Optical Absorption Studies Research Articles

Microwave-assisted synthesis of tentacles like Ag-doped copper oxide nano-ceramics: Structural, optical, and anti-campylobacter studies

The facile microwave approach has been adopted to successfully synthesize silver-doped CuO (ACO) and bare CuO (BCO) photocatalysts. Powder X-ray diffraction (PXRD), energy dispersive X-ray (EDX), field emission scanning electron microscopy (FESEM), optical absorption, current-voltage (I–V), and photoluminescence (PL) studies have been executed to evaluate the effects of silver doping on texture, microscopic characteristics, crystal structure, bandgap, electrical conductivity, and charge carrier separation. Nanostructured (nanorods) and narrow bandgap ACO photocatalyst was produced owing to the synergistic impact of surfactant-assisted synthesis and silver doping-induced bandgap regulation. The ACO photocatalyst was a better choice for antibacterial application than its counterpart (BCO photocatalyst) due to the induced characteristics that increased its specific surface area and light-harvesting capabilities. Photoinduced charge carriers (holes and free electrons) in the ACO photocatalyst are shown to separate at maximal extent according to the PL and I–V analyses, indicating their facile transport from the production site to the site of action. The synergistic effects owing to the tailored features enable the ACO photocatalyst to kill the Campylobacter bacteria and mineralize Acridine orange dye at a fast rate. The antimicrobial activity of the doped catalyst has a linear relationship with the applied catalyst dosage as the zone of inhibition (ZOI) increases from 14.2 mm to 22.7 mm when the ACO photocatalyst dosage was raised from 4 μg to 12 μg.

Broadband near infrared emission in Cr3+: Cs2AgBiCl6 double perovskite halides

Near Infra-red phosphors with broad emission band, high thermal stability and excellent efficiency are required in various bioimaging applications. In this study, we report the design and chemical reflux synthesis of a new family of Cr3+: Cs2AgBiCl6 double perovskites. In detail this article focuses on the impact of Cr3+ doping on the crystal structure, optical and vibrational properties of Cs2AgBiCl6 double perovskites. Different concentrations of Cr3+ doped Cs2AgBiCl6 double perovskites were analyzed using UV–Visible, Raman, Photoluminescence (PL), PL excitation, time resolved PL, X-ray photoelectron spectroscopy. Morphological analyses were carried out using scanning and transmission electron microscopies. XRD spectra reveals that the cubic symmetry remains unaltered upon Cr3+ doping. Raman and Rietveld analysis show that the Bi–Cl bond decreases upon Cr3+ incorporation. Optical absorption studies reveal a decrease in bandgap from 2.76 to 2.66 eV upon Cr3+ doping. PL spectroscopy of Cr 3+:Cs2AgBiCl6 exhibits a broad near infrared (NIR) emission band at 973 nm with an FWHM of 186 nm at 355 nm excitation. This demonstrates the promising prospects for NIR light emitting diode and bioimaging applications of Cr3+:Cs2AgBiCl6 double perovskites.

Panchromatic Light-Capturing Bis-styryl BODIPY-Perylenediimide Donor-Acceptor Constructs: Occurrence of Sequential Energy Transfer Followed by Electron Transfer.

Two wide-band-capturing donor-acceptor conjugates featuring bis-styrylBODIPY and perylenediimide (PDI) have been newly synthesized, and the occurrence of ultrafast excitation transfer from the 1 PDI* to BODIPY, and a subsequent electron transfer from the 1 BODIPY* to PDI have been demonstrated. Optical absorption studies revealed panchromatic light capture but offered no evidence of ground-state interactions between the donor and acceptor entities. Steady-state fluorescence and excitation spectral recordings provided evidence of singlet-singlet energy transfer in these dyads, and quenched fluorescence of bis-styrylBODIPY emission in the dyads suggested additional photo-events. The facile oxidation of bis-styrylBODIPY and facile reduction of PDI, establishing their relative roles of electron donor and acceptor, were borne out by electrochemical studies. The electrostatic potential surfaces of the S1 and S2 states, derived from time-dependent DFT calculations, supported excited charge transfer in these dyads. Spectro-electrochemical studies on one-electron-oxidized and one-electron-reduced dyads and the monomeric precursor compounds were also performed in a thin-layer optical cell under corresponding applied potentials. From this study, both bis-styrylBODIPY⋅+ and PDI⋅- could be spectrally characterizes and were subsequently used in characterizing the electron-transfer products. Finally, pump-probe spectral studies were performed in dichlorobenzene under selective PDI and bis-styrylBODIPY excitation to secure energy and electron-transfer evidence. The measured rate constants for energy transfer, kENT , were in the range of 1011 s-1 , while the electron transfer rate constants, kET , were in the range of 1010 s-1 , thus highlighting their potential use in solar energy harvesting and optoelectronic applications.

