The controllable synthesis of nanomaterials with unique morphologies and sizes has attracted increasing interest because of the shape-dependent properties of the nanomaterials. In this article, we report the synthesis of the new 6-fold-symmetrical AlN hierarchical nanostructures including urchinlike and flowerlike ones assembled by AlN nanoneedles through the chemical reaction between AlCl3 and NH3 with the vaporization temperature of AlCl3 between 120 and 165 °C and the reaction temperature higher than 1050 °C. The morphologies, sizes, and densities of the AlN nanostructures could be controllably modulated by changing the reaction temperature and the vapor pressure of AlCl3. The formation mechanism of the AlN hierarchical nanostructures has been discussed on the basis of the change in the AlCl3 vapor pressure and the morphological evolution of the intermediate products. The shape-dependent properties of the AlN products have been observed, and the urchinlike nanostructures showed better optical and field-emission properties in comparison with the flowerlike ones. These results indicate the potential applications of the 6-fold-symmetrical AlN hierarchical nanostructures in optoelectronic and field-emission devices.

Highly oriented SiC porous nanowire (NW) arrays on Si substrate have been achieved via in situ carbonizing aligned Si NW arrays standing on Si substrate. The resultant SiC NW arrays inherit the diameter and length of the mother Si NW arrays. Field emission measurements show that these oriented SiC porous NW arrays are excellent field emitter with large field emission current denstity at very low electric field. The in situ conversion method reported here might be exploited to fabricate NW arrays of other materials containing silicon.

ConspectusCarbon-based nanomaterials have been the focus of research interests in the past 30 years due to their abundant microstructures and morphologies, excellent properties, and wide potential applications, as landmarked by 0D fullerene, 1D nanotubes, and 2D graphene. With the availability of high specific surface area (SSA), well-balanced pore distribution, high conductivity, and tunable wettability, carbon-based nanomaterials are highly expected as advanced materials for energy conversion and storage to meet the increasing demands for clean and renewable energies. In this context, attention is usually attracted by the star material of graphene in recent years.In this Account, we overview our studies on carbon-based nanotubes to nanocages for energy conversion and storage, including their synthesis, performances, and related mechanisms. The two carbon nanostructures have the common features of interior cavity, high conductivity, and easy doping but much different SSAs and pore distributions, leading to different performances. We demonstrated a six-membered-ring-based growth mechanism of carbon nanotubes (CNTs) with benzene precursor based on the structural similarity of the benzene ring to the building unit of CNTs. By this mechanism, nitrogen-doped CNTs (NCNTs) with homogeneous N distribution and predominant pyridinic N were obtained with pyridine precursor, providing a new kind of support for convenient surface functionalization via N-participation. Accordingly, various transition-metal nanoparticles were directly immobilized onto NCNTs without premodification. The so-constructed catalysts featured high dispersion, narrow size distribution and tunable composition, which presented superior catalytic performances for energy conversions, for example, the oxygen reduction reaction (ORR) and methanol oxidation in fuel cells. With the advent of the new field of carbon-based metal-free electrocatalysts, we first extended ORR catalysts from the electron-rich N-doped to the electron-deficient B-doped sp2 carbon. The combined experimental and theoretical study indicated the ORR activity originated from the activation of carbon π electrons by breaking the integrity of π conjugation, despite the electron-rich or electron-deficient nature of the dopants.With this understanding, metal-free electrocatalysts were further extended to the dopant-free defective carbon nanomaterials. Moreover, we developed novel 3D hierarchical carbon-based nanocages by the in situ MgO template method, which featured coexisting micro–meso–macropores and much larger SSA than the nanotubes. The unique 3D architecture avoids the restacking generally faced by 2D graphene due to the intrinsic π–π interaction. Consequently, the hierarchical nanocages presented superior performances not only as new catalyst supports and metal-free electrocatalysts but also as electrode materials for energy storage. State-of-the-art supercapacitive performances were achieved with high energy density and power density, as well as excellent rate capability and cycling stability. The large interior space of the nanocages enabled the encapsulation of high-loading sulfur to alleviate polysulfide dissolution while greatly enhancing the electron conduction and Li-ion diffusion, leading to top level performance of lithium–sulfur battery. These results not only provide unique carbon-based nanomaterials but also lead to in-depth understanding of growth mechanisms, material design, and structure–performance relationships, which is significant to promote their energy applications and also to enrich the exciting field of carbon-based nanomaterials.

Correction for ‘Recent advances in understanding of the mechanism and control of Li2O2 formation in aprotic Li–O2 batteries’ by Zhiyang Lyu et al., Chem. Soc. Rev., 2017, DOI: 10.1039/c7cs00255f.

Aprotic Li–O2 batteries represent promising alternative devices for electrical energy storage owing to their extremely high energy densities. Upon discharge, insulating solid Li2O2 forms on cathode surfaces, which is usually governed by two growth models, namely the solution model and the surface model. These Li2O2 growth models can largely determine the battery performances such as the discharge capacity, round-trip efficiency and cycling stability. Understanding the Li2O2 formation mechanism and controlling its growth are essential to fully realize the technological potential of Li–O2 batteries. In this review, we overview the recent advances in understanding the electrochemical and chemical processes that occur during the Li2O2 formation. In the beginning, the oxygen reduction mechanisms, the identification of O2−/LiO2 intermediates, and their influence on the Li2O2 morphology have been discussed. The effects of the discharge current density and potential on the Li2O2 growth model have been subsequently reviewed. Special focus is then given to the prominent strategies, including the electrolyte-mediated strategy and the cathode-catalyst-tailoring strategy, for controlling the Li2O2 growth pathways. Finally, we conclude by discussing the profound implications of controlling Li2O2 formation for further development in Li–O2 batteries.

High quality metastable wurtzite γ-MnS nanowires were synthesized via solution–solid–solid (SSS) growth in a mixed solvent of 1-dodecylamine and 1-dodecanethiol at a specified volume ratio. The length of the γ-MnS nanowires can be conveniently and well controlled by adjusting the amount ratio of [(C4H9)2NCS2]2Mn precursors to Ag2S catalysts. The results suggest the great potential of the SSS growth in the synthesis of high quality metastable chalcogenide nanowires.

