Integrating efficient filtration and visible-light photocatalysis by loading Ag-doped zeolite Y particles on aluminia nanofiber membrane

Filtration membrane technology has already been employed to remove various organic effluents produced from the textile, paper, plastic, leather, food and mineral processing industries. To improve membrane efficiency and alleviate membrane fouling, an integrated approach is adopted that combines membrane filtration and photocatalysis technology. In this study, alumina nanofiber (AF) membranes with pore size of about 10 nm (determined by the liquid-liquid displacement method) have been synthesized through an in situ hydrothermal reaction, which permitted a large flux and achieved high selectivity. Silver nanoparticles (Ag NPs) are subsequently doped on the nanofibers of the membranes. Silver nanoparticles can strongly absorb visible light due to the surface plasmon resonance (SPR) effect, and thus induce photocatalytic degradation of organic dyes, including anionic, cationic and neutral dyes, under visible light irradiation. In this integrated system, the dyes are retained on the membrane surface, their concentration in the vicinity of the Ag NPs are high and thus can be efficiently decomposed. Meanwhile, the usual flux deterioration caused by the accumulation of the filtered dyes in the passage pores can be avoided. For example, when an aqueous solution containing methylene blue is processed using an integrated membrane, a large flux of 200 L m-2 h-1 and a stable permeating selectivity of 85% were achieved. The combined photocatalysis and filtration function leads to superior performance of the integrated membranes, which have a potential to be used for the removal of organic pollutants in drinking water.

Reduced graphene oxide and Ag wrapped TiO2 photocatalyst for enhanced visible light photocatalysis

A well-organised reduced graphene oxide (RGO) and silver (Ag) wrapped TiO2 nano-hybrid was successfully achieved through a facile and easy route. The inherent characteristics of the synthesized RGO-Ag/TiO2 were revealed through crystalline phase, morphology, chemical composition, Raman scattering, UV-visible absorption, and photoluminescence analyses. The adopted synthesis route significantly controlled the uniform formation of silver nanoparticles and contributed for the absorption of light in the visible spectrum through localized surface plasmon resonance effects. The wrapped RGO nanosheets triggered the electron mobility and promoted visible light shift towards red spectrum. The accomplishment of synergised effect of RGO and Ag well degraded Bisphenol A under visible light irradiation with a removal efficiency of 61.9%.

Cost-effective and eco-friendly synthesis of novel and stable N-doped ZnO/g-C3N4 core-shell nanoplates with excellent visible-light responsive photocatalysis

N-doped ZnO/g-C3N4 hybrid core–shell nanoplates have been successfully prepared via a facile, cost-effective and eco-friendly ultrasonic dispersion method for the first time. HRTEM studies confirm the formation of the N-doped ZnO/g-C3N4 hybrid core–shell nanoplates with an average diameter of 50 nm and the g-C3N4 shell thickness can be tuned by varying the content of loaded g-C3N4. The direct contact of the N-doped ZnO surface and g-C3N4 shell without any adhesive interlayer introduced a new carbon energy level in the N-doped ZnO band gap and thereby effectively lowered the band gap energy. Consequently, the as-prepared hybrid core–shell nanoplates showed a greatly enhanced visible-light photocatalysis for the degradation of Rhodamine B compare to that of pure N-doped ZnO surface and g-C3N4. Based on the experimental results, a proposed mechanism for the N-doped ZnO/g-C3N4 photocatalyst was discussed. Interestingly, the hybrid core–shell nanoplates possess high photostability. The improved photocatalytic performance is due to a synergistic effect at the interface of the N-doped ZnO and g-C3N4 including large surface-exposure area, energy band structure and enhanced charge-separation properties. Significantly, the enhanced performance also demonstrates the importance of evaluating new core–shell composite photocatalysts with g-C3N4 as shell material.

