Screening Doping Strategies To Mitigate Electron Trapping at Anatase TiO2 Surfaces

3D-Architected Alkaline-Earth Perovskites

3D ceramic architectures are captivating geometrical features with an immense demand in optics. In this work, an additive manufacturing (AM) approach for printing alkaline-earth perovskite 3D microarchitectures is developed. The approach enables custom-made photoresists suited for two-photon lithography, permitting the production of alkaline-earth perovskite (BaZrO3 , CaZrO3 , and SrZrO3 ) 3D structures shaped in the form of octet-truss lattices, gyroids, or inspired architectures like sodalite zeolite, and C60 buckyballs with micrometric and nanometric feature sizes. Alkaline-earth perovskite morphological, structural, and chemical characteristics are studied. The optical properties of such perovskite architectures are investigated using cathodoluminescence and wide-field photoluminescence emission to estimate the lifetime rate and defects in BaZrO3 , CaZrO3 , and SrZrO3 . From a broad perspective, this AM methodology facilitates the production of 3D-structured mixed oxides. These findings are the first steps toward dimensionally refined high-refractive-index ceramics for micro-optics and other terrains like (photo/electro)catalysis.

Real-time TEM observations of ice formation in graphene liquid cell

The study of ice nucleation and growth at the nanoscale is of utmost importance in geological and atmospheric sciences. However, existing transmission electron microscopy (TEM) approaches have been unsuccessful in imaging ice formation directly. Herein, we demonstrate how radical scavengers - such as TiO₂ - encased with water in graphene liquid cells (GLCs) facilitate the observation of ice nucleation phenomena at low temperatures. Atomic-resolution imaging reveals the nucleation and growth of cubic ice-phase crystals at close proximity to TiO₂-water nanointerfaces at low temperatures. Interestingly, both heterogeneously and homogeneously nucleated ice crystals exhibited this cubic phase. Ice crystal nuclei were observed to be more stable at the TiO₂-water nanointerface, as compared with crystals in the bulk liquid (homogeneous nucleation), suggesting the radical scavenging efficacy of TiO₂ nanoparticles mitigating the electron beam by-products. The present work demonstrates that the use of radical scavengers in GLC TEM shows great promise towards unveiling the nanoscale pathways for ice nucleation and growth dynamic events.

Interface Analysis of LiCl as a Protective Layer of Li1.3Al0.3Ti1.7(PO4)3 for Electrochemically Stabilized All-Solid-State Li-Metal Batteries

Regardless of the superiorities of Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP), such as stability against oxygen and moisture, high ionic conductivity, and low activation energy, its practical application in all-solid-state lithium metal batteries is still impeded by the formation of ionic-resistance interphase layers. Upon contact with Li metal, electron migration from Li to LATP causes the reduction of Ti⁴⁺ in LATP. As a result, an ionic-resistance layer will be formed at the interface between the two materials. Applying a buffer layer between them is a potential measure to mitigate this problem. In this study, we analyzed the potential role of LiCl to protect the LATP solid electrolyte through a first-principle study-based density functional theory (DFT) calculation. Density-of-states (DOS) analysis on the Li/LiCl heterostructure reveals the insulating roles of LiCl in preventing electron flow to LATP. The insulating properties begin at depths of 4.3 and 5.0 Å for Li (001)/LiCl (111) and Li (001)/LiCl (001) heterostructures, respectively. These results indicate that LiCl (111) is highly potential to be applied as a protecting layer on LATP to avoid the formation of ionic resistance interphase caused by electron transfer from the Li metal anode.

Predictive Removal of Interfacial Defect-Induced Trap States between Titanium Dioxide Nanoparticles via Sub-Monolayer Zirconium Coating

First principles modeling of anatase TiO₂ surfaces and their interfacial contacts shows that defect-induced trap states within the band gap arise from intrinsic structural distortions, and these can be corrected by modification with Zr(IV) ions. Experimental testing of these predictions has been undertaken using anatase nanocrystals modified with a range of Zr precursors and characterized using structural and spectroscopic methods. Continuous-wave electron paramagnetic resonance (EPR) spectroscopy revealed that under illumination, nanoparticle-nanoparticle interfacial hole trap states dominate, which are significantly reduced after optimizing the Zr doping. Fabrication of nanoporous films of these materials and charge injection using electrochemical methods shows that Zr doping also leads to improved electron conductivity and mobility in these nanocrystalline systems. The simple methodology described here to reduce the concentration of interfacial defects may have wider application to improving the efficiency of systems incorporating metal oxide powders and films including photocatalysts, photovoltaics, fuel cells, and related energy applications.

Formation of a Nanorod-Assembled TiO2 Actinomorphic-Flower-like Microsphere Film via Ta Doping Using a Facile Solution Immersion Method for Humidity Sensing

This study fabricated tantalum (Ta)-doped titanium dioxide with a unique nanorod-assembled actinomorphic-flower-like microsphere structured film. The Ta-doped TiO₂ actinomorphic-flower-like microsphere (TAFM) was fabricated via the solution immersion method in a Schott bottle with a home-made improvised clamp. The samples were characterised using FESEM, HRTEM, XRD, Raman, XPS, and Hall effect measurements for their structural and electrical properties. Compared to the undoped sample, the rutile-phased TAFM sample had finer nanorods with an average 42 nm diameter assembled to form microsphere-like structures. It also had higher oxygen vacancy sites, electron concentration, and mobility. In addition, a reversed double-beam photoacoustic spectroscopy measurement was performed for TAFM, revealing that the sample had a high electron trap density of up to 2.5 μmolg^-1. The TAFM showed promising results when employed as the resistive-type sensing film for a humidity sensor, with the highest sensor response of 53,909% obtained at 3 at.% Ta doping. Adding rGO to 3 at.% TAFM further improved the sensor response to 232,152%.