McCumber analysis of Er3+ doped Al-Ba-Li phosphate glasses for optical amplifier material

Er3+-doped Al-Ba-Li phosphate glasses were produced using a melt-quenching method. In the current study, the effect of doping Er3+ ion concentration for photoluminescence efficiency and developing compact optical amplifiers operating at 1.53 μm was investigated. The optical absorption study reveals that ten absorption peaks, showing highest intensity at 522 nm and 980 nm in UV-Vis and NIR regions, respectively. Mc-Cumber theory was used to conduct and analyse the optical absorption measurements obtained. The maximum intensity was observed at 4I13/2→4I15/2 transition in the wavelength range of 1400–1700 nm, which is monitored using 980 nm. The highest emission intensity was observed for 1.5 mol% of Er3+ ions doped glass in the NIR emission spectra. The McCumber theory was applied to evaluate the emission cross-sections; for 1.5 mol% of Er3+ ions, the emission cross-section and gain bandwidth value show 9.02 10−21 cm2 and 1.16 1025 cm3 respectively. When the concentration of Er3+ ions increased from 0.1, 0.5, 1.0, 1.5, and 2.0 mol%, respectively, the lifetime () of the 4I13/2 level reduced from 1.45, 1.19, 0.93, 0.72, and 0.63 ms. The population inversion values show larger than 0.4, suggesting their potential suitability in broadband amplifiers used in C-band communication for a typical population inversion of more than 40%. Phosphate glass doped with Er3+ ions at a concentration of 1.5 mol% doped glass being potential candidate for solid-state optical amplifying devices.

Enhanced performance of organic solar cells through incorporation of MoS2 nanoparticles in bulk heterojunction layer

In this work, MoS2 nanoparticles were incorporated in P3HT:PC61BM bulk heterojunction (BHJ) layers and the effect of their concertation on BHJ morphology and structure, its optical absorption spectra and photovoltaic characteristics of P3HT:PC61BM BHJ solar cells were investigated in detail. MoS2 nanoparticles were deposited by laser ablation of MoS2 powders in chlorobenzene. AFM and optical absorption study indicates that an addition of MoS2 nanoparticles in BHJ results in an improved crystallinity and a decreased defect density. The volt-ampere measurements of BHJ solar cells shows that the increase in the MoS2 concertation up to 0.5 wt% leads to the boost of photovoltaic characteristics and a champion device, based on BHJ with MoS2 nanoparticles at concertation 0.5 wt %, generated the power conversion efficiency about 3%, which 3 times higher than an efficiency of a device based on pristine BHJ. The impedance spectroscopy (IS) technique was used to understand the effect of MoS2 nanoparticles on the charge transport mechanism. IS study revealed that a resistance of the BHJ layer slightly and steadily grows by increasing the concentration of MoS2 nanoparticles, whereas a recombination resistance determining the charge recombination rate increases reaching maximum value at the MoS2 concertation of 0.5 wt %. A further growth in the MoS2 concertation results in a decrease of the recombination resistance indicating an acceleration of charge recombination rate. The influence of conductivity nature of MoS2 nanoparticles and their distribution in the BHJ layer on charge transport mechanism is also discussed.

