Gamma Radiation-Induced Degradation of Acetohydroxamic Acid (AHA) in Aqueous Nitrate and Nitric Acid Solutions Evaluated by Multiscale Modelling.

Acetohydroxamic acid (AHA) has been proposed for inclusion in advanced, single-cycle, used nuclear fuel reprocessing solvent systems for the reduction and complexation of plutonium and neptunium ions. For this application, a detailed description of the fundamental degradation of AHA in dilute aqueous nitric acid is required. To this end, we present a comprehensive, multiscale computer model for the coupled radiolytic and hydrolytic degradation of AHA in aqueous sodium nitrate and nitric acid solutions. Rate coefficients for the reactions of AHA and hydroxylamine (HA) with the oxidizing nitrate radical were measured for the first time using electron pulse radiolysis and used as inputs for the kinetic model. The computer model results are validated by comparison to experimental data from steady-state gamma ray irradiations, for which the agreement is excellent. The presented model accurately predicts the yields of the major degradation products of AHA: acetic acid, HA, nitrous oxide, and molecular hydrogen.

Two Birds with One Stone: Concurrent Ligand Removal and Carbon Encapsulation Decipher Thickness-Dependent Catalytic Activity.

A carbon shell encapsulating a transition metal-based core has emerged as an intriguing type of catalyst structure, but the effect of the shell thickness on the catalytic properties of the buried components is not well known. Here, we present a proof-of-concept study to reveal the thickness effect by carbonizing the isotropic and homogeneous oleylamine (OAm) ligands that cover colloidal MoS2. A thermal treatment turns OAm into a uniform carbon shell, while the size of MoS2 monolayers remains identical. When evaluated toward an acidic hydrogen evolution reaction, the calcined MoS2 catalysts deliver a volcano-type activity trend that depends on the calcination temperature. Rutherford backscattering spectrometry and depth-profiling X-ray photoelectron spectroscopy consistently provide an accurate quantification of the carbon shell thickness. The same variation pattern of catalytic activity and carbon shell thickness, aided by kinetic studies, is then persuasively justified by the respective limitations of electron and proton conductivities on the two branches of the volcano curve.

Synthesis of 2D anatase TiO2 with highly reactive facets by fluorine-free topochemical conversion of 1T-TiS2 nanosheets.

Two-dimensional (2D) anatase titanium dioxide (TiO2) is expected to exhibit different properties as compared to anatase nanocrystallites, due to its highly reactive exposed facets. However, access to 2D anatase TiO2 is limited by the non-layered nature of the bulk crystal, which does not allow use of top-down chemical exfoliation. Large efforts have been dedicated to the growth of 2D anatase TiO2 with high reactive facets by bottom-up approaches, which relies on the use of harmful chemical reagents. Here, we demonstrate a novel fluorine-free strategy based on topochemical conversion of 2D 1T-TiS2 for the production of single crystalline 2D anatase TiO2, exposing the {001} facet on the top and bottom and {100} at the sides of the nanosheet. The exposure of these faces, with no additional defects or doping, gives rise to a significant activity enhancement in the hydrogen evolution reaction, as compared to commercially available Degussa P25 TiO2 nanoparticles. Because of the strong potential of TiO2 in many energy-based applications, our topochemical approach offers a low cost, green and mass scalable route for production of highly crystalline anatase TiO2 with well controlled and highly reactive exposed facets.

First observation of radiolytic bubble formation in unstirred nano-powder sludges and a consistent model thereof.

Experiments involving the irradiation of water contained within magnesium hydroxide and alumina nanoparticle sludges were conducted and culminated in observations of an increased yield of molecular hydrogen when compared to the yield from the irradiation of bulk water. We show that there is a relationship linking this increased yield to the direct nanoscale ionization mechanism in the nanoparticles, indicating that electron emission from the nanoparticles drives new radiative pathways in the water. Because the chemical changes in these sludges are introduced by irradiation only, we have a genuinely unstirred system. This feature allows us to determine the diffusivity of the dissolved gas. Using the measured gas production rate, we have developed a method for modelling when hydrogen bubble formation will occur within the nanoparticle sludges. This model facilitates the determination of a consistent radiolytic consumption rate coinciding with the observations of bubble formation. Thus, we demonstrate a nanoscale radiation effect directly influencing the formation of molecular hydrogen.

