Facet junction engineering for enhanced SERS activity of Ag/Cu2O composite substrates.

Featuring ultra-high sensitivity and molecule-specific detection ability, surface-enhanced Raman scattering (SERS) is suitable for the rapid sensing of trace-level chemicals in biological, environmental, and agricultural samples. Although crystal facet junction engineering is a powerful tool to manipulate the optoelectronic properties of semiconducting materials, its correlation with the SERS sensing activity of noble metal/semiconductor composites has still not been clarified. In this work, Ag was deposited on Cu2O nanocrystals enclosed by different facets, including {100} (cube), {111} (octahedron), and {100}/{111} (truncated octahedron), and a detailed study of their SERS performance was carried out. It was found that Ag/truncated-octahedral Cu2O (Ag/Cu2O(J3)) exhibited superior SERS activity to Ag/cubic Cu2O (Ag/Cu2O(C)) and Ag/octahedral Cu2O (Ag/Cu2O(O)). The {100}/{111} facet junction in Cu2O can promote the separation and transfer of photogenerated charge carriers, which is beneficial for enhancing SERS sensing performance. Moreover, Ag/Cu2O(J3) has a higher content of oxygen vacancies, providing extra interfacial charge-transfer pathways to the analyte, which also contribute to improving the SERS activity. The low detection limit of Ag/Cu2O(J3) was 1 × 10-11 M for 4-nitrobenzenethiol, two orders of magnitude lower than that of Ag/Cu2O(C) and Ag/Cu2O(O). In addition, Ag/Cu2O(J3) could detect CV and R6G at concentrations down to 1 × 10-10 M and 1 × 10-8 M, respectively. The findings of this work can provide insightful information for designing metal/semiconductor substrates toward SERS sensing application by regulating the crystal facet junction.

CdS-based Schottky junctions for efficient visible light photocatalytic hydrogen evolution.

Heterojunctions photocatalysts play a crucial role in achieving high solar-hydrogen conversion efficiency. In this work, we mainly focus on the charge transfer dynamics and pathways for sulfides-based Schottky junctions in the photocatalytic water splitting process to clarify the mechanism of heterostructures photocatalysis. Sulfides-based Schottky junctions (CdS/CoP and CdS/1T-MoS2) were successfully constructed for photocatalytic water splitting. Because of the higher work function of CdS than that of CoP and 1T-MoS2, the direction of the built-in electric field is from CoP or 1T-MoS2 to semiconductor. Therefore, CoP and 1T-MoS2 can act as electrons acceptors to accelerate the transfer of photo-generated electron on the surface of CdS, thus improving the charge utilization efficiency. Meanwhile, CoP and 1T-MoS2 as active sites can also promote the water dissociation and lower the H+ reduction overpotential, thus contributing to the excellent photocatalytic hydrogen production activity (23.59 mmol·h-1·g-1 and 1195.8 mol·h-1·g-1 for CdS/CoP and CdS/1T-MoS2).

Designing highly efficient oxygen evolution reaction electrocatalyst of high-entropy oxides FeCoNiZrOx: Theory and experiment.

The correlations between the experimental methods and catalytic activities are urgent to be defined for the design of highly efficient catalysts. In this work, a new oxygen evolution reaction electrocatalyst of high-entropy oxide (HEO) FeCoNiZrOx was designed and analyzed by experimental and theoretical methods. On account of the shortened coordinate bond along with the increased annealing temperature, the atomic/electronic structures of active site were adjusted quantitatively with the aid of the pre-designed correlator of d electron density, which contributed to adjust the catalytic activity of HEO specimens. The prepared HEO specimen exhibited the low overpotentials of 245 mV at 10 mA cm-2 and 288 mV at 100 mA cm-2 with small Tafel slope of 35.66 mV dec-1, fast charge transfer rate, and stable electrocatalytic activity. This strategy would be adopted to improve the catalytic activity of HEO by adjusting the d electron density of transition metal ions with suitable preparation method.

Engineering the crystal facets of α-MnO2 nanorods for electrochemical energy storage: experiments and theory.

