Using inelastic neutron scattering spectroscopy to probe CO2 binding in grafted aminosilanes.

While a range of in situ characterisation techniques are available to probe CO2 adsorption processes, inelastic neutron scattering is scarcely used, primarily due to the reliance on hydrogeneous modes. Materials capable of adsorbing CO2, such as solid supported-amines contain a range of C-H and N-H species, which can be probed to explore the adsorption of CO2. Here we show the benefits of using inelastic neutron spectroscopy to probe CO2 adsorption with solid supported-amines, and the complementarity that can be achieved using different world-leading spectrometers.

Systematic in situ hydration neutron reflectometry study on Nafion thin films.

Reported herein is a neutron reflectometry (NR) study on hydrated Nafion thin films (∼30 nm) on a silicon substrate with native oxide. The Nafion morphology is investigated systematically across the whole relative humidity range using both H2O and D2O vapours to enable a comparative study. By utilising this systematic approach two key results have been obtained. The first is that by leveraging the strong positive scattering signal from the D2O vapour, a complete and systematic water adsorption isotherm (Type II) for a Nafion thin film is produced. Utilising the slight negative scattering signal of the H2O enabled the quantification of the hydration dependent evolution of the formation of Nafion/water lamellae near the substrate surface. The number of lamellae layers increases continuously with hydration, and does not form abruptly. We also report the effects of swelling on the thin films across the relative humidity ranges. The work reported should prove useful in quantifying other hydration dependent properties of Nafion thin films such as conductivity and understanding Nafion/semiconductor based devices, as well as showcasing a NR methodology for other hydrophilic polymers.

The low energy phonon modes of the hydrogenated and deuterated π-conjugated system 7,7,8,8-tetracyanoquinodimethane: an inelastic neutron scattering study.

The organic acceptor, 7,7,8,8-tetracyanoquinodimethane (TCNQ), has been the subject of much research over the past few decades. Due to the π-conjugation of the quinodal structure, TCNQ as well as TCNQ based charge transfer compounds, exhibit some remarkable conducting properties. We present a study of the neutral TCNQ using inelastic neutron spectroscopy (INS) and show the first temperature dependent INS data collected on TCNQ allowing us to probe the temperature dependence of the low energy vibrational states, which have been shown to have an effect on charge delocalisation. Computational calculations have been used to help understand the data and the combination with the INS allows us to understand the phonon states. A complimentary experiment on deuterated TCNQ was also performed that allows a brief comparison of the isotopic substitution. This work utilises novel techniques to aid the understanding of the bulk properties of TCNQ in its neutral state that can be fed into work on other TCNQ based materials, including the exploration of deuteration as a technique to tune the properties of the parent compound.

In situ illumination with inelastic neutron scattering: a study of the photochromic material cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (CMTE).

For the first time we successfully demonstrate in situ illumination of a photochromic material during an inelastic neutron scattering (INS) experiment at cryogenic temperatures. cis-1,2-Dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (CMTE) is a well-studied and commercially available photochromic compound that undergoes a cyclisation reaction when exposed to light with a wavelength of around 400 nm. CMTE was therefore used as a proof of concept material to demonstrate the new sample environment used on the TOSCA indirect geometry spectrometer. When combined with simulated INS spectra from DFT calculations, the resulting difference spectra were interrogated to obtain insight into how the photoisomerisation affects the vibrational modes of the material. It was found that rigidification of the molecule after illumination, caused by the cyclisation of the central six-membered ring in CMTE, led to a blue-shifting in the methyl group torsion modes as well as methyl group scissor modes.

Structure and Dynamics of the Superprotonic Conductor Caesium Hydrogen Sulfate, CsHSO4.

We have investigated caesium hydrogen sulfate, CsHSO4, in all three of its ambient pressure phases by total scattering neutron diffraction, inelastic neutron scattering (INS) and Raman spectroscopies and periodic density functional theory calculations. Above 140 °C, CsHSO4, undergoes a phase transition to a superprotonic conductor that has potential application in intermediate temperature fuel cells. Total scattering neutron diffraction data clearly show that all the existing structures of this phase are unable to describe the local structure, because they have either partial occupancies of the atoms and/or non-physical O-H distances. Knowledge of the local structure is crucial because it is this that determines the conduction mechanism. Starting from one of the previous models, we have generated a new structure that has no partial occupancies and reasonable O-H distances. After geometry optimisation, the calculated radial distribution function is in reasonable agreement with the experimental data, as are the calculated and observed INS and Raman spectra. This work is particularly notable in that we have measured INS spectra in the O-H stretch region above room temperature, which is extremely rare. The INS spectra have the enormous advantage that the electrical anharmonicity that complicates the infrared spectra is absent and the stretch modes are plainly seen.

Neutron Spectroscopy: An Under-Utilised Tool for Organic Electronics Research?

Neutron scattering is a well-established technique that has proven to be an invaluable tool in myriad fields of chemical and physical research. Neutrons offer unique ways to study in situ or operando functional materials due to their highly penetrating nature and specific interactions with the nuclei of different isotopes. While some neutron scattering techniques, such as neutron diffraction (ND), neutron reflectometry (NR), and small-angle neutron scattering (SANS), have already been heavily adopted by the scientific community for use in the research of organic electronics, there are a number of techniques that are far less widely used: spectroscopic neutron scattering. This article aims to highlight these "under-utilised" techniques, to emphasise their potential use within the field of organic electronics, and to increase awareness of their utility among new research communities.

The binding and fluorescence quenching efficiency of nitroaromatic (explosive) vapors in fluorescent carbazole dendrimer thin films.

