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NiFe (oxy)hydroxides (NiFeOOH) are considered as one of the most efficient electrocatalysts for water splitting. Although several strategies recently have been developed to fabricate NiFeOOH electrodes, such as electrodeposition, hydrothermal reaction, corrosion engineering, etc., it has been a challenge to directly tune the energy level, conductivity and surface properties (such as molecular adsorption) of NiFeOOH with less time-consuming and convenient synthesis to overcome the relatively sluggish reaction kinetics. Herein, we, for the first time, utilize charged droplets to synthesize NiFeOOH with abundant oxygen vacancies (Ov) via the convenient electrospraying (ESI) approach without necessitating additional post-treatments. Specifically, the optimized NiFeOOH delivers much lower overpotentials of 145 mV, 215 mV and 360 mV for the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER) and overall water splitting in 1 M KOH at 10 mA cm−2 with long-term durability over 180 h. The good performing electrocatalyst is much improved compared with the benchmark Pt/C/NF in the high current density region, RuO2/NF and other NiFe-based electrocatalysts. Simultaneously, the unique reaction environment of the droplet (confinement, superacid and desolvation) was further explored to understand the synthesis reaction acceleration process and related oxygen defect formation mechanisms. Also, we used density functional theory (DFT) to investigate the change of free energy of the reaction pathway caused by oxygen vacancies and understand the reaction mechanism of electrocatalysts prepared by different treatment methods. The calculation results show that the shortened Fe–O bonds and oxygen defects of NiFeOOH synergistically improve the interaction between the metal cations and the intermediate species and further accelerate the overall reaction kinetics. This work demonstrates that the charged microdroplet chemistry offers a novel means to expedite the design of powerful electrocatalysts.

Plasmonic metals under photoexcitation can generate energetic hot electrons to directly induce chemical reactions. However, the capability and fundamental insights of the transportation of these hot electrons at plasmonic metal-2D material interfaces remain unclear. Herein, hot-electron transfer at Au-graphene interfaces has been in situ studied using surface-enhanced Raman spectroscopy (SERS) with atomic layer accuracy. Combining in situ SERS studies with density functional theory calculations, it is proved that hot electrons can be injected from plasmonic Au nanoparticles to graphene and directly penetrate graphene to trigger photocatalytic reactions. With increasing graphene layers, the transportation of hot electrons decays rapidly and would be completely blocked after five layers of graphene. Moreover, the transfer of hot electrons can be modulated by applying an external electric field, and the hot-electron transfer efficiency under electrochemical conditions is improved by over three times in the presence of a monolayer of graphene. These fundamental understandings about hot-electron transfer provide insightful information to promote the design of more efficient plasmonic materials and devices.

The energy crisis is a continuing topic for all human beings, threatening the development of human society. Accordingly, harvesting energy from the surrounding environment, such as wind, water flow and solar power, has become a promising direction for the research community. Water contains tremendous energy in a variety of forms, such as rivers, ocean waves, tides, and raindrops. Among them, raindrop energy is the most abundant. Raindrop energy not only can complement other forms of energy, such as solar energy, but also have potential applications in wearable and universal energy collectors. Over the past few years, droplet-based electricity nanogenerators (DENG) have attracted significant attention due to their advantages of small size and high power. To date, a variety of fundamental materials and ingenious structural designs have been proposed to achieve efficient droplet-based energy harvesting. The research and application of DENG in various fields have received widespread attention. In this review, we focus on the fundamental mechanism and recent progress of droplet-based nanogenerators in the following three aspects: droplet properties, energy harvesting and self-powered sensing. Finally, some challenges and further outlook for droplet-based nanogenerators are discussed to boost the future development of this promising field.

Nanostructured piezoelectric semiconductors offer unprecedented opportunities for high-performance sensing in numerous catalytic processes of biomedical, pharmaceutical, and agricultural interests, leveraging piezocatalysis that enhances the catalytic efficiency with the strain-induced piezoelectric field. Here, a cost-efficient, high-performance piezo-electrocatalytic sensor for detecting l-ascorbic acid (AA), a critical chemical for many organisms, metabolic processes, and medical treatments, is designed and demonstrated. Zinc oxide (ZnO) nanorods and nanosheets are prepared to characterize and compare their efficacy for the piezo-electrocatalysis of AA. The electrocatalytic efficacy of AA is significantly boosted by the piezoelectric polarization induced in the nanostructured semiconducting ZnO catalysts. The charge transfer between the strained ZnO nanostructures and AA is elucidated to reveal the mechanism for the related piezo-electrocatalytic process. The low-temperature synthesis of high-quality ZnO nanostructures allows low-cost, scalable production, and integration directly into wearable electrocatalytic sensors whose performance can be boosted by otherwise wasted mechanical energy from the working environment, for example, human-generated mechanical signals.

