Au/Cu2O core/shell nanostructures with efficient photoresponses

Metal/semiconductor nanoscale heterojunctions are of pronounced attention because of their specific structures and properties that vary in their individual counterparts and anticipated applications in photo-driven fields. A modified facile method is reported in this connection for the formation of cuprous-oxide coated gold (Au/Cu₂O) nanostructures including octahedral, cuboctahedral and flower-like structures using cubic and multi-faceted gold nanostructures as the core material. The well-organized shape growth of the Cu₂O-shell is accomplished via an adequate adjustment of the ratio H₂O:NH₂OH^·HCl in the solution. The effect of nanoparticle's shape and thickness of the shell on the optical properties of truncated-octahedra, cuboctahedra and flower-like Au/Cu₂O nanostructures (having sizes within 90–230 nm) shows a bathochromatic shift in the surface plasmon resonance (SPR) band of the Au-core with the increase of shell thickness. A comparative study to correlate the photoluminescence analyses of core/shell nanostructures with their photocatalytic activities shows that truncated-octahedra and nanoflowers, bounded by {111} facets, are photocatalytically more active. On the other hand, cuboctahedra with more {100} catalytically inactive sites reveal a comparatively sharp emission peak. These photoresposes are also appeared to be affected by SPR coupling between plasmonic metal-core and semiconducting-shell.     

Optical properties of Cu+ centers in NaCl

Different aggregation-precipitation states of Cu⁺ have been characterized by absorption bands peaked at 305, 350 and 372–383 nm.The absorption bands at 372–383 nm, observed exclusively in the most doped crystal, have been associated with the Z₁₂, Z₃ excitons of CuCl microcrystals incorporated into the NaCl matrix Their positions shift to low energies with increasing concentration, as expected for a decrease in the stress over the precipitate.The Z1₁₂, Z₃ exciton bands of CuCl microcrystals precipitated in NaCl can be observed by the optical absorption spectrum without reaching saturation Therefore, this technique could be an alternative method to studies of CuCl thin-film depositions or reflectivity of CuCl single crystals.The red emission band observed at 600 nm is a long-lived emission (τ? 29 ms) at variance with the behavior reported for the Cu⁺ emission It is related to energy transfer processes from Cu⁺ to Mn²⁺.     

Phase transitions of the q-state Potts model on multiply-laced Sierpinski gaskets

We have investigated stress effects on yellow exciton states in a Cu₂O thin film sandwiched by MgO plates by measuring wavelength modulated (WM) absorption spectra and their thermal variations. In the WM absorption spectra, dispersive spectral structures owing to 2P~4P states in the yellow excitons are clearly resolved in the thin film sample. A stress due to the lattice mismatch between Cu₂O and MgO provides a large red shift of the band gap in the green excitonic system. However, in the yellow excitonic transitions, it is found that a red-shift of the 2P excitonic state is much smaller than that in the green excitonic system. This result suggests that a shallow potential minimum for the yellow excitons is built up in the Cu₂O thin film sandwiched by MgO plates.     

Oxide films of copper prepared by the oxidation of copper sulphide films

Copper sulphide films prepared by reactive evaporation, when heated in air at 500 K, oxidized to Cu₂O after a series of intermediate chemical transitions. Golden yellow coloured Cu₂O films showed a large absorption before the fundamental absorption edge. The optical band gap was found to be (2.29±0.02)eV. When these Cu₂O films were further heated they got converted to CuO and the optical band gap was found to be (2.17±0.02)eV.     

Photocatalytic performance of Cu2O and Ag/Cu2O composite octahedra prepared by a propanetriol-reduced process

Cu₂O and Ag/Cu₂O composite octahedra were synthesized via a hydrothermal reaction of CuAc₂ with urea in H₂O–propanetriol binary solution by adjusting the quantity of AgNO₃ at 180 °C for 10 h. The influence of reaction temperature and time on the morphology and phase of the products was investigated, and a possible growth mechanism of Cu₂O octahedron was also proposed. The photocatalytic activities of the as-prepared Cu₂O and Ag/Cu₂O octahedra for the degradation of methyl orange aqueous solution were studied. The results show that they are effective photocatalysts for the degradation of methyl orange, and the photocatalytic ability of Ag/Cu₂O composite is stronger than that of Cu₂O octahedra, which are expected to be useful in the treatment of wastewaters.     

