Carbon-Doped TiNb₂O₇ Suppresses Amorphization-Induced Capacity Fading

The limited capacity of graphite anodes in high-performance batteries has led to considerable interest in alternative materials in recent years. Due to its high capacity, titanium niobium oxide (TiNb2O7, TNO) with a Wadsley–Roth crystallographic sheared structure holds great promise as a next-generation anode material, but a comprehensive understanding of TNO’s electrochemical behavior is lacking. In particular, the mechanism responsible for the capacity fading of TNO remains poorly elucidated. Given its metastable nature (as an entropy-stabilized oxide) and the large volume change in TNO upon lithiation and delithiation, which has long been overlooked, the factors governing capacity fading warrant investigation. Our studies reveal that the structural weakness of TNO is fatal to the long-term cycling stability of TNO and that the capacity fading of TNO is driven by amorphization, which results in a significant increase in impedance. While nanostructuring can kinetically boost lithium intercalation, this benefit comes at the expense of capacity fading. Carbon doping in TNO can effectively suppress the critical impedance increase despite the amorphization, providing a possible remedy to the stability issue

Surface State-Assisted Delayed Photocurrent Response of Au Nanocluster/TiO₂ Photoelectrodes

Gold nanoclusters (NCs) can be used as sensitizers to extend the absorption capabilities of TiO2 as photoelectrodes. However, the adsorption of NCs also creates additional surface states on the TiO2 surface, which gives rise to intricacies in the understanding of various interfacial phenomena occurring in NC-sensitized TiO2. One of the complexities that have recently been discovered is the size-dependent hole-transfer mechanism. In this work, we reveal another anomalous behavior in the hole-transfer process that the hole scavenging ability of the electrolyte also plays a role in determining the hole-transfer mechanism in the NC-TiO2 system, which is unprecedented in other photoelectrode systems. In the presence of an efficient hole scavenger (Na2SO3), the hole transfer in Au18–TiO2 occurs directly through the highest occupied molecular orbital (HOMO) of Au18 NCs. However, in the presence of a less efficient hole scavenger (ethylenediaminetetraacetic acid), hole transfer in Au18–TiO2 does not occur through the HOMO and shifts to surface state-assisted hole transfer. Due to surface state charging, this surface state-assisted hole-transfer mechanism results in delayed photocurrent response in Au18–TiO2. Evidence for this exotic hole-transfer mechanism shift is provided by photoelectrochemical electrochemical impedance spectroscopy, and its implications are discussed

Sonochemical Synthesis of Nanosized Hollow Hematite

Nanosized hollow iron oxide was synthesized using high-intensity ultrasound with carbon nanoparticles as a template. The hollow iron oxide was characterized by TEM, STEM, EELS, XRD, Raman, Mössbauer, and SQUID analyses. The TEM examination, EDS, and EELS analyses demonstrated the formation of iron oxide with nanosized hollow cores. XRD, Raman, and Mössbauer studies revealed that the hollow iron oxide is the most thermodynamically stable iron oxide, hematite (α-Fe2O3). Magnetization measurements showed that the hollow nanospheres of hematite are weakly ferromagnetic. Upon annealing, the hollow hematite becomes more crystalline but still maintains the hollow structure. Without the use of the carbon template, only agglomerated amorphous iron oxide was obtained

CdSe Quantum Dot–Fullerene Hybrid Nanocomposite for Solar Energy Conversion: Electron Transfer and Photoelectrochemistry

The development of organic/inorganic hybrid nanocomposite systems that enable efficient solar energy conversion has been important for applications in solar cell research. Nanostructured carbon-based systems, in particular C60, offer attractive strategies to collect and transport electrons generated in a light harvesting assembly. We have assembled CdSe–C60 nanocomposites by chemically linking CdSe quantum dots (QDs) with thiol-functionalized C60. The photoinduced charge separation and collection of electrons in CdSe QD–C60 nanocomposites have been evaluated using transient absorption spectroscopy and photoelectrochemical measurements. The rate constant for electron transfer between excited CdSe QD and C60 increased with the decreasing size of the CdSe QD (7.9 × 109 s–1 (4.5 nm), 1.7 × 1010 s–1 (3.2 nm), and 9.0 × 1010 s–1 (2.6 nm)). Slower hole transfer and faster charge recombination and transport events were found to dominate over the forward electron injection process, thus limiting the deliverance of maximum power in CdSe QD–C60-based solar cells. The photoinduced charge separation between CdSe QDs and C60 opens up new design strategies for developing light harvesting assemblies