In the new era of internet of things, big data collection and analysis based on widely distributed intelligent sensing technology is particularly important. Here, we report a flexible and durable wood-based triboelectric nanogenerator for self-powered sensing in athletic big data analytics. Based on a simple and effective strategy, natural wood can be converted into a high-performance triboelectric material with excellent mechanical properties, such as 7.5-fold enhancement in strength, superior flexibility, wear resistance and processability. The electrical output performance is also enhanced by more than 70% compared with natural wood. A self-powered falling point distribution statistical system and an edge ball judgement system are further developed to provide training guidance and real-time competition assistance for both athletes and referees. This work can not only expand the application area of the self-powered system to smart sport monitoring and assisting, but also promote the development of big data analytics in intelligent sports industry., Intelligent sensing technologies gain interest for the internet of things and applications that require collection and analysis of big data. Here the authors report a flexible and durable wood-based triboelectric nanogenerator for self-powered sensing in athletic big data analytics.

The chemical reactivity and/or the diffusion of Ag atoms or ions during thermal processing can cause irreversible structural damage, hindering the application of Ag nanowires (NWs) in transparent c...

Understanding the role of the morphology and particle–particle interactions on the plasmonic properties is of significant importance for the development of nanomaterials with excellent optical properties. However, the preparation of precisely defined nanomaterials with sizes that span a large range and their controllable self-assembly still remain a great challenge. Here, a multistep seed-mediated method has been established for preparing uniform Ag nanocubes over a broad size range from nanoscale (50 nm) to microscale (1400 nm) and with different hierarchical nanostructures range from “zero-dimension” (“0D”) to “three-dimension” (“3D”). The influence of the size and the interactions between the Ag nanocubes on their surface-enhanced Raman scattering (SERS) properties have been systematically and quantitatively investigated. It is demonstrated through experiments and finite-difference time-domain (FDTD) calculations that the SERS activity is dependent on the matching of the nanocube size to the excitation wavelength. The optimal combinations are 80, 110 and 130 nm nanocubes with respect to 532, 638 and 785 nm excitation wavelength, respectively. Furthermore, the Raman enhancement of the Ag nanocube hierarchical nanostructures increases rapidly from “0D” to “3D”, due to the extra increase of the hot spots that is attributed to the out-of-plane plasmonic coupling realized in the “3D” hierarchical nanostructures. This work clearly illustrates the quantitative role of the size and dimension of Ag nanocubes on their SERS properties and provides fundamental information for the design of advanced nanomaterials with higher SERS sensitivity.

Carbon-based anode materials hold a promising future for sodium-ion batteries (SIBs) due to their natural abundance and low cost of development. In spite of carbon's important role in the commercialization of lithium-ion batteries (LIBs), further exploration is necessary in order to find high-performance, high-rate carbon anode materials for SIBs. A honeycomb-like, nitrogen-rich (17.72 at%), hierarchically porous, and highly disordered carbonaceous material (N-HC) with an expanded interlayer distance (0.44 nm in average) is synthesized by spray drying and subsequent pyrolysis under flowing NH3. The hierarchically porous structure and rich nitrogen doping result in a large specific surface area (722 m2 g−1), more defects and active sites, and greater functional interface accessibility for the active porous carbonaceous material and electrolyte. When N-HC is used as the anode material for SIBs, the batteries display favorable discharge capacities (255.9 mA h g−1 in the 3000th cycle at 500 mA g−1) and good capacitive-energy-storage behavior (67% at a scan rate of 0.5 mV s−1) with excellent high-rate performance and ultra-stable cyclability over 10,000 cycles at 5000 mA g−1. Our results show that the combination of the hierarchically porous structure and nitrogen doping leads to improved energy storage by increasing the capacitive energy storage, which enhances the high-rate performance of N-HC. To further enhance the performance of the material, an electrical pretreatment is employed to increase the initial Coulombic efficiency of N-HC to 79.5%, a record high for an SIB cell. A full cell with an N-HC anode and a Na3V2(PO4)3/C cathode shows a high capacity with a favorable cyclability (238.7 mA h g−1 after 100 cycles at 100 mA g−1 and a capacity retention of 95.3% compared to the second cycle).

Smart sensing devices with high stretchability and self-powered characteristics are essential in future generation wearable human-integrated applications. Here we report for the first time scalable synthesis and integration of selenium (Se) nanowires into wearable piezoelectric devices, and explore the feasibility of such devices for self-powered sensing applications, e.g., physiological monitoring. The ultrathin device can be conformably worn onto the human body, effectively converting the imperceptible time-variant mechanical vibration from the human body into distinguishable electrical signals, e.g., gesture, vocal movement, and radial artery pulse, through straining the piezoelectric Se nanowires. Our results suggest the potential of solution-synthesized Se nanowire a new class of piezoelectric nanomaterial for self-powered biomedical devices and opens doors to new technologies in energy, electronics, and sensor applications.

