Direct Experimental Observation of Facet‐Dependent SERS of Cu2O Polyhedra

Semiconductor‐based surface enhanced Raman scattering (SERS) has attracted great attention due to its excellent spectral reproducibility, high uniformity, and good anti‐interference ability. However, its relatively low SERS sensitivity still hinders its further developments in both performance and applications. Since the SERS is a peculiar surface effect, investigating the facet‐dependent SERS activity of semiconductor nanostructures is crucial to boost their SERS signals. Although the semiconductor facet‐dependent SERS effect is predicted via numerical calculations, convincing experimental evidence is scarce due to complicated and undefined surface conditions. In this work, three facet‐defined ({100}, {110}, and {111} facets) Cu₂O microcrystals (MCs) with clear surface atomic configuration are utilized to investigate the facet‐dependent SERS effect. The results from the Kelvin probe force microscopy measurements on single Cu₂O polyhedron, demonstrate that the facet‐dependent work function plays a crucial role in the interfacial charge transfer process. Comparing with the {110} and {111} facets, the {100} facet possesses the lowest electronic work function, which enables more efficient interfacial charge transfer. The simulation results further confirm that the {100}‐facets can transfer the most electrons from Cu₂O MCs to molecules due to its lowest facet work function, resulting in the largest increment of the molecular polarization.     

Polyhedral 30‐Faceted BiVO4 Microcrystals Predominantly Enclosed by High‐Index Planes Promoting Photocatalytic Water‐Splitting Activity

Unprecedented 30‐faceted BiVO₄ polyhedra predominantly surrounded by {132}, {321}, and {121} high‐index facets are fabricated through the engineering of high‐index surfaces by a trace amount of Au nanoparticles. The growth of high‐index facets results in a 3–5 fold enhancement of O₂ evolution from photocatalytic water splitting by the BiVO₄ polyhedron, relative to its low‐index counterparts. Theory calculations reveal that water dissociation is more energetically favorable on the high‐index surfaces than on the low‐index (010), (110), and (101) surfaces, which is accompanied by a notable reduction in the overpotential (0.77–1.14 V) for the oxygen evolution reaction. The apparent quantum efficiency of O₂ generation without an external electron supply reaches 18.3% under 430 nm light irradiation, which is an order of magnitude higher than that of the catalysts reported hitherto.     

Manipulation of Charge Transport by Metallic V13O16 Decorated on Bismuth Vanadate Photoelectrochemical Catalyst

Conductive metal oxides represent a new category of functional material with vital importance for many modern applications. The present work introduces a new conductive metal oxide V₁₃O₁₆, which is synthesized via a simplified photoelectrochemical procedure and decorated onto the semiconducting photocatalyst BiVO₄ in controlled mass percentages ranging from 25% to 37%. Owing to its excellent conductivity and good compatibility with oxide materials, the metallic V₁₃O₁₆‐decorated BiVO₄ hybrid catalyst shows a high photocurrent density of 2.2 ± 0.2 mA cm^?2 at 1.23 V versus reversible hydrogen electrode (RHE). Both experimental characterization and density functional theory calculations indicate that the superior photocurrent derives from enhanced charge separation and transfer, resulting from ohmic contact at the interface of mixed phases and superior electrical conductivity from V₁₃O₁₆. A Co–Pi coating on BiVO₄–V₁₃O₁₆ further increases the photocurrent to 5.0 ± 0.5 mA cm^?2 at 1.23 V versus RHE, which is among the highest reported for BiVO₄‐based photoelectrodes. Surface photovoltage and transient photocurrent measurements suggest a charge‐transfer model in which photocurrents are enhanced by improved surface passivation, although the barrier at the Co–Pi/electrolyte interface limits the charge transfer.     