Optical absorption spectrum of Cu2O-CaO-P2O5 glasses

Homogeneous CaO-P₂O₅ and Cu₂O-CaO-P₂O₅ glasses were prepared using a melt-quenched method under controlled conditions. The binary glasses were found to be colourless and transparent while the ternary glasses changed from light green to dark green as the Cu₂O content increased. From the absorption edge studies, the values of the optical band gap, E_opt and Urbach energy, ΔE were evaluated. The position of the absorption edge and hence the optical band gap were found to depend on the glass composition. Analysis of the optical band gap shows that for the binary glasses, the value increases as the content of CaO decreases, while for the ternary glasses, the value of the optical band gap increases as the content of the Cu₂O decreases. The density of the glasses was also measured and was found to increase with the increase in CaO and Cu₂O contents.     

Electronic excitations and optical response of metal nanocomposites under heavy ion implantation

The optical transmission and ion-induced luminescence under implantation of copper ions into quartz glass (a-SiO₂) have been measured to study the processes of formation of copper nanoparticles. It is shown that in situ measurements are more informative in comparison with the ordinary approach—investigation of the properties of ion-implanted nanocomposites only after implantation. A series of experiments was performed to prove that the ion-induced luminescence band at 545–550 nm is due to Cu⁺ ions dissolved in a-SiO₂. The combined use of in situ optical techniques makes it possible to monitor the states of implanted copper (metal nanoparticles and dissolved atoms) by the change in the optical absorption near the surface plasmon resonance of nanoparticles and by the intensity of ion-induced luminescence of Cu⁺ states in solid solution. It is shown that the optical bands of defects, dissolved copper, and nanoparticles can be separated within a simple linear approximation. Near the surface plasmon resonance and defect bands, ion-induced transient optical absorption has been revealed. The transient optical absorption near the surface plasmon resonance is explained by the temperature effect. The relationship between the electronic excitation, radiation-induced optical response, and the kinetics of nanoparticle formation is analyzed. Several stages of nanoparticle formation have been established: accumulation of implanted copper in solid solution, nucleation of nanoparticles, coalescence, growth of nanoparticles, and saturation of nanocomposites.     

Influence of Cu doping on the microstructure,optical properties and photoluminescence features of Cd0.9Zn0.1S nanoparticles

Cd_0.9−xZn_0.1Cu_xS (0≤x≤0.06) nanoparticles were successfully synthesized by a conventional chemical co-precipitation method at room temperature. Crystalline phases and optical absorption of the nanoparticles have been studied by X-ray diffraction (XRD) and UV–visible spectrophotometer. XRD confirms the phase singularity of the synthesized material, which also confirmed the formation of Cd–Zn–Cu–S alloy nanocrystals rather than separate nucleation or phase formation. Elemental composition was examined by the energy dispersive X-ray analysis and the microstructure was examined by scanning electron microscope. The blue shift of absorption edge below Cu=2% is responsible for dominance of Cu⁺ while at higher Cu concentration dominated Cu²⁺, d–d transition may exist. It is suggested that the addition of third metal ion (Cu²⁺/Cu⁺) is an effective way to improve the optical property and stability of the Cd_0.9Zn_0.1S solid solutions. When Cu is introduced, stretching of Cd–Zn–Cu–S bond is shifted lower wave number side from 678 cm⁻¹ (Cu=0%) to 671 cm⁻¹ (Cu=6%) due to the presence of Cu in Cd–Zn–S lattice and also the size effect. The variation in blue band emission peak from 456 nm (∼2.72 eV) to 482 nm (∼2.58 eV) by Cu-doping is corresponding to the inter-band radiation combination of photo-generated electrons and holes. Intensity of red band emission centered at 656 nm significantly increased up to Cu=4%; beyond 4% it is decreased due to the quenching of Cu concentration.     

过渡金属Fe或Ag掺杂K2Ti6O13的电子结构和光学性质

本文利用基于密度泛函理论（DFT）的第一性原理计算研究了它们的电子结构和光学性质.光学性质的计算结果和实验相一致.结果表明,Fe或Ag掺杂后,K₂Ti₆O₁₃的带隙中出现了杂质带且其带隙值变小,因而使掺杂后的K₂Ti₆O₁₃的吸收边发生红移并实现了其对可见光吸收.其中杂质带主要由Fe 3d态或Ag 4d态与Ti 3d态和O 2p态杂化而成.对于Fe掺杂的K₂Ti₆O₁₃,杂质带位于带隙中间,因此可以作为电子从价带跃迁到导带的桥梁.对于Ag掺杂的K₂Ti₆O₁₃,杂质带位于价带顶附近为受主能级,可以降低光生载流子的复合概率.实验和计算研究表明,通过Fe或Ag的掺杂可以实现了K₂Ti₆O₁₃对可见光的吸收,这对进一步研究K₂Ti₆O₁₃的光学性质具有重要意义.     

