Showing papers by "Prashant V. Kamat published in 2011"
TL;DR: Graphene-based assemblies are gaining attention as a viable alternate to boost the efficiency of various catalytic and storage reactions in energy conversion applications as discussed by the authors, and the use of reduced graphene oxide has already proved useful in collecting and transporting charge in photoelectrochemical solar cells, photocatalysis, and electrocatalysis.
Abstract: Graphene-based assemblies are gaining attention as a viable alternate to boost the efficiency of various catalytic and storage reactions in energy conversion applications. The use of reduced graphene oxide has already proved useful in collecting and transporting charge in photoelectrochemical solar cells, photocatalysis, and electrocatalysis. In many of these applications, the flat carbon serves as a scaffold to anchor metal and semiconductor nanoparticles and assists in promoting selectivity and efficiency of the catalytic process. Covalent and noncovalent interaction with organic molecules is another area that is expected to provide new frontiers in graphene research. Recent advances in manipulating graphene-based two-dimensional carbon architecture for energy conversion are described.
602 citations
TL;DR: Interestingly, the films which exhibited the fastest electron transfer rates were not the same as those which showed the highest photocurrent, suggesting that, in addition to electron transfer at the quantum dot-metal oxide interface, other electron transfer reactions play key roles in the determination of overall device efficiency.
Abstract: Quantum dot-metal oxide junctions are an integral part of next-generation solar cells, light emitting diodes, and nanostructured electronic arrays. Here we present a comprehensive examination of electron transfer at these junctions, using a series of CdSe quantum dot donors (sizes 2.8, 3.3, 4.0, and 4.2 nm in diameter) and metal oxide nanoparticle acceptors (SnO2, TiO2, and ZnO). Apparent electron transfer rate constants showed strong dependence on change in system free energy, exhibiting a sharp rise at small driving forces followed by a modest rise further away from the characteristic reorganization energy. The observed trend mimics the predicted behavior of electron transfer from a single quantum state to a continuum of electron accepting states, such as those present in the conduction band of a metal oxide nanoparticle. In contrast with dye-sensitized metal oxide electron transfer studies, our systems did not exhibit unthermalized hot-electron injection due to relatively large ratios of electron cooling rate to electron transfer rate. To investigate the implications of these findings in photovoltaic cells, quantum dot-metal oxide working electrodes were constructed in an identical fashion to the films used for the electron transfer portion of the study. Interestingly, the films which exhibited the fastest electron transfer rates (SnO2) were not the same as those which showed the highest photocurrent (TiO2). These findings suggest that, in addition to electron transfer at the quantum dot-metal oxide interface, other electron transfer reactions play key roles in the determination of overall device efficiency.
601 citations
TL;DR: Reduced graphene oxide (RGO)-Cu2S composite has now succeeded in shuttling electrons through the RGO sheets and polysulfide-active Cu2S more efficiently than Pt electrode, improving the fill factor by ∼75%.
Abstract: Polysulfide electrolyte that is employed as a redox electrolyte in quantum dot sensitized solar cells provides stability to the cadmium chalcogenide photoanode but introduces significant redox limitations at the counter electrode through undesirable surface reactions. By designing reduced graphene oxide (RGO)-Cu2S composite, we have now succeeded in shuttling electrons through the RGO sheets and polysulfide-active Cu2S more efficiently than Pt electrode, improving the fill factor by ∼75%. The composite material characterized and optimized at different compositions indicates a Cu/RGO mass ratio of 4 provides the best electrochemical performance. A sandwich CdSe quantum dot sensitized solar cell constructed using the optimized RGO-Cu2S composite counter electrode exhibited an unsurpassed power conversion efficiency of 4.4%.
448 citations
TL;DR: UV irradiation of TiO(2) nanoparticles in the presence of Ag(+) ions results in the quantitative reduction and deposition of silver on its surface, which causes a blue shift in the surface plasmon peak from 430 to 415 nm.
