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A general method for preparing nano-sized metal oxide nanoparticles with highly disordered crystal structure and their processing into stable aqueous dispersions is presented. With these nanoparticles as building blocks, a series of nanoparticles@reduced graphene oxide (rGO) composite aerogels are fabricated and directly used as high-power anodes for lithium-ion hybrid supercapacitors (Li-HSCs). To clarify the effect of the degree of disorder, control samples of crystalline nanoparticles with similar particle size are prepared. The results indicate that the structurally disordered samples show a significantly enhanced electrochemical performance compared to the crystalline counterparts. In particular, structurally disordered Ni x Fe y O z @rGO delivers a capacity of 388 mAh g -1 at 5 A g -1 , which is 6 times that of the crystalline sample. Disordered Ni x Fe y O z @rGO is taken as an example to study the reasons for the enhanced performance. Compared with the crystalline sample, density functional theory calculations reveal a smaller volume expansion during Li + insertion for the structurally disordered Ni x Fe y O z nanoparticles, and they are found to exhibit larger pseudocapacitive effects. Combined with an activated carbon (AC) cathode, full-cell tests of the lithium-ion hybrid supercapacitors are performed, demonstrating that the structurally disordered metal oxide nanoparticles@rGO||AC hybrid systems deliver high energy and power densities within the voltage range of 1.0-4.0 V. These results indicate that structurally disordered nanomaterials might be interesting candidates for exploring high-power anodes for Li-HSCs.

Multiferroic composite materials combining ferroelectric and ferromagnetic order at room temperature have great potential for emerging applications such as four-state memories, magnetoelectric sensors, and microwave devices. In this paper, we report an effective and facile liquid phase deposition route to create multiferroic composite thin films involving the spin-coating of nanoparticle dispersions of BaTiO 3 , a well-known ferroelectric, and CoFe 2 O 4 , a highly magnetostrictive material. This approach offers great flexibility in terms of accessible film configurations (co-dispersed as well as layered films), thicknesses (from 100 nm to several μm) and composition (5-50 wt % CoFe 2 O 4 with respect to BaTiO 3 ) to address various potential applications. A detailed structural characterization proves that BaTiO 3 and CoFe 2 O 4 remain phase-separated with clear interfaces on the nanoscale after heat treatment, while electrical and magnetic studies indicate the simultaneous presence of both ferroelectric and ferromagnetic order. Furthermore, coupling between these orders within the films is demonstrated with voltage control of the magnetism at ambient temperatures.

The preparation of macroscopic materials from two-dimensional nanostructures represents a great challenge. Restacking and random aggregation to dense structures during processing prevents the preservation of the two-dimensional morphology of the nanobuilding blocks in the final body. Here we present a facile solution route to ultrathin, crystalline Y2O3 nanosheets, which can be assembled into a 3D network by a simple centrifugation-induced gelation method. The wet gels are converted into aerogel monoliths of macroscopic dimensions via supercritical drying. The as-prepared, fully crystalline Y2O3 aerogels show high surface areas of up to 445 m(2)/g and a very low density of 0.15 g/cm(3), which is only 3% of the bulk density of Y2O3. By doping and co-doping the Y2O3 nanosheets with Eu(3+) and Tb(3+), we successfully fabricated luminescent aerogel monoliths with tunable color emissions from red to green under UV excitation. Moreover, the as-prepared gels and aerogels exhibit excellent adsorption capacities for organic dyes in water without losing their structural integrity. For methyl blue we measured an unmatched adsorption capacity of 8080 mg/g. Finally, the deposition of gold nanoparticles on the nanosheets gave access to Y2O3-Au nanocomposite aerogels, proving that this approach may be used for the synthesis of catalytically active materials. The broad range of properties including low density, high porosity, and large surface area in combination with tunable photoluminescence makes these Y2O3 aerogels a truly multifunctional material with potential applications in optoelectronics, wastewater treatment, and catalysis.

