Mg-doped hematite (α-Fe2O3) was synthesized by atomic layer deposition (ALD). The resulting material was identified as p-type with a hole concentration of ca. 1.7 × 1015 cm–3. When grown on n-type hematite, the p-type layer was found to create a built-in field that could be used to assist photoelectrochemical water splitting reactions. A nominal 200 mV turn-on voltage shift toward the cathodic direction was measured, which is comparable to what has been measured using water oxidation catalysts. This result suggests that it is possible to achieve desired energetics for solar water splitting directly on metal oxides through advanced material preparations. Similar approaches may be used to mitigate problems caused by energy mismatch between water redox potentials and the band edges of hematite and many other low-cost metal oxides, enabling practical solar water splitting as a means for solar energy storage.

•Solid-state supercapacitors made using ruthenium oxide coated silicon nanowires.•Fabrication process is compatible with silicon integrated circuit processing.•Ruthenium oxide provides a three order of magnitude increase in capacitance.•Specific capacitance scales with the total nanowire surface area.•High specific energies and cyclability without sacrificing power performance.Solid-state on-chip supercapacitors based on ruthenium oxide coated silicon nanowires were fabricated using a process that is compatible with silicon integrated circuit processing. Ordered arrays of silicon nanowires were fabricated using metal-assisted anodic etching (MAAE). Atomic layer deposition (ALD) was used to form a uniform coating of ruthenium oxide on high-aspect-ratio silicon nanowires at a moderate temperature of 290 °C. Coated nanowire electrodes were studied using cyclic voltammetry and charge-discharge tests in a neutral Na2SO4 electrolyte, and a specific capacitance of 19 mFcm−2 was achieved at 5 mVs−1. Solid state nanowire capacitors were then fabricated with symmetric face to face nanowire arrays separated by a polymer-based electrolyte. This device exhibited a specific capacitance as high as 6.5 mFcm−2 at 2 mVs−1. The full device was tested over 10000 cycles under galvanostatic charge-discharge at 0.4 mAcm−2, and showed a retention of 92% of the specific capacitance. The specific capacitance was found to scale with the total nanowire surface area, as controlled by controlling the aspect ratios of the wires. The solid state nanowire-based device also achieved high specific energies without sacrificing power performance.

Porous carbon materials are widely used in particulate forms for energy applications such as fuel cells, batteries, and (super) capacitors. To better hold the particles together, polymeric additives are utilized as binders, which not only increase the weight and volume of the devices, but also cause adverse side effects. We developed a wood-derived, free-standing porous carbon electrode and successfully applied it as a cathode in Li-O2 batteries. The spontaneously formed hierarchical porous structure exhibits good performance in facilitating the mass transport and hosting the discharge products of Li2O2. Heteroatom (N) doping further improves the catalytic activity of the carbon cathode with lower overpotential and higher capacity. Overall, the Li-O2 battery based on the new carbon cathode affords a stable energy efficiency of 65% and can be operated for 20 cycles at a discharge depth of 70%. The wood-derived free-standing carbon represents a new, unique structure for energy applications.

The addition of a co-catalyst onto the surface of a photocathode often greatly enhances the harvested photovoltage of the system. However, the true nature of how the catalyst improves the onset potential remains poorly understood. As a result, how to best utilize effective co-catalysts is still a limiting factor in achieving high performance earth abundant photoelectrochemical hydrogen evolution. Using intensity modulated photocurrent spectroscopy (IMPS), we have probed charge behaviors at the photoelectrode co-catalyst interface. We find that Pt drastically reduces charge recombination at the semiconductor liquid interface (SCLI). Further studies reveal that the onset potentials can be improved either by accelerating the reaction kinetics or reducing the recombination at the SCLI. The knowledge permits us to understand how earth abundant HER catalysts, such as CoP, behave at the SCLI. It is found that CoP is more effective at accelerating the reaction kinetics than reducing recombination.

Photoelectrochemical (PEC) reactions, such as water splitting, promise a direct route for solar-to-chemical energy conversion. Successful implementations of these reactions often require the combination of catalysts with photoelectrodes. How these catalysts improve the performance of photoelectrodes, however, is not well understood, making it difficult to further improve these systems for practical applications. Here, we present a systematic study that directly compares two water-oxidation catalysts (WOCs) on a hematite (α-Fe2O3)-based PEC system. We observe that when a thin layer of a heterogenized molecular Ir catalyst (het-WOC) is applied to a hematite photoanode, the system's performance is improved primarily due to improved charge transfer (>2 fold), while the surface recombination rate remains unchanged. In stark contrast, heterogeneous oxide catalysts (IrOx) improve the PEC performance of hematite by significantly reducing the surface recombination rate. These results suggest that the het-WOC provides additional charge-transfer pathways across the Fe2O3|H2O interface, while IrOx and similar bulk metal-oxide catalysts replace the Fe2O3|H2O interface with a fundamentally different one.

