Ioannis T. Papadas

Turn‐On Luminescence Sensing and Real‐Time Detection of Traces of Water in Organic Solvents by a Flexible Metal–Organic Framework

The development of efficient sensors for the determination of the water content in organic solvents is highly desirable for a number of chemical industries. Presented herein is a Mg(2+) metal-organic framework (MOF), which exhibits the remarkable capability to rapidly detect traces of water (0.05-5 % v/v) in various organic solvents through an unusual turn-on luminescence sensing mechanism. The extraordinary sensitivity and fast response of this MOF for water, and its reusability make it one of the most powerful water sensors known.

A high surface area ordered mesoporous BiFeO3 semiconductor with efficient water oxidation activity

Bismuth ferrite (BiFeO3) is an important multiferroic oxide material because of its unique magnetic and ferroelectric properties. Here, we synthesize for the first time a highly ordered mesoporous BiFeO3 semiconductor using tartaric acid-assisted growth of the BiFeO3 compound inside the pores of a carbon template. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and N2 physisorption measurements reveal that the template-free material possesses a three-dimensional hexagonal mesostructure with a large internal BET surface area (141 m2 g−1) and narrow sized pores (ca. 4 nm). Also, the pore walls comprise single-phase BiFeO3 nanocrystals according to the high-resolution TEM, electron diffraction and magnetic experiments. The mesoporous BiFeO3 shows high activity for the photocatalytic oxygen evolution reaction (OER) under UV-visible light (λ > 380 nm), affording an average oxygen evolution rate of 66 μmol h−1 g−1. We also show that the propensity of photogenerated holes for the OER can be significantly enhanced when 1 wt% Au nanoparticles are deposited on the BiFeO3 surface. The Au/BiFeO3 heterostructure exerts excellent OER activity (586 μmol h−1 g−1) and long-term cycling stability, raising the possibility for the design of effective and robust OER photocatalysts.

Rapid, green and inexpensive synthesis of high quality UiO-66 amino-functionalized materials with exceptional capability for removal of hexavalent chromium from industrial waste

We describe a new synthetic method for the isolation of the UiO-66 amino-functionalized material (called metal organic resin-1, MOR-1) and its composite with alginic acid (HA). MOR-1 can be prepared in high yield (∼70%) and purity within an hour via a reflux reaction of ZrCl4 and 2-amino-terephthalic acid in acifidied aqueous solution, whereas addition of sodium alginate to the fine suspension of MOR-1 resulting from the reflux synthesis affords the MOR-1-HA composite. This inexpensive, green and fast preparation method results in UiO-66 amino-functionalized materials (MOR-1 and MOR-1-HA) of the same quality and microporous features as those of compounds isolated with the slower solvothermal synthesis involving toxic and costly organic solvents. Field Emission-Scanning Electron Microscopy (FE-SEM) studies revealed that MOR-1 consists of spongy nanoparticles (150–300 nm in size), whereas MOR-1-HA nanoparticles are relatively compact. Thus, for the first time we could visualize the effect of alginic acid partially coating the surface of the MOR particles. The composite prepared by this method can be successfully utilized as a stationary phase, mixed with sand, in an anion-exchange column. The column shows excellent hexavalent chromium sorption properties and can be easily regenerated and reused several times with almost no loss of its initial Cr(VI) removal capacity. Remarkably, this ion exchange column is capable of eliminating Cr(VI) ions from chrome plating wastewater samples, thus indicating its potential for applications in industrial wastewater treatment.

Controllable Synthesis of Mesoporous Iron Oxide Nanoparticle Assemblies for Chemoselective Catalytic Reduction of Nitroarenes.

Iron(III) oxide is a low-cost material with applications ranging from electronics to magnetism, and catalysis. Recent efforts have targeted new nanostructured forms of Fe2O3 with high surface area-to-volume ratio and large pore volume. Herein, the synthesis of 3D mesoporous networks consisting of 4-5 nm γ-Fe2O3 nanoparticles by a polymer-assisted aggregating self-assembly method is reported. Iron oxide assemblies obtained from the hybrid networks after heat treatment have an open-pore structure with high surface area (up to 167 m(2)g(-1)) and uniform pores (ca. 6.3 nm). The constituent iron oxide nanocrystals can undergo controllable phase transition from γ-Fe2O3 to α-Fe2O3 and to Fe3O4 under different annealing conditions while maintaining the 3D structure and open porosity. These new ensemble structures exhibit high catalytic activity and stability for the selective reduction of aryl and alkyl nitro compounds to the corresponding aryl amines and oximes, even in large-scale synthesis.

