Browsing by Author "Rahman, KA"

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    Chromium segregation in Cr-doped TiO2 (rutile): impact of oxygen activity
    (Springer Nature, 2019-01-04) Rahman, KA; Sharma, N; Atanacio, AJ; Bak, T; Wachsman, ED; Moffit, M; Nowotny, J
    This work considers the effect of chromium surface segregation for polycrystalline Cr-doped TiO2 on surface vs. bulk defect disorder. It is shown that annealing of Cr-doped TiO2 (0.04 at% Cr) in the gas phase of variable oxygen activity at 1273 K results in a gradual transition in the valence of chromium at the surface from predominantly Cr3+ species in reduced conditions, p(O2) = 10−12 Pa, to comparable concentrations of both Cr3+ and Cr6+ species in oxidising conditions, p(O2) = 105 Pa. The reported data is considered in terms of defect equilibria leading to the formation of positively and negatively charged chromium in both the cation sub-lattice and interstitial sites. The derived theoretical models represent the effect of oxygen activity on the surface charge and the resulting electric field leading to migration mechanism of charged chromium species. © 2019 Springer Nature
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    Defect engineering of photosensitive oxide materials. Example of TiO2 solid solutions
    (Elsevier, 2018) Atanacio, AJ; Bak, T; Rahman, KA; Nowotny, J
    The imperative to protect the environment from increasingly apparent climate change imposes the urgent need to reduce the emissions of greenhouse gases to the atmosphere. This, consequently, results in intensification of research in the development of new materials and devices for the generation of energy that is environmentally clean. This work considers photosensitive oxide semiconductors for solar energy conversion by light-induced water oxidation. It has been documented that the performance of oxide semiconductors for solar-to-chemical energy conversion is determined by a range of defect-related properties, including the concentration of surface active sites, Fermi level, charge transport, electronic structure, and alignment of band edges with the energy level of the redox couple. The present work considers the research strategy in processing TiO2-based semiconductors, which are the promising candidates for a new generation of solar materials. It is shown that the performance-related properties of TiO2 and its solid solutions are determined by surface versus bulk defect disorder and the associated semiconducting properties. Therefore, the development of TiO2-based materials with enhanced performance could be based on using defect engineering for imposing optimized bulk versus surface properties. In this work, we discuss a range of defect-related properties of TiO2 and its solid solutions, such as electrical and optical properties and the related photocatalytic performance. We show that the phenomenon of segregation may be used as the technology for imposition of controlled surface versus bulk defect disorder that is required for processing the systems with optimized properties. Copyright © 2018 Elsevier Inc.
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    Toward sustainable energy: photocatalysis of Cr-doped TiO2: 1. electronic structure
    (Springer Nature, 2017-12-26) Rahman, KA; Bak, T; Atanacio, AJ; Ionescu, M; Nowotny, J
    The present chain of five papers considers the concept of defect engineering in processing TiO2-based photosensitive semiconductors for solar-to-chemical energy conversion. The papers report the effect of chromium on the key performance-related properties of polycrystalline TiO2 (rutile), including (i) electronic structure, (ii) chromium-related photocatalytic properties, (iii) oxygen-related photocatalytic properties, (iv) electrochemical coupling and (v) surface versus bulk properties. The present work reports the effect of chromium on defect disorder and the related electronic structure of TiO2, including the band gap and mid-gap energy levels. It is shown that chromium incorporation into the TiO2 lattice results in a decrease of the band gap from 3.04 eV for pure TiO2 to 1.4 and 1.3 eV, for Cr-doped TiO2 (1.365 at% Cr) after annealing at 1373 K in the gas phase of controlled oxygen activity, 21 kPa and 10−10 Pa, respectively. The optical properties determined using the ultraviolet-vis spectroscopy (in the reflectance mode) indicate that chromium incorporation results in the formation of mid-band energy levels. In this work, we show that chromium at and above the concentrations levels of 0.04 and 0.376 at% results in the formation of acceptor-type energy levels at 0.57 and 1.16 eV (above the valence band), respectively, which are related to tri-valent chromium in titanium sites and titanium vacancies, respectively. Application of well-defined protocol leads to the determination of data that are well reproducible. The new insight involves the determination of the band gap of TiO2 processed in the gas phase of controlled oxygen activity. © 2018 Springer Nature Switzerland AG
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    Toward sustainable energy: photocatalysis of Cr-doped TiO2: 2. effect of defect disorder
    (Springer Nature, 2017-12-26) Rahman, KA; Bak, T; Atanacio, AJ; Ionescu, M; Nowotny, J
