Dual Functionality of Surface States in Dictating Photocurrent Generation of Au Nanocluster-Sensitized TiO2 Electrodes.

Gold nanoclusters (Au NCs), composed of only a few atoms, exhibit molecule-like behavior due to their distinct electronic structures arising from quantum confinement effects. Unlike their plasmonic nanoparticle counterparts, these nonplasmonic Au NCs possess unique properties with significant potential for photosensitizer applications. While traditional and NC-based electrodes share architectural similarities, the photoelectrochemical (PEC) behavior of the latter diverges significantly. Sensitizing TiO2 with Au NCs introduces additional surface trap states. In contrast to conventional photosensitizers, where surface states typically have a negligible impact on hole transfer, these trap states actively mediate the charge transfer process in Au NC-sensitized TiO2 electrodes. In this study, we employed impedance spectroscopy to elucidate the role of surface trap states in photocurrent generation. Our investigation revealed that these states are critical in determining PEC performance, presenting a dichotomy: they facilitate charge transfer (enhancing PEC performance) while simultaneously promoting carrier recombination (limiting efficiency). We demonstrated that the judicious control of otherwise deleterious surface trap states can significantly boost photocurrent. Our findings highlight that the dual nature of surface trap states demands a comprehensive investigation to fully understand their intricate impact on PEC performance.

Surface Pinning of Mn by Oxidation State Control for the Synthesis of Cobalt-Free, Ni-Rich, Core/Shell Structured Cathode Materials.

Motivated by the increasing cost, environmental concerns, and limited availability of Co, researchers are actively seeking alternative cathode materials for lithium-ion batteries. A promising strategy involves structure-modified materials, such as a NiMn core/shell system. This design leverages the high energy density of a Ni-rich core while employing an Mn-rich shell to enhance interfacial stability by suppressing unwanted reactions with the electrolyte. This approach offers improved cycling stability and reduced reliance on Co. However, the interdiffusion of Mn ions between the core and shell remains a significant challenge during synthesis. This work presents a facile approach to address the issue of Mn interdiffusion in core/shell cathode materials. The study demonstrates that partial oxidation of the precursor during the drying stage effectively enhances the Mn oxidation state. This strategy successfully suppresses Mn interdiffusion during subsequent calcination, leading to the preservation of the core/shell architecture in the final cathode material. This optimized structure mitigates interfacial reactions, enhances chemomechanical properties, and reduces crosstalk, a major contributor to rollover failure. This work presents a novel approach for synthesizing high-performance core/shell cathode materials for next-generation lithium-ion batteries.

Amorphization-Driven Lithium Ion Storage Mechanism Change for Anomalous Capacity Enhancement.

Capacity fading as a function of lithiation/delithiation cycles is a major limitation of Li-ion batteries. Most Li storage materials are susceptible to this phenomenon due to the degradation of the crystal structure and particle integrity as a result of volume changes associated with lithiation/delithiation processes and/or irreversible redox reactions. However, some Li storage materials show an increase in capacity with an increase in cycles; this phenomenon has been termed "negative fading." Negative fading in Li host materials is usually associated with the additional charge storage at the particle/solid-electrolyte interface (SEI) layer, decomposition/formation of the SEI layer, or redox reactions of various Li species at the interface. In this work, we report the observation of negative fading in a newly discovered anode material, TiNbO4 (TNO), and reveal amorphization as a new mechanism for negative fading in Li host materials. This assertion was confirmed via a close relationship between changes in the crystal structure and the Li storage mechanism in TNO. Given that other titanium niobium oxide analogues (e.g., TiNb2O7) suffer from capacity loss due to amorphization, this unique electrochemical behavior of TNO may provide an interesting new direction to tune the titanium niobium oxides for high-performance, stable battery anodes.

Overview of the oxygen vacancy effect in bimetallic spinel and perovskite oxide electrode materials for high-performance supercapacitors.

