Proton transfer in bulk water using the full adaptive QM/MM method: integration of solute- and solvent-adaptive approaches.

The quantum mechanical/molecular mechanical (QM/MM) method is a hybrid molecular simulation technique that increases the accessibility of local electronic structures of large systems. The technique combines the benefit of accuracy found in the QM method and that of cost efficiency found in the MM method. However, it is difficult to directly apply the QM/MM method to the dynamics of solution systems, particularly for proton transfer. As explained in the Grotthuss mechanism, proton transfer is a structural interconversion between hydronium ions and solvent water molecules. Hence, when the QM/MM method is applied, an adaptive treatment, namely on-the-fly revisions on molecular definitions, is required for both the solute and solvent. Although several solvent-adaptive methods have been proposed, a full adaptive framework, which is an approach that also considers adaptation for solutes, remains untapped. In this paper, we propose a new numerical expression for the coordinates of the excess proton and its control algorithm. Furthermore, we confirm that this method can stably and accurately simulate proton transfer dynamics in bulk water.

Transparent Ta3 N5 Photoanodes for Efficient Oxygen Evolution toward the Development of Tandem Cells.

Photoelectrochemical water splitting is regarded as a promising approach to the production of hydrogen, and the development of efficient photoelectrodes is one aspect of realizing practical systems. In this work, transparent Ta3 N5 photoanodes were fabricated on n-type GaN/sapphire substrates to promote O2 evolution in tandem with a photocathode, to realize overall water splitting. Following the incorporation of an underlying GaN layer, a photocurrent of 6.3 mA cm-2 was achieved at 1.23 V vs. a reversible hydrogen electrode. The transparency of Ta3 N5 to wavelengths longer than 600 nm allowed incoming solar light to be transmitted to a CuInSe2 (CIS), which absorbs up to 1100 nm. A stand-alone tandem cell with a serially-connected dual-CIS unit terminated with a Pt/Ni electrode was thus constructed for H2 evolution. This tandem cell exhibited a solar-to-hydrogen energy conversion efficiency greater than 7 % at the initial stage of the reaction.

Particulate Photocatalyst Sheets Based on Carbon Conductor Layer for Efficient Z-Scheme Pure-Water Splitting at Ambient Pressure.

Development of sunlight-driven water splitting systems with high efficiency, scalability, and cost-competitiveness is a central issue for mass production of solar hydrogen as a renewable and storable energy carrier. Photocatalyst sheets comprising a particulate hydrogen evolution photocatalyst (HEP) and an oxygen evolution photocatalyst (OEP) embedded in a conductive thin film can realize efficient and scalable solar hydrogen production using Z-scheme water splitting. However, the use of expensive precious metal thin films that also promote reverse reactions is a major obstacle to developing a cost-effective process at ambient pressure. In this study, we present a standalone particulate photocatalyst sheet based on an earth-abundant, relatively inert, and conductive carbon film for efficient Z-scheme water splitting at ambient pressure. A SrTiO3:La,Rh/C/BiVO4:Mo sheet is shown to achieve unassisted pure-water (pH 6.8) splitting with a solar-to-hydrogen energy conversion efficiency (STH) of 1.2% at 331 K and 10 kPa, while retaining 80% of this efficiency at 91 kPa. The STH value of 1.0% is the highest among Z-scheme pure water splitting operating at ambient pressure. The working mechanism of the photocatalyst sheet is discussed on the basis of band diagram simulation. In addition, the photocatalyst sheet split pure water more efficiently than conventional powder suspension systems and photoelectrochemical parallel cells because H+ and OH- concentration overpotentials and an IR drop between the HEP and OEP were effectively suppressed. The proposed carbon-based photocatalyst sheet, which can be used at ambient pressure, is an important alternative to (photo)electrochemical systems for practical solar hydrogen production.

Trapped state sensitive kinetics in LaTiO2N solid photocatalyst with and without cocatalyst loading.

In addition to the process of photogeneration of electrons and holes in photocatalyst materials, the competitive process of trapping of these charge carriers by existing defects, which can both enhance the photocatalytic activity by promoting electron-hole separation or can deteriorate the activity by serving as recombination centers, is also very crucial to the overall performance of the photocatalyst. In this work, using femtosecond diffuse reflectance spectroscopy we have provided evidence for the existence of energetically distributed trapped states in visible-light responsive solid photocatalyst powder material LaTiO2N (LTON). We observe trapped state sensitive kinetics in bare-LTON. CoOx cocatalyst loading (2 wt % CoOx-LTON) shows effect on the kinetics only when presence of excess energy (for above bandgap excitation) results in the generation of surface carriers. Thus, the kinetics show appreciable excitation wavelength dependence, and the experimental results obtained for different λexc have been rationalized on this basis. In an earlier work by Domen and co-workers, the optimized CoOx/LTON has been reported to exhibit a high quantum efficiency of 27.1 ± 2.6% at 440 nm, the highest reported for this class of photocatalysts (J. Am. Chem. Soc. 2012, 134, 8348-8351). In the present work, the mechanism is addressed in terms of picosecond charge carrier dynamics.

