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  • 1.
    Araujo, C. Moysés
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Kapilashrami, Mukes
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Jun, Xu
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Jayakumar, Onattu D.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Nagar, Sandeep
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Wu, Yan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Århammar, Cecilia
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Belova, Lyubov
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Gehring, Gillian A.
    Rao, K. Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Room temperature ferromagnetism in pristine MgO thin films2010In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, no 23Article in journal (Refereed)
    Abstract [en]

    Robust ferromagnetic ordering at, and well above room temperature is observed in pure transparent MgO thin films (<170 nm thick) deposited by three different techniques. Careful study of the wide scan x-ray photoelectron spectroscopy rule out the possible presence of any magnetic contaminants. In the magnetron sputtered films, we observe magnetic phase transitions as a function of film thickness. The maximum saturation magnetization of 5.7 emu/cm(3) is measured on a 170 nm thick film. The films above 500 nm are found to be diamagnetic. Ab initio calculations suggest that the ferromagnetism is mediated by cation vacancies.

  • 2. Blomqvist, Andreas
    et al.
    Araujo, Carlos Moyses
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Scheicher, Ralph H.
    Srepusharawoot, Pornjuk
    Li, Wen
    Chen, Ping
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Hydrogen as promoter and inhibitor of superionicity: A case study on Li-N-H systems2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 2, p. 024304-Article in journal (Refereed)
    Abstract [en]

    Materials which possess a high lithium ion conductivity are very attractive for battery and fuel cell applications. Hydrogenation of the fast-ion conductor lithium nitride (Li3N) leads to the formation of lithium imide (Li2NH) and subsequently of lithium amide (LiNH2). Using ab initio molecular dynamics simulations, we carried out a comparative study of the Li diffusion in these three systems. The results demonstrate that hydrogen can work as both promoter and inhibitor of Li mobility, with the lowest transition temperature to the superionic state occurring in Li2NH. Furthermore, we show that the creation of Li vacancies strongly affects Li diffusion in Li3N, but not so in Li2NH. Finally, we explain our findings with the help of a simple model.

  • 3.
    Duan, Lele
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Organic Chemistry.
    Araujo, Carlos Moyses
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Ahlquist, Mårten S. G.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Sun, Licheng
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Organic Chemistry.
    Highly efficient and robust molecular ruthenium catalysts for water oxidation2012In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 109, no 39, p. 15584-15588Article in journal (Refereed)
    Abstract [en]

    Water oxidation catalysts are essential components of light-driven water splitting systems, which could convert water to H-2 driven by solar radiation (H2O + h nu -> 1/2O(2) + H-2). The oxidation of water (H2O -> 1/2O(2) + 2H(+) + 2e(-)) provides protons and electrons for the production of dihydrogen (2H(+) + 2e(-) -> H-2), a clean-burning and high-capacity energy carrier. One of the obstacles now is the lack of effective and robust water oxidation catalysts. Aiming at developing robust molecular Ru-bda (H(2)bda = 2,2'-bipyridine-6,6'-dicarboxylic acid) water oxidation catalysts, we carried out density functional theory studies, correlated the robustness of catalysts against hydration with the highest occupied molecular orbital levels of a set of ligands, and successfully directed the synthesis of robust Ru-bda water oxidation catalysts. A series of mononuclear ruthenium complexes [Ru(bda)L-2] (L = pyridazine, pyrimidine, and phthalazine) were subsequently synthesized and shown to effectively catalyze Ce-IV-driven [Ce-IV = Ce(NH4)(2()NO3)(6)] water oxidation with high oxygen production rates up to 286 s(-1) and high turnover numbers up to 55,400.

  • 4. Huang, L. M.
    et al.
    Araujo, Carlos Moyses
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Magnetic and electronic properties of 3d transition-metal-doped In2O3: An ab initio study2009In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 87, no 2Article in journal (Refereed)
    Abstract [en]

    The magnetic and electronic properties of the transition metal (TM) (V, Cr, Mn, Fe, Co, Ni, Cu) doped In2O3 have been theoretically studied by using the density functional theory. When two TM ions are placed close to each other (TM-TM distance of about 3.4 angstrom), the ferromagnetic ordering is found to be the lowest-energy configuration. The only exception is Fe, which possesses a half-filled 3d band. However, for further separation distance of about 7.2 angstrom, only Co, Ni and Cu ions (having more than half-filled 3d band) still prefer the ferromagnetic orientation, while V, Cr, or Mn ions (having less than half-filled 3d band) prefer antiferromagnetic ordering. The energies of the 3d band for TM ions show a decrease with increasing TM atomic number. For V, Cr and Mn, the 3d bands are merged with the conduction band, and show less hybridization with the host valence band; while for Co, Ni and Cu, the 3d bands show strong hybridization with the host valence band mainly formed by the oxygen 2p state. In this situation, polarized holes are formed on the oxygen sites close to the TM ions. Moreover, V-doped In2O3 is found to meet the requirements for a strong donor-mediated ferromagnetism.

