properties of In2O3-CuO Nanocomposite thin films prepared by chemical spray pyrolysis

The NO2gas sensor was synthesized from In2O3-CuO Nanocomposite thin films, deposited on glass substrate, with different mixing ratios (5, 15)Vol% of copper oxide, by chemical spray pyrolysis technique, at 400°C. The X-ray diffraction results showed that all the prepared films are polycrystalline, crystalline size in the range of (27.9 - 13.79 nm) with mixed ratio from 0 -15 Vol% of CuO respectively. From atomic force microscopy (AFM), the grain size ranged from (107.5 - 52.35 nm), and the prepared films have average roughness of (3.13-17.7 nm). The results of (UV-visible spectrometer) showed that the prepared films have high transmittance, and direct energy gap (3.5 -3.15) eV. The prepared films showed a high sensitivity to NO2with a concentration of 193ppm. For the unmixed films, the sensitivity reached (89.77%) with an equal response and recovery times of (24s), at 200°C operating temperature. The mixture thin films gave less sensitivity than unmixed indium oxide thin films.

Innovative visualization of the effects of crystal morphology on semiconductor photocatalysts. Tuning the Hückel polarity of the shape-tailoring agents: the case of Bi2WO6

Morphology was quantified for the first time; the rose-shape measured in “rosality-R_SDC” was found to be directly related with the activity and structural properties.

Porous Co3O4/SnO2 quantum dot (QD) heterostructures with abundant oxygen vacancies and Co2+ ions for highly efficient gas sensing and oxygen evolution reaction

Porous Co3O4/SnO2 quantum dot (QD) heterojunctions with a strong synergistic effect are successfully synthesized in this paper. Owing to the strong synergistic effect between Co3O4 and SnO2QDs, Co3O4/SnO2QD heterostructures possess more Co2+ ions for a faster Co2+/Co0 redox reaction in the process of sensing of reducing gases and electrochemical reactions, and more oxygen vacancies for more active sites and reduced charge transfer resistance on the surface. These advantages are demonstrated to significantly enhance the gas sensitivity to xylene and greatly improve the catalysis for the oxygen evolution reaction (OER). As a catalyst for the OER, Co3O4/SnO2QD (1 : 1) heterostructures exhibit the highest current density, lowest onset potential, largest active surface area and remarkable durability in alkaline electrolytes. The sensitivity of Co3O4/SnO2QD (1 : 1) heterostructures to 100 ppm xylene is almost 10 times higher than that of pure Co3O4 nanosheets and 3 times higher than that of SnO2QDs. In addition, Co3O4/SnO2QD (1 : 1) heterostructure sensors exhibit excellent gas selectivity, long-term stability and markedly high response to low concentrations of xylene at low operating temperatures.

Study on room temperature gas-sensing performance of CuO film-decorated ordered porous ZnO composite by In2O3 sensitization

For the first time, ordered mesoporous ZnO nanoparticles have been synthesized by a template method. The electroplating after chemical plating method was creatively used to form copper film on the surface of the prepared ZnO, and then a CuO film-decorated ordered porous ZnO composite (CuO/ZnO) was obtained by a high-temperature oxidation method. In₂O₃ was loaded into the prepared CuO film-ZnO by an ultrasonic-assisted method to sensitize the room temperature gas-sensing performance of the prepared CuO/ZnO materials. The doped In₂O₃ could effectively improve the gas-sensing properties of the prepared materials to nitrogen oxides (NO _x ) at room temperature. The 1% In₂O₃ doped CuO/ZnO sample (1 wt% In₂O₃-CuO/ZnO) showed the best gas-sensing properties whose response to 100 ppm NO _x reached 82%, and the detectable minimum concentration reached 1 ppm at room temperature. The prepared materials had a good selectivity, better response, very low detection limit, and high sensitivity to NO _x gas at room temperature, which would have a great development space in the gas sensor field and a great research value.

Role of vanadium oxide and palladium multiple loading on the sensitivity and recovery kinetics of tin dioxide based gas sensors

Improving the gas sensing properties by Pd and V₂O₅ co-loading on the SnO₂ attributed to the role of each additive.

"Crystallographic" holes: new insights for a beneficial structural feature for photocatalytic applications

One of the most fundamental aspects of the heterogeneous catalysis field is the manipulation of the catalysts' activity. In photocatalysis this is carried out by maximizing the right crystal plane of a semiconductor oxide. Until now, most of the papers have achieved this by a combination of different oxides, with noble metals and sometimes with carbon nanomaterials. In this work MWCNTs (multiwalled carbon nanotubes) were applied as "crystallization promoters" in a very simple, safe, one-step hydrothermal method. By this method TiO₂ nano/micro crystals with exposed {001} facets were obtained in the first step. The next episode in the crystal manipulation "saga" was the modification of the (001) crystallographic plane's structure by creating ordered/own faceted "crystallographic holes". These elements are capable of further enhancing the obtained activity of titania microcrystals to a higher extent, as shown by the UV driven photocatalytic phenol degradation experiments. The appearance of the holes was "provoked" by simple calcination and their presence and influence were demonstrated by XPS and HRTEM.

Surface-modification of SnO2 nanoparticles by incorporation of Al for the detection of combustible gases in a humid atmosphere

Aluminium-doped SnO₂ nanoparticles inhibited hydroxyl poisoning and enhanced the sensor response in a humid atmosphere.

Si doped highly crystalline mesoporous In2O3 nanowires: synthesis, characterization and ultra-high response to NOx at room temperature

The synthesized INW-2 has an ultrathin surface layer and high density defects. The special structure offers available active centers for gas/surface reactions. INW-2 sensor possesses the ultrahigh response and selectivity to NOx at room temperature.

A successive photocurrent transient study to probe the sub-band gap electron and hole traps in ZnO nanorods

Probing of the sub-band gap electron and hole traps in ZnO nanorods has been carried out using a simple technique of successive photocurrent transients.