Crystal facet engineering is an effective strategy for precisely regulating the orientations and electrochemical properties of metal oxides. However, the contribution of each crystal facet to pseudocapacitance is still puzzling, which is a bottleneck that restricts the specific capacitance of metal oxides. Herein, α-MnO2 nanorods with different exposed facets were synthesized through a hydrothermal route and applied to pseudocapacitors. XRD and TEM results verified that the exposure ratio of active crystal facets was significantly increased with the assistance of the structure-directing agents. XPS analysis showed that there was more adsorbed oxygen and Mn3+ on the active crystal facets, which can provide strong kinetics for the electrochemical reaction. Consequently, the α-MnO2 nanorods with {110} and {310} facets exhibited much higher pseudocapacitances of 120.0 F g-1 and 133.0 F g-1 than their α-MnO2-200 counterparts (67.5 F g-1). The theoretical calculations proved that the {310} and {110} facets have stronger adsorption capacity and lower diffusion barriers for sodium ions, which is responsible for the enhanced pseudocapacitance of MnO2. This study provides a strategy to enhance the electrochemical performance of metal oxide, based on facet engineering.

VASERP: An Adaptive, Lightweight, Secure, and Efficient RFID-Based Authentication Scheme for IoV.

With the rapid growth in wireless communication and IoT technologies, Radio Frequency Identification (RFID) is applied to the Internet of Vehicles (IoV) to ensure the security of private data and the accuracy of identification and tracking. However, in traffic congestion scenarios, frequent mutual authentication increases the overall computing and communication overhead of the network. For this reason, in this work, we propose a lightweight RFID security fast authentication protocol for traffic congestion scenarios, designing an ownership transfer protocol to transfer access rights to vehicle tags in non-congestion scenarios. The edge server is used for authentication, and the elliptic curve cryptography (ECC) algorithm and the hash function are combined to ensure the security of vehicles' private data. The Scyther tool is used for the formal analysis of the proposed scheme, and this analysis shows that the proposed scheme can resist typical attacks in mobile communication of the IoV. Experimental results show that, compared to other RFID authentication protocols, the calculation and communication overheads of the tags proposed in this work are reduced by 66.35% in congested scenarios and 66.67% in non-congested scenarios, while the lowest are reduced by 32.71% and 50%, respectively. The results of this study demonstrate a significant reduction in the computational and communication overhead of tags while ensuring security.

Experimental and theoretical evaluation of crystal facet exposure on the charge transfer and SERS activity of ZnO films.

Semiconductors exhibit great potential as a surface enhanced Raman scattering (SERS) substrate due to their low cost, good stability and biocompatibility. However, the extensive application of semiconductors has been restricted by their intrinsically low SERS sensitivity. It is urgently required to design uniform metal oxide substrates with enhanced charge transfer and SERS activity. Herein, three facet-defined ({101̄0}, {0001} and {101̄1}) ZnO films were synthesized via an electrodeposition procedure with the assistance of KCl or ethylenediamine. According to the results, the ZnO films with {0001} and {101̄1} exposed facets exhibit appreciable SERS enhancement factors (EFs) of 1.6 × 104 and 2.8 × 104 for 4-nitrobenzenethiol (4-NBT), as well as a relatively low limit of detection (LOD) down to 1 × 10-6 M and 5 × 10-7 M, respectively. Simultaneously, the electrodeposited ZnO films deliver good repeatability and SERS stability, with relative standard deviation (RSD) less than 6% and 85.2% of their original activity retained after 40 days. Theoretical calculations verified that the {0001} and {101̄1} facets can transfer more electrons from ZnO to the molecules on account of their low facet-related electronic work functions, thus generating the noticeable improvement of SERS activity. The current study provides theoretical and technical support for the crystal facet engineering and property improvement of semiconductors.

Sulfur Vacancy-Rich CuS for Improved Surface-Enhanced Raman Spectroscopy and Full-Spectrum Photocatalysis.

