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Li ionic conduction is decoupled from polymer segmental relaxations in PEO-ZIF QSSEs. Li ion conductivity is enhanced by tuning the pore architecture of the fillers.

The low ionic conductivity and electrode-electrolyte interface instability issues with solid polymer electrolytes jeopardize their electrochemical performances in lithium-ion batteries (LIBs). The use of quasi-solid-state electrolytes (QSSEs) with concentrated Li salt embedded inside the pore networks of metal organic frameworks (MOFs) can successfully address the aforementioned issues. Owing to the sieve effect of zeolitic imidazolate framework-8 (ZIF-8) towards selective cation permeability over anions through the interconnected pore network, a unique QSSE with LiTFSI salt concentrated in the ZIF-8 skeleton used as a filler in poly(ethylene oxide) has been synthesized. LiTFSI gets embedded inside the interconnected pore network of ZIF-8 that furnishes unhindered pathways for Li

The limited ionic conductivity of polymer electrolytes is a major issue for their industrial application. Enhancement of ionic conductivity in the poly(ethylene oxide), PEO, based electrolyte has been achieved by loading passive nanofillers such as SiO2 nanoparticles (NPs). To investigate the role of modifications in free volume characteristics and the polymer chain dynamics induced by the loading of passive fillers on the ionic conductivity of the PEO based ternary electrolyte, a systematic investigation has been carried out using positron annihilation and broadband dielectric spectroscopy. As a result of interfacial interactions, the loading of SiO2 NPs alters the semi-crystalline morphology of PEO resulting in a higher crystallinity at lower loadings due to the surface confinement of PEO chains, and the formation of smaller PEO crystallites at higher loadings due to interparticle nanoconfinement. These modifications are accompanied by a decrease in free volume fraction at the lowest loading (0.5 wt%) followed by an increase at higher loadings (≥2.0 wt%). The Almond–West formalism considering two different universalities in different temperature and frequency ranges has been used to explain the ion-conduction process at different NP loadings. The Li ion conductivity is observed to be maximum for a 5.0 wt% loading of SiO2 NPs. The enhancement in ionic conductivity is observed to be directly correlated with the free volume characteristics and segmental dynamics of the PEO matrix, confirming their role in ion transport in polymer electrolytes.

Structure, size, and defect manipulations have been found to be highly effective strategies in tuning the properties of optical functional materials. To investigate the same, nanocrystalline and bulk counterparts of Eu3+-doped scheelite BaWxMo1–xO4 solid solutions were synthesized and their optical properties were investigated. Positron annihilation lifetimes were found to be larger in the as-prepared nanocrystalline samples compared to the bulk, suggesting more defects in the nanocrystalline samples. The positron lifetimes increased on Eu3+ doping, suggesting the creation of more defects, with possibilities being W/Mo occupying Ba2+ sites or the co-inclusion of Na+ for charge compensation. The inclusion of Mo was found to inhibit host-to-dopant (Eu3+) energy transfer (HDET) in bulk samples, but very efficient HDET was seen in nanocrystalline samples. A higher degree of asymmetry around the Eu3+ in the nanocrystalline samples compared to the bulk samples was observed, attributed to the large defect density as well as the relaxations around the defects in the former, and this is also manifested as a lower unit cell volume. It was found that doped nanocrystalline scheelites of BaWxMo1–xO4 can be used as red phosphor, while the bulk sample can be used as a color-tunable phosphor in red, orange, yellow, and near white luminescence applications by varying the composition. This work really projects the importance of defect and doping manipulation on the optical properties of undoped and doped, pure and mixed tungstate–molybdate systems.

Poly(ethylene oxide), PEO, based nanocomposites are shown to have higher ionic conductivity compared to pure PEO. In order to investigate the exclusive role of free volume structure of amorphous region of PEO on its electrical conductivity, PEO-silica nanoparticles composite films have been prepared with different silica loadings. The free volume fraction of polymer composites measured using positron annihilation lifetime spectroscopy (PALS) was observed to increase nonlinearly with the increase in loading of silica. The increase in free volume on silica loading has been attributed to PEO-silica interphase as well as newly created amorphous PEO. At higher loading, positron annihilation from regions near silica particles has been confirmed by coincidence Doppler broadening measurements. Thermal analysis of polymer nanocomposites shows a decrease in crystallinity and crystallite melting temperature with the increase in silica loading. The PEO-silica nanocomposite films showed enhanced ac conductivity as well as mechanical properties (higher Young’s modulus and ductility). The observed changes in free volume structure and semicrystalline morphology of PEO matrix are attributed to the physical confinement imposed by presence of silica nanoparticles in PEO matrix. The present study shows that free volume measured using PALS has a direct relation with ac conductivity which is observed to increase drastically after a critical free volume fraction.