High volumetric energy density combined with high power density is highly desired for electrical double-layer capacitors. Usually the volumetric performance is improved by compressing carbon material to increase density but at the much expense of power density due to the deviation of the compressed porous structure from the ideal one. Herein the authors report an efficient approach to increase the density and optimize the porous structure by collapsing the carbon nanocages via capillarity. Three samples with decreasing sizes of meso- and macropores provide us an ideal model system to demonstrate the correlation of volumetric performance with porous structure. The results indicate that reducing the surplus macropores and, more importantly, the surplus mesopores is an efficient strategy to enhance the volumetric energy density while keeping the high power density. The optimized sample achieves a record-high stack volumetric energy density of 73 Wh L−1 in ionic liquid with superb power density and cycling stability.

The unique hierarchical nitrogen-doped carbon nanocages (hNCNC) are used as a new support to homogeneously immobilize spinel CoFe2O4 nanoparticles by a facile solvothermal method. The so-constructed hierarchical CoFe2O4/hNCNC catalyst exhibits a high oxygen reduction activity with an onset potential of 0.966 V and half-wave potential of 0.819 V versus reversible hydrogen electrode, far superior to the corresponding 0.846 and 0.742 V for its counterpart of CoFe2O4/hCNC with undoped hierarchical carbon nanocages (hCNC) as the support, which locates at the top level for spinel-based catalysts to date. Consequently, the CoFe2O4/hNCNC displays the superior performance to the CoFe2O4/hCNC, when used as the cathode catalysts in the home-made Al-air batteries. X-ray photoelectron spectroscopy characterizations reveal the more charge transfer from CoFe2O4 to hNCNC than to hCNC, indicating the stronger interaction between CoFe2O4 and hNCNC due to the nitrogen participation. The enhanced interaction and hierarchical morphology favor the high dispersion and modification of electronic states for the active species as well as the mass transport during the oxygen reduction process, which plays a significant role in boosting the electrocatalytic performances. In addition, we noticed the high sensitivity of O 1s spectrum to the particle size and chemical environment for spinel oxides, which is used as an indicator to understand the evolution of ORR activities for all the CoFe2O4-related contrast catalysts. Accordingly, the well-defined structure-performance relationship is demonstrated by the combination of experimental characterizations with theoretical calculations. This study provides a promising strategy to develop efficient, inexpensive and durable oxygen reduction electrocatalysts by tuning the interaction between spinel metal oxides and the carbon-based supports.Download high-res image (195KB)Download full-size image

•Heterogenous Co3O4-nanocube/Co(OH)2-nanosheet hybrid is prepared by a hydrothermal method.•Co3O4 nanocubes are highly uniform and are distributed on the individual Co(OH)2 nanosheets.•Unique nanostructures show significant advantages for flexible supercapacitor electrodes.A novel heterogenous Co3O4-nanocube/Co(OH)2-nanosheet hybrid is prepared by a controllable facile one-pot hydrothermal reaction. The resulting Co3O4 nanocubes are highly uniform in morphology, and are distributed uniformly on the individual Co(OH)2 nanosheets. Such unique nanostructural features show significant advantages for applications as flexible supercapacitor electrodes in terms of enhanced durability and capacitance. The as-prepared electrode has offered a large capacitance of 1164 F g−1 at 1.2 A g−1. When being paired with activated carbon, the resulting flexible all-solid-state device exhibited a maximum energy density of 9.4 mWh cm−3. It is worthwhile noting that this as-assembled device showed little capacitance decay after over 5000 cycles with 97.4% retention of its original specific capacitance. Such a high performance outperforms most metal oxides-based electrodes and shows the advantages of the hybrid strategy, which shed light on exploring robust and cheap electrode materials for high-performance flexible supercapacitors.A novel heterogenous Co3O4-nanocube/Co(OH)2-nanosheet hybrid is prepared by a controllable facile one-pot hydrothermal reaction, which shows unique nanostructural features and achieves high-performance flexible all-solid-state supercapacitors.Download high-res image (165KB)Download full-size image

•The correlation between Li2O2 growth mechanism and O2 adsorbability is established.•A strategy to realize a simultaneous large capacity and low overpotential is demonstrated.•The dominant role of the carbon additive on the discharge performance is revealed.Understanding and controlling the growth of the vital Li2O2 product, which is associated with intrinsic property of cathode surface, is essential to design effective cathode catalysts in Li-O2 batteries. Herein we establish the correlation between the Li2O2 growth model and the O2 adsorbability on cathode surface that determines the pathway of the first electron transfer to O2. The weak O2 adsorbability drives the solution growth model to form Li2O2 toroid, while the strong one drives the surface growth model to thin film. Based on this mechanism, we select the N-doped carbon nanocages as cathode to realize a simultaneous large discharge capacity and low charge overpotential by forming copious thin-film Li2O2, deriving from its high specific surface area and enhanced O2 adsorbability due to N-doping. Our study demonstrates an effective strategy to design advanced cathode catalysts in Li–O2 batteries and potentially other metal-air batteries.The correlation between the Li2O2 growth mechanism and the O2 adsorbability on cathode surface is established, and a strategy to design advanced cathode catalysts of Li-O2 batteries is demonstrated accordingly.Download high-res image (327KB)Download full-size image

Currently, iron nitride (Fe2N) and iron carbide (Fe3C), which are regarded as the new non-precious metal electrocatalysts for the oxygen reduction reaction (ORR) with high activity and stability in acidic medium, are attracting increasing attention. Herein, by systematic comparison of the ORR activities for Fe2N- and Fe3C-based catalysts designed with or without a solid nitrogen source, we found that only the former is highly active for ORR while the latter is quite poorly active despite their similar crystalline phases. This result indicates that the Fe–N related species are responsible for the high ORR activities of the Fe-based catalysts, similar to the case of Fe/N/C catalysts. Density functional theory calculations demonstrate that the Fe–N4/C moiety has a far superior ORR activity to that of Fe2N and Fe3C. The experimental and theoretical results mutually support that the high activities of the Fe-based catalysts originate from Fe–Nx/C moieties (x ≥ 4) rather than Fe2N or Fe3C phases, which is significant for exploring advanced Fe-based electrocatalysts.

