Soumik Sarkar

Photoselective excited state dynamics in ZnO-Au nanocomposites and their implications in photocatalysis and dye-sensitized solar cells.

Improving the performance of photoactive solid-state devices begins with systematic studies of the metal-semiconductor nanocomposites (NCs) upon which such devices are based. Here, we report the photo-dependent excitonic mechanism and the charge migration kinetics in a colloidal ZnO-Au NC system. By using a picosecond-resolved Förster resonance energy transfer (FRET) technique, we have demonstrated that excited ZnO nanoparticles (NPs) resonantly transfer visible optical radiation to the Au NPs, and the quenching of defect-mediated visible emission depends solely on the excitation level of the semiconductor. The role of the gold layer in promoting photolytic charge transfer, the activity of which is dependent upon the degree of excitation, was probed using methylene blue (MB) reduction at the semiconductor interface. Incident photon-to-current efficiency measurements show improved charge injection from a sensitizing dye to a semiconductor electrode in the presence of gold in the visible region. Furthermore, the short-circuit current density and the energy conversion efficiency of the ZnO-Au NP based dye-sensitized solar cell (DSSC) are much higher than those of a DSSC comprised of only ZnO NP. Our results represent a new paradigm for understanding the mechanism of defect-state passivation and photolytic activity of the metal component in metal-semiconductor nanocomposite systems.

Highly efficient ZnO/Au Schottky barrier dye-sensitized solar cells: Role of gold nanoparticles on the charge-transfer process

Summary Zinc oxide (ZnO) nanorods decorated with gold (Au) nanoparticles have been synthesized and used to fabricate dye-sensitized solar cells (DSSC). The picosecond-resolved, time-correlated single-photon-count (TCSPC) spectroscopy technique was used to explore the charge-transfer mechanism in the ZnO/Au-nanocomposite DSSC. Due to the formation of the Schottky barrier at the ZnO/Au interface and the higher optical absorptions of the ZnO/Au photoelectrodes arising from the surface plasmon absorption of the Au nanoparticles, enhanced power-conversion efficiency (PCE) of 6.49% for small-area (0.1 cm2) ZnO/Au-nanocomposite DSSC was achieved compared to the 5.34% efficiency of the bare ZnO nanorod DSSC. The TCSPC studies revealed similar dynamics for the charge transfer from dye molecules to ZnO both in the presence and absence of Au nanoparticles. A slower fluorescence decay associated with the electron recombination process, observed in the presence of Au nanoparticles, confirmed the blocking of the electron transfer from ZnO back to the dye or electrolyte by the Schottky barrier formed at the ZnO/Au interface. For large area DSSC (1 cm2), ~130% enhancement in PCE (from 0.50% to 1.16%) was achieved after incorporation of the Au nanoparticles into the ZnO nanorods.

Hematoporphyrin-ZnO nanohybrids : Twin applications in efficient visible-light photocatalysis and dye-sensitized solar cells

Light-harvesting nanohybrids (LHNs) are systems composed of an inorganic nanostructure associated with an organic pigment that have been exploited to improve the light-harvesting performance over individual components. The present study is focused on developing a potential LHN, attained by the functionalization of dense arrays of ZnO nanorods (NRs) with a biologically important organic pigment hematoporphyrin (HP), which is an integral part of red blood cells (hemoglobin). Application of spectroscopic techniques, namely, Fourier transform infrared spectroscopy (FTIR) and Raman scattering, confirm successful monodentate binding of HP carboxylic groups to Zn(2+) located at the surface of ZnO NRs. Picosecond-resolved fluorescence studies on the resulting HP-ZnO nanohybrid show efficient electron migration from photoexcited HP to the host ZnO NRs. This essential photoinduced event activates the LHN under sunlight, which ultimately leads to the realization of visible-light photocatalysis (VLP) of a model contaminant Methylene Blue (MB) in aqueous solution. A control experiment in an inert gas atmosphere clearly reveals that the photocatalytic activity is influenced by the formation of reactive oxygen species (ROS) in the media. Furthermore, the stable LHNs prepared by optimized dye loading have also been used as an active layer in dye-sensitized solar cells (DSSCs). We believe these promising LHNs to find their dual applications in organic electronics and for the treatment of contaminant wastewater.