Doping of ceria with trivalent lanthanides and pentavalent ions is known to influence its catalytic activity, optical properties and ionic conductivity. In this study, (Nb,Eu)-codoped CeO2 has been synthesized by hydroxide precipitation to study the synergistic effect of (Nb, Eu) codoping on the defect evolution and optical properties of CeO2. Through X-ray diffraction, it was seen that Nb5+ has limited solubility in CeO2 which could be enhanced by codoping with Eu3+. From Rietveld refinement of the X-ray diffraction data, the ceria lattice seems to expand on Eu3+ doping which could be explained taking into account the formation of oxygen vacancies (OVs) in stoichiometric ratios. The observed lower than expected contraction in the Nb5+ doped samples is attributed to lattice expansion due to the creation of oxygen interstitials (OIs). PL studies in terms of the ratio of electric dipole transition (EDT) to magnetic dipole transition (MDT) intensities have shown that Eu3+ lacks inversion symmetry at Ce4+ which invokes the formation of OVs for charge compensation. Interestingly, Nb5+ co-doping restored the inversion symmetry around the Eu3+ site by eliminating the formed OVs. This is achieved by ion pair matching of the dopants. This work has great relevance in understanding the role of defects and co-doping in designing highly efficient catalysts, solid oxide fuel cells and doped phosphors.

Zeolitic imidazolate framework-8 (ZIF-8) based membranes show poor selectivity for gas separation due to flexibility of the frameworks. Mixed linker frameworks (ZIF-7x-8) membranes are shown to have higher separation selectivity and low permeance of gases. In order to investigate the role of pores' interconnectivity and surface accessibility towards gases’ permeance and selectivity, variable energy positron spectroscopy (VEPS) measurements have been carried out for ZIF-7x-8 (x = 0, 20.6, 52.6, 79.4, 90.4, 97.2 and 100%) membranes. These membranes were deposited using fast current driven synthesis method to produce a stiffened phase (Cm) of ZIF-8. The frameworks have been characterized using XRD, FTIR, Raman spectroscopy and FE-SEM for their crystalline structure, chemical bonding and morphology. Depth dependent Doppler broadening measurements using VEPS show distinct variations in S-E and 3γ/2γ-E profiles of the membranes. The positronium diffusion length determined by fitting of S-E profiles considering a diffusion model shows a drastic reduction in pore interconnectivity and their surface accessibility for x = 20–97%. These modifications explain the reduction in gas permeance of ZIF-7x-8 based membranes. The present study confirms that membranes having x∼20% shows higher selectivity due to their ZIF-8_Cm crystal structure, reasonable pore interconnectivity and limited non-selective grain boundary structure diffusion.

Zeolitic Imidazolate Frameworks (ZIFs) show great promise in molecular separation as well as recognition owing to their crystalline morphology consisting of a well designed pore architecture. However, the sieving performance of ZIFs remains limited due to their fixed pore sizes and framework flexibility. In the present study, fast current driven synthesis (FCDS) has been used to prepare the stiffened phase of mixed ligands (2-methylimidazole and benzimidazole) based ZIFs having tunable pore sizes and morphologies. X-ray diffraction has been used to determine the crystal structure of the frameworks, which were observed to be crystalline through a wide range of ligand ratio (benzimidazole fraction: ∼0.206–0.972). The random distribution of both the ligands in the frameworks has been established through complementary methods viz. Raman spectroscopy, Fourier transform infrared and scanning electron microscopy. Morphology of the frameworks was observed to vary from micrometer size individual crystals to spherical aggregates of inter-grown nanocrystals to individual nanocrystals. Pore sizes determined using positronium lifetime measurements were observed to be consistent with the literature for pure phase of ZIF-7 and stiffened ZIF-8. The pore sizes were observed to vary with the benzimidazole ligand fraction confirming that mixed ligand strategy can be used to efficiently tune the pore sizes of the frameworks. Positronium diffusion investigation confirms that pore interconnectivity is inferior as compared to stiffened ZIF-8 but superior than ZIF-7 in these mixed ligand frameworks.

ZIF-8 nanoporous films of variable thicknesses, which are proposed to be very low-k dielectric films for chip industries, have been deposited on Si substrate using simple solution chemistry method. Contrary to the earlier reports, ZIF-8 crystals number density on the substrate in early deposition cycles increases leading to higher coverage of the substrate. The growth of crystal size or film thickness occurs in later deposition cycles. The deposition of ZIF-8 crystals is highly random, however a preferential growth in 002 orientation is observed. Synchrotron based X-ray photoelectron spectroscopy has confirmed Zn enrichment at surface of the films compared to imidazole linkers. Depth dependent Doppler broadening spectroscopy measurements utilizing the positronium diffusion process have unequivocally confirmed that the pores in ZIF-8 films are interconnected at micrometer scale leading to their accessibility from the outer surface. This type of pore architecture along with the surface enrichment might be responsible for adsorbate selectivity and very high adsorption capacity of these films as reported in literature. On thermal annealing, the pore architecture of these films begins to collapse at a much lower temperature (~373 K) than the decomposition temperature (~700 K) of ZIF-8 powder.