Electronic Structure and Band Alignments of Various Phases of Titania Using the Self-Consistent Hybrid Density Functional and DFT+U Methods

Silicon-containing nanomedicine and biomaterials: materials chemistry, multi-dimensional design, and biomedical application

The invention of silica-based bioactive glass in the late 1960s has sparked significant interest in exploring a wide range of silicon-containing biomaterials from the macroscale to the nanoscale. Over the past few decades, these biomaterials have been extensively explored for their potential in diverse biomedical applications, considering their remarkable bioactivity, excellent biocompatibility, facile surface functionalization, controllable synthesis, etc. However, to expedite the clinical translation and the unexpected utilization of silicon-composed nanomedicine and biomaterials, it is highly desirable to achieve a thorough comprehension of their characteristics and biological effects from an overall perspective. In this review, we provide a comprehensive discussion on the state-of-the-art progress of silicon-composed biomaterials, including their classification, characteristics, fabrication methods, and versatile biomedical applications. Additionally, we highlight the multi-dimensional design of both pure and hybrid silicon-composed nanomedicine and biomaterials and their intrinsic biological effects and interactions with biological systems. Their extensive biomedical applications span from drug delivery and bioimaging to therapeutic interventions and regenerative medicine, showcasing the significance of their rational design and fabrication to meet specific requirements and optimize their theranostic performance. Additionally, we offer insights into the future prospects and potential challenges regarding silicon-composed nanomedicine and biomaterials. By shedding light on these exciting research advances, we aspire to foster further progress in the biomedical field and drive the development of innovative silicon-composed nanomedicine and biomaterials with transformative applications in biomedicine.

First-Principles Band Alignments at the Si:Anatase TiO2 Interface

TiO₂ has been identified as a promising electron transport layer in Si solar cells. Experiments have revealed that the Si:TiO₂ interface undergoes structural changes depending on how it was fabricated. However, less is understood about the sensitivity of electronic properties, such as band alignments, to these changes. Here, we present first-principles calculations of band alignments between Si and anatase TiO₂, investigating different surface orientations and terminations. By calculating vacuum-level alignments, we observe a large band offset reduction of 2.5 eV for the O-terminated Si slab compared to other terminations. Furthermore, a 0.5 eV increase is found for the anatase (101) surface compared to (001). We compare the band offsets obtained through vacuum alignment with four different heterostructure models. Even though the heterostructure models contain an excess of oxygen, their offsets agree well with vacuum-level alignments using stoichiometric or H-terminated slabs, and the reduction in band offsets seen for the O-terminated Si slab is not observed. Additionally, we have investigated different exchange-correlation treatments including PBE + U, postprocessing GW corrections, and the meta-GGA rSCAN functional. We find that rSCAN provides more accurate band offsets than PBE, but further corrections are still required to achieve <0.5 eV accuracy. Overall, our study quantifies the importance of surface termination and orientation for this interface.

Understanding the Role of Rutile TiO2 Surface Orientation on Molecular Hydrogen Activation

Titanium oxide (TiO₂) has been widely used in many fields, such as photocatalysis, photovoltaics, catalysis, and sensors, where its interaction with molecular H₂ with TiO₂ surface plays an important role. However, the activation of hydrogen over rutile TiO₂ surfaces has not been systematically studied regarding the surface termination dependence. In this work, we use density functional theory (PBE+U) to identify the pathways for two processes: the heterolytic dissociation of H₂ as a hydride–proton pair, and the subsequent H transfer from Ti to near O accompanied by reduction of the Ti sites. Four stoichiometric surface orientations were considered: (001), (100), (110), and (101). The lowest activation barriers are found for hydrogen dissociation on (001) and (110), with energies of 0.56 eV and 0.50 eV, respectively. The highest activation barriers are found on (100) and (101), with energies of 1.08 eV and 0.79 eV, respectively. For hydrogen transfer from Ti to near O, the activation barriers are higher (from 1.40 to 1.86 eV). Our results indicate that the dissociation step is kinetically more favorable than the H transfer process, although the latter is thermodynamically more favorable. We discuss the implications in the stability of the hydride–proton pair, and provide structures, electronic structure, vibrational analysis, and temperature effects to characterize the reactivity of the four TiO₂ orientations.

Role of Heterojunction in Charge Carrier Separation in Coexposed Anatase (001)-(101) Surfaces

A heterojunction made by coexposed anatase (001)-(101) surfaces is studied using an explicit atomistic model of the interface via density functional theory. High photoactivity for this system has been demonstrated recently. Usually, the nature of a semiconductor heterojunction is evaluated by looking at band edges of the separate, noninteracting units, thus neglecting interfacial effects. Our results show non-negligible structural and electronic effects occurring at the junction, but because of the canceling nature of these effects, the alignment of the bands is qualitatively similar for the real interface and for the separated, noninteracting fragments. We also show from first principles that upon light absorption and electron excitation, the junction promotes charge carrier separation via localization of holes at O ions of the (001) side and electrons at Ti ions of the (101) side of the junction. This hinders recombination and is most likely the reason for high photoactivity.

Ti-fraction-induced electronic and magnetic transformations in titanium oxide films

Titanium dioxide has been widely used in modern industrial applications, especially as an effective photocatalyst. Recently, freestanding TiO₂ films with a markedly reduced bandgap of ∼1.8 eV have been synthesized, indicating that the dimension has a considerable influence on the bulk band gap (>∼3 eV) and enhances the adsorption range of visible light. Titanium oxide compounds have various stoichiometries and versatile properties. Therefore, it is very necessary to explore the electronic properties and functionalities of other titanium oxide films with different stoichiometries. Here, we combined structure searches with first-principle calculations to explore candidate Ti-O films with different stoichiometries. In addition to the experimentally synthesized TiO₂ film, the structure searches identified three new energetically and dynamically stable Ti-O films with stoichiometries of Ti₃O₅, Ti₃O₂, and Ti₂O. Calculations show that the Ti-O films undergo several interesting electronic transformations as the Ti fraction increases, namely, from a wide-gap semiconductor (TiO₂, 3.2 eV) to a narrow-gap semiconductor (Ti₃O₅, 1.80 eV) and then to metals (Ti₃O₂ and Ti₂O) due to the abundance of unpaired Ti_d electrons. In addition to the electronic transformations, we observed nonmagnetic (TiO₂) to ferromagnetic (Ti₃O₅, Ti₃O₂, and Ti₂O) transformations. Notably, the Ti₃O₅ film possesses both narrow-gap semiconductive and ferromagnetic properties, with a large magnetic moment of 2.0 µ_B per unit cell; therefore, this film has high potential for use in applications such as spintronic devices. The results highlight metal fraction-induced electronic and magnetic transformations in transition metal oxide films and provide an alternative route for the design of new, functional thin-film materials.