Photoelectrochemical (PEC) water splitting holds the potential to meet the challenges associated with the intermittent nature of sunlight. Catalysts have often been shown to improve the performance of PEC water splitting, but their working mechanisms are not well understood. Using intensity modulated photocurrent spectroscopy (IMPS), we determined the rate constants of water oxidation and recombination at the surface of three different hematite-based photoanodes. It was found that the best performing electrodes, in terms of photocurrent onset potential, exhibited the slowest water oxidation rate constants, which was a surprise. The performance of these photoelectrodes was enabled by the slow surface recombination. When amorphous NiFeOx, a water oxidation catalyst, was present, the rate of surface hole transfer actually slowed down; what was slowed more was the recombination rate at the hematite surface, resulting in better water oxidation performance. As such, NiFeOx primarily serves as a passivation layer rather than a catalytic layer. Together a better understanding of the role of catalytic overlayers for water oxidation has been achieved.

•A dual-electrolyte design is proposed and demonstrated for Mg-Br2 Battery applications.•Good stability for both the Mg anode and Br2 cathode is obtained.•High Columbic efficiencies (96%) are measured.•Up to 20 cycles of repeated discharge and recharge are achieved.Owing to its low cost and high volumetric capacity, Mg is a promising anode material for energy storage applications. Previous research has identified the cathode chemistry as a major challenge that must be addressed for further development of Mg-based batteries. In response to this challenge, here we show Br2-based conversion chemistry is a potential route toward rechargeable Mg-batteries. Compared with Mg-ion or Mg-air chemistries, the Mg-Br2 system features fast kinetics and good cyclability. To solve the issues of poor electrolyte stability, a non-aqueous, dual-electrolyte scheme was employed for this proof-of-concept demonstration. The anolyte consisted of Mg(TFSI)2 dissolved in a monoglyme and diglyme mixture. The catholyte was composed of Mg(TFSI)2 in PYR14TFSI ionic liquid mixed with active bromine species. When Mg was used as the anode, an open circuit voltage of 3.0 V (vs. Mg2+/Mg) was measured. The prototypical cell was successfully discharged and charged for over 20 cycles with consistently high coulombic efficiencies (ca. 96%).

As a promising high-capacity energy storage technology, Li–O2 batteries face two critical challenges, poor cycle lifetime and low round-trip efficiencies, both of which are connected to the high overpotentials. The problem is particularly acute during recharge, where the reactions typically follow two-electron mechanisms that are inherently slow. Here we present a strategy that can significantly reduce recharge overpotentials. Our approach seeks to promote Li2O2 decomposition by one-electron processes, and the key is to stabilize the important intermediate of superoxide species. With the introduction of a highly polarizing electrolyte, we observe that recharge processes are successfully switched from a two-electron pathway to a single-electron one. While a similar one-electron route has been reported for the discharge processes, it has rarely been described for recharge except for the initial stage due to the poor mobilities of surface bound superoxide ions (O2–), a necessary intermediate for the mechanism. Key to our observation is the solvation of O2– by an ionic liquid electrolyte (PYR14TFSI). Recharge overpotentials as low as 0.19 V at 100 mA/gcarbon are measured.

Solid solutions have been widely investigated for solar energy conversion because of the ease to control properties (e.g., band edge positions, charge carrier transport, and chemical stability). In this study, we introduce a new method to investigate intrinsic solar energy conversion properties of solid solutions through fabricating high-quality single-crystalline solid solution films by pulsed laser deposition. This method rules out external factors, such as morphology, crystalline grain size, orientation, density and distribution, surface area, and particle–particle or particle–conducting layer connection, that have plagued previous studies on solid solution photoelectrodes. Perovskite BiFeO3 (BFO) and SrTiO3 (STO) were chosen as “end” members of the solid solutions (i.e., (BFO)x(STO)1–x (0 ≤ x ≤ 1)). Optical and photoelectrochemical (PEC) properties of the solid solutions significantly varied with changing compositions. Among the six studied compositions, BFO:STO (3:1 molar ratio) exhibited the highest photocurrent density with the photovoltage of 1.08 V. The photoelectrode also produced stable photocurrent for 12 h. Faradaic efficiencies of H2 and O2 formation close to 100% were measured.

