Polyoxometalate-based molecular/nano composites: Advances in environmental remediation by photocatalysis and biomimetic approaches to solar energy conversion

Titania hollow spheres modified with tungstophosphoric acid with enhanced visible light absorption for the photodegradation of 4-chlorophenol

Titania hollow spheres were synthesized using silica nanospheres as the template. The core was removed using NaOH solution. They were subsequently impregnated with tungstophosphoric acid (TPA) solutions and annealed at two different temperatures (100 and 500 °C). These materials were characterized by several physicochemical techniques (XRD, BET, SEM, DRS, FT-IR, FT-Raman and ³¹P MAS-NMR). The ³¹P MAS-NMR and FT-IR characterization showed that the main species present in the samples was the [PW₁₂O₄₀]^3- anion, which was partially transformed into the [P₂W₂₁O₇₁]^6- anion during the synthesis and drying step. ³¹P MAS-NMR, and FT-Raman characterization revealed the evidence of a strong interaction between the Keggin anion of TPA and TiO₂ surfaces, possibly due to the formation of surface heteropolyacid-TiO₂ complexes. The DRS results showed that the absorption threshold onset continuously shifted to the visible region with increased TPA concentration and calcination at 500 °C. The enhanced visible light absorption could be related to the formation of a surface complex TPA Keggin anion-TiO₂. The catalytic activity of the materials in the photodegradation of 4-chlorophenol under UV and visible light irradiation increased when the TPA content and the calcination temperature of the samples were raised.

Ag/AgxH3-xPMo12O40 Nanowires with Enhanced Visible-Light-Driven Photocatalytic Performance

Photocatalysis, a promising technology platform to address the environmental problems, has been attracting considerable attention. In this paper, Ag/Ag_xH_3-xPMo₁₂O₄₀ (simplified as Ag/AgHPMo₁₂) nanowires have been synthesized by a facile solid reaction route and in situ photodeposited method. The results of SEM and TEM indicate that the diameters of AgHPMo₁₂ nanowires are about 45 ± 10 nm, and Ag nanoparticles with diameters in the range of 5-15 nm are uniformly anchored on the surface of AgHPMo₁₂ nanowires. The Ag content in the Ag/AgHPMo₁₂ composite was manipulated by the light irradiation time (Ag/AgHPMo₁₂-x; x stands for the irradiation time; x = 2, 4, 6, 8 h, respectively). With increasing irradiation time, the light absorption of as-synthesized samples in the visible region was gradually enhanced. The Ag/AgHPMo₁₂-4 exhibits the best photocatalytic performance for the degradation of methyl orange and reduction of Cr₂O₇^2- under visible-light (λ > 420 nm) irradiation. The study of the photocatalytic mechanism reveals that both Ag and AgHPMo₁₂ can be excited by visible light. The photoinduced electrons were transferred from AgHPMo₁₂ to metallic Ag, and combined with the Ag plasmonic holes. The Ag plasmonic electrons were trapped by O₂ to form ·O₂^-, or directly reduced Cr₂O₇^2- to Cr³⁺. Meanwhile, the ·O₂^- species and the photogenerated holes of AgHPMo₁₂ were used to oxidize MO or i-PrOH; thus, they showed highly efficient and recyclable photocatalytic performance for removing the organic and inorganic pollutants.

Crystallization at Nanodroplet Interfaces in Emulsion Systems: A Soft-Template Strategy for Preparing Porous and Hollow Nanoparticles

A heterophase method to prepare hollow and/or porous crystalline nanoparticles of metal oxides at room temperature is presented, taking cerium(IV) oxide and γ-iron(III) oxide (i.e., maghemite) as representative cases. The crystallization begins at the oil-water interface in aqueous nanodroplets of the precursor in inverse (water-in-oil) miniemulsion systems, and it may continue toward the inner part of the droplets. A poly(styrene-b-acrylic acid) block copolymer is used as a structuring agent because the ability of the carboxylic groups to bind metal ions improves the inorganic shell formation. A precipitating base is added from the continuous phase, generating hydroxide species at the interface that begin the crystallization. We analyze the effects of the synthetic parameters in terms of colloidal stability and morphology of the resulting materials. In the case of maghemite samples, the prepared dispersions of hollow particles present a distinct magnetofluidic behavior.

