The effect of organic cations on the electronic, optical and luminescence properties of 1D piperidinium, pyridinium, and 3-hydroxypyridinium lead trihalides.

We present a structural and optoelectronic study of 1D piperidinium, pyridinium, and 3-hydroxypyridinium lead trihalides. In contrast to the piperidinium and pyridinium species whose single inorganic chains [PbX31-]n are separated by organic cations, the 3-hydroxypyridinium compound is characterized by double inorganic chains. According to DFT the valence and conduction bands of the piperidinium lead trihalides are composed of occupied p-orbitals of the halogen anions and unoccupied p-orbitals of the Pb2+ cations. In contrast, the pyridinium species feature low-lying cationic energy levels formed from the cation's π*-orbitals. Thus, electronic transitions between the cationic energy levels and valence bands require less energy than valence to conduction band transitions in the case of piperidinium lead trihalides. The presence of an OH group in the pyridinium ring leads to a bathochromic shift of the cationic energy levels resulting in a decreased energy of transitions from the cationic energy levels to the valence band. Electronic transitions predicted by DFT are observable in experimental optical absorption and luminescence spectra. This study paves the way for creation of 1D perovskite-like structures with desired optoelectronic properties.

Pyridinium lead tribromide and pyridinium lead triiodide: quasi-one-dimensional perovskites with an optically active aromatic π-system.

Two novel hybrid organic-inorganic perovskites, pyridinium lead tribromide and pyridinium lead triiodide, are prepared. Based on the XRD data, we find that the compounds are quasi-one-dimensional crystals of Pna21 symmetry. DFT calculations demonstrate that the valence band of the new compounds is comprised of the occupied p-orbitals of the halogen anions, while their conduction bands CB and CB + 1 are composed mainly of the unoccupied π-orbitals of the aromatic pyridinium rings. The computed DOS matches well with the recorded XPS spectra. The pyridinium lead tribromide and pyridinium lead triiodide absorption peaks derived from the experimental DRS are located at ∼3.1 eV and ∼2.4 eV, respectively. An agreement between the experimentally and theoretically predicted absorption spectra is found. The synthesized compounds extend the range of quasi-one-dimensional organometallic perovskites suitable for optical and possibly electronic applications.

New insights into the plasmonic enhancement for photocatalytic H2 production by Cu-TiO2 upon visible light illumination.

Cu nanoparticles were deposited on the surface of commercial TiO2 nanoparticles (Cu-TiO2) using different methods aiming at the production of highly efficient visible light photocatalysts. Photocatalytic H2 evolution rates obtained from methanol/water mixtures revealed no significant influence of the presence of copper oxides on the photoreaction upon visible light illumination. The photocatalytic H2 production rates were evaluated upon illumination with different spectral ranges (≥420 nm or ≥500 nm) and the results evidenced that the visible light induced charge carrier formation on the Cu-TiO2 photocatalysts consists of two distinct pathways: the direct excitation of TiO2 and the induced excitation by the so-called surface plasmon resonance (SPR) effect of the Cu nanoparticles on the TiO2 surface. Both pathways are present when the full visible range of the spectrum is used (≥420 nm), while for illumination at longer wavelengths (≥500 nm), the photocatalytic activity is solely promoted by the Cu-SPR effect. Electron paramagnetic resonance (EPR) and laser flash photolysis measurements were performed to clarify the underlying mechanism of Cu-TiO2 photocatalysts upon visible light illumination.

Harvesting visible light with MoO3 nanorods modified by Fe(iii) nanoclusters for effective photocatalytic degradation of organic pollutants.