Li oxygen (Li–O2) batteries promise high energy densities but suffer from challenges such as poor cycling lifetime and low round-trip efficiencies. Recently, the instability of carbon cathode support has been recognized to contribute significantly to the problems faced by Li–O2 batteries. One strategy to address the challenge is to replace carbon materials with carbon-free ones. Here, we present titanium silicide nanonets (TiSi2) as such a new material platform for this purpose. Because TiSi2 exhibits no oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) activities, catalysts are required to promote discharge and recharge reactions at reduced overpotentials. Pd nanoparticles grown by atomic layer deposition (ALD) were observed to provide the bifunctionalities of ORR and OER. Their adhesion to TiSi2 nanonets, however, was found to be poor, leading to drastic performance decay due to Pd detachments and aggregation. The problem was solved by adding another layer of Co3O4, also prepared by ALD. Together, the Pd/Co3O4/TiSi2 combination affords the desired functionalities and stability. Li–O2 test cells that lasted more than 126 cycles were achieved. The reversible formation and decomposition of Li2O2 was verified by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), ferrocenium back-titration, and gas-chromatography and mass spectrometry (GC-MS). Our results provide a new material platform for detailed studies of Li–O2 operations for better understanding of the chemistries involved, which is expected to help pave the way toward practical Li–O2 battery realizations.Keywords: atomic layer deposition; catalyst; cathode; cobalt oxide; lithium oxygen battery; palladium; titanium disilicide

Si nanowires (SiNWs) are shown to absorb visible light to reduce Ni catalysts into Ni0 compounds, enabling alkyne carboxylation reactions with CO2 as a carbon feedstock. The reduced Ni catalysts are effective in CO2 fixation through a 4-octyne carboxylation reaction. The reduction potentials of the Ni catalysts can be tuned from -1.35 to -0.51 V (vs. saturated calomel electrode) by altering the binding ligands. The results shed light on the nature of charge transfer from SiNWs to the catalyst for this new class of photocatalytic reactions. By controlling the CO2 reduction potential of the catalysts with carefully ligand designs, it will bring more opportunities and options to realize the highly selective, effective and sustainable CO2 reduction in the future.硅纳米线在吸收光之后还原催化剂镍离子, 从而引发以二氧化碳为碳原料的炔烃羧基化反应. 镍催化剂的还原态产物在这 一固碳反应中能够非常高效地将4-辛炔羧酸化. 配位体的给电子基团和吸电子基团的能力不同, 将会影响催化剂的氧化还原电 位. 还原 电位由于构成镍催化剂的配位体不同, 控制在−1.35到−0.51 V(相对于甘汞电极电位)之间. 这种新的光催化反应本质上是 利用了硅纳 米线在光照时产生的光电子从半导体界面转移到催化剂的现象, 通过改变催化剂的配位体实现控制催化剂的还原电位. 此方法 为实现 设计可持续性高效的二氧化碳还原反应提供了更多的选择.

Photoelectrochemistry (PEC) holds potential as a direct route for solar energy storage. Its performance is governed by how efficiently photoexcited charges are separated and how fast the charges are transferred to the solution, both of which are highly sensitive to the photoelectrode surfaces near the electrolyte. While other aspects of a PEC system, such as the light-absorbing materials and the catalysts that facilitate charge transfer, have been extensively examined in the past, an underwhelming amount of attention has been paid to the energetics at the photoelectrode/electrolyte interface. The lack of understanding of this interface is an important reason why many photoelectrode materials fail to deliver the expected performance in harvesting solar energy in a PEC system. Using hematite (α-Fe2O3) as a material platform, we present in this Perspective how surface modifications can alter the energetics and the resulting consequences on the overall PEC performance. It has been shown that a detailed understanding of the photoelectrode/eletrolyte interfaces can contribute significantly to improving the performance of hematite, which enabled unassisted solar water splitting when combined with an amorphous Si photocathode.

The Li–O2 battery promises high capacity to meet the need for electrochemical energy storage applications. Successful development of the technology hinges on the availability of stable cathodes. The reactivity exhibited by a carbon support compromises the cyclability of Li–O2 operation. A noncarbon cathode support has therefore become a necessity. Using a TiSi2 nanonet as a high surface area, conductive support, we obtained a new noncarbon cathode material that corrects the deficiency. To enable oxygen reduction and evolution, Ru nanoparticles were deposited by atomic layer deposition onto TiSi2 nanonets. A surprising site-selective growth whereupon Ru nanoparticles only deposit onto the b planes of TiSi2 was observed. DFT calculations show that the selectivity is a result of different interface energetics. The resulting heteronanostructure proves to be a highly effective cathode material. It enables Li–O2 test cells that can be recharged more than 100 cycles with average round-trip efficiencies >70%.