Reactivity of Hydrogen-Related Electron Centers in Powders, Layers, and Electrodes Consisting of Anatase TiO2 Nanocrystal Aggregates

Anatase TiO₂ nanoparticle aggregates were used as model systems for studying at different water activities the reactivity of electron centers at semiconductor surfaces. The investigated surface conditions evolve from a solid/vacuum interface to a solid/bulk electrolyte interface. Hydrogen-related electron centers were generated either chemically-upon sample exposure to atomic hydrogen at the semiconductor/gas interface-or electrochemically-upon bias-induced charge accumulation at the semiconductor/electrolyte interface. Based on their corresponding spectroscopic and electrochemical fingerprints, we investigated the reactivity of hydrogen-related electron centers as a function of the interfacial condition and at different levels of complexity, that is, (i) for dehydrated and (partially) dehydroxylated oxide surfaces, (ii) for oxide surfaces covered by a thin film of interfacial water, and (iii) for oxide surfaces in contact with a 0.1 M HClO₄ aqueous solution. Visible (Vis) and infrared (IR) spectroscopy evidence a chemical equilibrium between hydrogen atoms in the gas phase and-following their dissociation-electron/proton centers in the oxide. The excess electrons are either localized forming (Vis-active) Ti³⁺ centers or delocalized as (IR-active) free conduction band electrons. The addition of molecular oxygen to chemically reduced anatase TiO₂ nanoparticle aggregates leads to a quantitative quenching of Ti³⁺ centers, while a fraction of ∼10% of hydrogen-derived conduction band electrons remains in the oxide pointing to a persistent hydrogen doping of the semiconductor. Neither trapped electrons (i.e., Ti³⁺ centers) nor conduction band electrons react with water or its adsorption products at the oxide surface. However, the presence of an interfacial water layer does not impede the electron transfer to molecular oxygen. At the semiconductor/electrolyte interface, inactivity of trapped electrons with regard to water reduction and electron transfer to oxygen were evidenced by cyclic voltammetry.

Hole Polaron Migration in Bulk Phases of TiO2 Using Hybrid Density Functional Theory

Understanding charge-carrier transport in semiconductors is vital to the improvement of material performance for various applications in optoelectronics and photochemistry. Here, we use hybrid density functional theory to model small hole polaron transport in the anatase, brookite, and TiO₂-B phases of titanium dioxide and determine the rates of site-to-site hopping as well as thermal ionization into the valance band and retrapping. We find that the hole polaron mobility increases in the order TiO₂-B < anatase < brookite and there are distinct differences in the character of hole polaron migration in each phase. As well as having fundamental interest, these results have implications for applications of TiO₂ in photocatalysis and photoelectrochemistry, which we discuss.

Tantalum-Doped TiO2 Prepared by Atomic Layer Deposition and Its Application in Perovskite Solar Cells

Tantalum (Ta)-doped titanium oxide (TiO₂) thin films are grown by plasma enhanced atomic layer deposition (PEALD), and used as both an electron transport layer and hole blocking compact layer of perovskite solar cells. The metal precursors of tantalum ethoxide and titanium isopropoxide are simultaneously injected into the deposition chamber. The Ta content is controlled by the temperature of the metal precursors. The experimental results show that the Ta incorporation introduces oxygen vacancies defects, accompanied by the reduced crystallinity and optical band gap. The PEALD Ta-doped films show a resistivity three orders of magnitude lower than undoped TiO₂, even at a low Ta content (0.8–0.95 at.%). The ultraviolet photoelectron spectroscopy spectra reveal that Ta incorporation leads to a down shift of valance band and conduction positions, and this is helpful for the applications involving band alignment engineering. Finally, the perovskite solar cell with Ta-doped TiO₂ electron transport layer demonstrates significantly improved fill factor and conversion efficiency as compared to that with the undoped TiO₂ layer.

Particle Consolidation and Electron Transport in Anatase TiO2 Nanocrystal Films

A sequence of chemical vapor synthesis and thermal annealing in defined gas atmospheres was used to prepare phase-pure anatase TiO₂ nanocrystal powders featuring clean surfaces and a narrow particle size distribution with a median particle diameter of 14.5 ± 0.5 nm. Random networks of these nanocrystals were immobilized from aqueous dispersions onto conducting substrates and are introduced as model systems for electronic conductivity studies. Thermal annealing of the immobilized films at 100 °C < T < 450 °C in air was performed to generate particle-particle contacts upon virtual preservation of the structural properties of the nanoparticle films. The distribution of electrochemically active electronic states as well as the dependence of the electronic conductivity on the Fermi level position in the semiconductor films was studied in aqueous electrolytes in situ using electrochemical methods. An exponential distribution of surface states is observed to remain unchanged upon sintering. However, capacitive peaks corresponding to deep electron traps in the nanoparticle films shift positive on the potential scale evidencing an increase of the trapping energy upon progressive thermal annealing. These peaks are attributed to trap states at particle-particle interfaces in the random nanocrystal network (i.e., at grain boundaries). In the potential region, where the capacitive peaks are detected, we observe an exponential conductivity variation by up to 5 orders of magnitude. The potential range featuring the exponential conductivity variation shifts positive by up to 0.15 V when increasing the sintering temperature from 100 to 450 °C. Importantly, all films approach a potential- and sintering-temperature-independent maximum conductivity of ∼10^-4 Ω^-1·cm^-1 at more negative potentials. On the basis of these results we introduce a qualitative model, which highlights the detrimental impact of electron traps located on particle-particle interfaces on the electronic conductivity in random semiconductor nanoparticle networks.