A Conversation with Al Bard

A Conversation with Al Bar

Galvanic Exchange on Reduced Graphene Oxide: Designing a Multifunctional Two-Dimensional Catalyst Assembly

The two-dimensional network of reduced graphene oxide (RGO) is decorated with silver and gold nanoparticles. The silver nanoparticles deposited on RGO by photocatalytic reduction are subjected to galvanic exchange with Au³⁺ ions to transform them into gold nanoparticles. This compositional change on the RGO surface demonstrates RGO’s versatile ability to anchor a wide array of nanoparticles and facilitate chemical transformations. Coupled with RGO’s unique ability to capture and transport electrons, galvanic exchange is used to contrive a two-dimensional nanocatalyst mat. Raman studies show that metal nanoparticles anchored on reduced graphene oxide facilitate enhancement of Raman bands. Using methyl viologen as a probe we elucidate the photocatalytic activity of the semiconductor–RGO–metal nanoassembly and highlight the mediation of RGO in charge transfer processes

CdS Nanowire Solar Cells: Dual Role of Squaraine Dye as a Sensitizer and a Hole Transporter

The squaraine dye (SQ) anchored onto CdS nanowires serves as a photosensitizing dye and a hole acceptor. This dual role of the squaraine dye has been successfully exploited in a nanowire solar cell to improve the photoconversion efficiency. Electrophoretic deposition of CdS NWs and CdS NWs+SQ composite onto conducting glass electrodes was performed to obtain robust photoanodes and evaluate the photovoltaic performance of nanowire solar cells (NWSCs). Whereas the sensitization property of the SQ extends the response of CdS NWSCs into the near-IR (NIR) region, its redox property facilitates shuttling of holes to the electrolyte and suppressing the charge recombination process. Transient absorption measurements confirm the formation of cation radical of the dye arising from these two processes. The dual role of the squaraine dye has enabled us to improve the power conversion efficiency of NWSCs by a factor of ∼20. Photoelectrochemical, spectroelectrochemical, and spectroscopic measurements provide insight into the multifaceted role of squaraine dye in improving the performance of NWSCs

Fortification of CdSe Quantum Dots with Graphene Oxide. Excited State Interactions and Light Energy Conversion

Graphene based 2-D carbon nanostructures provide new opportunities to fortify semiconductor based light harvesting assemblies. Electron and energy transfer rates from photoexcited CdSe colloidal quantum dots (QDs) to graphene oxide (GO) and reduced graphene oxide (RGO) were isolated by analysis of excited state deactivation lifetimes as a function of degree of oxidation and charging in (R)­GO. Apparent rate constants for energy and electron transfer determined for CdSe–GO composites were 5.5 × 10⁸ and 6.7 × 10⁸ s^–1, respectively. Additionally, incorporation of GO in colloidal CdSe QD films deposited on conducting glass electrodes was found to enhance the charge separation and electron conduction through the QD film, thus allowing three-dimensional sensitization. Photoanodes assembled from CdSe–graphene composites in quantum dot sensitized solar cells display improved photocurrent response (∼150%) over those prepared without GO

A Conversation with Henry Snaith

A Conversation with Henry Snait

Indium-Rich AgInS₂–ZnS Quantum DotsAg-/Zn-Dependent Photophysics and Photovoltaics

AgInS₂–ZnS solid solution quantum dots (QDs) prepared with varying Ag/Zn ratios demonstrate composition-dependent photophysical properties. Absorption and emission processes are extremely complex in these compounds because of easily formed crystallographic defects which serve as intraband gap states and provide additional excitation and relaxation pathways. In addition to valence to conduction band absorption, defect states located within the band gap are responsible for tail absorption in these nanoparticles and are assigned to Ag_In antisite defects. These AgInS₂–ZnS QDs display wavelength-dependent photoluminescence (PL) decays along with large Stokes shifts and long PL lifetimes, strongly suggesting that donor–acceptor pair recombination is the dominant radiative pathway. The excited-state interaction between AgInS₂–ZnS and TiO₂ is studied through the use of transient absorption spectroscopy, and a fast photoinduced electron-transfer rate constant of 5 × 10¹¹ s^–1 is determined. This interaction with TiO₂ is further probed by testing various compositions of AgInS₂–ZnS in liquid-junction solar cells, with the optimum device power conversion efficiency reaching 1.83%