Structure-property relationships and DFT studies of three quaternary chalcogenides: BaCeCuSe3, BaCeAgS3, and BaCeAgSe3

In this study, we present the syntheses of three chalcogenides, BaCeCuSe3, BaCeAgS3, and BaCeAgSe3, using the sealed-tube method. The single-crystal X-ray diffraction technique is employed to determine the structures of BaCeAgQ3 (Q = S/Se) for the first time. These isostructural Ag-containing compounds (space group: Cmcm) crystallize in the KCuZrS3 structure type. Rietveld refinement study of the BaCeCuSe3 sample confirms its orthorhombic Eu2CuS3 type structure (Pnma). The building blocks of these pseudo-two-dimensional structures are tetrahedral MQ4 (M = Cu/Ag) and octahedral CeQ6 polyhedra, which are conjoined to form ∞2[CeMQ3]2− layer. The optical absorption studies reveal the semiconducting nature of the BaCeMQ3 with bandgaps of 1.4(2) eV-1.5(2) eV consistent with their electronic structures calculated using DFT studies. The thermal conductivity (ktot) measurements showed extremely low ktot values (0.43–0.67 Wm−1K−1) for these samples at 773 K. Solar cells were fabricated with the chalcogenides and the highest efficiency of ∼4% was achieved with BaCeAgS3.

Magnetic, Optical, and Thermic Properties of SrLnCuSe3 (Ln = Dy, Ho, Er, Tm) Compounds

SrLnCuSe3 (Ln = Dy, Ho, Er, Tm) compounds crystallize in the Pnma and Cmcm orthorhombic space group and belong to the Eu2CuS3 and KCuZrS3 structural type, respectively. According to a single-crystal XRD study, the SrTmCuSe3 unit cell parameters are a = 4.0631 (4), b = 13.4544 (14), c = 10.4430 (10) Å, and V = 570.88 (10) Å3. All the studied SrLnCuSe3 samples in the temperature range of 1.77–300 K demonstrate paramagnetic behavior without any features pointing to magnetic ordering. The measured Curie constants coincide with the values expected for Ln3+ ions with good accuracy, which confirms the stoichiometric composition of the samples and the non-magnetic state of the copper ions, Cu1+ (S = 0). The conducted optical absorption study showed that the polycrystalline SrLnCuSe3 (Ln = Dy, Ho, Er, Tm) samples are semiconductors with a direct bandgap ranging from 2.14 eV to 2.31 eV. Two indirect bandgaps were revealed and explained by the presence of optical transitions to highly dispersive subbands in the conduction band. The compounds demonstrate two reversible phase transitions α⇆β, β⇆γ and an incongruent melting at 1606 K (Dy), 1584 K (Ho), 1634 K (Er), and 1620 K (Tm) associated with the formation of solid solutions of SrSe, Cu2-XSe, and Ln2Se3 binary compounds.

Studies of optical, photophysical, and thermal properties of poly(paraphenylenevinylene) (PPV) polymers with symmetric alkoxy side‐chain lengths for potential application in electronic devices

AbstractOptical, photophysical, and thermal properties of poly(paraphenylenevinylene) (PPV) derivatives of various symmetric alkoxyl substituents were studied with UV–vis and photoluminescence spectroscopy. Molar absorptivity coefficients of the symmetric monomers increased compared to that of hydroquinone and its derivatives. The symmetric polymers exhibited photoluminescence quantum yields in the range of 32%–39% and excited‐state lifetimes of 3.01–3.61 ns. The emission energy values for the calculated band gaps were in the range of 2.16–2.12 eV for the symmetric polymers. The photoluminescent characteristics of these compounds may show great potential in improving the emitting layers of electronic devices. The thermal behavior and heat changes of the symmetric dialkyl PPV polymers were monitored by TGA and DSC. Glass transition temperatures (Tg) for these polymers were observed as being in range of 67–108°C. The heat values of the symmetric dialkyl PPV polymers were found to be in range 220.41–599.98 J/g, and the entropy values of symmetric dialkyl PPV polymers were determined to be in range 0.567–1.754 J/ g °C.Highlights• Symmetric poly(phenylene vinylene) (PPV) derivatives were prepared.• UV–vis spectroscopy was used for optical absorption studies.• Photophysical properties were examined by photoluminescence spectroscopy.• TGA was used for the thermal behavior of symmetric dialkyl PPV polymers.• Heat changes of symmetric dialkyl PPV polymers were investigated by DSC.