Single molecules in an extension clamp: extracting rates and activation barriers.

When a macromolecule, held at a fixed end-to-end separation, undergoes conformational rearrangements, the fluctuating mechanical force generated by the molecule can be used as a reporter of the molecular conformational dynamics. We present an analytical framework for extracting the intrinsic rates of conformational transitions and the locations and heights of the rate-limiting barriers from such extension clamp measurements. The unique nature of the bias imposed by the extension clamp on the activation barriers allows access to biomolecular transitions currently not accessible in pulling experiments. A mapping rule is established between the outputs of different types of experiments, providing information about poorly accessible regions of the molecular landscape.

Biomolecules under mechanical stress: a simple mechanism of complex behavior.

The unfolding of a biomolecule by stretching force is commonly treated theoretically as one-dimensional dynamics along the reaction coordinate coincident with the direction of pulling. Here we explore a situation, particularly relevant to complex biomolecules, when the pulling direction alone is not an adequate reaction coordinate for the unfolding or rupture process. We show that in this case the system can respond to pulling force in unusual ways. Our theory points out a remarkably simple, but largely overlooked, mechanism of the complex responses of biomolecules to force. The mechanism originates from the basic property of the transition state to change its structure under applied force. A relationship is established between a key experimental observable--force-dependent lifetime--and the microscopic properties of the biomolecule in the form of an analytical solution to the problem of a force-induced molecular transition in two dimensions. The theory is applicable to biological contexts ranging from protein folding to ligand-receptor interactions.

Single-molecule rupture dynamics on multidimensional landscapes.

We explore emergent effects of multidimensionality of the free energy landscape on single-molecule kinetics under constant force. The proposed minimal model reveals the existence of a spectrum of unusual scenarios for the force-dependent lifetime, all of which are shown to occur on a free energy landscape with a single transition state. We present an analytical solution that governs single-molecule responses to a constant force and relates them to microscopic parameters of the system.

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.

Enhancement of Charge Separation and Hydrogen Evolution on Particulate La5Ti2CuS5O7 Photocathodes by Surface Modification.

Particulate La5Ti2CuS5O7 (LTC) photocathodes prepared by particle transfer show a positive onset potential of 0.9 V vs RHE for the photocathodic current in photoelectrochemical (PEC) H2 evolution. However, the low photocathodic current imposes a ceiling on the solar-to-hydrogen energy conversion efficiency of PEC cells based on LTC photocathodes. To improve the photocurrent, in this work, the surface of Mg-doped LTC photocathodes was modified with TiO2, Nb2O5, and Ta2O5 by radio frequency reactive magnetron sputtering. The photocurrent of the modified Mg-doped LTC photocathodes was doubled because these oxides formed type-II heterojunctions and extended the lifetimes of photogenerated charge carriers. The enhanced photocathodic current was attributed to hydrogen evolution at a positive potential of +0.7 V vs RHE. This work opens up possibilities for improving PEC hydrogen evolution on particulate photocathodes based on surface oxide modifications and also highlights the importance of the band gap alignment.

Durable hydrogen evolution from water driven by sunlight using (Ag,Cu)GaSe2 photocathodes modified with CdS and CuGa3Se5.

The photoelectrochemical properties of (Ag,Cu)GaSe2 (ACGSe) modified by deposition of CdS and CuGa3Se5 layers were investigated. The CdS and CuGa3Se5 layers formed p-n junctions with an appropriate band diagram on the surface of the electrode and they clearly increased the cathodic photocurrent and onset potential. The Pt, CdS, and CuGa3Se5 modified ACGSe (Pt/CdS/CuGa3Se5/ACGSe) with an appropriate thickness of CuGa3Se5 layers (ca. 100 nm) showed a cathodic photocurrent of 8.79 mA cm-2 at 0 VRHE and an onset potential of 0.62 VRHE (defined as cathodic photocurrent of 1.0 mA cm-2) under simulated sunlight irradiation in 0.1 M Na2HPO4 (pH 10). Pt/CdS/CuGa3Se5/ACGSe showed durable cathodic current under the observed reaction conditions and hydrogen was evolved for about 20 days.

Photoanodic and photocathodic behaviour of La5Ti2CuS5O7 electrodes in the water splitting reaction.