The photocatalytic performance of MoO3 is limited due to its weak visible light absorption ability and quick recombination of charge carriers. In the present work, we report the facile synthesis of Fe(iii)-grafted MoO3 nanorods using a hydrothermal method followed by an impregnation technique with the aim of enhancing the light harvesting ability and photocatalytic efficiency of MoO3. The prepared samples were characterized through the standard analytical techniques of XRD, SEM-EDS, TEM, XPS, UV-Vis-DRS, FT-IR, TG-DTA and PL spectrophotometry. XPS and TEM analyses reveal that Fe(iii) ions are successfully grafted onto the surface of the MoO3 nanorod with intimate interfacial contact. The photocatalytic performances of the prepared samples were investigated by studying the degradation of methylene blue (MB), rhodamine B (RhB) and 4-nitrophenol (4-NP) under visible light irradiation. The surface-modified MoO3 with Fe(iii) ions showed excellent photocatalytic activity towards the degradation of the above-mentioned pollutants, where Fe(iii) ions act as effective cocatalytic sites to produce hydroxyl radicals through multi-electron reduction of oxygen molecules. The improved photocatalytic activity could be ascribed to the effective separation of charge carriers and efficient production of hydroxyl radicals via the rapid capture of electrons by Fe(iii) through a well-known photoinduced interfacial charge transfer mechanism. Based on scavenger analysis study, a mechanism for the enhanced photocatalytic activity has been discussed and proposed. The concept of surface grafting onto large bandgap semiconductors with ubiquitous elements opens up a new avenue for the development of visible-light-responsive photocatalysts with excellent photocatalytic activity.

Sonochemical synthesis of porous NiTiO3 nanorods for photocatalytic degradation of ceftiofur sodium.

Porous NiTiO3 nanorods were synthesized through the sonochemical route followed by calcination at various temperature conditions. Surface morphology of the samples was tuned by varying the heat treatment temperature from 100 to 600°C. The synthesized NiTiO3 nanorods were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, diffused reflectance spectroscopy, photoluminescence spectroscopy and Brunauer-Emmett-Teller (BET) analyses. The characterization studies revealed that the NiTiO3 nanomaterial was tuned to porous and perfectly rod shaped structure during the heat treatment at 600°C. The porous NiTiO3 nanorods showed visible optical response and thus can be utilized in the photocatalytic degradation of ceftiofur sodium (CFS) under direct sunlight. The photoluminescence intensity of the porous NiTiO3 nanorods formed while heating at 600°C was lower than that of the as-synthesized NiTiO3 sample owing to the photogenerated electrons delocalization along the one dimensional nanorods and this delocalization resulted in the reduction of the electron-hole recombination rate. The photocatalytic degradation of ceftiofur sodium (CFS) was carried out using NiTiO3 nanorods under the direct sunlight irradiation and their intermediate products were analysed through HPLC to deduce the possible degradation mechanism. The porous NiTiO3 nanorods exhibited an excellent photocatalytic activity towards the CFS degradation and further, the photocatalytic activity was increased by the addition of peroxomonosulfate owing to the simultaneous generation of both OH and SO4-.

Ease synthesis of mesoporous WO3-TiO2 nanocomposites with enhanced photocatalytic performance for photodegradation of herbicide imazapyr under visible light and UV illumination.

Herein, we report the ease synthesis of mesoporous WO3-TiO2 nanocomposites at different WO3 contents (0-5wt%) together with their photocatalytic performance for the degradation of the imazapyr herbicide under visible light and UV illumination. XRD and Raman spectra indicated that the highly crystalline anatase TiO2 phase and monoclinic and triclinic of WO3 were formed. The mesoporous TiO2 exhibits large pore volumes of 0.267cm(3)g-1 and high surface areas of 180m(2)g(-1) but they become reduced to 0.221cm(3)g(-1) and 113m(2)g(-1), respectively upon WO3 incorporation, with tunable mesopore diameter in the range of 5-6.5nm. TEM images show WO3-TiO2 nanocomposites are quite uniform with 10-15nm of TiO2 and 5-10nm of WO3 sizes. Under UV illumination, the overall photocatalytic efficiency of the 3% WO3-TiO2 nanocomposite is 3.5 and 6.6 times higher than that of mesoporous TiO2 and commercial UV-100 photocatalyst, respectively. The 3% WO3-TiO2 nanocomposite is considered to be the optimum photocatalyst which is able to degrade completely (100% conversion) of imazapyr herbicide along 120min with high photonic efficiency ∼8%. While under visible light illumination, the 0.5% WO3-TiO2 nanocomposite is the optimum photocatalyst which achieves 46% photocatalytic efficiency.

Effect of polar and movable (OH or NH2 groups) on the photocatalytic H2 production of alkyl-alkanolamine: a comparative study.

