Colloidal Synthesis and Photophysics of M3 Sb2 I9 (M=Cs and Rb) Nanocrystals: Lead-Free Perovskites.

Herein we report the colloidal synthesis of Cs3 Sb2 I9 and Rb3 Sb2 I9 perovskite nanocrystals, and explore their potential for optoelectronic applications. Different morphologies, such as nanoplatelets and nanorods of Cs3 Sb2 I9 , and spherical Rb3 Sb2 I9 nanocrystals were prepared. All these samples show band-edge emissions in the yellow-red region. Exciton many-body interactions studied by femtosecond transient absorption spectroscopy of Cs3 Sb2 I9 nanorods reveals characteristic second-derivative-type spectral features, suggesting red-shifted excitons by as much as 79 meV. A high absorption cross-section of ca. 10-15  cm2 was estimated. The results suggest that colloidal Cs3 Sb2 I9 and Rb3 Sb2 I9 nanocrystals are potential candidates for optical and optoelectronic applications in the visible region, though a better control of defect chemistry is required for efficient applications.

Faceted metal and metal oxide nanoparticles: design, fabrication and catalysis.

The review addresses new advances in metal, bimetallic, metal oxide, and composite particles in their nanoregime for facet-selective catalytic applications. The synthesis and growth mechanisms of the particles have been summarized in brief in this review with a view to develop critical examination of the faceted morphology of the particles for catalysis. The size, shape and composition of the particles have been found to be largely irrelevant in comparison to the nature of facets in catalysis. Thus selective high- and low-index facets have been found to selectively promote adsorption, which eventually leads to an effective catalytic reaction. As a consequence, a high density of atoms rest at the corners, steps, stages, kinks etc on the catalyst surface in order to host the adsorbate efficiently and catalyze the reaction. Again, surface atomic arrangement and bond length have been found to play a dominant role in adsorption, leading to effective catalysis.

Imine (-CH=N-) brings selectivity for silver enhanced fluorescence.

Metal enhanced fluorescence (MEF) from organic molecules has been the focus of plasmonic/photonic research. In this regard, silver and gold particle stimulated fluorescence has received much attention. However, the involvement of small organic molecules in solution has not been accounted for in MEF. We have reported here that the aqueous alkaline solution of salicylaldehyde exhibits highly enhanced fluorescence at room temperature (λ(em) ∼ 420 nm, Stokes shift 120 nm, stability > a year) in the presence of Ag(I) or Au(III) after simple ageing for two days. The increased scattering cross-section of the in situ produced aggregated metal particles and the lightening rod effect of the metal aggregates that concentrate the electric field around the fluorophore (i.e., alkaline solutions of salicylaldehyde) are ascribed to such extraordinary fluorescence enhancement caused by Ag and Au particles. Finally, selective fluorescence enhancement due to silver particles alone has been made possible in the presence of ammonia or primary amines due to imine bond formation that eliminates Au enhanced fluorescence quantitatively. This finding allows us to design a highly selective Ag(I) sensor in the solution phase with a cheap and commercially available compound with LOD far below EPA-permissible levels. The imine induced selective silver enhanced fluorescence phenomenon becomes a general matter in our studies even with different Schiff bases.

Precursor salt assisted syntheses of high-index faceted concave hexagon and nanorod-like polyoxometalates.

This paper describes an effective method for a precursor salt assisted fabrication and reshaping of two different polyoxometalates [(NH4)2Cu(MoO4)2 (ACM) and Cu3(MoO4)2(OH)2 (CMOH)] into five distinctive shapes through straightforward and indirect routes. Explicit regulation of the structural arrangements of ACM and CMOH has been studied in detail with altered precursor salt concentration employing our laboratory developed modified hydrothermal (MHT) method. Morphologically different ACM 3D architectures are evolved with higher molybdate concentration, whereas 1D growth of CMOH is observed with increased copper concentration. Interesting morphological transformation of the products has been accomplished employing one precursor salt at a time without using any other foreign reagent. It has been proven that large ACMs become labile in the presence of incoming Cu(II) and NH4(+) ions of the precursor salts. A new strategy for the conversion of faceted ACMs (hexagonal plate, circular plate and hollow flower) to exclusive CMOH nanorods through a Cu(II) assisted reaction has been adopted. According to thermodynamic consideration, the synthesis of rare concave nanostructures with high index facet is still challenging due to their higher reactivity. In this study, concave hexagonal ACM with high index facet {hkl} has been successfully prepared for the first time from hexagonal ACM through simple etching with ammonium heptamolybdate (AHM), which is another precursor salt. Hexagonal ACM corrugates to a concave hexagon because of the higher reactivity of the {001} crystal plane than that of the {010} plane. It has been shown that high index facet exposed concave hexagonal ACM serves as a better catalyst for the photodegradation of dye than the other microstructures enclosed by low index facets.