The particulate semiconductor La5Ti2CuS5O7 (LTC) with a band gap energy of 1.9 eV functioned as either a photocathode or a photoanode when embedded onto Au or Ti metal layers, respectively. By applying an LTC/Au photocathode and LTC/Ti photoanode to, respectively, photoelectrochemical (PEC) water reduction and oxidation concurrently, zero-bias overall water splitting was accomplished under visible light irradiation. The band structures of LTC/Au and LTC/Ti calculated using a semiconductor device simulator (AFORS-HET) confirmed the critical role of the solid/solid junction of the metal back contact in the charge separation and PEC properties of LTC photoelectrodes. The prominently long lifetime of photoexcited charge carriers in LTC, confirmed by transient absorption spectroscopy, allowed the utilization of both photoexcited electrons and holes depending on the band structure at the solid/solid junction.

Exploring a free energy landscape by means of multidimensional infrared and terahertz spectroscopies.

A model for the dipolar crystal system is employed to explore a role of free energy landscape (FEL), in which dipolar molecules are posted on two-dimensional lattice sites with two-state libratinal dynamics. All dipole-dipole interactions are included to have frustrated interactions among the dipoles. For the regular and distorted lattice cases, the FEL is calculated from the interaction energies and the total polarizations for all possible dipolar states at various temperatures. At high temperatures, the shape of the calculated FEL is smooth and parabolic, while it becomes bumpy at low temperatures exhibiting multiple local minima. To study dynamical aspects of the system, the single flip dynamics and the single-double mixed flips dynamics of dipoles are examined from a master equation approach. As the observables of linear absorption and two-dimensional (2D) infrared, far infrared, and terahertz spectroscopies, the first- and third-order response functions of polarization are calculated for different physical conditions characterized by the FEL. While the linear absorption signals decay in time in a similar manner regardless of the FEL profiles, the 2D signals exhibit prominent differences for those profiles. This indicates that we may differentiate the FEL profiles by changing two-time valuables in 2D spectroscopy. As illustrated in the single-double flips case, the FEL study by means of 2D spectroscopy, however, relies on the dynamics which is set independently from the FEL. The Smoluchowski equation is applied to examine the description of the collective dynamics on the microscopically calculated FEL. We found that the one-dimensional and 2D signals calculated from the Smoluchowski equation agree with those from master equation only at temperatures where the FEL becomes parabolic shape.

Free energy landscapes of electron transfer system in dipolar environment below and above the rotational freezing temperature.

Electron transfer reaction in a polar solvent is modeled by a solute dipole surrounded by dipolar molecules with simple rotational dynamics posted on the three-dimensional distorted lattice sites. The interaction energy between the solute and solvent dipoles as a reaction coordinate is adopted and free energy landscapes are calculated by generating all possible states for a 26 dipolar system and by employing Wang-Landau sampling algorithm for a 92 dipolar system. For temperatures higher than the energy scale of dipole-dipole interactions, the free energy landscapes for the small reaction coordinate region have quadratic shape as predicted by Marcus [Rev. Mod. Phys. 65, 599 (1993)] whereas for the large reaction coordinate region, the landscapes exhibit a nonquadratic shape. When the temperature drops, small notched structures appear on the free energy profiles because of the frustrated interactions among dipoles. The formation of notched structure is analyzed with statistical approach and it is shown that the amplitude of notched structure depend upon the segment size of the reaction coordinate and is characterized by the interaction energy among the dipoles. Using simulated free energy landscapes, the authors calculate the reaction rates as a function of the energy gap for various temperatures. At high temperature, the reactions rates follow a bell shaped (inverted parabolic) energy gap law in the small energy gap regions, while it becomes steeper than the parabolic shape in a large energy gap regions due to the nonquadratic shape of the free energy landscape. The peak position of parabola also changes as the function of temperature. At low temperature, the profile of the reaction rates is no longer smooth because of the many local minima of the free energy landscape.

Free energy landscape analysis of two-dimensional dipolar solvent model at temperatures below and above the rotational freezing point.

Ionic solvation in a polar solvent is modeled by a central charge surrounded by dipolar molecules posted on two-dimensional distorted lattice sites with simple rotational dynamics. Density of states is calculated by applying the Wang-Landau algorithm to both the energy and polarization states. The free energy landscapes of solvent molecules as a function of polarization are depicted to explore the competition between the thermal fluctuation and solvation energy. Without a central charge, for temperatures higher than the energy scale of the dipole-dipole interactions, the energy landscape for the small polarization region exhibits a parabolic shape as predicted by Marcus [Rev. Mod. Phys. 65, 599 (1993)] for electron transfer reaction, while there is an additional quartic contribution to the landscape for the large polarization region. When the temperature drops, the simulated free energy landscapes are no longer smooth due to the presence of multiple local minima arising from the frustrated interaction among the dipoles. The parabolic contribution becomes negligible and the energy landscape becomes quartic in shape. For a strong central charge, the energy landscape exhibits an asymmetric profile due to the contributions of linear and cubic terms that arise from the charge-dipole interactions.