Reversible and Cyclical Transformations between Solid and Hollow Nanostructures in Confined Reactions of Manganese Oxide and Silica within Nanosized Spheres

Stratum-Confined Solid-State Reaction (SC-SSR) toward Colloidal Silicon-Based Hollow Nanostructures for Bioapplications

Silicon nanostructures (SiNSs) can provide multifaceted bioapplications; but preserving their subhundred nm size during high-temperature silica-to-silicon conversion is the major bottleneck. The SC-SSR utilizes an interior metal-silicide stratum space at a predetermined radial distance inside silica nanosphere to guide the magnesiothermic reduction reaction (MTR)-mediated synthesis of hollow and porous SiNSs. In depth mechanistic study explores solid-to-hollow transformation encompassing predefined radial boundary through the participation of metal-silicide species directing the in-situ formed Si-phase accumulation within the narrow stratum. Evolving thin-porous Si-shell remains well protected by the in-situ segregated MgO emerging as a protective cast against the heat-induced deformation and interparticle sintering. Retrieved hydrophilic SiNSs (<100 nm) can be conveniently processed in different biomedia as colloidal solutions and endocytosized inside cells as photoluminescence (PL)-based bioimaging probes. Inside the cell, rattle-like SiNSs encapsulated with Pd nanocrystals can function as biorthogonal nanoreactors to catalyze intracellular synthesis of probe molecules through C-C cross coupling reaction.

Nanocrystal Conversion Chemistry within Slit-like 2D Nanogap for High-Rate Cyclic Stability of Lithium-Ion Battery Anodes

Nanoscale optimization of late transition-metal oxides for fixing the reversible lithiation/delithiation mechanism with an in-depth mechanistic understanding of nanocrystal (NC) conversion chemistry is important for furthering next-generation Li-ion battery (LIB) technologies. Herein, 1 nm-thin Ni₃CoO_x (1 nm-NCO) nanosheets synthesized through isomorphic transformation of NiCo layered double hydroxides within a two-dimensional (2D)-SiO₂ envelope are chosen. The interconversion of metal/metal-oxide NCs under redox-switching thermal treatment, while retaining reversibility, inspired the accomplishment of identical consequences under the harsh operational conditions of LIB redox cycles by application of the thin-NCO-defined 2D nanospace. During charge/discharge cycles, 1 nm-NCO covered with an in situ formed solid-electrolyte-interphase layer enables fully reversible interconversion between the reactive NC redox pairs, as evidenced by detailed morphological and electrochemical analyses, thus providing high-rate capability with a specific capacity of 61.2% at 5.0 C relative to 0.2 C, outstanding cycle stability delivering a reversible capacity of 1169 mAh g^-1, and 913 mAh g^-1 with high average Coulombic efficiency (>99.2%) at 3.0 and 5.0 C for 1000 cycles, respectively, which has not been achieved with other transition-metal oxides. Such a nanospace-confinement effect on sustainability of reactive NCs to follow-up a highly reversible conversion reaction at fast charging in LIBs is operative within a slit-like ultrathin 2D nanogap from 1 nm-NCO only, as a relatively thicker 7 nm-NCO anode, with accompanying larger space available, has evidenced poor reversibility of NCs and inadequate cyclic stability under potential high-power density LIB application.

Solid-State Reaction Synthesis of Nanoscale Materials: Strategies and Applications

Nanomaterials (NMs) with unique structures and compositions can give rise to exotic physicochemical properties and applications. Despite the advancement in solution-based methods, scalable access to a wide range of crystal phases and intricate compositions is still challenging. Solid-state reaction (SSR) syntheses have high potential owing to their flexibility toward multielemental phases under feasibly high temperatures and solvent-free conditions as well as their scalability and simplicity. Controlling the nanoscale features through SSRs demands a strategic nanospace-confinement approach due to the risk of heat-induced reshaping and sintering. Here, we describe advanced SSR strategies for NM synthesis, focusing on mechanistic insights, novel nanoscale phenomena, and underlying principles using a series of examples under different categories. After introducing the history of classical SSRs, key theories, and definitions central to the topic, we categorize various modern SSR strategies based on the surrounding solid-state media used for nanostructure growth, conversion, and migration under nanospace or dimensional confinement. This comprehensive review will advance the quest for new materials design, synthesis, and applications.