    The present chain of five papers considers the concept of solar-to-chemical energy conversion using TiO2-based semiconductors. The series reports the effect of chromium on the key performance-related properties of polycrystalline TiO2 (rutile), including electronic structure, photocatalytic activity, intrinsic defect disorder, electrochemical coupling and surface versus bulk properties. In this work, we show that the effect of chromium on photocatalytic performance of TiO2 depends on its elemental content and the related defect disorder that is determined by oxygen activity in the oxide lattice. At high oxygen activity, chromium leads to enhanced photocatalytic performance only for dilute solid solutions (up to 0.04–0.043 at.% Cr). Higher chromium content results in a decrease of photocatalytic activity below that for pure TiO2, despite the observed substantial decrease of the band gap. The photocatalytic performance of Cr-doped TiO2 annealed in reducing conditions is low within the entire studied range of compositions. The obtained results led to derivation of a theoretical model representing the mechanism of the light-induced reactivity of TiO2 with water and the related charge transfer. The photocatalytic performance is considered in terms of a competitive effect of several key performance-related properties. The performance is predominantly influenced by the concentration of titanium vacancies acting as reactive surface sites related to anodic charge transfer. © 2018 Springer Nature
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    Towards sustainable energy. Photocatalysis of Cr-doped TiO2: 3. Effect of oxygen activity
    (Springer Nature, 2018-12-04) Rahman, KA; Bak, T; Atanacio, AJ; Ionescu, M; Nowotny, J
    The present chain of five papers considers the concept of solar-to-chemical energy conversion using TiO2-based semiconductors. The series reports the effect of chromium on the key performance-related properties of polycrystalline TiO2 (rutile), including electronic structure, photocatalytic activity, intrinsic defect disorder, electrochemical coupling and surface vs. bulk properties. The present work reports the effect of oxygen activity in the oxide lattice on photocatalytic activity of pure and Cr-doped TiO2 (0.04 at% Cr). Processing of specimens included annealing at 1273 K in the gas phase of controlled oxygen activity in the range 10−12 Pa < p(O2) < 105 Pa. We show that the increase of oxygen activity results initially in a decrease of photocatalytic activity, minimum around the n-p transition point, and then increase assuming maximum at p(O2) = 105 Pa. The obtained results are considered in terms of a theoretical model that explains the effect of defect disorder on the reactivity of TiO2 with water. The minimum of the photocatalytic activity corresponds to the n-p transition point. The maximum of performance at high p(O2) is determined by increased concentration of titanium vacancies forming surface active sites.© 2018 Springer Nature Switzerland AG
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    Towards sustainable energy. Photocatalysis of Cr-doped TiO2: 4. Electrochemical coupling
    (Springer Nature, 2018-01-06) Rahman, KA; Atanacio, AJ; Ionescu, M; Davis, J; Bak, T; Nowotny, J
    The present chain of five papers considers the concept of solar-to-chemical energy conversion using TiO2-based semiconductors. The series reports the effect of chromium on the key performance-related properties of polycrystalline TiO2 (rutile), including electronic structure, photocatalytic activity, intrinsic defect disorder, electrochemical coupling and surface versus bulk properties. This work reports the effect of photoelectrochemical coupling of both pure and Cr-doped TiO2 on photocatalytic partial water oxidation. The couples are annealed in oxidising and reducing conditions, at p(O2) = 105 Pa and p(O2) = 10−10 Pa, respectively. The performance of the coupled system may be enhanced, or retarded, by the electrical potential barrier that is formed between the couple components as a result of the difference in Fermi levels. In this work, we show that the potential barrier results in the effect of synergy when the charge transport within the couple components is high enough. This is the case for the couples annealed in reducing conditions. © 2018 Springer Nature Switzerland AG.
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    Towards sustainable energy: photocatalysis of Cr-doped TiO2. 5. Effect of segregation on surface versus bulk composition
    (Springer Nature, 2017-11-07) Rahman, KA; Bak, T; Atanacio, AJ; Ionescu, M; Liu, R; Nowotny, J
    The present chain of five papers considers the concept of solar-to-chemical energy conversion using TiO2-based semiconductors. The series reports the effect of chromium on the key performance-related properties of polycrystalline TiO2 (rutile), including electronic structure, photocatalytic activity, intrinsic defect disorder, electrochemical coupling and surface versus bulk properties. This work considers the effect of oxygen activity on segregation-induced surface versus bulk composition for both polycrystalline and single-crystal specimens of Cr-doped TiO2. It has been documented that annealing of Cr-doped TiO2 at 1273 K in oxidising conditions results in an enrichment and depletion of the surface layer with chromium. It is shown that the segregation-induced enrichment factor for single crystal is substantially larger than that for polycrystalline specimen. The effect is considered in terms of a theoretical model showing that surface segregation of solute in polycrystalline specimen is encumbered by its segregation to grain boundaries. It is also shown that the segregation-induced enrichment is profoundly influenced by oxygen activity. The new insight of this work involves (i) the determination of well-defined chromium segregation in Cr-doped TiO2, including single-crystal and polycrystalline specimens, after annealing in the gas phase of controlled oxygen activity, and (ii) identification of the predominant driving force of segregation of chromium in Cr-doped TiO2 that is based on electrostatic interactions between the low-dimensional surface structure (LDSS) and electrically charge segregating species. © 2018 Springer Nature Switzerland AG