| 1. |
|
|
| 2. |
- Knut, Ronny, et al.
(författare)
-
Magnetic and electronic characterization of highly Co-doped ZnO : An annealing study at the solubility limit
- 2010
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:9, s. 094438-
-
Tidskriftsartikel (refereegranskat)abstract
- We report on investigations of the electronic structure and magnetic properties of ZnO doped with 15 at. % Co and postgrowth annealed at temperatures ranging between 250 and 800°C. In particular, we demonstrate how the presence of Co3+, indicative of secondary phases, is manifested in spectroscopy. Through resonant photoemmision spectroscopy we have found that x-ray diffraction in some cases underestimates or does not reveal the presence of secondary phases, possibly due to unrelaxed structures or structural arrangements with sizes below the detection limit. The magnetic properties are in most cases understood by assuming small antiferromagnetic clusters but can also show a behavior indicative of ferromagnetic interactions.
|
|
| 3. |
- Kronawitter, C. X., et al.
(författare)
-
Titanium incorporation into hematite photoelectrodes : theoretical considerations and experimental observations
- 2014
-
Ingår i: Energy & Environmental Science. - : Royal Society of Chemistry (RSC). - 1754-5692 .- 1754-5706. ; 7:10, s. 3100-3121
-
Tidskriftsartikel (refereegranskat)abstract
- A theoretical and experimental perspective on the role of titanium impurities in hematite (alpha-Fe2O3) nanostructured photoelectrodes for solar fuel synthesis devices is provided. Titanium incorporation is a known correlate to efficiency enhancement in alpha-Fe2O3 cc photoanodes for solar water oxidation; here the relevant literature and the latest advances are presented and various proposed mechanisms for enhancement are contrasted. Available experimental evidence suggests that Ti incorporation increases net electron carrier concentrations in electrodes, most likely to the extent that (synthesis-dependent) charge compensating cation vacancies are not present. However, electron conductivity increases alone cannot quantitatively account for the large associated photoelectrochemical performance enhancements. The magnitudes of the effects of Ti incorporation on electronic and magnetic properties appear to be highly synthesis-dependent, which has made difficult the development of consistent and general mechanisms explaining experimental and theoretical observations. In this context, we consider how the electronic structure correlates with Ti impurity incorporation in alpha-Fe2O3 a from the perspective of synchrotron-based soft X-ray absorption spectroscopy measurements. Measurements are performed on sets of electrodes fabricated by five relevant and unrelated chemical and physical techniques. The effects of titanium impurities are reflected in the electronic structure through several universally observed spectral characteristics, irrespective of the synthesis techniques. Absorption spectra at the oxygen K-edge show that Ti incorporation is associated with new oxygen 2p-hybridized states, overlapping with and distorting the known unoccupied Fe 3d-O 2xp band of alpha-Fe2O3. This is an indication of mixing of Ti s and d states in the conduction band of alpha-Fe2O3. cc A comparison of spectra obtained with electron and photon detection shows that the effects of Ti incorporation on the conduction band are more pronounced in the near-surface region. Titanium L-2,L-3-edge absorption spectra show that titanium is incorporated into alpha-Fe2O3 as Ti4+ by all fabrication methods, with no long-range titania order detected. Iron L-2,L-3-edge absorption spectra indicate that Ti incorporation is not associated with the formation, of any significant concentrations of Fe2+, an observation common to many prior studies on this material system.
|
|
| 4. |
- Kumar, P. Anil, et al.