Alkaline Earth Metal Ion/Dihydroxy–Terephthalate MOFs: Structural Diversity and Unusual Luminescent Properties

Alkaline earth (group 2) metal ion organic frameworks (AEMOFs) represent an important subcategory of MOFs with interesting structures and physical properties. Five MOFs, namely, [Mg2(H2dhtp)2(μ-H2O)(NMP)4] (AEMOF-2), [Mg2(H2dhtp)1.5(DMAc)4]Cl·DMAc (AEMOF-3), [Ca(H2dhtp)(DMAc)2] (AEMOF-4), [Sr3(H2dhtp)3(DMAc)6]·H2O (AEMOF-5), and [Ba(H2dhtp)(DMAc)] (AEMOF-6) (H4dhtp = 2,5-dihydroxy-terepthalic acid; DMAc = N,N-dimethylacetamide; NMP = N-methylpyrrolidone), are presented herein. The reported MOFs display structural variety with diverse topologies and new structural features. Interestingly, AEMOF-6 is the first example of a Ba(2+)-H2dhtp(2-) MOF, and AEMOF-5 is only the second known Sr(2+)-H2dhtp(2-) MOF. Detailed photoluminescence studies revealed alkaline earth metal ion-dependent fluorescence properties of the materials, with the heavier alkaline earth metal ions exhibiting red-shifted emission with respect to the lighter ions at room temperature. A bathochromic shift of the emission was observed for the MOFs (mostly for AEMOF-3 and AEMOF-4) at 77 K as a result of excited state proton transfer (ESIPT), which involves an intramolecular proton transfer from a hydroxyl to an adjacent carboxylic group of the H2dhtp(2-) ligand. Remarkably, AEMOF-6 displays rare yellow fluorescence at room temperature, which is attractive for solid state lighting applications. To probe whether the alkaline earth metal ions are responsible for the unusual luminescence properties of the reported MOFs, the potential energy surfaces (PESs) of the ground, S0, and lowest energy excited singlet, S1, states of model complexes along the intramolecular proton transfer coordinate were calculated by DFT and TD-DFT methods.

Mesoporous assembled structures of Cu2O and TiO2 nanoparticles for highly efficient photocatalytic hydrogen generation from water

Photocatalytic water splitting to produce hydrogen using solar energy is a particularly attractive solution to increasing energy demands. However, to be of practical use, semiconductor electrodes need to be made of inexpensive, abundant elements and have a high, yet stable, photocatalytic H2-production activity. Here we report the first demonstration of 3D mesoporous networks of Cu2O and TiO2 nanoparticles as highly efficient photocatalysts for hydrogen generation from water. These assembled structures feature a highly accessible pore surface that exposes a large fraction of anatase TiO2 and Cu2O nanoparticles to electrolytes, and has a small grain size of the constituent nanocrystals, which lead to excellent activity for H2 evolution via a UV-visible light-driven reduction of protons. Catalytic results associated with optical UV-vis/NIR absorption and photoluminescence (PL) data indicated that the large separation of photogenerated electrons and holes at the Cu2O–TiO2 p–n junctions was the main reason attributed to the improved photochemical performance. Consequently, the mesoporous Cu2O/TiO2 catalyst containing ∼1.5 wt% Cu reaches an average H2 evolution rate of ∼542 μmol h−1 (or ∼36 133 μmol h−1 g−1) with an apparent quantum efficiency (QE) of 13.5% at 365 nm and an incident photon conversion efficiency of ∼4.1% under UV-visible light illumination (360–780 nm), which is one of the best HER activities among TiO2-based semiconductor systems reported to date.

