Highly ordered hollow oxide nanostructures: the Kirkendall effect at the nanoscale

Partial Chemicalization of Nanoscale Metals: An Intra-Material Transformative Approach for the Synthesis of Functional Colloidal Metal-Semiconductor Nanoheterostructures

Heterostructuring colloidal nanocrystals into multicomponent modular constructs, where domains of distinct metal and semiconductor phases are interconnected through bonding interfaces, is a consolidated approach to advanced breeds of solution-processable hybrid nanomaterials capable of expressing richly tunable and even entirely novel physical-chemical properties and functionalities. To meet the challenges posed by the wet-chemical synthesis of metal-semiconductor nanoheterostructures and to overcome some intrinsic limitations of available protocols, innovative transformative routes, based on the paradigm of partial chemicalization, have recently been devised within the framework of the standard seeded-growth scheme. These techniques involve regiospecific replacement reactions on preformed nanocrystal substrates, thus holding great synthetic potential for programmable configurational diversification. This review article illustrates achievements so far made in the elaboration of metal-semiconductor nanoheterostructures with tailored arrangements of their component modules by means of conversion pathways that leverage on spatially controlled partial chemicalization of mono- and bi-metallic seeds. The advantages and limitations of these approaches are discussed within the context of the most plausible mechanisms underlying the evolution of the nanoheterostructures in liquid media. Representative physical-chemical properties and applications of chemicalization-derived metal-semiconductor nanoheterostructures are emphasized. Finally, prospects for developments in the field are outlined.

Oxygen Plasma Induced Nanochannels for Creating Bimetallic Hollow Nanocrystals

Platinum-based metal catalysts are considered excellent converters in various catalytic reactions, particularly in fuel cell applications. The atomic structure at the nanocrystal surface and the metal interface both influence the catalytic performance, controlling the efficiency of the electrochemical reactions. Here we report the synthesis of Ag/Pt and Ag/Pd core/shell nanocrystals and insight into the formation mechanism of these bimetallic core/shell nanocrystals when undergoing oxygen plasma treatment. We carefully designed the oxidation treatment that determines the structural and compositional evolution. The accelerated oxidation-triggered diffusion of Ag toward the outer metal shell leads to the Kirkendall effect. After prolonged oxygen plasma treatment, most core/shell nanocrystals evolve into hollow spheres. At the same time, a minor fraction of the metal remains unchanged with a well-protected Ag core and a monocrystalline Pt or Pd shell. We hypothesize that the O2 plasma disturbs the Pt or Pd shell surface and introduces active O species that react with the diffused Ag from the inside out. Based on EDX elemental mapping, combined with several electron microscopic techniques, we deduced the formation mechanism of the hollow structures to be as follows: (I) the oxidation of Ag within the Pt or Pd lattice causes a disrupted crystal lattice of Pt or Pd; (II) nanochannels arise at the defect locations on the Pt or Pd shell; (III) the remaining Ag atoms pass through these nanochannels and leave a hollow crystal behind. Our findings deepen the understanding of interface dynamics of bimetallic nanostructured catalysts under an oxidative environment and unveil an alternative approach for catalyst pretreatment.

Degeneration Behavior of Cu Nanowires under Carbon Dioxide Environment: An In Situ/Operando Study

Copper (Cu) is a catalyst broadly used in industry for hydrogenation of carbon dioxide, which has broad implications for environmental sustainability. An accurate understanding of the degeneration behavior of Cu catalysts under operando conditions is critical for uncovering the failure mechanism of catalysts and designing novel ones with optimized performance. Despite the widespread use of these materials, their failure mechanisms are not well understood because conventional characterization techniques lack the necessary time and spatial resolution to capture these complex behaviors. In order to overcome these challenges, we carried out transmission electron microscopy (TEM) with a specialized in situ gas environmental holder, which allows us to unravel the dynamic behavior of the Cu nanowires (NWs) in operando. The failure process of these nanoscale Cu catalysts under CO₂ atmosphere were tracked and further rationalized based on our numerical modeling using phase-field methods.

