Crystallographic Faces Research Articles

Surface Hydroxyl OH Defects of η-Al2O3 and χ-Al2O3 by Solid State NMR, XRD, and DFT Calculations

Abstract For the first time, the detailed structure of χ-Al2O3 and η-Al2O3 surface has been established by implementing the NMR crystallography approach. The surface of η-Al2O3 has been demonstrated to be formed primarily by the (111) facets, while the χ-Al2O3 surface is a combination of (111) and (110) facets. This observation supports the block model of aluminum oxides previously proposed by Tsybulya and Kryukova [S. V. Tsybulya, G. N. Kryukova, Phys. Rev. B 77 (2008) 024112.]. The additional terminal OH groups, observed experimentally and not contributing to (111) and (110) theoretical calculations, are considered to be bonded to the tetrahedral aluminum sites. Their origin is related to the junctions of crystallographic faces of spinel building blocks, being a part of discussed model. Higher content of these terminal OH groups in χ-Al2O3 is a result of more junctions in the case of its more mosaic structure compared to η-Al2O3.

Three-layer model for the surface second-harmonic generation yield including multiple reflections

We present the three-layer model to calculate the surface second-harmonic generation (SSHG) yield. This model considers that the surface is represented by three regions or layers. The first layer is the vacuum region with a dielectric function ${\ensuremath{\epsilon}}_{v}(\ensuremath{\omega})=1$ from where the fundamental electric field impinges on the material. The second layer is a thin layer $(\ensuremath{\ell})$ of thickness $d$ characterized by a dielectric function ${\ensuremath{\epsilon}}_{\ensuremath{\ell}}(\ensuremath{\omega})$, and it is in this layer where the SSHG takes place. The third layer is the bulk region denoted by $b$ and characterized by ${\ensuremath{\epsilon}}_{b}(\ensuremath{\omega})$. Both the vacuum and bulk layers are semi-infinite. The model includes the multiple reflections of both the fundamental and the second-harmonic (SH) fields that take place at the thin layer $\ensuremath{\ell}$. We obtain explicit expressions for the SSHG yield for the commonly used $s$ and $p$ polarizations of the incoming $1\ensuremath{\omega}$ and outgoing $2\ensuremath{\omega}$ electric fields, where no assumptions for the symmetry of the surface are made. These symmetry assumptions ultimately determine which components of the surface nonlinear second-order susceptibility tensor $\mathbf{\ensuremath{\chi}}(\ensuremath{-}2\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega})$ are different from zero, and thus contribute to the SSHG yield. Then, we particularize the results for the most commonly investigated surfaces, the (001), (110), and (111) crystallographic faces, taking their symmetries into account. We use the three-layer model and compare it against the experimental results of a Si(111)$(1\ifmmode\times\else\texttimes\fi{}1)$:H surface, as a test case, and use it to predict the SSHG yield of a Si(001)$(2\ifmmode\times\else\texttimes\fi{}1)$ surface.

Parametric study on vapor-solid-solid growth mechanism of multiwalled carbon nanotubes

This study aimed at investigating the effect of the fluidized bed chemical vapor deposition (FBCVD) process parameters on growth mechanism, morphology and purity of the multiwalled carbon nanotubes (MWCNTs). Nanotubes were produced in a vertical FBCVD reactor by catalytic decomposition of ethylene over Al2O3 supported nano-iron catalyst buds at different flow rates. FESEM, TEM, Raman spectroscopy and TGA thermograms were used to elaborate the growth parameters of the as grown MWCNTs. As the growth process was driven by the process temperatures well below the iron-carbon eutectic temperature (1147 °C), the appearance of graphite platelets from the crystallographic faces of the catalyst particles suggested a solid form of the catalyst during CNT nucleation. A vapor-solid-solid (VSS) growth mechanism was predicted for nucleation of MWCNTs with very low activation energy. The nanotubes grown at optimized temperature and ethylene flow rate posed high graphitic symmetry, purity, narrow diameter distribution and shorter inter-layer spacing. In Raman and TGA analyses, small ID/IG ratio and residual mass revealed negligible ratios of structural defects and amorphous carbon in the product. However, several structural defects and impurity elements were spotted in the nanotubes grown under unoptimized process parameters.

