Presence Of Electron Acceptors Research Articles

Multiexciton Annihilation and Dissociation in Quantum Confined Semiconductor Nanocrystals

Recent reports of multiexciton generation (MEG), a process by which one absorbed photon generates multiple excitons, in lead chalcogenide nanocrystals (NCs) have intensified research interest in using this phenomenon to improve the efficiency of solar energy conversion. Practical implementation of MEG processes in solar cells and solar-to-fuel conversion devices requires the development of materials with higher MEG efficiencies and lower excitation thresholds than are currently available, as well as schemes for efficient multiexciton extraction before the ultrafast exciton-exciton annihilation occurs. This Account focuses on the extraction of multiexcitons by interfacial electron transfer in model NC-molecular acceptor complexes. We provide an overview of multiexciton annihilation and multiexciton dissociation (MED) processes in NC-acceptor complexes of (i) CdSe quantum dots (QDs), (ii) CdSe/CdS quasi-type II core/shell QDs, (iii) CdSe quantum confined nanorods (QRs), and (iv) PbS QDs. We show that ultrafast electron transfer to adsorbed molecular acceptors can efficiently dissociate multiexcitons generated by absorption of multiple photons in (i), (ii), and (iii). Compared to core-only CdSe QDs, the electron hole distributions in CdSe/CdS quasi-type II QDs and CdSe QRs significantly improve their MED efficiencies by simultaneously retarding Auger recombination and facilitating interfacial electron transfer. Finally, in PbS-methylene blue (MB(+)) complexes, we show that the presence of electron acceptors does not affect the MEG efficiency and electron transfer to MB(+) efficiently dissociates the multiple excitons generated in PbS QDs. Our findings demonstrate that ultrafast interfacial charge transfer can be an efficient approach for extracting multiexcitons, and wavefunction engineering in quantum confined NCs can further improve MED efficiency.

Bioremediation

Bioremediation

Multiple Exciton Generation and Dissociation in PbS Quantum Dot-Electron Acceptor Complexes

Multiple exciton generation (MEG) in quantum dots (QDs), a process by which one absorbed photon generates multiple electron-hole pairs, has provided exciting possibilities for improving the energy conversion efficiency of photovoltaic and photocatalytic devices. However, implementing MEG in practical devices requires the extraction of multiple charge carriers before exciton-exciton annihilation and the development of materials with improved MEG efficiency. In this report, using PbS QD/methylene blue complexes as a QD/electron acceptor model system, we demonstrate that the presence of electron acceptors does not affect the MEG efficiency of QDs and all generated excitons can be dissociated by electron transfer to the acceptor, achieving MEG and multiple exciton dissociation efficiencies of 112%. We further demonstrate that these efficiencies are not affected by the charging of QDs.

TiO2-assisted photodegradation of pharmaceuticals — a review

Abstract Pharmaceutical compounds have been detected in the environment and potentially arise from the discharge of excreted and improperly disposed medication from sewage treatment facilities. In order to minimize environmental exposure of pharmaceutical residues, a potential technique to remove pharmaceuticals from water is the use of an advanced oxidation process (AOP) involving titanium dioxide (TiO2) photocatalysis. To evaluate the extent UV/TiO2 processes have been studied for pharmaceutical degradation, a literature search using the keywords ‘titanium dioxide’, ‘photocatalysis’, ‘advanced oxidation processes’, ‘pharmaceuticals’ and ‘degradation’ were used in the ISI Web of Knowledge TM, Scopus TM and ScienceDirect TM databases up to and including articles published on 23 November 2011. The degradation rates of pharmaceuticals under UV/TiO2 treatment were dependent on type and amount of TiO2 loading, pharmaceutical concentration, the presence of electron acceptors and pH. Complete mineralization under particular experimental conditions were reported for some pharmaceuticals; however, some experiments reported evolution of toxic intermediates during the photocatalytic process. It is concluded that the UV/TiO2 system is potentially a feasible wastewater treatment process, but careful consideration of the treatment time, the loading and the type of TiO2 (doped vs. undoped) used for a particular pharmaceutical is necessary for a successful application (198 words).

