Search Results (10)

With the aim of reducing catalysts’ cost while maintaining high performance in water splitting, ZnO and RuO₂ were combined into composites with ZnO to RuO₂ mass ratios of 1:1, 2:1, and 10:1. The ZnO/RuO₂ composites were prepared by microwave processing of a suspension containing Zn(OH)₂ in situ precipitated onto RuO₂ powder, and subsequently thermally modified at 600 °C to promote heterojunction formation and alter the defect chemistry. Phase composition, crystal structure, morphology, and optical properties were analyzed in detail employing XRD, TEM/HRTEM, HAADF-STEM with EDS, PL and XPS spectroscopy. The photoelectrocatalytic (PEC) activity of the composites toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) was evaluated by linear sweep voltammetry in alkaline electrolyte (0.1 M NaOH, pH 13), before and after one hour of electrochemical system illumination. The analysis focused on surface and bulk oxygen vacancies, which may have a crucial impact in PEC activity, by (1) promoting charge separation and increasing the number of active sites thus enhancing PEC activity, or (2) acting as electron–hole traps and recombination centers, reducing the lifetime of photo-induced charge carriers and thus deteriorating PEC activity. The presented results demonstrate that the combination of ZnO with RuO₂ in a specific mass ratio, along with controlled defect structure, offers a worthwhile route for developing bifunctional, noble-metal-reduced catalysts for green hydrogen and oxygen production. Full article

The formation of intrinsic point defects in the N-sublattice of semi-insulating Mg-doped GaN crystals grown by the ammonothermal method (SI AT GaN:Mg) was investigated for the first time. The grown-in defects produced by the displacement of nitrogen atoms were experimentally observed as deep traps revealed by the Laplace transform photoinduced transient spectroscopy in the compensated p-type crystals with the Mg concentrations of 6 × 10¹⁸ and 2 × 10¹⁹ cm⁻³ and resistivities of ~10¹¹ Ωcm and ~10⁶ Ωcm, respectively. In both kinds of materials, three closely located traps with activation energies of 430, 450, and 460 meV were revealed. The traps, whose concentrations in the stronger-doped material were found to be significantly higher, are assigned to the (3+/+) and (2+/+) transition levels of nitrogen vacancies as well as to the (2+/+) level of nitrogen split interstitials, respectively. In the material with the lower Mg concentration, a middle-gap trap with the activation energy of 1870 meV was found to be predominant. The results are confirmed and quantitatively described by temperature-dependent Hall effect measurements. The mechanism of nitrogen atom displacement due to the local strain field arising in SI AT GaN:Mg is proposed and the effect of the Mg concentration on the charge compensation is discussed. Full article

Heterogeneous photocatalysts incorporating metal halide perovskites (MHPs) have garnered significant attention due to their remarkable attributes: strong visible-light absorption, tuneable band energy levels, rapid charge transfer, and defect tolerance. Additionally, the promising optical and electronic properties of MHP nanocrystals can be harnessed for photocatalytic applications through controlled crystal structure engineering, involving composition tuning via metal ion and halide ion variations, dimensional tuning, and surface chemistry modifications. Combination of perovskites with other materials can improve the photoinduced charge separation and charge transfer, building heterostructures with different band alignments, such as type-II, Z-scheme, and Schottky heterojunctions, which can fine-tune redox potentials of the perovskite for photocatalytic organic reactions. This review delves into the activation of organic molecules through charge and energy transfer mechanisms. The review further investigates the impact of crystal engineering on photocatalytic activity, spanning a diverse array of organic transformations, such as C–X bond formation (X = C, N, and O), [2 + 2] and [4 + 2] cycloadditions, substrate isomerization, and asymmetric catalysis. This study provides insights to propel the advancement of metal halide perovskite-based photocatalysts, thereby fostering innovation in organic chemical transformations. Full article