Deconstructing the Heterogeneity of Surface-Bound Catalysts: Rutile Surface Structure Affects Molecular Properties

Catalytic interfaces involving surface-bound molecular catalysts often exhibit a large structural heterogeneity from uncontrolled variation in surface morphology. Conventional spectroscopic techniques typically average over these different structural motifs within the sample, making it difficult to link the underlying surface morphology to the properties of the immobilized catalyst. Here we present the first direct comparison of the vibrational dynamics of a CO2 reduction catalyst bound to two different single-crystalline TiO2 surfaces, rutile (001) and (110), probed with transient surface-specific sum-frequency generation spectroscopy. We find that the change in surface structure between crystallographic faces alters both the vibrational frequency and relaxation time of the symmetric carbonyl stretching mode of the catalyst, with (001) displaying a lower frequency and longer relaxation time. This results from a change in the catalyst electronic structure and indicates that the molecular properties of the catalyst, likely including the catalytic properties, depend on the specific TiO2 surface to which it is bound. The comparison of the molecular properties on these two single crystal surfaces is an essential step toward understanding how semiconductor surface structure influences catalyst behavior and identifying optimal surface structures for improved catalytic performance.

H2 and N2 Remote Plasma Processing of Wurtzite‐Like Oxides: Implications for Energy Applications

Remote plasma processing is one of the most effective methods for the manipulation of intrinsic defects, doping, stoichiometry, and morphology of a large variety of oxides. Here, we discuss the interaction of polar ZnO and of LiGaO2 and LiAlO2 with atomic hydrogen and nitrogen produced by remote H2 and N2 plasmas. We show that the various crystallographic faces of those wurtzite‐like oxides are characterized by a different reactivity to hydrogen and nitrogen. Specifically, we demonstrate that the O‐polar ZnO is the most stable surface to hydrogen and, therefore, it should be the one of interest to preserve electrode stability in all those applications involving hydrogen. Conversely, we found that H2 remote plasma processing does not significantly alter Li‐based oxides LiGaO2 and LiAlO2, while their nitridation can be exploited to alter the Li composition and mobility.

Thin Water Films at Multifaceted Hematite Particle Surfaces.

Mineral surfaces exposed to moist air stabilize nanometer- to micrometer-thick water films. This study resolves the nature of thin water film formation at multifaceted hematite (α-Fe2O3) nanoparticle surfaces with crystallographic faces resolved by selected area electron diffraction. Dynamic vapor adsorption (DVA) in the 0-19 Torr range at 298 K showed that these particles stabilize water films consisting of up to 4-5 monolayers. Modeling of these data predicts water loadings in terms of an "adsorption regime" (up to 16 H2O/nm(2)) involving direct water binding to hematite surface sites, and of a "condensation regime" (up to 34 H2O/nm(2)) involving water binding to hematite-bound water nanoclusters. Vibration spectroscopy identified the predominant hematite surface hydroxo groups (-OH, μ-OH, μ3-OH) through which first layer water molecules formed hydrogen bonds, as well as surface iron sites directly coordinating water molecules (i.e., as geminal η-(OH2)2 sites). Chemometric analyses of the vibration spectra also revealed a strong correspondence in the response of hematite surface hydroxo groups to DVA-derived water loadings. These findings point to a near-saturation of the hydrogen-bonding environment of surface hydroxo groups at a partial water vapor pressure of ∼8 Torr (∼40% relative humidity). Classical molecular dynamics (MD) resolved the interfacial water structures and hydrogen bonding populations at five representative crystallographic faces expressed in these nanoparticles. Simulations of single oriented slabs underscored the individual roles of all (hydro)oxo groups in donating and accepting hydrogen bonds with first layer water in the "adsorption regime". These analyses pointed to the preponderance of hydrogen bond-donating -OH groups in the stabilization of thin water films. Contributions of μ-OH and μ3-OH groups are secondary, yet remain essential in the stabilization of thin water films. MD simulations also helped resolve crystallographic controls on water-water interactions occurring in the "condensation regime". Water-water hydrogen bond populations are greatest on the (001) face, and decrease in importance in the order (001) > (012) ≈ (110) > (014) ≫ (100). Simulations of a single (∼5 nm × ∼ 6 nm × ∼ 6 nm) nanometric hematite particle terminated by the (001), (110), (012), and (100) faces also highlighted the key roles that sites at particle edges play in interconnecting thin water films grown along contiguous crystallographic faces. Hydroxo-water hydrogen bond populations showed that edges were the preferential loci of binding. These simulations also suggested that equilibration times for water binding at edges were slower than on crystallographic faces. In this regard, edges, and by extension roughened surfaces, are expected to play commanding roles in the stabilization of thin water films. Thus, in focusing on the properties of nanometric-thick water layers at hematite surfaces, this study revealed the nature of interactions between water and multifaced particle surfaces. Our results pave the way for furthering our understanding of mineral-thin water film interfacial structure and reactivity on a broader range of materials.

