Nanoporous Si Research Articles

Effect of SiO2 passivation layer on nanoporous Si photocathode under the density of SiO2 passivation layer

This study presents the fabrication and comparative evaluation of SiO2/Si photoelectrodes employing two distinct SiO2 oxidation methods. Each SiO2/Si photoelectrodes was successfully generated based on uniform SiO2 layers a top nanoporous Si that have thicknesses of 2.7 nm via hydrogen peroxide oxidation (H2O2 oxidation) and 1.8 nm via nitric acid oxidation of Si (NAOS). Notably, the thin NAOS-formed SiO2 layer resulted in a measured suboxide density approximately 4.8 times lower (0.5192 × 1014 atoms/cm2) than that of the H2O2 oxidation (2.4969 × 1014 atoms/cm2). The presence of this SiO2 layer not only passivates defects but also improves electrical properties. As a result, the photocurrent density of the NAOS-formed SiO2/Si photoelectrode notably outperformed that of the H2O2-formed counterpart (about 1.7 times). This enhancement is attributed to the consistent passivation effects and improved electrical properties achieved due to NAOS-formed SiO2. This study contributes to a deeper understanding of how the SiO2 passivation layer can be utilized to improved photocurrent density.

Scale-dependent behavior of microstructure characteristics of laser powder bed fused TiB2/AlSi10Mg composite

The pre-alloyed TiB2/AlSi10Mg composite, a new high-strength aluminum alloy developed for laser powder bed fusion (LPBF) technology, offers promising applications in lightweight and multi-scaled structures. However, thermal behavior during LPBF is markedly scale-dependent, leading to microstructural variations that significantly affect the load-bearing capacity of multi-scaled structures. Therefore, this study systematically investigates the scale-dependent behavior of microstructure characteristics of this composite. Utilizing a hatching scanning strategy, it was found that the marginal zones of samples are predominantly composed of coarse Al cell and Al grain structures, contrasting with the fine microstructures in the central zones. With increasing structure scale, cell and grain structures in both the marginal and central zones become more refined, with cell sizes reducing by 49 %–72 % (∼3.02 µm → 0.86–1.55 µm). Particularly, the minimum-scaled structures also feature broken eutectic Si particles and nanopores. The essence is primarily due to the low heat dissipation with higher peak temperature and longer duration time at high temperatures in both the small-scale structures and marginal zones. Additionally, smaller structures correlate with reduced microhardness and tensile strength, accompanied by the “softening” of the marginal zones. The strength of the minimum-scaled structure is only half that of the standard sample. Our findings suggest a scale threshold of 2.0 mm for researching scale effect. Encouragingly, incorporating additional contour scanning significantly counteracts the adverse influence of the scale effect. Owing to the combined influence of extended inter-layer time and laser remelting, all samples demonstrate a distinctly refined microstructure. This results in consistently high levels of microhardness and strength, with the “hardening” of the marginal zones. Eventually, the relationship between mechanical properties and microstructure sizes is established. This study provides valuable insights into the innovative designs and engineering applications of multi-scaled structures in LPBF using various materials.

Multi-stage stabilization and high-strength nano-porous Si@C for simple fabrication of lithium-ion batteries

Multi-stage stabilization and high-strength nano-porous Si@C for simple fabrication of lithium-ion batteries

Stress-Relief Engineering in a N-Doped C-Modified Hierarchical Nanoporous Si Anode with a Microcurved Pore Wall Structure for Enhanced Lithium Storage.

The commercialization of alloy-type anodes has been hindered by rapid capacity degradation due to volume fluctuations. To address this issue, stress-relief engineering is proposed for Si anodes that combines hierarchical nanoporous structures and modified layers, inspired by the phenomenon in which structures with continuous changes in curvature can reduce stress concentration. The N-doped C-modified hierarchical nanoporous Si anode with a microcurved pore wall (N-C@m-HNP Si) is prepared from inexpensive Mg-55Si alloys using a simple chemical etching and heat treatment process. When used as the anode for lithium-ion batteries, the N-C@m-HNP Si anode exhibits initial charge/discharge specific capacities of 1092.93 and 2636.32 mAh g-1 at 0.1 C (1 C = 3579 mA g-1), respectively, and a stable reversible specific capacity of 1071.84 mAh g-1 after 200 cycles. The synergy of the hierarchical porous structure with a microcurved pore wall and the N-doped C-modified layer effectively improves the electrochemical performance of N-C@m-HNP Si, and the effectiveness of stress-relief engineering is quantitatively analyzed through the theory of elastic bending of thin plates. Moreover, the formation process of Li15Si4 crystals, which causes substantial mechanical stress, is investigated using first-principles molecular dynamic simulations to reveal their tendency to occur at different scales. The results demonstrate that the hierarchical nanoporous structure helps to inhibit the transformation of amorphous LixSi into metastable Li15Si4 crystals during lithiation.