Deformable energy devices capable of efficiently scavenging ubiquitous mechanical signals enable the realization of self-powered wearable electronic systems for emerging human-integrated technologies. Triboelectric nanogenerators (TENGs) utilizing soft polymers with embedded additives and engineered dielectric properties emerge as ideal candidates for such applications. However, the use of solid filler materials in the state-of-the-art TENGs limits the devices' mechanical deformability and long-term durability. The current structural design for TENGs faces the dilemma where the enhanced dielectric constant of the TENG's contact layer leads to an undesirable saturation of the surface charge density. Here, we present a novel scheme to address the above issues, by exploring a liquid-metal-inclusion based TENG (LMI-TENG) where inherently deformable core–shell LMIs are incorporated into wearable high-dielectric-constant polymers. Through a holistic approach integrating theoretical and experimental efforts, we identified the parameter space for designing an LMI-TENG with co-optimized output and mechanical deformability. As a proof of concept, we demonstrated an LMI-TENG based wireless media control system for a self-powered user interface. The device architecture and design scheme presented here provide a promising solution towards the realization of self-powered human-integrated technologies.

The ability of future electronics to derive operational power from the working environment is attractive for realizing self‐powered unattended electronics. Triboelectric nanogenerator (TENG) effectively harvests the otherwise wasted ubiquitous mechanical energy. Despite the recent advances in the design and development of various triboelectric devices, these devices' output still falls short in meeting the practical needs of directly powering electronics and sensors. Here, we review the recent progress in the design and optimization strategies for improving the TENG output performance, including but not limited to theoretical simulation, materials engineering, device engineering, and power management. The ubiquitous applications of the TENGs in micro/high‐voltage power source, multifunctional intelligence, blue energy harvesting, and self‐powered electrochemical system are also discussed. Finally, we provide our perspectives regarding the challenges and opportunities for the future development of triboelectric devices with engineered high‐performance and designer functionalities.

Two-dimensional (2-D) materials of atomic thickness have attracted considerable interest due to their excellent electrical, optoelectronic, mechanical, and thermal properties, which make them attractive for electronic devices, sensors, and energy systems. Scavenging the otherwise wasted energy from the ambient environment into electrical power holds promise to address the emerging energy needs, in particular for the portable and wearable devices. The versatile properties of 2-D materials together with their atomically thin body create diverse possibilities for the conversion of ambient energy. The present review focuses on the recent key advances in emerging energy-harvesting devices based on monolayer 2-D materials through various mechanisms such as photovoltaic, thermoelectric, piezoelectric, triboelectric, and hydrovoltaic devices, as well as progress for harvesting the osmotic pressure and Wi-Fi wireless energy. The representative achievements regarding the monolayer heterostructures and hybrid devices are also discussed. Finally, we provide a discussion of the challenges and opportunities for 2-D monolayer material-based energy-harvesting devices in the development of self-powered electronics and wearable technologies.

SiC oxidizes and is structurally unstable at temperatures above 1000 ​°C in a water-containing oxidative gaseous environment. Herein, we explore the hydrothermal stability of high-surface-area mesoporous SiC (m-SiC) prepared using Pluronic F-127 as structure directing agent, with and without boron doping as synthesized by a magnesiothermic reduction method (700 ​°C for 12 ​h). Under hydrothermal treatment at 800 ​°C in a water-saturated air stream, the undoped m-SiC is completely oxidized and reduced to amorphous SiO2. Lightly doped boron-substituted m-SiC (m-BxSiC, B/Si ​= ​0.04) is significantly more stable and retains the structural characteristics of m-SiC. m-BxSiC shows significant (300 ​°C) catalytic performance (CO conversion, HC conversion, and NO oxidation), better Pt dispersibility, and retention of a smaller Pt particle size. Studies of hierarchically porous m-BxSiC/PS prepared with F-127 and polystyrene dual templating further confirmed the enhancement by boron doping of the 800 ​°C hydrothermal stability and the 300 ​°C catalytic activity.

Development of electrode materials with high capability and long cycle life are central issues for lithium-ion batteries (LIBs). Here, we report an architecture of three-dimensional (3D) flexible silicon and graphene/carbon nanofibers (FSiGCNFs) with atomic-scale control of the expansion space as the binder-free anode for flexible LIBs. The FSiGCNFs with Si nanoparticles surrounded by accurate and controllable void spaces ensure excellent mechanical strength and afford sufficient space to overcome the damage caused by the volume expansion of Si nanoparticles during charge and discharge processes. This 3D porous structure possessing built-in void space between the Si and graphene/carbon matrix not only limits most solid-electrolyte interphase formation to the outer surface, instead of on the surface of individual NPs, and increases its stability but also achieves highly efficient channels for the fast transport of both electrons and lithium ions during cycling, thus offering outstanding electrochemical performance (2002 mAh g(-1) at a current density of 700 mA g(-1) over 1050 cycles corresponding to 3840 mAh g(-1) for silicon alone and 582 mAh g(-1) at the highest current density of 28 000 mA g(-1)).

