Our most recent papers...
461. How Does a SILAR CdSe Film Grow? Tuning the Deposition Steps to Suppress Interfacial Charge Recombination in Solar Cells
Becker, M. A.; Radich, J. G.; Bunker, B. A.; Kamat, P. V. J. Phys Chem. Lett. 2014, ASAP.
Successive Ionic Layer Adsorption and Reaction (SILAR) is a popular method of depositing the metal chalcogenide semiconductor layer on the mesoscopic metal oxide films for designing quantum dot sensitized solar cell (QDSSC) or Extremely Thin Absorber (ETA) solar cells. While this deposition method exhibits higher loading of light absorbing semiconductor layer than direct adsorption of pre-synthesized colloidal quantum dots, the chemical identity of these nanostructures and evolution of interfacial structure are poorly understood. We have now analyzed step-by-step SILAR deposition of CdSe films on mesoscopic TiO2 nanoparticle films using x-ray absorption near-edge structure analysis and probed the interfacial structure of these films. The film characteristics interestingly show dependence on the order in which the Cd and Se are deposited, and the CdSe-TiO2 interface is affected only during the first few cycles of deposition. Development of SeO2 passivation layer in the SILAR-prepared films to form TiO2/SeO2/CdSe junction facilitates an increase in photocurrents and power conversion efficiencies of quantum dot solar cells when these films were integrated as photoanodes in a photoelectrochemical solar cell.
460. Glutathione Capped Gold Nanoclusters as Photosensitizers. Visible Light Induced Hydrogen Generation in Neutral Water
Chen, Y.-S.; Kamat, P. V. J. Am. Chem. Soc. 2014, ASAP.
Glutathione capped metal nanoclusters (Aux-GSH NCs) which exhibit molecular like properties are employed as a photosen-sitizer for hydrogen generation in a photoelectrochemical cell (PEC) and a photocatalytic slurry reactor. The reversible re-duction (E0= -0.63 V vs. RHE) and oxidation (E0= 0.97 V and 1.51 V vs. RHE) potentials of these metal nanoclusters make them suita-ble for driving the water splitting reaction. When a mesoscopic TiO2 film sensitized by Aux-GSH NCs was used as the pho-toanode 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 Aux-GSH NCs in an aqueous slurry system and irradiating with visible light produced H2 at a rate of 0.3 mmole of hydrogen/hr/gram of Aux-GSH NCs. The rate of H2 evolution was significantly enhanced (~5 times) when a sacrificial donor, such as EDTA, was introduced into the sys-tem. 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.
459. CdSe-Graphene Oxide Light Harvesting Assembly. Size Dependent Electron Transfer and Light Energy Conversion Aspects
Krishnamurthy, S.; Kamat, P. V. ChemPhysChem 2014, ASAP.
Excited state interaction between CdSe quantum dots (QD) of different sizes (2.3, 3.2 and 4.2 nm diameter) and graphene oxide (GO) has been probed by depositing them as films on conducting glass electrodes. The emission of smaller CdSe QDs (2.3 nm) is quenched by GO three times faster than that of larger QDs (4.2 nm). Electrophoretic deposition method has allowed us to sequentially deposit single or multiple layers of different sized QDs and GO assemblies on conducting glass electrode and achieve the modulation of photoresponse in photoelectrochemical solar cells. Superior photoconversion efficiency through the incorporation of GO is attributed to the improved charge separation in the composite assembly.
458. Quantum Dot Solar Cells. Hole Transfer as a Limiting Factor in Boosting Photoconversion Efficiency
Kamat, P. V.; Christians, J. A.; Radich, J. G. Langmuir ASAP (Feature Article).
Semiconductor nanostructures are attractive for designing low cost solar cells with tunable photoresponse. The recent advances in size and shape selective synthesis have enabled the design of quantum dot solar cells with photoconversion efficiencies greater than 5%. In order to make them competitive with other existing thin film or polycrystalline photovoltaic technologies, it is important to overcome kinetic barriers for charge transfer at semiconductor interfaces. This feature focuses on the limitations imposed by slow hole transfer in improving solar cell performance and its role in the stability of metal chalcogenide solar cells. Strategies to improve the rate of hole transfer through surface modified redox relays offer new opportunities to overcome the hole transfer limitation. The mechanistic and kinetic aspects of hole transfer in Quantum Dot Solar Cells (QDSCs), Nanowire Solar Cells (NWSCs) and Extremely Thin Absorber (ETA) solar cells are discussed.
457. CdSeS Nanowires. Compositionally Controlled Band Gap and Exciton Dynamics
Kim J.-P.; Christians, J. A.; Choi, H.; Krishnamurthy, S.; Kamat, P. V. J. Phys. Chem. Lett. 5, 1103-1109.
CdS, CdSe and ternary CdSexS(1-x) are some of the most widely studied II-VI semiconductors due to their wide range of applications and promising performance in numerous systems. One-dimensional semiconductor nanowires offer the ability to conduct charges efficiently along the length of the wire which has potential charge transport benefits compared to nanoparticles. Herein, we report a simple, inexpensive synthetic procedure for high quality CdSeS nanowires where the composition can be easily modulated from pure CdSe to pure CdS by simply adjusting the Se:S precursor ratio. This allows for tuning of the absorption and emission properties of the nanowires across the visible spectrum. The CdSeS nanowires have a wurtzite crystal structure and grow along the  direction. As measured by femtosecond transient absorption spectroscopy, the short component of the excited state lifetime remains relatively constant at ~10 ps with increasing Se; however the contribution of this short lifetime component increased dramatically from 8.4% to 57.7% with increasing Se content. These CdSeS nanowires offer facile synthesis and widely adjustable optical properties, characteristics which give them broad potential applications in the fields of optoelectronics, and photovoltaics.