Hematite/Si Nanowire Dual-Absorber System for Photoelectrochemical Water Splitting at Low Applied Potentials

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

Layered Titanium Disilicide Stabilized by Oxide Coating for Highly Reversible Lithium Insertion and Extraction

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

Electrochemically Switchable Ring-Opening Polymerization of Lactide and Cyclohexene Oxide

An electrochemical method was developed for the redox switchable polymerization of lactide and cyclohexene oxide. Using a lithium reversible sacrificial electrode and a high surface area carbon working electrode, efficient transformation between formally iron­(II) and iron­(III) oxidation states of a bis­(imino)­pyridine iron alkoxide complex was possible, which led to the ability to activate the complex for ring opening polymerization reactions. In addition to serving as a redox trigger, an electrochemical toggle switch was developed in which the chemoselectivity for lactide and epoxide polymerization was altered <i>in situ</i>. These findings led to the synthesis of poly­(lactic acid-<i>b</i>-cyclohexene oxide) block copolymers in which the sequence of monomers incorporated is controlled by the electrical potential applied

Understanding Photocharging Effects on Bismuth Vanadate

Bismuth vanadate (BiVO₄) is a promising material for photoelectrochemical water oxidation. Recently, it has been shown that “photocharging” BiVO₄ results in an improved water oxidation performance. However, the understanding of how BiVO₄ is being improved has been lacking. Here we study the surface kinetics of BiVO₄ using intensity-modulated photocurrent spectroscopy and show that photocharging BiVO₄ results in both surface and bulk improvements. This result sheds light on how the surface charge transfer and bulk charge transport of BiVO₄ respond to illumination

Ionic-Diffusion-Driven, Low-Temperature, Solid-State Reactions Observed on Copper Sulfide Nanowires

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

A Nanonet-Enabled Li Ion Battery Cathode Material with High Power Rate, High Capacity, and Long Cycle Lifetime

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

Functionalizing Titanium Disilicide Nanonets with Cobalt Oxide and Palladium for Stable Li Oxygen Battery Operations

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

Site-Selective Deposition of Twinned Platinum Nanoparticles on TiSi₂ Nanonets by Atomic Layer Deposition and Their Oxygen Reduction Activities

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

Energetics at the Surface of Photoelectrodes and Its Influence on the Photoelectrochemical Properties

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

Investigation of Photoexcited Carrier Dynamics in Hematite and the Effect of Surface Modifications by an Advanced Transient Grating Technique

Photoexcited carrier dynamics in a hematite film with and without amorphous NiFeO<i>_x</i> on the surface was investigated using the heterodyne transient grating method. We found that two different electron/hole dynamics took place in the micro- and millisecond time regions and successfully assigned each component to the decay processes of electrons and holes trapped at surface states, respectively. It was also demonstrated that the amorphous NiFeO<i>_x</i> coating plays a crucial role in increasing the survival of the holes at the surface trap states, which was caused by the decrease in the surface recombination rate