Bimetallic spinel and perovskite metal oxide materials are advanced electrode materials for supercapacitor (SC) applications because of their low-cost, distinct crystal structures, eco-friendly nature, and high conductivity. However, they suffer from the disadvantages of poor ion-diffusion kinetics and pulverization issues during cyclability tests. Along with a deeper understanding of redox chemistry, the role of oxygen vacancies (OVs) in electrode materials to support the reaction kinetics for excellence in SCs must be clarified. In this review, we highlight for the first time the importance of OVs and summarize various design strategies for the preparation of advanced bimetallic spinel oxides and perovskites with improved electrochemical performances for SC applications. With new insights, we envision that the SC research community would endeavor to utilize the benefits of OVs effectively for the development of high-performance SCs.

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.

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.

Nanostructuring Matters: Stabilization of Electrocatalytic Oxygen Evolution Reaction Activity of ZnCo2O4 by Zinc Leaching.

Despite the enormous attention paid to cobalt oxide materials as efficient water splitting electrocatalysts, a deep understanding of their activity discrepancy is still elusive. In this work, we showed that stabilization of the internally generated oxygen evolution reaction (OER) active phase (oxyhydroxide) is crucial for ZnCo2O4 electrocatalysts. A systematic evaluation of the bulk and nanostructured ZnCo2O4 system concomitant with nanostructured Co3O4 showed that leaching of Zn is the driving force behind the near-surface transformation to the oxyhydroxide phase. The relative contribution to this near-surface reconstruction was found to be surface-sensitive. The electrochemical observations combined with Raman and impedance spectroscopy revealed that the good catalytic activity could be attributed to the formation of the cobalt oxyhydroxide phase, which was created by the dissolution of Zn from the nanostructured surface. Moreover, this study sheds light on previous contradicting postulates regarding the discrepancy of the OER activity of ZnCo2O4. Our finding regarding the formation of the OER active phase in spinel Zn-Co oxide will motivate researchers to focus more on the near-surface reconstruction behavior of cobalt-based oxide electrocatalysts in the future.

Exploring the Effect of Cation Vacancies in TiO2: Lithiation Behavior of n-Type and p-Type TiO2.

TiO2 offers several advantages over graphite as an anode material for Li-ion batteries (LIBs) but suffers from low electrical conductivity and Li-diffusion issues. Control over defect chemistry has proven to be an effective strategy to overcome these issues. However, defect engineering has primarily been focused on oxygen vacancies (VO). The role of another intrinsic TiO2 vacancy [i.e., titanium vacancies (VTi)] with regard to the Li+ storage behavior of TiO2 has largely evaded attention. Hence, a comparison of VO- and VTi-defective TiO2 can provide valuable insight into how these two types of defects affect Li+ storage behavior. To eliminate other factors that may also affect the Li+ storage behavior of TiO2, we carefully devised synthesis protocols to prepare TiO2 with either VO (n-TiO2) or VTi (p-TiO2). Both TiO2 materials were verified to have a very similar morphology, surface area, and crystal structure. Although VO provided additional sites that improved the capacity at low C-rates, the benefit obtained from over-lithiation turned out to be detrimental to cycling stability. Unlike VO, VTi could not serve as an additional lithium reservoir but could significantly improve the rate performance of TiO2. More importantly, the presence of VTi prevented over-lithiation, significantly improving the cycling stability of TiO2. We believe that these new insights could help guide the development of high-performance TiO2 for LIB applications.

Interfacial Engineering at Quantum Dot-Sensitized TiO2 Photoelectrodes for Ultrahigh Photocurrent Generation.

Metal oxide semiconductor/chalcogenide quantum dot (QD) heterostructured photoanodes show photocurrent densities >30 mA/cm2 with ZnO, approaching the theoretical limits in photovoltaic (PV) cells. However, comparative performance has not been achieved with TiO2. Here, we applied a TiO2(B) surface passivation layer (SPL) on TiO2/QD (PbS and CdS) and achieved a photocurrent density of 34.59 mA/cm2 under AM 1.5G illumination for PV cells, the highest recorded to date. The SPL improves electron conductivity by increasing the density of surface states, facilitating multiple trapping/detrapping transport, and increasing the coordination number of TiO2 nanoparticles. This, along with impeded electron recombination, led to enhanced collection efficiency, which is a major factor for performance. Furthermore, SPL-treated TiO2/QD photoanodes were successfully exploited in photoelectrochemical water splitting cells, showing an excellent photocurrent density of 14.43 mA/cm2 at 0.82 V versus the Reversible Hydrogen Electrode (RHE). These results suggest a new promising strategy for the development of high-performance photoelectrochemical devices.