Anisotropy in surface-initiated melting of the triclinic molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene: A molecular dynamics study.

Surface-initiated melting of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), a triclinic molecular crystal, was investigated using molecular dynamics simulations. Simulations were performed for the three principal crystallographic planes exposed to vacuum, with the normal vectors to the planes given by b × c, c × a, and a × b (where a, b, and c define the edge vectors of the unit cell), denoted as (100), (010), and (001), respectively. The best estimate of the normal melting temperature for TATB is 851 ± 5 K. The nature and extent of disordering of the crystal-vacuum interface depend on the exposed crystallographic face, with the (001) face exhibiting incomplete melting and superheating. This is attributed to the anisotropy of the inter-molecular hydrogen bonding and the propensity of the crystal to form stacking faults in directions approximately perpendicular to the (100) and (010) faces. For all three crystal orientations, formation of molecular vacancies in the lattice at the crystal-vacuum (or crystal-quasi-liquid layer) interface precedes the complete loss of order at the interface.

The ash heap of crystallography: restoring forgotten basic knowledge

A critical analysis of some basic notions often overlooked in crystallographic education is presented to correct some common oversights occurring both in the literature and in textbooks. The crystal forms (face forms), defined in terms of their geometric eigensymmetry, are 47 in number, not 48 as often found in the literature. The split of the dihedron into dome and sphenoid calls for the consideration of the physical properties of the faces building a form; in that case, however, the same criterion should be used for all forms. By taking the handedness of the faces as representative of the physical properties of the faces, the occurrence of 130 crystallographic face forms (97 affine face forms and 33 enantiomorphic pairs) is demonstrated. Next, the correct use of non-coprime Miller indices when a centred unit cell is adopted is shown, and the inconsistent multiplication of Miller indices in the Bravais–Friedel–Donnay–Harker law is pointed out. A geometric derivation of the reflection conditions is reviewed. Finally, the inconsistent presentation of metric restrictions imposed by the structural symmetry is pointed out and corrected.

Determination of Vickers microhardness in β-Ga2O3 single crystals grown from their own melt

The results of microhardness measurements of β-Ga2O3 single crystals for (001) crystallographic face are reported. The crystals were grown by the free crystallization with the Garnet-2M equipment. Microhardness values were determined by the Vickers method at varying loads. A four-sided diamond pyramid was used as an indenter. The average value of gallium oxide microhardness was equal to 8.91 GPa. We have carried out comparison of the values obtained with the microhardness for the other wide bandgap semiconductors epitaxial GaN layers grown on 6H-SiC and GaP layers grown on GaP:S. The findings are usable for machining process development of β-Ga2O3 single crystal substrates. In particular, silicon carbide and electrocorundum may be recommended for β-Ga2O3 machine processing.

Morphological control of TiO2 anatase nanoparticles: What is the good surface property to obtain efficient photocatalysts?

The richness of titanium dioxide sol–gel syntheses described in literature provided a set of four different morphologies of pure anatase nanoparticles to study the impact of the exposed surfaces on the photocatalytic efficiency of the corresponding material. The selection of the experimental parameters such as the temperature, the heating method or organic additives allowed the synthesis of pure anatase materials with significantly different shapes. A thorough microscopic study of these particles gave the exposed crystallographic faces. The photocatalytic activity of the different materials was estimated following the degradation of the rhodamine B dye under UV-light and significantly different behaviors were observed. In the applied photodegradation conditions, two samples were shown to be more efficient than the reference photocatalyst P25. The rationalization of these results was done through the study of the oxide surface properties, using FT-IR spectroscopy with pyridine as a surface probe and the EPR analysis of photogenerated radicals under UV light. The most efficient photocatalyst for rhodamine B degradation was found to be the morphology presenting the stronger acidic surface sites.

