Complex Photoconductivity Reveals How the Nonstoichiometric Sr/Ti Affects the Charge Dynamics of a SrTiO3 Photocatalyst

Enhance the stability of oxygen vacancies in SrTiO3 via metallic Ag modification for efficient and durable photocatalytic NO abatement

Oxygen vacancy engineering is an appealing strategy in the direction of photocatalytic pollutant purification. Unfortunately, the short lifetime of oxygen vacancies significantly limits photocatalytic efficiencies and their application. Herein, we report that such a scenario can be resolved via plasmonic silver metal modification SrTiO₃ containing oxygen vacancies, which can achieve a high NO removal rate of 70.0% and long stability. This outstanding photocatalytic activity can be attributed to the increased optical response range and carrier separation by metallic Ag with the unique character of localized surface plasmonic resonance (LSPR) effect. Moreover, the intrinsic mechanism of how the plasmonic metal could enhance the stability of oxygen vacancies is proposed. The plasmon-driven hot carriers inject SrTiO₃ support that promotes the regeneration of oxygen vacancies around the interface, meanwhile, the introduction of Ag nanoparticles prevents the oxygen vacancies from being filled by the reactant. This work elucidates the unique role of plasmonic metal in photocatalysis, providing an innovative idea for improving catalytic stability.

Octahedra-Tilted Control of Displacement Disorder and Dielectric Relaxation in Mn-Doped SrTiO3 Single Crystals

Strontium titanate SrTiO₃ (STO) is a canonical example of a quantum paraelectric, and its doping with manganese ions unlocks its potential as a quantum multiferroic candidate. However, to date, the specifics of incorporation of the manganese ion into the perovskite lattice and its impact on structure-property relationships are debatable questions. Herein, using high-precision X-ray diffraction of a Mn (2 atom %)-doped STO single crystal, clear fingerprints of the displacement disorder of Mn cations in the perovskite B-sublattice are observed. Moreover, near the temperature of the antiferrodistortive transition, the off-center Mn position splits in two, providing the unequal potential barrier's distribution for possible local atomic hopping. A link with this was found via analysis of the dielectric response that reveals two Arrhenius-type relaxation processes with similar activation energies (35 and 43 meV) and attempt frequencies (1 × 10¹¹ and ∼1.6 × 10¹⁰ Hz), suggesting similar dielectric relaxation mechanisms.

Charge Carrier Trapping during Diffusion Generally Observed for Particulate Photocatalytic Films

Photo-excited charge carriers play a vital role in photocatalysts and photovoltaics, and their dynamic processes must be understood to improve their efficiencies by controlling them. The photo-excited charge carriers in photocatalytic materials are usually trapped to the defect states in the picosecond time range and are subject to recombination to the nanosecond to microsecond order. When photo-excited charge carrier dynamics are observed via refractive index changes, especially in particulate photocatalytic materials, another response between the trapping and recombination phases is often observed. This response has always provided the gradual increase of the refractive index changes in the nanosecond order, and we propose that the shallowly trapped charge carriers could still diffuse and be trapped to other states during this process. We examined various photocatalytic materials such as TiO2, SrTiO3, hematite, BiVO4, and methylammonium lead iodide for similar rising responses. Based on our assumption of surface trapping with diffusion, the responses were fit with the theoretical model with sufficient accuracy. We propose that these slow charge trapping processes must be included to fully understand the charge carrier dynamics of particulate photocatalytic materials.

In situ photoacoustic analysis of near-infrared absorption of rhodium-doped strontium titanate photocatalyst powder

Near-infrared absorption of strontium titanate (SrTiO₃) doped with rhodium (Rh) was investigated by photoacoustic (PA) Fourier transform infrared spectroscopy. In the absence of an electron acceptor and the presence of a hole scavenger, the largest absorption change in the Rh valence state from tetravalent to trivalent was observed in Rh-doped SrTiO₃ prepared at 1473 K, which showed the highest activity for hydrogen evolution. PA measurements revealed the effective redox cycle mechanism between tetravalent and trivalent Rh ions in Rh-doped SrTiO₃.