Electrochemical energy storage (EES) devices combining high energy density with high power density are necessary for addressing the growing energy demand and environmental crisis. Nickel oxide (NiO) is a promising electrode material for EES owing to the ultrahigh theoretical specific capacity, but the practical values are far below the theoretical limit to date, with inferior rate and cycling performances. Herein, we report the novel mesostructured NiO/Ni composites, which consist of hetero-NiO/Ni components at nanoscale while displaying 3D porous architectures at mesoscale, with adjustable metallic Ni content in a wide range. The unique mesostructure boosts the EES performance of NiO to its theoretical limit with the ultrahigh specific capacity, high rate capability and stability. The superior performance is well correlated with the synergism of the high accessibility to electrolyte, short solid-state ion diffusion length, and much enhanced conductivity of the mesostructured NiO/Ni composites. This study demonstrates a new strategy likely applicable to other transition metal oxides in maximizing their potential in energy storage, i.e. by constructing the similar mesostructured metal-oxide/metal composites.

Iron/nitrogen/carbon (Fe/N/C) catalyst is so far the most promising non-precious metal electrocatalyst for oxygen reduction reaction (ORR) in acidic medium, whose performance depends closely on the synthesis chemistry. Herein, we report a MnOx-induced strategy to construct the Fe/N/C with highly exposed Fe–Nx active sites, which involves the uniform spreading of polyaniline on hierarchical N-doped carbon nanocages by a reactive-template polymerization, followed by the successive iron incorporation and polyaniline pyrolysis. The resulting Fe/N/C demonstrates an excellent ORR performance, including an onset potential of 0.92 V (vs. RHE), four electron selectivity, superb stability and immunity to methanol crossover. The excellent performance is well correlated with the greatly enhanced surface active sites of the catalyst stemming from the unique MnOx-induced strategy. This study provides an efficient approach for exploring the advanced ORR electrocatalysts by increasing the exposed active sites.

Pt-based electrocatalysts are the most popular for direct alcohol fuel cells, but their performances easily deteriorate for the oxygen reduction reaction (ORR) at the cathode because of the alcohol crossover effect. Herein, we report the novel Pt electrocatalyst encapsulated inside nitrogen-doped carbon nanocages (Pt@NCNC), which presents excellent alcohol-tolerant ORR activity and durability in acidic media, far superior to the Pt counterpart immobilized outside the nanocages (Pt/NCNC). The superb performance is correlated with the molecule-sieving effect of the micropores penetrating through the shells of the nanocages, which admit the small-sized oxygen and ions but block the large-sized alcohols into the nanocages. This mechanism is confirmed by examining the size dependence of ORR and alcohol oxidation activities by regulating the micropores sizes. This study provides a promising strategy to develop the superior alcohol-tolerant Pt-based ORR electrocatalyst in acidic media.

The electronic structures of carbon nanocages (CNCs) and nitrogen/phosphorus doped carbon nanocages (N-CNCs/P-CNCs) have been studied by X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and resonant X-ray emission spectroscopy (RXES). The doping configurations for N/P dopants are identified from the experiments. The results have shown that there are three major doping configurations for nitrogen but only one doping configuration for phosphorus. The nitrogen doping reveals the complex coexistence of graphite-like, pyridine-like and pyrrole-like configurations that are proved by density functional theory (DFT) simulations, while the phosphorus doping presents only the “graphite-like” configuration. The different configuration profiles result in less atomic structure ordering of N-CNCs than that of P-CNCs. XAS spectra obtained from both surface and bulk sensitive detection suggest different chemical environments between the interior and shell for all types of nanocages. The electronic structure modifications show significant difference between nitrogen and phosphorus doping from the DOS calculations.

Metal-free carbon-based nanomaterials have been widely studied as a type of excellent electrocatalyst for the oxygen reduction reaction (ORR) in an alkaline medium due to their great stability, wide availability, environmental acceptability, and strong resistance to poisonous gas. They usually show a much inferior activity in an acidic medium, and a strategy to modify sp2 carbon to facilitate the ORR in an acidic medium is critically needed. Herein, by taking carbon nanotubes (CNT) as the platform, and O2 chemisorption ability as the descriptor of activity in an acidic medium, a series of N, B, BOx, P and S mono- and multi-doped CNT(5,5) were studied using an ab initio modeling method. Eight configurations resulting from N mono-doping, BOx doping and B/N, BOx/N and P/N multi-doping are found to be potential active structures in an acidic medium due to their highly exothermic O2 chemisorption. These results indicate the great potential of sp2 carbon as an ORR electrocatalyst in an acidic medium, and also provide theoretical references for the exploration of carbon-based metal-free ORR electrocatalysts.

Prediction and design of various nanomaterials is a long-term dream in nanoscience and nanotechnology, which depends on the deep understanding on the growth mechanism. Herein, we report the successful prediction on the growth of AlN nanowires by nitriding Al69Ni31 alloy particles across the liquid-solid (β) phase region (1133–1638°C) based on the phase-equilibrium-dominated vapor-liquid-solid (PED-VLS) mechanism proposed in our previous study. All predictions about the growth of AlN nanowires, the evolutions of lattice parameters and geometries of the coexisting Al-Ni alloy phases are experimentally confirmed quantitatively. The preconditions for the applicability of the PED-VLS mechanism are also clarified. This progress provides the further evidence for the validity of the PED-VLS mechanism and demonstrates a practical guidance for designing and synthesizing different nanomaterials according to corresponding phase diagrams based on the insight into the growth mechanism.纳米材料的预测和设计是纳米科学与技术领域的长期梦想, 该梦想的实现有赖于对生长机理的深刻理解. 本文基于我们前期研究揭示 的相平衡主导的气-液-固(VLS)生长机理, 成功地预测了在1133~1638°C温区内通过氮化Al69Ni31合金颗粒生长AlN纳米线的过程, 有关AlN纳米 线的生长、共存Al-Ni合金相的晶格参数及形貌演变等预测均得到了定量化实验结果的证实, 并界定了相平衡主导的VLS生长机理的适用条件. 本文为相平衡主导的VLS生长机理的有效性提供了进一步的实验证据, 同时展示了在生长机理的指导下根据相图设计和制备纳米材料的一个 实例.

