PbO-modified TiO2 thin films: a route to visible light photocatalysts

Experimental and computational study of metal oxide nanoparticles for the photocatalytic degradation of organic pollutants: a review

Photocatalysis is a more proficient technique that involves the breakdown or decomposition of different organic contaminants, various dyes, and harmful viruses and fungi using UV or visible light solar spectrum. Metal oxides are considered promising candidate photocatalysts owing to their low cost, efficiency, simple fabricating method, sufficient availability, and environment-friendliness for photocatalytic applications. Among metal oxides, TiO2 is the most studied photocatalyst and is highly applied in wastewater treatment and hydrogen production. However, TiO2 is relatively active only under ultraviolet light due to its wide bandgap, which limits its applicability because the production of ultraviolet is expensive. At present, the discovery of a photocatalyst of suitable bandgap with visible light or modification of the existing photocatalyst is becoming very attractive for photocatalysis technology. However, the major drawbacks of photocatalysts are the high recombination rate of photogenerated electron-hole pairs, the ultraviolet light activity limitations, and low surface coverage. In this review, the most commonly used synthesis method for metal oxide nanoparticles, photocatalytic applications of metal oxides, and applications and toxicity of different dyes are comprehensively highlighted. In addition, the challenges in the photocatalytic applications of metal oxides, strategies to suppress these challenges, and metal oxide studied by density functional theory for photocatalytic applications are described in detail.

Controlling the Structural Properties and Optical Bandgap of PbO–Al2O3 Nanocomposites for Enhanced Photodegradation of Methylene Blue

In the present work, PbO-x wt% Al2O3 nanocomposites (where x = 0, 10, 20, 30, 40, 50, 60, 70, and 100 wt%) were prepared by a microwave irradiation method. Their structural parameters, morphology, and chemical bonds, were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier-transform infrared spectroscopy (FTIR). It was noticed that the produced phases have an orthorhombic crystal structure and the smaller average crystallite sizes were formed when the ratio of Al2O3 is 40 wt%. The FTIR analysis reveals the formation of various bonds between Al or Pb and O. The TEM analysis reveals that the PbO-x%Al2O3 composites (x = 20, 40, and 60), composed of dense particles, and their size are smaller compared to the pure Al2O3 sample. The optical bandgap obeys the direct allowed transition and decreases from 4.83 eV to 4.35 eV as the PbO ratio in the composites increases from 0 to 100%. The intensity of the photoluminescence emission, at the same wavelength, increases as the PbO ratio increases from 0% to 60% implying that increasing the PbO content increases the capacity of free carriers within the trap centers. The prepared composites are used as a catalyst to remove the methylene blue (MB) from the wasted water under UV-visible or visible light irradiations. The photocatalytic degradation of MB was investigated by applying various kinetic models. It was found that the PbO-30% Al2O3, and PbO-40% Al2O3 composites are the best ones amongst other compositions. Furthermore, the pseudo-second-order model is the best model for describing the deterioration mechanism among the models studied. The formed composites could be suitable for the degradation of organic dyes for water purification as well as applications that required a higher optical bandgap.

Zn and N Codoped TiO2 Thin Films: Photocatalytic and Bactericidal Activity

We explore a series of Zn and N codoped TiO₂ thin films grown using chemical vapor deposition. Films were prepared with various concentrations of Zn (0.4-2.9 at. % Zn vs Ti), and their impact on superoxide formation, photocatalytic activity, and bactericidal properties were determined. Superoxide (O₂^•-) formation was assessed using a 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium sodium salt (XTT) as an indicator, photocatalytic activity was determined from the degradation of stearic acid under UVA light, and bactericidal activity was assessed using a Gram-negative bacterium E. coli under both UVA and fluorescent light (similar to what is found in a clinical environment). The 0.4% Zn,N:TiO₂ thin film demonstrated the highest formal quantum efficiency in degrading stearic acid (3.3 × 10^-5 molecules·photon^-1), while the 1.0% Zn,N:TiO₂ film showed the highest bactericidal activity under both UVA and fluorescent light conditions (>3 log kill). The enhanced efficiency of the films was correlated with increased charge carrier lifetime, supported by transient absorption spectroscopy (TAS) measurements.

