Graphene‐Based Mesoporous SnO2 with Enhanced Electrochemical Performance for Lithium‐Ion Batteries

Graphene‐based metal oxides generally show outstanding electrochemical performance due to the superior properties of graphene. However, the aggregation of active metal oxide nanoparticles on the graphene surface may result in a capacity fading and poor cycle performance. Here, a mesostructured graphene‐based SnO₂ composite is prepared through in situ growth of SnO₂ particles on the graphene surface using cetyltrimethylammonium bromide as the structure‐directing agent. This novel mesoporous composite inherits the advantages of graphene nanosheets and mesoporous materials and exhibits higher reversible capacity, better cycle performance, and better rate capability compared to pure mesoporous SnO₂ and graphene‐based nonporous SnO₂. It is concluded that the synergetic effect between graphene and mesostructure benefits the improvement of the electrochemical properties of the hybrid composites. This facile method may offer an attractive alternative approach for preparation of the graphene‐based mesoporous composites as high‐ performance electrodes for lithium‐ion batteries.     

Electrochemical Method for Synthesis of a ZnFe2O4/TiO2 Composite Nanotube Array Modified Electrode with Enhanced Photoelectrochemical Activity

An electrode with intimate and well‐aligned ZnFe₂O₄/TiO₂ composite nanotube arrays is prepared via electrochemical anodization of pure titanium foil in fluorine‐containing ethylene glycol, followed by a novel cathodic electrodeposition method. The deposition of ZnFe₂O₄ is promoted in the self‐aligned, vertically oriented TiO₂ nanotube arrays but minimized at the tube entrances. Thus, pore clogging is prevented. Environmental scanning electron microscopy, energy‐dispersive X‐ray spectra, high‐resolution transmission electron microscopy, X‐ray diffraction patterns, and X‐ray photoelectron spectroscopy indicate that the as‐prepared samples are highly ordered and vertically aligned TiO₂ nanotube arrays with ZnFe₂O₄ nanoparticles loading. The TiO₂ nanotubes are anatase with the preferential orientation of <101> plane. Enhanced absorption in both UV and visible light regions is observed for the composite nanotube arrays. The current–voltage curve of ZnFe₂O₄‐loaded TiO₂ nanotube arrays reveals a rectifying behavior. The enhanced separation of photoinduced electrons and holes is demonstrated by surface photovoltage and photocurrent measurements. Meanwhile, the photoelectrochemical investigations verify that the ZnFe₂O₄/TiO₂ composite nanotube array modified electrode has a more effective photoconversion capability than the aligned TiO₂ nanotube arrays alone. In addition, the photoelectrocatalytic ability of the novel electrode is found enhanced in the degradation of 4‐chlorophenol.     

Simple Synthesis of Hollow Tin Dioxide Microspheres and Their Application to Lithium‐Ion Battery Anodes

Hollow tin dioxide (SnO₂) microspheres were synthesized by the simple heat treatment of a mixture composed of tin(IV ) tetrachloride pentahydrate (SnCl₄·5H₂O) and resorcinol–formaldehyde gel (RF gel). Because hollow structures were formed during the heat treatment, the pre‐formation of template and the adsorption of target precursor on template are unnecessary in the current method, leading to simplified synthetic procedures and facilitating mass production. Field‐emission scanning electron microscopy (FE‐SEM) images showed 1.7–2.5 μm sized hollow spherical particles. Transmission electron microscopy (TEM) images showed that the produced spherical particles are composed of a hollow inner cavity and thin outer shell. When the hollow SnO₂ microspheres were used as a lithium‐battery anode, they exhibited extraordinarily high discharge capacities and coulombic efficiency. The reported synthetic procedure is straightforward and inexpensive, and consequently can be readily adopted to produce large quantities of hollow SnO₂ microspheres. This straightforward approach can be extended for the synthesis of other hollow microspheres including those obtained from ZrO₂ and ZrO₂/CeO₂ solid solutions.     