Modulation of the photoelectrochemical behavior of Au nanocluster-TiO2 electrode by doping.

Despite the successful debut of gold nanoclusters (Au NCs) in solar cell applications, Au NCs, compared to dyes and quantum dots, have several drawbacks, such as lower extinction coefficients. Any modulation of the physical properties of NCs can have a significant influence on the delicate control of absorbance, energy levels, and charge separation, which are essential to ensure high power conversion efficiency. To this end, we systematically alter the optoelectronic structure of Au18(SR)14 by Ag doping and explain its influence on solar cell performance. Our in-depth spectroscopic and electrochemical characterization combined with computational study reveals that the performance-dictating factors respond in different manners to the Ag doping level, and we determine that the best compromise is the incorporation of a single Ag atom into an Au NC. This new insight highlights the unique aspect of NCs-susceptibility to atomic level doping-and helps establish a new design principle for efficient NC-based solar cells.

Lithiation Mechanism Change Driven by Thermally Induced Grain Fining and Its Impact on the Performance of LiMn2 O4 in Lithium-Ion Batteries.

The nature of precursors employed in the synthesis of lithium-ion battery cathode materials is a crucial performance-dictating factor. Therefore, it is of great importance to establish a way to manipulate the precursor and seek a comprehensive understanding of its influence on the electrochemical behavior of a targeted electrode material. A thermal route is herein demonstrated for the synthesis of lithium-excess LiMn2 O4 (LMO) by exploiting an intriguing thermal phenomenon, thermally induced grain fining, and sheds light on how it affects the mechanism and kinetics of lithiation, and, furthermore, the electrochemical behavior of LMO. Detailed insights into the lithiation mechanism and kinetics reveal that the use of a finely grained, porous Mn3 O4 , which possesses an open crystal structure, is a key to the success of incorporating excess Li. In addition, this in-depth electrochemical investigation verifies a very recent theoretical prediction of faster Li diffusion kinetics enabled by excess Li.

Role of Regeneration of Nanoclusters in Dictating the Power Conversion Efficiency of Metal-Nanocluster-Sensitized Solar Cells.

Metal nanoclusters (NCs) have emerged as feasible alternatives to dyes and quantum dots in light energy conversion applications. Despite the remarkable enhancement in power conversion efficiency (PCE) in recent years and the increase in the number of NCs available as sensitizers, a comprehensive understanding of the various interfacial charge-transfer, transport, and recombination events in NCs is still lacking. This understanding is vital to the establishment of design principles for an efficient photoelectrode that uses NCs. In this work, we carefully design a comparison study of two representative NCs, Au and Ag, based on transient absorption spectroscopy and electrochemical impedance spectroscopy, methods that shed light on the true benefits and limitations of NC sensitizers. Low NC regeneration efficiency is the most critical factor that limits the performance of metal-nanocluster-sensitized solar cells (MCSSCs). The slow regeneration that results from sluggish hole transfer kinetics not only limits photocurrent generation efficiency but also has a profound effect on the stability of MCSSCs. This finding calls for urgent attention to the development of an efficient redox couple that has a great hole-extraction ability and no corrosive nature. This work also reveals different interfacial behaviors of Au and Ag NCs in photoelectrodes, suggesting that utilizing the benefits of both types of NCs simultaneously by cosensitization or using AuAg alloy NCs may be one avenue for further PCE improvement in MCSSCs.

Ag(I)-Thiolate-Protected Silver Nanoclusters for Solar Cells: Electrochemical and Spectroscopic Look into the Photoelectrode/Electrolyte Interface.

