Showing papers by "Prashant V. Kamat published in 2012"

Mn-Doped Quantum Dot Sensitized Solar Cells: A Strategy to Boost Efficiency over 5%

Abstract: To make Quantum Dot Sensitized Solar Cells (QDSC) competitive, it is necessary to achieve power conversion efficiencies comparable to other emerging solar cell technologies. By employing Mn2+ doping of CdS, we have now succeeded in significantly improving QDSC performance. QDSC constructed with Mn-doped-CdS/CdSe deposited on mesoscopic TiO2 film as photoanode, Cu2S/Graphene Oxide composite electrode, and sulfide/polysulfide electrolyte deliver power conversion efficiency of 5.4%.

Manipulation of Charge Transfer Across Semiconductor Interface. A Criterion That Cannot Be Ignored in Photocatalyst Design

Abstract: The Perspective focuses on photoinduced electron transfer between semiconductor–metal and semiconductor–semiconductor nanostructures and factors that influence the rate of electron transfer at the interface. The storage and discharge properties of metal nanoparticles play an important role in dictating the photocatalytic performance of semiconductor–metal composite assemblies. Both electron and hole transfer across the interface with comparable rates are important in maintaining high photocatalytic efficiency and stability of the semiconductor assemblies. Coupled semiconductors of well-matched band energies are convenient to improve charge separation. Furthermore, semiconductor and metal nanoparticles assembled on reduced graphene oxide sheets offer new ways to design multifunctional catalyst mat. The fundamental understanding of charge-transfer processes is important in the future design of light-harvesting assemblies.

Boosting the efficiency of quantum dot sensitized solar cells through modulation of interfacial charge transfer.

Abstract: The demand for clean energy will require the design of nanostructure-based light-harvesting assemblies for the conversion of solar energy into chemical energy (solar fuels) and electrical energy (s...

Know thy nano neighbor. Plasmonic versus electron charging effects of metal nanoparticles in dye-sensitized solar cells.

Abstract: Neighboring metal nanoparticles influence photovoltaic and photocatalytic behavior of semiconductor nanostructures either through Fermi level equilibration by accepting electrons or inducing localized surface plasmon effects. By employing SiO2- and TiO2-capped Au nanoparticles we have identified the mechanism with which the performance of dye-sensitized solar cells (DSSC) is influenced by the neighboring metal nanoparticles. The efficiency of an N719 dye-sensitized solar cell (9.3%) increased to 10.2% upon incorporation of 0.7% Au@SiO2 and to 9.8% upon loading of 0.7% Au@TiO2 nanoparticles. The plasmonic effect as monitored by introducing Au@SiO2 in DSSC produces higher photocurrent. However, Au nanoparticles undergo charge equilibration with TiO2 nanoparticles and shift the apparent Fermi level of the composite to more negative potentials. As a result, Au@TiO2 nanoparticle-embedded DSSC exhibit higher photovoltage. A better understanding of these two effects is crucial in exploiting the beneficial aspect...

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

Abstract: 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.

Photoinduced Surface Oxidation and Its Effect on the Exciton Dynamics of CdSe Quantum Dots

Abstract: With increased interest in semiconductor nanoparticles for use in quantum dot solar cells there comes a need to understand the long-term photostability of such materials. Colloidal CdSe quantum dots (QDs) were suspended in toluene and stored in combinations of light/dark and N2/O2 to simulate four possible benchtop storage environments. CdSe QDs stored in a dark, oxygen-free environment were observed to better retain their optical properties over the course of 90 days. The excited state lifetimes, determined through femtosecond transient absorption spectroscopy, of air-equilibrated samples exposed to light exhibit a decrease in average lifetime (0.81 ns) when compared to samples stored in a nitrogen/dark environment (8.3 ns). A photoetching technique commonly used for controlled reduction of QD size was found to induce energetic trap states to CdSe QDs and accelerate the rate of electron–hole recombination. X-ray absorption near edge structure (XANES) analysis confirms surface oxidation, the extent of whi...

Sun-Believable Solar Paint. A Transformative One-Step Approach for Designing Nanocrystalline Solar Cells

Abstract: A transformative approach is required to meet the demand of economically viable solar cell technology. By making use of recent advances in semiconductor nanocrystal research, we have now developed a one-coat solar paint for designing quantum dot solar cells. A binder-free paste consisting of CdS, CdSe, and TiO2 semiconductor nanoparticles was prepared and applied to conducting glass surface and annealed at 473 K. The photoconversion behavior of these semiconductor film electrodes was evaluated in a photoelectrochemical cell consisting of graphene–Cu2S counter electrode and sulfide/polysulfide redox couple. Open-circuit voltage as high as 600 mV and short circuit current of 3.1 mA/cm2 were obtained with CdS/TiO2–CdSe/TiO2 electrodes. A power conversion efficiency exceeding 1% has been obtained for solar cells constructed using the simple conventional paint brush approach under ambient conditions. Whereas further improvements are necessary to develop strategies for large area, all solid state devices, this ...

