Molybdenum disulfide/silver/p-silicon nanowire heterostructure with enhanced photoelectrocatalytic activity for hydrogen evolution

Suitable semiconductor and its efficient coupling with catalysts is vital to hydrogen evolution reaction (HER). Herein, Ternary heterostructured MoS₂/Ag/p-type silicon nanowires (SiNWs) array photocathode are constructed by a simple two-step method, where Ag is self-reduced on SiNWs via Galvanic Displacement method and MoS₂ is subsequently loaded by direct thermal decomposition. Ag interfacial layer is introduced between Si and MoS₂ to facilitate the charge transfer and suppress the recombination of photo-generated electron-hole pairs. MoS₂/Ag/SiNWs exhibits an onset potential of 62 mV and photocurrent density of 50 mA cm^?2 at ?1.0 V_RHE, as well as good stability. Besides, MoS₂/Ag/SiNWs is capable of generating 325.9 μL hydrogen per minute. The superior HER catalytic activity of MoS₂/Ag/SiNWs is contributed to the improved charge transport at the solid–solid interfaces by virtue of Ag layer, allowing more electrons flow from SiNWs to MoS₂ and thus effectively separating the photoelectrons and holes. This work demonstrates the potential of novel heterostructure for robust and efficient photoelectrochemical HER.     

Formation,rapid thermal oxidation and passivation of solar grade silicon nanowires for advanced photovoltaic applications

Uniform and regular silicon nanowires (SiNWs) arrays are fabricated on both sides of solar grade silicons (SiGS) by silver assist-electrochemical etching. SiNWs arrays exhibit an excellent antireflection character with an overall reflectance of 2% in the range from 300 to 1000 nm. More importantly, the effective lifetimes of the symmetric SiNWs/Si structures decreased due to the high densities of dangling bonds and surface defects. Surface passivation to overcome lifetime degradation is realized by means of rapid thermal oxidation (RTO). Following rapid oxidation, Fourier Transform Infrared spectroscopy reveals that oxygen diffusion is enhanced inside silicon nanowires where the morphological structure is preserved during RTO. Moreover, it is shown that even the rapid thermal oxidation process is not effective to recover initial τ_eff due to the high density of imperfections involved during nanowires formation and the contamination level induced by silver. The interdiffusion between residual silver and metal contaminants in the core of the nanowire can probably limit the passivation effect due to the segregation of metal atoms at SiO₂ and to the redistribution of both impurities across the wire.     

CIS/CdS/ZnO/ZnO:Al modified photocathode for enhanced photoelectrochemical behavior under visible irradiation: Effects of pH and concentration of electrolyte solution

In this paper, the CuInS₂ films were firstly modified with CdS and CdS/ZnO/ZnO:Al/Au layers in order to improve the photoelectrochemical (PEC) water splitting efficiency. The CuInS₂ photoelectrode was synthesized by electrodeposition method as a facial and green method, on the FTO substrate. The effects of pH and concentration of Na₂S electrolyte solution on the photocurrent density of photoelectrode samples were studied. As a p-n junction photocathode, the CIS/CdS/ZnO/ZnO:Al/Au photoelectrode indicates the enhanced PEC activity. The photocurrent density of CIS/CdS/ZnO/ZnO:Al/Au photoelectrode reaches to 1.91 mA/cm², while is about 2.5 times higher than that for CuInS₂ film at pH = 8 (−0.6 V vs Ag/AgCl). The formation of a p-n junction at the CuInS₂ photoelectrode surface not only reduces the recombination of electron-hole pairs but also increases the PEC response and water splitting performance of the as-prepared CIS/CdS/ZnO/ZnO:Al/Au photoelectrode.     

Fabrication and electrochemical characterization of a vertical array of MnO2 nanowires grown on silicon substrates as a cathode material for lithium rechargeable batteries

A complementary metal-oxide-semiconductor (CMOS) compatible process for fabricating on-chip microbatteries based on nanostructures has been developed by growing manganese dioxide nanowires on silicon dioxide (SiO₂)/silicon (Si) substrate as a cathode material for lithium rechargeable batteries. High aspect-ratio anodized aluminum oxide (AAO) template integrated on SiO₂/Si substrates can be exploited for fabrication of a vertical array of nanowires having high surface area. The electrolytic manganese dioxide (EMD) nanowires are galvanostatically synthesized by direct current (dc) electrodeposition. The microstructure of these nanowire arrays is investigated by scanning electron microscopy and X-ray diffraction. Their electrochemical tests show that the discharge capacity of about 150 mAh g⁻¹ is maintained during a few cycles at the high discharge/charge rate of 300 mA g⁻¹.     