Exploring cheap and stable electrocatalysts to replace Pt for the oxygen reduction reaction (ORR) is now the key issue for the large-scale application of fuel cells. Herein, we report an alloyed Co–Mo nitride electrocatalyst supported on nitrogen-doped carbon nanocages (NCNCs) which combines the merits of cobalt nitride and molybdenum nitride, showing high activity comparable to that of cobalt nitride and progressively enhanced stability with the increase in the Mo ratio. The typical Co0.5Mo0.5Ny/NCNCs catalyst demonstrates excellent ORR performance in acidic medium with a high onset potential of 808 mV vs RHE, superior stability (>80% retention after 100 h of continuous testing in 0.5 mol L–1 H2SO4), a dominant four-electron catalytic process, and good immunity to methanol crossover. Together with the convenient and scalable preparation as well as the low cost, the alloyed Co–Mo nitride electrocatalyst shows great potential in application for fuel cells. This study also suggests a promising strategy to develop non-precious-metal ORR electrocatalysts in acidic medium: i.e., to construct the alloyed compounds by combining substances with respective high activity and high stability.Keywords: acidic medium; alloyed Co−Mo nitride; electrocatalysts; nonprecious metal; oxygen reduction reaction

While the field of carbon-based metal-free electrocatalysts for oxygen reduction reaction (ORR) has experienced great progress in recent years, the fundamental issue of the origin of ORR activity is far from being clarified. To date, the ORR activities of these electrocatalysts are usually attributed to different dopants, while the contribution of intrinsic carbon defects has been explored little. Herein, we report the high ORR activity of the defective carbon nanocages, which is better than that of the B-doped carbon nanotubes and comparable to that of the N-doped carbon nanostructures. Density functional theory calculations indicate that pentagon and zigzag edge defects are responsible for the high ORR activity. The mutually corroborated experimental and theoretical results reveal the significant contribution of the intrinsic carbon defects to ORR activity, which is crucial for understanding the ORR origin and exploring the advanced carbon-based metal-free electrocatalysts.Keywords: carbon nanocages; density functional calculations; electrocatalysis; intrinsic carbon defects; oxygen reduction reaction

•Novel 3D hierarchical carbon nanocages with high pore volume, network geometry and good conductivity.•High-loading confinement of ~80 wt% sulfur inside the nanocages.•Much alleviated polysulfides dissolution due to the confinement.•The high-rate performance for the high-sulfur-loading carbon-sulfur composites.•Well established correlation between the high performance and unique structure.Lithium–sulfur batteries are hindered by the low utilization of sulfur, short cycle life and poor rate capability which are severe challenges today. Herein we report a new kind of carbon–sulfur composites by infusing sulfur into the novel hierarchical carbon nanocages (hCNC) with high pore volume, network geometry and good conductivity. The designed S@hCNC composite with a high sulfur loading of 79.8 wt% presents the large capacity, high-rate capability and long cycle life, which could shorten the charging time for mobile devices from hours to minutes. The excellent performance derives from the unique mesostructure of hCNC that enables the encapsulation of high-loading sulfur inside the carbon nanocages to alleviate polysulfide dissolution, meanwhile much enhance the electron conduction and Li-ion diffusion.

Composition regulation of semiconductors can engineer their bandgaps and hence tune their properties. Herein, we report the first synthesis of ternary ZnxCd1−xS semiconductor nanorods by superionic conductor (Ag2S)-mediated growth with [(C4H9)2NCS2]2M (M = Zn, Cd) as single-source precursors. The compositions of the ZnxCd1−xS nanorods are conveniently tuned over a wide range by adjusting the molar ratio of the corresponding precursors, leading to tunable bandgaps and hence the progressive evolution of the light absorption and photoluminescence spectra. The nanorods present well-distributed size and length, which are controlled by the uniform Ag2S nanoparticles and the fixed amount of the precursors. The results suggest the great potential of superionic conductor-mediated growth in composition regulation and bandgap engineering of chalcogenide nanowires/nanorods.

Novel hierarchical carbon nanocages (hCNCs) are proposed as high-rate anodes for Li- and Na-ion batteries. The unique structure of the porous network for hCNCs greatly favors electrolyte penetration, ion diffusion, electron conduction, and structural stability, resulting in high rate capability and excellent cyclability. For lithium storage, the corresponding electrode stores a steady reversible capacity of 970 mAh·g−1 at a rate of 0.1 A·g−1 after 10 cycles, and stabilizes at 229 mAh·g−1 after 10,000 cycles at a high rate of 25 A·g−1 (33 s for full-charging) while delivering a large specific power of \(37 kW \cdot kg_{electrode^{ - 1} }\) and specific energy of \(339 Wh \cdot kg_{electrode^{ - 1} }\). For sodium storage, the hCNC reaches a high discharge capacity of ∼50 mAh·g−1 even at a high rate of 10 A·g−1.

