Influence of shape on the carrier relaxation dynamics of CsPbBr3 perovskite nanocrystals.

Lead halide perovskite nanocrystals (NCs) have recently emerged as a new class of functional materials for designing efficient light harvesting systems because of their unique photophysical properties. Here, we report the influence of different shapes on the relaxation dynamics of perovskite nanocrystals. The structural transformation of CsPbBr3 NCs from cubic shape to rod shape occurs by changing the solvent from toluene to dichloromethane (DCM). Rietveld analysis reveals that the crystallinity along with the preferred orientation (PO) of the orthorhombic phase plays a vital role for the unidirectional growth of rod shaped CsPbBr3 NCs in DCM. Time-resolved emission spectroscopy and ultrafast transient absorption spectroscopy are used to understand the photoinduced relaxation processes. Global and target analysis of femto-second transient absorption kinetics has been done to understand the individual excited-state species. The analysis reveals that trap states play an important role in the carrier relaxation dynamics of cubic and rod shaped NCs. The lifetime of the shallow trap (ST) changes from 25 ps to 45 ps and the lifetime of the deep trap (DT) state changes from 163 ps to 303 ps with changing the shape of the nanocrystals from cubic to rod. This work highlights the tuning of the crystal phase, shape and the exciton dynamics of CsPbBr3 NCs that would be beneficial for designing efficient photovoltaic devices.

Photoswitching and Thermoresponsive Properties of Conjugated Multi-chromophore Nanostructured Materials.

Conjugated multi-chromophore organic nanostructured materials have recently emerged as a new class of functional materials for developing efficient light-harvesting, photosensitization, photocatalysis, and sensor devices because of their unique photophysical and photochemical properties. Here, we demonstrate the formation of various nanostructures (fibers and flakes) related to the molecular arrangement (H-aggregation) of quaterthiophene (QTH) molecules and their influence on the photophysical properties. XRD studies confirm that the fiber structure consists of >95% crystalline material, whereas the flake structure is almost completely amorphous and the microstrain in flake-shaped QTH is significantly higher than that of QTH in solution. The influence of the aggregation of the QTH molecules on their photoswitching and thermoresponsive photoluminescence properties is revealed. Time-resolved anisotropic studies further unveil the relaxation dynamics and restricted chromophore properties of the self-assembled nano/microstructured morphologies. Further investigations should pave the way for the future development of organic electronics, photovoltaics, and light-harvesting systems based on π-conjugated multi-chromophore organic nanostructured materials.

Influence of size and shape on the photocatalytic properties of SnO2 nanocrystals.

Tuning the functional properties of nanocrystals is an important issue in nanoscience. Here, we are able to tune the photocatalytic properties of SnO2 nanocrystals by controlling their size and shape. A structural analysis was carried out by using X-ray diffraction (XRD)/Rietveld and transmission electron microscopy (TEM). The results reveal that the number of oxygen-related defects varies upon changing the size and shape of the nanocrystals, which eventually influences their photocatalytic properties. Time-resolved spectroscopic studies of the carrier relaxation dynamics of the SnO2 nanocrystals further confirm that the electron-hole recombination process is controlled by oxygen/defect states, which can be tuned by changing the shape and size of the materials. The degradation of dyes (90%) in the presence of SnO2 nanoparticles under UV light is comparable to that (88%) in the presence of standard TiO2 Degussa P-25 (P25) powders. The photocatalytic activity of the nanoparticles is significantly higher than those of nanorods and nanospheres because the effective charge separation in the SnO2 nanoparticles is controlled by defect states leading to enhanced photocatalytic properties. The size- and shape-dependent photocatalytic properties of SnO2 nanocrystals make these materials interesting candidates for photocatalytic applications.

Visible light photocatalysts from low-grade iron ore: the environmentally benign production of magnetite/carbon (Fe3O4/C) nanocomposites.

Magnetite (Fe3O4) nanoparticles coated with dextrose and gluconic acid possessing both super-paramagnetism and excellent optical properties have been productively synthesized through a straightforward, efficient and cost-efficient hydrothermal reduction route using Fe3+ as sole metal precursor acquired from accumulated iron ore tailings-a mining waste that usually represents a major environmental threat. Fe3O4/C nanocomposites were fully elucidated by FEGSEM and TEM, revealing a combination of platelets (<1 µm) capped by particles (<10 nm) and magnetite which was verified by XPS, which demonstrated also oxygen deficiency. A dextrose/gluconic acid coating was elucidated by Fourier transform-infrared (FT-IR) spectroscopy and thermogravimetric analysis (TGA). The Fe3O4/C nanocomposites were found to be superparamagnetic at room temperature. Meanwhile, their optical properties were investigated by UV-visible diffuse reflectance spectroscopy (UV-vis DRS) and photoluminescence (PL) spectroscopy; an Eg of 1.86 eV was determined, and emissions at 612 and 650 nm (ex. 250 nm) were consistent with the XPS identification of oxygen vacancies. The efficacy of the as-synthesized magnetically recoverable magnetite/carbon (Fe3O4/C) nanocomposites has been exhibited in the photocatalytic degradation of the toxic textile (industrial) dye bodactive red BNC-BS.

Microstructure characterization of biocompatible heterojunction hydrogen titanate-Ag2O nanocomposites for superior visible light photocatalysis and antibacterial activity.