Well-aligned single-crystalline silicon nanowire hybrid solar cells on glass

Hybrid solar cells are fabricated on the glass substrate using well-aligned single-crystalline Si nanowires (SiNWs) and poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM). Their key benefits are discussed. The well-aligned SiNWs are fabricated from Si wafer and transferred onto the glass substrate with the P3HT:PCBM. Such SiNWs provide uninterrupted conduction paths for electron transport, enhance the optical absorption to serve as an interesting candidate of the absorber, and increase the surface area for exciton dissociation. Our investigations show that SiNWs are promising for hybrid organic photovoltaic cells with improved performance by increasing the short-circuit current density from 7.17 to 11.61 mA/cm².     

Photocatalytic degradation of cationic and anionic dyes in water using hydrogen-terminated silicon nanowires as catalyst

The photocatalytic degradation of cationic (methylene blue (MB)) and anionic (methyl orange (MO)) dyes was investigated using hydrogen-terminated silicon nanowires (H-SiNWs) as photocatalysts. Several silicon nanowires samples with different morphologies were elaborated and the morphology was changed by acting on the silicon nanowires formation parameters such as substrate type, doping level, crystallographic orientation, silver deposition time and etching time. It was shown that the photocatalytic activity strongly depends on the morphology of SiNWs arrays. Indeed, it was found that n-type H-SiNWs elaborated on highly doped Si (100) substrates exhibit the highest photocatalytic activity for the degradation of MB. In addition, it was demonstrated that H-SiNWs are more efficient for the photodegradation of MO than MB in the solutions with pH values higher than the pH of zero charge point of silica (pH_pzc).     

Study of the Mo/CuInS2/ZnS system by TEM

This work presents results from a study carried out on the Mo/CuInS₂/ZnS stacked layers, using high-resolution transmission electron microscopy (HRTEM). This system will be used later for the fabrication of solar cells with Mo/CuInS₂/ZnS/TCO structure, where the layers conforming it will perform as an electrical contact, absorber layer and buffer layer, respectively. The layers of the Mo/CuInS₂/ZnS system were deposited sequentially on soda lime glass substrates. The Mo film was deposited by DC magnetron sputtering, the CuInS₂ (CIS) layer was grown by co-evaporation of precursors in a two-stage process and the ZnS was deposited by co-evaporation and by CBD (chemical bath deposition) using a solution containing zinc acetate, sodium citrate, ammonia and thiourea.The performed study provided significant information regarding crystalline structure, grain boundaries and defects visualization of each one of the layers as well as of the Mo/CuInS₂ and CuInS₂/ZnS interfaces.     

Enhancement of capacity of carbon-coated Si–Cu3Si composite anode using metal–organic compound for lithium-ion batteries

A carbon-coated Si–Cu₃Si composite material is prepared using silicon and copper(II) d-gluconate powders by simple mechanical milling and pyrolysis, and is investigated as an anode material for lithium-ion batteries. In this process, the Cu₃Si and pyrolyzed carbon uniformly adhere to the surface of the silicon particles. The cycling performance of the composite material exhibits a stable capacity of 850 mAh g⁻¹ for 30 cycles. The improved cycling performance is attributed to the fact that the copper silicide and pyrolyzed carbon provide both a better electrical contact with the current–collector and a buffering effect for the volume expansion–contraction during cycling.     

Cycleable graphite/FeSi6 alloy composite as a high capacity anode material for Li-ion batteries

FeSi₆/graphite composite was prepared by mechanical ball milling. The FeSi₆ alloy particles consist of an electrochemically active silicon phase and inactive phases FeSi₂, distributed uniformly in the graphite matrix. The composite anode offers a large reversible capacity (about 800 mAh g⁻¹) and good cycleability, due to the buffering effect of the inactive FeSi₂ phase and graphite layers on the volumetric changes of Si phase during lithium–Si alloying reaction. Since FeSi₆ alloy is a low-cost industrial material, this alloy compound provides a possible alternative for development of high capacity lithium-ion batteries.     

Epitaxial n-Si/p-CuInS2 heterojunction devices

We present and characterize the first epitaxial Si/CuInS₂ heterojunction devices. By means of molecular beam epitaxy (MBE), slightly copper-rich CuInS₂ epilayers are deposited on sulphur-terminated Si-(1 1 1) surfaces of n-type wafers. Both the quality of the substrate and the deposited epilayer are controlled in situ using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) and various other methods including transmission electron microscopy (TEM) are applied in the ex situ structural characterization. The heterojunction diodes are completed by the deposition of an indium-tin oxide (ITO) layer and of metallic contacts. Their electronic and structural properties are discussed.     

