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%.

Few-atom gold nanoclusters (NCs) exhibit moleculelike properties due to a discrete electronic structure driven by the quantum confinement effect. Unlike plasmonic Au particles, these nonplasmonic particles of diameter less than 2 nm, commonly referred to as nanoclusters, possess a distinct excited-state behavior that can offer a new opportunity to employ them as a photosensitizer. Their size-dependent excited-state behavior enables establishing logical designing principles to build up efficient light energy conversion systems. The photodynamics of thiolated Au NCs and efforts to exploit the Au NCs in light energy conversion applications discussed in this Review show new opportunities to utilize them as photosensitizers. Current bottlenecks in implementing thiolated Au NCs in light conversion applications and new strategies and future directions to address these limitations are also discussed.

It is well-known that platinum (Pt) is the best electrocatalyst currently available. However, its high cost and scarcity have hindered the commercialization of many green technologies that require the use of Pt electrocatalysts. To pave the way for mass production, the search for alternative electrocatalysts was initiated by researchers working in the fuel cell area, and it has been rapidly expanded to other energy applications in the past few years. Our discussion in this perspective starts with several reasons why Pt is the best electrocatalyst for many important electrochemical reactions such as the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER). Along with a brief introduction of other novel metal electrocatalysts (ruthenium and iridium compounds) used for the oxygen evolution reaction (OER), this perspective highlights recent advances in the development of non-Pt electrocatalysts for dye-/quantum-dotsensitized solar cells (DSSCs/QDSSCs) and water-splitting systems. We identify the key materials in each area that have shown promising results for replacing Pt as an electrocatalyst and discuss their pros and cons. The possible mechanisms responsible for the observed improvements in performance are also discussed. While many materials have shown encouraging electrocatalytic activity, the long-term stability under a variety of operating conditions remains as a critical issue that must be addressed. An improved theoretical understanding is also required to accelerate the progress in this area. The main challenge is to identify the active sites and operating mechanisms in order to intelligently design and synthesize better and more cost-effective electrocatalysts.

Investigating photoelectrode interfaces is challenging due to complex charge carrier pathways, and photodegradation aggravates this difficulty because interfacial properties are significantly altered by degradation. Unlike dyes and semiconductors that degrade into photoinactive materials, the photodegradation of Au nanoclusters (NCs) yields Au nanoparticles (NPs) that are photoactive. Besides, these NPs can form Schottky barriers with TiO2, which can affect interfacial band structures. Hence, the copresence of this photoactive nanoduo gives rise to unprecedented complexity in understanding the photoelectrochemical behavior of NC-sensitized photoelectrodes. In this work, we unveil that electron injection into TiO2 and subsequent electron trapping at deep surface trap states in TiO2, which are created by sensitization, play a vital role in the photodegradation. We also demonstrate that photocurrent can be enhanced through judicious control over photodegradation that would otherwise be deleterious. This photocurrent enhancement is attributed to multiple overlooked effects of Au NPs (plasmonic field enhancement and interfacial band bending).

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 NCsensitized 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.

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 lithiumion (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 (TiO 2 ). A systematic study on the effect of the carbon coating reveals that it is exclusively governed by the characteristics of the TiO 2 /carbon interface rather than by the nature of the carbon coating. Depending on their number, oxygen vacancies created by carbothermal reduction on the TiO 2 surface are detrimental to Li + diffusion in the TiO 2 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.

Optoelectronic properties of Au18(SR)14 are modulated by Ag doping, and its influence on photoelectrochemical performance is investigated. The best compromise for light conversion efficiency is made when a single Ag atom is incorporated.

Despite the potential of PbS quantum dots (QDs) as sensitizers for quantum-dot-sensitized solar cells (QDSSCs), achieving a high photocurrent density over 30 mA cm(-2) remains a challenging task in PbS-sensitized solar cells. In contrast to previous attempts, where Hg(2+)-doping or multi-step post-treatment is necessary, we are capable of achieving a high photocurrent exceeding 30 mA cm(-2) simply by manipulating the successive ionic layer adsorption and reaction (SILAR) method. We show that controlling temperature at which SILAR is performed is critical to obtain a higher and more uniform coverage of PbS QDs over a mesoporous TiO2 film. The deposition of a CdS inter-layer between TiO2 and PbS is found to be an effective means of ensuring high photocurrent and stability. Not only does this modification improve the light absorption capability of the photoanode, but it also has a significant effect on charge recombination and electron injection efficiency at the PbS/TiO2 interface according to our in-depth study using electrochemical impedance spectroscopy (EIS). The implication of subtle changes in the interfacial events via modified SILAR conditions for PbS-sensitized solar cells is discussed.