Nanostructured CeO2 photocatalysts: optimizing surface chemistry, morphology, and visible-light absorption.

Emerging photocatalytic applications of cerium dioxide (CeO2) include green hydrogen production, CO2 conversion to fuels, and environmental remediation of various toxic molecules. These applications leverage the oxygen storage capacity and tunable surface chemistry of CeO2 to photocatalyze the chosen reaction, but many open questions remain regarding the fundamental physics of photocatalysis over CeO2. The commonly ascribed 'bandgap' of CeO2 (∼3.1 eV) differs fundamentally from other photocatalytic oxides such as TiO2; UV light excites an electron from the CeO2 valence band into a 4f state, generating a polaron as the lattice distorts around the localized charge. Researchers often disregard the distinction between the 4f state and a traditional, delocalized conduction band, resulting in ambiguity regarding mechanisms of charge transfer and visible-light absorption. This review summarizes modern literature regarding CeO2 photocatalysis and discusses commonly reported photocatalytic reactions and visible light-sensitization strategies. We detail the often misunderstood fundamental physics of CeO2 photocatalysis and supplement previous work with original computational insights. The exceptional progress and remaining challenges of CeO2-based photocatalysts are highlighted, along with suggestions for further research directions based on the observed gaps in current understanding.

Significantly Enhanced Thermoelectric Performance of Graphene through Atomic-Scale Defect Engineering via Mobile Hot-Wire Chemical Vapor Deposition Systems.

Over the years, numerous studies have attempted to develop two-dimensional (2D) materials for improving both the applicability and performance of thermoelectric devices. Among the 2D materials, graphene is one of the promising candidates for thermoelectric materials owing to its extraordinary electrical properties, flexibility, and nontoxicity. However, graphene synthesized through traditional methods suffers from a low Seebeck coefficient and high thermal conductivity, resulting in an extremely low thermoelectric figure of merit (ZT). Here, we present an atomic-scale defect engineering strategy to improve the thermoelectric properties of graphene using embedded high-angle tilt boundary (HATB) domains in graphene films. These HATB domains serve as both energy filtering sites to filter out lower-energy charge carriers and scattering sites for phonons. Compared to the conventionally grown chemical vapor deposited graphene, the graphene with HATB domains shows an improved Seebeck coefficient (50.1 vs 21.1 µV K-1) and reduced thermal conductivity (382 vs 952 W m-1K-1), resulting in a ZT value that is ∼7 times greater at 350 K. This defect engineering strategy is promising not only for graphene-based materials but also for 2D materials, in general, where further research and optimization could overcome the limitations of conventional bulk thermoelectric materials in energy-harvesting systems.

Synthesis and applications of WO3 nanosheets: the importance of phase, stoichiometry, and aspect ratio.

Tungsten trioxide (WO3) is an abundant, versatile oxide that is widely explored for catalysis, sensing, electrochromic devices, and numerous other applications. The exploitation of WO3 in nanosheet form provides potential advantages in many of these fields because the 2D structures have high surface area and preferentially exposed facets. Relative to bulk WO3, nanosheets expose more active sites for surface-sensitive sensing/catalytic reactions, and improve reaction kinetics in cases where ionic diffusion is a limiting factor (e.g. electrochromic or charge storage). Synthesis of high aspect ratio WO3 nanosheets, however, is more challenging than other 2D materials because bulk WO3 is not an intrinsically layered material, making the widely-studied sonication-based exfoliation methods used for other 2D materials not well-suited to WO3. WO3 is also highly complex in terms of how the synthesis method affects the properties of the final material. Depending on the route used and subsequent post-synthesis treatments, a wide variety of different morphologies, phases, exposed facets, and defect structures are created, all of which must be carefully considered for the chosen application. In this review, the recent developments in WO3 nanosheet synthesis and their impact on performance in various applications are summarized and critically analyzed.

Simultaneous Enhancement of Thermopower and Electrical Conductivity through Isovalent Substitution of Cerium in Bismuth Selenide Thermoelectric Materials.

It is challenging to achieve highly efficient thermoelectric materials due to the conflicts between thermopower (Seebeck coefficient) and electrical conductivity. These parameters are the core factors defining the thermoelectric property of any material. Here, we report the use of isovalent substitution as a tool to decouple the interdependency of the Seebeck coefficient and the electrical properties of cerium-doped bismuth selenide thermoelectric material. With this strategy, we can achieve a simultaneous increase in both the electrical conductivity and the Seebeck coefficient of the material by tuning the concentration of cerium doping, due to formation of neutral impurities and consequently the improvement of carrier mobility. Our theoretical calculation reveals a downward shift of the valence band with cerium concentration, which influences the thermoelectric enhancement of the synthesized materials. Finally, an order of magnitude enhancement of the figure of merit is obtained due to isovalent substitution, thus providing a new avenue for enhancing the thermoelectric performance of materials.

2D and 3D nanostructuring strategies for thermoelectric materials.

