Mesoporous titania photocatalysts: Preparation, characterization and reaction mechanisms

Titanium dioxide is a very important semiconductor with a high potential for applications in photocatalysis, solar cells, photochromism, sensoring, and various other areas of nanotechnology. Increasing attention has recently been focused on the simultaneous achievement of high bulk crystallinity and the formation of ordered mesoporous TiO2 frameworks with high thermal stability. Mesoporous TiO2 has continued to be highly active in photocatalytic applications because it is beneficial for promoting the diffusion of reactants and products, as well as for enhancing the photocatalytic activity by facilitating access to the reactive sites on the surface of photocatalyst. This steady progress has demonstrated that mesoporous TiO2 nanoparticles are playing and will continue to play an important role in the protection of the environment and in the search for renewable and clean energy technologies. This review focuses on the preparation and characterisation of mesoporous titania, noble metals nanoparticles, transition metal ions, non-metal/doped mesoporous titania networks. The photocatalytic activity of mesoporous titania materials upon visible and UV illumination will be reviewed, summarized and discussed, in particular, concerning the influence of preparation and solid-state properties of the materials. Reaction mechanisms that are being discussed to explain these effects will be presented and critically evaluated. © The Royal Society of Chemistry 2011

Stabilization of free radical intermediates on metal oxide semiconductors surfaces

Free radicals intermediates are formed on the surface of metal oxide semiconductors, namely TiO_2 and Fe_2O_3, following the light-induced charge separation upon band-gap illumination. These radicals appear to adhere strongly to the surfaces of the small colloidal particles (diameters between 5 and 20 nm) employed in this study. A second electron-transfer step yielding the observed end-products therefore preceeds desorption of the reactive intermediates. Oxidation of methylviologen (1,1'-dimethyl-4,4'-bipyridylium chloride) and reduction of halothane (2-bromo-2-chloro-1,1,1- trifluoroethane) is thus found to proceed with high quantum yields and very specific mechanisms on colloidal TiO_2. The oxidation of sulfite on α-Fe_2O_3 particles yields the sulfite radical anion, SO_3^- , as initial intermediate. A subsequent electron transfer leading to the formation of sulfate occurs on the surface of the same catalyst particle

One-step synthesis of mesoporous platinum/titania nanocomposites as photocatalyst with enhanced photocatalytic activity for methanol oxidation

Mesostructured Pt/TiO2 nanocomposites have been synthesized at different Pt (0–2 wt%) and anatase/rutile ratios through simple one-step sol–gel reactions. The as-made mesostructured hybrids were subjected to H2 gas for 2 h at 450 °C to obtain ordered hexagonal mesoporous Pt/TiO2 nanocomposites with highly crystalline TiO2. Subsequently, the samples were calcined at 350 °C in air for 4 h to remove the surfactant. XRD data clearly show that biphasial anatase and rutile mixtures are formed by the addition of the Pt islands. The TEM results indicated that TiO2 and Pt, are partly in close contact; the lattice fringes exhibit the typical distances, i.e., Pt (111) (2.2 Å) and TiO2 (101) (3.54 Å). TiO2 nanoparticles with an average diameter of about 10 nm particles are not agglomerated and quite uniform in size and shape. Also, Pt nanoparticles are well dispersed and exhibit diameters of about 5–12 nm based on the Pt content. Our photocatalysts have been compared with colloidal 0.5 wt% Pt loaded onto commercial photocatalysts either UV-100 Hombikat or Aeroxide TiO2 P25 by the determination of the initial rate of HCHO formation generated by the photooxidation of CH3OH in aqueous suspensions to calculate the corresponding photonic efficiencies. The overall photocatalytic activities of 0.5 wt% Pt/TiO2 nanocomposites are significantly 3-times higher than that of colloidal 0.5 wt% Pt loaded onto commercial photocatalysts either UV-100 Hombikat or Aeroxide TiO2 P25. To the best of our knowledge, the measured photonic efficiency ξ = 15.5% of hexagonal mesostructured Pt/TiO2 nanocomposites is found to be among the highest ξ values reported up to now. The superiority of Pt/TiO2 is attributed to the bicrystalline (anatase/rutile) framework, large surface area, high crystallinity and mesoporous structure of Pt/TiO2 nanocomposites

Factors affecting the selectivity of the photocatalytic conversion of nitroaromatic compounds over TiO2 to valuable nitrogen-containing organic compounds