Nanoporous silicon fiber networks in a composite anode for all-solid-state batteries with superior cycling performance

All-solid-state batteries comprising Si anodes are promising materials for energy storage in electronic vehicles because their energy density is approximately 1.7 times higher than that of graphite anodes. However, Si undergoes severe volume changes during cycling, resulting in the loss of electronic and ionic conduction pathways and rapid capacity fading. To address this challenge, we developed composite anodes with a nanoporous Si fiber network structure in sulfide-based solid electrolytes (SEs) and conductive additives. Nanoporous Si fibers were fabricated by electrospinning, followed by magnesiothermic reduction. The total pore volume of the fibers allowed pore shrinkage to compensate for the volumetric expansion of Li12Si7, thereby suppressing outward expansion and preserving the Si-SE (or conductive additive) interface. The network structure of the lithiated Si fibers compensates for electronic and ionic conduction pathways even to the partially delaminated areas, leading to increased Si utilization. The anodes exhibited superior performance, achieving an initial Coulombic efficiency of 71%, a reversible capacity of 1474 mAh g−1, and capacity retention of 85% after 40 cycles with an industrially acceptable areal capacity of 1.3 mAh cm−2. The proposed approach can reduce the constraint pressure during charging/discharging and may have practical applications in large-area all-solid-state batteries.

Carbon-Free Conversion of SiO2 to Si via Ultra-Rapid Alloy Formation: Toward the Sustainable Fabrication of Nanoporous Si for Lithium-Ion Batteries.

Silicon has the potential to improve lithium-ion battery (LIB) performance substantially by replacing graphite as an anode. The sustainability of such a transformation, however, depends on the source of silicon and the nature of the manufacturing process. Today's silicon industry still overwhelmingly depends on the energy-intensive, high-temperature carbothermal reduction of silica─a process that adversely impacts the environment. Rather than use conventional thermoreduction alone to break Si-O bonds, we report the efficient conversion of SiO2 directly to Mg2Si by a microwave-induced Mg plasma within 2.5 min at merely 200 W under vacuum. The underlying mechanism is proposed, wherein electrons with enhanced kinetics function readily as the reductant while the "bombardment" from Mg cations and electrons promotes the fast nucleation of Mg2Si. The 3D nanoporous (NP) Si is then fabricated by a facile thermal dealloying step. The resulting hierarchical NP Si anodes deliver stable, extended cycling with excellent rate capability in Li-ion half-cells, with capacities several times greater than graphite. The microwave-induced metal plasma (MIMP) concept can be applied just as efficiently to the synthesis of Mg2Si from Si, and the chemistry should be extendable to the reduction of multiple metal(loid) oxides via their respective Mg alloys.

Direct Vibrational Stark Shift Probe of Quasi-Fermi Level Alignment in Metal Nanoparticle Catalyst-Based Metal-Insulator-Semiconductor Junction Photoelectrodes.

Photoelectrodes consisting of metal-insulator-semiconductor (MIS) junctions are a promising candidate architecture for water splitting and for the CO2 reduction reaction (CO2RR). The photovoltage is an essential indicator of the driving force that a photoelectrode can provide for surface catalytic reactions. However, for MIS photoelectrodes that contain metal nanoparticles, direct photovoltage measurements at the metal sites under operational conditions remain challenging. Herein, we report a new in situ spectroscopic approach to probe the quasi-Fermi level of metal catalyst sites in heterogeneous MIS photoelectrodes via surface-enhanced Raman spectroscopy. Using a CO2RR photocathode, nanoporous p-type Si modified with Ag nanoparticles, as a prototype, we demonstrate a selective probe of the photovoltage of ∼0.59 V generated at the Si/SiOx/Ag junctions. Because it can directly probe the photovoltage of MIS heterogeneous junctions, this vibrational Stark probing approach paves the way for the thermodynamic evaluation of MIS photoelectrodes with varied architectural designs.