Silica-Enveloped 2D-Sheet-to-Nanocrystals Conversion for Resilient Catalytic Dry Reforming of Methane

Here, lamellar confinement strategy is introduced for "sheet-to-nanocrystals (NCs)" conversion within a 2D-SiO₂ envelope, which constructs a catalytic nanocartridge holding a platoon of isolated and in-plane-aligned ultrasmall Ni-NCs, performing as a robust and coking-resistant catalytic system for dry reforming of methane. Overcoming the problem of unavoidable bulk crystal growth from multiple sheets-stack or sheet-on-open-support, silica bilayer-encasing tightly clamps the atomic-thin Ni(OH)₂ -nanosheet during thermal conversion and further hinders the migratory fusion of the resultant Ni-NCs. Upon heating-cooling cycle, the flapping silica envelope clutches the Ni-NCs like "eggs in a carton," subsequently, ensuring their thermal stability. Owing to the unique 2D-enveloped rigid architecture, Ni-NCs can circumvent sintering and coke deposition while tolerating the high temperatures (>700 °C) for long operation (>100 h), affording high conversions to syngas.

Silica Jar-with-Lid as Chemo-Enzymatic Nano-Compartment for Enantioselective Synthesis inside Living Cells

Nanodevices, harvesting the power of synthetic catalysts and enzymes to perform enantioselective synthesis inside cell, have never been reported. Here, we synthesized round bottom jar-like silica nanostructures (SiJARs) with a chemo-responsive metal-silicate lid. This was isolated as an intermediate structure during highly controlled solid-state nanocrystal-conversion at the arc-section of silica shell. Different catalytic noble metals (Pt, Pd, Ru) were selectively modified on the lid-section through galvanic reactions. And, lid aperture-opening was regulated by mild acidic conditions or intracellular environment which accommodated the metal nanocrystals and enzymes, and in turn created an open-mouth nanoreactor. Distinct from the free enzymes, SiJARs performed asymmetric aldol reactions with high activity and enantioselectivity (yield >99 %, ee=95 %) and also functioned as the artificial catalytic organelles inside living cells. This work bridges the enormous potential of sophisticated nanocrystal-conversion chemistry and advanced platforms for new-to-nature catalysis.

Intermetallic PtCu Nanoframes as Efficient Oxygen Reduction Electrocatalysts

Nanoframe alloy structures represent a class of high-performance catalysts for the oxygen reduction reaction (ORR), owing to their high active surface area, efficient molecular accessibility, and nanoconfinement effect. However, structural and chemical instabilities of nanoframes remain an important challenge. Here, we report the synthesis of PtCu nanoframes constructed with an atomically ordered intermetallic structure (O-PtCuNF/C) showing high ORR activity, durability, and chemical stability. We rationally designed the O-PtCuNF/C catalyst by combining theoretical composition predictions with a silica-coating-mediated synthesis. The O-PtCuNF/C combines intensified strain and ligand effects from the intermetallic PtCu L1₁ structure and advantages of the nanoframes, resulting in superior ORR activity to disordered alloy PtCu nanoframes (D-PtCuNF/C) and commercial Pt/C catalysts. Importantly, the O-PtCuNF/C showed the highest ORR mass activity among PtCu-based catalysts. Furthermore, the O-PtCuNF/C exhibited higher ORR durability and far less etching of constituent atoms than D-PtCuNF/C and Pt/C, attesting to the chemically stable nature of the intermetallic structure.

