© 2018 Elsevier B.V. Converting CO2 to fuels is required to enable the production of sustainable fuels and to contribute to alleviating CO2 emissions. In considering conversion of CO2, the initial step of adsorption and activation by the catalyst is crucial. In addressing this difficult problem, we have examined how nanoclusters of reducible metal oxides supported on TiO2 can promote CO2 activation. In this paper we present density functional theory (DFT) simulations of CO2 activation on heterostructures composed of clean or hydroxylated extended rutile and anatase TiO2 surfaces modified with chromia nanoclusters. The heterostructures show non-bulk Cr and O sites in the nanoclusters and an upshifted valence band edge that is dominated by Cr 3d- O 2p interactions. We show that the supported chromia nanoclusters can adsorb and activate CO2 and that activation of CO2 is promoted whether the TiO2 support is oxidised or hydroxylated. Reduced heterostructures, formed by removal of oxygen from the chromia nanocluster, also promote CO2 activation. In the strong CO2 adsorption modes, the molecule bends giving O–C–O angles of 127 - 132° and elongation of C–O distances up to 1.30 Å; no carbonates are formed. The electronic properties show a strong CO2–Cr–O interaction that drives the interaction of CO2 with the nanocluster and induces the structural distortions. These results highlight that a metal oxide support modified with reducible metal oxide nanoclusters can activate CO2, thus helping to overcome difficulties associated with the difficult first step in CO2 conversion.
Fronzi, M, Assadi, MHN & Hanaor, DAH 2019, 'Theoretical insights into the hydrophobicity of low index CeO2 surfaces', APPLIED SURFACE SCIENCE, vol. 478, pp. 68-74.View/Download from: UTS OPUS or Publisher's site
Fronzi, M, Tawfik, SA, Stampfl, C & Ford, MJ 2018, 'Magnetic properties of stoichiometric and defective Co9S8.', Physical chemistry chemical physics : PCCP, vol. 20, no. 4, pp. 2356-2362.View/Download from: UTS OPUS or Publisher's site
In this paper, we present a detailed study of the stoichiometric and reduced Co9S8 pentlandite magnetic properties, based on density functional theory. We analyze both its geometry and electronic properties and show that only by the inclusion of the Hubbard term it is possible to correctly describe d-d splitting, which is necessary to accurately characterize the Co9S8 spin configuration and its antiferromagnetic nature. We also analyze the effect of sulfur vacancies and predict the formation of ferromagnetic clusters that give local ferromagnetic character to non-stoichiometric Co9S8, which may explain the contradictory experimental results reported in the literature.
Gutiérrez Moreno, JJ, Fronzi, M, Lovera, P, O'Riordan, A & Nolan, M 2018, 'Stability of Adsorbed Water on TiO2-TiN Interfaces. A First-Principles and Ab Initio Thermodynamics Investigation', Journal of Physical Chemistry C, vol. 122, no. 27, pp. 15395-15408.View/Download from: UTS OPUS or Publisher's site
© 2018 American Chemical Society. Titanium nitride (TiN) surfaces can oxidize, and the growth of a TiOx layer on the surface along with the likely presence of water in the surrounding environment can modify the properties of this widely used coating material. The present density functional theory study, including Hubbard + U correction (DFT+U), investigates the stability of adsorbed water at TiO2-TiN interfaces with different defects that serve as a model for an oxide layer grown on a TiN surface. Surface free energy calculations show the stability of a perfect TiN-TiO2 interface at regular O pressures, while oxygen vacancy-rich TiO1.88-TiN is more favorable at reducing conditions. An isolated water is preferentially adsorbed dissociatively at perfect and oxygen-defective interfaces, while molecular adsorption is more stable at higher coverages. The adsorption energy is stronger at the oxygen-defective interfaces which arise from the high concentration of reduced Ti3+ and strong interfacial atomic relaxations. Ab initio atomistic thermodynamics show that water will be present at high coverage on TiO2-TiN interfaces at ambient conditions, and the pristine interface is only stable at very low pressure of O and H2O. The results of these DFT+U simulations are important for the fundamental understanding of wettability of interfacial systems involving metal oxides.