Nanoporous silicon photocathodes with [Co] molecular catalyst for enhanced solar-driven hydrogen generation

Efficient photoelectrochemical water splitting by nanoporous Si photocathode using Co(dmgH)2(py)Cl as H2-evolution catalyst under illumination of simulated sunlight.

Engineering thermal transport within Si thin films: The impact of nanoslot alignment and ion implantation

In recent years, nanoporous Si films have been intensively studied for their potential applications in thermoelectrics and the thermal management of devices. To minimize the thermal conductivity, ultrafine nanoporous patterns are required but the smallest structure size is largely limited by the spatial resolution of the employed nanofabrication techniques. Along this line, an effectively smaller characteristic length of a nanoporous film can be achieved with offset nanoslot patterns. Compared with periodic circular pores, the nanoslot pattern can achieve an even lower thermal conductivity, where a much smaller porosity is required using ultra-narrow nanoslots. The obtained low thermal conductivity can be understood from the thermally dead volume revealed by phonon Monte Carlo simulations. To further minimize the contribution from short-wavelength phonons, an additional 25% thermal conductivity reduction can be achieved with Ga ions implanted using a focused ion beam.

Failure analysis and design principles of silicon-based lithium-ion batteries using micron-sized porous silicon/carbon composite

Significant progress has been made toward overcoming fundamental challenges in developing a silicon (Si) anode for lithium-ion batteries (LIBs). However, much less work has been reported on design and failure analysis of these batteries for practical applications. In this work, we analyzed the main factors that affect the performance of a Si anode and the energy density of pouch cells using micron-sized, nanoporous Si coated by pitch-derived carbon (p-Si/C). The volumetric energy density of the Si anode depends heavily on these factors, such as the loading of p-Si/C in the anode, the calendering density of the anode, the first-cycle coulombic efficiency, and the capacity density of the anode. The effects of other factors on the cycling performance were also revealed by postmortem analysis of cycled anodes. We found that a stable p-Si/C electrode structure is critical to avoid the disintegration of the anode structural integrity and lithium plating and enable long-term cycling. Moreover, prelithiation of Si anodes increases energy density, while the degree of prelithiation is another factor to balance with the cycle life of Si-based full cells. We propose pathways and strategies for adoption of micron-sized p-Si/C anodes in LIBs for their practical applications.

The Size Effects of Si Particles on the Final Si@C Composite Anode

The particle size of Si raw materials is closely related with the stability and the cost of the final Si-based anode. Nevertheless, the detailed mechanism is still unclear. In the present study, Si particles with different sizes (100 and 500 nm and 1 μm) were used to prepare a nanoporous Si@C (P–Si–C) anode via Si–Mg alloy intermediate and carbon coating with Mg reduction of CO2. Contrary to the general idea, we found that P–Si–C synthesized with larger particle size of silicon exhibited better performance, and P–Si–C (1 μm) could maintain a specific capacity of 1741.1 mAh/g after 70 cycles at a current density of 0.5 A/g, with a capacity retention rate of 81.2%. It enlightens us that silicon–carbon composites can be prepared with low-cost large-grained silicon, which has significant guidance for industrial applications.

Enhanced Photoelectrochemical Water Splitting of Black Silicon Photoanode with pH‐Dependent Copper‐Bipyridine Catalysts

Since the water oxidation half-reaction requires the transfer of multi-electrons and the formation of O-O bond, it's crucial to investigate the catalytic behaviours of semiconductor photoanodes. In this work, a bio-inspired copper-bipyridine catalyst of Cu(dcbpy) is decorated on the nanoporous Si photoanode (black Si, b-Si). Under AM1.5G illumination, the b-Si/Cu(dcbpy) photoanode exhibits a high photocurrent density of 6.31 mA cm-2 at 1.5 VRHE at pH 11.0, which is dramatically improved from the b-Si photoanode (1.03 mA cm-2 ) and f-Si photoanode (0.0087 mA cm-2 ). Mechanism studies demonstrate that b-Si/Cu(dcbpy) has improved light-harvesting, interfacial charge-transfer, and surface area for water splitting. More interestingly, b-Si/Cu(dcbpy) exhibits a pH-dependent water oxidation behaviour with a minimum Tafel slope of 241 mV/dec and the lowest overpotential of 0.19 V at pH 11.0, which is due to the monomer/dimer equilibrium of copper catalyst. At pH ∼11, the formation of dimeric hydroxyl-complex could form O-O bond through a redox isomerization (RI) mechanism, which decreases the required potential for water oxidation. This in-depth understanding of pH-dependent water oxidation catalyst brings insights into the design of dimer water oxidation catalysts and efficient photoanodes for solar energy conversion.