Triboelectric nanogenerators (TENGs) have been invented as a new technology for harvesting mechanical energy, with enormous advantages. One of the major themes in their development is the improvement of the power output, which is fundamentally determined by the triboelectric charge density. Besides the demonstrated physical surface engineering methods to enhance this density, chemical surface functionalization to modify the surface potential could be a more effective and direct approach. In this paper, we introduced the method of using self-assembled monolayers (SAMs) to functionalize surfaces for the enhancement of TENGs' output. By using thiol molecules with different head groups to functionalize Au surfaces, the influence of head groups on both the surface potential and the triboelectric charge density was systematically studied, which reveals their direct correlation. With amine as the head group, the TENG's output power is enhanced by ∼4 times. By using silane-SAMs with an amine head group to modify the silica surface, this approach is also demonstrated for insulating triboelectric layers in TENGs. This research provides an important route for the future research on improving TENGs' output through materials optimization.

Developing lithium-ion batteries (LIBs)/sodium-ion batteries (SIBs) with high energy density is vital to meet increasingly demanding requirements for energy storage. The initial Coulombic efficiency (ICE) of LIBs and SIBs anode materials, which is associated with the amount of redundant cathode materials in full cells, is a key parameter for the improvement of energy density in batteries. Generally, the low ICE of anode materials is compensated by additional loading of cathode materials in current commercial LIBs. Nevertheless, because the specific capacity of common lithium-metal oxide cathodes is lower than that of anodes, an excessive amount of cathode materials (10%–15% for graphite anode material) must be added to overcome the insufficient ICE of anode materials, which leads to an appreciable reduction of energy density. Specifically, the reduction is about 5%–20% of total available capacity in commercial graphite electrodes; and, it can even be as high as 15–50% for next-generation high-capacity anode materials (such as Si and Sn). Much work has been devoted to exploring anode materials with high ICE in LIBs/SIBs; however, to the best of our knowledge, there does not yet exist a comprehensive review. Herein, we provide an overview of ICE of anode materials both in LIBs and SIBs. In this review, we first discuss the current understanding of the association between ICE and energy density. This is followed by a detailed assessment of the reasons of the initial capacity loss (low ICE) for various types of anode materials. A summary is then given of the growing number of methods and related fundamental mechanisms being used to enhance ICE. We conclude with a perspective on the current challenges and promising research directions that might lead to further improvements of the ICE and the fabrication of higher-energy-density batteries.

The rapid development of portable and wearable electronic devices has increased demand for flexible and efficient energy harvesting and storage units. Conventionally, these are built and used separately as discrete components. Herein, we propose a simple and cost-effective laser engraving technique for fabricating a flexible self-charging micro-supercapacitor power unit (SCMPU), by integrating a triboelectric nanogenerator (TENG) and a micro-supercapacitor (MSC) array into a single device. The SCMPU can be charged directly by ambient mechanical motion. We demonstrate the ability of the SCMPU to continuously power light-emitting diodes and a commercial hygrothermograph. This investigation may promote the development of sustainable self-powered systems and provide a promising new research application for supercapacitors.

Harvesting ambient mechanical energy is a key technology for realizing self-powered electronics, which has tremendous applications in wireless sensing networks, implantable devices, portable electronics, etc. The currently reported triboelectric nanogenerator (TENG) mainly uses solid materials, so that the contact between the two layers cannot be 100% with considering the roughness of the surfaces, which greatly reduces the total charge density that can be transferred and thus the total energy conversion efficiency. In this work, a liquid-metal-based triboelectric nanogenerator (LM-TENG) is developed for high power generation through conversion of mechanical energy, which allows a total contact between the metal and the dielectric. Due to that the liquid–solid contact induces large contacting surface and its shape adaptive with the polymer thin films, the LM-TENG exhibits a high output charge density of 430 μC m−2, which is four to five times of that using a solid thin film electrode. And its power density reaches 6.7 W m−2 and 133 kW m−3. More importantly, the instantaneous energy conversion efficiency is demonstrated to be as high as 70.6%. This provides a new approach for improving the performance of the TENG for special applications. Furthermore, the liquid easily fluctuates, which makes the LM-TENG inherently suitable for vibration energy harvesting.