Intrinsic low stability and short excited lifetimes associated with Ag nanoclusters (NCs) are major hurdles that have prevented the full utilization of the many advantages of Ag NCs over their longtime contender, Au NCs, in light energy conversion systems. In this report, we diagnosed the problems of conventional thiolated Ag NCs used for solar cell applications and developed a new synthesis route to form aggregation-induced emission (AIE)-type Ag NCs that can significantly overcome these limitations. A series of Ag(0)/Ag(I)-thiolate core/shell-structured NCs with different core sizes were explored for photoelectrodes, and the nature of the two important interfacial events occurring in Ag NC-sensitized solar cells (photoinduced electron transfer and charge recombination) were unveiled by in-depth spectroscopic and electrochemical analyses. This work reveals that the subtle interplay between the light absorbing capability, charge separation dynamics, and charge recombination kinetics in the photoelectrode dictates the solar cell performance. In addition, we demonstrate significant improvement in the photocurrent stability and light conversion efficiency that have not been achieved previously. Our comprehensive understanding of the critical parameters that limit the light conversion efficiency lays a foundation on which new principles for designing Ag NCs for efficient light energy conversion can be built.

Exploring the Capacitive Behavior of Carbon Functionalized with Cyclic Ethers: A Rational Strategy To Exploit Oxygen Functional Groups for Enhanced Capacitive Performance.

The presence of oxygen functional groups (OFGs) on a carbon surface is a double-edged sword in electric double-layer capacitors (EDLCs) because of their mixed influences on capacitance. Critical problems of common OFGs are greatly decreased electrical conductivity, steric hindrance limiting the migration of ions, and promoted self-discharge via faradaic reactions. To explore a new breakthrough to these long-standing problems, carbon electrodes selectively functionalized with cyclic ether groups (CEGs) are investigated with in-depth spectroscopic and electrochemical analyses. The in-plane CEGs embedded in the graphene matrix are greatly advantageous over conventional out-of-plane OFGs for EDLC performance because they can boost capacitance via pseudocapacitance while substantially minimizing all of the negative effects of traditional OFGs. This study also reveals that preserving the original sp2 carbon network during surface functionalization is crucial to maximizing the benefits of OFGs. These new insights call for the development of elaborate surface engineering strategies that can introduce functionalities with no significant damage to π-conjugation.

Designing Hierarchical Assembly of Carbon-Coated TiO2 Nanocrystals and Unraveling the Role of TiO2/Carbon Interface in Lithium-Ion Storage in TiO2.

Despite the many benefits of hierarchical nanostructures of oxide-based electrode materials for lithium-ion batteries, it remains a challenging task to fully exploit the advantages of such materials partly because of their intrinsically poor electrical conductivities. The resulting limited electron supply to primary particles inside secondary microparticles gives rise to significant variation in the lithium-ion (Li+) storage capability within the nanostructured particles. To address this, facile annealing, where in situ generated carbon-coated primary particles were assembled into porous microagglomerates, is demonstrated to prepare nanostructured titanium dioxide (TiO2). A systematic study on the effect of the carbon coating reveals that it is exclusively governed by the characteristics of the TiO2/carbon interface rather than by the nature of the carbon coating. Depending on their number, oxygen vacancies created by carbothermal reduction on the TiO2 surface are detrimental to Li+ diffusion in the TiO2 lattice, and structural distortion at the interface profoundly influences the Li+ (de)intercalation mechanism. This new insight serves as a stepping stone toward understanding an important yet often overlooked effect of the oxide/carbon interface on Li+ storage kinetics, thereby demanding more investigations to establish a new design principle for carbon-coated oxide electrode materials.

A Generalizable Top-Down Nanostructuring Method of Bulk Oxides: Sequential Oxygen-Nitrogen Exchange Reaction.