Controllably Engineering Mesoporous Surface and Dimensionality of SnO2 toward High‐Performance CO2 Electroreduction

Currently, the precise control of the architecture and surface of functional materials for high‐performance still remains a great challenge. Here, a feasible approach is presented to synchronously manipulate mesoporous surface and dimensionality of SnO₂ catalysts into hierarchically mesoporous nanosheets and nanospheres within one simple reaction system. By adjustment of the hydrophobic chain length of different fluorinated surfactants, 0D SnO₂ nanospheres with average size of 165 nm, and 2D SnO₂ ulthrathin nanosheets with thickness of 22.5 nm with the distinct dimensionalities are separately obtained (one stone, two birds), both of which are well decorated with ordered mesopore arrays on their surfaces (pore size of 16 nm). The following calcination gave rise to the formation of hierarchically mesopores (5 and 16 nm, respectively) with high crystallization and improved surface area (96.8 m² g^?1). The resultant mesoporous SnO₂ nanosheets as catalyst for CO₂ electroreduction reaction (CO₂ RR) exhibit excellent selectivity, with a high Faraday efficiency (FE) of HCOOH reaching up to 90.0% at ?1.3 V and C₁ FE of 97.4% at ?1.2 V versus reversible hydrogen electrode, as well as long‐term stability, which is among the best performance compared to reported SnO₂ materials, thanks to the collective contributions of the unique architecture and mesoporous structure.     

Block‐Copolymer‐Assisted One‐Pot Synthesis of Ordered Mesoporous WO3−x/Carbon Nanocomposites as High‐Rate‐Performance Electrodes for Pseudocapacitors

An ordered mesoporous tungsten‐oxide/carbon (denoted as m‐WO_3?x‐C‐s) nanocomposite is synthesized using a simple one‐pot method using polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) as a structure‐directing agent. The hydrophilic PEO block interacts with the carbon and tungsten precursors (resol polymer and WCl₆), and the PS block is converted to pores after heating at 700 °C under a nitrogen flow. The m‐WO_3?x‐C‐s nanocomposite has a high Brunauer–Emmett–Teller (BET) surface area and hexagonally ordered pores. Because of its mesoporous structure and high intrinsic density of tungsten oxide, this material exhibits a high average volumetric capacitance and gravimetric capacitance as a pseudocapacitor electrode. In comparison with reduced mesoporous tungsten oxide (denoted as m‐WO_3?x‐h), which is synthesized by a tedious hard template approach and further reduction in a H₂/N₂ atmosphere, m‐WO_3?x‐C‐s shows a high capacitance and enhanced rate performance, as confirmed by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy. The good performance of m‐WO_3?x‐C‐s is attributed to the high surface area arising from the mesoporous structure, the large interconnected mesopores, and the low internal resistance from the well‐dispersed reduced tungsten oxide and amorphous carbon composite structure. Here, the amorphous carbon acts as an electrical pathway for effective pseudocapacitor behavior of WO_3‐x.     

Growth of Aligned Square‐Shaped SnO2 Tube Arrays

Tin dioxide (SnO₂) box beams, or tubes with square or rectangular cross‐sections, are synthesized on quartz substrates using a combustion chemical vapor deposition (CVD) method in an open atmosphere at 850 °C to 1150 °C. The cross‐sectional width of the as‐synthesized SnO₂ tubules is tunable from 50 nm to sub‐micrometer depending on synthesis temperature. Each tubule is found to be a single crystal of rutile structure with four {110} peripheral surfaces and &#60;001&#62; growth direction. Although several growth patterns are observed for different samples, the basic growth mechanism is believed to be a self‐catalyzed, direct vapor–solid (VS) process, where most new material is incorporated into the bottom parts of the existing SnO₂ tubules through surface diffusion. The tubes are readily aligned in the direction perpendicular to the substrate surface to form tube arrays. These well‐aligned SnO₂ tubule arrays with tunable tube size could be the building blocks or templates for fabrication of functional nanodevices, especially those relevant to energy storage and conversion such as nanobatteries, nanofuel cells, and nanosensors. A gas sensor based on a single SnO₂ nanotubes demonstrated extremely high sensitivity to ethanol vapor.     