The photocatalytic production of molecular hydrogen (H2) from aqueous solutions of alkyl-alkanolamines (triethanolamine (tri-EOA), diethanolamine (di-EOA), ethanolamine (EOA), ethylamine (EA) and ethanol over Pt-loaded commercial (Sachtleben Hombikat UV100) nanoparticle titanium dioxide photocatalysts was studied. These photocatalysts were characterized using X-ray diffraction, Brunauer-Emmett-Teller, field-emission scanning electron microscopy, atomic force microscopy and UV-vis diffuse reflectance spectroscopy techniques. Effect of pH and temperature on photocatalytic hydrogen production of alkyl-alkanolamines was investigated. It was found for all molecules under study that the amount of hydrogen produced at a constant illumination time is a function of pH and temperature. At all temperatures investigated the rate of hydrogen production at pH 9 followed the order tri-EOAethanolamine >> di-EOA ≈ EOA > ethanol > EA. From the values of the photocatalytic rate constants at various temperatures, the apparent activation energies were determined for the hydrogen production. The observed dependence of the reaction rate on solution temperature cannot be related to light-driven reaction steps, because the band-gap energy of the semiconductor is too high for thermal excitation to become significant in the temperature range investigated.

The nature of chlorine-inhibition of photocatalytic degradation of dichloroacetic acid in a TiO2-based microreactor.

Photocatalytic degradation of dichloroacetic acid (DCA) was studied in a continuous-flow set-up using a titanium microreactor with an immobilized double-layered TiO2 nanoparticle/nanotube film. Chloride ions, formed during the degradation process, negatively affect the photocatalytic efficiency and at a certain concentration (approximately 0.5 mM) completely stop the reaction in the microreactor. Two proposed mechanisms of inhibition with chloride ions, competitive adsorption and photogenerated-hole scavenging, have been proposed and investigated by adsorption isotherms and electron paramagnetic resonance (EPR) measurements. The results show that chloride ions block the DCA adsorption sites on the titania surface and reduce the amount of adsorbed DCA molecules. The scavenging effect of chloride ions during photocatalysis through the formation of chlorine radicals was not detected.

Transition metal-modified zinc oxides for UV and visible light photocatalysis.

In order to use photocatalysis with solar light, finding more active and especially visible light active photocatalysts is a very important challenge. Also, studies of these photocatalysts should employ a standardized test procedure so that their results can be accurately compared and evaluated with one another. A systematic study of transition metal-modified zinc oxide was conducted to determine whether they are suitable as visible light photocatalysts. The photocatalytic activity of ZnO modified with eight different transition metals (Cu, Co, Fe, Mn, Ni, Ru, Ti, Zr) in three different concentrations (0.01, 0.1, and 1 at.%) was investigated under irradiation with UV as well as with visible light. The employed activity test is the gas-phase degradation of acetaldehyde as described by the ISO standard 22197-2. The results suggest that the UV activity can be improved with almost any modification element and that there exists an optimal modification ratio at about 0.1 at.%. Additionally, Mn- and Ru-modified ZnO display visible light activity. Especially the Ru-modified ZnO is highly active and surpasses the visible light activity of all studied titania standards. These findings suggest that modified zinc oxides may be a viable alternative to titanium dioxide-based catalysts for visible light photocatalysis. Eventually, possible underlying mechanisms are proposed and discussed.

Synthesis and photocatalytic activity of boron-doped TiO(2) in aqueous suspensions under UV-A irradiation.

Boron-doped TiO(2) photocatalysts were synthesized employing a sol-gel method. Boric acid was used as the boron source and titanium tetra-isopropoxide as the TiO(2) precursor, both dissolved in isopropanol. Nominal boron to titanium atomic ratios were in the range 0 to 4%. After the hydrolysis step, two different procedures for the recovery of TiO(2) were followed, based on either centrifugation of the resulting reaction mixture or evaporation of the solvent under reduced pressure, both followed by a subsequent calcination step performed at 400 or 500 degrees C. The photocatalytic efficiency of the synthesized photocatalysts was assessed by measuring the photocatalytic mineralization of dichloroacetic acid in aqueous suspensions under UV-A irradiation and it was compared to the corresponding efficiency of the commercial Degussa P 25 TiO(2). Photocatalytic efficiency of the synthesized catalysts was higher for the boron-doped TiO(2) synthesized at 2% boron to titanium nominal atomic ratio, centrifuged after the hydrolysis step followed by calcinations at 400 degrees C. However, all photocatalysts synthesized in this work showed lower photocatalytic activity than Degussa P 25 TiO(2), thus highlighting the need of further improvements of the proposed method.

