Shambhu Nath Jha

One-pot thioetherification of aryl halides with thiourea and benzyl bromide in water catalyzed by Cu-grafted furfural imine-functionalized mesoporous SBA-15

Surface functionalization of SBA-15 followed by its reaction with Cu(OAc)(2) has been carried out to develop a new Cu-grafted functionalized mesoporous material, which catalyzes one-pot three component coupling of different aryl halides with thiourea and benzyl bromide in aqueous medium to produce aryl thioethers in very good yields (80-88%).

Structural, optical and magnetic properties of sol–gel derived ZnO:Co diluted magnetic semiconductor nanocrystals: an EXAFS study

Structural, local structural, optical and magnetic properties of sol–gel derived Zn1−xCoxO (0 ≤ x ≤ 0.04) nanoparticles have been studied. The crystallite structure, size, and lattice strain have been estimated by X-ray diffraction (XRD) with Rietveld refinement and high-resolution transmission electron microscopy (HRTEM). The small linear increase in lattice parameter ‘a’ and decrease in lattice parameter ‘c’ have been observed which can be attributed to the small distortion of Zn tetrahedron. Extended X-ray Absorption Fine Structure (EXAFS) measurements show that Co-doping creates oxygen vacancies without causing any significant change in the host lattice structure. X-ray Absorption Near Edge Structure (XANES) measurements rule out the presence of metallic Co clusters in the samples. Raman spectroscopy has been employed to study the crystalline quality, structural disorder, and defects in the host lattice. The tetrahedral coordination of the oxygen ions surrounding the zinc ions and wurtzite structure has been studied by FTIR analysis. UV-Vis measurements have been used to study the effect of Co-doping on absorption spectra and hence on the band gap. The band gap initially decreases for low Co-concentration and increases with higher Co-concentration. The PL spectra show six peaks out of which the peak in the ultraviolet (UV) region has been assigned to the near band edge excitonic emission (NBE) and other peaks are related to different defect states. Room temperature ferromagnetism (weak) is observed and magnetization increases with increasing Co-concentration. The grain boundaries, oxygen vacancy and bound magnetic polarons (BMPs) jointly may be responsible for this room temperature ferromagnetism. Variation of resistivity with temperature shows that a thermally activated conduction (Arrhenius) mechanism is valid in the high temperature region whereas Motts variable-range hopping (VRH) mechanism is valid in the low temperature region.

Luminescence Properties of SrZrO3/Tb3+ Perovskite: Host-Dopant Energy-Transfer Dynamics and Local Structure of Tb3+

SrZrO3 perovskite (SZP) was synthesized using gel-combustion route and characterized systematically using X-ray diffraction and time-resolved photoluminescence techniques. A detailed analysis of the optical properties of Tb(3+) ions in SrZrO3 was performed to correlate them with the local environment of the lanthanide ions in this perovskite. Photoluminescence (PL) spectroscopy showed that emission spectrum consists of host as well as Tb(3+) emission indicating the absence of complete host-dopant energy transfer. On the basis of emission spectrum and PL decay study it was also observed that Tb(3+) is not homogeneously distributed in SrZrO3 perovskite; rather, it is occupying two different sites. It is corroborated using extended X-ray absorption fine structure studies that Tb(3+) is stabilized on both six-coordinated Zr(4+) and eight-coordinated Sr(2) site. The energies calculated using density functional theory (DFT) indicates that Tb occupation in Sr site is energetically more favorable than Zr site. The analysis of valence charge distribution also substantiated our structural stability analysis of site-selective Tb doping in SrZrO3. Time-resolved emission spectroscopy is employed to elucidate the difference in the spectral feature of Tb(3+) ion at Sr(2+) and Zr(4+) site. DFT-calculated density of states analysis showed that energy mismatch of Tb-d states with Zr-d and O-p states of SZP makes the energy transfer from host SZP to Tb(3+) ion difficult.

