Hydrostatic pressure-induced reversible phase transformation in iron oxide nanoparticles.

We report hydrostatic pressure-induced reversible phase transformation in maghemite γ-Fe2O3 nanoparticles (cubic → tetragonal → cubic) using an in situ diamond anvil cell (DAC) technique. Thermal arc plasma-synthesized nanoparticles, particularly in a He gas medium, exhibit the reversible phase transformation under pressure ranging from 0 to 2.58 GPa. Rietveld refinement reflects that cubic to tetragonal maghemite phase transformation coexists with a cubic metallic Fe phase at 0.55 GPa pressure. The generation of two new superlattice reflections at 6.93° and 8.11°, respectively, reflects the phase transformation. The presence of a core-shell-type nanostructure observed from transmission electron microscopy micrographs is found to exhibit a spin glass shell-type behavior. This triggers pressure-induced fluctuating magnetization and interparticle interaction-induced surface anisotropy and spin disorder with broken bonds, translational symmetry, and incomplete coordination. This leads to overcoming the nucleation barrier at the surface, subsequently denser nucleation sites and increased nucleation probability. This further leads to an atomic rearrangement and tetragonality in the maghemite phase. Furthermore, with increasing pressure, the reversible structural change, i.e. from the tetragonal to cubic maghemite phase, has been explained in the light of the "internal stress model". The grains are again forced back to the cubic phase via generation of uniform compression along the c-axis and tension along the a and b axes. The spin glass behavior of the core-shell nanostructure along with the "internal stress model" explain the whole reversible phase transformation phenomenon in the γ-Fe2O3 phase.

Silver Decoration of Vertically Aligned MoS2-MoOx Nanosheets: A Comprehensive XPS Investigation.

This study investigates the simultaneous decoration of vertically aligned molybdenum disulfide nanostructure (VA-MoS2) with Ag nanoparticles (NPs) and nitrogen functionalization. Nitrogen functionalization was achieved through physical vapor deposition (PVD) DC-magnetron sputtering using nitrogen as a reactive gas, aiming to induce p-type behavior in MoS2. The utilization of reactive sputtering resulted in the growth of three-dimensional silver structures on the surface of MoS2, promoting the formation of silver nanoparticles. A comprehensive characterization was conducted to assess surface modifications and analyze chemical and structural changes. X-ray photoelectron spectroscopy (XPS) showed the presence of silver on the MoS2 surface. Scanning electron microscopy (SEM) confirmed successful decoration with silver nanoparticles, showing that deposition time affects the size and distribution of the silver on the MoS2 surface.

Impact of Synthesis Parameters upon the Electronic Structure in PVD-Deposited CdxZn1-xO Composite Thin Films: An XPS-XANES Investigation.

The impact of different synthesis parameters, such as thickness, postsynthesis annealing temperature, and oxygen gas flow rate, upon the electronic structure is discussed in detail in the present experimental investigation. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) spectroscopy techniques are used to evaluate the surface electronic properties along with the presence and stability of the CdO2 surface oxide in CdxZn1-xO (x = 0.4) composite thin films. The thin films were synthesized with varying thicknesses using a Cd0.4Zn0.6O (CZO) ceramic and Cd0.4Zn0.6 (CZ) metallic targets and oxygen gas flow rates during magnetron sputtering. The Zn L3,2 edge and O K edge XANES spectra are affected by the oxygen gas flow rate. For the zero rate, an increase in intensity is observed in the Zn L3,2 edge, and notable changes occur in the overall spectral features of the O K edge. In the films synthesized in the presence of oxygen, highly probable O 2p → antibonding Zn 3d electronic transitions decrease the probability of the Zn 2p1/2 → antibonding Zn 3d electronic transition by filling the vacant antibonding Zn 3d states, leading to the reduction in overall intensity in the Zn L3,2 edge. Scanning electron microscopy reveals grain growth with increasing annealing temperature. The annealing induces orbital hybridization, generating new electronic states with higher transition probabilities and intensity enhancement in both Zn L3,2 and O K edges. The presence of the CdO2 surface phase is confirmed by analyzing the Cd 3d5/2 and O 1s XPS core levels. The CdO2 surface phase is observed in the films synthesized using the CZO target for all thicknesses, while the CZ target is only observed for higher thicknesses. Further postsynthesis annealing treatment results in the disappearance of the CdO2 phase. The CdO2 surface phase can be controlled by varying the film thickness and postsynthesis annealing temperature.