In Situ Current-Accelerated Phase Cycling with Metallic and Semiconducting Switching in Copper Nanobelts at Room Temperature

Here, a current-accelerated phase cycling by an in situ current-induced oxidation process was demonstrated to reversibly switch the local metallic Cu and semiconducting Cu₂O phases of patterned polycrystalline copper nanobelts. Once the Cu nanobelts were applied by a direct-current bias of ∼0.5 to 1 V in air with opposite polarities, the resistance between several hundred ohms and more than MΩ can be manipulated. In practice, the thickness of 60 nm with a moderate grain size inhibiting both electromigration and permanent oxidation is the optimized condition for reversible switching when the oxygen supply is sufficient. More than 40% of the copper localized beneath the positively biased electrode was oxidized assisted by the Joule heating, blocking the current flow. On the contrary, the reduction reaction of Cu₂O was activated by the thermally assisted electromigration of Cu atoms penetrating the interlayer at the reverse bias. Finally, based on a high on/off ratio, the fast switching and the scalable production, reusable feasibility based on copper nanobelts such as the memristor array was demonstrated.

Probing the Origin of Gold Dissolution and Tunneling Across Ni2P Shell Using in situ Transmission Electron Microscopy

Hybrid core-shell catalysts possess superior physiochemical properties that are closely related to their atomic structures. Here, we report novel diffusion phenomena in an Au-Ni₂P yolk-shell structure using in situ heating transmission electron microscopy (TEM) analysis. The Au yolks can dissolve completely into and diffuse across the Ni₂P shell at 500 °C, resulting in an inward volume expansion of the Ni₂P shell and shrinkage of the void. The dissolved Au is subsequently segregated, forming a new crystal on the outer layer of the shell. A detailed atomic-scale resolution imaging revealed that the outward Au diffusion and aggregation occur when the Au yolks are epitaxial to the Ni₂P shells. Theoretical analysis shows that the outward diffusion and deposition of Au atoms is primarily driven by the excess epitaxial strain energy.

Resolving single Cu nanoparticle oxidation and Kirkendall void formation with in situ plasmonic nanospectroscopy and electrodynamic simulations

Copper nanostructures are ubiquitous in microelectronics and heterogeneous catalysis and their oxidation is a topic of high current interest and broad relevance. It relates to important questions, such as catalyst active phase, activity and selectivity, as well as fatal failure of microelectronic devices. Despite the obvious importance of understanding the mechanism of Cu nanostructure oxidation, numerous open questions remain, including under what conditions homogeneous oxide layer growth occurs and when the nanoscale Kirkendall void forms. Experimentally, this is not trivial to investigate because when a large number of nanoparticles are simultaneously probed, ensemble averaging makes rigorous conclusions difficult. On the other hand, when (in situ) electron-microscopy approaches with single nanoparticle resolution are applied, concerns about beam effects that may both reduce the oxide or prevent oxidation via the deposition and cross-linking of carbonaceous species cannot be neglected. In response we present how single particle plasmonic nanospectroscopy can be used for the in situ real time characterization of multiple individual Cu nanoparticles during oxidation. Our analysis of their optical response combined with post mortem electron microscopy imaging and detailed Finite-Difference Time-Domain electrodynamics simulations enables in situ identification of the oxidation mechanism both in the initial oxide shell growth phase and during Kirkendall void formation, as well as the transition between them. In a wider perspective, this work presents the foundation for the application of single particle plasmonic nanospectroscopy in investigations of the impact of parameters like particle size, shape and grain structure with respect to defects and grain boundaries on the oxidation of metal nanoparticles.

Heterodimers for in Situ Plasmonic Spectroscopy: Cu Nanoparticle Oxidation Kinetics, Kirkendall Effect, and Compensation in the Arrhenius Parameters