Facile fabrication of silicon nanowires as photocathode for visible-light induced photoelectrochemical water splitting

We report the fabrication of one dimensional Silicon nanowires (Si NWs) using p-Si (100) substrate through facile two step metal assisted chemical etching (MACE) approach. The evolution of structural and optical properties of Si NWs by etching Si substrate was studied as a function of hydrogen peroxide (H₂O₂), a strong oxidation agent. The length of the NWs increased linearly with the H₂O₂ concentrations and reached maximum of 51 μm for etching of 60 min. The merits of metal free Si NWs as photocathode in the photoelectrochemical (PEC) neutral water splitting under the visible light was investigated. The performance of the photocathode highly depends on the morphology of Si nanostructure. A high density and well separated Si NWs fabricated by 0.6 M of H₂O₂ results in maximum photocurrent density of 6 mA cm^?2 with applied bias photocurrent conversion (ABPE) efficiency of 1.1% under visible light illumination.     

Structural and electrochemical study of the reaction of lithium with silicon nanowires

The structural transformations of silicon nanowires when cycled against lithium were evaluated using electrochemical potential spectroscopy and galvanostatic cycling. During the charge, the nanowires alloy with lithium to form an amorphous Li_xSi compound. At potentials <50 mV, a structural transformation occurs. In studies on micron-sized particles previously reported in the literature, this transformation is a crystallization to a metastable Li₁₅Si₄ phase. X-ray diffraction measurements on the Si nanowires, however, show that they are amorphous, suggesting that a different amorphous phase (Li_ySi) is formed. Lithium is removed from this phase in the discharge to form amorphous silicon. We have found that limiting the voltage in the charge to 70 mV results in improved efficiency and cyclability compared to charging to 10 mV. This improvement is due to the suppression of the transformation at low potentials, which alloys for reversible cycling of amorphous silicon nanowires.     

Research progress on Si/C composites as anode for lithium ion batteries

硅基负极材料具有比容量高、电压平台低、环境友好、资源丰富等优点,有望替代石墨负极应用于下一代高比能锂离子电池。但是硅的导电性较差,且在充放电过程中存在巨大的体积效应,极易导致电极极化、材料粉化、SEI膜重构、库仑效率低和容量持续衰减。硅和碳复合能很好地综合两者的优势,形成结构稳定、循环性好及容量高的负极材料。本文从不同维度的硅（SiNPs、SiNTs/SiNWs、SiNFs、Bulk Si）与碳复合这一角度,综述了硅碳复合材料在结构设计、制备工艺、电化学性能等方面的最新研究进展,并对未来的硅碳复合材料的研究工作进行了展望。     

Photovoltaic characteristics of silicon nanowire arrays synthesized by vapor-liquid-solid process

Silicon nanowire (SiNW) arrays were grown directly on a P type Si substrate, pre-deposited with gold catalyst, and then made into solar cell for photovoltaic characteristic measurement. Different growth conditions of SiNWs, including variation of the flow rate ratio of SiH₄ versus N₂, and the thickness of Au film, which can be sputtered into different size of nanoparticles, will be made in order to obtain an optimum photovoltaic conversion efficiency. The morphologies and crystalline structure of the nanowires are studied by SEM, TEM and XRD. The SiNW array surface is shown to have good antireflection property, and is expected to raise light absorption and short circuit current. The photovoltaic performance of the solar cells with SiNWs grown at different conditions is measured and discussed. More effort is still needed to raise the performance of SiNW solar cells.     

Deposition of CuInS2 films by electrostatic field assisted ultrasonic spray pyrolysis

We studied CuInS₂ (CIS) film growth using high electrostatic field assisted ultrasonic spray (HEFAUS) deposition. CIS films were fabricated with various precursors and substrate temperatures. All the as-sprayed CIS films were observed to be grown with mixed ordering mode, where chalcopyrite (CH) and CuAu (CA) orderings coexisted. It was found that application of additional sulfurization to sprayed CIS films induced re-crystallization of films accompanied by enhancement of CH ordering. After the post-sulfurization, the most improved film showed nearly the same CH-fraction as that for a CIS film which was made by sulfurization of sputtered Cu-In alloy film. These results indicate that our modified spray deposition could be used for fabrication of CIS photoabsorbing layer instead of high-cost vacuum-based process. All fabricated films were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscope and energy dispersive X-ray analysis measurements.     

Developing lithium ion battery silicon/cobalt core-shell electrodes for enhanced electrochemical properties

In this work, core-shell Si/Co composite powders were produced using an electroless deposition process. The effect of different concentrations of CoSO₄ in the plating bath was studied to provide the Co deposition and to reveal the structure on the surface of silicon powders. The surface morphology and the composition of the produced Si/Co composite powders were characterized using scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) analysis was performed to investigate the structure of the Si/Co composite powders. The discharge capacities of Si/Co composite electrodes were determined with cyclically testing, and resistivity of the produced Si/Co composite electrodes were studied using electrochemical impedance spectroscopy (EIS). The silicon/cobalt composite electrode produced using 40 g/L CoSO₄ exhibited the most stable capacity retention, and a discharge capacity of approximately 220 mAh/g was obtained after 15 cycles for this electrode. This study demonstrated that the conductivity of the electrodes was improved and the capacity retention of the Si/Co composite electrodes was increased by increasing the shell structured cobalt content on the surface of silicon powders due to the buffering effect of cobalt against huge volume changes during the lithiation and delithiation process.     