Structural,optical, FTIR and photoluminescence properties of Zn0.96−xCo0.04CuxO (x=0.03, 0.04 and 0.05) nanopowders

Un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn_0.96−xCo_0.04Cu_xO, x=0.03, 0.04 and 0.05) nanopowders have been synthesized by co-precipitation method. The synthesized samples have been characterized by powder X-ray diffraction, energy dispersive X-ray spectra, UV–Visible spectrophotometer and Fourier transform infrared spectroscopy. The calculated average crystalline size increases from 37.3 to 50.6 nm for un-hydrogenated samples from x=0.03 to 0.05 and it changes from 29.4 to 34.9 nm for hydrogenated samples. The change in lattice parameters, micro-strain, a small shift of X-ray diffraction peaks towards lower angles and reduction in energy gap reveal the substitution of Cu²⁺ ions into Zn–Co–O lattice. The hydrogenation effect reduces the particle size and induces the more uniform distribution of particles than the un-hydrogenated samples which is confirmed by SEM micrographs. Photoluminescence spectra of Zn_0.96−xCo_0.04Cu_xO system shows that red shift in near band edge ultraviolet emission from 393 to 403 nm with suppressing intensity and a blue shift in green band emission from 537 to 529 nm with enhancing intensity confirms the substitution of Cu into the Zn–Co–O lattice.     

Spectral studies on Cu ions in sodium–lead borophosphate glasses

Electron Paramagnetic Resonance (EPR) and optical absorption spectra of Cu²⁺ ions in sodium–lead borophosphate glasses doped with different concentrations of Cu²⁺ ions have been studied. EPR spectra of all the glass samples exhibit resonance signals characteristic of Cu²⁺ ions. The values of spin-Hamiltonian parameters indicate that the Cu²⁺ ions in sodium–lead borophosphate glasses are present in octahedral sites with tetragonal distortion. The optical absorption spectra of all the glass samples show a single broad band, which has been assigned to the ²B_1g→²B_2g transition of Cu²⁺ ions. The optical band gap energy (E_opt) and Urbach energy (ΔE) are calculated from their ultraviolet absorption edges. The emission bands observed in the ultraviolet and blue region are attributed to 3d⁹4s→3d¹⁰ triplet transition in Cu⁺ ion. The FT-IR spectra show that the glass system contains BO₃, BO₄ and PO₄ structural units.     

On the synthesis and properties of ternary copper oxide sulfides (Cu2O1–xSx)

We report on the sputter deposition of copper oxide sulfides Cu₂O_1–xS_x up to a composition of Cu₂O_0.61S_0.39. For higher sulfur contents the films first become amorphous and then change to Cu₂S. Within the range 0 < x < 0.39 the cubic crystal structure is maintained and the lattice constant changes linearly with the sulfur content. The composition of the films is measured by energy dispersive X‐ray fluorescence and suggests the formation of a random alloy. The transmission spectra show a clear red shift of the order of 450 meV (from Cu₂O to Cu₂O_0.61S_0.39) with an average transmission of 70%. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Water Evaporation Induced Conversion of CuSe Nanoflakes to Cu2−xSe Hierarchical Columnar Superstructures for High‐Performance Solar Cell Applications

In this work, a facile and low‐temperature water evaporation approach to prepare columnar superstructures consisting of face centered cubic (fcc) Cu_2?xSe nanoflakes stacked along 〈111〉 direction is reported. Formation of such unique stacked nanoflake assemblies is resulted from oriented attachment of isolated hexagonal CuSe nanoflakes along the 〈001〉 direction with a ripening effect driven by solvent evaporation, and then followed by a phase conversion into fcc Cu_2?xSe. Evolution from hexagonal CuSe nanoflakes to fcc Cu_2?xSe columnar superstructures results in obvious red‐shift of band‐gap absorption edge from 670 to 786 nm and dramatically decreased Raman resonance band intensity of the Se–Se stretching mode at 259 cm^?1 due to the phase conversion and composition variation. Remarkably, the Cu_2?xSe columnar superstructures are employed as low‐cost and highly efficient counter electrodes (CEs) in quantum dot sensitized solar cells, exhibiting excellent electrocatalytic activity for polysulfide electrolyte regeneration. A ZnSe/CdSe cosensitized solar cell using the Cu_2?xSe CE shows a significant increase in fill factor and short‐current density (J_SC) and yields a 128% enhancement in power conversion efficiency as compared to the traditional noble metal Pt CE.     