•Lamellar K2Co3(P2O7)2·2H2O nanocrystal whiskers are synthesized for the first time.•Inkjet printing was utilized to make a flexible solid-state device.•The assembled micro-device exhibits a high specific capacitance (6.0 F cm−3).•The device exhibits good rate/flexibility stability and long-cycling stability.•The device exhibits a maximun energy density of 0.96 mW h cm−3.A flexible all-solid-state asymmetric micro-supercapacitor based on lamellar (K2Co3(P2O7)2·2H2O) nanocrystal whiskers and graphene nanosheets was successfully fabricated by inkjet printing in a simple and cost-effective way. A facile method to synthesize lamellar K2Co3(P2O7)2·2H2O nanocrystal whiskers under a mild hydrothermal condition was also established. The assembled micro-device exhibited a high specific capacitance (6.0 F cm−3), good rate/mechanical stability and a long cycling stability (5000 cycles) with a maximun energy density of 0.96 mW h cm−3, demonstrating great promise for applications in flexible all-solid-state micro-supercapacitors.

Fischer–Tropsch synthesis (FTS) is a classical topic of great significance because of the approach of post-petroleum times. For decades, people have attempted to develop iron-based FTS catalysts with high selectivity for lower olefins. By means of the anchoring effect and the intrinsic basicity of nitrogen-doped carbon nanotubes (NCNTs), iron nanoparticles were conveniently immobilized on NCNTs without surface premodification. The so-constructed Fe/NCNTs catalyst presents superb catalytic performance in FTS with high selectivity for lower olefins of up to 46.7% as well as high activity and stability. The excellent performance is well-correlated with enhanced dissociative CO adsorption, inhibition of secondary hydrogenation of lower olefins, and promoted formation of the active phase of χ-Fe5C2. All of these merits result from participation of the nitrogen, as revealed by our experimental characterization. These results may lead to a new strategy for exploring advanced FTS catalysts with abundant N-doped carbon nanostructures.Keywords: basicity; Fischer−Tropsch synthesis; heterogeneous catalysis; iron nanoparticles; lower-olefin selectivity; nitrogen-doped carbon nanotubes

Two kinds of boron and nitrogen co-doped carbon nanotubes (CNTs) dominated by bonded or separated B and N are intentionally prepared, which present distinct oxygen reduction reaction (ORR) performances. The experimental and theoretical results indicate that the bonded case cannot, while the separated one can, turn the inert CNTs into ORR electrocatalysts. This progress demonstrates the crucial role of the doping microstructure on ORR performance, which is of significance in exploring the advanced C-based metal-free electrocatalysts.

Here we report the construction of CsI–AlN hybrid nanostructures by evaporating CsI onto preformed AlN nanocone arrays. The field emission performances of the hybrid samples are much improved due to the lower work function, better conductivity and more emission sites, depending on the size and density of the CsI nanoparticles. But the performance gradually decays after ca. 45 min stable emission due to the instability of CsI during field emission which has been neglected to date. The results suggest that further effort to prevent the loss of Cs species to improve the stability of CsI-decorated hybrid nanostructures is required for practical applications.

Herein we report the synthesis of vertically aligned AlN nanostructures on conductive substrates through the chemical reaction between AlCl3 and NH3 in the temperature range of 650–850 °C. The morphologies of the AlN nanostructures could be controllably modulated from cone-like to rod-like geometries by increasing the reaction temperature. The formation mechanism of the AlN nanostructures on the nitrified Ti substrates has been discussed based on the analysis of the intermediate products. The field emission (FE) property of AlN nanocones grown on the nitrified Ti substrate is better than that for AlN nanocones on Si substrate. The improvement of FE property can be attributed to the lower resistance between AlN nanocones and the nitrified Ti substrate because the conductive titanium nitride film can directly contact with AlN emitters while a high-resistive silica layer would easily form between Si substrate and AlN nanocones. These results indicate that the deposition of nanoscale filed emitters on conductive substrates is an effective way to improve the FE behavior, and may find potential applications in FE devices.Highlights► AlN nanocone arrays are promising field emitters. ► The substrates cannot influence the morphologies of AlN products. ► AlN products can evolve from cone-like to rod-like geometries with temperature. ► AlN nanocones on conductive substrate show better field emission properties. ► Better performance arising from better electrical contact.

AlGaN ternary alloys have unique properties suitable for numerous applications due to their tunable direct band gap from 3.4 to 6.2 eV by changing the composition. Herein we report a convenient chemical vapor deposition growth of the quasi-aligned AlxGa1−xN alloy nanocones over the entire composition range. The nanocones were grown on Si substrates in large area by the reactions between GaCl3, AlCl3 vapors, and NH3 gas under moderate temperature around 700 °C. The as-prepared wurtzite AlxGa1−xN nanocones have single-crystalline structure preferentially growing along the c-axis, with homogeneous composition distribution, as revealed by the characterizations of electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and selected area electron diffraction. The continuous composition tunability is also demonstrated by the progressive evolutions of the band edge emission in cathodoluminescence and the turn-on and threshold fields in field emission measurements. The successful preparation of AlxGa1−xN nanocones provides the new possibility for the further development of advanced nano- and opto-electronic devices.Keywords (): chemical vapor deposition; composition regulation; one-dimensional nanostructures; single phase; ternary AlGaN alloy

The vapor−liquid−solid (VLS) growth model has been widely used to direct the growth of one-dimensional (1D) nanomaterials, but the origin of the proposed process has not been experimentally confirmed. Here we report the experimental evidence of the origin of VLS growth. Al69Ni31 alloyed particles are used as “catalysts” for growing AlN nanowires by nitridation reaction in N2−NH3 at different temperatures. The nanowire growth occurs following the emergence of the catalyst droplets as revealed by in situ X-ray diffraction and thermal analysis. The physicochemical process involved has been elucidated by quantitative analysis on the evolution of the lattice parameters and relative contents of the nitridation products. These direct experimental results reveal that VLS growth of AlN nanowires is dominated by the phase equilibrium of the Al−Ni alloy catalyst. The in-depth insight into the VLS mechanism indicates the general validity of this growth model and may facilitate the rational design and controllable growth of 1D nanomaterials according to the corresponding phase diagrams.