Si–Ni alloy–graphite composite synthesized by arc-melting and high-energy mechanical milling for use as an anode in lithium-ion batteries

A Si–Ni alloy and graphite composite is synthesized by arc-melting followed by high-energy mechanical milling. The alloy particles consist of an electrochemically active silicon phase with inactive phases such as NiSi₂ and NiSi distributed uniformly on the surface of the graphite. The inactive phases can accommodate the large volume changes of Si during cycling of the composite as an anode material for lithium batteries. The cycle-life of the composite increases with increase in Si content. A large reversible capacity (about 800 mAh g⁻¹) and good cycleability suggest that the composite may prove to be an alternative to conventional graphite-based anode materials for lithium-ion secondary batteries.     

Stable and highly efficient MoS2/Si NWs hybrid heterostructure for photoelectrocatalytic hydrogen evolution reaction

We report, the fabrication of molybdenum disulphide (MoS₂) wrapped silicon nanowires (Si NWs) for visible light driven water splitting applications. The morphological and elemental studies ensure the vertical alignment of Si NWs wrapped with 2D layered MoS₂. The photoelectrocatalytic (PEC) results evidence the significant enhancement in performance of MoS₂/Si NWs based hybrid photocathode with ~300 mV (under reversible hydrogen electrode (RHE)) anodic shift in onset potential as that of pristine Si NWs (+0.194 V vs. RHE), and the current density of −26.5 mA/cm² was achieved at the applied bias of 0 V vs. RHE. Further, the electrochemical impedance studies ensure the interface resistance-free charge transfer between Si NWs and electrolyte via 2D MoS₂ layer which provokes rapid hydrogen production. The wrapping of Si NWs with MoS₂ protects the superlative photocathode from harsh acid electrolyte environment. The overgrown MoS₂ triangular particles with active sulphur edge sites are found to eventually augment the solar hydrogen evolution rate. Further, the PEC performance of our MoS₂/Si NWs is also comparable with stable Pt/Si NWs photoelectrode. It is note-worthy that, MoS₂/Si NWs hybrid heterostructure would be a potential candidate in future large scale, low cost and day-to-day solar water splitting applications.     

Cu5Si–Si/C composites for lithium-ion battery anodes

Cu₅Si–Si/C composites with precursor atomic ratio of Si:Cu = 1, 2 and 4.5 have been produced by high-energy ball-milling of a mixture of copper–silicon alloy and graphite powder for anode materials of lithium-ion battery. X-ray diffraction and scanning electron microscope measurements show that Cu₅Si alloy is formed after the intensive ball milling and alloy particles along with low-crystallite Si are interspersed in graphite uniformly. Cu₅Si–Si/C composite electrodes deliver a larger reversible capacity than commercialized graphite and better cyclability than silicon. The increase of copper amount in the composites decreases reversible capacity but improves cycling performance. Cu₅Si–Si/C composite with Si:Cu = 1 demonstrates an initial reversible capacity of 612 mAh g⁻¹ at 0.2 mA cm⁻² in the voltage range from 0.02 to 1.5 V. The capacity retention is respectively 74.5 and 70.0% at the 40th cycle at the current density of 0.2 and 1 mA cm⁻².     

Electrochemical properties of carbon-coated Si/B composite anode for lithium ion batteries

Carbon-coated Si and Si/B composite powders prepared by hydrocarbon gas (argon + 10 mol% propylene) pyrolysis were investigated as the anodes for lithium-ion batteries. Carbon-coated silicon anode demonstrated the first discharge and charge capacity as 1568 mAh g⁻¹ and 1242 mAh g⁻¹, respectively, with good capacity retention for 10 cycles. The capacity fading rate of carbon-coated Si/B composite anode decreased as the amounts of boron increased. In addition, the cycle life of carbon-coated Si/B/graphite composite anode has been significantly improved by using sodium carboxymethyl cellulose (NaCMC) and styrene butadiene rubber (SBR)/NaCMC mixture binders compared to the poly(vinylidene fluoride, PVdF) binder. A reversible capacity of about 550 mAh g⁻¹ has been achieved at 0.05 mAm g⁻¹ rate and its capacity could be maintained up to 450 mAh g⁻¹ at high rate of 0.2 mAm g⁻¹ even after 30 cycles. The improvement of the cycling performance is attributed to the lower interfacial resistance due to good electric contact between silicon particles and copper substrate.