Fronzi, M, Assadi, MHN & Ford, MJ 2018, 'Ab Initio Investigation of Water Adsorption and Hydrogen Evolution on Co9S8and Co3S4Low-Index Surfaces', ACS Omega, vol. 3, no. 9, pp. 12215-12228.View/Download from: UTS OPUS or Publisher's site
Copyright © 2018 American Chemical Society. We used density functional theory approach, with the inclusion of a semiempirical dispersion potential to take into account van der Waals interactions, to investigate the water adsorption and dissociation on cobalt sulfide Co9S8and Co3S4(100) surfaces. We first determined the nanocrystal shape and selected representative surfaces to analyze. We then calculated water adsorption and dissociation energies, as well as hydrogen and oxygen adsorption energies, and we found that sulfur vacancies on Co9S8(100) surface enhance the catalytic activity toward water dissociation by raising the energy level of unhybridized Co 3d states closer to the Fermi level. Sulfur vacancies, however, do not have a significant impact on the energetics of Co3S4(100) surface.
Assadi, MHN, Fronzi, M, Ford, M & Shigeta, Y 2018, 'High-performance Na ion cathodes based on the ubiquitous and reversible O redox reaction', Journal of Materials Chemistry A, vol. 6, no. 47, pp. 24120-24127.View/Download from: UTS OPUS or Publisher's site
© 2018 The Royal Society of Chemistry. Utilising reversible oxygen redox in Na and transition metal oxides offers unprecedented opportunities for the design of high voltage, high capacity and affordable cathodes for application in rechargeable Na-ion batteries. Through a judicious materials search and theoretical investigations, we identified new compounds with excellent energy storage properties that rely on oxygen states for charge compensation during the redox reaction. According to our predictions, Na2-xMoO4 demonstrates a voltage of 4.743 V and an energy density of 617.3 W h kg-1. These values exceed the performance of most commercialised Na-ion cathode materials. Furthermore, both Na4-xZr5O12, demonstrating a voltage of 3.583 V, and Na1-xPd2PO3, demonstrating a voltage of 2.630 V, exhibit a meagre absolute volume change of 1% during the sodiation/desodiation process. Because of this minor volume change, these compounds are suitable for all-solid-state battery applications. An examination of the electronic structures of these compounds reveals that O states are always present at the top of the valence band regardless of the presence of 4d transition metal species or their oxidation states. This feature is attributed to the exceedingly substantial 4d-2p hybridisation over the entire valence band which also prevents the bonding of oxidised O ions in the desodiated compounds, thus preventing irreversible oxygen loss.
Bishop, J, Fronzi, M, Elbadawi, C, Nikam, V, Pritchard, J, Fröch, JE, Duong, NMH, Ford, MJ, Aharonovich, I, Lobo, CJ & Toth, M 2018, 'Deterministic Nanopatterning of Diamond Using Electron Beams.', ACS nano, vol. 12, no. 3, pp. 2873-2882.View/Download from: UTS OPUS or Publisher's site
Diamond is an ideal material for a broad range of current and emerging applications in tribology, quantum photonics, high-power electronics, and sensing. However, top-down processing is very challenging due to its extreme chemical and physical properties. Gas-mediated electron beam-induced etching (EBIE) has recently emerged as a minimally invasive, facile means to dry etch and pattern diamond at the nanoscale using oxidizing precursor gases such as O2 and H2O. Here we explain the roles of oxygen and hydrogen in the etch process and show that oxygen gives rise to rapid, isotropic etching, while the addition of hydrogen gives rise to anisotropic etching and the formation of topographic surface patterns. We identify the etch reaction pathways and show that the anisotropy is caused by preferential passivation of specific crystal planes. The anisotropy can be controlled by the partial pressure of hydrogen and by using a remote RF plasma source to radicalize the precursor gas. It can be used to manipulate the geometries of topographic surface patterns as well as nano- and microstructures fabricated by EBIE. Our findings constitute a comprehensive explanation of the anisotropic etch process and advance present understanding of electron-surface interactions.
