Silver nanoparticles: Large scale solvothermal synthesis and optical properties

Silver nanoparticles have been successfully synthesized by a simple and modified solvothermal method at large scale using ethanol as the refluxing solvent and NaBH₄ as reducing agent. The nanopowder was investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS), UV-visible and BET surface area studies. XRD studies reveal the monophasic nature of these highly crystalline silver nanoparticles. Transmission electron microscopic studies show the monodisperse and highly uniform nanoparticles of silver of the particle size of 5 nm, however, the size is found to be 7 nm using dynamic light scattering which is in good agreement with the TEM and X-ray line broadening studies. The surface area was found to be 34.5 m²/g. UV-visible studies show the absorption band at ∼425 nm due to surface plasmon resonance. The percentage yield of silver nanoparticles was found to be as high as 98.5%.     

Hydroxy propyl cellulose capped silver nanoparticles produced by simple dialysis process

Silver (Ag) nanoparticles (∼6 nm) were synthesized using a novel dialysis process. Silver nitrate was used as a starting precursor, ethylene glycol as solvent and hydroxy propyl cellulose (HPC) introduced as a capping agent. Different batches of reaction mixtures were prepared with different concentrations of silver nitrate (AgNO₃). After the reduction and aging, these solutions were subjected to ultra-violet visible spectroscopy (UVS). Optimized solution, containing 250 mg AgNO₃ revealed strong plasmon resonance peak at ∼410 nm in the spectrum indicating good colloidal state of Ag nanoparticles in the diluted solution. The optimized solution was subjected to dialysis process to remove any unreacted solvent. UVS of the optimized solution after dialysis showed the plasmon resonance peak shifting to ∼440 nm indicating the reduction of Ag ions into zero-valent Ag. This solution was dried at 80 °C and the resultant HPC capped Ag (HPC/Ag) nanoparticles were studied using transmission electron microscopy (TEM) for their particle size and morphology. The particle size distribution (PSD) analysis of these nanoparticles showed skewed distribution plot with particle size ranging from 3 to 18 nm. The nanoparticles were characterized for phase composition using X-ray diffractrometry (XRD) and Fourier transform infrared spectroscopy (FT-IR).     

Synthesis of silver nanoparticles in a polyvinylpyrrolidone (PVP) paste, and their optical properties in a film and in ethylene glycol

Silver nanoparticles were synthesized in a paste of polyvinylpyrrolidone formed after mixing PVP with acetone and a small volume of aqueous silver nitrate under magnetic stirring. A film made with the material was characterized by UV-vis spectroscopy. The obtained spectrum shows a single peak at 438 nm, arising from the surface plasmon absorption of silver colloids. This result clearly indicates that silver nanoparticles are embedded in PVP. When the pre-treated PVP-Ag colloid is dissolved in ethylene glycol, the UV-vis spectrum of the resulting dispersion shows an absorption peak at 433 nm, whose maximum absorption blue shifts to 416 nm after 18 days of agitation. The silver nanoparticles have an average particle size of 4.12 nm. Because the IR band assigned to the carbonyl group of the PVP shifts to longer wavelengths, the interaction of this polymer with silver nanoparticles seems to take place through the carbonyl oxygen.     

Synthesis and anti-bacterial activity of Cu, Ag and Cu-Ag alloy nanoparticles: A green approach

Metallic and bimetallic nanoparticles of copper and silver in various proportions were prepared by microwave assisted chemical reduction in aqueous medium using the biopolymer, starch as a stabilizing agent. Ascorbic acid was used as the reducing agent. The silver and copper nanoparticles exhibited surface plasmon absorption resonance maxima (SPR) at 416 and 584 nm, respectively; while SPR for the Cu-Ag alloys appeared in between depending on the alloy composition. The SPR maxima for bimetallic nanoparticles changes linearly with increasing copper content in the alloy. Transmission electron micrograph (TEM) showed monodispersed particles in the range of 20 ± 5 nm size. Both silver and copper nanoparticles exhibited emission band at 485 and 645 nm, respectively. The starch-stabilized nanoparticles exhibited interesting antibacterial activity with both gram positive and gram negative bacteria at micromolar concentrations.     