In this work, the piezotronic effect is investigated for the first time in external electric fields ranging from 0 V·cm–1 to 2,000 V·cm–1 by using n-type ZnO nanowires supported by a flexible substrate. In the presence of an external electric field, the Schottky barrier height (SBH) is lowered by the image force, allowing more free carriers to pass through the metal-semiconductor junction and enhancing the screening effect on positive piezoelectric polarization charges. As the strength of the external electric field increases, the piezotronic effect is significantly suppressed and the metal-semiconductor contact finally exhibits Ohmic behavior. The experimental results show that devices can be classified into three groups, corresponding to low, moderate, and high carrier densities of the nanowires used. This work not only helps us to explicate the basic physical mechanism of the piezotronic effect in a harsh environment in an electric field but also provides guidelines for future design and fabrication of piezotronic devices.

Portable and flexible pressure sensors with highly sensitive and small size have great potential applications in areas such as wearable electronics, environmental monitoring, and medical equipment. Here, we demonstrate an integrated self-powered pressure sensing system made of a passive resistive pressure sensor and a triboelectric nanogenerator. Based on wrinkled and flexible polydimethylsiloxane films, the whole device is of sandwich structure with ultrahigh sensitivity to pressure (204.4kPa−1), which is more than one order of magnitude higher than all previously reported flexible pressure sensors. And our system exhibits a very low detection limit, rapid response time, and long-term stability. In addition, we built a self-powered, portable visualization system for semi-quantitative analysis of pressure, which can directly convert a pressure information to visual display.

To date, quite a few wearable electronics have entered the market, which are changing the life pattern of consumers. However, the limited lifetime and energy storage capacity have made rechargeable batteries the bottleneck in wearable technology, especially with the increase of number of wearable devices and their large distribution. To solve this problem, we demonstrate a woven-structured triboelectric nanogenerator (W-TENG) using commodity nylon fabric, polyester fabric, and conductive silver fiber fabric. With the advantage of being flexible, washable, breathable, wearable, and able to be triggered by a freestanding triboelectric layer, this W-TENG can move freely without any constraint and is suitable for wearable electronics. To demonstrate the potential applications of the W-TENG, the W-TENG is integrated into shoes, coats, and trousers to harvest different kinds of mechanical energy from human motion. This work presents a new approach in applying triboelectric nanogenerator to wearable devices.

The first application of an implanted triboelectric nanogenerator (iTENG) that enables harvesting energy from in vivo mechanical movement in breathing to directly drive a pacemaker is reported. The energy harvested by iTENG from animal breathing is stored in a capacitor and successfully drives a pacemaker prototype to regulate the heart rate of a rat. This research shows a feasible approach to scavenge biomechanical energy, and presents a crucial step forward for lifetime-implantable self-powered medical devices.

Transparent, flexible and highly efficient power sources are essential components of mobile electronics and optoelectronic devices. Here, based on the first generation of the transparent triboelectric nanogenerator (TENG), we demonstrate a simple and innovative design that can simultaneously improve the output performance and transmittance of the TENG. The improved TENG gives a maximum output up to 200 V and 7 μA at a current density of ∼0.78 μA cm−2. The TENG shows a high transmittance of ∼78%. To deeply understand the nature of the triboelectric effect, we investigated the influence of the UV–ozone treatment, surface properties, and surrounding environment on the output performance. Integrating the characterization results, we conclude that the tribocharge generation of the PDMS surface is probably due to the bond breaking of Si–O–Si groups, and is closely related to the surface properties and surrounding environment.

Portable and wearable personal electronics and smart security systems are accelerating the development of transparent, flexible, and thin-film electronic devices. Here, we report a transparent and flexible selfcharging power film (SCPF) that functions either as a power generator integrated with an energy storage unit or as a self-powered information input matrix. The SCPF possesses the capability of harvesting mechanical energy from finger motions, based on the coupling between the contact electrification and electrostatic induction effects, and meanwhile storing the generated energy. Under the fast finger sliding, the film can be charged from 0 to 2.5 V within 2094 s and discharge at 1 μA for approximately 1630 s. Furthermore, the film is able to identify personal characteristics during a sliding motion by recording the electric signals related to the person’s individual bioelectricity, applied pressing force, sliding speed, and so on, which shows its potential applications in security systems in touchpad technology.

Harvest mechanical energy with variable frequency and amplitude in our environment for building self-powered systems is an effective and practically applicable technology to assure the independently and sustainable operation of mobile electronics and sensor networks without the use of a battery or at least with extended life time. In this study, we demonstrated a novel and simple arch-shaped flexible triboelectric nanogenerator (TENG) that can efficiently harvesting irregular mechanical energy. The mechanism of the TENG was intensively discussed and illustrated. The instantaneous output power of single TENG device can reach as high as∼4.125 mW by a finger typing, which is high enough to instantaneously drive 50 commercial blue LEDs connected in series, demonstrating the potential application of the TENG for self-powered systems and mobile electronics.