Carbon nanotubes (CNTs) functionalized by metals have great potential for many applications. Our recent experimental results indicated that Pt-based nanoparticles could be directly immobilized onto nitrogen-doped carbon nanotubes (CNxNTs) through nitrogen mediation without pre-modification. The nanocomposite catalysts thus obtained exhibited excellent catalytic activity in direct methanol oxidation and oxygen reduction reaction, indicating their promising important application in fuel cells. In this study, the adsorptions of 12 different transition metals (TMs = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Pd, and Pt) on CNxNTs have been systematically studied using density functional theory calculations. And the interactions of the Pt/CNxNT catalyst with the common species CH3OH, HCHO and HCOOH involved in methanol oxidation have also been investigated in detail. The results indicate that nitrogen incorporation could efficiently enhance the binding energy of TMs on CNxNTs compared with the cases on CNTs. And the electronic states of the transition metals could also be modified, consequently the Pt/CNxNT catalyst is superior to the Pt/CNT catalyst for methanol oxidation, presenting improved performance for reactant adsorption. The theoretical understanding on the interactions between TMs and CNxNTs, and between reactants and Pt/CNxNT catalyst indicate obvious advantages of CNxNTs over CNTs for the construction and optimization of nanocomposite catalysts, which suggests great potential applications in many fields.

Taking the advantage of the inherent chemical activity arisen from the nitrogen incorporation for nitrogen-doped carbon nanotubes (NCNTs), we have developed a facile strategy for the construction of binary Pt–Ru/NCNT electrocatalysts. Alloyed Pt–Ru nanoparticles have been directly immobilized onto the outer surface of NCNTs without pre-modification due to the nitrogen participation. The Pt–Ru nanoparticles have a high dispersion, a narrow size distribution of 2.5–3.5 nm and tunable chemical composition. These catalysts have been evaluated for methanol oxidation and show good stability and better CO tolerance than the monometallic Pt/NCNT catalyst due to the bifunctional and electronic effects. The Pt5Ru5/NCNT catalyst shows superior electrocatalytic performance to the commercial Pt5Ru5/C catalyst. The easy fabrication and excellent performance of the NCNT-based Pt–Ru alloy catalysts indicate their potential application in direct methanol fuel cells.

X-ray diffraction (XRD), Mössbauer spectroscopy, and temperature-programmed reduction (TPR) were employed to investigate the dispersion and reduction behaviors of the Fe2O3/CuO/γ-Al2O3 system. The results indicated that: (1) the crystalline CuO particle in the CuO/γ-Al2O3 samples was redispersed during impregnating CuO/γ-Al2O3 samples with Fe(NO3)3 solutions; (2) two different dispersion states of surface iron species could be observed, i.e., State I corresponding to the iron(III) species located in the D layer on the surface of γ-Al2O3 and State II corresponding to those in the C layer. The dispersed states of surface iron(III) species were closely related to the iron loading amount; (3) the copper species located in the D layer of alumina surface was easily reduced and the copper species located in the C layer were more stable, which could be due to the influence of the iron(III) species in the different layers; (4) in the NO + CO reaction, the catalytic performances were enhanced due to the Cu–Fe synergism and the main active species in this system should be the surface-dispersed copper oxide species.The dispersed copper oxide and ferric oxide locate in the different layers of the preferentially exposed (1 1 0) plane of γ-Al2O3.

Large-scale arrays of aligned ZnO hexagonal nanorods with planar ends have been achieved on a bare native n-type (100) Si substrate via a simple electrochemical deposition in Zn(NH3)4(NO3)2 solution with a pure Zn sheet as the anode without using any catalysts, additives, and additional seed crystals. The ZnO hexagonal nanorods grow from the single-crystal seeds self-formed in the initial stage of the electrochemical deposition. The diameters and the lengths of the ZnO nanorods can be tailored by adjusting the electrolyte concentration and the electrodeposition duration. Field-emission measurements show that the ZnO nanorods with smaller diameters exhibit lower turn-on field and higher current density. The well-aligned ZnO hexagonal nanorods (on Si substrate) with tunable size and field emission performance may have potentials in the future of nanotechnology.

A new kind of carbon nitride (CNx) nanofiber has been prepared by the calcination of polypyrrole nanowires at 800 °C. The product maintained a wire-like morphology during calcination, and the pyrrolic nitrogen in the polypyrrole nanowires gradually changed to pyridinic and graphitic nitrogen as annealing temperature increased. These CNx nanofibers, prepared at 800 °C, have a nitrogen concentration of about 10%. Pt nanoparticles with average size of ∼3 nm could therefore be easily immobilized onto the CNx nanofibers because of the inherent chemical activity arising from the nitrogen incorporation. The Pt/CNx composite catalyst thus obtained has a large electrochemically active area and gives good electrocatalytic performance for methanol oxidation, both in activity and stability, suggesting it has potential application in fuel cells.

Patterned growth of AlN nanocones on a Ni-coated Si substrate is demonstrated through the reaction between AlCl3 and NH3 at 700 °C with Mo grid as a mask. The AlN nanocones are selectively deposited in the hollow region of the mask with diameters of ∼10 nm at the tips and 50−60 nm at the roots. The field-emission (FE) performance is effectively enhanced by the patterned growth mainly because of the decreased screening effect, and both turn-on and threshold fields are dramatically decreased, less than half of the corresponding ones for the unpatterned product with similar sizes. The results indicate that patterned growth is an efficient and reproducible way to enhance the FE performance of AlN nanocones, which could be applied to optimize the FE properties of other nanoscale field emitters.Keywords: aluminum nitride; field emission; one-dimensional nanostructure; patterned growth

One-dimensional ZnO nanostructures have been in situ grown on the conductive brass substrate by directly heating the Cu0.70Zn0.30 alloy foils in O2/Ar at 550−900 °C. The growth process and products have been well characterized by means of X-ray diffraction, Raman scattering, electron microscopy, X-ray photoelectron, and photoluminescence spectroscopy. The results indicate that more Zn species is lost from the brass foils at the higher temperatures. Zn species is exhausted, and a sharp diffraction peak shift appears around 750 °C due to the quick depletion. The structures of the ZnO products have been systematically regulated and the more oxygen vacancy is associated with the higher reaction temperature. The correlation in between the oxygen vacancy concentration and the green and exciton emission for these products has been experimentally demonstrated and well understood, which is important for luminescence applications.