Selective reductions using visible light photocatalysts of supported gold nanoparticles

Photocatalytic synthesis using visible light is a desirable chemical process because of its potential to utilize sunlight. Supported gold nanoparticles (Au-NPs) were found to be efficient photocatalysts and the effects of the supports were identified including CeO2, TiO2, ZrO2, Al2O3, and zeolite Y. In particular Au/CeO2 exhibited the high catalytic activity to reduce nitroaromatics to azo compounds, hydrogenate azobenzene to hydroazobenzene, reduce ketones to alcohols, and deoxygenate epoxides to alkenes at ambient temperatures, under irradiation of visible light (or simulated sunlight). The reac-tive efficiency depends on two primary factors: one is the light adsorption of catalysts and another is the driving ability of catalysts corresponding to the reactants. The light absorption by Au-NPs is due to surface plasmon resonance effect or inter-band electron transition; this is related to the reduction ability of the photocatalysts. Irradiation with shorter wavelengths can excite the conduction electrons in Au-NPs to higher energy levels and as a result, induce reduction with more negative reduction potentials. It is known when irradiated with light the Au-NPs can abstract hydrogen from isopropanol forming Au-H species on the Au-NP surface. Hence, we proposed that the active Au-H species will react with the N=O, N=N, C=O double bonds or epoxide bonds, which are weakened by the interaction with the excited electrons in the Au-NPs, and yield the final reductive products. The reacting power of the Au-H species depends on the energy of the excited electrons in Au-NPs: the higher the electronic energy, the stronger the reduction ability of the Au-H species. This finding demonstrates that we can tune the reduction ability of the photocatalysts by manipulating the irradiation wavelength.

A review of iron species for visible-light photocatalytic water purification

Iron species are one of the least toxic and least expensive substances that are photocatalytic in the visible region of the spectrum. Therefore, this article focuses on iron-based photocatalysts sensitive to visible light. Photo-Fenton reactions are considered with respect to those assisted by and involve the in situ production of H2O2. The possible role that photoactive iron species play by interacting with natural organic matter in water purification in the natural environment is considered. The review also considered photosensitization by phthalocyanines and the potential role that layered double hydroxides may have not only as catalyst supports but also as photosensitizers themselves. Finally, photocatalytic disinfection of water is discussed, and the desirability of standardized metrics and experimental conditions to assist in the comparative evaluation of photocatalysts is highlighted.

New Intermediate band sulphide nanoparticles acting in the full visible light range spectra as an active photocatalyst

Nowadays one of the challenges of materials science is to find new technologies that will be able to make the most of renewable energies. An example of new proposals in this field are the intermediate-band (IB) materials, which promise higher efficiencies in photovoltaic applications (through the intermediate band solar cells), or in heterogeneous photocatalysis (using nanoparticles of them, for the light-induced degradation of pollutants or for the efficient photoevolution of hydrogen from water). An IB material consists in a semiconductor in which gap a new level is introduced [1], the intermediate band (IB), which should be partially filled by electrons and completely separated of the valence band (VB) and of the conduction band (CB). This scheme (figure 1) allows an electron from the VB to be promoted to the IB, and from the latter to the CB, upon absorption of photons with energy below the band gap Eg, so that energy can be absorbed in a wider range of the solar spectrum and a higher current can be obtained without sacrificing the photovoltage (or the chemical driving force) corresponding to the full bandgap Eg, thus increasing the overall efficiency. This concept, applied to photocatalysis, would allow using photons of a wider visible range while keeping the same redox capacity. It is important to note that this concept differs from the classic photocatalyst doping principle, which essentially tries just to decrease the bandgap. This new type of materials would keep the full bandgap potential but would use also lower energy photons. In our group several IB materials have been proposed, mainly for the photovoltaic application, based on extensively doping known semiconductors with transition metals [2], examining with DFT calculations their electronic structures. Here we refer to In2S3 and SnS2, which contain octahedral cations; when doped with Ti or V an IB is formed according to quantum calculations (see e.g. figure 2). We have used a solvotermal synthesis method to prepare in nanocrystalline form the In2S3 thiospinel and the layered compound SnS2 (which when undoped have bandgaps of 2.0 and 2.2 eV respectively) where the cation is substituted by vanadium at a ?10% level. This substitution has been studied, characterizing the materials by different physical and chemical techniques (TXRF, XRD, HR-TEM/EDS) (see e.g. figure 3) and verifying with UV spectrometry that this substitution introduces in the spectrum the sub-bandgap features predicted by the calculations (figure 4). For both sulphide type nanoparticles (doped and undoped) the photocatalytic activity was studied by following at room temperature the oxidation of formic acid in aqueous suspension, a simple reaction which is easily monitored by UV-Vis spectroscopy. The spectral response of the process is measured using a collection of band pass filters that allow only some wavelengths into the reaction system. Thanks to this method the spectral range in which the materials are active in the photodecomposition (which coincides with the band gap for the undoped samples) can be checked, proving that for the vanadium substituted samples this range is increased, making possible to cover all the visible light range. Furthermore it is checked that these new materials are more photocorrosion resistant than the toxic CdS witch is a well know compound frequently used in tests of visible light photocatalysis. These materials are thus promising not only for degradation of pollutants (or for photovoltaic cells) but also for efficient photoevolution of hydrogen from water; work in this direction is now being pursued.

