Photocatalysts with internal electric fields

The photocatalytic activity of materials for water splitting is limited by the recombination of photogenerated electron-hole pairs as well as the back-reaction of intermediate species. This review concentrates on the use of electric fields within catalyst particles to mitigate the effects of recombination and back-reaction and to increase photochemical reactivity. Internal electric fields in photocatalysts can arise from ferroelectric phenomena, p-n junctions, polar surface terminations, and polymorph junctions. The manipulation of internal fields through the creation of charged interfaces in hierarchically structured materials is a promising strategy for the design of improved photocatalysts.

Localized nano-solid-solution induced by Cu doping in ZnS for efficient solar hydrogen generation

Visible-light-driven H₂evolution rate reached 1.03 mmol h⁻¹with a quantum yield of 26.2% over the localized Cu_1−xZn_xS nano-solid-solutions.

Interpenetrating network V2O5 nanosheets/carbon nanotubes nanocomposite for fast lithium storage

An interpenetrating V₂O₅ nanosheets/CNTs nanocomposite was fabricated by a freeze-drying method. The nanocomposite has excellent electrochemical properties for fast lithium storage.

Synthesizing nano-sized (∼20 nm) Li4Ti5O12 at low temperature for a high-rate performance lithium ion battery anode

This is the first report on the preparation of highly crystalline Li₄Ti₅O₁₂ with a small particle size of ∼20 nm using a low synthesis temperature (500 °C).

Porous inorganic nanostructures with colloidal dimensions: synthesis and applications in electrochemical energy devices

Porous colloidal nanostructures are ideal materials for batteries, supercapacitors, solar and fuel cells (electrochemical devices that operate on renewable energy).

Hydrogenated TiO(2) nanocrystals: a novel microwave absorbing material

Here, we report, for the first time, hydrogenated TiO2 nanocrystals as a novel and exciting microwave absorbing material, based on an innovative collective-movement-of-interfacial-dipole mechanism which causes collective-interfacial-polarization-amplified microwave absorption at the crystalline/disordered and anatase/rutile interfaces. This mechanism is intriguing and upon further exploration may trigger other new concepts and applications.

Electrospun TiO(2) fiber composite photoelectrodes for water splitting

This work has focused on the development of electrospun TiO2 fiber composite photoelectrodes for hydrogen production by water splitting. For comparison, similar photoelectrodes were also developed using commercial TiO2 (Aeroxide P25) nanoparticles (NPs). Dispersions of either fibers or P25 NPs were used to make homogenous TiO2 films on fluorine-doped SnO2 (FTO) glass substrates by a doctor blade (DB) technique. Scanning electron microscopy (SEM) analysis revealed a much lower packing density of the DB fibers, with respect to DB-P25 TiO2 NPs; this was also directly reflected by the higher photocurrent measured for the NPs when irradiating the photoelectrodes at a light intensity of 1.5AM (1 sun, 1000 W/m(2)). For a better comparison of fibers vs. NPs, composite photoelectrodes by dip-coating (onto FTO) TiO2 sol-gel (SG) matrixes containing an equal amount (5 or 20 wt %) of either fibers or P25 NPs were also investigated. It emerged that the photoactivity of the fibers was significantly higher. For composites containing 5 wt % TiO2 fibers, a photocurrent of 0.5 mA/cm(2) (at 0.23 V vs Ag/AgCl) was measured, whereas 5 wt % P25 NPs only provided 0.2 mA/cm(2). When increasing to 20 wt % fibers or NPs, the photocurrent decreased, because of the formation of microcracks in the photoelectrodes, because of the shrinkage of the sol-gel. The high photoactivity of the fiber-based electrodes could be confirmed by incident photon to current efficiency (IPCE) measurements. Remarkably, the IPCE of composites containing 5 wt % fibers was between 35% and 40% in the region of 380-320 nm, and when accounting for transmission/reflection losses, the absorbed photon to current efficiency (APCE) was consistently over 60% between 380 nm and 320 nm. The superior photoactivity is attributed to the enhanced electron transport in the electrospun fibers, with respect to P25 NPs. According to this study, it is clear that the electronic connectivity ensured by the sol-gel also contributes positively to the enhanced photocurrent.