High Cycle Stability of Nanoporous Si Composites in All-solid-state Lithium-ion Batteries

Stress relaxation of Si with large structural fluctuations is a critical challenge for its practical application in lithium-ion batteries (LIBs). In this study, nanoporous Si particles, which are prepared by Mg2Si reduction of mesoporous SiO2 spheres, are applied as an anode active material for all-solid-state LIBs (ASSLIBs) with a Li3PS4 solid electrolyte. Nanoporous Si half-cells exhibit an excellent cyclability with a high-capacity retention of about 90% at 50 cycles compared to non-porous Si half-cells below 20%. The cross-sectional characteristics of nanoporous and non-porous Si composite anodes are accurately compared using electrochemical impedance spectroscopy and field emission scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy. Based on these results, we conclude that the expansion/contraction of nanosized Si pores and the elastic deformation of Li3PS4 effectively relieve the structural stress derived from the volume change of Si particles/aggregates during lithiation and delithiation, resulting in high cycle stability. These findings provide valuable information for the rational design of Si-based anodes for high-performance ASSLIBs.

Extension of the two-layer model to heat transfer coefficient predictions of nanoporous Si thin films

Heat exchange between a solid material and the gas environment is critical for the heat dissipation of miniature electronic devices. In this aspect, existing experimental studies focus on non-porous structures such as solid thin films, nanotubes, and wires. In this work, the proposed two-layer model for the heat transfer coefficient (HTC) between a solid sample and the surrounding air is extended to 70-nm-thick nanoporous Si thin films that are patterned with periodic rectangular nanopores having feature sizes of 100–400 nm. The HTC values are extracted using the 3ω method based on AC self-heating of a suspended sample with better accuracy than steady-state measurements in some studies. The dominance of air conduction in the measured HTCs is confirmed by comparing measurements with varied sample orientations. The two-layer model, developed for nanotubes, is still found to be accurate when the nanoporous film is simply treated as a solid film in the HTC evaluation along with the radiative mean beam length as the characteristic length of the nanoporous film. This finding indicates the potential of increasing HTC by introducing ultra-fine nanoporous patterns, as guided by the two-layer model.

Performance improvement of nanoporous Si composite anodes in all-solid-state lithium-ion batteries by using acetylene black as a conductive additive

Si is the most promising anode active material for lithium-ion batteries owing to its high theoretical capacity and low operating potential. In this study, composite anodes consisting of nanoporous Si particles, Li3PS4 solid electrolyte, and a conductive additive (CA) in weight ratios of 4:6:x (x = 1, 2, 3, and 4) are fabricated and tested. The electrical conductivities are 4.1 × 10-4 S cm−1 and 6.8 × 10-4 S cm−1 at × = 1 and 4, respectively. In addition, the charge capacity increases in proportion with CA content from 2700 mAh g−1 to 3015 mAh g−1. These results indicate that new conduction paths are created in the nanoporous Si composite anodes upon adding a CA. To the best of our knowledge, this is the first study to examine the effect of a CA on the electrochemical performance of all-solid-state lithium-ion batteries with Si-based anodes. Our findings provide valuable information that should be helpful in meeting the energy demands of next-generation applications.

Flexible Porous Silicon/Carbon Fiber Anode for High-Performance Lithium-Ion Batteries.