Abstract: UV irradiation of TiO2 nanoparticles in the presence of Ag+ ions results in the quantitative reduction and deposition of silver on its surface. Continued UV irradiation following the deposition of Ag on the TiO2 surface causes a blue shift in the surface plasmon peak from 430 to 415 nm as these particles become charged with excess electrons. Under UV irradiation, both the charging and discharging of electrons occur at different rates, thus allowing the system to attain a steady state. Upon stopping the UV irradiation, a fraction of these electrons remain stored. The electron storage is dependent on the amount of Ag deposited on TiO2 nanoparticles with maximum capacity seen at 8.6 μM of Ag in a suspension containing 5.8 mM of TiO2. Such electron charging and discharging processes in semiconductor–metal composites need to be taken into account while evaluating the plasmon resonance induced effects in photocatalysis and photoelectrochemistry.
339 citations
TL;DR: In this article, the authors present the kinetics of hole transfer, characterization of the intermediates involved in the hole oxidation of sulfide ion, and the back electron transfer between sulfide radical and electrons injected into TiO2 nanoparticles.
Abstract: The presence of sulfide/polysulfide redox couple is crucial in achieving stability of metal chalcogenide (e.g., CdS and CdSe)-based quantum dot-sensitized solar cells (QDSC). However, the interfacial charge transfer processes play a pivotal role in dictating the net photoconversion efficiency. We present here kinetics of hole transfer, characterization of the intermediates involved in the hole oxidation of sulfide ion, and the back electron transfer between sulfide radical and electrons injected into TiO2 nanoparticles. The kinetic rate constant (107–109 s–1) for the hole transfer obtained from the emission lifetime measurements suggests slow hole scavenging from CdSe by S2– is one of the limiting factors in attaining high overall efficiency. The presence of the oxidized couple, by addition of S or Se to the electrolyte, increases the photocurrent, but it also enhances the rate of back electron transfer.
282 citations
TL;DR: In this paper, a kinetic adaption model was developed to elucidate both Langmuir-like submonolayer adsorption and QD aggregation processes on mesoscopic TiO2 films.
Abstract: Understanding CdSe quantum dot (QD) adsorption phenomena on mesoscopic TiO2 films is important for improving the performance of quantum dot sensitized solar cells (QDSSCs). A kinetic adsorption model has been developed to elucidate both Langmuir-like submonolayer adsorption and QD aggregation processes. Removal of surface-bound trioctylphosphine oxide as well as the use of 3-mercaptopropionic acid (MPA) as a molecular linker improved the adsorption of toluene-suspended QDs onto TiO2 films. The adsorption constant Kad for submonolayer coverage was (6.7 ± 2.7) × 103 M–1 for direct adsorption and (4.2 ± 2.0) × 104 M–1 for MPA-linked assemblies. Prolonged exposure of a TiO2 film to a CdSe QD suspension resulted in the assembly of aggregated particles regardless of the method of adsorption. A greater coverage of TiO2 was achieved with smaller QDs due to reduced size constraints. Ultrafast transient absorption spectroscopy demonstrated faster electron injection into TiO2 from directly adsorbed QDs (kET = 7.2 × ...
179 citations
TL;DR: Slower hole transfer and faster charge recombination and transport events were found to dominate over the forward electron injection process, thus limiting the deliverance of maximum power in CdSe QD-C(60)-based solar cells.