We present a study on three generations of fluorescent carbazole dendrimers that exhibit strong binding with nitroaromatic compounds accompanied by photoluminescence (PL) quenching, making them attractive sensing materials for the detection of explosives such as 2,4,6-trinitrotoluene (TNT). The absorption and release of vapors of the (deuterated) TNT analogue 4-nitrotoluene (pNT) from thin films of the dendrimers were studied with a combination of time-correlated neutron reflectometry and PL spectroscopy. When saturated with pNT the PL of the films was fully quenched and could not be recovered with flowing nitrogen at room temperature but only upon heating to 40-80 °C. Although the majority of the absorbed pNT could be removed with this method the recovered films were found to still contain a residual pNT concentration of ~0.1 molecules per cubic nanometer. However, the proportion of the PL recovered increased with generation with the third generation dendrimer exhibiting close to full recovery despite the presence of residual pNT. This result is attributed to a combination of two effects. First, the dendrimer films present a range of binding sites for nitroaromatic molecules with the stronger binding sites surviving the thermal recovery process. Second, there is a large decrease of the exciton diffusion coefficient with dendrimer generation, preventing migration of the excitation to the remaining bound pNT.

Vibrational spectra of neutral and doped oligothiophenes and polythiophene.

We have measured the infrared, Raman and inelastic neutron scattering (INS) spectra of a series of oligothiophenes (bithiophene, terthiophene, quarterthiophene, sexithiophene and octithiophene) and polythiophene, both pristine and after doping with iodine. The spectra of the pristine (i.e. neutral) systems show a rapid convergence towards the spectrum of polythiophene, such that the spectra of sexithiophene and octithiophene are almost indistinguishable from that of polythiophene. The spectra, in combination with periodic density functional theory calculations, have also provided the first complete assignment of polythiophene. In contrast to the infrared and Raman spectra that show dramatic changes on doping, the INS spectra show only small changes. Isolated molecule DFT calculations show that the molecular structures are not greatly modified on doping and since the INS spectrum largely depends on the structure, this does not change much. In contrast, as shown by others, the electronic structure is greatly modified and this accounts for the major changes in the infrared and Raman spectra.

In situ real-time neutron imaging of gaseous H2 adsorption and D2 exchange on carbon-supported Pd catalysts.

We use in situ neutron imaging to observe the adsorption/absorption of hydrogen within a packed catalyst bed of a Pd/C catalyst at a spatial and temporal resolution of ∼430 µm and a ∼9 s respectively. Additionally, the H2/D2 exchange process across the catalyst bed is followed in real time.

Understanding the ZIF-L to ZIF-8 transformation from fundamentals to fully costed kilogram-scale production.

The production of MOFs at large scale in a sustainable way is key if these materials are to be exploited for their promised widespread application. Much of the published literature has focused on demonstrations of preparation routes using difficult or expensive methodologies to scale. One such MOF is nano-zeolitic imidazolate framework-8 (ZIF-8) - a material of interest for a range of possible applications. Work presented here shows how the synthesis of ZIF-8 can be tracked by a range of methods including X-ray diffraction, thermo gravimetric analysis and inelastic neutron scattering - which offer the prospect of in-line monitoring of the synthesis reaction. Herein we disclose how the production of nano-ZIF-8 can be conducted at scale using the intermediate phase ZIF-L. By understanding the economics and demonstrating the production of 1 kg of nano-ZIF-8 at pilot scale we have shown how this once difficult to make material can be produced to specification in a scalable and cost-efficient fashion.

Control of zeolite microenvironment for propene synthesis from methanol.

Optimising the balance between propene selectivity, propene/ethene ratio and catalytic stability and unravelling the explicit mechanism on formation of the first carbon-carbon bond are challenging goals of great importance in state-of-the-art methanol-to-olefin (MTO) research. We report a strategy to finely control the nature of active sites within the pores of commercial MFI-zeolites by incorporating tantalum(V) and aluminium(III) centres into the framework. The resultant TaAlS-1 zeolite exhibits simultaneously remarkable propene selectivity (51%), propene/ethene ratio (8.3) and catalytic stability (>50 h) at full methanol conversion. In situ synchrotron X-ray powder diffraction, X-ray absorption spectroscopy and inelastic neutron scattering coupled with DFT calculations reveal that the first carbon-carbon bond is formed between an activated methanol molecule and a trimethyloxonium intermediate. The unprecedented cooperativity between tantalum(V) and Brønsted acid sites creates an optimal microenvironment for efficient conversion of methanol and thus greatly promotes the application of zeolites in the sustainable manufacturing of light olefins.

Solid-state dendrimer sensors: probing the diffusion of an explosive analogue using neutron reflectometry.

Determining how analytes are sequestered into thin films is important for solid-state sensors that detect the presence of the analyte by oxidative luminescence quenching. We show that thin (230 +/- 30 A) and thick (750 +/- 50 A) films of a first-generation dendrimer comprised of 2-ethylhexyloxy surface groups, biphenyl-based dendrons, and a 9,9,9',9'-tetra-n-propyl-2,2'-bifluorene core, can rapidly and reversibly detect p-nitrotoluene by oxidative luminescence quenching. For both the thin and thick films the photoluminescence (PL) is quenched by p-nitrotoluene by approximately 90% in 4 s, which is much faster than that reported for luminescent polymer films. Combined PL and neutron reflectometry measurements on pristine and analyte-saturated films gave important insight into the analyte adsorption process. It was found that during the adsorption process the films swelled, being on average 4% thicker for both the thin and thick dendrimer films. At the same time the PL was completely quenched. On removal of the analyte the films returned to their original thickness and scattering length density, and the PL was restored, showing that the sensing process was fully reversible.