Antiviral Properties against SARS-CoV-2 of Nanostructured ZnO Obtained by Green Combustion Synthesis and Coated in Waterborne Acrylic Coatings.

The COVID-19 pandemic has increased the need for developing disinfectant surfaces as well as reducing the spread of infections on contaminated surfaces and the contamination risk from the fomite route. The present work reports on the antiviral activity of coatings containing ZnO particles obtained by two simple synthesis routes using Aloe vera (ZnO-aloe) or cassava starch (ZnO-starch) as reaction fuel. After detailed characterization using XRD and NEXAFS, the obtained ZnO particles were dispersed in a proportion of 10% with two different waterborne acrylic coatings (binder and commercial white paint) and brushed on the surface of polycarbonates (PC). The cured ZnO/coatings were characterized by scanning electron microscopes (SEM) and energy-dispersive X-ray spectroscopy (EDS). Wettability tests were performed. The virucidal activity of the ZnO particles dispersed in the waterborne acrylic coating was compared to a reference control sample (PC plates). According to RT-PCR results, the ZnO-aloe/coating displays the highest outcome for antiviral activity against SARS-CoV-2 using the acrylic binder, inactivating >99% of the virus after 24 h of contact relative to reference control.

Spin and valence variation in cobalt doped barium strontium titanate ceramics.

In the present decade, owing to half-metallic ferromagnetism, controlled 3d transition metal-doping based defect engineering in oxide perovskites attracts considerable attention in the pursuit of spintronics. We aim to investigate the electronic structure of Co-doped barium strontium titanate (Ba0.8Sr0.2CoxTi1-xO3 where x = 0, 0.1, 0.2) solid solution. Structural, vibrational and microscopic properties indicate the cationic substitution of Co at the octahedral Ti position along with a displacive kind of tetragonal-to-cubic phase transformation. X-ray photoelectron spectroscopy evidences the reduction in the valence state from Co3+ to Co2+ and Ti K edge X-ray absorption spectroscopy endorses the higher lattice symmetry with increasing Co doping. Orbital hybridization triggered electron hopping between O 2p and Co eg orbitals results in a spin fluctuation from the occupation t62ge0g for x = 0.1 to the occupation t62ge1gL for x = 0.20 (L designates a hole in the O 2p shell) aligned state observed from density functional theory calculations. The dominating crystal field energy as compared to intra-atomic exchange (Hund) energy decides the spin-orbital degeneracy for the Co 3d orbital to induce spin fluctuations.

Biophysical and Computational Approaches to Unravel pH-Dependent Conformational Change of PspA Assist PspA-PspF Complex Formation in Yersinia enterocolitica.

In enteropathogen, Yersinia enterocolitica, the genes encoding phage shock proteins are organized in an operon (pspA-E), which is activated at the various types of cellular stress (i.e., extracytoplasmic or envelop stress) whereas, PspA negatively regulates PspF, a transcriptional activator of pspA-E and pspG, and is also involved in other cellular machinery maintenance processes. The exact mechanism of association and dissociation of PspA and PspF during the stress response is not entirely clear. In this concern, we address conformational change of PspA in different pH conditions using various in-silico and biophysical methods. At the near-neutral pH, CD and FTIR measurements reveal a ß-like conformational change of PspA; however, AFM measurement indicates the lower oligomeric form at the above-mentioned pH. Additionally, the results of the MD simulation also support the conformational changes which indicate salt-bridge strength takes an intermediate position compared to other pHs. Furthermore, the bio-layer interferometry study confirms the stable complex formation that takes place between PspA and PspF at the near-neutral pH. It, thus, appears that PspA conformational change in adverse pH conditions abandons PspF from having a stable complex with it, and thus, the latter can act as a trans-activator. Taken together, it seems that PspA alone can transduce adverse signals by changing its conformation.

Synthesis and Characterization of Ag/ZnO Nanoparticles for Bacteria Disinfection in Water.

In this study, two green synthesis routes were used for the synthesis of Ag/ZnO nanoparticles, using cassava starch as a simple and low-cost effective fuel and Aloe vera as a reducing and stabilizing agent. The Ag/ZnO nanoparticles were characterized and used for bacterial disinfection of lake water contaminated with Escherichia coli (E. coli). Characterization indicated the formation of a face-centered cubic structure of metallic silver nanoparticles with no insertion of Ag into the ZnO hexagonal wurtzite structure. Physicochemical and bacteriological analyses described in "Standard Methods for the Examination of Water and Wastewater" were used to evaluate the efficiency of the treatment. In comparison to pure ZnO, the synthesized Ag/ZnO nanoparticles showed high efficiencies against Escherichia coli (E. coli) and general coliforms present in the lake water. These pathogens were absent after treatment using Ag/ZnO nanoparticles. The results indicate that Ag/ZnO nanoparticles synthesized via green chemistry are a promising candidate for the treatment of wastewaters contaminated by bacteria, due to their facile preparation, low-cost synthesis, and disinfection efficiency.