Photocatalysis on supported gold and silver nanoparticles under ultraviolet and visible light irradiation

Studies of the optical properties and catalytic capabilities of noble metal nanoparticles (NPs), such as gold (Au) and silver (Ag), have formed the basis for the very recent fast expansion of the field of green photocatalysis: photocatalysis utilizing visible and ultraviolet light, a major part of the solar spectrum. The reason for this growth is the recognition that the localised surface plasmon resonance (LSPR) effect of Au NPs and Ag NPs can couple the light flux to the conduction electrons of metal NPs, and the excited electrons and enhanced electric fields in close proximity to the NPs can contribute to converting the solar energy to chemical energy by photon-driven photocatalytic reactions. Previously the LSPR effect of noble metal NPs was utilized almost exclusively to improve the performance of semiconductor photocatalysts (for example, TiO2 and Ag halides), but recently, a conceptual breakthrough was made: studies on light driven reactions catalysed by NPs of Au or Ag on photocatalytically inactive supports (insulating solids with a very wide band gap) have demonstrated that these materials are a class of efficient photocatalysts working by mechanisms distinct from those of semiconducting photocatalysts. There are several reasons for the significant photocatalytic activity of Au and Ag NPs. (1) The conduction electrons of the particles gain the irradiation energy, resulting in high energy electrons at the NP surface which is desirable for activating molecules on the particles for chemical reactions. (2) In such a photocatalysis system, both light harvesting and the catalysing reaction take place on the nanoparticle, and so charge transfer between the NPs and support is not a prerequisite. (3) The density of the conduction electrons at the NP surface is much higher than that at the surface of any semiconductor, and these electrons can drive the reactions on the catalysts. (4) The metal NPs have much better affinity than semiconductors to many reactants, especially organic molecules. Recent progress in photocatalysis using Au and Ag NPs on insulator supports is reviewed. We focus on the mechanism differences between insulator and semiconductor-supported Au and Ag NPs when applied in photocatalytic processes, and the influence of important factors, light intensity and wavelength, in particular estimations of light irradiation contribution, by calculating the apparent activation energies of photo reactions and thermal reactions.

Direct photocatalysis for organic synthesis by using plasmonic-metal nanoparticles irradiated with visible light

Recent advances in direct-use plasmonic-metal nanoparticles (NPs) as photocatalysts to drive organic synthesis reactions under visible-light irradiation have attracted great interest. Plasmonic-metal NPs are characterized by their strong interaction with visible light through excitation of the localized surface plasmon resonance (LSPR). Herein, we review recent developments in direct photocatalysis using plasmonic-metal NPs and their applications. We focus on the role played by the LSPR of the metal NPs in catalyzing organic transformations and, more broadly, the role that light irradiation plays in catalyzing the reactions. Through this, the reaction mechanisms that these light-excited energetic electrons promote will be highlighted. This review will be of particular interest to researchers who are designing and fabricating new plasmonic-metal NP photocatalysts by identifying important reaction mechanisms that occur through light irradiation.

Tuning the reduction power of supported gold nanoparticle photocatalysts for selective reductions by manipulating wavelength of visible light irradiation

Gold nanoparticles supported on CeO2 were found to be efficient photocatalysts for three selective reductions of organic compounds at ambient temperatures, under irradiation of visible light; their reduction ability can be tuned by manipulating the irradiation wavelength.

Visible light photocatalytic synthesis of fine organic chemicals with new photocatalysts

This project was a step forward in developing new recyclable photocatalysts for chemical reactions. These new photocatalysts can facilitate reactions by using visible light under moderate reaction conditions which is suitable for a sustainable, green and eco-friendly modern chemical industry. The outcome of the study greatly extended our understanding in metal nanoparticle photocatalysis, which reveals new photocatalytic mechanisms for the controlled transformation of chemical reactions. The prospect of sunlight irradiation driving chemical reactions may provide opportunity for the organic synthesis via a more controlled, simplified, and greener process in the future.