  • 5. Huang, L. M.
    et al.
    Silvearv, F.
    Araujo, Carlos Moyses
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Defect-induced strong ferromagnetism in Cr-doped In2O3 from first-principles theory2010In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 150, no 13-14, p. 663-665Article in journal (Refereed)
    Abstract [en]

    We demonstrate by means of first-principles calculations that the high Curie temperature observed in Cr-doped In2O3 is mediated by intrinsic p-type defects, namely In vacancies or O interstitials. Charge transfer from Cr 3d states to the hole states formed by these defects makes Cr ions in the mixed valence state, giving rise to a strong ferromagnetic coupling. Calculated formation energies of various defects also show that doping Cr in In2O3 could greatly lower the formation energies of p-type intrinsic defects even in oxygen-deficient growth conditions. These results advance our understanding of the underlying physics of diluted magnetic oxides.

  • 6. Nisar, J.
    et al.
    Almeida Silva, L.
    Gomes Almeida, C.
    Santos Mascarenhas, J. A.
    Wang, Baochang
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Moysés Araújo, Carlos
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Pepe, I.
    Souza de Almeida, J.
    Ferreira da Silva, A.
    Study of electronic and optical properties of BiTaO 4 for photocatalysis2012In: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 9, no 7, p. 1593-1596Article in journal (Refereed)
    Abstract [en]

    We present the optical absorption spectrum of BiTaO 4 using the photo acoustic spectroscopy (PAS) technique and first principles approach. Band gap have been estimated 2.65 and 2.45 eV using PAS method and DFT calculations, respectively. Position of reduction and oxidation level with respect to vacuum level are identified, which shows that BiTaO 4 can be used as photocatalyst for hydrogen production. Electronic structure is explained by plotting total density of states (TDOS).

  • 7. Nisar, J.
    et al.
    Araujo, Carlos Moyses
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Structural, electronic and energetic properties of water adsorbed on beta-Si3N4 (0001) surface: First-principles calculations2010In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 604, no 5-6, p. 617-622Article in journal (Refereed)
    Abstract [en]

    Structural, energetic and electronic properties of water molecules adsorbed on beta-Si3N4 (0 00 1) surface, at various coverages, are investigated using density functional theory. At low coverages (0 <= 0.5), it is found that all H2O molecules undergo spontaneous dissociation forming hydroxyl (OH) and imino (NH) groups where the reactive sites are identified, a result shown for the first time using ab initio theory. For higher coverages (0 > 0.5), only partial dissociation takes place where some of the molecules stay intact being bound via H-bond in good agreement with experimental findings. The driving force for the water dissociation has been identified to be dangling bonds on lower coordinated N and Si surface atoms showing that not all surface atoms are reactive corroborating with previous experimental findings.

  • 8. Nisar, J.
    et al.
    Araujo, Carlos Moyses
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Water Interaction with native defects on rutile TiO2 nanowire: Ab initio calculations2011In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 98, no 8, p. 083115-Article in journal (Refereed)
    Abstract [en]

    Adsorption of water molecules on stoichiometric and defective surfaces of rutile TiO2 nanowire oriented along the [(1) over bar 10] direction is investigated using density function theory calculations. We have investigated, in particular, O and Ti vacancies where energetic, structural, and electronic properties were evaluated. It was found that the water molecules interacting with O-vacancy undergo spontaneous dissociation, forming hydroxyl groups bound to Ti atoms and other OH groups formed by surface O and H-water. The same is not found in the case of perfect and Ti-vacancy containing nanowire. This dissociation of water molecules is due to charge transfer from neighboring Ti atom, which is polarized due to the O-vacancy.

  • 9. Nisar, Jawad
    et al.
    Wang, Baochang
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Araújo, Carlos Moysés
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    da Silva, Antonio Ferreira
    Kang, Tae Won
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Band gap engineering by anion doping in the photocatalyst BiTaO4: First principle calculations2012In: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487, Vol. 37, no 4, p. 3014-3018Article in journal (Refereed)
    Abstract [en]

    We have shown the effect of mono and co-doping of non-metallic anion atoms on the electronic structure in BiTaO4 using the first-principles method. It can improve the photocatalytic efficiency for hydrogen production in the presence of visible sunlight. It is found that the band gap of BiTaO4 has been reduced significantly up to 54% with different nonmetallic doping. Electronic structure analysis shows that the doping of nitrogen is able to reduce the band gap of BiTaO4 due to the impurity N 2p state at the upper edge of the valence band. In case of C or C-S doped BiTaO4, double occupied (filled) states have been observed deep inside the band gap of BiTaO4. The large reduction of band gap has been achieved, which increases the visible light absorption. These results indicate that the doping of non-metallic element in BiTaO4 is a promising candidate for the photocatalyst due to its reasonable band gap.