Templated assembly of BiFeO3 nanocrystals into 3D mesoporous networks for catalytic applications

The self-assembly of uniform nanocrystals into large porous architectures is currently of immense interest for nanochemistry and nanotechnology. These materials combine the respective advantages of discrete nanoparticles and mesoporous structures. In this article, we demonstrate a facile nanoparticle templating process to synthesize a three-dimensional mesoporous BiFeO₃ material. This approach involves the polymer-assisted aggregating assembly of 3-aminopropanoic acid-stabilized bismuth ferrite (BiFeO₃) nanocrystals followed by thermal decomposition of the surfactant. The resulting material consists of a network of tightly connected BiFeO₃ nanoparticles (∼6-7 nm in diameter) and has a moderately high surface area (62 m(2) g(-1)) and uniform pores (ca. 6.3 nm). As a result of the unique mesostructure, the porous assemblies of BiFeO₃ nanoparticles show an excellent catalytic activity and chemical stability for the reduction of p-nitrophenol to p-aminophenol with NaBH4.

Synthesis of Ordered Mesoporous CuO/CeO2 Composite Frameworks as Anode Catalysts for Water Oxidation

Cerium-rich metal oxide materials have recently emerged as promising candidates for the photocatalytic oxygen evolution reaction (OER). In this article, we report the synthesis of ordered mesoporous CuO/CeO2 composite frameworks with different contents of copper(II) oxide and demonstrate their activity for photocatalytic O2 production via UV-Vis light-driven oxidation of water. Mesoporous CuO/CeO2 materials have been successfully prepared by a nanocasting route, using mesoporous silica as a rigid template. X-ray diffraction, electron transmission microscopy and N2 porosimetry characterization of the as-prepared products reveal a mesoporous structure composed of parallel arranged nanorods, with a large surface area and a narrow pore size distribution. The molecular structure and optical properties of the composite materials were investigated with Raman and UV-Vis/NIR diffuse reflectance spectroscopy. Catalytic results indicated that incorporation of CuO clusters in the CeO2 lattice improved the photochemical properties. As a result, the CuO/CeO2 composite catalyst containing ~38 wt % CuO reaches a high O2 evolution rate of ~19.6 µmol·h−1 (or 392 µmol·h−1·g−1) with an apparent quantum efficiency of 17.6% at λ = 365 ± 10 nm. This OER activity compares favorably with that obtained from the non-porous CuO/CeO2 counterpart (~1.3 µmol·h−1) and pure mesoporous CeO2 (~1 µmol·h−1).

The effect of hole transporting layer in charge accumulation properties of p-i-n perovskite solar cells

The charge accumulation properties of p-i-n perovskite solar cells were investigated using three representative organic and inorganic hole transporting layer (HTL): (a) Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS, Al 4083), (b) copper-doped nickel oxide (Cu:NiOx), and (c) Copper oxide (CuO). Through impedance spectroscopy analysis and modelling, it is shown that charge accumulation is decreased in the HTL/perovskite interface, between PEDOT:PSS to Cu:NiOx and CuO. This was indicative from the decrease in double layer capacitance (Cdl) and interfacial charge accumulation capacitance (Cel), resulting in an increase to recombination resistance (Rrec), thus decreased charge recombination events between the three HTLs. Through AFM measurements, it is also shown that the reduced recombination events (followed by the increase in Rrec) are also a result of increased grain size between the three HTLs, thus reduction in the grain boundary area. These charge accumulation properties of the thre...

Room temperature nanoparticulate interfacial layers for perovskite solar cells via solvothermal synthesis

We present a solvothermal synthetic route to produce monodisperse CuO nanoparticles (NPs) in the range of 5–10 nm that can be used as a hole selective interfacial layer between indium tin oxide (ITO) and the perovskite active layer for p–i–n perovskite solar cells by spin casting the dispersions at room temperature. The bottom electrode interface modification provided by spherical CuO-NPs at room temperature promotes the formation of high quality perovskite photoactive layers with a large crystal size and strong optical absorption. Furthermore, it is shown that the nanoparticulate nature of the CuO hole transporting interfacial layer can be used to improve light manipulation within the perovskite solar cell device structure. The corresponding p–i–n CH3NH3PbI3-based solar cells show high Voc values of 1.09 V, which is significantly higher compared to the Voc values obtained with conventional PEDOT:PSS hole selective contact based perovskite solar cells.