Facile synthesis of SrTiO3 hollow microspheres built as assembly of nanocubes and their associated photocatalytic activity

Research Progress on Photocatalytic CO2 Reduction Based on Perovskite Oxides

Photocatalytic CO2 reduction to valuable fuels is a promising way to alleviate anthropogenic CO2 emissions and energy crises. Perovskite oxides have attracted widespread attention as photocatalysts for CO2 reduction by virtue of their high catalytic activity, compositional flexibility, bandgap adjustability, and good stability. In this review, the basic theory of photocatalysis and the mechanism of CO2 reduction over perovskite oxide are first introduced. Then, perovskite oxides' structures, properties, and preparations are presented. In detail, the research progress on perovskite oxides for photocatalytic CO2 reduction is discussed from five aspects: as a photocatalyst in its own right, metal cation doping at A and B sites of perovskite oxides, anion doping at O sites of perovskite oxides and oxygen vacancies, loading cocatalyst on perovskite oxides, and constructing heterojunction with other semiconductors. Finally, the development prospects of perovskite oxides for photocatalytic CO2 reduction are put forward. This article should serve as a useful guide for creating perovskite oxide-based photocatalysts that are more effective and reasonable.

Surfactant-Free Synthesis of Three-Dimensional Perovskite Titania-Based Micron-Scale Motifs Used as Catalytic Supports for the Methanol Oxidation Reaction

We synthesized and subsequently rationalized the formation of a series of 3D hierarchical metal oxide spherical motifs. Specifically, we varied the chemical composition within a family of ATiO3 (wherein "A" = Ca, Sr, and Ba) perovskites, using a two-step, surfactant-free synthesis procedure to generate structures with average diameters of ~3 microns. In terms of demonstrating the practicality of these perovskite materials, we have explored their use as supports for the methanol oxidation reaction (MOR) as a function of their size, morphology, and chemical composition. The MOR activity of our target systems was found to increase with decreasing ionic radius of the "A" site cation, in order of Pt/CaTiO3 (CTO) > Pt/SrTiO3 (STO) > Pt/BaTiO3 (BTO). With respect to morphology, we observed an MOR enhancement of our 3D spherical motifs, as compared with either ultra-small or cubic control samples. Moreover, the Pt/CTO sample yielded not only improved mass and specific activity values but also a greater stability and durability, as compared with both commercial TiO2 nanoparticle standards and precursor TiO2 templates.

Historical development and prospects of photocatalysts for pollutant removal in water

Photocatalysis, as a low-cost and environment friendly technology, has demonstrated a significant potential for water pollution purification; it has received extensive attention in recent decades. The key is the photocatalyst; a large number of photocatalysts have been developed. To better understand and further develop the photocatalysis technology for water treatment, this review summarizes its development over time. The development period is divided into four stages (1960s-1993, 1994-2000, 2001-2010, and 2011-present) to provide readers with a better understanding of the development characteristics, and causes and consequences of each historical stage. This review expounds the origin and development of photocatalysis and the obstacles encountered and overcome. It describes the development of mechanisms and methods to solve these problems in each time period. Moreover, it reviews the recent development of new photocatalysts, the concept of designing photocatalysts, and photocatalytic-coupling systems. Finally, it enumerates the problems that continue to exist in the application of photocatalysis technology, and highlights the key issues that must be addressed in future research. The review is aimed at providing the researchers with a deeper understanding of photocatalysis technology and encourage further development of the application of photocatalysis to water treatment.