The photocatalytic conversion of various nitroaromatic compounds in alcohols employing four different types of TiO2 (Sachtleben Hombikat UV100 as anatase, Crystal Global R34 as rutile, Evonik-Degussa Aeroxide P25 as an anatase-rutile mixture, and home-made mesoporous anatase) has been studied. The effect of platinization of these different types of TiO2 on the reaction sequence has also been investigated. Upon irradiation of an ethanolic solution of m-nitrotoluene, as a model reaction, in the presence of the bare photocatalyst, different products were obtained according to the applied photocatalyst. It was found that the surface properties of the photocatalyst play an important role in the reaction pathway and thus in the selectivity of the products. In all cases, a simultaneous reduction of the nitro compound and an oxidation of the alcohol are induced by the photogenerated electrons and holes, respectively. An imine is then produced upon condensation of the generated aldehyde and amino compounds. Rutile was found to be more selective towards the primary amino compound (m-toluedine) while anatase catalysts gave a mixture of m-toluidine and its imine (N-ethylidene-3-methylaniline). A cyclization reaction of the produced imine to generate methyl quinoline was observed when Aeroxide P25 was used as a photocatalyst. Employing platinized TiO2, the hydrogenation of the produced imine yielding N-alkylated products was found to occur in all cases. However, the selectivity towards the mono N-alkylated product was the best using platinized Hombikat UV100. This selectivity was found to be also influenced by the loaded amount of Pt, the platinization method, and the illumination time but not by the light intensity.© 2013 the Owner Societies

Photocatalytic activity and charge carrier dynamics of TiO2 powders with a binary particle size distribution

The effects of the particle size distribution on the charge carrier dynamics and the photocatalytic activity of mixed titanium dioxide (TiO2) powder samples were investigated in this work. Instead of the synthesis of the small semiconductor particles, the binary particle size distributions of the powders were obtained by mixing commercially available TiO2 powders with different particle sizes. The pure anatase samples (average diameters: 7, 20, and 125 nm, respectively) were created via ultrasound treatment and discreet drying. The photocatalytic activity of the powder samples was assessed by the degradation of nitric oxide (NO) and acetaldehyde in the gas phase. Furthermore, the charge carrier kinetics was determined using transient absorption spectroscopy following pulsed laser excitation. Importantly, a recently published model based on fractal dimensions was used to fit the transient signals of the photo generated charge carriers in the TiO2 powder samples. The effects of the particle size on the acetaldehyde degradation could be explained by the formation of agglomerates, which reduce the available surface area of smaller particles. The fast oxidation of acetaldehyde on the surface of TiO2 by direct hole transfer was further independent of the observed charge carrier lifetimes on the microsecond time scale. The photocatalytic NO degradation, on the other hand, increased for samples containing larger amounts of small particles. The corresponding photonic efficiencies correlated well with the charge carrier lifetimes determined by the time-resolved studies. Hence, it was concluded that a long charge carrier lifetime generally leads to higher fractional conversions of NO. The employed fractal fit function was proved to be beneficial for the kinetic analysis of charge carrier recombination in TiO2, in direct comparison with a second order fit function. © 2018 the Owner Societies

Reaction rate study of the photocatalytic degradation of dichloroacetic acid in a black body reactor

The light-induced degradation of dichloroacetic acid in aqueous suspensions containing the TiO2 photocatalyst Hombikat UV 100 was investigated. The reactions were performed in a black body reactor in which the rate of conversion, defined as the time derivative of the extent of conversion, is not affected by the light scattering properties of the photocatalysts. At sufficiently high concentrations of both the probe compound and the photocatalyst the rate of conversion was found to be unswayed by the initial concentration of the probe compound, the mass concentration of the photocatalyst, and the suspension volume. Thus, the chosen experimental conditions enable the determination of the rate of conversion and the quantum yield of the light induced degradation of dichloroacetic acid in aqueous photocatalyst suspension with sufficiently good reproducibility. The experimental procedure employed here seems to be generally applicable to determine rates of conversion and quantum yields that possibly allow a comparison of the activities of photocatalysts in aqueous suspensions

Latest progress in g-C3N4 based heterojunctions for hydrogen production via photocatalytic water splitting: A mini review

Graphitic carbon nitride based heterojunction photocatalysts have gained increasing attention in producing the clean energy source of hydrogen. Coupling carbon nitride (g-C3N4) with other semiconductor materials or metals as co-catalysts is considered as an effective strategy to overcome the drawbacks of g-C3N4 such as the quick recombination of photogenerated charges. In this review, the recent research advancements in the construction of g-C3N4-based heterojunctions as well as their different charge separation/transfer mechanisms will be systematically discussed, making special emphasis on the design and fabrication of type-II, Z-scheme, S-scheme and Schottky heterojunctions and their application towards H2 generation from water splitting. Finally, a summary and some crucial issues, which should be further resolved for developing advanced g-C3N4-based heterojunction photocatalysts, are presented