Surface Transformations of Lead Oxides and Carbonates Using First-Principles and Thermodynamics Calculations

Lead (Pb)-containing solids find widespread commercial use in batteries, piezoelectrics, and as starting materials for synthesis. Here, we combine density functional theory (DFT) and thermodynamics in a DFT + solvent ion model to compare the surface reactivity of Pb oxides and carbonates, specifically litharge, massicot, and cerussite, in contact with water. The information provided by this model is used to delineate structure-property relationships for surfaces that are able to release Pb as Pb²⁺. We find that Pb²⁺ release is dependent on pH and chemical bonding environment and go on to correlate changes in the surface bonding to key features of the electronic structure through a projected density of states analysis. Collectively, our analyses link the atomistic structure to i) specific electronic states and ii) the thermodynamics of surface transformations, and the results presented here can be used to guide synthetic efforts of Pb²⁺-containing materials in aqueous media or be used to better understand the initial steps in solid decomposition.

Mechanochemical synthesis of nanophotocatalysts SiO2/TiO2/Fe2O3: their structural, thermal and photocatalytic properties

In the study the mechanochemical synthesis was used to prepare photocatalytic materials based on TiO2, SiO2, and Fe2O3. During the preparation the impact of composition, milling speed, and calcination process on the properties of the composites was investigated. The structural and thermal properties of photocatalysts were determined using the N2 adsorption/desorption, XRD, and FT-IR/PAS methods. The thermal stability of the obtained materials was also examined (TG/DTG). Moreover, their photocatalytic activity was tested in relation to Methylene Blue at UV and Vis radiation. The results indicate that the mechanochemical synthesis in the high-energy planetary mill is an effective method for obtaining materials with photocatalytic properties at the UV and Vis radiation. It was shown that the removal process of MB may be described by the pseudo-first-order kinetics.

Activation of Water on MnOx-Nanocluster-Modified Rutile (110) and Anatase (101) TiO2 and the Role of Cation Reduction

Surface modification of titania surfaces with dispersed metal oxide nanoclusters has the potential to enhance photocatalytic activity. These modifications can induce visible light absorption and suppress charge carrier recombination which are vital in improving the efficiency. We have studied heterostructures of Mn4O6 nanoclusters modifying the TiO2 rutile (110) and anatase (101) surfaces using density functional theory (DFT) corrected for on-site Coulomb interactions (DFT + U). Such studies typically focus on the pristine surface, free of the point defects and surface hydroxyls present in real surfaces. In our study we have considered partial hydroxylation of the rutile and anatase surfaces and the role of cation reduction, via oxygen vacancy formation, and how this impacts on a variety of properties governing the photocatalytic performance such as nanocluster adsorption, light absorption, charge separation, and reducibility. Our results indicate that the modifiers adsorb strongly at the surface and that modification extends light absorption into the visible range. MnOx-modified titania can show an off-stoichiometric ground state, through oxygen vacancy formation and cation reduction spontaneously, and both modified rutile and anatase are highly reducible with moderate energy costs. Manganese ions are therefore present in a mixture of oxidation states. Photoexcited electrons and holes localize at cluster metal and oxygen sites, respectively. The interaction of water at the modified surfaces depends on the stoichiometry and spontaneous dissociation to surface bound hydroxyls is favored in the presence of oxygen vacancies and reduced metal cations. Comparisons with bare TiO2 and other TiO2-based photocatalyst materials are presented throughout.

Antimicrobial Properties of Copper-Doped ZnO Coatings under Darkness and White Light Illumination

We report the first antimicrobial study of transparent and robust Cu-doped ZnO coatings that displayed potent antimicrobial activity that resulted in bacterial (Escherichia coli) reduction below detection limits within 6 h of illumination via a white light source that is found in hospital environments. The same bacterial reduction rate was observed even under darkness for 4.0 atom % Cu-doped ZnO films thus providing an efficient 24 h disinfection. All films were produced via a novel, inexpensive, and easily scalable route and were also thoroughly analyzed for their material properties.

Chemical Vapor Deposition Synthesis and Optical Properties of Nb2O5 Thin Films with Hybrid Functional Theoretical Insight into the Band Structure and Band Gaps

Nb₂O₅ is an important material able to exist in many polymorphs with unique optical properties and morphologies that are dependent on the synthetic route. Here we report a novel ambient-pressure chemical vapor deposition route to Nb₂O₅ via aerosol-assisted chemical vapor deposition. The amorphous as-deposited films were annealed in air to obtain the the three most stable crystal structures: orthorhombic, tetragonal, and monoclinic. The films were thoroughly characterized for their material properties, and an in-depth study into the optical properties was carried out using state-of-the-art hybrid functional theory that allowed more insight into the optical properties of the materials.