Misses during Water Oxidation in Photosystem II Are S State-dependent

The period of four oscillation of the S state intermediates of the water oxidizing complex in Photosystem II (PSII) is commonly analyzed by the Kok parameters. The important miss factor determines the efficiency for each S transition. Commonly, an equal miss factor has been used in the analysis. We have used EPR signals which probe all S states in the same sample during S cycle advancement. This allows, for the first time, to measure directly the miss parameter for each S state transition. Experiments were performed in PSII membrane preparations from spinach in the presence of electron acceptor at 1 °C and 20 °C. The data show that the miss parameter is different in different transitions and shows different temperature dependence. We found no misses at 1 °C and 10% misses at 20 °C during the S(1)→S(2) transition. The highest miss factor was found in the S(2)→S(3) transition which decreased from 23% to 16% with increasing temperature. For the S(3)→S(0) transition the miss parameter was found to be 7% at 1 °C and decreased to 3% at 20 °C. For the S(0)→S(1) transition the miss parameter was found to be approximately 10% at both temperatures. The contribution from the acceptor side in the form of recombination reactions as well as from the donor side of PSII to the uneven misses is discussed. It is suggested that the different transition efficiency in each S transition partly reflects the chemistry at the CaMn(4)O(5) cluster. That consequently contributes to the uneven misses during S cycle turnover in PSII.

Photoinduced Electron Transfer Across a Molecular Wall: Coumarin Dyes as Donors and Methyl viologen and TiO2 as Acceptors

Coumarins C-153, C-480, and C-1 formed 1:2 (guest:host) complexes with a water-soluble cavitand having eight carboxylic acid groups (OA) in aqueous borate buffer solution. The complexes were photoexcited in the presence of electron acceptors (methyl viologen, MV(2+), or TiO(2)) to probe the possibility of electron transfer between a donor and an acceptor physically separated by a molecular wall. In solution at basic pH, the dication MV(2+) was associated to the exterior of the complex C-153@OA(2), as suggested by diffusion constants (~1.2 × 10(-6) cm(2)/s) determined by DOSY NMR. The fluorescence of C-153@OA(2) was quenched in the presence of increasing amounts of MV(2+) and Stern-Volmer plots of I(o)/I and τ(o)/τ vs [MV(2+)] indicated that the quenching was static. As per FT-IR-ATR spectra, the capsule C-153@OA(2) was bound to TiO(2) nanoparticle films. Selective excitation (λ(exc) = 420) of the above bound complex resulted in fluorescence quenching. When adsorbed on insulating ZrO(2) nanoparticle films, excitation of the complex resulted in a broad fluorescence spectrum centered at 500 nm and consistent with C-153 being within the lipophilic capsule interior. Consistent with the above results, colloidal TiO(2) quenched the emission while colloidal ZrO(2) did not.

Titanium Dioxide-Mediated Photcatalysed Degradation of Two Herbicide Derivatives Chloridazon and Metribuzin in Aqueous Suspensions

The aim of this paper is to find out the optimal degradation condition for two potential environmental pollutants, chloridazon and metribuzin (herbicide derivatives), employing advanced oxidation process using TiO2photocatalyst in aqueous suspensions. The degradation/mineralization of the herbicide was monitored by measuring the change in pollutant concentration and depletion in TOC content as a function of time. A detailed degradation kinetics was studied under different conditions such as types of TiO2(anatase/anatase-rutile mixture), catalyst concentration, herbicide concentration, initial reaction pH, and in the presence of electron acceptors (hydrogen peroxide, ammonium persulphate, potassium persulphate) in addition to atmospheric oxygen. The photocatalyst, Degussa P25, was found to be more efficient catalyst for the degradation of both herbicides as compared with two other commercially available TiO2powders like Hombikat UV100 and PC500. Chloridazon (CHL) was found to degrade more efficiently under acidic condition, whereas metribuzin (MET) degraded faster under alkaline medium. All three electron acceptors tested in this study were found to enhance the degradation rate of both herbicides.

