Origin of the overall water splitting activity of Ta3N5 revealed by ultrafast transient absorption spectroscopy

Tantalum nitride (Ta3N5) is one of the few visible light absorbing photocatalysts capable of overall water splitting (OWS), by which the evolution of both H2 and O2 is possible. Despite favourable energetics, realizing the OWS or efficient H2 evolution in Ta3N5 prepared by the nitridation of tantalum oxide (Ta2O5) or Ta foil remains a challenge even after 15 years of intensive research. Recently our group demonstrated OWS in Ta3N5 when prepared by the short time nitridation of potassium tantalate (KTaO3). To obtain a mechanistic insight on the role of Ta precursor and nitridation time in realizing OWS, ultrafast dynamics of electrons (3435 nm probe) and holes (545 nm probe) is investigated using transient absorption spectroscopy. Electrons decay majorly by trapping in Ta3N5 prepared by the nitridation of Ta2O5, which do not show OWS. However, OWS activity in Ta3N5 prepared by 0.25 hour nitridation of KTaO3 is particularly favoured by the virtually absent electron and hole trapping. On further increasing the nitridation time of KTaO3 from 0.25 to 10 hour, trapping of both electron and hole is enhanced which concurrently results in a reduction of the OWS activity. Insights from correlating the synthesis conditions—structural defects—carrier dynamics—photocatalytic activity is of importance in designing novel photocatalysts to enhance solar fuel production

Relation between charge carrier mobility and lifetime in organic photovoltaics

The relationship between charge carrier lifetime and mobility in a bulk heterojunction based organic solar cell, utilizing diketopyrrolopyrole- naphthalene co-polymer and PC71BM in the photoactive blend layer, is investigated using the photoinduced charge extraction by linearly increasing voltage technique. Light intensity, delay time, and temperature dependent experiments are used to quantify the charge carrier mobility and density as well as the temperature dependence of both. From the saturation of photoinduced current at high laser intensities, it is shown that Langevin-type bimolecular recombination is present in the studied system. The charge carrier lifetime, especially in Langevin systems, is discussed to be an ambiguous and unreliable parameter to determine the performance of organic solar cells, because of the dependence of charge carrier lifetime on charge carrier density, mobility, and type of recombination. It is revealed that the relation between charge mobility (μ) and lifetime (τ) is inversely proportional, where the μτ product is independent of temperature. The results indicate that in photovoltaic systems with Langevin type bimolecular recombination, the strategies to increase the charge lifetime might not be beneficial because of an accompanying reduction in charge carrier mobility. Instead, the focus on non-Langevin mechanisms of recombination is crucial, because this allows an increase in the charge extraction rate by improving the carrier lifetime, density, and mobility simultaneously

New Insights into the Mechanism of Visible Light Photocatalysis

ABSTRACT: In recent years, the area of developing visible-lightactive photocatalysts based on titanium dioxide has been enormously investigated due to its wide range of applications in energy and environment related fields. Various strategies have been designed to efficiently utilize the solar radiation and to enhance the efficiency of photocatalytic processes. Building on the fundamental strategies to improve the visible light activity of TiO2-based photocatalysts, this Perspective aims to give an insight into many contemporary developments in the field of visible-light-active photocatalysis. Various examples of advanced TiO2 composites have been discussed in relation to their visible light induced photoconversion efficiency, dynamics of electron− hole separation, and decomposition of organic and inorganic pollutants, which suggest the critical need for further development of these types of materials for energy conversion and environmental remediation purposes

The violent youth of bright and massive cluster galaxies and their maturation over 7 billion years

In this study, we investigate the formation and evolution mechanisms of the brightest cluster galaxies (BCGs) over cosmic time. At high redshift (z ∼ 0.9), we selected BCGs and most massive cluster galaxies (MMCGs) from the Cl1604 supercluster and compared them to low-redshift (z ∼ 0.1) counterparts drawn from the MCXC meta-catalogue, supplemented by Sloan Digital Sky Survey imaging and spectroscopy. We observed striking differences in the morphological, colour, spectral, and stellar mass properties of the BCGs/MMCGs in the two samples. High-redshift BCGs/MMCGs were, in many cases, star-forming, late-type galaxies, with blue broad-band colours, properties largely absent amongst the low-redshift BCGs/MMCGs. The stellar mass of BCGs was found to increase by an average factor of 2.51 ± 0.71 from z ∼ 0.9 to z ∼ 0.1. Through this and other comparisons, we conclude that a combination of major merging (mainly wet or mixed) and in situ star formation are the main mechanisms which build stellar mass in BCGs/MMCGs. The stellar mass growth of the BCGs/MMCGs also appears to grow in lockstep with both the stellar baryonic and total mass of the cluster. Additionally, BCGs/MMCGs were found to grow in size, on average, a factor of ∼3, while their average Sérsic index increased by ∼0.45 from z ∼ 0.9 to z ∼ 0.1, also supporting a scenario involving major merging, though some adiabatic expansion is required. These observational results are compared to both models and simulations to further explore the implications on processes which shape and evolve BCGs/MMCGs over the past ∼7 Gyr

Coexistence of femtosecond- and nonelectron-injecting dyes in dye-sensitized solar cells: inhomogeniety limits the efficiency

We performed a detailed and quantitative spectroscopic study of the electron injection dynamics for porphyrin as one of organic dyes at an adequate level to discuss the dye-sensitized solar cell performance. The electron injection kinetics and the electron injection yield for dye-sensitized TiO 2 electrodes in redox-containing electrolytes were measured by femtosecond transient absorption and picosecond fluorescence spectroscopy. By comparing the dynamics of two of the most studied porphyrins with those of a Ru complex (N719), we have directly elucidated that the short-circuit current for the porphyrin-sensitized solar cells is limited by the presence of excited dyes that are quenched in the subnanosecond time range without competing with the electron injection process, even though both porphyrins shows faster injection processes within the picosecond time range than N719. Therefore, it was clearly indicated the electron injection efficiency was mainly limited by the inhomogeniety, which should be carefully considered for further development of organic dye-sensitized solar cells. Copyright 2011 American Chemical Society

Plate-like Sm2Ti2S2O5 Particles Prepared by a Flux-Assisted One-Step Synthesis for the Evolution of O-2 from Aqueous Solutions by Both Photocatalytic and Photoelectrochemical Reactions

Sm2Ti2S2O5 (STSO) is a visible-light-responsive oxysulfide semiconductor photocatalyst with applications to water splitting. In this work, plate-like STSO particles were synthesized through a flux-assisted one-step method at various temperatures. The activities of these materials during photocatalytic and photoelectrochemical O-2 evolution from aqueous solutions were investigated. Single-phase STSO with a single crystal habit was produced at 923 K, which is approximately 200 K lower than the temperatures required for previously reported methods, such as solid-state reactions and thermal sulfurization under a H2S flow. The STSO sample synthesized at the optimal temperature exhibited an AQE of 1.3 +/- 0.2% at 420 nm during photocatalytic sacrificial O-2 evolution. This efficiency is twice the values reported for specimens prepared using conventional methods. An STSO/Ti/Sn electrode fabricated by the particle transfer method generated a photoanodic current and evolved O-2 by water oxidation with a Faradaic efficiency of approximately 70 +/- 7%. The synthesis temperature yielding the highest activity was lower for photocatalytic O-2 evolution than photoelectrochemical O-2 evolution. This work demonstrates the applicability for of the flux method to the synthesis of well-crystallized oxysulfides having various particle sizes and intended for different uses