Decatungstate acid improves the photo-induced electron lifetime and retards the recombination in dye sensitized solar cells

Decatungstate acid (DA) was utilized to modify TiO2 in the photoanode of dye sensitized solar cells. The photo-induced electron lifetime was evidently improved and the recombination was greatly inhibited. DA can introduce levels of impurities and lower the Fermi level through a doping effect and thus increase the photocurrent. Moreover, the improved charge carrier density can be found through external electric field surface photovoltage and Mott-Schottky plots. A 22.94% enhancement in photocurrent was achieved with little degradation in photovoltage, leading to a 10.28% increase in optic-to-electric power conversion efficiency.

Dual Role of Zirconium Oxoclusters in Hybrid Nanoparticles: Cross-Linkers and Catalytic Sites

Organic-inorganic hybrid nanoparticles are prepared by free-radical copolymerization of methyl methacrylate (MMA) with the structurally well-defined methacrylate-functionalized zirconium oxocluster Zr₄O₂(methacrylate)₁₂. The polymerization process occurs in the confined space of miniemulsion droplets. The formation of covalent chemical bonds between the organic and the inorganic counterparts improves the distribution of the guest species (oxoclusters) in the polymer particles, overcoming problems related to migration, leaching, and stability. Because of the presence of a high number of double bonds (12 per oxocluster), the oxoclusters act as efficient cross-linking units for the resulting polymer matrix, thus ruling its swelling behavior in organic solvents. The synthesized hybrid nanostructures are applied as heterogeneous systems in the catalytic oxidation of an organic sulfide to the corresponding sulfoxide and sulfone by hydrogen peroxide, displaying quantitative sulfide conversion in 4-24 h, with overall turnover numbers (TON) up to 8000 after 4 cycles.

Real-time spectroscopic monitoring of photocatalytic activity promoted by graphene in a microfluidic reactor

Photocatalytic microreactors have been utilized as rapid, versatile platforms for the characterization of photocatalysts. In this work, a photocatalytic microreactor integrated with absorption spectroscopy was proposed for the real-time monitoring of photocatalytic activity using different catalysts. The validity of this method was investigated by the rapid screening on the photocatalytic performance of a titanium oxide (TiO₂)-decorated graphene oxide (GO) sheet for the degradation of methylene blue under monochromatic visible irradiation. The sampling interval time could be minimized to 10 s for achieving real-time detection. The best photocatalytic activity was observed for an optimized TiO₂/GO weight mixing ratio of 7:11, with a reaction rate constant up to 0.067 min⁻¹. The addition of GO into TiO₂ enhances photocatalytic activity and adsorption of MB molecules. The synthetic reaction rate constant was up to approximately 0.11 min⁻¹, which was also the highest among the catalysts. The microreactor exhibited good sensitivity and reproducibility without weakening the performance of the photocatalysts. Consequently, the photocatalytic microreactor is promising as a simple, portable, and rapid screening tool for new photocatalysts.

An organo-functionalized metal–oxide cluster, [VIV6O6{(OCH2CH2)2N(CH2CH2OH)}6], with Anderson-like structure

The polyoxovanadium cluster compound, [VIV6O₆{(OCH₂CH₂)₂N(CH₂CH₂OH)}₆], is the isopolyoxovanadium analogue to cationic heteropolyoxovanadium species, such as [MVIV6O₆{(OCH₂CH₂)₂N(CH₂CH₂OH)}₆]⁺ (M = Li, Na). These isopolyoxovanadium and heteropolyoxovanadium clusters represent the first reported isopoly-heteropoly analogues in the polyoxometalate field.

H3PW12O40/TiO2–In2O3: a visible light driven type-II heterojunction photocatalyst for the photocatalytic degradation of imidacloprid

H₃PW₁₂O₄₀/TiO₂–In₂O₃, a visible light driven type-II heterojunction photocatalyst, displays enhanced photocatalytic activity for the degradation of imidacloprid.

Phosphotungstic acid supported on aminosilica functionalized perovskite-type LaFeO3 nanoparticles: a novel recyclable and excellent visible-light photocatalyst

In this research, a novel nanohybrid compound, LaFeO₃@SiO₂–NH₂/PTA has been prepared, in which H₃PW₁₂O₄₀ (abbreviated as PTA) was successfully anchored onto the surface of 3-aminopropylsilica modified LaFeO₃.

Investigation on the photoconductivity of polyoxometalates

The Keggin-type tungsten-series polyoxometalates photoconductivities and gas sensing performance for ethanol.