Built-in electric field-assisted surface-amorphized nanocrystals for high-rate lithium-ion battery

High-power batteries require fast charge/discharge rates and high capacity besides safe operation. TiO2 has been investigated as a safer alternative candidate to the current graphite or incoming silicon anodes due to higher redox potentials in effectively preventing lithium deposition. However, its charge/discharge rates are reluctant to improve due to poor ion diffusion coefficients, and its capacity fades quickly with rate as only thinner surface layers can be effectively used in faster charge/discharge processes. Here, we demonstrate that surface-amorphized TiO2 nanocrystals greatly improve lithium-ion rechargeable battery performance: 20 times rate and 340% capacity improvement over crystalline TiO2 nanocrystals. This improvement is benefited from the built-in electric field within the nanocrystals that induces much lower lithium-ion diffusion resistance and facilitates its transport in both insertion and extraction processes. This concept thus offers an innovative and general approach toward designing battery materials with better performance.

Relative photooxidation and photoreduction activities of the {100}, {101}, and {001} surfaces of anatase TiO2

The photoredox ability of the TiO2 {100}, {101}, and {001} surfaces is investigated by examining the trapping energies, trapping sites, and relative oxidation and reduction potentials of simulated photogenerated holes and electrons in the form of more realistic polaronic states on the basis of density functional electronic structure calculations. Our results enable us to re-estimate their relative photooxidation ({100} > {101} > {001}) and photoreduction ({100} > {101} > {001}) activities, which rectify the conventional understanding. The dual functions of the surface under coordinated atoms acting as active adsorption sites for adsorbates and hindering the population of electrons to the outermost surface layer are identified, and the specific surface geometric structures also play an important role in trapping holes and electrons through the ease of lattice distortion. In addition, we attribute the commonly low photocatalytic performance of the {101} surface to the large and similar trapping energies and adjacent trapping sites for electrons and holes, which result in high electron-hole recombination rates. However, the large difference in trapping energies for electrons and holes on different surfaces allows us to spatially gather electrons and holes on different surfaces by artificially designing the exposing facets of nanocrystals without resorting to the energy band potential difference between surfaces, thus expanding the ideas to improve the photocatalytic properties of materials through the regulation of crystal facets. Our present work can provide a helpful message for the design of more reactive photocatalytic TiO2 nanocrystals and the fabrication of other reactive photocatalysts.

Directional heat dissipation across the interface in anatase-rutile nanocomposites

Understanding the structures and properties of interfaces in (nano-)composites helps to reveal their important influence on reactivity and overall performance. TiO2 is a technologically important material, and anatase/rutile TiO2 composites have been shown to display enhanced photocatalytic performance over pure anatase or rutile TiO2. This has been attributed to a synergistic effect between the two phases, but the origin of this effect as well as the structure of the interface has not been established. Using Raman spectroscopy, here we provide evidence of distinct differences in the thermal properties of the anatase and rutile moieties in the composite, with anatase becoming effectively much warmer than the rutile phase under laser irradiation. With the help of first-principles calculations, we analyze the atomic structure and unique electronic properties of the composite and infer possible reasons for the directional heat dissipation across the interface.