The ability to study oxidation, reduction, and other chemical transformations of nanoparticles in real time and under realistic conditions is a nontrivial task due to their small dimensions and the often challenging environment in terms of temperature and pressure. For scrutinizing oxidation of metal nanoparticles, visible light optical spectroscopy based on the plasmonic properties of the metal has been established as a suitable method. However, directly relying on the plasmonic resonance of metal nanoparticles as a built-in probe to track oxidation has a number of drawbacks, including the loss of optical contrast in the late oxidation stages. To address these intrinsic limitations, we present a plasmonic heterodimer-based nanospectroscopy approach, which enables continuous self-referencing by using polarized light to eliminate parasitic signals and provides large optical contrast all the way to complete oxidation. Using Au-Cu heterodimers and combining experiments with finite-difference time-domain simulations, we quantitatively analyze the oxidation kinetics of ca. 30 nm sized Cu nanoparticles up to complete oxidation. Taking the Kirkendall effect into account, we extract the corresponding apparent Arrhenius parameters at various extents of oxidation and find that they exhibit a significant compensation effect, implying that changes in the oxidation mechanism occur as oxidation progresses and the structure of the formed oxide evolves. In a wider perspective, our work promotes the use of model-system-type in situ optical plasmonic spectroscopy experiments in combination with electrodynamics simulations to quantitatively analyze and mechanistically interpret oxidation of metal nanoparticles and the corresponding kinetics in demanding chemical environments, such as in heterogeneous catalysis.

Under Diffusion Control: from Structuring Matter to Directional Motion

Self-organization in synthetic chemical systems is quickly developing into a powerful strategy for designing new functional materials. As self-organization requires the system to exist far from thermodynamic equilibrium, chemists have begun to go beyond the classical equilibrium self-assembly that is often applied in bottom-up supramolecular synthesis, and to learn about the surprising and unpredicted emergent properties of chemical systems that are characterized by a higher level of complexity and extended reactivity networks. The present review focuses on self-organization in reaction-diffusion systems. Selected examples show how the emergence of complex morphogenesis is feasible in synthetic systems leading to hierarchically and nanostructured matter. Starting from well-investigated oscillating reactions, recent developments extend diffusion-limited reactivity to supramolecular systems. The concept of dynamic instability is introduced and illustrated as an additional tool for the design of smart materials and actuators, with emphasis on the realization of motion even at the macroscopic scale. The formation of spatio-temporal patterns along diffusive chemical gradients is exploited as the main channel to realize symmetry breaking and therefore anisotropic and directional mechanical transformations. Finally, the interaction between external perturbations and chemical gradients is explored to give mechanistic insights in the design of materials responsive to external stimuli.

Bimetallic Nanoparticle Oxidation in Three Dimensions by Chemically Sensitive Electron Tomography and in Situ Transmission Electron Microscopy

The formation of hollow-structured oxide nanoparticles is primarily governed by the Kirkendall effect. However, the degree of complexity of the oxidation process multiplies in the bimetallic system because of the incorporation of more than one element. Spatially dependent oxidation kinetics controls the final morphology of the hollow nanoparticles, and the process is highly dependent on the elemental composition. Currently, a theoretical framework that can predict how different metal elements result in different oxide morphologies remains elusive. In this work, utilizing a combination of state-of-the-art in situ environmental transmission electron microscopy and three-dimensional (3D) chemically sensitive electron tomography, we provide an in situ and 3D investigation of the oxidation mechanism of the Ni-Fe nanoparticles. The direct measurements allow us to correlate the 3D elemental segregation in the particles with the oxidation morphologies, that is, single-cavity or dual-cavity hollow structure, and multicavity porous structures. Our findings in conjunction with theoretical calculations show that metal concentration, diffusivity, and particle size are important parameters that dictate the mechanical and phase stabilities of the hollow oxide shell, which in turn determine its barrier properties and the final hollow oxide morphology. It sheds light on how to use multielemental oxidation to control morphology in nanomaterials and demonstrates the power of 3D chemical imaging.

A Novel Approach to Realize Si-Based Porous Wire-In-Tube Nanostructures for High-Performance Lithium-Ion Batteries

Hollow inorganic nanostructures have drawn great attention due to their fascinating features, such as large surface area, high loading capacity, and high permeability. The formation, characterization, and application of partially and entirely hollow structure by applying a Si-based reactive ion deposition and etching method on silicon nanowire as a template are reported. This fabrication technique is extended to a stainless steel substrate to be used as the binder-free anode for high capacity and high rate lithium-ion batteries. The electrochemical analyses exhibit that in addition to the high initial discharge capacity of 4125 mAh g^-1 at a rate of C/16, the best performing electrode shows discharge/charge capacity of as high as 3302.14/2832.1 mAh g^-1 , respectively, with an excellent charge capacity retention of 96.7% over 100 cycles at a rate density of 1 C. Even at a rate of 12 C, the as-designed structure is still able to deliver an impressive 1553 mAh g^-1 , which probably is attributed to fast lithium diffusion in its hollow part and high porosity of Si and alumina layer. It is proved that the change in hollowness ratio significantly affects capacity retention and average coulombic efficiency of the lithium-ion cells.

