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Recently, the rational design of non-precious metal electrocatalysts for highly efficient hydrogen evolution reaction (HER) in alkaline media has received considerable interests in sustainable and renewable energy researches. Herein, vertically aligned and interconnected NiS2/CoS2/MoS2 nanosheet arrays on Ni foam were prepared by a two-step procedure that conducted by the hydrothermal synthesis of Ni-Co molybdate nanosheet array as the precursor and followed by the vapor phase sulfurization to achieve in situ conversion. Basing on the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations, it can be found that the honeycomb-like structure of the Ni-Co molybdate nanosheet array was well preserved after the sulfurization process. The high-resolution TEM (HRTEM) characterization reveals that the NiS2/CoS2/MoS2 nanosheet array provided abundant well-exposed active edge sites and multiple heterointerfaces towards enhanced alkaline HER performance. Electrochemical studies demonstrated that the ultrathin NiS2/CoS2/MoS2 nanosheets exhibited excellent HER performance with an overpotential of 112 mV at 10 mA cm−2 and a smaller Tafel slope of 59 mV dec−1 in comparison with NiS2/MoS2 (155 mV and 89 mV dec−1) and CoS2/MoS2 (124 mV and 75 mV dec−1) samples by taking the advantage of the well-exposed multiple heterointerfaces. This work presents a facile and reliable synthetic strategy for the rational design of highly efficient electrocatalysts for the HER in alkaline solution based on non-precious metal sulfide nanocomposite.

CoS2/MoS2 heteronanosheet arrays (HNSAs) with vertically aligned flower-like architectures are fabricated through in situ topotactic sulfurization of CoMoO4 nanosheet array (NSA) precursors on conductive Ni foam. CoMoO4 NSAs are prepared by a self-template hydrothermal method without using any hard template and surfactant. Benefiting from a 3D flower-like architecture constituted by ultrathin nanosheets with abundant exposed heterointerfaces as highly active sites and predesigned void spaces, the as-synthesized CoS2/MoS2 HNSAs exhibit an excellent hydrogen evolution reaction (HER) performance with a low overpotential of 50 mV at 10 mA cm−2, and a small Tafel slope of 76 mV dec−1 in 1.0 M KOH, which outperforms most previously reported CoS2 and MoS2 based electrocatalysts with compositional or morphological similarity. This work demonstrates the great potential in developing high-efficiency and earth-abundant electrocatalysts for alkaline HER through heterointerface engineering and morphological design by utilizing transition metal molybdate as a promising platform.

Nanostructured intermediate band solar cells have been proposed as one of the promising candidates to surpass the limited efficiency of single-junction photovoltaic devices, while challenges still remain for realizing their practical implementation due to intricate carrier behaviors occurring in confined states. In this report, we have elucidated a detailed photo-carrier collection mechanisms in a quantum dot-in-well intermediate band solar cell, which is varied by effective electron intraband excitation process. Results from the photoluminescence response of quantum dots are discussed to examine the occupation state inside confined levels. Detailed analyses point out that diffusion of holes plays an important role in modulating the electrical generations.

We elucidate a photocarrier collection mechanism in intermediate band solar cells (IBSCs) with InAs-quantum dots (QDs)-in-an-Al0.3Ga0.7As/GaAs-quantum well structures. When the Al0.3Ga0.7As barrier is excited, the device electrical output can be varied by additional infrared light for the electron intraband optical transition in QDs. The photocurrent in IBSC with a single QDs-in-a-well structure shows a monotonic increase with the intraband-excitation density. Conversely, IBSC with a multilayered QDs-in-a-well structure exhibits a photocurrent reduction when electrons in QDs are optically pumped out. The simultaneously measured photoluminescence spectra proved that the polarity of QD states changes depending on the intraband-excitation density. We discuss the drift and diffusion current components and point out that the hole diffusion current is significantly influenced by carriers inside the confinement structure. Under strong intraband excitations, we consider an increased hole diffusion current occurs by blocking hole-capture in the quantum structures. This causes unexpected photocurrent reduction in the multilayered device.

