78 research outputs found
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 Au<sup>3+</sup> 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)ÂGO. Apparent
rate constants for energy and electron transfer determined for CdSe–GO
composites were 5.5 × 10<sup>8</sup> and 6.7 × 10<sup>8</sup> s<sup>–1</sup>, 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
Indium-Rich AgInS<sub>2</sub>–ZnS Quantum DotsAg-/Zn-Dependent Photophysics and Photovoltaics
AgInS<sub>2</sub>–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<sub>In</sub> antisite defects. These AgInS<sub>2</sub>–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 AgInS<sub>2</sub>–ZnS
and TiO<sub>2</sub> is studied through the use of transient absorption
spectroscopy, and a fast photoinduced electron-transfer rate constant
of 5 × 10<sup>11</sup> s<sup>–1</sup> is determined. This
interaction with TiO<sub>2</sub> is further probed by testing various
compositions of AgInS<sub>2</sub>–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<sub><i>x</i></sub>-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><sup>0</sup> = −0.63 V vs RHE) and oxidation
(<i>E</i><sup>0</sup> = 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 TiO<sub>2</sub> film sensitized by Au<sub><i>x</i></sub>-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/TiO<sub>2</sub> nanoparticles
with Au<sub><i>x</i></sub>-GSH NCs in an aqueous slurry
system and irradiating with visible light produce H<sub>2</sub> at
a rate of 0.3 mmol of hydrogen/h/g of Au<sub><i>x</i></sub>-GSH NCs. The rate of H<sub>2</sub> 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.,
Au<sub>25</sub>GSH<sub>18</sub>) with core-metal atoms display rapid
(<1 ps) as well as slower relaxation (∼200 ns) while homoleptic
clusters (e.g., Au<sub>10–12</sub>GSH<sub>10–12</sub>) 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, Au<sub>25</sub> < Au<sub>18</sub> < Au<sub>15</sub> < Au<sub>10–12</sub>. Gold clusters,
Au<sub>18</sub>GSH<sub>14</sub>, 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 TiO<sub>2</sub> 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 Sb<sub>2</sub>S<sub>3</sub>/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 Sb<sub>2</sub>S<sub>3</sub> (absorber) and CuSCN (hole conductor), we have constructed ETA solar cells exhibiting a power conversion efficiency of 3.3%. The hole transfer from excited Sb<sub>2</sub>S<sub>3</sub> into CuSCN, which limits the overall power conversion efficiency of these solar cells, is now independently studied using transient absorption spectroscopy. In the Sb<sub>2</sub>S<sub>3</sub> absorber layer, photogenerated holes are rapidly localized on the sulfur atoms of the crystal lattice, forming a sulfide radical (S<sup>–•</sup>) species. This trapped hole is transferred from the Sb<sub>2</sub>S<sub>3</sub> 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<sup>–•</sup> species in Sb<sub>2</sub>S<sub>3</sub>, providing direct evidence for the hole transfer dynamics in ETA solar cells. Elucidation of the hole transfer mechanism from Sb<sub>2</sub>S<sub>3</sub> to CuSCN represents a significant step toward understanding charge separation in Sb<sub>2</sub>S<sub>3</sub> solar cells and provides insight into the design of new architectures for higher efficiency devices
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