Stencil Lithography for Scalable Micro- and Nanomanufacturing

In this paper, we review the current development of stencil lithography for scalable micro- and nanomanufacturing as a resistless and reusable patterning technique. We first introduce the motivation and advantages of stencil lithography for large-area micro- and nanopatterning. Then we review the progress of using rigid membranes such as SiNx and Si as stencil masks as well as stacking layers. We also review the current use of flexible membranes including a compliant SiNx membrane with springs, polyimide film, polydimethylsiloxane (PDMS) layer, and photoresist-based membranes as stencil lithography masks to address problems such as blurring and non-planar surface patterning. Moreover, we discuss the dynamic stencil lithography technique, which significantly improves the patterning throughput and speed by moving the stencil over the target substrate during deposition. Lastly, we discuss the future advancement of stencil lithography for a resistless, reusable, scalable, and programmable nanolithography method.

From Galvanic to Anti-Galvanic Synthesis of Bimetallic Nanoparticles and Applications in Catalysis, Sensing, and Materials Science

The properties of two alloyed metals have been known since the Bronze Age to outperform those of a single metal. How alloying and mixing metals applies to the nanoworld is now attracting considerable attention. The galvanic process, which is more than two centuries old and involves the reduction of a noble-metal cation by a less noble metal, has not only been used in technological processes, but also in the design of nanomaterials for the synthesis of bimetallic transition-metal nanoparticles. The background and nanoscience applications of the galvanic reactions (GRs) are reviewed here, in particular with emphasis on recent progress in bimetallic catalysis. Very recently, new reactions have been discovered with nanomaterials that contradict the galvanic principle, and these reactions, called anti-galvanic reactions (AGRs), are now attracting much interest for their mechanistic, synthetic, catalytic, and sensor aspects. The second part of the review deals with these AGRs and compares GRs and AGRs, including the intriguing AGRs mechanism and the first applications.

Hollow CuxO (x = 2, 1) micro/nanostructures: synthesis, fundamental properties and applications

In this review, we comprehensively summarize the important advances in hollow Cu_xO micro/nanostructures, including the universal synthesis strategies, the interfacial Cu–O atomic structures as well as the intrinsic properties, and potential applications. Remarks on emerging issues and promising research directions are also discussed.

Enhanced adhesion and field emission of CuO nanowires synthesized by simply modified thermal oxidation technique

Metal oxide nanowires (NWs) can be easily grown by the thermal oxidation method, but the low adhesion between the NWs and the substrate restricts their practical applications in functional devices. In this work, the conventional hotplate technique is simply modified by introducing one or two stainless steel plates to supply a more stable oxidation environment, which is found to be beneficial to the growth and adhesion of CuO NWs on the Cu substrate. In detail, the Cu foils were heated on the hotplate directly, on one plate over the hotplate, and between two plates over the hotplate at 400 °C in ambient condition. It is found that the NWs obtained between two plates exhibit large length and diameter with moderate density. The sufficient activated oxygen, stable temperature, and proper temperature gradient configuration caused by the two plates accelerate the formation of CuO NWs, and result in the longest NWs with enhanced adhesion. The grain-boundary diffusion and Kirkendall effect are proposed to explain the mechanism of NWs growth and the formation of cracks. The NWs obtained between two plates also showed the best field emission properties, with lowest turn-on field (5.31 V μm(-1)) and threshold field (9.8 V μm(-1)). Excellent field emission properties and enhanced NW-substrate adhesion indicate that these NW arrays could be potentially used as the cathode of field emission displays.