Nitrogen-Doped Carbon Nanocoil Array Integrated on Carbon Nanofiber Paper for Supercapacitor Electrodes

Integrating a nanostructured carbon array on a conductive substrate remains a challenging task that presently relies primarily on high-vacuum deposition technology. To overcome the problems associated with current vacuum techniques, we demonstrate the formation of an N-doped carbon array by pyrolysis of a polymer array that was electrochemically grown on carbon fiber paper. The resulting carbon array was investigated for use as a supercapacitor electrode. In-depth surface characterization results revealed that the microtextural properties, surface functionalities, and degree of nitrogen incorporated into the N-doped carbon array can be delicately controlled by manipulating carbonization temperatures. Furthermore, electrochemical measurements showed that subtle changes in these physical properties resulted in significant changes in the capacitive behavior of the N-doped carbon array. Pore structures and nitrogen/oxygen functional groups, which are favorable for charge storage, were formed at low carbonization temperatures. This result showed the importance of having a comprehensive understanding of how the surface characteristics of carbon affect its capacitive performance. When utilized as a substrate in a pseudocapacitive electrode material, the N-doped carbon array maximizes capacitive performance by simultaneously achieving high gravimetric and areal capacitances due to its large surface area and high electrical conductivity

Memory Effect in Lithium Titanate Driven by Interfacial Oxygen Vacancies

Memory effect is undesirable abnormal voltage behavior in batteries that appears as a result of certain usage history. Al-doped Li4Ti5O12 (LTO), LiFePO4, and TiO2 that undergo phase transformations during lithiation/delithiation are known to possess a memory effect. Memory effect in these materials typically arises because of delayed onset of potential overshoot at the beginning of the delithiation voltage profile. In general, memory effect is associated with the sluggish electrochemical kinetics of phase transformation. Herein, we reveal a new source of memory effect in undoped LTO, which originates from an unstable LTO/electrolyte interface that can be produced by a distorted and defect-ridden particle surface. Electrochemical impedance spectroscopy shows that deep lithiation leads to variable resistance toward charge transfer, which produces a memory effect in the proceeding cycles. This work indicates the importance of controlling the particle/electrolyte interface to prevent an unprecedented memory effect in LTO

Solid-State Lithiation Reaction: What Is the Actual Lithiation Temperature?

Solid-State Lithiation Reaction: What Is the Actual Lithiation Temperature

Improved Photovoltaic Performance of Si Nanowire Solar Cells Integrated with ZnSe Quantum Dots

Introducing a ZnSe quantum dot (QD) layer over silicon nanowire (Si NW) solar cells considerably enhances external quantum efficiency (EQE) over broadband wavelengths. This is attributed to the combination of two major benefits of ZnSe QDs: superior light trapping and photon down-conversion. The integration of ZnSe QDs on the Si NW solar cell significantly reduces Fresnel reflection at the silicon/air interface because the refractive index of ZnSe QDs falls between those of Si and air. As a result, the refractive index mismatch at the interface can be alleviated. This decreases the Si NW length required for obtaining superior light absorption over 90%, which consequently leads to a substantial reduction in surface recombination loss. A remarkable enhancement of ∼30% in EQE around the absorption maximum of ZnSe QDs reveals that photon down-conversion by ZnSe QDs significantly contributed to EQE enhancement in a short-wavelength region. Our Si NW/ZnSe QDs hybrid solar cell showed nearly 13% improvement in power conversion efficiency compared to that of a bare Si NW counterpart, highlighting the feasibility of thin-layered semiconductor nanoparticles as a booster for highly efficient Si solar cells

Quantum Dots from Chemical Aerosol Flow Synthesis: Preparation, Characterization, and Cellular Imaging

CdTeSe and CdTeS ternary quantum dots (QDs) that fluoresce in the red to near-IR regions have been prepared using chemical aerosol flow synthesis (CAFS). Changing the composition ratio of Te to Se permits bandgap tuning of CdTeSe QDs, and replacing selenium with sulfur in the precursor solution increased the quantum efficiency of CdTe QDs up to ∼37%, because of better surface passivation of the CdS outer shell that grows on top of the CdTe core. ICP-MS and XPS analyses showed the internal structure of these ternary QDs had a gradient in the Se/Te concentration ratio with a CdTe-rich core. A simple phase transfer reaction rendered CdTeSe QDs water-soluble, and the water-soluble CdTeSe QDs were evaluated as a fluorescence labeling agent for intracellular imaging applications. A blue shift in the photoluminescence of CdTeSe QDs was observed over time both in living cells and in solutions under air exposed to light, which is attributed to photo-oxidation of the outer shell