Block‐Copolymer‐Templated Synthesis of Electroactive RuO2‐Based Mesoporous Thin Films

RuO₂‐based mesoporous thin films of optical quality are synthesized from ruthenium‐peroxo‐based sols using micelle templates made of amphiphilic polystyrene‐polyethylene oxide block copolymers. The mesoporous structure and physical properties of the RuO₂ films (mesoporous volume: 30%; pore diameter: ～30 nm) can be controlled by the careful tuning of both the precursor solution and thermal treatment (150–350 °C). The optimal temperature that allows control of both mesoporosity and nanocristallinity is strongly dependent on the substrate (silicon or fluorine‐doped tin oxide). The structure of the resulting mesoporous films are investigated using X‐ray diffraction, X‐ray photoelectron spectroscopy, and atomic force microscopy. Mesoporous layers are additionally characterized by transmission and scanning electron microscopy and ellipsometry while their electrochemical properties are analyzed via cyclic voltammetry. Thick mesoporous films of ruthenium oxide hydrates, RuO₂ · xH₂O, obtained using a thermal treatment at 280 °C, exhibit capacitances as high as 1000 ± 100 F g^?1 at a scan rate of 10 mV s^?1, indicating their potential application as electrode materials.     

Tin Oxide with Controlled Morphology and Crystallinity by Atomic Layer Deposition onto Graphene Nanosheets for Enhanced Lithium Storage

As one of the most promising negative electrode materials in lithium‐ion batteries (LIBs), SnO₂ experiences intense investigation due to its high specific capacity and energy density, relative to conventional graphite anodes. In this study, for the first time, atomic layer deposition (ALD) is used to deposit SnO₂, containing both amorphous and crystalline phases, onto graphene nanosheets (GNS) as anodes for LIBs. The resultant SnO₂‐graphene nanocomposites exhibit a sandwich structure, and, when cycled against a lithium counter electrode, demonstrate a promising electrochemical performance. It is demonstrated that the introduction of GNS into the nanocomposites is beneficial for the anodes by increasing their electrical conductivity and releasing strain energy: thus, the nanocomposite electrode materials maintain a high electrical conductivity and flexibility. It is found that the amorphous SnO₂‐GNS is more effective than the crystalline SnO₂‐GNS in overcoming electrochemical and mechanical degradation; this observation is consistent with the intrinsically isotropic nature of the amorphous SnO₂, which can mitigate the large volume changes associated with charge/discharge processes. It is observed that after 150 charge/discharge cycles, 793 mA h g^?1 is achieved. Moreover, a higher coulombic efficiency is obtained for the amorphous SnO₂‐GNS composite anode. This study provides an approach to fabricate novel anode materials and clarifies the influence of SnO₂ phases on the electrochemical performance of LIBs.     

Low-temperature carbon monoxide gas sensors based gold/tin dioxide

Tin dioxide nanocrystals were synthesized by a precipitation process and then used as the support for 2 wt.% gold/tin dioxide preparation via a deposition–precipitation method, followed by calcination at 200 °C. Thick films were fabricated from gold/tin dioxide powders, and the sensing behavior for carbon monoxide gas was investigated. The gold/tin dioxide was found to be efficient carbon monoxide gas-sensing materials under low operating temperature (83–210 °C). The Au/SnO₂ sensor with SnO₂ calcined at 300 °C exhibited better CO gas-sensing behavior than the SnO₂ calcined at other temperatures. The experimental results indicated the potential use of Au doped SnO₂ for CO gas sensing.     

Absorption Properties of Hybrid SnO<Subscript>2</Subscript> Nanocrystal-Carbon Nanotube Structures

Tin oxide (SnO₂) nanocrystals of a few nanometers are of great interest for electronic applications. Here we present a mini-arc plasma method to produce aerosol tin oxide nanocrystals at atmospheric pressure. The product SnO₂ nanocrystals are then assembled onto the external surface of carbon nanotubes (CNTs) to form hybrid SnO₂–CNT structures. The absorption properties of both the SnO₂ nanocrystals and the SnO₂–CNT hybrid structures have been characterized. Quantum size effects have been observed for as-produced SnO₂ nanocrystals. The intrinsic nanoparticle size selection during the assembly process results in a blue shift of the absorption spectrum for hybrid nanostructures.     