Bat-Erdene, M, Batmunkh, M, Tawfik, SA, Fronzi, M, Ford, MJ, Shearer, CJ, Yu, LP, Dadkhah, M, Gascooke, JR, Gibson, CT & Shapter, JG 2017, 'Efficiency Enhancement of Single-Walled Carbon Nanotube-Silicon Heterojunction Solar Cells Using Microwave-Exfoliated Few-Layer Black Phosphorus', Advanced Functional Materials, vol. 27, no. 48.View/Download from: UTS OPUS or Publisher's site
© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Carbon nanotube-silicon (CNT-Si)-based heterojunction solar cells (HJSCs) are a promising photovoltaic (PV) system. Herein, few-layer black phosphorus (FL-BP) sheets are produced in N-methyl-2-pyrrolidone (NMP) using microwave-assisted liquid-phase exfoliation and introduced into the CNTs-Si-based HJSCs for the first time. The NMP-based FL-BP sheets remain stable after mixing with aqueous CNT dispersion for device fabrication. Due to their unique 2D structure and p-type dominated conduction, the FL-BP/NMP incorporated CNT-Si devices show an impressive improvement in the power conversion efficiency from 7.52% (control CNT-Si cell) to 9.37%. Our density-functional theory calculation reveals that lowest unoccupied molecular orbital (LUMO) of FL-BP is higher in energy than that of single-walled CNT. Therefore, we observed a reduction in the orbitals localized on FL-BP upon highest occupied molecular orbital to LUMO transition, which corresponds to an improved charge transport. This study opens a new avenue in utilizing 2D phosphorene nanosheets for next-generation PVs.
Fronzi, M & Nolan, M 2017, 'First-principles analysis of the stability of water on oxidised and reduced CuO(111) surfaces', RSC Advances, vol. 7, no. 89, pp. 56721-56731.View/Download from: UTS OPUS or Publisher's site
© 2017 The Royal Society of Chemistry. We use first-principles density functional theory calculations including the Hubbard + U correction (PBE + U) on Cu-3d states to investigate the interaction of water with a CuO(111) surface. We compute adsorption energies and the stability of different water coverages, with a particular focus on the interaction of water with oxygen vacancy sites, and how vacancy stabilization occurs. We study the energetics, geometry and electronic structure of relevant configurations, finding that there are only small changes to the local geometry around the water adsorption site(s) and the electronic properties. The inclusion of van der Waals interactions has no significant impact on the stability of water on CuO(111). We extend the analysis to include realistic environmental conditions within the ab initio atomistic thermodynamics framework, which allows us to assess the stability of the water/copper-oxide system as a function of ambient conditions, and focus on three important surface processes: water adsorption/desorption on the stoichiometric surface, conditions for dissociation, and oxygen vacancy stabilization.
Fronzi, M & Nolan, M 2017, 'Surface modification of perfect and hydroxylated TiO2rutile (110) and anatase (101) with chromium oxide nanoclusters', ACS Omega, vol. 2, no. 10, pp. 6795-6808.View/Download from: UTS OPUS or Publisher's site
© 2017 American Chemical Society. We use first-principles density functional theory calculations to analyze the effect of chromia nanocluster modification on TiO 2 rutile (110) and anatase (101) surfaces, in which both dry/perfect and wet/hydroxylated TiO 2 surfaces are considered. We show that the adsorption of chromia nanoclusters on both surfaces is favorable and results in a reduction of the energy gap due to a valence band upshift. A simple model of the photoexcited state confirms this red shift and shows that photoexcited electrons and holes will localize on the chromia nanocluster. The oxidation states of the cations show that Ti 3+ , Cr 4+ , and Cr 2+ (with no Cr6+) can be present. To probe potential reactivity, the energy of oxygen vacancy formation is shown to be significantly reduced compared to that of pure TiO 2 and chromia. Finally, we show that inclusion of water on the TiO 2 surface, to begin inclusion of environment effects, has no notable effect on the energy gap or oxygen vacancy formation. These results help us to understand earlier experimental work on chromia-modified anatase TiO 2 and demonstrate that chromia-modified TiO 2 presents an interesting composite system for photocatalysis.
Tawfik, SA, Ali, S, Fronzi, M, Kianinia, M, Tran, TT, Stampfl, C, Aharonovich, I, Toth, M & Ford, MJ 2017, 'First-principles investigation of quantum emission from hBN defects.', Nanoscale, vol. 9, no. 36, pp. 13575-13582.View/Download from: UTS OPUS or Publisher's site
Hexagonal boron nitride (hBN) has recently emerged as a fascinating platform for room-temperature quantum photonics due to the discovery of robust visible light single-photon emitters. In order to utilize these emitters, it is necessary to have a clear understanding of their atomic structure and the associated excitation processes that give rise to this single photon emission. Here, we performed density-functional theory (DFT) and constrained DFT calculations for a range of hBN point defects in order to identify potential emission candidates. By applying a number of criteria on the electronic structure of the ground state and the atomic structure of the excited states of the considered defects, and then calculating the Huang-Rhys (HR) factor, we found that the CBVN defect, in which a carbon atom substitutes a boron atom and the opposite nitrogen atom is removed, is a potential emission source with a HR factor of 1.66, in good agreement with the experimental HR factor. We calculated the photoluminescence (PL) line shape for this defect and found that it reproduces a number of key features in the experimental PL lineshape.
