Glutathione-Capped Gold Nanoclusters as Photosensitizers. Visible Light-Induced Hydrogen Generation in Neutral Water

Glutathione-capped metal nanoclusters (Au_<i>x</i>-GSH NCs) which exhibit molecular-like properties are employed as a photosensitizer for hydrogen generation in a photoelectrochemical cell (PEC) and a photocatalytic slurry reactor. The reversible reduction (<i>E</i>⁰ = −0.63 V vs RHE) and oxidation (<i>E</i>⁰ = 0.97 and 1.51 V vs RHE) potentials of these metal nanoclusters make them suitable for driving the water-splitting reaction. When a mesoscopic TiO₂ film sensitized by Au_<i>x</i>-GSH NCs is used as the photoanode with a Pt counter electrode in aqueous buffer solution (pH = 7), we observe significant photocurrent activity under visible light (400–500 nm) excitation. Additionally, sensitizing Pt/TiO₂ nanoparticles with Au_<i>x</i>-GSH NCs in an aqueous slurry system and irradiating with visible light produce H₂ at a rate of 0.3 mmol of hydrogen/h/g of Au_<i>x</i>-GSH NCs. The rate of H₂ evolution is significantly enhanced (∼5 times) when a sacrificial donor, such as EDTA, is introduced into the system. Using metal nanoclusters as a photosensitizer for hydrogen generation lays the foundation for the future exploration of other metal nanoclusters with well-controlled numbers of metal atoms and capping ligands

Excited-State Behavior of Luminescent Glutathione-Protected Gold Clusters

The excited-state behavior of luminescent gold clusters provides new insights in understanding their photocatalytic activity in the visible region. The excited state of glutathione-protected gold nanoclusters (AuGSH), which is characterized by the long-lived excited state (τ = 780 ns), arises from the ligand-to-metal type transition. These AuGSH clusters are in a partially oxidized state (Au­(I)) and are readily reduced by chemical or electrochemical methods. Interestingly, a metal core transition with short-lived lifetime (τ < 3 ps) appears along with a longer lifetime in reduced AuGSH clusters. The role of the oxidation state of gold clusters in dictating the photocatalytic reduction of methyl viologen is discussed

Metal-Cluster-Sensitized Solar Cells. A New Class of Thiolated Gold Sensitizers Delivering Efficiency Greater Than 2%

A new class of metal-cluster sensitizers has been explored for designing high-efficiency solar cells. Thiol-protected gold clusters which exhibit molecular-like properties have been found to inject electrons into TiO₂ nanostructures under visible excitation. Mesoscopic TiO₂ films modified with gold clusters deliver stable photocurrent of 3.96 mA/cm² with power conversion efficiencies of 2.3% under AM 1.5 illumination. The overall absorption features and cell performance of metal-cluster-sensitized solar cells (MCSCs) are comparable to those of CdS quantum-dot-based solar cells (QDSCs). The relatively high open-circuit voltage of 832 mV and fill factor of 0.7 for MCSCs as compared to QDSCs show the viability of these new sensitizers as alternatives to semiconductor QDs and sensitizing dyes in the next generation of solar cells. The superior performance of MCSCs discussed in this maiden study lays the foundation to explore other metal clusters with broader visible absorption

All Solution-Processed Lead Halide Perovskite-BiVO₄ Tandem Assembly for Photolytic Solar Fuels Production

The quest for economic, large-scale hydrogen production has motivated the search for new materials and device designs capable of splitting water using only energy from the sun. Here we introduce an all solution-processed tandem water splitting assembly composed of a BiVO₄ photoanode and a single-junction CH₃NH₃PbI₃ hybrid perovskite solar cell. This unique configuration allows efficient solar photon management, with the metal oxide photoanode selectively harvesting high energy visible photons, and the underlying perovskite solar cell capturing lower energy visible-near IR wavelengths in a single-pass excitation. Operating without external bias under standard AM 1.5G illumination, the photoanode–photovoltaic architecture, in conjunction with an earth-abundant cobalt phosphate catalyst, exhibits a solar-to-hydrogen conversion efficiency of 2.5% at neutral pH. The design of low-cost tandem water splitting assemblies employing single-junction hybrid perovskite materials establishes a potentially promising new frontier for solar water splitting research

