I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[8952]
Abstract: The (011) termination of rutile TiO2 is reported to be particularly effective for photocatalysis. Here the structure of the interface formed between this substrate and water is revealed using surface X-ray diffraction. While the TiO2(011) surface exhibits a (2x1) reconstruction in ultra-high vacuum, this is lifted in the presence of a multilayer of water at room temperature. This change is driven by the formation of Ti-OH at the interface, which has a bond distance of 1.93 ± 0.02 Å. The experimental solution is in good agreement with density functional theory and first principles molecular dynamics calculations. These results point to the important differences that can arise between the structure of oxide surfaces in ultra-high vacuum and technical environments and will ultimately lead to an atomistic understanding of the photocatalytic process of water splitting on TiO2 surfaces.
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May 2019
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I07-Surface & interface diffraction
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Abstract: Surface X-ray diffraction (SXRD) has been employed to quantitatively determine the geometric structure of an X-ray-induced superhydrophilic rutile-TiO2(110)(1×1) surface. A scatterer, assumed to be oxygen, is found at a distance of 1.90 ± 0.02 Å above the fivefold co-ordinated surface Ti atom, indicating surface hydroxylation. Two more oxygen atoms, situated further from the substrate, are also included to achieve the optimal agreement between experimental and simulated diffraction data. It is concluded that these latter scatterers are from water molecules, surface localised through hydrogen bonding. Comparing this interfacial structure with previous studies suggests that the superhydophilicity of titania is most likely to be a result of the depletion of surface carbon contamination, coupled with extensive surface hydroxylation.
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Feb 2019
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I07-Surface & interface diffraction
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Abstract: We report a simple reduction of molybdenum oxide (β-MoO3) grown on reconstructed Si(100) by thermal annealing in ultra-high vacuum (UHV) using molecular beam epitaxy (MBE). By increasing the substrate temperature during deposition or the annealing temperature after growth, the morphologies of as-deposited structures were found to vary from nanoribbons (NRs) of β-MoO3 to nanoparticles (NPs) of Mo. The change in morphologies have been associated with a structural transition from β-MoO3 to MoO2 at 400°C and MoO2 to Mo at 750°C. The in-situ X-ray photoelectron spectroscopy (XPS) measurements revealed a shift of the Mo 3d peaks towards lower binding energies, representing the reduction in Mo oxidation states until a pure Mo 3d peak at 750°C was observed. The ex-situ KPFM measurements showed a decrease in the local work function (Φ) (from ≈ 5.27 ± 0.05 eV to ≈ 4.83 ± 0.05 eV) with increasing substrate temperature. A gradual reduction of the band gap from ≈ 3.32 eV for β-MoO3 NRs to zero band gap for Mo NPs is also observed during the annealing up to 750°C.
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Jan 2019
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[12083, 6663]
Open Access
Abstract: Liquid layering at heterogeneous solid/liquid interfaces is a general phenomenon, which provides structural templates for nucleation of crystalline phases on potent nucleants. However, its efficacy near poor nucleants is incompletely understood. Here we use a combination of X-ray crystal truncation rod analysis and ab initio molecular dynamics to probe the pre-nucleation liquid layering at the sapphire–aluminium solid/liquid interface. At the sapphire side, a ~1.6 aluminium-terminated structure develops, and at the liquid side, two pre-nucleation layers emerge at 950 K. No more pre-nucleation layer forms with decreasing temperature indicating that nucleation of crystalline aluminium through layer-by-layer atomic adsorption of liquid atoms is not favoured. Instead, the appearance of stochastically-formed nuclei near the substrate is supported by our experiments. Nucleation on poor nucleants is dominated by the stochastic nucleation events which are substantially influenced by the pre-nucleation layers that determine the surface structure in contact with the nuclei.
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Jan 2019
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I07-Surface & interface diffraction
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Open Access
Abstract: Understanding how diffusion takes place within nanocrystals is of great importance for their stability and for controlling their synthesis. In this study, we used the strain sensitivity of Bragg coherent diffraction imaging (BCDI) to study the diffusion of iron into individual gold nanocrystals in situ at elevated temperatures. The BCDI experiments were performed at the I-07 beamline at Diamond Light Source, UK. The diffraction pattern of individual gold nanocrystals was measured around the (11-1) Bragg peak of gold before and after iron deposition as a function of temperature and time. Phase retrieval algorithms were used to obtain real space reconstructions of the nanocrystals from their measured diffraction patterns. Alloying of iron with gold at sample temperatures of 300°C-500°C and dealloying of iron from gold at 600°C were observed. The volume of the alloyed region in the nanocrystals was found to increase with the dose of iron. However, no significant time dependence was observed for the structure following each iron deposition, suggesting that the samples reached equilibrium relatively quickly. The resulting phase distribution within the gold nanocrystals after the iron depositions suggests a contraction due to diffusion of iron. Our results show that BCDI is a useful technique for studying diffusion in three dimensions (3D) and alloying behaviour in individual crystalline grains.
