Quantum Efficiency Of Charge Separation Research Articles

Construction of novel direct Z-scheme AgIO4-g-C3N4 heterojunction for photocatalytic hydrogen production and photodegradation of fluorescein dye

Highly mesoporous AgIO4/g-C3N4 heterojunctions were synthesized by sonochemical route for exceptional photocatalytic degradation of fluorescein dye (FLU) and hydrogen production. AgIO4 nanoparticles are deposited on the corners and edges of the wrinkled sheets of g-C3N4. Mass ratio of AgIO4 acted a pivotal part in enhancing the photocatalytic reactivity. Due to the small difference between the valence band potential of g-C3N4 (EVB = +1.4 eV) and conduction band potential of AgIO4 (ECB = +1.08 eV), a perfect direct Z-scheme mechanism is constructed. Noticeably, the removal of fluorescein dye reach 98% and the rate of hydrogen produced is 23 mmolh−1 g−1. Taking the benefits of matching the band energy structure, AgIO4/g-C3N4 heterojunction enhances the recombination of the useless positive hole and electron in the inferior valence and conduction band of g-C3N4 and AgIO4, respectively through Z-scheme aspects. However, the holes and electrons in the higher valence and conduction band are preserved with strong oxidation and reduction power. This Z-scheme mechanism not only enhances the quantum efficiency of charge separation, but also, increase the oxidative and reductive power of the charge carrier. Positive holes and the superoxide radicals are the main reactive species confirmed the Z-scheme mechanism. This research work give a hot spot in synthesis of low cost novel photocatalyst for hydrogen production and wastewater treatment.

The influence of mesoscale physical structures in the phytoplankton taxonomic composition of the subsurface chlorophyll maximum off western Baja California

The distribution of the subsurface chlorophyll maximum (SCM) layer, its taxonomic phytoplankton composition, and the maximum quantum efficiency of charge separation of PSII (Fv/Fm) was investigated in the west coast off Baja California during October 2003. SCM characteristics were described and related to the hydrographic regime and the mesoscale physical structures present during this period. Seven groups of phytoplankton were detected in the SCM based on chemotaxonomic analysis of pigment fingerprints: diatoms, haptophytes, pelagophytes, prasinophytes, cryptophytes, Prochlorococcus and cyanobacteria. The distribution of these groups was heterogeneous and closely related to the circulation patterns characterized by the interaction of subarctic and tropical water. Eddies and meanders were detected in the study area and these structures exerted a direct response in the depth, chlorophyll concentration, and photosynthetic competence of phytoplankton in the SCM. A cyclonic eddy characterized by a high chlorophyll concentration (1.6mgm−3) and high values of Fv/Fm, (0.52) was detected in the northern zone of the study area. In the central zone, a cyclonic eddy (1.2mgm−3); and other structure resembling a mode-water eddy was located northwest of Cedros Island. This structure presented the highest chlorophyll concentration (1.8mgm−3) and high Fv/Fm (∼0.5). Chlorophyll concentration and the photosynthetic performance of the phytoplankton community was lower outside of these eddies. Cyanobacteria dominated the phytoplankton SCM community in these areas.

Impairment of benthic diatom adhesion and photosynthetic activity by 2E,4E-decadienal

Within biofilms, microorganisms are exposed to a wide range of chemicals released by phototrophic organisms. Those chemicals are likely to influence the dynamics and functioning of biofilms. 2E,4E-decadienal (DD) is a polyunsaturated aldehyde produced by diatoms which is known to induce adverse effects in many aquatic organisms. It has been shown to inhibit the adhesion and motility of one benthic diatom. The aim of this article was to determine if the effects of DD on diatom adhesion were widespread and if it could affect biofilm formation and functioning. The adhesion of 5 of 10 benthic diatom strains was strongly inhibited at 2.5 μg ml −1 DD. This indicates a high variability in diatom sensitivity to DD. Several experiments in microcosms showed that the presence of DD diffusing from a substrate decreased biofilm formation. This effect was dose-dependent and persisted for 72 h, though the molecule is highly volatile. Using a PHYTO-PAM fluorometer, we also showed that the effective quantum efficiency of charge separation of PSII of biofilms exposed to DD was negatively affected. This indicates a decrease in the efficiency of the photochemical processes. All these results suggest that the presence of DD-producing strains may have a significant impact on the composition and physiology of biofilms.