A new and effective chemical reduction method for preparation of nanosized silver powder and colloid dispersion

Nanosized uniform silver powders and colloidal dispersions of silver were prepared from AgNO₃ by a chemical reduction method involving the intermediate preparation of Ag₂O colloidal dispersion in the presence of sodium dodecyle sulfate CH₃(CH₂)₁₁OSO₃Na as a surfactant. Several reducing agents such as hydrazine hydrate (N₂H₄·H₂O), formaldehyde (HCOH) and glucose (C₆H₁₀O₅) have been found to be preferable in this study from a practical point of view. The silver powder with the 60-120 nm particle size and colloidal dispersion with the particles size 10-20 nm and 0.5-2.0 wt.% concentration were successfully synthesized.     

Synthesis of uniformly porous NiO/ZrO2 particles

Porous NiO-ZrO₂ particles were successfully synthesized using a spray-drying method with polystyrene latex (PSL: 400 nm) as a template and starting materials that included NiO powder (7 nm) and ZrO₂ sol (1.2 nm). Porous particles with an average diameter of 4.5 μm and nearly spherical, narrow pores with an average size of ∼300 nm were obtained from the precursor at a pH of 3.7. The Brunauer, Emmett and Teller (BET) surface area of the prepared particles was relatively high—about 27 m²/g. When the solution pH was increased to 9.7, the particle morphology became completely spherical, indicating that the morphology of prepared particles can be controlled by adjusting the pH. Calcinations at 900 and 1200 °C were carried out to estimate the thermal stability of the prepared particles, which had shrinkage of less than 36%. The existence of these pores means that various applications, such as electrodes and catalysts, will be possible for the prepared particles.     

Silver nanoparticles: Ultrasonic wave assisted synthesis, optical characterization and surface area studies

Silver nanoparticles have been successfully synthesized by the sonochemical route using sodium borohydride and sodium citrate as the reducing agents. The effect of the reducing agents on the particle size and morphology has been studied by carrying out the two reactions at the same ultrasound frequency (20 KHz). The strong reducing agent (NaBH₄) produced spherical silver nanoparticles of sizes 10 nm whereas sodium citrate led to much smaller silver nanoparticles of ~ 3 nm diameter. Powder X-ray diffraction studies reveal a high degree of crystallinity and monophasic silver particles. UV-Visible studies show the presence of a surface plasmon band at 405 nm. However the reflectance spectra give a broad band between 340 and 360 nm which is characteristic for the quasi-spherical silver nanoparticles. The specific surface area was found to be 2.6 and 13.1 m²/g and the pore radius was found to be 15.2 and 12.3 Å for silver nanoparticles obtained by the sodium borohydride and sodium citrate reduction respectively.     

Size-dependent optical and dielectric properties of nanocrystalline ZnS thin films synthesized via rf-magnetron sputtering technique

Nanocrystalline ZnS thin films have been synthesized by radio frequency magnetron sputtering technique on glass and Si substrates at a substrate temperature 300 K. X-ray diffraction and selected area electron diffraction studies confirmed the formation of nanocrystalline cubic phase of ZnS in the films, although the target material was hexagonal ZnS. The particle size, calculated from the XRD patterns of the thin films was found in the range 2.06-4.86 nm. TEM micrographs of the thin films revealed the manifestation of ZnS nanoparticles with sizes in the range 3.00-5.83 nm. UV-vis-NIR spectrophotometric measurements showed that the films were highly transparent (∼90%) in the wavelength range 400-2600 nm with a blue shift of the absorption edge. The direct allowed bandgaps have been calculated and they lie in the range 3.89-4.44 eV. The particle size, calculated from the shift of direct bandgap, due to quantum confinement effect lying in the range 3.23-5.60 nm, well support the TEM results. The room temperature photoluminescence spectra of the films showed two peaks centered around 315 and 450 nm. We assigned the first peak due to bandgap transitions while the latter was due to sulfur vacancy in the films. The composition analysis by energy dispersive X-rays also supported the existence of sulfur deficiency in the films. The dielectric property study showed high dielectric constant (85-100) at a higher frequency (>5 kHz).     