Visible light-driven cross-coupling reactions at lower temperatures using a photocatalyst of palladium and gold alloy nanoparticles

Palladium (Pd)-catalyzed cross-coupling reactions are among the most important methods in organic synthesis. We report the discovery of highly efficient and green photocatalytic processes by which cross-coupling reactions, including Sonogashira, Stille, Hiyama, Ullmann, and Buchwald–Hartwig reactions, can be driven with visible light at temperatures slightly above room temperature using alloy nanoparticles of gold and Pd on zirconium oxide, thus achieving high yields. The alloy nanoparticles absorb visible light, and their conduction electrons gain energy, which is available at the surface Pd sites. Results of the density functional theory calculations indicate that transfer of the light excited electrons from the nanoparticle surface to the reactant molecules adsorbed on the nanoparticle surface activates the reactants. When the light intensity was increased, a higher reaction rate was observed, because of the increased population of photoexcited electrons. The irradiation wavelength also has an important impact on the reaction rates. Ultraviolet irradiation can drive some reactions with the chlorobenzene substrate, while visible light irradiation failed to, and substantially improve the yields of the reactions with the bromobenzene substrate. The discovery reveals the possibility of using low-energy and -density sources such as sunlight to drive chemical transformations.

Catalytic transformation of aliphatic alcohols to corresponding esters in O2 under neutral conditions using visible-light irradiation

Selective oxidation of aliphatic alcohols under mild and base-free conditions is a challenging process for organic synthesis. Herein, we report a one-pot process for the direct oxidative esterification of aliphatic alcohols that is significantly enhanced by visible-light irradiation at ambient temperatures. The new methodology uses heterogenerous photocatalysts of gold–palladium alloy nanoparticles on a phosphate-modified hydrotalcite support and molecular oxygen as a benign oxidant. The alloy photocatalysts can absorb incident light, and the light-excited metal electrons on the surface of metal nanoparticles can activate the adsorbed reactant molecules. Tuning the light intensity and wavelength of the irradiation can remarkably change the reaction activity. Shorter wavelength light (<550 nm) drives the reaction more efficiently than light of longer wavelength (e.g., 620 nm), especially at low temperatures. The phosphate-exchanged hydrotalcite support provides sufficient basicity (and buffer) for the catalytic reactions; thus, the addition of base is not required. The photocatalysts are efficient and readily recyclable. The findings reveal the first example of using “green” oxidants and light energy to drive direct oxidative esterification of aliphatic alcohols under base-free, mild conditions.

Alloying gold with copper makes for a highly selective visible-light photocatalyst for the reduction of nitroaromatics to anilines

Finely control of product selectivity is an essential issue in organic chemical production. In the synthesis of functionalized anilines via reduction of the corresponding nitroarenes, the challenge is to selectively reduce only the nitro group in the presence of other reducible functional groups in nitroarene molecules at a high reaction rate. Normally, the nitroarene is reduced stepwise through a series of intermediates that remain as byproducts, increasing the aniline synthesis cost. Here we report that alloying small amounts of copper into gold nanoparticles can alter the reaction pathway of the catalytic reduction under visible-light irradiation at ambient temperature, allowing nitroaromatics to be transformed directly to anilines in a highly selective manner. The reasons for the high efficiency of the photocatalytic reduction under these comparatively benign conditions as well as the light-excited reaction mechanisms are discussed. This photocatalytic process avoids byproducts, exhibits a high reaction rate and excellent substituent tolerance, and can be used for the synthesis of many useful functionalized anilines under environmentally benign conditions. Switching of the reaction pathway simply by tailoring the bimetallic alloy NPs of the photocatalysts is effective for engineering of product chemoselectivity.

Heterojunctions between amorphous and crystalline niobium oxide with enhanced photoactivity for selective aerobic oxidation of benzylamine to imine under visible light

The formation of heterojunctions between two crystals with different band gap structures, acting as a tunnel for the unidirectional transfer of photo-generated charges, is an efficient strategy to enhance photocatalytic performance in semiconductor photocatalysts. The heterojunctions may also promote the photoactivity in the visible-light-response of any surface complex catalysts by influencing the transfer of photo-generated electrons. Herein, Nb2O5 microfibers, with a high surface area of interfaces between an amorphous phase and crystalline phase, were designed and synthesised by the calcination of hydrogen-form niobate while controlling the crystallization The photoactivity of these microfibers towards selective aerobic oxidation reactions was investigated. As predicted, the Nb2O5 microfibres containing heterojunctions exhibited the highest photoactivity. This could be due to the band gap difference between the amorphous phase and the crystalline phase, which shortened the charge mobile distance and improved the efficiency.