(författare)
-
All-alkoxide based deposition and properties of a multilayer La0.67Sr0.33MnO3/CoFe2O4/La0.67Sr0.33MnO3 film
- 2021
-
Ingår i: European Journal of Inorganic Chemistry. - : John Wiley & Sons. - 1434-1948 .- 1099-1948 .- 1099-0682. ; 2021:18, s. 1736-1744
-
Tidskriftsartikel (refereegranskat)abstract
- Single and multilayer films of La0.67Sr0.33MnO3 and CoFe2O4 were deposited by spin-coating. The all-alkoxide precursors allowed inorganic gel films of extreme homogeneity to be formed and converted to phase pure complex oxides at low temperatures. La0.67Sr0.33MnO3 films were made with La- and Ca-methoxy-ethoxides and Mn19O12(moe)(14)(moeH)(10) as precursors at 800 degrees C. The CoFe2O4 films were obtained at extremely low 275 degrees C, using a new CoFe2-methoxyethoxide precursor. The decomposition and microstructural development on heating was described by TG, TEM, XRD and IR spectroscopy. XRD showed no spurious phases and the unit-cell dimensions coincided quite well with literature values of the targeted phases. The structural, magnetic and electronic properties of these films established their phase purity and high quality with physical properties comparable to films deposited by physical deposition methods. The magnetic and magneto transport results are presented for single, bi- and tri- layer structures. The magnetically soft La0.67Sr0.33MnO3 layer was exchange coupled to the magnetically hard CoFe2O4 layer, giving rise to interesting switching behaviour in magnetism and magneto-transport properties.
|
|
| 5. |
|
|
| 6. |
- Westin, Gunnar, et al.
(författare)
-
All alkoxide route to manganate and coboltate perovskite films and powders : effects of processing parameters
- 2008
-
Ingår i: Journal of Sol-Gel Science and Technology. - : Springer. - 0928-0707 .- 1573-4846. ; 48:1-2, s. 194-202
-
Tidskriftsartikel (refereegranskat)abstract
- Manganate and cobaltate perovskites having the general formula ABO3 have many technologically important applications. Here we present all alkoxide based routes to manganate and coboltate perovskite films and nano-phase powders of the compositions; LCMO (La0.67Ca0.33MnO3), LSMO (La0.75Sr0.25MnO3), LNCMO (La0.33Nd0.33Ca0.33MnO3), LBSMO (La0.75Ba0.125Sr0.125MnO3) and LSCO (La0.50Sr0.50CoO3). The Mn and Co (oxo) methoxy-ethoxide precursors were prepared by reaction of MnCl2 or CoCl2 with 2 Kmoe in methoxy-ethanol-toluene. After hydrolysis of the alkoxide solutions by atmospheric air all systems produced X-ray and electron diffraction amorphous gels of high elemental homogeneity, and the IR spectra showed that they consisted of hydrated oxo-carbonates. Heating in air resulted in similar weight-loss curves for all studied gels passing; loss of H2O in the range 20–300 °C, decomposition of carbonate groups into oxide and CO2 in the temperature range 300–700 °C, and in some cases loss of a small amount of oxygen in the temperature range 700–1,000 °C. The pure perovskites were obtained at 690–770 °C with heat rates of typically 5–20 °C min−1 without annealing. Perovskites could also be obtained at 550 °C by annealing, but these perovskites are prone to be A-site ion inhomogeneous according to the TEM EDS studies, which was not the case for the perovskites heat-treated to at least the carbonate decomposition temperature. This A-site inhomogeneity is ascribed to sequential decomposition of carbonates due to their different thermal stabilities, which is probably a general feature also with other sol–gel precursors and low temperature annealing. High quality polycrystalline films were prepared on Si/SiO2/TiO2/Pt substrates with all compositions and high quality epitaxial films were prepared of LCMO (on 100 LaAlO3) and LSCO (on 100 SrTiO3). The colossal magneto resistance (CMR) of the epitaxial LCMO films of 32% (246 K) is in parity with PLD derived films. The conductivities of the epitaxial and polycrystalline LSCO films were 1.9 mΩcm (on STO) and 1.7 mΩcm (on α-Al2O3, respectively).
|
|
| 7. |
- Westin, Gunnar, et al.