Ultrafine Amorphous SnOx Embedded in Carbon Nanofiber/Carbon Nanotube Composites for Li‐Ion and Na‐Ion Batteries

Core–shell‐structured, ultrafine SnO_x/carbon nanofiber (CNF)/carbon nanotube composite films are in situ synthesized by electrospinning through a dual nozzle. The carbon shell layer functions as a buffer to prevent the separation of SnO_x particles from the CNF core, allowing full utilization of high‐capacity SnO_x in both Li‐ion and Na‐ion batteries. The composite electrodes reveal an anomalous Li‐ and Na‐ion storage mechanism where all the intermediate phases, like Li_xSn and Na_xSn alloys, maintain amorphous states during the entire charge/discharge process. The uniform dispersion on an atomic scale and the amorphous state of the SnO_x particles remain intact in the carbon matrix without growth or crystallization even after 300 cycles, which is responsible for sustaining excellent capacity retention of the electrodes. These discoveries not only shed new insights into fundamental understanding of the electrochemical behavior of SnO_x electrodes but also offer a potential strategy to improve the cyclic stability of other types of alloy anodes that suffer from rapid capacity decays due to large volume changes.     

Preparation of Mesoporous Sb‐, F‐, and In‐Doped SnO2 Bulk Powder with High Surface Area for Use as Catalyst Supports in Electrolytic Cells

The M‐doped tin oxides (M = Sb, F, and In) to be used as catalyst support are synthesized by using templating process with tetradecylamine (TDA) as the template, combined with a hydrothermal (HT) method to improve its thermal stability. The obtained materials are characterized by XRD, SAXS, TEM, EDX, SEM, and BET to study microstructure and physical properties, which have a mesoporous structure, small particle size, and high surface area (125–263 m² g^–1). The materials show an overall conductivity of 0.102–0.295 S cm^–1. Repetitive potential cycling is employed to characterize the electrochemical properties and stability. The M‐doped tin oxides are highly electrochemical stable compared to carbon black. From the observed results, it can be concluded that the combination of TDA and HT treatment are an effective synthetic method for designing mesoporous M‐doped tin oxide as catalyst supports.     

Design and Synthesis of Multifunctional Drug Carriers Based on Luminescent Rattle‐Type Mesoporous Silica Microspheres with a Thermosensitive Hydrogel as a Controlled Switch

A novel approach for the fabrication of multifunctional microspheres integrating several advantages of mesoporous, luminescence, and temperature responses into one single entity is reported. First, the hollow mesoporous silica capsules are fabricated via a sacrificial template route. Then, Gd₂O₃:Eu³⁺ luminescent nanoparticles are incorporated into the internal cavities to form rattle‐type mesoporous silica nanocapsules by an incipient‐wetness impregnation method. Finally, the rattle‐type capsules serve as a nanoreactor for successfully filling temperature‐responsive hydrogel via photoinduced polymerization to form the multifunctional composite microspheres. The organic–inorganic hybrid microspheres show a red emission under UV irradiation due to the luminescent Gd₂O₃:Eu³⁺ core. The in vitro cytotoxicity tests show that the samples have good biocompatibility, which indicates that the nanocomposite could be a promising candidate for drug delivery. In addition, flow cytometry and confocal laser scanning microscopy (CLSM) confirm that the sample can be effectively taken up by SKOV3 cells. For in vitro magnetic resonance imaging (MRI), the sample shows the promising spin‐lattice relaxation time (T₁) weighted effect and could potentially apply as a T₁‐positive contrast agent. This composite drug delivery system (DDS) provides a positive temperature controlled “on‐off”drug release pattern and the drug, indomethacin (IMC), is released fast at 45 °C (on phase) and completely shut off at 20 °C (off phase). Meanwhile Gd₂O₃:Eu³⁺ plays an important role as the luminescent tag for tracking the drug loading and release process by the reversible luminescence quenching and recovery phenomenon. These results indicate that the obtained multifunctional composite has the potential to be used as a smart DDS for biomedical applications.     

