Two-Orders-of-Magnitude Enhancement of SERS Activity via a Simple Surface Engineering of Quasi-Metal Single-Crystal Frameworks.

Beyond noble metals and semiconductors, quasi-metals have recently been shown to be noteworthy substrates for surface enhanced Raman spectroscopy, and their excellent quasi-metal surface-enhanced Raman spectroscopy (SERS) sensing has demonstrated a wider range of application scenarios. However, the underlying mechanism behind the enhanced Raman activity is still unclear. Here, we demonstrate that surface hydroxyls play a crucial role in the enhancement of the SERS activity of quasi-metal nanostructures. As a demonstration material, quasi-metallic MoO2 single-crystal frameworks rich in surface hydroxyls have been shown to have 100 times higher SERS activity than MoO2 single-crystal frameworks without hydroxyl functionalization, with a Raman enhancement factor of up to 7.6 × 107. Experimental and first-principles density-functional theory calculation results show that the enhanced Raman activity can be attributed to an effective interfacial charge transfer within the MoO2/OH/molecule system.

Build Borophite from Borophenes: A Boron Analogue Graphite.

Unlike graphene derived from graphite, borophenes represent a distinct class of synthetic two-dimensional materials devoid of analogous bulk-layered allotropes, leading to covalent bonding within borophenes instead of van der Waals (vdW) stacking. Our investigation focuses on 665 vdW-stacking boron bilayers to uncover potential bulk-layered boron allotropes through vdW stacking. Systematic high-throughput screening and stability analysis reveal a prevailing inclination toward covalently bonded layers in the majority of boron bilayers. However, an intriguing outlier emerges in δ5 borophene, demonstrating potential as a vdW-stacking candidate. We delve into electronic and topological structural similarities between δ5 borophene and graphene, shedding light on the structural integrity and stability of vdW-stacked boron structures across bilayers, multilayers, and bulk-layered allotropes. The δ5 borophene analogues exhibit metallic properties and characteristics of phonon-mediated superconductors, boasting a critical temperature near 22 K. This study paves the way for the concept of "borophite", a long-awaited boron analogue of graphite.

Synthesis of Graphdiyne Hollow Spheres and Multiwalled Nanotubes and Applications in Water Purification and Raman Sensing.

Controlling the structure of graphdiyne (GDY) is crucial for the discovery of new properties and the development of new applications. Herein, the microemulsion synthesis of GDY hollow spheres (HSs) and multiwalled nanotubes composed of ultrathin nanosheets is reported for the first time. The formation of an oil-in-water (O/W) microemulsion is found to be a key factor controlling the growth of GDY. These GDY HSs have fully exposed surfaces because of the avoidance of overlapping between nanosheets, thereby showing an ultrahigh specific surface area of 1246 m2 g-1 and potential applications in the fields of water purification and Raman sensing.

Superconducting boron allotrope featuring pentagonal bipyramid at ambient pressure.

Elemental boron has evoked substantial interest owing to its chemical complexity in nature. It can form multicenter bonds due to its electron deficiency, which induces the formation of various stable and metastable allotropes. The search for allotropes is attractive for finding functional materials with fascinating properties. Using first-principles calculations with evolutionary structure search, we have explored boron-rich K-B binary compounds under pressure. A series of dynamically stable structures (Pmm2 KB5, Pmma KB7, Immm KB9, and Pmmm KB10) containing boron framework with open channels are predicted, which can possibly be synthesized under high pressure and high temperature conditions. After the removal of K atoms, we obtain four novel boron allotropes, o-B14, o-B15, o-B36, and o-B10, which exhibit dynamical, thermal, and mechanical stability at ambient pressure. Among them, o-B14 contains an unusual B7 pentagonal bipyramid and appears in a bonding combination of seven-center-two-electron (7c-2e) B-B π bonds, which is the first time to be identified in three-dimensional boron allotropes. Interestingly, our calculation reveals that o-B14 can act as a superconductor with a Tc value of 29.1 K under ambient conditions.

ZnGeSe2 monolayer: water-splitting photocatalyst with ultrahigh solar conversion efficiency.