A combined theoretical and experimental study of the D-2-aminobutyric acid L-norvaline single crystal for efficient third-order nonlinear optical (NLO) applications

Here we report, the formation of an optically active single crystal of D-2-aminobutyric acid L-Norvaline (ABANV) by the slow evaporation technique by co-crystallization of amino acids D-2-aminobutyric acid (ABA) and L-Norvaline together in water. An X-ray diffraction technique has been used to determine the dimensions of the single crystal unit cells. The functional group of ABANV crystal was studied using FTIR spectroscopy in order to determine its chemical structure. The FT NMR spectrum was conducted to analyze the carbon and hydrogen environments of the grown crystal. In optical absorption studies, the crystal was shown to be transparent in the visible region with a lower cut off wavelength of 236 nm, which led to an optical band gap energy Eg of 5.25 eV. In addition, using TG/DSC techniques, we have investigated the thermal stability of the grown ABANV single crystal as well as their accurate melting point. A HOMO and LUMO analysis were performed to determine the electronic charge distribution and reactivity of the molecules within the crystal. First-order molecular hyperpolarizality (β) properties of the grown crystal have been determined using the B3LYP method. The scatter graph provides an explanation of the chemical bonding of ELF and LOL, as well as the inter and intra fragment interactions.

Investigations of structural modifications, physical and optical properties of lead boro-tellurite glasses doped with europium trioxide for possible optical switching applications

This manuscript intends to the structural modifications, and physical and optical properties of a set of heavy metal alkali boro-tellurite glasses doped with Eu2O3. These glasses were produced by a conservative melt-quenching method. The existence of non-crystalline properties in the glasses was ascertained by X-ray diffraction analysis. The structural modifications were noticed by MAS-NMR spectroscopic investigation. Physical properties such as density, molar volume, oxygen packing density, average boron-boron separation, interionic distance, and polaron radius have been calculated by a suitable approach. The optical absorption studies were made through UV-visible absorption spectroscopy in the 350 nm to 800 nm wavelength range. The optical properties namely, optical energy bandgap, Urbach energy, optical basicity, electronegativity, and electric susceptibility, were also determined by using appropriate methods. The MAS-NMR spectroscopic experiments reveal that fewer BO4 units are converted to BO3 units and those NBOs are turned into bridging oxygen at a lower rate. The optical refractive index values and optical dielectric constant range from 2.241 to 2.358, and 5.0220 to 5.5601, respectively. The obtained energy band gap values (Eg(d): 3.367 eV to 3.597 eV and Eg(ind) 2.109 eV to 2.863 eV) and other significant optical parameters suggest that the investigated glasses are potential candidates for europium-doped fiber amplifier applications and possible optical switching applications.

Stability and physical properties of the K, Na and Nb doped BiFeO3 lead-free system synthesized by the solid state route

A study of the structural, dielectric, and optical properties of the lead-free ceramic compound (1-x)BiFeO3–(x)(K0.5Na0.5)NbO3 [(1-x)BFO-(x)KNN] prepared by the solid-state reaction method is presented. The structural analysis was carried out by performing Rietveld refinement on the X-ray diffraction data showing a distorted rhombohedral structure of the R3c space group for all samples. The presence of all active Raman modes confirms this result. The effect of K, Na, and Nb incorporation in the BFO structure is clearly seen in the scanning electron microscopy micrographs of the samples, and energy dispersive x-ray spectroscopy (EDS) confirms the existence of all elements. Piezoelectric force microscopy analysis reveals a well-defined domain structure for the higher concentrations of KNN as well as ferroelectric switching behavior. The room temperature dielectric response increases with KNN doping. A small hump in the dielectric permittivity vs. temperature plots is observed near 350 °C and is ascribed to a para-antiferromagnetic phase transition at the Nèel temperature. An optical absorption study revealed that tuning of the bandgap energy is possible through controlled doping, as shown in the present case, where the bandgap varies from 2.16 eV for x = 0.0 to 1.93 eV for x = 0.08. The results of this investigation demonstrate that K, Na, and Nb doping is an effective way to enhance the dielectric and optical properties of BFO.

