Engineering the Surface/Interface Structures of Titanium Dioxide Micro and Nano Architectures towards Environmental and Electrochemical Applications

Titanium dioxide (TiO2) materials have been intensively studied in the past years because of many varied applications. This mini review article focuses on TiO2 micro and nano architectures with the prevalent crystal structures (anatase, rutile, brookite, and TiO2(B)), and summarizes the major advances in the surface and interface engineering and applications in environmental and electrochemical applications. We analyze the advantages of surface/interface engineered TiO2 micro and nano structures, and present the principles and growth mechanisms of TiO2 nanostructures via different strategies, with an emphasis on rational control of the surface and interface structures. We further discuss the applications of TiO2 micro and nano architectures in photocatalysis, lithium/sodium ion batteries, and Li–S batteries. Throughout the discussion, the relationship between the device performance and the surface/interface structures of TiO2 micro and nano structures will be highlighted. Then, we discuss the phase transitions of TiO2 nanostructures and possible strategies of improving the phase stability. The review concludes with a perspective on the current challenges and future research directions

Single atom Cu(I) promoted mesoporous titanias for photocatalytic Methyl Orange depollution and H 2 production

Tailoring the physicochemical properties and hence reactivity of semiconductor photocatalysts in a predictable fashion, remains a challenge to their industrial application. Here we demonstrate the striking promotional effect of incorporating single Cu(I) atoms, on aqueous phase photocatalytic dye degradation and H2 production over surfactant-templated mesoporous TiO2. X-ray absorption spectroscopy reveals that ultra-low concentrations of copper (0.02-0.1 wt%) introduced into the mesoporous TiO2 surface create isolated Cu (I) species which suppress charge recombination, and confer a six-fold photocatalytic promotion of Methyl Orange degradation and four-fold enhancement of H2 evolution. The impact of mesopore structure and photophysical properties on photocatalytic activity is also quantified for the first time: calcination increases mesopore size and nanocrystalline order, and induces an anatase to rutile phase transition that is accompanied by a decrease in the optical band gap, increased charge carrier lifetime, and a concomitant significant activity enhancement

The kinetics of metal oxide photoanodesfrom charge generation to catalysis

Generating charge carriers with lifetimes long enough to drive catalysis is a critical aspect for both photoelectrochemical and photocatalytic systems and a key determinant of their efficiency. This review addresses the charge carrier dynamics underlying the performance of metal oxides as photoanodes and their ability to drive photoelectrochemical water oxidation, alongside wider comparison with metal oxide function in photocatalytic and electrocatalytic systems. We start by highlighting the disparity between the ps–ns lifetimes of electron and holes photoexcited in bulk metal oxides versus the ms –s timescale of water oxidation catalysis. We go onto review recent literature of the dominant kinetic processes determining photoanode performance, namely charge generation, polaron formation and charge trapping, bulk and surface recombination, charge separation and extraction, and finally the kinetics of water oxidation catalysis. With each topic, we review current understanding and note areas of remaining uncertainty or controversy. We discuss the potential for material selection and examine approaches such as doping, nanostructuring, junction formation and/or co-catalyst deposition to enhance performance. Critically, we examine how such performance enhancements can be understood from analyses of carrier dynamics and propose design guidelines for further material or device optimisation

Tailoring the deposition of MoSe2 on TiO2 nanorods arrays via radiofrequency magnetron sputtering for enhanced photoelectrochemical water splitting