Ionic liquid-mediated microstructure regulations of layered perovskite for enhanced visible light photocatalytic activity

The presence of pollutants, e.g., pharmaceutical residues and industrial pollutants causes serious risks and irreversible damage to public health and ecological balance. Semiconductor-based photocatalysis is an attractive way to treat polluted water. Rational design and nanostructuring of semiconductors with visible light absorption and prominent surfaces could strengthen surface-interface reactions, resulting in improved photocatalytic degradation. Herein, layered structured perovskites Bi4Ti3O12 (BTO) were synthesized by an ionic liquid [1-butyl-3-methylimidazolium iodide (Bmim)I] assisted approach. The precise tuning of synthetic conditions allowed formations of various microstructures, including spherical nanoparticles, nanoplates and nanorods, respectively. The optical analyses demonstrated that samples were typically visible light absorbents with narrow band gap energies (2.96–2.73 eV), and displayed pronounced degradation for pharmaceutical residues under visible light illumination. The factors responsible for the high efficiency of BTO photocatalysts were discussed in terms of unique structure, optical alignment, dipole induced carrier separation and formation of active radicals. Among studied samples, the nanorod shaped BTO showed 1.31 and 1.46 times higher apparent rate constants for tetracycline and ibuprofen degradation than its counterparts (spherical nanoparticles and nanoplates), respectively. The better performance of nanorods was ascribed to their higher visible light harvesting ability. Importantly, BTO nanorods exhibited nonselective degradation activity for diverse pollutants of pharmaceutical residues and industrial contaminants. This work demonstrates the unique strategy of microstructure regulation and a wide range of applications of layered perovskites for environmental remediation

Recent progresses on metal halide perovskite-based material as potential photocatalyst

Recent years have witnessed an incredibly high interest in perovskite-based materials. Among this class, metal halide perovskites (MHPs) have attracted a lot of attention due to their easy preparation and excellent opto-electronic properties, showing a remarkably fast development in a few decades, particularly in solar light-driven applications. The high extinction coefficients, the optimal band gaps, the high photoluminescence quantum yields and the long electron–hole diffusion lengths make MHPs promising candidates in several technologies. Currently, the researchers have been focusing their attention on MHPs-based solar cells, light-emitting diodes, photodetectors, lasers, X-ray detectors and luminescent solar concentrators. In our review, we firstly present a brief introduction on the recent discoveries and on the remarkable properties of metal halide perovskites, followed by a summary of some of their more traditional and representative applications. In particular, the core of this work was to examine the recent progresses of MHPs-based materials in photocatalytic applications. We summarize some recent developments of hybrid organic–inorganic and all-inorganic MHPs, recently used as photocatalysts for hydrogen evolution, carbon dioxide reduction, organic contaminant degradation and organic synthesis. Finally, the main limitations and the future potential of this new generation of materials have been discussed. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Ruthenium-modified zinc oxide, a highly active vis-photocatalyst: the nature and reactivity of photoactive centres

We recently reported a highly active photocatalyst, ruthenium-modified zinc oxide, which was found to be able to utilise the red part of the visible light spectrum for photocatalytic reactions [Bloh et al., Environ. Sci. Pollut. Res., 2012, 19, 3688-3695]. However, the origin and mechanism of the observed activity as well as the nature of the photoactive centres are still unknown. Herein, we expand on that by reporting a series of experiments specifically designed to unravel the mechanism of the visible light induced photocatalytic reactions. The absolute potentials of the valence and the conduction band edge are identified by the combined use of electrochemical impedance and UV-vis diffuse reflectance spectroscopy. The conduction band electron and the valence band hole activity are assessed through a novel approach tracing their signature oxidative species, i.e., hydrogen peroxide and hydroxyl radicals, respectively. Oxygen reduction currents are measured at different potentials to investigate the role of molecular oxygen as an electron scavenger as well as the underlying reduction pathways. Additionally, the photocatalytic activity of the samples is verified using another (ISO standard) degradation test, the gas-phase oxidation of nitric oxide. The experimental results reveal that the employed synthetic route yields a unique mixture of ruthenium(VI)-doped zinc oxide and ruthenium(VI) oxide particles with both forms of the ruthenium playing their own independent role in the enhancement of the photocatalytic activity. The ruthenium ions acting as dopants enable a better charge separation as well as the absorption of red light resulting in the direct promotion of electrons from the Ru(VI)-species to the conduction band. Both, the conduction band electrons and the thus formed Ru(VII) subsequently participate in the degradation of the pollutant molecules. The ruthenium dioxide particles, on the other hand, act as catalysts increasing the efficiency of the reaction by improving the oxygen reduction properties of the material.BMBF/HelioClean/03X0069