Insight into the origin of ferromagnetism in Fe-doped ZnO diluted magnetic semiconductor nanocrystals: an EXFAS study of local structure

In this paper we have studied the structural, local structural and magnetic properties of sol–gel derived Zn1−xFexO (0 ≤ x ≤ 0.06) nanoparticles. The crystalline structure and crystallite size have been estimated by X-ray diffraction with Rietveld refinement and high-resolution transmission electron microscopy (HRTEM). Other structural and local structural properties have been studied by extended X-ray absorption fine structure (EXAFS)-, X-ray absorption near edge structure (XANES)- and Raman-analysis. Weak ferromagnetism is observed at room temperature and magnetization increases with increasing Fe-concentration. The oxygen vacancy assisted bound magnetic polarons (BMPs) and possibly the grain boundaries are responsible for this room temperature ferromagnetism. Variation of resistivity with temperature has also been studied. Appearance of ferromagnetism in ZnO:Fe nanoparticles may open the potential in bio-imaging and drug-delivery applications.

Correlation of structural and magnetic properties of Ni-doped ZnO nanocrystals

In order to gain insight into the magnetic behaviour of Ni-doped ZnO nanocrystals (NCs) the local structure was rigorously studied by extended x-ray absorption fine structure (EXAFS) along with the magnetic measurements. At low doping level of Ni, local structure remains identical to ZnO NCs with subtle increase in the bond length. EXAFS and the optical absorption measurements indicate valence of Ni to be +2. Undoped as well as Ni-doped ZnO NCs with very low Ni concentration are weakly ferromagnetic. Further increase in doping level yields paramagnetic NCs. The present studies clearly prove that ferromagnetic ordering in NCs is primarily a manifestation of point defects and incorporation of transition metal impurities plays a secondary role.

An insight into local environment of lanthanide ions in Sr2SiO4:Ln (Ln = Sm, Eu and Dy)

Sr2SiO4 is an important inorganic host for lanthanide-doped white-light-emitting diodes. In order to probe the local structure and symmetry around lanthanide ions in Sr2SiO4, detailed experimental and theoretical investigations have been carried out. Samples prepared via sol–gel methods were thoroughly characterized using X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) and photoluminescence (PL) spectroscopy. The local symmetries of SrO9 and SrO10 polyhedra were determined using emission spectroscopy with europium as the probe ion. Despite having the same oxidation state and comparable ionic radii, Sm3+, Dy3+ and Eu3+ behave differently in terms of their site occupation at the Sr sites of SrO9 and SrO10 polyhedra. While Eu replaces Sr in both polyhedra, Sm and Dy reside specifically at the 9-coordinated Sr sites only. Based on density functional theory (DFT) calculations, it has been established that for Dy and Sm, the strong metal–oxygen bonding leads to significant distortion and hence the destabilization of MO10 polyhedra in the host.

Nano-structured hybrid molybdenum carbides/nitrides generated in situ for HER applications

A nanohybrid material containing carbon-supported molybdenum carbide and nitride nanoparticles of size ranging from 8 to 12 nm exhibits excellent HER catalytic activity. This molybdenum based catalyst (MoCat) is designed as a highly efficient, low-cost (precious-metal-free), highly stable electrocatalyst for water electrolysis in acidic media, and synthesized using a simple methodology. These nanoparticles (β-Mo2C and γ-Mo2N) were produced in situ using a metal precursor and C/N source via a controlled solid state reaction. An overpotential of 96 mV for driving a current density of 10 mA cm−2 was measured for the MoCat catalyst, which is very close to that of commercially available Pt/C catalysts (61 mV).

Search for Origin of Room Temperature Ferromagnetism Properties in Ni-Doped ZnO Nanostructure

The origin of room temperature (RT) ferromagnetism (FM) in Zn1-xNixO (0< x < 0.125) samples are systematically investigated through physical, optical, and magnetic properties of nanostructure, prepared by simple low-temperature wet chemical method. Reitveld refinement of X-ray diffraction pattern displays an increase in lattice parameters with strain relaxation and contraction in Zn/O occupancy ratio by means of Ni-doping. Similarly, scanning electron microscope demonstrates modification in the morphology from nanorods to nanoflakes with Ni doping, suggests incorporation of Ni ions in ZnO. More interestingly, XANES (X-ray absorption near edge spectroscopy) measurements confirm that Ni is being incorporated in ZnO as Ni2+. EXAFS (extended X-ray absorption fine structure) analysis reveals that structural disorders near the Zn sites in the ZnO samples upsurges with increasing Ni concentration. Raman spectroscopy exhibits additional defect driven vibrational mode (at 275 cm-1), appeared only in Ni-doped samples and the shift with broadening in 580 cm-1 peak, which manifests the presence of the oxygen vacancy (VO) related defects. Moreover, in photoluminescence (PL) spectra, we have observed a peak at 524 nm, indicating the presence of singly ionized VO+, which may be activating bound magnetic polarons (BMPs) in dilute magnetic semiconductors (DMSs). Magnetization measurements indicate weak ferromagnetism at RT, which rises with increasing Ni concentration. It is therefore proposed that the effect of the Ni ions as well as the inherent exchange interactions arising from VO+ assist to produce BMPs, which are accountable for the RT-FM in Zn1-xNixO (0< x < 0.125) system.

