Photocatalytic Overall Water Splitting by SrTiO3 with Surface Oxygen Vacancies

Mechanistic understanding of the thermal-assisted photocatalytic oxidation of methanol-to-formaldehyde with water vapor over Pt/SrTiO3

Anaerobic thermal-assisted photocatalytic methanol conversion in the gas phase in the presence of water vapor has been suggested as an interesting way to generate formaldehyde as a valuable coupled product in addition to H2 production. Here, the reaction mechanism and photocatalyst deactivation are investigated in detail using in situ diffuse reflectance infrared fourier transform (DRIFTS) and electron paramagnetic resonance (EPR) spectroscopy. EPR shows that paramagnetic oxygen vacancies are not involved in the reaction mechanism over undoped SrTiO3. Instead, on an optimized 0.1 wt% Pt/SrTiO3 photocatalyst, methoxy species are formed by dissociative adsorption of methanol leading to formaldehyde formation while the formation of CO, CO2 (via a formate intermediate) and methyl formate occurs through three concurrent reactions from formyl species. Our findings suggest that CO adsorbed on Pt is a spectator species not perturbing the reaction kinetics, and deactivation is shown to be strongly correlated with the accumulation of formate groups on SrTiO3, which is more pronounced at high reaction temperatures. The mechanistic understanding provided here forms the basis for the further optimization of photocatalysts to increase methanol conversion and improve formaldehyde selectivity.

Tunable Photocatalytic Activity of CoFe Prussian Blue Analogue Modified SrTiO3 Core-Shell Structures for Solar-Driven Water Oxidation

This study presents a pioneering semiconductor-catalyst core-shell architecture designed to enhance photocatalytic water oxidation activity significantly. This innovative assembly involves the in situ deposition of CoFe Prussian blue analogue (PBA) particles onto SrTiO3 (STO) and blue SrTiO3 (bSTO) nanocubes, effectively establishing a robust p-n junction, as demonstrated by Mott-Schottky analysis. Of notable significance, the STO/PB core-shell catalyst displayed remarkable photocatalytic performance, achieving an oxygen evolution rate of 129.6 μmol g-1 h-1, with stability over an extended 9-h in the presence of S2O82- as an electron scavenger. Thorough characterization unequivocally verified the precise alignment of the band energies within the STO/PB core-shell assembly. Our research underscores the critical role of tailored semiconductor-catalyst interfaces in advancing the realm of photocatalysis and its broader applications in renewable energy technologies.

Flame Spray Pyrolysis Synthesis of Vo-Rich Nano-SrTiO3-x

Engineering of oxygen vacancies (Vo) in nanomaterials allows diligent control of their physicochemical properties. SrTiO3 possesses the typical ABO3 structure and has attracted considerable attention among the titanates due to its chemical stability and its high conduction band energy. This has resulted in its extensive use in photocatalytic energy-related processes, among others. Herein, we introduce the use of Flame Spray Pyrolysis (FSP); an industrial and scalable process to produce Vo-rich SrTiO3 perovskites. We present two types of Anoxic Flame Spray Pyrolysis (A-FSP) technologies using CH4 gas as a reducing source: Radial A-FSP (RA-FSP); and Axial A-FSP (AA-FSP). These are used for the control engineering of oxygen vacancies in the SrTiO3-x nanolattice. Based on X-ray photoelectron spectroscopy, Raman and thermogravimetry-differential thermal analysis, we discuss the role and the amount of the Vos in the so-produced nano-SrTiO3-x, correlating the properties of the nanolattice and energy-band structure of the SrTiO3-x. The present work further corroborates the versatility of FSP as a synthetic process and the potential future application of this process to engineer photocatalysts with oxygen vacancies in quantities that can be measured in kilograms.

Unravelling the interfacial water structure at the photocatalyst strontium titanate by sum frequency generation spectroscopy

The direct conversion of solar energy to hydrogen is considered as a possible method to produce carbon neutral hydrogen fuel. The mechanism of photocatalytic water splitting involves the chemical breakdown of water and re-assembly into hydrogen and oxygen at the interface of a photocatalyst. The selection rules of a suitable material are well established, but the fundamental understanding of the mechanisms, occurring at the interface between the catalyst and the water, remains missing. Using surface specific sum frequency generation spectroscopy, we present here characterisation of the interface between water and the photocatalyst strontium titanate (SrTiO3). We monitor the OH-stretching vibrations present at the interface. Their variations of intensities and frequencies as functions of isotopic dilution, pH and salt concentration provide information about the nature of the hydrogen bonding environment. We observe the presence of water molecules that flip their orientation at pH 5 indicating the point of zero charge of the SrTiO3 layer. These water molecules are oriented with their hydrogen away from the surface when the pH of the solutions is below 5 and pointing towards the surface when the pH is higher than 5. Besides, water molecules donating a H-bond to probably surface TiOH groups are observed at all pH.

CaCu3Ti4O12 nanoparticle-loaded cotton fabric for dual photocatalytic antibacterial and dye degradation applications

CaCu3Ti4O12 (CCTO) nanoparticles (NPs) were screen printed on pristine cotton fabric. The CCTO-coated fabric was characterized using attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), Raman, X-ray diffraction (XRD), x-ray photoelectron spectrometer (XPS), and field emission-scanning electron microscopy (FE-SEM). The modified fabric photocatalytic antibacterial and dye-degradation abilities were assessed. After 2 h of bacterial contact, unwashed CCTO-embedded cotton reduced E. coli and S. aureus by 95.1% and 94.3%, respectively. After 20 washing cycles, the modified fabric was able to eliminate S. aureus and E. coli by more than 85%. The cloth coated with CCTO-NPs degraded the methylene blue (MB) dye by 82% in 4 h, as opposed to the pure cotton's 11% degradation rate. The embedding of CCTO-NPs onto the cotton surface had minimal effect on fabric intrinsic properties like tensile strength, abrasion resistance, and water-vapor permeability.

