Colloidal synthesis offers a route to nanoparticles (NPs) with controlled composition and structural features. This Perspective describes the use of polyvinylpyrrolidone (PVP) to obtain such nanostructures. PVP can serve as a surface stabilizer, growth modifier, nanoparticle dispersant, and reducing agent. As shown with examples, its role depends on the synthetic conditions. This dependence arises from the amphiphilic nature of PVP along with the molecular weight of the selected PVP. These characteristics can affect nanoparticle growth and morphology by providing solubility in diverse solvents, selective surface stabilization, and even access to kinetically controlled growth conditions. This Perspective includes discussions of the properties of PVP-capped NPs for surface enhanced Raman spectroscopy (SERS), assembly, catalysis, and more. The contribution of PVP to these properties as well as its removal is considered. Ultimately, the NPs accessed through the use of PVP in colloidal syntheses are opening new applications, and the concluding guidelines provided herein should enable new nanostructures to be accessed facilely.

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Polyvinylpyrrolidone (PVP) in nanoparticle synthesis

Thin-film technology is widely implemented in numerous applications1. Although flat substrates are commonly used, we report on the advantages of using curved surfaces as a substrate. The curvature induces a lateral film-thickness variation that allows alteration of the properties of the deposited material2,3. Based on this concept, a variety of implementations in materials science can be expected. As an example, a topographic pattern formed of spherical nanoparticles4,5 is combined with magnetic multilayer film deposition. Here we show that this combination leads to a new class of magnetic material with a unique combination of remarkable properties: The so-formed nanostructures are monodisperse, magnetically isolated, single-domain, and reveal a uniform magnetic anisotropy with an unexpected switching behaviour induced by their spherical shape. Furthermore, changing the deposition angle with respect to the particle ensemble allows tailoring of the orientation of the magnetic anisotropy, which results in tilted nanostructure material.

http://absimage.aps.org/image/MAR06/MWS_MAR06-2005-000595.pdf

Magnetic multilayers on nanospheres

Fabrication of hybrid MOF-on-MOF heteroarchitectures can create novel and multifunctional platforms to achieve desired properties. However, only MOFs with similar crystallographic parameters can be hybridized by the classical epitaxial growth method (EGM), which largely suppressed its applications. A general strategy, called internal extended growth method (IEGM), is demonstrated for the feasible assembly of MOFs with distinct crystallographic parameters in an MOF matrix. Various MOFs with diverse functions could be introduced in a modular MOF matrix to form 3D core-satellite pluralistic hybrid system. The number of different MOF crystals interspersed could be varied on demand. More importantly, the different MOF crystals distributed in individual domains could be used to further incorporate functional units or enhance target functions.

Controllable Modular Growth of Hierarchical MOF-on-MOF Architectures

Two-dimensional (2D) materials have attracted great attention by researchers due to their fascinating properties and promising applications. However, the synthesis methods for few layers are usually difficult to expand to large area applications because of their low yield. In this paper, graphene-like MoSe2 nanosheets are successfully and scaleable synthesized by a facile and low-cost hydrothermal method under the synergy of PVP and graphene. The ultrathin MoSe2 nanosheets are typically 1–3 layers, which are confirmed by HRTEM. This unique structure makes this MoSe2 electrode material show superior activity toward the electrocatalytic hydrogen production with a low Tafel slope about 70 mV·dec–1. Furthermore, the synthesized graphene-like MoSe2 nanosheets had a high stability during the electrocatalytic process and we nearly cannot find the degradation after 1000 cyclic voltammetric sweeps.

Large-Scale Synthesis of Graphene-Like MoSe2 Nanosheets for Efficient Hydrogen Evolution Reaction

ZnCr2O4 nanocrystals with different sizes were synthesized by a coprecipitation and postcalcined method. X-ray powder diffraction and transmission electron microscopy showed that the nanocrystals annealed at different temperatures have the same crystal structure and their size increases with increasing annealing temperature. Cr and Zn K-edge X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) was used to investigate the change of the inversion parameter vs the annealing temperature. The inversion parameter is found to decrease with increasing annealing temperature. In particular, the sample annealed at 750 °C has a normal spinel structure, like bulk ZnCr2O4. On the contrary, the inversion parameter of the sample annealed at 300 °C can get 0.24. This study of ZnCr2O4 shows for the first time a variation of the inversion degree with annealing temperature, and provides a method to obtain ZnCr2O4 with different inversion parameters.

Cation Distribution in ZnCr2O4 Nanocrystals Investigated by X-ray Absorption Fine Structure Spectroscopy

CoFe2O4 particles were synthesized using metallic nitrates and polyvinylpyrolidone (PVP) using sol–gel method followed by calcination for 2 h at 960 °C. PVP performed as a surfactant and the effect of various concentrations of PVP on the resultant CoFe2O4 powder was studied. The resultant samples were characterized by XRD, TG/DSC, HR-SEM and VSM. X-ray diffraction results indicated the crystalline phase of CoFe2O4 particles and impurity phase of hematite was observed for higher PVP concentrations. SEM images demonstrated the influence of PVP concentration on the size of the particles. By VSM measurements, the variations in magnetic properties with respect to PVP concentration are studied. All the magnetic characteristics H
 c
 , M
 s
 and M
 r
 increased for 6 wt% and 15 wt% of PVP concentration. The CoFe2O4 particles synthesized with the optimum concentration of PVP may be very attractive for potential applications because of their outstanding magnetic properties (M
 s
 =81.1 Am2/kg, H
 c
 =831 Gauss).

/pdf/study-of-cofe2o4-particles-synthesized-with-various-q7rju8fuw0.pdf

V. Umapathy

Papers

Study of CoFe2O4 Particles Synthesized with Various Concentrations of PVP Polymer