Hematite prepared by atomic layer deposition (ALD) was found to exhibit photocurrents when illuminated by near-infrared light (λ = 830 nm), whose energy is smaller than the band gap of hematite. The phenomenon was inferred to be a result of valence band to surface state transition. The influence of surface states on the thermodynamics of the hematite/water interface was studied under open-circuit conditions. It was discovered that the equilibrium potential of the hematite surface was more negative than water oxidation potential by at least 0.4 V. With a NiFeOx coating by photochemical decomposition of organometallic precursors, the equilibrium potential of hematite was restored to water oxidation potential, and the photoresponse under 830 nm illumination was annihilated. Therefore, the states were rationalized by the chemical status at the electrode surfaces, and this hypothesis was supported by similar observations on other metal oxide electrodes such as TiO2.

In order for the future energy needs of humanity to be adequately and sustainably met, alternative energy techniques such as artificial photosynthesis need to be made more efficient and therefore commercially viable. On a grand scale, the energies coming to and leaving from the earth are balanced. With the fast increasing waste heat produced by human activities, the balance may be shifted to threaten the ecosystem in which we reside. To avoid such dire consequences, it is necessary to power human activities using energy derived from the incoming source, which is predominantly solar irradiation. Indeed, most life on the surface of the earth is supported, directly or indirectly, by photosynthesis that harvests solar energy and stores it in chemical bonds for redistribution. Being able to mimic the process and perform it at high efficiencies using low-cost materials has significant implications. Such an understanding is a major intellectual driving force that motivates research by us and many others.From a thermodynamic perspective, the key energy conversion step in natural photosynthesis happens in the light reactions, where H2O splits to give O2 and reactive protons. The capability of carrying out direct sunlight-driven water splitting with high efficiency is therefore fundamentally important. We are particularly interested in doing so using inorganic semiconductor materials because they offer the promise of durability and low cost. In this Account, we share our recent efforts in bringing semiconductor-based water splitting reactions closer to reality. More specifically, we focus on earth-abundant oxide semiconductors such as Fe2O3 and work on improving the performance of these materials as photoelectrodes for photoelectrochemical reactions.Using hematite (α-Fe2O3) as an example, we examine how the main problems that limit the performance, namely, the short hole collection distance, poor light absorption near the band edge, and mismatch of the band edge energetics with those of water redox reactions, can in principle be addressed by adding nanoscale charge collectors, forming buried junctions, and including additional light absorbers. These results highlight the power of forming homo- or heterojunctions at the nanoscale, which permits us to engineer the band structures of semiconductors to the specific application of water splitting. The key enabling factor is our ability to synthesize materials with precise control over the dimensions, crystallinity, and, most importantly, the interface quality at the nanoscale. While being able to tailor specific properties on a simple, earth-abundant device is not straightforward, the approaches we report here take significant steps towards efficient artificial photosynthesis, an energy harvesting technique necessary for the well-being of humanity.

Compared with competing technologies, rechargeable lithium ion batteries offer relative advantages such as high capacity and long cycle lifetime. Remarkable advances in the development of this technology notwithstanding significant performance improvements are still required to meet society's ever-growing electrical energy storage need. In particular, we long for devices with greater capacity, a higher power rate and a longer cycle lifetime. Aimed at solving challenges associated with poor charge transport within electrode materials, we have recently tested a new, nanonet-based material platform. The nanonet, made of TiSi2 (C49), is similar to the more commonly used porous carbon in that it has high surface area and good electrical conductivity. The key uniqueness of the nanonet lies in that its morphology is well-defined, permitting us to design and test various heteronanostructures. In essence, the TiSi2 nanonet can serve as a charge collector and a mechanical support for the construction of electrodes for a wide range of applications. We show that when combined with Si, an anode with superior performance is obtained. Similarly, a high-performance cathode is enabled by the TiSi2–V2O5 combination.

For many electrochemical reactions such as oxygen reduction, catalysts are of critical importance, as they are often necessary to reduce reaction overpotentials. To fulfill the promises held by catalysts, a well-defined charge transport pathway is indispensable. Presently, porous carbon is most commonly used for this purpose, the application of which has been recently recognized to be a potential source of concern. To meet this challenge, here we present the development of a catalyst system without the need for carbon. Instead, we focused on a conductive, two-dimensional material of a TiSi2 nanonet, which is also of high surface area. As a proof-of-concept, we grew Pt nanoparticles onto TiSi2 by atomic layer deposition. Surprisingly, the growth exhibited a unique selectivity, with Pt deposited only on the top/bottom surfaces of the nanonets at the nanoscale without mask or patterning. Pt {111} surfaces are preferably exposed as a result of a multiple-twinning effect. The materials showed great promise in catalyzing oxygen reduction reactions, which is one of the key challenges in both fuel cells and metal air batteries.Keywords: atomic layer deposition; catalyst; nanonets; oxygen reduction reaction; platinum; titanium disilicide