Modulation of defect-mediated energy transfer from ZnO nanoparticles for the photocatalytic degradation of bilirubin

Summary In recent years, nanotechnology has gained significant interest for applications in the medical field. In this regard, a utilization of the ZnO nanoparticles for the efficient degradation of bilirubin (BR) through photocatalysis was explored. BR is a water insoluble byproduct of the heme catabolism that can cause jaundice when its excretion is impaired. The photocatalytic degradation of BR activated by ZnO nanoparticles through a non-radiative energy transfer pathway can be influenced by the surface defect-states (mainly the oxygen vacancies) of the catalyst nanoparticles. These were modulated by applying a simple annealing in an oxygen-rich atmosphere. The mechanism of the energy transfer process between the ZnO nanoparticles and the BR molecules adsorbed at the surface was studied by using steady-state and picosecond-resolved fluorescence spectroscopy. A correlation of photocatalytic degradation and time-correlated single photon counting studies revealed that the defect-engineered ZnO nanoparticles that were obtained through post-annealing treatments led to an efficient decomposition of BR molecules that was enabled by Förster resonance energy transfer.

Dynamics of light harvesting in ZnO nanoparticles

We have explored light harvesting of the complex of ZnO nanoparticles with the biological probe Oxazine 1 in the near-infrared region using picosecond-time-resolved fluorescence decay studies. We have used ZnO nanoparticles and Oxazine 1 as a model donor and acceptor, respectively, to explore the efficacy of the Förster resonance energy transfer (FRET) in the nanoparticle-dye system. It has been shown that FRET from the states localized near the surface and those in the bulk of the ZnO nanoparticles can be resolved by measuring the resonance efficiency for various wavelengths of the emission spectrum. It has been observed that the states located near the surface for the nanoparticles (contributing to visible emission at lambda approximately 550 nm) can contribute to very high efficiency (>90%) FRET. The efficiency of light harvesting dynamics of the ZnO nanorods has also been explored in this study and they were found to have much less efficiency (approximately 40%) for energy transfer compared to the nanoparticles. The possibility of an electron transfer reaction has been ruled out from the picosecond-resolved fluorescence decay of the acceptor dye at the ZnO surface.

Role of central metal ions in hematoporphyrin-functionalized titania in solar energy conversion dynamics

In this study, we have investigated the efficacy of electron transfer processes in hematoporphyrin (HP) and iron hematoporphyrin ((Fe)HP) sensitized titania as potential materials for capturing and storing solar energy. Steady-state and picosecond-resolved fluorescence studies show the efficient photoinduced electron transfer processes in hematoporphyrin-TiO2 (HP-TiO2) and Fe(III)-hematoporphyrin-TiO2 (Fe(III)HP-TiO2) nanohybrids, which reveal the role of central metal ions in electron transfer processes. The bidentate covalent attachment of HP onto TiO2 particulates is confirmed by FTIR, Raman scattering and X-ray photoelectron spectroscopy (XPS) studies. The iron oxidation states and the attachment of iron to porphyrin through pyrrole nitrogen atoms were investigated by cyclic voltammetry and FTIR studies, respectively. We also investigated the potential application of HP-TiO2 and Fe(III)HP-TiO2 nanohybrids for the photodegradation of a model organic pollutant methylene blue (MB) in aqueous solution under wavelength dependent light irradiation. To further investigate the role of iron oxidation states in electron transfer processes, photocurrent measurements were done by using Fe(III) and Fe(II) ions in porphyrin. This work demonstrates the role of central metal ions in fundamental electron transfer processes in porphyrin sensitized titania and their implications for dye-sensitized device performance.

UVA radiation induced ultrafast electron transfer from a food carcinogen benzo[a]pyrene to organic molecules, biological macromolecules, and inorganic nano structures