Dominance of Plasmonic Resonant Energy Transfer over Direct Electron Transfer in Substantially Enhanced Water Oxidation Activity of BiVO4 by Shape‐Controlled Au Nanoparticles

The performance of plasmonic Au nanostructure/metal oxide heterointerface shows great promise in enhancing photoactivity, due to its ability to confine light to the small volume inside the semiconductor and modify the interfacial electronic band structure. While the shape control of Au nanoparticles (NPs) is crucial for moderate bandgap semiconductors, because plasmonic resonance by interband excitations overlaps above the absorption edge of semiconductors, its critical role in water splitting is still not fully understood. Here, first, the plasmonic effects of shape‐controlled Au NPs on bismuth vanadate (BiVO₄) are studied, and a largely enhanced photoactivity of BiVO₄ is reported by introducing the octahedral Au NPs. The octahedral Au NP/BiVO₄ achieves 2.4 mA cm^?2 at the 1.23 V versus reversible hydrogen electrode, which is the threefold enhancement compared to BiVO₄. It is the highest value among the previously reported plasmonic Au NPs/BiVO₄. Improved photoactivity is attributed to the localized surface plasmon resonance; direct electron transfer (DET), plasmonic resonant energy transfer (PRET). The PRET can be stressed over DET when considering the moderate bandgap semiconductor. Enhanced water oxidation induced by the shape‐controlled Au NPs is applicable to moderate semiconductors, and shows a systematic study to explore new efficient plasmonic solar water splitting cells.     

One‐Step Transfer and Integration of Multifunctionality in CVD Graphene by TiO2/Graphene Oxide Hybrid Layer

We present a straightforward method for simultaneously enhancing the electrical conductivity, environmental stability, and photocatalytic properties of graphene films through one‐step transfer of CVD graphene and integration by introducing TiO₂/graphene oxide layer. A highly durable and flexible TiO₂ layer is successfully used as a supporting layer for graphene transfer instead of the commonly used PMMA. Transferred graphene/TiO₂ film is directly used for measuring the carrier transport and optoelectronic properties without an extra TiO₂ removal and following deposition steps for multifunctional integration into devices because the thin TiO₂ layer is optically transparent and electrically semiconducting. Moreover, the TiO₂ layer induces charge screening by electrostatically interacting with the residual oxygen moieties on graphene, which are charge scattering centers, resulting in a reduced current hysteresis. Adsorption of water and other chemical molecules onto the graphene surface is also prevented by the passivating TiO₂ layer, resulting in the long term environmental stability of the graphene under high temperature and humidity. In addition, the graphene/TiO₂ film shows effectively enhanced photocatalytic properties because of the increase in the transport efficiency of the photogenerated electrons due to the decrease in the injection barrier formed at the interface between the F‐doped tin oxide and TiO₂ layers.     

New BiVO4 Dual Photoanodes with Enriched Oxygen Vacancies for Efficient Solar‐Driven Water Splitting

Bismuth vanadate (BiVO₄) is a promising photoanode material for photoelectrochemical (PEC) water splitting. However, owing to the short carrier diffusion length, the trade‐off between sufficient light absorption and efficient charge separation often leads to poor PEC performance. Herein, a new electrodeposition process is developed to prepare bismuth oxide precursor films, which can be converted to transparent BiVO₄ films with well‐controlled oxygen vacancies via a mild thermal treatment process. The optimized BiVO₄ film exhibits an excellent back illumination charge separation efficiency mainly due to the presence of enriched oxygen vacancies which act as shallow donors. By loading FeOOH/NiOOH as the cocatalysts, the BiVO₄ dual photoanodes exhibit a remarkable and highly stable photocurrent density of 5.87 mA cm^?2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination. An artificial leaf composed of the BiVO₄/FeOOH/NiOOH dual photoanodes and a single sealed perovskite solar cell delivers a solar‐to‐hydrogen conversion efficiency as high as 6.5% for unbiased water splitting.     