Multivariate comparison of photocatalytic properties of thirteen nanostructured metal oxides for water purification

Although the environmental photocatalysis is being developed for many years, the relationships between simple metal oxides have not been explored so far. In this study a multivariate comparison of thirteen nanostructured metal oxides (Bi₂O₃, CeO₂, Co₃O₄, Fe₂O₃, NiO, Pr₆O₁₁, SnO₂, SrTiO₃, TiO₂, WO₃, ZnFe₂O₄, ZnO and ZrO₂) was performed. The solution containing twenty-six psychotropic pharmaceuticals was used as a model mixture. In order to ensure the influence of the dissolved organic matter on the process, all the experiments were conducted in the river water. Simulated solar radiation was applied as the most environmentally relevant. The high-resolution LC-MS profiles, obtained for the photocatalytic samples after 1 h of irradiation, were then submitted to the multivariate chemometric analysis. Graphical representations of principal component analysis and hierarchical cluster analysis enabled visualization of the relationships between the studied oxides. The registered degradation profiles were compared qualitatively and discussed.

Investigation of the photolysis and TiO2, SrTiO3, H2O2-mediated photocatalysis of an antipsychotic drug loxapine - Evaluation of kinetics, identification of photoproducts, and in silico estimation of properties

The photolytic and photocatalytic transformation of loxapine with the use of H₂O₂, TiO₂ and SrTiO₃ under the simulated solar radiation was studied. A micro-scale method for simultaneous irradiation of multiple samples in photostability chamber was applied. RP-UHPLC-DAD coupled with ESI-Q-TOF mass spectrometer was used for the quantitative and qualitative analysis of the processes. Influence of catalysts concentration on kinetic parameters of loxapine photodecomposition was evaluated, and TiO₂ at medium concentration (100 mg L^-1) turned out to be the most effective. Sixteen transformation products were detected and their structures were elucidated. On the basis of the elucidated structures, computational evaluation of toxicity, bioconcentration and bioaccumulation factors as well as biodegradability of transformation products were conducted. The multivariate chemometric method (principal component analysis) was used to compare the calculated properties as well as the applied methods. Most of the transformation products were generally less toxic and more biodegradable than the parent compound.

A review on photocatalytic CO2 reduction using perovskite oxide nanomaterials

As the search for efficient catalysts for CO₂ photoreduction continues, nanostructured perovskite oxides have emerged as a class of high-performance photocatalytic materials. The perovskite oxide candidates for CO₂ photoreduction are primarily nanostructured forms of titanates, niobates, tantalates and cobaltates. These materials form the focus of this review article because they are much sought-after due to their nontoxic nature, adequate chemical stability, and tunable crystal structures, bandgaps and surface energies. As compared to conventional semiconductors and nanomaterial catalysts, nanostructured perovskite oxides also exhibit an extended optical-absorption edge, longer charge carrier lifetimes, and favorable band-alignment with respect to reduction potential of activated CO₂ and reduction products of the same. While CO₂ reduction product yields of several hundred μmol^-1 h^-1 are observed with many types of perovskite oxide nanomaterials in stand-alone forms, yield of such quantities are not common with semiconductor nanomaterials of other types. In this review, we present current state-of-the-art synthesis methods to form perovskite oxide nanomaterials, and procedures to engineer their bandgaps. This review also presents a comprehensive summary and discussion on crystal structures, defect distribution, morphologies and electronic properties of the perovskite oxides, and correlation of these properties to CO₂ photoreduction performance. This review offers researchers key insights for developing advanced perovskite oxides in order to further improve the yields of CO₂ reduction products.

Hydrothermal synthesis of strontium titanate: thermodynamic considerations, morphology control and crystallisation mechanisms

The hydrothermal/solvothermal method is one of the most versatile synthetic routes for producing a large number of compounds. The thermodynamic aspects, the control of morphology and the crystallisation mechanisms are reviewed and discussed in this highlight, with special emphasis on the synthesis of SrTiO₃, as a model system.