Hydrogen production through solar water-splitting offers a clean and renewable solution to tackle the ongoing issues of energy scarcity and environmental pollution. Here, the solar water-splitting performance of the ZnGeSe2 monolayer was explored via first-principles calculations. Our calculated results reveal that the ZnGeSe2 monolayer embodies stable configurations and semiconducting properties with direct bandgaps ranging from 1.23 to 1.60 eV under the biaxial strain from -1% to +2%. The generated holes and electrons of the ZnGeSe2 monolayer are separately distributed because of the intrinsic dipole. The calculated band edges of the ZnGeSe2 monolayer are demonstrated to be favorable for solar water-splitting. Additionally, the ZnGeSe2 monolayer exhibits strong optical absorption in the whole visible region. The hydrogen and oxygen evolution reactions can be accomplished without cocatalysts. Of particular significance, the solar to hydrogen (STH) efficiency of the ZnGeSe2 monolayer reaches up to 32%, far exceeding the economic value (10%). In light of these hallmarks, the ZnGeSe2 monolayer is demonstrated as an excellent water-splitting photocatalyst.

New oxide structures clearing up the origin of two-dimensional electron gas in AlGaN/GaN heterostructures.

Control over the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures is crucial for their practical applications in current semiconducting devices. However, the oxide surface structures inducing 2DEG are still ambiguous because oxide-stoichiometry (OS) matching structures possess occupied surface donor states at 1.0-1.8 eV below the conduction band minimum of bulk but are usually not available in energy than electron counting (EC) rule structures. In this work, a global optimization algorithm was introduced to explore the possible oxidation structures on GaN (0001) and AlN (0001) surfaces; the method was demonstrated to be available due to the fact that the reported oxidized structures were reproduced at each stoichiometry. Interestingly, the two similar oxide structures with close energy were found in each oxide-bilayer, which can be used to clarify the experimental observations of disordered surface oxide layers below 550 °C. Additionally, new stable oxidation structures with low surface energy were proposed. Interestingly, the new OS matching structures are proposed with remarkably lower energy than EC rule structures under cation-rich and oxygen-poor conditions, which is caused by the large formation enthalpy of Al2O3 and Ga2O3. Further electronic structure calculations demonstrate that the new OS structures possess highest occupied states above the half of the gap and are the origin of 2DEG in AlGaN/GaN heterostructures.

A Metallic Niobium Nitride with Open Nanocavities for Surface-Enhanced Raman Spectroscopy.

The construction of open hot-spot structures that facilitate the entry of analytes is crucial for surface-enhanced Raman spectroscopy. Here, metallic niobium nitride (NbN) three-dimensional (3D) hierarchical networks with open nanocavity structure are first found to exhibit a strong visible-light localized surface plasmon resonance (LSPR) effect and extraordinary surface-enhanced Raman scattering (SERS) performance. The unique nanocavity structure allows easy entry of molecules, promoting the utilization of electromagnetic hot spots. The NbN substrate has a lowest detection limit of 1.0 × 10-12 M and a Raman enhancement factor (EF) of 1.4 × 108 for contaminants. Furthermore, the NbN hierarchical networks possess outstanding environmental durability, high signal reproducibility, and detection universality. The remarkable SERS sensitivity of the NbN substrate can be attributed to the joint effect of LSPR and interfacial charge transport (CT).

Enhanced Thermoelectric Performance in Black Phosphorene via Tunable Interlayer Twist.

Twist-angle two-dimensional (2D) systems are attractive in their exotic and tunable properties by the formation of the moiré superlattices, allowing easy access to manipulating intrinsic electrical and thermal properties. Here, the angle-dependent thermoelectric properties of twisted bilayer black phosphorene (tbBP) by first-principles calculations are reported. The simulations show that significantly enhanced Seebeck coefficient and power factor can be achieved in p-type tbBP due to merging of the multi-valley electronic states and flat moiré bands. Moreover, the twisted layers bring in a strong anharmonic phonon scattering and thus very low lattice thermal conductivity of 4.51 W m-1  K-1 at 300 K. Consequently, a maximal ZT value can be achieved in p-type 10.11° tbBP along the armchair direction up to 0.57 and 1.06 at 300 and 500 K, respectively. The room-temperature ZT value along the zigzag direction is also significantly increased by almost 40 times compared to pristine BP when the twist angle is close to 70.68°. This work demonstrates a platform to manipulate thermoelectric performance in 2D materials by creating moiré patterns, leading tbBP as a promising eco-friendly candidate for thermoelectric applications.

Selective Preparation of Mo2N and MoN with High Surface Area for Flexible SERS Sensing.