The characteristics of the surface and interface of nanocomposites are important for exerting multi-functional properties and widening interdisciplinary applications. These properties are mainly depending on the electronic structures of materials. Some key factors, such as the surface, interface, grain boundaries, and defects take vital roles in the contribution of desired properties. Due to the excellent sensitivity of the QCM (quartz crystal microbalance) device, the surface and interface features of the nanocomposite were studied with the aid of the gas-response of the sensors (Sensor’s Gas-Sensitivity) in this work. To make full use of the visible light and part of NIR, a ZnO/MnS_x nanocomposite was constructed using hydrothermal synthesis for narrowing the bandgap width of wide bandgap materials. The results indicated that the absorbance of the resulting nanocomposite was extended to part of the NIR range due to the introduction of impurity level or defect level, although ZnO and MnS belonged to wide bandgap semiconductor materials. To explore the physical mechanism of light activities, the photoconductive responses to weak visible light (650 nm, etc.) and NIR (near-infrared) (808 nm, 980 nm, and 1064 nm, etc.) were studied based on interdigital electrodes of Au on flexible PET (polyethylene terephthalate) film substrate with the casting method. The results showed that the on/off ratio of ZnO/MnS_x nanocomposite to weak visible light and part of NIR light were changed by about one to five orders of magnitude, with changes varying with the amount of MnS_x nanoparticle loading due to defect-assisted photoconductive behavior. It illustrated that the ZnO/MnS_x nanocomposite easily produced photo-induced free charges, effectively avoiding the recombination of electrons/holes because of the formation of strong built-in electrical fields. To examine the surface and interface properties of nanocomposites, chemical prototype sensor arrays were constructed based on ZnO, ZnO/MnS_x nanocomposite, and QCM arrays. The adsorption response behaviors of the sensor arrays to some typical volatile compounds were examined under a similar micro-environment. The results exhibited that in comparison to ZnO nanosheets, the ZnO nanosheets/MnS_x nanocomposite increased adsorption properties to some typical organic volatile compounds significantly. It would have good potential applications in photo-catalysts, self-cleaning films, multi-functional coatings, and organic pollutants treatment (VOCs) of environmental fields for sustainable development. It provided some reference value to explore the physical mechanism of materials physics and photophysics for photo-active functional nanocomposites. Full article

Nanosized titanium dioxide (TiO₂) is currently being actively studied by the global scientific community, since it has a number of properties that are important from a practical point of view. One of these properties is a large specific surface, which makes this material promising for use in photocatalysts, sensors, solar cells, etc. In this work, we prepared photocatalysts based on TiO₂ nanotubes for converting carbon dioxide (CO₂) into energy-intensive hydrocarbon compounds. Efficient gas-phase CO₂ conversion in the prepared single-walled TiO₂ nanotube-Cu_xO composites was investigated. Parameters of defects (radicals) in composites were studied. Methanol and methane were detected during the CO₂ photoreduction process. In single-walled TiO₂ nanotubes, only Ti³⁺/oxygen vacancy defects were detected. The Cu²⁺ centers and O₂⁻ radicals were found in TiO₂ nanotube-Cu_xO composites using the EPR technique. It has been established that copper oxide nanoparticles are present in the TiO₂ nanotube-Cu_xO composites in the form of the CuO phase. A phase transformation of CuO to Cu₂O takes place during illumination, as has been shown by EPR spectroscopy. It is shown that defects accumulate photoinduced charge carriers. The mechanism of methane and methanol formation is discussed. The results obtained are completely original and show high promise for the use of TiO₂-Cu_xO nanotube composites as photocatalysts for CO₂ conversion into hydrocarbon fuel precursors. Full article

Two series of Sc³⁺- and Nb⁵⁺-doped TiO₂ (rutile) samples were synthesized and characterized by SEM, ICPE spectroscopy, XPS, and BET methods. Photocatalytic activity of the doped TiO₂ samples was tested in photocatalytic degradation of phenol. Dependences of the photocatalytic activities of the doped TiO₂ samples demonstrate a volcano-like behavior, indicating the existence of the optimal dopant concentrations to achieve the highest activity of photocatalysts. Remarkably, the optimal dopant concentrations correspond to the extrema observed in work function dependences on the dopant concentrations, that indicates a significant energy redistribution of the defect states within the bandgap of TiO₂. Such a redistribution of the defect states is also proven by the alterations of the optical and EPR spectra of the intrinsic Ti³⁺ defect states in TiO₂. Based on the analysis of the experimental results, we conclude that both Sc³⁺ and Nb⁵⁺ doping of TiO₂ results in redistribution of the defect states and the optimal dopant concentrations correspond to the defect structures, which are ineffective in charge carrier recombination, that ultimately leads to the higher photocatalytic activity of doped TiO₂. Full article