A thermal reaction route that induces grain fracture instead of grain growth is devised and developed as a top-down approach to prepare nanostructured oxides from bulk solids. This novel synthesis approach, referred to as the sequential oxygen-nitrogen exchange (SONE) reaction, exploits the reversible anion exchange between oxygen and nitrogen in oxides that is driven by a simple two-step thermal treatment in ammonia and air. Internal stress developed by significant structural rearrangement via the formation of (oxy)nitride and the creation of oxygen vacancies and their subsequent combination into nanopores transforms bulk solid oxides into nanostructured oxides. The SONE reaction can be applicable to most transition metal oxides, and when utilized in a lithium-ion battery, the produced nanostructured materials are superior to their bulk counterparts and even comparable to those produced by conventional bottom-up approaches. Given its simplicity and scalability, this synthesis method could open a new avenue to the development of high-performance nanostructured electrode materials that can meet the industrial demand of cost-effectiveness for mass production.

The influence of surface area, porous structure, and surface state on the supercapacitor performance of titanium oxynitride: implications for a nanostructuring strategy.

A recent surge of interest in metal (oxy)nitride materials for energy storage devices has given rise to the rapid development of various nanostructuring strategies for these materials. In supercapacitor applications, early transition metal (oxy)nitrides have been extensively explored, among which titanium oxynitride stands out due to its great potential for charge storage. Despite recent advances in supercapacitors based on titanium oxynitride, many underlying factors governing their capacitive performance remain elusive. In this work, nanostructured titanium oxynitride is prepared by firing an organic-inorganic hybrid precursor under a hot ammonia atmosphere, and the influence of its physical characteristics on the supercapacitor performance is investigated. New insights into the effects of surface area, porous structure, and surface state of titanium oxynitride on the supercapacitor performance are revealed through which a comprehensive understanding about the capacitive behavior of titanium oxynitride is provided. In addition, the implications of these insights for a nanostructuring strategy striving for higher capacitance and improved stability are discussed.

Control of morphology and defect density in zinc oxide for improved dye-sensitized solar cells.

While zinc oxide (ZnO) with a mesoporous network has long been explored for adsorption of dyes and as an electron-transporting medium in dye-sensitized solar cells (DSSCs), the performance of ZnO-based DSSCs remains unsatisfactory. Despite the importance of understanding the surface characteristics of ZnO in DSSC applications, most of the studies relevant to ZnO-based DSSCs are focused on the synthesis of unique nanostructures of ZnO. In this study, we not only introduce a novel disk-shaped ZnO nanostructure, but also provide insight into the distinctive surface properties of ZnO and its influence on DSSC performance. When utilized in DSSCs, the mesoporous ZnO nanodisk yields 60% better power conversion efficiency (PCE) compared to commercial ZnO nanoparticles, which is attributed to less surface and bulk trap densities as concluded by an in-depth open-circuit voltage decay (OCVD) analysis and electrochemical impedance spectroscopy (EIS). Another aspect that contributes to the higher PCE is the better connectivity of primary particles that join together to form secondary disk-shaped particles. Furthermore, a 40% improvement in the PCE was observed by coating the mesoporous ZnO nanodisk with TiO2, which results from the passivation of the surface defects that aid in suppressing the interfacial charge recombination.

Exploring Interfacial Events in Gold-Nanocluster-Sensitized Solar Cells: Insights into the Effects of the Cluster Size and Electrolyte on Solar Cell Performance.

Gold nanoclusters (Au NCs) with molecule-like behavior have emerged as a new light harvester in various energy conversion systems. Despite several important strides made recently, efforts toward the utilization of NCs as a light harvester have been primarily restricted to proving their potency and feasibility. In solar cell applications, ground-breaking research with a power conversion efficiency (PCE) of more than 2% has recently been reported. Because of the lack of complete characterization of metal cluster-sensitized solar cells (MCSSCs), however, comprehensive understanding of the interfacial events and limiting factors which dictate their performance remains elusive. In this regard, we provide deep insight into MCSSCs for the first time by performing in-depth electrochemical impedance spectroscopy (EIS) analysis combined with physical characterization and density functional theory (DFT) calculations of Au NCs. In particular, we focused on the effect of the size of the Au NCs and electrolytes on the performance of MCSSCs and reveal that they are significantly influential on important solar cell characteristics such as the light absorption capability, charge injection kinetics, interfacial charge recombination, and charge transport. Besides offering comprehensive insights, this work represents an important stepping stone toward the development of MCSSCs by accomplishing a new PCE record of 3.8%.

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.