Structural Engineering of BiVO4/CoFe MOF Heterostructures Boosting Charge Transfer for Efficient Photoelectrochemical Water Splitting

Boosting charge separation and transfer of photoanodes is crucial for providing high viability of photoelectrochemical hydrogen (H₂) generation. Here, a structural engineering strategy is designed and synthesized for uniformly coating an ultrathin CoFe bimetal-organic framework (CoFe MOF) layer over a BiVO₄ photoanode for boosted charge separation and transfer. The photocurrent density of the optimized BiVO₄/CoFe MOF(NA) photoanode reaches a value of 3.92 mA cm⁻² at 1.23 V versus reversible hydrogen electrode (RHE), up to 6.03 times that of pristine BiVO₄, due to the greatly increased efficiency of charge transfer and separation. In addition, this photoanode records one onset potential that is considerably shifted negatively when compared to BiVO₄. Transient absorption spectroscopy reveals that the CoFe MOF(NA) prolongs charge recombination lifetime by blocking the hole-transfer pathway from the BiVO₄ to its surface trap states. This work sheds light on boosting charge separation and transfer through structural engineering to enhance the photocurrent of photoanodes for solar H₂ production.     

Recent advances in titanium dioxide/graphene photocatalyst materials as potentials of energy generation

The properties of titanium dioxide (\(\hbox {TiO}_{2})\)/graphene/graphene oxides (GO) are examined in this study. These views summarize the recent theoretical and experimental novel approaches in the catalytic activity of \(\hbox {TiO}_{2}\)/graphene interface. Imperative results at a level of detail, suitable for upcoming experimental and theoretical researchers involved an overview of the enthralling characteristics of \(\hbox {TiO}_{2}\) and graphene composites were presented. Aspects like crystal lattice, electronic band structure and phonon dispersion, among others that were used to describe the properties of a \(\hbox {TiO}_{2}\) interface with pristine graphene and graphene dioxide among other composites are discussed. In particular, this review covers reactivity, binding energies, geometric structures as well as the photocatalytic activity of anatase \(\hbox {TiO}_{2}\) surfaces with graphene and graphene oxide with hybrid nanocomposites. These views also explore the understanding of the \(\hbox {TiO}_{2}\) interactions with graphene and possible applications. Finally, highlights on the challenges and proposed strategies in developing advanced photocatalytic semiconductor-based composites for water-splitting applications are provided.     

Facet Engineered Interface Design of Plasmonic Metal and Cocatalyst on BiOCl Nanoplates for Enhanced Visible Photocatalytic Oxygen Evolution

Integration of plasmonic metal and cocatalyst with semiconductor is a promising approach to simultaneously optimize the generation, transfer, and consumption of photoinduced charge carriers for high‐performance photocatalysis. The photocatalytic activities of the designed hybrid structures are greatly determined by the efficiencies of charge transfer across the interfaces between different components. In this paper, interface design of Ag‐BiOCl‐PdO_x hybrid photocatalysts is demonstrated based on the choice of suitable BiOCl facets in depositing plasmonic Ag and PdO_x cocatalyst, respectively. It is found that the selective deposition of Ag and PdO_x on BiOCl(110) planes realizes the superior photocatalytic activity in O₂ evolution compared with the samples with other Ag and PdO_x deposition locations. The reason was the superior hole transfer abilities of Ag‐(110)BiOCl and BiOCl(110)‐PdO_x interfaces in comparison with those of Ag‐(001)BiOCl and BiOCl(001)‐PdO_x interfaces. Two effects are proposed to contribute to this enhancement: (1) stronger electronic coupling at the BiOCl(110)‐based interfaces resulted from the thinner contact barrier layer and (2) the shortest average hole diffuse distance realized by Ag and PdO_x on BiOCl(110) planes. This work represents a step toward the interface design of high‐performance photocatalyst through facet engineering.     