Dealloying of gold-copper alloy nanowires: From hillocks to ring-shaped nanopores

We report on a novel fabrication approach of metal nanowires with complex surface. Taking advantage of nodular growth triggered by the presence of surface defects created intentionally on the substrate as well as the high tilt angle between the magnetron source axis and the normal to the substrate, metal nanowires containing hillocks emerging out of the surface can be created. The approach is demonstrated for several metals and alloys including gold, copper, silver, gold-copper and gold-silver. We demonstrate that applying an electrochemical dealloying process to the gold-copper alloy nanowire arrays allows for transforming the hillocks into ring-like shaped nanopores. The resulting porous gold nanowires exhibit a very high roughness and high specific surface making of them a promising candidate for the development of SERS-based sensors.

In situ conversion of nanostructures from solid to hollow in transmission electron microscopes using electron beam

With the current development of electron beam sources, the use of transmission electron microscopes is no more limited to imaging or chemical analysis but has rather been extended to nanoengineering. This includes the e-beam induced growth, etching and structural transformation of nanomaterials. In this review we summarize recent progress on the e-beam induced morphological transformation of nanostructures from solid to hollow. We provide a detailed account of the processes reported so far in the literature with a special emphasis on the mechanistic understanding of the e-beam induced hollowing of nanomaterials. Through an important number of examples, we discuss how one can achieve a precise control of such hollowing processes by understanding the fundamental mechanisms occurring at the atomic scale during the irradiation of solid nanostructures. Finally, we conclude with remarks and our own view on the prospective future directions of this research field.

Controlling the Formation of Nanocavities in Kirkendall Nanoobjects through Sequential Thermal Ex Situ Oxidation and In Situ Reduction Reactions

Controlling the porosity, the shape, and the morphology of Kirkendall hollow nanostructures is the key factor to tune the properties of these tailor-made nanomaterials which allow in turn broadening their applications. It is shown that by applying a continuous oxidation to copper nanowires following a temperature ramp protocol, one can synthesize cuprous oxide nanotubes containing periodic copper nanoparticles. A further oxidation of such nanoobjects allows obtaining cupric oxide nanotubes with a bamboo-like structure. On the other hand, by applying a sequential oxidation and reduction reactions to copper nanowires, one can synthesize hollow nanoobjects with complex shapes and morphologies that cannot be obtained using the Kirkendall effect alone, such as necklace-like cuprous oxide nanotubes, periodic solid copper nanoparticles or hollow cuprous oxide nanospheres interconnected with single crystal cuprous oxide nanorods, and aligned and periodic hollow nanospheres embedded in a cuprous oxide nanotube. The strategy demonstrated in this study opens new avenues for the engineering of hollow nanostructures with potential applications in gas sensing, catalysis, and energy storage.

Creating nanoporosity in silver nanocolumns by direct exposure to radio-frequency air plasma

Nanoporous materials are of great importance for a broad range of applications including catalysis, optical sensors and water filtration. Although several approaches already exist for the creation of nanoporous materials, the race for the development of versatile methods, more suitable for the nanoelectronics industry, is still ongoing. In this communication we report for the first time on the possibility of generating nanoporosity in silver nanocolumns using a dry approach based on the oxidation of silver by direct exposure to a commercially available radio-frequency air plasma. The silver nanocolumns are created by glancing angle deposition using magnetron sputtering of a silver target in pure argon plasma. We show that upon exposure to the rf air plasma, the nanocolumns transform from solid silver into nanoporous silver oxide. We further show that by tuning the plasma pressure and the exposure duration, the oxidation process can be finely adjusted allowing for precisely controlling the morphology and the nanoporosity of the silver oxide nanocolumns. The generation of porosity within the silver nanocolumns is explained according to a cracking-induced oxidation mechanism based on two repeated events occurring alternately during the oxidation process: (i) oxidation of silver upon exposure to the air plasma and (ii) generation of nanocracks and blisters within the oxide layer due to the high internal stress generated within the material during oxidation.