Hydrothermal derived nitrogen doped SrTiO3 for efficient visible light driven photocatalytic reduction of chromium(VI)

In this work, we report on the synthesis of nitrogen doped SrTiO₃ nanoparticles with efficient visible light driven photocatalytic activity toward Cr(VI) by the solvothermal method. The samples are carefully characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV–Vis diffuse reflectance spectroscopy and photocatalytic test. It is found that nitrogen doping in SrTiO₃ lattice led to an apparent lattice expansion, particle size reduction as well as subsequent increase of Brunner–Emmet–Teller surface area. The visible light absorption edge and intensity can be modulated by nitrogen doping content, which absorption edge extends to about 600 nm. Moreover, nitrogen doping can not only modulate the visible light absorption feature, but also have consequence on the enhancement of charge separation efficiency, which can promote the photocatalytic activity. With well controlled particle size, Brunner–Emmet–Teller surface area, and electronic structure via nitrogen doping, the photocatalytic performance toward Cr(VI) reduction of nitrogen doped SrTiO₃ was optimized at initial hexamethylenetetramine content of 2.

Fabrication of bimodal-pore SrTiO3 microspheres with excellent photocatalytic performance for Cr(VI) reduction under simulated sunlight

Solving the increasing contamination from toxic heavy metal ions in wastewater is an imperative issue in photocatalysis research area. In this study, three-dimensional (3D) porous SrTiO3 microspheres have been fabricated by a sol-gel-templating method using the agarose gel bead containing SrCO3 granules as the template. The resultant SrTiO3 microspheres, several tens of micrometers in diameter, exhibit a bimodal pore structure, in which the macropore about 70-150nm in size stems from SrCO3 granules and the mesopore about 3nm is formed via removing the agarose fiber embedded in the composite microspheres. The porous framework of SrTiO3 microspheres is assembled by regular single-crystalline SrTiO3 nanocubes with an edge length of 100±10nm. The highly interconnected porous network renders numerous pathways for the rapid mass transport, strong adsorption of reactants and multi-reflection of incident light. Moreover, the as-prepared SrTiO3 microspheres exhibit excellent photocatalytic performance for the Cr(VI) reduction under simulated sunlight, which can reduce nearly 100% Cr(VI) at pH 2 within 2h and retain a relatively high reduction ability after six recycles.

Synthesis and applications of porous non-silica metal oxide submicrospheres

A variety of metal oxide particles of spherical morphology from nano to micrometer size have been reviewed with a special emphasis on the appraisal of synthetic strategies and applications in biomedical, environmental and energy-related areas.

Morphology-controlled synthesis of SrTiO3/TiO2 heterostructures and their photocatalytic performance for water splitting

Different morphologies of SrTiO₃/TiO₂ heterostructures like nanocubes, nanoparticles, nanospheres, and nanofibers were synthesized via a facile hydrothermal process, using TiO₂ as both a template and precursor in Sr(OH)₂ solution.

Self-Template Synthesis of Porous Perovskite Titanate Solid and Hollow Submicrospheres for Photocatalytic Oxygen Evolution and Mesoscopic Solar Cells

We describe a general synthesis strategy, which combines sol-gel and hydrothermal processes, for the large-scale synthesis of porous perovskite titanates spheres with tunable particle size and inner structures. Amorphous hydrous TiO2 solid spheres (AHTSS) are first synthesized by a sol-gel method and are then used as precursor and template for the subsequent hydrothermal reaction with alkaline earth metal ions in an alkaline medium. This strategy can be generalized to synthesize porous spheres of various perovskite titanates (i.e., SrTiO3, BaTiO3, and CaTiO3) consisting of single-crystalline nanocubes. By controlling the textural properties (i.e., size, porosity, and structure) of AHTSS, perovskite titanates with tunable size and inner structures are selectively synthesized. The underlying formation mechanism is manifested by XRD and TEM to involve in situ crystallization or Ostwald ripening during the hydrothermal process. The obtained porous SrTiO3 spheres present superior performance in photocatalytic oxygen evolution and CdSe-sensitized mesoscopic solar cells.