Intermediate-band solar cell is one of the promising concepts to jump out the dilemma of limited efficiency for traditional single junction photovoltaic devices. Albeit fruitful explorations have been done on revealing its working principle and promoting output performance, optical and electrical match of outer system can sometimes get it complicated because of extra below-bandgap photon absorptions. In this study, we reported a voltage backtracking behavior under intensive bi/uni-color irradiation circumstance within this kind of cell. Two different mechanisms are separately disclosed here for the sake of deep understanding how it operates and by which it can better work.

Two-step photon up-conversion solar cell is a newly-proposed concept for high-conversion efficiency photovoltaics, in which a heterointerface of different semiconductor materials is delicately merged inside common single-junction cell structure. Recently, we have reported an enhancement of its photocurrent under a reverse bias condition due to the boosted carrier collection efficiency. To push it further, in this study, modulation-doping method is employed in order for a better elucidation of two-step photon up-conversion at the heterointerface. It is observed that the difference of the external quantum efficiency with additional infrared light irradiation on the TPU-SCs can be greatly enlarged by changing doping concentration selectively reflecting the importance of the electric field at the heterointerface.

Sensorineural hearing loss tops the list of most suffering disease for the sake of its chronic, spirit pressing, and handicapped features, which can happen to all age groups, from newborns to old folks. Laggard technical design as well as external power dependence of conventional cochlear implant cumbers agonized patients and restrict its wider practical application, driving researchers to seek for fundamental improvement. In this paper, we successfully proposed a novel bionic cochlear basilar membrane acoustic sensor in conjugation with triboelectric nanogenerator. By trapezoidally distributing nine silver electrodes on both two polytetrafluoroethylene membranes, a highly frequency-selective function was fulfilled in this gadget, ranging from 20 to 3000 Hz. It is believed to be more discernable with the increment of electrode numbers, referring to the actual basilar membrane in the cochlear. Besides, the as-made device can be somewhat self-powered via absorption of vibration energy carried by sound, which tremendously facilitates its potential users. As a consequence, the elaborate bionic system provides an innovative perspective tackling the problem of sensorineural hearing loss.

The generation of two-photon photocurrent plays an essential role in realizing intermediate-band solar cells (IBSCs) with high conversion efficiencies. This current generation process strongly depends on the photocarrier dynamics in the intermediate levels, which can sometimes give rise to a deficient output current unexpectedly. In this work, we investigated the two-photon photocurrent generation process in InAs quantum dot (QD)-in-well intermediate-band solar cells. The two-photon photocurrent is generated by an interband transition in the structure (for example, in the well or the QD) and a subsequent intraband transition in the QD, and we used two different light sources to separately control these transitions. We found that, in the case of QD interband excitation in a sample with multiple wells, the carrier collection efficiency does not simply increase with the intraband excitation intensity; in the range from about 0.08 to 0.5 W/cm2, the collection efficiency decreases with increasing intraband excitation density. A comparison between samples with different numbers of wells revealed that the repetition of carrier trapping and detrapping during the transport in a multi-well structure can effectively modulate the recombination rate. This modulation induces a reduction of the current yield under certain illumination conditions. We propose a model to explain this phenomenon and verify it by investigating the bias dependence of the two-photon photocurrent from the QD.

A two-step photon upconversion (TPU) solar cell is a single-junction solar cell with a suitably designed heterointerface in the intrinsic layer. At this interface, upconversion can take place by absorbing infrared light, which excites carriers to energies above the heterojunction barrier, and then an electric field extracts these upconverted carriers to the barrier material. In this work, we study this photoabsorption process in a TPU solar cell with a modulation-doped structure. The modulation doping technique enables us to control the electric field at the heterointerface, and we demonstrate that the efficiency of the TPU process strongly depends on the electric field at the heterointerface. While we show that the TPU process induced by infrared light leads to a significant increase in the external quantum efficiency, we also note that this TPU process is an adiabatic process. It is shown that the adiabatic intraband photoexcitation at the heterointerface is able to increase the open-circuit voltage even above the open-circuit voltage that we predict from the diode equation and the actual current increase. We demonstrate obvious voltage boost effects and elucidate the relationship between the current and voltage.