Yolk/shell nanoparticles: classifications, synthesis, properties, and applications

Core/shell nanoparticles were first reported in the early 1990s with a simple spherical core and shell structure, but the area is gradually diversifying in multiple directions such as different shapes, multishells, yolk/shell etc., because of the development of different new properties of the materials, which are useful for several advanced applications. Among different sub-areas of core/shell nanoparticles, yolk/shell nanoparticles (YS NPs) have drawn significant attention in recent years because of their unique properties such as low density, large surface area, ease of interior core functionalization, a good molecular loading capacity in the void space, tunable interstitial void space, and a hollow outer shell. The YS NPs have better properties over simple core/shell or hollow NPs in various fields including biomedical, catalysis, sensors, lithium batteries, adsorbents, DSSCs, microwave absorbers etc., mainly because of the presence of free void space, porous hollow shell, and free core surface. This review presents an extensive classification of YS NPs based on their structures and types of materials, along with synthesis strategies, properties, and applications with which one would be able to draw a complete picture of this area.

Synthesis of Hollow Nanotubes of Zn2SiO4 or SiO2: Mechanistic Understanding and Uranium Adsorption Behavior

We report a facile synthesis of Zn2SiO4 nanotubes using a two-step process consisting of a wet-chemical synthesis of core-shell ZnO@SiO2 nanorods followed by thermal annealing. While annealing in air leads to the formation of hollow Zn2SiO4, annealing under reducing atmosphere leads to the formation of SiO2 nanotubes. We rationalize the formation of the silicate phase at temperatures much lower than the temperatures reported in the literature based on the porous nature of the silica shell on the ZnO nanorods. We present results from in situ transmission electron microscopy experiments to clearly show void nucleation at the interface between ZnO and the silica shell and the growth of the silicate phase by the Kirkendall effect. The porous nature of the silica shell is also responsible for the etching of the ZnO leading to the formation of silica nanotubes under reducing conditions. Both the hollow silica and silicate nanotubes exhibit good uranium sorption at different ranges of pH making them possible candidates for nuclear waste management.

Nanorecycling: Monolithic Integration of Copper and Copper Oxide Nanowire Network Electrode through Selective Reversible Photothermochemical Reduction

Laser induced selective photothermochemical reduction is demonstrated to locally and reversibly control the oxidation state of Cu and Cu oxide nanowires in ambient conditions without any inert gas environment. This new concept of "nanorecycling" can monolithically integrate Cu and Cu oxide nanowires by restoring oxidized Cu, considered unusable for the electrode, back to a metallic state for repetitive reuse.

Controlling Disorder and Superconductivity in Titanium Oxynitride Nanoribbons with Anion Exchange

In recent years, conversion chemical reactions, which are driven by ion diffusion, emerged as an important concept for formation of nanoparticles. Here we demonstrate that the slow anion diffusion in anion exchange reactions can be efficiently used to tune the disorder strength and the related electronic properties of nanoparticles. This paradigm is applied to high-temperature formation of titanium oxynitride nanoribbons, Ti(O,N), transformed from hydrogen titanate nanoribbons in an ammonia atmosphere. The nitrogen content, which determines the chemical disorder through random O/N occupancy and ion vacancies in the Ti(O,N) composition, increases with the reaction time. The presence of disorder has paramount effects on resistivity of Ti(O,N) nanoribbons. Atypically for metals, the resistivity increases with decreasing temperature due to the weak localization effects. From this state, superconductivity develops below considerably or completely suppressed critical temperatures, depending on the disorder strength. Our results thus establish the remarkable versatility of anion exchange for tuning of the electronic properties of Ti(O,N) nanoribbons and suggest that similar strategies may be applied to a vast number of nanostructures.

Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I)

Postsynthetic chemical transformations of colloidal nanocrystals, such as ion-exchange reactions, provide an avenue to compositional fine-tuning or to otherwise inaccessible materials and morphologies. While cation-exchange is facile and commonplace, anion-exchange reactions have not received substantial deployment. Here we report fast, low-temperature, deliberately partial, or complete anion-exchange in highly luminescent semiconductor nanocrystals of cesium lead halide perovskites (CsPbX3, X = Cl, Br, I). By adjusting the halide ratios in the colloidal nanocrystal solution, the bright photoluminescence can be tuned over the entire visible spectral region (410-700 nm) while maintaining high quantum yields of 20-80% and narrow emission line widths of 10-40 nm (from blue to red). Furthermore, fast internanocrystal anion-exchange is demonstrated, leading to uniform CsPb(Cl/Br)3 or CsPb(Br/I)3 compositions simply by mixing CsPbCl3, CsPbBr3, and CsPbI3 nanocrystals in appropriate ratios.