Time-Resolved Changes in Dielectric Constant of Metal Halide Perovskites under Illumination

Despite their impressive performance as a solar absorber, much remains unknown on the fundamental properties of metal halide perovskites (MHPs). Their polar nature in particular is an intense area of study, and the relative permittivity (ε_r) is a parameter widely used to quantify polarization over a range of different time scales. In this report, we have exploited frequency-dependent time-resolved microwave conductivity (TRMC) to study how ε_r values of a range of MHPs change as a function of time, upon optical illumination. Further characterization of charge carriers and polarizability are conducted by femtosecond transient absorption and stimulated Raman spectroscopy. We find that changes in ε_r are roughly proportional to photogenerated carrier density but decay with a shorter time constant than conductance, suggesting that the presence of charge carriers alone does not determine polarization.

Evaluation-oriented exploration of photo energy conversion systems: from fundamental optoelectronics and material screening to the combination with data science

Light is a form of energy that can be converted to electric and chemical energies. Thus, organic photovoltaics (OPVs), perovskite solar cells (PSCs), photocatalysts, and photodetectors have evolved as scientific and commercial enterprises. However, the complex photochemical reactions and multicomponent materials involved in these systems have hampered rapid progress in their fundamental understanding and material design. This review showcases the evaluation-oriented exploration of photo energy conversion materials by using electrodeless time-resolved microwave conductivity (TRMC) and materials informatics (MI). TRMC with its unique options (excitation sources, environmental control, frequency modulation, etc.) provides not only accelerated experimental screening of OPV and PSC materials but also a versatile route toward shedding light on their charge carrier dynamics. Furthermore, MI powered by machine learning is shown to allow extremely high-throughput exploration in the large molecular space, which is compatible with experimental screening and combinatorial synthesis.

Highly efficient SrTiO3/Ag2O n-p heterojunction photocatalysts: improved charge carrier separation and enhanced visible-light harvesting

Strontium titanate (SrTiO₃) with perovskite structure has recently received significant attention in t he area of photocatalysis. However, challenges remain relating to its industrial applications; the high charge carrier recombination rate and low light-harvesting efficiency being the main two. Herein, a novel strategy based on fabrication of a typical n-p heterojunction has been proposed and the typical narrow-bandgap p-type semiconductor Ag₂O was chosen to be coupled with SrTiO₃ using a facile chemical precipitation method. The phase compositions, microstructures and optical properties of the prepared SrTiO₃/Ag₂O heterostructured photocatalysts have been systematically investigated with an x-ray diffractometer, scanning electron microscope, high resolution transmission electron microscope, x-ray photoelectron spectroscope and UV-vis spectrophotometer. The photocatalytic properties were evaluated through photodegradation of a common organic dye Rhodamine B (RhB). The results demonstrated that the heterostructured photocatalyst SrTiO₃/Ag₂O-0.15 outperformed pristine SrTiO₃ and Ag₂O. Specifically, the reaction rate of SrTiO₃/Ag₂O-0.15 is about 69 times and 4 times that of bare SrTiO₃ and Ag₂O respectively in photodegradation of RhB. The excellent photocatalytic performance was attributed to the synergetic effect between the improved visible-light harvesting efficiency and inhibited electron-hole recombination rate arising from the built-in electric field in a p-n heterojunction, as evidenced by the transient photocurrent and photoluminescence spectrum investigation. Furthermore, the excellent recyclability of the heterostructured photocatalyst was confirmed and holes were verified to be the major active species contributing to the overall degradation. Our findings demonstrate construction of p-n heterojunctions with narrow-bandgap semiconductors as a feasible avenue to promote overall photocatalytic efficiency, through simultaneously boosting charge-carrier separation and expanding photon-absorption range.