Doped SrTiO3 Research Articles

Colossal dielectric behavior of Nb-doped SrTiO3 single crystal

Crystals are usually used to explore the origin of material properties due to the fact that they have no grain boundaries and no pores. Although doped SrTiO3 crystal have remarkable dielectric properties, the origin of this improvement is still not fully understood. Herein, Nb5+ has been selected as a donor ion to study the dielectric properties of SrTiO3 due to its radius close to Ti4+. It has been found that the dielectric permittivity of the Nb-doped crystal is greater than 94,500 and the dielectric loss is maintained below 0.002 in the frequency range from 20 Hz to 0.2 MHz. X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy indicate that the dielectric properties of doped crystals were improved due to the existence of oxygen vacancies and there was no effect of interface polarization. It was beneficial for the Nb-doped crystal to be annealed in a N2 atmosphere resulting in oxygen loss more oxygen vacancies, and a higher concentration of Ti3+. The electron can be bound by VO•• and Ti4+ and form the defect dipole [Ti4+∙e − VO•• − Ti4+∙e], which showed a higher permittivity and lower dielectric loss. In summary, the origin and optimization of dielectric properties of SrTiO3-based materials can be deeply understood when from doped SrTiO3 crystals.

Practical liquid phase mixing-calcination synthesis of Nb5+ doped SrTiO3 nanoparticles with elevated photocatalysis

The mediocre photocatalytic activity and unpractical preparation methods of SrTiO3 have restricted its practical application as a photocatalyst. Herein, a practical liquid phase mixing-calcination method was designed to synthesize Nb5+ doped SrTiO3 (Nb-SrTiO3) with remarkably elevated photocatalytic activity. Nb-SrTiO3 nanoparticles were synthesized by mixing Sr(NO3)2, NbCl5 and tetrabutyl titanate in deionized water (DW), evaporating the solvent (DW) and calcining the evenly mixed precursors at 700 °C for 4 h. The as-synthesized Nb-SrTiO3 exhibited a photocatalytic activity 4.2 times that of SrTiO3 in the reduction of Cr(VI) under Xenon lamp irradiation. Moreover, Nb-SrTiO3 showed a rather good photocatalytic stability. Nb5+ doping can decrease the particle size and promote the photogenerated carrier separation and transfer of SrTiO3, thereby contributing to heightened photocatalytic activity of Nb-SrTiO3.

Effect of vacuum sintering on the microstructure and spectroscopic properties of Eu doped strontium titanate ceramics

In this work, we report on the optical properties of europium-doped SrTiO3, synthesized in situ via the sol-gel method as nanopowders and sintered ceramic bulks by heat treatment in vacuum. The structure and morphology of the Sr1-3x/2EuxTiO3 (x = 0.01–0.07) nanopowders and ceramics, concerning to the europium concentration, were characterized by X-ray powder diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The optical properties of as-prepared materials were also investigated. These analyses demonstrated that a temperature of 1425 °C in a vacuum of 10−4 Pa and a plateau of 4h are the optimal sintering conditions for Sr1-3x/2EuxTiO3 ceramics. Eu3+ doped SrTiO3 ceramics exhibit visible red photoluminescence characteristics. The emission of SrTiO3:Eu3+ phosphor with different amounts of Eu3+, excited by 395 nm and 465 nm light, was studied. The emission spectra exhibit characteristic luminescence from 5D0 → 7Fj (j = 0, 1, 2, 3, 4) intra-4f shell Eu3+ ion transitions. The most intense transitions are 5D0 → 7F1 (591 nm) for pumping at 395 nm and, 5D0 → 7F2 (615 nm) at 465 nm. The corresponding coordinates CIE for the Eu-doped SrTiO3 samples were calculated from the emission spectra obtained at 395 nm and 465 nm excitation wavelengths. This study demonstrates that the original protocol of materials engineering used is a simple and efficient method for manufacturing novel nanophosphors with enhanced reddish-orange emission. The as-obtained results indicated that the sol-gel synthesis combined with the sintering process in a vacuum at an appropriate temperature, plateau, could be a novel way to fabricate materials based on strontium titanate with reasonable optical properties.