There are growing interests in the development of bifunctional semiconducting nanostructures for photocatalysis and real-time monitoring of degradation process on catalysts. Defect engineering is a low-cost approach to manipulating the properties of semiconductors. Herein, we prepared CuS nanoplates by a hydrothermal method at increasing amounts of thioacetamide (CS-1, CS-2, and CS-3) and investigated the influence of sulfur vacancy (Vs) on surface-enhanced Raman spectroscopy (SERS) and photocatalysis performance. SERS intensity of 4-nitrobenzenethiol on CS-3 is 346 and 17 times that of CS-1 and CS-2, respectively, and enhancement factor is 1.34 × 104. Moreover, SERS is successfully applied to monitor the photodegradation of methyl orange. In addition, CS-3 also exhibited higher efficiency of Cr(VI) photoreduction than CS-1 and CS-2, and removal rate is 88%, 96%, and 73% under 2 h UV, 4 h visible, and 4 h near-infrared illumination, respectively. A systematic study including electron paramagnetic resonance spectra, photoelectrochemical measurements, and nitrogen adsorption isotherms were conducted to investigate the underlying mechanism. This work may help to understand the impact of vacancy defect on SERS and photocatalysis, and provide an effective and low-cost approach for the design of multifunctional materials.

CdIn2S4/In(OH)3/NiCr-LDH Multi-Interface Heterostructure Photocatalyst for Enhanced Photocatalytic H2 Evolution and Cr(VI) Reduction.

The development of highly active and stable photocatalysts, an effective way to remediate environment pollution and alleviate energy shortages, remains a challenging issue. In this work, a CdIn2S4/In(OH)3 nanocomposite was deposited in-situ on NiCr-LDH nanosheets by a simple hydrothermal method, and the obtained CdIn2S4/In(OH)3/NiCr-LDH heterostructure photocatalysts with multiple intimate-contact interfaces exhibited better photocatalytic activity. The photocatalytic H2 evolution rate of CdIn2S4/In(OH)3/NiCr-LDH increased to 10.9 and 58.7 times that of the counterparts CdIn2S4 and NiCr-LDH, respectively. Moreover, the photocatalytic removal efficiency of Cr(VI) increased from 6% for NiCr-LDH and 75% for CdIn2S4 to 97% for CdIn2S4/In(OH)3/NiCr-LDH. The enhanced photocatalytic performance was attributed to the formation of multi-interfaces with strong interfacial interactions and staggered band alignments, which offered multiple pathways for carrier migration, thus promoting the separation efficiency of photo-excited electrons and holes. This study demonstrates a facile method to fabricate inexpensive and efficient heterostructure photocatalysts for solving environmental problems.

Designing of Highly Efficient Oxygen Evolution Reaction Electrocatalysts Utilizing A Correlation Factor: Theory and Experiment.

The theoretical prediction of the catalytic activity is very beneficial for the design of highly efficient catalysts. At present, most theoretical descriptors focus on estimating the catalytic activity and understanding the enhancement mechanism of catalysts, while it is also quite important to find a factor to correlate the descriptors with preparation methods. In this work, a correlation factor, the d electron density of transition metal ions, was developed to correlate the d band center values of transition metal ions with the preparation methods of amorphization and Al introduction. According to the results of theoretical simulations, the correlation factor not only exhibited favorable linear relationships with the theoretical overpotentials of (CoFeAlx)3O4 and (CoFeAlx)3O4 + (CoFeAlx)OOH systems but also correlated with two preparation methods by altering the volume of systems. Based on theoretical guidance, the electrocatalytic activities of the prepared (CoFeAlx)3O4 specimens were gradually improved by the preparation methods of amorphization and Al introduction, and the Am-CoFeAl-2-10h specimen exhibited a low kinetic barrier of 268 mV, fast charge transfer rate, and stable electrocatalytic activity. This strategy could be applied to design highly efficient catalysts by adjusting the correlation factor of the active site with suitable preparation methods.

Experimental and Theoretical Investigation of the Effect of Oxygen Vacancies on the Electronic Structure and Pseudocapacitance of MnO2.