The n-type semiconductor tungsten oxide is readily dissolved in aqueous solution at pH > 4 and may be problematic in water splitting catalysis. We have reported that a tungsten oxide photoanode prepared by atomic layer deposition can be stabilized with a Mn-oxo compound for efficient photo water splitting at pH 4 and pH 7. However the molecular mechanism of water oxidation reaction in this robust catalytic system is not known. In this work, the mechanism for oxygen and hydrogen production by photo water splitting using Mn-oxo complex/tungsten oxide heteronanostructures was examined under different experimental conditions by X-ray photoelectron spectroscopy as well as gas chromatographic analysis, O-18 isotope measurements, and pH dependence of photocurrent. We found that the Mn(II) species plays an important role in the catalytic cycle of water oxidation in the Mn-oxo oligomer complex/tungsten oxide system and propose a working model of the Mn-oxo oligomer complex/tungsten oxide catalytic system in photo water splitting.Highlights► Mn-oxo oligomer complex/WO3 heterostructure mimics photosystem II water splitting. ► Mn-oxo oligomer complex/WO3 heterostructure is a robust catalyst for photo water splitting at pH 2–8. ► Mn(II) species is in the catalytic cycle of Mn/WO3 photo water splitting.

Confounded by global energy needs, much research has been devoted to convert solar energy to various usable forms, such as chemical energy in the form of hydrogen via water splitting. Most photoelectrodes, such as hematite, utilize UV and visible radiation, whereas ∼40% infrared (IR) energy remains unconverted. This work represents our initial attempt to utilize IR radiation, that is, adding rare-earth materials to existing photoelectrodes. A simple substrate composed of hematite film and rare-earth nanocrystals (RENs) was prepared and characterized. Spectroscopy evidence indicates that the RENs in the composite absorb IR radiation (980 nm) and emit at 550 and 670 nm. The emitted photons are absorbed by surrounding hematite films, leading to improvement of water splitting efficiency as measured by photocurrent enhancement. This initial work demonstrates the feasibility and concept of using RENs for utilizing more solar radiation, thus improving the efficiency of existing solar materials and devices.Keywords: energy conversion; hematite electrode; IR radiation; rare earth; solar water splitting; upconversion nanocrystals;

The discovery of new materials has played an important role in battery technology development. Among the newly discovered materials, those with layered structures are often of particular interest because many have been found to permit highly repeatable ionic insertion and extraction. Examples include graphite and LiCoO2 as anode and cathode materials, respectively. Here we report C49 titanium disilicide (TiSi2) as a new layered anode material, within which lithium ions can react with the Si-only layers. This result is enabled by the strategy of coating a thin (<5 nm) layer of oxide on the surface of TiSi2. This coating helped us rule out the possibility that the measured capacity is due to surface reactions. It also stabilizes TiSi2 to allow for the direct observation of TiSi2 in its lithiated and delithiated states. In addition, this stabilization significantly improved the charge and discharge performance of TiSi2. The confirmation that the lithium-ion storage capacity of TiSi2 is a result of its layered structure is expected to have major fundamental and practical implications.Keywords: electrode materials; energy storage; layered structures; lithium-ion battery; silicide

The performance of advanced energy conversion and storage devices, including solar cells and batteries, is intimately connected to the electrode designs at the nanoscale. Consider a rechargeable Li ion battery, a prevalent energy storage technology, as an example. Among other factors, the electrode material design at the nanoscale is key to realizing the goal of measuring fast ionic diffusion and high electronic conductivity, the inherent properties that determine power rates, and good stability upon repeated charge and discharge, which is critical to the sustainable high capacities. Here we show that such a goal can be achieved by forming heteronanostructures on a radically new platform we discovered, TiSi2 nanonets. In addition to the benefits of high surface area, good electrical conductivity, and superb mechanical strength offered by the nanonet, the design also takes advantage of how TiSi2 reacts with O2 upon heating. The resulting TiSi2/V2O5 nanostructures exhibit a specific capacity of 350 Ah/kg, a power rate up to 14.5 kW/kg, and 78.7% capacity retention after 9800 cycles of charge and discharge. These figures indicate that a cathode material significantly better than V2O5 of other morphologies is produced.Keywords: high power rate; lithium ion battery; long cycle time; titanium disilicide; vanadium oxide

As the most commonly encountered form of iron oxide in nature, hematite is a semiconducting crystal with an almost ideal bandgap for solar water splitting. Compelled by this unique property and other advantages, including its abundance in the Earth's crust and its stability under harsh chemical conditions, researchers have studied hematite for several decades. In this perspective, we provide a concise overview of the challenges that have prevented us from actualizing the full potentials of this promising material. Particular attention is paid to the importance of efficient charge transport, the successful realization of which is expected to result in reduced charge recombination and increased quantum efficiencies. We also present a general strategy of forming heteronanostructures to help meet the charge transport challenge. The strategy is introduced within the context of two material platforms, webbed nanonets and vertically aligned transparent conductive nanotubes. Time-resolved photoconductivity measurements verify the hypothesis that the addition of conductive components indeed increases charge lifetimes. Because the heteronanostructure approach is highly versatile, it has the potential to address other issues of hematite as well and promises new opportunities for the development of efficient energy conversion using this inexpensive and stable material.