Oxysulfide-Based Photocatalyst Sheet Modified with Silica Layer for Z-Scheme Overall Water Splitting at Atmospheric Pressure

Photocatalytic water splitting using particulate photocatalysts is considered as an ideal route to produce economically cheap renewable, and transportable H2 utilizing sunlight [1]. Photocatalyst sheets are directly scalable and have recently gained interest because a benchmark STH conversion efficiency of 1.0% was achieved at near ambient pressure using the one composed of Rh, La-codoped SrTiO3 and Mo-doped BiVO4 (BVO:Mo) as the H2- and O2- evolution photocatalysts (HEP and OEP), respectively. However, such systems absorb only a little portion of the sunlight due to the weak visible light absorption of the doped SrTiO3 [2]. To utilize longer wavelength visible light, our group has recently studied photocatalyst sheets composed of Ga-doped La5Ti2Cu0.9Ag0.1O7S5 (LTCA:Ga) (HEP) and BVO:Mo (OEP) with Au as conducting layer. The combination of LTCA:Ga/Au/BVO:Mo modified with Rh/Cr2O3 and CoO x as reduction and oxidation cocatalysts, respectively, achieved an STH efficiency of 0.4% under reduced pressure, but this was far lower than the oxide-based sheet system and the activity was decreased sharply against background pressure [3]. Considering intense visible light absorption of LTCA (λabs ~ 700 nm) and the associated maximum theoretical energy conversion efficiency for this combination, improvements in the net STH efficiency and activation under atmospheric pressure is highly indispensable.In this study, LTCA photocatalysts with different dopants were applied to the sheet system in combination with hydrothermally synthesized BVO:Mo and the resulting sheets were modified with CoO x /Cr2O3/Rh cocatalysts in an attempt to upgrade the water splitting activity. Moreover, surface modification onto LTCA-based photocatalyst sheets with a thin amorphous TiO2 and SiO2 layer were evaluated at elevated pressure and high temperature.Doped LTCA were synthesized by a conventional solid-state reaction in sealed evacuated quartz tubes similar to previous studies [3]. Precursors were blended inside a glove box. BVO:Mo (Mo 0.1 mol%) was synthesized by a hydrothermal method [4]. Photocatalyst sheet was fabricated by the particle transfer method [3]. SiO2 and TiO2 layers functioning as molecular sieves were deposited onto the cocatalyst-modified sheet from titanium tetraisopropoxide (TTIP) or tetraethyl orthosilicate (TEOS) by photo-decomposition and hydrolysis method, respectively, to prevent backward reactions at elevated pressure [5]. Photocatalytic water splitting reaction was carried out at different temperatures and pressures using a closed gas circulation system.Rh cocatalyst was first adsorbed and photo-deposited onto the doped LTCA/Au/BVO:Mo(h) sheet [3]. Gas evolution rates were significantly enhanced upon deposition of Cr2O3 layer as the oxygen reduction reaction (ORR), a backward reaction, was majorly suppressed promoted by uncovered Rh surface under the measured condition. Photocatalyst sheet composed of doped LTCA/Au/BVO:Mo(h) with optimal dopant amounts and cocatalysts loadings showed the H2 and O2 evolution rates of 110 and 54 µmol/h, respectively, under visible light irradiation and yielded an STH of 0.67% at 4 kPa and temperature 301 K [6]. The water splitting activity was improved by 1.6 times under reduced pressure compared to the previously studied LTCAGa/Au/BVO:Mo. However, this activity was highly sensitive against background pressure as nearly 90% reduced upon increasing pressure from 2 to 90 kPa owing to the predominating ORR occurring on LTCA and exposed Au surface. To suppress this backward reaction, TiO2 layer was coated to suppress ORR but the sheet failed to work at a temperature higher than 318 K due to the condensation of such layer. However, SiO2 layer coated sheet effectively suppressed ORR backward reaction and stably work under hot water and elevated pressure [6]. Consequently, under optimized condition SiO2 deposited sheet achieved an STH of 0.4% at 90 kPa at temperature 333 K.Doped LTCA materials were applied to the photocatalyst sheet systems as HEP along with BVO:Mo and Au layer as OEP and conductive layer, respectively, for Z-scheme pure water splitting under visible light. This combination achieved an STH of 0.67% at reduced pressure. This STH is considered as one of the highest values recorded for OWS systems involving non-oxide photocatalysts. The activity of the photocatalyst sheet was found negatively dependent onto background pressure due to the ORR on LTCA and Au layer. Surface modifications with SiO2 layer activated the sheet under atmospheric pressure. These findings will contribute to the improvement in the STH for other systems under realistic condition where OWS is likely to be conducted at an ambient pressure. References Nandy et al. Chem. Sci. 2021, 12, 9866-9884.Wang et al. Nat. Mater. 2016, 15, 611-615.Chen et al. ACS Catal. 2023, 13, 3285-3294.Li et al. Energy Environ. Sci., 2014, 7, 1369-1376.Pan et al. Angew. Chem. Int. Ed. 2015, 54, 2955-2959.Nandy et al. Joule, 2023, under revision.