Fabrication of superhydrophobic copper surface on various substrates for roll-off, self-cleaning, and water/oil separation.

Superhydrophobic surfaces prevent percolation of water droplets and thus render roll-off, self-cleaning, corrosion protection, etc., which find day-to-day and industrial applications. In this work, we developed a facile, cost-effective, and free-standing method for direct fabrication of copper nanoparticles to engender superhydrophobicity for various flat and irregular surfaces such as glass, transparency sheet (plastic), cotton wool, textile, and silicon substrates. The fabrication of as-prepared superhydrophobic surfaces was accomplished using a simple chemical reduction of copper acetate by hydrazine hydrate at room temperature. The surface morphological studies demonstrate that the as-prepared surfaces are rough and display superhydrophobic character on wetting due to generation of air pockets (The Cassie-Baxter state). Because of the low adhesion of water droplets on the as-prepared surfaces, the surfaces exhibited not only high water contact angle (164 ± 2°, 5 µL droplets) but also superb roll-off and self-cleaning properties. Superhydrophobic copper nanoparticle coated glass surface uniquely withstands water (10 min), mild alkali (5 min in saturated aqueous NaHCO3 of pH ≈ 9), acids (10 s in dilute HNO3, H2SO4 of pH ≈ 5) and thiol (10 s in neat 1-octanethiol) at room temperature (25-35 °C). Again as-prepared surface (cotton wool) was also found to be very effective for water-kerosene separation due to its superhydrophobic and oleophilic character. Additionally, the superhydrophobic copper nanoparticle (deposited on glass surface) was found to exhibit antibacterial activity against both Gram-negative and Gram-positive bacteria.

Intriguing manipulation of metal-enhanced fluorescence for the detection of Cu(II) and cysteine.

Commercially available salicylaldehyde, in alkaline medium, exhibits strong fluorescence after one hour of UV exposure in the presence of Ag(I) . The phenolic group of salicylaldehyde is converted into the quinone form under alkaline conditions in the presence of AgNO3 , resulting in aggregated Ag(0), which causes approximately 250 times fluorescence enhancement of the in situ produced quinone. Such high silver-enhanced-fluorescence (SEF) is selectively quenched by cysteine, arginine, histidine, methionine, and tryptophan. In contrast to the other amino acids, ageing brings selectivity of the cysteine-induced quenching effect. Interestingly, Cu(II) is found to be the only metal ion that exclusively regenerates the lost fluorescence. Thus, quenching and recovery of fluorescence (Turn Off/On) can be used for the selective and sensitive detection of cysteine as well as Cu(II) ions in one pot. Alteration of the electric field density around the fluorophore (lightening rod effect) and scattering/absorption cross-section have been proposed to account for the Off/On fluorescence.

Fluorescent Au(I)@Ag2/Ag3 giant cluster for selective sensing of mercury(II) ion.

Highly stable Au(I)(core)-Ag(0)(shell) particles have been synthesized in aqueous solution via a green chemistry pathway utilising sunlight irradiation. The shell of the particles is composed of fluorescent Ag2 and Ag3 clusters which make the large core-shell particles highly fluorescent. The Au(I) core of the particles offers long-term stability to the silver clusters, which are otherwise unstable in solution at room temperature, by the transfer of electron density from the shell. Successive additions of Hg(II) ions to the fluorescent solution cause efficient and selective quenching of the fluorescence with gradual red shifting of the emission peak. The metallophilic 5d(10)(Hg(2+))-4d(10)(Ag(δ+)) interaction as well as Hg(II) stimulated aggregation have been ascribed to causing the fluorescence quenching and red shift. The fluorescent Au(I)(core)-Ag(0)(shell) particles are a highly selective and sensitive sensing platform for the detection of Hg(II) down to 6 nM in the presence of various metal ions. The detection limit is far below the permissible level as determined by the EPA. Interferences due to Cu(II) and Fe(III) have been eliminated using Na2-EDTA and NH4HF2, respectively. The fluorescent particles are successfully transferred to various solvent systems making Hg(II) determination also possible in non-aqueous media. Finally, the temperature dependent fluorescence change with and without Hg(II) provides information about the metallophilic interaction.