Diffusive Formation of Hollow Mesoporous Silica Shells from Core-Shell Composites: Insights from the Hydrogen Sulfide Capture Cycle of CuO@mSiO2 Nanoparticles

Mesoporous silica is often employed as a coating material in core-shell nanoparticles to decrease the possibility of sintering or aggregation of the core particles. In this work, we discovered a surprising morphological transformation during the sulfidation and regeneration (oxidation) of core-shell CuO@mSiO₂ materials designed for H₂S capture. Although CuS cores were still encapsulated within the silica shells after in situ sulfidation, hollow silica shells formed during the regeneration step as CuO leached out of the shell and aggregated into larger particles. The successful sulfidation of pristine CuO@mSiO₂ was facilitated by the restraining effect of silica shells on lattice growth from CuO into CuS, and the mesopores allowed for volume expansion. The phase and morphology changes during the regeneration (oxidation) process leading to the hollow shells were investigated by X-ray diffraction and transmission electron microscopy. It was observed that the cores remained encaged during the disproportionation of CuS to Cu₂S, which is the first step in the oxidation of CuS. However, voids were generated when Cu₂S was oxidized and reacted with water generated from the condensation of silica. A possible mechanism for this transformation involves the outward diffusion of copper ions through the mesoporous silica, leading to the migration of core particles. This migration was further accelerated by the elevated temperature in the regeneration process and promoted by the formation of the copper sulfate hydroxide through the reaction with water. This work provides key insights into the chemical stability of such core-shell structures under the influence of diffusion-driven structural transformations.

Controlled Synthesis of Manganese Oxide Nanoparticles Encaged in Hollow Mesoporous Silica Nanoreactors and Their Enhanced Dye Degradation Activity

In this study, controlled synthesis of hollow mesoporous silica nanoreactors with small manganese oxide nanoparticles in their cavities (Mn _x O _y @HMSNs) is reported, and the dye degradation performance in the presence of hydrogen peroxide over Mn _x O _y @HMSNs is investigated. Specifically, triple ligands (a compound with three dipicolinic acid groups) were used to coordinate manganese ions to form negatively charged coordination complex networks, which further combine with positively charged copolymers to obtain metal ion-containing polymer micelles. Following silica deposition onto micellar coronas and calcinations simultaneously result in hollow mesoporous silica nanoreactors and manganese oxide nanoparticles in their cavities. In this work, the influences of synthetic parameters on the structures are studied in detail. The obtained Mn _x O _y @HMSNs show greatly enhanced activity and stability for a series of dye degradations. The performance enhancement is ascribed to their unique nanostructures, where mesoporous silica walls provide protection to the inner Mn _x O _y nanoparticles and the small size of the manganese oxide nanoparticles greatly enhances the dye degradation activity.

A new etching process for zinc oxide with etching rate and crystal plane control: experiment, calculation, and membrane application

The etching process can be a useful method for the morphology control of nanostructures. Using precise experiments and theoretical calculations, we report a new ZnO etching process triggered by the reaction of ZnO with transition metal cations and demonstrate that the etching rate and direction could be controlled by varying the kind of transition metal cation. In addition, the developed etching process was introduced to form a thin and uniform ZnO layer, which was utilized for the fabrication of an improved propylene-selective ZIF-8 membrane via conversion seeding and secondary growth.

Spatially Confined Formation and Transformation of Nanocrystals within Nanometer-Sized Reaction Media

The extensive research performed in the past two decades has enabled the production of a range of colloidal nanocrystals, mostly through solution-based procedures that generate and transform nanostructures in bulk-phase solutions containing precursors and surfactants. However, the understanding and control of each nanocrystal (trans)formation step during the synthesis are still complicated because of the high complexity of this process, in which multiple diverse events such as nucleation, subsequent growth, attachment, and ripening occur simultaneously in bulk suspensions. Unlike well-established solution-based methods, solid-state reactions, which had been at the forefront of traditional inorganic materials chemistry, are quite rarely utilized in the realm of nanomaterials because of the high temperatures required for most solid-state reactions, as a result of which the clusters and NCs are prone to migrate through the bulk reaction medium and sinter together uncontrollably into large particles. We have been pursuing the "nanospace-confined approach" to explore the use of a variety of solid and hollow silica nanoparticles as either solid-state or solution-phase reaction media to carry out the syntheses and transformations of nanocrystals in a unique microenvironment, partitioning the reactants, intermediates, and transition states from the rest of the bulk reaction medium. Such nanoconfined systems have the potential not only to enable efficient and selective nanocrystal conversion chemistries but also to provide fundamental understanding pertaining to the synthetic evolution of nanostructures and transient mechanistic steps. The unique spaces with sizes of a few tens of nanometers inside nanoconfined systems offer the opportunity to observe and elucidate novel deconvoluted chemical phenomena that are impossible to investigate in bulk systems, and comprehensive understanding of nanoconfined chemistry can be implicated in explaining and controlling the macroscopic chemical behaviors. This Account describes our focused research on developing spatially confined platforms for nanocrystal syntheses and transformations, highlighting our diversity-oriented strategy, namely, the "postdecoration approach", which results in the evolution of new nanocatalytic sites in a preformed cavity for diversifying and controlling their morphologies, number, density and combinations. We discuss key examples of the "nanoconfined solid-state conversion approach" that involve novel reactions of nanocrystals within thermally stable solid silica nanospheres to synthesize and transform complex hybrid nanocrystals with increased complexity and functionality. In addition, an enlightening discussion of the examples of nanocrystal syntheses and conversions in nanoconfined solutions inside enclosed and exposed cavities of silica nanospheres is included. Finally, the important applications of nanospace-confined systems in various fields are also briefly discussed.