Metal Doping in SrTiO3 Photocatalyst to Improve the Photocatalytic Activity for CO2 Conversion

Among various strategies toward finding new energy sources to replace conventional fossil fuels, photocatalytic conversion of CO2 into useful chemicals using water as an electron donor and a proton source has been increasingly attracting attentions because it can convert light energy to chemical energy. The photocatalytic conversion of CO2 involves photoabsorption (light harvesting), charge separation, and H2O oxidation, as well as the consumption of the photogenerated electrons for reduction. The evolution of H2 rather than CO is preferred when H2O is used as the electron donor, since the redox potential of CO2/CO (−0.51 V vs NHE, at pH 7) is more negative than that of H+/H2 (−0.41 V vs NHE, at pH 7) in an aqueous solution. Therefore, “high selectivity toward CO2 reduction” is one of the most important requirements for the photocatalytic system to achieve the CO2 conversion in water. Various mixed oxides including tantalum-based oxides, titanium-based oxides, gallium-based oxides, and niobium-based oxides were found to be excellent photocatalysts for the conversion of CO2 into CO in the presence of Ag cocatalyst. However, these materials show significant activity for CO evolution only when irradiated with deep region UV-light (λ < 300 nm). Therefore, a series of photocatalysts which can drive under the photoirradiation with UV-light at λ > 300 nm is strongly desired in the research area of the photocatalytic conversion of CO2. As continuous efforts to develop effective photocatalysts for the photocatalytic conversion of CO2 to CO in water, in this study, we investigated metal doping in the SrTiO3 photocatalyst, and evaluated the photocatalytic activity of the doped SrTiO3 photocatalysts for the conversion of CO2 to CO under UV-LED-light irradiation at 365 nm. SrTiO3 photocatalyst was prepared by a previously reported solid-state method (Journal of Power Sources, 2008, 183, 701−707). SrCO3 and TiO2 were ground for 15 min in a mortar and pestle, and the mixture was transferred to an alumina crucible to calcine at 1373 K for 10 h in air. The resulting powder was washed three times with hot ultrapure water and dried overnight at room temperature. M-doped SrTiO3 (M−SrTiO3, M = Al, Zn, Li, Mn, W, Ca, Y, and Mg) was synthesized via a flux method (ACS Applied Energy Materials, 2020, 3, 1468−1475.). Excess amount of SrCl2 flux was added to a mixture of the prepared SrTiO3 and a metal oxide as a doping source. The mixture was ground for 15 min in a mortar and pestle, transferred to an yttria crucible, and calcined at 1418 K for 15 h. The obtained powder was washed three times with hot ultrapure water, and dried overnight at 353 K. Photocatalytic activity of AgCo/M−SrTiO3 photocatalyst was evaluated by using an external-irradiation type reaction vessel in a quasi-flowing batch system. The photocatalyst powder was dispersed in 0.2 L of an aqueous NaHCO3 solution, and the dissolved air in the suspension was degassed by a flow of high-purity CO2 gas. CO2 gas was continuously bubbled into the reaction solution at a flow rate of 30 mL/min. The suspension was irradiated using a monochromatic UV-LED lamp at 365 nm (IRS-1000, CELL System Co., Ltd., Japan). The gaseous products in the outlet gas, e.g., CO, H2, and O2, were analyzed by gas chromatography (FID-GC with methanizer: CO, TCD-GC: H2 and O2). The attached figure shows a formation rate of CO (red), H2 (blue), and O2 (green) in the photocatalytic conversion of CO2 with H2O using the M−SrTiO3 photocatalysts (M = Al, Zn, Li, Mn, W, Ca, Y, and Mg) in the presence of AgCo cocatalyst. Zn−, Li−, Mn−, W−, Ca−, and Y−SrTiO3 showed extremely low photocatalytic activities for the conversion of CO2, whereas Al− and Mg−SrTiO3 produced certain amount of CO and O2. Mg−SrTiO3 exhibited the highest activity for CO2 conversion, which has been observed as a CO formation rate of ca. 20 μmol h−1. However, H2 evolution was thoroughly suppressed (0.055 μmol h−1) in the presence of AgCo/Mg−SrTiO3, where the selectivity toward CO evolution was higher than 99%. These results indicated that Mg-doping into SrTiO3 improved the photocatalytic activity for the conversion of CO2. Moreover, stoichiometric O2 evolution (8.9 μmol h−1) was found in the presence of AgCo/Mg−SrTiO3, suggesting that H2O acts as both an electron donor and a proton source for the photocatalytic conversion of CO2 in water. The apparent quantum efficiency (AQE) of AgCo/Mg−SrTiO3 photocatalyst in the photocatalytic conversion of CO2 under monochromatic UV-light irradiation (365 nm) was determined to be 0.05%. Figure 1

