Correlated Visible-Light Absorption and Intrinsic Magnetism of SrTiO₃ Due to Oxygen Deficiency: Bulk or Surface Effect?

The visible-light absorption and luminescence of wide band gap (3.25 eV) strontium titanate (SrTiO₃) are well-known, in many cases, to originate from the existence of natural oxygen deficiency in the material. In this study based on density functional theory (DFT) calculations, we provide, to the best of our knowledge, the first report indicating that oxygen vacancies in the bulk and on the surfaces of SrTiO₃ (STO) play different roles in the optical and magnetic properties. We found that the doubly charged state of oxygen vacancy (V_O²⁺) is dominant in bulk SrTiO₃ and does not contribute to the sub-band gap photoexcitation or intrinsic magnetism of STO. Neutral oxygen vacancies (V_O⁰) on (001) surfaces terminated with both TiO₂ and SrO layers induce magnetic moments, which are dependent on the charged state of V_O. The calculated absorption spectra for the (001) surfaces exhibit mid-infrared absorption (<0.5 eV) and sub-band gap absorption (2.5–3.1 eV) due to oxygen vacancies. In particular, V_O⁰ on the TiO₂-terminated surface has a relatively low formation energy and magnetic moments, which can explain the recently observed spin-dependent photon absorptions of STO in a magnetic circular dichroism measurement [Rice, W. D.; et al. Nat. Mater. 13, 481, 2014]

Unexpected Roles of Interstitially Doped Lithium in Blue and Green Light Emitting Y₂O₃:Bi³⁺: A Combined Experimental and Computational Study

To enhance the photoluminescence of lanthanide oxide, a clear understanding of its defect chemistry is necessary. In particular, when yttrium oxide, a widely used phosphor, undergoes doping, several of its atomic structures may be coupled with point defects that are difficult to understand through experimental results alone. Here, we report the strong enhancement of the photoluminescence (PL) of Y₂O₃:Bi³⁺ via codoping with Li⁺ ions and suggest a plausible mechanism for that enhancement using both experimental and computational studies. The codoping of Li⁺ ions into the Y₂O₃:Bi³⁺ phosphor was found to cause significant changes in its structural and optical properties. Interestingly, unlike previous reports on Li⁺ codoping with several other phosphors, we found that Li⁺ ions preferentially occupy interstitial sites of the Y₂O₃:Bi³⁺ phosphor. Computational insights based on density functional theory calculations also indicate that Li⁺ is energetically more stable in the interstitial sites than in the substitutional sites. In addition, interstitially doped Li⁺ was found to favor the vicinity of Bi³⁺ by an energy difference of 0.40 eV in comparison to isolated sites. The calculated DOS showed the formation of a shallow level directly above the unoccupied 6p orbital of Bi³⁺ as the result of interstitial Li⁺ doping, which may be responsible for the enhanced PL. Although the crystallinity of the host materials increased with the addition of Li salts, the degree of increase was minimal when the Li⁺ content was low (<1 mol %) where major PL enhancement was observed. Therefore, we reason that the enhanced PL mainly results from the shallow levels created by the interstitial Li⁺

Parallelized Reaction Pathway and Stronger Internal Band Bending by Partial Oxidation of Metal Sulfide–Graphene Composites: Important Factors of Synergistic Oxygen Evolution Reaction Enhancement

The electrocatalytic performance of transition metal sulfide (TMS)–graphene composites has been simply regarded as the results of high conductivity and the large surface/volume ratio. However, unavoidable factors such as degree of oxidation of TMSs have been hardly considered for the origin of this catalytic activity of TMS–graphene composites. To accomplish the reliable application of TMS-based electrocatalytic materials, a clear understanding of the thermodynamic stability of TMS and effects of oxidation on catalytic activity is necessary. In addition, the mechanism of charge transfer at the TMS–graphene interface must be studied in depth to properly design composite materials. Herein, we report a comprehensive study of the physical chemistry at the junction of a Co_1–<i>x</i>Ni_<i>x</i>S₂–graphene composite, which is a prototype designed to unravel the mechanisms of charge transfer between TMS and graphene. Specifically, the thermodynamic stability and the effects of oxidation of TMSs during the oxygen evolution reaction (OER) on the reaction mechanism are systematically investigated using density functional theory (DFT) calculations and experimental observations. Cobalt atoms anchored on pyridinic N sites in the graphene support form metal–semiconductor (SC) junctions, and the internal band bending at these junctions facilitates electron transfer from TMSs to graphene. The junction enables fast sinking of the excess electron from OH^– adsorbate. Partially oxidized amorphous TMS layers formed during the OER can facilitate adsorption and desorption of OH and H atoms, boosting the OER performance of TMS–graphene nanocomposites. From the DFT calculations, the enhanced electrocatalytic activity of TMS–graphene nanocomposites originates from two important factors: (i) increased internal band bending and (ii) parallelized OER pathways at the interface of pristine and oxidized TMSs

Si/Ge Double-Layered Nanotube Array as a Lithium Ion Battery Anode

Problems related to tremendous volume changes associated with cycling and the low electron conductivity and ion diffusivity of Si represent major obstacles to its use in high-capacity anodes for lithium ion batteries. We have developed a group IVA based nanotube heterostructure array, consisting of a high-capacity Si inner layer and a highly conductive Ge outer layer, to yield both favorable mechanics and kinetics in battery applications. This type of Si/Ge double-layered nanotube array electrode exhibits improved electrochemical performances over the analogous homogeneous Si system, including stable capacity retention (85% after 50 cycles) and doubled capacity at a 3<i>C</i> rate. These results stem from reduced maximum hoop strain in the nanotubes, supported by theoretical mechanics modeling, and lowered activation energy barrier for Li diffusion. This electrode technology creates opportunities in the development of group IVA nanotube heterostructures for next generation lithium ion batteries