Account of nitroarene reduction with size- and facet-controlled CuO-MnO2 nanocomposites.

In this work, we propose a systematic and delicate size- and shape-controlled synthesis of CuO-MnO2 composite nanostructures from time-dependent redox transformation reactions between Cu2O and KMnO4. The parental size and shape of Cu2O nanostructures are retained, even after the redox transformation, but the morphology becomes porous in nature. After prolonged reaction times (>24 h), the product shapes are ruptured, and as a result, tiny spherical porous nanocomposites of ∼100 nm in size are obtained. This method is highly advantageous due to its low cost, its easy operation, and a surfactant or stabilizing agent-free approach with high reproducibility, and it provides a facile but new way to fabricate porous CuO-MnO2 nanocomposites of varied shape and size. The composite nanomaterials act as efficient recyclable catalysts for nitroarene reduction in water at room temperature. The time-dependent reduction kinetics can be easily monitored by using UV-vis spectrophotometer. The catalytic system is found to be very useful toward the reduction of nitro compounds, regardless of the type and position of the substituent(s). Furthermore, it is revealed that CuO-MnO2 composite nanomaterials exhibit facet-dependent catalytic activity toward nitroarene reduction, where the (111) facet of the composite stands to be more active than that of the (100) facet. The results are also corroborated from the BET surface area measurements. It is worthwhile to mention that porous tiny spheres (product of 48 h reaction) exhibit the highest catalytic activity due to pronounced surface area and smaller size.

Aggregation of nitroaniline in tetrahydrofuran through intriguing H-bond formation by sodium borohydride.

The participation of sodium borohydride (NaBH4) in hydrogen bonding interactions and transient anion radical formation has been proved. Thus, the properties of NaBH4 are extended beyond the purview of its normal reducing capability and nucleophilic property. It is reported that ortho- and para-nitroanilines (NAs) form stable aggregates only in tetrahydrofuran (THF) in the presence of NaBH4 and unprecedented orange/red colorations are observed. The same recipe with nitrobenzene instead of nitroanilines (NAs) in the presence of NaBH4 evolves a transient rose red solution due to the formation of a highly fluorescent anion radical. Spectroscopic studies (UV-vis, fluorescence, RLS, Raman, NMR etc.) as well as theoretical calculations supplement the J-aggregate formation of NAs due to extensive hydrogen bonding. This is the first report where BH4(-) in THF has been shown to support such an aggregation process through H-bonding. It is further confirmed that stable intermolecular hydrogen bond-induced aggregation requires a geometrical match in both the nitro- and amino-functionalities attached to the phenyl ring with proper geometry. On the contrary, meta-nitroaniline remains as the odd man out and does not take part in such aggregation. Surprisingly, Au nanoparticles dismantle the J-aggregates of NA in THF. Explicit hydrogen bond formation in NA has been confirmed experimentally considering its promising applications in different fields including non-linear optics.

Morphology controlled synthesis of SnS2 nanomaterial for promoting photocatalytic reduction of aqueous Cr(VI) under visible light.

A mild, template free protocol has been demonstrated for SnS2 nanoflake formation at the gram level from SnCl2 and thioacetamide (TAA). The SnS2 nanoflakes congregate to nanoflowers and nanoyarns with variable TAA concentrations. BET measurements reveal that the synthesized nanomaterials are highly porous having very high surface area, and the nanoflower has higher surface area than the nanoyarn. The synthesized nanomaterial finds application for promoting photoreduction of extremely toxic and lethal Cr(VI) under visible light irradiation due to their porous nature. The nanoflowers photocatalyst is proved to be superior to nanoyarn due to the increased surface area and higher pore volume. It was also inferred that increased pH decreased the reaction rate. The present result suggests that the morphology-dependent photoreduction of Cr(VI) by SnS2 nanomaterial under visible light exposure will endorse a new technique for harvesting energy and purification of wastewater.

The tuning of metal enhanced fluorescence for sensing applications.