Nolan, M, Iwaszuk, A, Lucid, AK, Carey, JJ & Fronzi, M 2016, 'Design of Novel Visible Light Active Photocatalyst Materials: Surface Modified TiO2', ADVANCED MATERIALS, vol. 28, no. 27, pp. 5425-5446.View/Download from: Publisher's site
Fronzi, M, Iwaszuk, A, Lucid, A & Nolan, M 2016, 'Metal oxide nanocluster-modified TiO2 as solar activated photocatalyst materials', JOURNAL OF PHYSICS-CONDENSED MATTER, vol. 28, no. 7.View/Download from: Publisher's site
Fronzi, M, Daly, W & Nolan, M 2016, 'Reactivity of metal oxide nanocluster modified rutile and anatase TiO2: Oxygen vacancy formation and CO2 interaction', APPLIED CATALYSIS A-GENERAL, vol. 521, pp. 240-249.View/Download from: Publisher's site
Fronzi, M, Tateyama, Y, Marzari, N, Nolan, M & Traversa, E 2016, 'First-principles molecular dynamics simulations of proton diffusion in cubic BaZrO 3 perovskite under strain conditions', Materials for Renewable and Sustainable Energy, vol. 5, no. 4.View/Download from: Publisher's site
© 2016, The Author(s). First-principles molecular dynamics simulations have been employed to analyse the proton diffusion in cubic BaZrO3 perovskite at 1300 K. A non-linear effect on the proton diffusion coefficient arising from an applied isometric strain up to 2 % of the lattice parameter, and an evident enhancement of proton diffusion under compressive conditions have been observed. The structural and electronic properties of BaZrO3 are analysed from Density Functional Theory calculations, and after an analysis of the electronic structure, we provide a possible explanation for an enhanced ionic conductivity of this bulk structure that can be caused by the formation of a preferential path for proton diffusion under compressive strain conditions. By means of Nudged Elastic Band calculations, diffusion barriers were also computed with results supporting our conclusions.
Fronzi, M, Kimizuka, H & Ogata, S 2015, 'Atomistic investigation of vacancy assisted diffusion mechanism in Mg ternary (Mg-RE-M) alloys', COMPUTATIONAL MATERIALS SCIENCE, vol. 98, pp. 76-82.View/Download from: UTS OPUS or Publisher's site
Fronzi, M, Piccinin, S, Delley, B, Traversa, E & Stampfl, C 2014, 'CHx adsorption (x=1-4) and thermodynamic stability on the CeO2(111) surface: a first-principles investigation', RSC ADVANCES, vol. 4, no. 24, pp. 12245-12251.View/Download from: UTS OPUS or Publisher's site
Kimizuka, H, Fronzi, M & Ogata, S 2013, 'Effect of alloying elements on in-plane ordering and disordering of solute clusters in Mg-based long-period stacking ordered structures: A first-principles analysis', SCRIPTA MATERIALIA, vol. 69, no. 8, pp. 594-597.View/Download from: UTS OPUS or Publisher's site
Fronzi, M, Cereda, S, Tateyama, Y, De Vita, A & Traversa, E 2012, 'Ab initio investigation of defect formation at ZrO-CeO interfaces', Physical review B: Condensed matter and materials physics, vol. 86, no. 8.View/Download from: UTS OPUS or Publisher's site
The structural and electronic properties of low index (100) and (111) ZrO 2 -CeO 2 interfaces are analyzed on the basis of density functional theory calculations. The formation energy and relative stability of substitutional defects, oxygen vacancies, and vacancy-dopant complexes are investigated for the (100) orientation. By comparing these results with the ones obtained in bulk structures, we provide a possible explanation for the higher experimental ionic conductivity measured at the interface. © 2012 American Physical Society.
