The continuous combustion of non-renewable fossil fuels and depletion of existing resources is intensifying the research and development of alternative future energy options that can directly abate and process ever-increasing carbon dioxide (CO2) emissions. Since CO2 is a thermodynamically stable compound, its reduction must not consume additional energy or increase net CO2 emissions. Renewable sources like solar energy provide readily available and continuous light supply required for driving this conversion process. Therefore, the use of solar energy to drive CO2 photocatalytic reactions simultaneously addresses the aforementioned challenges, while producing sustainable fuels or chemicals suitable for use in existing energy infrastructure. Recent progress in this area has focused on the development and testing of promising TiO2 based photocatalysts in different reactor configurations due to their unique physicochemical properties for CO2 photoreduction. TiO2 nanostructured materials with different morphological and textural properties modified by using organic and inorganic compounds as photosensitizers (dye sensitization), coupling semiconductors of different energy levels or doping with metals or non-metals have been tested. This review presents contemporary views on state of the art in photocatalytic CO2 reduction over titanium oxide (TiO2) nanostructured materials, with emphasis on material design and reactor configurations. In this review, we discuss existing and recent TiO2 based supports, encompassing comparative analysis of existing systems, novel designs being employed to improve selectivity and photoconversion rates as well as emerging opportunities for future development, crucial to the field of CO2 photocatalytic reduction. The influence of different operating and morphological variables on the selectivity and efficiency of CO2 photoreduction is reviewed. Finally, perspectives on the progress of TiO2 induced photocatalysis for CO2 photoreduction will be presented.

/pdf/review-of-material-design-and-reactor-engineering-on-tio2-1lszlc3tvl.pdf

Review of material design and reactor engineering on TiO2 photocatalysis for CO2 reduction

Solar driven photoelectrochemical water splitting (PEC-WS) using semiconductor photoelectrodes represents a promising approach for a sustainable and environmentally friendly production of renewable energy vectors and fuel sources, such as dihydrogen (H2). In this context, titanium dioxide (TiO2) and iron oxide (hematite, α-Fe2O3) are among the most investigated candidates as photoanode materials, mainly owing to their resistance to photocorrosion, non-toxicity, natural abundance, and low production cost. Major drawbacks are, however, an inherently low electrical conductivity and a limited hole diffusion length that significantly affect the performance of TiO2 and α-Fe2O3 in PEC devices. To this regard, one-dimensional (1D) nanostructuring is typically applied as it provides several superior features such as a significant enlargement of the material surface area, extended contact between the semiconductor and the electrolyte and, most remarkably, preferential electrical transport that overall suppress charge carrier recombination and improve TiO2 and α-Fe2O3 photoelectrocatalytic properties. The present review describes various synthetic methods and modifying concepts of 1D-photoanodes (nanotubes, nanorods, nanofibers, nanowires) based on titania, hematite, and on α-Fe2O3/TiO2 heterostructures, for PEC applications. Various routes towards modification and enhancement of PEC activity of 1D photoanodes are discussed including doping, decoration with co-catalysts and heterojunction engineering. Finally, the challenges related to the optimization of charge transfer kinetics in both oxides are highlighted.

Photoanodes based on TiO2 and α-Fe2O3 for solar water splitting – superior role of 1D nanoarchitectures and of combined heterostructures

Photoelectrochemical (PEC) water splitting is a promising approach for solar-driven hydrogen production with zero emissions, and it has been intensively studied over the past decades. However, the solar-to-hydrogen (STH) efficiencies of the current PEC systems are still far from the 10% target needed for practical application. The development of efficient photoelectrodes in PEC systems holds the key to achieving high STH efficiencies. In recent years, crystal facet engineering has emerged as an important strategy in designing efficient photoelectrodes for PEC water splitting, which has yet to be comprehensively reviewed and is the main focus of this article. After the Introduction, the second section of this review concisely introduces the mechanisms of crystal facet engineering. The subsequent section provides a snapshot of the unique facet-dependent properties of some semiconductor crystals including surface electronic structures, redox reaction sites, surface built-in electric fields, molecular adsorption, photoreaction activity, photocorrosion resistance, and electrical conductivity. Then, the methods for fabricating photoelectrodes with faceted semiconductor crystals are reviewed, with a focus on the preparation processes. In addition, the notable advantages of the crystal facet engineering of photoelectrodes in terms of light harvesting, charge separation and transfer, and surface reactions are critically discussed. This is followed by a systematic overview of the modification strategies of faceted photoelectrodes to further enhance the PEC performance. The last section summarizes the major challenges and some invigorating perspectives for future research on crystal facet engineered photoelectrodes, which are believed to play a vital role in promoting the development of this important research field.