Stable coinage metal nanoparticles (NPs) have been synthesized individually in an aqueous alkaline solution from the corresponding metal salts as precursors using the condensation product (CP) of salicylaldehyde and triethylenetetramine as a reagent. Silver and gold NPs are obtained with and without light illumination but UV irradiation is essential for Cu(0)NP formation. During nanoparticle formation the CP is oxidized to OCP which eventually becomes a fluorophore and also a stabilizer for the in situ produced NPs. It has been observed that silver and gold particle formation kinetics is accelerated by UV exposure. Thus the ease of evolution of coinage metal NP formation relates to their nobility. The as prepared OCP solutions containing coinage metals exhibit a fluorescence contrast behaviour (fluorescence enhancement by Cu and Ag; quenching by AuNP) due to the match and mismatch of wave vectors. The electric field evident from the FDTD simulation abreast of the scattering cross section of the NPs governed from Mie theory as a consequence of surface plasmon coupled emission (SPCE), near field electromagnetic intensity enhancement and lightening rod effect concentrating the electric field around the fluorophore are responsible for the Cu and AgNPs stimulated fluorescence. Again, lossy surface waves are anticipated for efficient quenching by the AuNPs. The most unprecedented observation is 'Turn On' fluorescence which is reported here as a result of the substitution of Au(0) or Cu(0) by Ag(0). Finally, the preferential fluorescence enhancement helps the selective detection of Ag(i) and Cu(ii) well below the US Environmental Protection Agency (EPA) permissible level by tuning the experimental conditions.

Pure inorganic gel: a new host with tremendous sorption capability.

In aqueous medium at room temperature Ag(I) forms a metallogel with vanadate ions which doesn't have any carbon in the gel skeleton. The serendipitously discovered inorganic gel, which involves a reasonably large volume of water, is capable of sequestering gas, dye and toxic pollutants from water bodies.

Green synthesis and reversible dispersion of a giant fluorescent cluster in solid and liquid phase.

A water-soluble highly fluorescent silver cluster on Au(I) surface has been synthesized with green chemistry under sunlight. The evolution of the silver cluster is synergistic, demanding gold and glutathione. The fluorescent Au(I)core-Ag(0)shell particles are huge in size and at the same time they are robust. That is why they become a deliverable fluorescing solid upon drying. Again, the giant particles run into common water miscible solvents. As a result, the fluorescence intensity increases to a great extent without any alteration of emission maxima. In this respect, acetone has been found to be the best-suited solvent. To have a universal applicability of the fluorescent clusters, the particles in the water pool of a reverse micelle have been prepared to transfer the particles into different water immiscible solvents. The comparatively lower fluorescence intensity of the particles has been ascribed to a space confinement effect. Finally, giant-cluster-impregnated yellow-orange fluorescent polymer film and fluorescent cotton wool, as well as paper substrate, have been prepared. The antibacterial activity of the fluorescent particle has also been tested involving modified cotton wool and paper substrate for Gram-negative and -positive Escherichia coli and Staphylococcus aureus, respectively.

Measuring free-living physical activity in adults with and without neurologic dysfunction with a triaxial accelerometer.

OBJECTIVE: To investigate the reliability, validity, and utility of a triaxial accelerometer to measure physical activity in the free-living environment in adults with and without neurologic dysfunction. DESIGN: Repeated-measures design. SETTING: General community. PARTICIPANTS: Volunteer sample of 17 men and 30 women (age range, 28-91y) living in the community with stroke of greater than 6 months in duration (n=20), Parkinson disease (n=7), or multiple sclerosis (n=11), and healthy but sedentary controls (n=9). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Physical activity measured with the TriTrac RT3 accelerometer, 7-day recall questionnaire, and activity diary. RESULTS: The accelerometer reliably measured free-living physical activity (intraclass correlation coefficient, .85; 95% confidence interval, .74-.91; P=.000). The standard error of measurement indicated that a second test would differ from a baseline test by +/-23%. Mean daily RT3 data collected in the first 3 days differed significantly from that of the mean daily RT3 data collected over 7 days. The RT3 appeared to distinguish level of mobility better than the 7-day recall questionnaire, and participants found the RT3 to be a user-friendly and acceptable measure of physical activity. CONCLUSIONS: The triaxial accelerometer provided a stable measure of free-living physical activity, was found to distinguish between people with varying levels of mobility, and was well tolerated by participants. The results indicate that collecting data for 3 days was not reflective of data collected over 7 days.