Fronzi, M, Soon, A, Delley, B, Traversa, E & Stampfl, C 2009, 'Stability and morphology of cerium oxide surfaces in an oxidizing environment: A first-principles investigation', JOURNAL OF CHEMICAL PHYSICS, vol. 131, no. 10.View/Download from: Publisher's site
Fronzi, M, Piccinin, S, Delley, B, Traversa, E & Stampfl, C 2009, 'Water adsorption on the stoichiometric and reduced CeO2(111) surface: a first-principles investigation', PHYSICAL CHEMISTRY CHEMICAL PHYSICS, vol. 11, no. 40, pp. 9188-9199.View/Download from: Publisher's site
Arulsamy, AD & Fronzi, M 2008, 'Length-dependent resistance model for a single-wall carbon nanotube', Physica E: Low-Dimensional Systems and Nanostructures, vol. 41, no. 1, pp. 74-79.View/Download from: Publisher's site
The non-linear length-dependent resistance, R (l), observed in single-wall carbon nanotubes (SNTs) is explained through the recently proposed ionization energy (EI) based Fermi-Dirac statistics (iFDS). The length here corresponds to the carbon atoms number (N) along the SNT. It is also shown that Ry (ly) < Rx (lx) is associated with EIy < EIx, which can be attributed to different conducting properties in their respective y and x directions, or due to chirality. Crown Copyright © 2008.
Nolan, M, Rhatigan, S, Daly, W & Fronzi, M 2018, 'Design of Photocatalysts for CO2 Reduction from First Principles', 2018 IEEE 18TH INTERNATIONAL CONFERENCE ON NANOTECHNOLOGY (IEEE-NANO), 18th IEEE International Conference on Nanotechnology (IEEE-NANO), IEEE, Tyndall Natl Inst, Cork, IRELAND.
Fronzi, M, De Vita, A, Tateyama, Y & Traversa, E 2011, 'ZrO 2-CeO 2 interface properties: A first principle investigation', ECS Transactions, ECS Transactions (ECST), pp. 1203-1210.View/Download from: UTS OPUS or Publisher's site
In the present work we present a Density Functional Theory approach to investigate the structural and electronic properties of the low index ZrO 2 -CeO 2 interface. Optimizations of the crystal geometry for separate ZrO 2 and CeO 2 bulks as well as the interfaces are carried out and the structural morphology is analyzed. The energy of formation of the oxygen vacancies is analyzed at different values of the lattice parameter, in order to verify its dependency on the strain. This eventually allows us to identify the vacancy concentration difference between bulks and interfaces. Activation energy of the oxygen migration is also calculated in the optimized bulk as well as under strain condition as at the interfaces level, to identify eventual preferential migration channel. The effect of doping on the lattice geometry is analyzed for the low index interfaces in order to verify its influence on the morphologic disorder and consequently on vacancy concentration. ©The Electrochemical Society.
Esposito, V, Fronzi, M & Traversa, E 2006, 'Synthesis and densification of nanometric Ce0.8Sm0.2O1.9-delta', SOLID-STATE IONICS-2006, Symposium on Solid-State Ionics held at the 2006 MRS Fall Meeting, MATERIALS RESEARCH SOC, Boston, MA, pp. 169-+.
Esposito, V, Bac, HL, Fronzi, M & Traversa, E 2005, 'Synthesis, characterization and densification of Samaria Doped Ceria (SDC) ultra-fine powders', ECS Transactions, pp. 35-50.View/Download from: Publisher's site
Nanometric Samaria 20% mol Doped Ceria (SDC20) powders were synthesized by a co-precipitation method. Nano-sized SDC20 powders (crystal size 15 nm) were obtained by calcination at 600°C. The powders were formed by uniaxial pressure and then sintered. Sintering procedures were led both conventionally, at 1500°C for different sintering times (1, 5, and 10 h), and by fast firing process at different temperatures (1200, 1300, 1400, 1500, and 1600°C for 0.1 h). Different microstructures were obtained from the different thermal treatments. Electrochemical impedance spectroscopy (EIS) was used to determine total conductivity and to separate bulk and grain boundary impedance contributions. As expected, densification and electrical properties of the conventionally sintered samples showed to be mainly dependent on the average grain size. On the other hand, for the fast fired samples, lattice diffusion mechanism mainly controlled the densification, boundary arrangement and the resulting microstructures at various temperatures. copyright The Electrochemical Society.