Synergistic effect of Ag deposition and nitrogen doping in TiO2 for the degradation of phenol under solar irradiation in presence of electron acceptor

Ag deposited nitrogen doped TiO2 (Ag–TiO2−xNx) was prepared using sol gel titania by grinding it with stoichiometric amount of Urea as nitrogen source followed by the photoreduction of Ag+ to Ag0. These samples were characterized by powder X-ray diffraction (PXRD), UV-visible absorption spectroscopy, Fourier transformed infrared spectroscopy (FTIR), photoluminescence (PL) and scanning electron microscope (SEM). PL analysis of the samples indicated that the electron-hole recombination has been effectively inhibited after the Ag deposition on TiO2 and TiO2−xNx. Ag–TiO2−xNx exhibits much higher visible-light photocatalytic activity when compared with N-doped or pristine TiO2. The catalysts along with the oxidants can accelerate electron transfer process and inhibit the fast electron-hole recombination. The observed high process efficiency (Φλ) of Ag–TiO2−xNx particles compared to TiO2−xNx and pure TiO2 photocatalyst can be accounted to the synergistic effect of Ag loading along with N doping. Strongly interacting electron accepting species such as hydrogen peroxide and ammonium persulphate chemisorbed at the photocatalyst surface are said to act like surface states enabling inelastic transfer of electrons from the conduction band to the oxidizing species. The electronic states of different energies within the band gap have a major role in enhancing the efficiency.

Photo-catalytic degradation of toxic dye amaranth on TiO 2/UV in aqueous suspensions

The photo-catalytic degradation of an azo dye − Amaranth (AM) – has been investigated in TiO 2/UV aqueous suspensions. The results obtained from the experiments during H 2O 2/TiO 2 addition show that the highest decolorization rate is provided by the combination of (UV + TiO 2 + H 2O 2). The decolorization efficiencies were 17%, 26%, 38% and 64% in the runs UV, UV + H 2O 2, UV + TiO 2 and (UV + TiO 2 + H 2O 2) after approximately 100 min illumination periods, respectively. The observed dye degradation rates followed pseudo-first order kinetics with respect to the substrate concentration under the experimental conditions used. Different experimental conditions, such as temperature, pH and presence of electron acceptor were investigated. The temperature effect was investigated at the range of 293–313 K and it was observed that decolorization rate increased by the increase in temperature. Chemical oxygen demand and dye absorbance of the photodegraded dye solution substantially decreased. Effect of pH was also investigated and it was observed that the lower the pH the higher the degradation. In addition, an enhancement in the photodegradation rate was observed by the addition of hydrogen peroxide as an electron acceptor. The adsorption trends of Amaranth at various initial concentrations followed the Langmuir isotherm trend. This work adds to the global discussion on the role of the advanced oxidation processes in water treatment.

Characterization and emended description of Lactobacillus kunkeei as a fructophilic lactic acid bacterium

Lactobacillus kunkeei is an inhabitant of fructose-rich niches and is a potential member of the fructophilic lactic acid bacteria. In the present study, the phylogenetic and biochemical characteristics of the type strain and eight isolates of L. kunkeei, originating from wine, flowers and honey, were studied. The nine isolates, including the type strain, formed a well-defined phylogenetic subcluster based on the analysis of 16S rRNA gene sequences. The subcluster was not closely related to other subclusters in the Lactobacillus phylogenetic group. Biochemically, the eight new isolates showed typical fructophilic characteristics. The eight isolates grew poorly on glucose, but grew well on fructose. Good growth on glucose was only recorded in the presence of electron acceptors. The type strain of L. kunkeei differed from the other isolates only on the basis of poor growth on fructose. Although they belong to a group of obligately heterofermentative lactic acid bacteria, all nine isolates, including the type strain, produced almost equimolar amounts of lactic acid and acetic acid and very little ethanol from glucose. Eight of the isolates can thus be regarded as typical 'obligately' fructophilic lactic acid bacteria. Although the type strain of L. kunkeei was phenotypically slightly different from the other isolates, it possessed several important fructophilic characteristics. On the basis of the evidence gathered in this study, the type strain of L. kunkeei is recognized as a member of the 'obligately' fructophilic lactic acid bacteria.