Gamma Radiation-Induced Oxidation, Doping, and Etching of Two-Dimensional MoS2 Crystals.

Two-dimensional (2D) MoS2 is a promising material for future electronic and optoelectronic applications. 2D MoS2 devices have been shown to perform reliably under irradiation conditions relevant for a low Earth orbit. However, a systematic investigation of the stability of 2D MoS2 crystals under high-dose gamma irradiation is still missing. In this work, absorbed doses of up to 1000 kGy are administered to 2D MoS2. Radiation damage is monitored via optical microscopy and Raman, photoluminescence, and X-ray photoelectron spectroscopy techniques. After irradiation with 500 kGy dose, p-doping of the monolayer MoS2 is observed and attributed to the adsorption of O2 onto created vacancies. Extensive oxidation of the MoS2 crystal is attributed to reactions involving the products of adsorbate radiolysis. Edge-selective radiolytic etching of the uppermost layer in 2D MoS2 is attributed to the high reactivity of active edge sites. After irradiation with 1000 kGy, the monolayer MoS2 crystals appear to be completely etched. This holistic study reveals the previously unreported effects of high-dose gamma irradiation on the physical and chemical properties of 2D MoS2. Consequently, it demonstrates that radiation shielding, adsorbate concentrations, and required device lifetimes must be carefully considered, if devices incorporating 2D MoS2 are intended for use in high-dose radiation environments.

Inulinase immobilization on polyethylene glycol/polypyrrole multiwall carbon nanotubes producing a catalyst with enhanced thermal and operational stability.

This paper describes the development of a simple method for mixed non-covalent and covalent bonding of partially purified inulinase on functionalized multiwall carbon nanotubes (f-MWCNTs) with polypyrrole (PPy). The pyrrole (Py) was electrochemically polymerized on MWCNTs in order to fabricate MWCNTs/PPy nanocomposite. Two multiple forms of enzyme were bound to N-H functional groups from PPy and -COO- from activated MWCNTs to yield a stable MWCNTs/PPy/PEG immobilized preparation with increased thermal stability. Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) were used to confirm functionalization of nanoparticles and immobilization of the enzyme. The immobilization yield of 85% and optimal enzyme load of 345 µg protein onto MWCNTs was obtained. The optimum reaction conditions and kinetic parameters were established using the UV-Vis analytical assay. The best functional performance for prepared heterogeneous catalyst has been observed at pH 3.6 and 10, and at the temperatures of 60 and 80ºC. The half-life (t 1/2) of the immobilized inulinase at 60 and 80ºC was found to be 231 and 99 min, respectively. The reusability of the immobilized formulation was evaluated based on a method in which the enzyme retained 50% of its initial activity, which occurred after the eighteenth operation cycle.

Elucidating the impact of molecular packing and device architecture on the performance of nanostructured perylene diimide solar cells.

The performance of organic photovoltaic devices (OPV) with nanostructured polymer:perylene diimide (PDI) photoactive layers approaches the levels of the corresponding polymer:fullerene systems. Nevertheless, a coherent understanding of the difficulty for PDI-based OPV devices to deliver high power conversion efficiencies remains elusive. Here we perform a comparative study of a set of four different polymer:PDI OPV model systems. The different device performances observed are attributed to differences in the nanostructural motif of these composites, as determined by wide-angle X-ray scattering (WAXS) measurements. Long-range structural order in the PDI domain dictates (i) the stabilization energy and (ii) the concentration of the PDI excimers in the composites. The quenching of the PDI excimer photoluminescence (PL) is found to be insensitive to the former, but it depends on the latter. High PL quenching occurs for the low concentration of PDI excimers that are formed in PDI columns with a length comparable to the PDI excimer diffusion length. The stabilization of the PDI excimer state increases as the long-range order in the PDI domains improves. The structural order of the PDI domains primarily affects charge transport. Electron mobility reduces as the size of the PDI domain increases, suggesting that well-ordered PDI domains suffer from poor electronic connectivity. WAXS further reveals the presence of additional intermolecular PDI interactions, other than the direct face-to-face intermolecular coupling, that introduce a substantial energetic disorder in the polymer:PDI composites. Conventional device architectures with hole-collecting ITO/PEDOT:PSS bottom electrodes are compared with inverted device architectures bearing bottom electron-collecting electrodes of ITO/ZnO. In all cases the ZnO-functionalized devices surpass the performance of the conventional device analogues. X-ray photoelectron spectroscopy explains that in PEDOT: PSS-functionalized devices, the PDI component preferentially segregates closer to the hydrophilic PEDOT: PSS electrode, thus impeding the efficient charge extraction and limiting device photocurrent.