The growth mechanism of carbon nanotubes (CNTs) catalyzed by nickel catalyst with benzene precursor has been studied by using density functional theory calculations. The typical initial process of the graphene growth is considered as the formation of biphenyl on Ni(111) surface with two benzene molecules and the minimum energy pathway for this formation has been investigated in detail. It is found that the carbon−hydrogen bond could be selectively dissociated while the carbon−carbon connection still remained. Thus the dehydrogenated hexagonal rings are generated, which could form the graphene sheet on the catalyst surface through radical incorporation for sequential growth of CNTs. The calculations rationalize the six-membered-ring-based growth mechanism previously proposed according to in situ experimental results. The train of thought of this radical mechanism also should be enlightening for the synthesis of doped CNTs with some suitable heterocyclic precursors, which is of great importance for functionalization and wide applications.

In situ chloride-generated route has been extended to prepare AlN nanostructures on Si substrate in the moderate temperature range of 680−800 °C. Different AlN nanostructures, including polycrystalline films, nanoflowers, nanoballs as well as quasi-aligned nanowires, have been conveniently obtained by regulating reaction temperature or changing reaction procedure. On the basis of the morphological evolution of the products and the involved chemical reactions, the growth process of this route was well discussed accordingly, and the low-temperature advantage could be attributed to the generation of the intermediate AlCl. The interesting results in this study indicate the wide potential applications of the in situ chloride-generated route for preparing many other advanced nanomaterials.

Pt nanoparticles with sizes of 3–9 nm were well dispersed on carbon nitride (CNx) nanotubes without needing pre-surface modification on the CNx nanotubes due to the inherent chemical activity. The products were characterized by transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. All the experimental results revealed that Pt nanoparticles were immobilized on the CNx nanotubes due to the N-participation in the connection of Pt species with the support. The electrocatalytic property of the as-prepared Pt/CNx catalyst in methanol oxidation was examined by cyclic voltammetry. The results reveal that the so-constructed Pt/CNx catalyst has obvious catalytic activity, suggesting potential applications in fuel cells.

Hollow nanostructures have attracted increasing attention due to the unique properties and potential applications in many fields such as drug delivery, biomedical agents, energy storage, and reaction containers. In this paper, we report the preparation of cerium fluoride (CeF3) hollow nanostructures including nanocages, nanorings, nanococoons, and circular hollow disks through a facile hydrothermal process. The morphologies (hollow or solid) and cross sections (hexagonal or circular) of as-prepared nanostructures are dependent on the hydrothermal temperatures. All the precursors including KBrO3, Ce(IV) ions, H2SO4, and organic reducer (for example, citric acid or malonic acid), are necessary agents for the formation of CeF3 hollow nanostructures. On the basis of these experimental results, it is proposed that the growth microenvironment of CeF3 products is controlled by the Belousov−Zhabotinsky oscillating reaction, and the obtained hollow nanostructures could be regarded as the replicas of spatial patterns formed in the unstirred bromate−citric acid−Ce(IV)−H2SO4 oscillating system. The strong and uniform blue light emission of CeF3 hollow nanostructures implies their potential applications in light-emitting devices.

An in situ generated template method has been developed via the simple reaction of AlCl 3 with NH 3 for the synthesis of AlN hollow nanospheres with diameters of 80−400 nm and shell thickness of ∼15 nm. On the basis of the detailed characterization on the intermediate compounds, the preparation process has been elucidated which includes the first formation of the AlCl 3· xNH 3@AlN core−shell nanostructures followed by the decomposition of the inner compound around 1100 °C. This reaction route successfully extended the traditional CVD reaction of AlCl 3 with NH 3 from producing solid AlN powder to preparing AlN hollow nanospheres.

Controllable synthesis of well-shaped nanocrystals is of significant importance for understanding the surface-related properties as well as for the exploration of potential applications. Herein, CeO2 nanorods and nanocubes were selectively synthesized using cerium(III) chloride and cerium(III) nitrate as precursor, respectively. Counter anions of the cerium source were crucial to the shapes of the resulting products. Intriguingly, the as-synthesized nanorods could be converted into nanocubes by the addition of an appropriate amount of NO3− ions into the hydrothermal reaction. The NO3− ions are considered as both a capping agent and an oxidizer during the formation of CeO2 nanocubes. Moreover, the influences of several others anions are investigated. Br−, I−, and SO42− ions have similar roles to Cl− ions, which lead to the formation of nanorods. The introduction of BrO3− ions can bring on the generation of irregular nanoparticles because they can function as an oxidizer but not a capping agent. The anion-induced controllable growth process is simple and low cost, which makes this strategy potentially useful for the preparation of other faceted nanostructures.

Herein we demonstrate a convenient regulation of several aluminum nitride nanostructures through direct nitridation of aluminum precursor under different conditions. Different AlN nanostructures including conic nanoflowers, nanowires, quasi-aligned nanocones and polycrystalline thin film have been obtained on the Au-coated Si substrates just simply by decreasing the reaction temperature or changing the reaction procedure, and a four-stage growth mechanism is hereby deduced. The conic nanoflower composed of AlN nanocones is an interesting geometry. The photoluminescence and field emission measurement revealed that these AlN nanoflowers own a broad blue emission band and a rather good field emission property, suggesting the potential applications in light and field emission nanodevices.

Polycrystalline hollow AlN nanospheres with diameters ranging from 20 to 200 nm and shell thickness of about 10 nm were successfully synthesized through the reaction of irregular Al nanopowder with a CH4–NH3 mixture at around 1000 °C by the self-templated method. The products were well characterized by X-ray diffraction, high-resolution transmission electron microscopy, Raman spectroscopy and photoluminescence measurements. The photoluminescence properties of the hollow AlN nanospheres showed the routine blue emissions for AlN nanoparticles as well as an unusual green emission at around 533 nm, indicating the potential for luminescent devices. The synthesis mechanism was reasonably speculated and further supported by the similar synthesis of hollow AlN nanospheres from regular Al particles.