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Oct 2018
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I07-Surface & interface diffraction
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Linus
Pithan
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Dmitrii
Nabok
,
Caterina
Cocchi
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Paul
Beyer
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Giuliano
Duva
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Joseph
Simbrunner
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Jonathan
Rawle
,
Chris
Nicklin
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Peter
Schäfer
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Claudia
Draxl
,
Frank
Schreiber
,
Stefan
Kowarik
Diamond Proposal Number(s):
[12895]
Abstract: We present a combined experimental and theoretical study to solve the unit-cell and molecular arrangement of the tetracene thin film (TF) phase. TF phases, also known as substrate induced phases (SIPs), are polymorphs that exist at interfaces and decisively impact the functionality of organic thin films, e.g., in a transistor channel, but also change the optical spectra due to the different molecular packing. As SIPs only exist in textured ultrathin films, their structure determination remains challenging compared to bulk materials. Here, we use grazing incidence X-ray diffraction and atomistic simulations to extract the TF unit-cell parameters of tetracene together with the atomic positions within the unit-cell.
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Oct 2018
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[13089]
Abstract: Single crystal [0 0 1], [1 1 0], and [1 1 1]-oriented UO2 thin films were utilised to investigate the orientational dependence of radiolytic dissolution. The films were exposed to water in the presence of a synchrotron x-ray source to induce dissolution, with X-ray Reflectivity (XRR) used to observe changes in morphology as a function of exposure time. The [0 0 1] and [1 1 0]-oriented films were found to corrode at comparable rates, however, the [1 1 1] film was significantly more corrosion resistant, passivating after the initial 90 s dissolution period. This result shows the orientational dependence of UO2 dissolution, which may have important consequences for theoretical dissolution models.
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Sep 2018
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I07-Surface & interface diffraction
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Immad M.
Nadeem
,
Jon P. W.
Treacy
,
Sencer
Selcuk
,
Xavier
Torrelles
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Hadeel
Hussain
,
Axel
Wilson
,
David C.
Grinter
,
Gregory
Cabailh
,
Oier
Bikondoa
,
Chris
Nicklin
,
Annabella
Selloni
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Jorg
Zegenhagen
,
Robert
Lindsay
,
Geoff
Thornton
Diamond Proposal Number(s):
[8634, 11345]
Abstract: Elucidating the structure of the interface between natural (reduced) anatase TiO2 (101) and water is an essential step towards understanding the associated photo-assisted water splitting mechanism. Here we present surface X-ray diffraction results for the room temperature interface with ultra-thin and bulk water, which we explain by reference to density functional theory calculations. We find that both interfaces contain a 25:75 mixture of molecular H2O and terminal OH bound to titanium atoms along with bridging OH species in the contact layer. This is in complete contrast to the inert character of room temperature anatase TiO2 (101) in ultra-high vacuum. A key difference between the ultra-thin and bulk water interfaces is that in the latter, water in the second layer is also ordered. These molecules are hydrogen bonded to the contact layer, modifying the bond angles.
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May 2018
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[15207]
Open Access
Abstract: We report an in-situ study of stacking fault evolution in C60 thin films using grazing-incidence x-ray scattering. A Williamson-Hall analysis of the main scattering features during growth of a 15 nm film on glass indicates lattice strain as high as 6% in the first 5 nm of the film, with a decrease to 2% beyond 8 nm thickness. Deformation stacking faults along the {220} plane are found to occur with 68% probability and closely linked to the formation of a nanocrystalline powder-like film. Our findings, which capture monolayer-resolution growth, are consistent with previous work on crystalline and powder C60 films, and provide a crystallographic context for the real-time study of organic semiconductor thin films.
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Dec 2017
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[14220, 15207]
Open Access
Abstract: A sample environment to enable real-time X-ray scattering measurements to be recorded during the growth of materials by thermal evaporation in vacuum is presented. The in situ capabilities include studying microstructure development with time or during exposure to different environmental conditions, such as temperature and gas pressure. The chamber provides internal slits and a beam stop, to reduce the background scattering from the X-rays passing through the entrance and exit windows, together with highly controllable flux rates of the evaporants. Initial experiments demonstrate some of the possibilities by monitoring the growth of bathophenanthroline (BPhen), a common molecule used in organic solar cells and organic light emitting diodes, including the development of the microstructure with time and depth within the film. The results show how BPhen nanocrystal structures coarsen at room temperature under vacuum, highlighting the importance of using real time measurements to understand the as-deposited pristine film structure and its development with time. More generally, this sample environment is versatile and can be used for investigation of structure-property relationships in a wide range of vacuum deposited materials and their applications in, for example, optoelectronic devices and energy storage.
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Oct 2017
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