Abstract: The development of organic/inorganic hybrid nanocomposite systems that enable efficient solar energy conversion has been important for applications in solar cell research. Nanostructured carbon-based systems, in particular C(60), offer attractive strategies to collect and transport electrons generated in a light harvesting assembly. We have assembled CdSe-C(60) nanocomposites by chemically linking CdSe quantum dots (QDs) with thiol-functionalized C(60). The photoinduced charge separation and collection of electrons in CdSe QD-C(60) nanocomposites have been evaluated using transient absorption spectroscopy and photoelectrochemical measurements. The rate constant for electron transfer between excited CdSe QD and C(60) increased with the decreasing size of the CdSe QD (7.9 × 10(9) s(-1) (4.5 nm), 1.7 × 10(10) s(-1) (3.2 nm), and 9.0 × 10(10) s(-1) (2.6 nm)). Slower hole transfer and faster charge recombination and transport events were found to dominate over the forward electron injection process, thus limiting the deliverance of maximum power in CdSe QD-C(60)-based solar cells. The photoinduced charge separation between CdSe QDs and C(60) opens up new design strategies for developing light harvesting assemblies.
172 citations
TL;DR: A bottom-up strategy has been developed to construct a multiple electron transfer system composed of organic/inorganic ternary composites on a semiconducting electrode without impairing the respective donor-acceptor components.
Abstract: A bottom-up strategy has been developed to construct a multiple electron transfer system composed of organic/inorganic ternary composites (porphyrin, zinc oxide nanoparticles, reduced graphene oxide) on a semiconducting electrode without impairing the respective donor–acceptor components. The hierarchical electron transfer cascade system exhibited remarkably high photocurrent generation with an incident-photon-to-current efficiency of up to ca. 70%.
133 citations
TL;DR: Transient absorption and pulse radiolysis measurements provide further insight into the excited state interactions of squaraine dye with SiO, TiO, and TiO(2)/CdS/Al(2)O(3) films and interfacial electron transfer processes.
Abstract: The photoresponse of quantum dot solar cells (QDSCs) has been successfully extended to the near-IR (NIR) region by sensitizing nanostructured TiO2–CdS films with a squaraine dye (JK-216). CdS nanoparticles anchored on mesoscopic TiO2 films obtained by successive ionic layer adsorption and reaction (SILAR) exhibit limited absorption below 500 nm with a net power conversion efficiency of ∼1% when employed as a photoanode in QDSC. By depositing a thin barrier layer of Al2O3, the TiO2–CdS films were further modified with a NIR absorbing squaraine dye. Quantum dot sensitized solar cells supersensitized with a squariand dye (JK-216) showed good stability during illumination with standard global AM 1.5 solar conditions, delivering a maximum overall power conversion efficiency (η) of 3.14%. Transient absorption and pulse radiolysis measurements provide further insight into the excited state interactions of squaraine dye with SiO2, TiO2, and TiO2/CdS/Al2O3 films and interfacial electron transfer processes. The syn...
132 citations
TL;DR: In this article, the authors provided a mechanistic and kinetic insight into the catalytic role of IrO2 in the photogenerated hole transfer process in a colloidal TiO2 suspension in ethanol containing 5% acetic acid.
Abstract: Iridium oxide, a water oxidation cocatalyst, plays an important role in mediating the hole transfer process of a UV-irradiated TiO2 system. Spectroscopic identification of trapped holes has enabled their characterization in colloidal TiO2 suspension and monitoring of the transfer of trapped holes to IrO2. Titration of trapped holes with potassium iodide yields an estimate of three holes per particle during 7 min of UV irradiation of TiO2 suspension in ethanol containing 5% acetic acid. The hole transfer to IrO2 occurs with a rate constant of 6 × 105 s–1. Interestingly, IrO2 also catalyzes the recombination of trapped holes with reduced oxygen species. The results discussed here provide a mechanistic and kinetic insight into the catalytic role of IrO2 in the photogenerated hole transfer process.
102 citations
TL;DR: In this paper, tetra(4-aminophenyl) porphyrin (TAPP) with Ag nanoparticles is confirmed by ground-state absorption and Raman spectroscopy.