Design of Novel Visible Light Active Photocatalyst Materials: Surface Modified TiO2

Work on the design of new TiO2 based photocatalysts is described. The key concept is the formation of composite structures through the modification of anatase and rutile TiO2 with molecular-sized nanoclusters of metal oxides. Density functional theory (DFT) level simulations are compared with experimental work synthesizing and characterizing surface modified TiO2 . DFT calculations are used to show that nanoclusters of metal oxides such as TiO2 , SnO/SnO2 , PbO/PbO2 , ZnO and CuO are stable when adsorbed at rutile and anatase surfaces, and can lead to a significant red shift in the absorption edge which will induce visible light absorption; this is the first requirement for a useful photocatalyst. The origin of the red shift and the fate of excited electrons and holes are determined. For p-block metal oxides the oxidation state of Sn and Pb can be used to modify the magnitude of the red shift and its mechanism. Comparisons of recent experimental studies of surface modified TiO2 that validate our DFT simulations are described. These nanocluster-modified TiO2 structures form the basis of a new class of photocatalysts which will be useful in oxidation reactions and with a correct choice of nanocluster modified can be applied to other reactions.

Bismuth oxyhalides: synthesis, structure and photoelectrochemical activity

We report the synthesis and photoelectrochemical assessment of phase pure tetragonal matlockite structured BiOX (where X = Cl, Br, I) films.

We report the synthesis and photoelectrochemical assessment of phase pure tetragonal matlockite structured BiOX (where X = Cl, Br, I) films. The materials were deposited using aerosol-assisted chemical vapour deposition. The measured optical bandgaps of the oxyhalides, supported by density functional theory calculations, showed a red shift with the increasing size of halide following the binding energy of the anion p-orbitals that form the valence band. Stability and photoelectrochemical studies carried out without a sacrificial electron donor showed the n-type BiOBr film to have the highest photocurrent reported for BiOBr in the literature to date (0.3 mA cm^–2 at 1.23 V vs. RHE), indicating it is an excellent candidate for solar fuel production with a very low onset potential of 0.2 V vs. RHE. The high performance was attributed to the preferred growth of the film in the [011] direction, as shown by X-ray diffraction, leading to internal electric fields that minimize charge carrier recombination.

Solution based CVD of main group materials

This critical review focuses on the solution based chemical vapour deposition (CVD) of main group materials with particular emphasis on their current and potential applications. Deposition of thin films of main group materials, such as metal oxides, sulfides and arsenides, have been researched owing to the array of applications which utilise them including solar cells, transparent conducting oxides (TCOs) and window coatings. Solution based CVD processes, such as aerosol-assisted (AA)CVD have been developed due to their scalability and to overcome the requirement of suitably volatile precursors as the technique relies on the solubility rather than volatility of precursors which vastly extends the range of potentially applicable compounds. An introduction into the applications and precursor requirements of main group materials will be presented first followed by a detailed discussion of their deposition reviewed according to this application. The challenges and prospects for further enabling research in terms of emerging main group materials will be discussed.

Tungsten Doped TiO2 with Enhanced Photocatalytic and Optoelectrical Properties via Aerosol Assisted Chemical Vapor Deposition

Tungsten doped titanium dioxide films with both transparent conducting oxide (TCO) and photocatalytic properties were produced via aerosol-assisted chemical vapor deposition of titanium ethoxide and dopant concentrations of tungsten ethoxide at 500 °C from a toluene solution. The films were anatase TiO_2, with good n-type electrical conductivities as determined via Hall effect measurements. The film doped with 2.25 at.% W showed the lowest resistivity at 0.034 Ω.cm and respectable charge carrier mobility (14.9 cm³/V.s) and concentration (×10¹⁹ cm⁻³). XPS indicated the presence of both W⁶⁺ and W⁴⁺ in the TiO₂ matrix, with the substitutional doping of W⁴⁺ inducing an expansion of the anatase unit cell as determined by XRD. The films also showed good photocatalytic activity under UV-light illumination, with degradation of resazurin redox dye at a higher rate than with undoped TiO₂.

Aerosol assisted chemical vapour deposition of a ZrO2–TiO2 composite thin film with enhanced photocatalytic activity

ZrO₂–TiO₂ composite thin film deposited via aerosol assisted chemical vapour deposition showed enhanced photocatalytic activity compared to pure anatase TiO₂. The photocatalytic properties were determined using resasurin redox dye.

Nanochemistry-derived Bi2WO6 nanostructures: towards production of sustainable chemicals and fuels induced by visible light

The advances of Bi₂WO₆ nanostructures utilized in photocatalytic organic synthesis and fuel production under visible light are discussed and prospected.