  • 10. Srepusharawoot, P.
    et al.
    Blomqvist, A.
    Araujo, C. Moyses
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Scheicher, R. H.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Hydrogen binding in alkali-decorated iso-reticular metal organic framework-16 based on Zn, Mg, and Ca2011In: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487, Vol. 36, no 1, p. 555-562Article in journal (Refereed)
    Abstract [en]

    Hydrogen adsorption energies were investigated in three different types of iso-reticular Metal Organic Framework-16, Zn-/Mg-/Ca-MOF16, decorated with either Li, Na, or K. Concerning the binding strengths of the alkali metals, the density functional theory calculations reveal that Li is bound strongest to the host framework, followed by K and Na. Decoration with Li also results in the highest hydrogen adsorption energies among the studied alkali metals. Furthermore, Zn-MOF16 exhibits the highest hydrogen adsorption energies near the metal oxide cluster, while hydrogen binding strengths at organic linker sites do not differ substantially between Zn-/Mg-/Ca-MOF16. Based on these results, we conclude that for Metal Organic Framework-16, Li-decorated Zn-MOF16 appears to be the optimal choice for hydrogen storage among the nine combinations.

  • 11. Srepusharawoot, Pornjuk
    et al.
    Blomqvist, Andreas
    Araújo, C. Moysés
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Scheicher, Ralph H.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    One-dimensional polymeric carbon structure based on five-membered rings in alkaline earth metal dicarbides BeC2 and MgC22010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 12, p. 125439-Article in journal (Refereed)
    Abstract [en]

    We studied five alkaline earth dicarbide systems MAEC2 (where M-AE = Be-Ba) by using ab initio random structure search. For BeC2 and MgC2, the lowest energy and dynamically stable configuration consists of five-membered carbon rings connected to each other via an individual carbon atom, stabilized through the donation of electrons from the surrounding alkaline earth ions. For CaC2, SrC2, and BaC2, our study shows that the chain crystal structure is more stable than the predicted structure due to strains induced by the increasing size of alkaline earth metal ions. The reaction energies of the typical synthesis pathway are comparable to those calculated for the experimental reaction of the known chain-type structure. Finally, the proposed structure should be optically distinguishable due to a significantly narrower band gap.

  • 12.
    Århammar, Cecilia
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Araujo, C. Moyses
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Rao, K. Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Norgren, S.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Energetics and magnetic properties of V-doped MgO bulk and (001) surface: A GGA, GGA+U, and hybrid density functional study2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 13, p. 134406-Article in journal (Refereed)
    Abstract [en]

    In this work, a first-principles study of the energetic and magnetic properties of V-doped MgO is presented, where both the bulk and (001) surface were investigated. It is found that V assumes a high-spin state with a local moment of about 3 mu(B). In the bulk, the interaction between these local moments is very short ranged and the antiferromagnetic (AFM) ordering is energetically more favorable. The formation of V-V-Mg-V defect clusters is found to weaken the antiferromagnetic coupling in bulk MgO, degenerating the AFM and ferro-magnetic state. However, these clusters are high in energy and will not form at equilibrium conditions. By employing the GGA+U approach, with U=5 eV, the V 3d states on the (001) surface are shifted below the Fermi level, and a reasonable surface geometry was achieved. A calculation with the hybrid HSE03 functional, contradicts the GGA+U results, indicating that the V-MgO surface should be metallic at this concentration. From the energetics it is concluded that, at the modeled concentration, VxOy phases will limit the solubility of V in MgO at equilibrium conditions, which is in agreement with previous experimental findings. In order to achieve higher concentrations of V, an off-equilibrium synthesis method is needed. Finally, we find that the formation energy of V at the surface is considerably higher than in the bulk and V is thus expected to diffuse from the surface into the bulk of MgO.

  • 13.
    Århammar, Cecilia
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Moysés Araújo, C.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Energetics of Al doping and intrinsic defects in monoclinic and cubic zirconia: First-principles calculations2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 11, p. 115208-Article in journal (Refereed)
    Abstract [en]

    First-principles theory within the supercell approach has been employed to investigate Al doping and intrinsic defects in monoclinic and cubic zirconia. The effect of oxygen chemical potential and Fermi level on the formation energy and on the defect concentration have been taken into account. The formation of oxygen vacancies is found to be energetically more favorable in the cubic than in the monoclinic phase under the same oxygen chemical potential and Fermi energy. In both phases, substitutional Al decays from neutral charge state into the charge state -1, with the transition energy just above to the top of the valence band. Our findings indicate that by confining the Fermi energy to the region between the middle of the band gap and the bottom of the conduction band, high Al solubility could be achieved, although formation of Al is likely followed by the formation of interstitial oxygen. Furthermore, the concentration of Al with charge state -1 along with the equilibrium Fermi energy have been calculated in a self-consistent procedure. Here, the possible compensating defects with the relevant charge states have been considered. The obtained concentrations of Al and oxygen vacancies follow the experimental trend but underestimates experimental data. When the formation of defect clusters, composed by two substitutional Al and one oxygen vacancy, are considered, good quantitative agreement with experimental values of both Al and oxygen vacancy concentration is achieved. The results suggest that defect clusters will be formed as a result of Al doping in cubic phase of ZrO2, whereas the concentration of defect clusters is negligible in the monoclinic phase, both in accordance with experiment.

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