A toxic Pseudo-nitzschia bloom in Todos Santos Bay, northwestern Baja California, Mexico

A toxic Pseudo-nitzschia spp. bloom in the Todos Santos Bay area (31.8°N), Mexico, is described. This is the southernmost report of the presence of domoic acid (DA) in the California Current System and it is also the first report of the distribution of toxic Pseudo-nitzschia species and DA on the Baja California west coast. The maximum cell abundance of Pseudo-nitzschia was 3.02×105cellsL−1 and the maximum concentration of DA in particulate matter (pDA) was 0.86μgL−1. P. australis constituted the major proportion of cells identified as Pseudo-nitzschia. The environmental conditions associated with wind-driven upwelling were the cause for the accumulation of toxic cells. Maximum pDA and cell concentration were detected around 14°C. The ratio of the concentration of macronutrients seemed to be the important factor for the accumulation of P. australis. The highest cell abundance was detected in areas with a high Si(OH)4 to N ratio in the entire water column. Therefore, the relative increase of silicate concentration related to upwelling conditions was the probable cause for the accumulation of P. australis. Maximum photosystem II (PSII) quantum efficiency of charge separation (Fv/Fm) was negatively correlated to the pDA to fucoxanthin ratio. This ratio was used in this work as an index of cellular DA content. Therefore, the photosynthetic competence of the cells might be an important factor that affected their DA cellular content.

Maximum efficiency of charge separation of photosystem II of the phytoplankton community in the Eastern Tropical North Pacific off Mexico: A nutrient stress diagnostic tool?

Phytoplankton growth in oligotrophic regions might be constrained by a low photosynthetic capacity related to nutrient stress. A reduced photosynthetic capacity has been documented by low values of the maximum quantum efficiency of charge separation (Fv/Fm) of photosystem II (PSII). In the present work, spatial and temporal variations of Fv/Fm in phytoplankton samples of the Eastern Tropical North Pacific (ETNP) off Mexico were characterized to explore the potential of this parameter as a nutrient stress indicator. Our results show that Fv/Fm was lower than 0.5 in most of the samples and that in surface waters this parameter was highly variable. The variability of Fv/Fm was related to the time of sampling, and Fv/Fm changes followed a day/night cycle with maximum values at dawn and a strong reduction at noon related to down-regulation of PSII activity. Enrichment experiments showed that Fv/Fm did not increase after nitrate addition to surface samples. These results suggest that photosynthesis of the phytoplankton community in the ETNP dominated by picoplankton, of which 60–80% was represented by prokaryote biomass (Prochloroccocus and other cyanobacteria), was not rate-limited under natural conditions. Therefore, low Fv/Fm values associated with PSII down-regulation and inherent of specific phytoplankton species are not indicative of nutrient stress in picoplankton-dominated environments such as the ETNP off Mexico. These sources of variability that are not directly associated with the nutritional status of the phytoplankton community should be taken into account when using Fv/Fm as a diagnostic tool for nutrient limitation.

Influence of the TiCl4Treatment on Nanocrystalline TiO2Films in Dye-Sensitized Solar Cells. 2. Charge Density, Band Edge Shifts, and Quantification of Recombination Losses at Short Circuit

Chemical bath deposition of TiO2 from TiCl4 is an often used treatment that improves the photocurrent from dye-sensitized TiO2 solar cells. In this paper, charge density and kinetic data are used to show that the main effects of this treatment are an 80 mV downward shift in the TiO2 conduction band edge potential and a 20-fold decrease in the electron/electrolyte recombination rate constant. Together, these changes increase the quantum efficiency of charge separation at the interface, thus providing the observed increase in the photocurrent. The reduction in the recombination rate constant allows a greater concentration of electrons to accumulate at Voc, thus offsetting the Voc loss otherwise expected from the conduction band edge shift. Photocurrent transients and charge extraction data are used to show that the TiCl4 treatment has little effect on the transport of electrons at short circuit. The electron/electrolyte recombination rate constant at short circuit has been measured with the CCTPV (Constant Current Transient PhotoVoltage) technique. The results further confirm that any improvements in transport could not cause the beneficial effect of the TiCl4 treatment. Verification of the CCTPV technique is undertaken by comparison to transient absorption and by a model of the technique. Charge separation in dye-sensitized cells concerns two steps, charge injection and dye regeneration. Transient optical experiments to determine which process is improved by the TiCl4 treatment are discussed.