Glycine methyl ester based Schiff base bearing long alkoxy arm and its zinc(II) and copper(II) complexes: Synthesis, mesomorphism, photoluminescence, CT-DNA interaction and DFT study

A new photoluminescent mesogenic glycine methyl ester based Schiff base ligand and its zinc(II) and copper(II) complexes have been synthesised. The compounds were characterized by elemental analyses, Fourier transform infrared spectroscopy (FT-IR), 1H, 13C nuclear magnetic resonance (NMR), ultraviolet–visible spectroscopy (UV–Vis), ESI-MS, MALDI-TOF, EDX and thermogravimetric analysis (TGA). The polarizing optical microscopy (POM) and differential scanning calorimetry (DSC) study revealed that the ligand and zinc(II) complex possess SmecticA mesophase on heating but copper(II) complex is non-mesogenic. The optical absorption and fluorescence spectroscopy studies of zinc(II) complex indicate the existence of aggregation/ de-aggregation species in the presence of non-coordinating and coordinating solvents. The Density Functional Theory (DFT) study was carried out using LanL2DZ and 6–31 G (d, p) basis set with meta-hybrid Truhlar's functional M06–2X to obtain a stable, optimized structure, HOMO-LUMO energy gap and chemical hardness. As designed, the combination of both the –CN- and -COO- linkages plays significant role to influence the mesomorphic behaviour as well as biogenic properties of the compounds. The in vitro interactions of the compounds with CT-DNA has been investigated using UV–Vis and fluorescence spectroscopy. To the best of our knowledge, this is one of the few report on DNA binding studies of mesogenic amino acid Schiff base, which is having binding constant in the same range with the gold standard compound, ethidium bromide.

Enhanced ethanol vapor sensing by 2D CuO-ZnO composite at ambient conditions

Gas sensors find application in the fields of food packaging, medical, agriculture, industries, transport, storage, and warehousing, etc. An efficient gas sensor should be sensitive and selective towards analyte, have fast response and recovery time and operate under moderate conditions. The commercial sensors have an in-built heating element. Thus, there is scope for finding materials that can function as room temperature ethanol sensors. Here, a composite of two-dimensional copper oxide (CuO) with zinc oxide (ZnO) nanoparticles is reported as a potential ethanol (EtOH) vapor sensor under ambient conditions. CuO is synthesized with the help of liquid–liquid phase separation technique yielding architecturally aligned 2D structures. ZnO nanoparticles are synthesized via microwave assisted wet chemical route separately. X-ray diffraction studies show the phase formation of pure CuO and ZnO individually. The CuO and ZnO samples are ground mechanically in 1:1 wt ratio to form a composite referred to as CZC. The optical absorption studies show bandgap of CuO structures as 1.41 eV, ZnO as 3.13 eV, and of CZC as 1.24 eV. A slurry of the composite is then coated on a glass substrate to study the resistance changes after ethanol vapor exposure. It is observed that while pure CuO is almost unresponsive to EtOH vapors, ZnO has sluggish response, the CZC samples responded fast. The best average response time is observed to be ∼ 4 sec while recovery time is ∼ 12 sec. The sensors show good reversibility as they return to their initial state without applying any heat treatment. The change in EtOH sensing in composites is attributed to the formation of heterojunction between CuO-ZnO which facilitates faster electron transfer across the heterojunction and the presence of large surface area renders an increased number of adsorption sites.

A Hybrid S‐Scheme WS2/BiOI Heterojunction Photocatalyst for Wastewater Treatment

AbstractThere is a growing demand for efficient methods to decrease hazardous organic contaminants in wastewater. Herein, a novel S‐scheme WS2/BiOI heterojunction photocatalyst was prepared by a one‐pot solvothermal process. The structural morphology, crystal structure and optical absorption studies were studied. The sugar‐like nanocrystals of WS2 are deposited on 3D‐BiOI marigold flowers, providing high surface area with increased visible light absorption. The photocatalytic activity of the synthesized materials was studied by visible light degradation of Rhodamine‐B (RhB). The optimized 20 wt.% WS2 in the composite photocatalyst (band gap 2.25 eV) exhibited a 98.9 % photocatalytic degradation activity with a rate constant of 0.0940 min−1 with performance retained after 3 consecutive recycling runs. Photoelectrochemical studies revealed excellent stability, increased separation and migration ability of charge carriers with lower charge resistance in the optimized WS2/BiOI photocatalyst.