Morphology-controlled fabrication of Nb5+ doped SrTiO3 for promoting photocatalytic reduction of Cr(VI)

To achieve improved photocatalytic activity of Nb5+ doped SrTiO3 (Nb-SrTiO3), Nb-SrTiO3 nanomaterials with different morphologies were prepared via a one-step solvothermal method in ethylene glycol (EG)-H2O mixed solvents. The photocatalytic activities of the as-prepared Nb-SrTiO3 samples were evaluated by the reduction of Cr(VI) under xenon lamp irradiation. The effects of the ratio of VEG/VH2O on the morphology, optical and electronic properties, and photocatalytic activity of Nb-SrTiO3 were investigated. The results indicated that Nb-SrTiO3 with different shapes, including block, sphere, and flower-like structure, could be prepared by controlling the ratio of VEG/VH2O. The sphere-shaped Nb-SrTiO3-3 obtained with the ratio of VEG/VH2O being 4 exhibited stronger light absorption, more efficient separation and transfer of photo-generated charge carriers, and superior photocatalytic activity. Moreover, Nb-SrTiO3-3 can catalyze the reduction of almost 100 % Cr(VI) in the diluted Cr(VI)-containing passivation solution after light irradiation for 25 min, so it holds promise in Cr(VI) wastewater treatment.

Structural and dielectric properties of Y3+ doped SrTiO3: a novel anode materials for solid oxide fuel cell

An ongoing and elusive task is to create single phase anode materials for solid oxide fuel cells (SOFC). Compared to the typical Ni-YSZ cermet anode, perovskite oxide based anode material has a number of benefits. Here, we present the structural and electrical characteristics of Y3+ doped SrTiO3 ceramics that are used as the anode in solid oxide fuel cells. The samples with nominal composition x = 0 and 0.05 were synthesized by high energy ball milling. The powder X-ray diffraction pattern reveals that the synthesized samples crystallize into cubic crystal structure with space group Pm3m for both the compositions. The samples were sintered at a temperature of 1300 °C for 6h. The sintered density of the samples was found to be greater than 97%. The temperature dependent dielectric measurement studies reveal that dielectric constant increases with yttrium doping and shows relaxer behavior. A higher dielectric constant and electrical conductivity was observed for the Y-doped sample which fulfill the basic requirement for anode materials of solid oxide fuel cells.

X9R-type SrTiO3 based ceramics with colossal dielectric permittivity and low loss derived from defect engineering and high-energy ball milling

The miniaturization of electronic components has become a megatrend in the future, making colossal dielectric permittivity materials receive unprecedented attention. In this work, we proposed a universal route to achieve colossal dielectric permittivity, low loss factor and excellent dielectric temperature stability in rare earth doped perovskite oxide SrTiO3. Sm doped SrTiO3 ceramics prepared by high-energy ball milling exhibit ultrahigh dielectric permittivity 441,543, low loss factor 0.02, good dielectric temperature stability (<15 %) in the temperature range of −55–200 °C, and very high resistance 109Ω, which meets the EIA X9R standard, XPS and impedance spectra indicate that the electron-pinned defect-dipoles effect is the origin of colossal dielectric permittivity and low loss factor. This work shows that Sm-doped SrTiO3 activated by high-energy ball milling has broad prospects in the applications of thermally-stable miniaturized capacitors.