Synchronized energy and electron transfer processes in covalently linked CdSe-squaraine dye-TiO2 light harvesting assembly.

Abstract: Manipulation of energy and electron transfer processes in a light harvesting assembly is an important criterion to mimic natural photosynthesis. We have now succeeded in sequentially assembling CdSe quantum dot (QD) and squaraine dye (SQSH) on TiO2 film and couple energy and electron transfer processes to generate photocurrent in a hybrid solar cell. When attached separately, both CdSe QDs and SQSH inject electrons into TiO2 under visible–near-IR irradiation. However, CdSe QD if linked to TiO2 with SQSH linker participates in an energy transfer process. The hybrid solar cells prepared with squaraine dye as a linker between CdSe QD and TiO2 exhibited power conversion efficiency of 3.65% and good stability during illumination with global AM 1.5 solar condition. Transient absorption spectroscopy measurements provided further insight into the energy transfer between excited CdSe QD and SQSH (rate constant of 6.7 × 1010 s–1) and interfacial electron transfer between excited SQSH and TiO2 (rate constant of 1.2 ...

Realizing Visible Photoactivity of Metal Nanoparticles: Excited-State Behavior and Electron-Transfer Properties of Silver (Ag8) Clusters.

Abstract: Silver nanoclusters complexed with dihydrolipoic acid (DHLA) exhibit molecular-like excited-state properties with well-defined absorption and emission features. The 1.8 nm diameter Ag nanoparticles capped with Ag8 clusters exhibit fluorescence maximum at 660 nm with a quantum yield of 0.07%. Although the excited state is relatively short-lived (τ 130 ps), it exhibits significant photochemical reactivity. By introducing MV(2+) as a probe, we have succeeded in elucidating the interfacial electron transfer dynamics of Ag nanoclusters. The formation of MV(+•) as the electron-transfer product with a rate constant of 2.74 × 10(10) s(-1) confirms the ability of these metal clusters to participate in the photocatalytic reduction process. Basic understanding of excited-state processes in fluorescent metal clusters paves the way toward the development of biological probes, sensors, and catalysts in energy conversion devices.

Electron Hopping Through Single-to-Few-Layer Graphene Oxide Films. Side-Selective Photocatalytic Deposition of Metal Nanoparticles

Abstract: Single- to few-layer graphene oxide (GO) sheets have been successfully anchored onto TiO2 films using electrophoretic deposition. Upon UV illumination of TiO2–GO films, photogenerated electrons from TiO2 are captured by GO. These electrons are initially used in GO’s reduction, while additional electron transfer results in storage across its sp2 network. In the presence of silver ions, deposition of silver nanoparticles (NPs) is accomplished on the GO surface opposite the TiO2, thus confirming the ability of GO to transport electrons through its plane. Illumination-controlled reduction of silver ions allows for simple selection of particle size and loading, making these semiconductor–graphene–metal (SGM) films ideal for custom catalysis and sensor applications. Initial testing of SGM films as surface-enhanced resonance Raman (SERRS) sensors produced significant target molecule signal enhancements, enabling detection of nanomolar concentrations.

Origin of Reduced Graphene Oxide Enhancements in Electrochemical Energy Storage

Abstract: Reduced graphene oxide (RGO) has become a common substrate upon which active intercalation materials are anchored for electrochemical applications such as supercapacitors and lithium ion batteries. The unique attributes of RGO, including high conductivity and porous macrostructure, are often credited for enhanced cycling and capacity performance. Here we focus on probing the electrochemical response of α-MnO2/RGO composite used as an electrode in a lithium ion battery cell and elucidating the mechanistic aspects of the RGO on the commonly observed improvements in cycling and capacity. We find that electron storage properties of RGO enables better electrode kinetics, more rapid diffusion of Li+ to intercalation sites, and a greater capacitance effect during discharge. Further investigation of the length of the one-dimensional nanowire morphology of the α-MnO2 has allowed us to differentiate between the innate characteristics of the MnO2 and those of the RGO. RGO coupled with long nanowires (>5 μm) exhibite...

Getting your submission right and avoiding rejection

Abstract: Less than a decade ago, manuscripts were manually submitted to publishers. The cover letter, copies of the manuscript, and figures were placed in a large envelope and sent to the editorial office via snail mail. Authors submitting papers understood that failing to follow submission guidelines could result in the loss of precious time (back then, a fewweeks to amonth!) if the editorial office had to return the manuscript for any reason. As a result, authors tended to carefully adhere to journal Author Guidelines and faithfully follow them. Today, submission of a manuscript to any scientific journal takes only a few clicks on a computer. A manuscript can be submitted from anywhere the author’s office or home, a cafe,́ or even from an airline flight at 32 000 feet. The ease of online submissions, however, has contributed to several common mistakes, which can still cost time for the authors as the editorial staff work to correct mistakes. The most common mistake is incomplete or incorrect entry of author/coauthor information. A few simple checks prior to submission can help avoid such errors and resultant manuscript processing delays.