Direct synthesis and optical absorbance studies of the first representatives of 3-ylidenemalononitrile derivatives of 2-oxopyrroles containing a phenol moiety

A direct synthesis of previously unknown 2-[5-(4-hydroxyphenyl)-2-oxo-4-phenyl-1,2-dihydro-3H-pyrrol-3-ylidene]malononitriles was carried out, which are donor–acceptor chromophores bearing phenol moiety as an electron-donor fragment. A developed synthetic route was based on the initial reaction of methoxy-substituted aryl ketones with tetracyanoethylene and resulted in 4-(4-methoxyphenyl)-4-oxo-3-butane-1,1,2,2-tetracarbonitriles, which were further converted into corresponding 2-[5-(4-methoxyphenyl)-2-oxo-1,2-dihydro-3H-pyrrol-3-ylidene]malononitriles by the action of morpholine. Treatment of the latter with boron tribromide led to demethylation of the methoxy group and formation of the desired phenolic chromophores. The obtained phenol derivatives are intensely colored compounds found to be capable to reversible high-contrast phenol-phenolate (quinoid) transformation. Their solvatochromic behavior was directly related to the donor number (DN) of the solvent and well fitted to the Kamlet-Taft model.

ZnWO4 nanorod-colloidal SnO2 quantum dots core@shell heterostructures: Efficient solar-light-driven photocatalytic degradation of tetracycline

Zinc tungsten oxide (ZW) and colloidal SnO2 quantum dots (CS) were synthesized individually by hydrothermal and wet chemical methods. ZW-CS core@shell nanorods were prepared using a sonochemical method for the enhanced photocatalytic activity of tetracycline (TC) degradation. ZW-CS core@shell nanorods were systematically characterized by structural, morphological mapping and optical techniques. All characterization techniques were synchronized to confirm the construction of core@shell nanorods. Optical absorption studies indicate an increased light-capturing efficiency along with a reduced bandgap from 3.56 to 3.23 eV, which is further supported by photoluminescence. Mapping analysis from SEM and HR-TEM evidence the presence of elements as well as a core@shell nanostructure. The optimized sample of ZW-CS 1.0 shows improved photocatalytic degradation of TC under stimulated solar light. The TC degradation efficiency by ZW-CS 1.0 core@shell nanorods was about 97% within 2 h. The formation of core@shell nanorod structure might be the reason for the better photocatalytic tetracycline degradation performance.

Electronic properties and optical absorption study of Arsene with PtSe2 type-II heterostructure: based on the first-principles calculation

Electronic properties and optical absorption study of Arsene with PtSe2 type-II heterostructure: based on the first-principles calculation

Multichromophore Integrated Panchromatic AzaBODIPYs: Synthesis, Characterization, and Exploring the Role of AzaBODIPY as an Energy and Electron Relay in Photoinduced Processes.