Effect of doping SrTiO3 with Nb studied with wide-range spectroscopic ellipsometry

The low-temperature optical properties of a SrTiO3 crystal doped with 0.7% wt. Nb and a pure SrTiO3 reference crystal were studied using spectroscopic ellipsometry. The optical constants and dielectric functions were obtained in the spectral range of 0.8–8.8 eV at temperatures from 10 to 300 K, and the optical conductivity was obtained in the spectral range of 0.03–1 eV at 300 K. Analysis of the optical conductivity spectra in the infrared spectral range confirmed the presence of free electrons and additional absorption hump between 0.1 and 0.4 eV (806 and 3226 cm−1) in doped SrTiO3 and optical phonons in both doped and undoped SrTiO3. The performed analysis of optical absorption spectra in the range 0.8–8.8 eV revealed frustration of the indirect bandgap and an increase in the direct bandgap energy by ∼0.03 eV in Nb-doped SrTiO3, compared to undoped SrTiO3 over the whole temperature range. The energies of the peaks’ maxima of the dielectric function spectra did not significantly differ for doped and pure SrTiO3. For both Nb-doped and pure SrTiO3, temperature dependence of the direct bandgap energy and that of the index of refraction showed inflection between 100 and 150 K, which may be considered evidence of an antiferrodistortive phase transition from a cubic to a tetragonal structure.

Impact of La, Ni-doping on structural and electronic properties of SrTiO3 for photocatalytic water splitting

The study of the physical properties of doped SrTiO3 materials using the full potential linearised augmented plane wave (FP-LAPW) technique has revealed interesting findings. In particular, we explored the semiconducting behavior of La- and Ni-doped SrTiO3 alloys through band structure calculations. The results showed that Sr0.945La0.055TiO3 exhibited n-type semiconducting behavior with a bandgap of approximately 2.924 eV. In contrast, Ni-doped SrTiO3 demonstrated a reduction in band gap value of around 1.337 eV due to the emergence of 3d-Ti states in the middle of the SrTiO3 band gap. This unique characteristic makes SrNi0.055Ti0.945O3 a promising candidate for optoelectronic devices. Furthermore, the photo-electrochemical analysis revealed that Sr0.945La0.055TiO3 has a band edge position that is suitable for photocatalytic water splitting, making it a more attractive candidate for this application compared to SrNi0.055Ti0.945O3. These findings have important implications for the design and development of novel materials with specific properties for a range of technological applications.

Tunable-wavelength photoluminescence of a flexible transition metal doped oxide phosphor thin film

Near-infrared luminescence phosphors are key material basis to potential applications for light sources and optoelectronic devices. In particular, it is vital to tune the luminescent properties of these phosphors in a flexible and controllable manner. Here, we demonstrate that a flexural strain originated from bending can be used to modulate the photoluminescence of freestanding Ni2+ doped SrTiO3 membranes. The bent membranes show remarkable red-shift emissions, arising from the variations of the symmetry of host materials and the local crystal fields around the Ni2+ ions. In addition, the phosphor films show a reversible and stable wavelength modulation with remarkable anti-fatigue characteristics after 104 bending cycles. These results provide a potential routine to develop flexible strain-tunable devices for applications in optical amplifiers and other optoelectronics.

Enhanced Thermoelectric Performance of Rare-Earth-Free n-Type Oxide Perovskite Composite with Graphene Analogous 2D MXene.

Here, the first experimental demonstration on the effect of incorporating new generation 2D material, MXene, on the thermoelectric performance of rare-earth-free oxide perovskite is reported. The charge localization phenomenon is predominant in the electron transport of doped SrTiO3 perovskites, which deters from achieving a higher thermoelectric power factor in these oxides. In this work, it is shown that incorporating Ti3 C2 Tx MXene in a matrix of SrTi0.85 Nb0.15 O3 (STN) facilitates the delocalization of electrons resulting in better than single-crystal-like electron mobility in polycrystalline composites. A 1851% increase in electrical conductivity and a 1000% enhancement in power factor are attained. Besides, anharmonicity caused by MXene in the STN matrix has led to enhanced Umklapp scattering giving rise to lower lattice thermal conductivity. Hence, 700% ZT enhancement is achieved in this composite. Further, a prototype of thermoelectric generator (TEG) using only n-type STN + MXene is fabricated and a power output of 38mW is obtained, which is higher than the reported values for oxide TEG.