Electric field-assisted dissociation of singlet excitons in tris-(8-hydroxyquinolinato) aluminum (III)

Results based on the electric-field dependence of electro-modulated fluorescence and steady-state photoconduction in films of tris-(8-hydroxyquinolinato) aluminum (III) (Alq3) are consistent with a model where singlet excitons dissociate into electron–hole pairs through charge transfer state. Dissociation into free carriers or predominantly geminate recombination results from this intermediate state. The quantum efficiency of charge separation is in agreement with predictions based on a model using the three-dimensional Onsager theory of geminate recombination combined with volume (bimolecular) recombination of the photogenerated space charge. This model is used to account for the decrease in the electroluminescence quantum efficiency of Alq3-based organic light-emitting diodes at high electric fields.

A few molecules of zeaxanthin per reaction centre of photosystem II permit effective thermal dissipation of light energy in photosystem II of a poikilohydric moss.

The relationship between thermal dissipation of light energy (as indicated by the quenching of chlorophyll fluorescence), zeaxanthin availability and protonation reactions was investigated in the moss Rhytidiadelphus squarrosus (Hedw.) Warnst. In the absence of zeaxanthin and actinic illumination, acidification by 20% CO2 in air was incapable of quenching basal, so-called F0 fluorescence either in the moss or in spinach (Spinacia oleracea L.) leaves. However, 1-s light pulses given either every 40, 60 or 200 s increased thermal dissipation as indicated by F0 and Fm quenching in the presence of 20% CO2 in air in the moss, but not in spinach while reaction centres of photosystem II (PSII) were photochemically open. In the moss, a few short light pulses, which were separated by prolonged dark times, were sufficient to raise zeaxanthin levels in the presence of 20% CO2 in air. Simultaneously, quantum efficiency of charge separation in PSII was decreased. Increasing the CO2 concentration beyond 20% further decreased quantum efficiency even in the absence of short light pulses. Under conditions optimal for fluorescence quenching, one molecule of zeaxanthin per reaction centre of PSII was sufficient to decrease quantum efficiency of charge separation in PSII by 50%. Thus, in combination with a protonation reaction, one molecule of zeaxanthin was as efficient at capturing excitation energy as a photochemically open reaction centre. The data are discussed in relation to the interaction between zeaxanthin and thylakoid protonation, which enables effective thermal dissipation of light energy in the antennae of PSII in the moss but not in higher plants when actinic illumination is absent.

Photoinduced Electron Transfer in Porphyrin Quinone Donor/Acceptor Pairs: pH-Dependent Free Ion Yield

The protonation of 2,3-dimethoxy-5-methyl-1,4-benzoquinone (UQ0) has been determined to improve the quantum efficiency of charge separation between a zinc meso-tetra(4-sulfonatophenyl)porphine (MP) donor and UQ0 (UQ0H+ upon protonation) acceptor system. The primary effect of protonation is to increase the driving force of the initial photoinitiated electron transfer (ET). Although this does not alter the diffusion-limited rate of forward ET, it may have a substantial effect on the nature of the ion pair formed by the initial ET and the subsequent competition between free ion separation and back electron transfer (BET). Transient absorption data show that although the rates of ET from photoexcited 3MP* to UQ0 or UQ0H+ are diffusion limited, there is a >3-fold increase in free ion product yields upon UQ0 protonation. The results are assessed in terms of current electron transfer theories. It is argued that the increase in product yield is a result of BET for the reactions of UQ0H+ being deeper in the normal Marcus region. The importance of these results for understanding trends in the rates of BET is discussed.