Carbon Nanotube Based Inverted Flexible Perovskite Solar Cells with All‐Inorganic Charge Contacts

Organolead halide perovskite solar cells (PSC) are arising as promising candidates for next‐generation renewable energy conversion devices. Currently, inverted PSCs typically employ expensive organic semiconductor as electron transport material and thermally deposited metal as cathode (such as Ag, Au, or Al), which are incompatible with their large‐scale production. Moreover, the use of metal cathode also limits the long‐term device stability under normal operation conditions. Herein, a novel inverted PSC employs a SnO₂‐coated carbon nanotube (SnO₂@CSCNT) film as cathode in both rigid and flexible substrates (substrate/NiO‐perovskite/Al₂O₃‐perovskite/SnO₂@CSCNT‐perovskite). Inverted PSCs with SnO₂@CSCNT cathode exhibit considerable enhancement in photovoltaic performance in comparison with the devices without SnO₂ coating owing to the significantly reduced charge recombination. As a result, a power conversion efficiency of 14.3% can be obtained on rigid substrates while the flexible ones achieve 10.5% efficiency. More importantly, SnO₂@CSCNT‐based inverted PSCs exhibit significantly improved stability compared to the standard inverted devices made with silver cathode, retaining over 88% of their original efficiencies after 550 h of full light soaking or thermal stress. The results indicate that SnO₂@CSCNT is a promising cathode material for long‐term device operation and pave the way toward realistic commercialization of flexible PSCs.     

One‐Dimensional Hierarchical Structures Composed of Novel Metal Oxide Nanosheets on a Carbon Nanotube Backbone and Their Lithium‐Storage Properties

Some facile solution methods are developed to directly grow anatase TiO₂ nanosheets with exposed (001) facets and SnO₂ nanosheets onto a carbon nanotube backbone for the first time. TiO₂ nanosheets can also be grown on SnO₂@CNT coaxial nanocables. An electrochemical investigation shows that these unique one‐dimensional hierarchical nanostructures exhibit significantly enhanced lithium‐storage properties under different testing conditions by comparison to the pure metal oxide counterparts, and evidently suggests that improved electrochemical properties of TiO₂ and SnO₂ can be achieved through proper design of the nanostructure and chemical composition.     

Synthesis and Rate Performance of Monolithic Macroporous Carbon Electrodes for Lithium‐Ion Secondary Batteries

Three‐dimensionally ordered macroporous (3DOM) materials are composed of well‐interconnected pore and wall structures with wall thicknesses of a few tens of nanometers. These characteristics can be applied to enhance the rate performance of lithium‐ion secondary batteries. 3DOM monoliths of hard carbon have been synthesized via a resorcinol‐formaldehyde sol–gel process using poly(methyl methacrylate) colloidal‐crystal templates, and the rate performance of 3DOM carbon electrodes for lithium‐ion secondary batteries has been evaluated. The advantages of monolithic 3DOM carbon electrodes are: 1) solid‐state diffusion lengths for lithium ions of the order of a few tens of nanometers, 2) a large number of active sites for charge‐transfer reactions because of the material's high surface area, 3) reasonable electrical conductivity of 3DOM carbon due to a well‐interconnected wall structure, 4) high ionic conductivity of the electrolyte within the 3DOM carbon matrix, and 5) no need for a binder and/or a conducting agent. These factors lead to significantly improved rate performance compared to a similar but non‐templated carbon electrode and compared to an electrode prepared from spherical carbon with binder. To increase the energy density of 3DOM carbon, tin oxide nanoparticles have been coated on the surface of 3DOM carbon by thermal decomposition of tin sulfate, because the specific capacity of tin oxide is larger than that of carbon. The initial specific capacity of SnO₂‐coated 3DOM carbon increases compared to that of 3DOM carbon, resulting in a higher energy density of the modified 3DOM carbon. However, the specific capacity decreases as cycling proceeds, apparently because lithium–tin alloy nanoparticles were detached from the carbon support by volume changes during charge–discharge processes. The rate performance of SnO₂‐coated 3DOM carbon is improved compared to 3DOM carbon.     