Heterogeneous photocatalysed degradation of an insecticide derivative acetamiprid in aqueous suspensions of semiconductor

The photocatalytic degradation of an insecticide derivative, acetamiprid has been investigated in aqueous suspensions of titanium dioxide (TiO 2) as a function of irradiation time under a variety of conditions using UV–Vis and HPLC analysis techniques. The degradation kinetics were studied under different conditions such as types of TiO 2, catalyst and substrate concentration, reaction pH and in the presence of electron acceptors like hydrogen peroxide (H 2O 2), potassium bromate (KBrO 3), and potassium persulphate (K 2S 2O 8) besides molecular oxygen (O 2). The degradation rates were found to be strongly influenced by all the above parameters. The photocatalyst, Degussa P25 was found to be more efficient as compared to other TiO 2 powders used under these studies. The model compound was found to degrade more efficiently in alkaline pH and all electron acceptors enhanced the degradation rate. GC/MS analysis of the irradiated samples indicated the formation of several by-products. A probable mechanism for the formation of by-products has been proposed.

Surface modification of pyrolyzed carbon fibres by cyclic voltammetry and their characterization with XPS and dye adsorption

Abstract Commercial carbon fibres were pyrolyzed up to 1000 °C and were then electrochemically treated by cyclic voltammetry in aqueous electrolyte solutions of H 2 SO 4 , in two potential sweep ranges: a narrow region, N, and a wide region, W, avoiding and including water decomposition, respectively. The anodic and cathodic peaks were correlated with oxide formation and their partial reduction, respectively. The nature of oxygen containing groups on the fibre surfaces was determined by XPS. Wide scan spectra and high energy resolution spectra were recorded through the C 1s, O 1s, N 1s and S 2p photoelectron regions. The ability of the fibres to adsorb methylene blue and alizarin yellow dyes from their aqueous solutions indicates the presence of electron acceptor or donor groups on the fibres, respectively. The carbon fibres were classified into two categories. The first includes electrochemically untreated and treated in the N region, and the second those treated in the W region. The high oxygen concentration and effective dye adsorption on the carbon fibres in the second category indicates that their surfaces were effectively modified. The adsorption of dyes on carbon fibres constitutes a complementary method to XPS for an indirect estimation of oxygen and other groups present on the carbon fibre surfaces.

Detection of the D0→D1 transition of β-carotene radical cation photoinduced in photosystem II

The D0→D1 absorption band of a β-carotene radical cation in the near-infrared (NIR) region was detected for the first time in photosystem II (PSII). PSII-enriched membranes and isolated reaction center (RC) complexes (D1/D2/Cytb559) from spinach were illuminated at 80 and 150 K, respectively, in the presence of electron acceptors. In both preparations, UV-Vis-NIR difference spectra upon illumination exhibited a medium-intensity band at ∼1450 nm along with a strong band at ∼990 nm. The latter band has been assigned to the D0→D2 transition of the radical cation of the β-carotene in the secondary electron transfer pathway in PSII. These NIR bands disappeared at 210 K in the PSII membranes, and diminished their intensities in the RC complexes partially depleted of carotenoid. The absence or diminish of the β-carotene cation with little change in the formation of chlorophyll cations under these conditions were also confirmed by detecting light-induced FTIR difference spectra in the mid-IR region. From these results, it was concluded that the NIR band observed at ∼1450 nm arose from the D0→D1 transition of the β-carotene radical cation. It was shown that the observed band in the RC complexes was a mixture of the band of one β-carotene cation (Car507+) at 1464 nm and that of the other cation (Car489+) at a wavelength shorter than 1434 nm, indicating that the D0→D1 transition is sensitive to the protein environment. It is proposed that the position and the relative intensity of the D0→D1 band together with the well-known D0→D2 band can be useful monitors to investigate the properties of the radical cation and the molecular interaction of β-carotene in the PSII proteins.