Panchromatic azaborondipyrromethenes directly connected with N,N-ditolylaniline (TPA) and naphthalene (Naph) at 1,7- and/or 3,5-positions of the azaBODIPY platform, 1, 2, and 3, were synthesized and the role of the individual chromophore constituents in the photo-induced energy and electron transfer processes has been investigated. Optical absorption studies have indicated that, integrating the complementary absorbers, naphthalene and TPA moieties, into the azaBODIPY core yielded broad-band capturing dyes with the absorption ranging from 250-1000 nm. Parallel electrochemical studies revealed that TPA moiety in 1 and 2 is easier to oxidize when compared to azaBODIPY moiety, which are in congruent with the computational studies indicating that TPA moiety would behave as an electron donor and azaBODIPY as an acceptor in PET processes. Steady-state fluorescence studies indicated that the photo-excitation of the TPA moiety in 2 resulted in the occurrence of PET from 1TPA* to azaBODIPY generating (TPA)2+.-(azaBODIPY)-⸱ while that of naphthalene in 3 resulted in PEnT from 1(naphthalene)* to azaBODIPY forming (Naph)2+.-(azaBODIPY)*. Interestingly, in 1, excitation of naphthalene moiety resulted in sequential PEnT from 1(naphthalene)* to azaBODIPY followed by the PET from TPA to 1(azaBODIPY)* generating a charge-separated state, (TPA)2+.-(azaBODIPY)-⸱-(Naph)2. Fluorescence lifetime studies have indicated that the electron and energy transfer process occurred in nanosecond time scales.

Modulating interfacial charge redistribution of Ni2P/CuCo2S4 p-n nano-heterojunctions for efficient electrocatalytic overall water splitting

Fabricating effective yet inexpensive catalysts is an important target in the research of water electrolysis and clean energy generation. Key challenges still remaining in this area are the rich density of surface-active sites, efficient interfacial charge transfer and improved reaction kinetics. Herein, Ni2P/CuCo2S4 p-n junctions are constructed via an in situ hydrothermal growth of Ni2P nanoparticles on CuCo2S4 nanosheets. Extensive X-ray photoelectron, optical absorption and electrochemical spectroscopy studies coupled with density functional theory calculations provide a mechanistic understanding of the electrochemical behaviour of these catalysts. The integrated Ni2P/CuCo2S4 p-n junctions, owing to the intimate interfacial interactions, offer interesting possibilities to purposively modulate the electronic structure of active sites at the interface, and thus to improve the hydrogen adsorption energetics and electrochemical reaction kinetics. As a result, the catalyst with 30 wt% Ni2P content displays high intrinsic electrocatalytic activity, requiring overpotentials of 183 and 360 mV to deliver 10 mA cm−2 for HER and 40 mA cm−2 for OER in alkaline media, respectively, far lower than those of individual Ni2P (400 and 520 mV) and CuCo2S4 (348 and 380 mV), further showing remarkable durability for 30 h. In addition, an alkaline two-electrode water electrolyzer assembled by Ni2P/CuCo2S4 nano-heterojunctions exhibits a relatively low cell potential of 1.67 V at 10 mA cm−2. These Ni2P-modified CuCo2S4 heterostructures demonstrate great potential for renewable hydrogen production technologies, including water electrolysis.

High-pressure structural transition and magnetic phase diagram of CuB2O4

Noncentrosymmetric $\mathrm{Cu}{\mathrm{B}}_{2}{\mathrm{O}}_{4}$ is a prototypical system with interacting spin sublattices of different dimensionality, exhibiting diverse magnetic structures and magnetic phase transitions. Here we report pressure-induced structural evolution of $\mathrm{Cu}{\mathrm{B}}_{2}{\mathrm{O}}_{4}$ and its effect on the magnetic phase diagram. Powder x-ray diffraction (XRD), Raman scattering, and infrared phonon measurements have been performed under high pressure at room temperature. Although XRD shows lattice stability up to $\ensuremath{\sim}25\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, Raman and IR phonon measurements reveal structural anomaly near 3 GPa, followed by a structural phase transition at $\ensuremath{\sim}12\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Pressure evolution of the zero-phonon lines in the low-temperature optical absorption studies helps identifying the symmetry change of the Cu sites modifying the crystal-field energy levels at the structural transition. High-pressure magnetization measurements have been performed with magnetic field applied perpendicular to the tetragonal $c$ axis. With increasing pressure the saturation magnetization of the commensurate weak ferromagnetic phase increases with systematic reduction of its turnup critical field. Pressure-induced systematic suppression of the dc susceptibility in this phase is attributed to the reduced critical field, indicating pressure-driven enhanced stability of the weak ferromagnetic phase at high pressure. Whereas, below 10 K the critical field of the incommensurate-commensurate transition increases with pressure, indicating enhanced stability of the incommensurate helical order at high pressure.