Synthesis of PbS/poly (vinyl-pyrrolidone) nanocomposite

A simple solution growth method for synthesis of nanocomposite of PbS nanoparticles in poly(vinyl-pyrrolidone) (PVP) polymer is described. The nanocomposite is prepared from methanolic solution of lead acetate (PbAc), thiourea (TU) and PVP at room temperature (∼27 °C). Optical absorption spectrum of PbS/PVP nanocomposite solution shows strong absorption from 300 to 650 nm with significant bands at 400 and 590 nm which is characteristic of nanoscale PbS. Spin-coated nanocomposite films on glass have an absorption edge at ∼650 nm with band gap of 2.55 eV. Fourier transform infrared (FTIR) spectroscopy of PbS/PVP nanocomposite and PVP shows strong chemical bond between PbS nanoparticles and host PVP polymer. The transmission electron microscope (TEM) images reveal that 5-10 nm PbS particles are evenly embedded in PVP polymer. The formation of PbS is confirmed by selective area electron diffraction (SAED) of a typical nanoparticle.     

Anomalous electrical transport properties of nonstoichiometric nickel ferrite below room temperature

The transport properties of nonstoichiometric nickel ferrite nanoparticles synthesised by the co-precipitation method followed by mechanical milling is reported here. The particle size of ferrite phase in the ball milled samples is found to be ranging from ∼3.5 nm to ∼14 nm but in the un-milled sample it becomes ∼75 nm. A minimum in the conductivity has been observed in dc conductivity versus temperature variation while the activation energies of all the samples show an increasing trend with increasing milling time. The alternating current conductivity has been described by power law σ′(f,T) ∝ f^sTⁿ. The frequency exponent ‘s’ shows anomalous behavior, while the magnitude of the temperature exponent ‘n’ strongly depends on frequency. The dc and ac magnetoresistivities have been observed to be negative. Although the grain boundary contribution is predominated over grain contribution, the magnitude of both grain and grain boundary resistances reduce to lower value under the application of magnetic field.     

Fabrication of nanocomposite particles with superparamagnetic and luminescent functionalities

A new kind of superparamagnetic luminescent nanocomposite particles has been synthesized using a modified Stöber method combined with an electrostatic assembly process. Fe₃O₄ superparamagnetic nanoparticles were coated with uniform silica shell, and then 3-aminopropyltrimethoxysilane was used to terminate the silica surface with amino groups. Finally, negatively charged CdSe quantum dots (QDs) were assembled onto the surface of the amino-terminated SiO₂/Fe₃O₄ nanoparticles through electrostatic interactions. X-ray diffraction (XRD), transmission electron microscopy (TEM), microelectrophoresis, UV-vis absorption and emission spectroscopy and magnetometry were applied to characterize the nanocomposite particles. Dense CdSe QDs were immobilized on the silica surface. The thickness of silica shell was about 35 nm and the particle size of the final products was about 100 nm. The particles exhibited favorable superparamagnetic and photoluminescent properties.     

Selective synthesis, characterization, and photoluminescence study of YPO4:Eu nanorods and nanoparticles

Two different morphological varieties of YPO₄:Eu³⁺ (nanorods and nanoparticles) have been synthesized by convenient template-assisted routes. A surfactant-mediated synthesis route generated the nanorods, while a complex based precursor solution method led to the formation of the nanoparticles. The nanorods have an average diameter of ∼33 nm and an average length of ∼290 nm, whereas the spherical nanoparticles show an average diameter of ∼34 nm. Upon photoluminescence study of the as-prepared samples, the chromaticity of the dopant ion was found to be sensitive to the host morphology as well as the preparative strategy adopted for sample fabrication.     