Plasmonic nanostructures can profoundly affect the optical properties of light absorbers and emitters. Using quantum dot-modified-atomic force microscopy (AFM) tips as a nanoscale light source, the topological features of a Ag nanowire and its effect on the fluorescence intensity and lifetime of the QDs can be simultaneously imaged with AFM spatial resolution. Modeling of the QD–nanowire interaction and the contrasts in the fluorescence intensity and lifetime images suggests that this novel method can be used for direct high-resolution mapping of the electric field distributions and energy-transfer quenching near metallic nanostructures.

The appropriate choice of chemical composition of a metallic precursor, which produces the basic structure units in the growth process of nanocrystals, is a high priority in the synthesis of metal-especially Au-nanoparticles. In the present work, Au seeds (prepared by the reduction of Au3+ solution with NaBH4 in the presence of cetyltrimethylammonium bromide (CTAB)) have been used to initiate the growth of Au nanoparticles from two different Au precursors. When an aqueous Au+ solution prepared in the presence of CTAB micelles was treated with ascorbic acid in the presence of the gold seeds, a high yield (up to 92%) of gold nanoparticles was obtained. By varying the volume of the seed solution with a fixed amount of Au+, we can effectively control the morphological transformation of the resulting Au nanoparticles from cubes to octahedra. When an aqueous Au3+ solution was prepared in the presence of CTAB micelles and treated with ascorbic acid in the presence of the gold seeds, smaller yields of Au nanoparticles were obtained. A preliminary growth mechanism has been proposed based on the changes induced by varying the amount of ascorbic acid and the ratio of the concentration of Au+ to the number of seeds. Open image in new window

Metallic nanowires (NWs) possess significant potential for applications in integrated photonic and electronic devices at the nanoscale. Considering the manipulation of NWs and energy loss associated with surface plasmon polaritons (SPPs) modes which serve as signal carriers in the nanophotonic devices, NWs with large diameters are significant. In this work, we report a successive multi-step polyol process approach for the synthesis of ultra-thick silver nanowires (Ag NWs) and investigate their energy loss. Thin Ag NWs prepared in the first step are used as seeds for the further growth of thick Ag NWs in the subsequent steps, where Ag NWs with diameter as large as 1820 nm have been prepared. We further investigate the SPP propagation properties of these thick Ag NWs, and find that energy loss is decreased in Ag NWs with improved diameter. Our experimental results are important for the design and fabrication of SPP-based nanophotonic components and circuits.

Charges induced in triboelectric process are usually referred as a negative effect either in scientific research or technological applications, and they are wasted energy in many cases. Here, we demonstrate a simple, low cost and effective approach of using the charging process in friction to convert mechanical energy into electric power for driving small electronics. The triboelectric generator (TEG) is fabricated by stacking two polymer sheets made of materials having distinctly different triboelectric characteristics, with metal films deposited on the top and bottom of the assembled structure. Once subjected to mechanical deformation, a friction between the two films, owing to the nano-scale surface roughness, generates equal amount but opposite signs of charges at two sides. Thus, a triboelectric potential layer is formed at the interface region, which serves as a charge “pump” for driving the flow of electrons in the external load if there is a variation in the capacitance of the system. Such a flexible polymer TEG gives an output voltage of up to 3.3 V at a power density of ∼10.4 mW/cm 3 . TEGs have the potential of harvesting energy from human activities, rotating tires, ocean waves, mechanical vibration and more, with great applications in self-powered systems for personal electronics, environmental monitoring, medical science and even large-scale power.

Molecular assembly processes are in general much more complex than chemical synthesis. Their goal is the same, that to create a variety of new matter and new functional materials. In accordance with their similarities, the concept of catalysis, which is widely used in chemical synthesis, could be extended to assembly. Herein we suggest that the term “cassemblysis” be used when referring to increased rates and control during assembly. The role played by a cassemblyst in molecular assembly is similar to that played by a catalyst in a synthesis with high efficiency and selectivity. Furthermore, it may be helpful to classify some terms which are used often but without clarity in the literature, such as self-assembly and assisted self-assembly. Molecular assembly can be divided into two major types, self-assembly and assisted assembly. Most assembly processes belong to assisted assembly, which can be further divided into three categories: cassemblysis, co-assembly and field-assisted assembly. Of these three, cassemblysis is the most efficient way to create new materials (e.g., soft matter) above the molecular level. After cassemblysis, some assembly systems undergo a chemical coupling reaction which significantly enhances stability and prevents them from de-assembling. This increased stability is essential for practical applications. We call this cassembly-reaction process “catassemblysis”. We present some typical examples involving small molecules and large biological molecules to show that cassemblysis and catassemblysis are very common, especially in biological systems. We believe that photo-electro-cassemblysis is possible and that the concept of cassembly could be extended to controllable nanoparticle (or other nanostructure) assembly. Finally we discuss the primary mechanism of cassemblysis and catassemblysis. With emphasis on the experimental and theoretical methodologies of cassemblysis, controllable assembly could play a greater role in creating new matter and new functional materials.