High-capacity anode materials for lithium ion batteries (LIBs), such as spinel-type metal oxides, generally suffer from poor Li(+) and e(-) conductivities. Their drastic crystal structure and volume changes, as a result of the conversion reaction mechanism with Li, severely impede the high-rate and cyclability performance toward their practical application. In this article, we present a general and facile approach to fabricate flexible spinel-type oxide/reduced graphene oxide (rGO) composite aerogels as binder-free anodes where the spinel nanoparticles (NPs) are integrated in an interconnected rGO network. Benefiting from the hierarchical porosity, conductive network and mechanical stability constructed by interpenetrated rGO layers, and from the pillar effect of NPs in between rGO sheets, the hybrid system synergistically enhances the intrinsic properties of each component, yet is robust and flexible. Consequently, the spinel/rGO composite aerogels demonstrate greatly enhanced rate capability and long-term stability without obvious capacity fading for 1000 cycles at high rates of up to 4.5 A g(-1) in the case of CoFe2O4. This electrode design can successfully be applied to several other spinel ferrites such as MnFe2O4, Fe3O4, NiFe2O4 or Co3O4, all of which lead to excellent electrochemical performances.

Nonaqueous synthesis routes have emerged as a powerful platform for directly obtaining diverse metal oxide nanoparticles with high crystallinity and tunable compositions. The benzyl alcohol (BA) route, for example, has been applied toward dozens of oxides including binary, ternary, and even more complex multimetal systems. Here we compare anatase nanoparticles made from the BA route with the traditional hydrothermal route. XPS measurements indicated that the BA route resulted in more reduced Ti states, corresponding to additional oxygen vacancies. These defects resulted in additional trap states, slower recombination, and slower charge transport. The performance of BA anatase was improved by incorporating niobium intended to suppress oxygen vacancies. The higher performance Nb-containing films were post-treated to yield a 7.96% power conversion efficiency (AM 1.5), similar to the state-of-the-art hydrolytic TiO2 in the same configuration.

Dopant atoms are used to tailor the properties of materials. However, whether the desired effect is achieved through selective doping depends on the dopant distribution within the host material. The clustering of dopant atoms can have a deleterious effect on the achievable properties because a two-phase material is obtained instead of a homogeneous material. Thus, the examination of dopant fluctuations in nanodevices requires a reliable method to chemically probe individual atoms within the host material. This is particularly challenging in the case of functionalized nanoparticles where the characteristic length scale of the particles demands the use of a high-spatial-resolution and high-sensitivity technique. Here we demonstrate a chemically sensitive atomic resolution imaging technique which delivers direct site-specific information on the dopant distribution in nanoparticles. We employ electron energy-loss spectroscopy imaging in a scanning transmission electron microscope combined with multivariate statistical analysis to map the distribution of Ba dopant atoms in SrTiO(3) nanoparticles. Our results provide direct evidence for clustering of the Ba dopants in the SrTiO(3) nanoparticles outlining a possible explanation for the presence of polar nanoregions in the Ba:SrTiO(3) system. The results we present constitute the first example of site-specific atomic resolution spectroscopy of foreign atoms in doped nanoparticles and suggest a general strategy to ascertain the spatial distribution of impurity atoms in nanocrystals and hence improve the performance of nanoparticle-based devices.

A detailed study of kinetic and thermodynamic aspects in the microwave-assisted synthesis of ZnO nanoparticles from zinc acetate and benzyl alcohol is presented. The use of a nonaqueous sol-gel approach provides the unique opportunity to investigate simultaneously the organic reaction, that is, the esterification between acetate and benzyl alcohol, and the inorganic process, represented by the growth of the ZnO nanoparticles. Monitoring both the formation of the organic species as well as ZnO crystal size with time makes it possible to directly correlate the kinetics of the organic side reaction with the growth kinetics of the ZnO nanoparticles. The esterification reaction, which is the chemical basis for producing the monomers for ZnO formation, was found to be first order. The growth of the ZnO nanoparticles followed the Lifshitz-Slyozov-Wagner model for coarsening, pointing to a diffusion-limited process. Comparison of the microwave-mediated route with conventional heating showed that microwave irradiation greatly accelerates nanoparticle formation by (a) facilitating the dissolution of the precursor in the solvent, (b) increasing the rate constants for the esterification reaction by 1 order of magnitude, resulting in faster production of monomer and consequently in an earlier nucleation event, and (c) increasing the rate constants k(growth) for the crystal growth from 3.9 nm(3)/min (conventional heating) to 15.4 nm(3)/min (microwave heating).