Origin of Blue-Green Emission in α-Zn2P2O7 and Local Structure of Ln3+ Ion in α-Zn2P2O7:Ln3+ (Ln = Sm, Eu): Time-Resolved Photoluminescence, EXAFS, and DFT Measurements

Considering the fact that pyrophosphate-based hosts are in high demand for making highly efficient luminescence materials, we doped two visible lanthanide ions, viz. Sm3+ and Eu3+, in Zn2P2O7. Interestingly, it was oberved that pure Zn2P2O7 displayed blue-green dual emission on irradiation with ultraviolet light. Emission and lifetime spectroscopy shows the presence of defects in pyrophosphate samples which are responsible for such emission. DFT calculations clearly pinpointed that the electronic transitions between defect states located at just below the conduction band minimum (arises due to VO1+ and VO2+ defects) and valence band maximum, as well as impurity states situated in the band gap, can lead to dual emission in the blue-green region, as is also indicated by emission and lifetime spectra. X-ray absorption near edge spectroscopy (XANES) shows the stabilization of europium as well as samarium ion in the +3 oxidation state in α-Zn2P2O7. The fact that α-Zn2P2O7 has two different coordination numbers for zinc ions, i.e. five- and six-coordinate, the study of dopant ion distribution in this particular matrix will be an important step in realizing a highly efficient europium- and samarium-based red-emitting phosphor. Time resolved photoluminescence (TRPL) shows that both of these ions are heterogeneously distributed between five- and six-coordinated Zn2+ sites and it is the six-coordinated Zn2+ site which is the most favorable for lanthanide ion doping. Extended X-ray absorption fine structure (EXAFS) measurements also suggested that a six-coordinated zinc ion is the preferred site occupied by trivalent lanthanide ions, which is in complete agreement with TRPL results. It was observed that there is almost complete transfer of photon energy from Zn2P2O7 to Eu3+, whereas this transfer is inefficient and almost incomplete in case of Sm3+, which is indeed important information for the realization of pyrophosphate-based tunable phosphors.

Structural and optical properties of sol–gel derived Cr-doped ZnO diluted magnetic semiconductor nanocrystals: an EXAFS study to relate the local structure

Structural, local structural and optical properties of sol–gel derived Zn1−xCrxO (0 ≤ x ≤ 0.06) nanoparticles have been thoroughly studied by several complementary techniques. The crystallite structure, size, and lattice strain have been estimated by X-ray diffraction (XRD) with Rietveld refinement and high-resolution transmission electron microscopy (HRTEM). No significant change in lattice parameters a and c has been observed upon Cr doping, though crystallite size and tensile strain in the crystals change. Extended X-ray absorption fine structure (EXAFS) measurement shows that Cr doping creates oxygen vacancies without causing any significant change in the host lattice structure. X-ray absorption near edge structure (XANES) measurements rule out the presence of metallic Cr clusters in the samples. Raman spectroscopy has been employed to study the crystalline quality, structural disorder, and defects in the host lattice. XANES and FTIR results show that the local structure around Cr becomes increasingly octahedral with an increase in Cr doping concentration. UV-vis measurements have been used to study the effect of Cr-doping on absorption spectra and hence on the band gaps of the samples. The band gap initially increases for low Cr-concentration and then decreases with higher Cr-concentration. The PL spectra show many emissions including green emission, which is attributed to singly ionized oxygen vacancies, increases with an increase in Cr doping concentration in the samples.