Towards an Atomic-Scale Understanding of the Adsorbate-Driven Formation of High-Index Faces

Preferential adsorption takes place at the step ledge between adjacent crystal faces, which usually serve as active sites for breaking chemical bonds. In this paper, we present a structural model to interpret the habit modification of single crystals in terms of the step geometries relationship between crystal faces. A new series of high index faces parallel to the ledge between adjacent facets can be explicitly determined from the presence of the symmetry operators in the space group. The relative stability of these new faces undergoes a faceting transition, driven by the adsorbate-induced changes of the step configuration. Combined with the chemical bond-geometric approach, our predictions accurately reproduce the tapering evolutions of KDP crystals in the present of metallic ions. The current work provides a new insight on how changes affecting elementary steps on one face are translated into the emergence of a new crystallographic face.

Copper nanoparticles of well-controlled size and shape: a new advance in synthesis and self-organization.

Here, we report a new synthetic route for spherical small copper nanoparticles (CuNPs) with size ranging from 3.5 nm to 11 nm and with an unprecedented associated monodispersity (<10%). This synthesis is based on the reduction of an organometallic precursor (CuCl(PPh3)3) by tert-butylamine borane in the presence of dodecylamine (DDA) at a moderate temperature (50 to 100 °C). Because of their narrow size distribution, the CuNPs form long-range 2D organizations (several μm(2)). The wide range of CuNPs sizes is obtained by controlling the reaction temperature and DDA-to-copper phosphine salt ratio during the synthesis process. The addition of oleic acid (OA) after the synthesis stabilizes the CuNPs (no coalescence) for several weeks under a nitrogen atmosphere. The nature and the reactivity of the ligands were studied by IR and UV-visible spectroscopy. We thus show that just after synthesis the nanoparticles are coated by phosphine and DDA. After adding OA, a clear exchange between phosphine and OA is evidenced. This exchange is possible thanks to an acid-base reaction between the free alkylamine in excess in the solution and OA. OA is then adsorbed on the NPs surface in the form of carboxylate. Furthermore, the use of oleylamine (OYA) instead of DDA as the capping agent allows one to obtain other NP shapes (nanorods, triangles and nanodisks). We get evidence that OYA allows the selective adsorption of chloride ions derived from the copper precursor on the different crystallographic faces during the growth of CuNPs that induces the formation of anisotropic shapes such nanorods or triangles.

InGaAs (110) Surface Cleaning Using Atomic Hydrogen

A major obstacle facing III-V semiconductor based metal oxide semiconductor field effect transistors (MOSFETs) is the large density of trap states that exist at the semiconductor/oxide interface.[1] These trap states can pin the Fermi level preventing the MOSFET from acting as a switch in logic devices. Several sources of Fermi level pinning have been proposed including oxidation of the III-V substrate.[2, 3] In order to minimize the presence of III-V oxides it is crucial to employ either an ex-situ etch or to use an in-situ method such as atomic hydrogen cleaning.[4, 5] Although atomic H cleaning of III-V surfaces is well known, it has never been demonstrated on InGaAs (110) crystallographic faces. Furthermore, tri-gate field effect transistors (finFETs) have recently been employed in commercially available logic chips.[6] This unique device architecture allows for a reduction in short channel effects, minimization of the subthreshold swing, and a higher transconductance.[7] The InGaAs (110) surface would be the sidewalls of a vertically aligned (001) based finFETs.[8] Therefore, it is essential to find an in-situ method to efficiently remove any oxides or contamination from the (110) surfaces that is also compatible with the (001) surface. In this study, STM was employed to determine if atomic hydrogen can be used to remove the native oxide from air exposed InGaAs (110) samples. A post clean anneal was used to restore the surface to molecular beam epitaxy (MBE) levels of cleanliness.

Sepiolite-palygorskite polysomatic series: Oriented aggregation as a crystal growth mechanism in natural environments

A detailed microscopic study of sepiolite and palygorskite natural samples reveals that independently of the fiber length they all are composed of other minor width fibers until the minor units or the true crystals. They are prismatic crystals elongated along [001] with rhombus-like morphology in cross section, designated laths. Although their length can vary greatly, their width is always nanometric (~10–30 nm). The laths are grouped in a crystallographic arrangement, sharing edges or faces forming rods. Several rods, parallel to the c -axis of the fiber, form bundles. The laths are the smallest stable nucleated crystals. After the nucleation, the process of growth continues via a nonclassic crystal growth mechanism (aggregation of the nanolaths). Subsequently, the aggregated sepiolite and palygorskite natural crystals can continue growing, along the c -axis at the expense of the diffusion of molecular scale species throughout the solution. The morphology of the faces is controlled by the highly different attachment energy of the faces.