Reactions involving electron transfer (ET) and reactive oxygen species (ROS) play a pivotal role in carcinogenesis and cancer biochemistry. Our present study emphasizes UVA radiation induced ET reaction as one of the key aspects of a potential carcinogen, benzo[a]pyrene (BP), in the presence of a wide variety of molecules covering organic p-benzoquinone (BQ), biological macromolecules like calf-thymus DNA (CT-DNA), human serum albumin (HSA) protein, and inorganic zinc oxide (ZnO) nanorods (NRs). Steady-state and picosecond-resolved fluorescence spectroscopy have been used to monitor such ET reactions. Physical consequences of BP association with CT-DNA have been investigated through temperature-dependent circular dichroism (CD) spectroscopy. The temperature-dependent steady-state, picosecond-resolved fluorescence lifetime and anisotropy studies reveal the effect of temperature on the perturbation of such ET reactions from BP to biological macromolecules, highlighting their temperature-dependent association. Furthermore, the electron-donating property of BP has been corroborated by measuring wavelength-dependent photocurrent in a BP-anchored ZnO NR-based photodevice, offering new physical insights for the carcinogenic study of BP.

Ultrafast excited state deactivation of doped porous anodic alumina membranes

Free-standing, bi-directionally permeable and ultra-thin anodic aluminum oxide (AAO) membranes establish attractive templates (host) for the synthesis of nano-dots and rods of various materials (guest). This is due to their chemical and structural integrity and high periodicity on length scales of 5-150 nm which are often used to host photoactive nano-materials for various device applications including dye-sensitized solar cells. In the present study, AAO membranes are synthesized by using electrochemical methods and a detailed structural characterization using FEG-SEM, XRD and TGA confirms the porosity and purity of the material. Defect-mediated photoluminescence quenching of the porous AAO membrane in the presence of an electron accepting guest organic molecule (benzoquinone) is studied by means of steady-state and picosecond/femtosecond-resolved luminescence measurements. Using time-resolved luminescence transients, we have also revealed light harvesting of complexes of porous alumina impregnated with inorganic quantum dots (Maple Red) or gold nanowires. Both the Förster resonance energy transfer and the nano-surface energy transfer techniques are employed to examine the observed quenching behavior as a function of the characteristic donor-acceptor distances. The experimental results will find their relevance in light harvesting devices based on AAOs combined with other materials involving a decisive energy/charge transfer dynamics.

Engineering FRET-Based Solar Cells: Manipulation of Energy and Electron Transfer Processes in a Light Harvesting Assembly

From the prevalent interest in the advancement of renewable energy sources, dye-sensitized solar cells (DSSCs) have emerged as one of the front running prospects due mainly to a constructive balance between cost and efficiency. In this chapter, we will review our works on the utility of using Forster resonance energy transfer (FRET) in the light harvesting dynamics of zinc oxide (ZnO)-based nanomaterials, which has recently shown promise for significant improvement in various aspects of photoelectrochemical cells. Firstly, we have used ZnO nanoparticles (NPs) and Oxazine 1 as model donor and acceptor, respectively, to investigate the key ultrafast process of FRET in the NP–dye system. The consequence of the energy transfer on the performance of a model ZnO NP-based DSSC has also been explored by using well-known Ruthenium-based sensitizers N719 attached to ZnO NPs offering as an intrinsic co-sensitizer. By using a picosecond-resolved FRET technique, we have also demonstrated the role of the gold layer in promoting photoinduced charge transfer from ZnO–Au nanocomposite to a model contaminant methylene blue (MB). Due to the formation of the Schottky barrier at the ZnO–Au interface and the higher optical absorptions of the ZnO–Au photoelectrodes arising from the surface plasmon absorption of the Au NPs, enhanced power-conversion efficiency was achieved compared to bare ZnO-based DSSCs. Finally, potential co-sensitization of extrinsic sensitizer CdTe quantum dots (QDs) in ZnO nanorod (NR)-based DSSCs has been established where we have shown two major pathways by which CdTe QDs may contribute to the net photocurrent in a DSSC: (1) a direct injection of charge carriers from QDs to ZnO semiconductor via photoinduced electron transfer (PET) and (2) an indirect excitation of the sensitizing dye N719 molecules by funneling harvested light via FRET. Based on these advantages, the short-circuit current density and the photoconductivity of the QD-assembled DSSCs with distinct architectures are found to be much higher than DSSCs fabricated with N719 sensitizer only. As demonstrated, the multipath enhancement offered in this device architecture results in an increased and extended photo-response with respect to the individual materials employed. Further engineering of suitable donor acceptor pairs and optimization of charge separation in conjugated molecular blends has the potential to become a continuing avenue toward enhancing hybrid DSSC efficiencies.