NSF of China [21033007]; Ministry of Scientific and Technological Projects [2010IM040100]; 973 Program [2009CB930703]; NFFTBS [J0630429]

Plasmonic waveguides made of metal nanowires (NWs) possess significant potential for applications in integrated photonic and electronic devices. Energy loss induced by bending of a NW during light propagation is critical in affecting its performance as a plasmonic waveguide. We report the characterization of the pure bending loss in curved crystalline silver NW plasmonic waveguides by decoupling the energy loss caused by bending and propagation. The energy attenuation coefficiency due purely to bending was also determined, which exhibited an exponential relationship with the bending radius. Finite-difference-time-domain (FDTD) methods were utilized for theoretical simulations, which matched the experimental results well.

We have rationally synthesized and optimized catalytic nanoparticles consisting of a gold core, covered by a palladium shell, onto which platinum clusters are deposited (Au@Pd@Pt NPs). The amount of Pt and Pd used is extremely small, yet they show unusually high activity for electrooxidation of formic acid. The optimized structure has only 2 atomic layers of Pd and a half-monolayer equivalent of Pt (θPt ≈ 0.5) but a further increase in the loading of Pd or Pt will actually reduce catalytic activity, inferring that a synergistic effect exists between the three different nanostructure components (sphere, shell and islands). A combined electrochemical, surface-enhanced Raman scattering (SERS) and density functional theory (DFT) study of formic acid and CO oxidation reveals that our core–shell–cluster trimetallic nanostructure has some unique electronic and morphological properties, and that it could be the first in a new family of nanocatalysts possessing unusually high chemical reactivity. Our results are immediately applicable to the design of catalysts for direct formic acid fuel cells (DFAFCs).

National Natural Science Foundation of China[20663002]; Foundation of State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, China[200511]; Natural Science Foundation of Jiangxi Province, China[0620025]

Using a two-step seed-mediated growth method, we synthesized bimetallic nanoparticles (NPs) having a gold octahedron core and a palladium epitaxial shell with controlled Pd-shell thickness. The mismatch-release mechanism between the Au core and Pd shell of the NPs was systematically investigated by high-resolution transmission electron microscopy. In the NPs coated with a single atomic layer of Pd, the strain between the surface Pd layer and the Au core is released by Shockley partial dislocations (SPDs) accompanied by the formation of stacking faults. For NPs coated with more Pd (2 nm), the stacking faults still exist, but no SPDs are found. This may be due to the diffusion of Au atoms into the Pd shell layers to eliminate the SPDs. At the same time, a long-range ordered L1(1) AuPd alloy phase has been identified in the interface area, supporting the assumption of the diffusion of Au into Pd to release the interface mismatch. With increasing numbers of Pd shell layers, the shape of the Au-Pd NP changes, step by step, from truncated-octahedral to cubic. After the bimetallic NPs were annealed at 523 K for 10 min, the SPDs at the surface of the NPs coated with a single atomic layer of Pd disappeared due to diffusion of the Au atoms into the surface layer, while the stacking faults and the L1(1) Au-Pd alloyed structure remained. When the annealing temperature was increased to 800 K, electron diffraction patterns and diffraction contrast images revealed that the NPs became a uniform Au-Pd alloy, and most of the stacking faults disappeared as a result of the annealing. Even so, some clues still support the existence of the L1(1) phase, which suggests that the L1(1) phase is a stable, long-range ordered structure in Au-Pd bimetallic NPs.