Optical density and photonic efficiency of silica-supported TiO2 photocatalysts.

Over the last years, many research groups have developed supported TiO2-based materials in order to improve the engineering applications of photocatalytic technologies. However, not many attempts have been made to evaluate the optical behavior of these materials. This work focuses on the study of the photonic efficiencies of silica-supported TiO2 photocatalysts following the photodegradation of dichloroacetic acid (DCA) as model compound. Catalysts with different types of silica support and titania loadings were tested and their activity was found to be in correlation with the results of the clusters size distribution of the TiO2 nanocrystals. The photonic efficiency of the supported photocatalysts depends extremely on the optical density of the solid suspensions. Influence of the textural properties of the support and the titania loading on the optical density as well as on the photonic efficiency of the materials are discussed. The dependence of the absorption of radiation by the suspension on the catalyst concentration is also analyzed.

Heterogeneous photocatalysed degradation of two selected pesticide derivatives, triclopyr and daminozid in aqueous suspensions of titanium dioxide.

Heterogeneous photocatalysed degradation of two selected pesticide derivatives, triclopyr (1) and daminozid (2), has been investigated in aqueous suspensions of titanium dioxide by monitoring the change in substrate concentration employing the UV Spectroscopic analysis technique and depletion of Total Organic Carbon (TOC) content as a function of irradiation time. The degradation kinetics were studied under different conditions such as reaction pH, substrate and catalyst concentration, different types of TiO2 and in the presence of electron acceptors such as hydrogen peroxide (H2O2), potassium bromate (KBrO3) and ammonium persulphate (NH4)2S2O8 in addition to molecular oxygen. The degradation rates were found to be strongly influenced by all the above parameters. The photocatalyst Degussa P25 was found to be more efficient as compared with other photocatalysts. The pesticide derivative triclopyr (1) was found to degrade faster as compared to daminozid (2). An attempt was also made to identify the intermediate products formed during the photooxidation process using GC/MS analysis. Probable pathways for the formation of products have been proposed.

Heterogeneous photocatalysed reaction of three selected pesticide derivatives, propham, propachlor and tebuthiuron in aqueous suspensions of titanium dioxide.

Heterogeneous photocatalysed reaction of three selected pesticide derivatives such as propham (1), propachlor (2) and tebuthiuron (3) has been investigated in aqueous suspensions of titanium dioxide by monitoring the change in substrate concentration employing UV Spectroscopic analysis and depletion in Total Organic Carbon (TOC) content as a function of irradiation time. The degradation kinetics was studied under different conditions such as pH, catalyst concentration, substrate concentration, different types of TiO(2) and in the presence of electron acceptors such as hydrogen peroxide (H(2)O(2)), potassium bromate (KBrO(3)) and ammonium persulphate (NH(4))(2)S(2)O(8) besides molecular oxygen. The degradation rates were found to be strongly influenced by all the above parameters. The photocatalyst Degussa P25 was found to be more efficient as compared with other photocatalysts. The pesticide derivative propham (1) was found to degrade faster as compared to propachlor (2) and tebuthiuron (3). An attempt has also been made to identify the products formed during the photooxidation process through GC/MS analysis technique. All the model pollutants showed the formation of several intermediate products, which were identified on the basis of molecular ion and mass spectrometric fragmentation pattern. A probable mechanism for the formation of the products has been proposed.

Enhancement of photocatalytic activity by metal deposition: characterisation and photonic efficiency of Pt, Au and Pd deposited on TiO2 catalyst.

Pt, Au and Pd deposited TiO2 have been prepared and characterised by surface analytical methods such as surface area, XRD, and scanning electron micrograph and photophysical characterisation by diffuse reflectance spectroscopy. The photocatalytic activity of the doped catalysts was ascertained by the photo-oxidation of leather dye, acid green 16 in aqueous solution illuminated with low-pressure mercury lamp ( approximately 254 nm). The effect of metal contents on the photocatalytic activity was investigated. The highest photonic efficiency was observed with metal deposition level of less than 1 wt%.

Photocatalytic water treatment: fundamental knowledge required for its practical application.