We report the highest external quantum efficiency measured on hematite (α-Fe2O3) without intentional doping in a water-splitting environment: 46% at λ = 400 nm. This result was enabled by the introduction of TiSi2 nanonets, which are highly conductive and have suitably high surface areas. The nanonets serve a dual role as a structural support and an efficient charge collector, allowing for maximum photon-to-charge conversion. Without the addition of any oxygen-evolving catalysts, we obtained photocurrents of 1.6 and 2.7 mA/cm2 at 1.23 and 1.53 V vs RHE, respectively. These results highlight the importance of charge transport in semiconductor-based water splitting, particularly for materials whose performance is limited by poor charge diffusion. Our design introduces material components to provide a dedicated charge-transport pathway, alleviating the reliance on the materials’ intrinsic properties, and therefore has the potential to greatly broaden where and how various existing materials can be used in energy-related applications.

Using vertically aligned Cu2S nanowires as both physical templates and chemical sources, unique heteronanostructures were synthesized by solid-state conversion reactions at low temperatures. At temperatures as low as 105 °C and in the presence of H2S, segmented nanowires and rod-in-a-tube (RIT) structures were produced. The different morphologies were discovered to depend on the diffusivity of the ions from various metal coatings. In the case where the inward diffusion of outer metal is faster or roughly equivalent to that of Cu+ outward diffusion, incorporation and subsequent phase segregation occurred to yield segmented nanowires; in instances where Cu+ diffuses outward more quickly than the metal coating inward, the RIT morphology formed via a Kirkendall-like mechanism. The nanowire–Cu substrate interface was believed to play a unique and crucial role as either a reservoir of additional Cu or as a sink for out-diffusing Cu, depending on the nature of the reaction. Full conversion of Cu2S nanowires to wurtzite ZnS was also demonstrated, with the complete displacement of Cu back into the Cu substrate. These low-temperature, solid-state conversion reactions show promise as a possible route for synthesizing vertically aligned nanostructures with more complicated compositions.Keywords: conversion; copper sulfide; ionic diffusion; iron; nanorods; nanotube; nanowire; solid-state; zinc;

The titanium disicilicate (TiSi2) nanonet is a material with a unique two-dimensional morphology and has proven beneficial for energy conversion and storage applications. Detailed knowledge about how the nanonet grows may have important implications for understanding seedless nanostructure synthesis, in general, but is presently missing. Here, we report our recent efforts toward correcting this deficiency. We show that the TiSi2 nanonet growth is sensitive to the nature of the receiving substrates. High-yield nanonets are only obtained on those exhibiting no or low reactivities with Si. This result indicates that Si-containing clusters deposited on the substrate surfaces play an important role in the nanonet synthesis, and we suggest they serve to initiate the growth. The morphological complexity of the nanonet depends on the precursor concentrations but not on the growth durations. More TiCl4 results in nanonets with more complex structures. We understand that once a beam of a TiSi2 nanonet is formed, its sidewalls are resistant to branch formation. Instead, the tip of a beam is where a branch forms. This process is driven by the reactions between Ti- and Si-containing species. Building on this understanding, we demonstrate the creation of second-generation nanonets.Keywords: chemical vapor deposition; nanonets; nanostructures; seedless growth; titanium silicide

The performance of advanced energy conversion and storage devices, such as solar cells, supercapacitors, and lithium (Li) ion batteries, is intimately connected to the electrode design at the nanoscale. To enable significant developments in these research fields, we need detailed information about how the properties of the electrode materials depend on their dimensions and morphologies. This information is currently unavailable, as previous studies have mostly focused on understanding one type of morphology at a time. Here, we report a systematic study to compare the performance of nanostructures enabled by two platforms, one-dimensional nanowires and two-dimensional nanonets. The nanowires and nanonets shared the same composition (titanium disilicide) and similar sizes. Within the framework of Li ion battery applications, they exhibited different stabilities upon lithiation and delithiation (at a rate of 6 A/g), the nanonets-based nanostructures maintaining 90% and the nanowires-based ones 80% of their initial stable capacities after 100 cycles of repeated charge and discharge. The superior stability of the nanonets was ascribed to the two-dimensional connectivity, which afforded better structural stability than nanowires. Information generated by this study should contribute to the design of electrode materials and thereby enable broader applications of complex nanostructures for energy conversion and storage.Keywords: lithium ion batteries; nanonets; nanostructures; nanowires; silicide