Defect engineering is an effective way to modulate the intrinsic physicochemical properties of materials. In this work, δ-MnO2 with oxygen vacancies is fabricated by a simple oxidation or reduction process, and the relationship between the electronic structure and pseudocapacitance is systematically studied through experimental analysis and theoretical calculations. The peaks in the Raman spectra of the as-prepared samples are shifted compared with those of pure MnO2 and the Mn3+ /Mn4+ ratio and O species content also change after the introduction of oxygen vacancies. The optimized samples exhibit a better specific capacitance of 207 F g-1 after the oxidation process and 181.4 F g-1 after the reduction treatment compared with only 143.9 F g-1 for the pure MnO2 . The samples obtained through the oxidation or reduction process also retain 93.3 or 86.4 % of the initial capacity after 5000 cycles. The excellent properties are attributed to the enhanced conductivity and increased surface reactivity or electrochemically active sites. Theoretical calculations demonstrate that the presence of oxygen vacancies leads to an increase in the density of states, which improves the redox reaction of MnO2 . This study will provide a reference for exploring and designing highperformance pseudocapacitive materials.

Waste bones derived nitrogen-doped carbon with high micropore ratio towards supercapacitor applications.

Fabrication of high-performance electrodes from waste biomass has attracted increasing attention among the energy storage and conversion field. In this work, we have synthesized nitrogen-doped activated carbon by a simultaneous pyrolysis/activation method from waste bones. It is found that the specific surface area and pore structure of as-synthesized carbon depends on the carbonization temperature (500-800 °C), and the highest specific surface area is 1522 m2 g-1. The electrochemical properties of Pork bone, Blackfish bone, Eel bone based activated carbon (PBAC, BFAC, EBAC) mainly depend on their micro-/mesoporosity. Three samples PBAC-600, BFAC-600 and EBAC-600, which have higher ratio of micropore surface area and nitrogen content, exhibit enhanced specific capacitance of 263, 302 and 264F g-1 in 6 M KOH electrolyte. Furthermore, the assembled symmetric supercapacitors of PBAC-600 can deliver energy density as high as 7.0 and 26.2 Wh Kg-1 in the aqueous and ionic liquid electrolyte, respectively. Such excellent performance can be attributed to the microporous structure, reasonable pore size distribution and nitrogen self-doping of the activated carbon. This research indicates that waste bones have great potential for mass fabrication of the activated carbon electrodes for energy storage applications.

Unexpected monoatomic catalytic-host synergetic OER/ORR by graphitic carbon nitride: density functional theory.

Although single metal atoms (SMAs) have been extensively investigated as unique active sites in single-atom catalysts, the possible active sites of the host catalysts have been unfortunately neglected in previous studies. In single-atom catalysts, the SMAs can promote the chemical and catalytic activities of host atoms, which may act as secondary active sites, resulting in a significant synergistic effect on the catalytic performance. Using density functional theory calculations, we studied the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) on two different types of active sites: single metal (M1) atoms and the neighboring host atoms of several M1/g-C3N4 samples. The contributions of M1 and host atoms towards the reduction of the OER/ORR overpotentials of Fe1, Co1, Ni1, Cu1 and Zn1/g-C3N4, bifunctional electrocatalysts with the OER/ORR overpotentials of 0.50-0.70 V were investigated. Finally, new M1/g-C3N4 catalysts with high OER/ORR performances could be estimated based on the d-band centre of the M1 atoms in the future.

CuS/RGO hybrid photocatalyst for full solar spectrum photoreduction from UV/Vis to near-infrared light.

To make full use of the solar energy, it remains a great challenge for semiconductor photocatalysts to harvest the full solar light spectrum from ultraviolet (UV) to visible even the near infrared (NIR) wavelength. Here we show firstly the CuS/RGO (reduced graphene oxide) hybrid photocatalyst synthesized via a microwave assisted method with full solar light (UV-Vis-NIR) active for efficient Cr(VI) reduction. The CuS/RGO displays high absorption and catalytic activity in the UV, visible and even the NIR light regions. As co-catalyst, RGO can separate and inhibit the recombination of charge carriers, consequently improving the catalytic activity. Only 1wt% RGO emersions can reduce 90% of Cr(VI) under the radiation of light over the full spectrum. Findings may provide a new strategy and substance to expand the utilization range of solar light from UV to visible even the NIR energy.

(H, Li)Br and LiOH Solvation Bonding Dynamics: Molecular Nonbond Interactions and Solute Extraordinary Capabilities.