Particle morphological and roughness controls on mineral surface charge development

Effects of mineral particle morphology and roughness on potential determining ion (p.d.i.; H+, OH−) loadings achieved at synthetic lepidocrocite (γ-FeOOH) surfaces were predominantly investigated by potentiometry and thermodynamic modeling. Nanosized rod- (RL) and lath-shaped (LL) particles exhibiting different proportions of the same predominant crystallographic faces acquired largely comparable pH, ionic strength and counterion (NaCl, NaClO4) dependencies on p.d.i. loadings. These results supported previous claims that faces ideally containing proton silent sites only, are likely populated by additional proton active sites. This concept was supported further by results of roughened LL-like particles (LLR) also showing highly congruent pH-, ionic strength- and composition-dependent p.d.i. loadings with those of LL and RL. These loadings thereby correspond to maximal levels allowed by net attractive and repulsive forces at each solution composition, irrespective of particle morphology. Contrasting equilibration times required to achieve these loadings revealed considerably slower exchange of p.d.i. and electrolyte ions near the point of zero charge in the rough LLR than in the more idealized LL and RL particles.Thermodynamic modeling was used to test various concepts accounting for these results. The model made use of a novel framework capable of isolating electrostatic contributions from different faces, and of accounting for ion-specific double-layer properties within a single crystallographic face. These efforts made use of capacitance values for each electrolyte ions within the framework of a recently developed Variable Capacitance Model. Attempts at modeling all three particle types were used to suggest that the (010) face contains ∼0.9sitenm−2 of proton active sites, a value notably constrained by recently published Na+, Cl−, and ClO4− loadings derived by cryogenic X-ray photoelectron spectroscopy. The model presented in this work thus provides a means to predict p.d.i. loadings on multifaceted mineral particle surfaces, and can therefore be used to constrain further our understanding of mineral/water interface reactivity.

A new architecture of bowl-type mesoporous TiO2 via facile electrospray method

We demonstrate here the fabrication of a novel morphology of bowl-shaped TiO2 aggregates via a facile electrospray technique using TiO2 nanowires. These have been prepared by using lauric acid via solvothermal synthetic route. The modulation of surface energies of the different crystallographic faces through the use of lauric acid is the key parameter for the anisotropic growth. These aggregates have high specific surface area of up to 93.10m2/g, and relatively large pores with a mean size of 4.68nm. These porous structures can provide a high number of active adsorption sites and act as photo-related reaction centers. The prepared bowl-shaped TiO2 are in micrometer size and have good morphology and high specific surface area. A possible mechanism for the formation of bowl-shaped TiO2 aggregates is also proposed, based on the results. The light scattering properties of bowl-shaped architectures are characterized by the UV–Vis spectrometry. The superior light scattering abilities of bowl-shape makes them promising architectures for applications in the photovoltaic devices.

Controlled morphologies and growth direction of WO3 nanostructures hydrothermally synthesized with citric acid

Hexagonal tungsten oxide (h-WO3) nanorods have been synthesized by a hydrothermal process at 180°C using ammonium metatungstate and citric acid as starting materials. The phase constituent, morphology and growth direction of the h-WO3 nanocrystals were characterized by using XRD, SEM, HRTEM techniques. We found that the morphology, size and preferred orientation of the h-WO3 nanocrystals can be controlled by simply adjusting the amount of citric acid. In the absence of citric acid, 2-dimensional (2D) nanoplates or nanosheets 10–20nm thick were formed. With increasing the amount of citric acid, 1-dimensional (1D) nanorods or nanoneedles grew along [002] crystallographic orientation from some certain nanoplate crystallographic planes. The as-synthesized WO3 nanorods were up to 1μm in length and 40–50nm in diameter. It is suggested that the adsorption of carboxyl group on certain crystallographic faces leads to an anisotropic crystal growth and the formation of 1D morphology.

Raman spectroscopy and scanning electron microscopy characterizations of fission track method datable zircon grains.