Transparent electronic devices built on flexible substrates are expected to meet emerging technological demands for the next generation of flexible electronic and optoelectronic devices. A suitable energy source is a vital part for realizing fully self-powered systems. Energy harvesting and conversion technology from environment in which the system will be deployed is a promising route. Recently, we have introduced a new concept on the basis of triboelectrification and electrostatic induction for successfully converting mechanical energy into electric energy at an energy conversion efficiency as high as 50%. Subsequently, transparent triboelectric nanogenerators (TENGs) using indium tin oxide (ITO) or graphene as electrodes have been demonstrated, which are the only possible power source that light can penetrate through. However, there is a need for a low-cost and large area compatible technology for producing transparent TENGs with high-output power supply, aiming for applications such as touch screens. In our previous work, we fabricated a transparent TENG and pressure sensor by using polyester (PET) and polydimethylsiloxane (PDMS) films as flexible substrates to integrate ITO electrodes. It has also been demonstrated that the patterned surface is an effective method to improve the output performance of the TENG. However, realizing such a structure is not trivial in mass production and the patterned structure will reduce the transparency of the device due to the light scattering effect. Here, we present a simple, cost-effective and large-scalable method for fabrication of highly transparent TENGs. Although triboelectric effect has been known for thousands years, the underlying mechanism is actually very complex and still under debating. The competing possible mechanisms appear to include electron transfer, ion transfer, bond dissociation, chemical changes, and material transfer, and it is likely that different mechanisms may be involved depending on the specific materials and environmental conditions. Usually, it needs specialized equipment to characterize the charged surface and study the mechanism. It is worth noting that the output performance of TENG can directly reflect the capacity of the triboelectrification. Herein, we use this feature to investigate the mechanism of the triboelectrification by examining various factors on the performance of TENG. Furthermore, another meaningful outcome of our study is that we explore the mechanism from the microscopic and molecular perspective using the methods for surface characterization, which would be beneficial to the improvementand application of TENG at a large scale. References F. R. Fan, Z. Q. Tian, Z. L. Wang, Nano Energy 2012, 1, 328-334. F. R. Fan, L. Lin, G. Zhu, W. Z. Wu, R. Zhang, Z. L. Wang, Nano Lett. 2012, 12, 3109-3114. F. R. Fan, J. J. Luo, W. Tang, C. Y. Li, C. P. Zhang, Z. Q. Tian, Z. L. Wang, J. Mater. Chem. A 2014, 2, 13219-13225.

Surface-enhanced Raman scattering (SERS) is a powerful spectroscopy technique that can provide non-destructive and ultra-sensitive characterization down to single molecular level, comparable to single-molecule fluorescence spectroscopy. However, generally substrates based on metals such as Ag, Au and Cu, either with roughened surfaces or in the form of nanoparticles, are required to realise a substantial SERS effect, and this has severely limited the breadth of practical applications of SERS. A number of approaches have extended the technique to non-traditional substrates, most notably tip-enhanced Raman spectroscopy (TERS) where the probed substance (molecule or material surface) can be on a generic substrate and where a nanoscale gold tip above the substrate acts as the Raman signal amplifier. The drawback is that the total Raman scattering signal from the tip area is rather weak, thus limiting TERS studies to molecules with large Raman cross-sections. Here, we report an approach, which we name shell-isolated nanoparticle-enhanced Raman spectroscopy, in which the Raman signal amplification is provided by gold nanoparticles with an ultrathin silica or alumina shell. A monolayer of such nanoparticles is spread as 'smart dust' over the surface that is to be probed. The ultrathin coating keeps the nanoparticles from agglomerating, separates them from direct contact with the probed material and allows the nanoparticles to conform to different contours of substrates. High-quality Raman spectra were obtained on various molecules adsorbed at Pt and Au single-crystal surfaces and from Si surfaces with hydrogen monolayers. These measurements and our studies on yeast cells and citrus fruits with pesticide residues illustrate that our method significantly expands the flexibility of SERS for useful applications in the materials and life sciences, as well as for the inspection of food safety, drugs, explosives and environment pollutants.

Nearly monodisperse single-crystalline palladium (Pd) nanocubes and nanodendrites have been successfully prepared in aqueous solution at room temperature, for the first time, mainly by utilizing the equilibrium between the dissolution and precipitation of the Pd-cetyltrimethylammonium bromide (Pd-CTAB) complexes. The morphology of the obtained Pd nanocrystals can be tuned by the addition of foreign halide ions (Cl− and Br−). The corresponding selected area electron diffraction (SAED) and X-ray diffraction (XRD) patterns confirmed that the synthesis of nanocubes and nanodendrites are single-crystalline pure Pd structures with fcc crystal lattice. A preliminary formation mechanism based on the dissolution and precipitation of the Pd-CTAB complexes and competitive adsorption between different ions on the nanoparticle surface has been proposed.

Flexible nanogenerators that efficiently convert mechanical energy into electrical energy have been extensively studied because of their great potential for driving low-power personal electronics and self-powered sensors. Integration of flexibility and stretchability to nanogenerator has important research significance that enables applications in flexible/stretchable electronics, organic optoelectronics, and wearable electronics. Progress in nanogenerators for mechanical energy harvesting is reviewed, mainly including two key technologies: flexible piezoelectric nanogenerators (PENGs) and flexible triboelectric nanogenerators (TENGs). By means of material classification, various approaches of PENGs based on ZnO nanowires, lead zirconate titanate (PZT), poly(vinylidene fluoride) (PVDF), 2D materials, and composite materials are introduced. For flexible TENG, its structural designs and factors determining its output performance are discussed, as well as its integration, fabrication and applications. The latest representative achievements regarding the hybrid nanogenerator are also summarized. Finally, some perspectives and challenges in this field are discussed.