I–V characteristics of ZnO/Cu2O thin film n–i–p heterojunction

ZnO/Cu₂O thin film n–i–p heterojunctions were fabricated by magnetron sputtering. The microstructure, optical, and electrical properties of n-type (n‐) ZnO, insulating (i‐) ZnO, and p-type (p‐) Cu₂O films deposited on glass substrates were characterized by X-Ray diffraction (XRD), spectrophotometer, and the van der Pauw method, respectively. XRD results show that the mean grain size of i-ZnO film is much larger than that of n-ZnO film. The optical band gap energies of n-ZnO, i-ZnO, and p-Cu₂O film are 3.27, 3.47, and 2.00 eV, respectively. The carrier concentration of n-ZnO film is two orders of magnitude larger than that of p-Cu₂O film. The current–voltage (I–V) characteristics of ZnO/Cu₂O thin film n–i–p heterojunctions with different i-ZnO film thicknesses were investigated. Results show that ZnO/Cu₂O n–i–p heterojunctions have well-defined rectifying behavior. All ideality factors of these n–i–p heterojunctions are larger than 2.0. The forward bias threshold voltage and ideality factor increase when i-ZnO layer thickness increases from 100 to 200 nm. An energy band diagram was proposed to analyze the I–V characteristics of these n–i–p heterojunctions.     

Oxygen vacancy in N-doped Cu2O crystals: A density functional theory study

The N-doping effects on the electronic properties of Cu2O crystals are investigated using density functional theory. The calculated results show that N-doped Cu2O with or without oxygen vacancy exhibits different modifications of electronic band structure. In N anion-doped Cu2O, some N 2p states overlap and mix with the O 2p valence band, leading to a slight narrowing of band gap compared with the undoped Cu2O. However, it is found that the coexistence of both N impurity and oxygen vacancy contributes to band gap widening which may account for the experimentally observed optical band gap widening by N doping.     

Structural,magnetic and optical properties of diluted magnetic semiconductor (DMS) phase of Ni modified CuO nanoparticles

Control on the size of copper oxide (CuO) in the nano range is a highly motivating approach to study its multifunctional nature. The present investigation reports a sol-gel derived Ni doped CuO nanoparticles (Cu_1-xNi_xO). Rietveld refinement of the XRD spectra confirms the formation of single monoclinic phase of Cu_1-xNi_xO nanoparticles having crystallite size within the range of 19–21 nm. Raman spectra show the presence of characteristics Raman active modes and vibrational bands in the Cu_1-xNi_xO samples that corroborate the monoclinic phase of the samples as revealed by refinement of XRD data. The estimated band gap of pure CuO is found to be ∼1.43 eV, which decreases with the increase of dopant concentration into CuO matrix. This result is in line with estimated crystallite size. Magnetization curves confirm the weak ferromagnetic nature of Cu_1-xNi_xO nanoparticles which reveal the DMS phase. This weak magnetic nature may be induced in the samples due to the exchange interaction between the localized magnetic d-spins of Ni ions and carriers (holes or electrons) from the valence band of pristine CuO lattice. Replacement of Cu⁺² by Ni⁺² ions into the host CuO lattice induces the magnetization. The quantified value of squareness ratio (S < 0.5) confirms the inter-grain magnetic interactions in the Cu_1-xNi_xO nanoparticles which is also the reason of weak induced magnetization.     

Surface plasmon resonance in nano-gold antimony glass–ceramic dichroic nanocomposites: One-step synthesis and enhanced fluorescence application

A single-step melt-quench in situ thermochemical reduction technique has been used to synthesize a new series of Au° nanoparticles embedded antimony glass–ceramic (K₂O–B₂O₃–Sb₂O₃–ZnO) dichroic nanocomposites. X-ray and selected area electron diffractions manifest growth of Au° nanoparticles along (2 0 0) planes. The particle sizes obtained from X-ray diffraction patterns are found to vary in the range 4–21 nm. Dichroic behavior is attributed to the elliptical shape gold nanoparticles having aspect ratio 1.2, as observed from the transmission electron microscopy (TEM) images. The Au° nanoparticles exhibit surface plasmon resonance band (SPR) around 600 nm, which experiences red-shifts with increasing Au concentration. These nanocomposites when co-doped with Sm₂O₃ and excited at 949 nm, exhibit 2-fold intensification of 636 nm red emission transition (⁴G_5/2 → ⁶H_9/2) due to SPR induced local field enhancement of Au° nanoparticles and are promising materials for display applications.     