The Psb27 Protein Facilitates Manganese Cluster Assembly in Photosystem II

Photosystem II (PSII) is a large membrane protein complex that uses light energy to convert water to molecular oxygen. This enzyme undergoes an intricate assembly process to ensure accurate and efficient positioning of its many components. It has been proposed that the Psb27 protein, a lumenal extrinsic subunit, serves as a PSII assembly factor. Using a psb27 genetic deletion strain (Deltapsb27) of the cyanobacterium Synechocystis sp. PCC 6803, we have defined the role of the Psb27 protein in PSII biogenesis. While the Psb27 protein was not essential for photosynthetic activity, various PSII assembly assays revealed that the Deltapsb27 mutant was defective in integration of the Mn(4)Ca(1)Cl(x) cluster, the catalytic core of the oxygen-evolving machinery within the PSII complex. The other lumenal extrinsic proteins (PsbO, PsbU, PsbV, and PsbQ) are key components of the fully assembled PSII complex and are important for the water oxidation reaction, but we propose that the Psb27 protein has a distinct function separate from these subunits. We show that the Psb27 protein facilitates Mn(4)Ca(1)Cl(x) cluster assembly in PSII at least in part by preventing the premature association of the other extrinsic proteins. Thus, we propose an exchange of lumenal subunits and cofactors during PSII assembly, in that the Psb27 protein is replaced by the other extrinsic proteins upon assembly of the Mn(4)Ca(1)Cl(x) cluster. Furthermore, we show that the Psb27 protein provides a selective advantage for cyanobacterial cells under conditions such as nutrient deprivation where Mn(4)Ca(1)Cl(x) cluster assembly efficiency is critical for survival.

Electrode-less Measurement of Conductivity Transients in Poly(n-alkylthiophene)s induced by 193nm Photoexcitation

Transient electric conductivity of regioregular and random poly(n-alkylthiophene)s films were investigated by time-resolved microwave conductivity upon exposure to 193 nm laser. In the presence of electron acceptor: perylene carboxydiimide, the decays of charge carriers were decelerated while those induced by 355 nm were reported not to differ. We discuss the dependence of side chain length on signal intensity and decay rate.

Heterogeneous photocatalysed decolorization of two selected dye derivatives neutral red and toluidine blue in aqueous suspensions

The photocatalysed decolorization of two selected dye derivative neutral red ( 1 ) and toluidine blue ( 2 ) has been investigated in aqueous suspensions of titanium dioxide under a variety of conditions. The decolorization was studied by monitoring the change in concentration of organic dyestuff employing UV spectroscopic analysis technique as a function of irradiation time. The decolorization was investigated using various parameters such as, different types of TiO 2 , reaction pH, catalyst concentration, concentration of organic dyestuffs, and in the presence of electron acceptors like hydrogen peroxide (H 2 O 2 ) and potassium bromate (KBrO 3 ) besides molecular oxygen with an aim to determine the optimal decolorization condition for the decomposition of compounds under investigation.