γ-Mo2N and δ-MoN are the two most important molybdenum nitrides, but controllable preparation of them with high surface area has not been achieved. Herein, we achieved selective preparation of γ-Mo2N and δ-MoN. The key factor for the selective preparation of γ-Mo2N and δ-MoN is to control the crystal phase of the precursor MoO3. In H2O and NH3 mixed gas, the α-MoO3 nanoribbons are nitridated to obtain γ-Mo2N single-crystal porous nanobelts, while the h-MoO3 prisms are nitrided to obtain δ-MoN hierarchical porous columns. The corrosion effect of H2O plays a key role in the formation of single-crystal porous structure. The γ-Mo2N flexible membrane composed of the single-crystal porous nanobelts exhibits strong localized surface plasmon resonance and surface enhanced Raman scattering effect, which show highly sensitive response to polychlorinated phenol.

General Microwave Route to Single-Crystal Porous Transition Metal Nitrides for Highly Sensitive and Stable Raman Scattering Substrates.

The synthesis of metallic transition metal nitrides (TMNs) has traditionally been performed under harsh conditions, which makes it difficult to prepare TMNs with high surface area and porosity due to the grain sintering. Herein, we report a general and rapid (30 s) microwave synthesis method for preparing TMNs with high specific surface area (122.6-141.7 m2 g-1) and porosity (0.29-0.34 cm3 g-1). Novel single-crystal porous WN, Mo2N, and V2N are first prepared by this method, which exhibits strong surface plasmon resonance, photothermal conversion, and surface-enhanced Raman scattering effects. Different from the conventional low-temperature microwave absorbing media such as water and polymers, as new concept absorbing media, hydrated metal oxides and metallic metal oxides are found to have a remarkable high-temperature microwave heating effect and play key roles in the formation of TMNs. The current research results provide a new-concept microwave method for preparing high lattice energy compounds with high specific surface.

Nitrogen-Doped Titanium Monoxide Flexible Membrane for a Low-Cost, Biocompatible, and Durable Raman Scattering Substrate.

The development of low-cost, biocompatible, and durable high-performance substrates is an urgent issue in the field of surface-enhanced Raman scattering (SERS). Herein, by reducing and exfoliating the TiO2-layered nanoplates in the gas phase, nitrogen-doped titanium monoxide (N-TiO) ultrathin nanosheets composed of 2-3 single layers with a thickness of only ∼1.2 nm are synthesized. Compared with pure TiO, the oxidation resistance of N-TiO is greatly improved, in which the oxidation threshold is significantly increased from 187.5 to 415.6 °C. The N-TiO ultrathin nanosheets are found to have strong surface plasmon resonance in the visible region. These ultrathin N-TiO nanosheets can be easily assembled into a large-scale flexible membrane and exhibit remarkable SERS effects. Moreover, this low-cost flexible SERS substrate combines the high durability of noble-metal substrates and the high biocompatibility of semiconductor substrates.

Enhanced Thermoelectric Performance in Black Phosphorus Nanotubes by Band Modulation through Tailoring Nanotube Chirality.

Black phosphorus (BP) has attracted great attention for applications in thermoelectric devices, owing to its unique in-plane anisotropic electrical and thermal properties. However, its limited conversion efficiency hinders practical application. Here, the thermoelectric properties of 1D BP nanotubes (BPNTs) with different tube chirality are investigated using first-principles calculations and Boltzmann transport theory. The results reveal that variation of crystallographic orientation has a distinct impact on band dispersions, which provides a wide tunability of electronic transport. It is shown that (1,1)-oriented BPNT structure can yield an order-of-magnitude enhanced thermoelectric figure of merit ZT at room temperature (as high as 1.0), compared with the bulk counterpart. The distinct enhancement is attributed to the favorable multiple band structures that lead to high carrier mobility of 2430 cm2 V-1 s-1 . Further performance improvement can be realized by suitable doping, such as N-alloying, reaching an increase of room-temperature ZT by a factor of 3 over that of pristine BPNT. The work provides an applicable method to achieve band engineering design, and presents a new strategy of designing 1D BPNT that are promising candidates for flexible, eco-friendly, and high-performance thermoelectrics.

Two-dimensional type-II g-C3N4/SiP-GaS heterojunctions as water splitting photocatalysts: first-principles predictions.