Exploring earth-abundant and noble metal-free catalysts for water electrolysis is pivotal in renewable hydrogen production. Herein, a highly active electrocatalyst of nitrogen-doped porous carbon nanosheets coupled with Mo2C nanoparticles (Mo2C/NPC) was synthesized by a novel method with high BET surface area of 1380 m2 g−1 using KOH to activate carbon composite materials. The KOH plays a key role in etching out MoS2 to produce Mo precursor; simultaneously, it corrodes carbon to form porous structure and produce reducing gas such as H2 and CO. The resulting Mo2C/NPC hybrid demonstrated superior HER activity in acid solution, with the overpotential of 166 mV at current density of 10 mA cm−2, onset overpotential of 93 mV, Tafel slope of 68 mV dec−1, and remarkable long-term cycling stability. The present strategy may provide a promising strategy to fabricate other metal carbide/carbon hybrids for energy conversion and storage.

Additional file 1: Figure S1. SEM images of (a) c-MoS2, (b) MoS2@PDA, (c) MoS2@C and (d) Mo2C/NPC. Figure S2. TGA curve of the as-prepared Mo2C/NPC nanosheets. Figure S3. The calculation of ECSA for Mo2C/NPC. Figure S4. Chronoamperometric response at the potential of -0.166â V vs. the RHE. Table S1. Comparison of the exchange current density (j0) in acidic media for Mo2C/NPC with other non-noble metal electrocatalysts. (DOCX 932 kb)

Low power and multifunctional nonvolatile memories are promising candidates for processing massive data in the Internet of Things era. However, the storage states in conventional memory devices are under the restricted control by electrical or optical signals. Herein, a new multibit tribotronic nonvolatile memory (T-NVM) based on a graphene/hexagonal boron nitride/molybdenum disulfide van der Waals heterostructure and triboelectric nanogenerator (TENG) is proposed. The programming/erasing states can be modulated by the triboelectric potential, which is determined by changing the distance between the two triboelectrification layers. Under the modulation of external mechanical actions, the device exhibits a high on/off ratio of 105 via manipulating mechanical distance from − 0.2 mm to + 0.2 mm, a long retention time up to 6000 s, a stable switching behavior for over 100 cycles, and a multilevel data storage capability of 14 stages by different external stimuli. Furthermore, a memory inverter circuit employing the triboelectric potential as input signals can serve as the conversion of logical signals. This work proves the great potential of tribotronic devices for direct interaction with external environment in lower power and broadening diverse applications of human-robot interactions, self-powered wearable devices, and intelligent instrumentation.

Owing to the additional usage of sub-bandgap photons, the intermediate-band solar cell has been regarded as a promising device design to exceed the conversion limits of conventional photovoltaic devices. An output-voltage preservation is theoretically possible in this kind of device in the case of independent quasi-Fermi levels. This phenomenon manifests experimentally in a voltage recovery induced by supplementary two-step photon absorption processes. Here, we study the excitation-power and temperature dependences of the voltage performance in an intermediate-band solar cell containing InAs quantum dots in Al0.3Ga0.7As/GaAs quantum wells. The two-color photoexcitation method is used to separately control the interband and quantum dot-conduction band transitions. The output voltage is sensitive to the balance between the two excitation densities and the cell temperature. It is found that a strongly asymmetric irradiation can even lead to a voltage decrease. The temperature-dependent data suggest a faster electron–hole annihilation at lower temperatures. We introduce a new characteristic index to qualitatively evaluate the carrier loss in the intermediate band.

Smart energy textiles based on triboelectric nanogenerators (TENGs) are promising power supply and sensor arrays for wearable electronics and artificial intelligence. However, the challenges of the smart textile in large-scale industrialized production, facile and universal fabrication process, and enough robustness still remains. In this study, a textile-based triboelectric nanogenerator (t-TENG) is proposed through a facile and universal fabrication process. Liquid-metal/polymer core/shell fibers (LCFs) structure as the basic unit of TENG textiles, can be extensively achieved by pumping liquid metal into uniform ultrafine polymer hollow fibers, which contribute to the large-scale industrialized production of TENG textiles. In addition to gaining a high output, the t-TENG demonstrates excellent acid and alkali resistance, and high friction durability. Furthermore, profiting from the universal fabrication process, the polymer hollow fibers can be replaced by any type of fibers with various functions or colors according to practical applications (e.g., the silicone rubber could be an excellent choice for highly stretchable devices). Moreover, the fabricated t-TENGs demonstrate great capability in self-powered sensors for smart home controlling. All these advantages of the t-TENG exhibit its great potential for further application in wearable electronics and artificial intelligence and it will speed up the development of IoTs in the near future.