Terahertz time-domain spectroscopic ellipsometry of metallic strontium titanate

The coexistence of the polar instability and metallic conduction has attracted much attention. Ferroelectric soft optic modes and carrier electrons were investigated in A- and B-site heterovalent doped strontium titanate SrTiO3 crystals by the terahertz time-domain spectroscopic ellipsometry. The observed real and imaginary parts of a complex dielectric constant were fitted by the damped harmonic oscillator model for the ferroelectric soft optic mode and the Drude-Smith model for carrier electrons. The observed soft mode frequency increases as the La and Nb content increases. It is found that the square of the ferroelectric soft mode frequency linearly changes to the carrier electron concentration. This fact indicates that the carrier electrons suppress the ferroelectric instability and the ferroelectric soft mode frequency increases as the carrier concentration increases.

Optimization of Z-Scheme Photocatalytic Reactors for Solar Water Splitting

The U.S. Department of Energy recently announced its first Energy Earthshot on Clean Hydrogen, with a cost target of $1/kg-H2 by 2031. Assuming future utility-scale grid electricity prices from photovoltaics ($0.02/kWh), 80% of the cost of H2 would come from performing low-temperature water electrolysis at its thermoneutral voltage, with zero additional overpotential. This fact motivates alternative, less-expensive means of using light to generate mobile charge carriers than photovoltaics, and reactor designs with exceedingly low capital costs, like those we recently invented. Systems using low capital cost reactors benefit from low-voltage operation, which represents a paradigm shift from current state-of-the-art electrolyzers that aim to operate at high current densities. Analytical models predict that solar photocatalytic water splitting inherently operates at low voltages through use of an ensemble of optically thin photoabsorbers each operating at a low rate. Collectively the ensemble exhibits larger overall solar-to-hydrogen conversion efficiencies in comparison to optically thick designs. In efforts to attain these predicted higher efficiencies, we are performing detailed studies on the properties of state-of-the-art doped SrTiO3 and BiVO4 photocatalyst particles. During my talk, I will share our recent efforts in atomic-layer deposited ultrathin oxide coatings to impart redox selectivity and materials stability, single-photocatalyst-particle current–potential behavior and mobile charge carrier properties, and atomic-level information on dopant distributions and materials interfaces obtained from electron microscopies and X-ray spectroscopies. Collectively, our discoveries provide new design guidelines and additional research pathways for the development of effective composite materials to serve as active components in techno-economically viable artificial photosynthetic devices.

A DFT+U approach to doped SrTiO3 for solar harvesting applications

We present a Hubbard U optimized density functional theory based study of band gap engineered doped SrTiO3 suitable for light sensitive applications. We tune the Hubbard U parameter Uopt to match the experimental direct band gap of SrTiO3. We benchmarked the DFT+Uopt derived density of states, band structure and optical properties with that of sophisticated Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional simulations. The reliability of the DFT+U method is further tested in simulating stability of the cubic SrTiO3 from elastic tensor and density functional perturbation theory based phonon band structure calculations. Moreover, the DFT+Uopt simulated SrTiO3 Raman peaks are identified against the experimental observations found in existing literature. We red shift the energy band gap of SrTiO3 within the DFT+Uopt formalism from ultraviolet to visible range by incorporating different dopants such as Pt, S and Se which is consistent with recent HSE06 hybrid functional based simulations found elsewhere. The optical absorption simulations revealed steep absorption edges of doped SrTiO3 in the visible spectrum. Overall, the DFT+U approach successfully probed the potential of doped SrTiO3 in solar harvesting applications.

Superconductivity from energy fluctuations in dilute quantum critical polar metals

Superconductivity in low carrier density metals challenges the conventional electron-phonon theory due to the absence of retardation required to overcome Coulomb repulsion. Here we demonstrate that pairing mediated by energy fluctuations, ubiquitously present close to continuous phase transitions, occurs in dilute quantum critical polar metals and results in a dome-like dependence of the superconducting Tc on carrier density, characteristic of non-BCS superconductors. In quantum critical polar metals, the Coulomb repulsion is heavily screened, while the critical transverse optical phonons decouple from the electron charge. In the resulting vacuum, long-range attractive interactions emerge from the energy fluctuations of the critical phonons, resembling the gravitational interactions of a chargeless dark matter universe. Our estimates show that this mechanism may explain the critical temperatures observed in doped SrTiO3. We provide predictions for the enhancement of superconductivity near polar quantum criticality in two- and three-dimensional materials that can be used to test our theory.