Hydrogen trititanate (H2Ti3O7·2H2O) and hydrogen trititanate/Ag2O hybrid nanocomposites (NCs) with novel structure have been synthesized by a simple solvothermal route followed by Na+/H+ ion-exchange. Growths of hydrogen trititanate with nanofiber (HTNF) and nanotube (HTNT) morphologies and hydrogen trititanate-Ag2O (HTFAG and HTTAG) nanocomposites have been tailored by controlling the solvent media. Detailed microstructure characterization of all these samples have been carried out by Rietveld refinement of XRD data and analyzing FESEM/HRTEM micrographs and FTIR spectra. Band gap energies of all these semiconducting samples are obtained from UV-Vis absorption spectra. Both HTFAG and HTTAG NCs exhibit enhanced photocatalytic degradation of organic pollutant (Congo red dye) under visible light, in comparison to HTNF and HTNT respectively due to the formation of a heterojunction between H2Ti3O7·2H2O and Ag2O, which is supported by photoluminescence spectroscopy. HTFAG and HTTAG NCs also show superior antibacterial activity against both gram-negative (Escherichia coli) and gram-positive (Bacillus subtilis) bacteria compared to their pure counterparts. MTT assay reflects a sufficiently high percentage of cell viability and confirms the significant cytocompatibility of all the samples.

Postsynthesis Spontaneous Coalescence of Mixed-Halide Perovskite Nanocubes into Phase-Stable Single-Crystalline Uniform Luminescent Nanowires.

All inorganic mixed-halide perovskite, CsPb(Br xI1- x)3 (0 ≤ x ≤ 1), nanocrystals possess tunable photoluminescence with high quantum yield in the visible window. However, the photoluminescence degrades rapidly with postsynthetic aging due to the spontaneous ion separation and phase instability. Here we show that the postsynthetic aging of CsPb(Br xI1- x)3 nanocubes spontaneously forms highly uniform single-crystalline nanowires with a diameter of 9 ± 0.5 nm and length of up to several micrometers. The nanowires show bright photoluminescence with an absolute photoluminescence quantum yield of 41%. Rietveld refinement identifies the stable orthorhombic phase of the nanowires, implying a phase transition from the cubic crystallographic phase of the nanocubes during the morphology evolution. Transient absorption spectroscopy reveals a faster excited-state decay dynamic with a large exciton delocalization length in 1D nanowires. Our findings elucidate the insights into the postsynthesis morphology evolution of mixed-halide perovskite nanocrystals leading to luminescent nanowires with excellent crystal phase stability for potential optoelectronic applications.

White light emitting MgAl2O4:Dy3+,Eu3+ nanophosphor for multifunctional applications.

Feeble white emission with a low Colour Rendering Index (CRI) has become the principal gridlock for the extensive commercialization of phosphor converted white LEDs (pc-WLEDs). Fusion of red, green and blue emitting rare-earth (RE) ions in a suitable host can overcome these drawbacks but the energy migration between multiple RE ions at single excitation wavelength defines the key standpoint in designing such white light emitting phosphors. Apart from the abovementioned obstacles, recently traditional optical temperature sensors based on RE ions have faced difficulties due to their low relative sensitivity and large detection error. Keeping these points in mind, in this work, a series of MgAl2O4:Dy3+,Eu3+ nanophosphors are synthesized among which 2% Dy3+,0.2% Eu3+ doped MgAl2O4 nanophosphors demonstrate strong white emission with CIE co-ordinates of (0.31, 0.33), and high quantum yield (∼67%), which could be directly utilized for pc-WLED based solid state lighting devices. Detailed investigation of PL properties reveals that Eu3+ ions can be well sensitized by Dy3+ under near-ultraviolet excitation of 351 nm. Dexter's theory & Reisfeld's approximation are employed for an in-depth analysis of the inter-RE energy transfer (ET) mechanism, which signposts that the dipole-quadrupole interaction phenomenon is responsible for the ET process from Dy3+ to Eu3+. Additionally, the validated ET plays a pivotal role in demonstrating the self-referencing ratiometric temperature sensor behaviour supported by a distinct high temperature thermal quenching trend between Dy3+ and Eu3+ ions. Hence the obtained nanophosphors are highly promising for utilizing in WLED based solid state lighting and self-referencing ratiometric temperature sensor applications.

Facile synthesis of SnO2-PbS nanocomposites with controlled structure for applications in photocatalysis.

Recent studies have shown that SnO2-based nanocomposites offer excellent electrical, optical, and electrochemical properties. In this article, we present the facile and cost-effective fabrication, characterization and testing of a new SnO2-PbS nanocomposite photocatalyst designed to overcome low photocatalytic efficiency brought about by electron-hole recombination and narrow photoresponse range. The structure is fully elucidated by X-ray diffraction (XRD)/Reitveld refinement, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) surface area analysis, and transmission electron microscopy (TEM). Energy-dispersive X-ray spectroscopy (EDX) spectrum imaging analysis demonstrates the intermixing of SnO2 and PbS to form nanocomposites. A charge separation mechanism is presented that explains how the two semiconductors in junction function synergistically. The efficacy of this new nanocomposite material in the photocatalytic degradation of the toxic dye Rhodamine B under simulated solar irradiation is demonstrated. An apparent quantum yield of 0.217 mol min(-1) W(-1) is calculated with data revealing good catalyst recyclability and that charge separation in SnO2-PbS leads to significantly enhanced photocatalytic activity in comparison to either SnO2 or PbS.