Probing Interfacial Electrochemistry on a Co₃O₄ Water Oxidation Catalyst Using Lab-Based Ambient Pressure X‑ray Photoelectron Spectroscopy

The design and mechanistic understanding of efficient and low-cost catalysts for the oxygen evolution reaction (OER) are currently the focus of electrochemical water-splitting technology. Herein, we report the chemical transformations on the water-vapor/solid interface and catalytic performance of an OER catalyst consisting of Co₃O₄ nanoparticles on multiwalled carbon nanotubes (Co₃O₄–MWCNT). Using a specially constructed electrochemical cell incorporated to the lab-based ambient-pressure X-ray photoelectron spectroscopy (APXPS) to mimic <i>operando</i> conditions, we obtained experimental evidence for the formation of CoO­(OH) as the catalytically active phase on a Co₃O₄–MWCNT OER catalyst. Under water and applied potential conditions, CoO­(OH) is formed, enriching the surface of Co₃O₄ nanoparticles with subnanometer thickness, and oxidizing H₂O into O₂. However, immediately after the removal of the applied potential, the CoO­(OH) phase is converted back to Co₃O₄. This back-conversion from CoO­(OH) to Co₃O₄ is likely driven by locally concentrated protons (H⁺) in water vapor, which shows the necessity of an electrochemical bias to preserve the catalytically active phase. These results reveal the surface chemical identities of the Co₃O₄–MWCNT OER catalyst, which are in agreement with those obtained from in-situ APXPS studies of liquid/solid interfaces consisting of Co₃O₄ catalyst and disagree with those obtained from ex-situ ultrahigh vacuum (UHV) XPS. Thus, our results demonstrate the possibility of performing surface chemical analysis in simplified electrochemical systems and further reinforce the importance of performing mechanistic studies of electrochemical devices under in-situ conditions

Boosting the Photovoltage of Dye-Sensitized Solar Cells with Thiolated Gold Nanoclusters

Glutathione-capped gold nanoclusters (Au_<i>x</i>-GSH NCs) are anchored along with a sensitizing squaraine dye on a TiO₂ surface to evaluate the cosensitizing role of Au_<i>x</i>-GSH NCs in dye-sensitized solar cells (DSSCs). Photoelectrochemical measurements show an increase in the photoconversion efficiency of DSSCs when both sensitizers are present. The observed photoelectrochemical improvements in cosensitized DSSCs are more than additive effects as evident from the increase in photovoltage (Δ<i>V</i> as high as 0.24 V) when Au_<i>x</i>-GSH NCs are present. Electron equilibration and accumulation within gold nanoclusters increase the quasi-Fermi level of TiO₂ closer to the conduction band and thus decrease the photovoltage penalty. A similar beneficial role of gold nanoclusters toward boosting the <i>V</i>_oc and enhancing the efficiency of Ru­(II) polypyridyl complex-sensitized solar cells is also discussed

Ultrasmall α‑Fe₂O₃ Superparamagnetic Nanoparticles with High Magnetization Prepared by Template-Assisted Combustion Process

A template-assisted combustion-based method is developed to synthesize the ultrasmall (below 5 nm) α-Fe₂O₃ nanoparticles. The iron and ammonium nitrate are used as oxidizers, glycine as a “fuel” and mesoporous silica (SBA-15) as a template. Because of the ultralow sizes and high crystallinity, the combustion-derived α-Fe₂O₃ nanoparticles exhibit superparamagnetism in the temperature range of 70–300 K. The high specific surface area (132 m²/g) of α-Fe₂O₃ indicates the important role of surface magnetic spins resulting in remarkably high magnetization (21 emu/g) at 300 K