Morphological, electronic, and magnetic properties of multicomponent cobalt oxide nanoparticles synthesized by high temperature arc plasma.

Many technological applications demand large amount of nanoparticles with well-defined properties, which is feasible only by using large-scale production methods. In this framework, we have performed structural and local geometric investigations of cobalt oxide nanoparticles synthesized by high temperature arc plasma route in helium and in air atmosphere with different arc currents, a competitive and low cost technological approach to synthesize large quantity of different types of nanoparticles. The complex scenario of phase fraction, shape, size distribution and hysteresis loop features of high temperature arc plasma synthesis of nanoparticles can be determined by the arc current and the selected gas. X-ray diffraction patterns reveal a multicomponent phase formation containing cubic cobaltous oxide (CoO), cobaltic oxide (Co3O4) and metallic cobalt phases. The synthesis of different phases is confirmed by x-ray absorption spectroscopy measurements at the CoK-edge. Both extended x-ray absorption fine structure and x-ray absorption near edge structure analyses show the presence of metallic nanoparticles in He ambient at high arc current. Moreover, high-resolution transmission electron microscopy images and magnetic hysteresis loop measurements show that the mean particle size increases and the coercivity decreases with increasing arc current in air ambient due to the intense particle-particle interaction. At variance, in He ambient synthesized samples due to the high quenching rate and the high thermal conductivity, a multi-domain formation in which the nanoparticles' crystalline fraction decreases and a fluctuating coercivity due to core-shell structure is observed.

Large scale synthesis of copper nickel alloy nanoparticles with reduced compressibility using arc thermal plasma process.

Among the various methods employed in the synthesis of nanostructures, those involving high operating temperature and sharp thermal gradients often lead to the establishment of new exotic properties. Herein, we report on the formation of Cu-Ni metallic alloy nanoparticles with greatly enhanced stiffness achieved through direct-current transferred arc-thermal plasma assisted vapour-phase condensation. High pressure synchrotron X-ray powder diffraction (XRPD) at ambient temperature as well as XRPD in the temperature range 180 to 920 K, show that the thermal arc-plasma route resulted in alloy nanoparticles with much enhanced bulk modulus compared to their bulk counterparts. Such a behaviour may find an explanation in the sudden quenching assisted by the retention of a large amount of local strain due to alloying, combined with the perfect miscibility of the elemental components during the thermal plasma synthesis process.

Orbital hybridization-induced band offset phenomena in NixCd1-xO thin films.

Herein, we present the cationic impurity-assisted band offset phenomena in NixCd1-xO (x = 0, 0.02, 0.05, 0.1, 0.2, 0.4, 0.8, and 1) thin films and further discuss them based on orbital hybridization modification. The compositional and structural studies revealed that the cationic substitution of Cd2+ by Ni2+ ions leads to a monotonic shift in the (220) diffraction peak, indicating the suppression of lattice distortion, while the evolution of local strain with an increase in Ni concentration is mainly associated with the mismatch in the electronegativity of the Cd2+ and Ni2+ ions. In fact, Fermi level pinning towards the conduction band minimum takes place with an increase in the Ni concentration at the cost of electronically compensated oxygen vacancies, resulting in the modification of the distribution of carrier concentration, which eventually affects the band edge effective mass of the conduction band electrons and further endorses band gap renormalization. Besides, the appearance of a longitudinal optical (LO) mode at 477 cm-1, as manifested by Raman spectroscopy, also indicates the active involvement of electron-phonon scattering, whereas modification in the local coordination environment, particularly anti-crossing interaction in conjunction with the presence of satellite features and shake-up states with Ni doping, was confirmed by X-ray absorption near-edge and X-ray photoelectron spectroscopy studies. These results manifest the gradual reduction of orbital hybridization upon the incorporation of Ni, leading to a decrement in the band edge effective electron mass. Finally, the molecular dynamics simulation reflected a 13% reduction in the lattice parameter for the NiO thin film compared to the undoped film, while the projected density of states calculation further supports the experimental observation of reduced orbital hybridization with an increase in Ni concentration.