We demonstrate a cross−linked, 3D conductive network structure, porous silicon@carbon nanofiber (P−Si@CNF) anode by magnesium thermal reduction (MR) and the electrospinning methods. The P−Si thermally reduced from silica (SiO2) preserved the monodisperse spheric morphology which can effectively achieve good dispersion in the carbon matrix. The mesoporous structure of P–Si and internal nanopores can effectively relieve the volume expansion to ensure the structure integrity, and its high specific surface area enhances the multi−position electrical contact with the carbon material to improve the conductivity. Additionally, the electrospun CNFs exhibited 3D conductive frameworks that provide pathways for rapid electron/ion diffusion. Through the structural design, key basic scientific problems such as electron/ion transport and the process of lithiation/delithiation can be solved to enhance the cyclic stability. As expected, the P−Si@CNFs showed a high capacity of 907.3 mAh g−1 after 100 cycles at a current density of 100 mA g−1 and excellent cycling performance, with 625.6 mAh g−1 maintained even after 300 cycles. This work develops an alternative approach to solve the key problem of Si nanoparticles’ uneven dispersion in a carbon matrix.

Nanoporous germanium prepared by a mechanochemical reaction with enhanced lithium storage properties.

The cost-effective and facile fabrication of nanostructured germanium for lithium-ion batteries (LIBs) remains a grand challenge. Herein, nanoporous Z-Ge was generated via a facile two-step mechanochemical-etching reaction with Mg2Ge and ZnCl2. The prepared nanoporous Ge nanoparticles, as the anode for Li-Ge half cells, showed superior LIB performance, in terms of a high capacity, good rate capability, and good long-term stability of 700 cycles. Significantly, the mechanochemical reaction was extended to produce other nanoporous Ge or Si materials such as A-Ge, Z-Si, and A-Si via the mechanochemical reaction between Mg2Ge and AlCl3, Mg2Si and ZnCl2, and Mg2Si and AlCl3.

Negligibly Small Load Changes Observed in All-Solid-State Lithium-Ion Full Cells with Nanoporous Si Composite Anodes

Negligibly Small Load Changes Observed in All-Solid-State Lithium-Ion Full Cells with Nanoporous Si Composite Anodes

Thermal conduction and rectification phenomena in nanoporous silicon membranes.

Non-equilibrium molecular dynamics simulations have been applied to study thermal transport properties, such as thermal conductivity and rectification, in nanoporous Si membranes. Cylindrical pores have been generated in crystalline Si membranes with different configurations, including step-like, ordered and random pore distributions. The effect of interface and overall porosity on thermal transport properties has been investigated as well as the impact of the porosity profile on the direction of the heat current. The lowest thermal conductivity and highest thermal rectification for equal porosity have been found for a step-like pore distribution. Increasing interface porosity resulted in an increase of thermal rectification, which has been found to be systematically higher for random pore distribution with respect to an ordered one. Furthermore, a maximum in rectification of 5.5% has been found for a specific overall porosity (Φtot = 0.02) in samples with constant interface porosity and ordered pore distribution. This has been attributed to an increased effect of asymmetric interface boundary resistance resulting from increased fluctuations of the latter with altering temperature. The average value of the interface boundary resistance has been found to decrease with increasing porosity for samples with ordered pore distribution leading to a decrease in thermal rectification.

A Micrometer-Sized Silicon/Carbon Composite Anode Synthesized by Impregnation of Petroleum Pitch in Nanoporous Silicon.

Porous silicon (Si)/carbon nanocomposites have been extensively explored as a promising anode material for high-energy lithium (Li)-ion batteries (LIBs). However, shrinking of the pores and sintering of Si in the nanoporous structure during fabrication often diminishes the full benefits of nanoporous Si. Herein, a scalable method is reported to preserve the porous Si nanostructure by impregnating petroleum pitch inside of porous Si before high-temperature treatment. The resulting micrometer-sized Si/C composite maintains a desired porosity to accommodate large volume change and high conductivity to facilitate charge transfer. It also forms a stable surface coating that limits the penetration of electrolyte into nanoporous Si and minimizes the side reaction between electrolyte and Si during cycling and storage. A Si-based anode with 80% of pitch-derived carbon/nanoporous Si enables very stable cycling of a Si||Li(Ni0.5Co0.2Mn0.3)O2 (NMC532) battery (80% capacity retention after 450 cycles). It also leads to low swelling in both particle and electrode levels required for the next generation of high-energy LIBs. The process also can be used to preserve the porous structure of other nanoporous materials that need to be treated at high temperatures.