Heterogeneous Catalysis of Polyoxometalate Based Organic-Inorganic Hybrids

Organic–inorganic hybrid polyoxometalate (POM) compounds are a subset of materials with unique structures and physical/chemical properties. The combination of metal-organic coordination complexes with classical POMs not only provides a powerful way to gain multifarious new compounds but also affords a new method to modify and functionalize POMs. In parallel with the many reports on the synthesis and structure of new hybrid POM compounds, the application of these compounds for heterogeneous catalysis has also attracted considerable attention. The hybrid POM compounds show noteworthy catalytic performance in acid, oxidation, and even in asymmetric catalytic reactions. This review summarizes the design and synthesis of organic–inorganic hybrid POM compounds and particularly highlights their recent progress in heterogeneous catalysis.

Supported and unsupported nanomaterials for water and soil remediation: are they a useful solution for worldwide pollution?

Remediation technologies for wastes generated by industrial processes include coagulation, reverse osmosis, electrochemistry, photoelectrochemistry, advanced oxidation processes, and biological methods, among others. Adsorption onto activated carbon, sewage sludge, zeolites, chitosan, silica, and agricultural wastes has shown potential for pollutants' removal from aqueous media. Recently, nanoscale systems [nanoparticles (NPs) supported on different inorganic adsorbents] have shown additional benefits for the removal/degradation of several contaminants. According to the literature, NPs enhance the adsorption capacity of adsorbent materials and facilitate degradation of pollutants through redox reactions. In this review we analyzed relevant literature from 2011 to 2013, dealing with water and soil remediation by nanomaterials (NMs), either unsupported or supported upon inorganic adsorbents. Despite the outstanding reported results for some NMs, the analysis of the literature makes clear the necessity of more studies. There is lack of information about NMs regeneration and reusability, their large-scale application, and their efficiency in actual industrial wastewaters and contaminated soils. Additionally, little is known about NMs' life cycle, release of metal ions, disposal of pollutant loaded NMs, and their impacts on different ecosystems.

A light-harvesting polyoxometalate-polypyridine hybrid induces electron transfer as its Re(I) complex

A derivative of the Dawson polyoxometalate [P2V3W15O62](9-) functionalized with one remote bipyridine coordination site (2) has been synthesized and combined with the neutral {Re(CO)3Br} moiety. The new Re(I)-hybrid (3) was characterized by various analytical techniques. Hybrid 3 exhibits several redox processes on a wide range of potentials with reductions centered on V(V), W(VI) and the organic ligand in order of decreasing potential. Both units, the polyoxometalate and the transition metal complex, retain their intrinsic properties in the hybrid 3, which displays photosensitization in the UV region with tailing into high-energy visible region.

Photoelectrocatalytic performance of a titania-keggin type polyoxometalate-gold nanocomposite modified electrode in methanol oxidation

Aminosilicate sol-gel supported titania-keggin type polyoxometalate-gold nanocomposite materials (APS/(P25-PTA-Au)NCM) (APS, (3-aminopropyl)triethoxysilane; P25, Degussa-TiO2; PTA, Na3PW12O40·xH2O) were prepared by a simple chemical reduction method and characterized by diffuse reflectance spectroscopy, photoluminescence, x-ray diffraction, transmission electron microscopy and energy-dispersive x-ray analysis. The as-prepared APS/(P25-PTA-Au)NCM was used to fabricate the photoelectrode for a photoelectrochemical cell. The photoelectrocatalytic activity of the APS/(P25-PTA-Au)NCM modified photoelectrode in methanol oxidation was investigated. The APS/(P25-PTA-Au)NCM modified photoelectrode showed a higher photocurrent for methanol oxidation than control photoelectrodes. The modification of titania using PTA and Au nanoparticles significantly boosted the photoelectrocatalytic performance by a synergistic effect and thus improved the interfacial charge transfer processes. The presence of Au nanoparticles enhances the interfacial electron transfer process. The APS silicate sol-gel matrix acts as a very good support material for the preparation of the nanocomposite material and for preparation of the chemically modified electrode. This newly fabricated APS/(P25-PTA-Au)NCM modified photoelectrode could be a promising candidate for photoelectrochemical cells.

Catalytic epoxidation activity of keplerate polyoxomolybdate nanoball toward aqueous suspension of olefins under mild aerobic conditions

Catalytic efficiency of a sphere-shaped nanosized polyoxomolybdate {Mo132} in the aerobic epoxidation of olefins in water at ambient temperature and pressure in the absence of reducing agent is exploited which resulted good-to-high yields and desired selectivity.