Hydrogen production from water splitting by sunlight is a promising approach to solve the increasing energy and environmental crises, and the two-dimensional (2D) g-C3N4 monolayer is a red star in this realm. However, it suffers from low quantum efficiency caused by the fast combination of photogenerated electrons and holes. In this work, we investigate the electronic and photocatalytic properties of three newly proposed g-C3N4/SiP-GaS-α, -ß and -γ heterojunctions via first principles predictions. Theoretical results demonstrate that the three g-C3N4/SiP-GaS heterojunctions exhibit direct bandgaps of ∼2.2 eV, and have a type-II band alignment with the valence band maximum (VBM) located at the g-C3N4 layer and the conduction band minimum (CBM) at the SiP-GaS layer. Furthermore, their band edges straddle the redox potential of water in a wide range of biaxial strain. Their absorption coefficients are several times larger than that of most previously discovered 2D heterojunctions. Moreover, the in-built electric field adds a driving force to separate photogenerated electrons and holes. The oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) successfully take place on the g-C3N4 and SiP-GaS layers, respectively. Briefly, separated charge carriers, suitable band edges and strong visible-light absorbance, successful OER and HER enable the three g-C3N4/SiP-GaS heterojunctions to be promising water-splitting photocatalysts.

Predicted stable Li5P2 and Li4P at ambient pressure: novel high-performance anodes for lithium-ion batteries.

Lithium-rich phosphides have recently attracted considerable attention due to their potential application as high-capacity and high-rate anodes for lithium-ion batteries (LIBs). However, there is still short of the promising candidate thus far because of the poor electrical conductivity or huge volume change in the already known Li-P compounds. In this work, we report two novel Li-P states, Li5P2 and Li4P, stabilized under high pressures that are predicted to be quenchable down to ambient conditions by first-principles swarm structure calculations. The predicted P3m1 Li5P2 shows interesting features as a p-type semiconductor with an indirect band gap of 0.787 eV, possessing significant anisotropy properties in electrical transport, while R3[combining macron]m Li4P acts as a typical electride with metallic behavior at pressures of 0-82 GPa. More importantly, our calculations reveal that the theoretical capacities of Li5P2 and Li4P are predicted to reach 2164 and 3462 mA h g-1, respectively. Combined with the good electrical transport properties, the calculated volume expansion of Li5P2 (130%) is found to be much smaller than those of the previously reported Li-P compounds, indicating its potential as a high performance anode material for LIBs.

Alternative to Noble Metal Substrates: Metallic and Plasmonic Ti3O5 Hierarchical Microspheres for Surface Enhanced Raman Spectroscopy.

Compared with noble metals, improving the sensitivity of semiconducting surface-enhanced Raman scattering (SERS) substrates is of great significance to their fundamental research and practical application of Raman spectroscopy. In this paper, it is found that the SERS sensitivity is increased by 10 000 times by reducing the semiconducting TiO2 microspheres to quasi-metallic Ti3O5 microspheres. Its lowest detectable limit is up to 10-10 M, which may be the best among the non-noble metal substrates and even reaches or exceeds certain Au/Ag nanostructures to the best of our knowledge. This new type of non-noble metal SERS substrate breaks through the bottleneck of poor stability of conventional semiconductor substrate and can withstand high temperature oxidation at 200 °C and strong acid-base corrosion without performance degradation. Benefiting from its excellent ability of visible-light photocatalytic degradation of organic molecules, the substrate can be reused. Moreover, the new material also exhibits excellent photothermal conversion properties.

Plasmonic MoO2 Nanospheres as a Highly Sensitive and Stable Non-Noble Metal Substrate for Multicomponent Surface-Enhanced Raman Analysis.

Semiconductor-based surface-enhanced Raman spectroscopy is getting more and more attention because of its great price advantage. One of the biggest obstacles to the large-scale application of it is the poor stability. Here, we report that plasmonic MoO2 nanospheres can be used as a highly sensitive and stable semiconducting-substrate material for surface-enhanced Raman scattering (SERS). By using the MoO2 nanospheres as Raman substrates, a series of typical compounds with high attention can be accurately detected. This new non-noble metal substrate material shows a very high detection limit of 10-8 M, and exhibits great near-field enhancement with one of the highest enhancement factor of 4.8 × 106 reported to date. More importantly, the oxide with intermediate valence displays unexpected ultrahigh stability, which can withstand the corrosion of strong acid and strong alkali as well as 150 °C high temperature oxidation in air. Moreover, the accurate detection of multicomponent samples was also successful on this substrate. These results show that some simple metal oxides with intermediate valence may become sensitive and stable SERS substrate materials due to their abundant free electrons and structure that easily causes hot spots.

Hydrolysis of Sulfur Dioxide in Small Clusters of Sulfuric Acid: Mechanistic and Kinetic Study.