Reduction, aggregation and physicochemical properties of silver nanoparticles in propan-2-ol:cyclohexane mixtures induced by a high energy electron beam

Radiolytic reduction of silver and gold ions and subsequent formation of their aggregates have been studied in propan-2-ol:cyclohexane mixture using pulse radiolysis technique. The silver sol, produced on irradiation of Ag⁺ solution with a train of electron pulses, has been characterized using XRD and TEM. The size of the particles has been found to be in the range of 30-50 nm. The silver sol emit light with a maximum at 340 nm on irradiation with a high energy electron beam. The intensity of emission has been found to decrease with decrease in concentration of Ag particles. Formation of colloidal gold has also been observed on irradiation of NaAuCl₄ solution in propan-2-ol:cyclohexane by train of electron pulses. The particles so formed are oxidized on exposure to air. No light emission has been observed from Au sol.     

Size-controlled synthesis, microstructure and magnetic properties of Ni nanoparticles

Ni nanoparticles with different mean diameters of 15-83 nm were synthesized by solution reduction process. The size of Ni nanoparticles can be controlled by varying the concentration of NiCl₂·6H₂O and synthesis temperature. The samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS). Results show that the synthesized particles are single-phased Ni with a face-centered cubic crystal structure. Magnetic measurements indicate that Ni nanoparticles are ferromagnetic. The lattice constants and coercivities of the samples are size-dependent.     

Synthesis,microstructure and mechanical properties of Yttria Stabilized Zirconia (3YTZP) – Multi-Walled Nanotube (MWNTs) nanocomposite by direct in-situ growth of MWNTs on Zirconia particles

In this research, Yttria Stabilized Zirconia (3YTZP) – carbon nanotube (CNT) composites are fabricated by direct in-situ growth of CNTs on the Zirconia particles, followed by densification via the Spark Plasma Sintering (SPS) technique. Scanning electron microscopy analysis of the 3YTZP-CNT powders shows uniform distribution of CNTs without the formation of agglomerates frequently seen with the traditional ex-situ mixing of CNTs in ceramic compositions. The samples were sintered to nearly 100% theoretical density and with a finer grain size microstructure. High Resolution Transmission Electron Microscopy (HRTEM) and Raman Spectroscopy confirm CNT retention in the sintered nanocomposites up to 1600 °C. The flexural strength increases from ∼260 MPa for samples without CNTs sintered at 1600 °C to ∼312 MPa for samples with ∼4 wt.% CNTs sintered at the same temperature. A corresponding increase in the indentation fracture toughness is also observed for samples with ∼4 wt.% CNTs sintered at 1600 °C as compared to samples sintered at the same temperature without CNTs.     

Photoluminescence study of PVP capped CdS nanoparticles embedded in PVA matrix

Photoluminescence properties of polyvinyl pyrrolidone (PVP) capped cadmium sulphide (CdS) nanoparticles embedded in polyvinyl alcohol matrix (PVA) are reported. The PVP-CdS nanoparticles are prepared by non-aqueous method wherein cadmium nitrate is used as the cadmium source and hydrogen sulphide as the sulphur source. The synthesized nanoparticles are dispersed in polyvinyl alcohol (PVA) matrix and cast as self-standing flexible (PVP-CdS)-PVA films. The nanocomposites are characterized by optical absorption spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies. XRD and TEM studies show the formation of cubic CdS particles with average size ∼3-5 nm. Thermal studies, carried out to observe the changes in PVA matrix due to the incorporation of PVP-CdS nanoparticles show strong interaction between the polymer matrix and nanoparticles. The photoluminescence emission spectra of the nanocomposites show two peaks, at 502 and 636 nm, which are attributed to the band edge and surface defects respectively, of CdS nanoparticles. Effective surface capping with optimum concentration of polyvinyl pyrrolidone leads to the quenching of surface defect-related emission.     