A Generic Method for Preparing Hollow Mesoporous Silica Catalytic Nanoreactors with Metal Oxide Nanoparticles inside Their Cavities

We report a facile and generic method for the synthesis of hollow mesoporous silica nanoreactors (HMSNs) with small-sized metal oxide nanoparticles (NPs) inside their cavities. They were made by deposition of silica onto metal-containing charge-driven polymer micelles and subsequent calcination. The micelles consist of 1) negatively charged supramolecular polyelectrolyte chains of bis-ligand-bound metal ions, and 2) water-soluble, neutral/positive diblock copolymers. Owing to the facile coordination between transition-metal ion and the employed bidentate ligand, a series of HMSNs with <2 nm M_x O_y NPs inside cavities (M=Mn, Co, Ni, Cu, or Zn) were obtained by simply varying the metal ions inside the micelles. The developed method circumvents the pre- and post-synthesis of metal oxide NPs; after calcination, hollow mesoporous nanostructures containing small-sized metal oxide NPs inside their cavities are directly obtained. The Co_x O_y -functionalized HMSNs catalyze the degradation of various dyes with H₂ O₂ .

Monofacet-Selective Cavitation within Solid-State Silica-Nanoconfinement toward Janus Iron Oxide Nanocube

Here, a highly selective solid-state nanocrystal conversion strategy is developed toward concave iron oxide (Fe₃O₄) nanocube with an open-mouthed cavity engraved exclusively on a single face. The strategy is based on a novel heat-induced nanospace-confined domino-type migration of Fe²⁺ ions from the SiO₂-Fe₃O₄ interface toward the surrounding silica shell and concomitant self-limiting nanoscale phase-transition to the Fe-silicate form. Equipped with the chemically unique cavity, the produced Janus-type concave iron oxide nanocube was further functionalized with controllable density of catalytic Pt-nanocrystals exclusively on concave sites and utilized as a highly diffusive catalytic Janus nanoswimmer for the efficient degradation of pollutant-dyes in water.

Colloids of Holey Gd2 O3 Nanosheets Converted from Exfoliated Gadolinium Hydroxide Layers

This paper proposes a confined solid-state conversion approach using layered metal-hydroxides for the production of a colloidal suspension of porous 2D crystalline metal oxide layers with superior electrochemical H₂ O₂ sensing performance. This study investigates the conversion chemistry of delaminated layers of gadolinium hydroxide (LGdH), [Gd₂ (OH)₅ ]⁺ , encapsulated in a silica nanoshell that provides an antistacking and antisintering environment during the phase-transition at high temperature. Thermal treatment of the LGdH layers within the protected environment results in a dimensionally confined phase-transition into crystalline Gd₂ O₃ nanosheets with an isomorphic 2D structure. Furthermore, annealing at higher temperatures leads to the evolution of in-plane mesoporous structure on the Gd₂ O₃ nanosheet. Based on insight acquired from in-depth investigation, the evolution of in-plane porosity proceeds through the in-plane dominant silicate-formation reaction at the interface with the surrounding silica shell. Their 2D-anisotropic and mesoporous morphological features are preserved, producing a colloidal suspension of holey nanosheets that can be used to fabricate a thin and porous film through wet-coating deposition. This study also demonstrates the superior electrochemical H₂ O₂ sensing ability of the resultant porous Gd₂ O₃ film, which represents a ≈1000- and 10-fold enhancement of the detection limit and sensitivity, respectively, in comparison to previously reported Gd₂ O₃ films.