Artificial photosynthesis. 1. Photosensitization of titania solar cells with chlorophyll derivatives and related natural porphyrins

Colloidal TiO[sub 2] electrodes were photosensitized with derivatives of chlorophyll and related natural porphyrins resulting in light harvesting and charge separation efficiencies comparable to those in natural photosynthesis. The photocurrent action spectra of the electrodes correlate well with the absorption spectra of the dyes in solution. Incident photon to current efficiencies up to 83% are reached in the Soret peak at 400 nm with a 12-[mu]m-thick TiO[sub 2] film sensitized by copper mesoporphyrin IX, which corresponds to nearly unity quantum efficiency of charge separation when light reflection losses are taken into account. Photocurrent/voltage curves of TiO[sub 2] solar cells sensitized with copper chlorophyllin show an energy conversion efficiency of 10% for the red peak at 630 nm. Under simulated sunlight illumination, an open circuit photovoltage of 0.52 V and a short circuit current density of 9.4 mA/cm[sup 2] are measured. The overall energy conversion efficiency of the cell is 2.6% under these conditions, in part limited by ohmic losses at such high current densities. The comparison of different chlorophyll derivatives indicates that free carboxyl groups are important for adsorption and sensitization on TiO[sub 2]. However, conjugation of the carboxyl groups with the [pi] electron system of the chromophore is notmore » necessary for efficient electron transfer. Free bases, zinc, and even the nonfluorescent copper complexes of chlorophyllins and mesoporphyrin IX are efficient sensitizers for TiO[sub 2]. Cholanic acids as coadsorbates were found to be unique in improving both photocurrent and voltage of copper chlorophyllin sensitized cells. This effect is discussed by comparison with other coadsorbates. 23 refs., 3 figs., 1 tab.« less

Effect of replacing the primary quinone by different species on the ultrafast photosynthetic electron transfer in bacterial reaction centres

Using picosecond absorption spectroscopy it has been shown that in Rhodobacter sphaeroides reaction centres the substitution of the primary quinone acceptor (Q A), ubiquinone-10, by other quinone species (with redox potentials higher or lower than that of ubiquinone-10) has essentially no modifying effect on the reaction centre protein. The molecular relaxation processes that accompany the localization and stabilization of a photoexcited electron on the intermediate acceptor, bacteriopheophytin (I), are not affected, although the subsequent transfer of the electron from I to Q A is slowed down. Consequently, this leads to a lower quantum efficiency of charge separation between the photoactive pigment and Q A. Therefore the high rate of direct I − → Q A reaction is normally due to the specificity of the primary quinone species and its binding site in the reaction centre protein which provide optimum steric and chemical conditions for an effective interaction between I and Q A.

Problems of back electron transfer in electron transfer sensitization

The problems of back electron transfer in electron transfer sensitization were discussed and methods of preventing back electron transfer to improve the quantum efficiency of charge separation in homogeneous systems were proposed. Although complete quenching of an excited sensitizer by an electron donor or acceptor is not difficult, chemical yields of oxidized or reduced species are in general low. The importance of the coulombic effect was demonstrated for benzophenone/leuco crystal violet, pyrene/methylviologen (MV 2+)/ethylenediaminetetraacetic acid and phenothiazine/viologen analogue systems. Then the discussion was extended to the quenching of Ru(bpy) 3 2+* (bpy ≡ 2,2′-bipyridine) by organic acceptors and donors in acetonitrile. The magnitude of the quenching constant k q as a function of exoergicity could be explainable by the Rehm—Weller equation in both cases: Δ H ‡ is apparently negative for oxidative quenching (RuL 3 2+* + A → RuL 3 3+ + −; in contrast, for reductive quenching (RuL 3 2+* + D → RuL 3 + + D +) Δ H ‡ is normal and control k q. A detailed kinetic mechanistic study leads to the conclusion that the attraction between RuL 3 3+ and A − in oxidative quenching brings about back electron transfer to the excited state, and the charge separation yield is found is found to be lower than that for reductive quenching as would be expected. By modulation of the ligand structure from 2,2′-bipyridine to 2,2′-bipyrazine and other ligands, the redox properties of RuL 3 2+* can be modified. Thus, the photo-reaction of the RuL 3 2+/MV 2+/triethanolamine (TEOA) system proceeds via reductive quenching when L is 2,2-bipyrazine or some other ligands (RuL 3 2+* + TEOA → RuL 3 +; RuL 3 + + MV 2+ → RuL 3 2+ + MV +. The quantum yield of MV + formation approaches 100% by tris(2,2′-bipyrazine)-ruthenium(II) whereas the conventional Ru(bpy) 2 2+ sensitizer reacting via the oxidative quenching mechanism (RuL 3 2+* + MV 2+ → RuL 3 3+ + MV +; RuL 3 3+ + TEOA → RuL 3 2+) gives a quantum yield of MV + formation of only about 20%.