(författare)
-
Solution processing of conducting dense and porous films; doped ZnO and TiO2, and perovskites
- 2009
-
Ingår i: International Journal of Nanotechnology. ; 6:9, s. 828-849
-
Tidskriftsartikel (refereegranskat)abstract
- Soln. based processing of porous and dense films by all-alkoxide based precursor systems have been investigated and compared, with a focus on the connection between the precursors and heat-treatment, and the product oxide quality and structure. Three systems were investigated typically with TGA, DSC, XPS, IR spectroscopy, X-ray diffraction, TEM-EDS and SEM-EDS, but also with other advanced characterization techniques: (i) manganate and cobaltate perovskites of the LCMO (La0.67Ca0.33MnO3), LSMO (La0.75Sr0.25MnO3), LNCMO (La0.33Nd0.33Ca0.33MnO3), LBSM (La0.75Ba0.125Sr0.125MnO3) and LSCO (La0.50Sr0.50CoO3) compns. were investigated. Highly epitaxial films of LCMO and LSCO were prepd., and the CMR properties of the LCMO were comparable to those of PVD derived films. Polycryst. films were prepd. for all perovskites. (ii) Three routes to ZnO : Co/Al were investigated and compared; an acetate based route and two alkoxide based routes with different heat-treatments. With the acetate based and the alkoxide based route using hydrolysis in air, a max. Co doping of 6% was obtained, while in absence of CO2 and O2, the doping range could be extended to 20%Co. Both dense films and porous nano-structured films were prepd. The magnetic properties of the Co-doped films did not show any room-temp. ferro-magnetism. (iii) ZrO2 and NbO2.5 doped anatase TiO2 were prepd. by heat-treatment or hydrothermal routes. Alio-valent doping of up to more than 35% of NbO2.5 could be achieved in the anatase. It is believed that the pentavalent metal doping is compensated by metal vacancies, and as expected from this assumption, the materials have a low d. ZrO2 doping in titania led to an increase in the dye-sensitized solar cell efficiency and highly active and stable photo-catalysts.
|
|
| 8. |
- Westin, Gunnar, et al.
(författare)
-
Solution processing of conducting dense and porous films; doped ZnO and TiO2, and perovskites
- 2009
-
Ingår i: International Journal of Nanotechnology. - 1475-7435. ; 6:9, s. 828-849
-
Tidskriftsartikel (refereegranskat)abstract
- Solution based processing of porous and dense films by all-alkoxide based precursor systems have been investigated and compared, with a focus on the connection between the precursors and heat-treatment, and the product oxide quality and structure. Three systems were investigated typically with TGA, DSC, XPS, IR spectroscopy, X-ray diffraction, TEM-EDS and SEM-EDS, but also with other advanced characterisation techniques: (i) manganate and cobaltate perovskites of the LCMO (La0.67Ca0.33MnO3), LSMO (La0.75Sr0.25MnO3), LNCMO (La0.33Nd0.33Ca0.33MnO3), LBSM (La0.75Ba0.125Sr0.125MnO3) and LSCO (La0.50Sr0.50CoO3) compositions were investigated. Highly epitaxial films of LCMO and LSCO were prepared, and the CMR properties of the LCMO were comparable to those of PVD derived films. Polycrystalline films were prepared for all perovskites. (ii) Three routes to ZnO : Co/Al were investigated and compared; an acetate based route and two alkoxide based routes with different heat-treatments. With the acetate based and the alkoxide based route using hydrolysis in air, a maximum Co doping of 6% was obtained, while in absence of CO2 and O2, the doping range could be extended to 20%Co. Both dense films and porous nano-structured films were prepared. The magnetic properties of the Co-doped films did not show any room-temperature ferro-magnetism. (iii) ZrO2 and NbO2.5 doped anatase TiO2 were prepared by heat-treatment or hydrothermal routes. Alio-valent doping of up to more than 35% of NbO2.5 could be achieved in the anatase. It is believed that the pentavalent metal doping is compensated by metal vacancies, and as expected from this assumption, the materials have a low density. ZrO2 doping in titania led to an increase in the dye-sensitised solar cell efficiency and highly active and stable photo-catalysts.
|
|