The Kirkendall effect and nanoscience: hollow nanospheres and nanotubes

Hollow nanostructures are ranked among the top materials for applications in various modern technological areas including energy storage devices, catalyst, optics and sensors. The last years have witnessed increasing interest in the Kirkendall effect as a versatile route to fabricate hollow nanostructures with different shapes, compositions and functionalities. Although the conversion chemistry of nanostructures from solid to hollow has reached a very advanced maturity, there is still much to be discovered and learned on this effect. Here, the recent progress on the use of the Kirkendall effect to synthesize hollow nanospheres and nanotubes is reviewed with a special emphasis on the fundamental mechanisms occurring during such a conversion process. The discussion includes the oxidation of metal nanostructures (i.e., nanospheres and nanowires), which is an important process involving the Kirkendall effect. For nanospheres, the symmetrical and the asymmetrical mechanisms are both reviewed and compared on the basis of recent reports in the literature. For nanotubes, in addition to a summary of the conversion processes, the unusual effects observed in some particular cases (e.g., formation of segmented or bamboo-like nanotubes) are summarized and discussed. Finally, we conclude with a summary, where the prospective future direction of this research field is discussed.

Prospects of nanoscience with nanocrystals

Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today's strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenomena are constantly being discovered in the photophysics of NCs and in the electronic properties of NC solids. In this Nano Focus, we review the state of the art in research on colloidal NCs focusing on the most recent works published in the last 2 years.

Real-time imaging and elemental mapping of AgAu nanoparticle transformations

We report the controlled alloying, oxidation, and subsequent reduction of individual AgAu nanoparticles in the scanning transmission electron microscope (STEM). Through sequential application of electron beam induced oxidation and in situ heating and quenching, we demonstrate the transformation of Ag-Au core-shell nanoparticles into: AgAu alloyed, Au-Ag core-shell, hollow Au-Ag2O core-shell, and Au-Ag2O yolk-shell nanoparticles. We are able to directly image these morphological transformations in real-time at atomic resolution and perform energy dispersive X-ray (EDX) spectrum imaging to map changing elemental distributions with sub-nanometre resolution. By combining aberration corrected STEM imaging and high efficiency EDX spectroscopy we are able to quantify not only the growth and coalescence of Kirkendall voids during oxidation but also the compositional changes occurring during this reaction. This is the first time that it has been possible to track the changing distribution of elements in an individual nanoparticle undergoing oxidation driven shell growth and hollowing.

Nanoparticle conversion chemistry: Kirkendall effect, galvanic exchange, and anion exchange

Conversion chemistry is a rapidly maturing field, where chemical conversion of template nanoparticles (NPs) into new compositions is often accompanied by morphological changes, such as void formation. The principles and examples of three major classes of conversion chemical reactions are reviewed: the Kirkendall effect for metal NPs, galvanic exchange, and anion exchange, each of which can result in void formation in NPs. These reactions can be used to obtain complex structures that may not be attainable by other methods. During each kind of conversion chemical reaction, NPs undergo distinct chemical and morphological changes, and insights into the mechanisms of these reactions will allow for improved fine control and prediction of the structures of intermediates and products. Conversion of metal NPs into oxides, phosphides, sulphides, and selenides often occurs through the Kirkendall effect, where outward diffusion of metal atoms from the core is faster than inward diffusion of reactive species, resulting in void formation. In galvanic exchange reactions, metal NPs react with noble metal salts, where a redox reaction favours reduction and deposition of the noble metal (alloying) and oxidation and dissolution of the template metal (dealloying). In anion exchange reactions, addition of certain kinds of anions to solutions containing metal compound NPs drives anion exchange, which often results in significant morphological changes due to the large size of anions compared to cations. Conversion chemistry thus allows for the formation of NPs with complex compositions and structures, for which numerous applications are anticipated arising from their novel catalytic, electronic, optical, magnetic, and electrochemical properties.