Nanostructured WO3/BiVO4 Photoanodes for Efficient Photoelectrochemical Water Splitting

Nanostructured photoanodes based on well‐separated and vertically oriented WO₃ nanorods capped with extremely thin BiVO₄ absorber layers are fabricated by the combination of Glancing Angle Deposition and normal physical sputtering techniques. The optimized WO₃‐NRs/BiVO₄ photoanode modified with Co‐Pi oxygen evolution co‐catalyst shows remarkably stable photocurrents of 3.2 and 5.1 mA/cm² at 1.23 V versus a reversible hydrogen electrode in a stable Na₂SO₄ electrolyte under simulated solar light at the standard 1 Sun and concentrated 2 Suns illumination, respectively. The photocurrent enhancement is attributed to the faster charge separation in the electronically thin BiVO₄ layer and significantly reduced charge recombination. The enhanced light trapping in the nanostructured WO₃‐NRs/BiVO₄ photoanode effectively increases the optical thickness of the BiVO₄ layer and results in efficient absorption of the incident light.     

Thermodynamic modeling of the initial microstructural evolution of oxide films grown on bare copper

A thermodynamic model is presented that predicts the initial growth of either a (semi-) coherent crystalline oxide phase or an amorphous oxide phase (with a subsequent amorphous-to-crystalline transition) on a bare metal as function of the substrate orientation, growth temperature and film thickness. The model accounts for possible relaxation of growth stresses by plastic deformation. The direct formation and growth of semi-coherent, crystalline Cu₂O is predicted by application of the model to oxide overgrowth on bare Cu{111}, Cu{100} and Cu{110}. For oxide overgrowths on Cu{111} and Cu{110}, a square grid of misfit dislocations with a dislocation distance of about six Cu₂O unit cells would occur on the basis of the model calculations, which agrees with experimental observations reported for Cu{111} in the literature. On Cu{100} an array of misfit dislocations is formed along the single direction of high lattice mismatch.     

Facet‐Dependent Optical Properties of Semiconductor Nanocrystals

Recent observations of facet‐dependent electrical conductivity and photocatalytic activity of various semiconductor crystals are presented. Then, the discovery of facet‐dependent surface plasmon resonance absorption of metal–Cu₂O core–shell nanocrystals with tunable sizes and shapes is discussed. The Cu₂O shells also exhibit a facet‐specific optical absorption feature. The facet‐dependent electrical conductivity, photocatalytic activity, and optical properties are related phenomena, resulting from the presence of an ultrathin surface layer with different band structures and thus varying degrees of band bending for the {100}, {110}, and {111} faces of Cu₂O to absorb light of somewhat different wavelengths. Recently, it is shown that the light absorption and photoluminescence properties of pure Cu₂O cubes, octahedra, and rhombic dodecahedra also display size and facet effects because of their tunable band gaps. A modified band diagram of Cu₂O can be constructed to incorporate these optical effects. Literature also provides examples of facet‐dependent optical behaviors of semiconductor nanostructures, indicating that optical properties of nanoscale semiconductor materials are intrinsically facet‐dependent. Some applications of semiconductor optical size and facet effects are considered.     

Investigation of capacitance characteristics in metal/high-<Emphasis Type="Italic">k</Emphasis> semiconductor devices at different parameters and with and without interface state density (traps)

Capacitance vs. voltage (\(C{-}V\)) curves at AC high frequency of a metal–insulator–semiconductor (MIS) capacitor are investigated in this paper. Bi-dimensional simulations with Silvaco TCAD were carried out to study the effect of oxide thickness, the surface of the structure, frequency, temperature and fixed charge in the oxide on the \(C{-}V\) curves. We evaluate also the analysis of MIS capacitor structures by different substrate doping concentrations with and without interface state density at different temperatures (100, 300 and 600 K). These studies indicate that the doping substrate concentration and the traps enormously affect the high-frequency \(C{-}V\) curve behaviour. We also demonstrate that for low and high temperatures, the high-frequency \(C{-}V\) curves behaviour changes, indicating that the capacitance due to the substrate is significantly influenced in these conditions (bias and substrate doping concentration).     