Photocatalytic reduction synthesis of SrTiO3-graphene nanocomposites and their enhanced photocatalytic activity

SrTiO3-graphene nanocomposites were prepared via photocatalytic reduction of graphene oxide by UV light-irradiated SrTiO3 nanoparticles. Fourier transformed infrared spectroscopy analysis indicates that graphene oxide is reduced into graphene. Transmission electron microscope observation shows that SrTiO3 nanoparticles are well assembled onto graphene sheets. The photocatalytic activity of as-prepared SrTiO3-graphene composites was evaluated by the degradation of acid orange 7 (AO7) under a 254-nm UV irradiation, revealing that the composites exhibit significantly enhanced photocatalytic activity compared to the bare SrTiO3 nanoparticles. This can be explained by the fact that photogenerated electrons are captured by graphene, leading to an increased separation and availability of electrons and holes for the photocatalytic reaction. Hydroxyl (·OH) radicals were detected by the photoluminescence technique using terephthalic acid as a probe molecule and were found to be produced over the irradiated SrTiO3 nanoparticles and SrTiO3-graphene composites; especially, an enhanced yield is observed for the latter. The influence of ethanol, KI, and N2 on the photocatalytic efficiency was also investigated. Based on the experimental results, ·OH, h(+), and H2O2 are suggested to be the main active species in the photocatalytic degradation of AO7 by SrTiO3-graphene composites.

PACS

61.46. + w; 78.67.Bf; 78.66.Sq.

Garden-like perovskite superstructures with enhanced photocatalytic activity

By subjecting amorphous flower-like TiO2 to a facile hydrothermal synthesis in the presence of Sr(2+), garden-like perovskite SrTiO3 superstructures were achieved. The amorphous TiO2 was preformed using ZnO flowers as templates. Different three-dimensional SrTiO3 architectures were coexisted in the garden, including SrTiO3 flowers composed of several hollow sword-shaped petals, many sheet-shaped petals or numerous flake-shaped petals, and SrTiO3 grass consisting of a number of long blades. These SrTiO3 superstructures were simultaneously grown on fluorine-doped tin oxide (FTO) substrates. On the basis of a comprehensive study on the effects of growth time, temperature, initial concentrations of precursor, and pH, the formation of these various hierarchical architectures was attributed primarily to the dissolution of amorphous TiO2 and precipitation of perovskite crystals, followed by the Ostwald ripening process of perovskite nanocrystals and self-organization of perovskite building blocks. Interestingly, this approach can be readily extended to create other perovskite structures, including dendritic BaTiO3 and nest-like CaTiO3, as well as PbTiO3 transformed from plate-like pyrochlore Pb2Ti2O6 after post-thermal treatment. Garden-like SrTiO3 superstructures showed a superior photocatalytic performance when compared to other as-prepared semiconductors and perovskite materials (i.e., ZnO, TiO2, BaTiO3, CaTiO3 and PbTiO3), probably due to their intrinsic photocatalytic activity and special garden-like features with a coexistence of various structures that significantly facilitated the adsorption and diffusion of methyl blue (MB) molecules and oxygen species in the photochemical reaction of MB degradation.

Ni and p-Cu2O nanocubes with a small size distribution by templated electrodeposition and their characterization by photocurrent measurement

A method for the reproducible formation of Ni and Cu2O nanocubes with dimensions of 200-500 nm and a small size distribution is introduced. For this, the well-known templated electrodeposition technique was extended to cubic PMMA templates made by nanoimprint lithography. When making cubic templates in larger quantities, this method has the potential to become simple and cost-effective. This method was successfully used for the formation of Ni and p-Cu2O nanocubes as well as for the formation of segmented nanobars containing both phases. The lateral dimensions of the nanocubes exactly resembled the dimensions of the template, and the height could be varied by adjusting the deposition time. Nanocubes formed via this method can remain attached to the substrate or can be dispersed in solution. p-Cu2O is considered to be one of the most promising photocathode materials for solar water splitting. It is demonstrated that the activity of the p-Cu2O nanocubes for photocatalytic water splitting can be measured, and it was found that the nanocube morphology enhances the photocatalytic activity compared to thin films.