The deposition and hydrolysis reaction of SO2 + H2O in small clusters of sulfuric acid and water are studied by theoretical calculations of the molecular clusters SO2-(H2SO4)n-(H2O)m (m = 1,2; n = 1,2). Sulfuric acid exhibits a dramatic catalytic effect on the hydrolysis reaction of SO2 as it lowers the energy barrier by over 20 kcal/mol. The reaction with monohydrated sulfuric acid (SO2 + H2O + H2SO4 - H2O) has the lowest energy barrier of 3.83 kcal/mol, in which the cluster H2SO4-(H2O)2 forms initially at the entrance channel. The energy barriers for the three hydrolysis reactions are in the order SO2 + (H2SO4)-H2O > SO2 + (H2SO4)2-H2O > SO2 + H2SO4-H2O. Furthermore, sulfurous acid is more strongly bonded to the hydrated sulfuric acid (or dimer) clusters than the corresponding reactant (monohydrated SO2). Consequently, sulfuric acid promotes the hydrolysis of SO2 both kinetically and thermodynamically. Kinetics simulations have been performed to study the importance of these reactions in the reduction of atmospheric SO2. The results will give a new insight on how the pre-existing aerosols catalyze the hydrolysis of SO2, leading to the formation and growth of new particles.

Surfactant-free interfacial growth of graphdiyne hollow microspheres and the mechanistic origin of their SERS activity.

As a two-dimensional carbon allotrope, graphdiyne possesses a direct band gap, excellent charge carrier mobility, and uniformly distributed pores. Here, a surfactant-free growth method is developed to efficiently synthesize graphdiyne hollow microspheres at liquid‒liquid interfaces with a self-supporting structure, which avoids the influence of surfactants on product properties. We demonstrate that pristine graphdiyne hollow microspheres, without any additional functionalization, show a strong surface-enhanced Raman scattering effect with an enhancement factor of 3.7 × 107 and a detection limit of 1 × 10-12 M for rhodamine 6 G, which is approximately 1000 times that of graphene. Experimental measurements and first-principles density functional theory simulations confirm the hypothesis that the surface-enhanced Raman scattering activity can be attributed to an efficiency interfacial charge transfer within the graphdiyne-molecule system.

Enhancement of NO2 Gas Sensing Properties of Polypyrrole by Polarization Doping with DBS: Experimental and DFT Studies.

A new technique of polarization doping was adopted to improve NO2 gas sensing properties of the polypyrrole (PPy) sensor. PPy nanosheets polarization doped with sodium dodecyl benzenesulfonate (SDBS) were synthesized by low-temperature polymerization. The semiagglomerated PPy nanosheets were well-dispersed and a large specific surface areas due to the introduction of dodecyl benzenesulfonate (DBS). The DBS doped PPy sensor shows excellent NO2 sensing performance. Polarization of sulfonate ions to PPy enhanced the adsorption ability of NO2 with the synergistic effect of NO2. The adsorption energy (-0.676 eV) and electron transfer (0.521 |e|) of PPy to NO2 increased greatly after doping. An unoccupied electron state is observed in the valence band electron structure of PPy/DBS after the adsorption of NO2 by calculations of Density Functional Theory (DFT). The intermolecular synergy between NO2 and PPy/DBS causes a strong polarization, resulting in a high polarization potential, which enhances the NO2 sensing performance of PPy sensor. It is of great significance to develop NO2 detection device based on PPy that works at room temperature.

Novel Two-Dimensional ABX3 Dirac Materials: Achieving a High-Speed Strain Sensor via a Self-Doping Effect.

The pristine semimetal property of two-dimensional (2D) Dirac materials has limited their practical applications in today's electronic devices. Here we report a new type of 2D Dirac material, termed ABX3 (A = F, Cl, Br, or I; B = P or As; X = C or Si) monolayers. We demonstrate that 14 ABX3 monolayers possess good stability and high Fermi velocities. The FPC3, ClPC3, BrPC3, and FAsC3 monolayers exhibit a pristine n-type self-doping Dirac cone due to the interactions of electrons between the A-B units and C6 rings, which is beneficial for realizing high-speed carriers. Interestingly, the ClPSi3 monolayer exhibits remarkable responses to strain because a self-doping Dirac cone can be induced by relatively small in-plane biaxial strains (-5%), and the current-voltage (I-V) curves verified that the response strength is 11.57 times that of the graphene-based strain sensor at a bias of 1.10 V, indicating that the ClPSi3 monolayer could be used as a potential excellent strain sensor.