Twinning and orientation relationships of T-phase precipitates in an Al matrix

The recent increasing interest of T-phase in Al alloy has been switched to its twins. In this study, we employed high resolution transmission electron microscopy to study and compare the morphology and orientation relationships (OR) of T-phase and its twins in an Al–Cu–Mg–Mn alloy. It is found that T-phase tends to form on the {403}_Al habit planes and exhibit a rod-like shape, with it longitudinal axis, [010]_T, being parallel to the matrix [010]_Al direction. Three different OR types are determined between T-phase and Al matrix, namely, {200}_T〈010〉_T//{200}_Al〈010〉_Al (OR-I), {200}_T〈010〉_T// $ \{ 40\bar{3}\}_{\text{Al}} $ 〈010〉_Al (OR-II), and {200}_T〈010〉_T//{301}_Al〈010〉_Al (OR-III). OR-II is the most widely observed OR, while OR-I and III can form from the OR-II by twinning. During the twinning, the cross-section of T-phase transforms from a parallelogram-like shape into a shell-like shape. Further analyses on the shell-like T-twins strongly suggest that tenfold twins could form directly from the successive twinning of an individual T crystal.     

Strong Charge Transfer at 2H–1T Phase Boundary of MoS2 for Superb High‐Performance Energy Storage

Transition metal dichalcogenides exhibit several different phases (e.g., semiconducting 2H, metallic 1T, 1T′) arising from the collective and sluggish atomic displacements rooted in the charge‐lattice interaction. The coexistence of multiphase in a single sheet enables ubiquitous heterophase and inhomogeneous charge distribution. Herein, by combining the first‐principles calculations and experimental investigations, a strong charge transfer ability at the heterophase boundary of molybdenum disulfide (MoS₂) assembled together with graphene is reported. By modulating the phase composition in MoS₂, the performance of the nanohybrid for energy storage can be modulated, whereby remarkable gravimetric and volumetric capacitances of 272 F g^?1 and 685 F cm^?3 are demonstrated. As a proof of concept for energy application, a flexible solid‐state asymmetric supercapacitor is constructed with the MoS₂‐graphene heterolayers, which shows superb energy and power densities (46.3 mWh cm^?3 and 3.013 W cm^?3, respectively). The present work demonstrates a new pathway for efficient charge flow and application in energy storage by engineering the phase boundary and interface in 2D materials of transition metal dichalcogenides.     

Controlling Monomer Feeding Rate to Achieve Highly Crystalline Covalent Triazine Frameworks

The synthesis of highly crystalline covalent triazine frameworks (CTFs) with ultrastrong covalent bonds (aromatic C?N) from the triazine linkage presents a great challenge to synthetic chemists. Herein, the synthesis of highly crystalline CTFs via directly controlling the monomer feeding rate is reported. By tuning the feeding rate of monomers, the crystallization process can be readily governed in a controlled manner in an open system. The sample of CTF‐HUST‐HC1 with abundant exposed {001} crystal facets has the better crystallinity and thus is selected to study the effect of high crystallinity on photoelectric properties. Owing to the better separation of photogenerated electron–hole pairs and charge transfer, the obtained highly ordered CTF‐HUST‐HC1 has superior performance in the photocatalytic removal of nitric oxide (NO) than its lesser crystalline counterparts and g‐C₃N₄.     

3D Brochosomes‐Like TiO2/WO3/BiVO4 Arrays as Photoanode for Photoelectrochemical Hydrogen Production

An ideal photoelectrochemical (PEC) anode should process effective light absorption, charge transport, and separation efficiency. Here, a novel 3D brochosomes‐like TiO₂/WO₃/BiVO₄ array as an efficient photoanode by combining a colloid polystyrene sphere template and electrochemical deposition routes for PEC hydrogen generation is reported. The as‐fabricated 3D TiO₂/WO₃/BiVO₄ brochosomes photoanode yields excellent PEC performance with photocurrent densities of ≈3.13 and ≈4.27 mA cm^?2 with FeOOH/NiOOH catalyst, respectively, measured in 0.5 m Na₂SO₄ solution with 0.1 m Na₂SO₃ at 1.23 V versus reversible hydrogen electrode (RHE) under simulated AM1.5 light illumination, which is ≈6 times the reference sample of a planar WO₃/BiVO₄ film electrode. The significantly improved performance could be benefited from the ordered hollow porous structure that provides enhanced light absorption and efficient charge transport as well as improved charge separation efficiency by WO₃/BiVO₄ “host–guest” heterojunctions.     