Excimer laser deposited CuO and Cu<Subscript>2</Subscript>O films with third-order optical nonlinearities by femtosecond <Emphasis Type="Italic">z</Emphasis>-scan measurement

By controlling the oxygen pressure, single-phase CuO and Cu₂O thin films have been obtained on quartz substrates using a pulsed laser deposition technique. The structure properties and linear optical absorption of the films were characterized by X-ray diffraction and UV–VIS spectroscopy. By performing z-scan measurements using a femtosecond laser (800 nm, 50 fs), the real and imaginary parts of the third-order nonlinear susceptibility, Re χ ⁽³⁾ and Im χ ⁽³⁾, of the films were determined. Both CuO and Cu₂O films exhibited large optical nonlinearities, which is comparable to those in some representative semiconductor films such as ZnO and GaN films using femtosecond laser excitation. Compared with Cu₂O films, the CuO films showed larger third-order nonlinear optical effects under off-resonance excitation. Furthermore, the mechanisms of the optical nonlinearities in CuO and Cu₂O films are explained in the main text. It was suggested that the reasons of the difference in their nonlinear refractive effects may be related to the different electronic structure in CuO and Cu₂O materials.     

Luminescence characteristics of cobalt doped TiO2 nanoparticles

TiO₂ nanoparticles doped with two different concentrations of Cobalt, 0.02 and 0.04 mol, are prepared by sol–gel method. The crystalline phase of the doped and undoped nanoparticles and particle sizes are observed with X-ray diffraction and transmission electron microscope. FTIR confirms the bonding interaction of Co²⁺ in TiO₂ lattice framework. The UV absorption spectra of the doped material shows two absorption peaks in the visible region related to d–d electronic transitions of Co²⁺ in TiO₂ lattice. Compared to undoped TiO₂ nanoparticles, the cobalt doped samples show a red shift in the band gap. Steady state photoluminescence spectra give emission peaks related to oxygen defects. The decrease in the intensity ratio of UV/visible emission peaks confirms distortion of structural regularity and formation of defects after doping. The intensity ratio of different visible emission peaks is nearly same for undoped and 0.02 Co²⁺. However, this ratio decreases profoundly at 0.04 Co²⁺, due to concentration quenching effect. Photoluminescence excitation spectra, recorded at 598 nm emission wavelength, give different excitation peaks associated with oxygen vacancies and Co²⁺. Time resolved photoluminescence spectra give longer decay time for doped samples, indicating longer relaxation of conduction band electrons on the defect and on dopant sites.     

Electronic properties of Bi2O3 and MoO3 and relationships to oxidation catalysis

A semiempirical atom superposition and electron delocalization molecular orbital analysis of the bonding and electronic structure of MoO₃, oxygen deficient MoO₃, and the α, β, and δ phases of Bi₂O₃ has been made. It is found that both small — e.g. MoO₆ — and large — e.g. Mo₆O₂₄ — clusters are useful models for cation electronic structure within the theory used. From the calculations, an interpretation is given for all available optical and photoemission data for the oxides. The color, conductivity, and new photoemission peak of oxygen-deficient MoO₃ conducting bronzes are found to be due to the addition of electrons to the lowest of three Mo 5d bands which are empty in MoO₃. Weakly allowed d ← d transitions in the red are responsible for the color. Strongly allowed Mo 5d ← O 2p charge transfer excitations are responsible for the optical absorption above 3.2 eV. For the bismuth oxides, three occupied bands are found showing strong Bi 6s, 6p, 6d and O 2p hybridization. These bands have been seen experimentally. The highest band surprisingly has Bi 6p lone-pair character which is explained in terms of the relative Bi 6s and 6p and O 2p ionization potentials using perturbation theory. Rather similar electronic structures are found for the three phases despite their varying cation coordinations and structures. A charge transfer optical absorption edge at ～ 2.6 eV for the β form agrees well with observations reported in the literature, and similar edges should occur for the other phases. The cubic δ form has an unusual low-lying band suggesting absorption in the infrared. Our results provide insight into the surface properties of these oxides.