(Invited) Optimization of Z-Scheme Photocatalytic Reactors for Solar Water Splitting

The U.S. Department of Energy recently announced its first Energy Earthshot on Clean Hydrogen, with a cost target of $1/kg-H2 by 2031. Assuming future utility-scale grid electricity prices from photovoltaics ($0.02/kWh), 80% of the cost of H2 would come from performing low-temperature water electrolysis at its thermoneutral voltage, with zero additional overpotential. This fact motivates alternative, less-expensive means of using light to generate mobile charge carriers than photovoltaics, and reactor designs with exceedingly low capital costs, which we recently invented. Systems that use low capital cost reactors benefit from low-voltage operation, which represents a paradigm shift from current state-of-the-art electrolyzers that aim to operate at high current densities. Analytical models predict that solar photocatalytic water splitting inherently exhibits such low-voltage operation through use of an ensemble of optically thin photoabsorbers, which results in larger overall solar-to-hydrogen conversion efficiencies in comparison to optically thick designs. In efforts to attain these predicted higher efficiencies, we are performing detailed studies on the properties of state-of-the-art doped SrTiO3 photocatalyst particles. During my talk, I will share our recent efforts in atomic-layer deposited ultrathin oxide coatings to impart redox selectivity and materials stability, single-photocatalyst-particle current–potential behavior and mobile charge carrier properties, and atomic-level information on dopant distributions and materials interfaces obtained from electron microscopies and X-ray spectroscopies. Collectively our discoveries provide new design guidelines and additional research pathways for the development of effective composite materials to serve as active components in techno-economically viable artificial photosynthetic devices.

Epitaxial SrTiO3 films with dielectric constants exceeding 25,000

SignificanceSemiconductor interfaces are among the most important in use in modern technology. The properties they exhibit can either enable or disable the characteristics of the materials they connect for functional performance. While much is known about important junctions involving conventional semiconductors such as Si and GaAs, there are several unsolved mysteries surrounding interfaces between oxide semiconductors. Here we resolve a long-standing issue concerning the measurement of anomalously low dielectric constants in SrTiO3 films with record high electron mobilities. We show that the junction between doped and undoped SrTiO3 required to make dielectric constant measurements masks the dielectric properties of the undoped film. Through modeling, we extract the latter and show that it is much higher than previously measured.

Down conversions luminescent properties of Eu doped SrTiO3

The work presents down conversion (DC) luminescent properties of pure SrTiO3 (STO) and Eu3+ doped STO prepared by wet chemical reaction sintered at 1100°C. Prepared samples were characterized by FT-IR, UV absorption, XRD, EDX and fluorescent spectroscopy. The FT-IR spectrum of Eu3+ doped STO had absorption bands at 922, 972 and 538 cm-1 indicating the stretching vibration of Sr-O, Eu-O and Ti-O bond, respectively. XRD pattern of SrTiO3 sintered at 1100°C depicted a typical perovskite of cubic symmetry. The EDX spectra confirmed the uniform substitution of Eu3+ in STO. The optical absorption edge of pure SrTiO3 was in UV region. Emission properties were studied by exciting the pure STO and Eu doped STO by 280,325 and 396 nm wavelengths. Eu3+ doping enhanced the emission intensity if STO. Multiple peaks were observed in visible region ranging from 400 to 700m which might be due to defect states present in energy band gap. Therefore visible emissions were obtained from STO and Eu3+ doped STO by exciting with UV light. Visible emission from prepared STO and Eu3+ doped STO powder could be useful in solid state lighting and for display devices.

Transition Metal Doping Induces Ti3+ to Promote the Performance of SrTiO3 @TiO2 Visible Light Photocatalytic Reduction of CO2 to Prepare C1 Product.

Transition metal Fe, Co, Ni and Cu doped strontium titanate-rich SrTiO3 @TiO2 (STO@T) materials were prepared by hydrothermal method. The prepared doped materials exhibit better photocatalytic CO2 reduction to CH4 ability under visible light conditions. Among them, Fe-doped and undoped SrTiO3 @TiO2 under visible light conditions CO2 reduction products only CO, while M-STO@T (M=Co, Ni, Cu) samples converted CO2 to CH4 . The average methane yield of Ni-doped STO@T samples are as high as 73.85 μmol g-1 h-1 . The production of methane is mainly due to the increase in the response of the doped samples to visible light. And the increase in the separation rate of photogenerated electrons and holes and the efficiency of electron transport caused by the generation of impurity levels. The impurity level caused by Ti3+ plays an important role in the production of methane by CO2 visible light reduction. Ni doping effectively improves the photocatalytic performance of STO@T and CO2 reduction mechanism were explained.