MoSe2/1 D TiO2 nanorods (NRs) heterojunction assembly was systematically fabricated, and its photoelectrocatalytic properties were investigated. The fabrication process involves the growth of 1D TiO2 NRs arrays on FTO substrates using hydrothermal synthesis followed by the deposition of MoSe2 nanosheets on the TiO2 NRs using radiofrequency magnetron sputtering (RF magnetron sputtering). The photoelectrochemical properties of the heterojunction were explored and optimized as a function of the thickness of the MoSe2 layer, which was controlled by the sputtering time. The MoSe2 grows perpendicularly on TiO2 NRs in a 2D layered structure, maximizing the exposed active edges, an essential aspect that permits maximum exploitation of deposited MoSe2. Compared to pure TiO2 NRs, the heterojunction nanostructured assembly displayed excellent spectral and photoelectrochemical properties, including more surface oxygen vacancies, enhanced visible-light absorption, higher photocurrent response, and decreased charge transfer resistance. In particular, the sample synthesized by sputtering of MoSe2 for 90 s, i.e., MoSe2@TiO2-90 s, depicted the highest current density (1.86 mA cm−2 at 0.5 V vs. Ag/AgCl) compared to other samples. The excellent photoelectrochemical activity of the heterojunction stemmed from the synergy between tailored loading of MoSe2 nanosheets and the 1D structure of TiO2 NRs, which afford a high surface/volume ratio, effective charge separation, fast electron transfer, and easy accessibility to the MoSe2 active edges. These factors boost the catalytic activity.This work was made possible by NPRP Grant no. NPRP 12S-0304-190218 from the Qatar National Research Fund (a member of the Qatar Foundation). The statements made herein are solely the responsibility of the authors. Open Access funding provided by the Qatar National Library.Scopu

Barium titanate: photophysics, photocatalysis & the influence of the ferroelectric effect

Photocatalytic and photoelectrochemical water splitting processes remain hindered by fast recombination of photogenerated electrons and holes. Ferroelectric materials are increasingly being considered to address this issue; their internal electric fields have been shown to spatially separate electrons and holes, and thus should greatly reduce recombination rates. A kinetic understanding of the extent to which electron–hole recombination can be slowed in ferroelectric materials is essential to ascertain if they can play a significant role in achieving higher solar-driven water splitting efficiencies. The focus of this thesis is an experimental investigation of charge carrier dynamics in barium titanate (BaTiO3) to observe the effect of internal electric fields on recombination rates. Time-resolved spectroscopic techniques were used in conjunction with photocatalysis studies to determine whether ferroelectricity can significantly reduce recombination rates and lead to enhanced performance. It is found that, although the transient absorption spectrum of ferroelectric BaTiO3 is similar to previously reported metal oxides, the carrier lifetimes are significantly longer, indicating the potential for ferroelectrics to be used in devices limited by fast electron–hole recombination. In the first results chapter, the transient absorption spectrum of single crystal BaTiO3 is characterised under inert atmosphere over two timescales: femtosecond–nanosecond and microsecond–second. Absorption signals due to photogenerated holes and electrons are identified using electron and hole scavengers, respectively. Comparisons are drawn between BaTiO3 and other single crystal, but non-ferroelectric, metal oxides. It is found that, on timescales relevant for water oxidation, lifetimes in BaTiO3 are at least an order of magnitude longer. In the second results chapter, the origin of long carrier lifetimes in ferroelectric BaTiO3 is explored. When the polarisation is switched off by both temperature and nanostructuring, carrier lifetimes decrease by four orders of magnitude. Recombination rates in BaTiO3 exhibit a much stronger temperature dependence than other metal oxides, which is rationalised by considering the temperature dependence of the spontaneous polarisation. The third results chapter investigates the photocatalytic performance of BaTiO3 nanopowders. It is found that, in the presence of an electron scavenger, BaTiO3 photogenerated holes are reactive and can oxidise water to produce oxygen. Transient and photoinduced absorption spectroscopies indicated that hole accumulation in a BaTiO3 sample with a higher tetragonal (ferroelectric) content, which translates to higher rates of oxygen evolution. The final results chapter probes the influence of a ferroelectric BaTiO3 substrate on α-Fe2O3 thin films. Preliminary data suggests the internal field can penetrate through the film and slow electron–hole recombination rates in α-Fe2O3.Open Acces