We synthesized a unique heteronanostructure consisting of two-dimensional TiSi2 nanonets and particulate Si coating. The high conductivity and the structural integrity of the TiSi2 nanonet core were proven as great merits to permit reproducible Li+ insertion and extraction into and from the Si coating. This heteronanostructure was tested as the anode material for Li+ storage. At a charge/discharge rate of 8400 mA/g, we measured specific capacities >1000 mAh/g. Only an average of 0.1% capacity fade per cycle was observed between the 20th and the 100th cycles. The combined high capacity, long capacity life, and fast charge/discharge rate represent one of the best anode materials that have been reported. The remarkable performance was enabled by the capability to preserve the crystalline TiSi2 core during the charge/discharge process. This achievement demonstrates the potency of this novel heteronanostructure design as an electrode material for energy storage.

We report that TiSi2 nanonet exhibits considerable activities in the reversible lithiation and delithiation processes, although bulk-sized titanium silicide is known to be inactive when used as an electrode material for lithium ion batteries. The detailed mechanism of this unique process was studied using electrochemical techniques including the electrochemical impedance spectroscopy (EIS) method. By systematic characterizations of the Nyquist plots and comparisons with the microstructure examinations, we identified the main reason for the activities as the layered crystal structure that is found stable only in TiSi2 nanonets. The layer structure is characterized by the existence of a Si-only layer, which exhibits reactivity when exposed to lithium ions. Control studies where TiSi2 nanowires and TiSi2/Si heteronanostructures were involved, respectively, were performed. Similar to bulk TiSi2, TiSi2 nanowires show limited reactivity in lithium ion insertion and deinsertion; the EIS characteristics of TiSi2/Si heteronanostructures, on the other hand, are distinctly different from those of TiSi2 nanonets. The result supports our proposed TiSi2 nanonet lithiation mechanism. This discovery highlights the uniqueness of nanoscale materials and will likely broaden the spectrum of electrode material choices for electrochemical energy storage.Keywords: anode; electrochemical impedance spectroscopy; lithium ion battery; nanonets; titanium silicide

We present in this article our successes in synthesizing TiSi2 nanostructures with various complexities using a chemical vapor deposition (CVD) method. Attention has been paid to understanding the growth mechanism. The governing factor was found to be the surface energy differences between various crystal planes of orthorhombic TiSi2 (C54 and C49), because of their specific atomic arrangements of Si and Ti on the surfaces. This understanding has allowed us to control the growth morphologies and obtained one-dimensional (1D) nanowires, two-dimensional (2D) nanonets and three-dimensional (3D) complexes with rational designs by tuning the precursor chemical reactions. Careful studies of the atomically-resolved microstructures revealed the existence of distorted C54 phases in 3D complexes, which was attributed to playing the key role in the unique structure formation. These results are expected to shed light on metal silicide nanostructure growths broadly and to present opportunities for novel nanostructure syntheses.

Various silicon crystal structures with different atomic arrangements from that of diamond have been observed in chemically synthesized nanowires. The structures are typified by mixed stacking mismatches of closely packed Si dimers. Instead of viewing them as defects, we define the concept of hexagonality and describe these structures as Si polymorphs. The small transverse dimensions of a nanowire make this approach meaningful. Unique among the polymorphs are cubic symmetry diamond and hexagonal symmetry wurtzite structures. Electron diffraction studies conducted with Au as an internal reference unambiguously confirm the existence of the hexagonal symmetry Si nanowires.Cohesive energy calculations suggest that the wurtzite polymorph is the least stable and the diamond polymorph is the most stable. Cohesive energies of intermediate polymorphs follow a linear trend with respect to their structural hexagonality. We identify the driving force in the polymorph formations as the growth kinetics. Fast longitudinal elongation during the growth freezes stacking mismatches and thus leads to a variety of Si polymorphs. The results are expected to shed new light on the importance of growth kinetics in nanomaterial syntheses and may open up ways to produce structures that are uncommon in bulk materials.