Recently it has been found that one dimensional (1D) Ag nano-structures can be synthesized through oriented attachment (OA) growth. In OA growth of 1D structures, nanoparticles (NPs) attach to the growing nanorods (NRs) via either on-axis or off-axis attachment. However, the thermodynamic basis for understanding the preference of each growth mode has remained unexplored until now. In this paper, molecular static calculations are performed to investigate the van der Waals interactions (vdW) in both on-axis and off-axis attachments of 1D Ag nano-structures. The correlation of parameters including the size, aspect ratio (AR), crystalline orientation of NR, the inter-particle separation and the off-axis approaching angle, with both OA attachments is investigated in detail. The results show that off-axis attachment is thermodynamically favorable compared to on-axis attachment in a typical OA growth, and straight on-axis OA growth are typically realized by tuning the other aspects of an OA growth system. Interestingly, it is found that the off-axis growth is both precursor-size dependent and crystalline-orientation dependent.

In this study, molecular static calculation is carried out to evaluate the van der Waals interaction (vdW) associated with different oriented attachment (OA) growth systems involving 1D nanorods (NRs), 2D nanoplates (NPts) and 3D nanostructures (NSts) for the first time. Our results show that the vdW is, to a large extent, determined by the attaching area at all OA growth stages of nanocrystals. The vdW increases significantly as the OA growth varies from 1D NR–NP, end-to-end NR–NR assemblies to 2D side-by-side NR–NR/3D NPt–NPt assemblies. Our study reveals the fundamental details in vdW, one of the governing inter-particle interactions involved in OA growth of NCs, and facilitates the analytical understanding of the OA growth thermodynamics.

Porosity is the most significant parameter that determines the shelter effcet of a porous fence in the large open stock piles. Planar and a type of non-planar porous fences with different porosities have been performed experimentally to analyze the flow characteristics through the wind tunnel with roughness elements. Velocities were measured by a hot-wire anemometer and the experiment data were discussed. According to the experiment results, the optimum porosity of the planar porous fence is found to be around 0.264, and the optimum porosity of the non-planar porous fence is found to be about 0.273. With the similar shelter effect, the non-planar porous fences do have higher engineering application due to the superiority of the stiffness. Keywords-porosity; non-planar porous fence; wind tunnel experiment; wind reduction ratio

The microstructure of ferroelectric [PbZrxTi1� xO3/PbZryTi1� yO3]n epitaxial multilayers (x/y ¼ 0.2/0.4, 0.4/0.6) deposited on SrRuO3-coated SrTiO3 substrates by pulsed-laser deposition with different layer periodicity and layer thickness was characterized by means of transmission electron microscopy. Electron diffraction and contrast analysis revealed a very clear and well-separated layer sequence. The microstructures of PbZr0.2Ti0.8O3/ PbZr0.4Ti0.6O3 and PbZr0.4Ti0.6O3/PbZr0.6Ti0.4O3 multilayers show a similar tendency in the dependence on the individual layer thickness. Whereas with thick individual layers, tetragonal a-domains are confined to specific layers of the two types of multilayers, below a certain critical thickness of the individual layers, a-domains extend over the whole film. This indicates a transition into a uniform tetragonal lattice and strain state of the whole multilayer. Increasing the layer periodicity further, the interfaces in PbZr0.4Ti0.6O3/PbZr0.6Ti0.4O3 multilayers become rough, and complex a-domain configurations appear.

Multilayers consisting of two tetragonal compositions PbZr(0.2)Ti(0.8)O(3) and PbZr(0.4)Ti(0.6)O(3) were deposited onto a SrRuO(3) electrode grown on a vicinal (100) SrTiO(3) substrate. It has been shown by extensive structural investigations comprising transmission electron microscopy in conventional and high resolution mode, reciprocal space mapping and piezoresponse force microscopy that with decreasing layer thickness a transition from a-domains confined to individual layers to a-domains propagating through the whole film takes place. This is caused by the formation of a common strain state of all layers which is responsible for the observed enhancement of the electrical properties. These show a maximum in the product of remanent polarization and dielectric constant at a certain density of interfaces. If the interface density becomes too high the lattice distortion accompanying each interface deteriorates the properties of the multilayer structure.