Yolk/shell or 'rattle-typed' nanomaterials with nanoparticle cores inside hollow shells are interesting among the complex hollow nanostructures. Yolk/shell nanoparticles (YSNs) are promising functional nanomaterials for a variety of applications such as catalysis, delivery, lithium-ion batteries and biosensors due to their tailorability and functionality in both the cores and hollow shells. This feature article provides an overview of advances in this exciting area of YSNs, covering systematic synthesis approaches and key promising applications based on the literature and our own recent work. We present some strategies for the synthesis of YSNs with controllable sizes, compositions, geometries, structures and functionalities. Applications of these new materials in a wide range of potential areas are discussed including nanoreactors, biomedicine and lithium-ion batteries. Promising future directions of this active research field are also highlighted.

Figure S1. XRD patterns of the CeO 2 hydrothermal treated at 170 o C for (a) 12h, (b) 24h, (c) 48h, (d) 144h.

Commercial lead-based piezoelectric materials raised worldwide environmental concerns in the past decade. Bi 0.5 Na 0.5 TiO 3 -based solid solution is among the most promising lead-free piezoelectric candidates; however, depolarization of these solid solutions is a longstanding obstacle for their practical applications. Here we use a strategy to defer the thermal depolarization, even render depolarization-free Bi 0.5 Na 0.5 TiO 3 -based 0-3-type composites. This is achieved by introducing semiconducting ZnO particles into the relaxor ferroelectric 0.94Bi 0.5 Na 0.5 TiO 3 -0.06BaTiO 3 matrix. The depolarization temperature increases with increasing ZnO concentration until depolarization disappears at 30 mol% ZnO. The semiconducting nature of ZnO provides charges to partially compensate the ferroelectric depolarization field. These results not only pave the way for applications of Bi 0.5 Na 0.5 TiO 3 -based piezoceramics, but also have great impact on the understanding of the mechanism of depolarization so as to provide a new design to optimize the performance of lead-free piezoelectrics.

Most materials expand on heating, known as positive thermal expansion. There are some instances most of which have been discovered in the past decade to exhibit a negative thermal expansion (NTE). [1][2][3][4][5][6][7][8][9] The nature of NTE behavior originates from the effect of atomic vibrations, (e.g., the low-energy transverse mode (ice), 2 the coupled rotation of rigid polyhedra (ZrW 2 O 8 , Fe[Co-(CN) 6 ]), 1,4 and active vibration modes of carbon fullerenes and nanotubes), 7 from the effect of magnetic transition (Invar alloy), 3 or from the changes in electron configuration (Sm 2.72 C 60 , YbCuAl). 8 The occurrence of NTE materials immediately found their important technical applications in many fields, because the overall thermal expansion coefficient (TEC) could be tailored by introduction of NTE materials. 1,2 In particular, zero thermal expansion (ZTE) is very interesting, where the volume neither expands nor contracts with the temperature fluctuation. 3-6 The ZTE could be achieved to form composite by combining the materials with positive thermal expansion with NTE materials. However, the fabrication of ZTE composite is hampered by the poor thermal stability of NTE compounds. For example, ZrW 2 O 8 will be decomposed at a relatively low temperature (777°C). 1 The requirement of ZTE will be satisfied if the ZTE is available in a single phase. Up to now, rare materials exhibit the novel ZTE, such as Invar alloys and Fe-[Co(CN) 6 ]. 3,4 Moreover, the ZTE generally appears in a low temperature (below room temperature). The ZTE over a wider temperature range would be very useful for the applications.PbTiO 3 (PT) as an important perovskite-type multifunctional material exhibits a unique NTE in the perovskite family. 9,10 The unit cell volume of PT contracts over a wide temperature range in the ferroelectric phase (25-490°C) with an average intrinsic volumetric TEC (-1.99 × 10 -5°C-1 ). 9b The NTE of PT-based compounds can be controlled over a large range from -0.11 × 10 -5 to -3.92 × 10 -5°C-1 , which covers the range found in almost all other known NTE oxides. 9 However, a low or ZTE could only be achieved by sacrificing the temperature range, that is, reducing the Curie point (T C ), such as for Pb 0.80 La 0.20 TiO 3 (-0.11 × 10 -5°C -1 , 25-130°C). 9b It is a challenge to expand ZTE to the hightemperature range. On the basis of our previously studied PbTiO 3 -based compounds, we could only access a low expansion or ZTE by reducing the tetragonality (c/a), resulting in the decrease in the ZTE temperature range (region II in Figure 1). To obtain the ZTE in a wider temperature range, a kind of PbTiO 3 -based compound should be found in the region I where c/a is large and the absolute value of TEC is low (Figure 1). Recently, in the PbTiO 3 -BiMeO 3 (Me is cations with an average valence +3), the Bi substitution plays an unusual role in which both T C and c/a are considerably enhanced, owing to the strong coupling between the Pb/Bi cations and the B-site cations with strong ferroelectricity activity, suc...

The understanding of the negative thermal expansion (NTE) mechanism remains challenging but critical for the development of NTE materials. This study sheds light on NTE of ScF3, one of the most outstanding materials with NTE. The local dynamics of ScF3 has been investigated by a combined analysis of synchrotron-based X-ray total scattering, extended X-ray absorption fine structure, and neutron powder diffraction. Very interestingly, we observe that (i) the Sc-F nearest-neighbor distance strongly expands with increasing temperature, while the Sc-Sc next-nearest-neighbor distance contracts, (ii) the thermal ellipsoids of relative vibrations between Sc-F nearest-neighbors are highly elongated in the direction perpendicular to the Sc-F bond, indicating that the Sc-F bond is much softer to bend than to stretch, and (iii) there is mainly dynamically transverse motion of fluorine atoms, rather than static shifts. These results are direct experimental evidence for the NTE mechanism, in which the rigid unit is not necessary for the occurrence of NTE, and the key role is played by the transverse thermal vibrations of fluorine atoms through the "guitar-string" effect.

PbTiO(3)-based compounds are well-known ferroelectrics that exhibit a negative thermal expansion more or less in the tetragonal phase. The mechanism of negative thermal expansion has been studied by high-temperature neutron powder diffraction performed on two representative compounds, 0.7PbTiO(3)-0.3BiFeO(3) and 0.7PbTiO(3)-0.3Bi(Zn(1/2)Ti(1/2))O(3), whose negative thermal expansion is contrarily enhanced and weakened, respectively. With increasing temperature up to the Curie temperature, the spontaneous polarization displacement of Pb/Bi (δz(Pb/Bi)) is weakened in 0.7PbTiO(3)-0.3BiFeO(3) but well-maintained in 0.7PbTiO(3)-0.3Bi(Zn(1/2)Ti(1/2))O(3). There is an apparent correlation between tetragonality (c/a) and spontaneous polarization. Direct experimental evidence indicates that the spontaneous polarization originating from Pb/Bi-O hybridization is strongly associated with the negative thermal expansion. This mechanism can be used as a guide for the future design of negative thermal expansion of phase-transforming oxides.

Highly ordered mesoporous Cr(2)O(3) materials with high specific surface area and narrow pore size distribution were successfully prepared by a vacuum assisted impregnation method. Both 2-dimensional hexagonal and 3-dimensional cubic Cr(2)O(3) mesoporous replicas from SBA-15 and KIT-6 templates exhibit enhanced performance for gas sensors and lithium ion batteries, compared to the bulk Cr(2)O(3) counterpart.

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