Spectroscopic and morphological studies, designed to improve our understanding of the physicochemical phenomena that occur during zircon crystallization, are presented. The zircon fission track method (ZFTM) is used routinely in various laboratories around the world; however, there are some methodological difficulties needing attention. Depending on the surface fission track density observed under an optical microscope, the zircon grain surfaces are classified as homogeneous, heterogeneous, hybrid, or anomalous. In this study, zircon grain surfaces are characterized using complementary techniques such as optical microscopy (OM), micro-Raman spectroscopy, and scanning electron microscopy (SEM), both before and after chemical etching. Our results suggest that anomalous grains have subfamilies and that etching anisotropy related to heterogeneous grains is due to different crystallographic faces within the same polished surface that cannot be observed under an optical microscope. The improved methodology was used to determine the zircon fission track ages of samples collected from the Bauru Group located in the north of Paraná Basin, Brazil. A total of 514 zircon grains were analyzed, consisting of 10% homogeneous, about 10% heterogeneous, about 20% hybrid, and 60% anomalous grains. These results show that the age distributions obtained for homogeneous, heterogeneous, and hybrid grains are both statistically and geologically compatible.

Specificity of Growth Inhibitors and their Cooperative Effects in Calcium Oxalate Monohydrate Crystallization

The molecular recognition and interactions governing site-specific adsorption of growth inhibitors on crystal surfaces can be tailored in order to control the anisotropic growth rates and physical properties of crystalline materials. Here we examine this phenomenon in calcium oxalate monohydrate (COM) crystallization, a model system of calcification with specific relevance for pathological mineralization. We analyzed the effect of three putative growth inhibitors--chondroitin sulfate, serum albumin, and transferrin--using analytical techniques capable of resolving inhibitor-crystal interactions from interfacial to bulk scales. We observed that each inhibitor alters surface growth by adsorbing on to distinct steps emanating from screw dislocations on COM surfaces. Binding of inhibitors to different crystallographic faces produced morphological modifications that are consistent with classical mechanisms of layer-by-layer crystal growth inhibition. The site-specific adsorption of inhibitors on COM surfaces was confirmed by bulk crystallization, fluorescent confocal microscopy, and atomic force microscopy. Kinetic studies of COM growth at varying inhibitor concentrations revealed marked differences in their efficacy and potency. Systematic analysis of inhibitor combinations, quantified via the combination index, identified various binary pairings capable of producing synergistic, additive, and antagonistic effects. Collectively, our investigation of physiologically relevant biomolecules suggests potential roles of COM inhibitors in pathological crystallization and provides guiding principles for biomimetic design of molecular modifiers for applications in crystal engineering.

β-MnO2 nanorods with exposed tunnel structures as high-performance cathode materials for sodium-ion batteries

Sodium-ion batteries are being considered as a promising system for stationary energy storage and conversion, owing to the natural abundance of sodium. It is important to develop new cathode and anode materials with high capacities for sodium-ion batteries. Herein, we report the synthesis of β-MnO2 nanorods with exposed tunnel structures by a hydrothermal method. The as-prepared β-MnO2 nanorods have exposed {111} crystal planes with a high density of (1 × 1) tunnels, which leads to facile sodium ion (Na-ion) insertion and extraction. When applied as cathode materials in sodium-ion batteries, β-MnO2 nanorods exhibited good electrochemical performance with a high initial Na-ion storage capacity of 350 mAh g−1. β-MnO2 nanorods also demonstrated a satisfactory high-rate capability as cathode materials for sodium-ion batteries. β-MnO2 nanorods with exposed tunnel structures have been successfully synthesized by a hydrothermal method. The as-prepared β-MnO2 nanorods have exposed {111} crystal planes with a high density of (1 × 1) tunnels, leading to facile Na-ion insertion and extraction. When applied as cathode materials in Na-ion batteries, the β-MnO2 nanorods exhibited a superior electrochemical performance with a high initial Na-ion storage capacity of 350 mAh g−1.β-MnO2 nanorods also demonstrated an excellent high rate capability and a good cyclability. Sodium-ion batteries are promising alternatives to their lithium-ion counterparts for stationary energy storage and conversion. Sodium is more abundant than lithium, resulting in cheaper rechargeable batteries. However, this gain is coupled to lower densities and cyclabilities, leading researchers to focus on finding appropriate materials for electrodes. Guoxiu Wang and co-workers from Australia and South Korea have now prepared a cathode material with exposed tunnel structures that demonstrates good performances. Nanorods of β-MnO2 — a form of manganese dioxide — make attractive components for cathodes owing to their tunnel structure, which favors the storage and transport of sodium ions. Through a hydrothermal synthesis— whereby chemical precursors are heated in aqueous solution—Wang and colleagues fabricated nanorods with specific crystallographic faces (111) exposed. When these nanorods were used as battery cathodes, the team observed a high initial sodium storage capacity as well as the facile insertion and extraction of sodium ions.