We resolved the O:H-O bond transition from the mode of ordinary water to its hydration in terms of its phonon stiffness (vibration frequency shift Δω), order of fluctuation (line width), and number fraction (phonon abundance), fx(C) = Nhyd/Ntotal. The fx(C) follows fH(C) = 0, fLi(C) ∝ fOH(C) ∝ C, and fBr(C) ∝ 1 - exp(-C/C0) toward saturation with C being the solute concentration. The invariant dfx(C)/dC suggests that the solute forms a constantly sized hydration droplet without responding to interference of other ions because its hydrating H2O dipoles fully screen its electric field. However, the number inadequacy of the highly ordered hydration H2O dipoles partially screens the large Br-. The Br- then interacts repulsively with other Br- anions, which weakens its electric field and the fBr(C) approaches saturation at higher solute concentration. The consistency in the concentration trend of the fLiBr(C), the Jones-Dole viscosity η(C), and the surface stress of LiBr solution clarifies their common origin of ionic polarization. The resultant energy of the solvent H-O exothermic elongation by O: ⇔ :O repulsion and the solute H-O endothermic contraction by bond-order deficiency heats up the LiOH solution. An estimation of at least 0.15 eV (160% of the O:H cohesive energy of 0.1 eV) suggests that the H-O elongation is the main source heating up the solution, while the molecular motion, structure fluctuation, or even evaporation dissipates energy caped at 0.1 eV.

Compression icing of room-temperature NaX solutions (X = F, Cl, Br, I).

In situ Raman spectroscopy revealed that transiting H2O/NaX (∼64) solutions into an ice VI phase and then into an ice VII phase at a temperature of 298 K requires excessive pressures with respect to pure water. The increase of the critical pressures varies with the solute type in the Hofmeister series order: X = I > Br > Cl > F ∼ 0. The results suggest that the solute hydration creates electric fields that lengthen and soften the O:H nonbond and meanwhile shorten and stiffen the H-O bond through O-O Coulomb repulsion. Compression, however, does the opposite to solute electrification upon the O:H-O bond relaxation. Therefore, compression of the aqueous solutions recovers the electrification-deformed O:H-O bond first and then proceeds to the phase transitions, which requires excessive energy for the same sequence of phase transitions. Ice exclusion of solute disperses the frequencies of characteristic phonons and the critical pressures with unlikely new bond formation.

Atomic under-coordination fascinated catalytic and magnetic behavior of Pt and Rh nanoclusters.

Density functional theory (DFT) calculations with local spin density discrimination have been performed to examine the effect of atomic under-coordination on the catalytic and magnetic properties of cuboctahedral (CO) and marks decahedral (MD) structured Pt and Rh nanoclusters. Consistency between theoretical calculations and experimental observations confirmed the predictions based on the framework of bond-order-length-strength (BOLS) correlation and nonbonding electron polarization (NEP) notations. The BOLS-NEP notation suggests that the shorter-and-stronger bonds between under-coordinated atoms induce local densification and quantum entrapment of core electrons, which then polarize the otherwise conducting electrons and result in shifts of the binding energy. Such strong localization resolves the intriguing catalytic and magnetic attributes of Pt and Rh nanoclusters.

Skin-resolved local bond contraction, core electron entrapment, and valence charge polarization of Ag and Cu nanoclusters.

Consistency between density functional theory (DFT) calculations and experimental observations confirmed our predictions on the behaviour of local bonds, and the electron binding energy of cuboctahedral and Marks decahedral structures of Ag and Cu nanoclusters. The shorter and stronger bonds between under-coordinated atoms cause local densification and quantum entrapment of the core electrons, which polarize the otherwise conducting electrons (valence electrons). Such strong localization may result in extraordinary catalytic and plasmonic properties in Ag and Cu nanoclusters.

Morphological control and photoluminescence of zinc oxide nanocrystals.

Nanocrystals of the wide band gap semiconductor zinc oxide of controllable morphologies were synthesized by a simple thermal decomposition method. The predominating factor in determining the morphology (spheres, triangular prisms, and rods) was the solvent, selected on the basis of coordinating power. The nanoparticles were structurally analyzed, and the photoluminescence of each shape was compared. The intensity of the green band emission, common to many ZnO structures, was found to vary with morphology. The strongest green band intensity corresponded to the shape with the largest surface/volume ratio and could be attributed to surface oxygen vacancies. Control over the morphology of ZnO at the nanoscale is presented as a means to control the green band emission.