Role of the low-energy excited states in the radiolysis of aromatic liquids.

The contribution of the low-energy excited states to the overall product formation in the radiolysis of simple aromatic liquids--benzene, pyridine, toluene, and aniline--has been examined by comparison of product yields obtained in UV-photolysis and in γ-radiolysis. In photolysis, these electronic excited states were selectively populated using UV-light excitation sources with various energies. Yields of molecular hydrogen and of "dimers" (biphenyl, bibenzyl, dipyridyl for benzene, toluene, pyridine, respectively, and of ammonia and diphenylamine for aniline) have been determined, since they are the most abundant radiolytic products. Negligibly small production of molecular hydrogen in the UV-photolysis of aromatic liquids with excitation to energies of 4.88, 5.41, 5.79, and 6.70 eV and the lack of a scavenger effect suggest that this product originates from short-lived high-energy singlet states. A significant reduction in "dimer" radiation-chemical yields in the presence of scavengers such as anthracene or naphthalene indicates that the triplet excited states are important precursors to these products. The results for toluene and aniline suggest that efficient dissociation from the lowest-energy excited triplet state leads to noticeable "dimer" production. For benzene and pyridine, the lowest-energy triplet excited states are not likely to fragment into radicals because of the relatively large energy gap between the excited state level and corresponding bond dissociation energy. The "dimer" formation in the radiolysis of benzene and pyridine is likely to involve short-lived high-energy triplet states.

Ionization of amino-, thio- and hydroxy-naphtalenes via free (unhindered) electron transfer.

The electron transfer from various monosubstituted naphthyl derivatives (naphtols, NpOH; naphtylamines, NpNH2; and thionaphtols, NpSH) to parent n-BuCl radical cations was studied by means of pulse radiolysis. The experiments reveal the synchronous and direct formation of two types of transients: the metastable solute radical cation (NpXH(*+), X = heteroatom) and the corresponding heteroatom-containing radical (NpX(*)) in comparable amounts. This is explained in terms of the free (unhindered) electron transfer in nonpolar solvents, which is a bimolecular process reflecting femtosecond time scale events of intramolecular dynamic motions accompanied by significant changes of the electron distribution within the donor molecule.

Kinetic and energetic analysis of the free electron transfer.

In this paper, the bimolecular free (unhindered) electron transfer (FET) from various trityl-containing compounds to the solvent radical cations of n-BuCl is described. In good agreement with the previously studied cases, the FET involving trityl-derived compounds results in the formation of two different types of the radical cation, which undergo the subsequent fragmentation via two alternative reaction channels. This unusual effect is caused by the intramolecular rotational motion in the ground-state molecules around the arrow-marked bond Ar-//-X-CPh 3 (Ar = aromatic moiety; X = S, O, NH, CH 2), since such oscillations are directly connected with the electron distribution within the molecule. An unhindered electron jump from the donor trityl compound to the solvent radical cation, taking place in the subfemtosecond time range, generates the solute radical cation with the inherited geometry and the electron distribution of its precursor. Among the whole variety of produced radical cations, two extreme conformer states can be distinguished, namely, a planar and a twisted state. The planar type represents the structures with minimum energy, whereas the twisted type is destabilized by the increased value of the rotational barrier in the ionized state. The difference in the energetic profiles between planar and twisted radical cations plays a crucial role in their subsequent fragmentation. The planar radical cation follows the thermodynamically favored pathway generating ArX (*) and Ph 3C (+). A distinct part of the twisted radical cation dissociates faster than it relaxes into the more preferable planar conformation and, therefore, produces a thermodynamically unfavorable couple of products: ArX (+) and Ph 3C (*). This fragmentation channel is exclusively caused by FET. The undertaken quantum chemical calculations enable the judgment of the energetics of the different dissociation channels of the radical cations of the trityl derivatives.