Crystal facet engineering of photoelectrodes for photoelectrochemical water splitting

Solar driven photoelectrochemical water splitting (PEC-WS) using semiconductor photoelectrodes represents a promising approach for a sustainable and environmentally friendly production of renewable energy vectors and fuel sources, such as dihydrogen (H2). In this context, titanium dioxide (TiO$_2$) and iron oxide (hematite, $\alpha$-Fe$_2$O$_3$) are among the most investigated candidates as photoanode materials, mainly owing to their resistance to photocorrosion, non-toxicity, natural abundance, and low production cost. Major drawbacks are, however, an inherently low electrical conductivity and a limited hole diffusion length that significantly affect the performance of TiO$_2$ and $\alpha$-Fe$_2$O$_3$ in PEC devices. To this regard, one-dimensional (1D) nanostructuring is typically applied as it provides several superior features such as a significant enlargement of the material surface area, extended contact between the semiconductor and the electrolyte and, most remarkably, preferential electrical transport that overall suppress charge carrier recombination and improve TiO$_2$ and $\alpha$-Fe$_2$O$_3$ photo-electrocatalytic properties. The present review describes various synthetic methods, properties and PEC applications of 1D-photoanodes (nanotubes, nanorods, nanofibers, nanowires) based on titania, hematite, and on $\alpha$-Fe$_2$O$_3$/TiO$_2$ heterostructures. Various routes towards modification and enhancement of PEC activity of 1D photoanodes are also discussed including doping, decoration with co-catalysts and heterojunction engineering. Finally, the challenges related to the optimization of charge transfer kinetics in both oxides are highlighted.

Photoanodes Based on TiO$_2$ and $\alpha$-Fe$_2$O$_3$ for Solar Water Splitting Superior Role of 1D Nanoarchitectures and of Combined Heterostructures

Rechargeable lithium-ion batteries (LIBs) offer the advantages of having great electrical energy storage and increased continuous and pulsed power output capabilities, which enable their applications in grid energy storage and electric vehicles (EVs). For safety, high power and durability considerations, spinel Li4Ti5O12 is one of the most appealing potential candidate as an anode material for power LIBs due to its excellent cycling stability and thermal stability. However, there are still a number of challenges remaining for Li4Ti5O12 battery applications. Herein, an updated overview of the latest advances in Li4Ti5O12 research is provided and key challenges for its future development (i.e., fast-charging, specific capacity, swelling, interface chemistry, matching cathode and electrolyte as well as batteries design and manufacturing) are highlighted.

Challenges of Spinel Li4Ti5O12 for Lithium‐Ion Battery Industrial Applications

The rational construction of earth‐abundant and advanced electrocatalysts for oxygen evolution reaction (OER) is extremely desired and significant to seawater electrolysis. Herein, by directly etching Ni3S2 nanosheets through potassium ferricyanide, a novel self‐sacrificing template strategy is proposed to realize the in situ growth of NiFe‐based Prussian blue analogs (NiFe PBA) on Ni3S2 in an interfacial redox reaction. The well‐designed Ni3S2@NiFe PBA composite as precursor displays a unique spherical magic cube architecture composed of nanocubes, which even maintains after a phosphating treatment to obtain the derived Ni3S2/Fe‐NiPx on nickel foam. Specifically, in alkaline seawater, the Ni3S2/Fe‐NiPx as OER precatalyst marvelously realizes the ultralow overpotentials of 336 and 351 mV at large current densities of 500 and 1000 mA cm–2, respectively, with remarkable durability for over 225 h, outperforming most reported advanced OER electrocatalysts. Experimentally, a series of characterization results confirm the reconstruction behavior in the Ni3S2/Fe‐NiPx surface, leading to the in situ formation of Ni(OH)2/Ni(Fe)OOH with abundant oxygen vacancies and grain boundaries, which constructs the Ni3S2/Fe‐NiPx reconstruction system responsible for the remarkable OER catalytic activity. Theoretical calculation results further verify the enhanced OER activity for Ni3S2/Fe‐NiPx reconstruction system, and unveil that the Fe‐Ni2P/FeOOH as active origin contributes to the central OER activity.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/advs.202104846