Combined Spectroscopic and Topographic Characterization of Nanoscale Domains and Their Distributions of a Redox Protein on Bacterial Cell Surfaces

Redox protein nanoscale domains on the cell surface of a bacterium, Shewanella oneidensis MR1, grown in the absence and presence of electron acceptors, is topographically characterized using combined atomic force microscopy (AFM) and confocal surface enhanced Raman scattering (SERS) spectroscopy. The protruding nanoscale domains on the outer membrane of S. oneidensis were observed, as was their disappearance upon exposure to electron acceptors such as oxygen, nitrate, fumarate, and iron nitrilotriacetate (FeNTA). Using SERS spectroscopy, a redox heme protein was identified as a major component of the cell surface domains. This conclusion was further confirmed by the disappearance of Raman vibrational frequencies, characteristic of heme proteins, upon exposure of the cells to electron acceptors. Our experimental results from our AFM imaging and SERS spectroscopy, consistent with the literature, suggest the protruding nanoscale surface domains as heme-containing secretions. Our results on the distributions of redox proteins on microbial cell surfaces will be helpful for a mechanistic understanding of the behaviors of surface proteins and their interactions with redox environments.

Photocatalytic effects of titania supported nanoporous MCM-41 on degradation of methyl orange in the presence of electron acceptors

This study reports on the synthesis and photocatalytic evaluation of nanometer sized titania supported MCM-41 (TiMCM-41) by photodegrading methyl orange in the presence of electron acceptors such as peroxomonosulphate (PMS), peroxodisulphate (PDS) and H 2O 2 using visible-light irradiation to prove it is a better photocatalyst for advanced oxidation technologies (AOTs). This system is relatively inexpensive, reproducible, extremely stable and efficient in complete degradation of dye in aqueous solution. In order to obtain maximum information about the performance of TiMCM-41 catalyst, we did experiments under different operating conditions, i.e., variation of amount of catalyst, concentration of dye and electron acceptors. In addition to the above, a comparative study on the photocatalytic activities of colloidal TiO 2 was also made.

Experimental investigations by using electrochemical, steady state and time resolved spectroscopic tools on the photoreactions of disubstituted indoles in presence of tetracyanoquinodimethane (TCNQ) and a theoretical approach by using time-dependent density functional theory

The spectroscopic and photophysical properties of some dimethylindoles, 1,2-dimethylindole (12DMI) and 2,3-dimethylindole (23DMI) were measured in presence of electron acceptor, tetracyanoquinodimethane (TCNQ) in solvents of varying polarity by using electrochemical, steady state and time resolved spectroscopic techniques. Both from the theoretical considerations made by using time-dependent density functional theory (TD-DFT) and steady state polarization spectral measurements, it reveals the possibility of mixing of the two closely lying lowest electronic excited states 1L a (S2) and 1L b (S1) of DMIs. Though Stern–Volmer (SV) analysis of steady state measurements is unable to provide the information on the concurrent occurrences of static and dynamic processes involved, but electrochemical measurements coupled with time resolved spectroscopic investigations demonstrate that TCNQ may act as a potential electron acceptor in presence of dimethylindoles to undergo highly exergonic photoinduced electron transfer reactions in Marcus inverted region. Possibility of building up of various artificial or model photoactive systems with the linked DMI–TCNQ dyad systems is hinted at.

Heterogeneous photocatalytic degradation of an anthraquinone and a triphenylmethane dye derivative in aqueous suspensions of semiconductor

The photochemical reactions of two selected dye derivatives, such as acid blue 45 ( 1) and xylenol orange ( 2) have been investigated in aqueous suspensions of titanium dioxide (TiO 2) under a variety of conditions. The reactions were studied by monitoring the change in substrate concentration employing UV spectroscopic analysis technique as a function of irradiation time. The dye derivatives under investigation were found to degrade during irradiation in the presence of TiO 2 and oxygen. The degradation of the dye derivatives was studied under different conditions such as different types of TiO 2, reaction pH, catalyst concentration, substrate concentration, and in the presence of electron acceptors like hydrogen peroxide and potassium bromate, besides oxygen. The degradation rates were found to be strongly influenced by all the above parameters. The degradation of dyes was also investigated under sunlight and the efficiency of degradation was compared with that of the artificial light source. The photocatalyst Hombikat UV100 was found to be more efficient for the degradation of acid blue 45 ( 1). However, Degussa P25 showed better photocatalytic activity in case of xylenol orange ( 2).