Abstract:Anatase titanium oxide is important for its high chemical stability and photocatalytic properties, however, the latter are plagued by its large band gap that limits its activity to only a small percentage of the solar spectrum. In that respect, straining the material can reduce its band gap increasing the photocatalytic activity of titanium oxide. We apply density functional theory with the introduction of the Hubbard + U model, to investigate the impact of stress on the electronic structure of anatase in conj… Show more
“…The energy states at 2.4 eV are created from the hybridization of Cr-3d with Ti-3d orbitals. The results are in good agreement with other theoretical works [51]. In Figure 3b, the CrTi:TiO2 case is examined.…”
Section: Structural and Electronic Propertiessupporting
Titania (TiO2) is a key material used as an electron transport in dye-sensitized and halide perovskite solar cells due to its intrinsic n-type conductivity, visible transparency, low-toxicity, and abundance. Moreover, it exhibits pronounced photocatalytic properties in the ultra-violet part of the solar spectrum. However, its wide bandgap (around 3.2 eV) reduces its photocatalytic activity in the visible wavelengths’ region and electron transport ability. One of the most efficient strategies to simultaneously decrease its bandgap value and increase its n-type conductivity is doping with appropriate elements. Here, we have investigated using the density functional theory (DFT), as well as the influence of chromium (Cr), molybdenum (Mo), and tungsten (W) doping on the structural, electronic, and optical properties of TiO2. We find that doping with group 6 elements positively impacts the above-mentioned properties and should be considered an appropriate method for photocatalystic applications. In addition to the pronounced reduction in the bandgap values, we also predict the formation of energy states inside the forbidden gap, in all the cases. These states are highly desirable for photocatalytic applications as they induce low energy transitions, thus increasing the oxide’s absorption within the visible. Still, they can be detrimental to solar cells’ performance, as they constitute trap sites for photogenerated charge carriers.
“…The energy states at 2.4 eV are created from the hybridization of Cr-3d with Ti-3d orbitals. The results are in good agreement with other theoretical works [51]. In Figure 3b, the CrTi:TiO2 case is examined.…”
Section: Structural and Electronic Propertiessupporting
Titania (TiO2) is a key material used as an electron transport in dye-sensitized and halide perovskite solar cells due to its intrinsic n-type conductivity, visible transparency, low-toxicity, and abundance. Moreover, it exhibits pronounced photocatalytic properties in the ultra-violet part of the solar spectrum. However, its wide bandgap (around 3.2 eV) reduces its photocatalytic activity in the visible wavelengths’ region and electron transport ability. One of the most efficient strategies to simultaneously decrease its bandgap value and increase its n-type conductivity is doping with appropriate elements. Here, we have investigated using the density functional theory (DFT), as well as the influence of chromium (Cr), molybdenum (Mo), and tungsten (W) doping on the structural, electronic, and optical properties of TiO2. We find that doping with group 6 elements positively impacts the above-mentioned properties and should be considered an appropriate method for photocatalystic applications. In addition to the pronounced reduction in the bandgap values, we also predict the formation of energy states inside the forbidden gap, in all the cases. These states are highly desirable for photocatalytic applications as they induce low energy transitions, thus increasing the oxide’s absorption within the visible. Still, they can be detrimental to solar cells’ performance, as they constitute trap sites for photogenerated charge carriers.
“…Our calculations were performed with DFT + U model with the Hubbard-U parameter equal to 8.2 eV 33,34 . We calculated the band gap at 3.14 eV, in agreement with previous theoretical studies 33,34,44 , which show a band gap narrowing from 3 to 3.16 eV and close to the experimental value of 3.2 eV.Figure 3( a ) DOS of the undoped bulk anatase TiO 2 , ( b ) DOS of interstitially F - doped TiO 2 ( c ) DOS of F:TiO 2 with F substituting an Oxygen site and displacing it to an interstitial site. ( d ) DOS of the F:TiO 2 when F is a substitution to O, ( e ) The Density of States graph of the Cl-Doped TiO 2 with Cl as an interstitial, ( f ) The Density of States graph of the Cl:TiO 2 when Cl is a substitution to O.…”
Section: Resultsmentioning
confidence: 99%
“…A common way to reduce the band gap of TiO 2 is through doping with appropriate elements which has also significant impact on its electronic structure. A vast variety of literature reports previously demonstrated that by doping TiO 2 with nitrogen (N), halogens such as fluorine (F) and chlorine (Cl), or several transition metal ions such as zinc (Zn) or nickel (Ni), significant changes in its electronic structure are observed 28–34 . Particularly, the formation of mid gap states resulting in a band gap reduction was evident.…”
Titanium dioxide represents one of the most widely studied transition metal oxides due to its high chemical stability, non-toxicity, abundance, electron transport capability in many classes of optoelectronic devices and excellent photocatalytic properties. Nevertheless, the wide bang gap of pristine oxide reduces its electron transport ability and photocatalytic activity. Doping with halides and other elements has been proven an efficient defect engineering strategy in order to reduce the band gap and maximize the photocatalytic activity. In the present study, we apply Density Functional Theory to investigate the influence of fluorine and chlorine doping on the electronic properties of TiO2. Furthermore, we present a complete investigation of spin polarized density functional theory of the (001) surface doped with F and Cl in order to elaborate changes in the electronic structure and compare them with the bulk TiO2.
“…We present quantitative and physically intuitive arguments that explain the origins of such irregular η-driven effects on E g in terms of dielectric susceptibility and metal-oxygen bond length changes. It is worth noting that recent experimental and theoretical works have already proposed epitaxial strain as a means for tuning E g in some oxide materials such as TiO 2 [17,18], ZnO [16], SnO 2 [19], and CdO [20]. However, a clear and general understanding of how biaxial strain affects the photocatalytic performance of binary oxides is still missing.…”
Photocatalytic materials are pivotal for the implementation of disruptive clean energy applications such as conversion of H2O and CO2 into fuels and chemicals driven by solar energy. However, efficient and cost-effective materials able to catalyze the chemical reactions of interest when exposed to visible light are scarce due to the stringent electronic conditions that they must satisfy. Chemical and nanostructuring approaches are capable of improving the catalytic performance of known photoactive compounds however the complexity of the synthesized nanomaterials and sophistication of the employed methods make systematic design of photocatalysts difficult. Here, we show by means of first-principles simulation methods that application of biaxial stress, η, on semiconductor oxide thin films can modify their optoelectronic and catalytic properties in a significant and predictable manner. In particular, we show that upon moderate tensile strains CeO2 and TiO2 thin films become suitable materials for photocatalytic conversion of H2O into H2 and CO2 into CH4 under sunlight. The band gap shifts induced by η are reproduced qualitatively by a simple analytical model that depends only on structural and dielectric susceptibility changes. Thus, epitaxial strain represents a promising route for methodical screening and rational design of photocatalytic materials.
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