Nanoparticulate Perovskites for Photocatalytic Water Reduction

SrTiO₃ and BaTiO₃ nanoparticles (NPs) were activated using H₂O₂ or aqueous HNO₃, and pristine and activated NPs were functionalized with a 2,2'-bipyridine phosphonic acid anchoring ligand (1), followed by reaction with RuCl₃^.3H₂O and bpy, RhCl₃^.3H₂O and bpy, or RuCl₃^.3H₂O. The surface-bound metal complex functionalized NPs were used for the photogeneration of H₂ from water, and their activity was compared to related systems using TiO₂ NPs. The role of pH during surface complexation was found to be important. The NPs were characterized using Fourier transform infrared (FTIR) and solid-state absorption spectroscopies, thermogravimetric analysis mass spectrometry (TGA-MS), and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), and the dihydrogen generation was analyzed using gas chromatography-mass spectrometry (GC-MS). Our findings indicate that extensively functionalized SrTiO₃ or BaTiO₃ NPs may perform better than TiO₂ NPs for water reduction.

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.

In Tandem Control of La-Doping and CuO-Heterojunction on SrTiO3 Perovskite by Double-Nozzle Flame Spray Pyrolysis: Selective H2 vs. CH4 Photocatalytic Production from H2O/CH3OH

ABO₃ perovskites offer versatile photoactive nano-templates that can be optimized towards specific technologies, either by means of doping or via heterojunction engineering. SrTiO₃ is a well-studied perovskite photocatalyst, with a highly reducing conduction-band edge. Herein we present a Double-Nozzle Flame Spray Pyrolysis (DN-FSP) technology for the synthesis of high crystallinity SrTiO₃ nanoparticles with controlled La-doping in tandem with SrTiO₃/CuO-heterojunction formation. So-produced La:SrTiO₃/CuO nanocatalysts were optimized for photocatalysis of H₂O/CH₃OH mixtures by varying the La-doping level in the range from 0.25 to 0.9%. We find that, in absence of CuO, the 0.9La:SrTiO₃ material achieved maximal efficient photocatalytic H₂ production, i.e., 12 mmol g^-1 h^-1. Introduction of CuO on La:SrTiO₃ enhanced selective production of methane CH₄. The optimized 0.25La:SrTiO₃/0.5%CuO catalyst achieved photocatalytic CH₄ production of 1.5 mmol g^-1 h^-1. Based on XRD, XRF, XPS, BET, and UV-Vis/DRS data, we discuss the photophysical basis of these trends and attribute them to the effect of La atoms in the SrTiO₃ lattice regarding the H₂-production, plus the effect of interfacial CuO on the promotion of CH₄ production. Technology-wise this work is among the first to exemplify the potential of DN-FSP for scalable production of complex nanomaterials such as La:SrTiO₃/CuO with a diligent control of doping and heterojunction in a single-step synthesis.

An Overview of Polymer-Supported Catalysts for Wastewater Treatment through Light-Driven Processes

In recent years, alarm has been raised due to the presence of chemical contaminants of emerging concern (CECs) in water. This concern is due to the risks associated with their exposure, even in small amounts. These complex compounds cannot be removed or degraded by existing technologies in wastewater treatment plants. Therefore, advanced oxidation processes have been studied, with the objective of developing a technology capable of complementing the conventional water treatment plants. Heterogenous photocatalysis stands out for being a cost-effective and environmentally friendly process. However, its most common form (with suspended catalytic particles) requires time-consuming and costly downstream processes. Therefore, the heterogeneous photocatalysis process with a supported catalyst is preferable. Among the available supports, polymeric ones stand out due to their favorable characteristics, such as their transparency, flexibility and stability. This is a relatively novel process; therefore, there are still some gaps in the scientific knowledge. Thus, this review article aims to gather the existing information about this process and verify which questions are still to be answered.

Photocatalysis and perovskite oxide-based materials: a remedy for a clean and sustainable future

The massive use of non-renewable energy resources by humankind to fulfill their energy demands is causing severe environmental issues. Photocatalysis is considered one of the potential solutions for a clean and sustainable future because of its cleanliness, inexhaustibility, efficiency, and cost-effectiveness. Significant efforts have been made to design highly proficient photocatalyst materials for various applications such as water pollutant degradation, water splitting, CO2 reduction, and nitrogen fixation. Perovskite photocatalyst materials are gained special attention due to their exceptional properties because of their flexibility in chemical composition, structure, bandgap, oxidation states, and valence states. The current review is focused on perovskite materials and their applications in photocatalysis. Special attention has been given to the structural, stoichiometric, and compositional flexibility of perovskite photocatalyst materials. The photocatalytic activity of perovskite materials in different photocatalysis applications is also discussed. Various mechanisms involved in photocatalysis application from wastewater treatment to hydrogen production are also provided. The key objective of this review is to encapsulate the role of perovskite materials in photocatalysis along with their fundamental properties to provide valuable insight for addressing future environmental challenges.

In situ growth of metal-free snowflake-like 1D/2D phosphorus element heterostructures for photocatalytic overall pure-water splitting

In this work, we report a novel phosphorus element heterostructure with a snowflake-like morphology consisting of 1D rod-like black phosphorus (BP) and 2D flake-like red phosphorus (RP).