General Synthetic Strategy for Hollow Hybrid Microspheres through a Progressive Inward Crystallization Process

Hollow hybrid microspheres have found great potential in different areas, such as drug delivery, nanoreactors, photonics, and lithium-ion batteries. Here, we report a simple and scalable approach to construct high-quality hollow hybrid microspheres through a previously unexplored growth mechanism. Starting from uniform solid microspheres with low crystallinity, we identified that a hollowing process can happen through the progressive inward crystallization process initiated on the particle surface: the gradual encroachment of the crystallization frontline toward the core leads to the depletion of the center and forms the central cavity. We showed that such a synthetic platform was versatile and can be applicable for a large variety of materials. By using the production of Li4Ti5O12-carbon hollow hybrid microspheres as an example, we demonstrated that high-performance anode materials could be achieved through synthesis and structure control. We expect that our findings offer new perspectives in different areas ranging from materials chemistry, energy storage devices, catalysis, to drug delivery.

A waxberry-like SiO2@MnSiO3 core–shell nanocomposite synthesized via a simple solvothermal self-template method and its potential in catalytic degradation and heavy metal ion removal

A novel waxberry-like SiO₂@MnSiO₃ core–shell nanocomposite was facilely fabricated via the simple one-step thermal treatment of SiO₂ nanospheres, MnCl₂·4H₂O, ethylenediamine (EDA), and ethylene glycol (EG).

Solid-State Conversion Chemistry of Multicomponent Nanocrystals Cast in a Hollow Silica Nanosphere: Morphology-Controlled Syntheses of Hybrid Nanocrystals

During thermal transformation of multicomponent nanocrystals in a silica nanosphere, FeAuPd alloy nanocrystals migrate outward and thereby leave a cavity in the silica matrix. Oxidation then converts these nanocrystals back into phase-segregated hybrid nanocrystals, AuPd@Fe3O4, with various morphologies. The FeAuPd-to-AuPd@Fe3O4 transformation was cast by the in situ generated hollow silica mold. Therefore, the morphological parameters of the transformed AuPd@Fe3O4 are defined by the degree of migration of the FeAuPd in the hollow silica nanoshell. This hollow silica-cast nanocrystal conversion was studied to develop a solid state protocol that can be used to produce a range of hybrid nanocrystals and that allows for systematic and sophisticated control of the resulting morphologies.

Fabrication of Supported AuPt Alloy Nanocrystals with Enhanced Electrocatalytic Activity for Formic Acid Oxidation through Conversion Chemistry of Layer-Deposited Pt(2+) on Au Nanocrystals

The exploitation of nanoconfined conversion of Au- and Pt-containing binary nanocrystals for developing a controllable synthesis of surfactant-free AuPt nanocrystals with enhanced formic acid oxidation (FAO) activity is reported, which can be stably and evenly immobilized on various support materials to diversify and optimize their electrocatalytic performance. In this study, an atomic layer of Pt(2+) species is discovered to be spontaneously deposited in situ on the Au nanocrystal generated from a reverse-microemulsion solution. The resulting Au/Pt(2+) nanocrystal thermally transforms into a reduced AuPt alloy nanocrystal during the subsequent solid-state conversion process within the SiO2 nanosphere. The alloy nanocrystals can be isolated from SiO2 in a surfactant-free form and then dispersedly loaded on the carbon sphere surface, allowing for the production of a supported electrocatalyst that exhibits much higher FAO activity than commercial Pt/C catalysts. Furthermore, by involving Fe3O4 nanocrystals in the conversion process, the AuPt alloy nanocrystals can be grown on the oxide surface, improving the durability of supported metal catalysts, and then uniformly loaded on a reduced graphene oxide (RGO) layer with high electroconductivity. This produces electrocatalytic AuPt/Fe3O4/RGO nanocomposites whose catalyst-oxide-graphene triple-junction structure provides improved electrocatalytic properties in terms of both activity and durability in catalyzing FAO.