Abstract: Highly photoactive porphyrin is shown to form charge-transfer complex with silver nanoparticles. Complexation of tetra(4-aminophenyl) porphyrin (TAPP) with Ag nanoparticles is confirmed by ground-state absorption and Raman spectroscopy. Strong Raman enhancement indicates both electromagnetic and chemical enhancement. Evidence of chemical enhancement includes a selective enhancement of porphyrin Raman bands. Fast charge separation in the complex is indicated by ultrafast transient absorption and fluorescence upconversion measurements. The charge-separated state is shown to have a lifetime of 116 ± 6 ps. Porphyrin substituents are shown to play a role in the formation of charge-transfer complex.
TL;DR: In this paper, the electrons can be utilized to reduce Ag + ions and anchor silver nanoparticles on reduced graphene oxide (RGO), and the spectroscopic experiments that elucidate the quantitative electron transfer and transmission electron microscopy that highlights the potential of designing metal-RGO assemblies are discussed.
Abstract: Photochemically generated methyl viologen radicals undergo electron transfer with graphene oxide (GO) in ethanol suspensions. This charge transfer interaction results in the reduction of GO as well as storage of electrons. The stored electrons can be utilized to reduce Ag + ions and thus anchor silver nanoparticles on reduced graphene oxide (RGO). The spectroscopic experiments that elucidate the quantitative electron transfer and transmission electron microscopy that highlights the potential of designing metal–RGO assemblies are discussed.
TL;DR: In this article, the authors discuss representative papers published in the journals of Physical Chemistry A/B/C and Letters to provide a physical chemistry perspective on graphene-based nanostructures.
Abstract: C nanostructures have dominated advances in nanoscience and nanotechnology since the latter part of the 20th century. The 1996 Nobel Prize in Chemistry awarded jointly to Robert F. Curl, Jr., Sir Harold W. Kroto, and Richard E. Smalley “for their discovery of fullerenes” and the 2010 Nobel Prize in Physics awarded jointly to Andre Geim and Konstantin Novoselov “for groundbreaking experiments regarding the twodimensional material graphene” recognize the potential impact of fullerenes, carbon nanotubes, graphene, and other carbon nanostructures in future nanotechnology-based discoveries. Fullerenes, carbon nanotubes and nanofibers, and carbon quantum dots all consist of similar carbon atom networks, but exhibit significantly different properties that are dictated by size, shape, and chirality. Having established many of their electronic and optical properties during the last two decades, scientists switched attention to the parent structure, graphene. Physical chemistry has been at the forefront of disseminating a fundamental understanding of graphene and functionalized graphene structures. Several recent perspective articles highlight current progress and emerging issues related to graphene research. In this virtual issue, we discuss representative papers published in the Journals of Physical Chemistry A/B/C and Letters to provide a physical chemistry perspective on graphene-based nanostructures (Scheme 1). The electronic structure of graphene (flat carbon) exhibits properties that are different from its one-dimensional analog, viz., carbon nanotube. Graphene is considered to be a zero-bandgap semiconductor material. By application of an electric field or with suitable functionalization, the Fermi level can be altered to obtain n-doped or p-doped material. The early work of de Heer and coworkers highlighted electronic device applications based on nanopatterned epitaxial graphene. Ultrathin epitaxial graphite films grown by thermal decomposition on the (0001) surface of 6H-SiC exhibited remarkable two-dimensional electron gas behavior. This landmark JPC-B paper has garnered nearly 700 citations and is included in the citation of the 2010 Nobel Prize in Physics (http://nobelprize.org/nobel_prizes/physics/laureates/ 2010/sciback_phy_10_2.pdf).
01 Jan 2011
TL;DR: In this paper, the authors provide a description of the evolution of the solar cell explaining the theory and design behind three generations of photovoltaics and present an in-depth focus on quantum-dot-sensitized solar cells.
Abstract: This article provides a description of the evolution of the solar cell explaining the theory and design behind three generations of photovoltaics. An in-depth focus on quantum-dot-sensitized solar cells (a third-generation design) with emphasis on both theory and experimental findings is presented. The article concludes with a brief discussion of some future challenges facing quantum dot solar cells. Extensive referencing to relevant and recent literature is provided.