In situ synthesis of Zn2GeO4 hollow spheres and their enhanced photocatalytic activity for the degradation of antibiotic metronidazole

In this study, Zn2GeO4 hollow spheres were successfully fabricated by a template-engaged approach using zinc hydroxide carbonate (Zn4CO3(OH)6·H2O, ZHC) spheres as the template. During the hydrothermal process, Zn(2+) dissolved from the surface of the ZHC spheres could rapidly react with the HGeO3(-) in solution and the Zn2GeO4 outer shell was formed in situ. Moreover, the building units of the Zn2GeO4 hollow spheres could gradually transform from the nanoparticles into nanobundles with the increase of the reaction time. The photocatalytic degradation results indicate that the Zn2GeO4 hollow spheres exhibited high photocatalytic activity and excellent stability for the degradation of antibiotic metronidazole in solution. Finally, the radical species involved in the degradation process have been investigated by using the scavenger experiments.

Long-range and short-range structures of cube-like shape SrTiO3 powders: microwave-assisted hydrothermal synthesis and photocatalytic activity

We report herein a detailed study on the influence of microwave-assisted hydrothermal (MAH) treatment time on both long and short range structures around Ti atoms of SrTiO3 powders. Few studies have been carried out on short-order structural properties as well as the relationship between the local order and the SrTiO3 photocatalytic properties. We use X-ray diffraction to determine the long-range structure, while the local environment around the Ti atom is probed with X-ray absorption spectroscopy and the vibration frequencies are investigated by Raman spectroscopy. The faster crystallization of SrTiO3 powders provided by the MAH system resulted in large distortions of Ti-O bond lengths which remain unchanged even for a longer MAH treatment time. Despite the long-range structure being associated with ideal TiO6 clusters, X-ray absorption spectroscopy measurements identified the presence of undercoordinated TiO5 clusters. Compared with the reference bulk SrTiO3, the hierarchical SrTiO3 cube-like shape showed enhanced photocatalytic activity, which was associated with the presence of these TiO5 clusters. Field emission scanning electron microscopy (FE-SEM) revealed that the superstructures based on a cube-like shape are formed by an assembly process, becoming well defined as a function of MAH treatment time.

A survey of photocatalytic materials for environmental remediation

Heterogeneous photocatalysis is an advanced oxidation process which has been the subject of a huge amount of studies related to air cleaning and water purification. All these processes have been carried out mainly by using TiO(2)-based materials as the photocatalysts and ca. 75% of the articles published in the last 3 years is related to them. This review illustrates the efforts in the search of alternative photocatalysts that are not based on TiO(2), with some exceptions concerning particularly innovative modifications as nanoassembled TiO(2) or TiO(2) composites with active carbon, graphite and fullerene. Papers reporting preparation, characterization and testing of binary, ternary and quaternary compounds, have been reviewed. Despite many of these photocatalysts being effective for the photodecomposition of many pollutants, most of them do not allow a complete mineralization of the starting compounds, differently from TiO(2).

Visible light-driven photocatalysis of doped SrTiO3 tubular structure

SrTiO3 tubular structures co-doped with Cr and Ta were synthesized through a combination of solvothermal-hydrothermal processes. X-ray photoelectron spectroscopy (XPS) measurements of the oxidation state of Cr ions reveal that the formation of Cr6+ ions, which would serve as the non-radiative recombination centers for photogenerated electrons and holes, was suppressed without the process of high temperature hydrogen reduction. Compared to similar co-doped materials synthesized by solid-state reaction, (Cr, Ta) co-doped SrTiO3 tubular structures have significantly higher photocatalytic activity for hydrogen evolution as measured in an aqueous methanol solution under visible light irradiation.