Porous Spinel Zn(x)Co(3-x)O(4) hollow polyhedra templated for high-rate lithium-ion batteries

Nanostructured metal oxides with both anisotropic texture and hollow structures have attracted considerable attention with respect to improved electrochemical energy storage and enhanced catalytic activity. While synthetic strategies for the preparation of binary metal oxide hollow structures are well-established, the rational design and fabrication of complex ternary metal oxide with nonspherical hollow features is still a challenge. Herein, we report a simple and scalable strategy to fabricate highly symmetric porous ternary ZnxCo3-xO4 hollow polyhedra composed of nanosized building blocks, which involves a morphology-inherited and thermolysis-induced transformation of heterobimetallic zeolitic imidazolate frameworks. When tested as anode materials for lithium-ion batteries, these hollow polyhedra have exhibited excellent electrochemical performance with high reversible capacity, excellent cycling stability, and good rate capability.

Fabrication of polymer nanowires via maskless O2 plasma etching

In this paper, we introduce a simple fabrication technique which can pattern high-aspect-ratio polymer nanowire structures of photoresist films by using a maskless one-step oxygen plasma etching process. When carbon-based photoresist materials on silicon substrates are etched by oxygen plasma in a metallic etching chamber, nanoparticles such as antimony, aluminum, fluorine, silicon or their compound materials are self-generated and densely occupy the photoresist polymer surface. Such self-masking effects result in the formation of high-aspect-ratio vertical nanowire arrays of the polymer in the reactive ion etching mode without the necessity of any artificial etch mask. Nanowires fabricated by this technique have a diameter of less than 50 nm and an aspect ratio greater than 20. When such nanowires are fabricated on lithographically pre-patterned photoresist films, hierarchical and hybrid nanostructures of polymer are also conveniently attained. This simple and high-throughput fabrication technique for polymer nanostructures should pave the way to a wide range of applications such as in sensors, energy storage, optical devices and microfluidics systems.

Electron beam nanosculpting of Kirkendall oxide nanochannels

The nanomanipulation of metal nanoparticles inside oxide nanotubes, synthesized by means of the Kirkendall effect, is demonstrated. In this strategy, a focused electron beam, extracted from a transmission electron microscope source, is used to site-selectively heat the oxide material in order to generate and steer a metal ion diffusion flux inside the nanochannels. The metal ion flux generated inside the tube is a consequence of the reduction of the oxide phase occurring upon exposure to the e-beam. We further show that the directional migration of the metal ions inside the nanotubes can be achieved by locally tuning the chemistry and the morphology of the channel at the nanoscale. This allows sculpting organized metal nanoparticles inside the nanotubes with various sizes, shapes, and periodicities. This nanomanipulation technique is very promising since it enables creating unique nanostructures that, at present, cannot be produced by an alternative classical synthesis route.

Formation of bundle-shaped β-NaYF4 upconversion microtubes via Ostwald ripening

In this work, the uniform bundle-shaped microtubes composed of six half-pipes are synthesized for the first time in hydrothermal solutions via an intentional delayed phase transition pathway induced by Mn(2+) doping. The structural and kinetic factors that govern the phase and shape evolution of NaYF4 microcrystals have been carefully studied, and the influences of Mn(2+) to RE(3+) ratio, the amount of trisodium citrate, and the pH value in conjunction with the intrinsic character of RE(3+) ions on the phase and shape evolution are systematically discussed. It is found that the proper Mn(2+) to RE(3+) ratio is mainly responsible for delayed phase transition process and induces interior density gradient of solid aggregate for creating hollow bundle-shaped microtubes. While the amount of trisodium citrate and the pH value are the keys for the shape control of the NaYF4 microcrystals such as prismatic microtubes, prismatic short rods, thin plates, and particles. The up and downconversion emissions were obtained independent of whether α- or β-NaYF4:Er(3+)/Yb(3+) samples doped with Mn(2+), but the significant tuning of output color was only obtained in cube NaYF4 nanoparticles rather than in hexagonal microtubes via adjusting the amount of Mn(2+) ions. These spectral measurements and EDX analyses indicate that the distribution or concentration of Mn(2+) in hexagonal phase solid solution has changed, which supports Ostwald ripening growth mechanism and rules out agglomeration or oriented attachment growth mechanism. We designed crystal growth mode by simply addition of dopant may provide a versatile approach for fabricating a wide range of hollow nano/microcrystals and thus bring us a clearer understanding on the interaction between the dopant reagents and the nano/microcrystals.