Mechanistic Insight into the Yolk@Shell Transformation of MnO@Silica Nanospheres Incorporating Ni(2+) Ions toward a Colloidal Hollow Nanoreactor

As an effort to develop a simple and versatile synthetic strategy that contributes to the evolution of hollow nanostructures with increasing complexity and functionality, this research is devoted to study the hollow transformation within a nanosized solid matrix. Through an in-depth investigation of a hollowing process of MnO nanocrystals confined within a Ni(2+) incorporating silica nanosphere, a very distinct transformation pathway can be explained that produces the yolk@shell nanostructure with a single Ni nanocrystal inside a silicate nanoshell. The yolk@shell structure is developed by a mechanism combining different processes, including the formation of a (Ni0.1 Mn0.9 )O mixed-metal oxide and subsequent segregation of the reduced Ni. Furthermore, this study also devises a protocol to exploit the solid-state-synthesized powder for fabricating a colloidal hollow nanoreactor that can selectively catalyze the reduction of nitroarenes and be recycled via the magnetic process.

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.

Hollow and concave nanoparticles via preferential oxidation of the core in colloidal core/shell nanocrystals

Hollow and concave nanocrystals find applications in many fields, and their fabrication can follow different possible mechanisms. We report a new route to these nanostructures that exploits the oxidation of Cu(2-x)Se/Cu(2-x)S core/shell nanocrystals with various etchants. Even though the Cu(2-x)Se core is encased in a thick Cu(2-x)S shell, the initial effect of oxidation is the creation of a void in the core. This is rationalized in terms of diffusion of Cu(+) ions and electrons from the core to the shell (and from there to the solution). Differently from the classical Kirkendall effect, which entails an imbalance between in-diffusion and out-diffusion of two different species across an interface, the present mechanism can be considered as a limiting case of such effect and is triggered by the stronger tendency of Cu(2-x)Se over Cu(2-x)S toward oxidation and by fast Cu(+) diffusion in copper chalcogenides. As the oxidation progresses, expansion of the inner void erodes the entire Cu(2-x)Se core, accompanied by etching and partial collapse of the shell, yielding Cu(2-x)S(y)Se(1-y) concave particles.

Assembling metal oxide nanocrystals into dense, hollow, porous nanoparticles for lithium-ion and lithium-oxygen battery application

New dense hollow porous (DHP) metal oxide nanoparticles that are smaller than 100 nm and composed of Co3O4, FeOx, NiO and MnOx were prepared by densely assembling metal oxide nanocrystals based on the hard-template method using a carbon colloid as a sacrificial core. These nanoparticles are quite different from the traditional particles as their hollow interior originates from the stacking of nanocrystals rather than a spherical shell. The DHP nanoparticles preserve the intriguing properties of nanocrystals and possess desirable surface area and pore volume that enhance the active surface, which ultimately benefits applications such as lithium-ion batteries. The DHP Co3O4 nanoparticles demonstrated an enhanced capacity of 1168 mA h g(-1) at 100 mA g(-1)vs. 590 mA h g(-1) of powders and stable cycling performance greater than 250 cycles when used as an anode material. Most importantly, the electrochemical performance of DHP Co3O4 nanoparticles in a lithium-O2 battery was also investigated for the first time. A low charge potential of ∼4.0 V, a high discharge voltage near 2.74 V and a long cycle ability greater than 100 cycles at a delivered capacity of 2000 mA h g(-1) (current density, 200 mA g(-1)) were observed. The performances were considerably improved compared to recent results of mesoporous Co3O4, Co3O4 nanoparticles and a composite of Co3O4/RGO and Co3O4/Pd. Therefore, it would be promising to investigate such properties of DHP nanoparticles or other hollow metal (oxide) particles for the popular lithium-air battery.