1,720,958 research outputs found
Solvothermal synthesis of Sn<sub>3</sub>N<sub>4</sub> as a high capacity sodium-ion anode: theoretical and experimental study of its storage mechanism
No full textA new simple and scalable method to synthesise spinel-structured Sn3N4 has been developed using SnCl4 and LiNH2 precursors under solvothermal conditions. Nanocrystalline Sn3N4 with a crystallite size < 10 nm was produced and tested as anode material in sodium half cells, demonstrating a very high reversible (de-sodiation) capacity of ∼850 mA h g-1 measured over 50 cycles, the highest reported reversible capacity for a sodium anode apart from sodium itself. Ex situ X-ray absorption spectroscopy and X-ray diffraction show that the electrochemical reactions are reversible and that Sn3N4 is recovered upon re-oxidation. X-ray diffraction shows that the peaks associated with Sn3N4 reflections become narrower during discharge (reduction), evidencing that the smaller Sn3N4 particles are primarily involved in the electrochemical reactions, and broadening of the peaks is reversibly recovered upon oxidation. The analysis of the near edge X-ray absorption data (XANES) shows that the Sn oxidation state decreases during reduction and nearly recovers the initial value during oxidation. DFT calculations suggest that the insertion of Na into the Sn3N4 surface followed by substitution of tetrahedral Sn by Na is energetically favourable, and evidence of the removal of tetrahedral Sn from the spinel Sn3N4 structure is obtained from the analysis of the extended X-ray absorption fine-structure (EXAFS) measurements of reduced electrodes, which also show the recovery of the pristine structure at the end of oxidation. DFT also shows that Sn substitution by Na is only favourable at the Sn3N4 surface (not for bulk Sn3N4), in agreement with the electrochemical characterisation that shows that controlling the nanoparticle size is crucial to achieve full utilisation of Sn3N4 (and thus high capacity).</p
Dataset for: Solvothermal Synthesis of Sn3N4 as a High Capacity Sodium-Ion Anode: Theoretical and Experimental Study of its Storage Mechanism
No full textDataset supporting published paper:
Fitch, S, Cibin, G, Hepplestone, S, Garcia-Araez, N & Hector, AL 2020, 'Solvothermal Synthesis of Sn3N4 as a High Capacity Sodium-Ion Anode: Theoretical and Experimental Study of its Storage Mechanism' Journal of Materials Chemistry A.</span
Structuring Methods of Hybrid Perovskites for Solar Applications
Full text linkIn this thesis, structures and properties of hybrid perovskites are studied at the atomic scale, with a focus on their optoelectronic applications. Three-dimensional hybrid perovskites are mainly discussed in this work for solar photovoltaic cells and a two-dimensional hybrid perovskite for a photodetector in the last chapter of results. A solar cell made of methylammonium lead iodide (MAPI) or CHNHPbI has previously been measured to have excellent power conversion efficiency which was attributed to the material's band gap. This understanding of the relationship allows us to tune the band gap by mixing compositions so the theoretical efficiency of the solar cells can be optimised.
Here, we first investigate the structural, energetic and electronic properties of the potential candidates, MAPI, CHNHPbBr, CH(NH)PbI and CH(NH)PbBr (FAPB). The results suggest they can be present at various crystal phases, including narrow-band-gap cubic-like and wide-band-gap hexagonal phases. Yet, the hexagonal phase of these bulk perovskites is more favourable at room temperature so the higher band gaps limit the theoretical efficiency of the solar cells. %ranging from 1.55 to 1.85 eV.
Two methods of mixing are studied to improve the properties for solar applications, such as solid solution and superlattice.
In the solid solution, we take into account all studied 3D perovskites mentioned above as constituents and compared their hexagonal and cubic-like phases. The results show that while the intermixed cubic-like phases of all combinations remain to have band gaps within the range of those of the constituents, their structural stability improves, i.e. more energetically favourable than hexagonal structures at room temperature. Therefore, the solid solution is suggested to be used as an alternative to bulk perovskite to build the single-junction solar cell or used for the multi-junction solar cell to improve the measured efficiency.
In the superlattice, MAPI and FAPB are considered in various stacking layers. Both cubic and hexagonal phases of the heterostructures are examined in terms of structural, energetic and electronic properties. Promisingly, the cubic structures have shown improvement in band gaps and FAPB(2)/MAPI(1) even exhibits the optimal value of 1.34 eV that predicts the maximum theoretical efficiency of a single-junction solar cell. Moreover, these superlattices undergo a phase transition from hexagonal to cubic at various temperatures below the room temperature and the energy required to form a cubic interface is within the thermal excitation energy at room temperature. These findings conclude that superlattice structuring of hybrid perovskites will outcome the most efficient single-junction solar cell.
The layered two-dimensional perovskite studied as a photodetector is 4-fluoro-phenylethylammonium lead iodide. The bandgap is shown in good agreement with the experiment measurement by collaborators. We predict the Schottky barrier of the electron at the interface between the perovskite and Au contact. Several mechanisms are studied to explain the measured absorption peaks within the band gap and iodine vacancy in the perovskite shows a similar absorption peak.
The extent of this work has demonstrated two methods of structuring the mixed hybrid perovskites to examine the band gap and structural stability for solar cell applications. Both studies have shown the stability improves but the investigation of superlattice also successfully has predicted a mean to achieve the optimal band gap. Moreover, we have also studied the properties of the two-dimensional hybrid perovskite to support its experimental measurements in a photodetector device
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Instability of O3 perovskite surfaces induced by vacancy formation (=Ca,Sr,Ba; =Ti,Zr,Sn)
No full textThis work presents a first principles study of the (001) surface energetics of nine oxide perovskites, with a focus on the role of surface vacancies in determining termination stability. Additionally, investigation into the behaviour of vacancies as a function of depth from the surface in these perovskites, ABO (A=Ca, Sr, Ba; B=Ti, Zr, Sn), is carried out, and results are compared to formation of the vacancies in bulk. Combining results from these investigations reveals a general trend for all nine perovskites - the undefected AO surface is more energetically favourable to form than the BO2 surface. This dominance of the AO over the BO2 surface is further enforced by the phase diagrams of perovskite surfaces. However, A-site vacancies at the AO surface are far more favourable (1-2 eV lower in energy) than B-site vacancies at the BO2 surface. Charged vacancies only drive this further under oxygen-rich conditions, showing a smaller range of stability for the AO than the BO2 surface. These results indicates that, whilst the AO surface is easier to form, the BO2 surface will display better long term stability, making it more suitable for use in potential applications. This study furthers the understanding of oxide perovskite (001)-terminated surface stability, which will aid in surface growth and manufacturing.The authors thank the Leverhulme for funding this research via Grant No. RPG-2021-086. The work provided by N.T.T. was supported in part by the Government Office for Science and the Royal Academy of Engineering under the UK Intelligence Community Postdoctoral Research Fellowships scheme (Grant No. ICRF2425-8-148). Via our membership of the UK's HEC Materials Chemistry Consortium, which is funded by EPSRC (Grant No. EP/R029431), this work used the ARCHER2 UK National Supercomputing Service [94]. The authors would like to acknowledge the use of the University of Exeter High-Performance Computing (HPC) facility in carrying out this work. Data retrieved from the Materials Project for CaTiO3 (mp-4019), SrTiO3 (mp-5229), BaTiO3 (mp-5986), CaZrO3 (mp-4571), SrZrO3 (mp-4387), BaZrO3 (mp-3834), CaSnO3 (mp-4438), SrSnO3 (mp-2879), and BaSnO3 (mp-3163) from database version v2022.10.28Peer reviewe
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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