This article reviews our recent progress on ultra-high density nanowires (NWs) array-based electronics. The superlattice nanowire pattern transfer (SNAP) method is utilized to produce aligned, ultra-high density Si NW arrays. We fi rst cover processing and materials issues related to achieving bulk-like conductivity characteristics from 10 20 nm wide Si NWs. We then discuss Si NW-based fi eld-effect transistors (FETs). These NWs & NW FETs provide terrifi c building blocks for various electronic circuits with applications to memory, energy conversion, fundamental physics, logic, and others. We focus our discussion on complementary symmetry NW logic circuitry, since that provides the most demanding metrics for guiding nanofabrication. Issues such as controlling the density and spatial distribution of both p-and n-type dopants within NW arrays are discussed, as are general methods for achieving Ohmic contacts to both p-and n-type NWs. These various materials and nanofabrication advances are brought together to demonstrate energy effi cient, complementary symmetry NW logic circuits.

Hematite (α-Fe2O3) was grown on vertically aligned Si nanowires (NWs) using atomic layer deposition to form a dual-absorber system. Si NWs absorb photons that are transparent to hematite (600 nm < λ < 1100 nm) and convert the energy into additional photovoltage to assist photoelectrochemical (PEC) water splitting by hematite. Compared with hematite-only photoelectrodes, those with Si NWs exhibited a photocurrent turn-on potential as low as 0.6 V vs RHE. This result represents one of the lowest turn-on potentials observed for hematite-based PEC water splitting systems. It addresses a critical challenge of using hematite for PEC water splitting, namely, the fact that the band-edge positions are too positive for high-efficiency water splitting.

Mg-doped hematite (α-Fe2O3) was synthesized by atomic layer deposition (ALD). The resulting material was identified as p-type with a hole concentration of ca. 1.7 × 1015 cm–3. When grown on n-type hematite, the p-type layer was found to create a built-in field that could be used to assist photoelectrochemical water splitting reactions. A nominal 200 mV turn-on voltage shift toward the cathodic direction was measured, which is comparable to what has been measured using water oxidation catalysts. This result suggests that it is possible to achieve desired energetics for solar water splitting directly on metal oxides through advanced material preparations. Similar approaches may be used to mitigate problems caused by energy mismatch between water redox potentials and the band edges of hematite and many other low-cost metal oxides, enabling practical solar water splitting as a means for solar energy storage.

The CO2-reduction activity of two Re(I)–NHC complexes is investigated employing a silicon nanowire photoelectrode to drive catalysis. Photovoltages greater than 440 mV are observed along with excellent selectivity towards CO over H2 formation. The observed selectivity towards CO production correlates with strong adsorption of the catalysts on the photoelectrode surface.

Dual redox mediators (RMs) were introduced for Mg–O2 batteries. 1,4-Benzoquinone (BQ) facilitates the discharge with an overpotential reduction of 0.3 V. 5,10,15,20-Tetraphenyl-21H,23H-porphine cobalt(II) (Co(II)TPP) facilitates the recharge with an overpotential decrease of up to 0.3 V. Importantly, the two redox mediators are compatible in the same DMSO-based electrolyte.

Compared with competing technologies, rechargeable lithium ion batteries offer relative advantages such as high capacity and long cycle lifetime. Remarkable advances in the development of this technology notwithstanding significant performance improvements are still required to meet society's ever-growing electrical energy storage need. In particular, we long for devices with greater capacity, a higher power rate and a longer cycle lifetime. Aimed at solving challenges associated with poor charge transport within electrode materials, we have recently tested a new, nanonet-based material platform. The nanonet, made of TiSi2 (C49), is similar to the more commonly used porous carbon in that it has high surface area and good electrical conductivity. The key uniqueness of the nanonet lies in that its morphology is well-defined, permitting us to design and test various heteronanostructures. In essence, the TiSi2 nanonet can serve as a charge collector and a mechanical support for the construction of electrodes for a wide range of applications. We show that when combined with Si, an anode with superior performance is obtained. Similarly, a high-performance cathode is enabled by the TiSi2–V2O5 combination.

Photoelectrochemical (PEC) water splitting holds the potential to meet the challenges associated with the intermittent nature of sunlight. Catalysts have often been shown to improve the performance of PEC water splitting, but their working mechanisms are not well understood. Using intensity modulated photocurrent spectroscopy (IMPS), we determined the rate constants of water oxidation and recombination at the surface of three different hematite-based photoanodes. It was found that the best performing electrodes, in terms of photocurrent onset potential, exhibited the slowest water oxidation rate constants, which was a surprise. The performance of these photoelectrodes was enabled by the slow surface recombination. When amorphous NiFeOx, a water oxidation catalyst, was present, the rate of surface hole transfer actually slowed down; what was slowed more was the recombination rate at the hematite surface, resulting in better water oxidation performance. As such, NiFeOx primarily serves as a passivation layer rather than a catalytic layer. Together a better understanding of the role of catalytic overlayers for water oxidation has been achieved.