光触媒性能を高めるための層状チタン酸塩の構造変換

筑波大学University of Tsukuba博士（工学）Doctor of Philosophy in Engineering2021doctoral thesi

Synthetic strategies to nanostructured photocatalysts for CO2 reduction to solar fuels and chemicals

Artificial photosynthesis represents one of the great scientific challenges of the 21st century, offering the possibility of clean energy through water photolysis and renewable chemicals through CO2 utilisation as a sustainable feedstock. Catalysis will undoubtedly play a key role in delivering technologies able to meet these goals, mediating solar energy via excited generate charge carriers to selectively activate molecular bonds under ambient conditions. This review describes recent synthetic approaches adopted to engineer nanostructured photocatalytic materials for efficient light harnessing, charge separation and the photoreduction of CO2 to higher hydrocarbons such as methane, methanol and even olefins

Catalizadores nanoestructurados con aplicaciones medioambientales. Hidrodesoxigenación de biocombustibles y purificación de hidrógeno

Los soportes de titania se sintetizaron mediante un método sol-gel y mediante dos métodos hidrotermales marcadamente ácido y básico. El primer método condujo a la formación de un soporte mesoporoso que a su vez se calcinó a distintas temperaturas. El catalizador con una mezcla idónea de fases anatasa/rutilo (soporte calcinado a 600ºC), una alta dispersión de nanopartículas de oro y un tamaño de partícula idóneo condujo a los mejores resultados de conversión de CO al proporcionar mayor densidad de sitios activos. El segundo método condujo a la formación de nanovarillas de titania que permitieron una alta dispersión de las nanopartículas de oro. Los ensayos catalíticos demostraron que todos los catalizadores presentan una alta conversión de CO. Sin embargo, la alta fotoactividad condujo a una disminución de la selectividad hacia CO2 como resultado de la reacción competitiva de oxidación de H2 a H2O. Fecha de lectura de Tesis Doctoral: 13 diciembre 2018.La presente tesis plantea el desarrollo de sistemas catalíticos para el estudio de reacciones de interés energético y medioambiental. La primera parte se centra en la síntesis de catalizadores activos en reacciones de hidrodesoxigenación (HDO) de moléculas modelo presentes en el bio-oil derivado de la pirólisis de biomasa lignocelulósica para la mejora de biocombustibles. La segunda incluye el desarrollo de fotocatalizadores activos en la reacción de oxidación preferencial de CO en exceso de hidrógeno bajo irradiación solar simulada para la purificación de hidrógeno generado a bordo en dispositivos móviles, el cual se usa como alimento de celdas de combustible de membrana polimérica. Se han sintetizado una serie de fosfuros de metales de transición soportados sobre sílice comercial y pertenecientes a la triada del hierro (Fe, Co y Ni) que han resultado muy activos en la reacción de hidrodesoxigenación de fenol y dibenzofurano. Los resultados de caracterización revelaron que la relación molar inicial fósforo/metal es determinante en la estequiometría de la fase fosfuro formada, la acidez, el tamaño de partícula, el grado de exposición metálica en superficie y consecuentemente, en la actividad catalítica. Los resultados catalíticos reflejaron que los fosfuros ricos en metal, Ni2P y Fe2P, condujeron a los mejores resultados para los catalizadores de hierro y níquel, mientras que para la serie de fosfuros de cobalto el monofosfuro, CoP, condujo a las mayores actividades. Por otro lado, los resultados de selectividad revelaron que todos los fosfuros reaccionan vía hidrogenación/deshidratación, obteniéndose ciclohexano y biciclohexano tras HDO de fenol y dibenzofurano, respectivamente. Para la reacción de oxidación preferencial de CO en exceso de hidrógeno bajo irradiación solar iluminada a temperatura ambiente y presión atmosférica se han sintetizado fotocatalizadores basados en nanopartículas de oro depositadas sobre TiO2