To achieve self-cleaning at a low maintenance cost, we investigated the possibility of obtaining a sustainable hydrophilic surface of TiO₂ thin film. As the hydrophilicity of TiO₂ films fabricated by FTS has not yet been studied, we deposited TiO_x using FTS, and then TiO₂ was formed through additional treatment. Hydrophilic surfaces were obtained by thermoinduced and photoinduced methods. UV irradiation led to the conversion of Ti⁴⁺ to Ti³⁺ in the lattice structure and an increase in the number of OH groups on the surface, and annealing induced the formation of Ti³⁺ defect sites, as well as organic degradation and changes in the crystal structure. Through the annealing process, the water contact angle of as-deposited film was decreased from 78.7° to 35.7°, and crystallinity changed from amorphous to anatase. These changes contributed to the formation of a hydrophilic surface and reduced the water contact angle by up to 10.8°. After the formation of a hydrophilic surface through annealing and UV irradiation, the sample returned to its original state. We confirmed that the water contact angle of the returned sample was decreased through exposure to sunlight; it reduced the water contact angle of the returned sample by 15.2°. Thus, the results revealed that the crystallinity influences the hydrophilicity and its sustainability for TiO₂ films under sunlight. Full article

A solid-state Ultraviolet-photoreduction process of silver cations to produce Ag⁰ nanostructures on a mesoporous silica is presented as an innovative method for the preparation of efficient environmental anti-fouling agents. Mesoporous silica powder, contacted with AgNO₃, is irradiated at 366 nm, where silica surface defects absorb. The detailed characterization of the materials enables us to document the silica assisted photo-reduction. The appearance of a Visible (Vis) band centered at 470 nm in the extinction spectra, due to the surface plasmon resonance of Ag⁰ nanostructures, and the morphology changes observed in transmission electron microscopy (TEM) images, associated with the increase of Ag/O ratio in energy dispersive X-ray (EDX) analysis, indicate the photo-induced formation of Ag⁰. The data demonstrate that the photo-induced reduction of silver cation occurs in the solid state and takes place through the activation of silica defects. The activation of the materials after UV-processing is then tested, evaluating their antimicrobial activity using an environmental filamentous fungus, Aspergillus niger. The treatment doubled inhibitory capacity in terms of minimal inhibitory concentration (MIC) and biofilm growth. The antimicrobial properties of silver–silica nanocomposites are investigated when dispersed in a commercial sealant; the nanocomposites show excellent dispersion in the silicon and improve its anti-fouling capacity. Full article

In this study, the impact of the type of ligand at the surface of colloidal CdSe@CdS dot-in-rod nanostructures on the basic exciton relaxation and charge localization processes is closely examined. These systems have been introduced into the field of artificial photosynthesis as potent photosensitizers in assemblies for light driven hydrogen generation. Following photoinduced exciton generation, electrons can be transferred to catalytic reaction centers while holes localize into the CdSe seed, which can prevent charge recombination and lead to the formation of long-lived charge separation in assemblies containing catalytic reaction centers. These processes are in competition with trapping processes of charges at surface defect sites. The density and type of surface defects strongly depend on the type of ligand used. Here we report on a systematic steady-state and time-resolved spectroscopic investigation of the impact of the type of anchoring group (phosphine oxide, thiols, dithiols, amines) and the bulkiness of the ligand (alkyl chains vs. poly(ethylene glycol) (PEG)) to unravel trapping pathways and localization efficiencies. We show that the introduction of the widely used thiol ligands leads to an increase of hole traps at the surface compared to trioctylphosphine oxide (TOPO) capped rods, which prevent hole localization in the CdSe core. On the other hand, steric restrictions, e.g., in dithiolates or with bulky side chains (PEG), decrease the surface coverage, and increase the density of electron trap states, impacting the recombination dynamics at the ns timescale. The amines in poly(ethylene imine) (PEI) on the other hand can saturate and remove surface traps to a wide extent. Implications for catalysis are discussed. Full article

We study the spatio-temporal formation and spreading of the low-spin state (LS) during the thermal spin transition and the cooperative relaxation of the photo-induced metastable high spin (HS) state at low temperature, in the presence of a structural defect. The model is made of a two-dimensional rectangular-shaped lattice with discrete spins coupled by springs. The investigations are performed for a perfect lattice and a lattice with a hole (simulating the defect) with a fixed size. We found that the presence of the defect affects the thermal equilibrium by reducing the size of the thermal hysteresis at the transition, although the transition temperature remains unchanged. The study of the low-temperature relaxation of the defect-free lattice from HS to LS state indicated the existence of three different regimes of the growth process: (i) a first regime of growth from one corner of the rectangle along the width, then followed by (ii) a second regime of longitudinal propagation at almost constant velocity, and (iii) a third rapid regime when the system feels the surface or the border of the crystal. When a hole is injected inside the lattice, it results in (i) the deformation of the HS/LS interface’s shape when it approaches the defect position; and (ii) the slowing down of its propagation velocity. These results, which are in good agreement with available experimental data, are discussed in terms of elastic energy stored in the system during the relaxation process. Full article