Carbon-coated iron nanoparticles with diameters ranging from 5 to 50 nm and a few layers of graphitic shells have been synthesized in a mass scale by laser-induction complex heating evaporation. Through the studies on the dissolution behavior of the inner iron cores under the treatments of HCl and HNO3 acids at different conditions, a practical route with the combined treatments of HNO3 and HCl acids has been optimized to produce carbon nanocages. The nanocages thus obtained have some channels and are full of defects in the shells, as characterized by high-resolution transmission electron microscopy and Raman spectroscopy. Similar treatments should also be applicable to some other carbon-coated metal nanoparticles, e.g., Ni–C and Co–C, for the same purpose.

Hexagonal AlN (h-AlN) nanowires with an average diameter of around 15 nm have been prepared by an extended vapor–liquid–solid growth technique and characterized by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray analysis, Raman spectroscopy and field emission measurements. This preparation is a rather simple route for bulk fabrication of h-AlN nanowires. The promising field emission property observed for h-AlN nanowires points to the important application potential of this material.

Polytetrafluoroethylene (PTFE) films were treated by non-equilibrium microwave plasma of a mixture of water vapor and argon at low pressure. The properties of the PTFE surface were evaluated by means of contact angle measurements, adhesion strength measurements, attenuated total reflectance infrared spectrometry (ATR IR), and X-ray photoelectron spectrometry (XPS). The influence of some plasma parameters, such as microwave power and treatment time, has been studied. H2O/Ar plasma treatment led to a significant decrease of the water contact angle from 110.0° to 23.6° under the optimal condition, which resulted from the substantial surface defluoration and the introduction of an unusual amount of oxygen and polar functions as revealed by XPS and ATR IR spectra. When the treated samples were kept in air at room temperature, the contact angles increased markedly within a few days probably due to the evolution of chemical composition and structure of the treated PTFE surface during storage. Nevertheless, after the contact angle had reached a constant value of 60°, the plasma-treated PTFE still exhibited significantly improved wettability. Adhesion strength was improved from 26.7 to 583 N cm−2, a factor of 22, after plasma treatments.

High quality metastable wurtzite γ-MnS nanowires were synthesized via solution–solid–solid (SSS) growth in a mixed solvent of 1-dodecylamine and 1-dodecanethiol at a specified volume ratio. The length of the γ-MnS nanowires can be conveniently and well controlled by adjusting the amount ratio of [(C4H9)2NCS2]2Mn precursors to Ag2S catalysts. The results suggest the great potential of the SSS growth in the synthesis of high quality metastable chalcogenide nanowires.

Here we report the construction of CsI–AlN hybrid nanostructures by evaporating CsI onto preformed AlN nanocone arrays. The field emission performances of the hybrid samples are much improved due to the lower work function, better conductivity and more emission sites, depending on the size and density of the CsI nanoparticles. But the performance gradually decays after ca. 45 min stable emission due to the instability of CsI during field emission which has been neglected to date. The results suggest that further effort to prevent the loss of Cs species to improve the stability of CsI-decorated hybrid nanostructures is required for practical applications.

Pt nanoparticles with sizes of 3–9 nm were well dispersed on carbon nitride (CNx) nanotubes without needing pre-surface modification on the CNx nanotubes due to the inherent chemical activity. The products were characterized by transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. All the experimental results revealed that Pt nanoparticles were immobilized on the CNx nanotubes due to the N-participation in the connection of Pt species with the support. The electrocatalytic property of the as-prepared Pt/CNx catalyst in methanol oxidation was examined by cyclic voltammetry. The results reveal that the so-constructed Pt/CNx catalyst has obvious catalytic activity, suggesting potential applications in fuel cells.

Currently, iron nitride (Fe2N) and iron carbide (Fe3C), which are regarded as the new non-precious metal electrocatalysts for the oxygen reduction reaction (ORR) with high activity and stability in acidic medium, are attracting increasing attention. Herein, by systematic comparison of the ORR activities for Fe2N- and Fe3C-based catalysts designed with or without a solid nitrogen source, we found that only the former is highly active for ORR while the latter is quite poorly active despite their similar crystalline phases. This result indicates that the Fe–N related species are responsible for the high ORR activities of the Fe-based catalysts, similar to the case of Fe/N/C catalysts. Density functional theory calculations demonstrate that the Fe–N4/C moiety has a far superior ORR activity to that of Fe2N and Fe3C. The experimental and theoretical results mutually support that the high activities of the Fe-based catalysts originate from Fe–Nx/C moieties (x ≥ 4) rather than Fe2N or Fe3C phases, which is significant for exploring advanced Fe-based electrocatalysts.

Carbon nanotubes (CNTs) functionalized by metals have great potential for many applications. Our recent experimental results indicated that Pt-based nanoparticles could be directly immobilized onto nitrogen-doped carbon nanotubes (CNxNTs) through nitrogen mediation without pre-modification. The nanocomposite catalysts thus obtained exhibited excellent catalytic activity in direct methanol oxidation and oxygen reduction reaction, indicating their promising important application in fuel cells. In this study, the adsorptions of 12 different transition metals (TMs = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Pd, and Pt) on CNxNTs have been systematically studied using density functional theory calculations. And the interactions of the Pt/CNxNT catalyst with the common species CH3OH, HCHO and HCOOH involved in methanol oxidation have also been investigated in detail. The results indicate that nitrogen incorporation could efficiently enhance the binding energy of TMs on CNxNTs compared with the cases on CNTs. And the electronic states of the transition metals could also be modified, consequently the Pt/CNxNT catalyst is superior to the Pt/CNT catalyst for methanol oxidation, presenting improved performance for reactant adsorption. The theoretical understanding on the interactions between TMs and CNxNTs, and between reactants and Pt/CNxNT catalyst indicate obvious advantages of CNxNTs over CNTs for the construction and optimization of nanocomposite catalysts, which suggests great potential applications in many fields.