High temperature-mediated rocksalt to wurtzite phase transformation in cadmium oxide nanosheets and its theoretical evidence.

Herein, a high temperature-induced phase transformation (PT) in chemically grown CdO thin films is demonstrated, and its corresponding electronic origin further investigated by density functional theory. In particular, the cubic rocksalt to hexagonal wurtzite PT in the CdO thin film annealed at 900 °C was confirmed by X-ray diffraction (XRD), which was consistent with the high-resolution transmission electron microscopy (TEM) results. Moreover, atomic force microscopy and scanning electron microscopy clearly evidenced the morphological evolution via the formation of a nanosheet network in the wurtzite-phase CdO film. The high temperature treatment also led to a significant enhancement in the optical band gap from 2.2 to 3.2 eV, as manifested by UV-visible spectroscopy. The enhanced surface roughness of the nanosheet caused a deviation in the net dipole moment, which may break the polarizable bonds and help in reducing the average dielectric constant, resulting in a band gap opening for the transformed phase. Furthermore, X-ray absorption spectroscopy at the oxygen k-edge revealed a notable shift in the inflection point of the absorption edge, while the X-ray photoelectron spectroscopy (XPS) Cd 3d and O 1s spectra suggested a gradual reduction in the CdO2 phase with an increase in annealing temperature. In addition, different complementary techniques including Rutherford backscattering and Raman spectroscopy were exploited to understand the aforementioned PT and its structural correlation. Finally, molecular dynamics simulation together with density functional theory calculation suggested that the symmetry modification at the Brillouin zone boundary provides a succinct signature for the PT in the CdO thin film.

Correlations of charge neutrality level with electronic structure and p-d hybridization.

The formation of charge neutrality level (CNL) in highly conducting Cadmium oxide (CdO) thin films is demonstarted by the observed variation in the band gap upon annealing and doping. It may be explained by the observation that Tin (Sn) doping breaks the perfect periodicity of CdO cubic crystal structure and creates virtual gap states (ViGS). The level of local CNL resides at the branch point of ViGS, making the energy at which native defect's character changes from predominantly donor-like below CNL to predominantly acceptor-like above the CNL and a schematic band diagram is developed to substantiate the same. Further investigations using soft x-ray absorption spectroscopy (SXAS) at Oxygen and Cadmium edges show the reduction of Sn4+ to Sn2+. The analysis of the spectral features has revealed an evidence of p-d interaction between O 2p and Cd 4d orbitals that pushes the valence band minima at higher energies which is symmetry forbidden at г point and causing a positive valance band dispersion away from the zone centre in the г ~ L, K direction. Thus, origin of the CNL is attributed to the high density of the Oxygen vacancies as confirmed by the change in the local electronic structure and p-d hybridization of orbitals.

Electronic structure modification and Fermi level shifting in niobium-doped anatase titanium dioxide thin films: a comparative study of NEXAFS, work function and stiffening of phonons.

The electronic structure and tuning of work function (WF) by electronic excitations (EEs) induced by swift heavy ions (SHIs) in anatase niobium-doped titanium dioxide (NTO) thin films is reported. The densities of EEs were varied using 80 MeV O, 130 MeV Ni and 120 MeV Ag ions for irradiation. The EE-induced modifications in electronic structure were studied by O K-edge and Ti L3,2 edge absorption spectra using near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The reduction of hybridized O 2p and Ti 3d unoccupied states in the conduction band with a decrease in energy of the crystal field strength of ∼ 480 meV and the correlated effect on the decrease in the WF value of ∼ 520 meV upon increasing the total energy deposition in the lattice are evident from the study of NEXAFS and scanning Kelvin probe microscopy (SKPM), respectively. The observed stiffening in the low frequency Raman mode (LFRM) of ∼ 9 cm(-1) further validates the electronic structure modification under the influence of EE-induced strain in TiO6 octahedra. The reduction of hybridized valence states, stiffening behavior of LFRM and decrease in WF by nano-crystallization followed by amorphization and defects in NTO lattice are explained in terms of continuous, discontinuous amorphous ion tracks containing intestinally created defects and non-stoichiometry in the lattice. These studies are very appropriate for better insights of electronic structure modification during phase transformation and controlled Fermi level shifting, which plays a crucial role in controlling the charge carrier injection efficiency in opto-electronic applications and also provides a deeper understanding of the involved physical processes.