Effects of temperature and ferromagnetism on the γ-Ni/γ′-Ni3Al interfacial free energy from first principles calculations

The temperature dependencies of the γ(f.c.c.)-Ni/γ′-Ni₃Al(L1₂) interfacial free energy for the {100}, {110}, and {111} interfaces are calculated using first-principles calculations, including both coherency strain energy and phonon vibrational entropy. Calculations performed including ferromagnetic effects predict that the {100}-type interface has the smallest free energy at different elevated temperatures, while alternatively the {111}-type interface has the smallest free energy when ferromagnetism is absent; the latter result is inconsistent with experimental observations of γ′-Ni₃Al-precipitates in Ni–Al alloys faceted strongly on {100}-type planes. The γ(f.c.c.)-Ni/γ′-Ni₃Al interfacial free energies for the {100}, {110}, and {111} interfaces decrease with increasing temperature due to vibrational entropy. The predicted morphology of γ′-Ni₃Al(L1₂) precipitates, based on a Wulff construction, is a Great Rhombicuboctahedron (or Truncated Cuboctahedron), which is one of the 13 Archimedean solids, with 6-{100}, 12-{110}, and 8-{111} facets. The first-principles calculated morphology of a γ′-Ni₃Al(L1₂) precipitate is in agreement with experimental three-dimensional atom-probe tomographic observations of cuboidal L1₂ precipitates with large {100}-type facets in a Ni-13.0 at.% Al alloy aged at 823 K for 4096 h. At 823 K this alloy has a lattice parameter mismatch of 0.004 ± 0.001 between the γ(f.c.c.)-Ni-matrix and the γ′-Ni₃Al-precipitates.     

Amino‐Acid‐Induced Preferential Orientation of Perovskite Crystals for Enhancing Interfacial Charge Transfer and Photovoltaic Performance

Interfacial engineering of perovskite solar cells (PSCs) is attracting intensive attention owing to the charge transfer efficiency at an interface, which greatly influences the photovoltaic performance. This study demonstrates the modification of a TiO₂ electron‐transporting layer with various amino acids, which affects charge transfer efficiency at the TiO₂/CH₃NH₃PbI₃ interface in PSC, among which the l ‐alanine‐modified cell exhibits the best power conversion efficiency with 30% enhancement. This study also shows that the (110) plane of perovskite crystallites tends to align in the direction perpendicular to the amino‐acid‐modified TiO₂ as observed in grazing‐incidence wide‐angle X‐ray scattering of thin CH₃NH₃PbI₃ perovskite film. Electrochemical impedance spectroscopy reveals less charge transfer resistance at the TiO₂/CH₃NH₃PbI₃ interface after being modified with amino acids, which is also supported by the lower intensity of steady‐state photoluminescence (PL) and the reduced PL lifetime of perovskite. In addition, based on the PL measurement with excitation from different side of the sample, amino‐acid‐modified samples show less surface trapping effect compared to the sample without modification, which may also facilitate charge transfer efficiency at the interface. The results suggest that appropriate orientation of perovskite crystallites at the interface and trap‐passivation are the niche for better photovoltaic performance.     

Size and shape control of LiFePO4 nanocrystals for better lithium ion battery cathode materials

Lithium iron phosphate (LiFePO₄) is a potential high efficiency cathode material for lithium ion batteries, but the low electronic conductivity and single diffusion channel for lithium ions require good particle size and shape control during the synthesis of this material. In this paper, six LiFePO₄ nanocrystals with different size and shape have been successfully synthesized in ethylene glycol. The addition sequence Fe-PO₄-Li helps to form LiFePO₄ nanocrystals with mostly {010} faces exposed, and increasing the amount of LiOH leads to a decrease in particle size. The electrochemical performance of the six distinct LiFePO₄ particles show that the most promising LiFePO₄ nanocrystals either have predominant {010} face exposure or high specific area, with little iron(II) oxidation.