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

Size-Dependent Excited State Behavior of Glutathione-Capped Gold Clusters and Their Light-Harvesting Capacity

Glutathione-protected gold clusters exhibit size-dependent excited state and electron transfer properties. Larger-size clusters (e.g., Au₂₅GSH₁₈) with core-metal atoms display rapid (<1 ps) as well as slower relaxation (∼200 ns) while homoleptic clusters (e.g., Au_10–12GSH_10–12) exhibit only slower relaxation. These decay components have been identified as metal–metal transition and ligand-to-metal charge transfer, respectively. The short lifetime relaxation component becomes less dominant as the size of the gold cluster decreases. The long-lived excited state and ability to participate in electron transfer are integral for these clusters to serve as light-harvesting antennae. A strong correlation between the ligand-to-metal charge-transfer excited state lifetime and photocatalytic activity was evidenced from the electron transfer to methyl viologen. The photoactivity of these metal clusters shows increasing photocatalytic reduction yield (0.05–0.14) with decreasing cluster size, Au₂₅ < Au₁₈ < Au₁₅ < Au_10–12. Gold clusters, Au₁₈GSH₁₄, were found to have the highest potential as a photosensitizer on the basis of the quantum yield of electron transfer and good visible light absorption properties

Tandem-Layered Quantum Dot Solar Cells: Tuning the Photovoltaic Response with Luminescent Ternary Cadmium Chalcogenides

Photon management in solar cells is an important criterion as it enables the capture of incident visible and infrared photons in an efficient way. Highly luminescent CdSeS quantum dots (QDs) with a diameter of 4.5 nm were prepared with a gradient structure that allows tuning of absorption and emission bands over the entire visible region without varying the particle size. These crystalline ternary cadmium chalcogenides were deposited within a mesoscopic TiO₂ film by electrophoretic deposition with a sequentially-layered architecture. This approach enabled us to design tandem layers of CdSeS QDs of varying band gap within the photoactive anode of a QD solar cell (QDSC). An increase in power conversion efficiency of 1.97–2.81% with decreasing band gap was observed for single-layer CdSeS, thus indicating varying degrees of photon harvesting. In two- and three-layered tandem QDSCs, we observed maximum power conversion efficiencies of 3.2 and 3.0%, respectively. These efficiencies are greater than the values obtained for the three individually layered photoanodes. The synergy of using tandem layers of the ternary semiconductor CdSeS in QDSCs was systematically evaluated using transient spectroscopy and photoelectrochemistry

Trap and Transfer. Two-Step Hole Injection Across the Sb₂S₃/CuSCN Interface in Solid-State Solar Cells

In solid-state semiconductor-sensitized solar cells, commonly known as extremely thin absorber (ETA) or solid-state quantum-dot-sensitized solar cells (QDSCs), transfer of photogenerated holes from the absorber species to the p-type hole conductor plays a critical role in the charge separation process. Using Sb₂S₃ (absorber) and CuSCN (hole conductor), we have constructed ETA solar cells exhibiting a power conversion efficiency of 3.3%. The hole transfer from excited Sb₂S₃ into CuSCN, which limits the overall power conversion efficiency of these solar cells, is now independently studied using transient absorption spectroscopy. In the Sb₂S₃ absorber layer, photogenerated holes are rapidly localized on the sulfur atoms of the crystal lattice, forming a sulfide radical (S^–•) species. This trapped hole is transferred from the Sb₂S₃ absorber to the CuSCN hole conductor with an exponential time constant of 1680 ps. This process was monitored through the spectroscopic signal seen for the S^–• species in Sb₂S₃, providing direct evidence for the hole transfer dynamics in ETA solar cells. Elucidation of the hole transfer mechanism from Sb₂S₃ to CuSCN represents a significant step toward understanding charge separation in Sb₂S₃ solar cells and provides insight into the design of new architectures for higher efficiency devices