Direct Synthesis of Highly Stable Mesoporous Molecular Sieves Containing Zeolite Building Units

A novel, one‐step synthesis of a highly stable mesoporous molecular sieve (MMS‐H), which has a structure analogous to MCM‐48 but which contains zeolite building units, is reported. A variety of experimental techniques—X‐ray diffraction (XRD), N₂ adsorption/desorption, transmission electron microscopy (TEM), Fourier‐transform infrared (FTIR) spectroscopy, hyperpolarized ¹²⁹Xe NMR, and solid‐state ²⁷Al and ³¹P magic‐angle spinning (MAS) NMR spectroscopies—have been used to characterize the framework structure, porosity, and acidity of this novel mesoporous/microporous composite material, which is also found to possess superior thermal, hydrothermal, steam, and mechanical stabilities.     

Tin Dioxide Sensing Layer Grown on Tubular Nanostructures by a Non‐Aqueous Atomic Layer Deposition Process

A new atomic layer deposition (ALD) process for nanocrystalline tin dioxide films is developed and applied for the coating of nanostructured materials. This approach, which is adapted from non‐hydrolytic sol‐gel chemistry, permits the deposition of SnO₂ at temperatures as low as 75 °C. It allows the coating of the inner and outer surface of multiwalled carbon nanotubes with a highly conformal film of controllable thickness. The ALD‐coated tubes are investigated as active components in gas‐sensor devices. Due to the formation of a p‐n heterojunction between the highly conductive support and the SnO₂ thin film an enhancement of the gas sensing response is observed.     

Oxidation study of pure tin and its alloys via electrochemical reduction analysis

The oxidation of pure Sn and high Pb-Sn alloys was investigated under different oxidizing conditions of temperature and humidity. Both the chemical nature and the amount of oxides were characterized using electrochemical reduction analysis by measuring the electrolytic reduction potential and total transferred electrical charges. For pure tin, SnO grew faster under humid conditions than in dry air. A very thin (<10 Å) layer of SnO₂ was formed on the top surface under humid conditions. The mixture of SnO and SnO₂ was found for oxidation at 150°C. For oxidation of high Pb-Sn alloys, tin was preferentially oxidized on the surface, and tin content over the solubility limit suppressed the formation of lead oxide. For the evaporated Pb-3Sn alloy reflowed two times at 370°C under H₂ atmosphere, tin was enriched on the surface as tin dendrites and lead oxide was formed in the tin-depleted region that surrounds tin dendrites.     

Ultrathin Mesoporous NiCo2O4 Nanosheets Supported on Ni Foam as Advanced Electrodes for Supercapacitors

A facile two‐step method is developed for large‐scale growth of ultrathin mesoporous nickel cobaltite (NiCo₂O₄) nanosheets on conductive nickel foam with robust adhesion as a high‐performance electrode for electrochemical capacitors. The synthesis involves the co‐electrodeposition of a bimetallic (Ni, Co) hydroxide precursor on a Ni foam support and subsequent thermal transformation to spinel mesoporous NiCo₂O₄. The as‐prepared ultrathin NiCo₂O₄ nanosheets with the thickness of a few nanometers possess many interparticle mesopores with a size range from 2 to 5 nm. The nickel foam supported ultrathin mesoporous NiCo₂O₄ nanosheets promise fast electron and ion transport, large electroactive surface area, and excellent structural stability. As a result, superior pseudocapacitive performance is achieved with an ultrahigh specific capacitance of 1450 F g^?1, even at a very high current density of 20 A g^?1, and excellent cycling performance at high rates, suggesting its promising application as an efficient electrode for electrochemical capacitors.