Spectroscopic investigations of nanoporous SiO2 impregnated with Ag β-diketonates from supercritical solution of carbon dioxide

Porous materials based on SiO₂ containing nanoparticles of silver (NP Ag) are perspective objects for different medico-biological and optoelectronic applications. In the present work nanoporous glasses Vycor (pore size ∼4 nm) and synthetic opal matrices (OM, void size ∼40 nm) impregnated with β-diketonates of silver (Ag(hfac)COD) via solution of supercritical carbon dioxide were studied. Paramagnetic molecules Cu(hfac)₂ were used as spectroscopic probes permitting to obtain the information about the incorporation of similar organometallic molecules into these matrices. Spectra of absorption and electronic paramagnetic resonance (EPR) in the samples of porous SiO₂ containing Ag(hfac)COD were studied before and after heat treatment in air atmosphere. In both materials impregnated at temperature above 50 °C the absorption band at 420-430 nm (the plasmon resonance (PR) band)) typical for nanoparticles of metallic Ag is observed. This band increases in the intensity at heat treatment or at an increase in temperature of impregnation processing. Features of the formation of the PR band in each case depends on sizes and shape of voids in the samples as well as on conditions of heat treatment. The states of paramagnetic centers formed after Ag(hfac)COD introduction are analyzed.     

Synthesis of ZnO flowers and their photoluminescence properties

Flower-like ZnO nano/microstructures have been synthesized by thermal treatment of Zn(NH₃)₄²⁺ precursor in aqueous solvent, using ammonia as the structure directing agent. A number of techniques, including X-ray diffraction (XRD), field emission scan electron microscopy (FESEM), transmission electron microscopy (TEM), thermal analysis, and photoluminescence (PL) were used to characterize the obtained ZnO structures. The photoluminescence (PL) measurements indicated that the as-synthesized ZnO structures showed UV (∼375 nm), blue (∼465 nm), and yellow (∼585 nm) emission bands when they were excited by a He-Gd laser using 320 nm as the excitation source. Furthermore, it has been interestingly found that the intensity of light emission at ∼585 nm remarkably decreased when the obtained ZnO nanocrystals were annealed at 600 °C for 3 h in air. The reason might be the possible oxygen vacancies and interstitials in the sample decreased at high temperature.     

Microstructural characterization of amorphous and nanocrystalline boron nitride prepared by high-energy ball milling

Microstructural parameters like crystallite size, lattice strain, stacking faults and dislocation density were evaluated from the X-ray diffraction data of boron nitride (BN) powder milled in a high-energy vibrational ball mill for different length of time (2-120 h), using different model based approaches like Scherrer analysis, integral breadth method, Williamson-Hall technique and modified Rietveld technique. From diffraction line-broadening analysis of the successive patterns of BN with varying milling time, it was observed that overall line broadening was an operative cause for crystallite size reduction at lower milling time (∼5 h), whereas lattice strains were the prominent cause of line broadening at higher milling times (>19 h). For intermediate milling time (7-19 h), both crystallite size and lattice strain influence the profile broadening although their relative contribution vary with milling time. Microstructural information showed that after long time milling (>19 h) BN becomes mixture of nanocrystalline and amorphous BN. The accumulations of defects cause this crystalline to amorphous transition. It has been found that twin fault (β′) and deformation fault (α) significantly contributed to BN powder as synthesized by a high-energy ball-milling technique. Present study consider only three ball-milled (0, 2 and 3 h) BN powder for faults calculation because fault effected reflections (1 0 1, 1 0 2, 1 0 3) disappear with milling time (>3 h). The morphology and particle size of the BN powders before and after ball milling were also observed in a field emission scanning electron microscope (FESEM).     

Continuous production of phosphor YAG:Tb nanoparticles by hydrothermal synthesis in supercritical water

Phosphor YAG:Tb ((Y_2.7Tb_0.3)Al₅O₁₂) nano particles were synthesized by a hydrothermal method at supercritical conditions (400 °C and 30 MPa) using a flow reactor. Hydroxide sol solutions formed by stoichiometric aluminum nitrate, yttrium nitrate, terbium nitrate and potassium hydroxide solutions. The relationship between particle size and experimental variables including pH, concentration of coexistent ions and hydroxide sol were investigated. Particles were characterized by XRD, TEM and photo-luminescence measurements. Particle size of YAG:Tb became finer as pH was increased or potassium nitrate concentration of the starting metal salt solution was increased. By removing the coexisting ions (NO₃⁻, K⁺) from the metal salt solution, single phase YAG:Tb particles with 20 nm particle size were obtained. The emission spectra of YAG:Tb particles of 14 nm shows a blue shift.