This review attempts to present a state-of-the art overview of the underlying principles of photocatalysis that should not be neglected when designing pilot-scale facilities. In particular, the roles and properties of the primary charge carriers generated upon bandgap illumination within a photocatalyst particle, i.e. valence band hole and conduction band electron, are discussed. The chemical nature of the primary oxidant is critically assessed.

Photocatalysed degradation of a herbicide derivative, bromacil, in aqueous suspensions of titanium dioxide.

The photocatalysed degradation of a herbicide, 5-bromo-3-sec-butyl-6-methyl uracil (bromacil, 1) has been investigated in aqueous suspensions of titanium dioxide under a variety of conditions. The degradation was studied by monitoring the depletion in total organic carbon (TOC) content as a function of irradiation time. The degradation kinetics of the model compound was studied under different conditions such as pH, catalyst concentration, substrate concentration, different types of TiO2 and in the presence of electron acceptors such as hydrogen peroxide (H2O2) and potassium bromate (KBrO3) besides molecular oxygen. The degradation rate was found to be strongly influenced by all the above factors. Higher degradation rate was observed with Degussa P25 as compared with other photocatalysts. The addition of bromate ion has been found to enhance the degradation rate markedly. 5-Hydroxy-3-sec-butyl-6-methyl uracil (2) and diisopropyl urea (16) were identified as the degradation products by GC-MS analysis and probable pathways for the formation of the products have been proposed.

Determination of photonic efficiency and quantum yield of formaldehyde formation in the presence of various TiO2 photocatalysts.

Illumination of aqueous TiO2 suspentions yields hydroxyl radicals, which can be trapped by methanol producing formaldehyde (HCHO). In this work, the photonic efficiency and quantum yield of HCHO formation in colloidal TiO2 solutions and, P25 and UV 100 suspensions have been determined. Differences in photocatalytic activity of the three photocatalysts have been found and are discussed. The photonic efficiency of HCHO formation in the presence of P25 and UV 100 depends on the concentration of TiO2 and the pH. The critical concentration is 2.5 g/L. Below this, the photonic efficiency with P25 is higher than with UV 100, and vice versa. Optimum pH values for P25 and UV 100 giving the maximum photonic efficiency are 7.7 and 10.4, respectively. Compared to P25 and UV 100, the true quantum yield of HCHO formation in colloidal TiO2 solution varies a little with pH and virtually does not change with the amount of loading of TiO2. The true quantum yield varies as the inverse square root of light intensity. The quantum yield increases from 0.02 to 0.08 when the absorbed photon flux decreases from 8.1 x 10(-7) Ein/L s to 4.9 x 10(-8) Ein/L s. A simple model is presented to explain the experimental observation.

One electron reduction of CCl4 in oxygenated aqueous solutions: a CCl3O2-free radical mediated formation of Cl- and CO2.

Product yields have been determined after one-electron-induced reduction of CCl4 in aqueous solutions containing t-butanol and various concentrations of oxygen. It was shown that CCl3 radicals add oxygen to form CCl3O2 radicals, which eventually yield three chloride ions and CO2. A constant ratio of G(Cl-)/G(CO2) = 4 is found in solutions containing 1.5 X 10(-4) M or more oxygen. Competing reactions of the CCl3 radical increase this ratio at lower oxygen concentrations. The rate constant for the oxygen addition to CCl3 radicals was determined by pulse radiolysis to 3.3 X 10(9) M-1 s-1. Possible reaction mechanisms leading to the observed end products are discussed.

Metronidazole (Flagyl), misonidazole (Ro 07-0582), iron, zinc and sulphur compounds in cancer therapy.

The nitro radiosensitizers, metronidazole and misonidazole, have been shown to react rapidly with the sulphydryl compounds cysteine and cysteamine in the presence of ferrous ions. Similar reactions occur in the presence of copper ions but these are much slower. The initial interactions of the drugs and of oxygen with an iron-cysteine complex are extremely rapid: in the case of oxygen reaction half-lives of 27 ms have been measured. Misonidazole also reacts rapidly with glutathione in the presence of ferrous ions and is subsequently reduced: metronidazole is reduced only slowly if at all. These reactions, which have been found to be inhibited by high concentrations of zince ions, are discussed in the light of the known radiosensitizing and chemotherapeutic efficiencies of the nitro drugs and the side effect of peripheral neuropathy sometimes observed during their clinical use.