Spherical Ni3S2/Fe‐NiP x Magic Cube with Ultrahigh Water/Seawater Oxidation Efficiency

Cu3SnS4 with S vacancy and different ratios of Cu(I/II) and Sn(II/IV) was designed for photocatalytic CO2 reduction with high selectivity and activity in this study. The conduction band (CB) position dominated by the Sn(II) 5p orbital of Cu3SnS4 could be regulated via controlling the content of Sn(II). Cu(I) and Sn(IV) in crystal lattice acted as the adsorption sites of CO2 and H2O as demonstrated by Density Functional Theory (DFT) calculations, meanwhile Cu(I) had a strong adsorption ability to CO, which was conducive to further protonation for CH4 generation (CO2→COOH*→CO*→CHO*→CH2O*→CH3O*→CH4). S vacancy could result in the appearance of Cu(I) and Sn(II), which could successfully inhibit the electron-hole recombination and improve the reactivity (CH4 with yield of 22.65 μmol/g/h) and selectivity (CH4 ∼ 83.10 %). This work can shed some light on the synthetic method by controlling vacancy and elements to adjust CB position to increase reduction capability and selectivity.

Effect of S vacancy in Cu3SnS4 on high selectivity and activity of photocatalytic CO2 reduction

Enhanced photoelectrocatalytic performance of temperature-dependent 2D/1D BiOBr/TiO2-x nanotubes

Tuning the structural defects of graphite carbon nitride (g-C3N4) is an effective strategy to modify its band structure and promote charge separation, but it is still limited by complex and harsh p...

Fumaric Acid Assistant Band Structure Tunable Nitrogen Defective g-C3N4 Fabrication for Enhanced Photocatalytic Hydrogen Evolution

Interest in highly reactive anatase TiO2 with controllable facets has increased rapidly in recent years because its unique properties provide new insights into the interfacial chemistry of TiO2 in applications such as photocatalysis and in its use as an electrode material. We report here the fabrication of hetero-structured TiO2/SrTiO3 nanotube arrays with a preferred orientation to the anatase TiO2 {001} facet by a typical anodization and subsequent hydrothermal approach, followed by annealing. Based on XRD, FE-SEM, FE-TEM, XPS and Raman measurements, it is shown that TiO2 is partially converted into SrTiO3 attached to in situ TiO2 nanotubes. A hetero-structured interface of TiO2/SrTiO3 with preferred growth of the anatase TiO2 {001} facet was confirmed to be formed after the hydrothermal process. The resulting hetero-structured samples were used as photocatalysts to decolorize methylene blue in aqueous solution in a photocatalytic and photoelectrocatalytic process. The hetero-structured samples show enhanced photocatalytic properties compared with the reference TiO2 nanotube arrays.

Xin Tan

Papers

Spherical Ni3S2/Fe‐NiP x Magic Cube with Ultrahigh Water/Seawater Oxidation Efficiency

Effect of S vacancy in Cu3SnS4 on high selectivity and activity of photocatalytic CO2 reduction

Enhanced photoelectrocatalytic performance of temperature-dependent 2D/1D BiOBr/TiO2-x nanotubes

Fumaric Acid Assistant Band Structure Tunable Nitrogen Defective g-C3N4 Fabrication for Enhanced Photocatalytic Hydrogen Evolution

Hetero-structured TiO2/SrTiO3 nanotube array film with highly reactive anatase TiO2 {001} facets