Thermoelectric materials have attracted increased research attention as the implementation of various nanostructures has potential to improve both their performance and applicability. A traditional limitation of thermoelectric performance in bulk materials is the interconnected nature of the individual parameters (for example, it is difficult to decrease thermal conductivity while maintaining electrical conductivity), but through the rational design of nanoscale structures, it is possible to decouple these relationships and greatly enhance the performance. For 2D strategies, newly investigated materials such as graphene, transition metal dichalcogenides, black phosphorus, etc. are attractive thanks to not only their unique thermoelectric properties, but also potential advantages in ease of processing, flexibility, and lack of rare or toxic constituent elements. For 3D strategies, the use of induced porosity, assembly of various nanostructures, and nanoscale lithography all offer specific advantages over bulk materials of the same chemical composition, most notably decreased thermal conductivity due to phonon scattering and enhanced Seebeck coefficient due to energy filtering. In this review, a general summary of the popular techniques and strategies for 2D and 3D thermoelectric materials will be provided, along with suggestions for future research directions based on the observed trends.

Two-Dimensional WO3 Nanosheets Chemically Converted from Layered WS2 for High-Performance Electrochromic Devices.

Two-dimensional (2D) transitional metal oxides (TMOs) are an attractive class of materials due to the combined advantages of high active surface area, enhanced electrochemical properties, and stability. Among the 2D TMOs, 2D tungsten oxide (WO3) nanosheets possess great potential in electrochemical applications, particularly in electrochromic (EC) devices. However, feasible production of 2D WO3 nanosheets is challenging due to the innate 3D crystallographic structure of WO3. Here we report a novel solution-phase synthesis of 2D WO3 nanosheets through simple oxidation from 2D tungsten disulfide (WS2) nanosheets exfoliated from bulk WS2 powder. The complete conversion from WS2 into WO3 was confirmed through crystallographic and elemental analyses, followed by validation of the 2D WO3 nanosheets applied in the EC device. The EC device showed color modulation of 62.57% at 700 nm wavelength, which is 3.43 times higher than the value of the conventional device using bulk WO3 powder, while also showing enhancement of ∼46.62% and ∼62.71% in switching response-time (coloration and bleaching). The mechanism of enhancement was rationalized through comparative analysis based on the thickness of the WO3 components. In the future, 2D WO3 nanosheets could also be used for other promising applications such as sensors, catalysis, thermoelectric, and energy conversion.

Low-Cost Black Phosphorus Nanofillers for Improved Thermoelectric Performance in PEDOT:PSS Composite Films.

In recent years, two-dimensional black phosphorus (BP) has seen a surge of research because of its unique optical, electronic, and chemical properties. BP has also received interest as a potential thermoelectric material because of its high Seebeck coefficient and excellent charge mobility, but further development is limited by the high cost and poor scalability of traditional BP synthesis techniques. In this work, high-quality BP is synthesized using a low-cost method and utilized in a PEDOT:PSS film to create the first ever BP composite thermoelectric material. The thermoelectric properties are found to be greatly enhanced after the BP addition, with the power factor of the film, with 2 wt % BP (36.2 µW m-1 K-2) representing a 109% improvement over the pure PEDOT:PSS film (17.3 µW m-1 K-2). A simultaneous increase of mobility and decrease of the carrier concentration is found to occur with the increasing BP wt %, which allows for both Seebeck coefficient and electrical conductivity to be increased. These results show the potential of this low-cost BP for use in energy devices.

Solution-phase phosphorus substitution for enhanced oxygen evolution reaction in Cu2WS4.

Transition metal phosphides are among the most promising materials for achieving efficient electrocatalytic performance without the use of rare or expensive noble metals. However, previous research into phosphides for the hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) has focused on high-temperature vapor-phase processes, which are not practical for large-scale applications. Here, we introduce a simple, one-step solution-phase method of phosphide synthesis by modifying Cu2WS4 using triphenylphosphine (TPP), which serves to substitute S with P and transform the normally inactive basal plane of Cu2WS4 into a defect-rich, activated basal plane. The OER activity was significantly enhanced by phosphorus substitution, with the resulting Tafel slope of the sample with ∼8 at% phosphorus reaching 194 mV dec-1, a result close to that of the best OER catalyst (RuO2, 151 mV dec-1). The sample possessed stable OER performance, showing no degradation in current density over ∼24 hours (500 cycles), proving the robust and stable nature of the phosphorus substitution. These results open the possibility for further phosphide catalyst development using this low-cost, solution-phase method.

Analysis of contact resistance in single-walled carbon nanotube channel and graphene electrodes in a thin film transistor.

In this work, we present the experimental investigation on the contact resistance of graphene/single-walled carbon nanotube (SWCNT) junction using transfer length method with the simple equivalent circuit model. We find that p-n like junctions are formed in graphene/SWCNT transistors, and the contact resistance in the junction is observed to be ~ 494 and ~ 617 kΩ in case of metallic SWCNT (m-SWCNT) and semiconducting SWCNT (s-SWCNT), respectively. In addition, the contact resistance increases from 617 to 2316 kΩ as Vg increases from - 30 to - 10 V. Through our study, high carrier density induced from doping in both graphene and SWCNT leads to low contact resistance. This development of contact engineering, namely modulation of carrier density in the junction and contact length (Lcon) scaling shows the potential for all-carbon based electronics.

Fast P3HT Exciton Dissociation and Absorption Enhancement of Organic Solar Cells by PEG-Functionalized Graphene Quantum Dots.

PEG-functionalized graphene quantum dots (GQDs) are shown to promote fast exciton dissociation in organic solar cells. Short-chain PEG promotes the most favorable interaction with other organic layers, and the overall efficiency is improved by 36% when compared to the reference devices. The mechanism of enhancement is shown to be increased absorption due to fewer charges remain-ing in the bound state.