Photogeneration of hydrogen from water using CdSe nanocrystals demonstrating the importance of surface exchange

Unique tripodal S-donor capping agents with an attached carboxylate are found to bind tightly to the surface of CdSe nanocrystals (NCs), making the latter water soluble. Unlike that in similarly solubilized CdSe NCs with one-sulfur or two-sulfur capping agents, dissociation from the NC surface is greatly reduced. The impact of this behavior is seen in the photochemical generation of H2 in which the CdSe NCs function as the light absorber with metal complexes in aqueous solution as the H2-forming catalyst and ascorbic acid as the electron donor source. This precious-metal-free system for H2 generation from water using [Co(bdt)2](-) (bdt, benzene-1,2-dithiolate) as the catalyst exhibits excellent activity with a quantum yield for H2 formation of 24% at 520 nm light and durability with >300,000 turnovers relative to catalyst in 60 h.

A Bi2 WO6 -based hybrid photocatalyst with broad spectrum photocatalytic properties under UV, visible, and near-infrared irradiation

Near-infrared active photocatalytic properties of Bi2 WO6 nanosheets owing to the oxygen vacancies of the Bi2 WO6 nanosheets are reported. The broad spectrum photocatalyst, Bi2 WO6 -TiO2 nanobelt heterostructures, are obtained by assembling Bi2 WO6 nanocrystals on TiO2 nanobelts. The active light band of the novel hybrid photocatalyst with high photocatalytic activity covers full-spectrum solar light including the UV, visible, and near-infrared ranges.

Hydrogenation and disorder in engineered black TiO2

A new form of TiO2 which is black in color has been shown to exhibit high efficiency for photocatalytic reactions under solar radiation [X. Chen, L. Liu, P. Y. Yu, and S. S. Mao, Science 331, 746 (2011)]. However, the mechanism behind this disorder-engineering process is not fully understood. In this Letter, based on density functional theory, we describe the role of hydrogen in producing lattice disorder in the anatase nanocrystals. We clarify further that the highly localized nature of the midgap states results in spatial separation of photoexcited electrons and holes in black TiO2, and that accounts for its high photocatalytic efficiency.

Functionalization of SnO₂ crystals with a covalently-assembled porphyrin monolayer

The functionalization of micro- and nano-sized metal-oxide powders offers many advantages because of their large surface areas and, therefore, the large number of functional molecules that can be grafted onto the grain surfaces. Porphyrin molecules on large band-gap semiconducting metal oxides represent key materials for many different optical and electronic applications. Herein, we have proposed a general two-step procedure for the functionalization of metal-oxide crystals with dye-sensitizers. In particular, we functionalized SnO₂ nanoparticles with a monolayer of the bifunctional trichloro[4-(chloromethyl)phenyl]silane. Then, a monolayer of 5,10,15,20-tetrakis(4-hydroxyphenyl)-21H,23H-porphyne was covalently bound to the silanized SnO₂ grains. IR, UV/Vis, and luminescence measurements were used for optical characterization. The measured footprint of the grafted porphyrin molecules indicated total surface coverage of the grains. The surface electronic characterization was performed by using X-ray photoelectron spectroscopy. Emission measurements revealed two strong bands at 664.1 and 721.0 nm that were attributed to the porphyrin monolayer assembled on the surface of the SnO₂ crystals.

Leaf-architectured 3D hierarchical artificial photosynthetic system of perovskite titanates towards CO₂ photoreduction into hydrocarbon fuels

The development of an "artificial photosynthetic system" (APS) having both the analogous important structural elements and reaction features of photosynthesis to achieve solar-driven water splitting and CO₂ reduction is highly challenging. Here, we demonstrate a design strategy for a promising 3D APS architecture as an efficient mass flow/light harvesting network relying on the morphological replacement of a concept prototype-leaf's 3D architecture into perovskite titanates for CO₂ photoreduction into hydrocarbon fuels (CO and CH₄). The process uses artificial sunlight as the energy source, water as an electron donor and CO₂ as the carbon source, mimicking what real leaves do. To our knowledge this is the first example utilizing biological systems as "architecture-directing agents" for APS towards CO₂ photoreduction, which hints at a more general principle for APS architectures with a great variety of optimized biological geometries. This research would have great significance for the potential realization of global carbon neutral cycle.